CD-Laboratory for Continuous Solidification Processes

The main work done in this laboratory concerns problems in continuous casting of steel, and the mathematical modelling of different processes. In 1997, one part of this laboratory (head: Prof. Dr. W. Schneider, TU Vienna) was established at our institute after the appointment of Prof. Dr. U. Schaflinger (vice head of the laboratory) at TU Graz. The work in the two projects of this part was carried out in close cooperation with industrial partners. A) Clogging and impurities in tundishes: During continuous casting processes, impurities like deoxidation products in the melt are deposited on the walls of the tundish nozzle and the stopper. This phenomenon, termed clogging, causes obstruction of the flow and decreases the quality of the product. A detailed knowledge of the flow pattern within the tundish nozzle-stopper region is very important for the design of appropriate nozzle and stopper geometries. By means of a numerical simulation, the important influence of turbulence on the velocity field of the liquid metal in the nozzle region could be demonstrated. Most particles close to the walls are deposited when there is high turbulence. A new design for the nozzle-stopper region reduces turbulence, prevents recirculation, and permits a weaker lateral particle transport. A further mechanism may cause inherent problems through the formation of sediment layers at the nozzle walls: the suction of ambient air through the porous material of the submerged entry-nozzle. A specific test facility was built up to study the correlation between different parameters (flow-rate, stopper position) and the pressures prevailing in the nozzle-stopper region. On the other hand, the tundish itself is very important for the quality through separation of inclusions. During the present studies, the flow of liquid steel has been investigated by experiments in a water model with different flow-control devices and by numerical analysis for both isothermal and non-isothermal conditions. The experimental studies were intended to enlarge the mean residence time of impurities, and the results could be validated by the isothermal calculation. The tundish is not only a distribution vessel, but also a device to collect impurities in the slag, where they are transported by buoyancy. However, because of the limited residence time, very small deoxidation products do not reach the surface. These particles move through the nozzle into the mold, where they may cause problems of the abovementioned kinds. A constant mass flow can only be maintained by a permanent change of the stopper position. This results in reduced productivity and quality, which is particularly severe in aluminium-killed steel. Appropriate steel quality requires the removal of impurities in the tundish and, therefore, various efforts to enhance steel purity during the production process. The removal of inclusions is affected by a variety of parameters -- the geometry of the tundish and the resulting flow field highly influence the quality of the solid steel. The flow behaviour and the residence time of a tundish were studied experimentally in a water model by means of different visualisation methods. Furthermore, the concentration of a tracer in the tundish nozzle was measured. In addition, the temperature field was studied numerically by accounting for non-isothermal conditions. Experiments were performed in a full scale model of the tundish to investigate the influence of flow control devices on particle separation and residence time. It was found that it is absolutely necessary to increase the plug flow fraction and to minimize the stagnant regions in order to improve steel purity. Dam-weir combinations were compared to turbulence inhibitors, and quite different flow behaviour was obtained. The turbulence inhibitor creates a uniform flow field and increases the plug flow fraction, thereby increasing the mean residence time. B) Bubble formation on porous refractories: In metal casting, gas purging is used for purity improvement in order to fulfil todays high product quality demands. The impurities aggregate at the surface of the gas bubbles and are transported to the slag by the rising bubbles. This process is called flotation. Efficient flotation requires an optimal bubble size. The present work deals with refractory surface factors which influence substantially the volume of the generated bubbles. It is known that the bubble formation is strongly influenced by the wettability of the porous refractory by the melt. In our project we investigated the air bubble formation on aluminium, porous refractory, and Teflon surfaces of different structures in water. The Teflon surfaces were used because of similarity reasons, since the porous refractory is not wetted by the melt. The kinematic viscosity and the kinematic surface tension of water at room temperature and of liquid steel at 1600°C are nearly equal. Therefore, the influence of wettability, surface inclination, and roughness on bubble shape and volume could be studied in our water model. Our experimental arrangement consisted of a Perspex tank filled with distilled water. Various head-pieces were attached to the carrier through which the air was supplied. In order to study the influence of inclined surfaces, the carrier was connected to a rotatable rod allowing for a continuous change of the surface inclination. We investigated single orifices as well as patterns of many orifices, wettable and non-wettable surfaces, which are the most important parameters. For the industrial application in gas purging, the gas bubbles produced should be reasonably small in order to achieve a capacious net surface with a reasonable bubble rising time. A situation as observed on the wetted aluminium orifice pattern was found to be the ideal case, which can, however, not be achieved in reality because of the non-wettability of the refractory surface by the melt. Thus, the aim of the present project was to find out about the influence of surface structure on bubble formation and volume resulting in scopes for design.