Influence of Selective Surface Oxidation of Si and Mn on Fe-Zn Alloying Reaction on Hot-rolled Steel Sheets

Abstract

The Fe-Zn alloying reaction and the selective oxidation behavior of 0.7 mass% Si - 1.15 mass% Mn added hot-rolled steel annealed at 600-800°C were investigated by comparison with those of cold-rolled steel. The Fe-Zn reactivity of the hot-rolled steel improved from 600°C to 700°C, while, it deteriorated from 700°C to 800°C. Above 700°C, the amount of Fe-Si-Mn oxide on the steel surface increased with increasing temperature and deteriorated Fe-Zn reactivity. Below 700°C, a thin layer of Fe oxide which existed on the steel surface deteriorated Fe-Zn reactivity. This thin oxide layer was reduced by Si and Mn diffused from the steel substrate. As the temperature increased from 600°C to 700°C, Fe-Zn reactivity improved due to the formation of reduced iron on the steel surface. In the case of the cold-rolled steel, the same selective oxidation behavior and reduction mechanism of the Fe oxide were also confirmed. As a result, the Fe-Zn reactivity of the cold-rolled steel showed the same behavior as that of the hot-rolled steel. However, the Fe-Zn reactivity of the cold-rolled steel improved at a lower temperature than that of the hot-rolled steel. This can be explained by the faster diffusion rates of Si and Mn in the cold-rolled steel than in the hot-rolled steel. At the surface of the cold-rolled steel, reduction of the Fe oxide layer was promoted and the Fe-Zn reactivity of the cold-rolled steel was improved at a lower temperature.