An innovative pathway to produce high-performance quenching and partitioning steel through ultra-fast full austenitization annealing

Abstract

Quenching and partitioning (Q&P) treatment was conducted on a cold-rolled low-carbon steel using a quenching dilatometer. A novel phase-transformation pathway to produce high-performance Q&P steel by introducing fast-heating full austenitization annealing method was proposed in the present work. Different from conventional slow-heating intercritical annealing (4 K/s), an unexpected transition from austenite to ferrite occurs even upon rapid cooling (70 K/s) through ultra-fast full austenitization (300 K/s) annealing, corresponding to unstable carbon-depleted austenite. Ultra-fine high-carbon austenite retards the martensitic transformation to lower temperature regime upon further cooling, and lath-like martensite forms owing to the pinning effect from prior ferrite and ultra-fine austenite grain boundaries. Mechanical properties demonstrated that the full austenitization annealing method could enhance both the slow-heating QP-4 and fast-heating QP-300 samples from 980 MPa grade to a higher grade (1180 MPa) Q&P steel, and meanwhile, it should be noted that the total elongation up to 21.7 % was obtained for the QP-300 samples, higher than 16.5 % of the QP-4 samples. Although the yield strength of QP-300 samples slightly decreased, the product of strength and ductility strikingly improved from 19.5 GPa% to 25.7 GPa%.