Bake Hardening Behavior and Plastic–Strain–Dependent Elastic Modulus of Steel Sheets for Automotive Outer Panels

Steel sheets with bake hardening (BH) are widely used for automotive outer panels because of their high dent resistance due to a remarkable increase in yield strength induced by a paint baking process. In the dent resistance test, a panel is subjected to (i) forming and springback, (ii) BH, and (iii) concentrated loading that induces a local plastic deformation and springback. Corresponding to the above BH dent process, in this work, the effect of BH on the elastic-plastic behavior of 270D, 340BH, and 440BH steel sheets was investigated by performing uniaxial tension and compression experiments for tensile prestrained and bake-hardened specimens. Furthermore, the degradation of the elastic modulus of bake-hardened specimens with increasing plastic deformation was measured. The important finding was that the elastic modulus decreased by the plastic strain was recovered to the initial Young’s modulus by BH. Consequently, the plastic-strain-dependent elastic modulus including the BH effect was successfully expressed by the extended Yoshida–Uemori elastic model.