JPH0885815A - Method for effectively refining molten chromium-containing steel using decarburized slag - Google Patents

Abstract

PURPOSE: To reduce and recover produced chromium oxide in a method at a low cost in a decarburization-refining of molten chromium-containing steel and also, to effectively execute the reduction and the decarburization. CONSTITUTION: At the time of tapping the decarburization-refined molten steel in the decarburization-refining of the molten chromium-containing steel, the crude molten steel having >=1400 deg.C, >=0.2mass% [Si] concn. and >=1.2mass% [C] concn., is received under the condition of remaining slag containing 15-50mass% chromium oxide produced by the decarburization-refining in a furnace. In the case of being >=0.1mass% [Si] concn. in the crude molten steel, only inert gas is supplied from a bottom-blowing tuyere, and in the case of being <0.1mass% [Si] concn., oxygen gas is supplied from a top-blowing lance in addition of the inert gas from the bottom-blowing tuyere, to execute the reduction. By this method, since the remarkable lowerings of the unit consumptions of Si for reduction and dilute gas, and the shortening of the refining time are obtd., the refining cost can remarkably be reduced.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to the decarburization refining of molten steel containing chromium, in which the chromium oxides produced by the decarburization refining are reduced by an inexpensive method to recover chromium, and the reduction and decarburization are efficiently performed. The present invention relates to a method for refining molten steel containing chromium.

[0002]

2. Description of the Related Art Conventionally, 11mas such as stainless steel
As a decarburizing refining method for molten chromium-containing steel containing s% or more of chromium, there are a vacuum decarburizing method in which decarburization is carried out after the middle stage of decarburization (for example, [C] 0.5 mass% or less), and an atmosphere in which a diluting gas is blown Diluted decarburization, which lowers the CO partial pressure, is widely used. The former is generally called the VOD method, and the latter is A
It is called the OD method and the top-bottom blowing converter method. All of these methods are used in molten steel [C
It is intended to efficiently carry out decarburization while suppressing the oxidation loss of [r]. However, as the [C] concentration decreases, the oxidation of [Cr] is unavoidable.
r] The amount of oxidation increases.

Conventionally, in order to suppress the oxidation loss of [Cr] in molten steel, for example, in the VOD method, JP-A-55-89417 is used.
As disclosed in Japanese Patent Laid-Open Publication No. 55-152118 and Japanese Patent Laid-Open No. 55-152118, the oxygen supply amount is adjusted in accordance with the progress of decarburization, or the adjustment is performed under a vacuum of 100 Torr or less. Further, in the AOD method, a method of increasing the ratio of the diluent gas as the [C] concentration decreases is adopted.

In these methods, most of the role of slag is to suppress splash and melt loss of furnace refractory, and the oxygen source for decarburization is oxygen supplied as gas or oxygen in molten steel. Therefore, the oxidation of [Cr] in the molten steel by the supplied oxygen gas is unavoidable.
Oxidation of chromium oxide in the slag (generally (C
r ₂ O ₃ )) concentration increases, and the melting point of slag rises sharply. Chromium oxide concentration is 1 at the end of decarburization
The melting point becomes 5 to 50 mass%, the melting point becomes a high temperature of 1700 ° C. or higher, and the slag completely forms a solid phase.

Conventionally, this slag is used as an oxygen source for a decarburization reaction to reduce chromium oxide and recover [Cr], and for the decarburization end stage (eg, [C] 0.1 mass%).
In order to increase the decarburization reaction rate in
The method seeks to increase the vacuum and increase the gas injection amount. In addition, in the AOD method, vacuum refining may be applied as shown in JP-A-3-68713 and JP-A-4-254509. But,
In these methods, the slag forms a solid phase, and the decarburization rate is very slow compared to the initial decarburization (for example, [C] 0.5 mass% or more), and the decarburization amount is small. No effect has been obtained. Therefore, after the decarburization refining, in order to reduce and recover valuable metals such as chromium in the slag, a large amount of a reducing material such as Si or Al is introduced to carry out reduction refining, and then steel is tapped.

On the other hand, as a method for solving these problems, Japanese Patent Laid-Open No. 243711/1987 and Japanese Patent Laid-Open No.
The method described in Japanese Patent No. 73424 is disclosed. These methods are methods in which slag containing a large amount of chromium oxide is left in the furnace and reduced only by [C] in the newly molten steel. However, reduction with only [C] results in a slow reduction rate, so [C] in chromium oxide
The sufficient effect for recovering r] has not been obtained.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In decarburizing and refining molten chromium-containing steel using a converter type refining vessel such as D, when tapping the decarburized and refined molten steel, a slag containing a large amount of chromium oxide is left in the furnace. In the method of reducing with newly received crude molten steel, the reduction rate is maintained at a high level and is efficiently performed.

[0008]

The present invention advantageously solves the above-mentioned problems, and the gist thereof is as follows. (1) In decarburization refining of molten chromium-containing steel using a converter-type refining vessel such as AOD, when the decarburized refining-treated molten steel is tapped, the chromium oxide produced by the decarburization refining treatment is In the method of reducing the chromium oxide by [Si] and [C] in the newly-received crude molten steel, the slag containing 15 mass% or more and 50 mass% or less is left in the furnace at the time of tapping. When the Si] concentration is 0.1 mass% or more, an inert gas such as Ar or N ₂ is supplied from the bottom blowing tuyere, and the [Si] concentration is 0.1 mass%.
%, An inert gas is supplied from the bottom blowing tuyere, and oxygen gas is supplied from the top blowing lance. An efficient method for refining molten chromium-containing steel using a decarburizing slag.

(2) The decarburizing slag described in (1) above, wherein the flow rate of the oxygen gas supplied from the top blowing lance is adjusted according to the reduction rate of [C] of the chromium oxide in the slag. Efficient refining method for molten steel containing chromium. (3) The temperature of the newly obtained crude molten steel is 1400 ° C. or higher, the [Si] concentration is 0.2 mass% or higher, and [C].
An efficient method for refining molten chromium-containing steel using a decarburizing slag as described in (1) above, wherein the concentration is 1.2 mass% or more.

(4) After the completion of the reduction of the chromium oxide, only the slag is discharged from the refining vessel, and then the decarburization smelting of the chromium-containing molten steel is carried out. Efficient refining method of molten chromium-containing steel used. The present invention will be described in detail below. The method for refining molten chromium-containing steel of the present invention uses a converter type refining vessel such as AOD,
This is a refining process as illustrated in FIG. In FIG.
The decarburization refining up to the target [C] concentration is carried out in step 1, and the slag containing the chromium oxide produced in the decarburization refining step is left in the furnace, and only the decarburized refining molten steel is tapped. Then
The crude molten steel is newly received at, and the slag left in the furnace is reduced by the crude molten steel at, and only the slag reduced at is discharged. Then, return to the same process as decarburization refining in. Therefore, an efficient refining method can be obtained by repeating steps from.

In the figure, a is a furnace body, b is slag, c is molten steel, d is tuyere, e is a ladle, and f is a slag pot. According to the present invention, when the slag containing chromium oxide is reduced by the newly received crude molten steel, [Si] is contained in the crude molten steel in addition to [C].
Coexisting, and adjusting the [Si] concentration according to the amount of chromium oxide, and by changing the gas species to be blown in accordance with the [Si] concentration in the crude molten steel, efficient refining is performed. Is.

FIG. 2 shows the change over time in the chromium oxide concentration in the slag when the slag containing 30 mass% of chromium oxide was subjected to reduction treatment with the crude molten steel having a temperature of about 1500 ° C. 0 mass%, [Si] concentration of 0.
01 mass% and [C] concentration 2.0 mass%,
The case where the [Si] concentration is 0.30 mass% is shown for comparison. It can be seen that the inclusion rate of [Si] in the crude molten steel increases the reduction rate of chromium oxide concentration, that is, the reduction rate. Therefore, the reduction of slag containing chromium oxide is more efficient with [Si] and [C] than with [C] alone.

FIG. 3 shows about 15 slags containing chromium oxide.
The relationship between the chromium oxide concentration before reduction and the reduction index when the reduction treatment is performed for 10 minutes on the crude molten steel having a temperature of 00 ° C is shown. The reduction index is such that the chromium oxide concentration is 50 ma.
In the case of ss%, the average value of the reduction amount was set as 100 and indexed. Further, the [C] concentration in the crude molten steel before the reduction was 2.0 mass% and the [Si] concentration was 0.3 mass%. It was From Fig. 3, the chrome oxide concentration in the slag is 50 ma.
It can be seen that the reduction index sharply decreases at ss% or more. When the chromium oxide concentration is less than 15 mass%, the decarburization rate at the final stage of decarburization is low, and the amount of chromium oxide reduced when the slag is left after the decarburization is finished and the reduction treatment is carried out with newly received crude molten steel. Is smaller, the cost reduction effect is also smaller. Therefore, the concentration of chromium oxide in the slag that remains in the furnace after decarburization is 15mass.
It is necessary to be s% or more and 50 mass% or less.

FIG. 4 shows a slag containing about 40 mass% of chromium oxide at a temperature of about 1500 ° C. and a [C] concentration of about 2.0 mass.
[S in crude molten steel when reduction treatment is performed with s% of crude molten steel
i] The concentration and the ratio of reduction by [Si] in the crude molten steel to the total reduction amount are shown. From FIG. 4, [Si] concentration is 0.1 mas
It can be seen that if it is less than s%, the ratio of reduction by [Si] drops sharply. On the other hand, in the region where the ratio of reduction by [Si] is high, the ratio of reduction by [C] is small,
The amount of CO gas generated by the reduction by [C] is small. That is, when oxygen gas is blown in a region where the amount of CO gas generated is small, the oxygen gas is not effective because it acts on the oxidation of [Si] and [Cr] in the crude molten steel. Also,
Gas injection from the bottom blowing tuyere is effective in efficiently promoting the reaction between molten steel and slag, and gas injection from the top blowing lance is effective in reacting with the gas generated from inside the molten steel. . Therefore, when the [Si] concentration is 0.1 mass% or more, the inert gas such as Ar or N ₂ is blown from the bottom blowing tuyere to promote the [Si] reduction of the chromium oxide in the slag in the crude molten steel. Also, the [Si] concentration is 0.1 mass%
If the amount is less than the above, the reduction of chromium oxide by [C] in the crude molten steel is promoted by blowing an inert gas from the bottom blowing tuyere, and the amount of CO generated by flowing oxygen gas from the top blowing lance It is effective to promote the reaction (CO + 1 / 2O ₂ = CO ₂ ) and recover the heat of reaction.

FIG. 5 shows a slag containing about 40 mass% of chromium oxide at about 1500 ° C. and a [C] concentration of about 2.0 mass.
s%, [Si] concentration of approximately 0.3 mass% when subjected to reduction treatment with crude molten steel, the flow rate of oxygen gas and the index of oxidation amount of [Cr] by oxygen gas in the range of [Si] concentration of less than 0.1 mass% Shows the relationship. It should be noted that the oxygen gas was sprayed from the top blowing lance, and the oxygen gas flow rate with respect to the decarburization amount represents the oxygen gas flow rate blown when the decarburization of 1 kg progressed. ]
The oxidation amount index is a value obtained by indexing the average value of the [Cr] oxidation amount when the oxygen gas flow rate is 1.0 as 1.0. Figure 5
As can be seen from the above, the flow rate of oxygen gas supplied from the top blowing lance is the rate of reduction of chromium oxide by [C] (decarburization amount).
In order to suppress the oxidation of [Cr], it can be said that the amount of decarburization is about 1.3 times or less.

FIG. 6 shows the relationship between the temperature of the crude molten steel and the reduction index when the crude molten steel newly receiving the slag containing about 30 mass% of chromium oxide was subjected to the reduction treatment for 10 minutes. The reduction index is a value obtained by indexing the average value of the reduction amount of chromium oxide when the temperature of the crude molten steel is 1400 ° C. as 100. Further, the [C] concentration in the crude molten steel before the reduction was 2.0 mass% and the [Si] concentration was 0.3 mass%. As can be seen from FIG. 6, the temperature of the crude molten steel is 140
When the temperature is lower than 0 ° C, the reduction index sharply decreases, so the temperature is preferably 1400 ° C or higher.

In FIG. 7, a slag containing about 30 mass% of chromium oxide was left in the furnace, and about 150 new steel was received.
The relationship between the [Si] concentration in the crude molten steel and the reduction index when the reduction treatment is performed on the crude molten steel having a temperature of 0 ° C. for 10 minutes is shown. The reduction index is [Si] concentration of 0.20 mass%.
In the case of, the average value of the reduction amount of the chromium oxide is set as 100 and indexed. In addition, [C] in the crude molten steel before reduction
The concentration was 2.0 mass%. As can be seen from FIG. 7, when the [Si] concentration is less than 0.2 mass%, the reduction index decreases, so that the [Si] concentration in the crude molten steel is 0.2 m.
It is preferably ass% or more.

In FIG. 8, slag containing about 30 mass% of chromium oxide was left in the furnace, and about 150 new steel was received.
The relationship between the [C] concentration in the crude molten steel and the reduction index when the reduction treatment is performed on the crude molten steel having a temperature of 0 ° C. for 10 minutes is shown. The reduction index is a value obtained by indexing the average value of the reduction amount of chromium oxide in the case of [C] concentration of 1.2 mass% as 100. As can be seen from FIG. 8, the reduction index decreases when the [C] concentration is less than 1.2 mass%. Therefore, the [C] concentration in the crude molten steel is preferably 1.2 mass% or more.

After the reduction of the chromium oxide in the slag is completed, it is possible to carry out decarburization refining as it is. However, if decarburization refining is performed as it is, the amount of slag increases significantly and the amount of [Cr] oxidation in the molten steel also increases.
It is efficient to discharge only the slag from the refining vessel after the reduction of the chromium oxide, and then perform decarburization refining. As described above, in the decarburization refining of molten steel containing chromium, slag containing 15 mass% or more and 50 mass% or less of chromium oxide generated by the decarburization refining treatment is left in the furnace,
In order to efficiently proceed with the reduction of chromium oxide in the newly received crude molten steel, it is effective to change the type of gas blown according to the [Si] concentration in the crude molten steel. Conditions for further efficient reduction include [C] concentration in crude molten steel and [Si]
Suitable conditions such as concentration and temperature have been derived.

[0020]

In general, molten steel containing chromium such as stainless steel containing 11 mass% or more of chromium is decarburized to a [C] concentration of 0.1 mass% or less by decarburization refining. The decarburization reaction in this case is expressed by the equation (1), and the equilibrium constant K _co is expressed by the equation (2). C + O = CO (g) ... (1) K co = P co / (a c · a o) ... (2) where, a _c, a _o is active in the molten steel [C] and [O] The quantity, P _co, is indicated by the CO partial pressure (atm) in the atmosphere.

In the chromium-containing molten steel, [Cr] in the molten steel is a _c
In order to reduce a and a _o , the oxidation of [Cr] in the molten steel represented by the formula (3) is unavoidable as the concentration of [C] decreases, and P _co is reduced like the AOD method or the VOD method. Even if lowered, this reaction will proceed. 2 Cr +3 O = (Cr ₂ O ₃ ) (3) At the end of decarburization refining, the slag (Cr ₂ O ₃ ) concentration is 15
The slag is solidified with a high concentration of mass% or more.
[Cr] is a valuable metal and needs to be recovered. For this reason, until now, after the decarburization has been completed, a large amount of reducing materials such as Si and Al have been added to carry out reduction refining. Therefore, the refining cost was increased.

As a method for solving these problems, JP-A-62-243711 and JP-A-6-73424 are known.
The method described in the publication is disclosed. These methods are
This is a method in which slag containing a large amount of chromium oxide is left in the furnace and is reduced only by [C in newly received crude molten steel. The reaction in this case is represented by the equation (4). (Cr ₂ O ₃ ) +3 C = 2 Cr + 3CO (g) (4) The reaction of the formula (4) can obtain a certain speed in the liquid phase of the slag, but is very slow in the solid phase, and the reduction treatment It is inefficient because it takes a long time. Then, as a method of solving this problem, a method of reducing a part of chromium oxide in the slag with [Si] in the crude molten steel represented by the formula (5) was found.

2 (Cr ₂ O ₃ ) + 3Si = 4 Cr +3 (SiO ₂ ) ... (5) The reaction of the equation (5) is much faster than the reaction of the equation (4),
When [Si] and [C] are contained in the crude molten steel,
Apparently, the reaction of the formula (4) proceeds after the reaction of the formula (5) is completed. Both the reactions of the equations (4) and (5) are exothermic reactions, but the calorific value is small and the temperature of the molten steel gradually decreases because the heat is removed from the furnace body by blowing only the inert gas. CO gas is generated in the reaction of the equation (4), but by blowing oxygen gas onto the CO gas, the reaction of the following equation (6) proceeds. The calorific value of the reaction of the equation (6) is very large, and the molten steel temperature gradually rises, which enables efficient refining.

CO (g) + 1 / 2O ₂ (g) = CO ₂ (g) (6) As a condition for efficiently proceeding the reaction of the formula (6), the [Si] concentration in the crude molten steel is 0. It has been found that the oxygen gas is made to flow from the top blowing lance at less than 1 mass% and the oxygen gas flow rate is adjusted according to the decarburization amount. Furthermore, suitable conditions for the temperature and composition of the crude molten steel have been found as conditions for efficiently promoting the reactions of equations (4) and (5).

[0025]

[Example] SUS304 stainless steel (8 mass% N
i-18 mass% Cr) 60 ton treatment was carried out in the embodiment shown in FIG. At the end of decarburization refining, the target [C] concentration is all 0.05 mass%, and while decarburizing to the target [C] concentration, oxidation of [Cr] in molten steel progresses, and chromium oxide in slag The concentration increased. This slag was left in the furnace at the time of tapping, and reduction treatment was performed with newly received crude molten steel. The reduction treatment is [Si] concentration 0.1.
Up to mass%, an inert gas of Ar or N ₂ was blown from the bottom blowing tuyere at a flow rate of 2000 Nm ³ / Hr, and [S
i] When the concentration is less than 0.1 mass%, Ar will come out from the bottom blowing tuyeres.
Alternatively 2000 Nm ³ / Hr inert gas N _2, in accordance with the reduction amount by the advance measuring oxygen gas from the top lance (C), 300~2000Nm ³ / Hr
Shed in the range of. These reduction treatments took 10 minutes. After the reduction treatment, only the slag was discharged, and the decarburization refining of the crude molten steel was subsequently carried out.

Table 1 shows the conditions of the embodiment. The examples of the present invention were carried out such that the gas blowing conditions and the conditions such as the composition of the crude molten steel satisfied the conditions described above. No. of the comparative example.
No. 7 is a conventional method disclosed in JP-A-62-24371.
This is an example performed according to the method described in Japanese Patent Laid-Open No. 1-No. After completion of decarburization refining, 8 was added with Si to reduce the chromium oxide in the slag and subjected to reduction treatment,
This is an example in which molten steel and slag were tapped at the same time. 9-No. 11 is an example in which the processing conditions are outside the conditions of the present invention.

The results of the implementation are shown in Table 2. The values in the table are the numbers of the comparative example. The result of 7 is 100, and all values are proportionally converted. In the example of the present invention, since the reduction was carried out under the condition of promoting the reduction of the chromium oxide in the slag, no significant difference was observed in the refining time and the refining cost, but in the comparative example, no. In Example 8, the refining cost is very high.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

According to the method of the present invention, in refining molten steel containing chromium, a reduction in the basic unit of reducing Si and the basic unit of diluting gas are significantly reduced, and the reduction of chromium oxide is promoted. Since the time is shortened, the refining cost can be greatly reduced. Further, since it becomes possible to control the temperature of the crude molten steel during the reduction, decarburization refining after the reduction treatment can be efficiently performed. Further, since the slag is left in the furnace after the decarburization refining, the amount of slag inclusion in the molten steel is reduced, and the quality of the product is improved.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention, where is a decarburization refining step, is a step of leaving slag in the furnace to produce only molten steel, is a step of receiving crude molten steel, and is reduction of slag. A treatment process, a slag discharge process, and a decarburization refining process.

FIG. 2 is a diagram showing the effect of the reduction method of chromium oxides [Si] and [C] in the method of the present invention.

FIG. 3 is a diagram showing the reason for limiting the concentration of chromium oxide in the slag at the end of reduction by [Si] in the crude molten steel in the method of the present invention.

FIG. 4 is a diagram showing the reason for limiting the [Si] concentration in the crude molten steel for changing the gas blowing conditions in the method of the present invention.

FIG. 5 is a diagram showing the necessity of adjusting the flow rate of oxygen gas flowing from an upper blowing lance in the method of the present invention.

FIG. 6 is a diagram showing the reason for limiting the temperature of the crude molten steel newly received in the method of the present invention.

7] FIG. 7 shows the [S of the newly obtained crude molten steel in the method of the present invention]
i] It is a figure which shows the reason for limiting the density.

FIG. 8 is a diagram showing the reason for limiting the [C] concentration of the crude molten steel newly received in the method of the present invention.

[Explanation of symbols]

 a furnace body b slag c molten steel d tuyere e ladle f slag pan

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroaki Morishige 3434 Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Co., Ltd.

Claims (4)

[Claims] 1. In the decarburization refining of molten chromium-containing steel using a converter-type refining vessel such as AOD, when the decarburization-refined molten steel is tapped, chromium oxidation produced by the decarburization refining treatment is carried out. 15mass% or more and 50ma
In the method for reducing the chromium oxides by leaving slag containing ss% or less in the furnace at the time of tapping and reducing the chromium oxide with [Si] and [C] in the newly received crude molten steel,
i) When the concentration is 0.1 mass% or more, A from the bottom blown tuyere
An inert gas such as r, N ₂ or the like is supplied, and the [Si] concentration becomes 0.
If it is less than 1 mass%, an inert gas is emitted from the bottom tuyeres,
An efficient refining method of molten chromium-containing steel using a decarburizing slag, characterized by supplying oxygen gas from a top blowing lance. 2. The decarburizing slag according to claim 1, wherein the flow rate of the oxygen gas supplied from the upper blowing lance is adjusted according to the reduction rate of [C] of chromium oxide in the slag. Efficient method for refining molten steel containing chromium. 3. The temperature of the crude molten steel newly received is 1400.
At a temperature of ℃ or more and a [Si] concentration of 0.2 mass% or more,
The efficient method for refining molten chromium-containing steel using a decarburizing slag according to claim 1, wherein the [C] concentration is 1.2 mass% or more. 4. The decarburizing slag according to claim 1, wherein after the completion of the reduction of the chromium oxide, only the slag is discharged from the refining vessel, and then the chromium-containing molten steel is decarburized and refined. Efficient refining method for molten steel containing chromium.