Effects of Al on microstructure and tensile properties of C-bearing high Mn TWIP steel

The effects of Al on microstructure, stacking fault energy, tensile properties and fractured surface in Fe–18Mn–0.6C–(0–2)Al twinning-induced plasticity (TWIP) steels were systemically investigated. The Al addition suppressed the cementite precipitation during cooling after hot-rolling. The stacking-fault energy was linearly raised with a constant slope of 7.8 mJ m−2 per 1 wt.% Al. The Al addition increased the yield stress, reduction in area, uniform (eu), and post-uniform (epu) elongations, while it decreased the amount and rate of strain hardening and dynamic strain aging (DSA). In particular, although the epu of the TWIP steel without Al was almost zero, it was improved up to ∼7% by addition of 2 wt.% Al, which was comparable with those of dual phase and TRIP steels with a similar tensile strength of 780 MPa. In order to elucidate the reasons for the poor epu in C-bearing TWIP steel and for the prolonged epu by Al addition, the apparent absolute strain-rate sensitivity m=dσ/dlnε˙ of two TWIP steels with different Al concentrations of 0 and 2 wt.% was investigated at room temperature. Both TWIP steels had negative strain-rate sensitivity at a large strain (ε = 0.4) of just before necking. However, the Al addition increased the strain-rate sensitivity, resulting in improved epu because of reduced DSA by decreases in both activity and diffusivity of C in austenite.