JP3754271B2 - Butterfly valve and manufacturing method thereof - Google Patents

Description

【００１９】
【表２】

【００２０】
表2 に示すように、本発明の実施例によるものは、摩耗痕跡深さが２０μｍ以下と非常に少なく、かつ摩耗痕跡周囲には割れ・チッピングの欠損が何れの場合も認められず、耐摩耗性、耐欠損性共に優れるが、比較例の各弁体は本発明の実施例に比べて、破損や破断が発生するまでの短期間の摩耗痕跡深さ８０μｍ以上と大きく、その上、ヒビ等の欠損が発生しており、耐摩耗性、耐欠損性が不充分であることが確認された。
【００２１】
【発明の効果】
以上述べたように、本発明のβ- Ｓｉ₃ Ｎ₄ 相、Ｓｉ₂ Ｎ₂ Ｏ相及びＹ₂ Ｓｉ₂ Ｏ₇ 相により構成される窒化珪素質セラミックス焼結体を成形加工してなるバタフライ弁は、熱的安定性、機械的安定性に優れ、長期耐久性を有することから、高温高圧環境下での長期信頼性の非常に優れたバタフライ弁である。そして、例えば製鉄用高炉羽口から吹き込む熱風ガスの流量調整等の弁として、本発明のバタフライ弁を使用すれば、長期間熱風流量調整に供することができ、高炉等の安定操業による生産性向上と共に製造コスト低減に寄与すること大である。
【図面の簡単な説明】
【図１】実施例の高炉熱風ガスの支管風量調節用バタフライ弁( 全長500mm)の模式図。
【図２】実施例におけるバタフライ弁の設置状況を示す模式図。
【符号の説明】
1 …バタフライ弁の軸部
2 …バタフライ弁の弁板[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a butterfly valve for controlling the flow rate of hot hot air.
[0002]
[Prior art]
Conventionally, butterfly valves have been used as valves for adjusting the flow rate of hot air gas blown from the tuyere of an iron blast furnace heated to a high temperature of 1200 ° C. or higher. This butterfly valve is generally a valve body composed of a disc-shaped valve plate and two rotating shafts arranged so as to face the circumferential portion, and is a casing fluid made of a refractory heat insulating material. The valve plate is rotatably supported by a rotating shaft in the flow path, and the flow rate is controlled by adjusting the degree of opening of the flow path by the rotation of the valve plate. Because of this structure, the butterfly valve is constantly exposed to high-pressure and high-speed gas fluid in addition to high temperature, and a rotational load is applied when the valve body is rotated for air volume adjustment. Furthermore, the operating environment is extremely harsh, such that the temperature difference between the hot air flow path and the bearing portion of the rotating shaft is large and irregular vibrations are always received. For this reason, as a material of the butterfly valve, application of ceramics has been studied because metal cannot withstand practical use.
[0003]
Japanese Utility Model Publication No. 2-332944 proposes a valve body of a butterfly valve in which a valve plate and a shaft are integrally formed of silicon nitride, sialon, silicon carbide, zirconia, alumina or mullite ceramics. ing. Japanese Patent Laid-Open No. 9-42472 discloses that a valve plate and a shaft portion are integrally formed of ceramics having a bending strength at 1200 ° C. of 294 N / mm ² (30 kg / mm ² ) or more, and the end portion of the shaft portion is formed. A butterfly valve in which a metal shaft is fitted by shrink fitting is proposed, and silicon nitride-based or silicon carbide-based dense ceramics are disclosed as ceramics. Attempts have been made to apply ceramics with excellent heat resistance and bending strength in this way. However, when used for butterfly valves of blast furnace tuyeres for iron making, the durability is short and it will break within a short period of time. Basic problems regarding gender are not solved.
[0004]
So far, silicon nitride-based sintered bodies have excellent fracture toughness but have low high temperature strength, thermal shock resistance, thermal fatigue resistance and hardness. For example, in a system in which yttrium oxide and aluminum oxide are added, excellent thermal shock resistance is obtained, but there are cases where heat resistance, toughness, and mechanical strength at high temperatures are inferior. Therefore, a melilite mineral phase (Y ₂ O ₃ · Si ₃ N ₄ compound) was generated in the sintered body disclosed in JP-A-56-59674 for the purpose of improving the characteristics at high temperatures. Trial of silicon nitride sintered body and silicon nitride sintered body in which zirconium oxide + yttrium oxide + silicon oxide is added and zirconium oxide is precipitated in the sintered body disclosed in JP-A-62-202864 It is known that it is effective for improving high-temperature strength. Further, a nitridation in which a J-phase (Si ₂ N ₂ O 2Y ₂ O ₃ ) solid solution exists in a grain boundary phase in a sintered body containing rare earth oxide and zirconium oxide disclosed in JP-A-62-246865. Silicon sintered bodies have been tried and are known to be effective in improving heat resistance, oxidation resistance, and static fatigue characteristics. Further, in JP-A-3-155574, for the purpose of improving the high temperature strength characteristics, HfO ₂ was added as a sintering aid for sialon, and Y ₂ Hf ₂ O ₇ was produced as a grain boundary phase. -β 'Sialon is disclosed.
[0005]
However, although the above material has excellent high-temperature immediate breaking strength, it has not yet dramatically improved toughness and oxidation resistance while maintaining high-temperature strength, so particles in a severer use environment, particularly in high-temperature combustion flames. When applied to a structural member in which such a collision occurs, there are problems such as lack of reliability, which impedes practical application. Accordingly, there is a demand for materials that have improved oxidation resistance, thermal shock resistance and toughness in addition to improved high-temperature strength.
[0006]
[Problems to be solved by the invention]
In this way, in the environment where the butterfly valve is used, not only mere heat resistance and high temperature strength, but also particle collision damage, oxidation resistance, thermal fatigue caused by dust particles such as dust and refractory particles contained in the gas fluid It is necessary to have excellent physical and chemical stability and mechanical stability such as characteristics, mechanical durability against vibration during torsion during rotation and air flow fluctuation, and fracture resistance during handling. It has been desired to provide a butterfly valve having a material with excellent characteristics.
[0007]
Accordingly, the present invention solves the problems of the above-mentioned conventional ceramic butterfly valve, and has excellent physical and chemical stability, thermal stability, mechanical stability, and has a long-term durability, and its manufacture It aims to provide a method.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, the present inventors have intensively studied the crystal phase constituting the silicon nitride ceramic sintered body, and as a result, the β-Si ₃ N ₄ phase, the Si ₂ N ₂ O phase The inventors have found that a sintered body having excellent characteristics as a butterfly valve can be obtained when it is composed of three types of crystal phases of Y ₂ Si ₂ O ₇ phase, and has completed the present invention.
[0009]
That is, the present invention
(1) substantially β-Si ₃ N ₄ phase, a Si ₂ N ₂ O phase and Y ₂ Si silicon nitride sintered body consisting of ₂ O ₇ phase, Si ₂ N ₂ O phase and a grain boundary phase It is composed of Y ₂ Si ₂ O ₇ phase, β-Si ₃ N ₄ average crystal grain size is 1 to 3 μm, aspect ratio is 1.5 to 10, and β-Si ₃ N ₄ columnar crystal grains are entangled A butterfly valve characterized by being formed by processing what is an
(2) The composition of the silicon nitride-based sintered body is an Si ₂ N ₂ O phase having a composition of 0.1 to 3% by mass, a Y ₂ Si ₂ O ₇ phase of 4.9 to 12% by mass and the balance being β-Si _3. The butterfly valve according to (1), comprising N ₄ phases,
(3) The butterfly valve according to (1) or (2), wherein the relative density of the silicon nitride sintered body is 95% or more,
(4) A molded powder comprising _{3 to} 10% by mass of yttrium oxide (Y ₂ O ₃ ), 1 to 5% by mass of silicon oxide (SiO ₂ ), and the balance being silicon nitride (Si ₃ N ₄ ), and the molded product Is sintered in a temperature range of 1700 to 2000 ° C. in a nitrogen gas atmosphere,
(A) The temperature lowering rate in the temperature lowering process of sintering is 5 ° C./min or less,
In the temperature lowering process of (b), holding at a temperature range of 1350 to 1650 ° C. for 2 hours or more,
(C) performing a reheating treatment for 2 hours or more in a temperature range of 1350 to 1650 ° C. in a nitrogen atmosphere after sintering;
(1) to (3), wherein a silicon nitride sintered body in which a Si ₂ N ₂ O phase and a Y ₂ Si ₂ O ₇ phase are formed as grain boundary phases by at least one of A method for manufacturing the butterfly valve according to any one of the above ,
It is.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
As a result of intensive analysis of the wear state of ceramic butterfly valves that have been used in the past, the present inventors have found that when high-temperature and high-pressure gas fluids are circulated at high speed, materials with poor oxidation resistance are resistant to the surface. It was found that an oxide layer having poor wear properties was formed, and that the oxide layer was easily worn and consumed. In addition, chipping and cracking defects are often found around the worn part, and these defects are generated and propagated due to thermal fatigue and mechanical impacts associated with particle collisions in the fluid, leading to damage to the butterfly valve. I also found out. These wear and defects were particularly noticeable when the butterfly valve material was inferior in oxidation resistance and had low toughness and thermal shock resistance characteristics. Therefore, in order to use the butterfly valve stably for a long period of time, it is necessary to improve the wear resistance and fracture resistance at the same time. To that end, it is excellent in oxidation resistance and thermal shock resistance and is made of a tough material. It is essential to use ceramics. Unlike ceramic sintered bodies mainly composed of alumina, zirconia, etc., silicon nitride sintered bodies have excellent heat resistance and can maintain mechanical strength at high temperatures. It is most suitable as a material.
[0011]
Therefore, in order to improve these characteristics at the same time, silicon nitride-based sintered bodies composed of various crystal phases were produced, and the characteristics were evaluated. A conventional silicon nitride sintered body having a low melting point glass phase is inferior in oxidation resistance and thermal shock resistance at high temperatures. As a result of characteristic evaluation, it was found that a dense ceramic sintered body composed of β-Si ₃ N ₄ phase and Si ₂ N ₂ O phase and Y ₂ Si ₂ O ₇ phase as grain boundary phases has excellent characteristics. It was. In particular, a butterfly valve formed by processing a silicon nitride sintered body composed of β-Si ₃ N ₄ phase, Si ₂ N ₂ O phase and Y ₂ Si ₂ O ₇ phase has excellent oxidation resistance and thermal shock resistance, It has the characteristics of enhancing the static fatigue characteristics due to the temperature gradient generated in the valve body under the usage environment and the thermal stress fracture resistance characteristics associated with the rapid cooling during resting. In order to crystallize the Si ₂ N ₂ O phase and the Y ₂ Si ₂ O ₇ phase as the grain boundary phase, cooling is performed at a temperature lowering rate of 5 ° C / min or lower during the temperature lowering process of sintering, or 1350 ~ Either heat treatment at 1650 ° C. for 2 hours or more, or at least one of reheating treatment at 1350-1650 ° C. for 2 hours or more in a nitrogen atmosphere after sintering. When the Si ₂ N ₂ O phase and the Y ₂ Si ₂ O ₇ phase are precipitated in the temperature lowering process, the temperature decreasing rate is preferably 5 ° C./min or less, more preferably 2 ° C./min or less. When the cooling rate is faster than 5 ° C / min, the Si ₂ N ₂ O phase and the Y ₂ Si ₂ O ₇ phase are not sufficiently formed. In addition, when the holding temperature during the temperature lowering process and the holding temperature during the reheating process are less than 1350 ° C and higher than 1650 ° C, the Si ₂ N ₂ O phase and the Y ₂ Si ₂ O ₇ phase are also sufficient. Do not generate. Further, even when each holding time is less than 2 hours, the Si ₂ N ₂ O phase and the Y ₂ Si ₂ O ₇ phase are not generated. If the Si ₂ N ₂ O phase and the Y ₂ Si ₂ O ₇ phase are less than 0.1% and less than 4.9% in mass ratio, respectively, the porosity in the sintered body will be undesirably high, and if it exceeds 3% and 12%, β- The Si ₃ N ₄ crystal grains are not sufficiently entangled and the strength and toughness are lowered, which is not preferable. In addition, regarding the Si ₂ N ₂ O phase and the Y ₂ Si ₂ O ₇ phase, if the mass ratio of the Si ₂ N ₂ O phase is less than 0.1% of the total, the effect of contributing to mechanical strength is small, and if it exceeds 3%, β -It is not preferable because Si ₃ N ₄ crystal grains are not sufficiently entangled and strength and toughness are lowered. Similarly, if the mass ratio of the Y ₂ Si ₂ O ₇ phase is less than 4.9% of the total, the liquid phase at the α → β transition of Si ₃ N ₄ is small and the phase transition does not proceed smoothly, and if it exceeds 12%, β- This is not preferable because Si ₃ N ₄ crystal grains are not sufficiently entangled and strength and toughness are lowered. The silicon nitride sintered body obtained by the present invention has a β-Si ₃ N ₄ average crystal grain size of about 1 to 3 μm, an aspect ratio of about 1.5 to 10 and a β-Si ₃ N ₄ columnar shape. It has a structure in which crystal grains are intertwined, and high melting point Si ₂ N ₂ O phase and Y ₂ Si ₂ O ₇ phase are precipitated at the grain boundary, so it has high toughness while maintaining high strength up to high temperature. With a bending strength of 500MPa or higher at 1400 ° C in the atmosphere and a toughness value K _IC of 5MPam ^1/2 , it is suitable for butterfly valves that require characteristics in high-temperature environments. Can be used. Here, the Si ₂ N ₂ O phase has the same type of X-ray diffraction pattern as the Si ₂ N ₂ O crystal identified by the powder X-ray diffraction method, and is a compound composed of Si ₃ N ₄ and SiO _2. It is the most stable compound in a high-temperature oxidizing atmosphere. Similarly, the Y ₂ Si ₂ O ₇ crystal phase has the same type of X-ray diffraction pattern as the Y ₂ Si ₂ O ₇ crystal identified by the powder X-ray diffraction method, and is a compound composed of Y ₂ O ₃ and SiO ₂ Among them, it is the most stable compound in a high-temperature oxidizing atmosphere. The β-Si ₃ N ₄ crystal phase has the same type of X-ray diffraction pattern as the β-Si ₃ N ₄ crystal shown by the JCPDS card 333-1160. Furthermore, the relative density of the silicon nitride sintered body composed of the β-Si ₃ N ₄ phase, Si ₂ N ₂ O phase and Y ₂ Si ₂ O ₇ phase should be 95% or more of the theoretical density. Is desirable. If the relative density is less than 95%, the thermal stability and mechanical stability tend to be insufficient, and there is a high possibility that the effect of improving long-term durability is not observed.
[0012]
As the silicon nitride powder used in the present invention, Si ₃ N ₄ powder having an α-type crystal structure is preferable from the viewpoint of sinterability, but β-type or amorphous Si ₃ N ₄ powder is included. It doesn't matter. In order to obtain a sufficiently high density during sintering, fine particles having an average particle diameter of 1 μm or less are desirable. Since silicon nitride is a substance having strong covalent bonding and is often difficult to sinter alone, generally a sintering aid is added for densification. In the present invention, silicon oxide and yttrium oxide are used as the sintering aid. Here, yttrium oxide has a function of accelerating the crystal phase transition during the sintering of Si ₃ N ₄ alpha-Si ₃ from N ₄ phase to the beta-Si ₃ N ₄ phase in its molten in further beta-Si It is known to improve high temperature strength and toughness by promoting the growth of ₃ N ₄ columnar phase. The addition amounts of silicon oxide are preferably 1 to 5% by mass and yttrium oxide is preferably 3 to 10% by mass. When silicon oxide is less than 1% by mass, the liquid phase generation temperature at the time of sintering temperature rises and a sufficiently dense sintered body cannot be obtained, and the Si ₂ N ₂ O phase and the Y ₂ Si ₂ O ₇ phase Not formed. If it exceeds 5% by mass, the Y ₂ Si ₂ O ₇ phase is not formed, and a relatively low melting point SiO ₂ phase is formed, resulting in a decrease in mechanical strength at high temperatures. If the amount of yttrium oxide added is less than 3% by mass, melt formation is insufficient and the relative density is less than 95%, and densification does not proceed. When the added amount of yttrium oxide exceeds 10% by mass, the Y ₂ Si ₂ O ₇ phase is not formed, and a relatively low melting point Y ₂ SiO ₅ phase is formed. The mechanical strength of the obtained sintered body at high temperature And the oxidation resistance decreases. Both silicon oxide and yttrium oxide are preferably fine particles having an average particle size of 2 μm or less in order to obtain a homogeneous and high-density sintered body. These raw material powders used as sintering aids are relatively inexpensive, and are stable ceramic powders that do not change even during the mixing process in water. As a sintering method, various sintering methods such as a pressureless sintering method, a gas pressure sintering method, a hot isostatic pressing sintering method, a hot pressing sintering method, etc. are performed in an atmosphere containing nitrogen gas. A plurality of these sintering methods may be combined. The reason why sintering is performed in an atmosphere containing nitrogen gas is to suppress decomposition of Si ₃ N ₄ during sintering. Si ₃ N ₄ decomposes at about 1850 ° C or higher under 1 atmosphere of nitrogen gas, so when sintering at 1850 ° C or higher, the nitrogen gas pressure is the critical decomposition pressure of Si ₃ N _{4 at} the sintering temperature. Set as above. In addition, when manufacturing a large butterfly-shaped butterfly valve, it is possible to perform hot isostatic pressing in a nitrogen gas atmosphere after pressureless sintering in order to achieve sufficient densification. More preferred. As pressureless and hot isostatic pressing sintering conditions, the sintering temperature is desirably 1700 to 2000 ° C. When the temperature is less than 1700 ° C., a dense sintered body cannot be obtained, and it becomes difficult to generate sufficient residual stress in the vicinity of solid solution particles, so that a high toughness sintered body cannot be obtained. On the other hand, at a high temperature exceeding 2000 ° C., the β-Si ₃ N ₄ crystal grains become coarse and the strength is lowered, and high hardness and thermal shock resistance cannot be obtained. Further, if the holding time is less than 8 hours, the densification does not proceed sufficiently although it depends on the thickness of the molded body.
[0013]
【Example】
Next, examples of the present invention will be described together with comparative examples.
(Examples 1 to 3)
Silicon nitride (Si ₃ N ₄ ) powder (α conversion 97% or more, purity 99.7%, average particle size 0.3 μm), yttrium oxide (Y ₂ O ₃ ) powder (average particle size 1.5 μm), silicon oxide (SiO ₂ ) Powder (average particle size: 0.3 μm) was added in a predetermined amount (mass%) shown in Table 1, and purified water or acetone was used as a dispersion medium and kneaded for 24 hours in a ball mill with silicon carbide ceramics attached thereto. The amount of purified water or acetone added was 120 g with respect to 100 g of all ceramic powder raw materials.
[0014]
Next, the obtained mixed powder was molded and then sintered. The molding conditions were a pressure of 150 MPa by cold isostatic pressure, and a flat plate of 250 mm × 700 mm × 65 mm thick was molded. This was processed to obtain a molded body having a shape of two isometric shaft diameters of φ55 mm × length of 220 mm arranged so as to face the outer peripheral portion of the valve body diameter of φ220 mm × thickness of 28 mm. As sintering conditions, non-pressure sintering was carried out for 8 hours at the temperature shown in Table 1 in the flow of nitrogen gas. Furnace cooling was performed. For Example 3, after allowing to cool, the temperature was reheated to 1500 ° C. and held as shown in Table 1. From the obtained sintered body, as shown in FIG. 1, two isometric shaft portions of diameter φ40 mm × length 170 mm arranged so as to face the outer peripheral portion of the valve plate 2 having a diameter φ160 mm × thickness 20 mm. 1 was ground and subjected to an endurance test in hot air.
[0015]
Test pieces of various shapes were cut out from the obtained sintered body, and mechanical properties were evaluated. The bending strength was measured according to JIS R1601 at room temperature in the atmosphere and 1400 ° C. The hardness was measured as Vickers hardness with an indentation load of 98N. As for toughness, the fracture toughness value K _IC was measured at room temperature by the SEPB method of JIS R1607. In addition, the thermal shock resistance was evaluated by a rapid cooling temperature difference ΔT at which a bending test piece was heated to a predetermined temperature in the air and then rapidly cooled in water and the bending strength began to deteriorate. The sintered body density was measured as a relative density by the Archimedes method. The ratio of various crystal phases was determined according to a calibration curve obtained in advance from the X-ray diffraction peak height, and is shown in Table 1.
[0016]
Table 2 shows the various characteristics of the obtained sintered bodies. In the hot air gas ventilation test, the gas components were air + oxygen 3%, gas pressure 0.3 MPa, gas temperature 1200 ° C., tuyere ventilation speed 120 m / sec. As shown in Fig. 2, the valve body is fixed at an angle of 45 ° to the ventilation direction (valve = half-open), and after two months of ventilation, the depth of wear marks generated on the outer periphery of the valve body The height h was measured with a projection microscope. In addition, the presence or absence of damage around the wear trace, the chipping depth, and the crack depth were evaluated by fluorescent flaw detection and observation of the cross-section polished surface with an optical microscope.
[0017]
(Comparative Examples 4 to 5)
In Comparative Examples 4 to 5, the same raw materials as in Examples 1 to 3 were used and prepared in the same manner with purified water or acetone. However, when the sintering conditions at the time of cooling were inappropriate and the relative density was less than 95% (Comparison Example 4) is a comparative example in which the addition ratio of the sintering aid (Y ₂ O ₃ ) is inappropriate and the relative density is less than 95% (Comparative Example 5). These are shown together in Table 1. The materials of these comparative examples were also subjected to a ventilation test under the same conditions as in Examples 1 to 3, and the results are shown in Table 2.
[0018]
[Table 1]

[0019]
[Table 2]

[0020]
As shown in Table 2, according to the embodiment of the present invention, the wear trace depth is very small as 20 μm or less, and no cracks or chipping defects are observed around the wear trace. In comparison with the examples of the present invention, each valve body of the comparative example is large with a short wear mark depth of 80 μm or more until breakage or fracture occurs, and cracks etc. It was confirmed that there was insufficient wear resistance and chipping resistance.
[0021]
【The invention's effect】
As described above, the butterfly valve formed by molding the silicon nitride ceramic sintered body composed of the β-Si ₃ N ₄ phase, Si ₂ N ₂ O phase and Y ₂ Si ₂ O ₇ phase of the present invention. Is a butterfly valve with excellent long-term reliability in a high-temperature and high-pressure environment because of excellent thermal stability and mechanical stability and long-term durability. And, for example, if the butterfly valve of the present invention is used as a valve for adjusting the flow rate of hot air gas blown from the blast furnace tuyere for iron making, etc., it can be used for long-term hot air flow rate adjustment, and productivity improvement by stable operation of the blast furnace etc. At the same time, it greatly contributes to the reduction of manufacturing costs.
[Brief description of the drawings]
FIG. 1 is a schematic view of a butterfly valve (total length: 500 mm) for adjusting branch air volume of blast furnace hot air gas according to an embodiment.
FIG. 2 is a schematic diagram showing an installation situation of a butterfly valve in the embodiment.
[Explanation of symbols]
1… Butterfly valve shaft
2… butterfly valve plate

Claims (4)

Substantially β-Si ₃ N ₄ phase, Si ₂ N ₂ O phase and Y ₂ Si ₂ consists O ₇ phase A sintered silicon nitride, Si ₂ N ₂ O phase and Y ₂ Si as grain boundary phase It is composed of ₂ O ₇ phase, β-Si ₃ N ₄ average grain size is 1 to 3 μm, aspect ratio is 1.5 to 10, and β-Si ₃ N ₄ columnar crystal grains are intertwined. butterfly valve, characterized by comprising a certain thing by molding. The composition of the silicon nitride sintered body is a Si ₂ N ₂ O phase of 0.1 to 3% by mass, a Y ₂ Si ₂ O ₇ phase of 4.9 to 12% by mass, and the remainder is a β-Si ₃ N ₄ phase. The butterfly valve according to claim 1. The butterfly valve according to claim 1 or 2, wherein the silicon nitride sintered body has a relative density of 95% or more. A mixed powder composed of _{3 to} 10% by mass of yttrium oxide (Y ₂ O ₃ ), 1 to 5% by mass of silicon oxide (SiO ₂ ) and the balance of silicon nitride (Si ₃ N ₄ ) is molded, and the molded product is converted into nitrogen gas. Sintered in the temperature range of 1700-2000 ° C in the atmosphere,
(A) The temperature lowering rate in the temperature lowering process of sintering is 5 ° C./min or less,
In the temperature lowering process of (b), holding at a temperature range of 1350 to 1650 ° C. for 2 hours or more,
(C) performing a reheating treatment for 2 hours or more in a temperature range of 1350 to 1650 ° C. in a nitrogen atmosphere after sintering;
4. The silicon nitride sintered body in which a Si ₂ N ₂ O phase and a Y ₂ Si ₂ O ₇ phase are formed as grain boundary phases by at least one of the above, is formed and processed . A method for producing a butterfly valve according to claim 1.

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