Microstructure Characteristics, Mechanical Properties and Strain Hardening Behavior of Nitrogen-Bearing Fe-0.2 C-5Mn TRIP Steel

As a strong and low-priced austenite stabilizer, nitrogen (0.085 wt%) was deliberately added into a hot-rolled Fe-0.2C-5Mn TRIP steel for optimizing the strength-ductility balance. Nitrogen addition can not only increase austenite fraction (> 50%), but also favor the improvement of austenite stability due to its partition behavior from ferrite to austenite during intercritical annealing. Besides, the low annealing temperature (635 degrees C) and nitrogen addition result in the simultaneous precipitation of cementite (size, 21-396 nm) and carbonitride (size, 7-193 nm) in ferrite and at grain boundaries, which can effectively increase the yield strength through precipitation strengthening (33-63 and 41-61 MPa, respectively). Furthermore, the sufficient interstitial solute originated from N addition as well as the discontinuous TRIP effect benefits the strong serrated flow behavior and multi-peak strain hardening behavior. As a result, the strong TRIP effect together with precipitation strengthening is believed to be responsible for the excellent strength-ductility balance of present steel annealed for 3 h, demonstrating a high tensile strength of 1043 MPa, outstanding total elongation of 43.3% and superior product of strength and elongation (PSE) of 44.8 GPa%.