JP2709554B2 - Ladle electrode heating and refining method for molten steel - Google Patents

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 and refining (hereinafter referred to as LF) method as 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 increasingly severe with the sophistication and diversification of their utilization techniques, and the need for high-purity steel production has been increasing. In response to such demands for the production of high-purity steel, the steelmaking process has attempted to expand the hot metal pretreatment or secondary refining equipment. In particular, removal of molten steel S
For refining to measure inclusions and de-inclusions, refining with strong reduction and high basicity slag is necessary, and the introduction of the LF process makes this slag refining possible, significantly increasing the production process capacity of low S steel and high cleanliness steel. Improved.

As a conventional technique, in the LF process, a high slag basicity is secured and a thorough slag oxidation degree (Fe
It is generally practiced to reduce the amount of O + MnO) to promote the de-S reaction or reduce inclusions in the steel (for example, Japan Iron and Steel Institute: 126th and 127th Nishiyama Memorial Technical Course, 1988). . On the other hand, in recent years, even in steel types that require high cleanliness, S is actively added to improve the machinability of steel materials,
There is an increasing trend for steel types that require higher concentrations. However, as described above, in the conventional slag refining technology in the LF process, the purification of molten steel and the removal of S are simultaneously promoted, and in the high basicity operation for securing the current cleanliness, the molten steel of added S The rate of dissolution in the medium was low, and a large amount of S had to be added.

[0004]

In the conventional LF refining of a steel type having high cleanliness and machinability as described above, it is necessary to increase the basicity of slag in order to give priority to the high purification treatment for removing inclusions. However, it is necessary to add a large amount of S due to a decrease in the yield of S addition. In addition, in performing the LF treatment, it is necessary to add a high-grade slag forming material such as quicklime, alumina, and lightly-burned dolomite, and to further form a slag. There are issues to be improved.

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

[0006]

That is, in the present invention, in the LF treatment which is one of the secondary refining processes of molten steel, after removing S by a high basicity and a low oxidation degree slag, the high S content slag is obtained. Is recovered and reused in the refining of steel types that require high S steel to suppress the removal of S.
Add slag basicity to 3-5 by adding iO ₂ source
Control to the gist of LF refining method characterized by refining.

[0007]

[0008]

Conventionally, the LF treatment has been carried out in the step shown in the conventional method of FIG. 1, and a new slag material has been added each time. The present inventors, as shown in the method of the present invention in FIG. 1, have a high basicity (CaO / SiO ₂ ), a low oxidation degree (T.Fe + MnO) after de-S treatment by LF, and When the slag containing high S is reused in the processing that requires the suppression of the removal of S by LF, the yield of S addition is improved, and T [O] in the steel that generates inclusions in the molten steel is improved.
It has been found that the slag is generated more quickly than the conventional processing method, the LF total processing time is shortened, and the energy cost can be significantly reduced without increasing the conventional method.

That is, the S removal reaction in the LF step is described by the following equation. (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 of (S ²⁺ ) in the right side of equation (1) or the activity of [O] in molten steel must be increased, or the left side of equation (1) In slag (O ²⁺ )
It is effective to decrease the activity. Of these, [O]
An increase in the activity and a decrease in the (O ²⁺ ) activity are accompanied by an increase in the slag oxidation degree and a decrease in the basicity due to a decrease in (CaO) in the slag in the formula (2). It is physicochemically clear that increasing the activity of (S ²⁺ ) in the slag is most effective in suppressing the de-S reaction while maintaining the cleanliness.

[0010] Therefore, by reuse of the slag having a high S content after de-S, while maintaining the high basicity and low oxidation degree of the slag,
The activity of S in the slag can be increased, and the S removal reaction can be suppressed without lowering the cleanliness of the molten steel. The present inventors have confirmed the above-mentioned target reaction with an actual machine. As shown in FIG. 2, after the conventional method of adding and treating a new slag forming material such as quicklime, alumina, lightly burned dolomite, and the like, It was confirmed that the method of the present invention in which slag having a high basicity and a low oxidation degree was reused, which had a high S content of 0.25 to 0.35%, had a lower S removal rate of molten steel. Further, as shown in FIG. 2, the desulfurization rate of the molten steel decreases sharply from the vicinity of slag basicity 5 after the LF treatment, and as shown in FIG. . It has been found that the Fe concentration sharply increases. Therefore L
By increasing the S concentration in the slag by reusing the slag after the F de-S, maintaining the activity of S at a high level, and controlling the basicity in the slag to the above range, the de-S reaction is further increased. It can be suppressed efficiently and can be applied to the production of high-purity steel with a higher S content.

The basicity can be controlled by adding an SiO ₂ source such as silica stone, and the amount of addition can be easily determined from the composition and amount of the slag to be reused. By the way, the slag after the LF de-S suppression processing can be used again for the LF de-S processing. In this case, it is necessary to control the slag composition to a high basicity level by adding quicklime or the like.

Regarding the method of reusing the slag, it is considered that, in consideration of safety, a method of crushing after discharging and cooling, crushing, and adding the slag in a flexible container bag is not limited thereto.

[0013]

EXAMPLE Slag after LF treatment after removal of S and deoxygenation was recovered, and the recovered slag was reused at 10 kg / T for suppression of removal of S in LF. Table 1 shows the component systems of the steel types subjected to the de-S and oxygen reduction treatments and the component systems of the steel types subjected to the de-S suppression treatment. Table 2 shows the slag composition recovered after the de-S treatment and the slag composition and quality status after the slag was reused in the de-S control processing. Further, Table 3 also shows the results of performing the desulfurization suppression processing by the conventional method for comparison. Here, the conventional method refers to a method of newly adding a slag-making material such as quicklime, lightly-burned dolomite, and alumina to generate slag necessary for the treatment.

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

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

As will be understood from the above description, the present invention has the following effects. First, the removal of S during the LF treatment is suppressed, and the yield of added S is improved.
Further, the lowering of the S concentration in the steel in the RH or the continuous casting step, which is a lower step, is suppressed. That is, the present invention is effective for a steel type which needs to improve the machinability by increasing the S concentration in the steel to maintain the cleanness (reduction of the amount of inclusions) since the required basicity can be easily controlled. Becomes possible.

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

[Brief description of the drawings]

FIG. 1 is a diagram showing steps of a conventional method and a 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 shows slag basicity and TF in slag after LF treatment. It is a figure showing the relationship of Fe content.

Continued on the front page (72) Inventor Kazuo Isoda 12 Nakamachi, Muroran City, Hokkaido Nippon Steel Corporation Muroran Works (72) Inventor Kenji Tateno 12 Nakamachi, Muroran City, Hokkaido Nippon Steel Corporation Muroran Inside the ironworks (56) References JP-A-61-174314 (JP, A)

Claims (1)

(57) [Claims] 1. A ladle electrode heating refining method, which is molten steel secondary refining step, after the de-S process that due to the high basicity and low oxidation degree slag, and recovering the slag, a high S steel Escape from S
When reused in refining, the slag basicity is in the following range.
A ladle electrode heating and refining method for molten steel, characterized by adding a SiO ₂ source so as to satisfy the requirements. 3 ≦ slag basicity; (% CaO /% SiO ₂ ) ≦ 5 ”