JP7645121B2 - Alloy for R-T-B permanent magnet and method for producing R-T-B permanent magnet - Google Patents

Description

The present invention relates to an alloy for R-T-B permanent magnets and a method for manufacturing R-T-B permanent magnets.

Patent document 1 describes an invention relating to alloy flakes for rare earth magnets, characterized by having a thickness and surface roughness within a specific range. By setting the surface roughness of the casting rotating roll surface within a specific range, the fine R-rich phase region in the alloy flakes for rare earth magnets is reduced, improving the magnetic properties.

Patent Document 2 describes an invention relating to a method for producing rare earth-containing alloy flakes, characterized in that the shape of the casting surface of the rotating cast roll is a specific shape and the surface roughness of the casting surface of the rotating cast roll is within a specific range. The R-T-B alloy flakes produced by this production method have a reduced fine R-rich phase region and are excellent in homogeneity.

Patent document 3 describes an invention relating to a raw material alloy for R-T-B magnets, characterized in that the volume ratio of the area where secondary dendrite arms are formed is within a specific range. The formation of secondary dendrite arms refines the structure, so that the coercive force of the R-T-B sintered magnet obtained using the raw material alloy for R-T-B magnets is improved.

JP 2003-188006 A JP 2004-181531 A International Publication No. 2014/156181

The objective of the present invention is to provide an alloy for R-T-B permanent magnets that can be used to produce R-T-B permanent magnets with improved magnetic properties.

In order to achieve the above object, the present invention provides an R-T-B system permanent magnet alloy,
The alloy for an R-T-B system permanent magnet contains R, T and B, where R is a rare earth element, T is a transition metal element, and B is boron, and the area ratio of a fine linear R-rich phase-containing structure in a cross section is 0.5% or more and 20% or less.

The total area ratio of the fine linear R-rich phase-containing structure and the fine dotted R-rich phase-containing structure in the cross section may be 1.0% or more and 24.0% or less.

The total area ratio of the chill crystal structure, the fine linear R-rich phase-containing structure, and the fine dotted R-rich phase-containing structure in the cross section may be 1.0% or more and 27.0% or less.

The R content may be 29.0 mass% or more and 33.5 mass% or less, and the B content may be 0.70 mass% or more and less than 0.96 mass%.

The method for producing an R-T-B permanent magnet according to the present invention includes a step of crushing the above-mentioned R-T-B permanent magnet alloy.

This is a backscattered electron image of normal tissue. This is a backscattered electron image of chill crystal structure. 1 is a backscattered electron image of a structure containing fine dotted R-rich phase. 1 is a backscattered electron image of a structure containing fine dotted R-rich phase. 1 is a backscattered electron image of a structure containing dot-like R-rich phases. 1 is a backscattered electron image of a structure containing fine linear R-rich phases. 1 is a backscattered electron image of a structure containing fine linear R-rich phases. This is a backscattered electron image of a structure containing large dot-like R-rich phases. FIG.

The present invention will now be described based on the embodiments shown in the drawings.

<Structure of R-T-B based permanent magnet alloy>
The R-T-B system permanent magnet alloy according to this embodiment is an R-T-B system permanent magnet alloy having R, T and B, where R is a rare earth element, T is a transition metal element and B is boron, and in a cross section, the area ratio of a fine linear R-rich phase-containing structure described below is 0.5% or more and 20% or less.

In general, an RTB based permanent magnet alloy contains a main phase which is a columnar crystal having an R ₂ T ₁₄ B type crystal structure, and an R-rich phase which has a higher R content than the main phase.

As shown in the backscattered electron image of Figure 1, in the cross section of an R-T-B permanent magnet alloy, a clear difference can be confirmed between the main phase 11 and the R-rich phase 13, and the majority of the R-rich phase 13 is linear in shape (structure 7, hereafter referred to as normal structure 7). Most of the main phase 11 contained in normal structure 7 is columnar. In normal structure 7, the spacing between linear R-rich phases (hereafter referred to as linear R-rich phases) is 2 μm or more on average. Furthermore, normal structure 7 is a structure in which primary dendrites extend in the direction of solidification of the alloy, in other words, in the thickness direction of the alloy.

In addition to the normal structure 7, R-T-B alloys for permanent magnets also contain a structure that includes a main phase that is not columnar. A main phase that is not columnar is called a non-columnar crystal, and a structure that includes non-columnar crystals is called a non-columnar crystal structure. A non-columnar crystal structure is also a structure in which the primary dendrites do not necessarily extend in the direction of solidification of the alloy.

The non-columnar crystal structures contained in R-T-B permanent magnet alloys are mainly classified into six types when distinguishing between structure 1a and structure 1b, which will be described later. Below, the six types of non-columnar crystal structures are explained using drawings (backscattered electron images).

Figure 2 includes chill crystal structure 1, which includes structures 1a and 1b. Structure 1a is more different from normal structure 7 than structure 1b. Structure 1a shows no clear difference between the main phase and the R-rich phase. The brightness of structure 1a in the backscattered electron image is intermediate between that of the main phase and the R-rich phase. Furthermore, structure 1a shows smooth variations in shading in the backscattered electron image.

Structure 1b is closer to normal structure 7 than structure 1a. However, in structure 1b, no clear difference is observed between the main phase and the R-rich phase. The brightness of structure 1b in the backscattered electron image is intermediate between the main phase and the R-rich phase. Furthermore, the variation in shading in the backscattered electron image of structure 1b is smooth. Also, unlike structure 1a, structure 1b partially contains linear R-rich phase.

In the following description, when simply referring to chill crystal structure, no distinction is made between structure 1a and structure 1b.

Figures 3 and 4 contain a fine dot-like R-rich phase-containing structure 3 (hereinafter, sometimes referred to as structure 3). Note that different alloy pieces are observed in Figures 3 and 4. In structure 3, a clear difference is seen between the main phase and the R-rich phase. However, compared to the normal structure 7 shown in Figure 1, in structure 3, the R-rich phase is aggregated in dots and is excessively dense. Furthermore, compared to the dot-like R-rich phase-containing structure 4 described below, the dot-like R-rich phase is finer.

The dot-like R-rich phases contained in the structure 3 have a circle-equivalent diameter of 0.3 to 1.5 μm, and the number of dot-like R-rich phases having a circle-equivalent diameter of 0.3 to 1.5 μm per unit area is 0.25 pieces/ ^μm2 or more.

Furthermore, structure 3 can be cut into sections where the circle equivalent diameter and density of the dot-like R-rich phase are subtly different locally. The actual cut state is shown by dotted lines in Figure 4. The cut polygonal region has a long axis of 10 μm to 120 μm and a short axis of 5 μm to 80 μm. The long axis refers to the longest distance between two parallel lines that contact the polygon from both sides, and the short axis refers to the shortest distance between two parallel lines that contact the polygon from both sides.

From the above, a structure in which the number of dot-like R-rich phases having a circle-equivalent diameter of 0.3 to 1.5 μm per unit area is 0.25 pieces/ ^μm2 or more, and when cut into polygons, the major axis and minor axis are within the above-mentioned ranges, is defined as structure 3.

Figure 5 includes dot-like R-rich phase-containing structure 4 (hereinafter, sometimes referred to as structure 4). Structure 4 has a larger dot-like R-rich phase itself than structure 3. Furthermore, in the area where the dot-like R-rich phase aggregates, a phase that is darker than the main phase is present.

Figures 6 and 7 contain a fine linear R-rich phase-containing structure 5 (hereinafter sometimes referred to as structure 5). Note that different alloy pieces are observed in Figures 6 and 7. In structure 5, clear differences can be seen in the main phase and the R-rich phase. However, compared to the normal structure 7 shown in Figure 1, in structure 5, the linear R-rich phase is thin and excessively dense.

The length of the linear R-rich phase contained in structure 5 is 4 to 125 μm, the width is 0.3 to 10 μm, and the spacing between the linear R-rich phases is 1.0 to 1.8 μm.

Furthermore, structure 5 can be cut into sections where the length, width, and density of the linear R-rich phase are subtly different locally. The actual cut state is shown by dotted lines in Figure 7. The cut polygonal regions have a long axis of 30 μm to 200 μm and a short axis of 5 μm to 150 μm.

From the above, a structure that has linear R-rich phases with a length of 4 to 125 μm and a width of 0.3 to 10 μm, the spacing between the linear R-rich phases is 1.0 to 1.8 μm, and when cut into polygons, the major axis and minor axis exist within the range of the polygons that fall within the above ranges is defined as structure 5.

Figure 8 includes a large dot-like R-rich phase-containing structure 6 (hereinafter sometimes referred to as structure 6). In structure 6, the R-rich phase is aggregated to a large extent. The brightness of the main phase surrounding the aggregated R-rich phase is slightly higher. In addition, linear R-rich phases may be sandwiched between the large aggregated R-rich phases.

The seven normal structures and the six types of non-columnar crystal structures described above can be visually distinguished from the backscattered electron image of the cross section of the R-T-B permanent magnet alloy. There is no particular limit to the magnification of the backscattered electron image used to distinguish each structure. For example, it may be 200 to 500 times.

The inventors have found that an R-T-B system permanent magnet made using an R-T-B system permanent magnet alloy in which the area ratio of structure 5 is 0.5% or more and 20.0% or less has good magnetic properties. The area ratio of structure 5 may be 1.0% or more, or 3.4% or more. It may also be 19.2% or less.

The total area ratio of structure 3 and structure 5 may be 1.0% or more and 24.0% or less. It may be 1.1% or more, or 3.6% or more. It may be 23.4% or less, or 3.6% or more and 23.4% or less.

The total area ratio of structure 1a, structure 1b, structure 3, and structure 5 may be 1.0% or more and 27.0% or less. It may be 1.3% or more, 4.0% or more, or 4.3% or more. It may also be 26.2% or less.

The total area ratio of structure 1a, structure 1b, structure 3, structure 4, structure 5, and structure 6 may be 1.2% or more and 36.0% or less, or 1.4% or more and 30.3% or less.

<Composition of R-T-B based permanent magnet alloy>
There is no particular restriction on the composition of the R-T-B permanent magnet alloy. R represents at least one rare earth element. The rare earth elements refer to Sc, Y, and lanthanoid elements belonging to Group 3 of the long periodic table. The lanthanoid elements include, for example, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The rare earth elements are classified into light rare earth elements and heavy rare earth elements, and the heavy rare earth elements refer to Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and the light rare earth elements are rare earth elements other than the heavy rare earth elements. In this embodiment, from the viewpoint of suitably controlling the manufacturing cost and magnetic properties, Nd and/or Pr may be included as R. In addition, from the viewpoint of improving HcJ in particular, both light rare earth elements and heavy rare earth elements may be included. The content of the heavy rare earth element is not particularly limited, and the heavy rare earth element may not be contained. The content of the heavy rare earth element is, for example, 5 mass % or less (including 0 mass %).

The R content may be 29.0 mass% or more and 33.5 mass% or less. If the R content is small, the HcJ of the resulting R-T-B system permanent magnet tends to decrease. If the R content is large, the Br tends to decrease.

The B content may be 0.70% by mass or more, or 0.80% by mass or more. The upper limit of the B content may be less than 1.0% by mass, less than 0.96% by mass, or 0.90% by mass or less. When the B content is smaller than the stoichiometric ratio, specifically less than 1.0% by mass, the area ratio of each of the above structures is more likely to be within the above range. When the B content is small, HcJ is more likely to decrease. Furthermore, abnormal grain growth is more likely to occur when sintering at high temperatures. Furthermore, Hk/HcJ is less likely to increase when sintering at low temperatures. When the B content is large, abnormal grain growth is more likely to occur. And Br is more likely to decrease.

T is a transition metal element. It may be Fe alone or Fe and Co. The Co content may be 0% by mass or more and 2.0% by mass or less. If the Co content is small, abnormal grain growth is likely to occur when sintering at high temperatures. Also, it is difficult to increase Hk/HcJ when sintering at low temperatures. If the Co content is large, Br and HcJ decrease. Also, the R-T-B system permanent magnet according to this embodiment tends to be expensive.

The R-T-B system permanent magnet alloy may contain M. M is one or more selected from Cu, Ga, Zr and Al. There is no particular limit to the total content of M. It may be 0.1% by mass or more and 2.0% by mass or less.

There is no particular limit to the Cu content. For example, it may be 0.05% by mass or more and 0.50% by mass or less. By including 0.05% by mass or more of Cu, abnormal grain growth is less likely to occur when sintering at high temperatures. In addition, even when sintering at low temperatures, Hk/HcJ tends to be sufficiently high. By including 0.50% by mass or less of Cu, Br tends to be improved.

There is no particular limit to the Ga content. For example, it may be 0% by mass or more and 1.0% by mass or less. By including Ga, abnormal grain growth is less likely to occur when sintering at high temperatures. Furthermore, even when sintering at low temperatures, Hk/HcJ tends to be sufficiently high. Furthermore, HcJ also tends to be improved. By including 1.0% by mass or less of Ga, Br tends to be improved.

There is no particular limit to the Al content. For example, it may be 0.05% by mass or more and 1.00% by mass or less. By including 0.05% by mass or more of Al, HcJ is more likely to be improved. In addition, abnormal grain growth is less likely to occur even when sintering at high temperatures. Furthermore, Hk/HcJ is more likely to be sufficiently high even when sintering at low temperatures. By including 1.00% by mass or less of Al, Br is more likely to be improved.

There is no particular limit to the Zr content. For example, it may be 0.05% by mass or more and 1.00% by mass or less. By including 0.05% by mass or more of Zr, Hk/HcJ tends to be sufficiently high even when sintering at low temperatures. In addition, abnormal grain growth is less likely to occur even when sintering at high temperatures. By including 1.00% by mass or less of Zr, Br tends to be improved.

The Fe content is the substantial balance in the components of the R-T-B permanent magnet alloy. The Fe content being the substantial balance means that the total content of elements other than R, Fe, Co, B, and M is 1.0 mass% or less.

The R-T-B system permanent magnets produced using the alloy for R-T-B system permanent magnets according to this embodiment can be processed into any shape before use. The shape of the R-T-B system permanent magnet is not particularly limited, and can be any shape, such as a rectangular parallelepiped, hexahedron, flat plate, or columnar shape such as a square prism, or a C-shaped cylinder in the cross section of the R-T-B system permanent magnet.

In addition, R-T-B permanent magnets include both magnet products that are made by processing and magnetizing the magnet, and magnet products that are not magnetized.

<Method of manufacturing R-T-B based permanent magnet alloy>
As an example of a method for producing an RTB system permanent magnet alloy, a strip casting method using a casting apparatus shown in FIG. 9 will be described.

The casting apparatus shown in FIG. 9 has a chill roll 21 and a tundish 23. Although not shown in FIG. 9, the casting apparatus may also have well-known components of a casting apparatus, such as a refractory crucible and a collection container. There is no particular limit to the type of refractory crucible. Examples include an alumina crucible, a mullite crucible, and a zirconia crucible. There is no particular limit to the material of the chill roll 21. Examples include copper, copper alloys, copper with a plated or sprayed surface, and copper alloys with a plated or sprayed surface.

First, the raw metals are weighed out to obtain the desired alloy composition for the R-T-B permanent magnet alloy, and then mixed to obtain a raw material mixture.

Next, the obtained raw material mixture is loaded into a refractory crucible, and the loaded raw material mixture is melted to obtain a molten alloy. There are no particular limitations on the method for melting the raw material mixture. For example, a method in which the refractory crucible loaded with the raw material mixture is placed in a high-frequency vacuum induction furnace and heated can be mentioned.

Then, the obtained molten alloy is cast by a strip casting method using the casting device shown in FIG. 9 to obtain a cast alloy ribbon (alloy for R-T-B type permanent magnets). Specifically, the molten alloy 25 is supplied to a cooling roll 21, the inside of which is water-cooled, via a tundish 23. The molten alloy 25 supplied onto the cooling roll 21 is cooled and detached from the cooling roll 21 on the opposite side of the tundish 23, and is recovered as a cast alloy ribbon.

The surface of the cast alloy ribbon that contacts the chill roll 21 is the roll surface, and the surface opposite the roll surface is the free surface. The roll surface is cooled more rapidly by the chill roll 21 than the free surface. Here, non-columnar crystal structures such as structure 5 are more likely to form when the cooling rate is faster than that of normal structures. Among non-columnar crystal structures, chill crystal structures are more likely to form when the cooling rate is particularly fast. Therefore, chill crystal structures and non-columnar crystal structures are more likely to form on the roll surface than on the free surface. Note that the roll surface is on the left side in all of Figures 1 to 8.

There are no particular limitations on the position of the observation range for calculating the area ratio of each of the above structures, but the length parallel to the thickness direction is preferably a length that includes the entire R-T-B permanent magnet alloy, i.e., the thickness of the alloy. This is to ensure that both the vicinity of the roll surface and the vicinity of the free surface are included in the observation range. The length perpendicular to the thickness direction is preferably set to a length of at least 180 μm. In other words, the observation magnification is determined so that the observation range includes a range of at least 180 μm x (thickness of the alloy).

The area ratio of each structure varies mainly depending on the temperature of the molten alloy 25 (casting temperature), the composition of the molten alloy 25, the roll surface roughness Rz of the chill roll 21, the shape of the irregularities that affect the roll surface roughness Rz, the molten metal head pressure 27, the roll circumferential speed of the chill roll 21, and the supply speed of the molten alloy 25 to the chill roll 21 (supply speed per unit contact width between the molten alloy 25 and the chill roll 21). The temperature of the molten alloy 25 is, for example, 1300 to 1600°C. The roll surface roughness Rz of the chill roll 21 is, for example, 10 to 50 μm. The roll circumferential speed of the chill roll 21 is, for example, 0.5 to 2.5 m/s. The supply speed of the molten alloy 25 to the chill roll 21 is, for example, 0.5 to 2.5 kg/(min·cm).

The molten metal head pressure 27 will be explained below. The molten metal head pressure 27 refers to the depth of the molten alloy 25 in contact with the chill roll 21. The higher the molten metal head pressure 27, the stronger the pressure with which the molten alloy 25 is pressed against the chill roll 21 by its own weight. Therefore, the higher the molten metal head pressure 27, the stronger the adhesion between the molten alloy 25 and the chill roll 21, and the higher the heat transfer rate from the molten alloy 25 to the chill roll 21. At the same time, the length of the molten alloy 25 in contact with the chill roll 21 also changes. Therefore, if other conditions are the same, the higher the molten metal head pressure 27, the thicker the cast alloy ribbon will be. In practice, the molten metal head pressure 27 is selected appropriately in accordance with other casting conditions.

As mentioned above, the faster the cooling rate, the easier it is for non-columnar crystal structure (chilled crystal structure) to form. Therefore, the greater the molten metal head pressure 27, the greater the area proportion of non-columnar crystal structure (chilled crystal structure).

One method for increasing the molten metal head pressure 27 is to increase the amount of molten alloy 25 in the tundish 23. One method for increasing the amount of molten alloy 25 in the tundish 23 is to increase the speed at which the molten alloy 25 is supplied to the tundish 23. If the speed at which the molten alloy 25 is supplied to the tundish 23 is increased, the cast alloy ribbon will become thicker. One method for increasing the molten metal head pressure 27 without changing the thickness of the cast alloy ribbon is to increase the peripheral speed of the cooling roll 21.

In other words, when producing a cast alloy ribbon of the same thickness, if the supply rate of the molten alloy 25 is fast and the peripheral speed of the chill roll 21 is fast, the molten metal head pressure 27 will be large, and if the supply rate of the molten alloy 25 is slow and the peripheral speed of the chill roll 21 is slow, the molten metal head pressure 27 will be small.

<Method of manufacturing R-T-B based permanent magnets>
There is no particular limitation on the method for producing an R-T-B system permanent magnet using the R-T-B system permanent magnet alloy produced by the above method. For example, a method having the following steps may be mentioned.
(a) a grinding process for grinding an alloy for an R-T-B system permanent magnet; (b) a molding process for molding the obtained alloy powder; (c) a sintering process for sintering the molded body to obtain an R-T-B system permanent magnet; (d) an aging treatment process for ageing the R-T-B system permanent magnet; (e) a cooling process for cooling the R-T-B system permanent magnet; (f) a processing process for machining the R-T-B system permanent magnet; (g) a grain boundary diffusion process for diffusing a heavy rare earth element into the grain boundaries of the R-T-B system permanent magnet; and (h) a surface treatment process for surface-treating the R-T-B system permanent magnet.

[Crushing process]
The R-T-B based permanent magnet alloy is pulverized (pulverization process). The pulverization process includes a coarse pulverization process in which the alloy is pulverized to a particle size of several hundred μm to several mm, and a fine pulverization process in which the alloy is pulverized to a particle size of about several μm.

(Coarse grinding process)
The R-T-B system permanent magnet alloy is coarsely pulverized until the particle size is about several hundred μm to several mm (coarse pulverization step). This produces a coarsely pulverized powder of the R-T-B system permanent magnet alloy. The coarse pulverization can be performed, for example, by absorbing hydrogen into the R-T-B system permanent magnet alloy, and then releasing the hydrogen based on the difference in the amount of hydrogen absorbed between different phases, thereby causing self-destructive pulverization by dehydrogenation (hydrogen absorption pulverization).

In addition to using hydrogen absorption pulverization as described above, the coarse pulverization process may be performed in an inert gas atmosphere using a coarse pulverizer such as a stamp mill, jaw crusher, or Braun mill.

The oxygen concentration is adjusted by controlling the atmosphere in each manufacturing process. From the viewpoint of obtaining high magnetic properties, the amount of oxygen in the final R-T-B permanent magnet may be reduced. To achieve this, the oxygen concentration in each process from the crushing process to the sintering process described below may be set to 100 ppm or less.

(Fine pulverization process)
After the R-T-B permanent magnet alloy is coarsely pulverized, the obtained coarsely pulverized powder of the R-T-B permanent magnet alloy is finely pulverized until the average particle size is about several μm (fine pulverization step). In this way, finely pulverized powder of the R-T-B permanent magnet alloy is obtained. By further finely pulverizing the coarsely pulverized powder, finely pulverized powder having particles of, for example, 1 μm to 10 μm, or 3 μm to 5 μm can be obtained.

Fine pulverization is performed by further pulverizing the coarsely pulverized powder using a fine pulverizer such as a jet mill, a ball mill, a vibration mill, a wet attritor, etc., while appropriately adjusting conditions such as pulverization time. A jet mill is a method in which a high-pressure inert gas (e.g., _N2 gas) is released from a narrow nozzle to generate a high-speed gas flow, and the high-speed gas flow accelerates the coarsely pulverized powder of the raw alloy, causing collisions between the coarsely pulverized powder of the raw alloy and with a target or a container wall, thereby pulverizing the powder.

When the coarsely crushed powder of the raw alloy is finely crushed, adding a grinding aid such as zinc stearate, urea, stearic acid amide, or oleic acid amide can produce a finely crushed powder with a high degree of orientation during molding.

[Molding process]
The finely pulverized powder is molded into a desired shape (molding process). In the molding process, the finely pulverized powder is filled into a die surrounded by an electromagnet and pressurized to mold the finely pulverized powder into a desired shape. This is done while applying a magnetic field, which causes a specific orientation in the finely pulverized powder, and molding is performed in the magnetic field with the crystal axes oriented. This results in a molded body. The obtained molded body is oriented in a specific direction, resulting in an R-T-B system permanent magnet with stronger magnetic anisotropy.

The pressure applied during molding may be 30 MPa to 300 MPa. The magnetic field applied may be 950 kA/m to 1600 kA/m. The magnetic field applied is not limited to a static magnetic field, and may also be a pulsed magnetic field. A static magnetic field and a pulsed magnetic field may also be used in combination.

As for the molding method, in addition to the dry molding in which the finely pulverized powder is molded as is as described above, wet molding can also be used in which a slurry in which the finely pulverized powder is dispersed in a solvent such as oil is molded.

The shape of the compact obtained by compacting the finely pulverized powder is not particularly limited, and can be any shape depending on the desired shape of the R-T-B system permanent magnet, such as a rectangular parallelepiped, flat plate, column, ring, etc.

[Sintering process]
The compact obtained by molding in a magnetic field and forming into a desired shape is sintered in a vacuum or inert gas atmosphere to obtain an R-T-B system permanent magnet (sintering process). The sintering temperature needs to be adjusted depending on various conditions such as the composition, the pulverization method, the particle size and particle size distribution, etc. The compact is sintered, for example, by heating in a vacuum or in the presence of an inert gas at 1000°C to 1200°C for 1 hour to 48 hours. This causes liquid phase sintering of the finely pulverized powder, and an R-T-B system permanent magnet (a sintered body of an R-T-B system magnet) with an improved volume ratio of main phase particles is obtained. After sintering the compact to obtain a sintered body, the sintered body may be quenched from the viewpoint of improving production efficiency.

[Aging treatment process]
After sintering the compact, the R-T-B system permanent magnet is subjected to aging treatment (aging treatment step). After sintering, the R-T-B system permanent magnet obtained is subjected to aging treatment by holding it at a temperature lower than that at the time of sintering. The aging treatment may be, for example, a two-stage heating at a temperature of 700°C or higher and 1000°C or lower for 10 minutes to 6 hours, and then a one-stage heating at a temperature of 500°C to 700°C for 10 minutes to 6 hours, or a one-stage heating at a temperature of about 600°C for 10 minutes to 6 hours, and the treatment conditions are appropriately adjusted depending on the number of times the aging treatment is performed. Such aging treatment can improve the magnetic properties of the R-T-B system permanent magnet. The aging treatment step may also be performed after the processing step described below.

[Cooling process]
After the R-T-B system permanent magnet is subjected to aging treatment, the R-T-B system permanent magnet is rapidly cooled in an Ar gas atmosphere (cooling step). This makes it possible to obtain the R-T-B system permanent magnet according to this embodiment. The cooling rate is not particularly limited, and may be 30°C/min or more.

[Processing process]
The obtained R-T-B system permanent magnet may be processed into a desired shape as necessary (processing step). Processing methods include, for example, shaping such as cutting and grinding, and chamfering such as barrel polishing.

[Grain boundary diffusion process]
A heavy rare earth element may be further diffused into the grain boundaries of the processed R-T-B system permanent magnet (grain boundary diffusion step). There are no particular limitations on the method of grain boundary diffusion. For example, it may be carried out by attaching a compound containing a heavy rare earth element to the surface of the R-T-B system permanent magnet by coating or vapor deposition, etc., and then carrying out a heat treatment. It may also be carried out by carrying out a heat treatment on the R-T-B system permanent magnet in an atmosphere containing vapor of a heavy rare earth element. Grain boundary diffusion can further improve the HcJ of the R-T-B system permanent magnet.

[Surface treatment process]
The RTB system permanent magnet obtained by the above steps may be subjected to a surface treatment such as plating, resin coating, oxidation treatment, or chemical conversion treatment (surface treatment step).

In this embodiment, a processing step, a grain boundary diffusion step, and a surface treatment step are performed, but these steps do not necessarily have to be performed.

The R-T-B system permanent magnet according to this embodiment obtained in the above manner has good magnetic properties and a wide sintering temperature range. As a result, the R-T-B system permanent magnet according to this embodiment is a magnet that can be produced stably.

The R-T-B permanent magnet according to the present embodiment thus obtained is suitable for use as a magnet in, for example, a surface permanent magnet (SPM) rotating machine with a magnet attached to the rotor surface, an interior permanent magnet (IPM) rotating machine such as an inner rotor brushless motor, or a permanent magnet reluctance motor (PRM). Specifically, the R-T-B permanent magnet according to the present embodiment is suitable for use in spindle motors and voice coil motors for rotating the hard disks of hard disk drives, motors for electric and hybrid cars, motors for electric power steering in automobiles, servo motors for machine tools, motors for vibrators in mobile phones, motors for printers, motors for generators, and the like.

The present invention is not limited to the above-described embodiment, and can be modified in various ways within the scope of the present invention.

The invention will be described in more detail below with reference to examples, but the invention is not limited to these examples.

(Preparation of R-T-B based permanent magnet alloy)
Nd metal (purity 99% by mass or more), an alloy of Nd and Pr (didymium, purity 99% by mass or more), Tb metal (purity 99% by mass or more), ferroboron (Fe content 80% by mass, B content 20% by mass), Fe metal (purity 99% by mass or more), Co metal (purity 99% by mass or more), Zr metal (purity 99% by mass or more), Cu metal (purity 99% by mass or more), Al metal (purity 99% by mass or more), and Ga metal (purity 99% by mass or more) were weighed out to obtain the alloy composition shown in Table 1 below and mixed to obtain a raw material mixture. In Table 1, "T.RE" is the total content (mass%) of rare earth elements (Nd, Pr, Dy, and Tb), and "T.RL" is the total content (mass%) of light rare earth elements (Nd and Pr). "bal." is the remainder. The Fe content is expressed as "bal." to indicate that the Fe content is the remainder when the entire R-T-B system permanent magnet alloy containing elements other than the elements listed in Table 1 is taken as 100 mass %.

The obtained raw material mixture was loaded into an alumina crucible. The alumina crucible loaded with the raw material mixture was placed in a high-frequency vacuum induction furnace, and the atmosphere inside the high-frequency vacuum induction furnace was then replaced with Ar. The high-frequency vacuum induction furnace was then heated to melt the raw material mixture loaded into the alumina crucible, thereby obtaining a molten alloy. The obtained molten alloy was cast by a strip casting method using the casting device shown in Figure 9, to obtain a cast alloy ribbon (R-T-B system alloy for permanent magnets). Casting was performed in an Ar atmosphere.

The temperature of the molten alloy, the roll surface roughness Rz of the chill roll, and the molten metal head pressure were appropriately selected so as to obtain the R-T-B rare permanent magnet alloy shown in Table 2. Specifically, the temperature of the molten alloy (casting temperature) was selected within the range of 1300 to 1600°C, the roll surface roughness Rz of the chill roll was selected within the range of 5 to 50 μm, and the molten metal head pressure was selected within the range of 5 to 25 mm. The material of the chill roll was a copper alloy. The roll peripheral speed of the chill roll and the supply speed of the molten metal to the chill roll (supply speed per unit contact width between the molten metal and the chill roll) were adjusted to the values shown in Table 2 below.

(Measurement of the area ratio of each tissue)
The area ratio of each structure in the RTB system permanent magnet alloy was measured using an electron microscope (manufactured by JEOL Ltd.).

First, samples for evaluation were taken. Specifically, in the process in which one production lot of R-T-B permanent magnet alloy is supplied by the above-mentioned strip casting method, alloy pieces were sampled from the R-T-B permanent magnet alloy supplied at regular time intervals. Thirty alloy pieces were randomly taken from the R-T-B permanent magnet alloy, and each alloy piece was used as an evaluation sample.

Next, the thickness of the evaluation samples was measured. Of the 30 evaluation samples, the second, third, and fourth thickest samples were selected as thick samples, the 14th, 15th, and 16th thickest samples were selected as near-average samples, and the 27th, 28th, and 29th thickest samples were selected as thin samples.

Next, the area ratio of each texture in the cross section was measured for each of the nine selected evaluation samples. Specifically, the observation range was set at 350x magnification of the electron microscope. The observation range was selected from the observation surface of each evaluation sample in an average state. The length of the observation range in the thickness direction was set to the average thickness of each sample. This is to include both the roll surface and the free surface. The length perpendicular to the thickness direction was set to 270 μm. The area ratio of each texture was measured for the nine samples and averaged. The results are shown in Table 2. The "Texture 3 + 5" column in Table 2 represents the total area ratio of texture 3 and texture 5. The "Texture 1a + 1b + 3 + 5" column in Table 2 represents the total area ratio of texture 1a, texture 1b, texture 3, and texture 5.

(Preparation of R-T-B based permanent magnets)
The R-T-B alloy for permanent magnets was allowed to absorb hydrogen at room temperature, and then subjected to a hydrogen pulverization treatment (coarse pulverization) in which hydrogen was removed in a vacuum at 600° C. for 1 hour, to obtain an alloy powder (coarse pulverized powder). In this example, each step from the hydrogen pulverization treatment to sintering (fine pulverization and molding) was performed in an Ar atmosphere with an oxygen concentration of less than 50 ppm or in a vacuum.

Next, zinc stearate and stearic acid amide were added to the alloy powder as grinding aids and mixed using a Nauta mixer. The amount of zinc stearate added was 0.05 parts by mass per 100 parts by mass of the coarsely ground powder. The amount of stearic acid amide added was 0.05 parts by mass per 100 parts by mass of the coarsely ground powder. After that, fine grinding was performed using a jet mill to produce a finely ground powder with an average particle size of about 3.0 μm.

The resulting finely pulverized powder was filled into a die placed inside an electromagnet, and a magnetic field of 1200 kA/m was applied while a pressure of 120 MPa was applied to perform magnetic field molding, resulting in a compact.

The resulting compact was then sintered in a vacuum at 1050°C for 4 hours and then rapidly cooled to obtain a sintered body having the magnet composition shown in Table 1. The resulting sintered body was then subjected to a two-stage aging treatment at 900°C for 1 hour and at 500°C for 1 hour (both in an Ar atmosphere) to obtain an R-T-B system permanent magnet (R-T-B system sintered magnet).

[Magnetic properties]
The Br, HcJ and Hk/HcJ of each sample were measured using a B-H tracer. Br was measured for all samples at room temperature. In Examples 1 to 4 and Comparative Example 1, HcJ and Hk/HcJ were measured at room temperature. In the other Examples and Comparative Examples, HcJ and Hk/HcJ were measured at 150°C. Note that Hk in this example is the value of the magnetic field when the magnetization is Br x 0.9 in the second quadrant of the demagnetization curve.

From Examples 1 to 4 and Comparative Example 1, the R-T-B permanent magnets of Examples 1 to 4, which were produced using an alloy for an R-T-B permanent magnet in which the area ratio of structure 5 was 0.5% or more and 20% or less, had higher Br, comparable HcJ, and higher Hk/HcJ than the R-T-B permanent magnet of Comparative Example 1, which was produced using an alloy for an R-T-B permanent magnet produced under the same conditions except that the area ratio of structure 5 was outside the above range. In particular, the Hk/HcJ at room temperature was higher.

From Examples 5 to 11, Examples 6 to 11, in which the total area ratio of Structure 1a, Structure 1b, Structure 3, and Structure 5 in the R-T-B permanent magnet alloy is 4.0% or more and 27.0% or less, have a higher HcJ than Example 5, which was produced under the same conditions except that the total area ratio of Structure 1a, Structure 1b, Structure 3, and Structure 5 in the R-T-B permanent magnet alloy is less than 4.0%.

From Examples 6 to 11, R-T-B permanent magnets produced under the same conditions except that the total area ratio of Structure 3 and Structure 5 in the R-T-B permanent magnet alloy was changed from 1.0% to 24.0% showed a tendency for the HcJ to increase as the total area ratio of Structure 3 and Structure 5 in the R-T-B permanent magnet alloy increased.

In Comparative Example 2, the B content is 0.96 mass%, which is higher than the other Examples. Therefore, even under casting conditions similar to those of the other Examples, the area ratio of the non-columnar crystal structure, and in particular the area ratio of Structure 5, decreased. As a result, HcJ decreased compared to the Examples.

Reference Signs List 1: Chilled crystal structure 3: Micro-dot R-rich phase-containing structure 4: Dotted R-rich phase-containing structure 5: Micro-linear R-rich phase-containing structure 6: Large dotted R-rich phase-containing structure 7: Normal structure 11: Main phase 13: R-rich phase 21: Chill roll 23: Tundish 25: Molten alloy 27: Molten metal head pressure

Claims (5)

An R-T-B system alloy for a permanent magnet, comprising R, T and B, R being a rare earth element, T being a transition metal element and B being boron, the area ratio of a fine linear R-rich phase-containing structure in a cross section being 0.5% or more and 20.0% or less,
the total area ratio of the fine linear R-rich phase-containing structure and the fine dotted R-rich phase-containing structure is 3.6% or more and 23.4% or less;
In the fine linear R-rich phase-containing structure, the linear R-rich phases contained in the fine linear R-rich phase-containing structure have a length of 4 to 125 μm and a width of 0.3 to 10 μm, and the intervals between the linear R-rich phases are 1.0 to 1.8 μm, and when the fine linear R-rich phase-containing structure is locally cut into polygonal regions at locations where the lengths, widths and density of the linear R-rich phases differ, the polygonal regions have a major axis of 30 μm or more and 200 μm or less and a minor axis of 5 μm or more and 150 μm or less,
The R-T-B system permanent magnet alloy has a fine dotted R-rich phase-containing structure, in which the dotted R-rich phases contained in the fine dotted R-rich phase-containing structure have a size of 0.3 to 1.5 μm in terms of circle equivalent diameter, the number of the dotted R-rich phases per unit area is 0.25/μm or ^more , and when the fine dotted R-rich phase-containing structure is locally cut into polygonal regions at locations where the dotted R-rich phases have different circle equivalent diameters and density states, the polygonal regions have a major axis of 10 μm or more and a minor axis of 5 μm or more and 80 μm or less . An R-T-B system alloy for a permanent magnet, comprising R, T and B, R being a rare earth element, T being a transition metal element and B being boron, the area ratio of a fine linear R-rich phase-containing structure in a cross section being 0.5% or more and 20.0% or less,
a total area ratio of the chill crystal structure, the fine linear R-rich phase-containing structure, and the fine dotted R-rich phase-containing structure is 4.3% or more and 26.2% or less;
In the fine linear R-rich phase-containing structure, the linear R-rich phases contained in the fine linear R-rich phase-containing structure have a length of 4 to 125 μm and a width of 0.3 to 10 μm, and the intervals between the linear R-rich phases are 1.0 to 1.8 μm, and when the fine linear R-rich phase-containing structure is locally cut into polygonal regions at locations where the lengths, widths and density of the linear R-rich phases differ, the polygonal regions have a major axis of 30 μm or more and 200 μm or less and a minor axis of 5 μm or more and 150 μm or less,
The R-T-B system permanent magnet alloy has a fine dotted R-rich phase-containing structure, in which the dotted R-rich phases contained in the fine dotted R-rich phase-containing structure have a size of 0.3 to 1.5 μm in terms of circle equivalent diameter, the number of the dotted R-rich phases per unit area is 0.25/μm or ^more , and when the fine dotted R-rich phase-containing structure is locally cut into polygonal regions at locations where the dotted R-rich phases have different circle equivalent diameters and density states, the polygonal regions have a major axis of 10 μm or more and 120 μm or less and a minor axis of 5 μm or more and 80 μm or less . 3. An R-T-B system permanent magnet alloy according to claim 2, wherein a total area ratio of the fine linear R-rich phase containing structure and the fine dot R-rich phase containing structure is 3.6% or more and 23.4% or less. An R-T-B system alloy for permanent magnets according to any one of claims 1 to 3, in which the R content is 29.0 mass% or more and 33.5 mass% or less, and the B content is 0.70 mass% or more and less than 0.96 mass%. A method for producing an RTB system permanent magnet, comprising the step of pulverizing the RTB system permanent magnet alloy according to any one of claims 1 to 4.

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