JP2012142400A - METHOD OF MANUFACTURING ANTICORROSIVE R-Fe-B-BASED SINTERED MAGNET - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing an R-Fe-B-based sintered magnet which contains only a small amount of RH or does not contain the RH, and has an excellent corrosion resistance.SOLUTION: The method of manufacturing an R-Fe-B-based sintered magnet includes a step for performing heat treatment at 200-450°C in an atmosphere where the oxygen partial pressure is 1×10-1×10Pa, the water vapor partial pressure is lower than 1000 Pa and the ratio of the oxygen partial pressure and the water vapor partial pressure (oxygen partial pressure/water vapor partial pressure) is 1-20000 for an R-Fe-B-based sintered magnet containing 4.5 mass% or less of heavy rare-earth elements (at least one kind selected from RH:Dy and Tb). Temperature of the magnet is lowered from the heat treatment temperature at an average cooling rate of 650°C/hour or higher at least until 100°C.

Description

  The present invention relates to a method for producing a corrosion-resistant R—Fe—B based sintered magnet. More specifically, the present invention relates to a method for producing an R—Fe—B based sintered magnet having a low content of heavy rare earth elements (RH: at least one selected from Dy and Tb) or not containing RH and excellent in corrosion resistance. .

R-Fe-B-based sintered magnets represented by Nd-Fe-B-based sintered magnets are widely used today because they use resource-rich and inexpensive materials and have high magnetic properties. Although it is used in various fields, since it contains a highly reactive rare earth element: R, it has the property of being easily oxidized and corroded in the atmosphere. Therefore, the R—Fe—B based sintered magnet is usually used for practical use by forming a corrosion-resistant coating such as a metal coating or a resin coating on the surface thereof. However, it can be used as a drive motor for a hybrid vehicle or an electric vehicle. In the case where the magnet is embedded in a part, such as an IPM (Interior Permanent Magnet) motor incorporated in a compressor of an air conditioner, etc., such a corrosion-resistant coating is not necessarily provided on the surface of the magnet. It is not required to form. However, it is of course necessary to ensure the corrosion resistance of the magnet during the period from when the magnet is manufactured to when it is embedded in the part.
As described above, as a method of imparting corrosion resistance to the R—Fe—B based sintered magnet, a method of forming a corrosion resistant coating such as a metal coating or a resin coating on the surface is representative, but in recent years, oxidation A method of modifying the surface of a magnet by performing heat treatment (oxidation heat treatment) on the magnet in a neutral atmosphere has attracted attention as a simple technique for improving corrosion resistance. For example, Patent Document 1 and Patent Document 2 describe a method of forming an oxidizing atmosphere using oxygen and performing a heat treatment, and Patent Documents 3 to 6 use water vapor alone, or A method is described in which oxygen is combined with water vapor to form an oxidizing atmosphere and heat treatment is performed. However, even if surface modification is performed on the magnet by these methods, the surface of the magnet is finely affected by fluctuations in temperature and humidity, such as in transportation and storage environments where temperature and humidity are not controlled. In an environment where repeated condensation occurs repeatedly, sufficient corrosion resistance is not necessarily obtained. In Patent Documents 3 to 6, the water vapor partial pressure is preferably 10 hPa (1000 Pa) or more. When heat treatment is performed in an atmosphere with a high water vapor partial pressure, a large amount of hydrogen is generated as a by-product due to an oxidation reaction that occurs on the surface of the magnet, and the magnetic properties are reduced by occlusion and embrittlement of the hydrogen generated by the magnet. This has been clarified by the study of the present inventors. Therefore, the present inventors, as a better surface modification method for R-Fe-B based sintered magnets, oxygen partial pressure and water vapor partial pressure of less than 10 hPa, which is inappropriate in Patent Documents 3 to 6. Is a heat treatment method in an oxidizing atmosphere appropriately controlled, specifically, an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 0.1 Pa to 1000 Pa (excluding 1000 Pa) Patent Document 7 proposed a method of performing heat treatment at 200 ° C. to 600 ° C. in the atmosphere described above.

Japanese Patent No. 2844269 JP 2002-57052 A JP 2006-156853 A JP 2006-210864 A JP 2007-103523 A JP 2007-207936 A International Publication No. 2009/041639

By the way, when the R—Fe—B based sintered magnet is exposed to a high temperature in the usage environment, a countermeasure against a decrease in coercive force is required. As one of such measures, there is known a method for improving the coercive force by replacing a part of a light rare earth element (RL: at least one selected from Nd and Pr) contained as R with RH. . However, since RH is a scarce resource, it is required to reduce the amount used, and there is a problem that the residual magnetic flux density decreases when the replacement ratio of RL with RH increases. Thus, various methods for improving the coercive force while suppressing the decrease in the residual magnetic flux density with a smaller amount of RH used have been proposed, but such R—Fe—B based sintered magnets with a low RH content, RH Of course, it is necessary to secure corrosion resistance even in a magnet that does not contain.
Accordingly, an object of the present invention is to provide a method for producing an R—Fe—B based sintered magnet having a low RH content or containing no RH and having excellent corrosion resistance.

  In the process of conducting intensive studies in view of the above points, the present inventors applied a surface modification method for the R—Fe—B based sintered magnet proposed in Patent Document 7 to a magnet with a low RH content, RH When applied to a magnet that does not contain, the surface modification effect as expected can be obtained, but surprisingly, it has been found that the coercive force decrease that does not occur in a magnet with a high RH content occurs. Therefore, as a result of further studies to eliminate this phenomenon, it has been found that it is effective to eliminate this phenomenon by rapidly lowering the temperature of the magnet after performing the heat treatment under a predetermined condition.

The manufacturing method of the corrosion-resistant R—Fe—B based sintered magnet of the present invention completed based on the above knowledge, as described in claim 1, has an RH content of 4.5 mass% or less. For the sintered magnet, the oxygen partial pressure is 1 × 10 ³ Pa to 1 × 10 ⁵ Pa, the water vapor partial pressure is less than 1000 Pa, and the ratio of oxygen partial pressure to water vapor partial pressure (oxygen partial pressure / water vapor partial pressure) ) Includes a step of performing heat treatment at 200 ° C. to 450 ° C. in an atmosphere of 1 to 20000, and lowering the temperature of the magnet from the heat-treated temperature at an average cooling rate of 650 ° C./hour or more until reaching at least 100 ° C. It is characterized by performing.
The manufacturing method according to claim 2 is characterized in that, in the manufacturing method according to claim 1, the average cooling rate is 2000 ° C./hour or less.

  According to the present invention, excellent corrosion resistance is imparted to a R-Fe-B sintered magnet having a low RH content or no RH by performing surface modification without causing a reduction in coercive force. can do.

It is the schematic (side view) of an example of the continuous processing furnace which can be employ | adopted suitably for the manufacturing method of the corrosion-resistant R-Fe-B type sintered magnet of this invention.

The method for producing a corrosion-resistant R—Fe—B based sintered magnet of the present invention has an oxygen partial pressure of 1 × 10 ³ Pa to an R—Fe—B based sintered magnet having an RH content of 4.5 mass% or less. Heat treatment is performed at 200 ° C. to 450 ° C. in an atmosphere of 1 × 10 ⁵ Pa with a water vapor partial pressure of less than 1000 Pa and a ratio of oxygen partial pressure to water vapor partial pressure (oxygen partial pressure / water vapor partial pressure) of 1 to 20000. The temperature drop of the magnet from the temperature at which the heat treatment was performed is performed at an average cooling rate of 650 ° C./hour or more until reaching at least 100 ° C.

By to define the oxygen partial pressure in the step of performing a heat treatment with ^{1 × 10 3 Pa~1 × 10 5} Pa is the oxygen partial pressure is less than 1 × ¹⁰ 3 Pa, the oxygen content in the atmosphere is too low The surface modification of the magnet takes too much time, or the surface of the part of the magnet that contacts the holding member is not sufficiently modified, so that sufficient corrosion resistance and stability are not given to the part. This is because the contact mark with the holding member may remain. On the other hand, even if the oxygen partial pressure is made higher than 1 × 10 ⁵ Pa, the effect of surface modification of the magnet by increasing the oxygen partial pressure is not recognized so much, which may only lead to an increase in cost. Because. In order to perform the desired modification on the surface of the magnet more effectively and at low cost, the oxygen partial pressure is preferably 1 × 10 ³ Pa to 5 × 10 ⁴ Pa, and 1 × 10 ⁴ Pa to ³ × 10. ⁴ Pa is more desirable. The water vapor partial pressure is defined as less than 1000 Pa. If the water vapor partial pressure is 1000 Pa or more, the amount of water vapor in the atmosphere is excessive, and the surface of the magnet is modified to a stable one that exhibits excellent corrosion resistance. Because there is a fear that it cannot be done. In order to perform the desired modification on the surface of the magnet more effectively and at low cost, the water vapor partial pressure is desirably 700 Pa or less, and more desirably 45 Pa or less. The lower limit of the water vapor partial pressure is not particularly limited, but is usually 1 Pa. The ratio between the oxygen partial pressure and the water vapor partial pressure is defined as 1 to 20000. If the ratio is smaller than 1, the amount of water vapor with respect to the amount of oxygen in the atmosphere is too large, so that the surface of the magnet has excellent corrosion resistance. This is because there is a possibility that it cannot be reformed to a stable one. On the other hand, an atmosphere in which the ratio is greater than 20000 is a special environment and is not practical. In order to perform the desired modification on the surface of the magnet more effectively and at a low cost, the ratio is preferably 10 to 10,000, more preferably 300 to 5000, and further preferably 450 to 4000. The atmosphere in the processing chamber may be formed, for example, by individually introducing these oxidizing gases so as to have a predetermined partial pressure, or a dew point at which these oxidizing gases are included at a predetermined partial pressure. You may form by introduce | transducing the atmosphere which has. Further, an inert gas such as nitrogen gas or argon gas may coexist in the processing chamber. If the total pressure of the atmosphere is set to atmospheric pressure or a pressure in the vicinity thereof (specifically, for example, 9 × 10 ⁴ Pa to 1.2 × 10 ⁵ Pa), a predetermined atmosphere is not required without requiring special pressure adjusting means. Can be easily formed to improve the surface of the magnet.

  The temperature of the heat treatment is defined as 200 ° C. to 450 ° C. If the temperature is lower than 200 ° C., it may be difficult to perform the desired modification on the surface of the magnet, while the heat treatment temperature is higher than 450 ° C. This is because the magnetic properties of the magnet may be adversely affected. The temperature of the heat treatment is preferably 240 ° C to 430 ° C, and more preferably 280 ° C to 400 ° C. The heat treatment time is preferably 1 minute to 3 hours, more preferably 15 minutes to 2.5 hours. If the time is too short, it may be difficult to perform the desired modification on the surface of the magnet, while if the time is too long, the magnetic properties of the magnet may be adversely affected.

In addition, it is desirable to perform the process of raising the temperature of the magnet from room temperature to the temperature at which heat treatment is performed in an atmosphere having an oxygen partial pressure of 1 × 10 ³ Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 Pa to 100 Pa. By raising the temperature in such an atmosphere, moisture that is naturally adsorbed on the surface of the magnet is desorbed at an early stage, so that the moisture present on the surface of the magnet is It is possible to avoid adverse effects as much as possible. The average heating rate may be, for example, 100 ° C./hour to 2000 ° C./hour. In the present invention, “normal temperature” refers to the temperature (for example, room temperature) of the environment in which the R—Fe—B sintered magnet subjected to surface modification is placed at the start of temperature rise, and is illustrative. Means a temperature defined as 5 ° C. to 35 ° C. in JIS Z 8703 of the Japanese Industrial Standard.

  The step of lowering the magnet after the heat treatment, which is characterized in the present invention, is performed at an average cooling rate of 650 ° C./hour or more from the heat treatment temperature to at least 100 ° C. According to the study by the present inventors, the coercive force of the magnet whose surface-modified RH content is 4.5 mass or less by performing the heat treatment under the above-described conditions is large in the cooling rate in the temperature lowering process after the heat treatment. The coercive force is lowered when the cooling rate is low (depending on the cooling rate in the temperature range of 400 ° C. to 200 ° C.). This decrease in coercive force is a phenomenon that the present inventors have not predicted at all, and does not occur in a magnet having an RH content exceeding 4.5 mass%. Therefore, in the present invention, the temperature drop of the magnet from the temperature at which the heat treatment is performed is performed at an average cooling rate of 650 ° C./hour or more until reaching a temperature of at least 100 ° C., thereby suppressing a decrease in coercive force during the temperature drop after the heat treatment. . The upper limit of the average cooling rate is not particularly limited, but is preferably 2000 ° C./hour in order to lower the temperature at a low cost by a simple method. When the temperature of the magnet further decreases after reaching 100 ° C., the above average cooling rate may or may not be maintained. The step of lowering the temperature of the magnet that has been heat-treated is performed by adopting the same atmosphere as that used in the temperature-raising step when performing heat treatment of the magnet. It is desirable in that it can prevent the phenomenon that the magnetic properties deteriorate due to corrosion.

  The specific means for lowering the temperature of the magnet after the heat treatment at the above average cooling rate is not particularly limited. For example, the magnet is housed in a heat-resistant container made of a material such as SUS, Ti, Mo, Nb, etc., capable of controlling the atmosphere by circulating an atmosphere gas in the temperature raising process, heat treatment process, and temperature lowering process. When a heat-resistant container containing a magnet is housed in a processing chamber of a batch-type heat treatment furnace and the atmosphere inside the heat-resistant container is controlled, the flow rate of atmospheric gas used to lower the temperature of the magnet is increased or the temperature is increased. The temperature of the magnet can be lowered at a desired average cooling rate. The temperature drop of the magnet at the desired average cooling rate can be achieved by opening the processing chamber containing the heat-resistant container containing the magnet to the atmosphere, reducing the number of magnets contained in the heat-resistant container, and the heat-resistant container containing the magnet. It can also be carried out by a method of taking out from the heat treatment furnace and cooling separately. In addition, when performing a temperature raising process, a heat treatment process, and a temperature lowering process for a magnet using a batch-type heat treatment furnace that can change the environment in the processing chamber in which the magnet is accommodated to an environment for sequentially performing each process. The temperature lowering step is performed by introducing an atmospheric gas used for lowering the temperature of the magnet into the processing chamber, and the temperature of the magnet can be decreased at a desired average cooling rate by increasing the flow rate or decreasing the temperature. it can.

  Moreover, the temperature raising process, heat treatment process, and temperature lowering process for the magnet are divided into areas controlled to the environment for performing each process, and a continuous processing furnace (for example, a magnet can be sequentially moved to each area (for example, In the case of using a magnet as shown in FIG. 1, the temperature of the magnet can be decreased at a desired average cooling rate by appropriately controlling the temperature decreasing environment of the magnet in the temperature decreasing region. For example, in the continuous processing furnace shown in FIG. 1, each process is performed while moving the magnet from the left to the right in the figure by a moving means such as a belt conveyor. Arrows indicate the flow of the atmospheric gas in each region formed by an unillustrated air supply means and exhaust means. The inlet of the temperature rising region and the outlet of the temperature falling region are partitioned by, for example, an air curtain, and the boundary between the temperature rising region and the heat treatment region and the boundary between the heat treatment region and the temperature lowering region are partitioned by, for example, the flow of the atmospheric gas indicated by the arrows (these This may be done mechanically with a shutter). When such a continuous processing furnace capable of continuously performing surface modification with a stable quality on a large number of magnets is used, the temperature drop of the magnet at the desired average cooling rate is the same as that of the atmospheric gas used in the temperature drop region. This can be done by increasing the flow rate or lowering the temperature, or by adjusting the moving speed of the moving means.

  The thickness of the surface modification layer formed on the surface of the R—Fe—B based sintered magnet by the above process is preferably 0.5 μm to 10 μm. If the thickness is too thin, sufficient corrosion resistance may not be exhibited. On the other hand, if the thickness is too thick, the magnetic properties of the magnet may be adversely affected.

Examples of the R—Fe—B based sintered magnet to which the present invention is applied include those having an RH content of 4.5 mass% or less manufactured by the following manufacturing method.
An alloy containing 25 mass% or more and 40 mass% or less of rare earth element R, 0.6 mass% to 1.6 mass% of B (boron), the balance Fe and inevitable impurities is prepared. Here, R may contain a heavy rare earth element RH. Further, a part of B may be substituted by C (carbon), and a part of Fe (50 mass% or less) may be substituted by another transition metal element (for example, Co or Ni). Good. This alloy is suitable for a variety of purposes, including Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Ag, In, Sn, Hf, Ta, W, Pb, and You may contain about 0.01-1.0 mass% of the at least 1 sort (s) of additional element M selected from the group which consists of Bi.
The above-mentioned alloy can be suitably produced by rapidly cooling a molten raw material alloy by, for example, a strip casting method. Hereinafter, preparation of a rapidly solidified alloy by a strip casting method will be described.
First, a raw material alloy having the above composition is melted by high-frequency melting in an argon atmosphere to form a molten raw material alloy. Next, after holding this molten metal at about 1350 ° C., it is rapidly cooled by a single roll method to obtain, for example, a flake-shaped alloy ingot having a thickness of about 0.3 mm. The alloy slab thus produced is pulverized into, for example, 1 to 10 mm flakes before the next hydrogen pulverization treatment. In addition, the manufacturing method of the raw material alloy by a strip cast method is disclosed by US Patent 5,383,978 specification, for example.
[Coarse grinding process]
The alloy slab coarsely crushed into flakes is accommodated in the hydrogen furnace. Next, a hydrogen embrittlement treatment process (hereinafter sometimes referred to as “hydrogen pulverization treatment” or simply “hydrogen treatment”) is performed inside the hydrogen furnace. When the coarsely pulverized powder alloy powder after the hydrogen pulverization treatment is taken out from the hydrogen furnace, the takeout operation is preferably performed in an inert atmosphere so that the coarsely pulverized powder does not come into contact with the atmosphere. By doing so, it is possible to prevent the coarsely pulverized powder from oxidizing and generating heat, and to suppress the deterioration of the magnetic properties of the magnet.
By the hydrogen pulverization treatment, the rare earth alloy is pulverized to a size of about 0.1 mm to several mm, and the average particle size becomes 500 μm or less. After the hydrogen pulverization treatment, the embrittled raw material alloy is preferably crushed more finely and cooled. When the raw material is taken out in a relatively high temperature state, the cooling process time may be relatively long.
[Fine grinding process]
Next, the coarsely pulverized powder is finely pulverized using a jet mill pulverizer. A cyclone classifier is connected to the jet mill crusher used in the present embodiment. The jet mill pulverizer is supplied with the rare earth alloy (coarse pulverized powder) coarsely pulverized in the coarse pulverization step, and pulverizes in the pulverizer. The powder pulverized in the pulverizer is collected in a collection tank through a cyclone classifier. Thus, a fine powder of about 0.1 to 20 μm can be obtained. The pulverizer used for such fine pulverization is not limited to a jet mill, and may be an attritor or a ball mill. In grinding, a lubricant such as zinc stearate may be used as a grinding aid.
[Press molding]
In the present embodiment, for example, 0.3 mass% of a lubricant is added to and mixed with the magnetic powder produced by the above method in a rocking mixer, for example, and the surface of the alloy powder particles is coated with the lubricant. Next, the magnetic powder produced by the above-described method is molded in an orientation magnetic field using a known press machine. The intensity of the applied magnetic field is, for example, 1.0 to 1.7 Tesla (T). The molding pressure is set so that the green density of the molded body is, for example, about 4 to 4.5 g / cm ³ .
[Sintering process]
It carries out for 10 to 240 minutes with respect to said powder molded object at the temperature within the range of 1000-1200 degreeC, for example. Even if the step of holding at a temperature in the range of 650 to 1000 ° C. for 10 to 240 minutes and the step of further promoting the sintering at a temperature higher than the above holding temperature (for example, 1000 to 1200 ° C.) are sequentially performed. Good. During sintering, particularly when a liquid phase is generated (when the temperature is in the range of 650 to 1000 ° C.), the R-rich phase in the grain boundary phase begins to melt and a liquid phase is formed. Then, sintering progresses and a sintered magnet body is formed. After the sintering step, aging treatment (400 ° C. to 700 ° C.) and grinding for dimension adjustment may be performed.

  The surface-modified R—Fe—B sintered magnet produced by the production method of the present invention has excellent corrosion resistance imparted by an oxidation heat treatment, and a decrease in coercive force when the temperature is lowered after the heat treatment is suppressed. Therefore, for example, it is suitable for use in an IPM motor that is used as a drive motor for a hybrid vehicle or an electric vehicle, or incorporated in a compressor of an air conditioner. In addition, what is necessary is just to pass through the process of embedding a magnet in the inside of a rotor, when manufacturing an IPM motor using the surface-modified R-Fe-B system sintered magnet manufactured by the manufacturing method of this invention.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is limited to this and is not interpreted.

Example 1
Nd: 20.8, Pr: 5.9, Dy: 3.0, B: 0.93, Co: 2.0, Cu: 0.1, Al: 0.15, Ga: 0.07, balance: An alloy flake having a composition of Fe (unit: mass%) and having a thickness of 0.2 mm to 0.3 mm was prepared by strip casting.
Next, this alloy flake was filled in a container and accommodated in a hydrogen treatment apparatus. Then, the hydrogen treatment apparatus was filled with hydrogen gas at a pressure of 500 kPa, so that hydrogen was occluded in the alloy flakes at room temperature and then released. By performing such hydrogen treatment, the alloy flakes were embrittled and coarsely pulverized powder having a size of about 0.15 mm to 0.2 mm was produced.
After adding 0.04 mass% zinc stearate as a pulverization aid to the coarsely pulverized powder produced by the above hydrogen treatment and mixing, a pulverization step using a jet mill device is performed to obtain a fine particle diameter of about 3 μm. A powder was prepared.
The fine powder thus produced was molded by a press apparatus to produce a powder compact. Specifically, the powder particles were compressed in a magnetic field-oriented state in an applied magnetic field and pressed. Thereafter, the molded body was extracted from the press apparatus, and a sintering process was performed at 1050 ° C. for 4 hours in a vacuum furnace to obtain a sintered body block.
The obtained sintered body block was subjected to an aging treatment at 490 ° C. for 2.5 hours in a vacuum, and then the surface was ground to adjust the dimensions, and the thickness was 6 mm × length 7 mm × width 7 mm. A sintered magnet was obtained.

After the sintered magnet obtained by the above method was ultrasonically washed, the temperature was improved, the heat treatment step, and the temperature reduction step were performed using the continuous processing furnace having the configuration shown in FIG. . The temperature of the sintered magnet was measured by monitoring the temperature of a temperature measuring magnet equipped with a thermocouple.
(1) Temperature rising step Temperature rising from room temperature (meaning 25 ° C., the same applies hereinafter) to the temperature at which heat treatment is performed (420 ° C.) is carried out in the atmosphere with a dew point of −40 ° C. (oxygen partial pressure 20000 Pa, water vapor partial pressure 19 Pa, oxygen The reaction was performed at an average temperature increase rate of 500 ° C./hour in an atmosphere of partial pressure / water vapor partial pressure = 1052).
(2) Heat treatment step Heat treatment was performed at 420 ° C. for 30 minutes in an atmosphere with a dew point of 0 ° C. (oxygen partial pressure 20000 Pa, water vapor partial pressure 600 Pa, oxygen partial pressure / water vapor partial pressure = 33.3).
(3) Temperature-falling step In the same atmosphere as that employed in the temperature-raising step, the temperature was lowered from 420 ° C. to 100 ° C. at an average cooling rate of 690 ° C./hour. The average cooling rate was adjusted by adjusting the flow rate of the atmospheric gas used for cooling.

  The thickness of the modified layer formed on the surface of the sintered magnet by the above method was 1.7 μm. The thickness of the modified layer was determined by preparing a sample using an ion beam cross-section processing device (SM09010: manufactured by JEOL Ltd.) after embedding and polishing a surface-modified sintered magnet, and then using a field emission scanning electron microscope ( S-4300: manufactured by Hitachi High-Technology Corporation) was used for cross-sectional observation.

(Example 2)
The same method as in Example 1 except that the heat treatment step was performed at 420 ° C. for 20 minutes in the atmosphere of dew point −40 ° C. in the atmosphere (oxygen partial pressure 20000 Pa, water vapor partial pressure 19 Pa, oxygen partial pressure / water vapor partial pressure = 1052). The surface modification of the sintered magnet was performed. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.6 μm.

(Example 3)
A sintered magnet is accommodated in a SUS heat-resistant container equipped with a gas introduction path and a gas discharge path for controlling the atmosphere by circulating the atmosphere gas inside, and a heat-resistant container containing the sintered magnet is provided. After performing the temperature raising process and heat treatment process while controlling the atmosphere inside the heat-resistant container housed in the processing chamber of the batch type heat treatment furnace, the temperature-lowering process is performed and the atmosphere inside the heat-resistant container is the atmosphere for the temperature-lowering process. After that, the surface modification of the sintered magnet was performed in the same manner as in Example 1 except that the heat-resistant container was taken out of the heat treatment furnace and cooled separately to carry out at an average cooling rate of 1800 ° C./hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.7 μm.

Example 4
Nd: 19.8, Pr: 5.7, Dy: 4.3, B: 0.93, Co: 2.0, Cu: 0.1, Al: 0.15, Ga: 0.07, balance: Except that a sintered magnet was produced using an alloy flake having a composition of Fe (unit: mass%) having a thickness of 0.2 mm to 0.3 mm, and the heat treatment step was performed at 400 ° C. for 30 minutes. The surface modification of the sintered magnet was performed in the same manner as in No. 3. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.5 μm.

(Example 5)
Nd: 19.8, Pr: 5.7, Dy: 4.3, B: 0.93, Co: 2.0, Cu: 0.1, Al: 0.15, Ga: 0.07, balance: A sintered magnet was prepared using an alloy flake with a thickness of 0.2 mm to 0.3 mm having a composition of Fe (unit: mass%), and the heat treatment step was performed in an atmosphere with a dew point of −40 ° C. (oxygen partial pressure 20000 Pa, Example 1 except that it was performed at 420 ° C. for 20 minutes in an atmosphere of water vapor partial pressure 19 Pa and oxygen partial pressure / water vapor partial pressure = 1052) and that the temperature lowering step was performed at an average cooling rate of 750 ° C./hour. The surface modification of the sintered magnet was performed by the same method. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.8 μm.

(Example 6)
Thickness having a composition of Nd: 30.5, B: 0.95, Co: 0.9, Cu: 0.1, Al: 0.1, Ga: 0.1, balance: Fe (unit: mass%) Surface modification of the sintered magnet in the same manner as in Example 3 except that a sintered magnet was prepared using an alloy flake with a thickness of 0.2 mm to 0.3 mm and the heat treatment step was performed at 340 ° C. for 120 minutes. Went. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.2 μm.

(Example 7)
Nd: 23.0, Pr: 6.6, Dy: 1.2, B: 1.00, Co: 2.0, Cu: 0.1, Al: 0.2, balance: Fe (unit is mass%) ) And the heat treatment process was performed in the atmosphere (oxygen partial pressure 20000 Pa, water vapor partial pressure 19 Pa, oxygen Sintered magnet in the same manner as in Example 1 except that it was performed at 300 ° C. for 120 minutes in an atmosphere of partial pressure / water vapor partial pressure = 1052) and that the temperature lowering step was performed at an average cooling rate of 750 ° C./hour. The surface was modified. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 0.9 μm.

(Comparative Example 1)
The surface modification of the sintered magnet was performed in the same manner as in Example 1 except that the temperature lowering step was performed at an average cooling rate of 420 ° C./hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.6 μm.

(Comparative Example 2)
The surface modification of the sintered magnet was performed in the same manner as in Example 2 except that the temperature lowering step was performed at an average cooling rate of 530 ° C./hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.7 μm.

(Comparative Example 3)
The surface modification of the sintered magnet was performed in the same manner as in Example 4 except that the temperature lowering step was performed at an average cooling rate of 530 ° C./hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.6 μm.

(Comparative Example 4)
The surface modification of the sintered magnet was performed in the same manner as in Example 6 except that the temperature lowering step was performed at an average cooling rate of 490 ° C./hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.3 μm.

(Comparative Example 5)
The surface modification of the sintered magnet was performed in the same manner as in Example 7 except that the temperature lowering step was performed at an average cooling rate of 530 ° C./hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 0.9 μm.

(Reference Example 1)
Nd: 18.1, Pr: 5.2, Dy: 6.5, B: 0.95, Co: 2.0, Cu: 0.1, Al: 0.15, Ga: 0.07, balance: A sintered magnet was produced using an alloy flake having a composition of Fe (unit: mass%) having a thickness of 0.2 mm to 0.3 mm, a heat treatment step was performed at 400 ° C. for 30 minutes, and 690 ° C. The surface modification of the sintered magnet was performed in the same manner as in Example 1 except that the temperature lowering step was performed at an average cooling rate of / hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.6 μm.

(Reference Example 2)
Nd: 15.4, Pr: 4.3, Dy: 11.7, B: 0.95, Co: 2.0, Cu: 0.1, Al: 0.15, Ga: 0.07, balance: A sintered magnet was produced using an alloy flake having a composition of Fe (unit: mass%) having a thickness of 0.2 mm to 0.3 mm, a heat treatment step was performed at 400 ° C. for 30 minutes, and 690 ° C. The surface modification of the sintered magnet was performed in the same manner as in Example 1 except that the temperature lowering step was performed at an average cooling rate of / hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.7 μm.

(Reference Example 3)
The surface modification of the sintered magnet was performed in the same manner as in Reference Example 1 except that the temperature lowering step was performed at an average cooling rate of 420 ° C./hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.7 μm.

(Reference Example 4)
The surface modification of the sintered magnet was performed in the same manner as in Reference Example 2 except that the temperature lowering step was performed at an average cooling rate of 420 ° C./hour. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.6 μm.

(Evaluation of magnetic properties)
The intrinsic coercivity of the sintered magnet subjected to surface modification in each of Examples 1 to 7, Comparative Examples 1 to 5, and Reference Examples 1 to 4, and the intrinsic coercivity of the sintered magnet before surface modification In comparison, the intrinsic coercivity deterioration rate was calculated by the following formula. The results are shown in Table 1. The intrinsic coercive force was measured using a magnetometer (TPN-2-10: manufactured by Toei Kogyo Co., Ltd.).
Inherent coercive force deterioration rate (%) = ((A−B) / A) × 100
A: Intrinsic coercive force of sintered magnet before surface modification (average value of 20)
B: Intrinsic coercive force of sintered magnet after surface modification (average value of 20)

(Corrosion resistance evaluation)
For each sintered magnet subjected to surface modification in each of Examples 1 to 7, Comparative Examples 1 to 5, and Reference Examples 1 to 4, the temperature was 60 ° C. and the relative humidity was 90% under high temperature and high humidity conditions. A corrosion resistance test was conducted for 24 hours, and the presence or absence of surface rust after the test was examined by appearance observation. Table 1 shows the number of magnets on which surface rusting was observed among the 20 magnets subjected to the test.

(Summary)
As is clear from the reference examples, sintered magnets with a high RH content (specifically, 6.5 mass% or more) have an extremely low intrinsic coercive force deterioration rate regardless of the average cooling rate when the temperature is lowered after heat treatment. This is a numerical value, and no decrease in coercive force was observed when the temperature was lowered. On the other hand, as is clear from the examples and comparative examples, the sintered magnet with a low RH content has a high rate of deterioration of the intrinsic coercive force when the average cooling rate at the time of temperature reduction after the heat treatment is slow, Although a decrease in coercive force was observed, the rate of degradation of the intrinsic coercive force was improved by increasing the average cooling rate, and the decrease in coercive force during cooling could be suppressed. In addition, a decrease in coercive force due to a slow average cooling rate at the time of temperature decrease was observed in a sintered magnet having an RH content of at least 4.5 mass%, and the decrease in coercive force was suppressed at an average cooling rate of at least 650 ° C./hour. The effect was recognized (by a separate experiment). The corrosion resistance imparted by the surface modification of the sintered magnet was not different among Examples, Comparative Examples, and Reference Examples, and was excellent in all cases.

  INDUSTRIAL APPLICABILITY The present invention can provide a method for producing an R—Fe—B based sintered magnet having a low RH content or containing no RH and having excellent corrosion resistance without causing a reduction in coercive force during the process. In terms of industrial applicability.

Claims (2)

The oxygen partial pressure is 1 × 10 ³ Pa to 1 × for an R—Fe—B based sintered magnet having a heavy rare earth element (RH: at least one selected from Dy and Tb) content of 4.5 mass% or less. Heat treatment is performed at 200 ° C. to 450 ° C. in an atmosphere of 10 ⁵ Pa with a water vapor partial pressure of less than 1000 Pa and a ratio of oxygen partial pressure to water vapor partial pressure (oxygen partial pressure / water vapor partial pressure) of 1 to 20000. A corrosion-resistant R—Fe—B-based sintered magnet comprising a step of lowering the temperature of the magnet from the heat-treated temperature at an average cooling rate of 650 ° C./hour or more until reaching a temperature of at least 100 ° C. Manufacturing method.   The method according to claim 1, wherein the average cooling rate is 2000 ° C / hour or less.