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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 676; pp. 56 - 64
• Main Authors: Mandal, G., Ghosh, S.K., Bera, S., Mukherjee, S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 31.10.2016
• Subjects: Alloys; Bainite; Carbon; Enrichment; Ferrite; High strength steels; Martensite; Mechanical properties; Microstructure; Partitioning; Q&P treatment; Quadratic programming; Retained austenite; Q&P treatment; Mechanical properties; Ferrite; Retained austenite; Martensite; Alloys
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2016.08.094

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 30min 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 10.50wt%. It is evident that partitioning for 30min leads to good carbon enrichment (>1wt%) 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 (921–922MPa) 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.