JP2018117004A - Magnetic core, inductor, and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic core in which peeling, dropping or cracking of a flat power is less likely to occur with a ridge line part of an end face or a corner part to which the ridge line part is matched, defined as an origin, an inductor, and a manufacturing method thereof.SOLUTION: The present invention relates to a magnetic core consisting of a flat soft magnetic metal powder in 60 vol.% or more, a binder containing silicon nitride in 10 vol.% or more and 30 vol.% or less as a main component, and a compound magnetic substance including pores in 10 vol.% or more and 25 vol.% or less, being elastic and having a polygonal shape. A ridge line part and a corner part are rounded and an end of the soft magnetic metal powder in the rounded portion is folded.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a magnetic core, an inductor, and a manufacturing method thereof used in various electronic devices.

  In recent years, electronic devices are required to be smaller and have a lower voltage, and electronic components mounted on circuit boards, especially semiconductor elements and inductors in power supply systems, inevitably respond to higher currents. Improvement of heat resistance is regarded as important.

  As one of electronic parts that meet such a requirement, an inductor having a configuration as disclosed in Patent Document 1 is known. Patent Document 1 discloses an inductor in which a soft magnetic metal powder having a flat shape is bound by a binder component and a magnetic body having elasticity is used as a magnetic core, and a manufacturing method thereof.

  That is, a soft magnetic metal powder having a flat shape is mixed with a binder component to prepare a slurry, and the slurry applied by the die slot method or doctor blade method is dried to volatilize the solvent, and then the desired size is obtained. To obtain a plurality of sheets. After a plurality of sheets are pressed according to the configuration of the magnetic core and pressed to obtain a laminated body, holes and slits are formed in a predetermined position of the laminated body with a milling machine, etc. The magnetic body provided with is obtained.

JP-A-2006-39222

  The magnetic body can form a laminated body for each magnetic core. However, in consideration of application to mass production, a plurality of holes and slits constituting the magnetic core are formed in the laminated body, and then a cutting machine is formed. The method of cutting into pieces is preferable.

  The end face of the magnetic core has a problem in that the flat powder starting from the ridgeline portion or the corner portion where the ridgeline portions are united is likely to be peeled off or dropped off. If the flat powder is peeled or dropped or cracks occur in the magnetic core manufacturing process or inductor product mounting process, the quality of the product may be degraded.

  The end face formed by cutting or milling a composite magnetic body has sharp edges and corners, and when it comes into contact with surrounding objects in the machining and assembly mounting processes, stress is concentrated locally and becomes fine. Destruction may occur. In particular, a composite magnetic body that has been subjected to a heat treatment that decomposes the organic components of the binder is harder and more brittle than the state before the heat treatment. There is a case.

  A molded body obtained by laminating sheets of composite magnetic materials and subjected to heat treatment has a structure in which soft magnetic metal powders are stacked in an orderly manner, and is very firm and elastic. However, when an external force such as cutting or milling is applied, the connective tissue may be broken or peeled off at a portion having a relatively low bonding force, such as between sheets or between particles of soft magnetic metal powder. In addition, the end face formed by cutting or milling has sharp edges and corners, so that when it comes into contact with surrounding objects in the machining process or assembly / mounting process, local fracture of the stress occurs due to local stress concentration There is.

  If a part of the ridge line and corners of the magnetic core formed into a polyhedron shape and heat-treated are rounded, it is possible to alleviate the fine breakdown due to the local stress concentration to some extent. However, processing a solid molded body after heat treatment may cause delamination such as delamination due to the processing stress itself. In addition, since the cross-section of the structure in which soft magnetic metal powder, which is the starting point of peeling, is regularly stacked, is exposed on the end face of the composite magnetic body, simple rounding processing starts from the ridge or corner of the end face. It is difficult to prevent the flat powder from peeling off or falling off.

  Accordingly, an object of the present invention is to provide a magnetic core, an inductor, and a method for manufacturing the same, in which the flat powder starting from the edge portion of the end face or the corner portion where the ridge line portions are united, and the cracks are not easily generated. .

  According to the present invention, the soft magnetic metal powder is composed of a composite magnetic body obtained by binding a flat soft magnetic metal powder with a binder mainly composed of silicon oxide, and the composite magnetic body is 60% by volume or more of the soft magnetic metal powder. And 10% by volume or more and 30% by volume or less of the binder, and 10% by volume or more and 25% by volume or less of pores, having elasticity and a polyhedral shape, and at least a part of the ridgeline of the polyhedron The core is characterized in that the corners and corners are rounded, and at least a part of the rounded portion of the soft magnetic metal powder has a bent end.

  Moreover, according to the present invention, a magnetic core having a radius of curvature of the rounded portion of 0.15 mm or more can be obtained.

  Further, according to the present invention, a magnetic core having a radius of curvature of the rounded portion of 0.3 mm or more and 0.8 mm or less is obtained.

  In addition, according to the present invention, a magnetic core in which the end portion of the soft magnetic metal powder is bent is 10 degrees or more and less than 180 degrees.

  In addition, according to the present invention, a magnetic core having an angle at which the end of the soft magnetic metal powder is bent is 20 degrees or more and less than 180 degrees.

  Moreover, according to this invention, the said magnetic core can obtain a magnetic core provided with one or more penetration parts which penetrate the two surfaces which the said polyhedron opposes.

  Further, according to the present invention, a magnetic core having a rubber hardness of 92 or more and 96 or less according to ISO7619-type D can be obtained.

  Further, according to the present invention, a magnetic core having an electrical resistivity of 10 KΩ · cm or more can be obtained.

  In addition, according to the present invention, a magnetic core in which at least a part of the pores is impregnated with a thermoplastic or thermosetting resin can be obtained.

  In addition, according to the present invention, an inductor using the magnetic core including a coil connected to two opposing faces of the polyhedron via the through portion can be obtained.

  According to the present invention, the step of adjusting the composite magnetic body by mixing a flat soft magnetic metal powder and a binder mainly composed of silicon oxide, and rotating the composite magnetic body with a disk-shaped grindstone A step of cutting into individual pieces, a step of rounding at least a part of ridge lines and corners of the individual pieces by polishing, and an organic component of the binder of the composite magnetic body subjected to the polishing, In this way, a method for manufacturing a magnetic core is provided, which includes a step of disassembling.

  In addition, according to the present invention, the step of adjusting the composite magnetic body by mixing a flat soft magnetic metal powder and a binder mainly composed of silicon oxide, and the penetrating through the two opposing surfaces of the composite magnetic body A step of providing at least one portion, a step of rotating the composite magnetic body into pieces by rotating a disc-shaped grindstone, and rounding at least a part of the ridge line portions and corner portions of the pieces by polishing. A method of manufacturing a magnetic core is provided, including a step and a step of decomposing an organic component of the binder of the composite magnetic body subjected to the polishing process by heat treatment.

  In addition, according to the present invention, there is obtained an inductor manufacturing method, wherein a coil is formed by winding a conductor around the magnetic core.

  In addition, according to the present invention, there is obtained an inductor manufacturing method, wherein a coil is formed by connecting a conductor through two opposing surfaces of the magnetic core and the penetrating portion.

  The present invention is configured to round a ridge line portion and a corner portion of a composite magnetic body formed into a polyhedron shape, and to configure a part of the soft magnetic metal powder of the rounded portion so that the end portion is bent. is there. Due to the bending deformation of the end of the soft magnetic metal powder, it is avoided that the structure is stacked in an orderly manner, and it is difficult for chain peeling and dropping to start from the ridges and corners of the end face of the composite magnetic body. Become.

  The method of manufacturing a magnetic core according to the present invention is to produce a composite magnetic body by mixing a flat soft magnetic metal powder and a binder mainly composed of silicon oxide, and mold the barrel into a desired shape using various processing machines. The edge of the soft magnetic metal powder of the composite magnetic body still having flexibility is bent and curved, and then the composite magnetic body By decomposing the organic component of the binder by heat treatment, a magnetic core having desired characteristics, strength and heat resistance is obtained. An inductor can be formed by applying a coil winding to the magnetic core.

  In addition, as a method of manufacturing a magnetic core according to the present invention, after processing a composite magnetic body into a polyhedron shape, it is possible to provide one or more penetrating portions that penetrate two opposing surfaces of the polyhedron and then round by polishing. good. An inductor can be configured by forming a coil by connecting a conductor through two opposing surfaces of a magnetic core and a penetrating portion.

  In the step of cutting into individual pieces, an outer peripheral cutting machine or an inner peripheral blade that rotates a disk-shaped grindstone using diamond abrasive grains at a high speed to cut or grooving an object. A cutting machine can be used.

  Forming such as cutting before the heat treatment of the composite magnetic body is because the composite magnetic body after the heat treatment is very solid, hard and brittle, so that the processing method applicable to the post-processing is limited and the cost is high It is desirable to avoid damages during processing of the composite magnetic material (such as chipping at corners and generation of cracks between layers).

  According to the present invention, it is possible to provide a magnetic core, an inductor, and a method of manufacturing the same, which are less likely to cause peeling and dropping of flat powder starting from a ridge line part of an end face or a corner part where the ridge line parts are united, and a crack. Become.

It is an image which shows the end surface containing the ridgeline part of a magnetic core by the Example of this invention, and a corner | angular part. It is an image which shows the cross section containing the corner | angular part of the magnetic core by the Example of this invention. It is an image which shows the end surface containing the ridgeline part of a magnetic core by a comparative example, and a corner | angular part. It is an image which shows the cross section containing the corner | angular part of the magnetic core by a comparative example. It is an image which shows delamination of the magnetic core by a comparative example. It is a perspective view which shows an example of the magnetic core cut | disconnected in the piece in embodiment of this invention. It is a front view showing an example of an inductor in an embodiment of the invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment)
The composite magnetic body used in the magnetic core of the present embodiment is obtained by binding flat soft magnetic metal powder with a binder (insulating material) mainly composed of silicon oxide, and has a soft magnetic content of 60% by volume or more. It contains metal powder, 10% by volume or more and 30% by volume or less of binder, and 10% by volume or more and 25% by volume or less of pores, and has elasticity.

  The flat soft magnetic metal powder is produced, for example, by flattening a particulate soft magnetic metal powder using a ball mill or the like. As the soft magnetic metal powder, an Fe-based alloy having good magnetic properties is preferably used. Fe-Si based alloys, Fe-Si-Cr based alloys having good soft magnetic properties, or Fe-- called Sendust (registered trademark). It is preferable to consist of a Si-Al type alloy. When the soft magnetic metal powder contains Si and Al, the magnetocrystalline anisotropy constant and the magnetostriction constant are lowered and the magnetic properties are improved. Therefore, the ratio of Si may be 3 wt% or more and 18 wt% or less. Preferably, the Al ratio is preferably 1% by weight or more and 12% by weight or less.

  As a binder for binding the flat soft magnetic metal powder, for example, a methyl silicone resin or a methylphenyl silicone resin may be used as a main component of silicon oxide.

  To the soft magnetic metal powder, a binder, a solvent, and, if necessary, an additive such as a thickener are added and mixed to prepare a slurry. The slurry is applied to a release substrate, etc., dried by heating and cured to obtain a sheet-like composite magnetic body. The composite magnetic body in this state can be pressure-molded because the binder component retains flexibility, and a plurality of sheets of the composite magnetic body are pressed according to the configuration of the magnetic core, and the pre-molded body is pressed. To obtain a laminate. In order to obtain a magnetic core having a desired size, the laminated body is cut into pieces having a polyhedral shape.

  Further, depending on the configuration of the magnetic core, a penetrating portion penetrating through two opposing surfaces of the laminate, a slit-like long hole, or the like can be provided. If it is in the state of a preformed body, the forming process of the composite magnetic body is relatively easy, and means such as a drill, a milling machine, and a mold press can be used as appropriate for the processing of the penetrating part.

  After performing the necessary processing, the laminate of the composite magnetic material is cut into individual pieces having a desired size. For cutting, it is necessary to use a peripheral blade cutter or an inner blade cutter that rotates a disk-shaped grindstone using diamond abrasive grains at high speed and cuts or grooving the target object against the processing site. desirable.

  FIG. 6 is a perspective view showing an example of a magnetic core cut into individual pieces in the embodiment of the present invention. In the figure, a magnetic core 1 which is a composite magnetic body is provided with four through portions 2 at predetermined positions. The dimensions, number, shape, position, and the like of the penetrating portion 2 can be appropriately set as necessary, and may not be provided when the core 1 is directly wound.

  After the composite magnetic body is cut into individual pieces to form a polyhedron, polishing is performed to round the ridge line portion and the corner portion. Here, the term “polishing” refers to a processing method that physically grinds the surface of an object using abrasive grains. The abrasive grains may be free abrasive grains or fixed abrasive grains, and the processing itself may be dry or wet. . In this embodiment, barrel polishing will be described in detail. However, the processing method is not limited to barrel polishing. Sand blasting (shot blasting) in which abrasive grains are projected by air pressure or centrifugal force of an impeller, or magnetic media is magnetically applied. It is also possible to use magnetic polishing that is moved by.

  In barrel polishing, the processing proceeds to the corners and ridges where stress is concentrated and is in a so-called “rounded” state, which is suitable as a method for processing a magnetic core of the present invention. However, the processing covers the entire object. Therefore, shot blasting may be selected when local processing or high-speed processing is required.

  Barrel polishing is a processing method in which an object and media (abrasive material) are placed in a polishing tank and moved together to perform polishing. A rotary type that rotates the polishing tank and a vibration type that vibrates the polishing tank, each of which rotates automatically. There are an even number of polishing tanks that have an entire number of polishing tanks that revolve in the opposite direction of rotation, and a flow type that creates a fluid state on the inner wall of the fixed tank by rotating the bottom rotating disk (vortex type). It is used according to the characteristics of The rounding process of the composite magnetic body of the present invention can be performed by any method, but there is an advantage that a rotary barrel polishing apparatus is small and inexpensive, and the cost for introduction is low.

  The shape and diameter of the medium are appropriately selected depending on the processing conditions, but if a medium larger than the diameter of the penetrating part is used, the rounding of the penetrating part does not progress much. When the corner portion of the through portion is also rounded, it is necessary to select a medium having a diameter that is at least a fraction of the opening of the through portion. When it is not desired to round the penetrating portion at all, the composite magnetic body may be perforated or slitted after barrel polishing. Further, if a processing means having directionality in the movement of abrasive grains such as shot blasting is used, only a part or one side of the composite magnetic body can be polished.

  Desired magnetic properties and physical properties can be obtained by heat-treating the individual pieces of the composite magnetic material subjected to rounding processing at a high temperature (for example, 600 ° C. or higher). The heat treatment temperature here greatly exceeds the curing temperature (about 100 to 250 ° C.) due to the condensation reaction of the silicone resin, and the organic component of the binder decomposes to become vitreous mainly composed of silicon oxide. The temperature is sufficient to bind the metal powder. At this time, since the binder loses heat, voids are formed inside the structure of the composite magnetic body, thereby providing elasticity. The magnetic core thus manufactured can withstand reflow caused by a high temperature of about 260 ° C., and has excellent frequency characteristics and high electrical resistivity.

  Since the magnetic core of the present invention uses a composite magnetic material in which soft magnetic metal powder is bound by an insulating binder, it has a high electrical resistivity. Specifically, since it has an electric resistivity of 10 KΩ · cm or more, a coil can be formed by directly contacting the surface of a conductor that exhibits good insulation and does not have an insulating coating.

  The ratio of the pores (porosity) included in the composite magnetic body is preferably 10% by volume or more and 25% by volume or less, and the amount of binder in the slurry and the pressing force at the time of preforming the laminate are adjusted. By doing so, a desired porosity can be obtained. When the porosity is 10% by volume or more, the composite magnetic body has elasticity, and when the porosity is 25% by volume or less, it is possible to contain 60% by volume or more of soft magnetic metal powder that provides good magnetic properties. it can. The rubber hardness according to ISO7619-type D of such a magnetic core is 92 or more and 96 or less, and can be elastically deformed.

  The volume ratio of the binder component contained in the composite magnetic body is preferably 10% by volume to 30% by volume. When the volume ratio of the binder component is smaller than 10% by volume, the composite magnetic body does not have sufficient strength. Further, when the volume ratio of the binder component is larger than 30% by volume, the porosity cannot be made 10% by volume or more while the volume ratio of the soft magnetic metal powder is made 60% by volume or more.

  The inductor of the present invention can be manufactured by winding a conductor around the magnetic core obtained by this embodiment, or by connecting a conductor through two opposing surfaces of the magnetic core and a through portion to form a coil.

  FIG. 7 is a front view showing an example of an inductor in the embodiment of the present invention. In the drawing, a conductor is inserted into the penetrating portion 4 of the magnetic core 3 that is a composite magnetic body, and a first connecting portion 5 disposed on one surface of the magnetic core 3 and a second disposed on the other surface of the magnetic core 3. A coil can be formed by electrically connecting the connecting portion 6 and the terminal portion 7 to obtain an inductor.

(Example)
Examples of the present invention will be described below.

  FIG. 1 is an image showing an end surface including a ridge line part and a corner part of a magnetic core according to an embodiment of the present invention, and after cutting the composite magnetic body, the ridge line part and the corner part are rounded by barrel polishing and subjected to heat treatment. It is an image of the end face of the magnetic core. FIG. 2 is an image showing a cross section including the corners of the magnetic core according to the embodiment of the present invention, and shows the laminated state of the soft magnetic metal powder by further cutting the magnetic core of FIG. 1 and polishing the cut surface. It is a cross-sectional enlarged image.

  As a soft magnetic metal powder used for the composite magnetic body of this example, a gas atomized powder of Fe-Si-Al alloy having a median diameter (D50) of 55 μm was prepared, pulverized with a ball mill for 8 hours, and then at 700 ° C. in a nitrogen atmosphere. A heat treatment for 3 hours was performed to obtain a flat soft magnetic metal powder.

  The flat soft magnetic metal powder was observed using a scanning electron microscope, and the aspect ratio (D / t) was calculated from the major axis (D) of the powder and the thickness (t) of the thickest part. The average aspect ratio of the obtained powder was 20. According to the knowledge of the inventors, the average aspect ratio for obtaining magnetic and physical characteristics preferable as a magnetic core may be 10 or more.

  Next, the obtained flat soft magnetic metal powder is mixed with a methyl silicone resin as a binder, a polyacrylic ester as a thickener, and ethanol as a solvent to prepare a slurry, and PET is obtained by a die slot method. After coating on a (polyethylene terephthalate) film and drying at 60 ° C. for 1 hour to remove the solvent, the PET film was peeled off to obtain a composite magnetic sheet.

  A predetermined number of composite magnetic material sheets were laminated and sealed in a mold, and a pressure of 2 MPa was applied at a temperature of 150 ° C. for 1 hour, and pressure molding was performed to obtain a preform. The thickness of the composite magnetic body after preforming was 1.2 mm. Further, in order to provide a through portion that penetrates two opposing surfaces for forming a coil at a predetermined position of the preform, a predetermined number of through holes having a diameter of 1.2 mm are provided by drill cutting, and then the outer peripheral blade is cut. Using a machine, it was cut into 15 mm × 11 mm pieces. Depending on the shape of the coil and the configuration of the inductor, it may not be necessary to form a through hole or a slit-like long hole may be provided.

  The composite magnetic material as individual pieces was polished by a rotary barrel polishing machine. First, a composite magnetic body, media (abrasive material), compound (cleaning agent), and water were put into a polishing tank. The media was a SiC-based spherical carborundum having a diameter of 3 mm. The media material, shape, and size differ depending on the processing conditions. Although the optimum value of the ratio of the processed material and the medium varies depending on the processing conditions, the volume ratio of the medium 4 to the composite magnetic body 1 is used in this embodiment. The compound was added to water at a concentration of 1% and adjusted so as to create an appropriate space that would not interfere with the movement of the composite magnetic material and the media inside the polishing tank.

  Next, after rotating the barrel polishing machine at 45 rpm for 30 minutes, the contents were sieved to recover the composite magnetic material. Further, the rounded composite magnetic material was subjected to heat treatment at 650 ° C. for 1 hour in a nitrogen atmosphere, and the binder component was thermally decomposed to obtain the magnetic core of this example.

(Comparative example)
Hereinafter, a comparative example will be described.

  The process up to the step of cutting the composite magnetic body preform into individual pieces was made under the same conditions as in the example, and the high temperature heat treatment was performed without rounding barrel polishing to obtain a magnetic core of a comparative example.

  FIG. 3 is an image showing an end surface including a ridge line portion and a corner portion of a magnetic core according to a comparative example, and is an image of the end surface of the magnetic core subjected to high-temperature heat treatment after cutting the composite magnetic body. FIG. 4 is an image showing a cross section including the corners of the magnetic core according to the comparative example. The cross section of the magnetic core of FIG. 3 is further cut, and the cut surface is polished to show a laminated state of the soft magnetic metal powder. It is an image.

  100 magnetic cores of the examples and comparative examples were each produced, and the defect rate A (%) selected by visual inspection for damage such as delamination and cracks of the magnetic core generated from the cutting of the individual pieces to after the high-temperature heat treatment was assembled. Table 1 shows the defect rate B (%) of the magnetic core when processed for 10 minutes at 45 rpm with the configuration in which the media is excluded from the barrel polishing of the example in order to estimate the ease of breakage during handling from mounting to mounting. Shown in

  In Table 1, both the defect rate A and the defect rate B are high in the comparative example because of the processing stress at the time of cutting the piece, there are defects that cause delamination and cracks inside the composite magnetic body. Is considered to have been expanded by high-temperature heat treatment. FIG. 5 is an image showing the delamination of the magnetic core according to the comparative example, and the peeling and destruction expand from a portion having a relatively low bonding force starting from the end face, the ridge line portion, and the corner portion.

  In the example, the edge and corner of the magnetic core are rounded by barrel polishing, and the end of the soft magnetic metal powder in the rounded portion is bent as shown in FIG. This is considered to prevent the expansion of minute breakage that becomes the starting point of delamination, soft magnetic metal powder falling off, cracks and the like generated in the part.

  As mentioned above, although the Example of this invention was described, this invention is not limited to the structure of an Example, A change and correction of a structure are possible in the range which does not deviate from the summary of this invention. That is, various changes and modifications that can be made by those skilled in the art are also included in the present invention.

DESCRIPTION OF SYMBOLS 1, 3 Magnetic core 2, 4 Through part 5 1st connection part 6 2nd connection part 7 Terminal part

Claims (14)

  It consists of a composite magnetic body obtained by binding a flat soft magnetic metal powder with a binder containing silicon oxide as a main component, and the composite magnetic body includes 60% by volume or more of the soft magnetic metal powder and 10% by volume or more. 30% by volume or less of the binder and 10% by volume or more and 25% by volume or less of pores, having elasticity and having a polyhedral shape, and at least a part of the ridges and corners of the polyhedron are rounded. At least a part of the soft magnetic metal powder attached to the rounded portion is bent at the end.   The magnetic core according to claim 1, wherein a radius of curvature of the rounded portion is 0.15 mm or more.   The magnetic core according to claim 1, wherein a radius of curvature of the rounded portion is 0.3 mm or more and 0.8 mm or less.   The magnetic core according to any one of claims 1 to 3, wherein an angle at which the end portion of the soft magnetic metal powder is bent is 10 degrees or more and less than 180 degrees.   The magnetic core according to any one of claims 1 to 3, wherein an angle at which the end portion of the soft magnetic metal powder is bent is 20 degrees or more and less than 180 degrees.   The magnetic core according to any one of claims 1 to 5, wherein the magnetic core includes one or more penetrating portions penetrating through two opposing surfaces of the polyhedron.   The magnetic core according to any one of claims 1 to 6, wherein the magnetic core has a rubber hardness of 92 or more and 96 or less according to ISO7619-type D.   The magnetic core according to claim 1, wherein the magnetic core has an electrical resistivity of 10 KΩ · cm or more.   The magnetic core according to any one of claims 1 to 8, wherein at least a part of the pores is impregnated with a thermoplastic or thermosetting resin.   The inductor using the magnetic core according to any one of claims 6 to 9, comprising a coil connected to two opposing faces of the polyhedron through the through portion.   A step of mixing a flat soft magnetic metal powder and a binder mainly composed of silicon oxide to adjust a composite magnetic body; a step of rotating the composite magnetic body into pieces by rotating a disc-shaped grindstone; Rounding at least some ridges and corners of the individual pieces by polishing, and removing the volatile components of the binder of the composite magnetic body subjected to the polishing by heat treatment. A method of manufacturing a magnetic core, which is characterized.   A step of mixing a flat soft magnetic metal powder and a binder containing silicon oxide as a main component to prepare a composite magnetic body; and a step of providing one or more penetrating portions that penetrate two opposing surfaces of the composite magnetic body; A step of rotating the composite magnetic body into pieces by rotating a disk-shaped grindstone, a step of rounding at least a part of the ridge line and corner of the piece by polishing, and performing the polishing. A method of manufacturing a magnetic core, comprising: removing a volatile component of the binder of the composite magnetic body by heat treatment.   A method for manufacturing an inductor, wherein a coil is formed by winding a conductor around a magnetic core manufactured using the method for manufacturing a magnetic core according to claim 11.   A method for manufacturing an inductor, comprising: forming a coil by connecting a conductor through two through surfaces and two opposite surfaces of a magnetic core manufactured by using the magnetic core manufacturing method according to claim 12.