METHOD FOR PRODUCING RFeB SYSTEM SINTERED MAGNET

Abstract

A method for producing an RFeB system sintered magnet according to the present invention includes: a process (S1) of preparing a lump of HDDR-treated raw material alloy that contains a polycrystalline substance including crystal grains having an average grain size of 1 μm or less in terms of an equivalent circle diameter calculated from an electron micrograph image, by an HDDR treatment including steps of heating a lump of RFeB system alloy containing 26.5 to 29.5% by weight of the rare-earth element R, in a hydrogen atmosphere at a temperature between 700 and 1,000° C., and changing the atmosphere to vacuum while maintaining the temperature within a range from 750 to 900° C.; a process (S2) of preparing a lump of raw material alloy having a high rare-earth content by heating the lump of HDDR-treated raw material alloy at a temperature between 700 and 950° C. in a state where the HDDR-treated raw material alloy is in contact with a contact substance including a second alloy that contains the rare-earth element R at a higher content ratio than a content ratio of the rare-earth element R in the RFeB system alloy; a process (S3) of preparing raw material alloy powder by fine pulverization of the lump of raw material alloy having a high rare-earth content into powder having an average particle size of 1 μm or less; an orienting process (S4) including steps of placing the raw material alloy powder in a mold, and applying a magnetic field to the raw material alloy powder without conducting compression molding; and a sintering process (S5) including a step of heating the oriented raw material alloy powder at a temperature between 850 and 1,050° C.

Claims

1. A method for producing an RFeB system sintered magnet containing a rare-earth element R, Fe, and B as main components, the method comprising: a) a process of preparing a lump of HDDR-treated raw material alloy that contains a polycrystalline substance including crystal grains having an average grain size of 1 μm or less in terms of an equivalent circle diameter calculated from an electron micrograph image, by an HDDR treatment including steps of heating a lump of RFeB system alloy containing 26.5 to 29.5% by weight of the rare-earth element R, in a hydrogen atmosphere at a temperature between 700 and 1,000° C., and changing the atmosphere to vacuum while maintaining the temperature within a range from 750 to 900° C.; b) a process of preparing a lump of raw material alloy having a high rare-earth content by heating the lump of HDDR-treated raw material alloy at a temperature between 700 and 950° C. in a state where the HDDR-treated raw material alloy is in contact with a contact substance including a second alloy that contains the rare-earth element R at a higher content ratio than a content ratio of the rare-earth element R in the RFeB system alloy; c) a process of preparing raw material alloy powder by fine pulverization of the lump of raw material alloy having a high rare-earth content into powder having an average particle size of 1 μm or less; d) an orienting process including steps of placing the raw material alloy powder in a mold, and applying a magnetic field to the raw material alloy powder without conducting compression molding; and e) a sintering process including a step of heating the oriented raw material alloy powder at a temperature between 850 and 1,050° C.

2. The method for producing an RFeB system sintered magnet according to claim 1, wherein the lump of RFeB system alloy is prepared by a strip casting method.

3. The method for producing an RFeB system sintered magnet according to claim 1, wherein the contact substance is in a powdery form.

4. The method for producing an RFeB system sintered magnet according to claim 1, wherein the fine pulverization is performed by a jet mill method using helium gas.

5. The method for producing an RFeB system sintered magnet according to claim 1, wherein the second alloy contains Ga.

6. The method for producing an RFeB system sintered magnet according to claim 2, wherein the contact substance is in a powdery form.

7. The method for producing an RFeB system sintered magnet according to claim 2, wherein the fine pulverization is performed by a jet mill method using helium gas.

8. The method for producing an RFeB system sintered magnet according to claim 3, wherein the fine pulverization is performed by a jet mill method using helium gas.

9. The method for producing an RFeB system sintered magnet according to claim 6, wherein the fine pulverization is performed by a jet mill method using helium gas.

10. The method for producing an RFeB system sintered magnet according to claim 2, wherein the second alloy contains Ga.

11. The method for producing an RFeB system sintered magnet according to claim 3, wherein the second alloy contains Ga.

12. The method for producing an RFeB system sintered magnet according to claim 4, wherein the second alloy contains Ga.

13. The method for producing an RFeB system sintered magnet according to claim 6, wherein the second alloy contains Ga.

14. The method for producing an RFeB system sintered magnet according to claim 7, wherein the second alloy contains Ga.

15. The method for producing an RFeB system sintered magnet according to claim 8, wherein the second alloy contains Ga.

16. The method for producing an RFeB system sintered magnet according to claim 9, wherein the second alloy contains Ga.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0033] FIG. 1A is a flow chart showing processes of a method for producing an RFeB system sintered magnet according to examples of the present invention. FIG. 1B is a flow chart showing processes in Comparative Examples.

[0034] FIG. 2 is a graph showing a temperature history and a gas atmosphere during an HDDR treatment in the examples.

[0035] FIGS. 3A and 3B are backscattered electron images taken with an electron microscope, where FIG. 3A shows a lump of material alloy having a high rare-earth content prepared in one stage of a method for producing the RFeB system sintered magnet according to Example 2, and FIG. 3B shows the lump of HDDR-treated raw material alloy prepared in the previous stage.

[0036] FIGS. 4A and 4B are backscattered electron images showing a lump of HDDR-treated raw material alloy observed with an electron microscope, prepared in one stage of a method for producing an RFeB system sintered magnet according to Comparative Examples 1 and 2, respectively.

DESCRIPTION OF EMBODIMENTS

[0037] Examples of the method for producing an RFeB system sintered magnet according to the present invention are hereinafter described with reference to the drawings. It should be noted that the present invention is not limited to the following examples.

Method for Producing RFeB System Sintered Magnet According to Example 1

[0038] In Example 1, an RFeB system sintered magnet was produced using, as materials, a lump of RFeB system alloy and powder of a second alloy having the compositions shown in Table 1 below, by five processes as shown in FIG. 1A: the HDDR process (Step S1), rare earth grain boundary penetration process (Step S2), raw material alloy powder preparation process (Step S3), orienting process (Step S4), and sintering process (Step S5). In Table 1, “TRE” indicates the total content by percentage of the rare-earth elements (Nd and praseodymium (Pr) in Example 1) contained in the lump of RFeB system alloy.

TABLE-US-00001 TABLE 1 Composition of materials used in Example 1 (unit: % by weight) TRE Nd Pr B Cu Al Co Fe RFeB system 28.1 24.33 3.76 1.00 0.00 0.04 0.95 bal. alloy lump Second alloy 80.0 80.0 0.00 0.00 10.0 10.0 0.00 0.00 powder

[0039] The HDDR process is described with reference to the graph shown in FIG. 2. First, a lump of RFeB system alloy that had been prepared by a strip casting method, and had an equivalent circle diameter ranging from 100 μm to 20 mm was prepared. The lump of RFeB system alloy was made to occlude hydrogen sufficiently at room temperature, and then heated at 950° C. for 60 minutes in a hydrogen atmosphere of 100 kPa to decompose the Nd.sub.2Fe.sub.14B compound (main phase) in the lump of HDDR-treated raw material alloy into three phases of NdH.sub.2 phase, Fe.sub.2B phase, and Fe phase (Decomposition: “HD process” in FIG. 2). Next, with the hydrogen atmosphere being maintained, the temperature was decreased to 800° C., and then argon (Ar) gas was supplied for 10 minutes, with the temperature being maintained at 800° C., to remove hydrogen gas surrounding the lump of RFeB system alloy. Subsequently, the atmosphere was changed to vacuum, and the temperature was maintained at 800° C. for 60 minutes to desorb the hydrogen atoms in the form of gas from the NdH.sub.2 phase in the lump of RFeB system alloy so as to cause a recombination reaction of the Fe.sub.2B phase and the Fe phase (Desorption and Recombination: “DR process” in FIG. 2). After that, the temperature was decreased to room temperature by cooling the furnace. The lump of HDDR-treated raw material alloy was thus prepared. It should be noted that the purpose of decreasing the temperature from 950° C. to 800° C. upon transition from the HD process to the DR process in the HDDR operation is to prevent the crystal grains formed by the DR process from additional growth during this process. In the present example, the obtained lump of HDDR-treated raw material alloy was mechanically crushed into coarse powder with an equivalent circle diameter of 100 pmn or less using the Wonder Blender (manufactured by OSAKA CHEMICAL Co, Ltd.). Such coarse powder obtained by the crushing the lump is also regarded as the HDDR-treated raw material alloy in the present invention.

[0040] In the rare earth grain boundary penetration process, the coarsely crushed lump of HDDR-treated raw material alloy and the second alloy powder previously prepared by pulverizing the second alloy into powder having an average particle size of 4 μm by a jet mill method using nitrogen gas were mixed at the weight ratio of 95:5 and heated at a temperature of 700° C. for 10 minutes, to thereby prepare a lump of raw material alloy having a high rare-earth content.

[0041] In the raw material alloy powder preparation process, the lump of raw material alloy having a high rare-earth content was maintained in a hydrogen atmosphere at a temperature of 200° C. for five hours to embrittle the lump, and was subsequently pulverized into powder having an average particle size of 1 μm or less by a helium jet mill method, to thereby prepare raw material alloy powder.

[0042] In the orienting process, an organic lubricant was first mixed in the raw material alloy powder; the powder was placed in a mold at a filling density of 3.5 g/cm.sup.3; and a pulsed magnetic field of approximately 5 tesla was applied without conducting compression molding. In the subsequent sintering process, the raw material alloy powder being held in the mold was sintered by being heated in vacuum at a temperature of 940° C. for one hour without undergoing compression molding. After the sintering process, the obtained sintered body was heated for ten minutes in an argon atmosphere at the temperature at which the highest coercivity can be obtained within the range from 500° C. to 660° C. The obtained sintered body was machined to create a cylindrical RFeB system sintered magnet measuring 9.8 mm in diameter and 7.0 mm in length.

Method for Producing RFeB System Sintered Magnet According to Example 2

[0043] In Example 2, an RFeB system sintered magnet was produced using, as materials, a lump of RFeB system alloy and powder of a second alloy having the compositions shown in Table 2 below by basically the same processes as used in Example 1. The differences from Example 1, other than the compositions of materials, are listed below. [0044] The second alloy powder was prepared using the Wonder Blender instead of the jet mill method using nitrogen gas. Accordingly, the average particle size of the second alloy powder was larger than that of Example 1. [0045] The mixture ratio of the lump of HDDR-treated raw material alloy with the second alloy powder in the rare earth grain boundary penetration process was 94:6 in weight ratio, and the heating time was 30 minutes (the heating temperature was 700° C., i.e. the same as in Example 1). [0046] The sintering temperature in the sintering process was 860° C.

TABLE-US-00002 TABLE 2 Composition of materials used in Example 2 (unit: % by weight) TRE Nd Pr B Cu Al Co Fe RFeB system 27.6 27.47 0.07 1.10 0.00 0.04 0.00 bal. alloy lump Second alloy 80.0 80.0 0.00 0.00 10.0 10.0 0.00 0.00 powder *The composition of the second alloy powder was the same as used in Example 1.

Method for Producing RFeB System Sintered Magnet According to Examples 3 to 7

[0047] In Examples 3 to 7, as shown in Table 3 below, lumps of RFeB system alloy having the same composition (but different from those used in Examples 1 and 2) were used, and the second alloy powders having individual compositions were used. The composition of the second alloy powder in Example 3 was the same as used in Examples 1 and 2. The differences from Example 1 with respect to conditions other than the composition of the materials are listed below. [0048] The mixture ratio of the lump of HDDR-treated raw material alloy with the second alloy powder in weight ratio in the rare earth grain boundary penetration process was 95:5, and the heating time was 60 minutes (the heating temperature was 700° C., i.e., the same as in Example 1). [0049] The sintering temperature in the sintering process was 890° C. in Examples 3 and 4, and 880° C. in Examples 5 to 7.

TABLE-US-00003 TABLE 3 Composition of materials used in Examples 3 to 7 (unit: % by weight) TRE Nd Pr B Cu Al Co Ga Fe RFeB Common to 2.75 27.4 0.1 1.13 0 0.04 0.01 0 bal. system Examples 3 to 7 alloy lump Second Example 3 80.0 80.0 0 0 10.0 10.0 0 0 0 alloy Example 4 76.05 76.05 0 1.03 9.50 9.50 0 0 bal. powder Example 5 63.83 63.83 0 0 0.59 0 0 3.06 bal. Example 6 90.07 90.07 0 0 2.02 0 0 6.35 bal. Example 7 83.55 83.55 0 0 2.42 0 0 11.77 bal.

Method for Producing RFeB System Sintered Magnet According to Comparative Examples

[0050] In Comparative Examples, RFeB system sintered magnets were produced using lumps of two types of RFeB system alloys having composition shown in Table 3 below, by the four steps as shown in FIG. 1B, including the HDDR process (Step S91), raw material alloy powder preparation process (Step S93), orienting process (Step S94), and sintering process (Step S95). In the HDDR process, the lump of RFeB system alloy was subjected to the same HDDR treatment as in Examples 1 and 2, to prepare a lump of HDDR-treated raw material alloy. Subsequently, the operation immediately proceeded to the raw material alloy powder preparation process, without conducting any processes corresponding to the rare earth grain boundary penetration process conducted in Examples 1 and 2. In the raw material alloy powder preparation process, the lump of HDDR-treated raw material alloy was held in a hydrogen atmosphere at a temperature of 200° C. for five hours to embrittle the lump, and subsequently pulverized into powder having an average particle size of 1 μm or less by the helium jet mill method, to thereby prepare raw material alloy powder. The raw material alloy powder thus obtained was subjected to the orienting process and the sintering process in a similar manner to Examples 1 and 2. Thus, RFeB system sintered magnets according to Comparative Examples were obtained.

TABLE-US-00004 TABLE 4 Composition of RFeB system alloy lump used in Comparative Examples (unit: % by weight) TRE Nd Pr B Cu Al Co Fe Comparative 30.42 26.35 4.07 1.00 0.10 0.28 0.92 bal. Example 1 Comparative 32.59 28.23 4.36 1.00 0.10 0.26 0.96 bal. Example 2

Composition of Raw Material Alloy Powder in Examples and Comparative Examples

[0051] Table 4 shows the results obtained by measuring the composition at the stage of raw material alloy powder (which is considered to have a composition close to that of the obtained RFeB system sintered magnet) in Examples 1 and 2 as well as Comparative Examples 1 and 2. As for the TRE value, both Examples and Comparative Examples have higher TRE values than those of the main phase, i.e., 26 to 27% by weight (when the rare-earth elements R are Nd and Pr). In other words, the content ratio of the rare-earth elements R in the entire raw material alloy powder is higher than that of the main phase.

TABLE-US-00005 TABLE 5 Composition of raw material alloy powder (unit: % by weight) TRE Nd Pr B Cu Al Co Fe Example 1 30.61 27.00 3.61 0.94 0.49 0.54 0.88 bal. Example 2 31.16 31.10 0.06 0.99 0.64 0.61 0.00 bal. Comparative 30.05 26.00 4.04 0.97 0.10 0.28 0.89 bal. Example 1 Comparative 32.65 28.20 4.44 0.95 0.11 0.28 0.94 bal. Example 2

Coercivity of the RFeB System Sintered Magnets Obtained in Examples and Comparative Examples

[0052] The coercivity of the RFeB system sintered magnets obtained in Examples and Comparative Examples was measured. The results were as shown in Table 6 below. Saturation magnetization was also measured for Examples 3 to 7. As shown in Table 6, the coercivity in Examples is higher than those in Comparative Examples, although the sintered magnets were prepared under almost the same conditions in both Examples and Comparative Examples, except for the implementation of the rare earth grain boundary penetration process. The saturation magnetization in Examples 5 to 7 is higher than those of Examples 3 and 4. The coercivity in Examples 5 to 7 is as high as in other Examples. Examples 5 to 7 are the same as Examples 3 and 4 in terms of the composition of the lump of RFeB system alloy as well as the mixture ratio of the lump of RFeB system alloy with the second alloy powder, but different from Examples 3 and 4 in that the second alloy powder contains gallium (Ga). Thus, it is clarified that both high saturation magnetization and high coercivity can be achieved by additionally mixing Ga in the second alloy powder.

TABLE-US-00006 TABLE 6 Measured result of coercivity and saturation magnetization Coercivity Saturation (kOe) magnetization (kG) Example 1 15.5 — Example 2 16.4 — Example 3 15.65 14.36 Example 4 15.59 14.44 Example 5 14.87 15.31 Example 6 15.97 14.87 Example 7 16.08 14.82 Comparative Example 1 11.5 — Comparative Example 2 12.7 —

[0053] For an RFeB system sintered magnet prepared by a normal method without the HDDR process, the higher the TRE value is, the larger the volume of the rare-earth rich phase becomes. This improves the dispersibility of the rare-earth rich phase, and thus an intergranular grain boundary with a large grain-boundary width is readily formed, thereby improving the coercivity. By comparison, the results of Comparative Examples demonstrate that, in the case of an RFeB system sintered magnet prepared with the HDDR process, the coercivity cannot be improved by merely increasing the TRE values. The reason is as follows. Even if the TRE value is increased, a lamellae structure of the rare-earth rich phase remains after the HDDR process. This prevents the rare-earth rich phase from penetrating through the main phase grains each of which is sandwiched between the rare-earth rich phases, resulting in an uneven structure.

Electron Micrographs of Alloy Lumps Immediately Before Raw Material Alloy Powder Preparation Process in Example and Comparative Examples

[0054] Electron micrographs were taken for alloy lumps immediately before the raw material alloy powder preparation process in Example 2 and Comparative Examples 1 and 2, in order to ascertain reasons for the aforementioned difference in coercivity. The alloy lumps immediately before the raw material alloy powder preparation process are a lump of raw material alloy having a high rare-earth content in Example 2, and a lump of HDDR-treated raw material alloy in Comparative Examples 1 and 2. For Example 2, an electron micrograph was also taken for a lump of HDDR-treated raw material alloy.

[0055] FIG. 3A is an electron micrograph showing a lump of raw material alloy having a high rare-earth content according to Example 2. FIG. 3B is an electron micrograph showing a lump of HDDR-treated raw material alloy according to Example 2. FIG. 4A is an electron micrograph showing a lump of HDDR-treated raw material alloy according to Comparative Example 1. FIG. 4B is an electron micrograph showing a lump of HDDR-treated raw material alloy according to Comparative Example 2. A comparison of the electron micrographs of alloy lumps immediately before the raw material alloy powder preparation process, i.e. a comparison of the electron micrograph of FIG. 3A with those of FIG. 4A and FIG. 4B, demonstrates that white line-like portions between the gray grains are clearly observed in FIG. 3A which shows Example 2, whereas white dot-like portions are observed within the widespread gray areas in FIGS. 4A and 4B which show the Comparative Examples. This means that, in Example 2, the rare-earth rich phase including the second alloy is uniformly spread through the grain boundaries of the crystal grains (gray grains) in the lump of raw material alloy having a high rare-earth content, whereas, in Comparative Examples, the rare-earth rich phase is not uniformly spread through the grain boundaries, but localized at dot-like portions. This shows that, in Example 2, the rare-earth rich phase is uniformly diffused among the grains in the raw material alloy powder obtained by pulverizing the lump of raw material alloy having a high rare-earth content, and thus the rare-earth rich phase was uniformly diffused among the crystal grains in the RFeB system sintered magnet obtained by sintering such raw material alloy powder, forming intergranular grain boundaries having a large grain-boundary width. In contrast, the raw material alloy powder obtained by pulverizing the lump of HDDR-treated raw material alloy according to the Comparative Examples does not allow the rare-earth rich phase to be uniformly diffused among the grains, which also prevents the RFeB system sintered magnet obtained by sintering such raw material alloy powder from having the rare-earth rich phase uniformly diffused among the crystal grains. This is the likely reason why intergranular grain boundaries having a large grain-boundary width cannot be formed.

[0056] The electron micrograph of FIG. 3B, which shows the lump of HDDR-treated raw material alloy according to Example 2, shows almost no white portion. This is due to the fact that the lump of HDDR-treated raw material alloy (and the same alloy lump in the previous stage) used in Example 2 had a TRE value close to that of the main phase, which means that the HDDR-treated raw material alloy includes almost no rare-earth rich phase. Performing the rare earth grain boundary penetration process on such a lump of HDDR-treated raw material alloy that contains little rare-earth rich phase results in a lump of raw material alloy that contains a high amount of rare earth, with the rare-earth rich phase diffused through the grain boundaries of the crystal grains, as shown in FIG. 3A.