WO2012002412A1 - 表面改質された希土類系焼結磁石の製造方法 - Google Patents
表面改質された希土類系焼結磁石の製造方法 Download PDFInfo
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- WO2012002412A1 WO2012002412A1 PCT/JP2011/064865 JP2011064865W WO2012002412A1 WO 2012002412 A1 WO2012002412 A1 WO 2012002412A1 JP 2011064865 W JP2011064865 W JP 2011064865W WO 2012002412 A1 WO2012002412 A1 WO 2012002412A1
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- H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
- H01F41/02—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets
- H01F41/0253—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets for manufacturing permanent magnets
- H01F41/026—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets for manufacturing permanent magnets protecting methods against environmental influences, e.g. oxygen, by surface treatment
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- H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
- H01F41/005—Impregnating or encapsulating
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
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- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
- C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
- C23C8/10—Oxidising
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- H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
- H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
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- H—ELECTRICITY
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- H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
- H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
- H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
- H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
- H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
- H01F1/047—Alloys characterised by their composition
- H01F1/053—Alloys characterised by their composition containing rare earth metals
- H01F1/0536—Alloys characterised by their composition containing rare earth metals sintered
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
- B22F3/24—After-treatment of workpieces or articles
- B22F2003/248—Thermal after-treatment
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C2202/00—Physical properties
- C22C2202/02—Magnetic
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- H—ELECTRICITY
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- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
- H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
- H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
- H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
- H01F1/04—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials metals or alloys
- H01F1/047—Alloys characterised by their composition
- H01F1/053—Alloys characterised by their composition containing rare earth metals
- H01F1/055—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5
- H01F1/057—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B
- H01F1/0571—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes
- H01F1/0575—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes pressed, sintered or bonded together
- H01F1/0577—Alloys characterised by their composition containing rare earth metals and magnetic transition metals, e.g. SmCo5 and IIIa elements, e.g. Nd2Fe14B in the form of particles, e.g. rapid quenched powders or ribbon flakes pressed, sintered or bonded together sintered
Definitions
- the present invention is surface-modified with sufficient corrosion resistance and excellent magnetic properties even in an environment where temperature and humidity fluctuate, such as transportation environment and storage environment where temperature and humidity are not managed.
- the present invention relates to a method for producing a rare earth sintered magnet.
- Rare earth-based sintered magnets such as R-Fe-B-based sintered magnets typified by Nd-Fe-B-based sintered magnets use 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. Therefore, rare-earth sintered magnets are usually used for practical purposes by forming a corrosion-resistant coating such as a metal coating or a resin coating on the surface. However, they are used as drive motors for hybrid vehicles and electric vehicles, and are used for air conditioners.
- 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 7 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.
- the temperature and humidity may fluctuate, such as in transport and storage environments where temperature and humidity are not controlled. In an environment in which minute dew condensation is repeatedly generated on the surface, sufficient corrosion resistance is not necessarily obtained.
- the water vapor partial pressure is preferably 10 hPa (1000 Pa) or more.
- Patent Document 8 proposed a method of performing heat treatment at 200 ° C. to 600 ° C.
- Japanese Patent No. 2844269 Japanese Patent Laid-Open No. 2002-57052 JP 2006-156853 A JP 2006-210864 A JP 2007-103523 A JP 2007-207936 A JP 2008-244126 A International Publication No. 2009/041639
- an object of the present invention is to provide a method for producing a surface-modified rare earth sintered magnet having extremely excellent corrosion resistance and excellent magnetic properties even in an environment where temperature and humidity vary.
- the present inventors have intensively studied whether or not there is room for improvement in the surface modification method for the rare earth sintered magnet proposed in Patent Document 8, and as a result, the water vapor partial pressure is made as small as possible.
- the ratio of oxygen partial pressure to water vapor partial pressure is made larger than the ratio (1 to 400) considered suitable in Patent Document 8 to improve the corrosion resistance. I found out that I can do it.
- the manufacturing method of the surface-modified rare earth sintered magnet of the present invention completed based on the above knowledge has an oxygen partial pressure of 1 ⁇ 10 3 relative to the rare earth sintered magnet as described in claim 1.
- a step of performing a heat treatment is characterized in that, in the manufacturing method according to claim 1, the total pressure of the atmosphere is set to 9 ⁇ 10 4 Pa to 1.2 ⁇ 10 5 Pa.
- the manufacturing method according to claim 3 is the manufacturing method according to claim 1, wherein the temperature rise from room temperature to the temperature at which the heat treatment is performed and / or the temperature drop after the heat treatment is performed in the same atmosphere as the atmosphere in which the heat treatment is performed. It is characterized by being performed by.
- the surface-modified rare earth-based sintered magnet of the present invention is manufactured by the manufacturing method according to claim 1 as described in claim 4.
- the rare earth sintered magnet according to claim 5 is the rare earth sintered magnet according to claim 4, wherein the surface potential difference is within 0.35V.
- the rare earth sintered magnet according to claim 6 is the rare earth sintered magnet according to claim 4, wherein iron oxide substantially consisting of hematite and substantially R 2 O 3 are used as the constituent components of the modified layer.
- the R oxide which consists of is contained, It is characterized by the above-mentioned.
- the present invention it is possible to provide a method for producing a surface-modified rare earth sintered magnet having excellent corrosion resistance and excellent magnetic properties even in an environment where temperature and humidity fluctuate.
- the method for producing a surface-modified rare earth sintered magnet of the present invention has an oxygen partial pressure of 1 ⁇ 10 3 Pa to 1 ⁇ 10 5 Pa and a water vapor partial pressure of 45 Pa or less compared to the rare earth sintered magnet. And a step of performing heat treatment at 200 ° C. to 600 ° C. in an atmosphere having a ratio of oxygen partial pressure to water vapor partial pressure (oxygen partial pressure / water vapor partial pressure) of 450 to 20000. is there.
- the oxygen partial pressure is defined as 1 ⁇ 10 3 Pa to 1 ⁇ 10 5 Pa because if the oxygen partial pressure is less than 1 ⁇ 10 3 Pa, the amount of oxygen in the atmosphere is too small, and the surface modification of the magnet The quality of the magnet is too long, or the surface of the magnet that is in contact with the holding member is not sufficiently modified, so that sufficient corrosion resistance is not imparted to the portion or the contact mark with the holding member is left on the portion. This is because they may remain. On the other hand, even if the oxygen partial pressure is made higher than 1 ⁇ 10 5 Pa, the effect of improving the corrosion resistance by increasing the oxygen partial pressure is not recognized so much, and there is a risk that the cost may be increased. is there.
- the oxygen partial pressure is desirably 1 ⁇ 10 4 Pa to 3 ⁇ 10 4 Pa 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 defined as 45 Pa or less. When the water vapor partial pressure is higher than 45 Pa, the amount of water vapor in the atmosphere is too large, and a stable modified layer that exhibits excellent corrosion resistance is formed on the surface of the magnet. Because there is a fear that it cannot be done.
- 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 450 to 20000 because if the ratio is smaller than 450, the amount of water vapor relative to the amount of oxygen in the atmosphere is too large. This is because a stable modified layer that exhibits excellent corrosion resistance may not be formed on the surface of the magnet.
- an atmosphere in which the ratio is greater than 20000 is a special environment and is not practical. Accordingly, the ratio is desirably 500 to 10,000, and more desirably 600 to 5,000.
- 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
- the heat treatment temperature is defined as 200 ° C. to 600 ° C.
- the heat treatment temperature is preferably 240 ° C. to 500 ° C., more preferably 350 ° C. to 450 ° 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.
- the step of raising the temperature of the magnet from room temperature to the temperature at which heat treatment is performed is desirably performed in the same atmosphere as the atmosphere in which heat treatment is performed.
- the same atmosphere as the atmosphere in which heat treatment is performed 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 Adverse effects can be avoided as much as possible.
- the heat treatment can be performed continuously without changing the atmosphere in the processing chamber after the temperature is raised.
- the rate of temperature rise may be, for example, 100 ° C./hour to 2000 ° C./hour.
- normal temperature refers to the temperature (for example, room temperature) of the environment where the rare earth-based sintered magnet on which surface modification is performed starts to raise the temperature. It means a temperature specified as 5 ° C to 35 ° C in the industrial standard JIS Z 8703.
- the step of lowering the temperature of the magnet after the heat treatment is performed in the same atmosphere as the atmosphere in which the heat treatment is performed.
- the temperature in such an atmosphere it is possible to prevent a phenomenon that the surface of the magnet is condensed during the process and the magnet is corroded to deteriorate the magnetic characteristics.
- the step of raising the temperature of the magnet from room temperature to the temperature at which heat treatment is performed, the step of heat-treating the magnet, and the step of lowering the temperature of the magnet after heat treatment are sequentially performed in the respective processing chamber environments in which the magnets are accommodated It may be performed by changing to an environment for performing the process, or may be performed by dividing the processing chamber into regions controlled by the environment for performing each process and sequentially moving the magnets to the respective regions.
- FIG. 1 shows an example of a continuous processing furnace in which the above three steps are divided into regions controlled by the environment for performing each step, and a magnet is sequentially moved to each region. It is a schematic diagram (side view).
- 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 decreasing 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 decreasing 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). 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 located above the main phase on the surface of the magnet is composed of iron oxide mainly composed of hematite ( ⁇ -Fe 2 O 3 ) having excellent stability, and is located above the grain boundary triple point. Is composed of an R oxide mainly composed of R 2 O 3 having excellent stability. It is desirable that 75 mass% or more of iron oxide contained as a component of the modified layer is hematite.
- the ratio of hematite in iron oxide and the ratio of R 2 O 3 in R oxide can be analyzed by, for example, Raman spectroscopy.
- the thickness of the surface modification layer formed on the surface of the rare earth sintered magnet is desirably 0.5 ⁇ m to 10 ⁇ m. 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 rare earth sintered magnet to which the present invention is applied include an R—Fe—B sintered magnet manufactured by the following manufacturing method.
- An alloy containing a rare earth element R of 25 mass% to 40 mass%, B (boron) of 0.6 mass% to 1.6 mass%, the balance Fe and inevitable impurities is prepared.
- R may contain a heavy rare earth element RH.
- 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 About 0.01 mass% to 1.0 mass% of at least one additive element M selected from the group consisting of Bi may be contained.
- the above-mentioned alloy can be suitably produced by rapidly cooling a molten raw material alloy by, for example, a strip casting method.
- 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.
- 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.
- 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 ⁇ m or less.
- the embrittled raw material alloy is preferably crushed more finely and cooled.
- 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.
- a fine powder having a size of about 0.1 ⁇ m to 20 ⁇ m (typically an average particle size of 3 ⁇ m to 5 ⁇ m) 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.
- 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.
- the magnetic powder produced by the above-described method is molded in an orientation magnetic field using a known press machine.
- the strength of the applied magnetic field is, for example, 1.5 Tesla to 1.7 Tesla (T).
- the molding pressure is set so that the green density of the molded body is, for example, about 4.0 g / cm 3 to 4.5 g / cm 3 .
- the above powder compact is carried out at a temperature in the range of 1000 ° C. to 1200 ° C. for 10 minutes to 240 minutes. A step of holding for 10 minutes to 240 minutes at a temperature in the range of 650 ° C.
- a step of further promoting sintering at a temperature higher than the above holding temperature for example, 1000 ° C. to 1200 ° C.
- sintering particularly when a liquid phase is generated (when the temperature is in the range of 650 ° C. 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.
- aging treatment 400 ° C. to 700 ° C.
- grinding for dimension adjustment may be performed.
- the surface-modified rare earth-based 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 magnetic properties due to the oxidation heat treatment is suppressed. 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 rare earth type sintered magnet manufactured by the manufacturing method of this invention.
- Example 1 Nd: 18.5, Pr: 5.7, Dy: 7.2, B: 1.00, Co: 0.9, Cu: 0.1, Al: 0.2, balance: Fe (unit is mass%)
- the alloy flakes having a composition of 0.2) to a thickness of 0.2 mm to 0.3 mm were produced by strip casting.
- this alloy flake was filled in a container and accommodated in a hydrogen treatment apparatus.
- 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.
- the alloy flakes were embrittled and coarsely pulverized powder having a size of about 0.15 mm to 0.2 mm was produced.
- a pulverization step using a jet mill device is performed to obtain a fine particle diameter of about 3 ⁇ m.
- 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 and adjusted to a thickness of 6 mm ⁇ length of 7 mm ⁇ width of 7 mm, A sintered magnet was obtained by ultrasonic washing.
- the sintered magnet obtained by the above method was subjected to a temperature raising process, an oxidation heat treatment process, and a temperature lowering process in the following manner to perform surface modification.
- Oxidation heat treatment step Heat treatment was performed at 400 ° C for 30 minutes in an air atmosphere with a dew point of -35 ° C.
- Temperature reduction step The temperature was lowered from 400 ° C. to room temperature by natural cooling in an air atmosphere with a dew point of ⁇ 35 ° C.
- the thickness of the modified layer formed on the surface of the sintered magnet by the above method was 2.2 ⁇ m.
- 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 (hereinafter the same).
- Surface modification was carried out by the same method as that described above. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.9 ⁇ m.
- Example 3 Surface modification was performed in the same manner as in Example 1 except that the oxidation heat treatment step was performed at 340 ° C. for 2 hours. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.3 ⁇ m.
- Surface modification was carried out by the same method as that described above. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 2.2 ⁇ m.
- Surface modification was performed in the same manner as in 1.
- the thickness of the modified layer formed on the surface of the sintered magnet was 2.3 ⁇ m.
- Test Example 1 1000 sintered magnets were prepared, and surface modification was performed on 100 sintered magnets per treatment under the conditions of Example 1, and 1000 surface-modified firings were performed by a total of 10 treatments. A magnet was obtained. Similarly, 1000 surface-modified sintered magnets were obtained by a total of 10 treatments under the conditions of Examples 2 to 5 and Comparative Examples 1 to 4. The surface-modified sintered magnet thus obtained was subjected to a 24 hour accelerated corrosion resistance test under a high temperature and high humidity condition of a temperature of 60 ° C. and a relative humidity of 90%. The number of magnets that rusted was investigated. The results are shown in Table 1. Table 1 also shows the results of the above accelerated corrosion resistance test performed on 1000 sintered magnets before surface modification (reference example).
- Test Example 2 With reference to the neutral salt spray cycle test method based on JIS H8502-1999, only the dry and wet cycle tests excluding the salt spray were subjected to surface modification obtained in Examples 1 to 5 and Comparative Example 1. The test was performed on 10 sintered magnets (samples obtained in different lots) (number of cycles: 3 and 6), and the rating number after the test (corrosion defect evaluation based on JIS H8502-1999) was performed. A magnet having a rating number of 7 or more was determined to be an acceptable product, and a magnet having a rating number less than 7 was determined to be an unacceptable product, and the number of magnets determined to be unacceptable among the 10 magnets was examined. As a result, in all of Examples 1 to 5 and Comparative Example 1, the number of magnets determined to be rejected was zero.
- the surface modification method described in Patent Document 8 is a method for imparting excellent corrosion resistance to rare earth sintered magnets. Later, since no particular deterioration in magnetic properties was observed, it was confirmed that the practical requirements were sufficiently met, but the surface modification method of the present invention further imparted excellent corrosion resistance. It was a method, and no particular decrease in magnetic properties was observed after the test.
- the surface of the surface-modified sintered magnet obtained in Example 1 was analyzed using a Raman spectroscopic analyzer (Holo Lab 5000R: manufactured by KAISER OPTICAL SYSTEM).
- the modified layer formed on the surface of the sintered magnet in Example 1 includes, as constituent components, iron oxide substantially made of hematite and R oxide substantially made of R 2 O 3. I understood it.
- SPM- FIG. 3 shows a potential mapping image measured using 9600 (manufactured by Shimadzu Corporation). As is apparent from FIG.
- the surface potential distribution of the sintered magnet surface-modified by performing the treatment under the conditions of Example 1 is very uniform, and is in the range of ⁇ 0.10 V to ⁇ 0.34 V.
- the surface potential difference was 0.24V.
- the surface potential distribution of the sintered magnet before the surface modification was non-uniform, being in the range of ⁇ 0.13 V to ⁇ 0.60 V, and the surface potential difference was 0.47 V.
- the surface-modified sintered magnet obtained in Example 1 has extremely excellent corrosion resistance because potentiometric corrosion is effectively suppressed. It was considered.
- the modified layer located above the main phase is mainly composed of hematite having excellent stability.
- the modified layer composed of iron oxide and located above the grain boundary triple point is composed of R oxide mainly composed of R 2 O 3 which is excellent in stability, while surface modification is performed under the conditions of Comparative Example 1.
- the modified layer positioned above the grain boundary triple point has an R hydroxide other than R 2 O 3.
- Example 6 Nd: 16.2, Pr: 4.5, Dy: 9.1, B: 0.93, Co: 2.0, Cu: 0.1, Al: 0.15, Ga: 0.07, balance:
- a sintered magnet is obtained using an alloy flake having a composition of Fe (unit: mass%) and having a thickness of 0.2 mm to 0.3 mm, and a heating step, an oxidation heat treatment step, and a cooling step are performed at a dew point of ⁇ 51 ° C.
- the surface modification was carried out in the same manner as in Example 6 except that was carried out at 400 ° C. for 20 minutes.
- the thickness of the modified layer formed on the surface of the sintered magnet was 1.6 ⁇ m.
- Example 8 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: Obtaining a sintered magnet using an alloy flake having a composition of Fe (unit: mass%) having a thickness of 0.2 mm to 0.3 mm, and performing the temperature raising step at a temperature raising rate of 520 ° C./hour; The surface modification was performed in the same manner as in Example 5 except that the oxidation heat treatment step was performed at 420 ° C. for 20 minutes. As a result, the thickness of the modified layer formed on the surface of the sintered magnet was 1.8 ⁇ m.
- Test Example 3 An accelerated corrosion resistance test was performed in the same manner as in Test Example 1, and the number of rusted magnets out of 1000 sintered magnets of Examples 6 to 8 and Comparative Example 5 was examined. The results are shown in Table 2. As is apparent from Table 2, in Examples 6 to 8, no rusted magnets were present.
- the present invention is industrially advantageous in that it can provide a method for producing a surface-modified rare earth sintered magnet having extremely excellent corrosion resistance and excellent magnetic properties even in an environment where temperature and humidity fluctuate.
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Abstract
Description
上記の通り、希土類系焼結磁石に対して耐食性を付与する方法としては、その表面に金属被膜や樹脂被膜などの耐食性被膜を形成する方法が代表的であるが、近年、酸化性雰囲気下での熱処理(酸化熱処理)を希土類系焼結磁石に対して行うことによって磁石の表面を改質する方法が簡易耐食性向上技術として注目されている。例えば、特許文献1や特許文献2には、酸素を利用して酸化性雰囲気を形成して熱処理する方法が記載され、特許文献3~特許文献7には、水蒸気を単独で利用して、或いは、水蒸気に酸素を組み合わせて酸化性雰囲気を形成して熱処理する方法が記載されている。しかしながら、これらの方法で希土類系焼結磁石に対して表面改質を行っても、温度や湿度の管理がされていない輸送環境や保管環境などのような、温度や湿度が変動することで磁石の表面に微細な結露を繰り返し生じさせてしまう環境では必ずしも十分な耐食性が得られないこと、特許文献3~特許文献7においては、水蒸気分圧は10hPa(1000Pa)以上が好適とされているが、このような水蒸気分圧が高い雰囲気下で熱処理を行うと、磁石の表面で起こる酸化反応によって水素が副産物として大量に生成し、磁石が生成した水素を吸蔵して脆化することで磁気特性が低下してしまうことが本発明者らの検討によって明らかになった。そこで本発明者らは、希土類系焼結磁石に対するより優れた表面改質方法として、酸素分圧と、特許文献3~特許文献7において不適とされている10hPa未満の水蒸気分圧を適切に制御した酸化性雰囲気下での熱処理方法、具体的には、酸素分圧が1×102Pa~1×105Paで水蒸気分圧が0.1Pa~1000Pa(但し1000Paを除く)の雰囲気下、200℃~600℃で熱処理を行う方法を特許文献8において提案した。
そこで本発明は、温度や湿度が変動する環境においても極めて優れた耐食性を有するとともに、優れた磁気特性を有する表面改質された希土類系焼結磁石の製造方法を提供することを目的とする。
また、請求項2記載の製造方法は、請求項1記載の製造方法において、雰囲気の全圧を9×104Pa~1.2×105Paとすることを特徴とする。
また、請求項3記載の製造方法は、請求項1記載の製造方法において、常温から熱処理を行う温度までの昇温および/または熱処理を行った後の降温を、熱処理を行う雰囲気と同じ雰囲気下で行うことを特徴とする。
また、本発明の表面改質された希土類系焼結磁石は、請求項4記載の通り、請求項1記載の製造方法によって製造されてなることを特徴とする。
また、請求項5記載の希土類系焼結磁石は、請求項4記載の希土類系焼結磁石において、表面電位差が0.35V以内であることを特徴とする。
また、請求項6記載の希土類系焼結磁石は、請求項4記載の希土類系焼結磁石において、改質層の構成成分として、実質的にヘマタイトからなる酸化鉄と実質的にR2O3からなるR酸化物が含まれることを特徴とする。
25mass%~40mass%の希土類元素Rと、0.6mass%~1.6mass%のB(硼素)と、残部Feおよび不可避不純物とを包含する合金を用意する。ここで、Rは重希土類元素RHを含んでいてもよい。また、Bの一部はC(炭素)によって置換されていてもよいし、Feの一部は(50mass%以下)は、他の遷移金属元素(例えば、CoまたはNi)によって置換されていてもよい。この合金は、種々の目的により、Al、Si、Ti、V、Cr、Mn、Ni、Cu、Zn、Ga、Zr、Nb、Mo、Ag、In、Sn、Hf、Ta、W、Pb、およびBiからなる群から選択された少なくとも1種の添加元素Mを0.01mass%~1.0mass%程度含有していてもよい。
上記の合金は、原料合金の溶湯を例えばストリップキャスト法によって急冷して好適に作製され得る。以下、ストリップキャスト法による急冷凝固合金の作製を説明する。
まず、上記組成を有する原料合金をアルゴン雰囲気中において高周波溶解によって溶解し、原料合金の溶湯を形成する。次に、この溶湯を1350℃程度に保持した後、単ロール法によって急冷し、例えば厚さ約0.3mmのフレーク状合金鋳塊を得る。こうして作製した合金鋳片を、次の水素粉砕処理前に例えば1mm~10mmのフレーク状に粉砕する。なお、ストリップキャスト法による原料合金の製造方法は、例えば、米国特許第5、383、978号明細書に開示されている。
[粗粉砕工程]
上記のフレーク状に粗く粉砕された合金鋳片を水素炉の内部へ収容する。次に、水素炉の内部で水素脆化処理(以下、「水素粉砕処理」や単に「水素処理」と称する場合がある)工程を行う。水素粉砕処理後の粗粉砕粉合金粉末を水素炉から取り出す際、粗粉砕粉が大気と接触しないように、不活性雰囲気下で取り出し動作を実行することが好ましい。そうすれば、粗粉砕粉が酸化・発熱することが防止され、磁石の磁気特性の低下が抑制できるからである。
水素粉砕処理によって、希土類合金は0.1mm~数mm程度の大きさに粉砕され、その平均粒径は500μm以下となる。水素粉砕処理後、脆化した原料合金をより細かく解砕するとともに冷却することが好ましい。比較的高い温度状態のまま原料を取り出す場合は、冷却処理の時間を相対的に長くすればよい。
[微粉砕工程]
次に、粗粉砕粉に対してジェットミル粉砕装置を用いて微粉砕を実行する。本実施形態で使用するジェットミル粉砕装置にはサイクロン分級機が接続されている。ジェットミル粉砕装置は、粗粉砕工程で粗く粉砕された希土類合金(粗粉砕粉)の供給を受け、粉砕機内で粉砕する。粉砕機内で粉砕された粉末はサイクロン分級機を経て回収タンクに集められる。こうして、0.1μm~20μm程度(典型的には平均粒径3μm~5μm)の微粉末を得ることができる。このような微粉砕に用いる粉砕装置は、ジェットミルに限定されず、アトライタやボールミルであってもよい。粉砕に際して、ステアリン酸亜鉛などの潤滑剤を粉砕助剤として用いてもよい。
[プレス成形]
本実施形態では、上記方法で作製された磁性粉末に対し、例えばロッキングミキサー内で潤滑剤を例えば0.3mass%添加・混合し、潤滑剤で合金粉末粒子の表面を被覆する。次に、上述の方法で作製した磁性粉末を公知のプレス装置を用いて配向磁界中で成形する。印加する磁界の強度は、例えば1.5テスラ~1.7テスラ(T)である。また、成形圧力は、成形体のグリーン密度が例えば4.0g/cm3~4.5g/cm3程度になるように設定される。
[焼結工程]
上記の粉末成形体に対して、例えば、1000℃~1200℃の範囲内の温度で10分間~240分間行う。650℃~1000℃の範囲内の温度で10分間~240分間保持する工程と、その後、上記の保持温度よりも高い温度(例えば、1000℃~1200℃)で焼結を更に進める工程とを順次行ってもよい。焼結時、特に液相が生成されるとき(温度が650℃~1000℃の範囲内にあるとき)、粒界相中のRリッチ相が融け始め、液相が形成される。その後、焼結が進行し、焼結磁石体が形成される。焼結工程の後、時効処理(400℃~700℃)や寸法調整のための研削を行ってもよい。
Nd:18.5、Pr:5.7、Dy:7.2、B:1.00、Co:0.9、Cu:0.1、Al:0.2、残部:Fe(単位はmass%)の組成を有する厚さ0.2mm~0.3mmの合金薄片をストリップキャスト法により作製した。
次に、この合金薄片を容器に充填し、水素処理装置内に収容した。そして、水素処理装置内を圧力500kPaの水素ガスで満たすことにより、室温で合金薄片に水素吸蔵させた後、放出させた。このような水素処理を行うことにより、合金薄片を脆化し、大きさ約0.15mm~0.2mmの粗粉砕粉末を作製した。
上記の水素処理により作製した粗粉砕粉末に対し粉砕助剤として0.04mass%のステアリン酸亜鉛を添加し混合した後、ジェットミル装置による粉砕工程を行うことにより、粉末粒径が約3μmの微粉末を作製した。
こうして作製した微粉末をプレス装置により成形し、粉末成形体を作製した。具体的には、印加磁界中で粉末粒子を磁界配向した状態で圧縮し、プレス成形を行った。その後、成形体をプレス装置から抜き出し、真空炉により1050℃で4時間の焼結工程を行い、焼結体ブロックを得た。
得られた焼結体ブロックを真空中にて490℃で2.5時間の時効処理を行った後、その表面に対し研削加工を行い、厚さ6mm×縦7mm×横7mmに寸法調整し、超音波水洗を行うことで焼結磁石を得た。
(1)昇温工程
常温(25℃を意味する。以下同じ)から酸化熱処理を行う温度(400℃)までの昇温を、露点-35℃の大気(酸素分圧20000Pa,水蒸気分圧32Pa,酸素分圧/水蒸気分圧=625。以下同じ)の雰囲気下、500℃/時間の昇温速度で行った。
(2)酸化熱処理工程
露点-35℃の大気の雰囲気下、400℃で30分間の熱処理を行った。
(3)降温工程
露点-35℃の大気の雰囲気下、自然放冷にて400℃から常温まで行った。
昇温工程、酸化熱処理工程、降温工程を、露点-45℃の大気(酸素分圧20000Pa,水蒸気分圧11Pa,酸素分圧/水蒸気分圧=1818)の雰囲気下で行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは1.9μmであった。
酸化熱処理工程を340℃で2時間行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは1.3μmであった。
昇温工程、酸化熱処理工程、降温工程を、露点-32℃の大気(酸素分圧20000Pa,水蒸気分圧42Pa,酸素分圧/水蒸気分圧=476)の雰囲気下で行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは1.8μmであった。
昇温工程、酸化熱処理工程、降温工程を、露点-60℃の大気(酸素分圧20000Pa,水蒸気分圧2Pa,酸素分圧/水蒸気分圧=10000)の雰囲気下で行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは2.2μmであった。
昇温工程、酸化熱処理工程、降温工程を、露点0℃の大気(酸素分圧20000Pa,水蒸気分圧600Pa,酸素分圧/水蒸気分圧=33.3)の雰囲気下で行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは2.0μmであった。
昇温工程、酸化熱処理工程、降温工程を、露点10℃の大気(酸素分圧20000Pa,水蒸気分圧1230Pa,酸素分圧/水蒸気分圧=16.3)の雰囲気下で行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは2.3μmであった。
昇温工程、酸化熱処理工程、降温工程を、気温21℃×相対湿度63%の大気(酸素分圧20000Pa,水蒸気分圧1570Pa,酸素分圧/水蒸気分圧=12.7)の雰囲気下で行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは2.2μmであった。
真空熱処理炉を用いて、昇温工程、酸化熱処理工程、降温工程を、露点-60℃(水蒸気分圧2Pa)で圧力100Pa(0.75Torr)の減圧酸素雰囲気下(酸素分圧/水蒸気分圧=50)で行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは1.6μmであった。
焼結磁石を1000個用意し、実施例1の条件で1回の処理につき100個の焼結磁石に対して表面改質を行い、合計10回の処理によって1000個の表面改質された焼結磁石を得た。同様にして、実施例2~実施例5、比較例1~比較例4のそれぞれの条件でそれぞれ合計10回の処理によって1000個の表面改質された焼結磁石を得た。こうして得た表面改質された焼結磁石に対し、温度60℃×相対湿度90%の高温高湿条件下での24時間の耐食性加速試験を行った後、外観観察を行い、1000個の磁石のうち発錆した磁石の個数を調べた。結果を表1に示す。なお、表1には表面改質を行う前の焼結磁石1000個に対して上記の耐食性加速試験を行った結果をあわせて示す(参考例)。
JIS H8502-1999に基づく中性塩水噴霧サイクル試験方法を参考にし、塩水噴霧を除いた乾燥と湿潤だけのサイクル試験を、実施例1~実施例5と比較例1で得た表面改質された焼結磁石それぞれ10個(別々のロットで得たサンプル)に対して行い(サイクル数:3および6)、試験後のレイティングナンバ評価(JIS H8502-1999に基づく腐食欠陥評価)を行った。レイティングナンバが7以上の磁石を合格品、7未満の磁石を不合格品と判定し、10個の磁石のうち不合格品と判定された磁石の個数を調べた。その結果、実施例1~実施例5と比較例1のすべてにおいて不合格品と判定された磁石の個数は0個であった。
上記の試験例1の耐食性加速試験と試験例2の乾燥・湿潤サイクル試験の結果から、特許文献8に記載の表面改質方法は、希土類系焼結磁石に対する優れた耐食性付与方法であり、試験後に特段の磁気特性の低下も認められなかったことから、実用上の要求を十分に満たすものであることを確認することができたが、本発明の表面改質方法は、さらに優れた耐食性付与方法であり、試験後に特段の磁気特性の低下も認められなかった。
ラマン分光分析装置(Holo Lab 5000R:KAISER OPTICAL SYSTEM社製)を用いて実施例1で得た表面改質された焼結磁石の表面を分析したところ、実質的に検出された表面改質層の構成成分は安定性に優れるヘマタイトとR2O3のみであった(図2)。従って、実施例1において焼結磁石の表面に形成された改質層には、構成成分として、実質的にヘマタイトからなる酸化鉄と、実質的にR2O3からなるR酸化物が含まれることがわかった。また、別途、焼結磁石を湿式法によって鏡面加工した後、実施例1の条件で処理を行うことで得た表面改質された焼結磁石の表面電位分布を、走査型プローブ顕微鏡(SPM-9600:島津製作所社製)を用いて測定した電位マッピング像を図3に示す。図3から明らかなように、実施例1の条件で処理を行うことで表面改質された焼結磁石の表面電位分布は非常に均一であって、-0.10V~-0.34Vの範囲にあり、表面電位差は0.24Vであった。これに対し、表面改質を行う前の焼結磁石の表面電位分布は不均一であって、-0.13V~-0.60Vの範囲にあり、表面電位差は0.47Vであったことからすれば(電位マッピング像を図4に示す)、実施例1で得た表面改質された焼結磁石が極めて優れた耐食性を有するのは、電位差腐食が効果的に抑制されていることによるものと考えられた。
本発明者らは、鏡面加工した焼結磁石に対し、実施例1の条件で表面改質を行った場合、その主相の上部に位置する改質層は安定性に優れるヘマタイトを主体とする酸化鉄から構成され、粒界三重点の上部に位置する改質層は安定性に優れるR2O3を主体とするR酸化物から構成される一方で、比較例1の条件で表面改質を行った場合、実施例1の条件で表面改質を行った場合との相違点として、粒界三重点の上部に位置する改質層に、R2O3の他にR水酸化物などの不安定なR化合物と推察される化合物が存在することを、走査型電子顕微鏡とエネルギー分散型X線分析装置を用いた断面の組成分析、および、ラマン分光分析装置を用いた表面分析によって別途確認している。従って、実施例1と比較例1で得た表面改質された焼結磁石の耐食性加速試験の結果の相違は、磁石の表面にわずかに存在する粒界三重点の上部に位置する改質層の構成成分の相違によってもたらされているものと考えられた。
実施例1で得た表面改質された焼結磁石をロータの内部に埋め込む工程を経て、ハイブリッド自動車や電気自動車の駆動モータとして使用されるIPMモータを製造した。
Nd:16.2、Pr:4.5、Dy:9.1、B:0.93、Co:2.0、Cu:0.1、Al:0.15、Ga:0.07、残部:Fe(単位はmass%)の組成を有する厚さ0.2mm~0.3mmの合金薄片を用いて焼結磁石を得ることと、昇温工程、酸化熱処理工程、降温工程を、露点-51℃の大気(酸素分圧20000Pa,水蒸気分圧6Pa,酸素分圧/水蒸気分圧=3333)の雰囲気下で行うこと以外は実施例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは2.0μmであった。
昇温工程、酸化熱処理工程、降温工程を、露点-54℃の大気(酸素分圧20000Pa,水蒸気分圧4Pa,酸素分圧/水蒸気分圧=5000)の雰囲気下で行うことと、酸化熱処理工程を400℃で20分間行うこと以外は実施例6と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは1.6μmであった。
Nd:19.8、Pr:5.7、Dy:4.3、B:0.93、Co:2.0、Cu:0.1、Al:0.15、Ga:0.07、残部:Fe(単位はmass%)の組成を有する厚さ0.2mm~0.3mmの合金薄片を用いて焼結磁石を得ることと、昇温工程を520℃/時間の昇温速度で行うことと、酸化熱処理工程を420℃で20分間行うこと以外は実施例5と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは1.8μmであった。
昇温工程と降温工程を、露点-60℃の大気(酸素分圧20000Pa,水蒸気分圧2Pa,酸素分圧/水蒸気分圧=10000)の雰囲気下で行うこと以外は比較例1と同じ方法で表面改質を行った。その結果、焼結磁石の表面に形成された改質層の厚みは1.9μmであった。
試験例1と同じ方法で耐食性加速試験を行い、実施例6~実施例8と比較例5の焼結磁石それぞれ1000個のうち発錆した磁石の個数を調べた。結果を表2に示す。表2から明らかなように、実施例6~実施例8では発錆した磁石は存在しなかった。
Claims (6)
- 希土類系焼結磁石に対し、酸素分圧が1×103Pa~1×105Paで水蒸気分圧が45Pa以下であり、かつ、酸素分圧と水蒸気分圧の比率(酸素分圧/水蒸気分圧)が450~20000の雰囲気下、200℃~600℃で熱処理を行う工程を含んでなることを特徴とする表面改質された希土類系焼結磁石の製造方法。
- 雰囲気の全圧を9×104Pa~1.2×105Paとすることを特徴とする請求項1記載の製造方法。
- 常温から熱処理を行う温度までの昇温および/または熱処理を行った後の降温を、熱処理を行う雰囲気と同じ雰囲気下で行うことを特徴とする請求項1記載の製造方法。
- 請求項1記載の製造方法によって製造されてなることを特徴とする表面改質された希土類系焼結磁石。
- 表面電位差が0.35V以内であることを特徴とする請求項4記載の希土類系焼結磁石。
- 改質層の構成成分として、実質的にヘマタイトからなる酸化鉄と実質的にR2O3からなるR酸化物が含まれることを特徴とする請求項4記載の希土類系焼結磁石。
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