The influence of severe warm deformation on the microstructural evolution and flow behavior of a plain carbon steel has been investigated through torsion testing. The specimens were severe deformed within the ferritic region up to strain of 70 with constant strain rate of 0.1 s 1 − . Microstructural evolutions have been investigated using high resolution electron backscatter diffraction (EBSD). True stress-true strain curves exhibit a single and smooth maximum, followed by a slow but significant softening stage. A steady state is observed at very large strains. This finding suggests that the relative balance between comparable work hardening and dynamic work softening results in the occurrence of warm ductility during large deformation. The initial grain structure was equiaxed, but at low strains the grains become elongated in the torsion direction. However, at large strains the grain aspect ratio decreases and finally, further straining leads to the formation of new fine grains with high-angle boundaries, which become more equiaxed than the previous fragmented structure. The flow stress, as well as all the average microstructural parameters, then remains independent of strain. The mechanisms operating during such warm flow behavior and structure changes are discussed in detail.