JP2005220438A - Fe-Cr-Al BASED MAGNETIC POWDER, Fe-Cr-Al BASED MAGNETIC POWDER COMPACT, AND ITS PRODUCTION METHOD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide Fe-Cr-Al based magnetic powder whose surface can be coated with a substance having high electric resistance without depending on expensive chemical treatment, and which can attain high density when being made into a compact, to provide an Fe-Cr-Al based magnetic powder compact, and to provide its production method. <P>SOLUTION: The Fe-Cr-Al based magnetic powder is composed of, by mass, 1.0 to 30.0% Cr and 1.0 to 8.0% Al, and the balance substantially Fe. In the Fe-Cr-Al based magnetic powder compact, all the contact faces between respective particles composing the structure are substantially isolated by an oxide film containing alumina. The compact can be obtained, e.g., by subjecting the above magnetic powder to heating treatment in an oxidizing atmosphere at ≥800°C, and allowing an oxide film containing alumina of ≥20 mass% to self-form so as to be compacted by discharge plasma sintering. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention includes, for example, a Fe-Cr-Al magnetic powder and a Fe-Cr-Al magnetic powder that are raw materials for magnetic materials used in an alternating magnetic field, such as a stator or a rotor of an automobile motor. The present invention relates to a molded body and a manufacturing method thereof.

  Conventionally, magnetic materials used in alternating magnetic fields such as motor stators and rotors are made by punching electromagnetic steel sheets coated with an electrical insulating material on a surface into a predetermined shape and then stacking them (hereinafter referred to as stacking). Used as body). The purpose of coating an insulating material on the surface of an electrical steel sheet is to increase the electrical resistance in the thickness direction of the laminate, thereby suppressing eddy currents generated in an alternating magnetic field, increasing loss due to higher frequencies, and increasing relative permeability and It is to suppress a decrease in magnetic flux density, that is, deterioration of soft magnetism. However, due to the presence of this insulating layer, the stacked magnetic flux density in the vertical direction is low, and the magnetic circuit is restricted to a two-dimensional design.

In recent years, this magnetic steel sheet laminate is coated with an insulating material such as phosphate on the surface of magnetic powder typified by iron powder, and a binder resin is added to the insulating coated magnetic powder and solidified. Molded magnetic powder compacts have been developed (see, for example, non-patent documents 1 and 2). Since this magnetic powder molded body exhibits isotropic magnetic characteristics unlike a laminated body of electromagnetic steel sheets, it is an excellent technique in that the degree of freedom of the magnetic circuit is increased and three-dimensional circuit design can be applied.
Masao Ito, “Sintered Soft Magnetic Materials and Their Uses, and Application to Compaction Core Motors,” September, 2003 (2003), pp. 13-20 Yoshiyuki Shimada "Development of high-performance P / M soft magnetic materials" Materia (Vol. 42, No. 11 (2003), pages 801-805

  The magnetic powder molded bodies disclosed in Non-Patent Documents 1 and 2 described above are advantageous in that a three-dimensional magnetic circuit design can be applied because of their isotropic magnetic characteristics. Expensive chemical treatment is required to coat the surface of the insulating material.

  Moreover, when solidifying and molding the magnetic powder after the insulation coating, it is necessary to add a resin as a binder. For this reason, the structure | tissue of a magnetic powder molded object becomes a form typically shown in FIG. 8 (citing FIG. 9 of a nonpatent literature 2). That is, a space in which the resin 3 enters is required between the magnetic powder 1 covered with the insulating film 2 and the adjacent magnetic powder 1. This resin 3 reduces the density of the magnetic powder molded body. The decrease in density in the magnetic powder compact causes a decrease in magnetic flux density.

  The two problems listed here ((1) expensive chemical treatment for insulation coating, (2) density reduction of magnetic powder molded body due to resin addition) This is a problem when put to practical use in a rotor. The object of the present invention is to solve these two problems. That is, without adding a magnetic powder that can coat the powder surface with a material having high electrical resistance by an inexpensive method, a magnetic powder coated with the high electrical resistance material, and a binder resin, Is an electrically insulated magnetic powder molded body and a method for producing the same.

  According to the study of the present inventor, even if the material covering the surface of the magnetic powder is not a complete insulating material such as a phosphate compound or a resin described in Non-Patent Documents 1 and 2, the electrical resistance of the magnetic powder As long as the substance can increase the electric resistance of the magnetic powder molded body using the magnetic powder, it is often possible to suppress soft magnetic deterioration in a high frequency range. Therefore, the present inventor does not coat the surface of the magnetic powder with another insulating material due to an external element, but allows the surface of the magnetic powder to self-generate a high electrical resistance material by a chemical reaction of the magnetic powder itself. It has been found that the chemical treatment for coating can be simplified.

  As a magnetic powder capable of self-generation of this high electrical resistance substance, an Fe—Cr—Al based magnetic powder having an optimum component composition was found, and the present invention was achieved. More specifically, by oxidizing the above-described Fe—Cr—Al based magnetic powder, a high electrical resistance oxide film containing alumina on the surface can be generated, and Fe coated with the high electrical resistance oxide film. The present invention was reached by investigating and examining the surface state of the —Cr—Al-based magnetic powder, the structure and magnetic properties of the magnetic powder compact obtained by solidifying the magnetic powder, and the solidification molding method of the magnetic powder.

  That is, according to the present invention, the Fe—Cr—Al-based magnetism is characterized in that Cr: 1.0-30.0%, Al: 1.0-8.0% by mass, and the balance being substantially composed of Fe. It is a powder. Preferably, it is Fe—Cr—Al based magnetic powder containing one or two of Ti: 1.0% or less and Zr: 1.0% or less by mass%. And it is Fe-Cr-Al type magnetic powder by which the oxide film containing 20% or more of alumina by mass% was self-generated on the surface of said Fe-Cr-Al type magnetic powder, More preferably, it is by mass%. This is an Fe—Cr—Al based magnetic powder in which an oxide film containing 50% or more of alumina is self-generated.

  Further, the present invention is a Fe—Cr—Al based magnetic powder molded body in which all contact surfaces between particles constituting a structure are substantially isolated by an oxide film containing alumina, and bonded between the powders. The agent resin is not introduced.

  Furthermore, the present invention provides a Fe—Cr—Al based magnetic powder consisting of Cr: 1.0 to 30.0%, Al: 1.0 to 8.0%, and the balance substantially consisting of Fe, or Fe-Cr-Al-based magnetic powder containing one or two of Ti: 1.0% or less and Zr: 1.0% or less in the composition by mass is heated in an oxidizing atmosphere of 800 ° C. or higher. An Fe-Cr-Al magnetic powder molded body characterized in that an oxide film containing 20% by mass or more of alumina on the surface is self-generated, and then the heat-treated powder is solidified and molded. It is a manufacturing method, preferably a Fe-Cr-Al based magnetic powder molded body characterized by solidification molding by discharge plasma sintering.

  Since the Fe—Cr—Al based magnetic powder of the present invention can easily form a high electrical resistance oxide film containing alumina on the surface by oxidation treatment, it is possible to insulate the powder surface with an inexpensive method. it can. In addition, the Fe—Cr—Al based magnetic powder molded body of the present invention has a high density because no binder is added. Furthermore, since each particle constituting the magnetic powder molded body is covered with an oxide film containing alumina, the electric resistance is high. Therefore, it is expected to be a magnetic powder compact having a high magnetic flux density suitable for use in an alternating magnetic field, and can be expected to be applied to a stator or a rotor of a motor.

  As described above, the first feature of the present invention is that the Fe—Cr—Al based magnetic powder is adopted as the magnetic powder capable of self-generating a high electrical resistance oxide film containing alumina on the surface by oxidation treatment, and optimal for that. This is because the composition of various components has been clarified. The reason why the chemical composition of the Fe—Cr—Al based magnetic powder is specified will be described below. Unless otherwise specified, mass% is expressed as%.

・ Cr: 1.0-30.0%
When Cr heat-processes magnetic powder in oxidizing atmosphere, it plays the role which suppresses and prevents the oxidation of Fe. That is, in Fe-Al based magnetic powder not containing Cr, Fe is preferentially oxidized over Al, so that it is difficult to produce alumina in the oxide film on the surface, and conductive Fe oxide is produced. By containing an appropriate amount of Cr, diffusion of Fe to the surface is suppressed and preferential oxidation of Al occurs. Further, the addition of Cr is also effective for making the oxide particles constituting the oxide film finer, and thus making the structure of the oxide film dense and uniform. The range of the amount of Cr is set to 1.0 to 30.0%. When the amount is less than 1.0%, the amount of alumina produced on the surface after the oxidation treatment is small. Conversely, when the amount exceeds 30.0%, the amount of alumina produced is This is because the magnetic flux density of the magnetic powder compact is reduced. A more desirable range of the Cr content is 2.0 to 10.0%.

-Al: 1.0-8.0%
Al is an element necessary for generating alumina in the oxide film when the magnetic powder is oxidized. The range of Al content is set to 1.0 to 8.0%. When the amount is less than 1.0%, the amount of alumina produced on the surface of the powder after the oxidation treatment is small. This is because the magnetic flux density of the powder compact decreases. A more preferable range of the amount of Al is 3.0 to 6.0%.

Although the balance is substantially Fe, impurity elements such as C, Si, Mn, P, S, O, and N are contained. These elements may be contained in the following ranges as a range that does not adversely affect the magnetic properties of the magnetic powder compact, and are desirable regulated amounts.
C ≦ 0.10%, Si ≦ 0.50%, Mn ≦ 0.50%,
P ≦ 0.05%, S ≦ 0.05%, O ≦ 0.05%, N ≦ 0.05%

  The reason for including one or two of Ti: 1.0% or less and Zr: 1.0% or less as a preferable element will be described. Ti and Zr have an effect of strengthening the adhesion between the oxide film containing alumina formed on the surface as an internal oxide and the powder when the magnetic powder is oxidized (hereinafter referred to as an anchoring effect). is there. Preferably, any element is made 0.05% or more for obtaining the anchoring effect. However, in any element exceeding 1.0%, although the anchoring effect is large, the magnetic flux density of the magnetic powder molded body is lowered, so the above upper limit range is specified. More preferably, one or two of Ti: 0.10 to 0.50% and Zr: 0.10 to 0.50% may be contained.

The reason why the ratio of alumina on the surface of the Fe—Cr—Al magnetic powder coated with the high electrical resistance film is specified will be described. The reason why an oxide film containing 20% or more by mass of alumina is formed on the surface is to sufficiently increase the electric resistance when the magnetic powder is solidified and formed into a magnetic powder molded body. . If the oxide film has an alumina ratio of 20% or more, it can function as a high electrical resistance film. In this case, 20% or more means that the powder surface is quantitatively analyzed under an acceleration voltage of 15 kV using an energy-dispersive X-ray analyzer (hereinafter referred to as SEM-EDX) attached to the scanning electron microscope. In addition, among all the detected oxides (FeO, Cr ₂ O ₃ , Al ₂ O ₃ ), the mass ratio of Al ₂ O ₃ (alumina) is 20% or more. A more desirable range of the alumina ratio on the surface is 50% or more, more preferably 70% or more.

  The second feature of the present invention resides in a Fe—Cr—Al based magnetic powder molded body in which each particle is isolated by a film having a high electric resistance without adding a binder resin, and a method for producing the same. That is, to explain the reason for defining the structure of the magnetic powder compact, all the contact surfaces between the particles constituting the structure are substantially isolated by the oxide film containing alumina. This is to increase the electric resistance of the magnetic powder compact and reduce eddy current loss generated in an alternating magnetic field. And since the binder resin is not introduced between the powders, the molded body achieves high density.

  As a method for producing a Fe—Cr—Al based magnetic powder molded body, first, Cr: 1.0 to 30.0%, Al: 1.0 to 8.0% in the above-mentioned mass%, and the balance is substantial. Fe-Cr-Al-based magnetic powder composed of Fe, or Fe-Cr-Al containing one or two of Ti: 1.0% or less and Zr: 1.0% or less in this composition. The system magnetic powder may be heat-treated in an oxidizing atmosphere at 800 ° C. or higher to produce an oxide film containing alumina on the surface by 20% by mass or more, preferably 50% or more by mass. The reason why the heating temperature is set to 800 ° C. or more is that the heating temperature is effective for producing 20% or more of alumina on the surface of the magnetic powder. More preferably, it is 900 degreeC or more.

  Next, the magnetic powder on which the oxide film is formed is solidified and molded. When the magnetic powder is aggregated by heating at 800 ° C. or higher, a step of pulverizing the aggregate may be added. As a method of solidifying and molding the magnetic powder, discharge plasma sintering is desirable. The reason is that oxide films containing alumina produced on the powder surface can be sufficiently sintered without adding a resin as a binder. The Fe—Cr—Al based magnetic powder molded body formed by spark plasma sintering has a high density because no resin is added. Therefore, it can be expected to be a magnetic powder compact having a high magnetic flux density suitable for use in an alternating magnetic field, and application to a stator or rotor of a motor can be expected.

The following examples further illustrate the present invention.
After melting the mother alloy by vacuum melting, magnetic powders of three types of compositions were prepared using a gas atomizer. Their chemical composition (mass%) is shown in Table 1. No. in Table 1 1 and No. 2 corresponds to the Fe—Cr—Al based magnetic powder of the present invention. In Table 1, No. 3 is a Cr-free additive and is a comparative example of the present invention.

  These three types of magnetic powders were heat-treated for 1 hour in an atmospheric furnace maintained at temperatures between 900 and 1180 ° C., and then air-cooled to generate an oxide film on the powder surface. In addition, since powders aggregated by heat processing at each temperature between 900-1180 degreeC, the aggregate was grind | pulverized after air cooling. The powder surface after pulverization was observed and analyzed by SEM-EDX under an acceleration voltage of 15 kV. As an example of surface observation, No. 1 (invention) after heat treatment at 1180 ° C. No. 2 (invention) after heat treatment at 1100 ° C. 2 (invention) after heat treatment at 900 ° C. 3 to 6 show the observation results of 3 (comparative example) after heat treatment at 1180 ° C., respectively. Table 2 shows the analysis results of the oxide ratio in the powder after each heat treatment.

  No. of the present invention. No. 1 after heat treatment at 1180 ° C. (FIG. 3) and No. 1 of the present invention. 2 after heat treatment at 1100 ° C. (FIG. 4), the surface of the oxide film has a dense and uniform structure, and contains an oxide containing a high proportion of 79.3% and 73.0%, respectively. A film is formed (Table 2). In addition, No. 1 of the present invention. The oxide film of No. 2 after heat treatment at 900 ° C. (FIG. 5) has a thinner oxide film than the one after heat treatment at 1100 ° C. (FIG. 4). Although the field is observed, the film form is dense and uniform. Moreover, the alumina ratio in an oxide film is 25.9% and 20% or more (Table 2). In contrast, No. of the comparative example. In No. 3, the surface form of the oxide film was rough (FIG. 6), and the alumina ratio in the oxide film was detected only at 13.1% (Table 2).

  In order to investigate the change in electrical resistance due to the formation of an oxide film, three types of Fe—Cr—Al magnetic powder (No. 1 to 3) before heat treatment, and each of the heat treatment shown in FIGS. Samples of 2 g each were taken out from the four types of Fe-Cr-Al magnetic powders on which the oxide film was formed, filled in a pure copper jig for each magnetic powder, and compacted to have a diameter of about 11 mm and a thickness of about 3 mm. Seven types of green compacts were obtained. Since it is difficult to measure the electrical resistance of the magnetic powder itself, measuring the electrical resistance of the green compact filled with the magnetic powder in a pure copper jig in this way can be used as the electrical resistance of the magnetic powder. To do. Table 3 shows the results of determining the electrical resistance of each magnetic powder from a potential difference measurement at both ends by passing a DC current of 500 mA through both ends of each jig.

  From Table 3, in the state before performing heat processing, it is No. There is no great difference in the electrical resistance of the three types of magnetic powders 1 to 3. However, no. 1 and No. In the magnetic powder No. 2, the electrical resistance is increased by heat treatment. In particular, no. 1 at 1180 ° C. 2 was heat-treated at 1100 ° C., and the alumina ratio in the oxide film was increased to 79.3% and 73.0%, respectively, the increase in electrical resistance was large. No. Even when 2 is heat-treated at 900 ° C. and the alumina ratio in the oxide film is 25.9%, the effect of increasing the electrical resistance is also observed. On the other hand, no. Even if 3 is heat-treated at 1180 ° C., the alumina ratio in the oxide film is low, so the electrical resistance hardly increases.

  From the above Example 1, by setting the chemical composition of the powder within the specified range of the present invention, an oxide film having a high alumina ratio can be self-generated on the surface after the heat treatment. And by making this alumina ratio into the specified range (20% or more) of the present invention, there is an effect of increasing the electric resistance of the magnetic powder, and by making it the desirable range of 50% or more, it is possible to further increase the electric resistance. Was confirmed.

  In Table 3, three types of Fe—Cr—Al-based magnetic powders having an alumina ratio of 20% or more in the oxide film (No. 1 heated at 1180 ° C., No. 2 heated at 1100 ° C., No. 2 heated at 900 ° C. ), And No. not subjected to heat treatment. The magnetic powder 2 was filled into a graphite mold for molding. No binder resin was added to the mold. This filled powder is put in a vacuum chamber of a discharge plasma sintering machine, heated to 900-1100 ° C. under a pressure of 50 (MPa), heated and held for 5 minutes, and then pressure sintered. And a Fe—Cr—Al based magnetic powder compact having a diameter of 22 mm and a height of 5 mm was obtained. List the types of magnetic powder, the heating temperature during oxidation treatment, the heating temperature during discharge plasma sintering, and the density, true density, and relative density of the obtained Fe-Cr-Al-based magnetic powder compact. 4 shows. In addition, the true density in a table | surface has shown the density of the mother alloy of each magnetic powder.

  In the magnetic powder compacts A to D produced by the discharge plasma sintering, a high relative density of 90% or more is obtained in all cases, and the effect that no binder resin is added appears. A part of these magnetic powder compacts was cut out, embedded in a resin, mirror-polished, and subjected to a structure investigation. Among these, the optical microscope observation structure of the magnetic powder compact A and the distribution of each element (O, Al, Cr, Fe) by SEM-EDX are shown in FIGS. 1 and 2, respectively (in FIG. 2, the lack of each element). Part is black). From FIG. 1, it can be seen that substantially all of the particles constituting the structure are isolated by an oxide film, and from FIG. 2, the oxide film observed in FIG. 1 is an oxide film rich in alumina. Similarly, in the magnetic powder compacts B and C, all of the contact surfaces between the particles were substantially in the form of a structure isolated by an oxide film containing alumina. However, in the magnetic powder molded body D that is not subjected to heat treatment when powdered, no oxide film is formed on the contact surfaces between the particles, which is a comparative example of the present invention.

Next, an annular sample having a thickness of 4 mm, an outer diameter of 20 mm, and an inner diameter of 14 mm is cut out from the magnetic powder molded body B of the present invention and the magnetic powder molded body D of the comparative example. under the conditions of the applied magnetic field _{H m = 0.14 (a / m} ), it was measured relative permeability mu _r at frequency f = 1~100 (kHz). The frequency dependence of the permeability mu _r of the magnetic powder compact illustrated in FIG. In the magnetic powder molded body B of the present invention, even when the frequency is increased to 100 kHz, the relative permeability is not lowered due to the high frequency, that is, the soft magnetism is not deteriorated. On the other hand, in the magnetic powder molded body D of the comparative example, although the relative permeability is higher than that of the magnetic powder molded body B at 1 kHz on the low frequency side, the relative permeability is reduced as the frequency is increased.

  From the above Example 2, the magnetic powder of the present invention having an oxide film self-generated on the surface is formed by press-sintering using discharge plasma sintering without adding a binder resin at all. Confirm that it is possible to form a high-density powder molded body in which all the contact surfaces are isolated by an oxide film and that the oxide film present on the contact surface between each particle suppresses the deterioration of soft magnetism in the high frequency range. It was done. That is, even if the surface of the magnetic powder is not subjected to an expensive chemical treatment, the purpose of suppressing the deterioration of soft magnetism in a high frequency range can be achieved by an inexpensive method such as an oxidation treatment. This example overcomes the above-mentioned two problems ((1) expensive chemical treatment for insulation coating, (2) density reduction of magnetic powder molded body by addition of resin) and is practical. It can be applied as a magnetic powder compact.

  The present invention is excellent in that it can perform insulation coating of magnetic powder by an inexpensive method, and can obtain a high-density magnetic powder molded body without adding a binder, so that it can be used in an alternating magnetic field. It is a suitable magnetic material and can be expected to be applied to the stator and rotor of motors for automobiles.

It is an optical microscope photograph which shows the structure | tissue of the magnetic powder molded object of this invention. It is a mapping image which shows distribution of each element in the structure | tissue of the magnetic powder molded object of this invention. It is an electron micrograph which shows the surface form of the magnetic powder of this invention. It is an electron micrograph which shows the surface form of the magnetic powder of this invention. It is an electron micrograph which shows the surface form of the magnetic powder of this invention. It is an electron micrograph which shows the surface form of the magnetic powder of a comparative example. It is a figure which shows the frequency dependence of the relative permeability of the magnetic powder compact of this invention and a comparative example. It is a schematic diagram which shows the structure | tissue structure of the conventional magnetic powder molded object.

Explanation of symbols

1 Magnetic powder, 2 Insulating film, 3 Resin

Claims (7)

A Fe—Cr—Al based magnetic powder characterized by comprising Cr: 1.0 to 30.0% by mass%, Al: 1.0 to 8.0%, and the balance being substantially composed of Fe. 2. The Fe—Cr—Al based magnetic powder according to claim 1, wherein one or two of Ti: 1.0% or less and Zr: 1.0% or less are contained by mass%. The Fe-Cr-Al-based magnetic powder according to claim 1 or 2, wherein an oxide film containing 20% or more by mass of alumina is self-generated on the surface of the Fe-Cr-Al-based magnetic powder. Al magnetic powder. The Fe-Cr-Al magnetic powder according to claim 3, wherein an oxide film containing 50% or more by mass of alumina is self-generated. An Fe—Cr—Al based magnetic powder molded body characterized in that all of the contact surfaces between particles constituting the structure are substantially isolated by an oxide film containing alumina. Cr: 1.0-30.0% in mass%, Al: 1.0-8.0%, Fe—Cr—Al based magnetic powder consisting essentially of Fe, or the above composition in mass% A Fe—Cr—Al based magnetic powder containing one or two of Ti: 1.0% or less and Zr: 1.0% or less is heat-treated in an oxidizing atmosphere of 800 ° C. or more, and the surface has a mass. A method for producing a Fe—Cr—Al based magnetic powder molded body, wherein an oxide film containing 20% or more of alumina is self-generated, and then the heat-treated powder is solidified and molded. The method for producing a Fe-Cr-Al-based magnetic powder molded body according to claim 6, wherein solidification molding is performed by spark plasma sintering.