Microstructure and mechanical property connections for a punched non-oriented electrical steel lamination

Secondary manufacturing processes used on electrical steels, such as mechanical or laser cutting, have a detrimental effect on the magnetic properties and increase the losses for electric motors. When the process is mechanical, there is both work hardening, and residual stress induced near the cut edge. The knowledge of the type of deterioration near the cut edge and the degree of deterioration is important for designing electrical machines in terms of magnetic field and loss calculations. The present paper evaluates the effect of punching on the microstructure and mechanical properties of non-oriented electrical steel. Scanning Electron Microscopy (SEM) and Electron backscattered diffraction (EBSD) analysis was performed on the cross section of the punched non-oriented electrical steel to study the evolution of microstructure and crystallographic texture near the edge whereas nanoindentation was used to determine the hardness profile. The observed mechanical properties were related to the microstructure near the edge, which was highly heterogeneous. The microstructure of the punched steel consisted of elongated grains and shear bands with ultrafine grains of size smaller than 500 nm. The hardness was maximum in a region where shear bands were observed in the punched microstructure and the value obtained was 4.54 GPa. This hardness increase, with respect to the average hardness of undamaged steel (3.15 GPa), was attributed to various factors such as grain refinement, work hardening and residual stress. Finally, pop-in analysis was done to study the extent of work hardening due to punching and its effect on hardness. The zero pop-in region was extended to a distance of 200 µm from the edge in the fracture section whereas it was 50 µm in roll over section.