Effect Of Morphology Of Inner Oxide Layer On Formation Mechanism Of Forsterite Film Of Grain-oriented Silicon Steel During High Temperature Annealing

Due to its extremely strong Goss texture,Grain-oriented silicon steel has high magnetic properties when magnetizing along the rolling direction,and its core loss is lower compared to ordinary low-carbon steel,so it is often used as the core of transformers and motors.During the high temperature annealing stage,silicon steel is placed in coils in a bell furnace for annealing,which results in uneven heating of various parts of the silicon steel,and the content of crystalline water released from the Mg O coating is also different.As a result,the composition of the annealing atmosphere between the steel coils may differ,leading to the appearance of"exposed crystals"in the final bottom layer.In this paper,the influence of oxide layers with different morphologies on the formation of forsterite film was studied by changing the annealing atmosphere during the primary heating stage of high-temperature annealing.By refining the annealing temperature nodes and using various testing methods,the samples under the annealing conditions for producing high-quality forsterite film were thoroughly studied,and a process model for the formation of forsterite film on the surface of Grain-oriented silicon steel was established,with a view to providing guidance for industrial production.In this study,scanning electron microscopy,glow discharge spectroscopy,electron microprobe,and XPS were used to detect and study the annealed samples.The annealed samples were placed in an atmosphere with different hydrogen content and dew point temperature,and the morphology and phase distribution of the oxide layer and forsterite film on the surface of the samples annealed to different temperatures were analyzed.The main conclusions were as follows:（1）When the water hydrogen partial pressure ratio of the annealing atmosphere remains unchanged,with the gradual increase of the dew point temperature,the oxygen partial pressure in the annealing furnace increases,the oxide layer gradually thickens,and the adhesion property of the forsterite film increases,without falling off.Under low oxygen partial pressure,the forsterite film is complete and continuous,and magnesium aluminum spinel particles are formed below it;At higher oxygen partial pressures,relatively thin Fe O is formed in the oxide layer,and the content of iron particles inclusions in the forsterite film gradually increases,resulting in the formation of a discontinuous forsterite film and the presence of unreacted Si O₂ below some of the forsterite film.When the dew point temperature of the annealing atmosphere remains unchanged,with the increase of hydrogen content,the oxygen partial pressure in the annealing furnace decreases,and the thickness of the oxide layer on the surface of oriented silicon steel decreases.The forsterite film is discontinuous,and the forsterite film is relatively loose and easy to fall off locally.（2）As the annealing temperature increases,the thickness of the oxide layer begins to increase slightly,but due to the occurrence of solid state reactions,the thickness of the oxide layer begins to decrease at the later stage of annealing.The spherical Si O₂ in the oxide layer undergoes aging and decreases in number,while the deeper banded Si O₂ is locally thinned,fractured,and precipitated.The outermost side of the oxide layer forms a layer of（Fe,Mg）₂Si O₄.With the diffusion of Mg²⁺,Fe²⁺is gradually replaced,and the Mg₂Si O₄ gradually thickens and combines with the iron matrix in a"mosaic"manner.Due to the differences in the thermal expansion coefficients of the three phases of magnesium silicate,iron matrix,and magnesium aluminum spinel,when Al does not diffuse to the bonding interface between the magnesium silicate substrate and the matrix,but remains below the substrate and reacts with Mg₂Si O₄ in the matrix to generate magnesium aluminum spinel"pinned"particles,the substrate has good adhesion properties.