Effects of Si and its content on the scale formation on hot-rolled steel strips

The effect of silicon content (0.01–1.91 wt.%) on the oxidation kinetics and the structure of scales formed on silicon steels was investigated. A Thermal Gravimetric Analyzer (TGA) was used to measure the weight changes of various steels oxidized in dry air in the temperature range of 700–1000 °C. The results showed that both oxidation rate and scale thickness decreased with increasing silicon content in the steels. The scales that were formed on plain carbon steel and contained 0.51 wt.% Si mainly consisted of Fe2O3, Fe3O4, and Fe1−XO. A scale with banded oxide structure was also observed in the steel containing 0.51 wt.% Si. For the steel containing 1.14 wt.% Si, a scale with two distinct layers of oxides was found. The outer layer was Fe2O3 while the inner layer of the oxide contained a high amount of Si. The scale of the steel containing 1.91 wt.% Si was much thinner. Beneath the outer Fe2O3 layer, an oxide enriched with Si and Al was found at the scale/substrate interface, which improved the passivity of the steel. Nodular scale formation resulting from local breakdown of the oxide was found on some discrete surface spots, particularly for the steel containing 1.91 wt.% Si. Internal oxidation was also observed in the high Si-containing steels.