Iron powder for dust cores

Abstract

The present invention provides iron powder for dust cores that has excellent compressibility and low iron loss after formation. In the iron powder for dust cores, Si content is 0.01 mass % or less, apparent density is 3.8 g/cm.sup.3 or more, the ratio of iron powder particles with a particle size of 45 m or less is 10 mass % or less, the ratio of iron powder particles with a particle size of over 180 m and 250 m or less is less than 30 mass %, the ratio of iron powder particles with a particle size of over 250 m is 10 mass % or less, and the Vickers hardness (test force: 0.245 N) of a powder cross-section is 80 Hv or less.

Claims

1. An iron powder for dust cores, the iron powder comprising iron powder obtained by a water atomizing method, wherein: in the iron powder: Si content is 0.01 mass % or less; C content is 0.01 mass % or less; N content is 0.005 mass % or less; O content is 0.1 mass % or less; Al content is 0.01 mass % or less; P content is 0.01 mass % or less; S content is 0.01 mass % or less; Mn content is 0.1 mass % or less; Cr content is 0.1 mass % or less; an apparent density is 3.8 g/cm.sup.3 or more; a ratio of iron powder particles with a particle size of 45 m or less is more than 0 mass % and 5 mass % or less; a ratio of iron powder particles with a particle size of over 45 m and 180 m or less is 50 mass % or more and less than 100 mass %; a ratio of iron powder particles with a particle size of over 180 m and 250 m or less is less than 30 mass %; a ratio of iron powder particles with a particle size of over 250 m is 10 mass % or less; and a Vickers hardness (test force: 0.245 N) of a powder cross-section is 80 Hv or less.

2. The iron powder for dust cores according to claim 1, wherein: in the iron powder, Si content is more than 0 mass %.

3. The iron powder for dust cores according to claim 1, wherein: in the iron powder, Si content is 0.006 mass % or more.

4. The iron powder for dust cores according to claim 1, wherein: the ratio of iron powder particles with a particle size of over 180 m and 250 m or less is less than 25 mass %.

5. The iron powder for dust cores according to claim 1, wherein: the ratio of iron powder particles with a particle size of over 250 m is 5 mass % or less.

6. The iron powder for dust cores according to claim 1, wherein: the ratio of iron powder particles with a particle size of over 250 m is 0 mass %.

7. Dust cores including the iron powder according to claim 1.

Description

DETAILED DESCRIPTION

(1) The present invention will be described in detail below.

(2) First, the reasons for the numerical limitations of the present invention are described.

(3) (Si Content)

(4) If Si is included in molten steel, the pure iron powder obtained by a water atomizing method (also referred to simply as powder or iron powder) oxidizes at the time of water atomizing, and an oxide-based inclusion forms in the particle, thereby increasing hysteresis loss. Furthermore, fine Si oxides forming at the time of water atomizing and solute Si that does not oxidize at the time of atomizing harden the powder, causing the compressibility to lower. For these reasons, it is essential to reduce Si content insofar as possible. In the present invention, a range of 0.01 mass % or less is adopted. The content may also be 0 mass %.

(5) (Apparent Density)

(6) The iron powder is subjected to plastic deformation by press forming to yield a high-density green compact. As the amount of plastic deformation at this time of formation is smaller, the crystal grains after stress relief annealing grow coarse, and as described below, fine iron powder with a particle size of 45 m or less greatly increases hysteresis loss. Therefore, the amount of plastic deformation needs to be reduced insofar as possible.

(7) In order to reduce the amount of plastic deformation of the powder at the time of formation, the filling rate of the powder into the die needs to be increased. In the present invention, an apparent density of the powder of 3.8 g/cm.sup.3 or more is essential and an apparent density of 4.0 g/cm.sup.3 or more is preferable. The reason is that if the apparent density falls below 3.8 g/cm.sup.3, a large amount of stress is introduced into the powder at the time of formation, and the crystal grains after stress relief annealing are refined. Note that the apparent density is an index indicating the degree of the filling rate of the powder and can be measured with the experimental method prescribed in JIS Z 2504.

(8) (Amounts of Fine Powder and Coarse Powder)

(9) In the iron powder according to the present invention, particles with a particle size of over 45 m and 180 m or less are predominant (i.e. 50 mass % or more, with 100 mass % being acceptable). Since fine iron powder with a particle size of 45 m or less greatly increases hysteresis loss, however, such fine iron powder needs to be reduced insofar as possible, with an amount of 10 mass % or less being essential and 5 mass % or less being preferable. The amount may also be 0 mass %. Note that the ratio of 45 m or less iron powder can be calculated by sieving with a sieve prescribed in JIS Z 8801-1.

(10) Since the compressibility of coarse iron powder with a particle size of over 180 m is high, such coarse iron powder needs to be included at a particular ratio, yet excessive inclusion leads to an increase in eddy current loss. Therefore, iron powder with a particle size of over 180 m and 250 m or less needs to be set to less than 30 mass %, and iron powder exceeding 250 m needs to be set to 10 mass % or less.

(11) Iron powder with a particle size of over 180 m and 250 m or less is preferably set to 25 mass % or less, and iron powder exceeding 250 m is preferably set to 5 mass % or less. These amounts may also each be 0 mass %.

(12) (Vickers Hardness)

(13) If the powder is hard, a larger compacting pressure is required to increase the density of the green compact. Therefore, the powder needs to be softened insofar as possible, and it is essential that the hardness (Hv) in a Vickers hardness test with a test force of 0.245 N be 80 or less. Hv is preferably 75 or less. The Vickers hardness can be measured by the following method.

(14) First, the iron powder to be measured is mixed into thermoplastic resin powder. The resulting mixed powder is then injected into an appropriate mold and heated to melt the resin. The result is cooled and hardened to yield a resin solid containing iron powder. An appropriate cross-section of this resin solid containing iron powder is cut, and the resulting face is polished. After removing this polished, treated layer by corrosion, the hardness of the iron powder is measured using a micro Vickers hardness gauge (test force: 0.245 N (25 gf)). With one measurement point per particle, the hardness of at least ten particles is preferably measured, with the average then being taken. The powder that is measured needs to be of a size that can accommodate indentations and hence preferably has a powder particle size of 100 m or more. Other than the above-described points, measurement is performed in accordance with JIS Z 2244.

(15) Next, a representative method of manufacturing a product according to the present invention is described. Of course, a method other than the one described below may be used to obtain a product according to the present invention.

(16) The iron powder for dust cores in the present invention is obtained by a water atomizing method, and other than Si, C, O, S, and N, the molten steel has the composition of regular pure iron powder. The content of Si is set so that Si0.01 mass %. For deoxidation, C may be added beyond the composition of the pure iron powder, yet by a subsequent decarburization process, the final C content is preferably reduced to 0.01 mass % or less. Furthermore, since O, S, and N can be removed by performing annealing in a hydrogen atmosphere during a subsequent process, relatively large amounts of these elements as compared to the composition of the pure iron powder may be mixed in, yet if the amounts are too large, the burden for reduction annealing increases, and therefore insofar as possible the amounts are preferably brought close to the composition of pure iron powder.

(17) In this context, the composition of pure iron powder is a composition equivalent to 300A, which is a pure iron powder for powder metallurgy marketed by JFE Steel Corporation.

(18) Next, this powder is subjected to reduction annealing. Reduction annealing is preferably performed in a reductive atmosphere that includes hydrogen and is preferably performed at a temperature of 800 C. or more to less than 1100 C. for 1 h or more to 5 h or less. When the iron power after atomizing includes a large amount of C, reduction annealing is performed with water vapor included in the hydrogen. The amount of water vapor need not be restricted and may be modified appropriately in response to the amount of C in the iron powder. Water vapor is typically added to achieve a dew point of approximately 30 C. to 60 C.

(19) Since a portion of the iron powder after reduction annealing is agglomerated, the agglomeration is broken up by a crushing process, and the result is sieved so that particles of 45 m or less constitute 10 mass % or less. The iron powder may also be sieved appropriately so as to remove coarse powder. This sieving may be accomplished with a method using a sieve prescribed in JIS Z 8801-1.

(20) If the apparent density of the sieved iron powder is less than 3.8 g/cm.sup.3, separate particle size adjustment or spheroidization (for example, JP S64-21001 B2) may be applied to set the apparent density to 3.8 g/cm.sup.3 or more. When subjecting the iron powder to spheroidization, in order to remove stress during processing, stress relief annealing is preferably applied in a hydrogen atmosphere at a temperature of 700 C. to 850 C. for approximately 1 h to 5 h.

(21) In order to obtain a dust core from the iron powder produced as above, an insulating coating is preferably applied to the iron powder surface. This insulating coating may be any coating capable of maintaining insulation between particles. Examples of such an insulating coating include silicone resin; a vitreous insulating amorphous layer with metal phosphate or metal borate as a base; a metal oxide such as MgO, forsterite, talc, or Al.sub.2O.sub.3; or a crystalline insulating layer with SiO.sub.2 as a base.

(22) The iron powder to which the insulating coating has been applied is injected in a die and pressure formed to a shape with desired dimensions (dust core shape) to yield a dust core. The pressure formation method may be any regular formation method, such as cold molding, die lubrication molding, or the like. The compacting pressure and die temperature are appropriately determined in accordance with use. If the compacting pressure is increased, the green density increases. Hence, a compacting pressure of 981 MPa (10 t/cm.sup.2) or more is preferable, with 1471 MPa (15 t/cm.sup.2) or more being more preferable. On the other hand, no restriction is placed on the upper limit of the compacting pressure, yet in view of restrictions on manufacturing facilities, the upper limit may be approximately 1960 MPa (20 t/cm.sup.2).

(23) Even if the die temperature is increased, the green density rises. Therefore, the die temperature is preferably 80 C. or more and more preferably 100 C. or more. On the other hand, no restriction is placed on the upper limit of the die temperature, yet in view of restrictions on manufacturing facilities, the upper limit may be approximately 300 C.

(24) Of course, the above formation conditions may be changed appropriately in accordance with use. At the time of pressure formation, as necessary, a lubricant may be applied to the die walls or added to the powder.

(25) At the time of pressure formation, the friction between the die and the powder can thus be reduced, suppressing a reduction in the green density. Furthermore, the friction upon removal from the die can also be reduced, thereby preventing cracks in the green compact (dust core) at the time of removal. Preferable lubricants include metallic soaps such as lithium stearate, zinc stearate, and calcium stearate, and waxes such as fatty acid amide.

(26) After pressure formation, the dust core is subjected to heat treatment in order to reduce hysteresis loss due to stress relief and to increase the green compact strength. The heat treatment time is preferably in the range of 5 min to 120 min. Any of the following may be used without any problem as the heating atmosphere: the regular atmosphere, an inert atmosphere, a reductive atmosphere, or a vacuum. The atmospheric dew point may be determined appropriately in accordance with use. Furthermore, when raising or lowering the temperature during heat treatment, a stage at which the temperature is maintained constant may be provided.

EXAMPLES

(27) In the present Example, 11 types of pure iron powder obtained by a water atomizing method and having the characteristics in Table 1 were used. Other than the element Si, the following ranges were satisfied in all of the samples: C0.01 mass %, N0.005 mass %, O0.1 mass %, Al0.01 mass %, P0.01 mass %, S0.01 mass %, Mn0.1 mass %, and Cr0.1 mass %.

(28) TABLE-US-00001 TABLE 1 Particle size ratio (mass %) Apparent Over 180 m Vickers Sample Si content density 45 m and 250 m Over hardness number (mass %) (g/cm.sup.3) or less or less 250 m (Hv) Other Notes 1 0.006 4.3 3 21 0 73 Inventive example 2 0.008 4.2 6 21 0 76 Inventive example 3 0.019 4.2 5 22 0 78 Corresponds to PTL 1 Comparative example and PTL 2 4 0.027 4.2 3 21 0 80 Comparative example 5 0.066 4.3 2 23 0 83 Comparative example 6 0.137 4.2 4 20 0 89 Comparative example 7 0.006 4.3 13 17 0 72 Particle size out of range Comparative example (corresponds to PTL 3) 8 0.006 4.4 3 21 0 86 Comparative example 9 0.006 3.6 4 20 0 72 Comparative example 10 0.006 4.2 0 56 35 72 Corresponds to PTL 4 Comparative example 11 0.006 4.2 0 40 5 72 Corresponds to PTL 4 Comparative example

(29) An insulating coating was applied to the powders listed in Table 1 using silicone resin. After dissolving the silicone resin in toluene to produce a resin dilute solution such that the resin component is 0.9 mass %, the powder and the resin dilute solution were mixed so that the rate of addition of the resin with respect to the powder became 0.1 mass %. The result was then dried in the atmosphere. After drying, a resin baking process was performed in the atmosphere at 200 C. for 120 min to yield iron powders coated in silicone resin.

(30) These powders were then molded by die lubrication at a compacting pressure of 1471 MPa (15 t/cm.sup.2) to produce ring-shaped test pieces with an outer diameter of 38 mm, an inner diameter of 25 mm, and a height of 6 mm. After subjecting the produced test pieces to heat treatment in nitrogen at 600 C. for 45 min, winding was performed (primary winding, 100 turns; secondary winding, 40 turns). Magnetic flux density measurement with a DC magnetizing device (H=10000 A/m, DC magnetizing measurement device produced by METRON, Inc.) and iron loss measurement with an iron loss measurement device (1.0 T, 1 kHz, high-frequency iron loss measurement device produced by METRON, Inc.) were then performed.

(31) Table 2 shows the measurement results of density and magnetic properties of the green compact along with green density. In the present Example, the acceptance criterion for magnetic flux density was B.sub.1001.70 T, and the acceptance criterion for iron loss was W.sub.10/1K80 W/kg.

(32) Table 2 also lists the measurement results for the crystal grains.

(33) TABLE-US-00002 TABLE 2 Green Sample density Magnetic flux Iron loss number (g/cm.sup.3) density (T) (W/kg) Notes 1 7.72 1.71 72.1 Inventive example 2 7.71 1.70 73.2 Inventive example 3 7.67 1.62 82.8 Comparative example 4 7.65 1.61 81.5 Comparative example 5 7.57 1.58 86.7 Comparative example 6 7.61 1.56 119.5 Comparative example 7 7.63 1.60 96.1 Comparative example 8 7.58 1.58 91.6 Comparative example 9 7.68 1.71 99.7 Comparative example 10 7.73 1.72 85.0 Comparative example 11 7.72 1.72 82.0 Comparative example

(34) Table 2 shows that the inventive examples according to the present invention (sample numbers 1 and 2) not only have a high green density but also pass the acceptance criteria for both magnetic flux density (B.sub.100) and iron loss (W.sub.10/1K) and thus have excellent magnetic properties.

(35) By contrast, sample numbers 3 to 6, in which the Si content was greater than in the inventive examples, did not meet the acceptance criteria for either magnetic flux density or iron loss. From the results for sample numbers 3 to 6, it is clear that increasing the Si content tends to reduce the magnetic flux density and increase iron loss. The reason is thought to be that the powder hardens along with an increase in Si content and that fine oxides produced at the time of water atomizing increase.

(36) Sample number 7, which includes more iron powder with a particle size of 45 m or less than do the inventive examples, and sample number 8, which has high powder hardness, also did not meet the acceptance criteria for either magnetic flux density or iron loss.

(37) For sample number 7, it is inferred that the increase in fine powder led to a reduction in compressibility and to an increase in total iron loss due to an increase in hysteresis loss. On the other hand, for sample number 8, it was thought that hardness of the powder increased due to the fineness of crystal grains in the powder or to the accumulation of strain, which in turn was thought to lead to reduced compressibility and an increase in total iron loss due to an increase in hysteresis loss.

(38) Although sample numbers 9, 10, and 11 satisfied the acceptance criterion for magnetic flux density, they did not meet the acceptance criterion for iron loss.

(39) For sample number 9, it was thought that a large amount of strain accumulated during formation due to the reduction in apparent density, causing hysteresis loss to increase, thus resulting in increased iron loss. By contrast, sample numbers 10 and 11 had high compressibility due to the inclusion of much coarse powder and exhibited higher green density and magnetic flux density than did the inventive examples, yet it was thought that these samples did not meet the acceptance criterion for iron loss due to the coarse powder causing an increase in eddy current loss.