| SA537CL1 steel is widely used in the construction of large-scale LPG low-temperature storage tanks,which require steel to have good strength and excellent low-temperature impact toughness after normalizing;16Mn Cr5 is a steel for fine blanking and carburizing widely used in automobile transmission systems.The steel is required to have excellent plastic deformation processing ability after spheroidizing annealing,and to avoid the production of austenite mixed crystals during high-temperature carburizing.Aiming at the SA537CL1 and 16Mn Cr5 hot-rolled steel sheets provided by a steel mill,this article focuses on the evolution of the microstructure of SA537CL1 during normalizing and 16Mn Cr5 isothermal spheroidizing annealing and its influence on the mechanical properties,by obtaining a reasonable heat treatment process window to meet the stricter technical requirements of downstream customers,and provide theoretical guidance for production process optimization and material performance improvement.The main research results of this article are as follows:?Evolution of structure and properties of SA537CL1 steel plate during normalizing(1)There is Mn and Ni element segregation in the core of SA537CL1 steel plate(12mm thick),so that granular bainite structure will also appear in the core during normalizing air cooling,and it is impossible to eliminate through normalizing process,so it will reduce low-temperature impact toughness of normalized steel plate.(2)With the increase of the normalizing temperature and the extension of the holding time,the ferrite grain size of the test steel becomes larger,the volume fraction of ferrite decreases,the strength and elongation change little,and the low-temperature impact toughness decreases.(3)The low-temperature impact toughness is inversely proportional to the ferrite grain size.It is necessary to improve the low-temperature impact toughness of steel through the best matching of normalizing temperature and holding time,which can refining the ferrite grains.(4)The best normalizing process window for SA537CL1 steel plate(12mm thick)is:heating temperature is 850?870?,holding time is 5?40min,which can meet the user's strength and strict low-temperature impact energy requirements(Akv_-50??41J).?Evolution of microstructure and properties of 16Mn Cr5 steel plate during spheroidizing annealing(1)Through image analysis method,parameters such as carbide spheroidization rate and long and short axis ratio are obtained,which can quantitatively evaluate the spheroidization effect.(2)The reasonable isothermal spheroidizing annealing process window for 16Mn Cr5 steel plate(6mm thick)is:cold rolling deformation 20%?50%,using 770?780?×1?2h(first stage)and 700?×4?6h(second Stage)combined process.Uniform and fine spheroidized cementite can be obtained.The elongation is increased from 26.88%in the hot-rolled state to 42.23%after the spheroidization is degraded.It can carry out large deformation cold processing such as fine blanking,and meet the strict technical requirements of tensile strength?500MPa,yield strength?300 MPa and elongation?35%.(3)As the cold rolling deformation rate increases,the pearlite lamella spacing becomes smaller,the number of sub-grain boundaries increases,the driving force for spheroidization of cementite becomes larger,and the spheroidization process accelerates.Therefore,cold rolling deformation is beneficial to obtain uniform and fine spherical infiltration.Carbon body,plasticity is significantly improved.(4)Under the simulated carburizing condition of 930?×6h,it is found that 16Mn Cr5 steel has obvious austenite mixed crystal problem.Based on this,the following solutions are proposed:controlling the mass fraction of Al in the steel to 0.02%?0.055%,and at the same time,controlling N The mass fraction is 0.01%?0.018%,which make it form fine dispersed Al N in the structure and compound with a small amount of residual Ti and V microalloying elements to pin the grain boundaries,thereby preventing the coarsening and growth of austenite grains. |
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