Effect of carbon on isothermal reduction of high-strength steel oxide scale in 30%H2–N2 atmosphere

Abstract

To replace the traditional pickling process, an environmentally friendly gaseous reduction method was used for the removal of the oxide scale of steel in this work. Isothermal hydrogen reduction of high-strength steel was investigated in the temperature range from 500 °C to 800 °C in 30%H2–N2 atmosphere. The obtained results revealed a clear relationship between the oxide phase composition and the holding temperature during the heating stage, prior to hydrogen reduction. With increasing temperatures, the Fe2O3 and Fe3O4 components of the oxide scale gradually transform into FeO. When the sample was reduced under H2–N2 atmosphere, the surface morphology of the high-strength steel was different from that of the traditional hot-rolled steel strip. It was observed that the surface pore size and porosity increased with increasing temperatures and internal and external oxidation occurred simultaneously in the reduction layer. The addition of carbon to the reduction system decreased the water pressure and clearly promoted the reduction at 800 °C. The surface of the sample was reduced to irregular but uniformly distributed small particles of pure iron. Defects in the oxide scale such as cracks and gaps and carbon reacting with water play a vital role in the reduction process. Since the use of hydrogen reduction of hot-rolled strip eliminates the need for the acid pickling pollution process and the pre-oxidation process in the annealing of cold-rolled strips, it can significantly reduce the energy consumption of steel production and improve the production line efficiency. Thus, this approach is a promising green production technology in hot-dip galvanizing.