On the Formability of Automotive TRIP Steels 2003-01-0521
The issue of cost and weight reduction at optimum car crash safety is a driving force behind the growing use of advanced high strength steels, particularly in Europe and Japan. Recent developments in the availability of high strength steel (HSS) sheets in North America; in particular transformation induced plasticity (TRIP) steels, offer an attractive option to the automotive designer for weight reduction and improved safety performance. For example, the use of TRIP steels, as opposed to more conventional steel products such as high strength low alloy (HSLA), in some applications may result in up to 40% part weight reduction at similar vehicle crash performance. When the excellent formability of TRIP steel is considered at product design stage, it may also lead to reducing part count and tooling cost. In this paper the formability of TRIP steels of various gauges is assessed. Experimental forming limit curves (FLCs) are determined for T600 grade. These FLCs were compared to the conventional ASM FLCs calculated from n-value and sheet thickness. Results show that the experimental FLCs for TRIP steel are located within a range of approximately -1.0% to +0.5% as compared to the calculated FLC in the draw forming and plane strain forming modes. However, in the stretch forming mode the formability may exceed the conventional ASM curve by 10%. Bake hardenability resulting from a typical paint baking temperature cycle is determined for the tested material. Bendability results are also presented.