Fundamentals of cold formable HSLA steels

High Strength Low Alloy (HSLA) steels, or more precisely microalloyed low carbon steels, are the classic solution for the automotive industry when high strength and good cold formability is required. By adding small amounts of niobium, titanium, vanadium strength is increased by grain refinement, precipitation, and solid solution hardening without significantly impairing formability. These steels are supplied in different strength levels as hot or cold rolled grades; the annealing after cold rolling might be of the batch annealing or the continuous annealing type. The mechanical properties of HSLA steels are not only determined by their chemical composition but also by the processing parameters during hot rolling, cold rolling, and annealing. While the different yield strength levels can be easily reached by adding microalloying or solid solution strengthening elements, formability is strongly affected by the large number of fine precipitates, which retard recrystallization and texture formation and result in poor r-values. Thus the deep drawing properties are limited, while the stretch formability is attractive due to the pronounced strain hardening when the precipitates are distributed finely and homogeneously. The corrosion protection of HSLA sheet steels can be obtained by electro or by hot dip galvanizing; the later need a proper control of the recrystallization behavior in a hot dip galvanizing line. Recent developments are steels with isotropic plastic behavior and the processing of HSLA steels on compact strip mills to very thin gauges. The major benefit of HSLA steels are their processabilty in worldwide available classical hot and cold rolling facilities and their applicability for many different car body and under body parts. These steels represent the vast majority of currently used high strength steels in the automotive industry. (AU)