2006 Volume 92 Issue 7 Pages 457-463
The reduction in car weight has led to demand for higher strength steels. However, increasing strength generally has a negative effect on formability. The high strength steels being developed are divided broadly into two types: steels characterized by high uniform elongation such as DP and TRIP steels having a heterogeneous microstructure, and steels characterized by high local elongation such as bainite steel having a homogeneous microstructure. High strength steels with both high uniform elongation and local elongation are under investigation.
In this study, various levels of carbon content and hot-rolling reduction in DP steel containing Si, Mn, Cr, and Mo were examined. The effects of grain size and volume fraction of martensite on the mechanical properties of DP steel were studied. It was found that increasing reduction, i.e., decreasing the diameter of the dispersed martensite, improved the balance between strength and total elongation. The authors investigated the total elongation in terms of both uniform elongation and local elongation. Reduction increased the balance of strength and uniform elongation as well as the balance of strength and local elongation. The balance of strength and uniform elongation increased markedly with increasing carbon content when the reduction was large. The reason for these results is explained in terms of work-hardening theory and SEM observation of deformed DP steels. An ideal microstructure for uniform and local elongation in high strength steels is concluded to be macroscopically homogeneous and microscopically heterogeneous.