1stGen AHSS, AHSS, Press Hardened Steels, Steel Grades
Introduction
Press hardening steels are typically carbon-manganese-boron alloyed steels. 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.
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 12% (A80), 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 Some companies describe them 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 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. The latter grade is known as “multi-step hot forming steel” and could be abbreviated as, CR1500T-MS, PHS1500MS, PHSMS950Y1300T or similar.V-9
Figure 1: Stress-Strain Curves of PHS1500 before and after quenching* (re-created after Citations U-9, O-8, B-18).
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 tensile strength levels of 450 MPa and 550 MPa (noted as PQS450 and PQS550 in Figure 2) and higher ductility PHS grades with approximate 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 1.S-64, V-9
Table 1: Chemical composition limits for 22MnB5 (listed in wt.%).S-64, V-9
VDA239-500, a draft material recommendation from Verband Der Automotbilindustrie E.V. (VDA), is an attempt to further standardize hot stamping materials. The document has not been published as of early 2021. According to this draft 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 elongation to ensure cold formability.V-9 Figure 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 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.
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. 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 2.
Table 2: Chemical compositions of PHS grades with 1500 MPa tensile strength (all listed in wt.%).V-9
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 zine 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. As press hardened, the material has approximately 1000-1050 MPa yield strength and 1500 MPa tensile strength. Once bake hardened (170 °C, 20 minutes), yield strength may exceed 1100 MPa.K-22 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).
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).
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.
- 450–600 MPa tensile strength level and >15% total elongation, listed as PQS450 and PQS550.
- 1000–1300 MPa tensile strength level and >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 3. There is a working document by German Association of Automotive Industry (Verband der Automobilindustrie, VDA), which only specifies one of the PQS grades. In the draft standard, VDA239-500, PQS450 is listed as CR500T-LA (Cold Rolled, 500 MPa Tensile strength, Low Alloyed). Similarly, PQS550 is listed as CR600T-LA.V-9 Some OEMs may prefer to name these grades with respect to their yield and tensile strength together, as listed in Table 3.
Table 3: 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
PQS grades have been under development at least since 2002. In the earliest studies, PQS 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 there 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).
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), tailor welded blanks with PQS450, as shown Figure 10.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 10: Use of laser welded tailored PQS-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 11A.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 11B shows such components on the Mercedes C-Class.K-26
Figure 11: 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
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 12. 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 12: 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).
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 13. According to Mazda, the new material saved 4.8 kg per vehicle. The chemistry of the steel is Nb modified 30MnB5.H-33, M-28 Figure 14 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 13: Bumper beam reinforcements of Mazda CX-5 (SOP 2011) are the first automotive applications of higher strength boron steels.M-28
Figure 14: Performance comparison of bumper beams with PHS1500 and PHS1800.H-33
PHS 1800 is 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 DP 600 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.
Figure 15: 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 coating. 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 USIBOR 2000 in the Haval H6 SUV.V-12
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. 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, thinner coatings are preferred for bendability (see Table 5).W-28 More 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.
Table 4: Chemical compositions of higher strength PHS grades. “0” means it is known that there is no alloying element, while “-” means there is no information. “~” is used for typical values; otherwise, minimum or maximum are given. The terminology descriptions are not standardized. PQS names are based on their properties and grade names are 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.W-28, B-32, H-33, G-33, L-28, S-67, S-66, Y-12, B-33
Table 5: Mechanical properties of higher strength PHS grades. “~” is used for typical values; otherwise, minimum or maximum are given. Superscript PB means after paint bake cycle. The terminology descriptions are not standardized. PQS names are based on their properties and grade names are 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.W-28, B-32, H-33, G-33, L-28, S-67, S-66, Y-12
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:
- Stainless steels
- Medium-Mn steels
- Composite steels
Stainless Steels
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 16, martensitic stainless steels may have higher formability at elevated temperatures, compared to PHS1500 (22MnB5). Other advantages of stainless steels are:
- better corrosion resistanceM-37,
- potentially higher heating rates (i.e., induction heating) F-30,
- possibility of air hardening – allowing the multi-step process — as seen in Figure 17a H-42,
- high cold formability – allowing indirect process – as seen in Figure 17b.M-37
Disadvantages include (a) higher material cost, and (b) higher furnace temperature (up to around 1050-1150 °C).M-37, F-30 As of 2020, there are two commercially available stainless steel grades specifically developed for press hardening process.
Figure 16: Tensile strength and total elongation variation with temperature of (a) PHS1500 = 22MnB5M-38 and (b) martensitic stainless steel.M-36
Figure 17: (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 18 illustrates engineering stress-strain curves of the commercially available grades (1.6065 and 1.4064), and compares them with the 22MnB5 and a duplex stainless steel (Austenite + Martensite after press hardening). 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 18: 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.)
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 19: 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 19, 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 7, the furnace temperature could be further lowered 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 20, 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 20: 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 21, 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 and soaking time, and
- cooling rate.S-80
Figure 21: 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).
Table 7: Summary of mechanical properties of press hardenable Medium-Mn grades shown in Figure 18. 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.L-38, W-30, L-37, S-80
Composite Steels
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 22.R-14
Figure 22: 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 23. 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 23: Engineering Stress-Strain curves of core layer, outer layer and the composite steel (re-created after Citation P-26).
Table 8: Summary of composite steels and comparison with conventional PHS and PQS grades. Typical values are indicated with “~”. (Table re-created after Citation B-14).
* 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:
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Tailored Products
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Types of Tailored Products
Key materials characteristics for formed parts include strength, thickness, and corrosion protection. Tailored products provide opportunities to place these attributes where they are most needed for part function, and remove weight that does not contribute to part performance.
Welded Tailored Blanks (Tailor Welded Blanks)
Welded tailored blanks are a type of tailored product, as are patchwork blanks, tailor welded coils, tailor rolled coils, tailor rolled tubes and tailor welded tubes.
Laser welding is the most common approach to creating these products, but other options like resistance mash seam welding are possible. Mash seam welding requires that the blanks overlap, which adds mass. The heat affected zone (HAZ) for mash seam welds is considerably larger than that of laser welds. Compared with laser welding, the temperature reached when resistance mash seam welding is lower. This may improve formability due to reduced martensite formation, but the larger HAZ and thicker joint may negate this benefit. Non-linear welds are more challenging to produce with the mash seam approach. Welded Tailored Blanks refers to blanks made via either laser welding or mash seam welding. The following discussion centers on blanks and coils joined by lasers.
Butt-welding two or more flat sheets into a single blank creates a laser welded tailored blank (LWTB), also known as a tailor welded blank or laser welded blank (LWB). Steel grades and thickness may be (and usually are) different in the component sheets. Corrosion protection strategies may be different on each component of the welded blank. “Sub blanks” is another name for these component sheets.
Figure 1 shows a laser welded door ring with multiple grades and thicknesses. This technology allows for a reduction in panel thickness in non-critical areas, thus contributing to an overall mass reduction of the part. The lower strength product in the bottom section of the B-Pillar helps dissipate the crash energy in the event of a side impact, playing a key role in crash energy management.
Figure 1: Six-piece Laser Welded Tailored Blank with multiple grades and thickness.R-3
Patchwork Blank
An alternate approach to a laser welded tailored blank involves the creation of what is known as a patchwork blank, consisting of two or more blanks potentially having different thickness or grades. The unique characteristic is that the blanks are placed one on top of the other and (typically) spot welded together while still flat. The patched blank requires only one stamping operation, eliminating the need to join them after forming the components separately. This approach is satisfactory for both cold stamping and hot stamping. The correct number of spot-welds is important in terms of cost and safety. Furthermore, if the components shift relative to one another during the forming operation, the spot welds may shear. There is excellent fit between both parts after forming, allowing for easy application of additional spot welds if needed. Part size, the number and location of reinforcements, and available infrastructure are among the key considerations in deciding between a laser welded or patch blank approach. For example, patchwork blanks may be preferred if the reinforced area is relatively small with complex contours or located within the boundaries of the main blank.
Tailor Welded Coils
Rather than building blanks from individual components, a similar approach is to join entire coils together, edge to edge. These tailor welded coils (Figure 2) are used in coil-fed processes such as blanking, progressive die stamping, transfer press stamping, and roll forming operations. The basic process takes separate coils, prepares their edges for contiguous joining, and laser welds these together into one master coil. The new strip is either directly blanked or re-coiled for future blanking or use as feedstock for a continuous coil-fed stamping or roll forming line (Figure 3). Variations in strength, thickness, and coating occur across the width.
Figure 2: Production Process of Tailor Welded Coils.S-28
Figure 3: General usage of Tailor Welded Coils.S-28
Tailor Rolled Coils
Whereas tailor welded coils can vary in properties across the coil width, tailor rolled coils have variable thickness down the length of the coil. Production facilities for tailor rolled coils vary the gap between the rolls used for thickness reduction, allowing for different strip thicknesses in the direction of rolling (Figure 4). Accurate measuring and feedback control technology guarantees the strip thickness tolerances. A significant advantage to this approach results from the transition from one thickness to another with no joints or discontinuities, providing an efficient load path without stress risers. A tailor rolled coil can be either used for blanking operations (for stamping or tubular blanks), or as feedstock into a roll forming line.
Figure 4: Tailor Rolled Coils vary in thickness down the length of the coil.Z-5
After cold rolling to achieve the targeted thickness variation, coils are annealed to reset the mechanical properties. Tensile properties can be uniform in the blank, with only the thickness varying. Tailored properties are generated by adjusting the incoming coil thickness and the associated thickness reductions, leading to different degrees of recrystallization occurring in the annealing step.Z-14 TRBs used in press hardening do not need to be annealed first, and tailored properties can be produced with the appropriate PHS process.
Tailor Rolled Tubes
Tailor rolled coils are the feedstock to produce variable thickness tailor rolled tubes.
Tailor Welded Tubes
Conventional tube production involves roll forming strips to the desired shape and welding the free ends together to create a closed section. A tailor welded tube production process allows the designer to create complex variations in shape, thickness, strength, and coating (Figure 5)
Figure 5: Tailored tube production allows for differing thickness and strength within the same tube. A) 1-piece cylindrical tube with monolithic properties; B and C) 2-piece tailored tube with property variation down the length of the tube; D) 1-piece conical tube; E) 2-piece conical tube with a patchwork blank.
With hydroforming technology, the next step in tubular components is to bring the sheet metal into a shape closer to the design of the final component without losing tailored blank features (Figure 6).
Figure 6: Mechanical properties of tailored tubes are close to the original metal properties in the sheet condition. Hydroforming changes the properties based on the local deformation.G-8
Hydroformed conical tailored tubes offer automotive body engineers an additional approach to crash energy management while achieving a lightweight design. In frontal crash and side impacts the load paths have a key importance on the body design as they have a major bearing on the configuration of the structural members and joints. Figure 7 shows an example of a front-rail hydroformed prototype. The conical tailored tubes for this purpose take advantage of the high work hardening potential of TRIP steel.
Figure 7: Front-rail prototype based on a conical tube having 40 mm end to end difference in diameter.F-3
Benefits of Tailored Products in Automotive Body Construction
Figure 8 highlights some of the areas within the body structure where companies have considered transitioning to welded tailored blanks. Other tailored products may be suitable in other areas.
Figure 8: Applications suited for welded tailored blanks.A-31
Tailored products offer numerous advantages over the conventional approach involving the stamping and assembly of individual monolithic blanks which have a single grade, thickness, and coating, including:
Improved materials utilization
- Certain parts, like door rings (Figure 1), window frames, and door inner panels, have large cutout areas contributing to engineered scrap. Converting these to welded tailored blanks allows for optimized nesting of the individual components. Figure 9 presents an example of optimized nesting associated with body side aperture designs using a tailor welded blank. Reduced blank width requirements may allow for additional suppliers or use of master coils yielding slit mults. In the other extreme, blank dimensions larger than rolling mill capabilities are now feasible.
Elimination of reinforcement parts and reduced manufacturing infrastructure requirements
- In areas needing additional thickness for stiffness or crash performance, conventional approaches require stamping both the primary part and an additional smaller reinforcement and then spot welding the two parts together. The tailored product directly incorporates the required strength and thickness. Compared with a tailored product, the conventional approach requires twice the stamping time and dunnage, creates inventory, and adds the spot welding operation. Tolerance and fit-up issues appear when joining two formed parts, since their individual springback characteristics must be accommodated.
Part consolidation
- Similar to the benefits of eliminating reinforcements, tailored products may combine the function of what would otherwise be multiple distinct parts which would need to be joined.
Weight savings
- Conventional approaches to body-in-white construction requires individual parts to have flat weld flanges to facilitate spot welding. Combining multiple parts into a tailored product removes the need for weld flanges, and their associated weight.
Improved NVH, safety, and build quality
- Joining formed parts is more challenging than joining flat blanks first and then stamping. Tailored products have better dimensional integrity. Elimination of spot welds leads to a reduction in Noise, Vibration, and Harshness (NVH). A continuous weld line in tailored products means a more efficient load path.
Enhanced engineering flexibility
- Using tailored products provides the ability to add sectional strength in precise locations to optimize body structure performance.
Easily integrated with advanced manufacturing technologies for additional savings
- Tailored products incorporated into hot stamping or hydroforming applications magnify the advantages described here, and open up additional benefits.
Figure 9: Nesting optimization dramatically reduces engineered scrap.A-31
Design, Usage, and Troubleshooting Guidance
Edge Quality of Component Blanks Used to Create the Welded Blank
Minimal burr, maximized sheared edge stretchability, and maximized tool life are characteristics of good sheared edge quality in conventional blanking operations. When creating sub-blanks for assembly into a laser welded tailored blank, there is an additional critical characteristic: squared edges with high straightness are a prerequisite to avoid local gaps between the blank edges causing undercut in the weld bead and the associated loss of mechanical properties (Figure 10). Conventional shearing operations produce the edge seen in Figure 10a. Precision die blanking with tight clearances, as well as laser blanking, produce edges suitable for the blank welding operation (Figure 10b). Another option is to use a conventional shearing approach to produce a slightly oversize blank, and then use precision shears as the first step in the laser welding line. The advantage to this approach minimizes the impact of edge damage which might occur during transportation.
Figure 10: Edge condition and welding result. A) Poor edge resulting in increased residual butt gap and weld undercut; B) Optimum squared edge by precision die blanking or laser welding resulting in flush weld surfaces.S-29
Forming analysis of Welded Blanks
The weld joint and adjacent heat affected zone complicates the forming analysis of welded blanks. There are generally two scenarios that occur: failure perpendicular to the weld and failure parallel to the weld (Figure 10).
Figure 11: Major strain axis relative to weld line orientation. A) Weld line is perpendicular to the major strain axis; B) Weld line is parallel to the major strain axis.
Consider the scenario in Figure 11A, where the major strain is perpendicular to the weld line orientation. Failure occurs parallel to the weld in the lower strength, more ductile material. The restraining force provided by the higher strength material results in the weld line shifting towards that side.
With the weld seam moving towards the thicker and stronger metal, deformation occurs in thinner and weaker metal. The movement of the weld seam during stamping requires that a blankholder design with appropriate clearances so the welds can move through the blankholder during the drawing process. The punch-cavity clearances must allow for movement as well – otherwise a pinch-point is created and metal flow stops. These clearances are necessary for movement of the heavier gauge metal and the welds, but can also be a source of wrinkling on the lighter gauge side due to the lack of hold-down force in this area. Also, the potential exists for die wear in the locations in contact with weld seam. To counter the weld line motion to the higher strength side, consider allowing greater material flow from that side of the weld through a less severe draw bead design, reduced blankholder pressure or modified blank outline. These strategies help minimize the risk of this type of split.
Now consider the scenario in Figure 11B, where the major strain is parallel to the weld line orientation. Here, fracture occurs perpendicular to the weld line – typically within the weld itself – due to the poorer mechanical properties of the weld with respect to the base material. To avoid splits in the weld, orient the weld away from the major strain direction and away from areas with high strain concentration.
Use of Advanced High Strength Steels in Laser Welded Tailored Products
As sub-blanks are welded together to create the main blank, the beginning or ending of each of the weld lines are not in a steady state condition. As such, the start or stop action of the welding may create a stress riser. After the main blank is built-up, a scalloped cut can remove the affected areas so that only sections of uniform edge quality are formed in subsequent operations.
Laser welding is a high energy density process that focuses the heat in a very localized region as the laser travels at a high speed. This results in a narrow heat affected zone and the rapid quenching leads to the formation of a high-hardness martensite. Increased welding speeds generally require higher energy intensity at the weld which increases peak hardness.
Different welding parameters must be used for advanced high strength steels due to the presence of martensite in the base metal. The microstructure in the heat affected zone is a function of the local time/temperature profile, which changes with distance away from the weld as well as with the composition of the alloy in question. Laser welding some AHSS grades results in tempered martensite in the heat affected zone, leading to a locally softened region. Crash modeling should incorporate the influence of the softened region, which may help dissipate a portion of the crash energy.
However, this softened region creates an area for high strain concentration, leading to premature failure in martensite-containing AHSS grades. Higher strength dual phase steels have more martensite than lower strength dual phase grades, and therefore are more likely to generate HAZ soft zones and reduced formability when used in laser welded blanks. CP and TRIP steels have a lower incidence of HAZ soft zones due to higher alloying content. More information about laser welding is found here.
Blank Stacking, Destacking, and Feeding Problems
Depending on the thickness differences between the component blanks and their relative sizes, embossed dimples on the thinner gauge side can balance the thickness difference and to allow stable coil winding (in the case of tailor welded coil production) or stacking as they are blanked onto a lift. The stack weight and banding of the pallets may contribute to collapsing the dimples, so some companies choose to minimize the stack height.
Automatic destackers and feeders typically require uniform blanks and stacks for trouble-free operation. Dimples and cutouts may interfere with proper operation of overhead vacuum cups or magnetic belts. Moving or deactivating Vacuum cups may be necessary. Dimples should either be outside of the magnetic belt area or have their form away from the belt. Dimples may be a problem for some in-line blank washers and die-lube roll coaters.
Press and Tooling Considerations
When there is a large thickness difference within the component blanks, press tonnage and balance can be a problem. Press tonnage monitors are essential for this type of part, not only for protection of the press equipment, but also for good process control. If press balance is a problem, the tonnage monitors will indicate where the problem is. Supplemental balance devices in the die, such as nitrogen cylinders, may be appropriate, or the stamping may need to move to a larger press.
Tailored products often contain sub-blanks of mixed strength and thickness. The chosen die process must reflect those differences, and account for changes in side-wall curl, springback and drawability characteristics across the part. Mild steel can usually be formed with high strength steel die processes, but high strength steel will not always form satisfactorily with conventional die processes used with mild steels.
Tailor Welded Blanks Stamping Tryout Issues Checklist.A-20
Case Study: Deploying Tailored Blanks in Electric Vehicles
Many thanks to Isaac Luther, TWB Company, for providing this case study.
Laser-welded blanks (LWBs) allow the combination of different steel grades, thicknesses, and even coating types into a single blank. This results in stamping a single component with the right material in the right place for on-vehicle requirements. This technology allows the consolidation of multiple stampings into a single component.
One example is the front door inner. A two-piece design will have an inner panel and a reinforcement in the hinge area. As shown in Figure 12, a laser welded front door inner incorporates a thicker front section in the hinge area and a thinner rear section for the inner panel, providing on-vehicle mass savings. This eliminates the need for additional components, reducing the tooling investment in the program. This also simplifies the assembly process, eliminating the need to spot weld a reinforcement onto the panel.
Figure 12: Front Door Inner Stamped from a Laser Welded Blank
Today, large opportunities exist to consolidate components in a BEV in the battery structure. Design strategies vary from different automakers, including how the enclosure is constructed or how the battery mounts into the vehicle. The battery tray can have over 100 stamped components, including sealing surfaces, structural members, and reinforcementsM-68. As an idea, a battery tray perimeter could be eight pieces, four lateral and longitudinal members, and four corners. The upper and lower covers are two additional stamped components, for a total of ten stampings that make up the sealing structure of the battery tray. On a large BEV truck, that results in over 17m of external sealing surfaces.
Part consolidation in the battery structure provides cost savings in material requirements and reduced investment in required tooling. Another benefit of assembly simplification is improved quality. Fewer components mean fewer sealing surfaces, resulting in less rework in the assembly process, where every battery tray is leak-tested.
The deep-drawn battery tub is a consolidated lower battery enclosure and perimeter. This can be seen in Figure 13; a three-piece welded blank incorporates a thicker and highly formable material at the ends and in the center section, either a martensitic steel for intrusion protection or a low-cost mild steel. This one-piece deep-drawn tub reduces the number of stampings and sealing surfaces, resulting in a more optimized and efficient design when considered against a multi-piece assembly. In the previous example of a BEV truck, the deep-drawn battery tub would reduce the external sealing surface distance by 40%. To validate this concept, component level simulations of crash, intrusion, and formability were conducted. As well as a physical prototype built that was used for leak and thermal testing Y-14 with the outcomes proving the validity of this concept, as well as developing preliminary design guidelines. Additional work is underway to increase the depth of the draw while minimizing the draft angle on the tub stamping.
Figure 13: Deep Battery Tub Stamped from a Laser Welded Blank
In most BEVs today, the passenger compartment has a floor structure common in an ICE vehicle. However, the BEV also has a top cover on the battery assembly that, in most cases, is the same size as the passenger compartment floor. In execution of part consolidation, the body floor and battery top cover effectively seal the same opening and can be consolidated into one component. An example is shown below, where seat reinforcements found on the vehicle floor are integrated into the battery top cover, and the traditional floor of the vehicle is removed. Advanced high-strength steels are used in different grades and thicknesses. Figure 14 show what the laser welded battery top cover looks like on the assembly.
Figure 14: Battery Top Cover from a Laser Welded Blank
Vehicle assembly can also be radically simplified as front seats are mounted on the battery before being installed in the vehicle as shown in Figure 15, the ergonomics of the assembly operation are improved by increased access inside the passenger compartment through the open floor.
Figure 15: Ergonomics of the Assembly Operation
Cost mitigation is more important than ever before, with reductions in piece cost and investment and assembly costs being important. At the foundation BEVs currently have cost challenges in comparison to their ICE counterparts, however the optimization potential for the architecture remains high, specifically in part consolidation. Unique concepts such as using laser welded blanks for deep-drawn battery tubs and integrated floor/battery top covers are novel approaches to improve challenges faced with existing BEV designs. Laser welded blank applications throughout the body in white and closures remain relevant in BEVs, providing further part consolidation opportunities.
Thanks go to Isaac Luther for his contribution of this case study. Luther is a senior product engineer on the new product development team at TWB Company. TWB Company provides tailor-welded solutions in North America. In this role, Isaac is responsible for application development in vehicle body and frame applications and battery systems. Isaac has a Bachelor of Science in welding engineering from The Ohio State University.
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