Insight into reduction mechanism of the oxide scale formed on hot-rolled advanced high-strength steel in 5%H2–N2 atmosphere

Hydrogen reduction of oxide scale, as an environmentally friendly technology, is required to replace the conventional acid pickling process and provides a promising green production route for hot-dip galvanizing. In order to understand the hydrogen reduction mechanism of oxide scale in advanced high strength steel, the cross-section morphology, Mn distribution, and decarburization have been studied with the reduction time of 1~10 minutes in a 5%H2–N2 atmosphere at 900°C. Results show a clear relationship between the surface iron layer and the wavy reduction at the interface. The thickness of the surface iron layer remains almost unchanged throughout the whole reduction process, whereas the reduction at the oxide scale/steel interface plays a dominant role. Because the surface reduced iron is thin enough, oxygen can continue to diffuse away from the reduced iron/(Fe,Mn)1-xO interface, which provides a driving force for inward iron diffusion in the opposite direction of the oxygen potential gradient, crossing through the oxide scale and depositing at the interface. Three points are proposed to explain the reasons for the rapid growth of reduced iron at the interface, dissolved carbon and internal oxidation consuming oxygen released from (Fe,Mn)1-xO decomposition, carbon reacting with water vapor, and microcracks acting as efficient rapid channels of gaseous products. Meanwhile, Mn moves upward along with (Fe,Mn)1-xO, due to the faster growth of interface reduced iron, which reveals Mn migration from the perspective of reduction evolution.