Microstructural evolution and tensile properties of Fe-11Mn-10Al-1.2C medium-Mn lightweight steel

In the present study, the variations of microstructure and tensile properties were investigated as a function of aging time using a Fe-11Mn-10Al-1.2C medium-Mn lightweight steel. The 1000 degrees C-annealed specimen revealed an austenite matrix containing nano-sized particles of intra-granular (IG) kappa-carbide and a small amount of alpha-ferrite. After aging at 550 degrees C for 3 min, pre-existing IG kappa-carbide grew up rapidly and relatively coarse kappa-carbide formed at the gamma-austenite grain boundary (GB). With aging for 10 min, the C-depleted zones, close to the coarsened GB kappa-carbide particles, were transformed to filmy grain boundary ferrite (GBF). When aging time increased to 30 min, the eutectoid reaction (gamma ->alpha+ kappa) occurred preferentially at the coarse particles of GB kappa-carbide. The eutectoid reaction continued with increasing aging time up to 720 min, and finally the specimen had a eutectoid matrix (alpha+kappa) with a few of alpha-ferrite. The annealed specimen possessed high strengths (yield strength (YS) of 1242 MPa and ultimate tensile strength (UTS) of 1314 GPa) with moderate elongation (El) of 22.2%. The YS value of aged specimens was improved to similar to 1.35 GPa by the growth of IG kappa-carbide particles after aging for 3 min, and increased further up to similar to 1.54 GPa by the formation of eutectoid structure after aging for 720 min.