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 (e _u), and post-uniform (e _pu) elongations, while it decreased the amount and rate of strain hardening and dynamic strain aging (DSA). In particular, although the e _pu 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 e _pu in C-bearing TWIP steel and for the prolonged e _pu by Al addition, the apparent absolute strain-rate sensitivity m=dσ/dlṅ 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 ( = 0.4) of just before necking. However, the Al addition increased the strain-rate sensitivity, resulting in improved e _pu because of reduced DSA by decreases in both activity and diffusivity of C in austenite.