Abstract: This paper investigated the effect of the calcium oxide (CaO) content in magnesium oxide (MgO) coatings on the quality of the forsterite substrate formed on grain⁃oriented silicon steel. Using scanning electron microscopy (SEM), the study examined the impact of different CaO concentrations on both the substrate morphology and the surface condition of the finished steel strip. The results demonstrated that the CaO content in MgO was a critical factor for the final substrate quality. Specifically, as the CaO mass fraction was increased from 0.2 % to 0.5 %, the hydration rate of the MgO layer post⁃drying rose from 1.01 % to 1.92 %. The increase in CaO corresponded with a coarsening of MgO particles and poorer uniformity in the substrate layer, which macroscopically manifested as surface oxidation on the steel strip. The mechanism suggests that higher CaO content introduces more water into the MgO slurry. Consequently, during the high⁃temperature annealing phase in a bell furnace, steam trapped at the coil edges as it escapes between layers leads to additional oxidation along the strip edges, causing the observed defects.

Key words: magnesium oxide, calcium content, grain?oriented silicon steel, forsterite substrat