JP2639413B2 - Method for preventing breakage of graphite electrode socket during electric melting and refining of metal in arc furnace and breakage prevention device provided for the method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Object of the Invention> The present invention relates to a method for preventing a graphite electrode from being broken during an arc furnace refining operation and a device for preventing a broken electrode from being used for the method.

2. Description of the Related Art Recently, in electric arc melting and refining of steel and other metals, a water-cooled type in which the inside of an upper electrode is cooled by cooling water in order to suppress oxidation consumption of a graphite electrode outer peripheral surface due to a decrease in electrode unit consumption. A non-consumable electrode is constructed, a graphite electrode is connected to the lower end of the non-consumable electrode via a nipple as a consumable electrode, and a method and a device for refining while cooling the lower graphite electrode by the upper non-consumable electrode are proposed. Have been.

However, in this case, when the graphite electrode consumed by use is removed and a new graphite electrode is connected, the graphite electrode is moved off-line from the arc electric furnace and the consumed graphite electrode is removed from the nipple. Thereafter, when a new graphite electrode is connected, a nipple is attached to the upper electrode composed of the non-consumable electrode, a new graphite electrode is attached to the nipple, and the nipple is transferred from the off-line to the arc electric furnace. Since the transfer between the off-line and the off-line at the time of this exchange is extremely heavy labor, even though the oxidative consumption of the graphite electrode is prevented, the above-mentioned non-consumable electrode method is not practically used at present.

On the other hand, conventionally, metal electrodes such as steelmaking are generally refined by sequentially connecting graphite electrodes from above. In this case, the connection of the electrodes can be performed on an electric arc furnace, and there is no complexity of transferring off-line. Even in this case, cooling water is continuously sprayed to the outer peripheral surface of the upper graphite electrode to cool it, thereby suppressing oxidation consumption of the lower graphite electrode and reducing the unit consumption of the electrode. Has also been proposed. However, when the graphite electrodes are sequentially connected from the top and the cooling water is sprayed on the surface of the graphite electrode, the cooling water is sprayed on the graphite electrode and the graphite electrode is cooled. Since the nipple to be provided is in a high temperature state as it is, such a difference in the temperature conditions causes not a few breakage accidents at the nipple and the bottom of the socket. Therefore, it is desired to prevent such an accident.

Even when the cooling water is not sprayed on the surface of the graphite electrode, a breakage accident of the socket portion or the like easily occurs due to a difference in thermal expansion coefficient based on a temperature difference between the graphite electrode and the nipple.

Problems to be Solved by the Invention First, the mechanism of occurrence of breakage in a socket portion corresponding to a connection portion of graphite electrodes connected to each other has been conventionally described by stress concentration by a stress concentration coefficient.

However, paying attention to the fact that the mechanism of occurrence cannot be explained by this stress concentration, the present invention has elucidated this mechanism from another point of view, and has been established based on the elucidated results.

That is, according to the clarified results, cracks first occur in the upper graphite electrode of the pair of graphite electrodes, and in particular, in the upper graphite electrode, the lower end surface in contact with the upper end surface of the lower graphite electrode. And does not occur on the upper end surface of the lower graphite electrode. From this point, in order to prevent initial cracks occurring on the lower end surface of the upper graphite electrode, a fastening ring is fitted to a position including the lower end surface on the outer peripheral portion of the upper graphite electrode, and a cooling medium is attached to the fastening ring. To give a radially compressive force to the lower end surface of the upper graphite electrode. Thus, a method for preventing a breakage accident and a device for preventing the same are proposed.

<Structure of the Invention> Means for Solving the Problems and Their Actions That is, the present invention provides a method for refining a metal in an electric arc furnace, which includes a pair of upper and lower graphite electrodes connected via a nipple. At the lower end of the graphite electrode, a tightening ring is fitted only to the outer peripheral surface including the lower end surface, and a cooling medium is sprayed on the tightening ring, and the lower end surface of the upper graphite electrode is directed radially toward the center by the tightening ring. It is characterized by applying a compressive force to

The configuration of these means and their effects will be specifically described below with reference to the drawings.

FIG. 1 is a front view of an example of a prevention device used in carrying out the present invention, FIG. 2 (a) is a longitudinal sectional view taken along line AA, and FIG. 2 (b). FIG. 3 is a cross-sectional view taken along the line BB of FIGS. 3 (a), (b) and (c).
FIG. 3 (a) is an explanatory view of a breakage accident occurrence mechanism. FIG. 3 (a) is a vertical sectional view of a graphite electrode connected by a nipple, and FIG.
FIG. 3B is a cross-sectional view taken along line CC of FIG.
FIG. 3 (c) is a cross-sectional view taken along line DD of FIG. 3 (a), and FIGS. 4 and 5 are front views showing a part of a protection device according to another embodiment in cross section. .

First, in FIGS. 1, 2A and 2B, reference numeral 1 denotes an upper graphite electrode, and the upper end of the upper graphite electrode 1 is gripped by an electrode holder (not shown) as in the conventional example. ing. For connection of the electrodes, a nipple (not shown in FIGS. 1, 2 (a) and 2 (b) but previously shown in FIG. 3 ( In a), the nipple is screwed into a socket at the upper end of the lower graphite electrode 2, and thus the two graphite electrodes 1 and 2 are connected via the nipple 5. You.

At the time of such connection, a notch 1a is formed in an annular shape on the outer peripheral surface including the lower end surface in contact with the lower graphite electrode 2 to be connected at the lower end of the upper graphite electrode 1, and the notch 1a is formed.
2A, the lower end surface of the lower ring electrode 3 is aligned with the lower end surface of the upper graphite electrode 1 by aligning the lower end surface of the lower ring electrode 3 with the lower end surface of the upper graphite electrode 1 as shown in FIG. The outer periphery of the portion is not enclosed, and only the lower end portion of the upper graphite electrode 1 is tightened with the tightening ring 3 including the lower end surface.
Besieged by.

That is, the fastening ring 3 is fitted only on the outer peripheral surface of the lower end of the upper graphite electrode 1. In this case, as shown in FIGS. 2 (a) and 2 (b), a notch 1a reaching the lower end is formed in an annular shape on the outer peripheral surface of the lower end of the upper graphite electrode 1, and the annular notch 1a is formed. Is fitted with the tightening ring 3.

At this time, the tightening ring 3 is heated at a high temperature and expanded in advance, and the tightening ring 3 is seated at the annular notch 1a. Thereafter, the tightening ring 3 is rapidly cooled and fitted.

In this manner, the fastening ring 3 is fitted only to the lower end of the upper graphite electrode 1 and cooling water is sprinkled on the fastening ring 3 to suppress the thermal expansion of the fastening ring 3, thereby
A force directed from the radial direction toward the center, that is, a compressive force is applied to the upper end portion of the upper graphite electrode 1, especially the lower end surface, thereby preventing cracks generated on the lower end surface of the upper graphite electrode.

When the tightening ring 3 fitted to the lower end of the upper graphite electrode 1 is cooled by cooling water, the upper end surface of the lower graphite electrode 2 may be in contact with the lower surface of the tightening ring 3, and The upper end surface of the graphite electrode 2 further cools down the cooling water sprayed toward the tightening ring 3 along the outer peripheral surface of the lower graphite electrode 2, so that the lower graphite electrode 2 is cooled,
This also prevents the lower graphite electrode 2 from being oxidized and consumed.

Hitherto, many means for reinforcing a connection portion of a graphite electrode have been reported. However, in these reports, as noted above,
The mechanism of breakage of the electrode socket has not been elucidated in general, and in particular, this point has not been taken into account, despite the fact that the connection is exposed to high temperatures and the thermal effect is remarkable. For this reason, even if many means for reinforcing the connecting portion have been proposed, they remain in the proposal stage and have not yet been put to practical use.

From the above, the present inventors conducted a fundamental study of the mechanism of the breakage of the socket portion, not only from the viewpoint of stress concentration, and found that the mechanism is as follows.

That is, in FIG. 3 (b), the nipple 5 is at a high temperature even if the outer peripheral surface of the upper graphite electrode 1 is cooled in order to prevent oxidation and consumption. For this reason, the lower end surface of the upper graphite electrode 1 is pushed outward from the center in the radial direction by the nipple 5, and the lower end surface of the upper graphite electrode 1
A tensile stress directed outward in the radial direction acts. This tensile stress is generated at the lower end surface of the upper graphite electrode 1 (a).
When the allowable tensile strength of the upper graphite electrode 1 is exceeded (generally, the graphite material has a large compressive strength but a very low tensile strength), cracks 4 are first generated in the directions from (a) to (b). This first crack 4
Rises sequentially from the lower end surface of the upper graphite electrode 1, propagates in the direction of the central axis of the upper graphite electrode sequentially with the rise, and reaches a portion in contact with the final thread of the nipple 5. When this point is reached, further, as shown in FIG. 3 (c), cracks 4a are generated in the horizontal direction also in the places shown in (c) and (d), and a breakage of the socket portion appears. .

Based on the mechanism of occurrence of the broken socket part accident thus clarified, in the present invention, in order to prevent this accident,
On the lower end surface of the upper graphite electrode, the cracks 4 that occur temporarily, that is, initially, are prevented, thereby preventing a breakage accident.

As a means for preventing this, a compressive stress toward the center in the radial direction is generated by the tightening ring 3 on the upper graphite electrode 1 in order to overcome the tensile stress toward the outside in the radial direction of the upper graphite electrode.

In this case, even if the compressive stress is not always equal to the tensile stress, even if it becomes larger than the generated tensile stress,
The graphite material has no problem because it has a higher compressive strength than a tensile strength.

That is, in order to apply compressive stress during operation by the tightening ring 3, even if the use condition of the graphite electrode is 1000 ° C. or more,
At a high temperature, it is necessary that (i) there is no gap between the fastening ring 3 and the graphite electrode 1 due to the difference in thermal expansion, and (ii) the hot strength is sufficient.

In this regard, in the present invention, in consideration of the difference in thermal expansion under high temperature, the tightening ring 3 is heated and shrink-fitted in advance, and the notch 1a of the graphite electrode 1 is cooled to suppress the thermal expansion. When it is made of a metal material or the like, water is sprayed on the tightening ring 3 when the electrode is used, and cooled.

With such an operation, the above (i) and (ii)
Condition can be sufficiently maintained.

At this time, the heating temperature at the time of fitting the fastening ring 3 is preferably 200 to 700C, particularly preferably 500 to 600C.

Further, as described above, when the fastening ring 3 provided near the socket portion of the upper graphite electrode 1 is sprayed and cooled to suppress thermal expansion, the lower graphite electrode 2 can also be cooled by the cooling water falling, and thereby the electrode base can be cooled. A large unit reduction can be achieved.

That is, the upper and lower graphite electrodes 1 and 2 are made of graphite, a material having extremely good thermal conductivity. For this reason, the cooling water sprinkled on the tightening ring 3 is considerably cooled by descending along the outer peripheral surface of the lower graphite electrode 2, is also cooled from the end surface by the tightening ring 3, and the oxidation consumption at the tip is considerably suppressed. For example, when about 10% of the length of the upper graphite electrode 1 is maintained in a black state, the oxidative consumption of the lower graphite electrode 2 is considerably suppressed, and the ratio of the reduced electrode basic unit is 12% or more. And greatly improved.

The fastening ring 3 can be made of any material having sufficient heat resistance and considerable strength. For example, it can be composed of metal materials such as iron, steel, aluminum, and alloys of these metals. In the case of these materials, they expand during operation and lose the fastening effect, so that it is necessary to spray cooling water. In contrast, carbon fibers, silicon carbide fibers, and materials reinforced with these fibers, such as CCM (carbon-carbon composite), CFRP (carbon fiber reinforced plastic), FRM (fiber reinforced metal), etc. In the case of construction, other than the FRM, the tightening effect of the other materials is hardly impaired as the temperature increases.

Further, the fastening ring 3 can be provided as shown in FIG. 4 or FIG. 5 without being fitted as described above.

That is, in the example shown in FIG. 4, in the two graphite electrodes 1 and 2 connected via a nipple, a tightening ring 3 is fitted to the outer periphery including the lower end surface of the upper graphite electrode 1, and the upper part is Cover the lower end surface of the graphite electrode 1.
In this case, a notch is not formed, but if the degree of fitting is increased by the tightening ring 3, a sufficient compressive stress can be applied to the upper graphite electrode 1.

In the example shown in FIG. 5, an outer peripheral surface is formed in a tapered shape at a lower end portion including a lower end surface of the upper graphite electrode 1, and a tightening ring 3 having an inversely tapered inner surface is fitted therein, and the tightening ring 3 is formed. Covers the lower end surface of the upper graphite electrode 1. In this example, the tightening ring 3 can be smoothly fitted into the upper graphite electrode 1 without any gap, and a compressive stress can be uniformly and reasonably applied to the graphite electrode by the wedge effect.

EXAMPLE A graphite electrode having an outer diameter of 20 inches was used, and four types of tightening rings 3 were used to cut out the lower end of the upper graphite electrode 1 as shown in FIGS. 1, 2 (a) and 2 (b). A part 1a was formed, and four types of tightening rings were fitted therein.

For these examples, the flexural strength of the graphite electrode socket at room temperature was 100% higher than that of the comparative example without the tightening ring.
The results shown in Table 1 were obtained based on the above.

In addition, the tightening rings are all carbon steel pipes, and the shapes are divided into the following two forms.

Tightening ring A: outer diameter 510 mm, inner diameter 504 mm, height 40 mm Tightening ring B: outer diameter 516 mm, inner diameter 507 mm, height 30 mm For comparison, a case where no tightening ring is used is also shown as a comparative example.

From the above results, when the tightening ring is installed, the transverse rupture strength at room temperature is about
It turned out that it improved by about 50%.

The bending strength is a value obtained by multiplying a bending moment including a contact end face with the lower graphite electrode, with a fulcrum at a point where the final thread of the nipple contacts the socket portion of the upper graphite electrode. That is, a bending moment is applied to the comparative example having no tightening ring, and the value of the bending moment when the nipple is broken is determined. This value is set to 100%, and the other values are shown as the comparison values.

Next, based on the results shown in Table 1, 50 practical tests were carried out in an actual steelmaking arc furnace using the fastening ring A (structural carbon steel pipe).

At this time, when the cooling water was sprayed on the tightening ring 3 for operation, a breakage rate of about 3% occurred in the conventional example, but according to the present invention, no breakage accident occurred.

<Effects of the Invention> As described above in detail, the present invention, among a pair of graphite electrodes to be connected, fits a tightening ring only on the outer periphery including the lower end surface at the lower end portion of the upper graphite electrode,
A cooling medium is sprayed on the tightening ring, thereby applying a compressive force directed radially toward the center at the lower end surface of the upper graphite electrode. Therefore, cracks near the end surface of the upper graphite electrode socket, which occur temporarily in the arc furnace, can be completely prevented, and breakage of the graphite electrode can be completely prevented.

Further, the cooling of the tightening ring by spraying the cooling water can suppress the oxidative consumption of the lower graphite electrode.

[Brief description of the drawings]

FIG. 1 is a front view of an example of a prevention device used in practicing the present invention, FIG. 2 (a) is a longitudinal sectional view taken along the line AA,
FIG. 2 (b) is a cross-sectional view taken along the line BB, and FIGS. 3 (a), (b) and (c) are explanatory views of a breakage accident occurrence mechanism, and FIG. FIG. 3 (b) is a longitudinal sectional view of a graphite electrode connected by a nipple, and FIG.
FIG. 3 (c) is a cross-sectional view taken along line CC of FIG.
FIG. 4 and FIG. 5 are front views showing a part of a preventive device according to another embodiment in cross section. Symbols 1, 2 ... graphite electrode 3 ... fastening ring

Claims (2)

(57) [Claims] 1. A pair of upper and lower graphite electrodes connected via a nipple during metal refining in an electric arc furnace, and tightened only to the outer peripheral surface including the lower end surface at the lower end of the upper graphite electrode. At the time of an arc furnace refining operation, a ring is fitted, a cooling medium is sprayed on the tightening ring, and a compressive force is applied radially toward the center at the lower end surface of the upper graphite electrode by the tightening ring. How to prevent graphite electrode breakage accident. 2. A pair of upper and lower graphite electrodes connected via a nipple during metal refining of an electric arc furnace, only on the outer periphery including the lower end surface at the lower end of the upper graphite electrode. An arc furnace refining operation characterized by fitting a clamping ring which is annular and to which a cooling medium is sprayed and which applies a compressive force radially directed toward the center at the lower end face of the upper graphite electrode. Graphite electrode breakage accident prevention equipment.