JP7729124B2 - Method for manufacturing RTB based sintered magnet - Google Patents

Description

This application relates to a method for producing R-T-B based sintered magnets.

R-T-B based sintered magnets (R is a rare earth element that must contain at least one selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal that must contain Fe; and _B is boron) are composed of a main phase of a compound having an _R2Fe14B crystal structure, a grain boundary phase located at the grain boundaries of this main phase, and a compound phase formed by the influence of trace additive elements and impurities. R-T-B based sintered magnets exhibit high remanence B _r (hereinafter sometimes simply referred to as "B _r ") and high coercivity H _cJ (hereinafter sometimes simply referred to as "H _cJ "), and are known as the highest-performance permanent magnets. For this reason, R-T-B based sintered magnets are used in a variety of motors in the automotive field, such as electric vehicles (EVs, HVs, PHVs), renewable energy fields such as wind power generation, home appliances, and industrial fields. R-T-B based sintered magnets are an essential material for making these motors smaller, lighter, more efficient, and more energy-efficient (improving energy efficiency). Furthermore, R-T-B based sintered magnets are used in the drive motors of electric vehicles, and the replacement of internal combustion engine vehicles with electric vehicles contributes to the prevention of global warming by reducing greenhouse gases such as carbon dioxide (reducing fuel and exhaust gases). In this way, R-T-B based sintered magnets are making a significant contribution to the realization of a clean energy society.

R-T-B based sintered magnets are manufactured through processes such as preparing R-T-B based alloy powder, press-molding the R-T-B based alloy powder to create a compact, and sintering the compact.

Patent Document 1 discloses an example of such an R-T-B based sintered magnet.

International Publication No. 2013/008756

In recent years, the coercive force H _cJ and squareness ratio (H _k /H _cJ ) of R-T-B based sintered magnets have increased due to the development of materials and improvements in manufacturing methods. However, it has been found that when the compact is in the form of a thin sheet, the sintered body can warp.

Embodiments of the present disclosure provide a method for producing R-T-B based sintered magnets that can solve these problems.

In an exemplary embodiment, the method for producing an R-T-B based sintered magnet disclosed herein includes the steps of: preparing a compact of an R-T-B based alloy powder containing a rare earth element (R is a rare earth element and always includes at least one selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal and always includes Fe); and sintering the compact in a sintering furnace. The compact has a sheet shape with a thickness of 8 mm or less. The sintering step includes a first heating step in which the set temperature of the sintering furnace is increased to a first temperature at a first heating rate; a second heating step in which the set temperature of the sintering furnace is increased to a second temperature higher than the first temperature at a second heating rate lower than the first heating rate; and a first sintering temperature holding step in which sintering of the compact proceeds at the second temperature. The second heating rate is 0.1°C/min or more and 1.5°C/min or less, and the difference between the first temperature and the second temperature is 20°C or more.

In one embodiment, the molded body has a sheet surface large enough to encompass a square with a side length of 20 mm.

In one embodiment, the second temperature is greater than 900°C and less than or equal to 1100°C.

In one embodiment, the difference between the first temperature and the second temperature is 100°C or less.

In one embodiment, the difference between the first temperature and the second temperature is 20°C or more and 50°C or less.

In one embodiment, the first temperature increase rate is 2.0°C/min or more.

In one embodiment, the sintering process includes a temperature-lowering process in which, after the first sintering temperature holding process, the set temperature of the sintering furnace is lowered to a third temperature lower than the second temperature; a process in which the set temperature of the sintering furnace is held at the third temperature; a third temperature-raising process in which the set temperature of the sintering furnace is raised to a fourth temperature higher than the third temperature; and a second sintering temperature holding process in which sintering of the molded body is further promoted at the fourth temperature.

In one embodiment, the time for which the second temperature is held in the first sintering temperature holding step is less than half the time for which the fourth temperature is held in the second sintering temperature holding step.

In one embodiment, the duration of the second sintering temperature holding step is 1 hour or more and 20 hours or less.

In one embodiment, the third temperature is 700°C or higher and 900°C or lower.

In one embodiment, the composition of the R-T-B alloy powder includes R: 28% by mass or more and 35% by mass or less, B: 0.8% by mass or more and 1.20% by mass or less, and T: 61.5% by mass or more, where [B] is the content of B in mass% and [T] is the content of T in mass% and satisfies 14[B]/10.8<[T]/55.85.

Embodiments of the present disclosure provide a method for manufacturing an R-T-B based sintered magnet that can suppress warping during the sintering process.

FIG. 1A is a flow chart illustrating the manufacturing process of the present disclosure. FIG. 1B is a flow chart illustrating the sintering process of the present disclosure. FIG. 2 is a diagram showing a schematic example of a set temperature profile of the sintering furnace in the sintering step of this embodiment. FIG. 3 is a flowchart showing another example of the sintering process according to the present disclosure. FIG. 4 is a diagram schematically showing another example of the set temperature profile of the sintering furnace in the sintering step of this embodiment. FIG. 5 is a diagram showing a schematic diagram of still another example of the set temperature profile of the sintering furnace in the sintering step of this embodiment. FIG. 6 is a diagram showing a schematic diagram of still another example of the set temperature profile of the sintering furnace in the sintering step of this embodiment. FIG. 7 is a diagram schematically showing yet another example of the set temperature profile of the sintering furnace in the sintering step of this embodiment.

Below, an embodiment of a method for producing an R-T-B based sintered magnet according to the present disclosure will be described.

In the R-T-B based sintered magnet according to this embodiment, R is a rare earth element and always contains at least one element selected from the group consisting of Nd, Pr, and Ce. Furthermore, T is at least one transition metal and always contains Fe.

The RTB based sintered magnet of this embodiment is produced as shown in FIG. 1A .
A step (A) of preparing a compact of R-T-B-based alloy powder containing a rare earth element;
a sintering step (B) of sintering the formed body in a sintering furnace;
The molded body has a sheet shape with a thickness of 8 mm or less.

The atmosphere in the sintering step (B) is, for example, an inert gas such as nitrogen gas or a rare gas, or a vacuum.

The sintering step (B) is as shown in FIG.
A first temperature-raising step (S10) of raising the set temperature of the sintering furnace to a first temperature at a first temperature-raising rate;
A second temperature-raising step (S20) of raising the set temperature of the sintering furnace to a second temperature higher than the first temperature at a second temperature-raising rate lower than the first temperature-raising rate;
A first sintering temperature holding step (S30) of sintering the molded body at a second temperature;
Includes.

The second temperature increase rate in the second temperature increase step (S20) is 0.1°C/min or more and 1.5°C/min or less. The difference between the first temperature and the second temperature is 20°C or more. Hereinafter, the first temperature will be referred to as T1 and the second temperature will be referred to as T2. The unit of temperature is [°C].

R-T-B based sintered magnets are composed of crystal grains of the main phase, Nd ₂ Fe ₁₄ B (ferromagnetic), and intermetallic compounds, such as boron (B)-rich B-rich phase and Nd-rich phase, located at the grain boundaries of the main phase crystal grains. The sintering reaction proceeds through the formation of a liquid phase involving these phases contained in the powder particles that make up the compact. While the densification reaction does not occur when the amount of liquid phase is insufficient, as the amount of liquid phase increases with increasing temperature, the densification reaction progresses rapidly. During the sintering process, the liquid phase, which is generated when part of the intermetallic compounds in the powder particles melts, modifies the surfaces of the main phase crystal grains or reduces oxides, thereby promoting particle bonding and densification.

This sintering reaction is sensitive to temperature. Therefore, if the heating rate of the sintering furnace is set too high, the temperature and temperature change of the compact will be uneven across different regions. As a result, sintering proceeds unevenly across different regions, resulting in variations in the shrinkage rate depending on the surface or interior of the compact. This uneven shrinkage rate can lead to significant warpage in the final sintered body, particularly in sheet-shaped compacts. The inventors' research has shown that simply lowering the heating rate not only lengthens the sintering process time and reduces mass productivity, but also fails to adequately suppress warpage. Further research has revealed that dividing the heating process into two stages, with a relatively high heating rate in the first stage and a temporary reduction in the heating rate in the second stage before reaching the temperature (second temperature T2) of the first sintering temperature holding step (S30), is effective in suppressing warpage. When performing this two-stage temperature increase, desirable results cannot be obtained if the difference (T2-T1) between the first temperature T1 and the second temperature T2 is too small, or if the temperature increase rate in the second stage is too high or too low. For this reason, the second temperature increase rate in the second temperature increase step (S20) is set to between 0.1°C/min and 1.5°C/min, and the difference between the first and second temperatures is set to 20°C or more.

In an embodiment of the present disclosure, the second temperature T2 is, for example, greater than 900°C and less than or equal to 1100°C. Outside this range, sintering may not proceed properly, resulting in deterioration of the magnetic properties. In one embodiment, the difference between the first temperature T1 and the second temperature T2 (T2 - T1) is preferably 100°C or less, and more preferably 20°C or more and 50°C or less. If the difference between the first temperature T1 and the second temperature T2 is too great, productivity may deteriorate. If the first temperature T1 is too close to the second temperature T2, it becomes difficult to fully achieve the effect of suppressing warpage by temporarily reducing the heating rate. Furthermore, the first heating rate is preferably 2.0°C/min or more. If the first heating rate is too low, the sintering process takes too long, reducing mass productivity.

Next, with reference to Figure 2, an example of each of the above steps S10, S20, and S30 will be described in more detail. This figure is a graph with time on the horizontal axis and temperature on the vertical axis, and schematically shows an example of the temperature profile of the heat treatment object (green body and sintered body) during the sintering process. The temperatures shown are the set temperatures of the sintering furnace, measured by a thermometer such as a thermocouple installed in the sintering furnace. The actual temperature of the green body or sintered body does not need to exactly match the set temperature of the sintering furnace; a deviation of ±5°C or less is acceptable between the two.

In Figure 2, the thick solid line shows the relationship between temperature and time. Time is the elapsed time from the start of the temperature increase for sintering. The elapsed time is measured in hours, for example, but can also be measured in minutes or seconds. As mentioned above, the temperature is the set temperature of the sintering furnace specified by the temperature control program, but is substantially equal to the temperature of the atmosphere inside the sintering furnace. The thick solid line in the figure is composed of straight line segments, but the actual temperature or set temperature may fluctuate in a curved manner.

In the example of Figure 2, the set temperature increases monotonically at a first heating rate of 2.0°C/min or more, and the heating rate is constant. However, the heating rate does not always need to be constant when heating from room temperature. To volatilize lubricants, hydrogen (during hydrogen pulverization), and other oils contained in the compact, the compact may be maintained at a temperature of approximately 200°C for 1 hour to 10 hours. For this reason, the first heating rate in this disclosure is defined as the heating rate up to the first temperature T1 in the range of 700°C or above. The graph in Figure 2 includes horizontal lines representing temperatures of 700°C and 1100°C.

As mentioned above, the first temperature T1 is lower than the second temperature T2, and the temperature difference (T2 - T1) is preferably 100°C or less. T2 - T1 is more preferably 20°C or more, and even more preferably 20°C or more and 50°C or less. It is even more preferably 25°C or more and 40°C or less. By having the difference between the first and second temperatures be 20°C or more and the second temperature rise rate from the first temperature T1 to the second temperature T2 be 0.1°C/min or more and 1.5°C/min or less, it is believed that the temperature distribution in the heat treatment object can approach a uniform distribution before the first sintering temperature holding step (S30) in which sintering progresses, thereby suppressing warping. If the second temperature rise rate from the first temperature T1 to the second temperature T2 exceeds 1.5°C/min, for example, 2.0°C/min or more, warping often becomes noticeable.

In the first sintering temperature holding step (S30), the time t for holding the second temperature T2 is, for example, 30 minutes or more and 30 hours or less. In the first sintering temperature holding step (S30), it is not necessary to continuously hold the set temperature of the sintering furnace at a constant level of the second temperature T2.

The sintering step (B) is as shown in FIG.
a temperature-reducing step (S40) of lowering the set temperature of the sintering furnace to a third temperature lower than the second temperature after the first sintering temperature holding step (S30);
A step (S50) of maintaining the set temperature of the sintering furnace at a third temperature;
A third temperature-raising step (S60) of raising the set temperature of the sintering furnace to a fourth temperature higher than the third temperature;
A second sintering temperature holding step (S70) of further promoting sintering of the molded body at the fourth temperature;
may also include:

Examples of the above steps S40-S70 will be described in more detail below with reference to Figures 4 to 7. Similar to Figure 2, these figures are graphs with time on the horizontal axis and temperature on the vertical axis, and schematically show examples of the temperature profile of the heat treatment object (green body and sintered body) during the sintering process.

First, let's look at Figure 4. In Figure 4, the thick solid line shows the relationship between temperature and time. In the example of Figure 4, the set temperature of the sintering furnace is raised to the second temperature T2 as explained with reference to Figure 2. Therefore, the steps after the first sintering temperature holding step (S30) will be explained in detail.

In the temperature-reducing step (S40) that follows the first sintering temperature holding step (S30), the set temperature of the sintering furnace is first lowered to a third temperature T3, which is lower than the second temperature T2. The temperature-reducing rate at this time is, for example, 2°C/min to 5°C/min. Next, in step (S50), the set temperature of the sintering furnace is maintained at the third temperature T3.

In the third heating step (S60), the set temperature of the sintering furnace is raised to a fourth temperature T4, which is higher than the third temperature T3. In the second sintering temperature holding step (S70), sintering of the compact is further promoted at the fourth temperature T4. In this example, the sintering step is carried out in two stages. The first sintering temperature holding step (S30) may be referred to as the first-stage sintering step, and the second sintering temperature holding step (S70) may be referred to as the second-stage sintering step. The time of the first sintering temperature holding step (S30) may be referred to as the first sintering time t1, and the time of the second sintering temperature holding step (S70) may be referred to as the second sintering time t2. In one embodiment, the first sintering time t1 is 30 minutes or more and 2 hours or less, and the second sintering time t2 is 1 hour or more and 15 hours or less.

In a preferred embodiment, the second temperature T2 and the fourth temperature T4 are both greater than 900°C and less than 1100°C. In the example of FIG. 4, the second temperature T2 is higher than the fourth temperature T4, but as shown in FIG. 5, the second temperature T2 and the fourth temperature T4 may be equal. In a preferred embodiment, the second temperature T2 is, for example, greater than or equal to 1040°C and less than 1080°C, and the fourth temperature T4 is, for example, greater than or equal to 1020°C and less than 1060°C.

According to the inventors' investigations, by dividing the sintering process into two stages, it is sometimes possible to achieve good _HcJ and _Hk / _HcJ without extending the sintering time. Preferably, the first sintering time t1 is 30 minutes or more and 1 hour or less, and the second sintering time t2 is 1 hour or more and 8 hours or less. In particular, when the first temperature T1 is higher than the second temperature T2 as shown in FIG. 4, it is preferable to set the first sintering time t1 shorter than the second sintering time t2. In this case, for example, it is preferable that the first sintering time t1 be half or less of the second sintering time t2.

In this embodiment, a step (S50) of maintaining the set temperature at a third temperature T3 is carried out between the first-stage sintering step (S30) and the second-stage sintering step (S70). In this disclosure, the time t0 during which the set temperature remains below 900°C during the cooling step is defined as the "cooling time." Therefore, this cooling time t0 includes the temperature-drop period from 900°C to the third temperature T3 during the temperature drop from the second temperature T2, and the transition time from the third temperature T3 to 900°C during the temperature rise from the third temperature T3. It is preferable that the difference between the second temperature T2 and the third temperature T3 is 50°C or more. In other words, it is preferable that the third temperature T3 is 50°C or more lower than the second temperature T2. The temperature drop rate in the temperature drop step (S40) is arbitrary, and as shown in Figure 6, the temperature may be lowered at a relatively slow rate.

Furthermore, the third temperature T3 may be in the range of 700°C to 900°C, or may be at room temperature, as shown in Figure 7, so long as it is 900°C or less. From the perspective of shortening the time required for the cooling process and improving mass productivity, the third temperature T3 may be set, for example, in the range of 800°C to 900°C.

<RTB system sintered magnet>
R is a rare earth element and must contain at least one element selected from the group consisting of Nd, Pr, and Ce. Preferably, a combination of rare earth elements represented by Nd-Dy, Nd-Tb, Nd-Dy-Tb, Nd-Pr-Dy, Nd-Pr-Tb, Nd-Pr-Dy-Tb, Nd-Ce-Dy, Nd-Ce-Tb, Nd-Ce-Dy-Tb, Nd-Pr-Ce-Dy, Nd-Pr-Ce-Tb, or Nd-Pr-Ce-Dy-Tb is used.

Among the R elements, Dy and Tb are particularly effective in improving _HcJ . In addition to the above elements, other rare earth elements such as La may be contained, and misch metal or didymium may also be used. Furthermore, R does not have to be a pure element; it may contain impurities unavoidable during manufacturing within the industrially available range. The content is, for example, 28% by mass or more and 35% by mass or less. Preferably, the R content of the R-T-B based sintered magnet is 31% by mass or less (27% by mass or more and 31% by mass or less, preferably 29% by mass or more and 31% by mass or less). By keeping the R content of the R-T-B based sintered magnet at 31% by mass or less and the oxygen content at 400 ppm to 4000 ppm (preferably 400 ppm to 2500 ppm, more preferably 400 ppm to 2000 ppm), the generation of oxidized R is reduced. This allows for better magnetic properties to be obtained.

T is at least one transition metal and includes iron, which must contain Fe (including cases where T consists essentially of iron), and up to 50% by mass of this may be replaced with cobalt (Co) (including cases where T consists essentially of iron and cobalt). Co is effective in improving temperature characteristics and corrosion resistance, and R-T-B alloy powders may contain up to 10% by mass of Co. The content of T may account for the remainder of R and B, or R, B, and M, which will be described later.

The B content may be any known content, and a preferred range is, for example, 0.8% by mass to 1.2% by mass. If the B content is less than 0.8% by mass, a high _HcJ may not be obtained, and if the B content exceeds 1.2% by mass, _Br may decrease. Note that a portion of B can be substituted with C (carbon).

In addition to the above elements, an M element can be added to improve _HcJ . The M element is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta, and W. The amount of M element added is preferably 5.0 mass% or less. This is because if it exceeds 5.0 mass%, Br may decrease. Inevitable impurities are also acceptable.

The N (nitrogen) content in R-T-B based sintered magnets is preferably 50 ppm or more and 600 ppm or less. By pulverizing the magnet so that the N (nitrogen) content is 50 ppm or more and 600 ppm or less, it is possible to improve pulverization while suppressing degradation of magnetic properties due to nitriding. A nitrogen content of 50 ppm or more and 400 ppm or less is more preferable, and a nitrogen content of 100 ppm or more and 300 ppm or less is most preferable. This is because it is possible to further improve pulverization while suppressing degradation of magnetic properties due to nitriding. Furthermore, the C (carbon) content in R-T-B based sintered magnets is preferably 50 ppm or more and 1300 ppm or less.

An example of the composition of the RTB based sintered magnet in this embodiment is shown below.
R: 28% by mass or more and 35% by mass or less,
B: 0.8% by mass or more and 1.2% by mass or less,
The composition contains 61.5 mass% or more of T, where [B] is the content of B expressed in mass%, and [T] is the content of T expressed in mass%, and the formula 1 satisfies the following: 14[B]/10.8<[T]/55.85
Satisfy.

When producing an RTB based sintered magnet having a composition that satisfies formula 1, by dividing the sintering process into the two stages described above, it is possible to achieve good H _cJ and H _k /H _cJ without extending the sintering time.

By satisfying the relational expression 1: 14[B]/10.8<[T]/55.85, the B content is lower than that of a typical R-T-B based sintered magnet. In a typical R-T-B based sintered magnet, [T]/55.85 (atomic weight of Fe) is lower than ₁₄ [B]/10.8 (atomic weight of _B ) so that the soft magnetic phase R ₂ T ₁₇ phase is not generated in addition to the main phase R 2 T 14 B phase.

<(1) Example of a process for preparing coarsely pulverized powder of an RTB-based sintered magnet alloy>
In this embodiment, the process of preparing a coarsely pulverized powder of an alloy for an R-T-B based sintered magnet can include a process of preparing an alloy for an R-T-B based sintered magnet and a process of coarsely pulverizing this alloy by, for example, a hydrogen pulverization method.

The following is an example of a method for producing an alloy for R-T-B based sintered magnets.

First, an alloy ingot can be obtained by ingot casting, in which a metal or alloy previously prepared to have the composition described above is melted and poured into a mold. Alternatively, alloy flakes can be produced by a rapid cooling method, typically strip casting or centrifugal casting, in which the molten metal is brought into contact with a single roll, twin rolls, rotating disk, or rotating cylindrical mold, and rapidly cooled, producing a solidified alloy that is thinner than the alloy produced by the ingot method.

In embodiments of the present disclosure, materials produced by either the ingot method or the quenching method can be used, but production by a quenching method such as strip casting is preferred. The thickness of quenched alloys produced by the quenching method is typically in the range of 0.03 mm to 1 mm and in the form of flakes. The molten alloy begins to solidify at the surface where it contacts the chill roll (the roll contact surface), and crystals grow columnarly from the roll contact surface in the thickness direction. Compared to alloys (ingot alloys) produced by conventional ingot casting (mold casting), quenched alloys are cooled in a shorter time, resulting in a finer structure and smaller crystal grain size. They also have a larger grain boundary area. Because the R-rich phase spreads widely within the grain boundaries, the quenching method provides excellent dispersion of the R-rich phase. Therefore, hydrogen pulverization makes it easier for the alloy to fracture at the grain boundaries. By hydrogen pulverizing the quenched alloy, the size (average particle size) of the hydrogen pulverized powder (coarsely pulverized powder) can be reduced to, for example, 1.0 mm or less, preferably 10 μm to 500 μm.

<(2) Example of a process for obtaining fine powder>
In the process of obtaining a fine powder in this embodiment, the coarsely pulverized powder is supplied to a jet mill device whose pulverization chamber is filled with an inert gas, and the coarsely pulverized powder is pulverized to obtain a fine powder. In this process, for example, a fine powder (RTB-based alloy powder) having an average particle size of 2.0 μm or more and 4.5 μm or less can be obtained. The process of obtaining such a fine powder can be carried out, for example, using a jet mill pulverization system.

The increase in oxygen content of the R-T-B based sintered magnet due to processes after pulverization (mainly the process of producing a sintered body from the fine powder) is preferably 50 ppm to 300 ppm, and more preferably 50 ppm to 200 ppm. To achieve this, it is preferable to perform wet pressing in a magnetic field or magnetic pressing in an inert gas atmosphere, as described below, and then sinter the resulting compact. The average particle size of the fine powder obtained in the process of obtaining the fine powder is preferably 2.0 μm to 3.5 μm. Reducing the average particle size makes it possible to improve magnetic properties.

<(3) Example of a process for producing a compact of fine powder>
In the process of producing a compact, it is preferable to form the compact by pressing in an inert gas atmosphere or wet pressing from the viewpoint of suppressing oxidation when pressing in a magnetic field. In particular, wet pressing coats the surfaces of the particles constituting the compact with a dispersant such as an oil, suppressing contact with oxygen and water vapor in the atmosphere. This prevents or suppresses oxidation of the particles by the atmosphere before, during, or after the pressing process.

When wet pressing in a magnetic field, a slurry is prepared by mixing fine powder with a dispersant, which is then supplied to the cavity in the mold of the wet pressing device and press-molded in a magnetic field.

Dispersion Medium The dispersion medium is a liquid in which the RTB-based alloy powder can be dispersed to obtain a slurry.

Preferred dispersion media for use in this disclosure include mineral oils and synthetic oils.

Preparation of Slurry A slurry can be obtained by mixing the obtained RTB-based alloy powder with a dispersion medium.

The mixing ratio of the R-T-B alloy powder and the dispersion medium is not particularly limited, but the concentration of the R-T-B alloy powder in the slurry is preferably 70% or more (i.e., 70 mass% or more). This is because, at a flow rate of 20 to 600 cm ³ /sec, the R-T-B alloy powder can be efficiently supplied into the cavity and excellent magnetic properties can be obtained. The concentration of the R-T-B alloy powder in the slurry is preferably 90% or less by mass. The method for mixing the R-T-B alloy powder and the dispersion medium is not particularly limited. The R-T-B alloy powder and the dispersion medium may be prepared separately, and then the R-T-B alloy powder and the dispersion medium may be weighed out and mixed in predetermined amounts. Furthermore, when dry-pulverizing coarsely pulverized powder using a jet mill or the like to obtain an R-T-B alloy powder, a container containing the dispersion medium may be placed at the R-T-B alloy powder outlet of the pulverizing device such as the jet mill, and the pulverized R-T-B alloy powder may be directly recovered into the dispersion medium in the container to obtain a slurry. In this case, it is preferable that the inside of the container is also filled with an atmosphere of nitrogen gas and/or argon gas, and the obtained R-T-B system alloy powder is directly recovered in the dispersion medium without being exposed to the atmosphere to form a slurry. Furthermore, it is also possible to obtain a slurry consisting of the R-T-B system alloy powder and the dispersion medium by wet-pulverizing the coarsely pulverized powder while it is held in the dispersion medium using a vibration mill, ball mill, attritor, or the like.

The slurry thus obtained can be molded using a known wet press to obtain a molded block of the desired size and shape.

In one embodiment of the present disclosure, the resulting block of compact is sliced, for example by wire sawing, to form a plurality of compacts having a sheet shape and a thickness of 8 mm or less (e.g., 4 mm to 7 mm). Each compact has a sheet surface large enough to encompass a square with a side length of 20 mm.

For example, a sheet shape means that the minimum dimension of the main surface (the surface with the largest area) is at least twice the thickness. The sheet surface may have a rectangular shape, for example, with a length of 70 to 150 mm and a width of 30 to 150 mm. Note that the shape and size are not limited to this example.

Next, this sheet-shaped compact is sintered to obtain a sintered body.

<(4) Example of sintering process>
Next, the compact is sintered to obtain a sintered body.
A first temperature-raising step (S10) of raising the set temperature of the sintering furnace to a first temperature at a first temperature-raising rate;
A second temperature-raising step (S20) of raising the set temperature of the sintering furnace to a second temperature higher than the first temperature at a second temperature-raising rate lower than the first temperature-raising rate;
A first sintering temperature holding step (S30) of sintering the molded body at a second temperature;
Includes.

The compact may be sintered in vacuum or using an inert gas such as helium or argon.

The sintered body thus obtained is preferably subjected to heat treatment, which can improve its magnetic properties. Heat treatment conditions such as heat treatment temperature and time can be well-known. For example, the sintered body is heat-treated by heating it for at least one hour at a temperature lower than the second temperature T2 and the fourth temperature T4 (e.g., 400°C to 800°C). The R-T-B based sintered magnet thus obtained is then subjected to grinding and polishing, surface treatment, and magnetization processes as necessary to produce the final R-T-B based sintered magnet.

In a preferred embodiment, the method for producing an R-T-B based sintered magnet disclosed herein includes a diffusion step in which a heavy rare earth element RH (RH is at least one of Tb, Dy, and Ho) is diffused from the surface to the interior of the sintered body. Diffusing the heavy rare earth element RH from the surface to the interior of the sintered body can efficiently increase the coercive force.

The warpage can be measured, for example, by known three-dimensional measurement. The magnitude of the warpage can be defined, for example, by measuring the height from the base plate to the magnet of a sheet-shaped sintered body placed on a base plate so that the surfaces perpendicular to the thickness direction are in contact with each other using a three-dimensional measuring machine at multiple points (e.g., 10 points) including both ends and the center of the surface, and then determining the difference between the highest and lowest points.

The present disclosure will be explained in more detail using examples, but the present disclosure is not limited thereto.

Experimental Example 1
Powders of alloys for R-T-B based sintered magnets were prepared, with a composition of 25.0% by mass of Nd, 4.0% by mass of Pr, 0.89% by mass of B, 0.1% by mass of Cu, 0.5% by mass of Ga, 0.9% by mass of Co, and the remainder being Fe. These powders were used to produce powder compact blocks in a wet press. The resulting compact blocks were sliced using a wire saw to produce sheet-shaped compacts measuring 85 mm long, 50 mm wide, and 5.5 mm thick (5.5 mm being the magnetic field orientation direction). The resulting sheet-shaped compacts were sintered under the conditions shown in Tables 1 and 2 to produce sintered bodies. Nos. 1 to 5 in Table 1 represent the pattern for the first sintering step only (the pattern in Figure 2), while Nos. 6 to 8 in Table 2 represent the pattern for both the first and second sintering steps (the pattern in Figure 4). Nos. 1 to 5 in Table 1 represent the pattern for the first sintering step only (the pattern in Figure 2). 2 includes a first temperature-raising step (S10) in which the set temperature of the sintering furnace is raised to a first temperature (1030°C) at a first temperature-raising rate (5°C/min), a second temperature-raising step (S20) in which the set temperature of the sintering furnace is raised to a second temperature (1080°C) higher than the first temperature (1030°C) at a second temperature-raising rate (1.2°C/min) lower than the first temperature-raising rate (5°C/min), and a first sintering temperature holding step (holding time 60 minutes) (S30) in which sintering of the compact at the second temperature (1080°C) is progressed. Nos. 1, 3 to 5 in Table 1 are similarly described. Note that No. 1 in Table 1 was sintered at 1080°C for 60 minutes.

No. 6 in Table 2 was performed under the same conditions as No. 1 with regard to the first and second temperatures and holding times (S10-S30). Following the first sintering temperature holding step (S30), a temperature-lowering step (S40) was performed to lower the set temperature of the sintering furnace to a third temperature (650°C) lower than the second temperature (1080°C), a step (S50) was performed to hold the set temperature of the sintering furnace at the third temperature (650°C), a third temperature-raising step (S60) was performed to raise the set temperature of the sintering furnace to a fourth temperature (1050°C) higher than the third temperature (650°C), and a second sintering temperature holding step (holding time: 240 minutes) (S70) was performed to further sinter the compact at the fourth temperature (1050°C). The same applies to No. 7 and No. 8 in Table 2. The amount of warpage was measured for the resulting sintered compact. The amount of warpage was measured by placing a sheet-shaped sintered body on a base plate so that the surface perpendicular to the thickness direction (85 mm x 50 mm surface) was in contact with the magnet. The height from the base plate to the magnet was measured using a coordinate measuring machine at 10 points, including both ends and the center of the surface, and the difference between the highest and lowest points was determined. The results are shown in Tables 1 and 2. The "Sintering Time" in Tables 1 and 2 also lists the time required for these sintering processes. No. 1 in Table 1 means that the sintering time was approximately 4 to 5 hours. No. 2 to 8 in Tables 1 and 2 are similarly listed.

As shown in Tables 1 and 2, the warpage of the inventive examples was suppressed to 1.20 mm or less. Furthermore, the comparative examples, Nos. 5 and 8, required longer sintering times than the inventive examples, significantly reducing mass productivity.

The manufacturing method for R-T-B based sintered magnets disclosed herein can be used to manufacture permanent magnets used in a wide variety of applications, including voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and various other motors, as well as home appliances.

Claims (8)

a step of preparing a compact of an R-T-B type alloy powder containing a rare earth element (R is a rare earth element and always contains at least one selected from the group consisting of Nd, Pr, and Ce, and T is at least one transition metal and always contains Fe);
a sintering step of sintering the compact in a sintering furnace;
Including,
The molded body has a sheet shape with a thickness of 8 mm or less,
The sintering step comprises:
a first heating step of raising the set temperature of the sintering furnace to a first temperature at a first heating rate;
a second temperature-raising step of raising the set temperature of the sintering furnace to a second temperature higher than the first temperature at a second temperature-raising rate lower than the first temperature-raising rate;
a first sintering temperature holding step of sintering the compact at the second temperature;
Including,
The first temperature rise rate is defined as a temperature rise rate until the set temperature of the sintering furnace reaches the first temperature T1 in a range of 700°C or higher, and is 2.0°C/min or higher;
the second temperature rise rate is 0.1°C/min or more and 1.5°C/min or less,
The difference between the first temperature and the second temperature is 20°C or more and 50°C or less .
A method for producing an RTB based sintered magnet. The method for producing an R-T-B based sintered magnet described in claim 1, wherein the compact has a sheet surface large enough to encompass a square with a side length of 20 mm. The method for producing an R-T-B based sintered magnet according to claim 1 or 2, wherein the second temperature is greater than 900°C and less than or equal to 1100°C. The sintering step comprises:
a temperature-reducing step of reducing the set temperature of the sintering furnace to a third temperature lower than the second temperature after the first sintering temperature-holding step;
maintaining the set temperature of the sintering furnace at the third temperature;
a third temperature-raising step of raising the set temperature of the sintering furnace to a fourth temperature higher than the third temperature;
a second sintering temperature holding step of further promoting sintering of the compact at the fourth temperature;
Including,
The method for producing the RTB based sintered magnet according to any one of claims 1 to 3 . a time for which the temperature is held at the second temperature in the first sintering temperature holding step is equal to or less than half a time for which the temperature is held at the fourth temperature in the second sintering temperature holding step;
The method for producing the RTB based sintered magnet according to claim 4 . The time for the second sintering temperature holding step is 1 hour or more and 20 hours or less.
The method for producing an RTB based sintered magnet according to claim 4 or 5 . 7. The method for producing a sintered RTB based magnet according to claim 4 , wherein the third temperature is 700°C or higher and 900°C or lower. The composition of the R-T-B alloy powder is
R (R is a rare earth element and must contain at least one selected from the group consisting of Nd, Pr, and Ce): 28 mass% or more and 35 mass% or less, B: 0.8 mass% or more and 1.2 mass% or less, T (T is at least one transition metal and must contain Fe): 61.5 mass% or more, where [B] is the content of B expressed in mass% and [T] is the content of T expressed in mass%, 14[B]/10.8<[T]/55.85
Satisfy the
The method for producing the RTB based sintered magnet according to any one of claims 1 to 7 .