JPH09256024A - Method for preventing powdering of slag in electric arc furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the powdering of molten iron slag for stainless steel in an electric arc furnace without lowering desulfurizing facility. SOLUTION: In a method in which, after melting raw material and slag of the molten iron for stainless steel in the electric arc furnace, the molten iron and slag are tapped into a ladle, and after executing the stirring in the ladle, the slag is removed, the basicity of the slag before tapping the molten iron and slag is made to 1.4-2.2 and Si concn. in the molten iron is adjusted in the range from 0.02 CR% to 0.07 CR% according to the chromium oxide concn. (CR%) in the slag, and the slag and the molten iron are stirred under controlling the oxygen atmosphere in the ladle, and the basicity is lowered by oxidizing Si in the molten iron with the chromium oxide in the slag. In this case, B2 O3 containing material as a reforming agent is added in the ratio of<= 0.45wt% in terms of B2 O3 per the slag quantity. The ioorking environment for the slag treatment is improved and the slag treatment cost is reduced, and also, the reformed slag can be used as a resorce.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to preventing slag from being pulverized when smelting stainless steel hot metal in an electric arc furnace.

[0002]

2. Description of the Related Art In steelmaking using an electric arc furnace, various scraps are generally used as raw materials.
In addition to this, in recent years, many oxide raw materials such as steelmaking dust, sludge, ore are also used. Even when the molten iron is smelted in an electric arc furnace, such oxides are used in order to recycle the by-products in the steel mill and to reduce the raw material consumption by recovering useful metals. This leads to an increase in the amount of slag produced during melting,
This causes an increase in power cost for melting and makes it difficult to adjust the slag composition. Even in operations using these raw materials, after the raw materials have burned out, the refining and component / temperature adjustment processes are completed in the shortest possible time in order to improve productivity and reduce electric power costs, and then the tapping process is carried out. It is desirable to transport to. Therefore, the slag or oxide component generated in the furnace may remain unreacted and may remain ununiform, or the refining reaction may not be carried out sufficiently while being non-uniform. In these refining processes, desulfurization action is important in order to deal with S mixed from the raw material, but in order to secure desulfurization ability of slag from such a situation, it is necessary to add excess CaO to deal with it. Has been done. As a result, the amount of slag increases more and more, and its basicity (CaO / SiO ₂ ) tends to be high, and the basicity often exceeds 2.5. Further, in the field of ordinary steel, it is said that reducing slag is particularly likely to be pulverized during cooling, but also in the field of stainless steel, as the use of oxide raw materials increases, it is possible to reduce and recover useful components. For,
It is necessary to maintain the reducing atmosphere in the final stage of operation in the electric furnace. For this reason, stainless hot metal slag is generated in a reducing atmosphere with high basicity, and its pulverization phenomenon is unavoidable. Such slag is usually sent to a processing step for recovering the metal lost in the slag and reusing it as a flux. However, as described above, these slags are easily pulverized and are weathered and disintegrated during the treatment to generate a large amount of pulverized slag. For this reason, it is difficult to handle, and it significantly deteriorates the working environment of the slag processing plant, etc. Since it is powdery, it cannot be used for road ballasts, construction aggregates, etc., and there is no way to recycle it. For,
Combined with the large amount, they are spending enormous costs on subsequent disposal.

Such a property of the steelmaking slag is that CaO and SiO ₂ which are the main components in the slag are 2CaO.SiO ₂
It exists in the form of crystals, and because it undergoes a large volume change during the crystal transition during cooling, it collapses to a powder state. On the other hand, it is a borate-based stabilizer (modifier).
Is added to the slag to form a 2CaO.SiO ₂ crystal structure that does not cause a large volume change at the phase transition to prevent slag pulverization (Kawasaki Steel Technical Report Vol. 18, No. 18). .1 (1986), p.20-2.
4, JP-A-64-37444, JP-A-63-79
743, JP-A-1-259114, and JP-A-3-23243). However, for high basicity and reducing stainless hot metal slag, the modifier B
_Since it is necessary to add a large amount of ₂ O _3, and B ₂ O ₃ containing is expensive as a mineral resource, it is uneconomical to increase the amount used. Further, as a result of various studies by the present inventors, B ₂ O was added to stainless steel hot slag having high basicity and reducing property.
Although the powdering prevention effect by the addition of ₃ is recognized, a uniform dissolution and dispersion action in the slag is strictly required, and it is practically difficult to satisfy the addition conditions and exert the effect. Further, it was revealed that the higher the basicity of such slag, the lower the dusting preventing effect by addition. On the other hand, such a slag dusting phenomenon is
Since it is due to the 2CaO.SiO ₂ crystals in the slag as described above, it is sufficient to increase the amount of addition of the SiO ₂ component in the slag composition. However, decreasing the basicity by adding SiO ₂ has a high melting point. Not only is it uneconomical to heat the SiO ₂ raw material to require heating energy, but such a decrease in basicity reduces the desulfurization ability of the slag, and under the above refining conditions, a sufficient desulfurization reaction is required. Can't hope to do.

[0004]

DISCLOSURE OF THE INVENTION The present invention has been devised to solve such problems, and to maintain the desulfurization ability of slag in an electric arc furnace and prevent the slag from being pulverized. With the goal. Also used B ₂ O
_The purpose is to reduce the amount of ₃ added to a minimum and make it work effectively, and to reduce the raw material unit.

[0005]

Means for Solving the Problems In order to achieve the object, the present invention melts raw materials for stainless hot metal and slag in an electric arc furnace, and then taps / slags to a ladle and agitates in the ladle. In the method of removing slag after the operation, the slag containing B ₂ O
_{The three} substances were added in a ratio of 0.45% by weight or less in terms of B ₂ O _{3 with} respect to the amount of slag, and the slag basicity before tapping / slag was set to 1.4 to 2.2, and in the hot metal The Si concentration of [Si] = [Si] =, depending on the chromium oxide concentration (CR%) in the slag.
The slag and hot metal are agitated under the control of oxygen atmosphere in the ladle within the range of 0.02 · CR to 0.07 · CR%, and the basicity is lowered by the oxidation of the hot metal Si by the chromium oxide in the slag. It is characterized by

[0006] The slag produced during the production of molten stainless steel in an electric arc furnace has high basicity and reducibility as described above, but even if such high basicity slag can satisfy the desulfurization ability, it remains as it is. When tapping / slagging, the pulverization phenomenon as described above occurs during cooling, which is a problem. Although various studies have been conducted on the influence of basicity on the pulverization phenomenon, generally, the pulverization phenomenon becomes more remarkable as the basicity increases. On the other hand, when the basicity is lowered, the desulfurization ability of the core is lowered, and the hot metal becomes unusable. On the other hand, the present inventors adjusted the hot metal [Si] and the slag basicity before tapping, then tapped it in a ladle and immersed it in a ladle under the condition that controlled the oxygen in the atmosphere. It has been found that by performing gas agitation by, for example, the desulfurization effect can be sufficiently exhibited even if the slag basicity before tapping is relatively low.

In the refining reaction in the ladle, the hot metal [Si] reduces the chromium oxide in the slag and is itself oxidized to become a SiO ₂ source. Therefore, the slag basicity at the time of slag removal in the final process. Will be further reduced. Also,
This means that, as described above, when the slag basicity is high, the effect as a stabilizer of B ₂ O _{3 that} cannot exhibit sufficient effect for preventing pulverization is enhanced. Therefore, these effects are combined, and a remarkable effect is exhibited in preventing slag dusting.

In these ladle refining processes, when the oxygen concentration in the atmosphere rises, [Si] is oxidized and consumed by oxygen during stirring, and the ratio used for the reduction reaction of Cr ₂ O _{3 in} the slag decreases. . According to the results of studies by the present inventors, it is known that the oxygen concentration in the atmosphere and the reduction reaction rate are related to each other, and that the reduction reaction rate remarkably decreases as the oxygen concentration increases. This means that stirring for a long time is required to obtain the target reduction rate, but if stirring for a long time, the temperature of the hot metal will drop significantly, which is a disadvantage. Will occur. In addition to the reduction reaction of oxides in slag by stirring for a long time in an oxidizing atmosphere, [S
The consumption loss of i] means an increase in Si cost. Here, by stirring the hot metal and the slag,
A desulfurization reaction occurs at the interface, and the desulfurization reaction proceeds in association with the reduction reaction of an acidic component mainly containing chromium oxide in the slag. That is, it is important for desulfurization to reduce the concentration of chromium oxide, which is the oxygen potential of the reaction system, to reduce the concentration. Therefore, it is effective to stir in an atmosphere of low oxygen concentration as described above.
As a means for increasing the desulfurization ability, there is a method of increasing CaO to increase the basicity itself. However, this is contrary to the object of the present invention, so rather, even if the basicity is decreased, the chromium oxide concentration is decreased. The purpose is to achieve the desired desulfurization by exerting the above effect. By comprehensively examining the merit of recovering Cr by reduction of chromium oxide and the effect of reducing powdering by lowering basicity, and the demerit of lowering Si consumption and temperature, the target chromium oxide can be obtained with the maximum allowable stirring time. Efficiency required to achieve reduction of carbon (amount of Si used for reduction of oxides in slag / total Si consumption)
However, due to the relationship between the oxygen concentration in the atmosphere, the reduction of chromium oxide (chromium oxide concentration in slag after stirring) and the Si efficiency, it is possible to keep the oxygen concentration in the ladle atmosphere at 10 VOL.% Or less. desirable.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The electric arc furnace used in the present invention may be an arc furnace for melting iron, steel or non-ferrous metal using a carbon or graphite electrode, and may be of an AC, DC or monopolar type, or a multipolar type. It doesn't matter the format. The slag generated when the molten iron for stainless steel is smelted in an electric arc furnace is used as a raw material and various raw materials such as various scraps as described above.
Since various kinds of wastes such as various kinds of smelting dust produced in the factory are used, the components are various, and CaO is 35 to 56% and SiO ₂ is 20 to 40%.
%, Wherein the CaF ₂ 2.0 to 12%, other, MgO,
It contains Al ₂ O ₃ , Cr ₂ O ₃ , MnO and the like.

Examples of the chromium oxide source in the slag include chromium ore, steelmaking dust, hot rolled scale and sludge. The chromium-containing oxide raw material is introduced before or during the start of energization in the electric furnace operation. In the present invention, this chromium oxide is used as an oxidizing agent of [Si] to reduce the basicity, and at the same time, the merit of chromium recovery can be expected by reducing the chromium oxide. As the Si source, there are scraps of raw materials, Si and Si alloys in ferroalloy, and various Si alloys such as ferrosilicon are suitable for adjustment. In the present invention, after adjusting the slag basicity and hot metal [Si] before tapping, tapping into a ladle and performing gas stirring with an immersion lance in an atmosphere with controlled oxygen concentration, an efficient chromium oxide is obtained. It is possible to reduce the amount of slag powdered due to the decrease in basicity simultaneously with the reduction of slag. In order to control the oxygen concentration, the ladle has a lid,
At the start of stirring, it is desirable to introduce a seal gas such as Ar into the ladle and replace it with air before starting the gas stirring. Further, by continuously introducing the seal gas into the ladle even during stirring, the chromium oxide can be reduced more efficiently. As the inert gas, for example, nitrogen or argon can be used. From the economical aspect, it is preferable to use nitrogen, but depending on the steel type, nitrogen pickup may be harmful, and in such a case, argon is used. Further, a reducing gas such as H ₂ or CO ₂ may be mixed with the inert gas. A method of blowing an inert gas into a metal bath using an immersion lance is desirable because the strength of stirring can be freely set depending on the flow rate and the immersion depth. The flow rate of gas blown from the lance is 200 to 1500 NL /
Although the immersion depth of the lance is 50 to 200 cm, optimum conditions are set depending on the amount of slag (slag layer thickness), the amount of hot metal, the target [S], and the like. Further, by changing the lance shape, the nozzle diameter, and the number of nozzles, the optimum slag / metal mixed state can be obtained. In addition, gas may be blown from a nozzle or a porous plug provided at the bottom of the pot, or mechanical stirring may be performed.

As described above, in the present invention, the range of the hot metal [Si] and slag basicity before tapping and the conditions of stirring in the ladle are optimized to oxidize the hot metal Si by the chromium oxide in the slag. The basicity before tapping from the ladle is further reduced, and the slag has a reduced powdering action. By setting such slag conditions, it becomes possible to prevent the slag from being pulverized by adding a small amount of a slag modifier thereto. That is, in order to agglomerate the slag, a modifier containing B ₂ O ₃ as conventionally used is added to the slag. The addition method is important in order to effectively operate the expensive modifier in a small amount. In the present invention, it is desirable to put in the ladle at the time of tapping / slagging, preferably on the tapping flow. Further, it may be charged in advance into the slag in the electric furnace before tapping / slagging. At this time, the modifier is added at a ratio of 0.45% or less in terms of B ₂ O _{3 with} respect to the amount of slag. Conventionally, at least 0.5% or more was required for the reducing slag of stainless steel, but in the present invention, B ₂ O is used.
₃ By optimizing various conditions before and after addition and further reducing the basicity of the slag before tapping by stirring the pan,
Due to the uniform dispersion of B ₂ O _{3 in} the slag by stirring,
It is possible to completely agglomerate the slag with a smaller addition amount than before.

When the pickup of B becomes a problem, the B is put into the slag pan at the time of removing the slag after stirring the ladle. Since there is no hot metal, the temperature tends to decrease after the addition of the modifier, which may make it difficult to uniformly dissolve the modifier in the slag. In such a case, it is desirable to stir promptly as soon as the modifier is added to the slag pan, but there is no particular restriction on the atmosphere, so stir the inert gas or air, depending on the case. It is performed using oxygen gas or the like. You may mechanically stir with a paddle of other suitable material. As the B ₂ O ₃ -containing modifier, a B-containing mineral such as colemanite or borax, which has been appropriately pulverized and the particle size thereof has been adjusted, or those which have been appropriately subjected to beneficiation and concentration treatment may be used. Alternatively, it is also possible to use a B ₂ O ₃ -containing slag whose physical properties are artificially adjusted and whose particle size is adjusted, as in the case of existing commercial products. Also, the input method is
It may be packed into a minimum unit in a bag and then charged, or may be charged as a powder by using a shooter exclusively for powder. Alternatively, it is also desirable to inject using a powder blowing device aiming at uniform dissolution and stirring effect at the time of addition.

After adjusting the hot metal [Si] and slag basicity before tapping in this way, tapping is done in a ladle, and the atmospheric oxygen concentration in the ladle is controlled to 10 VOL. Hot metal [Si] before tapping when agitating
The effects of slag basicity and slag on the slag pulverization rate and hot metal [S] are shown in FIGS. In addition, as the definition of the pulverization rate, the pan dedicated to the slag was turned over, the slag was discharged, the slag was sampled from several places at arbitrary places, and the weight ratio under the sieve was passed through a 42-mesh sieve as the powder rate. .

As shown in FIG. 1, which shows the relationship between the hot metal [Si] and the pulverization rate, when [Si] is less than 0.02 · CR%,
Due to the shortage of the SiO ₂ source, the basicity does not sufficiently decrease and the slag dusting cannot be reduced. On the other hand, when it exceeds 0.07 · CR%, SiO ₂ becomes excessive and the basicity is lowered too much.
In addition to insufficient desulfurization, there is also a case where the material is re-sulfurized to, for example, 130 ppm or more, which is the target value of [S], after stirring. Therefore, [Si] depends on the chromium oxide concentration in the slag,
Within the optimum range above.

In the present invention, the slag basicity before tapping is 1.
The most effective range is from 4 to 2.2. FIG.
The effect of slag basicity on the pulverization rate and [S] was shown. When the slag basicity is less than 1.4, the pulverization rate is low, but the desulfurization ability is significantly reduced. On the contrary, if the basicity exceeds 2.2, the pulverization rate remarkably increases, which is not desirable. Therefore, the [Si] concentration in the hot metal before tapping / slagging is [S] depending on the Cr ₂ O ₃ concentration (CR%) in the slag.
i] = 0.02 · CR to within 0.07 CR%,
It can be seen that setting the slag basicity (CaO / SiO ₂ ) to 1.4 to 2.2 is a condition for achieving both desulfurization ability and pulverization prevention of the hot metal hot slag. When smelting stainless steel hot metal, the more costly the chromium oxide raw material is used as the chromium source, the more cost effective, but the power cost, productivity,
Considering various conditions such as slag intensity, there is an optimum range. Chromium oxide concentration (CR%) in slag before tapping
The optimum range of 5 to 20% is desirable, and this can be used as a guide for the amount of chromium oxide used. Hereinafter, specific examples will be described.

Example A variety of scraps and scraps were produced in a 90 ton electric arc furnace.
Chromium-containing oxide raw materials such as steelmaking dust were melted to produce molten iron for stainless steel. The basicity before tapping was adjusted by supplying CaO within the range of 1.4 to 2.2, supplying ferrosilicon as the Si source, and [Si] depending on the Cr ₂ O ₃ concentration in the slag. Was adjusted. For example, for each charge, CH. No. CR% (Cr ₂ O ₃ ) for 101
Since it was 11%, [Si] = 0.03.CR (=
0.33%), CH. No. In 102, CR% (C
Since r ₂ O ₃ ) was 13%, [Si] = 0.03
・ CR (= 0.39%). After adjusting and confirming the metal components, a sample was taken and analyzed immediately before tapping. next,
It was tapped / slagged in a dedicated ladle, and gas stirring was carried out for 10 minutes by an immersion lance. Before starting agitation, install a special lid with a heat resistant brick lined inside the ladle, and pour Ar gas as a seal gas through the gas introduction pipe into the furnace installed in the lid to adjust the oxygen concentration in the ladle to 10 vol. .% Or less, and stirring was performed with N ₂ gas. After completion of stirring, the hot metal was tapped into a ladle exclusively for slag, and the hot metal was conveyed to the converter in the next step. After cooling the slag, the ladle was turned over at the slag processing plant to confirm the solidified slag. Note that CH.
No. In Nos. 101 to 103, in the electric furnace, the modifier was simultaneously added to the slag amount in terms of B ₂ O ₃ at the same time as CaO.
% Was added. In addition, CH. No. In 104 to 106, the modifier was continuously added to the ladle during the tapping / slagging operation using a special shooter. The modifier used was a B ₂ O ₃ substance produced by treating borax, and its addition amount was 0.4% based on the amount of slag in terms of B ₂ O ₃ . In addition, CH. N
o. In 107 to 108, the modifier was placed in the slag pot in advance. The addition amount of the modifying agent, based on the amount of slag in terms _of B ₂ O ₃ is from 0.3 to 0.45 percent. After cooling the slag, the ladle was turned over at the slag processing plant to discharge the slag, and the state of agglomeration was observed. The results are shown in Table 1. All of them were lumpy and could be completely prevented from being pulverized.

Comparative Example 1 In the same manner as in Example 1, various scraps and chromium oxide raw materials such as steelmaking dust were melted in a 90 ton electric arc furnace to produce molten pig iron for stainless steel. Cr in slag
[Si] was adjusted with ferrosilicon according to the result of the ₂ O ₃ concentration analysis. Here, CH. No. In 114 to 117, [Si] a _{0.02 · CR (% Cr 2 O} 3) ~0.
07 · CR (% Cr ₂ O ₃ ) outside the range,
CH. No. For 118 to 121, 0.05 / CR (% C
r ₂ O ₃ ). Also, the slag basicity was adjusted by supplying CaO, but CH. No. 114-117, it is set to 1.8, and CH. No. In 118 to 121, CaO was consciously supplied in excess and in shortage, and 1.4 to 2.2.
It was out of the range. After adjusting and confirming the metal components, samples were taken and analyzed before tapping. Then, as in the example,
After tapping / slagging to a dedicated ladle and performing gas stirring for 10 minutes with an immersion lance, the tapping was carried out to a ladle dedicated to slag, and the hot metal was conveyed to the converter in the next step. After cooling the slag, the ladle was tumbled at the slag processing plant to investigate the state of the solidified slag. The modifying agent was CaO in the electric arc furnace at the same time as B ₂ O ₃ with respect to the amount of slag.
4% was added. The results are shown in Table 1, where [Si] is 0.0
Agglomeration could not be achieved by a charge less than 2 · CR (% Cr ₂ O ₃ ) or a charge having a high basicity of 2.3 or more. Also, [Si] was added to 0.07 · CR (% Cr ₂
Agglomeration was possible with a charge exceeding O ₃ ) and a charge with a basicity of less than 1.4, but [S] could not be reduced to the target value after stirring.

Comparative Example 2 Various scraps and oxide raw materials were melted in a 90 ton electric arc furnace in the same manner as in the example, and molten pig iron for stainless steel was melted. However, the basicity and [Si] before the tapping as in the example were not adjusted. Next, tapping / slagging was carried out in a ladle containing a modifier for preventing pulverization. The modifier used was the same B ₂ O ₃ substance as in the example, and the amount added It was set to 0.4 to 0.7% relative to the amount of slag in terms _of B ₂ O _3. Further, gas agitation by the immersion lance in the ladle was not performed. The slag was removed to a dedicated ladle, and after cooling, the ladle was tumbled at the slag processing plant to discharge the slag, and the state of agglomeration was investigated. The result is 0.45% as shown in Table 1. In the following, it was not possible to surely agglomerate.

[0019]

[0020]

As described above, according to the present invention, the working environment of slag treatment is improved by preventing pulverization without lowering the desulfurization ability of the electric arc furnace hot metal slag, and the slag treatment cost is reduced. In addition, it is possible to promote resource utilization. In addition, the recovery of chromium from chromium oxide and the inclusion of B ₂
The amount of the O ₃ modifier added can be reduced.

[Brief description of drawings]

FIG. 1 shows the relationship between the [Si] before tapping, the pulverization rate, and the hot metal [S] after stirring.

[Fig. 2] Basicity and powdering ratio before tapping and hot metal [S] after stirring
connection of

Claims (1)

[Claims] 1. A method in which the raw material for stainless hot metal and slag are melted in an electric arc furnace, then tapped / slagged in a ladle, stirred in the ladle, and then slag removed, in which B ₂ is contained in the slag.
O ₃ substance is 0.45 in terms of B ₂ O _{3 with} respect to the amount of slag.
In addition to adding at a ratio of not more than wt%, the slag basicity before tapping / slagging is set to 1.4 to 2.2, and the Si concentration in the hot metal is adjusted according to the chromium oxide concentration (CR%) in the slag. Si]
= 0.02 · CR to 0.07 · CR%, the slag and hot metal are stirred in a ladle under the control of an oxygen atmosphere, and the base is obtained by oxidizing the hot metal [Si] with chromium oxide in the slag. A method for preventing pulverization of electric arc furnace slag, which is characterized by decreasing the degree of slag.