JP7714892B2 - Method for manufacturing RTB based sintered magnet - Google Patents

Description

The present invention relates to a method for producing R-T-B based sintered magnets.

Rare earth sintered magnets have been in high demand in recent years, and among them, R-T-B sintered magnets (R is at least one rare earth element, including at least one of Nd and Pr; T is primarily Fe; B is boron) are known as the highest performance magnets, and are used in a variety of motors and home appliances, including voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs, etc.), and motors for industrial equipment. R-T-B sintered magnets contribute to energy conservation and reduced environmental impact by making various motors smaller and lighter.

Such R-T-B based sintered magnets are manufactured, for example, through the steps of preparing an R-T-B based alloy powder, forming the R-T-B based alloy powder into a compact, and sintering the compact. When sintering the compact, the compact is placed on a support plate made of a high-melting-point metal material such as molybdenum and the sintering container is heated to a temperature of approximately 1000°C to 1100°C. During this process, deformation of the sintered body may occur due to the direct influence of the sintering container, as well as the influence of oxygen, carbon, etc.

Patent Document 1 proposes placing a getter material in the sintering case to remove oxygen from the sintering atmosphere, thereby suppressing deformation of the sintered body of a rare earth sintered magnet after sintering.

Japanese Patent Application Laid-Open No. 2002-25842

Although the method described in Patent Document 1 can reduce the effects of oxygen and carbon, it may not be sufficient to prevent deformation of the sintered body.
An embodiment of the present disclosure provides a method for producing an RTB based sintered magnet that suppresses deformation of the sintered body of a rare earth sintered magnet after sintering.

In an exemplary embodiment, a method for producing an R-T-B based sintered magnet according to the present disclosure includes the steps of: preparing an R-T-B based alloy powder (R is a rare earth element and always includes at least one selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal element and always includes Fe); compacting the R-T-B based alloy powder to produce a rectangular parallelepiped compact; and chamfering an edge of at least one surface of the compact.
The method includes a step of placing the chamfered molded body on a support plate so that the chamfered surface of the edge portion comes into contact with the support plate, and a step of sintering the molded body placed on the support plate.

In one embodiment, when the shortest side of the rectangular parallelepiped molded body is defined as the thickness, the chamfered surface of the edge is a surface perpendicular to the thickness direction.

In one embodiment, the thickness is 7 mm or less.

Embodiments of the present disclosure can provide a method for manufacturing an R-T-B based sintered magnet that suppresses deformation of the sintered body of a rare earth sintered magnet after sintering.

FIG. 2 is an explanatory diagram showing an example of a rectangular parallelepiped molded body according to the present disclosure. FIG. 2 is an explanatory diagram showing an example of a rectangular parallelepiped molded body before chamfering. FIG. 10 is an explanatory diagram showing an example of a rectangular parallelepiped molded body after chamfering. FIG. 10 is an explanatory diagram showing a state in which the molded body is placed on a support plate so that the chamfered edge surface comes into contact with the support plate.

The inventors conducted a detailed study of the effects of oxidation during sintering of rectangular solid bodies. They found that because the edges of the body are more susceptible to oxidation than the center, the thickness of the edge portions is greater than the thickness of the center. Therefore, the center of the body is lifted using the edge portions (all edges around the surface of the body that contacts the support plate) of the surface of the body that contacts the support plate on which the body is placed during sintering as fulcrums. This center portion sags under its own weight during sintering, resulting in deformation (warping) of the resulting sintered body. Furthermore, they found that this tendency is particularly pronounced in thin (7 mm or less) flat rectangular solids. Based on these findings, further study revealed that deformation can be suppressed even for thin (7 mm or less) flat rectangular solids by chamfering at least one edge of the body and placing the body on the support plate so that the chamfered surface contacts the support plate. This is thought to be because by chamfering the edges of the surface of the molded body that comes into contact with the support plate, it is possible to eliminate the fulcrum at the center of the molded body that lifts it up.

(RTB based sintered magnet)
The RTB based sintered magnet of the present disclosure has, for example, the following composition.
R: 27-35 mass%,
B: 0.80 to 1.20 mass%,
Ga: 0 to 1.0 mass%,
Cu: 0 to 0.5 mass%,
T: Contains 60 mass% or more.

(R: 27-35 mass%)
R is a rare earth element and must contain at least one element selected from the group consisting of Nd, Pr, and Ce. If the R content is less than 27 mass%, a liquid phase may not be sufficiently generated during the sintering process, making it difficult to sufficiently densify the sintered body. On the other hand, if the R content exceeds 35 mass%, grain growth may occur during sintering, potentially reducing _HcJ . The R content is preferably 29.5 to 33.0 mass%. If the R content is within this range, a higher B _r can be obtained.

(B:0.80~1.20mass%)
If B is less than 0.80 mass%, B _r may decrease. On the other hand, if B exceeds 1.20 mass%, H _cJ may decrease. The B content is preferably 0.88 to 0.90 mass%. If B is in this range, a higher H _cJ can be obtained.

(Ga: 0 to 1.0 mass%)
The Ga content is preferably 0 to 1.0 mass%, more preferably 0.2 to 0.7 mass%, and within this range, a higher _HcJ can be obtained.

(Cu: 0 to 0.50 mass%)
The Cu content is preferably 0 to 0.50 mass%, more preferably 0.05 to 0.30 mass%. If the Cu content is in this range, a higher _HcJ can be obtained.

(T: 60mass% or more)
T is at least one transition metal element, and always includes Fe.
If the T content in the sintered magnet is less than 60 mass%, the magnetic properties may be significantly degraded. The T content is preferably 61.5 to 69.5 mass%. Furthermore, when the total amount of T is 100 mass%, up to 10 mass% of that can be substituted with Co. For example, 90 mass% of the total amount of T can be Fe and 10 mass% can be Co. Alternatively, the total amount of T (100 mass%) may be Fe. Although the inclusion of Co can improve corrosion resistance, if the amount of Co substituted exceeds 10 mass% of Fe, a high _Br may not be obtained.
The sintered magnet of the present invention may further contain any other elements.

The method for producing a sintered RTB based magnet will now be described.
(1) Step of Preparing RTB-Based Alloy Powder Metals or alloys of the respective elements are prepared so as to achieve the target composition, and these are then subjected to strip casting or the like to produce alloy flakes.
The obtained flake-shaped alloy is subjected to hydrogen pulverization to reduce the size of the coarsely pulverized powder to, for example, 1.0 mm or less. The coarsely pulverized powder is then finely pulverized using a jet mill or the like to obtain a finely pulverized powder (alloy powder) having a median diameter _d50 (a value obtained by laser diffraction using an airflow dispersion method) of, for example, 2.5 μm≦d50≦4.5 μm. Note that a known lubricant may be used as an auxiliary agent in the coarsely pulverized powder before jet mill pulverization, and in the alloy powder during and after jet mill pulverization. d50 can be measured using an airflow dispersion laser diffraction method (in accordance with JIS Z 8825: 2013 revised edition). In other words, in this disclosure, d50 refers to the particle size (median diameter) at which the cumulative particle size distribution (volume basis) from the small particle size side is 50%.
In this disclosure, d50 is measured using a particle size distribution measuring device "HELOS &RODOS" manufactured by Sympatec. Dispersion pressure: 4 bar
Measurement range: R2
Calculation mode: HRLD
The d50 measured under the conditions is shown.

(2) Step of Preparing a Rectangular Compact: The obtained alloy powder is subjected to magnetic field compaction to obtain a compact. Any known magnetic field compaction method may be used for magnetic field compaction, including a dry compaction method in which dried alloy powder is inserted into a mold cavity and compacted while a magnetic field is applied, and a wet compaction method in which a slurry containing the alloy powder is poured into a mold cavity and compacted while the slurry's dispersion medium is discharged. An example of a rectangular compact in the present disclosure is shown in FIG. 1. The edge portions of at least one surface of the compact in the present disclosure refer to all edges 2 surrounding surface 1 of compact 100 in FIG. 1, for example. Furthermore, the thickness in the present disclosure refers to the shortest side of the rectangular solid, e.g., side 3 in FIG. 1. The dimensions of the rectangular compact may be, for example, 20 mm long x 10 mm wide x 5 mm thick. In the present disclosure, deformation can be suppressed even for thin, flat rectangular compacts, so the thickness of the compact is preferably 7 mm or less, and more preferably 5 mm or less.

(3) Step of chamfering the edge of at least one surface of the molded body The edge of at least one surface of the obtained molded body is chamfered. FIG. 2A is an explanatory diagram showing an example of a rectangular parallelepiped molded body before chamfering, and FIG. 2B is an explanatory diagram showing an example of a rectangular parallelepiped molded body after chamfering. As shown in FIGS. 2A and 2B, by chamfering the edge of surface 4 (all edges 5 around surface 4) of molded body 200 in FIG. 2A, the edge of surface 4 of molded body 200 is chamfered as shown in FIG. 2B. The chamfering is preferably performed by cutting to a depth of 0.5 mm to 2 mm (C0.5 to C2 mm) from the tip of the corner of the edge. There is no particular restriction on the chamfering method. It may be performed by a known method. For example, it may be scraped off using a spatula or knife.

(4) Step of Placing the Compacted Body on a Support Plate In step (3), the compacted body with at least one chamfered surface is placed on a support plate. FIG. 3 is an explanatory diagram showing the state in which the chamfered compact is placed on a support plate so that the chamfered edge contacts the support plate. In the present disclosure, as shown in FIG. 3, the compacted body 200 is placed on the support plate 6 so that the surface 4 having the chamfered edge contacts the support plate 6. This prevents the center of the compact from lifting during sintering, thereby suppressing deformation of the resulting sintered rare earth sintered magnet. Preferably, as shown in FIG. 3, the chamfered edge surface is perpendicular to the thickness direction. Because the surface perpendicular to the thickness direction, which is the shortest side, is more susceptible to deformation, chamfering this surface more reliably suppresses deformation of the resulting sintered body. Examples of materials for the support plate include molybdenum and stainless steel.

(5) Sintering Step The compact placed on the support plate is sintered. Sintering may be performed by a known method. For example, the compact placed on the support plate is placed in a sintering vessel and heated to 1000°C to 1100°C for sintering. In order to prevent oxidation due to the atmosphere during sintering, sintering is preferably performed in a vacuum atmosphere or in an atmospheric gas. The atmospheric gas used is preferably an inert gas such as helium or argon.

(6) Heat Treatment Step The sintered magnet obtained according to the embodiment of the present disclosure may be subjected to a heat treatment to further improve its magnetic properties. For example, a first-stage heat treatment may be performed at a temperature lower than the sintering temperature (400°C to 600°C). Alternatively, a first heat treatment may be performed at a relatively high temperature (700°C to the sintering temperature) followed by a second heat treatment at a relatively low temperature (400°C to 600°C) (two-stage heat treatment). A specific example of a two-stage heat treatment may include a first heat treatment at a temperature of 750°C to 850°C for approximately 5 to 500 minutes, and a second heat treatment at a temperature of 440°C to 550°C for approximately 5 to 500 minutes. Between the first and second heat treatments, the magnet may be cooled to room temperature or to a temperature of 440°C to 550°C.

The resulting sintered magnet may be subjected to machining such as grinding to achieve the final product shape. In this case, heat treatment may be performed before or after machining. Furthermore, the resulting sintered magnet may be subjected to a surface treatment. This surface treatment may be any known surface treatment, such as Al vapor deposition, Ni electroplating, or resin paint.

The following examples provide further details, but the present disclosure is not limited to them.

Example 1
An R-T-B alloy powder having a composition of Nd: 24.5 mass%, Pr: 5.5 mass%, B: 0.93 mass%, Cu: 0.3 mass%, Ga: 0.5 mass%, Co: 0.45 mass%, Al: 0.12 mass%, Zr: 0.05 mass%, and the balance Fe was prepared. The particle size d50 of the powder was 3.2 μm. A compact was produced using these powders in a wet press. The dimensions of the obtained compact were a rectangular parallelepiped with a length of 82.3 mm, a width of 49.0 mm, and a thickness of 5.3 mm. The density of the obtained compact was approximately 4.3 Mg/m ^3. The edge portions of the 82.3 mm long x 49.0 mm wide surface (the surface perpendicular to the thickness direction) of the obtained compact were chamfered. The chamfering was performed by using a spatula to remove a 1.0 mm depth (C1.0 mm) from the tip of the corner at the edge. The chamfered compact was then placed on a molybdenum support plate so that the chamfered surface of the edge was in contact with the support plate. The compact placed on the support plate was then sintered (a temperature was selected that would sufficiently cause densification by sintering) to produce sintered compact A (RTB-based sintered magnet), an example of the present invention. As a comparative example, sintered compact B (RTB-based sintered magnet) was produced in a similar manner, except that the compact was not chamfered.

The amount of deformation of the resulting R-T-B based sintered magnets was examined. The amount of deformation was measured using a three-dimensional measuring machine (Nikon NEXIV). The measurement showed that the amount of deformation of sintered body A, an example of the present invention in which the compact was chamfered, was 0.2 mm, while the amount of deformation of sintered body B, a comparative example in which the compact was not chamfered, was 0.6 mm. Therefore, the deformation of the sintered body of the example of the present invention (sintered body A) is significantly suppressed compared to the comparative example (sintered body B).

1, 4 Faces 2, 3, 5 Side 6 Support plate 100, 200 Molded body

Claims (2)

A step of preparing an R-T-B system alloy powder (R is a rare earth element and must contain at least one selected from the group consisting of Nd, Pr, and Ce, and T is at least one transition metal element and must contain Fe);
a step of forming a flat rectangular parallelepiped compact having a thickness of 5 mm to 7 mm by compacting the R-T-B based alloy powder while applying a magnetic field ;
chamfering all corners where at least one surface perpendicular to the thickness direction of the molded body intersects with a surface parallel to the thickness direction to a depth of 0.5 mm to 2 mm from the tip of the corner;
placing the molded body on a support plate so that the surface perpendicular to the thickness direction, all of the corners of which have been chamfered, comes into contact with the support plate;
sintering the compact placed on the support plate;
A method for producing an RTB based sintered magnet, comprising: The method for producing an R-T-B based sintered magnet according to claim 1, wherein the chamfer is a C-chamfer.