Manufacturing precision welded tubes typically involves continuous roll forming followed by a longitudinal weld typically created by high frequency (HF) induction welding process known as electric resistance welding (ERW) or by laser welding.
Tubular components can be a cost-effective way to reduce vehicle mass and improve safety. Closed sections are more rigid, resulting in improved structural stiffness. Automotive applications include seat structures, cross members, side impact beams, bumpers, engine subframes, suspension arms, and twist beams. All AHSS grades can be roll formed and welded into tubes with large D/t ratios (tube diameter / wall thickness); tubes having 100:1 D/t with a 1mm wall thickness are available for Dual Phase and TRIP grades.
Figure 1: Automotive Applications for Tubular Components.A-35
As roll formed and welded tubes are used with mounting brackets and little else in some Side Intrusion Beams (Figure 2), or they can be used as a precursor to hydroforming, such as the Engine Cradle shown in Figure 3.
Figure 2: Side intrusion beams made from a welded tube with mounting brackets.S-34
Figure 3: The stages of a Hydroformed Engine Cradle: A) Straight Tube, B) After bending; C) After pre-forming; D) Hydroformed Engine Cradle. S-35
The processing steps of tube manufacturing affect the mechanical properties of the tube, increasing the yield strength and tensile strength, while decreasing the total elongation. Subsequent operations like flaring, flattening, expansion, reduction, die forming, bending and hydroforming must consider the tube properties rather than the properties of the incoming flat sheet.
The work hardening, which takes place during the tube manufacturing process, increases the yield strength and makes the welded AHSS tubes appropriate as a structural material. Mechanical properties of welded AHSS tubes (Figure 4) show welded AHSS tubes provide excellent engineering properties. AHSS tubes are suitable for structures and offer competitive advantage through high-energy absorption, high strength, low weight, and cost efficient manufacturing
Figure 4: Anticipated Properties of AHSS Tubes; A) Yield Strength, B) Total Elongation.R-1
The degree of work hardening, and consequently the formability of the tube, depends both on the steel grade and the tube diameter/thickness ratio (D/T) as shown in Figure 5. The degree of work hardening influences the reduction in formability of tubular materials compared with the as-produced sheet material. Furthermore, computerized forming-process development utilizes the actual true stress-true strain curve of steel taken from the tube, which is influenced by the steel grade, tube diameter, and forming process.
Figure 5: Examples of true stress – true strain curves for AHSS tubes made from Dual Phase Steel with 590 MPa minimum tensile strength.A-36
Bending AHSS tubes follows the same laws that apply to ordinary steel tubes. Splitting, buckling, and wrinkling must all be avoided. As wall thickness and bend radius decrease, the potential for wrinkling or buckling increases.
One method to evaluate the formability of a tube is the minimum bend radius. An empirically derived formulaS-36 for the minimum Centerline Radius (CLR) considers both tube diameter (D) and total elongation (A) determined in a tensile test with a proportional test specimen, and assumes tube formation via rotary draw bending:
The formula shows that a bending radius equal to the tube diameter (1xD) requires a steel with 50% elongation. Successfully bending low elongation material needs a greater bend radius. Consider, for example, a dual phase steel grade where the elongation of a sample measured off the tube is 12.5%. Here, the minimum bend radius is 4 times the tube diameter. The tube bending method, the use and type of mandrels, and choice of lubrication may all affect the CLR.
The engineering strain on the outer surface can be estimated as the tube diameter (D) divided by twice the Centerline Radius (CLR):
For example, if you are bending a 40 mm diameter tube around a centerline radius of 100mm, the engineering strain on the outer surface is approximately 40/[2*100] = 20%. As a rough estimate, successful bending requires the tube to have a minimum elongation value from a tensile test in excess of this amount. Otherwise, a larger radius or modifications to the forming process is needed.
Springback is related to the elastic behavior of the tube. Yield strength variation between production batches can lead to variation in the amount of springback. A rule of thumb is that a variation of ± 10 MPa in yield strength causes a variation of approximately ± 0.1° in the bending radius. In addition, a high frequency weld which is harder and stronger than the base material causes a maximum of approximately ± 0.1° variation in bending radius.S-36
The bending behavior of tube depends on both the tubular material and the bending technique. The weld seam is also an area of non-uniformity in the tubular cross section, and therefore influences the forming behavior of welded tubes. The recommended procedure is to locate the weld area in a neutral position during the bending operation.
Figures 6 and 7 provide examples of the forming of AHSS tubes. The discussion on Tailored Products describes tailored tubes, which may be further hydroformed.
Figure 6: DP steel bent to 45 degrees with centerline bending radius of 1.5xD using booster bending. Steel properties in the tube: 610 MPa yield strength, 680 MPa tensile strength, 27% total elongation. R-1
Figure 7: Hydroformed Engine Cradle made from a dual phase steel welded tube by draw bending with centerline bending radius of 1.6xD and a bending angle greater than 90 degrees. Steel properties in the tube: 540 MPa yield strength, 710 MPa tensile strength, 34% total elongation. R-1
Key Points
Due to the cold working generated during tube forming, the formability of the tube is reduced compared to the as-received sheet.
The work hardening during tube forming increases the YS and TS, thereby allowing the tube to be a structural member.
Successful bending requires aligning the targeted radii with the available elongation of the selected steel grade.
The weld seam should be located at the neutral axis of the tube, whenever possible during the bending operation.
Tensile testing cannot be used to determine bendability, since these are different failure modes. Failure in bending is like other modes limited by local formability in that only the outermost surface must exceed the failure criteria.
ASTM E290A-26, ISO 7438I-8, and JIS Z2248J-5 are some of the general standards which describe the requirements for the bend testing of metals. In a Three-Point Bend Test, a supported sample is loaded at the center point and bent to a predetermined angle or until the test sample fractures. Failure is determined by the size and frequency of cracks and imperfections on the outer surface allowed by the material specification or the end user.
Variables in this test include the distance between the supports, the bending radius of the indenter (sometimes called a pusher or former), the loading angle which stops the test, whether the loading angle is determined while under load or after springback, and the crack size and frequency resulting in failure.
For automotive applications, the VDA238-100V-4 test specification is increasingly used. Here, sample dimension, punch tip radius, roller spacing, and roller radius are all constrained to limit variability in results. Figure 1 shows a schematic of the test.
Figure 1: Schematic of Bend Testing to VDA238-100, with Bending Angle Definition.
This video, courtesy of Universal Grip Company,U-5 describes the support rollers in the VDA238-100 test.
Calculation of the bending angle is not always straightforward. Bending formulas such as that shown within VDA238-100 assume perfect contact between the sheet metal and the punch radius. However, experimental evidence exists showing this contact does not always occur, especially in AHSS grades.
Figure 2 presents one example testing DP600 where the punch radius is larger than the radius on the bent sheet, leading to a physical separation between the punch and sheet.L-12
This physical separation also has implications for standardized bendability characterizations. A common measure of bendability is the punch radius to sheet thickness ratio, rPUNCH/t. In higher strength grades where this punch-sheet-liftoff is likely to occur, this may lead to an overestimation of how safe a design is when the punch radius may be measurably larger (less severe) than the tighter, more extreme radius actually experienced on the sheet.
Figure 2: DP600 After Testing to VDA238-100. Note punch radius is larger than radius in bent sheet resulting in separation.L-12
Furthermore, bending tests do not always result in a round bent sheet shape and constant thickness around the punch tip, especially when testing 980MPa tensile strength steel grades and higher which have low strain hardening capability. Figure 3 shows pronounced flattening and thinning of the sheet below the punch tip after bending, occurring primarily on the side opposite the punch stretched in tension. Efforts to replicate this phenomenon in simulation have failed, since the underlying mechanism is not yet fully understood.
Figure 3: Flattening and Thinning Behavior after Bending.L-12
Results from bend testing are typically reported as the smallest R/T (the ratio between the die radius and the sheet thickness) that results in a crack-free bend. Many steel companies report minimum bend test limits for various grades and certain automakers include minimum bend test requirements in their specifications as well. Different steel companies and automakers may have different bend test methods and/or requirements, so it is important to understand those requirements and procedures to better match the material characteristics with the customer’s design and process expectations. The test methods could involve a bend of 60°, 90°, 180° as well as various radii, die materials, speeds, etc.
Figure 4 shows etched cross sections of different grades bend to either 0T (fold flat) or 0.5T radii for reference purposes.
Figure 4: Etched cross sections of various grades. Top row, left: 0T bend of DP350/600; Top row, right: 0T bend of HSLA450/550; Bottom left: 0.5T bend of TRIP 350/600; Bottom center: 0T bend of TRIP 350/600; Bottom right: 0.5T bend of DP 450/800.K-1
The ISO 16630 Hole Expansion Test and the VDA238-100 bend test are among the few standardized tests to characterize local formability, the term which describes when part and process design, in addition to sheet metal properties like strength and elongation, influence the amount of deformation the metal can undergo prior to failure.
As schematically shown in Figure 1, the stress state at the edge of the hole expansion test specimen is influenced by friction, contact forces, and out-of-plane deformation, and therefore the hole expansion ratio as determined by ISO 16630 is not solely a material characteristic.
Figure 1: Parameters influencing stress state at edge of hole expansion test specimen.K-68
Researchers have developed alternate tests to further investigate process parameters and more clearly understand and optimize non-steel related variables. Also important is the investigation of different strain states from the ones seen in the hole expansion and bending tests. Comparisons of the stretched or bent edge performance evaluating different process parameters using the same material help to better define optimum process parameters. Repeating the testing with different AHSS grades confirms if similar trends exist across different microstructures and strengths. Standardization of these alternate tests has not yet occurred, so use caution when comparing specific values from different studies.
Hole Tension Test (In-Plane Deformation)
Microstructural damage in the shear affected zone reduces edge ductility. Damage has been evaluated with a modified tensile dogbone containing a central hole prepared by shearing or reaming, as shown in Figure 2. In contrast with a hole expansion test using a conical punch, the researchers described this as a Hole Tension test, which determines failure strain as a function of grade and edge preparation.
A two-dimensional (2-D) edge tension test, also called Half-A-Dogbone test, also evaluates edge stretchability. There are multiple versions of this type of test, but they all are based on the same concept. Like a standard tensile test, the 2-D edge tension test pulls a steel specimen in tension until failure. Unlike a standard tensile test where both sides of the tensile specimen are milled into a “dog bone”, the 2D tension test uses half of a dogbone with different preparation methods for the straight edge and the edge containing the reduced section (Figure 3). The chosen preparation method for each face is a function of the parameter being investigated (ductility, strain, burr, and shear affected zone for example). Potential edge preparation methods include laser cutting, EDM, water jet cutting, milling, slitting or mechanical cutting at various trim clearances, shear angles, rake angles or with different die materials.
Figure 3: 2-D Edge Tension Test Sample. Note the edges are prepared differently based on the targeted property evaluated.
The hole expansion test is prone to scatter in the results. Many replicates are usually tested to improve the robustness of any conclusions. In addition, the test averages the strains over the entire circumference, and as such is lower than the peak strain immediately surrounding the location of crack initiation.
The Sheared Edge Tensile test (SET)W-25 has many merits: it does not require many samples, sample preparation is relatively easy, and the test itself can be performed on a regular tensile test machine. However, the test itself can be complicated to execute, partially because it requires stopping the test immediately after a load drop of 0.5% to 4% from peak load. Full separation of the specimen after failure is to be avoided since it can cause substantial distortion of the cross section along the failure.
This test was modified to eliminate the need to monitor and control based on load drop by using non-contact optical strain measurements facilitated by DIC (digital image correlation), creating what is termed the Sheared Edge Tensile – Improved test (SETi).A-92
A benefit of the use of DIC with SETi is that local peak strains can be captured, allowing for determination of critical strain measurements that can be fed into forming simulations.
More information about SETi can be found in Citations A-92 and A-93.
Side Bending Test (In-Plane Deformation)
Instead of a dogbone or half-dogbone, some studies use a rectangular strip without a reduced section. Bending performance can be evaluated with a rectangular strip having one finished edge and one trimmed edge while preventing out-of-plane buckling, as shown in Figure 4.G-7
Figure 4: The side-bending test expands a trimmed edge over a rolling pin until detection of the first edge crack.G-7
Another type of in-plane side bending test is described in Citation K-68. The set-up shown in Figure 5 delays necking, allowing for determination of the hardening curve above uniform elongation. A key addition to the fixture is the anti-bucking mechanism. Unlike a standard hole expansion test, there is no friction and contact stress on the edge. The strain gradient is controlled by changing the beam height of the sample (Figure 6). An indication of the measurable dimensions after deformation is seen in Figure 7.
Figure 6: Sample geometry for in-plane bending test.K-69
Figure 7: Deformed sample after in-plane bending test.K-68
The in-plane bending test provides more details of fracture strain in a desired material orientation relative to the rolling direction compared with the axisymmetric hole expansion test. In the hole expansion test, the weakest direction determines the result due to inherent rotational symmetry in the test. With potential anisotropy in the martensite morphology found in DP steels, the ability to improve characterization in specific orientations may be useful.
The in-plane bending test was used to show a DP800 sheared edge parallel to the transverse direction has a lower fracture strain than a sheared edge parallel to the rolling direction. Furthermore, a sharper cutting tool improves edge ductility.K-68 All tests allowed for two weeks between cutting and testing; over that two week period edge fracture strain is influenced by the time between cutting and testing, but stabilizes after two weeks.A-91
This test also was used to explore void distribution and the uniformity of plastic deformation in sheared edges.K-70 This study shows that the blanking process creates an inhomogeneous void distribution in the thickness direction. As the deformation increases during the subsequent in-plane bending test, the micro-cracks initiate at the burr region and grow towards the rollover region. Once they entirely pass the thickness of the material, they grow further, away from the edge.
No voids are present in the relatively smoother rollover and burnish regions of the sheared edge, whereas the rougher fracture region and burr contain many voids. The majority of voids are a result of martensite cracking and separation of ferrite–martensite interfaces.
Overall, the initial void volume fraction inside the severely hardened layer of the as-cut edge plays a more critical role than roughness in edge ductility. Removing the volume from edge with high void density (in this case, 40 microns) is enough to significantly increase the edge ductility. It is not necessary to remove the complete shear-affected zone, which for this study is on the order of 500 microns.
Half-Specimen Dome Test (Out-of-Plane Deformation)
Deformation in these three tests occur in the plane of the sheet. Typical hole expansion tests, like production stampings, deform the sheet metal perpendicular to the plane of the sheet. However, hole expansion testing does not always give consistent test results. The half-specimen dome test (HSDT) also attempts to replicate this 3-dimensional forming mode (Figure 8), and appears to be more repeatable likely due to creating a straight cut rather than round hole.
In the HSDT, a rectangular blank is prepared with one edge having the preparation method of interest, like sheared with a certain clearance or laser cut or water-jet cut. The sample is then clamped with the edge to be evaluated over a hemispherical punch. The punch then strains the clamped sample creating a dome shape, with the test stopping with the first crack appears at the edge. Edge stretchability is quantified by measuring dome height or edge thinning or other characteristics.
Figure 8: Half-Specimen Dome Test sample. Arrow points to edge crack.S-12
Edge Flange Test (Out-of-Plane Deformation)
Flanging limits depend on the part contour, edge quality, and material properties. Non-optimal flange lengths – either too long or too short – will lead to fracture. Different tools can assess the influence of flange length, including the one shown in Figure 9 from Citation U-3.
Figure 9: Tool design to investigate flange length before fracture. Flange height: 40mm in left image, 20mm in right image.U-3
Physical tests using this tool show that optimizing sample orientation relative to the rolling direction leads to longer flange lengths before splitting.U-3Figure 10 highlights the results from testing DP800.
Figure 10: Flange height limits as a function of orientation in DP800.U-3
Effect of Time on Edge Ductility
The time passed between cutting and deforming has a large influence on the test results.
Edge ductility test results are impacted by the amount of elapsed time between cutting and deforming. Citation A-91 documented this effect on two coils each of galvanized or bare dual phase steels having different chemistry that were evaluated with Hole Expansion Capacity (HEC), Sheared Edge Tensile – Improved (SETi), and BMW’s Kantenriss Empfindlichkeitstest (translated to edge crack sensitivity test, abbreviated as KRE) tests.
The difference between testing a few hours versus a few days after cutting can be a reduction of up to 10 HEC%, or a true strain of 0.1. Property values appear to be stable when tested within 4 hours after cutting or after 24 hours after cutting, at the high and low values, respectively. It was also found that this effect pertains only to zinc coated materials – no edge ductility reduction over time was found in bare steels.
A practical implication of this time effect is that a galvanized part formed in a coil-fed press line should have fewer problems with edge ductility than if blanks were prepared offline in advance, all other things being equal.
At least one automotive OEM recognizes this and puts timing restrictions on the hole expansion evaluation test procedure, stating that there must be a delay of “24 h ± 1 h at room temperature” after punching but before the hole expansion test begins to avoid the aging effect.
Making a second cut to remove the work hardened zone produced during the first cut, called shaving or pre-piercing, can improve the edge ductility. See the bottom of our page on shearing for more information. This practice may have the additional benefit of reducing or eliminating the time delay sensitivity associated with stretching of galvanized cut edges.
Although Citation A-91 evaluated only coated and uncoated DP steels, similar conclusions were drawn in another study covering dual phase (DP), complex phase (CP), and high strength low alloy (HSLA) grades,S-126 and another study covering DP steels of different strength levels.W-46
However, another studyC-44 observed a decrease in hole expansion capacity for galvanized complex phase steel, uncoated martensitic steel, and one galvanized dual phase grade. Yet the effect was absent in galvanized bake hardening (BH), uncoated transformation induced plasticity aided bainitic-ferritic (TBF), and another uncoated dual phase grade. A related studyC-45 concluded that the advanced high strength grades with good edge ductility were the most susceptible to a decrease in edge stretchability with aging.
Influence of Starting Dimension
In addition to the time between edge creation and testing, the starting reference dimension of the analyzed region influences the results. Edge strains of samples from CR440Y780T-DP were evaluated using four tests: two with a relatively large strain reference length, and two associated with smaller strain reference lengths. It is found that the measured edge formability increases with a smaller strain reference length.S-127
This is similar to the effect seen in a comparison of A80 and A50 tensile bars, where A80 – having the longer reference length – is associated with lower elongation values than on an A50 bar of the same material.
The strain reference length in physical testing should correspond to the element size in associated forming simulation.
Figure 11: Limiting edge strains of the same material tested and evaluated differently. The red bar is from a conventional ISO 16630 test, except a dome was used to expand the hole instead of a conical punch. The lighter blue bar are the results from the same test, but a non-contact DIC method was used to measure the limiting strains instead of the change in hole diameter.S-127
The Hole Expansion test (HET) quantifies the edge stretching capability of a sheet metal grade having a specific edge condition. Many variables affect hole expansion performance. By understanding the microstructural basis for this performance, steelmakers have been able to create new grades with better edge stretch capability.
Multiphase microstructures with large hardness differences between the phases, specifically islands of the very hard martensite surrounded by a softer ferrite matrix, may crack along the ferrite-martensite interface (Figure 1). The larger the size of the initiated damage site (due to edge shearing), the smaller the critical stress required for crack propagation.M-6 The microstructure and damage are key components of the Shear Affected Zone, or SAZ.
Figure 1: Features and mechanisms of damage initiation and propagation in Dual Phase steel.M-6
One metallurgical approach to improve sheared edge stretchability is targeting a homogeneous microstructure. Steel suppliers have engineered product offerings like complex phase steel, where extensive grain refinement (reducing the size of the ferrite and martensite grains) is achieved. Consequently, the size of the initial damage resulting from shearing is reduced, raising the critical stress for crack propagation to higher levels and reducing the likelihood for crack propagation. Additionally, reducing the difference in hardness between the soft ferrite phase and the hard martensite phase improves the hole expansion ratio. Changes in chemistry, hot rolling conditions and intercritical annealing temperatures are some of the methods used to achieve this. Such metallurgical trends can include a single phase of bainite or multiple phases including bainite and removal of large particles of martensite. This trend is shown in Figure 2, adapted from Citation M-11.
Figure 2: Hole Expansion as a Function of Strength and Microstructure. Adapted from Citation M-11.
An example of the impact of these modifications is shown in a paper published by C. Chiriac and D. HoydickC-10, where a 1 mm DP780 galvannealed steel was modified to produce a grade with improved hole expansion to achieve greater resistance to local formability failures such as edge fracture and shear fracture. These changes were made while retaining the same base metal chemistry and the same fraction of martensite in the structure, and resulted in similar tensile strength and total elongation but with a 50% increase in hole expansion (Table I and Figure 3). The key difference is a lower martensite hardness, and a smaller difference between the hardness of the martensite and ferrite. The modified DP grade has more homogeneous distribution of martensite with smaller ferrite and martensite grains (Figure 4).
Table I: Comparison of a conventional DP780 steel with a similar chemistry modified to improve hole expansion.C-10
Figure 3: Improvement in Hole Expansion improves with grade modifications and edge quality. DMR = drilled, milled, and reamed hole; EDL = Edge Ductility Loss index, the ratio of the hole expansion of the DMR hole to that of the punched hole.C-10
Figure 4: Comparison of the microstructure of a conventional DP780 steel (left) with a similar chemistry modified to improve hole expansion (right). Overall, there is the same fraction of martensite in both grades, but the modified chemistry has finer features.C-10
A presentation at a 2020 conferenceK-16 described a study which compared DP780 from six different global suppliers. Hole expansion tests were done on 1.4 mm to 1.5 mm mm thick samples prepared with either a sheared edge at 13% clearance, a sheared edge with 20% clearance, or a machined edge. Not surprisingly, the machined edge with minimal work hardening outperformed either of the sheared edge conditions. However, when considering only the machined edge samples, the hole expansion ratio ranged from below 30% to more than 70% (Figure 5). Presumably the only difference was the microstructural characteristics of the six DP780 products.
Figure 5: Variation in hole expansion performance from DP780 from 6 global suppliers.K-56
The microstructural differences that enhance local formability characteristics may be detrimental to global formability characteristics and vice versa. Conventional dual phase steels, with a soft ferrite matrix surrounding hard martensite islands, excel in applications where global formability is the limiting scenario. These steels have a low YS/TS ratio and high total elongation. However, the interface between the ferrite and martensite is the site of failures that limit the sheared edge extension of these grades. On the other end of the spectrum, fully martensitic grades are the highest strength steels available. These have a high YS/TS ratio, and low total elongation. Having only a single phase helps these grades achieve surprisingly high hole expansion values considering the strength, as seen in Figure 6.
Figure 6: Hole Expansion as a Function of Edge Quality and Microstructure. Adapted from Citation.H-7
Knowing that a higher volume fraction of martensite is needed to increase strength, combined with the awareness that minimizing the hardness differences between microstructural phases is needed to increase hole expansion (Figure 7), allows steelmakers to fine-tune their chemistry and mill processing to target specific balances of strength, tensile elongation, and cut edge expandability as measured in a tensile test.
Figure 7: Improved Hole Expansion by Reducing the Hardness Difference between Ferrite and Martensite.H-8
This expands the selection of grades from which manufacturers can choose. Traditional material selection and identification may have been based on tensile strength to satisfy structural requirements – DP980 is a dual phase steel with 980MPa minimum tensile strength. However, newly engineered grade options offer users an extra level of refinement depending on the functional needs of the part. Products can be specified as needing high tensile elongation, high hole expansion, or a balance of these two. In the example shown in Figure 8, note that all 3 grades have nearly identical tensile strength.
Figure 8: Engineered Microstructures Achieve Targeted Product Characteristics. (Data from Citations N-8 and F-5)
The influence of microstructure and the hardness differences between the phases is also seen in the hole expansion values of AHSS grades at strengths below 980 MPa. A study from 2016 shows the impact of a small amount of martensite on a ferrite-bainite microstructure.N-9 Both products compared had a microstructure of 80% ferrite. In one product, the remaining phase was only bainite, while the other had both martensite and bainite. The presence of just 8% martensite was sufficient to decrease the hole expansion capacity by 40%. (Figure 9).
Figure 9: High Hardness Differences in Microstructural Phases Decrease Edge Ductility at All Strength Levels. Adapted from Citation N-9.
Rolling direction may also influence edge fracture sensitivity on some multiphase AHSS grades. When testing a sample, edge fractures may occur first at the hole edge along the rolling direction, which corresponds to a tensile axis in the transverse direction. If the chosen grade exhibits this behavior, locate stretch flanges perpendicular to the rolling direction when possible during die and process development to increase resistance to edge fracture. If this is not practical, identify locations where inserting scallops/notches in the stretch flange will not negatively impact the part structure, fit or die processing.
During die development and die try-out, it is important to use the production-intent AHSS grade – not just one that has the same tensile strength. Blank orientation relative to the rolling direction in these trials must also be production-intent. Often the blank die is the last completed die, so prototype blanks may be prepared by laser, EDM, water jet or even by hand during tryout. These cutting methods will have different sheared edge extension, as measured by the hole expansion test, compared with the production-intent shearing. These differences may be sufficiently significant to prevent replication of production conditions in tryout.
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.
![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)
