JP2001107104A - High permeability iron-base magnetic metal powder for powder magnetic core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high permeability iron-base magnetic metal powder for a powder magnetic core. SOLUTION: The iron-base magnetic metal powder is constituted of secondary particles prepared by agglomeration of a plurality of primary particles. The secondary particles are composed of agglomerates of a plurality of flat or acicular primary particles or agglomerates in which a plurality of string type primary particles are tangled into sponge shape.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an iron-based magnetic metal powder used as a material for a dust core having a high magnetic permeability in a high frequency range, used for a choke coil or the like.

[0002]

2. Description of the Related Art A choke coil used in a high frequency range (10 kHz to 1 MHz) includes a ferrite sintered body magnetic core,
A dust core or the like is used. Among them, the ferrite sintered body core has a low saturation magnetic flux density. On the other hand, a dust core manufactured by mixing and compressing magnetic metal powder with an adhesive has a higher saturation magnetic flux density than a ferrite sintered body core and does not require sintering at high temperatures. In addition, the manufacturing cost is lower than that of the ferrite sintered body magnetic core.

A magnetic core used for a choke coil is used in an environment where an external magnetic field acts. A magnetic material such as a ferrite sintered body serving as a magnetic core of a choke coil or a magnetic metal powder inside a dust core is required to be magnetized in the direction of an external magnetic field. The ease of magnetization at this time is the magnetic permeability (permeability = magnetic flux density / the strength of the external magnetic field), and a high magnetic permeability is required. By the way, generally, as a result of the magnetization generated in the magnetic body, a magnetic field (reverse magnetic field) in the direction opposite to the external magnetic field is formed around the magnetic body, so that the external magnetic field is weakened, the magnetization of the magnetic body itself is hindered, There is a problem that the magnetic flux density decreases and the magnetic permeability decreases. To improve the magnetic permeability, it is necessary to reduce the demagnetizing field.

The demagnetizing field varies depending on the shape of the magnetic metal powder. When the demagnetizing field has a highly anisotropic shape such as a needle shape or a flat shape, and the direction of magnetization in the magnetic body coincides with the major axis direction. descend. In the case of a dust core obtained by molding a magnetic metal powder having such a shape and orienting the major axis so as to be aligned in one direction, conditions for applying a magnetic field in the major axis direction of each metal powder When used in, the demagnetizing field inside the dust core becomes smaller,
The magnetic permeability of the dust core becomes higher.

As a method of flattening a magnetic metal powder, a molten metal is powdered by an atomizing method disclosed in Japanese Patent Application Laid-Open No. H11-140512, and cooling of a rotating body is performed while the metal powder is still molten. There are known a method of flattening a metal powder by colliding with a body (1), a method of flattening a magnetic metal powder by crushing with a vibrating ball mill, and a method of flattening the metal powder disclosed in Japanese Patent No. 1647608. I have.
Powder magnetic cores manufactured using these flat magnetic metal powders have improved magnetic permeability, but have a problem that magnetic permeability in a high-frequency region is low.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic metal powder capable of producing a dust core maintaining a high magnetic permeability up to a high frequency range.

[0007]

Means for Solving the Problems The first invention of the present invention is:
In an iron-based magnetic metal powder comprising secondary particles in which a plurality of primary particles are aggregated, the secondary particles are (1) an aggregate of a plurality of flat primary particles, (2) an aggregate of a plurality of needle-like primary particles, Or (3) an iron-based magnetic metal powder for a dust core having a high magnetic permeability, which is an aggregate in which a plurality of string-like primary particles are entangled to form a sponge.

A preferred first invention is an iron-based magnetic metal powder for a dust core having a high magnetic permeability, wherein the secondary particles have an average particle diameter of 40 to 100 μm.

A first aspect of the present invention is a powder magnetic core having a high magnetic permeability, characterized in that the primary particles have an average minor axis length of 1 to 5 μm in cross section and an average aspect ratio of 2 to 100. It is an iron-based magnetic metal powder.

[0010] Furthermore, a preferred first invention is the iron-based magnetic metal for dust cores having a high magnetic permeability, wherein the sponge-like iron-based magnetic metal powder is obtained by reducing iron oxide. Powder.

In a preferred first invention, the flat primary particles are obtained by flattening atomized iron powder, and the iron-based magnetism for a dust core having a high magnetic permeability is provided. Metal powder.

Further, a preferred first invention is characterized in that the needle-like primary particles are obtained by cutting a smelted steel material, and the iron-based magnetism for a dust core having a high magnetic permeability is provided. Metal powder.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION A secondary particulate iron-based magnetic metal powder obtained by assembling a plurality of primary particles can be sieved to adjust its size to a certain value or less. Its average particle size is 40-100
μm, preferably 60 to 80 μm. Average particle size is 1
If it exceeds 00 μm, the dust core obtained by molding this
The eddy current inside the secondary particles becomes excessive, and the initial permeability (μ _i
/ μ ₀ ) decreases and iron loss increases, making it unsuitable for applications such as choke coils. When the average particle size is less than 40 μm, the secondary particulate magnetic metal powder has a reduced compression moldability, resulting in a reduced density of the dust core and a reduced magnetic permeability, which is also used in applications such as choke coils. Becomes unsuitable.

The primary particles of the present invention are flat, needle-like or cord-like, and these shapes are defined by the ratio of the average major axis length of the primary particle cross section to the average minor axis length orthogonal to the primary particle section. Defined by the defined average aspect ratio. The major axis length and the minor axis length of the primary particles are determined by embedding the secondary particulate metal powder in the resin product, cutting the resin product, further polishing the cut surface, and arbitrarily measuring the length of the secondary particle observed on the cut surface. It is measured by observing the cross section of the secondary particulate metal powder with an optical microscope and observing the cross section of a plurality of primary particles constituting the secondary particulate metal powder cross section.

FIG. 1a shows a cross section of a secondary particle formed by a plurality of flat primary particles, FIG. 1b shows a cross section of a secondary particle formed by a plurality of acicular primary particles, and FIG. The schematic diagram showing the cross section of the secondary particle which a plurality of primary particles are entangled is shown. As shown in each figure, the major axis length and the minor axis length were measured for each of a plurality of primary particles constituting the secondary particles, and the average was obtained. The axis length. For the cord-like primary particles, the long axis was not a straight line, so the longest length was the long axis length and the shortest width was the short axis length, as shown in FIG. 1c.

The preferred primary particles have an average aspect ratio of 2
~ 100. If the average aspect ratio is less than 2,
The dust core obtained by press-molding the secondary particulate metal powder,
The internal demagnetizing field is not sufficiently reduced. On the other hand, if the average aspect ratio exceeds 100, uniform processing (flattening / needle processing) of the primary particles becomes difficult, and processing costs increase, which is not preferable.

The average minor axis length is 1 to 5 μm, preferably 2 to 5 μm. If the average short axis length exceeds 5 μm, the dust core obtained by molding the same cannot have a sufficient reduction in the internal demagnetizing field. On the other hand, if the average short axis length is less than 1 μm,
Processing of primary particles is not preferable because it increases costs.

The average major axis length of the primary particles is 2 from the average minor axis length range of the primary particles (1 to 5 μm) and the preferred range of the average aspect ratio of the primary particles (2 to 100).
５００500 μm. However, as is apparent from the schematic diagrams of the secondary particles shown in FIGS. 1A to 1C, in order to realize a preferable range of the average particle size of the secondary particles, the average major axis length of the primary particles is It is necessary that the average particle size of the particles be equal to or smaller than the average particle size. Therefore, since the upper limit of the average major axis length of the primary particles inside the secondary particles is limited by the average particle diameter of the secondary particles, it is understood that the range must be 2 to 100 μm. More preferably, it is in the range of 2 to 80 μm. The average major axis length of the primary particles is adjusted by sieving the primary particles.

The secondary particles formed by assembling a plurality of primary particles are generally produced as follows. The flat primary particles are manufactured by mechanically flattening atomized iron powder by subjecting it to a pulverizing process such as a ball mill, an attritor, or a disk mill, or by performing a rolling process. The atomized iron powder is manufactured by spraying gas or water jet on molten steel and pulverizing,
The inside is dense and the outer shape is a nearly spherical powder. Atomized iron powder contains C, Co, Ni, Si, Al and other unavoidable impurities. To the extent that the compressibility of the metal powder is not reduced, these components may be added to molten steel before powderization for the purpose of reducing iron loss and improving magnetic permeability.

The flat primary particles are formed into secondary particles by binding the particles together by, for example, granulation. FIG. 2 is an electron micrograph (400 times magnification) showing a state in which flat atomized iron powder (primary particles) are bound together to form secondary particles.

The above-mentioned binding is to prevent the primary particles from separating from each other unless a force is applied. This is a state in which the primary particles are adhered in a point-like manner (as shown in FIG. 4, not in the entire surface or in the entire line, but in the point-like spread). This involves (1) pressure bonding, (2) bonding between individual primary particles with an adhesive or the like, (3) temporary sintering of the material constituting the primary particles between the primary particles and bonding by diffusing. Is realized by: Here, press bonding means, for example, a method in which a large number of primary particles are made to flow at high speed in a fluidized bed using gas, and the particles collide with each other. And the like.

Examples of the adhesive include water-insoluble substances such as polyvinyl alcohol, methylcellulose, and polyacrylamide, and water-insoluble substances such as paraffin wax, polyethylene glycol, polyvinylpyrrolidone, hydroxypropylcellulose, ethylcellulose, acetylcellulose, nitrocellulose, and acetic acid. Vinyl resins and the like are preferred. Further, thermosetting resins such as epoxy resin, phenol resin and imide resin, and thermoplastic resins such as polyamide and fluororesin are also suitable.

The term “temporary sintering” means that heat treatment is performed by appropriately controlling the temperature, atmosphere, and dew point. Is oxidized so that the surfaces are insulated from each other even if contact by pressure occurs. FIG. 2 is an electron micrograph (400-fold magnification) of secondary particles obtained by sintering and flattening flat atomized iron powders (primary particles).

The secondary particles composed of the acicular primary particles are preferably produced by binding the acicular primary particles in the same manner as in the production of the secondary particles composed of the flat primary particles. The needle-like primary particles can be obtained by cutting a molten metal material or by cutting a wire material.

The secondary particles composed of string-like primary particles can be obtained by reducing an iron-based metal oxide with CO. When the iron-based metal oxide is reduced, the reduced metal crystals grow into a plurality of strings, which are entangled during the growth to form sponge-like secondary particles.

When the reduction proceeds rapidly, it is possible to suppress the growth of the string-like primary particles in the radial direction and increase the average aspect ratio. For rapid reduction, means for increasing the reduction temperature or means for increasing the CO gas concentration are effective.
The reduced iron powder usually contains C, Co, Ni, Mn, Si, Al and other unavoidable impurities.

The sponge-like magnetic metal powder is in the form of secondary particles in which the primary particles are aggregated as described in the present invention, and the desired magnetic metal powder can be obtained without further treatment or processing for binding. Properties can be obtained. Of course, processing for binding may be performed. FIG. 3 is an electron micrograph (500 times magnification) of the sponge-like magnetic metal powder (secondary particles) obtained in the present invention. It is observed that a plurality of string-like primary particles are entangled to form secondary particles.

The plurality of primary particles obtained by the above method are combined.
Magnetic metal powder consisting of secondary particles adhered to
It is more compacted into a dust core, but the compression pressure is 286-686.
MPa is preferred. When using pure iron-based metal powder,
Magnetic core density of 6.0 Mg / m ^ThreeAbove, preferably 6.9
Mg / m^ThreeIf it is above, a particularly high magnetic permeability is exhibited.

Next, the reason why the magnetic permeability of a dust core made of a secondary particulate magnetic metal powder formed by binding a plurality of primary particles having a high average aspect ratio is improved will be described. When the cross section of the dust core obtained from the sponge-like metal powder is polished and observed, the string-like primary particles are well oriented in the direction perpendicular to the pressing direction. From the cross section of the dust core using the secondary particles having the flat magnetic metal powder or the needle-shaped magnetic metal powder as the primary particles, the long axis of the flat primary particles or the needle-shaped primary particles is in the pressing direction. It is observed that it is vertically oriented. This is presumed to be because the cord-like, needle-like, or flat primary particles are easily deformed by pressurization, and the major axis is oriented in a direction perpendicular to the pressurization direction.

In addition, in a toroidal core composed of a ring core often used for a noise filter core or the like, an internal magnetic field is generated in a circumferential direction. Therefore, when pressure is applied along the center axis direction of the circle, the major axes of a plurality of primary particles constituting the magnetic metal powder (secondary particles) constituting the dust core are oriented in the circumferential direction, and the toroidal core is formed. , And the internal demagnetizing field decreases.

[0031]

EXAMPLES (Examples 1 and 2 and Comparative Example 1) A carbon material and lime were mixed and adjusted with iron oxide, and the iron oxide was rapidly reduced by CO gas generated by heating. Iron powder was obtained (Inventive Examples 1 and 2). In addition, reduction was performed at a conventional reduction rate to obtain reduced iron powder for powder metallurgy (Comparative Example 1). These were sieved and classified to obtain iron powder having an average particle size shown in Table 1.

The above iron powder is embedded in a resin product, cut, and
After polishing the cross section, the cross section of the secondary particles of the iron powder was observed using an optical microscope, and the average long major axis length and the average short minor axis length of the string primary particles constituting the sponge secondary particles were measured.
The average aspect ratio of the cord-like primary particles was determined as the ratio of the average major axis length to the average minor axis length. Table 1 shows the average particle size of the secondary particles, the average minor axis length of the primary particles, the average aspect ratio, and the apparent density.

Next, each of the iron powders was compressed at a pressure of 686 MPa.
The outer diameter is 38 mm and the inner diameter is 25
A ring-shaped dust core having a thickness of 6.2 mm and a thickness of 6.2 mm was formed.
At the frequency of 10 kHz, the obtained powder magnetic core was measured for its initial AC permeability μ _i / μ _o (μ _o : vacuum permeability) and green compact density. The AC initial permeability is a limit value given by the following equation (where H: AC magnetic field strength, B: AC magnetic flux density) near the origin of the initial magnetization curve.

[0034]

(Equation 1)

The initial AC permeability was measured by using an impedance meter (Hewlet) for a core obtained by winding a formal-coated wire having a diameter of 0.6 mm around the powder magnetic core 11 times at equal intervals.
t-Packard HP-4284A) using JIS C256
It measured by the method based on No. 1. AC permeability is 10kHz
_Was calculated as the critical frequency _fcr . Since the critical frequency shows a higher value as the iron loss in the high frequency region is smaller, it becomes an index indicating the iron loss in the high frequency region.

The iron loss is determined by winding a formal-coated conductor having a diameter of 0.6 mm on the dust core used for measuring the AC initial permeability as a primary side wire and a secondary side wire at equal intervals 40 times each. About the sample thus obtained, use a BH analyzer (Hewlett-Pa
The measurement was carried out under the conditions of a frequency of 10 kHz and a magnetic flux density of 0.1 T using a Cckard E-5060A).

The density of the dust core was calculated by dividing the weight determined by the scale by the volume determined using the outer diameter, inner diameter, and thickness measured by a micrometer. Table 1 shows the characteristics of the dust cores.

In each case, the magnetic iron powders of Inventive Examples 1 and 2 and Comparative Example 1 were observed such that a plurality of string-like primary particles were entangled to form secondary particles inside the particles. .

The average minor axis length of the cord-like primary particles of Invention Examples 1 and 2 is 5.0 μm or less, and is smaller than 11.0 μm or less of Comparative Example 1. The average aspect ratio of each cord-like primary particle is 6, 5, and 3, respectively.

The AC initial permeability of the cord-like primary particles of Invention Examples 1 and 2 is 80 or more, which is larger than 76 of Comparative Example 1. The core loss of the string-like primary particles of Invention Examples 1 and 2 was 36 W, respectively.
/ Kg, 35 W / kg, 41 W / kg of Comparative Example 1
Was lower. The critical frequencies of the string-like primary particles of Inventive Examples 1 and 2 are 0.80 MHz and 0.60 MHz, respectively, which is higher than 0.48 MHz of Comparative Example 1.

As described above, the dust cores of Invention Examples 1 and 2 using iron powder in which the average short-axis length of the string-like primary particles is 5.0 μm or less are comparative examples of the prior art. As compared with the dust core of No. 1, the dust core has a high AC initial permeability, a low iron loss, a high critical frequency, and is excellent in high frequency characteristics.

The iron loss was reduced and the critical frequency was increased because the diameter of the primary particles was reduced, so that the eddy current loss inside the primary particles was reduced and that the primary particles were formed on the surface of the primary particles. It is presumed that the eddy current loss between the primary particles was reduced by being insulated from each other by the oxide films.

[0043]

(Inventive Examples 3 to 4, Comparative Example 2) Water atomized iron powder was flattened with a ball mill and classified with a sieve having a mesh size of 45 μm to obtain flat primary particles. Furthermore, presintering was performed at 800 ° C. for 1 hour in a hydrogen atmosphere containing water vapor (steam dew point: 60 ° C.). Thereafter, the iron powder of the obtained cake was crushed and classified with a sieve having an opening of 180 μm to obtain iron powder of secondary particles to which flat primary particles were bound (Invention Example 3).

An average diameter of a wrought bar of mild steel was cut to 4.
Acicular primary particles of 0 μm were obtained. This was pre-sintered under the same conditions as in Invention Example 3 to obtain secondary particles to which needle-like primary particles were bound (Invention Example 4). Table 2 shows the powder characteristics of the iron powder (secondary particles) and the average aspect ratio of the primary particles. For comparison,
Table 2 shows the characteristics of atomized iron powder for powder metallurgy (Comparative Example 2) and iron powder obtained by flattening the iron powder (Comparative Example 3). The iron powder of Comparative Example 3 was processed by a vibration ball mill, classified with a sieve having an opening of 180 μm, and further heat-treated at 800 ° C. for 1 hour in a hydrogen atmosphere containing water vapor (steam dew point: 60 ° C.). These iron powders were formed into a dust core in the same procedure as in Inventive Example 1, and the characteristics were measured in the same manner. The results are shown in Table 2.

From the observation of the inside of the secondary particles, a cross section of flat primary particles was observed in the iron powder of Invention Example 3. The average diameter of the flat primary particles was 3.6 μm, and the average aspect ratio was 5. Further, needle-like primary particles were confirmed in the iron powder of Invention Example 4. The average diameter of the acicular primary particles is 4.0.
μm, and the average aspect ratio was 10. In the iron powders of Comparative Examples 2 and 3, a structure in which the inside of the iron powder was further divided was not confirmed. When the width of the iron powder in Comparative Examples 2 and 3 was regarded as the primary particle diameter, the values were 20.0 μm and 1 respectively.
It was 7.0 μm. The aspect ratios were 1 and 6, respectively.

Next, the density of each dust core was measured. The dust core of Inventive Example 3 had a density lower by 0.07 to 0.09 Mg / m ³ than the dust cores of Comparative Examples 2 and ³ . The dust core of Inventive Example 4 was equivalent to Comparative Example 2. The AC initial magnetic permeability of the dust core of Invention Example 3 was the same as Comparative Example 2,
Despite having a lower density than the dust core of No.3,
80, and that of the dust core of Invention Example 4 is 88
It was even bigger.

In addition to increasing the density of the dust core,
It is preferable to provide a structure that reduces the internal demagnetizing field of the particles inside the green compact, which is clear from a comparison between Comparative Example 2 and Invention Examples 3 and 4. Flat primary particles obtained by flattening atomized iron powder, or primary particles having a shape that reduces the internal demagnetizing field, such as needle-shaped primary particles obtained by cutting an ingot, are formed by binding. It can be seen that a powder magnetic core having a higher AC initial permeability can be obtained by using iron powder having a secondary particle structure and having an average diameter of the flat or acicular primary particles of 5.0 μm or less.

Further, as can be seen from the comparison between Comparative Example 3 and Invention Examples 3 and 4, when a secondary particle structure composed of primary particles having a high average aspect ratio is provided, iron produced by a conventionally known atomizing method is provided. A dust core having a higher magnetic permeability than the powder was obtained.

The iron losses of Invention Examples 3 and 4 were 36 W /
kg, 35 W / kg, lower than 45 W / kg in Comparative Example 2 and 41 W / kg in Comparative Example 3. The critical frequencies of Inventive Examples 3 and 4 were higher than those of Comparative Example 2, indicating that the secondary particle structure is effective for improving the critical frequency.

That is, even if the raw material is an internal dense iron powder, it is shaped into such a shape that the internal demagnetizing field is reduced, and then they are bound by temporary sintering to have a secondary particle structure. As compared with the case of using the raw iron powder as it is, a dust core having a lower iron loss and a higher AC initial permeability was obtained.

[0052]

[0053]

Since the magnetic metal powder of the present invention has a small demagnetizing field, it is formed into a dust core having a high magnetic permeability in a high frequency range, and is suitable for a choke coil or the like.

[Brief description of the drawings]

FIG. 1A is a schematic view of a cross section obtained by embedding and polishing a secondary particulate magnetic metal powder formed by assembling a plurality of flat primary particles in a resin product. (B) is a schematic diagram of a cross section obtained by embedding and polishing a secondary particulate magnetic metal powder composed of a plurality of needle-like primary particles in a resin product. (C) is a schematic diagram of a cross section obtained by embedding and polishing a secondary particulate magnetic metal powder in which a plurality of string-like primary particles are entangled in a resin product.

FIG. 2 is an electron micrograph (magnification: 400) of a magnetic metal powder in which a plurality of flat primary particles are aggregated to form secondary particles.
Times).

FIG. 3 is an electron micrograph (500 × magnification) of magnetic metal powder in which a plurality of string-like primary particles are entangled to form sponge-like secondary particles.

FIG. 4 is a schematic diagram illustrating the binding between primary particles.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K017 AA03 BA06 CA03 DA04 EA04 EA06 EB01 EH18 FB05 4K018 BA15 BB01 BB04 BC08 BC11 BD01 KA44 5E041 AA11 AA19 CA02 HB05 HB08 NN06

Claims (6)

[Claims] An iron-based magnetic metal powder comprising secondary particles in which a plurality of primary particles are aggregated, wherein the secondary particles are (1) an aggregate of a plurality of flat primary particles, and (2) a plurality of acicular primary particles. An iron-based magnetic metal powder for a dust core having a high magnetic permeability, wherein the iron-based magnetic powder is an aggregate of particles or (3) an aggregate in which a plurality of string-like primary particles are entangled to form a sponge. 2. The secondary particles have an average particle size of 40 to 100 μm.
2. The iron-based magnetic metal powder for a dust core having a high magnetic permeability according to claim 1, wherein m is m. 3. An average minor axis length of a cross section of the primary particles is 1 to 5.
The iron-based magnetic metal powder for a dust core having a high magnetic permeability according to claim 1 or 2, wherein the average magnetic aspect ratio is 2 to 100 µm. 4. The sponge-like iron-based magnetic metal powder is obtained by reducing iron oxide.
The iron-based magnetic metal powder for a dust core having a high magnetic permeability according to any one of the above. 5. The dust core having a high magnetic permeability according to claim 1, wherein said flat primary particles are obtained by flattening atomized iron powder. For iron-based magnetic metal powder. 6. The dust core having a high magnetic permeability according to claim 1, wherein the needle-like primary particles are obtained by cutting a smelted steel material. For iron-based magnetic metal powder.