Investigation of microstructural evolution and bainite transformation kinetics of multi-phase steel

In this study, multi-phase steel with a mixture of ferrite, bainite, retained austenite and martensite is obtained after specific heat treatments, which include intercritical annealing, isothermal holding and final quenching. The microstructural constituents and individual phase fractions are precisely quantified based on multi-scale characterizations. The focus is on bainite transformation kinetics and its impact on the microstructural evolution, which is systematically analyzed according to the assumption of displacive modeling theory. Bainite transformation begins with austenite grain-boundary nucleation and continues through autocatalytic nucleation based on nucleation kinetics of bainitic sub-units. A small discrepancy between the modeling and experimental results is indicative of carbon partitioning into austenite, which reduces the driving force and/or the rate parameter kappa for bainite transformation. In addition, the continuous carbon enrichment in austenite decelerates the bainite transformation and prolongs the duration time. Moreover, the maximum carbon concentration in austenite is limited by the To line. Austenite with a lower carbon content but higher volume fraction will not transform to bainite at higher isothermal bainite transformation temperatures, and such mechanically unstable austenite is more likely to transform to martensite during final quenching. As a result, with increasing isothermal holding temperature, the bainite and retained austenite fractions decrease while the martensite fraction increases in the sepcimens after final quenching.