Factors influencing austenite stability during Inter-critical annealing and effect on mechanical properties of low-density medium Mn steels

Addition of lightweight elements to medium-Mn steels has been seen as a potential method to reduce the density of advanced high strength steels (AHSS). Reduction in density of steels, especially in the automotive sector, leads to weight reduction of automobile and hence contributes towards improved fuel efficiency and lower emission of harmful gases. Present work investigates the addition of 3 % Al, with and without 1 % Si, to Fe–8Mn-0.2C medium-Mn steel system aiming to not only obtain an improved combination of strength and ductility, but also investigate the austenite stability. Hot-rolled steels were subjected to intercritical annealing (IA) followed by water quenching. Presence of Al and/or Si helped in broadening the IA window, thus allowing flexibility for selection of IA temperature. Uniaxial tensile test results show that the Si-free steel after IA performed better in terms of obtaining a product of strength and elongation (PSE) value of 61 GPa% as compared to PSE value of 41 GPa% for Si-containing steel. Microstructural characterization using electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) revealed the presence of $\alpha$-ferrite and austenite with negligible amount of martensite in both steels. However, the fraction of retained austenite was found to be significantly higher in Si-free steel (45 %) as compared to the Si-containing steel (19 %). Superior mechanical performance of Si-free steel is attributed to its higher retained austenite fraction and relatively slower TRIP effect. The role of IA temperature in determining the stability of austenite against transformation to martensite during cooling was investigated with the help of thermodynamic equilibrium predictions assisted with Koistinen-Marburger model for retained austenite calculations. Results indicate that additional factors related to local microstructural heterogeneity such as partitioning of elements and grain size differences may have contributed to the higher than expected austenite stability. Further, mechanical response of the obtained microstructural constituents is discussed in terms of the mechanical stability of retained austenite. Analysis related to the transformation kinetics of retained austenite revealed a lower value of mechanical stability parameter for Si-free steel, thus explaining the slower TRIP effect.