JP2626775B2 - Fixed electrode for plasma arc - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a fixed electrode for generating a plasma arc, and more particularly, to a plasma torch when a molten steel in a tundish is plasma-heated in a continuous casting process. The present invention relates to a fixed electrode provided in a tundish to generate a plasma arc between molten steels.

(Conventional technology) In the continuous casting method, in order to prevent carbon segregation and the like and to produce high-quality steel, it is required to reheat the molten steel in the tundish and maintain the molten steel temperature at a predetermined temperature.

Conventionally, various methods, such as induction heating and electric heating, have been proposed for reheating molten steel in a tundish. One of them is plasma heating.

In plasma heating, high-density heat is generated by a mechanically and electrically converged arc, and the heat is used to reheat molten steel. This plasma arc includes a transfer arc generated between the main electrode and the molten steel, and a non-transfer arc generated between the main electrode and the nozzle in the torch, but high output is possible to reheat the molten steel in the tundish. Transfer arc method is adopted. In this transfer arc method,
There are a DC plasma system using a DC power supply and an AC plasma system using an AC power supply. However, the AC plasma method requires a plurality of torches, so the initial cost increases.

On the other hand, the DC plasma method shown in FIG. 5 has the advantage that only one torch is required, but on the other hand, it gives a potential difference between the main electrode 2 in the torch 1 and the molten steel 3 in the tundish. 4 must be installed.

According to this DC plasma method, at the time of ignition, a current is supplied between the nozzle 5 and the main electrode 2 in the torch 1 by the auxiliary power supply 6, and a starter arc 7 is generated between the nozzle 5 and the main electrode 2 by a non-transfer arc method. Next, a main arc 9 is generated between the main electrode 2 and the molten steel 3 by a transfer arc method using a DC power supply 8. At this time, if the voltage drop due to the electric resistance of the fixed electrode 4 is large, the arc is interrupted when the starter arc 7 and the main arc 9 are switched. Therefore, the material of the fixed electrode 4 must be a material having a small electric resistance. In addition, the performance of the fixed electrode 4 is required to be durable against molten steel 3 and slag in the tundish and not to pollute the molten steel 3.

Conventionally, a steel electrode has been used for the fixed electrode.

(Problems to be solved by the invention) However, the fixed electrode made of steel is significantly damaged by molten steel,
There is a drawback that the fixed electrode must be replaced frequently, resulting in poor workability.

The present invention has been made in view of the above problems, and has as its object to provide a fixed electrode that is excellent in durability, has low electric resistance, and does not stain molten steel.

The present inventor, in order to achieve this object, carbon, magnesia and carbon-based refractories, silicon carbide and carbon-based refractories, zirconia, zirconia and carbon-based refractories, etc. As a result of repeated studies on various refractories, it has been found that refractories containing alumina and carbon as main components have extremely excellent properties as fixed electrodes disposed in a tundish.

(Means for Solving the Problems) The present invention has been made based on the above findings, and has a structure in which a refractory mainly composed of alumina and carbon, which is formed into a hollow rod and sintered, and an internal And a shaft core made of a good conductor having a long hole communicating with the outer peripheral surface, and a good conductor powder filled in the long hole, and the shaft core is fitted in a hollow portion of the refractory. The fixed electrode is the gist.

(Action) The refractory mainly composed of alumina and carbon has a small electric resistance, the voltage drop at the fixed electrode is extremely small, and the voltage drop hardly affects the plasma arc. Moreover, this refractory has excellent durability and has little risk of fouling the molten steel in the tundish.

When the temperature of the fixed electrode rises in the tundish, a gap is formed between the shaft core and the refractory due to the difference in the coefficient of expansion between the shaft and the refractory. Since it is buried by melting out on the outer peripheral surface, the conductivity between the two can be kept good.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. First
FIG. 1 is a cross-sectional view showing one embodiment of the present invention.
Is composed of a steel shaft 11 and a refractory 12 mainly composed of alumina and carbon. A screw portion 11a is formed in a part of the outer peripheral surface of the shaft core 11, and an elongated hole 11b penetrating from one end to the other end is formed in the inside. A plurality of branch holes 11c are connected to the elongated hole 11b, and the elongated hole 11c communicates the outer peripheral surface of the shaft core 11 with the elongated hole 11c.

The refractory 12 has a hollow rod structure with one end opened,
A threaded portion 12b screwed with the threaded portion 11a is formed on the threaded portion 12a, and a head portion 12c is formed on the other end, and is integrally fitted to the shaft core 11.

The soot powder 14 is filled in the long hole 11b, the space 13 defined inside the tip of the shaft core 11 and the inside of the head portion 12c.

Since the shaft core 11 and the refractory 12 have different expansion coefficients, a high temperature causes a gap between the shaft core 11 and the refractory, but this gap is formed by the elongated hole 11b and the soot powder 14 in the empty space 13 formed by the shaft core 11. Is melted out on the outer peripheral surface of the refractory, and the conductivity between the shaft core 11 and the refractory 12 is kept good even at a high temperature.

The refractory 12 is obtained by forming a raw material powder having a composition of 29 to 32% of carbon (C), 41 to 45% of alumina (Al ₂ O ₃ ), and 25 to 27% of silica (SiO ₂ ) into a hollow rod shape by a rubber press or the like. After molding and sintering, the head portion 12c is manufactured by grinding to a predetermined shape and dimensions.

FIG. 2 shows a characteristic curve of the temperature and the resistivity of the refractory 12 obtained by the experiment.

As is well known, the relational expression of R = ρ · l / A holds between the resistance value R and the resistivity ρ. Where l is the length of the resistor and A is the cross-sectional area. In the case of the fixed electrode 10 shown in FIG. 1, the diameter of the fixed electrode is 120 mm, the thickness of the refractory is 36 mm, and the depth at which the fixed electrode 10 is immersed in molten steel is 400 mm. When the resistance value is calculated by applying the rate ρ = 5.91 × 10 ⁻³ (Ωcm), R = 1.4 × 10 ⁻⁵ Ω. Therefore,
When the maximum value of the current supplied to the fixed electrode 10 is 1.6 K, the voltage drop of the fixed electrode 10 is an extremely low value of about 0.023 V, and there is no possibility that the voltage drop adversely affects the plasma arc.

Further, the refractory 12 having the above composition has excellent durability against erosion due to molten steel, and furthermore, there is little danger that carbon of the composition component will melt into the molten steel, so that it is injected not only as a fixed electrode but also from a tundish into a mold. It is also suitable for use as a stopper for controlling the flow rate of molten steel.

FIG. 3 shows a tundish for continuous casting in which the fixed electrode 10 having the above-mentioned structure is also used as a stopper.

In FIG. 3, reference numeral 20 denotes a ladle, from which molten steel is supplied to a tundish 30 from a nozzle 21 provided at the bottom. The tundish 30 is covered with a lid 31 for keeping heat, and the lid 31 has a heating chamber 32 formed therein. The plasma torch 1 described with reference to FIG. 5 penetrates the upper surface of the heating chamber 32.
It faces the molten steel 3 in 30. A molten steel outlet 33 is formed on the bottom surface of the tundish 30, and the molten steel 3 is connected to the molten steel outlet.
It is injected into a mold 35 through an immersion nozzle 34 attached to 33.

The fixed electrode 10 is provided to penetrate the lid 31 and face the molten steel outlet 33. The fixed electrode 10 is slidably mounted in the up-down direction. By moving the fixed electrode 10 toward and away from the molten steel outlet 33, the flow rate of the molten steel 3 flowing out of the molten steel outlet 33 to the mold 35 can be controlled. . In addition, fixed electrode
A DC power supply (not shown) is connected between the plasma torch 1 and 10.

The molten steel 3 supplied from the ladle 20 to the tundish 30
The heating chamber 32 is heated by a plasma arc radiated from the plasma torch 1 and is maintained at a predetermined temperature. And molten steel 3 flowing from the tundish 30 to the mold 35
Is controlled by sliding the fixed electrode 10 vertically.

According to the tundish 30 for continuous casting having such a structure,
Since the fixed electrode 10 also serves as a stopper, the structure of the tundish 30 is simplified, and this is particularly advantageous in remodeling an existing tundish to one capable of plasma heating.

Next, FIG. 4 shows a characteristic curve obtained by an experiment for another embodiment of the refractory mainly composed of alumina and carbon.

Refractories showing this characteristic curve are carbon (C) 28-31.
%, Alumina (Al ₂ O ₃ ) 52-56%, silica (SiO ₂ ) 5
It has a composition of 7% and silicon carbide (SiC) 4-5%. This refractory has a slightly higher resistivity than the refractory of the characteristic curve shown in FIG. 2, but when a fixed electrode is manufactured using the refractory, the effect of the voltage drop on the plasma arc is as described in the previous example. As in the examples, the results were also negligibly low, and satisfactory results were obtained with respect to durability and contamination of molten steel.

In the above-described embodiment, an iron core was used for the shaft core 11.However, a carbon electrode may be used instead.In addition to tin, the powder to be filled in the elongated hole 11b may be made of aluminum or heated. It is possible to use expanded graphite or the like which expands.

(Effect of the Invention) Since the fixed electrode for a plasma arc according to the present invention uses a refractory containing alumina and carbon as main components, the voltage drop is small, the durability is excellent, and the molten steel in the tundish may be contaminated. Also less. Moreover, it is also possible to use the fixed electrode as a tundish stopper,
The structure of the tundish for plasma heating can be simplified.

[Brief description of the drawings]

1 is a sectional view showing an embodiment of the present invention, FIG. 2 is a temperature-resistivity characteristic curve diagram of a refractory used in the present embodiment, and FIG. 3 is a sectional view of a tundish provided with the present embodiment. FIG. 4 is a temperature-resistivity characteristic curve diagram of another refractory, and FIG. 5 is an explanatory diagram of a DC plasma system. 10: fixed electrode, 11: shaft core, 11b: long hole, 12: refractory, 12a: hollow part.

Claims (1)

(57) [Claims] 1. A refractory having alumina and carbon as main components, which is formed and sintered into a hollow rod shape, and a shaft core made of a good conductor having a long hole communicating between the inside and the outer peripheral surface thereof. A fixed electrode for a plasma arc, wherein a long conductor is filled with a good conductor powder, and the shaft core is fitted in a hollow portion of the refractory.