Determination of the forming limits of martensitic chromium steels in hot sheet forming

Hot forming of martensitic chromium steels in the field of sheet metal forming has various process as well as component-side advantages compared to the press hardening of conventional used manganese-boron steels, such as a lower critical cooling rate, a higher adjustable elongation at fracture with simultaneously high strengths and the better corrosion resistance of the material. Hot metal sheet forming and, in particular, press hardening involve a complex interaction between blank temperature, tool temperature and process time, which significantly influences microstructure development, final component properties, residual stresses and distortion. Therefore, numerical modelling is essential for an efficient process design. Especially the higher material costs when using martensitic chromium steels compared to manganese-boron steels make a numerical process design indispensable in order to save resources, trials and thus costs. To date, no commercial simulation or material models are known for modelling the hot sheet metal forming of martensitic chromium steels. For this purpose, a comprehensive material characterisation of the material used must be carried out and the obtained data must be modelled using suitable mathematical approaches. For the determination of the forming capacity of sheet materials, the use of forming limit curves is state of the art. Since the flow behaviour and forming limit of a material depend on temperature, conventional forming limit curves recorded at room temperature cannot be used. Therefore, the further aim of this project is to record forming limit curves at high temperature. The material model in combination with the determined forming limit curves enables the forming capacity during hot sheet forming and the resulting component properties after cooling to be calculated realistically. This enables the potential of the new material to be fully exploited.