JPH10189236A - Ac-dc compatible arc furnace and its operating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an arc furnace capable of efficiently operating by arranging a rectifying device between an upper electrode and a bottom electrode of the arc furnace and a secondary side transformer of an electric facility through a switch, and supplying either one of an AC power and a DC power by switching over the switch. SOLUTION: In an arc furnace having a plurality of upper electrodes 3 and a furnace bottom electrode part 4, a plurality of switches 6 are arranged between the upper electrode 3 and a secondary side transformer 5 and connected to them. A rectifying device 7 is connected in parallel to each switch 6, the output sides of each rectifying device 7 are connected to the upper electrode 3 and the furnace bottom electrode part 4, and by switching over the switch 6, AC power is directly supplied to the upper electrode 3 and the furnace bottom electrode part 4, and DC power rectified with the rectifying device 7 is supplied to them through the rectifying device 7. The arc furnace with high efficiency, capable of switching the AC power and the DC power during operation is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an arc furnace, and more particularly to an arc furnace capable of switching between AC and DC during operation, maximizing the advantages of both, and consequently enabling efficient operation. The present invention relates to a furnace, and is advantageously used, for example, as a steelmaking arc furnace.

[0002]

2. Description of the Related Art Steelmaking arc furnaces are broadly classified into AC arc furnaces using AC as a power source and DC arc furnaces using DC. Among them, the AC arc furnace is a furnace that has been used more often than ever, and technologies such as large capacity and UHP operation have been introduced. However, i) large flicker, ii) so-called hot spot, in which the furnace wall near the electrode is melted and damaged due to the difference in reactance of each phase, power imbalance and electromagnetic force acting between the arc currents of each phase. On the other hand, there is a problem that a cold spot where the melting of the scrap does not easily proceed occurs, and iii) when the arc is broken due to the collapse of the charged material or the like, the operator has to manually perform the re-ignition. .

On the other hand, a DC arc furnace has a furnace body structure having a movable electrode on the furnace as a negative electrode and a positive electrode on a furnace bottom, and is a type of furnace in which power is supplied by a DC converter using a thyristor rectifier. In addition, in addition to the arc voltage control by the automatic raising and lowering of the movable electrode, high-speed control of the arc current by the thyristor rectifier improves the control characteristics of the arc, so that a large flicker reduction effect is obtained and no-trip operation is possible. However, since this DC arc furnace has an electrode on the furnace bottom, the furnace bottom electrode and the refractory around the furnace bottom electrode are greatly eroded. In addition, since it is necessary to leave hot heel (pre-charged molten steel) in order to ensure conductivity at the time of ignition, there is also a problem that the amount and composition of hot heel causes variation in refining characteristics of the next charge. . On the other hand, DC arc furnaces have a longer arc length than AC arc furnaces, so they have higher heat transfer efficiency during the melting period, but less heat transfer efficiency during the refining period. There is also a problem that it may be worn.

[0004] As described above, the AC arc furnace and the DC arc furnace have conflicting advantages and disadvantages.
Both AC arc furnaces and DC arc furnaces have been actively developed to solve these remaining problems, but at present, arc furnaces that are satisfactory in all aspects have not been obtained. is there.

[0005]

SUMMARY OF THE INVENTION Therefore, a first object of the present invention is to make the most of the advantages of such an AC arc furnace and a DC arc furnace, to solve the problems advantageously, and to realize an efficient operation. It is to propose an arc furnace capable of performing the above.

[0006] In the operation of the arc furnace, reduced iron is used as a part of the main raw material, if necessary, to dilute the tramp elements. Heretofore, reduced iron has been charged into a scrap basket together with other scraps, and has been charged together with the other scraps from the scrap basket into the furnace. However, since the reduced iron has a uniform shape as compared with other scraps, the bulk density becomes high when the reduced iron is charged in a lump in the furnace, so that the reduced iron is likely to be fused and agglomerated. For this reason, undissolved reduced iron may occur. In addition, since reduced iron has a high C content of 1 to 2%, bumping was likely to occur during the progress of dissolution.

[0007] In addition, auxiliary materials such as quicklime and dolomite have been batch-loaded from the furnace immediately before or immediately after scrap charging, or charged together with scrap from a scrap basket. However, in these methods, the added auxiliary material accumulates in the scrap and melts after the refining period, and the presence of the scrap limits the rate at which the auxiliary material falls, so the auxiliary material becomes massive. In addition, there is a problem that slag formation is delayed.

Accordingly, a second object of the present invention is to propose an arc furnace which enables charged reduced iron and auxiliary materials to be introduced into the furnace without causing a problem such as agglomeration. is there.

[0009]

An arc furnace according to the present invention comprises:
An arc furnace having a plurality of upper electrodes and a furnace bottom electrode, in which a rectifier is provided in parallel between these electrodes and a secondary transformer of electric equipment via a switch, and the AC is obtained by switching the switch. An AC / DC combined arc furnace characterized by being capable of supplying either electric power or DC power, and an arc furnace having a plurality of upper electrodes and a furnace bottom electrode, wherein these electrodes and electric equipment are provided. A converter and an inverter are provided between the secondary transformer and an AC / DC combined arc furnace characterized by being able to supply either AC power or DC power. It is advantageous to provide the gap with a feed hole for the raw material. Further, the method for operating the arc furnace of the present invention includes:
A method of operating an AC / DC combined arc furnace using the AC / DC combined arc furnace described above, wherein AC power is supplied during ignition and refining, and DC is supplied during melting. The present invention relates to a method for operating an arc furnace, a method for operating an arc furnace combined with AC and DC, comprising a plurality of upper electrodes and a furnace bottom electrode, and providing a converter and an inverter between these electrodes and a secondary transformer of electric equipment. The method for operating an AC / DC combined arc furnace characterized by changing the frequency of AC power to be supplied during operation by the inverter, comprising a plurality of upper electrodes and a furnace bottom electrode; A method of operating an AC / DC combined arc furnace in which a converter and an inverter are provided between the secondary transformer and the inverter, and the inverter supplies AC power having a steep zero-crossing edge. An operation method of an arc furnace combined with an alternating current and a direct current, comprising a plurality of upper electrodes and a bottom electrode, and capable of supplying either one of an AC power and a DC power to these electrodes. The method for operating an arc furnace combined with an alternating current and a direct current, characterized in that a raw material is charged from a raw material charging hole formed in a furnace lid when direct current power is supplied.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The arc furnace of the present invention has a configuration in which AC power and DC power can be switched during operation. Thus, it is possible to supply electric power in an optimal form at the time of ignition, the melting period, and the refining period.

Switching between AC power and DC power is performed by:
When the power frequency is achieved by a device combining a converter and an inverter, the power frequency can be made lower or higher than the commercial frequency by the action of the inverter. Further, the AC waveform can be made steeper by the inverter. By the action of these inverters, when the AC power is supplied to the electrode as described above, the electrode can be easily fired by setting the frequency higher than the commercial frequency (for example, 1 MHz or more). Also,
Similarly, during the refining period when AC power is supplied, by setting the frequency lower than the commercial frequency, the arc impedance can be kept low even when a reactor is installed to reduce flicker, and stable operation is achieved. It becomes possible. Further, by setting the frequency to a low frequency during the refining period, a phenomenon in which the side surface of the electrode is preferentially worn due to a skin effect is mitigated.

In addition, by making the waveform of the AC power into a waveform having a steep zero-crossing edge by the inverter, arc breakage is less likely to occur even in the refining period in which the AC operation is performed. This is particularly effective because if the frequency is set to a low frequency, arc breakage tends to occur.

Next, in the arc furnace of the present invention, a hole for introducing reduced iron or auxiliary material may be provided in the gap between the upper electrodes in the furnace lid. With this configuration, the reduced iron and auxiliary materials (quick lime, dolomite, etc.) can be continuously or intermittently charged into the furnace during operation. In addition to avoiding deposition and lumps, it is also ensured to be charged near the electrode, in other words, near the arc spot, so that the melting proceeds rapidly. In particular, in an arc furnace having a plurality of upper electrodes as in the present invention, when DC power is supplied to these electrodes (melting period), the arcs of the respective electrodes attract each other by Lorentz force, The reduced iron and the auxiliary raw materials charged into the furnace through the charging holes are immediately heated to a high temperature by this arc, and thus dissolve more quickly.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described more specifically with reference to the drawings. FIG. 1 shows an example of the arc furnace of the present invention in a main part. In the figure, reference numeral 1 denotes a furnace body, 2 denotes a furnace lid, and 3 denotes an upper electrode which can move forward and backward through the furnace lid 2;
The book is a set. Reference numeral 4 denotes a furnace bottom electrode. A switch 6 is provided between the electrode 3 and the secondary transformer 5. A rectifier 7 and an upper electrode 3 are connected to the switch 6, and the output side of the rectifier 7 is connected to the upper electrode 3 and the furnace bottom electrode 4. By switching the switch 6, the AC power from the secondary transformer can be supplied to the electrodes directly or after being rectified and converted into DC power. It should be noted that devices that are usually provided in the arc furnace, such as an automatic electrode control device and a flicker reduction device, are not shown.

In the arc furnace illustrated by such a configuration,
In the same furnace, it is possible to switch between DC and AC during operation, so that the advantages of both the DC arc furnace and the AC arc furnace can be effectively utilized, and the harmful effects of each arc furnace can be effectively improved. . Specifically, first, at the time of ignition after the charging of the raw material, the switch 5 is connected to the AC side to select AC. As a result, conduction can be achieved without a hot heel, and variations in refining characteristics due to the amount and composition of the hot heel are reduced. Next, in the melting period of the scrap or the like after ignition, the switch 5 is switched to the DC side, and the rectifier 6 rectifies the AC with a thyristor, for example, and then supplies DC to the furnace upper electrode and the furnace bottom electrode. Thus, current and voltage control can be easily performed, and a no-trip operation can be achieved while reducing flicker. Moreover, by selecting a direct current during this melting period, a long arc operation is performed, so that the arc flare easily heats up the scrap on the furnace wall side,
Uniform dissolution becomes possible. On the other hand, during the refining period of the flat bath after the melting, the switch 5 is switched to the AC side, and is used again as an AC arc furnace. As a result, a furnace wall attack due to arc deflection such as a DC arc furnace can be avoided, and a short arc operation is performed. As a result, heat loss to the furnace wall is reduced, and the heat transfer efficiency of the arc to the molten steel is improved.

FIG. 2 shows another embodiment of the present invention. FIG.
In this embodiment, a converter 8 and an inverter 9 are provided in series between the upper electrode 3 and the secondary transformer 5, and the output of the inverter 9 is supplied to the upper electrode 3 on one side and the furnace bottom electrode 4 on the other side. Connect to each. With such a configuration,
When the DC converted by the converter 8 is output as it is without being converted by the inverter 9, the furnace top electrode 3 and the furnace bottom electrode 4
DC can be applied to the
If the DC converted by the above is converted into AC by the inverter 9, it is possible to apply AC to the electrodes. Therefore, similarly to the embodiment shown in FIG. 1, it is possible to switch between AC power and DC power during operation.

In addition, in the arc furnace shown in FIG. 2, when an AC power is applied, an arbitrary low frequency or a high frequency Alternatively, AC power having a waveform with a steep zero-crossing edge can be applied. Therefore, at the time of ignition, by supplying high-frequency power of 1 MHz or more, the atmosphere between the upper electrode and the charged material is ionized,
Since the dielectric breakdown voltage is reduced, ignition is facilitated.

In the refining period, 50
Even if a reactor is provided to prevent flicker by applying a low frequency power of less than 5 Hz, preferably 5 to 30 Hz, the arc impedance is reduced to 10 mΩ or less (for example, 4 to 30 Hz).
Can be as easy as 10 mΩ and the arc is stable. This is for the following reason. When installing a reactor,
Since the impedance of the power supply circuit increases sharply, it is required to increase the voltage in order to secure a predetermined arc current. As a result, a high arc impedance operation is likely to be performed. On the other hand, if the power frequency is reduced by the inverter 9, a predetermined arc current can be ensured without increasing the circuit voltage, so that the benefits of reducing flicker by the reactor and the arc impedance can be reduced. It can be maintained until. Further, by supplying low-frequency power during the refining period, the skin effect of the upper electrode 3 is also reduced, so that the wear of the upper electrode 3 is reduced.

In addition, the arc 9 can be suppressed by the inverter 9 by supplying AC power to the electrode with a steep zero-crossing edge during the refining period, thereby reducing the operator's monitoring load and work load. Operational efficiency is improved and power consumption is reduced. This is because if the zero-crossing edge is a steep AC power, the polarity between the electrode and the raw material metal is rapidly reversed, so that the arc break can be effectively suppressed. Fig. 3 (a) shows the AC power with a steep zero-crossing edge.
A rectangular wave as shown in FIG. 3 is a typical example. However, the trapezoidal waveform shown in FIG. 4B is also effective because the zero-crossing edge is steeper than the sine wave which is commercial power.

Moreover, when low-frequency AC power is supplied by the inverter 9 during the refining period as in the present invention, arc breakage is likely to occur. However, if the AC power is sharp at the zero-crossing edge, the low-frequency AC power can be reduced. However, since arc breakage is less likely to occur, the above-described effects of preventing flicker, maintaining low arc impedance, suppressing electrode erosion, and preventing arc breakage can be obtained.

FIG. 4 shows a main part of another embodiment of the present invention. In FIG. 4A, illustration of the upper electrode 3 is omitted. FIG. 2B is a plan view. In the embodiment shown in FIG. 4, in the arc furnace shown in FIGS. 1 and 2, a feed hole 10 for raw material is provided at the center of the furnace lid 2, and a feeder 11 connected to the feed hole 10 and a feeder 11 are provided. A hopper 12 for reduced iron and a hopper 13 for auxiliary raw materials are provided so as to be connected to the feed hole 10 so that reduced iron or auxiliary raw materials can be charged through the charging holes 10.

As described above, since the charging hole 10 is provided at a position close to the electrode 3, the reduced iron introduced into the furnace from the charging hole 10 is reduced by the arc generated at the tip of the electrode 3. Since it dissolves immediately, there is no problem of bulking or undissolved, and bumping does not occur. The auxiliary raw materials (quick lime, dolomite, etc.) introduced into the furnace from the charging hole 10 also rapidly progress to slagging without agglomeration or slagging delay. In addition, such an auxiliary material has a great effect if it is charged during the melting period for supplying DC power, but it goes without saying that advantageous effects can be obtained even if it is supplied during the refining period for supplying AC power. .

Here, using the arc furnace (100 t) of the present invention shown in FIG. 1, AC power is supplied at the time of ignition, and immediately after ignition, a switch is switched to supply DC power to supply the charged material. Melting was performed, and when this melting operation was completed, a switch was switched to supply AC power, and the power consumption was 340 kWh / t. This is the power consumption rate of 370 kWh / t when operating in a conventional DC arc furnace and 380 kWh in power consumption when operating in a conventional AC arc furnace.
It is effectively improved compared to h / t. In addition, when the flicker index when the operation was performed in the conventional AC arc furnace was 1, the flicker index was 0.5 in the DC arc furnace, but the flicker index was 0.5 in the arc furnace of the present invention. The same excellent flicker control effect as the furnace was obtained.

Next, the arc furnace (10) of the present invention shown in FIG.
Using 0t), AC power is supplied at the time of ignition, and immediately after the ignition, the inverter is turned off and DC power is supplied to dissolve the charged material. When the melting operation is completed, the inverter is turned on. When a rectangular wave, 30 Hz AC power was supplied, the power consumption was 325 kWh / t. This is the power consumption of 370 when operating in a conventional DC arc furnace.
It is effectively improved compared to kWh / t and the unit power consumption of 380 kWh / t when operating with the conventional AC arc furnace.
In addition, when the flicker index when the operation was performed in the conventional AC arc furnace was 1, the flicker index was 0.5 in the DC arc furnace, but the flicker index was 0.5 in the arc furnace of the present invention. The same excellent flicker control effect as the furnace was obtained. In addition, the basic unit of electrode is 1.7 kg / t
This is an improvement over the conventional AC arc furnace of 2.0 kg / t, if not as much as 1.3 kg / t of the conventional DC arc furnace.

The arc furnace (10) shown in FIG.
In the furnace lid of 0 t), a charging hole shown in FIG. 4 was provided, and reduced iron was continuously charged from this charging hole over the melting period so that the mixing ratio with respect to the total main raw material amount was 30%. There was no undissolved residue, and bumping frequency was 1%.
On the other hand, for comparison, a conventional DC arc furnace (100 t)
Reduced iron (mixing ratio 30%) from scrap basket,
Melting operation was carried out after charging the furnace together with other scraps. As a result, the unmelted frequency was 12%, and the bumping frequency was 5%.

The arc furnace (10) of the present invention shown in FIG.
0 t) was provided with a charging hole shown in FIG. 4 in the furnace lid, and quicklime was continuously charged through the charging hole over the melting period.
The basic unit of quicklime was 21 kg / t. In addition, (P) / [P] before tapping was 37. On the other hand, for comparison, a conventional direct current arc furnace (100 t) was charged with quicklime together with scrap in advance in the furnace and then subjected to a melting and refining operation, and the basic unit of quicklime was 23 kg / t. Before tapping (P)
/ [P] was 31.

[0027]

According to the arc furnace of the present invention, since the power supplied to the electrode can be switched between AC power and DC power during operation, it is optimal for each of the firing, melting and refining periods. And an excellent arc furnace operation having the advantages of the conventional DC arc furnace and AC arc furnace.

[Brief description of the drawings]

FIG. 1 is a view showing an example of an arc furnace of the present invention in a main part.

FIG. 2 is a diagram showing another example of the arc furnace of the present invention in a main part.

FIG. 3 is a diagram showing an example of a waveform of AC power applied to the arc furnace of the present invention.

FIG. 4 is a view showing another example of the arc furnace of the present invention in a main part.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Furnace body 2 Furnace lid 3 Upper electrode 4 Furnace bottom electrode part 5 Secondary transformer 6 Switch 7 Rectifier 8 Converter 9 Inverter 10 Input hole 11 Feeder 12 Hopper for reduced iron 13 Hopper for auxiliary material

Continuation of the front page (72) Inventor Nakato Sanka 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. No address) Inside the Kawasaki Steel Works, Mizushima Works

Claims (7)

[Claims] 1. An arc furnace having a plurality of upper electrodes and a furnace bottom electrode, wherein a rectifier is provided in parallel between these electrodes and a secondary transformer of electric equipment via a switch, and An AC / DC combined arc furnace characterized in that one of AC power and DC power can be supplied by switching a switch. 2. An arc furnace having a plurality of upper electrodes and a furnace bottom electrode, wherein a converter and an inverter are provided between these electrodes and a secondary transformer of the electric equipment, and an AC power and a DC power are provided. An AC / DC combined arc furnace characterized in that either one can be supplied. 3. The combined arc and direct current arc furnace according to claim 1, wherein a raw material charging hole is provided in a gap between the upper electrodes in the furnace lid. 4. A method for operating an AC / DC combined arc furnace using the combined AC / DC arc furnace according to claim 1 or 2, wherein AC power is supplied during ignition and refining, and DC power is supplied during melting. A method for operating a DC / AC combined arc furnace characterized by supplying. 5. A method for operating an AC / DC combined arc furnace, comprising a plurality of upper electrodes and a furnace bottom electrode, wherein a converter and an inverter are provided between these electrodes and a secondary transformer of the electric equipment. A method for operating an AC / DC combined arc furnace, characterized in that the frequency of AC power supplied is changed during operation by the inverter. 6. A method for operating an AC / DC combined arc furnace comprising a plurality of upper electrodes and a furnace bottom electrode, wherein a converter and an inverter are provided between these electrodes and a secondary transformer of the electric equipment. A method for operating an AC / DC combined arc furnace, characterized in that the inverter supplies AC power having a waveform with a steep zero-crossing edge. 7. A method for operating an AC / DC combined arc furnace having a plurality of upper electrodes and a furnace bottom electrode and capable of supplying either one of AC power and DC power to these electrodes, comprising supplying DC power. A method of operating an arc furnace combined with an alternating current and a direct current, characterized in that a raw material is sometimes charged from a raw material charging hole formed in a furnace lid.