Patent Application
20170018342
The present invention relates to an R-T-B based sintered magnet, and a method for producing same.
An R-T-B-based sintered magnet including an R2T14B type compound as a main phase (R is at least one of rare-earth elements and indispensably includes Nd, and T is a transition metal element and indispensably includes Fe) has been known as a permanent magnet with the highest performance among permanent magnets, and has been used in various motors such as a voice coil motor (VCM) for hard disk drive, a motor for electric cars (EV, HV, PHV, etc.), and motors for industrial apparatuses, and home appliances.
The R-T-B based sintered magnet is mainly composed of a main phase made of an R2T14B compound, and a grain boundary phase located on a grain boundary portion of this main phase. The R2T14B compound as the main phase is a ferromagnetic material having high magnetization and forms the backbone of properties of the R-T-B based sintered magnet.
In the R-T-B-based sintered magnet, a coercive force HcJ (hereinafter sometimes simply referred to as “HcJ”) decreases at an elevated temperature to cause irreversible thermal demagnetization. Therefore, when used particularly in motors for electric cars (or motors for hybrid cars), there is a need to maintain high HcJ even at an elevated temperature. To suppress irreversible thermal demagnetization at an elevated temperature, namely, to maintain high HcJ even at an elevated temperature, there is a need to obtain higher HcJ at room temperature.
It has been known that HcJ is improved when light rare-earth elements RL (mainly, Nd and/or Pr) included in R of an R2T14B compound as a main phase are partially substituted with heavy rare-earth elements RH (mainly, Dy and/or Tb) in the R-T-B based sintered magnet. With increasing the amount of heavy rare-earth elements RH substitution, HcJ is improved.
To increase HcJ, numerous heavy rare-earth elements (mainly, Dy) have hitherto been added to the R-T-B-based sintered magnet. However, there arose a problem that a residual magnetic flux density Br (hereinafter sometimes simply referred to as “Br”) decreases. Therefore, there has recently been employed a method in which heavy rare-earth elements are diffused from the surface into the inside of the R-T-B-based sintered magnet to thereby increase the concentration of the heavy rare-earth elements at the outer shell part of main phase crystal grains, thus obtaining high HcJ while suppressing a reduction in Br.
However, Dy has problems such as unstable supply and price fluctuations because of restriction of the producing district. Therefore, there is a need to develop technique for improving HcJ of the R-T-B-based sintered magnet without using heavy rare-earth elements such as Dy as much as possible (by reducing the usage amount as much as possible).
Patent Document 1 mentions that the amount of B is limited to a relatively small amount in a specific range as compared with a conventionally used R-T-B-based alloy, and one or more metal elements M selected from among Al, Ga and Cu are included to form an R2T17 phase, and also a volume fraction of a transition metal-rich phase (R6T13M) formed from the R2T17 phase as a raw material is sufficiently secured to obtain an R-T-B-based rare-earth sintered magnet having a high coercive force while suppressing the content of Dy.
Patent Document 2 discloses that the amount of B is made smaller than that of a conventional R-T-B-based alloy and amounts of B, Al, Cu, Co, Ga, C, and O are adjusted in a predetermined range, and also atomic ratio of Nd and Pr to B, and atomic ratio of Ga and C to B satisfy a specific relation to obtain high residual magnetic flux density and coercive force.
There have been proposed various methods in which a metal, an alloy, or a compound, each containing heavy rare-earth elements RH, as an improving means for HcJ of an R-T-B based sintered magnet are supplied to a surface of the R-T-B based sintered magnet by a specific means, and heavy rare-earth elements RH are diffused into the inside of the magnet by a heat treatment, and also light rare-earth elements RL at the outer shell portion of an R2T14B compound are substituted with heavy rare-earth elements RH to thereby improve HcJ while suppressing a reduction in Br.
For example, Patent Document 3 discloses a method in which a bulk body including an R—Fe—B-based rare-earth sintered magnet body and heavy rare-earth elements RH (at least one selected from the group consisting of Dy, Ho, and Tb) are disposed in a treatment chamber and then heated to a temperature of 700° C. or higher and 1,000° C. or lower to thereby diffuse heavy rare-earth elements RH into the inside of the R—Fe—B-based rare-earth sintered magnet body while supplying heavy rare-earth elements RH to a surface of the R—Fe—B-based rare-earth sintered magnet body from the bulk body.
Furthermore, Patent Document 4 mentions that an R-T-B-based alloy containing 4 to 10% by mass of Dy is mixed with a high melting point compound having a melting point of 1,080° C. or higher (an oxide, a boride, a carbide, a nitride, or a silicate of one selected from the group consisting of Al, Ga, Mg, Nb, Si, Ti, and Zr), and then the mixture is molded and sintered to obtain a high coercive force without increasing the Dy concentration, thus enabling suppression of degradation of magnetic properties such as magnetization (Br) due to the addition of Dy.
However, the inventors of the present invention have found that a sintered magnet with the composition in which the amount of B is made smaller than that of a common R-T-B based sintered magnet (made smaller than a stoichiometric ratio of the amount of B of the R2T14B type compound) and Ga is added, as mentioned in Patent Documents 1 and 2, has a problem that only a slight change in the amount of B leads to a significant change in HcJ.
For example, only slight change in the amount of B by 0.01% by mass may leads to change in HcJ by 100 kA/m. Whereas, in a common R-T-B based sintered magnet (including B in the amount larger than a stoichiometric ratio of the amount of B of the R2T14B type compound), HcJ scarcely changes even if B changes by 0.1% by weight.
Therefore, there is a need for the R-T-B based sintered magnet, in which the amount of B is made smaller than that of the common R-T-B based sintered magnet, and Ga is added, to control the amount of B at high precision of 0.01% by mass so as to suppress a change in HcJ. However, it is significantly difficult to control the amount of B at precision of 0.01% by mass when a raw material alloy is melted and subjected to casting, in a mass-production facility.
One embodiment according to the present invention (first embodiment) has been made so as to solve these problems and an object thereof is to provide an R-T-B based sintered magnet that causes little change in HcJ to a change in the amount of B, and also has high Br and high HcJ.
Problems according to another embodiment will be described below.
There is a need to produce an R-T-B-based alloy with new composition that is different from before in Patent Document 1, so that there is a need to find out optimum conditions of melting and casting conditions of alloys, pulverization conditions, sintering conditions, and heat treatment conditions all over from the beginning. When the respective conditions are different from current production conditions, there arise problems, that is, there is a need to change various conditions of each facility each time a new R-T-B-based alloy is produced, leading to an increase in man-hours and costs during production.
Furthermore, according to Patent Document 1, an R-T-B based sintered magnet having higher HcJ than before. However, use of Dy is indispensable to satisfy high HcJ required when used in motors for electric cars, motors for hybrid cars, and the like. Therefore, there is nothing to do but to apply a method in which heavy rare-earth elements are supplied from a surface of an R-T-B based sintered magnet and diffused into the inside, as disclosed in Patent Document 3, so as to reduce the usage amount of Dy.
However, when the method disclosed in Patent Document 3 is applied to an R-T-B-based rare-earth sintered magnet disclosed in Patent Document 1, there arises a problem such as drastic decrease in squareness ratio Hk/HcJ (hereinafter sometimes simply referred to as “Hk/HcJ”. Hk is the value of H at the position where J becomes a given proportion of the value to the value of Jr [residual magnetization=Br] in a second quadrant of a J [magnitude of magnetization]−H [intensity of magnetic field]curve. In the R-T-B based sintered magnet, 0.9×Jr [0.9×Br] is often used as a given proportion of the value).
According to Patent Document 4, a high coercive force can be obtained without increasing Dy concentration. Because of significantly large amount of Dy included in an R-T-B-based alloy (4 to 10% by mass in an R-T-B-based alloy), it is impossible to satisfy users' requirements of improving HcJ without causing degradation of Br, without using heavy rare-earth elements RH as much as possible.
Furthermore, in Patent Document 4, an oxide, a boride, a carbide, a nitride, or a silicate of one selected from the group consisting of Al, Ga, Mg, Nb, Si, Ti, and Zr is used as a high melting point compound. There is a possibility that oxygen, boron, carbon, nitrogen, silicon, and the like included in the compounds remain in a magnet even after sintering to thereby cause degradation of magnetic properties of the thus obtained magnet.
An object of the other one embodiment (second embodiment) according to the present invention is to provide an R-T-B based sintered magnet having high HcJ and high Hk/HcJ at low cost while suppressing a reduction in Br without using heavy rare-earth elements RH as much as possible.
An aspect 1-1 of the first embodiment according to the present invention is directed to an R-T-B based sintered magnet, wherein the composition represented by the following formula (1) satisfies the following inequality expressions (2) to (9):
uRwBxGazAlvCoqTigFejM (1)
(R is at least one of rare-earth elements and indispensably includes Nd, M is an element except for R, B, Ga, Al, Co, Ti, and Fe, and u, w, x, z, v, q, g, and j are expressed in terms of % by mass)
29.0≦u≦32.0 (2)
(heavy rare-earth elements RH account for 10% by mass or less of the R-T-B based sintered magnet)
0.93≦w≦1.00 (3)
0.3≦x≦0.8 (4)
0.05≦z≦0.5 (5)
0≦v≦3.0 (6)
0.15≦q≦0.28 (7)
60.42≦g≦69.57 (8)
0≦j≦2.0 (9)
and, when the value obtained by dividing g by the atomic weight of Fe is g′, the value obtained by dividing v by the atomic weight of Co is v′, the value obtained by dividing z by the atomic weight of Al is z′, the value obtained by dividing w by the atomic weight of B is w′, and the value obtained by dividing q by the atomic weight of Ti is q′, the following inequality expressions (A) and (B) are satisfied.
0.06≦(g′+v′+z′)−(14×(w′−2×q′)) (A)
0.10≧(g′+v′+z′)−(14×(w′−q′)) (B)
An aspect 1-2 of the first embodiment according to the present invention is directed to the R-T-B based sintered magnet according to Aspect 1-1, wherein 0.18≦q≦0.28.
An aspect 1-3 of the first embodiment according to the present invention provides the R-T-B based sintered magnet according to the aspect 1-1 or 1-2, which has a structure in which:
an R2T14B compound (R is at least one of rare-earth elements and indispensably includes Nd, and T is at least one of transition metal elements and indispensably includes Fe),
an R6T13A compound (R is at least one of rare-earth elements and indispensably includes Nd, T is at least one of transition metal elements and indispensably includes Fe, and A is at least one of Ga, Al, Cu and Si and indispensably includes Ga), and
a boride of Ti coexist.
An aspect 1-4 of the first embodiment according to the present invention provides the R-T-B based sintered magnet according to any one of the aspects 1-1 to 1-3, wherein an area ratio of the R6T13A compound in an arbitrary cross section of the R-T-B based sintered magnet is 2% or more.
An aspect 2-1 of the second embodiment according to the present invention is a method for producing an R-T-B based sintered magnet, which includes the steps of:
preparing an alloy powder including:
R: 27 to 35% by mass (R is at least one of rare-earth elements and indispensably includes Nd),
B: 0.9 to 1.0% by mass,
Ga: 0.15 to 0.6% by mass, and
balance T (T is at least one of transition metal elements and indispensably includes Fe) and inevitable impurities;
preparing a powder of a hydride of Ti;
mixing the alloy powder with the powder of a hydride of Ti so as to adjust the amount of Ti included in 100% by mass of the mixed powder after mixing to 0.3% by mass or less to thereby prepare the mixed powder;
molding the mixed powder to prepare a molded body;
sintering the molded body to prepare an R-T-B based sintered magnet material; and
subjecting the R-T-B based sintered magnet material to a heat treatment.
An aspect 2-2 of the present invention is directed to the method for producing an R-T-B based sintered magnet according to the aspect 2-1, which includes the steps of:
preparing an RH diffusion source comprising a metal, an alloy, or a compound, each containing Dy and/or Tb, in place of the step of subjecting the R-T-B based sintered magnet material to a heat treatment;
subjecting to an RH supply and diffusion treatment of supplying Dy and/or Tb of the RH diffusion source to the R-T-B based sintered magnet material, and diffusing Dy and/or Tb; and
subjecting the R-T-B based sintered magnet material after the RH supply and diffusion treatment step to a heat treatment.
An aspect 2-3 of the present invention is directed to the method for producing an R-T-B based sintered magnet according to the aspect 2-1 or 2-2, which includes:
B: 0.91 to 1.0% by mass.
An aspect 2-4 of the present invention is directed to the method for producing an R-T-B based sintered magnet according to any one of the aspects 2-1 to 2-3, wherein the R-T-B based sintered magnet has a structure in which:
an R2T14B compound (R is at least one of rare-earth elements and indispensably includes Nd, and T is at least one of transition metal elements and indispensably includes Fe),
an R6T13M compound (R is at least one of rare-earth elements and indispensably includes Nd, T is at least one of transition metal elements and indispensably includes Fe, and M is at least one of Ga, Al, Cu and Si and indispensably includes Ga), and
a boride of Ti coexist.
An aspect 2-5 of the present invention is directed to the method for producing an R-T-B based sintered magnet according to the aspect 2-4, wherein an area ratio of the R6T13M compound in an arbitrary cross section of the R-T-B based sintered magnet is 1% or more.
An aspect 2-6 of the present invention is directed to the method for producing an R-T-B based sintered magnet according to the aspect 2-5, wherein an area ratio of the R6T13M compound in an arbitrary cross section of the R-T-B based sintered magnet is 2% or more.
According to one embodiment of the present invention, it is possible to provide an R-T-B based sintered magnet that causes little change in HcJ to a change in the amount of B, and also has high Br and high HcJ.
According to the other one embodiment of the present invention, it is possible to provide an R-T-B based sintered magnet having high HcJ and high Hk/HcJ at low cost while suppressing a reduction in Br without using heavy rare-earth elements RH as much as possible.
Embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description below, if necessary, the terms indicative of the specific direction or position (for example, “upper”, “lower”, “right”, “left”, and other words including these words) are used for easy understanding of the present invention with reference to the drawings. The meanings of the terms are not intended to restrict the scope of the present invention.
The inventors have intensively studied and found that sintered magnet shows a change in HcJ to a change in the amount of B is suppressed. The sintered magnet has a composition, in which titanium is added so as to adjust to the content in a specific range to form a boride of titanium during the production process, whereby, the amount of B obtained by subtracting the amount of B, consumed as a result of bonding with Ti during the production process, from the amount of B of the entire R-T-B based sintered magnet (hereinafter, residual amount of B, which does not form a boride with Ti, is sometimes referred to as an effective amount of B “amount of Beff”) is made smaller than the amount of B of the entire common R-T-B based sintered magnet (less than the amount of B of a stoichiometric ratio of an R2T14B type compound); and Ga and the like are also added. The inventors have also confirmed that, when such addition of Ti is performed, high Br and high HcJ are obtained, as well as the effect recognized in a sintered magnet in which the amount of B is made smaller than a stoichiometric ratio of an R2T14B type compound, and Ga is added.
The inventors have confirmed that a boride of Ti (TiB and/or TiB2) is formed in an R-T-B based sintered magnet according to this embodiment. In this embodiment, a boride of Ti is formed so that the amount of Beff becomes less than the amount of B of a common R-T-B based sintered magnet. A mechanism in which inclusion of a predetermined content of Ti enables suppression of a change in HcJ even if the amount of B changes, proposed by the inventors based on the above-mentioned confirmation, is as follows. Note that the mechanism mentioned below is not intended to limit the scope of the invention according to this embodiment.
As mentioned above, it is possible for a sintered magnet, which employs the composition in which the amount of B is made smaller than that of a common R-T-B based sintered magnet (less than the amount of B of a stoichiometric ratio of an R2T14B type compound), and also Ga is added, to obtain high HcJ.
The reason is considered as follows. That is, when the amount of B is less than a stoichiometric ratio of an R2T14B type compound, a R2T17 phase is formed because of excessive R and T. Usually, magnetic properties are quickly degraded with the decrease of the amount of B. However, if the magnet composition contains Ga, an R-T-Ga phase (a representative example is an R6T13A compound) is formed in place of the R2T17 phase, thus obtaining high HcJ.
As used herein, “R-T-Ga phase” includes those including: R: 20 atomic % or more and 35 atomic % or less, T: 55 atomic % or more and 75 atomic % or less, and Ga: 3 atomic % or more and 15 atomic % or less, and a typical example is an R6T13Ga compound. The R-T-Ga phase sometimes includes, as inevitable impurities, Al, Si, Cu and the like, and the phase can be defined as an R6T13A compound (R is at least one of rare-earth elements and indispensably includes Nd, T is at least one of transition metal elements and indispensably includes Fe, A is at least one of Ga, Al, Cu, and Si and indispensably includes Ga). For example, the compound is sometimes an R6T13(Ga1-i-y-sAliSiyCus) compound.
However, as mentioned above, in the sintered magnet, which employs the composition in which the amount of B is made smaller than that of a common R-T-B based sintered magnet, and also Ga is added, HcJ significantly changes when the amount of B changes. The reason is considered as follows. That is, since the amount of the R-T-Ga phase formed significantly changes according to how much the amount of B become less than a stoichiometric ratio of an R2T14B type compound (how much do R and T become excessive), it is considered that dependency of HcJ on the amount of B increases.
Whereas, the inventors have intensively studied and found that, when the amount of Beff is made smaller than the amount of B of a stoichiometric ratio of an R2T14B type compound by adding Ti to form a boride (TiB and/or TiB2), it is possible to reduce dependency of HcJ on the amount of B of the entire magnet.
The reason is considered as follows. That is, like this embodiment, when the amount of Beff is made smaller than the amount of B of a common R-T-B based sintered magnet, by forming a boride of Ti in an R-T-B based sintered magnet with the composition in which the amount of B is more than the amount of B determined from a stoichiometric ratio of an R2Fe14B type compound, the addition of Ga leads to suppression of formation of an R2T17 phase to form an R-T-Ga phase, thus improving HcJ. At this time, when the amount of B of the entire magnet composition changes to the amount of B of a stoichiometric ratio of an R2T14B type compound, a formation ratio of TiB to TiB2 changes. Namely, when there is a small difference between the amount of B of the entire magnet composition and the amount of B determined from a stoichiometric ratio of an R2T14B type compound (namely, smaller amount of B contained), TiB is formed in a larger amount than that of TiB2. In contrast, when there is a large difference between the amount of B of the entire magnet composition and the amount of B determined from a stoichiometric ratio of an R2T14B type compound (namely, larger amount of B contained), TiB2 is formed in a larger amount than that of TiB. As mentioned above, a B-rich Ti boride (TiB2) is formed as B increases, while a B-poor Ti boride (TiB) is formed as B decreases. Therefore, it is possible to reduce a change in the amount of B (amount of Beff), which does not form a Ti compound with Ti in the magnet, even if the amount of B of the entire magnet changes. As a result, it is possible to reduce a change in the formation amount of an R-T-Ga phase to a change in the amount of B, thus enabling suppression of a change in HcJ.
Based on the above-mentioned these considerations, the inventors have further studied and found that it is possible to adjust the formation amount of an R-T-Ga phase in an appropriate range when the amount of Ti and the amount of B satisfy the formula (A) and the formula (B), thus making it possible to obtain high Br and high HcJ while suppressing a change in HcJ to a change in the amount of B:
0.06≦(g′+v′+z′)−(14×(w′−2×q′)) (A)
0.10≧(g′+v′+z′)−(14×(w′−q′)) (B)
where g′ is the value obtained by dividing g by the atomic weight of Fe (55.845), v′ is the value obtained by dividing v by the atomic weight of Co (58.933), z′ is the value obtained by dividing z by the atomic weight of Al (26.982), w′ is the value obtained by dividing w by the atomic weight of B (10.811), and q′ is the value obtained by dividing q by the atomic weight of Ti (47.867).
The formula (A) and the formula (B) will be described below.
When the amount of Beff is lower than a stoichiometric ratio of the R2T14B type compound, Fe, and Co and Al capable of easily replacing a Fe site of a main phase become excessive (namely, the total amount of Fe, Co, and Al becomes more excessive than the amount of T of a stoichiometric ratio of the R2T14B type compound). Therefore, when the entire Ti becomes TiB2 (namely, when Ti is bonded with the largest number of B), there is a need that [(g′+v′+z′)−(14×(w′−2×q′))](the total of Fe, Co, and Al that does not form a main phase) is larger than 0 (Fe, Co, and Al becomes excessive) so as to make the amount of Beff less than the amount of B of a stoichiometric ratio of an R2T14B type compound. It is the formula (A) defining that the total of Fe, Co, and Al, which do no form this main phase, is 0.06 or more. It is possible to appropriately form an R-T-Ga phase by adjusting to 0.06 or more. It is possible to determine the formula (A) by calculating using, as analytical values of Fe(g), Co(v), Al(z), B(w), and Ti(q), the values (g′, v′, z′, w′, and q′) divided by each atomic weight of Fe, Co, Al, B, and Ti. The same shall apply to the formula B mentioned below.
When the total of Fe, Co, and Al, which do not form a main phase, is less than 0.06, it may become impossible to obtain high HcJ because of too small phase proportion of an R-T-Ga phase.
Furthermore, in this embodiment, the formula (B) defines that [(g′+v′+z′)−(14×(w′−q′))](the total of Fe, Co, and Al that does not form a main phase) is 0.10 or less when the entire Ti becomes TiB (namely, when Ti is bonded with the smallest number of B). When the total of Fe, Co, and Al, which do not form a main phase, exceeds 0.10, it may become impossible to obtain high Br because of too high proportion of an R-T-Ga phase.
As mentioned above, the R-T-B based sintered magnet of this embodiment may have a structure in which an R2T14B compound, an R6T13A compound, and a boride of Ti (TiB2 or TiB, and TiB2) coexist. In a preferable aspect, in the R-T-B based sintered magnet of this embodiment, an R6T13A compound is included in an area ratio of 2% or more in an arbitrary cross section thereof. It is possible to determine the area ratio of the R6T13A compound by analyzing an image of a reflected electron image (BSE image) of an arbitrary cross section of the R-T-B based sintered magnet, which is observed by a field emission-type scanning electron microscope (FE-SEM), using commercially available an image processing software, as shown in the below-mentioned Examples.
As used herein, “arbitrary cross section” means an arbitrary cross section selected based on rational expectations that typical feature of an R-T-B based sintered magnet according to the present invention, like a cross section including the center portion, and does not include a cross section selected arbitrarily so as not to disclose the feature of the present invention.
The composition of an R-T-B based sintered magnet according to this embodiment will be described in detail below.
As mentioned above, in this embodiment, Ti is added to form a boride of Ti, thus reducing the amount of Beff to less than the amount of B of a common R-T-B based sintered magnet, and Ga is included. Whereby, an R-T-Ga phase is formed on a grain boundary and, even if the content of heavy rare-earth elements such as Dy is suppressed, high HcJ can be obtained.
The composition of an R-T-B based sintered magnet according to this embodiment can be represented by the formula (1):
uRwBxGazAlvCoqTigFejM (1)
(R is at least one of rare-earth elements and indispensably includes Nd; M is an element except for R, B, Ga, Al, Co, Ti, and Fe; and u, w, x, z, v, q, g, and j are expressed in terms of % by mass)
Hereinafter, a description will be made of the composition ranges of individual elements, namely, numerical value ranges of u, w, x, z, v, q, g, and j.
R in the R-T-B based sintered magnet of this embodiment is at least one of rare-earth elements, and indispensably includes Nd. In the R-T-B based sintered magnet according to this embodiment, since high Br and high HcJ can be obtained without using heavy rare-earth elements RH, the addition amount of RH can be reduced even when higher HcJ is required, RH can be typically set at 10% by mass or less, and preferably 5% by mass or less.
As shown in the inequality expression (2), the content of R is in a range of 29.0% by mass to 32.0% by mass.
29.0≦u≦32.0 (2)
When the content of R is less than 29.0% by mass, it may be impossible to ensure R required to form a sufficient amount of an R-T-Ga phase, thus failing to obtain high HcJ. When the content exceeds 32.0% by mass, the proportion of a main phase is reduced, thus failing to obtain high Br.
AS shown in the inequality expression (3), the content of B is in a range of 0.93% by mass to 1.00% by mass.
0.93≦w≦1.00 (3)
The content of B of less than 0.93% by mass leads to excessive reduction in the amount of Beff and precipitation of an R2T17 phase, thus failing to obtain high HcJ, or the proportion of a main phase is reduced, thus failing to obtain high Br. When the content exceeds 1.00% by mass, an R-T-Ga phase is not sufficiently formed, thus failing to obtain high HcJ.
As shown in the inequality expression (4), the content of Ga is in a range of 0.3% by mass to 0.8% by mass.
0.3≦x≦0.8 (4)
The content of Ga of less than 0.3% by mass leads to excessively small formation amount of an R-T-Ga phase, thus failing to allow an R2T17 phase to disappear and to obtain high HcJ. When the content exceeds 0.8% by mass, the unnecessary Ga exists, and thus the proportion of a main phase may be reduced, leading to a reduction in Br.
As shown in the inequality expression (5), the content of Al is in a range of 0.05% by mass to 0.5% by mass.
0.05≦z≦0.5 (5)
Inclusion of Al enables an improvement in HcJ. Al may be included as inevitable impurities, or included by positive addition. When the content of Al exceeds 0.5% by mass, Br may be reduced. Al is included in the amount of 0.05% by mass or more and 0.5% by mass or less as the total amount of Al included as inevitable impurities and Al added positively.
As shown in the inequality expression (6), the content of Co is 3.0% by mass or less.
0≦v≦3.0 (6)
Co may be included in the amount of 3.0% by mass or less. Co is effective for improvement of temperature properties and improvement of corrosion resistance. When the content of Co exceeds 3.0% by mass, it may be impossible to obtain high Br.
As shown in the inequality expression (7), the content of Ti is in a range from 0.15% by mass to 0.28% by mass.
0.15≦q≦0.28 (7)
When the content of Ti is less than 0.15% by mass, it may be impossible to suppress a change in HcJ due to a change in the amount of B. When the content exceeds 0.28% by mass, the proportion of a main phase may be reduced, thus failing to obtain high Br. As shown in the inequality expression (10) mentioned below, the content is preferably in a range of 0.18% by mass or more and 0.28% by mass or less. It is possible to suppress a change in HcJ due to a change in the amount of B.
0.18≦q≦0.28 (10)
As shown in the inequality expression (8), the content of Fe is in a range of 60.42% by mass to 69.57% by mass.
60.42≦g≦69.57 (8)
When the content of Fe is less than 60.42% by mass, the proportion of a main phase may be reduced, thus failing to obtain high Br. When the content exceeds 69.57% by mass, excessive R-T-Ga phase may be formed, leading to a reduction in the proportion of a main phase, thus failing to obtain high Br.
M is an element except for R, B, Ga, Al, Co, Ti, and Fe.
As shown in the inequality expression (9), 2.0% by mass or less of an element M except for R, B, Ga, Al, Co, Ti, and Fe may be included in total.
0≦g≦2.0 (9)
Namely, the inequality expression (9) indicates that an optional element (that may be plural kinds of elements) and inevitable impurities (excluding Al when Al corresponds to inevitable impurities) may be included in the total amount of up to 2.0% by mass for the purpose of improving properties of the thus obtained R-T-B based sintered magnet.
It is possible to include, as an element capable of improving properties of an R-T-B based sintered magnet, for example, Cu, Ni, Ag, Au, Mo, and the like in the amount in a range of 0% by mass to 2.0% by mass.
It is particularly preferred to include Cu. Higher HcJ can be obtained by including Cu. The content of Cu is more preferably 0.05% by mass or more and 1.0% by mass or less.
In one of preferred embodiments of M, M is composed of inevitable impurities (as mentioned above, Cu is preferably included). Examples of inevitable impurities in the R-T-B based sintered magnet of this embodiment include inevitable impurities, which are usually contained in raw materials used industrially, such as a didymium alloy (Nd—Pr alloy), electrolytic iron, ferroboron, and the like. Examples of these inevitable impurities include Cr, Mn, Si, and the like. Furthermore, examples of inevitable impurities in the production process include oxygen (O), nitrogen (N), carbon (C), and the like. Preferably, the amount of O is in a range of 600 to 8,000 ppm, the amount of N is 800 ppm or less, and the amount of C is in 1,000 ppm or less.
It is possible to employ high-frequency inductively coupled plasma emission spectrometry (ICP emission spectrometry, ICP-OES) in evaluation of u, w, x, z, v, q, g, and j, which are contents (% by mass) of R, B, Ga, Al, Co, Ti, Fe, and M shown in the formula (1), respectively. For example, a gas fusion-infrared absorption method can be employed for evaluation of the amount of oxygen. For example, a gas fusion-thermal conductivity method can be employed for evaluation of the amount of nitrogen. For example, a gas analyzer by a combustion infrared absorption method can be employed for evaluation of the amount of carbon.
An example of a method for producing an R-T-B based sintered magnet of this embodiment will be described below. The method for producing an R-T-B based sintered magnet includes a step of obtaining an alloy powder, a molding step, a sintering step, and a heat treatment step. Each step will be described below.
Metals or alloys of the respective elements are prepared so as to obtain the above-mentioned composition, followed by melting and further casting to obtain an alloy having a predetermined composition. Typically, a flaky alloy is produced using a strip casting method. The flaky alloy thus obtained is subjected to hydrogen grinding to obtain a coarsely pulverized powder having a size of 1.0 mm or less. Next, the coarsely pulverized powder is finely pulverized by a jet mill to obtain a finely pulverized powder (alloy powder) having a grain size D50 (median size on a volume basis obtained by a laser diffraction method using an air flow dispersion method) of 3 to 7 μm. One kind of an alloy powder (alloy powder alone) may be used as the alloy powder. It is also possible to use a so-called two-alloy method in which two or more kinds of alloy powders are mixed and then pulverized to obtain an alloy powder (mixed alloy powder). Alternatively, an alloy powder may be produced so as to obtain the composition of this embodiment, using a known method. A known lubricant may be used as a pulverization assistant in a coarsely pulverized powder before jet mill pulverization, or an alloy powder during and after jet mill pulverization.
Regarding the addition of Ti, in the production of a raw material alloy using a strip casting method, when a molten metal for subjecting to casting is obtained, Ti is added in the form of Ti metal, a Ti alloy, or a Ti-containing compound to obtain a molten metal containing Ti, followed by solidification. Alternatively, Ti may be added in the form of Ti metal, a Ti alloy, or a Ti-containing compound before finishing of molding and after preparation of a raw material alloy. The method includes, for example, a method in which a hydride of Ti (TiH2, etc.) is added to an alloy powder before and after hydrogen grinding, or after jet mill pulverization.
Using the alloy powder thus obtained, molding under a magnetic field is performed to obtain a molded body. The molding under a magnetic field may be performed using known optional methods of molding under a magnetic field including a dry molding method in which a dry alloy powder is loaded in a cavity of a mold and then molded while applying a magnetic field, and a wet molding method in which a slurry containing the alloy powder dispersed therein is injected in a cavity of a mold and then molded under a magnetic field while discharging a dispersion medium of the slurry.
The molded body is sintered to obtain a sintered magnet. A known method can be used to sinter the molded body. To prevent oxidation from occurring due to an atmosphere during sintering, sintering is preferably performed in a vacuum atmosphere or an atmospheric gas. It is preferable to use, as the atmospheric gas, an inert gas such as helium or argon.
The sintered magnet thus obtained is preferably subjected to a heat treatment for the purpose of improving magnetic properties. Known conditions can be employed for heat treatment temperature, heat treatment time, and the like. To impart a final product shape to the sintered magnet, the magnet may be subjected to machining such as grinding. In that case, the heat treatment may be performed before or after machining. The sintered magnet may also be subjected to a surface treatment. The surface treatment may be a known surface treatment, and it is possible to perform surface treatments, for example, Al vapor deposition, Ni electroplating, resin coating, and the like.
This embodiment is characterized in that a predetermined amount of a powder of a hydride of Ti (hereinafter sometimes referred to as “Ti hydride powder”) is added to an alloy powder with the composition, which is almost the same as in a conventional R-T-B based sintered magnet (composition containing R, B, Ga, Fe and the like, the amount of B being high [0.9 to 1.0% by mass] as compared with the sintered magnet of Patent Document 1). Whereby, it is possible to provide an R-T-B based sintered magnet having high HcJ and high Hk/HcJ while suppressing a reduction in Br without using heavy rare-earth elements RH as much as possible.
The reason why the R-T-B based sintered magnet according to this embodiment has high HcJ and high Hk/HcJ while suppressing a reduction in Br is not clear. However, it is considered that the addition of a Ti hydride powder leads to formation of an R6T13M compound (typically, an Nd6Fe13Ga compound) and a boride of Ti (typically, a TiB2 compound) during sintering and/or a heat treatment.
According to this embodiment, since an alloy powder with the composition, which is almost the same as that of a conventional R-T-B based sintered magnet, is used, HcJ does not cause significant change (quick reduction) by a slight change in the amount of B. There is not a need to use a new alloy, a new step, and the like, and existing production conditions can be basically applied as they are. Therefore, it becomes possible to provide a sintered magnet having high HcJ, which is equal to or higher than that of the sintered magnet of Patent Document 1, at low cost.
The R-T-B based sintered magnet according to this embodiment enables suppression of a reduction in Hk/HcJ due to a RH supply and diffusion treatment. This reason is not also clear, but is considered that the addition of a Ti hydride powder leads to formation of an R6T13M compound and a boride of Ti during sintering and/or a heat treatment, similarly as mentioned above.
Meanwhile, in Patent Document 4, oxygen, boron, carbon, nitrogen, silicon, and the like included in a high melting point compound (an oxide, a boride, a carbide, a nitride, or a silicate of one selected from the group consisting of Al, Ga, Mg, Nb, Si, Ti, and Zr) may remain in the magnet even after sintering, leading to a reduction in magnetic properties of the thus obtained magnet. The Ti hydride powder used in this embodiment is decomposed into Ti and H2 (hydrogen) during the sintering step, and hydrogen is released into a sintering furnace from the magnet, and discharged finally out of the sintering furnace. Therefore, magnetic properties may be scarcely degraded.
As mentioned above, according to this embodiment, it is possible to provide an R-T-B based sintered magnet, which has high HcJ equal to or higher than that of the sintered magnet of Patent Document 1, and also has high Hk/HcJ while suppressing a reduction in Br at low cost without using heavy rare-earth elements RH as much as possible.
The present embodiment will be described below. In the following description of this embodiment, a treatment of supplying heavy rare-earth elements RH as an RH diffusion source to a surface of an R-T-B based sintered magnet material and diffusing RH into the inside of the R-T-B based sintered magnet material, as mentioned in Patent Document 3, refers to an “RH supply and diffusion treatment”. A treatment of carrying out the RH supply and diffusion treatment, and diffusing RH into the inside of the R-T-B based sintered magnet material without supplying RH refers to an “RH diffusion treatment”. Furthermore, a heat treatment applied to an R-T-B based sintered magnet material after sintering, and a heat treatment applied after the RH supply and diffusion treatment or the RH diffusion treatment refers simply to a “heat treatment”. The R-T-B based sintered magnet before a heat treatment refers to an “R-T-B based sintered magnet material”, and the R-T-B based sintered magnet after a heat treatment refers to an “R-T-B based sintered magnet”.
In the step of preparing an alloy powder, the composition of the alloy powder includes:
R: 27 to 35% by mass,
B: 0.9 to 1.0% by mass,
Ga: 0.15 to 0.6% by mass, and
the balance T and inevitable impurities.
In the composition, when the content of each element is less than the lower limit of the above-mentioned range, or exceeds the upper limit, it sometimes becomes impossible to obtain an R-T-B based sintered magnet having high HcJ and high Hk/HcJ while suppressing a reduction in Br. The amount of B is more preferably 0.91 to 1.0% by mass. The amount of Ga is preferably in a range of 0.2 to 0.6% by mass, more preferably 0.3 to 0.6% by mass, still more preferably 0.4 to 0.6% by mass, and most preferably 0.4 to 0.5% by mass.
R is at least one of rare-earth elements and indispensably includes Nd. Examples of the rare-earth element except for Nd include Pr. It is also possible to contain a small amount of at least one of Dy, Tb, Gd, and Ho. The content of at least one of Dy, Tb, Gd, and Ho is preferably 1.0% by mass or less based on the entire R-T-B based sintered magnet. B can be partially substituted with C. T is at least one of transition metal elements and indispensably includes Fe. Examples of the transition metal element except for Fe include Co. It is also possible to contain a small amount of V, Cr, Mn, Ni, Zr, Nb, Mo, Hf, Ta, W, and the like.
It is also possible to contain Cu and Al as the element except for the above-mentioned elements. Cu and Al may be positively added for the purpose of improving magnetic properties, and it is also possible to make use of these elements contained in raw material used, and those which are indispensably introduced in the production process of an alloy powder (raw material containing Cu and Al as impurities may be used). Each of the contents of Cu and Al (total amount in each element of the amount added positively and the amount included as inevitable impurities) is preferably 0.5% by mass or less.
In the step of preparing an alloy powder, after weighing raw materials of each element so as to adjust to the above composition, a powder is prepared by a known production method. For example, an alloy is prepared by a strip casting method, and a coarsely pulverized powder is obtained from the thus obtained alloy by a hydrogen grinding method. Alternatively, a finely pulverized powder is obtained from the coarsely pulverized powder using a jet mill. The alloy powder may be either a coarsely pulverized powder or a finely pulverized powder.
Commercially available Ti hydride powders can be employed. The grain size of commercially available Ti hydride powder is, for example, about 50 μm in terms of D50 which is a volume median value obtained by a laser diffraction method using an air flow dispersion method. A Ti hydride powder is a very stable substance as compared with a state of metal (Ti metal) and also can be pulverized by a jet mill. Therefore, there is an advantage that it is possible to handle comparatively safely even if commercially available Ti hydride powder is finely pulverized by a jet mill to give a finely pulverized powder (5 μm or less in terms of D50).
As mentioned above, in Patent Document 4, oxygen, boron, carbon, nitrogen, silicon, and the like included in a high melting point compound (an oxide, a boride, a carbide, a nitride, or a silicate of one selected from the group consisting of Al, Ga, Mg, Nb, Si, Ti, and Zr) may remain in the magnet even after sintering, thus degrading magnetic properties of the thus obtained magnet. The Ti hydride powder used in this embodiment is decomposed into Ti and H2 (hydrogen) during the sintering step, and hydrogen is released into a sintering furnace from the magnet, and discharged finally out of the sintering furnace. Therefore, there is an advantage that magnetic properties may be scarcely degraded. Whereby, it is possible to suppress an increase in the content of oxygen, the content of carbon and the content of nitrogen of the R-T-B based sintered magnet and, for example, it is possible to produce an R-T-B based sintered magnet having oxygen content of 2,000 ppm or less, carbon content of 1,500 ppm or less, and nitrogen content of 1,000 ppm or less, thus enabling further improvement in magnetic properties.
The thus prepared alloy powder and Ti hydride powder mentioned above are mixed so as to adjust the content of Ti in 100% by mass of the mixed powder after mixing to 0.3% by mass or less to give a mixed powder. When the content of Ti in 100% by mass of the mixed powder after mixing exceeds 0.3% by mass, it becomes impossible to obtain an R-T-B based sintered magnet having high HcJ and high Hk/HcJ while suppressing a reduction in Br. The mixing amount of Ti is preferably in a range of 0.05 to 0.3% by mass, more preferably 0.12 to 0.3% by mass, still more preferably 0.18 to 0.3% by mass, and most preferably 0.22 to 0.3% by mass. Mixing is preferably performed by mixing an alloy powder composed of a coarsely pulverized powder with an (unpulverized) Ti hydride powder, followed by fine pulverization using a jet mill. Fine pulverization after mixing enables uniform mixing, and preparation of a mixed powder composed of a finely pulverized powder of an alloy powder and a Ti hydride powder in the same step as usual without adding a new step. As a matter of course, an alloy powder and a Ti hydride powder are separately subjected to fine pulverization, followed by mixing using a known mixing means to prepare a mixed powder. In this case, mixing may be performed by a dry or wet method.
The mixed powder is molded to obtain a molded body. Molding is performed by a known molding means. For example, it is possible to apply a dry molding method in which a dry alloy powder is loaded in a cavity of a mold and then molded while applying a magnetic field, or a wet molding method in which a slurry containing the alloy powder dispersed therein is injected in a cavity of a mold and then molded under a magnetic field while discharging a dispersion medium of the slurry.
The molded body is sintered to obtain an R-T-B based sintered magnet material (sintered body). Sintering is performed by a known sintering means. It is possible to apply a method of sintering in a vacuum atmosphere or inert gas (helium, argon, etc.) atmosphere at a sintering temperature of 1,000° C. or higher and 1,180° C. or lower, for about 1 to 10 hours as a sintering time.
The R-T-B based sintered magnet material is subjected to a heat treatment to obtain an R-T-B based sintered magnet.
Known conditions can be applied to temperature, time, atmosphere, and the like of a heat treatment. For example, it is possible to employ conditions in which a heat treatment is performed at only a comparatively low temperature (400° C. or higher and 600° C. or lower) (single-stage heat treatment), or at a comparatively low temperature (400° C. or higher and 600° C. or lower) after a heat treatment at a comparatively elevated temperature (700° C. or higher and sintering temperature or lower (for example, 1050° C. or lower) (two-stage heat treatment). Examples of preferred conditions include conditions in which a heat treatment is applied at 730° C. or higher and 1020° C. or lower for 5 about minutes to 500 minutes and, after cooling to (cooling to room temperature, or 440° C. or higher and 550° C. or lower), a heat treat is performed at 440° C. or higher and 550° C. or lower for about 5 minutes to 500 minutes. The heat treatment is preferably performed in an atmosphere such as a vacuum atmosphere or an inert gas (helium, argon, etc.) atmosphere.
When an RH supply and diffusion treatment is applied so as to further improve HcJ of the R-T-B based sintered magnet, the following step is carried out in place of applying a heat treatment to the R-T-B based sintered magnet material.
It is possible to apply, as the step of preparing an RH diffusion source comprising a metal, an alloy, or a compound, each containing Dy and/or Tb, the step disclosed in a known RH supply and diffusion treatment such as disclosed in Patent Document 3.
It is possible to apply, as the step of applying an RH supply and diffusion treatment of supplying Dy and/or Tb of an RH diffusion source to an R-T-B based sintered magnet material, followed by diffusion, a known RH supply and diffusion treatment such as disclosed in Patent Document 3. The RH supply and diffusion treatment may be performed by a method of diffusing heavy rare-earth elements RH from an RH diffusion source into the inside of an R-T-B based sintered magnet material while supplying to a surface thereof, like Patent Document 3, or a method of allowing metal, an alloy or a compound, each containing RH, to exist on a surface of an R-T-B based sintered magnet material in advance by film formation (dry method or wet method) or application, and diffusing the metal, alloy or compound into the inside of the R-T-B based sintered magnet material by a heat treatment.
An RH diffusion treatment may be performed for the purpose of further diffusing Dy and/or Tb, which is supplied into the inside of an R-T-B based sintered magnet material by the RH supply and diffusion treatment, into the inside. After carrying out the RH supply and diffusion treatment, heating is performed without newly supplying Dy and/or Tb from the RH diffusion source, in the RH diffusion treatment. For example, when the RH supply and diffusion treatment is carried out and then an RH diffusion treatment is performed, the RH diffusion treatment is preferably carried out at 700° C. or higher and 1,000° C. or lower, and more preferably 800° C. or higher and 950° C. or lower, under conditions that Dy and/or Tb is/are not supplied newly from the RH supply source. Alternatively, when the RH supply and diffusion treatment and then only an R-T-B based sintered magnet material is recovered, the treatment is preferably subjected to the R-T-B based sintered magnet material in a vacuum under an atmospheric pressure or less or inert gas atmosphere preferably at 700° C. or higher and 1,000° C. or lower, and more preferably at 800° C. or higher and 950° C. or lower. The treatment hour is, for example, in a range of about 10 minutes to 24 hours, and more preferably about 1 hour to 6 hours. By the RH diffusion treatment, diffusion of Dy and/or Tb occurs in the inside of the R-T-B based sintered magnet material, and Dy and/or Tb supplied in the vicinity of a surface layer is/are further diffused into the inside, thus enabling an increase in HcJ of the entire magnet.
A heat treatment is applied to an R-T-B based sintered magnet material obtained in an RH supply and diffusion treatment step (RH diffusion step may be performed after the RH supply and diffusion treatment step) to obtain an R-T-B based sintered magnet. This heat treatment is the same as the heat treatment (6).
As mentioned above, by adding a Ti hydride powder, an R6T13M compound (typically, an Nd6Fe13Ga compound) and a boride of Ti (typically, a TiB2 compound) are formed in sintering and/or a heat treatment (including the case of subjecting to an RH supply and diffusion treatment, and a heat treatment in place of the step of subjecting to a heat treatment). Namely, the R-T-B based sintered magnet obtained by a method for producing an R-T-B based sintered magnet of this embodiment has a structure in which an R2T14B compound, an R6T13M compound and a boride of Ti coexist.
In the R2T14B compound, R is at least one of rare-earth elements and indispensably includes Nd. Examples of the rare-earth element except for Nd include Pr. It is possible to further contain a small amount of at least one of Dy, Tb, Gd, and Ho. T is at least one of transition metal elements and indispensably includes Fe. Examples of the transition metal element except for Fe include Co. B can be partially substituted with C.
In the R6T13M compound, R is at least one of rare-earth elements and indispensably includes Nd. Examples of the rare-earth element except for Nd include Pr. It is possible to further contain a small amount of at least one of Dy, Tb, Gd, and Ho. T is at least one of transition metal elements and indispensably includes Fe. Examples of the transition metal element except for Fe include Co. M is mainly Ga. The R6T13M compound is typically an Nd6Fe13Ga compound. The R6T13M compound has a La6Co11Ga3 type crystal structure. The R6T13M compound is sometimes an R6T13−αM1+α compound (α is typically 2 or less) according to a state thereof. When using only Ga as M, the compound may be R6T13−α(Ga1-x-y-zCuxAlySiz)1+α in case an R-T-B based sintered magnet contains Al, Cu, and Si.
In the R-T-B based sintered magnet obtained by a method for producing an R-T-B based sintered magnet of this embodiment, an R6T13M compound is included in an area ratio of 1% or more in an arbitrary cross section thereof. Furthermore, when having higher HcJ, the R6T13M compound is included in an area ratio of 2% or more. It is possible to determine the area ratio of the R6T13M compound by analyzing an image of a reflected electron image (BSE image) of an arbitrary cross section of the R-T-B based sintered magnet, which is observed by a field emission-type scanning electron microscope (FE-SEM), using commercially available an image processing software, as shown in the below-mentioned Examples.
A boride of Ti is typically a TiB2 compound. A TiB compound sometimes exists together with the TiB2 compound. Examples of Patent Document 4 mention that, when a high melting point compound is TiC, TiC reacts with B in a material of an R-T-B-based rare-earth permanent magnet during sintering to form TiB2, which exists on grain boundaries. However, carbon (C) separated from TiC may remain in the magnet even after sintering, thus degrading magnetic properties of the thus obtained magnet. It is considered that the R6T13M compound is scarcely formed since the content of Ga is 0.08% by mass in Examples of Patent Document 4. Therefore, it is considered that there has never been obtained an R-T-B based sintered magnet having a structure in which an R2T14B compound, an R6T13M compound, and a boride of Ti coexist, in Patent Document 4.
Nd metal, Pr metal, ferroboron alloy, Ga metal, Cu metal, Al metal, electrolytic Co, Ti metal, and electrolytic iron (any of metals has a purity of 99% by mass or more) were mixed so as to obtain a given composition, and then these raw materials were melted and subjected to casting by a strip casting method to obtain a flaky raw alloy having a thickness of 0.2 to 0.4 mm. The flaky raw alloy thus obtained was subjected to hydrogen embrittlement in a hydrogen atmosphere under an increased pressure and then subjected to a dehydrogenation treatment of heating to 550° C. in vacuum and cooling to obtain a coarsely pulverized powder.
To the coarsely pulverized powder thus obtained, zinc stearate was added as a lubricant in the proportion of 0.04% by mass based on 100% by mass of the coarsely pulverized powder, followed by mixing. Using an air flow-type pulverizer (jet milling machine), the mixture was subjected to dry pulverization in a nitrogen gas flow to obtain a finely pulverized powder (alloy powder) having a grain size D50 of 4 μm. In this Test Example, the oxygen amount of the sintered magnet thus obtained finally was adjusted to about 0.1% by mass by adjusting the oxygen concentration in a nitrogen gas during pulverization to 50 ppm or less. The grain size D50 is the value obtained by a laser diffraction method using an air flow dispersion method (median size on a volume basis).
To the finely pulverized powder mentioned above, zinc stearate was added as a lubricant in the proportion of 0.05% by mass based on 100% by mass of the finely pulverized powder, followed by mixing. The mixture was molded under a magnetic field to obtain a molded body. A molding device used was a so-called perpendicular magnetic field molding device (transverse magnetic field molding device) in which a magnetic field application direction and a pressuring direction are perpendicular to each other.
The molded body thus obtained was sintered by retaining in vacuum at 1,070° C. to 1,090° C. for 4 hours, and then rapid cooled to obtain a sintered magnet.
The sintered magnet had a density of 7.5 Mg/m3 or more. The component analysis results of the thus obtained sintered magnet are shown in Table 1. The respective components in Table 1 were determined using high-frequency inductively coupled plasma emission spectrometry (ICP-OES). O (amount of oxygen) was measured using a gas fusion-infrared absorption method, N (amount of nitrogen) was measured using a gas fusion-thermal conductivity method, and C (amount of carbon) was measured by a combustion infrared absorption method, using a gas analyzer. The amount of R (u) in Table 1 is the value obtained by summing up the amounts of Nd and Pr, and the amount of M (j) is the value obtained by summing up the amounts of Cu, Cr, Mn, Si, 0, N, and C as elements except for R, B, Ga, Al, Co, Ti, and Fe, using ICP-OES. The same shall apply in Tables 3, 5, and 7 mentioned below. Using the values shown in Table 1, (g′+v′+z′)−(14×(w′−2×q′)) of the formula (A) and (g′+v′+z′)−(14×(w′−q′)) of the formula (B) were calculated. The columns “Formula A” and “Formula B” in Table 1 were filled with “G” (Good) when the calculated value is within the range of present invention, while the columns in Table 1 were filled with “B” (Bad) when the calculated value deviates from the range of present invention. The same shall apply to Tables 3, 5, and 7 mentioned below. As shown in Table 1, each of samples Nos. 1 to 3, 4 to 6, 7 to 9, 10 to 11, 12 to 15, and 16 to 17 has the almost same composition, except that the samples differ in the amount of B.
The sintered magnet thus obtained was subjected to a heat treatment of retaining at 900 to 1,000° C. for 2 hours and cooling to room temperature, followed by retaining at 500° C. for 2 hours and cooling to room temperature. The sintered magnet thus obtained after the heat treatment was machined to produce samples of 7 mm in length×7 mm in width×7 mm in thickness, magnetized under a pulse magnetic field of 3.2 MA/m, and then Br and HcJ of each sample were measured by a B-H tracer. The measurements results are shown in Table 2. Component analysis and gas analysis of each R-T-B based sintered magnet subjected to the measurement of Br and HcJ were performed. The results were the same as those of component analysis and gas analysis of the R-T-B based sintered magnet material in Table 1.
Furthermore, a change in HcJ to a change in the amount of B in each of samples Nos. 1 to 3, 4 to 6, 7 to 9, 10 to 11, 12 to 15, and 16 to 17 was determined in the following manner.
First, a difference in the amount of B between the lowest amount of B and the highest amount of B among the respective samples (among the almost same compositions except for the amount of B) was determined, and a difference between the lowest HcJ and the highest HcJ was determined, and then the difference in HcJ was divided by the difference in the amount of B, thus determining how much does HcJ change when the amount of B changes by 0.01% by mass. For example, the change in HcJ in samples Nos. 4 to 6 was determined in the following manner.
First, in samples Nos. 4 to 6, the lowest amount of B is 0.90% by mass of sample No. 4, the highest the amount of B is 0.95% by mass of sample No. 6, the lowest HcJ is 1,278 kA/m of sample No. 6, and the highest HcJ is 1,509 kA/m of sample No. 4. When the amount of B changes from 0.90% by mass to 0.95% by mass (changes by 0.05% by mass), HcJ changes from 1,508 kA/m to 1,278 kA/m (changes by 230 kA/m), so that, when the amount of B changes by 0.01% by mass, HcJ changes by 46 kA/m (230/(0.05≦x≦100)). In the same manner, the change in HcJ to the change in the amount of B in samples Nos. 1 to 3, 7 to 9, 10 to 11, 12 to 15, and 16 to 17 was also determined. The results are shown in the column “ΔHcJ/0.01B” in Table 2. ΔHcJ/0.01B in Table 6 mentioned below was determined in the same manner.
As shown in Table 2, samples Nos. 7 to 9, 10 to 11, 12 to 15, and 16 to 17, which are samples of Examples according to this embodiment, exhibit ΔHcJ/0.01B of 24 kA/m or less, namely, small change in HcJ to the change in the amount of B, and also exhibit high Br and high HcJ. Meanwhile, samples Nos. 1 to 3, and 4 to 6 in which the amount of Ti deviates from the range of this embodiment exhibit ΔHcJ/0.01B B of 46 kA/m or more, namely, the change in HcJ to the change in the amount of B is larger than that of samples of Examples. Therefore, when the amount of B increases, HcJ decreases (for example, 1,260 kA/m in sample No. 3), thus failing to obtain high HcJ. As is apparent from samples Nos. 10 to 11, 12 to 15, and 16 to 17 which are samples of Examples according to this embodiment, when the amount of Ti is 0.18% by mass or more, ΔHcJ/0.01B is 12 kA/m or less, namely, the change in HcJ to the change in the amount of B is small further.
Nd metal, Pr metal, ferroboron alloy, Ga metal, Cu metal, Al metal, electrolytic Co, Ti metal, and electrolytic iron (any of metals has a purity of 99% by mass or more) were mixed so as to obtain a given composition, and then these raw materials were melted and subjected to casting by a strip casting method to obtain a flaky raw alloy having a thickness of 0.2 to 0.4 mm. Using the flaky raw alloy thus obtained, a coarsely pulverized powder was prepared in the same manner as in Test Example 1. The coarsely pulverized powder thus obtained was subjected to dry pulverization in the same manner as in Test Example 1 to obtain a finely pulverized powder (alloy powder) having a grain size D50 of 4 μm. In the same manner as in Test Example 1, the mixture was molded under a magnetic field to obtain a molded body. The molded body thus obtained was sintered by retaining in vacuum at 1,080° C. for 4 hours, and then rapid cooled to obtain a sintered magnet. The sintered magnet had a density of 7.5 Mg/m3 or more.
The component analysis results of the sintered magnet thus obtained are shown in Table 3. The respective components in Table 3 were determined using high-frequency inductively coupled plasma emission spectrometry (ICP-OES). O (amount of oxygen) was measured by a gas fusion-infrared absorption method, N (amount of nitrogen) was measured by a gas fusion-thermal conductivity method, and C (amount of carbon) was measured by a combustion infrared absorption method, using a gas analyzer. The results of the formula (A) and the formula (B) calculated from analytical values of ICP-OES are shown in Table 3. The sintered magnet thus obtained was subjected to the same heat treatment as in Test Example 1. The sintered magnet thus obtained after the heat treatment was machined to produce samples of 7 mm in length×7 mm in width×7 mm in thickness, magnetized under a pulse magnetic field of 3.2 MA/m, and then Br and HcJ of each sample were measured by a B-H tracer. The measurements results are shown in Table 4. Component analysis and gas analysis of each R-T-B based sintered magnet subjected to the measurement of Br and HcJ were performed. The results were the same as those of component analysis and gas analysis of the R-T-B based sintered magnet material in Table 3. The measurement results are shown in Table 4.
Sample No. 18 shown in Table 3 has almost the same composition as that of sample No. 9, which is sample of Example shown in Test Example 1, except that the formula (A) is not satisfied. As shown in Table 4HcJ is significantly decreased to 1,341 KA/m as compared with 1,444 kA/m of sample No. 9 when the relationship between Ti and B deviates from the range of the present invention even if the amount of Ti is within the range of the present invention.
Nd metal, Pr metal, ferroboron alloy, Ga metal, Cu metal, Al metal, electrolytic Co, and electrolytic iron (any of metals has a purity of 99% by mass or more) were mixed so as to obtain a given composition, and then these raw materials were melted and subjected to casting by a strip casting method to obtain a flaky raw alloy having a thickness of 0.2 to 0.4 mm. The flaky raw alloy thus obtained was subjected to hydrogen embrittlement in a hydrogen atmosphere under an increased pressure and then subjected to a dehydrogenation treatment of heating to 550° C. in vacuum and cooling to obtain a coarsely pulverized powder. To the coarsely pulverized powder thus obtained, zinc stearate was added as a lubricant in the proportion of 0.04% by mass based on 100% by mass of the coarsely pulverized powder, followed by mixing. Using an air flow-type pulverizer (jet milling machine), the mixture was subjected to dry pulverization in a nitrogen gas flow to obtain a finely pulverized powder (alloy powder) having a grain size D50 of 4 μm. In this Test Example, the oxygen amount of the sintered magnet thus obtained finally was adjusted to about 0.1% by mass by adjusting the oxygen concentration in a nitrogen gas during pulverization to 50 ppm or less. The grain size D50 is the value obtained by a laser diffraction method using an air flow dispersion method (median size on a volume basis).
To the finely pulverized powder mentioned above, 0.22% by mass of a TiH2 powder having a grain size D50 of 10 μm or less was added and zinc stearate was added as a lubricant in the proportion of 0.05% by mass based on 100% by mass of the finely pulverized powder, followed by mixing. The mixture was molded under a magnetic field to obtain a molded body. A molding device used was a so-called perpendicular magnetic field molding device (transverse magnetic field molding device) in which a magnetic field application direction and a pressuring direction are perpendicular to each other.
The molded body thus obtained was sintered by retaining in vacuum at 1,040° C. for 4 hours, and then rapid cooled to obtain a sintered magnet.
The sintered magnet had a density of 7.5 Mg/m3 or more. The component analysis results of the thus obtained sintered magnet are shown in Table 5. The respective components in Table 5 were determined using high-frequency inductively coupled plasma emission spectrometry (ICP-OES). O (amount of oxygen) was measured using a gas fusion-infrared absorption method, N (amount of nitrogen) was measured using a gas fusion-thermal conductivity method, and C (amount of carbon) was measured by a combustion infrared absorption method, using a gas analyzer. The results of the formula (A) and the formula (B) calculated from analytical values of ICP-OES are shown in Table 5. As shown in Table 5, each of samples Nos. 19 to 22 has almost the same composition, except that the samples differ in the amount of B.
The sintered magnet thus obtained was subjected to a heat treatment of retaining at 900 to 1,000° C. for 2 hours and cooling to room temperature, followed by retaining at 500° C. for 2 hours and cooling to room temperature. The sintered magnet thus obtained after the heat treatment was machined to produce samples of 7 mm in length×7 mm in width×7 mm in thickness and magnetized under a pulse magnetic field of 3.2 MA/m, and then Br and HcJ of each sample were measured by a B-H tracer. The measurements results are shown in Table 6. Component analysis and gas analysis of each R-T-B based sintered magnet subjected to the measurement of Br and HcJ were performed. The results were the same as those of component analysis and gas analysis of the R-T-B based sintered magnet material in Table 5. Furthermore, the change in HcJ to the change in the amount of B in samples Nos. 19 to 22 is shown in ΔHcJ/0.01B of Table 6.
As shown in Table 6, sample of Example according to this embodiment exhibits small change in ΔHcJ/0.01B of 6 kA/m, and also has high Br and high HcJ.
Nd metal, Pr metal, ferroboron alloy, Ga metal, Cu metal, Al metal, electrolytic Co, and electrolytic iron (any of metals has a purity of 99% by mass or more) were mixed so as to obtain a given composition, and then these raw materials were melted and subjected to casting by a strip casting method to obtain a flaky raw alloy having a thickness of 0.2 to 0.4 mm. The flaky raw alloy thus obtained was subjected to hydrogen embrittlement in a hydrogen atmosphere under an increased pressure and then subjected to a dehydrogenation treatment of heating to 550° C. in vacuum and cooling to obtain a coarsely pulverized powder. To the coarsely pulverized powder thus obtained, zinc stearate was added as a lubricant in the proportion of 0.04% by mass based on 100% by mass of the coarsely pulverized powder, followed by mixing. Using an air flow-type pulverizer (jet milling machine), the mixture was subjected to dry pulverization in a nitrogen gas flow to obtain a finely pulverized powder (alloy powder) having a grain size D50 of 4 μm. In this Test Example, the oxygen amount of the sintered magnet thus obtained finally was adjusted to about 0.1% by mass by adjusting the oxygen concentration in a nitrogen gas during pulverization to 50 ppm or less. The grain size D50 is the value obtained by a laser diffraction method using an air flow dispersion method (median size on a volume basis).
To the finely pulverized powder mentioned above, 0.1 to 0.28% by mass of a TiH2 powder having a grain size D50 of 10 μm or less was added and zinc stearate was added as a lubricant in the proportion of 0.05% by mass based on 100% by mass of the finely pulverized powder, followed by mixing. The mixture was molded under a magnetic field to obtain a molded body. A molding device used was a so-called perpendicular magnetic field molding device (transverse magnetic field molding device) in which a magnetic field application direction and a pressuring direction are perpendicular to each other.
The molded body thus obtained was sintered by retaining in vacuum at 1,040° C. for 4 hours, and then rapid cooled to obtain a sintered magnet.
The sintered magnet had a density of 7.5 Mg/m3 or more. The results of component analysis and gas analysis ((O (amount of oxygen), N (amount of nitrogen), and C (amount of carbon)) of the thus obtained sintered magnet are shown in Table 7. The respective components in Table 7 were determined using high-frequency inductively coupled plasma emission spectrometry (ICP-OES). O (amount of oxygen) was measured using a gas fusion-infrared absorption method, N (amount of nitrogen) was measured using a gas fusion-thermal conductivity method, and C (amount of carbon) was measured by a combustion infrared absorption method, using a gas analyzer. The results of the formula (A) and the formula (B) calculated from analytical values of ICP-OES are shown in Table 7. As shown in Table 7, samples Nos. 23 to 26 and 27 to 28 have almost the same composition, except that the samples differ in the amount of Ti.
The sintered magnet thus obtained was subjected to a heat treatment of retaining at 900 to 1,000° C. for 2 hours and cooling to room temperature, followed by retaining at 500° C. for 2 hours and cooling to room temperature. The sintered magnet thus obtained after the heat treatment was machined to produce samples of 7 mm in length×7 mm in width×7 mm in thickness and magnetized under a pulse magnetic field of 3.2 MA/m, and then Br and HcJ of each sample were measured by a B-H tracer. The measurements results are shown in Table 8. Component analysis and gas analysis of each R-T-B based sintered magnet subjected to the measurement of Br and HcJ were performed. The results were the same as those of component analysis and gas analysis of the R-T-B based sintered magnet material in Table 7. The measurement results are shown in Table 8.
As shown in Table 8, samples of Comparative Examples, which do not satisfy either the formula (A) or the formula (B), exhibit HcJ decreased significantly as compared with samples of Examples satisfying both formulas.
Regarding sample No. 25 (Example), after cutting using a cross section polisher (device name: SM-09010, manufactured by JEOL, Ltd.), a reflected electron image of a cut cross section was obtained using FE-SEM (device name: JSM-7001F, manufactured by JEOL, Ltd.) at a magnification of 2,000 times is shown in
As shown in
Furthermore, regarding sample No. 20 and sample No. 21, each having almost the same composition, except that the samples differ in the amount of B, after cutting using a cross section polisher (device name: SM-09010, manufactured by JEOL, Ltd.), a reflected electron image of a cut cross section was obtained using FE-SEM (device name: JSM-7001F, manufactured by JEOL, Ltd.) at a magnification of 20,000 times is shown in
Arbitrary cross sections of the R-T-B based sintered magnet of samples Nos. 13 and 15 of Table 1 and samples Nos. 20, 21, and 25 of Table 3 (all samples are samples of Examples of this embodiment) were subjected to mirror polishing, and then the mirror surfaces thereof were partially subjected to ion beam processing by a cross section polisher (SM-09010, manufactured by JEOL. Ltd.). Next, the polished surfaces thereof were observed (accelerating voltage: 5 kV, working distance: 4 mm, TTL mode, magnification: 2,000 times) by a field emission-type scanning electron microscope (FE-SEM) (JSM-7001F, manufactured by JEOL. Ltd.). The reflected electron images (BSE image) by FE-SEM were analyzed using image processing software (Scandium, manufactured by OLYMPUS SOFT IMAGING SOLUTIONS GMBH), and an area ratio of the t6T13A compound (typically Nd6Fe13Ga compound) was determined. The BSE image by FE-SEM is displayed more rightly as a mean atomic number of elements constituting the region increases, while the BSE image is displayed more darkly as a mean atomic number of elements constituting the region decreases. For example, a grain boundary phase (rare-earth-rich phase) is lightly displayed, while the main phase (R2T14B phase) and oxide are darkly displayed. The R6T13A compound is displayed with intermediate brightness. Regarding analysis by the image processing software, after making a graph in which the horizontal axis shows brightness of the BSE image and the vertical axis shows frequency (area) by image processing, the R6T13A compound was searched by energy dispersive X-ray spectrometry (EDS) and an area ratio of the R6T13A compound was determined by correspondence to specific brightness in the graph. Wideness of the visual field of a reflected electron image (BSE image) by FE-SEM was 45 μm×60 μm. The results are shown in Table 10.
As shown in Table 10, in the R-T-B based sintered magnet of this embodiment, the R6T13A compound is included in the arbitrary cross section in an area ratio of 2% or more.
Nd metal, Pr metal, Dy metal, ferroboron alloy, Ga metal, Cu metal, Al metal, electrolytic Co, Ti metal, and electrolytic iron (any of metals has a purity of 99% by mass or more) were mixed so as to obtain a given composition shown in Table 11, and then these raw materials were melted and subjected to casting by a strip casting method to obtain a flaky raw alloy having a thickness of 0.2 to 0.4 mm. The flaky raw alloy thus obtained was subjected to hydrogen embrittlement in a hydrogen atmosphere under an increased pressure and then subjected to a dehydrogenation treatment of heating to 550° C. in vacuum and cooling to obtain a coarsely pulverized powder.
To the coarsely pulverized powder thus obtained, zinc stearate was added as a lubricant in the proportion of 0.04% by mass based on 100% by mass of the coarsely pulverized powder, followed by mixing. Using an air flow-type pulverizer (jet milling machine), the mixture was subjected to dry pulverization in a nitrogen gas flow to obtain a finely pulverized powder (alloy powder) having a grain size D50 of 4 μm. In this Test Example, the oxygen amount of the sintered magnet thus obtained finally was adjusted to about 0.1% by mass by adjusting the oxygen concentration in a nitrogen gas during pulverization to 50 ppm or less. The grain size D50 is the value obtained by a laser diffraction method using an air flow dispersion method (median size on a volume basis).
To the finely pulverized powder mentioned above, zinc stearate was added as a lubricant in the proportion of 0.05% by mass based on 100% by mass of the finely pulverized powder, followed by mixing. The mixture was molded under a magnetic field to obtain a molded body. A molding device used was a so-called perpendicular magnetic field molding device (transverse magnetic field molding device) in which a magnetic field application direction and a pressuring direction are perpendicular to each other.
The molded body thus obtained was sintered by retaining in vacuum at 1,090° C. to 1,110° C. for 4 hours, and then rapid cooled to obtain a sintered magnet.
The sintered magnet had a density of 7.6 Mg/m3 or more. The component analysis results of the thus obtained sintered magnet are shown in Table 11. The respective components in Table 11 were determined using high-frequency inductively coupled plasma emission spectrometry (ICP-OES). O (amount of oxygen) was measured using a gas fusion-infrared absorption method, N (amount of nitrogen) was measured using a gas fusion-thermal conductivity method, and C (amount of carbon) was measured by a combustion infrared absorption method, using a gas analyzer. The amount of R (u) in Table 11 is the value obtained by summing up the amounts of Nd, Pr, and Dy, and the amount of M (j) is the value obtained by summing up the amounts of Cu, Cr, Mn, Si, 0, N, and C as elements except for R, B, Ga, Al, Co, Ti, and Fe, using ICP-OES. The values (g′, v′, z′, w′, and q′) obtained by dividing each of the analytical values of Fe(g), Co(v), Al(z), B(w), and Ti(q) by the atomic weight of Fe, Co, Al, B, and Ti were calculated and, using the values, (g′+v′+z′)−(14×(w′−2×q′)) of the formula (A) and (g′+v′+z′)−(14×(w′−q′)) of the formula (B) were calculated. The columns “Formula A” and “Formula B” in Table 11 were filled with “G” (Good) when the calculated value is within the range of present invention, while the columns in Table 11 were filled with “B” (Bad) when the calculated value deviates from the range of present invention. As shown in Table 11, each of samples Nos. 40 to 43 and 44 to 47 has the almost same composition, except that the samples differ in the amount of B.
The sintered magnet thus obtained was subjected to a heat treatment of retaining 1,000° C. for 2 hours and cooling to room temperature, followed by retaining at 500° C. for 2 hours and cooling to room temperature. The sintered magnet thus obtained after the heat treatment was machined to produce samples of 7 mm in length×7 mm in width×7 mm in thickness and magnetized under a pulse magnetic field of 3.2 MA/m, and then Br of each sample was measured by a B-H tracer and HcJ of each sample was measured by a pulse B-H tracer. The measurements results are shown in Table 12. Component analysis and gas analysis of each R-T-B based sintered magnet subjected to the measurement of Br and HcJ were performed. The results were the same as those of component analysis and gas analysis of the R-T-B based sintered magnet material in Table 12. Furthermore, the change in HcJ to the change in the amount of B is shown in ΔHcJ/0.01B of Table 12.
As shown in Table 12, sample of Example according to this embodiment exhibits small change in ΔHcJ/0.01B of 14 kA/m and 11 kA/m, and also has high Br and high HcJ.
After weighing raw materials of each element so as to obtain an alloy composition shown in A and B of Table 13, alloys were prepared by a strip casting method. Each of the alloys thus obtained was coarsely pulverized by a hydrogen grinding method to obtain a coarsely pulverized powder. The coarsely pulverized powder was mixed with TiH2 so that the composition of the mixed powder obtained after mixing with the coarsely pulverized powder of the thus obtained alloy A becomes the composition shown in samples Nos. 2 to 6 in Table 14 to prepare a mixed powder (mixed powder of a coarsely pulverized powder). Sample No. 1 is a coarsely pulverized powder of an alloy A and sample No. 7 is a coarsely pulverized powder of an alloy B, and TiH2 is not mixed in both samples. Mixed powders of samples Nos. 2 to 6 and coarsely pulverized powders of samples Nos. 1 and 7 were finely pulverized by a jet mill to prepare mixed powders of samples Nos. 2 to 6 (mixed powders of finely pulverized powders), and finely pulverized powders of samples Nos. 1 and 7, having a grain size D50 (volume median value obtained by a laser diffraction method using an air flow dispersion method, the same shall apply hereinafter) of 4.2 μm,
The mixed powders of samples Nos. 2 to 6 and finely pulverized powders of samples Nos. 1 and 7 were molded at a magnetic field intensity of 0.8 MA/m under a pressure of 49 MPa (0.5 ton/cm2) using a perpendicular magnetic field molding device (transverse magnetic field molding device) to obtain two molded bodies of 12 mm in thickness×26 mm in width×55 mm in length for each (width direction is magnetic field application direction). The molded body thus obtained was sintered at 1,030° C. for 4 hours to obtain two R-T-B based sintered magnet materials for each based on mixed powders of samples Nos. 2 to 6 and finely pulverized powders of samples Nos. 1 and 7 (hereinafter sometimes referred to as an “R-T-B based sintered magnet material of sample No. **”, the same shall apply hereinafter).
To measure magnetic properties of R-T-B based sintered magnets of samples Nos. 1 to 7, each one of two R-T-B based sintered magnet materials of samples Nos. 1 to 7 was subjected to a heat treatment in a vacuum atmosphere at a temperature of 880° C. for 3 hours, cooled and then subjected to a heat treatment in a vacuum atmosphere at 500° C. for 2 hours. Each of the thus obtained R-T-B based sintered magnets based on R-T-B based sintered magnet materials of samples Nos. 1 to 7 (hereinafter sometimes referred to as an “R-T-B based sintered magnet material of sample No. **”, the same shall apply hereinafter) was cut, polished, and then processed into pieces of 7.0 mm in thickness×7.0 mm in width×7.0 mm in length. After processing, magnetic properties of R-T-B based sintered magnets of samples Nos. 1 to 7 were measured by a B-H tracer. The measurement results are shown in Table 15. In Hk/HcJ, Hk is the value of H at the position where J becomes the value of 0.9×Jr (Jr is residual magnetization, Jr=Br) in a second quadrant of a J (magnitude of magnetization)−H (intensity of magnetic field) curve (the same shall apply hereinafter).
As is apparent from Table 15, R-T-B based sintered magnets obtained by mixing an alloy powder with TiH2, followed by molding, sintering, and a heat treatment (samples Nos. 2 to 6, Invented Examples) exhibit HcJ improved significantly as compared with those obtained by mixing with no TiH2 powder (sample No. 1, Comparative Example). It is also apparent that HcJ is particularly improved when the amount of Ti included in 100% by mass of the mixed powder is in a range of 0.22 to 0.27. Furthermore, although Br is slightly reduced by the addition of TiH2, a reduction in Br does not significantly contribute to the effect of improving HcJ. Namely, HcJ is improved while suppressing a reduction in Br. Furthermore, all samples have high Hk/HcJ such as 0.98. Sample No. 7 is Reproduction Example of Patent Document 1 and the amount of B is lower than that of the other sample (0.88% by mass). As shown in Table 15HcJ and Br of the R-T-B based sintered magnet of sample No. 7 before RH supply and diffusion treatment are almost the same as those of this embodiment.
Next, each one of two R-T-B based sintered magnet materials of samples Nos. 1 to 7 was cut, polished, and then processed into pieces of 7.4 mm in thickness×7.4 mm in width×7.4 mm in length. After processing, regarding each of R-T-B based sintered magnet materials of samples Nos. 1 to 7, seven kinds of laminates were prepared by laminating a RH diffusion source made of sheet-like Dy metal, a retaining member, an R-T-B based sintered magnet material, a retaining member, and an RH diffusion source made of sheet-like Dy metal on a Mo sheet in this order. A plain-woven wire netting made of Mo was used as the retaining member. The seven kinds of laminates were charged into a heat treatment furnace and then subjected to an RH supply and diffusion treatment in a vacuum atmosphere under a pressure of 0.1 Pa at a temperature of 880° C. for 5.5 hours. After cooling the inside of the furnace, only R-T-B based sintered magnet materials of samples Nos. 1 to 7 were taken out. After the RH supply and diffusion treatment, R-T-B based sintered magnet materials of samples Nos. 1 to 7 were subjected to an RH diffusion treatment in a vacuum atmosphere at a temperature of 880° C. for 5 hours, cooled, and then subjected to a heat treatment in a vacuum atmosphere at 500° C. for 2 hours to obtain R-T-B based sintered magnets of No. 1 to 7. The entire surface of the thus obtained R-T-B based sintered magnets of samples Nos. 1 to 7 was cut by 0.2 mm each to thereby process into pieces of 7.0 mm in thickness×7.0 mm in width×7.0 mm in length. After processing, magnetic properties of R-T-B based sintered magnets of samples Nos. 1 to 7 were measured by a pulse B-H tracer. The measurement results are shown in Table 16.
As is apparent from Table 16, R-T-B based sintered magnets obtained by subjecting an R-T-B based sintered magnet material, which is obtained by mixing an alloy powder with TiH2, followed by molding and further sintering, to an RH supply and diffusion treatment, an RH diffusion treatment, and a heat treatment (samples Nos. 2 to 6, Invented Examples) have high HcJ as compared with those obtained by mixing with no TiH2 powder (sample No. 1, Comparative Example). It is also apparent that the thus obtained R-T-B based sintered magnets have high Br and high Hk/HcJ since Br and Hk/HcJ are slightly reduced even after the RH supply and diffusion treatment. Meanwhile, the R-T-B based sintered magnet of sample No. 7, which is Reproduction Example of Patent Document 1, exhibits Hk/HcJ decreased significantly as compared with that before RH supply and diffusion.
After weighing raw materials of each element so as to obtain an alloy composition shown in C of Table 17, alloys were prepared by a strip casting method. Each of the alloys thus obtained was coarsely pulverized by a hydrogen grinding method to obtain a coarsely pulverized powder. The coarsely pulverized powder was mixed with TiH2 so that the composition of the mixed powder obtained after mixing with the coarsely pulverized powder of the thus obtained alloy C becomes the composition shown in Table 18 to prepare a mixed powder of samples Nos. 8 to 11 (mixed powder of a coarsely pulverized powder). Mixed powders of samples Nos. 8 to 11 were finely pulverized by a jet mill to prepare mixed powders of samples Nos. 8 to 11 (mixed powders of finely pulverized powders) having a grain size D50 of 4.2 μm.
In the same manner as in Example 1, mixed powders of samples Nos. 8 to 11 were molded and sintered, and then two R-T-B based sintered magnet materials for each based on mixed powders of samples Nos. 8 to 11 were prepared. To measure magnetic properties of R-T-B based sintered magnets of samples Nos. 8 to 11, each one of two R-T-B based sintered magnet materials of samples Nos. 8 to 11 was subjected to the same heat treatment and processing as in Example 1. Magnetic properties of the thus obtained R-T-B based sintered magnets of samples Nos. 8 to 11 were measured by a B-H tracer. The measurement results are shown in Table 19.
This Example is an example in which the amount of B (changed from 0.95 to 0.93), the amount of Ga (changed from 0.4 to 0.2), and the amount of Co (changed from 0.5 to 2.0) in the composition of the alloy A of Example 1 are made to be different. As shown in Table 19, although magnetic properties are slightly inferior as compared with magnetic properties of the R-T-B based sintered magnet based on the alloy A, excellent magnetic properties are obtained.
Next, each one of two R-T-B based sintered magnet materials of samples Nos. 8 to 11 was processed into the same shape as in Example 1, and then subjected to an RH supply and diffusion treatment, an RH diffusion treatment, and a heat treatment in the same manner as in Example 1. The thus obtained R-T-B based sintered magnets of samples Nos. 8 to 11 were processed in the same manner as in Example 1, and the magnetic properties were measured by a pulse B-H tracer. The measurement results are shown in Table 20.
As is apparent from Table 20, R-T-B based sintered magnets obtained by subjecting an R-T-B based sintered magnet material, which is obtained by mixing an alloy powder with TiH2, followed by molding and further sintering, to an RH supply and diffusion treatment, have high HcJ and high Hk/HcJ while suppressing a reduction in Br.
After weighing raw materials of each element so as to obtain an alloy composition shown in D to F of Table 21, alloys were prepared by a strip casting method. Each of the alloys thus obtained was coarsely pulverized by a hydrogen grinding method to obtain a coarsely pulverized powder. The coarsely pulverized powder of the thus obtained alloys D to F was mixed with TiH2 so that the composition of the mixed powder obtained after mixing with the coarsely pulverized powder of the thus obtained alloys becomes the composition shown in Table 22 to prepare mixed powders of samples Nos. 13 to 15, 17 to 20, and 22 to 25 (mixed powders of coarsely pulverized powders). Sample No. 12 is a coarsely pulverized powder of an alloy D, sample No. 16 is a coarsely pulverized powder of an alloy E, sample No. 21 is a coarsely pulverized powder of an alloy F, and TiH2 is not mixed in any of these samples. The above-mentioned mixed powders and coarsely pulverized powders were finely pulverized by a jet mill to prepare mixed powders of samples Nos. 13 to 15, 17 to 20, and 22 to 25 (mixed powders of finely pulverized powders) and finely pulverized powders of samples Nos. 12, 16, and 21, having a grain size D50 of 4.2 μm.
Mixed powders of samples Nos. 13 to 15, 17 to 20, and 22 to 25, and finely pulverized powders of samples Nos. 12, 16, and 21 were molded and then sintered in the same manner as in Example 1 to prepare R-T-B based sintered magnet materials based on mixed powders of samples Nos. 13 to 15, 17 to 20, and 22 to 25, and finely pulverized powders of samples Nos. 12, 16, and 21.
To measure magnetic properties of R-T-B based sintered magnets of samples Nos. 12 to 25, R-T-B based sintered magnet materials of samples Nos. 12 to 25 were subjected to the same heat treatment and processing as in Example 1. Magnetic properties of the thus obtained R-T-B based sintered magnets of samples Nos. 12 to 25 were measured by a B-H tracer. The measurement results are shown in
This Example is an example in which the amount of B of an alloy is changed. As is apparent from
As shown in
A coarsely pulverized powder of an alloy E of Example 3 was mixed with each powder of TiH2, TiO2, TiB2, TiC, and TiN so that Ti included in 100% by mass of the mixed powder becomes 0 to 0.3 (TiH2 becomes 0 to 0.31, and TiO2 becomes 0 to 0.18) after mixing, and then subjected to fine pulverization, molding, sintering, and a heat treatment in the same manner as in Example 1 to obtain an R-T-B based sintered magnet. HcJ of the thus obtained R-T-B based sintered magnet was measured by a B-H tracer. The measurement results are shown in
As is apparent from
Regarding the R-T-B based sintered magnet of sample No. 18 of Example 3, the structure was observed by e FE-TEM (field emission-type transmission electron microscope, HF-2100, manufactured by Hitachi High-Technologies Corporation.). The results (DF-STEM image) are shown in
To identify the compound, composition analysis of a standard sample of an R6T13M compound and a standard sample of a boride of Ti was performed by EDS in the same manner as mentioned above. The results are shown in Table 26. Commercially available TiB2 was used as a standard sample of a boride of Ti. Just in case, it was confirmed by X-ray diffraction using an X-ray diffractometer that commercially available TiB2 must be a TiB2 compound. The results of X-ray diffraction are shown in
The results of composition analysis by EDS of the site “a” in Table 25 and the results of a diffraction pattern which characterizes a crystal structure of electron diffraction of the site “a” shown in
The results of composition analysis of Table 25 to Table 27 and the results of a diffraction pattern which characterizes a crystal structure of electron diffraction of the site “b” shown in
Furthermore, the results of composition analysis of Table 25 to Table 27, and the results of a diffraction pattern which characterizes a crystal structure of electron diffraction of the site “c” shown in
As mentioned above, the addition of a Ti hydride powder leads to formation of an R6T13M compound (typically Nd6Fe13Ga compound) and a boride of Ti (typically, a TiB2 compound) in sintering and/or heat treatment. Namely, it is apparent that an R-T-B based sintered magnet obtained by a method for producing an R-T-B based sintered magnet of this embodiment has a structure in which an R2T14B compound, an R6T13M compound, and a boride of Ti coexist.
An arbitrary cross section of R-T-B based sintered magnets of samples Nos. 1 to 7 of Example 1 (R-T-B based sintered magnet subjected to neither an RH supply and diffusion treatment nor an RH diffusion treatment) were subjected to mirror polishing, and the mirror surfaces thereof were partially subjected to ion beam proceeding by a cross section polisher (SM-09010, manufactured by JEOL. Ltd.). Next, the polished surfaces were observed (accelerating voltage: 5 kV, working distance: 4 mm, TTL mode, magnification: 2,000 times) by a field emission-type scanning electron microscope (FE-SEM), JSM-7001F, manufactured by JEOL. Ltd.). A reflected electron image (BSE image) was analyzed by image processing software (Scandium, manufactured by OLYMPUS SOFT IMAGING SOLUTIONS GMBH) using FE-SEM to thereby determine an area ratio of an R6T13M compound (typically, an Nd6Fe13Ga compound). The BSE image by FE-SEM is displayed more rightly as a mean atomic number of elements constituting the region increases, while the BSE image is displayed more darkly as a mean atomic number of elements constituting the region decreases. For example, a grain boundary phase (rare-earth-rich phase) is lightly displayed, while the main phase (R2T14B phase) and oxide are darkly displayed. The R6T13M compound is displayed with intermediate brightness. Regarding analysis by image processing software, after making a graph in which the horizontal axis shows brightness of the BSE image and the vertical axis shows frequency (area) by image processing, the R6T13M compound was searched by energy dispersive X-ray spectrometry (EDS) and an area ratio of the R6T13M compound was determined by correspondence to specific brightness in the graph. This analysis was performed with respective to a BSE image of different five visual fields on a cross section (wideness of each visual field is 45 μm×60 μm), and the value was regarded as an area ratio of the R6T13M compound. The results are shown in Table 28.
As mentioned above, R-T-B based sintered magnets obtained by mixing an alloy powder with TiH2, followed by molding, sintering, and a heat treatment (samples Nos. 2 to 6, Invented Examples) have a structure in which an R2T14B compound, an R6T13M compound, and a boride of Ti coexist. As shown in Table 28, an R6T13M compound exists in an area ratio of 1% or more, and when the R6T13M compound exists in an area ratio of 2% or more, particularly high HcJ is exhibited. Meanwhile, regarding those obtained by mixing with no TiH2 powder (sample No. 1, Comparative Example) and sample No. 7 which is Reproduction Example of Patent Document 1 (Comparative Example), although an R6T13M compound exists in an area ratio of 1% or more, a boride of Ti is not formed. It is considered that an R-T-B based sintered magnet according to this embodiment has high HcJ and high Hk/HcJ while suppressing a reduction in Br because of a structure in which an R2T14B compound, an R6T13M compound and a boride of Ti coexist, and the existing amount of the R6T13M compound.
Regarding R-T-B based sintered magnets of samples Nos. 8 to 11 of Example 2 (R-T-B based sintered magnets subjected to neither RH supply and diffusion treatment nor RH diffusion treatment), an area ratio of an R6T13M compound was determined in the same manner as in Example 6. The results are shown in Table 29.
As mentioned above, R-T-B based sintered magnets obtained by mixing an alloy powder with TiH2, followed by molding, sintering and a heat treatment (samples Nos. 8 to 11, Invented Examples) have a structure in which an R2T14B compound, an R6T13M compound and a boride of Ti coexist and, as shown in Table 29, an R6T13M compound exists in an area ratio of 1% or more.
After weighing raw materials of each element so as to obtain an alloy composition shown in G and H of Table 30, alloys were prepared by a strip casting method. Each of the alloys thus obtained was coarsely pulverized by a hydrogen grinding method to obtain a coarsely pulverized powder. The coarsely pulverized powder was mixed with TiH2 so that the composition of the mixed powder obtained after mixing with the coarsely pulverized powder of the thus obtained alloy G becomes the composition shown in samples Nos. 48 to 52 in Table 31 to prepare a mixed powder (mixed powder of a coarsely pulverized powder). Sample No. 47 is a coarsely pulverized powder of an alloy G and sample No. 53 is a coarsely pulverized powder of an alloy H, and TiH2 is not mixed in both samples. Mixed powders of samples Nos. 48 to 52 and coarsely pulverized powders of samples Nos. 47 and 53 were finely pulverized by a jet mill to prepare mixed powders of samples Nos. 48 to 52 (mixed powders of finely pulverized powders) and finely pulverized powders of samples Nos. 47 and 53, having a grain size D50 (volume median value obtained by a laser diffraction method using an air flow dispersion method, the same shall apply hereinafter) of 4.2 μm.
Mixed powders of samples Nos. 48 to 52 and coarsely pulverized powders of samples Nos. 47 and 53 were molded and then sintered in the same manner as in Example 1 to prepare R-T-B based sintered magnet materials based on mixed powders of samples Nos. 48 to 52 and coarsely pulverized powders of samples Nos. 47 and 53. To measure magnetic properties of R-T-B based sintered magnets of samples Nos. 47 to 53, R-T-B based sintered magnet materials of samples Nos. 47 to 53 were subjected to a heat treatment and processing in the same manner as in Example 1. Magnetic properties of the thus obtained R-T-B based sintered magnets of samples Nos. 47 to 53 were measured by a B-H tracer. The measurement results are shown in Table 32. An area ratio of an R6T13M compound was determined in the same manner as in Example 6. The results are shown in Table 32.
This Example is an example in which the composition of the alloy A of Example 1 was changed, especially the amount of Ga was increased from 0.4% by mass to 0.5% by mass. As is apparent from Table 32, R-T-B based sintered magnets obtained by mixing an alloy powder with TiH2, followed by molding, sintering, and a heat treatment (samples Nos. 48 to 52, Invented Examples) have high HcJ as compared with an R-T-B based sintered magnet obtained by mixing with no TiH2 powder (sample No. 47, Comparative Example). Meanwhile, an R-T-B based sintered magnet of sample No. 53, which is Reproduction Example of Patent Document 1, exhibits the same HcJ and Br as those of Invented Examples, but exhibits significantly decreased Hk/HcJ.
An R-T-B based sintered magnet according to this Example has high HcJ of 1,500 kA/m or more while suppressing a reduction in Br, the amount of Ti being in a range of 0.22 to 0.27. For example, when a comparison is made between sample No. 50 of this Example in which the amount of Ti is the same as 0.22 and sample No. 3 of Example 1HcJ is improved by about 50 kA/m while Br is scarcely decreased.
Furthermore, as mentioned above, R-T-B based sintered magnets obtained by mixing an alloy powder with TiH2, followed by molding, sintering, and a heat treatment (samples Nos. 48 to 52, Invented Examples) have a structure in which an R2T14B compound, an R6T13M compound, and a boride of Ti coexist and, as shown in Table 32, the R6T13M compound exists in an area ratio of 2% or more.
After weighing raw materials of each element so as to obtain an alloy composition shown in I and J of Table 33, alloys were prepared by a strip casting method. Each of the alloys thus obtained was coarsely pulverized by a hydrogen grinding method to obtain a coarsely pulverized powder. The coarsely pulverized powder was mixed with TiH2 so that the composition of the mixed powder obtained after mixing with the coarsely pulverized powder of the thus obtained alloy I becomes the composition shown in samples Nos. 55 to 59 in Table 34 to prepare a mixed powder (mixed powder of a coarsely pulverized powder). Sample No. 54 is a coarsely pulverized powder of an alloy I and sample No. 60 is a coarsely pulverized powder of an alloy J, and TiH2 is not mixed in both samples. Mixed powders of samples Nos. 55 to 59 and coarsely pulverized powders of samples Nos. 54 and 60 were finely pulverized by a jet mill to prepare mixed powders of samples Nos. 55 to 59 (mixed powders of finely pulverized powders) and finely pulverized powders of samples Nos. 54 and 60, having a grain size D50 (volume median value obtained by a laser diffraction method using an air flow dispersion method, the same shall apply hereinafter) of 4.2 μm.
Mixed powders of samples Nos. 55 to 59 and coarsely pulverized powders of samples Nos. 54 and 60 were molded and then sintered in the same manner as in Example 1 to prepare R-T-B based sintered magnet materials based on mixed powders of samples Nos. 55 to 59 and coarsely pulverized powders of samples Nos. 54 and 60. To measure magnetic properties of R-T-B based sintered magnets of samples Nos. 54 to 60, R-T-B based sintered magnet materials of samples Nos. 54 to 60 were subjected to a heat treatment and processing in the same manner as in Example 1. Magnetic properties of the thus obtained R-T-B based sintered magnets of samples Nos. 54 to 60 were measured by a B-H tracer. The measurement results are shown in Table 35. An area ratio of an R6T13M compound was determined in the same manner as in Example 6. The results are shown in Table 35.
This Example is an example in which the amount of Al of the alloy G of Example 8 was increased from 0.1% by mass to 0.3% by mass. As is apparent from Table 35, R-T-B based sintered magnets obtained by mixing an alloy powder with TiH2, followed by molding, sintering, and a heat treatment (samples Nos. 55 to 59, Invented Examples) have high HcJ as compared with an R-T-B based sintered magnet obtained by mixing with no TiH2 powder (sample No. 54, Comparative Example). Meanwhile, an R-T-B based sintered magnet of sample No. 60, which is Reproduction Example of Patent Document 1, exhibits the same HcJ and Br as those of Invented Examples, but exhibits significantly decreased Hk/HcJ.
An R-T-B based sintered magnet according to this Example has HcJ of about 1,500 kA/m when the amount of Ti is 0.19% by mass, and also has high HcJ of about 1,500 kA/m or more when the amount of Ti is in a range of 0.22 to 0.27% by mass. Furthermore, as mentioned above, the R-T-B based sintered magnet according to this Example has a structure in which an R2T14B compound, an R6T13M compound, and a boride of Ti coexist. As shown in Table 35, in samples Nos. 56 to 59 having higher HcJ in which an R6T13M compound exists in an area ratio of 1.9% or more, the R6T13M compound exists in an area ratio of 2% or more.
This application claims priority based on Japanese Patent Application No. 2014-037838, filed on Feb. 28, 2014 and Japanese Patent Application 2014-198073, filed on Sep. 29, 2014, the disclosure of which is incorporated by reference herein.
The R-T-B based sintered magnet obtained by the present invention can be suitably employed in various motors such as voice coil motors (VCM) of hard disk drives, motors for electric cars, and motors for hybrid cars, and home appliances.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-037838 | Feb 2014 | JP | national |
| 2014-198073 | Sep 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/055874 | 2/27/2015 | WO | 00 |
