JP2013168648A - Method of manufacturing dust core, and dust core manufactured by that method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a dust core the strength of which can be enhanced without lowering the density significantly, and to provide a dust core.SOLUTION: The method of manufacturing a dust core includes a step for obtaining a dust compact by compression molding a mixture of an iron-based soft magnetic powder for dust compact having a phosphoric acid-based chemical conversion coating on the surface thereof and a lubricant, a heat treatment step 1 for heating the dust compact at 300°C-500°C, a heat treatment step 2 for heating the dust compact at 500°C-700°C in an oxidizing atmosphere, and a heat treatment step 3 for heating the dust compact at 300°C-450°C for 30-120 minutes.

Description

  The present invention relates to a method for manufacturing a dust core and a dust core obtained by using the 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. By covering the iron powder particles with the electric insulator, the iron powder particles are bonded to each other through the electric insulator, so that the mechanical strength of the dust core obtained by using this is improved.

  Until now, as a material for forming such an electrical insulator, there has been disclosed a technique using a highly heat-resistant silicone resin or a glassy compound obtained from phosphoric acid or the like (Patent Document 1).

  In addition, 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 a powder magnetic core with high mechanical strength is required more than ever.

Japanese Patent No. 2710152 Japanese Patent No. 4044591

  The present invention has been made paying attention to the above-described circumstances, and the object thereof is to provide a method for manufacturing a dust core capable of improving strength without greatly reducing the density, and a dust core. Is to provide.

  The method for producing a powder magnetic core according to the present invention, which has solved the above problems, comprises mixing an iron-based soft magnetic powder for a powder compact having a phosphoric acid-based chemical film on the surface of the iron-based soft magnetic powder and a lubricant. A molding step of compression-molding the mixture to obtain a green compact, a heat treatment step 1 in which the green compact is heated at 300 ° C. to 500 ° C., and then in an oxidizing atmosphere, over 500 ° C. 700 The present invention includes a heat treatment step 2 for heating at a temperature of not higher than ° C. and a heat treatment step 3 for heating at a temperature of 300 to 450 ° C. for 30 to 120 minutes.

  In the present invention, it is also preferable that the mixture further contains at least one oxygen source releasing compound selected from the group consisting of sugar alcohol, metal hydroxide, metal peroxide, percarbonate, and oxidizing agent. It is an aspect.

  In another preferred embodiment, the iron-based soft magnetic powder for a green compact has a silicone resin film on the phosphoric acid-based chemical conversion film.

It is also a preferred embodiment that the lubricant is a polyhydroxycarboxylic acid amide.
The present invention also includes a dust core obtained by the above manufacturing method.

  The present invention also includes a powder magnetic core obtained by compression-molding iron-based soft magnetic powder. The powder magnetic core is composed of 5.0 to 15% by volume of magnetite and 3.0% or less (0 volume) of wustite. %). Moreover, it is also a preferable embodiment that the dust core further contains hematite and the volume ratio of hematite to magnetite is 0.05 to 0.25.

  According to the production method of the present invention, it is possible to provide a dust core having high mechanical strength without greatly reducing the density.

  As described above, a manufacturing method for improving the mechanical strength of a dust core by heat-treating a mixture obtained by coating iron powder with an electrical insulator is known. In order to improve the mechanical strength, earnest research was repeated. As a result, it was found that among the iron oxides produced by heat treatment, magnetite and hematite contribute to increasing the strength, but wustite is a factor for reducing the strength. Then, by appropriately controlling the heat treatment conditions, it was found that wustite can be converted to magnetite and the mechanical strength can be further improved, and the present invention has been achieved.

  Hereinafter, the manufacturing method of the powder magnetic core which concerns on this invention, and the powder magnetic core obtained by this manufacturing method are demonstrated.

[Molding process]
The dust core of the present invention is manufactured by appropriately heat-treating the dust compact. The green compact can be obtained by compression molding a mixture of an iron-based soft magnetic powder for a green compact having a phosphoric acid-based chemical conversion film on the surface of the iron-based soft magnetic powder and a lubricant.
[Iron-based soft magnetic powder]
The iron-based soft magnetic powder is a ferromagnetic iron-based powder, specifically, pure iron powder, iron-based alloy powder (Fe-Al alloy, Fe-Si alloy, Sendust, Permalloy, etc.), iron-based amorphous powder. A powder etc. are mentioned. 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 μm to 250 μm that gives a cumulative particle size distribution of 50% in the particle size distribution evaluated by the sieving method is obtained. The base soft magnetic powder preferably has a particle size (median diameter) of about 50 μm to 150 μm.

[Phosphate-based chemical conversion coating]
The iron-based soft magnetic powder for compacted compacts (hereinafter sometimes referred to as “iron powder for compacted compacts”) has a phosphate conversion coating on the surface of the iron-based soft magnetic powder. Thereby, electrical insulation can be provided to the iron powder for a compacting body.

  The phosphoric acid-based chemical film is a glassy film formed using a compound containing P. The composition is not particularly limited, but it is preferably a glassy film formed using a compound containing Co, Na, S, or a compound containing Cs and / or Al in addition to P. . This is because these elements inhibit oxygen from forming a semiconductor with Fe during the heat treatment step 2 to be described later, thereby reducing the specific resistance.

  Among the above elements, P forms a chemical bond with the iron-based soft magnetic powder surface through oxygen. If the P content is low, the amount of chemical bonding between the surface of the iron-based soft magnetic powder and the phosphoric acid-based chemical film becomes insufficient, and a strong film may not be formed. On the other hand, when there is too much P content, P which does not participate in a chemical bond may remain unreacted, and the bond strength may be lowered.

  Co, Na, S, Cs, and Al have an effect of inhibiting Fe and oxygen from forming a semiconductor during the heat treatment step 2 and suppressing a decrease in specific resistance. Co, Na, and S maximize the effect of the combined addition. Further, either one of Cs and Al may be used. If excessive addition of Co, Na, S, Cs, and Al not only keeps the relative balance during compound addition, it also inhibits the formation of chemical bonds between P and the iron-based soft magnetic powder surface via oxygen. There are things to do. The phosphoric acid-based chemical conversion film may further contain Mg or B.

[Method of forming phosphoric acid-based chemical conversion film]
The method for forming a phosphoric acid-based chemical conversion film on the surface of the iron-based soft magnetic powder is not particularly limited. For example, it can be obtained by mixing 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 magnetic powder, and then evaporating the solvent as necessary.
Examples of the solvent 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 ). Examples of the compound for making the phosphoric acid-based chemical conversion film have the above composition include Co ₃ (PO ₄ ) ₂ (Co and P sources), Co ₃ (PO ₄ ) ₂ .8H ₂ O (Co and P Source), 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 source), Cs ₂ SO ₄ (Cs and S source), H ₂ SO ₄ (S source), MgO (Mg source), H ₃ BO ₃ (B source) and the like can be used.

[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. Moreover, the thermal stability of the insulating film can be improved by forming a Si—O bond having excellent heat resistance. The silicone resin is not particularly limited as long as the above effects can be obtained.

[Method of forming silicone resin film]
The method for forming the silicone resin film on the phosphoric acid-based chemical film is not particularly limited. For example, a silicone resin solution in which a silicone resin is dissolved in an alcohol, a petroleum-based organic solvent such as toluene or xylene, and an iron-based soft magnetic powder having a phosphoric acid-based chemical film (hereinafter referred to as “phosphate-based film-forming iron”). Sometimes called “powder”). Then, if necessary, the silicone solvent film can be formed by evaporating the organic solvent.

[lubricant]
In the present invention, an iron-based soft magnetic powder for powder compacts (iron powder for powder compacts) having a phosphoric acid-based chemical conversion film (or a silicone resin film on the phosphoric acid-based chemical conversion film) on the surface of the iron-based soft magnetic powder. ) And a lubricant to form a mixture. By this action of the lubricant, the frictional resistance between the iron powders during compression molding of the mixture or between the iron powder and the inner wall of the mold can be reduced, and the mold can be prevented from galling and heat generation during molding. The lubricant is not particularly limited as long as it has such an action, and conventionally known lubricants can be used alone or in combination.

  A conventionally well-known thing can be used as a lubricant. Specifically, metal salt powder of stearic acid such as zinc stearate, lithium stearate, calcium stearate, fatty acid amide such as polyhydroxycarboxylic acid amide, ethylenebisstearylamide and (N-octadecenyl) hexadecanoic acid amide, paraffin, Examples thereof include wax, natural or synthetic resin derivatives, and the like. Of these, polyhydroxycarboxylic acid amides and fatty acid amides are preferred. These lubricants may be used alone or in combination of two or more.

Examples of the polyhydroxycarboxylic acid amide include C _m H _{m + 1} (OH) _m —CONH—C _n H _{2n + 1} (m is 2 or 5, n is an integer of 6 to 24 described in WO2005 / 068588. ).

More specifically, the following polyhydroxycarboxylic acid amides may be mentioned.
_{(1) n-C 2 H} 3 (OH) 2 -CONH-n-C 6 H 13
(N-hexyl) glyceric acid amide (2) n-C ₂ H ₃ (OH) ₂ -CONH-n-C ₈ H ₁₇
(N-octyl) glyceric acid amide (3) n-C ₂ H ₃ (OH) ₂ -CONH-n-C ₁₈ H ₃₇
(N-octadecyl) glyceric acid amide (4) n-C ₂ H ₃ (OH) ₂ -CONH-n-C ₈ H ₃₅
(N- octadecenyl) glyceric acid amide _{(5) n-C 2 H} 3 (OH) 2 -CONH-n-C 22 H 45
(N-docosyl) glyceric acid amide (6) n-C ₂ H ₃ (OH) ₂ -CONH-n-C ₂₄ H ₄₉
(N-tetracosyl) glyceric acid amide (7) n-C ₅ H ₆ (OH) ₅ -CONH-n-C ₆ H ₁₃
(N- hexyl) gluconic acid amide _{(8) n-C 5 H} 6 (OH) 5 -CONH-n-C 8 H 17
(N- octyl) gluconic acid amide _{(9) n-C 5 H} 6 (OH) 5 -CONH-n-C 18 H 37
(N- octadecyl) gluconic acid amide _{(10) n-C 5 H} 6 (OH) 5 -CONH-n-C 18 H 35
(N- octadecenyl) gluconic acid amide _{(11) n-C 5 H} 6 (OH) 5 -CONH-n-C 22 H 45
(N- docosyl) gluconic acid amide _{(12) n-C 5 H} 6 (OH) 5 -CONH-n-C 24 H 49
(N-tetracosyl) gluconic acid amide

[Oxygen source releasing compound]
The mixture preferably further contains an oxygen source releasing compound. When the green compact is heated, oxygen sources such as oxygen, water, and hydrogen peroxide are released from the oxygen source releasing compound, and the oxidation of the iron powder surface for the green compact is further promoted. In particular, by mixing the oxygen source releasing compound so that it is also disposed inside the green compact, oxidation of the iron powder surface for the green compact proceeds in the green compact during the heat treatment step. To do.

  As a result, in the dust core of the present invention, the bond between the iron powder surface for dust compacts and the insulating film (for example, phosphoric acid-based chemical film) is strengthened, and the bond between the insulating films is also strengthened. , Mechanical strength is improved. Moreover, the specific resistance (insulating property) of the dust core is also improved.

  In order to effectively exhibit such effects, the total amount of the mixture of iron powder for compacted compacts (iron-based soft magnetic powder, phosphoric acid-based chemical conversion film, silicone resin film), lubricant, and oxygen source releasing compound The oxygen source releasing compound is preferably contained in an amount of 0.01% by mass or more, more preferably 0.05% by mass or more. However, if the amount of the oxygen source releasing compound is increased, it is contrary to the densification of the green compact, so that it is preferably 0.8% by mass or less, more preferably 0.6% by mass or less.

  The oxygen source releasing compound is not particularly limited as long as it releases an oxygen source such as oxygen, water and hydrogen peroxide by heating. For example, erythritol, glycerol, isomalt, lactitol, maltitol, mannitol, sorbitol, xylitol Sugar alcohol that releases water by heating, etc .; water is released by heating magnesium hydroxide, aluminum hydroxide, calcium hydroxide, manganese hydroxide, iron hydroxide, cobalt hydroxide, nickel hydroxide, copper hydroxide, etc. Metal hydroxide; Metal peroxide that releases oxygen by heating lithium peroxide, sodium peroxide, zinc peroxide, etc .; Percarbonate that releases hydrogen peroxide that decomposes into water and oxygen by heating sodium peroxide Salts: Oxidizing agents such as nitrate anion, nitrite anion, chlorate anion It is. Examples of the counter ion (cation) of the anion of the oxidizing agent include lithium ion, sodium ion, potassium ion, ammonium ion, calcium ion, strontium ion, and barium ion. These oxygen source releasing compounds may be used alone or in combination of two or more.

[Compression molding]
The green compact is obtained by compression molding the above mixture. The compression molding method is not particularly limited, and a conventionally known method can be employed. The compression molding conditions are not particularly limited, and the surface pressure may be adjusted so as to obtain a dust core having a desired density (for example, about 490 MPa to 1960 MPa).

  In this invention, a high intensity | strength powder magnetic core can be manufactured by heat-processing the compacting body obtained by compression molding in the following heat processing processes 1-3.

[Heat treatment step 1]
In the production method of the present invention, the compacted body after compression molding is first heated at 300 ° C. or more and 500 ° C. or less. By this step, the lubricant can be thermally decomposed and removed while preventing the insulating film (phosphoric acid-based chemical film and / or silicone resin film) from being destroyed.

  When an oxygen source releasing compound is added, oxygen source such as oxygen, water, hydrogen peroxide, etc. is slowly released from the oxygen source releasing compound by the heating, so that compacting can be performed for a long time. Evaporation and scattering of the lubricant can be promoted while suppressing the blockage of the oxygen source path between the iron powders inside the body.

  Specifically, the heat treatment step 1 is performed, for example, by putting a green compact into a pressure vessel, filling the gas in the vessel to saturate the inside of the vessel, and then keeping the inside of the vessel within the above temperature range. The method performed by heating is mentioned.

  The atmosphere of the heat treatment step 1 is not particularly limited, and may be any of an oxidizing atmosphere such as air, oxygen, ozone, and water vapor, a rare gas such as nitrogen, helium, and argon, and an inert atmosphere such as vacuum. Each atmosphere may contain other gases as long as the purpose of the heat treatment step 1 is not impaired.

  Further, when the temperature of the heat treatment step 1 is set lower than 300 ° C., the effect of decomposing and removing the lubricant becomes insufficient. Therefore, the temperature of the heat treatment step 1 is set to 300 ° C. or higher, preferably 330 ° C. or higher. On the other hand, if the temperature becomes too high, the compacted body undergoes oxidation and volume expansion occurs before the lubricant is sufficiently decomposed and removed, thereby blocking the lubricant volatilization path and Not fully removed. Therefore, oxygen supplied into the molded body becomes insufficient, and oxidation inside the molded body is hindered. When such a thing occurs, the mechanical strength (breaking strength) of the molded body deteriorates. Therefore, the temperature of the heat treatment step 1 is set to 500 ° C. or lower, preferably 480 ° C. or lower, more preferably 450 ° C. or lower.

  The heating time in the heat treatment step 1 is not particularly limited as long as the purpose of the heat treatment can be achieved. When the heating time is short, the above-mentioned effect by the heat treatment step 1 may not be sufficiently enjoyed. Therefore, the heating time is preferably 10 minutes or more, more preferably 20 minutes or more, preferably 250 minutes or less, more preferably 200 minutes or less, and even more preferably 150 minutes or less.

[Heat treatment step 2]
In the manufacturing method of this invention, following the said heat processing process 1, the process (heat processing process 2) heated at 500 degreeC or more and 700 degrees C or less in an oxidizing atmosphere is included. By the said process, the oxidation of the iron powder for compacting inside a compacting body is accelerated | stimulated, while a magnetite and a hematite are produced | generated, and the coupling | bonding of an iron powder surface and a phosphoric acid system chemical film becomes strong. Further, the bonding between adjacent insulating films is strengthened, and the mechanical strength of the obtained dust core is improved.

  Further, when an oxygen source releasing compound is added, the heating can further promote oxidation inside the green compact by the oxygen source released from the oxygen source releasing compound.

  In the present invention, each heat treatment step may be performed continuously or once cooled to a predetermined temperature. When changing the atmosphere in the container, for example, after the heat treatment step 1 is completed, the green compact is once cooled, and then the pressure-resistant container is replaced with an oxidizing gas, and the container is saturated with an oxidizing gas. Heat treatment step 2 may be performed by heating the inside of the container to a predetermined temperature. In the case where the atmosphere is not changed, it is desirable that, for example, the heat treatment step 1 and the heat treatment step 2 are continuously performed. That is, after the heat treatment step 1, the heat treatment step 2 may be performed by raising the temperature to a predetermined temperature without cooling.

  The atmosphere in the heat treatment step 2 needs to be performed in an oxidizing atmosphere in order to promote the oxidation of the iron powder, and if performed in an inert atmosphere such as nitrogen, the oxidation becomes insufficient and high mechanical strength is obtained. It becomes difficult. The heat treatment step 2 may be performed in the above-described oxidizing atmosphere, and may be an air atmosphere from the viewpoint of manufacturing cost.

  When the heat treatment temperature in the heat treatment step 2 is too low, the iron powder may be insufficiently oxidized, or it may take a long time to advance the oxidation to the inside of the green compact. If the heat treatment temperature is too high, the interface strength between the iron powder and the phosphoric acid-based chemical conversion film may be reduced, and the mechanical strength of the dust core may be reduced. Further, when the insulating film is exposed to high temperature, it may be thinned and the insulating property may be lowered. The heating temperature in the heat treatment step 2 is over 500 ° C., preferably 520 ° C. or higher, more preferably 550 ° C. or higher, 700 ° C. or lower, preferably 680 ° C. or lower, more preferably 650 ° C. or lower, and further preferably 625 ° C. It is as follows.

  The heating time in the heat treatment step 2 is not particularly limited as long as the purpose of the heat treatment can be achieved. Of course, when the heating time is too short, the above-described effects of the heat treatment step 2 may not be sufficiently enjoyed. Therefore, the heating time is preferably 10 minutes or more, more preferably 15 minutes or more. In addition, the heating time is preferably longer from the viewpoint of distortion removal. However, when high-temperature heat treatment is performed for a long time, the insulating property may be lowered due to the thinning of the insulating film as described above. Accordingly, the heating time is preferably 60 minutes or less, more preferably 50 minutes or less, and even more preferably 40 minutes or less.

  Moreover, if the heating temperature and heating time in the heat treatment step 2 are appropriately controlled, a dust core containing hematite that contributes to high strength can be obtained. Specifically, the heating temperature in the heat treatment step 2 is set to 520 ° C. or more and 625 ° C. or less, and heat treatment is performed for a heating time of 35 minutes or more, so that hematite can be generated. Specifically, the volume of hematite with respect to magnetite A dust core having a ratio of 0.05 to 0.25 can be obtained.

[Heat treatment step 3]
In the manufacturing method of this invention, following the said heat processing process 2, the process (heat treatment process 3) further heated at 300 degreeC or more and 450 degrees C or less for 30 minutes or more and 120 minutes or less is included. By this process, wustite is converted into magnetite to reduce wustite that causes low strength, and magnetite that contributes to high strength can be increased, so that the mechanical strength of the obtained dust core is further improved. Specifically, as will be described later, it is possible to obtain a dust core having a magnetite volume ratio of 5.0 to 15% and a wustite volume ratio of 3.0% or less (including 0%).

  By performing the heat treatment step 3 within a predetermined temperature range, it is possible to convert wustite to magnetite while preventing the already generated magnetite and hematite from being destroyed. When the heat treatment step 3 is performed at a temperature lower than 300 ° C., wustite cannot be converted to magnetite, and high strength cannot be achieved. In addition, when the heat treatment step 3 is performed at a temperature higher than 450 ° C., wustite may newly grow. The heating temperature in the heat treatment step 3 is preferably 320 ° C. or higher, more preferably 340 ° C. or higher, preferably 430 ° C. or lower, more preferably 410 ° C. or lower.

  The heating time is 30 minutes or more and 120 minutes or less in the heat treatment temperature range. When the heating time is short, wustite cannot be sufficiently converted to magnetite, and the mechanical strength may not be increased. Therefore, the heating time is preferably 40 minutes or more, more preferably 50 minutes or more. From the viewpoint of promoting the production of magnetite from wustite, a longer heating time is preferable. However, when high-temperature heat treatment is performed for a long time, wustite may newly grow. Preferably it is 110 minutes or less, More preferably, it is 90 minutes or less.

  The atmosphere of the heat treatment step 3 is not particularly limited, and may be any of an oxidizing atmosphere such as air, oxygen, ozone, and water vapor, a rare gas such as nitrogen, helium, and argon, and an inert atmosphere such as vacuum. Each atmosphere may contain other gases as long as the purpose of the heat treatment step 3 is not impaired.

[Dust core]
After heat-treating the green compact, the powder magnetic core of the present invention can be obtained by cooling to room temperature.

  As mentioned above, in the manufacturing method of this invention, if the heat processing process 1-the heat processing process 3 are performed on above-described conditions, the powder magnetic core which has mechanical strength higher than before can be manufactured.

  Specifically, it is possible to obtain a dust core having magnetite of 5.0 to 15% by volume and wustite of 3.0% by volume or less (including 0% by volume).

  If the volume fraction of magnetite is too low, sufficient strength cannot be obtained, and if the volume fraction is too high, the strength is rather lowered. Accordingly, the magnetite content is 5.0% by volume or more, preferably 5.2% by volume or more, more preferably 5.5% by volume or more, and 15% by volume or less, preferably 13% by volume or less, more preferably 11% by volume. % Or less.

  Further, since the strength decreases as the volume fraction of wustite increases, it is 3.0% by volume or less, preferably 1.0% by volume or less, more preferably not contained at all (0% by volume).

  Furthermore, since hematite contributes to high strength, it is preferable that hematite is contained in the dust core. At this time, if the volume ratio of hematite to magnetite is too low, a sufficient strength improvement effect cannot be achieved, whereas if the volume ratio of hematite is too high, the strength may be lowered. Therefore, the volume ratio of hematite to magnetite is preferably 0.05 to 0.25.

  The volume ratio (average value) of magnetite, hematite, and wustite can be obtained by measuring the fracture surface (arbitrary plural positions) of the dust core by the X-ray diffraction method.

  The remaining structure other than magnetite, hematite, and wustite is iron, but from the viewpoint of securing mechanical strength, the volume ratio of magnetite, hematite, and wustite may be appropriately controlled. The configuration is not particularly limited.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

(Molding process)
As iron powder for powder compacts, an iron base for compact compacts having a phosphoric acid-based chemical film on the surface of pure iron powder (iron-based soft magnetic powder) and a silicone resin film on the phosphoric acid-based chemical film. Soft magnetic powder (“ML35D” manufactured by Kobe Steel) was used.

  Polyhydroxycarboxylic acid amide lubricant as a lubricant is added and mixed so as to be 0.2% by mass with respect to the total amount of the mixture of iron powder for compacting body, lubricant, and oxygen source releasing compound. Got.

  For some examples, the oxygen source releasing agent (oxygen source releasing compound) described in Table 1 is further added to the iron powder for a green compact formed with a silicone resin film, the lubricant, and the oxygen source releasing compound. A mixture obtained by adding and mixing 0.1% by mass with respect to the total amount of the mixture was obtained.

  This mixture was filled in a mold and compression molded at room temperature (25 ° C.) at a surface pressure of 784 MPa to obtain a green compact (length 31.75 mm × width 12.7 mm × height about 5 mm).

(Heat treatment process)
Subsequently, heat treatment steps 1 to 3 were performed on the compacted body under the conditions described in Table 1 (No. 11 to 14 did not perform heat treatment step 3). After completion of the heat treatment step 3, the powder core was manufactured by cooling (furnace cooling) to room temperature in the container. In the heat treatment step 1 and the heat treatment step 2, the average rate of temperature rise to a predetermined temperature was controlled to be about 10 ° C./min. Further, the average cooling rate from the heat treatment temperature 2 to the predetermined temperature of the heat treatment temperature 3 was controlled to be about 10 ° C./min. In addition, each heat processing process was performed continuously (For example, in No. 1, after heat processing process 1 was performed at 300 degreeC, it heated up at 550 degreeC and performed heat processing process 2).

(Evaluation)
Table 2 shows the results of measuring the density, bending strength, and volume ratio of magnetite (Fe ₃ O ₄ ), hematite (Fe ₂ O ₃ ), and wustite (FeO) for each of the produced dust cores.

(density)
The density was calculated from the actual measurement values of the mass and size of the dust core. In the present invention, the case where the density was 7.40 g / cm ³ or more was evaluated as acceptable.

(Folding strength)
The bending strength of the dust core was subjected to a three-point bending test (in accordance with JPMA M 09-1992 of the Japan Powder Metallurgy Industry Association) to evaluate the mechanical strength. For the measurement, a tensile tester (“AUTOGRAPH AG-5000E” manufactured by Shimadzu Corporation) was used, and the distance between the fulcrums was set to 25 mm. The case where the bending strength was 70 MPa or more was evaluated as acceptable, and the case where it was less than 70 MPa was evaluated as unacceptable.

(Volume ratio)
The volume ratio of magnetite, hematite, and wustite contained in the dust core was measured using the test piece after the bending test. Specifically, X-ray diffraction measurement was performed by irradiating the surface exposed by fracture in the bending test, and the volume ratio of magnetite, hematite, and wustite was determined. As a measuring device, an X-ray diffractometer RAD-RU300 manufactured by Rigaku Corporation was used. Measurement was performed at a measurement angle (2θ) of 15 to 110 ° using Kα rays with a Co target and a monochromator. The X-ray irradiation area on the measurement surface was about 10 mm wide × 15 mm long, and a total of three locations at the center and both ends of the fracture surface were evaluated. Peak fitting is applied to the peak derived from Fe ₃ O _4, the peak derived from Fe ₂ O ₃ , the peak derived from FeO and the peak derived from Fe, and magnetite (Fe ₃ O ₄ ), hematite (Fe ₂ O ₃ ), and wustite ( The volume fraction of FeO) was determined.

  No. satisfying the production conditions of the present invention (heat treatment steps 1 to 3). In 1 to 10, oxidation progressed to the inside of the dust core, and a dust core with high mechanical strength was obtained. In particular, No. 1 to which an oxygen source releasing agent (oxygen source releasing compound) was added. In Nos. 6 to 10, no oxygen source releasing agent was added. Since oxidation was promoted as compared with 1 to 5, a dust core having higher mechanical strength was obtained.

  No. In Nos. 11 to 16, since the heat treatment step 2 was heat-treated at a heating temperature and a heating time suitable for producing hematite, magnetite and hematite were produced, and dust cores having high mechanical strength were obtained. Specifically, no. 11 to 16 are No. respectively. The bending strength was improved as compared with 1, 6-10.

  On the other hand, No. which does not satisfy the production conditions of the present invention. From 17 to 30, the mechanical strength of the dust core could not be increased.

  Specifically, No. in which the heat treatment step 3 was not performed. In 17-20, the wustite could not be reduced, and the mechanical strength of the dust core was low.

  No. in which the heating temperature in the heat treatment step 1 was low. In Nos. 21 and 22, the lubricant was not sufficiently decomposed, and the oxidation did not proceed sufficiently in the subsequent heat treatment step, so that the magnetite was small and the mechanical strength of the dust core was low.

  On the other hand, the heating temperature in heat treatment step 1 was high. In Nos. 23 and 24, since the oxidation of the molded body progressed before the lubricant was sufficiently removed, the oxidation of the molded body was inhibited, and sufficient magnetite could not be secured, resulting in low mechanical strength.

  No. in which the heating temperature in heat treatment step 2 was low. In No. 25, since sufficient oxidation was not performed, sufficient magnetite could not be secured, and the mechanical strength was low.

  On the other hand, the heating temperature in heat treatment step 2 was high. In Nos. 26 and 27, the oxidation proceeded excessively, the magnetite became excessive, and the mechanical strength was low.

  No. 1 in which the atmosphere in the heat treatment step 2 is nitrogen. In 28 and 29, since the oxidation was not sufficiently performed, sufficient magnetite could not be secured, and the mechanical strength was low.

  No. in which the heating time in heat treatment step 3 was short. In 30, wustite could not be reduced and the mechanical strength was low.

Claims (7)

A molding step in which a mixture obtained by mixing an iron-based soft magnetic powder and a lubricant for a green compact having a phosphoric acid-based chemical film on the surface of the iron-based soft magnetic powder is compression-molded to obtain a green compact; and
A heat treatment step 1 in which the green compact is heated at 300 ° C. or higher and 500 ° C. or lower;
Next, a heat treatment step 2 of heating in an oxidizing atmosphere at over 500 ° C. and below 700 ° C .;
Next, heat treatment step 3 of heating at 300 ° C. to 450 ° C. for 30 minutes to 120 minutes,
The manufacturing method of the powder magnetic core characterized by including.   2. The mixture according to claim 1, wherein the mixture further comprises at least one oxygen source releasing compound selected from the group consisting of sugar alcohols, metal hydroxides, metal peroxides, percarbonates, and oxidizing agents. Manufacturing method of a dust core.   The method for producing a dust core according to claim 1 or 2, wherein the iron-based soft magnetic powder for dust compacts has a silicone resin film on the phosphoric acid-based chemical conversion film.   The method for producing a dust core according to any one of claims 1 to 3, wherein the lubricant is polyhydroxycarboxylic amide.   A dust core obtained by the production method according to claim 1.   A powder magnetic core obtained by compression-molding iron-based soft magnetic powder, wherein magnetite is 5.0 to 15% by volume and wustite is 3.0% by volume or less (including 0% by volume). Powder magnetic core.   The dust core according to claim 6, wherein the dust core further includes hematite, and a volume ratio of hematite to magnetite is 0.05 to 0.25.