Effect of partial and full austenitisation on microstructure and mechanical properties of quenching and partitioning steel

A novel high-strength steel has been made through thermo-mechanical controlled processing with the finish rolling temperature of 750 °C followed by air cooling. Subsequently, both partial austenitisation at 800 °C and fully austenitisation at 930 °C have been attempted for equal duration of 30 min prior to one step quenching and partitioning (Q&P) at 345 °C below MS temperature (365 °C). As-rolled steel reveals ferrite-bainite-martensite microstructures with a good combination of strength and ductility. After Q&P, all the specimens have exhibited the multiphase microstructures comprising ferrite, lath microstructure (martensite and bainite), and retained austenite with the volume fractions of up to 10.50 wt%. It is evident that partitioning for 30 min leads to good carbon enrichment (>1 wt%) of the austenite phase from the neighbouring martensite or bainite which might be due to fast partitioning kinetics and possible suppression of carbides through a combination of Si and Al additions. The attractive combination of tensile strength (921-922 MPa) and ductility (25-26% total elongation) along with low yield ratio (0.63-0.69) are attributed to ferrite and lath microstructures along with the thin film like carbon enriched retained austenite obtained after Q&P process.