Dispensing – Molten metal dispensing – Gaseous fluid engages molten metal
Reexamination Certificate
2000-11-21
2002-12-10
Kastler, Scott (Department: 1742)
Dispensing
Molten metal dispensing
Gaseous fluid engages molten metal
C266S220000
Reexamination Certificate
active
06491190
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to components for foundry and steel mill applications and, more particularly, to nozzles typically found in ladles and tundishes used for teeming molten metals.
BACKGROUND OF THE INVENTION
Ladles and tundishes used for teeming molten metal, for example, steel, require an outlet or outlets at the bottom thereof to direct the flow of the molten steel into a subsequent stage, e.g. a tundish, an inner mold, or continuous casting molds. These outlets are typically formed with special nozzles made of refractory material having good corrosion and erosion resistance. Control of the casting rates of the molten steel is generally carried out by means of either a stopper rod assembly or a slide gate system, both of which include similar refractory material. Nozzles for directing the flow of teeming molten steel generally have an inner insert or lining which has a bore for directing the flow of the molten steel therethrough.
Known nozzles made of alumina-silica, chrome-alumina, alumina-graphite or zirconia-graphite refractories have an inherent problem. Such materials have an affinity for impurities in steel, especially in aluminum kilned steels. Thus, when teeming molten steel from a tundish, the refractory material of the nozzle is susceptible to attack from non-ferrous constituents of the steel, particularly aluminum. These elements, which are present in molten steel, readily combine with oxygen at the temperature of molten steel to form oxides which collect and build-up in the bore of the nozzle which can ultimately lead to blockage of the bore. Oxidation of these reactive particles is exacerbated by the substantial aspiration of air into the nozzle which arises as a result of the vacuum generated by the molten steel as it enters into and flows down through the nozzle.
In an attempt to solve the blockage problem created by deposit build-ups, it is known to provide a nozzle which comprises a porous or permeable nozzle that is made of a refractory material, for example magnesium oxide (“MgO”). The permeable MgO nozzle is often encased in a metal housing for structural support. The metal housing has a fitting or port for the admission of an insert gas, for example, argon, into the nozzle. The gas flows first into a manifold around the nozzle. The manifold may be a relatively large chamber defined by the space between the nozzle and the inside of the metal housing. In other nozzles, the gas manifold may be one or more axial and/or circumferential distribution channels. Such distribution channels are often located adjacent an area in the nozzle bore where blockage tends to occur.
Gas flow through the permeable nozzle provides an important benefit. As the inert gas flows into the bore, the gas forms a thin boundary layer between the surface of the bore and the molten steel flowing therethough. The gas film in the bore first retards the buildup of deposits in the bore as the molten steel flows therethough. Thus, the gas layer protects the nozzle from attack or build up of non-ferrous particles in the molten steel. Further, the gas film “lubricates” the surface of the bore of the nozzle, thereby further facilitating the flow of gas therethough. The gas flow provides a further advantage of creating a positive pressure around the molten steel flow. That positive pressure diminishes the magnitude of the vacuum generated by the flow of steel through the nozzle and prevents the introduction of air into the molten steel, thereby preventing oxidation of the steel.
While known permeable MgO nozzles perform very well, there are still opportunities for even better performance. Problems have arisen both in manufacturing the nozzle and in service. MgO has a very high coefficient of thermal expansion in comparison to other refractory ceramic materials. A permeable nozzle made with a high purity form of MgO has a relatively thick wall. During its manufacture, the nozzle is exposed to a firing cycle; and the nozzle has a tendency to crack due to a temperature differential that occurs across the nozzle wall. Extreme measures must be taken to reduce this tendency. The cracks may be visible or subsurface and thus may not be discovered until the nozzle is machined. Thus, a nozzle may be rejected for cracks at different stages of its manufacturing process. When in service, cracks in the nozzle disrupt the uniform flow of gas through the insert and the formation of a uniform gas film within the bore of the nozzle. With some cracks, molten steel may penetrate the nozzle and the gas manifold. Thus, cracks in the nozzle diminish the effectiveness of the gas flow and shorten the life of the nozzle. In worst case situations, cracks may result in a structural failure of the nozzle.
The same high thermal expansion of MgO can cause other problems in some tundish gate systems. When the nozzle is heated as a result of the molten steel pouring though it, the nozzle expands. Because it is held in place in the radial direction by other refractories, axial expansion occurs forcing it down against the tundish gate system below it. Proper operation of the gate system is sometimes impaired due to the pressure applied by the expanded nozzle. Therefore, there is a need to provide a permeable nozzle made of MgO that can be reliably manufactured without thermal expansion problems.
Known permeable nozzles made with MgO have another weakness. During manufacture, the permeable nozzle is machined so that a uniform space or gap of approximately 0.060 inch to 0.200 inch is defined between the nozzle and the inner surface of the cylindrical metal can or housing surrounding the nozzle. A thin, uniform layer of a cementitious refractory mortar is inserted in a space or gap to secure the metal housing to the nozzle. A conventionally known air-drying mortar or a phosphoric-acid containing mortar is used. Thus, the metal housing encases the nozzle and, together with the mortar, structurally reinforces the nozzle. It is intended that the metal housing and mortar form a relatively airtight barrier over the gas manifolds in the nozzle. The cementitious refractory mortar is a heat setting mortar; and in the process of heat curing, the mortar tends to shrink. Adhesion to the smooth, machined nozzle body and the metal housing are challenged, and separation from either, or both, of these joints can occur resulting in gas leaks to the atmosphere. In addition, mortars are not dense refractories, but in themselves tend to be somewhat porous, especially when thinned to the degree of fluidity required to fill the small space between the nozzle body and the metal housing. The mortar shrinkage sometimes results in a crack which will cause a gas leak to atmosphere, and any gas leakage through the mortar dilutes the gas flow through the nozzle. Attempts to provide a thicker cementitious layer can result in excessive shrinkage and a release of the mortar layer from the nozzle and/or the metal housing. The above-described nozzle structure is expensive to manufacture and may result in nozzles that have weakened or defective gas flow systems. Consequently, there is a need for a nozzle construction that is generally less costly but provides a more reliable and consistent gas flow system.
SUMMARY OF THE INVENTION
The present invention provides a permeable nozzle that functions as a reliable conduit for pouring molten metal from a tundish or other receptacle over a long period of time. The permeable nozzle of the present invention is less expensive to manufacture, easier to check for gas leaks and has a greater thermal shock resistance than known permeable nozzles. The more controllable and reliable operation of the permeable nozzle of the present invention makes it especially useful in the continuous casting of steel.
According to the principles of the present invention and in accordance with the described embodiment, the present invention provides a permeable nozzle for directing a flow of molten metal from a tundish or other receptacle. The nozzle comprises about 50%-80% of about a 20-48×100 mesh MgO, abou
Kastler Scott
Wood Herron & Evans L.L.P.
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