JP2962150B2 - Bottom electrode for DC arc furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bottom electrode for a DC arc furnace for steelmaking.

[0002]

2. Description of the Related Art FIG. 6 is a sectional view showing the structure of a conventional bottom electrode for a DC arc furnace. In FIG. 6, 1 is a furnace bottom,
1a is a refractory laid on the bottom of the furnace, 2 and 3 are conductors that receive electric power from a power supply (not shown) and flow current, 4 is a terminal having a cooling air introduction path 4a, 5 is a base, 6 is a support base, 7 is a contact pin penetrating the support base 6 and projecting its head to the furnace bottom, 8 is an electrode block filled with an irregular refractory around the contact pin 7, 9 is a graphite electrode, 11 is molten steel. is there. As is clear from FIG. 6, the structure of the conventional furnace bottom electrode has a large number (about 200) of contact pins 7 and a base 5 for joining them at almost the center of the furnace bottom 1. It is connected to a power supply via a conductor 2. The contact pin 7 is surrounded by an electrode block 8, and the electrode block 8 is supported by a support base 6 and is provided at the center of the furnace bottom 1.

[0003] The conventional DC arc furnace bottom electrode has the above-described structure, and current is applied to the conductor 2, the conductor 3, the terminal 4, and the base 5.
Then, it flows to the graphite electrode 9 through the contact pin 7 to form an arc, and the raw material such as iron chips is melted by the heat of the arc to become molten steel 11 and the refining operation is performed. At this time, the cooling air passes through the cooling air introduction passage 4a, passes between the base 5 and the support base 6, and cools the contact pins 7.

[0004] The above-mentioned conventional DC arc furnace bottom electrode has the following problems. (1) If the electrode block 8 wears out with the lapse of the melting time and the remaining thickness becomes less than a certain thickness, it must be replaced. For this reason, it has been proposed to open the furnace when the state of wear of the electrode block 8 reaches a certain level, and to repair the extension of a large number of contact pins 7 exceeding 200. It is usually difficult to replace the entire electrode block 8 together with the pins 7, and there is a limit to extending the life of the electrodes. (2) In a DC arc furnace, current drift may occur due to fluctuations in electric resistance. When drift occurs, current concentrates on the contact pins 7 present in the drift range, so that current density becomes abnormally high. In addition, the Joule heat generated in the contact pins 7 accelerates the wear of the contact pins 7 and the electrode block 8 and shortens the life of the DC arc furnace bottom electrode.

Techniques similar to the present invention for solving this problem include Japanese Patent Publication No. Sho 62-46183 (hereinafter referred to as Prior Art 1) and Japanese Patent Laid-Open Publication No. Hei 2-13785 (hereinafter referred to as Prior Art 2). ).

In the technique of the first prior art, the life of the hearth is reduced from one month to two months by installing an electrode block 8 made of a highly refractory magnesium refractory brick on the upper surface of the stamp at the hearth of the DC arc furnace. It has been extended to 3 months.

In the technique of the prior art example 2, when repairing the bottom electrode, the molten steel is left in the concave portion above the eroded bottom electrode, and is immersed in the molten steel so that the upper and lower parts are prepared in advance. The repair electrode member with the steel pin protruding is placed on the damaged bottom electrode, and the repair material is stamped around the repair electrode member, eliminating the need for replacement of the bottom electrode. In addition, the repair time is shortened.

[0008]

In the technique of the first prior art,
Since the bottom of the furnace is cooled and released every two to three months to replace the electrode block on the top of the stamp, the life of the bottom electrode is prolonged, but since the furnace is cooled and released each time and the operation stops,
This results in lower operating efficiency.

In the technique of the prior art 2, the repair electrode member manufactured in advance is immersed in the remaining molten steel and is placed on the furnace bottom electrode which has been damaged and damaged. Molten steel or slag may be interposed between the large number of steel pins projecting downward and the large number of steel pins of the melted hearth electrode. In this case, since it becomes difficult for the current to flow through the pin in which the slag is interposed, when the repair is performed by the method of the preceding example 2, drift is likely to occur. In addition, the electric drift generated in the pin in this way is difficult to disperse to other pins, and the pin causes local wear due to excessive Joule heat generated by the drift, further shortening the life of the electrode. Cause you to

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and it is an object of the present invention to provide a bottom electrode for a DC arc furnace which can easily repair the electrode and does not cause a current drift. It is intended for.

[0011]

SUMMARY OF THE INVENTION The present invention relates to a direct current arc furnace, which comprises a steel cylinder having a thickness of 30 to 60 mm and a cross section having an average current density of 0.5 A / mm ² or less. A bottom electrode for a DC arc furnace, wherein the bottom electrode is joined to a support base below the bottom electrode, and the inside and outside of the steel cylinder are made of a refractory.

[0012]

In the furnace bottom electrode for a DC arc furnace of the present invention,
Since the average current density flowing through the electrode conductor is 0.50 A / mm ² or less, excessive Joule heat is not generated in the electrode conductor, so that the electrode is less worn.

When an arc is generated between the upper graphite electrode and the furnace bottom electrode conductor, the lower electrode conductor is of a steel cylindrical type, so that current drift is less likely to occur. Further, even if a drift occurs, the arc is easily dispersed and horizontally moved on the circumference of the cylindrical electrode, and does not remain in a local area. For this reason,
Since there is no local excessive Joule heat, the hearth electrode does not suffer from local wear.

Further, the reason why the thickness of the steel cylinder is set to 30 to 60 mm is that the steel cylindrical electrode is consumed by a long operation, and the upper end of the electrode is lower than the surrounding electrode block, so that the electrode is recessed in a groove shape. When the groove is damaged, it is extremely easy to insert the repair steel cylinder by setting the width of the groove to 30 mm or more. Also,
When the thickness is 60 mm or less, even if molten steel generated due to melting of the scrap or melting of the electrode conductor itself flows in the groove, the flow of the molten steel is suppressed low, and damage to the electrode block is reduced.

Further, a repair steel cylindrical electrode is inserted into the above-mentioned groove according to the damage depth of the electrode block and placed on the damaged electrode. The repair of the electrodes is completed. For this reason, the repair time is greatly reduced as compared with the conventional bottom electrode using contact pins.

[0016]

1 is a sectional view showing the structure of a bottom electrode for a DC arc furnace according to an embodiment of the present invention, FIG. 2 is a plan view taken along the line AA in FIG. 1, and FIG. FIG. 3B is a plan view of a cross section.
In these figures, reference numerals 1 to 9 and 11 are the same as those in FIG. 6 of the conventional apparatus. Reference numeral 10 denotes a connecting column connecting the base 5 and the support base 6 provided side by side. Reference numeral 5a denotes a rectifying plate formed on the base 5 for effectively flowing the cooling air flowing from the cooling air introduction passage 4a; The cylindrical bottom electrode is a steel cylindrical electrode that is combined to form an integral unit, and has a plate thickness of 30 mm to 60 mm and a cross-sectional area of an average current density of 0.5 A / mm ² or less. As shown in FIG. 2, the inside and outside of the steel cylindrical electrode 12 are filled with a refractory material of a magnesium type and are solidified by an electrode block 8.

Since the structure of the bottom electrode for a DC arc furnace according to the present invention is as described above, the current is supplied from the conductor 2, the conductor 3, the terminal 4, and the base 5 to the steel base through the connecting pillar 10 via the support base 6. It flows to the cylindrical electrode 12 and forms an arc between the cylindrical electrode 12 and the graphite electrode 9, and the scrap heat is melted by the heat of the arc to form molten steel 11, and the refining operation is performed. At this time, as shown in FIG. 3, the cooling air is guided from the cooling air introduction path 4a through the gap between the base 5 and the support base 6 formed by the connecting columns 10 by the rectifying plate 5a, and passes through the support base 6. While cooling, the steel cylindrical electrode 12 is cooled via the support base 6.

FIG. 4 shows that the current density is 0.30 to 0.60 A
/ Mm ² is calculated by changing the temperature of the cylindrical electrode conductor when it is changed in the range of ² mm. The horizontal axis is the height (distance) of the electrode conductor from the bottom of the substrate, and the vertical axis is the temperature of the electrode conductor at this height. Is shown.

As the conditions for the heat transfer calculation, the outer diameter of the cylindrical electrode conductor is 2000 mm, the plate thickness is 45 mm, and the height is 1000 m.
m, and the temperature of the cylindrical electrode conductor was calculated by a one-dimensional model in the longitudinal direction. The temperature of the molten steel is 1650 ° C. Considering the heat conduction by the molten steel from above the electrode due to the molten steel temperature and the Joule heat generated in the electrode due to the current, there is no heat conduction between the cylindrical electrode and the refractory. The base was calculated with air-cooled conditions.

In FIG. 4, curve A represents a current density of 0.30.
A / mm ² , curve B shows 0.40 A / mm ² , curve C shows 0.50 A / mm ² , and curve D shows the heat transfer result of 0.60 A / mm ² . FIG. 4 shows that the current density was 0.30 A /
In mm ² , the conductor temperature rises almost linearly as the height from the base bottom increases, but the current density is 0.4
At 0 A / mm ² , the rate of increase in the electrode conductor temperature is greater than the rate of change in height (distance) from the bottom surface of the substrate, slightly deviating from the above linear relationship. 0.50 current density
In the curve C of A / mm ² , the above tendency is remarkable, and the temperature of the upper part of the electrode conductor when the height H from the bottom surface of the base is in the range of 750 mm to 950 mm is slightly higher than the molten steel temperature of 1650 ° C. Further, a curve D having a current density of 0.60 A / mm ²
In the above, the height from the bottom surface of the substrate is 450 mm to 950
The temperature of the upper part of the electrode conductor is higher than the molten steel temperature of 1650 ° C. over a wide range of mm.

The above heat transfer calculation results can be understood as follows. First, the closer to the upper end of the electrode conductor, the greater the thermal effect from the molten steel, so that the electrode conductor temperature increases. Further, as the current density increases, the Joule heat generated in the electrode conductor also increases, so that the electrode conductor temperature increases. In addition, since the electric resistance increases as the metal conductor temperature increases, Joule heat generated above the electrode conductor increases. Therefore, since the temperature of the upper part of the electrode conductor exceeds the melting point of the steel electrode conductor (generally, the melting point of pure iron is lower than 1530 ° C.), the higher the current density becomes, the more the upper conductor melts during arc melting.

Here, the current density of the curve C is 0.50 A / m
If the m ² and limits, is kept substantially linear relationship to the electrode conductor height and temperature, since no electrode conductor upper melts over a wide range, without being damaged at a time until the internal electrode conductor The electrode wear rate can be kept low.

At the same time, the average current density is 0.50 A / mm
^When it is ² or less, the temperature rise of the refractory constituting the inside and outside of the electrode conductor can be suppressed to be low, so that the wear of the electrode block refractory can be reduced.

In order to confirm the effect of the present invention, the average current density was 0.3 A / mm ² , 0.
A test for dissolving scrap for a long time at three levels of 5 A / mm ² and 0.6 A / mm ² was conducted, and the average electrode damage rate per dissolution time was investigated. Here, the outer diameter of the cylinder is 200
A steel cylindrical electrode having a thickness of 0 mm, a thickness of 45 mm, and a height of 1000 mm was used, and a magnesia refractory was used as a refractory material inside and outside the cylinder. As a result, if the damage rate index in the case of 0.3 A / mm ² is 1.0, 0.5 A / mm 2
² and 1.2 were obtained at 0.6 A / mm ² , respectively. In addition, the so-called localized damage, in which damage concentrates on a specific pin observed with a conventional pin-type bottom electrode, is 0.3 to
It was not observed in the range of 0.6 A / mm ² .

As a method for repairing the damaged electrode, a repair steel cylindrical electrode having a length corresponding to the damage depth of the electrode block was inserted into the groove and mounted on the damaged electrode. After that, the inside and outside of the cylinder were filled with magnesia-based amorphous refractories, and the repair of the furnace bottom electrode was completed. As a result, the repair time was significantly reduced as compared with the conventional furnace bottom electrode using contact pins.

In addition to the above-described electrode repair method, as shown in FIG. 5, the electrode block 8 having the groove-shaped damage is scraped into a gentle concave shape until the upper end of the steel cylinder 12 appears, and then the worn height is reduced. In addition, the repair steel cylinder 12a is placed,
Next, a repair method may be adopted in which the repair steel cylinder 12a is wrapped and filled inside and outside with a repair refractory 8a. This method can also greatly reduce repair time.

[0027]

According to the present invention, as described above, the average current density flowing through the bottom electrode for a DC arc furnace is 0.5 A / mm ^2.
By using a steel cylinder having the following cross-sectional area and a plate thickness of 30 mm to 60 mm, the life of the furnace bottom electrode can be improved, and the current can be repaired without causing a current drift during operation. An extremely easy furnace bottom electrode for a DC arc furnace can be obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view of a bottom electrode for a DC arc furnace according to one embodiment of the present invention.

FIG. 2 is a plan view of an AA cross section in FIG.

FIG. 3 is a plan view of a BB section of FIG. 1;

FIG. 4 is a curve diagram showing a relationship between an electrode conductor depth and a temperature depending on a current density.

FIG. 5 is a cross-sectional view of a cylindrical hearth electrode showing an electrode replenishment repair process.

FIG. 6 is a sectional view showing the structure of a conventional bottom electrode for a DC arc furnace.

[Explanation of symbols]

 1. Furnace bottom 2. Conductor 3. Conductor 4. Terminal 4a. 4. Cooling air introduction path Base 5a. Current plate 6. Support base 7. 7. Contact pin Electrode block 8a. Refractory for repair 9. Graphite electrode 10. Connecting pillar 11. Molten steel 12. Steel cylindrical electrode 12a. Repair steel cylindrical electrode

Claims (1)

(57) [Claims] 1. A direct current arc furnace having a plate thickness of 30 to 60.
A steel cylinder having a sectional area of 0.5 mm / mm ² and an average current density of 0.5 A / mm ² or less was joined on a support base below the furnace bottom electrode, and the inside and outside of the steel cylinder were made of a refractory. A bottom electrode for a DC arc furnace, comprising: