Abstract
An oxide scale layer always forms at the strip surface during the hot rolling process. Its properties have a large impact on surface quality. The most important features of the oxide layer are its thickness, composition, structure, adherence and coherence. Temperature, time and gas atmosphere determine the growth of oxide layers. In this paper, the high temperature oxidation properties of ultra low carbon steels are discussed in terms of oxide growth mechanism, kinetics and phase morphology. The oxidation kinetics of ultra-low carbon steel (ULC) in air, its scale structure and composition were investigated over the temperature range 923-1473K. Oxidation experiments were performed either under controlled atmosphere or in air, to analyse the oxidation process during strip production. A first series of experiments was carried out in an electric furnace at temperatures ranging from 923 to 1473K, for times between 16 and 7200s. A second series was carried out in a device especially designed to control the atmosphere. After heating under pure nitrogen, the samples were oxidised in air at temperatures between 923-1323K for various oxidation times. Thus treated specimens were characterised by metallography and their scale thickness was measured under the optical microscope. Scale morphology was studied and scale composition confirmed by EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscattered Diffraction) analysis. Results show that scale growth under controlled atmosphere is significantly faster than under non controlled conditions, additionally the adherence of the scale formed in the laboratory device was significantly better than the other one. It is clear that scale thickness and constitution depend strongly on the oxidation potential of atmosphere. Computed parabolic activation energies (Ea) values are in good agreement with those found in the literature.
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