Study On Thermo Mechanical Control Process And Heat Treatment Of Medium Manganese Steel Plate

With the continuous development of human society,oil,coal and other non-renewable resource reserves continue to decrease,and therefore the direction of human resources development gradually shifted from land to offshore.Aiming at the problems of high alloy cost,complicated production process and strong dependency of key parts of high strength steel import for offshore platform,this paper adopted C-Mn micro-alloying technology along with TMCP and refined heat treatment technology to develop a typical 30mm thick 690MPa grade medium manganese steel plate for offshore platform,lowering production costs and saving energy on the basis of ensuring high performance.Through the exploration of the microstructure evolution,mechanical property combination and strengthening as well as toughening mechanism of tested steels after different heat treatment process,the experimental steel production process was optimized,which provided the necessary technological foundation and theoretical support for the actual industrial production.The main contents are as follows:(1)The results of tested steel A of different tempering temperature showed that the yield strength and the yield ratio of the experimental steel decreased with the increase of the tempering temperature,and the elongation increased with the increase of tempering temperature.The tested steel reached the optimum overall performance when tempered at 650?.Its yield strength reached 840MPa and tensile strength was 900MPa.The impact energy at room temperature reached 250J,and the impact energy at-60? was above 130J.The material properties were much higher than the development requirement.(2)The results of tested steel B of different tempering time showed that with the increase of tempering time,the impact energy of the experimental steel increased gradually,and the yield as well as tensile strength and the yield ratio decreased with the increase of the tempering time.Experimental steel B's overall performance was ideal after being tempered for 60min,and the impact energy at room temperature was 200J.Its yield strength was about 725MPa,and the yield ratio was about 0.89 with an elongation of 27.4%.When the tested steel C was tempered at 650? for 100 min,its tensile strength reached 860MPa,the yield strength is 0.87 and the elongation was 33%.The TRIP effect in the tested steel was quite obvious.(3)The experimental results of studying the change regulation of retained austenite volume fraction showed that the retained austenite volume fraction increased with the increasing of the tempering temperature,and the maximum austenite volume fraction was up to 29%.The content of C and Mn in austenite decreased with the increase of tempering temperature,resulting in the decrease of retained austenite stability in the experimental steel.Therefore,it is easy for the retained austenite to spontaneously transform into martensite at low temperature or small load.(4)Study of retained austenite stability found that the determinants of retained austenitic stability lies on several factors,including the location,shape and size of the austenite.The film-shaped retained austenite has stronger mechanical stability than the massive retained austenite,and some particular individual austenite grains are protected by the transformation of the austenite grain around to prevent its phase transformation.The smaller the residual austenite grain is,the stronger the stability becomes.The retained austenite with lower stability will transform into martensite early at low temperature or low load,which will increase the local strength of the material,but it will not contribute much to the elongation of the material.(5)The XRD phase characterization of the impacted specimen showed that the austenite in the stress concentration part of the experimental steel undergone phase transformation under rapid load,but not all retained austenite transformed.In this process,the retained austenite transformed into martensite,which increased the material's crack initiation energy and crack propagation energy,thus increasing the impact toughness of the material.The higher the volume fraction of retained austenite is,the stronger the impact toughness is.(6)It is found through studying of processing map at a typical strain of 0.6 that the hot processing map based on the dynamic material model theory as well as the Prasad instability criterion have fine applicability in the ultra-low carbon low alloy medium manganese steel.The optimal hot working parameters range of the experimental steel was drawn,and the microstructure of the three unstable zones in the hot processing map was analyzed and the causes of the instability were analyzed.(7)The results showed that the dynamic recrystallization can be observed in the range of temperature from 1050? to 1150? and strain rate from 0.01 to 0.1s-1.The dissipation coefficient of the material in this region was between 30%and 40%,and the microstructure of hot deformation tested steels was fine and uniform.The parameters of this process area have certain guiding value for the actual hot working process parameter setting.