JP2000038613A - Method for refining molten steel having small quantity of dust under reduced pressure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refining method of molten steel having small quantity of dust under reduced pressure, in which the development of the dust is little at the time of decarburize-refining under reduced pressure and the sticking and the deposit to a duct etc., in an immersion tube, a vacuum vessel and an exhaust system are prevented and the wearing of refractory is restrained and the stable decarburizing can be executed. SOLUTION: In the refining method of the molten steel under reduced pressure, in which the decarburize-refining is executed under reduced pressure by dipping one piece of immersion tube 13 into the molten steel 12 in a ladle 11, while blowing inert gas from the bottom part 18 in the ladle 11, the pressure in the immersion tube 13 is reduced so that the vacuum degree becomes low vacuum, such as 100-750 Torr, at the initial stage of the decarburize-refining to execute the decarburizing, and then, the decarburizing is executed by reducing the pressure in the immersion tube 13 so that the vacuum degree becomes the high vacuum, such as <100 Torr, after reaching <150 ppm carbon concn.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for vacuum refining molten steel capable of reducing the generation of dust and preventing a reduction in decarburization speed, etc., when producing extremely low carbon molten steel by vacuum refining.

[0002]

2. Description of the Related Art As a method of reducing the carbon or nitrogen concentration of molten steel, RH, DH and V using vacuum (reduced pressure) are used.
OD and the like are widely used. However, these methods can reduce the carbon and nitrogen concentrations of molten steel to a certain extent, but it is particularly difficult to produce molten steel in which carbon (C) and the like are reduced to an extremely low concentration range. Therefore, as a method for increasing the refining efficiency under reduced pressure to produce extremely low carbon molten steel, for example, a cylindrical dip tube described in JP-A-51-55717 is immersed, and the inside of the immersion tube is immersed. A method has been proposed in which pressure is reduced and refining is performed by supplying an inert gas into molten steel from an inert gas blowing hole provided at the bottom of a ladle. Further, a cylindrical immersion pipe having a cell active area of 10% or more of the total molten steel surface area described in JP-A-6-212242 is immersed, the inside is depressurized, and impregnated into the molten steel from the bottom of the ladle. A method of decarburizing and refining molten steel has been proposed in which a foam active surface with relatively strong agitation is formed in a range of 30 to 80% of the bubble active area of the surface under reduced pressure while supplying an active gas.

[0003]

However, in the refining method described in JP-A-51-55717, the carbon and excess free oxygen in the molten steel are reduced during the initial decompression refining and stirring. Dust such as metal ingots, metal ingots, and metal ingots are generated by the CO gas generated by the rapid reaction and the supplied inert gas. And the like are attached or deposited. The adhesion and accumulation of the dust are re-dissolved at a time when the carbon concentration in the molten steel is reduced by decarburization, thereby causing an increase in the carbon concentration in the molten steel (carbon pickup). This increase in carbon concentration in molten steel
Reducing the relative decarburization rate, extending the processing time, causing wear of refractories such as ladles and immersion pipes, and in some cases, scattering exhaust dust and closing exhaust ducts, etc. Hinders Further, when the carbon concentration of the molten steel during the treatment increases, the carbon concentration reached when the decarburization refining is completed also increases, and there is a problem that it becomes difficult to produce extremely low carbon. Further, the refining method described in JP-A-6-212242 has a high decarburization rate, and thus has an advantage that molten steel of 7 ppm or less can be melted in a short time. However, since the strong stirring region is formed in the range of 30 to 80% of the bubble active area, within this range, the CO generated by the rapid reaction of the carbon in the molten steel and the excess free oxygen as described above. Due to the gas and the supplied inert gas, dust such as lumps of metal, metal particles, fine metal particles, etc. is generated, causing adhesion and deposition on immersion pipes, vacuum tanks, exhaust system ducts, etc. In some cases, the duct of the exhaust system may be blocked. In addition, as described in
As in the refining method described in JP-A-55717, an increase in the carbon concentration in the molten steel (carbon pick-up) occurs, the decarburization rate relatively decreases, and the treatment time is prolonged. There is a problem that the refractory is worn away, the carbon concentration reached at the end of the decarburization refining becomes high, and it becomes difficult to produce extremely low carbon molten steel.

[0004] The present invention has been made in view of such circumstances, and reduces the generation of dust during processing to prevent adhesion and deposition to dip tubes, vacuum tanks, exhaust system ducts, and the like, and to prevent refractories. It is an object of the present invention to provide a method for refining molten steel with reduced dust, which can suppress the wear and perform stable decarburization.

[0005]

According to the present invention, there is provided a semiconductor device comprising:
The vacuum refining method of molten steel with a small amount of dust described is a method of depressurizing and refining molten steel in which one dipping tube is immersed in molten steel in a ladle to perform decarburization refining in a decompressed state. While blowing the gas, in the initial stage of the decarburization refining, the inside of the immersion pipe is decompressed so as to have a low vacuum of 100 to 750 torr, and then the carbon concentration reaches less than 150 ppm. Vacuum degree is 10
The inside of the dip tube is decompressed and decarbonized so as to have a high vacuum of less than 0 torr. Here, the initial vacuum degree of the decarburization refining is 7
In the low vacuum region exceeding 50 torr, the decarburization rate due to the inert gas blown from the bottom of the ladle is reduced, and it takes time to process, thereby increasing the wear of refractories such as immersion tubes and ladle. I do. In addition, the initial vacuum degree of decarburization refining is 1
When the degree of vacuum becomes lower than 00 torr, the decarburization rate by the inert gas blown from the bottom of the ladle becomes too large, and the release of the CO gas generated by the rapid reaction between the carbon in the molten steel and excess free oxygen occurs. As a result, dust scattering and bumping occur.

[0006] The reduced-pressure refining method of molten steel with low dust according to claim 2 is the method of reducing pressure of refining molten steel with low dust according to claim 1, wherein the oxygen concentration of the molten steel at the time of starting the decarburization refining is 200-200. Adjust to 800 ppm. If the oxygen concentration at the start of decarburization refining is less than 200 ppm,
Since the absolute amount of oxygen reacting with carbon in the molten steel is insufficient, the decarburization reaction does not sufficiently occur, and the ultimate carbon concentration of the decarburization refining increases.
On the other hand, when the oxygen concentration exceeds 800 ppm, the absolute amount of oxygen that reacts with the carbon in the molten steel under reduced pressure increases, and bumping and dust are generated, and the relative decarburization rate decreases due to the increase in the carbon concentration in the molten steel. I do.

A third aspect of the present invention is directed to the method of reducing the pressure of molten steel having a small amount of dust in the ladle according to the first or second aspect. 0.1 to 0.7 of the total surface area of molten steel
Having.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention. FIG. 1 is a cross-sectional view of a vacuum refining apparatus 10 to which a method of vacuum refining molten steel with less dust according to an embodiment of the present invention is applied. First, a vacuum refining apparatus 10 includes a ladle 11 made of steel and lined with a refractory (not shown), a dip tube 13 dipped in molten steel 12 in the ladle 11, and a dip tube 13.
, A dip tube 13 and a vacuum tank 14
An exhaust duct 15 connected to an ejector for evacuating the interior and reducing the pressure, a storage hopper 16 and an addition chute 17 for adding alloyed iron and the like into the immersion pipe 13 are provided. Further, the bottom portion 18 of the ladle 11 is provided with a porous plug 19 for blowing an inert gas into the ladle 11. Reference numeral 12a denotes a molten metal surface of the molten steel 12 in the immersion pipe 13, and reference numeral 13a denotes a flange for joining the immersion pipe 13 and the vacuum chamber 14 with fastening means such as bolts and nuts.

Next, a method of vacuum refining molten steel with little dust using the vacuum refining apparatus 10 will be described. Ladle 1
First, the carbon concentration was adjusted to 2 by a refining furnace (not shown) such as a converter.
150 tons of molten steel 12 decarburized and refined to 00 to 500 ppm is received, and an argon gas, which is an example of an inert gas, is introduced into the molten steel 12 from the porous plug 19 by 0.6 to 15 NL.
The immersion pipe 13 was immersed in the molten steel 12 while blowing /(min./ton of molten steel), and the pressure in the immersion pipe 13 and the vacuum chamber 14 was reduced to 100 to 750 torr. In addition, this ladle 1
The conditions of 1 and the immersion pipe 13 were such that the ratio of the internal surface area (S ₁ ) of the molten steel immersion part of the immersion pipe 13 / the total molten steel surface area (S) in the ladle 11 was 0.1 to 0.7. this is,
If (S ₁ ) / (S) is less than 0.1, the bubble active surface (the discharge surface of argon gas bubbles) formed by the argon gas blown into the immersion tube 13 becomes narrow, and the progress of decarburization is hindered. , (S ₁ ) / (S) is larger than 0.7, the workability such as sampling is impaired. For this reason, when (S ₁ ) / (S) is 0.15 to 0.65, more preferable results can be obtained. Then, the ladle 11 is supplied by the argon gas supplied from the porous plug 19.
The molten steel 12 inside is stirred by the flow shown by the arrow in FIG.
The decarburization reaction is promoted by the bubble activated surface of the argon gas expanded on the molten metal surface 12a of the molten steel 12 in the immersion tube 13, and decarburization is performed. In this decarburization, the carbon concentration in the molten steel 12 is 150
In the range of ppm or more, the degree of vacuum in the immersion tube 13 is 1
Since the pressure is adjusted so as not to be a high vacuum of less than 00 torr, a rapid reaction between carbon in the molten steel and free oxygen can be suppressed, and scattering of dust accompanying release of CO gas can be prevented. On the other hand, the decarburization rate can be maintained at a good value since the carbon concentration in the molten steel 12 is in a high region, and can reach a predetermined carbon concentration in a short time.

[0010] In particular, by adjusting the value of the free oxygen concentration in the molten steel 12 to 200 to 800 ppm at the start of the decarburization refining, the release of CO gas, which is generated when the excess free oxygen reacts with the carbon, is suddenly generated. And the generation of dust consisting of metal ingots, metal ingots, and fine metal ingots caused by the bubble breaking action of argon gas bubbles can be suppressed.
Adhesion and accumulation on the vacuum tank 14 and the exhaust system duct 15 can be prevented, and operation obstruction due to blockage of the exhaust system duct and the like due to scattered dust can be avoided. By preventing dust from adhering to or accumulating on the immersion tube 13 and the vacuum tank 14, it is possible to suppress the re-dissolution of the dust and an increase in the carbon concentration in the molten steel 12 (carbon pickup). Prevents the speed from dropping and enables stable decarburization in a short time.
Carbon concentrations of less than 0 ppm can easily be reached.

Next, when the carbon concentration in the molten steel 12 is 150 pp
m, the exhaust of the ejector is increased, and the inside of the immersion tube 13 and the vacuum chamber 14 is reduced to less than 100 torr (10
The pressure is reduced to a high degree of vacuum of 0 torr (0.1 torr), and decarburization is continuously performed while blowing an argon gas into the molten steel 12 at 0.6 to 15 NL / (minute / ton of molten steel). In this decarburization, when the first decarburization is performed, the value of free oxygen in the molten steel 12 is adjusted to 200 to 800 ppm, so that the amount of free oxygen that reacts with carbon is sufficient, and argon gas is blown. With the agitation of the molten steel 12, the decarburization reaction is actively generated by the bubble activated surface of the molten metal surface 12a of the molten steel 12 in the immersion pipe 13, and reaches a very low carbon concentration of 7 ppm or less by short-time refining. it can. For this reason, 100 tons from low-vacuum decarburization refining with reduced dust generation
It is more preferable that the carbon concentration in the molten steel 12 to be switched to the high-vacuum degree of decarburization refining of less than rr is 140 to 30 ppm because the ultimate carbon concentration at the end of the decarburization refining can be stably reduced. Further, the ratio of the inner surface area (S ₁ ) of the molten steel immersion part of the immersion pipe 13 / the total molten steel surface area (S) in the ladle 11 to 0.1 to 0.1
Since it is set to 0.7, the bubble activation surface (the discharge surface of the argon gas bubble) in the immersion tube 13 formed by the blown argon gas can be maintained in an appropriate range, and the carbon concentration reached by the final refining can be reduced. Can be lowered stably. When the final decarburization refining is completed, the ejector is stopped, and the inside of the immersion pipe 13 and the vacuum chamber 14 are double-pressurized to atmospheric pressure. After that, the molten steel 12 is continuously cast into a slab and subjected to rolling or the like.

[0012]

Next, an embodiment of the method for vacuum refining molten steel with little dust according to the present invention will be described. Using a converter, decarbonize the carbon concentration to 300 ppm and reduce the oxygen concentration to 20 ppm.
Into the ladle 11 containing 150 tons of molten steel 12 adjusted to 0 ppm, 5 NL of argon gas was inserted from the porous plug 19.
/ (Minutes / ton of molten steel) while blowing the inner surface area of the molten steel immersion portion of the immersion pipe with respect to the total molten steel surface area in the ladle 11 (S
₁ / S) was immersed in an immersion tube 13 having an inner diameter of 0.35. Then, the pressure in the immersion pipe 13 and the vacuum tank 14 was reduced to perform decarburization refining, and dust generation, decarburization speed, and attained carbon concentration (carbon concentration at the end of the decarburization refining) were investigated.

As shown in Table 1, first, Example 1 uses a dip tube 13 in which the inner surface area of the molten steel dipping portion of the dip tube 13 with respect to the total surface area of the molten steel 12 in the ladle 11 is 0.35. The carbon concentration before the decarburization treatment is 300ppm to 150p
pm, the first decarburization refining is performed at a vacuum degree of 200 torr, and then the carbon concentration of the molten steel 12 becomes 1
At the time when the concentration became less than 50 ppm, the final degree of decarburization refining was carried out by setting the degree of vacuum to a high degree of vacuum of 2 torr. As a result, the degree of vacuum was not changed according to the carbon concentration of the comparative example (always 5).
Compared to the case of (torr), the dust generation index is greatly reduced to 0.6, and the decarburization rate index is also 1.4 times as good as that for decarburization, and the ultimate carbon concentration can be reduced to 5 ppm. Excellent (○) results were obtained as a comprehensive evaluation. In Example 2, the carbon concentration before the decarburization treatment was 300 using the immersion pipe 13 in which the inner surface area of the molten steel immersion portion of the immersion pipe 13 with respect to the total surface area of the molten steel 12 in the ladle 11 was 0.50.
from 6 ppm to 150 ppm.
The first decarburization refining was performed at 00 torr, and then, when the carbon concentration of the molten steel 12 became less than 150 ppm, the final decarburization refining was performed at a vacuum of 5 torr. As a result, compared to the comparative example, the dust generation index was significantly reduced to 0.5, the decarburization rate index was 1.3 times as good as that of the decarburization, and the carbon concentration reached 7 ppm. The results could be reduced, and excellent (優 れ) results were obtained as an overall evaluation.

[0014]

[Table 1]

On the other hand, in Comparative Examples, the degree of vacuum was not changed in accordance with the carbon concentration between 300 ppm before the decarburization treatment and the end of the decarburization refining. A large amount of dust was generated, and the decarburization rate index was considerably reduced to 1.0, and the reached carbon concentration was 10 ppm, resulting in a poor (X) result as a comprehensive evaluation.

In the first and second embodiments, the oxygen concentration was adjusted to 200 to 800 ppm. However, as another embodiment, the carbon concentration before the decarburization treatment was set to 300 ppm without particularly adjusting the oxygen concentration in the molten steel 12. The first decarburization smelting was performed at a vacuum of 600 ppm from a range of
Next, when the carbon concentration of the molten steel 12 became less than 150 ppm, the degree of vacuum was set to 5 torr, and final decarburization refining was performed. As a result, compared with the comparative example where the degree of vacuum was not changed according to the carbon concentration, the dust generation index was significantly reduced to 0.6, and the decarburization rate index was 1.2 times as high as that of the comparative example. Charcoal was carried out, the reached carbon concentration could be reduced to 8 ppm, and an excellent (）) result was obtained as an overall evaluation.

The embodiment of the present invention has been described above.
The present invention is not limited to the above-described embodiment, and all changes in conditions that do not depart from the gist are within the scope of the present invention. For example, a relational expression between the concentration of free oxygen in the molten steel and the carbon concentration is obtained based on the experimental results, and the free oxygen concentration can be predicted from the carbon concentration at that time. As a method for adjusting the free oxygen concentration in molten steel, Al, Al
It is also possible to increase the free oxygen concentration in the molten steel by adding a deoxidizing agent such as an alloy or a Si alloy, or by blowing oxygen or an oxygen-containing gas onto the molten steel 12.

[0018]

The vacuum refining method for molten steel with little dust according to any one of claims 1 to 3 is characterized in that the degree of vacuum in the immersion pipe at the initial stage of the decarburization refining is 100 to 750 torr while blowing inert gas from the bottom of the ladle. Degas with low vacuum,
100 to after the carbon concentration reaches less than 150ppm
Since decarburization is performed at a high vacuum of less than rr, the generation and scattering of dust during processing is reduced, and dust is prevented from adhering and accumulating on dip tubes, vacuum tanks, exhaust system ducts, etc. The decarburization can be performed by suppressing the wear of the steel.

In particular, in the vacuum refining method for molten steel with low dust according to the second aspect, the oxygen concentration of the molten steel at which decarburization refining is started is adjusted to 200 to 800 ppm, so that dust generated during decarburization refining is reduced. As a result, decarbonization can be promoted to lower the reached carbon concentration.

According to the third aspect of the present invention, since the inner surface area of the molten steel immersion portion of the dip tube is set to 0.1 to 0.7 of the total surface area of the molten steel in the ladle, the decarburization is performed. The reaction can be promoted to achieve a very low carbon concentration in a short time.

[Brief description of the drawings]

FIG. 1 is a sectional view of a vacuum refining apparatus according to a first embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Decompression refining apparatus 11 Ladle 12 Molten steel 12a Metal surface 13 Dip tube 13a Flange 14 Vacuum tank 15 Exhaust duct 16 Storage hopper 17 Addition chute 18 Bottom part 19 Porous plug

Continued on the front page F-term (reference) 4K013 BA02 CA02 CA11 CA23 CC02 CE02 CE05 CE08 CE09 DA03 DA05 DA12 DA13 DA17 FA02 FA04

Claims (3)

[Claims] 1. A method of vacuum refining molten steel in which one dipping tube is dipped in molten steel in a ladle to perform decarburization refining under reduced pressure, while blowing an inert gas from the bottom of the ladle,
In the early stage of the decarburization refining, the degree of vacuum is 100 to 750.
The inside of the immersion tube is decompressed to reduce the pressure to a low vacuum of torr, and then decarbonization is performed. After the carbon concentration reaches less than 150 ppm, the pressure in the immersion tube is reduced to a high vacuum of less than 100 torr. Pressure smelting method for molten steel with little dust, characterized by decarburization. 2. The method according to claim 1, wherein the oxygen concentration of the molten steel at the start of the decarburization refining is adjusted to 200 to 800 ppm. 3. The low dust content according to claim 1, wherein an inner surface area of the molten steel immersion portion of the immersion tube has 0.1 to 0.7 of a total surface area of the molten steel in the ladle. Vacuum refining method for molten steel.