  A novel medium Mn steel of composition Fe-12Mn-4.8Al-2Si-0.32C-0.3V was
manufactured with 1.09 GPa yield strength, 1.26 GPa tensile strength and 54%
elongation. The thermomechanical process route was designed to be industrially
translatable and consists of hot and then warm rolling before a 30 min
intercritical anneal. The resulting microstructure comprised of coarse
elongated austenite grains in the rolling direction surrounded by necklace
layers of fine austenite and ferrite grains. The tensile behaviour was
investigated by in-situ neutron diffraction and the evolution of microstructure
studied with Electron Backscattered Diffraction (EBSD). It was found that the
coarse austenite grains contributed to the first stage of strain hardening by
transforming into martensite and the fine austenite necklace grains contributed
to the second stage of strain hardening by a mixture of twinning and
transformation induced plasticity (TWIP and TRIP) mechanisms. This hierarchical
deformation behaviour contributed to the exceptional ductility of this steel.
