Microstructure evolution of forsterite film during the high-temperature annealing process in grain-oriented silicon steel

Abstract

Forsterite films play an important role in enhancing the adhesion of the insulation layer and the magnetic properties of grain-oriented silicon steel. To clarify the microstructure evolution of forsterite film, an experiment involving high-temperature annealing at different temperatures ranging from 850 °C to 1150 °C was performed. Additionally, the reaction processes between MgO and the oxides formed in the decarburization oxide layer, such as SiO₂ and Fe₂SiO₄, were investigated. The initial formation temperature of the forsterite film is found to be within the range of 900 °C – 950 °C. The composition analysis reveals a decrease in the magnesium content from the surface toward the interior of the oxide layer. Up to 1100 °C, there is no longer a transition region between MgO and Fe₂SiO₄ in the uppermost oxide layer, showing that the surface of oxide layer is composed entirely of forsterite. Simultaneously, with the diffusion of Mg²⁺, SiO₂ in the subsurface region reacts with Mg²⁺ and is thus transformed into forsterite. The enrichment of aluminum, silicon, and oxygen in the bottom part of the oxide layer at 1100 °C implies the potential formation of mullite. These findings provide valuable insights to tune and control the forsterite film in grain-oriented silicon steel.