JPS6364485B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Conventionally, reduced iron is melted using an arc furnace to produce molten steel, and the molten steel is utilized. When producing molten steel as described above, the molten steel accumulates in the deep part of the furnace, and a layer of slag is generally formed above the molten steel. In the present invention, reduced iron is sequentially charged into the slag, and in the process of directing the reduced iron through the slag layer to the molten steel, the reduced iron is melted and separated into a slag element and a molten steel element. The present invention relates to a method of controlling the arc furnace in order to obtain the molten steel.

When treating reduced iron as described above, the temperature of the molten steel layer is generally required to be maintained at a substantially constant temperature for the following reasons. That is, (a) If the temperature of molten steel becomes too low, it becomes impossible to maintain the original properties of molten steel, that is, the molten state, and it solidifies.

(b) If the temperature of molten steel is too high, it will cause damage to the furnace and increase energy loss.

Furthermore, the temperature of the slag is required to be maintained at a substantially constant temperature for the following reasons.
That is, (c) If the temperature of the slag becomes too low, the ability to dissolve reduced iron decreases, and the reduced iron directly enters the high-temperature molten steel. Then, carbon and oxygen react rapidly in the high-temperature molten steel and expand (CO boiling), causing an explosive phenomenon that is extremely dangerous.

(d) If the temperature of the slag is too high, not only will the energy loss increase significantly, but the damage to the furnace will also increase significantly.

From these viewpoints, it is necessary to maintain the temperatures of the molten steel and the slag layer at appropriate temperatures. However, the temperatures of the molten steel and slag vary due to variations in power supply voltage and ambient temperature of the furnace. In this case, with conventional control methods that involve adjusting the power supply or adjusting the arc length, if an attempt is made to correct the temperature of the molten steel or slag, the temperature of the other will follow suit, causing one to change. There was a problem that if one set the temperature to the proper temperature, the other would deviate from its own proper temperature.

Therefore, the present invention aims to eliminate the above-mentioned problems, and provides a method for controlling an arc furnace in which the temperatures of both slag and molten steel can be controlled individually to maintain each at an appropriate temperature. It is something.

The drawings showing the embodiments of the present application will be described below. In FIGS. 1 and 2, an arc furnace 1 is comprised of a concave furnace body 2 and a furnace lid 3 that closes an opening at the top of the furnace body 2. As shown in FIGS. As is well known, the furnace body 2 is
A refractory material 5 is placed on the inner surface of a furnace shell 4 made of steel or other material.
It is formed with the following. In the internal space of the furnace body 2, in its operating state, molten steel 6 and slag 7 covering the upper surface of the molten steel 6 exist as shown in FIG. A molten steel temperature detector 8 for detecting the temperature of molten steel 6 and a slag temperature detector 9 for detecting the temperature of slag 7 are attached to the furnace body 2. For example, thermocouples are used as these temperature detectors 8 and 9. The furnace body 2 is also provided with a tapping trough 10 for flowing out the molten steel 6, as is well known. On the other hand, the furnace cover 3 is made of steel and has a water-cooled structure, or is made of refractory material. A plurality of electrodes 12 for arc generation are provided at the center of the furnace lid 3 via an annular insulating material 11. The electrodes 12 are each moved up and down by an electrode lifting device (not shown). In FIG. 2, the electrode 12 on the left side is shown in a lowered state, and the electrode on the right side is shown in a slightly raised state. The furnace lid 3 also has a charging chute 1 for charging reduced iron.
3 is installed. This charging chute 13 is designed to be able to charge reduced iron near the center of the furnace body 2.

Next, FIG. 3 shows the power supply to the arc furnace 1 and its control system. That is, the power from the high voltage or extra high power source 21 is passed through the disconnector 22 and the circuit breaker 23.
It is guided to the primary side of the furnace converter 24 through the. The furnace transformer 24 is equipped with a tap changer 25 to adjust the output on its secondary side. The secondary output of the furnace transformer 24 is supplied to the electrode 12 in the arc furnace 1 . On the other hand, a part of the electric power passing through the disconnector 22 is sent to an arc furnace control panel 28 and an automatic electrode control device 29 via an instrument transformer 27 to operate them. The arc furnace control panel 28 receives the detected current values from the current transformers 30 and 31 provided on the primary and secondary sides of the furnace transformer 24 and the detected voltage value from the voltage detection terminal 32. Then, the tap changer 25 is switched so that the appropriate power required is supplied to the electrode 12 of the arc furnace 1. Further, the control panel 28 shuts off the circuit breaker 23 in an emergency.
Further, the automatic electrode control device 29 inputs the current detection value from the current transformer 31 and the voltage detection value from the voltage detection terminal 32 to control the electrode lifting device 33 for raising and lowering the electrode 12. This places the electrode 12 at a height where an arc of appropriate length occurs.

Next, the operation of the arc furnace 1 shown in FIGS. 1 and 2 will be explained. First, initial charging into the furnace body 2 is performed. For this initial charging, scrap or a mixture of scrap and reduced iron is used because reduced iron alone is difficult to dissolve. The charging amount is 2
For example, about 20% of the total charge amount, for example, about 20 tons in the case of a 100-ton furnace. In addition, if the initial charging is only scrap, batch charging similar to the general scrap steelmaking method may be used, but if scrap and reduced iron are mixed, the charging material will stick to the inner wall of the furnace body 2. It is better to build it up in the center of the bottom of the furnace body 2 to prevent it. If the initial charging is carried out as described above, after covering the furnace lid 3, electric power is supplied to the electrode 12 to generate an arc, and the scrap charged in the initial charging or the mixture of scrap and reduced iron is dissolve. Incidentally, for the above-mentioned initial charging, a method may be used in which a small amount of the molten steel from the previous time is left.

Once the initial charge as described above is melted and a molten material is formed at the bottom of the furnace body 2, power is continued to be supplied to the electrode 12 to continue generating the arc 14 from the electrode 12, and the Reduced iron 15 is continuously charged from the chute 13 as shown in FIG.
The reduced iron 15 charged in the above manner may be in the form of pellets, small lumps, or briquettes. In the steady state of operation of the furnace, where reduced iron is continuously charged and melted, molten steel 6 accumulates at the bottom of the space inside the furnace body 2, and slag 7 is on top of it, as shown in FIG. It is in a floating state. In this case, the specific gravity of the molten steel 6 is 7~
At around 7.5, the temperature is around 1580℃,
The specific gravity of the slag 7 is 1.5 to 3 (2.5 to 3 when the input of reduced iron is stopped), and its temperature is around 1600°C. The temperature of the molten steel 6 is set within a range from a low temperature that allows it to be used as molten steel (does not solidify) to a high temperature that does not cause excessive loss of thermal energy or damage to the inner wall surface of the furnace body 2. The temperature of the slag 7 is set within a range from a low temperature that does not reduce the melting ability of reduced iron to a high temperature that does not cause excessive loss of thermal energy or damage to the inner wall surface of the furnace body 2. Slag 7
The temperature varies depending on the composition of the slag. In the above steady state, the input chute 13
The reduced iron 15 introduced from the slag first enters the slag 7. In the slag 7, the reduced iron 15 is melted into a molten steel element (iron) and a slag element (slag).
Separate into Since the molten steel element has a large specific gravity, it moves downward and mixes into the molten steel 6. The slag element also mixes into the slag 7 around it.

Next, in the above operating state, molten steel 6 and slag 7
The operation for changing the temperature of is performed as follows. That is, changing the power input to the electrode 12 (this operation is performed, for example, using the control panel 28) or raising and lowering the electrode 12 (this operation is performed, for example, using the electrode control device 29). The length of the arc 14 formed between the lower end of the electrode 12 and the molten steel 6 is changed. Next, the operation will be explained based on FIG. FIG. 4 is a graph showing the relationship between the power supplied to the electrode 12, the length of the arc 14, and the temperature changes of the molten steel 6 and slag 7 when the amount of reduced iron 15 input is 1 ton per minute. Incidentally, since the arc 14 is generated inside the furnace, it is difficult to visually observe it. Therefore, as a means of estimating the length of the arc 14, the power factor (the power factor of the electric power supplied to the electrode 12), which has a constant relationship with the length of the arc, is generally used. is scaled by the power factor value. (The relationship between arc length and power factor is such that the longer the arc, the higher the power factor, and the shorter the arc, the lower the power factor.) (1) First of all, , when the reduced iron 15 is constantly charged and melted in the arc furnace 1 and the temperatures of the molten steel 6 and slag 7 are kept constant, the relationship between the supplied power and the power factor is as follows. Curve of temperature change at 0℃/min for molten steel and 0℃/min for slag
The state is at the intersection point (A) with the temperature change curve for 20 minutes.

(2) If the slag temperature detector 9 detects that the temperature of the slag 7 has decreased and you want to increase only the temperature of the slag 7, for example by 3°C/min,
Increase the supplied power and lengthen the arc length. That is, the supplied power and power factor are respectively increased to the point shown in (B). (At this time, the electrode 12 is raised as shown on the right side in FIG. 2, and the length of the arc 14 generated between the electrode 12 and the molten steel 6 becomes longer.) The furnace is operated in this state. By doing so, only the temperature of the slag 7 increases while the temperature of the molten steel 6 remains the same. When the slag temperature detector 9 detects that the temperature of the slag 7 has reached the desired temperature, the electric power and power factor are returned to the original point (A).
Thereafter, the molten steel 6 and slag 7 can be maintained at the changed temperature.

(3) If it is desired to lower only the temperature of the slag 7, reduce the supplied power and shorten the arc length. In other words, the supplied power and power factor are expressed as (C) in Figure 4, for example.
Lower it to the point indicated by . All that remains is to perform the same operations as in (2) above.

(4) If you want to increase only the temperature of molten steel 6 (for example, 2°C/min) because the molten steel temperature detector 8 has detected that the temperature of molten steel 6 has decreased,
Increase the supplied power and shorten the arc length. That is, the electric power and power factor are set to the state shown in (D). By operating the furnace in this state, only the temperature of the molten steel increases at a rate of 2°C/min. When the molten steel temperature detector 8 detects that the temperature of the molten steel has reached the desired temperature, the electric power and power factor are returned to the state shown in (A), and the molten steel 6 and slag 7 are It is possible to maintain the achieved temperature state.

(5) If you want to lower only the temperature of molten steel 6,
Decrease the power supply and increase the arc length. That is, the electric power and power factor are set to the state shown in (E), for example. After that, the same operation as in (4) above can be performed.

The above has described the case where the temperature of the slag or molten steel is changed by changing both the power supplied to the electrode 12 and the arc length. However, depending on the temperature change required for each (for example, lowering one and increasing the other, or increasing or decreasing the temperature with a predetermined temperature change, respectively), the power supply or arc length may vary. It is also possible to control only one of them to change the temperature of the molten steel and slag to predetermined values, respectively.

On the other hand, if there is a change in the amount of reduced iron input to the arc furnace 1 (the amount input per certain period of time), the temperature of the slag 7 or molten steel 6 will change if the operation is performed at the same amount of power or arc length as before. It ends up. In such a case, in order to correct the temperature change and return to the original temperature state, the relationship between the supplied power and arc length at the changed input amount of reduced iron and the temperature change of molten steel and slag (for example, The same operation as above may be performed based on the graph shown in FIG. 5 which shows the above relationship when the input amount of reduced iron is 1.2 tons per minute.

The above operations can be performed by calculating the relationships shown in Figures 4 or 5 in advance based on the various ratings of the arc furnace, or by determining such relationships experimentally. , the arc furnace control panel 28 or the automatic electrode control device 2 is controlled based on the detected temperatures of the temperature detectors 8 and 9 by a computer or other control device that stores these relationships in advance.
9 is preferably carried out under automatic control. However, while the temperatures of the molten steel 6 and slag 7 are being detected by the molten steel and slag temperature detectors 8 and 9, the power supply and the arc length (elevating and lowering the electrode 12) are manually adjusted as described above. It's okay to get old.

As described above, in this invention, reduced iron is charged, it is separated into a molten steel element and a slag element, and the molten steel element is mixed into the molten steel 6, so that the molten steel 6 is gradually increased. There are features that make it possible.

Furthermore, the present invention has the advantage that by generating an arc 14 between the electrode 12 and the molten steel 6, the temperatures of the slag 7 and the molten steel 6 can be maintained.

Furthermore, the excellent feature of the present invention is that when melting the above-mentioned reduced iron, fluctuations in the input power due to fluctuations in the power supply voltage, changes in the ambient temperature of the furnace, and temperature of the atmosphere inside the furnace due to dust collection of the furnace are avoided. Slag7 or molten steel6 due to changes in
The point is that if a change in temperature occurs, it can be corrected appropriately. That is, (1) When the temperature of the slag 7 decreases, the molten steel 6
(2) When the temperature of slag 7 rises, the temperature of molten steel 6 can be increased.
(3) When the temperature of molten steel 6 decreases, the temperature of slag 7 can be lowered while maintaining the same temperature.
(4) When the temperature of molten steel 6 rises, the temperature of slag 7 can be increased.
It has an epoch-making feature that only the temperature of the molten steel 6 can be lowered while maintaining the temperature of the molten steel 6 as it is.

This allows the temperatures of both the slag layer 7 and the molten steel 6 to be set to appropriate temperatures, and as a result, the reduced iron 15 can be melted at an appropriate temperature on the slag 7 side, and the furnace body can be prevented from being damaged by the slag. At the same time, on the molten steel 6 side, there is an excellent utility in that it is possible to maintain the molten state of the molten steel in an optimal state with less energy loss and less damage to the furnace.

[Brief explanation of drawings]

The drawings show an embodiment of the present application, and FIG. 1 is a plan view of an arc furnace, FIG. 2 is a cross-sectional view taken along the line ``-'', and FIG.
The diagrams are power supply and control system diagrams, Figures 4 and 5.
The figure is a graph showing the relationship between supplied power and power factor and temperature changes of molten steel and slag. 2... Furnace body, 6... Molten steel, 7... Slag, 12
...Electrode, 14...Arc.

Claims (1)

[Claims] 1. In the melting space formed inside the furnace body,
Molten steel and slag covering the upper surface of the molten steel are placed, and an electrode capable of generating an arc is disposed above the molten steel so as to be movable up and down. While generating an arc to heat the slag and molten steel, reduced iron is charged into the melting space to melt the reduced iron and decompose it into a slag element and a molten steel element, and the slag element is dissolved in the slag. During the process of mixing the molten steel elements into the molten steel, the temperatures of the molten steel and slag are detected by a molten steel temperature detector and a slag temperature detector, respectively, and when the temperature of the slag decreases, the temperature of the molten steel and slag are detected by the electrodes. By increasing the power supplied to the slag and elevating the electrode to lengthen the arc length, the temperature of the slag is raised while the temperature of the molten steel remains constant. By lowering the power supplied to the molten steel and lowering the electrode to shorten the arc length, the temperature of the slag is lowered while keeping the temperature of the molten steel constant. By increasing the supplied power and lowering the electrode to shorten the arc length, the temperature of the molten steel is increased while keeping the slag temperature constant, and when the temperature of the molten steel rises, the supply to the electrode is increased. A method of controlling an arc furnace characterized by lowering the temperature of molten steel while keeping the temperature of the slag constant by lowering the electric power and raising the electrode to lengthen the arc length.