Comparative study on microstructures and mechanical properties of Q&P steels prepared with hot-rolled and cold-rolled C–Si–Mn sheets

In this study, we investigate the microstructures and mechanical properties of quenching and partitioning (Q&P) steels prepared with hot-rolled (HR) and cold-rolled (CR) sheets at different annealing temperatures (partial austenitisation). The ferrite and retained austenite are present in laths and blocks in HR-Q&P samples but are mainly blocky in CR-Q&P samples. Compared with CR-Q&P, the HR-Q&P samples have higher content of retained austenite, better elongation and higher product of strength and elongation (PSE). The lath-shaped retained austenite in HR-Q&P samples with a higher ratio of surface to volume is conducive to homogenization of carbon in austenite grains, thus improving the thermal stability and content of retained austenite. The lath-shaped retained austenite transforms into martensite obviously until the strain exceeds 5% and exhibits higher mechanical stability than coarse blocky retained austenite. When annealed at 810 °C, HR-Q&P and CR-Q&P samples present best PSEs. Indeed, the PSE of HR-810°C-Q&P sample is 62.8% higher than that of CR%-810°C-Q&P. The better elongation and higher PSE possessed by HR-810°C-Q&P sample are linked with the higher content of retained austenite and the presence of two kinds of retained austenite with different mechanical stability. The retained austenite with different mechanical stability in HR-810°C-Q&P sample, contributes to continuous transformation during tensile deformation, thereby improving the elongation. Meanwhile, the lath-shaped structures can delay the formation of voids and microcracks, thereby improving the elongation.