Effect of annealing temperature and time on microstructure evolution of 0.2C-5Mn steel during intercritical annealing process

Microstructure evolution of 0.2C-5Mn steel during the intercritical annealing at different temperatures was studied. It was found that the microstructure gradually evolved from martensite structure into a structure consisted of austenite and ferrite during the intercritical annealing process. The retained austenite volume fraction reached the maximum value of 36.5% after about 10 min annealing at 680 degrees C. It was also found that carbides precipitated during initial annealing stage and gradually dissolved during following annealing process for all annealing temperatures; and interestingly, the fresh martensite was found, indicating that part of the newly forming austenite was unstable and would transform into martensite when quenching. Based on the microstructural analysis and the calculation by Thermo-Calc software, it was proposed that the different evolution behaviours of the microstructure during annealing were not only controlled by the thermal kinetics of the austenite, but also affected significantly by the thermal stability of the austenite.