JPH0649528A - Vacuum decarburization refining method of extra-low carbon steel - Google Patents

Abstract

PURPOSE:To efficiently refine an extra-low carbon steel without lowering a decarburization rate down to an extra-low carbon region. CONSTITUTION:This refining method for the molten steel 3 consists in making combination use of the formation of an bubble active surface by a gaseous substance and the molten steel circulating function by a gas lift by immersing a container 2 of a large-diameter straight cylindrical shape into the molten steel 3 in a ladle 1 tapped from a refining furnace, such as converter or electric furnace, reducing the pressure in this straight cylindrical immersion chamber and supplying the gaseous substance from the bottom of the steel bath. The blowing of the gas from the bottom is deviated and the inert gas is blown in a range of the amt. (GS) of the gas of the immersion pipe/the amt. (GB) of the gas in the low part=1.0 to 3.0 from the position of 500 to 1500mm under the surface of the steel bath from a gas blowing nozzle 5 provided on the half periphery of the immersion pipe on the descending flow side on an opposite side. In addition, the bubble active surface of >=10% of the total surface area of the molten steel is formed within the vacuum chamber, by which the extra- low carbon steel is efficiently melted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an efficient refining method for ultra-low carbon steel which enables efficient refining without lowering the decarburization rate to the ultra-low carbon range.

[0002]

2. Description of the Related Art As a vacuum decarburization method for ultra-low carbon steel, R
H and DH are widely used. However, if the carbon concentration is 2
If it is reduced to 0 ppm or less, the decarburization rate will slow down,
There was a problem that it took a long time. In order to solve this, a method of increasing the flow rate of Ar gas for reflux in RH, expanding the diameter of the dipping pipe, or increasing the molten steel reflux rate by increasing the tank ascending / descending rate in DH is usually taken. However, among these methods, Ar for reflux
Increasing the gas flow rate has a limit because it shortens the life of the refractory, and increasing the immersion pipe diameter has a limit due to dimensional constraints, and increasing the tank ascending / descending speed also has a limit from the followability of molten steel.

Materials and processes, Volume 3 (199
0), 168, a method of increasing the reaction interfacial area by blowing Ar gas into the tank in RH is presented, but in order to obtain an effect in an extremely low carbon concentration range, 50 N
It requires a large amount of gas injection of 1 / (ton · min) or more, and causes a violent splash in the tank, which causes a problem of significantly impairing operability. Further, according to Japanese Patent Laid-Open No. 57-200514, there is disclosed a method in which a gas for reflux is blown from the bottom of a ladle in RH, but a large amount of gas is used in order to exert an effect in an extremely low carbon concentration region. When introduced, there is a problem that the refractory wear is severe because bubbles collide with the lower end of the refractory in the immersion pipe.

On the other hand, Japanese Unexamined Patent Publication No. 53-67605 discloses a decompression refining furnace in which a cylindrical tube is immersed to reduce the pressure in the tube. In this method, molten steel in the tube and the tube are treated during processing. Multiple times for the purpose of mixing with external molten steel,
Since repeated depressurization / double pressure is applied, the time for which the molten steel reaction surface area is exposed to a high vacuum is short, and there is a problem that a long time is required in the case of melting extremely low carbon steel. On the other hand, JP-A-51-
Japanese Patent No. 55717 discloses a decompression refining furnace in which a cylindrical tube is immersed to decompress the inside of the tube, and then Ar gas is blown into the porous brick at the bottom of the ladle. However, as shown in these, only the method of sucking molten steel into a cylindrical dip tube and introducing an inert gas from the gas injection hole provided at the bottom of the ladle can stably desorb the ultra-low carbon region. Since it cannot be charcoal, it has not been put to practical use.In addition, this method alone cannot stably suppress the generation of splashes during processing, and because it entrains converter slag, it also enables stable melting of high-cleanliness steel. There is a problem that it is difficult.

[0005]

As described above, in the case of the method shown in Volume 3 (1990), 168 of materials and processes, there is a problem that a violent splash occurs. The method disclosed in Japanese Patent Laid-Open No. 57-200514 has a problem that the wear of the refractory is severe. Further, in the method disclosed in Japanese Patent Laid-Open No. 53-67605, the molten steel reaction surface area is exposed to a high vacuum for a short time because repeated depressurization / double pressure is applied during the treatment. Has a problem that it takes a long time. Further, by the methods disclosed in JP-A-53-67605 and JP-A-51-55717, stable decarburization to an extremely low carbon region is achieved even if the reflux of molten steel is positively improved. In addition, there is a problem that splash generation during processing cannot be stably suppressed, and stable melting of high cleanliness steel is difficult.

Therefore, the object of the present invention is to reduce the decarburization rate to an extremely low carbon region in a short time without causing problems such as severe splash, wear of refractory and deterioration of cleanliness. It is to enable efficient refining without doing so.

[0007]

In order to solve the above-mentioned problems of the prior art, the inventors of the present invention immerse a large diameter immersion pipe in molten steel in a ladle under vacuum refining, and I applied for a decarburization method in which a gas body was supplied to form a bubble active surface at 10% or more of the total molten steel surface. Further, as a result of intensive studies on decarburization under vacuum, the present invention has found that bubbles within a certain depth on the vacuum surface side are effective for promoting bubble decarburization. Obtained. The present invention is based on this finding.

[0008] The gist is that a large-diameter straight-body-shaped container is immersed in molten steel in a ladle that has been tapped from a refining furnace such as a converter or an electric furnace, and In the refining method of molten steel that decompresses and supplies the gas body from the bottom of the steel bath, and forms the bubble activated surface by the gas body and the molten steel circulation function by the gas lift, the gas injection from the lower part is eccentric and the opposite side 500 to 15 below the surface of the steel bath from a plurality of gas injection nozzles provided around the dip tube on the downflow side of
Inert gas from the position of 00 mm, gas flow rate of immersion pipe (G
G in relation to the ratio of _S) and the lower portion the gas flow rate (G _B)
Blowing in the range of _S / G _B = _1.0~3.0, and, by forming a 10% or more bubbles active surface of the molten steel total surface area in the vacuum chamber, to the carbon concentration in the molten steel and less 6ppm It is in.

[0009]

The present invention is based on promoting the bubble decarburization described below. Since the inert gas blown from the bottom of the ladle is subjected to the static pressure of molten steel, the CO partial pressure is relatively high and the decarburization reaction is difficult to proceed. By performing various experiments, the present inventors reduce the influence of static pressure exerted on the blown bubbles from the molten steel, that is, the CO partial pressure at the interface between the bubbles and the molten steel becomes sufficiently low to promote bubble decarburization. It was found that the area was within 1500 mm below the vacuum surface. That is, bubble decarburization does not proceed even if gas is blown from a position deeper than 1500 mm. Now, considering the case where gas is blown from the lower part of the ladle, the gas blown from the lower part floats due to the difference in specific gravity from the molten steel. And the bubbles rise,
Bubble decarburization proceeds when it reaches the region where it is not affected by static pressure, but the gas blown from the lower part always forms an upward flow, so the time it stays in this region is very short, Therefore, it is difficult to proceed the bubble decarburization with the gas supplied from the lower part.

Therefore, as a method for efficiently causing bubble decarburization, the present inventor decenters gas injection from the lower part and injects an inert gas from the half circumference of the dip tube on the opposite downflow side. Have invented a method for significantly improving bubble decarburization. According to this method, the gas supplied from the bottom of the ladle forms a bubble activated surface on the surface of the vacuum chamber to promote the surface decarburization, and then the gas blown from the dipping tube causes the bubble decarburization to occur. Therefore, the overall decarburization reaction has a very large decarburization reaction rate.

As described above, as a result of examining various conditions for accelerating the surface decarburization and then the bubble decarburization, the gas injection from the lower part of the ladle was eccentric and the opposite was observed. Side downflow side, the inert gas is introduced from a position of 500 to 1500 mm below the surface of the steel bath from a plurality of gas injection nozzles provided on the half circumference of the immersion pipe, the immersion pipe gas flow rate (GS) and the lower part gas flow rate (G _S ). G _S / G _B = 1.0~ in relation ratio between _B)
It was clarified that this can be achieved by blowing in the range of 3.0 and forming a bubble activated surface of 10% or more of the total surface area of molten steel in the vacuum chamber.

As shown in FIG. 1, when gas is injected from the gas injection nozzle 5 from a position other than the center position of the bottom of the ladle 1, a circulating flow of the molten steel 3 is formed along with the floating bubbles. At this time, when gas is blown from the gas blowing nozzle 5 from the half circumference of the dipping tube 2 on the side where the downward flow is formed on the side opposite to the gas blowing position, the inert gas blown from the dipping tube is discharged from the bottom. The downflow formed by the supplied gas balances the buoyancy of the bubbles. That is, the upflow of the bubbles is suppressed by the downflow, so that the bubbles stay at a depth where bubble decarburization easily proceeds, and The bubbles are dispersed due to the collision between the rectified downward flow and the rising bubbles, so that the decarburization progresses dramatically. Further, at this time, when the gas blown from the dip tube is shallower than 500 mm, the rectified downward flow is not formed, so the blown bubbles immediately float up and the bubble decarburization proceeds. Disappear. Furthermore, when the blowing position is deeper than 1500 mm, bubbles are entrained in the bottom of the bath (a region where bubble decarburization does not occur) in the descending flow, and then, in order to rapidly ascend on the fast ascending flow, the bubble decarburization is performed. Progress becomes worse.

Regarding the ratio of the gas flow rate from the bottom to the gas flow rate from the dip tube, if the gas flow rate from the bottom is too high, the downward flow is too fast and the bubbles are entrained in the bath lower part, resulting in bubble desorption. The progress of charcoal is hindered, and if it is too small, surface decarburization will not proceed and other obstacles will occur. Further, if the gas flow rate from the immersion pipe is too large, the bubbles coalesce and coarsen, and the buoyancy becomes large and the bubbles float up immediately when blown, hindering the progress of bubble decarburization and generating a large amount of splash. Therefore, the adhesion of the metal in the tank will be severe and the operability will be deteriorated. If the amount of gas from the dipping pipe is too small, the amount of bubble decarburization itself will decrease.

[0014] Thus, G _S / G _B as the ratio of the gas flow from the dip tube (G _S) and the gas flow rate from the bottom (G _B)
The optimum range is 1.0 to 3.0.

[0015]

EXAMPLE As shown in FIG. 1, in a ladle 1, a dip tube 2, a molten steel 3, a porous plug 4 and a gas injection nozzle 5,
Table 1 shows an example of using a 175 ton scale vacuum refining apparatus. Blown carbon concentration in the converter is 0.03〜
0.04%, bottom blown Ar gas flow rate is uniform, 300N
Vacuum refining was performed at 1 / min.

As can be seen from Table 1, the method of the present invention is a very excellent method for melting ultra low carbon steel.

[0017]

[Table 1]

[0018]

EFFECTS OF THE INVENTION By using the present invention, it is possible to efficiently carry out refining without causing a violent splash, and in a short time treatment without lowering the decarburization rate to an extremely low carbon concentration range. Became.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an embodiment of a vacuum refining method according to the present invention.

FIG. 2 is a diagram showing a nozzle arrangement of a dip tube.

FIG. 3 is a diagram showing a relationship between a gas injection depth from an immersion pipe and a decarburization rate constant.

[Fig. 4] Immersion tube gas flow rate (G _S ) / Lower part gas flow rate (G _S )
_B ) Diagram showing the relationship between ratio and decarburization rate constant.

FIG. 5 is a comparison diagram of changes over time (in semi-logarithmic display) in decarburization behavior between the conventional method and the method of the present invention.

[Explanation of symbols]

1 ... Ladle 2 ... Immersion pipe 3 ... Molten steel 4 ... Porous plug 5 ... Gas injection nozzle

Claims (1)

[Claims] 1. A large-diameter straight-body-shaped container is immersed in molten steel in a ladle that has been tapped from a refining furnace, the interior of the straight-body immersion tank is decompressed, and a gas body is supplied from the bottom of the steel bath. In the decarburizing decarburization refining process for molten steel, which uses both the formation of bubble active surfaces by the gas body and the circulation function of molten steel by gas lift, the gas injection from the lower part is eccentric, and it is installed on the half circumference of the dipping pipe on the opposite side and from a plurality of gas blowing nozzles, G in the inert gas from the position of the steel bath surface under 500～1500Mm, relationship of the ratio of the immersion tube gas flow rate (G _S) and the lower portion the gas flow rate (G _{B) S} / blowing in the range of G _B = 1.0 to 3.0, and, for efficient ultra-low carbon steel and forming a 10% or more bubbles active surface of the molten steel total surface area into the vacuum chamber vacuum degassing Charcoal refining method.