Yield Stress Anisotropy by Straining and Baking of Bake-hardenable Steel

Abstract

Bake-hardenable steel sheets have been applied in exterior body panels of automobiles for thickness reduction without deteriorating the panel dent-resistance. The increase in flow yield stress due to bake hardening depends largely on the amount of prestrain and strain path changes, before and after baking. In the present study, the strain-path effects on bake hardening properties have been examined by changing the direction of subsequent tensile tests after uniaxial tension, plane-strain and equi-biaxial stretching as prestrain for a ferrite based bake-hardenable steel sheet. The yield stress showed a strong anisotropy by prestraining. However, the anisotropy caused by prestraining was weakened by bake hardening treatments. The Taylor-theory calculations of yield stress based on the texture measurements revealed that the evolution of texture due to the prestraining is not responsible for the strong anisotropy prestrained materials observed in the experiments. The strain path changes can be characterized by a scalar product of previous and current strain-rate mode tensors. The subsequent yield stress after strain path changes of different combination could be qualitatively estimated by using this parameter, which showed a correlation between the strong anisotropy of the yield stress after prestraining and the microstructural evolution in the prestraining.