Therefore, the permanent lining, in turn, is coated with a wearable and disposable lining of a suitable refractory composition applied to the permanent lining by gunning, spraying or trowelling. The disposable tundish coating or tundish lining is in direct contact with the molten metal in the tundish and thus, acts as a thermal and chemical barrier, which protects the permanent lining from exposure to the molten metal and extends its life. After one or more heats of molten steel are put through the tundish, the disposable liner is replaced. The refractory composition of the tundish coating mass is set to keep its coefficient of thermal expansion different from that of the permanent liner so that "deskulling" or removal of the disposable coating material becomes easy.
The role of tundish in the continuous casting process evolved from that of a buffer between the ladle and mould to being a grade separator and also a device for removing unwanted inclusions through metallurgical processes / chemical reactions. The tundish is, in fact, an intermediate process reservoir which receives a large quantity of molten metal from conveying ladles exiting a furnace in which actual melting of the ore occurs, and which then transfers the molten metal to a casting mold.
The refractory compositions of tundish coating or disposable tundish working lining are based on refractory materials such as magnesite, doloma, dolomite, dunite, or mixtures thereof. In addition to the refractory aggregate, these refractory compositions often include various binders, density reducing filler materials, and plasticizers, as well as a stiffening agent and a set retarder that can be added in predetermined weight ratios to achieve optimal performance during spray application of the composition.
Given below are two different refractory compositions both of which provide corrosion resistant, sprayable refractory lining mass that combines the strength, thermal conductivity, and density reducing properties with the advantages of alkali metal free, basic refractory compositions, such as magnesia and dunite (olivine). Tundish coating mass (disposable tundish working lining) of the following two refractory compositions (formulations) are being supplied and used in one large integrated steel plant in India and a few other medium scale plants.
Both of the refractory compositions have excellent rapid heating, thermal insulative, vertical adhesion and deskullability properties. The compositions are useful as can be applied either by gunning, spraying, or trowelling over the permanent lining of a tundish, such as is used in continuous molten metal casting processes.
Compared with thermal insulting plate, tundish coating mass not only reduces the consumption of refractory per ton of steel, but also improves the purity and property of molten steel.
The tundish itself is generally a steel vessel which is lined with several layers of a refractory composition. A permanent lining, generally of refractory bricks or castables, serves as an inner lining to protect the vessel. Although these permanent lining materials are highly resistant to elevated temperatures, contact with molten metal and slag, and numerous heating and cooling cycles during the processing of molten metals can degrade the permanent refractory lining, causing the necessity for its frequent replacement.
Also read: Continuous Casting Tundish Lining Refractories: Practices, Advantages and Disadvantages