W-44. Y. Wu, A. Luo, and C. Wang, “Experimental research and application of cold cutting to hot stamping parts based on multiphase microstructure,” Baosteel Technical Research, Volume 17, Number 3, September 2023, Page 1; https://doi.org/10.3969/j.issn.1674-3458.2023.03.001
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous ramifications impacting multiple departments. The considerations 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.
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.
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 microstructure and tensile properties of that part from a general steel chemistry typically described as 22MnB5.
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.
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 – the stress-strain curve of a 3ʳᵈ Gen QP980 steel is presented in Figure 1. 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 grades from 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, with some applications shown in Figure 2.
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.
Newer PHS options with a modified chemistry and subsequent processing differences can reach nominal strength levels of 2000 MPa. Other options are available with additional ductility at strength levels of 1000 MPa or 1200 MPa. A special class called Press Quenched Steels have even higher ductility with strength as low as 450 MPa.
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.
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 (Figure 3) 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 (Figure 4) 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.
Specific welding parameters need to be developed for each combination of material type and thickness. In general, Press Hardening Steels require more demanding process conditions. One important factor is electrode force, which is typically higher than needed to weld cold formed steels of the same thickness. The actual recommended force depends on the strength level and the thickness of the steel. Of course, strength and thickness affects the welding machine/welding gun force capability requirement.
The welding current level, and more importantly, the current range are both important. The current range is one of the best indicators of welding process robustness, so it is sometimes described as the welding process window. Figure 5 indicates the relative range of current required for spot welding different steel types. A smaller process window may require more frequent weld quality evaluations, such as for adequate weld size, necessitating more frequent corrective actions to address the discovered quality concern.
Figure 5: Relative Current Range (process windows) for Different Steel Types
When resistance spot welding coated steels, the coating must be removed 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 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.
Cross sectioned spot welds made on press hardened steel with different coating weights of an Aluminum -Silicon coating is presented in Figure 6. Note the displaced coating at the periphery of weld. Figure 7 shows the difference in current range required to produce acceptable welds associated with these different coating weights. The thicker coating shows a smaller current range. In addition, the press-hardened Al-Si coating has a much higher melting point than the zinc coatings typically found on cold stamped steels, making it more difficult to displace from the weld area.
Figure 6: Aluminum -Silicon coated press hardened steels.O-16
Figure 7: Influence of Aluminum-Silicon coating weight on welding range.O-16
Cold-formable, coated, Advanced High Strength Steels are widely used in automotive applications. One welding issue these materials encounter is increased hardness in the weld area, that may result in brittle fracture of the weld. Another issue is their sensitivity to Liquid Metal Embrittlement (LME) cracking.
Both issues are discussed in detail in the Joining section of the WorldAutoSteel AHSS Guidelines website and the WorldAutoSteel Phase 2 Report on LME.
The most effective solution for these issues is using current pulsation during the welding cycle, schematically described in Figure 8.
Figure 8: Nugget growth differences in Single Pulse vs. Multi-Pulse Welding
Pulsation of the current allows much better control of the heat generation and weld nugget development. Pulsation variables include the number of pulses (typically 2 to 4), current, time for each pulse, and the cool time between the pulses.
Pulsation during Resistance Spot Welding is beneficial for press hardening steels, coated cold stamped steels of all grades, and multi-material stack-ups. Information about multi-sheet, multi-material stack-ups can be – as described in our articles on 3T/4T and 5T Stack-Ups
For more information about PHS grades and processing, see our Press Hardened Steel Primer.
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.
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.
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.
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.
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.
Danny Schaeffler is the Metallurgy and Forming Technical Editor of the AHSS Applications Guidelines available from WorldAutoSteel. He is founder and President of Engineering Quality Solutions (EQS). Danny wrote the monthly “Science of Forming” and “Metal Matters” column for Metalforming Magazine, and provides seminars on sheet metal formability for Auto/Steel Partnership and the Precision Metalforming Association. He has written for Stamping Journal and The Fabricator, and has lectured at FabTech. Danny is passionate about training new and experienced employees at manufacturing companies about how sheet metal properties impact their forming success.
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.
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.
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 microstructure and tensile properties of that part from a general steel chemistry typically described as 22MnB5.
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.
For more information, see our Press Hardened Steel Primer to learn more about PHS grades and processing!
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.
Danny Schaeffler is the Metallurgy and Forming Technical Editor of the AHSS Applications Guidelines available from WorldAutoSteel. He is founder and President of Engineering Quality Solutions (EQS). Danny wrote the monthly “Science of Forming” and “Metal Matters” column for Metalforming Magazine, and provides seminars on sheet metal formability for Auto/Steel Partnership and the Precision Metalforming Association. He has written for Stamping Journal and The Fabricator, and has lectured at FabTech. Danny is passionate about training new and experienced employees at manufacturing companies about how sheet metal properties impact their forming success.
There are different ways to classify automotive steels. One is a metallurgical designation providing some process information. Common designations include lower-strength steels (interstitial-free and mild steels); conventional high strength steels, such as bake hardenable and high-strength, low-alloy steels (HSLA); and Advanced High-Strength Steels (AHSS) such as dual phase and transformation-induced plasticity steels. Additional higher strength steels include press hardening steels and steels designed for unique applications that have improved edge stretch and stretch bending characteristics.
A second classification method important to part designers is strength of the steel. This document will use the general terms HSLA and AHSS to designate all higher strength steels. The principal difference between conventional HSLA steels and AHSS is their microstructure. Conventional HSLA steels are single-phase ferritic steels with a potential for some pearlite in C-Mn steels. AHSS are primarily steels with a multiphase microstructure containing one or more phases other than ferrite, pearlite, or cementite – for example martensite, bainite, austenite, and/or retained austenite in quantities sufficient to produce unique mechanical properties. Some types of AHSS have a higher strain hardening capacity resulting in a strength-ductility balance superior to conventional steels. Other types have ultra-high yield and tensile strengths and show a bake hardening behavior.
AHSS include all martensitic and multiphase steels having a minimum specified tensile strength of at least 440 MPa. Those steels with very high minimum specified tensile strength are sometimes referred to as Ultra High Strength Steels (UHSS). Several companies choose 980 MPa as the threshold where “Ultra” high strength begins, while others use higher thresholds of 1180 MPa or 1270 MPa. There is no generally accepted definition among the producers or users of the product. The difference between AHSS and UHSS is in terminology only – they are not separate products. The actions taken by the manufacturing community to form, join, or process is ultimately a function of the steel grade, thickness, and mechanical properties. Whether these steels are called “Advanced” or “Ultra” does not impact the technical response.
Third Generation, or 3rd Gen, AHSS builds on the previously developed 1st Gen AHSS (DP, TRIP, CP, MS, and PHS) and 2nd Gen AHSS (TWIP), with global commercialization starting around 2020. 3rd Gen AHSS are multi-phase steels engineered to develop enhanced formability as measured in tensile, sheared edge, and/or bending tests. Typically, these steels rely on retained austenite in a bainite or martensite matrix and potentially some amount of ferrite and/or precipitates, all in specific proportions and distributions, to develop these enhanced properties.
Historically, HSLA steels were described by their minimum yield strength. Depending on the region, the units may have been ksi or MPa, meaning that HSLA 50 and HSLA 340 both describe a High Strength Low Alloy steel with a minimum yield strength of 50 ksi = 50,000 psi ≈ 340 MPa. Although not possible to tell from this syntax, many of the specifications stated that the minimum tensile strength was 70 MPa to 80 MPa greater than the minimum yield strength.
Development of the initial AHSS grades evolved such that they were described by their metallurgical approach and minimum tensile strength, such as using DP590 to describe a dual phase steel with 590 MPa tensile strength. Furthermore, when Advanced High Strength Steels were first commercialized, there was often only one option for a given metallurgical type and tensile strength level. Now, for example, there are multiple distinct dual phase grades with a minimum 980 MPa tensile strength, each with different yield strength or formability.
To highlight these different characteristics throughout this website, each steel grade is identified by whether it is hot rolled or cold rolled, minimum yield strength (in MPa), minimum tensile strength (in MPa), and metallurgical type. Table 1 lists different types of steels.
Table 1: Different Types of Steels and Associated Abbreviations.
As an example, CR-500Y780T-DP describes a cold rolled dual phase steel with 500 MPa minimum yield strength and 780 MPa minimum ultimate tensile strength. There is also another grade with the same minimum UTS, but lower yield strength: CR440Y780T-DP. If the syntax is simply DP780, the reader should assume either that the referenced study did not distinguish between the variants or that the issues described in that section applies to all variants of a dual phase steel with a minimum 780 MPa tensile strength.
Another syntax issue is the presentation of the strength (yield or tensile), and whether it is rounded to the nearest 10 or 50 MPa. For example, consider DP980 compared with DP1000. Both forms represent essentially the same grade. In Europe, this steel may be described as having a tensile strength of 100 kgf/mm2, corresponding to 981 N/mm2 (981 MPa), and expressed as DP980. In Asia, the steel may be referred to as 100K (an abbreviation for 100 kgf/mm2). In other parts of the world, it may be rounded to nearest 50 MPa, as DP 1000. This naming approach applies to many grades, with some shown in Table 2. In some cases, although the OEM specification may list the steel as DP800 (for example), the minimum tensile strength requirement may still be 780 MPa. Furthermore, independent of the chosen naming syntax, the steel company will supply to the actual specification requirements, and will use different process controls to meet a 780 MPa minimum compared with an 800 MPa minimum.
Table 2: Syntax Related to AHSS Strength Levels
Press hardening steels sometimes require a different syntax. Some OEMs will use a similar terminology as described above. For example: CR-950Y1300T-PH (PH stands for Press Hardenable or Press Hardened) or CR-950Y1300T-MB (MB stands for Manganese-Boron steel) can describe the same cold rolled press hardening steel with 950 MPa minimum yield strength and 1300 MPa minimum tensile strength after completing the press hardening operation. Other specifications may show suffixes which highlight the forming process used, such as -DS for direct hot stamping, -IS for indirect hot stamping and MS for a multi-step process. Furthermore, sources may describe this product focused on its typical tensile strength as PHS1500T. The abbreviation PQS (Press Quenched Steel) is typically used for grades that do not harden after hot stamping. These may be noted as PQS450 and PQS550, where the numbers stand for the approximate minimum tensile strength after the hot stamping cycle (see the section on Grades With Higher Ductility on the linked page).
Generally, elongation (a measure of ductility) decreases as strength increases. Plotting elongation on the vertical axis and strength on the horizontal axis leads to a graph starting in the upper left (high elongation, lower strength) and progressing to the lower right (lower elongation, higher strength). This shape led to the colloquial description of calling this the banana diagram.
Figure 1: A generic “banana” diagram comparing strength and elongation.
With the continued development of advanced steel options, it is no longer appropriate to describe the plethora of options as being in the shape of a banana. Instead, with new grades filling the upper right portion (see Figure 2), perhaps it is more accurate to describe this as the football diagram as the options now start to fall into the shape of an American or Rugby Football. Officially, it is known as the steel Global Formability Diagram.
Figure 2: The Global Formability Diagram comparing strength and elongation of current and emerging steel grades. Click here for a high resolution download. Source: Courtesy of WorldAutoSteel
Even this approach has its limitations. Elongation is only one measure of ductility. Other ductility parameters are increasingly important with AHSS grades, such as hole expansion and bendability. BillurB-61 proposed a diagram comparing the bend angle determined from the VDA238-100 testV-4 with the yield strength for various press hardened and press quenched steels.
Figure 3: VDA Bending Angle typically decreases with increasing yield strength of PHS/PQS grades.B-61
Figure 4 shows a local/global formability map sometimes referred to as the Hance Diagram named after the researcher who proposed it.H-16 This diagram combines measures of local formability (characterized by true fracture strain) and global formability (characterized by uniform elongation), providing insight on different characteristics associated with many steel grades and helping with application-specific material grade selection. For example, if good trim conditions still create edge splits, selecting materials higher on the vertical axis may help address the edge-cracking problems. Likewise, global formability necking or splitting issues can be solved by using grades further to the right on the horizontal axis.
Figure 4: The Local/Global Formability Map combines measures of local formability (true fracture strain) and global formability (uniform elongation) to highlight the relative characteristics of different grades. In this version from Citation D-12, the colors distinguish different options at each tensile strength level.
Previous AHSS Application Guidelines showcased a materials portfolio driven by the FutureSteelVehicle (FSV) program, with more than twenty new grades of AHSS acknowledged as commercially available by 2020. The AHSS materials portfolio continues to grow, as the steel industry responds to requirements for high strength, lightweight steels. Table 3 reflects available AHSS grades as well as grades under development and nearing commercial application. The Steel Grades page provides details about these grades and their applications.
Table 3: Commercially available AHSS Grades and grades under development for near-term application.
Grade names shown in Italicized Bold were available in FutureSteelVehicle.
For all but PQS/PHS, Grade Name indicates the minimum yield strength, minimum tensile strength, and the type of AHSS.
PQS/PHS grade name indicates nominal tensile strength.
The min EL column indicates a typical minimum total elongation value, which may vary based on test sample shape, gauge length, and thickness.
The typ λ column indicates a typical hole expansion percentage, which may vary based on test sample diameter, punch shape, edge preparation techniques, and several other factors.
Zinc coatings may lower these hole expansion values by 10%-15%.
The ISO 16630 test for hole expansion is known to produce results with more scatter than desired.
Globally, many companies and consortia are attempting to converge on a robust edge stretch characterization procedure that produces meaningful results without undue scatter.
See the pages on Edge Stretch Characterization for more information.
You are advised to contact your steel supplier for specific guidance on the products available and the associated properties.
Global automakers create steel specification criteria suited for their vehicle targets, manufacturing infrastructure, and other constraints. Although similar specifications exist at other companies, perfect overlap of all specifications is unlikely. Global steelmakers have different equipment, production capabilities, and commercial availability.
Minimum or typical mechanical properties shown on this web page and throughout this site illustrates the broad range of AHSS grades that may be available. Properties of hot rolled steels can differ from cold rolled steels. Coating processes like hot dip galvanizing or galvannealing subjects the base metal to different thermal cycles that affect final properties. Test procedures and requirements have a regional or OEM influence, such as preference to using tensile test gauge length of 50 mm or 80 mm, or specifying minimum property values parallel or perpendicular to the rolling direction.
Steel users must communicate directly with individual steel companies to determine specific grade availability and the specific associated parameters and properties, such as: