Analysis of Inclusions in the Entire Smelting Process of High-Grade Rare Earth Non-Oriented Silicon Steel

Rare earth can modify inclusions in non-oriented silicon steel which is harmful to magnetic properties. This study focused on the 3.1% Si non-oriented silicon steel under industrial production conditions. Samples were taken during the stages before and after addition of rare earth ferrosilicon alloy in Ruhrstahl-Heraeus (RH) unit, different pouring time in tundish, and continuous casting slab. This study systematically examined the morphology, composition, and size distribution of inclusions throughout the smelting process of non-oriented silicon steel by scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS), and thermodynamic analysis at liquid steel temperature and thermodynamic analysis of equilibrium solidification. The research results demonstrated that the rare earth treatment ultimately modifies the original Al2O3 inclusions in the non-oriented silicon steel into REAlO3 and RE2O2S inclusions, while also aggregating AlN inclusions to form composite inclusions. After rare earth modification, the average size of the inclusions decreases. In the RH treatment process, the inclusions before the addition of rare earth ferrosilicon alloy are mainly AlN and Al2O3. After the addition of rare earth ferrosilicon alloy, the inclusions are mainly RES and REAlO3. In the tundish and continuous casting, the rare earth content decreased, and the rare earth inclusions transform into RE2O2S and REAlO3. For the size of inclusions, after adding rare earth ferrosilicon alloy, the average size of inclusions rapidly decreased from 16.15 μm to 2.65 μm and reach its minimum size 2.16 μm at the end of RH treatment. When the molten steel entered the tundish, the average size of inclusions increased slightly and gradually decreased with the progress of pouring. The average size of inclusions in the slab is 5.79 μm. Phase stability diagram calculation indicates the most stable rare earth inclusion is Ce2O2S in molten steel. Thermodynamic calculations indicated that Al2O3, Ce2O2S, Ce2S3, AlN, and MnS precipitate sequentially during the equilibrium solidification process of molten steel.