JP6384752B2 - Magnetic core and coil component using the same - Google Patents

Description

  The present invention relates to a magnetic core configured using Fe-based soft magnetic alloy powder and a coil component configured by winding a coil around the magnetic core.

  Conventionally, coil parts such as inductors, transformers and chokes have been used in a wide variety of applications such as home appliances, industrial equipment, and vehicles. The coil component includes a magnetic core and a coil wound around the magnetic core. In recent years, as power supply devices such as electronic devices have been downsized, the demand for coil parts that are small and low in profile and can be used for large currents has increased, and metallic magnetic powder with high saturation magnetic flux density has been developed. Adoption of the used powder magnetic core is progressing. As the metal magnetic powder, for example, soft magnetic alloy powder such as Fe-Si is used. In addition to the general structure in which a coil is wound around a powder magnetic core obtained by pressure molding, the coil and soft magnetic alloy powder are integrated into the coil component in order to meet the requirements for small size and low profile. A molded structure (coil enclosing structure) is also employed.

  A powder magnetic core obtained by compacting a soft magnetic alloy powder such as Fe-Si is higher in saturation magnetic flux density than a ferrite magnetic core, but has a low electrical resistivity because it is an alloy powder. Therefore, a method of increasing the insulation between the soft magnetic alloy powders, such as forming after forming an insulating coating on the surface of the soft magnetic alloy powder, is applied. For example, Patent Document 1 discloses an example in which an Fe—Cr—Al based soft magnetic alloy powder is used as a magnetic powder capable of self-generation of a high electrical resistance material serving as an insulating coating. In Patent Document 1, a magnetic powder is oxidized to produce an oxide film with high electrical resistance on the surface of the soft magnetic alloy powder, and the soft magnetic alloy powder is solidified and formed by discharge plasma sintering to form a dust core. Have gained.

  On the other hand, in Patent Document 2, a compact formed of Fe and Si, and a soft magnetic alloy particle group containing Cr or Al, which is a metal element that is easier to oxidize than Fe, is heat-treated at 400 ° C. to 900 ° C. A method and a magnetic core in which particles are bonded through an oxide layer formed by the heat treatment are disclosed. The object is to obtain a magnetic core with high permeability and high saturation magnetic flux density without requiring high pressure during molding.

JP 2005-220438 A JP 2011-249836 A

  The configuration described in Patent Document 1 is a manufacturing method that requires complicated equipment and a lot of time, although high pressure is not required at the time of molding. In addition, since a process for pulverizing the powder aggregated after the oxidation treatment of the soft magnetic alloy powder is required, the process becomes complicated. In addition, although it has been shown that the electrical resistance is improved by about 2.5 times by the oxide film, the resistance value itself is only about several mΩ regardless of the presence or absence of the oxide film, It is not satisfactory when the electrodes are directly formed on the surface of the magnetic core.

In addition, the magnetic core described in Patent Document 2 has a specific resistance exceeding 1 × 10 ³ Ω · m according to the heat treatment conditions described in the examples, but the breaking stress does not reach 100 MPa, and the ferrite core The strength was comparable. By raising the heat treatment temperature to 1000 ° C., the breaking stress is improved to 20 kgf / mm ² (196 MPa), but the specific resistance is significantly reduced to 2 × 10 ² Ω · cm (2 Ω · m). That is, it has not yet achieved both high specific resistance and high strength.

  Further, since the dust cores disclosed in Patent Documents 1 and 2 use metal-based magnetic powder, it is important to ensure not only insulation but also corrosion resistance. However, Patent Documents 1 and 2 do not consider the corrosion resistance at the magnetic core, and a technique for improving the corrosion resistance is required.

  In view of the above problems, an object of the present invention is to provide a magnetic core that has high strength, high specific resistance, and high corrosion resistance and can be manufactured by a simple method, and a coil component using the magnetic core.

The magnetic core of the present invention is a magnetic core having a structure in which Fe-based soft magnetic alloy grains are dispersed, wherein the Fe-based soft magnetic alloy grains include Al and Cr, and the Fe-based soft magnetic alloy grains are formed of the grains. It is bonded via an oxide layer formed on the surface, and a ^* (D65) of the L ^* a ^* b ^* color system is −3.0 to 1.0 in the color difference measurement of the surface. , B ^* (D65) satisfies -7.0 to 1.0. The specific resistance of the magnetic core is preferably 1.0 × 10 ³ Ω · m or more.

Further, the magnetic core has two planes parallel to each other on the surfaces at both ends in one direction,
The absolute value of the difference between the two planes a ^* (D65) is preferably 0.5 or less.

  The coil component according to the present invention includes the magnetic core and a coil wound around the magnetic core.

  According to the present invention, it is possible to provide a magnetic core having high strength, high specific resistance, and high corrosion resistance and a coil component using the same, which can be manufactured by a simple method.

1 is an external perspective view showing an embodiment of a magnetic core according to the present invention. It is a SEM photograph which shows an example of the structure | tissue which the magnetic core which concerns on this invention has. It is an expansion schematic diagram of the structure | tissue of a magnetic core. It is a figure which shows other embodiment of the magnetic core which concerns on this invention. It is a process flow figure for explaining an embodiment of a manufacturing method of a magnetic core concerning the present invention. It is a figure of element distribution of the section of a magnetic core. It is a TEM photograph near the grain boundary of the cross section of a magnetic core. It is a figure which shows the relationship between the salt spray test result of a magnetic core, and a ^* (D65) and b ^* (D65).

Hereinafter, although the embodiment of the magnetic core concerning the present invention is described concretely, the present invention is not limited to this. FIG. 1 is an external view showing an embodiment of a magnetic core according to the present invention. FIG. 2 is an SEM photograph showing the cross-sectional structure of the magnetic core, and FIG. 3 is an enlarged schematic view of the grain boundary portion between Fe-based soft magnetic alloy grains. The magnetic core 1 according to the present invention has a structure in which Fe-based soft magnetic alloy grains 2 are dispersed. The structure in which Fe-based soft magnetic alloy grains are dispersed is a structure formed by an aggregate of Fe-based soft magnetic alloy grains. The Fe-based soft magnetic alloy particles 2 contain Al and Cr, and the Fe-based soft magnetic alloy particles 2 are bonded to each other through an oxide layer 3 formed on the surface of the particles. More specifically, the oxide layer 3 is an oxide layer in which the ratio of Al to the sum of Fe, Al, and Cr is higher than the inner alloy phase in terms of mass ratio. The oxide layer 3 is formed by heat-treating a compact of Fe-based soft magnetic alloy powder and oxidizing the Fe-based soft magnetic alloy powder. Further, the magnetic core according to the present invention has an L ^* a ^* b ^* colorimetric system in which a ^* (D65) is −3.0 to 1.0, and b ^* (D65) is It satisfies at least one of −7.0 to 1.0.
With these configurations, the effects described below can be obtained.

The Fe-based soft magnetic alloy powder (Fe-based soft magnetic alloy particles in the magnetic core) used when producing the magnetic core is an Fe—Al—Cr soft magnetic alloy powder containing Al and Cr. Such Fe—Al—Cr-based soft magnetic alloy powder is superior in corrosion resistance compared to Fe—Si-based alloy powder. Furthermore, Fe—Al—Cr alloy powder is more easily plastically deformed than Fe—Si or Fe—Si—Cr alloy powder. Therefore, according to the Fe—Al—Cr soft magnetic alloy powder, a magnetic core having a high space factor and strength can be obtained even at a low molding pressure. Therefore, the enlargement and complexity of the molding machine can be avoided. In addition, since molding can be performed at a low pressure, damage to the mold is suppressed and productivity is improved.
The shape of the magnetic core is not limited to the toroidal shape shown in FIG. 1, and various shapes such as a U shape, an E shape, and a drum shape can be applied. From the viewpoint of utilizing the characteristics of high strength, the configuration according to the present invention has a columnar part 4 for winding a conducting wire as shown in FIG. 4, and a flange part 5 on one end side or both end sides of the columnar part. It is preferable to apply to a drum type magnetic core. The configuration of the drum type magnetic core is not particularly limited. For example, the shape of the collar portion is not limited to a disc shape, and a square plate shape, a polygonal shape, or an irregular shape can be used.

The oxide layer 3 formed on the surface of the Fe-based soft magnetic alloy grains 2 not only bonds the alloy grains, but also exhibits the function as an insulating layer between the Fe-based soft magnetic alloy grains and the effect of improving the corrosion resistance. sell. Furthermore, in the color difference measurement with the D65 light source on the surface, a ^* (D65) of the L ^* a ^* b ^* color system is −3.0 to 1.0, and b ^* (D65) is −7.0. By providing at least one of ˜1.0, a magnetic core particularly excellent in corrosion resistance is provided. a ^* (D65) and b ^* (D65) reflect the surface state of the magnetic core. One of the important features of the present invention is that a magnetic core having excellent corrosion resistance can be obtained by limiting the a ^* (D65) or b ^* (D65) to the above range.

What is necessary is just to measure a ^* (D65) and b ^* (D65) in the plane with the largest area in which a color difference measurement is possible among several surfaces which comprise the surface of a magnetic core. For example, a plane having the largest area in the case of a rectangular parallelepiped shape, a U-type / E-type plane in the case of U-type / E-type, an annular plane in the case of a toroidal shape, and a plane outside the collar portion in the case of drum-type (5a, 5b in FIG. 4). Except for the shape having a flange on only one end side of the columnar part, such planes are usually paired as two parallel planes on the magnetic core surfaces at both ends in one direction. In addition, even when there is a recess or a step for accommodating the coil terminal on the outer surface of the flange, the entire surface is regarded as a flat surface.

a ^* (D65) and b ^* (D65) depend on the temperature, holding time, atmosphere, arrangement, and the like when the compact of the Fe-based soft magnetic alloy powder is heat-treated. In particular, a ^* (D65) and b ^* (D65) tend to be high when the heat treatment temperature is high and the heat treatment is excessively advanced. When the a ^* (D65) or b ^* (D65) is increased due to excessive progress of the heat treatment, the corrosion resistance decreases and the resistivity of the magnetic core tends to decrease. Therefore, it is important to have at least one of a ^* (D65) being −3.0 to 1.0 and b ^* (D65) being −7.0 to 1.0 as the characteristics of the magnetic core. It is. In addition, since a ^* (D65) and b ^* (D65) far from the inherent properties of the Fe-based soft magnetic alloy powder cannot be taken, the lower limit of a ^* (D65) and b ^* (D65) is −3. 0 and −7.0.

On the other hand, in order to suppress variations in corrosion resistance and specific resistance, it is preferable that the entire core surface is uniformly processed in the heat treatment of the magnetic core. However, the entire core surface is uniform depending on the shape of the magnetic core, the arrangement of the magnetic core during the heat treatment, etc. In some cases, a ^* (D65) is not uniform. This is due to, for example, the difference in the heat treatment state between the mounting surface of the molded body during heat treatment and the free surface facing it. As for a ^* (D65) and b ^* (D65) on the surface of the magnetic core, the free surface tends to be smaller than the mounting surface on the heat treatment jig.
The absolute value of the difference between the two planes a ^* (D65) is preferably 0.5 or less. The two planes are separated from each other, and in the manufacturing process of the magnetic core, one plane is often the placement surface and the other plane is the free surface, and the difference such as a ^* (D65), that is, variation. Is likely to occur. Therefore, by setting the difference of a ^* (D65) between the two planes within the above range, it is ensured that the variation in the corrosion resistance of the magnetic core surface is suppressed. More preferably, the absolute value of the difference between b ^* (D65) of the two planes is also 0.5 or less.

Next, a preferable form is demonstrated below about the structure of magnetic cores other than the structure which concerns on a color difference measurement.
The magnetic core according to the present invention is suitable for achieving both high specific resistance and high strength. Therefore, it is preferable to obtain a specific resistance of 1.0 × 10 ³ Ω · m or more by applying the configuration of the magnetic core. A specific resistance of 1.0 × 10 ⁴ Ω · m or more can also be obtained. The crushing strength is preferably 120 MPa or more, and a crushing strength of 150 MPa or more can also be obtained.
In addition to the strength, the high-frequency characteristics are improved because the alloy grains constituting the magnetic core are fine. From this viewpoint, it is preferable that the number ratio of alloy grains having a maximum diameter exceeding 40 μm in the cross-sectional observation image of the magnetic core is less than 1.0%. The number ratio of alloy grains having a maximum diameter exceeding 40 μm is evaluated in a visual field range of at least 0.04 mm ² or more.

  The average thickness of the oxide layer after the heat treatment is preferably 150 nm or less, and more preferably 100 nm or less. The average thickness of this oxide layer is observed with a transmission electron microscope (TEM), for example, by observing a cross section of the magnetic core at a magnification of 600,000, and a substantially parallel outline of adjacent Fe-based soft magnetic alloy grains in the observation field is confirmed. The thickness of the portion where the Fe-based soft magnetic alloy grains are closest to each other (minimum thickness) and the thickness of the portion which is farthest apart (maximum thickness) are measured, and the thickness is calculated as the arithmetic average. Specifically, it is preferable to perform the measurement in the vicinity of the middle part between the triple points of the grain boundaries. If the thickness of the oxide layer is large, the spacing between Fe-based soft magnetic alloy grains becomes wide, leading to a decrease in permeability and an increase in hysteresis loss, and an increase in the proportion of oxide layers containing nonmagnetic oxides. As a result, the saturation magnetic flux density may decrease. On the other hand, if the thickness of the oxide layer is small, the eddy current loss may increase due to the tunnel current flowing through the oxide layer. Therefore, the average thickness of the oxide layer is preferably 10 nm or more. A more preferable average thickness of the oxide layer is 30 to 80 nm.

  The magnetic permeability required to configure the magnetic core can be determined according to the application. For inductor applications, for example, the initial permeability at 100 kHz is preferably 20 or more. More preferably, it is 30 or more, More preferably, it is 38 or more.

Hereinafter, the manufacturing method of the magnetic core and the characteristics of the magnetic core related thereto will be specifically described. FIG. 5 is a process flow for explaining an embodiment of a method of manufacturing a magnetic core according to the present invention. This manufacturing method is a manufacturing method of a magnetic core having a structure in which Fe-based soft magnetic alloy powder is dispersed, and is obtained through a first step of mixing Fe-based soft magnetic alloy powder and a binder, and the first step. A second step of molding the obtained mixture, and a third step of heat-treating the molded body obtained through the second step.
When using Fe-Al-Cr-based alloy powder as the Fe-based soft magnetic alloy powder, an insulating oxide layer can be formed on the surface of the Fe-based soft magnetic alloy powder by the heat treatment in the third step. Therefore, it is possible to omit the step of forming the insulating oxide before molding, and the method for forming the insulating coating is simplified, so that productivity is improved in this respect. Further, with the formation of the oxide layer, Fe-based soft magnetic alloy powders are bonded together via the oxide layer, and a high-strength magnetic core is obtained.

  Of the embodiments of the method for manufacturing the magnetic core, first, the Fe-based soft magnetic alloy powder used in the first step will be described. Hereinafter, unless otherwise specified, the content and percentage are based on mass ratio. The Fe-based soft magnetic alloy contains Fe as a main component having the highest content among the components constituting the soft magnetic alloy, and contains Al and Cr as subcomponents. That is, Fe, Al, and Cr are three main elements having a high content ratio. As long as the magnetic core can be configured, the contents of Fe, Al, and Cr are not particularly limited, but a preferable configuration will be described below.

Fe is a main magnetic element constituting the Fe-based soft magnetic alloy powder. As long as the magnetic core can be formed, the content is not particularly limited. However, from the viewpoint of securing a high saturation magnetic flux density, the Fe content is preferably 80% by mass or more.
Cr and Al are elements that improve corrosion resistance and the like. The Cr and Al contents are not particularly limited as long as the magnetic core can be formed. From the viewpoint of improving corrosion resistance, the Cr content is preferably 1.0% by mass or more, and more preferably 2.5% by mass or more. On the other hand, since the saturation magnetic flux density tends to decrease as the amount of nonmagnetic Cr increases, the Cr content is preferably 9.0% by mass or less, more preferably 7.0% by mass or less, and further preferably 4.%. 5% by mass or less.
Further, as described above, Al is also an element that improves corrosion resistance, and contributes particularly to the formation of the surface oxide of Fe-based soft magnetic alloy powder. From this viewpoint, the content of Al is preferably 2.0% by mass or more, more preferably 3.0% by mass or more, and further preferably 5.0% by mass or more. On the other hand, since the saturation magnetic flux density tends to decrease as the amount of nonmagnetic Al increases, the Al content is preferably 10.0% by mass or less, more preferably 8.0% by mass or less, and even more preferably 6. 0% by mass or less. Moreover, since Al contributes to the improvement of the space factor, it is more preferable to use Fe-based soft magnetic alloy powder having a higher Al content than Cr.

The Fe-based soft magnetic alloy powder can contain magnetic elements such as Co and Ni, and nonmagnetic elements other than Al and Cr. For example, the Fe-based soft magnetic alloy powder may contain Zr from the viewpoint of improving the specific resistance. If the content of Zr is too small, the effect is not sufficient, and if it is too large, the magnetic permeability and the like may be lowered. Therefore, the content is preferably 0.01 to 1.0% by mass.
The Fe-based soft magnetic alloy powder may contain Si, Mn, C, P, S, O, N, etc. as inevitable impurities. That is, the Fe-based soft magnetic alloy powder may contain Al and Cr, with the balance being Fe and inevitable impurities. The contents of such inevitable impurities are Si ≦ 1.0% by mass, Mn ≦ 1.0% by mass, C ≦ 0.05% by mass, O ≦ 0.3% by mass, N ≦ 0.1% by mass, P ≦ 0.02 mass% and S ≦ 0.02 mass% are preferable. Among these, since Si may affect the specific resistance and the like, it is more preferable to regulate to Si <0.5% by mass. The Si amount is more preferably 0.4% by mass or less.

  The average particle diameter of the Fe-based soft magnetic alloy powder (here, the median diameter d50 in the volume cumulative particle size distribution is used) is not particularly limited. For example, an Fe group having an average particle diameter of 1 μm or more and 100 μm or less. Soft magnetic alloy powder can be used. By reducing the average particle size, the core loss and the high frequency characteristics are improved, so the median diameter d50 is more preferably 30 μm or less, and even more preferably 15 μm or less. On the other hand, when the average particle size is small, the magnetic permeability tends to be low, so the median diameter d50 is more preferably 5 μm or more. It is more preferable to remove coarse particles from the Fe-based soft magnetic alloy powder using a sieve or the like. In this case, it is preferable to use Fe-based soft magnetic alloy powder that is at least under 32 μm (that is, has passed through a sieve having an opening of 32 μm).

  The form of the Fe-based soft magnetic alloy powder is not particularly limited, but it is preferable to use granular powder represented by atomized powder from the viewpoint of fluidity and the like. Atomizing methods such as gas atomization and water atomization are suitable for producing powders of alloys that are highly malleable and ductile and difficult to grind. The atomization method is also suitable for obtaining a substantially spherical Fe-based soft magnetic alloy powder.

  Next, the binder used in the first step will be described. The binder binds the powders during molding and gives the molded body the strength to withstand handling after molding. Although the kind of binder is not specifically limited, For example, various organic binders, such as polyethylene, polyvinyl alcohol, an acrylic resin, can be used. The organic binder is thermally decomposed by heat treatment after molding. Therefore, an inorganic binder such as a silicone resin that solidifies and remains after the heat treatment and binds the powders may be used in combination. However, in the method of manufacturing a magnetic core according to the present invention, the oxide layer formed in the third step functions to bind Fe-based soft magnetic alloy powders, and thus the use of the above inorganic binder is omitted. Thus, it is preferable to simplify the process.

  The amount of the binder added may be an amount that can reach between the Fe-based soft magnetic alloy powders and ensure a sufficient compact strength. On the other hand, if the amount is too large, the density and strength are lowered. From this viewpoint, the amount of the binder added is preferably 0.25 to 3.0 parts by weight with respect to 100 parts by weight of the Fe-based soft magnetic alloy powder, for example.

  The mixing method of the Fe-based soft magnetic alloy powder and the binder in the first step is not particularly limited, and conventionally known mixing methods and mixers can be used. In a state where the binder is mixed, the mixed powder is an agglomerated powder having a wide particle size distribution due to its binding action. By passing the mixed powder through a sieve using, for example, a vibration sieve or the like, a granulated powder having a desired secondary particle size suitable for molding can be obtained. As the granulation method, a wet granulation method such as spray-drying granulation can be employed. Of these, spray-drying granulation using a spray dryer is preferred. According to this, substantially spherical granules can be obtained, and the time of exposure to heated air is short, and a large amount of granules can be obtained. Further, it is preferable to add a lubricant such as stearic acid or stearate in order to reduce friction between the powder and the mold in the case of pressure molding. The addition amount of the lubricant is preferably 0.1 to 2.0 parts by weight with respect to 100 parts by weight of the soft magnetic material powder. The lubricant can be applied to the mold.

  Next, the 2nd process of shape | molding the mixture obtained through the 1st process is demonstrated. The mixture obtained in the first step is preferably granulated as described above and subjected to the second step. The granulated mixture is pressure-molded into a predetermined shape such as a toroidal shape or a rectangular parallelepiped shape using, for example, a molding die. When Fe-Al-Cr soft magnetic alloy powder is used as the Fe-based soft magnetic alloy powder, the space factor (relative density) of the dust core can be increased at a low pressure, and the strength of the dust core is also improved. It is preferable to make the space factor of the soft magnetic material powder in the dust core subjected to the heat treatment within the range of 80 to 90% by utilizing such action. The reason why such a range is preferable is that the magnetic characteristics are improved by increasing the space factor, but if the space factor is excessively increased, the equipment and cost are increased. More preferably, the space factor is 82 to 90%, and more preferably 83 to 88%.

  The molding in the second step may be room temperature molding or warm molding performed by heating to such an extent that the binder does not disappear. Further, the preparation method and the molding method of the mixture are not limited to those described above. For example, instead of pressure molding using a mold, sheet molding can be performed, and the obtained sheet can be laminated and pressure-bonded to obtain a molded body for a laminated magnetic core. In this case, the mixture is adjusted to a slurry state and supplied to a sheet forming machine such as a doctor blade.

  Next, the 3rd process of heat-processing the molded object obtained through the said 2nd process is demonstrated. In order to relieve stress strain introduced by molding or the like and obtain good magnetic properties, the molded body that has undergone the second step is subjected to heat treatment. By this heat treatment, an oxide layer having a higher ratio of Al to the sum of Fe, Al, and Cr than the inner alloy phase is formed on the surface of the Fe-based soft magnetic alloy powder. This oxide layer is grown by reacting Fe-based soft magnetic alloy powder and oxygen by heat treatment, and is formed by an oxidation reaction exceeding the natural oxidation of Fe-based soft magnetic alloy powder. Such heat treatment can be performed in an atmosphere in which oxygen exists, such as in the air or in a mixed gas of oxygen and an inert gas. Further, the heat treatment can be performed in an atmosphere in which water vapor exists, such as in a mixed gas of water vapor and inert gas. Of these, heat treatment in the air is simple and preferable.

  The Fe-based soft magnetic alloy powder is oxidized by the heat treatment, and an oxide layer is formed on the surface. At this time, Al in the Fe-based soft magnetic alloy powder is concentrated in the surface layer, and the oxide layer has a higher ratio of Al to the sum of Fe, Al, and Cr than the internal alloy phase. Typically, compared to the internal alloy phase, the ratio of Al among constituent metal elements is particularly high, and the ratio of Fe is low. Furthermore, microscopically, in the oxide layer at the grain boundary between the Fe-based soft magnetic alloy powders, a region having a higher Fe ratio than the vicinity of the alloy phase is formed on the center side of the layer. By forming such an oxide layer, the insulation and corrosion resistance of the Fe-based soft magnetic alloy powder are improved. Moreover, since this oxide layer is formed after forming a molded object, it contributes also to the coupling | bonding of Fe group soft magnetic alloy powder through this oxide layer. A high-strength magnetic core can be obtained by combining Fe-based soft magnetic alloy powders through the oxide layer.

The heat treatment in the third step may be performed at a temperature at which the oxide layer is formed. A magnetic core having excellent strength can be obtained by such heat treatment. Furthermore, the heat treatment in the third step is preferably performed at a temperature at which the Fe-based soft magnetic alloy powder is not significantly sintered. When the Fe-based soft magnetic alloy powder is significantly sintered, a part of the oxide layer having a high Al ratio is surrounded by the alloy phase and is isolated in an island shape. Therefore, the function as an oxide layer separating the base alloy phases of the Fe-based soft magnetic alloy powder is lowered, and the core loss is also increased. The specific heat treatment temperature is preferably in the range of 600 to 900 ° C. from the viewpoint of forming the oxide layer. However, an L ^* a ^* b ^* color system a ^* (D65) is −3.0 to 1.0, and b ^* (D65) is −7.0 to 1.0 to obtain a magnetic core having excellent corrosion resistance. Therefore, a range of less than 800 ° C. is preferable. More preferably, it is the range of 700-750 degreeC. The holding time in the above temperature range is appropriately set depending on the size of the magnetic core, the processing amount, the allowable range of variation in characteristics, and the like, and is preferably 0.5 to 4 hours, for example.

Depending on the heat treatment conditions in the third step, a ^* (D65) and b ^* (D65) of the L ^* a ^* b ^* color system in the color difference measurement on the surface of the magnetic core change. As described above, a ^* (D65) or the like varies depending on the heat treatment temperature, and the difference between a ^* (D65) and b ^* (D65) may vary depending on the region even in one individual depending on the shape and arrangement of the molded body. In order to suppress variation in a ^* (D65) and b ^* (D65) in the third step, the molded body can be heat-treated by being supported by points or lines instead of surfaces. For example, a method of placing a molded body on a mesh-like jig, a method of placing a drum-type magnetic core having a disk-shaped flange or a toroidal magnetic core on a flat jig with the outer peripheral surface (curved surface) down Can be adopted.
The heat treatment method in the third step adopting this arrangement is not limited to the case of using Fe—Al—Cr soft magnetic alloy powder as the Fe-based soft magnetic alloy powder, but Fe—Cr—Si soft magnetic alloy powder. As in the case of using, etc., Fe-based soft magnetic alloy grains can be widely applied to magnetic cores bonded through an oxide layer formed on the surface of the grains. That is, as a method for producing a magnetic core having a structure in which Fe-based soft magnetic alloy grains are dispersed, a first step of mixing Fe-based soft magnetic alloy powder and a binder, and a mixture obtained through the first step And a third step of supporting the molded body obtained through the second step with a point or a line and heat-treating the Fe-based soft magnetic alloy powder by the heat treatment. It is possible to apply a manufacturing method in which an oxide layer is formed on the surface of the metal and Fe-based soft magnetic alloy powders are bonded to each other through the oxide layer. Also in this case, it is expected that variation in a ^* (D65) and b ^* (D65) on the surface of the magnetic core is reduced.

It is possible to add other steps before and after the first to third steps. For example, a preliminary step of forming an insulating film on the soft magnetic material powder by heat treatment or sol-gel method may be added before the first step. However, in the method for manufacturing a magnetic core according to the present invention, the oxide layer can be formed on the surface of the Fe-based soft magnetic alloy powder by the third step. It is more preferable to simplify. In addition, the oxide layer itself is not easily plastically deformed. Therefore, by adopting the above-described process of forming an oxide layer rich in Al after molding, Fe-based soft magnetic alloy powder (Fe-Al-Cr soft magnetic alloy powder) has in the molding of the second step. High moldability can be used effectively.
Further, after the third step, the color difference measurement of the surface is performed, and a ^* (D65) of the L ^* a ^* b ^* color system is −3.0 to 1.0, b ^* (D65) By providing the step of confirming that at least one of the values of -7.0 to 1.0 is satisfied, a magnetic core having excellent corrosion resistance can be selected.

  A coil component is provided using the magnetic core and a coil wound around the magnetic core. The coil may be configured by winding a conductive wire around a magnetic core, or may be configured by winding it around a bobbin. A coil component having such a magnetic core and a coil is used as, for example, a choke, an inductor, a reactor, or a transformer. The frequency band in which the magnetic core and the coil component are used is not particularly limited. For example, the frequency band is 1 kHz or higher, and use in a frequency band of 100 kHz or higher is also preferable. Further, the magnetic core and the coil component are not limited to the static inductor but can be applied to a rotating machine such as a motor or a generator.

As described above, the magnetic core may be manufactured in the form of a powder magnetic core formed by pressing only the Fe-based soft magnetic alloy powder mixed with a binder or the like as described above, or manufactured in a form in which a coil is arranged inside. Also good. For example, a powder magnetic core having a coil encapsulating structure can be manufactured by integrally pressing an Fe-based soft magnetic alloy powder and a coil. In the case of a laminated magnetic core, the coil is wound inside the magnetic core.
Further, an electrode for connecting the end of the coil is formed on the surface of the magnetic core having a plane for forming the electrode by a technique such as plating or baking, and the coil winding end is connected to the electrode to form a coil. Parts can also be configured.

(Evaluation of material superiority)
First, a study was conducted to confirm the superiority in magnetic properties and strength of the magnetic core using the Fe—Al—Cr soft magnetic alloy powder.
As the Fe-based soft magnetic alloy powder, a Fe-Al-Cr soft magnetic alloy powder was used, and a dust core was produced as follows. Such an alloy powder is a granular atomized powder, the composition of which is Fe-4.9% Al-3.9% Cr in mass percentage, containing 0.2% by mass of Si as a main impurity element, Impurity elements were less than that. The atomized powder was classified with a 440 mesh (aperture 32 μm) sieve, and the Fe-based soft magnetic alloy powder that passed through the sieve was used for mixing. The average particle diameter (median diameter d50) of the Fe-based soft magnetic alloy powder that passed through the sieve was measured with a laser diffraction / scattering particle size distribution analyzer (LA-920, manufactured by Horiba, Ltd.). The average particle size was 9.8 μm.

  A ratio of 2.5 parts by weight of PVA (Poval PVA-205 manufactured by Kuraray Co., Ltd .; solid content: 10%) as a binder (0.25 parts by weight as solid content) with respect to 100 parts by weight of the Fe-based soft magnetic alloy powder. And mixed. This mixed powder was dried at 120 ° C. for 10 hours, and the dried mixed powder was passed through a sieve to obtain granulated powder. To this granulated powder, zinc stearate was added and mixed at a ratio of 0.4 parts by weight with respect to 100 parts by weight of the Fe-based soft magnetic alloy powder to obtain a mixture for molding.

  The obtained mixed powder was pressure-molded at room temperature with a molding pressure of 0.74 GPa using a press. The obtained molded body has a toroidal shape having an inner diameter of 7.8 mm, an outer diameter of 13.5 mm, and a height of 4.3 mm. The obtained molded body was heat-treated in air at a temperature of 750 ° C. and a holding time of 1.0 hour to obtain a dust core (No. 1).

For comparison, Fe-Cr-Si based soft magnetic alloy powder (Fe-4.0Cr-3.5% Si in mass percentage) was mixed and pressure-molded to obtain a toroidal shaped compact. Obtained (No 2).
When producing the No. 2 powder magnetic core, an acrylic resin binder (Polysol AP-604 solid content 40% by Showa Polymer Co., Ltd.) was used as the binder, and 100 parts by weight of the soft magnetic alloy powder. It mixed in the ratio of 2.0 weight part. Molding was performed at a molding pressure of 0.91 GPa, and heat treatment was performed at 800 ° C.

The density of the dust core produced by the above steps was calculated from its dimensions and mass, and the space factor (relative density) was calculated by dividing the density of the dust core by the true density of the Fe-based soft magnetic alloy. As the true density of the alloy, a measured density value of an alloy ingot having the same composition prepared by melting is used. Further, a load was applied in the radial direction of the toroidal powder magnetic core, the maximum load P (N) at the time of fracture was measured, and the crushing strength σr (MPa) was obtained from the following equation.
σr = P (D−d) / (Id ² )
(Where D is the outer diameter (mm) of the magnetic core, d is the radial thickness (mm) of the magnetic core, and I is the height (mm) of the magnetic core.)
Furthermore, 15 turns of the winding were wound on each of the primary side and the secondary side, and the core loss Pcv was measured under the conditions of a maximum magnetic flux density of 30 mT and a frequency of 300 kHz with a BH analyzer SY-8232 manufactured by Iwatatsu Measurement Co., Ltd. The initial permeability μi was measured at a frequency of 100 kHz by winding a conducting wire 30 turns around the toroidal powder magnetic core and using 4284A manufactured by Hewlett-Packard Company.
In addition, a conductive adhesive was applied to two opposing planes of the magnetic core sample, and after drying and solidification, the specific resistance (electric resistivity) was evaluated as follows. A resistance value R (Ω) was measured by applying a DC voltage of 50 V using an electrical resistance measuring device (8340A manufactured by ADC Corporation). The planar area A (m ² ) and thickness t (m) of the magnetic core sample were measured, and the specific resistance ρ (Ω · m) was calculated by the following equation.
Specific resistance ρ (Ω · m) = R × (A / t)

As shown in Table 1, the No. 1 dust core produced using the Fe—Al—Cr soft magnetic alloy powder was compared with the No. 2 dust core using the Fe—Cr—Si soft magnetic alloy powder. The space factor and permeability increased significantly. In addition, the crushing strength of the No. 1 dust core was 200 MPa or more, which was more than twice the value of the No. 2 dust core produced at a high molding pressure. That is, it was found that the configuration using the Fe-Al-Cr Fe-based soft magnetic alloy powder is extremely advantageous in obtaining excellent crushing strength and magnetic properties when applying simple pressure forming. .
Further, the No. 1 powder magnetic core produced using the Fe—Al—Cr soft magnetic alloy powder has a specific resistance compared to the No. 2 powder magnetic core produced using the Fe—Cr—Si soft magnetic alloy powder. It was also found that a dust core using Fe-Al-Cr soft magnetic alloy powder is particularly excellent in achieving both high strength and high specific resistance.

  For the No. 1 powder magnetic core, a cross-sectional observation of the powder magnetic core was performed using a scanning electron microscope (SEM / EDX), and the distribution of each constituent element was simultaneously examined. The results are shown in FIG. FIG. 6A is an SEM image. It can be seen that a phase having a black color tone is formed on the surface of the Fe-based soft magnetic alloy grains 2 having a light gray color tone. The Fe-based soft magnetic alloy particles in the magnetic core correspond to the Fe-based soft magnetic alloy powder subjected to mixing. When the ratio of the number of alloy grains having a maximum diameter of 40 μm or more was determined in a cross-sectional observation image 1000 times larger than the magnetic core by SEM, it was 0%. FIGS. 6B to 6D are mappings showing the distribution of Fe (iron), Al (aluminum), and O (oxygen), respectively. The brighter the color, the greater the number of target elements. Since a clear concentration distribution could not be confirmed for Cr, illustration of Cr was omitted.

  From FIG. 6, the surface (grain boundary) of the Fe-based soft magnetic alloy grains is rich in oxygen, and oxides are formed, and the soft magnetic alloy grains are bonded to each other through the oxides. I understand. Further, the Fe-based soft magnetic alloy grain surface has a lower Fe concentration than the inside, and Cr does not show a clear concentration distribution. On the other hand, the concentration of Al on the surface of Fe-based soft magnetic alloy grains is remarkably high. From these facts, it was confirmed that an oxide layer having a higher ratio of Al to the sum of Fe, Al, and Cr than the inner alloy phase was formed on the surface of the Fe-based soft magnetic alloy grains. Before the heat treatment, the concentration distribution of each constituent element as shown in FIG. 6 was not observed, and it was also found that the oxide layer was formed by the heat treatment. In addition, oxides are not only formed in layers at grain boundaries, but are also formed in the shape of triplets.

  FIG. 7 is a TEM (transmission electron microscope) photograph in which the cross section of the magnetic core of No. 1 was observed at a higher magnification (600,000 times). In the TEM photograph, the belt-like portion that crosses in the vertical direction is the oxide layer 3 at the grain boundary, and is located so as to be adjacent to each other through the grain boundary. 2. It was 60 nm when the average thickness of the grain boundary was evaluated in the observation visual field.

  The part from which a color tone differs by the center part (center part of an oxide layer) of a grain boundary and the vicinity of Fe group soft magnetic alloy grain is confirmed. In the vicinity of the Fe-based soft magnetic alloy grain, the diameter is approximately 5 nm away from the surface of the alloy grain appearing as the contour of the cross section (first point) and the central part (second point) of the grain boundary. Composition analysis was performed in a 1 nm region. Table 2 shows the results of the composition analysis of the oxide layer at the grain boundary by TEM / EDX.

Fe, Al, Cr, Si and O were confirmed at the grain boundaries. At the first point, the ratio of Al to the sum of Fe, Al, and Cr is higher than the ratio of each of Fe and Cr, and at the second point, the ratio of Fe to the sum of Fe, Al, and Cr is the ratio of each of Al and Cr. Higher than. That is, as shown in the schematic diagram of FIG. 3, the oxide layer 3 includes a first region 3a in which Al is concentrated more than Fe and Cr, and a second region 3b in which Fe is concentrated more than the nonferrous metal. And the first region 3a was found to be on the Fe-based soft magnetic alloy grain 2 side.
It is considered that the configuration related to the above-described oxide layer confirmed by SEM observation and TEM observation contributes to high electrical resistivity, low core loss, and high strength. That is, since the Al oxide has high insulation, it is presumed that the Al oxide is formed at the grain boundaries of the soft magnetic alloy grains, thereby contributing to ensuring insulation and reducing core loss. Further, as shown in FIGS. 6 and 7, soft magnetic alloy grains are bonded through an oxide layer at the grain boundary, and it is considered that such a configuration contributes to an improvement in strength.

Example 1
Next, using an atomized powder of Fe—Al—Cr soft magnetic alloy, a heat treatment condition is changed in the temperature range including the heat treatment temperature of 750 ° C., a dust core is produced, and the color difference of the surface is measured. It was. The composition of the Fe-Al-Cr soft magnetic alloy powder used is Fe-5.0% Al-4.0% Cr (composition A) and Fe-5.0% Al-4.0% Cr in mass percentage. -0.4% Zr (composition B), and contained the same inevitable impurities as the Fe-Al-Cr soft magnetic alloy powder used in the No. 1 magnetic core. The average particle diameter (median diameter d50) was 10.6 μm and 9.8 μm, respectively. The granulation was performed by spray drying with a spray dryer (hot air drying granulator) with the addition amount of the PVA set to a ratio of 1.0 part by weight in terms of solid content. The average particle size of the granulated powder was about 65 μm.

Molding was performed at a molding pressure of 0.74 GPa as in No1. For color difference measurement and corrosion resistance evaluation, a drum-type magnetic core having an outer diameter of Φ6 mm, a height of 5 mm, a columnar portion of φ3 mm, and a collar portion of 1 mm is prepared for composition A, and an outer diameter of Φ14 mm is high for composition B. A cylindrical magnetic core having a thickness of 3.5 mm was produced. For both compositions A and B, a magnetic core similar to the evaluation of No. 1 described above was also prepared for evaluation of magnetic properties and the like. The heat treatment was performed at a holding temperature of 700 to 800 ° C. and a holding time of 1 to 4 hours. The other production conditions were the same as in the case of the No. 1 magnetic core.
The color difference of the magnetic core surface is measured using a spectrophotometer CM-3500d manufactured by Konica Minolta Co., Ltd., observation light source: D65 light source, optical system: d / 8, measurement method: reflection measurement (SCE), observation field of view: 2 ° L ^* (D65), a ^* (D65), and b ^* (D65) of the L ^* a ^* b ^* color system were evaluated under the conditions of a visual field and a measurement diameter of Φ8 mm (or Φ3 mm). Further, based on JIS Z2371 (2000), a salt spray test was conducted by using a 5% NaCl aqueous solution and exposing the magnetic core at 35 ° C. for 24 hours. Corrosion resistance was evaluated by the presence or absence of rust generation after the salt spray test. These evaluation results are shown in Table 3 together with the evaluation results of magnetic characteristics and the like performed with the toroidal magnetic core. FIG. 8 shows the relationship between a ^* (D65) and b ^* (D65) and the salt spray test result (whether rust is generated). In addition, about a ^* etc. of Nos. 3-5 which are drum type magnetic cores, a color difference measurement is performed with respect to each pair of buttocks (a total of 10 surfaces) of five magnetic cores that are randomly arranged and heat-treated, and the average is obtained. Calculated.

As shown in Table 3, a ^* (D65) and b ^* (D65) changed when the heat treatment conditions, particularly the heat treatment temperature changed. The dust cores produced by heat treatment under the conditions of a holding temperature of 700 to 800 ° C. and a holding time of 1 to 4 hours all had good crumbling strength, initial permeability, and core loss. On the other hand, the magnetic cores of Nos. ^{3 to} 13 had a specific resistance of 1.0 × 10 ³ Ω · m or more. However, the magnetic cores of No. 14 having a long holding time at 800 ° C. were conducted and 1.0 × A specific resistance of 10 ³ Ω · m or more was not ensured.
Further, when the heat treatment temperature was increased to 800 ° C. or higher, the crushing strength and the like were good, but rust was generated in the salt spray test. The magnetic cores No. 3 to No. 14 exhibit sufficient corrosion resistance under normal use conditions and neglected environments, but a ^* (D65) is −3.0 to 1.0 as in No. 3 to No. 11, b ^* It was found that a magnetic core having particularly excellent corrosion resistance can be obtained by satisfying at least one of (D65) being -7.0 to 1.0. Of these, the cores of Nos. 5 and 8 to 11 in which a ^* (D65) is minus (less than 0.0) and b ^* (D65) is in the range of −7.0 to 6.0 are more than 170 MPa. Good crushing strength was exhibited.
In addition, although a ^* (D65) and b ^* (D65) of the powder magnetic cores of Nos. 3 to 5 shown in Table 3 are average values measured on 10 surfaces as described above, all measurement surfaces of Nos. 3 to 5 are used. And b ^* (D65): − 7.0 to 1.0 were satisfied, and a ^* (D65): − 3.0 to 1.0 was satisfied on all measurement surfaces of Nos. 4 and 5.

(Example 2)
Next, a drum-type magnetic core was produced under the same conditions as the No. 3 magnetic core of Example 1 described above, except for the arrangement method during heat treatment. In the heat treatment, one plane (first plane) is placed as a mounting surface on a flat jig (No15), and the outer circumference (curved surface) of the disc-shaped flange is placed down, that is, the outer circumference. The arrangement was made such that the surface was in line contact with a flat jig (No. 16). The color difference measurement was performed on the first plane and the plane outside the other collar (second plane) in the same manner as in Example 1 described above. The results are shown in Table 4. The magnetic core of No. 16 is arranged so that the drum-shaped axis is parallel to the plane of the plate jig, and the first plane and the second plane are not distinguished in terms of processing conditions. Is.

In the same salt spray test as in Example 1, the occurrence of rust was not confirmed. As shown in Table 4, in the magnetic core of No. 15 heat-treated using the flat surface of the collar as a mounting surface, the mounting surface (first plane) and the free surface (second plane) have a ^* (D65) And b ^* (D65) both increased slightly. On the other hand, in the No. 16 magnetic core, the absolute value of the difference of a ^* (D65) and the absolute value of the difference of b ^* (D65) were both 0.5 or less, and the variation was very small. That is, it has been clarified that supporting the molded body with a wire and heat-treating is particularly effective in reducing variations in the surface state of the magnetic core.

1: Magnetic core 2: Fe-based soft magnetic alloy grains 3: Oxide layer 4: Columnar portion 5: Gutter

Claims (4)

A magnetic core having a structure in which Fe-based soft magnetic alloy grains are dispersed,
The Fe-based soft magnetic alloy grains include Al and Cr;
The Fe-based soft magnetic alloy grains are bonded together through an oxide layer formed on the surface of the grains,
In measuring the color difference of the surface, a ^* (D65) of the L ^* a ^* b ^* color system is −3.0 to 1.0, and b ^* (D65) is −7.0 to 1.0. A magnetic core characterized by satisfying at least one of being. The magnetic core according to claim 1, wherein a specific resistance of the magnetic core is 1.0 × 10 ³ Ω · m or more. The magnetic core has two planes parallel to each other on the surfaces at both ends in one direction,
3. The magnetic core according to claim 1, wherein an absolute value of a difference between the a ^* (D65) of the two planes is 0.5 or less.   A coil component comprising the magnetic core according to claim 1 and a coil wound around the magnetic core.