JP6731633B2 - Method for manufacturing Mn-Al permanent magnet - Google Patents

Description

The present invention relates to the production how the Mn-Al permanent magnets.

In recent years, permanent magnets called neodymium magnets (Nd-Fe-B magnets) have been widely used for applications such as motors of hybrid cars. Although the neodymium magnet has a high coercive force, it has a problem that the coercive force decreases as the temperature rises when a motor or the like is used. With respect to this problem, in order to satisfy the requirement of high temperature and high coercive force, the rare element Dy element is added to the Nd element.

As a substitute for such a neodymium magnet, there is a demand for a material having high magnet characteristics, especially at high temperatures, without adding a rare element. In this regard, since Mn-based permanent magnet materials are highly elementally superior, various materials have been studied for practical use.

Examples of Mn-based permanent magnet materials include MnBi as disclosed in Patent Document 1.

In addition, Mn-Al is known as an alloy-based magnet and has an advantage that it is excellent in workability. The characteristic of the magnetic force lies between the ferrite magnet and the neodymium magnet.

However, it is difficult to stabilize Mn-Al because the τ phase which is a permanent magnet is a non-equilibrium phase. Therefore, the magnetic force actually obtained as the Mn-Al permanent magnet was very small.

On the other hand, in order to stabilize the τ phase, it has been practiced to add a C element in an amount of several wt% to obtain a Mn-Al-C permanent magnet as in Patent Document 2.

JP, 2005-63725, A JP, 10-270224, A

Although various studies have been made on Mn-based permanent magnet materials, the demand for materials satisfying high temperature and high coercive force has been increasing as described above. Further, as demand increases, it is also required to be able to easily manufacture.

Therefore, it is expected to provide a new manufacturing method and a permanent magnet that replaces the conventional manufacturing method of Mn-Al-C for Mn-Al-based permanent magnets.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for manufacturing a Mn-Al permanent magnet that does not require addition of a C element .

In order to achieve the above object, the method for producing an Mn-Al permanent magnet according to the first aspect of the present invention is
A step of performing a first heat treatment on a material composed of Mn, Al, and unavoidable impurities at a temperature at which a Mn-Al-based ε-phase is formed;
A step of performing a second heat treatment while applying a magnetic field having a magnetic flux density of 10 T or more to the material in which the ε phase is generated, and generating the τ phase.

The temperature of the second heat treatment is a temperature equal to or lower than the Curie temperature of the material,
It may be that.

The temperature of the second heat treatment is 300 to 350°C.
It may be that.

The time of the second heat treatment is 24 hours or more,
It may be that .

According to the present invention, a method for producing a Mn-Al permanent magnet that does not require addition of C element can be obtained .

It is a flowchart which shows the manufacturing process of the Mn-Al permanent magnet which concerns on embodiment of this invention. It is a binary system equilibrium phase diagram of Mn-Al. It is a figure which shows typically the phase transformation in the manufacturing process of the Mn-Al permanent magnet which concerns on embodiment of this invention. It is a reference drawing which shows typically a phase transformation in the manufacturing process of a Mn-Al permanent magnet when a magnetic field is not applied at the time of heat treatment. (A) And (b) is a reference drawing which shows typically phase transformation in the manufacturing process of the permanent magnet of a MnBi sintered compact. It is a graph which shows the magnetization curve when the temperature and magnetic field at the time of heat processing differ from each other. (A)-(c) is a graph which shows the magnetization curve when the temperature at the time of heat processing is changed mutually.

Hereinafter, a method for manufacturing a permanent magnet according to an embodiment of the present invention will be described with reference to the flowchart of FIG.

First, the raw material of the permanent magnet is synthesized (step S10). There are two types of raw materials for the permanent magnet in the present embodiment, Mn and Al. In addition, it is permissible to include a trace amount of unavoidable impurities that may be mixed in during the manufacturing process.

The permanent magnet according to the present embodiment is a binary material of Mn-Al and has a τ phase of a CuAu structure which is a permanent magnet. Therefore, the ratio of Mn in the raw material is in a range that can be the τ phase of the Mn-Al system.

Figure 2 shows the binary phase diagram of Mn-Al (Source: Inorganic Material Database (AtomWork, http://crystdb.nims.go.jp/) and Binary Alloy Phase Diagrams, II ed. (1990)) .. For the purpose of explanation in this specification, an additional description is added to the cited binary system equilibrium diagram.

In FIG. 2, when the ratio of Mn is approximately 48.5 to 59.5 at %, a substantially rectangular region at 840° C. or lower is a region that can be a τ phase that becomes a magnet (τ phase generation region). However, since the τ phase is a non-equilibrium phase, it cannot be generated by ordinary heat treatment. In the τ phase generation region, Mn ₁₁ Al ₁₅ (γ phase) and Mn (β phase) become equilibrium phases.

Therefore, the mixing of the raw materials is performed within the range of the τ phase generation region. However, as will be described later, in the manufacturing process, ε phase (Mn _0.55 Al _0.45 ) having an hcp (hexagonal close-packed) structure is generated by heat treatment, so that it is about 55 at%.

Then, the said raw material is melt|dissolved in a furnace (step S20). The temperature at the time of melting is the temperature at which the liquid phase (L) is reached in the binary system equilibrium diagram of FIG. 2, and is 1400° C., for example. The melting time is determined based on the amount of the raw material and the conditions such as the equipment of the furnace so that the raw material is sufficiently dissolved.

Then, the melted raw material is cast in a predetermined mold to manufacture an ingot (step S30). The method for producing the ingot may be a method used for a known Mn-Al-C permanent magnet, but may be any other method suitable for being subjected to the heat treatment described later.

Subsequently, the ingot is subjected to solution heat treatment (first heat treatment) to obtain a material in which the ε phase is generated (step S40). The temperature of the heat treatment is a temperature at which the ε phase can be generated, and is, for example, 1000 to 1100°C. The heat treatment time is determined based on the conditions such as the amount of ingot and the equipment of the furnace so that the ε phase is sufficiently generated. When the predetermined heat treatment is completed, the material in which the ε phase is generated is quenched by quenching or the like.

Subsequently, heat treatment (second heat treatment) is performed while applying a magnetic field to the material in which the ε phase is generated (step S50).

The temperature of the heat treatment may be a temperature at which a τ phase that is phase-transformed from the ε phase is generated, and is, for example, 350°C. However, if the temperature of the heat treatment is too high, the τ phase is generated once, but with the heat treatment over time, the β phase and γ phase of the equilibrium phase are generated from the ε phase and the nonequilibrium τ phase. I will end up. Further, the proportion of β phase and γ phase generated from ε phase also increases. Therefore, due to these phase transformations, it may be difficult to manufacture a permanent magnet having a sufficient magnetic force, that is, a sufficient fraction of the τ phase. If the temperature of the heat treatment is too low, it takes too much time to generate the τ phase, which is not preferable. In order for the phase transformation to proceed sufficiently within a finite heat treatment time, the temperature is preferably 300° C. or higher.

Therefore, the preferable heat treatment temperature for manufacturing the permanent magnet is the Curie temperature (380° C.) or less in order to effectively stabilize the magnet phase by the magnetic field, and the time required for sufficient formation of the τ phase in the heat treatment is set. Considering this, the temperature may be 350° C., for example. The heat treatment time is determined in consideration of the temperature of the heat treatment and is, for example, 48 hours.

The strength of the magnetic field to be applied may be a value that is preferable for the generation of the τ phase, and may be 15T, for example.

When the predetermined heat treatment is completed, the material in which the τ phase is generated is cooled by furnace cooling or liquid cooling. In this case, it is preferable to use a method in which the phase transformation from the τ phase to the β phase and the γ phase is suppressed.

After that, the material in which the τ phase is generated is subjected to finish processing (step S60) to complete a desired permanent magnet product.

Next, the reason why the magnetic field is applied during the second heat treatment in the present embodiment will be described.

In the Mn-Al-based material, when the generated ε phase is heat-treated at a temperature corresponding to the second heat treatment without applying a magnetic field, as shown in FIG. 3, a phase transformation to a τ phase which is a non-equilibrium phase is performed. To do. Further, the ε phase also undergoes phase transformation into the β phase and γ phase which are equilibrium phases. In this case, even if the ratio of the τ phase generated is higher than the ratios of the β phase and the γ phase, the τ phase is reduced by further phase transformation into the β phase and the γ phase with the passage of time. Therefore, it has been difficult to use the Mn-Al permanent magnet as a practical permanent magnet having a high magnetic force by an ordinary heat treatment.

On the other hand, applying the magnetic field during the second heat treatment as in this embodiment makes it possible to selectively generate and stabilize the τ phase of the nonequilibrium phase having ferromagnetism. Is. The behavior in this second heat treatment is schematically shown in FIG.

In the second heat treatment in which a magnetic field is applied, the ratio of the τ phase generated from the ε phase becomes significantly larger than that of the β phase and the γ phase. Further, since the phase transformation from the τ phase to the β phase and the γ phase is suppressed as compared with the case of FIG. 3, it is possible to secure a large amount of the τ phase remaining in the material as compared with the conventional manufacturing method. it can.

However, even if the ratio is low as shown in FIG. 4, the τ phase is changed to the β phase and the γ phase. Therefore, it is considered that the τ phase gradually decreases when the heat treatment is performed for an excessively long time. To be

Here, the difference from the prior art MnBi permanent magnet (Patent Document 1) will be described.

As shown in FIG. 5A, the mixture of Mn powder and Bi powder is sintered at 280° C., for example, to produce MnBi as a compound. Further, as shown in FIG. 5(b), when the sintering temperature is lowered to, for example, 250° C., MnBi is usually not produced. In Patent Document 1, by sintering the mixture while further applying a magnetic field under the temperature condition of FIG. 5B, the chemical reaction to MnBi is promoted as shown in FIG. 5A. Further, magnetic anisotropy is obtained in MnBi according to the direction of the magnetic field during sintering.

On the other hand, since Mn-Al of the present embodiment is an alloy, its meaning is significantly different from the application of a magnetic field to the reaction of the compound. When the heat treatment is performed from the ε phase of the hcp structure without applying a magnetic field, the non-ferromagnetic β phase and γ phase are generated, so that the magnet does not serve as a magnet. In this embodiment, by applying a magnetic field, the non-equilibrium ferromagnetic τ phase is selectively generated and stabilized as described above.

Further, the difference from the widely used manufacturing method of the Mn-Al-C permanent magnet will be described.

In the case of a Mn-Al-C permanent magnet, extrusion processing at about 700° C. is performed after the first heat treatment (corresponding to step S40 in FIG. 1) that produces the ε phase. When imparting uniaxial magnetic anisotropy to the permanent magnet, upsetting is further performed at about 700° C. in the extrusion axis direction. After that, finish processing (corresponding to step S60 in FIG. 1) is performed to manufacture a permanent magnet.

As described above, the Mn-Al-C permanent magnet needs to be mechanically processed at a temperature of about 700° C., which is higher than that of the present embodiment. On the other hand, in the present embodiment, the addition of carbon can be omitted, and instead of hot mechanical processing, it is possible to perform processing in a furnace at a relatively low temperature of the Curie temperature or lower, so the manufacturing method Can be greatly simplified.

As described above, according to this embodiment, it is possible to manufacture a high-magnetism permanent magnet with two types of Mn and Al without using C element as a raw material. Further, it is possible to cope with a heat treatment apparatus which does not require extrusion processing at a high temperature to generate the τ phase and is at a relatively low temperature below the Curie temperature and does not require equipment such as extrusion processing.

Hereinafter, representative examples of the present invention will be shown to clarify the present invention more specifically, but the present invention is not limited by the description of such examples. It goes without saying that. In addition to the embodiments described below, the present invention includes various changes, modifications, improvements, etc. based on the knowledge of those skilled in the art, in addition to the specific description above, without departing from the spirit of the present invention. It should be understood that it can be added.

(Example 1)
The raw materials for the Mn element and the Al element were prepared so that Mn was 55 at% of the whole. These raw materials were blended, melted by high frequency, and then rapidly cooled with a rod-shaped mold having a diameter of 10 mm. The rod-shaped sample thus obtained was cut into a disk having a thickness of 2 mm to obtain a disk-shaped sample. The disc-shaped sample was subjected to solution treatment at 1100° C. for 1 day and then rapidly cooled in ice water (first heat treatment).

Then, the disk-shaped sample was subjected to heat treatment for generating a magnet phase (τ phase) at heat treatment temperatures of 300° C. and 350° C. in a magnetic field of 15 T, 10 T and 0 T, respectively (second heat treatment). The heat treatment time was set to 12 h, 24 h, 48 h, and 96 h.

After the second heat treatment, the magnetization of each sample was measured at 27° C. in the maximum external magnetic field of 1.5 T. The result is shown in FIG.

In FIG. 6, the horizontal axis represents the heat treatment time (Annealing time (hour)), and the vertical axis represents the magnetization at 1.5 T (Magnetization (emu/g)). From FIG. 6, the effect of selective formation of the τ phase clearly appeared and the magnetic force was improved in any of the samples heat-treated in the magnetic field of 10T and 15T. In addition, even in the heat treatment at 300° C., where the heat treatment temperature is low and the generation takes time, the effect of selective formation of the τ phase by the magnetic field is considered to be low. It can be seen that the performance (magnetic force) as a permanent magnet is clearly improved in the case of doing so as compared with the case of not applying the magnetic field (0T).

(Example 2)
A disk-shaped sample in which the ε phase was generated by the first heat treatment was prepared in the same manner as in Example 1. These disk-shaped samples were subjected to the second heat treatment for 4 hours at the initial phase transformation from the ε phase to the τ phase. The temperature of the heat treatment is 300°C, 350°C and 400°C. Further, at each temperature, the magnetic field strength applied during the heat treatment was set to two cases of 0T and 15T.

After the second heat treatment, the magnetization curve was acquired for each sample. The maximum external magnetic field was 1.2 T, and the measurement was performed at room temperature. The results are shown in FIGS. 7(a) to 7(c).

7A to 7C, the horizontal axis represents the external magnetic field (Magnetic field (T)) and the vertical axis represents the magnetization (Magnetization (emu/g)). 7(a) shows the measurement results at 300° C., FIG. 7(b) at 350° C., and FIG. 7(c) at 400° C. In each figure, the magnetic fields applied during the heat treatment are 0T and 15T. And 2 cases are described.

In the case of 300° C. in FIG. 7A, it is understood that both 0T and 15T have a linear shape, and no clear magnetic field effect appears at the initial stage of phase transformation (4 h). That is, it can be said that the τ phase is hardly generated in any case.

In the case of 350° C. in FIG. 7B, the magnetization curve of 0T is almost linear, whereas a clear magnetization curve of 15T appears. That is, it can be said that the τ phase is selectively generated by applying the magnetic field of 15T. Moreover, since 350° C. is a temperature near the Curie temperature (380° C.) of Mn—Al, it is considered that the difference from 0T was more prominent.

In the case of 400° C. in FIG. 7C, a large magnetization was observed, but at 0T and 15T, the magnetization curves were almost overlapped. This indicates that the generation of the τ phase is dominated by that generated by temperature rather than that caused by the application of the magnetic field. However, since the temperature factor is dominant, at a temperature higher than the Curie temperature, the rate of phase transformation from the τ phase to the β phase and the γ phase becomes high even if a magnetic field is applied, so long-time heat treatment is required. It is considered that the remaining τ phase decreases with time and the magnetic force decreases.

From the comparison of FIGS. 7A to 7C, it can be seen that the magnetization is clearly improved by the heat treatment in the magnetic field. It can be said that the heat treatment temperature is preferably 350° C., which is slightly lower than the Curie temperature among the three types of the present embodiment and is expected to stabilize the τ phase by magnetic energy.

Claims (4)

A step of performing a first heat treatment on a material composed of Mn, Al, and unavoidable impurities at a temperature at which a Mn-Al-based ε-phase is formed;
Subjecting the material in which the ε phase is generated to a second heat treatment while applying a magnetic field having a magnetic flux density of 10 T or more, and generating the τ phase.
A method for producing a Mn-Al permanent magnet, comprising: The temperature of the second heat treatment is a temperature equal to or lower than the Curie temperature of the material,
The method for producing a Mn-Al permanent magnet according to claim 1, wherein. The temperature of the second heat treatment is 300 to 350°C.
The method for producing a Mn-Al permanent magnet according to claim 2, wherein. The time of the second heat treatment is 24 hours or more,
The method for producing a Mn-Al permanent magnet according to claim 2 or 3, characterized in that .