Boron nitride-containing material and method thereof

Abstract: A boron-nitride-containing material of multi-component system is obtained at a low cost by heating a mixed powder containing a boride and an oxide in a nitrifying atmosphere, whereby a part or the whole of the oxide is reduced with the element bonded to boron of the boride to convert the raw materials to one or more kinds of an oxide having less bonded oxygen, an acid nitride, a nitride, a carbide, and a boride and also boron nitride is formed. Also, a sintered product of a boron-nitride-containing material is obtained by packing a mixed powder containing borides in a heat-resistant mold and heating the packed powders in a nitrifying atmosphere while restraining the packed powders by the mold. A sintered product is obtained in which neither expansion, nor deformation, occurred and no cracks formed.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a boron-nitride-containing material and a method thereof. 2. Description of the Related Arts Hexagonal boron nitride is known to be a chemically stable material which has a high heat conductivity, an excellent electrical insulating property, and an excellent lubricating property and does not react with a molten product of iron, copper, nickel, zinc, gallium, arsenic, glass, cryolite, etc. Also, hexagonal boron nitride is stable in air until 950°C and in an inert gas or nitrogen gas atmosphere until 2,200 °C. Also, hexagonal boron nitride has the feature that machining such as cutting, grinding, etc., can be easily performed as in the case of metals. Boron nitride itself or material containing boron nitride has been supplied for various uses by utilizing that feature. The uses as the powder of hexagonal boron nitride, has been used as additives for plastics, lubricants, mold releasing agents for aluminum die casting and a glass molding, etc. Also, sintered hexagonal boron nitride, has been used for insulating parts, heat-resistant parts, crucibles for molten metals, brake rings for horizontal continuous casting, heat-emitting members, setters for sintering the powder moldings of metals or ceramics, mold materials, etc. Also the application thereof for refractories for casting, such as upper nozzles, immersion nozzles, etc., has been attempted. As a method of synthesizing the powder of hexagonal boron nitride having a wide use as described above, the following methods are known. (1) A method of synthesizing the powder thereof by heating borax and urea to a temperature of at least 800 °C in an ammonia atmosphere as described in JP-B-38-1610 ( the term "JP-B" as used herein means " examined Japanese Patent Publication " ). (2) A method of synthesizing the powder thereof by heating a mixture of boric acid or boron oxide and calcium phosphate in an ammonia atmosphere as descried in JP-B-42-24669. (3) A method of synthesizing the powder thereof by heating boric acid and a nitrogen-containing compound ( urea, melamine, dicyandiamide, etc. ) to a temperature of at least 1600 °C as described in JP-B-48-14559. Also, a boron nitride powder synthesized at a low temperature of 1400 °C or less contains unreacted materials, boron oxide, and B-N-O series intermediates in addition to boron nitride ( BN ) and also the boron nitride powder has an insufficient crystallization and is chemically unstable. Thus, the crude boron nitride powder is once cooled and washed with water followed by drying, and subjected to a heat treatment again for purification and crystallization. As one of these methods, a method of mixing the powder with a carbonaceous powder and heating the mixture to a temperature of at least 1500°C in an ammonia gas is known as described in JP-A-61-256905 (the term "JP-A" as used herein means "unexamined Japanese patent application"). The boron nitride thus obtained has a difficult sintering property and does not shape a sintered product by itself. Thus, for producing a sintered product of boron nitride, it is general to add a sintering aid to such a boron nitride powder and apply thereto a hot press and, for example, a method of adding a borate of an alkaline earth metal to boron nitride and hot-pressing the mixture is known as described in JP-B-49-40124. For the production of a sintered material of a boron-nitride-containing complex material, a hot press method is applied. For example, a method of obtaining a BN-TiB 2 -AlN three-component series container for metal vapor deposition by hot-pressing a mixed powder composed of titanium diborate, boron nitride, boron carbide and metallic aluminum in vacuum is disclosed in JP-A-62-139866. However, the hot-pressing method is problematic in that the production efficiency is poor and the method can be applied to a simple-shape part only and the industrial utilization of the method is limited. For example, it is very difficult to produce a long product such as a thermocouple protective tube of a boron-nitride-containing complex material. Thus, a technique capable of normal-pressure sintering of these parts by using a complex material of boron nitride and other material(s) as such parts and also, at the same time, improving the sintering property thereof has been developed. For example, JP-A-61-132564 discloses a method of CIP molding a mixed powder of BN, Al 2 O 3 , and B 2 O 3 and sintering the mixed powder in a non-oxidizing atmosphere at normal pressure. In this way normal-pressure sintering is applied to the boron-nitride-containing complex material capable of being normal-pressure sintered as described above, the fault ( the low productivity and the restriction on the shape of the parts ) in the hot press method can be avoided. Also, a method wherein a phenol resin is added, as a binder, to a boron-nitride-containing material, the material is molded, and thereafter the material is bonded with residual carbon obtained by thermal decomposition is known. For example, JP-A-3-268849 discloses a method of adding from 8 to 15 % by weight a liquid phenol resin to a mixed powder of boron nitride and partially stabilized zirconia followed by kneading and decomposing the phenol resin by sintering the kneaded product at a temperature of from 1,000°C to 1,200 °C to obtain an immersion nozzle for continuous casting. Now, as described above, for the hexagonal boron nitride material, the market expansion of the powder or the sintered product has been expected owing to its specific characteristics, and the practical use thereof has proceeded owing to the technology as described above but the market thereof has not progressed at present as expected. The main cause thereof is that the hexagonal boron nitride powder is expensive. The reason that the hexagonal boron nitride powder is expensive is that since the purity of boron nitride as the product is low because borax and boron oxide as the raw materials are softened in the course of the reaction during which they become glassy, which hinders the nitriding, and also the nitriding and the crystallization cannot be simultaneously carried out. Therefor, for the production of the hexagonal boron nitride powder, many steps are required as described above and the production steps become troublesome. Thus, a method of forming hexagonal boron nitride in the sintered product without using the expensive hexagonal boron nitride powder has been proposed. For example, JP-A-4-325461 disclosed that by molding a mixed powder of silicon and B 4 C and heating the mixed powder in a nitrogen gas atmosphere, a sintered product containing BN and C formed by the nitriding of B 4 C, SiC formed by the carbonization of silicon, and Si 3 N 4 formed by the nitriding of silicon can be produced. Also, JP-A-5-201771 discloses a method of forming a boron-nitride-containing sintered product by reaction sintering. Practically, using at least one material selected from the group consisting of silicon, SiB 4 , SiB 6 , AlB 2 , AlB 12 , CaB 6 , silicon oxide, and alumina as the starting powders, a molding having a shape resembling the final shape is obtained by casting and the molding is nitrided for from 5 to 100 hours, at a temperature of from 1,000 °C to 1,400 °C, and a nitrogen gas pressure of from 500 to 1,000 mbar or is nitrided for from 1 to 20 hours at a nitrogen gas pressure of from 2 to 20 bar. By forming Si 3 N 4 from silicon and forming a nitride of an element bonded to boron and BN from the boride ( SiB 6 shapes Si 3 N 4 and BN, and AlB 12 shapes AlN and BN ) in the nitriding step, Si 3 N 4 , BN, and AlN are formed. After the nitriding step, by sintering the products formed in the temperature range of from 1,500 °C to 1,900 °C, β'-sialon is formed from Si 3 N 4 , AlN, and previously added Al 2 O 3 is formed and silicon oxynitride ( Si 2 N 2 O ) is formed from Si 3 N 4 and previously added SiO 2 , whereby a sintered product composed of β'-sialon, acid silicon nitride, and boron nitride as the main components is obtained. In this case, it is described that, the carbon content of the boride is preferably required to be 0.1% or less and also B 4 C is excluded from the objects. Also, it is described that oxide(s) such as alumina, silicon dioxide, mullite, zirconia, yttrium oxide, YAG (yttrium-aluminum garnet 3Y 2 O 3 · 5Al 2 O 3 ), calcium oxide, and magnesium oxide may be added to the starting materials, but B 2 O 3 must not be present. Also, it is limited that these oxide additives are not reacted with the remaining starting powders in the nitriding step and during the sintering treatment. Both JP-A-4-325461 and JP-A-5-201771 disclose that a sintered product of the complex material containing silicon nitride and boron nitride is obtained by the nitriding of silicon and the boride(s) as described above. Also, it is known that the powders are formed prior to the nitriding step as described in the patent publications mentioned above and, in particular, it is described in the latter that a shaped material having a shape resembling the final shape is formed by casting the mixture of the starting materials and then applying nitriding to the shaped product. As described above, the market expansion of the hexagonal boron nitride material is expected as the powder or the sintered product thereof owing to the specific characteristics of the material but the market thereof has not expanded as expected by reason that the hexagonal boron nitride powder is expensive. In regard to the problem, as to the sintered product, a method of using a boride powder in place of the hexagonal boron nitride powder as a raw material and forming a hexagonal boron nitride by the nitriding of a molding containing the boride powder to produce a complex material containing boron nitride is proposed in JP-A-4-325461 and JP-A-5-201771 as described above, and if an inexpensive boride is obtained, the method leads the solving of the cost problem described above. However, the method is applied to a limited composition system only to obtain the sintered material of the complex material containing silicon nitride and boron nitride by the nitriding of metallic silicon and a boride and does not project the various possibilities of a boron-nitride-containing complex material. Also, when various reactions given in JP-A-4-325461 and JP-A-5-201771 are carried out, a problem occurs in that the product exhibits a volume increase of from 20 to 80%. That is, as described in the patent publications, when the starting materials are molded and the foregoing reaction is performed on to the molding in the state of the free surface thereof, the molding is expanded and deformed and in this step, cracks occur in many cases. Accordingly, it is difficult to obtain a sintered product capable of being used practically as it is. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a boron-nitride-containing material which is capable of obtaining the boron-nitride-containing material at a low cost and also to provide the boron-nitride-containing material obtained by the method. To attain the aforementioned object, the present invention provides a method for producing boron nitride containing sintered body comprising the steps of preparing a mixed powder which comprises 10 to 70 wt.% B 4 C and 30 to 90 wt.% Al; packing the mixed powder in a packing ratio of 30 to 70% into a heat-resistant mold, the packed material being restrained by the heat-resistant mold; and heating the mixed powder in the heat-resistant mold in a nitriding atmosphere to produce a boron nitride and AIN, or AION. Also, the present invention provides a boron nitride containing sintered body produced by the aforementioned method. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view exhibiting a restrained mold used in a example of the present invention; FIG. 2 is a view exhibiting the closing of the cavities of the mold by screwing an upper cover having screw portions onto the mold; FIG. 3 is a cross-sectional view exhibiting a sintered product obtained by dissolving the mold of FIG. 2; FIG. 4 is a view exhibiting a molding produced by a conventional method; and FIG. 5 is a view showing a sintered product produced by a conventional method. DESCRIPTION OF THE EMBODIMENT The inventors previously proposed a method of producing a boron-nitride-containing inorganic material, which comprises nitriding a filled or formed product of an inorganic powder containing at least 20% a boride of at least one kind of an element belonging to Groups II to IV of the periodic table to convert at least 50% by weight of boride to the foregoing element or a nitride of the element (JP-A-7-048105). It is described that by utilizing a method of nitriding a boride, the sintered product of a boron nitride-containing material can be produced without using a sintering aid. However, in order to ensure the production of a practically usable sintered body without causing expansion, deformation, or the formation of cracks by the method, it is inevitable to apply the hot press. However, as described above, the application of the hot press is limited to a simple shape in which hot pressing is applied in one direction. Also, the range of component systems to which it is applicable is broader than those shown in JP-A-4-325461 and JP-A-5-201771 but the applicable component systems are not yet sufficient. The present invention has been accomplished based on this knowledge, can be applied to very wide range of component systems, can ensure the production of a practically usable sintered product without causing expansion, deformation, or the formation of cracks without the use of a hot press, and can produce widely usable sintered products for parts of various shapes. Now, the present invention will be specifically described. There is no particular restriction on the boride of the starting material but a boride having at least one of the elements belonging to Groups II to IV of the periodic table is preferred. The boride of the element belonging to Group II of the periodic table includes, for example, BeB 2 , BeB 6 , BeB 12 , MgB 2 , CaB 6 , SrB 6 and BaB 6 . The boride of the element belonging to Group III of the periodic table includes, for example, AlB 2 , AlB 12 , ScB 2 , YB 2 , YB 6 , YB 12 , LnB 6 , and LnB 12 (wherein Ln is lanthanoid). The boride of the element belonging to Group IV of the periodic table includes, for example, B 4 C, B 13 C 12 , SiB 4 , SiB 6 , SiB 12 , TiB 2 , ZrB 2 , HfB 2 , ThB 4 , and ThB 6 . The boride of the element belonging to Group V of the periodic table includes, for example, VB, VB 2 , NbB 2 , TaB, and TaB 2 . The boride of the element belonging to Group VI of the periodic table includes, for example, CrB, CrB 2 , MoB, MoB 2 , WB, W 2 B 5 , WB 4 , UB 4 , and UB 12 . Also, the boride of the starting material for use in the present invention is more preferably one or more kinds of the borides selected from MgB 2 , CaB 6 , AlB 2 , AlB 12 , TiB 2 , ZrB 2 , and B 4 C. In the boron-nitride-containing material for use in the present invention, it is preferred that the content of boron nitride is from 5 to 100% by weight. This is because if the content thereof is less than 5% by weight, it is difficult to impart the characteristics of boron nitride described above to the boron-nitride-containing material. By disintegrating the porous boron-nitride-containing material by means of a hammer mill, a jaw crusher, a vessel-rotating ball mill, an epicyclic ball mill, a vibration ball mill, a stirrer-containing ball mill, etc., the powder thereof can be easily obtained. Since boron nitride formed by the reaction of a mixed powder including a boride and Al imparts to the material the property of being disintegrated, the powder of the boron-nitride containing material can be economically obtained by the method described above. The porosity of the foregoing porous material is preferably from 10 to 70 %, and more preferably from 50 to 70 %. The reason is that as the higher the porosity is, the easier crushing is made, while if the porosity is too high, the productivity in the nitriding step is reduced. When the reaction described above is practiced, the products exhibit a volume increase of from 20 to 80 %. When the starting material is molded by a known means to first give it the shape and the foregoing reaction is applied to the molding in the state of the free surface thereof, the molding is expanded and deformed. Also, in many cases, cracks occur in this step. Accordingly, it is difficult to use the sintered product as a part as it is. This problem is solved by packing the mixed powder containing the boride in a heat-resistant mold, if necessary, applying a load to the packing to make the packing ratio 30 to 70 %, also, if necessary, closing the packing openings to substantially control the shape of the packing with the heat-resistant mold, disposing the heat sensitive mold in a furnace, and heating the packing in a nitriding atmosphere to convert the content to the sintered product of the boron-nitride-containing material. The advantages obtained by using the heat-resistant mold are that a sintered product without cracks is obtained, the molding step can be omitted, since the shape of the final product or a shape resembling the final shape is obtained, a post-working for setting the shape of the final product can be omitted or greatly reduced, mass production is possible by disposing many heat-resistant molds in the furnace different from a hot press for producing a single product having a simple shape, and a product having a complicated shape can be produced. In this case, it is preferred that the heat-resistant mold is composed of a material containing at least 80 % by weight of graphite or hexagonal boron nitride. This is because both of the materials are thermally stable and reluctantly react with nitrogen at the temperature range suitable for the foregoing reaction and such an effect is obtained by containing at least 80 % by weight of the foregoing material. Also, the reason for defining the packing ratio of the mixed powder to 30 to 70 % is that if the packing ratio is less than 30 %, even with the volume increase with the nitriding reaction, it is difficult to obtain a bulk density capable of ensuring sufficient strength, while if the packing ratio is over 70%, the porosity is reduced by the volume increase with the nitriding reaction to clog the passageways for entering a gas, whereby the progress of the reaction becomes insufficient, the heat-resistant mold is damaged by the excessive volume increase, and also the dissolution of the heat-resistant mold becomes impossible. In the present invention, provided is a method of producing a boron-nitride-containing material, which comprises heating a mixed powder containing at least from 10 to 70 wt.% of B 4 C and from 30 to 90 wt.% of Al, preferably to a temperature of from 1,300°C to 2,000°C in a nitriding atmosphere, and the material obtained contains BN, AIN and C. In the embodiment of this invention, C may be crystalline or noncrystalline. That is, the material wherein at least AlN and BN are detected through X-ray diffraction. The material has the feature that the material comprising AlN is excellent in the molten steel resistance and that BN and C each are excellent in the molten slag resistance and is resistant to being corroded by molten steel or a molten slag. The material is produced using a mixed powder containing at least from 10 to 70 % by weight of B 4 C and from 30 to 90 % by weight of Al as the raw materials. This is because if the composition of the raw material is outside the range described above, the content of BN becomes excessive or too small and AlN becomes excessive or too small, whereby the characteristics of the boron-nitride containing material described above are not sufficiently obtained. In nitriding, it is preferred to heat the raw materials to a temperature of from 1,300 °C to 2,000 °C. If the heating temperature is lower than 1,300 °C, the nitriding of B 4 C becomes difficult and also if the heating temperature is higher than 2,000 °C, the loss of the raw materials by vaporization becomes large; both of which are undesirable. A boron-nitride-containing material containing at least from 22 to 90 % by weight of BN, from 8 to 68 % by weight of AlN, or AlON and from 2 to 9 % by weight of C can be produced. The material composed of AlN and AlON each excellent in the molten steel resistance, and BN and C each excellent in the slag resistance, and has a feature that the material is resistant to being corroded with molten steel and a slag, but if the composition of the raw materials is outside of the range of the composition, it becomes undesirably difficult to obtain both the molten steel resistance and the slag resistance. The boron-nitride-containing material can be also produced by known powder sintering without using the method of this invention described above. There is no restriction on the production method of the boron-nitride-containing material but it is more preferred to product the boron-nitride-containing material by the method of this invention. This is because BN, AlN, AlON, and C are all materials which are resistant to being sintered and thus require an expensive sintering aid such as Y 2 O 3 , etc., for enabling sintering but according to the method of this invention, the transfer of atoms actively occurs in the step of the nitriding and sintering proceeds without the addition of the sintering aid. In this case, for forming AlON, for example Al 2 O 3 may be added to the starting materials in addition to Al. AlON is the general term of the solid solution of Al, 0, and N and there is no particular restriction on the composition thereof. Also, by further adding SiC to the starting materials, the slag resistance can be more improved. That is, such an effect is obtained by the boron-nitride-containing material containing at least from 22 to 90 % by weight of BN, from 4 to 48 % by weight of AlN or AlON, and from 6 to 30 % by weight of SiC. SiC exhibits a very excellent slag resistance, has the effect of improving the slag resistance of BN, and by adding from 6 to 30 % by weight of SiC, such an effect is effectively obtained. However, if the amount of SiC is less than 6 % by weight, such characteristics of SiC can not be obtained and if the amount of higher than 30 % by weight, the molten steel resistance is deteriorated. In the boron-nitride-containing materials of this invention described above, the boron-nitride-containing material containing at least from 0.4 to 20 % by weight of CaB 6 , from 19 to 50 % by weight of BN, and from 20 to 80 % by weight of AlN is excellent in corrosion resistance, in particular, in molten steel resistance and slag resistance. The material has both the characteristics of AlN excellent in the excellent molten steel resistance and the characteristics of BN excellent in the slag resistance and precise sintering of the BN portion having a difficult sintering property becomes possible by means of CaB 6 , whereby the strength of the sintered product is also improved. To obtain the characteristics of the constituting components, it is preferred that the boron-nitride-containing material contains at least from 0.4 to 20 % by weight of CaB 6 , from 19 to 50 % by weight of BN, and from 30 to 80 % by weight of AlN. EXAMPLE The examples of the present invention will be explained below while comparing them, if necessary, with the comparative examples. Example 1 A mixture of 27.8 % by weight of a CaB 6 powder having particle sizes of not larger than 44 µm, 29.3 % by weight of a B 4 C powder having particle sizes of not larger than 44 µm, and 42.9 % by weight of Al having particle sizes of not larger than 74 µm was packed in a graphite vessel having a cavity of a diameter of 60 mm and a height of 50 mm in an amount of 50 g and the packed powders were lightly pressed by applying a surface pressure of 2 MPa thereto via a graphite plate having a diameter of 60 mm and a thickness of 5 mm. The graphite vessel was disposed in a graphite heating furnace, after heating the graphite vessel at a temperature-raising rate of 15 °C/minute up to 1,60 0 °C while evacuating by means of a vacuum pump, a nitrogen gas was introduced to make the nitrogen gas pressure 1.0 MPa ( absolute pressure ), and after maintaining the system for 8 hours at 1,600 °C, the graphite vessel was allowed to cool. The weight of the sintered product obtained was 87.1 g, the bulk density thereof was 2.24 g/cm 3 , and the relative amount of voids was 12.0 %. Also, through X-ray diffraction, BN, AlN and CaCN 2 were identified. The composition presumed by the calculation was 52.8 % by weight of BN, 37.4 % by weight of AlN, and 9.8 % by weight of CaCN 2 . From the sintered product, two samples each obtained by splitting the product to 1/4 thereof and one sample obtained by splitting the product to 1/2 thereof were prepared, the former was supplied to a corrosion resistance test and from the latter, a test piece of 3 mm x 4 mm x 40 mm for a bending test was cut. On one of the samples for the corrosion test was placed cubic 13 Cr steel having a side length of 5 mm, on another sample for the corrosion test were placed tablet-formed powders ( SiO 2 : 35.2 % by weight, Al 2 O 3 : 4.1 % by weight, and CaO: 27.5 % by weight ) each having a diameter of 5 mm and a height of 5 mm, they were heated at a temperature-raising rate of 15 °C/minute up to 1,550 °C, and after maintaining them for 2 hours at 1,550 °C, they were allowed to cool. Also, after melting and solidifying, the angle of contact with the 13Cr steel and the angle of contact with the slag formed from the tablet-formed powders were measured. The angle of contact with the 13Cr steel was 110 ° and the angle of contact with the slag was 88 ° , which confirmed that the BN-containing material exhibited a good corrosion resistance to molten steel. Also, the result of the 3- point bending test of a span length of 30 mm was 32 MPa, which confirmed that the material was at a level at which is usable as a refractory for casting. Example 2 A mixture of 13.5 % by weight of a CaB 6 powder having particle sizes of not larger than 44 mm, 43.7 % by weight of a B 4 C powder having particle sizes of not larger than 44 µm, and 42.8 % by weight of an Al powder having particle sizes of not larger than 74 µm was packed in a graphite vessel having a cavity of a diameter of 60 mm and a height of 50 mm in an amount of 50 g and the packed powders were lightly pressed by applying a surface pressure of 10 MPa thereto via a graphite plate having a diameter of 60 mm and a thickness of 5 mm. The graphite vessel was disposed in a graphite heating furnace, after heating the graphite vessel at a temperature-raising rate of 15 °C/minute up to 1,600 °C while evacuating by means of a vacuum pump, a nitrogen gas was introduced to make the nitrogen gas pressure 1.0 MPa ( absolute pressure ), and after maintaining the system for 8 hours at 1,600 °C, the graphite vessel was allowed to cool. The weight of the sintered product obtained was 83.4 g, the bulk density thereof was 2.14 g/cm 3 , and the relative amount of voids was 5.0 %. Also, through X-ray diffraction, BN, AlN and CaB 6 were identified. The composition presumed by the calculation was 47.2 % by weight of BN, 39.0 % by weight of AlN, 8.1 % by weight of CaB 6 , and 5.7 % by weight of C. From the sintered product, two samples each obtained by splitting the product to 1/4 thereof and one sample obtained by splitting the product to 1/2 thereof were prepared, the former was supplied to a corrosion resistance test and from the latter, a test piece of 3 mm x 4 mm x 40 mm for a bending test was cut. On one of the samples for the corrosion test was placed cubic 13 Cr steel having a side length of 5 mm, on another sample for the corrosion test were placed tablet-formed powders ( SiO 2 : 35.2 % by weight, Al 2 O 3 : 4.1 % by weight, and CaO: 27.5 % by weight ) each having a diameter of 5 mm and a height of 5 mm, they were heated at a temperature-raising rate of 15 °C/minute up to 1,550 °C, and after maintaining them for 2 hours at 1,550 °C, they were allowed to cool. Also, after melting and solidifying, the angle of contact with the 13Cr steel and the angle of contact with the slag formed from the tablet-formed powders were measured. The angle of contact with the 13Cr steel was 110 ° and the angle of contact with the slag was 86 ° , which confirmed that the BN-containing material exhibited a good corrosion resistance to molten steel. Also, the result of the 3 point bending test of a span length of 30 mm was 34 MPa, which confirmed that the material was made usable as a refractory for casting. Example 3 A mixture of 18.2 % by weight of a CaB 6 powder having particle sizes of not larger than 44 µm, 19.1 % by weight of a B 4 C powder having particle sizes of not larger than 44 µm, 34.7 % by weight of a baddeleyite powder having particle sizes of not larger than 44 µm, and 28.2 % by weight of Al having particle sizes of not larger than 74 µm was packed in a graphite vessel having a cavity of a diameter of 60 mm and a height of 50 mm in an amount of 50 g and the packed powders were lightly pressed by applying a surface pressure of 10 MPa thereto via a graphite plate having a diameter of 60 mm and a thickness of 5 mm. The graphite vessel was disposed in a graphite heating furnace, after heating the graphite vessel at a temperature-raising rate of 15 °C /minute up to 1,600 °C while evacuating by means of a vacuum pump, a nitrogen gas was introduced to make the nitrogen gas pressure 1.0 MPa ( absolute pressure ), and after maintaining the system for 8 hours at 1,600 °C, the graphite vessel was allowed to cool. The weight of the sintered product obtained was 74.0 g, the bulk density thereof was 2.59 g/cm 3 , and the relative amount of voids was 11.8 %. Also, through X-ray diffraction, BN, AlN, 3Ca0·17ZrO 2 , and CaCN 2 were identified. The composition presumed by the calculation was 40.5 % by weight of BN, 28.7 % by weight of AlN, 23.3 % by weight of 3CaO · 17ZrO 2 , and 7.5 % by weight CaCN 2 . From the sintered product, two samples each obtained by splitting the product to 1/4 thereof and one sample obtained by splitting the product to 1/2 thereof were prepared, the former was supplied to a corrosion resistance test and from the latter, a test piece of 3 mm x 4 mm x 40 mm for a bending test was cut. On one of the samples for the corrosion test was placed cubic 13Cr steel having a side length of 5 mm, on another sample for the corrosion test were placed tablet-formed powders (SiO 2 : 35.2 % by weight, Al 2 O 3 : 4.1 % by weight, and CaO: 27.5 % by weight ) each having a diameter of 5 mm and a height of 5 mm, they were heated at a temperature-raising rate of 15 °C/minute up to 1,550 °C, and after maintaining them for 2 hours at 1,550 °C, they were allowed to cool. Also, after melting and solidifying, the angle of contact with the 13Cr steel and the angle of contact with the slag formed from the tablet-formed powders were measured. The angle of contact with the 13Cr steel was 100 ° and the angle of contact with the slag was 96 ° , which confirmed that the BN-containing material exhibited a good corrosion resistance to molten steel and molten slag. Also, the result of the 3 point bending test of a span length of 30 mm was 25 MPa, which confirmed that the material was made usable as a refractory for casting. Example 4 A mixture of 50.5 % by weight of a B4C powder having particle sizes of not larger than 44 µm and 49.5 % by weight of an Al powder having particle sizes of not larger than 74 µm was packed in a graphite vessel having a cavity of a diameter of 60 mm and a height of 50 mm in an amount of 50 g and the packed powders were lightly pressed by applying a surface pressure of 10 MPa via a graphite plate having a diameter of 60 mm and a thickness of 5 mm. The graphite vessel was disposed in a graphite heating furnace, after heating the graphite vessel at a temperature-raising rate of 15 °C/minute up to 1,600 °C while evacuating by means of a vacuum pump, a nitrogen gas was introduced to make the nitrogen gas pressure 1.0 MPa ( absolute pressure ), and after maintaining the system for 8 hours at 1,600 °C, the graphite vessel was allowed to cool. The weight of the sintered product obtained was 65.7 g, the bulk density thereof was 1.93 g/cm 3 , and the relative amount of voids was 7.1 %. Also, through X-ray diffraction, BN, AlN, and C were identified. The composition presumed by the calculation was 51.4 % by weight of BN, 42.4 % by weight of AlN, and 6.2 % by weight of C. From the sintered product, two samples each obtained by splitting the product to 1/4 thereof and one sample obtained by splitting the product to 1/2 thereof were prepared, the former was supplied to a corrosion resistance test and from the latter, a test piece of 3 mm x 4 mm x 40 mm for a bending test was cut. On one of the samples for the corrosion test was placed cubic 13 Cr steel having a side length of 5 mm, on another sample for the corrosion test were placed tablet-formed powders (SiO 2 : 35.2 % by weight, Al 2 O 3 : 4.1 % by weight, and CaO: 27.5 % by weight) each having a diameter of 5 mm and a height of 5 mm, they were heated at a temperature-raising rate of 15 °C/minute up to 1,550 °C, and after maintaining them for 2 hours at 1,550 °C, they were allowed to cool. Also, after melting and solidifying, the angle of contact with the 13Cr steel and the angle of contact with the slag formed from the tablet-formed powders were measured. The angle of contact with the 13Cr steel was 110 ° and the angle of contact with the slag was 90 ° , which confirmed that the BN-containing material exhibited a good corrosion resistance to molten steel and molten slag. Also, the result of the 3 point bending test of a span length of 30 mm was 110 MPa, which confirmed that the material was made usable as a refractory for casting. Example 5 A mixture of 49.1 % by weight of a B 4 C powder having particle sizes of not larger than 44 µm, 11.1% by weight of a SiO 2 powder having particles sizes of not larger than 10 µm, and 39.8% by weight of an Al powder having particle sizes of not larger than 74 µm was packed in a graphite vessel having a cavity of a diameter of 60 mm and a height of 50 mm in an amount of 50 g and the packed powders were lightly pressed by applying a surface pressure of 10 MPa thereto via a graphite plate having a diameter of 60 mm and a thickness of 5 mm. The graphite vessel was disposed in a graphite heating furnace, after heating the graphite vessel at a temperature-raising rate of 15 °C /minute up to 1,600 °C while evacuating by means of a vacuum pump, a nitrogen gas was introduced to make the nitrogen gas pressure 1.0 MPa ( absolute pressure ), and after maintaining the system for 8 hours at 1,600 °C, the graphite vessel was allowed to cool. The weight of the sintered product obtained was 80.0 g, the bulk density thereof was 2.44 g/cm 3 , and the relative amount of voids was 6.2 %. Also, through X-ray diffraction, BN, AlN, SiC, and C were identified. The composition presumed by the calculation was 55.1 % by weight of BN, 37.5 % by weight of AlN, 4.6 % by weight of SiC, and 2.5 % by weight of C. The residual carbon in this case corresponded to 38 % of C constituting B 4 C of the starting materials. From the sintered product, two samples each obtained by splitting the product to 1/4 thereof and one sample obtained by splitting the product to 1/2 thereof were prepared, the former was supplied to a corrosion resistance test and from the latter, a test piece of 3 mm x 4 mm x 40 mm for a bending test was cut. On one of the samples for the corrosion test was placed cubic 13 Cr steel having a side length of 5 mm, on another sample for the corrosion test were placed tablet-formed powders ( SiO 2 : 35.2 % by weight, Al 2 O 3 : 4.1 % by weight, and CaO: 27.5 % by weight ) each having a diameter of 5 mm and a height of 5 mm, they were heated at a temperature-raising rate of 15 °C/minute up to 1,550 °C, and after maintaining them for 2 hours at 1,550 °C, they were allowed to cool. Also, after melting and solidifying, the angle of contact with the 13Cr steel and the angle of contact with the slag formed from the tablet-formed powders were measured. The angle of contact with the 13Cr steel was 110 ° and the angle of contact with the slag was 104° , which confirmed that the BN-containing material exhibited a good corrosion resistance to molten steel and molten slag. Also, the result of the 3 point bending test of a span length of 30 mm was 98 MPa, which confirmed that the material was made usable as a refractory for casting. Example 6 A mixture of 33.4 % by weight of a B 4 C powder having particle sizes of not larger than 44 µm, 40.3 % by weight of a baddeleyite powder having particles sizes of not larger than 44 µm, 12.1 % by weight of a SiO 2 powder having particle sizes of not larger than 10 µm, and 14.2 % by weight of an Al powder having particle sizes of not larger than 74 µm was packed in a graphite vessel having a cavity of a diameter of 60 mm and a height of 50 mm in an amount of 50 g and the packed powders were lightly pressed by applying a surface pressure of 10 MPa thereto via a graphite plate having a diameter of 60 mm and a thickness of 5 mm. The graphite vessel was disposed in a graphite heating furnace, after heating the graphite vessel at a temperature-raising rate of 15 °C/minute up to 1,600 °C while evacuating by means of a vacuum pump, a nitrogen gas was introduced to make the nitrogen gas pressure 1.0 MPa ( absolute pressure ), and after maintaining the system for 8 hours at 1,600 °C, the graphite vessel was allowed to cool. The weight of the sintered product obtained was 64.8 g, the bulk density thereof was 1.89 g/cm 3 , and the relative amount of voids was 21.1 %. Also, through X-ray diffraction, BN, ZrO 2 ( monoclinic system ) AlN, SiC, and ZrN were identified. The composition presumed by the calculation was 46.3 % by weight of BN, 25.0 % by weight of ZrO 2 ( monoclinic system ), 18.5 % by weight of AlN, 6.2 % by weight of SiC, and 4.0 % by weight of ZrN. From the sintered product, two samples each obtained by splitting the product to 1/4 thereof and one sample obtained by splitting the product to 1/2 thereof were prepared, the former was supplied to a corrosion resistance test and from the latter, a test piece of 3 mm x 4 mm x 40 mm for a bending test was cut. On one of the samples for the corrosion test was placed cubic 13 Cr steel having a side length of 5 mm, on another sample for the corrosion test were placed tablet-formed powders ( SiO 2 : 35.2% by weight, Al 2 O 3 : 4.1 % by weight, and CaO: 27.5 % by weight ) each having a diameter of 5 mm and a height of 5 mm, they were heated at a temperature-raising rate of 15 °C/minute up to 1,550 °C, and after maintaining them for 2 hours at 1,550 °C, they were allowed to cool. Also, after melting and solidifying, the angle of contact with the 13Cr steel and the angle of contact with the slag formed from the tablet-formed powders were measured. The angle of contact with the 13Cr steel was 100 ° and the angle of contact with the slag was 105° , which confirmed that the BN-containing material exhibited a good corrosion resistance to molten steel and molten slag. Also, the result of the 3 point bending test of a span length of 30 mm was 100 MPa, which confirmed that the material was made usable as a refractory for casting. Example 7 A mixed powder was prepared by mixing 50.5 % by weight of a B 4 C powder having particle sizes of not larger than 44 µm and 49.5 % by weight of an Al powder having particle sizes of not larger than 74 µm. On the other hand, as shown in Fig. 1, a core 1 of a graphite mold having an annular cavity 4 was constructed with an outer frame 2 having a screw portion 3. A BN powder was coated on the contact portion with the raw material powders as a mold-releasing agent. The volume of the cavity was 240 cm 3 . The mixed powder 5 described above was packed in the cavity, the packed powders were pressed using a hand via a plate and then the powder was additionally supplied thereto to pack a total of 353 g of the powders into the cavity. Then, as shown in Fig. 2, an upper cover 6 having a screw was screwed to close the cavity. The graphite mold was disposed in a graphite heating furnace, after heating the graphite mold at a temperature-raising rate of 15 °C/minute up to 1,600 °C while evacuating by means of a vacuum pump, a nitrogen gas was introduced to make the nitrogen gas pressure 1.0 MPa ( absolute pressure ), and after heating the system for 8 hours at 1,600 °C, the graphite mold was allowed to cool. The nitrogen gas was introduced into the packed powders through the screw portion 3 of the graphite mold and the connected portion of the core 1 and the upper frame 2. When the graphite mold was decomposed, a sintered product 7 shown in Fig. 3 was obtained. The shape of the sintered product was the completely transferred shape of the cavity shape. The weight thereof was 462 g, the bulk density was 1.92 g/cm 3 , and the relative amount of voids was 7.2%. Comparative Example 1 A mixed powder was prepared by mixing 50.5 % by weight of a B 4 C power having particle sizes of not larger than 44 µm and 49.5 % by weight of an Al powder having particle sizes of not larger than 74 µm. The mixed powder was mono-axially press molded at a molding pressure of 30 MPa to provide a sintered product having a size of 20 mm x 23 mm x 50 mm and a weight of 35 g. Fig. 4 exhibits the shape of the sintered product 8 thus molded. The sintered product was disposed in a graphite heating furnace. After heating the sintered product at a temperature-raising rate of 15 °C/minute up to 1,600 °C while evacuating by a vacuum pump, a nitrogen gas was introduced to make a nitrogen gas pressure 1.0 MPa ( absolute pressure ), and after maintaining the product for 8 hours at 1,600 °C, the sintered product was allowed to cool. Thus, a sintered product 9 shown in Fig. 5 was obtained. The sintered product was largely deformed from the shape of the molding and the volume was increased. The weight was 46 g. When cut the sintered product, the presence of many cracks was observed. Thus, it was confirmed that when the molding was nitrified as it was, the molding was expanded, deformed, and cracked, whereby the production of parts was made very difficult. Comparative Example 2 A test piece of 30 mm x 30 mm x 20 mm was cut from the slag contact portion ( composition: 68 % by weight of ZrO 2 , 3 % by weight of SiC 3 , and 23 % by weight of C ) of an immersion nozzle for continuous casting for a steel plate, tablets of the slag having the same compositions and sizes as in Examples 1 to 6 each were placed on the center of the test piece as in Examples 1 to 6 , they were heated under the same conditions as in the examples ( retained for 2 hours in an Ar atmosphere at 1,550 °C ) and cooled slowly. The slag on the test piece thus treated almost wholly permeated into the inside the test piece and thus the angle of contact could not be measured. Thus, it was confirmed that the conventional immersion nozzle which was used at present was very liable to be corroded by a molten slag as compared with the boron-nitride-containing material of this invention which was resistant to being wetted with a molten slag. In addition, the production conditions and the characteristics in Examples 1 to 6 described above are shown in following Tables 1 to 2. Example 1 2 Starting Material Composition (wt%) CaB 6 27.8, B 4 C 29.3, Al 42.9 CaB 6 13.5, B 4 C 43.7, Al 42.8 Packed Weight (Graphite vessel inside diameter 50mm)(g) 50.0 50.0 Nitrification Condition N 2 Absolute Pressure MPa 1.0 1.0 Raising Temp. (°C/min) 15 15 Retention Temp. (°C) 1600 1600 Retention Time (hr.) 8 8 Sintered Product Weight (g) 87.1 83.4 Bulk Density (g/cm 3 ) 2.24 2.10 Porosity (%) 12.0 5.0 X-Ray Diffraction BN 52.8, AlN 37.4, CaCN 2 9.8 BN 47.2, AlN 39.0, CaB 6 8.1, C 5.7 Molten Steel Resistance (degree) 110 110 Molten Slag Resistance (degree) 88 86 3-Point Bending Strength (MPa) 32 34 Example 3 4 5 6 Starting Material Composition (wt%) CaB 6 18.2, B 4 C 19.1, AL 28.0, ZrO 2 34.7 B 4 C 50.5, Al 49.5 SiO 2 11.1, B 4 C 49.1, Al 39.8 SiO 2 12.1, B 4 C 33.4, ZrO 2 40.3, Al 14.2 Packed Weight (Graphite vessel inside diameter 50mm)(g) 50.0 50.0 50.0 50.0 Nitrification Condition N 2 Absolute Pressure MPa 1.0 1.0 1.0 1.0 Raising Temp. (°C/min) 15 15 15 15 Retention Temp. (°C) 1600 1600 1600 1600 Retention Time (hr.) 8 8 8 8 Sintered Product Weight (g) 74.0 65.7 80.0 64.8 Bulk Density (g/cm 3 ) 2.59 1.93 2.44 1.89 Porosity (%) 11.8 7.1 6.2 21.1 X-Ray Diffraction BN 40.5, AlN 28.7, 3CaO · 17ZrO 2 23.3, CaCN 2 BN 51.4, AIN 42.4, C 6.2 BN 55.1, AlN 37.8, SiC 4.6, C 2.5 BN 46.3, ZrO 2 (m) 25.0, AlN 18.5, SiC 6.2, ZrN Molten Steel 7.5 4.0 Resistance (degree) 100 110 110 100 Molten Slag Resistance (degree) 96 90 104 105 3-Point Bending Strength (MPa) 25 110 98 100

Claims: Method for producing a boron-nitride-containing sintered body comprising the steps of: preparing a mixed powder which comprises 10 to 70 wt.% B 4 C and 30 to 90 wt.% Al; packing the mixed powder in a packing ratio of 30 to 70% into a heat-resistant mold, the packed material being restrained by the heat-resistant mold; and heating the mixed powder in the heat-resistant mold in a nitriding atmosphere to produce a sintered body comprising boron nitride and AlN or AlON. Method of claim 1, wherein the mixed powder is heated to a temperature of 1300 to 2000°C. Boron-nitride-containing sintered body obtainable by the method of claim 1 or 2. Boron-nitride-containing sintered body of claim 3, wherein the boron-nitride-containing sintered body comprises AlN, BN and C. Boron-nitride-containing sintered body of claim 3, wherein the boron-nitride-containing sintered body comprises 22 to 90 wt.% BN, 8 to 68 wt.% AlN or AlON and 2 to 9 wt.% C. Boron-nitride-containing sintered body of claim 3, wherein the boron-nitride-containing sintered body comprises 22 to 90 wt.% BN, 4 to 48 wt.% AlN or AlON and 6 to 30 wt.% SiC.

Inventors: NISHIO HIROAKI, YOSHIDA HIROYUKI, NISHIO, HIROAKI, YOSHIDA, HIROYUKI, NISHIO, HIROAKI, C/O INT. PROP. DEPT., NKK CORP., YOSHIDA, HIROYUKI, C/O INT. PROP. DEPT., NKK CORP.
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1995
Main classification: C01B21
IPC codes: C04B35, C01B21