Effects of Carbon and Silicon on Static/Dynamic Mechanical Properties of 780 MPa Grade Dual Phase Steel

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 10(3)/s is of importance. In this study, 0.110%-C1.44%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: 10(3)/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 X 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.