JPH0722174A - Dc arc generating method - Google Patents

Abstract

PURPOSE:To make cooling of arcs simply and effectively and give arcs a high voltage through utilization of the heat pinch effect by introducing gas to between a flow regulating cylinder and an electrode, and allowing the gas to jet out from the peripheral part of the foremost of electrode. CONSTITUTION:DC arcs A of transition type are generated by an electrode between its foremost and an object to be heated 2. The electrode 1 is connected with the negative pole of the power supply 5, while its positive pole is connected with the object 2. On the outside of the electrode 1, a gas flow regulating cylinder 7 surrounding the electrode 1 is installed with a certain gap reserved in between, and into it a gas G is introduced from the base end side of the electrode 1. The gas G is allowed to jet toward the tip from the periphery of the foremost of electrode 1 and surrounds the arcs A. The cylinder 7 produces a gas flow toward the object surrounding the arcs A generated between the electrode 1 and object 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for generating a DC arc used as a heating source in the fields of melting, smelting and steelmaking.

[0002]

2. Description of the Related Art Plasma or arc is used as a heating source in the fields of melting, smelting and steelmaking.
Plasma is used when cleanliness is required, the potential gradient in the vicinity of the torch is higher than that of an arc, and a high voltage can be easily obtained. Therefore, it is easy to increase the output by increasing the voltage.
However, there is a drawback in that the plasma generating cheech is expensive and maintenance is complicated. On the other hand, the arc generating electrode is cheaper than the plasma generating torch, structurally simple, and easy to maintain.

When plasma is used for heating, an example of DC plasma stress is known. For example, there is a transfer-type plasma that generates an arc between a solid plasma torch and an object to be heated ¹⁾ . Here, the plasma torch side is usually the cathode,
The heating object side is the anode.

On the other hand, when an arc is used for heating, a solid electrode of graphite is generally used ²⁾ , and the electrode is usually the cathode and the side of the object to be heated is the anode.

1) Examples of Potential for Industr
ial Applications: HGMuller, ISPC-9 p50〜59,1989 2) Recent trends in arc furnace equipment for steelmaking and melting technology: Noda et al., Iron and Steel 1977 (1991) No. 6, p6-12

The present invention deals with arc heating, which simplifies equipment and maintenance as compared with plasma heating.

In arc heating, the electric power applied to the arc is represented by the product of current and voltage. Therefore, in order to increase the input power, it is necessary to increase the current and the voltage.

Among the methods of increasing the applied power, the method of increasing the current requires that the wiring inside and outside the power source be made large in diameter, and when the current becomes large, the current density at the electrodes increases, resulting in severe electrode consumption. There's a problem. The increase in output by increasing the voltage is advantageous in this respect. However, since the arc has a lower potential gradient in the vicinity of the torch than the plasma, some measure is required to increase the output by increasing the voltage.

As a method of increasing the arc voltage, there is a method of cooling the surface of the arc and increasing the voltage by the thermal pinch effect. A method of increasing the arc voltage by increasing the arc length is also known.

Among these methods of increasing the arc voltage, the method of cooling the arc surface utilizing the thermal pinch effect is as follows:
Since a device such as a gas feeding nozzle for cooling tends to be complicated, a simple cooling method is required.

A first object of the present invention is to easily and efficiently cool the arc and utilize the thermal pinch effect to increase the voltage of the arc.

In the method of increasing the arc voltage by increasing the arc length, the instability of the arc occurs as the arc length increases, and therefore it is necessary to stabilize it.

A second object of the present invention is to suppress the instability of the arc that occurs when the arc length is increased to increase the applied power and a higher voltage is achieved.

Instability of the arc when the arc length is increased occurs as follows.

While the arc length is long, a plasma stream is generated near the cathode spot, flows from the cathode toward the anode, shapes the arc column, and stabilizes the arc.

As the arc length becomes longer, as shown in FIG. 3, both the cathode point c (usually the pole point on the side of the electrode 1) and the anode point a (usually the pole point on the side of the object to be heated 2) are very small spots. Therefore, since the arc column A becomes thicker than these spot diameters, a difference occurs in the pressure generated by the magnetic force, and the plasma air flows P1 and P2 are generated. That is, the plasma flow P is directed from the cathode point c toward the anode and from the anode point a toward the cathode P.
1 and P2 respectively occur. As a result, the plasma columns P1 and P2 collide with each other in the arc column A to form a complicated flow field.

Further, the anode point a generally has a property of being easily moved, and instantly moves to a place where it is more likely to occur. This movement of the anode point a gives the arc column A a bend.

Due to these causes, the balanced arc column A also comes out of the stable state. Lines of magnetic force are generated around the arc column A due to the current flowing in the arc. Once the arc column A is bent, the line density of the magnetic field becomes high inside the bend and the magnetic pressure becomes large, while the line outside the bend. Then, the magnetic line density becomes low and the magnetic pressure becomes small. As a result, as shown in FIG. 4, a force F that pushes the arc column A from the inside to the outside of the bend is generated due to the difference in magnetic pressure.
Therefore, the turbulence is further promoted and the arc becomes unstable.

This unstable arc damages surrounding equipment such as refractories and poses a problem. Occasionally, arc breakage occurs, which seriously affects the operation. In addition, the rated voltage of the power supply must be set to a high value in consideration of the fluctuation of the arc voltage due to the instability of the arc, and a high-voltage power supply is prepared and it is forced to be used with low efficiency. This is because arc breakage occurs in which the arc voltage reaches the rated voltage of the power supply.

Therefore, it is essential to stabilize the arc in the method of increasing the arc voltage for increasing the arc length.

As a method for stabilizing the DC arc, there is a fluid method in which a hollow electrode 3 is used as shown in FIG. 5 and a gas G is ejected from a hollow hole 4 thereof to extend and stabilize the arc A ^3). .

Also, a magnetic method has been proposed in which a plurality of electrodes are used, an arc is generated at each electrode, and the attractive force between the arcs is used to restrain and stabilize the arcs ⁴⁾ .

3) Japanese Patent Laid-Open No. 51-121409 4) Japanese Patent Application No. 3-216385

When the shape of the electrode is hollow and the arc G is generated while the gas G is ejected from the hollow hole 4 of the electrode 3,
This gas G assists the flow in the direction from the electrode 3 to the object to be heated 2 and suppresses the plasma air flow P2 in the opposite direction, whereby the arc A can be easily stabilized. However, as shown in FIG. 6, the cathode point c is generated from a point deviated from the central axis of the electrode 3 and is located at the edge of the gas ejection port. Therefore, the arc A is ejected from the electrode 3 as shown in FIG. The effect of stabilization is small because it does not efficiently flow in the flow of gas G.

Further, in the method of preparing a plurality of pairs of electrodes and power supplies and generating a plurality of arcs to stabilize each other's arcs by the magnetic attraction effect, when the power supplies are effective or the installation space for the power supplies is limited. Is disadvantageous. That is, the fluid method is more advantageous in terms of cost and space.

Therefore, it is desired to suppress the instability of the arc, which is caused when the arc length is increased and the voltage is increased in order to increase the input power, by another fluid method.

In arc heating, the amount of heat that contributes to heating is the product of the output of the arc and the heating efficiency. Therefore, in addition to increasing the output of the arc, the heat generated by the arc is efficiently applied to the object to be heated. It is also important.

In the present invention, a value obtained by dividing the amount of heat received by the object to be heated by the amount of heat generated by the arc is called "heating efficiency", and the improvement of the heating efficiency is the third object.

The heat generated in the arc column is used for direct heating / radiant heating of the object to be heated, heating of atmospheric gas (loss), heating of peripheral equipment (radiation loss), and the like. In the case of an arc generated by a conventional solid electrode, as described above, there is a flow in the direction from the object to be heated to the electrode, the direct heating efficiency is poor, and the heating efficiency is low. Therefore, a method for improving heating efficiency is required.

[0030]

The problems to be solved by the present invention are summarized as follows.

The arc is cooled easily and efficiently, and the voltage of the arc is increased by utilizing the thermal pinch effect.

The arc is stabilized by extending the arc length and increasing the voltage.

Increase heating efficiency.

[0034]

According to the present invention, when a transfer type DC arc is generated between an electrode tip and an object, a gas rectifying cylinder is arranged so as to surround the electrode, and the rectifying cylinder and the electrode are connected to each other. A gist of a direct current arc generation method is characterized in that a gas is introduced between the electrodes and the introduced gas is ejected from the peripheral portion of the electrode tip.

One embodiment of the method of the present invention is shown in FIG.
An example of the structure of the electrodes is shown in FIG.

In the method of the present invention, an inexpensive consumable solid electrode 1 made of, for example, graphite is used, and its tip and the heating object 2 are used.
A transfer type direct current arc A is generated between and. The electrode 1 is connected to the negative pole of the power source 5, and the positive pole of the power source 5 is connected to the heating object 2. Here, the heating target 2 is a container 6
Molten steel and the like housed inside.

On the outside of the electrode 1, a gas rectifying cylinder 7 surrounding the electrode 1 is installed with a gap. Gas G is introduced into the gap from the base end side of the electrode 1. The introduced gas G is ejected from the periphery of the tip portion of the electrode 1 toward the tip side and surrounds the arc A.

The shape of the flow straightening cylinder 7 may be a square cylinder or a cylinder as shown in FIG. 2, or a cylinder of another shape. The point is that it can form a gas flow to the heating target side that surrounds the arc A generated between the electrode 1 and the heating target 2.

The gas to be used is not particularly limited, but an inert gas such as Ar is used when cleaning of the object to be heated is required. However, in order to increase the voltage of the arc, it is preferable to use a gas of polyatomic molecules such as N ₂ , CO ₂ , and O ₂ because the potential gradient of the arc column becomes large.
However, in consideration of prolonging the life of the electrode, use of O ₂ gas causes oxidative consumption, which is not preferable. As described above, it is desired to select the gas type according to the situation. Table 1 summarizes the gas selection indexes.

[0041]

[Table 1]

With respect to the flow rate of gas, the larger the flow rate, the greater the effect of the present invention. The upper limit is determined according to economy, device capacity, etc., and is not particularly limited.

Regarding the size of the rectifying cylinder, the effect of the present invention can be further enhanced when the rectifying cylinder approaches the inner electrode and covers the electrode tip extension. However, when the electrode is brought close to the electrode, the rectifying cylinder approaches the arc column and may be damaged. Therefore, it is necessary to take measures such as water cooling, which is not preferable from the viewpoint of avoiding complication of the torch shape. Also, in the case of a shape that covers the electrode tip extension part, it is difficult to perform the arc start by touch start, so perform the arc start with the electrode exposed and keep the electrode tip extension part covered during operation. Ingenuity is needed. Also, if the cover is too extreme, a good electrical conductor cannot be used as the material of the cylinder. This is because the arc is generated from the tip of the cylinder (metal or the like).
It is desirable to set the specific dimensions of the flow straightening cylinder in consideration of the above.

As for the material of the rectifying cylinder, since it is used in the vicinity of the arc column, a material having a high melting point and excellent in thermal shock resistance is preferable in consideration of use at high temperature. Further, for high efficiency heating, it is preferable not to perform water cooling of the flow straightener if possible.

Examples of the material of the flow straightening cylinder are as follows. High melting point metals such as W and Mo. Although it has a high melting point, it is easily oxidized, and therefore it is not preferable to use it in an atmosphere containing O ₂ . High alloy Although it has a lower melting point than refractory metals, it is highly adaptable to the atmosphere. Water-cooled tube Water-cooled using a material with good thermal conductivity such as copper.

Besides, in order to improve the characteristics, it is also effective to coat the inner surface of the cylinder with a high temperature resistant heat insulating material such as ceramics (for example, ZrO ₂ —Y ₂ O ₃ ). With respect to the above, there is an effect of preventing a decrease in heating efficiency (reduction of loss).

[0047]

In the method of the present invention, the following effects can be obtained for cooling and stabilizing the arc and increasing the heating efficiency.

Regarding Cooling of Arc According to the method of the present invention, the gas is introduced between the electrode and the rectifying cylinder, and the gas is ejected from the peripheral portion of the electrode tip, whereby the arc column can be efficiently cooled, and the arc voltage is generated by the thermal pinch effect. Can be increased.

For comparison, the structure of the plasma arc generating torch is shown in FIG. 8 is a torch for plasma arc generation,
Reference numeral 9 is a water cooling nozzle, 10 is an anode, and 11 is a cathode.

The plasma arc generating torch is a torch having an efficient arc cooling function, but usually has a complicated torch structure. When the plasma arc passes through a water-cooled nozzle, the arc surface is sufficiently cooled and the arc voltage is increased by the thermal pinch effect. However, as a heater used in the field of iron making, the torch and the object to be heated are often used with a distance of about 100 mm or more.

Assuming that the plasma torch is used at a sufficient distance from such a torch, even if there is a great effect of the thermal pinch in the vicinity of the torch in the plasma torch, the effect exerted on the entire arc. Will be small.
Therefore, when a long arc is to be generated, it is not necessary to exhibit the thermal pinch effect of the arc as rapidly as the narrowing of the arc column by the nozzle performed by the plasma torch.

Industrially, it is not necessary to have a structure as complicated as that of a plasma torch, and electrode wear and the like noticeably affect the plasma arc characteristics, so that it is necessary to replace expensive parts with graphite. Consumable electrodes represented by electrodes can be used, and arc heating, which has a simple structure and is easy to repair, is advantageous.

Also, in the case of arc heating, when a hollow electrode is used and gas is ejected from the hollow hole, the effect of cooling the arc column can be obtained. However, the cooling is performed from the arc column and from one side, and since the arc column itself can be moved in the direction away from the gas flow (outer side), its effect as compared with the case of cooling from the periphery of the method of the present invention. Is inferior.

Regarding Arc Stabilization According to the method of the present invention, gas is introduced between the rectifying cylinders and ejected from the periphery of the electrode tip, so that the arc is efficiently placed on the flow of the ejected gas, and the arc is applied from the electrode side. It is possible to stabilize the arc by supporting the plasma airflow to the object and suppressing the reverse plasma airflow generated from the object side.

Since the arc column itself is constrained by the gas ejected from the peripheral portion of the electrode tip, the range of movement of the anode point (usually the pole point on the object side) is narrowed, and the cause of instability is reduced. Therefore, the arc can be stabilized more effectively than when the gas is ejected from the hollow hole of the hollow electrode.

Further, in the case of a plasma torch, the nozzle portion may be sharply narrowed down, and when a long arc is to be generated, the gas flow generated by conditions such as the gas flow rate contributes to the stability of the arc. It may not be done.

Regarding the increase of heating efficiency According to the method of the present invention, gas is introduced between the electrode and the rectifying cylinder and is ejected from the peripheral portion of the electrode tip, so that the flow in the vicinity of the arc column is directed from the electrode to the object to be heated. Since the arc is blown and the arc itself follows the flow, the efficiency of direct heating of the object to be heated is increased, and the heating efficiency can be increased.

The gas released from the electrode side to the side of the heating target changes the flow direction to the outside in the vicinity of the heating target. In the method of the present invention, since there is a gas flow around the arc column, the change in the flow hardly affects the arc column. In contrast,
In the case of the method of ejecting gas from the hollow hole of the hollow electrode,
Since the arc column is caused to flow laterally by the gas flow to the outside in the vicinity of the object to be heated, the heating (direct heating) due to the arc column being directly blown onto the object to be heated becomes inefficient.
The heating efficiency is not as excellent as the method of the present invention.

[0059]

EXAMPLES Examples and comparative examples of the present invention will be described below.

[0060]

[Table 2]

Table 2 shows an outline of the experimental apparatus. A power supply having a rated voltage of 400 V and an output current of 300 A was used. The test electrode is made of graphite and has a cylindrical straightening tube (inner diameter 16 mmφ) shown in FIG. 1 (outer diameter 8 mmφ), a conventional solid electrode without a straightening tube (outer diameter 8 mmφ), Four types were used: a hollow electrode having an outer diameter of 8 mmφ and a single through hole of 4 mmφ provided on the central axis, and a DC plasma electrode having a nozzle diameter of 6 mm shown in FIG. The atmosphere in which the arc was generated was 100% Ar and the arc current was 300A. Further, a carbon steel block was used as the object to be heated.

In the experiment, the arc start was carried out by touch start in which the electrode was once brought into contact with the object to be arced and then separated, and then the arc length was gradually extended to measure the arc voltage. In the case of the plasma torch, the arc voltage was measured by shortening the distance to the object and gradually increasing the distance.

Effect of Increasing Arc Voltage By the method of the present invention, the arc can be efficiently cooled, and higher voltage and higher output can be achieved. The data are shown below.

FIG. 9 shows the arc voltage for each electrode when the arc length is 50 mm.

The arc voltage was 32 V in the conventional solid electrode having no rectifying cylinder. When a hollow electrode was used and Ar gas was introduced into the electrode at 50 liter / min, the arc voltage was 34V. Using a solid electrode surrounded by a rectifying cylinder,
In the case of the method of the present invention in which Ar gas was introduced at 50 liter / min between the electrode and the rectifying cylinder, the arc voltage was 45V. In the case of a plasma torch using Ar gas of 50 liter / min as the plasma gas, the voltage is 50 V, which is 11% superior to the method of the present invention.

FIG. 10 shows the arc voltage for each electrode when the arc length is 100 mm.

With a conventional solid electrode, the arc voltage is 40V.
Met. When a hollow electrode was used and Ar gas was introduced into the electrode at a rate of 50 l / min, the arc voltage was 43V.
Use an electrode surrounded by a rectifying cylinder, and use Ar between the electrode and the rectifying cylinder.
In the case of the method of the present invention in which gas was introduced at 50 liter / min, the arc voltage was 74V. In the case of a plasma torch using Ar gas of 50 liter / min as the plasma gas, the arc voltage is 78 V, which is 5% compared with the method of the present invention.
Stay on top of.

As described above, according to the method of the present invention, the arc voltage can be increased and the output can be increased as compared with the conventional method. Compared with the case where the conventional solid electrode is used, the increase is 41% when the arc length is 100 mm. Further, as compared with the plasma torch, the longer the arc length is, the less advantageous the plasma torch is, and the significance of use of the arc generating method of the present invention is increased.

About Arc Stabilizing Effect The method of the present invention can stabilize the arc, and can increase the voltage and the output by increasing the arc length. The data are shown below.

FIG. 11 shows the relationship between the variation of the arc voltage and the time at a certain arc length. If the arc becomes unstable, the arc voltage fluctuates in this way. The degree of arc instability appears as the fluctuation range of the arc voltage. Also, arc breakage occurs when the fluctuating arc voltage reaches the upper limit of the supply voltage of the power supply. Therefore, during operation, the operating value of the arc voltage must be set so as not to exceed the upper limit of the supply voltage of the power source minus the fluctuation range.

In the following experiments, the fluctuation range of the arc voltage divided by the minimum value of the arc voltage at that time was used as the fluctuation ratio of the arc voltage, which was used as a standard for evaluating the stability of the arc.

FIG. 12 shows the examination result. Conventional solid electrodes are the least stable, followed by hollow electrodes. The same degree of stability is obtained when an arc is generated by the method of the present invention and when it is generated by a plasma torch. As described above, the method of the present invention showed excellent stability as compared with the case of using the conventional electrode. Assuming that the heating environment allows a fluctuation ratio of up to 20%, the arc length is 120 mm for the conventional solid electrode and 16 for the hollow electrode.
Can only be used up to 0 mm. With the method of the present invention, it is possible to use up to 250 mm, which is more stable than 230 mm in the case of the plasma torch.

The arc voltage at these usage limits is
As shown in FIG. 13, the conventional solid electrode, the hollow electrode, the solid electrode with a rectifying cylinder, and the plasma torch are arranged in this order in the order of 4
The arcs produced by the method of the present invention showed the best results at 2V, 49V, 131V and 128V.

By thus stabilizing the arc, not only damage to the peripheral equipment of the object to be heated can be reduced, but also the output can be greatly increased by increasing the upper limit of the arc length increase. Using the method of the present invention, an output improvement of 212% was achieved when using a conventional solid electrode.

Regarding heating efficiency improvement effect The heating efficiency can be improved by the method of the present invention.

A carbon steel block was heated for 5 minutes using an arc having an arc length of 100 mm, and the calorie obtained was measured using a calorimeter. The heating efficiency was calculated by dividing the amount of heat by the amount of electric power input. FIG. 14 shows the result of heating efficiency for each electrode.

When the conventional solid electrode was used for heating, the heating efficiency was 36%. A in the electrode using a hollow electrode
When r gas was introduced at 50 liter / min, it was 43%. Using an electrode having a rectifying cylinder, A is provided between the electrode and the rectifying cylinder.
Heating efficiency is 49% when introducing 50 l / min of r gas
Became. Use a plasma torch and set the plasma gas to A
When r gas was used at 50 l / min, it was 47%.

As described above, the heating efficiency can be improved by the method of the present invention.

[0079]

The DC arc generating method of the present invention has the following effects. The arc can be cooled efficiently, the arc voltage can be increased by the heat pinch effect, and high output can be achieved.

The arc can be stabilized. This makes it possible to use a long arc, so that the arc length can be lengthened to increase the output by increasing the voltage of the arc. As the arc has less fluctuation, damage to surrounding equipment can be reduced,
Since the fluctuation of the arc voltage is small, the margin of the arc voltage with respect to the rated voltage of the power supply can be reduced, and the power supply can be used highly efficiently.

The heating efficiency can be improved and the amount of heat loss can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic view showing one embodiment of the method of the present invention.

FIG. 2 is a schematic view showing a structural example of an electrode used in the method of the present invention.

FIG. 3 is a schematic diagram showing a plasma airflow generated near a pole.

FIG. 4 is a schematic diagram showing a force acting on an arc due to a difference in magnetic pressure.

FIG. 5 is a schematic diagram showing an arc stabilizing action when a hollow electrode is used.

FIG. 6 is a schematic view showing positions of cathode spots on a hollow electrode.

FIG. 7 is a schematic diagram showing an arc shape when a hollow electrode is used.

FIG. 8 is a schematic view showing the structure of a plasma torch.

FIG. 9 is a graph comparing arc voltages when the arc length is 50 mm.

FIG. 10 is a graph comparing arc voltages when the arc length is 100 mm.

FIG. 11 is a graph showing how the arc voltage fluctuates due to arc instability.

FIG. 12 is a graph comparing arc stability.

FIG. 13 is a graph comparing arc voltages at the maximum arc length when the arc voltage fluctuation ratio is 20% or less.

FIG. 14 is a graph comparing heating efficiencies.

[Explanation of symbols]

 1 Solid Electrode 2 Object to be Heated 3 Hollow Electrode 5 Power Supply 7 Gas Rectifier A Arc G Gas

Claims (1)

[Claims] 1. A gas rectifying cylinder is arranged so as to surround the electrode when a transfer DC arc is generated between the electrode tip and an object, and gas is introduced between the rectifying cylinder and the electrode. A method for generating a direct current arc, characterized in that gas is ejected from the periphery of the electrode tip.