Effect of Cu and Mo on the kinetics of isothermal phase transformations in advanced multiphase medium-Mn steels designed for the QP process

The current study aims to investigate the effect of Cu and Mo, commonly found in steel scrap, on the kinetics of isothermal phase transformations in medium-Mn steels designed for the quenching and partitioning (QP) process. Three steels were designed with a base steel composition of 0.18C-4Mn-0.8Al- 0.5Si (wt.%). The second steel contained an additional 0.3 wt.% Mo, while the third one included both 0.3 wt.% Mo and 1.0 wt.% Cu. The phase transformation kinetics at different isothermal temperatures (ranging from 480 to 210 °C) were analysed using the JMatPro software and experimentally verified by a high-resolution Baehr DIL805 dilatometer. As a result, the shorter bainite incubation time was predicted by JMatPro as compared to dilatometric experiments. This discrepancy is probably associated with software databases lacking the kinetic parameter corrections accounting for the Mo-and Cu-induced solute drag effect in the investigated multi-alloyed medium-Mn steels. The determined time–temperature–transformation (TTT) diagrams showed that Mo and Cu additions shifted the bainitic transformation to longer times. Microstructural analysis revealed mixed martensitic–bainitic structures with varying proportions, whereas martensite was more dominant in Mo- and Mo+Cu-alloyed steels. Subsequently, QP treatments were optimized and verified using dilatometry. Quenching at 270 °C and partitioning at 400 °C for 120-1800 s were used to cover different time scenarios during the heat treatment. SEM evidenced details of a tempered martensite morphology and RA films. The microscopic studies were completed by hardness measurements, with the highest hardness registered for the Mo+Cu steel (∼391 HV) at short and intermediate partitioning times.