Oxidation of low carbon steel in multicomponent gases: Part II. Reaction mechanisms during reheating
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I.
INTRODUCTION
REHEAT furnaces in the steel industry are used to reheat steel stocks prior to hot rolling. The furnace under investigation was a four-zone, direct-fired, walking-beam steel reheat furnace[1] (Figure 1). The furnace can use either natural gas, coke oven gas, synthetic gas (a mixture of methane, air, and coke oven gas), blast furnace gas, or fuel oil. The steel is heated by radiation from furnace walls and roof and by convective heat transfer from the hot combustion products. Slabs, normally at room temperature, are charged and heated continuously as they are moved along the furnace to an exit temperature of 1230 7C to 1250 7C. During this reheating period, the steel surface reacts with the gaseous products of combustion, CO2, H2O, and free oxygen (resulting from excess air), to form a layer of oxides (scale). From economic considerations, it is desirable to minimize scale formation so as to maximize steel yield. However, some degree of scaling is required in order to remove surface defects and oscillation marks produced in the continuous caster. In Part I of this series of articles[2] isothermal and nonisothermal laboratory experiments were carried out for oxidation of low carbon steel in various gases comprising O2, CO2, H2O and N2. It was found that the two most important parameters affecting oxidation rates were temperature and concentration of free oxygen in the oxidizing atmosphere. The work showed that CO2 and H2O at concentration levels H.T. ABULUWEFA, Postdoctoral Fellow, and R.I.L. GUTHRIE, Macdonald Professor, FRSC, and Director, MMPC, are with the McGill Metals Processing Centre, Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H2A 2A7. F. AJERSCH, Professor, is with the Department of Metallurgy and Materials Engineering, Ecole Polytechnique, 2900 Edouard-Montpetit, Montreal, Quebec, Canada H3C 3A7. Manuscript submitted April 4, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
found in the reheat furnace contributed little to the magnitude of oxidation rates. During the initial oxidation periods, rates were found to be most effected by mass transport of oxygen from the gas mixture to the reaction surface, through a gaseous boundary layer at the metal surface.[3] Subsequent rates of oxidation followed a parabolic rate law, where the diffusion of metal ions through the scale layer became rate controlling. In other work, Minaew et al.[4] investigated the effect of combustion products from natural gas, coke oven gas, and blast furnace gas, burned with air at 0.7 to 1.1 of stoichiometric requirements, on the oxidation rates of various carbon steels in the temperature range 800 to 1250 7C. They concluded that maximum scaling occurred in the combustion products from coke oven gas when burned with 1.1 of stoichiometric air. Such combustion products contained the highest concentrations of H2O. Little work has been reported on the oxidation of steels during heating cycles.[5,6] Hemsath and Vereech[5] studied the economics of minimum scale reheating by c
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