JP7622574B2 - Manufacturing method of rare earth magnet - Google Patents

Description

The present disclosure relates to a method for producing a rare earth magnet, and in particular to a method for producing a rare earth magnet having a main phase with a crystal structure of R ₂ T ₁₄ B type, where R is a rare earth element and T is a transition metal element.

R-Fe-B rare earth magnets have a main phase and a grain boundary phase that exists around the main phase. The main phase is a magnetic phase that has an R ₂ T ₁₄ B type crystal structure. This main phase allows high remanent magnetization to be obtained. For this reason, R-Fe-B rare earth magnets are often used in motors.

When permanent magnets, including R-Fe-B rare earth magnets, are used in motors, the permanent magnets are placed in an external magnetic field environment that changes periodically. As a result, the permanent magnets can be demagnetized by an increase in the external magnetic field. When using permanent magnets in motors, it is required that they are not demagnetized as much as possible in response to an increase in the external magnetic field. A demagnetization curve indicates the degree of demagnetization in response to an increase in the external magnetic field, and a demagnetization curve that satisfies the above requirements has a square shape. For this reason, satisfaction of the above requirements is said to have excellent squareness.

Since motors generate heat during operation, the permanent magnets used in motors are required to have high magnetic properties at high temperatures. In this specification, high temperatures in relation to magnetic properties mean temperatures in the range of 130 to 200°C, particularly 140 to 180°C, unless otherwise specified.

To improve the magnetic properties of R-Fe-B rare earth magnets at high temperatures, it is useful to increase the Curie temperature by substituting part of the Fe with Co. For example, Non-Patent Document 1 discloses that in an R-Fe-B rare earth magnet in which Nd is selected as R, the Curie temperature can be increased by substituting part of the Fe with Co.

Kazuhiko Mashima et al., "Effect of Co Addition on Sintering Characteristics of Nd2Fe14B Magnet Alloy," Powder and Powder Metallurgy, Vol. 39, No. 10, pp. 855-859, 1992.

When a part of Fe is simply replaced with Co, as in the R-Fe-B rare earth magnet disclosed in Non-Patent Document 1, the Curie temperature rises and the residual magnetization at high temperatures improves, but the squareness decreases. For this reason, the inventors have found that an R-Fe-B rare earth magnet in which the decrease in squareness is suppressed even when a part of Fe is replaced with Co is desired. Since there is a high correlation between squareness at room temperature and squareness at high temperatures, in this specification, unless otherwise specified, "squareness" refers to squareness at room temperature. In addition, squareness is evaluated by the squareness ratio expressed as Hr/Hc. Hc is the coercive force, and Hr is the magnetic field at 5% demagnetization. The magnetic field at 5% demagnetization refers to the magnetic field in the second quadrant (demagnetization curve) of the hysteresis curve when the magnetization is 5% lower than the residual magnetization (magnetic field when the applied magnetic field is 0 kA/m).

This disclosure has been made to solve the above problems. The purpose of this disclosure is to provide a method for manufacturing rare earth magnets that can suppress a decrease in squareness even when part of the Fe is replaced with Co.

The present inventors have conducted extensive research to achieve the above object, and have completed the method for producing a rare earth magnet according to the present disclosure. The method for producing a rare earth magnet according to the present disclosure includes the following aspects.
<1> preparing a rare earth magnet precursor having a main _{phase with an R2T14B type} crystal structure (where R is a rare earth element and T is a transition metal element) and a grain boundary phase present around the main phase; and diffusing and penetrating a modifier containing a rare earth element into the rare earth magnet precursor.
Including,
The rare earth magnet precursor contains Co, and
The molar ratio x of La to the total rare earth elements in the modifier satisfies 0.09<x<0.19;
A method for manufacturing rare earth magnets.
<2> The method according to item <1>, wherein the main phase has an average grain size of 1 to 10 μm.
<3> The rare earth magnet precursor has an overall composition in terms of molar ratio represented by the formula R ¹ _y (Fe _(1-z) Co _z ) _(100-y-w-v) B _w M ¹ _v , where R ¹ is one or more elements selected from the group consisting of Nd, Pr, Gd, Tb, Dy, and Ho, M ¹ is one or more elements selected from the group consisting of Ga, Al, Cu, Au, Ag, Zn, In, and Mn, and an unavoidable impurity element,
12.0≦y≦20.0,
0.10≦z≦0.30,
5.0≦w≦20.0, and 0≦v≦2.0
The method according to claim 1 or 2, wherein the compound is represented by the formula:
<4> The method according to item <3>, wherein R ¹ is one or more elements selected from the group consisting of Nd and Pr.
<5> The method according to any one of <1> to <3>, wherein the composition of the modifier is represented by the formula (R ² _(1-x) La _{x (1-s)} M ² _s (wherein R ² is one or more elements selected from the group consisting of Nd, Pr, Gd, Tb, Dy, and Ho, M ² is a metal element other than a rare earth element that is alloyed with R ² , and an unavoidable impurity element), and 0.09<x<0.19 and 0.05≦s≦0.40.
<6> The method according to item <5>, wherein ^R2 is one or more elements selected from the group consisting of Nd and Tb.
<7> The method according to item <5> or <6>, wherein ^M2 is Cu and an unavoidable impurity element.

According to the manufacturing method of rare earth magnets disclosed herein, the ferromagnetic phase containing Co in the grain boundary phase can be demagnetized by diffusing and infiltrating a modifier containing a predetermined percentage of La into the interior of a rare earth magnet precursor in which part of the Fe has been replaced with Co. As a result, it is possible to suppress a decrease in the squareness of the resulting R-T-B rare earth magnet. In other words, according to the present disclosure, it is possible to provide a manufacturing method of R-T-B rare earth magnets that can suppress a decrease in squareness even when part of the Fe has been replaced with Co.

FIG. 1A is an explanatory diagram illustrating a schematic structure of a rare earth magnet precursor used in the manufacturing method of the present disclosure. FIG. 1B is an explanatory diagram showing an enlarged portion of FIG. 1A enclosed by a dashed line. FIG. 2A is an explanatory diagram that illustrates a schematic structure of a rare earth magnet obtained by the manufacturing method of the present disclosure. FIG. 2B is an explanatory diagram showing an enlarged portion of FIG. 2A enclosed by a dashed line. FIG. 3A is an explanatory diagram that shows a schematic diagram of the structure of a rare earth magnet after diffusion infiltration of a modifier having an insufficient La content. FIG. 3B is an explanatory diagram showing an enlarged portion of FIG. 3A enclosed by a dashed line. FIG. 4A is an explanatory diagram that shows a schematic diagram of the structure of a rare earth magnet after diffusion and infiltration of a modifier containing an excessive La content. FIG. 4B is an enlarged explanatory diagram of the portion indicated by the dashed line in FIG. 4A. FIG. 5 is an explanatory diagram showing a schematic diagram of a cooling device used in the strip casting method. FIG. 6 is a graph showing the relationship between the molar ratio x of La to the total rare earth elements in the modifier and the squareness ratio (Hr/Hc) for each sample. FIG. 7 is a graph showing the demagnetization curves of each sample. FIG. 8 is an explanatory diagram showing an SEM image and the results of area analysis of the sample of Example 1. FIG. 9 is an explanatory diagram showing an SEM image and the results of area analysis of the sample of Comparative Example 1. FIG. 10 is an explanatory diagram showing an SEM image and the results of area analysis of the sample of Comparative Example 5.

Below, an embodiment of the manufacturing method for rare earth magnets disclosed herein (hereinafter referred to as the "manufacturing method of the present disclosure") will be described in detail. Note that the embodiment described below does not limit the manufacturing method of the present disclosure.

Without being bound by theory, the findings regarding the reason why the manufacturing method of the present disclosure can obtain an R-T-B rare earth magnet with suppressed deterioration of squareness will be explained using drawings. FIG. 1A is an explanatory diagram showing a schematic structure of a rare earth magnet precursor used in the manufacturing method of the present disclosure. FIG. 1B is an explanatory diagram showing an enlarged view of the portion indicated by the dashed line in FIG. 1A. FIG. 2A is an explanatory diagram showing a schematic structure of a rare earth magnet obtained by the manufacturing method of the present disclosure. FIG. 2B is an explanatory diagram showing an enlarged view of the portion indicated by the dashed line in FIG. 2A. FIG. 3A is an explanatory diagram showing a schematic structure of a rare earth magnet after diffusion infiltration of a modifier with an insufficient La content. FIG. 3B is an explanatory diagram showing an enlarged view of the portion indicated by the dashed line in FIG. 3A. FIG. 4A is an explanatory diagram showing a schematic structure of a rare earth magnet after diffusion infiltration of a modifier with an excessive La content. FIG. 4B is an explanatory diagram showing an enlarged view of the portion indicated by the dashed line in FIG. 4A.

As shown in Figures 1A and 1B, the rare earth magnet precursor 50 used in the manufacturing method of the present disclosure comprises a main phase 10 and a grain boundary phase 20 that exists around the main phase 10. The grain boundary phase 20 has an adjacent portion 22 where two main phases 10 are adjacent to each other, and a triple point 24 that is surrounded by three main phases 10.

The main phase 10 is a phase having an R ₂ T ₁₄ B type crystal structure. In the grain boundary phase 20, various phases having a crystal structure richer in R than the main phase 10 are present. Among the phases of the grain boundary phase 20, the phase having an RT ₂ type crystal structure exhibits ferromagnetic behavior. When a part of the Fe in the rare earth magnet precursor 50 is replaced with Co, the phase 26 having an RT ₂ type crystal structure is likely to exist in the grain boundary phase 20, particularly in the adjacent portion 22. Such a phase 26 having an RT ₂ type crystal structure inhibits magnetic separation between the main phases 10, reduces the coercive force, and leads to a decrease in squareness.

When a certain proportion of La is present in the grain boundary phase 20 of the rare earth magnet precursor 50, the phase 26 having the _RT2 type crystal structure can be reduced.

Therefore, a modifier containing a predetermined ratio of La is diffused and infiltrated into the inside of the rare earth magnet precursor 50 as shown in Figures 1A and 1B. The modifier mainly diffuses and infiltrates into the grain boundary phase 20 of the rare earth magnet precursor 50. Therefore, as shown in Figures 2A and 2B, in the rare earth magnet 100 after the modifier is diffused and infiltrated, the phase 26 having the _RT2 type crystal structure that existed in the adjacent portion 22 of the rare earth magnet precursor 50 of Figures 1A and 1B is almost eliminated. It is considered that a phase that is even more R-rich than the phase 26 having the _RT2 type crystal structure is formed in the adjacent portion 22. It is also considered that a phase that is even more R-rich than the phase 26 having the _RT2 type crystal structure is formed in the triple point 24. These phases that are more R-rich than the phase 26 having the RT2 type crystal structure contribute to magnetically separating the main phases 10 from each other, thereby suppressing a decrease in coercivity and suppressing a decrease in squareness.

In contrast, when a modifier with an insufficient La content is diffused and infiltrated into the rare earth magnet precursor 50 as shown in Figures 1A and 1B, the phase ₂₆ having the RT2 type crystal structure present in the adjacent portion 22 cannot be sufficiently eliminated. Therefore, as shown in Figures 3A and 3B, in the rare earth magnet 100 after the modifier is diffused and infiltrated, many phases 26 having the _RT2 type crystal structure remain in the adjacent portion 22. As a result, the decrease in coercivity cannot be suppressed, and the decrease in squareness cannot be suppressed. It is considered that the diffusion and infiltration of the modifier forms a phase richer in R than before the diffusion and infiltration of the modifier at the triple point 24, but this does not suppress the decrease in coercivity and squareness.

In addition, when a modifier containing an excessive amount of La is diffused and infiltrated into the inside of the rare earth magnet precursor 50 as shown in FIG. 1A and FIG. 1B, a phase 26 having an _RT2 type crystal structure is reformed, particularly at the triple point 24, as shown in FIG. 4A and FIG. 4B. In order to eliminate the phase 26 having an _RT2 type crystal structure by the diffusion and infiltration of the modifier, the grain boundary phase 22 must dissolve the single Fe generated by the elimination. However, when the solid solution becomes saturated, the phase 26 having an _RT2 type crystal structure is reformed. Then, the coercive force and the squareness are reduced by the reformed phase 26 having an _RT2 type crystal structure. In addition, the excess La is present as a La phase 28 in the adjacent portion 22.

For this reason, by diffusing and penetrating a modifier containing a certain percentage of La into the interior of the rare earth magnet precursor 50, it is possible to obtain an R-T-B rare earth magnet with reduced deterioration in squareness.

To suppress the formation of phase 26 having an _RT2 type crystal structure, it is sufficient that rare earth magnet precursor 50 contains a predetermined percentage of La, even without diffusing and infiltrating the modifier as described above. Such rare earth magnet precursor 50 is obtained by cooling a molten metal containing a predetermined percentage of La, and at that time, main phase 10 also contains La. When main phase 10 contains a light rare earth element, the remanence decreases. Because La is a light rare earth element, such rare earth magnet precursor 50 has low remanence.

In contrast, in the manufacturing method disclosed herein, La is supplied to the grain boundary phase 20 by diffusion and penetration of the modifier. Therefore, the rare earth magnet precursor 50 used in the manufacturing method disclosed herein does not need to contain La. For this reason, the manufacturing method disclosed herein is advantageous in that it can suppress the decrease in coercivity and the decrease in squareness without decreasing the residual magnetization.

The constituent elements of the manufacturing method disclosed herein, based on the findings described above, are described below.

<Production Method>
The manufacturing method of the present disclosure includes a rare earth magnet precursor preparation step, a modifier diffusion and infiltration step, and optionally a heat treatment step. Each step will be described below.

<Rare earth magnet precursor preparation process>
First, a rare earth magnet precursor is prepared. As shown in FIG. 1, a rare earth magnet precursor 50 used in the manufacturing method of the present disclosure includes a main phase 10 and a grain boundary phase 20. The grain boundary phase 20 is a main phase. It exists around phase 10.

The main phase has an R ₂ T ₁₄ B type crystal structure. R is a rare earth element, and T is a transition metal element. In this specification, unless otherwise specified, the rare earth elements are 17 elements, namely Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Of these, Sc, Y, La, and Ce are light rare earth elements, unless otherwise specified. Furthermore, Pr, Nd, Pm, Sm, and Eu are medium rare earth elements, unless otherwise specified. And, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu are heavy rare earth elements, unless otherwise specified. In general, heavy rare earth elements are highly rare, and light rare earth elements are less rare. The rarity of medium rare earth elements is between that of heavy rare earth elements and light rare earth elements. Here, Sc is scandium, Y is yttrium, La is lanthanum, Ce is cerium, Pr is praseodymium, Nd is neodymium, Pm is promethium, Sm is samarium, Eu is eurobium, Gd is gadolinium, Tb is terbium, Dy is dysprosium, Ho is holmium, Er is erbium, Tm is thulium, Yb is ytterbium, and Lu is ruthenium.

The rare earth magnet precursor is obtained by cooling a molten metal having the composition. The details of the manufacturing method of the rare earth magnet precursor 50 will be described later. The particle size of the main phase 10 in the rare earth magnet precursor 50 depends on the cooling rate of the molten metal. If the cooling rate of the molten metal is slow, many phases having an RT ₂ type crystal structure are formed in the grain boundary phase 20. Therefore, when using the rare earth magnet precursor 50 obtained at such a cooling rate, the manufacturing method of the present disclosure is particularly useful. The average particle size of the main phase in the rare earth magnet precursor 50 obtained at such a cooling rate may be 1.0 μm or more, 3.0 μm or more, 5.0 μm or more, or 6.0 μm or more, and may be 10.0 μm or less, 8.0 μm or less, 6.0 μm or less, or 6.5 μm or less.

The average grain size of the main phase is measured as follows. A certain region is defined in a scanning electron microscope image or a transmission electron microscope image observed from the direction perpendicular to the easy axis of magnetization, and multiple lines are drawn perpendicular to the easy axis of magnetization for the main phase present in this certain region, and the diameter (length) of the main phase is calculated from the distance between the intersecting points within the main phase particle (cutting method). When the cross section of the main phase is close to a circle, it is converted to the diameter equivalent to the projected area circle. When the cross section of the main phase is close to a rectangle, it is converted to a rectangular approximation. The _D50 value of the diameter (length) distribution (particle size distribution) obtained in this way is the average grain size.

The rare earth magnet precursor contains Co. This can increase the Curie temperature of the rare earth magnet obtained by the manufacturing method of the present disclosure, and as a result, can improve the magnetic properties at high temperatures. The main element of the rare earth magnet precursor is typically Fe, and a part of the Fe is substituted with Co. The ratio of Co to Fe, in terms of molar ratio, can be, for example, 0.10 or more, 0.12 or more, 0.14 or more, or 0.16 or more, and can be 0.30 or less, 0.28 or less, 0.26 or less, 0.24 or less, 0.22 or less, or 0.20 or less. Note that Co is cobalt, and Fe is iron.

From the viewpoint of easily obtaining _{an R2T14B type} crystal structure and easily obtaining high remanent magnetization, the rare earth element in the rare earth magnet precursor is preferably one or more elements selected from the group consisting of Nd, Pr, Gd, Tb, Dy, and Ho. Typically, a portion of the rare earth element in the rare earth magnet precursor 50 may be replaced with an inexpensive light rare earth element. When a light rare earth element is contained, the remanent magnetization decreases, so the content ratio (substitution ratio) of the light rare earth element may be appropriately determined in consideration of the desired remanent magnetization.

The overall composition of the rare earth magnet precursor as described above can be expressed by the following formula, for example. Note that the overall composition of the rare earth magnet precursor refers to the combined composition of all of the main phases and grain boundary phases in the rare earth magnet precursor.

The composition of the rare earth magnet precursor can be expressed, for example, in molar ratio, by the formula R ¹ _y (Fe _(1-z) Co _z ) _(100-y-w-v) B _w M ¹ _v . In this formula, R ¹ is y moles, the total of Fe and Co is (100-y-w-v) moles, B is w moles, and M ¹ is v moles. Also, Fe _(1-z) Co _z means that, relative to the total of Fe and Co, there is (1-z) Fe and z Co in molar ratio.

In the above formula, R ¹ may be one or more elements selected from the group consisting of Nd, Pr, Gd, Tb, Dy, and Ho. In the above formula, Fe is iron, Co is cobalt, and B is boron. Also, M ¹ may be one or more elements selected from the group consisting of Ga, Al, Cu, Au, Ag, Zn, In, and Mn, as well as unavoidable impurity elements. Ga is gallium, Al is aluminum, Cu is copper, Au is gold, Ag is silver, Zn is zinc, In is indium, and Mn is manganese.

The constituent elements of the rare earth magnet precursor represented by the above formula are explained below.

R ^1
R1 is an essential component for the rare earth magnet obtained by the manufacturing method of the present disclosure. As described above, ^R1 is preferably one or more elements selected from the group consisting of Nd, Pr, Gd, Tb, Dy, and Ho. ^R1 is a constituent element of the main phase (phase having an _R2T14B type crystal structure). From the viewpoint of the balance between remanence and coercivity and price _, ^R1 is preferably one or more elements selected from the group consisting of Nd and Pr. Didymium may be used as ^R1 when Nd and Pr are coexisting.

Content of ^R1 In the above formula, the content of ^R1 is represented by y and satisfies 12.0≦y≦20.0. The value of y is the content relative to the entire rare earth magnet precursor, and corresponds to mol % (atomic %).

If y is 12.0 or more, the rare earth magnet precursor does not contain a large amount of αFe phase, and a sufficient amount of the main phase (phase having an R ₂ T ₁₄ B type crystal structure) can be obtained. From this viewpoint, y may be 12.5 or more, 13.0 or more, or 13.5 or more. On the other hand, if y is 20.0 or less, the grain boundary phase does not become excessive. From this viewpoint, y may be 19.0 or less, 18.0 or less, 17.0 or less, 16.0 or less, 15.0 or less, 14.0 or less, or 13.9 or less.

・B
B constitutes the main phase (a phase having an R ₂ T ₁₄ B type crystal structure) and affects the abundance ratio of the main phase and the grain boundary phase.

The content ratio of B is represented by w in the above formula. The value of w is the content ratio relative to the entire rare earth magnet precursor, and corresponds to mol% (atomic%). If w is 20.0 or less, a rare earth magnet precursor in which the main phase and the grain boundary phase are appropriately present can be obtained. From this viewpoint, w may be 18.0 or less, 16.0 or less, 14.0 or less, 12.0 or less, 10.0 or less, 8.0 or less, 6.0 or less, or 5.8 or less. On the other hand, if w is 5.0 or more, it is difficult for a large amount of phases having a crystal structure of Th ₂ Zn ₁₇ type and/or Th ₂ Ni ₁₇ type to be generated, and as a result, the formation of a phase having a crystal structure of R ₂ T ₁₄ B type is less inhibited. From this viewpoint, w may be 5.2 or more, 5.4 or more, 5.5 or more, or 5.7 or more.

・^M1
M1 is an element that can be contained within a range that does not impair the properties of the rare earth magnet obtained by the manufacturing method of the present disclosure. ^M1 may contain unavoidable impurity elements. In this specification, unavoidable impurity elements refer to impurity elements contained in raw materials or impurity elements mixed in during the manufacturing process, etc., whose inclusion cannot be avoided, or whose avoidance would lead to a significant increase in manufacturing costs. Impurity elements mixed in during the manufacturing process include elements that are contained within a range that does not affect the magnetic properties due to manufacturing reasons. In addition, unavoidable impurity elements include rare earth elements other than the rare earth elements selected as ^R1 and ^R2 that are inevitably mixed in for the reasons described above. ^R2 will be described later.

Examples of element ^M1 that can be contained within a range that does not impair the properties of the rare earth magnet obtained by the manufacturing method of the present disclosure include one or more elements selected from the group consisting of Ga, Al, Cu, Au, Ag, Zn, In, and Mn. As long as these elements are present at or below the upper limit of the content of ^M1 , these elements do not substantially affect the magnetic properties. Therefore, these elements may be treated as equivalent to inevitable impurity elements. In addition to these elements, ^M1 may contain inevitable impurity elements. As ^M1 , one or more elements selected from the group consisting of Ga, Al, and Cu, as well as inevitable impurity elements, are preferred.

In the above formula, the content ratio of ^M1 is represented by v. The value of v is the content ratio relative to the entire rare earth magnet precursor, and corresponds to mol% (atomic%). If the value of v is 2.0 or less, the magnetic properties of the rare earth magnet obtained by the manufacturing method of the present disclosure will not be impaired. From this perspective, v may be 1.5 or less, 1.0 or less, 0.65 or less, 0.6 or less, or 0.5 or less. Since it is impossible to completely eliminate Ga, Al, Cu, Au, Ag, Zn, In, and Mn as well as unavoidable impurity elements as ^M1 , there is no practical problem even if the lower limit of v is 0.05, 0.1, or 0.2.

Fe
Fe is a main component constituting the main phase (phase having an R ₂ T ₁₄ B type crystal structure) and the grain boundary phase. A part of Fe is substituted with Co.

・Co
Co is an element that substitutes for part of Fe in the main phase and grain boundary phase, which improves the Curie point of the rare earth magnet obtained by the manufacturing method of the present disclosure and improves the magnetic properties at high temperatures, in particular the remanent magnetization at high temperatures.

Molar ratio of Fe to Co In the above formula, the molar ratio of Fe to Co is (1-z):z. If z is 0.1 or more, the magnetic properties at high temperatures due to the increase in the Curie point, particularly the improvement in residual magnetization at high temperatures, is significantly observed. From this viewpoint, z may be 0.12 or more, 0.14 or more, or 0.16 or more. On the other hand, if z is 0.30 or less, the phase having the RT ₂ type crystal structure can be effectively eliminated by diffusing and infiltrating a modifier containing a predetermined proportion of La. From this viewpoint, z may be 0.28 or less, 0.26 or less, 0.24 or less, 0.22 or less, or 0.20 or less. In addition, since Co is expensive, it is advantageous to be in the above range.

Total content of Fe and Co The total content of Fe and Co is the remainder of R ¹ , B, and M ¹ described above, and is expressed as (100-y-w-v). As described above, the values of y, w, and v are the content ratios relative to the entire rare earth magnet precursor, so (100-y-w-v) corresponds to mol % (atomic %). When y, w, and v are within the ranges described above, a main phase 10 and grain boundary phase 20 as shown in FIG. 1A are obtained.

A method for producing a rare earth magnet precursor can be applied to the production of a rare earth magnet precursor by using a well-known method for producing an RTB rare earth magnet. This method is outlined below.

The molten metal having the composition of the rare earth magnet precursor is cooled. Examples of methods for cooling the molten metal include a method using a book mold and a strip casting method. The strip casting method is preferred from the viewpoint of obtaining a stable cooling rate and being able to continuously cool a large amount of molten metal. Here, the strip casting method is explained using drawings. Figure 5 is an explanatory diagram that shows a schematic diagram of a cooling device used in the strip casting method.

The cooling device 70 includes a melting furnace 71, a tundish 73, and a cooling roll 74. Raw materials are melted in the melting furnace 71 to prepare a molten metal 72 having the above-mentioned composition. The molten metal 72 is supplied to the tundish 73 at a constant rate. The molten metal 72 supplied to the tundish 73 is supplied to the cooling roll 74 by its own weight from the end of the tundish 73.

The tundish 73 is made of ceramics or the like, and can temporarily store the molten metal 72 that is continuously supplied from the melting furnace 71 at a predetermined flow rate, and can straighten the flow of the molten metal 72 to the cooling roll 74. The tundish 73 also has the function of adjusting the temperature of the molten metal 72 just before it reaches the cooling roll 74.

The cooling roll 74 is made of a material with high thermal conductivity, such as copper or chromium, and the surface of the cooling roll 74 is chrome plated or the like to prevent corrosion by the high-temperature molten metal. The cooling roll 74 can be rotated in the direction of the arrow at a predetermined rotational speed by a drive device (not shown).

To obtain a main phase having an average grain size of 1 to 10 μm, the cooling rate of the molten metal may be, for example, 5° C./sec or more, 10° C./sec or more, or 10 ² ° C./sec or more, and 5×10 ³ ° C./sec or less, 10 ³ ° C./sec or less, or 5° C.×10 ² ° C./sec or less. To cool the molten metal at such a rate, the peripheral speed of the cooling roll 74 may be 0.5 m/s or more, 1.0 m/s or more, or 1.5 m/s or more, and 3.0 m/s or less, 2.5 m/s or less, or 2.0 m/s or less.

The temperature of the molten metal when it is supplied from the end of the tundish 73 to the cooling roll 74 may be 1350°C or more, 1400°C or more, or 1450°C or more, and may be 1600°C or less, 1550°C or less, or 1500°C or less.

The molten metal 72 is cooled and solidified on the outer circumference of the cooling roll 74, becomes a magnetic alloy 75, peels off from the cooling roll 74, and is collected by a collection device (not shown). The magnetic alloy 75 is typically in the form of a thin ribbon or flakes. When cooling the molten metal using the strip casting method, an inert gas atmosphere is preferable to prevent oxidation of the molten metal. The inert gas atmosphere includes a nitrogen gas atmosphere.

The ribbon or flakes obtained as described above are pulverized to obtain magnetic powder. The pulverization method is not particularly limited, but examples include a method in which the magnetic ribbon or flakes are coarsely pulverized and then further pulverized with a jet mill and/or a cutter mill. Examples of the coarse pulverization method include a method using a hammer mill and a method in which the magnetic ribbon and/or flakes are pulverized by hydrogen embrittlement. These methods may be combined.

The magnetic powder obtained as described above is sintered at 900 to 1100°C to obtain a sintered body. In order to sinter without pressure and increase the density of the sintered body, sintering is performed at high temperatures for a long period of time. The sintering temperature may be, for example, 900°C or higher, 950°C or higher, or 1000°C or higher, and 1100°C or lower, 1050°C or lower, or 1040°C or lower. By sintering at such temperatures, so-called liquid phase sintering occurs, in which sintering progresses while part of the magnetic powder is in a liquid phase. The sintering time may be, for example, 1 hour or more, 2 hours or more, 3 hours or more, or 4 hours or more, and 24 hours or less, 18 hours or less, 12 hours or less, or 6 hours or less. In order to suppress oxidation of the magnetic powder during sintering, the sintering atmosphere is preferably an inert gas atmosphere. The inert gas atmosphere includes a nitrogen gas atmosphere.

To improve the density of the sintered body, the magnetic powder may be compressed before sintering, and the compressed powder may be sintered. The compacting pressure during compaction may be, for example, 50 MPa or more, 100 MPa or more, 200 MPa or more, or 300 MPa or more, and 1000 MPa or less, 800 MPa or less, or 600 MPa or less. To impart anisotropy to the sintered body, the magnetic powder may be compressed while a magnetic field is applied. The magnetic field applied may be 0.1 T or more, 0.5 T or more, 1.0 T or more, 1.5 T or more, or 2.0 T or more, and may be 10.0 T or less, 8.0 T or less, 6.0 T or less, or 4.0 T or less.

The modifier is diffused and infiltrated into the rare earth magnet precursor obtained by the method described above. This is explained next.

Modifier diffusion and penetration process
A modifier containing rare earth elements is diffused and infiltrated into the interior of the rare earth magnet precursor. This modifier contains La, and the molar ratio x of La to the total rare earth elements in the modifier satisfies 0.09<x<0.19. By setting the molar ratio x of La in the above-mentioned range, it is possible to eliminate the phase having the _RT2 type crystal structure present in the grain boundary phase of the rare earth magnet precursor. As a result, it is possible to suppress the decrease in coercivity and the decrease in squareness.

The melting point of rare earth elements is generally high. Modifiers are typically composed of alloys of rare earth elements and transition metal elements other than rare earth elements. Therefore, the melting point of such modifiers is lower than the melting point of the rare earth elements. Therefore, the modifiers can typically be melted at temperatures lower than the melting point of the rare earth elements, and the molten modifier can diffuse and penetrate into the interior of the rare earth magnet precursor.

The content ratio of the transition metal elements other than the rare earth elements in the modifier may be appropriately determined so that the melt of the _modifier can diffuse and penetrate into the inside of the rare earth magnet precursor. Except for lowering the melting point of the modifier, the transition metal elements other than the rare earth elements in the modifier hardly contribute to the elimination of the phase having the _RT2 type crystal structure present in the grain boundary phase of the rare earth magnet precursor. Therefore, the elimination of the phase having the RT2 type crystal structure present in the grain boundary phase of the rare earth magnet precursor may be mainly considered with respect to the molar ratio x of La to the total rare earth elements in the modifier.

From the viewpoint of completely eliminating the phase having an _RT2 type crystal structure present in the grain boundary phase of the rare earth magnet precursor, the molar ratio x of La may be more than 0.09, 0.10 or more, 0.11 or more, or 0.12 or more. From the viewpoint of preventing the phase having an _RT2 type crystal structure present in the grain boundary phase of the rare earth magnet precursor from being reformed, the molar ratio x of La may be less than 0.19, 0.18 or less, 0.17 or less, 0.16 or less, 0.15 or less, 0.14 or less, or 0.13 or less.

If the composition of the modifier described so far were expressed as a formula, it would look something like this:

The composition of the modifier can be expressed, for example, in molar ratio, by the formula (R ² _(1-x) La _x ) _(1-s) M ² _s . In this formula, the total of R ² and La is (1-s) moles, and M ² is s moles. Also, R ² _(1-x) La _x ) means that, relative to the total of R ² and La, there is (1-x) R ² and x La in molar ratio.

In the above formula, ^R2 may be one or more elements selected from the group consisting of Nd, Pr, Gd, Tb, Dy, and Ho. ^M2 may be a metal element other than a rare earth element that is alloyed with ^R2 , or an inevitable impurity element.

The constituent elements of the modifier represented by the above formula are explained below.

^R2
R2 is a main element of the modifier. As described above, ^R2 may be one or more elements selected from the group consisting of Nd, Pr, Gd, Tb, Dy, and Ho, and magnetically separates the main phases to improve the coercive force. Therefore, among the above rare earth elements, it is preferable that at least a part of ^R2 is a heavy rare earth element. For these reasons, it is preferable that ^R2 is one or more elements selected from the group consisting of Nd and Tb. When Nd and Pr are coexisted as ^R2 , didymium may be used.

・La
As explained above, La contributes to the elimination of a phase having an _RT2 type crystal structure present in the grain boundary phase of a rare earth magnet precursor. The proportion of La is as explained above, and x in the above formula may be greater than 0.09, 0.10 or more, 0.11 or more, or 0.12 or more, or may be less than 0.19, 0.18 or less, 0.17 or less, 0.16 or less, 0.15 or less, 0.14 or less, or 0.13 or less.

・^M2
M2 is a metal element other than a rare earth element that is alloyed with ^R2 and an inevitable impurity element, and typically ^M2 is an alloy element and an inevitable impurity element that lowers the melting point of ^R2 _{(1-s) M2s} ^below the melting point of ^R2 . Examples of ^M2 include one or more elements selected from Cu, Al, Co, and Fe, and inevitable impurity elements. From the viewpoint of lowering the melting point of ^R2 (1- _{s )} ^M2s , Cu is preferred as ^M2 .

Molar ratio of R ² and M ² R ² and M ² constitute an alloy having a composition represented by the formula R ² _(1-s) M ² _s in molar ratio. s may be 0.05 to 0.40. If s is 0.05 or more, the modifier can be diffused and infiltrated into the inside of the rare earth magnet precursor at a temperature that can avoid coarsening of the main phase. From this viewpoint, s is preferably 0.10 or more, more preferably 0.15 or more. On the other hand, if s is 0.40 or less, after the modifier is diffused and infiltrated into the inside of the rare earth magnet precursor, the content of M ² remaining in the grain boundary phase of the rare earth magnet is suppressed, which contributes to suppressing the decrease in remanence magnetization. From this viewpoint, s may be 0.35 or less, 0.30 or less, 0.25 or less, 0.20 or less, or 0.18 or less.

There are no particular limitations on the method of preparing the modifier having the above-mentioned composition, and well-known methods can be adopted. For example, a method of obtaining a thin strip and/or flakes from a molten metal having the composition of the modifier using a liquid quenching method or a strip casting method is exemplified. In this method, since the molten metal is quenched, there is little segregation in the modifier, which is preferable. In addition, a method of preparing the modifier is, for example, casting a molten metal having the composition of the modifier into a mold such as a book mold. In this method, a large amount of the modifier can be obtained relatively easily. In order to reduce the segregation of the modifier, it is preferable that the book mold is made of a material with high thermal conductivity. In addition, it is preferable to suppress the segregation by subjecting the cast material to a homogenizing heat treatment. In addition, a method of preparing the modifier is to charge the raw material of the modifier into a container, arc-melt the raw material in the container, and cool the molten material to obtain an ingot. In this method, even if the melting point of the raw material is high, the modifier can be obtained relatively easily. To reduce segregation of the modifier, it is preferable to subject the ingot to homogenization heat treatment.

There are no particular limitations on the method for diffusing and penetrating the modifier into the rare earth magnet precursor, and any well-known method can be used. A typical method involves bringing the modifier into contact with the rare earth magnet precursor to obtain a contact body, heating the contact body, and diffusing and penetrating the molten modifier into the rare earth magnet precursor.

There are no particular limitations on the form of the contact body, so long as the rare earth magnet precursor is in contact with the modifier. Examples of the form of the contact body include a form in which a rare earth magnet precursor is in contact with a ribbon and/or flake modifier obtained by a strip casting method, or a form in which a modifier powder made by pulverizing strip cast material, book mold material, and/or arc melting and solidifying material is in contact with the rare earth magnet precursor.

There are no particular limitations on the diffusion temperature, so long as the modifier diffuses into the rare earth magnet precursor and the main phase does not become coarse. Typically, the diffusion temperature is equal to or higher than the melting point of the modifier and equal to or lower than the sintering temperature at which the rare earth magnet precursor is obtained. The diffusion temperature may be, for example, 750°C or higher, 775°C or higher, or 800°C or higher, and may be 1000°C or lower, 950°C or lower, 925°C or lower, or 900°C or lower.

During the diffusion and penetration of the modifier, in order to suppress oxidation of the rare earth magnet precursor and/or the modifier, the diffusion and penetration atmosphere is preferably an inert gas atmosphere. Examples of the inert gas atmosphere include a nitrogen gas atmosphere.

The diffusion and penetration of the modifier may also serve as a heat treatment, which will be described later. In this case, the heating and cooling conditions of the modifier may be appropriately determined with reference to the heat treatment conditions. This allows the diffusion and penetration of the modifier to magnetically separate the main phases and align the interfaces between the main phases and grain boundary phases, improving the magnetic properties, particularly the coercive force.

In the manufacturing method of the present disclosure, a modifier containing a predetermined ratio of La is diffused and infiltrated into the inside of the rare earth magnet precursor, thereby eliminating the phase having the RT ₂ type crystal structure. In the rare earth magnet precursor before the modifier is diffused and infiltrated, the coercive force is very small, but by diffusing and infiltrating rare earth elements other than La in the modifier, the main phases are magnetically separated from each other, and the coercive force is significantly improved. Therefore, the amount of the modifier diffused and infiltrated can be about the same as that of the conventional modifier. The amount of the modifier diffused and infiltrated may be, for example, 0.1 parts or more, 0.5 parts or more, 1.0 parts or more, 1.5 parts or more, 2.0 parts or more, 2.3 parts or more, 2.5 parts or more, 3.0 parts or more, or 10.0 parts or less, 9.0 parts or less, 8.0 parts or less, 7.0 parts or less, 6.0 parts or less, 5.0 parts or less, 4.0 parts or less, or 3.5 parts or less, relative to 100 parts of the rare earth magnet precursor. In particular, the amount of the modifier diffused and penetrated may be 2.0 parts or more, 2.3 parts or more, 2.5 parts or more, or 3.0 parts or more, and may be 6.0 parts or less, 5.5 parts or less, or 5.0 parts or less.

<Heat treatment process>
When the grain size of the main phase is 1 to 10 μm, the rare earth magnet precursor may be optionally heat-treated before and/or after the diffusion and penetration of the modifier to align the interface between the main phase and the grain boundary phase and improve the magnetic properties, especially the coercive force. In order to suppress oxidation of the rare earth magnet precursor during the heat treatment, the heat treatment atmosphere is preferably an inert gas atmosphere. The inert gas atmosphere includes a nitrogen gas atmosphere. Such heat treatment may include heat treating the rare earth magnet precursor at 450 to 650° C. for 30 to 180 minutes, preferably at 500 to 600° C. for 45 to 90 minutes.

《Transform》
In addition to what has been described so far, the manufacturing method of the present disclosure can be modified in various ways within the scope of the contents described in the claims. In the above description, the rare earth magnet precursor is a sintered body, but this is not limited to this, and for example, a so-called two-alloy method can be adopted. Specifically, a powdered rare earth magnet precursor is prepared, mixed with a modifier powder, and sintered. The sintering temperature and sintering time can be appropriately determined so as to achieve both pressureless sintering and diffusion and penetration of the modifier. The sintering temperature may be, for example, 900°C or higher, 950°C or higher, or 1000°C or higher, and 1100°C or lower, 1050°C or lower, or 1040°C or lower. The sintering time may be, for example, 1 hour or more, 2 hours or more, 3 hours or more, or 4 hours or more, and 24 hours or less, 18 hours or less, 12 hours or less, or 6 hours or less. In order to suppress oxidation of the rare earth magnet precursor and the modifier during sintering, the sintering atmosphere is preferably an inert gas atmosphere. The inert gas atmosphere includes a nitrogen gas atmosphere.

The manufacturing method of the present disclosure will be explained in more detail below with reference to examples and comparative examples. Note that the manufacturing method of the present disclosure is not limited to the conditions used in the following examples.

<Sample preparation>
Samples of Example 1 and Comparative Examples 1 to 8 were prepared according to the following procedure.

A strip cast material (magnetic ribbon) having the composition shown in Tables 1-1 and 1-2 was prepared. This strip cast material was coarsely crushed by hydrogen embrittlement, and then further crushed using a jet mill to obtain a magnetic powder. The magnetic powder was pressureless sintered (pressureless liquid phase sintering) at 1050°C for 4 hours. After sintering, the sintered body was cooled to room temperature to obtain a rare earth magnet precursor. The modifier ribbon was brought into contact with the rare earth magnet precursor to obtain a contact body. The contact body was heated at 950°C for 165 minutes to diffuse and infiltrate the modifier into the rare earth magnet precursor. The rare earth magnet precursor after diffusion and infiltration was then cooled to room temperature at a rate of 1.0°C/min, and then heat-treated at 550°C for 60 minutes to obtain each sample. However, the heat treatment temperature for the sample of Comparative Example 8 was 500°C.

"evaluation"
The magnetic properties of each sample were measured at 27° C. using a vibrating sample magnetometer (VSM).

Each sample was observed using a scanning electron microscope (SEM) to determine the average grain size of the main phase. In addition, surface analysis was performed for Fe, Co, Nd, Tb, and La using a scanning electron microscope-energy dispersive X-ray spectroscope (SEM-EDX).

The results are shown in Tables 1-1 and 1-2 and Figures 6 to 10. Figure 6 is a graph showing the relationship between the molar ratio x of La to the total rare earth elements in the modifier and the squareness ratio (Hr/Hc) for each sample. Figure 7 is a graph showing the demagnetization curves for each sample. Figure 8 is an explanatory diagram showing SEM images and area analysis results for the sample of Example 1. Figure 9 is an explanatory diagram showing SEM images and area analysis results for the sample of Comparative Example 1. Figure 10 is an explanatory diagram showing SEM images and area analysis results for the sample of Comparative Example 5. In the area analysis results of Figures 8 to 10, the areas where the concentration of the relevant element is high are bright fields.

As can be seen from Table 1 and Figures 6 to 7, it can be understood that the decrease in squareness is suppressed when x is in the range of 0.09<x<0.19. In addition, in Figures 8 to 10, the parts where the surface analysis result of Fe is a dark field and the surface analysis result of Co is a bright field are parts where a phase having an RT ₂ type crystal structure exists. Such parts are only observed slightly in the sample of Example 1. In contrast, in the sample of Comparative Example 1 in which a modifier containing substantially no La was diffused and infiltrated, such parts are observed in many adjacent parts. In addition, in the sample of Comparative Example 5 in which a modifier containing an excessive amount of La was diffused and infiltrated, such parts are observed in many triple points, and it is considered that a phase having an RT ₂ type crystal structure is reformed. In addition, it is observed that an excess of La exists in the adjacent parts.

These results confirm the effectiveness of the manufacturing method disclosed herein.

REFERENCE SIGNS LIST 10 Main phase 20 Grain boundary phase 22 Adjacent portion 24 Triple point 26 Phase having _RT2 type crystal structure 28 La phase 50 Rare earth magnet precursor 70 Cooling device 71 Melting furnace 72 Molten metal 73 Tundish 74 Chill roll 75 Magnetic alloy 100 Rare earth magnet

Claims (2)

preparing a rare earth magnet precursor having a main _{phase having a crystal structure of R2T14B type} (where R is a rare earth element and T is a transition metal element) and a grain boundary phase existing around the main phase; and diffusing and penetrating a modifier containing a rare earth element into the rare earth magnet precursor;
Including,
The rare earth magnet precursor has an overall composition in terms of molar ratio represented by the formula R ¹ _y (Fe _(1-z) Co _z ) _(100-y-w-v) B _w M ¹ _v , where R ¹ is one or more elements selected from the group consisting of Nd and Pr, M ¹ is one or more elements selected from the group consisting of Ga, Al, and Cu, and unavoidable impurity elements,
12.0≦y≦20.0,
0.10≦z≦0.30,
5.0≦w≦20.0, and
0≦v≦2.0
) is expressed as
The average grain size of the main phase is 1 to 10 μm,
The composition of the modifier is represented by the formula (R ² _(1-x) La _{x (1-s)} M 2 ^s ₍ wherein R ² is one or more elements selected from the group consisting of Nd and Tb, M ² is Cu and an unavoidable impurity element, and 0.09<x<0.19 and 0.05≦s≦0.40) in molar ratio.
A method for manufacturing rare earth magnets. y is equal to or greater than 13.5 and equal to or less than 13.9;
w is equal to or greater than 5.7 and equal to or less than 5.8;
v is equal to or greater than 0.2 and equal to or less than 0.5;
x is equal to or greater than 0.12 and equal to or less than 0.13;
s is 0.15 or more and 0.20 or less;
The method for producing the rare earth magnet according to claim 1.

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