Improving mechanical properties and retained-austenite stability of a medium carbon Q&P steel by adjusting phase ratio

In this study we explored mechanisms for improving the mechanical properties and retained-austenite (RA) stability of a Q&P steel by adjusting the phase ratio. With increasing annealing temperature, the yield strength of the steel increased and the ductility firstly increased and then decreased. These observations are closely related to the specific phase constituent and retained austenite stability produced by the Q&P process. The hard phase (tempered martensite, bainite, martensite-austenite island/secondary martensite) fraction increased with increasing annealing temperature, and the RA size and morphology changed from large, blocky to small, blocky and lath-like. At 800 °C, the phase ratio of RA, bainite, ferrite, and initial martensite is 2:1:2.2:2, which produced the superior combinations of yield strength (σ_y), tensile strength (σ_TS), and ductility (δ) (e.g. σ_y = 960 MPa, σ_TS = 1450 MPa, δ = 28%; or σ_y = 1320 MPa, σ_TS = 1540 MPa, δ = 14%). The superior mechanical properties are attributed to the coordinated deformation between hard phase and soft phase and TRIP effect.