Manufacturing method for RTB sintered magnets

Abstract

To suppress warpage that occurs when a powder compact with a sheet shape is sintered.SOLUTION: A method for manufacturing an R-T-B based sintered magnet includes a step of preparing a compact of R-T-B based alloy powder including a rare earth element, and a sintering step of sintering the compact in a heat treatment furnace. The compact has a sheet shape with a thickness of 8 nm or less. The sintering step includes: a first temperature-raising step of raising a preset temperature to a first temperature at a first heating rate; a second temperature-raising step of raising a preset temperature 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 of allowing the sintering of the compact to proceed at the second temperature. The second heating rate is 0.1°C / min or more and 1.5°C / min or less, and a difference between the first temperature and the second temperature is 20°C or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present application relates to a method for producing an RTB based sintered magnet. [Background technology]

[0002] RTB sintered magnets (R is a rare earth element and must contain at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal and must contain Fe, and B is boron) are classified into RFe 14 It consists of a main phase of a compound with a B-type 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. RTB-based sintered magnets have a high residual magnetic flux density B r (Hereafter, simply "B r ") and high coercive force H cJ (Hereafter, simply "H cJ "), and are known as the highest-performance permanent magnets. For this reason, RTB sintered magnets are used in a variety of motors in fields including electric vehicles (EVs, HVs, and PHVs) and other automotive applications, renewable energy applications such as wind power generation, home appliances, and industrial applications. RTB sintered magnets are essential materials for making these motors smaller, lighter, more efficient, and more energy-efficient (improving energy efficiency). RTB sintered magnets are also used in the drive motors of electric vehicles, and the replacement of internal combustion engines with electric vehicles contributes to the prevention of global warming by reducing greenhouse gases such as carbon dioxide (fuel and exhaust gas emissions). In this way, RTB sintered magnets are making a significant contribution to the realization of a clean energy society.

[0003] RTB sintered magnets are manufactured through a process that includes preparing an RTB alloy powder, press-molding the RTB alloy powder to produce a compact, and sintering the compact.

[0004] Patent Document 1 discloses an example of such an RTB based sintered magnet. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2013 / 008756 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, the development of materials and improvements in manufacturing methods for RTB-based sintered magnets have led to the development of a coercive force H cJ and squareness ratio (H k / H cJ ) has increased, but it has been found that when the molded body is in the form of a thin sheet, there is an issue of warping of the sintered body.

[0007] An embodiment of the present disclosure provides a method for producing an RTB based sintered magnet that can solve these problems. [Means for solving the problem]

[0008] In an exemplary embodiment, the method for producing an RTB sintered magnet according to the present disclosure includes the steps of preparing a compact of an RTB alloy powder containing a rare earth element (R is a rare earth element and must include at least one selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal and must include 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 higher and 1.5°C / min or lower, and the difference between the first temperature and the second temperature is 20°C or higher.

[0009] In one embodiment, the molded body has a sheet surface large enough to encompass a square with a side length of 20 mm.

[0010] In one embodiment, the second temperature is greater than 900°C and equal to or less than 1100°C.

[0011] In one embodiment, the difference between the first temperature and the second temperature is 100° C. or less.

[0012] In one embodiment, the difference between the first temperature and the second temperature is 20°C or more and 50°C or less.

[0013] In one embodiment, the first temperature increase rate is 2.0° C. / min or more.

[0014] In one embodiment, the sintering step includes a temperature lowering step in which the set temperature of the sintering furnace is lowered to a third temperature lower than the second temperature after the first sintering temperature holding step, a step in which the set temperature of the sintering furnace is held at the third temperature, a third temperature raising step 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 step in which sintering of the molded body is further promoted at the fourth temperature.

[0015] In one embodiment, the time for which the second temperature is held in the first sintering temperature holding step is equal to or less than half the time for which the fourth temperature is held in the second sintering temperature holding step.

[0016] In one embodiment, the time period for the second sintering temperature holding step is 1 hour or more and 20 hours or less.

[0017] In one embodiment, the third temperature is 700°C or higher and 900°C or lower.

[0018] In one embodiment, the composition of the RTB alloy powder includes 28% by mass or more and 35% by mass or less of R, 0.8% by mass or more and 1.20% by mass or less of B, and 61.5% by mass or more of T, 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. [Effects of the Invention]

[0019] According to the embodiments of the present disclosure, it is possible to provide a method for producing an RTB based sintered magnet that can suppress warping during the sintering process. [Brief explanation of the drawings]

[0020] [Figure 1A] FIG. 1A is a flow chart illustrating the manufacturing process of the present disclosure. [Figure 1B] FIG. 1B is a flow chart illustrating the sintering process of the present disclosure. [Figure 2] FIG. 2 is a diagram schematically showing an example of a set temperature profile of the sintering furnace in the sintering step of this embodiment. [Figure 3] FIG. 3 is a flowchart showing another example of the sintering process according to the present disclosure. [Figure 4] 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. [Figure 5] FIG. 5 is a diagram schematically showing yet another example of the set temperature profile of the sintering furnace in the sintering step of this embodiment. [Figure 6] FIG. 6 is a diagram schematically showing yet another example of the set temperature profile of the sintering furnace in the sintering step of this embodiment. [Figure 7] 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. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, an embodiment of a method for producing an RTB based sintered magnet according to the present disclosure will be described.

[0022] In the RTB based sintered magnet according to this embodiment, R is a rare earth element and always includes at least one selected from the group consisting of Nd, Pr, and Ce, and T is at least one transition metal and always includes Fe.

[0023] The RTB based sintered magnet of this embodiment is produced as shown in FIG. 1A . A step (A) of preparing a compact of an RTB-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.

[0024] The atmosphere in the sintering step (B) is, for example, an inert gas such as nitrogen gas or a rare gas, or a vacuum.

[0025] The sintering step (B) is as shown in FIG. 1B. 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 compact at a second temperature; Includes.

[0026] 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].

[0027] RTB sintered magnets are made of NdFe 14It is composed of B-phase (ferromagnetic) crystal grains and intermetallic compounds such as boron (B)-rich 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. When the amount of liquid phase is insufficient, the densification reaction does not occur, but as the amount of liquid phase increases with increasing temperature, the densification reaction progresses rapidly. During the sintering process, some of the intermetallic compounds within the powder particles melt, creating a liquid phase that modifies the surfaces of the main phase crystal grains or reduces oxides, thereby promoting particle bonding and densification.

[0028] This sintering reaction is sensitive to temperature. Therefore, if the heating rate of the sintering furnace is too high, the temperature and temperature change of the compact will be uneven across different parts. As a result, sintering will proceed unevenly across different parts, resulting in different shrinkage rates on the surface or inside of the compact. This uneven shrinkage rate can lead to significant warpage in the final sintered compact, 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 such a 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 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 set to 20°C or more.

[0029] 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.

[0030] Next, an example of each of the above steps S10, S20, and S30 will be described in more detail with reference to FIG. 2. This figure is a graph showing 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) in 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 strictly match the set temperature of the sintering furnace; a deviation of ±5°C or less between the two is allowed.

[0031] In Figure 2, the thick solid line indicates the relationship between temperature and time. The time is the elapsed time from the start of the temperature rise for sintering. The elapsed time is measured in units of, for example, hours, but may 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.

[0032] In the example of FIG. 2, the set temperature increases monotonically at a first temperature increase rate of 2.0°C / min or more, and the temperature increase rate is constant. However, the temperature increase rate does not need to be constant when increasing the temperature 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, for example, approximately 200°C for 1 hour to 10 hours. For this reason, the first temperature increase rate in this disclosure is defined as the temperature increase rate until the first temperature T1 is reached in the range of 700°C or higher. The graph in FIG. 2 includes horizontal lines representing temperatures of 700°C and 1100°C.

[0033] As described 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. Furthermore, it is even more preferably 25°C or more and 40°C or less. When the difference between the first temperature and the second temperature is 20°C or more and the second temperature rise rate from the first temperature T1 to the second temperature T2 is 0.1°C / min or more and 1.5°C / min or less, 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, and warpage is thought to be suppressed. When 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, warpage often occurs significantly.

[0034] In the first sintering temperature holding step (S30), the time t for holding at 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 continue to hold the set temperature of the sintering furnace at a constant level of the second temperature T2.

[0035] 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 sintering the compact at the fourth temperature; may also include:

[0036] 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 showing 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) in the sintering step.

[0037] First, let us refer to Figure 4. In Figure 4, the thick solid line indicates the relationship between temperature and time. In the example of Figure 4, the set temperature of the sintering furnace is increased 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.

[0038] In the temperature lowering step (S40) performed following the first sintering temperature holding step (S30), the set temperature of the sintering furnace is temporarily lowered to a third temperature T3 that is lower than the second temperature T2. The temperature lowering 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 held at the third temperature T3.

[0039] In the third temperature-raising 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 called the first-stage sintering step, and the second sintering temperature holding step (S70) may be called the second-stage sintering step. The time of the first sintering temperature holding step (S30) may be called the first sintering time t1, and the time of the second sintering temperature holding step (S70) may be called 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.

[0040] 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 to each other. 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.

[0041] According to the study by the present inventors, by dividing the sintering process into two stages, it is possible to obtain a good H cJ and H k / H cJ In some cases, the above can be achieved. 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 make 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 is half or less of the second sintering time t2.

[0042] In this embodiment, a step (S50) of maintaining the set temperature at a third temperature T3 is performed 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. The difference between the second temperature T2 and the third temperature T3 is preferably 50°C or more. In other words, the third temperature T3 is preferably 50°C or more lower than the second temperature T2. The temperature-drop rate in the temperature-dropping step (S40) is optional, and as shown in FIG. 6, the temperature may be dropped at a relatively slow rate.

[0043] Also, if the third temperature T3 is 900°C or lower, it may be in the range of 700°C or higher and 900°C or lower, or as shown in FIG. 7, it may be at room temperature level. From the viewpoint of shortening the time required for the cooling process and improving mass productivity, the third temperature T3 can be set, for example, within the range of 800°C or higher and 900°C or lower.

[0044] <R-T-B sintered magnet> R is a rare earth element and necessarily contains at least one 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, Nd-Pr-Ce-Dy-Tb is used.

[0045] Among R, Dy and Tb are particularly effective in improving H cJ In addition to the above elements, other rare earth elements such as La may be contained, and mischmetal or didymium can also be used. Also, R may not be a pure element and may contain manufacturing-inseparable impurities within an industrially available range. The content is, for example, 28 mass% or more and 35 mass% or less. Preferably, the R content of the R-T-B sintered magnet is 31 mass% or less (27 mass% or more and 31 mass% or less, preferably 29 mass% or more and 31 mass% or less). By setting the R content of the R-T-B sintered magnet to 31 mass% or less and the oxygen content to 400 ppm or more and 4000 ppm or less (preferably 400 ppm or more and 2500 ppm or less, more preferably 400 ppm or more and 2000 ppm or less), the generation of oxidized R is reduced. Therefore, higher magnetic properties can be obtained.

[0046] T is at least one transition metal and includes iron, which necessarily contains Fe (including cases where T consists essentially of iron), and up to 50% by mass of T may be substituted 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 RTB 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.

[0047] The content of B may be a known content, and for example, a preferred range is 0.8 mass % to 1.2 mass %. If it is less than 0.8 mass %, high H cJ If it exceeds 1.2 mass%, B may not be obtained. r In addition, a part of B can be substituted with C (carbon).

[0048] In addition to the above elements, H cJ To improve the properties, an M element can be added. 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. If it exceeds 5.0 mass%, Br may decrease. Inevitable impurities are also acceptable.

[0049] The N (nitrogen) content in the RTB sintered magnet 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 pulverizability while suppressing deterioration of magnetic properties due to nitriding. The nitrogen content is more preferably 50 ppm or more and 400 ppm or less, and most preferably 100 ppm or more and 300 ppm or less. This is because it is possible to further improve pulverizability while suppressing deterioration of magnetic properties due to nitriding. Furthermore, the C (carbon) content in the RTB sintered magnet is preferably 50 ppm or more and 1300 ppm or less.

[0050] An example of the composition of the RTB based sintered magnet according to 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, When T contains 61.5 mass% or more, [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 Equation 1 Satisfy.

[0051] 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 obtain a good H cJ and H k / H cJ can be achieved.

[0052] By satisfying the relationship 1: 14[B] / 10.8<[T] / 55.85, the B content is lower than that of typical RTB sintered magnets. Typical RTB sintered magnets contain R2T, the main phase. 14 In addition to the B phase, there is a soft magnetic phase, R2T 17 To prevent the formation of any phase, [T] / 55.85 (atomic weight of Fe) is less than 14[B] / 10.8 (atomic weight of B).

[0053] <(1) Example of a process for preparing coarsely pulverized powder of alloy for RTB sintered magnets> In this embodiment, the step of preparing a coarsely pulverized powder of an alloy for an RTB based sintered magnet can include a step of preparing an alloy for an RTB based sintered magnet and a step of coarsely pulverizing this alloy by, for example, a hydrogen pulverization method.

[0054] The following is an example of a method for producing an alloy for an RTB based sintered magnet.

[0055] First, an alloy ingot can be obtained by ingot casting, in which a metal or alloy previously prepared to have the above-mentioned composition is melted and poured into a mold. Alternatively, alloy flakes can be produced by a rapid cooling method, such as 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 then solidified into a thinner alloy than the alloy produced by the ingot method.

[0056] In the 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 from the surface that 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. Furthermore, the grain boundary area is larger. Because the R-rich phase spreads widely within the grain boundaries, the quenching method provides excellent dispersibility of the R-rich phase. Therefore, fracture at the grain boundaries is more likely to occur when using hydrogen pulverization. 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.

[0057] <(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 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.

[0058] The increase in oxygen content of the RTB sintered magnet due to the steps after pulverization (mainly the steps 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 step of obtaining the fine powder is preferably 2.0 μm to 3.5 μm. Reducing the average particle size makes it possible to improve the magnetic properties.

[0059] <(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.

[0060] When wet pressing in a magnetic field is performed, a slurry is prepared by mixing a dispersion medium with fine powder, and the slurry is supplied to a cavity in a mold of a wet pressing device and press-molded in a magnetic field.

[0061] ·Dispersion medium The dispersion medium is a liquid in which the RTB alloy powder can be dispersed to obtain a slurry.

[0062] Preferred dispersion media for use in the present disclosure include mineral oils and synthetic oils.

[0063] · Preparation of slurry The obtained RTB alloy powder is mixed with a dispersion medium to obtain a slurry.

[0064] The mixing ratio of the RTB alloy powder and the dispersion medium is not particularly limited, but the concentration of the RTB alloy powder in the slurry is preferably 70% or more (i.e., 70% by mass or more) in terms of mass ratio. 3 This is because a flow rate of 1 / sec allows the RTB alloy powder to be efficiently supplied into the cavity and excellent magnetic properties to be obtained. The concentration of the RTB alloy powder in the slurry is preferably 90% or less by mass. The method for mixing the RTB alloy powder and the dispersion medium is not particularly limited. The RTB alloy powder and the dispersion medium may be prepared separately, and then weighed and mixed in predetermined amounts. Alternatively, when dry-pulverizing coarsely pulverized powder using a jet mill or the like to obtain RTB alloy powder, a container containing the dispersion medium may be placed at the RTB alloy powder outlet of the jet mill or other grinding device, and the RTB alloy powder obtained by grinding may be directly recovered in the dispersion medium in the container to obtain a slurry. In this case, the container is preferably also filled with a nitrogen and / or argon gas atmosphere, and the obtained RTB alloy powder is directly recovered in the dispersion medium without being exposed to the air to form a slurry. Furthermore, it is also possible to wet-pulverize the coarsely pulverized powder while it is held in the dispersion medium using a vibrating mill, ball mill, attritor, or the like to obtain a slurry consisting of the RTB alloy powder and the dispersion medium.

[0065] The slurry thus obtained is molded in a known wet press to obtain a molded block having a predetermined size and shape.

[0066] In one embodiment of the present disclosure, the resulting compact block is sliced, for example by wire sawing, to form a plurality of compacts having a sheet shape with a thickness of 8 mm or less (e.g., 4 mm to 7 mm), each having a sheet surface large enough to encompass a square with a side length of 20 mm.

[0067] For example, a sheet shape is one in which 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.

[0068] Next, the sheet-shaped compact is sintered to obtain a sintered body.

[0069] <(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.

[0070] The compact may be sintered in vacuum or using an inert gas such as helium or argon.

[0071] The sintered body thus obtained is preferably subjected to a heat treatment. Heat treatment can improve the magnetic properties. Known conditions can be used for the heat treatment temperature, heat treatment time, and other heat treatment conditions. For example, the sintered body is heat treated by heating it for one hour or more at a temperature (e.g., 400°C to 800°C) lower than the second temperature T2 and the fourth temperature T4. The RTB based sintered magnet thus obtained is then subjected to grinding and polishing steps, surface treatment steps, and magnetization steps as necessary to produce the final RTB based sintered magnet.

[0072] In a preferred embodiment, the method for producing an RTB-based sintered magnet of the present disclosure includes a diffusion step of diffusing a heavy rare-earth element RH (RH is at least one of Tb, Dy, and Ho) from the surface of the sintered body to the interior. Diffusing the heavy rare-earth element RH from the surface to the interior of the sintered body can efficiently increase the coercive force.

[0073] The warpage can be measured, for example, by a known three-dimensional measurement method. The magnitude of the warpage is defined by, for example, 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 (for example, 10 points) including both ends and the center of the surface, and determining the difference between the highest and lowest points. [Example]

[0074] The present disclosure will be explained in more detail by way of examples, but the present disclosure is not limited thereto.

[0075] Experimental Example 1 An alloy powder for RTB-based sintered magnets was 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 then 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 compacts. Samples 1 to 5 in Table 1 represent the first-stage sintering process (the pattern shown in Figure 2), while samples 6 to 8 in Table 2 represent the first- and second-stage sintering processes (the pattern shown in Figure 4). No. 2 in Table 1 represents a sample that underwent a first temperature-raising step (S10) in which the set temperature of the sintering furnace was increased 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 was increased 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) was promoted. The same applies to Nos. 1, 3, and 5 in Table 1. Note that No. 1 in Table 1 represents a sample sintered at 1080°C for 60 minutes.

[0076] No. 6 in Table 2 was performed under the same conditions as No. 1 with respect to the first and second temperatures and holding times (S10-S30). After 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 Nos. 7 and 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 determining the difference between the highest and lowest points. 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. Nos. 2 to 8 in Tables 1 and 2 are listed in the same way.

[0077] [Table 1]

[0078] [Table 2]

[0079] As shown in Tables 1 and 2, the warpage of the examples of the present invention was suppressed to 1.20 mm or less. In addition, the comparative examples Nos. 5 and 8 required a longer sintering time than the examples of the present invention, which significantly reduced mass productivity. [Industrial Applicability]

[0080] The method for producing an RTB-based sintered magnet according to the present disclosure can be used to produce 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

1. 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.

2. 2. The method for producing a sintered RTB based magnet according to claim 1, wherein the compact has a sheet surface large enough to encompass a square with a side length of 20 mm.

3. 3. The method for producing a sintered RTB based magnet according to claim 1, wherein the second temperature is higher than 900°C and not higher than 1100°C.

4. 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.

5. 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.

6. 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. 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.

8. 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.

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