CHARACTERISATION OF OXIDES ON HIGH-SI STEEL BY EBSD

Although high temperature oxidation of steel has been widely studied, the oxide's microstructure has not yet been fully described. Characterising the oxide scale is of considerable significance because of its impact on the surface quality of the final product. Characterisation of the scale layers and scale/steel interfaces in terms of phase morphology, texture, grain structure and chemical composition is fundamental for a better understanding of the oxide growth, the influence of the steel substrate and the subsequent material's response to further processing. Silicon has a beneficial effect on the oxidation resistance of steel at temperatures up to 1170 degrees C. The repartition of silicon between scale and substrate during oxidation generates specific surface properties inherited from the corresponding oxidation characteristics. The growth of a Fe2SiO4-layer at the metal-scale interface decreases the overall oxidation rate. On the other hand, at temperatures above 1170 degrees C a liquid eutectic mixture of wustite (FeO) and fayalite (Fe2SiO4) is formed at the innermost layer between the scale and the steel surface, provoking catastrophic oxidation.A thorough characterization of the microstructure of iron oxides is tedious because of the difficulty in imaging with conventional optical or electron microscopy, and further increasing the Si-content complicates the study even more. Recently, electron backscattered diffraction (EBSD) has proved to be a powerful method to investigate different phases in oxide scales, providing valuable information on microstructure and orientation relationship between steel and scale layers. Results of EBSD measurements of oxide scales formed on Si steel are reported. Conventional oxidation experiments on a 2 m.-% Si steel in an open air resistance furnace carried out below and above the eutectic temperature of 1170 degrees C show the influence of Si on steel oxidation. Oxide scales with complex chemical compositions, such as fayalite, can be clearly distinguished in terms of image quality (IQ) parameter.Although high temperature oxidation of steel has been widely studied, the oxide's microstructure has not yet been fully described. Characterising the oxide scale is of considerable significance because of its impact on the surface quality of the final product. Characterisation of the scale layers and scale/steel interfaces in terms of phase morphology, texture, grain structure and chemical composition is fundamental for a better understanding of the oxide growth, the influence of the steel substrate and the subsequent material's response to further processing. Silicon has a beneficial effect on the oxidation resistance of steel at temperatures up to 1170 degrees C. The repartition of silicon between scale and substrate during oxidation generates specific surface properties inherited from the corresponding oxidation characteristics. The growth of a Fe2SiO4-layer at the metal-scale interface decreases the overall oxidation rate. On the other hand, at temperatures above 1170 degrees C a liquid eutectic mixture of wustite (FeO) and fayalite (Fe2SiO4) is formed at the innermost layer between the scale and the steel surface, provoking catastrophic oxidation.A thorough characterization of the microstructure of iron oxides is tedious because of the difficulty in imaging with conventional optical or electron microscopy, and further increasing the Si-content complicates the study even more. Recently, electron backscattered diffraction (EBSD) has proved to be a powerful method to investigate different phases in oxide scales, providing valuable information on microstructure and orientation relationship between steel and scale layers. Results of EBSD measurements of oxide scales formed on Si steel are reported. Conventional oxidation experiments on a 2 m.-% Si steel in an open air resistance furnace carried out below and above the eutectic temperature of 1170 degrees C show the influence of Si on steel oxidation. Oxide scales with complex chemical compositions, such as fayalite, can be clearly distinguished in terms of image quality (IQ) parameter.P