Extrusion die for hot-worked magnet and method for manufacturing hot-worked magnet using the same

Abstract

To provide an extrusion die for a hot-working magnet capable of preventing cracking of the hot-working magnet and a manufacturing method for hot-working magnets using the same.SOLUTION: The method for manufacturing a hot-working magnet includes a hot working step (step S3). Since the cross-sectional area of a plastic working portion 12 of an extrusion die 10 is gradually decreasing from a starting portion 12a to a terminal portion 12b, the pressurization against a compact is not loosened during the hot-working step, thereby effectively preventing the generation of cracks.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an extrusion die for hot-worked magnets and a method for manufacturing hot-worked magnets using the same.

Background Art

[0002] Conventionally, as a type of permanent magnet, an R-T-B-based permanent magnet having excellent magnetic properties is known and widely used. The R-T-B-based permanent magnet is roughly classified into two types, one is a sintered magnet manufactured by a powder metallurgy method, and the other is a hot-worked magnet manufactured by a hot plastic working method.

[0003] Manufacturing methods for hot-worked magnets include a die upsetting method, a broaching forging method, a backward extrusion method, a forward extrusion method, etc. Among these, the forward extrusion method is suitable for manufacturing hot-worked magnets used in high-efficiency motors such as IPM. The properties of hot-worked magnets are greatly affected by plastic deformation during hot working, and in the forward extrusion method, they can be greatly affected by the shape of the extrusion die that bears the plastic deformation.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the manufacturing method of hot-worked magnets according to the prior art, no consideration has been given to the cracks that occur in the manufactured hot-worked magnets, and the cracks could not be sufficiently suppressed. When cracks occur in a hot-worked magnet, the residual magnetic flux density Br may decrease as the main phase volume fraction decreases. In addition, a local demagnetizing field increases starting from the crack, making it easier for reversal nuclei to occur. As a result, the coercive force H cJ may decrease. Therefore, the inventors have conducted extensive research on the cracks that occur in hot-worked magnets and have newly found a technique capable of suppressing the cracks.

[0006] One aspect of the present invention aims to provide an extrusion die for hot-worked magnets capable of suppressing cracks in hot-worked magnets and a method for manufacturing hot-worked magnets using the same.

Means for Solving the Problems

[0007] An extrusion die for hot-worked magnets according to one embodiment of the present invention is an extrusion die for hot-worked magnets having a starting end face and a terminating end face facing each other and including a plastic working portion extending from the starting end face to the terminating end face, wherein the cross-sectional area of the plastic working portion in a cross-section orthogonal to the facing direction of the starting end face and the terminating end face gradually decreases from the starting end portion at the starting end face toward the terminating end portion at the terminating end face.

[0008] In the above extrusion die for hot-worked magnets, since the cross-sectional area of the plastic working portion gradually decreases from the starting end portion toward the terminating end portion, when used in the manufacture of hot-worked magnets, the pressure applied to the formed body during hot working gradually increases. That is, the pressure applied to the formed body does not loosen during hot working, and the occurrence of cracks caused by the loosening of the pressure is effectively suppressed.

[0009] The extrusion die for hot-worked magnets according to another embodiment has a ratio of the area of the terminating end portion to the area of the starting end portion of the plastic working portion of 60 to 90%. When used in the manufacture of hot-worked magnets, a hot-worked magnet having high magnetic properties can be obtained. In addition, the occurrence of cracks is further suppressed, thereby obtaining a hot-worked magnet having high magnetic properties.

[0010] The extrusion die for hot-worked magnets according to another form has an end face shape in which the starting end portion of the plastic working portion extends in one direction, and also has an end face shape in which the ending end portion of the plastic working portion extends in one direction.

[0011] The extrusion die for hot-worked magnets according to another form is such that, when viewed from the facing direction between the starting end face and the ending end face of the extrusion die, the first direction in which the end face shape of the starting end portion of the plastic working portion extends and the second direction in which the end face shape of the ending end portion of the plastic working portion extends intersect. In this case, a large plastic deformation can be imparted to the molded body.

[0012] The extrusion die for hot-worked magnets according to another form is such that the end face shape of the starting end portion and the end face shape of the ending end portion of the plastic working portion are rectangular.

[0013] The extrusion die for hot-worked magnets according to another form is such that, in the plastic working portion, from the end face shape of the starting end portion to the end face shape of the ending end portion, the lengths of the respective sides of the rectangle of the end face shape change exponentially. In this case, from the starting end portion at the starting end face of the plastic working portion toward the ending end portion at the ending end face, the cross-sectional area of the plastic working portion can be linearly reduced. Therefore, the pressure applied to the molded body increases at a constant rate from the starting end portion to the ending end portion of the plastic working portion, and the occurrence of cracks can be further suppressed.

[0014] The extrusion die for hot-worked magnets according to another form is such that the end face shape of the ending end portion of the plastic working portion is partially annular.

[0015] The method for manufacturing a hot-worked magnet according to one form of the present invention is a method for manufacturing a hot-worked magnet using the above-described extrusion die for hot-worked magnets, and includes a hot-working step of hot-working a molded body obtained by molding magnetic powder with the above-described extrusion die to obtain a hot-worked magnet.

[0016] In the method for manufacturing the hot-worked magnet, during the hot-working in the hot-working step, the pressure applied to the molded body does not relax, and the occurrence of cracks due to the relaxation of the pressure is effectively suppressed.

Advantages of the Invention

[0017] According to one aspect of the present invention, there is provided an extrusion die for hot-worked magnets that can suppress cracks in hot-worked magnets, and a method for manufacturing hot-worked magnets using the same.

Brief Description of the Drawings

[0018]

Figure 1

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Modes for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description, the same reference numerals are used for the same elements or elements having the same function, and duplicate descriptions are omitted. (First Embodiment)

[0020] The extrusion die 10 for hot-worked magnets according to the first embodiment will be described with reference to FIGS. 1 to 3. The extrusion die 10 has a start end face 10a and an end face 10b facing each other. In this embodiment, the extrusion die 10 has a cylindrical outer shape, and both the start end face 10a and the end face 10b are circular. In this embodiment, the start end face 10a and the end face 10b are parallel to each other. The extrusion die 10 is made of a high heat-resistant material (for example, nickel-based superalloy (for example, Inconel (registered trademark)), molybdenum, etc.).

[0021] The extrusion die 10 includes a plastic working portion 12 extending from the start end face 10a to the end face 10b. The plastic working portion 12 has a start end portion 12a at the start end face 10a and an end end portion 12b at the end face 10b.

[0022] The start end portion 12a of the plastic working portion 12 has an end face shape extending in one direction when viewed from the facing direction of the start end face 10a and the end face 10b. The end face shape of the start end portion 12a in this embodiment is rectangular.

[0023] Hereinafter, for convenience of explanation, the facing direction of the start end face 10a and the end face 10b is defined as the Z direction, the direction in which the end face shape of the start end portion 12a of the plastic working portion 12 extends is defined as the X direction, and the direction orthogonal to the Z direction and the X direction is defined as the Y direction.

[0024] In the extrusion die 10, the cross-sectional area in the X-Y cross-section of the plastic working portion 12 gradually decreases substantially linearly from the start end portion 12a toward the end end portion 12b.

[0025] The end portion 12b of the plastic working part 12 has an end face shape extending in one direction when viewed from the facing direction of the start end face 10a and the end face 10b. The end face shape of the end portion 12b in the present embodiment is rectangular. While the end face shape of the start end portion 12a extends in the X direction (that is, the long side is along the X axis), the end face shape of the end portion 12b extends in the Y direction (that is, the long side is along the Y axis). When viewed from the facing direction of the start end face 10a and the end face 10b, the X direction (the first direction) in which the end face shape of the start end portion 12a extends and the Y direction (the second direction) in which the end face shape of the end portion 12b extends intersect, and more specifically, they are orthogonal. The plastic working part 12 can also be expressed as having the long side (or major axis) and the short side (or minor axis) swapped between the rectangular end face of the start end portion 12a and the rectangular end face of the end portion 12b. The end face of the start end portion 12a and the end face of the end portion 12b are in a torsional positional relationship.

[0026] As shown in FIG. 1, the extrusion die 10 uses a punch 20 having the same dimensions (or slightly shorter) as the end face shape of the start end portion 12a of the plastic working part 12 to forwardly extrude the above-described molded body disposed on the start end face 10a toward the end face 10b in the Z direction. Thereby, a strip-shaped hot working magnet having the same cross-sectional shape as the end face shape of the end portion 12b of the plastic working part 12 is obtained. The strip-shaped hot working magnet is appropriately cut to a predetermined width.

[0027] Inside the extrusion die 10, the contour of the plastic working part 12 is formed by curves as shown in FIGS. 2 and 3.

[0028] In the Y-Z cross-section (the cross-section taken along line I-I in FIG. 2) shown in FIG. 3(a), the width of the start end portion 12a of the plastic working part 12 (that is, the short side length of the rectangular end face) gradually expands exponentially toward the end portion 12b and coincides with the width of the end portion 12b (that is, the long side length of the rectangular end face) at the end face 10b. That is, the contour lines 14A and 14B of the plastic working part 12 in the Y-Z cross-section are both curves that can be expressed by exponential functions.

[0029] In the X-Z cross-section shown in Fig. 3(b) (the cross-section taken along line II-II in Fig. 2), the width of the starting end 12a of the plastic working portion 12 (i.e., the length of the long side of the rectangular end face) gradually narrows exponentially as it approaches the terminating end 12b, and coincides with the width of the terminating end 12b (i.e., the length of the short side of the rectangular end face) at the end face 10b. That is, both the contour lines 16A and 16B of the plastic working portion 12 in the X-Z cross-section are curves that can be expressed by exponential functions.

[0030] In a hot-worked magnet, when cracks occur inside, the magnetization at that part decreases, and the magnetization per unit volume decreases. As a result, the residual magnetic flux density decreases. By using the above-described extrusion die 10 in the manufacture of a hot-worked magnet, cracks in the hot-worked magnet can be suppressed, so that a decrease in the residual magnetic flux density can be suppressed.

[0031] Also, in the part where cracks have occurred in the hot-worked magnet, a demagnetizing field is generated, similar to the magnet surface, and it becomes the starting point of magnetization reversal. As the number of cracks in the hot-worked magnet increases, the starting points of magnetization reversal increase, so that the coercive force of the hot-worked magnet decreases. According to the above-described method for manufacturing a hot-worked magnet, cracks that become the starting points of magnetization reversal can be suppressed, so that a decrease in the coercive force can be suppressed.

[0032] Note that by setting the ratio (area reduction rate) of the area of the terminating end 12b to the area of the starting end 12a of the plastic working portion 12 to 60 - 90% (86.8% as an example), a hot-worked magnet having high magnetic properties (for example, coercive force) can be obtained. In addition, the generation of cracks is further suppressed, thereby obtaining a hot-worked magnet having high magnetic properties (for example, residual magnetic flux density Br, coercive force H cJ ).

[0033] Further, the end face of the starting end portion 12a and the end face of the ending end portion 12b of the plastic working portion 12 may have a parallel positional relationship (for example, both extending in the X direction) instead of a twisted positional relationship. When the end face of the starting end portion 12a and the end face of the ending end portion 12b of the plastic working portion 12 are in a twisted positional relationship, a relatively large plastic deformation can be caused when the molded body passes through the plastic working portion 12, and a hot-worked magnet having high magnetic properties (for example, coercive force) can be obtained.

[0034] Furthermore, when the lengths of the respective sides of the rectangle of the end face shape change exponentially from the end face shape of the starting end portion 12a to the end face shape of the ending end portion 12b of the plastic working portion 12, the cross-sectional area of the plastic working portion 12 can be linearly reduced from the starting end portion 12a at the starting end face 10a toward the ending end portion 12b at the ending end face 10b. Therefore, the pressure applied to the molded body increases at a constant rate from the starting end portion 12a to the ending end portion 12b of the plastic working portion 12, and the generation of cracks can be further suppressed. (Second Embodiment)

[0035] The extrusion die 10A for the hot-worked magnet according to the second embodiment will be described with reference to FIGS. 4 to 7. The shape of the plastic working portion 12A of the extrusion die 10A is different from that of the extrusion die 10 according to the first embodiment, and the rest is the same as or similar to the extrusion die 10.

[0036] The starting end portion 12a of the plastic working portion 12A has an end face shape extending in one direction when viewed from the facing direction of the starting end face 10a and the ending end face 10b of the extrusion die 10A, and more specifically, has a rectangular end face shape. As shown in FIG. 5(a), the starting end portion 12a of the plastic working portion 12A has one short side length of L1, a long side length of L2, and the other short side length of L3. In the starting end portion 12a of the plastic working portion 12A, the short side lengths L1 and L3 are the same length, and as an example, it is 1.0 mm. In the starting end portion 12a of the plastic working portion 12A, the long side length L2 is 2.0 mm as an example.

[0037] The terminal end portion 12b of the plastic working portion 12A has a partial annular end face shape when viewed from the facing direction between the start end face 10a and the end end face 10b of the extrusion die 10A. The partial annular shape of the end face shape of the terminal end portion 12b is, more specifically, a semi-annular shape with an opening angle θ of the inner arc being 180 degrees. As shown in Fig. 5(b), the outer arc length of the terminal end portion 12b of the plastic working portion 12A is L1, the end side length is L2, and the inner arc length is L3. The curvature radius R1 of the inner arc is, for example, 0.65 mm, and in this case, the inner arc length L3 (= R1 × π) is approximately 2.0 mm. The curvature radius R2 of the outer arc is, for example, 1.15 mm, and in this case, the outer arc length L1 (= R2 × π) is approximately 3.6 mm. The end side length L2 (= R2 - R1) is, for example, 0.5 mm.

[0038] In the plastic working portion 12A, the shape and dimensions of the contour gradually change between the rectangular end face of the start end portion 12a and the semi-annular end face of the terminal end portion 12b. More specifically, one short side (length L1) of the start end portion 12a gradually changes to the outer arc of the terminal end portion 12b, the pair of long sides of the start end portion 12a gradually change to the pair of end sides of the terminal end portion 12b respectively, and the other short side (length L3) of the start end portion 12a gradually changes to the inner arc of the terminal end portion 12b.

[0039] Fig. 6 is a graph showing the change in the contour dimensions L1, L2, and L3 of the plastic working portion 12A, with the vertical axis indicating the contour dimension (mm) and the horizontal axis indicating the distance (depth) from the start end face 10a. As shown in the graph of Fig. 6, the contour dimensions L1 and L3 increase monotonically from the start end portion 12a to the terminal end portion 12b, and the contour dimension L2 decreases monotonically from the start end portion 12a to the terminal end portion 12b.

[0040] Fig. 7 is a graph showing the change in the cross-sectional area of the plastic working portion 12A, with the vertical axis indicating the area ratio when the area of the start end portion 12a is taken as 100% and the horizontal axis indicating the distance (depth) from the start end face 10a. As shown in the graph of Fig. 7, the cross-sectional area of the plastic working portion 12A gradually decreases from the start end portion 12a towards the terminal end portion 12b.

[0041] Thus, similar to the extrusion die 10 described above, in the extrusion die 10A, since the cross-sectional area of the plastic working part 12A gradually decreases from the start end 12a to the end end 12b, the pressure applied to the molded body during the hot working process does not loosen halfway, and thereby the generation of cracks can be effectively suppressed.

[0042] In addition, in the above-described extrusion die 10A, the end face shape of the end end 12b of the plastic working part 12A was a partial annular shape (i.e., a semi-annular shape) with the opening angle θ of the inner arc being 180 degrees, but it may also be a partial annular shape with the opening angle θ being narrower than 180 degrees. The opening angle θ may be 120 degrees or less, and may also be 90 degrees or less.

[0043] As described above, the embodiments of the present invention have been explained, but the present invention is not necessarily limited to the above-described embodiments, and various changes are possible without departing from the gist thereof.

[0044] For example, the end face shapes of the start end and the end end of the plastic working part are not limited to rectangular shapes, and may be elliptical shapes extending in one direction, or may be circular shapes, U-shaped, or V-shaped. (Method for manufacturing a hot working magnet)

[0045] A method for manufacturing a hot working magnet using the above-described extrusion dies 10 and 10A will be described along the flowchart shown in FIG. 8. Hereinafter, a method for manufacturing a neodymium magnet (neodymium iron boron-based magnet) having an R2T 14 B crystal as the main phase will be described.

[0046] In the R-T-B series permanent magnet, R represents a rare earth element. The permanent magnet contains at least neodymium (Nd) as a rare earth element. The permanent magnet may contain other rare earth elements in addition to Nd. The other rare earth elements may be at least one selected from the group consisting of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). In the R-T-B series permanent magnet, T represents a transition metal element. The permanent magnet contains at least iron (Fe) as a transition metal element. The permanent magnet may contain only Fe as a transition metal element. The permanent magnet may contain both Fe and cobalt (Co) as transition metal elements. In the R-T-B series permanent magnet, B is boron.

[0047] When manufacturing a hot-worked magnet, first, the magnet material as a raw material is crushed into magnetic powder (step S1). The crushing can be performed, for example, by a cutter mill or a propeller mill, and can be performed, for example, in an argon gas atmosphere (or a nitrogen gas atmosphere). The particle size of the magnetic powder obtained by crushing is, for example, about 100 to 300 μm. The magnetic powder is not finely crushed down to the dimensional level of neodymium magnet crystals (1 μm or less, for example, several tens to several hundreds of nm), and has a polycrystalline structure composed of a plurality of neodymium magnet crystals.

[0048] The magnetic powder obtained in step S1 is formed by a compression molding machine to obtain a molded body (step S2). The molding is performed in a nitrogen gas atmosphere (or an argon gas atmosphere), at a high temperature of 800 °C or lower (as an example, 750 °C), at a press pressure of 200 MPa or lower, for several tens of seconds. By molding, a dense molded body is obtained. However, in the state of this molded body, the magnet particles are randomly oriented, and the magnetization easy axis directions are not aligned.

[0049] The formed body obtained in step S2 is hot-worked by a forward extrusion method to obtain a hot-worked magnet (step S3). The hot working is performed in a nitrogen gas atmosphere (or an argon gas atmosphere, air) at a high temperature of 800°C or lower (for example, 750°C) and a press pressure of 100 MPa or lower for several tens of seconds. The above-described extrusion dies 10 and 10A can be used for this hot working. (Example)

[0050] Here, an experiment conducted by the inventors regarding the cross-sectional areas of the plastic working parts 12 and 12A of the extrusion dies 10 and 10A will be described.

[0051] As samples 1 and 2, extrusion dies were prepared in the same manner as the above-described extrusion die 10, having a rectangular end face at the starting end of the plastic working part of 22 mm × 11 mm, a rectangular end face at the ending end with the long side and the end side swapped of 7 mm × 30 mm, and a thickness of 20 mm. In sample 1, as shown in FIG. 9, the contour dimensions (length in the X direction and length in the Y direction) of the plastic working part were changed exponentially, and in sample 2, as shown in FIG. 10, the contour dimensions of the plastic working part were changed linearly. In the graphs of FIGS. 9 and 10, the vertical axis indicates the contour dimension, and the horizontal axis indicates the distance (depth) from the starting end face.

[0052] In sample 1, as shown in FIG. 11, the cross-sectional area of the plastic working part gradually decreases almost linearly from the starting end to the ending end. On the other hand, in sample 2, as shown in FIG. 12, the cross-sectional area of the plastic working part first gradually increases from the starting end to the ending end, reaches the maximum cross-sectional area near the middle between the starting end and the ending end, and then gradually decreases. In the graphs of FIGS. 11 and 12, the vertical axis indicates the area ratio when the area of the starting end is taken as 100%, and the horizontal axis indicates the distance (depth) from the starting end face. In the graph of FIG. 11, the ratio of the area of the ending end to the area of the starting end of the plastic working part is 86.8%.

[0053] Then, hot working of the formed body was performed using samples 1 and 2 to obtain hot-worked magnets. As a result, no cracks were confirmed in the hot-worked magnet obtained using sample 1, but cracks were scattered in the hot-worked magnet obtained using sample 2.

[0054] As Sample 3, an extrusion die was prepared in the same manner as the above-described extrusion die 10A, where the rectangular end face at the starting end of the plastic working part was 20 mm × 10 mm, and the semi-annular end face at the terminating end had an inner diameter of 13 mm, a thickness of 5 mm, and an opening angle of the inner arc of 180 degrees. In Sample 3, as shown in FIG. 6, the contour dimensions of the plastic working part were changed. In the graph of FIG. 6, the vertical axis represents the contour dimensions, and the horizontal axis represents the distance (depth) from the starting end face.

[0055] In Sample 3, as shown in FIG. 7, the cross-sectional area of the plastic working part gradually decreases from the starting end to the terminating end. In the graph of FIG. 7, the vertical axis represents the area ratio when the area of the starting end is taken as 100%, and the horizontal axis represents the distance (depth) from the starting end face. In the graph of FIG. 7, the ratio of the area of the terminating end to the area of the starting end of the plastic working part is 70.6%.

[0056] Then, hot working of the molded body was performed using Sample 3 to obtain a hot-worked magnet. As a result, no cracks were confirmed in the hot-worked magnet obtained using Sample 3.

[0057] This is because when the cross-sectional area of the plastic working part gradually decreases without increasing even once from the starting end to the terminating end as in Samples 1 and 3, the pressure on the molded body during hot working gradually increases, so the pressure does not loosen during the process. However, when the cross-sectional area of the plastic working part increases even slightly as in Sample 2, the pressure loosens, and it is considered that cracks caused by the loosening of the pressure occurred.

Explanation of Reference Numerals

[0058] 10, 10A... Extrusion die, 10a... Starting end face, 10b... Terminating end face, 12, 12A... Plastic working part, 12a... Starting end part, 12b... Terminating end part.

Claims

Claim 1: An extrusion die for hot working magnets, comprising a starting end face and a terminating end face facing each other, and a plastic working portion extending from the starting end face to the terminating end face, wherein in a cross-section orthogonal to the facing direction of the starting end face and the terminating end face, the cross-sectional area of the plastic working portion gradually decreases from the starting end portion at the starting end face to the terminating end portion at the terminating end face; the starting end portion of the plastic working portion has an end face shape extending in one direction, and the terminating end portion of the plastic working portion also has an end face shape extending in one direction; when viewed from the facing direction of the starting end face and the terminating end face of the extrusion die, a first direction in which the end face shape of the starting end portion of the plastic working portion extends intersects a second direction in which the end face shape of the terminating end portion of the plastic working portion extends; the end face shape of the starting end portion and the end face shape of the terminating end portion of the plastic working portion are rectangles; from the end face shape of the starting end portion to the end face shape of the terminating end portion of the plastic working portion, the lengths of the respective sides of the rectangle of the end face shape change exponentially; the plastic working portion is an extrusion die for hot working magnets in which the length exponentially decreases from the long side of the rectangle in the end face shape of the starting end portion to the short side of the rectangle in the end face shape of the terminating end portion, and the length exponentially increases from the short side of the rectangle in the end face shape of the starting end portion to the long side of the rectangle in the end face shape of the terminating end portion. Claim 2 The extrusion die for hot working magnets according to claim 1, wherein the cross-sectional area of the plastic working portion gradually decreases linearly from the starting end portion to the terminating end portion. Claim 3 The extrusion die for hot working magnets according to claim 1 or 2, wherein the ratio of the area of the terminating end portion to the area of the starting end portion of the plastic working portion is 60 to 90%. Claim 4 A method for manufacturing a hot working magnet using the extrusion die for hot working magnets according to any one of claims 1 to 3, the method comprising a hot working step of obtaining a hot working magnet by hot working a formed body obtained by molding magnetic powder with the extrusion die. A method for manufacturing a hot working magnet, including a hot working step of obtaining a hot working magnet by hot working a formed body obtained by molding magnetic powder with the extrusion die.

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