Bainitic transformation during the two-step quenching and partitioning process in a medium carbon steel containing silicon

A study of 40SiMnNiCr steel subjected to a two-step quenching and partitioning process (Q&P) is presented. The result suggests that strength variation of Q&P steels during the two-step Q&P process was a cumulative effect of increase of retained austenite fraction, decrease of carbon supersaturation of virgin martensite, and particularly much of lower bainite formation. A trade-off between high strength and good ductility of two-step Q&P steels can be tailored and adjusted by controlling lower bainite fraction. The final amount of austenite was influenced by the transformation kinetics of lower bainite during the partitioning process.