Microstructural evolution and mechanical properties of Ni-containing light-weight medium-Mn TRIP steel processed by intercritical annealing

Ni was deliberately added into a light-weight medium-Mn TRIP steel for further improving comprehensive mechanical properties through the precipitation of NiAl intermetallics. Ni addition results in the precipitation of 2-140 nm sized NiAl particles at phase boundaries and the interior of both austenite and ferrite, and more easily inside ferrite. Additionally, Ni addition benefits not only the increase of austenite fraction, ranging from 43% to 57%, but also the improvement of austenite stability because of the partition behavior of Ni from ferrite to austenite during annealing. Similar to Ni-free light-weight medium-Mn TRIP steels, martensitic transformation (TRIP effect) governed by austenite content and stability is the main deformation mechanism in the experimental steel. However, due to Ni addition, NiAl particles can increase yield strength through the precipitation strengthening. And the occurrence of twinning-induced plasticity (TWIP) effect has been confirmed, which contributes to the ductility. The present steel annealed at 680 degrees C demonstrates high yield strength of 855 MPa, ultimate tensile strength of 961 MPa and superior total elongation of 53%, which is ascribed to the combined action of precipitation strengthening, strong TRIP effect and minor TWIP effect.