The role of deformation twinning in the formation of a fine-grained structure in cold-rolled 310S steels has been investigated by using transmission electron microscopy. When 310S steels were rolled at room temperature, a deformation twin appeared in the early stage of the rolling process. A fine lamellar structure consisting of the deformation twin and matrix (T-M) developed with increased rolling strain. At moderate strain, micro shear bands appeared in the area of the T-M lamellar structure, and the shear bands grew and multiplied during further cold-rolling. Such multiplication of shear bands destroyed the T-M lamellar structure, which caused the development of a fine-grained structure. In shear bands, a highly misoriented structure with a submicron grain size was generated. In this paper, the mechanism of the transmutation process from the T-M lamellae into the fine-grained structure is discussed. Particular emphasis is laid on the role of deformation twinning in the formation of the fine-grained structure. (c) 2006 Springer Science + Business Media, Inc.