Effect of prior deformation on microstructural evolution and retained austenite stability in bainite–martensite structures of continuously cooled low-carbon steel

This study investigates the effects of prior deformation and continuous cooling rate on microstructure and retained austenite stability in low-carbon bainite/martensite steel. Continuous cooling was applied to laboratory-scaled specimens at cooling rates of 1–8 °C/s under deformed and undeformed conditions. The results demonstrated that under undeformed conditions, increasing the cooling rate led to a progressive transformation from bainite to martensite. At 8 °C/s cooling rate, a fully martensitic microstructure was achieved whereas lower rates yielded mixed bainitic–martensitic structures. Deformation in austenite region significantly accelerated transformation kinetics, as evidenced by a leftward shift in the deformation-modified continuous cooling transformation diagram. The deformed specimen cooled at 4 °C/s exhibited nearly similar microstructural features and hardness values with the undeformed specimen cooled at 2 °C/s. In undeformed conditions, retained austenite volume fraction increased with cooling rate. However, deformation prior to cooling substantially reduced retained austenite content, despite identical thermal histories. The findings highlight the importance of controlling both deformation and cooling on tailoring final microstructures and mechanical properties without the need for additional heat-treatments. The results provide an insight into optimizing energy-efficient and low-emission forging processes aimed at improving uniformity in components with varying cross-sections.