CARBON-CONTAINING REFRACTORY ARTICLE HAVING A PROTECTIVE COATING

Abstract: A carbon-containing refractory article having a protective coating is disclosed. The protective coating reduces oxidation of the article during preheating and casting operations. A high temperature wetting aid comprising a metal oxide, the metal being selected from the group consisting of arsenic, antimony, bismuth, lead, vanadium and tin, improves wetting of the protective coating on the article, and thereby reduces imperfections in the fused protective coating. The coating protects the article from oxidation and decarburization, which may result from using the article at high temperatures in an oxidising atmosphere. The wetting aid is a metal oxide, such as bismuth oxide, that reduces the surface tension of the protective coating at high temperature and promotes spreading of the protective coating over the surface of the article.

Description: Carbon-containing refractory article having a protective coating. Description. BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a carbon-containing refractory article having a protective coating. More particularly, the invention describes a protective coating that reduces carbon oxidation. Description of the Prior Art Refractory articles, which are used in the continuous casting of steel, often comprise refractory oxides and a significant amount of elemental carbon. Carbon can improve the article's resistance to thermochemical and thermomechanical stresses when contacting molten steel. For example, sub-entry shrouds (SES) transfer molten steel from a tundish to a mold, and often comprise 5-40 wt.% graphite. Carbon is susceptible to oxidation, which can seriously impair the performance of the refractory article. Elevated temperatures accelerate oxidation and can occur at least during the firing, preheating and casting operations. A carbon-containing article is often fired in a reducing, non-oxidizing atmosphere, so that oxidation cannot then occur. In contrast, a reducing atmosphere is impracticable in preheating and casting operations. To inhibit oxidation, the article is often covered with a protective coating that is relatively impervious to oxygen. Protective coatings can be formed from solids or liquids that fuse to form a protective, oxygen barrier. Importantly, the protective coating should "wet" and completely coat the article without defects or "pinholes". Pinholes in a protective coating allow oxygen to contact and oxidize the carbon- containing refractory article. Unfortunately, carbon itself can act to inhibit wetting by a protective coating. Coatings that are not designed for carbon-containing refractories also attempt to reduce pinholes and improve wetting. For example, U.S. Pat. No. 5,013,697 to Akhtar teaches a sealing glass for the semiconductor industry that uses up to 30 wt.% bismuth oxide to lower fusing temperatures below 400°C and improve the wettability of the glaze on ceramic substrates, especially metal coated ceramics. A sealing glass bonds two ceramic surfaces together forming an hermetically sealed container that is resistant to breakage and destruction. A lower fusing temperature reduces damage to the semiconductor. Akhtar does not teach a protective coating for carbon-containing substrates, a reduction in pinholing, or use in high temperature applications. EP-A-0252621 does describe a protective coating for a carbon-containing refractory article. The coating comprises a solid, particulate glaze for an electric arc furnace graphite electrode. The glaze comprises 20-40 wt.% of a graphite-wetting, fusible material, 35-70 wt.% of a refractory filler, and 10-25 wt.% of a metal or metalloid, which is oxidizable to a refractory oxide while the electrode is at an elevated temperature. The graphite-wetting, fusible material is a boron compound or a boron- containing glass, such as a borosilicate glass. The metal or metalloid is preferably aluminum, and is intended to react with any oxygen before the oxygen reaches the refractory article. The solid particulate glaze is sprayed and fuses onto the hot electrode. Similar solid compositions including alkali and boron oxides are also known from the document DE-A1-4216934. These compositions are trowelled, rammed or sprayed. Spraying, trowelling or ramming cannot easily and fastly coat complex geometries or cavities, including, for example, the bore of a pour tube. Other protecting composition that also comprises boron oxide are disclosed in the USP 3,460,987. These compositions however cannot be used directly on a substrate that contains less than 5% of boron oxide and are therefore not useful for the most carbon containing refractory articles used in the glass or metal industry. Applicant herein incorporates by reference U.S. Pat. No. 5,856,015 to Buchanan, which teaches a protective coating for carbon-containing articles. The protective coating is an aqueous suspension comprising clay, glass frit, powders of aluminum, silicon or their alloys, and an inhibitor. The frit is a borosilicate, borophosphate or phosphate glass having a softening point below 600°C. The powders are described as anti-oxidants, which inhibit oxidation of carbon. The inhibitor is described as decreasing the reactivity of the powder with water. A reaction between the inhibitor and water would evolve hydrogen gas causing bubbles and pinholes in the coating. The inhibitor includes several acids and salts, and is preferably sodium triphosphate. Benefits claimed include improved storage life and promotion of a more even protective coating. Despite the addition of wetting aids and inhibitors, pinholing and the resultant oxidation can still occur on carbon-containing refractory articles, especially articles further comprising alumina, magnesia and zirconia. A need persists for a protective coating that reduces pinholing on a carbon-containing refractory article at high temperature and protects the article from oxidation during preheating and casting operations. SUMMARY OF THE INVENTION The present invention relates to a carbon-containing refractory article having a coating that protects the article from oxidation. In a broad aspect, the article comprises a form of carbon, preferably graphite, and at least one other refractory component, such as, for example, an oxide, carbide or nitride. The protective coating comprises silica, boron oxide, alumina and a wetting aid. Alternatively, calcia or magnesia may substitute for all or part of the alumina. Most commonly, the carbon-containing refractory article is described as an article that contacts a stream of molten metal during casting, such as, for example, a shroud, nozzle, monoblock or stopper rod. Alternatively, the article may be a crucible or other refractory article for containing or contacting molten metal. The protective coating is described as particularly effective on difficult to wet refractory compositions, such as articles comprising magnesia and zirconia. The wetting aid is described as reducing the surface energy of the protective coating at elevated temperature, thereby improving wetting and reducing pinholing and oxidation of the carbon-containing refractory. In one aspect, the wetting aid is described as a metal oxide of metals selected from the group consisting of arsenic, antimony, bismuth, tin, vanadium and lead. Preferably, the metal oxide is bismuth oxide and is present at 10 wt.% or less. In a further embodiment of the invention, the protective coating also comprises a flux, which improves fluidity of the protective coating. The flux may be an oxide of an alkali metal, and preferably will be an oxide of lithium, sodium or potassium. Other details, objects and advantages of the invention will become apparent as the following description of a present preferred method of practicing the invention proceeds. DETAILED DESCRIPTION OF INVENTION The invention relates to a carbon-containing refractory article having a protective coating. The refractory article includes any article comprising carbon in an amount where oxidation of the carbon would have a significant impact on the article's performance. Typically, the refractory article comprises 2-40 wt.% carbon and carbon is in the form of graphite. Such articles are also frequently carbon-bonded and intended to directly contact molten metal. In addition to carbon, the article will comprise at least one refractory component, such as, for example, a refractory oxide, carbide or nitride. The choice of such refractory components is known to one skilled in the art. Examples of the refractory article include submerged entry nozzles, submerged entry shrouds, slide gate plates, stopper rods, graphite electrodes, crucibles and other components used in the steel and metallurgical industry. A refractory article may also include "monoblock" articles, which are unitary pieces comprising a slide gate plate fixedly secured to a tube, such as a nozzle or shroud. The protective coating includes a minor portion of a high temperature wetting aid, and a major portion comprising suitable oxides, such as silica, boron oxide and oxides of aluminum, calcium or magnesium. The major portion of the present invention is substantially described in U.S. Pat. No. 5,856,015, the teaching of which Applicant hereby incorporates by reference. The major portion is formed from particles of refractory oxides, particularly lead-free and barium-free glass comprising borosilicate, borophosphate or phosphate having a dilatometric softening point below 600°C. The major portion has 40-80 wt.% silica, 5-20 wt.% boron oxide and 5-20 wt.% of at least one refractory oxide selected from the group consisting of alumina, calcia and magnesia. Additionally, the major portion may have up to 4 wt.% cobalt oxide. The major portion is commonly present in the protective coating up to about 99 wt.%. The major portion may also contain an antioxidant that sacrificially oxidizes before elemental carbon in the refractory article. The inclusion of an antioxidant may recommend the use of an inhibitor, which delays oxidation of the antioxidant. The antioxidant and inhibitor may comprise up to about 30 wt.% of the major portion. Suitable antioxidants include powders of a metal, metalloid, carbides, nitrides or combinations thereof. The term "powder" includes both powder and flake materials. The powder preferably has a particle size in the range from 1 to 250 μm, and most preferably from 10 to 50 μm. Larger particle sizes are preferred when a thicker protective coating is desired. Common antioxidants include aluminum, magnesium, boron, calcium, silicon, and carbides of silicon, calcium, zirconium, boron, tantalum and titanium. The high temperature wetting aid will improve the wetting of the protective coating on a carbon-containing refractory article at temperatures above around 300°C. Improved wetting reduces pinholing and increases the protective coating's ability to protect the article from oxidation. Without intending to be bound by this explanation, the wetting aid may decrease the surface energy of the protective coating, thereby improving wetting, reducing pinholing and the diffusion of oxygen to the carbon- containing refractory article. The wetting aid comprises an oxide of a metal selected from the group consisting of arsenic, antimony, bismuth, tin, vanadium and lead. Bismuth oxide is particularly useful as a wetting aid. It will be appreciated that these metals may often be present both in their reduced state and as an oxide. Firing of the carbon-containing refractory article in a reducing atmosphere may contribute to the reduced metal being present. As is customary in the industry, the presence of the reduced metal is reported in weight percent as if it were an oxide. A sufficient amount of wetting aid will be present to improve wetting and reduce pinholing. An effective level depends on the particular conditions, such as, for example, the composition of the refractory article, the thickness of the protective coating, and the fusion temperature. An upper limit exists where the amount of wetting aid impairs the barrier properties of the major portion and allows oxygen to diffuse though the coating to the carbon-containing refractory article. Preferably, the amount of wetting aid will be in the range from 0.5 to 10 wt.% of the protective coating. The protective coating may also comprise a flux that decreases the viscosity of the protective coating at high temperatures. A lower viscosity increases fluidity and permits the protective coating to seal more easily minor imperfections in the coating. The flux should not deleteriously react with the other components of the protective coating, especially the antioxidant. The level of flux addition depends on the particular properties desired and may readily be determined by one skilled in the art. Oxides and fluorides of alkali metals, such as lithium, sodium and potassium, are especially useful fluxes. Fluxes are most effective in a range up to about 5 wt.%; although, up to about 20 wt.% may be used. The protective coating may be applied to the refractory article by any suitable application method, including, for example, slip casting, immersion, dry or wet spraying, or cementing. Depending on the method, additives such as a liquid carrier and rheology modifiers may be needed. The method will affect the type and amount additives required. Preferably, the wetting aid and major portion will be suspended in a liquid carrier. A rheology modifier may be used to thicken and stabilize the suspension in the liquid carrier, thereby facilitating coating of the suspension onto the refractory article. A liquid carrier will most commonly be water, but may be other suitable liquids, such as alcohols and other organic solvents. A sufficient amount of liquid carrier will be needed to suspend the wetting aid, major portion, and rheology modifier so that the protective coating may be coated onto the refractory article. The amount of liquid carrier and rheology modifier can be experimentally determined by combining with other components of the protective coating until the desired consistency is achieved. Example A carbon-bonded stopper rod was made having a protective coating. The body of the stopper rod was carbon-bonded and comprised alumina and at least 20 wt.% graphite. The nose of the stopper rod comprised magnesia and at least 20 wt.% graphite. The protective coating consisted essentially of the following composition: Material wt.% Silica 62 Boron oxide 13 Cobalt oxide 1 Potassium oxide 2 Sodium oxide 6 Lithium oxide 2 Calcium oxide 3 Alumina 9 Bismuth oxide 2 The protective coating was applied to the stopper rod as an aqueous suspension and prior to heating the article in an oxidizing atmosphere in excess of 800°C. The protective coating appeared to completely wet the stopper rod and no pinholes were observed. The stopper rod with the protective coating was heated in air at 1100°C for a period of 90 minutes. No oxidation or decarburization was noted. In contrast, a stopper rod without the protective coating exhibited extensive oxidation and embrittled as the carbon-bonding deteriorated. Obviously, numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described.

Claims: Claims. 1. Carbon-containing refractory article having a protective coating comprising : (a) 40-80 wt.% silica; (b) 5-20 wt.% boron oxide; (c) 5-20 wt.% of an oxide selected from the group consisting of alumina, calcia and magnesia; and (d) 0.5-10 wt.% high-temperature wetting aid, characterized in that the high-temperature wetting aid comprises a metal oxide and the metal is selected from the group consisting of arsenic, antimony, bismuth, lead, vanadium and tin. 2. Article according to claim 1, characterized in that the metal oxide is bismuth oxide. 3. Article according to claim 1 or 2, characterized in that the protective coating also comprises up to about 30 wt.% of an antioxidant selected from the group consisting of metals, metalloids, carbides, nitrides and combinations thereof. 4. Article according to claim 3, characterized in that the antioxidant is selected from the group consisting of aluminum, magnesium, silicon, boron, silicon carbide, boron nitride, aluminum nitride and combinations thereof. 5. Article according to any one of claims 1 to 4, characterized in that the protective coating also comprises up to about 20 wt.% of a flux. 6. Article according to claim 5, characterized in that the flux is an oxide of an alkali metal. 7. Article according to claim 6, characterized in that the alkali metal is selected from the group consisting of sodium, potassium and lithium. 8. Article according to any one of claims 1 to 7, characterized in that the article is carbon-bonded. 9. Article according to any one of claims 1 to 8, characterized in that the article is selected from the group consisting of a stopper rod, a shroud, a nozzle, a slide gate plate, a monoblock and a crucible.

Inventors: BRANDY GILBERT, DESVIGNES CECILE, BRANDY, GILBERT, DESVIGNES, CECILE
Applicants: BRANDY GILBERT, DESVIGNES CECILE, VESUVIUS CRUCIBLE CO, BRANDY, GILBERT, DESVIGNES, CECILE, VESUVIUS CRUCIBLE COMPANY
Application year: 2000
Main classification: C03C8
IPC codes: C03C8, C04B41