JP2014218699A - Method of producing rare earth sintered magnet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a rare earth sintered magnet which is excellent in moldability and lubricity of a mold in a molding step, compared with conventional production methods, and eliminates the need of a decarbonization treatment for removing solvent, resulting in improved efficiency of steps.SOLUTION: In a method of producing a rare earth sintered magnet that comprises charging an alloy powder for rare earth magnets into a cavity formed with a mold and a lower punch, molding in a magnetic field by uniaxial compression and heat-treating the molding to produce a rare earth sintered magnet, a liquid mold lubricant comprising 95-99.5 mass% of a fatty acid ester with a melting point or a freezing point of 15°C or lower and 0.5-5 mass% of a 10-24C fatty acid with a melting point of 30°C or higher is sprayed onto the mold as particles of an average particle size of 1-50 μm, and then the alloy powder for rare earth magnet is charged into the cavity to mold in a magnetic field so that a molding is obtained, which is then heat-treated.

Description

  The present invention relates to a method for producing a rare earth sintered magnet, and more particularly to a method for lubricating a mold used when a rare earth magnet alloy powder is molded in a magnetic field.

  Since rare earth magnets typified by Nd magnets have high magnetic properties, they have been widely used in recent years for various motors and sensors used in hard disks, air conditioners, hybrid vehicles, and the like. .

  Rare earth magnets are usually manufactured through the following steps by powder metallurgy. First, the raw materials are blended so as to have a predetermined composition, and an alloy is prepared by melting and casting using a high-frequency melting furnace or the like, and the alloy is roughly pulverized by a jaw crusher, a brown mill, a pin mill, hydrogenation, etc. Furthermore, it is finely pulverized by a jet mill or the like to obtain a fine powder having an average particle diameter of 1 to 10 μm. Next, in order to impart magnetic anisotropy, the fine powder is formed into a desired shape in a magnetic field to produce a molded body, and sintered and heat-treated to obtain a sintered magnet.

  As a molding method in a magnetic field in the production of a rare earth magnet by a general powder metallurgy method, a die molding is performed in which fine powder is filled into a cavity formed from a die and a lower punch and uniaxially pressed. In this mold forming, since the fine powder is compression-molded in the mold, the friction between the mold and the fine powder or the sliding punch is very large. For this reason, wrinkles may occur on the inner surface of the mold, and the generated wrinkles may be transferred to the molded body, and the wrinkles may be the starting point and cause fine cracks or chips in the molded body. In addition, the friction leads to an increase in the punching pressure generated when the punch is slid from the mold, which not only easily causes cracking, cracking, chipping, etc. of the molded body, but in the worst case, the punch, It may cause damage to the mold and failure of the molding machine.

  To reduce mold friction and prevent wrinkles, the inner surface of the mold is mirror-polished, a hard material such as cemented carbide is used as the mold material, and it adheres to the inner surface of the mold every one or several shots. However, it is not possible to obtain a sufficient effect.

  In addition, the moldability is improved by a method of mixing at least one of stearic acid, zinc stearate, and bisamide with a fine powder as a lubricant in advance (Patent Document 1: JP-A-4-214803). Generation | occurrence | production of the wrinkles etc. to a molded object by reducing the friction of powder and a metal mold | die can be suppressed. However, although the lubrication effect between the fine powder and the mold is recognized, it does not contribute to the lubrication of the mold and the punch, and if the amount of the lubricant added to the fine powder is increased to further enhance the lubrication effect, the lubrication Problems arise in magnetic properties such as a decrease in coercive force and deterioration in squareness due to a large amount of carbon present in the agent remaining in the sintered magnet.

  The most effective way to reduce mold friction is to lubricate the mold with a lubricant. As the mold lubricant, a solution in which a fatty acid such as stearic acid or behenic acid or an ester thereof is dissolved in a solvent consisting of a simple substance or a mixture of ethanol, normal propyl alcohol, isopropyl alcohol, etc., methyl caproate, methyl caprylate , A solution composed of a saturated fatty acid and a hydrocarbon solvent (Patent Document 2: JP 2000-109903 A) and the like are used. However, when a large amount of the above-mentioned mold lubricant is used, the lubricity is improved, but the mold lubricant penetrates into the molded body and a large amount of carbon remains after sintering, which may deteriorate the magnet characteristics. . In addition, if the above-mentioned mold lubricant is less than the required amount, the amount of carbon remaining in the sintered body is reduced, but the lubricity is poor, and molding defects such as cracks, cracks and chipping of the molded product are problematic. Become. When the above mold lubricant is used in an optimum amount, the lubricity will be good, but the residual carbon content in the sintered magnet will not affect coercive force and squareness. And a decarbonization process for volatilizing a hydrocarbon solvent at a predetermined temperature is required. In addition, since the solvent that has volatilized may accumulate in the vacuum pump and reduce the vacuum capacity, it is necessary to use more expensive equipment such as a dry pump instead of a rotary pump that is normally used as a vacuum pump. It is difficult to rationalize the process and its facilities.

JP-A-4-214803 JP 2000-109903 A

  The present invention has been made in view of the above circumstances, and has superior moldability and mold lubricity in the molding process than conventional manufacturing methods, and further eliminates the need for decarbonization treatment to remove the solvent, thereby making the process more efficient. It is an object of the present invention to provide a method for producing a rare earth sintered magnet that can be converted into a magnet.

  As a result of intensive studies to solve the above problems, the present inventors have found that the melting point or the freezing point is 95 to 99.5% by mass of a fatty acid ester having 10 to 24 carbon atoms and the fatty acid having 10 to 24 carbon atoms and a melting point of 30 ° C. or more. After spraying a mold lubricant consisting of 0.5 to 5% by mass as particles with an average particle diameter of 1 to 50 μm on the mold, filling the cavity with alloy powder for rare earth magnet and molding in a magnetic field, It has been found that the manufacturing method is excellent in moldability in the process and lubricity of the mold. In addition, since no solvent is used, it has been found that decarbonization treatment for removing the solvent is unnecessary, and it is effective for improving the efficiency of the process, and the present invention has been made.

That is, the present invention provides the following method for producing a rare earth sintered magnet.
[1] In a method of manufacturing a rare earth sintered magnet by filling rare earth magnet alloy powder into a cavity formed by a mold and a lower punch, then forming it by uniaxial pressing in a magnetic field, and then heat-treating it. When molding in a mold, the melting point or freezing point is comprised of 95 to 99.5% by mass of a fatty acid ester having a melting point of 15 ° C. or less and 0.5 to 5% by mass of a fatty acid having 10 to 24 carbon atoms and a melting point of 30 ° C. or more. After spraying a liquid mold lubricant as a particle having an average particle diameter of 1 to 50 μm on a mold, the cavity is filled with the alloy powder for rare earth magnets, and molded in a magnetic field to obtain a molded body. A method for producing a rare earth sintered magnet, characterized by heat treatment.
[2] The rare earth sintered magnet according to [1], wherein the mold lubricant is sprayed into the cavity at a spray amount of 1 × 10 ⁻⁴ to 1 × 10 ⁻² kg / m ^2. Method.
[3] The rare earth firing according to [1] or [2], wherein the mold lubricant has a kinematic viscosity at 20 ° C. of 2 × 10 ⁻⁶ to 1 × 10 ⁻⁵ m ² / s. A manufacturing method of a magnet.

  According to the method for producing a rare earth sintered magnet of the present invention, since it is excellent in moldability and mold lubricity, a stable and high quality rare earth sintered magnet can be provided, and further, decarbonization treatment can be performed. Since it is unnecessary, the efficiency of the process can be improved and the industrial utility value is extremely high.

It is a figure which shows the relationship between the number of shaping | molding shots of Example 1 and Comparative Example 1, and a molded object density.

Below, the manufacturing method of the rare earth sintered magnet which concerns on this invention is demonstrated.
The method for producing a rare earth sintered magnet according to the present invention comprises filling rare earth magnet alloy powder into a cavity formed by a mold and a lower punch, then forming it by uniaxial pressing in a magnetic field, and then heat-treating it. In the method for producing a sintered magnet, when molding a mold in a magnetic field, 95 to 99.5% by mass of a fatty acid ester having a melting point or freezing point of 15 ° C. or lower and a fatty acid having 10 to 24 carbon atoms and a melting point of 30 ° C. or higher. After spraying a liquid mold lubricant consisting of 0.5 to 5% by mass as particles having an average particle diameter of 1 to 50 μm onto the mold, the cavity is filled with the alloy powder for rare earth magnets and molded in a magnetic field. Thus, a molded body is obtained and then heat-treated.

Here, the rare earth permanent magnet to which the present invention is applied is particularly preferably an Nd-based rare earth sintered magnet, and its composition is R (R is one or two selected from Nd, Pr, Dy, Tb and Ho). The above rare earth element) is 20 to 35% by mass, Co is 15% by mass or less, B is 0.2 to 8% by mass, and Ni, Nb, Al, Ti, Zr, Cr, V, Mn, Mo, Preferably at least one element selected from Si, Sn, Ga, Cu and Zn is 8% by mass or less, the remainder is made of Fe and inevitable impurities, and is finely pulverized by a jet mill or the like. The alloy fine powder for rare earth magnets of 10 μm is molded using a mold in a magnetic field.
In addition, the average particle diameter (average fine particle diameter) of the pulverized rare earth alloy fine powder can be obtained, for example, as a weight average value (or median diameter) by a laser light diffraction method.

  In the present invention, when the fine powder is molded using a mold in a magnetic field, the melting point or the freezing point is 95 to 99.5% by mass of a fatty acid ester having a melting point of 15 ° C. or less and the carbon number is 10 to 24 and the melting point is 30 ° C. or more. A solution containing 0.5 to 5% by mass of the fatty acid (100% by mass in total of fatty acid ester and fatty acid) is used as a mold lubricant.

  That is, the fatty acid ester used in the present invention has a melting point or freezing point of 15 ° C. or lower, and preferably 5 ° C. or lower. This is because the mold lubricant is liquefied in the working environment of 20 to 25 ° C. If it is a liquid fatty acid ester in the working environment of 20 to 25 ° C., the melting point or the lower limit of the freezing point is not limited. Moreover, if it is fatty acid ester whose melting | fusing point or freezing point is 15 degrees C or less, it will not restrict | limit especially individually or its mixture, For example, methyl laurate (melting | fusing point 5 degreeC), methyl oleate (melting point -20 degreeC), etc. Can be mentioned.

  The fatty acid used in the present invention has a carbon number of 10 to 24, preferably 14 to 22, a melting point of 30 ° C. or higher, preferably 50 ° C. or higher, and must be solid at 20 to 25 ° C. which is a working environment. When the number of carbon atoms of the fatty acid is less than 10, it becomes liquid at 20 to 25 ° C. which is a working environment, and when it exceeds 24, it is solid at 20 to 25 ° C. which is a working environment, but it is difficult to obtain and expensive. Further, if the melting point is less than 30 ° C., sufficient effects in the present invention cannot be obtained. The upper limit of the melting point is not limited as long as it is a fatty acid having 10 to 24 carbon atoms and a solid fatty acid at 20 to 25 ° C. which is a working environment. Further, any fatty acid having 10 to 24 carbon atoms and a melting point of 30 ° C. or higher is not particularly limited alone or a mixture thereof. Examples thereof include stearic acid (melting point 69.6 ° C.) and behenic acid (melting point 81.5). ° C).

The mold lubricant used in the present invention is a solution comprising 95 to 99.5% by mass of the fatty acid ester and 0.5 to 5% by mass of the fatty acid, and dissolves a solid fatty acid in a liquid fatty acid ester. It is liquid in a use environment of 20 to 25 ° C. When the amount of the fatty acid ester is less than 95% by mass and the amount of the fatty acid exceeds 5% by mass, the dissolution amount of the fatty acid with respect to the fatty acid ester is exceeded, resulting in undissolved residue. This is not preferable because it causes clogging of the spray nozzle. Moreover, when the fatty acid ester exceeds 99.5 mass% and the fatty acid is less than 0.5 mass%, the lubricity of the mold tends to decrease. Therefore, the mold lubricant is composed of 95 to 99.5% by mass of fatty acid ester and 0.5 to 5% by mass of fatty acid, preferably 95 to 99% by mass of fatty acid ester and 1 to 5% by mass of fatty acid. Shall.
By using a solution composed of a fatty acid ester and a fatty acid as a mold lubricant, excellent lubricity and moldability can be obtained by a synergistic effect combining fluid lubrication and boundary lubrication.

In the present invention, the mold lubricant is sprayed onto the mold as particles having an average particle diameter of 1 to 50 μm, preferably 5 to 30 μm. When the mold lubricant particles have an average particle diameter of less than 1 μm, the specific surface area of the mold lubricant particles increases, so even if the fatty acid ester has a low vapor pressure, the volatilization amount cannot be ignored. Since the particles become so light that they rise when sprayed, it is difficult to adjust the spray amount of the mold lubricant to be deposited in the cavity. In addition, when the average particle diameter exceeds 50 μm, it is difficult to finely adjust the spray amount, and it becomes smaller than the target spray amount and the mold lubricity is lowered, resulting in poor molding, or more than the target spray amount. As a result, a large amount of carbon remains in the magnet after sintering, thereby deteriorating the magnet characteristics.
In addition, the average particle diameter of the metal lubricant particles can be obtained, for example, as a weight average value (or median diameter) by a laser light diffraction method.

  Further, as a method for spraying the mold lubricant, a method in which the hydraulic pressure and nitrogen pressure of the mold lubricant are controlled by a low-pressure precision regulator and sprayed from the spray nozzle into the cavity is preferable.

At this time, the amount of the mold lubricant sprayed into the cavity is sprayed per unit area of the cavity inner area composed of the mold and the lower punch, regardless of the cavity depth and the shape of the mold. The amount is preferably 1 × 10 ⁻⁴ to 1 × 10 ⁻² kg / m ² . When the spray amount is less than 1 × 10 ⁻⁴ kg / m ² , the lubricity of the mold is lowered, so that the release pressure is increased, and molding defects such as cracks, cracks and chipping of the molded body are likely to occur. In addition, when the spray amount exceeds 1 × 10 ⁻² kg / m ² , the moldability and lubricity are good, but the coercive force decreases due to the large amount of carbon components in the lubricant remaining in the sintered magnet. In addition, there is a risk of problems in magnetic properties such as deterioration of squareness.

Further, the kinematic viscosity at 20 ° C. of the mold lubricant is preferably 2 × 10 ⁻⁶ to 1 × 10 ⁻⁵ m ² / s, and 4 × 10 ⁻⁶ to 1 × 10 ⁻⁵ m ² / s. More preferably. When the kinematic viscosity at 20 ° C. is less than 2 × 10 ⁻⁶ m ² / s, the mold lubricant adhering to the cavity flows and the adhering state is not maintained, so that the lubricity may be lowered. ^Yes , if it exceeds 1 × 10 ⁻⁵ m ² / s, the kinematic viscosity is too high, and thus there is a possibility that stable spraying cannot be performed due to poor fluidity.
The kinematic viscosity is a value measured at 20 ° C. using an Ostwald viscometer.

After spraying the mold lubricant onto the mold under the above conditions, the alloy powder for rare earth magnet is filled in the cavity, and a magnetic field of 1.0 to 2.5 T is applied, and a pressure of 20 to 200 MPa is applied. A compact is obtained by applying pressure and molding in a magnetic field.
The molded body obtained under the above conditions was sintered in a high-vacuum or non-oxidizing atmosphere gas such as argon in a heat treatment furnace at 1,000 to 1,200 ° C. for 1 to 10 hours, A rare earth magnet is obtained by performing a heat treatment in a vacuum or in a non-oxidizing atmosphere gas such as argon at a temperature lower than the sintering temperature, preferably at a temperature of 400 to 700 ° C.

  According to the above method for producing a rare earth sintered magnet, since it is excellent in moldability and mold lubricity, a stable and high quality rare earth sintered magnet can be provided, and decarbonization treatment is unnecessary. Therefore, the efficiency of the process can be improved and the industrial utility value is extremely high.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following examples, the kinematic viscosity is a value measured using an Ostwald viscometer at 20 ° C. Further, the average fine particle diameter of the pulverized alloy powder was determined as a weight average value by a laser light diffraction method, and the average particle diameter of the sprayed mold lubricant was determined as a median diameter by a laser light diffraction method.

[Example 1, Comparative Example 1]
Nd magnet having Nd: 31.0 mass%, Co: 1.0 mass%, B: 1.0 mass%, Al: 0.2 mass%, Cu: 0.2 mass%, Fe: balance The alloy was coarsely pulverized by hydrogenation and finely pulverized by a jet mill to produce a fine powder having an average fine particle diameter of 3.0 μm, and a compact was obtained by molding in a magnetic field. Specifically, the mold has a shape of 20 × 40 mm and a cavity depth of 50 mm. After spraying a mold lubricant on the mold under the conditions shown in Table 1, the rare earth alloy fine powder is filled and molded. A molded body was obtained by molding in a magnetic field at a pressure of 100 MPa. Here, the steps from the metal lubricant spraying to forming in a magnetic field were repeated 300 times (number of forming shots 300), and the relationship between the number of forming shots and the density of the obtained formed body was investigated. The result is shown in FIG.
The kinematic viscosity at 20 ° C. of the mold lubricant used in Example 1 was 6 × 10 ⁻⁶ m ² / s.

  As is clear from FIG. 1, in Example 1, a substantially constant molded body density was obtained through 300 shots of molding, whereas in Comparative Example 1, there was a tendency to decrease the molded body density from about 50 shots. . Furthermore, since a sharp drop occurred from around 100 shots, molding was stopped at 170 shots. This indicates that in Comparative Example 1, the molding pressure is insufficient due to increased friction with the mold, that is, the lubricity is insufficient. On the other hand, it can be seen that Example 1 maintains good lubricity.

[Examples 2 and 3, Comparative Examples 2 and 3]
Nd magnet having Nd: 31.0 mass%, Co: 1.0 mass%, B: 1.0 mass%, Al: 0.2 mass%, Cu: 0.2 mass%, Fe: balance The alloy was coarsely pulverized by hydrogenation and finely pulverized by a jet mill to produce a fine powder having an average fine particle diameter of 3.0 μm, and a compact was obtained by molding in a magnetic field. Specifically, the mold has a shape of 20 × 20 mm and a cavity depth of 40 mm. After spraying a mold lubricant on the mold under the conditions shown in Table 2, the rare earth alloy fine powder is filled and molded. A molded body was obtained by molding in a magnetic field at a pressure of 100 MPa and a drawing pressure shown in Table 2. The obtained molded body was sintered in a heat treatment furnace in vacuum at 1,060 ° C. for 3 hours, and then subjected to low temperature heat treatment in vacuum at 500 ° C. for 3 hours to produce a sintered magnet. Table 3 shows the magnetic properties of the sintered magnets produced.
The kinematic viscosity at 20 ° C. of the mold lubricant used in this example was 4 × 10 ⁻⁶ m ² / s.

Table 2 shows that in Examples 2 and 3 and Comparative Example 3, there is no difference in the appearance and the punching pressure of the molded body. In Comparative Example 2, the punching pressure is about 30% higher than the others, and the molded body is broken. This indicates that the lubricity and moldability are poor.
The amount of carbon 1 in the sintered body in Table 3 indicates the amount of carbon in the region from the side of the sintered body to the inside of about 1 mm, and the amount of carbon 2 indicates the amount of carbon in the central portion of the sintered body. As shown in Table 3, Examples 2 and 3 and Comparative Example 2 are good with no difference in magnetic properties, but Comparative Example 3 is inferior in coercive force and squareness. This is considered to be due to the influence of the amount of carbon remaining in the sintered body due to the mold lubricant because the carbon amount of the sintered body of Comparative Example 3 is large as a whole, and especially the carbon amount 1 is large.
As described above, from Tables 2 and 3, Examples 2 and 3 are good in both formability and magnetic properties, but in Comparative Example 2, there is a molding defect due to insufficient mold lubrication. It can be seen that the magnetic properties are deteriorated due to excessive mold lubricant.

Claims (3)

  In the method of manufacturing rare earth sintered magnet by filling cavities formed by rare earth magnet with mold and lower punch, molding by uniaxial pressing in magnetic field and then heat treating When molding, liquid gold consisting of 95 to 99.5% by mass of a fatty acid ester having a melting point or freezing point of 15 ° C. or less and 0.5 to 5% by mass of a fatty acid having 10 to 24 carbon atoms and a melting point of 30 ° C. or more. After spraying the mold lubricant as particles having an average particle diameter of 1 to 50 μm onto the mold, the cavity is filled with the alloy powder for rare earth magnets and molded in a magnetic field to obtain a molded body, which is then heat-treated. A method for producing a rare earth sintered magnet. 2. The method for producing a rare earth sintered magnet according to claim 1, wherein the mold lubricant is sprayed into the cavity at a spraying amount of 1 × 10 ⁻⁴ to 1 × 10 ⁻² kg / m ² . 3. The method for producing a rare earth sintered magnet according to claim 1, wherein the mold lubricant has a kinematic viscosity at 20 ° C. of 2 × 10 ⁻⁶ to 1 × 10 ⁻⁵ m ² / s. .

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