Surface defects formed on Si-containing steels in hot-rolling process depend largely on the character of the scale composed of FeO and Fe
2SiO
4 (FeO+Fe
2SiO
4 oxide scale) during de-scaling. The formation of FeO+Fe
2SiO
4 oxide scale is influence by internal oxide precipitates. In order to elucidate quantitatively the internal oxidation, it is required to analyze the distributions of number, radius and volume of internal oxide precipitates in internal oxidation zone (IOZ).
Internal oxidation of Fe–1 mass%Si alloy was examined at 1473K in Ar–17.6%H
2–12.2%H
2O where an outer scale of FeO does not form. The growth rate of IOZ was obeyed the parabolic rate law, indicating that the rate-determining step was the diffusion of oxygen through alloy. SiO
2 precipitated from the internal oxidation front to 0.23±0.02 of normalized thickness of IOZ, and Fe
2SiO
4 precipitated from the position to the gas/alloy interface, depending on the chemical potential distribution of oxygen. Fe
2SiO
4 is formed by the reaction between SiO
2, Fe and dissolved oxygen in IOZ. The number of precipitates in unit volume is reciprocally proportional to depth in IOZ, suggesting that the grain growth of oxide obviously affected the distribution. The radius of the precipitates was proportional to the cubic root of depth so that the volume of the precipitate was proportional to depth. These results elucidated that the volume fraction of precipitates in IOZ was constant.
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