Automotive steels can be divided into a number of ways. The first classification method is a metallurgical group that provides certain amount of process information. General classifications include high- strength steel (HSS), low-strength steel, and the latest AHSS steel.
HSS are bake-hardenable steels that contain carbon-manganese and are characterized by high strength and low alloy content. Low-strength steels comprise mild and interstitial-free steels, while the AHSS includes twinning-induced plasticity, transformation-induced plasticity, dual phase, ferritic-bainitic, martensitic, and complex phase steels.
Other higher strength steels for the automotive industry comprise steels that are subjected to hot-formed and post-formed heat treatments, and steels that are developed for specific applications such as stretch and edge stretch bending.
The second classification method includes the strength of the steel, which holds significant importance to part designers. Therefore, general terms like HSS and AHSS are used in this article to specify steels with higher strength. However, this classification system has a drawback with the current development of new grades of steel. As a result, a TRIP or DP steel can have strength grades covering two or more strength ranges.
The third classification method provides the range of forming parameters or mechanical properties of different types of steels, such as hole expansion ratio (λ) or total elongation, work hardening exponent (n-value). For instance, Figure 1 compares the total elongation with the tensile strength of the present type of steel. These parameters are viral in virtual forming analyses and press shop operations.
Figure 1. Global formability diagram for today’s AHSS grades (includes comparison of traditional low-strength and high-strength steels).
HSS and AHSS Grade Steels
Both HSS and AHSS steels have a different microstructure. HSS are one-phase ferritic steels with a possibility to form pearlite in carbon-manganese steels, while AHSS are steels with a microstructure comprising a phase barring cementite, ferrite or pearlite; for instance austenite, bainite, martensite and/or retained austenite in adequate quantities to create special mechanical properties.
In addition, there are certain types of AHSS that exhibit a higher strain hardening capacity. This results in excellent strength and ductility when compared to that of traditional steels. Other types of AHSS are also available that exhibit excellent tensile strengths and high yields, and display a bake hardening behavior.
All AHSS grade steels are manufactured by controlling the cooling rate and chemistry from the austenite or from the austenite and ferrite phase. This is performed either in the cooling part of the continuous annealing furnace (for hot-dip coated products) or on the hot mill’s runout table (for hot-rolled products). Research has given way to novel chemical and processing combinations that have helped in producing not only additional grades of steel, but also aided in promoting enhanced properties in each type of AHSS.
Terminology of Steel Products
In view of the fact that the terminology utilized to categorize steel products differs significantly across the globe, the steels were defined using the World Auto Steel format. Each grade of steel is recognized by metallurgical type, minimum tensile strength (in MPa), and minimum yield strength (in MPa). For instance, DP 500/800 refers to a dual phase steel having a minimum yield strength and a ultimate tensile strength of 500MPa and 800MPa, respectively. This type of classification system was first used by the ULSAB-AVC programme.
The basic metallurgy of traditional low- and high-strength steels is well known by manufacturers and users alike. Given the fact that the metallurgy and processing of AHSS grades is rather new when compared to that of traditional steels, this article describes steels to provide a fundamental understanding of how their unmatched mechanical properties originate from their exclusive processing and structure.
Download the Advanced High-Strength Steels Applications Guidelines free here, to learn more about the metallurgy, forming and joining of these new steels.
This information has been sourced, reviewed and adapted from materials provided by WorldAutoSteel (World Auto Steel).
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