One of noble grain refinement methods to attain ferrite grain size of around 1m in plain C-Mn steel has been so-called super short interval multi-pass rolling (SSMR) process, which is characterized by finish-rolling above Ae3, extreme short interpass time and then rapid cooling. The transformation mechanisms are demonstrated by the microstructural observation of an fcc Ni-Fe model alloy and C-Mn steels. In the Ni-Fe alloy hot-rolled by SSMR, equiaxed and cellar array of dislocation walls with low and high angle misorientations are observed in shear directions without recrystallization. The average spacing between the dislocation walls in the Ni-Fe alloy is in good agreement with the ferrite grain size of the C-Mn steel by SSMR process. On the other hand, in the C-Mn steel quenched to bypass ferrite transformation, the fine ferrite nuclei have been identified along intra-granular shear directions in the quenched martensite. It is concluded that the dislocation substructure in heavily deformed austenite by SSMR process have high potency as inherited ferrite nucleation