JP7600531B2 - Manufacturing method of RTB based sintered magnet - Google Patents

Description

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

R-T-B sintered magnets (R is at least one rare earth element and always contains Nd, and T is at least one transition metal element and always contains Fe) are known as the most high-performance permanent magnets, and are used in a wide variety of applications, including voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EV, HV, PHV), motors for industrial equipment, and various other motors and home appliances.

R-T-B based sintered magnets are composed of a main phase consisting mainly of R ₂ T ₁₄ B compounds, and grain boundary phases (two-particle grain boundaries or multiple-point grain boundaries) located at the grain boundaries of this main phase. The R ₂ T ₁₄ B compound that is the main phase is a ferromagnetic material with high saturation magnetization and anisotropic magnetic field, and forms the basis of the characteristics of R-T-B based sintered magnets.

R-T-B based sintered magnets are manufactured, for example, through the steps of preparing alloy powder, press-molding the alloy powder to produce a powder compact, and sintering the powder compact. The alloy powder is manufactured, for example, by the following method. First, an alloy is manufactured from molten metals of various raw materials by an ingot method or a strip casting method. The obtained alloy is subjected to a crushing process to obtain an alloy powder having a predetermined particle size distribution. This crushing process usually includes a coarse crushing process and a fine crushing process, the former of which utilizes, for example, the hydrogen embrittlement phenomenon, and the latter of which uses, for example, an airflow crusher (jet mill).

Such R-T-B based sintered magnets have a lower coercive force H _cJ (hereinafter sometimes simply referred to as "H _cJ ") at high temperatures, causing irreversible thermal demagnetization. Therefore, particularly when used in electric vehicle motors, there is a demand for even higher H _cJ at room temperature in order to maintain a high H _cJ even at high temperatures.

Conventionally, large amounts of heavy rare earth elements (mainly Dy) have been added to sintered R-T-B magnets to improve _HcJ , but this has the problem of reducing the remanence B _r (hereinafter sometimes simply referred to as "B _r "). For this reason, in recent years, a method has been adopted in which heavy rare earth elements are diffused from the surface to the interior of a sintered R-T-B magnet, concentrating the heavy rare earth elements in the outer periphery of the main phase crystal grains, thereby obtaining a high _HcJ while suppressing the reduction in B _r .

However, heavy rare earth elements such as Dy are expensive and have problems such as price fluctuations, etc. Therefore, there is a demand for technology that improves the _HcJ of R-T-B based sintered magnets while minimizing the amount of heavy rare earth elements such as Dy used.

Patent Document 1 describes that by reducing the B content compared to that of a normal R-T-B based alloy and adding one or more metal elements M selected from Al, Ga, and Cu to form an R ₂ T ₁₇ phase, and by sufficiently ensuring the volume fraction of a transition metal-rich phase (R ₆ T ₁₃ M) formed using the R ₂ T ₁₇ phase as a raw material, it is possible to obtain an R-T-B based rare earth sintered magnet with high coercivity while suppressing the Dy content.

Furthermore, methods for improving performance such as improving _HcJ other than the method described in Patent Document 1 include, for example, making the structure finer and reducing the oxygen content. There is also a demand for R-T-B based sintered magnets to be cost-effective as well as to have high performance. Examples of methods for reducing cost include improving the crushing efficiency.

Patent Document 2 discloses a method for improving the efficiency of grinding by using a jet mill to grind materials using a humidified inert gas stream with a dew point of -20°C to 0°C. A similar method is also described in Patent Document 3.

International Publication No. 2013/008756 Japanese Patent Application Publication No. 8-148317 Japanese Patent Application Publication No. 6-140220

When producing R-T-B based sintered magnets with a reduced oxygen content, for example an oxygen content of 3000 ppm or less, high purity nitrogen gas, for example, is used as the inert gas to prevent oxidation of the powder particles during the grinding process.

According to the study by the present inventors, when jet mill pulverization is performed using an inert gas such as high-purity nitrogen gas, it is found that the expected high performance cannot be achieved by reducing the oxygen content. In addition, when trying to improve performance by making the powder finer, the pulverization efficiency is sacrificed. Regarding the pulverization efficiency, there are methods disclosed in Patent Documents 2 and 3, but the configurations disclosed in Patent Documents 2 and 3 are technologies for increasing the oxygen content to more than 3000 ppm in order to suppress reactivity, and cannot be applied to improving performance by reducing oxygen content. The embodiment of the present disclosure provides a manufacturing method for an R-T-B magnet that reduces the heavy rare earth element content to obtain high B _r and high H _cJ while suppressing the deterioration of the pulverization efficiency during jet mill pulverization even if the pulverized particle size of the fine powder is reduced.

In a non-limiting exemplary embodiment, the method for producing an R-T-B based sintered magnet disclosed herein is a method for producing an R-T-B based sintered magnet (R is at least one rare earth element and always includes Nd, and T is Fe or Fe and Co), comprising the steps of preparing an additive alloy powder containing R1: (R1 is at least one rare earth element, and 50 mass% or more of the total R1 is Pr) 33 to 69 mass%, B: 0.2 to 0.8 mass%, Cu: 0.8 to 3.0 mass%, Ga: 1.8 to 10 mass%, and T: 15 to 60 mass%, and preparing a main alloy powder containing R: 28.5 to 33.0 mass%, B: 0.80 to 1.0 mass%, Ga: 0.1 to 0.4 mass%, and T: 64 to 70 mass%. preparing a mixed alloy powder containing 1 to 16 mass % of the additive alloy powder and 84 to 99 mass % of the main alloy powder; supplying the mixed alloy powder to a jet mill device having a crushing chamber filled with an inert gas to crush the mixed alloy powder to obtain a fine powder having an average particle size of 2.0 μm to 4.5 μm; and producing a sintered body of the fine powder, wherein the Pr content of the additive alloy powder is greater than the Pr content of the main alloy powder, the inert gas is humidified, and the R-T-B based sintered magnet has an oxygen content of 1000 ppm to 3000 ppm, R: 28.5 mass % to 33.0 mass %, B: 28.5 mass % to 33.0 mass %, and : 0.85 mass% or more and 0.91 mass% or less, Cu: 0.05 mass% or more and 0.50 mass% or less, Ga: 0.3 mass% or more and 0.7 mass% or less, and T: 63 mass% or more and 70 mass% or less (T is Fe or Fe and Co), where the content (mass%) of T is [T] and the content (mass%) of B is [B], the relationship [T]/55.85>14×[B]/10.8 is satisfied.

In one embodiment, the R content of the R-T-B based sintered magnet is 31 mass% or less.

In one embodiment, the inert gas is nitrogen gas.

In one embodiment, the process for producing a sintered body of the fine powder includes a process for producing a powder compact from the fine powder by wet pressing in a magnetic field or by pressing in a magnetic field in an inert gas atmosphere, and a process for sintering the powder compact.

In one embodiment, the average particle size of the fine powder in the process of obtaining the fine powder is 2.0 μm or more and 3.5 μm or less.

According to an embodiment of the present disclosure, it is possible to provide a method for producing an R-T-B based magnet that achieves high B _r and high H _cJ by reducing the heavy rare earth element content while preventing a decrease in the pulverization efficiency during jet mill pulverization even when the pulverized particle size of the fine powder is reduced.

FIG. 1 is a diagram showing a schematic configuration example of an RTB based sintered magnet alloy crushing system 1000 according to this embodiment.

As a result of the study, the inventors of the present invention found that in the case of producing an R-T-B based sintered magnet with a reduced oxygen content, when the powder particles are reduced in size in the pulverization process, in addition to the deterioration of the pulverization efficiency, the powder particles are nitrided by an inert gas (particularly when dry nitrogen gas is used as the inert gas) in the pulverization process, and the desired improvement in magnetic properties due to the reduction in the size of the pulverized particles cannot be obtained. As a result of further study, the inventors of the present invention found that the deterioration of the powder particles due to the inert gas can be reduced by using a humidified inert gas. This is believed to be because the formation of an oxide film on the surface of the powder particles makes it possible to prevent the introduction of the inert gas (particularly nitrogen gas) into the powder particles, thereby suppressing the deterioration (nitridation) of the powder particles due to the inert gas. It has been known in the past that the pulverization efficiency is deteriorated when the powder particles are reduced in size in the pulverization process, and that this deterioration can be improved by using a humidified inert gas flow (for example, Patent Document 2 and Patent Document 3). However, when pulverizing using a humidified inert gas flow, it was thought that the powder particles would be oxidized and the magnetic properties would deteriorate, and so when manufacturing an R-T-B based sintered magnet with a reduced oxygen content in order to improve its magnetic properties, pulverization using a humidified inert gas flow was not actively carried out in an attempt to make the pulverized particles smaller (for example, the oxygen content of the fine powder in Patent Document 2 is relatively high at 4500 ppm and 4900 ppm, and Patent Document 3 does not disclose the oxygen content). However, the present inventors conducted extensive research based on the above-mentioned knowledge that the deterioration of powder particles due to an inert gas can be reduced by using a humidified inert gas, and as a result, they unexpectedly found that, in the finally obtained R-T-B based sintered magnet, when the powder particles are humidified and pulverized so that the oxygen content falls within a specific range, it is possible to suppress both the deterioration (nitridation) of the powder particles and the deterioration of the magnetic properties due to oxidation caused by humidification. Generally, the process after pulverization in which the oxygen content of an R-T-B based sintered magnet increases is mainly the process of forming a sintered body by molding and sintering fine powder, but the increase in the oxygen content of an R-T-B based sintered magnet is small (for example, 50 ppm to 300 ppm). Therefore, the oxygen content of an R-T-B based sintered magnet can be adjusted by the pulverization process. Furthermore, as a result of research, the inventors of the present invention found that when a mixed alloy powder obtained by mixing a main alloy powder of a specific composition with an additive alloy powder is subjected to the humidified pulverization of the present disclosure, _HcJ can be improved compared to pulverization without humidification. That is, the present disclosure has discovered that by using an additive alloy powder and a main alloy powder of a specific composition to set the composition of the R-T-B based sintered magnet within a specific range, and further by humidifying and pulverizing the R-T-B based sintered magnet in the pulverization step so that the oxygen content of the resulting R-T-B based sintered magnet is within a specific range (1000 ppm or more and 3000 ppm or less, preferably 1000 ppm or more and 2500 ppm or less) to reduce the powder particles (average particle size of 2.0 μm or more and 4.5 μm or less, preferably average particle size of 2.0 μm or more and 3.5 μm or less), it is possible to obtain an R-T-B based sintered magnet that has a high B _r and high H _cJ by reducing the content of heavy rare earth elements while reducing the deterioration of magnetic properties due to oxidation and nitridation in the pulverization step and preventing a deterioration in pulverization efficiency during jet mill pulverization.

<Method of manufacturing R-T-B based sintered magnet>
The method for producing an R-T-B based sintered magnet according to the present disclosure includes the steps of:
(1) preparing an additive alloy powder;
(2) preparing a primary alloy powder;
(3) preparing a mixed alloy powder;
(4) supplying the mixed alloy powder to a jet mill device having a pulverizing chamber filled with an inert gas to pulverize the mixed alloy powder to obtain a fine powder having an average particle size of 2.0 μm to 4.5 μm;
(5) preparing a sintered body of the fine powder;
The inert gas is humidified. The average particle size (d50) can be measured by an air-dispersion laser diffraction method.

<RTB-based sintered magnet>
The R-T-B based sintered magnet of the present disclosure has an oxygen content of 1000 ppm or more and 3000 ppm or less by mass. By setting the oxygen content to 1000 ppm or more and 3000 ppm or less, the fine powder of (4) above can be obtained. In the process for obtaining the above-mentioned powder, it is possible to suppress the deterioration of magnetic properties due to the nitridation of powder particles caused by the inert gas caused by insufficient humidification of the inert gas, or the deterioration of magnetic properties due to the oxidation of powder particles caused by humidification. To obtain higher magnetic properties, the oxygen content of the RTB based sintered magnet is preferably 1000 ppm or more and 2500 ppm or less, and more preferably 1000 ppm or more and 2000 ppm or less.

Furthermore, the R-T-B based sintered magnet according to the present invention (sometimes simply referred to as "sintered magnet") has the following characteristics:
R: 28.5 to 33.0% by mass,
B: 0.85 to 0.91% by mass,
Cu: 0.05 to 0.50% by mass,
Ga: 0.3 to 0.7 mass%; and T: 63 to 70 mass%;
When the content (mass %) of T is [T] and the content (mass %) of B is [B], the R-T-B based sintered magnet satisfies [T]/55.85>14×[B]/10.8.

The above composition reduces the amount of B compared to general R-T-B sintered magnets, and contains Ga and the like. This allows the R-T-Ga phase to be generated particularly at the multi-point grain boundaries, and the R-Cu-Ga phase to be generated particularly at the two-grain grain boundaries, and the content of heavy rare earth elements can be reduced to obtain an R-T-B sintered magnet with high B _r and high H _cJ . Here, the R-T-Ga phase is typically a phase consisting of Nd ₆ Fe ₁₃ Ga compound. The R ₆ T ₁₃ Ga compound has a La ₆ Co ₁₁ Ga ₃ type crystal structure. Depending on the state, the R ₆ T ₁₃ Ga compound may be an R ₆ T _13-δ Ga _1+δ compound (δ is typically 2 or less). For example, when a relatively large amount of Cu and Al is contained in an R-T-B based sintered magnet, it may be of the formula R ₆ T _13-δ (Ga _1-x-y Cu _x Al _y ) _1+δ . The R-Cu-Ga phase is an R-Ga phase in which part of the Ga has been replaced with Cu, and contains 70% by mass or more and 95% by mass or less of R, 5% by mass or more and 30% by mass or less of Ga, and 20% by mass or less of T (Fe) (including 0), and an example of this is an R ₃ (Ga,Cu) ₁ compound.

The individual components contained in R-T-B based sintered magnets are described in detail below.

(R: 28.5-33.0% by mass)
R is at least one rare earth element and necessarily includes Nd.
The content of R (R amount) is 28.5 to 33.0 mass %. If R is less than 28.5 mass %, densification during sintering may be difficult. If it exceeds 100% by mass, the main phase ratio decreases, and there is a risk that a high _Br cannot be obtained.
The amount of R is preferably 31% by mass or less, and more preferably 29 to 31% by mass. If R is in this range, a higher B _r can be obtained.

(B: 0.85-0.91% by mass)
The B content (B amount) of the sintered magnet is 0.85 to 0.91 mass %. If the B amount is less than 0.85 mass %, the R ₂ T ₁₇ phase is generated, resulting in a high H _cJ If the content exceeds 0.91 mass %, the amount of the RT-Ga phase produced may be too small to obtain a high _HcJ .

(Cu: 0.05 to 0.50% by mass)
The Cu content (Cu amount) of the sintered magnet is 0.05 to 0.50 mass %. If the Cu amount is less than 0.05 mass %, it may be difficult to obtain a high _HcJ . If the content exceeds 0.50 mass %, the sinterability may deteriorate, making it difficult to obtain a high _HcJ .

(Ga: 0.3 to 0.7% by mass)
The Ga content (Ga amount) of the sintered magnet is 0.3 to 0.7 mass %. If the Ga amount is less than 0.3 mass %, the amount of the RT-Ga phase produced is small. If the content is too high, the R ₂ T ₁₇ phase cannot be eliminated, and a high H _cJ may not be obtained. If the content exceeds 0.7 mass%, unnecessary Ga will be present, and the main phase ratio will be This may result in a decrease in B _r .

(T: 63-70% by mass)
T is Fe or Fe and Co.
The content of T (T amount) is 63.0% by mass to 70% by mass. If the content of T is less than 63.0% by mass or exceeds 70% by mass, there is a risk that _Br may be significantly reduced. be.

The content of B satisfies the following formula (1).
[T]/55.85>14×[B]/10.8
By satisfying formula (1), the content of B becomes smaller than that of a general R-T-B based sintered magnet. A general R-T-B based sintered magnet has a composition in which [ _{T]/55.85 (atomic weight of Fe) is smaller than 14×[B]/10.8 (atomic weight of B) ([T] is the content of Fe in mass%) so that the soft magnetic phase R 2 T 17 phase} is not generated other than the main phase R 2 T 14 B phase. Unlike a general R-T-B based sintered magnet, the R-T-B based sintered magnet of the present invention is specified by formula (1) so that [T]/55.85 is larger than 14×[B]/10.8. Note that the main component of T in the R-T-B based sintered magnet of the present invention is Fe, so the atomic weight of Fe is used.

(Inevitable impurities and other elements)
Furthermore, the R-T-B system sintered magnet of the present invention may contain Cr, Mn, Si, La, Ce, Sm, Ca, Mg, etc. as inevitable impurities that are usually contained in didymium alloys (Nd-Pr), electrolytic iron, ferroboron, etc. Such elements may be contained in a total amount of, for example, about 2.0 mass% or less. At this level, it is sufficiently possible to obtain an R-T-B system sintered magnet having high B _r and high H _cJ . Furthermore, examples of inevitable impurities that may be mixed in during the manufacturing process include O (oxygen), N (nitrogen), and C (carbon). Furthermore, the R-T-B system sintered magnet of the present disclosure may further contain one or more other elements (elements added intentionally other than inevitable impurities). For example, such elements may include small amounts (each about 0.2 mass % or less) of Ag, Zn, In, Sn, Ti, Ge, Y, H, F, P, S, V, Ni, Mo, Hf, Ta, W, Nb, Zr, etc.

The oxygen content of the R-T-B based sintered magnet is in the range of 1000 ppm to 3000 ppm (preferably 1000 ppm to 2500 ppm). Both nitridation of the powder particles and oxidation due to humidification can be suppressed to obtain high magnetic properties. As will be described later, the amount of oxygen contained in the R-T-B based sintered magnet can be adjusted by the crushing process.
The R-T-B based sintered magnet of the present invention may contain R, B, Cu, and Ga in the above-mentioned ranges, with the balance being Fe and unavoidable impurities. In other words, it is possible to obtain an R-T-B based sintered magnet that contains only B, R, Cu, Ga, Fe, and unavoidable impurities, and does not contain any other intentionally added elements.

(1) Step of Preparing Additive Alloy Powder In this step, the additive alloy powder to be used in the production of the sintered magnet is prepared.
The additive alloy powder having a predetermined composition described later can be produced by a method similar to the known method for producing R-T-B based sintered magnets. For example, flake-shaped alloy flakes are produced by an ingot method using a metal mold casting or a strip casting method in which a molten alloy is quenched using a cooling roll. The obtained flake-shaped alloy flakes are subjected to hydrogen pulverization to obtain a coarsely pulverized powder (additive alloy powder) having a size of, for example, 1.0 mm or less.

The composition of the additive alloy powder is:
R1: 33 to 69% by mass,
B: 0.2 to 0.8% by mass,
Cu: 0.8 to 3.0% by mass,
Ga: 1.8 to 10% by mass,
T: 15 to 60 mass%.

The reasons for limiting the amount of each element contained in the additive alloy powder are described below.

(R1: 33-69% by mass)
R1 is at least one rare earth element, and 50 mass % or more of all R1 is Pr.

By making Pr 50% or more by mass of the entire R1, grain boundaries containing Pr are more likely to form at the grain boundaries of the resulting R-T-B based sintered magnet, making it possible to obtain high magnetic properties.

The content of R1 in the additive alloy powder (R1 amount) is 33 to 69 mass%. If the R1 amount is less than 33 mass%, the R1 amount is too small relative to the R ₂ T ₁₄ B stoichiometric composition, so there is a risk that the R-Ga phase and the R-Ga-Cu phase may not be easily generated. If the R1 amount exceeds 69 mass%, the R1 amount is too large, so there is a risk of oxidation of R1, which may lead to a decrease in magnetic properties and a risk of fire, which may cause problems in production.

The amount of R1 is preferably 40 to 60 mass%. The Pr content of the additive alloy powder is set to be higher than the Pr content of the main alloy powder.

(B: 0.2-0.8% by mass)
The content of B in the additive alloy powder (B amount) is 0.2 to 0.8 mass %. B reacts with R and T to form an R ₂ T ₁₄ B type compound, which is the main phase. If the B content is less than 0.2 mass %, the amount of R ₂ T ₁₄ B type compound produced is small, and the R ₂ T ₁₇ phase is produced in the added alloy powder. As a result, the _HcJ of the finally obtained sintered magnet decreases. If the B content exceeds 0.8 mass %, the B content in the main alloy powder must be reduced. There is a risk that the R ₂ T ₁₇ phase will be generated, lowering the H _cJ of the final sintered magnet.
The amount of B is preferably 0.2 to 0.7 mass %.

(Cu: 0.8-3.0% by mass)
The Cu content (Cu amount) of the additive alloy powder is 0.8 to 3.0 mass %. If the Cu amount is less than 0.8 mass %, the Cu amount of the finally obtained sintered magnet may be too low. If the _Cu content exceeds 3.0 mass %, the sinterability of the mixed alloy powder containing the additive alloy powder and the main alloy powder deteriorates, and the sintering property becomes poor. There is a risk that the _HcJ of the magnet will decrease.
The Cu content is preferably 1.5 to 2.6 mass %.

(Ga: 1.8-10% by mass)
The Ga content of the additive alloy powder is 1.8 to 10 mass %. If the Ga content is less than 1.8 mass %, the Ga content in the main alloy powder must be increased. If the content of the RT-Ga phase in the alloy powder exceeds 10 mass %, the _HcJ of the finally obtained sintered magnet may decrease. Such a phase may be generated, lowering the _HcJ of the final sintered magnet.
The Ga content is preferably 3 to 8 mass %.

(T: 15-60% by mass)
T is Fe or Fe and Co.
The content of T in the additive alloy powder is 15 to 60 mass %. T is an element necessary for reacting with R and T to produce the R ₂ T ₁₄ B type compound which is the main phase. If the amount of T is less than 15 mass %, the amount of R ₂ T ₁₄ B type compound produced is small, and therefore the H _cJ of the finally obtained sintered magnet is reduced. If the amount of T in the powder is reduced, the R ₂ T ₁₇ phase will be generated in the main alloy powder, which may lower the H _cJ of the final sintered magnet.
The amount of T is preferably 20 to 50% by mass.

(Inevitable impurities and other elements)
The additive alloy powder may contain Cr, Mn, Si, La, Ce, Sm, Ca, Mg, etc. as inevitable impurities. The elements listed above as inevitable impurities may also be intentionally added. Such elements may be contained in a total amount of, for example, about 1.0 mass% or less. This amount is sufficient to obtain an R-T-B based sintered magnet with a high _HcJ . Furthermore, examples of inevitable impurities that may be mixed in during the manufacturing process include O (oxygen), N (nitrogen), and C (carbon). The R-T-B based sintered magnet of the present invention may also contain one or more other elements (elements that are intentionally added other than inevitable impurities). For example, such elements may contain small amounts (each about 0.1 mass% or less) of Ag, Zn, In, Sn, Ti, Ge, Y, H, F, P, S, V, Ni, Mo, Hf, Ta, W, Nb, Zr, etc.

Note that multiple types of additive alloy powders may be prepared as long as they are within the composition range of the additive alloy powder described above. In this case, the multiple types of additive alloy powders should total 1 to 16 mass % when the mixed alloy powder is 100 mass %.

(2) Step of Preparing Main Alloy Powder In this step, the main alloy powder used in producing the sintered magnet is prepared.
The main alloy powder can be produced by the same method as the additive alloy powder. For example, flake-shaped alloy flakes are produced by an ingot method using a metal mold casting or a strip casting method in which a molten alloy is quenched using a chill roll. The obtained flake-shaped alloy flakes are then hydrogen-pulverized to obtain a coarsely pulverized powder (main alloy powder) having a size of, for example, 1.0 mm or less.

The composition of the main alloy powder is adjusted so as to contain R, B, Ga, and T within the following ranges.
R: 28.5 to 33.0% by mass,
B: 0.80 to 1.00% by mass,
Ga: 0.1 to 0.4 mass%, and T: 64 to 70 mass%

The reasons for limiting the amounts of each element contained in the main alloy powder are described below.

(R: 28.5-33.0% by mass)
R is at least one rare earth element and necessarily includes Nd.
The R content (R amount) of the main alloy powder is 28.5 to 33.0 mass %. If the R amount is less than 28.5 mass %, there is a risk that _HcJ will decrease. If the content exceeds 33.0 mass %, _Br may decrease.

(B: 0.80 to 1.00% by mass)
The content of B in the main alloy powder (B amount) is 0.80 to 1.10 mass %. B reacts with R and T to form an R ₂ T ₁₄ B type compound, which is the main phase. If the B content is less than 0.80 mass %, the amount of R ₂ T ₁₄ B type compound produced is small, and the R ₂ T ₁₇ phase is likely to be produced in the added alloy powder. Therefore, the _HcJ of the finally obtained sintered magnet may decrease. If the B content exceeds 1.10 mass %, the B content in the added alloy powder must be reduced. There is a risk that an R ₂ T ₁₇ phase will be formed in the alloy powder, lowering the H _cJ of the final sintered magnet.

(Ga: 0.1-0.4% by mass)
The Ga content (Ga amount) of the main alloy powder is 0.1 to 0.4 mass %. If the Ga amount is less than 0.1 mass %, the R-Ga phase and the R-Ga-Cu phase are not formed. If the amount of Ga produced is too small, the _HcJ may decrease. If the amount of Ga produced exceeds 0.4 mass %, an R-T-Ga phase may be produced in the main alloy powder, and the final sintered powder may have a low HcJ. There is a risk that the _HcJ of the magnet will decrease.

(T: 64-70% by mass)
T is Fe or Fe and Co.
The T content (T amount) of the main alloy powder is 64 to 70 mass %. If the T amount is less than 64 mass %, there is a risk of a rapid drop in _HcJ . If the T amount exceeds 70 mass %, If the temperature is too high, the R ₂ T ₁₇ phase may be generated, resulting in a decrease in H _cJ .
When the total amount of T is taken as 100 mass %, for example, 0 to 10 mass % of T may be replaced with Co. In other words, 90 to 100 mass % of the total amount of T is Fe, and 0 % by weight is Co.

(Inevitable impurities and other elements)
The main alloy powder may contain Cr, Mn, Si, La, Ce, Sm, Ca, Mg, etc. as inevitable impurities. The elements listed above as inevitable impurities may also be intentionally added. Such elements may be contained in a total amount of, for example, about 2.0 mass% or less. This amount is sufficient to obtain an R-T-B based sintered magnet with a high _HcJ . Furthermore, examples of inevitable impurities that may be mixed in during the manufacturing process include O (oxygen), N (nitrogen), and C (carbon). The R-T-B based sintered magnet of the present invention may also contain one or more other elements (elements that are intentionally added other than the inevitable impurities). For example, small amounts (each about 0.2 mass% or less) of Ag, Zn, In, Sn, Ti, Ge, Y, H, F, P, S, V, Ni, Mo, Hf, Ta, W, Nb, Zr, etc. may be contained as such elements.

Note that multiple types of main alloy powders may be prepared as long as they fall within the composition range of the main alloy powder described above. In this case, the total amount of the multiple types of main alloy powders should be 84 to 99 mass % when the mixed alloy powder is 100 mass %.

(3) Step of preparing mixed alloy powder The additive alloy powder and the main alloy powder are mixed to prepare a mixed alloy powder. When the mixed alloy powder is taken as 100 mass%, 1 to 16 mass% of the additive alloy powder and 84 to 99 mass% of the main alloy powder are mixed.
If the amount of the added alloy powder mixed is less than 1 mass%, the amount of the added alloy powder is too small, and the formation of the R-T-Ga phase cannot be suppressed, which may result in a decrease in _HcJ . If the amount of the added alloy powder mixed exceeds 16 mass%, the _Br may decrease. The mixed alloy powder is prepared by mixing the added alloy powder and the main alloy powder in a known device such as a V-type mixer. The mixed alloy powder is then fed to a jet mill device and pulverized to obtain fine powder with an average particle size of 2.0 μm to 4.5 μm.

(4) A process of supplying the mixed alloy powder to a jet mill device whose grinding chamber is filled with an inert gas to grind the mixed alloy powder, thereby obtaining a fine powder having an average particle size of 2.0 μm or more and 4.5 μm or less <Crushing system>
First, a grinding system that can be used in the manufacturing method of the R-T-B based sintered magnet according to the present disclosure will be illustrated with reference to Fig. 1. Fig. 1 is a diagram showing a schematic configuration example of a grinding system 1000 in this embodiment. In this example, the R-T-B based sintered magnet alloy grinding system 1000 includes a jet mill device 100, a cyclone collection device 200, and a bag filter device 300.

The jet mill device 100 receives the material to be ground from a raw material tank (not shown) through a raw material input pipe 34. The material to be ground in this disclosure is a mixed alloy powder, and has, for example, an average particle size of 10 μm or more and 500 μm or less. The average particle size (d50) in this disclosure can be measured by an airflow dispersion laser diffraction method (in accordance with JIS Z 8825: 2013 revised edition). In other words, in this specification, the average particle size means the particle size (median diameter) at which the cumulative particle size distribution (volume basis) from the small particle size side is 50%.

The average particle size (d50) in the embodiment of the present disclosure was measured using a particle size distribution measuring device "HELOS &RODOS" manufactured by Sympatec. Dispersion pressure: 4 bar
Measurement range: R2
Calculation mode: HRLD
This indicates the d50 measured under the above conditions.

The raw material input pipe 34 is provided with multiple valves, and the internal pressure of the jet mill device 100 is appropriately maintained by opening and closing the valves. The material to be ground that is introduced into the jet mill device 100 is finely ground by collisions between the materials to be ground due to high-speed injection of inert gas from the nozzle tube 36 and collisions with a collision plate installed to efficiently advance the grinding. A humidification tube is connected to the nozzle tube 36 to add moisture to the inert gas.

The powder of alloys for R-T-B sintered magnets is active and easily oxidized. For this reason, the gas used in the jet mill device 100 is generally a dry (high purity) inert gas such as nitrogen, argon, or helium with a dew point of -60°C or less, in order to avoid the risk of heat generation and fire and to reduce the oxygen content as an impurity in order to improve the performance of the magnet. However, in the embodiment of the present disclosure, the grinding is performed in a humidified state in which moisture is intentionally introduced into such an inert gas. More details on this point will be given later.

The powder particles (fine powder) finely ground inside the jet mill device 100 are carried by the ascending air current and guided from the upper outlet to the inlet pipe 20 of the cyclone collector 200. Coarse particles that are not sufficiently ground are separated by a classification rotor installed to classify coarse particles of a median diameter (d50) or more, remain inside the jet mill device 100, and are subjected to a further crushing process by collision. The classification of these coarse particles can be performed using a classification rotor or centrifugation using a swirling flow. In this way, the material to be ground (mixed powder) fed into the jet mill device 100 is crushed into fine powder with a particle size distribution of an average particle size (median diameter: d50) of 2.0 μm to 4.5 μm, and then moved to the cyclone collector 200.

The cyclone collector 200 is used to separate the powder from the airflow carrying the powder. Specifically, the mixed alloy powder (coarsely crushed powder) is crushed in a jet mill in the front stage, and the fine powder generated by the crushing is supplied to the cyclone collector 200 through the inlet pipe 20 together with the gas used for crushing. A mixture of an inert gas (crushing gas) and the crushed fine powder forms a high-speed airflow and is sent to the cyclone collector 200. The cyclone collector 200 is used to separate the crushing gas and the fine powder. The fine powder separated from the crushing gas is collected by the powder collector 50 through the exhaust port 40. The crushing gas is supplied to the bag filter device 300 through the outlet pipe 30. The bag filter device 300 collects very small particles, and clean gas is discharged to the outside through the exhaust port 32. It is also possible to use a bag filter for this type of solid-gas separation without using the cyclone collector 200, but this would have a significant impact on the environment and safety, such as the dispersion of fine powder into the air due to damage to the filter. A bag filter may also be used to separate fine particles from the gas separated by the cyclone collector.

One of the distinctive features of the present disclosure is that the humidified grinding is performed so that the oxygen content of the R-T-B based sintered magnet is in the range of 1000 ppm to 3000 ppm. This makes it possible to suppress both the nitridation of the powder particles caused by grinding and the oxidation caused by humidification, thereby obtaining high magnetic properties. As mentioned above, the increase in the oxygen content of the R-T-B based sintered magnet due to the processes after grinding (mainly the process of producing a sintered body from the fine powder) is usually small (for example, 50 ppm to 300 ppm). Therefore, it is possible to adjust the amount of oxygen contained in the R-T-B based sintered magnet by the grinding process.

Specifically, the humidified inert gas in step (2) is obtained by adding moisture to the inert gas in an amount of, for example, 0.5 g to 6.0 g per kg of the mixed alloy powder (coarsely crushed powder).

The dew point in the grinding chamber and the amount of mixed alloy powder supplied to the jet mill device depend on the grinding time and the size of the jet mill device, but in one preferred embodiment, the inert gas is humidified so that the dew point during grinding is -65°C or higher and -30°C or lower. In yet another preferred embodiment, the rate at which the mixed alloy powder is supplied to the jet mill device is 35 kg/hour or higher and 180 kg/hour or lower.

Examples of inert gases are nitrogen, argon, helium, and mixtures thereof. Among them, nitrogen is the most preferable because it is available at a low cost as a high-purity gas. Therefore, in a preferred embodiment, the inert gas is nitrogen gas. However, as a result of the study by the present inventors, when jet mill grinding is performed using an inert gas made of nitrogen, it was found that when the average particle size of the resulting fine powder is 4.5 μm or less, the magnetic properties begin to deteriorate due to nitridation. In particular, it was found that when the average particle size is 3.5 μm or less, the deterioration of the magnetic properties due to nitridation may become significant. However, according to the embodiment of the present disclosure, since grinding is performed in an appropriately adjusted humidified atmosphere, even if nitrogen gas is used as the inert gas, it is possible to suppress both nitridation and oxidation. This is thought to be because, even if the inert gas in the grinding chamber is mainly nitrogen, by humidifying it so that it contains a specific adjusted amount of moisture, the active surface of the particles that appear by fine grinding can be thinly oxidized before it is nitrided. The increase in oxygen content of the R-T-B based sintered magnet due to the process after pulverization (mainly the process of producing a sintered body from the fine powder) is preferably 50 ppm to 300 ppm, and more preferably 50 ppm to 200 ppm. To achieve this, as described below, wet pressing in a magnetic field or pressing in a magnetic field in an inert gas atmosphere is performed, and the resulting molded body is sintered. It is more preferable that the average particle size of the fine powder in the process of obtaining the fine powder is 2.0 μm to 3.5 μm. By reducing the average particle size, it is possible to improve the magnetic properties.

(5) Step of preparing a sintered body of fine powder In a preferred embodiment, the step of preparing a sintered body of fine powder includes a step of preparing a powder compact from the fine powder by pressing in a magnetic field, and a step of sintering the powder compact. In the case of pressing in a magnetic field, it is preferable to form the powder compact by pressing in an inert gas atmosphere or by wet pressing from the viewpoint of suppressing oxidation. In particular, in the case of wet pressing, the surface of the particles constituting the powder compact is covered with a dispersant such as an oil agent, and contact with oxygen or water vapor in the air is suppressed. Therefore, it is possible to prevent or suppress oxidation of the particles by the air before, during, or after the pressing step.

When wet pressing in a magnetic field is performed, a slurry is prepared by mixing fine powder with a dispersion medium, which is then fed into the cavity of a mold in a wet pressing device and pressed into a magnetic field.

Dispersion Medium The dispersion medium is a liquid in which the alloy powder can be dispersed to obtain a slurry.

A preferred dispersion medium for use in the present disclosure may be mineral oil or synthetic oil. Although the type of mineral oil or synthetic oil is not specified, if the kinetic viscosity at room temperature exceeds 10 cSt, the increased viscosity may strengthen the bonding force between the alloy powders, which may adversely affect the orientation of the alloy powder during wet molding in a magnetic field. For this reason, the kinetic viscosity of the mineral oil or synthetic oil at room temperature is preferably 10 cSt or less. Furthermore, if the fractional point of the mineral oil or synthetic oil exceeds 400°C, it becomes difficult to remove the oil after obtaining the compact, and the amount of residual carbon in the sintered body may increase, resulting in a deterioration of the magnetic properties. Therefore, the fractional point of the mineral oil or synthetic oil is preferably 400°C or less. Vegetable oil may also be used as the dispersion medium. Vegetable oil refers to oil extracted from plants, and the type of plant is not limited to a specific plant.

Preparation of Slurry The obtained alloy powder and a dispersion medium are mixed to obtain a slurry.

The mixing ratio of the alloy powder and the dispersion medium is not particularly limited, but the concentration of the alloy powder in the slurry is preferably 70% or more by mass ratio. This is because at a flow rate of 20 to 600 cm ³ /sec, the alloy powder can be efficiently supplied into the cavity and excellent magnetic properties can be obtained. The concentration of the alloy powder in the slurry is preferably 90% or less by mass ratio. The method of mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and the dispersion medium may be prepared separately, and the alloy powder and the dispersion medium may be weighed and mixed in a predetermined amount. In addition, when the mixed alloy powder is dry-pulverized with a jet mill or the like to obtain a fine powder, a container containing the dispersion medium may be placed at the alloy powder outlet of a pulverizing device such as a jet mill, and the fine powder obtained by pulverization may be directly collected in the dispersion medium in the container to obtain a slurry. In this case, it is preferable that the inside of the container is also made of nitrogen gas and/or argon gas, and the obtained alloy powder is directly collected in the dispersion medium without being exposed to the air to obtain a slurry. The slurry obtained in this manner is molded with a known wet press device to obtain a molded body having a predetermined size and shape. This compact is sintered to obtain a sintered body.

Sintering Step Next, the compact is sintered to obtain a rare earth sintered magnet body (sintered body).
The sintering of the compact is preferably carried out at a pressure of 0.13 Pa (10 ^-3 Torr) or less, more preferably 0.07 Pa (5.0 x 10 ^-4 Torr) or less, at a temperature in the range of 1000°C to 1150°C. To prevent oxidation due to sintering, residual gas in the atmosphere may be replaced with an inert gas such as helium or argon. The obtained sintered body is preferably subjected to a heat treatment. The magnetic properties can be improved by the heat treatment. Known conditions for the heat treatment, such as the heat treatment temperature and heat treatment time, can be adopted. The rare earth sintered magnet body thus obtained is subjected to a grinding/polishing process, a surface treatment process, and a magnetization process as necessary, to complete the final rare earth sintered magnet.

The present disclosure will be described in more detail with reference to examples, but is not limited thereto.

Experimental Example 1
Alloys were produced by strip casting so as to have compositions (mass%) of R-T-B based sintered magnets shown in Samples No. 1 and 2 in Table 1 (excluding O, C, and N). In Table 1 and Table 6 described later, the units of the compositions of O, C, and N are ppm (mass ratio) rather than mass %. The obtained alloys were coarsely pulverized by hydrogen pulverization to obtain coarsely pulverized powders. The average particle size of the coarsely pulverized powders was measured. The average particle size was in the range of 200 μm to 400 μm. In this disclosure, the average particle size means the particle size (median diameter) at which the cumulative particle size distribution (volume basis) from the small particle size side is 50%. The average particle size (d50) was measured using a particle size distribution measuring device "HELOS &RODOS" manufactured by Sympatec Co., Ltd. Dispersion pressure: 4 bar
Measurement range: R2
Calculation mode: Measurement was performed under HRLD conditions.

Next, the additive alloy and main alloy were prepared by strip casting so as to have compositions approximately similar to those of No. a in Table 2 and No. A to C in Table 3. The obtained alloys were coarsely crushed by hydrogen crushing to obtain additive alloy powder and main alloy powder. The average particle size of the obtained alloy powders was in the range of 200 μm to 400 μm. The components of the obtained alloy powder were determined. Each component was measured by ICP emission spectrometry. The measurement results are shown in Tables 2 and 3. Next, the additive alloy powder and main alloy powder were mixed in a V-type mixer under the mixing conditions in Table 4. No. 3 in Table 4 is a mixture of additive alloy powder No. a and main alloy powder No. A, and is a mixed alloy powder containing 5 mass% of additive alloy powder (main alloy powder is 95 mass%). Nos. 4 to 7 are described in the same manner.

The coarsely pulverized powders of samples No. 1 and No. 2 and the mixed alloy powders of No. 3 to 7 were each put into the jet mill device 100 of FIG. 1 and pulverized to obtain fine powders. The pulverization conditions are shown in Table 5. No. 2 in Table 5 was humidified and pulverized by adding 1.4 g of moisture per kg of the coarsely pulverized powder to an inert gas, and the amount of the coarsely pulverized powder supplied to the jet mill device was 64.1 kg/h. No. 1 and No. 3 to 7 are similarly described (No. 1 and No. 3 were pulverized without humidification). In this example, nitrogen gas was used as the inert gas. The average particle size of the obtained fine powder is shown in Table 5. The fine powder was immersed in a mineral oil having a fractional point of 250° C. and a kinetic viscosity of 2 cSt at room temperature in a nitrogen atmosphere to prepare a slurry. The slurry concentration was 85% by mass. The obtained slurry was molded (wet molding) in a magnetic field to obtain a molded body. The molding device used was a so-called rectangular magnetic field molding device (horizontal magnetic field molding device) in which the magnetic field application direction and the pressure direction are perpendicular to each other. The obtained molded body was sintered in a vacuum at 1040°C (a temperature at which densification by sintering is sufficiently occurred) for 4 hours to obtain a sintered body. The density of the sintered body was 7.5 Mg/m3 ^or more. The sintered body was further subjected to a heat treatment in which it was held at 800°C for 2 hours and then cooled to room temperature, and then held at 500°C for 2 hours and then cooled to room temperature to obtain a sintered body (RTB-based sintered magnet). The components of the obtained sintered magnet were determined. The contents of Nd, Pr, Tb, B, Co, Al, Cu, Ga, and Zr were measured by ICP emission spectrometry. Furthermore, O (oxygen content) was measured using a gas fusion-infrared absorption method, N (nitrogen content) was measured using a gas fusion-thermal conduction method, and C (carbon content) was measured using a gas analyzer using a combustion-infrared absorption method. The results are shown in Tables 1 and 6. Note that No. It was confirmed that all of the R-T-B based sintered magnets Nos. 1 to 7 satisfied the condition of the present disclosure, [T]/55.85>14×[B]/10.8. The sintered magnets were machined to prepare samples with dimensions of 7 mm in length, 7 mm in width, and 7 mm in thickness, and the B _r and H _cJ of each sample were measured using a B-H tracer. The measurement results are shown in Table 7.

As shown in Table 1 (RTB-based sintered magnets made from single alloy powder) and Table 6 (RTB-based sintered magnets made from mixed alloy powder obtained by mixing additive alloy powder and main alloy powder), Nos. 1 to 7 have almost the same composition except for C, O, and N, and the average particle size (3.4 μm) of the fine powder obtained by jet mill pulverization is also the same. As shown in Table 3, all of the inventive examples (Nos. 4 to 6) have higher magnetic properties than Nos. 1 and 3, which were not humidified and pulverized. It has been believed that magnetic properties deteriorate when the oxygen content increases with almost the same composition and particle size. However, as shown in Nos. 3 and 4 to 6, if the oxygen content is within the range of the R-T-B-based sintered magnet of the present disclosure, the magnetic properties are improved. Furthermore, even if humidified and pulverized, if the oxygen content is outside the range of the R-T-B-based sintered magnet of the present disclosure, the magnetic properties deteriorate as shown in No. 7. Also, Comparing Nos. 1 and 2 (magnets made from single alloy powders without and with humidification) with Nos. 3 and 4 (magnets made from the mixed alloy powder of the present disclosure without and with humidification), Nos. 3 and 4 have better B _r and H _cJ . This shows that the magnetic properties can be further improved by subjecting the mixed alloy powder, which is a mixture of the additive alloy powder and the main alloy powder of the present disclosure, to humidification and grinding.

The manufacturing method of the R-T-B based sintered magnets disclosed herein can be used as permanent magnets for a wide variety of applications, including voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and other motors and home appliances.

100: Jet mill device, 200: Cyclone collection device, 300: Bag filter device

Claims (5)

A method for producing an R-T-B based sintered magnet (R is at least one rare earth element and necessarily contains Nd; T is Fe or Fe and Co; when Co is contained, 90 to 100 mass % of the total amount of T is Fe and 0 to 10 mass % is Co), comprising the steps of:
R1: (R1 is at least one rare earth element, and 50 mass% or more of the total of R1 is Pr) 33 to 69 mass%,
B: 0.2 to 0.8% by mass,
Cu: 0.8 to 3.0% by mass,
Ga: 1.8 to 10% by mass,
T: 15 to 60% by mass,
Inevitable impurities, and
At least one of Ag, Zn, In, Sn, Ti, Ge, Y, H, F, P, S, V, Ni, Mo, Hf, Ta, W, Nb, and Zr: 0 to 0.1 mass % or less;
preparing an additive alloy powder comprising :
R: 28.5 to 33.0% by mass,
B: 0.80 to 1.0% by mass,
Ga: 0.1 to 0.4% by mass,
T: 64 to 70% by mass,
Inevitable impurities, and
At least one of Ag, Zn, In, Sn, Ti, Ge, Y, H, F, P, S, V, Ni, Mo, Hf, Ta, W, Nb, and Zr: 0 to 0.2 mass% or less;
preparing a primary alloy powder consisting of :
preparing a mixed alloy powder containing 1 to 16 mass % of the additive alloy powder and 84 to 99 mass % of the main alloy powder;
supplying the mixed alloy powder to a jet mill having a milling chamber filled with an inert gas to mill the mixed alloy powder, thereby obtaining a fine powder having an average particle size of 2.0 μm or more and 4.5 μm or less;
and preparing a sintered body of the fine powder.
The Pr content of the additive alloy powder is greater than the Pr content of the main alloy powder,
The inert gas is humidified,
the R-T-B based sintered magnet has an oxygen content of 1000 ppm or more and 3000 ppm or less by mass ratio,
R: 28.5% by mass or more and 33.0% by mass or less,
B: 0.85% by mass or more and 0.91% by mass or less,
Cu: 0.05% by mass or more and 0.50% by mass or less,
Ga: 0.3% by mass or more and 0.7% by mass or less ,
T : 63% by mass or more and 70% by mass or less (T is Fe or Fe and Co )
Inevitable impurities, and
At least one of Ag, Zn, In, Sn, Ti, Ge, Y, H, F, P, S, V, Ni, Mo, Hf, Ta, W, Nb, and Zr: 0 to 0.2 mass% or less;
It consists of:
A method for producing an R-T-B based sintered magnet, which satisfies [T]/55.85>14×[B]/10.8, where [T] is the content (mass%) of T and [B] is the content (mass%) of B. The method for producing an R-T-B based sintered magnet according to claim 1, wherein the R content of the R-T-B based sintered magnet is 31 mass% or less. The method for producing an R-T-B based sintered magnet according to claim 1 or 2, wherein the inert gas is nitrogen gas. The step of preparing a sintered body of the fine powder includes:
preparing a powder compact from the fine powder by wet pressing in a magnetic field or by pressing in a magnetic field in an inert gas atmosphere;
sintering the powder compact;
A method for producing a sintered R-T-B based magnet according to any one of claims 1 to 3, comprising: The method for producing an R-T-B based sintered magnet according to any one of claims 1 to 4, wherein the average particle size of the fine powder in the step of obtaining the fine powder is 2.0 μm or more and 3.5 μm or less.

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