  A medium Mn steel of composition Fe-4.8Mn-2.8Al-1.5Si-0.51C (wt.\%) was
processed to obtain two different microstructures representing two different
approaches in the hot rolling mill, resulting in equiaxed vs. a mixed equiaxed
and lamellar microstructures. Both were found to exhibit a simultaneous
TWIP$+$TRIP plasticity enhancing mechanism where deformation twins and
$\alpha'$-martensite formed independently of twinning with strain. Interrupted
tensile tests were conducted in order to investigate the differences in
deformation structures between the two microstructures. A constitutive model
was used to find that, surprisingly, twinning contributed relatively little to
the strength of the alloy, chiefly due to the fine initial slip lengths that
then gave rise to relatively little opportunity for work hardening by grain
subdivision. Nevertheless, with lower high-cost alloying additions than
equivalent Dual Phase steels (2-3 wt\% Mn) and greater ductility, medium-Mn
TWIP$+$TRIP steels still represent an attractive area for future development.
