JPH0513166A - Dc electric furnace with upper electrode and furnace bottom electrode - Google Patents

Abstract

PURPOSE:To prevent DC arcs in a furnace from being made nonuniform by affecting them with the magnetic field generated around feed cables to furnace bottom electrodes of the DC electric furnace. CONSTITUTION:A furnace bottom electrode unit 1 with many small-diameter furnace bottom electrodes 30 is buried at the furnace bottom of a DC electric furnace. Three feed cables 5a, 5b, 5c are uniformly dispersed and arranged at the angle 120 deg. on a conducting, air-cooled pipe 34 provided at the center section of the furnace bottom electrode unit 1, and the current and voltage are independently fed to the cables 5a, 5b, 5c under control. DC arcs in the furnace are unified in polarity by the magnetic field generated around the feed cables 5a, 5b, 5c. The occurrence of cold spots on the furnace wall is reduced, and the fusing of scrap is accelerated.

Description

Detailed Description of the Invention

[0001]

[Industrial application] Melting of metal by direct current arc,
The present invention relates to a DC electric furnace having an upper electrode and a bottom electrode for refining.

[0002]

2. Description of the Related Art Electric furnaces include an alternating current electric furnace and a direct current electric furnace. In the alternating current electric furnace, three graphite upper electrodes are inserted from above the furnace, and the above electrodes are mainly inserted through scrap or molten steel. An arc is generated between them, and in a DC electric furnace, one graphite upper electrode is usually inserted, and a DC arc is generated using the furnace bottom as the other electrode.

Since the AC electrode has three electrodes, the upper structure of the furnace becomes complicated, and the three-phase arc is bent outward by mutual electromagnetic force, resulting in a large amount of radiated heat and poor thermal efficiency. Also, the arc wall bends the furnace wall. Is locally damaged. Further, not only the electrode consumption amount is large, but also noise is large and flickers are severe. On the other hand, a DC electric furnace generally has fewer upper electrodes, so the area around the electrodes above the furnace is simpler, and compared with an AC electric furnace, the unit consumption of graphite electrodes and power consumption and flicker are reduced. Although it has the advantage that it can be expected, there is a problem in the life and safety of the bottom electrode.

Published by Japan Industrial Furnace Association, Industrial Heating Furnace, Vol.25
(1988), No. 2, P24-33, as described in a paper entitled "Current State and Future of DC Arc Furnace", as a bottom electrode of a DC electric furnace, for refractory lined in the bottom of the furnace. Air cooling method of small diameter multi-electrode with many small diameter electrodes buried upright, and water cooling method of large diameter electrode with one or three large diameter steel rods installed upright on the furnace bottom It has been known.

FIG. 7 shows a conventional example of a large-diameter electrode type DC electric furnace in which the upper electrode 18 made of graphite is 3 through the furnace lid 12.
This is inserted and the bottom 16 is made of steel rod and is a water-cooled bottom electrode.
Three 30 are erected upright in the molded refractory. In this case, the maximum diameter of the bottom electrode 30 is about 250 mmφ.
The thyristor 25 faces the three upper electrodes 18 respectively.
Each furnace bottom electrode 30 constitutes an independent electrode control circuit, and controls the voltage and current. With such a configuration, for example, when the total transformer capacity of the furnace transformer 22 is 60 MVA, each of them controls the input power within the range of 20 MVA, so that three arcs are generated in the steady state. Reference numeral 21 indicates a transformer transformer.

In such a large-diameter bottom electrode type DC electric furnace, when all three bottom electrodes 30 become non-conductive due to slag adhesion or the like, one of the upper electrodes 18 is connected to the anode side and the upper electrode is connected. It is possible to melt the scrap in the furnace by generating an arc between 18. However, such a measure has a merit that the measure for the current non-conduction can be relatively easily taken though the scrap melting time becomes long because the input power is lowered.

On the other hand, however, in the large-diameter electrode system shown in FIG. 7, since the upper electrode 18 is three as in the conventional AC electric furnace, three systems are required for the electrode support arm, the electrode lifting device and the electrode control circuit. However, there is a problem that the equipment is complicated and the equipment cost and maintenance cost are inevitably high. Further, as shown in FIG. 8, since the positions of the three upper electrodes 18 in the furnace body 10 are asymmetrical with respect to the furnace wall, cold spots or hot spots are generated on the furnace wall, and scrap does not melt uniformly. Moreover, there is a problem that the small ceiling 12a (see FIG. 7) of the furnace lid 12 surrounded by the upper electrode 18 is extremely quickly damaged by the radiant heat of the arc, and furthermore, the scrap non-melting portion A is generated in the cold spot. Therefore, extra power is applied to dissolve and remove the non-dissolved portion A. As a result, there is a problem in that the time required from tap to tap molten steel from the furnace is extended, and the power consumption, electrode, and refractory basic unit are increased, resulting in high cost.

FIG. 9 is a schematic cross-sectional view showing a conventional example of a small-diameter multi-electrode air-cooling type DC electric furnace, in which a furnace body 10 includes a furnace lid 12 and a furnace wall 1.
4, consisting of a furnace bottom 16, one graphite electrode 18 (two or three in some cases) is inserted through the furnace lid 12, and a water cooling panel 20 is attached to the furnace wall 14. ing. At one end of the bottom 16 of the furnace, a tap hole 24 for tapping the molten steel after refining is provided, and at the other end of the bottom 16, an outlet 23 for discharging slag is provided. A large number of small-diameter furnace bottom electrodes 30 made of steel rods are embedded in the furnace bottom 16 and the furnace body 10
Can be tilted left and right by a tilting device (not shown) such as a hydraulic cylinder. A stopper 26 for stopping the discharge of the molten steel is provided directly below the tap hole 24 so as to be openable and closable.

The small-diameter bottom electrode 30 embedded in the bottom of the furnace has, for example, a furnace volume of 130 t / a large number (about 200) in a heating furnace and 1
A book is a steel round bar with a diameter of about 40 mm and is embedded upright in the furnace bottom 16 between refractory materials 28 'lined with a stamp.The furnace bottom electrodes 30 form the anode of the electrode circuit. A graphite electrode 18 protruding from the furnace lid 12 faces as a cathode. In the case of this method, the diameter of the bottom electrode 30 is 40 mm
φ is the maximum limit.

As shown in FIG. 10, a stamp material 28 'is formed around the small-diameter furnace bottom electrode 30, and the upper end surface of the furnace bottom electrode 30 is exposed on the upper surface of the stamp material 28'. The lower end reaches the electrode support plate 32 which is projected from the bottom plate 16a to the outside of the furnace and is spaced apart from the bottom plate 16a, and is fixed by the tightening nut 7. Further, the lower portion of the furnace bottom electrode 30 is cooled by supplying cooling air between the electrode support plate 32 and the bottom plate 16a from an air cooling tube 34 made of a conductor connected to the electrode support plate 32. Normally, the stamp material 28 'on the bottom plate 16a and the electrode support plate 32 serving as a cooling plate are the bottom electrodes 30
, And these can be replaced as blocks. The insulator 4 includes a bottom plate 16a and a furnace bottom iron skin 16 '.
Is insulated from. The power supply cable 5 is a water-cooled type, and the current supply route is the power supply cable 5 → air cooling pipe 34 → electrode support plate 32 → furnace bottom electrode 30 → molten steel → scrap when the hot heel (molten steel) is formed. → Becomes the upper electrode 18.

As shown in FIG. 9, the electric power supplied through the power receiving / transforming transformer 21 of the electric power supply circuit is converted into a voltage of about 200 to 800 V by the furnace transformer 22 and then supplied to the thyristor 25. . The thyristor 25 is provided in a one-system electrode control circuit that connects the upper electrode 18 and the furnace bottom electrode 30, and melting control by a DC electric furnace is performed by one system. The voltage control is performed by the position control of the upper electrode 18 by an electrode lifting device (not shown), and the current control is performed by the control of the thyristor 25.

Since there is only one upper electrode 18 in this way, the area around the electrode above the furnace is simple, and not only can the reduction of the basic unit of the upper electrode 18 and the basic unit of electric power be expected, but also the melting control of the DC electric furnace. It has the advantage of being easy to control because it can be performed by one system. Here, a DC electric furnace having an air-cooling type bottom electrode 30 in which a large number of small-diameter round steel rods are buried in the bottom shown in FIG.
There are about 200 steel rods up to about mmφ, and many of these bottom electrodes 30 are embedded in the stamped refractory 28 'and all are connected and attached to one electrode support plate 32. An electric current is collectively supplied from one power supply cable 5 to each furnace bottom electrode 30 via the electrode support plate 32. The DC electric furnace shown in FIG. 9 has the following problems because of its configuration and the air cooling system.

(1) When the scrap is melted and the refining charge is repeated by direct current, the small-diameter furnace bottom electrode is melted by heat input from the molten steel and Joule heat generated by the internal current. There is a limit to increasing the diameter of the bottom electrode because of its low heat removal ability, and the upper limit is about 40 mmφ. (2) Since the power is supplied to many bottom electrodes at once, it is impossible to finely control the current to the electrodes.

(3) Further, since the number of bottom electrodes is large, the degree of slag adhering to the upper electrodes of the bottom increases, and when the current becomes non-conducted, the remaining bottom remains when the supply current is constant. An excessive current will flow through the electrodes, which will adversely affect the furnace operation. (4) There are too many furnace bottom electrodes, and it is virtually impossible to monitor the state of electrode dissolution for each one with a thermocouple.

(5) Since the above items (1) to (4) are related to each other, the average current density per furnace bottom electrode is only about half that of the water cooling system, which is inefficient. (6) Because of the large number of bottom electrodes, only stamped refractories can be installed between the electrodes, so the wear rate is faster and the life of the bottom electrodes is shorter than that of brick refractories. (7) The direction of the arc generated in the furnace depends on the magnetic field generated around the power supply cable that supplies current to the electrodes, but its directionality is affected. Since the current is supplied to the electrodes all at once, the direction of the arc is determined by the arrangement of the power supply system, and the reduction of hot spots and cold spots cannot be achieved.

[0016]

The direction of the arc generated in the DC electric furnace is influenced by the magnetic field generated around the power supply cable for supplying the current to the furnace bottom electrode, but its direction is large. Since the electric current is supplied to the small-diameter bottom electrode or the small-diameter bottom electrode unit collectively via one power supply cable, the power supply cable route, that is, the layout of the power supply system, has a small degree of freedom and the arc direction is fixed. However, there is a drawback that hot spots and cold spots cannot be reduced.

The present invention solves the above problems of the prior art,
An upper electrode and a bottom electrode that reduce the non-uniformity of the direction of the arc generated in the DC electric furnace, prevent hot spots and cold spots on the furnace wall, and evenly melt the scrap in the furnace. An object of the present invention is to provide a direct current electric furnace equipped with.

[0018]

The present invention for achieving the above object provides a direct current electric furnace having an upper electrode and a bottom electrode for melting a metal by a direct current arc, and a furnace bottom of the direct current electric furnace is Embedded with one large-diameter bottom electrode or at least one group of small-diameter bottom electrode units, and connecting a plurality of power supply cables to each of the above-mentioned large-diameter bottom electrode or small-diameter bottom electrode unit independently The upper part is characterized in that the power supply routes of the respective power supply cables are dispersed and arranged so that the directivity of the DC arc generated in the furnace is made uniform by the magnetic field generated around these power supply cables. It is a DC electric furnace equipped with electrodes and a bottom electrode.

Further, in the present invention, each of the plurality of power supply cables is provided with a current control thyristor circuit for individually controlling the current, so that the directivity of the DC arc generated in the furnace is made uniform. It is preferable that each of the plurality of power supply cables is provided with an ammeter for measuring the value of the current so that the value of the current flowing through each cable is individually controlled.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. First, FIG. 1 is a sectional view of an air-cooled DC electric furnace according to the present invention. The same parts as those of the conventional example shown in FIG. A large number of furnace bottom electrodes 30 made of steel rods with a small diameter are embedded in the furnace bottom 16 facing one upper electrode 18, but in the present invention, a large number of small-diameter furnace bottom electrodes 30 are grouped. One small-diameter electrode unit 1 is embedded upright in a refractory 28 'on the bottom 16 of the furnace. Thus, each furnace bottom electrode 30 in the small-diameter electrode unit 1 forms the anode of the electrode circuit, and the upper electrode 18 protruding below the furnace lid 12 faces this anode as a cathode. The bottom plate 16a in the small-diameter electrode unit 1 is separated and insulated by the insulator 4.

As shown in FIGS. 2 and 3, in the small-diameter electrode unit 1, a stamp refractory material is provided around the furnace bottom electrode 30.
28 'is placed and the upper end of the bottom electrode 30 is made of stamp material.
It is exposed on the upper surface of 28 '. Further, the lower end portion of the furnace bottom electrode 30 is projected from the respective bottom plates 16a to the outside of the furnace and is fixed to an electrode support plate 32 corresponding to a unit provided apart from the bottom plates 16a. The furnace bottom electrode 30 is cooled by supplying it between the electrode support plate 32 and the bottom plate 16a from an air-cooling pipe 34 made of a conductor and connected to each electrode support plate 32. The current supply route is, when hot heel (molten steel) is formed, the power supply cable 5 corresponding to the electrode unit 1 → air cooling pipe 34 → electrode support plate 32 → bottom electrode 30 → molten steel 8 →
The scrap 9 becomes the upper electrode 18.

As shown in FIG. 1, the voltage is transformed to about 200 to 800 V by the furnace transformer 22 supplied through the power receiving and transforming transformer 21 of the power supply circuit, and then one upper electrode is formed.
18 and three thyristors 25 arranged in parallel are connected by the power supply cable 2, and the three thyristors 25 and the electrode supporting plate 32 of one electrode unit 1 are three via the air cooling pipes 34. Are connected by power supply cables 5a, 5b, 5c to form an electrode control circuit, and an ammeter 35 is provided on each cable 5a, 5b, 5c.

In the present invention, as shown in FIGS. 2 and 3, the feeding cables 5a, 5b, 5c for feeding the bottom electrode 30 of the small-diameter bottom electrode unit 1 are connected to the conductive air-cooling pipe 34 below the small-diameter electrode unit 1. Are distributed at 120 ° angles with respect to each other, and power is supplied to a large number of bottom electrodes 30 from the power supply cables 5a, 5b, 5c through the air cooling pipes 34 and the electrode support plates 32.

In FIG. 1, the portions on the right side of the two alternate long and short dash lines, that is, the power receiving and transforming transformer 21, the furnace transformer 22, the thyristor 25, and the ammeter 35 are fixedly arranged in the electric chamber and left of the two alternate long and short dashed lines. The portion (1) is constructed so as to be tilted integrally with the furnace body (10), and a flexible cable (6) is connected between them. Next, the operation of the present invention will be described.
After the scrap is charged into the furnace body 10, electric power is supplied from the cable 2 to the upper electrode 18 while controlling the current by each thyristor 25, and the upper electrode 18 and the electrode block are adjusted while adjusting the vertical position of the upper electrode 18. The scrap is melted by an arc generated by adjusting the current / voltage applied to the furnace bottom electrode 30 of No. 1 through the power supply cables 5a, 5b, 5c. At this time, the current flowing through each furnace bottom electrode 30 is measured by an ammeter 35 provided on each of the power supply cables 5a, 5b and 5c.

In the scrap melting period, the scrap remains unmelted due to the hanging or dropping of the scrap or the non-uniformity of the direction of the arc due to the magnetic field formed on the power supply cables 5a, 5b, 5c. The current supplied to the bottom electrode 30 of No. 1 is ammeter 35
The thyristor 25 finely controls the amount of current flowing through the power supply cables 5a, 5b, 5c so that the unmelted scrap is not locally generated while the scrap is melted.

As described above, in the present invention, since the three power supply cables 5a, 5b, 5c are evenly distributed near the lower part of the small-diameter electrode unit, the power supply cables 5a, 5b, 5c are energized by energization. The non-uniformity of the direction of the DC arc generated in the furnace under the influence of the magnetic field generated around the furnace is corrected, cold spots and hot spots generated on the furnace wall are prevented, and rapid scrap melting is achieved. It

FIG. 4 shows another embodiment of the air-cooled small-diameter bottom electrode according to the present invention, in which a large number of bottom electrodes 30 are grouped into three groups.
The set of small-diameter electrode units 1 is vertically erected on the bottom 16 of the furnace and buried in equal parts. And the conductive air cooling tube of each small diameter electrode unit 1
Three power supply cables 5a to 5c, 5d to f, and 5g to i are connected to the 34 at an angle of 120 ° in a distributed arrangement. A thyristor 25 and an ammeter 35 are provided in each power supply cable, and the individual control is the same. Due to such distributed arrangement of the plurality of power supply cables, the same operational effect as that of the above-described embodiment can be obtained also in a DC electric furnace having an air-cooled small-diameter bottom electrode in which a plurality of small-diameter electrode units 1 are set.

Further, FIG. 5 shows that one large-diameter bottom electrode 30 is attached to the bottom of the furnace.
The figure shows the case of a water-cooled type embedded in 16, in which four power supply cables 5a to 5d are dispersed and arranged at an angle of 90 °.
It is connected to the large diameter bottom electrode 30 of the book. In addition, FIG. 6 shows that each of the three large-diameter bottom electrodes 30 has an angle of 120.
c, 5d to f, and 5g to i are connected in a distributed arrangement. Similar effects can be obtained in a DC electric furnace having such a water-cooled large-diameter bottom electrode.

The number and distribution of the power supply cables used are not limited to those distributed at equal angles as described above, and the arrangement and the number of furnace bottom electrodes or the conditions of the upper electrodes as well as the furnace body It may be decided as appropriate according to the characteristics. When melting and refining steel scrap using a DC electric furnace having an air-cooled small-diameter bottom electrode under the conditions shown in Table 1, as shown in FIGS. 1 to 3, three power supply cables are connected to each other.
Smelting and refining of scrap were carried out according to an example of the present invention in which the electrodes are energized to the bottom electrode in a dispersed arrangement at an angle of 120 °, and a conventional example in which energization is performed with only one power supply cable shown in FIGS. 9 and 10. The operation results are shown in Table 2.

As shown in Table 2, according to the example of the present invention, the direction of the DC arc generated in the furnace can be controlled more easily than in the case of the conventional example. The generation of spots is resolved and the power consumption rate is reduced.
It was possible to reduce by 10 kwh / t.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

As described above, according to the present invention, by providing a plurality of power supply cables for supplying power to the bottom electrode of the furnace, and by arranging the power supply cables on the route in which the direction nonuniformity of the arc generated in the furnace is minimized, It is possible to suppress the occurrence of hot spots and cold spots. In addition, each feed cable is equipped with a thyristor that controls the current individually and an ammeter that measures the current value to control the direction of the arc generated in the furnace. It is also possible to promote the melting of molten scrap and suppress the generation of hot spots and cold spots.

As a result, it is possible to achieve an improvement in productivity by reducing the power consumption rate and shortening the melting time, and the resulting effect is very large.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the overall arrangement of a DC electric furnace having a small diameter bottom electrode according to an embodiment of the present invention.

FIG. 2 is a sectional view showing a small diameter furnace bottom electrode unit according to an embodiment of the present invention.

FIG. 3 is a plan view showing the arrangement of power supply cables as seen from the direction of arrows AA in FIG.

FIG. 4 is a plan view showing the arrangement of power supply cables to a plurality of small-diameter furnace bottom electrode units according to the present invention.

FIG. 5 is a plan view showing the arrangement of power supply cables to the large-diameter furnace bottom electrode according to the present invention.

FIG. 6 is a plan view showing an arrangement of power supply cables for a plurality of large-diameter furnace bottom electrodes according to the present invention.

FIG. 7 is a sectional view showing the overall arrangement of a DC electric furnace having a large-diameter bottom electrode according to a conventional example.

FIG. 8 is a plan view showing a scrap unmelted portion of the DC electric furnace according to the conventional example of FIG.

FIG. 9 is a cross-sectional view showing the overall arrangement of a DC electric furnace having a small-diameter furnace bottom electrode according to a conventional example.

FIG. 10 is a cross-sectional view showing a small-diameter furnace bottom electrode unit according to a conventional example.

[Explanation of symbols] 1 electrode unit 2 power supply cable 4 insulator 5 power supply cable 8 Molten steel 9 scrap 12 Furnace lid 14 Furnace wall 16 hearth 18 Upper electrode 20 water cooling panel 21 Substation transformer 22 Furnace transformer 23 Waste outlet 24 Steel tap 25 Thyristor 26 Stopper 28 Furnace bottom refractory 30 Furnace bottom electrode 32 Electrode support plate 34 Air-cooled tube 35 ammeter

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Tadaaki Iwamura             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works

Claims (3)

[Claims] 1. A direct current electric furnace comprising an upper electrode and a bottom electrode for melting a metal by means of a direct current arc, wherein at least one large diameter bottom electrode or at least one group of bottom electrodes of the direct current electric furnace is provided. A small-diameter bottom electrode unit is embedded, and a plurality of feeding cables are connected to each of the large-diameter bottom electrode or the small-diameter bottom electrode unit to feed power independently, and the furnace is generated by the magnetic field generated around these feeding cables. A DC electric furnace having an upper electrode and a furnace bottom electrode, characterized in that the power supply routes of the respective power supply cables are dispersed and arranged so as to make the directivity of the DC arc generated in the inside uniform. 2. The current control thyristor circuit for individually controlling the current is provided for each of the plurality of power supply cables.
A DC electric furnace equipped with the above described upper electrode and furnace bottom electrode. 3. A DC electric furnace comprising an upper electrode and a furnace bottom electrode according to claim 1, wherein each of the plurality of power supply cables is provided with an ammeter for measuring a current value.