Modeling of tailor hardened boron steel for crash simulation

The automotive industry is continuously working on the weight reduction of their vehicles, while maintaining or even improving the crashworthiness in accordance with increasing safety demands. This has favored the development of innovative materials and production processes. The hot stamping process, in which heated blanks are simultaneously formed and quenched in cooled dies, is one of these recent innovations. Due to the high cooling rates during quenching, an ultra high material strength of up to 1500 MPa can be obtained. By locally reducing the cooling rate during quenching, e.g. by using partially heated dies, regions of lower strength with increased ductility can be introduced. The resulting, so-called tailor hardened components are made out of a single sheet of metal and feature precisely defined zones of varying strength and ductility. To be able to fully exploit the possibilities of tailor hardened components, it is important to attain accurate predictive models of their crash response. In this thesis, a hardness-dependent material model for quench-hardenable, boron-alloyed steel 22MnB5 is presented. The material model is able to predict the material properties both in homogeneous areas of different strengths as well as in hardness transition zones. The model is designed to provide an accurate prediction of the deformation behavior of the material up to the point of fracture. Moreover, it includes a fracture prediction criterion that accounts for the complex loading paths experienced by the material in the event of a crash. An important aspect that has to be considered when using high strength steels for crash-relevant structural components is the change in microstructure in the proximity of thermal joints. During resistance spot welding, thermodynamically unstable phases of the hot formed material transform into softer microstructures. Under complex loading conditions, such as the simultaneous bending and stretching of a B-pillar in a side crash, strains might localize in the so-called softened heat-affected zones, which ultimately can lead to fracture. The hardness-based material model is extended to account for these effects. The presented model can be used in crashworthiness simulations to accurately capture the fracture behavior of resistance spot welded, tailor hardened 22MnB5.