JP2007266026A - Manufacturing method of rare-earth sintered magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a rare-earth sintered magnet capable of restraining a decrease in magnetic characteristics, and obtaining low-cost and high-characteristic rare-earth sintered magnet. <P>SOLUTION: Fine pulverization and formation in a magnetic field are performed under a low-oxygen atmosphere having an oxygen concentration of not more than 100 ppm. In this case, before the fine pulverization, a low-viscosity lubricant of not more than 10 mPa s, such as octanoic acid, octanoic acid ethyl, octanoic acid methyl, lauric acid ethyl, and lauric acid butyl is added to raw material alloy powder, thus restraining the amount of nitrogen contained in the powder and suppressing a decrease in magnetic characteristics. Processes for fine pulverization and formation in a magnetic field are performed in a nitrogen gas atmosphere, thus reducing oxygen concentration to not more than 100 ppm and restraining manufacturing costs. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a rare earth sintered magnet.

  Among the rare earth sintered magnets, R-T-B rare earth sintered magnets are excellent in magnetic properties, and Nd, which is the main component, is abundant in resources and relatively inexpensive, so demand is increasing year by year. is doing. Although research and development for improving the magnetic characteristics of the R-T-B rare earth sintered magnet has been energetically performed, higher performance is always desired.

  One technique for further improving the performance of an R-T-B rare earth sintered magnet is to reduce the oxygen content of the rare earth sintered magnet. Therefore, processes from pulverization to sintering are performed in a low oxygen atmosphere such as in an inert gas such as argon or nitrogen, or in a vacuum (see, for example, Patent Documents 1 and 2).

JP 2002-57052 A Japanese Patent No. 3240034

However, when nitrogen gas is selected as the inert gas, the amount of nitrogen contained in the powder increases particularly during the pulverization process, leading to a decrease in magnetic properties.
In addition, when argon gas is selected, there is a problem that argon is not suitable for mass production because it is expensive and affects the manufacturing cost.
An object of this invention is to provide the manufacturing method of the rare earth sintered magnet which can suppress the fall of a magnetic characteristic and can obtain the rare earth sintered magnet of a high characteristic at low cost.

Accordingly, the method for producing a rare earth sintered magnet of the present invention is such that a raw material alloy powder having a predetermined composition is added with a lubricant having an organic substance as a component and an viscosity of 10 mPa · s or less in an atmosphere having an oxygen concentration of 100 ppm or less. A finely pulverizing step in which the pulverized powder is finely pulverized in a state in which the pulverized powder is obtained in a magnetic field by molding the pulverized powder obtained in the fine pulverizing step in an atmosphere having an oxygen concentration of 100 ppm or less; And a sintering step for sintering.
Thus, when fine pulverization and molding in a magnetic field are performed in a low oxygen atmosphere with an oxygen concentration of 100 ppm or less, the viscosity of the lubricant added before the fine pulverization step is as low as 10 mPa · s. Is effective in reducing the amount of nitrogen contained in the pulverized powder. In particular, the present invention is effective when the fine pulverization step and the molding step in a magnetic field are performed in a nitrogen gas atmosphere.
Such lubricants include fatty acids or fatty acid esters, and specifically, octanoic acid, ethyl octoate, methyl octoate, ethyl laurate, butyl laurate and the like are suitable.
Further, in the present invention, prior to the sintering step, the molded body may further include a lubricant removing step of removing the lubricant by heat treatment under an atmosphere gas containing hydrogen (H ₂ ). preferable. Thereby, the amount of residual carbon can be reduced and the magnetic characteristics can be improved. At this time, it is preferable to perform heat processing at 100-550 degreeC.

  According to the present invention, when fine pulverization and molding in a magnetic field are performed in a low oxygen atmosphere, the amount of nitrogen contained in the pulverized powder can be suppressed, so that deterioration in magnetic properties can be suppressed. In addition, since the present invention is particularly effective when nitrogen is used to realize a low oxygen atmosphere, it is possible to obtain a rare earth sintered magnet having high characteristics at low cost.

Hereinafter, embodiments of the present invention will be described in detail by taking a method for producing a rare earth sintered magnet as an example.
Rare earth sintered magnets are usually produced through the basic steps of producing a raw material alloy, grinding the raw material alloy, forming the pulverized powder in a magnetic field, and sintering the compact. Hereinafter, the manufacturing method will be described in the order of steps including the lubricant removal treatment step which is a characteristic part of the present invention.

  The raw material alloy can be produced by a strip casting method or other known melting methods in a vacuum or an inert gas, preferably in an Ar atmosphere. In the strip casting method, a molten metal obtained by melting a raw metal in a non-oxidizing atmosphere such as an Ar gas atmosphere is ejected onto the surface of a rotating roll. The melt rapidly cooled by the roll is rapidly solidified in the form of a thin plate or flakes (scales). This rapidly solidified alloy has a homogeneous structure with a crystal grain size of 1 to 50 μm. The raw material alloy can be obtained not only by the strip casting method but also by a melting method such as high frequency induction melting.

  The raw material alloy is subjected to a grinding process. The pulverization process includes a coarse pulverization process and a fine pulverization process. First, the raw material alloy is coarsely pulverized until the particle size becomes about several hundred μm. The coarse pulverization is preferably performed in an inert gas atmosphere using a stamp mill, a jaw crusher, a brown mill or the like. Prior to coarse pulverization, it is effective to perform pulverization by allowing the raw material alloy to store hydrogen and then discharging it. This hydrogen pulverization can be regarded as coarse pulverization, and mechanical coarse pulverization can be omitted. In this case, for example, the raw material alloy obtained by the strip casting method is subjected to hydrogen pulverization in a state of being cut into a size of several mm to several tens mm.

  After the coarse pulverization process, the process proceeds to the fine pulverization process. A jet mill is mainly used for the fine pulverization, and the coarsely pulverized powder having a particle size of about several hundred μm is made into a fine powder having an average particle size of 2.5 to 6 μm, preferably 3 to 5 μm. The jet mill releases a high-pressure inert gas from a narrow nozzle to generate a high-speed gas flow, accelerates the coarsely pulverized powder with this high-speed gas flow, collides with the coarsely pulverized powder, and collides with the target or the container wall. It is a method of generating a collision and crushing.

Here, at the raw material alloy stage, an alloy mainly composed of the R ₂ T ₁₄ B phase (low R alloy) and an alloy containing more R than the low R alloy (high R alloy) are used. After pulverization, these can be mixed to obtain a fine powder. In that case, 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.

Prior to the fine pulverization, about 0.01 to 0.5 wt% of a lubricant containing an organic substance is added. As a result, fine powder having a desired particle diameter can be efficiently produced in the fine pulverization step, and in addition to the effect that fine powder having high orientation can be obtained at the time of molding in the next magnetic field, nitrogen contained in the fine pulverized powder The amount can be reduced. As the lubricant to be added before pulverization, it is preferable to use a fatty acid or a fatty acid ester. Among them, it is particularly preferable to use a lubricant having a viscosity of 10 mPa · s (10 cP) or less. Such lubricants include octanoic acid, ethyl octoate, methyl octoate, ethyl laurate, butyl laurate and the like.
Further, a lubricant can be added after pulverization. The lubricant added after pulverization is added to improve the degree of orientation.

The fine powder obtained as described above is subjected to molding in a magnetic field. The molding in the magnetic field may be performed at a pressure of about 0.5 to 2.0 ton / cm ² (50 to 200 MPa) in a magnetic field of 10.1 to 25 kOe (800 to 1975 kA / m). The applied magnetic field is not limited to a static magnetic field, and a pulsed magnetic field can be used. Furthermore, the direction of the magnetic field to be applied may be either a direction parallel to the pressing direction or a direction orthogonal to the pressing direction.

The molded body obtained as described above contains the lubricant described above. Since this lubricant reacts with a rare earth element, the amount of the rare earth element is insufficient as an R—Fe—B based sintered magnet, thereby deteriorating magnetic properties. In addition, when a large amount of lubricant is contained, shrinkage during sintering becomes non-uniform in the sintered body and there is a risk of deformation after sintering.
Therefore, it is preferable to perform the lubricant removal treatment for removing the lubricant by heat treatment. At this time, if the lubricant removal treatment is performed on the molded body under an atmosphere gas containing hydrogen, the amount of carbon remaining in the molded body is rapidly reduced compared to the lubricant removal treatment in a vacuum or an inert gas atmosphere. can do. Moreover, it is preferable to hold | maintain the heat processing for a lubricant removal process in the temperature range of 100-550 degreeC. This is because the effect of removing the lubricant cannot be sufficiently obtained when the temperature is lower than 100 ° C., and the effect is saturated when the temperature exceeds 550 ° C. Here, holding in the temperature range of 100 to 550 ° C. is not limited to holding the molded body at a constant temperature in the temperature range, and the molded body is heated to any temperature in the temperature range for a predetermined time. It only has to be. Therefore, various forms such as a form in which the temperature is continuously increased over 100 to 550 ° C. and a form in which the temperature is raised stepwise in the range of 100 to 550 ° C. are included. A preferable heat treatment temperature is 100 to 450 ° C., and a more preferable heat treatment temperature is 100 to 400 ° C.

  If the holding time of the heat treatment for the lubricant removing process is short, the effect of removing the lubricant is insufficient, while if the holding time is too long, the effect of removing the lubricant is saturated. Therefore, the heat treatment holding time is preferably 0.5 to 10 hours, and more preferably 1 to 7 hours.

The molded body that has been subjected to the above lubricant removal treatment is subjected to sintering. Sintering is performed in a vacuum or an inert gas atmosphere, preferably in a vacuum. Sintering conditions need to be adjusted according to various conditions such as composition, pulverization method, difference in average particle size and particle size distribution, etc., but a dense sintered body can be maintained at a temperature of 1000 to 1100 ° C. for about 1 to 10 hours. Can be obtained.
After sintering, the obtained sintered body can be subjected to an aging treatment. This process is an important process for controlling the coercive force. When the aging treatment is performed in two stages, holding for a predetermined time at 750 to 950 ° C. and 500 to 700 ° C. is effective. Further, since the coercive force is greatly increased by heat treatment at 500 to 700 ° C., the aging treatment at 500 to 700 ° C. is preferably performed when the aging treatment is performed in one stage.

  In order to obtain high magnetic properties, the atmosphere in each step from hydrogen pulverization (recovery after pulverization) to sintering (put into the sintering furnace) is suppressed to an oxygen concentration of less than 100 ppm, and in a low oxygen atmosphere Perform each process.

The present invention is preferably applied to an RTB-based sintered magnet represented by RTB (R is one or more rare earth elements, and T is Fe or Fe and Co).
The RTB-based sintered magnet contains 25 to 37 wt% of rare earth element (R). Here, R has a concept including Y. Therefore, one or two of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Selected from more than species. If the amount of R is less than 25 wt%, the R ₂ T ₁₄ B phase, which is the main phase of the RTB-based sintered magnet, is not sufficiently generated, and α-Fe having soft magnetism is precipitated and retained. The magnetic force is significantly reduced. On the other hand, when R exceeds 37 wt%, the volume ratio of the R ₂ T ₁₄ B phase, which is the main phase, decreases, and the residual magnetic flux density decreases. Further, R reacts with oxygen, the amount of oxygen contained increases, and accordingly, the R-rich phase effective for the generation of coercive force decreases, leading to a decrease in coercive force. Therefore, the amount of R is set to 25 to 37 wt%. A preferable amount of R is 28 to 35 wt%.

Further, the RTB-based sintered magnet to which the present invention is applied 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. On the other hand, when B exceeds 4.5 wt%, the residual magnetic flux density tends to decrease. Therefore, the upper limit of B is set to 4.5 wt%. A preferable amount of B is 0.5 to 1.5 wt%, and a more preferable amount of B is 0.8 to 1.2 wt%.
The RTB-based sintered magnet to which the present invention is applied can contain Co of 5.0 wt% or less (excluding 0), preferably 0.1 to 3.0 wt%. Co forms the same phase as Fe, but is effective in improving the Curie temperature and the corrosion resistance of the grain boundary phase.

  The RTB-based sintered magnet to which the present invention is applied allows the inclusion of other elements. For example, elements such as Al, Cu, Zr, Ti, Bi, Sn, Ga, Nb, Ta, Si, V, Ag, and Ge can be appropriately contained. On the other hand, it is preferable to reduce impurity elements such as oxygen, nitrogen, and carbon as much as possible. In particular, the amount of oxygen that impairs magnetic properties is preferably 2000 ppm or less, more preferably 1000 ppm or less. This is because when the amount of oxygen is large, the rare-earth oxide phase, which is a nonmagnetic component, increases and the magnetic properties are deteriorated.

  The R-T-B system sintered magnet has been described above. However, those skilled in the art can apply the present invention to other rare earth sintered magnets, and other sintered bodies other than magnets. It is clear from the above description or the description of the following examples.

The following raw material alloys were produced by strip casting.
Composition: 28.0 wt% Nd-0.1 wt% Pr-0.01 wt% Co-0.1 wt% Al-0.04 wt% Cu-1.0 wt% B-0.2 wt% Zr-bal. Fe
The obtained raw material alloy was hydrogen crushed to obtain raw material alloy coarse powder. The lubricant shown in Table 1 was added to the raw material alloy coarse powder. At this time, the amount of lubricant added was 0.1 wt%. The viscosity of the added lubricant was measured prior to the addition. The viscosity was measured using “DV-1 + LV series” manufactured by Brookfield. The temperature at the time of measurement was 20.0 ° C., and the spindle and the rotational speed were changed according to the respective viscosities.
In this example, after the hydrogen pulverization treatment, the atmosphere of each step from the collection of the raw material alloy coarse powder to the introduction to the sintering furnace for sintering is an oxygen concentration of less than 100 ppm by substitution with a nitrogen atmosphere. It is suppressed to.

Next, using an airflow pulverizer (jet mill), pulverization was performed in a high-pressure nitrogen gas atmosphere so that the average particle diameter was about 4 μm.
The nitrogen analysis value of the obtained fine powder was measured. The measurement results are shown in Table 1.

  By adding octanoic acid, ethyl octoate, methyl octoate, ethyl laurate, and butyl laurate that are liquid and have a viscosity of less than 10 mPa · s to solid octoamide and oleic amide, the amount of nitrogen is reduced. It can be seen that it can be reduced. On the other hand, when lauric acid, oleic acid, methyl oleate, and ethyl oleate having a viscosity exceeding 10 mPa · s while being liquid are added, nitrogen is compared with the case where solid octanoic acid amide or oleic acid amide is used. It was confirmed that the amount of decrease was small.

Subsequently, in the same manner as in Example 1, octanoic acid having a viscosity of less than 10 mPa · s (Example) and solid octanoic acid amide (Comparative Example) were used (the addition amount was 0.1 wt%), and the average was used. A fine powder having a particle size of 3.5 to 6.3 μm was prepared.
And the nitrogen analysis value of the obtained fine powder was measured. The measurement results are shown in FIG.

  As shown in FIG. 1, when pulverization is performed in a low-oxygen nitrogen atmosphere, when octanoic acid amide is used, the nitrogen amount tends to increase as the particle size of the fine powder becomes finer. On the other hand, when octanoic acid having a viscosity of less than 10 mPa · s is used, it can be seen that an increase in the amount of nitrogen can be suppressed regardless of the refinement of the fine particle diameter.

Further, the fine powder was molded in a magnetic field to obtain a molded body having a predetermined shape. In molding in a magnetic field, the fine powder was molded at a molding pressure of 1.5 ton / cm ² (147 MPa) in a magnetic field of 22 kOe (1750 kA / m). The magnetic field direction was perpendicular to the pressing direction. The size of the molded body was 20 mm × 18 mm × 12 mm.

The obtained molded body was sintered and aged to obtain a sintered body. Sintering was performed under the condition of holding at 1090 ° C. for 4 hours in vacuum, and the aging treatment was a two-stage aging treatment of holding at 900 ° C. for 1 hour in an Ar atmosphere and then holding at 540 ° C. for 1 hour.
With respect to the obtained sintered body, the holding force HcJ was measured. The result is shown in FIG.

  When octanoic acid amide is used, the coercive force HcJ tends to improve as the particle size of the fine powder becomes smaller as a whole, but the coercive force HcJ tends to decrease in the region where the fine particle size becomes finer. . From the relationship with FIG. 1, this is considered to be influenced by the amount of nitrogen in the fine powder. On the other hand, when octanoic acid having a viscosity of less than 10 mPa · s is used, the coercive force HcJ is improved as the particle size is reduced regardless of the particle size of the fine powder, particularly in a region where the fine particle size is 4 μm or less. The coercive force HcJ is remarkably increased as compared with the case of using octanoic acid amide.

Subsequently, 0.1 wt% of octanoic acid having a viscosity of less than 10 mPa · s was added in the same manner as in Example 1 to produce a fine powder having an average particle size of 4 μm to obtain a molded body.
And the lubricant removal process was performed about the obtained molded object. The condition is that after raising the temperature to 100, 200, 300, 400, 500, and 600 ° C. in an argon atmosphere, replacement with an atmosphere gas containing 95% or more of hydrogen is performed, and heating is performed for 2 hours in that state. It is to process.
The obtained molded body was sintered and aged to obtain a sintered body. Sintering was performed under the condition of holding at 1090 ° C. for 4 hours in vacuum, and the aging treatment was a two-stage aging treatment of holding at 900 ° C. for 1 hour in an Ar atmosphere and then holding at 540 ° C. for 1 hour.
For comparison, a sintered body obtained by sintering and aging the molded body without preparing the lubricant was prepared.
About the obtained sintered compact, the amount of residual carbon was measured. The result is shown in FIG. In FIG. 3, the temperature indicated at 0 ° C. is the amount of residual carbon when the lubricant removal process is not performed.

  As shown in FIG. 3, it can be seen that the residual carbon amount is reduced by the lubricant removal treatment, and particularly when the lubricant removal treatment temperature is 100 to 500 ° C., the residual carbon amount is significantly lower than 600 ppm.

For other lubricants, the effect of the lubricant removal treatment was confirmed as in Example 3.
In the same manner as in Example 1, 0.1 wt% each of octanoic acid, ethyl octoate, methyl octoate, ethyl laurate, and butyl laurate having a viscosity of less than 10 mPa · s was added, and the average particle size was 4 μm. A fine powder was produced to obtain a molded body.
And the lubricant removal process was performed about the obtained molded object. The condition is that after raising the temperature to 180 ° C. in an argon atmosphere, replacement with an atmospheric gas containing 95% or more of hydrogen is performed, and heat treatment is performed in that state for 2 hours.
For comparison, a sintered body obtained by sintering and aging the molded body without preparing the lubricant was prepared.
The obtained molded body was sintered and aged to obtain a sintered body. Sintering was performed under the condition of holding at 1090 ° C. for 4 hours in vacuum, and the aging treatment was a two-stage aging treatment of holding at 900 ° C. for 1 hour in an Ar atmosphere and then holding at 540 ° C. for 1 hour.
With respect to the obtained sintered body, magnetic properties (residual magnetic flux density Br, coercive force HcJ), residual oxygen amount, residual carbon amount, and residual nitrogen amount were measured. The results are shown in Table 2.

  As shown in Table 2, even when any lubricant is used, octanoic acid, ethyl octoate, methyl octoate, methyl laurate, butyl laurate, and the lubricant removal treatment reduces the residual carbon content. The coercive force HcJ was found to be improved, and the effectiveness of the lubricant removal process was confirmed.

Moreover, the difference by the addition amount of a lubricant was confirmed.
In the same manner as in Example 1, octanoic acid having a viscosity of less than 10 mPa · s and lauric acid having a viscosity of more than 10 mPa · s were added in the amounts shown in Table 3 to obtain fine powder having an average particle size of 4 μm. It produced and the molded object was obtained.
And the lubricant removal process was performed about the obtained molded object. The condition is that after raising the temperature to 180 ° C. in an argon atmosphere, replacement with an atmospheric gas containing 95% or more of hydrogen is performed, and heat treatment is performed in that state for 2 hours.
The obtained molded body was sintered and aged to obtain a sintered body. Sintering was performed under the condition of holding at 1090 ° C. for 4 hours in vacuum, and the aging treatment was a two-stage aging treatment of holding at 900 ° C. for 1 hour in an Ar atmosphere and then holding at 540 ° C. for 1 hour.
For comparison, a sintered body obtained by sintering and aging the molded body without preparing the lubricant was prepared.
About the obtained sintered compact, the amount of residual carbon and the amount of residual nitrogen were measured. The results are shown in Table 3.

  As shown in Table 3, octanoic acid having a viscosity of less than 10 mPa · s has a sufficient nitrogen content reduction effect with an addition amount of 0.1 wt%. On the other hand, in lauric acid, although the addition amount is 0.2 wt%, the effect of reducing the amount of nitrogen equivalent to octanoic acid can be obtained, but the residual carbon amount tends to increase as the addition amount increases, and the viscosity is increased. It can be seen that lauric acid exceeding 10 mPa · s cannot obtain an amount of nitrogen and carbon equivalent to or less than that of octanoic acid.

It is a figure for comparing the relationship between a particle size and the amount of nitrogen according to the viscosity of a lubricant. It is a figure for comparing the relationship between a particle size and a coercive force similarly. It is a figure which shows the relationship between lubricant removal process temperature and residual carbon amount.

Claims (5)

A fine pulverization step of finely pulverizing the raw material alloy powder having a predetermined composition in an atmosphere having an oxygen concentration of 100 ppm or less and having an organic substance as a constituent element and a lubricant having a viscosity of 10 mPa · s or less;
In an atmosphere with an oxygen concentration of 100 ppm or less, the pulverized powder obtained in the fine pulverization step is molded in a magnetic field, and a molding step in a magnetic field to obtain a molded body,
A sintering step of sintering the molded body;
A method for producing a rare earth sintered magnet.   2. The method for producing a rare earth sintered magnet according to claim 1, wherein the fine pulverizing step and the forming step in a magnetic field are performed in a nitrogen gas atmosphere so that the oxygen concentration is 100 ppm or less.   The method for producing a rare earth sintered magnet according to claim 1, wherein the lubricant is a fatty acid or a fatty acid ester.   The method for producing a rare earth sintered magnet according to any one of claims 1 to 3, wherein the lubricant is added to the raw material alloy powder prior to the fine pulverization step. Prior to the sintering step, the molded body further includes a lubricant removing step of removing the lubricant by heat-treating the compact under an atmosphere gas containing hydrogen (H ₂ ). 5. The method for producing a rare earth sintered magnet according to any one of 4 above.

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