On Formability Assessment of the Automotive Dual Phase Steels

The issue of improving car crash energy management and maintaining cost and weight reduction are the driving forces behind the growing use of advanced high strength steels, particularly in Europe and Japan. Recent developments in the manufacture of high strength steel (HSS) sheets in North America, in particular Dual Phase (DP) steels, offers an attractive option to the automotive designer for weight reduction and improved safety performance. For example, the use of dual phase steels, as opposed to more conventional steel products such as high strength low alloys (HSLA), in some cases may result in up to 40% part weight reduction at similar vehicle crash performance. In this paper the formability of commercially produced hot-dipped dual phase steels of various gages and grades is assessed. Forming Limit Curves (FLCs) are determined for commercial DP590 and DP780 grades. These FLCs were compared to the conventional ASM FLCs calculated from n-value and sheet thickness. Results show that the experimental FLCs for DP materials are located within a range of approximately -1.5% to +0.5% as compared to the calculated FLC. However, the overall shapes of the experimental curves are similar to the conventional ones. It has been found that for some testing conditions the experimental points are located significantly above the theoretical curve. Bake hardenability resulting from a typical paint baking temperature cycle are determined for all tested DP materials. Bendability results are also presented.