Tailoring the microstructure by prior treatment to achieve 37.6 GPa·% PSE in a room-temperature quenched and partitioned steel

Abstract

In this work, different initial microstructures, including deformed cold-rolled structure, quenched martensite, granular bainite, and lathy bainite, were intentionally designed through cold rolling and prior treatment. Meanwhile, a novel room-temperature quenching and partitioning (RT-Q&P) process was proposed to further enhance strength-ductility combination of Cu-Ni bearing Q&P. The present study aims to investigate the influences of initial microstructures on the formation kinetics of austenite, the mechanical stability of RA, microstructure evolution, and mechanical performance in Q&P steel treated by RT-Q&P process. Compared to sample without prior treatment, samples with prior treatment showed a larger austenization degree due to more nucleation sites on martensite/bainite matrix. DICTRA simulation indicates the lathy bainite matrix with low C and low Mn is more conducive to the refinement of parent austenite, thus leading to fine and dispersive TM/A. During tensile deformation, the fine and dispersive lath-structures can effectively alleviate strain localization on the interfaces which resulted from the deformability difference between the soft and hard phases, thereby delaying crack initiation and propagation. After RT-Q&P treatment, the outstanding mechanical properties with UTS of 1152 MPa, TEL of 32.6 %, YS of 754 MPa, and PSE of 37.6 GPa·% was achieved in sample with lathy bainite matrix, far exceeding the properties of traditional Q&P steels, which was mainly attributed to higher work-hardening ability of fine and dispersive structure and sustained TRIP effect of RA.