JPH07123083B2 - Cast rare earth-method for manufacturing iron-based permanent magnets - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cast rare earth-iron permanent magnet.

[Summary of the Invention] The present invention does not crush and sinter a cast ingot,
A rare earth-iron-based permanent magnet is obtained by magnetically hardening only by heat treatment after casting so that the casting macrostructure becomes columnar crystals.

[Prior Art] Conventionally, the following three methods have been reported for producing an R-Fe-B magnet.

(1) Sintering method based on powder metallurgy (Reference 1).

(2) Using a quenching ribbon manufacturing apparatus used for producing an amorphous alloy, a quenching thin piece having a thickness of about 30 μm is formed, and the thin piece is made into a magnet by a resin bonding method (reference document 2).

(3) A method of mechanically orienting the same thin piece used in the method of (2) by a two-step hot pressing method (reference document 2).

Reference 1 M. Sagawa, S. Fujimura, N. Togawa, H. Yamamot
o and Y. Matsuura; J.Appl.Phya.Vol.55 (6), 15 March
1984, P2083 Reference 2 RWLee; Appl.Phys.Lett.Vol.46 (8), 15
April 1985, P790 The above-mentioned prior art will be described along with the literature. First, in the sintering method (1), an alloy ingot is produced by melting and casting, and is crushed into magnet powder having a particle size of about 3 μm. Magnet powder is kneaded with a binder that serves as a molding aid,
It is press-molded in a magnetic field to form a molded body. The compact is sintered in argon at a temperature around 1100 ° C. for 1 hour and then rapidly cooled to room temperature. The coercive force is further improved by post-processing at a temperature around 600 ° C after sintering.

(2) First, R- at the optimum rotation speed of the quenching ribbon manufacturing apparatus.
Create a quenched ribbon of Fe-B alloy. The obtained ribbon has a thickness of 30μ
It is in the shape of a ribbon of m, and is a collection of polycrystals with a diameter of 1000Å or less. The ribbon is brittle and easily cracked, and the crystal grains are isotropically distributed, so that it is magnetically isotropic. If this ribbon is made to have an appropriate grain size and kneaded with a resin and press-molded, it is possible to fill about 85 volume% with a pressure of about 7 ton / cm ² .

In the manufacturing method of (3), first, a ribbon-shaped quenched ribbon or a strip of ribbon is placed at about 700 in a vacuum or in an inert atmosphere.
Place in graphite or other heat-resistant press mold preheated to ℃. Uniaxial pressure is applied when the ribbon reaches the desired temperature. The temperature and time are not specified,
T = 725 ± 25 ° C, pressure is P for sufficient plasticity.
About 1.4 ton / cm ² is suitable. At this stage the magnets are generally isotropic although they are slightly oriented in the pressing direction. The next hot pressing is performed with a mold having a large area. Most commonly, press at 700 ° C and 0.7 ton / cm ² for a few seconds. Then, the sample becomes 1/2 of the initial thickness, the easy axis of magnetization is oriented parallel to the pressing direction, and the alloy becomes anisotropic. These steps can produce a dense and anisotropic R-Fe-B magnet by this method called a two-stage hot-press procedure. In addition, the crystal grains of the ribbon ribbon made by the first melt spinning method should be smaller than the grain size when it shows the maximum coercive force, and later the crystal grains become coarse during hot pressing. Make sure the particle size is optimal.

[Problems to be Solved by the Invention] Although the R-Fe-B magnets can be produced for some time by the above-mentioned conventional techniques, the production methods utilizing these techniques have the following drawbacks. There is. In the sintering method of (1), it is essential to make the alloy into a powder, but since the R-Fe-B alloy is very active with respect to oxygen, if powdered, excessive oxidation will occur, resulting in sintering. The oxygen concentration in the body is inevitably high. Also, when molding the powder, a molding aid, such as zinc stearate, must be used, which is removed beforehand during the sintering process, but a few tenths of the form of carbon in the magnet body. Will remain. This carbon significantly deteriorates the magnetic performance of R-Fe-B. The green body is the green body after press molding by adding a molding aid. It is very fragile and difficult to handle. Therefore, it is a great disadvantage that it takes a lot of time and effort to put them neatly in the sintering furnace. Due to these drawbacks, generally speaking, the production of R-Fe-B system sintered magnets requires not only expensive equipment, but also poor production efficiency and high magnet production costs. I will end up. Therefore,
It is hard to say that this is a manufacturing method that can bring out the low cost of raw materials for R-Fe-B magnets.

The manufacturing methods (2) and (3) use a vacuum melt spinning device. This device is currently very unproductive,
Moreover, it is expensive. In (2), since it is isotropic in principle, it is a low energy product, and since the squareness of the hysteresis loop is not good, it is disadvantageous in terms of temperature characteristics and use. The method (3) is a unique method in which hot pressing is used in two steps, but it cannot be denied that it will be very inefficient when actually considering mass production. Further, as a drawback of the magnets (1) and (3), the mechanical strength is low. These magnets are magnets that were originally in the form of powder or ribbon, and were sintered or compression bonded at high temperature. Therefore, chipping is likely to occur in handling and handling becomes very difficult.

The method for producing an R-Fe-B magnet according to the present invention solves these drawbacks, and an object thereof is to provide a high-performance magnet at low cost.

[Means for Solving Problems] The present invention relates to a method for producing a cast rare earth-iron-based permanent magnet, specifically, 8 to 25 atomic% of R, 2 to 8 atomic% of B, and the balance. Melts an alloy consisting of iron and other impurities inevitable in production, casts the cast macrostructure into columnar crystals, and then heat-treats the cast ingot at a temperature of 500 ° C. or more to magnetically It is characterized by being cured.

As described above, the sintering method and the quenching method, which are existing manufacturing methods for rare earth-iron-based permanent magnets, have major drawbacks such as difficulty in powder management by pulverization, poor productivity, and low mechanical strength. is doing. In order to improve these drawbacks, the present inventors have started research on an alloy capable of obtaining a coercive force in the bulk state, and it is possible to obtain the coercive force in the bulk state in the above composition. When the cast structure is columnar crystal at this time, coercive force is easily obtained, and since it becomes an anisotropic magnet by utilizing the anisotropy of the columnar crystal, rather than using an equiaxed crystal, The inventors have invented that a higher performance permanent magnet can be obtained. In this method, since it is not necessary to crush the cast ingot, there is no need to perform the strict atmosphere control as in the sintering method, and the equipment cost is greatly reduced. Further, by magnetizing the alloy in the as-cast state, it does not go through the powder state, and as a result, the mutual coupling of the crystal grains becomes very strong and the mechanical strength is greatly improved. There is Hiroaki Miho et al. (The Japan Institute of Metals, Autumn Talk, 1985, Lecture No. (544)) in the same system, but this study is not only different from the present invention in the composition range, but also in the macro organization. No mention is made of any change in performance due to the above, and the performance is greatly inferior to the present invention. Also, after magnetically hardening, the secondary processing to obtain the desired shape is very easy in this system because its bending strength and compressive strength are greater than those of conventional samarium-cobalt rare earth magnets. .

As for the composition of the conventional R-Fe-B system magnet, as typified by Reference 1, R ₁₅ Fe ₇₇ B ₈ was considered to be the optimum composition. This composition has a composition of R · B in comparison with the composition R _11.7 Fe _82.4 B _{5.9 in} which the main phase R ₂ Fe ₁₄ B compound of the R—Fe—B system magnet is expressed in atomic percentage.
It has shifted to the side rich in both elements. This is explained from the point that not only the main phase but also a non-magnetic phase called Rrich system / Brich phase is necessary to obtain the coercive force. On the contrary, in the composition according to the present invention, on the contrary, a peak value is present when the composition shifts to the side with less B. This is a feature of this alloy. Firstly, the crystal grains become finer when the amount of B is reduced, and secondly, the crystal grains become finer because the molten metal for forming good columnar crystals is rapidly cooled. Therefore, it is considered that the composition range is peculiar to the magnet according to the present invention having the nucleation type coercive force mechanism.

The use of columnar crystals as a permanent magnet material has been carried out not only in Alnico magnets but also in rare earth magnet-based samarium-cobalt magnets. is doing. (T.Shimoda et al., Proceedings of the fifth intern
ational Workshop on Rare Earth-Cobalt Permanent M
agrets.1981.P595) Also in the present invention, obtaining columnar crystals in the cast state is an important point for high performance magnetization. That is, the process of obtaining coercive force by heat treatment is due to diffusion, and like samarium-cobalt, columnar crystals are more likely to obtain coercive force.
Further, the present magnet has a property that the easy axis of magnetization is oriented in a plane perpendicular to the columnar crystals, so that an in-plane anisotropic magnet can be produced by using the columnar crystals.

The reasons for limiting the composition of the permanent magnet according to the present invention will be described below. As rare earths, Y, La, Ce, Pr, Nd, Sm, Eu, G
The candidates are d, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and one or more of these can be used in combination. The highest magnetic properties are obtained with Pr. Therefore, Pr, Pr-Nd alloy, Ce-Pr-Nd alloy, etc. are practically used. In addition, a small amount of additional elements, such as heavy rare earth element Dy,
Tb, Al, Mo, Si, etc. are effective in improving the coercive force. R-
Main phase of Fe-B based magnet is R ₂ Fe ₁₄ B. Therefore, if R is less than 8 atomic%, the above compound is no longer formed and a cubic crystal structure having the same structure as α-iron is obtained, so that high magnetic properties cannot be obtained. On the other hand, if R exceeds 25 atomic%, the non-magnetic Rrich phase increases and the magnetic properties deteriorate remarkably. Therefore, the range of R is
8-25 atom% is suitable.

B is an essential element for forming the R ₂ Fe ₁₄ B phase, and 2
If it is less than atomic%, a high coercive force cannot be expected because it becomes a rhombohedral R-Fe system. However, when 8 atom% or more is added like a magnet produced by the conventional sintering method, on the contrary, the coercive force in the cast state cannot be obtained. Therefore, the amount of B is preferably in the range of 2 to 8 atomic%.

Example 1 A manufacturing process diagram according to the present invention is shown in FIG. First, an alloy having a desired composition is melted in an induction furnace and cast in an iron mold to form columnar crystals. Next, in order to obtain an in-plane anisotropic magnet, secondary processing was performed so that the plane perpendicular to the columnar crystal was the measurement direction, and a sample was prepared. Further heat treatment is applied to magnetically harden. In this example, the Pr ₁₄ Fe ₈₂ B ₄ composition is taken as a representative composition, and the coercive force iH depends on the heat treatment temperature, time and macrostructure.
I caught the change of c. As shown in Fig. 2, 800-1000 ℃
IHc also increases with increasing temperature and time. This indicates that the mechanism of iHc is diffusion-dominated rather than specific phase precipitation. Further, the caustic, equiaxed crystal sample as a comparative example has a very small coercive force even though it is heat-treated at 1000 ° C. The main phase of this magnet is R ₂ Fe ₁₄ B, which is a peritectic reaction Fe + L → R ₂ Fe ₁₄ B in which the iron phase is the primary crystal from the molten metal, and the primary crystal size largely depends on the cooling rate. Therefore, it is considered that equiaxed crystals with a slow cooling rate have large primary crystals and become coarse, and it takes time to diffuse into the main phase.

[Example 2] A composition as shown in Table 1 was dissolved and a magnet was produced by the method shown in Fig. 1. However, all annealing treatments were performed at 1000 ° C. for 24 hours.

The results obtained are shown in Table 2.

[Example 3] Next, using the alloys of sample Nos. 2 and 6 and 7 of Example 2,
A sintered magnet was produced based on Reference Document 1, and a sample having a length of 36 mm, a width of 4 mm and a thickness of 3 mm was cut out based on JIS R1601 and the bending strength was compared with that of the product of the present invention. The results are shown in Table 3. No. 2 is a representative composition according to the present invention, No. 7 is a composition near the optimum composition of the sintering method according to Reference 1, and No. 6 is an intermediate composition. It can be seen from Table 3 that the mechanical strength of the present invention is superior regardless of the composition.

[Effects of the Invention] As described above, according to the present invention, it is possible to obtain a coercive force in a bulk state in a composition range where it is difficult to obtain a coercive force by a conventional sintering method, which is excellent in mechanical strength, It is possible to obtain a simplified cast rare earth-iron magnet.

[Brief description of drawings]

FIG. 1 is a manufacturing process drawing of the R—Fe—B system magnet of the present invention. Figure 2 shows the change in coercive force of Pr ₁₄ Fe ₈₂ B ₄ alloy by heat treatment.

Claims (1)

[Claims] 1. Atomic percentage of R8-25% (where R is Y
At least one kind of rare earth element containing B), B2 to 8%, and the balance of iron and other alloys unavoidable in production are melted and cast so that the casting macrostructure becomes columnar crystals. A method for producing a cast rare earth-iron-based permanent magnet, characterized in that a cast ingot is heat-treated at a temperature of 500 ° C. or higher to magnetically harden to obtain a cast magnet made of an alloy lump.