Investigating the microstructure, mechanical properties and hole expansion of advanced high strength steel with the approach of application in the car body

In recent years, with the increase in competition between automobile companies and the establishment of government laws to further reduce fuel consumption and, as a result, the requirement to reduce vehicle weight, the third generation of advanced strength steels have received much attention in the automotive industry due to their better tensile strength and ductility than the first generation and lower cost than the second generation. In this research, a new candidate of the third generation of advanced high-strength steels, with the name of 112HR, was design, production, and introduced and its mechanical properties were investigated and compared with SAPH440 steel, which is currently used in the domestic car industry and in the car cabin. Uniaxial tensile test was performed in three directions of rolling, perpendicular to rolling and 45-degree angle to rolling. 112HR steel with a tensile strength of 1190 MPa and an increase in length of 54.5% and a ductility index of more than 60 GPa% has shown better properties than SAPH440 steel with a tensile strength of 450 MPa and elongation of 43%. But in the tensile test of grooved sample and hole expansion, which is a three-dimensional stress, 112HR steel performed worse, so that the cavity expansion rate of SAPH440 steel was about 3.76 times higher than that of 112HR steel. The reason for this difference is the transformation properties of austenite to martensite during deformation in 112HR steel and the low shear strength of martensite. The transformation of austenite to martensite was confirmed by X-ray diffraction and optical and electron microscopic investigations in 112HR steel.