Comparative Study On Flow Behavior And Dynamic Recrystallization Process Of 300M Steel Under Tension-compression Stress

In the process of die forging,the material will go through complex stress state.Previous studies have shown that the microstructure evolution behavior and material flow behavior of die forging materials are different under different stress states.So it is important to study the microstructure evolution behavior and material flow behavior of 300M steel under different stress states for formulating forging process parameters of large forgings and ensuring the forming quality.In this dissertation,high temperature tensile test of 300M steel was carried out to establish the constitutive model and dynamic recrystallization model.At the same time,the established model will be compared with the existing models of flow behavior and material structure evolution of 300M steel under high temperature compression.The main research contents are as follows:Firstly,the high temperature tensile tests of 300M steel were conducted in the temperature range of 900¹150?and strain rate range of 0.01s^-110s^-1 by the Gleeble 3500thermal simulator,and the true stress-strain curves were obtained by necking correction.Comparing the true stress-strain curves of tension and compression,it is found that the tensile stress-strain curves show a dynamic recovery type,while the compression stress-strain curves show a dynamic recrystallization type.The tensile stress is bigger than the compressive stress under the same deformation condition.The Arrhenius constitutive equation was established according to the true stress-strain curve,compared with the constitutive equation under compression,it is found that the deformation activation energy of 300M steel of tension deformation is smaller than that of compression stress.This is because more dynamic recrystallization occurs in compression deformation than in tension deformation,and more energy is consumed in dynamic recrystallization.Then,the Optical Microscope observation was used to observe the microstructures of300M steel after tension deformation.Unlike other materials,it was observed that obvious dynamic recrystallization occurred in 300M tensile specimens,and the dynamic recrystallization phenomenon increased with the increase of temperature and the decrease of strain rate.The dynamic recrystallization grain size was calculated by line intercept method.The dynamic recrystallization volume fraction model and grain size model were established.Comparing with the dynamic recrystallization volume fraction model under different stress states,the dynamic recrystallization volume fraction of tension is smaller than that of compression under the same deformation condition,which is consistent with the conclusion of previous studies on the constitutive model.Finally,the in-situ investigation of 300M steel in high temperature tension process was carried out.The dynamic recrystallization mechanism of 300M steel was analyzed by real-time observation of microstructure evolution in high temperature tensile deformation process.It is found that strain-induced grain boundary migration and twinning promote dynamic recrystallization during tension process.At the same time,the microstructure observed by in-situ investigation was compared with that observed by traditional metallographic method,it is found that the results obtained by tradition metallographic method and in-situ investigation can be well matched.