Evolution mechanisms of MgO-Al2O3 in non-oriented silicon steel

The evolution mechanisms of MgO-Al2O3 in the process of calcium treatment in non-oriented silicon steel were studied by laboratory experiments, with focus on the evolution process in the early and middle of calcium treatment. A small amount of MgO-Al2O3 surrounded by AlN was occasionally detected besides many pure spinels before calcium treatment. CaS or CaO formed as a tran-sient inclusion in the MgO-Al2O3 surface 1.5 min after calcium treatment. The composition from the inner part to outer part of inclu-sions was MgO-Al2O3, CaO-MgO-Al2O3 and CaO-Al2O3 in order 4.5 min after calcium treatment. 10 min after calcium treatment, unreacted MgO-Al2O3 spinels disappeared, and simultaneously CaO-MgO-Al2O3 and CaO-Al2O3 inclusions became homogeneous in composition. The phase stability diagrams of Al2O3/MgO-Al2O3/MgO and MgO-Al2O3/xCaO·yAl2O3 were calculated using thermo-dynamic data. The result showed that it was difficult to modify MgO-Al2O3 spinels with very high aluminum content. In combination with the scanning electron microscopy images, energy-dispersive spectra and element mappings of typical inclusions, the evolution route of an inclusion was described as MgO or Al2O3→MgO-Al2O3→CaO-MgO-Al2O3→CaO-Al2O3, and the alumina content occa-sionally showed a sudden increase besides a common decrease in the middle part of the product.