Additive Manufacturing (AM) has been associated with the future of manufacturing since its inception. While it does hold several advantages in complex geometries and low-volume production, modern AM systems have yet to make significant in-roads with direct-print parts in industries like automotive manufacturing which are characterized by large volumes of relatively large metal parts. With typical volumes of 1,000+ vehicles per line per day, the required throughput of automotive factory lines outstrips what is available in current AM systems. Further, the large volume of parts allows for rapid amortization of capital equipment such as tools, dies, and stamping presses, circumventing one of the primary advantages of AM: reduced per-piece cost. However, if the focus is shifted from direct-print parts to manufacturing the supporting infrastructure, tools and dies, the economic benefits are regained. Using AM to manufacture forming tools for Advanced High-Strength Steels (AHSS) and Press Hardened Steels (PHS) brings the opportunity for reduced tooling lead-time, reduced tooling cost, and optimization of the tools for weight, strength, and thermal management.
Metal AM: Application in Traditional Forming
While AM enables the flexible production of tools with lead-time reduction and minimal economic impactsG-35, W-29, the production cost for the metal AM tools is significantly higher compared to polymer AM tools and, in some cases, metal tools manufactured by conventional methods. However, cost of the AM tools can be mitigated through topology optimization.A-61 With topology optimization, parts with freeform geometry can be designed that are optimized against a specific objective, for example weight, strength, or stiffness. By reducing overall tool material while maintaining strength, build time on the AM system can be reduced, thereby lowering tool cost.
Potential applications of metal AM forming tools are in prototype construction or small series production, e.g. holders, flanges or medium-size adapters and reinforcing plates.S-74 AM methods have also been utilized for insertion of beads or other geometries for reinforcing/increasing the stiffness of tools.L-36 Cost typically prevents metal AM tools from being used in low-volume cold forming applications where the main tool body is printed, however, high wear components and insert applications have demonstrated significant lead-time savings over traditional manufacturing methods.L-36 Metal AM may be considered in cold forming applications where lead-time is at a premium and cost concerns are secondary.
However, in instances where complex internal structures are required, the increased cost of metal AM is outstripped by the benefits it can provide over conventionally manufactured tools. One such example is a metal AM tool for white goods that utilizes high performance stainless steel for the forming surface and less expensive mild steel for the underlying structure. The resulting die, Figure 6, was constructed from less material, reducing overall machining time required to create the finish die surface.P-25
Figure 6: Metal AM die under construction and after nitriding.P-25
Metal AM: Application in Hot Stamping
An important advantage of hot forming is that it requires low-forming loads and enables forming parts with high strength and minimal springback. However, the high temperatures required to form the material and the precise cooling required to ensure desirable component properties necessitate advanced tooling designs.
Bulk materials used for fabricating hot stamping dies require special properties. The tool material must exhibit high tensile strength, hardness, good corrosion resistance, a low thermal expansion coefficient, and high thermal conductivity.N-19 Traditionally, casting and machining are used to manufacture hot stamping tools, however, in recent years AM has gained significant traction due to the design freedom that it offers, especially when it comes to fabricating tools with conformal cooling channels. Reducing porosity is one of the primary remaining challenges to maximizing mechanical properties and achieving good build quality in AM components. Conventionally manufactured hot tool steels demonstrate properties of at least 1300 MPa tensile strength, 50 HRC hardness, 18 J of impact toughness and 22 W/mK of thermal conductivity. Selected AM materials should demonstrate at least these properties in order to be considered a reliable alternative.
Hot stamp tooling with conformal cooling channels has been demonstrated with both Directed Energy Deposition (DED) and and Powder-Bed Fusion (PBF) AM processes. With DED processes, it is possible to attain minimum channel diameters as low as 3 mm and a minimum wall thicknesses of 2 mm. Unlike drilling straight holes, as done with traditional tool manufacturing, it is possible to design and fabricate complex cooling channels inside the die that results in homogeneous temperature distribution within the tool and the stamped parts. The improved temperature distribution leads to lower cycle times in hot stamping and subsequent improvement in process efficiency, reducing overall production costs. DED also has been combined with subtractive processes to create a hybrid manufacturing process.C-21 One example includes hot stamping dies manufactured by machining and additively building inserts with conformal cooling ducts.M-35 As a result, the additively manufactured channels cooled six times faster than the conventional drilled channels. In another example, manufactured injection molds with conformal cooling ducts by combining direct metal rapid tooling and machining.A-62
PBF processes are also used to integrate conformal cooling channels into forging dies and hot stamping tools. Regardless of the AM method, development of the internal network channels can be aided by topology optimization, a tool that offers great flexibility in designing non-intuitive, novel, and complex parts with high performance at reduced material cost.G-36 In addition to optimizing for mechanical objectives, topology optimization can also be defined such that it designs products considering performance criteria across multiple domains such as thermal and mechanical. Such multi-objective topology optimization is a powerful tool in designing metal AM tooling that takes advantage of the optimized thermal and mechanical performance made possible through AM processes.
This is an excerpt of a full Guidelines article entitled, “Additive Manufacturing for Sheet Metal Forming Tools,” which is based on a project conducted in partnership between Honda Development & Manufacturing of America, LLC and The Ohio State University. This excerpt focuses on metal AM in traditional forming and hot stamping, while the full article surveys the use of polymer and metal AM for forming tools and discusses the benefits and challenges with respect to their use in manufacturing AHSS and PHS sheet metal components. Be sure to read the full article for the much more detail.
Press hardening steels (PHS) are typically carbon-manganese-boron alloyed steels, specifically designed for hot forming process in the automotive industry. They are also commonly known as:
Press Hardening Steels (PHS)
Hot Press Forming Steels (HPF), a term more common in Asia
Boron Steel: although the name may also refer to other steels, in automotive industry boron steel is typically used for PHS
Hot Formed Steel (HF), a term more common in Europe.
Press Quenched Steels (PQS) are basically low-alloy steels, however they are specifically designed to have consistent mechanical properties after hot forming process, even at very wide cooling rate ranges.
The most common PHS grade is PHS1500. In Europe, this grade is commonly referred to as 22MnB5 or 1.5528. As received, it has ferritic-pearlitic microstructure and a yield strength between 300-600 MPa depending on the cold working. The tensile strength of as received steel can be expected to be between 450 and 750 MPa. Total elongation must be over a minimum of 10% (A80, with this minimum possibly different for A50 or A), but depending on coating type and thickness may well exceed 18% (A80), see Figure 1*. Thus, the grade can be cold formed to relatively complex geometries using certain methods and coatings. When hardened, it has a minimum yield strength of 950 MPa and tensile strength typically around 1300-1650 MPa, Figure 1.B-14 After hardening, the total elongation requirement changes from OEM to OEM. Some OEM’s require miniature specimens and ask for elongation of these specific specimens (such as A30) which cannot be comparable with A50 or A80. Typically about 3.5% to 6% minimum total elongation is required, depending on the thickness of the sheet and the type of the specimen (A30, A50 or A80) V-9.
Figure 1: Stress-Strain Curves of PHS1500 before and after quenching * (re-created after U-9, O-8, B-18)
AHSS grades are almost always named after their “minimum” tensile strength. For example, DP590 is a steel with “minimum” tensile strength of 590 MPa. However, PHS and PQS grades may be named after their “typical” tensile strength level. Thus, PHS1500 may have a minimum tensile strength of 1300 MPa, as shown in Table 1. Some companies name PHS and PQS grades with their yield and tensile strength levels, such as PHS950Y1500T. It is also common in Europe to see this steel as PHS950Y1300T, and thus aiming for a minimum tensile strength of 1300 MPa after quenching. The numbers in the commercial names may also significantly differ from the minimum and/or typical. Thus, it is always important to check the specifications to see if the numbers used in the name are showing minimum or typical strength levels.
* A is the elongation after break for a proportional specimen with L0 = 5,65 √S0
** The minimum requirement may be dependent on material thickness.
*** A50, ISO Type 1 elongation after break.
“~” is used for typical values
Superscript PB means after paint bake cycle.
The PHS1500 name may also be used for the Zn-coated 20MnB8 or air hardenable 22MnSiB9-5 grades. The former is known as “direct forming with pre-cooling steel” and could be abbreviated as CR1500T-PS, PHS1500PS, PHSPS950Y1300T or similar (PS standing for Pre-Cooled Stamping). The latter grade is known as “multi-step hot forming steel” and could be abbreviated as, CR1500T-MS, PHS1500MS, PHSMS950Y1300T or similar (MS standing for Multi-Step Stamping).V-9
In the last decade, several steel makers introduced grades with higher carbon levels, leading to a tensile strength between 1800 MPa and 2000 MPa. Hydrogen induced cracking (HIC) and weldability limit applications of PHS1800, PHS1900 and PHS2000, with studies underway to develop practices which minimize or eliminate these limitations.
Lastly, there are higher energy absorbing, lower strength grades, which have improved ductility and bendability. These fall into two main groups: Press Quenched Steels (PQS) with approximate minimum tensile strength levels of 450 MPa and 550 MPa (noted as PQS450 and PQS550 in Figure 2) and higher ductility PHS grades with approximate minimum tensile strength levels of 1000 and 1200 MPa (shown as PHS1000 and PHS1200 in Figure 2).
Apart from these grades, other grades are suitable for press hardening. Several research groups and steel makers have offered special stainless-steel grades and recently developed Medium-Mn steels for hot stamping purposes. Also, one steel maker in Europe has developed a sandwich material by cladding PHS1500 with thin PQS450 layers on both sides.
Figure 2: Stress-strain curves of several PQS and PHS grades used in automotive industry, after hot stamping for full hardening* (re-created after Citations B-18, L-28, Z-7, Y-12, W-28, F-19, G-30).
PHS Grades with Tensile Strength Approximately 1500 MPa
Hot stamping as we know it today was developed in 1970s in Sweden. The most used steel since then has been 22MnB5 with slight modifications. 22MnB5 means, approximately 0.22 wt-% C, approximately (5/4) = 1.25% wt-% Mn, and B alloying.
The automotive use of this steel started in 1984 with door beams. Until 2001, the automotive use of hot stamped components was limited to door and bumper beams, made from uncoated 22MnB5, in the fully hardened condition. By the end of the 1990s, Type 1 aluminized coating was developed to address scale formation. Since then, 22MnB5 + AlSi coating has been used extensively.B-14
Although some steel makers claim 22MnB5 as a standard material, it is not listed in any international or regional (i.e., European, Asian, or American) standard. Only a similar 20MnB5 is listed in EN 10083-3.T-26, E-3 The acceptable range of chemical composition for 22MnB5 is given in Table 2.S-64, V-9
Table 2: Chemical composition limits for 22MnB5 (CR1500T-MB-DS/IS or HR1500T-MB-DS/IS) (listed in wt.%).S-64, V-9
[wt. %]
C
Mn
B
Si
P
S
Al
Ti
Nb
Cr
Mo
Ni
Cu
N
Min.
0.20
1.10
0.0020
0
0
0
0.015
0.02
–
0
0
0
0
0
Max.
0.25
1.50
0.0050
0.50
0.025
0.005
0.080
0.050
–
0.25
0.35
0.10
0.20
0.01
VDA239-500, a material recommendation from Verband Der Automotbilindustrie E.V. (VDA), is an attempt to further standardize hot stamping materials. The document was first published in December 2021. According to this standard, 22MnB5 may be delivered coated or uncoated, hot or cold rolled. Depending on these parameters, as-delivered mechanical properties may differ significantly. Steels for the indirect process, for example, has to have a higher minimum total elongation to ensure cold formability.V-9Figure 1 shows generic stress-strain curves, which may vary significantly depending on the coating and selected press hardening process.
For 22MnB5 to reach its high strength after quenching, it must be austenitized first. During heating, ferrite begins to transform to austenite at “lower transformation temperature” known as Ac1. The temperature at which the ferrite-to-austenite transformation is complete is called “upper transformation temperature,” abbreviated as Ac3. Both Ac1 and Ac3 are dependent on the heating rate and the exact chemical composition of the alloy in question. The upper transformation temperature (Ac3) for 22MnB5 is approximately 835-890 °C.D-21, H-30Austenite transforms to other microstructures as the steel is cooled. The microstructures produced from this transformation depends on the cooling rate, as seen in the continuous-cooling-transformation (CCT) curve in Figure 3. Achieving the “fully hardened” condition in PHS grades requires an almost fully martensitic microstructure. Avoiding transformation to other phases requires cooling rates exceeding a minimum threshold, called the “critical cooling rate,” which for 22MnB5 is 27 °C/s. For energy absorbing applications, there are also tailored parts with “soft zones”. In these soft zones, areas of interest will be intentionally made with other microstructures to ensure higher energy absorption.B-14
Figure 3: Continuous Cooling Transformation (CCT) curve for 22MnB5 (Published in Citation B-19, re-created after Citations M-25, V-10).
Once the parts are hot stamped and quenched over the critical cooling rate, they typically have a yield strength of 950-1200 MPa and an ultimate tensile strength between 1300 and 1700 MPa. Their hardness level is typically between 470 and 510 HV, depending on the testing methods.B-14
Once automotive parts are stamped, they are then joined to the car body in body shop. The fully assembled body known as the Body-in-White (BIW) with doors and closures, is then moved to the paint shop. Once the car is coated and painted, the BIW passes through a furnace to cure the paint. The time and temperature for this operation is called the paint bake cycle. Although the temperature and duration may be different from plant to plant, it is typically close to 170 °C for 20 minutes. Most automotive body components made from cold or hot formed steels and some aluminum grades may experience an increase in their yield strength after paint baking. The so-called Bake-Hardening Index (yield strength increase due to paint baking) is calculated based on EN 10325 or OEM standards.
In Figure 4, press hardened 22MnB5 is shown in the red curve. In this particular example, the proof strength was found to be approximately 1180 MPa. After processing through the standard 170 °C – 20 minutes bake hardening cycle, the proof strength increases to 1280 MPa (shown in the black curve).B-18 Most studies show a bake hardening increase of 100 MPa or more with press hardened 22MnB5 in industrial conditions.B-18, J-17, C-17
Figure 4: Bake hardening effect on press hardened 22MnB5. Named as BH0, is shown since there is no cold deformation pre-strain. (re-created after Citation B-18).
There are two modified versions of the 22MnB5 recently offered by several steel makers: 20MnB8 and 22MnSiB9-5. Both grades have higher Mn and Si compared to 22MnB5, as shown in Table 3.
Table 3: Chemical compositions of PHS grades with 1500 MPa tensile strength (all listed in wt.%).V-9
[wt. %]
C
Mn
B
Si
22MnB5
0.20-0.25
1.10-1.50
0.0020-0.0050
≤0.50
20MnB8
0.17-0.23
1.70-2.50
0.0020-0.0050
≤0.50
22MnSiB9-5
0.20-0.25
2.00-2.40
0.0015-0.0040
1.00-1.40
Both of these relatively recent grades are designed for Zn-based coatings and are designed for different process routes. For these reasons, many existing hot stamping lines would require some modifications to accommodate these grades.
20MnB8 has been designed for a “direct process with pre-cooling”. The main idea is to solidify the Zn coating before forming, eliminating the possibility that liquid zinc fills in the micro-cracks on the formed base metal surface, which in turn eliminates the risk of Liquid Metal Embrittlement (LME). The chemistry is modified such that the phase transformations occur later than 22MnB5. The critical cooling rate of 20MnB8 is approximately 10 °C/s. This allows the part to be transferred from the pre-cooling stage to the forming die without any phase transformations. As press hardened, the material has a typical yield strength of 1050 MPa and 1500 MPa typical tensile strength. Once bake hardened (170 °C, 20 minutes), yield strength may exceed 1100 MPa.K-22, V-24 This steel may be referred to as PHS950Y1300T-PS (Press Hardening Steel with minimum 950 MPa Yield, minimum 1300 MPa Tensile strength, for Pre-cooled Stamping) or CR1500T-MB-PS (Cold Rolled, typical 1500 MPa Tensile strength, Manganese-Boron alloyed, Pre-cooled Stamping) V-9.
22MnSiB9-5 has been developed for a transfer press process, named as “multi-step”. As quenched, the material has similar mechanical properties with 22MnB5 (Figure 5). As of 2020, there is at least one automotive part mass produced with this technology and is applied to a compact car in Germany.G-27 Although the critical cooling rate is listed as 5 °C/s, even at a cooling rate of 1 °C/s, hardness over 450HV can be achieved, as shown in Figure 6.H-27 This allows the material to be “air-hardenable” and thus, can handle a transfer press operation (hence the name multi-step) in a servo press. This material is also available with Zn coating.B-15 This steel may be referred to as PHS950Y1300T-MS (Press Hardening Steel with minimum 950 MPa Yield, minimum 1300 MPa Tensile strength, for Multi-Step process) or CR1500T-MB-MS (Cold Rolled, 1500 MPa typical Tensile strength, Manganese-Boron alloyed, Multi-Step Process).
Figure 5: Engineering stress-strain curves of 1500 MPa level grades (re-created after Citations B-18, G-29, K-22)
Figure 6: Critical cooling rates of 1500 MPa level press hardening steels (re-created after Citations K-22, H-31, H-27)
Grades with Higher Ductility
Press hardened parts are extremely strong, but cannot absorb much energy. Thus, they are mostly used where intrusion resistance is required. However, newer materials for hot stamping have been developed which have higher elongation (ductility) compared to the most common 22MnB5. These materials can be used in parts where energy absorption is required. These higher energy absorbing, lower strength grades fall into two groups, as shown in Figure 7. Those at the lower strength level are commonly referred to as “Press Quenched Steels” (PQS). The products having higher strength in Figure 7 are press hardening steels since they contain boron and do increase in strength from the quenching operation. The properties listed are after the hot stamping process.
Typical tensile strength levels of 450 and 550 MPa, with minimum 12% total elongation, listed as PQS450 and PQS550.
950 to 1250 MPa tensile strength level and minimum 5% total elongation, listed as PHS1000 and PHS1200.
Figure 7: Stress-strain curves of several PQS and PHS grades used in automotive industry, after hot stamping for full hardening* (re-created after Citations B-18, Y-12).
Currently none of these grades are standardized. Most steel producers have their own nomination and standard, as summarized in Table 4. There is a document by German Association of Automotive Industry (Verband der Automobilindustrie, VDA), which specifies the incoming properties of these grades. In the standard, VDA239-500, the steel shown here as PQS450 is listed as CR500T-LA (Cold Rolled, 500 MPa typical Tensile strength, Low Alloyed). Similarly, PQS550 in this document is listed as CR600T-LA. PHS1000 and PHS1200 in this document is similar to VDA239-500’s CR1100T-MB (Cold Rolled, MPa typical Tensile strength, Manganese-Boron alloyed).V-9 Some OEMs may prefer to name these grades with respect to their yield and tensile strength together, as listed in Table 4.
Table 4: Summary of Higher Ductility grades. The terminology descriptions are not standardized.
Higher Ductility grade names are based on their properties and
terminology is derived from a possible chemistry or OEM description.
The properties listed here encompass those presented in multiple sources
and may or may not be associated with any one specific commercial grade.Y-12, T-28, G-32, M-71, S-115
Terminology
VDA 239-500
Equivalent
OEM Nomenclature
Proof Strength
(Rp0.2)
[MPa]
Tensile Strength
(Rm or σUTS)
[MPa]
Elongation
(A*)
[%]
VDA Bending
Angle **
(α) [°]
PQS450
6Mn3
CR500T-LA
PQS340Y410T
PSC340Y460T
340-500
410-650
13
>120
PQS550
6Mn6
CR600T-LA
HS550T/370Y-MP
PQS370Y550T
PSC370Y550T
370-600
550-800
12
>85
PHS1000
8MnB7
CR1100T-MB
HS1000T/800Y-MP
PHS800Y1000T
PSC750Y950T
750-1000
950-1250
7
>80
PHS1200
12MnB6
>75
9MnCr
~850
~1080
~6
~80
Higher energy absorbing grades have been under development at least since 2002. In the earliest studies, PHS 1200 was planned.R-11 Between 2007 and 2009, three new cars were introduced in Europe, having improved “energy absorbing” capacity in their hot stamped components. VW Tiguan (2007-2016) and Audi A5 Sportback (2009-2016) had soft zones in their B-pillars (Figure 8B and C). Intentionally reducing the cooling rate in these soft zone areas produces microstructures having higher elongations. In the Audi A4 (2008-2016) a total of three laser welded tailored blanks were hot stamped. The soft areas of the A4 B-pillars were made of HX340LAD+AS (HSLA steel, with AlSi coating, as delivered, min yield strength = 340 MPa, tensile strength = 410-510 MPa) as shown in Figure 8A. After the hot stamping process, HX340LAD likely had a tensile strength between 490 and 560 MPaS-65, H-32, B-20, D-22, putting it in the range of PQS450 (see Table 3). Note that these were not the only cars to have tailored hot stamped components during that time.
Figure 8: Earliest energy absorbing hot stamped B-pillars: (A) Audi A4 (2008-2016) had a laser welded tailored blank with HSLA material; (B) VW Tiguan (2007-2015) and (C) Audi A5 Sportback (2009-2016) had soft zones in their B-pillars (re-created after Citations H-32, B-20, D-22).
A 2012 studyK-25 showed that a laser welded tailored B-Pillar with 340 MPa yield strength HSLA and 22MnB5 had the best energy absorbing capacity in drop tower tests, compared to a tailored (part with a ductile soft-zone) or a monolithic part, Figure 9. As HSLA is not designed for hot stamping, most HSLA grades may have very high scatter in the final properties after hot stamping depending on the local cooling rate. Although the overall part may be cooled at an average 40 to 60 °C/s, at local spots the cooling rate may be over 80 °C/s. PQS grades are developed to have stable mechanical properties after a conventional hot stamping process, in which high local cooling rates may be possible.M-26, G-31, T-27
Figure 9: Energy absorbing capacity of B-pillars increase significantly with soft zones or laser welded tailored blank with ductile material (re-created after Citation K-25).
Conventional High-Strength and Advanced High-Strength Steels are not designed for hot stamping process. HSLA340 (minimum yield strength, as delivered) and CMn440 steel (Carbon-manganese alloyed, minimum 440 MPa tensile strength at delivery) may be softer than their as-delivered condition when heated over austenitization temperature and slowly cooled at 15 °C/s cooling rate. Furthermore, if the local cooling rate is over 60 to 80 °C/s, a significant increase in hardness (see Figure 10) and sharp decrease in elongation may be observed.D-22, T-27 PQS550 and PHS1000 have relatively more stable mechanical properties at high cooling rates.S-116, S117
Figure 10: Vickers hardness variation of several cold stamping steels after austenitization and at different cooling rates (re-created using data from Citations D-22, S-116, and S117).
PQS grades have been in use at latest since 2014. One of the earliest cars to announce using PQS450 was VolvoXC90. There are six components (three right + three left), laser welded tailored blanks with PQS450, as shown Figure 11.L-29 Since then, many carmakers started to use PQS450 or PQS550 in their car bodies. These include:
Fiat 500X: Patchwork supported, laser welded tailored rear side member with PQS450 in crush zonesD-23,
Fiat Tipo (Hatchback and Station Wagon versions): similar rear side member with PQS450B-14,
Renault Scenic 3: laser welded tailored B-pillar with PQS550 in the lower sectionF-19,
Chrysler Pacifica: five-piece front door ring with PQS550 in the lower section of the B-Pillar areaT-29, and
Chrysler Ram: six-piece front door ring with PQS550 in the lower section of the B-Pillar area.R-3
Figure 11: Use of laser welded tailored blanks with PQS and PHS grades in 2nd generation Volvo XC90 (re-created after Citation L-29).
Several car makers use PQS grades to facilitate joining of components. The B-Pillar of the Jaguar I-PACE electric SUV is made of PQS450, with a PHS1500 patch that is spot welded before hot stamping, creating the patchwork blank shown in Figure 12a.B-21 Early PQS applications involved a laser welded tailored blank with PHS 1500. Since 2014, Mercedes hot stamped PQS550 blanks not combined with PHS1500. Figure 12b shows such components on the Mercedes C-Class.K-26
Figure 12: Recent PQS applications: (a) 2018 Jaguar I-PACE uses a patchwork B-pillar with PQS450 master blank and PHS1500 patchB-21, (b) 2014 Mercedes C-Class has a number of PQS550 components that are not laser welded to PHS1500.K-26
PHS1000 is also used for energy absorbing purposes, as well as facilitating weldability. Figure 13 shows some of the recent examples of PHS1000 usage.
Figure 13: PHS1000 applications: (a) Door ring of Tesla Model Y (SOP 2020)B-79,A-84; (b) Door ring of Voyah Dream (SOP 2022)H-70.
Whereas the microstructure of PHS with a targeted strength of 1500 MPa is martensite, the microstructure of PQS grades contain a combination of ferrite, martensite, and bainite. This indicates that these sections require different thermal profiles.
A publication from 2023 compares the effects of two thermal profiles on the properties of a PQS grade with a mating PHS 1500 grade, as indicated in Table 5. There is little effect on the PHS 1500 properties, but the second profile shows better ductility and a smaller thickness of the Al-Si alloy layer – both of which are preferred.L-74
Table 5: Performance comparison of two processes.L-74
Process
HC370 / 550HS + AS
HC950 / 1300HS + AS
Yield Strength
(MPa)
Tensile Strength
(MPa)
Elongation
(%)
Al-Si Layer Thickness
(µm)
Yield Strength
(MPa)
Tensile Strength
(MPa)
Elongation
(%)
Al-Si Layer Thickness
(µm)
1
509
640
14.00
14
1189
1500
6.74
18
2
365
606
24.72
8
1180
1492
7.72
8
PHS Grades over 1500 MPa
The most commonly used press hardening steels have 1500 MPa tensile strength, but are not the only optionsR-11, with 4 levels between 1700 and 2000 MPa tensile strength available or in development as shown in Figure 14. Hydrogen induced cracking (HIC) and weldability problems limit widespread use in automotive applications, with studies underway to develop practices which minimize or eliminate these limitations.
Figure 14: PHS grades over 1500 MPa tensile strength, compared with the common PHS1500 (re-created after Citations B-18, W-28, Z-7, L-30, L-28, B-14, O-15).
Mazda Motor Corporation was the first vehicle manufacturer to use higher strength boron steels, with the 2011 CX-5 using 1,800MPa tensile strength reinforcements in front and rear bumpers, Figure 15. According to Mazda, the new material saved 4.8 kg per vehicle. The chemistry of the steel is Nb modified 30MnB5.H-33, M-28Figure 16 shows the comparison of bumper beams with PHS1500 and PHS1800. With the higher strength material, it was possible to save 12.5% weight with equal performance.H-33
Figure 15: Bumper beam reinforcements of Mazda CX-5 (SOP 2011) are the first automotive applications of higher strength (>1500 MPa) press hardening steels.M-28
Figure 16: Performance comparison of bumper beams with PHS1500 and PHS1800 (re-created after H-33).
PHS 1800 is also used in the 2022 Genesis Electrified G80 (G80EV) and the new G90, both from Hyundai Motor. A specialized method lowering the heating furnace temperature by more than 50℃ limits the penetration of hydrogen into the blanks, minimizing the risk of hydrogen embrittlement. L-64.
MBW 1900 is the commercial name for a press hardening steel with 1900 MPa tensile strength. An MBW 1900 B-pillar with correct properties can save 22% weight compared to 590DP and yet may cost 9% less than the original Dual-Phase design.H-34 Ford had also demonstrated that by using MBW 1900 instead of PHS 1500, a further 15% weight could be saved.L-30 Since 2019, VW’s electric vehicle ID.3 has two seat crossbeams made of MBW 1900 steel, as seen in Figure 15.L-31 The components are part of MEB platform (Modularer E-Antriebs-Baukasten – modular electric-drive toolkit) and may be used in other VW Group EVs. MBW 1900 can be ordered as uncoated or AS Pro coated (Aluminum-Silicon with Magnesium).T-50
Figure 17: Underbody of VW ID3 (part of MEB platform).L-31
USIBOR 2000 is the commercial name given to a steel grade similar to 37MnB4 with an AlSi (AS) coating. Here, it will be described as PHS2000+AS.. Final properties are expected only after paint baking cycle, and the parts made with this grade may be brittle before paint bake.B-32 In June 2020, Chinese Great Wall Motors started using PHS2000+AS in the Haval H6 SUV.V-12
Since 2023, PHS2000+AS has been used in the A-pillar of Stellantis’ Maserati Grecale SUV. The design allowed downgauging from 1.4 mm PHS1500 to 1.3 mm PHS2000 with comparable crash performance. This weight savings corresponds to 0.8 kg per car. B-80 In late 2023, Toyota introduced the 2nd generation C-HR. This vehicle has a 2 GPa B-pillar with no additional reinforcement. The previous generation had PHS1500 B-pillar with a secondary reinforcement also from PHS1500. The new design not only saved 3.5 kg per vehicle, but also saved cost.A-83See Figure 18 for these applications. PHS2000+AS is also used as a patch in the door ring of Voyah Dream, as pictured in Figure 13b.H-70
Figure 18: PHS2000+AS applications: (a) A-pillar of Maserati GrecaleB-80 and (b) B-pillar of Toyota C-HR.D-45
HPF 2000, another commercial name, is used in a number of component-based examples, and also in the Renault EOLAB concept car.L-28, R-12 An 1800 MPa grade is under development.P-22 Docol PHS 1800, a commercial grade approximating 30MnB5, has been in production, with Docol PHS 2000 in development.S-66 PHS-ultraform 2000, a commercial name for a Zn (GI) coated blank, is suited for the indirect process.V-11
General Motors China, together with several still mills across the country, have developed two new PHS grades: PHS 1700 (20MnCr) and PHS2000 (34MnBV). 20MnCr uses Cr alloying to improve hardenability and oxidation resistance. This grade can be hot formed without a coating, and thus named as coating free PHS (CFPHS). The furnace has to be conditioned with N2 gas. The final part has high corrosion resistance, approximately 9% total elongation (see Figure 12) and high bendability (see Table 4). 34MnBV on the other hand, has a thin AlSi coating (20g/m2 on each side). Compared with the typical thickness of AlSi coatings (30 to 75 g/m2 on each side), thinner coatings are preferred for bendability (see Table 6).W-28More information about these oxidation resistant PHS grades, as well as a 1200 MPa version intended for applications benefiting from enhanced crash energy absorption, can be found in Citation L-60.
A research team, including Volkswagen Group Innovation, have recently developed 37SiB6 steel. They named the grade as SIBORA (Silicon – Boron alloyed steel with Retained Austenite). The grade when hot stamped has a yield strength of approximately 1600 MPa and a tensile strength of 2050 MPa. The microstructure has 3-5% retained austenite, giving it a very high total elongation of 10% (A80, measured on an ISO Type 2, 20×80 tensile specimen). The research team also developed a process called BQP, Bainitizing, Quenching and Partitioning. With this method, several strength-ductility levels can be achieved from the same alloy.O-15 BQP process is explained in the next section under Single Alloy Concepts.
Figure 19: 37SiB6, also known as SIBORA, before and after hot stamping (re-created after O-15).
Single Alloy Concepts
In 2021, a research group from China came up with the Uni-Steel proposal, where the whole car body can be built by using a single alloy concept. The material is similar to 22MnB5 but has higher Cr and Si content, and some Nb alloying. When hot stamped, the material would have a yield strength of 1400 MPa (typical) and tensile strength of 1600 MPa (thus named as PHS1600 in Figure 20a). When different heat treatments are applied, the material can be as soft as 420 MPa yield / 600 MPa tensile. This is named as HSLA in Figure 20a. The authors specified 6 different process routes, ending up 6 different material specs as shown in Figure 20. There was also a car body design made of single alloy, different strength levels, as shown in Figure 20c.L-68
Figure 20: Uni-Steel concept: (a) Different variants, b) their engineering stress-strain curves, (c) a concept BIW design using these variants (re-created after L-68).
SIBORA, or 37SiB6, can also be processed using a Bainitizing, Quenching and Partitioning (BQP) process to generate different variants. In the BQP process, the sheet metal is first austenitized around 930°C and then rapidly cooled to bainitizing temperature, between 360 and 460°C, as seen in Figure 21a. The bainitizing temperature and time affect tensile properties. By tailoring both variables, tensile strength can be altered between 1150 and 2050 MPa, total elongation can be altered between 19% and 9%, Figure 21b.O-15
Figure 21: (a) Schematic time-temperature curve of BQP process, (b) several variants of SIBORA (37SiB6) using hot stamping and BQP process routes (both re-created after L-15).
Other Steels for Press Hardening Process
In recent years, many new steel grades are under evaluation for use with the press hardening process. Few, if any, have reached mass production, and are instead in the research and development phase. These grades include:
Studies of press hardening of stainless steels primarily focus on martensitic grades (i.e., AISI SS400 series).M-36, H-42, B-40, M-37, F-30 As seen in Figure 22, martensitic stainless steels may have higher formability at elevated temperatures, compared to PHS1500 (22MnB5). Other advantages of stainless steels are:
No scale formation at high temperatures, no controlled atmosphere is required in the furnaceL-70
better corrosion resistance of the final partM-37,
allow higher heating rates (i.e., induction heating), since the blanks are uncoated (no rules for coating diffusion) F-30,L-70
possibility of air hardening – allowing the multi-step process — as seen in Figure 23aH-42,
Disadvantages include (a) higher material cost, (b) higher furnace temperature (up to around 1050-1150 °C – see Table 6), and (c) high Cr content would significantly reduce weldability.M-37, F-30 As of 2025, there are only two commercially available stainless steel grades specifically developed for press hardening process.A-85At least one more stainless steel maker have also developed a grade for hot stamping (420C), however, it was not listed in their commercial offerings, as of 2025.
Figure 22: Tensile strength and total elongation variation with temperature of (a) PHS1500 = 22MnB5M-38 and (b) martensitic stainless steel.M-36
Figure 23: (a) Critical cooling rate comparison of 22MnB5 and AISI SS410 (re-created after Citation H-42, (b) Room temperature forming limit curve comparison of DP600 and modified AISI SS410 (re-created after Citation M-37.
Final mechanical properties of stainless steels after press hardening process are typically superior to 22MnB5, in terms of elongation and energy absorbing capacity. Figure 24 illustrates engineering stress-strain curves of the two commercially available grades (1.6065 and 1.4064), and compares them with the 22MnB5 and a duplex stainless steel (Austenite + Martensite after press hardening and tempering). The duplex grade was also developed by a stainless steel maker, but is still not commercially available. These grades may also have bake hardening effect, abbreviated as BH0, as there will be no cold deformation.B-40, M-37, F-30
Figure 24: Engineering Stress-Strain curves of press hardened stainless steels, compared with 22MnB5 (re-created after Citations B-40, M-37, F-30, B-41).
Table 6: Summary of mechanical properties of press hardenable stainless steel grades. Typical values are indicated with “~”. (Table generated from Citations B-40, M-37, F-30, and D-46.)
Terminology
Furnace
Temperature
(Theat)
[°C]
Secondary
Tempering
Proof Strength
(Rp0.2)
[MPa]
Tensile Strength
(Rm or σUTS)
[MPa]
Elongation
(A*)
[%]
VDA Bending
Angle **
(α) [°]
1.4034
420C
1150
400°C, 5 min.
1100-1300
1700-1850
12-16
–
1.4003
410L
950
–
~830
~1020
~7
–
1.4021
420
1030
–
~1125
~1750
~5
–
1.4021
420
1030
600°C, 10 min.
~930
~1090
~9
–
1.6065
Modified 410
1050
–
~1200
~1600
≥7
~70
1.6065
Modified 410
950
–
~870
~1130
≥10
~100
** VDA Bending angle may depend on thickness and method of measurement (α0 or αM)
Other than these, there is also a study where 27MnB5 was cladded with AISI 304 on both top and bottom. This study is explained in detail in Composite Steels section.
Medium-Mn Steels
Medium-Mn steels typically contain 3 to 12 weight-% manganese alloying.D-27, H-30, S-80, R-16, K-35 Although these steels were originally designed for cold stamping applications, there are numerous studies related to using them in the press hardening process as well.H-30 Several advantages of medium-Mn steels in press hardening are:
Austenitization temperature may be significantly lower than compared to 22MnB5, as indicated in Figure 19.H-30, S-80 Thus, using medium-Mn steels may save energy in heating process.M-39 Lower heating temperature may also help reducing the liquid-metal embrittlement risk of Zn-coated blanks. It also may reduce oxidation and decarburization of uncoated blanks.S-80
Martensitic transformation can occur at low cooling rates. Simpler dies could be used with less or no cooling channels. In some grades, air hardening may be possible. Thus, multi-step process could be employed.S-80, B-14
Some retained austenite may be present at the final part, which can enhance the elongation, through the TRIP effect. This, in turn, improves toughness significantly.S-80, B-14
Figure 25: Effect of Mn content on equilibrium transformation temperatures (re-created after Citations H-30, B-14).
The change in transformation temperatures with Mn-alloying was calculated using ThermoCalc software.H-30 As seen in Figure 25, as Mn alloying is increased, austenitization temperatures are lowered.H-30 For typical 22MnB5 stamping containing 1.1 to 1.5 % Mn, furnace temperature is typically set at 930 °C in mass production. The multi-step material 22MnSiB9-5 has slightly higher Mn levels (2.0 to 2.4 %), so the furnace temperature could be reduced to 890 °C. As also indicated in Table 6, the furnace temperature could be further lowered to 650°C in hot forming of medium-Mn steels.
A study in the EU showed that if the maximum furnace temperature is 930 °C, which is common for 22MnB5, natural gas consumption will be around 32 m3/hr. In the study, two new medium-Mn steels were developed, one with 3 wt.% Mn and the other with 5 wt% Mn. These grades had lower austenitization temperature, and the maximum furnace set temperature could be reduced to 808 °C and 785 °C, respectively. Experimental data shows that at 808 °C natural gas consumption was reduced to 19 m3/hr, and at 785 °C to 17 m3/hr.M-39 In Figure 26, experimental data is plotted with a curve fit. Based on this model, it was estimated that by using 22MnSiB9-5, furnace gas consumption may be reduced by 15%.
Figure 26: Effect of maximum furnace set temperature (at the highest temperature furnace zone) on natural gas consumption (raw data from Citation M-39).
Lower heating temperature of medium-Mn steels may also help reducing the liquid-metal embrittlement risk of Zn-coated blanks. It also may reduce oxidation and decarburization of uncoated blanks.S-80
Medium-Mn steels may have high yield-point elongation (YPE), with reports of more than 5% after hot stamping. Mechanical properties may be sensitive to small changes in temperature profile. As seen in Figure 27, all studies with medium-Mn steel have a unique stress-strain curve after press hardening. This can be explained by:
differences in the chemistry,
thermomechanical history of the sheet prior to hot stamping,
heating rate, heating temperature (see Figure 28), and soaking time, and
secondary heat treatments such as quenching and partitioning.W-41
In a recent study, various heating temperatures were examined. As seen in Figure 28, both the stress-strain behavior and the phase fractions change significantly with heating temperature.W-42
Figure 27: Engineering Stress-Strain curves of several press hardened medium-Mn steels, compared with 22MnB5. See Table 7 for an explanation of each tested material (re-created after Citations S-80,L-37, W-30, L-38).
Figure 28: Engineering stress-strain curves of 4% Mn steel, heated to different temperature (760. 800 and 840°C). Phase fractions are also affected by the heating temperature (re-created after W-42).
Table 7: Summary of mechanical properties of press hardenable Medium-Mn grades shown in Figure 27.
Typical values are indicated with “~”.
Toughness is calculated as the area under the engineering stress-strain curve.
Items 4 and 5 also were annealed at different temperatures and therefore have different thermomechanical history.
Note that these grades are not commercially available.
Based on Citations L-38, W-30, L-37, S-80, and W-42.
TriBond ® is the name given to a family of steel composites.T-32 Here, three slabs (one core material (60 to 80% of the thickness) and two cladding layers) are surface prepared, stacked on top of each other, and welded around the edges. The stack is hot rolled to thickness. Cold rolling could also be applied. Initially, TriBond ® was designed for wear-resistant cladding and ductile core materials.
The original design was optimized for hot stamping.B-14 The core material, where bending strains are lower than the outer layers, is made from generic 22MnB5 (PHS1500). Outer layers are made with PQS450. The stack is cold rolled, annealed and AlSi coated.Z-9 Two grades are developed, differing by the thickness distribution between the layers, as shown in Figure 29.R-14
Figure 29: Sample microsections of the conventional hot stamping grade PHS1500+AS, the high strength composite Tribond® 1400 and the high energy absorbing composite Tribond® 1200. The Tribond® 1200 microsection is experimental and is taken from Citation R-14. The other two images are renditions created by the author for explanation purposes. (re-created after Citations R-14, R-15)
Total elongation of the composite steel is not improved, compared to PHS1500, as shown in Figure 30. The main advantage of the composite steels is their higher bendability, as seen in Table 8. Crashboxes, front and rear rails, seat crossmembers and similar components experience axial crush loading in the event of a crash. In axial crush, Tribond® 1200 saved 15% weight compared to DP780 (CR440Y780T-DP). The bending loading mode effects B-pillars, bumper beams, rocker (sill) reinforcements, side impact door beams, and similar components during a crash. In this bending mode, Tribond® 1400 saved 8 to 10% weight compared to regular PHS1500. Lightweighting cost with Tribond® 1400 was calculated as €1.50/kgsaved.G-37, P-26
Figure 30: Engineering Stress-Strain curves of core layer, outer layer and the composite steel (re-created after Citation P-26).
In 2021, two German universities developed a similar composite steel, this time the core was 27MnCrB5-2 with stainless steel 1.4301 (AISI 304) claddings at the top and bottom. The details of the study is summarized in Table 8.K-63
Table 8: Summary of composite steels and comparison with conventional PHS and PQS grades.
Typical values are indicated with “~”. (Table re-created after Citations B-14 and K-63.)
Grade
Composition
through
thickness
Proof Strength
(Rp0.2)
[MPa]
Tensile Strength
(Rm or σUTS)
[MPa]
Elongation
(A80)
[%]
Bending
angle (α) [°]
PQS450
100% PQS450
~400
~550
~17
140-155
Tribond ® 1200
20% PQS450
60% PHS1500
20% PQS450
≥730
≥1100
≥5
≥135
Tribond ® 1400
10% PQS450
80% PHS1500
10% PQS450
≥890
≥1300
≥5
≥75
PHS1500
100% PHS1500
≥1000
≥1400
≥5
≥55
AISI304
100% AISI 304
~300
~640
–
–
Composite Steel
12.5% AISI 304
75% 27MnCrB5-2
12.5% AISI304
~820
1300
–
–
27MnCrB5-2
100% ‘7MnCrB5-2
~1020
~1500
–
–
* Graphs in this article are for information purposes only. Production materials may have different curves. Consult the Certified Mill Test Report and/or characterize your current material with an appropriate test (such as a tensile, bending, hole expansion, or crash test) test to get the material data pertaining to your current stock.
For more information on Press Hardened Steels, see these pages:
Press hardening (also commonly known as hot stamping or hot press forming) combines forming and heat treatment technology to create high strength complex shapes. Elevated temperature allows for production of complex shapes. This can be achieved by 11 different processes. The two most common processes are the indirect and direct processes. To summarize:
Forming from the “as delivered” ferritic-pearlitic microstructure (i.e., at ❶ in Figure 1). The formed part is later heated in a special furnace and quenched in a cooled press die. This process is commonly known as “Indirect Hot Stamping”, “Form Hardening” or “Post Form Heat Treatment”.
Forming after heating a blank to over 880 °C to create an austenitic microstructure (i.e., at ❷ in Figure 1). The process is known as “Direct Hot Stamping”, “Press Hardening” or “Hot Press Forming”. Although we define Press Hardening as a synonym to “Direct Hot Stamping” in this text, the term Press Hardening is also used throughout these AHSS Application Guidelines in a broader sense to describe 11 different hot forming and quenching processes, including Direct Hot Stamping.
Selection of the process type usually depends on the coating and part complexity.
The required press forces for Press Hardening are relatively low – compared to the final part strength – and springback is significantly reduced, if not eliminated. The parts get their final properties during the quenching cycle.
In Press Hardening, OEMs require the final mechanical properties within the formed part to be guaranteed by the part supplier. In contrast, in cold stamping, the mechanical properties of the incoming sheet steel must be guaranteed by the steel supplier.
If the cooling rate is faster than the critical cooling rate of the steel, the final microstructure will be almost fully martensitic producing what is called the “Full Hard” (FH) condition. Depending on a part design, “soft zones” may be required. In this case, the heat treatment is controlled such that the soft zones have other phases such as bainite and in some cases ferrite and pearlite. The process of tailoring the microstructure distribution is called as “tailored tempering” (a common term in Europe) or “tailored properties”. Parts with tailored properties may be described as a “multi-strength part” or a “tailored-part”.
Figure 1: A visual overview of the microstructural and property changes occurring during the press hardening process. In the direct press hardening process, forming is done at state ❶, whereas the indirect process occurs at ❷. Re-created after B-14.
PHS1500 (see PHS Grades with TS approximately 1500 MPa on link page): refers to the first and most commonly used PHS grade 22MnB5 or the recently developed grades like 20MnB8 used in the “pre-cooled direct process” or the air-hardening 22MnSiB9-5 used in the “multi-step process.” These grades have a typical tensile strength of 1,500 MPa (possibly a range of 1300-1650 MPa) after quenching.
Some automotive components may require higher stiffness, strength, or energy absorbing capacity in a confined local area. One method to achieve these characteristics is to spot weld separate reinforcement panels to the main component. A strategy for improving energy absorption in high-strength components involves joining a second part made from a more ductile but (typically) lower strength material. Neither of these approaches are ideal in terms of manufacturing efficiency and product/process optimization.
Tailored parts are the term given to those parts that may have zones with different thickness, chemistry, and/or heat treatment, resulting in a reduced number of components, weight reduction, and/or lower costs. These goals are achieved through part consolidation and by reducing or in some cases even eliminating joining operations.
In cold stamping operations, tailored parts (tailored products) are typically produced at the incoming coil or blank level, but are typically called “tailored blanks” or the specific process/product produced:
Laser Welded Tailored Blanks (LWTB, also known as Tailor Welded Blanks, or TWB) or Tailor Welded Coils (TWC – not common in press hardening),
Tailor Rolled Blanks (TRB) or Tailor Rolled Coils (TRC),
Tailor Welded Tubes (TWT) or Tailor Rolled Tubes (TRT), or
Patchwork Blanks,
In press hardened components, a single component may be press hardened such that it has what are known as soft zones, or areas of lower hardness possessing increased ductility. The tailored processing of these multi-strength parts can be achieved byB-14:
Controlling the incoming blank temperature, (Tailored Heating, pre-process)
Controlling the quenching rate, (Tailored Quenching, during process)
Partially tempering (Tailored Tempering, post-process). Note that Tailored Tempering is a term used for Tailored Parts, especially in texts translated from the German language.
These are typically called tailored parts or tailor processed parts. Lastly it is also possible to combine two techniques, that is, making a tailored part using a tailored blank.
Laser Welded Tailored Blanks (LWTB) are blanks that are produced by laser butt welding of two or more sub-blanks, as shown in Figure 1. In the industry, the terms Tailor Welded Blanks (TWB) and Laser Welded Blanks (LWB) are also used interchangably.M-46
Figure 1: Steps of making a press hardened laser welded tailored blank (re-created after Citations B-14 and A-8).
Laser welded tailored blanks consist of at least two sub-blanks with:
Different thicknesses, allowing for use of thinner sheet steels in areas of the component having less rigorous loading requirements. Using thinner sheets saves weight.
Different grades, optimizing the energy absorption and intrusion resistance characteristics in each area of the same part (such as an automotive B-pillar, see Figure 6b).
A combination of both.
Laser welded tailored blanks with the same thickness and grade are used to create blanks having dimensions larger than mill rolling or processing capabilities. Another reason for such a tailored blank could be optimizing the material yield ratio by nesting (Figure 10c).
Laser welded tailored blanks offer many paths to weight savings and cost reduction, including:
Reducing the number of parts in the subassembly, such as the need for reinforcements;
Reducing the number of required forming tools, welding fixtures, etc.; and
Improved raw material utilization by sub-blank nesting optimization (see Figure 1 and Figure 10c).
The weld area of press hardened laser welded tailored blanks may not transform to martensite, and therefore may show a significant reduction in the hardness. This can be attributed to weld quality and quenching rate.B-47, W-3
Blanked edge geometry of the sub-blanks (notches, underfillings and weld seam pollution) affects weld quality. Separating the blanking into two operations, rough blanking and precision blanking, may improve blanked edge geometry and the resulting weld quality.M-46
As seen in Figure 1, Aluminium-Silicon (AS) coated sub-blanks may require a secondary ablation operation. The AS coating is removed (ablated) near the weld edge, typically by using a laser. When this AS coating is not removed and filler wire is not used, the aluminium from the coating may pollute the weld. When welding two AS coated PHS1500 sub-blanks together, an aluminium-polluted weld may have significantly lower hardness, as shown in Figure 2. A part made of such a blank will fail at the weld zone, both in quasi-static and dynamic conditions.E-8
Figure 2: Effect of ablation and filler wire on hardness distribution around the laser weld of equal thickness PHS1500+AS150 (re-created after Citation E-8).
Prior to 2023, the European Standard for Laser Welded Tailored Blanks (LWTB), EN 10359D-2, covered only LWTBs for cold stamping materials. This standard was expanded to include press hardened laser welded blanks in 2023. The new standard clearly defines that the average hardness in the weld zone shall not drop below the soft base material’s average hardness minus 30 HV, Figure 2. Al-polluted weld cannot be accepted. The standard also defines a maximum hardness in the weld zone. This cannot be more than 15% of the hardness of the stronger base material if the UTS of the stronger base material is over 1200 MPa. If the LTWB has a maximum UTS lower than 1200 MPa, the weld seam is allowed to have a maximum average hardness of 500 HV. In the weld zone, a minimum of 9 hardness measurements are required to get the average.E-16
Another common type of laser welded tailored blank is where a press hardening steel (typically PHS1500) is welded to a press quenched steel (PQS 450 or 550). Lower strength and higher ductility should be observed in the PQS region. Without ablation, a hardness drop is observed in AS coated welded blank, seen in Figure 3a. In quasi-static tests, fracture was observed in the PQS base metal. In dynamic tests, the part failed at the weld zone. When ablation is applied, a B-pillar with a PQS base absorbs more energy compared to the welded blank without ablation.E-8 In uncoated and Zn-coated steels, ablation is not required since there is no concern about aluminium pollution in the weld.A-68, M-2Figure 3b shows the hardness distribution in the weld seam of galvanized sub-blanks.
Figure 3: Hardness distribution in PHS-PQS laser welded blanks. a) AS coated sub-blanks with and without ablation (re-created after Citation E-8); b) Galvanized sub-blanks without ablation (re-created after Citation M-2). Note that the initial thicknesses of sub-blanks are different.
There are two methods of ablation. Full ablation removes the AS coating and the interdiffusion layer (IDL) in their entirety. In contrast, partial ablation removes only the AS coating, but the IDL remains intact. Full ablation may result in oxidation and decarburization in the weld seam.E-8, W-3
In addition to weld pollution, the hardness drop in the weld seam could also be caused by the local quenching rate. When a welded blank is made using sub-blanks with different thicknesses, misalignment (Δx in Figure 4a) may lower the quenching rate. Misalignment greater than 2 mm could cause over 30% hardness drop, from approximately 500 HV to less than 350 HV.B-47 A filler wire with high-C content could reduce the critical cooling rate, as shown in the Figure 4b. In a particular example using a filler wire containing 0.3% C presented in this image, the critical cooling rate was reduced to approximately 13 °C/s. Due to the high-C content, a 20% increase in the weld seam hardness may be possible,E-8 as indicated in Figure 2.
Figure 4: a) Misalignment of the blank in the die could cause lower quenching rate in the weld seam (re-created after Citation B-47); b) A high-carbon filler wire may reduce the critical cooling rate (re-created after Citation E-8).
The fourth generation Audi A4 (2008-2016 also known as B8) contained some of the earliest applications of press hardened laser welded tailored blanks. The car had five components made of laser welded tailored blanks: tunnel reinforcement, left/right B-pillar reinforcements, and left/right rear rails, as shown in Figure 5. As PQS grades were not commercially available at that time, High-Strength Low-Alloy (HSLA) steels were used for energy absorbing applications. As delivered, HX340LAD + AS, had a minimum 340 MPa yield strength. Press hardened parts and their final mechanical properties are shown in Figure 5.S-65
Figure 5: PHS applications in Audi A4 (2008-2016). The car had a total of three different components and five parts using laser welded tailored blanks (figure and table re-created using data and images from Citations S-65, D-11, V-21, W-5, and S-13).
In Citation K-25, using a laser welded tailored blank resulted in the highest energy absorbing capacity of a B-pillar reinforcement. In this study, PHS1500 (22MnB5) was laser welded to a HC340LA (uncoated HSLA steel with minimum 340 MPa incoming yield strength). Such a welded blank could absorb 3.3 kJ energy without fracture, whereas a monolithic (same thickness, same hardness all around) PHS1500 failed at 2.3 kJ (see Figure 6). PHS1500 with soft zones (see the Tailored Properties section below) passed a 2.3 kJ test but failed at 3.3 kJ.
Figure 6: Energy absorbing capacity of B-pillars increase significantly with soft zones or laser-welded ductile material (re-created after Citation K-25).
Conventional High-Strength Steels are not designed for hot stamping process. HSLA 340 and 410 MPa grades (minimum yield strength, as delivered) and CMn440 steel (Carbon-manganese alloyed, minimum 440 MPa tensile strength at delivery) may be softer than their as-delivered condition when heated over austenitization temperature and slowly cooled at 15 °C/s cooling rate. Furthermore, if the local cooling rate is over 60 to 80 °C/s, a significant increase in hardness (see Figure 7) and sharp decrease in elongation may be observed.D-22, T-27
Figure 7: Vickers hardness variation of several cold stamping steels after austenitization and at different cooling rates (re-created using data from Citation D-22).
Development of PQS grades started around 2007, targeting consistent mechanical properties over a wide range of cooling rates. Currently, typical laser welded blank applications of PQS450 and PQS550 in the automotive industry include B-pillars, front rails, and rear rails. One such car with LWTB components is the 2nd generation Volvo XC90 (2014-Present). The car has a total of 152 kg hot stamped parts, with approximately 132 kg of PHS1500 and 20 kg PQS450, comprising 33% and 5% of the BIW (excluding doors and closures), respectively. The XC90 has a total of six hot stamped welded blanks (three left and three right), as seen in Figure 8.L-29, L-8 More details about welded blanks with PQS450 and PQS550 are presented in the Grades with Higher Ductility Section within our article on PQS Grades.
Figure 8: Use of laser welded PQS-PHS grades in the 2nd generation Volvo XC90 (re-created after Citation L-29).
Recently PHS1000 and PHS1200 grades have been developed. The yield and tensile strength of these grades increase with hot stamping, and as such are considered press hardening steels (PHS, but not PQS). Y-12, G-30 More details about these grades are presented in the Grades with Higher Ductility Section within our article on PHS and PQS Grades. Renault conducted an experimental study in 2021 to replace PHS1500-PQS550 laser welded tailored blanks with those made from a PHS2000-PHS1000 combination. As seen in Figure 9, the new materials can absorb the same amount of energy with less intrusion. At the same level of intrusion, the energy absorbing capacity improves by 30%.B-62
Figure 9: Stroke vs. energy curves of representative sub-assemblies, emulating B-pillar (re-created after Citation B-62).
Laser welded tailored blanks may also be used to create larger blanks that may not be otherwise possible or economically feasible.M-4 Door rings represent one such application for hot stamping, as introduced by ArcelorMittal in 2010.A-17 A prototype door ring was produced in 2012, using four sub-blanks, including one PQS550, as shown Figure 10a. The part measured approximately 1500 mm long and 1250 mm high.B-63, T-1 May 2013 saw the first application of a hot stamped door ring with the introduction of the 3rd generation Acura MDX, running from 2013-2020. The vehicle used a two sub-blank LWB door ring, both PHS1500, with thicknesses of 1.2 mm and 1.6 mm. Through sub-blank nesting optimization, material utilization was improved to 63%. Details can be seen in Figures 10b and 10c.M-46
Figure 10: Door rings. a) one of the earliest concepts from 2010T-1; b) the first mass produced door ring of the 2013 Acura MDX; c) sub-blank nesting to improve the material utilization.M-46
For door ring manufacturing, a higher tonnage press with larger bolster area may be required, as well as a wider furnace and heavier capacity transfer systems. In most hot stamping lines, typically two or four parts are formed and quenched in one stroke (known as 2-out or 4-out) to improve productivity and reduce the total cost per piece. Due to the large size and additional requirements, door ring manufacturing is typically 1-out. However, as the part itself replaces four components (A and B pillars, hinge pillar and rocker reinforcement), it can be as cost effective as a 4-out hot stamping operation.W-6
Although not common, the Acura TLX (1st generation 2015-2021) and Hyundai Santa Fe (since 2018, 4th generation) utilize single piece (not from a welded blank) door rings with 1.4 mm and 1.1 mm thicknesses respectively. B-14, H-4 The 2nd generation Acura TLX (2021-present) has the door-ring of the 1st generation model as a carryover.L-61
Since its inception, laser welded door rings have been used in several Honda / Acura models. The number of sub-blanks was increased to 4 with the 2nd generation Honda Ridgeline (2017-present). This was the first door ring application in a pick-up truck.B-52 The Chrysler Pacifica started production in 2017 with 5 sub-blanks, as shown in Figure 11a, including PQS550 for crash energy absorption.T-19 The 5th generation RAM 1500 pick-up truck, which debuted in 2018, has a six sub-blank door ring, as seen in Figure 11b.R-3 In 2018, Acura RDX became the first car to have inner and outer door rings made of PHS1500 laser welded blanks. As seen in Figures 11c and 11d, five and four sub-blanks were used respectively for the inner and outer door rings, all PHS1500. This design further allowed downgauging and lightweighting.R-26
Figure 11: Laser welded door ring applications: (a) Chrysler Pacifica (SOP 2017) has five sub-blanks (recreated after Citation T-19); (b) RAM 1500 (SOP 2018) has six sub-blanks (re-created after Citation R-3); Acura RDX was the first car to have two door rings: (c) inner and (d) outer, both with four sub-blanks of PHS1500 (re-created after R-26).
Tesla Model Y uses a door ring with 4 sub-blanks, including PHS 1000 for the energy absorbing area of the B-pillar.B-79, A-84 One of the most complicated door rings is found in the Voyah Dream, a Chinese minivan that started production in 2022. The main door-ring consists of 5 sub-blanks laser welded to each other. The base of the B-pillar is again PHS1000. In addition to these 5 sub-blanks, 2 patches are also spot welded, where one of these is PHS1900.H-70 Figure 13 of our page on PHS Grades shows these door rings.
Double door rings (see Figure 12 and Figure 13) have been long discussed. One of the earliest applications was found in Tesla Cybertruck (SOP 2023). This was the first in-house hot stamping done at Tesla’s own hot stamping lines.B-81 The second application was found in the compact BEV car, GAC Aion UT (see Figure 13a) in 2025. The full-size SUV Lynk & Co 900 may have the largest double door ring in production as of 2025 (Figure 13b). The hot stamped part has all 4 pillars (A, B, C, and D) integrated into a single piece.
Figure 12: Double door ring of Tesla Cybertruck: (a) approximate dimensions of the inner part (image re-created after M-72), (b) schematic of outer ring and (c) inner ring (re-created after F-49).
Figure 13: Double door ring applications: (a) GAC Aion UT (SOP 2025)G-50, (b) Lynk & Co 900 (SOP 2025)P-30.
A study from 2023 showed that, it was possible to save 22.2 kg per vehicle by using inner and outer double door ring – compared to a legacy design. The total part number was reduced from 14 to 2, also reducing significant area from the body shop. Material utilization was improved to 70% (by means of sub-blank nesting optimization). The total steel usage was reduced from 170.7 kg to 129.1 kg (an almost 25% improvement).G-51 Another study from 2024 showed that B-pillar-less designs can be realized with a hot stamped double door ring.B-82
Tailor Rolled Blanks
Tailor Rolled Blanks (TRB), also known as flexibly rolled blanks (FRB) or variable thickness rolled blanks (VTRB) are produced by a secondary cold rolling of an already cold rolled and possibly coated coil. In this secondary cold rolling, the roll gap is adjusted during the process so that the thickness can be varied (tailored) locally, shown in Figure 14a. TRBs can be an alternative to “same material-different thickness” welded blanks.B-14 Contrary to an LWTB, thickness changes are not abrupt, but instead are continuous. Thus, TRBs do not have stress concentration due to the notch effect. Problems associated with weld quality in welded blanks (pollution, geometry, quenching rate, etc.) do not apply to TRBs since laser welding is not utilized to create local thickness changes.H-7
Tailor rolled blanks are typically named by their thicknesses from head-to-tail, and symmetrical sections with same thicknesses are written once. For example, the B-pillars of previous generation Ford Focus (2011-2018), as shown in Figure 14b, has five thicknesses in nine zones. This blank would be named as: 1.35-2.30-2.10-2.40-2.70. The process starts with a coil slightly thicker than 2.70 mm, and thickness reductions up to 50% would be completed during the tailor rolling process. The typical slope in the Thickness Transition Zones (TTZ) are 1:100, meaning 1 mm change in thickness would require a 100 mm long TTZ. Different slopes could also be utilized.Q-7, H-8
Figure 14: a) Principle of tailor rolling process (re-created after Citation Z-5); b) thickness profile and nesting of a B-pillar in a tailor rolled coil (re-created after Citation Q-7).
The tailored rolling process squeezes and thins any coating, and possibly damages the coating as well.T-4 For this reason, TRBs are typically used in dry areas. Because of similar reasons, in AS coated TRB applications, AS150 (75 g/m2 on each side, Al-Si coating) is preferred instead of thinner coatings such as AS80.
One of the first press hardened TRB applications was the B-pillar reinforcement of the BMW X5 (2nd generation, 2006-2013). The application saved 4 kg/car, compared to a monolithic press hardened part.P-1 Other applications include: heel piece of MQB (Modularer Querbaukasten, translating from German to “Modular Transversal Toolkit”) platform cars – covering many VW Group cars with transverse engine orientation, since 2012S-107, front crossmember of MLB Evo (Modularer Längsbaukasten, translating from German to “Modular Longitudinal Toolkit”) platform cars – covering Audi vehicles with longitudinal engine orientation, since 2015H-44, and roof crossmember of the 10th generation Honda Accord (2017-present).M-7Figure 15 summarizes these. Many other OEMs use tailor rolled blanks in press hardening, some recent applications include, but not limited to:
2024 Li Mega (A pillar uppers, B-pillars, Hinge pillars)C-42
Figure 15: Several TRB applications in recent vehicles (re-created after Citations H-44, P-1, S-107, and M-7).
Patchwork Blanks
In a patchwork blank, one or more “patch blanks” (reinforcements) are overlapped with a “master blank” and spot welded. The spot-welded blanks are then heated in a furnace and hot stamped as a single piece in one stroke. The final part will have increased thickness in the areas of interest. A patchwork blank may reduce the need for post-forming assemblies of reinforcements, as seen in Figure 16. Since the spot welds are also austenitized and quenched, their hardness distribution is typically better than spot welding after hot stamping, as shown clearly in Figure 17.B-20, U-12, N-3
Figure 16: Master blank and patch geometries of a sample B-pillar: (a) before, and (b) after spot welding, (c) after hot stamping. (re-created after Citations B-14 and L-52)
Figure 17: Hardness distribution in a spot weld, comparing when spot welding is done before or after press hardening. (Re-created after Citations B-14 and U-12)
Patchwork blanks allow for the possibility of reducing the number of forming tools and the associated fixed costs. Stamping and post-process joining costs may be reduced as well, leading to a variable cost reduction. Depending on how the part is engineered, a weight savings may be achieved. These benefits come at the expense of the additional blanking operation to create the patch blanks, and the pre-process welding stations.U-12, T-42
Optimizing the initial geometry of the patch blank helps reduce these costs. One approach is to use a one-step inverse simulation in the early planning / feasibility phase. In this method, the initial outline is estimated based on deformation theory of plasticity, requiring only relatively short CPU-times (in the order of a few minutes using a modern PC), with low accuracy (up to 3 mm deviation is common). A trim optimization method is recommended during the design phase of the patch blank blanking dies. In this method, an incremental solver is used with an initially assumed blank outline. Typically, the result of one-step solution is used for the first iteration. The software then compares the outline of the patch after forming and calculates the differences with the desired geometry. Then the initial geometry is modified accordingly, and another forming simulation is carried out. These iterations continue until the deviation is less than the set tolerances. For example, in a B-pillar patch optimization, ±0.25 mm deviation may be achieved in two to three iterations.W-8, Z-12, S-108
Reducing the number of spot welds also reduces the cost of the patch blank. Minimizing the number of spot welds may also reduce the cycle time in welding stations. In some cases, it may also affect the number of spot-welding stations — thus, the initial fixed cost. However, severe wrinkles may form if using an insufficient number of spot welds. Using finite element analysis may assist in finding the optimum number of spot welds for formability.A-19 In some cases, although the part could be hot formed with a smaller number of spot welds without any problems, more spot welds are applied for crash performance.U-12
Some of the earliest patchwork PHS applications were used in the B-pillars of 3rd generation Volvo V70 (2007-2016) and Fiat 500 (2007-2024). In the Volvo V70, a total of 46 spot welds were used to create the patchwork blank, Figure 18a. Both blanks were uncoated PHS1500, with a 1.4 mm thick master blank and a 2.0 mm thick patch.L-53 In the Fiat 500, the master blank was 2 mm thick, supported by a 1 mm thick patch, both AS coated, as seen in Figure 18b.Z-13 In recent years, patchwork PHS blanks have been used in more car bodies, including but not limited to several parts in the 2nd generation Volvo XC90 (2014-Present)L-29, rear rail of the Fiat 500X (2014-2024)M-45, B-pillar of the Opel Astra K (2015-Present)K-8, B-pillar of the 2nd generation Range Rover EvoqueF-1, and several Subaru models.U-12, A-73
In the rear rail of the Fiat 500X (2015-2024), the master blank is a laser welded tailored blank, consisting of 1.5 mm PHS1500 and 1.6 mm PQS450 sub-blanks. The patch blank is 1.5 mm thick PHS1500, spot welded onto the PHS1500 portion of the laser welded tailored blank.M-45 A similar design with different thicknesses was also used in Fiat Tipo/Egea (2015-present), as shown in Figure 18c.B-14 For the Opel Astra, the master blank is a 1.3 mm thick PHS1500 with soft zones (see the Tailored Properties discussion below). The patch is a TRB with 1.00-1.95-1.00 thickness distribution.K-8 A more recent application is seen in Voyah Dream minivan’s door ring. The door ring is consisting of 5 sub-blanks laser welded to each other and two patches spot welded to the main blank. The details can be seen in Figure 18d.H-70
Figure 18: Sample automotive applications of patchwork PHS: B-pillar reinforcements of (a) 2007 Volvo V70 (re-created after Citation N-4), (b) 2007 Fiat 500 (re-created after Citation Z-13); and (c) rear rail of 2015 Fiat Tipo/Egea (Citation T-43, recreated after Citation B-14); and (d) door ring of 2022 Voyah Dream (re-created after Citation H-70).
Jaguar I-PACE is an aluminium-intensive electric SUV making its debut in 2018. In this car, the B-pillar reinforcement is made up of a patchwork blank. Contrary to most earlier applications, the master blank is a PQS450, which could be joined easily to the rest of the body by mechanical joining. The patch is PHS1500, which improves the side impact and roof crush performance.B-21 In 2018, a global Tier 1 supplier showed the possibility of using PHS2000 master blank and patch for a rear bumper beam.N-6 Using PHS1900 as a patch is also a common method – since weldability of this material after hot stamping may be problematic (See Figure 18d).
Improvements in patchwork blank technology includes the weld type and quality. Conventional resistance spot welding has been used in making patchwork blanks. There are studies on using remote laser welding for this purpose as well. In one study, joining a patchwork blank with approximately 50 welds was completed in 35 seconds using 2.2 kW laser power, and in 23 seconds using 2.8 kW.L-54 Another study showed that when laser welding is used with AS-coated blanks, weld strength is reduced by approximately 40% compared to uncoated blanks.G-1
“Overlap patch blanks” are a sub-set of patchwork blanks. As seen in Figure 19a, instead of a master and patch blanks, two (or more) sub-blanks are spot welded over an “overlap region” to create a blank like a laser welded tailored blank. The technology was initially applied in cold stamped components.P-4 Recently an international tier 1 supplier developed door rings and floor panels made from overlap patch blanks that were press hardened. As seen in Figure 19 b and c, a door ring can be created using 5 sub-blanks, including one PQS (shown in green).G-3
Figure 19: Overlap patch blanks: (a) schematic of a B-pillar blank (re-created after B-75), (b) door ring concept from outer view, and (c) inner view.G-3
One of the benefits of using overlap patch blanks is the ability to build up larger welded blanks of Al-Si coated steel without the need to employ ablation technology. The overlapped sub-blanks can be resistance spot welded together, thereby avoiding the risk of aluminum polluting the weld pool that ablation would otherwise mitigate.
Patches can be engineered to increase stiffness in critical locations, and the spot welds provide easy adjustment to both the blank and weld as needed (Figure 20).S-111
Figure 20: Overlap patch blanks created with resistance spot welding eliminates the need to use laser welding and ablation techniques.S-111
Tailored Properties
Tailored properties is a term used for the technology to make a part with hard and soft zones. Hard zones are nearly 100% martensitic, whereas soft zones have a lower percentage of martensite. This type of part may be called a “multi-strength part”. In Europe, the term “tailored tempering” may be used to denote a part with tailored properties or the technique to make such a part. In this article, the “tailored tempering” term is specifically used to describe a part which was press hardened as a whole and later locally softened to modify properties in specific areas.
Soft zones may be used for several reasons:
To improve crashworthiness: Local areas with higher ductility aid in crash energy absorption. An example B-pillar is shown in Figure 6. This type of usage is very common in B-pillars, front rails, and rear rails, as shown in Figure 8. The first application for this purpose was realized in the B-pillar of the first-generation VW Tiguan (2007-2018).S-13 The technology is also used in rear rails. Both applications are shown in Figure 21. The technology is also used in rear rails of 10th generation Honda Civic (2015-2021)C-22, and 10th generation Honda Accord (2017-2024). In this particular application, shown in Figure 22, soft zones were designed such that the rear frame would deform in a pre-defined manner and absorb the crash energy efficiently.C-22, M-7, K-52 Tailored parts are used for improved energy absorption in numerous models from Audi, BMW, Ford, Honda, Mercedes and others.B-14
Figure 21: Example uses of soft zones for improved energy absorption: (a) first application was in 1st generation (2007-2018) VW Tiguan’s B-pillars (re-created after Citations V-22 and M-8), (b) a more recent application in 2013 Ford Escape’s rear rails (known as Ford Kuga in EU, sold between 2013 and 2019) (re-created after Citation M-59).
Figure 22: Rear rail assembly of Honda Civic (10th gen., 2015-2022): (a) Isometric view of the assembly, (b) bottom view of the frame, during rear crash condition (re-created after Citation K-52). A similar design was also employed in Honda Accord.M-7
To improve weld/joint strength: When base metal hardness is over 350 HV, the heat affected zone (HAZ) in the spot weld may be the weakest point of an assembly.B-20 Several other studies have proven the hardness drop and early fractures around spot weld of fully hardened parts, as summarized in Figure 23. When flanges are induction tempered (see the Tailored Tempering discussion below, a B-pillar assembly may absorb 30% more energy than a fully hardened B-pillar.H-61, F-2 In multi-material mix cars, such as the 2nd generation Audi Q7 (2015-present), “soft flanges” can be used for mechanical joining the PHS B-pillar reinforcement to aluminium components. Hemming of aluminium, around the PHS, can also be used to join the components.H-62 Since development of TemperBox® and Thermal Printer, soft zones for improving weld quality has significantly increased. Several new applications include Renault 5 BEV.G-53 and Mercedes EQE SUV BEV.D-47, as seen in Figure 24.
Figure 23: When spot welding is done on a soft zone: (a) hardness distribution would not have a soft HAZ, and (b) early fractures at spot welds are not observed (re-created after Citations B-14, H-61, and B-64).
Figure 24: Soft zone applications to improve weld quality: (a) Mercedes EQE SUVD-47, and (b) Renault 5 BEVG-53.
For secondary bending operations: Tailored tempering (softening areas of interest after a fully hardened press hardening process) may be used in bumper beams, where a secondary bending may be required to form an inner flange.L-40
To facilitate trimming/piercing: Although not very common, local soft zones may reduce the force/energy requirements and improve the cutting tool life if hard trimming will be used.L-55
In addition to the engineered use of soft zones, it is possible that they could be produced unintentionally. While the theoretical production press hardening process results in fully martensitic content, the real product may have a multiphase microstructure. For example, there must be good and continuous contact between the sheet and the stamping tools to achieve sufficient heat extraction such that the critical cooling rate is met. However, there may be unexpected regions of poor contact preventing the required rapid cooling. This might occur at the vertical sidewalls of a channel section, or after tooling wear creates an air gap between the punch and cavity. As such, the martensite volume fraction in production PHS may not be 100%. This leads to changes in strength-ductility properties, as well as the cutting force requirements: as the martensite percentage decreases, so does the force required to cold cut processed PHS. These are reflected in Table 1.W-44
Table 1: Mechanical properties and cutting force requirements of 22MnB5 as a function of the martensite volume fraction.W-44
Phase Composition
Yield Strength(MPa)
Tensile Strength(MPa)
Elongation
(%)
Max Cutting Force
(kN)
100% Martensite
1151
1675
8.40
54.42
87% M + 13% B
1003
1529
7.23
53.31
68% M + 32% B
767
1328
9.50
45.23
52% M + 48% B
604
1057
10.28
37.09
23% M + 77% B
567
976
13.83
32.66
100% Bainite
550
809
17.39
28.69
There are three methods to create the soft zones leading to tailored properties:B-14
Tailored heating during austenitization of the blank (typically achieved in the furnace),
Tailored quenching after austenitization (can be achieved in tempering stations or in the forming die),
Tailored tempering after fully hardening a part (after the press hardening process).
1) Tailored Heating (Pre-Process)
In tailored heating, areas of interest (the soft zones) are not fully austenitized. The critical heating temperature has been reported as 750 °C by several researchers. When heated below 750 °C and hot stamped, the part has a tensile strength of approximately 600 MPa and over 15% total elongation. As seen in Figure 25, mechanical properties will stay relatively constant with heating temperatures between 650 and 750 °C. Above this critical heating temperature, hardness (almost directly proportional with tensile strength) may increase significantly.K-53
Figure 25: Effect of blank heating temperature on hardness and converted tensile strength of PHS1500 (re-created after Citation K-53).
There are several methods to achieve tailored heating. In the direct process, where an undeformed blank is being heated, there were four main methods proposed:
Using a divided furnace,
Masking soft zones in furnace,
Heating by segmented contact plates, and
Conduction heating with controlled current flow.
A divided roller hearth furnace may have gas or electric heating for the first half of its length, ensuring a uniform temperature distribution during heating. In the second half of the furnace length (soaking zone), there may be several electric heating zones across the furnace width direction that can be set to different temperatures. To simplify the schematic, Figure 26 shows a two-zone divided furnace. In the soaking zone, five-zone furnaces were already available as early as 2011. By 2018, furnaces with 32 zones were industrially used to make parts for several German OEMs.H-47, E-12, O-13
Figure 26: Divided furnace concept (simplified with two zones): (a) temperature setting in the furnace affects the temperature distribution in the soft and hard zones; (b) in the tailored soaking area, up to 32 zones may be realized (re-created after Citations B-14, E-12, and O-13).
As heating of the blank in furnace is mostly achieved by radiation, an insulating mask may reduce the local temperature in the soft zones. Ceramic insulators or machined steel blocks may be used for masking purposes. Areas that are not masked will be heated above the austenitization temperature, whereas the masked areas will be at lower temperatures.N-3Figure 27 shows a schematic of the process. In addition to masking duty, the inlay should have enough heat capacity to absorb the heat from the blank. When steel inlays (masks) are used, they should be thicker than the blank to have the necessary heat capacity. Stainless steels could be used to avoid scaling of the steel inlay.B-65, B-66
Figure 27: (a) Using masking for tailored heating (re-created after Citations B-14 and N-3); (b) an example mask and blank from Citation K-54.
Although not commonly used for mass production, it was proven that contact plates may be used in tailored heating. Blanks are isolated from the environment during contact plate heating, significantly reducing oxidation on uncoated blanks. Fraunhofer IWU in Chemnitz, Germany, has developed a lab-scale contact plate heater that can generate soft zones. In the hard zones (those heated over 900 °C), the heating rate may be as high as 300 °C/s. The heater and a sample blank are shown in Figure 28.S-109, G-48
Figure 28: Tailored heating in contact plate heating: (a) right after the heating before the discharge, (b) a tailor heated blank with dimensions and approximate temperatures (re-created after Citation S-109).
Two different strategies were developed to generate tailored heat blanks using conductive heating while ensuring no current passes through the targeted soft zones.M-60 These approaches are applicable only to rectangular blanks. Researchers in Hanover University improved the technology to heat non-rectangular blanks with tailored temperature distribution. In a sample (non-rectangular) B-pillar blank, temperature was kept at 950 °C in the heated zones and approximately at 700 °C in the soft zones. Significant temperature drops were observed in the proximity of electrodes, resulting in non-uniform heating.B-67 Neither of the techniques are used in mass production for tailored parts.
In the indirect hot stamping process, the parts are formed prior to heating. Thus, it is not practical to apply any of the earlier strategies to get a cold zone in the part. For such components, soft zones are generated by using machined steel blocks known as absorption masses, which have high heat capacity to absorb the heat from the blank. As seen in Figure 29, correctly sized absorption masses keep the soft zones below 750 °C. When quenched, these areas have approximately 500 MPa tensile strength, over 20% total elongation (A50) and over 150° bending angle according to VDA bending test. The tailored parts have narrow transition zones, and are spot weldable, both in hard and soft zones. The technology is used in the B-pillar reinforcements of several BMW models.M-2, K-53, R-27
Figure 29: Tailored heating of galvanized PHS1500 in the indirect process: (a) Blank temperature evolution in hard and soft zones, in a roller hearth furnace using absorption mass in the soft zone; (b) hardness distribution and approximate tensile strength in hard and soft zones (re-created after Citation K-53).
Tailored heating technologies are beneficial for their energy efficiency, as the soft zones are heated to lower temperatures. The technology may be applied to uncoated and Zn-coated blanks; however, AS-coated blanks are at risk for incomplete coating diffusion in the soft zones. For these reasons, similar technologies (excluding conduction heating) also are used in a secondary heating device after the furnace.O-13 These techniques are listed in the Intermediate Pre-Cooling section below.
2) Tailored Quenching
In tailored quenching methods, the whole blank is austenitized in the furnace and the cooling rate is controlled such that the soft zones cannot develop high percentages of martensite. This can be achieved by two main process routes:
Intermediate pre-cooling, where a secondary furnace is employed where the temperature of hard zones is maintained, but soft zones are allowed to cool.
In-die cooling, where a fully austenitized blank with uniform temperature distribution is placed on the tool, but the part is cooled at different cooling rates through several process routes.
Intermediate Pre-Cooling
Complete coating diffusion may not occur in AS-coated blanks subjected to tailored heating profiles. To ensure the full coating diffusion and uniformity of the coating all around the blank, the blanks must be fully austenitized and soaked in the furnace for a dwell time. One of the earliest approaches kept the hard zones in the roller hearth furnace, while extending the soft zones out of the furnace. This technology produced a part with two zones only, with a linear transition zone (Figure 30). AS-coating is fully developed for weldability and e-coat adhesion. Tailored properties are reproducible. For this furnace-extending method, no extra investment is necessary other than automation programming.L-56
Figure 30: Simplest pre-cooling technology: extending the soft zones out of the furnace. (a) Schematic of extending out of furnace (not to scale, from Citation B-55), (b) B-pillars made by this technology.A-74
Intermediate pre-cooling can also be done in a divided furnace. In this case, contrary to Figure 26a, the uniform heating temperature is set over 885 °C. The soft zone area is then set to a lower temperature and thus pre-cooled. The rear rails of the 2013 Ford Escape (also known as Kuga) shown in Figure 21b are produced with this technique.M-59
Most of the tailored heating strategies discussed so far are suitable only for larger soft zone areas, but not for small areas. Intermediate pre-cooling by extending out of furnace and pre-cooling using a divided furnace strategy can only produce a two-zone tailored part, such as in Figure 31b. Since 2011, there has been an interest in producing three-zone tailored parts. By 2015, the Audi Q7 employed a three zone B-pillar with soft flanges for joining purposes. Soft zones for spot weld purposes are also commonly employed.H-62, A-74, B-68
Figure 31: Tailored B-pillar evolution: (a) monolithic, (b) two-zones tailored, (c) three-zones tailored, (d) three zones + soft flanges, (e) three zones + soft spots (re-created after Citations A-74, B-68, P-3).
To address these challenges, several intermediate cooling systems have been developed. AP&T uses multi-layer furnaces, with an addition of a TemperBox®. The blanks are austenitized in the multi-layer furnace. Before being fed into the press, the blanks are first moved into another layer (the TemperBox®) where re-heating is done with masking. Masked areas cool below 700 °C, whereas the unmasked areas are re-heated to 930 °C. The cycle time varies between 30 and 70 seconds, depending on the thickness of the blanks (Figure 32). For continuous production, one TemperBox® supports five-chamber furnaces.K-41
Figure 32: Time-temperature evolution in the TemperBox®. (re-created after Citation K-41).
Similar technologies have been developed by other furnace makers: Schwartz has developed a thermal printer which can be a stand-alone unit or installed at the end of a roller hearth furnace.L-56
In-Die Tailored Cooling
In this process, the blanks are fully austenitized in the furnace, but the cooling rate is locally adjusted. Areas with a local cooling rate over 27 °C/s are expected to transform to nearly 100% martensite. In soft zones, cooling rates should be lower than this critical number. The cooling rate is a function of the thermal contact conductance (see Figure 34a) and the temperature gradient (ΔT) between the tool surface and the blank. Thus, lower cooling rates can be achieved byB-14, M-61:
Heated die inserts,
Die relief method, or
Local die inserts with low thermal conductivity.
If a segment of the die is heated, sections of the blank in contact with this area have a smaller temperature gradient (ΔT), leading to reduced heat flow and lower cooling rates. It is possible for this phenomenon to occur unintentionally if the dies are not cooled efficiently and hot spots are observed.B-14
In the automotive industry, heated die inserts are used typically between 300 °C and 550 °C. Electric cartridge heaters are commonly used, Figure 33a. If the inserts are heated over 420 °C (the martensite start temperature for 22MnB5), no martensite formation occurs while the blank is in contact with the dies. For productivity purposes, sheets should stay in the dies as short as possible. After industrial quenching times (10-15 seconds), soft zones may still have phase transformation during air cooling in the exit conveyor. This may cause distortion in the final part. One simulation study found that 80 seconds of air cooling was needed to transform all the austenite into other phases.B-14, M-61, B-69, B-70
Figure 33: Tailored parts with heated die inserts: (a) Simulation model with cooling channels in hard zones and heating in soft zones, (b) phase transformation may continue in soft zones.B-70
This process has been applied as early as 2009 (if not earlier) in the Audi A5 Sportback.B-20 The car had a three-zone B-pillar, similar to the sketch in Figure 31c. Since then, several complicated geometries have been realized in an industrial scale with “heated die insert” technology. In 2015, the 10th generation Honda Civic was equipped with complicated rear rails, shown in Figure 22. These components are also made with heated inserts.C-22 Also debuting in 2015, the Audi Q7 was equipped with B-pillar reinforcements with soft band and soft flanges (similar to Figure 31d).H-62
Another method to get lower cooling rates is to reduce the contact pressure or introduce an air gap between the blank and the die. As seen in Figure 34a, as the contact is lost, thermal contact conductance (hc, the amount of heat passing through the unit area of blank to the tool) is reduced significantly. For example, at 5 MPa contact pressure, hc is equal to 1.5 kW/m2°K. As soon as the contact is lost, the value is less than 0.3 kW/m2°K.B-70 A Schematic showing an “air gap” design for soft flanges is presented in Figure 34c and compared with a conventional die in Figure 34b.C-4
Figure 34: (a) Thermal contact conductance is less than 0.3 kW/m2°K, once there is an air gap (Citation B-55, raw data from CitationsO-14 and M-62); (b) schematic of a conventional press hardening die, (c) introducing air gap to obtain soft flanges.C-4
Use of insulated die inserts is another method to obtain tailored cooling. These reduce the heat flow from the blank to the die. Typical hot forming tool steels have a heat conductivity of 27-32 W/m2°K. When ceramic insulators with less than 6 W/m2°K conductivity are used (Figure 35a), the inserts will heat over 200 °C after a few strokes. In the meantime, tool steel temperature is around 60 °C, as they can dissipate more heat energy. The strength in the soft zones may be as low as 650 MPa, corresponding to approximately 200 HV hardness.K-55 The method may not be feasible for mass production, as the first few parts will not have the same strength/elongation level until a “steady-state” is achieved, shown in Figure 35b and 35c. In real production conditions, the production may be halted for maintenance, safety, or work hours reasons, thus every re-start will have several “out-of-spec” parts.
Figure 35: Insulating inserts: (a) experimental die set at TU Graz, (b) hardness distribution of the first part, (c) hardness distribution after a few cycles (re-created after Citations B-14 and K-55).
3) Tailored Tempering (Post-Process Annealing)
The last method for obtaining tailored properties is to produce soft zones by annealing a fully hardened part. This can be done by induction or laser, as seen in Figure 36. Post-process annealing is relatively simple to implement, as the blank is heated and quenched uniformly in press hardening line. Annealing is added as a follow-up operation, which adds cost, but gives flexibility. The number of soft zones, their geometries and mechanical properties can be varied during the project timeline. Soft zones could be adjusted for different cars/variants that share the same component but require different soft zones.B-14, G-48, J-22
With this approach, however, final properties of soft zones may vary significantly depending on the temperature-time curves. Several studies have shown yield strength may spread from 450 MPa to 1300 MPa, and tensile strength between 550 and 1350 MPa. In addition, geometric distortion may also occur, since the heating and cooling is done in a local area. Finally, surface and coating conditions may change, affecting weldability, corrosion resistance and/or e-coat adhesion.L-40, M-61, B-72
BMW has been using induction annealed B-pillars in their 3-Series Sedan/Touring (2011-2019) and X5 SUV (2013-2018)R-27, and possibly other vehicles. Volvo has studied the technology with induction annealing, Figure 36a.H-61 Benteler has been using induction annealing for secondary bending of bumper beams.L-40 Gestamp evaluated laser tempering on prototype parts, around 2016, Figure 36b.B-71 Since 2022, Gestamp has been using laser tempering, initially starting with Nissan Ariya BEV.N-29
Figure 36: Post-process tailored annealing can be done by: (a) inductionH-61, and/or (b) laser.B-71
Post Hardening Processing: Trimming
Options include setting the trim line with a developed blank, laser cutting, soft zones, hard trimming, and hot cutting.
A developed blank might be appropriate for areas which can accommodate larger tolerances.
Soft zone development to aid in easier trimming and joining is discussed in the prior section.
Laser trimming leads to improved fatigue strength and raises the failure strains, but suffers from relatively long cycle times, high capital investment and operation and maintenance cost. More powerful high-end lasers measurably reduce cycle times.
Hard trimming is not usually the best long-term option for high volume applications since the hardness of the PHS part can be about the same as the hardness of trim steels. A harder, more wear resistant tool steel would now fail by chipping.
Hard trimming creates burrs, large shear zone, and microcracks in the fracture zone. Each of these lowers the fracture strain required for failure, which lowers component crashworthiness.
Hard trimming of fully hardened PHS needs extra consideration since the scale formed on uncoated PHS may lead to abrasive wear and Al-Si coatings may stick to the tool creating galling conditions. Uncoated blanks may be hard trimmed after sandblasting. PVD coatings improve wear resistance on trimming surfaces. World-class operations which use hard trimming specify advanced powder metallurgy tool steels with advanced tool coatings.
In-die hot cutting may occur after heating but before forming, and is usually limited to approximately 90° cuts. Microstructural changes occurring during the heating and cooling cycle influences flange position tolerances.
Hot cutting may also occur after forming but before quenching. This approach requires less force and causes reduced damage to the tooling and dies since the steel is softer to cut. One challenge is that the formed part is cooling during the cutting operation, with the part changing dimensions as it cools. Hot cutting improves cycle time and may reduce capital investment. Grains of fine ferrite rather than martensite form at hot sheared edge, reducing the delayed fracture risk associated with hydrogen embrittlement.
Honda developed this process to both trim the part as well as pierce holes during hot stamping to replace time- and energy-intensive laser cutting. (Figure 37, Figure 38). Their approach uses a high-speed hydraulic system, and further reduces cooling time by spraying the part with water (see Direct Water Quenching in PHS Processes). The first production application was the 2012 Honda N-Box Center Pillar Reinforcement. H-49, S-112
Figure 37: Honda N-Box Center Pillar Reinforcement created with In-Die Trimming during Hot Stamping.H-49
Figure 38: Internal Die Trimming Process at Honda. (re-created after Citations H-49 and T-40).
In its simplest explanation hot stamping consists of five operations: (1) blanking (or cutting-to-length), (2) forming, (3) heating, (4) cooling (quenching) and (5) trimming/piercing. Each process route listed below has a distinct order or type of these operations.
In most sources, hot stamping is explained with only two processes: Direct hot stamping (also known as Press Hardening) and indirect hot stamping (also known as Form Hardening). Although we define Press Hardening as a synonym to “Direct Hot Stamping” in this context, the term Press Hardening is also used throughout these AHSS Application Guidelines in a broader sense to collectively describe the different hot forming and quenching processes, including these 11 processes for part manufacturing:
Direct Process (Blanking > Heating > Forming > Quenching > Trimming > [Surface Conditioning if uncoated]) a. Direct Process with Direct Water Quenching
The most common process route in hot stamping is still the direct process (also known as press hardening).D-20 Here, previously cut blanks are heated typically in a roller hearth or a multi-chamber furnace to over 900 °C to create a fully austenitic microstructure. Depending on the material handling system, transfer from the furnace to the press may take up 6 to 10 seconds.B-14 During this time, the blank may cool down to 700 °C.G-24 Forming is done immediately after the blanks are transferred on the die, and should be completed before the blank cools below 420 °C.G-24, K-18 The blanks are formed in hot condition (state ❷ in Figure 1), and quenched in the same die to achieve the required properties. For 22MnB5 steel, if the quenching rate is over 27 °C/s, the part will transform to almost 100% martensite. For productivity purposes, higher cooling rates are often realized.K-18 Typical cycle times for a direct process with the 22MnB5 chemistry could be between 10 and 20 seconds, depending on the thickness.B-14 Global R&D efforts target improvements in cycle time.
The process is typically used for bare/uncoated steels (UC) or AlSi coated steels (AS). Zn coated blanks (GI or GA) are not suitable for direct process, as pure Zn melts around 420 °C and GA (Zn-Fe) coatings around 530-780 °C. (see Figure 3)G-25 If forming is done with liquid Zn over the blank, microcracks may fill with Zn and lower the fatigue strength of the final part significantly.K-20 A recently developed alloy minimizes these concerns, as explained in the “Pre-cooled Direct Process” section below.
Figure 1: A visual overview of the microstructural and property changes occurring during the press hardening process. In direct press hardening process, forming is done at state ❶, whereas the indirect process occurs at ❷. Re-created after B-14.
Typical Al-Si coatings prevent scale formation and decarburization at elevated temperatures. The aluminum-rich coating contains 7% to 11 wt.% Si, and acts as a barrier to offer corrosion resistance during service.F-14 In automotive industry, typical coating weights are AS150 (75 g/m2 coating on each side) or AS80 (40 g/m2 coating on each side).A-51 Refer to our page on Al-Si coatings for more details.
Formed parts must be trimmed and pierced to final geometry. In the direct process, the most common trimming method is laser cutting. The capital expense and cycle times associated with laser trimming factor into overall part cost calculations. In most plants, every hot stamping line has 3 to 5 laser trimming machines.B-14
When using uncoated blanks, controlled atmosphere in the furnace helps avoid excessive decarburization and scale formation. Surface scale locally changes the critical cooling rate, alters the metal flow and friction, and leads to premature tool wear. When uncoated material is hot stamped, a surface conditioning step like shot blasting may be required after forming to remove the scale.A-52Varnish coatings may also be used with direct hot stamping.
The grades used with the direct process may be referred to as CR1500T-MB-DS (Cold Rolled steel with typical 1500 MPa Tensile strength, Manganese-Boron alloyed for Direct [Hot] Stamping).
Direct Water Quenching
In conventional direct process, quenching is done by pumping water through the cooling channels drilled (or cast) in upper and lower tools. Since 2012, Japanese OEM’s and tier suppliers are commonly using a modification to this process, called “direct water quenching.”T-40 In this method, upper and/or lower tools are drilled with nozzles shown in Figure 2. The technique can be used with coated blanks. N-27
Figure 2: Direct water quenching: (a) schematic of the nozzles [N-27], (b) blank on the lower tool before deformation [T-51].
This technique may lead to productivity improvements: On a 1.4 mm steel, the 10-second quenching time associated with a conventional approach was reduced to 2.5 seconds with direct water quenching.N-28
Quenching time would be especially longer with thick blanks. When patchwork blanks are used, the thickest portion of the part dictates the quenching time duration. One Japanese OEM claims that at 3.8 mm thickness, water quenching may increase the part per minute by 5 times.K-64
Typically used for galvanized blanks, indirect hot stamping, also known as form hardening, starts by cold forming the part (at ❶ in Figure 1) in a transfer press or a tandem transfer line. The direct process is limited in that only one forming die can be used– which also doubles as quenching die. However, the indirect process can accommodate multiple die stations, allowing for the production of more complicated geometries, even those with undercuts. The part has almost the final shape exiting the cold forming press, where piercings and trimming could also be completed. The formed parts are then heated in a special furnace and quenched in a second die set.B-14,K-21,F-15
BMW 7 Series (2008-2015, codenamed F01) was the first car to have Zn-coated indirect hot stamped steel in its body-in-white.P-20 Zn-based coatings are favored for their cathodic protection. Zn-coated blanks may develop a thin oxide layer during heating, even if a protective atmosphere is used in the furnace. This layer helps preventing evaporation of the Zn (pure Zn evaporates at 907 °C at 1 atm. pressure), but must be removed before welding and painting. To achieve this, sandblasting, shot blasting or dry-ice (CO2) blasting are typically used.F-14, F-15 The grades for indirect process may be referred to as CR1500T-MB-IS (Cold Rolled steel with typical 1500 MPa Tensile strength, Manganese-Boron alloyed for Indirect [Hot] Stamping).
The indirect process cannot be applied to Al-Si coated blanks, as they have a hard but brittle intermetallic layer which would crack during cold deformation.F-14
In this process, as summarized in Figure 3, some of the forming occurs at the cold stage [❶ in Figure 1]. The semi-formed part then is heated in the furnace, significantly deformed to a final shape [❷ in Figure 1] and subsequently quenched in the same die. This process had found greater use in Europe, especially for deep drawn parts such as transmission tunnels. To avoid scale formation in the furnace and hot forming, a special varnish-type coating is commonly used. The final part must be surface cleaned with a process like shot blasting before welding to remove the varnish coating.S-63 Since the early 2010s, the process has been replaced by the direct process of Al-Si-coated blanks.N-15
Figure 3: Summary of “hybrid process” where deformation is done both at cold and hot conditions.B-14
A galvannealed (GA) coating primarily contains zinc and iron, and solidifies at temperatures between 530 °C and 782 °C, depending on the zinc content, as shown in Figure 4. Liquid Metal Embrittlement (LME) is not a concern if forming is done in the absence of liquid zinc.G-25 Hensen et al. conducted several studies heating galvannealed 22MnB5 blanks to 900 °C, but forming after a pre-cooling stage. As seen in Figure 5, the microcrack depth is significantly reduced when the forming starts at lower temperatures.H-26
Figure 4: Temperature limit to ensure absence of Zn-rich liquid (re-created after Citations G-25 and G-26)
Figure 5: Crack depth reduces significantly if the forming is done at lower temperature (re-created after Citation H-26)
In the pre-cooled direct process, the blank is heated above the austenitization temperature (approximately 870-900 °C), and kept in the furnace for a minimum soaking time of 45 seconds. Once the blank leaves the furnace, it is first pre-cooled to approximately 500 °C and then formed. Typical 22MnB5 cannot be formed at this temperature due to two reasons: (1) its formability would be reduced and (2) forming could not be completed before the start of martensite formation at approximately 420 °C.K-22, V-8
The development of a “conversion-delayed” hot stamping grade (see PHS Grades with approximately 1500 MPa TS), commonly known as 20MnB8, addresses these concerns. This steel has lower carbon (0.20%, as the number 20 in 20MnB8 implies), but higher Mn (8/4 = 2%). This chemistry modification slows the kinetics of the phase transformation compared with 22MnB5 – the critical cooling rate of 20MnB8 is approximately 10 °C/s. This allows the part to be transferred from pre-cooling stage to the forming die.
Then the blank is transferred to “pre-cooling stage” in less than 10 seconds. Precooling must be done at a rate over 20 °C/s, until the blank is cooled to approximately 500 °C. Then the part is transferred from the pre-cooling device to the press in less than 7 seconds. The forming is done in one hit in a hydraulic or servo-mechanical press, which can dwell at the bottom. The cooling rate after pre-cooling is advised to be over 40 °C/s. The final part may have zinc oxides and surface cleaning may be required.K-22, V-8 The grade may be referred to as CR1500T-MB-PS (Cold Rolled steel with typical 1500 MPa Tensile strength, Manganese-Boron alloyed, for Pre-cooled Stamping process).
Recently, several researchers have shown that pre-cooling may be used for drawing deeper partsO-6 or to achieve better thickness distribution of the final part.G-24 Since formed parts are typically removed from the press at approximately 200 °C, a pre-cooled part may require shorter time to quench, thus may increase the parts per minute.G-24
Multi-Step Process
(Blanking > Heating > Pre-cooling > Forming and Trimming > Air Quenching)
22MnSiB9-5 (see PHS Grades with approximately 1500 MPa TS) is a new steel grade developed by Kobe SteelH-27 for a transfer press process, named as “multi-step”. This steel has higher Mn and Si content, compared to typical 22MnB5. As quenched, the material has similar mechanical properties with 22MnB5. As of 2020, there is at least one automotive part mass produced with this technology and is applied to a compact car in Germany.G-27 Although critical cooling rate is listed as 2.5 °C/s, even at a cooling rate of 1 °C/s, hardness over 450HV can be achieved.H-27 This critical cooling rate allows the material to be “air-hardenable” and thus, can handle a transfer press operation (hence the name multi-step) in a servo press. This material is available only with Zn coating and requires a pre-cooling step before the transfer press operation.B-15 The grade may be referred to as CR1500T-MB-MS (Cold Rolled steel with typical 1500 MPa Tensile strength, for Multi-Step process).
Also known as inline hardening, this process is used to make profiles with constant cross sections and linear shapes. It is also possible to have closed profiles (tubes and similar) with this technology by adding a laser welding to the line (see Figure 6a). The process has been successfully used in many car bodies. Typical uses are: cross members, roof bows, side impact door beams, bumpers (with no sweep), front crash components and similar.G-28, H-28, F-16
Figure 6: Roll form PHS: (a) steps of the lineH-28, (b) photo of the induction heated area.G-28
The heating is typically done with induction heating, see Figure 6b. In one of the installations, the first induction coil operates at 25 kHz and the second at 200 kHz. The total heating power was approximately 700 kW and the line can run as fast as 6 m/s. It was found that if lubrication, speed and bending radius can be optimized, AlSi coated blanks could also be cold roll formed. However, they are not suitable for induction heating and may require a different process, such as form fixture hardening.K-23
The main difference between roll form PHS and form fixture hardening is the secondary “hot bending and forming” in the press. Here, cold roll formed profiles are cut-to-length and heated in a furnace. Heated profiles are then transferred to a press die, where sweep bending and/or further forming operations are completed. The parts are subsequently quenched in the same press die, similar to direct process. A typical line layout can be seen in Figure 7a. The secondary forming makes variable sections possible, as seen in Figure 7b. As the parts are cold roll formed and furnace heated, uncoated, Zn-coated and AlSi-coated (with precautions not to crack AlSi) blanks may be used in this process.H-28, K-23
Figure 7: Form fixture hardening: (a) schematic of a lineK-23, (b) bumper beam of Ford Mustang (2004-2014) made by this process..L-26
Form fixture hardening parts have been used in low volume cars such as Porsche 911 or Bentley Mulsanne. In some cars, form fixture hardening was used to manufacture the A-pillar of the convertible (cabriolet) versions of high-volume cars, especially in Europe. Most of these applications involved uncoated boron alloyed tubes (similar to 22MnB5).H-28 The 5th generation Ford Mustang (2004-2014) had form fixture hardened bumper beams in the front and rear, as seen in Figure 7b.L-26 The form fixture hardening process allows for use of AlSi coatings, since the steel goes through a furnace rather than an induction hardening step. Special care must be taken in cold roll-forming process to ensure the AlSi coating is not damaged.K-23
3-Dimensional Hot Bending and Quenching (3DQ)
(Cut-to-length tube > Local induction heating > 3-D Bending > Direct Water Quenching > Piercing)
In the 3DQ process, a tubular profile with constant cross section is quickly heated by induction heaters. By using movable roller dies, the part is bent. As the material is fed, water is sprayed on the induction heated portion of the tube to quench and harden it. The schematic of the process and the material strength through the process is illustrated in Figure 8. It is also possible to replace the movable roller dies with an industrial robot to bend and twist the tubular part.T-25
Figure 8: Schematic of 3DQ system (re-created after Citation T-25)
In January 2013, Mazda announced that the ISOFIX connection in the rear seats of a Premacy MPV (known as Mazda 5 in some markets) model was produced by this method, as shown in Figure 9a.M-24 In 2016, Honda started production of the sports car NSX (known as Acura NSX in some markets). This vehicle’s A-pillars were produced by 3DQ process, as shown in Figure 9b.H-29
Details of induction heating parameters, robot programming, cooling systems are explained in detail in reference L-71. The technology has been used on uncoated blanks, where a secondary e-coating may be used for corrosion prevention.T-52 In 2019, an academic study showed the feasibility of using Zn coated blanks in the 3DQ process.R-10
Form Blow Hardening / Hot Gas Metal Forming
(Cut-to-length tube or roll formed and welded profile > Heating > Pressure forming > Quenching > Piercing)
In hot gas metal forming, the tube or roll formed closed profile is heated first and placed onto a die set. The ends of the tube are sealed and pressurized gas or granular medium is forced inside the tubular blank. The forming forces are applied by the high pressure built inside the tube.C-16 It is also possible to end-feed material as in the case of (cold) tube hydroforming. After the deformation, the part is quenched either with water (form blow hardening) or by the air inside and the surface of the tool cavity (hot gas metal forming). In the latter case, similar to direct process, a water-cooling channel system inside the die inserts are typically required.K-23
Fraunhofer IWU has developed a hot gas metal forming setup in which both forming and quenching are done by compressed air. As shown in Figure 10a, the internal pressure can be increased to 70 MPa (700 bars) in only 6 seconds. The tools are cooled with internal cooling channels, Figure 10b. The parts produced with this technique have hardness values between 460 and 530 HV. Crashbox and camshafts are among the parts produced.L-27, N-16
Figure 10: Blow forming and quenching with air: (a) change of pressure in the tube and temperature of the tube, (b) simulation of heat transfer to the dies and cooling channels (recreated after Citation N-16)
In 2011, Spanish car maker SEAT published a study on form blow hardening process. In this study, they replaced the A-pillar and cantrail assembly of the SEAT León (Mk2, SOP 2005) with one form blow hardened part. The results were summarized asO-7:
7.9kg weight reduction per car,
Sheet material utilization increased from 40 to 95%,
Number of components in the assembly on one side of the car reduced from 5 to 2, and the roof rail was eliminated.
One advantage of this technology is the possibility to use the same die set for different wall thickness tubes. By doing so, parts can be produced for different variants of a car (i.e., coupe and cabrio, or North American spec. vs. emerging market spec.). This information applies to monolithic (i.e., same thickness throughout the tube) and tailor rolled/welded tubes as well.F-16 In 2017, tubular parts are hot gas formed by using 1900 MPa PHS tubes for customer trials.F-17
Since 2018, form blow hardening is being used in the Ford FocusB-16 and Jeep Wrangler.B-17 In the Ford Focus, a tailor rolled tube with thicknesses between 1.0 and 1.8 mm is used in Europe, whereas in China it is a monolithic (same thickness everywhere) 1.6 mm thick tube.F-16
A recent study using 42SiCr10 steel tubes of 2.5 mm wall thickness showed that the final part may have as high as 2500 MPa tensile strength. The heat treatment is summarized in Figure 11. When the tubes are removed from the dies (at around 195 °C) and heated in a furnace for 10 minutes at 260 °C, the total elongation is improved to over 20%, while the tensile strength is reduced to about 2000 MPa, as shown in Figure 12.W-43.
Figure 11: Process route of HMGF, including an additional Q&P process (re-created after W-43).
Figure 12: 42SiCr10 tubes after hot metal gas forming and additional partitioning process (re-created after W-43).
Steel Tube Air Forming process (STAF) is a modified and enhanced version of hot gas metal forming. In the STAF process, a metal tube is bent in a small press at room temperature. The preformed tube is transferred to the main press, where it is heated to the critical temperature using electrical conduction (Joule heating) by passing current through the tube. The first step creates the flanges where the press closes on the partially air blow formed tube. In the second step, air pressure completes the process by forming the desired cross section and overall shape.
As seen in Figure 13, the parts made with the STAF process can have a flange area for further welding/joining to other car body components. Some peripheral parts can be integrated into a single STAF part, improving productivity and manufacturing cost. The continuous closed cross section is created without the need for spot welding, improving stiffness and further reducing manufacturing costs. These factors combine to result in mass savings compared with conventional hot formed components, as indicated in Figure 14. F-18,F-41,F-42
Figure 13: The Steel Tube Air Forming process compared with other manufacturing approaches. STAF integrates flange formation without the need for additional spot welding.F-42 HSS stands for High-Strength Steel and may refer to conventional HSS or Advanced High-Strength Steels (AHSS).
Figure 14: The STAF process may reduce part count, assembled weight, and manufacturing complexity compared with other manufacturing approaches.F-41
The following video, kindly provided by Sumitomo Heavy Industries, highlights the STAF process along with associated benefits.F-41
In 2020, a Chinese company developed a new process route, named as vacuum hot stamping. In this technology, uncoated blanks are used. Parts are heated in a vacuum furnace (10-5 to 10-3 atm), resulting in a very thin (< 2μm) oxide layer and a decarburization layer of approximately 30 μm. The parts are formed in conventional hot stamping tools and press. The formed parts are first laser cut and then electro-galvanized. An additional tempering may be applied. Finally the coated parts are oiled for storage purposes. The process route is shown in Figure 15.A-86, A-87
Figure 15: Process route of vacuum hot stamping.A-87
The technique allows using uncoated blanks, thus has the following advantages over AlSi coated blanks:
Higher bending angle
No contamination on the furnace rollers
Fast heating possible
Ablation is not required for LWTB preparation
Since final part is Zn coated, has better corrosion resistance
Compared to uncoated blanks without vacuum forming, the advantages can be listed as:
Thin layer of oxides
No shot blasting required
Final part is Zn coated, has high corrosion resistance.
![Figure 6: Metal AM die under construction and after nitriding. [REFERENCE 39]](https://ahssinsights.org/wp-content/uploads/2021/02/Figure_6ab.svg)
![Figure 13: PHS1000 applications: (left) Door ring of Tesla Model Y (SOP 2020) [Citations B-79 and A-84], (right) Door ring of Voyah Dream (SOP 2022). [Citation H-70].](https://ahssinsights.org/wp-content/uploads/2025/11/PHS-Grade-RevFig-13.svg)
![Figure 28: Engineering stress-strain curves of 4% Mn steel, heated to different temperature (760. 800 and 840°C). Phase fractions are also affected by the heating temperature (re-created after [CITATION 20]).](https://ahssinsights.org/wp-content/uploads/2025/09/PHS-Grades-Figure-28.svg)
![Figure 12: Double door ring of Tesla Cybertruck: (a) approximate dimensions of the inner part (image re-created after [3]M-72), (b) schematic of outer ring and (c) inner ring (re-created after [4]=F-49).](https://ahssinsights.org/wp-content/uploads/2025/09/PHS-TailoredProps-Figure-12.svg)
![Figure 13: Double door ring applications: (a) GAC Aion UT (SOP 2025) [5]=G-50, (b) Lynk & Co 900 (SOP 2025) [6]=P-30.](https://ahssinsights.org/wp-content/uploads/2025/09/PHS-TailoredProps-Figure-13.svg)
![Figure 18: Sample automotive applications of patchwork PHS: B-pillar reinforcements of (a) 2007 Volvo V70 (re-crated after Citation N-4), (b) 2007 Fiat 500 (re-created after Citation Z-13); and (c) rear rail of 2015 Fiat Tipo/Egea (Citation T-43, recreated after Citation B-14); and (d) door ring of 2022 Voyah Dream (re-created after Citation [13]).H-70](https://ahssinsights.org/wp-content/uploads/2025/09/PHS-TailoredProps-Figure-18.svg)
![Figure 24: Soft zone applications to improve weld quality: (a) Mercedes EQE SUV [15]=D-47, and (b) Renault 5 BEV [14]=G-53.](https://ahssinsights.org/wp-content/uploads/2025/09/PHS-TailoredProps-Figure-24.svg)
![Figure 2: Direct water quenching: (a) schematic of the nozzles [N-27], (b) blank on the lower tool before deformation [T-51].](https://ahssinsights.org/wp-content/uploads/2025/09/2-PHS-ProdMethods-Fig-2.svg)
![Figure 15: Process route of vacuum hot stamping [A-87]](https://ahssinsights.org/wp-content/uploads/2025/09/2-PHS-ProdMethods-Fig-15.svg)