JP2003293024A - Method for operating electric furnace - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for operating an electric furnace by which iron oxide amount discharged from the electric furnace in a slag-foaming period is reduced at less than the conventional method and the iron yield can be improved. <P>SOLUTION: When a melting period for heating and melting various kinds of raw materials charged into the electric furnace with arc, an oxidizing period for oxidizing impurity elements by charging slag-making agent and performing the oxygen-blowing into melt-down molten steel and a reducing period for simultaneously blowing oxygen gas and carbonaceous material into a slag layer while discharging the generated oxide as the slag to the outside of the furnace, not only to reduce the iron oxide in the slag, but also, to develop the slag- foaming, are passed through in order, to produce the molten steel, the blowing amount of the carbonaceous material and/or a tilting angle of the electric furnace, are changed and adjusted so as to cover the foamed slag height as less as possible with the arc. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating an electric furnace, and more particularly to a steelmaking technique using an electric arc furnace.

[0002]

2. Description of the Related Art Generally, steelmaking using an arc type electric furnace has a melting stage in which various raw materials are charged into the electric furnace, and then the raw materials are heated and melted by an arc generated by applying an electric current, and a molten metal melted down. Injecting a slag-forming agent into the molten steel and blowing oxygen into it to oxidize the impurity elements, and to raise the temperature of the molten steel, and to remove the generated oxide as slag outside the furnace, The reduction period for adjustment (the oxidation period and the reduction period are collectively referred to as refining period) is sequentially passed. Among them, in the reduction period, the slag is used to coat the arc to improve the heat-transfer efficiency of the arc to the molten steel, and in order to improve the iron yield, the iron oxide in the slag is reduced, and as shown in FIG. A carbon-containing powder (hereinafter referred to as a carbonaceous material 3) is blown together with oxygen gas through a lance 2 inserted in 1 to generate a so-called "slag forming" phenomenon in which CO gas is generated to foam slag ( See JP-A-58-73714 and JP-A-2-107712). That is, compared with the generation rate of CO gas, the retention time of bubbles in the slag is increased to make bubbles, the slag layer 1 is apparently thickened, and at the same time, the tilt angle of the electric furnace 4 is maintained at a certain value, and the exhaust gas is discharged. The slag is naturally discharged from the slag mouth 5.

[0003]

Since the slag forming by blowing the carbonaceous material reduces the iron oxide in the slag, it was certainly effective in improving the iron yield. However, in the slag forming, since the slag containing iron oxide is also discharged from the electric furnace, the effect of improving the iron yield is not so large under the present circumstances. In addition, there was a large variation in iron yield.

In view of such circumstances, it is an object of the present invention to provide an electric furnace operating method capable of improving the iron yield by reducing the amount of iron oxide discharged from the electric furnace during the slag forming period. I am trying.

[0005]

In order to achieve the above-mentioned object, the inventor reviewed the conventional operation method,
I noticed that I focused only on the promotion of forming and did not consider the weight of discharged slag at all. Then, if the amount of discharged slag is constantly measured, it is thought that a good countermeasure technique can be developed based on the measured value, and earnest efforts have been made, and the results have been embodied in the present invention.

That is, according to the present invention, a melting period in which various raw materials charged in an electric furnace are heated and melted by an arc, and a slag-making agent is charged into the melted molten metal and oxygen is blown into the molten metal to oxidize an impurity element. And a reduction period that not only reduces the iron oxide in the slag by blowing carbonaceous material into the slag layer at the same time as discharging the generated oxide as slag out of the furnace, but also forming slag. In the operating method of the electric furnace to melt the molten steel through sequentially, from the start of the reduction period to a few minutes before the end, the height of the slag to be formed, the carbon material so that the arc is covered at a minimum, It is a method of operating an electric furnace, characterized by changing and adjusting a blowing amount and / or a tilt angle of the electric furnace. In this case, the height of the slag to be formed may be determined by constantly measuring the amount of slag discharged from the electric furnace and comparing the measured value with a target value.

According to the present invention, the amount of iron oxide discharged as slag from the electric furnace during the reduction period can be significantly reduced as compared with the conventional case, and the improvement of iron yield can be achieved.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below, including the background of the invention.

First, the inventor used the three-phase AC electric furnace having a capacity of 60 tons shown in FIG.
A test operation was performed to melt 60 tons of 95 carbon steel. At that time, during the reduction period, the carbonaceous material is blown into the slag layer to cause slag forming, and to maintain the minimum slag height at which the arc cannot be seen by the operator's eyes, An attempt was made to discharge slag while adjusting the tilting angle of the furnace.

The raw material is iron scrap, and the slag forming agent is Ca.
O: 50 to 70% by mass, SiO ₂ : 30 to 50% by mass, and the flow rate of oxygen gas is 30 to 40 (standard state) m ³ / min in the oxidation period and 20 to 30 (standard state) in the reduction period. ) M ³
/ Min. As the carbonaceous material, coke powder having a particle size of 5 mm or less was used.

As a result, the iron oxide in the slag can be sufficiently reduced and the iron yield can be improved by 1.6%, as compared with the conventional operation in which the target slag height is not adjusted by the carbonaceous material injection amount. Therefore, such an operating method is the first present invention.

However, as shown in FIG. 2, when the weight of the slag discharged into the ladle 6 was constantly measured by the load cell while being placed on the trolley 7, the temporal change of the accumulated weight was as follows.
As shown in FIG. In other words, the change with time of the cumulative weight is not a smooth straight line but a large upward-sloping curve, suggesting that the slag discharge work has not yet been performed satisfactorily. The reason is that the minimum slag height at which the arc cannot be seen is visually determined by the operator.

Therefore, in order to make a further improvement, the inventor decided to utilize the cumulative weight of the slag, which is constantly measured, rather than the operator visually judging the slag height. For that purpose, a target value (a straight line having a slope as a weight change rate) is required for the change with time of the cumulative weight, and the target value is a wave from the change curve of the cumulative weight with time shown in FIG. We decided to assume a straight line (shown by the dotted line). Then, when the operation of adjusting the slag height based on the change with time of the cumulative weight was applied to the steel types of the above-described test operation, the slag discharge amount was stabilized as the target, and the iron yield was further improved. Of course, the adjustment of the carbon material blowing amount and / or the tilt angle of the furnace is performed in advance by obtaining the relationship between the change amount of the slag cumulative weight and the carbon material blowing amount and / or the tilt angle of the furnace.
I made use of that relationship.

However, since the height of the slag depends on the capacity of the electric furnace, the type and amount of steel to be melted, and the amount of the slag additive to be added, the target value can be applied to the melting of all steel types. Not a thing. Therefore, the inventor sets the target value as
The second operation of the present invention was completed by conducting a test operation under standard conditions for each type of steel to be melted, measuring and determining the change in cumulative weight at that time.

[0015]

[Example] A three-phase AC electric furnace having a capacity of 60 tons is provided with a ladle 6 and a dolly 7 having a load cell as shown in FIG. 2 so that the accumulated weight of slag to be discharged can be constantly measured online. I chose

First, iron scrap as a raw material is put into the furnace, and the energization of the electrode 8 is started. Since the melting was completed 30 minutes after the start of the energization, the smelting agent was charged into the melted-down molten metal, and oxygen gas was blown into the molten metal to oxidize and refine the impurity elements. Thereafter, while discharging the produced oxide as slag to the ladle 6 provided outside the furnace, not only the oxygen gas is blown into the slag layer 1 at the same time as the carbonaceous material 3 having the above-mentioned particle size to reduce the iron oxide but also the slag. Reduction smelting that causes forming of the slag in the inside was performed, and a desired molten steel was melted. At that time, in the reduction period, the operation method according to the present invention in which the slag height that covers the minimum of the arc is judged visually or by the discharged slag weight and the target value is adjusted by the carbonaceous material injection amount and the tilting angle of the furnace is applied. , The obtained iron yield was compared with the result of the conventional operation (without adjusting the slag height). Table 1 shows the main operating conditions and iron yield for each molten steel type. The same slag forming agent and carbonaceous material as those used in the test operation were used.

From Table 1, it is clear that, according to the present invention, the iron yield is improved by an average of at least 1.5 to 2.2% as compared with the conventional case in any steel grade.

[0018]

[Table 1]

[0019]

As described above, according to the present invention, the height of slag forming during the reduction period can be adjusted to the extent that the arc is covered with the minimum. At that time, it is more effective to measure the cumulative weight of the discharged slag and adjust it to a target value, and the slag reduced in iron oxide is mainly discharged. As a result, the iron yield could be improved more than ever before.

[Brief description of drawings]

FIG. 1 is a diagram showing a change with time of a cumulative weight of slag discharged in a reduction period.

FIG. 2 is a cross-sectional view showing a three-phase arc type electric furnace.

[Explanation of symbols]

1 slag layer 2 Lance 3 carbon materials 4 electric furnace 5 waste outlet 6 ladle 7 dolly

Claims (2)

[Claims] 1. A melting period in which various raw materials charged in an electric furnace are heated and melted by an arc, an oxidizing period in which a slag-forming agent is charged into a melted-down molten metal, and oxygen is blown into the molten metal to oxidize an impurity element. While discharging the generated oxide as slag out of the furnace, carbonaceous material is blown into the slag layer at the same time as the oxygen gas to reduce the iron oxide in the slag, as well as a reducing period that causes the slag to form In the operating method of the electric furnace for smelting, from the start of the reduction period to a few minutes before the end, the height of the slag to be formed is set so that the arc is covered at a minimum and the carbon material blowing amount and / or Alternatively, the electric furnace operating method is characterized by changing and adjusting the tilt angle of the electric furnace. 2. The height of the slag to be formed is
The method for operating an electric furnace according to claim 1, wherein the amount of slag discharged from the electric furnace is constantly measured, and the measured value is compared with a target value to make a judgment.