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 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 but 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 this oxide deteriorated Fe–Zn reactivity. Below 700°C, a thin layer of Fe oxide on the steel surface deteriorated Fe–Zn reactivity. This oxide layer was reduced by Si and Mn that diffused from the steel substrate. Therefore, 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, and as a result, the Fe–Zn reactivity of the cold-rolled steel showed behavior similar to 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. That is, reduction of the surface Fe oxide layer by diffused Si and Mn proceeded at a lower temperature, and as a result, the Fe–Zn reactivity of the cold-rolled steel also improved at a lower temperature.