Characterization of Full-size Inclusions in High Grade Non-oriented Silicon Steel

The morphology and quantity of full-size inclusions in 3.0% Si−0.85% Al−0.27% Mn high grade non-oriented silicon steel billet and finished plate were detected and analyzed with bulk electrolysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The sources of main inclusions were analyzed based on the types of inclusions. The results show that the content of large inclusions in the casting billet is 2.18 mg/10 kg, and the large inclusions causing linear defects on the surface of non-oriented silicon steel finished plates come from composite inclusions of protective slag, top slag and refractory materials. The micro inclusions of 0.50–5.00 μm in the finished plate mainly include composite inclusions such as Al₂O₃, mCaO•nAl₂O₃, mAl₂O₃•nSiO₂, mMgO•nAl₂O₃, AlN, and sulfides, with diverse morphology distribution. The average size of fine inclusions of 0.1–1.0 μm is 0.295 μm, with a distribution density of 4.95×10⁴ particles/mm², and with size mainly distributed in the range of 0.10–0.30 μm. The precipitates smaller than 500 nm are mainly spherical or nearly spherical MnS and Cu₂S, rectangular AlN and TiN, and some small oxides with complex morphologies, with size mainly distributed at 50–200 nm. The density of precipitates with size of 50–100 nm is 5.23×10⁵ particles/mm². The fine oxides generated during the temperature drop and secondary oxidation processes of molten steel, Cu₂S precipitated during subsequent heat treatment, and AlN, TiN, and MnS not precipitated during the homogenization process all precipitate as fine inclusions, which will pin grain boundaries and seriously affect grain growth, thereby deteriorating the magnetic properties of non-oriented silicon steel.