Effect of Partitioning Treatment on the Mechanical Property of Fe-0.19C-1.47Mn-1.50Si Steel with Refined Martensitic Microstructure

In order to understand the effect of microstructural features on the mechanical property, quenching and partitioning (Q&P) and quenching and tempering (Q&T) treatments were carried out on a cold-rolled low-carbon Fe-0.19C-1.47Mn-1.50Si steel sheet. It has been shown that because of the rolling in advance, the grain size of prior austenite was dramatically reduced, which resulted in a great decrease in martensite packet/block size and an increase in dislocation density in martensite in the as-quenched state. However, there was no obvious change in average lath size. Different from Q&T treatment, Q&P not only stabilized a large amount of retained austenite, but also led to a serious carbon depletion in martensite as revealed by X-ray diffraction and three-dimensional-atom-probe analyses. In Q&T and Q&P samples, refining martensitic microstructure improves both the strength and impact toughness markedly but does not affect the elongation very much. Compared with Q&T sample, Q&P one is softer due to the existence of considerable amount of retained austenite and less carbon content in martensite, i.e., it has higher elongation and impact toughness but lower strength. Analyses indicated that the strength loss caused by carbon depletion in martensite is critical which has even completely covered up the strengthening effect of microstructural refinement. On the other hand, the carbon depletion in martensite is more essential in improving impact toughness, comparing the role of microstructural refinement and the existence of more retained austenite. Through a combination of rolling and Q&P processes, the refined Q&P microstructure was achieved for a greatly improved product of strength and elongation and a much lower ductile-to-brittle transition temperature.