CN113451033A

CN113451033A - Method for producing R-T-B sintered magnet

Info

Publication number: CN113451033A
Application number: CN202110324228.6A
Authority: CN
Inventors: 国吉太
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2020-03-27
Filing date: 2021-03-26
Publication date: 2021-09-28
Also published as: JP7468058B2; JP2021155811A

Abstract

The technical problem to be solved by the invention is as follows: provided is a method for producing an R-T-B sintered magnet, wherein the machining allowance of a sintered body can be reduced by suppressing dimensional variations of the sintered body. The solution is as follows: a method for producing an R-T-B sintered magnet, comprising: a grinding step for preparing powder of an alloy for R-T-B sintered magnets; a molding step of preparing a powder compact using the powder; a cutting step of cutting the powder molded body into a plurality of molded body pieces; and a sintering step of sintering each of the plurality of molded body pieces to produce a plurality of sintered bodies, wherein in the cutting step, a cutting interval of the powder molded body is set based on powder physical property data of the powder used for producing the powder molded body and powder molded body physical property data of the powder molded body.

Description

Method for producing R-T-B sintered magnet

Technical Field

The present invention relates to a method for producing an R-T-B sintered magnet.

Background

An R-T-B system sintered magnet (R is a rare earth element and must contain at least 1 selected from Nd, Pr and Ce, T is at least 1 of transition metals and must contain Fe, B is boron) is produced by sintering a rare earth element having R₂Fe₁₄Main phase of compound of type B crystal structure and located in grain boundary part of the main phaseA grain boundary phase and a compound phase generated by the influence of a trace amount of an additive element or an impurity. R-T-B sintered magnet exhibiting high residual magnetic flux density B_r(hereinafter, it may be abbreviated as "B" in some cases_r") and a high coercivity H_cJ(hereinafter, it may be abbreviated as "H" in some cases_cJ") and has excellent magnetic properties and is therefore known as the highest performing magnet of permanent magnets. Accordingly, R-T-B sintered magnets have been used in a wide variety of applications such as Voice Coil Motors (VCM) for hard disk drives, motors for electric vehicles (EV, HV, PHV), motors for industrial equipment, and various motors and household electric appliances.

Such an R-T-B sintered magnet can be produced, for example, by a step of preparing an alloy powder, a step of press-molding the alloy powder to produce a powder compact, and a step of sintering the powder compact. The alloy powder is produced, for example, by the following method.

First, an alloy is produced from a melt of various raw metals by a method such as an ingot casting method or a strip casting method. The obtained alloy is subjected to a pulverization step to obtain an alloy powder having a predetermined particle size distribution. The pulverization step usually includes a coarse pulverization step which is carried out by utilizing, for example, a hydrogen embrittlement phenomenon, and a fine pulverization step which is carried out by, for example, a jet mill.

The sintered body obtained in the step of sintering the powder compact is then subjected to mechanical processing such as grinding and cutting, and is singulated into a desired shape and size. More specifically, first, a powder compact having a size larger than that of the final magnet product is produced by compression molding R-T-B-based rare earth magnet powder using a press apparatus. Then, after the powder compact is made into a sintered body by a sintering step, the sintered body is ground by, for example, a cemented carbide blade saw, a rotary grindstone, or the like to give a desired shape. For example, a sintered body having a block shape is first prepared, and then the sintered body is sliced with a blade saw or the like to cut out a plurality of plate-like sintered body portions.

However, since a sintered body of a rare earth alloy magnet such as an R-Fe-B magnet is extremely hard and brittle and has a large machining load, high-precision grinding is a difficult operation and the machining time is long. Therefore, the machining process has been a large cause of an increase in manufacturing cost.

In order to solve such a problem, patent document 1 describes a technique of processing a magnet powder compact by using a wire saw before sintering. A wire saw is a processing technique in which a saw wire moving in one or two directions is pressed against a powder compact to be processed, and the powder compact is ground or cut by abrasive grains located between the saw wire and the powder compact. This technique cuts a powder compact which is extremely softer than the sintered compact and is easier to process, and therefore can significantly reduce the time required for cutting.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2003-303728

Disclosure of Invention

Technical problem to be solved by the invention

Since the R-Fe-B sintered magnet contains rare earth elements which are expensive and rare, further improvement in the utilization efficiency (yield) of the material is required. When the powder compact after the cutting process is sintered, shrinkage occurs, and the size of the sintered compact is reduced to, for example, about 60 to 70% of the size of the powder compact. Since the shrinkage rate varies in this case, the sintered compact has a varying size even when the sintered compact has the same size.

Embodiments of the present invention provide a method for producing an R-T-B sintered magnet that can solve such problems.

Technical solution for solving technical problem

In an exemplary embodiment, a method for producing an R-T-B sintered magnet according to the present invention includes: a grinding step of preparing a powder of an alloy for R-T-B sintered magnets (R is a rare earth element and must contain at least 1 kind selected from Nd, Pr, and Ce, T is at least 1 kind of a transition metal and must contain Fe, and B is boron); a molding step of preparing a powder compact using the powder; a cutting step of cutting the powder molded body into a plurality of molded body pieces; and a sintering step of sintering each of the plurality of molded body pieces to produce a plurality of sintered bodies, wherein in the cutting step, a cutting interval of the powder molded body is set based on powder physical property data of the powder used for producing the powder molded body and powder molded body physical property data of the powder molded body.

In one embodiment, the powder property data is data on the composition and particle size of the powder.

In one embodiment, the physical property data of the powder compact is density data of the compact.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the embodiment of the present invention, the dimensional variation of the sintered body after sintering can be suppressed, and the size of the sintered body can be brought close to the target value. Thus, a method for producing an R-T-B sintered magnet, in which the material utilization efficiency is further improved, can be provided.

Drawings

Fig. 1 is a flowchart showing the main steps of the manufacturing method according to the embodiment of the present invention.

Fig. 2 is a perspective view schematically showing an example of the powder formed body before cutting.

Fig. 3 is a perspective view schematically showing the relationship between the molded body piece 14 before sintering and the sintered body 18 after sintering.

Fig. 4 is a perspective view showing an example of a configuration of a wire saw device that can be used in the embodiment of the present invention.

Fig. 5 is a sectional view schematically showing a section of the saw wire.

Description of the symbols

10: workpiece, 20: fixing base, 30a, 30b, 30 c: roller, 40: saw wire, 100: a wire saw device.

Detailed Description

When a plurality of molded body pieces are prepared by cutting the powder molded body, and then the respective molded body pieces are sintered to produce a sintered body, the above-described shrinkage occurs. The difference in size between the molded body and the sintered body with respect to the size of the molded body is defined as "shrinkage rate". The shrinkage rate is, for example, about 30%, but the specific value may vary depending on parameters such as the composition and particle size of the powder and the molding density of the powder molded article. The size of the obtained sintered body may vary by, for example, about 2 to 5% depending on the variation in shrinkage ratio.

When a powder compact is cut by a wire saw or the like, divided into a plurality of compact pieces, and then sintered, conventionally, the shrinkage rate due to sintering is collected as basic data, and the width of cutting (cutting interval) of the powder compact is set according to the maximum value of the collected shrinkage rate. In other words, assuming that the maximum shrinkage occurs, a margin (margin) is added to the cutting interval. As a result, when the shrinkage rates vary, the sintered body (the sintered body having the smallest size) having the largest shrinkage has a size equal to or larger than the target size. For a relatively large sintered body exceeding the target size, the sintered body is ground by machining in a large amount, thereby enabling size adjustment. However, when the dimensions of the sintered body are adjusted by such machining, the material use efficiency and the material yield are reduced, and the mass productivity is deteriorated. Since the R-T-B system sintered magnet contains precious rare earth elements, it is undesirable to increase the amount of the ground sintered body, i.e., the machining allowance.

In the method for producing an R-T-B sintered magnet according to the present invention, when a powder compact is cut and divided, the shrinkage rate of the powder compact is estimated or predicted based on the "powder physical property data" and the "powder compact physical property data", and the width of the cut is set according to the shrinkage rate. This can suppress variation in the size of the sintered body and reduce the machining allowance of the sintered body. That is, according to the method for producing an R-T-B sintered magnet of the present invention, since an optimum width can be set for each powder compact, it is not necessary to set the cutting interval of the powder compact according to the maximum value of the shrinkage rate, and the margin (margin) added to the cutting interval can be suppressed. Further, since the cutting width is set for each powder compact, an excess cutting portion (a powder compact remaining after the step of cutting into a plurality of compact pieces is completed) may be generated after the powder compact is cut. Even in this case, the remaining cut portions and the cut powder generated during cutting have the same composition and size as the other particles contained in the powder compact, and therefore can be sufficiently reused. Therefore, the remaining cut portions and the cut powder are reused as the R-Fe-B-based rare earth magnet powder for obtaining the powder compact, and thus the material utilization efficiency and the material yield can be prevented from being lowered.

The powder physical property data can be collected by sampling a part of the powder used for producing the powder compact and measuring the physical property data. The physical property data of the powder compact can be collected by measuring the physical property data for each molding or for each batch (several hundreds). In a preferred embodiment, the powder property data includes data on the composition and particle size of the powder. The physical property data of the powder compact includes density data of the compact. The composition and particle size of the powder and the density of the compact are preferred examples of parameters for specifying the shrinkage of the powder compact caused by sintering. By preparing data indicating the relationship between such parameters and shrinkage rates based on actual measurement, the shrinkage rate of the powder compact can be estimated or predicted based on the set parameters or the values of the measured parameters. For example, the width of the powder compact can be set to be optimum by comparing the past actual data indicating the relationship between the powder physical property data (for example, the composition and particle size of the powder) and the shrinkage factor of the powder compact physical property data (for example, the density of the powder compact) with these physical property data of the powder compact to be cut.

When the value of the parameter included in the data is changed or has changed, the shrinkage rate of the powder compact can be estimated or predicted from the relationship between the parameter and the shrinkage rate after the change. For example, in the case where the density of the molded body of the powder molded body taken out from the powder compacting apparatus is measured and a change in the measured value from an initial set value is detected during mass production, the size of the molded body piece, that is, the cutting interval of the powder molded body can be corrected based on the shrinkage rate corresponding to the new measured value.

In addition, when the composition and the particle size of the powder are changed by design, the shrinkage after the change in design can be predicted based on the above data, and the cutting interval corresponding to the shrinkage can be changed.

The content of such data can be updated by acquiring the measured values of the parameters in the process of mass production.

Hereinafter, an embodiment of the method for producing an R-T-B sintered magnet according to the present invention will be described. As shown in the flowchart of fig. 1, the method for producing an R-T-B sintered magnet according to the present embodiment includes:

a grinding step (S10) for preparing powder of the alloy for R-T-B sintered magnets;

a molding step (S20) for producing a powder molded body using the powder;

a cutting step (S30) of cutting the powder compact into a plurality of compact pieces;

and a sintering step (S40) of sintering each of the plurality of molded body pieces to produce a plurality of sintered bodies.

The cutting step (S30) further includes: and a step (S35) for setting the cutting interval of the powder compact on the basis of the powder physical property data of the powder used for the production of the powder compact and the powder compact physical property data of the powder compact.

Next, an example of each of the steps S10 to S40 described above will be described with reference to fig. 1.

First, in step S10, powder of an alloy for R-T-B sintered magnets is prepared. The details of the composition of the alloy for R-T-B sintered magnets, the method for producing the powder, and the like will be described later.

In step S20, a powder compact is produced using the powder prepared in step S10. The powder compact can be produced by, for example, a wet or dry powder compacting apparatus.

Fig. 2 is a perspective view schematically showing an example of the powder formed body. In fig. 2, for reference, an XYZ coordinate system including X, Y, and Z axes orthogonal to each other is shown. The powder compact 10 shown in the figure has a rectangular parallelepiped block shape. In fig. 2, the direction M of the orienting magnetic field is shown by an arrow. This direction M is referred to as a "magnetic field orientation direction", and when powder of the alloy for an R-T-B-based sintered magnet is pressed to produce the powder compact 10, an orientation magnetic field is applied to the powder to orient the direction of each powder particle in the magnetic field orientation direction M. Finally, magnetization is carried out in a direction parallel to the magnetic field orientation direction M.

Reference is again made to fig. 1.

Next, in step S30, the powder compact 10 is cut, and the powder compact 10 is divided into a plurality of compact pieces 14. The powder compact 10 is not a sintered body but a compact (green compact) of powder before sintering. In a preferred embodiment, the powder compact 10 can be cut with a wire saw. The details of the wire saw are described later.

In the example of fig. 2, the powder compact 10 may first be cut into a plurality of plate-like portions extending along a plane (YZ plane) including the magnetic field orientation direction M of the powder compact 10. Thereafter, each plate-like portion is cut so as to cross the magnetic field orientation direction M of the powder compact, and finally divided into a plurality of compact pieces 14. In fig. 2, the cut section is shown in broken lines for reference. The order or the form of cutting is not limited to one example, and cutting may be performed in another order. In fig. 2, as the dimension of the molded body sheet 14, a dimension T, Z in the axial direction of a dimension W, Y in the X-axis direction is described. Dimension W, T, L corresponds to the "width", "thickness", and "length" of the formed body piece 14, respectively. The shape of the powder compact 10 is not limited to a rectangular parallelepiped, and may be a cylinder or another shape. In the example of fig. 2, the cut sections of the powder compact 10 are orthogonal to each other, but the invention is not limited to this example. The final molded body piece 14 may have a plate shape or a bar shape extending in 1 direction.

In the embodiment of the present invention, when the powder compact 10 is cut and divided into the plurality of compact pieces 14, the cutting interval of the powder compact 10 is set based on the powder physical property data of the powder used for producing the powder compact 10 and the powder compact physical property data of the powder compact 10 (step S35 in fig. 1). Details of the process S35 will be described later.

Next, in step S40, each of the plurality of molded body pieces 14 is sintered to produce a plurality of sintered bodies 16. The sintering conditions may be defined by sintering temperature, sintering time, and the like. Since the sintering conditions have an influence on the shrinkage rate of the powder compact, the sintering conditions can be used together with the above data to estimate or predict the shrinkage rate.

Fig. 3 is a perspective view schematically showing the relationship between the molded body piece 14 before sintering and the sintered body 18 after sintering. In the right part of fig. 3, as the dimension of the sintered body 18, a dimension Ws in the X-axis direction, a dimension Ts in the Y-axis direction, and a dimension Ls in the Z-axis direction are described. The dimensions Ws, Ts, Ls correspond to the "width", "thickness", and "length" of the sintered body 18, respectively. As shown in fig. 3, the molded body piece 14 shrinks by sintering, and becomes a sintered body 18 with a reduced size.

In the embodiment of the present invention, the sintered body size can be brought close to the target value by sampling and measuring the composition and particle size of the powder and the density of the molded body during mass production, managing the variation in the shrinkage rate, and correcting the cutting interval as needed.

The method for producing the R-T-B sintered magnet according to the present embodiment will be described in detail below. In this embodiment, an embodiment of a method for producing an R-T-B sintered magnet will be described.

S10: process for preparing powder of alloy for R-T-B sintered magnet

Composition of alloy for R-T-B sintered magnet

R is a rare earth element and must contain at least 1 selected from Nd, Pr and Ce. It is preferable to use a combination of rare earth elements represented by Nd-Dy, Nd-Tb, Nd-Dy-Tb, Nd-Pr-Dy, Nd-Pr-Tb, and Nd-Pr-Dy-Tb.

Among R, Dy and Tb exert an effect of improving HcJ in particular. In addition to the above elements, other rare earth elements such as La may be contained, and cerium alloy (misch metal) and didymium may be used. R may not be a pure element, and may contain impurities unavoidable in production within a range industrially available. The content is, for example, 27 mass% or more and 35 mass% or less. The R content of the R-T-B sintered magnet is preferably 31 mass% or less (27 mass% to 31 mass%, preferably 29 mass% to 31 mass%). The R-T-B sintered magnet has an R content of 31 mass% or less and an oxygen content of 500ppm to 3500 ppm.

T is at least 1 kind of transition metal and must contain Fe, and 50% or less of Fe may be replaced by cobalt (Co) in terms of mass ratio (including a case where T substantially consists of iron and cobalt). Co is effective for improving temperature characteristics and corrosion resistance, and the alloy powder may contain Co in an amount of 10 mass% or less. The content of T may occupy R and B or R, B and the remainder of M described later.

The content of B may be a known content, and is preferably in the range of 0.85 to 1.2% by mass, for example. If the content is less than 0.85% by mass, a high HcJ may not be obtained, and if the content exceeds 1.2% by mass, Br may be reduced. In addition, a part of B can be replaced with C (carbon).

In addition to the above elements, an M element may be added for increasing HcJ. The M element is one or more selected from Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta and W. The amount of the element M added is preferably 5.0% by mass or less. This is because if it exceeds 5.0 mass%, Br may be reduced in some cases. In addition, inevitable impurities may be allowed.

< Process for producing alloy for R-T-B sintered magnet

The production process of the alloy for R-T-B sintered magnets is exemplified. The alloy ingot can be obtained by melting a metal or alloy adjusted to the above composition in advance and casting the alloy into a mold by an ingot casting method. Further, the molten metal may be brought into contact with a single roll, a twin roll, a rotary disc, a rotary cylinder mold, or the like to be rapidly cooled, and an alloy flake may be produced by a rapid cooling method typified by a strip casting method or a centrifugal casting method for producing a solidified alloy thinner than an alloy produced by an ingot casting method.

In the embodiment of the present invention, a material produced by any of an ingot casting method and a quenching method can be used, and the material is preferably produced by a quenching method such as a strip casting method. The quenched alloy produced by the quenching method has a thickness of usually 0.03mm to 1mm and has a thin sheet shape. The alloy melt is solidified from the surface (roll contact surface) in contact with the cooling roll, and crystals grow in a columnar shape in the thickness direction from the roll contact surface. The quenched alloy is cooled in a short time, as compared with an alloy (ingot alloy) produced by a conventional ingot casting method (die casting method), and thereby the structure is refined and the crystal grain size is small. Further, the area of the grain boundary is large. Since the R-rich phase spreads widely in the grain boundary, the R-rich phase of the quenching method is excellent in dispersibility. Therefore, the grain boundaries are easily broken by the hydrogen pulverization method. By hydrogen-pulverizing the quenched alloy, the size of the hydrogen pulverized powder (coarse pulverized powder) can be set to, for example, 1.0mm or less. The coarsely pulverized powder thus obtained was pulverized by a jet mill.

< Process for preparing powder of alloy for R-T-B sintered magnet >

The powder of the alloy for R-T-B sintered magnets is active and easily oxidized. Therefore, as the gas used for the jet mill, an inert gas such as nitrogen, argon, helium or the like is used in order to reduce the oxygen content as an impurity to improve the performance of the magnet in order to avoid the risk of heat generation and ignition.

The pulverized material (coarsely pulverized powder) to be fed to the jet mill is, for example, a fine powder having a particle size distribution in which the average particle size (median diameter: d50) is 2.0 μm or more and 4.5 μm or less is collected by a cyclone collector. Cyclone trapping devices are used to separate powder from a gas stream that transports the powder. Specifically, coarsely pulverized powder of the alloy for R-T-B-based sintered magnets is pulverized in a jet mill in the preceding stage, and fine powder produced by the pulverization is supplied to a cyclone collector together with gas used for the pulverization. The mixture of the inert gas (pulverized gas) and the pulverized fine powder forms a high-speed gas flow, and is sent to the cyclone collector. The cyclone collector is used to separate these pulverized gas and fine powder. The fine powder separated from the pulverized gas is collected in a powder trap.

S20: process for producing powder molded article

Next, a powder compact is produced from the fine powder by pressing in a magnetic field. In the case of pressing in a magnetic field, it is preferable to form a powder compact by pressing or wet pressing in an inert gas atmosphere from the viewpoint of suppressing oxidation. In particular, in wet pressing, the surfaces of the particles constituting the powder compact are covered with a dispersant such as an oil agent, and contact with oxygen and water vapor in the atmosphere is suppressed. Therefore, oxidation of the pellets by the atmosphere before and after the pressing process or during the pressing process can be prevented or suppressed.

In the case of wet pressing in a magnetic field, a slurry in which a dispersion medium is mixed with fine powder is prepared, supplied to a cavity in a mold of a wet pressing apparatus, and press-molded in a magnetic field. The powder shaped bodies thus formed have, for example, 4Mg/m³Above 5Mg/m³The following density of the molded article.

Dispersing media

The dispersion medium is a liquid capable of obtaining a slurry by dispersing the alloy powder in the inside thereof.

Preferred examples of the dispersion medium used in the present invention include mineral oil and synthetic oil. Although the type of mineral oil or synthetic oil is not particularly limited, if the kinematic viscosity at room temperature exceeds 10cSt, the binding force between the alloy powders is increased due to the increased viscosity, and the orientation of the alloy powders may be adversely affected when wet molding is performed in a magnetic field. Therefore, the kinematic viscosity of the mineral oil or the synthetic oil at room temperature is preferably 10cSt or less. Further, if the fractionation temperature of the mineral oil or the synthetic oil exceeds 400 ℃, it becomes difficult to remove the oil after obtaining the powder compact, and the amount of residual carbon in the sintered body increases, and the magnetic properties may be deteriorated. Therefore, the fractionation temperature of the mineral oil or the synthetic oil is preferably 400 ℃ or lower. In addition, vegetable oils may also be used as the dispersion medium. The vegetable oil is an oil extracted from a plant, and the kind of the plant is not limited to a specific plant.

Preparation of the slurry

By mixing the obtained alloy powder with a dispersion medium, a slurry can be obtained.

The mixing ratio of the alloy powder and the dispersion medium is not particularly limited, and the ratio in the slurryThe concentration of the alloy powder is preferably 70% by mass or more (i.e., 70% by mass or more). This is because the length of the groove is 20 to 600cm³The alloy powder can be efficiently supplied into the cavity at a flow rate of one second, and excellent magnetic characteristics can be obtained. The concentration of the alloy powder in the slurry is preferably 90% by mass or less. The method of mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and the dispersion medium are prepared separately, and they are weighed and mixed in predetermined amounts. In addition, when the coarsely pulverized powder is dry-pulverized by a jet mill or the like to obtain an alloy powder, a container to which a dispersion medium is added may be disposed at an alloy powder discharge port of a pulverizing device such as a jet mill, and the alloy powder obtained by pulverization may be directly collected in the dispersion medium in the container to obtain a slurry. In this case, it is preferable that the inside of the container is also made into an atmosphere of nitrogen and/or argon, and the obtained alloy powder is recovered in the dispersion medium as it is without being brought into contact with the atmosphere to form a slurry. Alternatively, the coarsely pulverized powder may be wet-pulverized in a state of being held in a dispersion medium using a vibration mill, a ball mill, an attritor, or the like, to obtain a slurry composed of the alloy powder and the dispersion medium.

The slurry thus obtained is molded by a known wet press apparatus to obtain a powder compact having a predetermined size and shape. Conventionally, this powder compact is usually sintered to obtain a sintered body, but in the present embodiment, as described below, the powder compact is cut and divided into a plurality of compact pieces before sintering.

S30: cutting the powder molded body into a plurality of molded body pieces

In the embodiment of the present invention, a step (S30) of cutting the powder compact into a plurality of compact pieces is performed before the sintering step. The cutting can be suitably performed by a wire saw. The wire saw comprises: a multi-wire saw for dividing the powder compact into 3 or more compact pieces simultaneously along a plurality of cut sections, and a single-wire saw for sequentially dividing the powder compact along each cut section. In the embodiment of the present invention, any wire saw may be used.

Here, a configuration example of a wire saw device (multi-wire saw device) that can be used in the cutting step will be described with reference to fig. 4. Fig. 4 is a perspective view showing an example of the configuration of a wire saw device 100 according to an embodiment of the present invention. The wire saw device 100 in this example is a multi-wire saw device. In the figure, for reference, an xyz coordinate system including x, y, and z axes orthogonal to each other is shown. In this example, the xy-plane is horizontal and the z-axis is oriented in the vertical direction.

The wire saw device 100 of fig. 4 has

rollers

30a, 30b, 30c arranged with their central axes of rotation parallel to each other and one continuous saw wire 40. The powder compact 10 prepared in the step (S10) is supported by the fixing base 20.

The fixing base 20 moves up and down in the z-axis direction in a state where the powder compact 10 is fixed. The up-and-down movement can be performed by a driving device not shown. The driving device may be driven by a hydraulic cylinder or may be operated by a motor.

The

rollers

30a, 30b, and 30c are disposed at predetermined intervals such that the axis of the rotation center is positioned at the apex of the triangle when viewed in the direction parallel to the x-axis. The

rollers

30a, 30b, 30c are provided with a plurality of grooves on their respective side surfaces. The saw wire 40 is wound in sequence around the plurality of grooves of the

rollers

30a, 30b, 30 c. The center-to-center spacing (pitch) of the plurality of grooves defines the width (cutting interval) of the element divided by cutting with the wire saw.

As described above, the cutting step (S30) includes: and a step (S35) for setting the cutting interval of the powder compact on the basis of the powder physical property data of the powder used for the production of the powder compact and the powder compact physical property data of the powder compact. Therefore, when the wire saw device 100 according to the present embodiment is used, the center intervals of the plurality of grooves are determined so that the cutting intervals set based on the powder physical property data of the powder used for producing the powder compact and the powder compact physical property data of the powder compact can be realized. When the cutting interval is changed, the roller is replaced by a roller having grooves at a center interval corresponding to the cutting interval. In the case of using the single wire saw device, the position of the saw wire 40 in the X-axis direction may be moved in units of cutting intervals with respect to the powder compact 10, and then the cutting may be performed sequentially.

Both ends of the saw wire 40 are wound around, for example, a recovery reel not shown. During cutting, the

rollers

30a, 30b, and 30c and the collection reel rotate. The direction of rotation of the

rollers

30a, 30b, 30c depends on their arrangement and the way the saw wire 40 is hung. In the wire saw device 100 shown in fig. 4, the

rollers

30a, 30b, and 30c rotate in the same direction.

When the predetermined length of the saw wire 40 is wound around one of the recovery reels, the recovery reels and the

rollers

30a, 30b, and 30c are rotated in the opposite directions. This causes the saw wire 40 to move in the reverse direction, and this operation is repeated, whereby the saw wire 40 can be reciprocated (moved).

The saw wire 40 is, for example, a fixed abrasive wire. Specifically, a saw wire in which abrasive grains having high hardness suitable for cutting a high-hardness material are fixed to a wire material by electrodeposition can be used. Abrasive grains of high hardness, also referred to as superabrasive grains, are typically diamond abrasive grains.

Fig. 5 schematically shows a cross section of the saw wire 40. The saw wire 40 includes a wire (core wire) 42, abrasive grains 44 on the outer peripheral surface of the wire 42, and a fixing layer 46. The anchor layer 46 is formed of a plating metal such as Ni. The abrasive grains 44 are positioned on the surface of the wire rod 42, and the abrasive grains 44 and the surface of the wire rod 42 around the abrasive grains 44 are entirely covered with the fixing layer 46, whereby the abrasive grains 44 can be fixed to the wire rod 42. The fixation of the abrasive particles 44 may also be achieved by other methods. The abrasive grains 44 have an average particle diameter of, for example, 1 μm to 24 μm.

The step of cutting the powder compact 10 with a wire saw is preferably performed in a state where the powder compact 10 is immersed in a liquid. When the powder compact 10 is formed by wet pressing, a preferable example of the liquid is a dispersion medium such as an oil agent (mineral oil or synthetic oil) used for wet pressing.

When the powder compact 10 is processed by the wire saw device 100, the powder particles constituting the powder compact 10 are removed as cutting powder from the portion cut by the abrasive grains 44 of the saw wire 40. The cutting powder obtained from the powder compact 10 before sintering is recovered and directly mixed with fine powder for producing a powder compact, so that the cutting powder can be easily reused.

In a preferred embodiment, the diameter of the wire 42 (see fig. 5) of the saw wire 40 is, for example, 140 μm to 240 μm. When the diameter of the wire rod 42 is less than 140 μm, the strength is insufficient, and the wire rod 42 is stretched during cutting. The larger the diameter of the wire rod 42, the more the dischargeability of the cutting powder is improved, but the amount of the cutting powder is increased, and therefore, it is preferably 240 μmm or less.

The moving speed of the wire 40 (wire linear speed) may be set to a range of 100 m/min to 500 m/min, for example. On the other hand, the workpiece conveying speed (the moving speed of the fixing base 20 in the z-axis direction of fig. 4) may be set to a range of, for example, 100 mm/min to 600 mm/min. The tension applied to the saw wire 40 is, for example, 2.0kg to 3.0 kg.

From the viewpoint of rapidly discharging the cutting powder, it is desirable to perform wire saw processing (cutting in oil) while immersing the powder compact 10 in a dispersion medium (mineral oil or synthetic oil) used in the production of the powder compact by wet pressing. When wire saw processing is performed in the air, it is desirable to spray the same oil as the dispersion medium onto the portion (cut portion) where the powder compact 10 and the saw wire 40 are in contact.

In the present embodiment, the cutting interval is set based on the above-described data prepared in advance. Therefore, a molded body piece having a size corresponding to shrinkage occurring in the subsequent sintering step can be suitably produced. When the composition, particle size, or the like of the powder used for producing the powder compact is changed or when the density of the compact is changed, the set value of the cutting interval may be changed based on the above data. As a result, a molded body sheet having a size corresponding to the actual shrinkage rate can be produced.

In the cutting step, the powder particles constituting the powder compact become cutting powder and fall off. When a hard sintered body obtained by sintering a powder compact is cut, the cutting powder is particles or a combination of particles that undergo grain growth by sintering or a change in composition by a chemical reaction, and therefore, it is not possible to mix them with a powder of an R-T-B-based alloy for sintered magnets and reuse them. On the other hand, if the cutting powder is obtained from the powder compact before sintering, the composition and the size are the same as those of other particles contained in the powder compact, and thus the cutting powder can be sufficiently reused. Therefore, it is preferable to recover and reuse the cutting powder.

S40: a step of sintering the respective molded body pieces to produce a sintered body

Next, the molded body piece obtained by the cutting step described above is sintered to obtain an R-T-B-based sintered magnet (sintered body). The sintering step of the molded body sheet may be, for example, 0.13Pa (10 Pa)^－3Torr) or less, preferably 0.07Pa (5.0X 10)^－4Torr) or less, for example, at a temperature in the range of 1000 to 1150 ℃. In order to prevent oxidation due to sintering, the residual gas of the atmosphere may be replaced with an inert gas such as helium or argon.

The molded body piece shrinks during sintering, and the size of the resulting sintered body is reduced from the size of the molded body piece. However, in the present embodiment, since the molded body piece having a size corresponding to the shrinkage occurring in the sintering step can be suitably produced, the size of the sintered body after shrinkage has a value close to a target value. For example, the error between the size of the sintered body and the target value can be suppressed to 1% or less.

In the sintered body after step S40, an element such as a heavy rare earth element may be diffused from the surface of the sintered body to the inside thereof. Additional heat treatment such as aging treatment may be performed. By such diffusion and heat treatment, the magnetic properties can be improved. The heat treatment conditions such as the heat treatment temperature and the heat treatment time may be known conditions. The sintered body thus obtained is subjected to finishing such as grinding and polishing and surface treatment as necessary, and then subjected to a magnetization step to complete a final R-T-B sintered magnet.

< example >

Preparing a material having Nd: 24.5 mass%, Pr: 4.5 mass%, B: 0.90 mass%,Cu: 0.1 mass%, Ga: 0.4 mass%, Co: 1.0 mass% and the balance Fe. Particle diameter D of the powder₅₀And was 4 μm. Further, the particle diameter D was measured by an air-jet dispersion type laser diffraction method (modified according to JIS Z8825: 2013)₅₀. These powders were used to prepare a powder compact by a wet press apparatus. The size of the powder compact obtained was 100 mm. times.60 mm. times.90 mm (90mm is the magnetic field orientation direction). The density of the obtained powder compact was 4.5Mg/m³Left and right. The obtained powder compact is cut at a cutting interval set under conditions a and B described below to obtain a plurality of compact pieces. The dimensions of the molded body piece were 100 mm. times.60 mm. times.7.5 mm (7.5mm is the magnetic field orientation direction). The obtained molded body piece is sintered (a temperature at which densification by sintering is sufficiently generated is selected), thereby producing a sintered body.

(Condition A)

Condition a is the current setting method. As a previous experiment, a plurality of powder molded bodies were produced, the shrinkage rates were obtained by sintering the powder molded bodies, the shrinkage rates were collected as basic data, and the width of the cut powder molded body was set in accordance with the maximum value of the collected shrinkage rates.

(Condition B)

Condition B is the setting method of the present invention. First, powder physical property data of the powder used for producing the powder compact is collected. Specifically, the composition and particle size of the powder were determined. Next, physical property data of the powder compact was collected. Specifically, the compact density of the powder compact was measured. From these measurement data, the width of the powder compact was set based on the past actual data showing the relationship between the powder physical property data and the physical property data of the powder compact and the shrinkage rate.

The dimensions of the sintered bodies produced under the conditions a and B in the magnetic field orientation direction (7.5mm) were measured 20 times each, and the difference between the maximum dimension and the minimum dimension was determined to determine the dimensional variation of the sintered bodies. In addition, the dimension in the magnetic field orientation direction is most likely to be distorted in other dimension portions. Then, the difference from the dimensional target value of the sintered body (dimensional target value of 7.5mm in the magnetic field orientation direction) was obtained. Under condition A, the dimensional deviation of the sintered body was 0.22mm, and the difference from the target value was 3%. On the other hand, under condition B, the dimensional variation of the sintered body was 0.07mm, and the difference from the target value was 1%. Thus, under condition B, dimensional variations in the sintered body are suppressed, and the difference from the target value is small.

Claims

1. A method for producing an R-T-B sintered magnet, comprising:

a step of pulverizing powder of an alloy for R-T-B sintered magnets, wherein R is a rare earth element and must contain at least 1 kind selected from Nd, Pr, and Ce, T is at least 1 kind of a transition metal and must contain Fe, and B is boron;

a molding step of producing a powder compact using the powder;

a cutting step of cutting the powder compact into a plurality of compact pieces; and

a sintering step of sintering each of the plurality of molded body pieces to produce a plurality of sintered bodies,

in the cutting step, a cutting interval of the powder compact is set based on powder physical property data of the powder used for producing the powder compact and powder compact physical property data of the powder compact.

2. The method of manufacturing an R-T-B sintered magnet according to claim 1, wherein:

the powder physical property data is data of the composition and the granularity of the powder.

3. The method of manufacturing an R-T-B sintered magnet according to claim 1 or 2, wherein:

the physical property data of the powder forming body is the density data of the forming body.