Microstructure evolution and mechanical behavior of a novel hot-galvanized Q&P steel subjected to high-temperature short-time overaging treatment

Abstract

A novel hot-galvanized quenching and partitioning (Q&P) steel (0.225C-0.85Si-2.02Mn-0.91Al, in wt.%) with excellent strength-ductility balance was fabricated by intercritical annealing followed by high-temperature short-time overaging treatment. The resulting microstructure evolution and its relationship with mechanical properties were investigated using dilatometry, SEM, EBSD, TEM, XRD, nanoindentation and tensile testing, and compared with samples subjected to conventional quenching and austempering (QAT) process. The combined effects of structural subdivision by primary martensite and carbon partitioning simultaneously from martensite and bainite lead to the retention of larger amounts of retained austenite (RA) constituents with different morphologies, sizes and stabilities at room temperature. On the contrary, the QAT samples have coarser martensite-austenite (M-A) islands and smaller fraction of RA due to relatively insufficient partition and bainitic transformation kinetics. As compared to QAT treatment, the better mechanical properties with a yield strength (YS) of 736 MPa, tensile strength up to 1020 MPa, total elongation (TEL) of around 24%, and product of strength and elongation (PSE) above 24 GPa% can be achieved after the Q&P process, and particularly, more ductile tempered martensite and refined M-A particles result in a simultaneous increase of YS and TEL. The instantaneous work-hardening exponent vs. true strain curve of Q&P samples displays a small peak followed by a long plateau region before break, which can be attributed to a less active and sustained TRIP effect.