Study On Hot Deformation Behavior Of M50NiL Steel

M50NiL steel is a new generation engine bearing gear steel which developed from M50 steel.After carburizing/nitriding treatment,the steel has hard surface layer and high fracture toughness of the core section and its fatigue life greatly improved compared with M50 steel.The forging process of M50 NiL steel is narrow and the forgings' grain size is hard to control.Grain size directly affects the performance of forging,therefore,studying hot deformation behavior of M50 NiL steel has great significance to develop appropriate forging process and thus control forging performance.Through heating and holding tests,hot compression tests and metallographic experiments,the law of M50 NiL steel austenite grain growth behavior,hot deformation behavior and dynamic recrystallization microstructure evolution were investigated,moreover,austenitic grain growth model,true stress-true strain curves constitutive model and DRX model were also established.The austenite grain growth behavior of M50 NiL steel were studied by heating and holding test and metallographic experiment.Results show that the austenite grain size of M50 NiL steel increase with the increase of heating temperature and holding time.The austenite grain grow up slowly when the heating temperature is in the range of900~1100?.While the heating temperature is 1100~1200?,the austenite grain growth is more rapid.The austenite grain growth,when the holding time is less than 30 min,is faster than the holding time is more than 30 min.Moreover,the austenite grain growth model was obtained by using Anelli model and fitting the grain size,which can predict the austenite grain growth behavior well.M50NiL steel hot deformation behavior was studied by by hot compression test.The results show that the flow stress decreases with the increase of deformation temperature and the decrease of strain rate,and the microstructure are more prone to DRX.There were no DRX happened when the heating temperature is 900?.However,the DRX happened at all strain rate when the temperature is 1000?,but the DRX was divided into complete DRX and uncomplete DRX.When the temperature higer than1100?,the microstructure are substantially present complete DRX and the DRX grain size increases with the increase of temperature and the decrease of strain rate.Besides,afew DRX grain grow abnormally at the strain rate of 0.01s-1 and the deformation temperature of 1150? and 1200?.Constitutive model of true stress-true strain curves based on Zener-Hollomon parameter,the work hardening-dynamic recovery constitutive equation,DRX constitutive equation,critical strain model,peak strain model,DRX model,and DRX grain size model were established by fitting the true stress-true strain curves and DRX grain size.