Effect of Prior Cold Reduction of C-Si-Mn Hot-Rolled Sheet on Microstructures and Mechanical Properties after Quenching and Partitioning Treatment

This paper studies the microstructures and mechanical properties of quenching and partitioning (Q&P) samples prepared with 35% and 75% cold reduction sheets at an annealing temperature of 810 degrees C (intercritical temperature). The results indicate that prior cold reduction could significantly influence the ferrite recovery and recrystallization during intercritical annealing, which changes the size and distribution of the ferrite and retained austenite in the Q&P samples. Compared with the 75%-Q&P sample, the 35%-Q&P sample had smaller recrystallized ferrite and retained austenite grains, a higher volume fraction of retained austenite, and a more uneven size distribution of retained austenite. The 35%-Q&P sample presented better total elongation and a higher product of strength and elongation (PSE) than the 75%-Q&P sample. The higher total elongation was related to the higher content and uneven size distribution of retained austenite for they strengthened the TRIP effect and improved the uniform elongation of the sample. The results proved that Q&P steel prepared with a cold-rolled sheet with lower reduction exhibits a better combination of strength and plasticity due to the fact that lower reduction can delay the growth rate of austenite and recrystallized ferrite grains during the intercritical annealing stage.