Microstructure Evolution Of G31 And G50 Ultra High Strength Steel And Its Effect On Mechanical Properties

After the size increment,ultrahigh-strength steel with medium-low concentration of alloying element got different microstructural in different area because of different cooling rate.So the microstructural and the mechanical properties of forge piece got complex variation.On the other hand,the structures after heat treatment are mainly non-equilibrium structure,such as martensite and banite,even high-temperature tempering can't get equilibrium microstructure.The mechanical properties of forge piece got complex variation due to the non-equilibrium structure affect the structure and properties after final heat treatment through mechanism such as inheritance.This thesis' study objects are medium-alloy element-ultrahigh strength steel G31 and G50.The effect of different cooling rate after quenching from austenitizing to phase transition product and mechanical properties after the following low-temperature tempering was studied by controlling the cooling process.Next,simulate the cooling process of large forgings and the variation of temperature field on different location of corss section by finite element simulation software ANSYS,to get the laws between forgings dimension and cooling rate.At last,annealing acceleration has been studied of the experimental steel.The austenitizing temperature and annealing time's effect on perlitic transformation ability have been discussed,and a new convenient way to get equilibrium structure and interdict inheritance of coarse austenitic grain has been mastered.Cooling the G31 ultra-high strength steel with the cooling rate between 30.0?3.5?/min after austenitizing obtain different phase transition product and different strength and toughness after the following low-temperature tempering.The study result shows that lower the cooling rate still lead to the forming of martensite,but the strength is higher than conventional oil quenching,ductility and plasticity are lower.The forming of 25%-30%percent low bainite before the martensitic transformation increase the strength after low-temperature tempering,and don't lower the fracture toughness;but the forming of pearlite and upper bainite in phase transformation structure rapid deterioration the strength and toughness after low-temperature tempering.The phase transition product of G50 ultra-high strength steel cooling between 30.0?3.5?/min after austenitizing,and its effect on strength and toughness after the following low-temperature tempering.The study results shows that cooling the experimental steel by the cooling rate of 30.0?/min?15.0?/min?7.0?/min after 830?all get martensite structure;cooling with the rate of 3.5?/min obtained 50%lower banite,obtained the martensite/banite duplex phase structure.The overall mechanical properties of 3.5?/min are the best.G50 ultra-high strength steel have a higher alloy element content,especially the Ni content,leading to a better martensite formation ability and hardenability 'compared with G31,being more suitable for core mechanical property requirement after forging enlargement.A proper temperature austenitizing can accelerate the annealing process,can easily get the equilibrium structures such as pearlite.This method can make a contribution to lower the annealing hardness of ultra-high strength steel,interdict structural inheritance,and eliminate the coarse austenite grain come from smelting,solidification and forging.