The Effect Of Cr-Mo Microalloying On The Microstructure,Mechanical Properties And Austenite Stability Of 0.2C-7Mn Medium Mn Steel

Medium-Mn steels have attracted a lot of attention for their low economic cost and superior comprehensive mechanical properties in automotive industrial applications.In present study,a Cr and Mo microalloying medium Mn steel was designed to study the microstructure and mechanical properties under different quenching temperature and isothermal time conditions.And the influence of microalloying on the amount and stability of retained austenite was analyzed.The main research contents and results are as follows:After the experimental steel is intercritically heat-treated in the dual phase zone,a matrix structure mainly consisted by lath ferrite and austenite is obtained.The amount and stability of retained austenite show the same changing trend with the increase of quenching temperature and the extension of isothermal time.The addition of Cr and Mo effectively reduced the grain size of the lath structure,and reduced the retained austenite amount and its stability.The addition of Cr and Mo effectively improves the strength of medium-Mn steel through precipitation hardening and solid solution strengthening,and maintains the ductility equivalent to that of the reference steel,thus letting the Cr-Mo microalloyed steel exhibiting a higher strong product of tensile strength and elongation(PSE).After Annealing at 750 ℃ for 1 h and tempering at 200 ℃ for 20 min,the Cr-Mo microalloyed experimental steel obtained the best comprehensive mechanical properties,with a tensile strength of 1272 MPa,an elongation of 50%,and a PSE of up to 63 GPa%.With the extension of the isothermal time,the PSE of the experimental steel increased first and then decreased,and reached the maximum at 30 min.With further analysis of its strengthening and toughening mechanism,it is found that the excellent mechanical properties of Cr-Mo microalloyed steel are mainly related to the amount and stability of retained austenite.Even though the addition of Cr and Mo reduced the amount of retained austenite in the structure,the carbides formed by them are mainly distributed on the ferrite matrix,providing a precipitation hardening effect and causing the retained austenite to bear more strain partitioning during the deformation process,thereby accelerating the transformation to martensite and causing a stronger Transformation-induced-plasticity(TRIP)effect.The quenching temperature has a greater influence on the TRIP effect than the isothermal time.The samples quenched at 750 ℃ for 30 min～6 h all showed persistent TRIP effect in the second stage of their work hardening behavior,while the TRIP effect of samples quenched at other temperatures was obviously insufficient.