Enhanced strength-ductility synergy in cold-rolled Bainitic steel: Insights into the influence of slow cooling

Abstract

The microstructural evolution for varied cooling strategies is investigated in a cold rolled bainitic steel. Special attention is given to the effect of slow cooling on bainitic transformation kinetics as well as the microstructure and properties. The results indicate that the introduction of a slow cooling stage resulted in the formation of a small amount of ferrite, which slowed the subsequent bainitic transformation. With the decreasing of austempering temperature, bainitic transformation kinetics slows down. As the austempering temperature decreased to below Ms, the initial rate of bainite transformation is accelerated. Whereas, the introduction of pre-formed martensite could only accelerate the bainitic transformation kinetics in the early stage rather than the overall stage. The incorporation of the slow cooling stage significantly improved ductility due to the strain accommodation between soft and hard phase as well as the increased fraction of RA, yielding a higher combination of strength and ductility.