The bake hardening mechanism of dual-phase silicon steels under high pre-strain

Abstract

In this paper, the effect of silicon content in the range of 0.34–2.26 wt% was investigated on the mechanism of bake hardening of dual-phase steels. It was observed that with increasing silicon content before the bake hardening process, the yield and ultimate tensile strength decreased. However, uniform and total elongation increased. Bake hardening value increased initially with the silicon content. However, it decreased with further increasing silicon content, which this was discussed based on the microstructural evolutions during bake hardening. It was seen that the martensite phase during the bake hardening process was tempered significantly by increasing in the silicon content. The changes in the yield and tensile strength due to increase in silicon content were proportional to the bake hardening behavior. The total elongation after bake hardening improved with increasing silicon content to 1.51%. However, it was reduced with further increasing silicon content. Silicon partitioning in the ferrite in the bake-hardened samples was higher than unbaked samples and thus the ferrite solid solution strengthening increased during bake hardening. By comparing mechanical properties of dual-phase steels before and after the bake hardening process, it was found that the bake hardening process has a positive effect on the yield strength, while its effect on the tensile strength was negative in the sample with the high silicon content. The ductility of all samples increased significantly after bake hardening, indicating a useful effect of bake hardening on ductility. Fractography also displayed that the fracture of all bake-hardened samples is ductile type.