WO2015097517A1

WO2015097517A1 - Forward extrusion forging apparatus and forward extrusion forging method

Info

Publication number: WO2015097517A1
Application number: PCT/IB2014/002794
Authority: WO
Inventors: Tetsuya Shoji; Daisuke Ichigozaki; Dai KOBUCHI; Masahiro Tanaka
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2013-12-26
Filing date: 2014-12-16
Publication date: 2015-07-02
Also published as: JP2015123463A

Abstract

A die (1) of a forward extrusion forging apparatus (10) includes a first region (2) where a punch (6) slides, a bent portion (3), and a second region (4). The punch (6) slides in the first region (2) to extrude a workpiece into the second region (4), thereby forging the workpiece. When two sides forming the rectangular cross-section of the first region (2) are respectively a first side (2a) having a length t1, and a second side (2b) having a length t2, and two sides forming the rectangular cross-section of the second region (4) are respectively a third side (4a) having a length t3 and corresponding to the first side (2a), and a fourth side (4b) having a length t4 and corresponding to the second side (2b), at least one of a relationship of t3>t1 and a relationship of t4>t2 is satisfied.

Description

FORWARD EXTRUSION FORGING APPARATUS AND FORWARD EXTRUSION

FORGING METHOD

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a forward extrusion forging apparatus and a forward extrusion forging method.

[0002] In hot forging, there are roughly two processing forms. One of the two processing methods is a method of manufacturing a compact using so-called rearward extrusion (a compact is manufactured while extruding a target material in a direction opposite an extruding direction of a punch), the method including: placing a target material into a die; and extruding a part of the target material into a hollow portion of an extruding punch while pressing the target material with use of the extruding punch to reduce the thickness thereof. The Other one of the two processing methods is a method of manufacturing a compact using so-called forward extrusion (a compact is manufactured while extruding a target material in an extruding direction of a punch), the method including: placing a target material into a die having a hollow portion; and extruding a part of the target material from the hollow portion of the die while pressing the target material with use of a punch that does not have a hollow portion, to reduce the thickness thereof.

[0003] Rare earth magnets made using rare earth elements such as lanthanoid are called permanent magnets and are used for driving motors of hybrid vehicles, electric vehicles, and the like as well as motors included in hard disks and MRIs.

[0004] As an index indicating magnet performance of these rare earth magnets, for example, residual magnetization (residual magnetic flux density) and coercive force may be used. Along with a decrease in the size of a motor and an increase in current density, the amount of heat generation increases, and thus the demand that rare earth magnets to be used should have high heat resistance has further increased. Accordingly, one of the important research issues in this technical field is how to hold magnetic properties of a magnet when being used at a high temperature.

[0005] Examples of the rare earth magnets include commonly-used sintered magnets in which a grain size of crystal grains (main phase) constituting a structure thereof is approximately 3 μιη to 5 μτη; and nanocrystalline magnets in which crystal grains are refined into nano grains with size of approximately 50 nm to 300 nm. Among these, currently, nanocrystalline magnets have attracted attention because they make it possible to reduce the addition amount of expensive heavy rare earth elements (or to eliminate the necessity of adding heavy rare earth elements), and to realize the refinement of crystal grains.

[0006] An example of a method of manufacturing a rare earth magnet will be briefly described. For example, a method of manufacturing a rare earth magnet (oriented magnet) is commonly used, this method including: rapidly solidifying Nd-Fe-B molten metal to produce fine powder; press-forming the fine powder into a rare earth magnet precursor (sintered compact); and performing hot working on this sintered compact so as to impart magnetic anisotropy thereto.

[0007] In this, hot working, the above-described forward extrusion or rearward extrusion may be used. More specifically, in the rearward extrusion, a sintered compact is placed in a die and is extruded and pressed by an extruding punch having a hollow portion, and a rare earth magnet is manufactured while the sintered compact is extruded in a direction opposite an extruding direction. On the other hand, in the forward extrusion, a sintered compact is placed in a die and is pressed by a punch that does not have a hollow portion, and a part of the sintered compact is extruded from the hollow portion of the die, thereby manufacturing a rare earth magnet. No matter which extrusion method is used, in the rare earth magnet which is produced by being pressed by the punch, anisotropy is generated in a direction perpendicular to a pressing direction of the punch.

[0008] There are a plurality of kinds of extrusion methods. Hereinafter, a forward extrusion method of related art will be described.

[0009] In a forward extrusion apparatus of related art, a die which is a constituent element of the forward extrusion apparatus has a hollow portion extending in one extruding direction, and a sintered compact placed into this hollow portion is extruded in one direction by a punch so as to be forged. A modification example of this forward extrusion is an extrusion method which is generally called an E-CAP method. In the method, an extruding direction is changed at a right angle at an intermediate position, and a workpiece is extruded in a hollow portion where a cross-sectional dimension does not change, so that shearing deformation is imparted to the workpiece.

[0010] Japanese Patent Application Publication No. 2008-91867 (JP 2008-91867 A) describes a forward extrusion apparatus in which a sintered compact placed into a hollow portion of a die is extruded in one direction by a punch so as to be forged. However, the hollow portion described in JP 2008-91867 A has a cross-sectional shape in which a cross-sectional width (cross-sectional width in a Y direction) is enlarged, and the other cross-sectional width (cross-sectional width in an X direction) perpendicular to the Y direction is reduced to be curved and narrowed. That is, the cross-sectional dimensions of the hollow portion of the apparatus change.

[0011] Thus, the cross-section of the opening of the die of the forward extrusion apparatus described in JP 2008-91867 A has a complicated shape in which a part of the cross-section of the hollow portion is reduced in width to be curved and narrowed. Therefore, it is difficult to manufacture the apparatus, and it is apparent that the manufacturing cost of the apparatus is high. In addition, when a sintered compact changes to a rare earth magnet due to forward extrusion, and the thickness of the initial sintered compact changes, the change in the thickness of the sintered compact is three-dimensionally complicated due to the three-dimensionally complicated shape of the cross-section of the hollow portion. Accordingly, it is extremely difficult to design a change in thickness, a processing rate, and the like when the sintered compact changes to the rare earth magnet.

SUMMARY OF THE INVENTION

[0012] The invention provides a forward extrusion forging apparatus in which a hollow portion provided in a die has simple cross-sectional shapes such that an increase in the manufacturing cost is suppressed and a rare earth magnet having superior magnetic characteristics can be manufactured, and a forward extrusion forging method in which the forward extrusion forging apparatus is used.

[0013] A first aspect of the invention relates to a forward extrusion forging apparatus including: a die that is bent and has a hollow portion; and a punch that slides in the die, wherein the die includes a first region where the punch slides, a second region that is positioned ahead of the first region in an extruding direction, and a bent portion that connects the first region to the second region; a workpiece is placed into the first region, and the punch slides in the first region to extrude the workpiece into the second region, thereby forging the workpiece; a shape of a cross-section of each of the first region and the second region perpendicular to the extruding direction is rectangular; and when two sides that form the rectangular cross-section of the first region and are perpendicular to each other are respectively a first side having a length tl , and a second side having a length t2, and two sides that form the rectangular cross-section of the second region and are perpendicular to each other are respectively a third side having a length t3 and corresponding to the first side, and a fourth side having a length t4 and corresponding to the second side, at least one of a relationship of t3>tl and a relationship of t4>t2 is satisfied.

[0014] The forward extrusion forging apparatus according to the above-described aspect of the invention has two major characteristics. With regard to the first characteristic, unlike an apparatus of related art, the die which is a constituent element is bent and the die has the first region where the punch slides, the second region that is positioned ahead of the first region in the extruding direction, and the bent portion that connects the first region to the second region. With regard to the second characteristic, the cross-sectional shapes of the first region and the second region of the die are simple rectangular shapes, and when the two sides that form the rectangular cross-section of the first region and are perpendicular to each other are respectively the first side having the length tl, and the second side having the length t2, and the two sides that form the rectangular cross-section of the second region and are perpendicular to each other are respectively the third side having the length t3 and corresponding to the first side, and the fourth side having the length t4 and corresponding to the second side, at least one of the relationship of t3>tl and the relationship of t4>t2 is satisfied. With the forging apparatus which includes the die having the above two characteristic configurations, the cross-sectional shapes of the opening portion provided in the die are simple rectangular shapes, and thus an increase in the cost of manufacturing the apparatus is suppressed. Further, when the workpiece placed into the die is a sintered compact that is a rare earth magnet precursor, a shearing force is applied to the sintered compact while passing through the bent portion of the opening portion, and thus a shearing strain is introduced thereinto. In the second region of the die which is positioned ahead of the bent portion in the extruding direction, at least a part of the second region is wider than the first region, and thus crystals of the extruded sintered compact have a high orientation degree and are aligned. Accordingly, a rare earth magnet having superior magnetic properties, in particular, high residual magnetization can be manufactured.

[0015] In the specification, the meaning of the phrase "the cross-sections are rectangular" includes not only a case where the cross-sections of the first region and the ·> second region perpendicular to the extruding direction are rectangular, but also a case where the cross-sections thereof are square.

[0016] In addition, the meaning of the phrase "at least one of the relationship of t3>tl and the relationship of t4>t2 is satisfied" includes a configuration where the relationship of t3>tl and the relationship of t4>t2 are satisfied, a configuration where the relationship of t3>tl and the relationship of t4 = t2 are satisfied, and a configuration where the relationship of t3>tl and the relationship of t4<t2 are satisfied. That is, when the size relationship between the lengths of the third and first sides corresponding to each other satisfies t3>tl , the size relationship between the lengths of the fourth and second sides corresponding to each other may satisfy, conversely, t4<t2.

[0017] For example, when a side of the second region of the die which extends in the horizontal direction is the third side, and a side which is perpendicular to the third side and extends in the vertical direction is the fourth side, the length t3 of the third side is longer than the length tl of the first side of the first region corresponding to the third side (t3>tl). In addition, when the length t2 of the second side and the length t4 of the fourth side are the same (t4 = t2), in the sintered compact which is forward-extruded from the first region into the second region, the length (that is, thickness) in the height direction does not change, and only the length in the width direction changes from the length tl to the length t3. Therefore, the crystals having a flat shape constituting the sintered compact, which are shifted to the second region and to which a shearing force is applied and thus a shearing strain is introduced at the bent portion, are aligned in the horizontal direction in the cross-section of the second region. Accordingly, the crystals are oriented in the vertical direction. In this configuration, when a rare earth magnet is manufactured from a sintered compact by forward extrusion, only the width changes without a change in thickness between the sintered compact and the rare earth magnet, and the crystals are oriented and aligned. Thus, the configuration is preferable because the number of parameters required to design the product can be reduced and it is easy to design the product.

[0018] In addition, with regard to "the bent state" of the die, the angle of the extruding direction in the second region with respect to the extruding direction in the first region is larger than 0° and less than 180°. However, the angle is preferably 90° or less and more preferably 10° to 30° (for example, approximately 20°).

[0019] When the punch slides in the first region to extrude the workpiece placed in the first region, this extrusion may be performed using an actuator such as a motor or a cylinder mechanism or may be manually performed.

[0020] In the above-described aspect, the length of the side of the first region, which is shorter than the corresponding side of the second region, may gradually increase from an intermediate position of the first region to the bent portion.

[0021] When the workpiece is a sintered compact, the following can be assumed. For example, when the length sharply increases from the length tl to the length t3 in a region extending from the first side of the first region to the third side of the second region through the bent portion, it is practically difficult to orient, in a desirable manner, the crystals of the sintered compact to which a shearing force is applied at the bent portion.

[0022] Therefore, for example, when it is assumed that there is no change (difference) between the lengths t2, t4 of the second side and the fourth side and there is a change (difference) in length only between the first side and the third side, the die is designed such that the length of the first side gradually increases from the intermediate position of the first region to the bent portion and is the same as the length t3 of the third side at the bent portion. As a result, when the sintered compact is shifted from the first region to the second region, the width of the sintered compact gradually increases in a direction along the first side, from the intermediate position of the first region, a shearing force is applied and thus a shearing strain is introduced into the sintered compact at the bent portion. Thus, the sintered compact, which already has a length equal to the length t3 of the third side, is shifted to the second region. As a result, the crystals are smoothly oriented.

[0023] For example, in a case where the relationship of t3>tl and the relationship of t4>t2 are satisfied, in the first region, the lengths of the first side and the second side gradually increase toward the bent portion. In addition, for example, in a case where the relationship of t3>tl and the relationship of t4<t2 are satisfied, in the first region, the length of the first side gradually increases toward the bent portion, and the length of the second side gradually decreases toward the bent portion.

[0024] A second aspect of the invention relates to a forward extrusion forging method. The method includes: preparing a forward extrusion forging apparatus, wherein the forward extrusion forging apparatus includes a die that is bent and has a hollow portion, and a punch that slides in the die; the die includes a first region where the punch slides, a second region that is positioned ahead of the first region in a extruding direction, and a bent portion that Connects the first region to the second region; a workpiece is placed into the first region, and the punch slides in the first region to extrude the workpiece into the second region, thereby forging the workpiece; a shape of a cross-section of each of the first region and the second region perpendicular to the extruding direction is rectangular; and when two sides that form the rectangular cross-section of the first region and are perpendicular to each other are respectively a first side having a length tl , and a second side having a length t2, and two sides that form the rectangular cross-section of the second region and are perpendicular to each other are respectively a third side having a length t3 and corresponding to the first side, and a fourth side having a length t4 and corresponding to the second side, at least one of a relationship of t3>tl and a relationship of t4>t2 is satisfied; and manufacturing a forged product by placing the workpiece into the die and sliding the punch to forge the workpiece.

[0025] In the forward extrusion forging method according to the second aspect of the invention, the above-described forward extrusion forging apparatus is used. The forward extrusion forging apparatus includes the die including the bent portion, wherein the cross-sectional shapes thereof behind and ahead of the bent portion are rectangular, and at least one of the relationship of t3>tl and the relationship of t4>t2 is satisfied. Thus, when the workpiece is a sintered compact that is a rare earth magnet precursor, it is possible to manufacture a rare earth magnet having superior magnetic properties due to a high orientation degree, by performing extrusion forging using the forward extrusion forging apparatus.

[0026] The rare earth magnet manufactured by the forging method according to the second aspect of the invention includes a nanocrystalline magnet in which a grain size of a main phase (crystals) constituting a structure thereof is approximately 200 nm or less; a magnet having a grain size of 300 nm or more; a sintered magnet having a grain size of 1 μιη or more; and a bonded magnet in which crystal grains are bonded through a binder resin. In this case, it is preferable that the size of the main phase of magnetic power before hot working be adjusted such that an average maximum size (average maximum grain size) of the main phase of a rare earth magnet which is manufactured as a final product is approximately 300 nm to 400 nm or less.

[0027] An example of a method of manufacturing a rare earth magnet will be described. A quenched thin band .(quenched ribbon) which includes fine crystal grains is prepared by liquid quenching. Magnetic powder for a rare earth magnet is prepared by, for example, crushing the quenched ribbon. This magnetic powder is filled into, for example, a die and is sintered while being pressed by a punch so as to be bulked. As a result, an isotropic sintered compact is produced.

[0028] For example, this sintered compact has a metallographic structure that includes a RE-Fe-B-based main phase (RE: at least one of Nd and Pr, more specifically, one element or two or more elements selected from among Nd, Pr, and Nd-Pr) with a nanocrystalline structure and a grain boundary phase of an RE-X alloy (X: metal element) present around the main phase.

[0029] A rare earth magnet that is an oriented magnet is manufactured by placing the above-described sintered compact into the apparatus and performing hot extrusion forging (hot working) in which magnetic anisotropy is imparted to the sintered compact.

[0030] As can be understood from the above description, with the forward extrusion forging apparatus and the forward extrusion forging method according to the above-described aspects of the invention, a sintered compact that is a rare earth magnet precursor is placed into the die as the workpiece, and a shearing force is applied to the sintered compact at the bent portion of the die, and crystals constituting the extruded sintered compact have a high orientation degree and are aligned in the second region of the die which is positioned ahead of the bent portion in the extruding direction. Accordingly, a rare earth magnet having high residual magnetization can be manufactured. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 A is a longitudinal cross-sectional view illustrating a forward extrusion forging apparatus according to Embodiment 1 of the invention and FIG. IB is a cross-sectional view taken along an arrow IB-IB of FIG. 1 A;

FIG. 2A is a longitudinal cross-sectional view illustrating a state where a sintered compact as a workpiece is extruded forward so as to be forged using the forward extrusion forging apparatus according to Embodiment 1 and FIG. 2B is a cross-sectional view taken along an arrow IIB-IIB of FIG. 2 A;

FIG. 3 A is a longitudinal cross-sectional view illustrating a forward extrusion forging apparatus according to Embodiment 2 and FIG. 3B is a longitudinal cross-sectional view illustrating a forward extrusion forging apparatus according to Embodiment 3;

FIG. 4A is a diagram illustrating a microstructure of a sintered compact and FIG. 4B is a diagram illustrating a microstructure of a rare earth magnet; and

FIG. 5 is a graph illustrating the results of CAE analysis for determining a dimensional change at a time when a workpiece is shifted from a first region to a second region.

DETAILED DESCRIPTION OF EMBODIMENTS

[0032] Hereinafter, a forward extrusion forging apparatus and a forward extrusion forging method according to embodiments of the invention will be described with reference to the drawings.

[0033] (Forward Extrusion Forging Apparatus according to Embodiment) FIGS.

1A and IB are longitudinal cross-sectional views illustrating a forward extrusion forging apparatus according to Embodiment 1 of the invention. A forging apparatus 10 illustrated in the drawings includes a die 1 having a hollow portion 5, a punch 6 that is slidable (in a XI direction) in a first region 2 in the hollow portion 5 of the die 1, and a heating device (not illustrated) that heats an actuator (not illustrated) that slides the punch 6 and the die 1 during forging.

[0034] The die 1 includes a first region 2 that has a longitudinal direction LI (workpiece extruding direction in which a workpiece is extruded) extending in a vertical direction; a second region 4 that is provided at an angle Θ1 (90°) with respect to the first region 2 and has a longitudinal direction L2 (workpiece extruding direction) extending in a horizontal direction; and a bent portion 3 that connects the first region 2 to the second region 4.

[0035] A workpiece (not illustrated) placed into the hollow portion 5 is extnided forward in the hollow portion 5 in the workpiece extruding direction by the punch 6 that is positioned behind the workpiece in the workpiece extruding direction and slides along an inner peripheral surface of the first region 2 in the hollow portion 5. Then, the workpiece is extruded into the second region 4 through the bent portion 3 so as to be forged.

[0036] The shape of the cross-section of each of the first region 2 and the second region 4 perpendicular to the longitudinal direction thereof is rectangular. In the longitudinal cross-section view of the die 1 seen from a side in FIG. 1A, one side (second side 2b) that forms the rectangular cross-section of the first region 2 has a length t2, and one side (fourth side 4b) that forms the rectangular cross-section of the second region 4 has a length t4. In the vertical cross-sectional view of the die 1 seen from the front in FIG. IB, the other side (first side 2a) that forms the rectangular cross-section of the first region 2 has a length tl,_.and the other side (third side 4a) that forms the rectangular cross-section of the second region 4 has a length t3. In the example illustrated in the drawings, a relationship of t3>tl and t4 = t2 is satisfied.

[0037] However, in the forging apparatus according to the invention, as long as at least one of a relationship of t3>tl and a relationship of t4>t2 is satisfied with regard to the third side 4a corresponding to the first side 2a and the fourth side 4b corresponding to the second side 2b, the effects of the forging apparatus according to the invention can be obtained. Accordingly, a configuration having a relationship of t3>tl and a relationship of t4>t2, a configuration having a relationship of t3>tl and a relationship of t4 = t2, and a configuration having a relationship of t3>tl and a relationship of t4<t2 may be employed.

[0038] In the configuration having a relationship of t3>tl and t4 = t2 as illustrated in the drawings, in the case where a workpiece to be extruded and forged is a sintered compact that is a rare earth magnet precursor, when the sintered compact is extruded from the first region to the second region through the bent portion by forward extrusion, there is no change (difference) between the widths of the sintered compact and a rare earth magnet (because t4 is equal to t2 (t4 = t2)), and there is only a change (difference) between the thicknesses thereof. Therefore, crystals are oriented in a direction. Accordingly, the number of parameters required to design the product can be reduced and therefore it is easy , to design the product. Thus the configuration having a relationship of t3>tl and t4 = t2 is preferable.

[0039] The die 1 illustrated in the drawings includes a gradually increasing portion 2" in which the length of the first side 2a of the first region 2 corresponding to the third side 4a of the second region 4 having a relatively long length gradually increases from an intermediate position 2' of the first region 2 to the bent portion 3 (the length of the first side 2a of the first region 2, which is shorter than the corresponding third side 4a of the second region 4, gradually increases from the intermediate position 2' of the first region 2 to the bent portion 3).

[0040] For example, when a workpiece W is a sintered compact as illustrated in FIGS. 2A and 2B, the following can be assumed. For example, when the length sharply increases from the length tl to the length t3 in a region extending from the first side 2a of the first region 2 to the third side 4a of the second region 4 through the bent portion 3, it is practically difficult to orient, in a desirable manner, crystals of the sintered compact to which a shearing force S is applied at the bent portion 3. Therefore, the die 1 is designed such that the length of the first side 2a gradually increases from the intermediate position 2' of the first region 2 to the bent portion 3 and is equal to the length t3 of the third side 4a at the bent portion 3. As a result, when the sintered compact is shifted from the first region 2 to the second region 4, the width of the sintered compact gradually increases in a direction along the first side 2a from the intermediate position of the first region 2, a shearing force is applied and thus a shearing strain is introduced into the sintered compact at the¹ bent portion 3. Thus, the sintered compact, which already has a length equal to the length t3 of the third side 4a, is shifted to the second region 4. As a result, crystals are smoothly oriented.

[0041] In FIG. 2B, each of crystals, which form a rare earth magnet extruded into the second region by forward extrusion forging, has an elongated flat shape, and the crystals are aligned such that longitudinal direction of the crystals extends in a direction along the third side 4a. Thus, the crystals are oriented in a direction along the fourth side 4b. - '

[0042] FIG 3 A is a longitudinal cross-sectional view illustrating a forward extrusion forging apparatus according to Embodiment 2 and FIG. 3B is a longitudinal cross-sectional view illustrating a forward extrusion forging apparatus according to Embodiment 3. A forging apparatus 10A illustrated in FIG. 3 A includes a die 1A in which the first region 2 and a second region 4 A are connected by the bent portion 3, and an angle Θ2 of the second region 4A with respect to the first region 2 is less than 90°, A forging apparatus 10B illustrated in FIG. 3B includes a die IB in which the first region 2 and a second region 4B are connected by the bent portion 3, and an angle 03 of the second region 4B with respect to the first region 2 is more than 90° and less than 180°.

[0043] Thus, in the forward extrusion forging apparatus according to the invention, the die which is a constituent element of the forward extrusion forging apparatus is bent at the intermediate position at an angle more than 0° and less than 180°. As a result, when a workpiece passes through the bent portion, a shearing force is applied and thus a shearing strain can be introduced into the workpiece. /Further, at least one of two sides of the second region corresponding to two sides that form the rectangular cross-section of the first region is configured to be long. As a result, when a workpiece is a sintered compact that is a rare earth magnet precursor, the orientation (alignment) of crystals that form the sintered compact can be promoted.

[0044] When a workpiece is a sintered compact that is a rare earth magnet precursor, a microstructure of this sintered compact is illustrated in FIG. 4 A, and a microstructure of a rare earth magnet, which is obtained by performing extrusion forging (hot working) on the sintered compact using the forging apparatus 10 illustrated in the drawings, is illustrated in FIG. 4B.

[0045] As illustrated in FIG. 4A, the sintered compact has an isotropic crystal structure in which regions among nanocrystalline particles MP (main phase) are filled with a grain boundary phase BP.

[0046] On the other hand, as illustrated in FIG. 4B, the nanocrystalline particles MP have a flat shape, and the boundary face , which is substantially parallel to an anisotropic axis is curved or bent. Thus, this rare earth magnet has high magnetic anisotropy. [0047] (Method of Manufacturing Rare Earth Magnet and Forward Extrusion Forging Method according to Embodiment) Next, a method of manufacturing a rare earth magnet will be briefly described together with a forward extrusion forging method. In a furnace (not illustrated) with an Ar gas atmosphere in which the pressure is reduced to, for example, 50 kPa or less, an alloy ingot is melted by high-frequency heating using a single-roll melt spinning method, and molten metal having a composition of a rare earth magnet is injected to a copper roll R to prepare a quenched thin band (quenched ribbon), and this quenched ribbon is crushed.

[0048] Among particles of the crushed quenched ribbon, particles having a maximum particle size of approximately 200 nm or less are selected. These particles are filled into a cavity which is defined by a cemented carbide die and a cemented carbide punch sliding in the hollow portion. Next, the particles are heated by causing a current to flow therethrough in a pressing direction while being pressed with the cemented carbide punch. As a result, a quadrangular prism-shaped sintered compact is prepared, the sintered compact including: a Nd-Fe-B-based main phase (having a grain size of approximately 50 nm to 200 nm) with a nanocrystalline structure; and a grain boundary phase of a Nd-X alloy (X: metal element) present around the main phase.

[0049] The Nd-X alloy constituting the grain boundary phase is an alloy of Nd and at least one of Co, Fe, Ga, and the like and is in a Nd-rich state. For example, one alloy or a mixture of two or more alloys selected from among Nd-Co, Nd-Fe, Nd-Ga, Nd-Co-Fe, and Nd-Co-Fe-Ga may be used.

[0050] As illustrated in FIG. 4A, the sintered compact has an isotropic crystal structure in which regions among the nanocrystalline particles MP (main phase) are filled with the grain boundary phase BR

[0051] After the quadrangular prism-shaped sintered compact is manufactured, the sintered compact is placed into the first region 2 of the die 1 that forms the forging apparatus 10 illustrated in FIGS. 1A and IB. In this state, forward extrusion forging (hot working) is performed on the sintered compact to impart magnetic anisotropy to the sintered compact, and thus, a rare earth magnet (oriented magnet) is manufactured. By performing hot working including extrusion forging, in a manufactured rare earth magnet, as illustrated in FIG. 4B, the nanocrystalline particles MP have a flat shape, and the boundary face which is substantially parallel to an anisotropic axis is curved or bent. Thus, the rare earth magnet has high magnetic anisotropy.

[0052] The manufactured rare earth magnet has a metallographic structure that includes a RE-Fe-B-based main phase (RE: at least one of Nd and Pr) and a grain boundary phase of an RE-X alloy (X: metal element) present around the main phase. In the rare earth magnet, a content ratio of RE is 29 mass% < RE < 32 mass%, and an average grain size of the main phase of the manufactured rare earth magnet is preferably 300 nm. Since the content ratio of RE is in the above-described range, an effect of suppressing cracking during hot working can be further increased, and a high orientation degree can be ensured. In addition, since the content ratio of RE is in the above-described range, it is possible to ensure the size of the main phase at which high residual magnetic flux density can be ensured.

[0053] In addition, the coercive force of the manufactured rare earth magnet

(oriented magnet) may be further increased by grain boundary diffusion of a heavy rare earth metal such as Dy, and a modified alloy that does not contain a heavy rare earth metal such as a Nd-Cu alloy, a Nd-Al alloy, a Pr-Cu alloy, or a Pr-Al alloy, in the rare earth magnet. For example, the eutectic temperature of the Nd-Cu alloy is approximately 520°C, the eutectic temperature of the Pr-Cu alloy is approximately 480°C, the eutectic temperature of the Nd-Al alloy is approximately 640°C, and the eutectic temperature of the Pr-Al alloy is approximately 650°C. Since the eutectic temperatures of the above modified alloys are significantly lower than a range of 700°C to 1000°C in which crystal grains constituting the nanocrystalline magnet are coarsened, the modified alloys are particularly preferably used when the rare earth magnet is a nanocrystalline magnet.

[0054] (CAE Analysis for Verifying Improvement of Magnetic Properties and Results thereof) The present inventors performed Computer-Aided Engineering (CAE) analysis using the following method. That is, predetermined amounts of magnet materials (an alloy composition was Fe-30Nd-0.93B-4Co-0.4Ga by mass%) were mixed, the mixture was melted in an Ar atmosphere, and the molten metal was injected from an orifice of a crucible into a rotating roll made of Cu to be quenched. As a result, a quenched thin band (quenched ribbon) was manufactured.

[0055] This quenched ribbon was crushed with a cutter mill in an Ar atmosphere to obtain rare earth alloy powder having a particle size of 0.2 mm or less. Next, the rare earth alloy powder was placed into a mold having a size of 20 mmx20 mmx40 mm, and upper and lower portions thereof were sealed with a cemented carbide punch. This mold was set in a chamber and was pressed by applying a load of 400 MPa thereto under a reduced pressure of 10^" Pa and then immediately heating the mold to 650°C using a high-frequency coil. After the pressing, the mold was held at this state for 60 seconds, and a bulk body was extracted from the mold as a test piece for hot working. A compression test was conducted on this test piece at, a temperature of 750°C and a strain rate of 0.01/sec to 1/sec. In addition, by using physical property data on the obtained material as "INPUT", CAE analysis was performed regarding an E-CAP method of related art (in which an apparatus including a bent portion as in the case of the forging apparatus according to the invention is used) and the forward extrusion forging method according to the invention. In addition, magnetic properties were evaluated based on a strain distribution obtained by the CAE analysis. Further, a dimensional change in the forward extrusion forging method according to the invention was also determined. The following Table 1 shows the analysis results regarding the magnetic properties and the dimensional change, and FIG. 5 further illustrates the analysis results regarding the dimensional change. (Table 1)

[0056] With regard to residual magnetization (Br), it is known from a comparison between the experiment and the CAE analysis that residual magnetization can be estimated based on the equivalent plastic strain and the amount of elongation of the third side of the second region in the length direction. It was estimated that the residual magnetization in the E-CAP method of related art was 1.25 T and the residual magnetization in the forging method according to the invention was 1.32 T, based on the equivalent plastic strain and the amount of elongation of the third side of the second region in the length direction in each of the methods compared to each other.

[0057] With regard to coercive force (He), it is known that coercive force substantially correlates to only the equivalent plastic strain. It was estimated that the coercive force in the E-CAP method of related art was 15 kOe and the coercive force in the forging method according to the invention was 16 kOe, based on the equivalent plastic strain in each of the methods compared to each other.

[0058] With regard to dimensional change, in the E-CAP method of related art, there was no change in cross-sectional dimension in a region extending from the first region to the second region through the bent portion. Therefore, both the length of the first side of the first region and the length of the third side of the second region were 5 mm.

[0059] On the other hand, as illustrated in FIG. 5, in the forging method according to the invention, the length of the third side of the second region was measured at three positions PI, P2, and P3 located in this order from above in the height direction, and the average value was 7 mm.

[0060] Accordingly, when normalization was performed using 5 mm in the

E-CAP method of related art, a dimensional change ratio in the forging method according to the invention was determined to be 1.4.

[0061] Hereinabove, the embodiments of the invention have been described, with reference to the drawings. However, the invention is not limited to specific configurations in the embodiments, and design changes and the like which are made without departing from the scope of the invention are included in the invention.

Claims

CLAIMS:

1. A forward extrusion forging apparatus comprising:

a die that is bent and has a hollow portion; and

a punch that slides in the die, wherein:

the die includes a first region where the punch slides, a second region that is positioned ahead of the first region in an extruding direction, and a bent portion that connects the first region to the second region;

a workpiece is placed into the first region, and the punch slides in the first region to extrude the workpiece into the second region, thereby forging the workpiece;

a shape of a cross-section of each of the first region and the second region perpendicular to the extruding direction is rectangular; and

when two sides that form the rectangular cross-section of the first region and are perpendicular to each other are , respectively a first side having a length tl , and a second side having a length t2, and two sides that form the rectangular cross-section of the second region and are perpendicular to each other are respectively a third side having a length t3 and corresponding to the first side, and a fourth side having a length t4 and. corresponding to the second side, at least one of a relationship of t3>tl and a relationship of t4>t2 is satisfied.

2. The forward extrusion forging apparatus according to claim 1 , wherein the length of the side of the first region, which is shorter than the corresponding side of the second region, gradually increases from an intermediate position of the first region to the bent portion.

3. The forward extrusion forging apparatus according to claim 1, wherein the relationship of t3>tl and a relationship of t4 - 12 are satisfied.

4. The forward extrusion forging apparatus according to claim 3, wherein the first side gradually increases from an intermediate position of the first region to the bent portion.

5. A forward extrusion forging method comprising:

preparing a forward extrusion forging apparatus, wherein the forward extrusion forging apparatus includes a die that is bent and has a hollow portion, and a punch that slides in the die; the die includes a first region where the punch slides, a second region that is positioned ahead of the first region in an extruding direction, and a bent portion that connects the first region to the second region; a workpiece is placed into the first region, and the punch slides in the first region to extrude the workpiece into the second region, thereby forging the workpiece; a shape of a cross-section of each of the first region and the second region perpendicular to the extruding direction is rectangular; and when two sides that form the rectangular cross-section of the first region and are perpendicular to each other are respectively a first side having a length tl , and a second side having a length t2, and two sides that form the rectangular cross-section of the second region and are perpendicular to each other are respectively a third side having a length t3 and corresponding to the first side, and a fourth side having a length t4 and corresponding to the second side, at least one of a relationship of t3>tl and a relationship of t4>t2 is satisfied; and

manufacturing a forged product by placing the workpiece into the die and sliding the punch to forge the workpiece.

6. The forward extrusion forging method according to claim 5, wherein:

the workpiece is a sintered compact produced by press-forming powder that is a rare earth magnet material;

the powder includes a RE-Fe-B-based main phase and a grain boundary phase of a RE-X alloy present around the main phase;

RE represents at least one of Nd and Pr;

X represents a metal element; and a rare earth magnet that is the forged product is manufactured by performing hot working in which anisotropy is imparted to the sintered compact by forward extrusion forging.