Investigation of Elastic Modulus Degradation and Recovery with Time and Baking Process for Deformed Automotive Steel Sheets

This study investigated the elastic recovery phenomenon with elapsed time and heat treatment after the plastic deformation of automotive steel sheets. A conventional uniaxial loading–unloading-loading test was modified to include the elapsed time and heat treatment conditions. The elastic behaviors of five automotive steel sheets, MART1500, TRIP1180, EDDQ, DP980, and BH340, were characterized after applying various pre-strains, specific elapsed time conditions, and heat treatments. The elastic behaviors were quantitatively analyzed using the conventional chord modulus definition and a new elastic modulus definition representing the initial elastic characteristics. It was observed that the elastic behavior of BH steel was the most sensitive to elapsed time and heat treatment in terms of recovery owing to the bake-hardening effect. The elastic moduli of the MART1500 and TRIP1180 steel recovered somewhat after heat treatment, whereas no recoveries of EDDQ and DP980 steels were observed. Phenomenological modeling of the recovery process was also performed. The Yoshida–Uemori modulus model was applied to the experimental results for elastic degradation. This model was then extended to consider the elapsed time and heat treatment, and the elastic recovery behaviors of the different steels were captured successfully. Graphical Abstract