Influence of Isothermal Treatment Prior to Initial Quenching of Q&P Process on Microstructure and Mechanical Properties of Medium Mn Steel

The influence of isothermal treatment prior to the initial quenching step of Q&P process was investigated in 0.26C – 4.15Mn – 1.39Si steel. Compared to the microstructure of the Q&P processed steel: a mixture of martensite and retained austenite, the microstructure subjected to isothermal treatment before initial quenching step (BQ&P) consists of bainitic ferrite, martensite and retained austenite. Formation of bainitic ferrite during isothermal treatment refined the retained austenite grain size and made the filmy morphology prevalent in the final microstructure. Besides, carbon effectively enriched into the austenite adjacent the bainitic ferrite. The change in size, morphology and carbon content enhanced the mechanical stability of austenite in BQ&P processed steel; it has a beneficial influence on improving tensile elongation for a given tensile strength level. Moreover, controlling austenite characteristics by isothermal treatment is advantageous on the stretch-flangeability; hole expansion ratio is evaluated to be 22.2% for BQ&P processed steel, which is 2.7 time higher than that for the conventional Q&P processed one.