Improving strength–ductility synergy in a low-alloy steel via quenching and isochronal C partitioning

In the conventional quenching and partitioning (Q&P) process, the so-called film-like austenite, which is generally fabricated by decreasing the quenching temperatures, oftentimes contains a high C content of up to 1.2 wt.%, which is unfavorable for improving the ductility. Here, we combine the quenching with isochronal partitioning (Q&IP) to obtain the newly Q&IP steel, featuring the film-like austenite with a slightly reduced C content of 0.98 wt.%. As compared with the traditional Q&P steel fabricated by quenching and isothermal partitioning, the Q&IP steel exhibits higher yield and ultimate tensile strengths (1168 ± 8.7 MPa and 1722 ± 10.2 MPa, respectively) and good ductility (with a uniform elongation of 13.2 ± 0.2%), due to the combination of the enhanced dislocation plasticity, the higher back stress hardening, and the durable transformation-induced plasticity effect. Applying a thermo-kinetic theory of generalized stability, we further demonstrate that the increased strength and good plasticity in the Q&IP steel come from the phase transformations with high thermodynamic driving forces and high generalized stability.