Microstructure and mechanical properties of a mixed Si-Al TRIP-assisted steel subjected to continuous galvanizing heat treatments

Heat treatments with an isothermal bainitic transformation temperature of 465 °C compatible with the continuous galvanizing process were performed on a 0.2C-1.5Mn-1.0Si-0.5Al (wt%) TRIP-assisted steel. The effect of intercritical annealing temperature and isothermal bainitic transformation (IBT) time at 465 °C on mechanical properties and microstructure were determined. The best combination of strength and ductility was obtained for the lower intercritical annealing temperature of 750 °C for an isothermal bainitic transformation time of 90 s at 465 °C. However, after intercritical annealing at a higher temperature of 797 °C, optimum mechanical properties were obtained for a shorter isothermal bainitic transformation time of 60 s at 465 °C, likely due to the lower C content of the intercritical austenite bringing about faster transformation kinetics. It was concluded that varying the intercritical temperature and IBT time can play an important role in the development of a suitable process window to obtain the desired mechanical properties in low alloy TRIP-assisted steels given the constraints associated with the continuous galvanizing process.