Detection and measurement of asymmetric flow in the mould and assessment of its effect on product quality of continuously cast slabs

This report details the work carried out during the collaborative project between Corus UK Ltd (formerly British Steel Limited) Teesside Technology Centre, MEFOS, BFI and IRSID titled The Detection and Measurement of Asymmetric Flow in the Mould and the Assessment of its Effect on Product "Quality of Continuously Cast Slab". The flow of steel in the mould of continuous slab casting machines, which can be extremely complex, has a large impact on final product quality leading to entrainment of argon bubbles, flux powder and slag in the solidifying strand and in the worst case to breakouts. A detailed knowledge of flow conditions throughout the casting sequence will be of importance for further process optimisation and improved cast product cleanness; in particular finding solutions to the problem of preferential and oscillating asymmetric flow in the mould. Much theoretical work has been carried out on the flow patterns in the mould using mathematical and water modelling but there is little reliable experimental evidence to back up this work. This project aimed to develop advanced simulation methods in addition to instrumentation systems to detect and measure the flow of steel in the mould of slab casters using thermocouples and electromagnetic measuring devices. Corus carried out trials with two types of temporary sensor, the melting steel balls and the Karman vortex meter. In trials carried out with the melting steel balls in the pilot plant with a steel flow of known velocity, this method seemed to work reasonably well. Subsequent thermal analysis of melting times and comparison with the results of other workers gave inconsistent results. The initial Corus design of Karman vortex probe was not found to work well in trials carried out in the water model. A Karman vortex probe purchased from Electro-Nite was evaluated in the pilot plant at TTC using molten steel. The calibration of the probe was not found to follow the theory, but agreed with the calibration provided by Electro-Nite. Trials were also carried out in the water model with probes of different diameters and strip thickness. Three successful trials were carried out on the Scunthorpe slab caster. The probes worked well and the trials were confirmed by visual observations. For only two out of the three trials mould temperature data were available. A correlation was found between the velocity data and temperature data for a limited period only and this exhibited a delay. The probes have a relatively short life and considerably more data are needed to calibrate thermocouple based systems. Data were collected by Cows from end plates instrumented with additional thermocouples. These data illustrated large variations in temperature between end plates, as well as complexities in temperature variation and oscillations in flow. The level of these variations was found to increase with increasing casting speed. IRSID developed a new sensor which was used to develop an asymmetric flow index and carried out physical modelling work. The sensor was based on the torque exerted by the hydrodynamic forces on a cylindrical probe immersed in the mould. The probe not only detects the magnitude of the flow, but the direction. This detected change in flow to three different patterns during one sequence.