Pre-rolling effects on microstructural evolution and mechanical properties of hot- and cold-rolled Q&P specimens: an experimental study

Hot-rolled (HR) and cold-rolled (CR, 30% CR and 60% CR) 0.18C-1.35Si-2.48Mn sheets were subjected to intercritical (760°C) and complete (900°C) austenitization, followed by quenching and partitioning (Q&P) treatment. The effects of pre-rolling on the microstructural evolution and mechanical properties of the Q&P steels were analyzed. The results showed that after intercritical annealing of the HR-Q&P sample, ferrites and retained austenites (RA) existed in two morphologies, namely, blocky and lath-like. By contrast, the 30%CR-Q&P and 60%CR-Q&P specimens were mostly blocky, which was attributed to higher stored energy in CR sheets, the recrystallization of most ferrite after intercritical annealing, and the ability to grow into equiaxed crystals. After complete austenitization, lath martensites were the dominant microstructure in both, accompanied by some blocky and film-like RA. Compared with the CR-Q&P specimens, film-like RA accounted for a higher proportion in the HR-Q&P specimens. Compared with the CR-Q&P specimens, the HR-Q&P specimens had higher elongation and a higher product of strength and elongation (PSE). The HR-Q&P samples after intercritical annealing had a higher PSE, which might be related to their higher RA content and the presence of lath-like RA. Carbon was distributed more evenly in the lath-like RA, resulting in higher mechanical stability. The HR-Q&P samples after complete austenitization had a higher PSE, which might be related to their higher RA content and a higher proportion of film-like RA. The film-like RA exhibited a higher mechanical stability than the blocky RA, which was favorable for enhancing the TRIP effect.