Hot Deformation Behavior Of 316LN-Mn Austenitic Stainless Steel

316LN-Mn austenitic stainless steel was selected to produce the shell of Toroidal Field coil due to its superior mechanical properties at a low temperature of 4.2 K,meeting the requirements for the material used to fabricate the shell in the China Fusion Engineering Testing Reactor.The coil shell is a large forging,but the deformation resistance of 316LN-Mn austenitic stainless steel at high temperature is very high,and it is easy to crack during the forging process,which poses a great challenge to the actual production.Therefore,this project is carried out to study the hot deformation behavior of 316LN-Mn austenitic stainless steel under high temperature conditions in response to the above problem.This paper uses a simulated thermal deformation method to study the hot deformation behavior of the material.By using the Gleeble-3800 thermal simulation experimental machine to conduct single and double pass thermal compression experiments on the material respectively,the effects of deformation parameters such as deformation temperature and strain rate on the flow behavior of the material under high temperature deformation conditions are analyzed,the mathematical models of thermal deformation constitutive equations and dynamic-static recrystallization are established,and the processing maps of 316LN-Mn austenitic stainless steel under different deformation conditions are drawn.In addition,samples under different deformation conditions and heat treatment conditions are characterized with the help of various characterization equipment such as metallographic microscopes to analyze the effect of each parameter on the microstructure of the material and reveal the softening mechanism of the material during thermal deformation,the law of microstructure change in the thermal deformation gap and the grain growth law after hot deformation.As a result,during thermal deformation of 316LN-Mn austenitic stainless steel,discontinuous dynamic recrystallization is used as the nucleation mechanism for new grains,with a small amount of continuous dynamic recrystallization occurring.The magnitude of the flow stress after hot compression decreases with increasing temperature and decreasing strain rate.In addition,the flow stress in the second pass after two-pass compression decreases with the increase of the strain and the increase of the inter-pass time.Based on the results of stress-strain curves after single-pass and two-pass compression,the hot deformation constitutive equation,the critical equation for dynamic recrystallization and the kinetic equation for static recrystallization of 316LN-Mn austenitic stainless steel were established respectively and verified the accuracy of all established models.The processing map of 316LN-Mn austenitic stainless steel is constructed according to the dynamic material model,and the theoretical guidance of the constructed processing map for practical production is demonstrated by observing the organization after deformation under the hot deformation parameters corresponding to the four characteristic regions in the processing map.316LN-Mn austenitic stainless steel has an optimal thermal processing window of 1107-1160°C for deformation temperature and 0.005-0.026 s^-1 for strain rate.The grain size of 316LN-Mn austenitic stainless steel showed a positive correlation with both temperature and holding time,and the grain growth behavior of the material was more sensitive to temperature.A grain growth model was also established,and its accuracy was verified.