Effects of quenching temperature on bainite transformation, retained austenite and mechanical properties of hot-galvanized Q&P steel

Over the last decade, demand has increased for developing the hot-galvanized quenching and partitioning (Q&P) steel to overcome the disadvantages associated with vehicle safety, fuel consumption and corrosion resistant. The present work aims to elucidate the effects of quenching temperature on phase transformation kinetics, microstructure evolutions and mechanical properties of a hot-galvanized Q&P steel (0.225C-0.85Si-2.02Mn-0.91Al, in wt.%) by combining modeling and experimental research. Using dilatometry, SEM, EBSD, EPMA, TEM, PED, XRD and Image-Pro Plus (IPP) software, we quantitatively investigated the microstructure evolution at different quenching temperatures. Results indicated that a larger fraction of primary martensite at lower quenching temperature could strongly promote subsequent bainite transformation kinetics, which is attributed to more martensite-austenite interfaces and defect density. By fitting the dilatometer curves and establishing the equation of transformation rate vs. quenching temperature, a Kolmogorov-Johnson-Mehl-Avram (KJMA) equation was established to describe insufficient bainite formation kinetics during high-temperature short-time overaging. Furthermore, a modified CCE model taking into account intercritical ferrite and short-time bainite transformation was proposed and the predicted RA fractions are more consistent with the experimental values. As the quenching temperature decreases, small-sized blocky RA along martensite boundaries and filmy RA between martensite laths increase, while coarse lath/blocky RA inside bainite structures or at bainite boundaries decreases gradually. In addition, the YS decreases from 763 MPa to 431 MPa with the increase of quenching temperature, while the UTS varies in a narrow range between 967 MPa and 1036 MPa. A stable TEL of 22.6–25.7% can be obtained at a wide range quenching temperature (150–275 °C), which is attributed to the joint effects of TRIP effect and multiphase structure. This research would be of guiding significance for the industrial practice.