JP7779184B2 - 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.

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

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

International Publication No. 2013/008756

In recent years, _HcJ and the squareness ratio _Hk / _HcJ have increased thanks to the development of materials and improvements in manufacturing methods for R-T-B based sintered magnets, but variations in manufacturing conditions can sometimes cause unexpected decreases. As a result of the inventors' investigations, it has been found that such fluctuations in _HcJ and the squareness ratio _Hk / _HcJ depend on the method of heat treatment performed after the sintering process.

Embodiments of the present disclosure provide a method for producing R-T-B based sintered magnets that can solve the above 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 alloy powder containing a rare earth element; sintering the compact to produce a sintered body; a first heat treatment step of heat-treating the sintered body in a heat treatment furnace at a first temperature of 300°C or higher but lower than 400°C for 30 minutes or higher but not exceeding 900 minutes; and a second heat treatment step of heat-treating the sintered body in the heat treatment furnace at a second temperature of 400°C or higher but not exceeding 500°C for 30 minutes or higher but not exceeding 900 minutes. The difference between the first temperature and the second temperature is 50°C or higher. The alloy powder has a composition containing R (R is a rare earth element and must contain at least one selected from the group consisting of Nd, Pr, and Ce), T (T is at least one selected from the group consisting of Fe, Co, Al, Mn, and Si and must contain Fe), B, and at least one selected from the group consisting of Cu, Ga, Ni, Ag, Zn, and Sn, and the molar ratio of T to B [T]/[B] is greater than 14.0.

In one embodiment, the first temperature is less than 390°C and the second temperature is 460°C or higher.

In one embodiment, the rate of temperature rise from the first temperature to the second temperature is 0.5°C/min or more and 30°C/min or less.

According to the embodiments of the present disclosure, it is possible to suppress variations in H _cJ and squareness ratio H _k /H _cJ due to the heat treatment process carried out after the sintering process, and to stably produce high-performance RTB based sintered magnets.

FIG. 1 is a flow chart illustrating the sintering process of the present disclosure. FIG. 2 is a diagram schematically showing an example of a temperature profile of a heat treatment furnace in the low-temperature heat treatment step of this embodiment. FIG. 3 is a diagram showing a typical example of a temperature profile of a heat treatment furnace in a conventional low-temperature heat treatment process.

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.
A step (S10) of preparing a compact of alloy powder containing a rare earth element;
A sintering step (S20) of sintering the compact to produce a sintered body;
A first heat treatment step (S30) of heat treating the sintered body in a heat treatment furnace at a first temperature of 300 ° C. or higher and lower than 400 ° C. for 30 minutes or longer and 900 minutes or shorter;
A second heat treatment step (S40) of heat treating the sintered body in a heat treatment furnace at a second temperature of 400°C or higher and 500°C or lower for 30 minutes or higher and 900 minutes or lower;
Includes.

Here, the difference between the first temperature and the second temperature is 50°C or more. Hereinafter, the first heat treatment step (S30) and the second heat treatment step (S40) may be collectively referred to as the "low-temperature heat treatment step" or the "two-stage low-temperature heat treatment step." The term "low temperature" here refers to a temperature that is sufficiently lower than the heat treatment temperature (e.g., 950°C or higher and 1100°C or lower) in the sintering step (S20).

The composition of the alloy powder in the embodiment of the present disclosure contains R, T, B, and at least one selected from the group consisting of Cu, Ga, Ni, Ag, Zn, and Sn, and the molar ratio of T to B [T]/[B] is greater than 14.0. Here, R is a rare earth element and always contains at least one selected from the group consisting of Nd, Pr, and Ce. T is Fe, Co,
The element is at least one selected from the group consisting of Al, Mn, and Si, and always contains Fe.

R-T-B based sintered magnets are composed of crystal grains of the Nd ₂ Fe ₁₄ B phase (ferromagnetic), which is the main phase, and intermetallic compounds such as boron (B)-rich B-phase and Nd-rich phases 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, it progresses as the amount of liquid phase increases with increasing temperature. 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.

Furthermore, various elements have been added to the raw materials or diffused from the outside to the inside of the sintered body to modify the grain boundaries of the sintered body, thereby increasing _HcJ and the squareness ratio _Hk / _HcJ . For example, the success of improving _HcJ and the squareness ratio _Hk / _HcJ by modifying the grain boundaries depends on the type and amount of compound phases present at the grain boundaries, which in turn depend on the conditions of the low-temperature heat treatment step performed after the sintering process. The inventors' studies have found that the low-temperature heat treatment step performed after the sintering process can cause unexpected fluctuations in _HcJ and the squareness ratio _Hk / _HcJ , and this phenomenon is particularly pronounced when the molar ratio of T to B [T]/[B] in an R-T-B based sintered magnet exceeds 14.0 and elements such as Cu and Ga are added to modify the grain boundaries. The condition that the molar ratio of T [T]/[B] exceeds 14.0 indicates that the amount of B is relatively small compared to the amount of T used to form the main phase (R ₂ T ₁₄ B compound). In this way, by lowering the B content compared to that of a typical R-T-B based sintered magnet and then adding elements such as Cu and Ga, a high H _cJ can be obtained. However, as mentioned above, there was a problem in that the low-temperature heat treatment performed after the sintering process caused unexpected fluctuations in H _cJ and squareness ratio H _k /H _cJ . Furthermore, as a result of extensive research by the present inventors, it was found that the fluctuations in H _cJ and squareness ratio H _k /H _cJ were caused by fluctuations in the temperature distribution of the sintered body depending on the position of the sintered body in the heat treatment furnace during the low-temperature heat treatment process.

In the manufacturing method disclosed herein, by devising a low-temperature heat treatment step performed after the sintering step, the uniformity of the temperature distribution in the heat treatment furnace is improved, thereby expanding the heat treatment temperature range in which high _HcJ and squareness ratio _Hk / _HcJ can be achieved, thereby solving this problem. Specifically, in the first heat treatment step (S30), the sintered body is heat-treated at a first temperature of 300°C or higher but lower than 400°C for 30 minutes or longer but 900 minutes or shorter, thereby improving the uniformity of the temperature distribution of the sintered body in the heat treatment furnace. The low-temperature heat treatment temperature required to achieve high _HcJ and squareness ratio _Hk / _HcJ is 400°C or higher. Therefore, at the first temperature of 300°C or higher but lower than 400°C in the first heat treatment step (S30), various reactions that would normally occur in the sintered body due to low-temperature heat treatment are suppressed and hardly proceed. Therefore, the first heat treatment step (S30) functions to make the temperature distribution in the heat treatment furnace more uniform without changing the values of _HcJ and squareness ratio _Hk / _HcJ of the sintered body. Preferably, the temperature in the first heat treatment step (S30) is 350°C or higher and lower than 400°C. Next, in the second heat treatment step (S40), which is the original low-temperature heat treatment for increasing _HcJ and squareness ratio _Hk / _HcJ , the sintered body is heat-treated at a second temperature of 400°C or higher and 500°C or lower for 30 minutes or longer and 900 minutes or shorter, thereby exerting the various effects required in the low-temperature heat treatment step on the sintered body and making it possible to increase the values of _HcJ and squareness ratio _Hk / _HcJ .

It has also been found that dividing the low-temperature heat treatment process into two stages in this way makes it possible to shorten the time required for the low-temperature heat treatment. This is because, when low-temperature heat treatment is performed in a conventional heat treatment furnace with an uneven temperature distribution, there are positions in the heat treatment furnace where the temperature is too high or too low, and the H _cJ and squareness ratio H _k /H _cJ of the sintered body at such positions tend to decrease significantly if the set temperature in the heat treatment furnace deviates from the target value. Therefore, in conventional low-temperature heat treatment processes, it was necessary to narrow the range (margin) of the heat treatment temperature that serves as the control target and precisely control the heat treatment temperature relative to the target temperature. However, according to the two-stage low-temperature heat treatment process of the present disclosure, the values of H _cJ and squareness ratio H _k /H _cJ are less likely to vary depending on the position in the heat treatment furnace, making it possible to widen the temperature range (margin) centered on the target temperature. Furthermore, since the second heat treatment step (S40) can be performed under conditions where the temperature variation in the heat treatment furnace is small, even if the heat treatment temperature of the second heat treatment step (S40), i.e., the second temperature, is set to a relatively high temperature (e.g., 470°C or higher or 480°C or higher) within the range of 400°C or higher and 500°C or lower, a decrease in _HcJ and squareness ratio _Hk / _HcJ is suppressed. Increasing the second temperature makes it possible to shorten the time required for the second heat treatment step (S40). These features are suitable for mass production in factories.

Before the low-temperature heat treatment step, a high-temperature heat treatment step may be performed at a temperature relatively higher than that of the low-temperature heat treatment (700°C or higher and the sintering temperature or lower). In particular, when manufacturing an R-T-B based sintered magnet in which the molar ratio [T]/[B] of T exceeds 14.0 and elements such as Cu and Ga are added, it is preferable to perform high-temperature heat treatment before the low-temperature heat treatment step (i.e., before the first heat treatment step (S30) and the second heat treatment step (S40)). This allows for a more reliable attainment of a high _HcJ . The time period for the high-temperature heat treatment step is preferably 30 minutes or longer and 900 minutes or shorter. The term "high temperature" here refers to a temperature that is sufficiently higher than that of the first heat treatment step (S30) and the second heat treatment step (S40) in the low-temperature heat treatment.

In the field of R-T-B based sintered magnets, _Hk , which is one of the parameters that define the squareness ratio _Hk / _HcJ , is generally determined as follows. That is, if the strength of magnetization is "J," the remanent magnetization is " _Jr (= _Br )," and the magnetic field strength is "H," the reading on the H axis in the second quadrant of the J-H curve at the position where J is 0.9× _Jr is used. The value obtained by dividing _Hk by _HcJ on the demagnetization curve, _Hk / _HcJ = _Hk (kA/m)/ _HcJ (kA/m)×100(%), is defined as the squareness ratio.

Next, examples of the above steps S30 and S40 will be described in more detail with reference to Figures 2 and 3. These figures are graphs with the horizontal axis representing time and the vertical axis representing temperature, and schematically show examples of the temperature profile (heat pattern) of a heat treatment furnace in a low-temperature heat treatment process. The temperature of the heat treatment furnace is measured using a thermometer such as a thermocouple installed in the heat treatment device. It is preferable that the actual temperature of the sintered body (object temperature) matches the reading (measured temperature value) shown by the thermometer inside the sintering furnace, but this does not necessarily have to match exactly. A difference of about ±10°C between the two (object temperature) and the measured temperature value inside the sintering furnace is acceptable, and the effects of the present disclosure can be obtained.

First, let's look at Figure 2. Figure 2 is a diagram that schematically illustrates an example of the temperature profile (heat pattern) of the heat treatment furnace in the low-temperature heat treatment process of this embodiment. In Figure 2, the thick solid line indicates the relationship between temperature and time. Time is the elapsed time from the start of the low-temperature heat treatment process. The elapsed time is measured in hours, for example, but can also be measured in minutes or seconds. The temperature is the value measured by the thermometer in the heat treatment furnace, but is essentially equal to the set temperature specified by the temperature control program. The thick solid line in the diagram 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, "temperature" increases linearly and monotonically from room temperature to the first temperature T1, and the temperature increase rate is constant. However, the temperature increase rate does not have to be constant.

In the first heat treatment step (S30), heat treatment is performed at a first temperature T1 for a first time t1. This heat treatment may be referred to as "preparatory heating." After the first heat treatment step (S30), the temperature is raised from the first temperature T1 to a second temperature T2. Then, in the second heat treatment step (S40), heat treatment is performed at the second temperature T2 for a second time t2. Here, the first temperature T1 is equal to or higher than 300°C and lower than 400°C, and the second temperature T2 is equal to or higher than 400°C and lower than 500°C. The difference between the first temperature T1 and the second temperature T2 (T2 - T1) is equal to or higher than 50°C. The first time t1 and the second time t2 are each equal to or higher than 30 minutes and equal to or lower than 900 minutes.

Next, let us refer to FIG. 3 . FIG. 3 is a diagram schematically illustrating an example of a temperature profile (heat pattern) of a heat treatment furnace in a conventional low-temperature heat treatment process. In the example of FIG. 3 , the temperature of the heat treatment furnace is raised, for example, from room temperature to a temperature T0 for the low-temperature heat treatment process. Thereafter, the low-temperature heat treatment process is performed by maintaining the temperature of the heat treatment furnace at temperature T0 for a time t0. During the process of raising the temperature of the heat treatment furnace, the temperature inside the heat treatment furnace is not necessarily uniform and may vary significantly depending on the position. Such variations in temperature distribution increase as the temperature rise time increases and as the temperature rise rate increases. Therefore, in the example of FIG. 3 , at the time when the set temperature of the heat treatment furnace should have reached the target temperature T0, the actual temperature inside the heat treatment furnace is not uniform, with some parts reaching the target temperature T0 and others being higher or lower than temperature T0. Such temperature variations are expected to decrease as the low-temperature heat treatment process progresses, but when the temperature variations are large, the temperature T0 is 400°C or higher, for example, 450°C, and therefore various reactions that increase _HcJ and the squareness ratio _Hk / _HcJ occur non-uniformly within the sintered body.

However, according to an embodiment of the present disclosure, as shown in FIG. 2, after the step of raising the temperature from room temperature to the first temperature T1, a first heat treatment step (S30) is performed at the first temperature T1 below 400°C before the first heat treatment step (S40), which is essentially a low-temperature heat treatment step. This reduces the temperature variation within the heat treatment furnace, in other words, the temperature variation among multiple sintered bodies placed in the heat treatment furnace. Furthermore, when raising the temperature from the first temperature T1 close to 400°C to the second temperature T2 above 400°C, the temperature rise is small, making it unlikely that further temperature variation will occur.

According to experiments conducted by the inventors, the time required for the temperature rise from the first temperature T1 to the second temperature T2 is preferably 5 minutes or more and 30 minutes or less. This time can be appropriately determined depending on the magnitude of T2-T1. Furthermore, the temperature rise rate from the first temperature T1 to the second temperature T2 is preferably 0.5°C/min or more and 30°C/min or less. In this way, when the second heat treatment step (S40) is started, each sintered body in the heat treatment furnace has reached approximately the same temperature (target temperature), making it possible to appropriately perform the subsequent low-temperature heat treatment in the second heat treatment step (S40). Therefore, the _HcJ and squareness ratio _Hk / _HcJ of the finally obtained sintered bodies have small variations between sintered bodies and within each sintered body.

According to the inventors' investigations, the first temperature is preferably less than 390°C, and the second temperature is preferably 460°C or higher. When heat treatment is performed at such temperatures, the first heat treatment step (S30) can be performed for a relatively short time of 30 to 90 minutes, enabling the temperature distribution in the heat treatment furnace in the second heat treatment step (S40) to be sufficiently uniform. Furthermore, if the second temperature is 460°C or higher, the effect of low-temperature heat treatment, which increases _HcJ and the squareness ratio _Hk / _HcJ , can be fully achieved even with a relatively short time of 30 to 90 minutes. The heat treatments in the first heat treatment step (S30) and the second heat treatment step (S40) are preferably performed in a reduced-pressure atmosphere. This is because this can prevent the sintered body from being oxidized or nitrided during the heat treatment. After the second heat treatment step (S40), the temperature is lowered from the second temperature to, for example, room temperature. At this time, a step of cooling the sintered body at a temperature lowering rate of, for example, 5°C/min or higher may be performed.

<RTB-based 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-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, and may contain impurities unavoidable during production 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).

T contains iron (including cases where T consists essentially of iron), and up to 50% by mass of this iron 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 the alloy powder 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, 0.9% to 1.2% by mass. If it is less than 0.9% by mass, a high _HcJ may not be obtained, and if it 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, for example, 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. In addition, unavoidable impurities are also acceptable.

The composition of the R-T-B based sintered magnet in this embodiment contains R (R is a rare earth element and must contain at least one selected from the group consisting of Nd, Pr, and Ce), T (T is at least one selected from the group consisting of Fe, Co, Al, Mn, and Si and must contain Fe), B, and at least one selected from the group consisting of Cu, Ga, Ni, Ag, Zn, and Sn, and the molar ratio of T to B [T]/[B] is greater than 14.0.

<(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 having an average particle size of 2.0 μm to 4.5 μm can be obtained. The process of obtaining such a fine powder can be carried out, for example, using a jet mill pulverization system.

<(3) Example of a process for producing a compact of fine powder>
In a preferred embodiment, the step of producing a sintered body of fine powder includes the steps of producing a powder compact from the fine powder by pressing in a magnetic field and sintering the powder compact. When pressing in a magnetic field, it is preferable to form the powder compact by pressing in an inert gas atmosphere or wet pressing from the viewpoint of suppressing oxidation. The compact is then sintered to obtain a sintered body.

<(4) Example of sintering process>
The compact is then sintered to obtain a rare earth sintered magnet body (sintered body). In the sintering process of this embodiment, the compact is sintered at a temperature of 950°C or higher and 1100°C or lower, and then cooled to, for example, room temperature, at a rate of 10°C/min or lower. If the sintering temperature is lower than 950°C, the sintered density will be insufficient and a high B _r will not be obtained. Therefore, the sintering temperature for the compact according to this embodiment is 1000°C or higher, and preferably 1020°C or higher. Furthermore, if the sintering temperature exceeds 1100°C, rapid grain growth of the main phase will occur, making it impossible to obtain an R-T-B based sintered magnet with a high H _cJ and a high H _k /H _cJ through subsequent heat treatment. Therefore, the sintering temperature for the compact according to this embodiment is 1100°C or lower, and preferably 1080°C or lower.

The sintering of the compact can be performed by a known method. The sintering of the compact is preferably performed under a pressure of 0.13 Pa (10 ⁻³ Torr) or less, more preferably 0.07 Pa (5.0×10 ⁻⁴ Torr) or less. To prevent oxidation during sintering, residual gas in the atmosphere can be replaced with an inert gas such as helium or argon.

<(5) Example of low-temperature heat treatment process>
As described with reference to FIG. 3, the low-temperature heat treatment step in this embodiment is as follows:
A first heat treatment step (S30) of heat treating the sintered body in a heat treatment furnace at a first temperature of 300 ° C. or higher and lower than 400 ° C. for 30 minutes or longer and 900 minutes or shorter;
A second heat treatment step (S40) of heat treating the sintered body in a heat treatment furnace at a second temperature of 400°C or higher and 500°C or lower for 30 minutes or higher and 900 minutes or lower;
Includes.

By performing this two-stage low-temperature heat treatment process, it is possible to obtain R-T-B based sintered magnets with high H _cJ and high H _k /H _cJ in a good yield, compared to when the temperature of the heat treatment furnace is controlled using the temperature profile shown in the example of Figure 3.

The low-temperature heat treatment process can be performed using either a continuous furnace or a batch furnace.

The rare earth sintered magnet body obtained in this way is then subjected to grinding and polishing processes, surface treatment processes, and magnetization processes as necessary to produce the final rare earth 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 present disclosure will be explained in more detail using examples, but the present disclosure is not limited thereto.

Experimental Example 1
A metal or alloy was prepared to achieve the target composition of an R-T-B based sintered magnet: 24.0% by mass Nd, 7.0% by mass Pr, 0.89% by mass B, 0.1% by mass Cu, 0.5% by mass Ga, 0.9% by mass Co, and the remainder Fe. Alloy flakes were obtained by strip casting. The obtained alloy flakes were then subjected to hydrogen pulverization to obtain coarsely pulverized powder. The coarsely pulverized powder was then fed to a jet mill and pulverized to obtain a fine powder (alloy powder containing rare earth elements). The fine powder (alloy powder containing rare earth elements) was then compacted in a magnetic field to obtain a compact. The compacting device used was a so-called perpendicular magnetic field compacting device (horizontal magnetic field compacting device), in which the magnetic field application direction and the pressure direction are perpendicular to each other.

The resulting compact was then sintered (a temperature sufficient to cause densification by sintering was selected (approximately 1000°C)) to produce a sintered compact. The resulting sintered compact was then subjected to high-temperature heat treatment. The high-temperature heat treatment was performed at 900°C for 120 minutes. The sintered compact after the high-temperature heat treatment was then subjected to low-temperature heat treatment under the conditions shown in Table 1. Condition A in Table 1 refers to a first heat treatment step in which the sintered compact was heat-treated in a heat treatment furnace at a first temperature of 380°C for 90 minutes, and a second heat treatment step in which the sintered compact after the first heat treatment was heat-treated in a heat treatment furnace at a second temperature of 460°C for 90 minutes. The difference between the first and second temperatures was 80°C. Conditions B and C are similarly described. The temperature rise rate from the first temperature to the second temperature was 10°C/min. Conditions D to F were conventional low-temperature heat treatment processes, with heat treatment carried out at 460°C to 490°C for 180 minutes each.

The obtained sintered bodies (samples No. 1 to 6) were subjected to a component analysis, and the results were equivalent to the target composition (the molar ratio of T [T]/[B] exceeded 14.0). The components were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES). Furthermore, after machining the sintered bodies (No. 1 to 6), the properties (B _r and H _cJ ) of each sample were measured using a B-H tracer, and the squareness ratio H _k /H _cJ was calculated based on the measurement results. The results of H _cJ and squareness ratio H _k /H _cJ are shown in Table 2.

As shown in Table 2, Samples No. 1 to 3 under conditions A to C of the present disclosure (the temperature of the second heat treatment step after the first heat treatment was 460°C to 490°C) all exhibited high _HcJ and squareness ratio _Hk / _HcJ at all heat treatment temperatures (460°C to 490°C). In contrast, Samples No. 4 to 6 under conditions D to F of the comparative examples (the temperature of the low-temperature heat treatment step without the first heat treatment was 460°C to 490°C) exhibited high _HcJ and squareness ratio _Hk / _HcJ for Sample No. 4 (heat treatment temperature 460°C), but when the heat treatment temperature changed to 475°C or 490°C for Samples No. 5 and 6, the _HcJ and squareness ratio _Hk / _HcJ decreased, unlike the present invention examples. For these reasons, the manufacturing method of the present disclosure can expand the heat treatment temperature range in which high _HcJ and squareness ratio _Hk / _HcJ can be achieved, suppressing variations _{in HcJ} and squareness ratio Hk/ _HcJ due to the heat treatment process, and enabling the stable production of high-performance R-T-B based sintered magnets.

Experimental Example 2
A molded body was obtained in the same manner as in Example 1. Next, the obtained molded body was sintered (a temperature at which sufficient densification by sintering occurred (approximately 1000°C) was selected) to produce a sintered body from the molded body. Next, the obtained sintered body was subjected to high-temperature heat treatment in the same manner as in Example 1. Next, the sintered body after the high-temperature heat treatment was subjected to low-temperature heat treatment under the conditions shown in Table 1. The temperature rise rate from the first temperature to the second temperature was 10°C/min.

The obtained sintered bodies (samples No. 7 to 10) were subjected to a component analysis, and the results were equivalent to the target composition (the molar ratio of T [T]/[B] exceeded 14.0). The components were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES). Furthermore, after machining the sintered bodies (samples No. 7 to 10), the properties (B _r and H _cJ ) of each sample were measured using a B-H tracer, and the squareness ratio H _k /H _cJ was calculated based on the measurement results. The results of H _cJ and squareness ratio H _k /H _cJ are shown in Table 2.

As shown in Table 4, even when the time (30 to 90 minutes) or temperature (380° C. and 350° C.) of the first heat treatment step was changed under the conditions of the present disclosure, high H _cJ and squareness ratio H _k /H _cJ were obtained in all cases.

The manufacturing method for R-T-B based sintered magnets disclosed herein can be used to produce permanent magnets for 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 (3)

preparing a compact of alloy powder containing a rare earth element;
a sintering step of sintering the compact to produce a sintered body;
a first heat treatment step of heat treating the sintered body in a heat treatment furnace at a first temperature of 300°C or higher and lower than 400°C for 30 minutes or longer and 900 minutes or shorter;
a second heat treatment step of heat treating the sintered body in the heat treatment furnace at a second temperature of 400°C or higher and 500°C or lower for 30 minutes or higher and 900 minutes or lower;
Including,
a difference between the first temperature and the second temperature is 50°C or more;
The composition of the alloy powder is
R (R is a rare earth element and necessarily includes at least one selected from the group consisting of Nd, Pr, and Ce), T (T is at least one selected from the group consisting of Fe, Co, Al, Mn, and Si and necessarily includes Fe), B, and at least one selected from the group consisting of Cu, Ga, Ni, Ag, Zn, and Sn, and the molar ratio of T to B, [T]/[B], is greater than 14.0;
A method for producing an RTB based sintered magnet. the first temperature is less than 390°C;
The second temperature is 460°C or higher.
The method for producing the RTB based sintered magnet according to claim 1. The temperature rise rate from the first temperature to the second temperature is 0.5°C/min or more and 30°C/min or less.
The method for producing the RTB based sintered magnet according to claim 1 or 2.