JP5262902B2 - Method for producing surface-modified rare earth sintered magnet - Google Patents

Description

  The present invention relates to a method for producing a rare earth sintered magnet having sufficient corrosion resistance and excellent magnetic properties even in an environment where humidity varies, such as a transportation environment and a storage environment where humidity control is not performed.

  Rare earth-based sintered magnets such as R-Fe-B-based sintered magnets typified by Nd-Fe-B-based sintered magnets are made of resource-rich and inexpensive materials and have high magnetic properties. However, since it contains a highly reactive rare earth metal: R, it has the property of being easily oxidized and corroded in the atmosphere. Accordingly, rare earth-based sintered magnets are usually put to practical use by forming a corrosion-resistant coating such as a metal coating or a resin coating on the surface, but the magnet is embedded in a component like an IPM (Interior Permanent Magnet) motor. In the case of the embodiment to be used, it is not always necessary to form such a corrosion-resistant film on the surface of the magnet. 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. Therefore, as a method for ensuring the corrosion resistance of the rare earth-based sintered magnet in such a period, a method for modifying the surface of the magnet by performing a heat treatment in an oxidizing atmosphere has been proposed. It attracts attention as a simple anti-corrosion technology that can achieve the above object.

  The oxidizing atmosphere necessary for surface modification of the rare earth sintered magnet by oxidative heat treatment is formed using oxygen (see, for example, Patent Document 1 and Patent Document 2), and water vapor. It may be formed using For example, Patent Documents 3 to 6 describe a method of forming an oxidizing atmosphere using water vapor alone or combining water vapor with oxygen.

Japanese Patent No. 2844269 JP 2002-57052 A JP 2006-156853 A JP 2006-210864 A JP 2007-103523 A JP 2007-207936 A

Corrosion of the magnet in the period from when the rare earth sintered magnet is manufactured to when it is embedded in the part depends on the environment in which the magnet is placed. In particular, fluctuations in humidity repeatedly cause fine condensation on the surface of the magnet, which accelerates the corrosion of the magnet. As a result of verifying the usefulness of the simple corrosion resistance improvement technique described in the above-mentioned patent document, the present inventor does not always have sufficient corrosion resistance in an environment where the humidity fluctuates greatly even when any technique is adopted. In Patent Documents 3 to 6, the water vapor partial pressure is preferably 10 hPa (1000 Pa) or more. However, when heat treatment is performed in an atmosphere having such a high water vapor partial pressure, it occurs on the surface of the magnet. It has been clarified that magnetic properties are deteriorated when hydrogen is generated in large quantities as a by-product by the oxidation reaction, and the hydrogen generated by the magnet is occluded and embrittled.
Therefore, the present invention has an object to provide a method for producing a rare earth sintered magnet in which sufficient corrosion resistance is imparted by an oxidation heat treatment even in an environment where the humidity varies, and a decrease in magnetic properties due to the oxidation heat treatment is suppressed. And

  As a result of intensive studies in view of the above points, the inventor of the present invention appropriately controlled an oxygen partial pressure and a water vapor partial pressure of less than 10 hPa, which is inappropriate in Patent Documents 3 to 6. The rare earth-based sintered magnet surface-modified by performing heat treatment under appropriate temperature control based on the amount of Co substitution of Fe in the magnet has sufficient corrosion resistance even in an environment where humidity varies, It was found that the deterioration of magnetic properties due to heat treatment was suppressed.

The manufacturing method of the surface-modified rare earth sintered magnet of the present invention completed based on the above knowledge is as described in claim 1, wherein the rare earth sintered magnet is 25 mass% to 40 mass% rare earth. Element: R, 0.6 mass% to 1.6 mass% B (some of which may be replaced by C), 0 mass% to 1.0 mass% Al, Si, Ti, V At least one additive element selected from the group consisting of Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Ag, In, Sn, Hf, Ta, W, Pb, and Bi: M The remainder has a composition consisting of Fe partially substituted by Co and unavoidable impurities, with 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 In an atmosphere of 1000 Pa (excluding 1000 Pa), the magnet Fe When the amount of Co substitution is 0.01% to 1.2% by mass (excluding 1.2% by mass) of the whole magnet, it is 200 ° C. to 400 ° C. (excluding 400 ° C.). When the amount of Co substitution is 1.2 mass% to 2.5 mass% of the whole magnet, it includes a step of performing heat treatment at 400 ° C. to 600 ° C.
The method for producing a surface-modified rare earth-based sintered magnet according to claim 2 is the method for producing a surface-modified rare earth-based sintered magnet according to claim 1, wherein the oxygen partial pressure and the water vapor partial pressure are The ratio (oxygen partial pressure / water vapor partial pressure) is 1 to 400.
The method for producing a surface-modified rare earth-based sintered magnet according to claim 3 is the method for producing a surface-modified rare earth-based sintered magnet according to claim 1 or 2, wherein the temperature at which heat treatment is performed from room temperature. Is performed in an atmosphere having an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 × 10 ⁻³ Pa to 100 Pa.
The method for producing a surface-modified rare earth-based sintered magnet according to claim 4 is the method for producing a surface-modified rare earth-based sintered magnet according to any one of claims 1 to 3, wherein: A heat treatment is performed after surface grinding is performed on the surface.
The method for producing a surface-modified rare earth-based sintered magnet according to claim 5 is the method for producing a surface-modified rare earth-based sintered magnet according to claim 4, wherein the number is # 60 to # 400. Surface grinding is performed using a grindstone having a grain size.
Moreover, the surface-modified rare earth-based sintered magnet of the present invention is manufactured by the method for manufacturing a surface-modified rare earth-based sintered magnet according to claim 1, as described in claim 6. Features.
The surface-modified rare earth-based sintered magnet according to claim 7 is the surface-modified rare earth-based sintered magnet according to claim 6, wherein the surface-modified portions are sequentially arranged from the inside of the magnet. From a surface modified layer having a main layer containing R, Fe, B and oxygen, an amorphous layer containing at least R, Fe and oxygen, and an outermost layer containing iron oxide mainly composed of hematite as a constituent component It is characterized by becoming .

  According to the present invention, it is possible to provide a method for producing a rare earth-based sintered magnet in which sufficient corrosion resistance is imparted by an oxidation heat treatment even in an environment where the humidity varies, and a decrease in magnetic properties due to the oxidation heat treatment is suppressed. it can.

It is the schematic (side view) of an example of the continuous processing furnace suitable for manufacture of the surface-modified rare earth-based sintered magnet of the present invention. It is a photograph which shows the result of the cross-sectional observation using the field emission type | mold scanning electron microscope of the surface-modified magnet body test piece in Example 1. FIG. It is a chart which shows the result of having analyzed the outermost layer which comprises the surface-modified part (surface modification layer) of the surface-modified magnet body test piece from the surface using the X-ray-diffraction apparatus. It is a graph which shows the result of having investigated the influence which the heat processing in the reference example 1 has on the magnetic characteristics of the rare earth sintered magnet.

In the method for producing a surface-modified rare earth sintered magnet according to the present invention, the rare earth sintered magnet comprises 25 mass% to 40 mass% of rare earth element: R, 0.6 mass% to 1.6 mass%. B (however, part thereof may be substituted by C), 0% by mass to 1.0% by mass of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb , Mo, Ag, In, Sn, Hf, Ta, W, Pb, and Bi, at least one additive element selected from the group consisting of M, the balance being Fe partially substituted by Co, and unavoidable It has a composition composed of impurities, and has 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 (however, excluding 1000 Pa). Replacement amount is 0.01% to 1.2% by weight of the entire magnet In the case of (except 1.2 mass%), the amount of Co substitution of Fe in the magnet is 1.2 mass% to 2.5 mass% of the whole magnet at 200 ° C to 400 ° C (excluding 400 ° C). In this case, the method includes a step of performing a heat treatment at 400 ° C. to 600 ° C. Whether the amount of Co substitution of Fe in the magnet is less than 1.2% by mass or 1.2% by mass or more of the whole magnet in an oxidizing atmosphere in which the oxygen partial pressure and the water vapor partial pressure of less than 10 hPa are appropriately controlled By performing heat treatment under appropriate temperature control using as an index, surface modification that exhibits excellent corrosion resistance can be effectively performed on the magnet, and hydrogen caused by the presence of excess water vapor It is possible to suppress a decrease in magnetic properties of the magnet that accompanies mass production.

In order to perform the desired modification on the surface of the rare earth sintered magnet more effectively and at low cost, the oxygen partial pressure is preferably 5 × 10 ³ Pa to 5 × 10 ⁴ Pa, and 1 × 10 ⁴ Pa. -4 × 10 ⁴ Pa is more desirable. The water vapor partial pressure is preferably 250 Pa to 900 Pa, and more preferably 400 Pa to 700 Pa. The ratio of oxygen partial pressure to water vapor partial pressure (oxygen partial pressure / water vapor partial pressure) is preferably 1 to 400, and more preferably 5 to 100. The oxidizing 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 these oxidizing gases are included at a predetermined partial pressure. You may form by introduce | transducing the atmosphere which has a dew point. Further, an inert gas such as nitrogen or argon may coexist in the processing chamber.

  The heat treatment temperature is 200 ° C. to 400 ° C. (excluding 400 ° C.) when the amount of Co substitution of Fe in the magnet is less than 1.2% by mass of the whole magnet, but preferably 250 ° C. to 380 ° C. 300 to 370 degreeC is more desirable. If the treatment is performed at a temperature of less than 200 ° C., the surface of the rare earth sintered magnet may be difficult to be modified. On the other hand, if the treatment is performed at a temperature of 400 ° C. or more, the magnetic properties of the magnet will be adversely affected. There is a risk that the modified layer formed may fall off due to excessive influence or modification of the magnet surface. Further, when the amount of Co substitution of Fe in the magnet is 1.2% by mass or more of the whole magnet, 400 ° C. to 600 ° C. is adopted, but 405 ° C. to 550 ° C. is desirable, and 410 ° C. to 480 ° C. is more desirable. Surprisingly, it is adopted for magnets whose amount of Co substitution of Fe in the magnet is 1.2% by mass or more of the whole magnet, and for magnets whose amount of Co substitution of Fe in the magnet is less than 1.2% by mass of the whole magnet. For example, if heat treatment is performed at 300 ° C. to 400 ° C. (excluding 400 ° C.), the magnetic properties of the magnet are adversely affected. However, if heat treatment is performed at 420 ° C. to 480 ° C., the magnetic properties of the magnet tend to be improved. is there. However, if the treatment is performed at a temperature exceeding 600 ° C., the magnetic properties of the magnet may be adversely affected, and the modified layer formed due to excessive modification of the magnet surface may fall off. The processing time is preferably 1 minute to 3 hours.

The temperature rise from room temperature (for example, 10 ° C. to 30 ° C.) to the heat treatment temperature is performed in an atmosphere having an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 × 10 ⁻³ Pa to 100 Pa. It is desirable to do. If the temperature raising step is performed in the air without controlling the atmosphere, for example, an oxidation reaction due to moisture contained in the air occurs at the time of temperature rising, and the magnetic characteristics of the magnet are reduced due to the large amount of hydrogen generated. There is a risk of inviting. In addition, since the amount of moisture contained in the atmosphere varies depending on the season, there is a risk that surface modification with stable quality throughout the year cannot be performed on the magnet. On the other hand, since the above atmosphere contains moderate oxygen and water vapor, the temperature raising process itself has a favorable effect on the surface modification of the magnet, and imparts excellent corrosion resistance to the magnet and suppresses deterioration of the magnetic properties. Contribute to. The rate of temperature increase from room temperature to the heat treatment temperature is preferably 100 ° C./hour to 1800 ° C./hour, and the temperature increase time is preferably 20 minutes to 2 hours. After the magnet is heated to the heat treatment temperature, it may be immediately transferred to the heat treatment step, or after the magnet is held for a while (for example, 1 to 60 minutes) in the atmosphere of the temperature increase step, the heat treatment step may be performed. Good.

The temperature lowering after the heat treatment is also desirably performed in an atmosphere having an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 × 10 ⁻³ Pa to 100 Pa. By lowering the temperature in such an atmosphere, it is possible to prevent the surface of the magnet from condensing and causing corrosion during the process.

  The temperature raising process, the heat treatment process, and the temperature lowering process may be performed by sequentially changing the environment in the processing chamber in which the magnet is accommodated, or the processing chamber is divided into regions controlled by the respective environments, and the magnet is divided into each region. You may carry out by moving sequentially.

  FIG. 1 (a) shows a continuous processing furnace in which the temperature raising process, the heat treatment process, and the temperature lowering process can be performed by dividing the interior into regions controlled by the respective environments and moving the magnets sequentially to each region. It is a schematic diagram (side view) of an example. In the continuous processing furnace shown in FIG. 1 (a), each processing is performed while moving the magnet from the left to the right in the drawing by 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). FIG.1 (b) is a figure which shows the temperature change of the magnet which moves the inside of the continuous processing furnace shown to Fig.1 (a). If such a continuous processing furnace is used, surface modification with stable quality can be continuously performed for a large number of magnets.

The modified layer formed on the surface of the rare earth-based sintered magnet by the above steps is, in order from the inside of the magnet, a main layer containing R, Fe, B and oxygen, and an amorphous layer containing at least R, Fe and oxygen. And at least three outermost layers containing iron oxide mainly composed of hematite (α-Fe ₂ O ₃ ) as a constituent component. When the composition of the main layer in the surface-modified layer is compared with the composition of the magnet (material) that is not surface-modified, the Fe content is decreased and the oxygen content is increased. It is 2.5 mass%-15 mass%. The main layer in the surface modified layer may have an R-concentrated layer having a length extending in the lateral direction of 0.5 μm to 30 μm and a thickness of 50 nm to 400 nm. This R-concentrated layer is presumed to be formed by precipitation of R in the work strain part existing in the magnet, reinforcing the decrease in the strength of the magnet due to degranulation, etc. It is thought that it contributes to the improvement of the adhesive strength with the interposed parts. As for the outermost layer in the surface modified layer, it is desirable that 90% by mass or more of iron oxide contained as a constituent component is hematite. More preferably, it is 95 mass% or more, More preferably, it is 98 mass% or more. The fact that iron oxide contains hematite in a high ratio and does not contain magnetite (Fe ₃ O ₄ ) as much as possible contributes to imparting excellent corrosion resistance by performing surface modification of the magnet. By performing heat treatment in an oxidizing atmosphere in which the oxygen partial pressure and the water vapor partial pressure of less than 10 hPa are appropriately controlled, the outermost layer in the surface modified layer is composed of iron oxide containing hematite in a high ratio. Can be. In contrast, when heat treatment is performed in an atmosphere having a high water vapor partial pressure as described in Patent Documents 3 to 6, iron oxide constituting the outermost layer in the surface modified layer is magnetite. Contains at a high ratio. This is considered to be the reason why the methods described in these patent documents cannot perform surface modification that exhibits sufficient corrosion resistance in an environment where the humidity fluctuates greatly. The ratio of hematite in iron oxide contained as a constituent component in the outermost layer can be obtained by analyzing from the magnet surface by, for example, Raman analysis. The amorphous layer located between the main layer and the outermost layer in the surface modified layer is a portion where stable crystals were not formed when R or Fe contained in the magnet was converted into an oxide by an oxidation reaction. It is thought that.

  The thickness of the surface modification layer formed on the surface of the rare earth sintered magnet 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. The thickness of the main layer in the surface modification layer is preferably 0.4 μm to 9.9 μm, and more preferably 1 μm to 7 μm. The thickness of the amorphous layer is preferably 100 nm or less, more preferably 70 nm or less (the lower limit is preferably 10 nm, for example). The thickness of the outermost layer is desirably 10 nm to 300 nm, and more desirably 50 nm to 200 nm.

  Further, surface grinding may be performed on the magnet surface before performing the oxidation heat treatment. By adding such a step, the surface composition of the magnet is made uniform, thereby making it possible to perform uniform oxidation heat treatment on the entire surface of the magnet, and making the outermost layer uniform with a high surface coverage by hematite. be able to. The surface coverage by hematite is desirably 90% or more, and more desirably 95% or more. The surface grinding can be performed using a known surface grinding machine or a double-head grinding machine. It is desirable that the grindstone used has a grain size of # 60 to # 400. If the count is less than # 60 (if the grain size is too coarse), the magnet surface may be ground more than necessary, which may adversely affect the dimensional accuracy of the magnet. On the other hand, if the count exceeds # 400 (If the particle size is too fine), the surface composition of the magnet may become insufficiently uniform. The rotational speed of the grindstone is desirably 600 rpm to 2000 rpm, and the feeding speed of the magnet to the grinding machine is desirably 0.1 m / min to 5 m / min. The surface grinding process may be performed after the grinding for adjusting the size of the magnet is performed by another method. However, by performing the grinding for the dimension adjustment of the magnet by the surface grinding process, the size adjustment of the magnet and the magnet can be performed. The surface composition can be made uniform at the same time.

The rare earth sintered magnet to which the present invention is applied includes, for example, an R—Fe—B sintered magnet manufactured from an alloy corresponding to the composition by the following manufacturing method.
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 an average particle size of 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. In this way, a fine powder of about 0.1 to 20 μm (typically an average particle size of 3 to 5 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 this embodiment, for example, 0.3 wt% of a lubricant is added to and mixed with the magnetic powder produced by the above method in a rocking mixer, 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, 1.5 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]
With respect to said powder molded body, the step of holding at a temperature in the range of 650 to 1000 ° C. for 10 to 240 minutes, and then sintering at a temperature higher than the above holding temperature (for example, 1000 to 1200 ° C.). It is preferable to sequentially perform the further steps. 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.

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

(Example 1) Rare earth sintered magnet in which the amount of Co substitution of Fe in the magnet is 1.2 mass% or more of the whole magnet (Part 1)
Nd: 18.5, Pr: 5.1, Dy: 8.3, B: 1.00, Co: 2.0, Nb: 0.13, Al: 0.1, Ga: 0.07, Cu: An alloy flake having a composition of 0.1, balance: Fe (unit: mass%) and having a thickness of 0.2 to 0.3 mm was produced 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 a hydrogen treatment, the alloy flakes were embrittled to produce an amorphous powder having a size of about 0.15 to 0.2 mm.
After adding 0.04 wt% zinc stearate as a grinding aid to the coarsely pulverized powder produced by the above hydrogen treatment and mixing, an average powder particle size of about 3 μm is obtained by performing a pulverization step with a jet mill device. A fine 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 surface of the obtained sintered body block is subjected to surface grinding using a surface grinding machine (manufactured by Daisho Seiki Co., Ltd.) (grinding wheel count: # 100, grinding wheel rotation speed: 1500 rpm, magnet to grinding machine) A sintered magnet (hereinafter referred to as “magnet body test piece”) whose dimensions were adjusted to a thickness of 6 mm × length 7 mm × width 7 mm was obtained.
The magnet specimen was washed with alcohol and then subjected to an aging treatment at 460 ° C. for 8 hours in a vacuum. As a result of measuring the magnetic characteristics of this magnet body test piece using a magnetometer (TPM-2-10: manufactured by Toei Kogyo Co., Ltd., hereinafter the same), the intrinsic coercive force was 2050 kA / m.
Next, with respect to the magnet body test piece subjected to the aging treatment, at 410 ° C. in an atmosphere of dew point 0 ° C. in the atmosphere (oxygen partial pressure 20000 Pa, water vapor partial pressure 600 Pa, oxygen partial pressure / water vapor partial pressure = 33.3). By performing heat treatment for 2 hours, a surface-modified magnet body test piece was obtained. The temperature of the magnet specimen from room temperature to the heat treatment temperature is about 900 ° C./hour in an atmosphere with a dew point of −40 ° C. (oxygen partial pressure 20000 Pa, water vapor partial pressure 12.9 Pa). (The temperature rising time was 25 minutes). Further, the temperature drop after the heat treatment was performed in the same atmosphere. After this magnet body test piece is resin-filled and polished, a sample is prepared using an ion beam cross-section processing apparatus (SM09010: manufactured by JEOL Ltd.), and a field emission scanning electron microscope (S-4300: manufactured by Hitachi High-Technologies Corporation) is used. The results of cross-sectional observation are shown in FIG. As is apparent from FIG. 2, at this observation point, the thickness of the modified layer formed on the surface of the magnet test piece is about 5.9 μm, and this modified layer is composed of a plurality of layers, at least the main layer. It was found that there was a layer and an outermost layer having a thickness of about 170 nm. Further, in the modified layer, a layered structure composed of R having a thickness of about 100 nm and a length of about 5 μm (an R-concentrated layer having an R composition of 85% by mass or more) extends in the horizontal direction (approximately the same as the surface of the magnet body). It was confirmed that the film was formed in a parallel direction. Table 1 shows the results of analyzing the composition of the main layer in the modified layer and the composition of the material (magnet body test piece) using an energy dispersive X-ray analyzer (Genesis 2000: manufactured by EDAX). As is clear from Table 1, it was found that the main layer in the modified layer had a very high oxygen content while the Fe content was lower than that of the raw material. Furthermore, as a result of performing cross-sectional observation near the surface of the surface-modified magnetic body test piece using a transmission electron microscope (HF2100: manufactured by Hitachi High-Technology Corporation), the main layer and thickness are selected at the selected observation point. It was found that there was a layer with a thickness of about 70 nm between the outermost layers of about 250 nm. This layer was found to be amorphous (by electron diffraction analysis). As a result of analyzing the composition of the amorphous layer and the outermost layer in the modified layer using an energy dispersive X-ray analyzer (EDX: manufactured by NORAN), there is almost no R in the outermost layer in the modified layer. It was found that it was composed of iron oxide, and the amorphous layer was composed of a composite oxide of R and Fe. Moreover, the result of having analyzed the outermost layer in the modified layer of the surface-modified magnetic body test piece from the surface using an X-ray diffractometer (RINT2400: manufactured by Rigaku) is shown in FIG. As is apparent from FIG. 3, it was found that the outermost layer in the modified layer was a layer mainly composed of hematite (♦ in the figure: peak of hematite). The outermost layer mainly composed of hematite is formed by oxidizing part of the main phase (R ₂ Fe ₁₄ B) of the raw material by heat treatment, so that Fe flows out of the main phase and is oxidized. Was guessed. Furthermore, as a result of analyzing the outermost layer in the modified layer of the surface-modified magnetic body test piece from the surface using a Raman spectroscopic analyzer (manufactured by Holo Lab 5000R: KAISER OPTICAL SYSTEM), a constituent component is formed on the outermost layer. It was found that all of the iron oxide contained as (100% by mass) is hematite and the surface coverage by hematite is 96.2%. Further, as a result of measuring the magnetic properties of the surface-modified magnetic body specimen using a magnetometer, the intrinsic coercive force was 2052 kA / m, and no deterioration of the magnetic properties due to oxidation heat treatment was observed.

(Example 2) Rare earth-based sintered magnet in which the amount of Co substitution of Fe in the magnet is 1.2 mass% or more of the whole magnet (part 2)
The sintered body block obtained by the same method as in Example 1 was subjected to an aging treatment under the same conditions as in Example 1, and then surface grinding was performed under the same conditions as in Example 1 to obtain a thickness of 6 mm × length of 7 mm × A sintered magnet whose dimensions were adjusted to 7 mm in width (hereinafter referred to as “magnet body test piece”) was obtained. As a result of measuring the magnetic characteristics of this magnet body test piece with alcohol after washing with alcohol, the intrinsic coercive force was 2047 kA / m. After the magnet body test piece was washed with alcohol, heat treatment was performed under the same conditions as in Example 1 except that the heat treatment time was 30 minutes, thereby obtaining a surface-modified magnet body test piece. When this magnet body test piece was evaluated in the same manner as in Example 1, the modified layer formed on the surface of the magnet body test piece had a thickness of about 1.7 μm, and the configuration was obtained in Example 1. It was found that this was the same as the modified layer in the surface-modified magnetic body specimen (thickness of the outermost layer: about 70 nm). As a result of measuring the magnetic properties of the surface-modified magnetic body specimen using a magnetometer, the intrinsic coercive force was 2045 kA / m, and almost no deterioration of the magnetic properties due to oxidation heat treatment was observed.

(Example 3) Rare earth sintered magnet in which the amount of Co substitution of Fe in the magnet is 1.2 mass% or more of the entire magnet (part 3)
Nd: 16.4, Pr: 4.7, Dy: 9.4, B: 1.00, Co: 2.0, Al: 0.15, Ga: 0.07, Cu: 0.1, balance: An alloy flake having a composition of Fe (unit: mass%) and having a thickness of 0.2 to 0.3 mm was produced 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 a hydrogen treatment, the alloy flakes were embrittled to produce an amorphous powder having a size of about 0.15 to 0.2 mm.
After adding 0.04 wt% zinc stearate as a grinding aid to the coarsely pulverized powder produced by the above hydrogen treatment and mixing, an average powder particle size of about 3 μm is obtained by performing a pulverization step with a jet mill device. A fine powder was made and recovered in mineral oil to prevent oxidation.
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 degreasing process at 200 ° C. for 2 hours and a sintering process at 1050 ° C. for 4 hours were performed in a vacuum furnace to obtain a sintered body block.
The obtained sintered body block was subjected to an aging treatment under the same conditions as in Example 1, and then subjected to surface grinding under the same conditions as in Example 1, and the dimensions were adjusted to 6 mm in thickness × 7 mm in length × 7 mm in width. A sintered magnet (hereinafter referred to as “magnet body specimen”) was obtained. As a result of measuring the magnetic properties of this magnet body test piece with alcohol after washing with alcohol, the intrinsic coercive force was 2397 kA / m.
Next, after the magnet body test piece was washed with alcohol, the surface-modified magnet was subjected to heat treatment under the same conditions as in Example 1 except that the heat treatment temperature was 420 ° C. and the heat treatment time was 30 minutes. A body specimen was obtained. When this magnet body test piece was evaluated in the same manner as in Example 1, the modified layer formed on the surface of the magnet body test piece had a thickness of about 2.3 μm, and the configuration was obtained in Example 1. It was found that this was the same as the modified layer in the surface-modified magnetic body specimen (the thickness of the outermost layer: about 85 nm). As a result of measuring the magnetic properties of this surface-modified magnetic body specimen using a magnetometer, the intrinsic coercive force was 2395 kA / m, and almost no deterioration of the magnetic properties due to oxidation heat treatment was observed.

(Comparative example 1) Rare earth-based sintered magnet in which the amount of Co substitution of Fe in the magnet is 1.2 mass% or more of the whole magnet The same conditions as in Example 1 for the sintered body block obtained by the same method as in Example 1 A sintered magnet (hereinafter referred to as “magnet body test piece”) whose dimensions were adjusted to a thickness of 6 mm × length 7 mm × width 7 mm was obtained. After this magnet body test piece was washed with alcohol, an aging treatment was performed under the same conditions as in Example 1. As a result of measuring the magnetic properties of this magnet body test piece using a magnetometer, the intrinsic coercive force was 2050 kA / m. Next, the magnet body test piece subjected to the aging treatment was subjected to heat treatment under the same conditions as in Example 1 except that the heat treatment temperature was set to 350 ° C., thereby obtaining a surface-modified magnet body test piece. . When this magnet body test piece was evaluated in the same manner as in Example 1, the modified layer formed on the surface of the magnet body test piece had a thickness of about 1.6 μm, and the configuration was obtained in Example 1. It was found that this was the same as the modified layer in the surface-modified magnetic body test piece (the thickness of the outermost layer: about 60 nm). However, as a result of measuring the magnetic properties of the surface-modified magnetic body specimen using a magnetometer, the intrinsic coercive force was 1961 kA / m, and a significant deterioration in magnetic properties due to oxidation heat treatment was recognized.

Evaluation by dry / wet cycle test:
With reference to the neutral salt spray cycle test method based on JIS H8502-1999, a cycle test (cycle number: 3) only for drying and wetting without salt spray was performed in Examples 1 to 3 and Comparative Example 1, respectively. It performed with respect to the obtained magnetic body test piece by which surface modification was carried out, and the rating number evaluation (corrosion defect evaluation based on JISH8502-1999) after the test was implemented. The results are shown in Table 2. Table 2 also shows the evaluation results of magnet body test pieces (before heat treatment) subjected to the aging treatment obtained by the same method as in Example 1 (reference example).

  As is apparent from Table 2, the magnet body test piece subjected to surface modification by the method of the present invention in Examples 1 to 3 has excellent magnetic properties as described above, and also has a dry / wet cycle test. Later, it had sufficient corrosion resistance. On the other hand, the magnet body test piece subjected to surface modification by the method of Comparative Example 1 had sufficient corrosion resistance even after the dry / wet cycle test, but as described above, the magnetic properties were greatly deteriorated by the oxidation heat treatment. Was recognized. The above results show that the surface composition of the magnet is made uniform by performing surface grinding on the surface of the magnet before performing the oxidation heat treatment, thereby performing a uniform oxidation heat treatment on the entire surface of the magnet. A main layer having at least an oxygen content higher than that of the material, an amorphous layer composed of a composite oxide of R and Fe, and iron oxide mainly composed of stable hematite. It was thought that it contributed that the modified layer which consists of the structure which has the outermost layer as a structural component was formed over the whole surface of a magnet. In addition, the layered structure consisting of R confirmed in the modified layer has R flowing out from the main phase due to the decomposition of a part of the main phase of the material by heat treatment, and the grain boundary components that have become liquid phase by heat treatment. It was speculated that it was formed by being supplied to the crack part slightly generated in the modified layer due to the difference in thermal expansion coefficient between the raw material and the modified layer. It was thought that it contributed to the corrosion resistance of the layer.

(Example 4) Rare earth-based sintered magnet in which the amount of Co substitution of Fe in the magnet is less than 1.2% by mass of the whole magnet Nd: 18.6, Pr: 5.5, Dy: 7.1, B: 1. An alloy flake having a composition of 00, Co: 0.9, Al: 0.2, Cu: 0.1, balance: Fe (unit: mass%) and having a thickness of 0.2 to 0.3 mm is obtained by strip casting. Produced.
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 a hydrogen treatment, the alloy flakes were embrittled to produce an amorphous powder having a size of about 0.15 to 0.2 mm.
After adding 0.04 wt% zinc stearate as a grinding aid to the coarsely pulverized powder produced by the above hydrogen treatment and mixing, an average powder particle size of about 3 μm is obtained by performing a pulverization step with a jet mill device. A fine 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 surface of the obtained sintered block was subjected to surface grinding under the same conditions as in Example 1, and the sintered magnet (hereinafter referred to as “magnet test piece”) whose dimensions were adjusted to a thickness of 6 mm × length of 7 mm × width of 7 mm. Called). The magnet specimen was washed with alcohol, and then subjected to an aging treatment at 490 ° C. for 2.5 hours in a vacuum. As a result of measuring the magnetic properties of this magnet body test piece in the same manner as in Example 1, the intrinsic coercive force was 2092 kA / m. Next, the magnet body test piece subjected to the aging treatment was subjected to heat treatment under the same conditions as in Example 1 except that the heat treatment temperature was set to 350 ° C., thereby obtaining a surface-modified magnet body test piece. . When this magnet body test piece was evaluated in the same manner as in Example 1, the modified layer formed on the surface of the magnet body test piece had a thickness of about 1.9 μm, and the configuration was obtained in Example 1. It was found that this was the same as the modified layer in the surface-modified magnetic body specimen (thickness of the outermost layer: about 80 nm). As a result of measuring the magnetic properties of the surface-modified magnetic body test piece in the same manner as in Example 1, the intrinsic coercive force was 2085 kA / m, and almost no deterioration of the magnetic properties due to the oxidation heat treatment was observed.

(Reference Example 1) Examination of the relationship between the amount of Co substitution of Fe in a magnet and the effect on the magnetic properties of heat treatment A magnet specimen (hereinafter referred to as “magnet body”) subjected to an aging treatment obtained by the same method as in Example 1. Test piece 1 ”) and a magnet test piece (hereinafter referred to as“ magnet test piece 2 ”) subjected to the aging treatment obtained by the same method as in Example 4 to 240 ° C. to 460 ° C. Magnetic properties after heat treatment for 2 hours in vacuum at any temperature within the range are measured using a magnetometer (TPM-2-10: manufactured by Toei Kogyo Co., Ltd.), and the magnetic properties before heat treatment are performed. The effect of heat treatment on the magnetic properties was investigated. The results are shown in FIG. In addition, the vertical axis | shaft of FIG. 4 is a deterioration rate of intrinsic | native holding force, and was calculated | required with the following numerical formula.
Inherent coercive force deterioration rate (%) = ((A−B) / A) × 100
A: Intrinsic coercive force before heat treatment, B: Intrinsic coercivity after heat treatment As is apparent from FIG. 4, the magnetic body test piece 1 and the magnetic body test piece 2 affect the magnetic properties of the heat treatment depending on the temperature. The magnetic properties of the magnet specimen 2 are deteriorated by heat treatment in a temperature range (400 ° C. or higher) that does not adversely affect the magnetic characteristics of the magnet specimen 1, whereas the magnetic characteristics of the magnet specimen 2 are deteriorated. It was found that the magnetic properties of the magnet test piece 1 deteriorated by heat treatment in a temperature range (less than 400 ° C.) that does not adversely affect the magnetic field. As a result of further detailed examination based on this knowledge, when the amount of Co substitution of Fe of the magnet is 1.2% by mass or more of the whole magnet, the magnetic characteristics depending on the heat treatment temperature similar to the magnet specimen 1 are shown. Since it was found that the change was observed, the heat treatment temperature was less than 400 ° C., using as an index whether the Co substitution amount of Fe in the magnet was less than 1.2% by mass or 1.2% by mass or more of the whole magnet. It was found that the heat treatment can be performed without adversely affecting the magnetic properties by determining whether the temperature is 400 ° C. or higher.

The present invention can provide a method for producing a rare earth-based sintered magnet in which sufficient corrosion resistance is imparted by an oxidation heat treatment even in an environment where the humidity varies, and a decrease in magnetic properties due to the oxidation heat treatment is suppressed. Has industrial applicability.

Claims (7)

A method for producing a surface-modified rare earth sintered magnet, wherein the rare earth sintered magnet comprises 25 mass% to 40 mass% rare earth element: R, 0.6 mass% to 1.6 mass% B. (However, a part thereof may be substituted by C), 0% by mass to 1.0% by mass of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, At least one additional element selected from the group consisting of Mo, Ag, In, Sn, Hf, Ta, W, Pb, and Bi: M, the remainder being Fe partially substituted by Co, and inevitable impurities In the atmosphere of oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and water vapor partial pressure of 0.1 Pa to 1000 Pa (excluding 1000 Pa), the substitution of Co with Fe in the magnet The amount is 0.01 mass% to 1.2 mass% of the whole magnet. However, in the case of excluding 1.2% by mass), the Co substitution amount of Fe in the magnet is 1.2% to 2.5% by mass of the whole magnet at 200 ° C. to 400 ° C. (excluding 400 ° C.). In some cases, a method for producing a surface-modified rare earth-based sintered magnet comprising a step of performing a heat treatment at 400 ° C. to 600 ° C.   2. The method for producing a surface-modified rare earth sintered magnet according to claim 1, wherein the ratio of oxygen partial pressure to water vapor partial pressure (oxygen partial pressure / water vapor partial pressure) is 1 to 400. The temperature rise from room temperature to the temperature at which heat treatment is performed is performed in an atmosphere having an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 × 10 ⁻³ Pa to 100 Pa. The method for producing a surface-modified rare earth sintered magnet according to claim 1 or 2.   The method for producing a surface-modified rare earth sintered magnet according to any one of claims 1 to 3, wherein the surface of the magnet is subjected to surface grinding and then heat treatment is performed.   5. The method for producing a surface-modified rare earth sintered magnet according to claim 4, wherein surface grinding is performed using a grindstone having a grain size of # 60 to # 400.   A surface-modified rare earth sintered magnet manufactured by the method for producing a surface modified rare earth sintered magnet according to claim 1. The surface-modified part is composed of a main layer containing R, Fe, B and oxygen, an amorphous layer containing at least R, Fe and oxygen, and iron oxide mainly composed of hematite in order from the inside of the magnet. The surface-modified rare earth-based sintered magnet according to claim 6, comprising a surface-modified layer having at least three outermost layers .