High Strength-Ductility Nb-microalloyed Low Martensitic Carbon Steel：Novel Process and Mechanism

A low-carbon Nb-microalloyed Fe-Mn-Si-based steel was treated by a novel quenching-partitioning-tempering（Q-P-T） process as a modified quenching and partitioning（Q＆P） process. After processed by Q-P-T treatment,this steel exhibits excellent mechanical properties such as high product of strength and elongation. The addition of Nb markedly raises both the yield strength and tensile strength of Q-P-T martensitic steel, especially the yield strength, which can be attributed to the strong grain refinement strengthening and precipitation strengthening of Nb. The Nb addition can also lead to a little increase in ductility. The Nb-microalloyed steel treated by Q-P-T process displays much higher ductility than that treated by traditional quenching and tempering（Q＆T） process. The mechanisms of Q-P-T process on ductility enhancement were fully analyzed and can be attributed to high quenching temperature and considerable amount of retained austenite.