JPH04168206A - Manufacture of metal powder - Google Patents

Abstract

PURPOSE:To manufacture metal powder for obtaining a powder compacting body having high density by periodically changing flow rate of molten metal descending flow poured from a molten metal nozzle arranged at bottom part of a molten metal vessel and blowing high pressure fluid under the same condition to this descending flow. CONSTITUTION:A tool steel is charged into the molten metal vessel 11 and heated with high frequency coil 12 to make this the molten metal 13 and this is poured from the molten metal nozzle 14 arranged at the bottom part of molten metal vessel 11 to make this the molten metal descending flow 18. To this molten metal descending flow 18, high pressure water is injected from an annular high pressure water injecting pipe 16, and by forming conical water film 17 and this is made to drip-state 19 to form the metal powder. Then, a solenoid value 15 is set so as to surround the circumference of the molten metal nozzle 14, and by periodically applying electromagnetic force upward to the molten metal 13, molten metal flow rate is periodically varied and high pressure water is blown under the same condition. By this method, the metal powder having particle size distribution containing almost the same quantity from small particle diameter and to large particle diameter be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing metal powder, which is used as a raw material for powder metallurgy, electrical materials, etc., and particularly has a high compact density.

[Prior Art] In powder metallurgy, in which compacted metal powder is sintered to form a product in a predetermined shape, reduced powder obtained by reducing metal oxides, chlorides, etc. to a metallic state has been conventionally used. This reduced powder generally has a low apparent density and excellent cold moldability. However, since a reduction step is required, many steps and time are required to obtain metal powder, resulting in poor productivity.

Therefore, the atomization method has been adopted as a method that can relatively easily produce a large amount of metal powder. In this method, metal powder is produced by spraying a high-pressure fluid such as water or gas onto a downward flow of molten metal, breaking the molten metal into fine droplets, and rapidly cooling and solidifying the molten metal.

The metal powder obtained by the atomization method has relatively advanced spheroidization and has a small specific surface area. Therefore, the apparent density is high (and the cold formability is poor).

In order to eliminate this drawback, for example, Japanese Patent Application Laid-Open No. 58-819
Publication No. 03 proposes improving cold formability by subjecting metal powder obtained by water atomization to reduction annealing to form a complex shape in which the powders are adhesively bonded to each other. Further, in Japanese Patent Application Laid-open No. 59-59810, after the raw copper powder that aggregated during reduction annealing is crushed, the upper part of the sieve is further crushed to a slight degree.

The present inventors also developed a method of producing metal powder with excellent cold formability by controlling the atomization conditions, and proposed it in Japanese Patent Application No. 2-373, Japanese Patent Application No. 2-374, etc. did. Furthermore, Japanese Patent Application No. 2-423 proposes producing metal powder with controlled particle size by regulating the mass flow rate, pressure, and ejection speed of high-pressure fluid in relation to the flow rate of the downward flow of molten metal. did.

[Problems to be Solved by the Invention] When metal powder is compacted into a predetermined shape and sintered, the resulting sintered body has mechanical properties such as strength and toughness that change depending on its density. - If you go fishing on a boat and sinter a powder compact with a small density, the metallurgical bond during sintering will be insufficient, resulting in a structure with many bores inside the sintered compact, resulting in zero mechanical strength. When the sintered body is used for applications requiring the following characteristics, the required properties cannot be satisfied due to this structure.

Furthermore, when cold rolling is performed after sintering a powder-rolled green compact, many cracks occur in the rolled metal strip starting from the bore or its surroundings due to the brittle structure. As a result, the proportion of products that are discarded as defective increases, making it impossible to produce products that satisfy the required characteristics at a high yield.

In order to avoid the occurrence of such a brittle structure, it is necessary to obtain a high-density sintered body. To achieve this, it is necessary to increase the density of the green compact before sintering.By the way, when applying the latest control technology, it is necessary to divide the molten metal flow into fine droplets to produce metal powder. It is possible to perform atomization under almost constant conditions.As a result, the obtained metal powder has a slight variation, but the
It is produced with a fixed particle size. However, when metal powder with a fixed particle size is compacted to form a compact, there is a limit to improving the density of the compact.

The present invention was devised to solve these problems, and by periodically varying the atomization conditions, the particle size distribution of the obtained metal powder is widened and a high-density green compact is obtained. The purpose is to produce metal powders that can be used.

[Means for Solving the Problems] In order to achieve the object, the metal powder manufacturing method of the present invention sprays a high-pressure fluid onto the downward flow of the molten metal to break the molten metal into fine droplets, and cools the droplets. When turning metal powder into metal powder, the flow rate of the molten metal descending from a molten metal nozzle provided at the bottom of the molten metal container is periodically changed, and the high-pressure fluid is sprayed under the same conditions to the molten metal descending. Features.

As means for changing the flow rate of the downward flow of the molten metal, a method of periodically changing the electromagnetic braking force, the effective static pressure of the molten metal, etc. can be adopted.

For example, an electromagnetic valve is attached to a molten metal nozzle attached to a molten metal container, and upward electromagnetic force is periodically applied to the downward flow of molten metal. This reduces the flow rate of molten metal when applying electromagnetic force.
It is possible to increase the flow rate of molten metal when no addition is made.
The magnitude of the applied electromagnetic force may be periodically varied.

Alternatively, the upper chamber containing the molten metal container and/or
Alternatively, the atmospheric pressure of the lower chamber through which the downward flow of molten metal flows out is periodically changed. In accordance with this variation in atmospheric pressure, the effective static pressure applied to the molten metal near the molten metal nozzle changes, and the flow rate of the molten metal flowing out from the molten metal nozzle changes periodically.

Furthermore, by utilizing the surface tension between the molten metal and the molten metal nozzle,
It is also possible to periodically change the flow rate of the molten metal flowing out from the molten metal nozzle. In this case, the molten metal nozzle is formed of a ceramic material with low wettability to the molten metal, and the cross-sectional area of the molten metal flow path formed in the molten metal nozzle is made relatively small.

[Function] In order to obtain a high-density compact, metal powders with different particle sizes have been blended in the past. Therefore,
It is necessary to store and handle the metal powder by particle size classification, making the process of preparing the powder mixture time-consuming and troublesome. In the present invention, by artificially and periodically varying the manufacturing conditions in the same powder manufacturing process, a metal powder with a plurality of polymerized particle size distributions is manufactured, and the obtained metal powder is directly processed into a high-quality powder. This allows it to be molded into a dense powder compact.

The flow rate of the molten metal descending from the molten metal container varies periodically. Atomization is performed on this molten metal downward flow under the same conditions. Note that during atomization, high-pressure fluid such as water, inert gas, liquefied gas, endothermic gas, etc. can be used.

When the flow rate of the molten metal downward is small, the pressure of the high-pressure fluid acts as a large breaking force on the molten metal per unit weight.

Therefore, the droplets separated from the molten metal have a fine particle size. On the other hand, when the flow rate of the molten metal descending is large, the breaking force of the high-pressure fluid acting per unit weight becomes small. therefore,
The droplets separated from the molten metal have a relatively large particle size.

Therefore, by periodically changing the flow rate of the molten metal downward, metal powders with different particle size distributions can be produced in the same process. Therefore, the obtained metal powder is a mixture of powder particles having a relatively small particle size distribution and powder particles having a relatively large particle size distribution.

By changing the atomization conditions, it is also possible to obtain metal powder in which powder particles from small diameters to large diameters are distributed in approximately the same ratio. However, in this case, disadvantages such as unstable atomization conditions and formation of metal powder with a small specific surface area or an extremely large particle size tend to occur.

[Example] Hereinafter, an example will be described in which the present invention is applied to manufacturing a metal powder by a water atomization method and subjecting the metal powder to powder rolling, annealing, and rolling to manufacture a band-shaped metal body.

In this example, the atomization device shown schematically in FIG. 1 was used. Moreover, high-speed tool steel equivalent to JIS standard 5KH51 was used as a material manufactured into metal powder by water atomization.

A molten metal 13 was prepared by placing 10 kg of tool steel in a molten metal container 11 and maintaining it at a superheat degree 100'C higher than the liquidus line by a high frequency coil 12.

A molten metal nozzle 14 is incorporated into the bottom wall of the molten metal container 11. As the molten metal nozzle 14, a ceramic nozzle made of boron nitride (BN) having low wettability with respect to the molten metal 13 and having a diameter of 5 mm was used. Also,
An electromagnetic valve 15 surrounds the molten metal nozzle 14.
was placed.

As the electromagnetic valve 15, a commercially available linear motor-type electromagnetic coil was used, and it was set to apply an upward electromagnetic force to the molten metal 13 flowing down inside the molten metal nozzle 14.

An annular high-pressure water injection pipe 16 was arranged at a position 80 mm away from the lower end of the molten metal nozzle 14. The high-pressure water injection pipe 16 has a plurality of concentrically formed high-pressure water injection pipes (not shown) that are slightly downwardly inclined on the inner peripheral side thereof. The high-pressure water blown out from the high-pressure water injection hole forms a conical water film 17 facing downward. The high-pressure water injection pipe 16 was positioned so that the apex of the conical water film 17 coincided with the center of the downward flow 18 of the molten metal flowing down from the molten metal nozzle 14.

When no braking force was applied by the electromagnetic valve 15, a downward flow 18 of the molten metal flowed out from the molten metal nozzle 14 at a mass flow rate of 150 g/sec. In addition, when an upward electromagnetic force of 3xlo'N/m'' was applied, the mass flow rate of the downward flow of molten metal 18 became as small as 60 g/sec.Then, after continuing to apply electromagnetic force for 3 seconds, the state of no addition was reached. Electromagnetic force was intermittently applied to the molten metal descending flow 18, with one cycle of allowing the molten metal downward flow 18 to freely fall for 3 seconds.

High-pressure water at a temperature of 20°C was sprayed onto the molten metal descending flow 18 at a mass flow rate of 3,300 g/sec, a jetting pressure of 220 kgf/cm", and a jetting speed of 120 m/sec. This high-pressure water caused the molten metal descending flow 18 to become fine particles. The metal powder was divided into droplets 19 and solidified to become a metal powder.The obtained metal powder was subjected to a quenching effect after being mechanically divided by high-pressure water, so it was irregularly shaped particles with large irregularities and had a relatively large size. It had a large surface area.

The obtained metal powder was passed through a sieve and its particle size distribution was examined. The results are shown in FIG.

The particle size distribution of the metal powder A obtained by spraying high-pressure water under the above-mentioned conditions onto the descending flow of molten metal 18 that was freely falling from the molten metal nozzle 14 without applying electromagnetic force is as shown in FIG. , a normal distribution centered on the particle size of 55 μm. Further, the particle size distribution of metal powder B obtained when electromagnetic force was continuously applied was a normal distribution centered at a particle size of 35 μm.

On the other hand, the metal powder C of this example, which was manufactured while periodically applying electromagnetic force, has both particle size distributions added, and the powder particles over a wide range of 20 μm to 801 Lm are almost the same. It was included in the content.

Next, these metal powders A to C were powder rolled, and the density of the compact was investigated. The results are shown in FIG.

As the powder rolling mill, a pair of rolling rolls having a roll diameter of 400 mm were used, which were arranged parallel to each other with a roll gap of 1.0 mm. A powder reservoir with a width of 180 mm is provided between these rolling rolls, and the metal powder supplied to the powder reservoir is applied with a pressing force of 3xlO" kgf/c.
m" to produce a belt-shaped powder compact with a width of 180 mm and a plate thickness of 2.0 mm.

The compact obtained from metal powder A has a theoretical density of 100%.
The density was 78%. This is considered to be because, as shown in FIG. 2, the particle size of metal powder A is relatively large, and the voids between the powder particles in the compacted state are large. On the other hand, the density of the green compact obtained from metal powder B was 80%. This is considered to be due to the fact that although the particle size of the metal powder B is relatively small and the voids between the powder particles in the compacted powder state are small, a large number of voids exist throughout the entire compacted powder body.

On the other hand, the compact obtained from metal powder C of this example had a high density of 87%. This means that metal powder C has a particle size distribution that exists in almost the same proportion from small diameters to large diameters.
This is thought to be due to relatively small powder particles filling the spaces between relatively large powder particles.

After sintering these compacts at a temperature of 1200°C, the plate thickness was 1.
When cold-rolled to a thickness of 2-mm, the resulting band-shaped metal bodies had significantly different crack occurrence rates depending on the type of metal powder A to C. That is, in the band-shaped metal body using metal powders A and B as starting materials, many cracks were detected both on the surface layer and inside. Moreover, a large rack was generated during cold rolling, and a situation occurred in which cold rolling could not be continued. Therefore, the tensile strength is 30kgf/mm2.
The rate at which products satisfying the above required characteristics can be obtained is 60%.
%Met.

On the other hand, in the band-shaped metal body obtained from metal powder C,
Only a few cracks occurred, and a product satisfying the required characteristics could be manufactured with a high yield of 90%. When increasing the density of the green compact in this way, a metallurgical bonding reaction progresses inside the green compact during sintering, repairing the voids and preventing cracks from occurring during cold rolling. The metal phases are crimped together.As a result,
It becomes possible to manufacture a band-shaped metal body with excellent properties with high productivity.

In this example, the atomization device shown in FIG. 4 was used.

The inside of the powdering tank 21 was divided into an upper chamber 23 and a lower chamber 24 by a partition plate 22.

A gas introduction pipe 25 and an exhaust pipe 26 for blowing inert gas were opened in the upper chamber 23.

The gas introduction pipe 25 is connected to an inert gas supply source, and a flow rate adjustment valve 27 is attached in the middle. An inert gas having a predetermined pressure is introduced into the upper chamber 23 through the gas introduction pipe 25. The other exhaust pipe 26
is connected to an exhaust pump (not shown), and is further provided with an exhaust valve 28 that turns on and off the outflow of the inert gas in the upper chamber 23.

The lower chamber 24 is also provided with a gas introduction pipe 29 and an exhaust pipe 30 in order to maintain the inside at a predetermined inert atmosphere pressure. A valve similar to the flow rate adjustment valve 27 may be attached to the gas introduction pipe 29.

The molten metal container 31 was placed on the partition plate 22.

Further, the molten metal nozzle 32 provided on the bottom wall of the molten metal container 31 is inserted through the partition plate 22 into the lower chamber 24.
I made him appear. As the molten metal nozzle 32, a ceramic nozzle made of boron nitride similar to that in Example 1 was used. Then, as in Example 1, 10 kg of high-speed tool steel equivalent to JIS standard 5KH51 was placed in the molten metal container 31, and maintained at a superheat degree 100'C higher than the liquidus line by the high frequency coil 33 to prepare the molten metal 34. .

At a lower position 80 mm away from the lower end of the molten metal nozzle 32,
An annular high-pressure water injection pipe 35 is arranged. Then, the conical water belly 3 formed by the high pressure water injected from the high pressure water injection pipe 35
The apex of 6 is the downward flow 37 of the molten metal flowing down from the molten metal nozzle 32
The high pressure water injection pipe 35 was positioned so as to coincide with the center of. The descending molten metal flow 37 is divided by high-pressure water and then rapidly cooled and solidified to become metal powder 39.

Metal powder 39 is contained in a pan 40 located at the bottom of lower chamber 24 . After one charge of molten metal 34 has been atomized, or when a predetermined amount of metal powder 39 has been stored, the pan 40 is taken out of the powdering tank 2 and the metal powder 39 is recovered.

In this powder manufacturing process, by adjusting the flow rate of the inert gas sent into the upper chamber 23 via the gas introduction pipe 25 and the flow rate of the gas discharged from the exhaust pipe 26,
The atmospheric pressure in the upper chamber 23 was varied. In other words, as shown in Figure 5, the maximum pressure is 1.2 atm to the minimum 0.8 atm.
The atmospheric pressure in the upper chamber 23 was periodically varied so that the amplitude between atmospheric pressures was repeated every minute. Note that the atmospheric pressure in the lower chamber 24 was maintained at a constant value of 1 atmosphere.

When the flow rate of the molten metal descending flow 37 flowing out from the molten metal nozzle 32 was measured under these conditions, the flow rate changed periodically as shown in FIG. 5 in response to fluctuations in atmospheric pressure. Although there is a slight time delay in the change in the flow rate of the molten metal compared to the periodic change in atmospheric pressure, it changes from 180 g/sec to 1 in the same period.
It varied between 20g/sec.

High-pressure water was sprayed under the same conditions as in Example 1 to the molten metal descending flow 37 whose flow rate fluctuated in this way to perform atomization. As shown in FIG. 6, the obtained metal powder had a wide region containing powder particles having different particle sizes in approximately the same ratio.

When a band-shaped metal body was manufactured from this metal powder in the same manner as in Example 1, a product with a green compact density as high as 85% and an extremely low cracking rate was obtained.

In addition, the strength of the rolled metal strip is that the tensile strength is 3
It was 2 kgf/am2.

In this embodiment, the atmospheric pressure in the upper chamber 23 is periodically varied. In this case, the pressure applied to the molten metal 34 near the molten metal nozzle 32 is the static pressure of the molten metal 34 existing in the molten metal container 3i plus the atmospheric pressure.

This pressure acts as a force to cause the molten metal 34 to flow out through the molten metal nozzle 32, so that the molten metal 34 flows downward in response to fluctuations in atmospheric pressure.
7 flow rate changes.

However, by similarly varying the atmospheric pressure in the lower chamber 24, the flow rate of the molten metal downward flow 37 can be changed periodically. In this case, lower chamber 2
When the atmospheric pressure of the molten metal 34 decreases, the resistance applied to the molten metal 34 flowing down inside the molten metal nozzle 32 decreases, and a relatively large amount of the molten metal 34 flows out. On the other hand, as the atmospheric pressure in the lower chamber 24 increases, the resistance increases and the flow rate of the molten metal downward flow 37 decreases.

As a means for varying the atmospheric pressure in the lower chamber 24, a method is adopted in which the opening degree and operation of the on-off valves of the flow rate control valves etc. attached to the gas introduction pipe 29 and/or the exhaust pipe 30 are controlled according to a predetermined program. be able to. However, the atmospheric pressure in the lower chamber 24 is
This affects atomization, which breaks up the liquid into droplets 38. Therefore, if the atmospheric pressure is suddenly changed, there is a drawback that the operating conditions become unstable.

! Wataru 1: In this example, an atomization device was used in which the electromagnetic valve 15 was removed from the equipment configuration shown in FIG. As the molten metal nozzle 14, a ceramic nozzle made of boron nitride and having a diameter of 3 mm was used. Since the diameter of the molten metal nozzle 14 is small, the flow resistance when the molten metal 13 passes through the nozzle is large. Therefore, after a relatively large amount of molten metal 13 flows out due to surface tension, a relatively small amount of molten metal descends 18 flows down along the inner wall of the nozzle hole until the pressure above the nozzle hole is increased to a constant value.

This repetition causes the molten metal downward flow 18 to pulsate.

For example, under conditions in which the molten metal container 11 is maintained in an inert atmosphere of 1 atm and the high-pressure water injection pipe 16 is maintained in an inert atmosphere of 1 atm, the molten metal downward flow 18 whose flow rate increases and decreases at an average cycle of 3 seconds. Obtained. At this time, the maximum flow rate is 50g/
seconds, the lowest value was 10 g/sec.

High-pressure water was sprayed onto this pulsating downward flow of molten metal 18 under the same conditions as in Example 1 to break it into droplets 19. As a result, the obtained metal powder has a particle size ranging from 10 μm to 50 μm.
It had a particle size distribution in which the content was 80% or more in the range up to m.

When a band-shaped metal body was produced from this metal powder in the same manner as in Example 1, a product with an extremely low crack occurrence rate was obtained. The tensile strength of the rolled metal strip was 34 kgf/cm''.

In this example, a ceramic nozzle made of boron nitride was used as the molten metal nozzle 14. but,
The material for the molten metal nozzle is not limited to this, but as long as it is a material that has low wettability to molten metal, silicon nitride can be used.

Various materials can be used, such as Sialon. Further, the nozzle hole formed in the molten metal nozzle 14 is also appropriately windowed in consideration of the wettability depending on the material. However, molten metal 1
It is preferable that the aperture diameter is 4 mm or less so that the molten metal nozzle 3 does not flow out from the molten metal nozzle 14 in a steady state. However, if the diameter is extremely small, the smooth outflow of the molten metal 13 will be inhibited, so the lower limit is set to 1 mm.

In Examples 1 to 3 above, the case where the obtained metal powder was powder rolled and sintered to form a band-shaped sintered body was explained.

However, the method is not limited to this, and even if a normal sintering method is applied, a green compact with high density can be similarly obtained, resulting in a sintered compact with excellent properties.

[Effects of the Invention] As explained above, in the present invention, the flow rate of the downward flow of molten metal flowing out from the molten metal nozzle is periodically varied, and high-pressure water is sprayed against the downward flow of the molten metal under certain conditions. Performing atomization.

Therefore, powder particles formed by dividing a downward flow of molten metal with a small flow rate have a relatively small particle size, and powder particles formed by dividing a downward flow of molten metal with a large flow rate have a relatively large particle size. The particle size distribution of the obtained metal powder is a superposition of these particle sizes. In other words, the particle size distribution is such that particles from small to large particles are contained in approximately the same amount. Therefore, when compacting this metal powder, a high-density green compact can be obtained, and a sintered compact with excellent mechanical properties such as tensile strength and toughness can be manufactured.

[Brief explanation of drawings]

Figure 1 shows an outline of the atomization device used in Example 1 of the present invention, Figure 2 is a graph showing the influence of the addition of electromagnetic force on the particle size of metal powder, and Figure 3 is a graph showing the particle size of metal powder. A graph showing that the green compact density changes depending on the distribution, FIG. 4 shows the atomization device used in Example 2,
FIG. 5 is a graph showing the relationship between atmospheric pressure and molten metal flow rate, and FIG. 6 is a graph showing the particle size distribution of the metal powder obtained in Example 2. Figure 2 Particle size (μm) BC Type of metal powder Figure 4 Figure 5 Time (minutes) Particle size (μm)

Claims (4)

[Claims] (1) When high-pressure fluid is sprayed onto the downward flow of the molten metal to break the molten metal into fine droplets and the droplets are cooled to form metal powder, the molten metal flows down from the molten metal nozzle provided at the bottom of the molten metal container. A method for producing metal powder, comprising periodically changing the flow rate of the downward flow and spraying the high-pressure fluid onto the downward flow of the molten metal under the same conditions. (2) A method for producing metal powder, characterized in that the flow rate change according to claim 1 is performed by periodically turning on/off or increasing/decreasing an upward electromagnetic force generated by an electromagnetic valve attached to a molten metal nozzle. . (3) The flow rate change according to claim 1 is carried out by periodically changing the atmospheric pressure of the upper chamber housing the molten metal container or the lower chamber through which the molten metal descends. Production method. (4) A method for producing metal powder, characterized in that the flow rate change according to claim 1 is performed through a molten metal nozzle made of a ceramic material having low wettability with respect to the molten metal and having a small diameter flow path cross section.