High-temperature Oxidation Behavior and Scale Morphology of Si-containing Steels

The Fe–Si alloy system is very important in steel production because silicon is often added to high-strength steel, heat-resistance steel, and other steel types. However, Si-containing steels form un-removable scales on the surface at high temperatures, and the scales allow unfavorable surface bruises in hot-rolled steels. Although many studies have been conducted on the oxidation of Si-containing steel at high temperatures, the oxidation behavior and scale morphology have not been clarified adequately. In this study, the high-temperature oxidation behavior and scale morphology of Si-containing steels (0, 0.5, 1.5, and 3.0 mass%) exposed to the air and an LNG combustion gas atmosphere at 1 373 K and 1 473 K were investigated. The external scale and internal scale formed on Si-containing steels were composed of laminated α-Fe2O3 and Fe3O4 and a mixture of FeO and Fe2SiO4, respectively. The subscale may have been composed of vitreous SiO2. The kinetics of the external and internal scale growth followed 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 the grain boundaries but also the twin boundaries of the steel matrix, and isolated intragranular oxide particles were formed in the steel grains near the internal scale.