JPH06145764A - Molten steel ladle electrode heating refining method - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to an LF treatment method, which is a secondary refining process of molten steel, and recovers slag after LF treatment having a high S content,
By recycling this to LF refining of high S steel,
Suppression of S removal, reduction of LF slag material, reduction of slag time. [Structure] In the LF treatment, which is the secondary refining process for molten steel, after performing de-S treatment with high basicity / low oxidation degree slag, the slag is recovered and the type of steel that requires suppression of de-S in high S steel Recycle to LF treatment to improve the yield of added S and reduce the amount of LF flux used. Further, when performing the de-S suppressing treatment using the recycled slag, by adding a SiO ₂ source such as silica stone and controlling the slag basicity in the range of 3 to 5, the de-sintering is controlled while controlling the de-S. Promote things.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ladle electrode heating refining (hereinafter referred to as LF) method which is a secondary refining process for molten steel.

[0002]

2. Description of the Related Art In recent years, the quality requirements for steel materials have become more severe as the utilization technology has become more sophisticated and diversified, and the need for producing high-purity steel is increasing more and more. In response to such demands for producing high-purity steel, in the steelmaking process, hot metal pretreatment or secondary refining equipment has been expanded. Especially the removal of molten steel from S
In addition, for refining with removing inclusions, refining with strong reduction and high basicity slag is necessary, and this slag refining becomes possible by introducing the LF process, and the production process capacity of low S steel and high cleanliness steel is greatly improved. Improved.

As a conventional technique, in the LF process, a high degree of slag basicity is ensured and a thorough slag oxidation degree (Fe
It is generally practiced to promote the de-S reaction or reduce inclusions in steel by reducing O + MnO) (for example, Iron and Steel Institute of Japan: 126th 127th Nishiyama Memorial Technical Course, 1988). . On the other hand, in recent years, even in steel types requiring high cleanliness, S is positively added in order to improve the machinability of steel materials.
There is an increasing trend in steel grades that require higher concentrations. However, as described above, the conventional slag refining technology in the LF process promotes the cleaning of molten steel and the removal of S at the same time, and in the high basicity operation for ensuring the current cleanliness, the molten steel of added S is melted. The dissolution rate in the solution was low and it was necessary to add a large amount of S.

[0004]

As described above, in the conventional LF refining of steel grades having high cleanliness and machinability, the slag basicity must be increased in order to prioritize the high purification treatment for the purpose of removing inclusions. However, it is necessary to add a large amount of S because the yield of S addition is reduced. In addition, when performing LF treatment, it is necessary to secure a sufficient heat allowance of molten steel before refining by adding expensive slag material such as quick lime, alumina, light burned dolomite, etc. There are issues to be improved.

An object of the present invention is to provide an LF operating method capable of solving the problems of the conventional method.

[0006]

Means for Solving the Problems That is, according to the present invention, in the LF treatment, which is one of the secondary refining processes for molten steel, after de-S is performed with slag having a high basicity and a low oxidation degree, the slag having a high S content is obtained. Is recovered and reused in the refining of steel types that require high S steel to suppress S removal.
The main point is the refining method.

Further, the present invention provides a slag base by refining clean steel requiring reduction of T [O] in steel by adding a SiO ₂ source such as silica stone when reusing the high S-containing slag. The gist is an LF refining method characterized by refining while controlling the degree to 3 to 5.

[0008]

In the past, the LF treatment was performed in the step shown in the conventional method of FIG. 1, and a new slag-making material was added every time. The present inventors have a high basicity (CaO / SiO ₂ ), a low degree of oxidation (T.Fe + MnO) after deS treatment in LF, as shown in the method of the present invention in FIG. When slag with a high S content is reused for the treatment that requires suppression of S removal in LF, the S-added yield is improved and inclusions in molten steel form T [O] in steel.
It was found that the generation of slag is quicker, the LF total processing time is shortened, and the energy cost can be significantly reduced, without increasing the value of slag.

That is, the de-S reaction in the LF process is described by the following formula. [S] + (O ²⁺ ) = (S ²⁺ ) + [O] ……………… (1) [S] + (CaO) = (CaS) + [O] ……………… ( 2) Therefore, in order to suppress the de-S reaction of molten steel, the activity in slag (S ²⁺ ) or molten steel [O] on the right side of formula (1) should be increased, or on the left side of formula (1). During slag (O ²⁺ )
It is effective to reduce the activity of. Of these, [O]
The increase in activity and the decrease in (O ²⁺ ) activity are accompanied by a decrease in basicity due to an increase in the slag oxidation degree and a decrease in (CaO) in the slag of the formula (2). It is physicochemically clear that increasing the activity of (S ²⁺ ) in the slag is most effective for suppressing the de-S reaction while maintaining cleanliness.

Therefore, by reusing the slag having a high S content after de-S, while maintaining the high basicity and the low oxidation degree of the slag,
It becomes possible to increase the activity of S in the slag, and it is possible to suppress the de-S reaction without lowering the cleanliness of molten steel. After confirming the above-mentioned target reaction with an actual machine, the inventors of the present invention showed that after de-S after the conventional method in which new slag materials such as quick lime, alumina and light burned dolomite were added and treated as shown in FIG. It was confirmed that the S content removal rate of the molten steel is lower in the method of the present invention in which slag having a high basicity and a low oxidation degree having a high S content of 0.25 to 0.35% is reused. Further, as shown in FIG. 2, the de-S removal rate of molten steel decreases sharply from around the slag basicity of 5 after LF treatment, and as shown in FIG. ． It was found that the Fe concentration sharply increased. Therefore L
By increasing the S concentration in the slag by reusing the slag after F-de-S removal, keeping the S activity at a high level, and controlling the basicity in the slag within the above range, the S-reaction can be further promoted. It can be efficiently suppressed and can be applied to the production of high-cleanliness steel having a higher S content.

The basicity can be controlled by adding S such as silica stone if necessary.
It can be controlled by adding an iO ₂ source, and the addition amount can be easily determined from the composition and amount of the slag to be reused. By the way, the slag subjected to the LF desulfurization suppressing treatment can be used again for the LF desulfurization treatment, and in this case, it is necessary to control the slag composition to a high basicity level by adding quick lime or the like.

Regarding the method of reusing slag, the method of crushing after slag / cooling and crushing and adding by FIBC back packing is considered to be the optimum method in consideration of safety, but it is not limited to this.

[0013]

[Examples] The slag after the LF treatment, which was subjected to S-depletion and oxygen-deficient refining, was recovered, and 10 kg / T of the recovered slag was reused for the S-reduction suppression treatment in the LF. Table 1 shows the composition system of the steel types that have been subjected to the S-removing and oxygen-reducing treatment and the composition system of the steel types that have been subjected to the S-reducing suppression treatment. Table 2 shows the slag composition recovered after the de-S treatment and the slag composition and the quality status after the slag is reused for the de-S suppression treatment. Further, Table 3 also shows, for comparison, the actual results of performing the S removal suppression processing by the conventional method. Here, the conventional method refers to a method in which slag material such as quick lime, light burned dolomite, and alumina is newly added to generate slag necessary for the treatment.

As can be seen from the results shown in Table 3, the cleanliness of molten steel (T [O] level) according to the method of the present invention is comparable to that of the conventional method as compared with the conventional method, and no problem is observed. In the method of the present invention, since the slagging time is shortened by using the reused slag, the LF total treatment time is shortened, the LF-added S yield is improved, and the amount of quicklime etc. used as a slag-making material is also reduced. did.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

As can be seen from the above description, the present invention has the following effects. First, the de-S reaction during LF treatment is suppressed, and the S yield to be added is improved.
Further, it is to suppress the decrease of S concentration in steel in RH which is a lower step or continuous casting step. That is, the present invention is effective for the steel types that need to improve the machinability by increasing the S concentration in the steel, and since the required basicity can be easily controlled, the cleanliness is maintained (the amount of inclusions is reduced). Is possible.

Second, the amount of added flux in the LF process can be significantly reduced, and thirdly, slag can be quickly turned into slag, the LF processing time can be shortened, and energy or refractory costs can be reduced. Is to be brought.

[Brief description of drawings]

FIG. 1 is a diagram showing the steps of a conventional method and the method of the present invention in comparison.

FIG. 2 is a diagram showing the relationship between the slag basicity after LF treatment and the S removal rate.

FIG. 3: Slag basicity after LF treatment and T. It is a figure showing the relationship of Fe content rate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hajime Isoda 12 Nakamachi, Muroran City, Hokkaido Inside Nippon Steel Co., Ltd. Muroran Works (72) Inventor Kenji Tateno 12 Nakamachi, Muroran City, Hokkaido Muroran Co., Ltd. Inside the steelworks

Claims (2)

[Claims] 1. In a ladle electrode heating refining method, which is a secondary refining process for molten steel, after de-S treatment with slag having a high basicity and a low oxidation degree, the slag is recovered and deoxidized with high S steel. A ladle electrode heating and refining method for molten steel, characterized in that it is reused for refining steel types that require S suppression. 2. A SiO ₂ source is added so that the slag basicity satisfies the following range when the S removal-suppressing refining is performed using the recovered Slag after the S removal treatment according to claim 1. Molten steel ladle electrode heating refining method. 3 ≦ slag basicity; (% CaO /% SiO ₂ ) ≦ 5