Mixed Rate Law Research Articles

Discussion of Theoretical Methods to Predict Scale Thickness in Hot Rolling of Steel Strips

Oxidation of iron during hot rolling is inevitable and scale layers are formed on the strip. There are different rate laws of oxidation of iron to predict the scale thickness, in which parabolic rate law and mixed rate law are usually used in hot rolling of strips. As the strip passes through the roll gap, the scale layer can plastically be deformed. Due to limited plasticity of the oxide, cracks through the scale layer can be formed in every roll gap. They open and form new free metallic surfaces which are then oxidized in the interstand. By means of numerical simulations different rate laws of oxidation are compared and the scale deformation with different cases is taken into account. Sensitivity of some rolling parameters to prediction of the scale thickness is discussed in the present paper. Numerical results show that the parabolic rate law overestimates the scale thickness as compared with the mixed rate law and the mixed rate law is not sensitive to the deformation extent of scale.

The scaling of a Fe-20Cr alloy in H2-H2O-H2S mixtures

The scaling of an Fe-20Cr alloy has been studied in H2-H2O-H2S mixtures between 973 and 1223 K. According to a simplified phase diagram, Cr2O3 and FeS should be the thermodynamically stable compounds in the gas mixtures chosen. The reaction followed a mixed rate law between linear and parabolic, indicating that the reaction rate at the scale-gas interface was comparable with the diffusion rate in the scale. At a constant H2/H2S ratio the scaling rate initially decreased slightly with increasing water-vapor pressure. A further increase of the water-vapor pressure resulted in localized sulfide formation, while the other parts of the surface were covered with a Cr2O3 film. Only Cr2O3 formed above a critical water-vapor content. Three zones could be distinguished when a sulfide scale is formed. The outer zone consisted of practically pure FeS; the intermediate zone was a solid solution of (Fe,Cr)S, partially decomposed to FeCr2S4 and metal during cooling; and the inner zone contained small Cr2O3 inclusions in an (Fe,Cr)S matrix.