JPH0885813A - 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. The chromium oxide is reduced with [Si] and [C] in the crude molten steel. 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]

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

[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) under a reduced pressure, and a diluting gas blowing atmosphere Diluted decarburization, which lowers the CO partial pressure, is widely used. The former is generally called the VOD method, and the latter is called the AOD method and the bottom-blown converter. All of these methods are intended to efficiently progress decarburization while suppressing the oxidation loss of [Cr] in the molten steel after the middle stage of decarburization. However, as the [C] concentration decreases, the oxidation of [Cr] is inevitable, and the [Cr] oxidation amount increases.

Conventionally, in order to suppress the oxidation loss of molten steel (Cr), 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 Publication No. 55-152118, the oxygen supply amount is adjusted according to the progress of decarburization, or is adjusted under a vacuum of 100 Torr or more. 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 the oxygen source for decarburization is oxygen supplied as gas or oxygen in molten steel. For this reason,
Oxidation of [Cr] in the molten steel by the supplied oxygen gas is unavoidable, and the oxidation of [Cr] increases the concentration of chromium oxide (generally referred to as (Cr ₂ O ₃ )) in the slag and The melting point of is rising rapidly. Chromium oxide concentration reaches 15 to 50 mass% at the end of decarburization,
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, a large effect is not obtained because 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). For this reason, 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 added, and reduction refining is carried out before tapping.

On the other hand, a method for solving these problems is
JP-A-62-243711 and JP-A-6-73.
No. 424. 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, the reduction with [C] alone does not provide a sufficient effect for recovering [Cr] in chromium oxide, because the reduction rate is slow.

[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 by newly received crude molten steel, it is to maintain the reduction rate at a high level and to perform efficiently.

[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 A slag containing 15 mass% or more and 50 mass% or less is left in the furnace at the time of tapping, and the chromium oxide is reduced by [Si] and [C] in newly received crude molten steel having a temperature of 1400 ° C. or more. An efficient refining method for molten chromium-containing steel using a decarburizing slag.

(2) [Si] in newly melted crude molten steel
Concentration is 0.2 mass% or more, [C] concentration is 1.2 ma
An efficient method for refining molten chromium-containing steel using the decarburizing slag described in the above item (1), which is ss% or more. (3) After completion of the reduction treatment of the chromium oxide, only the slag is discharged, and then the decarburization refining of the molten steel containing chromium is performed, so that the decarburizing slag described in (1) above is used efficiently. Refining method for molten steel containing chromium.

The present invention will be described in detail below. The method for refining molten chromium-containing steel of the present invention is a refining process as illustrated in FIG. 1 using a converter type refining vessel such as AOD. As shown in FIG. 1, decarburization refining to a target [C] concentration is performed, and slag containing chromium oxide produced by decarburization refining is left in the furnace, and only decarburized refining-processed molten steel is tapped. Then, the crude molten steel is newly received, the slag is reduced by the crude molten steel, and only the slag is discharged. Then, return to the same process as decarburization refining in. Therefore,
By repeating from to, an efficient refining method can be obtained.

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].
By coexisting with, refining is efficiently performed.

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 the 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 30 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 when it exceeds ss%. 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. Becomes smaller, the cost reduction efficiency also becomes smaller. Therefore, the concentration of chromium oxide in the slag that remains in the furnace after decarburization is 15 mass.
% Or more and 50 mass% or less is required.

FIG. 4 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. 4, the temperature of the crude molten steel is 140
If the temperature is lower than 0 ° C, the reduction index sharply decreases. Therefore, the temperature of the crude molten steel needs to be 1400 ° C or higher.

In FIG. 5, 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 a value indexed with an average value of the reduction amounts of chromium oxides being 100 when the [Si] concentration is 0.2 mass%. Further, the [C] concentration in the crude molten steel before the reduction was 2.0 mass%. As can be seen from FIG. 5, the reduction index sharply decreases when the [Si] concentration is less than 0.2 mass%. Therefore, the [Si] concentration in the crude molten steel needs to be 0.2 mass% or more.

In FIG. 6, 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 [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. Further, the [Si] concentration in the crude molten steel before the reduction was 0.25 mass%. As can be seen from FIG. 6, when the [C] concentration is less than 1.2 mass%, the reduction index sharply decreases, so that the [C] concentration in the crude molten steel is 1.2.
Mass% or more is required.

The higher the [C] and [Si] concentrations in the crude molten steel, the more advantageous the reduction of the chromium oxide in the slag, but these concentrations are determined by the melting or refining treatment in the previous step. Because it cannot be arbitrarily determined. However, the [C] concentration is 1.2 mass% or more,
It is necessary to perform an operation operation that secures a [Si] concentration of 0.2 mass% or more.

Next, after the reduction treatment 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, so only the slag is discharged after the reduction treatment of chromium oxide is completed. It is efficient to carry out decarburization refining subsequently.

From the above, in the decarburization refining of molten chromium-containing steel, the chromium oxide produced by the decarburization refining treatment is
In order to allow the slag containing not less than 50 mass% and not more than 50 mass% to remain in the furnace and reduce the chromium oxide in the newly received crude molten steel, the crude molten steel has a temperature of 1400 ° C or higher,
It is efficient to reduce with [Si] and [C]. Furthermore, in order to perform the reduction efficiently, the reduction treatment is performed with [Si] concentration of 0.2 mass% or more and [C] concentration of 1.2 mass% or more, and after the reduction treatment of the chromium oxide is completed, only the slag is discharged, It is then necessary to carry out decarburization refining.

[0020]

In general, molten chromium-containing steel containing 11 mass% or more of chromium such as stainless steel is decarburized to a [C] concentration of 0.1 mass% or less by blowing oxygen gas.
The decarburization reaction in this case is represented by the equation (1), and the equilibrium constant K
_co is expressed by equation (2).

[0021]C+O= CO (g) (1) K_co= P_co/ (A_c・ A_o)… (2)  Where a_c・ A_oIs the activity of [C] and [O] in the molten steel.
Quantity, P_coIndicates the CO partial pressure (atm) in the atmosphere.

In the chromium-containing molten steel, [Cr] in the molten steel is a
_In order to reduce _c and a _o , the oxidation of [Cr] in the molten steel represented by the formula (3) is inevitable with the decrease of the [C] concentration, and P _cO as in the AOD method or the VOD method is _unavoidable. Even if is lowered, this reaction proceeds. 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 concentration becomes higher than mass% and the slag solidifies.
[Cr] is a valuable metal and needs to be recovered. For this reason, until the end of decarburization, 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 shown. These methods are methods 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 (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. It has been found that, as the reaction of the formula (5) progresses, the melting point of the slag lowers and becomes a liquid phase or a semi-liquid phase, and the reaction of the formula (4) also efficiently proceeds. Further, as conditions for efficiently promoting the reactions of the formulas (5) and (4), suitable conditions for the temperature and composition of the crude molten steel have been found.

[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 a reduction treatment was performed with newly received crude molten steel. Reduction treatment is Ar or N ₂
Of inert gas at a flow rate of 2000 Nm ³ / Hr,
It was treated for 10 minutes. After the reduction process, only slag is discharged,
Subsequently, decarburization refining of the crude molten steel was carried out.

Table 1 shows the conditions of the embodiment. The examples of the present invention were carried out so as to satisfy the above-mentioned conditions. No. of the comparative example. No. 6 is a conventional method disclosed in JP-A-62-2
This is an example performed according to the method described in Japanese Patent No. 43711,
No. of the comparative example. No. 7 of Comparative Example is an example in which molten steel and slag were simultaneously tapped after performing reduction treatment by adding Si to reduce chromium oxide in slag after completion of decarburization refining. 8, 9 and 10 are examples 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 6 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 7, the refining cost is very high.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

According to the method of the present invention, in the refining of molten steel containing chromium, the reduction Si basic unit and the dilution gas basic unit are significantly reduced, and the reduction of chromium oxide is promoted. As a result, the refining cost can be significantly reduced. 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 upper limit of chromium oxide in the slag left in the furnace in the method of the present invention.

FIG. 4 is a diagram showing the reason for limiting the temperature of the crude molten steel newly received in the method of the present invention.

[Fig. 5] Fig. 5 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. 6 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 Hiroshi Iwasaki 3434 Shimada, Hikari City, Yamaguchi Prefecture Inside the Nippon Steel Co., Ltd. Hikari Steel Co., Ltd. Inside the Kogaku Steel Works

Claims (3)

[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
Slag containing ss% or less is left in the furnace at the time of tapping, and
An efficient chromium-containing molten steel using a decarburizing slag, characterized in that the chromium oxide is reduced by [Si] and [C] in newly received crude molten steel having a temperature of 00 ° C or higher. Refining method. 2. A newly obtained crude molten steel has a [Si] concentration of 0.2 mass% or more and a [C] concentration of 1.2 mass%.
% Or more, and an efficient method for refining molten chromium-containing steel using the decarburizing slag according to claim 1. 3. The efficient use of the decarburizing slag according to claim 1, wherein after the reduction treatment of the chromium oxide is completed, only the slag is discharged, and then the decarburizing refining of the molten steel containing chromium is performed. Refining method for molten chromium-containing steel.