JP2003247016A - Method for decarburizing molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for decarburizing molten steel with which a carburizing reaction efficiency in the molten steel can be improved compared with that of the conventional method by eliminating a dead zone developed in the molten steel in a ladle with an RH vacuum degassing vessel disposed therein. <P>SOLUTION: When the molten steel is decarburized while circulating the molten steel between the RH vacuum degassing vessel and the ladle by disposing the RH vacuum degassing vessel in the molten steel held in the ladle, an immersion lance for blowing gas into the molten steel is arranged between the outer peripheral surface of the immersion tubes in the RH vacuum degassing vessel and the inner wall of the ladle, and the gas is blown through this immersion lance to stir the molten steel. In this case, it is desirable that the blowing gas through the above immersion lance is nitrogen or argon, or the dipping depth (d) of the above immersion tubes into the molten steel is adjusted so as to satisfy the following formula according to the inner radius (r) in this immersion tube. 0.9r≤d≤1.1r. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decarburizing molten steel, more specifically, decarburizing molten iron in a converter, tapping the formed molten steel to a ladle, and then vacuum degassing the molten steel. The present invention relates to a technique for further reducing the carbon content by performing decarburization treatment with a device.

[0002]

2. Description of the Related Art Conventionally, molten iron is decarburized and refined in a converter to obtain molten steel, which is then tapped into a ladle and then decarburized using a vacuum degassing device to reduce the carbon content of the molten steel. Further reduction (this is called secondary refining) is being carried out. Various techniques have been proposed in order to efficiently carry out the decarburization reaction in this secondary refining.

For example, Japanese Patent Laid-Open No. 4-131316 discloses a vacuum degassing tank 4 having two dip pipes 3 for raising and lowering the molten steel 2 held in a ladle 1, as shown in FIG.
Using a so-called "RH type vacuum degassing tank" in which the molten steel 2 is circulated between the ladle 1 and the ladle 1, a technique for improving the flow of the molten steel 2 by blowing an inert gas 5 from the bottom of the ladle 1 during the degassing process. Disclosure. The gas injection from the bottom of the ladle is usually performed by filling the bottom with a porous refractory 6 such as a porous plug. However, the porous refractory material 6 is not only restricted in the flow rate of gas blown due to clogging, but also has a problem that it requires a great deal of labor for replacement or has a short replacement cycle. . Further, since the embedding position of the porous refractory material 6 is usually uniquely determined at the time of design, for example, the molten steel flow is stagnant in the ladle 1 and the decarburization is difficult to proceed in the area ( Even if there is a "dead zone", the only way to eliminate it is to increase the flow rate of the gas blown from the bottom, and there is a problem that the increase in the flow rate accelerates the temperature drop of the molten steel 2. .

In order to improve the efficiency of the decarburization reaction of the molten steel 2 in the vacuum degassing tank 4, the inventors of the present invention first disclosed in Japanese Patent Laid-Open No.
As proposed in Japanese Patent Application Laid-Open No. 77518/1995, “The upper blowing lance 7 is inserted into the RH vacuum degassing tank 4 from above, and oxygen gas is blown into the molten steel 2 through the upper blowing lance 7 to decarburize it (FIG. 5). Technology) is effective. However, RH
The vacuum degassing tank 4 has a portion where the molten steel 2 (or slag) comes into contact with the atmosphere between the two immersion pipes 3 and between the outer peripheral surface of the immersion pipe 3 and the inner wall of the ladle 1, The flow of the molten steel 2 becomes relatively small in that portion, and the “dead zone” is likely to occur. The existence of this "dead zone" has been a source of prolonging the decarburization time or inducing poor decarburization of molten steel.

[0005]

In view of such circumstances, the present invention eliminates the dead zone that occurs in molten steel in the ladle provided with the RH vacuum degassing tank, and further improves the decarburization reaction efficiency of molten steel. It is an object of the present invention to provide a decarburizing method for molten steel that can be increased.

[0006]

Means for Solving the Problems The inventor has conducted extensive studies in order to achieve the above object, and realized the results in the present invention.

That is, according to the present invention, when the RH vacuum degassing tank is provided in the molten steel held in the ladle, and the decarburization is performed while circulating the molten steel between the RH vacuum degassing tank and the ladle, An immersion lance for blowing gas into the molten steel between the outer peripheral surface of the immersion pipe of the RH vacuum degassing tank and the inner wall of the ladle is provided, and the gas is blown through the immersion lance to stir the molten steel. This is a decarburizing method for molten steel.

In this case, the gas blown through the immersion lance is nitrogen or argon, or the penetration depth (d) of the immersion pipe into the molten steel is determined as follows depending on the inner diameter (r) of the immersion pipe. It is preferable to adjust so as to satisfy the formula.

0.9r.ltoreq.d.ltoreq.1.1r In addition, the oxygen gas is blown to the molten steel through an upper blowing lance inserted in the RH vacuum degassing tank,
Alternatively, the immersion lance is movable up and down, and the immersion position is ± in reference to the center of the RH vacuum degassing tank in plan view.
It is better to make it movable so as to adjust within a range of 60 °.

According to the present invention, the gas injection lance is inserted into the dead zone of the molten steel formed between the inner wall of the ladle and the outer peripheral surface of the immersion pipe of the RH vacuum degassing tank, so that the gas is injected. The zone disappears, and the time required for decarburizing the entire molten steel can be significantly shortened compared to the conventional time.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below, including the background of the invention.

First, the inventor conducted a simulation by a flow analysis when decarburizing molten steel in a ladle with a vacuum degassing equipment, and investigated the relationship between the carbon concentration in molten steel and each time in each portion of the ladle. did. As a result, as one of the causes of extending the decarburization time of the entire molten steel, the top and end of the ladle (at the wall)
It was confirmed that there is a region (a dead zone) where the flow of molten steel is small. Then, subsequently, a measure for eliminating these dead zones and shortening the decarburization time of the entire molten steel was examined.

As a concrete means, as shown in FIG. 1, a lance for injecting gas into the molten steel 2 between the outer peripheral surface of the dip tube 3 and the inner wall of the ladle 1 (hereinafter referred to as dipping lance 9). The present invention completes the present invention in which the inert gas and the active gas are blown into the molten steel 2 to stir the dead zone formed in the molten steel 2 above the ladle 1.

Usually, when the RH vacuum degassing tank 4 is used, a small diameter pipe 8 is provided separately for the gas for circulating the molten steel 2 (referred to as reflux gas 10) toward the dipping pipe 3 also called riser pipe. In addition to this, in addition to this, the vacuum degassing tank 4
In some cases, as described above, the upper blowing lance 7 of gas is inserted from above, and oxygen gas is blown through the upper blowing lance 7 to decarburize the molten steel. Therefore,
In the present invention, these gases are also blown, and then gas blowing into the molten steel between the outer peripheral surface of the dipping pipe and the inner wall of the ladle is also used. This is because the decarburization reaction becomes more efficient.

In addition, the inventors of the present invention, in the process of performing the above simulation, the deeper the immersion pipe 3 penetrates into the molten steel 2, the more "dead zone" the upper part of the ladle 1.
Have been found to expand. The expansion of the dead zone increases the time required for uniform decarburization of the molten steel 2.
On the other hand, if the depth of the dipping pipe 3 is too shallow, the slag floating in the molten steel 2 at the upper part of the ladle 1 easily enters the vacuum degassing tank 4 through the rising pipe, which causes the slag mixture. As a result, the cleanliness of molten steel 2 is deteriorated.

Therefore, the inventor has found that the molten steel 2 of the immersion pipe 3
Optimization of the immersion depth inside was also investigated. And
Regarding the immersion depth of the immersion pipe 3, the ratio of the immersion depth (d) into the molten steel of the immersion pipe and the inner diameter (r) of the immersion pipe shown in FIG. It was found that it affects the time required for decarburization of molten steel. Therefore, the inventor also studied to eliminate this effect, and thought that it is better to keep the above ratio within the following range as a measure to prevent the above-mentioned slag inclusion and the dead zone from expanding. This has also been added to the present invention.

0.9r ≦ d / r ≦ 1.1 That is, when the immersion depth (d) of the immersion pipe 3 is less than 0.9 of its inner diameter (r), the ratio of generation of defective steel due to slag inclusion increases. On the other hand, when the immersion depth (d) exceeds 1.1 of the inner diameter (r), the dead zone expands and the decarburization treatment time increases.

Further, the dead zone is mainly the ladle 1.
Occurs in the upper part and the end part (upward pipe side) of the molten steel 2 because the downward flow of the molten steel 2 due to the gas injection from the immersion lance 9 according to the present invention is maximized. It has been found that it is effective to move 9 in a plan view or to move up and down in the vertical direction. Then, as shown in FIG. 2, it is also added to the present invention that the immersion lance 9 is moved in the direction of the arrow in a plan view or is moved up and down. In this case, a well-known moving means of the immersion lance 9 may be used.
The present invention is not particularly limited.

Here, with respect to the ascending / descending distance of the immersion lance 9 in the vertical direction, the distance from the surface of the molten steel 2 downward is 50.
In the range of 0 to 2000 mm, and the movement cycle is 1
It is desirable that the time is about 2 minutes. This limitation is due to the fact that the dead zone generation area is usually within 1500 mm downward from the upper surface of the molten steel, and
This is because if bubbling treatment with gas is performed in a range shallower than 500 mm from the upper surface of the molten steel 2, splash generation of the molten steel 2 increases or the slag on the molten steel is agitated to lower the cleanliness thereof.

When oxygen is supplied to the molten steel in the RH vacuum degassing tank for decarburization, the method of blowing oxygen gas to the molten steel via the upper blowing lance is most preferably used as described above. As another method, oxygen gas is blown into the molten steel from the tuyere provided on the lower side of the RH vacuum degassing tank, and a known decarburization accelerating method such as spraying a metal oxide onto the molten steel from an upper blowing lance is applied. May be.

[0021]

EXAMPLE As shown in FIG. 1, molten steel 2 was tapped from a converter (not shown) and a RH vacuum degassing tank 4 was set in a ladle 1 having a capacity of 280 tons. The molten steel decarburizing method was carried out. In addition, as a comparison, an operation was performed without using the immersion lance 9 according to the present invention. The operation results are as shown in Table 1. In addition, "Operation" in Table 1 refers to immersion lance 9
Is fixed in a dead zone where the flow of molten steel 2 is the smallest (position at a depth of 1.5 m from the molten metal surface on the rising pipe side), and argon gas is blown at a flow rate of 200 liters per minute. In addition, the "operation" in Table 1 is such that the immersion lance 9 is moved up and down at a cycle of once every 1/2 minute, and the immersion lance 9 is moved once per half along the inner circumference of the ladle 1. It is an operation result at the time of rocking in the molten steel 2 in a cycle of minutes.

[0022]

[Table 1]

From Table 1, when the immersion lance 9 according to the present invention is used and the immersion lance 9 is not moved in plane or moved up and down (operation), the decarburization time is shortened by 2 minutes as compared with the comparative example. It is clear that you can. Further, the immersion lance 9 is vertically moved up and down (cycle once / 2 minutes, moving stroke: depth of 500 mm to 2000 mm from the upper surface of the molten steel in the ladle), and further in plan view with the center of the vacuum degassing tank 4 as a reference. ± 6
In the present invention (operation) in which the immersion lance 9 is moved at an angle of 0 °, the decarburization time can be shortened by 3 minutes as compared with the comparative example.
It became possible to decarburize stably. Therefore, by shortening the decarburization treatment time, the temperature drop of the molten steel during the treatment in the RH vacuum degassing tank can be suppressed, so that the tapping temperature from the converter, which is the upstream process, can be reduced by 3 ° C. became. In Table 1, the ratio of the immersion depth of the immersion pipe 3 to the inner diameter (d /
r) is shown, but all the examples of the present invention were operated at 1.0. Further, the carbon concentration after the decarburization treatment reached 15 ppm or less in all examples, as shown in FIG.

[0024]

As described above, according to the present invention, the RH
It is possible to eliminate the dead zone that occurs in the molten steel in the ladle provided with the vacuum degassing tank, and to further improve the decarburization reaction efficiency of the molten steel as compared with the conventional case.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an RH vacuum degassing tank in which a molten steel decarburizing method according to the present invention is carried out.

FIG. 2 is a plan view illustrating a moving range of the immersion lance according to the present invention.

FIG. 3 is a diagram showing a relationship between a carbon concentration in molten steel and a vacuum processing time according to a comparative example and an embodiment of the present invention.

FIG. 4 is a cross-sectional view of an RH vacuum degassing tank for explaining the immersion depth (d) and the immersion pipe inner diameter (r) of the immersion pipe into the molten steel.

FIG. 5 is a cross-sectional view showing an RH vacuum degassing tank in which a conventional molten steel decarburizing method is carried out.

[Explanation of symbols]

1 ladle 2 Molten steel 3 immersion pipe 4 vacuum degassing tank 5 Inert gas 6 Porous refractory 7 Top blowing lance 8 small diameter pipe 9 Immersion lance 10 Return gas

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshihisa Kitano             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Chiba Steel Works, Ltd. (72) Inventor Kazuyuki Kato             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Chiba Steel Works, Ltd. F-term (reference) 4K013 BA02 CA01 CA02 CA04 CA12                       CA15 CA16 CC01 CE01 CE04                       CE05

Claims (5)

[Claims] 1. An RH vacuum degassing tank is provided in the molten steel held in a ladle, and when decarburizing while circulating the molten steel between the RH vacuum degassing tank and the ladle, the RH vacuum degassing is performed. A immersion lance for blowing gas into the molten steel between the outer peripheral surface of the immersion pipe of the gas tank and the inner wall of the ladle is provided, and the gas is blown through the immersion lance to stir the molten steel. Charcoal method. 2. The gas blown through the immersion lance is nitrogen or argon.
Method for decarburizing molten steel described. 3. The penetration depth (d) of the immersion pipe into the molten steel
The method for decarburizing molten steel according to claim 1 or 2, wherein is adjusted according to the inner diameter (r) of the immersion pipe so as to satisfy the following formula. 0.9r ≦ d ≦ 1.1r 4. The method for decarburizing molten steel according to claim 1, wherein oxygen gas is blown onto the molten steel through an upper blowing lance inserted in the RH vacuum degassing tank. 5. The immersion lance is vertically movable and is movable so as to adjust the immersion position within a range of ± 60 ° with respect to the center of the RH vacuum degassing tank in plan view. The method for decarburizing molten steel according to any one of claims 1 to 4.