JPH0237702A - R-fe-b-m sintered magnet - Google Patents

Abstract

PURPOSE:To improve corrosion resistance by specifying residual stress of a surface layer and making a rich layer of rare earth elements substantially not exist on the surface layer. CONSTITUTION:The content of R (R is at least one kind of rare earth elements containing Y) is made 8 to 20% and the residual stress of a surface layer is made smaller than 10kg/mm<2>. An R rich layer is made not to exist on the surface layer substantially. Although one or more kinds of Y can be used for R, R is mainly composed of particularly Nd, Pr in view of magnetic characteristics. A part of it may be substituted with Ce or Mischmetal for the purpose of lowering cost price. For uses requiring resistance to heat, a part of it may be substituted with Dy, Tb, etc. As a result, the corrosion resistance of a permanent magnet is improved.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention selects the amount of rare earth elements added within a specific range and modifies the surface layer to significantly improve corrosion resistance while maintaining high magnetic properties. The present invention relates to an improved high performance R-Fe-BM based sintered magnet.

[Prior Art] In recent years, Sm, Co1I rare earth (hereinafter abbreviated as R), iron, and thread (hereinafter R-Fe) have been used instead of SmCo-based magnets, which are expensive and have concerns about raw material supply. (Abbreviated as -B series.) There are great expectations for permanent magnets, and they are attracting attention as a new material.

It is known that the magnetism of this R-Fe-B-based magnet is generated by an intermetallic compound represented by RFeB (R = 12. Fe = 82. B = 6% when converted to atomic percentage) (Japanese Patent Application Laid-Open No. 59-222564).

The known basic R--Fe--B based magnet contains 8 to 30% R and 2 to 20% boron in terms of molecular percentage.

Consisting of a composition range in which the balance is iron (Special Publication No. 61-342)
(See Publication No. 42). According to the examples of the publication, a typical composition with excellent magnetic properties is Nd=14, Fe=79. B
==7at%, and other compositions with excellent magnetic properties also have compositions around this range.

In other words, in conventional sintered magnets, the amount of R has been increased more than necessary, but the reason is that Nd, which is often used as R, forms a eutectic with Fe at Nd = about 70at%, which has a low melting point (640°C). This is to obtain a dense sintered body by liquid phase sintering, taking advantage of the property that

Also, R is active and has a high affinity for oxygen.

This is because there was oxidation loss during the process of crushing, forming, and sintering the ingot, and it was necessary to compensate for this loss. Further, the R oxide produced by oxidation was undesirable because it deteriorated the magnetic properties.

Furthermore, when an R-Fe-B-based magnet is considered as a magnetic material for industrial use, it has been pointed out that the magnetic properties deteriorate due to a processed damaged layer. That is, with the high performance and miniaturization of magnetic circuits in computer equipment, etc. in recent years, R
-Fe-B based magnets have been in the spotlight, but in order to meet these demands, in order to make the magnet thinner, remove surface irregularities and distortions, or remove the surface oxidation layer,
Furthermore, in order to incorporate it into a magnetic circuit, it was necessary to cut the entire surface or a part of the magnet body.

Therefore, it has been pointed out that the process-altered layer remaining after processing deteriorates the magnetic properties. Therefore, an invention was made in which hard powder is injected together with pressurized gas to remove the surface layer, and immediately after obtaining a clean surface, an A1 vapor deposited film is deposited (Japanese Patent Application Laid-Open No. 61-270308). It should be noted that in accordance with the invention, it is not enough to simply remove the process-affected layer, and a vapor deposition step is required immediately afterward.

Alternatively, it would be desirable to have an invention in which the ground surface is coated with a deposited layer consisting of body-centered cubic crystals containing Nd as a main component.
-264157). However, although this invention is effective in improving magnetic properties, there is a problem in that the R deposited layer on the magnet surface oxidizes and the magnetic properties deteriorate again, and an oxidation prevention layer such as a metal layer or an alloy layer is required. (Japanese Unexamined Patent Publication No. 188745/1983).

However, the various inventions that require the above-mentioned vapor deposited layer have the disadvantage that their essential industrial applicability is limited. This is because the vapor deposition process is a reaction carried out in a high-vacuum container, so there are inevitably restrictions on the size of the materials that can be processed, and it is difficult to deposit them uniformly.

Therefore, no invention has been found that easily satisfies both magnetic properties on an industrial scale.

However, although it is only the magnetic property, the process-altered layer is 500~
There has been an invention in the past that attempts to remove it by aging treatment at 900°C (Japanese Patent Application Laid-Open No. 140108/1983), but what can be done by this treatment? The publication states that the squareness of the demagnetization curve was improved by

[Problems to be Solved by the Invention] However, the composition of the magnet is not described in the above-mentioned Japanese Patent Application Laid-open No. 61-140108, and although there are examples of the effect of improving magnetic properties, the corrosion resistance is There is nothing written about it. Therefore, although the structure of the invention is unclear, according to research by the present inventors as described later, when such a structure is adopted, an R-rich layer is formed on the surface, resulting in an extremely low corrosion resistance. This is easily predicted, and as in the invention described in JP-A-61-264157, it is necessary to form an anti-oxidation coating on the surface.

This is because R is selectively oxidized to oxygen, and corrosion progresses deep into the magnet body starting from there.

Accordingly, one object of the present invention is to provide an R--Fe--B based sintered magnet that has a simple structure, significantly improved corrosion resistance, and excellent magnetic properties.

[Means for Solving the Problems] In order to solve these problems, the present invention provides B of 2 to 28% in atomic percentage and R between a predetermined composition box (wherein R is Y).
at least one rare earth element containing
In the R-Fe-B-M system sintered magnet, the R-Fe-B-M system sintered magnet consists essentially of the following:

The content of R is 8 to 20%, and the residual stress of the surface layer is lo
The object of the present invention is to provide an R-Fe-B-M based sintered magnet characterized by improved corrosion resistance due to the fact that the corrosion resistance is less than "kg/mm" and there is substantially no R-rich layer in the surface layer.

That is, the present inventor first determined that the composition range of R is 8 to 30.
It has been discovered that in the composition of conventional R-Fe-B-based magnets, where at% has been considered appropriate, corrosion resistance can be significantly improved by limiting the composition to 8 to 15% in terms of atomic percentage.

The reasons for limiting the components in the present invention, except for R, are the same as those conventionally known (for example, see JP-A-59-132104). That is, 2 to 28% B, 50% or less Go,
It can contain predetermined additional elements. Predetermined additive elements include At, Ti, v, Cr, Mn, Zr, Hf,
Nb, Ta.

Addition of known additive elements such as Mo, Ge, Sb, Sn, Bi, Ni, and W does not have any adverse effect on the effects of the present invention.

In the present invention, one or more types of R including Y can be used, but Nd is particularly preferred due to its magnetic properties.

Mainly composed of Pr, it may be partially replaced with Ce or Mitsushi metal for the purpose of cost reduction, and heavy R such as Dy and Tb may be partially replaced for applications requiring heat resistance.

In the present invention, if R is less than 8% in atomic percentage, the coercive force will be less than 1 k Oe, which is undesirable, and if it exceeds 20%, the corrosion resistance will be significantly reduced, which is not preferred.

The reason for this is not yet clear, but it is thought to be related to the appearance of massive rare earth-rich phases at grain boundaries. This massive R-enriched phase has a low melting point and is trapped between the dendrite crystals during ingot casting.
This is thought to be due to dendritic segregation caused by the sintering process, or residual during liquid phase sintering during sintering. According to the research of the present inventor, the R content is 2
When it exceeds 0%, there is a tendency for R-rich lumps to increase suddenly.

In the present invention, if the heat treatment temperature is lower than the eutectic point temperature, the effect cannot be exhibited, and if it exceeds the sintering temperature, the crystal grains become coarsened, which is not preferable.

Generally, on the surface of an R--Fe--B based magnet after processing, there is a processing-affected layer of about 10 to 30 .mu.m, as shown in the schematic cross-sectional view of FIG. After heat treatment, the residual stress is 50 kg/mm at around 600-700℃ as shown in Figure 2.
” to IQkg/mm” and the N in the surface layer
It can be seen that the characteristic X-ray peak intensity of d increases rapidly. At this time, an Nd-rich phase is observed in the surface layer, as shown in FIG. 1(b). This is similar to the "sweating phenomenon" of low-melting metals, which is well known in powder metallurgy.The melting point of Nd is 8
This is because the melting point is 40°C, but as described above, the melting point decreases to 640°C in the eutectic composition with Fe.

Therefore, although the residual stress due to processing is released by heat treatment, the corrosion resistance is drastically reduced due to this R-rich phase. This is because R has an extremely high affinity with oxygen.

Therefore, in the present invention, it is necessary to remove the R-rich layer on the surface layer. As the method, conventionally known means can be used. For example, puff polishing, electrolytic polishing 1 mechanical brushing, pickling, etc. can be used. In the case of pickling, a 2% HN O, m solution is particularly preferred. If a strong acid such as H2SO4 is used, corrosion will be too severe and this is not preferred.

Furthermore, in the present invention, since the content of R is limited to the necessary minimum, a manufacturing method that can prevent oxidation is preferred in order to reduce the S conversion loss as much as possible. For example, it is necessary to use hydrogen pulverized powder that absorbs hydrogen to make it easier to crush, and to work in an inert gas atmosphere. Deoxygenation by vacuum can also be used.

Furthermore, the corrosion resistance can be further improved by applying a laminated coating of a chemical conversion coating and a resin coating to the magnet body obtained by the present invention (Japanese Patent Application No. 62-239424), which has already been filed by the applicant.

The present invention will be explained below with reference to Examples.

[Example] (Example 1) An alloy having a composition of 14% Nd, 88% Nb, 1.2% Nb, and the balance Fe was prepared by arc melting in atomic %.The obtained ingot was roughened using a stamp mill and a disk mill. After pulverizing and adjusting to 32 mesh or less, it was finely pulverized with a jet mill. The grinding medium is nitrogen gas, and the grinding particle size is 3,
5 p m (F, S, S, S). Here F, S
, S, S are F1scher's 5ub-3ieveS
The particle size is shown using an Izer measuring device (air permeation method).

The obtained finely pulverized powder was subjected to magnetic field molding in a magnetic field of 15 kOe (the direction of pressure and the direction of the magnetic field were perpendicular). The molding pressure is 2 tons/cm2. The obtained compact was sintered at 1100° C. for 1 hour in an argon atmosphere, and after sintering, it was rapidly cooled in an argon stream.

Next, from the obtained permanent magnet body, 22 x 15 x 4
A test piece of m weight was cut out and heat treated at 800° C. for 1 hour in an argon atmosphere in order to modify the process-affected layer. Next, in order to remove the Nd-rich layer on the surface layer, 2% H
It was immersed in NO, aqueous solution for 5 minutes and pickled. As a comparative example,
The one that has been heat treated.

Thereafter, the same treatment as that according to the present invention was carried out.

Next, the epoxy resin is positively ionized by reaction with an acid in an aqueous solution, and the epoxy resin is electrically connected to the permanent magnet at a temperature of 28°C and a voltage of 150V for 3 minutes using a permanent magnet as a cathode and a 5US316 material as an anode. A crosslinking reaction was caused by heat curing, resulting in solidified coating (electrodeposition coating). The coating film thickness at this time was 20 μm. In the case of the comparative example, electrodeposition coating was performed in the same manner.

Place the permanent magnet obtained in this way in a constant temperature and humidity chamber,
Appearance, taping peel test, oxidation weight gain measurement, and solvent resistance test were conducted before and after the test. Here, the taping peel test is a test method in which a specific cellophane tape with a width of 18 mm is attached and peeled off, and the peeling state of the film is visually observed.
This test measures the change in mold breaking (increase in weight due to water content and oxidation) when held for 0 hours. An electronic balance was used for the measurement, and after the humidity test, the weight was measured at 30° C. and 40% RH for 2 hours, and then left in the air for 1 hour. Solvent resistance test was performed after painting, 1.1. I Using trichloroethane and IPA (isopropyl alcohol), the appearance was visually inspected after immersion at 30° C. for 24 hours, and a rubbing test (reciprocating friction test) was performed 400 times (200 reciprocations).

Results 1 According to the present invention, the results of appearance, taping peel test, solvent resistance test, and rubbing test were extremely good. The oxidation level after 600 hours of storage reached 0.07 mg, compared to 0.94 g/cm'' in the comparative example.
There was a remarkable corrosion resistance effect of /cm2.

(Example 2) In the composition of Example 1, the Nd content was 7 to 21 at%.
(Fe9. was adjusted so that the total amount was 100%), and the relationship between Nd content and corrosion resistance was investigated. The heat treatment and surface treatment were the same as in Example 1. However, the evaluation of corrosion resistance here is based on P CT (Pre
It was evaluated using the Sure Cooker Te5t) test. The evaluation standard was the durability time until corrosion occurred visually after the PCT test. Combine the results with the magnetic properties and
As shown in the table, when the Nd content is less than 8 at%, the magnetic properties are not sufficient, and when it exceeds 20%, the durability time of the PCT test is significantly deteriorated. Preferably 10-15%
The range satisfies both magnetic properties and corrosion resistance.

Table 1 [Effects of the Invention] According to the present invention, the corrosion resistance of R-Fa-B permanent magnets, which had been insufficient in the past, was significantly improved while maintaining good magnetic properties.

[Brief explanation of the drawing]

Figure 1 is a diagram schematically showing the state of the process-affected layer according to the present invention, and Figure 2 is a diagram showing the residual stress in the surface layer due to heat treatment, Nd
, t is a diagram showing changes in t. Scratch writing on r side (changed in content) Fig. 1 Heat treatment before processing and after processing

Claims (1)

[Claims] B in an atomic percentage of 2 to 28% and R in a predetermined composition range (
However, in the R-Fe-B-M system sintered magnet, which is substantially composed of R (at least one kind of rare earth element containing Y) and the remainder Fe (part of which can be replaced by an additional element M containing Co), the above-mentioned R content is 8-20%, residual stress in surface layer is 10
kg/mm^2 or less and has improved corrosion resistance due to the substantial absence of an R-rich layer in the surface layer.