Quantitative Analysis on Internal Oxide Precipitates in Fe–1 mass%Si Alloy Formed at 1473K in Ar–17.6%H2–12.2%H2O

Abstract

Surface defects formed on Si-containing steels in hot-rolling process depend largely on the character of the scale composed of FeO and Fe2SiO4 (FeO+Fe2SiO4 oxide scale) during de-scaling. The formation of FeO+Fe2SiO4 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%H2–12.2%H2O 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. SiO2 precipitated from the internal oxidation front to 0.23±0.02 of normalized thickness of IOZ, and Fe2SiO4 precipitated from the position to the gas/alloy interface, depending on the chemical potential distribution of oxygen. Fe2SiO4 is formed by the reaction between SiO2, 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.