Structural refinement in interstitial free (IF) steels has been obtained by three different methods: (i) deformation by cold or warm rolling, (ii) martensitic transformation and (iii) a combination of a martensitic transformation and plastic deformation. For all these processes, the refinement is discussed in terms of grain subdivision by high angle boundaries and dislocation boundaries on length scales from the micrometer level to the nanometer dimension. The characteristics of the subdividing boundaries are discussed, leading to the formulation of strength-structural relationship for IF steel in the deformed state.