JP2024050383A - Manufacturing method of r-t-b-based sintered magnet, and r-t-b-based sintered magnet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing an R-T-B-based sintered magnet capable of achieving high H_cJ while reducing usage of RH or without using RH, and the R-T-B-based sintered magnet.

SOLUTION: A manufacturing method of an R-T-B-based sintered magnet includes: a sinter step of obtaining a sintered body by sintering a compact formed from an R-T-B-based alloy power; and a diffusion step of disposing a diffusion source containing R₁ on a surface of the sintered body and performing diffusion processing, thereby diffusing R₁ in the diffusion source from the surface of the sintered body into the inside. In the manufacturing method of the R-T-B-based sintered magnet, the sinter step includes: preliminary sintering of performing heating to a pressurization starting temperature Tp at 700°C or higher and 850°C or lower under an inert gas environment lower than 10 kPa; and main sintering of performing heating from the pressurization starting temperature Tp to a sintering temperature Ts1 under the inert gas environment of 10 kPa or higher and 100 kPa or lower and further keeping the sintering temperature Ts1 for a predetermined time.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2024,JPO&INPIT

Description

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

R-T-B sintered magnets (R is at least one rare earth element, T is at least one transition metal element and always contains Fe, and B is boron) are known as the most high-performance permanent magnets, and are used in a variety of motors, including motors for electric vehicles (EV, HV, PHV, etc.), industrial equipment, and home appliances.

R-T-B based sintered magnets are composed of a main phase made mainly of R ₂ T ₁₄ B compounds and a grain boundary phase 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.

At high temperatures, the coercive force H _cJ (hereinafter sometimes simply referred to as "H _cJ ") of R-T-B based sintered magnets decreases, causing irreversible thermal demagnetization. For this reason, R-T-B based sintered magnets used in electric vehicle motors in particular are required to have a high H _cJ at high temperatures, and sintered magnets that meet this requirement have a high H _cJ even at room temperature.

It is known that in R-T-B based sintered magnets, when a part of the light rare earth elements RL (e.g., Nd and Pr) contained in R in the R ₂ T ₁₄ B compound is replaced with heavy rare earth elements RH (e.g., Dy and Tb), the H _cJ is improved. As the amount of RH replaced increases, the H _cJ improves. However, RH, particularly Tb and Dy, has problems such as unstable supply and large price fluctuations due to the fact that the amount of resource available is small and the production areas are limited. For this reason, in recent years, it has been desired to produce R-T-B based sintered magnets with high H _cJ and high residual magnetic flux density B _r (hereinafter sometimes simply referred to as "B _r ") without using RH as much as possible.

Patent Documents 1 and 2 disclose methods for producing RTB based sintered magnets that have high H _cJ and high B _r even when the amount of RH such as Tb and Dy used is reduced.
In the method of producing a sintered magnet disclosed in Patent Document 1, an alloy containing R, Ga, and Cu is diffused into an R-T-B based alloy sintered compact at a temperature of 450° C. or higher and 600° C. or lower, forming a thick two-particle boundary between main phase grains, thereby obtaining an R-T-B based sintered magnet having a high _HcJ .

In the method of producing a sintered magnet disclosed in Patent Document 2, in an R-T-B based sintered magnet obtained by diffusing a diffusion source containing R ₁ (R ₁ is at least one type of rare earth element) into a sintered body, when the density of the sintered body is d ₁ and the density of the diffused R-T-B based sintered magnet is d ₂ , by making d ₂ 7.3 g/cm ³ or more and 7.8 g/cm ³ or less and d ₁ /d ₂ 0.975 or more and 0.995 or less, an R-T-B based sintered magnet having high B _r and high H _cJ can be obtained.

Furthermore, Patent Document 3 discloses a method for producing an RTB based sintered magnet that is less susceptible to cracking.
The method of manufacturing a sintered magnet disclosed in Patent Document 3 includes a crushing step in which an alloy block, which is the raw material for the sintered magnet, is crushed by a method including a hydrogen crushing method, and a sintering step in which the alloy powder obtained in the crushing step is sintered by heating it to a predetermined sintering temperature, and is characterized in that in the sintering step, the alloy powder is heated in an inert gas atmosphere at a pressure higher than atmospheric pressure (e.g., 120 kPa (about 1.2 atm)) up to a predetermined pressurized maintenance temperature that is equal to or higher than the hydrogen desorption temperature (e.g., about 70°C) and lower than the sintering temperature. By applying a pressure higher than atmospheric pressure, it is possible to suppress cracking of the sintered magnet caused by the sudden desorption of hydrogen gas.

Patent Document 4 discloses a method for producing a rare earth sintered magnet, which includes a milling step of milling a raw alloy to prepare an alloy fine powder, a compacting step of compressing the alloy fine powder under application of a magnetic field to obtain a compact, and a sintering step of heat-treating the compact to obtain a sintered body. A lubricant is added in the milling step.
The sintering step heats the compact by increasing the temperature to a predetermined temperature that is equal to or higher than the decomposition temperature of the lubricant and equal to or lower than the sintering temperature of the sintered body, and then maintaining the temperature for a predetermined time, while performing the temperature increase and maintaining the temperature in an inert gas atmosphere of 10 kPa to 100 kPa (atmospheric heat treatment step). Here, the decomposition temperature of the lubricant is, for example, 200 to 400° C.
After the atmospheric heat treatment step, the atmosphere is switched to a vacuum atmosphere, and the temperature is raised to the sintering temperature of the sintered body (vacuum heat treatment step).
The manufacturing method of Patent Document 4 can reduce the concentration of carbon (derived from the lubricant) that adversely affects magnetic properties.

International Publication No. 2016/133071 JP 2021-150547 A International Publication No. 2014/123079 JP 2021-182623 A

In recent years, there has been a demand for ever-improved magnetic properties (high B _r and high H _cJ ) without using RH as much as possible, particularly in electric vehicle motors, etc. In particular, there is a demand for R-T-B based sintered magnets that can achieve a sufficiently high H _cJ by further reducing the amount of RH used, which has a large effect on H _cJ , and preferably without using RH at all.

An object of embodiments of the present invention is to provide a method for producing a sintered R-T-B based magnet that can achieve a high _HcJ using a reduced amount of RH or no RH at all, and to provide a sintered R-T-B based magnet.

Aspect 1 of the present invention is
A method for producing an R-T-B based sintered magnet (R is at least one selected from the group consisting of Nd, Pr and Ce, T is at least one transition metal element and necessarily contains Fe, and B can be partially substituted with C), comprising the steps of:
a sintering step of sintering the compact formed from the R-T-B based alloy powder to obtain a sintered body;
a diffusion step of disposing a diffusion source containing R ₁ (R ₁ is one or more of rare earth elements) on the surface of the sintered body and performing a diffusion treatment, thereby diffusing the R ₁ in the diffusion source from the surface to the inside of the sintered body,
The sintering step comprises:
Pre-sintering by heating to a pressure start temperature Tp of 700° C. or more and 850° C. or less in an inert gas atmosphere of less than 10 kPa;
and (c) main sintering, in which the mixture is heated from the pressurization start temperature Tp to a sintering temperature Ts1 in the inert gas atmosphere of 10 kPa or more and 100 kPa or less, and then held at the sintering temperature Ts1 for a predetermined period of time.

Aspect 2 of the present invention is
In the method for producing a sintered RTB based magnet according to aspect 1, the pre-sintering is carried out in an inert gas atmosphere of 500 Pa or less.

Aspect 3 of the present invention is
After the sintering step and before the diffusion step,
A cooling step of cooling from Ts1 to a cooling temperature Tc;
The method for producing a sintered RTB system magnet according to aspect 1 or 2, further comprising a second sintering step of heating from the cooling temperature Tc to a second sintering temperature Ts2.

Aspect 4 of the present invention is
The diffusion source includes one or more selected from the group consisting of Pr and Nd as _R1 ,
A method for producing a sintered RTB system magnet according to any one of Aspects 1 to 3, wherein the total content of Pr and Nd in the diffusion source is from 30% by mass to 97% by mass.

Aspect 5 of the present invention is
The R ₁ in the diffusion source further includes one or more selected from the group consisting of Tb and Dy;
A method for producing a sintered RTB system magnet according to Aspect 4, wherein the total content of Tb and Dy in the diffusion source is from 1 mass % to 50 mass %.

Aspect 6 of the present invention is
The diffusion source further includes M (M is at least one selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Ag, In, and Sn),
In the method for producing a sintered RTB system magnet according to any one of Aspects 1 to 5, the diffusion step diffuses the _R1 and the M in the diffusion source from the surface of the sintered body to the inside.

Aspect 7 of the present invention is
The diffusion source contains one or more elements selected from the group consisting of Cu and Ga as M,
A method for producing a sintered RTB system magnet according to aspect 6, wherein the total content of Cu and Ga in the diffusion source is from 2 mass % to 39 mass %.

Aspect 8 of the present invention is
An R-T-B based sintered magnet (R is at least one selected from the group consisting of Nd, Pr and Ce, T is at least one transition metal element and necessarily contains Fe, and B can be partially replaced by C),
The squareness ratio Hk/ _HcJ is 85% or more,
The R concentration gradually decreases from the magnet surface to a depth of 200 μm.
In a cross-sectional view, the porosity calculated from a structure observation image at a position 500 μm deep from the magnet surface is 0.03 area % or more and 0.5 area % or less.

Aspect 9 of the present invention is
Aspect 9 is an RTB based sintered magnet according to aspect 8, having a density of 7.4×10 ³ kg/m ³ or more and 7.6×10 ³ kg/m ³ or less.

Aspect 10 of the present invention is
The R necessarily contains Pr,
When the entire R-T-B based sintered magnet is taken as 100 mass %,
The content of R is 27% by mass or more and 33% by mass or less,
The content of B is 0.85% by mass or more and 0.94% by mass or less,
The RTB based sintered magnet according to aspect 8 or 9, wherein the T content is 61.5 mass % or more.

According to the method for producing a sintered R-T-B based magnet according to an embodiment of the present invention, it is possible to provide a method for producing a sintered R-T-B based magnet that can achieve a high _HcJ with a reduced amount of RH used or without using RH, and to provide a sintered R-T-B based magnet.

FIG. 1 is a diagram showing a schematic example of a temperature profile in a sintering step, a cooling step, and a second sintering step of a molded body. 2 is an FE-SEM image (backscattered electron image) of Example No. F1.

It is known that when R ₁ (R ₁ is one or more rare earth elements) is diffused from a diffusion source into a low-density R-T-B based alloy sintered compact (hereinafter sometimes simply referred to as a "sintered compact"), the diffusion of R ₁ into the sintered compact is promoted, and an R-T-B based sintered magnet with high H _cJ is obtained (Patent Document 2).
As a result of intensive research conducted by the inventors based on this finding, they discovered for the first time that a low-density sintered body can be produced by sintering a compact in an inert gas atmosphere and controlling the pressure of the inert gas in accordance with the heating temperature, and that by subjecting this sintered body to a diffusion treatment, an R-T-B based sintered magnet (hereinafter sometimes simply referred to as a "sintered magnet") with improved _HcJ can be obtained, as compared to a case in which a conventional low-density sintered body is subjected to the diffusion treatment, and thus completed the present invention.

The method for producing an R-T-B based sintered magnet according to an embodiment of the present invention (hereinafter sometimes simply referred to as "the method for producing a sintered magnet") is described in detail below.

(Embodiment 1)
In the method for producing a sintered magnet according to the first embodiment, R in the R-T-B based sintered magnet (sintered magnet) is at least one element selected from the group consisting of Nd, Pr and Ce, T is at least one element among transition metal elements and necessarily contains Fe, and B (boron) can be partially substituted with C.

When the entire R-T-B based sintered magnet is taken as 100 mass%, the R content is preferably 27 mass% or more and 33 mass% or less. When the entire R-T-B based sintered magnet is taken as 100 mass%, the B content is preferably 0.8 mass% or more and 1.2 mass% or less, and more preferably 0.85 mass% or more and 0.94 mass% or less. When the entire R-T-B based sintered magnet is taken as 100 mass%, the T content is preferably 61.5 mass% or more.
By setting the contents of R, B, and T within these ranges, a higher _HcJ can be obtained.

When the entire T is taken as 100 mass %, 50 mass % or less of T may be Co. Co is effective in improving temperature characteristics and corrosion resistance.
Furthermore, an M1 element may be added to improve _HcJ . For example, the M1 element is one or more elements 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. When the entire R-T-B based sintered magnet is taken as 100 mass%, the total amount of the M1 element added is preferably 3.0 mass% or less.
Other elements, such as unavoidable impurities like O and N, may also be tolerated.
The method for producing a sintered magnet is as follows:
a sintering step of sintering the compact formed from the R-T-B based alloy powder to obtain a sintered body;
and a diffusion step of arranging a diffusion source containing R ₁ (R ₁ is one or more of rare earth elements) on the surface of the sintered body and performing a diffusion treatment, thereby diffusing the R ₁ in the diffusion source from the surface to the inside of the sintered body.

The manufacturing method of sintered magnets further includes the following steps prior to the sintering step:
A step of preparing an R-T-B based alloy (alloy preparation step);
A step of obtaining a powder (fine powder) of an R-T-B alloy (a pulverizing step); and A step of obtaining a compact from the powder (fine powder) of the R-T-B alloy (a molding step).
may include any one or more of the following:

In the method for producing a sintered magnet, two-stage sintering may be performed.
For example, after the sintering step and before the diffusion step,
a cooling step of cooling to a cooling temperature Tc;
The method may further include a second sintering step of heating from the cooling temperature Tc to a second sintering temperature Ts2.

The method for producing a sintered magnet may further include a step of preparing a diffusion source (diffusion source preparation step) prior to the diffusion step.

Each process is described in detail below.

<Step A: Alloy Preparation Step>
In step A, an R-T-B alloy is prepared. Here, R in the R-T-B alloy is at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal element, and necessarily contains Fe, and B may be partially substituted with C. When the entire R-T-B alloy is taken as 100 mass%, R is preferably contained in a total of 27 mass% to 33 mass%, and more preferably contained in a total of 27 mass% to 31 mass%. This is because if R is less than 27 mass%, sintering may not be possible, and if it exceeds 33 mass%, B _r may be significantly reduced. In addition, boron (B) is preferably contained in a total of 0.8 mass% to 1.2 mass%. If it is less than 0.8 mass%, sufficient H _cJ may not be obtained, and if it exceeds 1.2 mass%, B _r may be reduced. When the entire T is taken as 100 mass%, T may be Co in an amount of 50 mass% or less. Co is effective in improving temperature characteristics and corrosion resistance. The content of T in the entire R-T-B alloy may be the remainder (excluding inevitable impurities) of R and boron (B) or R, B, and M1 described later. The M1 element can be added to improve _HcJ . The M1 element is, for example, 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. When the entire R-T-B alloy is taken as 100 mass%, the total amount of the added M1 element is preferably 3.0 mass% or less. In addition, other elements such as inevitable impurities such as O and N can also be tolerated.

R-T-B alloys can be produced by known methods, such as producing an ingot by melting the raw materials and pouring them into a mold, producing flakes by strip casting, producing ribbons by ultra-quenching, and producing powder by atomization. The produced alloy may be heat-treated to increase the crystal grain size and reduce heterogeneous phases. The produced or heat-treated alloy may also be subjected to hydrogen embrittlement treatment.

<Step B: Grinding step>
The alloy obtained in step A is pulverized to obtain a powder (fine powder) of an RTB type alloy.
The pulverization step may include preliminary pulverization (coarse pulverization) and fine pulverization. The preliminary pulverization may be performed by a known method such as a jaw crusher, a hammer mill, or a roller mill. The fine pulverization may be performed by a known method such as a jet mill, a stamp mill, or a ball mill.

A grinding aid may be added during the fine grinding process to improve the efficiency of the fine grinding process. Known grinding aids such as zinc stearate may be used as the grinding aid. In order to suppress oxidation of the powder and reduce the risk of fire or explosion, the grinding process is carried out in a gas such as nitrogen (N ₂ ), argon (Ar), or helium (He). A small amount of air, water, or oxygen may be mixed into the inert gas to improve the handling of the fine powder after grinding.

The fine powder obtained in the milling process may be either fine powder obtained from one type of alloy (single alloy powder) or fine powder obtained by mixing fine powders obtained by milling two or more types of alloys (mixed alloy powder). The latter method is known as the two-alloy method.

The particle size of the fine powder is preferably 2 μm or more and 12 μm or less in terms of D ₅₀ (volume-based median diameter of particles when the cumulative frequency is 50%) obtained by the laser diffraction method using the airflow dispersion method. If _{D 50} is 1 μm or more, the risk of fire is low and the mold is unlikely to be damaged during molding. Also, if D ₅₀ is 20 μm or less, the decrease in _HcJ can be suppressed.
The _D50 of the alloy fine powder is more preferably 2 μm or more and 6 μm or less. When _{the D50} is 2 μm or more and 6 μm or less, it becomes easy to adjust the sintering temperature and sintering time when producing a low-density sintered body, and the uniformity of the structure existing before the sintering process can be obtained, so that a sintered body with a higher _HcJ can be obtained while suppressing a decrease in B _r .

Note that if a method that produces fine powder, such as atomization, is used in the alloy preparation process, the crushing process may be omitted.

<Step C: Molding step>
The obtained powder (fine powder) of the R-T-B alloy is molded to obtain a molded body. In order to orient the fine powder during molding, it is preferable to apply a magnetic field during molding. For molding, known methods can be used, including a dry molding method in which dried fine powder is inserted into a mold cavity and molded, and a wet molding method in which a slurry (alloy powder dispersed in a dispersion medium) is injected into a mold cavity and molded while discharging the dispersion medium of the slurry.
During molding, a binder may be added to the fine powder, for example, a known lubricant such as zinc stearate.

<Step D1: Sintering step (first sintering step)>
In the sintering step (first sintering step), the obtained compact is sintered to obtain a sintered body.
Solid phase sintering or liquid phase sintering is carried out in an inert gas atmosphere, such as Ar or He, with Ar being particularly preferred.

The sintering step (first sintering step) will be described with reference to FIG.
The sintering process includes preliminary sintering and main sintering. In this specification, the period from the start of heating the sintered body until a predetermined temperature (pressurization start temperature Tp) is reached (the period from t0 to t1 in FIG. 1) is referred to as "preliminary sintering", and the period from the time the pressurization start temperature Tp is reached until the holding at the sintering temperature Ts1 is completed (the period from t1 to t3 in FIG. 1) is referred to as "main sintering".

(Pre-sintering)
In the pre-sintering, hydrogen that may be contained in the fine powder and binder that may be contained in the compact are removed (dehydrogenation, debindering). Hydrogen may be introduced when hydrogen embrittlement treatment is performed in the alloy preparation process. The binder is added optionally in the compacting process. In order to improve the efficiency of dehydrogenation and debindering, the pre-sintering is performed in an inert gas atmosphere of less than 10 kPa. In this way, the pre-sintering is performed by heating to a pressure start temperature Tp of 700°C or more and 850°C or less in an inert gas atmosphere of less than 10 kPa.
The pre-sintering is preferably carried out in an inert gas atmosphere of 500 Pa or less, and more preferably in an inert gas atmosphere of 100 Pa or less, so that the efficiency of dehydrogenation and binder removal can be further improved.

The average heating rate for pre-sintering can be, for example, 0.1°C/min to 2.0°C/min. The "average heating rate for pre-sintering" is calculated by dividing the temperature difference (Tp-T0) between the heating start temperature T0 and the pressurization start temperature Tp by the heating time from the heating start temperature T0 to the pressurization start temperature Tp (t1-t0 in Figure 1). The heating rate for pre-sintering does not have to be constant, and may further include a process of holding the temperature at a constant temperature.

(Main sintering)
In the main sintering, the material is heated from the pressurization start temperature Tp to a sintering temperature Ts1, and then held at the sintering temperature Ts1 for a predetermined time (the period from t2 to t3 in FIG. 1). The main sintering is performed in the inert gas atmosphere of 10 kPa to 100 kPa. Preferably, the main sintering is performed in the inert gas atmosphere of 20 kPa to 80 kPa. This makes it possible to more reliably improve _HcJ .

In this sintering, by setting the pressure of the inert gas to 10 kPa or more and 100 kPa or less, the compact can be sintered in a state where the inert gas is contained inside. As a result, pores are formed inside the obtained sintered body, and the density of the sintered body decreases. It is known that a low-density sintered body can improve _HcJ by facilitating the diffusion of _R1 from the diffusion source (Patent Document 2). However, surprisingly, it was confirmed that the low-density sintered body produced by the sintering process of this embodiment has an easier diffusion of _R1 than the low-density sintered body obtained in Patent Document 2, and has a higher effect of improving _HcJ .

The reason why _R1 is easily diffused in the low-density sintered body obtained by the sintering process of this embodiment is not clear, but it is presumed to be due to the following mechanism.
It is believed that vacancies remain in the crystal structure in low-density sintered bodies. When _R1 diffuses to fill these vacancies during diffusion, it becomes easier for _R1 to diffuse, and as a result, it is believed that _HcJ can be improved compared to the case of diffusion in a normal sintered body. However, when vacancies are generated in the crystal structure by lowering the sintering temperature, grain boundaries may not be formed uniformly because the sintering temperature is not appropriate. As a result, grain boundaries are not formed uniformly in the sintered magnet after diffusion, and as a result, it may be difficult to improve _HcJ .
In contrast, in the present embodiment, sintering is performed by setting the pressure of the inert gas within a specific pressure range from a specific temperature, thereby generating vacancies in the crystal structure while maintaining an appropriate sintering temperature. This makes it possible to prevent uneven grain boundaries caused by a drop in sintering temperature, while facilitating the diffusion of _R1 through the vacancies, which is believed to result in a high _HcJ .

In this sintering, the average heating rate from the pressurization start temperature Tp to the sintering temperature Ts1 can be, for example, 0.5°C/min to 10°C/min. The "average heating rate from the pressurization start temperature Tp to the sintering temperature Ts1" is calculated by dividing the temperature difference between the pressurization start temperature Tp and the sintering temperature Ts1 (Ts1-Tp in Figure 1) by the heating time (t2-t1) from the pressurization start temperature Tp to the sintering temperature Ts1. The heating rate from the pressurization start temperature Tp to the sintering temperature Ts1 does not have to be constant, and may further include a period during which the temperature is held at a constant temperature.

The sintering temperature Ts1 is preferably set in the range of 920° C. to 1100° C., and more preferably in the range of 920° C. to 1080° C. The sintering temperature Ts1 may be a substantially constant temperature (within a range of ±5° C.) during the holding time (the period from t2 to t3 in FIG. 1), or may vary in the range of 900° C. to 1100° C., or in the range of 920° C. to 1080° C.
The retention time (t2 to t3) is preferably set within the range of 30 minutes to 2 hours.

<Step D2: Cooling step and second sintering step>
It is possible to obtain a sintered body from the molded body by performing only the sintering step (first sintering step) of step D1, but it is also possible to obtain a sintered body by performing a second sintering step in addition to the sintering step (first sintering step) (so-called two-stage sintering).
In the two-stage sintering, after the sintering step D1 (first sintering step) and before the diffusion step F described later,
a cooling step of cooling to a cooling temperature Tc;
By further including a second sintering step of heating from the cooling temperature Tc to a second sintering temperature Ts2, two-stage sintering can be performed.
By subjecting the compact to two-stage sintering, a more uniform two-particle grain boundary can be formed, and the _HcJ of the final sintered magnet can be further improved.

The cooling step and the second sintering step are preferably carried out in an inert gas atmosphere of 10 kPa or more and 100 kPa or less.

(Cooling process)
The cooling step refers to a step carried out during the period from t3 to t5 in FIG.
After the holding time at the sintering temperature Ts1 is completed, the material is cooled from the sintering temperature Ts1 to a cooling temperature Tc (period t3 to t4 in FIG. 1), and is held at the cooling temperature Tc for a predetermined time (period t4 to t5).

The cooling temperature Tc is preferably set to 900°C or less, and more preferably within the range of 700°C to 900°C. The holding time at the cooling temperature Tc is optional, but can be, for example, 1 minute to 200 minutes.

The average cooling rate from the sintering temperature Ts1 to the cooling temperature Tc can be, for example, 1°C/min or more and 200°C/min or less. The "average cooling rate from the sintering temperature Ts1 to the cooling temperature Tc" is calculated by dividing the temperature difference between the sintering temperature Ts1 and the cooling temperature Tc (Ts1-Tc) by the cooling time from the sintering temperature Ts1 to the cooling temperature Tc (t4-t3 in FIG. 1). The cooling rate from the sintering temperature Ts1 to the cooling temperature Tc does not have to be constant, and may further include a period during which the temperature is held at a constant temperature.

(Second sintering step)
The second sintering step refers to a step carried out during the period from t5 to t7 in FIG.
After the holding time at the cooling temperature Tc is completed, the material is heated from the cooling temperature Tc to the second sintering temperature Ts2 (period t5 to t6 in FIG. 1), and is held at the second sintering temperature Ts2 for a predetermined time (period t6 to t7).
Then, cool to room temperature.

The second sintering temperature Ts2 can be set in the range of 900° C. to 1060° C. (preferably over 900° C.). The second sintering temperature Ts2 is preferably set to a temperature equal to or lower than the sintering temperature Ts1. The second sintering temperature Ts2 may be a substantially constant temperature (within a range of ±5° C.) during the holding time (the period from t6 to t7 in FIG. 1) or may vary within a range of 10° C. to 20° C.
The holding time (t6 to t7) at the second sintering temperature Ts2 is preferably set within a range of 1 hour to 8 hours. In addition, the second sintering temperature Ts2 is preferably a temperature lower than the sintering temperature Ts1.

The average heating rate from the cooling temperature Tc to the second sintering temperature Ts2 can be, for example, 0.5°C/min to 10°C/min. The "average heating rate from the cooling temperature Tc to the second sintering temperature Ts2" is calculated by dividing the temperature difference (Ts2-Tc) between the cooling temperature Tc and the second sintering temperature Ts2 by the heating time (t6-t5) from the cooling temperature Tc to the second sintering temperature Ts2. The heating rate from the cooling temperature Tc to the second sintering temperature Ts2 does not have to be constant, and may further include a period during which the temperature is held at a constant temperature.

The density _d1 of the sintered body produced in the sintering step (and optionally the second sintering step) is preferably, for example, 7.4×10 ³ kg/m ³ or more and 7.6×10 ³ kg/m ³ or less (=7.4 g/cm ³ or more and 7.6 g/cm ³ or less).

<Step E: Diffusion Source Preparation Step>
In the diffusion source preparation step, a diffusion source having a predetermined component composition is prepared. The diffusion source is used to diffuse elements contained in the diffusion source into the sintered body in the diffusion step described below.
The diffusion source includes _R1 ( _R1 is one or more of rare earth elements). _R1 diffuses into the sintered compact and replaces the rare earth elements in the main phase to improve the anisotropic magnetic field, and penetrates into the grain boundary between two grains to magnetically separate the main phase grains.

The diffusion source may contain, as _R1 , one or more elements selected from the group consisting of Pr and Nd, which are light rare earth elements RL.
Among rare earth elements, Pr and Nd are elements that diffuse relatively easily to deep parts, and are also elements necessary for thickening the grain boundary between two grains. In many cases, the rare earth elements constituting the main phase of the sintered body are Nd and Pr, so there is little concern about a decrease in the anisotropic magnetic field due to the substitution of Nd and Pr for the rare earth elements of the main phase.

By including an appropriate amount of light rare earth element RL, a sufficiently high _HcJ can be achieved even when the content of heavy rare earth element RH (eg, Tb and Dy) is low or no heavy rare earth element RH is included.
In this embodiment, by setting the atmosphere during sintering in step D1 (sintering step) to be the inert gas atmosphere of 10 kPa or more and 100 kPa or less, it is possible to improve the _HcJ of the sintered magnet that contains a light rare earth element RL and no heavy rare earth element RH.

The total content of Pr and Nd in the diffusion source is preferably 30% by mass or more and 97% by mass or less.
When the total content of Pr and Nd is 30 mass% or more, the effect of improving _HcJ is particularly high. When the total content of Pr and Nd is 97 mass% or less, the liquid phase generation temperature does not become too high, the liquid phase is easily formed, and the wettability is good.

The diffusion source _R1 includes one or more elements selected from the group consisting of Pr and Nd, and may also include other rare earth elements. The other rare earth elements may further include, for example, one or more elements selected from the group consisting of Tb and Dy, which are heavy rare earth elements RH. By diffusing the heavy rare earth elements RH, the anisotropic magnetic field of the sintered magnet can be significantly improved. However, it is preferable that the content of the heavy rare earth elements RH is as small as possible.
The total content of Tb and Dy in the diffusion source is preferably 1% by mass or more and 50% by mass or less. When the total content of Tb and Dy is 1% by mass or more, a sufficient effect of improving _HcJ can be obtained. When the total content of Tb and Dy is 50% by mass or less, the amount of RH used can be reduced.

The diffusion source may further contain M (M is at _least one selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Ag, In and Sn) together with R 1. When an alloy of R ₁ and M is used as the diffusion source, the temperature at which the liquid phase is generated is often lowered. This allows the processing temperature during diffusion to be lowered, and the wettability of the liquid phase is improved, making it easier to diffuse the diffusion source into the sintered body. Note that R ₁ and M in the diffusion source may exist in the form of a fluoride, hydride, or oxide.

The diffusion source preferably contains one or more selected from the group consisting of Cu and Ga as M, and the total content of Cu and Ga in the diffusion source is 2% by mass or more and 39% by mass or less. An alloy of Cu, Ga and a rare earth element has a relatively low melting point and good wettability of the liquid phase. It is also known that Cu and Ga react with a rare earth element and an iron group transition metal element to form a compound of La ₆ Co ₁₁ Ga ₃ type crystal structure. This compound has a relatively low magnetization, and Fe present in the grain boundary of two grains is used when this compound is formed, so that the main phase of the sintered body can be magnetically separated by weakening the magnetism of the grain boundary of two grains, and it also plays a role in increasing the coercive force. When the total content of Cu and Ga in the diffusion source is 2% by mass or more, the liquid phase generation temperature does not become too high, the liquid phase is easily formed, and the wettability is good. When the total content of Cu and Ga in the diffusion source is 39% by mass or less, the effect of improving _HcJ by the rare earth element and other elements can be sufficiently obtained.

The diffusion source can be produced by known methods such as producing an ingot by melting the raw material and pouring it into a mold, producing flakes by strip casting, producing ribbons by ultra-quenching, producing powder by atomization, and producing a solution containing a diffusion element. The produced diffusion source may be hydrogen-treated for the purpose of embrittlement. The produced or hydrogen-treated diffusion source may be pulverized to make it easier to handle. The pulverization method may be a known method such as a jaw crusher, hammer mill, roller mill, jet mill, stamp mill, or ball mill.

<Step F: Diffusion Step>
In the diffusion step, a diffusion source containing _R1 ( _R1 is one or more rare earth elements) is placed on the surface of the sintered body and diffusion treatment is performed, whereby the _R1 in the diffusion source can be diffused from the surface of the sintered body to the inside.
In the case where the diffusion source further contains M in addition to _R1 , the _R1 and M in the diffusion source are diffused from the surface to the inside of the sintered body by the diffusion step.
Incidentally, _R1 and M in the diffusion source may be diffused in the state of a fluoride, a hydride, or an oxide.

In the diffusion step, a diffusion source is placed on the surface of the sintered body, and the diffusion source is heated in contact with the surface of the sintered body (diffusion treatment). During the diffusion step, the diffusion source may be in continuous contact with the sintered body, or may be in intermittent contact as in barrel treatment.
The diffusion source may be disposed on the surface of the sintered body by a thin film forming method such as sputtering or vapor deposition.
The diffusion treatment can be carried out at a temperature equal to or lower than the sintering temperature, for example, at 850° C. to 950° C. for 1 hour to 50 hours.

When the _R1 is diffused from the diffusion source into the sintered body, the concentration of _R1 has a profile that is high on the surface of the sintered body and decreases toward the inside of the sintered body.

The amount of _R1 diffused from the diffusion source to the sintered body, _ΔR1 (mass%), is preferably 0.98 mass% or more when the mass of the R-T-B based sintered magnet is taken as 100 mass%, thereby making it possible to further improve the _HcJ of the diffused sintered magnet.
The diffusion amount _ΔR1 is determined by subtracting the total content (mass %) of R1 contained in the sintered magnet before diffusion from the total content (mass %) of R1 contained in the sintered magnet after diffusion.

＜Other＞
The sintered magnet after the diffusion treatment may be subjected to a heat treatment for the purpose of improving _HcJ , etc. The heat treatment is preferably performed in a vacuum atmosphere or in an inert gas such as argon or helium to prevent oxidation due to the atmosphere. The obtained sintered magnet may be subjected to known mechanical processing such as cutting or machining, or known surface treatment such as plating to impart corrosion resistance.

The sintered magnet obtained by the manufacturing method of embodiment 1 has a high _HcJ and is highly effective in improving HcJ compared to a sintered magnet manufactured by _{a conventional method using the same raw materials and diffusion source. In particular, when R1 does not contain a heavy rare earth element, the effect of improving HcJ is remarkable} .

(Embodiment 2)
The second embodiment relates to a sintered R-T-B magnet produced by the production method according to the first embodiment.
The R-T-B based sintered magnet obtained by the manufacturing method of embodiment 1 has, for example, a squareness ratio Hk/ _HcJ of 85% or more, a gradual decrease in R concentration in the range from the magnet surface to a depth of 200 μm, and a pore area ratio (porosity) calculated from a structure observation image at a position 500 μm deep from the magnet surface in a cross-sectional view of 0.03 area % to 0.5 area %. This configuration allows the magnet to have a higher _HcJ than sintered magnets manufactured by conventional methods using the same raw materials and diffusion sources.

The R-T-B based sintered magnet according to the second embodiment has a squareness ratio Hk/H _cJ of 85% or more. By making the squareness ratio Hk/H _cJ 85% or more, it is possible to have a high H _cJ even at high temperatures. In the field of R-T-B based sintered magnets, the parameter Hk measured to determine the squareness ratio Hk/H _cJ is generally taken as the reading on the H axis at the position where J is 0.9×J _r (J _r is the residual magnetization, J _r =B _r ) in the second quadrant of the J (magnetization strength)-H (magnetic field strength) curve. The squareness ratio Hk/H _cJ is defined as the value obtained by dividing this Hk by the H _cJ of the demagnetization curve (Hk/H _cJ =Hk(kA/m)/H _cJ (kA/m)×100(%)).

In the R-T-B based sintered magnet according to embodiment 2, the R concentration gradually decreases from the magnet surface to a depth of 200 μm. A sintered magnet having such a concentration distribution can be obtained by carrying out the above-mentioned diffusion step (disposing a diffusion source containing _R1 on the surface of the sintered body and carrying out a diffusion process).

The R concentration in the range from the surface to a depth of 200 μm can be confirmed by line analysis (line analysis) of the cross section of the sintered magnet from the magnet surface toward the center of the magnet to a depth of 200 μm using energy dispersive X-ray spectroscopy (EDX) or electron probe microanalyzer (EPMA). It is preferable to measure in a direction perpendicular to the surface and at a cross section at least 200 μm away from the edge of the surface. When performing line analysis on a cross section, the measurement is performed in a direction perpendicular to the outer periphery of the measurement cross section (corresponding to the surface of the magnet).

Compared to conventional R-T-B based sintered magnets, the R-T-B based sintered magnet according to embodiment 2 has more voids at a depth of 500 μm from the magnet surface. This is believed to be due to the fact that, in the production method according to embodiment 1, the sintering is carried out in the inert gas atmosphere of 10 kPa or more and 100 kPa or less.
In the manufacturing method according to the first embodiment, a low-density sintered body containing voids is obtained by carrying out the sintering under the above conditions. At this time, it is assumed that the voids are filled with an inert gas. In the subsequent diffusion step, _R1 diffuses from the diffusion source so as to fill the voids, but the diffusion of _R1 into the voids is somewhat hindered by the inert gas filled in the voids.

On the other hand, in the conventional method for producing a low-density sintered body, the sintering is performed in a vacuum, so it is assumed that the pores of the low-density sintered body are in a vacuum state, which promotes the diffusion of _R1 into the pores in the subsequent diffusion step.

The present inventors have found for the first time that the effect of differences in the atmosphere during sintering on the _R1 diffusion during diffusion can be confirmed at a depth of approximately 500 μm from the surface in the final sintered magnet product.
In the sintered magnet according to embodiment 1, voids remain at a depth of approximately 500 μm from the surface, whereas in the sintered magnet produced by the conventional manufacturing method, voids are barely detectable at a depth of approximately 500 μm from the surface.

As a result of careful investigation by the inventors, it was found that the sintered magnet obtained by the manufacturing method according to embodiment 1 has a porosity of 0.03% to 0.5% by area, calculated from a structural observation image at a depth of 500 μm from the magnet surface, in a cross-sectional view. In other words, it can be said that a sintered magnet with a porosity in the above range, calculated from a structural observation image at a depth of 500 μm from the magnet surface, is manufactured by the manufacturing method according to embodiment 1.

A sintered magnet having a porosity of 0.03 area % or more and 0.5 area % or less, calculated from a structural observation image at a depth of 500 μm from the magnet surface, has a higher H _cJ than a sintered magnet produced by a conventional method using the same raw materials and diffusion source (with a porosity of about 0 area % to 0.01 area %). If the porosity is less than 0.03 area %, the effect of the present disclosure may not be obtained, and if it exceeds 0.5 area %, B _r may decrease. A porosity of 0.05 area % or more and 0.4 area % or less is more preferable. A sintered magnet having a higher B _r and a higher H _cJ can be obtained.

In addition, since a sufficient amount of _R1 is diffused from the diffusion source disposed on the magnet surface into the shallow portion from the magnet surface, there is a possibility that almost all voids will be eliminated even in a sintered magnet manufactured by the manufacturing method according to embodiment 1.
On the other hand, since the amount of _R1 reaching the deep portion from the magnet surface from the diffusion source arranged on the magnet surface is insufficient, it is presumed that pores remain in both the sintered magnet produced by the production method according to embodiment 1 and the sintered magnet having pores produced by the conventional production method.

The method for measuring the porosity will be described.
The porosity is determined by observing the cross section of the sintered magnet with an SEM, and dividing the area of the pores determined by image analysis of each image by the area of the field of view. The field of view is a rectangle with one side of 150 μm to 300 μm. Preferably, SEM observation is performed in a rectangular field of view of 240 μm × 180 μm.
A cross-sectional SEM image of the sintered magnet (No. F1) produced in the example is shown in Figure 2. As shown in Figure 2, the light grey parts are the main phase, the white parts are grain boundaries, and the black parts are voids. In the image analysis, the area of the black parts is measured and divided by the field of view area to determine the porosity (area %). When determining the porosity, only voids with a maximum dimension of 0.2 μm or more were considered.
Each area may be calculated as the number of pixels.

The observation position is determined so that a position 500 μm deep from the outer periphery of the cross section (corresponding to the surface of the sintered magnet) approximately coincides with the intersection of the diagonals of the rectangular field of view.
When the cross section of the sintered magnet is substantially rectangular, it is preferable to set the field of view so that one side of the field of view is parallel to one side of the cross-sectional outline.When the field of view is rectangular, it is preferable to set the field of view so that the long side of the field of view is parallel to one side of the cross-sectional outline.
When the cross section of the sintered magnet is not substantially rectangular (e.g., circular), it is preferable to set the field of view so that one side of the field of view is parallel to the radius of the cross-sectional outline.When the field of view is rectangular, it is preferable to set the field of view so that the short side of the field of view is parallel to the radius of the cross-sectional outline.

It is preferable to perform SEM observations on a cross section perpendicular to the surface and at least 500 μm away from the edge of the surface. The depth from the magnet surface is measured from the outer periphery (surface) in a direction perpendicular to the outer periphery of the measured cross section (corresponding to the magnet's surface).

The embodiments of the present disclosure will be described in more detail with reference to examples, but are not limited to these examples.

(Experimental Example 1)
Each element was weighed and cast by strip casting so that the sintered body had the composition shown in Table 1 No. 1, to obtain a flake-shaped alloy. The obtained flake-shaped alloy was hydrogen embrittled in a pressurized hydrogen atmosphere, and then subjected to a dehydrogenation treatment in which it was heated and cooled in a vacuum to obtain a coarsely pulverized powder. The obtained coarsely pulverized powder was then pulverized using an airflow pulverizer (jet mill device) to obtain an alloy powder with a _D50 of 3.6 μm.

The alloy powder was mixed with 0.4 parts by mass of lubricant per 100 parts by mass of the finely pulverized powder, and then molded in a magnetic field to obtain a green body. The molding device used was a so-called perpendicular 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 under the conditions shown in Table 2. The sintering conditions listed in Table 2 will be explained in detail using No. C as an example. For No. C, the molded body prepared to have the composition of No. 1 in Table 1 was pre-sintered by heating it to the pressurization start temperature Tp: 850°C in an inert gas atmosphere of 30 Pa, and then heated from the pressurization start temperature Tp: 850°C to the sintering temperature Ts1: 1060°C in an inert gas atmosphere of 20 kPa, and then sintered at the sintering temperature Ts1: 1060°C for 240 minutes (Min). The sintering conditions for the other Nos. A, B, D, E, and F are also listed in Table 2.

Nos. A, B, C, and E were not sintered in two stages, and all of the sintered bodies were cooled to room temperature (around 25°C) after the main sintering. Nos. D and F were sintered in two stages, and all of them were sintered in two stages, and after the main sintering, a cooling step was performed to cool the body to a cooling temperature Tc of 700°C, and a second sintering step was performed to heat the body from the cooling temperature Tc of 700°C to a second sintering temperature Ts2 of 1050°C. The holding time at the cooling temperature Tc of 700°C in Nos. D and F was 60 minutes, and the holding time at the second sintering temperature was 240 minutes. In all of Nos. A to F, the heating start temperature T0 for pre-sintering was 25°C.
The components of the obtained sintered bodies Nos. A to F were measured. All of the compositions were almost the same. The component results are shown in Table 1. The components in Table 1 were measured using high-frequency inductively coupled plasma atomic emission spectrometry. The density of each sintered body was also measured. The measurement results are shown in Table 2. The density of the sintered body was determined by the Archimedes method using ion-exchanged water.

Next, a diffusion source was prepared. Each element was weighed so as to obtain the composition shown in No. a and b in Table 3, and the raw materials were melted and a ribbon or flake-shaped alloy (diffusion source) was obtained by a single-roll ultra-quenching method. The composition of the obtained diffusion source is shown in Table 3. Each component in Table 3 was measured using high-frequency inductively coupled plasma atomic emission spectrometry.

The sintered bodies of No. A to F in Table 2 were cut and machined to form cubes with sides of 7.2 mm. PVA was applied as an adhesive to the entire surface of the processed R-T-B magnet material by dipping. A diffusion source was attached to the entire surface of the sintered body coated with the adhesive, and the sintered body was subjected to a diffusion treatment in which the sintered body was heated at 920°C for 10 hours in a reduced pressure argon atmosphere controlled to 100 Pa using a vacuum heat treatment furnace, and then cooled. The sintered body after the diffusion treatment was subjected to a heat treatment in which the sintered body was heated to 500°C in a reduced pressure argon atmosphere controlled to 100 Pa using a vacuum heat treatment furnace. The entire surface of each sample after the heat treatment was machined using a surface grinder to obtain a cubic sample (RTB sintered magnet) of 7.0 mm x 7.0 mm x 7.0 mm. The residual magnetic flux density B _r and coercive force H _cJ of the obtained samples were measured using a B-H tracer. The results are shown in Table 4.

As shown in Table 4, the first group including Nos. A1 to F1 used No. a in Table 3 as the diffusion source, and the second group including Nos. A2 to F2 used No. b in Table 3 as the diffusion source. When the measurement results are compared between samples using the same diffusion source (i.e., between Nos. A1 to F1 in the first group, or between Nos. A2 to F2 in the second group), the samples of the invention in both groups have improved _HcJ compared to the samples of the comparative example.

The porosity and density of the R-T-B based sintered magnets No. A1, D1, and F1 in Table 4 were determined. In addition, as a comparative example, the R-T-B based sintered magnet No. G1 was produced in the same manner as the R-T-B based sintered magnet No. D1, except that the pressure in the main sintering process was changed from 20 kPa, which is within the range of the present invention, to 300 Pa, which is outside the range of the present invention. The porosity and density of the R-T-B based sintered magnet No. G1 were determined.

The porosity was determined as follows.
The 7.0mm x 7.0mm x 7.0mm cubic specimen used for measuring the residual magnetic flux density B _r and the coercive force H _cJ was cut into two equal parts at a cross section perpendicular to the plane (M-plane) perpendicular to the orientation direction. The cut surface was mechanically polished, and then ion beam milled using a Hitachi Hi-Tech ArBlade 5000. The crystal structure was observed from a direction perpendicular to the orientation direction using a field emission scanning electron microscope (FE-SEM). The FE-SEM used was a JSM7800F manufactured by JEOL Ltd., and the observation conditions were an acceleration voltage of 5 kV, a working distance of 4 mm, and an irradiation current of 15. The crystal structure was observed in a region 3 to 4 mm from the sample side perpendicular to the M-plane and 350 to 650 μm away perpendicularly from the M-plane, and two backscattered electron images with a field of view size of 240 μm x 180 μm were obtained per specimen.

Using the image analysis software "ImageJ", the number of pixels in the area of the pores (black parts in Figure 2) in each image was counted, and the value was divided by the number of pixels in the entire image, 1,228,800, to obtain the porosity (area %). The porosity values obtained from the two images of each sample were arithmetically averaged to obtain the porosity of each sample. Note that when calculating the porosity, only pores with a maximum dimension of 0.2 μm or more were considered.

The results of the density and porosity are shown in Table 5. Furthermore, the squareness ratio Hk/ _HcJ was measured for each of the magnet samples No. A1, D1, F1 and G1. The results are shown in Table 5.
Each of the magnet samples No. A1, D1, F1 and G1 was cut along a plane parallel to one of the surfaces and passing near the center of the magnet, and a line analysis was performed on the cross section from the magnet surface to near the center of the magnet using an EPMA. It was confirmed that in all samples, R (Tb and Pr in this magnet sample) gradually decreased (gradually became less concentrated) from the magnet surface to a depth of 200 μm.

As shown in Table 5, the R-T-B based sintered magnets of the present invention all had a squareness ratio Hk/ _HcJ of 85% or more, and a porosity of 0.03 area% or more and 0.5 area% or less. In contrast, all of the comparative examples were outside the range of the present disclosure. Furthermore, in No. A1, which had a low sintering temperature Ts1, the density of the sintered body before diffusion was 7.50 g/cm ³ (7.50×10 ³ kg/m ³ ), and the density of the R-T-B based sintered magnet after diffusion was 7.56 g/cm ³ (7.56×10 ³ kg/m ³ ), so the density was higher after diffusion. In contrast, in the examples of the present invention, no difference in density was observed between before and after diffusion. Furthermore, No. G1 had a lower _HcJ than No. D1 of the present invention.

Claims (10)

A method for producing an R-T-B based sintered magnet (R is at least one selected from the group consisting of Nd, Pr and Ce, T is at least one transition metal element and necessarily contains Fe, and B can be partially substituted with C), comprising the steps of:
a sintering step of sintering the compact formed from the R-T-B based alloy powder to obtain a sintered body;
a diffusion step of disposing a diffusion source containing R ₁ (R ₁ is one or more of rare earth elements) on the surface of the sintered body and performing a diffusion treatment, thereby diffusing the R ₁ in the diffusion source from the surface to the inside of the sintered body,
The sintering step comprises:
Pre-sintering by heating to a pressure start temperature Tp of 700° C. or more and 850° C. or less in an inert gas atmosphere of less than 10 kPa;
heating the mixture from the pressurization start temperature Tp to a sintering temperature Ts1 in the inert gas atmosphere of 10 kPa or more and 100 kPa or less, and then maintaining the mixture at the sintering temperature Ts1 for a predetermined period of time. The method for producing an R-T-B based sintered magnet according to claim 1, wherein the pre-sintering is carried out in an inert gas atmosphere of 500 Pa or less. After the sintering step and before the diffusion step,
A cooling step of cooling from Ts1 to a cooling temperature Tc;
2. The method for producing a sintered R-T-B based magnet according to claim 1, further comprising a second sintering step of heating from the cooling temperature Tc to a second sintering temperature Ts2. The diffusion source includes one or more selected from the group consisting of Pr and Nd as _R1 ,
2. The method for producing a sintered RTB based magnet according to claim 1, wherein the total content of Pr and Nd in the diffusion source is from 30% by mass to 97% by mass. The R ₁ in the diffusion source further includes one or more selected from the group consisting of Tb and Dy;
5. The method for producing a sintered R-T-B magnet according to claim 4, wherein the total content of Tb and Dy in the diffusion source is from 1% by mass to 50% by mass. The diffusion source further includes M (M is at least one selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Ag, In, and Sn),
2. The method for producing an R-T-B based sintered magnet according to claim 1, wherein said diffusion step diffuses said _R1 and said M in said diffusion source from the surface of said sintered body to the inside. The diffusion source contains one or more elements selected from the group consisting of Cu and Ga as the M,
7. The method for producing a sintered R-T-B magnet according to claim 6, wherein the total content of Cu and Ga in the diffusion source is from 2 mass % to 39 mass %. An R-T-B based sintered magnet (R is at least one selected from the group consisting of Nd, Pr and Ce, T is at least one transition metal element and necessarily contains Fe, and B can be partially replaced by C),
The squareness ratio Hk/ _HcJ is 85% or more,
The R concentration gradually decreases from the magnet surface to a depth of 200 μm.
An R-T-B based sintered magnet, wherein the porosity calculated from a structure observation image taken at a position 500 μm deep from the magnet surface in a cross-sectional view is 0.03 area % or more and 0.5 area % or less. 9. The R-T-B based sintered magnet according to claim 8, having a density of 7.4×10 ³ kg/m ³ or more and 7.6×10 ³ kg/m ³ or less. The R necessarily contains Pr,
When the entire R-T-B based sintered magnet is taken as 100 mass %,
The content of R is 27% by mass or more and 33% by mass or less,
The content of B is 0.85% by mass or more and 0.94% by mass or less,
10. The R-T-B based sintered magnet according to claim 8, wherein the T content is 61.5 mass % or more.

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