CN114334413A

CN114334413A - Method for producing R-T-B sintered magnet

Info

Publication number: CN114334413A
Application number: CN202111074295.3A
Authority: CN
Inventors: 村田刚志
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2020-09-28
Filing date: 2021-09-14
Publication date: 2022-04-12
Also published as: JP2022054683A; JP7232390B2

Abstract

The invention provides a method for producing an R-T-B sintered magnet without preparing an inert atmosphere. The method for producing an R-T-B sintered magnet of the present invention comprises: 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 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, wherein the cutting step includes a step of cutting the powder molded body immersed in a liquid by a traveling metal wire.

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 one element selected from Nd, Pr and Ce, T is at least one transition metal and must contain Fe, B is boron) is formed by using a sintered magnet having R₂Fe₁₄Main phase of compound having B-type crystal structure, grain boundary phase located in grain boundary part of the main phase, and compound generated by influence of trace additive element or impurityPhase composition. R-T-B sintered magnet exhibiting high residual magnetic flux density B_r(hereinafter, it may be referred to as "B" only_r") and a high coercivity H_cJ(hereinafter, it may be referred to as "H" only_cJ") and has excellent magnetic characteristics, and therefore, is known as the magnet having the highest performance among permanent magnets. Accordingly, the R-T-B sintered magnet is used in various applications such as a Voice Coil Motor (VCM) for a hard disk drive, a motor for an electric vehicle (EV, HV, PHV), a motor for industrial equipment, and various motors and home electric appliances.

Such an R-T-B sintered magnet is 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 by, for example, 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 generally includes a coarse pulverization step utilizing, for example, a hydrogen embrittlement phenomenon, and a fine pulverization step using, for example, a jet mill (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 compact having a size larger than that of the final magnet product is produced by compression molding R-Fe-B-based rare earth magnet powder using a press apparatus. Then, after the molded body is made into a sintered body by a sintering step, the sintered body is ground by, for example, a cutter saw made of cemented carbide, 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 knife saw or the like, thereby cutting 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 sintered 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. In addition, a material portion lost by processing is inevitably generated. Therefore, the machining process becomes a factor of increasing the manufacturing cost.

In order to solve the former problem, for example, patent document 1 describes a technique of processing a magnet molded body using a wire saw before sintering. A wire saw is a machining technique in which a saw wire running in one direction or two directions is pressed against a molded body to be machined, and the molded body is ground or cut by abrasive grains located between the saw wire and the molded body. According to this technique, the powder compact which is in a state of being exceptionally softer than the sintered compact and easily processed is cut, and therefore, the time required for the cutting process is significantly shortened.

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

Patent document 1 discloses a wire saw using a saw wire having an outer diameter of 0.1mm to 1.0mm and abrasive grains fixed to the saw wire, and processing a powder compact in an inert gas atmosphere in which the oxygen concentration is adjusted to 5% to 18% in the entire molar ratio. When wire saw processing is performed in an inert atmosphere in which the oxygen concentration is controlled in this manner, the facility and management thereof become complicated, and the mass productivity is poor.

Embodiments of the present invention provide a method for producing a novel R-T-B sintered magnet, which enables a wire saw process that does not require the preparation of an inert atmosphere.

Technical solution for solving technical problem

The present invention provides a method for manufacturing an R-T-B sintered magnet, which, in an exemplary embodiment, includes: a grinding step of preparing a powder of an alloy for an R-T-B sintered magnet (wherein R is a rare earth element and must include at least one element selected from Nd, Pr, and Ce, T is at least one transition metal and must include Fe, and B is boron); a molding step of preparing 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, wherein the cutting step includes a step of cutting the powder molded body immersed in a liquid by a traveling metal wire.

In one embodiment, in the cutting step, the metal wire rod has a traveling speed of 300 m/min or more.

In one embodiment, in the cutting step, the tension of the metal wire rod is 3kgf or more.

In one embodiment, in the cutting step, a cutting speed in a direction perpendicular to a traveling direction of the metal wire rod is 100 mm/min or more.

In one embodiment, the step of preparing the powder compact includes a step of forming the powder by wet pressing.

In one embodiment, the wet pressing is performed by adding the same kind of liquid as the liquid in the cutting step to the powder.

In one embodiment, the method further includes a step of recovering, from the liquid, particles of the powder shaved off from the powder compact in the cutting step.

Effects of the invention

According to the embodiment of the present invention, the cutting can be performed by the wire saw without preparing the inert atmosphere, and the mass productivity is excellent. In addition, the abrasive grains detached from the wire saw can be prevented from being mixed into the cutting powder. Therefore, the cutting powder generated from the powder compact can be easily reused in the cutting step by the wire saw for the production of the magnet, and the characteristics of the high-performance magnet can be maintained and the production cost can be reduced.

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 the structure of a wire saw device used in the embodiment of the present invention.

Fig. 3A is a front view for explaining a process of cutting the powder compact immersed in the liquid by the wire saw.

Fig. 3B is a front view for explaining a step of cutting the powder compact immersed in the liquid by the wire saw of the metal wire rod.

Fig. 4A is a side view for explaining a process of cutting the powder compact immersed in the liquid by the wire saw.

Fig. 4B is a side view for explaining a process of cutting the powder compact immersed in the liquid by the wire saw.

Fig. 5A is a side view for explaining a process of cutting the powder compact immersed in the liquid by the wire saw.

Fig. 5B is a side view for explaining a process of cutting the powder compact immersed in the liquid by the wire saw.

Fig. 6A is a schematic view showing a cut surface formed in the powder compact 10 by a wire saw.

Fig. 6B is a view schematically showing a cut surface formed in the powder compact 10 by a wire saw.

Fig. 6C is a view schematically showing a cut surface formed in the powder compact 10 by a wire saw.

Fig. 7 is a graph showing how the saw wire running speed and the cutting speed affect the shape of the molded body piece.

Fig. 8 is a graph showing how the saw wire running speed and the cutting speed affect the shape of the molded body piece.

Description of the symbols

10 … powder molded bodies; 20 … fixing base; 30a, 30b, 30c … rollers; 40 … saw wire; 50 … support means; 60 … liquid; 70 … grooves; 100 … wire saw device.

Detailed Description

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 step (S10) of pulverizing a powder of an alloy for R-T-B sintered magnets (R is a rare earth element and must contain at least one element selected from Nd, Pr, and Ce, T is at least one transition metal and must contain Fe, and B is boron);

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;

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) includes a step of cutting the powder compact immersed in the liquid by the traveling metal wire.

According to the method for producing an R-T-B sintered magnet of the present invention, since the powder compact is cut with the wire saw in a state of being immersed in the liquid, it is not necessary to prepare an inert atmosphere. Examples of the liquid that can be used in the embodiment of the present invention are oils such as mineral oil and synthetic oil.

Conventionally, in order to cut a powder compact with a wire saw, it is considered necessary to bring hard abrasive grains fixed to the surface of a metal wire constituting the wire saw into contact with the powder compact and scrape off a part of the powder compact by friction. However, as a result of experiments conducted by the present inventors, it was found that when a traveling metal wire rod is brought into contact with a powder compact immersed in a liquid, the powder compact can be ground and cut by using only the metal wire rod to which abrasive grains are not fixed. As a result of the investigation by the inventors, it has been found that the powder particles constituting the powder compact can be scraped off by generating a high-speed liquid flow (jet flow) in a region where the metal wire rod traveling at a speed in a predetermined range contacts the powder compact and in the vicinity thereof. It is considered that a part of the powder particles scraped off from the powder compact is sandwiched between the metal wire rod and the powder compact with the liquid flowing at a high speed, and the powder compact exhibits the same grinding function as the free abrasive particles, thereby promoting the cutting of the powder compact. From the mechanism of cutting the powder compact with the saw wire in a liquid, it is considered that the shape and form of the surface of the saw wire are not particularly limited. In other words, the surface of the saw wire can also be as smooth as a normal piano wire.

In the cutting step, the running speed of the wire is preferably 300 m/min or more, and the tension of the wire at this time is preferably 3kgf (29.4N) or more, for example, 15kgf (147N) or less. When the running speed of the saw wire is less than 300 m/min, a sufficient flow rate required for cutting the powder compact cannot be obtained, and when the tension of the saw wire is less than 3kgf, the saw wire is bent, and the flatness of the cut surface may be lowered. When the tension of the saw wire exceeds 15kgf, problems such as breakage of the saw wire may occur. In the cutting step, the cutting speed (workpiece conveying speed) in the direction perpendicular to the running direction of the saw wire is preferably 100 mm/min or more. This is because, when the cutting speed is less than 100 mm/min, the time required for the cutting step becomes long, and the production efficiency is lowered.

When the diameter of the saw wire is 200 μm or more, the running speed of the saw wire can be 500 m/min or more. The higher the speed of travel of the saw wire, the higher the cutting speed can be. For example, when the diameter of the saw wire is 250 μm and the running speed of the saw wire is 500 m/min or more, the cutting speed can be 250 mm/min or more.

One of the advantages of cutting the powder compact in a liquid is that the temperature rise due to frictional heat in the portion where the powder compact and the wire saw are in contact with each other can be suppressed, and the generated heat is easily dispersed in the liquid. In the present embodiment, if the powder molded body which has a high temperature due to the generated frictional heat in the atmosphere reacts with oxygen or water vapor in the atmosphere, the oxygen concentration in the finally obtained sintered magnet increases and the magnet characteristics deteriorate, and such a problem can be avoided.

Another advantage of cutting the powder compact in a liquid is that the powder particles shaved off from the powder compact by the wire saw are precipitated in the liquid and easily recovered. In a preferred embodiment, the step of preparing the powder compact includes a step of molding the powder by wet pressing. In this case, the wet pressing is preferably performed by adding the same kind of liquid as the liquid in the cutting step to the powder. This is because the particles of the powder scraped off from the powder compact in the cutting step are easily recovered from the liquid and reused.

Further, it was found that the powder compact could be cut even if the cutting speed of the wire saw was directed horizontally, if the wire saw was in a liquid. At least a part (for example, the upper surface) of the powder compact may have irregularities due to the powder pressing step, and it is necessary to cut or polish the surface by a process after the sintering step. According to the embodiment of the present invention, since the step of cutting or polishing can be eliminated, the characteristics of the high-performance magnet can be maintained, and the manufacturing cost can be reduced.

An example of a wire saw device that can be used in the above-described manufacturing method will be described with reference to fig. 2. Fig. 2 is a perspective view showing an example of the configuration of a wire saw device 100 according to an embodiment of the present invention. For reference, the X, Y and X axes are shown as being orthogonal to each other. 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. 2 includes:

rollers

30a, 30b, 30c arranged such that the central axes of rotation are parallel to each other; and a continuous saw wire 40. The

rollers

30a, 30b, 30c are each rotatably supported by the support device 50. The support device 50 can be moved in the vertical direction (positive and negative directions of the Z axis) by a drive device (not shown). The driving device may be driven by a hydraulic cylinder or may be operated by an electric motor. Further, since the cutting is performed along a horizontal direction (X-axis direction) described later, the support device 50 may be moved in the horizontal direction.

The powder compact 10 produced in the molding step (S20) is fixed to the fixing base 20 by a not-shown jig and is disposed inside the tank 70 for storing the liquid 60. In fig. 2, the grooves 70 are shown in dashed lines and the height of the surface of the liquid 60 is shown in dotted lines. In the example of fig. 2, the entire powder compact 10 is immersed in the liquid 60. Instead of the support device 50 moving in the vertical and horizontal directions, the fixing base 20 may be configured to move in the vertical and horizontal directions.

Specific examples of the step of producing the powder compact 10 will be described later. Note here that the powder compact 10 is not a sintered body, but a green compact of powder before sintering. The powder compact is obtained by molding a powder of an alloy for an R-T-B sintered magnet (R is a rare earth element and must contain at least one selected from Nd, Pr, and Ce, T is at least one transition metal and must contain Fe, and B is boron) by wet pressing or dry pressing in an oriented magnetic field.

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. A plurality of grooves are provided on the side surfaces of the

rollers

30a, 30b, and 30 c. The saw wire 40 is wound in sequence around a plurality of grooves provided in the

rollers

30a, 30b, and 30 c. The center-to-center spacing (pitch) of the plurality of grooves defines the width of the element divided by cutting with the wire saw. Both ends of the saw wire 40 are wound around, for example, a wire rewinding shaft not shown.

The saw wire 40 of the embodiment of the present invention is a metal wire rod having no abrasive grains fixed to the surface thereof. In the related art wire saw, a saw wire has a wire (core wire) and abrasive grains on an outer peripheral surface of the wire. The average particle diameter of the abrasive grains is, for example, several μm to several tens μm. A typical example of such abrasive grains is artificial diamond, which has a hardness higher than that of rare earth alloys. Unlike the conventional saw wire, the saw wire 40 of the present embodiment is made of a metal material such as carbon steel, and can be used without being stretched even if a tension of 3kgf or more, for example, is applied in the cutting step. The metal wire material that can be used for the saw wire 40 may be, for example, a piano wire, a high-tension steel wire, or the like. The surface of the saw wire 40 may also be plated. The diameter of the saw wire 40 is, for example, in the range of 100 μm to 350 μm, preferably in the range of 200 μm to 300 μm. When the diameter of the saw wire 40 is less than 100 μm, there is a problem in that the saw wire 40 is elongated in cutting due to insufficient strength. The larger the diameter of the saw wire 40 is, the more the discharge of the cutting dust is improved, but the amount of the cutting dust is increased, and therefore, it is preferably 350 μm or less.

At the time of cutting, the

rollers

30a, 30b, 30c and the recovery bobbin rotate. The direction of rotation of the

rollers

30a, 30b, 30c depends on their arrangement and the way the saw wire 40 is suspended. In the wire saw device 100 shown in fig. 2, the

rollers

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

If a prescribed length of saw wire 40 is wound on a take-up reel, the take-up reel and the

rollers

30a, 30b, 30c are rotated in reverse. This operation is repeated by moving the wire 40 in the reverse direction, and the wire 40 can be reciprocated (moved).

In the present embodiment, the step of cutting the powder compact 10 by the saw wire 40 is performed in a state where the powder compact 10 is immersed in the liquid 60. When the powder compact 10 is a powder compact formed by wet pressing, a preferable example of the liquid 60 is an oil agent of the same kind as a dispersion medium such as an oil agent (mineral oil or synthetic oil) used in wet pressing.

When the powder compact 10 is processed by the wire saw device 100, the powder particles constituting the powder compact 10 become cutting powder and fall off from the portion cut by the saw wire 40. These cutting powders are portions where powder particles constituting the powder compact 10 are detached from the powder compact 10, and each particle does not have a rough fracture surface like cutting powders (chips) of metal. The shape and size of the particles constituting the cutting powder scraped off from the powder compact before sintering by the saw wire are the same as those of the powder particles used for producing the powder compact 10. The present inventors studied to reuse the cutting powder. In the case of cutting a hard sintered body obtained by sintering a powder compact, the cutting powder is particles or a combination of particles in which grain growth or composition changes due to a chemical reaction by sintering. Therefore, even if they are mixed with the powder of a rare earth magnet and reused, there is a high possibility that the magnet characteristics are deteriorated. 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 therefore, the cutting powder is easily reused.

According to the study of the present inventors, it has been found that when a conventional saw wire with abrasive grains is used, the magnet characteristics may be deteriorated when the rare earth alloy powder particles scraped off from the powder compact 10 are recovered and a sintered magnet is produced from the rare earth alloy powder compact including the recovered powder particles. This is because the recovered powder contains abrasive grains detached from the saw wire 40. A typical example of the material of the abrasive grains is diamond, which is made of carbon. It is known that the mixing of the diamond particles causes pores (voids) in the sintering process, and the magnetic properties (particularly, corrosion resistance) can be deteriorated. However, when the wire saw 40 made of a metal wire material without abrasive grains is used, the recovered powder (cutting powder) does not contain abrasive grains, and a high-performance magnet can be manufactured with high yield.

In addition, when the powder compact 10 is produced by wet pressing, if wire saw processing is performed in the same type of oil as the dispersant, the recovered powder (cutting powder) can be used as it is for wet pressing, and the production efficiency is increased.

The method for producing the R-T-B sintered magnet according to the present embodiment will be described in detail below.

S10: grinding process

In the grinding step (S10), powder of an alloy for R-T-B sintered magnets is prepared. The composition of the alloy for an R-T-B sintered magnet, the production process of the alloy, and the preparation process of the alloy powder will be described in order below.

Composition of alloy for R-T-B sintered magnet

R is a rare earth element and must contain at least one 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-Tb, Nd-Ce-Dy, Nd-Ce-Tb, Nd-Ce-Dy-Tb, Nd-Pr-Ce-Dy, Nd-Pr-Ce-Tb, and Nd-Pr-Ce-Dy-Tb.

In R, Dy and Tb are improved by H_cJThe aspect (1) exerts an effect. In addition to the above elements, other rare earth elements such as La may be contained, and cerium alloy (mischmetal) or didymium may be used. R may not be a pure element, and may contain impurities unavoidable in production within an industrially available range. 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%). By mixing R in R-T-B sintered magnetThe amount is 31 mass% or less, and the oxygen content is 500ppm to 3500ppm (preferably 500ppm to 3200ppm, and more preferably 500ppm to 2500 ppm), higher magnetic properties can be obtained.

T contains iron (including a case where T actually consists of iron), and 50% or less thereof may be substituted with cobalt (Co) in terms of a mass ratio (including a case where T actually consists of iron and cobalt). Co is effective in 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 for example, 0.9 to 1.2% by mass is a preferable range. If the content is less than 0.9% by mass, a high H content may not be obtained_cJWhen it exceeds 1.2% by mass, B_rSometimes decreasing. Part of B can be substituted with C (carbon).

In addition to the elements mentioned above, to increase H_cJAn M element may be added. 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 when it exceeds 5.0 mass%, B_rSometimes decreasing. In addition, inevitable impurities can be also allowed.

The content of N (nitrogen) in the R-T-B sintered magnet is preferably 50ppm to 600 ppm. The content of C (carbon) in the R-T-B sintered magnet is preferably 50ppm to 1000 ppm.

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

The production process of the alloy for R-T-B sintered magnet is exemplified. An alloy ingot can be obtained by an ingot casting method in which a metal or an alloy having the above-described composition is melted and placed in a mold. Further, the alloy flakes can be produced by a quenching method typified by a strip casting method or a centrifugal casting method, in which a melt is rapidly quenched by being brought into contact with a single roll, a twin roll, a rotating disk, a rotating cylinder mold, or the like, to produce 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 one of an ingot casting method and a quenching method may 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 is generally in the range of 0.03mm to 1mm in thickness and in the form of a thin sheet. The alloy melt starts to solidify from the contact surface of the cooling roll (roll contact surface), and crystals grow in a columnar shape in the thickness direction from the roll contact surface. The quenched alloy is cooled in a shorter time than an alloy (ingot alloy) produced by a conventional ingot casting method (die casting method), and therefore, the structure is refined and the crystal grain size is small. In addition, the area of grain boundaries is large. The R-rich phase spreads widely in the grain boundary, and therefore, the dispersibility of the R-rich phase is excellent when the quenching method is used. Therefore, the hydrogen pulverization method is likely to cause fracture at grain boundaries. By hydrogen-pulverizing the quenched alloy, the size of hydrogen pulverized powder (coarse pulverized powder) can be, for example, 1.0mm or less. The coarsely pulverized powder thus obtained is finely pulverized by, for example, a jet mill.

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

The powder of a rare earth 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 can be used in order to avoid heat generation, risk of ignition, reduction of oxygen content as an impurity, and to achieve high performance of the magnet.

The object to be pulverized (coarsely pulverized powder) charged into the jet mill is pulverized into a fine powder having a particle size distribution with an average particle size (median diameter: d50) of 2.0 μm or more and 4.5 μm or less, for example, and then moved to a cyclone collector. Cyclone trapping devices are used to separate powder from a gas stream carrying the powder. Specifically, coarsely pulverized powder of an alloy for an R-T-B system sintered magnet is pulverized by 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 by a powder collector.

S20: shaping step

In the molding step (S20), a powder compact is produced using the powder obtained in the pulverization step (S10).

In the present embodiment, a powder compact is produced from the above powder by pressing in a magnetic field. When the pressing is performed in a magnetic field, the powder compact is preferably formed by pressing in an inert gas atmosphere or wet pressing from the viewpoint of suppressing oxidation. In particular, in wet pressing, the surfaces of the particles constituting the powder compact are coated with a dispersant such as an oil solution, and contact with oxygen or water vapor in the atmosphere is suppressed. Therefore, oxidation of the particles 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 molded body thus formed has, for example, 4g/cm³Above 5g/cm³The following densities.

Dispersing media

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

Preferred dispersion media for use in the present invention include mineral oils and synthetic oils. Although the type of the mineral oil or the synthetic oil is not particularly limited, when the kinematic viscosity at room temperature exceeds 10cSt, the viscosity increases, so that the bonding force between the alloy powders is increased, and the orientation of the alloy powders during wet molding in a magnetic field may be adversely affected. Therefore, the kinematic viscosity of the mineral oil or the synthetic oil at room temperature is preferably 10cSt or less. When the fractionation temperature of the mineral oil or the synthetic oil exceeds 400 ℃, it is difficult to remove the oil from the molded body, and the amount of residual carbon in the sintered body may increase to lower the magnetic properties. 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 one.

Preparation of the slurry

The obtained alloy powder was mixed with a dispersion medium to obtain a slurry.

The mixing ratio of the alloy powder and the dispersion medium is not particularly limited, and the concentration of the alloy powder in the slurry 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 may be prepared separately, and the alloy powder and the dispersion medium may be 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 containing a dispersion medium 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 into the dispersion medium in the container to obtain a slurry. In this case, it is preferable that an atmosphere of nitrogen and/or argon is also formed in the container, and the obtained alloy powder is directly recovered in the dispersion medium without contacting the atmosphere to prepare a slurry. Alternatively, the coarsely pulverized powder may be wet-pulverized while being held in a dispersion medium by 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 to obtain a powder compact having a predetermined size and shape. In the conventional technique, the powder compact is usually sintered to obtain a sintered body, but in the present embodiment, the powder compact is divided by a wire saw before sintering as described below.

S30: cutting step

In the cutting step (S30), the powder compact is cut and divided into a plurality of compact pieces.

The cutting of the powder compact in this step is performed by a wire saw device shown in fig. 2, for example. Fig. 3A and 3B are front views for explaining a process of cutting the powder compact 10 immersed in the liquid 60 by the saw wire 40. Fig. 3A shows a state before the cutting step is started, and fig. 3B shows a state in the middle of the cutting step. The broken line in the powder compact 10 shown in fig. 3B schematically shows the position of the saw wire 40 in cutting the powder compact 10. When the position of the saw wire 40 indicated by the broken line moves downward from the upper surface of the powder compact 10 and reaches the bottom surface of the powder compact 10, the powder compact 10 is divided into a plurality of compact pieces.

In the illustrated example, the saw wire 40 travels in the Y-axis direction at a predetermined speed, and moves in a direction (negative direction of the Z-axis) perpendicular to the travel direction of the saw wire 40. The direction orthogonal to the running direction of the saw wire 40 is the cutting direction, and the speed in this direction (cutting speed) is set to 100 mm/min or more, for example. In the example shown in fig. 3B, the traveling saw wire 40 moves in the negative direction of the Z axis with respect to the powder compact 10 in a stationary state, but the powder compact 10 may be raised in the positive direction of the Z axis together with the fixing base 20.

Fig. 4A and 4B are side views for explaining a process of cutting the powder compact 10 immersed in the liquid 60 by the saw wire 40. Fig. 4A shows a state before the cutting process is started, and fig. 4B shows a state in the middle of the cutting process. In the illustrated example, one powder compact 10 is divided into 8 compact pieces.

The diameter of the saw wire 40 is, for example, 100 μm to 350 μm. The traveling speed of the saw wire 40 (saw wire linear speed) may be set to a range of, for example, 100 m/min to 800 m/min. On the other hand, the cutting speed (the conveying speed of the saw wire with respect to the powder compact 10 in the negative direction of the Z axis in fig. 2) 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, 3kgf to 15 kgf. The tension can be adjusted by, for example, adjusting the distance of the roller 30c relative to the

rollers

30a and 30 b. The powder compact 10 can be divided into compact pieces having a thickness of, for example, about 1 to 10mm by wire saw cutting. As shown in fig. 4B, the thickness of the molded body piece is determined according to the interval of the saw lines 40 and the diameter of the saw lines 40.

By carrying out the wire saw process in a liquid, there is also an advantage of facilitating the discharge of the cutting powder. Further, as described above, by performing the immersion (cutting in oil) of the powder compact 10 in a state of being immersed in a dispersion medium (mineral oil or synthetic oil) used in the production of the powder compact 10 by wet pressing, it is possible to recover powder particles precipitated in a liquid in the wire saw process and reuse the recovered powder particles as they are in the molding step.

Fig. 5A and 5B are side views for explaining a process of cutting the powder compact 10 immersed in the liquid 60 in a horizontal direction by the saw wire 40. In the illustrated example, the

rollers

30a, 30b, and 30c are moved in the horizontal direction relative to the powder compact 10 in the cutting step. Before the steps described with reference to fig. 3A to 4B are performed, the surface of the powder compact 10 can be made flat by performing a horizontal incision with the saw wire 40. At least a part (for example, an upper surface) of the surface of the powder compact 10 may have irregularities due to the powder compacting step. For example, after filling the hole of the die of the powder compacting apparatus with powder, before pressing the powder with the punch, a "filter cloth" is disposed between the punch and the powder, and a dispersant (oil agent) can be discharged through the filter cloth. In this case, unevenness can be formed on the upper surface of the obtained powder compact by the filter cloth.

In the embodiment of the present invention, since the uneven surface is cut off by a saw wire before the sintering step, the step of cutting or polishing for flattening can be omitted after the sintering step.

Fig. 6A to 6C are schematic views showing cut surfaces formed on the powder compact 10 by a wire saw. By the step (first treatment step) described with reference to fig. 5A and 5B, the traveling saw wire 40 moves along the broken line 11c of fig. 6A with respect to the powder compact 10 immersed in the liquid 60, thereby thinly cutting the rough surface region 10T of the powder compact 10 and forming the first cut surface 11 orthogonal to the Z-axis direction. Then, by performing the step (second treatment step) described with reference to fig. 4A and 4B, a plurality of second cut surfaces 12 intersecting the first cut surfaces 11 are formed. In the second processing step, the second cut surface 12 is formed by moving the running saw wire along the broken line 12 c. The first and second processing steps may be performed by using the same wire saw device, or may be performed by using different wire saw devices. In other words, the second processing step may be performed by the same wire saw in a state of being immersed in the same liquid as the liquid in which the powder compact is immersed in the first processing step, or may be performed by a different wire saw in a state of being immersed in a different liquid.

In the example shown in fig. 6A to 6C, the first cut surface 11 is parallel to the horizontal plane, and the second cut surface 12 is orthogonal to the first cut surface 11. The directions of the first cut surface 11 and the second cut surface 12 are not limited to this example.

S40: sintering step

In the sintering step (S40), each of the plurality of molded body pieces is sintered to produce a plurality of sintered bodies. That is, the respective molded body pieces cut by the wire saw process described above are sintered to obtain an R-T-B-based sintered magnet (sintered body). The sintering step of the molded body sheet can be performed at, for example, 0.13Pa (10 Pa)^－3Torr) or less, preferably 0.07Pa (5.0X 10)^－4Torr) or less, at a temperature in the range of, for example, 1000 to 1150 ℃. In order to prevent oxidation due to sintering, the residual gas in the atmosphere may be replaced with an inert gas such as helium or argon. The obtained sintered body is preferably subjected to an additional heat treatment such as aging treatment. By this heat treatment, the magnetic properties can be improved. The heat treatment conditions such as the heat treatment temperature and the heat treatment time can be known. The R-T-B sintered magnet thus obtained is subjected to grinding, polishing, surface treatment and magnetization as necessary to complete the final R-T-B sintered magnet.

In a preferred embodiment, the method for producing an R-T-B sintered magnet according to the present invention further includes a diffusion step of diffusing a heavy rare earth element RH (RH is at least one of Tb, Dy, and Ho) from the surface of the sintered body into the interior. When the heavy rare earth element RH is diffused from the surface of the sintered body to the inside, the coercive force can be effectively increased. The method of the diffusion step is not particularly limited. A known method can be employed.

(examples)

So as to reach the Nd: 22.6%, Pr: 7.8%, B: 0.9%, Co: 0.5%, Al: 0.1%, Cu: 0.2%, Ga: 0.4% (all by mass%), and the balance of Fe, and the alloy was produced by strip casting. The obtained alloy was subjected to hydrogen pulverization to obtain a coarsely pulverized powder.

Next, to the obtained coarsely pulverized powder, zinc stearate as a lubricant was added in an amount of 0.04 mass% relative to 100 mass% of the coarsely pulverized powder, and after mixing, the mixture was dry-pulverized in a nitrogen gas flow using a jet mill to obtain a particle diameter D₅₀A 4 μm fine powder (alloy powder). The above-described finely pulverized powder was immersed in a mineral oil having a fractionation temperature of 250 ℃ and a kinematic viscosity of 2cSt at room temperature in a nitrogen atmosphere to prepare a slurry. The slurry concentration was 85 mass%. The slurry obtained was molded in a magnetic field (wet molding) to prepare a powder molded body. The size of the powder compact was 80 mm. times.45 mm. times.60 mm.

The powder compact was divided into 8 compact pieces by a wire saw (a metal wire rod made of piano wire) having a diameter of 250 μm. The cutting with the wire saw was performed in a state where the powder compact was immersed in a liquid (the same liquid as the above-mentioned mineral oil used in the molding). Each powder compact was cut by 8 saw wires (multi-saw wire) running in parallel. The tension applied to the wire during cutting was 10kg, and the roller interval was 250 mm.

Fig. 7 is a graph showing how the wire travel speed and the cutting speed affect the shape of the formed body piece. The horizontal axis of the graph represents the wire travel speed [ m/min ] and the vertical axis represents the cut-in speed [ mm/min ]. The term "x" in the graph means that "cracks" are generated in a part of the molded body piece divided by cutting with a wire saw, and the term "good" means that such cracks are not generated in the molded body piece and the molded body piece can be divided into good shapes.

When a saw wire having a diameter of 250 μm is used, a molded body piece having no crack can be obtained at a cutting speed of 100 to 150 mm/min at a travel speed of 300 m/min. Further, at a traveling speed of 500 m/min, a molded body sheet free from cracks can be obtained at a cutting speed of 250 mm/min. Further, at a running speed of 700 m/min, the saw wire was not bent during cutting even at a cutting speed of 400 mm/min, and a molded body piece without cracks could be obtained.

In addition, when a saw wire having a diameter of 160 μm is used, the sheet can be divided into good formed pieces when both the running speed and the cutting speed are low. Since the smaller the diameter of the saw wire, the more easily the saw wire is extended and easily bent, it is considered that when the saw wire is moved at a high speed by applying a high tension, cracks or chips are easily generated at the time of cutting the powder compact. Therefore, the diameter of the saw wire (metal wire rod) is preferably 200 μm or more. Further, the larger the diameter of the saw wire, the more the cutting work increases, but normal cutting can be performed.

Further, it was confirmed that for comparison, even if the powder compact left in the air is intended to be cut only with the metal wire rod, the cutting cannot be performed normally, and the contact between the metal wire rod and the powder compact which are traveling needs to be performed in a liquid (preferably oil).

Fig. 8 shows the results of an experiment in which the upper surface region of the powder compact was cut in oil in the horizontal transverse direction by one saw wire as shown in fig. 5A and 5B. The "cross feed" is the horizontal cross cut speed and the "line speed" is the speed of travel of the saw wire. When a saw wire having a diameter of 250 μm is used, a molded body piece having no crack can be obtained at a cutting speed of 100 to 300 mm/min at a running speed of 300 m/min. Further, if the traveling speed is 500 m/min, a molded body sheet free from cracks can be obtained at a cutting speed of 300mm to 500/min. Further, even at a traveling speed of 700 m/min, a molded body sheet free from cracks can be obtained at a cutting speed of 500 mm/min.

In order to cut the vicinity of the upper surface of the powder compact by the "lateral conveyance", the powder compact preferably has sufficient "hardness". Hardness of powder molded bodyThe degree can be evaluated from, for example, the molding pressure and density at the time of powder molding. It is found that the density of the powder molded article in the presence of the powder in the air (excluding the liquid such as oil) is less than 4g/cm³This may cause a problem that the cut surface becomes uneven. Therefore, the density of the powder compact is preferably 4g/cm³The above.

Claims

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

a step of preparing a powder of an alloy for R-T-B sintered magnets, wherein R is a rare earth element and must contain at least one element selected from Nd, Pr, and Ce, T is at least one 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,

the cutting step includes a step of cutting the powder compact immersed in a liquid by a traveling metal wire.

2. The method of manufacturing an R-T-B sintered magnet according to claim 1, wherein: in the cutting step, the metal wire rod has a running speed of 300 m/min or more.

3. The method of manufacturing an R-T-B sintered magnet according to claim 1 or 2, wherein: in the cutting step, the tension of the metal wire rod is 29.4N or more.

4. The method for producing an R-T-B sintered magnet according to any one of claims 1 to 3, wherein:

in the cutting step, the cutting speed in the direction perpendicular to the traveling direction of the metal wire rod is 100 mm/min or more.

5. The method for producing an R-T-B sintered magnet according to any one of claims 1 to 4, wherein:

the step of preparing the powder compact includes a step of molding the powder by wet pressing.

6. The method of manufacturing an R-T-B sintered magnet according to claim 5, wherein: the wet pressing is performed by mixing the powder with a liquid of the same kind as the liquid in the cutting step.

7. The method for producing an R-T-B sintered magnet according to any one of claims 1 to 6, wherein:

further comprising a step of recovering the particles of the powder shaved off from the powder compact in the cutting step from the liquid.