2008 Volume 94 Issue 8 Pages 305-312
Various types of high strength steel have been developed to improve the impact safety and reduce the weight of cars. For the steels used in anti-crash equipments, mechanical properties, especially, at high strain rates such as 103/s is of importance. In this study, 0.110%C–1.44%Si–1.29Mn–0.65%Cr–0.29%Mo containing hot rolled dual phase steel with 780 MPa grade in tensile strength was employed as a base steel and the effects of carbon and silicon on static (strain rate: 10−3/s)/ dynamic (strain rate: 103/s) mechanical properties of the dual phase steel were investigated. Carbon and silicon contents were changed in a range of 0.076–0.190% and 1.44–2.39%, respectively. Grain size of the steels was varied by hot rolling reduction: 53% (named coarse grain process), 73% (middle grain process) and 88% (fine grain process). Dynamic absorbed energy up to 10% tensile strain had a linear relationship with tensile strength, regardless of microstructures, i.e., neglecting carbon and silicon contents, and hot rolling conditions. All absorbed energy to fracture had a close relationship with tensile strength–ductile balance parameter (tensile strength×total elongation), reflecting microstructural change through chemical and rolling conditions. All the processed 0.190% C steels, and the fine grain processed 1.93% Si and 2.39% Si steels showed the highest all absorbed energy of all the steels tested. The 0.190% C steel was characterized by almost 100% martensite with some content of retained austenite, and the 1.93% Si and 2.39% Si steels were fine grained ferrite+martensite. It was found that carbon improves all absorbed energy through increase in volume fraction of martensite and silicon raises it through solid solution hardening of ferrite matrix.