JP2005286174A - R-t-b-based sintered magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an R-T-B-based sintered magnet that can obtain a high coercive force by constituting the magnet in a fine crystalline structure even when the oxygen content of the sintered body of the magnet is low and, at the same time, has a wide sintering temperature width. <P>SOLUTION: The R-T-B-based sintered magnet is composed of the sintered body which is provided with main-phase crystal grains composed of an R<SB>2</SB>T<SB>14</SB>B phase (wherein, R is one, two, or more kinds of elements selected from among rare-earth elements and T is one, two, or more kinds of elements selected from among transition metal elements including Fe or Fe and Co) and a grain boundary phase containing more R than the main-phase crystal grains do and contains an Nb-Fe compound in the crystal boundary phase. The R-T-B-based sintered magnet can be obtained by causing an alloy for forming the crystal boundary phase to contain Nb at the time of obtaining the R-T-B sintered magnet by using the mixing method. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to R (R is one or more elements selected from rare earth elements), T (T is one or more elements selected from transition metal elements including Fe or Fe and Co) ) And B (boron) as main components.

Among sintered magnets, RTB-based sintered magnets are excellent in magnetic properties, and Nd and Fe, which are main components, are abundant in resources and inexpensive. Demand for aircraft is increasing year by year. Nd-T-B characteristics have been continuously improved from the beginning of development. In recent years, when producing high-performance Nd-T-B, methods for reducing the amount of impurities, particularly oxygen, in the permanent magnet body have been intensively performed.
In order to reduce the oxygen content of the sintered magnet, it is important to suppress the oxidation of the material in each process as much as possible as well as the oxygen content of the raw material. However, low-oxygen materials are prone to abnormal grain growth during the sintering process, and high-quality magnets cannot be obtained. This is because the oxide contained in the magnet suppresses the growth of crystal grains.
In addition, since the magnetic properties of an R-T-B sintered magnet obtained by sintering depend on the sintering temperature, the temperature range is wide with respect to the sintering temperature in order to obtain the desired magnetic properties. It is important for industrial production scale.
JP-A-2002-75717 (Patent Document 1) discloses a magnet alloy in a sintering process by uniformly dispersing fine Zr-B, Nb-B, and Hf-B compounds in an RTB-based rare earth permanent magnet. Have been reported to suppress grain growth and improve magnetic properties and sintering temperature range.

JP 2002-75717 A Japanese Patent Publication No. 5-31807 Japanese Patent No. 3254229

According to Patent Document 1, the sintering temperature range is expanded by dispersing and precipitating the MB compound. However, in Example 3-1, disclosed in Patent Document 1, the sintering temperature width is as narrow as about 20 ° C. Therefore, it is desirable to further widen the sintering temperature range in order to obtain high magnetic characteristics in a mass production furnace or the like.
Accordingly, an object of the present invention is to suppress the growth of crystal grains and improve the magnetic properties, particularly the coercive force, in an RTB-based sintered magnet with a small amount of oxygen in which crystal grains are likely to grow. In addition, an object of the present invention is to provide an RTB-based sintered magnet having a high coercive force and a wide sintering temperature range.

In recent years, when manufacturing a high-performance RTB-based sintered magnet, a mixing method in which various metal powders and alloy powders having different compositions are mixed and sintered has been proposed (for example, Japanese Patent Publication No. 5-31807). Gazette (patent document 2), patent 3254229 gazette (patent document 3)). This mixing method typically includes an R ₂ T ₁₄ B intermetallic compound (R is one or more elements selected from rare earth elements, T is a transition metal element including Fe or Fe and Co). An alloy for forming a main phase mainly composed of one or more selected elements) and an alloy for forming a grain boundary phase existing between the main phases are mixed. Here, the main phase forming alloy is sometimes referred to as a low R alloy because the R content is relatively small. On the other hand, an alloy for forming a grain boundary phase is sometimes called a high-R alloy because of its relatively high R content.
When the present inventor contains Nb in a high R alloy and obtains an R-T-B sintered magnet by using a mixing method, it is effective for improving the coercive force and widening the sintering temperature range. I found.

The present invention is based on the above knowledge, and R ₂ T ₁₄ B phase (R is one or more elements selected from rare earth elements, T is selected from transition metal elements including Fe or Fe and Co) A main phase crystal grain composed of one or two or more elements) and a grain boundary phase containing more R than the main phase crystal grain, and a sintered body containing an Nb-Fe compound in the grain boundary phase. It is an RTB-based sintered magnet.
In the present invention, since the Nb—Fe compound has a high melting point, the coarsening of the main phase crystal grains in the sintering process can be suppressed. As a result, the sintered structure becomes fine and the coercive force can be improved.

In the present invention, the effect of improving the coercive force by containing Nb in the low R alloy is effective when the amount of oxygen contained in the sintered body is as low as 2000 ppm or less.
In the RTB-based sintered magnet of the present invention, Nb is desirably contained in the range of 0.1 to 2 wt%, and R: 25 to 35 wt% and B: 0.5 to 4.5 wt%. 1 or 2 of Al and Cu, 0.02 to 0.6 wt%, Nb: 0.1 to 2 wt%, Co: 4 wt% or less (excluding 0), and the balance substantially consisting of Fe It is desirable to do.

  According to the RTB-based sintered magnet of the present invention, a high coercive force can be obtained by forming a fine crystal structure even when the sintered body has a low oxygen content. Moreover, according to the RTB-based sintered magnet of the present invention, it has a wide sintering temperature range.

<Organization>
First, the structure of the RTB-based sintered magnet, which is a feature of the present invention, will be described.
As is well known, the RTB-based sintered magnet according to the present invention includes a main phase crystal grain composed of an R ₂ T ₁₄ B compound and a grain boundary phase containing more R than the main phase crystal grain. And at least. The present invention is characterized in that Nb is present in the grain boundary phase. In particular, this Nb is present in the grain boundary phase as an Nb-Fe compound. The Nb—Fe compound has a high melting point of 1400 to 1600 ° C. Thus, when the high melting point compound exists in the grain boundary phase, coarsening of the main phase crystal grains in the sintering process can be suppressed, and as a result, the structure after sintering can be made fine. As a result, a high coercive force can be obtained.

<Chemical composition>
Next, a desirable chemical composition of the RTB-based sintered magnet according to the present invention will be described. The chemical composition here refers to the chemical composition after sintering.
The RTB-based sintered magnet of the present invention contains 25 to 35 wt% of R.
Here, R has a concept including Y, and one or two selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, and Y More than a seed element. When the amount of R is less than 25 wt%, the generation of R ₂ T ₁₄ B main phase crystal grains that are the main phase of the R-T-B sintered magnet is not sufficient. For this reason, α-Fe or the like having soft magnetism is precipitated, and the coercive force is remarkably lowered. On the other hand, when the amount of R exceeds 35 wt%, the volume ratio of R ₂ T ₁₄ B main phase crystal grains constituting the main phase decreases, and the residual magnetic flux density decreases. On the other hand, when the amount of R exceeds 35 wt%, R reacts with oxygen, and the amount of oxygen contained increases, and as a result, the R-rich phase effective for the generation of coercive force decreases and the coercive force decreases. Therefore, the amount of R is set to 25 to 35 wt%. A desirable amount of R is 28 to 33 wt%, and a more desirable amount of R is 29 to 32 wt%.

  Since Nd is abundant in resources and relatively inexpensive, it is preferable that the main component as R is Nd. Further, the inclusion of Dy is effective in improving the coercive force because it increases the anisotropic magnetic field. Therefore, it is desirable that Nd and Dy are selected as R and the total of Nd and Dy is 25 to 35 wt%. In this range, the amount of Dy is preferably 0.1 to 12 wt%. It is desirable to determine the amount of Dy within the above range depending on which of the residual magnetic flux density and the coercive force is important. That is, when it is desired to obtain a high residual magnetic flux density, the Dy amount is preferably 0.1 to 3 wt%, and when it is desired to obtain a high coercive force, the Dy amount is desirably 3 to 12 wt%.

  The RTB-based sintered magnet of the present invention contains 0.5 to 4.5 wt% of boron (B). When B is less than 0.5 wt%, a high coercive force cannot be obtained. However, when B exceeds 4.5 wt%, the residual magnetic flux density tends to decrease. Therefore, the upper limit is 4.5 wt%. A desirable amount of B is 0.5 to 1.5 wt%, and a more desirable amount of B is 0.8 to 1.2 wt%.

  The RTB-based sintered magnet of the present invention can contain one or two of Al and Cu in the range of 0.02 to 0.6 wt%. By containing one or two of Al and Cu within this range, it is possible to increase the coercive force, corrosion resistance, and temperature characteristics of the obtained rare earth permanent magnet. In the case of adding Al, a desirable amount of Al is 0.03 to 0.3 wt%, and a more desirable amount of Al is 0.05 to 0.25 wt%. In addition, in the case of adding Cu, the amount of Cu is 0.3 wt% or less (excluding 0), desirably 0.15 wt% or less (not including 0), and the more desirable amount of Cu is 0.03 to 0 0.08 wt%.

  The RTB-based sintered magnet of the present invention contains 0.1 to 2 wt% Nb. When reducing the oxygen content in order to improve the magnetic properties of the RTB-based sintered magnet, Nb exhibits the effect of suppressing the abnormal growth of the main phase crystal grains during the sintering process. Make the body tissue uniform and fine. Therefore, Nb has a remarkable effect when the amount of oxygen is low. A desirable amount of Nb is 0.2 to 1.7 wt%, and a more desirable amount is 0.3 to 1.5 wt%.

  The RTB-based sintered magnet of the present invention has an oxygen content of 2000 ppm or less. When the amount of oxygen is large, the oxide phase, which is a nonmagnetic component, increases and the magnetic properties are deteriorated. Therefore, in the present invention, the amount of oxygen contained in the sintered body is set to 2000 ppm or less, desirably 1500 ppm or less, and more desirably 1000 ppm or less. However, when the oxygen amount is simply reduced, the oxide phase having the effect of suppressing grain growth is insufficient, and grain growth easily occurs in the process of obtaining a sufficient density increase during sintering. Therefore, in the present invention, a predetermined amount of Nb that exhibits the effect of suppressing abnormal growth of main phase crystal grains during the sintering process is contained in the RTB-based sintered magnet, particularly in the grain boundary phase. However, even if Nb is present in the main phase crystal grains, the effect of the present invention is not hindered.

  The RTB-based sintered magnet of the present invention contains 4 wt% or less (not including 0) of Co, desirably 0.1 to 2 wt%, and more desirably 0.3 to 1 wt%. Co forms the same phase as Fe, but is effective in improving the Curie temperature and improving the corrosion resistance of the grain boundary phase.

<Manufacturing method>
Next, the suitable manufacturing method of the RTB system sintered magnet by this invention is demonstrated.
In the present embodiment, R according to the present invention is made using an alloy mainly composed of R ₂ T ₁₄ B main phase grains (low R alloy) and an alloy containing more R than the low R alloy (high R alloy). -A TB sintered magnet is manufactured.
First, a low R alloy and a high R alloy are obtained by strip casting of the raw metal in a vacuum or an inert gas, preferably an Ar atmosphere, and other methods. As the raw material metal, rare earth metals or rare earth alloys, pure iron, ferroboron, and alloys thereof can be used. The obtained raw material alloy is subjected to a solution treatment as necessary when there is solidification segregation. The conditions may be maintained for 1 hour or longer in a region of 700 to 1500 ° C. in a vacuum or Ar atmosphere.

A characteristic feature of the present invention is that Nb is added from a high R alloy. As described in the section of <structure>, this adds Nb from a high R alloy to generate an Nb-Fe compound in the grain boundary phase and suppress abnormal growth of main phase crystal grains.
As long as the low R alloy is mainly composed of R ₂ T ₁₄ B main phase grains, it is not necessary to limit the composition thereof, but it is desirable that R be in the range of 25 to 35 wt%. Moreover, as for a high R alloy, it is desirable to make R into the range of 35-55 wt%.
In addition to R, T, and B, the low R alloy can contain Cu and Al. At this time, the low R alloy constitutes an R-Cu-Al-T (Fe) -B alloy. Further, the high R alloy can contain Cu, Co and Al in addition to R and T (Fe). At this time, the high R alloy constitutes an R—Cu—Co—Al—Nb—T (Fe—Co) based alloy.

  After the low R and high R alloys are made, each of these master alloys is ground separately or together. The pulverization process includes a coarse pulverization process and a fine pulverization process. First, each mother alloy is coarsely pulverized until the particle size becomes about several hundred μm. Coarse pulverization can be performed in an inert gas atmosphere using mechanical means such as a stamp mill, a jaw crusher, and a brown mill. Further, coarse pulverization can be performed by occluding hydrogen without using mechanical means. Furthermore, after hydrogen storage, coarse pulverization can be performed by mechanical means. Which method is used depends on the form of the master alloy. It is desirable to dehydrogenate after hydrogen storage. This is because hydrogen is an impurity for the RTB-based sintered magnet.

  After the coarse pulverization process, the process proceeds to the fine pulverization process. In the fine pulverization, a jet mill is mainly used, and a coarsely pulverized powder having a particle size of about several hundreds of μm is pulverized until the average particle size becomes 3 to 5 μm. The jet mill opens a high-pressure non-oxidizing gas (for example, nitrogen gas) from a narrow nozzle to generate a high-speed gas flow, accelerates the coarsely pulverized powder with this high-speed gas flow, This is a method of crushing by generating a collision with a target or a container wall.

  When the low R alloy and the high R alloy are separately pulverized in the fine pulverization step, the finely pulverized low R alloy powder and high R alloy powder are mixed in a nitrogen atmosphere. The mixing ratio of the low R alloy powder and the high R alloy powder may be about 80:20 to 97: 3 by weight. Similarly, the mixing ratio when the low R alloy and the high R alloy are finely pulverized together may be about 80:20 to 97: 3 by weight. By adding about 0.01 to 0.3 wt% of an additive such as zinc stearate at the time of fine pulverization, a fine powder having high orientation during molding can be obtained.

  Next, the mixed powder composed of the low R alloy powder and the high R alloy powder is pressure-molded in a state where the crystal axis is oriented by applying a magnetic field. The forming in the magnetic field may be performed at a pressure of 70 to 150 MPa in a magnetic field of 940 to 1400 kA / m.

  After molding in a magnetic field, the compact is sintered in a vacuum or an inert gas atmosphere. Although it is necessary to adjust sintering temperature by various conditions, such as a composition, a grinding | pulverization method, a particle size, and a particle size distribution difference, what is necessary is just to sinter at 1000-1100 degreeC for about 1 to 5 hours.

  After sintering, the obtained sintered body can be subjected to an aging treatment. The aging treatment is important for controlling the coercive force. When the aging treatment is performed in two stages, it is effective to hold for a predetermined time near 800 ° C. and 600 ° C. When the heat treatment at around 800 ° C. is performed after sintering, the coercive force increases, which is particularly effective in the mixing method. In addition, since the coercive force is greatly increased by the heat treatment at around 600 ° C., the aging treatment at around 600 ° C. is preferably performed when the aging treatment is performed in one stage.

Next, the present invention will be described in more detail with specific examples. In the following, the RTB-based sintered magnet according to the present invention will be described by dividing it into Example 1 to Example 2. However, since the prepared raw material alloy and each manufacturing process are common, this point is first introduced. Let me explain.
1) Raw material alloys Low R alloys and high R alloys having the compositions shown in Table 1 were prepared by strip casting.
2) Pulverization Step After occluding hydrogen at room temperature, a hydrogen pulverization treatment was performed in which dehydrogenation was performed at 600 ° C. for 1 hour in an Ar atmosphere.
In order to obtain high magnetic properties, in this experiment, the atmosphere of each process from hydrogen treatment (recovery after pulverization) to sintering (put into the sintering furnace) to suppress the amount of oxygen in the sintered body to 2000 ppm or less Is suppressed to an oxygen concentration of less than 100 ppm. Hereinafter, it is referred to as an oxygen-free process.
A low R alloy and a high R alloy which have been subjected to hydrogen storage treatment, and further additives are mixed. The type of the additive is not particularly limited, and any additive that contributes to improvement in grindability and orientation during molding may be appropriately selected. In this example, zinc stearate is added in an amount of 0.05 to 0. 1 wt% was mixed. The additive may be mixed for about 5 to 30 minutes using, for example, a Nauter mixer.
Thereafter, fine grinding was performed using a jet mill until the alloy powder had an average particle size of about 4.0 μm.
Of course, the mixing step and the fine pulverization step of the low R alloy, the high R alloy and the additive are both performed by an oxygen-free process.

  Before fine pulverization, a plurality of types of low R alloys can be prepared and mixed so as to have a desired composition (particularly, Nb amount) as necessary to obtain a desired composition. The mixing in this case may be performed only for about 5 to 30 minutes using, for example, a Nauta mixer.

3) Molding step The obtained fine powder is molded in a magnetic field. Specifically, molding was performed at a pressure of 120 MPa in a magnetic field of 1200 kA / m to obtain a molded body. This step was also performed by an oxygen-free process.
4) Sintering and aging process This molded body was sintered in vacuum at 1020 to 1080 ° C for 4 hours and then rapidly cooled. Next, the obtained sintered body was subjected to a two-stage aging treatment of 800 ° C. × 1 hour and 540 ° C. × 1 hour (both in an Ar atmosphere).

Using the low R alloy A1, low R alloy A2, high R alloy B1, high R alloy B2, and high R alloy B3 shown in Table 1, 24.6 wt% Nd-6.0 wt% Dy-0.5 wt% Co- 0.2 wt% Al-0.07 wt% Cu-1.0 wt% B- (0, 0.25, 0.50, 0.75, 1.0, 1.5, 2.0, 3.0) wt % Nb-bal. It mix | blended so that it might become the final composition of Fe. Thereafter, each mixture was subjected to hydrogen pulverization treatment and then pulverized in a jet mill. Then, after molding in a magnetic field, sintering (holding for 4 hours) at 1060 ° C. (when Nb content is 0.75 wt%, sintering is also performed at 1020 ° C., 1040 ° C., and 1080 ° C.). Aged.
About the obtained RTB system sintered magnet, the residual magnetic flux density (Br), the coercive force (HcJ), and the squareness ratio (Hk / HcJ) were measured with a BH tracer. Hk is the external magnetic field strength when the magnetic flux density is 90% of the residual magnetic flux density in the second quadrant of the magnetic hysteresis loop. Moreover, about the obtained RTB system sintered magnet, the amount of oxygen contained in the sintered body was measured. The above measurement results are shown in Table 2.

  FIG. 1 shows the relationship between the sintering temperature, the residual magnetic flux density (Br), the coercive force (HcJ), and the squareness ratio (Hk / HcJ). FIG. 2 shows the relationship between the Nb amount, the residual magnetic flux density (Br), the coercive force (HcJ), and the squareness ratio (Hk / HcJ).

In FIG. 1, particularly a graph showing the relationship between the sintering temperature and the coercive force (HcJ), the sintered magnet containing Nb has a higher coercive force (HcJ) than the sintered magnet containing no Nb. It can be seen that the decrease in the coercive force (HcJ) due to the change in the sintering temperature of the sintered magnet containing Nb is smaller than the decrease in the coercive force (HcJ) due to the change in the sintering temperature of the sintered magnet not containing Ni. That is, it can be seen that by adding Nb, the coercive force (HcJ) is improved and the sintering temperature range is widened.
However, as shown in FIG. 2, the coercive force (HcJ) increases as the Nb amount increases, but the residual magnetic flux density (Br) tends to decrease. Moreover, when the amount of Nb exceeds 2 wt%, the squareness ratio (Hk / HcJ) becomes lower than when Nb is not contained. Therefore, in the present invention, in order to ensure the magnetic characteristics, particularly the residual magnetic flux density (Br) and the squareness ratio (Hk / HcJ), the Nb content is desirably 2.0 wt% or less.

  FIG. 3 shows an SEM (scanning electron microscope, 1000 times) image of a fractured surface of a sintered magnet having a sintering temperature of 1060 ° C., compared with a sintered magnet not containing Nb (FIG. 3A). It can be seen that a sintered magnet containing 0.75 wt% Nb (FIG. 3B) shows a finer crystal structure.

  FIG. 4 shows a TEM (transmission electron microscope) image of a sintered magnet containing 0.75 wt% Nb and sintered at 1060 ° C. In FIG. 4, it is confirmed that a region surrounded by a dotted line is a grain boundary phase, and a plurality of compounds containing Nb and Fe exist in the grain boundary phase. It is understood that a fine crystal structure as shown in FIG. 3 can be obtained when this Nb—Fe compound suppresses the growth of main phase crystal grains.

  An R-T-B sintered magnet was obtained in the same manner as in Example 1 except that the alloy shown in Table 1 was used so that the final composition shown in Table 3 was obtained. Similarly, the magnetic characteristics were measured. The results are also shown in Table 3.

  As shown in Table 3, it can be seen that the coercive force (HcJ) can be improved by increasing the amount of Al and the amount of Cu. It can also be seen that the coercive force (HcJ) can be improved by increasing the amount of Dy. Therefore, according to the present invention, in addition to the effect of improving the coercive force (HcJ) due to the presence of the Nb—Fe compound in the grain boundary phase, the coercive force (HcJ) is particularly improved by adjusting the amounts of Al, Cu and Dy. R-T-B system sintered magnet having a high C can be obtained. Furthermore, it can be seen that the magnetic characteristics fluctuate by increasing / decreasing the R (Nd + Dy) amount and the B amount.

It is a graph which shows the relationship between the sintering temperature in Example 1, residual magnetic flux density (Br), coercive force (HcJ), and squareness ratio (Hk / HcJ). It is a graph which shows the relationship between Nb amount in Example 1, residual magnetic flux density (Br), coercive force (HcJ), and squareness ratio (Hk / HcJ). 2 is a SEM (scanning electron microscope, 1000 times) image of a fractured surface of an RTB-based sintered magnet in Example 1. FIG. 2 is a TEM (transmission electron microscope) image of an RTB-based sintered magnet in Example 1. FIG.

Claims (4)

From the R ₂ T ₁₄ B phase (R is one or more elements selected from rare earth elements, T is one or more elements selected from transition metal elements including Fe or Fe and Co) R-T-, comprising a main phase crystal grain and a grain boundary phase containing more R than the main phase crystal grain, and comprising a sintered body containing an Nb-Fe compound in the grain boundary phase. B-based sintered magnet.   2. The RTB-based sintered magnet according to claim 1, wherein the sintered body has an oxygen content of 2000 ppm or less.   The RTB-based sintered magnet according to claim 1 or 2, wherein the sintered body contains 0.1 to 2 wt% of Nb.   The sintered body is R: 25 to 35 wt%, B: 0.5 to 4.5 wt%, one or two of Al and Cu, 0.02 to 0.6 wt%, Nb: 0.1 to 2 wt% %, Co: 4 wt% or less (excluding 0), and the balance being substantially composed of Fe, the RTB-based sintered magnet according to claim 1.

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