JP4671024B2 - Manufacturing method of rare earth sintered magnet - Google Patents

Description

  The present invention relates to a method for producing a rare earth sintered magnet typified by an Nd—Fe—B system.

Rare earth sintered magnets (hereinafter simply referred to as sintered magnets) are widely used as high-performance magnets, and the demand for them increases with the miniaturization of various electronic devices and the increase in electronic devices in automobiles. Yes. In general, the higher the degree of orientation of the magnet, the higher the residual magnetic flux density. For this reason, a magnetic field is often applied to the raw material powder during molding, and compression molding is often performed while the raw material powder is oriented (so-called magnetic field molding).
At this time, in order to improve the orientation of the raw material powder with respect to the magnetic field, a lubricant may be added to the raw material powder.
In addition, before forming in a magnetic field as described above, a lubricant may be added to improve pulverization in a process of pulverizing a raw material alloy with a jet mill or the like to obtain a raw material powder (for example, Patent Documents). 1).

JP-A-8-111308 (Claims)

  By the way, when the kind and addition amount of the lubricant are adjusted in order to obtain high magnetic characteristics, the strength of the molded body is lowered and the yield of the molded body is lowered. On the other hand, if the lubricant is adjusted so as to increase the strength of the compact, it is difficult to improve the magnetic properties of the finally obtained sintered magnet.

  The present invention has been made based on such a technical problem, and an object of the present invention is to provide a method for producing a rare earth sintered magnet having a molded body having high strength and high magnetic properties.

The method for producing a rare earth sintered magnet of the present invention comprises a step of obtaining a pulverized powder by pulverizing a raw alloy powder for a rare earth sintered magnet , and applying a magnetic field to the pulverized powder, followed by pressure forming to obtain a compact. And a step of sintering the molded body, comprising a tetrafluoroethylene-based compound A, a general formula R ₁ —COO—R ₂ , R ₁ —OH and (R ₁ —COO) _n M Compound B represented by any one type (where R ₁ is C _n H _{2n + 1} ; R ₂ is H, C _n H _{2n + 1} or C _n H _2n-1 O _n-1 ; M is a metal; n is And a step of obtaining a molded body and a step of sintering the molded body . At this time, the addition timing of the lubricant may be in the state of the raw material alloy powder before pulverization, but is preferably after the pulverization process of the raw material powder or after the pulverization of the raw material powder. In particular, it is preferable to add the lubricant by introducing the granular lubricant into the pulverizing means together with the raw material alloy powder during the pulverization process of the raw material alloy powder.

Here, the compound A is polytetrafluoroethylene.
In the compound B, R ₁ is C _n H _{2n + 1} , wherein n is 10 or more, specifically a hydrocarbon having 17 carbon atoms. Such compound B is at least one compound selected from the group consisting of stearic acid, glyceryl monostearate, zinc stearate and stearyl alcohol.
In order to increase the strength of the compact and increase the magnetic properties of the sintered magnet, the total amount of compound A added and compound B added is 0.05 to 0.1 wt% .
Further, the particle diameters of Compound A and Compound B are each preferably 800 μm or less.

According to the present invention, a compound represented by _{one of the} group consisting of tetrafluoroethylene-based compound A and the general formula R ₁ —COO—R ₂ , R ₁ —OH and (R ₁ —COO) _n M B (where R ₁ is C _n H _{2n + 1} ; R ₂ is H, C _n H _{2n + 1} or C _n H _2n-1 O _n-1 ; M is a metal; n is an integer) By adding to the pulverized powder, the strength of the compact and the magnetic properties of the finally obtained sintered magnet are ensured while ensuring the pulverization of the raw material alloy in the pulverization process and the orientation of the pulverized powder in the magnetic field forming process It becomes possible to make it high.

Hereinafter, the present invention will be described in detail based on embodiments.
The present invention can be applied to, for example, a rare earth sintered magnet, particularly an RTB-based sintered magnet.
This RTB-based sintered magnet contains 25 to 37 wt% of a rare earth element (R). Here, R in the present invention has a concept including Y, and therefore 1 of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. It is selected from species or two or more species. If the amount of R is less than 25 wt%, the R ₂ T ₁₄ B phase, which is the main phase of the R-T-B system sintered magnet, is not sufficiently generated, and α-Fe having soft magnetism is precipitated and retained. The magnetic force is significantly reduced. On the other hand, when R exceeds 37 wt%, the volume ratio of the R ₂ T ₁₄ B phase, which is the main phase, decreases, and the residual magnetic flux density decreases. Further, R reacts with oxygen, the amount of oxygen contained increases, and accordingly, the R-rich phase effective for the generation of coercive force decreases, leading to a decrease in coercive force. Therefore, the amount of R is set to 25 to 37 wt%. A desirable amount of R is 28 to 35 wt%, and a more desirable amount of R is 29 to 33 wt%.

The RTB-based sintered magnet contains 0.5 to 4.5 wt% of boron (B). When B is less than 0.5 wt%, a high coercive force cannot be obtained. On the other hand, when B exceeds 4.5 wt%, the residual magnetic flux density tends to decrease. Therefore, the upper limit of B is set to 4.5 wt%. A desirable amount of B is 0.5 to 1.5 wt%, and a more desirable amount of B is 0.8 to 1.2 wt%.
This RTB-based sintered magnet contains Co of 2.0 wt% or less (excluding 0), desirably 0.1 to 1.0 wt%, and more desirably 0.3 to 0.7 wt%. can do. Co forms the same phase as Fe, but is effective in improving the Curie temperature and improving the corrosion resistance of the grain boundary phase.

The RTB-based sintered magnet can contain one or two of Al and Cu in the range of 0.02 to 0.5 wt%. By including one or two of Al and Cu in this range, it is possible to increase the coercive force, increase the corrosion resistance, and improve the temperature characteristics of the obtained RTB-based sintered magnet. In the case of adding Al, the desirable amount of Al is 0.03 to 0.3 wt%, and the more desirable amount of Al is 0.05 to 0.25 wt%. Further, in the case of adding Cu, the desirable amount of Cu is 0.15 wt% or less (not including 0), and the more desirable amount of Cu is 0.03 to 0.12 wt%.
Furthermore, this RTB-based sintered magnet allows the inclusion of other elements. For example, elements such as Zr, Ti, Bi, Sn, Ga, Nb, Ta, Si, V, Ag, and Ge can be appropriately contained. On the other hand, it is desirable to reduce impurity elements such as oxygen, nitrogen, and carbon as much as possible. In particular, the amount of oxygen that impairs magnetic properties is preferably 5000 ppm or less, more preferably 3000 ppm or less. This is because when the amount of oxygen is large, the rare-earth oxide phase, which is a nonmagnetic component, increases and the magnetic properties are deteriorated.

The present invention is not limited to the R-T-B based sintered magnet as described above, but can be applied to other rare earth sintered magnets. For example, the present invention can be applied to an R—Co based sintered magnet.
The R—Co based sintered magnet contains R, one or more elements selected from Fe, Ni, Mn, and Cr, and Co. In this case, it preferably further contains Cu or one or more elements selected from Nb, Zr, Ta, Hf, Ti and V, and particularly preferably from Cu and Nb, Zr, Ta, Hf, Ti and V. Containing one or more selected elements. Among these, in particular, an intermetallic compound of Sm and Co, preferably an Sm ₂ Co ₁₇ intermetallic compound, is the main phase, and a subphase mainly composed of SmCo ₅ exists at the grain boundary. The specific composition may be appropriately selected according to the production method, required magnetic characteristics, and the like. For example, R: 20 to 30 wt%, particularly about 22 to 28 wt%, Fe, Ni, Mn, and Cr Above: about 1 to 35 wt%, one or more of Nb, Zr, Ta, Hf, Ti and V: 0 to 6 wt%, especially about 0.5 to 4 wt%, Cu: 0 to 10 wt%, especially 1 to 10 wt% To the extent, Co: the balance composition is desirable.
The R-T-B sintered magnet and the R-Co sintered magnet have been described above, but the present invention does not prevent application to other rare earth sintered magnets.

Hereinafter, a method for producing a rare earth sintered magnet according to the present invention will be described in the order of steps.
The raw material alloy can be produced by a strip casting method or other known melting methods in a vacuum or an inert gas, preferably an argon atmosphere. In the strip casting method, a molten metal obtained by melting a raw material metal in a non-oxidizing atmosphere such as an argon gas atmosphere is jetted onto the surface of a rotating roll. The melt rapidly cooled by the roll is rapidly solidified in the form of a thin plate or flakes (scales). This rapidly solidified alloy has a homogeneous structure with a crystal grain size of 1 to 50 μm. The raw material alloy can be obtained not only by the strip casting method but also by a melting method such as high frequency induction melting. In order to prevent segregation after dissolution, for example, it can be solidified by pouring into a water-cooled copper plate. An alloy obtained by the reduction diffusion method can also be used as a raw material alloy.
When obtaining an RTB-based sintered magnet, a so-called alloy using a R ₂ T ₁₄ B crystal grain (low R alloy) and an alloy containing more R than a low R alloy (high R alloy) is used. A mixing method can also be applied to the present invention.

The raw material alloy is subjected to a grinding process. In the case of the mixing method, the low R alloy and the high R alloy are pulverized separately or together. The pulverization process includes a coarse pulverization process and a fine pulverization process.
First, in the coarse pulverization step, the raw material alloy is coarsely pulverized to a particle size of about several hundred μm to obtain coarsely pulverized powder (raw material alloy powder). The coarse pulverization is desirably performed in an inert gas atmosphere using a stamp mill, a jaw crusher, a brown mill or the like. Prior to coarse pulverization, it is effective to perform pulverization by allowing hydrogen to be stored in the raw material alloy and then releasing it. The hydrogen releasing treatment is performed for the purpose of reducing hydrogen as an impurity as a rare earth sintered magnet.

  After the coarse pulverization process, the process proceeds to the fine pulverization process. A jet mill is mainly used for fine pulverization. By pulverizing the coarsely pulverized powder, the average particle size is preferably 2.5 to 6 μm, and the strength of the compact to be described later is increased, and the magnetic properties of the sintered magnet are improved. More preferably, a finely pulverized powder (ground powder) of 3 to 5 μm is obtained in order to increase the height. The jet mill releases a high-pressure inert gas from a narrow nozzle to generate a high-speed gas flow, accelerates the coarsely pulverized powder with this high-speed gas flow, collides with the coarsely pulverized powder, and collides with the target or the container wall. It is a method of generating a collision and crushing.

  At this time, a lubricant is added to the raw material alloy powder in the fine grinding step. Lubricating and orientation during molding can be improved by adding a lubricant. The lubricant is preferably added to the raw material alloy powder before the start of the fine pulverization process. However, the lubricant may be added during the fine pulverization process or after the fine pulverization process and before the granule preparation process or in the magnetic field. It may be added to the finely pulverized powder before the molding step.

The lubricant is a tetrafluoroethylene-based compound A and a compound B represented by one of the group consisting of the general formulas R ₁ —COO—R ₂ , R ₁ —OH and (R ₁ —COO) _n M ( Wherein R ₁ is C _n H _{2n + 1} ; R ₂ is H, C _n H _{2n + 1} or C _n H _2n-1 O _n-1 ; M is a metal; n is an integer.
As the tetrafluoroethylene-based compound A, for example, polytetrafluoroethylene ((CF ₂ -CF ₂ ) n) in which tetrafluoroethylene is polymerized is preferable, but other compounds are copolymerized with polytetrafluoroethylene. It may be.

The compound B contained in the lubricant is, for example, a fatty acid compound or alcohol, and specifically includes higher fatty acids having 10 or more carbon atoms, higher fatty acid esters, higher fatty acid metal salts, higher alcohols, and the like. Among them, the compound is preferably a compound in which R ₁ is C _n H _{2n + 1} (n is 10 or more), for example, a compound in which R _{1 is} a hydrocarbon having 17 and 18 carbon atoms. Specific examples include stearic acid (C ₁₇ H ₃₅ O—O—H), glyceryl monostearate (C ₁₇ H ₃₅ —COO—C ₃ H ₇ O ₂ ), zinc stearate ((C ₁₇ H ₃₅ —COO) ⁻ ₂ Zn ²⁺ ) and stearyl alcohol (C ₁₈ H ₃₇ —O—H). Of these, stearic acid is preferred. As the compound having an R ₂ —O— group, only one type of compound may be used, but a plurality of compounds may be used.

The lubricant is preferably added in the form of granules. In that case, the lubricant is added by introducing a granular lubricant into a jet mill for pulverizing the raw material alloy powder.
The particle size of the lubricant is preferably 1000 μm or less, more preferably 800 μm or less, particularly preferably about 500 μm or less in order to increase the strength of the molded body described later and to increase the magnetic properties of the sintered magnet. In order to make the lubricant have the above particle diameter, it is preferable to grind the lubricant and classify it with a sieve or the like. In order to pulverize the lubricant, it is preferable that the lubricant is frozen using, for example, liquid nitrogen, and pulverized in that state with a pulverization mill or the like.

The addition amount of the lubricant is preferably as much as possible from the viewpoint of improving the grindability, but is preferably as small as possible from the viewpoint of the magnetic properties and the strength of the molded body . Increase the strength of the formed features, in order to increase the magnetic properties of the sintered magnet, the sum of the amount and the addition amount of Compound B Compound A, and 0.05~0.1wt%.

  In the case of the mixing method, the timing of mixing the two kinds of alloys is not limited. However, when the low R alloy and the high R alloy are separately pulverized in the pulverizing step, the pulverized low R alloy powder is used. And high R alloy powder in a nitrogen atmosphere. The mixing ratio of the low R alloy powder and the high R alloy powder may be about 80:20 to 97: 3 by weight. The mixing ratio when the low R alloy and the high R alloy are pulverized together is the same.

The granular granulated powder obtained as described above (or finely pulverized powder when the granule preparation step is omitted) is filled in a mold cavity and subjected to molding in a magnetic field.
The molding pressure in the magnetic field molding may be in the range of 0.3 to 3 ton / cm ² (30 to 300 MPa). The molding pressure may be constant from the beginning to the end of molding, may be gradually increased or gradually decreased, or may vary irregularly. The lower the molding pressure is, the better the orientation is. However, if the molding pressure is too low, the strength of the molded body is insufficient and handling problems occur. Therefore, the molding pressure is selected from the above range in consideration of this point. The final relative density of the molded body obtained by molding in a magnetic field is usually 50 to 60%.
The applied magnetic field may be about 12 to 20 kOe (960 to 1600 kA / m). Further, the applied magnetic field is not limited to a static magnetic field, and may be a pulsed magnetic field. A static magnetic field and a pulsed magnetic field can also be used in combination.

  The molded body obtained by molding in a magnetic field is subjected to binder removal processing. This is to prevent a decrease in magnetic properties due to carbon residue. The binder removal treatment is preferably performed under a predetermined heat treatment condition in a hydrogen atmosphere.

  After the binder removal treatment, the compact is sintered in a vacuum or an inert gas atmosphere. Although it is necessary to adjust sintering temperature by various conditions, such as a composition, a grinding | pulverization method, a difference in an average particle diameter, and a particle size distribution, what is necessary is just to sinter at 1000-1200 degreeC for about 1 to 10 hours in a vacuum.

  Now, after sintering, the obtained sintered body can be subjected to an aging treatment. This process is an important process for controlling the coercive force. When the aging treatment is performed in two stages, holding for a predetermined time at 750 to 950 ° C. and 500 to 700 ° C. is effective. When the heat treatment in the vicinity of 800 ° C. is performed after sintering, the coercive force increases, which is particularly effective in the mixing method. In addition, since the coercive force is greatly increased by heat treatment near 600 ° C., when aging treatment is performed in one stage, it is preferable to perform aging treatment near 600 ° C.

<Manufacture of rare earth sintered magnets>
First, the molded object used as evaluation object and the rare earth sintered magnet which sintered this were produced. The composition of the raw material alloy was Nd 24.5 wt%, Pr 6.0 wt%, Dy 1.8 wt%, Co 0.5 wt%, Al 0.2 wt%, Cu 0.07 wt%, B 1.0 wt%, and the balance Fe. The raw material metal or alloy was blended so as to have the above composition, and the raw material alloy thin plate was melted and cast by a strip casting method.

  The obtained raw material alloy thin plate is hydrogen pulverized and then mechanically coarsely pulverized by a brown mill to obtain raw material alloy coarse powder. As a lubricant (grinding aid), 0.05 wt% each of polytetrafluoroethylene and Compound B shown in Table 1 were added to the raw material alloy coarse powder as a compound A. Next, fine pulverization was performed using an airflow pulverizer (jet mill) in a high-pressure nitrogen gas atmosphere so that D50 = 4.1 μm, thereby obtaining rare earth alloy powder. Here, D50 is a particle size at which the cumulative volume ratio is 50% by measuring the particle size distribution of the finely pulverized powder.

  The obtained powder was molded in a magnetic field to obtain a molded body having a predetermined shape. In molding in a magnetic field, the powder was molded at a molding pressure of 147 MPa in a magnetic field of 1200 kA / m. The magnetic field direction is a direction perpendicular to the pressing direction. Two types of dimensions, 20 mm × 18 mm × 6.5 mm and 20 mm × 18 mm × 13 mm, were obtained for the compacts. And the bending strength was measured with the following method as the intensity | strength of a molded object using the former molded object.

  The bending strength measurement was performed according to Japanese Industrial Standard JIS R 1601. Specifically, as shown in FIG. 1, a molded body 11 having a shape of 20 mm × 18 mm × 6.5 mm is placed on two round bar-shaped supports 12 and 13, and a central position on the molded body 11 is placed. A round bar-shaped support 14 was placed on and a load was applied. The direction in which the bending pressure was applied was the pressing direction. The radius of the round bar-shaped supports 12, 13, and 14 was 3 mm, the distance between fulcrums was 10 mm, and the load point moving speed was 0.5 mm / min. The longitudinal direction of the molded body 11 and the support 14 were arranged so as to be parallel to each other. The measurement was carried out with 10 samples.

  Furthermore, the magnetic characteristics were evaluated. A molded product having a shape of 20 mm × 18 mm × 13 mm was obtained as an evaluation sample. This molded body was sintered and aged at 1030 ° C. for 4 hours, 900 ° C. for 1 hour and 530 ° C. for 1 hour. The surface of the obtained sintered body was ground to obtain a rectangular parallelepiped sample. This sample was evaluated for magnetic properties using a BH tracer.

Further, as a comparative example, a sample was prepared in the same manner as in Example 1 except that only 0.1 wt% of compound B shown in Table 1 was added without adding polytetrafluoroethylene, and a compact and a sintered magnet were obtained. The strength and magnetic properties were evaluated.
The results of Examples and Comparative Examples are shown in Table 1.

As shown in Table 1, in the comparative example, Br exceeded 13.2 kG, but the strength of the compact was below 0.9 MPa.
On the other hand, when polytetrafluoroethylene and compound B shown in Table 1 are added in combination, Br exceeds 13.2 kG, and the strength of the molded body also exceeds 1.1 MPa, which combines high molded body strength and high magnetic properties. It was confirmed that Moreover, it can be seen that the obtained magnetic properties are equivalent to the magnetic properties in the comparative example, and by adding polytetrafluoroethylene, the strength of the molded body can be increased without significantly reducing the magnetic properties. Was confirmed.
Thus, by adding a lubricant to the raw material alloy in the fine pulverization step, the strength of the compact is high while ensuring the pulverization property of the raw material alloy in the pulverization step and the orientation of the pulverized powder in the forming step in the magnetic field, Furthermore, the sintered magnet finally obtained has a high magnetic property.

It is a figure explaining the bending strength measuring method according to Japanese Industrial Standards JISR1601.

Claims (3)

Pulverizing raw alloy powder for rare earth sintered magnet to obtain pulverized powder;
Tetrafluoroethylene-based compound A and compound B represented by any one of the group consisting of general formulas R ₁ —COO—R ₂ , R ₁ —OH and (R ₁ —COO) _n M (where R ₁ is C _n H _{2n + 1} ; R ₂ is H, C _n H _{2n + 1} or C _n H _2n-1 O _n-1 ; M is a metal; n is an integer) and a magnetic field is applied to the pulverized powder. And a step of obtaining a molded body by pressure molding;
A step of sintering the molded body,
Compound A is polytetrafluoroethylene,
The compound B, Ri least one compound der selected from stearic acid, glyceryl monostearate, the group consisting of zinc stearate and stearyl alcohol,
The manufacturing method of the rare earth sintered magnet characterized by making the sum total of the addition amount of the said compound A and the addition amount of the said compound B 0.05-0.1 wt% . 2. The method for producing a rare earth sintered magnet according to claim 1, wherein the particle diameters of the compound A and the compound B are 800 μm or less. The method for producing a rare earth sintered magnet according to claim 1, wherein the rare earth sintered magnet is an R-T-B based sintered magnet.

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