JPH0610028A - Production of ultralow carbon steel - Google Patents

Abstract

PURPOSE:To quicken the decarburizing speed and to improve production efficiency by adding a decarburizing agent so that soluble oxygen quantity becomes the specific value at the latter stage of the decarburization for which carbon content in the molten steel becomes a specific value or lower, at the time of executing the production of an ultralow carbon steel by an RH type vacuum degassing refining method. CONSTITUTION:The molten steel 5 is circulated between a ladle 4 and a vacuum vessel 1 to decarburize the molten steel 5 in the ladle 4. Then, at the latter stage of the decarburization for which the carbon content in the molten steel 5 becomes <=50ppm, the deoxidizing agent, e.g. such as Al is added through a trough 7 so that the soluble oxygen quantity in the molten steel 5 sucked up into the vacuum vessel 1 becomes in the range of 100-200ppm. Further, deoxidized product, such as Al2O3, is generated and the product is made as the nuclei in CO bubbles to promote the decarburizing reaction. By this method, the carbon content in the molten steel is reduced less than an ordinary case, and the damage to the lining refractory of the immersion tubes is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an ultra low carbon steel by an RH type vacuum degassing refining method.

[0002]

2. Description of the Related Art The RH type vacuum degassing refining method is widely used as a method for industrially producing ultra-low carbon steel. In producing ultra-low carbon steel by such an RH-type vacuum degassing refining method, in recent years, higher quality and higher productivity have been required, and various methods have been proposed for that purpose.

For example, Japanese Patent Publication No. 62-6611 discloses RH.
When manufacturing ultra-low carbon steel by the vacuum degassing refining method, the dip tube connected to the bottom wall of the molten steel refining vacuum tank is formed into an elliptical cross-sectional shape, A method of increasing the cross-sectional area, thereby increasing the circulation amount of molten steel in the ladle with the vacuum chamber, and improving the removal rate of impurities in the steel (hereinafter referred to as prior art)
Is disclosed.

[0004]

However, according to the prior art, even if the removal rate of impurities can be improved to some extent, it cannot be expected to be greatly improved, and further, the refractory lining the dip tube is severely damaged. Therefore, the processing cost is increased.

Therefore, an object of the present invention is to solve the above-mentioned problems and to accelerate the decarburization rate when producing an ultra-low carbon steel by the RH type vacuum degassing refining method, whereby the carbon content in the molten steel is increased. It is an object of the present invention to provide a method capable of efficiently manufacturing an ultra-low carbon steel by reducing the above-mentioned amount compared with the conventional method and by less damaging the refractory lining of the immersion pipe.

[0006]

[Means for Solving the Problems] The present inventors have conducted extensive studies to solve the above-mentioned problems. As a result, when manufacturing ultra-low carbon steel by the RH vacuum degassing refining method, the amount of dissolved oxygen in the molten steel is within the range of 100 to 200 ppm at the latter stage of decarburization when the carbon content of the molten steel becomes 50 ppm or less. It was found that the decarburization reaction is promoted by adding a deoxidizing agent to the molten steel so that

The present invention has been made on the basis of the above-mentioned findings, and an ascending pipe and a descending pipe vertically connected to the bottom wall of a molten steel refining vacuum chamber so as to project downward are described above. It is immersed in molten steel contained in a ladle located below the vacuum tank, and while depressurizing the inside of the vacuum tank, an inert gas is blown from the rising pipe to move the molten steel in the ladle to the rising pipe. Through the vacuum tank, degass the molten steel in the vacuum tank, and the molten steel sucked in the vacuum tank is returned to the ladle through the downcomer, the molten steel, In the method of circulating between the ladle and the vacuum tank, thus decarburizing the molten steel in the ladle, to produce an ultra-low carbon steel, the carbon content of the molten steel is
It is characterized in that a deoxidizer is added to the molten steel so that the amount of dissolved oxygen in the molten steel falls within the range of 100 to 200 ppm in the latter stage of decarburization at 50 ppm or less.

[0008]

The decarburization reaction by the RH type vacuum degassing refining process proceeds by CO bubbles generated by the reaction of carbon and oxygen in the molten steel. According to the theory of heterogeneous nucleation, CO bubbles are likely to be generated at the non-uniform interface of molten steel, that is, at the contact surface between the refractory lining of the vacuum chamber, the riser pipe, the downcomer pipe and the ladle and the molten steel.

When the refining progresses and the amount of carbon in the molten steel decreases as in the latter stage of decarburization, and the reaction force between carbon and oxygen decreases, it is necessary to maintain the above reaction. , It is necessary to positively cause the nucleation of CO bubbles. As a means for allowing a substance in which CO bubble nuclei are easily generated to exist in the molten steel, it is conceivable to blow an oxide or the like directly into the molten steel in the ladle.
It is not necessarily an effective method from the viewpoint of the injection depth of oxide into molten steel and efficiency.

Therefore, in the present invention, a deoxidizing agent such as aluminum (Al) is added to the molten steel at the latter stage of decarburization when the carbon content of the molten steel becomes 50 ppm or less, and alumina (Al ₂ This produces a deoxidation product such as O ₃ ) and forms the deoxidation product as a nucleus of CO bubbles, thereby promoting the decarburization reaction. Since the above-mentioned deoxidation product is dispersed and produced over the entire molten steel, the decarburization reaction is efficiently promoted.

However, when the molten steel is completely killed by adding a deoxidizing agent to the molten steel,
There is no oxygen that should contribute to the decarburization reaction. Therefore, it is necessary to leave dissolved oxygen in the molten steel to the extent that it does not hinder the decarburization reaction. From this point of view, the amount of dissolved oxygen in molten steel is 1
It should be limited to the range of 00 to 200 ppm. If the amount of dissolved oxygen is less than 100 ppm or more than 200 ppm, the decarburization reaction will decrease.

As the deoxidizing agent added to the molten steel, at least one of aluminum, titanium, silicon, manganese, zirconium and the like is used. The deoxidizer may be added to the molten steel sucked up in the vacuum tank or may be added to the molten steel in the ladle.

The addition of the deoxidizing agent makes the carbon content in the molten steel 50%.
It is necessary to perform it in the latter stage of decarburization when it becomes below ppm.
If a deoxidizer is added to molten steel with a carbon content of more than 50 ppm,
While the reaction force between carbon and oxygen is maintained to some extent, the amount of dissolved oxygen is reduced, and as a result of the decrease in the reaction force, there arises a problem that the decarburization reaction decreases.

FIG. 1 is a schematic vertical sectional view showing an example of an apparatus for carrying out the method of the present invention. As shown in FIG. 1, an ascending pipe 2 and a descending pipe 3 are vertically connected to a bottom wall 1a of a molten steel refining vacuum tank 1 so as to project downward. The lower part of each of the ascending pipe 2 and the descending pipe 3 is
Molten steel 5 contained in a ladle 4 located below the vacuum tank 1.
It is immersed in. An inert gas blowing pipe 6 is connected to the rising pipe 2. On one side wall 1b of the vacuum chamber 1, there is provided a gutter 7 having one end connected to a hopper 8 for adding an oxidizing agent into the molten steel 5 sucked up in the vacuum chamber 1. 9 is for reducing the pressure in the vacuum chamber 1,
A conduit connected to a vacuum pump.

While decompressing the inside of the vacuum chamber 1, an inert gas is blown through the blow pipe 6 connected to the rising pipe 2 to move the molten steel 5 in the ladle 4 into the vacuum chamber 1 through the rising pipe 2. Molten steel 5 sucked up and sucked into the vacuum chamber 1
Is returned to the inside of the ladle 4 through the downcomer pipe 3, the molten steel 5 is circulated between the ladle 4 and the vacuum tank 1, and the molten steel 5 in the ladle 4 is decarburized. In the latter stage of decarburization when the carbon content of the molten steel 5 became 50 ppm or less, in the molten steel 5 sucked up in the vacuum tank 1,
The amount of dissolved oxygen in the molten steel is 100 to 20 by using the deoxidizer with the gutter 7.
Add it so that it is within the range of 0 ppm. As a result, as described above, the added deoxidizing agent produces a deoxidizing product, and this deoxidizing product serves as a nucleus of CO bubbles to accelerate the decarburization reaction.

[0016]

EXAMPLES Next, the present invention will be described in more detail by way of examples in comparison with comparative examples. Molten steel processing capacity shown in Fig. 1
A 250 ton RH type vacuum degassing and refining device was used to decarburize molten steel having the chemical composition shown in Table 1 below.

The pressure in the vacuum chamber 1 was reduced to 1 torr or less, and argon gas was blown at a rate of 2000 Nl / min through a blowing pipe 6 connected to the rising pipe 2 to melt the molten steel 5 in the ladle 4.
The molten steel 5 sucked up into the vacuum tank 1 through the rising pipe 2 and the molten steel 5 sucked up into the vacuum tank 1 through the descending pipe 3 is ladle 4
After returning to the inside, the molten steel 5 was circulated between the ladle 4 and the vacuum chamber 1 to decarburize the molten steel 5 in the ladle 4.

In the latter stage of decarburization when the carbon content of the molten steel becomes 50 ppm or less, in the molten steel 5 sucked up in the vacuum chamber 1,
Aluminum powder as a deoxidizer was added from the gutter 7 so that the amount of dissolved oxygen in the molten steel was in the range of 100 to 200 ppm.

FIG. 2 is a graph showing the relationship between the amount of dissolved oxygen in molten steel and the decarburization rate when aluminum powder is added in this manner. In FIG. 2, the horizontal axis represents the amount of dissolved oxygen in molten steel, and the vertical axis represents the decarburization rate.

The decarburization reaction was evaluated by the following formula (1) in the region where the carbon content in the molten steel was about 50 ppm. d [C] / dt = -Kc [C] --- (1) However, C: carbon amount in molten steel, Kc: decarburization rate constant, t: time.

From FIG. 2, the amount of dissolved oxygen in the molten steel is 100 to 20.
It is clear that the decarburization reaction becomes extremely fast in the range of 0 ppm. In FIG. 2, the shaded area is a conventional example in which no deoxidizer is added.

[0022]

As described above, according to the method of the present invention, the decarburization rate is remarkably increased in the production of the ultra-low carbon steel by the RH type vacuum degassing refining method. The carbon content can be reduced as compared with the conventional one, and an extremely low carbon steel can be efficiently produced, which brings about an industrially useful effect.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view showing an example of an apparatus for carrying out the method of the present invention.

FIG. 2 is a graph showing the relationship between the amount of dissolved oxygen in molten steel and the decarburizing rate when decarburizing molten steel by the method of the present invention.

[Explanation of symbols]

 1 vacuum tank, 2 riser pipe, 3 downcomer pipe, 4 ladle, 5 molten steel, 6 gas injection pipe, 7 gutter, 8 hopper, 9 conduits.

Claims (2)

[Claims] 1. An ascending pipe and a descending pipe vertically connected to a bottom wall of a molten steel refining vacuum tank so as to project downward are housed in a ladle located below the vacuum tank. Immersing in molten steel, while depressurizing the inside of the vacuum tank, blowing an inert gas from the rising pipe, sucking the molten steel in the ladle into the vacuum tank through the rising pipe, the molten steel in the vacuum tank Degassed inside, and the molten steel sucked into the vacuum tank, returned to the ladle through the downcomer pipe, the molten steel is circulated between the ladle and the vacuum tank, Thus, in the method of decarburizing the molten steel in the ladle, to produce an ultra-low carbon steel, in the latter stage of decarburization when the carbon content of the molten steel becomes 50 ppm or less, the amount of dissolved oxygen in the molten steel is 100 ~ It is a special feature to add a deoxidizer to the molten steel so that it falls within the range of 200 ppm. To method for producing a ultra-low carbon steel. 2. The method according to claim 1, wherein at least one of aluminum, titanium, silicon, manganese, calcium and zirconium is used as the deoxidizer.