JP2004072082A - Rare earth sintered magnet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rare earth sintered magnet excellent in corrosion resistance and sinterability, and to provide its manufacturing method. <P>SOLUTION: A rare earth sintered magnet has a main phase of R<SB>2</SB>T<SB>14</SB>Q type tetragonal compound(wherein; R denotes at least one kind of rare earth element; T denotes at least one kind of transition metal element essentially including Fe; Q denotes boron and/or carbon), and a grain boundary phase which surrounds the main phase. The R<SB>2</SB>T<SB>14</SB>Q type tetragonal compound in the main phase contains Cr in which part of Fe is substituted and carbon in which a part of boron is substituted as essential elements. The carbon concentration of the main phase is higher than that of the grain boundary phase. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a rare earth sintered magnet and a method for manufacturing the same.
[0002]
[Prior art]
Rare earth alloy sintered magnets (permanent magnets) are generally produced by pressing a rare earth alloy powder, sintering the resulting powder compact, and subjecting it to aging treatment. At present, two types of rare earth / cobalt magnets and rare earth / iron / boron magnets are widely used in various fields. Above all, rare earth / iron / boron magnets (hereinafter referred to as “R—Fe—B magnets.” R is a rare earth element containing Y, Fe is iron, and B is boron) are various magnets. It has the highest maximum magnetic energy product among them, and its price is relatively low, so it is actively used in various electronic devices.
[0003]
R-Fe-B based sintered magnets are mainly ₂ Fe ₁₄ It is composed of a main phase composed of a tetragonal compound of B, an R-rich phase composed of Nd or the like, and a B-rich phase. R-Fe-B based sintered magnets are described in, for example, Patent Documents 1 and 2.
[0004]
Conventionally, an ingot casting method has been used to produce an R-Fe-B-based alloy serving as such a magnet. According to a general ingot casting method, a rare earth metal, an electrolytic iron, and a ferroboron alloy, which are starting materials, are subjected to high-frequency melting, and the obtained molten metal is cooled relatively slowly in a mold to produce an alloy ingot.
[0005]
In recent years, the molten alloy has been cooled relatively quickly by contacting it with the inner surface of a single roll, twin roll, rotating disk, or rotating cylindrical mold, etc., and from the molten alloy, a solidified alloy ("alloy flake" A quenching method typified by a strip casting method and a centrifugal casting method for producing the quenching method has attracted attention. The thickness of the alloy piece produced by such a quenching method is generally in the range of about 0.03 mm or more and about 10 mm or less. According to the quenching method, the molten alloy begins to solidify from the surface in contact with the chill roll (roll contact surface), and crystals grow columnar from the roll contact surface in the thickness direction. As a result, the quenched alloy produced by the strip casting method or the like has an R size of about 0.1 μm to about 100 μm in the short axis direction and about 5 μm to about 500 μm in the long axis direction. ₂ Fe ₁₄ B crystal phase and R ₂ Fe ₁₄ It has a structure containing an R-rich phase dispersed and present at the grain boundaries of the B crystal phase. The R-rich phase is a non-magnetic phase in which the concentration of the rare-earth element R is relatively high, and its thickness (corresponding to the width of the grain boundary) is about 10 μm or less.
[0006]
The quenched alloy has a relatively short time (cooling rate: 10) as compared with an alloy (ingot alloy) produced by a conventional ingot casting method (die casting method). ² ° C / sec or more, 10 ⁴ (° C./sec or less), the structure is refined and the crystal grain size is small. Further, since the area of the grain boundary is large and the R-rich phase is widely spread in the grain boundary, there is an advantage that the dispersibility of the R-rich phase is excellent. Because of these characteristics, a magnet having excellent magnetic properties can be manufactured by using a quenched alloy.
[0007]
The alloy powder to be subjected to press molding is obtained by crushing an alloy lump produced by the above method or the like by, for example, a hydrogen storage method and / or various mechanical crushing methods (for example, a disk mill is used). It is obtained by finely pulverizing a coarse powder (for example, an average particle diameter of 10 μm to 500 μm) by, for example, a dry pulverization method using a jet mill.
[0008]
The average particle size of the R-Fe-B-based alloy powder to be subjected to press molding is preferably in the range of 1.5 m to 6 m from the viewpoint of magnetic properties. The “average particle size” of the powder refers to a mass median diameter (MMD) unless otherwise specified.
[0009]
[Patent Document 1]
U.S. Pat. No. 4,770,723
[Patent Document 2]
U.S. Pat. No. 4,792,368
[Patent Document 3]
JP-A-4-268051
[0010]
[Problems to be solved by the invention]
The rare earth sintered magnet produced by the above method has a problem that it has poor corrosion resistance and is very easily rusted. In order to improve the corrosion resistance, it is necessary to form a protective film on the surface of the sintered magnet by plating or vapor deposition. Since such a protective film forming step has a disadvantage of increasing the manufacturing cost, it is strongly required to improve the corrosion resistance of the magnet alloy itself.
[0011]
The present applicant has found that by introducing Cr into the main phase, the corrosion resistance of the rare earth sintered magnet is improved, and this is disclosed in Patent Document 3. However, when a sufficient amount of Cr is added to improve the corrosion resistance, the residual magnetic flux density B _r And as a result, the maximum magnetic energy product (BH) _max Is also reduced. There is also a problem that the corrosion resistance of the grain boundaries in the sintered magnet is not sufficiently improved by the addition of Cr.
[0012]
On the other hand, it has been proposed that carbon (C) is concentrated in the grain boundary phase of the sintered magnet and the main phase is surrounded by a grain boundary phase having a relatively high C concentration, thereby improving corrosion resistance (Patent) Reference 3). According to this technique, there is a problem that added C easily bonds to a rare earth element R such as Nd at a grain boundary, and the sinterability is reduced.
[0013]
The present invention has been made in view of the above points, and a main object of the present invention is to provide a rare earth sintered magnet excellent in corrosion resistance and sinterability, and a method of manufacturing the same.
[0014]
[Means for Solving the Problems]
The rare earth sintered magnet of the present invention has a R ₂ T ₁₄ A main phase of a Q-type tetragonal compound (R is at least one rare earth element, T is at least one transition metal element essential for Fe, Q is boron and / or carbon), and a grain boundary surrounding the main phase Rare earth sintered magnet having a phase and the R ₂ T ₁₄ The Q-type tetragonal compound contains, as essential elements, Cr partially substituted for Fe and carbon partially substituted for boron, and the carbon concentration of the main phase is higher than the carbon concentration of the grain boundary phase. It is characterized by the following.
[0015]
In a preferred embodiment, the grain boundary phase contains Co that accounts for 50 atomic% or more and 90 atomic% or less of the entire grain boundary phase.
[0016]
In a preferred embodiment, the grain boundary phase is R ₃ Contains Co compound.
[0017]
In a preferred embodiment, the R in the main phase ₂ T ₁₄ The Q-type tetragonal compound contains Co in which a part of Fe is substituted as an essential element.
[0018]
In a preferred embodiment, the R content is 12 atomic% or more and 18 atomic% or less, the T content is 60 atomic% or more and 88 atomic% or less, and the Cr content is 0.1 atomic% or more and 2.4 atomic% or less. , B content is 0.5 atomic% or more and 13 atomic% or less, and C content concentration is 0.4 atomic% or more and 4.5 atomic% or less.
[0019]
The rare earth sintered magnet of the present invention has a R ₂ T ₁₄ A main phase of a Q-type tetragonal compound (R is at least one rare earth element, T is at least one transition metal element essential for Fe, Q is boron and / or carbon), and a grain boundary surrounding the main phase And a rare earth sintered magnet having ₂ T ₁₄ The natural electrode potential of the Q-type tetragonal compound is −0.75 V or more.
[0020]
In a preferred embodiment, the R ₂ T ₁₄ The difference between the natural electrode potential of the Q-type tetragonal compound and the natural electrode potential of the grain boundary phase is 0.6 V or less.
[0021]
The method for producing a rare earth magnet according to the present invention comprises the steps of: ₂ T ₁₄ A main phase of a Q-type tetragonal compound (R is at least one rare earth element, T is at least one transition metal element essential for Fe, Q is boron and / or carbon), and a grain boundary surrounding the main phase A method for producing a rare earth sintered magnet having a phase ₂ T ₁₄ A step of preparing a powder of a main phase alloy containing 50% by volume or more of a Q-type tetragonal compound and containing Cr, boron, and carbon as essential elements, and a liquid phase alloy containing R and Co; Sintering the powder, thereby producing a rare earth sintered magnet in which the carbon concentration of the main phase is higher than the carbon concentration of the grain boundary phase.
[0022]
In a preferred embodiment, the content of R in the alloy for the main phase is 11 to 16 at%, the content of T is 60 to 87 at%, and the content of Cr is 0.2 at% or more. The content of B is 2.5 at% or less, the content of B is 1 to 14 at%, and the content of C is 0.5 to 5.0 at%.
[0023]
In a preferred embodiment, as the main phase alloy, a first alloy containing 0.8% by mass or more and 1.0% by mass or less of Q and a 1.2% by mass or more and 1.4% by mass or less of Q are contained. And a second alloy to be used. It is preferable that the weight ratio between the first alloy and the second alloy is set in a range of 3: 1 to 1: 3.
[0024]
In a preferred embodiment, the concentration of R in the liquid phase alloy is 60 to 80 atomic%, and the Co concentration is 20 to 40 atomic%.
[0025]
In a preferred embodiment, a ratio of the liquid phase alloy to the total of the main phase alloy and the liquid phase alloy is set in a range from 2% by volume to 20% by volume.
[0026]
In a preferred embodiment, a step of preparing a molten metal of the raw material alloy for the main phase alloy and a step of cooling and solidifying the molten metal of the raw material alloy at a rate of 100 ° C./second or more and 10,000 ° C./second or less are further included. Including.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
The rare earth sintered magnet of the present invention has an R ₂ T ₁₄ A main phase of a Q-type tetragonal compound (R is at least one rare earth element, T is at least one transition metal element essential for Fe, Q is boron and / or carbon), and a grain boundary phase surrounding the main phase This is a rare earth sintered magnet having: Here, the rare earth element includes Y (yttrium).
[0028]
The main feature of the present invention is that R ₂ T ₁₄ The point that the Q-type tetragonal compound contains, as essential elements, Cr in which Fe is partially substituted and carbon (C) in which boron (B) is partially substituted. It is at a point higher than the C concentration in the boundary phase.
[0029]
In this specification, “Cr in which a part of Fe is substituted” means R ₂ T ₁₄ It means that Cr exists at the same kind of site as the site of Fe constituting the crystal of the Q-type tetragonal compound, and it is not always necessary that the Fe atom is actually replaced by the Cr atom at the specific site. There is no. Similarly, “C in which a part of B is substituted” means R ₂ T ₁₄ It means that Cr exists at the same kind of site as the site where B constituting the crystal of the Q-type tetragonal compound exists, and it is not always necessary that the B atom is actually replaced by a C atom at a specific site. There is no.
[0030]
According to the sintered magnet of the present invention having such a configuration, the amount of Cr added is suppressed (for example, 2 atomic% or less of the entire magnet) so as not to cause significant deterioration of the magnet characteristics, and the simultaneous addition of C is effective. Corrosion resistance can be significantly increased without causing a decrease in sinterability.
[0031]
A preferred method for adding Cr and C and making the concentrations of Cr and C in the main phase higher than those in the grain boundary phase will be described in detail later. Here, first, by adding Cr and C, R ₂ T ₁₄ The mechanism by which the corrosion resistance of the Q-type tetragonal compound is improved will be described in detail based on experimental results.
[0032]
[Corrosion resistance and natural electrode potential]
Generally, corrosion of a metal is accelerated by the presence of an impurity metal having a potential higher than that of the metal. This is because the impurity metal becomes a local cathode and the battery reaction easily occurs. For this reason, it is considered that increasing the natural electrode potential of the alloy constituting the rare earth magnet is effective for increasing the corrosion resistance. On the other hand, it is considered that the corrosion of the rare earth sintered magnet proceeds by a battery reaction due to a potential difference between the main phase and the grain boundary phase.
[0033]
However, there is no guide for quantitative evaluation on what kind of additive element should be added and how much to improve the corrosion resistance. Then, the present inventor quantitatively evaluated the potential of various alloys constituting the main phase and the grain boundary phase by measuring the polarization curve, and analyzed the structure of the rare earth sintered magnet which is hard to corrode (it is hard to rust).
[0034]
FIG. 1 shows a schematic configuration of a potentiostat device that can be used for measuring a natural electrode potential of various metals or alloys. FIG. 1 shows electrons (e) when a sample (Sample) was used as an anode electrode. ⁻ ) Is schematically shown.
[0035]
The measurement of the natural electrode potential of the alloy can be performed by an electrokinetic method using the apparatus shown in FIG. Specifically, a sample alloy electrode (Sample) and a reference electrode (counter electrode) made of Pt are immersed in the solution, and the voltage applied between both electrodes is changed. As the applied voltage increases, part of the alloy constituting the sample alloy electrode is ionized, and electrons are emitted into the sample alloy electrode. The electrons move to the counter electrode (a reference electrode made of Pt). At this time, the number of electrons passing through the ammeter is measured as a current density, and the measured value with respect to the applied voltage is plotted on a graph to obtain a polarization curve.
[0036]
All the measured values of the natural electrode potential in this specification were obtained under the following measurement conditions.
[0037]
・ Sample surface area is 1cm ² The polarization width is set to 2.0 V, the average roughness of the sample surface is set to 0.02 μm or less, and the voltage sweep speed is set to 5 mV / sec.
[0038]
・ Hg as reference electrode ₂ Cl ₂ And a Pt electrode as a counter electrode.
[0039]
-In order to suppress the cathodic reaction due to dissolved oxygen, the dissolved oxygen is expelled by bubbling the solution with nitrogen for 10 minutes or more before measurement. During the measurement, a nitrogen gas flow is performed on the liquid surface in order to prevent oxygen from being dissolved into the solution from the atmosphere.
[0040]
-2.5% Na as a solution ₂ SO ₄ (PH 6.4) and set the temperature of the solution to 20 ° C.
[0041]
FIG. 2 shows an example of a polarization curve obtained under such measurement conditions. FIG. _11.8 Fe _82.2 B _6.0 3 shows a polarization curve of the alloy. The vertical axis in the graph of FIG. 2 indicates the current density between the electrodes in the potentiostat device, and the horizontal axis indicates the potential of the sample electrode. As the potential of the sample electrode is increased from a low level to a high level, the current density gradually decreases, and at a certain potential, the polarization curve shows a minimum value. The potential at which the current density becomes minimal is called "natural electrode potential (corrosion potential)". When a natural electrode potential is applied, the electrode reaction shown in FIG. 1 is in an equilibrium state.
[0042]
The higher the natural electrode potential of the alloy, the more difficult it is for the battery reaction to proceed. When the potential is increased to a level higher than the natural electrode potential, the current density increases again. This is because when the potential is lower than the natural electrode potential, the cathode reaction proceeds at the sample electrode, and when the potential becomes higher than the natural electrode potential, the anodic reaction proceeds at the sample electrode.
[0043]
Conventionally, it has been known that the corrosion resistance of a sintered magnet is improved by adding Co to a raw material alloy of an R—Fe—B-based rare earth sintered magnet. However, the details of the reason have not been clarified until now.
[0044]
The present inventors have confirmed that the addition of Co increases the natural electrode potential by actually measuring the polarization curve of the Nd-Fe-B-based rare earth magnet alloy to which Co is added. FIG. _11.8 Fe _82.2 B _6.0 Nd in which part of Fe in the alloy is replaced with Co _11.8 Fe _82.2-x Co _x B _6.0 3 shows a polarization curve of an alloy (x = 0, 5, 10, 20, 50). As is evident from FIG. 3, as the Co substitution amount (x) increases, Nd _11.8 Fe _82.2-x Co _x B _6.0 The natural electrode potential of the alloy is increasing. The reason is that the standard oxidation-reduction potential of Fe is -0.440V, whereas the standard oxidation-reduction potential of Co is -0.277V, so that Co is less likely to cause an electrochemical reaction than Fe. It is considered that the anodic reaction itself is suppressed by the substitution with Co.
[0045]
When Co is added to the raw material alloy of the rare earth sintered magnet, Co is also present in the grain boundary phase of the rare earth sintered magnet, and various intermetallic compounds in which Co and the rare earth element R are combined in the grain boundary phase are formed. It is thought that there is. If the difference between the natural electrode potential of the intermetallic compound present in such a grain boundary phase and the natural electrode potential of the alloy constituting the main phase is large, even if the natural electrode potential of the main phase alloy is high, the main phase and the grain boundary Since the battery reaction proceeds with the phase, the corrosion resistance may be deteriorated.
[0046]
FIG. 4 shows the natural electrode potential of an intermetallic compound considered to be formed in the grain boundary phase of a sintered magnet finally obtained by adding Co to a raw material alloy, and a part of Fe is replaced by Co. Main phase alloy (Nd _11.8 Fe _82.2-x Co _x B _6.0 ) Indicates the natural electrode potential. From FIG. 4, it is possible to evaluate the susceptibility of each of the alloy constituting the main phase and the alloy constituting the grain boundary phase to corrosion.
[0047]
First, the main phase alloy (Nd _11.8 Fe _82.2-x Co _x B _6.0 Attention is paid to the natural electrode potential of (2). As can be seen from FIG. 4, in a region where the amount of Co substitution x is 50 atomic% or less, the natural electrode potential increases with an increase in the amount of Co substitution x.
[0048]
Next, reference is made to FIG. FIG. 5 shows an alloy in which a part of Fe is replaced by Co (Nd _11.8 Fe _82.2-x Co _x B _6.0 2 shows the relationship between the change in weight and the natural electrode potential in a high-temperature and high-humidity test (temperature: 80 ° C., relative humidity: 90%) for the alloy (alloy). As can be seen from FIG. 5, the weight change of the alloy decreases as the natural electrode potential increases. This change in weight indicates the degree of corrosion in the alloy, and the smaller the change in weight, the less the corrosion.
[0049]
From the above, it can be seen that as the natural electrode potential of the alloy increases due to the Co substitution, the corrosion hardly progresses.
[0050]
Next, with reference to FIG. 4 again, the relationship between the natural electrode potential of the intermetallic compound formed in the grain boundary phase and the natural electrode potential of the main phase alloy will be examined. As shown in FIG. 5, the natural electrode potential of Nd metal (Nd-metal) is as low as about -1.40 V, whereas Nd constituting the main phase is low. _11.8 Fe _82.2 B _6.0 Is relatively high at about -0.82V. Therefore, the main phase alloy is Nd _11.8 Fe _82.2 B _6.0 If it is simply assumed that the grain boundary phase alloy is Nd metal, the natural electrode potential difference between them is as large as about 0.7 V, and corrosion due to battery reaction is likely to occur. On the other hand, when about 30 atomic% of Co is added to the raw material alloy, the main phase alloy becomes Nd _11.8 Fe _82.2-x Co _x B _6.0 And the grain boundary phase alloy is Nd ₃ A large amount of Co (natural electrode potential: -0.66 V) is included. In this case, the potential difference between the main phase and the grain boundary phase is small, and corrosion due to the battery reaction between the two is unlikely to occur.
[0051]
As described above, when Co is added, a part of Fe is replaced by Co in the main phase, thereby increasing the natural electrode potential of the main phase, and in the grain boundary phase, the natural electrode potential is higher than that of Nd metal. A high Nd-Co-based compound is easily generated. Then, since the difference in the natural electrode potential between the two is small, it is considered that corrosion that causes a battery reaction is unlikely to occur.
[0052]
From the above results, it can be seen that corrosion can be suppressed by increasing the natural electrode potential of the main phase alloy and reducing the difference in the natural electrode potential between the main phase alloy and the grain boundary phase alloy.
[0053]
From the above considerations, the present inventor has succeeded in obtaining a rare earth sintered magnet having higher corrosion resistance than before by properly selecting a combination of additional metal elements other than Co. More specifically, the present inventor has constructed a main phase of a rare earth sintered magnet by an alloy in which a part of Fe in Cr, Ni, and / or Al is replaced and a part of B is replaced by C. Then, they found that the corrosion resistance was significantly improved, and came to the present invention. Hereinafter, this point will be described in more detail.
[0054]
First, as the base alloy, Nd _11.8 Fe _82.2-x M _x B _6.0 An alloy (M = Co, Ni, Cr, or Al) was prepared, and the natural electrode potential of the obtained alloy was measured. FIG. 6 shows the relationship between the replacement amount x and the natural electrode potential. FIG. 6 shows the following.
[0055]
-The natural electrode potential increases with an increase in the substitution amount x.
[0056]
When the replacement amount x is constant, the highest natural electrode potential can be obtained when replacing Fe with Cr.
[0057]
-Even when Ni is substituted for Fe, if the substitution amount x is 5 atomic% or more, a natural electrode potential equivalent to that when Fe is substituted for Co can be obtained.
[0058]
Next, a description will be given of how the natural electrode potential is further increased by partially substituting Fe with an additive element such as Co and substituting part of B with C.
[0059]
7 to 9 show alloys Nd obtained by substituting Fe with Co, Ni, and Cr, respectively. _11.8 Fe _82.2-x M _x B _6.0 The natural electrode potential of the alloy (M = Co, Ni, Cr, or Al) and the alloy Nd in which a part of B is replaced by C in addition to their substitution _11.8 Fe _82.2-x M _x B _6.0-y C _y The natural electrode potential of the alloy (M = Co, Ni, Cr, or Al) is shown. As can be seen from FIGS. 7 to 9, in any case, the B substitution with C further increases the natural electrode potential. In particular, when Cr and C are added, that is, when Nd _11.8 Fe _82.2-x Cr _x B _6.0-y C _y The natural electrode potential of the alloy is highest.
[0060]
As described above, when a large amount of Cr is added to the raw material alloy, the residual magnetic flux density B _r And as a result, the maximum magnetic energy product (BH) _max There was a problem that also decreased. However, if part of B of the main phase alloy is replaced by C, it becomes possible to reduce the amount of Cr added while maintaining the corrosion resistance at a high level. However, if the added C is not effectively used for the B substitution of the main phase alloy and is present in a large amount in the grain boundary phase of the sintered magnet, as described above, the rare earth element, C, and impurities are contained in the grain boundary phase. O ₂ And there is a problem that sinterability deteriorates due to bonding. When the sinterability is reduced, the sintering step needs to be performed at a high temperature for a long time, so that the crystal grains of the sintered magnet are coarsened and the magnet characteristics are deteriorated. Therefore, simply adding C to the raw material alloy does not solve the problems of the prior art. Therefore, in the present invention, a structure that does not cause a decrease in sinterability is realized by introducing C at a high concentration into the main phase instead of the grain boundary phase.
[0061]
(Embodiment)
Hereinafter, a preferred embodiment of the method for producing a rare earth sintered magnet according to the present invention will be described.
[0062]
[Raw material alloy]
First, the raw material alloy for the main phase is melted by high frequency melting in an argon atmosphere to form a molten alloy. It is preferable to use an alloy having the following composition as such an alloy for the main phase.
[0063]
R concentration is 11 atomic% or more and 16 atomic% or less,
The content of T is not less than 60 atomic% and not more than 88 atomic%,
Cr concentration is 0.2 atomic% or more and 2.5 atomic% or less,
The concentration of B is 1 atomic% or more and 14 atomic% or less,
The concentration of C is 0.5 atomic% or more and 5.0 atomic% or less.
[0064]
In the present embodiment, since Cr and C are concentrated in the main phase, Nd ₂ Fe ₁₄ An alloy obtained by adding Cr and carbon to an alloy having a composition close to the stoichiometric composition of the B-type compound is used as the main phase alloy.
[0065]
Next, after maintaining the alloy melt at 1350 ° C., the alloy melt is rapidly cooled by a single roll method to obtain a flake-like alloy ingot having a thickness of, for example, about 0.3 mm. The rapid cooling condition at this time is, for example, a roll peripheral speed of about 1 m / sec, a cooling speed of 500 ° C./sec, and a supercooling of 200 ° C. The quenched alloy slab thus produced is pulverized into flakes having a size of 1 to 10 mm before the next hydrogen pulverization. Thus, R ₂ T ₁₄ It is possible to prepare an alloy for the main phase that contains 50% by volume or more of the Q-type tetragonal compound and Cr, B, and C as essential elements.
[0066]
If the Cr concentration in the alloy for the main phase (mass ratio to the entire alloy for the main phase) is less than 0.2 atomic%, the effect of improving the corrosion resistance is not exhibited, so the Cr concentration is 0.2 atomic%. It is preferable that it is above. Conversely, if the concentration of Cr exceeds 2.5 atomic%, the residual magnetic flux density B of the sintered magnet _r Therefore, the Cr concentration is preferably 2.5 atomic% or less. Therefore, the preferable range of the Cr concentration is 0.2 to 2.5 atomic%, and the more preferable range is 0.3 to 2.0 atomic%.
[0067]
If the C concentration (mass ratio to the entire main phase alloy) is less than 0.5 atomic%, the effect of improving corrosion resistance does not appear, so the C concentration may be 0.5 atomic% or more. preferable. Conversely, when the concentration of C exceeds 4.5 atomic%, the residual magnetic flux density B _r Therefore, the C concentration is preferably 4.5 atomic% or less. Therefore, a preferable range of the C concentration is 0.5 atomic% or more and 4.5 atomic% or less, and a more preferable range is 1.0 atomic% or more and 4.0 atomic% or less.
[0068]
The alloy for the main phase having the above composition may contain, in addition to Cr and C, 0.1 atomic% or more and 10 atomic% or less of Co, Ni, and / or Al and Cu. In particular, the addition of Co is effective for further improving the corrosion resistance.
[0069]
It is not preferable that the added C bonds with the element R under the rare earth element. However, if the added C is formed by a quenching method such as a strip casting method, the generation of the RC compound can be suppressed. Specifically, if the melt of the raw material alloy having the above composition is quenched and solidified at a rate of 100 ° C./second or more and 10,000 ° C./second or less, the added C is less likely to be bonded to the rare earth element R, and is efficiently combined with B. Replace.
[0070]
The main phase alloy may include a plurality of alloys having different compositions. For example, a first alloy having a total amount of B and C (Q content) of 0.8% by mass or more and 1.0% by mass or less, and a total amount of B and C (Q content) of 1.2% by mass or more A second alloy of 1.4% by mass or less may be used as the main phase alloy. In this case, a step of mixing the first alloy and the second alloy is required. This mixing may be performed during the fine pulverization step of the main phase alloy or may be performed during the coarse pulverization step. Is also good.
[0071]
By mixing the first and second alloys having different Q contents, the R produced from the first alloy is mixed. ₂ T ₁₇ Since the phase is combined with extra (B or C) derived from the second alloy, the added C tends to remain in the main phase. Therefore, it is possible to suppress the generation of the Nd-OC compound generated when C of the alloy for the main phase flows out to the grain boundary phase, and to prevent a decrease in magnetic properties.
[0072]
Further, by adding the second alloy having a relatively high Q content to the first alloy, the composition ratio of B and C is increased. ₂ T ₁₇ Phase is R ₂ T ₁₄ It changes to the B phase, and the effect of improving the magnet properties (coercive force) is also obtained.
[0073]
In this embodiment, in addition to the main phase alloy, a liquid phase alloy having an R content of 60 at% to 80 at% and a Co content of 20 to 40 at% is prepared. . The liquid phase alloy melts earlier than the main phase alloy in the sintering process, and contributes to the progress of liquid phase sintering. The liquid phase alloy will eventually constitute the grain boundary phase of the sintered magnet.
[0074]
The present invention is characterized in that Cr and C (particularly C) are concentrated in the main phase, while keeping the C concentration in the grain boundary phase as low as possible. This is because when the C concentration in the grain boundary phase increases, the sinterability decreases. Therefore, it is preferable to use an alloy to which C is not intentionally added as the liquid phase alloy.
[0075]
Further, in the present invention, in order to reduce the difference in the natural electrode potential between the main phase and the grain boundary phase, an R-Co alloy mainly containing the rare earth elements R and Co is used as the liquid phase alloy. By doing so, an intermetallic compound having a natural electrode potential of −0.70 V or more is easily generated in the grain boundary phase of the final sintered magnet. As the grain boundary phase, R ₃ Preferably, the alloy for liquid phase contains R ₃ It is desirable to have a composition that easily produces Co. For this reason, it is preferable that the concentration of the rare earth element R in the alloy for liquid phase be set to 60 at% to 80 at%, and the concentration of Co to 20 to 40 at%. Specifically, an alloy containing 60 atomic% of Nd and 40 atomic% of Co can be used.
[0076]
R in the liquid phase alloy plays an important role in generating a liquid phase, and Co combines with the rare earth element R to contribute to the generation of a compound whose natural electrode potential is close to that of the main phase. If Co in the liquid phase alloy is less than 20 atomic%, the natural electrode potential of the final grain boundary phase will not be sufficiently high, and the natural electrode potential difference between the main phase and the grain boundary phase will increase. Corrosion resistance is not sufficiently exhibited. On the other hand, if the Co in the liquid phase alloy exceeds 40 atomic%, the ferromagnetic RCO will be added to the grain boundary phase of the finally obtained sintered magnet. ₂ Are easily generated, which deteriorates the magnet characteristics, which is not preferable.
[0077]
The liquid phase alloy having the above composition can be produced by a rapid cooling method such as a strip casting method, similarly to the main phase alloy described above.
[0078]
Even if elements other than rare earth elements and Co (for example, C, B, Cr, etc.) are present in the liquid phase alloy, there is no problem as long as the ratio is 30 atomic% or less of the entire liquid phase alloy.
[0079]
[First crushing step]
A plurality of raw material packs (for example, made of stainless steel) are filled with the flakes of the alloy for the main phase and the alloy for the liquid phase which are roughly pulverized in a flake shape, and mounted on a rack. Thereafter, the rack on which the raw material pack is mounted is inserted into the hydrogen furnace. Next, the lid of the hydrogen furnace is closed, and a hydrogen embrittlement treatment (hereinafter sometimes referred to as “hydrogen pulverization treatment”) step is started. The hydrogen crushing process is performed according to, for example, a temperature profile shown in FIG. In the example of FIG. 10, first, the evacuation process I is performed for 0.5 hours, and then the hydrogen storage process II is performed for 2.5 hours. In the hydrogen storage process II, hydrogen gas is supplied into the furnace, and the inside of the furnace is set to a hydrogen atmosphere. The hydrogen pressure at that time is preferably about 200 to 400 kPa.
[0080]
Subsequently, after performing the dehydrogenation process III under a reduced pressure of about 0 to 3 Pa for 5.0 hours, the cooling process IV of the raw material alloy is performed for 5.0 hours while supplying argon gas into the furnace.
[0081]
In the cooling process IV, when the atmosphere temperature in the furnace is relatively high (for example, when the temperature exceeds 100 ° C.), an inert gas at room temperature is supplied into the hydrogen furnace and cooled. Then, at the stage when the temperature of the raw material alloy is lowered to a relatively low level (for example, when the temperature is 100 ° C. or lower), an inert gas cooled to a temperature lower than normal temperature (for example, room temperature minus about 10 ° C.) is introduced into the hydrogen furnace 10. Supplying is preferable from the viewpoint of cooling efficiency. The supply amount of argon gas is 10 to 100 Nm ³ / Min.
[0082]
When the temperature of the raw material alloy is lowered to about 20 to 25 ° C., an inert gas at substantially normal temperature (a temperature lower than room temperature but a difference from room temperature within a range of 5 ° C. or less) is blown into the hydrogen furnace, and the raw material is cooled. It is preferable to wait until the temperature of the mixture reaches the normal temperature level. By doing so, when the lid of the hydrogen furnace is opened, a situation in which dew condensation occurs inside the furnace can be avoided. If moisture is present inside the furnace due to dew condensation, the moisture freezes and evaporates in the evacuation step, which makes it difficult to increase the degree of vacuum and increases the time required for the evacuation step I, which is not preferable.
[0083]
When the coarsely pulverized alloy powder after hydrogen pulverization is taken out of the hydrogen furnace, it is preferable to perform the takeout operation in an inert atmosphere so that the coarsely pulverized powder does not come into contact with the atmosphere. This prevents the coarsely pulverized powder from oxidizing and generating heat, and improves the magnetic properties of the magnet. Next, the coarsely pulverized raw material alloy is filled into a plurality of raw material packs and mounted on a rack.
[0084]
By the hydrogen pulverization, the alloy for the main phase and the alloy for the liquid phase are both pulverized to a size of about 0.1 mm to several mm, and the average particle size is 500 μm or less. After hydrogen pulverization, it is preferable that the embrittled raw material alloy is further finely pulverized and cooled by a cooling device such as a rotary cooler. In the case where the raw material is taken out in a relatively high temperature state, the time of the cooling process using a rotary cooler or the like may be made relatively long.
[0085]
A large amount of Nd is exposed on the surface of the coarsely pulverized powder produced by hydrogen pulverization, and is in a state of being easily oxidized.
[0086]
The coarsely pulverized powder of the main phase alloy and the liquid phase alloy may be produced by simultaneously subjecting the main phase alloy and the liquid phase alloy to hydrogen embrittlement, as described above. The alloy for the main phase and the alloy for the liquid phase may be roughly pulverized separately.
[0087]
[Second grinding step]
After mixing the coarsely pulverized powder of the alloy for the main phase and the coarsely pulverized powder of the alloy for the liquid phase produced in the first pulverizing step, the mixed powder is finely pulverized using a jet mill pulverizer. A cyclone classifier is connected to the jet mill pulverizer used in the present embodiment.
[0088]
The jet mill pulverizer receives the supply of the rare earth alloy (coarse pulverized powder) coarsely pulverized in the first pulverization step, and pulverizes it in the pulverizer. The powder pulverized in the pulverizer is collected in a recovery tank via a cyclone classifier.
[0089]
Hereinafter, this will be described in more detail.
[0090]
The coarsely pulverized powder introduced into the pulverizer is wound up in the pulverizer by an inert gas injected at a high speed from an internal nozzle, and swirls in the pulverizer together with a high-speed airflow. Then, the objects to be ground are finely ground by mutual collision.
[0091]
The finely pulverized powder particles are guided to the classifying rotor by the ascending airflow and classified by the classifying rotor, and the coarse powder is pulverized again. The powder pulverized to a predetermined particle size or less is introduced into the classifier body of the cyclone classifier. In the main body of the classifier, relatively large powder particles having a predetermined particle size or more are deposited in a recovery tank provided at a lower portion, and the ultrafine powder is discharged to the outside through an exhaust pipe together with an inert gas stream.
[0092]
In the present embodiment, oxygen (20,000 ppm or less, for example, about 10,000 ppm) is slightly mixed into the inert gas introduced into the jet mill pulverizer. Thereby, the surface of the finely pulverized powder is appropriately oxidized so that rapid oxidation and heat generation do not occur when the finely pulverized powder comes into contact with the atmosphere. In order to obtain excellent magnet properties, it is preferable that the amount of oxygen in the powder is adjusted in a range of 2,000 ppm to 8,000 ppm by weight.
[0093]
If the amount of oxygen in the powder exceeds 8000 ppm and becomes too large, in the next sintering step, the rare earth element will be consumed for the generation of the oxide, and the amount of the rare earth element contributing to the liquid phase generation will decrease. As a result, the sinterability is reduced and the magnet characteristics are deteriorated due to the decrease in the main phase ratio, which is not preferable.
[0094]
In the present embodiment, the average particle size (FSSS particle size) of the powder is set to 1.5 μm or more and 10 μm or less, more preferably 2 μm or more and 6 μm or less (for example, 3 μm).
[0095]
In consideration of the preferable volume ratio of the main phase and the grain boundary phase in the final sintered magnet, the volume ratio of the liquid phase alloy to the whole alloy is set in the range of 2% to 20%. Is preferred.
[0096]
[Press molding]
In the present embodiment, for example, 0.3 wt% of a lubricant is added to and mixed with the mixed powder produced by the above method in a rocking mixer, and the surface of the alloy powder particles is coated with the lubricant. As the lubricant, a fatty acid ester diluted with a petroleum solvent can be used. In this example, methyl caproate is used as the fatty acid ester, and isoparaffin is used as the petroleum solvent. The weight ratio between methyl caproate and isoparaffin is, for example, 1: 9. Such a liquid lubricant coats the surface of the powder particles, exerts an antioxidant effect on the particles, and also has a function of improving the orientation during pressing and the powder moldability (the density of the molded body becomes uniform, and Eliminating defects such as cracks).
[0097]
The type of the lubricant is not limited to the above. As the fatty acid ester, other than methyl caproate, for example, methyl caprylate, methyl laurate, methyl laurate and the like may be used. As the solvent, a petroleum solvent represented by isoparaffin, a naphthenic solvent, or the like can be used. The timing of adding the lubricant is arbitrary, and may be, for example, any of before, during, or after the fine pulverization by a jet mill pulverizer. Instead of the liquid lubricant or together with the liquid lubricant, a solid (dry) lubricant such as zinc stearate may be used.
[0098]
Next, the magnetic powder produced by the method described above is molded in an orientation magnetic field using a known press. A preferable range of the applied orientation magnetic field is 0.5 T or more and 8 T or less (for example, 1.1 T). Thus, the density is, for example, 3.5 g / cm ³ 5.0 g / cm or more ³ The following (for example, 4.2 g / cm ³ ) To produce a compact.
[0099]
[Sintering process]
Next, this compact is sintered. The sintering can be performed, for example, by holding at 1080 ° C. for about 4 hours in an argon atmosphere. Instead of performing the sintering process under such conditions, after holding at a temperature in the range of 650 to 1000 ° C. for 10 to 240 minutes, sintering is performed at a temperature higher than the above holding temperature (for example, 1000 to 1100 ° C.). Further progress may be made (two-stage sintering). When the temperature is 650 to 1000 ° C., the powder of the liquid phase alloy is melted preferentially and a liquid phase is generated. If a two-stage sintering process is performed, the sintering proceeds efficiently. This is because the processing time can be shortened and grain growth during the sintering step can be suppressed.
[0100]
The rare earth sintered magnet thus obtained (sintering density: for example, 7.60 g / cm ³ ) Is R ₂ T ₁₄ A rare earth sintered magnet having a main phase of a Q-type tetragonal compound and a grain boundary phase surrounding the main phase, wherein the R ₂ T ₁₄ The Q-type tetragonal compound contains, as essential elements, Cr partially substituted for Fe and carbon partially substituted for boron, and the carbon concentration of the main phase is higher than the carbon concentration of the grain boundary phase. It has the characteristic of being expensive. For this reason, the natural electrode potential of the main phase rises to -0.75 V or more, and the corrosion resistance of the main phase itself is improved.
[0101]
The grain boundary phase of the rare earth sintered magnet contains Co, which accounts for 50 atomic% or more and 90 atomic% or less of the entire grain boundary phase, and particularly, R ₃ Contains Co compound. As a result, the natural electrode potential of the grain boundary layer also rises to -0.75 V or more, and the difference in the natural electrode potential between the main phase and the grain boundary phase is suppressed to 0.6 V or less. Therefore, the occurrence of corrosion due to the battery reaction between the main phase and the grain boundary phase is also prevented.
[0102]
As described above, the natural electrode potentials of the main phase and the grain boundary phase are both -0.75 V or more, and the difference in the natural electrode potential between the main phase and the grain boundary phase is 0.6 V or less. Therefore, the corrosion resistance is remarkably improved, and even if the surface of the sintered magnet is not covered with the protective film, the magnet can be realized for long-term practical use. The natural electrode potential of the main phase is preferably -0.82 V or more, and more preferably -0.8 V or more.
[0103]
Further, a compound in which R and C are bonded is hardly generated in the grain boundary phase, and sintering is easy. For this reason, since the crystal grains in the sintered magnet are not coarsened and the generation of a compound that deteriorates the magnet properties is suppressed, excellent magnet properties (for example, residual magnetic flux density B _r : 1.1 T or more, coercive force iHc: 900 kA / m or more).
[0104]
In the embodiment described above, since the sintered magnet is manufactured by a method using two kinds of alloys having different compositions (two-alloy method), Cr and C are more efficiently contained in the main phase than in the grain boundary phase. It is easy to concentrate. However, the sintered magnet of the present invention is not limited to this, and may be manufactured by other methods.
[0105]
(Example)
As the main phase alloy, Nd: 12.35 at%, Fe: 75.92 at%, B: 3.20 at%, C: 3.20 at%, Cr: 2.13 at%, Co: 3. A strip cast alloy A having a composition of 20 at% was prepared, and a strip cast alloy B having a composition of Nd: 60 at% and Co: 40 at% was prepared as a liquid phase alloy.
[0106]
The alloys A and B were mixed at a volume ratio of 9: 1, coarsely pulverized by hydrogen embrittlement, and then finely pulverized by a jet mill to prepare finely pulverized powder having an average particle diameter of 3.0 μm.
[0107]
Next, this finely pulverized powder was compression-molded in an orientation magnetic field of 1.1 T, and the molding density was 4.0 g / cm. ³ Was produced. This molded body was subjected to a sintering step at 1075 ° C. for 4 hours in an argon atmosphere. The resulting sintered body density is 7.55 g / cm. ³ Met. In the main phase, part of Fe was replaced by Cr and part of boron was replaced by C, and the C concentration in the main phase was higher than the carbon concentration in the grain boundary phase.
[0108]
When the magnet properties of the sintered magnet were measured, the residual magnetic flux density B _r Was 1.3 T and the coercive force iHc was 1280 kA / m. The sintered magnet hardly corroded (rusted) even in a high-temperature and high-humidity experiment at a temperature of 80 ° C., a relative humidity of 90% and a time of 500 hours or more.
[0109]
【The invention's effect】
According to the present invention, the natural electrode potential of the main phase alloy is increased by introducing Cr and C into the main phase, and the natural electrode potential of the R-Co compound or the like in the grain boundary phase is reduced to the natural electrode potential of the main phase. By forming a close compound, a sintered magnet excellent in both sinterability and corrosion resistance can be provided. As a result, it is possible to obtain a rare earth sintered magnet that does not rust without forming a surface protective film.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a device (potentiometer) for measuring a natural electrode potential.
FIG. 2 shows Nd _11.8 Fe _82.2 B _6.0 5 is a graph showing a polarization curve of an alloy.
FIG. 3 shows Nd _11.8 Fe _82.2-x Co _x B _6.0 It is a graph which shows the polarization curve of an alloy (x = 0, 5, 10, 20, 50).
FIG. 4 is a main phase alloy (Nd in which a part of Fe is substituted by Co). _11.8 Fe _82.2-x Co _x B _6.0 5) is a graph showing the relationship between the natural electrode potential of the intermetallic compound that can be contained in the grain boundary phase and the amount of Co substitution.
FIG. 5 is a graph showing a relationship between a weight change amount by a corrosion test and a natural electrode potential.
FIG. 6 is a graph showing a relationship between a natural electrode potential and a substitution amount in a main phase alloy in which Fe is substituted by Co, Ni, Cr, and Al.
FIG. 7 is a graph showing a relationship between a substitution amount and a natural electrode potential in a Co-substituted alloy and a Co + C-substituted alloy.
FIG. 8 is a graph showing the relationship between the amount of substitution and the natural electrode potential in Ni-substituted alloys and Ni + C-substituted alloys.
FIG. 9 is a graph showing the relationship between the substitution amount and the natural electrode potential in the Cr-substituted alloy and the Cr + C-substituted alloy.
FIG. 10 is a graph showing a process of a hydrogen pulverization process (an embrittlement process).

Claims (13)

A main phase of an R ₂ T ₁₄ Q-type tetragonal compound (R is at least one rare earth element, T is at least one transition metal element in which Fe is essential, and Q is boron and / or carbon); A rare earth sintered magnet having a grain boundary phase surrounding
The R ₂ T ₁₄ Q-type tetragonal compound in the main phase contains, as essential elements, Cr in which a part of Fe is substituted and carbon in which a part of boron is substituted.
The rare earth sintered magnet according to claim 1, wherein a carbon concentration of the main phase is higher than a carbon concentration of the grain boundary phase. 2. The rare earth sintered magnet according to claim 1, wherein the grain boundary phase contains Co that accounts for 50 atomic% or more and 90 atomic% or less of the entire grain boundary phase. The rare earth sintered magnet according to claim 2, wherein the grain boundary phase includes an R ₃ Co compound. The rare-earth sintered magnet according to any one of claims 1 to 3, wherein the R ₂ T ₁₄ Q-type tetragonal compound in the main phase contains, as an essential element, Co in which a part of Fe is substituted. The concentration of R is 12 atomic% or more and 18 atomic% or less,
The content of T is not less than 60 atomic% and not more than 88 atomic%,
The concentration of Cr is 0.1 atomic% or more and 2.4 atomic% or less,
B concentration is 0.5 atomic% or more and 13 atomic% or less,
The rare earth sintered magnet according to any one of claims 1 to 4, wherein the C concentration is 0.4 atomic% or more and 4.5 atomic% or less. A main phase of an R ₂ T ₁₄ Q-type tetragonal compound (R is at least one rare earth element, T is at least one transition metal element in which Fe is essential, and Q is boron and / or carbon); A rare earth sintered magnet having a grain boundary phase surrounding
A rare-earth sintered magnet, wherein the natural electrode potential of the R ₂ T ₁₄ Q-type tetragonal compound is −0.75 V or more. Rare earth sintered magnet according to claim 6, wherein the difference between the natural electrode potential of the grain boundary phase and the natural electrode potential of said R _{2 T} 14 _Q type tetragonal compound is 0.6V or less. A main phase of an R ₂ T ₁₄ Q-type tetragonal compound (R is at least one rare earth element, T is at least one transition metal element in which Fe is essential, and Q is boron and / or carbon); A method for producing a rare earth sintered magnet having a grain boundary phase surrounding
A main phase alloy containing R ₂ T ₁₄ Q type tetragonal compound in an amount of 50% by volume or more and containing Cr, boron, and carbon as essential elements, and a powder of a liquid phase alloy containing R and Co are included. The process of preparing,
Sintering the powder, thereby producing a rare earth sintered magnet having a carbon concentration of the main phase higher than a carbon concentration of the grain boundary phase;
A method for producing a rare earth magnet containing: In the main phase alloy,
R concentration is 11 atomic% or more and 16 atomic% or less,
The concentration of T is 60 atomic% or more and 87 atomic% or less,
Cr concentration is 0.2 atomic% or more and 2.5 atomic% or less,
The concentration of B is 1 atomic% or more and 14 atomic% or less,
The method for producing a rare-earth sintered magnet according to claim 8, wherein the content of C is 0.5 atom% or more and 5.0 atom% or less. As the main phase alloy, a first alloy containing 0.8% by mass or more and 1.0% by mass or less of Q, and a second alloy containing 1.2% by mass or more and 1.4% by mass or less of Q The method for producing a rare earth sintered magnet according to claim 9 for use. In the liquid phase alloy,
R concentration is 60 atom% or more and 80 atom% or less,
The method for producing a rare-earth sintered magnet according to claim 9, wherein the Co content concentration is 20 atomic% or more and 40 atomic% or less. The ratio of the alloy for a liquid phase to the total of the alloy for a main phase and the alloy for a liquid phase is set in a range of 2% by volume or more and 20% by volume or less. Manufacturing method of rare earth sintered magnet. Preparing a molten metal of the raw material alloy for the main phase alloy,
Cooling the molten metal of the raw material alloy at a speed of 100 seconds or more and 10,000 ° C./second or less, and solidifying;
The method for producing a rare earth sintered magnet according to any one of claims 8 to 12, further comprising:

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