JP3375185B2 - Method for producing silicon nitride - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inexpensive method for producing high α-type silicon nitride which can be easily pulverized. [0002] In recent years, from the viewpoint of energy saving and high energy efficiency, silicon nitride sintered bodies have been studied as engine parts of automobiles such as turbo rotors, valves and swirl chambers and various industrial machine parts. Since these are used under severe conditions, the conditions required for the silicon nitride powder are also strict as follows. (1) The α phase is mainly used. (2) Submicron fine particles. (3) High purity. (4) Inexpensive. Various methods for producing silicon nitride have been proposed so far, and these are roughly classified into the following four methods. (A) A direct nitriding method in which metallic silicon is nitrided using a reaction gas such as nitrogen or ammonia. (B) A reduction nitriding method in which silica is nitrided using a reducing agent such as carbon and a reaction gas. (C) An imide thermal decomposition method for thermally decomposing silicon diimide generated from silicon tetrachloride. (D) A gas phase method in which a gas such as monosilane or silicon tetrachloride is reacted with a gas such as ammonia by heating such as laser or plasma. [0004] Of these, the most popular method at present is the direct nitriding method. In the direct nitriding method, the produced ingot is pulverized into silicon nitride powder, and the powder characteristics are strongly affected by the ingot. An ingot is an aggregate of silicon nitride particles synthesized from metal silicon powder. The metal silicon powder as a main raw material is usually formed into a compact for improvement in handling properties or charged as it is in a nitriding furnace. Is a relatively strong aggregate, or ingot. [0005] There are wet methods and dry methods for grinding ingots, but they have advantages and disadvantages. In wet grinding, post-processes such as purification, filtration, drying, and disintegration of the pulverized material are required. In addition, grinding of the hard material to be pulverized, such as silicon nitride, is performed for a long time. As a result, a purification step for removing mixed media and increased surface oxygen becomes indispensable.
Furthermore, this purification step is expensive because it is an acid treatment.
In contrast, dry grinding does not have such a problem,
The specific surface area does not increase so much, and there are problems such as mixing of abrasion powder of the medium and a large increase in surface oxygen. Unless classified to reduce the yield, coarse particles of several tens μm or more remain. Even if the coarse particles slightly remain, they become large defects of the sintered body and reduce strength and toughness. To cope with such a problem, there have been many proposals for controlling a nitriding reaction to produce a silicon nitride powder having a high α ratio and a high specific surface area. For example, control of the reaction gas partial pressure and the reaction temperature (for example, JP-A-48-102100, JP-A-49-94600, JP-A-52-11798, etc.), control of the heating rate (for example, Publication No. 4-54607,
JP-A-54-24300, JP-A-63-17203, etc.), addition of a nitriding catalyst (for example, JP-A-2-248309), control of porosity of a raw material compact (for example, -88109)
And so on, but not enough. As described above, according to the conventional method, relatively inexpensive silicon nitride which satisfies the above conditions (1) to (4) and can be used industrially is manufactured. It was difficult to do so, and the emergence of new technologies was awaited. The present inventors have conducted various studies in order to respond to such a demand, and as a result, completed the present invention. [0008] That is, the present invention provides a method for producing a nitride material having a thickness d containing metallic silicon powder in an atmosphere containing nitrogen and / or ammonia under the following conditions (1) to (4): And a method for producing silicon nitride. (1) d / 10 from the surface of the nitriding raw material up to the nitriding rate of 30%
(2) The reaction rate is 4% / hr or less. (3) The reaction rate at a nitriding rate of 10 to 90% is 0.5%.
(4) The rate of increase in the reaction rate at a nitridation rate of less than 50% should be 0.6% / hr ² or less. Hereinafter, the present invention will be described in more detail. The strict control of both the oxidation and nitridation reactions of metallic silicon is required to produce a suitable ingot. The oxidation reaction of metallic silicon occurs at a lower temperature than the nitriding reaction, and occurs thermodynamically in preference to the nitriding reaction under a partial pressure of oxygen and water vapor in a normal nitriding atmosphere, and particularly proceeds near the surface of the ingot. As a result, a uniform nitriding reaction is inhibited, and it becomes difficult to produce silicon nitride having a low oxygen content. Conventionally, the oxygen content of metallic silicon is limited (for example, Japanese Patent Application Laid-Open No. 63-151603), the amount of oxygen in the atmosphere (for example, Japanese Patent Application Publication No. 2-18284) and the amount of moisture (for example, 221617), but these are all aimed at producing low-oxygen-containing silicon nitride, and are not intended to produce uniform and easily pulverized ingots. It was not strict. On the other hand, it is known to control the nitridation reaction rate (for example, Japanese Patent Application Laid-Open No. 56-50170), but the oxidation reaction is not controlled, and the control of the metal silicon grain size is not sufficient. Therefore, the reaction proceeded unevenly in the ingot, and sufficient results could not be obtained even if the average reaction rate between single or multiple ingots was controlled. The present invention is to produce high α-type silicon nitride which is easily pulverized by simultaneously controlling the oxidation reaction and the nitridation reaction. First, the nitriding raw material used in the present invention is obtained by mixing 0 to 200 parts by weight, preferably 40 to 100 parts by weight of silicon nitride powder as an aggregate with 100 parts by weight of metal silicon powder. Or it is used as a molded article. The smaller the bulk density of the nitriding raw material, the easier the reaction control becomes, but the operating efficiency of the nitriding furnace is reduced.
It is preferable 1.2 g / cm ³ and particularly 0. 6~1.0g / cm ^3. The metal silicon powder used in the present invention is:
It is desirable that the powder is coarse powder in order to reduce the amount of oxygen present therein, and that the oxygen content is 0.5% by weight or less and the specific surface area is 1 m ² / g or less, particularly 0.7 m ² / g or less. Is preferred. However, if the particle size is too large, it will remain unnitrided, so that it is preferably 60 μm or less, particularly 45 μm or less. On the other hand, metallic silicon powder having a large specific surface area tends to cause a runaway reaction at an early stage.
It is desirable that the amount of the fine particles having a particle size of not more than μm is small, and the content thereof is not more than 5% by weight, and particularly those having a particle size in a range of 3 to 45 μm are preferable. On the other hand, since it is difficult to separate the silicon nitride powder of the aggregate after nitriding, the oxygen content is at most 2% by weight, preferably 1.5% by weight or less, especially 1.2% by weight. % By weight or less and a specific surface area of 4 m ² / g or more, particularly preferably 6 m ² / g or more. Also, in order to prevent the generation of coarse particles from the aggregate when crushing the ingot, an aggregate that does not contain easily crushable or coarse particles is used, and further, a low oxygen content, high-purity silicon nitride powder is produced. In such a case, a low oxygen content and high purity aggregate is used. By grinding or mixing metal silicon and silicon nitride in a non-oxidizing atmosphere such as nitrogen, hydrogen, or argon, a nitriding raw material having a specific surface area of 1 m ² / g or more can be used. In this case, sufficient consideration should be given to the contamination of impurities during the grinding and mixing, especially impurities due to the wear of the media, and the oxidation of metallic silicon.When manufacturing high-purity products, use media made of silicon nitride. However, it is desirable to carry out in a non-oxidizing and low humidity atmosphere. A feature of the present invention is that the thickness of the nitriding material is d
In this case, nitriding is performed under the following conditions (1) to (4). Here, the thickness d is a length in a direction far from the surface of the molded body of the nitriding raw material or the surface of the powder layer filled in the container, and is, for example, a radius for a spherical body. First, the condition (1) is that the oxygen content from the surface of the nitriding raw material to a depth of d / 10 is less than 2% by weight up to a nitriding rate of 30%. The oxidation reaction proceeds in the thickness direction because oxygen and moisture in the atmosphere react with metal silicon, and the oxidation reaction is particularly likely to be oxidized up to a depth of d / 10. Are made non-uniform. Particularly in the initial stage up to a nitriding ratio of 30%, the reaction is unstable and a runaway reaction easily occurs. As a result of various investigations, it was found that the uniform nitriding reaction was significantly inhibited by the nitriding raw material layer having an oxygen content of 2% by weight or more. The oxygen content up to a depth of 10 is less than 2% by weight, especially less than 1.5% by weight. A specific method of controlling such an oxidation reaction is as follows.
The purpose is to properly select the conditions of the nitriding raw material, atmosphere, and furnace material. The oxygen content in the ingot includes one contained in the nitriding raw material and one taken in from the atmosphere. If both are reduced, it can be reduced. In the prior art (for example, Japanese Patent Publication No. 2-18284, Japanese Patent Publication No. 2221617), the oxygen concentration and the water concentration in the nitriding furnace were controlled. The ratio is important. The measurement of the amount of oxygen and the amount of moisture in the atmosphere in the present invention can be performed by measuring the oxygen concentration and the water concentration and multiplying them by the gas volume of the nitriding furnace. The gas volume of the nitriding furnace is different from the normally used effective volume,
This is the volume including the heat insulating material, the heating element, and the like, and is the atmosphere that affects the reaction, that is, the volume from the inlet to the outlet of the reaction gas. This volume can be calculated from the increase in the furnace pressure when the outlet is shut off and the fixed gas is supplied into the furnace. In the present invention, the sum of the amount of oxygen and the amount of water in the nitriding furnace is 0.1 to 0.1% of the metal silicon content in the nitriding raw material.
It is desirable that the content be 2% by weight or less. For example, in a nitriding furnace having a volume of 1 m ³ , 100 ppm of oxygen and 1000 pp
m is measured, oxygen 0.14 g, water 0.8
Since 0 g means a total of 0.94 g, if the metal silicon content in the nitriding raw material is 1 kg, it becomes 0.094% by weight. The smaller this ratio is, the better, but 0.2%
When the content is less than 2% by weight, the oxygen content at a depth of up to d / 10 of the nitriding raw material can be controlled to less than 2% by weight. In such an atmosphere, if the nitridation rate is 30% or more at which the nitridation reaction is stable, oxidation of the ingot hardly occurs, and the apparent volume of the nitriding raw material hardly changes due to nitriding. The oxygen content up to d / 10 of the ingot can be replaced by that at a nitriding rate of 30%. That is, when the nitriding raw material is composed of 100 parts by weight of metal silicon and 50 parts by weight of silicon nitride powder, the weight increase at a nitriding rate of 30% is 13% with respect to the nitriding raw material, and is 44% with complete nitriding.
By measuring the oxygen content X% by weight of the ingot after nitriding, the oxygen content Y% by weight at a nitriding rate of 30% by weight can be calculated according to Y = 1.44X / 1.13. In the present invention, the control of the amount of oxygen and the amount of moisture in the atmosphere can also be performed by circulating a gas of low oxygen and low moisture to replace the inside of the furnace. However, ordinary furnace materials used in nitriding furnaces contain a large amount of oxygen, oxides, moisture, etc., and when heated, they tend to be released, so raise the temperature to reduce the effect. It is necessary to circulate a large amount of gas for a long time while replacing the inside of the furnace, which increases the cost. On the other hand, according to a method in which gas in the furnace is taken out by a pump, a blower, a compressor, or the like, passed through a deoxidizer or a dehumidifier, and then returned to the furnace, it is advantageous in cost. In this case, the oxidation reaction proceeds near the surface of the nitriding raw material when the amount of oxygen and moisture in the atmosphere increases, even if the temperature is higher than 300 ° C., even temporarily, and the capacity of the deoxidizer or dehumidifier is increased. It is important to control the rate of temperature rise in accordance with the amount of oxygen and moisture in the atmosphere. In order to reduce the amount of oxygen and moisture in the atmosphere, it is necessary to consider the structure of the nitriding furnace. In particular, the inner lining of the furnace is made of bricks or castables such as alumina or magnesia, or carbon or silicon nitride. It is desirable to use a non-oxide-based material such as a molded product and reduce the amount of oxygen and water generated from the furnace material to 1500 ppm or less, particularly 1000 ppm or less at the maximum. A large amount of air and moisture is adsorbed on alumina fiber, oxide-based ceramic fiber, rock wool, and the like, so use of a large amount thereof is not recommended. As for the heating element, molybdenum, tungsten, or the like having a purity of 99% by weight or more is desirable. In the present invention, by including hydrogen gas in the atmosphere, the surface oxide layer of the metal silicon particles can be reduced and removed, and a uniform nitriding reaction can be performed.
The hydrogen gas concentration is preferably 5% by volume or more, but if it is too high, the nitrogen concentration becomes insufficient and the nitridation reaction is inhibited. In the final nitridation after the nitriding is almost completed, the effect of the hydrogen gas is not so large, and the nitriding ratio is 95
% Or more, the hydrogen gas concentration may be less than 5% by volume. Part or all of the hydrogen gas can be replaced with ammonia. Suitable nitriding raw materials for reducing oxygen taken in from the nitriding raw materials have been described above. Next, a description will be given of the condition (2) in which the reaction rate is 4% / hr or less. The reaction rate in the present invention means that the metal silicon content in the nitriding raw material charged in the nitriding furnace is reduced by the nitriding reaction for a unit time. Per-consumption rate. For example, when a nitriding reaction is performed on a nitriding raw material containing 1 kg of metallic silicon, a reaction rate of 1% / hr means that 10 g of metallic silicon is nitrided per hour. Strictly speaking, the reaction rate should be determined based on the amount of metal silicon reacting in a unit of minute time, but its measurement is technically difficult. The time for the measurement is desirably as short as possible, and the time is 30 minutes or less, preferably 10 minutes or less, particularly 5 minutes or less. To determine the reaction rate, a simple and accurate method is to measure the amount of reaction gas consumed within a certain period of time and convert it to the weight assuming that 1.5 mole times of metal silicon has been consumed. In this case, the amount of the reaction gas consumed can be measured by determining the difference between the amount of the reaction gas supplied to the nitriding furnace and the amount of the reaction gas discharged outside the system, and the reaction discharged to the outside of the system can be measured. When there is no gas, the measurement can be performed by measuring changes in the gas composition in the furnace, changes in the furnace pressure, and the like within a certain period of time. For example, assuming that the consumed gas amount of 22.4 liters (1 mol) is measured in 5 minutes in terms of the standard state, the amount of metal silicon reacted during that time is 1.5 mol (42 g). Then, it becomes 504 g. At this time, if the filling amount of the metal silicon in the nitriding raw material is 20 kg, the reaction rate is 2.
5% / hr. Since the nitridation of metallic silicon is a large exothermic reaction, the calorific value increases as the reaction rate increases,
There is a variation in the reaction between the portion where heat is easily stored and the portion where heat is easily released. A large amount of heat causes sintering of the generated silicon nitride primary particles to be difficult to pulverize. In the present invention, the reaction rate is controlled to 4% / hr or less, preferably 3% / hr or less, with respect to the filling amount of the nitriding raw material. On the other hand, when the reaction rate is reduced, the reaction time is inevitably increased, and the furnace efficiency is reduced. For example, 1% /
The reaction time takes 100 hours if nitriding at an average rate of hr, and 1000 hours at 0.1% / hr. In the present invention, in order to achieve a practical furnace efficiency, a nitriding ratio of 1
The condition (3) is that the reaction rate at 0 to 90% is 0.5% / hr or more. The reaction rate may be less than 0.5% / hr in the reaction region other than the nitridation rate of 10 to 90%, that is, in the initial stage and the final stage of the reaction, but it does not matter. Since the nitridation rate in the present invention is an integral value of the reaction rate, it can be obtained from the integrated amount of consumed metallic silicon. The last condition (4) is to control the increase in the reaction rate. The increase in the reaction rate in the present invention refers to an increase in the above reaction rate per hour. For example, the reaction rate for a certain 10 minutes is 1.5% / hr,
If the reaction rate per minute is 1.6% / hr, the reaction rate increases by 0.1% / hr in 10 minutes, so the increase per hour is 0.6% / hr ² . A large increase in the reaction rate indicates that the nitridation is proceeding at an accelerated rate. Even if the reaction rate is 4% / hr or less, the reaction is not sufficiently controlled, and Can not achieve. Since such a phenomenon is more remarkable in the first half of the nitridation than in the second half, in the present invention, the increase in the reaction rate at a nitridation rate of less than 50% is strictly 0.6%.
/ Hr ² or less, preferably 0.5% / hr ² or less. In the present invention, the above-mentioned nitriding raw material conditions and reaction conditions are combined to work together to control the nitriding reaction and the oxidation reaction, thereby producing an easily crushable and high α-type silicon nitride ingot. be able to. The reaction gas used in the present invention is nitrogen and / or ammonia.
These are usually mixed with an inert gas such as argon or a hydrogen gas for controlling the reaction. To produce silicon nitride powder from the silicon nitride ingot produced according to the present invention, grinding is performed.
For the pulverization, either a wet method or a dry method can be employed, but the dry method is well suited for achieving the object of the present invention.
In this case, from the viewpoint of reducing oxygen and increasing the specific surface area, in a non-oxidizing atmosphere such as nitrogen, ammonia, and argon, a grinding aid capable of preventing aggregation of the silicon nitride powder, for example, triethylamine, n-butylamine, methyl ethyl ketone ,
It is preferable to pulverize by adding acetonitrile and the like,
The amount used is about 0.2 to 3% by weight. As the pulverizing medium, those containing silicon nitride as a main component are desirable. EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples. Examples 1 to 4 Comparative Examples 1 to 8 100 parts by weight of commercially available high-purity metallic silicon having the maximum particle size shown in Table 1 was mixed with silicon nitride powder (trade name “SN-9FW” manufactured by Denki Kagaku Kogyo Co., Ltd.). ) 20 parts by weight, and mixed by a ball mill using silicon nitride balls,
1 kg of this was filled into a 16 × 16 cm silicon nitride container to a thickness of 6 cm to obtain a nitriding raw material. This nitriding raw material is placed in a nitriding furnace whose gas volume has been measured in advance, and after evacuating and replacing with nitrogen gas, it is confirmed that the oxygen concentration is less than 200 ppm and the water concentration is less than 1000 ppm. And hydrogen gas to supply 20% by volume of nitrogen, 30% by volume of hydrogen, and 5% of argon.
The temperature was adjusted to an atmosphere of 0% by volume and the temperature was raised. The nitriding furnace uses a molybdenum heating element with alumina foam brick lining, and takes out the atmospheric gas at a rate of 200 Nl / min, heat exchanges, deoxygenates, dehydrates, and then returns to the furnace. It has something. Adjustment of the sum of the oxygen concentration and the water concentration and the sum of the oxygen amount and the water amount were performed by changing the temperature raising rate. Table 1 shows the results. The reaction rate was determined by measuring the amount of gas at the inlet and outlet of the nitriding furnace every 5 minutes with an integrating flow meter, and using the difference as the amount of consumed gas to determine the amount of metal silicon that participated in the reaction. The reaction rate is adjusted by adjusting the nitrogen concentration and temperature in the atmosphere.If you want to increase the reaction rate, increase the nitrogen concentration while raising the temperature.On the other hand, if you want to decrease the reaction rate, stop the temperature increase and lower the nitrogen concentration. Was. The hydrogen concentration was determined by setting the outlet gas to 10
It was measured by gas chromatography every minute. Table 2 shows these conditions. After the nitriding rate reached 100%, the temperature was released to room temperature while flowing nitrogen gas, and the ingot was taken out. It is sorted into a part having a depth of up to d / 10 (that is, a part having a depth of up to 6 mm from the surface) and another part, and the oxygen content of the part having a depth of up to d / 10 and an ingot obtained by mixing the two. The overall average oxygen content was measured. The ingot was coarsely and medium crushed to 0.2 mm or less in a mortar made of silicon nitride, and then added with 1.5% by weight of triethylamine as a grinding aid in a ball mill using balls made of silicon nitride. It was pulverized for a time to produce a silicon nitride powder. Various physical properties of the obtained ingot and silicon nitride powder were measured as follows. Table 3 shows the results. (1) Maximum particle diameter: Measured with a Cirrus granulometer using a dilute slurry ultrasonically dispersed in water for 20 minutes. (2) Oxygen content: Measured using a tin capsule with an oxygen / nitrogen simultaneous analyzer manufactured by LECO. (3) Specific surface area: Measured by a flow-type one-point method using a helium-nitrogen mixed gas as a standard gas with a cantasorb manufactured by Yuasa Ionics. (4) Residue of coarse particles: 200 g of a sample was added to 500 ml of water, and ultrasonically dispersed for 30 minutes.
The operation of sieving water with a sieve of μm was repeated three times, and the ratio of the dry weight of the residue on the sieve to the sample weight was measured. (5) Amount of residual metallic silicon: Diffraction line intensity I _a111 of silicon (111) plane and α of silicon nitride were _determined by powder X-ray diffraction.
The ratio of the phases I _a102 and I _a210 and the β phase I _b101 and I _b210 was calculated by the following equation. Residual metal silicon _{= I a111 / (I a102 +} I a210 + I b101
+ I _b210 ) (6) α conversion: calculated from the diffraction line intensities I _a102 and I _a210 of the α phase (102) plane and the (210) plane and I _b101 and I _{b210 of the} β phase by powder X-ray diffraction _according to the following equation. did. α conversion = [(I _a102 + I _a210 ) / (I _a102 + I _a210 + I
_b101 + _Ib210 )] × 100% Further, the sinterability of the silicon nitride powder was evaluated as follows. Y _{2 in} 100 parts by weight of silicon nitride powder
O ₃ powder 5 parts by weight of Al ₂ O ₃ powder 3 parts by weight of 1,
A 50% by weight slurry aqueous solution of the mixed powder is prepared using an organic binder and granulated with a spray drier.
After drying and die press molding, CIP molding of 2 ton / cm ² was performed. Sinter it at 1800 ° C for 4 hours
The 4-point bending strength at room temperature was measured according to R1601. Table 3 shows the results. [Table 1] (Note) The oxygen content at a depth up to d / 10 of the nitriding raw material at a nitriding rate of 30% was calculated from the oxygen content of an ingot having a nitriding rate of 100%. [Table 2] (Note) The maximum value of the reaction rate is a value at a nitriding ratio of 0 to 100%. The minimum value of the reaction rate is 10-90%
Is the value at. The rate of increase of the reaction rate is as follows.
This is the maximum value at 0%. The reaction time is 0 to 1
This is the time required between 00%. The maximum value and the minimum value of the hydrogen concentration in the atmosphere are values at a nitriding ratio of 0 to 95%. [Table 3] From Tables 1 to 3, it can be seen that the silicon nitrides of Examples 1 to 4 produced according to the present invention had a specific surface area of 10 m ² / g or more by ordinary dry pulverization, and no high-alpha particles were observed. Percent silicon nitride powder can be produced,
The strength of the sintered body obtained by sintering it was sufficiently high. On the other hand, metallic silicon (Comparative Examples 2 to 5)
3) The depth up to the nitriding raw material d / 10 at a nitriding rate of 30% because the oxygen concentration and moisture concentration in the atmosphere (Comparative Example 4) and the hydrogen concentration in the atmosphere (Comparative Example 8) were not properly adjusted. In the case where the oxygen content is more than 2% by weight and deviates from the range of the present invention, the produced silicon nitride powder contains a large amount of residual metallic silicon and coarse particles, and the sinter produced by using the same. Body strength will be small. In Comparative Example 5 in which the maximum value of the reaction rate is not appropriate and in Comparative Example 6 in which the initial rate of increase in the reaction rate is not appropriate, only an ingot having a low specific surface area in which the residual metal silicon is observed at a low α-rate is required. However, the properties of the silicon nitride powder were not good, and the strength of the sintered body was smaller than that of the example. Further, since the maximum particle diameter of metal silicon is large (Comparative Example 1) and the concentration of hydrogen in the atmosphere is high (Comparative Example 7), the minimum value of the reaction rate at a nitriding rate of 10 to 90% according to the present invention. If the reaction time is out of the range, the reaction time becomes extremely long, and if the time for gas replacement, heating, and furnace cooling is included, 1
Approximately two weeks were required for the nitriding treatment of the batch, resulting in poor productivity. According to the present invention, a relatively inexpensive metallic silicon is used, and a complicated post-treatment such as a special pulverizing step or a wet refining treatment is not required. High α-type silicon nitride powder can be manufactured.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-56-50170 (JP, A) JP-A-63-170267 (JP, A) JP-A-54-24300 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C01B 21/068 C04B 35/626

Claims (1)

(57) [Claim 1] A nitriding raw material having a thickness d containing metal silicon powder is nitrided under an atmosphere containing nitrogen and / or ammonia under the following conditions (1) to (4). A method for producing silicon nitride. (1) d / 10 from the surface of the nitriding raw material up to the nitriding rate of 30%
(2) The reaction rate is 4% / hr or less. (3) The reaction rate at a nitriding rate of 10 to 90% is 0.5%.
(4) The rate of increase in the reaction rate at a nitridation rate of less than 50% is set to 0.6% / hr ² or less.