Reverse Transformation Of Deformation Induced Martensite In High Manganese Austenitic Steel

High manganese austenitic steel has received extensive attention in the automotive industry owing to its high tensile strength,excellent work hardening ability,good plastic and toughness,and high energy absorption ability.In addition,due to its lack of low-temperature ductile-brittle transition,non-magnetic and low-cost,it has good application prospects in LNG transportation and storage tank steel and strong magnetic field structural steel.However,the low yield strength of high manganese austenitic steel has become a major problem that limits its application.It is necessary to strengthen the high manganese austenitic steel without sacrificing plasticity.In this study,high-density nano-twins and ε martensite were introduced into high-manganese austenitic steel through plastic deformation,followed by a lowtemperature and short-time annealing process.By controlling the reverse transformation behavior of ε martensite,the material has excellent strength–ductility combination.The microstructure evolution of cold-rolled deformed Fe-25 Mn steel mainly includes the formation of twins,the formation of ε martensite and the formation of shear bands.With the increase of cold rolling deformation,more and more areas in the austenite grains of the sample underwent twinning deformation,which increases the density of twins,and the overall content of ε martensite phase increased significantly.The ε martensite phase content of the samples with50% deformation decreased relative to the samples with 40% deformation.In the large deformation stage,the twins were saturated.The material began to coordinate the deformation in the form of local shear,which leads the twinning continuity to be broken.Thermal stability and reverse phase transformation behavior of ε martensite under different cold rolling and annealing processes has been investigated.The thermal stability of ε martensite increases significantly with the increase of cold rolling deformation.The reverse phase transformation temperature of the sample at the small deformation stage is mainly below 200℃,while the reverse phase transformation temperature of the sample at the large deformation stage is above 300°C.The process of multiple annealing and prolonging the annealing time could not cause the ε martensite to undergo significant reverse phase transformation.Increasing the annealing temperature is an effective means to make the ε martensite undergo a complete reverse phase transformation.Increasing cold rolling reduction will significantly reduce the thermal stability of ε martensite.Different microstructure evolution and mechanical property changes of samples with different cold-rolled deformations after annealing at 300℃ for 10 mins has been investigated.The grain size,twin size and dislocation density of the samples with different cold rolling deformations did not change significantly after annealing.Due to the difference in thermal stability of ε martensite,the samples with less than 40% deformation occurred ε martensite reverse phase transformation,and the samples with deformation above 40% hardly underwentε martensitic reverse phase transformation.The reverse phase transformation of ε martensite will reduced the yield strength of the material.Since the reverse transformation of ε martensite did not occur in samples above 40% deformation,the yield strength hardly decreased after annealing.A complete stress-induced ε martensite reverse phase transformation occurred during tension deformation,which increases the plasticity from 7% to 14%.The amount of cold rolling deformation did not affect the occurrence of stress-induced ε martensite reverse phase transformation during the tensile deformation.Annealing treatment is the key condition for stress-induced ε martensite reverse phase transformation.In summary,through cold rolling combined with a simple low-temperature short-time annealing process,Fe-25 Mn steel with excellent comprehensive mechanical properties can be successfully prepared without any microalloying elements.In the traditional dual-phase materials of austenite and ε martensite,ε martensite is considered to be a phase that is difficult to coordinate plastic deformation,and heat treatment is often required to reduce its phase content to increase plasticity.Nevertheless,it will reduce the yield strength of the material.In this study,the thermal stability of ε martensite was used to prevent reverse phase transformation during heat treatment,thereby maintained the yield strength of the material.Stress-induced εmartensite reverse phase transformation occurred during the tensile deformation will double the plasticity of the material.