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

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 degrees C to 1150 degrees 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 degrees C - 950 degrees C. The composition analysis reveals a decrease in the magnesium content from the surface toward the interior of the oxide layer. Up to 1100 degrees 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 degrees C implies the potential formation of mullite. These findings provide valuable insights to tune and control the forsterite film in grain-oriented silicon steel.