Scaling behaviour of steels in simulated reheat furnace environments

The reheating of steel billets or slabs prior to rolling normally takes place in an oxidizing atinosphere containing O-2, H2O and CO2. At elevated temperatures the steel reacts readily with this atmosphere, leading to scale formation and significant yield losses. In order to decrease the rate of scaling, a decrease in the percentage of combustion air is desirable. The effectiveness of this approach has been tested by measuring scaling rates in a number of atmospheres corresponding to different fuel-air ratios. Oxidation scaling tests where conducted oh a number of commercial steels in laboratory simulated combustion products from natural gas with 112%, 99%, and 95% theoretical air. The results showed that all steels oxidised according to fast parabolic kinetics in the 112% hyperstoichiometric environment. In the 99% and 95% sub-stoichiometric environments, however, there was a sharp drop in scaling rate, and all steels oxidised according to linear kinetics. Under these conditions, the reactions were found to be controlled by surface processes. Substantial reductions in scale losses are shown to be available. The extent of oxidation under anisothermal conditions is shown to be predictable from isothermal kinetic data.