JP2011082365A - R-t-b-based sintered magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an R-T-B-based sintered magnet with superior magnetization capable of achieving high magnetization, even in a low magnetization magnetic field. <P>SOLUTION: In the R-T-B-based sintered magnet for improving its magnetization, the ratio of the number of crystal particles wherein no R<SB>2</SB>O<SB>3</SB>compound exists even between adjoining two particles, to the total number of crystal particles forming the sintered magnet is 50 to 100%. Preferably, the ratio is 70 to 100%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an R-T-B type sintered magnet with good magnetization that can achieve high magnetization even with a low magnetization field.

  R-T-B sintered magnets (R is a rare earth element including Y, T is Fe or Fe and Co, and B is boron) for applications such as rotary motors, linear motors, and voice coil motors (VCM) Widely used.

  R-T-B system sintered magnets for motors often employ so-called assembly magnetization that is magnetized after being incorporated in a motor in an unmagnetized state. In addition, it may be difficult to apply a magnetic field having a strength necessary for sufficient magnetization. A magnet with insufficient magnetization does not have the desired magnetic properties (particularly residual magnetic flux density).

  Various techniques have been proposed for improving magnetization so as to obtain a highly magnetized R-T-B sintered magnet that can achieve a large magnetization even in a low magnetization field.

In Patent Document 1, two types of R ₂ T ₁₄ B-based alloys having different ratios of light rare earth elements and heavy rare earth elements in the rare earth element R are prepared, mixed, pulverized, and sintered. and _R 2 _{T 14} B phase is large heavy rare-earth element RH into a heavy rare-earth element RH is less _R 2 _{T 14} B phase, and _R 2 _{T 14} B phase containing the heavy rare-earth element RH in their intermediate amount A technique for producing a mixed RTB-based sintered magnet is disclosed.

JP 2002-356701 A

And in the patent literature 1, the crystal grains heavy rare-earth element RH is often _R 2 _{T 14} B phase, containing a heavy rare-earth element RH is less _R 2 _{T 14} B phase, the heavy rare-earth element RH in their intermediate amount _{R 2} Although the magnetism is improved by the RTB-based sintered magnet in which the T ₁₄ B phase is mixed, it is difficult to obtain the desired magnetization particularly in the assembly magnetization.

In the R-T-B based sintered magnet, the present inventor is closely related to the magnetization by the presence or absence of the R ₂ O ₃ compound between _two grains of adjacent crystal grains (R ₂ T ₁₄ B phase). Based on this knowledge, the present invention has been completed.

In the R-T-B based sintered magnet, the ratio of the number of crystal grains in which no R ₂ O ₃ compound exists between any two adjacent crystal grains is 50% to 100%. It is an object of the present invention to provide an R-T-B based sintered magnet with good magnetization.

In the R-T-B system sintered magnet according to the present invention, no R ₂ O ₃ compound is present in any of two adjacent crystal grains with respect to the total number of crystal grains constituting the sintered magnet. An RTB-based sintered magnet having a crystal grain number ratio of 50% to 100%.

  In a preferred embodiment of the present invention, the ratio is 70% to 100%.

  According to the present invention, it is possible to provide an R-T-B sintered magnet having good magnetization that can achieve high magnetization even with a low magnetization field.

It is a SEM photograph in the sintered magnet cross section of an Example. It is a SEM photograph in the sintered magnet cross section of an Example. It is a TEM photograph of the sintered magnet of an Example. It is a SEM photograph in the cross section of the sintered magnet of a comparative example. It is a TEM photograph of the sintered magnet of a comparative example.

[Organization]
In the R-T-B system sintered magnet according to the present invention, no R ₂ O ₃ compound is present in any of two adjacent crystal grains with respect to the total number of crystal grains constituting the sintered magnet. The ratio of the number of crystal grains is 50% to 100%. R ₂ O ₃ compound, refers to R ₂ O ₃ compounds present between two particles of the crystal grains, R ₂ O ₃ compounds present in the grain boundary triple point are not the eligible claim 1. That is, it is considered that the R ₂ O ₃ compound existing at the grain boundary triple point is not related to magnetism.

In either between the crystal grains of 2 grains adjacent even count the number of crystal grains does not exist R ₂ O ₃ compound, R ₂ O ₃ compound either between 2 particles adjacent crystal grains are present The ratio of the number of crystal grains in which the R ₂ O ₃ compound does not exist is calculated with respect to the total number of crystal grains.

The R ₂ O ₃ compound existing between the two grains of the crystal grains tends to become granular crystals and expands the grain boundary of the two grains (there is an R ₂ O ₃ compound lumps between the two grains, so-called The tissue shape looks like an aneurysm). When the magnet is magnetized, the R ₂ O ₃ compound existing between the two grains of the crystal grains becomes the starting point of the occurrence of a reverse magnetic domain in a low magnetic field, and the R ₂ O ₃ compound existing between the crystal grains and the two grains It is thought that the magnetism of the magnet changes greatly depending on the existence. )

When the ratio of the number of crystal grains in which no R ₂ O ₃ compound is present to the total number of crystal grains is 50% or more, the magnetization with respect to the magnetizing magnetic field having the same strength is better than when the ratio is less than 50%.

Preferably, the ratio of the number of crystal grains in which no R ₂ O ₃ compound is present to the total number of crystal grains is 70% or more and 100% or less.

[composition]
Among the compositions of the RTB-based sintered magnet, R is a rare earth element including Y, and is preferably 27.3 mass% or more and 31.2 mass% or less. , Dy, Tb, which are heavy rare earth elements RH, or both can be contained as required. When R is less than 27.3 mass%, sintering becomes difficult and a soft magnetic phase is generated, which may reduce the coercivity of the R-T-B system sintered magnet. On the other hand, if R exceeds 31.2% by mass, the residual magnetic flux density of the RTB-based sintered magnet is lowered.

  B is preferably in the range of 0.92% by mass to 1.15% by mass. If the amount of B is less than 0.92% by mass, a soft magnetic phase may be generated and the coercive force of the RTB-based sintered magnet may be reduced. On the other hand, when B exceeds 1.15 mass%, the residual magnetic flux density of the RTB-based sintered magnet is lowered.

  T is the balance and is one or two of Fe, Fe and Co, and the Co content is preferably 20% by mass or less. When Co exceeds 20 mass% in T, the residual magnetic flux density of the RTB-based sintered magnet decreases.

  The additive element M includes at least one of Al, Si, Ti, V, Cr, Mn, Ni, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, Cu, and W. Also good. The addition amount is preferably 2.0% by mass or less.

  Below, the manufacturing method of the RTB system sintered magnet by this invention is explained in full detail.

[Raw material alloy]
The raw material alloy can be obtained by an ordinary ingot casting method, strip casting method, direct reduction method or the like. Here, in the sintered magnet, in order to suppress the formation of the R ₂ O ₃ compound between two grains of crystal grains, it is preferable to reduce the oxygen content immediately after the preparation of the R—T—B system alloy.

[Coarse grinding]
The rough pulverization of the raw material alloy is preferably a hydrogen embrittlement treatment. This is a method in which fine cracks are generated in the alloy and pulverized using the embrittlement phenomenon and volume expansion phenomenon of the alloy accompanying hydrogen storage. In the raw material alloy of the present invention, the difference in the hydrogen storage amount between the main phase and the R-rich phase, that is, the difference in the volume change amount causes cracks, so the probability of cracking at the grain boundary of the main phase increases. Thereby, the load in the pulverization process is reduced, and the uptake of impurities including oxygen is suppressed.

The hydrogen embrittlement treatment is performed by exposing to pressurized hydrogen for a certain period of time. Furthermore, after that, the temperature may be raised to 400 ° C. or higher and 800 ° C. or lower so that excessive hydrogen is released. The coarse powder after hydrogen embrittlement treatment is very active because it contains many cracks and the specific surface area is greatly increased, and the amount of oxygen increases significantly when handled in the atmosphere. , He,
It is desirable to handle in an inert gas such as Ar or a mixed gas thereof. Further, since a nitriding reaction may occur at a high temperature, it is preferable to handle in an Ar atmosphere if possible.

[Fine grinding]
In the fine pulverization step, dry pulverization using an airflow pulverizer can be used. In this case, in general, nitrogen or an inert gas containing nitrogen is used to suppress oxidation, but He does not contain nitrogen to suppress both oxidation and nitridation in order to prevent deterioration of magnetic properties. , Ar or a mixed gas thereof is preferably used.

  Another fine pulverization method is a wet pulverization method. Specifically, a ball mill or an attritor can be used. In this case, it is possible to select a grinding medium, a solvent, and an atmosphere so that impurities such as oxygen and carbon are not taken in over a predetermined amount. For example, a bead mill that stirs at a high speed using a very small-diameter ball can be miniaturized in a short time, so that the influence of impurities can be reduced, which is preferable for obtaining finely pulverized powder used in the present invention.

  Furthermore, once coarsely dry pulverizing with an airflow pulverizer and then wet pulverizing with a bead mill, the amount of oxygen taken into the powder can be reduced because pulverization is possible in a short time.

  The solvent used in the wet pulverization is selected in consideration of the reactivity with the raw material alloy, the oxidation deterrence, and the ease of removal before sintering. For example, organic solvents, particularly saturated hydrocarbons such as paraffin are preferred.

If the particle size distribution of the finely pulverized powder is wide, a sintered body having a uniform structure cannot be obtained, and the R ₂ O ₃ compound may be easily generated. Accordingly, the particle size is preferably 1 μm or less in terms of standard deviation (σ).

  In the conveyance and storage after the pulverization process, it is important to suppress oxidation of the powder. To do this, the pulverized powder is collected in a solvent such as oil, or the finely pulverized powder is managed in a container controlled with an inert gas (He, Ar, or a mixed gas thereof) so that the amount of oxygen is extremely small. It is good to do.

  In the present invention, when a sintered magnet is produced using a plurality of types of R-T-B type alloys having different components, a plurality of types of R-T-B type alloys having different components are separately pulverized into powder. Alternatively, a plurality of types of RTB-based alloys having different components may be mixed after coarse pulverization and then finely pulverized into powder.

  In this embodiment, the RTB-based alloy powder produced by the pulverization method is added and mixed with an appropriate amount of an antioxidant such as zinc stearate in a rocking mixer, for example, and the alloy powder particles are added with the antioxidant. It is preferable to coat the surface.

[Molding]
A known method can be used for the molding method of the present invention. For example, it is a method in which the finely pulverized powder is pressure-molded with a mold in a magnetic field. In order to minimize the uptake of oxygen and carbon, it is desirable to minimize the use of lubricants. When the lubricant is used, a highly volatile lubricant that can be degreased before or during the sintering step may be selected from known ones.

  As a measure for suppressing the oxidation of the finely pulverized powder, it is preferable that the finely pulverized powder is mixed with a solvent to form a slurry, and the slurry is subjected to molding in a magnetic field. In this case, considering the volatility of the solvent, a low molecular weight hydrocarbon that can be completely volatilized in a vacuum of, for example, 250 ° C. or lower in the subsequent sintering process can be selected. In particular, saturated hydrocarbons such as paraffin are preferable. When forming a slurry, the finely pulverized powder may be directly collected in a solvent to form a slurry.

[Sintering]
The atmosphere in the sintering process is an inert gas atmosphere in vacuum or at atmospheric pressure or lower. The inert gas here refers to Ar and / or He gas.

  In order to sufficiently remove the lubricant and solvent used in the pulverization process and the molding process, the temperature is 300 ° C. or less, the time is 30 minutes or more and 8 hours or less, in a vacuum or in an inert gas at atmospheric pressure or less. It is preferable to sinter after holding and degreasing. The degreasing treatment can be performed independently of the sintering step, but it is preferable to continuously sinter after the degreasing treatment from the viewpoints of processing efficiency, oxidation prevention and the like. In the degreasing step, it is preferable in terms of degreasing efficiency to be performed in an inert gas atmosphere at or below the atmospheric pressure. Moreover, in order to perform a degreasing process efficiently, the heat processing in a hydrogen atmosphere can also be performed.

  In the sintering process, a gas release phenomenon from the molded body is observed during the temperature rising process of the molded body. The gas release is mainly the release of hydrogen gas introduced in the hydrogen embrittlement treatment step. Since the liquid phase is generated only after the hydrogen gas is released, it is preferable to release the hydrogen gas sufficiently. For example, in the course of the temperature rising process, the temperature ranges from 700 ° C. to 850 ° C. for 30 minutes to 4 hours. It is preferable to hold.

The holding temperature at the time of sintering is, for example, 960 ° C. or higher and 1100 ° C. or lower. If it is less than 960 ° C., the release of the hydrogen gas is insufficient and a liquid phase necessary for the sintering reaction cannot be obtained sufficiently, and the sintering reaction does not proceed with the composition of the present invention. That is, a sintered density of 7.5 Mg / m ³ or more cannot be obtained.

Here, the oxygen content of the sintered body is preferably 0.10% by mass or less. When the content is 0.10% by mass or less, rare earth elements present in a large amount in the liquid phase component in the sintering process have an affinity for oxygen. Therefore, the rare earth elements are preferentially bonded to oxygen and become R ₂ O ₃ compounds. This is because the number remaining in the boundary is suppressed. The oxygen amount is preferably 0.05% by mass or less.

[Heat treatment]
After completion of the sintering process, after cooling to 300 ° C. or less, heat treatment can be performed again in the range of 400 ° C. or more and sintering temperature or less to increase the coercive force. This heat treatment may be performed multiple times at the same temperature or at different temperatures. In addition, the coercive force may be improved by slow cooling after holding at the heat treatment temperature.

[processing]
In order to obtain a predetermined shape and size, the RTB-based sintered magnet of the present invention can be subjected to general machining such as cutting and grinding.

[surface treatment]
The RTB-based sintered magnet of the present invention is preferably subjected to a surface coating treatment for rust prevention. For example, Ni plating, Sn plating, Zn plating, Al vapor deposition film, Al alloy vapor deposition film, resin coating, etc. can be performed.

[Magnetic]
For magnetization, a method of applying a pulsed magnetic field or a method of applying a static magnetic field can be applied. In consideration of ease of handling, the sintered magnet is usually magnetized by the above-mentioned method after assembling the magnetic circuit. Of course, the magnet can be magnetized by itself.

[Example 1]
Nd with a purity of 99.5% by mass or more, Tb, Dy, electrolytic iron, low carbon ferroboron alloy with a purity of 99.9% by mass or more, and other target elements are added in the form of an alloy with pure metal or Fe. The alloy having the composition was melted and cast by a strip casting method to obtain plate alloys A and B having a thickness of 0.3 mm to 0.4 mm. This RTB-based alloy was heat-treated at 850 ° C. in a vacuum atmosphere for 1 hour. .

  The alloys A and B were mixed as raw materials and hydrogen embrittled in a hydrogen pressurized atmosphere, and then heated and cooled in vacuum to 600 ° C. to obtain alloy coarse powder. 0.05% zinc stearate by mass ratio was added to and mixed with the coarse powder.

  The RTB-based alloy A has a composition of Nd 30.0%, B 0.94%, Co 0.87%, Ga 0.09%, Al 0.12%, and Cu 0.09% balance in mass%. Made of iron. The R-T-B alloy B has a main composition of Nd 20.4% by mass%, Dy 10% B 0.96%, Co 0.89%, Ga 0.09%, Al 0.09%. , Cu 0.09% The balance iron.

  The alloy A and the alloy B were mixed at a ratio of 9: 1. An appropriate amount of lubricant was added during mixing. The mixed alloy is recovered in a solvent composed of saturated hydrocarbons. During the recovery, Ar gas whose atmosphere was controlled so that the amount of oxygen and the amount of nitrogen were extremely reduced was flowed.

  Next, dry pulverization is performed in a mixed gas flow of Ar and He using an airflow pulverizer (jet mill device), and the standard deviation (σ) is 1 μm and the particle diameter D50 is 3.0 μm. B-based alloys A and B were prepared. At this time, the oxygen concentration in the pulverized gas is controlled to 50 ppm or less. The particle diameter D50 is a value obtained by a laser diffraction method using an airflow dispersion method.

  The mixed powder thus prepared was wet-molded in a magnetic field to prepare a compact. The magnetic field at this time was a static magnetic field of approximately 0.8 MA / m, and the applied pressure was 5 MPa. The magnetic field application direction and the pressing direction are orthogonal to each other.

Next, this compact was sintered in a vacuum at a temperature range of 1020 ° C. for 2 hours. The density after sintering was 7.5 Mg / m ³ . The obtained sintered magnet was heat-treated at 600 ° C. for 1 hour in an Ar atmosphere and cooled.

  Thereafter, the sintered magnet was mechanically processed to obtain a magnet sample having dimensions of 5 mm × 10 mm × 10 mm. The orientation direction is 5 mm.

  When the composition of the sintered magnet was examined, the composition was Nd 28.6% in mass%, Dy 1.0% B 0.94%, Co 0.88%, Ga 0.08%, Al 0.1%, Cu The balance was 0.09% iron, and the average crystal grain size (equivalent circle diameter) was 3.3 μm. The oxygen content was 980 ppm.

The surface of the prepared sample was processed with FIB using Ga ions, magnified 3000 times at an acceleration voltage of 2 kV with an SEM (ULTRA55 made by Carl Zeiss), and observed as shown in FIG. The ratio of the number of crystal grains in which no R ₂ O ₃ compound was present to the total number of crystal grains included in the visual field having an area of 5.0 mm ² was 60%. Here, whether or not the R ₂ O ₃ compound is present between the two particles was visually determined. When the R ₂ O ₃ compound was present between the two particles, it was confirmed that the width between the two particles was widened and varied as compared with the case where the R ₂ O ₃ compound was not present. When present, the width between the two particles was 15 nm to 30 nm, and there was a portion of maximum 100 nm. When the compound was not present, it was less than 5 nm.

  The evaluation of the magnetization was based on a method in which the magnetic flux of the sample was measured with a search coil after the sample was magnetized with a predetermined magnetic field generated by a pulse current in an air-core coil. The magnetization rate at 636 kA / m, 955 kA / m, 1273 kA / m, and 1591 kA / m was examined by setting the magnetization rate at a magnetization magnetic field of 1591 kA / m to 100%, and found to be 20% at 636 kA / m and 955 kA / m. The magnetization rate was 80%, 97% at 1273 kA / m, and 100% at 1591 kA / m. Further, when the residual magnetic flux density and the coercive force were measured with a BH tracer, the residual magnetic flux density was 1.43 T and the coercive force was 1250 kA / m.

[Example 2]
The composition of the alloy A of Example 1 was Nd 30.9% by mass, Pr 0.1%, B 0.93%, Co 0.87% Ga 0.07%, Al 0.08%, Cu 0.07. % Balance iron, and the composition of alloy B is Nd 21.2%, Dy 9.9%, B 0.94%, Co 0.88%, Ga 0.07%, Al 0.06%, Cu 0 by mass%. 0.09% balance iron, and the composition of the sintered magnet is Nd 29.2% by mass, Pr 0.2%, Dy 1.1% B 0.93%, Co 0.87% Ga 0.08%, Al Except for 0.09%, Cu 0.07%, and the remaining iron, the surface of the sample prepared in the same manner as in the example was processed with FIB and expanded 3000 times with SEM at an acceleration voltage of 2 kV. When observed, the ratio of the number of crystal grains in which no R ₂ O ₃ compound is present to the whole crystal grains as shown in FIG. Was 80%.

The average crystal grain size (equivalent circle diameter) was 3.2 μm. The oxygen content was 770 ppm. Here, whether or not the R ₂ O ₃ compound is present between the two particles was visually determined. When the R ₂ O ₃ compound was present between the two particles, it was confirmed that the width between the two particles was widened and varied as compared with the case where the R ₂ O ₃ compound was not present. When present, the width between the two particles was 15 nm to 30 nm, and there was a portion of maximum 100 nm. When the compound was not present, the thickness was 10 nm or less.

  When the magnetization rates at 636 kA / m, 955 kA / m, 1273 kA / m, and 1591 kA / m were examined, 20% at 636 kA / m, 80% at 955 kA / m, 98% at 1273 kA / m, and 1591 kA / m. The magnetization rate was 100%.

  Further, when the residual magnetic flux density and the coercive force were measured with a BH tracer, the residual magnetic flux density was 1.44 T and the coercive force was 1240 kA / m.

[Example 3]
The composition after sintering is Nd 28.6. %, Dy 1.0% B 0.94%, Ga 0.08%, Al 0.1%, one type of RTB-based alloy powder that becomes the balance iron, and the oxygen content of the sintered body In order to make it 500 ppm or less, it was produced in the same manner as in Example 1 except that the melting chamber was made an oxygen-free atmosphere and the inside was melted with an alumina crucible coated with boron nitride to prevent mixing of impurities containing oxygen. When the surface of the sample was processed with FIB and observed with an SEM at a magnification of 3000 times at an acceleration voltage of 2 kV, the ratio of the number of crystal grains in which no R ₂ O ₃ compound was present to the entire crystal grains was 100%. The oxygen content of the sintered magnet was 400 ppm. The average crystal grain size (equivalent circle diameter) was 3.3 μm. The width between the two particles was less than 5 nm.

Next, the sintered magnet of the sample was cut, and when two particles of the sintered magnet were confirmed by TEM analysis by EELS map from the cross section, no R ₂ O ₃ compound was confirmed between the two particles as shown in FIG. It was.

[Comparative Example 1]
A sintered magnet was prepared in the same manner as in Example 1 except that the RTB-based alloy of Example 1 was not subjected to heat treatment and was dry pulverized with nitrogen gas. When the composition of the sintered magnet was examined, the composition was Nd 28.7% in mass%, Dy 1.0% B 0.94%, Co 0.87%, Ga 0.09%, Al 0.11%, Cu The balance was 0.09% iron, and the average crystal grain size (equivalent circle diameter) was 3.3 μm. The oxygen content was 1900 ppm. When the surface of the sample produced in the same manner as in Example 1 was processed with FIB and observed with an SEM at a magnification of 3000 times at an acceleration voltage of 2 kV, R ₂ O _{3 with} respect to the total number of crystal grains as shown in FIG. The ratio of the number of crystal grains in which no compound was present was 10%. Most of the grain boundary thicknesses were 15 nm to 25 nm, and the width varied.

  Further, when the magnetization rates at 636 kA / m, 955 kA / m, 1273 kA / m, and 1591 kA / m were examined, 15% at 636 kA / m, 65% at 955 kA / m, 85% at 1273 kA / m, and 1591 kA / m. The magnetization rate was 100% at m. Further, when the residual magnetic flux density and the coercive force were measured with a BH tracer, the residual magnetic flux density was 1.42 T and the coercive force was 1178 kA / m.

When the sintered magnet was cut and the two-particle interface of the sintered magnet was confirmed from the cross section by TEM analysis using EELS map, it was observed that there was an R ₂ O ₃ compound in a circled region in FIG. .

  The present invention produces an RTB-based sintered magnet with good magnetization that can achieve high magnetization even with a low magnetization field.

Claims (2)

In the R-T-B system sintered magnet,
The ratio of the number of crystal grains in which no R ₂ O ₃ compound is present in any of the two adjacent crystal grains to the total number of crystal grains constituting the sintered magnet is 50% to 100%. -TB sintered magnet.   The RTB-based sintered magnet according to claim 1, wherein the ratio is 70% to 100%.