JP2006108475A - Process for producing soft magnetic material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a sintered body having a fine and rigid high electric resistance layer on the surface of soft magnetic powder through a simple process using inexpensive soft magnetic powder principally comprising iron as a material. <P>SOLUTION: Fe-Si alloy powder is heated in weak oxidizing atmosphere to form an SiO<SB>2</SB>oxide film on the surface and after being pressed, it is calcinated in weak oxidizing atmosphere to produce a sintered body. Surface oxidation process is carried out in weak oxidizing atmosphere of steam, or the like, to oxide Si selectively thus forming a thin oxide film of high electric resistance, which is then further calcinated in weak oxidizing atmosphere. The oxide film cracked at the time of press molding is repaired by repeating oxidation in pressurization atmosphere and degassing in pressure reduction atmosphere thus obtaining a high quality sintered body. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a soft magnetic material applied to a solenoid actuator, a core material of a transformer, and the like. More specifically, the present invention relates to a soft magnetic material obtained by firing iron-based soft magnetic powder whose surface is covered with an oxide film having a high electrical resistance. It is related with the method of manufacturing.

  In order to make a solenoid valve used in a fuel injection system of an internal combustion engine high-speed response, a soft magnetic material that is a core material of an actuator is required to have a high saturation magnetic flux density and a high magnetic permeability. Soft magnetic materials used for such applications are generally manufactured by sintering powder, and iron-based soft magnetic powders that are inexpensive and have a high saturation magnetic flux density are generally used as raw material powders. At this time, in order to reduce the loss (iron loss) due to eddy current of the obtained soft magnetic material, a grain boundary segregation layer with high electrical resistance was formed in the sintered structure, and high permeability and high It is necessary to make it a strong sintered body.

  Therefore, in recent years, soft magnetic powder materials with an insulating film formed on the surface of soft magnetic powder have been used for the purpose of increasing the magnetic permeability and lowering iron loss of the soft magnetic material, and sintering the press-molded product. Technology for producing soft magnetic materials has been studied. As a prior art, for example, there is Patent Document 1, and the surface of the mother phase particles made of Fe-based magnetic metal is covered with a second substance having a high electrical resistance and a high permeability such as ferrite, and further a third having a high electrical resistance. There has been proposed a composite soft magnetic powder material covered with an insulating film made of any of the above materials.

In the production method shown in Patent Document 1, first, an Fe-based atomized alloy powder is immersed in an aqueous solution of NiCl ₂ and ZnCl ₂ to adsorb metal ions, and then oxidized in air to cause a ferritization reaction. A soft magnetic Ni—Zn ferrite thin film (second material) is formed on the powder surface. Further, Al is sputtered in a nitrogen atmosphere to form an insulating film (third material) containing AlN as a main component on the Ni—Zn ferrite thin film. In this way, a composite magnetic powder having a three-layer structure is prepared, and then a B ₂ O ₃ powder is added to the composite magnetic powder to obtain a molding material. After this is pressure-molded into a predetermined shape, it is sintered at 1000 ° C. while being pressed by a hot press method to produce a soft magnetic material sintered body.
JP-A-5-36514 (Pages 2 to 3 etc.)

  However, in the above conventional manufacturing method, it is necessary to cover the surface of the atomized alloy powder with a plurality of different substances, and the process of impregnating and oxidizing the solution to form the Ni-Zn ferrite thin film is repeated, or the insulating film is formed. In order to form it, the process of performing sputtering of Al in a nitrogen atmosphere takes time, and the manufacturing cost increases. Moreover, in the method of forming an insulating film by covering the surface of the raw material powder with another material, such as sputtering of Al, the insulating film tends to be thick, and a nano-level thin film is uniformly formed. Is difficult. For this reason, the magnetic material density in the soft magnetic member is lowered, the saturation magnetic flux density is lowered, and the magnetic characteristics are deteriorated.

  On the other hand, when the insulating film is thinned in order to improve the magnetic characteristics, there is a possibility that cracking may occur in the insulating film on the surface of the soft magnetic powder due to the pressing pressure when the soft magnetic powder is pressed. If the insulating film is damaged, there is a problem that the insulation between the soft magnetic powders is lowered and the iron loss (eddy current loss) of the sintered soft magnetic material is increased.

  The present invention has been made in view of such circumstances, and a thin high-resistance insulating film is formed on the surface of an inexpensive iron-based powder, and damage such as cracks occurs in the insulating film. It is an object of the present invention to obtain a sintered soft magnetic material that satisfies the requirements of high saturation magnetic flux density, high magnetic permeability, low iron loss, high strength, and productivity at the same time.

  In order to achieve the above object, in the method for producing a soft magnetic powder material according to claim 1, an oxide film is formed on the surface of the soft magnetic powder mainly composed of iron (surface oxidation step), and then press-molded. A molded body having a predetermined shape (press molding process) is fired to form a sintered body of a soft magnetic material (sintering process). In this sintering step, an oxidation treatment in which the compact is brought into contact with a weak oxidizing atmosphere gas in which a weak oxidizing gas is mixed with an inert gas and a degassing treatment in which the atmospheric gas is discharged out of the system are alternately repeated.

  When a soft magnetic material is produced by sintering a soft magnetic powder having an oxide film formed on the surface, if the oxide film is thin, there is a risk of breakage during press molding. Thereafter, by supplying a weak oxidizing gas in the firing process, the powder surface can be oxidized again to fill in cracks and repair the oxide film. At this time, an element having high oxidation reactivity is selectively oxidized by using a weak oxidizing atmosphere.

  And it can discharge | emit by performing supply of a weak oxidizing gas, and a deaeration process alternately. Thereby, the weak oxidizing gas is supplied to the inside of the compact during the next oxidation treatment, and the oxide film on the surface of the soft magnetic powder can be re-formed with the entire compact. In addition, since the oxidation rate is moderately suppressed, a dense oxide film with a high electrical resistance can be formed on the powder surface layer.

  This makes it easier than ever to ensure insulation between soft magnetic powders, reduce losses (iron loss) due to eddy currents, and increase the magnetic material density by reducing the thickness of the oxide film. The characteristics can be improved. Therefore, the requirements for high saturation magnetic flux density, high magnetic permeability, low iron loss, high strength, and productivity can be satisfied at a high level at the same time.

  In the method according to claim 2, in the surface oxidation step, soft magnetic alloy powder containing iron as a main component and containing a second element having higher oxidation reactivity than iron is used. The soft magnetic alloy powder is heated in a weak oxidizing atmosphere in which a weak oxidizing gas is mixed with an inert gas, and the second element on the powder surface layer portion is mainly subjected to an oxidation reaction, so that the second element is formed on the surface. The oxide film is formed.

  Preferably, when a soft magnetic alloy powder containing a second element having a high oxidation reactivity is used as a raw material powder and an oxidation reaction is performed in a weakly oxidizing atmosphere, iron is oxidized in the surface layer portion of the soft magnetic alloy powder. Only the second element that is suppressed and more easily oxidizes is selectively oxidized. Moreover, since the oxidation rate is moderately suppressed, a dense and strong nano-level insulating thin film is formed on the surface of the high-purity iron-based soft magnetic alloy powder. Furthermore, even if a crack or the like occurs in the oxide film of the second element in the press molding process, it is repaired in the sintering process as described above, so that the surface layer has a dense and thin high resistance layer with a small particle size. A powder sintered body can be manufactured by a simple process.

  In the method according to claim 3, in the surface oxidation step, soft magnetic alloy powder containing a second element having iron as a main component and higher oxidation reactivity than iron is mixed with a weak oxidizing gas in an inert gas. Oxidation treatment for heating in a weakly oxidizing atmosphere and reduction treatment for heating in a reducing atmosphere are alternately performed to mainly cause the second element in the powder surface layer portion to undergo an oxidation reaction, thereby forming an oxide film of the second element Form.

  A soft magnetic alloy powder containing a second element having high oxidation reactivity is used as a raw material powder, and after performing an oxidation reaction in a weakly oxidizing atmosphere, a reduction reaction in a reducing atmosphere may be repeated. it can. By doing so, it is possible to promote the oxidation of the second element in the surface layer while suppressing the progress of oxidation to the inside, and to form a surface oxide film with higher purity and higher electrical resistance. As a result, it is possible to more effectively reduce the iron loss and improve the magnetic properties of the magnetic material.

  The second element is at least one selected from substances having higher oxidizability than iron typified by Si, Ti, Al, and Cr.

  These elements are suitable as raw materials for oxide films because the Gibbs free energy ΔG during the oxidation reaction is smaller than that of iron and the oxidation reaction easily proceeds.

  In the method according to claim 5, the oxidation treatment in the sintering step is performed in a pressurized atmosphere of atmospheric pressure or higher, and during the degassing treatment, the atmospheric gas is exhausted out of the system to reduce the pressure to atmospheric pressure or lower. Good.

  By performing the oxidation treatment in a pressurized atmosphere, a weak oxidizing gas can be evenly supplied to the surface of the soft magnetic powder inside the compact, and the effect of repairing the oxide film can be improved. Further, by reducing the atmospheric pressure to the atmospheric pressure or lower by deaeration treatment, the product gas of the reduction reaction that proceeds simultaneously with the oxidation reaction can be drawn from the inside of the molded body and discharged together with the atmospheric gas to the outside of the system. Therefore, the effect of promoting the oxidation of the surface of the soft magnetic powder can be obtained without hindering the supply of the weak oxidizing gas due to these gases remaining in the molded body.

  In the method of Claim 6, a sintering process shall be performed on 400-1100 degreeC temperature conditions.

  The sintering process is performed by raising the temperature to a temperature at which the press-molded body of the soft magnetic powder can be sintered at a temperature higher than the temperature at which the effect of re-forming the oxide film can be obtained while suppressing the reaction of iron by the weak oxidizing gas. . The sintering temperature differs depending on the raw material powder, and if it is an iron-based soft magnetic powder, it is usually preferable to set the temperature range to about 1100 ° C. or less.

  In the method according to claim 7, in the sintering step, first, an oxide film is re-formed by alternately repeating an oxidation treatment and a deaeration treatment under a temperature condition of 400 to 600 ° C. (first step). Next, the soft magnetic powder is sintered under a temperature condition of 600 to 1100 ° C. (second step).

  Preferably, in the first step, the soft magnetic powder is brought into contact with the weak oxidizing gas at a relatively low temperature to repair the surface oxide film, so that the surface of the iron-based soft magnetic alloy powder has a dense and strong nanostructure. A level insulating thin film is formed again. Thereafter, by raising the temperature to the sintering temperature in the second step, a high permeability and high strength sintered body having a high electric resistance grain boundary segregation layer is obtained.

  In the method according to claim 8, the weak oxidizing gas is water vapor or dinitrogen monoxide gas.

In the oxidation with water vapor, the oxidation reaction proceeds together with the reduction reaction of H ₂ O, so the reaction rate is slower than in the atmosphere. In particular, since the oxidation reaction of iron is almost in an equilibrium state and hardly proceeds, only the second element that is more easily oxidized can be selectively oxidized. In the case of dinitrogen monoxide gas, the same reaction form is taken.

  In the method according to claim 9, the weakly oxidizing gas is water vapor and is mixed in the inert gas so that the relative humidity at room temperature is higher than 50%.

  Specifically, when steam is used, a weakly oxidizing atmosphere can be easily formed. In particular, when the oxidation is performed in a high-humidity atmosphere exceeding 50%, the above-described effect can be easily obtained.

  In the method according to claim 10, the weakly oxidizing gas is water vapor and mixed in the inert gas so that the relative humidity at room temperature is 70% to 100%.

  Preferably, when oxidation is performed in a steam atmosphere with higher humidity, the oxide number density of the generated oxide film can be increased, and a dense and high electric resistance thin film can be formed.

  In the method according to claim 11, the surface oxidation step is performed under a temperature condition of 400 to 600 ° C.

  When the atmospheric temperature is lower than the above range, the free energy change ΔG <0 in the iron oxidation reaction system by the weak oxidizing gas, and the reaction suppressing effect is lowered. Moreover, when higher than the said range, although the oxidation of a 2nd element will advance easily, there exists a possibility that the characteristic of the magnetic material obtained may fall. By setting the content in the above range, an oxide film having a high oxide number density, a dense and high electric resistance can be formed.

  In the method of claim 12, the soft magnetic powder is an atomized alloy powder having an average particle size of 0.1 to 500 μm.

  By reducing the surface oxide film thickness described above, it is possible to reduce the particle size of the soft magnetic powder. Therefore, by using atomized particles with good compressibility and a fine particle size of 0.1 to 500 μm, the soft magnetic member Can be strengthened, and the degree of freedom in forming during molding increases.

Hereinafter, the best mode for carrying out the present invention will be described based on specific examples.
FIG. 1 shows a process for producing a soft magnetic material according to the present invention, a process (1) for preparing a soft magnetic alloy powder as a raw material, and a surface oxidation for forming an oxide film by surface oxidizing the soft magnetic alloy powder. A step (2), a press molding step (3) in which a soft magnetic alloy powder having an oxide film formed on the surface thereof is press-molded to form a molded body having a desired shape, and a binder removal step (4) for removing the binder of the press-molded body ), And sintering steps (5) and (6) to sinter the binder-free molded body to obtain a sintered body of a soft magnetic material.

(1): Raw material powder preparation step In the present invention, the soft magnetic alloy powder as the raw material is a powder containing iron (Fe) as a main component and a second element having a higher oxidation reactivity than iron. Examples of the second element include Si, Ti, Al, Cr, and the like. An alloy containing at least one or more selected from these elements, specifically, an Fe-Si alloy, Fe-Ti, and the like. Powders of alloys, Fe—Al alloys, Fe—Cr alloys, Fe—Al—Si alloys and the like are used. Among these, the Fe—Si alloy has a composition ratio of, for example, Fe: 95 to 99.9% and Si: 0.1 to 5%, and the Fe—Al alloy has, for example, Fe: 92.5 to 97. Fe: Al-Si alloy having a composition ratio of 0.5%, Al: 2.5-7.5%, for example, Fe: 90-97%, Al: 3.5-6.5%, Si: Those having a composition ratio of 0.1 to 5% can be used.

Here, in general, the composition ratio of Si, Al, and the like is determined by the following three factors (i) to (iii),
(I) In order to improve the magnetic characteristics, it is better that there is less Si, Al or the like.
(Ii) It is within the solid solution limit where no intermetallic compound is formed.
(Iii) The film thickness of the oxide film is not less than the film thickness that can ensure the target electric resistance value.
Is determined in consideration of For example, in order to improve the magnetic characteristics of (i), the composition ratio of these elements should be 2% or less, preferably 1% or less, and the minimum composition capable of forming a sufficient oxide film within this range. Select a ratio. In FIG. 1, an alloy powder (Fe-1% Si) containing only Si in Fe is shown as an example. Two or more of the above soft magnetic alloy powders may be mixed and used.

  As the soft magnetic alloy powder as a raw material, atomized particles prepared by an atomization method in which a molten alloy is pulverized using a spray medium such as water or an inert gas are preferably used. Since the atomized alloy powder has high purity and good compressibility, a soft magnetic material having high density and good magnetic properties can be realized. The average particle size of the soft magnetic alloy powder is usually 0.1 to 500 μm, preferably 100 to 200 μm, and is pulverized using a pulverizer (attritor) so as to have a desired average particle size. In this pulverization step, a highly active fracture surface is formed on the surface of the soft magnetic alloy powder. The raw material for the production of soft magnetic alloy powder is the one before annealing (annealing) so that it can be easily pulverized. The water should be cooled.

  In addition, when using the atomized particles prepared by the atomization method described above, or using the powder particles pulverized by using the above-described pulverizer (attritor) alone, the softening material as the raw material is used. Magnetic alloy powder may be obtained.

(2): Surface oxidation step Next, an oxide film is formed on the surface of the soft magnetic alloy powder as a raw material. This surface oxidation process is performed in a weakly oxidizing atmosphere in which a weakly oxidizing gas is mixed with an inert gas, and the soft magnetic alloy powder is heated to a high temperature to mainly oxidize the second element in the surface layer portion. Let Nitrogen (N ₂ ) gas or the like is preferably used as the inert gas, and water vapor (H ₂ O) is preferably used as the weak oxidizing gas, for example. By setting the atmosphere to be weakly oxidizing, the oxidation of Fe is suppressed, and the second element that is more easily oxidized can be selectively oxidized to form an oxide film of the second element. When the Fe—Si alloy powder illustrated in FIG. 1 is oxidized with water vapor (H ₂ O), Si, which is the second element, is selectively oxidized on the powder surface, and the high electricity covering the powder surface is obtained. A resistive SiO ₂ oxide film is formed with a thin film thickness of about several nm, for example.

Here, a mechanism of surface oxidation of the Fe—Si alloy powder in a weakly oxidizing atmosphere will be described. FIG. 2 shows a comparison of oxidation reactivity of Fe and Si in an oxygen (O ₂ ) atmosphere and a water vapor (H ₂ O) atmosphere. The oxidation reaction formula in each atmosphere of Fe and Si is as follows.
In the case of oxidation with oxygen (O ₂ ) 2Fe + O ₂ → 2FeO (Formula 1)
Si + O ₂ → SiO ₂ (Formula 2)
In the case of oxidation with water vapor (H ₂ O) Fe + H ₂ O → FeO + H ₂ (Formula 3)
Si + 2H ₂ O → SiO ₂ + H ₂ (Formula 4)

The vertical axis in FIG. 2 represents the Gibbs free energy change ΔG in each reaction system. As ΔG increases, oxidation becomes difficult. In FIG. 2, the oxidation of Fe is less likely to occur than Si, and the oxidation reaction with water vapor (H ₂ O) (formulas 3 and 4) rather than the oxidation reaction with oxygen (O ₂ ) (formulas 1 and 2). Is unlikely to occur. Further, in the oxidation with oxygen (O ₂ ), in both cases of Fe and Si, the free energy after the reaction is lower than that before the reaction, and the state is more stable. That is, the Gibbs free energy ΔG is both negative, and Si having a larger absolute value of ΔG is more easily oxidized, but the reactions of Formulas 1 and 2 proceed.

On the other hand, in the oxidation with water vapor (H ₂ O), the absolute value of Gibbs free energy ΔG is smaller than the oxidation with oxygen (O ₂ ) in both cases of Fe and Si. In particular, since the Gibbs free energy ΔG before and after the reaction of Fe is almost 0, the reaction of Formula 3 hardly proceeds and only the reaction of Formula 4 proceeds.

Therefore, when oxidizing with water vapor (H ₂ O), a SiO ₂ oxide film can be selectively formed while suppressing oxidation of Fe. As shown in FIG. 2, in the oxidation of Fe by water vapor (H ₂ O), the Gibbs free energy −ΔG is close to 0 in the entire temperature range, and particularly in the temperature range of about 400 ° C. or higher, The free energy −ΔG becomes substantially 0, and the effect of suppressing the oxidation of Fe is enhanced. In addition, in the oxidation of Si with water vapor (H ₂ O), the reduction reaction of H ₂ O proceeds simultaneously, so that the reaction is less likely to proceed than in an oxygen (O ₂ ) atmosphere, and the oxidation proceeds at an appropriate rate. For this reason, the oxidation does not proceed to the inside, the magnetic material density is kept high, and a uniform SiO ₂ oxide film is formed at a high density on the powder surface layer portion to form a dense thin film having a high electrical resistance of about several nanometers. it can.

Thus, as the weak oxidizing gas, a gas that is an oxygen compound gas and that undergoes a reduction reaction simultaneously with the oxidation reaction is suitable. The same effect can be obtained by using, for example, dinitrogen monoxide (N ₂ O) gas as a gas having the same reaction form.

When the weak oxidizing gas is water vapor (H ₂ O), the relative humidity at room temperature is preferably higher than 50% when water vapor is mixed into the atmosphere. In general, the higher the atmospheric humidity, the thicker the oxide film formed, and the oxide film does not grow sufficiently under low humidity conditions. The higher the humidity, the more the oxidation reaction of the second element such as Si and Al in the powder surface layer portion is promoted, and the oxide number density in the oxide film is increased, thereby obtaining a dense and high electric resistance insulating oxide film. . Preferably, it is good to mix so that it may become 70-100% (relative humidity) high humidity at normal temperature. If the atmospheric humidity is near 100%, an oxide film having a high oxide number density and a sufficient thickness can be obtained, and a target electric resistance can be ensured.

  As a heating means in the surface oxidation step, a general heating furnace such as an electric furnace is used. For example, when forming an oxide film with an electric furnace, the film thickness of the oxide film may be adjusted according to the atmospheric temperature (heating temperature), the heating time, the Si content or the Al content of the soft magnetic alloy powder. The ambient temperature is usually suitably set within a range of 400 to 600 ° C. By setting the ambient temperature to 400 ° C. or higher, the Gibbs free energy change ΔG of the iron oxidation reaction can be made close to 0, and the effect of suppressing iron oxidation can be obtained. When the atmospheric temperature is increased, the formation of the oxide film is likely to proceed, but the characteristics of the obtained magnetic material may be deteriorated. Preferably, the ambient temperature is in the range of 400 to 550 ° C.

3 and 4 show an example of surface oxidation of soft magnetic alloy powder by the above method. As shown in FIG. 3 (a), by using Fe-1% Si atomized alloy particles as a raw material powder and heating an average particle size of about 100 μm in an inert high-humidity atmosphere. The surface was oxidized. FIG. 4B is an oxide film generator used at this time, in which a container containing raw material powder is arranged in the center of the furnace core tube located in the electric furnace (see FIG. 4A), An atmosphere gas in which water vapor (H ₂ O) was mixed with nitrogen (N ₂ ) gas with a humidifier to achieve a relative humidity of 100% (normal temperature) was introduced into the furnace core tube at a predetermined flow rate. When the inside of the electric furnace was heated to a temperature of 450 ° C. with a temperature-controlled thermocouple and oxidized for 2 hours, a SiO ₂ oxide film having a thickness of 5 nm was formed on the surface of the Fe-1% Si alloy powder. It was.

FIG. 3B shows the state of formation of an oxide film in the surface layer portion of Fe-1% Si atomized alloy particles. As indicated by 1 to 3 in the figure, instead of supplying oxygen (O ₂ ) to the powder surface, when water vapor (H ₂ O) is supplied by the above device, as described above, the powder surface layer portion is more than Fe. The reaction between Si and H ₂ O, which are easily oxidized, proceeds. Then, since the Si concentration on the surface is lowered, Si diffuses from the inside to the surface and reacts with H ₂ O to be selectively oxidized. On the other hand, as indicated by 4 to 5 in the figure, Fe having a relatively high concentration moves so as to be pushed back into the inside, and the oxidation of Fe is suppressed. As a result, the surface of the Fe-1% Si alloy powder is uniformly covered with the SiO ₂ oxide film.

Further, unlike the case of oxidation with oxygen (O ₂ ), when the Fe—Si alloy powder is oxidized with water vapor (H ₂ O), as described above, the oxidation reaction of Si and H ₂ O on the powder surface. The production reaction of H ₂ due to the reduction of is proceeded simultaneously. Under such conditions, the oxidation rate is moderately controlled and the progress of oxidation toward the inside is suppressed, so that a selective oxide film of SiO ₂ can be formed at a high density. Therefore, even when the surface oxide film is a thin film of about 5 nm as in the embodiment of FIG.

  By performing surface oxidation in an inert high-humidity atmosphere in this way, a dense and high electrical resistance insulating nano thin film can be formed on the surface layer portion of the soft magnetic alloy powder.

(3): Press forming step In FIG. 1, the soft magnetic alloy powder having the SiO ₂ oxide film formed on the surface is then subjected to a press forming step. Here, first, a molding material in which a soft magnetic alloy powder having a surface oxide film is blended with a binder and a solvent and kneaded sufficiently is prepared. As the binder, for example, camphor with high adhesiveness and slip property is used for densification. As the solvent, an organic solvent such as acetone may be used. The soft magnetic alloy powder molding material is poured into a mold and subjected to pressure compression molding to obtain a densified compact with a predetermined shape. The press pressure may be about 980 Pa (10 ton / cm ² ), for example. The soft magnetic alloy powder on which the surface oxide film is formed can be directly compression-molded.

In the course of this press forming process, the Fe—Si alloy powder particles are deformed and compressed so as to fill the gaps between each other. For this reason, there is a possibility that breakage such as cracks may occur in the SiO ₂ oxide film on the surface at the contact portion of the powder. This repair process will be described later.

(4): Debinding process The molded body obtained by the press molding process is in a state where Fe-1% Si particles having an oxide film on the surface are bound with a binder, and before being subjected to the sintering process, the binder Etc. are desirable. Specifically, the press-molded body of soft magnetic alloy powder is removed by heating with, for example, an electric furnace to vaporize the binder and the solvent. The heating temperature is preferably about 50 to 100 ° C., for example.

(5): Sintering process (first process)
Next, this debinding binder is fired to obtain a sintered body of a soft magnetic material. However, since the SiO ₂ oxide film formed in the surface oxidation process of the previous process is as thin as several nanometers, and it is glassy and brittle, there is a crack in the oxide film on the surface during the process of powder deformation in the press molding process. It can happen. Therefore, in the present invention, in the sintering step, the debonded molded body is brought into contact with a weakly oxidizing atmosphere gas to repair cracks and the like generated in the surface oxide film.

Specifically, in the first step, the soft magnetic alloy powder compact is heated in a weak oxidizing atmosphere in which a weak oxidizing gas is mixed with an inert gas, and the atmospheric gas is discharged out of the system. The deaeration process is repeated alternately. Here, examples of the inert gas used during the oxidation treatment include nitrogen (N ₂ ) gas, and examples of the weak oxidizing gas include water vapor (H ₂ O) and dinitrogen monoxide (N ₂ O). ) Gas is preferably used. By this oxidation treatment, water vapor (H ₂ O) or the like is supplied to the surface of the soft magnetic alloy powder, and an atmosphere for forming an oxide film can be obtained again.

When the weak oxidizing gas is water vapor (H ₂ O), the relative humidity at room temperature is preferably higher than 50% when water vapor is mixed into the atmosphere. In general, the higher the atmospheric humidity, the easier the oxidation reaction proceeds. When the humidity is low, the oxidation reaction does not proceed. Therefore, it is preferable to mix them so that the humidity is 70 to 100% (relative humidity) at room temperature. Similar to the surface oxidation step, the higher the humidity, the more the oxidation reaction of the second element such as Si or Al at the crack end of the surface oxide film is promoted, and the repair effect is enhanced. In addition, the oxide number density in the re-formed oxide film is increased, and a dense and high electrical resistance insulating oxide film can be obtained. If the atmospheric humidity is near 100%, an oxide film having a high oxide number density and a sufficient thickness can be obtained, and a target electric resistance can be ensured.

The atmospheric pressure during the oxidation treatment is preferably a pressurized atmosphere of atmospheric pressure or higher. By performing the oxidation treatment in a pressurized atmosphere, the diffusion of weak oxidizing gas into the molded body is promoted, and water vapor (H ₂ O) or the like is evenly supplied to the soft magnetic alloy powder surface inside the molded body. The effect of repairing the oxide film can be improved.

  As a heating means during the oxidation treatment, a general heating furnace such as an electric furnace is used. The ambient temperature can be appropriately set within the range of 400 to 600 ° C., as in the surface oxidation step. By setting the ambient temperature to 400 ° C. or higher, the Gibbs free energy change ΔG of the iron oxidation reaction can be made close to 0, and the effect of re-forming the oxide film while suppressing iron oxidation can be obtained. If the atmospheric temperature is higher than 600 ° C., sintering may proceed without the oxide film being sufficiently repaired. Preferably, the surface of the soft magnetic alloy powder can be covered again with a strong electrically insulating thin film by keeping the atmospheric temperature in the range of 450 to 550 ° C. and maintaining it for a predetermined time.

The deaeration process is performed by discharging atmospheric gas in the system to the outside using a vacuum pump or the like. The atmospheric pressure during the deaeration process may be lower than the atmospheric pressure during the oxidation process, and is usually a reduced pressure atmosphere of atmospheric pressure or lower. At the time of oxidation treatment, the reduction reaction proceeds simultaneously with the oxidation reaction on the surface of the soft magnetic alloy powder, so that hydrogen (H ₂ ), which is a product gas of the reduction reaction, is sucked out of the compact and discharged out of the system by degassing treatment. be able to.

Preferably, the subsequent oxidation treatment can be performed more effectively by performing the deaeration treatment prior to the oxidation treatment. The atmospheric temperature during the deaeration treatment is the same as the atmospheric temperature during the oxidation treatment, and can be appropriately set within a range of 400 to 600 ° C. Preferably, the inside of the system is heated to 450 to 550 ° C. and maintained for a predetermined time while reducing the pressure, and then the oxidation treatment is performed by introducing a weak oxidizing atmosphere gas into the system. Further, at the start of the sintering process, first, the compact can be reduced in a reducing atmosphere such as hydrogen (H ₂ ) gas in the system.

FIG. 5 is a diagram showing the relationship between the atmospheric pressure and the mean free path of molecules, and it can be seen that the mean free path of molecules increases as the atmospheric pressure decreases. That is, the lower the atmospheric pressure during the degassing process, the easier the weak oxidizing gas diffuses during the subsequent oxidation process. In addition, by increasing the atmospheric temperature, the molecules can move easily, which is more effective. A vacuum atmosphere is preferably used to increase the mean free path of water vapor (H ₂ O) molecules and enhance the effect of promoting diffusion into the molded body.

  The number of repetitions of degassing treatment and oxidation treatment (one degassing treatment + one oxidation treatment is one cycle) is usually about 1 to 10 cycles, preferably 2 cycles or more. The treatment time in one cycle is not particularly limited, and for example, the degassing treatment may be appropriately combined in the range of about 1 to 60 minutes and the oxidation treatment in the range of about 1 to 60 minutes.

(6): Sintering process (second process)
Thereafter, the binder-molded body on which the surface oxide film is re-formed is heated to, for example, 600 to 1100 ° C. and held in a weakly oxidizing atmosphere for a predetermined time, whereby a soft magnetic material sintered body is obtained. And

  According to the sintering method of the present invention, a weak oxidizing gas is intermittently supplied to the soft magnetic powder compact in the first step, and the oxide film on the surface is efficiently repaired. A sintered body of quality soft magnetic material can be obtained.

  FIG. 6 and FIG. 7 show an embodiment of the sintering process of the soft magnetic alloy powder by the above method. As a sample for sintering, a molded body of Fe-1% Si alloy powder that has been surface-oxidized in the step of FIG. 3A is used, and this is fixed to a pedestal in an electric furnace of the sintering apparatus shown in FIG. did. FIG. 6A shows a change in processing temperature in the sintering process, and FIG. 6B shows details of the processing in the first process.

In the first step, first, a nitrogen (N ₂ ) -5% hydrogen (H ₂ ) mixed gas is introduced into an electric furnace, heated to 450 ° C., and reduced for a predetermined time (for example, 1 hour) in a pressurized atmosphere. Processing was performed. Next, the atmospheric gas in the system was discharged using a vacuum pump (RP) and maintained for a predetermined time (for example, 30 minutes). After this vacuum degassing treatment, steam (H ₂ O) is mixed in the system with a humidifier so that the relative humidity becomes 100% (room temperature), and oxidation treatment is performed for a predetermined time (for example, 45 minutes) in a pressurized atmosphere. Was done. This vacuum degassing treatment and pressure oxidation treatment were repeated for 4 cycles, and vacuum degassing treatment and reduction treatment were further performed.

In this way, by repeating the cycle of the vacuum degassing treatment and pressurized steam supply, to facilitate the discharge of feed and hydrogen (H ₂₎ molecules of water vapor (H ₂ O), molecules of the powder surface, SiO surface ₂ The oxide film is repaired. Next, in the second step, the temperature in the system was raised to 880 ° C., and the firing was performed by maintaining for a predetermined time. Thereafter, annealing (annealing) was performed while gradually lowering the temperature, and a formed body in a series of sintering steps was secured.

Thus, according to the method of the present invention, the surface oxide film of the Fe—Si alloy powder can be surely repaired to the inside of the molded body in the sintering step, and it can be coated again with a strong electrically insulating nano thin film. Therefore, it is possible to obtain a sintered body of soft magnetic alloy powder having a low iron loss and having a dense and high resistance insulating film mainly composed of inexpensive Fe. Further, even if the SiO ₂ oxide film formed in the surface oxidation process is a thin film of about 5 nm, sufficient insulation can be secured, so that the density of the magnetic material in the soft magnetic material is increased to increase the saturation magnetic flux density, High permeability can be realized and magnetic characteristics can be improved. In addition, it is possible to reduce the particle size of the soft magnetic powder by reducing the thickness of the oxide film. For example, by setting the average particle size of the soft magnetic powder to a fine particle size of 0.01 to 10 μm, the following Hall Petch's law can be obtained. As can be seen, the strength can be increased.
Hall-Petch's law: σy = σ0 + k · d ^-1/2
Here, σy is the yield stress, k is a constant, and d is the particle size of the soft magnetic powder.
Furthermore, the manufacturing process is simple and the productivity is excellent. The sintered body of the soft magnetic material thus obtained is useful as various soft magnetic parts such as a solenoid valve of an internal combustion engine and a core material of a transformer.

  FIG. 8A compares the depth from the surface layer of the surface oxide film and the oxide number density when the relative humidity at room temperature is 100% and 50% in an atmosphere in which water vapor is mixed in an inert gas. It is shown. As shown in the figure, under the condition of relative humidity of 50%, the surface oxide number density is reduced, a good oxide film is not formed, and oxidation is progressing to the inside. It can be seen that it has a great influence. In general, the atmospheric humidity and the thickness of the oxide film formed are in a relationship as shown in FIG. 8B, and the oxide film does not grow sufficiently under low humidity conditions. If the atmospheric humidity is about 70% or more, an almost sufficient oxide film thickness can be obtained. Preferably, if the atmospheric humidity is near 100%, high oxide number density and high electrical resistance can be obtained. It can be seen that an oxide film can be realized.

In the above manufacturing method, the atmosphere in the surface oxidation step is only a weak oxidizing atmosphere, but as shown in FIG. 9, an oxidation treatment in a weak oxidizing atmosphere in which a weak oxidizing gas is mixed in an inert gas, An oxide film can also be formed by alternately performing a reduction treatment in a reducing atmosphere. Here, the oxidation treatment is performed in the same manner as described above, and the soft magnetic alloy powder is heated to 400 to 600 ° C., preferably 450 to 600 ° C. in a weak oxidizing atmosphere in which a weak oxidizing gas is mixed with an inert gas. Heat to a high temperature of 550 ° C. Nitrogen (N ₂ ) gas or the like is used as the inert gas, and water vapor (H ₂ O) is used as the weak oxidizing gas, for example, and the relative humidity at room temperature is higher than 50%, preferably 70 to Try to be 100%.

By this oxidation treatment, the soft magnetic alloy powder having an oxide film formed on the surface is subsequently heated to a high temperature of 400 to 600 ° C., preferably 450 to 550 ° C. in a reducing atmosphere to perform the reduction treatment. For example, hydrogen (H ₂ ) gas is preferably used as the reducing gas. As described above, when the reduction treatment is performed after the oxidation treatment, the surface layer portion is exposed to the reducing atmosphere, so that the diffusion of oxygen to the inside is suppressed, and only the surface layer portion can be highly purified. Presumed to be.

  Therefore, by repeating the oxidation treatment and the reduction treatment, the purity of the formed surface oxide film can be improved, and a denser and thinner oxide film with high electrical resistance can be uniformly formed. A sintered body can be obtained.

It is a figure for demonstrating an example of the manufacturing process of the soft-magnetic material by the method of this invention. It is a figure which shows the free energy change (DELTA) G of the oxidation reaction of Fe and Si for demonstrating the mechanism of the surface oxidation by the method of this invention. (A) is a figure for demonstrating the surface oxidation process of Fe-Si powder by the method of this invention, (b) is a figure for demonstrating the mechanism of surface oxidation, and is an enlarged view of the Fe-Si powder surface. (A) is the elements on larger scale of (b), (b) is the whole block diagram of the oxide film production | generation apparatus used by the surface oxidation process of this invention. It is a figure which shows the relationship between the oxidation process atmospheric pressure in the sintering process of this invention method, and the mean free path of a molecule | numerator. (A) is a figure for demonstrating the temperature conditions of the 1st process of a sintering process by this invention method, and a 2nd process, (b) is a figure which shows the repetition example of the oxidation process and deaeration process in a 1st process. is there. It is a whole block diagram of the sintering apparatus used at the sintering process of this invention. (A) is a figure which shows the relationship between the depth from the surface layer of an oxide film and oxide number density when atmospheric humidity is set to 100% and 50%, (b) is the case where an atmospheric humidity is changed and an oxide film is formed It is a figure which shows the relationship between atmospheric humidity and the oxide film thickness formed. It is a figure for demonstrating the other example of the surface oxidation process by the method of this invention.

Claims (12)

A surface oxidation step of forming an oxide film on the surface of the soft magnetic powder containing iron as a main component;
A press molding process in which a soft magnetic powder having an oxide film formed on the surface thereof is press-molded to form a molded body having a predetermined shape;
Sintering the soft magnetic powder molded body to have a sintered body of a soft magnetic material,
In the sintering step, an oxidation treatment in which the compact is brought into contact with a weak oxidizing atmosphere gas in which a weak oxidizing gas is mixed with an inert gas and a degassing treatment in which the atmospheric gas is discharged out of the system are alternately repeated. A method for producing a soft magnetic material.   In the surface oxidation step, the soft magnetic alloy powder containing the second element having iron as a main component and higher oxidation reactivity than iron is heated in a weak oxidizing atmosphere in which a weak oxidizing gas is mixed in an inert gas. The method for producing a soft magnetic material according to claim 1, wherein the second element in the powder surface layer part is mainly oxidized to form an oxide film of the second element on the surface.   In the surface oxidation step, the soft magnetic alloy powder containing the second element having iron as a main component and higher oxidation reactivity than iron is heated in a weak oxidizing atmosphere in which a weak oxidizing gas is mixed in an inert gas. The oxidation treatment to be performed and the reduction treatment to be heated in a reducing atmosphere are alternately performed to mainly oxidize the second element in the powder surface layer portion to form an oxide film of the second element on the surface. The method for producing a soft magnetic material according to claim 1.   4. The soft magnetic material according to claim 2, wherein the second element is at least one selected from substances having a higher oxidation property than iron typified by Si, Ti, Al, and Cr. Production method.   5. The soft magnetism according to claim 1, wherein the oxidation treatment in the sintering step is performed in a pressurized atmosphere of atmospheric pressure or higher and the pressure is reduced to atmospheric pressure or lower by the degassing treatment. A method of manufacturing the material.   The method for producing a soft magnetic material according to any one of claims 1 to 5, wherein the sintering step is performed under a temperature condition of 400 to 1100 ° C.   The sintering step includes a first step of re-forming the oxide film by alternately repeating the oxidation treatment and the deaeration treatment under a temperature condition of 400 to 600 ° C, and a temperature condition of 600 to 1100 ° C. The method for producing a soft magnetic material according to claim 1, further comprising a second step of sintering the soft magnetic powder.   The method for producing a soft magnetic material according to claim 1, wherein the weak oxidizing gas is water vapor or dinitrogen monoxide gas.   The soft magnetic material according to any one of claims 1 to 8, wherein the weak oxidizing gas is water vapor and is mixed into the inert gas so that the relative humidity at room temperature is higher than 50%. Method.   The soft magnetic material according to any one of claims 1 to 9, wherein the weakly oxidizing gas is water vapor and is mixed into the inert gas so that the relative humidity at room temperature is 70% to 100%. Manufacturing method.   The method for producing a soft magnetic material according to any one of claims 1 to 10, wherein the surface oxidation step is performed under a temperature condition of 400 to 600 ° C.   The method for producing a soft magnetic material according to claim 1, wherein the soft magnetic powder is an atomized alloy powder having an average particle size of 0.1 to 500 μm.

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