High Temperature Oxide Scale Morphology and Secondary Cooling Property of Cr Bearing Low Alloy Steel

The high temperature oxidation behavior in air atmosphere and air-mist cooling property of 1 ton ingot are investigated using plain carbon steel (P.C. steel), chromium bearing steel (Cr steel) and chromium and nickel bearing steel (Cr-Ni steel) in order to clarify the effect of surface oxidation on cooling property. The results obtained are summarized as follows. At 1573K, Cr enriched outer scale layer and internal oxide consist of Fe2SiO4 eutectic oxide are formed on Cr steel, whereas characteristic subscale of Ni bearing steel is formed on Cr-Ni bearing steel. These characteristics disappear at lower oxidation temperature. At mist cooling, cooling rate, and hence heat transfer coefficient of Cr steel is significantly reduced compared with P.C. steel and Cr-Ni steel. Characteristics of scale morphology and cooling property due to chromium bearing disappear by Ni addition. Comparing with other cooling experiments, specimen dimension has significant effects on cooling properties. As air-mist cooling intensity is mild in this work, insulation effect of surface scale or subscale layer is insufficient to explain the cooling property. It is considered that the morphology change of cast surface is caused by the formation of chromium oxide layer at the inside of outer scale and hence prevention of FeO or Fe2SiO4 formation which clamp scale layer.