Isothermal Reduction Behavior of Oxide Scale on the Surface of Hot-Rolled Strip Steel Under Different Hydrogen Concentrations

The oxide scale on the surface of hot-rolled low-carbon steel strips is subjected to isothermal reduction in 10 vol%H₂–Ar and 20 vol%H₂–Ar environments to simulate the reduction process that occurs in a continuous annealing furnace. The influence of hydrogen concentration on the reduction kinetics and the microstructural evolution of the oxide scale after reduction at temperatures ranging from 450 to 850 °C for a duration of 20 min are investigated in detail. The mass changes of the oxide scale in the two gases are quantified using a thermogravimetric analyzer. This data is then employed to calculate the reduction rate constant and the apparent activation energy. To examine the microstructure and element distribution, electron probe microanalysis and energy-dispersive spectrometry are employed. An novel approach is also undertaken to assess the reduction degree of the oxide scale by measuring surface microhardness. In the findings, it is indicated that an increase in hydrogen concentration served primarily to accelerate the reduction reaction within the temperature ranges of 450–550 and 800–850 °C. Meanwhile, the mechanism of physical transformation of oxide scale, the microstructure of reduction layer, and hydrogen concentration on reduction efficiency under different reaction stages are proposed.