Effect of different matrix structure in medium-Mn quenching and partitioning steel on austenite stability and work hardening behavior

The "one-step" quenching and partitioning (Q&P) process combined with the annealing process is applied to medium-Mn steel. The effects of different annealing temperatures on the microstructure, C-element diffusion and mechanical properties of the experimental steels were systematically studied. When the annealing temperature varies from 810 degrees C to 600 degrees C, the martensite-austenite island of the test steel gradually decreased or disappeared, the polygonal blocky ferrite increased, and the large lath-like retained austenite became flake-like (film-like). Meanwhile, C that martensite provides can be taken full advantage by retained austenite due to the relatively small amount of martensite in the test steel, and the sustained transformation induced plasticity (TRIP) effect could be achieved by such retained austenite, resulting in a good mechanical property. However, the test steels treated at too high annealing temperatures (>= 700 degrees C) are difficult to occur TRIP effect because of the over C enrichment in retained austenite in the steel. In summary, excellent comprehensive mechanical properties were obtained for the test steel annealed at 670 degrees C with "one-step" Q&P process, with an ultimate tensile strength of 1130 MPa and total elongation of 26.80%.