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 °C and aging at 550 °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 °C and aging produces a microstructure consisting of nano-sized κ-carbides and Mo₂C carbides within equiaxed austenite grains and submicron-sized Mo₆C carbides along grain boundaries. Further increasing annealing temperature to 950 °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 °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 °C increases the strain hardening rate effectively.