JP2823972B2 - Molten metal outflow nozzle and method for producing metal powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten metal outflow nozzle used for producing a metal powder by an atomizing method and a method for producing a metal powder having a uniform particle size with a high yield.

[0002]

2. Description of the Related Art In powder metallurgy in which a compacted metal powder is sintered into a product having a predetermined shape, a reduced powder obtained by reducing a metal oxide or chloride to a metal state has been conventionally used. This reduced powder generally has a low apparent density and is excellent in cold moldability. However, since a reduction step is required, many steps and time are required until a metal powder is obtained, resulting in poor productivity.

Therefore, the atomizing method has been adopted as a method for producing a large amount of metal powder relatively easily. In the atomization method, a metal powder is produced by spraying a high-pressure fluid such as water or gas onto a downward flow of molten metal, dividing the molten metal into fine droplets, quenching, and solidifying. (JP-A-61-207
No. 502, JP-A-2-43306, etc.)

When a metal powder is produced by a water atomization method, a reaction between a molten metal and water is inevitable, and an oxide film is formed on the surface of the obtained powder particles. In this regard, in the gas atomizing method using an inert gas such as nitrogen, no reaction occurs with the molten metal, and powder particles having a metallic surface can be obtained.

[0005]

The gas atomization method comprises:
This is advantageous in that the reactivity with the molten metal is low, but the shearing force applied to the molten metal is small. Therefore, in order to shear the molten metal into droplets having the same size as that of the water atomization method, it is necessary to extremely increase the gas pressure. Further, in gas atomization, generally, in order to make effective use of gas pressure, a gas ejection nozzle is often arranged immediately near the lower end of a molten metal nozzle, and a gas flow is ejected in a downward inverted conical shape. At this time, the suction force due to the gas flow acts on the molten metal descending flow in the molten metal nozzle. Therefore, as the gas flow becomes high pressure, that is, as the gas flow velocity increases, the attractive force acts more, and the mass of the molten metal descending flow increases accordingly. The flow rate increases.

On the other hand, the flow rate of the molten metal descending from the molten metal nozzle is affected by the level of the molten metal in the molten metal holding vessel.
That is, as the amount of molten metal in the molten metal holding container decreases, the static pressure of the molten metal applied near the molten metal nozzle decreases. Thereby, the flow rate of the molten metal flowing down from the molten metal nozzle is reduced.

In particular, the gas atomization method uses a gas having a smaller heat capacity than the water atomization method.
The influence of such a change in the flow rate of the molten metal tends to be large. As a result, the obtained metal powder has a non-uniform particle size.

The present invention has been devised in order to solve such a problem. In the gas atomization, the molten metal is positively utilized by utilizing the suction force generated by the gas flow on the downward flow of the molten metal. An object of the present invention is to cancel a flow rate change accompanying a change in the amount of molten metal in a holding container and obtain a powder having a uniform particle size.

[0009]

In order to achieve this object, a molten metal outflow nozzle according to the present invention blows a high pressure gas onto a molten metal descending flow to divide the molten metal into fine droplets and cool the droplets. When the metal powder is used as the metal powder, a gas inflow hole communicating the inside of the melt nozzle with the external space is formed in the side wall of the melt nozzle, and a shutter mechanism capable of changing the opening area of the gas inflow hole is provided. And

In the method of producing metal powder, when the molten metal is caused to flow down by using the molten metal nozzle, the high pressure gas is blown while changing the opening area of a gas inflow hole for communicating the inside of the nozzle with an external space. It is characterized in that the molten metal is atomized by the method.

[0011] As a gas ejection nozzle for spraying a high-pressure gas to the molten metal, a nozzle arranged around the molten metal nozzle and having a plurality of gas ejection holes concentrically opened so as to eject the high-pressure gas in an inverted conical shape. Is used. Alternatively, instead of forming a plurality of gas ejection holes, a gas ejection nozzle having a circumferential gas ejection slit centered on the molten metal nozzle may be formed in the gas ejection nozzle.

[0012]

By providing a gas inlet hole communicating with the external space on the side wall of the molten metal nozzle and providing a shutter mechanism for changing the opening area of the gas inlet hole, the suction force due to the gas flow ejected from the gas outlet hole is reduced. Works on the downward flow of the molten metal at a constant rate according to the opening area. Therefore, when adjusting the suction force so as to cancel the flow rate change of the molten metal descending flow that fluctuates according to the level of the molten metal in the molten metal holding container, the molten metal can flow down from the molten metal nozzle at a constant flow rate.

The suction force is adjusted by changing the opening area of the gas inflow hole provided on the side wall of the molten metal nozzle. For example, when the flow rate of the molten metal descending flow decreases due to a decrease in the level of the molten metal in the molten metal holding container, the suction force acting on the molten metal descending flow is increased by reducing the opening area,
If the flow rate is increased to cancel the change, the flow rate of the molten metal descending flow is always kept constant.

Since the high-pressure gas is blown against the downward flow of the molten metal having a constant flow rate in this manner, the atomizing conditions are stabilized, and the particle size distribution of the obtained metal powder becomes sharp with little variation. As a result, it is possible to obtain a metal powder having a desired particle size in a high yield.

As the atomizing apparatus, for example, an apparatus having a facility structure schematically shown in FIG. 1 is used. The molten metal 13 to be atomized is stored in the molten metal holding container 11,
The superheat degree is maintained at 100 ° C. higher than the liquidus line by the high frequency coil 12. The molten metal outflow nozzle 14 according to the present invention is incorporated in the bottom wall of the molten metal holding container 11. In the molten metal outflow nozzle 14, a gas inflow hole 16 for communicating the inside of the molten metal outflow nozzle 14 with an external space is formed in a side wall immediately below the bottom wall of the molten metal holding container 11. Further, a shutter 17 that can change the opening area S of the gas inflow hole 16 is provided outside the gas inflow hole 16. Further, as a material of the molten metal outflow nozzle 14, it is preferable to use ceramics or the like having low wettability to the molten metal 13.

A gas jet nozzle 20 is arranged below the molten metal outflow nozzle 14 so as to surround the molten metal outflow nozzle 14. An annular distribution chamber 21 is provided inside the gas ejection nozzle 20.
Is provided, and a gas introduction pipe 22 connected to a gas supply source (not shown) opens to the distribution chamber 21. Also,
A plurality of gas ejection holes 23 are formed concentrically from the distribution chamber 21 at an angle inclined downward toward the molten metal outflow nozzle 14, and the ejected high-pressure gas flow 26 is provided immediately below the lower end of the molten metal outflow nozzle 14. An inverted conical flow having an apex is formed. Also, the opening 2 of the gas ejection hole 23
4 are located on the same horizontal plane as shown in FIG.

As shown in FIG. 3A, the gas ejection holes 23 are formed at equal intervals on a circumference centered on the molten metal outflow nozzle 14. Alternatively, as shown in FIG. 3B, a circumferential gas ejection slit 25 may be formed in the gas ejection nozzle 20.

Using a molten metal outflow nozzle 14 having a diameter of 4 mm,
FIG. 4 shows a change in the mass flow rate of the molten metal descending flow 15 when the flow area of the gas flow 26 is 90 liter / second and the opening area S of the gas inflow hole 16 is changed from 0 to ²⁰ mm ² . In addition,
At this time, the projecting length L of the molten metal outflow nozzle 14 was 8 mm.

As shown in FIG. 4, when the molten metal outflow nozzle 14 according to the present invention is used, the mass flow rate of the molten metal descending flow 15 changes at a constant rate by changing the opening area S of the gas inflow hole 16. However, it can be seen that the mass flow rate control of the molten metal descending flow 15 is sufficiently possible.

[0020]

EXAMPLE A high-speed tool steel equivalent to JIS standard SKH51 was used as a material manufactured by atomization into a metal powder. 10 kg of tool steel is stored in the molten metal holding container 11,
The molten metal 13 was prepared by maintaining the superheat degree by 100 ° C. higher than the liquidus line by the high frequency coil 12. Molten holding container 1
A molten metal outflow nozzle 14 having a gas inflow hole 16 and a shutter 17 capable of changing an opening area S of the gas inflow hole 16 was provided on the bottom wall of the first metal melt. The molten metal outflow nozzle 14 is made of boron nitride (BN) having a small wettability to the molten metal 13 and has a diameter of 4 mm.

The gas descending flow 15 flowing out of the molten metal outflow nozzle 14 was atomized by blowing nitrogen gas from a gas ejection nozzle 20. Gas flow 2 at this time
The flow rate of No. 6 was 90 liter / sec. The gas flow 26
Has a vertical angle of 30 degrees.

As a means for adjusting the opening area S of the gas inlet 16, the level of the molten metal 13 in the molten metal holding vessel 11 is detected by a liquid level gauge 30, and based on the detection result, the gas inlet 1 is detected.
The method of controlling the opening area S of No. 6 was adopted. That is,
The liquid level detected by the liquid level gauge 30 is input to the control circuit 31 as a detection signal a. The control circuit 31 receives, as a reference signal b, a relationship between a predetermined level of the molten metal surface and the amount of molten metal flowing out, and a relationship between an opening area S of the gas inflow hole 16 described later and a suction force.

The input detection signal a is compared with a reference signal b, and the suction area applied to the molten metal descending flow 15 flowing down the molten metal nozzle 14 is adjusted so that the flow rate is constant. Calculate S. The calculation result was input to a shutter driving device (not shown) as a control signal c, and the opening area S was controlled by moving the shutter 17.

The molten metal level is such that the molten metal 13 is
As it flows out through the pipe, the pressure drops as shown in FIG. Since the static pressure of the molten metal applied to the vicinity of the molten metal nozzle 14 is reduced with the decrease of the molten metal level, the molten metal 13
The force to flow down becomes weak. Therefore, the gas inflow hole 16
When a conventional melt nozzle having no melt flow was used, the flow rate of the melt descending flow 15 from the melt nozzle 14 decreased as shown by the flow rate I in FIG.

Therefore, in order to counteract this decrease in the flow rate, the opening area S of the gas inflow hole 16 formed in the molten metal nozzle 14 was adjusted stepwise from 20 m ² to 0 m ² . As a result, the suction force by the high-pressure gas increases, and the molten metal descending flow 1
5 were promoted. As a result, as shown as the flow rate II in FIG. 5, a significant decrease in the flow rate was suppressed even at the end of the outflow when the level of the molten metal dropped.

Under these conditions, the jet pressure of the gas stream 26 is set to 10
FIG. 6 shows the particle size distribution of the product powder at 0 kgf / cm ² . Also, the particle size distribution of the product powder when the conventional melt nozzle is used under the same conditions is shown as a comparative example. The powder produced by the method according to the present invention has an average particle size of 60 μm
And a particle size range usable for powder rolling, that is, 30 to 8
The powder yield between 0 μm was 85%.

On the other hand, when atomizing under the same conditions using a conventional molten metal outflow nozzle which does not have the gas inflow hole 16 and the shutter 17 which can change the opening area S of the gas inflow hole 16, the obtained metal is obtained. The particle size distribution of the powder, as shown as a comparative example in FIG.
The particle size was slightly reduced to μm, but the sharpness of the particle size distribution was poor, and the yield of the powder between 30 and 80 μm was only 67%.

As is clear from this comparison, by adjusting the opening area S of the gas inflow hole 16 formed in the molten metal nozzle 14, it is possible to efficiently obtain metal powder having a uniform particle size.

In the above example, the molten metal nozzle 1
Although the static pressure of the molten metal applied to the molten metal 13 flowing down 4 is obtained from the level of the molten metal, a change in the static pressure of the molten metal can be calculated from a change in weight by attaching a load cell to the molten metal holding container 11 instead.

[0030]

As described above, according to the present invention, a gas inflow hole is formed in the molten metal outflow nozzle and the opening area of the gas inflow hole is adjusted to reduce the flow of the molten metal downflow by the atomizing gas flow. Suction is controlled to suppress fluctuations in the flow rate of the melt downflow. As a result, the high-pressure gas is blown onto the downflow of the molten metal having a constant mass flow rate and is atomized, so that the shearing force and the quenching effect applied to the molten metal are averaged, and a high-yield metal powder having a uniform particle size is obtained. It becomes possible.

[Brief description of the drawings]

FIG. 1 schematically shows an atomizing device used in the present invention.

FIG. 2 shows a relationship between a molten metal outflow nozzle and a gas ejection nozzle according to the present invention.

FIG. 3 shows a gas ejection nozzle in which a plurality of gas ejection holes are arranged concentrically and a gas ejection nozzle in which a circumferential gas ejection slit is formed.

FIG. 4 is a graph showing a relationship between an opening area of a gas inflow hole and a mass flow rate of a downflow of a molten metal.

FIG. 5 shows changes over time in operating conditions during atomization.

FIG. 6 is a graph showing the particle size distribution of the obtained metal powder.

[Description of sign]

11 Molten holding container 12 High frequency coil
13 molten metal 14 molten metal outflow nozzle 15 molten metal descending flow
16 Gas inlet 17 Shutter 18 Droplet
Reference Signs List 20 gas ejection nozzle 21 distribution chamber 22 gas introduction pipe
23 Gas ejection hole 24 Opening 25 Gas ejection slit
26 High-pressure gas flow l Length of the molten metal outflow nozzle 14 protruding from the opening surface θ Apex angle of the cone formed by the gas flow 26 S Opening area of the gas inflow hole

Continuation of the front page (56) References Japanese Utility Model Sho 51-133320 (JP, U) Japanese Patent Publication No. 2-15601 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22F 9 / 08

Claims (2)

(57) [Claims] 1. A high-pressure gas is blown onto a downflow of a molten metal to divide the molten metal into fine droplets, and when the droplets are cooled to form metal powder, a molten metal nozzle provided at the bottom of the molten metal container is used. Then, the inside of the melt nozzle is communicated with the external space,
And a shutter mechanism for changing an opening area of the gas inflow hole, the gas inflow hole being provided in a side wall portion of the nozzle and communicating the inside of the molten metal nozzle with an external space, and a shutter mechanism. 2. A method in which a high-pressure gas is blown onto a downward flow of a molten metal to divide the molten metal into fine droplets, and when the droplets are cooled to form metal powder, the molten metal flows down from a molten metal nozzle provided at the bottom of the molten metal container. A method for producing metal powder, characterized in that an opening area of a gas inflow hole formed in a side wall portion of the molten metal nozzle is changed during gas blowing so that a flow rate of the molten metal descending flow is constant.