Control Of Heat Treatment Process And Microstructure And Mechanical Properties Of Medium Manganese Steel Plate With High Strength And Toughness

Currently,one of the main ways of high strength steel plates production is to add the moderate amount of micro-alloying elements in the traditional C-Mn steel or C-Mn-Si steel and combine with TMCP process,which aims to refine the grains and obtain the improvement of strength and toughness simultaneously through controlling the solution and precipitation behavior of micro-alloying elements and the transformation behavior of microstrUctures.However,studies on the ultra-low carbon medium manganese steel plate are seldom reported home and abroad.Fe-0.01C-5.3Mn-1.53Si steel is developed to be studied in this paper and a new type steel plate with good matching of strength and toughness is obtained using a reasonable hot-rolling and heat treatment process,which room temperature microstructure is laths-like ferrite and retained austenite.Several experimental content and conclusions are as follows:(1)The Acl and Ac3 temperature and static CCT curve of the experimental steel are obtained by thermal expansion method;The intercritical tempering process of the experimental steel is simulated using the automatic phase transformation instrument and we find that the austenite inverse transformation completion time is more than 3h when treated by DQ-T process while the completion time decrease to 70min when treated by DQ-LT process,which indicate that the DQ-LT process can significantly shorten the intercritical tempering time.(2)The hot-rolled direct quenching(DQ)steel has higher yield strength,tensile strength and yield strength ratio,but the elongation and impact energy at room temperature is relatively low.While after treated by the best DQ-T process(640 ?-8h),a significant improvement of the mechanical properties of the experimental steel is obtained with the yield and tensile strength is 560 MPa and 865 MPa,respectively,the break elongation is 25%and the PSE and impact energy reach to 21.63GPa%and 181 J,respectively.(3)As for the DQ-LT process,the intercritical austenitization annealing temperature and intercritical tempering time can significantly affect the yield strength of the experimental steel.The yield strength decreases constantly,tensile strength,break elongation and PSE decreases before increasing as the intercritical austenitization annealing temperature rises,but the change is not obvious,the impact energy at room temperature increases first then decreases.As the intercritical tempering time increases,the yield strength decreases constantly,tensile strength,break elongation and PSE has no obvious change and the impact energy at room temperature increases first then decreases.The optimal parameters of the DQ-LT process is 700?-0.5h-Q-640?-2.5h,which implies that hot-rolled plate is intercritical annealed at 700? for 0.5h,followed by quenching and intercritical tempering at 640?for 2.5h.The optimal matching of strength and toughness is obtained with the yield and tensile strength is 635 MPa and 820 MPa,respectively,the break elongation is 29%and the PSE and impact energy reach to 23.78GPa%and 210J,respectively.(4)Contrast with the DQ-T(640?-8h)process,the low temperature toughness of the experimental steel has an obvious improvement after treated by the DQ-LT(700 ?-0.5h-Q-640?-2.5h)process.The impact energy remained at about 101J when tested at-80?and the ductile brittle transition temperature is under-80 ?.(5)The toughening mechanism of the DQ-LT process contrast to the DQ-T process is:Increasing the crack initiation energy by further purifying the matrix microstructure and improving the material plastic;The microstructure of the experimental steel is optimized,which is beneficial to relieve the stress concentration,prevent the crack instability propagation and improve the crack stability propagation energy to a certain extent;Rising the rates of large angle grain boundaries,refining the grain size effectively and strengthening the ability to passivate instable crack and change its extension path to increase the crack instability propagation energy.(6)Contrast with the DQ-T process,the DQ-LT process can significantly shorten the intercritical tempering time and optimize the matching of strength and toughness,therefore determine the DQ-LT process as the optimum heat treatment process.