5th International High Manganese Steel Conference - HMNS 2022

As medium manganese steels are often treated in the intercritical domain to obtain bimodal retained austenite and ferrite microstructures, the application of a quenching and partitioning treatment to a medium manganese steel also promotes the retention of austenite at room temperature via carbon partitioning from martensite to austenite and by the austenite stabilizing effect of manganese in solution. The resulting microstructures are complex and they exhibit interesting mechanical properties that contrast with that of traditional Q&P steels, reaching high strength levels (1600 MPa) and ductilities (20%). However, the medium manganese Q&P microstructures are sensitive to the mechanical contrast brought by the presence of fresh martensite, often leading to early fractures. This research provides an understanding on the contribution of each constituent phase on the global mechanical properties by applying post-tempering treatments on a 5 wt.% Q&P steel. Three microstructure-types with different phases proportions were systematically investigated to assess the role primary martensite, retained austenite and fresh martensite on the work-hardening and ductility. The constituent phases were characterized with XRD, EBSD, TEM, EDX and nanoindentation. The mechanical contrast between phases was found to be a major responsible for the rapid transformation of metastable retained austenite to martensite during straining leading to early fractures. The reduction of the mechanical contrast between phases by post-tempering treatment led to the deployment of different degrees of serrations on the tensile curve, allowing to highlight qualitative conditions for the occurrence of this phenomenon.