Magnesium Non-metallic Inclusions in Non-oriented Electrical Steel Sheets

During routine metallographic investigations of some fully processed, non-oriented, electrical steel sheets, the typical MnS inclusions were not observed in the microstructures. In the MS-type sulphides, the manganese was substituted by magnesium. A systematic ex-situ characterisation of the non-metallic, magnesium-containing inclusions was carried out and the origin of the inclusions was proposed. The inclusions' chemistry and morphology were investigated by light microscopy and field-emission scanning electron microscopy. The non-metallic, magnesium-containing inclusions were classified as complex sulphides, oxides and spinels. Magnesium was also detected as being co-precipitated with other non-metallic inclusions, like nitrides. The form of the co-precipitated magnesium inclusions was predetermined by the shape of the thermodynamically most stable inclusions.