JP7548688B2 - RTB based sintered magnet - Google Patents

Description

This disclosure relates to R-T-B based sintered magnets.

R-T-B based sintered magnets (R is at least one rare earth element including at least one of Nd and Pr, T is Fe or Fe and Co, and 90% or more by mass of T is Fe) are composed of a main phase made of a compound having an R ₂ T ₁₄ B crystal structure and a grain boundary phase located at the grain boundaries of this main phase, and are known as the magnets with the highest performance among permanent magnets.

For this reason, they are used in a wide variety of applications, including voice coil motors (VCMs) in hard disk drives, motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and various motors in home appliances.

As applications expand in this way, motors for electric vehicles, for example, may be exposed to high temperatures of around 100°C, and stable operation even at high temperatures is required.

However, R-T-B based sintered magnets have a problem in that their coercive force H _cJ (hereinafter sometimes simply referred to as "H _cJ ") decreases at high temperatures, causing irreversible thermal demagnetization. For example, when an R-T-B based sintered magnet is used in an electric vehicle motor, H _cJ decreases when used at high temperatures, and there is a risk that the motor will not operate stably. For this reason, there is a demand for R-T-B based sintered magnets that have a high H _cJ even at high temperatures.

Conventionally, heavy rare earth elements RH (mainly Dy) have been added to R-T-B based sintered magnets to improve H _cJ , but this has the problem of reducing the remanence B _r (hereinafter sometimes simply referred to as "B _r "). Furthermore, Dy has problems such as an unstable supply due to limited production areas, and its price can fluctuate greatly. Therefore, there is a demand for technology that can improve the H _cJ of R-T-B based sintered magnets while using as little heavy rare earth elements RH as possible, such as Dy.

As an example of such a technique, Patent Document 1 discloses that an R-T-B based sintered magnet with high coercivity can be obtained while suppressing the Dy content by forming an R ₂ T ₁₇ phase by lowering the B content compared to normal R-T- _B based alloys and adding one _or more metal elements M selected from Al, Ga, and Cu, and by sufficiently ensuring the volume fraction of the transition metal-rich phase (R-T-Ga phase) formed using the R 2 T 17 phase as a raw material.

International Publication No. 2013/008756

Although the RTB based sintered magnet described in Patent Document 1 has an improved _HcJ , it is still insufficient to satisfy recent demands.

Therefore, the present disclosure provides an RTB based sintered magnet that uses as little heavy rare earth element RH as possible and has a high coercive force H _cJ at high temperatures (eg, 100° C.).

In an exemplary embodiment, the R-T-B based sintered magnet of the present disclosure contains R: 28.5 mass % or more and 33.0 mass % or less (R is at least one rare earth element and includes at least one of Nd and Pr), B: 0.85 mass % or more and 0.91 mass % or less, Ga: 0.35 mass % or more and 0.75 mass % or less, Cu: 0.05 mass % or more and 0.50 mass % or less, Mn: 0.03 mass % or more and 0.15 mass % or less, and T: 61.5 mass % or more and 70.0 mass % or less (T is Fe or Fe and Co, and 90 mass % or more of T is Fe), and satisfies the following formula (1):
14[B]/10.8<[T]/55.85 (1)
([B] is the content of B in mass%, and [T] is the content of T in mass%)

In one embodiment, the RTB based sintered magnet does not contain a heavy rare earth element (except for unavoidable impurities), and has H _cJ ≧880 kA/m at 100° C. and Br ≧1.32 T at 22.5° C.
In one embodiment, the RTB based sintered magnet contains 1.0 mass % or less of Tb, and at 100° C., H _cJ ≧880+168 [Tb] kA/m, and at 22.5° C., B _r ≧1.32−0.024 [Tb]T.
([Tb] is the Tb content in mass%)

According to the embodiments of the present disclosure, it is possible to provide an RTB based sintered magnet that has a high coercive force H _cJ at high temperatures (eg, 100° C.) while using as little heavy rare earth element RH as possible.

1 is an explanatory diagram showing the relationship between the Mn content and _HcJ at 100° C. for Nos. 1 to 10 of Experimental Example 1.

As a result of extensive investigations, the present inventors have found that a high HcJ can be obtained even at high temperatures by further adding a specific range of Mn to an R-T-B based sintered magnet having the specific R, B, Ga, and Cu contents of the present disclosure, in particular an extremely narrow specific range of B content. This is believed to be because the temperature _coefficient is improved by further adding Mn to an R-T-B based sintered magnet having a B content within the specific composition range of the present disclosure.

[RTB-based sintered magnet]
The R-T-B based sintered magnet of the present disclosure is
R: 28.5% by mass or more and 33.0% by mass or less (R is at least one rare earth element, including at least one of Nd and Pr);
B: 0.85% by mass or more and 0.91% by mass or less,
Ga: 0.35% by mass or more and 0.75% by mass or less,
Cu: 0.05% by mass or more and 0.50% by mass or less,
Mn: 0.03% by mass or more and 0.15% by mass or less,
T: 61.5 mass% or more and 70.0 mass% or less (T is Fe or Fe and Co, and 90 mass% or more of T is Fe);
The following formula (1) is satisfied.

14[B]/10.8<[T]/55.85 (1)
([B] is the content of B in mass%, and [T] is the content of T in mass%)
Each composition will be described in detail below.

(R: 28.5-33.0% by mass)
R is at least one rare earth element, including at least one of Nd and Pr. The content of R is 28.5 to 33.0 mass%. If the content of R is less than 33.0 mass%, it may be difficult to achieve densification during sintering, and if it exceeds 33.0 mass%, the main phase ratio may decrease, resulting in a decrease in _Br . %, preferably 29.5 to 32.5 mass %. When R is in this range, a higher B _r can be obtained.

(B: 0.85-0.91% by mass)
The content of B is 0.85 to 0.91 mass %. The RTB based sintered magnet contains B within the range of the present disclosure and further contains Mn within a specific range described below. By doing so, the temperature coefficient is improved, and a high _HcJ can be obtained even at high temperatures. In addition, a part _of B can be replaced with C.
Furthermore, the content of B satisfies the following formula (1).

14[B]/10.8<[T]/55.85 (1)

By satisfying formula (1), the content of B becomes smaller than that of a typical R-T-B based sintered magnet. In order to prevent the formation of the _soft magnetic _{phase R2T17 phase other than the R2T14B phase} , [T]/55.85 (atomic weight of Fe) is set to 14[B]/10.8 (atomic weight of B). ([T] is the content of T in mass %) The R-T-B system sintered magnet of the present invention is different from the general R-T-B system sintered magnet. Unlike the sintered magnet, the formula (1) specifies that [T]/55.85 is greater than 14[B]/10.8. Since the main component of T in the magnet is Fe, the atomic weight of Fe was used.

(Ga: 0.35 to 0.75% by mass)
The Ga content is 0.35 to 0.75 mass %. If the Ga content is less than 0.35 mass %, the temperature coefficient is not improved and a high _HcJ cannot be obtained at high temperatures. Furthermore, the amount of the RT-Ga phase produced is small, and the R ₂ T ₁₇ phase cannot be eliminated, so that a high H _cJ value cannot be obtained at room temperature. If it exceeds 0.75 mass %, unnecessary Ga will be present, and there is a possibility that the main phase ratio will decrease and _Br will decrease.

(Cu: 0.05-0.50% by mass)
The Cu content is 0.05 to 0.50 mass %. If the Cu content is less than 0.05 mass %, it may be impossible to obtain a high _HcJ at room temperature and at high temperatures. If it exceeds 50 mass %, the sinterability may deteriorate, making it difficult to obtain a high _HcJ at room temperature and at high temperatures.

(Mn: 0.03 to 0.15% by mass)
The Mn content is 0.03 to 0.15 mass%. By limiting the B content to the above range and further including 0.03 to 0.15 mass% of Mn, The temperature coefficient is improved, and a high _HcJ can be obtained at high temperatures. If the Mn content is less than 0.03 mass %, the temperature coefficient is not improved, and a high _HcJ cannot be obtained at high temperatures. If the mass ratio exceeds 0.15, the B _r may decrease.

(T: 61.5% by mass to 70.0% by mass)
T is Fe or Fe and Co, and 90 mass % or more of T is Fe. By containing Co, the corrosion resistance can be improved, but if the amount of Co substituted exceeds 10 mass % of T, If the content of T is less than 61.5 mass %, a high B _r may not be obtained. The content of T is 61.5 mass % or more and 70.0 mass % or less, and satisfies the above formula (1). When the content is less than 61.5 mass %, the B _r may be significantly decreased. Preferably, T is the remainder.

Furthermore, the R-T-B system sintered magnet of the present disclosure 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. Furthermore, examples of inevitable impurities during the manufacturing process include O (oxygen), N (nitrogen), C (carbon), Al, etc. Furthermore, the R-T-B system sintered magnet of the present disclosure may contain one or more other elements (elements added intentionally other than inevitable impurities). For example, such elements may contain small amounts (about 0.1 mass% each) of Ag, Zn, In, Sn, Ti, Ge, Y, H, F, P, S, V, Ni, Mo, Hf, Ta, W, Nb, Zr, etc. Furthermore, the elements listed above as inevitable impurities may be intentionally added. Such elements may be contained in a total amount of, for example, about 1.0 mass%. If the temperature is within this range, it is quite possible to obtain an RTB based sintered magnet that has a high _HcJ at high temperatures.

The R-T-B based sintered magnet having the composition according to the present embodiment described above is manufactured by, for example, a process of preparing an alloy powder, a molding process of molding the alloy powder to obtain a compact, a sintering process of sintering the compact to obtain a sintered body, and a heat treatment process of subjecting the sintered body to a heat treatment.

[Method of manufacturing R-T-B based sintered magnets]
Each step will be described below.

(Step of producing alloy powder)
Metals or alloys (molten raw materials) of the respective elements are prepared so that the R-T-B based sintered magnet has the above-mentioned specific composition, and flake-shaped raw alloy is produced by strip casting or the like. The flake-shaped raw alloy is then coarsely pulverized by hydrogen pulverization or the like to prepare coarsely pulverized powder with an average particle size of 1.0 mm or less. The coarsely pulverized powder is then finely pulverized by a jet mill or the like in an inert gas to obtain a finely pulverized powder (raw alloy powder) with a particle size D ₅₀ of, for example, 3 to 5 μm. A known lubricant may be added as an auxiliary agent to the coarsely pulverized powder before the jet mill pulverization, and to the alloy powder during and after the jet mill pulverization.

(Molding process)
The obtained raw alloy powder is subjected to compaction in a magnetic field to obtain a compact. The compaction in a magnetic field may be performed by any known method including a dry compaction method in which a dried alloy powder is inserted into a cavity of a die and compacted while a magnetic field is applied, and a wet compaction method in which a slurry is injected into a cavity of a die and compacted while the dispersion medium of the slurry is discharged.

(Sintering process)
The compact is sintered to obtain a sintered body (sintered magnet). A known method can be used for sintering the compact. In order to prevent oxidation due to the atmosphere during sintering, sintering is preferably carried out in a vacuum atmosphere or in an inert gas. The inert gas used is preferably an inert gas such as helium or argon.

(Heat treatment process)
The obtained sintered magnet is preferably subjected to a heat treatment for the purpose of improving the magnetic properties. The heat treatment temperature, heat treatment time, and the like can be known conditions. For example, heat treatment (one-stage heat treatment) may be performed only at a relatively low temperature (400°C or more and 600°C or less), or heat treatment at a relatively high temperature (700°C or more and sintering temperature or less (e.g., 1050°C or less)) may be performed, followed by heat treatment (two-stage heat treatment) at a relatively low temperature (400°C or more and 600°C or less). Preferred conditions include a heat treatment at 730°C or more and 1020°C or less for about 5 to 500 minutes, cooling (to room temperature or to 440°C or more and 550°C or less), and then a further heat treatment at 440°C or more and 550°C or less for about 5 to 500 minutes. The heat treatment atmosphere is preferably a vacuum atmosphere or an inert gas (helium, argon, etc.).

The obtained sintered magnet may be subjected to machining such as grinding in order to obtain the final product shape. In this case, the heat treatment may be performed before or after the machining. Furthermore, the obtained sintered magnet may be subjected to a surface treatment. The surface treatment may be a known surface treatment, such as Al vapor deposition, Ni electric plating, or resin paint.

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

Experimental Example 1

Each element was weighed so that the composition of the R-T-B based sintered magnet was approximately the composition of No. 1 to No. 12 in Table 1, and the alloy was cast by strip casting to prepare a quenched alloy. The quenched alloy thus obtained was subjected to hydrogen embrittlement in a pressurized hydrogen atmosphere, and then to a dehydrogenation treatment in which the alloy was heated to 550C in a vacuum and cooled, to obtain a coarsely pulverized powder. Next, 0.04 mass% of zinc stearate was added to the coarsely pulverized powder thus obtained as a lubricant relative to 100 mass% of the coarsely pulverized powder, and the mixture was mixed and dry-pulverized in a nitrogen gas stream using an airflow pulverizer (jet mill device) to obtain a finely pulverized powder (alloy powder) with a particle size _D50 (median diameter) of 4 μm.

The obtained alloy powder was mixed with a dispersion medium to prepare a slurry. Normal dodecane was used as the solvent, and methyl caprylate was mixed as the lubricant. The concentration of the slurry was 70% by mass of alloy powder, 30% by mass of dispersion medium, and 0.16% by mass of lubricant relative to 100% by mass of alloy powder. The slurry was molded in a magnetic field to obtain a compact. The magnetic field during molding was a static magnetic field of 0.8 MA/m, and the pressure was 5 MPa. 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 pressure direction are perpendicular to each other.

The obtained compact was sintered in a vacuum at 1000°C to 1090°C (a temperature at which densification by sintering is sufficient was selected for each sample) for 4 hours, and then quenched to obtain a sintered body. The density of the obtained sintered body was 7.5 Mg/ ^m3 or more. The obtained sintered body was held in a vacuum at 800°C for 2 hours, quenched to room temperature, and then held in a vacuum at 430°C to 530°C (a temperature at which good coercivity is obtained was selected for each sample) for 2 hours, and then cooled to room temperature, to obtain a R-T-B based sintered magnet.

The components of the obtained R-T-B based sintered magnet are shown in Table 1. Note that each component in Table 1 (other than O, N, and C) was measured using inductively coupled plasma optical emission spectroscopy (ICP-OES). The O (oxygen) content was measured using a gas fusion-infrared absorption method, the N (nitrogen) content was measured using a gas fusion-thermal conduction method, and the C (carbon) content was measured using a gas analyzer using the combustion-infrared absorption method.
The degree of satisfaction of formula (1) is shown in Table 1. Here, "◯" means that formula (1) is satisfied, and "×" means that formula (1) is not satisfied.
The results of measuring the magnetic properties of the obtained R-T-B based sintered magnets are shown in Table 2. In Table 2, "22.5°C B _r " and "22.5°C H _cJ " are the values of B _r and H _cJ at room temperature (22.5°C), and "100°C B _r " and "100°C H _cJ " are the values of B _r and H _cJ at high temperature (100°C). These B _r and H _cJ values were measured by machining the R-T-B based sintered magnets to produce samples of 7 mm x 7 mm x 7 mm using a BH tracer. Furthermore, the temperature coefficient (β: 22.5 to 100°C) was determined as follows.
Temperature coefficient=( _HcJ at 100° C.− _HcJ at 22.5° C.)/ _HcJ at 22.5° C./(100° C.−22.5° C.)×100%
The smaller the absolute value of the temperature coefficient, the more improved the temperature coefficient is.

As shown in Table 2, the invention examples (Nos. 7, 8, 9, and 12) that satisfy the composition of the R-T-B based sintered magnets of the present disclosure all have a higher H _cJ at high temperatures (100°C) than the comparative examples (which are outside the composition range of the present disclosure). Furthermore, in Example 1, none of the invention examples contain a heavy rare earth element (excluding unavoidable impurities), and at 100°C, H _cJ ≧880 kA/m and at 22.5°C, Br ≧1.32 T, providing better magnetic properties than the comparative examples. Furthermore, as shown by the temperature coefficients in Table 2, the invention examples all have smaller absolute values of temperature coefficients than the comparative examples.
FIG. 1 shows the relationship between the Mn content and H _cJ at 100°C for Nos. 1 to 10. In FIG. 1, squares (■) are examples of the present invention, and triangles (▲) are comparative examples. As shown in Nos. 1 to 5 in FIG. 1, when the B content of the R-T-B based sintered magnet is outside the range of the present disclosure (Nos. 1 to 3 and 5 have B contents outside the range, and No. 4 has formula (1) outside the range), the H _cJ at high temperatures is hardly improved even if the Mn content is increased. On the other hand, as shown in Nos. 6 to 10, when the B content of the R-T-B based sintered magnet is within the range of the present disclosure, the H _cJ at high temperatures is greatly improved when the Mn content exceeds 0.02 mass% (No. 6). Note that No. 10 (Mn: 0.20 mass%) has a high H _cJ at high temperatures, but as shown in Table 2, B _r is reduced.

Experimental Example 2

The R-T-B based sintered magnets were produced in the same manner as in Example 1, except that each element was weighed so that the composition of the R-T-B based sintered magnets would be approximately the composition of No. 13 to No. 15 in Table 3. The composition and magnetic properties of the obtained R-T-B based sintered magnets were measured in the same manner as in Example 1. The respective results are shown in Tables 3 and 4.

When Tb is contained in an R-T-B based sintered magnet, B _r decreases and H _cJ improves according to the Tb content. In this case, B _r decreases by about 0.024 T at 22.5° C. when 1 mass % of Tb is contained. Also, H _cJ increases by about 168 kA/m at 100° C. when 1 mass % of Tb is contained. Therefore, in the present disclosure, when no heavy rare earth element is contained as described above, H _cJ ≧880 kA/m at 100° C. and B _r ≧1.32 T at 22.5° C. is satisfied, and when Tb is contained, H _cJ ≧880+168 [Tb] kA/m at 100° C. and B _r ≧1.32-0.024 [Tb] T at 22.5° C. is satisfied.

As shown in Tables 3 and 4, Nos. 13 and 14, which are invention examples, contain 1.0 mass % or less of Tb, and have high magnetic properties, with H _cJ ≧880+168 [Tb] kA/m at 100° C. and B _r ≧1.32−0.024 [Tb] T at 22.5° C. Furthermore, the invention examples all have small absolute values of the temperature coefficients, similar to the invention example of Example 1.

Claims (8)

R: 28.5% by mass or more and 33.0% by mass or less (R is at least one rare earth element, including at least one of Nd and Pr);
B: 0.85% by mass or more and 0.91% by mass or less,
Ga: 0.35% by mass or more and 0.75% by mass or less,
Cu: 0.05% by mass or more and 0.50% by mass or less,
Mn: 0.03% by mass or more and 0.15% by mass or less,
T: 61.5 mass% or more and 70.0 mass% or less (T is Fe or Fe and Co, and 90 mass% or more of T is Fe);
The Ga content is greater than the Cu content,
An RTB based sintered magnet satisfying the following formula (1):

[T]/55.85-14[B]/10.8≧0.063 (1)
([B] is the content of B in mass%, and [T] is the content of T in mass%) The R-T-B sintered magnet according to claim 1, in which the Ga content is at least 1.4 times the Cu content. The R-T-B sintered magnet according to claim 1 or 2, in which the Ga content is 3.7 times or more the Cu content. The R-T-B based sintered magnet according to any one of claims 1 to 3, further comprising 0.2 mass% or less of O. An R-T-B based sintered magnet according to any one of claims 1 to 4, in which T is Fe and Co. The R-T-B based sintered magnet according to any one of claims 1 to 5, containing 29.5 mass% or more and 32.5 mass% or less of R. It does not contain heavy rare earth elements (except for unavoidable impurities), and has _HcJ ≧880 kA/m at 100° C. and Br≧1.32 T at 22.5° C.;
The RTB based sintered magnet according to any one of claims 1 to 6. 7. The R-T-B system sintered magnet according to claim 1, containing 1.0 mass % or less of Tb, having H _cJ ≧880+168 [Tb]kA/m at 100° C. and B _r ≧1.32-0.024 [Tb]T at 22.5° C.
([Tb] is the Tb content in mass%)