Effect of Annealing Atmosphere and Heating Rate on the Internal Oxidized Zone and Forsterite Film During Annealing

The use of Hi-B steel in the manufacture of transformer cores can save raw materials and greatly reduce the volume and energy consumption of the transformer. A film of forsterite on the steel surface provides a certain degree of insulation and forms by the reaction between the internal oxidized zone (IOZ) and the MgO coating during the high-temperature annealing. Therefore, the IOZ morphology is the key to affect the film morphology. This paper studies the effect of annealing atmosphere and heating rate on the morphology evolution of IOZ during the first heating stage of high-temperature annealing. The main factors affecting IOZ morphology are discussed in the context of the influence of its morphology evolution on the film morphology. The results show the change trend of IOZ thickness on the steel with MgO coating is completely different from that without MgO coating. The IOZ thickness in the steel with or without MgO coating jointly affects the forsterite film thickness.