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The microstructures of a SS400 steel after thermomechanical control process (TMCP) in an industrial production were observed by optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that the size of ferrite grains was 4～5μm, and the volume fraction of ferrite was around 70%. The types of the ultrafine ferrite grains were analyzed and the strengthening mechanisms were discussed. The results show that the ultrafine ferrite grains came from three processes, i.e.deformation induced ferrite transformation (DIFT), dynamic recrystallization of ferrite and accelerated cooling process. The increase in the strength of the material was mainly due to the grain refining.