Comparison on mechanical properties and microstructure of a C-Mn-Si steel treated by quenching and partitioning (Q&P) and quenching and tempering (Q&T) processes

A steel containing Fe-0.20C-1.57Mn-1.55Si-0.0029P-0.0018S (wt%) was treated by quenching and partitioning (Q&P) and quenching and tempering (Q&T) process. The resulted mechanical properties indicate that the Q&P process may bring on an excellent combination of strength and elongation for the steel, of which the product was between 14.5 and 20.4 GPa%. The morphological relationship between the retained austenite and martensite was characterized as well. It was found that the TRIP effect of retained austenite plays an important role in enhancing the work hardening rate and instantaneous n-value at high strain level, which further increases the uniform elongation. Fractography of samples treated by Q&P process showed mainly ductile mechanism, while a small percentage of brittle fracture exists in the samples treated by Q&T process. Possible isothermal decomposition of austenite at quenching temperature (QT) and partitioning temperature (PT) and its effect on carbon partitioning was discussed.