Papers by Author: X. Vanden Eynde

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.

Abstract: Steel strip surface oxidation during hot mill processing represents an industrial and environmental problem: secondary oxide is removed after roughing, but tertiary oxide scales already start to form before entering the finishing stands. Their properties affect the final steel surface quality and its response to further processing. Controlling the oxide layer growth kinetics and mechanical properties can make pickling easier and improve downstream behaviour. A thin wustite-dominated scale layer (<20 μm) is created under controlled conditions in an original laboratory device adequately positioned in a compression test machine to investigate plane strain compression. A first series of oxidation tests were performed on a ULC steel grade to measure the kinetics of oxide scale growth. The samples were first heated up under a protective atmosphere (nitrogen), before being oxidised in air at different temperatures for various oxidation times. These experiments can be considered fair quantitative and qualitative simulations of scale growth as it occurs in a hot strip mill, insofar as the results thus obtained are in good agreement with the literature. After the oxide growth, plane strain compression (PSC) was performed immediately to simulate the hot rolling process. The oxide layers were characterised before and after compression tests by optical and secondary electron microscopy. As expected, the oxide is seen to deform during compression. The obtained oxide layers exhibit good adhesion to the substrate and homogeneity over the thickness, even after compression.