You’ll find most of this content as part of our page on Press Hardening Steel Grades, but this month, we want to highlight it in our AHSS Insights blog. Thanks to Eren Billur, Ph.D., President, Billur Metal Form, for providing this information.
What Are Press Hardened Steels and How Are They Made?
Press hardening is a special hot forming process, where the part is quenched in a forming die to receive its high hardness. It has been used in the automotive industry for over 40 years now.
The most common press hardening steel (PHS) is 22MnB5, a low carbon steel with Manganese-Boron alloying. Since it achieves a typical tensile strength of 1500 MPa after heat treatment, this material is mostly named as PHS1500 or CR1500T-MB (Cold Rolled, 1500 MPa typical Tensile strength, Manganese-Boron alloyed).B-14, V-19
The Direct Press Hardening Process
The direct press hardening process involves heating the blanks over 900°C (1650°F) in an industrial furnace. The blanks are then removed from the furnace and quickly transferred to a forming die. The formed parts are not removed immediately. Instead, they are kept under force in a water-cooled tool set for quenching. With a 22MnB5 steel, the quenched part typically reaches 1500 MPa tensile strength. B-14
Coatings and Early Process Enhancements
Over the years, the first improvement on the 22MnB5 material was the application of an aluminum-silicon coating around early 2000’s. The addition of the coating did not affect its strength or elongation but improved the process as it eliminated scale formation during forming and quenching. AlSi coating however limited the process to the aforementioned direct process.B-14
Zinc Coatings and the Indirect Process
Some OEMs, especially in Europe, wanted to use Zn-based coatings for corrosion protection. The typical 22MnB5 is available with hot-dip galvanized (GI) and galvannealed (GA) coatings. Liquid metal embrittlement (LME) with Zn-based coatings is avoided with an indirect press hardening process. Forming is done at ambient temperatures, with the part subsequently heated and quenched in a press tool. These materials and techniques have been available since 2008.P-5
Figure 1 summarizes the most common 22MnB5 grades and coatings before and after the press hardening process.
Figure 1: 22MnB5 before and after the hot stamping and quenching cycle. The incoming material is similar to HSLA 380 or DP600 and can be cold formed if needed. After hot stamping, typical tensile strength is around 1500 MPa (re-created after: B-18, O-8, U-9).
Measuring Performance: VDA Standards
In 2010, German Association of the Automotive Industry (VDA) developed a new bending test (238-100) to evaluate energy absorbing capacity of PHS and PQS grades.U-9 This test gave a “bending angle” measurement, which replaced – to some extent – the use of “total elongation” value for energy absorbing calculations.
PHS1800 and Beyond
In 2011, a Japanese steel maker developed the first 1800 MPa (typical) tensile strength material. The material was AlSi coated with a modified, higher carbon, 30MnB5 chemistry and Nb alloying.H-33 One Japanese OEM applied the material in their bumper beams. The higher strength allowed using 1.4 mm thick PHS1800 material, instead of 1.6 mm PHS1500.M-28
Tailored Solutions for Specific Applications
A German OEM designed a B-pillar with a PHS1500 upper section, laser welded to a lower section formed from HSLA 340LA (340 MPa yield strength) steel for improved energy absorption.S-13 It was later found that typical HSLA steels not designed for hot stamping process may show significant variation in mechanical properties depending on the cooling rate.D-22
The Rise of Press Quenched Steels (PQS)
Steel companies subsequently developed “Press Quenched Steels” (PQS) which are also HSLA but have been specifically modified to achieve consistent material properties at varying cooling rates.H-69 PQS grades are not hardenable, even after hot stamping and quenching cycle.
Expanding Options: Pre-Cooled and Composite Steels
In 2015, a steel company in Europe developed 20MnB8 with GI coating. Chemistry with slightly lower carbon and higher manganese allowed forming to be done at lower temperatures. The company developed a new process route where the heated blank is first pre-cooled to around 500°C (930°F) and then formed and quenched – solving any LME concerns. The grade’s mechanical properties are nearly identical to 22MnB5 after quenching.K-21 Thus, it may be called PHS1500, but to differentiate the material, they are typically named CR1500T-MB-PS (PS stands for Pre-cooled Stamping).V-9
Improving Formability and Weldability
In 2016, two different composite steels were developed for hot stamping. These are 3-layers, hot rolled cladded grades with PQS on the outer skin and PHS1500 in the core. These were 1200 and 1400 MPa tensile strength level grades, with significantly improved bendability. There is only a commercial name for this material. To avoid using those names, the grades may be referred to as PHS1200 Sandwich and PHS1400 Sandwich.L-68
Around 2016, steel makers started developing another PHS grade which has 1000-1200 MPa tensile strength after quenching. The grade had almost similar elongation with PHS1500 (almost 5%), but higher bendability (75° vs. 50°). These grades also have lower metallurgical notch effects when spot welded. The material may be named CR1100T-MB.V-9
Multi-Step and Air-Hardening Innovations
In 2019, a Japanese steel company developed “air-hardening” 22MnSiB9-5 alloy with GA coating. After hot forming and quenching, the material had mechanical properties almost equivalent to 22MnB5. Thus, this material can also be named as PHS1500. Since the material is air-hardenable, meaning that it hardens even at very low cooling rates, it can be hot formed in a multi-station servo-mechanical-transfer press [16]. The technique is then named as “multi-step hot forming”, with the grade referred to as CR1500T-MB-MS (the last MS stands for Multi-Step).V-9
Ultra High Strength and the VDA Naming System
Since 2020, steel companies rolled out 1900 or 2000 MPa (typical) tensile strength materials. These grades are now commonly referred to as CR1900T-MB. These grades are already available uncoated, AlSi coated or GA coated.V-9
In 2021, VDA published a new standard (239-500), which standardizes the naming, chemistry and mechanical properties of PHS and PQS grades. All the grades shown in Figure 2 (excluding the sandwich) are named based on this VDA standard.V-9
Figure 2: Stress-strain curves of commercially available PHS and PQS grades after quenching. (re-created after: B-18, Y-12, R-14).
The UniSteel Concept: One Alloy, Many Properties
In late 2021, researchers from China came up with a concept of using one chemistry (a modified 22MnB5) combined with different thermal processes to tailor the production of differing mechanical properties. Thus, it became possible to make a whole car from the same alloy, named as “UniSteel”.The different properties and their use areas are shown in Figure 3. The research was published in Science magazine.L-68
Figure 3: UniSteel concept: (a) material usage in a car body, (2) mechanical properties after heat treatments (re-created after L-68).
The Future: BQP and SIBORA Development
In 2025, a German consortium developed a new grade 37SiB6 and a new process route called Bainitizing, Quenching and Partitioning (BQP). Similar to the Chinese UniSteel concept, the new SIBORA (Silicon Boron with Retained Austenite) material can have various strength and elongation levels. Both the process and resulting mechanical properties are given in Figure 4. Different strength levels can be achieved by changing the bainitizing temperature between 360 and 460°C (680 and 860°F).O-15
Figure 4: (a) the BQP process (shown here is 360°C bainitizing temperature), (b) the mechanical properties after PHS or BQP processes (re-created after O-15).
We encourage you to visit this steel grades page to learn more about these grades available for Press Hardening, and head to this PHS and PQS Overview page for our PHS Primer. Thank you to Eren Billur for providing this information.
Thanks go to Eren Billur, Ph.D. for his contribution of this article to the AHSS Insights blog. Eren Billur is the Technical Manager of Billur Makine and Billur Metal Form, based in Ankara, Turkey, specializing in advanced sheet metal forming technologies. He holds a Ph.D. in Mechanical Engineering from The Ohio State University and has extensive experience in material characterization, sheet metal forming processes, and finite element simulations. Eren has contributed significantly to the understanding and application of hot stamping and advanced high-strength steels (AHSS) in the automotive industry. He is a regular columnist for MetalForming Magazine’s “Cutting Edge” column and has authored numerous scientific papers and book chapters, including contributions to the WorldAutoSteel AHSS Applications Guidelines. Passionate about advancing manufacturing knowledge, Eren provides engineering consulting, training, and simulation services worldwide, helping manufacturers optimize forming processes and successfully implement new-generation AHSS materials.
Liquid Metal Embrittlement (LME) during Resistance Spot Welding (RSW) can cause cracks when welding advanced high strength steels. Recent advances in steel metallurgy, resistance spot welding processing and accompanying simulation tools have substantially improved the way that LME can be handled in industrial practice. This article gives a brief overview of easy measures to implement when LME might potentially occur during production.
Introduction
During resistance spot welding of zinc-coated advanced high strength steels (AHSS) liquid metal embrittlement -related cracking may be observed. Since LME is often associated with a reduction of steel’s mechanical properties, it is desired to control its occurrence during production. An exemplary LME crack, forced with increased weld heat and deliberate electrode misalignment, is shown below.
Figure 1: A typical LME crack created under laboratory conditions by deliberately increasing the welding time and introducing 5° electrode tilt
Over the past several years, LME has been a a focus in welding research. It is now well-understood to the degree that it can be predicted and avoided with easy measures. Below is an overview of four key steps to address the potential of LME during automotive production.
Obtain the latest steel grades from your steel supplier
Over the past decade, steel producers have released AHSS with improved chemical compositions, helping to significantly reduce the occurrence of LME iIt is beneficial to talk with steel suppliers and ask about their latest AHSS grades, as these are likely far less sensitive to LME than previously tested grades. A recent study commissioned by WorldAutoSteel demonstrated that all five chosen material stack-ups from current production data did not show any LME even under exacerbated conditions. Only by choosing an especially difficult material stack-up could LME be forced to appear at all to conduct the study.
Read up on the current state of research for LME
WorldAutoSteel has published two studies on liquid metal embrittlement: One focused on lab conditions and the second on real-life stamped components. These studies provide an overview of all aspects of LME and how to manage and avoid LME issues.
Establish in-house testing protocols to gauge the sensitivity of your material stack-ups
To investigate LME in-house, it’s critical to establish a testing protocol that forces the cracks to appear and allows for comparison of different steels, stack-ups and welding parameters. as there There is currently no industry-wide agreed-upon testing standard.
Still, there is a good selection of well-documented procedures to choose from. The easiest procedure is to increase the welding time until cracks start to appear – keep in mind that you need to remove the zinc coating before you can observe any cracks on the surface.
Other methods are based on so-called “Gleeble testing” or on deliberately introducing imperfections like tilted electrodes or large gaps into the welds. As you establish a testing procedure in your lab, you can use it to evaluate LME occurrence in the stack-ups that you want to implement into body-in-whites.
Think about implementing LME mitigation strategies in your most difficult welds
Suitable measures should always be adapted to the specific use case. Generally, the most effective measures for LME prevention or mitigation are:
Avoidance of excessive heat input (e.g. excess welding time, current)
Avoidance of sharp edges on spot welding electrodes; instead use electrodes with larger working plane diameter, while not increasing nugget-size
Employing extended hold times to allow for sufficient heat dissipation and lower surface temperatures
Avoidance of improper welding equipment (e.g. misalignments of the welding gun, highly worn electrodes, insufficient electrode cooling)
These measures can be implemented in the planning stage and in an ongoing production environment to increase the LME-free process windows.
In Conclusion
While Liquid Metal Embrittlement may present a challenge when welding AHSS, it’s no longer an unpredictable threat. Thanks to advancements in steel development, welding techniques, and testing methods, manufacturers have the tools they need to reliably mitigate LME during production.
Staying informed, working closely with steel suppliers, implementing smart testing protocols, and applying targeted welding strategies can help automakers maintain both strength and quality in AHSS joints. With this proactive approach, LME doesn’t have to stand in the way of innovation in automotive manufacturing.
Tensile testing is one of the most basic formability characterization methods available. Results from tensile testing are a key input into metal forming simulations, but if the right information isn’t included, the simulation will not accurately reflect material behavior.
Metal forming simulation is particularly beneficial on the value-added parts made from advanced high strength steels, since accurate simulations allow for optimal processing with minimal recuts … at least when the right information is used as inputs.
Tensile Testing
During tensile testing, a standard sample shape called a dogbone is pulled in tension. Load and displacement are recorded, and which are then converted to a stress-strain curve. Strength is defined as load divided by cross-sectional area. Exactly when the cross-sectional area is measured during the test influences the results.
Before starting the pull, it’s easiest to measure the width and thickness of the test sample.
Engineering Stress-Strain Curve
At any load, the engineering stress is the load divided by this initial cross-sectional area. Engineering stress reaches a maximum at the Tensile Strength, which occurs at an engineering strain equal to Uniform Elongation. After that point, engineering stress decreases with increasing strain, progressing until the sample fractures.
However, metals get stronger with deformation through a process known as strain hardening or work hardening. As a tensile test progresses, additional load must be applied to achieve further deformation, even after the “ultimate” tensile strength is reached. Understanding true stress and true strain helps to address the need for additional load after the peak strength is reached.
During the tensile test, the width and thickness shrink as the length of the test sample increases. Although these dimensional changes are not considered when determining the engineering stress, they are of primary importance when determining true stress. At any load, the true stress is the load divided by the cross-sectional area at that instant.
True Stress-Strain Curve
The true stress – true strain curve gives an accurate view of the stress-strain relationship, one where the stress is not dropping after exceeding the tensile strength stress level.
True stress is determined by dividing the tensile load by the instantaneous area.
True stress-strain curves obtained from tensile bars are valid only through uniform elongation due to the effects of necking and the associated strain state on the calculations. Inaccuracies are introduced if the true stress-true strain curve is extrapolated beyond uniform strain, and as such a different test is needed. Biaxial bulge testing has been used to determine stress-strain curves beyond uniform elongation. Optical measuring systems based on the principles of Digital Image Correlation (DIC) are used to measure strains. The method by which this test is performed is covered in ISO 16808.
Stress-strain curves and associated parameters historically were based on engineering units, since starting dimensions are easily measured and incorporated into the calculations. These are the values you see on certified metal properties, also called metal cert sheets that you get with your steel shipments.
True stress and true strain provide a much better representation of how the material behaves as it is being deformed, which explains its use in computer forming and crash simulations.
It’s much more challenging to get accurate dimensional measurements once the test has started unless there are multiple loops of the operator stopping the test, remeasuring, then restarting the pull. This is not a practical approach.
Fortunately, there are equations that relate engineering units to true units. Conventional stress-strain curves generated in engineering units can be converted to true units for inclusion in simulation software packages.
As the industry moves to more value-added stampings, metal forming simulation is done on nearly every part. The value-added nature of parts made from advanced high strength steels requires best practices be used throughout – otherwise the results from simulation drift further away from matching reality, leading to longer development times and costly recuts.
The discussions relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid formed.
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous factors. This blog covers some important considerations related to welding these materials for automotive applications. Most important is the discussion on Resistance Spot Welding (RSW) as it is the dominating process in automotive manufacturing.
Setting Correct Welding Parameters for Resistance Spot Welding
Specific welding parameters need to be developed for each combination of material type and thickness. In general, the Hot Press (HP) steels require more demanding process conditions. One important factor is electrode force which should be higher for the HP steel than for cold press type steel of the same thickness. The actual recommended force will depend on the strength level, and the thickness of the steel. Of course, this will affect the welding machine/welding gun force capability requirement.
Another important variable is the welding current level and even more important is the current range at which acceptable welds can be made. The current range is weldability measurement, and the best indicator of the welding process robustness in the manufacturing environment and sometime called proceed window. Note the relative range of current for different steel types. A smaller process window may require more frequent weld quality evaluation such as for weld size.
Relative Current Range (process windows) for Different Steel Types
The Effect of Coating Type on Weldability
In all cases of resistance spot welding coated steels, it is imperative to move the coating away from the weld area during and in the beginning of the weld cycle to allow a steel-to-steel weld to occur. The combination of welding current, weld time and electrode force are responsible for this coating displacement.
For all the coated steels, the ability of the coating to flow is a function of the coating type and properties, such as electrical resistivity and melting point, as well as the coating thickness.
An example of cross sectioned spot welds made on Hot Press Steel with Aluminum -Silicon coating is shown below. It shows two coating thicknesses and the displaced coating at the periphery of weld. This figure also shows the difference in current range for the different coating thickness. The thicker coating shows a smaller current range. In addition, the Al-Si coating has a much higher melting point than the zinc coatings on the cold stamped steels, making it more difficult to displace from the weld area.
Hot Press Steel with Aluminum -Silicon
Liquid Metal Embrittlement and Resistance Spot Welding
Cold-formable, coated, Advance High Strength Steels such as the 3rd Generation Advanced HighStrength Steels are being widely used in automotive applications. One welding issue these materials encounter is the increased hardness in the weld area, that sometime results in brittle fracture of the weld.
Another issue is their sensitivity to Liquid Metal Embrittlement (LME) cracking. These two issues are discussed in detail on the WorldAutoSteel AHSS Guidelines website and our recently released Phase 2 Report on LME.
Resistance Spot Welding Using Current Pulsation
The most effective solution for the issues described above is using current pulsation during the welding cycle. A schematic description is shown below.
The pulsation allows much better control of the heat generation and the weld nugget development. The pulsation variables include the number of pulses (typically 2-4), the current level and time for each pulse, and the cool time between the pulses.
In summery, pulsation (and sometime current upslope) in Resistance Spot Welding proved to be beneficial for the following applications:
PHS steels
Coated Cold Stamped steels
Cold stamped Advance High Strength Steels
Multi materials stack-ups – As described in our articles here on 3T/4T and 5T Stack-Ups
Thanks is given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University and Technical Editor – Joining, AHSS Application Guidelines, for this article.
Grade Options and Corrosion Protection Considerations When Deciding How A Part Gets Formed
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous ramifications impacting multiple departments. Our prior blog on this topic highlighted the equipment differences and the property development differences between each approach. We continue this blog series, now focusing on grade options and corrosion protection.
The discussions below relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid-formed.
Grade Options for Cold Stamped or Hot Formed Steel
There are two types of parts needed for vehicle safety cage applications: those with the highest strength that prevent intrusion, and those with some additional ductility that can help with energy absorption. Each of these types can be achieved via cold stamping or hot stamping.
When it comes to cold stamped parts, many grade options exist at 1000 MPa that also have decent ductility. The advent of the 3rd Generation Advanced High Strength Steels adds to the tally. Most of these top out at 1200 MPa, with some companies offering cold-formable Advanced High Strength Steels with 1400 or 1500 MPa tensile strength. The chemistry of AHSS grades is a function of the specific characteristics of each production mill, meaning that OEMs must exercise diligence when changing suppliers.
Figure 1: Stress-strain curve of industrially produced QP980.W-35
Martensitic gradesfrom the steel mill have been in commercial production for many years, with minimum strength levels typically ranging from 900 MPa to 1470 MPa, depending on the grade. These products are typically destined for roll forming, except for possibly those at the lower strengths, due to limited ductility. Until recently, MS1470, a martensitic steel with 1470 MPa minimum tensile strength, was the highest strength cold formable option available. New offerings from global steelmakers now include MS1700, with a 1700 MPa minimum tensile strength, as well as MS 1470 with sufficient ductility to allow for cold stamping. Automakers have deployed these grades in cold stamped applications such as crossmembers and roof reinforcements.
Figure 2: Cold-Stamped Martensitic Steel with 1500 MPa Tensile Strength used in the Nissan B-Segment Hatchback.K-57
Until these recent developments, hot stamping was the primary option to reach the highest strength levels in part shapes having even mild complexity. Under proper conditions, a chemistry of 22MnB5 could routinely reach a nominal or aim strength of 1500 MPa, which led to this grade being described as PHS1500, CR1500T-MB, or with similar nomenclature. Note that in this terminology, 1500 MPa nominal strength typically corresponds to a minimum strength of 1300 MPa.
The 22MnB5 chemistry is globally available, but the coating approaches discussed below may be company-specific.
The spectrum of grades available for cold-stamped and hot formed steel parts allows automakers to fine-tune the crash energy management features within a body structure, contributing to steel’s “infinite tune-ability” capability which gives automotive engineers design flexibility and freedoms not available from other structural materials.
Corrosion Protection
Uncoated versions of a grade must take a different chemistry approach than the hot dip galvanized (GI) or hot dipped galvannealed (HDGA) versions since the hot dip galvanizing process acts as a heat treatment cycle that changes the properties of the base steel. Steelmakers adjust the base steel chemistry to account for this heat treatment to ensure the resultant properties fall within the grade requirements.
Figure 3: Schematic of a typical hot-dipped galvanizing line with galvanneal capability.
This strategy has limitations as it relates to grades with increasing amounts of martensite in the microstructure. Complex thermal cycles are needed to produce the highly engineered microstructures seen in advanced steels. Above a certain strength level, it is not possible to create a GI or HDGA version of that grade.
For example, when discussing fully martensitic grades from the steel mill, hot dip galvanizing is not an option. If a martensitic grade needs corrosion protection, then electrogalvanizing is the common approach since an EG coating is applied at ambient temperature, which is low enough to avoid negatively impacting the properties. Automakers might choose to forgo a galvanized coating if the intended application is in a dry area that is not exposed to road salt.
Figure 4: Schematic of an electrogalvanizing line.
For press hardening steels, coatings serve multiple purposes. Without a coating, uncoated steels will oxidize in the austenitizing furnace and develop scale on the surface. During hot stamping, this scale layer limits efficient thermal transfer and may prevent the critical cooling rate from being reached. Furthermore, scale may flake off in the tooling, leading to tool surface damage. Finally, scale remaining after hot stamping is typically removed by shot blasting, an off-line operation that may induce additional issues.
Using a hot dip galvanized steel in a conventional direct press hardening process (blank -> heat -> form/quench) may contribute to liquid metal embrittlement (LME). Getting around this requires either changing the steel chemistry from the conventional 22MnB5 or using an indirect press hardening process that sees the bulk of the part shape formed at ambient temperatures followed by heating and quenching.
Those companies wishing to use the direct press hardening process can use a base steel having an aluminum-silicon (Al-Si) coating, providing that the heating cycle in the austenitizing furnace is such that there is sufficient time for alloying between the coating and the base steel. Welding practices using these coated steels need to account for the aluminum in the coating, but robust practices have been developed and are in widespread use.
Equipment, Responsibilities, and Property Development Considerations When Deciding How A Part Gets Formed
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous ramifications impacting multiple departments. Over a series of blogs, we’ll cover some of the considerations that must enter the discussion.
The discussions relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid-formed.
Equipment
There is a well-established infrastructure for cold stamping. New grades benefit from servo presses, especially for those grades where press force and press energy must be considered. Larger press beds may be necessary to accommodate larger parts. As long as these factors are considered, the existing infrastructure is likely sufficient.
Progressive-die presses have tonnage ratings commonly in the range of 630 to 1250 tons at relatively high stroke rates. Transfer presses, typically ranging from 800 to 2500 tons, operate at relatively lower stroke rates. Power requirements can vary between 75 kW (630 tons) to 350 kW (2500 tons). Recent transfer press installations of approximately 3000 tons capacity allow for processing of an expanded range of higher strength steels.
Hot stamping requires a high-tonnage servo-driven press (approximately 1000 ton force capacity) with a 3 meter by 2 meter bolster, fed by either a roller-hearth furnace more than 30 m long or a multi-chamber furnace. Press hardened steels need to be heated to 900 °C for full austenitization in order to achieve a uniform consistent phase, and this contributes to energy requirements often exceeding 2 MW.
Integrating multiple functions into fewer parts leads to part consolidation. Accommodating large laser-welded parts such as combined front and rear door rings expands the need for even wider furnaces, higher-tonnage presses, and larger bolster dimensions.
Blanking of coils used in the PHS process occurs before the hardening step, so forces are low. Post-hardening trimming usually requires laser cutting, or possibly mechanical cutting if some processing was done to soften the areas of interest.
That contrasts with the blanking and trimming of high strength cold-forming grades. Except for the highest strength cold forming grades, both blanking and trimming tonnage requirements are sufficiently low that conventional mechanical cutting is used on the vast majority of parts. Cut edge quality and uniformity greatly impact the edge stretchability that may lead to unexpected fracture.
Responsibilities
Most cold stamped parts going into a given body-in-white are formed by a tier supplier. In contrast, some automakers create the vast majority of their hot stamped parts in-house, while others rely on their tier suppliers to provide hot stamped components. The number of qualified suppliers capable of producing hot stamped parts is markedly smaller than the number of cold stamping part suppliers.
Hot stamping is more complex than just adding heat to a cold stamping process. Suppliers of cold stamped parts are responsible for forming a dimensionally accurate part, assuming the steel supplier provides sheet metal with the required tensile properties achieved with a targeted microstructure.
Suppliers of hot stamped parts are also responsible for producing a dimensionally accurate part, but have additional responsibility for developing the microstructureand tensile properties of that part from a general steel chemistry typically described as 22MnB5.
Property Development
Independent of which company creates the hot formed part, appropriate quality assurance practices must be in place. With cold stamped parts, steel is produced to meet the minimum requirements for that grade, so routine property testing of the formed part is usually not performed. This is in contrast to hot stamped parts, where the local quench rate has a direct effect on tensile properties after forming. If any portion of the part is not quenched faster than the critical cooling rate, the targeted mechanical properties will not be met and part performance can be compromised. Many companies have a standard practice of testing multiple areas on samples pulled every run. It’s critical that these tested areas are representative of the entire part. For example, on the top of a hat-section profile where there is good contact between the punch and cavity, heat extraction is likely uniform and consistent. However, on the vertical sidewalls, getting sufficient contact between the sheet metal and the tooling is more challenging. As a result, the reduced heat extraction may limit the strengthening effect due to an insufficient quench rate.
Thanks are given to Eren Billur, Ph.D., Billur MetalForm for his contributions to the Equipment section, as well as many of the webpages relating to Press Hardening Steels at www.AHSSinsights.org.
Thanks go to Eren Billur, Ph.D. for his contribution of this article to the AHSS Insights blog. Eren Billur is the Technical Manager of Billur Makine and Billur Metal Form, based in Ankara, Turkey, specializing in advanced sheet metal forming technologies. He holds a Ph.D. in Mechanical Engineering from The Ohio State University and has extensive experience in material characterization, sheet metal forming processes, and finite element simulations. Eren has contributed significantly to the understanding and application of hot stamping and advanced high-strength steels (AHSS) in the automotive industry. He is a regular columnist for MetalForming Magazine’s “Cutting Edge” column and has authored numerous scientific papers and book chapters, including contributions to the WorldAutoSteel AHSS Applications Guidelines. Passionate about advancing manufacturing knowledge, Eren provides engineering consulting, training, and simulation services worldwide, helping manufacturers optimize forming processes and successfully implement new-generation AHSS materials.
