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Definitions

• This invention relates to ceramic compositions which are of particular value in the handling and casting of high melting temperature metals such as iron or steel.
• carbon bonded ceramics also known as black refractories
• examples of such articles are pouring nozzles for molten metal-containing vessels such as ladles or tundishes, and shrouds which surround the metal stream flowing from one vessel to another.
• These carbon bonded ceramics are formed from a mixture of graphite, one or more oxides such as alumina, magnesia and zirconia, and a binder such as a phenolic resin or pitch which will decompose to produce a carbon bond.
• the above carbon bonded ceramic materials suffer from a number of disadvantages. They have poor thermal shock resistance and tend to crack, so that it is necessary to treat articles such as nozzles and shrouds in some way so as to minimise the thermal shock produced when the articles are heated rapidly to elevated temperatures.
• the materials also have low oxidation resistance as they contain a relatively high proportion of carbon, mainly in the form of graphite.
• the materials also suffer from additional disadvantages in specific applications.
• the outer surface of a nozzle is susceptible to attack by slag present on the surface of the molten metal in which the nozzle is immersed (known as slag line attack), and the bore of a nozzle tends to become clogged in use due to the build up of alumina, when casting aluminium killed steel.
• a carbon bonded ceramic material consisting of a mixture of boron nitride, zirconium diboride and at least one other refractory material, is particularly useful as an alternative to conventional graphite-containing carbon bonded ceramics for the production of articles used for the handling and casting of molten metals such as steel
• a ceramic composition comprising 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
• the other refractory material may be for example a refractory metal, an oxide, a carbide, a bo ⁇ de or a nitride
• the refractory metal may be for example boron
• Suitable refractory oxides include aluminium oxide, zirconium oxide, magnesium oxide, yttrium oxide, calcium oxide, chromium oxide and silicon oxide More than one oxide may be used, and the oxide may be a mixed refractory oxide such as mullite
• suitable carbides include silicon carbide, boron carbide, aluminium carbide and zirconium carbide More than one carbide may be used
• bo ⁇ des include titanium diboride and calcium hexabo ⁇ de
• suitable nitrides include silicon nitride, aluminium nitride, titanium nitride, zirconium nitride and sialon More than one bo ⁇ de and more than one nitride may be used
• the ceramic composition comprises a mixture of boron nitride, zirconium diboride and zirconium oxide, and the ceramic composition preferably contains 5 - 70 % by weight of boron nitride more preferably 15 - 50 % by weight, 5 - 60 % by weight of zirconium diboride, more preferably 15 - 50 % by weight, and 5 - 80 % by weight of zirconium oxide, more preferably 10 - 60 % by weight
• the ceramic composition comprises a mixture of boron nitride, zirconium diboride and aluminium oxide, and the ceramic composition preferably contains 5 - 70 % by weight of boron nitride, more preferably 15 - 50 % by weight, 5 - 60 % by weight of zirconium diboride, more preferably 15 - 50 % by weight, and 10 - 70 % by weight of aluminium oxide, more preferably 15 - 60 % by weight
• the proportion of each of the components of the ceramic composition is expressed as percentage by weight based on the total weight of the ceramic composition, excluding the carbon bond
• the organic binder which decomposes to produce a carbon bond may be for example a phenol-formaldehyde resin such as a novolac or a resol phenol-formaldehyde resin, a urea-formaldehyde resin a melamine-formaldehyde resin, an epoxy resin, a furane resin or pitch
• the organic binder is preferably a phenol-formaldehyde resin, and it is preferred that the resin is used in the form of a liquid
• a powdered phenolic resin can be used but it is necessary to dissolve the resin in a suitable solvent, such as furfural, in order to mix the resin with the other components and produce the ceramic composition
• the amount of liquid phenolic resin used will usually be of the order 5 - 25%, preferably 10 - 15% by weight, based on the total of the other components, and after production of the ceramic composition, the composition will usually contain 2 - 12% by weight, preferably of the order of 5% by weight, of carbon produced by decomposition of the resin, based on the total weight of the ceramic composition
• the ceramic compositions of the invention may be produced by first mixing together particles of the boron nitride, the zirconium diboride and the other refractory material , and then adding the liquid resin and mixing until the mixture of the particles and the resin is homogeneous It may be necessary to heat the mixture
• the ceramic compositions of the invention may be used for other applications, for example in the melting and handling of glass or in the melting, handling and casting of relatively low melting temperature metals such as aluminium and its alloys, the compositions are particularly useful for use in the handling and casting of high melting temperature metals such as iron or steel
• each of the three components of the ceramic compositions of the invention confers particular properties on the compositions
• the boron nitride makes the compositions non-wetting in the presence of molten steel or molten slag, and hence when used for example in a composition which is used for a casting nozzle will prevent clogging of the nozzle due to alumina build up
• the boron nitride makes the compositions resistant to thermal shock, and helps to protect the compositions from oxidation
• the zirconium diboride confers erosion resistance, gives protection against oxidation at higher temperatures ( up to about 1250° C) than does the boron nitride, and improves the resistance of the compositions to attack by molten slag.
• both the aluminium oxide and the zirconium oxide improve the resistance of the composition to attack by molten steel
• Examples of applications for the ceramic compositions of the invention in the handling and casting of steel are lining materials, and nozzles and shrouds, such as those used in continuous casting
• the zirconium oxide-containing composition described above is particularly suitable for forming that part of a nozzle which in use is at the boundary between the surface of molten steel and molten slag which lies on top of the steel
• the aluminium oxide-containing composition described above is particularly suitable for forming the inside of a nozzle, since it can readily be co-pressed with an alumina-graphite material which forms the rest of the nozzle, and it prevents build up of alumina and clogging of the nozzle
• compositions may be used to form the whole nozzle if desired, it is preferred to use them only to form portions of the nozzles as described
• the remainder of the nozzles can then be formed from a conventional carbon bonded ceramic material such as a carbon bonded alumina and graphite mixture
• compositions were prepared as in Table 1 below The amount of each of the refractory components is expressed as percentage by weight based on the total, and the amount of liquid resin is expressed as percentage by weight of the total of the refractory components Table 1
• Ceramic compositions according to the invention were produced by first mixing together particulate boron nitride, particulate zirconium diboride and, if present particulate aluminium oxide, zirconium oxide and silicon carbide in an intensive mixer and then adding a liquid phenol-formaldehyde resin, and mixing until the mixture of the particles and the resin was homogeneous
• the boron nitride was a refractory grade containing up to 7% by weight of oxygen and had a particle size of less than 10 microns, and the zirconium diboride had a particle size of less than 45 microns
• the aluminium oxide and zirconium oxide Were both 50/50 w/w of particles of less than 500 microns and particles of less than 53 microns
• the silicon carbide had a particle size of less than 150 microns
• the resin was a liquid novolac phenol-formaldehyde resin having a solids content of 60% by weight
• the mixture of particles and liquid resin was heated to reduce the liquid content of the resin to render the mixture suitable for forming
• the mixture was then formed into test specimens by cold isostatic pressing of the mixture in a mould After forming the specimens were stripped from the mould, and heated for 1 hour at 200° C heated to cure and cross-link the resin. Finally the test specimens were heated at 900° C to pyrolyse the resin and produce a carbon bond.
• compositions 1 , 2, 3, and 4 from Example 1 were tested to assess their resistance to molten steel in comparison with a conventional carbon bonded alumina-graphite material, by measuring their corrosion rate when immersed in molten steel at 1650° C
• Rods 50 mm in diameter and 300 mm in length were made by isostatic pressing using the method described in Example 1 , and their diameter was accurately measured The rods were then held in jigs, and immersed for one hour in molten steel in an induction furnace At the end of the test the diameter of the rods was remeasured
• compositions 6, 7, and 8 from Example 1 were tested to assess their resistance to molten slag in comparison with a carbon bonded zirconia graphite material, by measuring their corrosion rate when immersed in molten slag at 1580° C
• Rods of the same dimensions as those in Example 1 were made using the method described in Example 1 , and their diameter was accurately measured A borosilicate glass was sprinkled on to the surface of molten steel in an induction furnace, and allowed to melt to form a slag The rods were then held in jigs and immersed in the molten steel for one hour At the end of the test the diameter of the rods was remeasured in the area which had been in contact with the molten slag
• Example 1 All eight compositions from Example 1 were tested to assess their resistance to oxidation, by measuring their oxidation rate at 1200° C at various time intervals Disc shaped specimens 30 mm in diameter and 10 mm high were made by the method described in Example 1. The specimens were weighed and placed in an electric oven for various times, and then removed, cooled and reweighed.
• Compositions 1 and 3 were tested in comparison with a conventional carbon bonded alumina-graphite material to assess their ability to suppress clogging due to alumina build up when used to form the inside surface of a nozzle though which molten steel is cast.
• Tubular nozzles having an outside diameter of 50 mm, an inside diameter of 15 mm and a length of 300m were made using the method described in Example 1
• the nozzles were immersed in aluminium killed steel having an aluminium content of 0 2% by weight After immersion of the nozzles, oxygen was bubbled into the steel and the nozzles were agitated continuously to distribute the oxygen After 30 minutes the tests were concluded and the nozzles were removed The nozzles were then sectioned and inspected to assess the build up of alumina
• composition 3 showed no clogging, and while composition 1 did show some clogging the material was considerably better than the alumina-graphite material
• Example 5 Four compositions were prepared as in Table 5 below using the method described in Example 1
• the boron nitride, zirconium diboride, aluminium oxide and zirconium oxide which were used were the same as those which were used in Example 1
• the titanium diboride, boron and calcium hexabo ⁇ de were powders of particle size less than 50 microns
• the magnesium oxide had a particle size of 53 to 500 microns
• the amount of each component is expressed in the same manner as in Example 1
• compositions were tested to assess their resistance to molten slag using the method described in Example 3. and they were tested to assess their resistance to oxidation using the method described in Example 4.
• a mixture was prepared having the following composition by weight -
• the mixture of the ceramic components was mixed with 6 5 % by weight, based on the total weight of the four ceramic components, of a liquid novolac phenol-formaldehyde resin having a solids content of 60 % by weight as described in Example 1
• a slag containing 7 % by weight of fluoride was melted on top of molten steel held at 1650 °C in a 250 kg capacity high frequency induction heating furnace
• the rods were then held in jigs, and tested by immersing them in the molten steel for two hours to assess their resistance to thermal shock, the degree of penetration of molten steel and slag, and the rate of corrosion at the slag/metal interface Similar rods made from a carbon bonded zirco ⁇ ia-graphite material were tested in a similar manner Both types of rod had adequate thermal shock resistance and resistance to penetration, but the rods made from the composition according to the invention was superior in terms of its rate of corrosion at the slag/metal interface
• the carbon bonded zirconia-graphite rods had a corrosion rate of 3 05 mm per hour at the slag line whereas the rods made form the composition according to the invention had a corrosion rate of only 0 95 mm per hour
• a mixture was prepared having the following composition by weight -
• the mixture of the ceramic components was mixed with 7 5 % by weight, based on the total weight of the three ceramic components, of a liquid novalac phenol-formaldehyde resin having a solids content of 60 % by weight as described in Example 1
• the rods were then held in jigs and immersed in aluminium killed steel containing 0 05 to 0 1 % by weight aluminium in a 250 kg capacity high frequency induction heating furnace
• aluminium killed steel containing 0 05 to 0 1 % by weight aluminium in a 250 kg capacity high frequency induction heating furnace
• 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 argon gas was also used to protect the surface of the steel
• the temperature of the molten steel was 1570 to

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• 1996
  • 1996-07-05 GB GBGB9614188.2A patent/GB9614188D0/en active Pending
• 1997
  • 1997-06-24 EP EP97932897A patent/EP0909263A1/de not_active Ceased
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• 1999
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