CN1225079A - ceramic composition - Google Patents

Abstract

Ceramic compositions which are of particular value in the handling or casting of steel, for example as lining materials or for producing nozzles or shrouds used in continuous casting, comprise a mixture of particles of boron nitride, zirconium diboride and at least one other refractory material, bonded together by carbon produced by the decomposition of an organic binder such as a resin or pitch. The other refractory material may be for example a refractory metal, an oxide, a carbide, a boride or a nitride. Zirconium oxide containing compositions comprising 5-70% by weight boron nitride, 5-60% by weight zirconium diboride and 5-80% by weight of zirconium oxide are particularly suitable for forming at least that part of a nozzle which in use is at the slag line in a molten steel vessel. Aluminum oxide containing compositions comprising 5-70% by weight boron nitride, 15-50% by weight zirconium diboride and 10-70% by weight aluminum oxide are particularly suitable for forming the inside of nozzles as they resist alumina build up and prevent clogging of the nozzles.

Description

ceramic composition

This invention relates to ceramic compositions which are of particular value for the processing and casting of refractory metals such as iron and steel.

The use of carbon cemented ceramics, also known as ferrous refractories, to manufacture articles for handling and casting molten metals such as steel is well known. Examples of such articles are spouts for vessels containing molten metal such as ladles, tundishes and shields surrounding the flow of metal from one vessel into another. These carbon-cemented ceramics consist of a mixture of graphite, one or more oxides such as alumina, magnesia, and zirconia, and a binder such as phenolic resin or pitch, which decomposes to produce carbon glued.

The carbon cemented ceramics described above suffer from a series of disadvantages. Its thermal shock resistance is poor and it is easy to break, so that it is necessary to treat products such as sprues, shields, etc. in a certain way to minimize the thermal shock generated when it is quickly heated to a higher temperature degree. This type of material is also mainly in the form of graphite due to its high carbon content, which makes it low in oxidation resistance. This type of material presents additional disadvantages in specific applications. For example, when the sprue is immersed in molten metal, its outer surface is easily corroded by the slag existing on the surface of the molten metal. When pouring aluminum-deoxidized steel, the cavity of the sprue is easy to be blocked due to the formation of alumina.

It has now been found that a carbon cemented ceramic material consisting of a mixture of boron nitride, zirconium diboride and at least one other refractory material is a particularly suitable alternative to conventional graphite-containing carbon cemented ceramics for the manufacture of processing and Casting a product of molten metal such as steel.

A first feature of the present invention is to provide a ceramic composition comprising a mixture of granular boron nitride, zirconium diboride and at least one other refractory material, the composition resulting from the decomposition of an organic binder Carbon Lei glue.

The other refractory material can be, for example, a refractory metal, an oxide, a carbide, a boride or a nitride.

The refractory metal may be, for example, boron.

Examples of suitable refractory oxides include alumina, zirconia, magnesia, ytterbium oxide, calcia, chromia and silica. More than one oxide may be used and the oxide may be a mixed refractory oxide such as mullite.

Examples of suitable carbides include silicon carbide, boron carbide, aluminum carbide and zirconium carbide. More than one carbide may be used.

Examples of suitable borides include titanium diboride and calcium hexaboride, examples of suitable nitrides include silicon nitride, aluminum nitride, titanium nitride, zirconium nitride and aluminum nitride-alumina-silicon oxide ceramics Material. More than one boride and more than one nitride may be used.

According to a preferred embodiment of the present invention, the ceramic composition contains a mixture of boron nitride, zirconium diboride and zirconia, and the ceramic composition preferably contains 5-70% by weight, more preferably 1 5-50% % (weight) boron nitride, 5-60% (weight), more preferably 15-50% (weight) zirconium diboride and 5-80% (weight), more preferably 10-60% (weight) oxide zirconium.

According to another preferred embodiment of the present invention, the ceramic composition contains a mixture of boron nitride, zirconium diboride and alumina, and the ceramic composition preferably contains 5-70% by weight, more preferably 15% -50% (weight) boron nitride, 5-60% (weight), more preferably 15-50% (weight) zirconium diboride, 10-70% (weight), more preferably 15-60% (weight) ) alumina.

In the above-listed embodiments, the proportions of each component of the ceramic composition are expressed as percentages of the total weight of the ceramic composition without considering carbon bonding.

The organic binder that decomposes to produce the glue can be, for example, a type of phenolic resin such as novolac or resole resin, urea-formaldehyde resin, melamine-formaldehyde resin, epoxy resin, furan resin or pitch.

The organic binder is preferably a phenolic resin, and the resin is preferably used in a liquid state. Granular phenolic resins can be used, but the resin must be dissolved in a suitable solvent, such as furfural, and the resin mixed with the other components to form the ceramic composition. The amount of used liquid phenolic resin is generally 5-25% of other total components, preferably 10-15% (weight), after making ceramic composition, this composition generally contains and accounts for 2-12% (weight) of ceramic composition ), preferably 5% carbon, which is produced by resin decomposition.

The ceramic composition of the present invention can be made by first mixing particles of boron nitride, zirconium diboride and another refractory material, then adding the liquid resin and stirring until the particles and resin are uniformly mixed. The mixture may need to be heated to reduce the liquid content of the resin and make the mixture suitable for molding. The mixture is then formed into the desired shape, preferably by isostatically pressing the mixture in a suitable mould. After molding, the form is heated at about 150-300°C for 1 hour to cure and crosslink the resin, and then heated at about 700-1200°C to pyrolyze the resin to produce carbon bonding.

Although, the ceramic composition of the present invention can be used in other fields, for example for melting and processing glass, or for melting, processing and casting lower melting point metals such as aluminum and its alloys, the composition of the present invention is particularly suitable for processing and cast high melting point metals such as iron or steel.

Each of the three components of the ceramic composition of the present invention imparts specific properties to the composition when used to process and cast metals such as steel. Boron nitride renders the composition non-wetting in the presence of molten steel or molten slag, and thus is used in the sprue composition to prevent clogging of the sprue by the formed alumina. In addition, boron nitride imparts thermal shock resistance to the composition and helps render the composition resistant to oxidation. Zirconium diboride imparts resistance to abrasion and oxidation at higher temperatures than boron nitride (up to about 1250°C), and improves the composition's resistance to slag attack. In a preferred embodiment, both alumina and zirconia improve the ability of the composition to resist attack by molten steel.

In order to improve the oxidation resistance of the composition at higher temperatures such as about 1400°C, it is desirable to add silicon carbide and/or titanium diboride at least as a third refractory to the composition in an amount of 5-20% by weight of the composition. part of the substance.

Examples of applications of the ceramic composition of the present invention in steel processing and casting are as bushing materials and sprues and shields for continuous casting. The above-mentioned zirconia-containing composition is particularly suitable for making a sprue portion for use at the interface between the surface of the molten steel and the slag floating on the upper portion of the molten steel. The alumina-containing composition mentioned above is particularly suitable for making the interior of the sprue, because it is easy to laminate with the alumina-graphite material, which makes up the rest of the sprue, and it prevents the formation and pouring of alumina. Blockage of the mouth. Although these compositions can be used to make the entire sprue if desired, it is preferred to use them to make the sprue region as described above. The remainder of the sprue can be made from conventional carbon-bonded ceramic materials, such as a mixture of carbon-bonded alumina and graphite.

The following examples serve to illustrate the invention:

example 1

A series of compositions prepared are listed in Table 1. The amount of each refractory component is expressed in weight percent of the total amount, and the amount of liquid resin is expressed in weight percent of the total amount of refractory components.

Table 1 Composition number BN ZrB ₂ A1 ₂ O _{3 ZrO2} SiC resin 1 20 45 35 - - 10 2 25 40 35 - - 13 3 30 35 35 - - 15 4 20 35 45 - - 10 5 30 35 30 - 5 15 6 15 35 - 50 - 7 7 15 25 - 60 - 7 8 50 35 - 15 - 20

According to the invention, the ceramic composition is prepared by first mixing granular silicon nitride, granular zirconium diboride, if present, granular aluminum oxide, zirconium oxide and silicon carbide in an intensive mixer, and then Add the liquid phenolic resin and mix until the particles and resin are evenly combined.

The boron nitride is a refractory grade which contains 7% by weight oxygen and has a particle size of less than 10 microns and the zirconium diboride has a particle size of less than 45 microns. The particles of both alumina and zirconia were 50% by weight smaller than 500 microns and 50% by weight smaller than 53 microns. Silicon carbide has a particle size of less than 150 microns.

The resin was a liquid novolak with a solids content of 60% by weight.

The mixture of pellets and liquid resin is heated to reduce the liquid content of the resin and make the mixture suitable for molding. The mixture was then placed in a mold and pressed into specimens by cold isostatic pressure. After the sample was formed, it was taken out of the mold and heated at 200°C for 1 hour to cure and crosslink the resin. Finally, the samples were heated at 900°C to pyrolyze the resin to produce carbon bonding.

Example 2

Compositions 1, 2, 3 and 4 in Example 1 were immersed in molten steel at 1650°C, and their corrosion rates were measured to evaluate their ability to resist molten steel and compare with ordinary carbon-bonded alumina-graphite materials.

According to the method described in Example 1, the rod with a diameter of 50 mm and a length of 300 mm was pressed isostatically, and its diameter was accurately measured. The rods are clamped and immersed in molten steel in an induction furnace for 1 hour, and the diameter of the rods is measured at the end of the test.

The results obtained are listed in Table 2

Table 2 Composition number Corrosion rate (mm/hour) 1 0.3 2 0.2 3 0.1 4 0.6 Alumina/graphite 2

Example 3

Compositions 6, 7 and 8 in Example 1 were immersed in slag at 1580°C and their corrosion rates were measured to evaluate their ability to resist slag and compare with carbon bonded zirconia-graphite materials.

Prepare a rod of the same size as Example 2 by the method described in Example 1, and accurately measure its diameter. Borosilicate glass is sprayed on the surface of molten steel in an induction furnace and allowed to form slag. Then clamp the rod with a clamp and immerse in molten steel for 1 hour. At the end of the test, the diameter of the rod in the area in contact with the slag was measured again.

The obtained results are listed in Table 3

table 3 Composition number Corrosion rate (mm/hour) 6 2 7 2.5 8 0.5 Zirconia/graphite 4

Example 4

The oxidation rates of all eight compositions in Example 1 were measured at different time intervals at 1200° C. to evaluate their oxidation resistance.

A disc sample with a diameter of 30 mm and a height of 10 mm was prepared as described in Example 1. The samples were weighed and placed in an electric furnace, removed at various times, cooled, and re-weighed.

The results are shown in Table 4 in terms of sample weight change mg/cm ² /hour.

Table 4 Composition number 2 hours 26 hours 130 hours 1 0.97 -0.14 0.0001 2 2.77 -0.37 -0.00005 3 1.74 -0.60 -0.00002 4 5.54 2.97 -0.0015 5 0.63 0.20 0.00001 6 15.10 1.89 0.00025 7 10.73 1.69 0.00008 8 0.54 0.29 0.00003

As shown in Table 4, the oxidation rate decreased sharply with time and practically reached zero after 130 hours. This can be explained by the intrinsic inert state oxidation phenomenon of the composition.

Example 5

Compositions 1 and 3 were used to fabricate the inner surface of the sprue through which molten steel was poured, and their ability to inhibit plugging caused by alumina accumulation was evaluated and compared with ordinary carbon bonded alumina-graphite materials.

A tubular sprue with an outer diameter of 50 mm, an inner diameter of 15 mm and a length of 300 mm was prepared by the method described in Example 1. The sprue is immersed in a steel fully deoxidized with aluminum with an aluminum content of 0.2% by weight. After immersing in the sprue, blow oxygen into the steel and shake the sprue constantly to distribute the oxygen. The test was terminated after 30 minutes, and the sprue was removed and cut open to check for alumina accumulation.

Alumina-graphite material plugging is severe. Composition 3 has no clogging, while composition 1 has some clogging, but it is significantly better than the alumina-graphite material.

Example 6

Four compositions were prepared as described in Example 1 and are listed in Table 5. The boron nitride, zirconium diboride, alumina and zirconia used were the same as those used in Example 1. Titanium diboride, boron and calcium hexaboride are powders with a particle size of less than 50 microns. The particle size of magnesium oxide is 53-500 microns. The expression method of each component quantity is the same as Example 1.

table 5 components Composition 9 Composition 10 Composition 11 Composition 12 BN 40 20 10 40 ZrB ₂ 35 30 35 30 TiB ₂ 15 15 10 15 B 10 10 - - Al ₂ O ₃ - 20 10 - _ZrO2 - - - 15 CaB ₆ - 5 - - MgO - - 35 - resin 18 15 15 20

The method described in Example 3 was tested and evaluated for its ability to resist slag, and the method described in Example 4 was tested and evaluated for its ability to resist oxidation.

The obtained results are shown in Table 6. The results of the oxidation resistance test are expressed in mg/cm ² /hour of sample weight change.

Table 6 Composition number Corrosion rate (mm/hour) 2 hours 26 hours 130 hours 9 0.4 0.3 0.2 0 10 0.8 0.4 0.3 0 11 0.7 20 4 0.01 12 1 0.4 0.2 0

Example 7

A mixture of the following compositions was prepared:

Boron nitride 20% (weight)

Zirconium diboride 20% (weight)

Zirconia 55% (weight)

Silicon carbide 5% (weight)

All four components are as described in Example 1.

This mixture of ceramic components was mixed with 6.5% by weight, based on the total weight of the four ceramic components, of a liquid novolak resin having a solids content of 60% as described in Example 1.

Prepare a ceramic test rod with a diameter of 4 cm and a length of 30 cm by the method described in Example 1, and accurately measure the diameter of the rod.

Slag containing 7% by weight of fluoride was melted on top of molten steel in a high-frequency induction heating furnace with a capacity of 250 kg maintained at 1650°C.

The bars were then clamped and immersed in molten steel for 2 hours to evaluate their thermal shock resistance, penetration of molten steel and slag, and oxidation rate at the slag/metal interface. Similar rods of carbon cemented zirconia-graphite material were prepared and similarly tested. Both types of rods have good enough thermal shock resistance and penetration resistance. However, rods made from the compositions of the present invention are superior in terms of corrosion rate at the slag/metal interface. The corrosion rate of the carbon bonded zirconia-graphite rod in the slag boundary was 3.05mm/hour, while the corrosion rate of the rod made of the composition of the present invention was only 0.95mm/hour.

Example 8

A mixture of the following compositions was prepared:

Boron nitride 25% (weight)

Zirconium diboride 20% (weight)

Aluminum oxide 55% (weight)

All three components are as described in Example 1.

This mixture of ceramic components was mixed with 7.5% by weight of the total weight of the three ceramic components of a liquid novolac resin having a solids content of 60% by weight as described in Example 1.

Prepare a ceramic test rod with a diameter of 4 cm and a length of 30 cm by the method described in Example 1.

Then clamp the bar with a clamp, and immerse it in the fully deoxidized steel containing 0.05-0.1% (by weight) aluminum in a high-frequency induction heating furnace with a capacity of 250 kg. The surface of the molten steel was covered with a layer of rice husks, and in order to prevent excessive oxidation of the steel during the test, the surface of the steel was protected with argon gas. The temperature of the molten steel is 1570-1580°C, and the immersion time is 2 hours. Similar tests were carried out with similar test bars prepared from a carbon-bonded alumina-graphite material.

At the end of the test, the accumulation of alumina on the surface of the rods prepared from the composition of the invention was significantly lower than the accumulation on the surface of the rods prepared with carbon cemented alumina-graphite material.

Claims (15)

1. A ceramic composition comprising a particulate mixture of boron nitride, zirconium diboride, and at least one other refractory substance, bonded together by carbon produced by decomposition of an organic binder. 2. The ceramic composition of claim 1, wherein the at least one refractory substance is a refractory metal, oxide, carbide, boride or nitride. 3. 2. The ceramic composition of claim 2, wherein the refractory metal is boron. 4. The ceramic composition according to claim 2, characterized in that the oxide is one or more of alumina, zirconia, magnesia, ytterbium oxide, calcium oxide, chromium oxide and silicon oxide. 5. The ceramic composition according to claim 2, characterized in that the carbide is one or more of silicon carbide, boron carbide, aluminum carbide and zirconium carbide. 6. The ceramic composition according to claim 2, characterized in that the boride is titanium diboride and/or calcium hexaboride. 7. The ceramic composition according to claim 2, characterized in that the nitride is one or more of silicon nitride, aluminum nitride, titanium nitride, zirconium nitride and aluminum nitride-alumina-silicon oxide ceramic materials. 8. The ceramic composition according to claim 4, characterized in that, based on the total weight of the ceramic composition disregarding carbon bonding, the composition contains 5-70% by weight of boron nitride, 5-60% by weight of diboron Zirconium oxide and 5-80% (weight) zirconia. 9. The ceramic composition of claim 8, characterized in that the composition contains 15-50% by weight of boron nitride, 15-50% by weight of zirconium diboride and 10-60% by weight of zirconium oxide . 10. The ceramic composition according to claim 4, characterized in that, based on the total weight of the ceramic composition without considering carbon bonding, the composition contains 5-70% by weight of boron nitride, 5-60% by weight of diboride Zirconium and 10-70% by weight of alumina. 11. 10. Ceramic composition according to claim 10, characterized in that the composition contains 15-50% by weight of boron nitride, 15-50% by weight of zirconium diboride and 15-60% by weight of alumina. 12. The ceramic composition according to one of claims 1-11, characterized in that the organic binder is novolak resin, resol resin, urea-formaldehyde resin, melamine-formaldehyde resin, epoxy resin or pitch. 13. 12. Ceramic composition as claimed in one of claims 1-12, characterized in that it contains 2-12% by weight of carbon produced by decomposition of the organic binder. 14. 13. Ceramic composition according to one of claims 1-13, characterized in that at least part of the other refractory substances are silicon carbide and/or titanium diboride. 15. 14. Ceramic composition according to claim 14, characterized in that it contains 5-20% by weight of boron carbide and/or titanium diboride.