Microstructure and mechanical properties of partially ferritic Q&P steels

The quenching and partitioning (Q&P) heat treatment is a promising way to produce third generation advanced high strength sheet steels consisting of martensite and retained austenite. For an improvement of their mechanical properties, ferrite can be introduced into the microstructure by annealing in the intercritical (IC) temperature region. An alternative heat treatment for producing partially ferritic Q&P steels is investigated in this study. In this heat treatment cycle, the ferrite is introduced by a slow gas jet (SJ) cooling after full austenitization. The differences between IC and SJ treated conditions were studied and compared to a state without ferrite for the same low carbon steel. The mechanical properties were obtained by tensile testing and correlated with the microstructures, as analyzed by light optical and scanning electron microscopy. The local mechanical properties were investigated by nanoindentation measurements. The results demonstrate that a strong increase in elongation can be achieved by both partially ferritic heat treatments, but the IC one results in a better combination of local and global formability.