JPH11302080A - Powdery boron nitride mixture and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a powdery boron nitride mixture having good fillability and sinterability and suitable for use as a material of BN-base refractories. SOLUTION: A boron carbide powder filled body is nitrided at a temp. of >=1,300 deg.C in a nitrogen atmosphere under >=1 kg/cm<2> partial pressure of nitrogen to obtain a sintered body contg. 50-85 wt.% BN. This sintered body is comminuted to obtain the objective powdery boron nitride mixture having >=40 μm max. particle diameter, good fillability and sinterability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a boron nitride mixed powder suitable for producing a BN refractory.

[0002]

2. Description of the Related Art Hexagonal boron nitride has high thermal conductivity, excellent electrical insulation, and excellent lubricating properties.
It is known as a chemically stable material that does not react with molten materials such as nickel, zinc, gallium, arsenic, glass, cryolite, and the like. It is stable up to 950 ° C. in air and up to 2200 ° C. in inert gas, and has a low Young's modulus, so it is easily elastically deformed and resistant to thermal shock. Further, since it has low hardness, it has a feature that, like metal, machining such as cutting and grinding can be easily performed. Taking advantage of such features, boron nitride is used for various purposes as a simple substance or a boron nitride-containing composite material. For use as a sintered body, it is used for insulating parts, heat-resistant parts, crucibles for molten metal, break rings for horizontal continuous casting, heat-radiating parts, setters for sintering metal or ceramic powder compacts, molds, and the like.

[0003] Application to refractories such as upper nozzles and immersion nozzles has also been attempted. For example, JP-A-63-84750 relates to a continuous casting nozzle, and particularly relates to a continuous casting suitable for continuous casting of deoxidized steel.
Disclosed is a nozzle for continuous casting, which comprises 10 parts by weight of aluminum nitride, 10 to 40 parts by weight of aluminum nitride, and 10 to 30 parts by weight of graphite. The effect of the invention is that a nozzle in which boron nitride, aluminum nitride, and graphite are blended at a predetermined ratio has a low wettability to molten steel, so that it is possible to prevent inclusions from adhering to the inner surface of the nozzle. Also, Interceram, Special Issue (1987), page 70,
52.6 wt% as a nozzle for continuous casting of boron nitride
BN, 27.0 wt% AlN, 2.0 wt% SiO ₂ ,
A composition of 17.5 wt% in total of C and SiC is disclosed. The porosity is 18.8%. Also, JP-A-04-143
No. 049 discloses that a specific calcium zirconate-based clinker, boron nitride, graphite, and a refractory raw material are limited to a limited weight%.
By using a continuous casting nozzle containing the nozzle, it is possible to prevent nozzle blockage due to alumina adhesion and to perform stable casting for a long time.

On the other hand, for MgO-C refractories, Japanese Patent Application Laid-Open No. 04-275973 mentions the addition of boron nitride. That is, MgO, which overcomes a major drawback of graphite, lack of oxidation resistance, has excellent oxidation resistance and corrosion resistance
To obtain -C refractories, one or more of the non-oxidizing boron compound such as B ₄ C and BN as the medium of resin or pitch between the layers or on the surface of the graphite B ₂ 0 ₃ in terms of It has been proposed to use composite graphite containing 5 to 40% by weight. By this, graphite itself has an antioxidant function, and the oxidation resistance and corrosion resistance of the MgO-C refractory containing the same are improved.

Japanese Patent Application Laid-Open No. 06-238409 relates to a ceramic mold for continuous casting, in which a portion of the inner wall surface where molten steel starts to solidify is an AlN sintered body, and the outer periphery of the AlN sintered body is a BN-based sintered body. A ceramic mold characterized by being covered with an outer wall of a body is disclosed. 19 wt% AlN-80 wt% BN-1 wt% Y as BN-based sintered body of outer wall
₂ O ₃ , and 19 wt% AlN-80 wt% BN-1w
The composition of t% MgO is shown. That is, AlN-
Main component of AlN constituting the BN based refractory material, using Y ₂ O _3, MgO as a binder for binding the particles of BN. Oxide binders are the main raw material powders AlN, BN
Reacts with the oxides of Al ₂ O ₃ and B ₂ O ₃ covering the surface of the alloy to form intergranular composite oxides, which reduce the intergranular distance and promote sintering shrinkage. Thus, an AlN-BN refractory material having a relatively high density and excellent air oxidation resistance can be obtained.

The boron nitride powder used for the BN-based refractory described above is generally prepared by mixing borax or boric anhydride with urea or dicyandiamide and nitriding in a synthesis furnace in an ammonia or nitrogen stream. Manufactured in. Since the boron nitride obtained in this manner contains many impurities and is very fine and difficult to handle, it is subjected to high-purity and crystallization treatments. For example, Japanese Patent Publication No. 05-35084 discloses 800
The coarse boron nitride fine powder obtained by synthesizing at about 1200 ° C. to about 1200 ° C. is placed in a non-oxidizing atmosphere of 20 Torr or more for 12
Heat at a temperature of 00-1800 ° C. As a result, grain growth is caused to some extent, and 10 nm to 5 μm depending on temperature.
With a boron nitride having a primary particle size of about range, the conversion of impurities such as B-O-N to BN and B ₂ O _3. Next, BN and the like are removed by heating to 1600 to 2000 ° C. in a non-oxidizing atmosphere of 20 Torr or less. This document describes that impurities such as B ₂ O ₃ are evaporated and removed without causing grain growth to control the primary particle diameter to a desired size. Although a step of achieving grain growth by crystallization of the ultrafine powder is performed, the resulting BN particle size is still small.

[0007]

The conventional boron nitride powder has an average particle size of 10 μm or less and a maximum particle size of 44 μm.
The specific surface area is as large as ² m ² / g or more. For this reason, as the amount of the boron nitride powder increases, it becomes necessary to significantly increase the amount of the binder. Further, the conventional boron nitride powder grows in the course of the crystallization treatment and becomes a plate-like particle with large anisotropy, so that the packing density is hardly increased, and the tap packing density is 0.8 g / cm ^3. Below and low. For this reason, if the amount of boron nitride is increased, the porosity of the filler increases. As described above, it is difficult to obtain a sintered body having a high density because of the fine powder and the highly anisotropic particle shape. further,
Boron nitride is difficult to sinter.

The present invention has been made in view of the above circumstances, and provides a boron nitride mixed powder having good filling properties and sinterability suitable for the production of BN-based refractories, and a method for producing the same.

[0009]

When boron carbide (B ₄ C) is heated in a nitrogen atmosphere, nitriding proceeds according to the following equation, and BN
And C are generated. This can be predicted from the chemical equilibrium data. It can also be expected that this is a very large exothermic reaction.

B ₄ C + 2N ₂ → 4BN + C The inventor of the present invention has found that, when a boron carbide powder filler is heated in a nitrogen atmosphere, a weight increase corresponding to nitriding occurs, and a part of the filler may be sintered to form a block. This led to the completion of the present invention.

That is, as the boron nitride mixed powder as described above, BN and B ₄ C
And the maximum particle size of BN is 40 μm or more. According to a second aspect of the present invention, there is provided the boron nitride mixed powder according to the first aspect, wherein BN and B ₄
The composition of C is BN 50-85 wt%, B ₄ C 5-40 w
t%.

According to a third aspect of the present invention, there is provided a method for producing a boron nitride mixed powder, comprising the steps of: nitriding a boron carbide powder filler to obtain a sintered body containing at least BN and B ₄ C; Pulverizing the sintered body to obtain a boron nitride mixed powder. According to a fourth aspect of the present invention, in the boron nitride mixed powder according to the third aspect, the composition of BN and B ₄ C is BN 50 to 85 w.
t4 and B4C in the range of 5 to ₄₀ wt%.

According to a fifth aspect of the present invention, the step of obtaining the sintered body according to the third aspect is characterized in that nitriding is performed in a nitrogen atmosphere at a temperature of 1300 ° C. or more and a nitrogen partial pressure of 1 kg / cm ² or more. And

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. In the production method of the present invention, the reaction for nitriding the boron carbide powder filling body proceeds such that the BN in the product is 50 to 85 wt% based on the weight of the product (100 wt%). Here, the reason for setting the content to 50 to 85 wt% is as follows. That is, since the main purpose is to generate BN and use it as a raw material of a BN-based refractory, the reaction proceeds so that BN in the product becomes 50 wt% or more. On the other hand, when unreacted B ₄ C remains, a mixture of BN, C, and B ₄ C is obtained, but it is better that a part of unreacted B ₄ C remains. This is because when B ₄ C is contained in the refractory, it has a beneficial effect on the refractory. B ₄ C is oxidized by air from 500 ° C and B ₂ O ₃
And CO gas are produced, but the B ₂ O ₃ melt is beneficial because it forms a film on the refractory surface and suppresses the progress of oxidation. In addition, since B ₄ C has an effect of promoting sintering of BN which is difficult to sinter, it is preferable that B ₄ C contains 5 to ₄₀ wt% to sinter BN of 50 wt% or more. 5 wt% in product by nitriding boron carbide powder packing
BN needs to be limited to 85 wt% in order to allow B ₄ C to remain. From the above, it is desirable that BN in the product of the nitriding reaction be 50 to 85 wt%.

The finer the boron carbide powder, the higher the reaction rate and the higher the proportion of BN in the product. BN50wt
%, The particle size of the boron carbide powder is 74 μm
It is preferable that the following weight ratio is 90% or more. More preferably, the weight ratio of 44 μm or less is set to 90% or more.

This reaction is preferably carried out at a temperature of 1300 ° C. or higher. This is because if the temperature is less than 1300 ° C., sintering does not occur even if the temperature of the filler rises due to heat generated by the reaction, and a massive product cannot be obtained. Increasing the nitriding temperature increases the proportion of lumps in the product, and at 2200 ° C. almost all lumps. Therefore, it does not make much sense to select an ambient temperature exceeding 2200 ° C.

The nitriding atmosphere may be N ₂ alone or a mixed gas, for example, an ammonia decomposition gas. It is preferable to use a nitrogen atmosphere in which the partial pressure of N ₂ is 1 kg / cm ² or more. This is because if the N ₂ partial pressure is less than 1 kg / cm ² , the reaction rate becomes too low and sintering becomes insufficient.

Thus, a boron nitride sintered body containing 50 to 85% by weight of BN is obtained. By grinding this boron nitride sintered body, the boron nitride mixed powder according to the present invention is obtained. Excessive pulverization is not preferable because BN-specific plate-like particles appear and the packing density decreases. A powder having a high packing density can be obtained by appropriate pulverization.
The standard is to set the maximum particle diameter of the boron nitride powder to 40 μm or more. Thus, tap filling density 1.0g /
It is possible to obtain a boron nitride mixed powder having a filling property of not less than ³ cm ³ .

The boron nitride mixed powder thus obtained has a good filling property and a good sinterability by containing B ₄ C, so that it can be effectively used as a raw material of a BN refractory.

Next, the use of the boron nitride mixed powder as a raw material of the BN refractory will be described. The boron carbide powder that has passed through a predetermined sieve is filled in a graphite tray, which is placed in an electric resistance heating furnace and maintained at 1300 ° C. while maintaining a vacuum.
The temperature is raised to the above-mentioned predetermined temperature, nitrogen gas is introduced at that temperature, the pressure is raised to a predetermined pressure of 1 kg / cm ² (absolute pressure) or more, and maintained for a predetermined time. After allowing to cool, the chunks and powder are separated from the tray and taken out. The lump is crushed with a jaw crusher and crushed through a vibrating crushing sieve so that the top size is 40 μm or more. The boron nitride mixed powder thus obtained is used as a raw material for a BN-based refractory.

FIG. 1 shows an apparatus for controlling the flow rate of molten steel sent from a tundish to a mold. This apparatus raises and lowers the flow rate of molten steel by moving up and down the gap between the cap nozzle 3 and the long stopper 4 constituting the flow path at the upper end of the immersion nozzle 2 attached to the furnace bottom 1 of a tundish, which is a molten steel container. It is a device for adjusting. The cap nozzle 3 is required to be made of a material such as alumina, to which nonmetallic inclusions contained in molten steel are unlikely to adhere.
BN refractories are suitable for this purpose.

Therefore, a method for manufacturing a cap nozzle of a BN refractory to which the boron nitride mixed powder according to the present invention is applied will be described. As shown in FIG. 2, a molding die 5 is filled with a mixed powder of an Al source powder and the above-mentioned boron nitride mixed powder, an upper lid 7 is attached, and then pressurized by CIP to consolidate the filler 6. A mold is obtained by releasing the mold.

This compact is placed in a nitriding furnace, heated while pressurizing with nitrogen, kept at a predetermined temperature, and cooled. Thus, a sintered body containing AlN-BN as a main component, that is, B
The cap nozzle 3 of the N-based refractory is obtained.

FIG. 3 shows an immersion nozzle 8 used for continuous casting of a slab. The molten steel flows into the mold from two discharge holes 10 through the inner hole 9 from above. The immersion nozzle 8 is such that non-metallic inclusions contained in molten steel, such as alumina, hardly adhere to and grow on the inner hole 9 and the discharge hole 10.
Is required not to be affected by mold powder. B
N-based refractories serve this purpose.

Therefore, a method for manufacturing a BN-based refractory immersion nozzle to which the boron nitride mixed powder according to the present invention is applied will be described. As shown in FIG. 4, a sleeve 12 made of graphite,
The lower cover 13 and the core 14 are assembled. The cavity is filled with a mixed powder of the Al source powder and the boron nitride mixed powder to obtain a filling body 15 and an upper lid 16 made of graphite is attached.

The packed body 15 is placed in a nitriding furnace while being kept in the graphite container, the temperature is increased while pressurizing with nitrogen, and the temperature is maintained at a predetermined temperature and then cooled. Thus, the obtained sintered body 15 containing AlN-BN as a main component is taken out, the discharge hole 10 shown in FIG. 3 is opened, and the BN refractory immersion nozzle 8 is obtained.

[0027]

EXAMPLES Example 1 A boron nitride powder was produced by the above-described method (maximum particle size: 61 to 74 μm). That is, 32
100.0 kg of boron carbide powder passed through a 0 mesh sieve
Was prepared. This was filled in a graphite tray, placed in a furnace, heated in vacuum to 1600 ° C., introduced with nitrogen at this temperature, and kept at 0.1 kg / cm ² (gauge pressure) for 2 hours. After cooling, the lump and the powder were taken out of the tray. Lump is 12
1.5 kg and 42.0 kg of powder. The composition estimated from the weight increase is 70.5 wt% BN, 21.0 wt%
B ₄ C and 8.5 wt% C. Among these, the lumps were crushed by a jaw crusher and crushed by a vibration crushing sieve to obtain a crushed powder in which 44% or more accounted for 5 wt%. The maximum particle size of this powder was found by sieving to be between 61 and 74 μm. The boron nitride mixed powder 50
packing density when vibration Shi decrease in fill volume has subsided vibration table put g graduated cylinder, i.e. the tap bulk density was 1.05 g / cm ^3.

A cap nozzle is made using the Al source powder and the pulverized boron nitride mixed powder as main raw materials. Al atomized powder having a metal Al content of 99.97 wt%, a metal Al content of 79.0 wt%, and a metal Mg content of 2.1 wt as an Al source powder
% Al polishing powder was used. Here, the Al polishing powder is a powder obtained by polishing aluminum scrap.

100 wt%, 105 μm or less, 37 μm
The above is Al atomized powder having a particle size of 20 wt%, 147 μm
Al whose particle size has been adjusted so that m is 95 wt% or more.
Polishing powder, 30.0 wt% of each powder of bulk pulverized boron nitride powder of 4 wt% or more of 5 wt%, 50.0 wt%
% And 20.0 wt%. This mixed powder was filled into a mold for a cap nozzle by the method described above, and CI
Pressure was applied at a molding pressure of 300 kg / cm ² using P to obtain a molded body. The packing density was 1.70 g / cm ³ .
The molded body was heated to 1750 ° C. under pressurized nitrogen at a nitrogen pressure of 9.1 kg / cm ² , kept for 3 hours, and allowed to cool. Thus, a sound sintered body was obtained. Bulk density 2.69 g /
cm ³ , 77.9 wt% AlN, 10.5 wt% BN, and a four-point bending strength of 13.0 kg / mm ^2.
Sufficient characteristics can be expected as a cap nozzle for N-based refractories.

The cap nozzle was subjected to actual casting of 13Cr steel, and continuous casting was performed at 5 charges, but no sign of nozzle clogging due to adhesion of alumina occurred. After use, the nozzle was collected and observed. As a result, no deposits were deposited on the inner hole surface, and the result was good.

Example 2 Al source powder and boron nitride mixed powder (maximum particle size: 61 to 74 μm, tap filling density: 1.0)
An immersion nozzle is made using 5 g / cm ³ ) as a main raw material. 10
100 wt% for 5 μm or less, 20 wt% for 37 μm or more
40.0 wt% each of two types of powders: an Al atomized powder having a particle size of 99.97 wt% and a metal Al content of 99.97 wt%;
% And 60.0 wt%. The mixed powder was filled in a mold by the method described above, and the packing density was 1.40 g / cm ^3.
Was obtained. This packed body is filled with a nitrogen pressure of 9.1 kg / c.
The temperature was raised to 1750 ° C. under pressurized nitrogen of m ² , kept for 3 hours, and allowed to cool. Thus, a sound sintered body was obtained. Bulk density 1.95 g / cm ³ , AlN 50.2 wt%, BN
With a composition of 35.4 wt%, a four-point bending strength of 1.48 kg /
mm ² , which can be expected to have sufficient characteristics as a BN-based refractory immersion nozzle.

This immersion nozzle was used for actual casting of low-carbon Al-killed steel, and continuous casting was performed at 5 charges, but no sign of nozzle clogging due to alumina adhesion was found.
After use, the nozzles were collected and observed. No deposits were deposited on both the inner surface and the discharge holes, and there was no wear. In addition, erosion by the mold powder on the outer surface was also slight.

(Example 3) Using the same raw materials as in Example 2, Al atomized powder and lump-crushed boron nitride powder were mixed at a ratio of 25.0 wt% and 75.0 wt%, respectively.
The mixed powder was filled in a mold by the method described above, and the packing density was 1.
A filling of 26 g / cm ³ was obtained. This packed body was heated to 1750 ° C. under pressurized nitrogen at a nitrogen pressure of 9.1 kg / cm ² , kept for 3 hours, and allowed to cool. Thus, a sound sintered body was obtained. Bulk density 1.67 g / cm ³ , AlN33.
It has a composition of 5 wt% and BN of 47.3 wt%, has a four-point bending strength of 1.46 kg / mm ² , and can be expected to have sufficient characteristics as a BN-based refractory immersion nozzle.

The immersion nozzle was used for actual casting of low-carbon Al-killed steel, and continuous casting was performed at 5 charges. However, no sign of nozzle clogging due to adhesion of alumina was found.
After use, the nozzles were collected and observed. No deposits were deposited on both the inner surface and the discharge holes, and there was no wear. In addition, erosion by the mold powder on the outer surface was also slight.

Example 4 Using the same raw materials as in Example 2, Al atomized powder and lump-crushed boron nitride powder were mixed at a ratio of 15.0 wt% and 85.0 wt%, respectively.
The mixed powder was filled in a mold by the method described above, and the packing density was 1.
A packing of 20 g / cm ³ was obtained. This packed body was heated to 1750 ° C. under pressurized nitrogen at a nitrogen pressure of 9.1 kg / cm ² , kept for 3 hours, and allowed to cool. Thus, a sound sintered body was obtained. Bulk density 1.52 g / cm ³ , AlN 21.
It has a composition of 0 wt% and BN of 56.1 wt%, has a four-point bending strength of 1.24 kg / mm ² , and can be expected to have sufficient characteristics as a BN-based refractory immersion nozzle.

This immersion nozzle was used for actual casting of low-carbon Al-killed steel, and continuous casting was performed at 5 charges, but no sign of nozzle clogging due to alumina adhesion was found.
After use, the nozzles were collected and observed. No deposits were deposited on both the inner surface and the discharge holes, and there was no wear. In addition, erosion by the mold powder on the outer surface was also slight.

(Comparative Example 1) An immersion nozzle is manufactured using Al source powder and BN source powder as main raw materials. Metal A as Al source powder
99.97wt% for 1 minute, 100wt for 105μm or less
%, An aluminum atomized powder having a particle size of 20 wt% of 37 μm or more, and a commercially available fine and high-purity BN powder produced by a conventional technique as a BN source powder were selected. That, BN purity 99.7 wt%, more particle diameter 37μm was used an average particle size 10 [mu] m, BN powder with inferior filling of the tap bulk density 0.30 g / cm ³ at 0 wt%.

The Al atomized powder and this BN powder were mixed at a ratio of 15.0 wt% and 58.0 wt%, respectively.
This mixed powder was filled in a mold by the method described above, and the packing density was set at 0.
A filling of 85 g / cm ³ was obtained. This packed body was heated to 1750 ° C. under pressurized nitrogen at a nitrogen pressure of 9.1 kg / cm ² , kept for 3 hours, and allowed to cool. Many cracks were observed in the vertical and horizontal directions in the sintered body thus obtained. The bulk density is as low as 1.07 g / cm ³ and the four-point bending strength is 0.18 k.
g / mm ² . Table 1 shows the raw material powders, production conditions and production results of Examples 1 to 4 and Comparative Example 1.

[0039]

[Table 1]

[0040]

As described above, according to the present invention, a boron nitride powder having a good filling property can be obtained, so that the use of the powder enables the density of the sintered body to be increased and a BN-based refractory having high strength to be obtained. .

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an apparatus for controlling a flow rate of molten steel sent from a tundish to a mold.

FIG. 2 is an explanatory view of a method for manufacturing a BN-based refractory cap nozzle to which the boron nitride powder according to the present invention is applied.

FIG. 3 is an explanatory view of an immersion nozzle used for continuous casting of a slab.

FIG. 4 is an explanatory view showing a method for manufacturing a BN-based refractory immersion nozzle to which the boron nitride powder according to the present invention is applied.

[Explanation of symbols]

1. Furnace bottom, 2. Immersion nozzle, 3. Cap nozzle, 4. Long stopper, 5. Mold, 6. Filler, 7. Top lid, 8. .. Submerged nozzle, 9 ... bore, 1
0 ... discharge hole, 11 ... outer surface, 12 ... sleeve, 1
3 ... Lower lid, 14 ... Core, 15 ... Filler, 16 ...
Top lid

Claims (5)

[Claims] 1. A method comprising at least BN and B ₄ C,
A boron nitride mixed powder having a maximum particle size of N of 40 μm or more. 2. The composition of BN and B ₄ C is BN 50-85.
The boron nitride mixed powder according to claim 1, wherein the content is ₅ wt% and B4C is _{5 to} 40 wt%. 3. A step of obtaining a sintered body containing at least BN and B ₄ C by nitriding the boron carbide powder-filled body, and a step of pulverizing the sintered body to obtain a boron nitride mixed powder. A method for producing a boron nitride mixed powder. _4. The composition of BN and B ₄ C is BN 50-85.
_{4. The} method for producing a boron nitride mixed powder according to claim 3, wherein the content of B ₄ C is _{5 to} 40 wt%. 5. 5. The process for producing a boron nitride mixed powder according to claim 3, wherein the step of obtaining the sintered body includes nitriding in a nitrogen atmosphere at a temperature of 1300 ° C. or more and a nitrogen partial pressure of 1 kg / cm ² or more. Method.