Effects of nitrogen content on TRIP of Fe–20Cr–5Mn–xN duplex stainless steel

Near Ni-free duplex stainless steels (D-SS) containing the different N contents of 0.1–0.35 wt.% were tensile-tested. Microstructure of D-SSs consisted of a contiguous band structure of coarse ferrite and fine austenite. The austenite fraction increased almost linearly as the N content increased. All D-SSs exhibited transformation induced plasticity (TRIP), resulting in an enhancement of both strength and ductility, especially over 1 GPa and 60% for the 0.35 N D-SS. Strain induced martensite (SIM) that causes TRIP nucleated at not only in-grain ɛ martensite band intersections but austenite grain boundaries (AGBs) and annealing twin boundaries (TBs). Growth of SIM at AGBs and TBs was found to be faster than that of in-grain SIM. The apparent SIM fraction increased as the N content increased (correspondingly, the austenite fraction), leading to a better strain hardening. The overall strain hardening was dominated by the amount of accumulated SIM and the strain hardening of the remaining untransformed austenite, not the SIM transformation rate. Analysis using the SIM fraction normalized to the initial austenite fraction revealed that the SIM transformation was delayed as the N content increased due to the austenite stabilization increase. Also, it was found that regardless of the austenite fraction, there is a critical SIM fraction showing the transition of the flow mechanism of remaining untransformed austenite.