Factors influencing the tensile behavior of a Fe–28Mn–9Al–0.8C steel

High Mn–Al–C steels were recently reappraised as the promising material for advanced auto body system by their excellent combination of strength and ductility over 60,000 MPa% associated with microband-induced plasticity. This study was intended to examine the factors affecting the tensile properties of high Mn–Al–C steels. For this purpose, tensile tests and microstructural analyses were performed on a Fe–28Mn–9Al–0.8C steel at various conditions. As usual, strength increased and elongation decreased with decreasing the grain size. The strain hardening rate of the fine-grained steel remained unchanged to the medium strain level but that of the coarse-grained steel continuously increased to the high strain level, resulting in exceptional ductility. Fully austenitic steel obtained by high temperature solution treatment exhibited the continuous increase of strain hardening rate while that of the steel solution-treated at low temperature decreased with increasing strain due to the bimodal grain size distribution and the presence of submicron ferrite. In the strain rates of 2 × 10−4 s−1 to 10−1 s−1, strength was relatively insensitive to the strain rate but elongation decreased with increasing the strain rate, indicating that plastic deformation was mainly achieved by thermal activation in this strain rate range. In the temperature range of 25–450 °C, both strength and elongation decreased with increasing temperature except at 300 °C where serrated flow occurred with extended ductility. The flow characteristics at 300 °C were rationalized in terms of dynamic strain aging associated with carbon exhaustion.