Study On Q-P-T Process And Strengthening-Toughening Mechanism Of Low Carbon Alloy Steel

Since the 21 st century,China’s industrial development has become more and more rapid,and people’s performance requirements for steel materials have become more stringent.Therefore,it is of great significance to develop high-performance steel with high strength and high plasticity.In this paper,24 CSiMnNiCrMoV low-carbon alloy steel is selected as the research object,and QPT heat treatment is carried out on the alloy steel.OM,SEM,TEM,XRD,EDS and tensile tests are adopted to study the influence of heat treatment process parameters on the microstructure and mechanical properties of the material,and then the strengthening and toughening mechanism is revealed.The low-carbon alloy is heated to 900℃ for complete austenitization and heat preservation for 30 min,austempered in salt bath solution at 240℃ for 5 min,tempered at280-360℃ for 60 min,and finally cooled to room temperature.The effect of tempering temperature on microstructure and properties of low carbon alloy steel was studied.The results show that with the increase of tempering temperature,the yield strength of the experimental steel gradually decreases,the tensile strength decreases first and then increases,while the elongation shows a trend of increasing first and then decreasing.When the tempering temperature is 340℃,the mechanical properties of the experimental steel reach the best combination,i.e.the tensile strength is 1806 MPa,the yield strength is 1495 MPa,the elongation is 17.7%,and the strong plastic deposition can reach 32.92 GPa·%.Compared with the traditional QT process,the strong plastic deposition is increased by about 75%.The change rule of strengthening and toughening performance with temperature is closely related to the multiphase structure obtained from experimental steel,which consists of lath bainite,martensite,retained austenite and carbide.With the increase of tempering temperature,the content of secondary martensite with higher carbon content gradually decreases,while bainite gradually grows from the grain boundary to the grain interior,and its size becomes coarser.The content of retained austenite increases with the increase of tempering temperature,its morphology gradually changes from thin film to block,and its mechanical stability gradually decreases.In addition,cementite and vanadium carbide particles formed on the martensite matrix also contribute significantly to the strength of the material,but the effect will gradually decrease with the increase of temperature.In QPT process,isothermal treatment in two-phase zone was added and a certain amount of ferrite soft phase was introduced.The effects of isothermal treatment in two-phase zone and tempering partition temperature on microstructure and properties of experimental steel were studied.Ferrite can not only coordinate martensite deformation but also postpone TRIP effect of retained austenite during the stress process.The properties of ferrite,bainite,martensite,retained austenite,carbide and other multiphase structures can be optimized by adjusting their morphology,size and proportion.When the tempering temperature is 300℃,the mechanical properties reach the best combination,the tensile strength is 1508 MPa,the yield strength is 1048 MPa,the elongation is 22.4%,and the strong plastic product is 33.7 GPa·%.