Consideration of phase transformations in numerical simulation of press hardening

The importance of high-strength steel concepts for car bodies has increased in the last years due to the need of reduction in weight and enhanced crash safety. It is possible to produce components with a much higher strength, e.g. a tensile strength of about R-m = 1500 MPa, compared to cold forming processes when using press hardening of boron alloyed heat-treatable steels. Moreover, parts with complex shapes can be realized. Numerical simulation by means of Finite-Element Method has become an indispensable tool for process design and construction. But for a more realistic prediction of the resulting component properties, for instance residual stresses and distortion, it is essential to consider the complex effects of phase transformation within the simulation. Because it is not a standard task currently, a material model was implemented in the commercial FE-Code LS-Dyna. The diffusion-controlled phase transformation is modelled with Johnson-Mehl-Avrami equation for isothermal transformation. The formation of martensite is described by Koistinen-Marburger equation for diffusionless transformation. The latent heat caused by austenite decomposition is also considered by implementation of a thermal model via a user subroutine. The needed isotherm time-temperature-transformation diagram is approximated by a diagram of related steel. These approaches are applied to a simple model process. In this process a round sheet metal is formed and subsequently quenched by cooled tools, therefore a thermal-mechanical sequential coupled simulation of a model process is implemented. The transport from furnace to the press and the closing of the tools are simulated in order to get a realistic temperature distribution in the sheet metal at the beginning of the forming process. The tools are modelled as deformable bodies and heat-transfer is taken into account. The simulation results show that nearly the whole austenite is transformed into martensite after the cooling phase.