Analysis of phase transformation thermodynamics and kinetics and its relationship to structure-mechanical properties in a medium-Mn high strength steel

The difference in transformation mechanisms between the normal and reverse phase transformation in medium Mn steel was revealed using thermodynamic and kinetic analysis. The results indicate that the austenite formation during the reverse phase transformation is a fast reaction, significant amount of 46.9 vol% retained austenite is obtained after subjected partial reversion at 650 degrees C x 1 h. At the nucleation stage, the lath-like structure, dislocations, martensitic packet boundaries, and grain boundaries provide high-density nucleation sites for austenite formation. The critical temperature of austenite formation is reduced by the enrichment of Mn and C. However, no ferrite transformation occurred during the normal phase transformation (650 degrees C x 6 h isothermal treatment and subsequent furnace-cooling process). At the nucleation stage, the undersaturated Mn and C in the prior austenite grain interior provide insufficient concentration fluctuation, leading to a low nucleation rate. At the phase growth stage, the atomic mobility of Mn and C in the parent austenite (control the ferrite growth) is 1/69 and 1/282 than that in ferrite (control the austenite growth) at 650 degrees C. The differences in nucleation site density, driving force, and element diffusion rate lead to significant variations in phase transformation behaviors. The excellent mechanical property with the product of strength and elongation (PSE) exceeding 20 GPa % is achieved via the sustained transformation-induced plasticity (TRIP) effect by metastable retained austenite over a large strain range.