RTB-based sintered magnet

Abstract

RTB-based sintered magnet comprising an RTB-based compound as main phase grains (2), wherein the RTB-based connection R2T 14 B with a crystal structure characterized by tetragonal R2T 14 B is shaped, R represents at least one rare earth element, T represents one or more iron family elements comprising Fe or the combination of Fe and Co, the Zr content contained in the RTB-based sintered magnet is 0.3 mass% to 2.0 mass%, the main phase grains (2) comprise Zr, the RTB-based sintered magnet comprises main phase grains (2) in which, in the cross section of each of the main phase grains (2), the Zr mass concentration in a peripheral region (6) in each of the main phase grains (2) is 70% or less of that in the central region (4) in each of the main phase grains (2), and the edge area lies on a line 22a-23a and a line 22b-23b; if a random straight line is introduced which passes through the main phase grain (2) and also crosses a center of gravity (21) of the main phase grain (2), points at which the random straight line and an outermost edge intersect are point 22a and point 22b, a length of a segment 22a-22b is L, a point which is at a distance of 0.25 x L from point 22a on the segment 22a-22b is point 23a and a point which is at a distance of 0.25 x L from point 22b on the segment 22a-22b is point 23b.

Description

[0001] The present invention relates to an RTB-based sintered magnet having a rare earth element (R), at least one iron family element (T) with Fe or, as necessary, the combination of Fe and Co, and boron (B) as its main components. BACKGROUND

[0002] The RTB-based sintered magnet exhibits excellent magnetic properties and is therefore used in the voice coil motor (VCM) in a hard disk drive, various motors such as the motor equipped in a hybrid electric vehicle, household electrical appliances, or the like.

[0003] Active research and development has been conducted to improve the magnetic properties of RTB-based sintered magnets. For example, Patent Document 1 reported that adding 0.02 to 0.5 at% of Cu to RTB-based rare earth permanent magnets can optimize the magnetic properties and also improve the thermal treatment conditions. However, the method described in Patent Document 1 cannot achieve sufficiently high magnetic properties, such as high coercive force (HcJ) and high residual magnetic flux density (Br), required in a high-performance magnet.

[0004] To make the RTB-based sintered magnet a magnet with further improved performance, the oxygen content in the alloy must be reduced. However, if the oxygen content in the alloy is reduced, abnormal grain growth is likely to occur during the sintering process, resulting in a reduced squareness ratio or a significant drop in coercivity. Since the oxides formed by the oxygen in the alloy prevent grains from growing, the reduction in the oxygen content in the alloy is likely to cause abnormal grain growth.

[0005] Accordingly, a method is being investigated to optimize the magnetic properties by adding new elements to RTB-based sintered magnets containing Cu. Patent Document 2 reports that Zr and / or Cr are added to provide high coercive force and high residual magnetic flux density.

[0006] Similarly, Patent Document 3 reports uniformly dispersing and precipitating a finely divided ZrB compound, NbB compound, or HfB compound in an RTB-based rare-earth permanent magnet containing Co, Al, and Cu, as well as Zr, Nb, or Hf. This prevents grain growth during the sintering process to improve the magnetic properties and sintering temperature range.

[0007] In order to reduce the amount of rare heavy rare earth elements used, such as Dy or Tb, a method has recently been introduced in which the main phase grains in RTB-based sintered magnets are micronized to improve the coercive force. However, if the main phase grains in the sintered magnet are to be micronized, the particle size of the finely pulverized raw material powder must be reduced. If the particle size of the finely pulverized powder is reduced, abnormal grain growth tends to occur during the sintering process. Therefore, if the finely pulverized powder with a small particle size is used as the raw material, the sintering temperature must be low, resulting in a relatively long sintering time, which results in a significant decrease in productivity.As a process using finely pulverized powder with such a small particle size and conducting the sintering process under the same conditions as a conventional process, it is considered necessary to further increase the amount of Zr added, as an element with a high effect in preventing abnormal grain growth. However, increasing the amount of Zr added will cause technical problems: the residual magnetic flux density will decrease, and the desired good properties cannot be obtained. Patent specifications Patent specification 1: JP H01 - 219 143 A Patent document 2: JP 2000 - 234 151 A Patent Document 3: JP 2002 - 75 717 A Patent document 4: WO 2013 / 191 276 A1 describes a sintered magnet. Patent document 5: US 2004 / 0 094 237 A1 describes an R-Fe-B sintered magnet. SUMMARY

[0008] The present invention has been made in view of the above conditions, and the object of the present invention is to provide an RTB-based sintered magnet having good magnetic properties by minimizing the deterioration of the magnetic properties and inhibiting grain growth.

[0009] To solve this problem, the present inventors investigated the conditions required for inhibiting grain growth by adding Zr. As a result, it was discovered that, although it is conventionally believed that grain growth can be inhibited by depositing a Zr-based compound, such as ZrB, at the grain boundary of the sintered magnet, the presence of Zr in the main-phase grains also produces an inhibitory effect on grain growth. It was further discovered that a high residual magnetic flux density and a high coercive force can be achieved by providing a structure in which the Zr mass concentration in the peripheral region of the main-phase grain is lower than that in the central region of the main-phase grain.

[0010] The mechanism has not been fully determined and is considered as follows. That is, when the Zr-based compound is deposited at the grain boundary in a conventional manner, only the proportion of the non-magnetic phase in the grain boundary increases, resulting in a reduced residual magnetic flux density. In contrast, when Zr is present in the main phase grains, as proposed in the present invention, an increase in the non-magnetic phase in the grain boundary can be prevented, and the decrease in residual magnetic flux density can be inhibited. On the other hand, when Zr is present in the main phase grains, Zr forms a solid solution in the RTB-based compound, and the anisotropic magnetic field decreases in intensity. In this regard, the coercive force generally decreases.However, it is considered that when a structure is formed in which the Zr concentration in the peripheral region of the main phase grain is lower than that in the central region as described in the present invention, a high coercive force is provided along with the inhibiting effect on the abnormal grain growth by inhibiting the intensity decrease of the anisotropic magnetic field near the surface of the main phase grain and also inhibiting the nucleation of the magnetic reversal on the surface of the main phase grain.

[0011] The present invention is based on the above-mentioned discovery. The RTB-based sintered magnet of the present invention is characterized by containing an RTB-based compound as the main phase grains, wherein the Zr content of the RTB-based sintered magnet is 0.3 mass% to 2.0 mass%, the main phase grains contain Zr, and the RTB-based sintered magnet contains main phase grains in which, in the cross sections of the main phase grains, the Zr mass concentration in the peripheral region of the main phase grains is 70% or less of that in the central region of the main phase grains.

[0012] Grain growth during the sintering process can be inhibited in the RTB-based sintered magnet of the present invention. Meanwhile, the RTB-based sintered magnet exhibits a high remanent magnetic flux density and a high coercive field strength.

[0013] The RTB-based sintered magnet of the present invention preferably contains main phase grains in which the Zr mass concentration in the peripheral region of the main phase grains is 40% or less of that in the central region of the main phase grains. With such a distribution of the Zr mass concentration in the main phase grains, the coercive force of the RTB-based sintered magnet can be further increased.

[0014] Preferably, in the RTB-based sintered magnet of the present invention, the Zr mass concentration in the peripheral region of the main phase grains is 0.15 mass% or less. When the Zr mass concentration in the peripheral region of the main phase grains is at such a low level, the coercive force of the RTB-based sintered magnet can be further increased.

[0015] According to the present invention, an RTB-based sintered magnet having good magnetic properties can be provided by minimizing the decrease of magnetic properties and also by inhibiting grain growth. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic sectional view of the RTB-based sintered magnet according to the present invention. Fig. 2 is a schematic sectional view of the main phase grains of the RTB-based sintered magnet according to the present invention. Fig. 3 is a flowchart showing an example of the method for manufacturing the RTB-based sintered magnet of the present invention. Fig. Figure 4 is a backscattered electron image showing the cross section of the RTB-based sintered magnet obtained from Example 1. Fig.Figure 5 shows the result of quantitative analysis of the Zr concentration by ESMA in a main phase grain in the RTB-based sintered magnet of Example 1 along a straight line crossing the center of gravity of the grain. DETAILED DESCRIPTION OF EMBODIMENTS

[0016] Hereinafter, the present invention will be described based on the embodiments shown in the drawings. <R-T-B-basierter gesinterter Magnet>

[0017] The embodiments of the RTB-based sintered magnet of the present invention will be described. As in the Fig. 1, the RTB-based sintered magnet in the present embodiment includes a plurality of main phase grains 2 and grain boundary phases 8 existing in the grain boundary of the main phase grains.

[0018] The main phase grain 2 is composed of an RTB-based compound. The RTB-based compound is R2T 14B with a tetragonal R2T 14 B shaped crystal structure.

[0019] R represents at least one rare earth element. The rare earth element refers to Sc, Y, and lanthanide elements, which belong to the third group in the long periodic table. The lanthanide elements include, for example, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and the like. The rare earth elements are classified as light rare earths and heavy rare earths. The heavy rare earth element refers to Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, while the light rare earth element refers to the other rare earth elements.

[0020] In the present embodiment, T represents one or more iron family elements, including Fe or the combination of Fe and Co. T can be only Fe, or Fe partially replaced by Co. When part of Fe is replaced by Co, temperature characteristics can be improved without deteriorating magnetic properties.

[0021] In the RTB-based compound of the present embodiment, part of B can be replaced with carbon (C). In this case, the manufacturing of the magnet becomes simple, and production costs can be reduced. Furthermore, the amount of C used to replace B is essentially an amount that has no effect on the magnetic properties.

[0022] The RTB-based compound of the present embodiment may also contain various known additive elements. Specifically, at least one element selected from the group consisting of Ti, V, Cu, Cr, Mn, Ni, Zr, Nb, Mo, Hf, Ta, W, Al, Ga, Si, Bi, and Sn may be included.

[0023] In the present embodiment, the main phase grains 2 contain Zr. If the main phase grains 2 contain Zr, grain growth can be inhibited during sintering, even when using a pulverized raw material powder with a small particle size. The presence of Zr in the main phase grains can be confirmed by analyzing Zr in the main phase grain region in the cross-section of the sintered magnet using an analytical method such as one using ESMA (electron beam microanalysis).

[0024] In the present embodiment, main phase grains are included as the main phase grain, in which the Zr mass concentration in the peripheral region 6 in the main phase grains is lower than that in the central region 4 in the main phase grains. Fig.2 is a schematic view showing the method for measuring the Zr mass concentration in both the peripheral region and the central region of the main phase grain in the present embodiment. First, the center of gravity 21 of a main phase grain is determined by image analysis on the cross-section of the main phase grain to be measured. The position where the center of gravity 21 of the main phase grain is located can be determined by mapping the cross-sectional image of the main phase grain onto the XY plane and then averaging the X values ​​and Y values ​​of all pixels within the main phase grain. Then, a random straight line is introduced into the cross-section of the main phase grain, which passes over the main phase grain and also crosses the center of gravity 21 of the main phase grain. The points where this straight line and the outermost circumferential line intersect are defined as point 22a and point 22b.If the length of the segment 22a-22b is defined as L, a point 0.25 × L away from point 22a on the segment 22a-22b is defined as point 23a, and a point 0.25 × L away from point 22b is defined as point 23b. Next, using an analysis method such as ESMA, the Zr mass concentration along the segment 22a-22b is quantitatively analyzed with a certain interval between two analysis points. The average of the Zr mass concentrations at the analysis points on the segment 23a-23b is defined as Mc, and the average of the Zr mass concentrations of the analysis points on the segment 22a-23a and the segment 22b-23b is defined as Ms. Thus, Mc is specified as the Zr mass concentration in the central region of the main phase grain, and Ms is specified as the Zr mass concentration in the peripheral region of the main phase grain.Furthermore, in the analysis, the interval between two adjacent analysis points used in the quantitative analysis of the Zr mass concentration along the route 22a-22b is set such that the central region and the peripheral region of the main phase grain, respectively, have four or more analysis points. In the present embodiment, when Ms (i.e., the Zr mass concentration in the peripheral region of the main phase grain) measured through the above-mentioned steps is 70% or less of Mc (i.e., the Zr mass concentration in the central region of the main phase grain), it is determined that the Zr mass concentration in the peripheral region of the main phase grain is lower than that in the central region of the main phase grain.

[0025] As described above, since the main phase grains are comprised of main phase grains in which the Zr mass concentration in the peripheral region of the main phase grains is 70% or less of that in the central region of the main phase grain, the decrease in the residual magnetic flux density and the decrease in the coercive force associated with the increased Zr content can be inhibited. Furthermore, grain growth during sintering can be prevented even when using a pulverized raw material powder with a small particle size.

[0026] The ratio (Ms / Mc) of Ms (i.e., the Zr mass concentration in the peripheral region of the main phase grain) to Mc (i.e., the Zr mass concentration in the central region of the main phase grain) is preferably 40% or less. With such a ratio, a high coercive force can be easily achieved.

[0027] The Zr mass concentration in the peripheral region of the main phase grain (Ms) is preferably 0.15 mass% or less. When the Zr mass concentration in the peripheral region of the main phase grain is at such a low level, the nucleation of magnetization reversal on the surface of the main phase grains can be prevented, thus further improving the coercive force.

[0028] The RTB-based sintered magnet in the present embodiment can be manufactured, for example, as described below. Specifically, an alloy in which Zr is dissolved in the RTB-based main phase compound to form a solid solution is produced by controlling the casting conditions during the casting of the raw material alloy and further controlling the manufacturing conditions in the manufacturing process, such as the sintering pattern.

[0029] In the present embodiment, not all main phase grains constituting the RTB-based sintered magnet need to have a structure with the Zr mass concentration distribution mentioned above. In other words, the main phase grains with such a structure should account for 30% or more of the total main phase grains. If less than 30% of main phase grains with such a structure are included, it is difficult to achieve the full effect of the present invention.

[0030] In the present embodiment, the sectional area of ​​each main phase grain in the section parallel to the c-axis in the RTB-based sintered magnet is calculated by a method such as image processing, and the circular diameter of this sectional area (i.e., the equivalent circular diameter) is defined as the grain size of the main phase grain at this section. Further, the grain size of the main phase grain (whose cross-sectional area is cumulatively 50% of the total cross-sectional area accumulated by the main phase grain with a small cross-sectional area) is defined as the average grain size of the main phase grains. The average grain size of the main phase grains is preferably 4.0 μm or less. If the average grain size of the main phase grains is larger than 4.0 μm, the coercive force tends to decrease.Furthermore, the average grain size of the main phase grains is preferably 1.5 µm or larger. If the average grain size is less than 1.5 µm, it is likely that main phase grains with the above-mentioned Zr mass concentration distribution cannot be successfully formed. Furthermore, in view of improving magnetic properties, the average grain size of the main phase grains is particularly preferably 1.5 µm or more and 3.5 µm or less.

[0031] In the present embodiment, Zr may also be present in the grain boundary phase 8 in addition to the main phase grain 2. Zr may be present in the grain boundary phase 8 in the form of, for example, a Zr-based compound such as ZrB, ZrC, and the like.

[0032] The R content in the RTB-based sintered magnet of the present embodiment is 25 mass% or more and 35 mass% or less, and preferably 29 mass% or more and 34 mass% or less. If the R content is less than 25 mass%, the generation of the RTB-based compound, which is the main phase of the RTB-based sintered magnet, is insufficient. Thus, soft magnetic materials such as α-Fe may be deposited, and the magnetic properties may deteriorate. Furthermore, in the present embodiment, the content of the heavy rare earth element contained as R is preferably 1.0 mass% or less, in view of cost reduction and avoiding resource risks.

[0033] The B content in the RTB-based sintered magnet of the present embodiment is 0.5 mass% or more and 1.5 mass% or less. When the B content is less than 0.5 mass%, the coercive force HcJ tends to decrease. When the content is more than 1.5 mass%, the residual magnetic flux density Br tends to decrease.

[0034] Furthermore, in the present embodiment, the B content in the RTB-based sintered magnet is preferably 0.7 mass% or more and 0.95 mass% or less, and more preferably 0.75 mass% or more and 0.90 mass% or less. With a reduced B content compared to that in the conventional RTB-based sintered magnet, an effect is produced in that Zr hardly migrates into the grain boundary and can easily be present in the main phase grains. The reason for this is not yet clear at this stage. It can be presumed that the defects in B are generated in the RTB-based compound, which is the main phase, so that Zr is easily dissolved in the RTB-based compound to form a solid solution.

[0035] As described above, T represents at least one iron family element including Fe or the combination of Fe and Co. The Fe content in the RTB-based sintered magnet of the present embodiment is essentially the remainder of the constituent elements for the RTB-based sintered magnet, and part of the Fe may be replaced with Co. The Co content is preferably 0.3 mass% or more and 4.0 mass% or less, and more preferably 0.5 mass% or more and 3.0 mass% or less. When the Co content exceeds 4 mass%, the residual magnetic flux density tends to decrease. Furthermore, the RTB-based sintered magnet in the present embodiment tends to be more expensive. On the other hand, when the Co content is less than 0.3 mass%, the corrosion resistance tends to deteriorate.

[0036] The RTB-based sintered magnet of the present embodiment must contain Zr. In the present embodiment, the Zr content is 0.3 mass% or more and 2.0 mass% or less. If the content is less than 0.3 mass%, the grain growth inhibition effect cannot be sufficiently obtained. If the content is more than 2.0 mass%, the residual magnetic flux density Br tends to decrease.

[0037] The RTB-based sintered magnet in the present embodiment preferably contains Ga. The Ga content is preferably 0.05 to 1.5 mass%, and more preferably 0.3 to 1.0 mass%. With Ga, an effect is produced in that Zr can hardly migrate into the grain boundary and is likely to be present in the main phase grains. The reason is considered to be the same as in the case where the B content is reduced. Specifically, Ga is solid-solved in the RTB-based compound of the main phase, resulting in changes in the crystal lattices, and thus Zr is readily solid-solved in the RTB-based compound. When the Ga content is less than 0.05 mass%, it becomes difficult for Zr to enter the main phase grains. Thus, the effect of the present invention is likely to be hardly produced.Furthermore, when the Ga content is above 1.5 mass%, the remanent magnetic flux density usually decreases.

[0038] The RTB-based sintered magnet in the present embodiment preferably contains Cu. The Cu content is preferably 0.05 to 1.5 mass%, and more preferably 0.3 to 1.0 mass%. When Cu is included, the resulting magnet has high coercive force and high corrosion resistance, and its temperature properties are also improved. When the Cu content is higher than 1.5 mass%, the residual magnetic flux density tends to decrease. Furthermore, when the Cu content is less than 0.05 mass%, the coercive force tends to decrease.

[0039] The RTB-based sintered magnet in the present embodiment preferably contains Al. When Al is included, the resulting magnet has high coercive force and high corrosion resistance, and its temperature properties are also improved. The Al content is preferably 0.03 mass% or more and 0.6 mass% or less, and more preferably 0.05 mass% or more and 0.4 mass% or less.

[0040] Other additive elements than those mentioned above may be included in the RTB-based sintered magnet of the present embodiment. Specifically, Ti, V, Cr, Mn, Ni, Nb, Mo, Hf, Ta, W, Si, Bi, Sn, Ca, and the like can be cited as examples.

[0041] A certain amount of oxygen (O) may be contained in the RTB-based sintered magnet of the present embodiment. This certain amount varies depending on other parameters and can be determined appropriately. The oxygen content is preferably 500 ppm or more in view of corrosion resistance. Furthermore, the content is preferably 2000 ppm or less in consideration of magnetic properties.

[0042] The carbon (C) content in the RTB-based sintered magnet according to the present embodiment is preferably 500 ppm or more and 3000 ppm or less, and more preferably 1200 ppm or more and 2500 ppm or less. When the carbon content is more than 3000 ppm, the magnetic properties of the resulting RTB-based sintered magnet tend to deteriorate. On the other hand, when the content is less than 500 ppm, alignment during pressing in a magnetic field becomes difficult. Since carbon is mainly added by means of lubricant during pressing, the content can be adjusted by controlling the amount of lubricant.

[0043] Furthermore, a certain amount of nitrogen (N) may be contained in the RTB-based sintered magnet according to the present embodiment. This certain amount varies depending on other parameters and can be determined appropriately. The nitrogen content is preferably 100 to 2000 ppm in view of magnetic properties.

[0044] The RTB-based sintered magnet of the present embodiment is usually used after being processed into any shape. The shape of the RTB-based sintered magnet according to the present embodiment is not particularly limited; it can be a columnar shape such as a cuboid, a hexahedron, a tabular shape, a quadrangular prism, and the like. A cross-sectional shape of the RTB-based sintered magnet can be any shape, such as a C-shaped cylindrical shape. Regarding a quadrangular prism, the quadrangular prism can be one with a rectangular base or one with a square base.

[0045] Furthermore, the RTB-based sintered magnet according to the present embodiment includes both a magnet product in which the present magnet has been magnetized after processing and a magnet product in which the present magnet has not been magnetized. <Herstellungsverfahren für R-T-B-basierten gesinterten Magneten>

[0046] An example of the method for manufacturing the RTB-based sintered magnet of the present embodiment having the above-mentioned structure will be described with reference to the drawings. Fig. 3 is a flowchart showing an example of the manufacturing method of the RTB-based sintered magnet according to the present embodiment. As shown in Fig. 3, a method for manufacturing the RTB-based sintered magnet according to the present embodiment includes the following steps. (a) An alloy manufacturing step in which an alloy is manufactured (step S11); (b) A pulverization step in which the alloy is pulverized (step S12); (c) A pressing step in which the alloy powder is pressed (step S13); (d) A sintering step in which the green compact is sintered to provide an RTB-based sintered magnet (step S14); (e) An aging treatment step in which the RTB-based sintered magnet is subjected to an aging treatment (step S15); (f) A cooling step in which the RTB-based sintered magnet is cooled (step S16); (g) A processing step in which the RTB-based sintered magnet is processed (step S17); (i) A grain boundary diffusion step in which a heavy rare earth element is diffused into the grain boundary of the RTB-based sintered magnet (step S18); (j) A surface treatment step in which the RTB-based sintered magnet is subjected to a surface treatment (step S19). [Alloy manufacturing step: Step S11]

[0047] In manufacturing the RTB-based sintered magnet of the present embodiment, a raw material alloy constituting the RTB-based sintered magnet is first prepared (an alloy manufacturing step (step S11)). In this alloy manufacturing step (step S11), the raw material metals corresponding to the composition of the RTB-based sintered magnet of the present embodiment are melted under vacuum or in an inert gas atmosphere such as Ar gas. Then, they are cast to provide the alloy having a desired composition. Furthermore, in the present embodiment, a single alloying method using only one type of alloy is described. However, a two-alloying method in which the raw material powder is prepared by casting two types of alloys and then mixing them may also be used.

[0048] The raw material metal can be, for example, a rare earth metal or rare earth alloy, pure iron, ferroboron, and furthermore, an alloy or compound thereof. The casting method for casting the raw material metals can be, for example, an ingot casting method, a strip casting method, a split die casting method, a centrifugal casting method, or the like. In particular, the strip casting method is preferable.

[0049] In the present embodiment, since Zr must be present in the main phase grains of the RTB-based sintered magnet, Zr in the alloy phase must be dissolved in the RTB-based compound of the main phase to form a solid solution. To produce such an alloy, when using the strip casting method, the molten metal temperature at which the raw material metals are melted and the cooling rate must be controlled. The optimal conditions vary depending on the alloy composition. Specifically, the molten metal temperature is preferably set within a range of 1450°C to 1550°C, which is higher than the conventional temperature, and the cooling rate is controlled to be 1500°C / sec or higher. [Pulverization step: Step S12]

[0050] Then, the alloy obtained after casting is pulverized (a pulverization step (step S12)). This pulverization step (step S12) includes a coarse pulverization step (step S12-1) in which the alloy is pulverized to a particle size of several hundred µm to several mm, and a fine pulverization step (step S12-2) in which fine pulverization is performed to a particle size of several µm. (Coarse pulverization step: Step S12-1)

[0051] The alloy obtained after casting is coarsely pulverized to provide a particle size of several hundred µm to several mm (the coarse pulverization step (step S12-1)). Thus, the coarsely pulverized powder of the alloy is obtained. Coarse pulverization can be performed as follows: First, hydrogen is stored in the alloy. Then, the hydrogen is released based on the difference in the amount of hydrogen stored between the different phases. With dehydrogenation, self-collapsing pulverization (hydrogen storage pulverization) is performed.

[0052] Further, in addition to the above-mentioned hydrogen storage pulverization, the coarse pulverization step (step S12-1) can also be carried out using a coarse pulverizer such as a crusher, a jaw crusher, a Brown mill, and the like in an inert gas atmosphere.

[0053] Furthermore, to provide good magnetic properties, the atmosphere in each step, from the pulverization step (step S12) to the sintering step (step S15), preferably has a low oxygen concentration. The oxygen concentration can be adjusted by controlling the atmosphere in each manufacturing step. If the oxygen concentration is high in each manufacturing step, the rare earth element in the alloy powder oxidizes, and R oxide is generated. The R oxide is directly deposited in the grain boundary without being reduced in the sintering process, resulting in reduced Br in the obtained RTB-based sintered magnet. Therefore, the oxygen concentration in each step is preferably, for example, 100 ppm or less. (Fine pulverization step: Step S12-2)

[0054] After the alloy is coarsely pulverized, the resulting coarsely pulverized powder is finely pulverized to provide an average particle size of approximately several μm (a fine pulverization step (step S12-2)). In this way, the finely pulverized powder of the alloy is then obtained. When the coarsely pulverized powder is further finely pulverized, a finely pulverized powder having a particle size of 0.1 μm or more and 5 μm or less, and more preferably 1 μm or more and 3 μm or less, can be obtained.

[0055] Fine pulverization is performed by appropriately adjusting conditions such as pulverization time and the like, and at the same time, further pulverizing the coarsely pulverized powder using a fine pulverizer such as a jet mill, a bead mill, and the like. The jet mill is used to perform the pulverization process as follows. The jet mill discharges inert gas (e.g., N2 gas) at high pressure from a narrow nozzle to generate a high-speed gas stream. The coarsely pulverized powder is accelerated by this high-speed gas stream, causing collision between the coarsely pulverized powder particles or collision between the coarsely pulverized powder and a target or the wall of a container.

[0056] When a jet mill is used to obtain finely pulverized powder with a small particle size, the pulverized powder has a very high surface activity. Thus, the pulverized powders are likely to reagglomerate with each other or adhere to the wall of a container, and thus the yield tends to decrease. Therefore, by adding a pulverizing aid such as zinc stearate, oleic acid amide, and the like during the fine pulverization of the coarsely pulverized powder, the powder can be prevented from reagglomerating or adhering to the wall of a container. Thus, the finely pulverized powder with a high yield can be obtained. Furthermore, by adding the pulverizing aid in this way, a finely pulverized powder that can be easily aligned during the pressing step can be obtained.The amount of pulverizing aid to be added varies depending on the particle size of the finely pulverized powder or the type of pulverizing aid to be added, and is preferably about 0.1 mass% to 1 mass%. [Pressing step: Step S13]

[0057] After fine pulverization, the finely pulverized powder is pressed to have a desired shape (a pressing step (step S13)). In the pressing step, the finely pulverized powder of the alloy is filled into a die surrounded by an electromagnet, and then pressure is applied. In this way, the finely pulverized powder is pressed to provide any desired shape. During this time, a magnetic field is applied, and a predetermined orientation is created in the raw material powder by the applied magnetic field. Then, the raw material powder is pressed with the crystal axis aligned by the magnetic field. Thus, a green compact is obtained. Since the resulting green compact is oriented in a specified direction, an anisotropic RTB-based sintered magnet with stronger magnetism can be provided.

[0058] The pressure applied during the pressing step is preferably between 30 MPa and 300 MPa. The intensity of the applied magnetic field is preferably between 950 kA / m and 1600 kA / m. The applied magnetic field is not limited to a magnetostatic field; it can also be a pulsed magnetic field. Furthermore, a magnetostatic field and a pulsed magnetic field can be used in combination.

[0059] In addition to the dry pressing method as described above in which the finely pulverized powder is directly pressed, the pressing method may also be a wet pressing method in which a slurry obtained by dispersing the raw material powder in a solvent such as an oil is pressed.

[0060] The shape of the green compact obtained by pressing the finely pulverized powder is not particularly limited and may be any shape such as a cuboid, a flat shape, a columnar shape, a ring shape, and the like according to the desired shape of the RTB-based sintered magnet. [Sintering step: Step S14]

[0061] The green compact, which has been pressed in a magnetic field to a desired shape, is sintered under vacuum or in an inert atmosphere to obtain an RTB-based sintered magnet (a sintering step (step S14)). The green compact is sintered by subjecting it to a thermal treatment under vacuum or in an inert atmosphere at 900°C or more and 1200°C or less for one hour or more and 30 hours or less. This performs liquid-phase sintering in the finely pulverized powder, and then an RTB-based sintered magnet (a sintered body of RTB-based magnet) with an increased volume ratio occupied by the main phase is obtained.

[0062] In the present embodiment, the peripheral region with a low Zr mass concentration is easily formed in the main phase grains by controlling the cooling rate after the sintered body is kept at the sintering temperature for a certain period of time during the sintering step. Specifically, it is preferable that the sintered body be cooled slowly from the sintering temperature to 800°C and then rapidly cooled. The cooling rate from the sintering temperature to 800°C is preferably set at 2°C / minute to 6°C / minute.

[0063] The reason why the peripheral region with a low Zr mass concentration in the main phase grains easily forms when the cooling rate is controlled in the manner described above is not entirely clear. The mechanism is hypothesized as follows. (1) With the control of individual elements and casting conditions, Zr is dissolved in the RTB-based compound of the main phase under solid solution formation before sintering. (2) At sintering temperature, the grain boundary phase becomes a liquid phase, and part of the main phase grains dissolves to form a liquid phase, so that the sintering process continues. (3) When the sintered body is cooled from the sintering temperature, the RTB-based compound is redeposited from the liquid phase onto the surface of the main phase grains. If the cooling rate is relatively fast, Zr is likely to enter the RTB-based compound. Conversely, if the cooling rate is slow, it is difficult for Zr to enter the RTB-based compound. The Zr that has not entered the RTB-based compound is deposited in the grain boundary phase as a Zr-based compound. (4) With the above-mentioned process, Zr dissolved in the initial alloy phase through solid solution formation will remain directly in the central region of the main phase grains. On the other hand, Zr is formed in the peripheral region by redeposition from the liquid phase, and its concentration will decrease. Thus, the structure with a concentration distribution of Zr in the main phase grains is formed. [Aging treatment step: Step S15]

[0064] After the green compact is sintered, the RTB-based sintered magnet is subjected to an aging treatment (an aging treatment step (step S15)). After the sintering step, the RTB-based sintered magnet is subjected to aging treatment by keeping the RTB-based sintered magnet at a temperature lower than that during sintering. For example, the aging treatment can be performed either in two stages or in a single stage. In the two-stage heat treatment, the RTB-based sintered magnet is heated at 700°C or more and 900°C or less for 1 hour to 3 hours, and then further heated at 500°C to 700°C for 1 hour to 3 hours. In the single-stage heat treatment, the RTB-based sintered magnet is heated at about 600°C for 1 hour to 3 hours.The treatment conditions can be appropriately adjusted based on the number of times the aging treatment is to be performed. With such an aging treatment, the magnetic properties of the RTB-based sintered magnet can be improved. Furthermore, the aging treatment step (step S15) can be performed after a machining step (step S17) or a grain boundary diffusion step (step S18). [Cooling step: Step S16]

[0065] After the RTB-based sintered magnet is subjected to aging treatment, the RTB-based sintered magnet is rapidly cooled in an Ar atmosphere (a cooling step (step S16)). Thus, the RTB-based sintered magnet according to the present embodiment is obtained. The cooling rate is not particularly limited, and it is preferably 30°C / min or higher. [Processing step: Step S17]

[0066] The obtained RTB-based sintered magnet can be machined to have a desired shape, if necessary (a machining step: step S17). The machining method can be, for example, a shaping process such as cutting, grinding, and the like, and a chamfering process such as barrel polishing and the like. [Grain boundary diffusion step: Step S18]

[0067] A step in which the heavy rare earth element is further diffused into a grain boundary of the processed RTB-based sintered magnet may be included (a grain boundary diffusion step: step S18). The grain boundary diffusion may be performed by adhering a compound containing the heavy rare earth element to the surface of the RTB-based sintered magnet by coating, deposition, or the like, followed by a thermal treatment, or alternatively, by subjecting the RTB-based sintered magnet to a thermal treatment in an atmosphere containing a vapor of the heavy rare earth element. With this step, the coercive force of the RTB-based sintered magnet can be further improved. [Surface treatment step: Step S19]

[0068] Surface treatment such as plating, resin coating, oxidation treatment, chemical conversion treatment, and the like can be applied to the RTB-based sintered magnet obtained from the above steps (a surface treatment step (step S19)). Thus, corrosion resistance can be further improved.

[0069] Also, although the machining step (step S17), the grain boundary diffusion step (step S18), and the surface treatment step (step S19) are performed in the present embodiment, these steps do not necessarily have to be performed.

[0070] The RTB-based sintered magnet according to the present embodiment is manufactured as described above, and the treatment is completed. Further, a magnet product can be obtained by magnetizing the resulting magnet.

[0071] In the RTB-based sintered magnet thus obtained according to the present embodiment, since the main phase grains include main phase grains in which the Zr mass concentration in the peripheral region of the main phase grain is lower than that in the central region of the main phase grain, the decrease in the residual magnetic flux density and the decrease in the coercive force associated with the increased Zr content can be inhibited. Furthermore, grain growth during sintering can be prevented even when a pulverized raw material powder with a small particle size is used.

[0072] The RTB-based sintered magnet of the present embodiment can be suitably used as a magnet in, for example, a surface permanent magnet (SPM) type rotating electric machine having a magnet mounted on the surface of a rotor, an internal permanent magnet (IPM) type rotating electric machine such as an inner rotor type brushless motor, a PRM (permanent magnet reluctance motor), or the like. Specifically, the RTB-based sintered magnet according to the present embodiment is applicable to a spindle motor for a hard disk drive or a voice coil motor in a hard disk drive, a motor for an electric vehicle or a hybrid car, an electric power steering motor in an automobile, a servo motor for a machine tool, a motor for a vibrator of a mobile phone, a motor for a printer, a motor for a generator, and the like. Examples

[0073] In the following, the present invention will be described in more detail by means of examples. <Herstellung eines R-T-B-basierten gesinterten Magnets> (Example 1)

[0074] First, a raw material alloy capable of providing a sintered magnet with a composition (Composition A, i.e., 24.50 mass% Nd - 7.00 mass% Pr - 0.50 mass% Co - 0.45 mass% Ga - 0.20 mass% Al - 0.20 mass% Cu - 0.86 mass% B - 1.00 mass% Zr - balance Fe) was prepared by a strip casting process. The casting process was carried out at a molten metal temperature of 1500°C and a cooling rate of about 2000°C / minute. Furthermore, balance refers to the amount of residue when the entire composition is to be 100 mass%.

[0075] Next, the hydrogen pulverization treatment (i.e., coarse pulverization) was performed. Specifically, after storing hydrogen in the raw material alloy at room temperature, dehydrogenation was performed at 500°C for 1 hour in an Ar atmosphere.

[0076] Furthermore, in the present example, each of the steps from the hydrogen pulverization treatment to the sintering step (the fine pulverization and pressing step) was carried out in an Ar atmosphere, with the oxygen concentration therein being less than 50 ppm (the same conditions were applied in the following examples and comparative examples).

[0077] Next, 0.3 mass% of oleic acid amide was added to the resulting coarsely pulverized powder as a pulverization aid. The mixture was then blended using a Nauta mixer. Fine pulverization was then performed using a jet mill to obtain a finely pulverized powder with an average particle size of approximately 2.8 µm.

[0078] Subsequently, the resulting finely pulverized powder was filled into a mold mounted inside an electromagnet, and pressing was performed under an applied pressure of 120 MPa in a magnetic field of 1200 kA / m. Thus, a green compact was obtained.

[0079] The green compact was then sintered in a vacuum at 1070°C for 8 hours. After sintering, the temperature was slowly cooled to 800°C at a cooling rate of 4°C / minute, followed by rapid cooling at a cooling rate of 40°C / minute to room temperature. Thus, a sintered body (an RTB-based sintered magnet) was obtained. Next, a two-stage aging treatment was performed on the obtained sintered body at 850°C for 1 hour and then at 500°C for 1 hour (both in an Ar atmosphere). Thus, the RTB sintered magnets of Examples 1 to 6 were obtained. (Examples 2 to 4 and comparative example 1)

[0080] The RTB-based sintered magnets of Examples 2 to 4 and Comparative Example 1 were obtained as in Example 1, except that the cooling rate during the cooling process from the sintering temperature to 800°C was set to the values ​​listed in Table 1.

[0081] The surface of a cross section of each RTB-based sintered magnet was milled by ion etching to eliminate the influence of oxidation on the outermost surface or the like, and then the cross section of the RTB-based sintered magnet was evaluated by ESMA (electron beam microanalysis). Fig. Figure 4 shows the backscattered electron image of a cross section in the RTB-based sintered magnet of Example 1. The darker part represents the main phase grains in dark contrast. A main phase grain in the backscattered electron image of the Fig.4 was applied to the Zr concentration along a straight line passing through the center of gravity of the grain (the dotted line in Fig. 4) is quantitatively analyzed with an interval of 0.3 µm between two adjacent analysis points, and the results are presented in Fig. 5. It was confirmed that Mc (i.e., the Zr mass concentration in the central region of the main phase grain) was 0.84 mass% and Ms (i.e., the Zr mass concentration in the peripheral region of the main phase grain) was 0.14 mass%. Furthermore, it was confirmed that the ratio (Ms / Mc) of Ms (the Zr mass concentration in the peripheral region of the main phase grain) to Mc (the Zr mass concentration in the central region of the main phase grain) was 70% or less.

[0082] The same analysis was performed on each RTB-based sintered magnet of Examples 2 to 4 and Comparative Example 1, and the results are shown in Table 1. The Ms / Mc value increased as the cooling rate during the cooling process from the sintering temperature to 800°C became faster. In Comparative Example 1, in which the cooling rate during the cooling process from the sintering temperature to 800°C was set at 40°C / minute, the Ms / Mc reached a value higher than 70%. [Table 1] Cooling rate during the cooling process to 800°C (°C / min.) Oxygen content (ppm) Carbon content (ppm) Zr mass concentration in the central region (Mc) (mass%) Zr mass concentration in the edge region (Ms) (mass%) Ms / Mc(%) Magnetic properties HcJ difference compared to the reference example (kA / m) Br (mT) HcJ (kA / m) Example 1 4 910 1510 0, 84 0,14 17% 1351 1685 93 Example 2 6 860 1490 0,89 0,22 25% 1353 1654 62 Example 3 8 930 1530 0,95 0,38 40% 1349 1650 58 Example 4 12 870 1520 0,99 0,69 70% 1350 1622 30 Comparison example 1 40 900 1510 1,01 0,82 81% 1348 1592

[0083] Each of the RTB-based sintered magnets obtained in Examples 1 to 4 and Comparative Example 1 was subjected to composition analysis using X-ray fluorescence analysis and inductively coupled plasma mass spectrometry (ICP-MS). As a result, it was confirmed that the composition of each of the RTB-based sintered magnets was almost the same as the target composition. Furthermore, the oxygen content was measured by an inert gas fusion non-dispersive infrared absorption method, and the carbon content was measured by an oxygen-air combustion infrared absorption method. The results regarding the oxygen content and carbon content are shown in Table 1.

[0084] For each RTB-based sintered magnet of Examples 1 to 4 and Comparative Example 1, the average grain size of the main phase grains was estimated. Regarding the average grain size of the main phase grains, the cross-section of a sample was ground and then examined through an optical microscope, and the cross-sectional image was subjected to image analysis software to determine the grain size distribution in the main phase grains. In each of the sintered magnets, the average grain size of the main phase grains was 3.3 µm.

[0085] The magnetic properties of the RTB-based sintered magnets obtained in Examples 1 to 4 and Comparative Example 1 were determined using a BH tracer. The residual magnetic flux density Br and the coercive field strength HcJ were measured as magnetic properties. The results from the measurement of the residual magnetic flux density Br and the coercive field strength HcJ in each RTB-based sintered magnet were shown in Table 1. The coercive field strength differences between the RTB-based sintered magnets of Examples 1 to 4 and the RTB-based sintered magnet of Comparative Example 1 were also shown in Table 1. It was confirmed that the RTB-based sintered magnets of Examples 1 to 4 had a higher coercive field strength HcJ than that of Comparative Example 1. (Examples 5 to 9 and comparative examples 2 to 6)

[0086] The RTB-based sintered magnets of Examples 5 to 9 were manufactured as in Example 1, except that the raw material alloys were prepared by strip casting to provide sintered magnets having compositions B to F as shown in Table 2. Further, the RTB-based sintered magnets of Comparative Examples 2 to 6 were manufactured as in Comparative Example 1, except that the raw material alloys were prepared by strip casting to provide sintered magnets having compositions B to F as shown in Table 2. [Table 2] Composition (mass%) Corresponding examples Corresponding comparison examples Nd Pr Dy (T.RE) Co Ga Al Cu B Zr Fe CompositionA 24,50 7, 00 0,00 31,50 0,50 0,45 0,20 0,20 0,86 1,00 rest Examples 1 to 4 Comparison example 1 CompositionB 31,00 0,00 0,00 31,00 1,50 0,70 0,10 0,10 0,82 0,60 rest Example 5 Comparison example 2 CompositionC 24, 00 7, 00 1, 00 32,00 1,00 0,30 0,20 0,30 0,90 1,30 rest Example 6 Comparison example 3 CompositionD 30, 50 0, 00 2, 00 32,50 2,00 1,00 0,20 0,50 0,75 0,30 rest Example 7 Comparison example 4 CompositionE 24, 00 6,00 0, 00 30,00 0,50 0, 60 0,20 1,00 0,80 0,50 rest Example 8 Comparison example 5 CompositionF 24,50 7,50 0,00 32,00 1,00 0, 80 0,60 0,60 0,78 2,00 rest Example 9 Comparison example 6 CompositionG 24,50 7, 00 0,00 31,50 0,50 0,45 0,20 0,20 0,86 0,25 rest Comparison example 7 CompositionH 24,50 7, 00 0,00 31,50 0,50 0,45 0,20 0,20 0,86 2,50 rest Comparison example 8

[0087] For each RTB-based sintered magnet from Examples 5 to 9 and Comparative Examples 2 to 6, an analysis of the Zr mass concentration in the main phase grains was performed similarly to Example 1. The results were shown in Table 3. All Ms / Mc values ​​in the RTB-based sintered magnets from Examples 5 to 9 were 70% or less. In contrast, the Ms / Mc values ​​in the RTB-based sintered magnets from Comparative Examples 2 to 6 were higher than 70%. [Table 3] composition Oxygen content (ppm) Carbon content (ppm) Average grain size of the main phase grains (µm) Zr mass concentration in the central region (Mc) (mass%) Zr mass concentration in the edge region (Ms) (mass%) Ms / Mc(%) Magnetic properties HcJ difference compared to the reference example (kA / m) Br (mT) HcJ (kA / m) Example 5 B 760 1400 3,5 0,47 0,11 23% 1338 1723 110 Comparison example 2 810 1420 3,5 0,61 0,55 90% 1332 1613 DT -- Example 6 C 850 1960 2,2 0, 98 0,38 39% 1340 1692 65 Comparison example 3 860 1970 2,2 1,33 1,02 77% 1332 1627 Example 7 D 1030 1620 2,8 0,28 0,16 57% 1281 2034 37 Comparison example 4 1060 1600 2,8 0,30 0,26 87% 1275 1997 DT Example 8 E 1150 1210 4,0 0,43 0,16 37% 1371 1594 73 Comparison example5 1210 1230 4,0 0,49 0,42 86% 1359 1521 Example 9 F 1400 2480 1,5 1,76 0,47 27% 1302 1834 78 Comparison example6 1350 2520 1,5 1, 92 1,52 79% 1295 1756

[0088] The composition analysis of each RTB-based sintered magnet from Examples 5 to 9 and Comparative Examples 2 to 6 was performed similarly to Example 1. As a result, it was confirmed that the composition of each of the RTB-based sintered magnets was substantially the same as the target composition (each composition as shown in Table 2). Furthermore, the oxygen content, carbon content, and average grain size of the main phase grains were also analyzed as in Example 1. The results are shown collectively in Table 3.

[0089] As in Example 1, the magnetic properties of the RTB-based sintered magnets of Examples 5 to 9 and Comparative Examples 2 to 6 were similarly evaluated. The results were shown in Table 3. Comparing the RTB-based sintered magnets in Examples 5 to 9 with those in Comparative Examples having the same composition, it could be seen that the RTB-based sintered magnet of the Examples has a higher coercive force than that of the Comparative Examples having the same composition. (Comparison examples 7 and 8)

[0090] The RTB-based sintered magnets of Comparative Examples 7 to 8 were prepared as in Example 1, except that the raw material alloys were prepared by strip casting to provide sintered magnets having compositions G and H as shown in Table 2. Furthermore, composition G is the same as composition A in Example 1 except that the Zr content was changed to 0.25 mass%, and composition H is the same as composition A in Example 1 except that the Zr content was changed to 2.5 mass%.

[0091] The composition analysis of each RTB-based sintered magnet from Comparative Examples 7 and 8 was performed similarly to Example 1. As a result, it was confirmed that the composition of both RTB-based sintered magnets was substantially the same as the target composition (each composition as shown in Table 2). Furthermore, the oxygen content, carbon content, and average grain size of the main phase grains were also analyzed as in Example 1. The results are shown in Table 4. In the sample from Comparative Example 7 with a lower Zr content, abnormal grain growth occurred during the sintering process, and the average grain size of the main phase grains became extremely large compared to that in Example 1. [Table 4] composition Zr content (mass%) Oxygen content (ppm) Carbon content (ppm) Average grain size of the main phase grains (µm) Magnetic properties Br(mT) HcJ(kA / m) Comparison example 7 G 0,25 880 1530 6,2 1386 912 Comparison example 8 H 2,5 870 1520 3,2 1136 1442 Example 1 A 1,0 910 1510 3,3 1351 1685

[0092] The magnetic properties of each RTB-based sintered magnet obtained in Comparative Examples 7 and 8 were estimated similarly to those in Example 1. The results were shown in Table 4 along with the results of Example 1. It can be seen that the magnet of Comparative Example 7 with a lower Zr content exhibits a significant reduction in coercive force due to the influence of abnormal grain growth compared to Example 1. Furthermore, the magnet of Comparative Example 8 with a higher Zr content results in a significant decrease in the residual magnetic flux density. DESCRIPTION OF REFERENCE SYMBOLS 2 main phase grain 4 Central area 6 Marginal area 8 Grain boundary phase

Claims

[1] RTB-based sintered magnet comprising an RTB-based compound as main phase grains (2), wherein the RTB-based connection R2T 14 B with a crystal structure characterized by tetragonal R2T 14 B is shaped, R represents at least one rare earth element, T represents one or more iron family elements comprising Fe or the combination of Fe and Co, the Zr content contained in the RTB-based sintered magnet is 0.3 mass% to 2.0 mass%, the main phase grains (2) comprise Zr, the RTB-based sintered magnet comprises main phase grains (2) in which, in the cross section of each of the main phase grains (2), the Zr mass concentration in a peripheral region (6) in each of the main phase grains (2) is 70% or less of that in the central region (4) in each of the main phase grains (2), and the edge area lies on a line 22a-23a and a line 22b-23b; if a random straight line is introduced which passes through the main phase grain (2) and also crosses a center of gravity (21) of the main phase grain (2), points at which the random straight line and an outermost edge intersect are point 22a and point 22b, a length of a segment 22a-22b is L, a point which is at a distance of 0.25 x L from point 22a on the segment 22a-22b is point 23a and a point which is at a distance of 0.25 x L from point 22b on the segment 22a-22b is point 23b. [2] The RTB-based sintered magnet according to claim 1, comprising main phase grains (2) in which, in the cross section of each of the main phase grains (2), the Zr mass concentration in the peripheral region (6) in each of the main phase grains (2) is 40% or less of that in the central region (4) in each of the main phase grains (2). [3] The RTB-based sintered magnet according to claim 1 or 2, wherein the Zr mass concentration in the peripheral region (6) of each main phase grain (2) is 0.15 mass% or less.

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