Influence of pre-tempering treatment on microstructure and mechanical properties in quenching and partitioning steels with ferrite-martensite start structure

Abstract

The low carbon micro-alloyed steel with start microstructure of ferrite-martensite was pre-tempered to be ferrite-tempered martensite microstructure before cold rolling. Then an identical quenching and partitioning process was applied to the cold rolled sheet with start microstructures of deformed ferrite-martensite (Q&P-UFC) and deformed ferrite-tempered martensite (Q&P-T). It revealed that the austenitization of Q&P-T was retarded and much more austenite with higher stability was obtained after annealing. In both cases, the austenite formed at recrystallized martensite boundaries was significantly fine, while that formed at recrystallized ferrite boundaries was much coarser. Although the retained austenite fraction (5–6%) and corresponding carbon content (1.1–1.2 wt.%) were identical in both cases, a significant Mn-enrichment of retained austenite was only observed in Q&P-T related to the pre-existed Mn-enriched cementite and hence the corresponding TRIP effect in Q&P-T occurred at relatively larger strain. Moreover, abundant martensite variant combination with twin boundary was observed and exhibited dispersive distribution in Q&P-T. The work hardening behaviors of both cases showed three-stage variation, successively ascribed to the dislocation glide of ferrite, TRIP effect of retained austenite and dislocation intersection/tangle of ferrite and martensite, respectively. Both samples exhibited excellent combination of strength and ductility with UTS larger than 1000 MPa and TEL larger than 20%, and Q&P-UFC was favorable to obtain high strength level, while Q&P-T possessed better ductility and excellent combination of strength and ductility.