Bainitic transformation for press hardening applications: A multiscale FE-MPF coupled approachWednesday (26.09.2018) 14:30 - 14:45 S1/01 - A2 Part of:
The use of press-hardened parts in the automotive industry has continuously increased in the last decades. By means of pre-heated tools (350-550 °C) bainitic microstructure is produced improving ductility. Quenching and forming are performed simultaneously in this process. Hence, bainitic transformation occurs under high stresses and in pre-deformed austenite, involving the displacive transformation of undercooled austenite into bainitic ferrite, accompanied by carbon diffusion and carbides precipitation. In this work, the material response of the 22MnB5 manganese-boron steel grade is characterized using a thermomechanical simulator (Gleeble® 3500), dilatometer tests and electron backscatter diffraction (EBSD) analysis. The experimental results demonstrate significant influence of process parameters on the final microstructure and thus the mechanical properties. The bainitic variant selection (“Magee effect”) is examined using EBSD prior-austenite reconstruction techniques. To understand the phase evolution and mechanical response of the material during such complex thermo-mechanical processes, a coupling between macroscopic finite element (FE) simulations and a mesoscopic multi-phase field (MPF) model is proposed, where special focus is placed on the thermodynamic consistency of the multiscale approach. The stress-induced migration of phase boundaries, the partitioning of carbon and the consequent carbide precipitation are modeled by means of the MPF method, in a mesoscopic representative volume element (RVE). Macroscopically incremental results, such as stress, strain and temperature, are provided to the meso-model (the MPF domain) as boundary conditions, while the homogenized phase fraction and constitutive response obtained in the MPF domain is given back to the FE model for further calculation.