JPH0473591A - Furnace bottom electrode for electric furnace - Google Patents

Abstract

PURPOSE:To ensure a contact pin excellent in conductivity, mechanical strength, and high-impact properties by incorporating the contact pin divided longitudi nally into a plurality of parts thereof, the contact pin being of zirconium boride ceramic. CONSTITUTION:A contact pin 12 is obtained by assembling the four division pins 30 of a fan-shaped (or square) cross section into a columnar shape (or pyramidal) as a whole, and binding the division pins. Further, zirconium ceramic is employed as the pin 12 of a furnace bottom electrode. Thereby, the pin 12 is reduced in wear speed in view of material quality, and is durable for a long period use with its life prolonged 10 times the conventional life. Further, for the structure of the pin, by dividing the pin 12 into a plurality partial pins each divided pin is made thin even though the diameter of the pin is large and uniform and dense texture formation is achievable to improve conductivity, mechanical strength, and high-impact properties.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to an electric furnace used for melting and refining steel, particularly a bottom electrode of a DC electric furnace.

"Prior Art" A DC electric furnace used for steelmaking is equipped with a bottom electrode, which is an anode, at the bottom of the furnace, and a graphite electrode, which is a cathode, at the top of the furnace. During operation, scrap and auxiliary raw materials are charged into an electric furnace, and a DC arc is generated between the bottom electrode and the top graphite electrode to convert electrical energy into thermal energy, which is applied to the scrap and melted. do.

Direct current arc furnaces have the following advantages compared to three-phase (alternating current) arc furnaces, so it is expected that the number of them in operation will increase in the future.

■Since there is only one cathode, there is less surface area of the graphite metal to be consumed (Also, due to the cathode characteristics, there is less stress on the tip of the electrode, so there is less wear on the graphite electrode, and the consumption rate of the graphite cathode is approximately 50% smaller. can.

■The noise during melting is 110db compared to a normal AC furnace with the same capacity.
On the other hand, it is small at 90db or less.

■Since there is only one cathode and the arc flies downward approximately vertically, the temperature distribution is relatively uniform, and hot spots that accelerate the consumption of surrounding furnace materials do not occur.

■Since there is no induction loss that cannot be avoided with AC, and energy efficiency is good, melting time and refining time are shortened, and the power consumption rate is small.

Generally, in a DC electric furnace, the anode (bottom electrode) in contact with molten metal and the furnace material around the cathode are worn out over the course of operation. This wear is mainly caused by erosion by molten metal, and is particularly noticeable in metal contact pins used as furnace bottom electrodes. Usually, when the length of the contact pin reaches its usable limit, the bottom electrode needs to be replaced.

Conventional furnace bottom electrodes have vertically long and comparatively thin contact pins (for example, approximately 40 mmφ) made of conductive metal such as low carbon steel (mild steel), and these contact pins are covered with an iron shell to protect them. The case is filled with magnesia stamp material (a type of monolithic refractory) so as to surround the contact pins.

``Problems to be solved by the invention'' However, in conventional furnace bottom electrodes, when the amount of molten steel remaining in the furnace during tapping becomes small, the slag floating on the surface of the molten steel comes into contact with the magnesia-based stamp material. This produces a compound with a low melting point, resulting in significant wear and tear on the stamp material. That is, in this case, the wear rate of the magnesia stamp material is as fast as 0.5 to 1.0 mm per hour (in particular, the central part of the furnace bottom electrode wears out before the peripheral part, and it takes about 700 heats (one heat is (corresponding to about 1 hour of operation), i.e. the electrodes must be replaced after about - months of use, i.e.
The lifespan of the furnace bottom electrode determines the repair period for DC electric furnaces, and the current situation is that furnaces are repaired frequently.

Furthermore, when replacing the furnace bottom electrode, there are the following problems. entered the furnace and removed the worn-out furnace bottom electrode in an extremely hot environment.
Installation of new hearth bottom electrodes and construction of monolithic refractories will be carried out. This replacement work requires approximately 8 hours of work time in addition to the time required to cool the furnace, reducing the productivity of this zero furnace. Also, due to the thermal stress during cooling,
The wear and tear of the furnace material around the repaired area is accelerated, and the current unit of furnace material increases further.

Incidentally, normally, the lifespan of the furnace materials other than the hearth electrode is about one year, and it is desired to extend the lifespan of the hearth electrode to the same level as the lifespan of the other parts.

The present invention has been made in view of the problems of the prior art described above, and its purpose is to provide a hearth electrode for an electric furnace that has a low wear rate (and a long life).

"Structure of the Invention" In order to achieve the above object, the hearth electrode for an electric furnace of the present invention includes a vertically long contact pin whose exposed upper end portion is brought into contact with an object to be heated and heated by electricity, and a A hearth electrode for an electric furnace, comprising a refractory disposed so as to surround a lower part following an upper end, and a connection part to a power source provided at a lower end of the contact pin, the contact pin comprising: It is characterized by assembling contact pins made of zirconium boride ceramics that are vertically divided into a plurality of pieces (hereinafter abbreviated as divided pins).

In a preferred embodiment of the electric furnace bottom electrode of the present invention,
The number of vertical divisions of the contact pin is 3 to 7.

In another preferred embodiment of the bottom electrode for an electric furnace of the present invention, the divided bins are bound together with a metal band or sleeve surrounding the divided bins.

In another preferred embodiment of the electric furnace bottom electrode of the present invention, at least the upper end of the contact pin is covered with a metal cap.

In another preferred embodiment of the electric furnace bottom electrode of the present invention, a metal plate is inserted into the seam of the divided bins.

In another preferred embodiment of the bottom electrode for an electric furnace of the present invention, the contact pin has a columnar body having a hole vertically penetrating the center thereof when the divided contact pins are assembled, and The hole is filled with a monolithic refractory made of zirconium boride.

In another preferred embodiment of the bottom electrode for an electric furnace of the present invention, the ridge of the divided bin is chamfered or rounded.

In another preferred embodiment of the bottom electrode for an electric furnace of the present invention, at least a portion of the refractory surrounding the lower portion of the contact pin is a shaped refractory.

In another preferred embodiment of the bottom electrode for an electric furnace of the present invention, at least a portion of the refractory surrounding the lower portion of the contact pin is a zirconium boride refractory.

In another preferred embodiment of the bottom electrode for an electric furnace of the present invention, the shaped refractory is a magnesia graphite refractory.

In another preferred embodiment of the bottom electrode for an electric furnace of the present invention, the lower end of the contact pin is held by a connecting member made of a combination of a metal member with a large coefficient of thermal expansion and a metal member with a small coefficient of thermal expansion, and is connected to a power source. It is connected.

"For Inventory" In the present invention, zirconium boride ceramics is used as the material for the contact pin in the bottom electrode for an electric furnace. Zirconium boride ceramics have a melting point of 300
Sintered material has a high temperature of 0°C or higher, exhibits excellent corrosion resistance against slag and molten metal, especially molten steel, and has conductivity equivalent to that of currently used mild steel, that is, the level of conductivity required for electrodes. You can build your body. Therefore, by using zirconium boride ceramics for contact pins, the wear rate is low, especially when used for steel manufacturing.
A contact pin that can withstand long-term use and has a long life can be obtained.

By the way, thick contact pins cannot be used with current contact pins such as mild steel bundles because they have poor corrosion resistance against molten steel. Zirco boride: This restriction is overcome with a laminate contact pin, but since ceramics are used, various ingenuity is required. The larger the cross-sectional area of the contact bottle, or in other words, the thicker the diameter, the larger the conductive cross section of the bottle, so the current load per bottle can be increased and the output can be increased.

However, when making contact pins from zirconium boride ceramics, if the diameter is increased, the sintering of the ceramic will not be uniform on the surface and inside, resulting in an uneven structure and a dense sintered body. Because I can't do it,
The conductivity and mechanical strength are low (and it is not possible to obtain satisfactory performance as a contact pin.Also, even if a thick and dense contact pin can be made, it will not be able to withstand thermal stress due to temperature distribution during temperature rise and fall.) The weak ones yell.

In the furnace bottom electrode of the present invention, the contact pin is assembled into a plurality of vertically divided contact pins made of zirconium boride ceramics, so even if the contact pin has a large diameter as a whole, each The wall thickness of the divided bottles can be made thinner. Therefore, when firing the zirconium boride contact pin, it is possible to obtain a uniform and dense structure. In this way, multiple divided bottles of zirconium boride ceramics, which have a uniform and dense structure, are assembled to form a single thicker contact pin, so even large contact pins with large diameters can be It becomes possible to obtain a contact pin that maintains good electrical conductivity of zirconium ceramics and also has excellent thermal shock resistance. 7. By increasing the diameter of the contact pin, it is possible to increase the current load on one contact pin and obtain a large capacity output.

In a preferred embodiment of the furnace bottom electrode of the present invention, the number of vertical divisions of the contact pin is 3 to 7, so that a contact pin of sufficient thickness can be formed and is easy to manufacture.

In addition, when the divided bottles are assembled, they are tied together with a metal band or sleeve that surrounds them, making it easier to handle when attaching electrodes, etc. The conductivity between the two becomes better.

In addition, by covering at least the upper end of the contact pin with a metal cap, the surface of the contact pin is directly exposed to the atmosphere and is heated and oxidized, especially when the temperature rises after replacing the contact pin. It is possible to prevent the zirconium from forming into zirconium, and it is possible to prevent the conductivity of the surface that comes into contact with the molten steel from decreasing. Furthermore, it is possible to increase the strength against mechanical shock when scrap is introduced into the furnace.

In addition, when assembling divided bins, by inserting a metal plate at the joint between them, the conductivity between the divided bins can be further improved, and the current load of each divided bin can be equalized, making it possible to It becomes possible to flow current.

In addition, by making the contact pin form a columnar body with a hole that passes through the center in the vertical direction when the divided bottles are assembled and arranged in one order, the wall thickness of the individual divided bottles can be made more uniform. It is possible to sinter the zirconium ceramics evenly, and by filling the holes with zirconium boride monolithic refractories, durability is imparted, and thermal expansion between the filled refractories and This is preferable in that thermal stress due to the difference does not occur in the contact pin.

In addition, by chamfering or rounding the twill portion of the divided bottle, it is possible to avoid corner chips during handling.
In addition, when the temperature rises, the ridges are rapidly heated and thermal stress is generated due to the temperature distribution, thereby preventing damage such as chipping of the corners.

In the hearth electrode for electric furnaces of the present invention, each of the contact pins is made thicker, so the space between the contact pins becomes wider, making it possible to install a shaped refractory, and instead of a shaped refractory. Compared to shaped refractories, it is more durable because it can be manufactured at a higher density, and by lining at least a portion of the space between the contact pins with shaped refractories, the life of the bottom electrode can be further extended.

In addition, by making at least a portion of the refractory surrounding the lower part of the contact pin a zirconium boride refractory or a magnesia graphite shaped refractory, the wear rate is low (it can withstand long-term use and has a long service life). This results in a furnace bottom electrode that can be further extended.

Furthermore, the lower end of the contact pin is held by a connecting member that combines a metal member with a large coefficient of thermal expansion and a metal member with a small coefficient of thermal expansion, and is connected to a power supply, so that the coefficient of thermal expansion is lower than that of ordinary metal materials. Tightening due to the difference in thermal expansion between the zirconium boride ceramic and the metal connection part, which is known to cause noise, cancels out the loosening of the part, and even if the connection part becomes high temperature, the connection part of the contact pin to the power supply will become loose. This can be prevented.

"Example" FIG. 1 shows an outline of a cross section of an example of a bottom electrode for an electric furnace according to the present invention.

A furnace bottom electrode 11 is fitted into the center of the furnace bottom of the DC electric furnace. The furnace bottom electrode 11 is unitized and surrounded by block bricks 20 provided at appropriate locations on the furnace lining refractory 21. A monolithic refractory 22 made of magnesia is filled between the case 19 of the hearth electrode 11 and the block bricks 20. Note that the block bricks 20 are made of magnesia graphite shaped refractories. Hearth electrode J1
The water-cooled cable 17 connected to is connected to the anode side of a DC power source (not shown).

On the other hand, the cathode side of the DC power source is connected to a graphite electrode (not shown). The graphite electrode penetrates the lid of the DC electric furnace,
Its tip faces the object to be heated in the furnace. Note that a power supply with a capacity of 120,000 amperes or more is normally used.

The cable 17 connected to the hearth bottom electrode 11 is connected to the electrode terminal 16
Further, the terminal 16 is connected to the cooling air introduction pipe 15.
It is connected to the current collector plate 14 via. A substrate 13 is provided directly above the current collector plate 14 . The substrate 13 and current collector plate 14 are mounted substantially horizontally and in parallel. Board 1
3 is supported by the iron shell of the furnace body via a bracket 23. The substrate 13 is insulated from the furnace body by a bracket 23 which is an insulating member. Then, a plurality of contact pins 12 (for example, 40) made of zirconium boride ceramics that are divided and assembled are attached to the board work 3.
The lower ends of the straws are connected to and supported by the current collector plate 14, and are erected parallel to each other. A steel case 19 is provided on the upper surface of the substrate 13 so as to surround a group of contact pins 12 embedded in a refractory 18.

In addition, a refractory material 18 is installed inside the case 19,
Most of the lower part of the contact pin 12 except for its upper end is embedded in a refractory 18 . In this example,
The refractory 18 has a two-layer structure in which the lower layer is made of a stamp material made of magnesia and the upper layer is made of an amorphous refractory made of zirconium boride. The thickness of the refractory material 18 is, for example, 70 to 100 cm including the upper and lower layers, and the upper end portion of the contact pin 12 is configured to protrude from the upper surface of the refractory material 18 by a certain length. As the zirconium boride monolithic refractory, for example, those shown in Table 1 are preferably employed.

The ZrB2 content of the zirconium boride refractory used in Table 1 is preferably 90 wt% or more in order to ensure corrosion resistance. This zirconium boride monolithic refractory undergoes sintering at a temperature of about 1500° C. or higher during use and becomes electrically conductive, so that it functions as a part of the furnace bottom electrode.

Next, the contact pin 12 in the hearth bottom electrode 11 will be explained. 2 to 17 show various forms of separately assembled contact pins as examples of the present invention.

The contact pin 12 shown in FIG. 2 is made by assembling three divided bins 30 each having a fan-shaped cross section to form a cylindrical shape as a whole, and covering the outer periphery with a metal sleeve 31 to bind the pin.

In this case, the sleeve 31 is shaped to cover up to the top surface of the contact pin 12, and not only binds the divided bins 30 together, but also prevents the contact pin 12 from being damaged by impact when the scrap is inserted. The contact pin 12 in FIG.
Four divided bottles 30 each having a fan-shaped cross section are assembled to form a cylindrical shape as a whole, and the outer periphery is covered with a metal sleeve 3 and bound together. The contact pin 12 in FIG.
Four divided bins 30 with a square cross section are assembled to form a regular square prism as a whole, and the outer periphery is covered with a metal sleeve 31 and bound together. The contact pin 12 shown in FIG. 5 is made by assembling six divided bins 30 each having a regular triangular cross section to form a regular hexagonal column as a whole, and the outer periphery of the pin 12 is covered with a metal sleeve 31 and bound together. Contact pin 1 in Figure 6
2, by assembling three divided bins 30 with a hexagonal cross section,
It has a polygonal column shape as a whole, and is bound by covering the outer periphery with a metal sleeve 31. In this way, split bin 3
By assembling an arbitrary number of 0's, contact pins 12 of various shapes can be obtained.

The contact pin 12 shown in FIG. 7 is formed by assembling four divided pins 30 each having a fan-shaped cross section to form a cylindrical shape as a whole, with a circular hole 32 penetrating vertically in the center, and the outer periphery is made of metal. It is covered with a sleeve 31 and tied together. This circular hole is filled with a monolithic refractory to prevent molten metal from leaking from the hole. The contact pin 12 shown in FIG. 8 differs from the contact pin 12 shown in FIG. 7 in that the hole 32 in the center has a square cross-sectional shape. The contact pin 12 shown in FIG. 9 is made by assembling six divided bins 30 with fan-shaped cross sections to form a cylindrical shape as a whole, with a regular hexagonal hole 32 penetrating vertically in the center, and on the outer periphery. It is covered with a metal sleeve 31 and tied together. Contact pin 1 in Figure 10
2 is 4 whose cross section is a square with one corner cut out.
Two divided bottles 30 are assembled to form a regular square prism as a whole, and a square hole 32 is formed vertically through the center, and the outer periphery is covered with a metal sleeve 31 and bound together. The contact pin 12 shown in FIG.
Three divided bins 30 each having a rhombic cross-section with one corner cut out are assembled to form a regular hexagonal prism as a whole, with a triangular hole 32 penetrating vertically in the center, and a metal part on the outer periphery. It is covered with a sleeve 31 and tied together.

The contact pin 12 shown in FIG. 12 is the same as the contact pin 12 shown in FIG.

The contact pin 12 shown in FIG. 13 is the contact pin 12 shown in FIG. 4 in which a metal plate 33 is inserted into the contact surface of each divided bottle 30. The contact pin 12 shown in FIG. 14 is the contact pin 1 shown in FIG.
2, a metal plate 33 is inserted into the mating surface of each divided bottle 3o. Contact pin 12 shown in FIG.
This is the contact pin 2 shown in FIG. 10 in which a metal plate 33 is inserted into the mating surface of each divided bottle 30. In these contact pins 12, the metal plate 33 is softer than the divided bottles made of zirconium boride, so the conductivity due to contact between the divided bottles 30 is increased, and the current density as a whole of the contact pins 12 is made uniform. Contributes to increasing current carrying capacity.

Contact pin 2 shown in FIG. 16 is the contact pin 12 shown in FIG. 7, with a chamfer 33 formed on the ridgeline portion of each split pin 30. Contact pin 12 shown in FIG. 17, contact pin 12 shown in FIG.
In this case, the ridgeline portion of each divided bin 30 is rounded 34.

Generally, in ceramic members, thermal stress due to temperature gradients is likely to occur at corners and ridges during heating and cooling, and these parts often crack or chip. For this reason, chamfers 33 and rounded edges 34 are provided on the ridgeline of each divided bin 30.
By applying this, it is possible to alleviate the occurrence of thermal stress when the temperature is raised or lowered, and furthermore, it is possible to prevent cracking or chipping during handling. Note that the hole 32 formed in the center of the contact pin 12 and the gaps formed by the chamfering 33 and rounding 34 are filled with a monolithic refractory made of zirconium boride, etc., so that the contact pin 12 as a whole is It is preferable to improve the conductivity and durability of the material.

In addition, in the above example, the divided bottle 3D is the metal sleeve 3.
1, but instead of the sleeve 31, a metal band or the like may be used to bind the divided bottles.

Further, the sleeve 31 may not cover the entire contact pin 30 in the axial direction, but may be attached only partially to the upper end, middle, or lower end.

As the material of the contact pin 12, for example, a sintered body of zirconium boride as shown in Table 2 can be used.

Table 2 This sintered body of zirconium boride has physical properties as shown in Table 3.

(Hereinafter, blank space) Table 3 In the present invention, as mentioned above, the contact pin I2 is
Since it is constructed by assembling a plurality of divided bins 30 divided in the vertical direction, the conductive cross-sectional area of one contact pin can be made thick (and the diameter of the contact pin 12 as a whole can be made thick (but each division Each of the bottles 30 is relatively thin.Therefore, when the zirconium boride ceramic is sintered to form the divided bottles 30, a sintered body with a uniform and dense structure can be obtained.

Therefore, a sintered body with excellent conductivity, mechanical strength, etc. can be obtained, and by assembling a plurality of divided bottles 30 made of such a sintered body, a contact pin 12 with excellent performance such as conductivity and durability can be obtained. can be obtained.

In addition, the advantage of using thick contact pins is that there is no need to use many thin contact pins, so the space between the contact pins can be widened, and it can be used as a refractory to fill in the spaces between the contact pins instead of monolithic refractories. This also means that it will be possible to construct shaped refractories with high properties.

FIG. 18 shows an example of a connection structure between a metal cap 46 and a power source of contact pins 12 bound together by a band 48;
A cross section is shown.

As shown in FIG. 1, the contact pin 12 is arranged so as to penetrate the refractory 18 and the substrate 13.

In FIG. 18, the lower portion of the contact pin 12 is held on the substrate 13 via a metal connection holder 41. In FIG.

This connection holder 41 is a cylindrical body 4 fixed to the substrate 13.
2, a split ring 43 disposed inside this cylindrical body 42 , an intermediate ring 44 that abuts on the split ring 43 inside the cylindrical body 42 , and a split ring 43 connected to the contact pin 12 via this intermediate ring 44 . It is composed of a presser screw 45 that is brought into pressure contact. The contact pin 12 extends vertically through these members.

Inside the cylindrical body 42, there is a tapered wall 42 that expands downward.
a is formed, and the upper outer periphery of the split ring 43 is in contact with this tapered wall 42a. The split ring 43 is divided into three or four parts in the circumferential direction. Also,
The intermediate ring 44 is in contact with the lower outer periphery of the split ring 43. The retaining screw 45 presses the split ring 43 upward via the intermediate ring 44 by being screwed into the lower opening of the cylindrical body 42 . As a result, the split ring 43
is pressed inward along the tapered wall 42a to hold the outer periphery of the contact pin 12 and electrically connect it.

In the above, the thermal expansion coefficient of the cylindrical body 42 is relatively smaller than that of the contact pin, but since the split ring 43 is formed of a metal with a relatively large thermal expansion coefficient, the thermal expansion coefficient of the cylindrical body 42 is relatively smaller than that of the contact pin. The contact pin 12 has a structure like
When the electric furnace is operated with the bottom electrodes installed and the holder 41 tightened securely, the holder 41 also becomes high in temperature due to heat transfer from the top. At this time, the contact pin 12 made of zirconium boride ceramics and the metal connection holder 4
1, there is a difference in the coefficient of thermal expansion, so there is a risk that the tightening force by the connection holder 41 will be loosened and the electrical connection will be broken. However, since the split ring 43 is made of a metal with a large coefficient of thermal expansion, when the temperature rises, the split ring 43 expands relatively within the cylindrical body 42, and the loosening of the holding force of the bottle 12 is canceled out even at high temperatures. It will be done.

Note that the holding structure of the contact pin 12 is not limited to the above structure, and various structures can be adopted.

For example, the lower end of the contact pin 12 made of a sintered body of zirconium boride is threaded, and a threaded metal contact pin whose thermal expansion coefficient is close to that of zirconium boride is connected to this. This metal contact pin can also be fixed to the substrate 13.

Further, in FIG. 18, a metal cap 46 is attached to the upper end of the contact pin 12 that protrudes from the upper surface of the monolithic refractory 18, and a metal band is attached to the middle part of the contact pin. . When the surface of a zirconium boride sintered body is oxidized, it becomes zirconium oxide and loses its electrical conductivity. At the start of operation after replacing the contact pin 12, the contact pin 12 is heated while being directly exposed to the atmosphere. For this reason, contact pin 1
A metal cap 46 is attached to the upper end of 2 to prevent oxidation at the start of operation.

Further, when scrap is introduced into the furnace, the steel scrap is attracted by a magnet and dropped near the bottom electrode of the furnace, so there is a strong mechanical shock when it is introduced, which may damage the contact pin 12. metal cap 4
6 functions as a buffer against such mechanical impact and helps prevent damage to the contact pin.

Note that the contact pins 12 shown in FIGS. 2 to 17 include
A metal sleeve 31 is placed on the outer periphery of each case, but if this metal sleeve 31 is shaped to cover the top surface of the contact pin 12, it can also serve as the function of the metal cap 46 described above. .

Test example: The inner diameter of the furnace is approximately 300 mm as shown in the cross-sectional outline in Figure 19.
A mock test furnace of mmφ was manufactured and the practicality and durability of the furnace bottom electrode using contact pins assembled in sections according to the present invention were investigated.

In the figure, 51 is an induction coil, 52 is a metal case, 53 is a monolithic refractory, 54 is a contact pin for testing, 55 is an upper electrode, 56 is a copper terminal, 57 is molten steel, 58 is a cable, and 59 is a press. Molded magnesia graphite refractory, 4
2 is a cylindrical body, 43 is a split ring, 41 is a connection holder, 6
2 is an insulating material, and 63 is a cap made of mild steel.

The contact pin 54 is manufactured, for example, as follows. That is, coarse grains of 4 to 14 meshes of ZrB2 and 1
Medium grains of 4 mesh or less, fine grains of 150 mesh or less, and natural graphite are mixed with ZrB 2 with a purity of 95% or more.
Particles of 2 to 200 meshes are blended so that 10% by weight natural graphite is 4% by weight, phenol resin (resol type) is added to this, kneaded and granulated, and then shaped into a predetermined shape using an isostatic press. It is molded and fired at a temperature of 2000°C or higher under normal pressure. The contact pins 54 to be used for the test were formed using the above-mentioned method to form divided bottles in the shape of a round bar divided into four in the vertical direction, processed the mating surfaces after firing, and bound the four divided bottles into a 150φ x 400 piece. 50mmφ in mm
A cylindrical contact pin with a through hole was prepared.

The contact pin 54 and the upper electrode 55 for comparison were made of 100 ml, which were integrally molded and fired in the same manner as above.
A round bar with a diameter of 400 mm was used.

These two types of contact pins 54 and upper electrodes 55 have their tips covered with caps 63 made of mild steel with a thickness of 0.5 mm.
covered with. Further, a connection portion with a power source was formed in the same manner as in FIG. 18 except that a bracket 62, which is an insulator, was provided at the lower end of each contact pin.

The contact pin 54 was arranged so as to penetrate the bottom of the case 52, and was embedded and fixed in the refractory 53. The hole in the center of the contact pin 54 was filled with a monolithic refractory made of zirconium boride.

After constructing the simulated test furnace in this way, each contact pin 54 was tested. First, an ingot of 5S41 steel with a size of about 10 mm was placed in a furnace, and the induction coil 51 was energized to heat it by high-frequency induction, and the ingot was melted while being added. During this time, the bottom of the furnace was forcedly cooled with air using a blower. When the upper electrode 55 above was brought into contact with the molten steel during melting and the resistance between the electrodes was measured, it was 0.03Ω or less.

After that, when the induction power was interrupted and the power supply was connected to the upper electrode 55 and the contact pin 54, the voltage was 1500V at about 25V.
A current of A was able to flow. As a result, it was confirmed that electricity could be stably passed between the electrodes no matter which contact pin 54 was used.

Thereafter, molten steel was tapped from an outlet steel provided at the lower side of the furnace (not shown), the tap was closed, and while the steel ingot was again introduced, melting was performed again using high frequency dielectric. After the molten steel was made, the resistance between the upper electrode 55 and the contact pin 54 was measured, and it was 0.03Ω or less for all contact pins. No increase in inter-electrode resistance was observed.

However, poor connections were occasionally observed at the connection between the upper electrode and the cable, so the connection terminals were retightened and a good contact condition was confirmed before measurement of interelectrode resistance and conduction test.

After this, the upper electrode was removed and further heated by induction for about 16 hours.
The molten state at 00° C. was maintained for 2 hours, and then the furnace was tilted to pour out all of the molten steel. After cooling, the state of corrosion of the contact pin 54 and the refractory was examined.

As a result, no cracks were observed in any of the contact pins 54 made of zirconium boride in the examples and comparative examples.
Almost no wear or oxidation of operating surfaces was observed. Furthermore, electrical continuity was maintained at the connection between the contact pin and the power supply without causing any loosening. Concerning the refractories, progress of erosion was observed in the monolithic refractories, but the erosion in the press-formed shaped refractories embedded in the monolithic refractories was slight.

From the results of the above tests, it was confirmed that zirconium boride contact pins that were assembled in sections could be used without any problems in the same way as contact pins that were not divided. It was also confirmed that shaped refractories are more durable than monolithic refractories.

"Effects of the Invention" As explained above, in the bottom electrode of the present invention, by using zirconium boride ceramics as the contact pin of the bottom electrode, it is possible to extend the life of the contact pin by more than 10 times compared to the conventional one. gender has been confirmed.

Furthermore, by configuring the contact pin as a plurality of divided bottles assembled in the vertical direction, even if the diameter of the contact pin is increased, each divided bottle will be relatively thin, and it will be uniform and uniform during sintering. It becomes possible to form a dense structure, and a contact pin with good conductivity, mechanical strength, and thermal shock resistance can be constructed. In this way, the characteristics of the contact pin (contact pin 1
Since the diameter of the book can be made thicker, contact pin 1
The power load per book can be high. Therefore, a large amount of current can be passed with a relatively small number of contact pins, and it is also possible to construct a highly durable shaped refractory material around the contact pins.

By using press-formed shaped refractories as the refractories to fill the contact pins, it is possible to significantly extend the life of the entire hearth electrode, and it is possible to match the lifespan of the furnace lining refractory and the hearth electrode. was confirmed, making it possible to significantly reduce the number of furnace repairs.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view showing an embodiment of the bottom electrode of an electric furnace according to the present invention, FIG. 2(a), (b), FIG. 3(a),
b), Fig. 4(a), (b), Fig. 5(a), (b)
), Figure 6 (a), (b), Figure 7 (aL (b),
Figure 8(a). (b), Figure 9 (a), (b), Figure 10 (a),
(b), FIG. 11(a), and (b) are plan views and partial front views showing different examples of contact pins used in the hearth electrode of the present invention, FIG. 12, FIG. 13, and FIG. 14. , FIG. 15, FIG. 16, and FIG. 17 are cross-sectional views showing other different examples of contact pins used in the hearth electrode of the present invention, and FIG. 18 is cross-sectional views showing contact pins used in the hearth bottom electrode of the present invention. FIG. 19 is a partial cross-sectional view showing an example of a connection structure with a power source, and FIG. 19 is a vertical cross-sectional view showing an outline of a bottom electrode simulation test furnace. In the figure, 11 is a hearth bottom electrode, I2 is a contact pin, 13 is a substrate, 14 is a current collector plate, 15 is a cooling air introduction pipe, 16 is an electrode terminal, 17 is a cable, 18 is a refractory, and 19 is a steel Case 20 is block brick, 21 is furnace lining refractory, 22 is monolithic refractory joint material, 30 is divided bottle, 31
32 is a metal sleeve, 32 is a hole, 33 is a metal plate, 4 is a connection holder, and 46 is a metal cap. sheep? 1 Closed drawing 1) ¥ 1 Simple drawing / 2 Easy drawing / 3 Fig. 3 Fig. Fig. 7 / 7 Koppu drawing Yu δ Fig. 2 Fig. 10 Fig. Horn drawing / 4b Leopard/a 1￥I

Claims (1)

[Scope of Claims] (1) A vertically long contact pin whose exposed upper end is brought into contact with an object to be heated and heated by electricity, and a contact pin arranged so as to surround a lower part of the contact pin that continues from the exposed upper end. A bottom electrode for an electric furnace, comprising a refractory and a connection part to a power source provided at the lower end of the contact pin, the contact pin being formed of a plurality of pieces made of zirconium boride ceramic in the vertical direction. A hearth electrode for an electric furnace, characterized in that it is an assembly of divided contact pins. (2) The hearth electrode for an electric furnace according to claim 1, wherein the number of vertical divisions of the contact pin is 3 to 7. (3) The hearth electrode for an electric furnace according to claim 1 or 2, wherein the divided contact pins are bound together with a metal band or sleeve surrounding the divided contact pins. (4) The hearth electrode for an electric furnace according to any one of claims 1 to 3, wherein at least an upper end portion of the contact pin is covered with a metal cap. (5) The hearth electrode for an electric furnace according to any one of claims 1 to 4, wherein a metal plate is inserted into the seam of the divided contact pins. (6) In any one of claims 1 to 5, the contact pin, when the divided contact pins are assembled, forms a columnar body having a hole passing through the center in the vertical direction, and A bottom electrode for electric furnaces that is filled with amorphous refractory made of zirconium boride. (7) The hearth electrode for an electric furnace according to any one of claims 1 to 6, wherein the ridge portions of the divided contact pins are chamfered or rounded. (8) The hearth electrode for an electric furnace according to any one of claims 1 to 7, wherein at least a portion of the refractory surrounding the lower part of the contact pin is a shaped refractory. (9) The hearth electrode for an electric furnace according to any one of claims 1 to 8, wherein at least a portion of the refractory surrounding the lower part of the contact pin is a zirconium boride refractory. (10) The hearth electrode for an electric furnace according to claim 8, wherein the shaped refractory is a magnesia graphite refractory. (11) In any one of claims 1 to 10, the lower end of the contact pin is held by a connecting member made of a combination of a metal member with a large coefficient of thermal expansion and a metal member with a small coefficient of thermal expansion, and is connected to a power source. Hearth electrode for electric furnaces.