Effect Of Heat Treatment On Microstructure And Properties Of Hot/cold Rolled Medium-Mn Steel

With the global energy crisis and environmental degradation increasing,lightweight and safety have become the development trend of the new generation of automobile manufacturing.As a new type of high-strength plastic steel,medium-Mn steel has the advantages of low density,low cost and good impact energy absorption,playing an active role in dealing with environmental and energy issues.Hence,research and development of medium-Mn steel has become an important research direction for steel researchers at home and abroad.In this paper,several testing methods are used,such as optical microscopy(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),JMat Pro 7.0 simulation software and mechanical properties testing.Meanwhile,the effects of the quenching and tempering(Q&T)process and the critical annealing(IA)process on the microstructure and mechanical properties of the different rolling conditions medium-Mn steel(0.48C-10.2Mn-2.2Al-0.7Si-0.75V-0.03Ni)are studied.The specific conclusions obtained are summarized as following:(1)The as-hot-rolled medium-Mn steel obtained a yield strength from 761 MPa to 1169 MPa,a tensile strength from 1073 MPa to 1334 MPa,and an elongation of more than 9% after quenching from 650 °C to 800 °C and a tempering temperature at 200 °C.The microstructure consists of dislocation/twin martensite,retained austenite,ferrite,and nanometer precipitates.With the increase of quenching temperature,the yield strength and tensile strength of steel increased by 408 MPa and 61 MPa,respectively,because the martensite content and dislocation density increased with the increase of quenching temperature.When the quenching temperature is 750 °C,the microstructure has 35% retained austenite,and the stability is moderate,the conversion rate is 31.4%,showing better comprehensive mechanical properties.(2)The as-cold-rolled medium-Mn steel obtained an ultra-high tensile strength from 1535 MPa to 1750 MPa and average elongation of 9.8%.After inter-critical annealing treatment in the dual-phase zone,the medium-Mn steel can obtain a yield strength from 997 MPa to 1239 MPa,a tensile strength from 1349 MPa to 1445 MPa,and an elongation from 15% to 27%.Its microstructure consists of equiaxed austenite and ferrite and nano-precipitates on the matrix.As the annealing temperature increases,the yield strength and tensile strength of the steel decrease by 242 MPa and 96 MPa,respectively,but the elongation increases by 12%,because the increasing size in austenite and ferrite grain,the increasing carbon content in austenite the decrease in the content of precipitates,and the decrease in dislocation density.When the annealing temperature is 680 °C,89% of austenite remains in the structure,and the austenite transformation rate is 39.3%,which has a good elongation.(3)After annealing at 680 °C,the cold-rolled medium-Mn steel has a tensile strength of 1349 MPa,an elongation of 27%,and the product of strength and elongation reached 36.4 GPa %,meeting the requirements of the third-generation automotive steel.