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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 microstructure and 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.
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.
![Martensite]()
1stGen AHSS, AHSS, Steel Grades
Martensitic steels are characterized by a microstructure that is mostly all martensite, but possibly also containing small amounts of ferrite and/or bainite (Figure 1 and 2). Steels with a fully martensitic microstructure are associated with the highest tensile strength – grades with a tensile strength of 2000 MPa is commercially available, and higher strength levels are under development.
Figure 1: Schematic of a martensitic steel microstructure. Ferrite and bainite may also be found in small amounts.
Figure 2: Microstructure of MS 950/1200
To create MS steels, the austenite that exists during hot-rolling or annealing is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. Adding carbon to MS steels increases hardenability and strengthens the martensite. Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel are also used in various combinations to increase hardenability.
These steels are often subjected to post-quench tempering to improve ductility, so that extremely high strength levels can be achieved along with adequate ductility for certain forming processes like Roll Forming.
Figure 3 shows MS950/1200 compared to HSLA. Engineering and true stress-strain curves for MS steel grades are presented in Figures 4 and 5.
Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and MS 950/1200.
Figure 4: Engineering stress-strain curves for a series of MS steel grades.S-5 Sheet thicknesses: 1.8 mm to 2.0 mm.
Figure 5: True stress-strain curves for a series of MS steel grades.S-5 Sheet thicknesses: 1.8 mm to 2.0mm.
In addition to being produced directly at the steel mill, a martensitic microstructure also can be developed during the hot stamping of press hardening steels.
Examples of current production grades of martensitic steels and typical automotive applications include:
| MS 950/1200 |
Cross-members, side intrusion beams, bumper beams, bumper reinforcements |
| MS 1150/1400 |
Rocker outer, side intrusion beams, bumper beams, bumper reinforcements |
| MS 1250/1500 |
Side intrusion beams, bumper beams, bumper reinforcements |
Some of the specifications describing uncoated cold rolled 1st Generation martensite steel (MS) are included below, with the grades typically listed in order of increasing minimum tensile strength. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements.
- ASTM A980M, with Grades 130 [900], 160 [1100], 190 [1300], and 220 [1500]A-23
- VDA 239-100, with the terms CR860Y1100T-MS, CR1030Y1300T-MS, CR1220Y1500T-MS, and CR1350Y1700T-MSV-3
- SAE J2745, with terms Martensite (MS) 900T/700Y, 1100T/860Y, 1300T/1030Y, and 1500T/1200YS-18
Case Study: Using Martensitic Steels
as an Alternative to Press Hardening Steel
– Laboratory Evaluations
Martensitic steel grades provide a cold formed alternative to hot formed press hardening steels. Not all product shapes can be cold formed. For those shapes where forming at ambient temperatures is possible, design and process strategies must address the springback which comes with the high strength levels, as well as eliminate the risk of delayed fracture. The potential benefits associated with cold forming include lower energy costs, reduced carbon footprint, and improved cycle times compared with hot forming processes.
Highlighting product forms achievable in cold stamping, an automotive steel Product Applications Laboratory formed a Roof Center Reinforcement from 1.4 mm CR1200Y1470T-MS using conventional cold stamping rather than roll forming, Figure 6. Using cold stamping allows for the flexibility of considering different strategies when die processing which may result in reduced springback or incorporating part features not achievable with roll forming.
Figure 6: Roof Center Reinforcement cold stamped from CR1200Y1470T-MS martensitic steel.U-1
Cold stamping of martensitic steels is not limited to simpler shapes with gentle curvature. Shown in Figure 7 is a Center Pillar Outer, cold stamped using a tailor welded blank containing CR1200Y1470T-MS and CR320Y590T-DP as the upper and lower portion steels.U-1
Figure 7: Center Pillar Outer stamped at ambient temperature from a tailor welded blank containing 1470 MPa tensile strength martensitic steel.U-1
Another characteristic of martensitic steels is their high yield strength, which is associated with improved crash performance. In a laboratory environment, crash behavior is assessed with 3-point bending moments. A studyS-8 determined there was a correlation between sheet steel yield strength and the 3-point bending deformation of hat shaped parts. Based on a comparison of yield strength, Figure 8 shows that CR1200Y1470T-MS has similar performance to hot stamped PHS-CR1800T-MB and PHS-CR1900T-MB at the same thickness and exceeds the frequently used PHS-CR1500T-MB. For this reason, there may be the potential to reduce costs and even weight with a cold stamping approach, providing appropriate press, process, and die designs are used.
Figure 8: Effect of Yield Strength on Bending Moment. The right image shows the typical yield strength range of CR1030Y1300T-MS and CR1200Y1470T-MS as well as typical yield strength values of several Press Hardened Steels.S-8
Case Study: Martensitic Steels as an Alternative to
Press Hardening Steel – Automotive Production Examples
with Springback Mitigation Strategies
Recent years have seen some applications typically associated with press hardening steels transition to a cold stamped martensitic steel, CR1200Y1470T-MS. One such example is found in the third-generation Nissan B-segment hatchback (2020 start of production), which uses 1.2 mm thick CR1200Y1470T-MS as the material for the Second Cross Member Reinforcement.K-45
Using the carbon equivalent formula Ceq=C+Si/30+Mn/20+2P+4S K-45, the newly developed martensitic grade has a carbon equivalent value of 0.28, which is lower than the 0.35 associated with the conventional PHS grade of comparable tensile strength, 22MnB5 (PHS 1500T). The lower carbon equivalent value is expected to translate into easier welding conditions. Furthermore, conventional mechanical trimming and piercing equipment and techniques work with the cold formable martensitic grade, whereas parts formed from press hardening steels typically require laser trimming or other more costly approaches. An evaluation of delayed fracture found no evidence of this failure mode.
Figure 9 highlights this reinforcement, with its placement on the cross member and in the vehicle shown in red. The varying elevation of this part, combined with a non-uniform cross section at the outermost edges, help control springback, but makes roll forming significantly more challenging if that were the cold forming approach.
Figure 9: Cold-Stamped Martensitic Steel with 1500 MPa Tensile Strength used in the Nissan B-Segment Hatchback.K-57
Unbalanced stresses in stamped parts lead to several types of shape fixability issues collectively called springback. In hat shape wall sections, shape fixing beads sometimes referred to as stake beads (see Post Stretch information at this link) mitigate sidewall curl by imparting a tensile stress state on both the top and bottom sheet surfaces and increasing the rigidity. Springback control to limit flexing down the length of longitudinally curved parts requires a different technique. Here, the root cause is the stress difference between the tensile stress at the punch top and the compressive stress at the flange at bottom dead center of the press stroke. Figure 10 presents schematics of the stress distribution when the punch is located at bottom dead center of the press stroke, and the shape fixability issue after load removal.
Figure 10: Cold-Stamped Martensitic Steel With 1500 MPa Tensile Strength Used in the Lexus NXJ-24
A patented approach known as Stress Reverse Forming™T-44 improved dimensional accuracy in the second-generation Lexus NX (2021 start of production) center roof reinforcement, cold stamped from martensitic steel, CR1200Y1470T-MS.J-24 Figure 11 shows different views of this part.
Figure 11: Left Image: Springback Differences Exist in Coils at the Low and High End of the Strength Specification; Right Image: Stress Reverse Forming™ Process Reduces Sensitivity to Springback (Images Adapted from Citation T-29)
Stress Reverse Forming™T-44 uses the principles of the Bauschinger Effect to reverse the direction of the forming stresses during a restrike forming process to achieve a final part closer to the targeted dimensions.T-29 Parts processed with this two-step approach are first over-formed to a smaller radius of curvature than the final part shape. Removing the part from the tool after this first forming step results in the stress distribution seen in the left image in Figure 10. The unique aspect of this approach comes from the second forming step where the tool shape forces the punch top into slight compression while the lower flange is put into slight tension. The tool shape used in this stage contains a slightly greater radius of curvature than the targeted part shape. As shown in Citation T-29, this process appears to be equally effective at all steel strengths.
Without effective countermeasures, springback increases with part strength. Related to this is the springback difference between coils having strength at the lowest and highest ends of the acceptable property range. This can lead to substantial differences in springback between coils completely within specification. However, after using effective countermeasures such as Stress Reverse Forming™ described in Citation T-29, springback differences between coils are minimized, which leads to increased dimensional accuracy and more consistent stamping performance. This phenomenon is shown schematically in Figure 12. Furthermore, unlike conventional stamping approaches, the amount of springback in parts made with this approach does not increase with steel strength.
Figure 12: Left Image: Springback Differences Exist in Coils at the Low and High End of the Strength Specification; Right Image: Stress Reverse Forming™ Process Reduces Sensitivity to Springback (Images Adapted from Citation T-29)
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Metallurgy
Basis
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.
Nomenclature
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).
Graphical Presentation
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.
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.
Grade Portfolio
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. The min EL column indicates a typical minimum total elongation value, which may vary based on test sample shape, gauge length, and thickness. PQS/PHS grade name indicates nominal tensile strength.
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:
- Chemical composition specifications,
- Mechanical properties and ranges,
- Thickness and width capabilities,
- Hot-rolled, cold-rolled, and coating availability,
- Joining characteristics.
1stGen AHSS, AHSS, Steel Grades
Dual Phase (DP) steels have a microstructure consisting of a ferritic matrix with martensitic islands as a hard second phase, shown schematically in Figure 1. The soft ferrite phase is generally continuous, giving these steels excellent ductility. When these steels deform, strain is concentrated in the lower-strength ferrite phase surrounding the islands of martensite, creating the unique high initial work-hardening rate (n-value) exhibited by these steels. Figure 2 is a micrograph showing the ferrite and martensite constituents.
Figure 1: Schematic of a Dual Phase steel microstructure showing islands of martensite in a matrix of ferrite.
Figure 2: Micrograph of Dual Phase Steel
Hot rolled DP steels do not have the benefit of an annealing cycle, so the dual phase microstructure must be achieved by controlled cooling from the austenite phase after exiting the hot strip mill finishing stands and before coiling. This typically requires a more highly alloyed chemistry than cold rolled DP steels require. Higher alloying is generally associated with a change in welding practices.
Continuously annealed cold-rolled and hot-dip coated Dual Phase steels are produced by controlled cooling from the two-phase ferrite plus austenite (α + γ) region to transform some austenite to ferrite before a rapid cooling transforms the remaining austenite to martensite. Due to the production process, small amounts of other phases (bainite and retained austenite) may be present.
Higher strength dual phase steels are typically achieved by increasing the martensite volume fraction. Depending on the composition and process route, steels requiring enhanced capability to resist cracking on a stretched edge (as typically measured by hole expansion capacity) can have a microstructure containing significant quantities of bainite.
The work hardening rate plus excellent elongation creates DP steels with much higher ultimate tensile strengths than conventional steels of similar yield strength. Figure 3 compares the engineering stress-strain curve for HSLA steel to a DP steel curve of similar yield strength. The DP steel exhibits higher initial work hardening rate, higher ultimate tensile strength, and lower YS/TS ratio than the HSLA with comparable yield strength. Additional engineering and true stress-strain curves for DP steel grades are presented in Figures 4 and 5.
Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and DP 350/600K-1
Figure 4: Engineering stress-strain curves for a series of DP steel grades.S-5, V-1 Sheet thicknesses: DP 250/450 and DP 500/800 = 1.0mm. All other steels were 1.8-2.0mm.
Figure 5: True stress-strain curves for a series of DP steel grades.S-5, V-1 Sheet thicknesses: DP 250/450 and DP 500/800 = 1.0mm. All other steels were 1.8-2.0mm.
DP and other AHSS also have a bake hardening effect that is an important benefit compared to conventional higher strength steels. The extent of the bake hardening effect in AHSS depends on an adequate amount of forming strain for the specific chemistry and thermal history of the steel.
In DP steels, carbon enables the formation of martensite at practical cooling rates by increasing the hardenability of the steel. Manganese, chromium, molybdenum, vanadium, and nickel, added individually or in combination, also help increase hardenability. Carbon also strengthens the martensite as a ferrite solute strengthener, as do silicon and phosphorus. These additions are carefully balanced, not only to produce unique mechanical properties, but also to maintain the generally good resistance spot welding capability. However, when welding the higher strength grades (DP 700/1000 and above) to themselves, the spot weldability may require adjustments to the welding practice.
Examples of current production grades of DP steels and typical automotive applications include:
| DP 300/500 |
Roof outer, door outer, body side outer, package tray, floor panel |
| DP 350/600 |
Floor panel, hood outer, body side outer, cowl, fender, floor reinforcements |
| DP 500/800 |
Body side inner, quarter panel inner, rear rails, rear shock reinforcements |
| DP 600/980 |
Safety cage components (B-pillar, floor panel tunnel, engine cradle, front sub-frame package tray, shotgun, seat) |
| DP 700/1000 |
Roof rails |
| DP 800/1180 |
B-Pillar upper |
Some of the specifications describing uncoated cold rolled 1st Generation dual phase (DP) steel are included below, with the grades typically listed in order of increasing minimum tensile strength and ductility. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements.
- ASTM A1088, with the terms Dual phase (DP) steel Grades 440T/250Y, 490T/290Y, 590T/340Y, 780T/420Y, and 980T/550YA-22
- EN 10338, with the terms HCT450X, HCT490X, HCT590X, HCT780X, HCT980X, HCT980XG, and HCT1180XD-6
- JIS G3135, with the terms SPFC490Y, SPFC540Y, SPFC590Y, SPFC780Y and SPFC980YJ-3
- JFS A2001, with the terms JSC590Y, JSC780Y, JSC980Y, JSC980YL, JSC980YH, JSC1180Y, JSC1180YL, and JSC1180YHJ-23
- VDA 239-100, with the terms CR290Y490T-DP, CR330Y590T-DP, CR440Y780T-DP, CR590Y980T-DP, and CR700Y980T-DPV-3
- SAE J2745, with terms Dual Phase (DP) 440T/250Y, 490T/290Y, 590T/340Y, 6907/550Y, 780T/420Y, and 980T/550YS-18
1stGen AHSS, 3rdGen AHSS, AHSS, Steel Grades
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The microstructure of Transformation Induced Plasticity (TRIP) steels contains a matrix of ferrite, with retained austenite, martensite, and bainite present in varying amounts. Production of TRIP steels typically requires the use of an isothermal hold at an intermediate temperature, which produces some bainite. Higher silicon and carbon content of TRIP steels result in significant volume fractions of retained austenite in the final microstructure. Figure 1 shows a schematic of TRIP steel microstructure, with Figure 2 showing a micrograph of an actual sample of TRIP steel. Figure 3 compares the engineering stress-strain curve for HSLA steel to a TRIP steel curve of similar yield strength.
Figure 1: Schematic of a TRIP steel microstructure showing a matrix of ferrite, with martensite, bainite and retained austenite as the additional phases.
Figure 2: Micrograph of Transformation Induced Plasticity steel.
Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and TRIP 350/600.K-1
During deformation, the dispersion of hard second phases in soft ferrite creates a high work hardening rate, as observed in the DP steels. However, in TRIP steels the retained austenite also progressively transforms to martensite with increasing strain, thereby increasing the work hardening rate at higher strain levels. This is known as the TRIP Effect. This is illustrated in Figure 4, which compares the engineering stress-strain behavior of HSLA, DP and TRIP steels of nominally the same yield strength. The TRIP steel has a lower initial work hardening rate than the DP steel, but the hardening rate persists at higher strains where work hardening of the DP begins to diminish. Additional engineering and true stress-strain curves for TRIP steel grades are shown in Figure 5.
Figure 4: TRIP 350/600 with a greater total elongation than DP 350/600 and HSLA 350/450. K-1
Figure 5: Engineering stress-strain (left graphic) and true stress-strain (right graphic) curves for a series of TRIP steel grades. Sheet thickness: TRIP 350/600 = 1.2mm, TRIP 450/700 = 1.5mm, TRIP 500/750 = 2.0mm, and Mild Steel = approx. 1.9mm. V-1
The strain hardening response of TRIP steels are substantially higher than for conventional HSS, resulting in significantly improved formability in stretch deformation. This response is indicated by a comparison of the n-value for the grades. The improvement in stretch formability is particularly useful when designers take advantage of the improved strain hardening response to design a part utilizing the as-formed mechanical properties. High n-value persists to higher strains in TRIP steels, providing a slight advantage over DP in the most severe stretch forming applications.
Austenite is a higher temperature phase and is not stable at room temperature under equilibrium conditions. Along with a specific thermal cycle, carbon content greater than that used in DP steels are needed in TRIP steels to promote room-temperature stabilization of austenite. Retained austenite is the term given to the austenitic phase that is stable at room temperature.
Higher contents of silicon and/or aluminum accelerate the ferrite/bainite formation. These elements assist in maintaining the necessary carbon content within the retained austenite. Suppressing the carbide precipitation during bainitic transformation appears to be crucial for TRIP steels. Silicon and aluminum are used to avoid carbide precipitation in the bainite region.
The carbon level of the TRIP alloy alters the strain level at which the TRIP Effect occurs. The strain level at which retained austenite begins to transform to martensite is controlled by adjusting the carbon content. At lower carbon levels, retained austenite begins to transform almost immediately upon deformation, increasing the work hardening rate and formability during the stamping process. At higher carbon contents, retained austenite is more stable and begins to transform only at strain levels beyond those produced during forming. At these carbon levels, retained austenite transforms to martensite during subsequent deformation, such as a crash event.
TRIP steels therefore can be engineered to provide excellent formability for manufacturing complex AHSS parts or to exhibit high strain hardening during crash deformation resulting in excellent crash energy absorption.
The additional alloying requirements of TRIP steels degrade their resistance spot-welding behavior. This can be addressed through weld cycle modification, such as the use of pulsating welding or dilution welding. Table 1 provides a list of current production grades of TRIP steels and example automotive applications:
Table 1: Current Production Grades Of TRIP Steels And Example Automotive Applications.
Some of the specifications describing uncoated cold rolled 1st Generation transformation induced plasticity (TRIP) steel are included below, with the grades typically listed in order of increasing minimum tensile strength and ductility. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements.
• ASTM A1088, with the terms Transformation induced plasticity (TRIP) steel Grades 690T/410Y and 780T/440YA-22
• JFS A2001, with the terms JSC590T and JSC780TJ-23
• EN 10338, with the terms HCT690T and HCT780TD-18
• VDA 239-100, with the terms CR400Y690T-TR and CR450Y780T-TRV-3
• SAE J2745, with terms Transformation Induced Plasticity (TRIP) 590T/380Y, 690T/400Y, and 780T/420YS-18
Transformation Induced Plasticity Effect
Austenite is not stable at room temperature under equilibrium conditions. An austenitic microstructure is retained at room temperature with the combined use of a specific chemistry and controlled thermal cycle. Deformation from sheet forming provides the necessary energy to allow the crystallographic structure to change from austenite to martensite. There is insufficient time and temperature for substantial diffusion of carbon to occur from carbon-rich austenite, which results in a high-carbon (high strength) martensite after transformation. Transformation to high strength martensite continues as deformation increases, as long as retained austenite is still available to be transformed.
Alloys capable of the TRIP effect are characterized by a high ductility – high strength combination. Such alloys include 1st Gen AHSS TRIP steels, as well as several 3rd Gen AHSS grades like TRIP-Assisted Bainitic Ferrite, Carbide Free Bainite, and Quench & Partition Steels.
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