Safety-relevant components in automobiles (e. g. B-pillars or crash boxes) require materials with special properties. These are e. g. high strength combined with sufficient residual deform-ability and high energy absorption in the event of a crash. They are of great importance, e. g. for protection of occupants of a car. Targeted adjustment of a mixed microstructure and/or a gradation in components can provide a material with these properties. Therefore, the overall objective of this project is the simulation-based process control of a thermo-mechanical material processing and the resulting (graded) microstructure of the used material as well as its mechanical properties.
As an example of a coupled process, the press hardening of ultra-high-strength boron-manganese steel 22MnB5 is investigated. The target structure should be a mixture of fine-grained martensite and bainite with high strength and increased ductility compared to purely martensitic microstructure. The focus of this research project is the direct press hardening process. In this context, the influence of non-uniform process variables on the resulting micro-structure and properties of the workpiece has to be studied and described. Different austenitization and cooling strategies not only lead to a varying mixed microstructure but also to a varying grain size in the polycrystalline material, which influences the material behavior decisively, too.
To carry out suitable experiments for generation of graded structures, the construction of a new testing rig is planned. Further, an existing macroscopic model for multiphase transformations is adapted to the press hardening process, extended and applied for simulation of the process. The extensions include the consideration of carbide phases, the influence of austenitizing temperature and predeformation on the phase transformation as well as the average grain size in the material and its development. The purpose of the last-named extension is the modeling of Hall-Petch effect, i.e. the influence of the grain sizes on the flow stress.
The research project will be complemented by proper experiments for the targeted setting of graded microstructures, planned close co-operation between applicants. The data from the experiments are then used for parameter identification of the material model resulting from the research project. The new testing rig allows the generation of a hierarchical data structure, which in turn supports the stability assessment of the material parameters. Finally, the validation of the resulting material model is planned with aid of suitable demonstrators in the form of mini-B-pillars, produced via press hardening.