Analysis of Hot Press Formed Parts with Tailored Strength

Abstract

Tailored properties in hot press forming process were investigated with particular emphasis 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 method uses two sheets with different hardenabilities, which result in graded mechanical properties after heat treatment in the hot press forming process. The latter uses a single sheet but different heat treatment conditions. Dilatometric analyses were conducted to study strength differentials for various cooling rates after heat treatment. U-draw bending tests were performed to evaluate the influence of a number of process parameters, namely, blank and tool temperatures, and cooling time, on the mechanical properties of the product. 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, the microstructure evolution and the associated deformation behavior under HPF conditions. 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 for the PQ-HPF approach. In addition finite element simulation and forming tests for a newly designed lab scaled B-pillar were carried out to evaluate the formability and weld-line displacement.