Thermal conditions of a press hardening process to obtain tailored properties

By press hardening of boron-manganese-alloyed steels, extraordinary strength can be achieved. Accordingly, the process is applied for different crash relevant components in the car body structure. Aiming for an outstanding energy dissipation, sections with lower ultimate strength to the benefit of an increased elongation are required. This can be achieved by a tailored tempering process, where the uniform heating and segmented cooling figured out to be the most appropriate process route on an industrial scale. Due to the non-uniform distribution of temperature during the quenching phase and the uneven level of thermal strains in the final cooling subsequent to the forming itself, a higher level of distortion was observed. Thus, a detailed understanding of the thermal effects and flux during the whole process chain is a key condition in order to achieve stable mechanical as well as geometrical properties.
Within the recent research project gratefully supported by FOSTA, this detailed analysis of the thermal evolution of the different part sections of a tailor-tempered part were elaborated. By the repetitive crosscheck between process-integrated measurement and FEA prediction, the microstructural evolution and geometric accuracy were investigated. The relation between elastic spring back, thermal shrinkage and their interaction towards parts stress state and geometric deflection was examined. By the more detailed understanding of the underlying mechanisms, the route is opened for well-aimed compensation strategies.
While the portion of press-hardened and in particular tailor-tempered components is established and stable in the car body structure, the ongoing development towards large-sized BIW-components such as the door ring concept come into the competition. This amplifies the relevance of the component accuracy and thus of the presented topic.