Automotive steels can be classified in several different ways. One is a metallurgical designation. Common designations include low-strength steels (interstitial-free and mild steels); conventional HSS (carbonmanganese, bake hardenable, high-strength interstitial-free, and high-strength, low-alloy steels); and the newer types of AHSS (dual phase, transformation-induced plasticity, complex phase, and martensitic steels). Additional higher strength steels for the automotive market include ferritic-bainitic, twinning-induced plasticity, hot-formed, and post-forming heat-treated steels.
A second classification method important to part designers is strength of the steel. Therefore, this document will use the general terms HSS and AHSS to designate all higher strength steels. In contrast, much of the current literature uses narrowly defined ranges to categorize different steel strength levels. One such system defines High-Strength Steels (HSS) as yield strengths from 210 to 550 MPa and tensile strengths from 270–700 MPa, while Ultra-High-Strength Steels (UHSS) steels have yield strengths greater than 550 MPa and tensile strengths greater than 700 MPa. These arbitrary ranges suggest discontinuous changes in formability when moving from one category to another. However, data show property changes are a continuum across the entire span of steel strengths. In addition, many steel types have a wide range of grades covering two or more strength ranges.
A third classification method presents various mechanical properties or forming parameters of different steels, such as total elongation, work hardening exponent n, or hole expansion ratio . As an example, Figure 1-1 compares total elongations – a steel property related to formability – for the different metallurgical types of steel. Figure 1-1A shows the lower strength steels in dark grey and the traditional HSS steels in light grey. Some of the early AHSS steel ellipses have colour instead of shades of gray. Figure 1-1B highlights some of the newer higher strength steels for the automotive market. Figures 1-1A and 1-1B illustrate only the relative comparison of different steel grades – not specific property ranges of each type.
Figure 1-1A – Schematic of AHSS steels (shown in colour) compared to low strength steels (dark grey) and traditional HSS (light grey).
Figure 1-1B – Schematic of newer higher strength steels utilizing unique chemistries, processing, and microstructure to gain more specific properties and forming characteristics.
The principal difference between conventional HSS and AHSS is their microstructure. Conventional HSS are single phase ferritic steels. AHSS are primarily multi-phase steels, which contain ferrite, martensite, bainite, 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 behaviour.
Since the terminology used to classify steel products varies considerably throughout the world, this document uses a combination of methods to define the steels. Each steel grade is identified by metallurgical type, yield strength (in MPa), and tensile strength (in MPa). As an example, DP 500/800 means a dual phase steel type with 500 MPa minimum yield strength and 800 MPa minimum ultimate tensile strength. This classification system was used in the ULSAB-AVC (UltraLight Steel Auto Body – Advanced Vehicle Concepts) Program.
Table 1-1 illustrates a range of AHSS grades used in the ULSAB-AVC body-structure concept design.
Table 1-1 – Examples of Steel Grade Properties from ULSAB-AVC
YS and UTS are minimum values
Tot. EL (Total Elongation) is a typical value for a broad range of thicknesses and gage lengths.
It is important to note that different specification criteria have been adopted by different automotive companies throughout the world and that steel companies have different production capabilities and commercial availability. Therefore, typical mechanical properties are shown above simply to illustrate the broad range of AHSS grades that may be available. It is imperative to communicate directly with individual steel companies to determine specific grade availability and the specific associated parameters and properties, such as:
- Mechanical properties and ranges.
- Thickness and width capabilities.
- Hot-rolled, cold-rolled, and coating availability.
- Chemical composition specifications.
If you’d like to learn more about applying advanced high-strength steels to vehicle applications, download a copy of the Advanced High-Strength Steel Application Guidelines (AHSS Guidelines), which you can download from the link at the top right of this page.