Thermo-mechanical-metallurgical modeling for hot-press forming in consideration of the prior austenite deformation effect

In this study, a prior austenite grain refinement model was incorporated into semi-empirical diffusive transformation kinetics for application to hot-press forming. In particular, the kinetics equations were modified to include the effects of boron addition and austenite deformation on transformation behaviors during forming. To simulate the hot-press forming process, a thermo-mechanical-metallurgical model was formulated implicitly and implemented into the finite element program ABAQUS using the user subroutines UMAT and UMATHT. This nonconventional finite element modeling is appropriate to consider thermal- and transformation-associated strains. The proposed model was validated through simple finite element simulation examples, i.e., dilatometry simulation with and without external loading, and hot torsion and quenching of a rod. Finally, the hot-press forming of a U-channel-type part was simulated to study the effect of austenite deformation on the phase kinetics, hardness and residual stress. The simulation results showed that the austenite deformation had considerable influence on the final strength and residual stress distribution in the hot-press formed sheet, which resulted from an increase in ferritic phases due to the modified kinetics. In particular, the austenite deformation effect was more noticeable in the side-wall region of the U-channel where plastic deformation was the most severe.