JPH10102134A - Method for desulfurizing molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the blowing quantity of desulfurizing agent and the treating time of desufurization and to enable the control of Sol.Al content after desulfurizing by obtaining the blowing quantity of the desulfurizing agent according to S content before desulfurizing and a target S content after desulfurizing. SOLUTION: Previous to the desulfurizing treatment of molten steel, the blowing quantity of the desulfurizing agent is obtd. from the S content before desulfurizing the molten steel during raising the temp. and the target S content after desulfurizing. A reducing ratio of the Sol.Al during the desulfurizing treatment is obtd. from the blowing quantity of the desulfurizing agent. Then, the target Sol.Al content before desulfurizing which can achieve the target Sol.Al value satisfying a product characteristic, is inversely calculated, and after reducing the Sol.Al content in the molten steel to the inversely calculated value or lower, the desulfurizing treatment is started. By this method, the Sol.Al content after desulfurizing can be controlled to the target value and the oxygen blowing for combustion-removing Al after desulfurizing can be omitted. Therefore, the shortening of the treating time and the improvement of the steel quality can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for desulfurizing molten steel, in which a fine powdery desulfurizing agent is blown into molten steel during RH vacuum degassing, whereby desulfurization can be performed effectively.

[0002]

2. Description of the Related Art In recent years, high-grade steel, low-alloy steel, and special steel have increasingly strict quality requirements on the user side, and in particular, low sulfide and extremely low sulfide requirements.
Therefore, the sulfur (hereinafter referred to as S) content is particularly 0.00
For the production of extremely low sulfur steel at 1% or less, the development of a more efficient molten steel desulfurization method is desired. Generally, the method of desulfurizing molten iron is roughly classified into hot metal desulfurization performed in the hot metal stage and molten steel desulfurization performed in the molten steel stage. In order to melt ultra-low sulfur steel having an S content of 0.001% or less, Hot metal desulfurization alone is not sufficient, and it is necessary to combine hot metal desulfurization with steel desulfurization.

[0003] Conventionally, as a desulfurization method for molten steel desulfurization, an injection method in which a desulfurizing agent powder is blown into a ladle together with a carrier gas, or a gas blowing forced stirring in which a gas is blown into molten steel after adding a desulfurizing agent and stirred. Methods and the like have been generally adopted. However, these methods are intended to increase the reaction interface area between the molten steel and the desulfurizing agent and to effectively promote the reaction. However, these methods lower the temperature of the molten steel and reduce the nitrogen (hereinafter referred to as N) from the atmosphere. The pickup) is large.

As a method for solving these problems, a method has been developed in which a desulfurizing agent is added during the vacuum degassing process of the RH method comprising a heat treatment step, a vacuum degassing step, and a component fine adjustment step. The simplest method of adding a desulfurizing agent during the vacuum degassing process by the RH method is a method in which a desulfurizing agent is introduced from an alloy iron addition port provided in the middle of a vacuum tank.
In order to prevent the desulfurizing agent from being sucked into the exhaust system, it is necessary to increase the particle size of the desulfurizing agent, which is disadvantageous in terms of reaction efficiency.

[0005] Further, as an improvement method of the above, a fine powdery desulfurizing agent and a carrier gas are supplied to the ascending flow of molten steel from a blowing lance of a desulfurizing agent opened at the lower center of a dip pipe for guiding the ascending flow of molten steel in RH equipment. Method (Japanese Patent Laid-Open No. 58-37)
No. 112), a method in which a powder desulfurizing agent is directly blown into molten steel by a carrier gas through a desulfurizing agent blowing tuyere provided below a molten steel bath surface in a vacuum degassing tank (Japanese Patent Laid-Open No. 61-130).
No. 413), the upper surface of the bath of the reaction tank is kept under reduced pressure or an inert gas atmosphere, and the inert gas is used as a carrier gas without causing slag to substantially exist.
aO, CaF ₂ and MgO, and other unavoidable components. MgO is 10 to 60% by weight, and the weight ratio is {CaF
₂ / (CaO + CaF ₂ )｝ × 100 = 20 to 80% of the first treating agent is blown into the bath, and before and / or after the blowing, the main components are CaF ₂ and CaO, and the balance is Consists of unavoidable components, and the above Ca
CaF ₂ concentration in the main component consisting of F ₂ and CaO is 20
A method of blowing a second treating agent having a weight percent or less into the bath by using an inert gas as a carrier gas (Japanese Patent Laid-Open No. 62-19 / 1987).
No. 6317) has been proposed.

As a method for obtaining a high desulfurization rate, a powder desulfurizing agent is directly blown into molten steel by a carrier gas through a desulfurizing agent blowing tuyere provided below a molten steel bath surface in a vacuum degassing tank. At the time, after Al was introduced, t = 0.02W + 5 ...
(Minutes) t: Time (minutes) from the introduction of Al to the start of blowing of the desulfurizing agent, W: a method of blowing a powder desulfurizing agent after elapse of at least t minutes represented by the amount of molten steel (ton) 62-2052
No. 20), when the deoxidized molten steel is subjected to vacuum degassing in an RH facility, a desulfurizing agent is blown upward together with a carrier gas from the upper part of the vacuum tank using a lance to desulfurize, and then the Al in the molten steel is burned. Removal method (JP-A-6-29)
No. 9229) has been proposed.

[0007]

Problems to be Solved by the Invention
112, JP-A-61-130413, JP-A-62-196317 and JP-A-62-205
In any of the methods disclosed in Japanese Patent Publication No. 220, it is necessary to flow gas so that molten steel does not enter from the powder injection port even during the time when the desulfurizing agent is not injected, which poses a problem in terms of cost and maintaining the degree of vacuum. Have.

[0008] The method disclosed in Japanese Patent Application Laid-Open No. Hei 6-299229 discloses a method in which Sol. In order to perform desulfurization treatment after adding Al to be 0.100% or more, it is necessary to inject oxygen for burning off Al in the component fine adjustment step after desulfurization. ,
There is a concern that reversion S from the slag to the molten steel may occur, and quality deterioration such as poor surface properties due to inclusions at the product stage.

An object of the present invention is to solve the above-mentioned disadvantages of the prior art and eliminate the need for flowing gas from the powder injection port except when the desulfurizing agent is blown, and to remove Al combustion in the fine component adjustment step after desulfurization. It is an object of the present invention to provide a method for desulfurizing molten steel that does not require oxygen injection for the purpose.

[0010]

According to the method for desulfurizing molten steel of the present invention, the [S] content before desulfurization and the target [S] content after desulfurization are determined from the correlation between the desulfurization rate and the amount of desulfurizing agent injected. The amount of the desulfurizing agent to be blown is determined in accordance with the amount of the desulfurizing agent to be blown and the amount of the desulfurizing agent during the desulfurization treatment. The target after desulfurization [Sol. [Sol.] Before desulfurization corresponding to the [Al] content.
Al] content was back calculated and the back calculated [Sol. Al]
After reducing the content to below the content, desulfurization treatment is to be started. As described above, in the vacuum degassing process of the RH method including the heat treatment process, the vacuum degassing process, and the component fine adjustment process, the [S] before desulfurization was determined from the correlation between the desulfurization rate and the amount of the desulfurizing agent injected. By obtaining the desulfurizing agent injection amount according to the content rate and the target [S] content after desulfurization, it is possible to reduce the desulfurizing agent injection amount by suppressing excessive desulfurization and shorten the desulfurization treatment time. Further, the desulfurizing agent blowing amount and the desulfurization during the desulfurization treatment were determined. The target after desulfurization [S
ol. [Sol.] Before desulfurization corresponding to the [Al] content. A
l], the content was calculated backward, and the calculated content [Sol. Al] content, the desulfurization treatment is started, and then the desulfurized [Sol. A
l] Target content [Sol. [Al] content can be controlled, oxygen blowing for Al combustion removal after desulfurization can be omitted, processing time can be shortened, and steel quality can be improved.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The desulfurization reaction mechanism by blowing a desulfurizing agent into molten steel is based on the movement of S at the interface between the desulfurizing agent and the molten steel, as shown in FIG. The larger the amount of the desulfurizing agent injected, the greater the reaction interface and the greater the amount of the desulfurizing agent absorbed [S].
The desulfurization rate increases. The desulfurization rate in FIG. 2 is determined by ([S] content before desulfurization− [S] content after desulfurization) ÷ [S] content before desulfurization × 100 (%). According to the following formula (1), desulfurization has a correlation with [O] concentration in molten steel, and the desulfurization ability increases as the [O] concentration in molten steel decreases. (O ²⁻ ) + [S] = [O] + (S ²⁻ ) (1) where []: in molten steel, (): in flux and slag

When a CaO-based flux is blown into molten steel, [Al] and [O] in the molten steel react at the interface between the powder and the molten steel, and Al ₂ O ₃ is easily generated. This is Al ₂ O ₃
Forms a low-melting inclusion, so that both the effect of accelerating the reaction by lowering the melting point of the desulfurizing agent and the effect of accelerating desulfurization by lowering the [O] concentration in the molten steel are obtained. That is, FIG.
As shown in the figure, the amount of the desulfurizing agent injected and [Sol. A
l] There is a correlation with the reduction rate, and as the desulfurizing agent injection amount increases, the reaction amount of [Al] and [O] in the molten steel increases,
During desulfurization [Sol. Al] reduction rate increases. Therefore, the desulfurizing agent injection amount is set based on the [S] content before desulfurization and the target [S] content after desulfurization, and the desulfurizing agent injection amount and the desol. The target after desulfurization [Sol. [S] before desulfurization corresponding to the [Al] content
ol. Al] content can be calculated backward.

In the present invention, the desulfurizing agent injection amount is set based on the [S] content before desulfurization in the molten steel and the target [S] content after desulfurization, whereby the desulfurization treatment based on the desulfurizing agent injection amount is performed. Sol. The target after desulfurization [So
l. [So] in molten steel before desulfurization corresponding to the [Al] content
l. Al] content was back calculated and the back calculated [Sol. A
[Sol.l] below the [Sol. After the [Al] content is reduced, desulfurization treatment is started. As a result, it is possible to reduce the amount of the desulfurizing agent blown by suppressing excessive desulfurization and to shorten the desulfurization treatment time, and to reduce the amount of [So
l. Al] content [Sol. [Al] content can be controlled, oxygen blowing for Al combustion removal after desulfurization can be omitted, processing time can be shortened, and steel quality can be improved.

In the present invention, it is desirable that the lance used for blowing the desulfurizing agent upward can be cooled using cooling water or the like. If an uncooled lance is used, lance erosion,
It is expected that troubles such as inability to raise and lower the lance due to the adhesion of the metal occur, and it becomes impossible to accurately maintain the distance between the lance and the molten steel bath surface. Also, when using a non-cooled lance, if the nozzle at the tip melts even if the entire lance is sound, the flow velocity of the carrier gas changes and the blowing direction of the powder changes. May not be maintained.

[0015]

【Example】

Embodiment 1 Hereinafter, a method for desulfurizing molten steel of the present invention will be described in detail with reference to FIG. 1 showing an embodiment. FIG. 1 is an overall explanatory diagram of the main parts of the RH equipment. Using the RH equipment shown in FIG. 1, after adding a ferromanganese alloy (high carbon ferromanganese or low carbon ferromanganese) in an amount corresponding to a predetermined Mn level which has been blown in a converter, the deoxidized state is obtained. Appropriate slag reforming treatment is performed on molten steel 2 that has been tapped into ladle 1.
Then, the immersion pipes 3 and 4 are immersed in the molten steel 2 in the ladle 1, and the inside of the vacuum tank 5 is sucked from the exhaust pipe 6 and the pressure is reduced to suck the molten steel 2 into the vacuum tank 5. Thereafter, Ar gas is blown from a reflux gas injection tuyere 7 provided inside the immersion pipe 3 (rising pipe), and the molten steel 2 in the immersion pipe 3 is raised based on the principle of gas lift, and descends from the immersion pipe 4 (downcoming pipe). Then, the molten steel 2 is refluxed.

After the reflux of the molten steel 2 is stabilized, the water cooling lance 8 is lowered vertically from the upper part of the vacuum chamber 5 to
After the oxygen is blown from a nozzle provided at the tip of the furnace to raise the temperature, a desulfurizing agent is blown together with the Ar gas of the carrier gas during the vacuum degassing process, and the molten steel 2 is desulfurized. Prior to the desulfurization treatment, the [S] content of the molten steel 2 before desulfurization and the target [S] content after desulfurization during the temperature raising treatment were determined as shown in FIG.
The amount of the desulfurizing agent to be blown is determined on the basis of the amount of the desulfurizing agent blown, and [Sol. Al] The reduction rate is determined and the target [S] for satisfying the predetermined product characteristics is obtained.
ol. Al] value can be achieved before desulfurization [Sol. A
l] content is calculated backward, and [Sol. [Al] content was reduced to a value equal to or less than the value calculated backward.

Therefore, in the method for desulfurizing molten steel of the present invention, the amount of the desulfurizing agent to be injected is determined from the [S] content before desulfurization and the target [S] content after desulfurization. It is possible to reduce the charging amount and the desulfurization treatment time. In the method for desulfurizing molten steel according to the present invention, the amount of [So
l. Al] reduction rate, and a target [Sol. Al] value can be achieved before desulfurization [Sol. [Al] content is calculated backward to obtain [So
l. Al] content is reduced to a value equal to or less than the value obtained by back calculation,
Since the desulfurization treatment is started, [Sol. Al] content [Sol. Al] content can be controlled,
Oxygen injection for Al combustion removal after desulfurization can be omitted,
It is possible to shorten the processing time and improve the quality of steel.

Example 2 Using a 160-ton scale RH facility shown in FIG. 1 of Example 1 above, the ferromanganese alloy (high carbon ferromanganese or After the addition of low carbon ferromanganese), the molten steel 2 that had been tapped into the ladle 1 in a deoxidized state was subjected to an appropriate slag reforming treatment. And the temperature of the ladle 1 1600-1
The immersion tubes 3 and 4 are immersed in the molten steel 2 at 640 ° C., the inside of the vacuum chamber 5 is sucked from the exhaust pipe 6 and the pressure is reduced to 2 Torr or less.
The molten steel 2 was sucked into the vacuum chamber 5. Thereafter, Ar gas is blown at 1000 l / min from the reflux gas injection tuyere 7 provided inside the immersion pipe 3 (rising pipe), the molten steel 2 in the immersion pipe 3 is raised based on the gas lift principle, and the immersion pipe 4 ( tube)
And the molten steel 2 was refluxed.

After the reflux of the molten steel 2 is stabilized, as shown in Table 1, based on the [S] content before desulfurization and the target [S] content after desulfurization, the desulfurizing agent blowing amount is 2 to 9 kg based on FIG. /
ton and the desulfurizing agent blowing amount 2 to 9 kg /
ton [Sol. Al] reduction amount, and a target [Sol. Al] value can be achieved before desulfurization [Sol. Al] content is calculated backward, and [Sol. When the [Al] content is reduced to a value equal to or less than the back calculated value, the water-cooling lance 8 is lowered vertically from the upper part of the vacuum chamber 5, and Ar gas 5 Nl / min of a carrier gas is injected from a nozzle provided at the tip of the water-cooling lance 8,
Table 1 blow amount of desulfurizing agent that shows a (particle size of about 0.15 mm) was blown over 3 to 13 minutes at a spray pressure of 5 kg / cm ^2, after which then refluxed molten steel 2 3 minutes, the molten steel 2 desulfurization Processing completed. Table 1 shows the results.

[0020]

[Table 1]

As shown in Table 1, the [S] content in the molten steel after the desulfurization treatment could be reduced to almost the target [S] content. In addition, in the molten steel after the desulfurization treatment [Sol. Al]
The content is almost the target [Sol. Al] content in the molten steel [Sol. [Al] The removal of the Al combustion by blowing over O _{2 to} reduce the content can be eliminated.

[0022]

According to the method for desulfurizing molten steel of the present invention, the [S] content in molten steel can always be reduced to almost the target [S] content to prevent excessive desulfurization. It is possible to reduce the time and the time required for the desulfurization treatment accordingly. In addition, in the molten steel after the desulfurization treatment [Sol. Al] content is almost targeted [Sol. Al] content in the molten steel [Sol. Al] O ₂ for content reduction
Al combustion removal by top blowing can be eliminated.

[Brief description of the drawings]

FIG. 1 is an overall explanatory diagram of a main part of an RH facility used in an embodiment.

FIG. 2 is a graph showing a relationship between a desulfurizing agent blowing amount and a desulfurization rate.

FIG. 3 shows the injection amount of desulfurizing agent and molten steel during desulfurization [Sol. A
1] A graph showing the relationship with the reduction rate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladle 2 Molten steel 3, 4 Immersion pipe 5 Vacuum tank 6 Exhaust pipe 7 Reflux gas injection tuyere 8 Water cooling lance

Claims (1)

[Claims] 1. A desulfurization method for a molten steel in which a desulfurizing agent is blown up together with a carrier gas from above the vacuum tank using a lance when the deoxidized molten steel is subjected to vacuum degassing by a RH facility. From the correlation between the blowing amounts, the [S] content before desulfurization and the target [S] content after desulfurization are determined, and the desulfurizing agent blowing amount according to the target [S] content is determined. . The target after desulfurization [Sol. [Sol.] Before desulfurization corresponding to the [Al] content.
Al] content was back calculated and the back calculated [Sol. Al]
A desulfurization method for molten steel, wherein the desulfurization treatment is started after the content is reduced to the content or less.