Enhancements on strength of body structure due to bake hardening effect on hot stamping steel

Abstract

Body structure projects are designed to develop lightweight structures, which result in low fuel consumption and emissions for internal combustion engines and longer battery life in the case of electric vehicles. The structures need to be lightweight but also strong in order to provide maximum safety to the occupants. These premises have led to the development of special materials such as the hot stamping steels. The most used steel in this process, quenched 22MnB5, also exhibits the bake hardening effect: its yield stress increases after being exposed at temperatures close to 200 °C (typical heat treatment of paint lines in the automotive industry). The aim of this study was to verify the improvement in the mechanical strength of a body structure due to the bake hardening effect to which the hot-stamped steel components were submitted. Samples of annealed 22MnB5 steel were submitted to tensile and chemical analysis. A second group of the same steel was subjected to an additional heat treatment in a laboratory furnace simulating the heating that occurs in the body paint line. Subsequently, the same tests were performed with the samples from the first group. Tensile tests evidenced the increase of 6.5% in the yield stress of the samples that suffered the bake hardening effect. The stress-strain diagram curves were used as the input to the side crash simulation program. The simulations demonstrated that the body structure whose hot-stamped parts were subjected the bake hardening effect had a 2% reduction in side crash intrusion compared to body structure where the parts did not suffer this effect. In the case of the lateral protection bar test, the bake hardening effect provided a 5% increase in the maximum bending load.