Influence of Nb Addition on Microstructure and Mechanical Properties of Medium-Mn Low-Density Steels

The microstructural modifications and mechanical properties are investigated in two kinds of hot-rolled Fe-8Mn-6Al-0.15C (0Nb steel) and Fe-8Mn-6Al-0.15C-0.02Nb (0.02Nb steel) medium-Mn low-density steels, subjected to an intercritical annealing treatment. Two experimental alloys presented an extraordinary bimodal duplex microstructure made of coarse delta-ferrite grains and fine-grained (FG) {intrecritical ferrite+retained austenite} regions after annealing at 800-1000 degrees C for 1h. In the 0.02Nb steel annealed at 700 degrees C in particular, martensite lath and niobium-contained M5C2-type carbides in spherical nanoscale shape are found. The 0.02Nb steel exhibited excellent mechanical properties of tensile strength of 940MPa and elongation of 36%, which are 11.1% and 12.5% higher than those of 0Nb steel. The addition of Nb in 0.02Nb steel led to partitioning of C and Mn from austenite and retardation of the gamma/alpha transformation. The improvement of the mechanical properties in 0.02Nb steel is mainly attributed to the sufficient transformation induced plasticity (TRIP) effect during tensile behavior resulted from the higher austenite volume fraction and lower austenite stability.