High-temperature Oxidation and Subscale Morphology of Si Containing Steels

The oxidation behavior of Si-containing steels at high temperature has been the subject of numerous investigations. In many cases, the addition of Si to the steel in sufficient amounts to form an external film of SiO ₂ or other Si-rich oxide on the steel surface has resulted in extremely slow oxidation rates. However, the steel in which Si content is insufficient forms an external scale and an internal scale on the steel surface and a subscale in the steel interior which are less protective. In order to suppress the growth of the scales on the hot slab which causes surface bruise of hot rolled Si-containing steel sheets, many researchers paid attention to the de-scaling characteristics of Si-containing steels. However, the effects of Si content on the oxidation kinetics, scale composition and scale structure, especially three-dimensional scale morphology formed on the Si-containing steels at high temperatures are not necessarily understood adequately. In this study, the high-temperature oxidation behavior and scale morphology of (0, 0.5, 1.5, 3.0mass%) Si-containing steels exposed to the flowing air and the LNG combustion gas atmosphere at 1373K or 1473K were investigated. The external scale and the internal scale formed on Si-containing steels were composed of the laminated α-Fe ₂O ₃ and Fe ₃O ₄ and the mixture of FeO and Fe ₂SiO ₄, respectively. The subscale may be composed of vitreous SiO ₂. The kinetics of the external and internal scale formation were followed by parabolic rate laws. The SEM images of the three-dimensional subscale extracted from the oxidized steels by means of an organic solvent system dissolution technique showed that an intergranular oxide formed thin walls along not only grain boundaries but also twin boundaries of the steel matrix and an intragranular oxide formed isolated particles in the steel grains near the internal scale.