Abstract:
The metal coating line at New Zealand Steel relies on a large electric radiant furnace to heat steel strip before hot-dip galvanising in a continuous process. The temperature evolution of the strip inside the furnace is vital in ensuring the speci ed mechanical properties are achieved for a range of steel products. Ductile products require high temperatures su cient to cause recrystallisation of the steel microstructure, while stronger products must be heated without causing recrystallisation. Strip dimensions and desired properties are changed often and irregularly during operation, and these changes and associated furnace control actions cause changes in furnace and strip temperatures and rate of heat transfer over several di erent time scales. Accurate control of temperature is di cult because temperature measurement devices are strongly a ected by re ected radiation in the furnace cavity. The furnace is often operating during transient temperature conditions, as control actions take e ect very slowly compared to the the rate of change of operational targets. Understanding of the transient behaviour of this system of interrelated, nonlinear variables can be improved using modelling to calculate furnace and strip temperatures as a result of control actions in real time, which cannot otherwise be measured or predicted. It is shown that a three-dimensional model is capable of accurately calculating furnace temperatures changing over both time and location, requiring minimal simpli cation of the physical system, but is computationally expensive. Radiative heat exchange in the furnace cavity causes signi cantly increased temperature along the edges of the steel strip, which can cause reject product due to localised softening. It was found that furnace thermocouples are strongly a ected by re ected radiation, so that furnace wall temperatures be may signi cantly hotter than measured. A simpli ed, coupled temperature-metallurgical model was shown to accurately calcui late both furnace and strip temperatures and metallurgical changes, while the 3D model provides understanding of e ects not explicitly modelled in the simpli ed model. The simpli ed model is used for optimisation of furnace operational parameters, to improve plant throughput and energy e ciency while maintaining desired metallurgical properties, which is demonstrated by application to common products at NZ Steel.