Evolution of non-metallic inclusions in non-oriented electrical steel: industrial observations and laboratory test results

Non-oriented electrical steel is widely applied as a core material for various electrical machines due to its excellent soft magnetic properties. Besides grain size, precipitates, defects, and grain orientation, non-metallic inclusions (NMIs) significantly affect the magnetic properties of non-oriented electrical steel by inhibiting grain growth, distorting the lattice, and hindering domain wall motion. This study investigates changes in the inclusion landscape of non-oriented electrical steel during the manufacturing process. Accordingly, industrial samples from different production steps were analyzed using automated SEM/EDS measurements. The detected NMIs in each sample were compared with respect to elemental composition, number per mm², and mean equivalent circular diameter (ECD). Additionally, laboratory trials in a resistance-heated Tammann-type furnace with raw material from industry were carried out, examining the influence of Ce and La on the inclusion landscape. For the industrial samples, the number of NMIs per mm² decreases with ongoing processing. Furthermore, a slight increase in the mean ECD is observed. The distribution of NMI classes changed from primarily oxides in the ladle furnace sample to comparable amounts of sulfides, nitrides, oxide–sulfides and nitride–sulfides in the final product. The laboratory samples alloyed with Ce and La showed a reduction in NMIs per mm² as well as an increase in mean ECD compared to the input material, which suggests an improvement of the magnetic properties.