Research On The Microstructure And Mechanical Property Of07MnMoVR Steel In Quenching And Tempering Heat Treatment

Recently, with the development of national economy, progress of hydraulic electrogenerating and petroleum reserve has been made, and various kinds of liquefied gas were produced, stored and transported. The demand of pressure vessel steel is growing. With the mass production of the pressure vessel steel, its quality is attracting increasing attention. Research on pressure vessel steel, which can improve the level of production technology of high-quality and crack free quenching and tempering steel, can also bring considerable economic benefits for the enterprise.In this paper, firstly the composition of07MnMoVR test steel was designed in the laboratory, secondly rolling and tempering process of factory was simulated, then quenching and tempering heating tratement of07MnMoVR steel was studied. The performance index of test steel meets GB1989-2011. The main contents in this thesis are as follows.(1) Thermodynamic software Thermo-Calc was used to calculate equilibrium phase and precipitated phase of the test steel, and Jmatpro was used to simulate material Property of the test steel. On the condition of no changing the equilibrium phase in the equilibrium phase diagram, the contents of Cr, Mn, Mo, Ni after adjusted were0.21%、1.50%、0.22%、0.30%。(2) Expansion experiment was used to measure test steel static CCT curves and dynamic CCT curves. Austenite transformation law under different cooling conditions of the test steel was studied and analyzed.(3) Through the analysis of the test steel microstructure after quenching and tempering, the microstructure of test steel after quenching consist of granular bainite, lath bainite and residual austenite, the microstructure after tempering was composed of ferrite and carbide. With the increasing of tempering temperature and time, lath structure coarsened and gradually disappeared; ferrites recovered and recrystallized, the boundary of polygonal ferrite became straight, grain tended to be homogenized. Cementite spherized and grew up gradually, and then gathered at the grain boundaries. (4) Through the analysis of the experimental steel’mechanical Property after quenching and tempering, with the increasing of quenching temperature, tensile strength and yield strength increased, and the elongation and impact power increased first and then decreased. When the time of quenching is beyond1H, mechanical Property stabilized. Tensile strength and yield strength of test steel decreased with the increasing of tempering temperature, and decreased with the increasing of tempering time. The elongation and impact energy increased with the tempering temperature increasing, and increased with the tempering time increased.(5) According to the research of microstructure and Property of test steel in quenching and tempering process, it was known that the best quenching and tempering process of07MnMoVR was:quenching heating temperature was910℃, the time was1H; tempering temperature was630～650℃, the time was1-2H. With these craft, tensile strength was640MPa, yield strength was580-630MPa, the elongation was beyond26%, impact power was160～180J, Which provides a reference for the actual production of the steel factory.