Micromagnetic techniques have been considered as a potential non-destructive evaluation (NDE) method for microstructural characterization and stress/strain measurements in ferritic steels [1–14]. The effect of tensile deformation on micromagnetic parameters has been studied by many researchers [8–14]. Most of these studies have been done only in the elastic region and the obtained relation between magnetic parameters and the applied stress has been used to determine the residual stresses, considering that the residual stress level will not exceed the yield stress of the material. These studies have not taken into account the microstructural changes due to dislocations generation by the plastic deformation.