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1. IN6129/DELNP/2014 - POWDER FOR MAGNETIC CORE AND POWDER MAGNETIC CORE

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[Document Name] Specification[Title of the Invention ] Powder for magnetic core and powder magnetic core[Technical Field][0001]The present invention relates to a powder for a magnetic core and a powder magnetic core. [Background Art][0002]As is well known, for example, a power source circuit, which is used by being incorporated into, for example, an electric product and a mechanical product, is mounted with a transformer, a step-up transformer, a rectifier, and the like, which include various coil components each formed of a magnetic core and a winding as main parts, such as a choke coil, a power inductor, and a reactor. In order to respond to a request for low power consumption with respect to the electric product and the mechanical product on the background of increasing consciousness of energy saving in recent years, there is a demand for improvements in magnetic characteristics of the magnetic core to be used frequently in the power source circuit. Further, in recent years, with increasing consciousness of a global warming issue, there has been an increasing demand for a hybrid electric vehicle (HEV), which can suppress consumption of fossil fuel, and an electric vehicle (EV), which does not directly consume fossil fuel. Running performance and the like of the HEV and the EV depend on performance of a motor. Therefore, there is also a demand for improvements in magnetic characteristics of a magnetic core (a stator core or a rotor core) to be incorporated into various motors.[0003]Hitherto, as the magnetic core, a so-called laminated magnetic core in which steel plates (magnetic steel plates) whose surface is covered with an insulating coating film are laminated through intermediation of an adhesive layer has been widely used. However, such laminated magnetic core has a low degree of freedom of a shape and is difficult to respond to a request for miniaturization and a complicated shape. Thus, there has been developed a so-called powder magnetic core obtained by subjecting a soft magnetic metal powder (metal powder having a small coercive force and a large magnetic permeability, which is generally a metal powder containing iron as a main component) whose surface is covered with an insulating coating film to compression molding. The powder magnetic core has been mounted on various products.[0004]Meanwhile, as one of the effective means for improving the magnetic characteristics of the magnetic core, there is given means for decreasing the coercive force of the magnetic core. This is because, as the coercive force is decreased, a magnetic permeability increases whereas a hysteresis loss (iron loss) decreases. The coercive force of the powder magnetic core depends on, for example, a particle diameter, impurity content, and strain amount of the soft magnetic metal powder forming a powder for molding into a powder magnetic core (powder for a magnetic core). As one of the effective means for easily obtaining a powder magnetic core having a small coercive force, there is given means for removing a strain (crystal strain) accumulated in the soft magnetic metal powder during powder production, during compression molding into a compact, and the like. In order to properly remove the strain, it is necessary to heat the compact at a recrystallization temperature or more of the metal powder (metal) for a predetermined period of time. For example, in the case of molding a powder for a magnetic core including a pure iron powder and an insulating coating film covering a surface of the pure iron powder into a compact, it is necessary to heat the compact at 600°C or more, preferably 650°C or more, more preferably 700°C or more. Note that, a heating temperature and a heating time are appropriately adjusted depending on a purity of the soft magnetic metal powder to be used and the like.[0005]Thus, the insulating coating film for covering the surface of the soft magnetic metal powder desirably has high heat resistance. The reason for this is as described below. When the heat resistance of the insulating coating film is insufficient, the insulating coating film is damaged, decomposed, peeled, and the like along with heating treatment, and hence the heating treatment cannot be performed at high temperature at which the strain accumulated in the soft magnetic metal powder can be removed properly. Note that, as specific examples of the insulating coating film having high heat resistance, there are known an insulating coating film having a two-layered structure formed of a high-resistance substance and a phosphate-based chemically treated coating film covering the surface of the high-resistance substance (Patent Literature 1), an insulating coating film formed of an alkoxide coating film made of an Al-Si-O-based composite oxide and a silicone resin coating film formed on the alkoxide coating film (Patent Literature 2), an insulating coating film formed of an insulating layer of at least one kind selected from an oxide, a carbonate, and a sulfate, and a silicone resin layer formed on the insulating layer (Patent Literature 3), and the like. [Prior Art Documents] [Patent Documents][0006][Patent Document 1] Japanese Patent Application Laid-open No. 2001-85211[Patent Document 2] Japanese Patent No. 4589374[Patent Document 3] Japanese Patent Application Laid-open No. 2010-43361 [Summary of the Invention ] [Problem to be solved by the Invention] [0007]However, in the powder for a magnetic core having an insulating coating film disclosed in Patent Literatures 1 to 3, it is difficult to obtain a powder magnetic core having high magnetic characteristics, in particular, a high magnetic flux density for the following reason. The magnetic flux density of a powder magnetic core increases as the density of the powder magnetic core increases. However, in the case where the insulating coating film has a two-layered structure as described above, the thickness of the insulating coating film is likely to be large, and molding into a powder magnetic core (compact) at a high density becomes difficult accordingly. Further, it is not easy to strictly control the thickness of a phosphate-based chemically treated coating film or a silicone coating film, and it is much more difficuh to control the thickness at a nano-order level as requested in recent years. Further, when the insulating coating film has a two-layered structure, it takes much labor for forming a coating film, which causes an increase in cost of a powder for a magnetic core and a powder magnetic core.[0008]In view of the above-mentioned circumstances, a main object of the present invention is to provide a powder for a magnetic core, comprising a soft magnetic metal powder and an insulating coating film covering a surface of the soft magnetic metal powder, which enables the low-cost production of an insulating coating film capable of exhibiting high heat resistance and insulating performance even with a small thickness and enables the low-cost production of a powder magnetic core excellent in magnetic characteristics. [Means for solving the Problem][0009]The inventors of the present invention earnestly conducted studies. As a result, the inventors of the present invention paid attention to various characteristics of silicate layers forming a swellable layered clay mineral and found that an insulating coating film capable of exhibiting high heat resistance and insulating performance even with a small thickness can be formed at low cost through use of the silicate layers forming a swellable layered clay mineral as a material for forming an insulating coating film, thereby achieving the present invention.[0010]That is, a powder for a magnetic core according to the present invention devised for achieving the above-mentioned object comprises: a soft magnetic metal powder; and an insulating coating film covering a surface of the soft magnetic metal powder, in which the insulating coating film comprises an aggregate of silicate layers obtained by cleaving a swellable layered clay mineral. Note that, the phrase "silicate layers obtained by cleaving a swellable layered clay mineral" as used herein is used synonymously with the phrase "silicate layers forming a swellable layered clay mineral and cleaved from the swellable layered clay mineral."[0011]The swellable layered clay mineral is a kind of phyllosilicate in which silicate layers having a negative charge are laminated through intermediation of alkali metal cations or alkaline earth metal cations, and in the atmosphere, the negative charge of the silicate layers is neutralized with the metal cations interposed between the silicate layers, whereby the balance of charges, that is, the laminate structure of the silicate layers and the metal cations is kept in a stable state. On the other hand, when a swellable layered clay mineral of this kind is soaked in an appropriate solvent (strictly speaking, is stirred after the soaking), a solution is easily produced in which silicate layers forming the swellable layered clay mineral are dispersed while being cleaved completely. That is, when a swellable layered clay mineral is soaked in an appropriate solvent, a solution is produced in which silicate layers having a negative charge and metal cations having a positive charge are dispersed while being separated completely. Therefore, when a soft magnetic metal powder is soaked in the above-mentioned solution, the silicate layers dispersed in the solution while being cleaved completely are successively deposited (accumulated) on the surface of the soft magnetic metal powder, while hydroxyl groups on the surface of the soft magnetic metal powder are bonded to metal cations dispersed in the solution through ion exchange. [0012]1 "1Then, a silicate generally has as high a volume resistivity as 10 Qcm or more. Therefore, when the silicate layers (forming a swellable layered clay mineral and cleaved from the swellable layered clay mineral) are deposited on the surface of a soft magnetic metal powder, an aggregate of the deposited silicate layers can form an insulating coating film covering the surface of the soft magnetic metal powder. In addition, the silicate has as high a decomposition temperature as 800°C or more. Further, the silicate layers each have a plate shape in which an aspect ratio (=maximum diameter/thickness) calculated by dividing the maximum diameter by the thickness is at least 100 or more, and the thickness is stably kept at from about 1 to several nm. From the above, when an insulating coating film is formed of an aggregate of silicate layers obtained by cleaving a swellable layered clay mineral, the insulating coating film having high heat resistance and insulating performance even with a small thickness can be formed with good accuracy. Thus, according to the present invention, a powder for a magnetic core can be produced at low cost, in which a soft magnetic metal powder is covered with an insulating coating film capable of exhibiting high heat resistance and insulating performance even with a small thickness.[0013]Note that, silicate layers in an amount greater than necessary are deposited and accumulated on the surface of a metal powder, depending on the soaking time of the soft magnetic metal powder in the above-mentioned solution, the concentration of the solution, and the like. Even in such case, silicate layers ionically bonded to cations of an alkali metal, an alkaline earth metal, or the like are cleaved easily in a state in which a solvent is present, and hence the silicate layers can be easily removed, as compared to silicate layers ionically bonded to a soft magnetic metal powder. Therefore, in the case where silicate layers are deposited in an amount greater than necessary, the laminated silicate layers are subjected to delamination to reduce the thickness of an insulating coating film, for example, merely by exposing the silicate layers to running water. That is, according to the configuration of the present invention, the thickness of an insulating coating film can be controlled easily with good accuracy, and hence there is also an advantage in that an insulating coating film having a small thickness and less variation in thickness (having a substantially uniform thickness) can be obtained easily.[0014]As the swellable layered clay mineral that may be preferably used in the above-mentioned configuration, a swellable smectite-group mineral, which is a cation-exchange type swellable layered clay mineral, or a swellable mica-group mineral may be given. Specific examples of the swellable smectite-group mineral that may be used may include hectorite, montmorillonite, saponite, stevensite, beidellite, nontronite, and bentonite. In addition, specific examples of the swellable mica-group mineral that may be used may include Na-type tetrasilicic fluormica, Li-type tetrasilicic fluormica, Na-type fluortaeniolite, Li-type fluortaeniolite, and vermiculite.[0015]In the above-mentioned configuration, the soft magnetic metal powder can be used without any problems irrespective of a production method by which the soft magnetic metal powder is produced. Specifically, various metal powders produced by known production methods, such as a reduced powder produced by a reduction method, an atomized powder produced by an atomizing method, and an electrolytic powder produced by an electrolytic method, can be used. Note that, of those, an atomized powder, which has a relatively high purity, is excellent in removal property of a strain, and is excellent in moldability, is desirably used for the following reasons. As the purity becomes higher, the recrystallization temperature decreases and the removal property of a strain is enhanced. Therefore, a powder magnetic core having a small coercive force is likely to be obtained. Further, as the moldability becomes more excellent, a compact having a high density, and a powder magnetic core having a high magnetic flux density can be obtained more easily.[0016]In the above-mentioned configuration, when a soft magnetic metal powder having a small particle diameter of less than 30 ^m is used as the soft magnetic metal powder, the flowability of the powder in a molding die (cavity) decreases, which makes it difficult to obtain a powder magnetic core having a high density, and further a hysteresis loss (iron loss) of a powder magnetic core increases. On the other hand, when a soft magnetic metal powder having a large particle diameter of more than 100 ^m is used as the soft magnetic metal powder, an eddy current loss (iron loss) of a powder magnetic core increases. Thus, from the viewpoint of obtaining a powder magnetic core having a high magnetic flux density and a small iron loss, a soft magnetic metal powder having a particle diameter of 30 |j,m or more and 100 ixm or less is preferably used as the soft magnetic metal powder. Note that, the term "particle diameter" as used herein refers to a number average particle diameter (the same applies to the following).[0017]In the above-mentioned configuration, it is desired that the thickness of the insulating coating film be small so as to increase the magnetic flux density of a powder magnetic core obtained by subjecting the powder for a magnetic core to compression molding or the like (so as to enable high-density molding into a compact), as long as an eddy current can be effectively prevented from flowing between adjacent powders for a magnetic core (metal powders). Thus, the thickness of the insulating coating film is desirably 1 nm or more and 500 nm or less, more desirably 1 nm or more and 100 nm or less, still more desirably 1 nm or more and 20 nm or less. Note that, as described above, the thickness of the insulating coating film can be controlled easily according to the configuration of the present invention.[0018]A powder magnetic core to be obtained by heating a compact of a raw material powder containing the powder for a magnetic core described above as a main component is excellent in magnetic characteristics. This is because the insulating coating film forming the powder for a magnetic core is formed of an aggregate of silicate layers excellent in heat resistance having a decomposition temperature of 800°C or more, and hence even when heating treatment is performed at high temperature (equal to or more than the recrystallization temperature of a soft magnetic metal) at which the strain accumulated in the soft magnetic metal powder can be removed properly, the insulating coating film is not damaged, decomposed, or peeled, for example. Further, the silicate layer forming the insulating coating film is bonded to an adjacent silicate layer through a condensation reaction, when heated at a predetermined temperature (temperature substantially equal to the recrystallization temperature of a soft magnetic metal) or more. Thus, when the compact is heated at an appropriate temperature or more, a powder magnetic core excellent in various strengths (mechanical strength, chipping resistance, etc.) as well as magnetic characteristics can be obtained.[0019]In the above-mentioned configuration, when the powder magnetic core is increased in relative density to 93% or more, the versatility of the powder magnetic core is enhanced because the magnetic characteristics and further mechanical strength and chipping resistance of the powder magnetic core are enhanced sufficiently. The relative density as used herein is represented by the following relational expression.Relative density=(Density of entire powder magnetic core/True density) x 100 [%]Note that, the true density refers to a theoretical density in the case where no pores are present inside a raw material.[0020]As the raw material powder for obtaining the powder magnetic core (compact), the powder for a magnetic core mixed with an appropriate amount of a solid lubricant can be used for the following reason. When an appropriate amount of a solid lubricant is mixed with the powder for a magnetic core, the friction between the powders for a magnetic core can be reduced during molding into the compact. Therefore, the damage, peeling, and the like of the insulating coating film caused by the friction between the powders for a magnetic core can also be prevented as much as possible, in addition to the ease of obtaining a compact having a high density. Specifically, it is desired that a raw material powder containing 0.3 to 7 vol% of a solid lubricant, with the balance being a powder for a magnetic core, be used.[0021]The powder magnetic core into which the powder for a magnetic core according to the present invention is molded has a high degree of freedom of a shape and is excellent in magnetic characteristics and various strengths. Therefore, the powder magnetic core can be preferably used as a magnetic core for a motor for vehicles typified by automobiles and railroad vehicles or as a magnetic core for power source circuit components such as a choke coil, a power inductor, and a reactor. [Effects of the Invention ][0022]10 As described above, according to one embodiment of the present invention, the powder for a magnetic core, including a soft magnetic metal powder and an insulating coating film covering the surface of the soft magnetic metal powder, which enables the low-cost formation of an insulating coating film capable of exhibiting high heat resistance and insulating performance even with a small thickness, can be provided. This enables a powder magnetic core excellent in magnetic characteristics and various strengths to be obtained at low cost. [Brief Description of the Drawings][0023][FIGS. 1] FIG. 1(a) is a view schematically illustrating a powder for a magnetic core according to the present invention, FIG. 1(b) is a view schematically illustrating a production step of the powder for a magnetic core illustrated in FIG. 1(a), and FIG. 1(c) is a view schematically illustrating a state in which an insulating coating film is being formed.[FIGS. 2] FIGS. 2(a) and 2(b) are views schematically illustrating major parts of a compression molding step.[FIGS. 3] FIG. 3(a) is a view schematically illustrating a part of a compact to be obtained through the compression molding step and FIG. 3(b) is a view schematically illustrating a part of a powder magnetic core to be obtained through a heating step.[FIG. 4] FIG. 4 is a plan view of a stator core as an example of the powder magnetic core.[FIG. 5] FIG. 5 is a table showing production conditions for each ring-shaped test body used in a confirmation test.[FIG. 6] FIG. 6 is a table showing test results of the confirmation test. [Modes for carrying out the Invention][0024]11 Hereinafter, embodiments of the present invention are described with reference to the drawings.[0025]A powder for a magnetic core 1 according to the present invention includes a soft magnetic metal powder 2 and an insulating coating film 3 covering the surface of the soft magnetic metal powder 2, as illustrated in FIG. 1(a). The insulating coating film 3 is formed of an aggregate of silicate layers 4 forming a swellable layered clay mineral, as illustrated in FIG. 1(c). The powder for a magnetic core 1 is a powder for molding into a powder magnetic core, for example, a stator core 20 (see FIG. 4) to be used, for example, by being incorporated into a stator of a motor. A powder magnetic core is produced mainly through a powder production step of generating the powder for a magnetic core 1, a compression molding step of obtaining a compact of the powder for a magnetic core 1, and a heating step of subjecting the compact to heating treatment successively. Hereinafter, each step is described in detail with reference to the drawings.[0026]FIG. 1(b) schematically illustrates a part of the powder production step of generating the powder for a magnetic core 1 illustrated in FIG. 1(a). The powder production step involves soaking a soft magnetic metal powder 2 in a solution 11 containing a material for forming the insulating coating film 3 filling a container 10, and performing drying treatment for removing a liquid component of the solution 11 adhering to the surface of the soft magnetic metal powder 2, thereby obtaining the powder for a magnetic core 1 including the soft magnetic metal powder 2 and the insulating coating film 3 covering the surface of the soft magnetic metal powder 2. Note that, as the thickness of the insulating coating film 3 increases, it becomes more difficult to obtain the compact 5 (see FIG. 3(a)) having a high density, and in addition, the magnetic permeability of a powder magnetic core 6 decreases.12 On the other hand, as the thickness of the insulating coating film 3 decreases, the magnetic permeability of the powder magnetic core 6 can be enhanced more, but when the thickness of the insulating coating film 3 is too small, the insulating coating film 3 is liable to be damaged and the like when the powder for a magnetic core 1 is compressed during the compression molding step and an eddy current may flow between the powders for a magnetic core 1 (soft magnetic metal powders 2) adjacent to each other. Therefore, the thickness of the insulating coating film 3 is preferably 1 nm or more and 500 nm or less, more preferably 1 nm or more and 100 nm or less, still more preferably 1 nm or more and 20 nm or less.[0027]As the soft magnetic metal powder 2, an iron powder having a purity of 97% or more is preferably used, and a pure iron powder is more preferably used, because, in general, a higher purity is more advantageous for obtaining a powder magnetic core having a small coercive force. Note that, any other known soft magnetic metal powder such as a silicon alloy (Fe-Si) powder, a sendust (Fe-Al-Si) powder, or a permendur (Fe-Co) powder can also be used.[0028]In addition, the soft magnetic metal powder 2 can be used without any problems irrespective of a production method by which the soft magnetic metal powder 2 is produced. Specifically, any of a reduced powder produced by a reduction method, an atomized powder produced by an atomizing method, and an electrolytic powder produced by an electrolytic method may be used. Note that, of those, an atomized powder, which has a relatively high purity, is excellent in removal property of a strain, and fiirther, is excellent in moldability and is easily molded into a compact having a high density, is preferably used. The atomized powder is roughly classified into a water atomized powder produced by a water atomizing method and a gas atomized powder produced by a gas atomizing method. The water13 atomized powder is excellent in moldability as compared to the gas atomized powder, and hence the compact having a high density and the powder magnetic core having a high magnetic flux density are likely to be obtained. Thus, in the case of using the atomized powder as the soft magnetic metal powder 2, it is most preferred to select and use, in particular, the water atomized powder.[0029]Further, as the soft magnetic metal powder 2, a soft magnetic metal powder having a particle diameter of 30 \im or more and 100 nm or less is used for the following reason. When the soft magnetic metal powder 2 to be used has a small particle diameter of less than 30 ^m, the flowability in the molding die (cavity) to be used in the compression molding step described below is degraded, and hence it becomes difficult to obtain the compact 5 having a high density and the powder magnetic core 6 having a high magnetic flux density. In addition, a hysteresis loss (iron loss) of the powder magnetic core 5 increases. On the other hand, when the soft magnetic metal powder 2 to be used has a large particle diameter of more than 100 )xm, an eddy current loss (iron loss) of the powder magnetic core 5 increases.[0030]The solution 11 containing a material for forming the insulating coating film 3 is produced by loading an appropriate amount of the swellable layered clay mineral into an appropriate solvent (for example, water or an organic solvent). Herein, the swellable layered clay mineral is a kind of phyllosilicate in which silicate layers having a negative charge are laminated through intermediation of alkali metal cations or alkaline earth metal cations, and in the atmosphere, the negative charge of the silicate layers is neutralized with metal cations interposed between the silicate layers, whereby the balance of charges, that is, the laminate structure of the silicate layers is kept in a stable state. On the other hand, when the swellable layered clay mineral is soaked in an appropriate solvent, followed by stirring, the solution 1114 in which the silicate layers are dispersed while being cleaved completely is produced easily. That is, when the swellable layered clay mineral is soaked in an appropriate solvent, followed by stirring, the solution 11 in which the silicate layers having a negative charge and the metal cations having a positive charge are dispersed while being completely separated from each other is produced.[0031]As the swellable layered clay mineral, a swellable smectite-group mineral, which is a cation-exchange type swellable layered clay mineral, or a swellable mica-group mineral can be preferably used. Specific examples of the swellable smectite-group mineral that may be used may include hectorite, montmorillonite, saponite, stevensite, beidellite, nontronite, and bentonite. hi addition, specific examples of the swellable mica-group mineral that may be used may include Na-type tetrasilicic fluormica, Li-type tetrasilicic fluormica, Na-type fluortaeniolite, Li-type fluortaeniolite, and vermiculite. Note that, in addition to the swellable smectite-group mineral or the swellable mica-group mineral, a layered silicate mineral having a similar structure to those of these minerals, or a substitution product, derivative, or modified product thereof can be used. One kind of the swellable layered clay minerals may be used alone, or two or more kinds thereof may be used as a mixture.[0032]Note that, the silicate layers forming the smectite-group mineral each have a plate shape in which an aspect ratio (=maximum diameter/thickness) calculated by dividing the maximum diameter by the thickness is at least 200 or more, and the thickness is stable at about 1 nm. Further, the silicate layers forming the mica-group mineral each have a plate shape in which the aspect ratio is at least 100 or more, and the thickness is stably kept at about 2nm.[0033]15 When the soft magnetic metal powder 2 is soaked in the solution 11 produced in the above-mentioned aspect, the silicate layers 4 dispersed in the solution 11 while being cleaved completely are successively deposited and accumulated on the surface of the soft magnetic metal powder 2, as illustrated in FIG. 1(c), while hydroxyl groups on the surface of the soft magnetic metal powder 2 are bonded to metal cations dispersed in the solution 11 through ion exchange.[0034]A silicate has as high a volume resistivity as 10 Qxm or more. Therefore, when the soft magnetic metal powder 2 is taken out from the solution 11 and drying treatment for removing a liquid component of the solution 11 is conducted after the silicate layers 4 are deposited on the surface of the soft magnetic metal powder 2, the insulating coating film 3 for covering the surface of the soft magnetic metal powder 2 is formed of an aggregate of the deposited silicate layers 4. hi addition, the silicate has as high a decomposition temperature as 800°C or more, and further, as described above, the silicate layers each have a thin plate shape in which an aspect ratio (=maximum diameter/thickness) is at least 100 or more, and the thickness thereof is stably kept at about several nm. Therefore, the insulating coating film 3 formed of an aggregate of the silicate layers 4 obtained by cleaving the swellable layered clay mineral has (can exhibit) high heat resistance and insulating performance even with a small thickness. Thus, according to the present invention, the powder for a magnetic core 1, in which the surface of the soft magnetic metal powder 2 is covered with the insulating coating film 3 capable of exhibiting high heat resistance and insulating performance even with a small thickness, can be formed easily at low cost.[0035]Note that, the silicate layers 4 in an amount greater than necessary are deposited and accumulated on the surface of the soft magnetic metal powder 2, depending on the soaking16 time of the soft magnetic metal powder 2 in the solution 11, the concentration of the solution, and the like. However, the silicate layers 4 ionically bonded to cations of an alkali metal, an alkaline earth metal, or the like are easily cleaved in a state in which a solvent is present, and hence the silicate layers 4 can be easily removed, as compared to the silicate layers 4 ionically bonded to the soft magnetic metal powder 2. Therefore, in the case where the silicate layers 4 are deposited in an amount greater than necessary, the laminated silicate layers 4 are subjected to delamination to reduce the thickness of the insulating coating film 3, for example, merely by exposing the silicate layers 4 to running water. That is, according to the configuration of the present invention, the thickness of the insulating coating film 3 can be controlled easily with good accuracy, and hence there is also an advantage in that the insulating coating film 3 having a small thickness and less variation in thickness (having a substantially uniform thickness) can be obtained easily.[0036]Next, in a compression molding step schematically illustrated in FIGS. 2(a) and 2(b), the compact 5 schematically illustrated in FIG. 3(a) is obtained through compression molding by using a molding die having a die 12 and a punch 13 disposed coaxially. A raw material powder 1' to be used in the molding into the compact 5 may be formed only of the powder for a magnetic core 1 obtained in the above-mentioned powder production step. Herein, the compact 5 is obtained through compression molding by using the raw material powder 1' containing an appropriate amount of a solid lubricant such as graphite, molybdenum disulfide, or zinc stearate, with the balance being the powder for a magnetic core 1. Accordingly, when the raw material powder 1' contains a solid lubricant, the fiiction between the powders for a magnetic core 1 can be reduced during compression molding into the compact 5. Therefore, the damage and the like of the insulating coating film 3 caused by the friction between the powders for a magnetic core 1 can also be prevented as much as possible, in17 addition to the ease of obtaining the compact 5 having a high density.[0037]Note that, in the case where the blending amount of the solid lubricant occupying the raw material powder 1' is too small, specifically, in the case where the blending amount of the solid lubricant is less than 0.3 vol% when the total amount of the raw material powder 1' is defined as 100 vol%, the above-mentioned advantages exhibited by mixing the solid lubricant cannot be effectively obtained. Further, in the case where the blending amount of the solid lubricant is too large, specifically, in the case where the blending amount of the solid lubricant is more than 7 vol%, the occupying amount of the solid lubricant in the raw material powder r becomes too large, and consequently it becomes difficult to obtain the compact 5 having a high density and the powder magnetic core 6. Thus, in the case of compression molding into the compact 5 through use of the raw material powder 1' containing a solid lubricant, it is desired that the raw material powder 1' containing 0.3 to 7 vol% of a solid lubricant be used, with the balance being the powder for a magnetic core 1.[0038]In the above-mentioned configuration, as illustrated in FIGS. 2(a) and 2(b), the raw material powder 1' was filled into the cavity of the molding die, and then subjected to compression molding into the compact 5 by relatively moving the punch 13 so as to be close to the die 12. The molding pressure is set to a pressure at which the contact area between the powders for a magnetic core 1 adjacent to each other can be increased, for example, 600 MPa or more, more preferably 800 MPa or more. Thus, as schematically illustrated in FIG. 3(a), the compact 5 having a high density in which the powders for a magnetic core 1 are in strong contact with each other is obtained. Note that, in the case where the molding pressure is too high, the insulating coating film 3 is damaged or the like to have its insulating property decreased, in addition to a decrease in durability life of the molding die. Thus, it is desired that the molding pressure be set to 600 MPa or more and 2,000 MPa or less.[0039]The compact 5 obtained through the compression molding step is transferred to a heating step. In the heating step, the compact 5 in an atmosphere of inert gas (for example, nitrogen gas) or under a vacuum is heated at a temperature equal to or more than the recrystallization temperature and equal to or less than the melting point of the soft magnetic metal powder 2. Thus, the powder magnetic core 6 having a high density (see FIG. 3(b)) from which a strain (crystal strain) accumulated in the soft magnetic metal powder 2 through the compression molding step or the like has been properly removed, specifically, the powder magnetic core 6 having a relative density of 93% or more is obtained. In the case of using a pure iron powder as the soft magnetic metal powder 2, the strain can be removed properly by performing heating treatment at 650°C or more for a predetermined period of time. Herein, the heating treatment with respect to the compact 5 is performed at 700°C for 1 hour. In this case, the insulating coating film 3 is not damaged, decomposed, peeled, and the like even when the compact 5 is subjected to the heating treatment in the above-mentioned aspect, because the decomposition temperature of the silicate layers 4 (silicate) forming the insulating coating film 3 is 800°C or more.[0040]In addition, the strain accumulated in the soft magnetic metal powder 2 is properly removed from the powder magnetic core 6 obtained by performing the above-mentioned heating treatment, and the powder magnetic core 6 becomes excellent in magnetic characteristics. Specifically, the powder magnetic core 6 can be obtained in which the magnetic flux density is 1.65 T or more and the maximum magnetic permeability is 800 or more in an environment of a DC magnetic field of 10,000 A/m, and the iron loss is less than 140 W/kg under the condition of a frequency of 1,000 Hz/a magnetic flux density of IT in an19 AC magnetic field.[0041]Further, when heating treatment is performed at the above-mentioned heating temperature, each silicate layer 4 forming the insulating coating film 3 is bonded to the adjacent silicate layer 4 through a condensation reaction, simultaneously with the removal of the strain accumulated in the soft magnetic metal powder 2. Therefore, the powder magnetic core 6 with mechanical strength and chipping resistance enhanced sufficiently can be obtained. Specifically, the powder magnetic core 6 having a radial crushing strength of 120 MPa or more and a rattler measured value, which is an indicator of chipping resistance, of less than 0.06% can be obtained.[0042]In the foregoing, the powder for a magnetic core 1 according to the embodiment of the present invention and the powder magnetic core 6 produced through use of the powder for a magnetic core 1 have been described. However, the powder for a magnetic core 1 and the powder magnetic core 6 can be appropriately modified within the range not departing from the gist of the present invention.[0043]For example, in the compression molding into the compact 5, die lubrication may be performed. Thus, the friction force between the inner wall surface of the molding die and the raw material powder 1' (powder for a magnetic core 1) is reduced, and hence the compact 5 can be rendered dense further easily. The die lubrication can be performed, for example, by applying a lubricant such as zinc stearate to an inner wall surface of a molding die, or by subjecting an inner wall surface of a molding die to surface treatment and covering the inner wall surface with a lubricant coating film.[0044]20 The powder magnetic core 6 obtained as described above has sufficiently enhanced various strengths such as mechanical strength and chipping resistance in addition to the magnetic characteristics, as described above. Therefore, the powder magnetic core 6 can be preferably used as motors for vehicles having a high rotation speed and a high acceleration and being exposed to vibration constantly, such as automobiles and railroad vehicles, and as magnetic cores of components for power source circuits, such as a choke coil, a power inductor, and a reactor. Specifically, the powder magnetic core 6 according to the present invention can be used as the stator core 20 as illustrated in FIG. 4. The stator core 20 illustrated in FIG. 4 is used by being integrated, for example, with a base member forming a stationary side of various motors, and includes a cylindrical portion 21 having an attachment surface with respect to the base member and a plurality of protrusions 22 extending radially fi-om the cylindrical portion 21 to the outside in a radial direction, a coil (not shown) being wound around the outer circumference of the protrusions 22. The powder magnetic core 6 has a high degree of freedom of a shape, and hence even the stator core 20 having a complicated shape as illustrated in FIG. 4 can be easily mass-produced. [Examples][0045]In order to verify the usefulness of the present invention, ring-shaped test pieces having the configuration of the present invention (Examples 1 to 13) and ring-shaped test pieces not having the configuration of the present invention (Comparative Examples I to 3) were subjected to confirmation tests for measuring and calculating the following evaluation items: (1) density; (2) electric resistivity of an insulating coating film; (3) magnetic flux density; (4) maximum magnetic permeability; (5) iron loss; (6) radial crushing strength; and (7) rattler value. Based on the test resuhs, the evaluation for the respective items (1) to (7) was performed on a three-point scale. Note that, in the measurement of (2) electric21 resistivity of an insulating coating film, an electric resistivity of an insulating coating film formed on an iron plate, not of an insulating coating film formed on a ring-shaped test piece, was measured, with the details described below. The purpose of this is to measure the electric resistivity of an insulating coating film itself correctly. Then, the performance of each ring-shaped test piece was evaluated by a total value (total score) of evaluation points of the items (3) to (5) as indicators of magnetic characteristics and the items (6) and (7) as indicators of strength. Hereinafter, first, a method for measurement and calculation of the evaluation items (1) to (7) and evaluation points thereof are described in detail. [0046](1) DensityThe size and weight of each ring-shaped test piece were measured, and the density thereof was calculated fi"om the measurement results. The following evaluation points were given to the ring-shaped test piece in accordance with the calculated values.3 points: 7.5 g/cm^ or more2 points: 7.4 g/cm or more and less than 7.5 g/cm1 point: less than 7.4 g/cm^[0047](2) Electric resistivity of insulating coating filmAn insulating coating film was formed on the surface of an iron plate having dimensions of 50 mm longx50 mm widex5 mm high by the same procedure as that for forming an insulating coating film on a powder for a magnetic core to be used in production of a ring-shaped test piece, and the electric resistivity of the insulating coating film was measured with a resistivity meter (Hiresta UP/Loresta GP manufactured by Mitsubishi Chemical Analytech Co., Ltd.). The following evaluation points were given to each insulating coating film in accordance with the measured value.22 3 points: 10^ Qcm or more2 points: 10 Qxm or more and less than 10 Qcm1 point: less than 10 ft-cm[0048](3) Magnetic flux densityThe magnetic flux density [T] at a magnetic field of 10,000 A/m was measured with a DC B-H measurement unit (SK-110 type manufactured by Metron Inc.). The following evaluation points were given in accordance with the measured value.3 points: 1.65 T or more2 points: 1.6 T or more and less than 1.65 T1 point: less than 1.6 T[0049](4) Maximum magnetic permeabilityThe maximum magnetic permeability at a magnetic field of 10,000 A/m was measured with a DC B-H measurement unit (SK-110 type manufactured by Metron Inc.). The following evaluation points were given in accordance with the measured value.3 points: 800 or more2 points: 500 or more and less than 8001 point: less than 500[0050](5) Iron lossThe iron loss [W/kg] at a frequency of 1,000 Hz was measured with an AC B-H measurement unit (B-H analyzer SY-8218 manufactured by Iwatsu Test Instruments Corporation). The following evaluation points were given in accordance with the measured value.23 3 points: 100 W/kg or more and less than 140 W/kg2 points: 140 W/kg or more and less than 200 W/kg1 point: 200 W/kg or more[0051](6) Radial crushing strengthA compression force (compression speed: 1.0 mm/min) in a reduced diameter direction was applied to an outer circumferential surface of each ring-shaped test piece through use of a precision universal tester Autograph AG-XPlus manufactured by Shimadzu Co., Ltd., and a compression force at a time when the ring-shaped test piece was broken divided by a broken cross-sectional area was defined as radial crushing strength [MPa]. The following evaluation points were given in accordance with the calculated value.3 points: 140 MPa or more2 points: 120 MPa or more and less than 140 MPa1 point: less than 120 MPa[0052](7) Rattler value (weight reduction ratio)Compliant with "Rattler value measurement method for metal compact" stipulated under the specification JPMA PI 1-1992 of Japan Powder Metallurgy Association. Specifically, a ring-shaped test piece loaded into an activity wheel of a rattler measurement unit was rotated 1,000 times, and thereafter, the weight reduction ratio [%] of the ring-shaped test piece was calculated as a rattler value as an indicator of chipping resistance. The following evaluation points were given in accordance with the calculated value.3 points: less than 0.06%2 points: 0.06% or more and less than 0.10%1 point: 0.10% or more24 [0053]Next, a method of producing a ring-shaped test piece according to Examples 1 to 13 is described.[Example 1]An atomized iron powder manufactured by Wako Pure Chemical Industries, Ltd. as a soft magnetic metal powder was classified to obtain an atomized iron powder having a particle diameter of from 30 to 100 fxm. Then, the iron powder was soaked in an aqueous solution in which 0.3 mass% of hydrophilic hectorite manufactured by Wako Pure Chemical Industries, Ltd. was dispersed while being cleaved completely, and thereafter, the resultant was stirred for about 3 minutes while its foaming was prevented. Then, the procedure of discharge of the hectorite aqueous solution, washing with pure water, and heating (drying) at 80°C for 24 hours in a vacuum thermostat chamber was performed to produce a powder for a magnetic core including the atomized iron powder and an insulating coating film having a thickness of 10 nm covering the surface of the atomized iron powder. Then, a raw material powder containing 2.1 vol% of zinc stearate as a solid lubricant, with the balance being the above-mentioned powder for a magnetic core, was loaded into a molding die, and molded at a molding pressure of 1,200 MPa into a ring-shaped compact having an outer diameter of 17 mm, an inner diameter of 10 mm, and a thickness of 6 mm. Finally, the ring-shaped compact was heated at 700°C for 1 hour in a nitrogen atmosphere to obtain a ring-shaped test piece of Example 1.[Example 2]An iron powder obtained in the same way as in Example 1 was soaked in an aqueous solution in which 0.3 mass% of montmorillonite [trade name: Bengel A ("Bengel" is a trademark)] manufactured by Hojun Co., Ltd. was dispersed while being cleaved completely, and thereafter, the resultant was stirred for about 3 minutes while its foaming was prevented.25 Then, the same procedure as that of Example 1 was performed to produce a powder for a magnetic core including an atomized iron powder and an insulating coating film having a thickness of 10 nm covering the surface of the atomized iron powder. Then, a ring-shaped test piece of Example 2 was obtained in the same way as in Example 1.[Example 3]An iron powder obtained in the same way as in Example 1 was soaked in an aqueous solution in which 0.3 mass% of hydrophilic synthetic mica manufactured by Wako Pure Chemical Industries, Ltd. was dispersed while being cleaved completely, and thereafter, the resultant was stirred for about 3 minutes while its foaming was prevented. Then, the same procedure as that of Example 1 was performed to produce a powder for a magnetic core including an atomized iron powder and an insulating coating film having a thickness of 20 nm covering the surface of the atomized iron powder. Then, a ring-shaped test piece of Example 3 was obtained in the same way as in Example 1.[Example 4]An iron powder obtained in the same way as in Example 1 was soaked in an ethanol solution in which 0.3 mass% of lipophilic smectite [trade name: LUCENTITE SPN ("LUCENTITE" is a trademark)] manufactured by Co-op Chemical Co., Ltd. was dispersed while being cleaved completely, and thereafter, the resultant was stirred for about 3 minutes while its foaming was prevented. Then, the procedure of discharge of the lipophilic smectite ethanol solution, washing with ethanol, and heating at 80°C for 24 hours in a vacuum thermostat chamber was performed to produce a powder for a magnetic core including an atomized iron powder and an insulating coating film having a thickness of 20 nm covering the surface of the atomized iron powder. Then, a ring-shaped test piece of Example 4 was obtained in the same way as in Example 1.[Example 5]26 An iron powder obtained in the same way as in Example 1 was soaked in an aqueous solution in which 0.3 mass% of hydrophilic hectorite manufactured by Wako Pure Chemical Industries, Ltd. was dispersed while being cleaved completely, and thereafter, the resultant was stirred for about 3 minutes while its foaming was prevented. Then, the procedure of discharge of the hectorite aqueous solution and heating at 80°C for 24 hours in a vacuum thermostat chamber was performed (that is, "washing step with pure water" was omitted) to produce a powder for a magnetic core including an atomized iron powder and an insulating coating film having a thickness of 500 nm covering the surface of the atomized iron powder. Then, a ring-shaped test piece of Example 5 was obtained in the same way as in Example 1.[Example 6]A production method (production procedure) for a ring-shaped test piece of Example 6 was performed in substantial conformance with Example I. In Example 6, instead of the atomized iron powder, an electrolytic iron powder was used as a soft magnetic metal powder forming a powder for a magnetic core.[Example 7]An atomized iron powder manufactured by Wako Pure Chemical Industries, Ltd. as a soft magnetic metal powder was classified to provide an atomized iron powder having a particle diameter of 100 [im or more. Then, a ring-shaped test piece of Example 7 was obtained in the same way as in Example 1.[Example 8]A production method for a ring-shaped test piece of Example 8 was performed in substantial conformance with Example 1. In Example 8, a raw material powder for molding into a compact in which the blending ratio of zinc stearate was 0.35 vol% was used.[Example 9]A production method for a ring-shaped test piece of Example 9 was performed in27 substantial conformance with Example 1. In Example 9, a raw material powder for molding into a compact in which the blending ratio of zinc stearate was 7.0 vol% was used.[Example 10]A production method for a ring-shaped test piece of Example 10 was performed in substantial conformance with Example 1. In Example 10, the molding pressure in the molding into a compact was set to 600 MPa.[Example 11]A production method for a ring-shaped test piece of Example 11 was performed in substantial conformance with Example 1. In Example 11, the molding pressure in the molding into a compact was set to 800 MPa.[Example 12]A production method for a ring-shaped test piece of Example 12 was performed in substantial conformance with Example 1. In Example 12, the conditions for heating treatment of a compact were 500°Cxl hour.[Example 13]A production method for a ring-shaped test piece of Example 13 was performed in substantial conformance with Example 1. In Example 13, the conditions for heating treatment of a compact were 550°Cxl hour.[0054]Finally, a method of producing a ring-shaped test piece according to Comparative Examples 1 to 3 is described.[Comparative Example 1]An iron powder obtained in the same way as in Example 1 was soaked in an aqueous solution containing 0.5 mass% of manganese phosphate hydrate, and thereafter, the resultant was stirred for about 10 minutes while its foaming was prevented. After that, the procedure28 of discharge of the manganese phosphate aqueous solution and heating (drying) at 80°C for 24 hours in a vacuum thermostat chamber was performed to produce a powder for a magnetic core including an atomized iron powder and a manganese phosphate coating film (insulating coating film) having a thickness of 2,000 nm covering the surface of the atomized iron powder. Then, a ring-shaped test piece of Comparative Example 1 was obtained in the same way as in Example 1.[Comparative Example 2]An iron powder obtained in the same way as in Example 1 was soaked in an ethanol solution containing 0.5 mass% of titanium methoxide manufactured by Alfa Aesar, and thereafter, the resultant was stirred for about 2 minutes while its foaming was prevented. Then, the procedure of discharge of the titanium methoxide ethanol solution and heating (drying) at 80°C for 24 hours in a vacuum thermostat chamber was performed to produce a powder for a magnetic core in which titanium (thickness: 2,000 nm) as a precursor of an insulating coating film adhered to the surface of the iron powder. Then, a ring-shaped test piece of Comparative Example 2 was obtained in the same way as in Example 1. Note that, titanium adhering to the surface of the iron powder became titanium oxide (insulating coating film) along with the heating treatment performed with respect to a compact.[Comparative Example 3]An iron powder obtained in the same way as in Example 1 was soaked in a solution in which a silicone resin was dissolved in an organic solvent, and thereafter, the resultant was dried to produce a powder for a magnetic core including an iron powder and a silicone coating film having a thickness of 5,000 nm covering the surface of the iron powder. Then, a ring-shaped test piece of Comparative Example 3 was obtained in the same way as in Example 1.[0055]29 FIG. 5 shows a summary of the respective production methods of Examples 1 to 13 and Comparative Examples 1 to 3, and FIG. 6 shows evaluation points of (1) density, (2) electric resistivity of an insulating coating film, (3) magnetic flux density, (4) maximum magnetic permeability, (5) iron loss, (6) radial crushing strength, and (7) rattler value in each of Examples and Comparative Examples, and total values of the evaluation points of the evaluation items (3) to (7). As apparent from FIG. 6, the total score in any of Examples 1 to 13 was higher than that of Comparative Examples 1 to 3. Of those, the total scores of Examples 1 to 4 were particularly high. On the other hand, the total scores of Comparative Examples 1 to 3 did not reach 10 points and Comparative Examples 1 to 3 were all inferior to Examples in strength in particular.[0056]It is considered that the total scores (evaluations) of Examples 1 to 4 were particularly high for the following reasons (a) to (f).(a) An atomized iron powder excellent in moldability is used.(b) A metal powder having a particle diameter of from 30 to 100 |xm is used.(c) A raw material powder with an appropriate amount of a solid lubricant mixed therein is molded into a compact.(d) The molding pressure of a compact is proper.(e) The conditions for heating treatment of a compact are proper.(f) The thickness of an insulating coating film is from 10 to 20 nm.In addition, the density of a compact was increased for the above-mentioned reasons (a), (c), (d), and (f), and as a result, the evaluation points of (3) magnetic flux density, (6) radial crushing strength, and (7) rattler value were enhanced. Further, the above-mentioned reason (b) contributed to the reduction in (5) iron loss. Further, the reduction in coercive force and the enhancement of strength of an insulating coating film (compact) were achieved30 for the above-mentioned reason (e), and as a result, the evaluation points of (4) maximum magnetic permeability, (5) iron loss, (6) radial crushing strength, and (7) rattler value were enhanced.[0057]On the other hand, regarding Comparative Examples 1 to 3, the following is considered. First, in Comparative Examples 1 and 3, the heat resistance of an insulating coating film was low, and hence the insulating coating film was damaged and the like along with the heating treatment performed with respect to a compact, and as a result, an iron loss increased remarkably. Further, it is considered that the thickness of an insulating coating film was as large as 2,000 nm and 5,000 nm in Comparative Examples 1 and 3, respectively, and hence (1) density and (3) magnetic flux density were low, and (6) radial crushing strength and (7) rattler value were unsatisfactory. Next, in Comparative Example 2, it is considered that an insulating coating film was not able to be formed with good accuracy, and a powder for a magnetic core in which part of the surface of a metal powder was exposed to the outside was mixed, with the resuh that an eddy current was generated between powders and (5) iron loss increased. Further, it is considered that the thickness of the insulating coating film of Comparative Example 2 was as large as 2,000 nm in the same way as in Comparative Example 1, and hence (1) density and (3) magnetic flux density were low, and (6) radial crushing strength and (7) rattler value were unsatisfactory.[0058]It is verified fi-om the foregoing confirmation test resuhs that the powder for a magnetic core according to the present invention is very useful for obtaining a powder magnetic core excellent in magnetic characteristics and various strengths. [ Description of Symbols][0059]31 1 powder for a magnetic core r raw material powder2 soft magnetic metal powder3 insulating coating film4 silicate layer5 compact6 powder magnetic core 20 stator core Scope of