Abstract
Tailored properties in hot press forming process (HPF) were investigated with focus on phase transformation characteristics and deformation behavior of constituent base materials for the Tailor Welded Blank (TWB-HPF) and partial quenching (PQ-HPF) approaches. The former uses sheets with different hardenabilities, which results in graded mechanical properties after HPF. The latter uses a single sheet but different heat treatment conditions, which eventually induce the same effect as the first approach. Preliminary dilatometric experiment was conducted to investigate the phase transformation characteristics of the two base metals for TWB-HPF. U-channel was formed by draw bending with the two approaches. For PQ-HPF, several initial tool temperatures varying from room to 450°C were investigated. Thermo-mechanical finite element (FE) analyses of the HPF process with metallurgical considerations in the constitutive model were performed for parts exhibiting tailored properties. These analyzes included the evaluation of heat transfer characteristics and the microstructure evolutions under HPF conditions. A thermodynamic calculation program was used to determine the thermo-physical, physical and metallurgical properties for various microstructures and temperatures. FE analyzes of the hot press forming with phase transformation considerations were conducted using the commercial software DEFORMTM-3D with the DEFORMTM-HT module. The combined experimental and simulation results provided an understanding, on the one hand, of the role of phase transformation in strengthening the material and reducing springback for the TWB-HPF approach and, on the other hand, of temperature on phase transformation characteristics of HPF steel for the PQ-HPF approach.
