JP4180049B2 - R-T-B permanent magnet - Google Patents

Description

  The present invention relates to an R-T-B system permanent magnet having a plating film formed on the surface and a plating film.

R—T—B system permanent magnet having R ₂ T ₁₄ B type intermetallic compound as the main phase (R is one or more of rare earth elements including Y, and T is essential for Fe or Fe and Co) 1 type or 2 types or more of transition metal elements) are used in various electric devices because they have excellent magnetic properties and Nd as a main component is abundant in resources and relatively inexpensive.
R-T-B permanent magnets having excellent magnetic properties also have some technical problems. One of them is corrosion resistance. That is, the R-T-B permanent magnet is inferior in corrosion resistance because R and Fe, which are main constituent elements, are easily oxidized. Therefore, a protective film for preventing corrosion is formed on the magnet surface. As the protective film, a resin coating, a chromate film, or plating is adopted. In particular, a method of forming a metal plating layer typified by Ni plating, Cu plating, or Sn plating on the surface is corrosion resistance, wear resistance, etc. It is excellent in and widely used.

  Proposals have been made to improve the corrosion resistance of the metal plating layer formed on the surface of the RTB-based permanent magnet. For example, in Japanese Patent No. 2941446 (Patent Document 1), a Ni plating layer containing 0.001 to 0.01 wt% of S is provided as a lower layer, and 0.001 to 1.0 wt% of S is contained thereon. The Ni plating layer is provided as an upper layer, and the upper Ni plating layer contains 0.01 wt% or more of S than the lower Ni plating layer, thereby improving the corrosion resistance. Patent Document 1 shows that by providing two Ni plating layers having the above S content relationship, the upper layer is anodized to generate an anodic effect, thereby preventing the lower layer from being corroded. .

Japanese Patent No. 2941446

  According to Patent Document 1, improved corrosion resistance can be imparted to the RTB-based permanent magnet. However, according to the study by the present inventors, it is not easy to control the S content in the plating film. S contained in the plating film is mainly derived from the brightener added to the plating bath. However, when the type of brightener is specified, it is difficult to freely set the amount of S contained in the plating film. Therefore, it must be said that the method for improving the corrosion resistance by controlling the S content in the plating film has little versatility with respect to the actual production of the R-T-B permanent magnet.

Further, hardness is listed as a characteristic required for the plating film. Depending on the manufacturing process or application of the R-T-B system permanent magnet, a considerable stress may be applied to the plated film surface, or wear resistance may be required for the surrounding environment. Because there is. However, no effective method for improving the hardness of the plating film has been proposed so far. In particular, the present inventors have not found a proposal for improving hardness including corrosion resistance, which is an essential characteristic required for a plating film.
The present invention has been made on the basis of such a technical problem, and is easily applied to the production of an actual R-T-B system permanent magnet, and is also effective in ensuring hardness. An object of the present invention is to provide an R-T-B system permanent magnet provided with the plating film.

The inventors of the present invention have confirmed that C is an effective element for improving the hardness of the plating film while easily controlling the content in the plating film for improving the corrosion resistance of the plating film. It has also been found that the adhesion of the plating film can be improved by containing a predetermined amount of C in the plating film. That is, the present invention relates to an R ₂ T ₁₄ B compound (where R is one or more of rare earth elements including Y, and T is one or more transition metal elements essential for Fe, Fe and Co). ) And a magnet body made of a sintered body including at least a grain boundary phase containing more R than the main phase crystal grains, and a C content covering the surface of the magnet body, Cc (wt %)), An electrolytic Ni plating film satisfying 0.005 <Cc ≦ 0.2 wt%. The electrolytic Ni plating film includes a first plating layer disposed on the surface of the magnet body, a first The second plating layer includes a second plating layer disposed on the plating layer, and the first plating layer has an S content below the detection limit by combustion in an oxygen stream-infrared absorption method, and the second plating layer is more than the first plating layer. R-T- with a low C content in the range of 0.048 wt% to 0.1 wt% The above problem is solved by a B-based permanent magnet.
In the present invention, the C content of the plating film is preferably set to 0.007 ≦ Cc ≦ 0.18wt%.

  According to the present invention, the corrosion resistance of the plating film can be improved by using C, whose content in the plating film can be easily controlled. The inclusion of a predetermined amount of C in the plating film can also improve the hardness of the plating film and the adhesion to the magnet body. In particular, as in the present invention, when the plating film is formed of a plurality of layers having different C contents, the effect becomes remarkable.

Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.
As shown in FIG. 1, the RTB-based permanent magnet 1 of the present invention includes a magnet body 2 and a plating film 3 coated on the surface of the magnet body 2. The present invention is characterized by the plating film 3, and the plating film 3 can impart excellent corrosion resistance to the plating film 3 by containing 0.005 <Cc ≦ 0.2 wt% of C. In addition, the plating film 3 containing such an amount of C has an effect of improving hardness and can improve the adhesion of the plating film 3 to the magnet body 2. If the C content is not more than 0.005 wt% (including zero), the above effect cannot be obtained. On the other hand, if the C content exceeds 0.2 wt%, cracks occur in the plating film 3 and it becomes impossible to ensure corrosion resistance. Accordingly, in the present invention, the amount of C contained in the plating film 3 is set to 0.005 <Cc ≦ 0.2 wt%. A preferable amount of C contained in the plating film 3 is 0.007 ≦ Cc ≦ 0.18 wt%, and a more preferable amount of C is 0.007 ≦ Cc ≦ 0.15 wt%.

  The plating film 3 containing 0.005 <Cc ≦ 0.2 wt% of C is effective for improving the corrosion resistance, improving the adhesion to the magnet body 2 and further improving the hardness. However, C contained in the plating film 3 has an effect of suppressing the growth of the structure constituting the plating film 3, particularly the growth in the plane direction. For this reason, the structure of the plating film 3 is refined and densified. It is estimated that the corrosion resistance and adhesion are improved. Similarly, it is presumed that the refinement of the structure contributed to the improvement in hardness.

In particular, as shown in FIG. 2, the present invention includes a first plating layer 3a disposed on the surface side of the magnet body 2 and a second plating layer 3b disposed on the first plating layer 3a. The difference in C content between the first plating layer 3a and the second plating layer 3b is 0.048 wt% or more and 0.1 wt% or less, which contributes to improvement of corrosion resistance. The reason why the corrosion resistance decreases when the difference in the C content between the first plating layer 3a and the second plating layer 3b exceeds 0.1 wt% is not clear, but the first plating layer 3a and the first plating layer 3a caused by the difference in the C amount It is presumed that the difference in the particle size of the two plating layers 3b occurs, causing mismatching in the vicinity of the interface and reducing the corrosion resistance. The difference in C content between the first plating layer 3a and the second plating layer 3b is more desirably 0.08 wt% or less.
The content of C contained in the first plating layer 3a and the second plating layer 3b may be uniform in the entire area of the first plating layer 3a and the second plating layer 3b, or the content may vary. Also good.

The present invention you a metal constituting the plating film 3 and Ni. This is because Ni has excellent corrosion resistance when the plating film 3 of the RTB-based permanent magnet 1 is configured. Of course, further improvement in corrosion resistance can be achieved by applying the present invention.

  Furthermore, the plating film 3 can also be configured by laminating a plurality of the same type of metal. For example, after forming one layer (first plating layer) of Ni plating on the magnet body 2, Ni plating (second plating layer) can be further laminated. When the plating film 3 has a plurality of layers, it is particularly preferable to form a plurality of Ni plating layers. The number of stacked layers is not limited to two or three layers, and may be four or more.

In the present invention, when the plating film 3 has a plurality of layers (two or more layers), when the C content of each plating layer is Cc (wt%), the range is 0.005 <Cc ≦ 0.2 wt%. It is necessary. Further, in the two plating layers that are in direct contact, the difference in C content is 0.048 wt% or more and 0.1 wt% or less, and more preferably 0.08 wt% or less.

The C content in the plating film 3 is in the range of 0.005 <Cc ≦ 0.2 wt% specified in the present invention, and the difference in C content between the first plating layer 3a and the second plating layer 3b is 0.048 wt. Although the method of making it into 0.1 to 0.1 wt% is not ask | required, C content in the plating film 3 is controllable by adjusting the following matters.
The C content in the plating film 3 can be varied by changing the number of C—C bonds in the plating bath. Specifically, the C content in the plating film 3 can be controlled by changing the type of the organic functional group in the plating bath. For example, the C content in the plating film 3 can be changed by changing HCHO, which is a kind of semi-brightening agent that may be contained in the plating bath, to CH ₃ CHO and further to C ₂ H ₅ CHO. Further, the C content in the plating film 3 can also be changed by changing the concentration of the brightening agent that may be contained in the plating bath. Brighteners include sulfonates such as sodium 1,5 naphthalene disulfonate, sodium 1,3,6 naphthalene trisulfonate, paratoluenesulfonamide, saccharin, formaldehyde, 1,4 butynediol, propargyl alcohol, ethylene cyanide Hydrine or the like can be used.

  As another method capable of controlling the C content in the plating film 3, there is a method of changing the current density applied to the plating bath during the plating step. Although the C content varies depending on the additive added to the plating bath, generally, the C content in the formed plating film 3 can be increased by increasing the current density. Therefore, when it is desired to increase the C content in the plating film 3, the current density of the plating bath during the plating process may be increased. Conversely, when it is desired to reduce the C content in the plating film 3, the current density of the plating bath during the plating process may be reduced.

  The thickness of the plating film 3 is preferably in the range of 1 to 30 μm. If the thickness of the plating film 3 is less than 1 μm, sufficient corrosion resistance cannot be obtained even if the present invention is applied. Moreover, even if the thickness of the plating film 3 exceeds 30 μm, the corrosion resistance effect is saturated, and the volume of the magnet body 2 occupying the R-T-B system permanent magnet 1 is reduced, so that per unit volume. Magnetic properties will be degraded. This decrease in magnetic characteristics becomes more prominent as the R-T-B permanent magnet 1 becomes smaller. Furthermore, the thickness of the plating film 3 is preferably 5 to 25 μm.

Next, the magnet body 2 will be described. When an R-T-B permanent magnet is used as the magnet body 2, the effect of the present invention is remarkable. As described above, the R-T-B permanent magnet is inferior in corrosion resistance. Hereinafter, a preferable chemical composition of the R—T—B system permanent magnet will be described.
The RTB-based permanent magnet contains 27.0 to 35.0 wt% of rare earth element (R). Here, the rare earth element has a concept including Y, and therefore R of the present invention is La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu and It is selected from one or more of Y. When the amount of the rare earth element in the magnet body 2 is less than 27.0 wt%, α-Fe having soft magnetism is precipitated, and the coercive force is significantly reduced. Moreover, if it is less than 27.0 wt%, the sinterability is inferior. On the other hand, if the rare earth element exceeds 35.0 wt%, the amount of R-rich phase is increased and the corrosion resistance is deteriorated, and the volume ratio of R ₂ T ₁₄ B crystal grains as the main phase is lowered, and the residual magnetic flux density is reduced. descend. Therefore, the amount of rare earth elements is 27.0 to 35.0 wt%. A preferable amount of the rare earth element is 28.0 to 33.0 wt%, and a more preferable amount of the rare earth element is 29.0 to 31.0 wt%.

  Among R, Nd and Pr are preferably the main component as a rare earth element is Nd or Pr because Nd and Pr have the best balance of magnetic properties and are abundant and relatively inexpensive. Dy and Tb have a large anisotropic magnetic field and are effective in improving the coercive force. Therefore, it is preferable that Nd, Pr, Dy, and Tb are selected as R, and the total of Nd, Pr, Dy, and Tb is 27.0 to 35.0 wt%. The amount of Dy or Tb is preferably determined 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 total amount of Dy and Tb is 0.1 to 4.0 wt%, and when a high coercive force is desired, the total amount of Dy and Tb is 4.0 to 12%. 0.0 wt% is preferable.

  The RTB-based permanent magnet constituting the magnet body 2 contains 0.5 to 2.0 wt% of boron (B). When B is less than 0.5 wt%, a high coercive force cannot be obtained. However, when B exceeds 2.0 wt%, the residual magnetic flux density tends to decrease. Therefore, the upper limit is 2.0 wt%. A preferable amount of B is 0.5 to 1.5 wt%, and a more preferable amount of B is 0.9 to 1.1 wt%.

  The R-T-B system permanent magnet constituting the magnet body 2 is Nb: 0.1 to 2.0 wt%, Zr: 0.05 to 0.25 wt%, Al: 0.02 to 2.0 wt%, The inclusion of one or more of Co: 0.3 to 5.0 wt% and Cu: 0.01 to 1.0 wt% is allowed. These are positioned as elements that substitute a part of Fe.

  The present invention allows elements other than the above elements to be contained. For example, it is preferable for the present invention to contain Ga, Bi, and Sn as appropriate. Ga, Bi, and Sn are effective in improving the coercive force and the temperature characteristics of the coercive force. However, excessive addition of these elements leads to a decrease in residual magnetic flux density, so 0.02 to 0.2 wt% is preferable. Further, for example, one or more of Ti, V, Cr, Mn, Ta, Mo, W, Sb, Ge, Ni, Si, and Hf can be contained.

Next, a method for manufacturing the magnet body 2 will be described.
As is well known, the RTB-based permanent magnet constituting the magnet body 2 has a main phase composed of R ₂ Fe ₁₄ B crystal grains and a grain boundary phase containing more R than the main phase. And a sintered body containing at least. A suitable manufacturing method for obtaining this sintered body will be described.

The raw material alloy can be produced by strip casting or other known melting methods in a vacuum or an inert gas, preferably in an Ar atmosphere. The R-T-B system according to the present invention is a so-called mixing method using an alloy mainly composed of R ₂ Fe ₁₄ B crystal grains (low R alloy) and an alloy containing more R than the low R alloy (high R alloy). The same applies to the production of permanent magnets.

  The raw material alloy is subjected to a grinding process. In the case of the mixing method, the low R alloy and the high R alloy are pulverized separately or together. 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 occluding hydrogen in the raw material alloy and then releasing it. This hydrogen pulverization can be regarded as coarse pulverization, and mechanical coarse pulverization can be omitted.

  After the coarse pulverization process, the process proceeds to the fine pulverization process. A jet mill is mainly used for fine pulverization, and a coarsely pulverized powder having a particle size of about several hundreds of μm has an average particle size of 2 to 10 μm, preferably 3 to 8 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.

In the case of the mixing method, the timing of mixing the two kinds of alloys is not limited. However, when the low R alloy and the high R alloy are separately pulverized in the pulverizing step, the pulverized low R alloy powder is used. And the high R alloy powder are preferably mixed in an inert gas 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. The mixing ratio when the low R alloy and the high R alloy are pulverized together is the same. By adding about 0.01 to 0.3 wt% of a grinding aid such as zinc stearate at the time of fine grinding, fine powder with high orientation can be obtained at the time of molding in the next magnetic field.
The fine powder obtained as described above is subjected to molding in a magnetic field. The forming in the magnetic field may be performed at a pressure of about 68.6 to 147 MPa (0.7 to 1.5 t / cm ² ) in a magnetic field of about 960 to 1600 kA / m (12 to 20 kOe).

  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, the difference of an average particle diameter, and particle size distribution, what is necessary is just to sinter at 1000-1100 degreeC for about 1 to 10 hours. You may perform the process which removes the grinding | pulverization adjuvant, gas, etc. which are contained in the molded object before a sintering process. After sintering, the obtained sintered body can be subjected to an aging treatment. This process is an important process for controlling the coercive force. In the case where the aging treatment is performed in two stages, holding for a predetermined time at around 800 ° C. and around 600 ° C. is effective. 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.

  After obtaining the sintered body, the plating film 3 is formed. The plated film 3 according to the present invention may be formed by either electrolytic plating or electroless plating, but is more preferably formed by electrolytic plating. This is because it is easy to control the C content. When performing electroplating, the process (pretreatment) before performing electroplating is performed with respect to a sintered compact. In this pretreatment, for example, barrel processing, degreasing, water washing, etching (for example, nitric acid), and water washing are performed after processing the sintered body to a predetermined shape and accuracy. This process is merely an example, and is not an element that limits the present invention. Next, a plating film 3 is formed by electrolytic plating. After the plating film 3 is formed, it is washed with water and dried to complete a series of steps for forming the plating film 3 by electrolytic plating.

The film formation of the plating film 3 will be further described.
As typical plating conditions when an electrolytic nickel plating film is formed as the plating film 3, the following can be applied. However, the following is only an example and does not limit the present invention.
(1) Plating bath: nickel sulfate, ammonium chloride, boric acid pH: 5.6 to 5.8
Temperature: 20-30 ° C
Current density: 0.5 to 5 A / dm ²
(2) Plating bath (Watt bath): Nickel sulfate, nickel chloride, boric acid pH: 4.5 to 5.5
Temperature: 40-60 ° C
Current density: 1-7 A / dm ²
(3) Plating bath: nickel sulfamate, nickel bromide, boric acid pH: 4.0-5.0
Temperature: 40-50 ° C
Current density: 1-15 A / dm ²

As typical plating conditions when an electrolytic copper plating film is formed as the plating film 3, the following can be applied. However, the following is only an example and does not limit the present invention.
(1) Plating bath: copper pyrophosphate trihydrate, potassium pyrophosphate, ammonia pH: 8-10
Temperature: 50-60 ° C
Current density: 2-6 A / dm ²
(2) Plating bath: copper salt, phosphate, aliphatic phosphonic acid compound, metal hydroxide pH: 9.5 to 10.5
Temperature: 55-65 ° C
Current density: 1-10 A / dm ²

When an electrolytic Sn plating film is formed as the plating film 3, any one of a ferrostan method, a halogen method, and an alkali method can be employed. The ferrostan method and the halogen method are plating methods using an acidic bath, and Sn is precipitated from Sn ²⁺ . In the ferrostan method, tin phenol sulfonate is used, and in the halogen method, stannous chloride is used. In the alkaline method, sodium stannate is the main component, and Sn is precipitated from Sn ²⁺ .

  In the above, the example which applies the plating film 3 by this invention to a RTB type permanent magnet was demonstrated. However, the plating film 3 according to the present invention is not limited to the use as a protective film for an R-T-B system permanent magnet, but other rare earth permanent magnets that require corrosion resistance, and other members that require corrosion resistance. Needless to say, it can be used as a protective film.

A ribbon-like alloy having a predetermined composition was produced by strip casting. This thin strip alloy was occluded with hydrogen at room temperature, and then heated to about 400 to 700 ° C. in an Ar atmosphere to perform dehydrogenation, thereby obtaining a coarse powder.
The coarse powder was pulverized using a jet mill. Milling was performed using a high-pressure N ₂ gas flow through the jet mill was replaced with N ₂ gas. The average particle size of the obtained fine powder was 4.0 μm. In addition, before performing fine grinding, 0.01 to 0.10 wt% of zinc stearate was added as a grinding aid.

The obtained fine powder was molded at a pressure of 98 MPa (1.0 ton / cm ² ) in a magnetic field of 1200 kA / m (15 kOe) to obtain a molded body. The molded body was sintered in a vacuum at 1030 ° C. for 4 hours and then rapidly cooled. Next, the obtained sintered body was subjected to a two-stage aging treatment of 850 ° C. × 1 hour and 540 ° C. × 1 hour (both in an Ar atmosphere). When the composition of the sintered body was analyzed, 26.5 wt% Nd-5.9 wt% Dy-0.25 wt% Al-0.5 wt% Co-0.07 wt% Cu-1.0 wt% B-Fe. It had a composition of bal.

  The obtained RTB-based permanent magnet was processed to prepare a sample having a size of 30 mm × 40 mm × 5 mm. This sample was barrel-polished and then subjected to alkali degreasing, nitric acid cleaning, and alkali ultrasonic cleaning. After drying the sample, the surface of the sample was plated with Ni under the conditions shown in Table 1. In addition, Test No. a and b are single-layer Ni platings, test nos. In c to h, multiple layers (two layers) of Ni plating were performed. In addition, Test No. c to e varied the C content of the first plating layer and the second plating layer by adjusting the film forming conditions of the first plating layer and the second plating layer. In addition, Test No. f to h varied the C content of the second plating layer by adjusting the current density of the second plating layer.

After the Ni plating was completed, the formed plating film was evaluated. Evaluation items and evaluation methods are shown below. The evaluation results are shown in Table 2.
<Plating film thickness measurement>: The film thickness was measured with a fluorescent X-ray microscopic part thickness meter. The film thickness was measured at the center of the plane of the sample, and the average value of the five samples was taken.
<Plating film composition analysis>: Only the plating film was peeled off, and the amounts of C and S were analyzed using combustion in an oxygen stream-infrared absorption method. Test No. For c to h (two-layer plating), a single layer sample obtained by plating a sintered sample (magnet body) under the same conditions as the first plating layer, and plating under the same conditions as the second plating layer A single layer sample was used. This is because in the case of two-layer plating, it is difficult to analyze the composition by separating the first plating layer and the second plating layer.
<Hardness>: Vickers hardness was measured with a Vickers hardness meter. The average value of 5 samples.
<Corrosion resistance>: 120 ° C., 100% RH, 2 atm. The sample surface state (swelling and rust) after being held for 100 hours under the following conditions was visually observed. The 20 samples were evaluated based on the ratio of the number of samples in which swelling and rust occurred.
<Adhesiveness>: 10 mm wide, 30-40 μm deep, 20 mm long cuts are placed in parallel on the plating film, and the two cuts are connected by a cut of the same depth. Only the plating film was peeled off vertically, and the peeling force at that time was measured. The average value of 5 samples.

  In Table 2, test no. Although a and b are single layer Ni plating, test No. with a small C content of 0.005 wt%. a is inferior in corrosion resistance and adhesion of the plating film. On the contrary, test No. with a large C content of 0.220 wt% was obtained. As for b, the crack generate | occur | produced in the plating film. In addition, test No. in which the crack occurred. For b, hardness, corrosion resistance, and adhesion were not evaluated.

  Test No. c to e are C contents in the range of 0.005 to 0.2 wt% recommended in the present invention for both the first plating layer and the second plating layer, and the second plating layer is more preferable than the first plating layer. Has a low C content. However, test no. As for e, the difference in C content between the first plating layer and the second plating layer (| first plating layer−second plating layer |) is as large as 0.115 wt%. Thus, when the difference of C content of a 1st plating layer and a 2nd plating layer is large, it turns out that corrosion resistance is inferior. Also, the hardness is low. Furthermore, test no. When c and d are compared, it can be seen that the smaller the difference in C content between the first plating layer and the second plating layer, or the higher the C content in the second plating layer, the harder the plating film.

  Test No. f to h are obtained by adjusting the current density at the time of forming the second plating layer. It can be seen that if the current density during film formation is increased, the C content contained in the plating layer increases. And test no. From f to h, it can be seen that the hardness of the plating film improves as the difference in C content between the first plating layer and the second plating layer decreases or as the C content in the second plating layer increases. .

It is a figure which shows the structure of the RTB type | system | group permanent magnet by this invention. It is a figure which shows the structural example of the plating film by this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... R-T-B system permanent magnet, 2 ... Magnet element body, 3 ... Plating film, 3a ... 1st plating layer, 3b ... 2nd plating layer

Claims (2)

Main phase comprising R ₂ T ₁₄ B compound (where R is one or more of rare earth elements including Y and T is one or more transition metal elements in which Fe, Fe and Co are essential) A magnet body composed of a sintered body including at least crystal grains and a grain boundary phase containing more R than the main phase crystal grains;
An electrolytic Ni plating film that covers the surface of the magnet body, where C content is Cc (wt%), and 0.005 <Cc ≦ 0.2 wt%;
With
The electrolytic Ni plating film includes a first plating layer disposed on the magnet body surface side and a second plating layer disposed on the first plating layer,
The content of S in the first plating layer is below the detection limit by combustion in an oxygen stream-infrared absorption method,
The RTB -based permanent magnet is characterized in that the second plating layer has a C content smaller than that of the first plating layer in a range of 0.048 wt% to 0.1 wt%.   2. The RTB-based permanent magnet according to claim 1, wherein a C content Cc (wt%) of the first plating layer satisfies 0.098 ≦ Cc (wt%). 3.

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