Inclusions Removal by Gas Bubbles in Steel Continuous Casting Tundish

"Small inclusions removal from molten steel in a continuous casting tundish by gas bubbles, which were generated in the turbulent flow region by introducing inert gas, was analyzed theoretically and the inclusion-bubble adhesion and flotation process was also observed by high-speed photo-graph in water model. Results showed that small inclusions in the range of 20-50µm removal from molten steel by bubbles flotation involves three stages: contact, adhesion and flotation. The inclusion-bubble adhesion processes include two ways: inclusions adhering to the upper part of the bubble and inclusions adhering to the lower part of the bubble. As the bubble floats up, the number of inclusions adhering to the bubble are increased. The proper size of bubbles, which could enhance small inclusions removal efficiency, should be controlled in the range of 1-2mm.IntroductionReducing inclusions content in molten steel is a very important way to improve quality of steel, and one of the main tasks in steel making process.A variety of methods have been used for inclusions removal in molten steel. At present, the main approaches to remove inclusions are gas stirring, slag cleaning, filtering, centrifugal separating, and by means of ultrasonic or electromagnetic field. Among these techniques, gas stirring is the most widely and effectively method for inclusions removal. Inclusions in molten steel will be aggregated and generated to form larger size inclusions, and subsequently floated to the surface of molten steel by buoyancy; meanwhile, some inclusions will be adhered to bubbles and floated together to metal-slag interface where inclusions are absorbed by the covering slag layer. Some research work showed that flow behaviors of small bubbles are different from that of big ones [1-2], and smaller bubbles are more efficiency for fine inclusions removal in molten steel.There are many methods to generate bubbles. During steelmaking process, highly turbulent flow in the ladle shroud can split introduced inert gas into small bubbles, which enhances the removal of fine inclusions from molten steel. Turbulent flow of molten steel injected into tundish from ladle shroud has a high speed and it is a strong turbulent area in continuous casting. The gas can be introduced into the shroud below the slide gate. The high turbulence flow would break up the gas into small gas bubbles. Fine inclusions will be easily adhered to small bubbles and subsequently floated to the surface of molten steel. Generation of small bubbles in ladle shroud have been studied in some literatures [3-4]. Some research have analyzed the fine inclusions removal with injecting gas from the ladle shroud by mathematical simulation, and the results show that the refining method can obviously improve the removal efficiency of fine inclusions in tundish [5-8]."