Multiphase precipitation behavior and tensile properties of a Fe-Mn-Al-Mo-C austenitic lightweight steel

In order to improve the strength and strain hardening rate, multiple precipitates were introduced by annealing at 850-950 degrees C and aging at 550 degrees C after hot or cold rolling in a Fe-26Mn-8Al-3Mo-1.2C austenitic lightweight steel. The results show that the combination of rolling, annealing at 850 degrees C and aging produces a microstructure consisting of nano-sized kappa-carbides and Mo2C carbides within equiaxed austenite grains and submicron-sized Mo6C carbides along grain boundaries. Further increasing annealing temperature to 950 degrees C reduces the precipitation of Mo2C and decreases the density of dislocation. The yield strengths increase by -300-500 MPa, and ultimate tensile strengths increase by -200-400 MPa by comparison with the steel processing by solid solution and two-step aging. Cold rolling before heat treatments leads to an improvement in strength compared to hot rolling, mainly due to grain boundary strengthening and Mo2C precipitation strengthening. Decreasing annealing temperature after rolling from 950 to 850 degrees C increases strength primarily ascribed to Mo2C precipitation strengthening and dislocation strengthening. The introduction of a large number of fine Mo2C precipitates distributed evenly within the steel matrix through cold rolling and subsequent annealing at 850 degrees C increases the strain hardening rate effectively.