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Please cite this article in press as: Hippchen, P., et al., Modelling kinetics of phase transformation for the indirect hot stamping process to focus on car body parts with tailored properties. J. Mater. Process. Tech. (2015), http://dx. a b s t r a c tTo design the indirect hot stamping process, a finite element method (FEM) based prediction of the part geometry and the mechanical properties is required. In case of indirect hot stamping processes, producing car body parts with tailored properties, cooling paths occur causing diffusionless and diffusion controlled phase transformations. The volume expansion caused by the phase transformation of face-centred cubic (fcc) into body-centred cubic (bcc) and the martensitic formation of body-centred tetragonal (bct) leads to transformation induced strains that are important for the calculation of overall stresses in hot stamped car body parts. To calculate the strain and stress state correctly, it is necessary to model the diffusionless and diffusion controlled phase transformation phenomena, taking into account the boundary conditions of indirect hot stamping processes. The existing material models are analysed and extended in order to improve their prediction accuracy in calculating the amount and distribution of ferrite, perlite, bainite and martensite during the whole process of annealing. For industrial use the new approaches are implemented in the FE-code LS-DYNA 971 (Livermore Software Technology Corporation, 2006).

Press hardening is a well-established production process in the automotive industry today. The actual trend of this process technology points towards the manufacturing of parts with tailored properties. Since the knowledge of the mechanical properties of a structural part after forming and quenching is essential for the evaluation of for example the crash performance, an accurate as possible virtual assessment of the production process is more than ever necessary. In order to achieve this, the definition of reliable input parameters and boundary conditions for the thermo-mechanically coupled simulation of the process steps is required. One of the most important input parameters, especially regarding the final properties of the quenched material, is the contact heat transfer coefficient (IHTC). The CHTC depends on the effective pressure or the gap distance between part and tool. The CHTC at different contact pressures and gap distances is determined through inverse parameter identification. Furthermore a simulation strategy for the subsequent steps of the press hardening process as well as adequate modeling approaches for part and tools are discussed. For the prediction of the yield curves of the material after press hardening a phenomenological model is presented. This model requires the knowledge of the microstructure within the part. By post processing the nodal temperature history with a CCT diagram the quantitative distribution of the phase fractions martensite, bainite, ferrite and pearlite after press hardening is determined. The model itself is based on a Hockett-Sherby approach with the Hockett-Sherby parameters being defined in function of the phase fractions and a characteristic cooling rate.

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