Partitioning-time dependent evolution of retained austenite and its impact on microstructure and mechanical properties in lean TRIP-assisted steel

TRIP-assisted multiphase steels are gradually finding employment in the automotive industry for their excellent strain-hardening characteristics. In the current study, in a comparatively lean composition steel, different combinations of multiple phases such as ferrite, austenite and martensite, were obtained by heat-treatment, consisting of Inter-critical annealing followed by partitioning treatment for different holding times. It was observed that, with increasing isothermal holding time, the volume fraction of retained austenite increases. Electron backscattered diffraction (EBSD) and transmission electron microscope (TEM) studies revealed that during partitioning at 400°C, austenite decomposition occurs primarily by the growth of existing ferrite through interface migration, and also involved bainitic transformation features that are difficult to distinguish. As a consequence, the retained austenite is enriched with carbon and get stabilized against martensitic transformation at room temperature. The characteristics of the retained austenite was studied using in-situ high energy X-ray diffraction and differential scanning calorimetry. The mechanical properties have been found to be strongly influenced by the volume fraction of the microstructural constituents. The higher volume fraction of martensite has been observed to give rise to higher strength and the higher ductility is found to be related to higher amount of retained austenite. The mechanical balance between high strength and high ductility has been achieved with a high-volume fraction (around 20 %) of austenite. This comprehensive study implies that an adequate amount of retained austenite ensures the TRIP effect, while interaction among the phases present results in the achievement of excellent global mechanical properties.