JPS6272102A - Iron powder for magnetic dust core used at high frequency and manufacture thereof - Google Patents

Abstract

PURPOSE:To enable the titled iron powder to have a high saturation magnetic flux density, a high permeability and excellent frequency characteristics by a method wherein the flake-like iron powder having the specific oxygen content, the specific average aspect ratio and the average grain diameter is formed, and a heat treatment is performed at the specific temperature in a reducing atmosphere. CONSTITUTION:The titled iron powder contains oxygen of 0.15-0.5wt% and the remaining part is composed of inevitable impurities and Fe. The average grain diameter of the iron powder is in the range of 40-170mum, and its average aspect ratio is in the range of 4-25. Reduce iron powder or atomized iron powder is used as the starting material of the iron powder having the above-mentioned constitution. The parameter such as the grain size of the material to be processed, the quantity of iron powder to be processed, the quantity of lubricant auxiliary agent, period of processing and the like are properly set, and the iron powder is formed into the desired grain size and shape. In the heat treatment process, the atmosphere, the temperature, the period, the condition of removal of the strain due to working and the growth of crystal grains utilizing the strain due to working, the maintenance of shape and grain size of the iron powder after processing, and the condition wherein no condensation is generated on the iron powder are established. The heat treatment is performed at 550-850 deg.C in a non-oxidizing atmosphere or a reducing atmosphere, and the flake-like iron powder is formed.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention is applicable to powder magnetic cores 1 such as reactor cores and noise filter cores that require high saturation magnetic flux density, high magnetic permeability, and good frequency characteristics. This article relates to iron powder as a raw material and its manufacturing method.

[Conventional technology]

As is well known, after mixing iron powder with a binder such as resin,
The powder magnetic core obtained by pressure molding and heat curing of a binder if necessary has excellent frequency characteristics compared to the so-called M-layer core made of ordinary metal magnetic materials such as punched and laminated electric iron plates. It has advantages such as low manufacturing cost, and compared to ferrite, it is not only low cost but also has excellent temperature stability, high saturation magnetic flux density, and small fluctuation in magnetic permeability with respect to applied magnetic field. Due to its characteristics, it is used in reactor fist cores, noise filter cores, etc.

By the way, the most important characteristic required of a powder magnetic core using iron powder is high magnetic permeability in the high frequency range.
If we define the magnetic permeability extrapolated to the DC magnetic permeability as the DC magnetic permeability, and define the frequency at which the magnetic permeability becomes 80% of the DC magnetic permeability as the limit frequency, then (L) The DC magnetic permeability is large.

2. The critical frequency is large; becomes essential.

The DC permeability is greatly influenced by the density of the magnetic core and the effective demagnetizing field, and increases as the density increases and the effective demagnetizing field decreases.

On the other hand, the limit frequency increases as the eddy current loss decreases, and is considered to be proportional to the apparent resistivity of the magnetic core and inversely proportional to the DC magnetic permeability.

According to previous studies by the inventors and other reports, it is possible to improve the critical frequency to some extent by appropriately selecting the type and amount of binder added, but on the other hand, the DC ai rate decreases. It is clear that the characteristics of the magnetic core cannot be significantly improved without improving the properties of the iron powder itself.

A method of satisfying the conditions (1) and (2) above by controlling the properties of iron powder is a method of using reduced iron powder with appropriate particle size and apparent density (for example, JP-A-6O-21301).
has already been proposed. However, since conventional reduced iron powder is isotropic with respect to the axis that is perpendicular to the magnetic path and the two sleeves that are perpendicular to the magnetic path, the demagnetizing field is large and there is room for improvement in DC permeability. I'm leaving it behind.

Also, from this point of view, a method of bonding flake-like magnetic materials with a bonding material (for example, Japanese Patent Application Laid-Open No. 119801/1983) has been proposed, but
has not been done at all.

In addition, the electrolytic iron powder that has been used conventionally is comprehensively superior to other powders in terms of (1) (D and (1)), but -
It is extremely expensive compared to iron powder for shell powder metallurgy, and the above characteristics are not necessarily sufficient.

As described above, the reality is that there is no iron powder that is excellent in both properties and cost as iron powder for powder magnetic cores used in high frequencies.

[Problem that the invention seeks to solve]

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a low-cost flaky iron powder for powder magnetic cores having excellent frequency characteristics and a method for producing the same. .

[Means for solving problems]

The present inventors have investigated the type of raw material iron powder, the manufacturing conditions, and the flaky iron powder produced by annealing after crushing low-priced iron powder such as reduced iron powder or atomized iron powder as a raw material. The present invention was completed by investigating the relationship between powder properties and magnetic properties through detailed experiments.

That is, in order to achieve the object, the present invention has the following characteristics as a flaky iron powder for powder magnetic core.

(a) Contains 0.15 to 0.5 wt% of oxygen, with the remainder consisting of unavoidable impurities and Fe.

(b) The average aspect ratio (total average of the ratio of the average thickness of the particles to the average particle diameter as determined by sieve analysis) is in the range of 4 to 25.

(c) The average particle size is between 40 and 170 m.

Regarding the manufacturing method of such iron powder, (d) the starting material iron powder is reduced iron powder, atomized iron powder, or a mixture thereof, and (e) the average particle diameter and average aspect ratio after crushing and before annealing are determined respectively. 40-170 lm, process to 4-25.

(f) Limiting the annealing temperature to 550 to 850°C.

[Effect]

Hereinafter, the reasons for the limitations of the present invention and their effects will be explained in order.

First, impurities contained in flaky iron powder will be described.

Containing acid JS:u,' needs to be limited to 0.15 to 0.5 wt% for the following reasons.

Most of the oxygen contained is distributed on the surface of iron powder particles,
It improves the insulation between iron powder particles and reduces compressibility.

That is, if the upper limit of 0.5 wt% is exceeded, the green powder density will drop significantly, and as a result, the DC permeability will drop to such an extent that it cannot withstand use.

On the other hand, when the lower limit of 0.15 wt% is reached, the oxygen film becomes completely unable to provide interparticle insulation.
Eddy current loss increases and the limit frequency decreases.

Further, when attempting to reduce oxygen to 0.15 wt% in the annealing process, iron powder particles often strongly aggregate during annealing. A large number of these agglomerates remain even after disintegration, and seriously impede interparticle insulation. Moreover, once the flaky iron powder has agglomerated during the annealing process, it is extremely difficult to dissociate, and even if it is forcibly crushed, it has the negative effect of peeling off the surface oxides of the grains and changing the shape to a non-flake-like shape. There are many things.

For the above reasons, the oxygen content of flaky iron powder is 0.
It is limited to 15 to 0.5 wt%.

Next, the reason why the average aspect ratio is limited to 4 to 25 will be explained.

When flaky iron powders differing only in their average aspect ratios are molded using a constant binder clay and a constant pressure, as the average aspect ratio increases, the demagnetizing field decreases and the DC permeability increases, but the contact between the particles increases. As the area increases and the apparent resistivity of the magnetic core decreases, the limit frequency decreases. but,
By appropriately adjusting the amount of binder and molding pressure for each iron powder, both the DC permeability and limit frequency characteristics can be made larger than those of conventional iron powders.

However, if the average aspect ratio is less than 4, the flaking effect will be small, while if the average aspect ratio exceeds 25, the contact area between particles will become extremely large, making it difficult to adjust the binder 1.1 or molding pressure. However, it is never possible to obtain a higher limit frequency than with conventional iron powder.

Therefore, the average aspect ratio needs to be in the range of 4-25. More preferably, the average aspect ratio is 6 to 20.

The reason why the average particle size was limited to 40-170 μm is as follows.

When the average particle size is less than 40 gm, the DC permeability becomes extremely low due to low compressibility. On the other hand, when it exceeds 170 gm, the thickness of the flaky iron powder becomes large, so that the eddy current flowing inside the particles becomes large, and as a result, the limit frequency decreases.

Therefore, the average particle size must be in the range 40-170 gm, where the particle size is determined by the size of the sieve mesh determined by sieve analysis or similar methods.

The manufacturing method will be explained below.

Possible starting materials include reduced iron powder, atomized iron powder, electrolytic iron powder, and carbonyl iron powder, but electrolytic iron powder and carbonyl iron powder are expensive.

This is excluded because it does not meet the purpose of the present invention. The term "reduced iron powder" as used herein refers to iron powder obtained by reducing iron oxide.

When reduced iron powder or atomized iron powder is used, as is clear from the later examples, the difference due to the difference in starting materials is small. This is because both starting materials have a difference in shape expressed by apparent density and Although there is a difference in the amount of impurities, the former is no longer significant after the pulverization process, and the latter has little influence as long as a certain degree of density is applied.

For the above reasons, in the present invention, the starting material is limited to reduced iron powder or atomized iron powder.

The processing and heat treatment methods will be explained below.

Processing is carried out using ordinary vibrating ball mills, attritors, stamp mills, etc., and by appropriately determining parameters such as raw material particle size, iron powder treatment; It can be made into any shape.

Processing is done with an average grain size of 40-170gm and an f-average aspect ratio of 4.
~25 must be carried out.

However, in order to make the flaky iron powder of the present invention 11>, in the heat treatment process,
There are 8 concave holes for setting the time.

History) It is necessary to set conditions to remove processing strain and to grow crystal grains using processing strain.

(■ It is necessary to maintain the iron powder shape and particle size as much as possible after processing, and to set conditions in which iron powder agglomeration does not substantially occur. This is because flaky iron powder is suitable for general powder metallurgy. This is because it aggregates more easily than iron powder, and this aggregation significantly lowers the limit frequency.

Rough heat treatment is performed in a non-oxidizing or Ω-based atmosphere at 550 °C.
It is necessary to carry out at a temperature of ~850°C.

At low temperatures, such as less than 550'C, removal of processing strain and growth of crystal grains are often insufficient, resulting in a decrease in DC permeability. Although this method can be used even at low temperatures by holding it for a long time, it is not an effective annealing method and is contrary to the purpose of the present invention. Therefore, the lower limit of the heat treatment temperature is 550°C.
It is stipulated that

On the other hand, when the heat treatment temperature exceeds 850° C., the flaky iron powder is strongly agglomerated, and it is virtually impossible to dissociate the agglomeration by crushing, which is a subsequent process, and the limit frequency is significantly lowered. Therefore, the upper limit of the annealing temperature is defined as 850°C.

Only by setting the conditions as described above can the flaky iron powder for dust cores of the present invention be obtained.

As a matter of course, the sieving may be carried out after heat treatment to adjust the average particle size to 40 to 200 m by a process or the like.

〔Example〕

Examples of the present invention will be described below.

Example-1 Mill scale reduced iron powder (iron powder A) having the characteristics shown in Table 1
) and atomized iron powder (iron powder B) were collected. The characteristics of iron powders C and D, which were ground using a dry attritor using iron powders A and B as starting materials, are listed in Table 1. Furthermore, using a tube furnace, in ammonia decomposition gas (dew point 20'C),
Annealing is performed by holding at 750°C for 1 hour, and the obtained sintered cake is crushed to produce iron powders E and F having the properties shown in Table 2.
(Iron powders obtained by annealing C and D, respectively) were obtained.

The test piece was made of flaky iron powder, epoxy resin powder and 1
After mixing wt% zinc stearate, a ring piece with an outer diameter of 38 mm, an inner diameter of 25 mm, and a height of 6.5 mm was molded at a pressure of 7 t/cm', and the resin was cured at 150°C in the atmosphere for 1 hour. I made it.

Magnetic properties were determined by winding a ring test piece and measuring the frequency dependence of magnetic permeability using an impedance analyzer.
The DC permeability and limit frequency were determined.

By the way, since the optimal amount of resin to be added and mixed differs depending on the type of iron powder, iron powder cannot be properly evaluated with a constant amount of resin.Therefore, the commonly used DC magnetic permeability 7
The quality of the iron powder is determined based on the limit frequency obtained when the resin amount is adjusted to 5±1.

Table 2 shows the limit frequencies when the DC magnetic permeability of this example is 75±1.

For comparison, the limit frequencies obtained by a similar sample preparation method and a similar magnetic property measurement method using electrolytic iron powder are also noted.

From Table 2, it can be seen that flaky iron powder obtained using reduced iron powder or atomized iron powder as a starting material is superior to electrolytic iron powder. Note that reduced iron powder is even slightly better.

The method for producing a ring-shaped test piece and the method for measuring the limit frequency in the following Examples and Comparative Examples are the same.

Example 2 Using the starting material $4B, flaky iron powder with different amounts of oxygen was produced in the same manner as in Example 1 except for the heat treatment conditions. The difference between the heat treatment conditions used here and those of the example process is only one holding time. Table 3 shows the properties and limit frequency values of the obtained iron powders of the present invention and for comparison.

In the comparative example, when the oxygen rI3: was O, l 2 wt%, the amount of resin was increased compared to the others in order to adjust the DC permeability to 75 ± 1, but due to the poor insulation properties of the iron powder. The limit frequency is low.

When the oxygen concentration was 0.58 wt%, the compressibility was too low and the DC permeability did not reach 75±1.

Table 3 (Note) Example of unmeasurable limit frequency - 3 Apparent density 2.53 g 7'cm', average density 78 gm
The mill scale reduced powder was processed for various times using a dry photographing ball mill, and wood 51 light and comparative flaky iron powder with different average aspect ratios were produced in the same L order as in Example 1. .

Its properties and limit frequencies are shown in Table 3! +<S・ For iron powder with an average aspect ratio of 3, the effective demagnetizing field is large and a large amount of resin cannot be added, so the limit frequency is low.

On the other hand, iron powder with an average aspect ratio of 30 can have a large amount of resin added to it, but the cross-sectional area of contact with the iron powder is too large and cannot be effectively insulated. , the limit frequency is extremely low.

Example 4 Flaky iron powder was produced in the same manner as in Example 1 using iron powder with an apparent density of 2.75 g/crnj and an average particle diameter of 120 Jim, and then sieved to reduce the average particle size. Flake-like iron powder with different diameters was obtained.

Table 3 shows the properties and limit frequency values of iron powders of the present invention and comparative iron powders having different average particle diameters.

Flake-like iron powder with an average particle size of 22 gm has low compressibility and a large effective demagnetizing field, so that almost no resin could be added to obtain a DC permeability of 75±1. As a result, the limit frequency is lower than that without invention.

Regarding the flaky iron powder with an average particle size of 189 ALm, the limit frequency is extremely low even though more resin was added than the iron powder of the present invention for DC permeability adjustment. This is because, for large iron powders, most of the eddy current loss is due to parasitic currents flowing within each iron powder particle, and the effect of strengthening interparticle insulation is small.

Example-5 Iron powder C obtained in the middle of Example-1 and subjected to only pulverization treatment was annealed at various temperatures in H2 gas, and then subjected to the same treatment as in Example 1, and the results are shown in Table 4. Flaky iron powders of the present invention and for comparison were obtained having the properties and limit frequencies shown below.

When annealing at 900°C, sintering was so strong that flaky iron powder could not be obtained to the extent that the average aspect ratio could be determined even by crushing it strongly. low.

On the other hand, when annealing at 500'C, almost no resin can be added to adjust the DC magnetic permeability due to residual cutting strain and insufficient growth of crystal grains, resulting in the limit frequency has declined extremely.

〔Effect of the invention〕

The present invention provides high saturation magnetic flux density, high permeability and good lI
Compared to electrolytic iron powder, which is currently most commonly used as a raw material for powder magnetic cores that require high frequency characteristics, such as reactor cores and noise filters, flakes have superior characteristics and are lower in cost. We were able to obtain iron powder.

Claims (1)

[Claims] 1. Contains 0.15 to 0.5 wt% of oxygen, the remainder consists of unavoidable impurities and Fe, has an average particle size in the range of 40 to 170 μm, and has an average aspect ratio in the range of 4 to 25. Iron powder for powder magnetic cores. 2. Process reduced iron powder, atomized iron powder, or a mixture thereof to have an average particle size of 40 to 170 μm and an average aspect ratio of 4 to 25, and process it under a non-oxidizing or reducing atmosphere to
A method for producing flaky iron powder for powder magnetic cores, which comprises heat-treating at 850°C and adjusting the amount of oxygen.