Today’s AHSS for Automotive
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 article 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.
For more details on AHSS metallurgy, 3rd Gen steels and much more, visit the new online AHSS Application Guidelines for a full database of information. See Defining Steels, particularly, for a more details on that topic.