Corus Research, Development &Technology, ljmuiden I The NetherlandsSurface quality and downstream processability of hot rolled strip depend on tertiary scale thickness and composition at all process stages, from secondary descaling to the cold coil. TGA oxidation experiments provided the basis for quantitative modelling of scale growth during finishing hot rolling of low carbon steels (with or without Si or Pl. Finishing temperature (FT), gauge, and (particularly at the edges) coiling temperature (CT) are the main influence factors with respect to scale thickness. Based on HTXRD oxidation trials and microscopic analyses, trendlines are presented that roughly predict the final scale composition as a function of CT, for the coil centre and the edges. It appears that a CT of 400-600°C, in combination with a coil centre position, is favourable for decomposed wustite, which is the preferred component for a good scale ad-and cohesion. Retention of wustite, the most preferred phase in the roll bite but very brittle at room temperature, is promoted by relatively high CTs (particularly with a coil quench) and by very low CTs. Hematite is formed at the edges in case of a high CT. This extremely hard phase is unwanted at any stage.

The oxygen bearing gases of the atmosphere in a reheating furnace oxidise the feedstock producing scale on the surface of the metal and, for high carbon steels, cause decarburisation of the surface layers. Modelling of these effects has to take into account the competitive nature of the two processes, scale formation and decarburisation, and complications that arise from changes in the controlling mechanism. Initially, the rate of scale formation may be controlled by effects in the gas phase, or nucleation of scale on the metal surface. Subsequently, scale growth is influenced by the gas composition and may be disturbed by the accumulation of oxidation products at the scalemetal interface. Voids and cracks within the scale may either inhibit solid state diffusion within the scale or provide channels for oxidising gas to access the scale-metal interface. For decarburisation, complications arise if a ferrite rim creates a step in the carbon profile or if retention of carbon monoxide within the scale provides a thermodynamic barrier to the reaction. This paper considers these complications and how they may be handled in mathematical models.

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