A low-cost and easy-to-produce C–Mn–Cr automotive steel for both cold and hot forming is presented in this paper. The alloying element Cr was used to replace Mn in medium-Mn steel and instead of B in hot-formed steel, in order to achieve microstructure control and hardenability improvement, replacing the residual austenite-enhanced plasticization with multidimensional enhanced plasticization through multiphase microstructure design, grain refinement, and dispersion enhancement of second-phase particles. The products of strength and elongation for the cold-formed and hot-formed steel were 20 GPa·% and 18 GPa·%, respectively, while the tensile strengths were more than 1000 MPa and 1500 MPa, respectively. This new automotive steel was also characterized by good oxidation resistance. The mechanisms of strength and plasticization of the experimental automotive steel were analyzed.

The effect of austenitizing temperature and aging treatment on the microstructure and mechanical properties of two new cold-rolled automotive steel plates (20Mn2Cr and 20Mn2CrNb) was investigated by using isothermal heat treatment, optical microscope, scanning electron microscope, microhardness tester, and tensile testing machine. The results show that as the austenitizing temperature increased, the original austenite grain sizes of both steels increased. The original austenite grain size of 20Mn2CrNb was smaller than that of 20Mn2Cr. The microhardness of 20Mn2CrNb gradually decreased with increasing aging temperature, while the hardness of 20Mn2Cr varied irregularly. The mechanical properties of 20Mn2Cr were better than those of 20Mn2CrNb under the same heat-treatment process. The effect of heat treatment on microstructure and mechanical properties was related to the martensite content, dislocation density, and precipitation of second-phase particles.