Effect of Surface Oxidation on Decarburization of a Fe-3%Si Steel during Annealing

The main purpose of decarburization annealing is to reduce the mass fraction of carbon in Fe-3%Si steels to avoid magnetic aging in silicon steels. Decarburization annealing experiments are carried out at different annealing temperatures, PH2O/PH2 and times. The morphological evolution of oxidized layer on steel surface, the changes of carbon and oxygen concentrations in a Fe-3% Si steel, and the influence of the oxidation of alloying elements on the decarburization are analyzed. Results show that decarburization rate of the tested silicon steel is the highest when annealing temperature is 1103 K and PH2O/PH2 is 0.374. When PH2O/PH2 and annealing temperature are lower than that of optimum annealing condition, increasing annealing temperature or PH2O/PH2 result in the increase of the convective mass transfer coefficient between steel and atmosphere, promoting the mass transfer and intensifying decarburization reaction. On the other hand, the amount of oxide particles also increases with further increasing PH2O/PH2 and annealing temperature, resulting in the hindrance of the diffusion of carbon atoms in the oxidized layer and slowdown of decarburization.