Elastic-plastic constitutive model for accurate springback prediction in hot press sheet forming

The present study investigates the effect of phase transformation plasticity on the springback of hot press formed parts. Employing an implicit finite element formulation to take phase transformations during rapid cooling from fully austenitic phase and their related thermo-mechanical behavior into account, two validations - (1) the loaded dilatometry problem which induces significant plastic deformation even at lower stress levels than the classical yield stress, and (2) the 2-D draw bending problem, which introduces high temperature forming and subsequent quenching - are presented and compared with known experimental data. The study showed that the magnitude of springback predicted by considering the phase transformation plasticity and the temperature change induced volumetric strain agreed well with date from experiments, while the results determined with the conventional elastic-plasticity theory in which only plastic deformation by external load was used, had considerable deviation for the springback profile after hot press forming. The negligible springback amount after hot press forming, which has been frequently reported in many experimental observations, is due to the stress relaxation by the addition of abnormal transformation plasticity.