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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. [ABSTRACT FROM AUTHOR]

The oxidation behavior of SUS430 ferritic stainless steel is studied during isothermal oxidation at 1100 °C for 2 h under a mixture of 10, 30, and 50% water vapor and air, and oxidation kinetics curves are drawn. The surface morphology and phase composition of oxide scale are analyzed using X‐Ray diffractometer, scanning electron microscope, and energy dispersive spectrometer. The oxidation kinetic curves under three different humidities are S‐shaped and include three stages: induction period, acceleration period, and deceleration period. The controlling steps under 10% water vapor and air are nucleation and growth, while under 30% and 50% water vapor and air are both phase boundary control reactions. The induction period becomes shorter and the breakaway oxidation occurs earlier with increasing water vapor content. The structure of oxide scale produced is layered, with the outer layer being iron oxide and the inner layer being mainly the Fe–Cr spinel phase. The volatilization of Cr and the growth stress of the Cr2O3 layer cause the formation of nodules, and a large number of cracks are generated in oxide scale. The cracks provide diffusion paths, accelerating the diffusion of Fe ions and electrons, as well as the erosion of the substrate. [ABSTRACT FROM AUTHOR]

The reduction behavior of the oxide scales with two different microstructures in the gas of H2:Ar = 1:2 at 400–625 °C is investigated in detail. The mass change in the scale is measured by thermogravimetric analysis, and a function between reduction degree, reduction temperature, and reaction time is established. The experiment results show that the increase in temperature has an accelerating influence on the reduction reaction of the scale with eutectoid (Fe3O4/α‐Fe) until reaching 550 °C, and then the further increase in temperature decelerates the reduction. For the scale without eutectoid, the reduction rate at 450 °C is extremely high, and then the rate decreases with the increase in temperatures. Additionally, the reduction rate in the scale with eutectoid is faster than that without eutectoid at the temperature range of this study. At 400–550 °C, a large number of phase interfaces appear within the scale with eutectoid, which can provide diffusion channels for hydrogen atoms and water molecules within the Fe oxides. When the temperature reaches 575–625 °C, the phase transformation in the scale (Fe3O4 + α‐Fe → FeO) is accompanied by the reduction reaction, which is believed to be responsible for the lower reaction rate. [ABSTRACT FROM AUTHOR]