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. 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. 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 tensile strength, and whether it is shown as the actual minimum or a rounded-up value. For example, consider DP780 compared with DP800. Both forms represent essentially the same grade. Some sources may describe the grade as DP800 as a colloquial simplification. It is possible that the OEM specification calls out a minimum 800 MPa tensile strength, but most specifications show the value as a minimum 780 MPa tensile strength. A steel company will supply to the actual specification, and will use different process controls to meet a 780 MPa minimum compared with an 800 MPa minimum.
Press hardening steels sometimes require a different syntax. Some OEMs will use a similar terminology as described above, for example: CR-950Y1300T-PH describes a 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 and -IS for indirect hot stamping. Furthermore, sources may describe this product focused on its typical tensile strength as PHS1500T.
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.
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 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.