JP5159751B2 - Manufacturing method of dust core and dust core obtained by this manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a high resistivity core and a dust core obtained by using this method.

  It is important that the powder magnetic core for electromagnetic parts has good handling properties in the manufacturing process and has sufficient mechanical strength that does not break during winding to form a coil. In consideration of these points, in the dust core field, a technique for coating iron powder particles with an electrical insulator is known. Thus, since iron powder particle | grains are adhere | attached via an electrical insulator by coat | covering iron powder particle | grains with an electrical insulator, the mechanical strength improves the powder magnetic core obtained using this.

  Until now, a technique using a silicone resin having high heat resistance as a material for forming such an electrical insulator has been developed. In addition, a technique using a glassy compound obtained from phosphoric acid or the like as an electrical insulator (forming material) other than resin has been known for a long time (Patent Document 1).

  Furthermore, the present applicant forms a phosphoric acid-based chemical conversion film containing a specific element and a silicone resin film in this order on the surface of the iron-based soft magnetic powder, thereby achieving a high magnetic flux density, a low iron loss, and a high machine. Has successfully provided a powder magnetic core with sufficient strength and has already received a patent (Patent Document 2).

  However, the demand for higher performance of the powder magnetic core is further increased compared with the time of filing of Patent Document 2, and higher magnetic flux density and higher mechanical strength are required than ever before.

  To improve the magnetic flux density, it is effective to form a compacted body at a high density. To reduce iron loss, especially hysteresis loss, it is necessary to anneal at a high temperature to release the distortion of the compacted body. It is considered effective. However, since the insulating film deteriorates during high-temperature annealing, there is a problem that the insulating property is lowered after annealing and eddy current loss is increased.

  In order to solve this problem, in Patent Document 3, the annealing temperature and annealing time, and the temperature lowering conditions are controlled to suppress an increase in eddy current loss due to thermal degradation of the insulating film of the soft magnetic member within an allowable range. However, a method for efficiently reducing hysteresis loss due to annealing is described.

Japanese Patent No. 2710152 Japanese Patent No. 4044591 JP 2009-16701 A

  Since the invention described in Patent Document 3 uses only a silicone resin for forming an insulating film, it is effective for an iron-based soft magnetic powder having a phosphoric acid-based chemical film that the present inventors have been studying. That wasn't true. Therefore, in the present invention, a method for producing a dust core that exhibits high electrical insulation even after high-temperature annealing, has high magnetic flux density, and can exhibit high mechanical strength has been found. The object was to provide a dust core.

The present invention, which has been able to solve the above-mentioned problems, is a method of compacting an iron-based soft magnetic powder for a dust core having a phosphoric acid-based chemical conversion film on the surface of the iron-based soft magnetic powder, and then subjecting the compact to a heat treatment. In the method of manufacturing a dust core,
The heat treatment temperature is 550 ° C. or higher, the heat treatment time is longer than 20 minutes, and the compacted body after the heat treatment is cooled in a liquid medium.

  As the liquid medium, water or oil is preferably used, and the temperature of the liquid medium is preferably 30 ° C. or less. Moreover, it is more preferable that the time required until the temperature of the green compact after the heat treatment is lowered to 400 ° C. is less than 3 minutes.

  The present invention also includes a dust core obtained by the method for producing a dust core of the present invention.

  According to the manufacturing method of the present invention, it was possible to manufacture a dust core having high electrical insulation, that is, high specific resistance without increasing eddy current loss (corresponding to coercive force) during high-temperature annealing. . Therefore, the dust core obtained by the production method of the present invention has a high performance that satisfies all of the required characteristics of high magnetic flux density, low iron loss, and high mechanical strength.

  The features of the production method of the present invention are that the heat treatment temperature in the annealing step is 550 ° C. or higher and the heat treatment time is longer than 20 minutes, and that the compacted body is cooled in the liquid medium after the heat treatment is completed. .

  When the present inventors examined, the insulation fall in the distortion removal annealing process of a compacting body is that the phosphoric acid type | system | group chemical conversion film (insulation film) of the iron powder surface becomes thin with heating, and long The main cause was the local short circuit between the iron powders caused by heating over time. Moreover, since the influence of the thinning of the phosphoric acid-based chemical conversion film becomes remarkable at approximately 350 ° C. or higher, the time during which the green compact is maintained at 350 ° C. or higher can be shortened as much as possible. It was considered important. However, in order to take the distortion of the green compact, the green compact needs to be 550 ° C. or higher. Therefore, in the present invention, after the strain is removed at a sufficient temperature, the green compact is cooled to a temperature at which the thinning of the phosphoric acid-based chemical film does not occur, thereby suppressing the thinning of the phosphoric acid-based chemical film. Succeeded. Hereinafter, the present invention will be described in detail.

[Heat treatment (annealing)]
In the present invention, in the annealing process for strain removal, that is, the heat treatment process, the green compact is heat-treated at 550 ° C. or more for more than 20 minutes. When the temperature is lower than 550 ° C. or when the time is short, an increase in hysteresis loss caused by molding cannot be sufficiently reduced. The heat treatment temperature is preferably 570 ° C. or higher, and more preferably 590 ° C. or higher. The upper limit of the heat treatment temperature is not particularly limited, but is preferably 700 ° C. and more preferably 650 ° C. in order to suppress the thinning of the iron phosphate film. The heat treatment time is preferably 25 minutes or more, and more preferably 27 minutes or more. The heat treatment time is preferably longer, but if high-temperature heat treatment is performed for a long time, the phosphoric acid-based chemical conversion film is thinned and the insulating property is lowered as described above. 60 minutes or less is more preferable, and 35 minutes or less is particularly preferable. The heat treatment atmosphere is not particularly limited, but it is preferably performed in an inert gas atmosphere such as nitrogen.

  After the heat treatment, it is necessary to quickly cool (rapidly cool) the green compact. This is for suppressing the thinning of the phosphoric acid-based chemical conversion film and preventing the insulating property from being lowered. Therefore, in the present invention, cooling is performed using a liquid medium. A faster cooling rate is preferred. As an indication of the cooling rate, although it depends on the size of the green compact, it is preferable that the time until the temperature of the green compact after the heat treatment is lowered to 400 ° C. is less than 3 minutes. It is more preferable that the time until the temperature of the green compact after the heat treatment is lowered to 350 ° C. is less than 3 minutes. Thinning of the phosphoric acid-based chemical conversion film can be more reliably suppressed.

  Although it does not specifically limit as a liquid medium, From the surface of safety | security or cost, water or oil is preferable. As the oil, quenching oil that is liquid at normal temperature (25 ° C.) is suitable.

  Examples of the cooling method using the liquid medium include a method of immersing the green compact in the liquid medium bath, a method of pouring the liquid medium over the green compact, and the like. This is preferable because it can be easily reused.

  As the amount of the liquid medium to be used is larger and the temperature is lower, the cooling efficiency is improved. However, the amount of the liquid medium and the temperature may be appropriately selected in view of cost. In terms of temperature, it is preferable to use a liquid medium that remains unheated and uncooled in terms of cost. When cooling is completed, a dust core is obtained.

  By performing heat treatment for strain relief under the above-described method and cooling with a liquid medium after heat treatment, high electrical insulation, that is, high specific resistance without increasing eddy current loss (corresponding to coercive force) is achieved. It is possible to manufacture a dust core having

  Hereinafter, the suitable embodiment of the compacting body used for manufacture of a powder magnetic core in this invention is demonstrated.

[Iron-based soft magnetic powder]
The iron-based soft magnetic powder used in the present invention is a ferromagnetic iron-based powder. Specifically, pure iron powder, iron-based alloy powder (Fe-Al alloy, Fe-Si alloy, Sendust, Permalloy, etc.) , And iron-based amorphous powders. These iron-based soft magnetic powders can be produced, for example, by reducing molten iron (or molten iron alloy) into fine particles by an atomizing method, and then reducing and grinding. In such a production method, an iron-based soft magnetic powder having a particle size (median diameter) of about 20 to 250 μm that gives a cumulative particle size distribution of 50% in the particle size distribution evaluated by the sieving method can be obtained. The base soft magnetic powder preferably has a particle size (median diameter) of about 50 to 150 μm.

[Phosphate-based chemical conversion coating]
The iron powder for a green compact used in the present invention has a phosphoric acid-based chemical conversion film. Thereby, electrical insulation can be provided to the iron powder for a compacting body.

  The composition of the phosphoric acid-based chemical film is not particularly limited as long as it is a glassy film formed using a compound containing P. In addition to P, a compound containing Co, Na, and S is also included. Or it is preferable that it is a glassy film | membrane formed using the compound containing Cs and / or Al. This is because these elements prevent oxygen from forming a semiconductor with Fe during heat treatment (annealing) to lower the specific resistance.

  In the case where the phosphoric acid-based chemical film is a glassy film formed using a compound containing Co or the like other than P, the content of these elements is 100 masses of iron powder for a green compact. %, P is 0.005 to 1% by mass, Co is 0.005 to 0.1% by mass, Na is 0.002 to 0.6% by mass, and S is 0.001 to 0.2% by mass. % Is preferred. Moreover, it is preferable that Cs is 0.002-0.6 mass% and Al is 0.001-0.1 mass%. Also when Cs and Al are used in combination, it is preferable that each be within this range.

  Among the above elements, P forms a chemical bond with the iron-based soft magnetic powder surface through oxygen. Therefore, when the amount of P is less than 0.005% by mass, the amount of chemical bonding between the surface of the iron-based soft magnetic powder and the phosphoric acid-based chemical film is insufficient, and a strong film may not be formed. On the other hand, when the amount of P exceeds 1% by mass, P that is not involved in chemical bonding remains unreacted, which may reduce the bonding strength, which is not preferable.

  Co, Na, S, Cs, and Al have an effect of inhibiting Fe and oxygen from forming a semiconductor during heat treatment (annealing) and suppressing a decrease in specific resistance. Co, Na, and S are combined to maximize the effect. Further, either one of Cs and Al may be used, but the lower limit value of each element is the minimum amount for exerting the effect of combined addition of Co, Na, and S. In addition, if Co, Na, S, Cs, and Al are added more than necessary, the relative balance cannot be maintained during the composite addition, but the oxygen-mediated P and the iron-based soft magnetic powder surface It is thought to inhibit the formation of chemical bonds.

  The phosphoric acid-based chemical conversion film of the present invention may contain Mg or B. The content of these elements is preferably 0.001 to 0.5% by mass for both Mg and B as the amount in 100% by mass of iron powder for a green compact.

  The thickness of the phosphoric acid-based chemical conversion film of the present invention is preferably about 1 to 250 nm. If the film thickness is thinner than 1 nm, the insulating effect may not be exhibited. On the other hand, if it exceeds 250 nm, the insulating effect is saturated and it is not desirable from the viewpoint of increasing the density of the green compact. A more preferable film thickness is 10 to 50 nm.

[Method of forming phosphoric acid-based chemical conversion film]
The iron powder for a green compact used in the present invention may be produced in any form. For example, a solution in which a compound containing P is dissolved in a solvent composed of water and / or an organic solvent, and an iron-based soft powder. After mixing with magnetic powder, it can be obtained by evaporating the solvent if necessary.

  Examples of the solvent used in this step include water, hydrophilic organic solvents such as alcohol and ketone, and mixtures thereof. A known surfactant may be added to the solvent.

Examples of the compound containing P include orthophosphoric acid (H ₃ PO ₄ ). In addition, examples of the compound for making the phosphoric acid-based chemical conversion film have the above composition include, for example, Co ₃ (PO ₄ ) ₂ (Co and P sources), Co ₃ (PO ₄ ) ₂ .8H ₂ O ( Co and P sources), Na ₂ HPO ₄ (P and Na sources), NaH ₂ PO ₄ (P and Na sources), NaH ₂ PO ₄ .nH ₂ O (P and Na sources), Al (H ₂ PO ₄ ) ₃ (P and Al sources), Cs ₂ SO ₄ (Cs and S sources), H ₂ SO ₄ (S sources), MgO (Mg sources), H ₃ BO ₃ (B sources) and the like can be used. Among these, when dihydrogen phosphate sodium salt (NaH ₂ PO ₄ ) is used as a P source or Na source, the density, strength, and specific resistance of the obtained green compact are excellent in a well-balanced manner.

  The amount of the compound containing P with respect to the iron-based soft magnetic powder may be such that the composition of the phosphoric acid-based chemical conversion film to be formed falls within the above range. For example, a solution of a compound containing P prepared so that the solid content is about 0.01 to 10% by mass and a compound containing an element to be included in the film as necessary is 100 mass of iron-based soft magnetic powder. The composition of the phosphoric acid-based chemical film formed by adding about 1 to 10 parts by mass to the part and mixing with a known mixer, ball mill, kneader, V-type mixer, granulator or the like. It can be within the above range.

  Moreover, you may dry at 150-250 degreeC under air | atmosphere, pressure reduction, or a vacuum after the said mixing process as needed. After drying, a sieve having an opening of about 200 to 500 μm may be passed. By passing through the said process, the iron powder for compacting bodies in which the phosphoric acid type chemical film was formed is obtained.

[Silicone resin film]
The iron powder for a green compact of the present invention may further have a silicone resin film on the phosphoric acid-based chemical conversion film. Thereby, at the time of completion | finish of the bridge | crosslinking and hardening reaction of a silicone resin (at the time of compression), powders couple | bond together firmly. In addition, an Si—O bond having excellent heat resistance is formed, and an insulating film having excellent thermal stability is obtained.

As a silicone resin, if the curing is slow, the powder is sticky and the handling property after film formation is poor, so the bifunctional D unit (R ₂ SiX ₂ : R is an organic group, X is a hydrolyzable group) Preferably has a large number of trifunctional T units (RSiX ₃ : R, X is as defined above). However, if a large amount of tetrafunctional Q units (SiX ₄ : X is the same as above) is contained, the powder will be firmly bound at the time of pre-curing and the subsequent molding process cannot be performed. It is not preferable. Accordingly, a silicone resin having a T unit of 60 mol% or more is preferable, a silicone resin having 80 mol% or more is more preferable, and a silicone resin having all T units is most preferable.

  Further, as the silicone resin, a methylphenyl silicone resin in which the above R is a methyl group or a phenyl group is common, and it is said that the heat resistance is higher when it has more phenyl groups, but it is adopted in the present invention. Under such high-temperature heat treatment (annealing) conditions, the presence of phenyl groups was not very effective. It is thought that the bulkiness of the phenyl group disturbs the dense glassy network structure and reduces the thermal stability and the compound formation inhibitory effect with iron. Therefore, in the present invention, it is preferable to use a methylphenyl silicone resin having a methyl group of 50 mol% or more (for example, KR255, KR311, etc. manufactured by Shin-Etsu Chemical Co., Ltd.), and 70 mol% or more (for example, manufactured by Shin-Etsu Chemical Co., Ltd.). KR300 and the like, and methyl silicone resins having no phenyl group (for example, KR251, KR400, KR220L, KR242A, KR240, KR500, KC89, etc. manufactured by Shin-Etsu Chemical Co., Ltd., SR2400 manufactured by Toray Dow Corning) Etc.) is most preferred. The ratio and functionality of the methyl group and phenyl group of the silicone resin (film) can be analyzed by FT-IR or the like.

  The adhesion amount of the silicone resin film is 0.05 to 0.3% by mass when the iron powder for a compacting body in which the phosphoric acid-based chemical film and the silicone resin film are formed in this order is 100% by mass. It is preferable to adjust so that it becomes. When the amount is less than 0.05% by mass, the insulation is inferior and the electric resistance is lowered. However, when the amount is more than 0.3% by mass, it is difficult to achieve a high density of the obtained green compact.

  The thickness of the silicone resin film is preferably 1 to 200 nm. A more preferred thickness is 20 to 150 nm. The total thickness of the phosphoric acid-based chemical film and the silicone resin film is preferably 250 nm or less. If it exceeds 250 nm, the decrease in magnetic flux density may increase.

[Method of forming silicone resin film]
Formation of the silicone resin film is, for example, an iron-based soft magnetic powder having a silicone resin solution in which a silicone resin is dissolved in alcohols, petroleum-based organic solvents such as toluene and xylene, and a phosphoric acid-based chemical film. (Hereinafter, it may be simply referred to as “phosphoric acid-based film-forming iron powder” for the sake of convenience.) And then, if necessary, the organic solvent is evaporated.

  The amount of the silicone resin added to the phosphoric acid-based film-forming iron powder is not particularly limited as long as the amount of the formed silicone resin film is within the above range. For example, about 0.5 to 10 parts by mass of a resin solution prepared so that the solid content is about 2 to 10% by mass is added to and mixed with 100 parts by mass of the phosphoric acid-based chemical film forming iron powder. What is necessary is just to dry. If the amount is less than 0.5 parts by mass, mixing may take time or the film may become non-uniform. On the other hand, if it exceeds 10 parts by mass, drying may take time or drying may be insufficient. The resin solution may be appropriately heated. The same mixer as described above can be used.

  In the drying step, it is desirable to sufficiently evaporate the organic solvent by heating to a temperature at which the organic solvent used volatilizes and below the curing temperature of the silicone resin. A specific drying temperature is preferably about 60 to 80 ° C. in the case of the alcohols and petroleum organic solvents described above. After drying, it is preferable to pass through a sieve having an opening of about 300 to 500 μm in order to remove aggregated lumps.

  After drying, the iron powder for a green compact on which a silicone resin film is formed (hereinafter, sometimes simply referred to as “silicone resin film-forming iron powder”) is heated to pre-cure the silicone resin film. It is recommended. The pre-curing is a process for terminating the softening process at the time of curing the silicone resin film in a powder state. By this preliminary curing treatment, the flowability of the silicone resin film-forming iron powder can be ensured during warm molding (about 100 to 250 ° C.). As a specific method, a method of heating the silicone resin film-forming iron powder in the vicinity of the curing temperature of the silicone resin for a short time is simple, but a method using a drug (curing agent) can also be used. The difference between pre-curing and curing (complete curing that is not preliminary) is that the pre-curing process can be easily crushed without completely solidifying the powder, whereas In the high temperature heat curing process to be performed, the resin is cured and the powders are bonded and solidified. The strength of the molded body is improved by the complete curing treatment.

  As mentioned above, after pre-curing the silicone resin, it can be crushed to obtain a powder with excellent fluidity, which can be poured into the mold during sand compaction like sand. become able to. If it is not pre-cured, for example, powders may adhere to each other during warm molding, and it may be difficult to charge the mold in a short time. In practical operation, the improvement of handling is very significant. It has also been found that the specific resistance of the resulting dust core is greatly improved by pre-curing. Although this reason is not clear, it is thought that it may be because the adhesiveness with the iron powder at the time of curing increases.

  When pre-curing is performed by a short-time heating method, the heat treatment is preferably performed at 100 to 200 ° C. for 5 to 100 minutes. 10-30 minutes is more preferable at 130-170 degreeC. Even after preliminary curing, it is preferable to pass through a sieve as described above.

[lubricant]
The iron powder for a green compact of the present invention may further contain a lubricant. The action of this lubricant can reduce the frictional resistance between the iron powder when compressing the iron powder for compacted compacts, or between the iron powder and the inner wall of the mold, and it can reduce the mold galling and heat generation during molding. Can be prevented. In order to effectively exhibit such an effect, it is preferable that the lubricant is contained in an amount of 0.2% by mass or more in the total amount of iron powder for a green compact. However, when the amount of the lubricant is increased, it is against the densification of the green compact, so that it is preferable to keep the amount to 0.8% by mass or less. Further, when compression molding is performed, a lubricant is applied to the inner wall surface of the mold and then molded (mold lubrication molding), and the amount of lubricant may be less than 0.2% by mass.

  As the lubricant, conventionally known ones may be used. Specifically, stearic acid metal salt powder such as zinc stearate, lithium stearate, calcium stearate, and paraffin, wax, Examples thereof include natural or synthetic resin derivatives.

[Compression molding]
A compacting body is obtained by compression-molding the iron powder for compacting bodies. The compression molding method is not particularly limited, and a conventionally known method can be employed.

A suitable condition for compression molding is a surface pressure of 490 MPa to 1960 MPa, more preferably 790 MPa to 1180 MPa. In particular, when compression molding is performed under conditions of 980 MPa or more, it is easy to obtain a dust core having a final density of 7.50 g / cm ³ or more, and a dust core having high strength and good magnetic properties (magnetic flux density) is obtained. Since it is obtained, it is preferable. The molding temperature can be either room temperature molding or warm molding (100 to 250 ° C.). It is preferable to perform warm molding by mold lubrication molding because a powder magnetic core with higher strength can be obtained. As a measure of strength, it is preferable that the bending strength measured by the measurement method in Examples described later is 100 MPa or more.

[Dust core]
After obtaining the green compact, heat treatment and subsequent cooling with a liquid medium are performed under the conditions described above. A dust core is obtained after cooling.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention. Unless otherwise specified, “part” means “part by mass” and “%” means “% by mass”.

Experimental Example Pure iron powder (manufactured by Kobe Steel; Atmel 300NH; particle size 80-100 μm) was used as the soft magnetic powder.

As a treatment solution for an iron phosphate chemical conversion film, water: 50 parts, Na ₂ HPO ₄ : 30 parts, H ₃ PO ₄ : 10 parts, (NH ₂ OH) ₂ · H ₂ SO ₄ : 10 parts, Co ₃ (PO ₄ ) ₂ : 10 parts were mixed, and a treatment solution diluted 10 times with water was used. 50 cc of this treatment liquid was added to 1 kg of the above pure iron powder, mixed for 30 minutes or more using a V-type mixer, dried in the atmosphere at 200 ° C. for 30 minutes, and passed through a sieve having an opening of 300 μm.

  Next, a silicone resin “KR220L” (manufactured by Shin-Etsu Chemical Co., Ltd.) having a methyl group of 100 mol% and a T unit of 100 mol% is dissolved in toluene to produce a resin solution having a solid content concentration of 4.8%. did. This resin solution was added to and mixed with the above iron powder so that the resin solid content was 0.15%, dried in an oven furnace at 75 ° C. for 30 minutes in the atmosphere, and then sieved with a sieve having an opening of 300 μm. I passed. Thereafter, preliminary curing was performed at 150 ° C. for 30 minutes.

  Subsequently, Zn stearate was dispersed in alcohol and applied to the surface of the mold, and then iron powder for a compacting body was added, and compacting was performed at a surface pressure of 980 MPa at room temperature (25 ° C.). The molded body dimensions are 31.75 mm × 12.7 mm and the height is about 5 mm. Thereafter, annealing was performed at 600 ° C. for 1 minute or 30 minutes in a nitrogen atmosphere. The heating rate was about 5 ° C./min. After the heat treatment, furnace cooling, cooling with room temperature (non-heated) water (water quench), cooling with room temperature (non-heated) oil (made by Idemitsu Kosan Co., Ltd .; “Dafney Quench B”) (oil quench) was performed.

  The density, coercive force, specific resistance, and bending strength of the obtained molded product were measured and shown in Table 1. The measuring method is as follows.

[density]
The mass and size of the molded body were measured and calculated.

[Coercivity]
Using a BH curve tracer “BHS-40S” manufactured by Riken Denshi Co., Ltd., measurement was performed at 25 ° C. with a maximum applied magnetic field (B) of 100 (Oe).

[Resistivity]
The specific resistance was measured by the 4-terminal method. Measurement was performed in a four-terminal resistance measurement mode using “RM-14L” manufactured by Rika Denshi Co., Ltd. as the probe and a digital multimeter “VOAC-7510” manufactured by Iwasaki Tsushinki Co., Ltd. as the measuring instrument. The measurement was performed by setting the distance between the terminals to 7 mm, the probe stroke length to 5.9 mm, the spring load to the 10-S type, and pressing the probe against the measurement sample.

[Folding strength]
The bending strength test was performed by a three-point bending test in accordance with JPMA M 09-1992 (Japan Powder Metallurgy Industry Association; bending strength test method for sintered metal materials). A tensile tester (“AUTOGRAPH AG-5000E” manufactured by Shimadzu Corporation) was used for the strength measurement, and the distance between fulcrums was set to 25 mm.

[Cooling time]
While the thermocouple was inserted into the test piece and the temperature was examined, the time required for the compact to reach 400 ° C. after the heat treatment was completed was measured.

  As apparent from Table 1, since the molding conditions themselves are the same, the density of each molded body is hardly changed. However, in Comparative Example 1, since furnace cooling with a slow temperature drop rate was performed, it took about 40 minutes for the compact to reach 400 ° C. As a result, thinning of the phosphoric acid-based chemical conversion film occurred. Thus, it can be seen that the insulation is lowered and the specific resistance is greatly reduced. In Comparative Example 2, the heat treatment time was too short to reduce the hysteresis loss, and the coercive force remained high.

  In Examples 1 and 2, the coercive force, specific resistance, and bending strength were excellent in a well-balanced manner by combining sufficient heat treatment time and cooling (quenching) with a liquid medium.

  According to the method for manufacturing a dust core of the present invention, a dust core that can achieve high magnetic flux density, low coercive force (low iron loss), and high mechanical strength can be manufactured. This dust core is useful as a rotor of a motor or a core of a stator.

Claims (5)

In a method for producing a dust core by compacting an iron-based soft magnetic powder for a powder magnetic core having a phosphoric acid-based chemical conversion film on the surface of the iron-based soft magnetic powder, and then subjecting the compact to a heat treatment.
A method for producing a dust core, wherein the heat treatment temperature is 550 ° C. or more, the heat treatment time is longer than 20 minutes, and the dust compact after the heat treatment is finished is cooled in a liquid medium.   The production method according to claim 1, wherein water or oil is used as the liquid medium.   The production method according to claim 1 or 2, wherein the liquid medium is 30 ° C or lower.   The manufacturing method according to any one of claims 1 to 3, wherein a time required until the temperature of the green compact after the heat treatment is lowered to 400 ° C is less than 3 minutes.   A dust core obtained by the manufacturing method according to claim 1.