JP2024047520A - Manufacturing method of RTB based sintered magnet - Google Patents

Abstract

To provide a method of producing an R-T-B sintered magnet, with which, even if press-molding in a magnetic field is performed using mixed powder for sintered magnets, the mixed powder being obtained by strong stirring, an obtained molded body has sufficient strength and an obtained R-T-B sintered magnet has high magnetic characteristics.SOLUTION: A method of producing an R-T-B sintered magnet includes: a mixing/stirring step of stirring alloy powder for R-T-B sintered magnets containing R (rare earth element), T (Fe or Fe and Co) and B, a solid lubricant comprising one or both of a fatty acid ester and a fatty acid metal salt, and paraffin wax using a stirring blade to obtain mixed powder for sintered magnets; a step of press-molding in a magnetic field, in which the mixed powder for sintered magnets is press-molded in a magnetic field to obtain a molded body for R-T-B sintered magnets; and a sintering step of sintering the molded body for R-T-B sintered magnets. In the mixing/stirring step, the amount of the solid lubricant is 0.025% or more by a mass ratio with respect to the alloy powder for R-T-B sintered magnets, the amount of the paraffin wax is 0.010% or more by a mass ratio with respect to the alloy powder for R-T-B sintered magnets, the total amount of the solid lubricant and the paraffin wax is 0.150% or less by a mass ratio with respect to the alloy powder for R-T-B sintered magnets, the amount of the paraffin wax is 0.10 or more and 0.80 or less by a mass ratio with respect to the total amount of the solid lubricant and the paraffin wax, and stirring using the stirring blade is strong stirring with a circumferential velocity of 7 m/s or more.SELECTED DRAWING: None

Description

This disclosure relates to a method for producing R-T-B based sintered magnets.

Sintered magnets containing rare earth elements such as neodymium, also known as R-T-B sintered magnets (R is a rare earth element, and T is Fe or Fe and Co), have the highest maximum magnetic energy product among various practical magnets and are relatively inexpensive, so they are used in many applications including various electronic devices and motors for electric vehicles, fuel cell vehicles, and hybrid vehicles.

Furthermore, RTB based sintered magnets are required to have high magnetic properties, in particular high residual magnetic flux density (hereinafter sometimes referred to as "B _r ") and sufficient coercive force (hereinafter sometimes referred to as "H _cJ ").
In order to obtain such excellent magnetic properties, various measures such as adjusting the composition have been taken. As one of the measures, a technology focusing on the process of preparing a mixed powder for sintered magnets (alloy powder material) used to obtain a compact in press molding in a magnetic field is disclosed in Patent Document 1. Specifically, Patent Document 1 discloses that when an alloy powder for R-T-B based sintered magnets (alloy powder) is mixed with a solid lubricant to obtain a mixed powder for sintered magnets, excellent magnetic properties can be obtained by stirring while maintaining an apparent volume that is no more than twice the apparent volume of the alloy powder for R-T-B based sintered magnets.

JP 2005-336592 A

However, when press molding is performed in a magnetic field using the mixed powder for sintered magnets obtained by conventional strong stirring as described in Patent Document 1, the strength of the resulting molded body is insufficient, and there is a problem that the molded body may crack when it is released from the mold used for press molding.

This disclosure has been made in light of these circumstances, and one of its objectives is to provide a method for producing R-T-B based sintered magnets that has sufficient strength when press-molded in a magnetic field using a mixed powder for sintered magnets obtained by strong stirring, and that allows the resulting R-T-B based sintered magnets to have high magnetic properties, particularly high residual magnetic flux density.

A first aspect of the present invention is a mixing and stirring process for obtaining a mixed powder for a sintered magnet by stirring, with a stirring blade, an alloy powder for an R-T-B based sintered magnet, which contains R (a rare earth element), T (Fe or Fe and Co), and B, a solid lubricant consisting of one or more of phosphorous acid, a fatty acid, a phosphite ester, a fatty acid ester, and a fatty acid metal salt, and paraffin wax,
the amount of said solid lubricant is 0.025% or more by mass ratio relative to said alloy powder for R-T-B system magnets, the amount of said paraffin wax is 0.010% or more by mass ratio relative to said alloy powder for R-T-B system magnets, and the total amount of said solid lubricant and said paraffin wax is 0.150% or less by mass ratio relative to said alloy powder for R-T-B system magnets,
the amount of the paraffin wax is 0.10 or more and 0.80 or less in terms of mass ratio with respect to the total amount of the solid lubricant and the paraffin wax;
A mixing and stirring step in which the stirring using the stirring blade is strong stirring at a peripheral speed of 7 m/sec or more;
a magnetic field press molding step of press-molding the mixed powder for sintered magnet in a magnetic field to obtain a compact for an R-T-B magnet;
a sintering step of sintering the R-T-B magnet green body;
The present invention relates to a method for producing a sintered RTB based magnet, comprising the steps of:

Aspect 2 of the present invention is a method for producing an R-T-B based sintered magnet according to aspect 1, in which the compact is ring-shaped.

Aspect 3 of the present invention is a method for producing an R-T-B based sintered magnet according to aspect 2, in which the strength of the magnetic field applied in the magnetic field press molding process is 0.7 T or less.

Aspect 4 of the present invention is a method for producing an R-T-B based sintered magnet according to any one of aspects 1 to 3, in which the paraffin wax and part or all of the solid lubricant are mixed with coarsely pulverized powder for R-T-B based sintered magnets, which has a particle size larger than that of the alloy powder for R-T-B based sintered magnets, and then the coarsely pulverized powder for R-T-B based sintered magnets is obtained using a jet mill.

According to an embodiment of the present invention, it is possible to provide a method for manufacturing an R-T-B based sintered magnet, in which a molded body obtained by press molding in a magnetic field using a mixed powder for a sintered magnet obtained by strong stirring (stirring using a stirring blade at a peripheral speed of 7 m/sec or more) has sufficient strength, and the obtained R-T-B based sintered magnet has high magnetic properties, particularly high residual magnetic flux density.

FIG. 1 is a graph showing the B _r -H _cJ balance of the samples of the example and the comparative example in which a magnetic field of 0.4 T was applied during press molding in a magnetic field.

The present inventors have focused on the mixing and stirring process in which an alloy powder for an RTB based sintered magnet and a solid lubricant are mixed and stirred to obtain a mixed powder for a sintered magnet.
The inventors have found a mixing and stirring process for obtaining a mixed powder for sintered magnets under the conditions that satisfy the following: (1) adding paraffin wax to the conventionally known alloy powder for R-T-B magnets and solid lubricant; (2) making the amount of paraffin wax 0.010% or more by mass relative to the alloy powder for R-T-B magnets, making the amount of solid lubricant 0.025% or more by mass relative to the alloy powder for R-T-B magnets, and making the total amount of the solid lubricant and paraffin wax 0.150% or less by mass relative to the alloy powder for R-T-B magnets; (3) making the amount of paraffin wax 0.10 or more and 0.80 or less by mass relative to the total amount of the paraffin wax and solid lubricant; and (4) making the strong stirring using a stirring blade and making the peripheral speed of the stirring blade 7 m/sec or more.
The inventors also discovered that the mixed powder for sintered magnets obtained in this mixing and stirring process is used to perform press molding in a magnetic field to obtain a green body having sufficient strength, and that the R-T-B based sintered magnet obtained by sintering the green body has high B _r and sufficient H _cJ , which led to the embodiments of the present invention.

The method for producing a sintered RTB based magnet according to an embodiment of the present invention will now be described in detail.
1. Mixing and Stirring Step In the mixing and stirring step, the alloy powder for the RTB based sintered magnet, the solid lubricant, and the paraffin wax are vigorously stirred to obtain a mixed powder for the sintered magnet.
First, the alloy powder for sintered RTB based magnets, the solid lubricant and the paraffin wax used will be described.

<Alloy powder for R-T-B based sintered magnets>
The alloy powder for an RTB based sintered magnet (R is a rare earth element) may have the following composition:
Nd may account for 50% or more by mass of R, and Pr may also be included. If Pr accounts for 50% or more by mass of R, there is a risk that the high-temperature H _cJ will be significantly reduced, so Pr is set to 50% or less by mass of R. In addition, a light rare earth element (RL) other than Nd and Pr, such as Ce, may be included. In addition, a heavy rare earth element RH, such as one or more elements selected from the group consisting of Tb, Dy, and Ho, may be included. However, since the B _r of the finally obtained R-T-B based sintered magnet decreases if RH is included in excess, it is preferable that RH is 10% or less by mass in total of the alloy powder for R-T-B based sintered magnets.
The total rare earth element R content in the alloy powder for a sintered R-T-B based magnet is preferably 27% by mass or more and 31% by mass or less, and more preferably 29% by mass or more and 31% by mass or less. If R is less than 27% by mass, sintering may not be possible, and if it exceeds 31% by mass, B _r may decrease significantly.
In this specification, the term "rare earth element" includes Y.

T is Fe or Fe and Co. When Co is contained, Co may account for 50% or less by mass of T. Co is effective for improving temperature characteristics and corrosion resistance, and the alloy powder for an R-T-B based sintered magnet may contain Co in an amount of 10% or less by mass.
The content of T may be the balance (excluding inevitable impurities) of R and boron (B) or R, B and M described below.

The boron (B) content may be any known content, and for example, a preferred range is 0.9% to 1.2% by mass of the alloy powder for R-T-B based sintered magnets. If it is less than 0.9% by mass, sufficient _HcJ may not be obtained, and if it exceeds 1.2% by mass, _Br may decrease. Note that a part of B can be replaced with C (carbon).

In addition to the above elements, M element can be added to improve _HcJ . M element is one or more selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta and W. The amount of M element added is preferably 2.0 mass% or less in total. In addition, other elements such as unavoidable impurities such as O and N can also be tolerated. In one preferred embodiment, the other elements are only unavoidable impurities. However, other elements may be intentionally included for the purpose of improving desired characteristics, etc. The other elements, including unavoidable impurities, are preferably 2 mass% or less of the alloy powder for sintered magnets, more preferably 1 mass% or less.

The alloy powder for RTB based sintered magnets may be obtained by using known methods, such as the following:
An alloy ingot produced by ingot casting or an alloy lump of alloy flakes produced by a quenching method typified by a strip casting method is coarsely pulverized, for example, by a hydrogen pulverization method and/or various mechanical pulverization methods (methods using a stamp mill, a jaw crusher, a Braun mill, or the like), to obtain a coarsely pulverized powder (coarsely pulverized powder for R-T-B based sintered magnets). The obtained coarsely pulverized powder may be finely pulverized, for example, by a jet mill, to obtain an alloy powder for R-T-B based sintered magnets.

The average particle size _D50 of the coarsely pulverized powder is preferably within the range of 10 μm to 500 μm. The average particle size _D50 of the alloy powder for sintered R-T-B based magnets to be subjected to press molding in a magnetic field is preferably within the range of 1 μm or more and 10 μm or less, and in order to ensure excellent compressibility, the average particle size _D50 of the alloy powder for sintered R-T-B based magnets is preferably 3 μm or more, and more preferably within the range of 3 μm or more and 6 μm or less. From the viewpoint of press moldability and magnetic properties, it is preferable that the average particle size is within the above range.

<Solid Lubricant>
The solid lubricant is composed of one or more of phosphorous acid, fatty acid, phosphite ester, fatty acid ester, and fatty acid metal salt. Preferred fatty acids include stearic acid and lauric acid. Preferred solid lubricants include zinc stearate. Use of the solid lubricant can improve the orientation of the alloy powder for R-T-B based sintered magnets during press molding in a magnetic field, and as a result, can improve the magnetic properties of the finally obtained R-T-B based sintered magnets.
In this specification, the term "solid lubricant" includes the case where one or both of a fatty acid ester and a fatty acid metal salt are provided in a liquid state in a volatile solvent, and the solvent evaporates during press molding in a magnetic field, leaving the lubricant in a solid state. In this case, the "amount of solid lubricant" refers to the amount of lubricant in a solid state remaining after the solvent has evaporated.

<Paraffin wax>
Paraffin wax is a chain saturated hydrocarbon represented by the general formula C _n H _2n+2 , where n is 20 or more and 40 or less. It may be either a simple substance with only one value for n or a mixture with multiple values for n. The melting point of paraffin wax may be in the range of 46 to 77°C, although this may vary depending on the molecular weight and the mixing ratio. Similarly, the boiling point may be in the range of 200 to 400°C, although this may vary depending on the molecular weight and the mixing ratio.

<Adding solid lubricant and paraffin wax to alloy powder for R-T-B sintered magnets>
In order to carry out the strong stirring described below, a solid lubricant and paraffin wax are added to the alloy powder for an RTB based sintered magnet.
The amount of solid lubricant added is 0.025% or more by mass relative to the alloy powder for R-T-B magnets (when 0.025% by mass is added to the alloy powder for R-T-B magnets, 0.025% by mass of solid lubricant is added to 100% of the alloy powder for R-T-B magnets). If the amount of solid lubricant added to the alloy powder for R-T-B magnets is less than 0.025% by mass, the solid lubricant cannot be coated more uniformly on the surface of the alloy powder for R-T-B magnets even when stirring as described below is performed, and sufficient magnetic properties cannot be obtained.

The amount of paraffin wax added must be at least 0.010% by mass relative to the alloy powder for R-T-B magnets. If the amount of paraffin added to the alloy powder for R-T-B magnets is less than 0.010% by mass, sufficient strength cannot be obtained for the molded body.

The total amount of solid lubricant and paraffin wax added is 0.150% or less by mass relative to the alloy powder for R-T-B magnets. If the total amount of solid lubricant and paraffin wax added exceeds 0.150% by mass relative to the alloy powder for R-T-B magnets, there is a risk of adversely affecting the magnetic properties.

The amount of paraffin wax added is set to a mass ratio of 0.10 to 0.80, preferably 0.10 to 0.60, based on the total amount of paraffin wax and solid lubricant added. By keeping the amount of paraffin wax added within this range, it is possible to achieve both high strength for the compact and excellent magnetic properties for the R-T-B based sintered magnet.

The addition of the solid lubricant and the paraffin wax may be performed after the alloy powder for R-T-B magnets is obtained. Alternatively, all or a part of the solid lubricant to be added may be added to the coarsely pulverized powder (coarsely pulverized powder for R-T-B sintered magnets) before the alloy powder for R-T-B magnets is obtained, and then finely pulverized to add the solid lubricant to the alloy powder for R-T-B magnets. Similarly, all or a part of the paraffin wax to be added may be added to the coarsely pulverized powder (coarsely pulverized powder for R-T-B sintered magnets) before the alloy powder for R-T-B magnets is obtained, and then finely pulverized to add the paraffin wax to the alloy powder for R-T-B magnets.

In one preferred embodiment, only solid lubricants and paraffin wax are added to the alloy powder for R-T-B sintered magnets. However, substances other than solid lubricants and paraffin wax may be added to the alloy powder for R-T-B sintered magnets, such as lubricants with lubricating properties other than the solid lubricants described above. In this case, the amount of substances other than solid lubricants and paraffin wax added is preferably 2% or less by mass, and more preferably 1% or less, of the total amount of the alloy powder for R-T-B sintered magnets, the solid lubricant, and the paraffin wax.

<Strong mixing>
As described above, the alloy powder for R-T-B sintered magnets is added with the solid lubricant and the paraffin wax, that is, the alloy powder for R-T-B sintered magnets, the solid lubricant, and the paraffin wax are mixed together, and strong stirring is performed to obtain a mixed powder for sintered magnets. Strong stirring is performed by rotating the stirring blade at a peripheral speed of 7 m/sec or more, preferably 10 m/sec or more. This allows sufficient shear force to be applied to the mixture of the alloy powder for R-T-B sintered magnets, the solid lubricant, and the paraffin wax, and the surface of the alloy powder for R-T-B sintered magnets can be uniformly coated with the solid lubricant and the paraffin wax. As a result, both high strength of the compact and high magnetic properties of the R-T-B sintered magnets can be achieved. Examples of equipment for strong stirring include, but are not limited to, a miller from Iwatani Corporation, a cyclomix from Hosokawa Micron Corporation, and a super mixer from Kawata Corporation.

2. Press Molding in a Magnetic Field The obtained mixed powder for sintered magnet is press molded in a magnetic field to obtain a compact for R-T-B magnet (molded body). Press molding in a magnetic field may be performed using any known method. For example, the mixed powder for sintered magnet may be supplied to a cavity of a die, and a mold and punch inserted from one or both ends of the cavity of the die may be used to apply compressive stress to the mixed powder for sintered magnet while a magnetic field is being applied to obtain a molded body. The obtained molded body may then be released from the die. The molded body according to the embodiment of the present invention has sufficient strength, so that problems such as the molded body breaking or cracking during release from the die can be effectively suppressed.

The shape of the compact is not limited to a block shape such as a rectangular parallelepiped, and may be any shape, such as a ring shape (doughnut shape). In this specification, the term "ring shape" refers not only to a shape in which the sintered magnet mixed powder is filled over the entire 360° of the void portion in plan view where no sintered magnet mixed powder is present, but also to a shape that includes a portion of the 360° that is not filled with the sintered magnet mixed powder (for example, a Landolt ring shape in plan view).

Since the stronger the strength of the applied magnetic field, the higher the degree of orientation and the improved magnetic properties, in one preferred embodiment the strength of the applied magnetic field is 1.0 T or more. Furthermore, the strength of a magnetic field that can be applied by a normal electromagnet is up to about 1.5 T, and applying a magnetic field exceeding this strength requires special equipment. From this perspective, it is preferable that the strength of the applied magnetic field is 1.5 T or less.

For example, in the case of a ring shape, the central gap is not formed as a cavity in the die, but metal is placed in this portion so that the mixed powder for sintered magnets does not enter this portion. In this way, depending on the shape of the compact, it is necessary to use a specially shaped die, and in that case, the strength of the magnetic field that can be applied to the cavity is limited to 0.7 T or less. However, even when the strength of the magnetic field to be applied is limited in this way, by using the manufacturing method according to this embodiment, it is possible to obtain an R-T-B based sintered magnet with higher magnetic properties than when using a conventional manufacturing method that applies a magnetic field of the same strength.
Therefore, in another preferred embodiment, the strength of the applied magnetic field is 0.7 T or less.

Even if the shape of the compact is a block shape or the like and there is no need to use a specially shaped mold, the production equipment can generally be simplified if the strength of the magnetic field applied is low. In the manufacturing method according to an embodiment of the present invention, even if the shape does not require a specially shaped mold, it is possible to obtain an R-T-B based sintered magnet with high magnetic properties by applying a low-strength magnetic field of 0.7 T or less using simple production equipment.

3. Sintering Step The resulting molded body is sintered to obtain an RTB based sintered magnet.
The sintering of the compact can be performed by the same method as the known method for producing sintered R-T-B magnets. In order to prevent oxidation due to the atmosphere during sintering, it is preferable to replace the atmospheric gas with an inert gas such as helium or argon. Sintering may also be performed in a vacuum.
The sintering temperature is not particularly limited, but may be 950 to 1150° C., and is preferably 960 to 1100° C.

The manufacturing method according to the embodiment of the present invention may include steps other than those described above as necessary. Examples of such additional steps include degreasing, surface coating, cutting and/or polishing for dimensional adjustment, and heat treatment to improve magnetic properties and other properties, such as diffusing one or both of the heavy rare earth element RH and the light rare earth element RL from the surface.

Experimental Example 1
1. Sample Preparation The following will explain the embodiments of the present invention in more detail with reference to examples. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above and below-described aims, and all of these are included in the technical scope of the embodiments of the present invention.

An alloy having a predetermined composition was prepared, and then coarsely crushed and finely crushed by a jet mill to prepare five types of alloy powders for sintered R-T-B magnets (alloy powders). Table 1 shows the compositions and average particle sizes _D50 of these alloy powders for sintered R-T-B magnets. In Table 1, the total content of all rare earth elements is shown as "TRE," and the contents of each of the rare earth elements Nd, Pr, and Dy are also shown.

These RTB-based sintered magnet alloy powders, solid lubricant, and paraffin wax were mixed and stirred to obtain mixed powders for sintered magnets. The preparation conditions for the mixed powders for sintering for each sample are shown in Table 2.
In Table 2, conditions that deviate from the embodiments of the present invention are underlined or marked with "-" to indicate that they are not applicable.

Paraffin wax was added before fine grinding, i.e., to the alloy powder after coarse grinding. Solid lubricants were added both before and after fine grinding. The "Paraffin wax" and "Solid lubricants" columns under "Added before fine grinding" in Table 2 show the mass ratios of paraffin wax and solid lubricants added to the alloy powder before fine grinding, respectively, to the alloy powder. Similarly, the "Solid lubricants" column under "Added after fine grinding" in Table 2 shows the mass ratios of solid lubricants added to the alloy powder after fine grinding, respectively. The "Paraffin wax" and "Solid lubricants" columns under "Total amount" show the total amounts before and after fine grinding, i.e., the mass ratios of paraffin wax and solid lubricants added to the alloy powder for R-T-B magnets. Also, "Solid lubricants + paraffin wax total" shows the total mass ratio of solid lubricants and paraffin wax to the mass of alloy powder for R-T-B magnets. Furthermore, "PWAX/(PWAX+solid lubricant)" indicates the mass ratio of the amount of paraffin wax added to the total amount of paraffin wax and solid lubricant added.

Zinc stearate was used as the solid lubricant. Paraffin wax with a melting point of 49-63°C and a boiling point of 237-321°C was used.

After adding the solid lubricant and paraffin wax to the alloy powder for R-T-B sintered magnets as described above, stirring was performed under the conditions shown in Table 2. "Mold mixing" in Table 2 is a conventional mixing method using a V-type mixer, and does not qualify as strong stirring. On the other hand, strong stirring was performed using a stirring blade. For samples that underwent strong stirring, the "Type" column of "V-stirring conditions" in Table 2 states "Strong stirring" along with the time for which strong stirring was performed, and the circumferential speed of the stirring blade is recorded in the "Circumferential speed" column.

The mixed powder for sintered magnets obtained in this way was press-molded in a magnetic field to produce R-T-B magnet compacts (green bodies). The applied magnetic field was varied from 0.2 T, 0.3 T, 0.4 T, 0.6 T, and 1.0 T, and samples were produced at each magnetic field strength. The green bodies were block-shaped, with dimensions after sintering of 11 mm x 26 mm x 13 mm.

Furthermore, for the four samples of Comparative Examples 1-1 and 1-2 and Examples 1-1 and 1-2, ring-shaped compacts were produced so that the dimensions after sintering were an outer diameter of 27 mm, an inner diameter of 21 mm, and a height of 17 mm.

The obtained R-T-B magnet green body sample was sintered at 1,060°C under an argon atmosphere of 500 Pa to obtain an R-T-B sintered magnet sample.

2. Evaluation of Characteristics 1) Flexural Strength A flexural test was carried out on a block-shaped molded product of each sample (a molded product in an applied magnetic field of 0.4 T), and the flexural strength obtained is shown in Table 3.
The flexural strength test was performed by three-point bending with a support distance of 22.1 mm, and the bending stress at which the sample broke was taken as the flexural strength. If the flexural strength of a molded product is 200 mN/ ^mm2 or more, the molded product will not usually crack when it is released from the mold, so a flexural strength of 200 mN/mm2 ^or more was considered to be pass, and a flexural strength of less ^than 200 mN/ ^mm2 was considered to be fail. When the flexural strength is less than 200 mN/mm2, the flexural strength value in Table 3 is underlined.

2) Quality of ring-shaped molded body As described above, for the four samples of Comparative Examples 1-1 and 1-2 and Examples 1-1 and 1-2 in which ring-shaped molded bodies were produced, the presence or absence of cracks in the ring-shaped molded body after demolding was confirmed. Samples with cracks were marked with "X" in the "Ring Molding Quality" column in Table 3, while samples without cracks were marked with "O" in the same column. In addition, if no experiments or measurements were performed, the column was left blank. From the results of Table 3, it can be seen that the technical common sense that cracks will not occur if the flexural strength is 200 mN/ ^mm2 or more is valid.

3) Magnetic properties For each block-shaped sample, B _r and H _cJ were measured using a BH tracer. The obtained values are shown in Table 3. In Table 3, descriptions such as "0.2T orientation" indicate that a magnetic field having the magnitude described before "orientation" was applied and press molding was performed in the magnetic field.

The following can be seen from Table 3.
All of the example samples had a sufficient strength, with a flexural strength of 200 mN/ ^mm2 , and the resulting RTB based sintered magnets had high B _r and sufficient H _cJ .

In Comparative Example 1-1, the amount of paraffin wax was too small relative to the total amount of paraffin wax and solid lubricant, and strong stirring was not performed, so that the B _r was generally lower than that of the example samples using the same alloy powder.
In Comparative Example 1-2, strong stirring was performed, but the amount of paraffin wax was too small relative to the total amount of paraffin wax and solid lubricant, resulting in a low strength of the compact.

In Comparative Example 1-3, the amount of solid lubricant is too small, and the amount of paraffin wax is too large relative to the total amount of paraffin wax and solid lubricant, resulting in a lower B _r than the Example sample using the same alloy powder.
In Comparative Examples 2-1, 3-1 and 4-1, strong stirring was not performed, and therefore, the B _r was lower than that of the Example samples using the same alloy powder.

In Comparative Example 5-1, no paraffin wax was used and strong stirring was not performed, resulting in a lower B _r than in the Example sample using the same alloy powder.

An example showing that the example samples have sufficient _HcJ is shown using Figure 1. Figure 1 is a graph showing the B _{r -HcJ} balance of the example and comparative example samples when a magnetic field of 0.4 T was applied during press molding in a magnetic field. Samples with low flexural strength are excluded. It can be seen that the example samples tend to have higher _HcJ than the comparative examples when they have the same level of B _r .
The graph line is drawn to connect the highest B _r for the same H _cJ in the examples and comparative examples. This shows that all the examples have a higher B _r than the line for the comparative samples.

Experimental Example 2
An alloy having a predetermined composition was prepared, and then coarsely crushed and finely crushed by a jet mill to prepare four types of alloy powders for R-T-B based sintered magnets (alloy powders). Table 4 shows the compositions and average particle sizes _D50 of these alloy powders for R-T-B based sintered magnets.

These R-T-B sintered magnet alloy powders were mixed with solid lubricant and paraffin wax and stirred to obtain mixed powders for sintered magnets. The preparation conditions for the mixed powders for sintering for each sample are shown in Table 5.

Paraffin wax was added before fine grinding, i.e., to the alloy powder after coarse grinding. Solid lubricants were added both before and after fine grinding, or after fine grinding. The "Paraffin wax" and "Solid lubricants" columns under "Added before fine grinding" in Table 5 show the mass ratios of paraffin wax and solid lubricants added to the alloy powder before fine grinding, respectively, to the alloy powder. Similarly, the "Solid lubricants" column under "Added after fine grinding" in Table 5 shows the mass ratios of solid lubricants added to the alloy powder after fine grinding, respectively. The "Paraffin wax" and "Solid lubricants" columns under "Total amount" show the total amount before and after fine grinding, i.e., the mass ratios of paraffin wax and solid lubricants added to the alloy powder for R-T-B magnets. Also, "Solid lubricants + paraffin wax total" shows the total mass ratio of solid lubricants and paraffin wax to the mass of alloy powder for R-T-B magnets. Furthermore, "PWAX/(PWAX+solid lubricant)" indicates the mass ratio of the amount of paraffin wax added to the total amount of paraffin wax and solid lubricant added.

Zinc stearate was used as the solid lubricant. Paraffin wax with a melting point of 49-63°C and a boiling point of 237-321°C was used.

After adding the solid lubricant and paraffin wax to the alloy powder for R-T-B sintered magnets as described above, stirring was carried out under the conditions shown in Table 5. "V-type mixing" in Table 5 is a conventional mixing method using a V-type mixer, and does not qualify as strong stirring. On the other hand, strong stirring was carried out using a stirring blade. For samples that underwent strong stirring, the "Type" column of "Stirring conditions" in Table 5 states "Strong stirring" along with the time for which strong stirring was carried out, and the circumferential speed of the stirring blade is recorded in the "Circumferential speed" column.

The mixed powder for sintered magnets obtained in this way was press-molded in a magnetic field to produce R-T-B magnet compacts (green bodies). The applied magnetic field was varied from 0.2 T, 0.3 T, 0.4 T, 0.6 T, and 1.0 T, and samples were produced at each magnetic field strength. The green bodies were block-shaped, with dimensions after sintering of 11 mm x 26 mm x 13 mm.

Furthermore, for the two samples of Comparative Example 7-1 and Example 7-1, ring-shaped compacts were produced so that the dimensions after sintering were an outer diameter of 27 mm, an inner diameter of 21 mm, and a height of 17 mm.

The obtained R-T-B magnet green body sample was sintered at 1,060°C under an argon atmosphere of 500 Pa to obtain an R-T-B sintered magnet sample.

2. Evaluation of Characteristics 1) Flexural Strength A flexural test was carried out on a block-shaped molded product of each sample (a molded product in an applied magnetic field of 0.4 T), and the flexural strength obtained is shown in Table 6.

2) Quality of ring-shaped molded body As described above, for the two samples of Comparative Example 7-1 and Example 7-1 in which ring-shaped molded bodies were produced, the presence or absence of cracks in the ring-shaped molded body was confirmed after demolding. Since there were no cracks in any of the samples, they were marked as "good".

3) Magnetic Properties For each block-shaped sample, B _r and H _cJ were measured using a BH tracer. The obtained values are shown in Table 6.

The following can be seen from Table 6.
All of the example samples had a sufficient strength, with a flexural strength of 200 mN/ ^mm2 , and the resulting RTB based sintered magnets had high B _r and sufficient H _cJ .
Comparing Comparative Example 6-1 and Example 6-1, Comparative Example 7-1 and Example 7-1, Comparative Example 8-1 and Example 8-1, and Comparative Example 9-1 and Example 9-1, which used the same alloy powder, the example samples generally have higher B _r .

Claims (4)

A mixing and stirring process for obtaining a mixed powder for a sintered magnet by stirring an alloy powder for an R-T-B based sintered magnet, which contains R (rare earth element), T (Fe or Fe and Co), and B, a solid lubricant consisting of one or more of phosphorous acid, fatty acid, phosphite ester, fatty acid ester, and fatty acid metal salt, and paraffin wax with a stirring blade,
the amount of the solid lubricant is 0.025% or more by mass ratio relative to the alloy powder for the R-T-B based sintered magnet, the amount of the paraffin wax is 0.010% or more by mass ratio relative to the alloy powder for the R-T-B based sintered magnet, and the total amount of the solid lubricant and the paraffin wax is 0.150% or less by mass ratio relative to the alloy powder for the R-T-B based sintered magnet,
the amount of the paraffin wax is 0.10 or more and 0.80 or less in terms of mass ratio with respect to the total amount of the solid lubricant and the paraffin wax;
A mixing and stirring step in which the stirring using the stirring blade is strong stirring at a peripheral speed of 7 m/sec or more;
a magnetic field press molding step of press-molding the mixed powder for sintered magnet in a magnetic field to obtain a compact for an R-T-B magnet;
a sintering step of sintering the R-T-B magnet green body;
The method for producing an RTB based sintered magnet includes the steps of: The method for producing an R-T-B based sintered magnet according to claim 1, wherein the compact is ring-shaped. The method for producing an R-T-B based sintered magnet according to claim 2, wherein the strength of the magnetic field applied in the magnetic field press molding process is 0.7 T or less. The method for producing an R-T-B based sintered magnet according to any one of claims 1 to 3, in which the paraffin wax and part or all of the solid lubricant are mixed with coarsely pulverized powder for R-T-B based sintered magnets, which has a larger average particle size than the alloy powder for R-T-B based sintered magnets, and then the alloy powder for R-T-B based sintered magnets is obtained using a jet mill.

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