JPH085249A - Furnace floor of dc electric furnace and execution method therefor - Google Patents

Abstract

PURPOSE:To obtain a furnace floor having low electric reistance by using scaly graphite powdered materials or graphitized carbon fiber felt or a mixture of the both as joint materials for a joint between a metal plate and a permanent brick and for a joint between the permanent brick and a wear brick. CONSTITUTION:A furnace floor of a DC electric furnace is constructed with an electrode plate made of a metal (mainly, copper plate) forming a furnace bottom, electrically conductive bricks (mainly, burned MgO-C bricks) with which the electrode plate is lined and which are usually stacked in two stages on the upper part of the electrode plate, and a stamp material constituting the furnace floor. An electric current is caused to flow through these layers. In this case, for a joint between the metal plate that is the electrode and a permanent brick and for a joint between the permanent brick and a wear brick, a scaly graphite powdered material having a mean particle size of 0.12 to 0.5mm or graphitized carbon fiber felt is used as a joint material. Further, the burned MgO-C bricks containing 8 to 25wt.% of graphite are used as the permanent and wear bricks.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hearth of a DC electric furnace in which the hearth is constructed of conductive bricks.

[0002]

2. Description of the Related Art A direct current electric furnace generates a direct current arc between a bottom electrode installed on the bottom of a furnace and a movable electrode inserted through a furnace lid, the heat of which melts scrap to produce metal such as steel. It is a furnace for melting and refining. The overall structure of the DC electric furnace is shown in FIG. As shown in FIG. 1, the hearth of the electric furnace is a metal electrode plate (usually an anode
Mainly a copper plate), a conductive brick (mainly a calcined MgO-C brick) lined in two stages on top of it, and a stamping material that constitutes the hearth, and a current flows through these layers. . At this time, to reduce the heat generation inside the hearth and to generate more heat inside the melt,
It is desirable that the hearth brick has a low electrical resistance.

An example of the structure of the furnace bottom is shown in FIG. 1. The lower perm brick and the upper wear brick are composed of, for example, 450 mm thick MgO-C brick containing conductive graphite. The hearth surface is made of, for example, a 100 mm thick MgO-C stamp material applied on a wear brick. The MgO-C brick contains flake graphite, which is a conductive substance, and reduces the electric resistance of the brick itself. However, not only the internal resistance of the brick itself, but also the electrical contact of the lower perm brick and the copper plate, and the upper wear brick and the perm brick have a great effect on the electrical resistance of the entire hearth. However, no studies have so far been made on a desirable joint material.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems and provides good and reproducible results between the perm brick and the wear brick, and between the wear brick and the metal electrode plate. An object of the present invention is to provide a hearth of a direct current electric furnace that ensures electric conductivity and is durable.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems and uses flake graphite as a joint material. Further, since the flake graphite has lubricity, in consideration of the safety of the work at the time of construction of the hearth brick, it has been found that the electric resistance can be reduced even if the graphitized fiber felt is used, and the invention described below is obtained. Came to do.

(1) The invention of claim 1 is a joint between a metal plate which is an electrode forming the lowermost part of the hearth of an electric furnace and a conductive perm brick arranged above the metal plate, and this perm. A hearth of a DC electric furnace characterized by using scaly graphite powder or graphitized carbon fiber felt as a joint material in a joint between a brick and a conductive wear brick on the brick. .

(2) In the invention of claim 2, the joint material is scaly graphite powder, and the average particle size of the scaly graphite powder is 0.
The hearth of the DC electric furnace according to claim 1, having a diameter of 12 to 0.5 mm.

(3) The invention of claim 3 is a furnace for a DC electric furnace according to claim 1 or 2, wherein the perm brick and the wear brick are fired MgO-C bricks containing 8 to 25% by weight of graphite. floor.

(4) According to the invention of claim 4, a dry flaky graphite powder is sprinkled to a thickness of 2.0 to 10 mm on the upper surface of a metal plate which is an electrode forming the lowermost part of the electric hearth. A graphite powder layer is formed, and then a conductive perm brick is stacked on the first graphite powder layer, and then dry scale-like graphite powder having a thickness of 2.0 to 10 mm is stacked on the perm brick. Is applied to form a second graphite powder layer, and an electrically conductive wear brick is further stacked on the second graphite powder layer to provide an electric furnace hearth construction method.

(5) The invention according to claim 5 has a thickness of 3. on the upper surface of the metal plate which is the electrode forming the lowermost part of the electric furnace hearth.
Lay the first graphitized fiber felt of 0-10mm,
Next, a conductive perm brick is stacked on the first graphitized fiber felt, and then a second graphitized fiber felt having a thickness of 3.0 to 10 mm is laid on the perm brick, and then the second graphitized fiber felt. A method for constructing an electric hearth hearth, which is characterized in that a conductive wear brick is placed on a fiber felt.

[0011]

First, considering the electric resistance of the entire hearth, the resistance value of two sheets of calcined MgO-C brick of about 450 mm thickness is, for example, about 1 Ω (calculated value) per unit surface area (cm ² ). On the other hand, the resistance value of the joint material is about 5Ω (estimated value). That is, the resistance of the joint material accounts for 80 to 90% of the total.
Therefore, it is extremely important to reduce the electric resistance at the joint.

The electrical resistance of the joint material varies depending on the material and the filling degree. Therefore, it is necessary that the joint material itself has a low electric resistance and that the joint material is a material that can be uniformly and easily filled. A desirable joint material is required to have the following characteristics. It has heat resistance and conductivity. Good oxidation resistance. There should be no uneven filling between the curved copper plate and the perm brick during brick construction. Desirably, workers should not slip during construction.

Therefore, the electrical resistance test described below was carried out, and natural flake graphite and graphitized fiber felt were selected as joint materials. The specific resistance of natural flake graphite is as small as about 10 ^-3 Ωcm in the direction along the open wall,
In addition to having a specific resistance about 100 times in the perpendicular direction, it has a characteristic that its oxidation resistance is relatively large. Of course, when it is used as a joint material, it exhibits conductivity, but since it is slippery, it is deposited on a plate of a metal electrode, for example, to a thickness of about 10 mm, and bricks are deposited thereon. When put on, it is compressed and averaged by its weight, and some flaky graphite powder flows out to its surroundings, and an appropriate amount, for example, a phase with a thickness of about 3 mm, makes the gap between the metal plate and the brick even. Remains filled.

The specific resistance of the graphitized fiber felt varies depending on the density. For example, the specific resistance of 0.1 g / cc is 1 to
It is 10 Ω · cm. Also, the oxidation resistance is relatively good. The felt has a thickness of 7 to 20 mm and a relatively high density (0.
If a joint material of 05-0.2 g / cc) is selected, the felt will be crushed by the weight of the bricks and the conductivity will increase, ensuring electrical contact. Further, when the felt is used, it does not scatter like scaly graphite powder, the work site is not slippery, and the safety problem that the worker slips does not occur. The graphitized fiber felt may be a prepreg impregnated with a binder having a large amount of residual carbon such as phenol resin.

In order to select the joint material, a test apparatus as shown in FIG. 2 was devised, and the electric resistances of various joint materials were compared. Various joint materials were filled between ^two iron plates (37 × 52 mm ² ) having a thickness of 5 mm, and the electric resistance was measured. The results are shown in Table 1.

As shown in Table 1, the contact between iron plates is the lowest when there is no joint material between the joints. However, in an actual furnace, there is a curved surface, and the occurrence of gaps is unavoidable, so joint materials that improve the contact state are required. As the joint material, the electric resistance of the scaly graphite powder and the graphitized fiber felt may be low. Further, in the case of the flake graphite powder, it is considered that the contact resistance between particles constituting the joint material is much larger than the resistance inside the particles due to the influence of the particle size.

Therefore, as a result of testing flake graphite having various particle diameters, it was found that it is preferable to use those having a coarse particle diameter, but it was found that those having a particle diameter of about 0.1 mm or more can be used. However, if the grain size is too coarse, the contact area will be small, so the desirable maximum grain size is about 0.5 mm. On the other hand, carbon black is considered to have a large electric resistance mainly because the particle size is too small.

On the other hand, the carbon fiber which is a long fiber and the graphitized fiber felt obtained by processing it into a felt shape should have a small number of contact points, and therefore the electrical resistance should be low. The electrical resistance was lower. This is because the graphitized fiber felt is formed by processing carbon fibers into a felt shape, so that the fibers are present in the direction perpendicular to the brick surface, and the number of contact points can be substantially reduced.

This is considered to be because, in the case of scale-like graphite powder, the scales are oriented perpendicularly to the loading direction and are folded and densely packed, whereas the carbon fibers are insufficiently dense. In this respect, it is considered that the graphitized fiber felt in which the graphitized fibers are densely woven to some extent in advance has obtained a low resistance value.

[0020]

[Table 1]

From the above, the scaly graphite powder having a relatively coarse particle diameter and the graphitized fiber felt are desirable as the joint material, and these may be used alone or in combination. (Claim 1).

Next, since the coarser particle size of the scaly graphite powder is more conductive as a joint material, it is desirable to use one having an average particle size of 0.12 mm or more. Usually, the average particle size is easy to obtain. It is desirable to use one having a diameter of 0.5 mm or less (claim 2).

Next, both the perm brick and the wear brick are required to have good conductivity. From this point, the amount of graphite is 8
It is desirable that the amount is at least wt%, and the graphite is preferably flake graphite.
Phenol resin is used as the binder, but it is desirable to use calcined MgO-C brick obtained by calcining and carbonizing the phenol resin because it has insulating properties. If the graphite content exceeds 25% by weight, it will be consumed by oxidation and the thermal conductivity will be high, and the heat insulation of the hearth will be impaired. Therefore, in order to secure the heat insulation of the hearth, it is desirable that the graphite content of the perm brick is smaller than that of the wear brick (claim 3).

In the construction method using flake graphite,
It is 2.0 mm or more because it is difficult to secure uniform conductivity in the joints unless the flake graphite powder is spread to a certain degree. If it is too thick, it will take time to install and the flake graphite powder will be worn more. Therefore, it should be 10 mm or less. When MgO-C bricks are lined on the spread scaly graphite layer, the thickness of the scaly graphite layer is about 1 / th of the thickness when spread.
Compressed to 4. The appropriate spraying thickness can be made small as long as the surfaces sandwiching the joint have few irregularities (claim 4).
In the present invention, the average particle size of the flake graphite powder is measured by a method of classifying with a sieve to obtain a weight cumulative distribution and obtaining a particle size at which the cumulative weight becomes 50% by weight.

Graphitized fiber felt is superior to scaly graphite powder in that it is less likely to scatter during construction, does not slip easily in the work place, and is easy to construct. Further, the thickness of the graphitized fiber felt is a value in which the density is converted to that of 0.05 g / cc. If the thickness is less than 3 mm, the graphitized fiber felt is easily torn at the time of construction, and the unevenness of the density becomes large after the construction. Since it becomes difficult to obtain conductivity, the thickness is preferably 3 mm or more, and the material cost is increased if the thickness is too thick, so it is preferably 10 mm or less (claim 5).

[0026]

Example: The content of flake graphite was about 20% by weight and about 10%.
450% thick MgO-C bricks in weight% were used for wear bricks and perm bricks, respectively, both between the anode copper metal plate and the perm brick, and between the perm brick and the wear brick. Average particle size
0.3 mm flaky graphite powder is sprinkled to a thickness of about 8 mm (the thickness of the bricks after stacking is about 2 mm) and piled up, and then a MgO-C stamp material is placed on top of the wear bricks.
It was constructed to a thickness of 100 mm to construct a hearth. When iron scrap was melted in this DC electric furnace, it was confirmed that the electric resistance of the hearth was sufficiently small and that the operation could be continued stably. In addition, since the joint material does not harden and harden after use, the additional effect of facilitating the dismantling of the hearth brick was obtained.

In the above example, the resistance value per unit area of the hearth was about 4.9 Ω · cm ² . Calculated from this value, the resistance of the MgO-C brick part (including stamp material) is 0.8 Ω · cm ² , and the resistance value between joints is about 4 Ω · cm.
²

Next, an example in which graphitized fiber felt is used as a joint material will be given. A commercially available graphitized fiber felt (trade name Dona Carbo Felt) with a thickness of 10 mm,
Basis weight 500 g / m ² , density 0.05 g / cm ³ , specific resistance 2
One layer of Ω · cm was laid between the metal plate made of copper as an anode and the perm brick, and between the perm brick and the wear brick, to make a joint material. Wear bricks and perm bricks have a thickness of 450 mm with the content of the above flake graphite being approximately 20% by weight and approximately 10% by weight, respectively.
This is a MgO-C brick. The electric resistance value in this case was slightly larger than that in the case of using flake graphite as a joint material,
When iron scrap was similarly melted in this DC electric furnace, it was confirmed that the operation could be continued stably.

[0029]

As described above, according to the present invention, the scaly graphite powder or the graphitized fiber felt is used as the joint material both between the metal plate and the perm brick and between the perm brick and the wear brick. In the DC electric hearth, the electric resistance of the entire hearth can be made sufficiently small, and the durability of the hearth is also good.
The power consumption per on can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing a hearth structure of a DC electric furnace.

FIG. 2 is a diagram showing a method for measuring an electric resistance of a joint material.

Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location F27D 11/08 A 8926-4K (72) Inventor Tsukasa Sugano 12 Nakamachi, Muroran-shi, Hokkaido Mitsubishi Steel Muroran special steel stock Muroran Manufacturing Company (72) Inventor Yoshimi Nakamori 12 Nakamachi, Muroran City, Hokkaido Mitsubishi Steel Muroran Special Steel Co., Ltd. Muroran Manufacturing Co., Ltd.

Claims (5)

[Claims] 1. A joint between a metal plate, which is an electrode forming the lowermost part of the hearth of an electric furnace, and a conductive perma brick disposed above the metal plate, and the perma brick and the electric conductivity on the perimeter brick. In a joint between a wear brick, scaly graphite powder or graphitized carbon fiber felt,
Alternatively, a hearth of a DC electric furnace is characterized by using a mixture of scaly graphite powder and graphitized carbon fiber felt as a joint material. 2. The hearth of a DC electric furnace according to claim 1, wherein the joint material is scaly graphite powder, and the average particle size of the scaly graphite powder is 0.12 to 0.5 mm. 3. The hearth of a DC electric furnace according to claim 1, wherein the perm brick and the wear brick are fired MgO—C bricks containing 8 to 25% by weight of graphite. 4. Dry scale-like graphite powder is added to the upper surface of a metal plate which is an electrode forming the lowermost part of the hearth
Form a first graphite powder layer scattered to a thickness of 10 mm, then stack conductive perma brick on the first graphite powder layer, and then dry scale-like on the perma brick Electricity characterized by spreading graphite powder to a thickness of 2.0 to 10 mm to form a second graphite powder layer, and further stacking conductive wear bricks on the second graphite powder layer. Construction method of hearth hearth. 5. A first graphitized fiber felt having a thickness of 3.0 to 10 mm is laid on the upper surface of a metal plate which is an electrode forming the lowermost part of the electric furnace hearth, and then the first graphitized fiber felt is placed on top. Stack conductive perm bricks on the
Next, an electric furnace furnace characterized in that a second graphitized fiber felt having a thickness of 3.0 to 10 mm is laid on the perm brick, and conductive wear bricks are further stacked on the second graphitized fiber felt. Floor construction method.