Effect of Post-Deformation Cooling on Microstructure and Mechanical Properties of a Cu-Containing High-Strength Low-Alloy (HSLA) Steel

A Cu-Containing high-strength low-alloy (HSLA) steel was subjected to single-stage near-isothermal forging followed by different post-deformation cooling rates. The effect of different cooling media such as water, air and furnace on the microstructure evolution and mechanical characteristics of the HSLA steel was evaluated. The hierarchical microstructures of HSLA steel were investigated by different microstructural characteristic techniques to draw a correlation between microstructure and mechanical properties. The HSLA steel exhibits predominantly martensitic microstructure with or without bainite irrespective of the cooling media. However, the morphology of the martensite was found to be dependent on the cooling rate. The effect of cooling medium on grain size was systematically shown by reconstruction of prior austenite grain boundaries from electron back-scattered diffraction investigation. Faster cooling results in higher yield and tensile strength due to predominant martensitic structure while relatively slower cooling rates result in comparatively lower strength values with a corresponding increase in ductility. This is attributed to the presence of retained austenite in the HSLA steel with decreasing cooling rate. An impact toughness of ≥ 50 J was obtained at subambient temperature of − 40 °C irrespective of the cooling rate.