Emissivity and oxidation evolution in reheating furnace environments

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Reheating furnaces are an essential part of Tata Steel Europe's hot mills. Reheating furnaces are heated by gas burners and aim to heat the steel slabs to their rolling temperature with a known temperature profile through the thickness. Reheating furnaces rely predominantly on heat being radiatively transferred to the slabs. Since steel is prone to oxidation, the slabs grow an oxide layer on their surface. This has two major effects for the heat transfer from the furnace to the slab: It changes the emissivity of the surface, and therefore the power the slab absorbs from the furnace. It introduces an insulating layer on the surface of the slab, which slows thermal diffusion into the slab. The oxide layer thickness can be approximately calculated for simple steels, however the evolution of emissivity with time and temperature as well as its dependency on the oxide thickness or topology of the oxide surface is unknown. The emissivity of oxide scales is known to be affected by a number of parameters, including the composition of the atmosphere, steel grade, temperature, steel surface condition and time. Some of these parameters are within our control, for example the composition of the atmosphere in the reheating furnace, the temperature cycle or the steel composition. In this project, we would like to understand how temperature cycle and eventually furnace atmosphere affects the surface condition of the oxide scale, and therefore the emissivity. The first industrial aim is to be able to introduce accurate emissivity values in the furnace model as a function of time and temperature, the second aim is to extrapolate this prediction to different steel compositions. An additional aim, if time permits, is to understand how to control the emissivity to maximise it and improve furnace efficiency. The objectives of the project include: Develop and validate an experimental setup that can measure the emissivity of oxide scales as a function of temperature and time. Measure emissivity as a function of temperature and evolution in time for HSLA and low carbon grades heated in an environment representative of a reheating furnace atmosphere. Relate emissivity changes to changes in the oxide layer, e.g. morphological, structural, or compositional changes. Formulate the relation emissivity, time, temperature, and steel composition to be used in the industrial model. Suggest, if time permits, the optimised furnace parameters, or slab surface modifications to achieve maximum emissivity on the slabs.