The study of austenite reverse transformation and grain refinement mechanisms in medium manganese steel during intercritical annealing process

The intercrictical annealing (IA) process is the key technique for achieving high-strength medium Mn steel (MMnS). This paper analyzes the grain refinement of austenite and alpha '-martensite/ferrite during the IA process of cold-rolled MMnS using EBSD (include high-temperature in-situ EBSD) and TEM characterization techniques. The results indicate that the grain refinement mechanisms of austenite and alpha '-martensite/ferrite are different. Grain refinement is primarily related to the austenite reverse transformation (ART) in alpha '-martensite/ferrite. The in-situ high-temperature EBSD results show that the alpha '-martensite/ferrite exhibits a grain orientation inheritance after IA process. The content of the alpha '-martensite decreases from 62 % to 24.2 %. The average grain size decreases from 2.05 mu m to 1.53 mu m. The high strain energy in the linear distribution of the lath alpha '-martensite leads to accelerated diffusion of elements at that location. The accelerated rate of austenite reversion transformation causes the boundary separation of the lath structure, leading to the formation of multiple equiaxed grains. The grain refinement mechanism of austenite is more complex. It is the result of the combined effects of ART and recrystallization. Martensite undergoes a diffusionless phase transformation with a holding time of 2 min, which results in the formation of austenite with high strain energy. A large number of subgrains appear within the austenite with a high dislocation density. The diffusion-type phase transformation leads to the direct nucleation of new austenitic grain around the untransformed alpha '-martensite with a holding time exceeds 5 min.