JPH04314818A - Vacuum degassing apparatus and production of extremely low carbon steel using this apparatus - Google Patents

Abstract

PURPOSE:To improve vacuum degassing refining reaction by forming a bottom part horizontal cross sectional plane in a vacuum vessel containing an upper tube and a downtake tube for molten steel to the specific shape in a circulating type vacuum degassing apparatus. CONSTITUTION:In the bottom part horizontal cross sectional plane containing the uptake tube 4-1 and the downtake tube 4-2 for the molten steel 7 arranged at the bottom part of vacuum degassing vessel 2 in the vacuum degassing refining apparatus, by forming to elliptic shape so that value of a/b becomes 1.1-3.0 at the time of using a for distance in the direction of interval of the uptake tube 4-1 and the downtake tube 4-2 and b for distance in the right angle direction to the above and further, by forming the shape of circulating tubes for the molten steel so that ratio of a2/b2 becomes 1.1-3.0 at the time of using a2 for distance at the right angle to the direction of interval of the uptake tube and downtake tube and b2 for distance in the right angle direction to the above, the molten steel in the vacuum degassing vessel 2 is circulated between a ladle 1 and this vessel all around, and the vacuum degassing refining is efficiently executed.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a vacuum degassing device for improving the efficiency of the degassing reaction when refluxing molten steel in a ladle, and a vacuum degassing device for molten ultra-low carbon steel using the device. This is an attempt to propose a manufacturing method.

[0002] In recent years, the method of performing secondary refining in a ladle using vacuum degassing treatment has become widespread due to the advancement of high-grade steel, and it is becoming increasingly popular for decarburization and decarbonization of ultra-low carbon steel, high purity steel, low hydrogen steel, etc. It is used in acid, dehydrogenation, etc., but it is desired to increase the degassing reaction efficiency in order to produce higher quality steel and improve efficiency. In the production of low carbon steel, it is desired to shorten the decarburization treatment time and reduce the achieved carbon concentration.

0003

[Prior Art] Until now, in the vacuum degassing tank of a reflux type vacuum degassing device, the horizontal cross-sectional inner contour shape of the bottom of the vacuum degassing tank (hereinafter simply referred to as the inner contour shape) has been circular; , when molten steel is refluxed between a vacuum degassing tank and a ladle via a pair of reflux pipes (rising pipe and downcomer pipe), the flow velocity distribution of molten steel is Inevitably, there will be areas where unevenness occurs, stagnation occurs, and the residence time in the vacuum degassing tank becomes longer. Slag containing Al2O3 etc. floats and stagnates, which inhibits the degassing reaction, so not only does the degassing reaction rate decrease even at the same reflux rate, but the stagnant molten steel containing this eddy current acts as a resistance and slows down the reflux rate. There is a problem that the temperature of molten steel decreases due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to a decrease in the temperature of the molten steel due to an increase in the residence time in the vacuum degassing tank.

As a method for improving the degassing reaction efficiency, for example, Japanese Patent Laid-Open No. 2-247315 and No. 2-247317 disclose that a vacuum degassing tank having a circular inner contour shape is used, and the diameter of the bottom of the vacuum degassing tank is A vacuum degassing tank and a vacuum degassing device have been disclosed which increase the reaction area of molten steel in the vacuum degassing tank. However, these disclosed examples do not solve the above problem because the inner contour shape of the bottom of the vacuum degassing tank is circular, and on the contrary, by increasing the diameter, the flow velocity distribution of molten steel is The area of the stagnation portion became more uneven and the area of the stagnation portion became larger, further aggravating the above problem.

Furthermore, Ichimura, Furugaki, Hiraoka, Nobemoto, Akamatsu,
Mimura, Hashimoto, Tetsu to Hagane, '84-S977P. 255
(1984) discloses a high-speed decarburization treatment technology that accelerates the decarburization reaction rate by increasing the flow rate of reflux gas and expanding the cross-sectional area of the immersion tube (reflux tube). In this case, an elliptical type immersion tube is used to increase its cross-sectional area. However, even in this disclosed example, there is nothing to suggest about the stagnation of the molten steel in the vacuum chamber, and therefore it does not eliminate the factor that inhibits the decarburization reaction due to the stagnation of the molten steel.

[0006]

[Problems to be Solved by the Invention] This invention proposes a vacuum degassing device having a suitable shape in order to advantageously solve the above-mentioned problems and increase the degassing reaction efficiency, and also aims to provide ultra-low carbon gas using the device. The purpose is to propose a method for melting steel.

[0007]

[Means for Solving the Problems] This invention provides that the flow velocity distribution of refluxing molten steel becomes non-uniform in a vacuum degassing tank, and stagnation of the molten steel occurs depending on the position. Focusing on the fact that the reaction efficiency is being reduced, the aim is to improve the degassing reaction efficiency by making the flow velocity distribution of molten steel uniform and eliminating stagnation areas.

That is, the gist is a reflux-type vacuum degassing apparatus for molten steel having a vacuum degassing tank equipped with a riser pipe and a downcomer pipe as reflux pipes, the vacuum degassing equipment being provided at least at the bottom of the vacuum degassing tank. A vacuum degassing device (first invention) in which the inner contour shape of the horizontal cross section of the gas tank is an elongated closed curve in the direction of the interval between the ascending pipe and the descending pipe,

000
9] In the first invention, the inner contour shape of the horizontal cross section consisting of a long and narrow closed curve at the bottom of the vacuum degassing tank has an aspect ratio of 1.
1 to 3.0 (second invention),

[0010] The vacuum degassing device according to the first invention or the second invention, wherein the horizontal cross-sectional inner surface contour shape of each of the rising pipe and the downcomer pipe is an elongated closed curve orthogonal to the direction of the interval between the rising pipe and the downcomer pipe. (Third invention),

[0011]
It has a vacuum degassing tank equipped with a rising pipe and a downcomer pipe as a reflux pipe, and the shape of the horizontal cross-sectional inner surface of the vacuum degassing tank at least at the bottom of the vacuum degassing tank is such that the interval between the rising pipe and the downcomer pipe is Using a vacuum degassing device consisting of a long and narrow closed curve in the direction of This is a method for producing ultra-low carbon steel (fourth invention) that makes the flow velocity distribution of molten steel uniform.

[0012] Here, the elongated closed curve of the inner surface contour shape refers to an ellipse including an ellipse and similar curved shapes, and the aspect ratio of the inner surface contour shape consisting of this elongated closed curve is: In the case of the bottom of the vacuum degassing tank, a/b is defined as a/b, where a is the length across the gap between the riser pipe and the downfall pipe, and b is the span length in the direction perpendicular to this. In the case of a downcomer pipe, a2 /b where the length perpendicular to the direction of the spacing between the riser pipe and the downcomer pipe is a2, and the length perpendicular to this is b2.
2. The aspect ratio of the inner contour shape of the vacuum degassing tank consisting of a long and narrow closed curve is 1.1.
If it is less than 3.0, it is virtually the same as a circular shape, and there is no effect in reducing the stagnation area of molten steel.On the other hand, if it exceeds 3.0, there will be equipment problems such as conflicts with the reflux pipe and ladle dimensions, making it impractical. Not on point. Therefore, its aspect ratio is 1.
1 to 3.0, preferably 1.2
A range of 2.0 to 2.0 is desirable.

[0013]Although the aspect ratio of the inner contour shape of the return pipe (rising pipe and descending pipe) consisting of a long and narrow closed curve is not particularly limited, the maximum for a practical machine is 4.5.
That's about it. Note that the length a2 perpendicular to the direction of the interval between the rising pipe and the downcomer pipe may be made as large as permitted by equipment constraints.

[0014]

[Operation] The greatest feature of this invention is that the inner contour shape of the bottom of the vacuum degassing tank is an elongated closed curve in the direction of the gap between the rising pipe and the downcomer pipe. The inner surface contour shape is a closed curve elongated in a direction perpendicular to the direction of the interval between the ascending pipe and the descending pipe.

[0015] By doing so, the flow velocity distribution of the molten steel in the vacuum degassing tank is made uniform and the stagnation portion is reduced. Therefore, the amount of slag floating on the molten steel surface that inhibits the degassing reaction is reduced, and furthermore, the degassing reaction efficiency can be improved, such as by increasing the reflux rate and preventing a drop in the molten steel temperature.

Furthermore, in addition to these, in order to further improve the degassing reaction efficiency, the degassing reaction interfacial area can be expanded by widening the interval between the rising pipe and the downcomer pipe. In other words, when the inner contour shape of the bottom of the vacuum degassing tank is circular, which has been conventionally used, in order to improve the degassing reaction efficiency,
Attempting to expand the degassing reaction interface area by simply enlarging the circular cross-sectional area or by widening the interval between the rising pipe and the descending pipe will increase the stagnation area of the molten steel, reduce the reflux rate, and increase the molten steel. However, in this invention, the inner contour shape of the bottom of the vacuum degassing tank is elongated and closed in the direction of the ascending pipe and the descending pipe. By forming a curve, the stagnation area of molten steel is reduced and the above-mentioned problems associated with this do not occur, so the gap between the rising pipe and the descending pipe can be widened and the degassing reaction interfacial area can be expanded without any adverse effects. can do.

Furthermore, the above effect is further enhanced by forming the inner contours of the rising pipe and the downcomer pipe into elongated closed curves perpendicular to the direction of the spacing between the rising pipe and the downcomer pipe.

[0018]

[Embodiment] First, the apparatus will be explained. As an example, a vertical cross-sectional view of a vacuum degassing tank and a ladle is shown in FIG. In FIG. 1, 1 is a ladle and 2 is a vacuum degassing tank. In this vacuum degassing tank 2, 3 is the bottom of the vacuum degassing tank, and reflux pipes 4-1 (ascending pipe) and 4-2 (descending pipe) are attached to the bottom. In addition, there is a monitoring window 5 at the top, 6
is an exhaust port connected to a vacuum pump (not shown). The molten steel 7 in the ladle 1 flows through the gas inlet (
Vacuum degassing tank 2 whose pressure is reduced by a vacuum pump connected to the exhaust port 6 by blowing reflux gas from
Then, it is sucked up in the direction of arrow 8 via the rising pipe 4-1. This is based on the principle of an air lift pump. The molten steel 7 that has entered the vacuum degassing tank bottom 3 moves through the vacuum degassing tank while being degassed, descends in the direction of the arrow 9 via the downcomer pipe 4-2, and returns to the ladle 1. In this way, the molten steel 7 continuously circulates between the ladle 1 and the vacuum degassing tank bottom 3 via the reflux pipes 4-1 and 4-2.

In the present invention, the inner contour shape of the vacuum degassing tank bottom 3 is an elongated closed curve in the direction of the interval between the ascending pipe 4-1 and the descending pipe 4-2; The inner contour shape of the ascending pipe 4-1 and the descending pipe 4-2 is an elongated closed curve orthogonal to the direction of the interval between the rising pipe 4-1 and the descending pipe 4-2.

[0020] Next, these will be further explained. 2, 3, and 4 show cross-sectional views of the bottom of the vacuum degassing tank of a conventional example and an example adapted to the present invention.

FIG. 2 is a cross-sectional view showing a conventional example in which the inner contour of the vacuum degassing tank bottom 3 is circular and the inner contour of the reflux pipe 4 is also circular. A stagnation part occurs.

On the other hand, FIG. 3 shows the bottom part 3 of the vacuum degassing tank.
This is a cross-sectional view showing an example of adapting the present invention in which the inner contour shape of the vacuum degassing tank 3 is oval and the inner contour shape of the reflux pipe 4 is circular. This can prevent stagnation of molten steel.

FIG. 4 is a cross-sectional view showing an example of adapting the present invention in which the inner contour shape of the vacuum degassing tank bottom 3 is oval and the inner contour shape of the reflux pipe 4 is also oval. 3
As in the case of , stagnation of molten steel can be prevented.

As described above, according to the present invention, the flow velocity distribution of the refluxed molten steel at the bottom of the vacuum degassing tank, which is a factor inhibiting the degassing reaction, is made uniform, and the occurrence of stagnation can be prevented, thereby improving the degassing reaction efficiency. can be done.

The mounting positions of the ascending pipe 4-1 and the descending pipe 4-2 are close to the inner surfaces of both ends of the vacuum degassing tank bottom 3 in the direction of the distance between the ascending pipe and the descending pipe (C in FIG. 3). C shown in Fig. 3 is within a range of 200 mm to 500 mm without any practical problems.
mm is appropriate, and by doing so, the riser pipe 4-1
The space between the degassing tube 4-2 and the downcomer pipe 4-2 can be made large, which not only allows the degassing reaction interface area to be expanded without causing any adverse effects.
This is also advantageous against the stagnation of molten steel that inevitably occurs at both ends of the bottom 3 of the vacuum degassing tank.

[0026] Next, the results of investigation on the amount of reflux and amount of decarburization of molten steel when using a conventional vacuum degassing device and a vacuum degassing device adapted to the present invention will be described below.

The specifications of the equipment used in this investigation, the composition of the molten steel, etc. are listed below. ○ Vacuum degassing device RH vacuum degassing device ○ Vacuum degassing tank bottom inner contour shape/conventional example Shape: circular, diameter: 3.2 m/compatible example
Shape: Oval, width (a in Figure 3): 3.4 m, length (
Figure 3 b): 3.0m, aspect ratio a/b = 1.13○
Reflux pipe inner contour shape/shape 1: circular, diameter: 0.6 m, cross-sectional area: 0.28
2 m2, shape 2: circular, diameter: 1.0 m, cross-sectional area:
0.785 m2・Shape 3: Oval, horizontal (a2 in Figure 4
): 1.2 m, vertical (b2 in Figure 4): 0.8 m
(Aspect ratio a2/b2 = 1.5), cross-sectional area: 0.8
2 m2 ○ Combination of vacuum degassing tank and reflux pipe, and distance between centers of reflux pipes (rising pipe and descending pipe) Case 1: Using a reflux pipe of shape 1 in a conventional vacuum degassing tank Case 2: Distance between reflux tube centers: 1.35m/Case 2
: When a reflux tube of shape 1 is used in the vacuum degassing tank of the compatible example, the distance between the centers of the reflux tubes: 1.70 m・Case 3: When the reflux tube of shape 2 is used in the vacuum degassing tank of the compatible example, Distance between reflux tube centers: 1.70 m Case 4: When a reflux tube of shape 3 is used in the vacuum degassing tank of the compatible example, the distance between reflux tube centers:
1.95m○ Amount of molten steel 300t (melted in a converter) ○ Molten steel temperature 1590-1610℃ ○ Initial composition of molten steel C: 0.03-0.05 wt%, Mn: 0.1
~0.2 wt%, P:0.025 ~0.030
wt%, S: 0.003 ~ 0.005 wt%, O: 0.06
~0.07 wt %○ Ultimate vacuum degree in vacuum degassing 0.3 ~ 0.5 Torr

■Results of reflux amount investigation Case 1 using a conventional vacuum degassing tank and a reflux pipe of shape 1, and case 2 using a compatible vacuum degassing tank and a reflux pipe of shape 1. The relationship between the amount of refluxed gas and the amount of molten steel refluxed was investigated. These survey results are summarized in Figure 5.

As is clear from this figure, when the vacuum degassing tank conforming to the present invention in case 2 is used, the effect is remarkable even though the diameter of the reflux tube is the same; Compared to the case of using a vacuum degassing tank, the amount of molten steel returned is increased by 40 to 50%.

■Results of investigation on amount of decarburization Degassing was performed for 20 minutes using case 1 using a conventional vacuum degassing tank and cases 2, 3, and 4 using a vacuum degassing tank compatible with the present invention. The molten steel C was measured after decarburization treatment. These measurement results are summarized in FIG. 6.

FIG. 6 shows the relationship between decarburization time and molten steel C. As is clear from this figure, in case 1 using a conventional vacuum degassing tank, molten steel C is 300 to 5.
00 wt ppm to 15 by decarburization for 20 minutes.
However, when a vacuum degassing tank conforming to the present invention is used, the decrease is large, and in Case 2, which uses the same reflux pipe diameter as Case 1, it is 8 to 20 wt ppm. 12 wt ppm, 5-9w for case 3
t ppm, and case 4 using an oval reflux tube.
In the case of 4 to 8 wt ppm, it is greatly reduced.

These results show that by using the vacuum degassing device compatible with the present invention, the degassing reaction efficiency is greatly improved and the carbon concentration achieved can be reduced. The device has shown to be very effective, especially in the production of ultra-low carbon steel.

[0033]

[Effects of the Invention] The main feature of the present invention is that the inner contour shape of the bottom of the vacuum degassing tank of the reflux type vacuum degassing device is an elongated closed curve in the direction of the spacing between the reflux tubes. By using this inventive device, the reflux rate of molten steel is improved, and the degassing reaction efficiency is improved, i.e., decarburization, deoxidation,
Efficiency of dehydrogenation etc. is improved. By performing decarburization using the device of the present invention, ultra-low carbon steel can be easily produced in a short time. Further, the apparatus of the present invention can be advantageously used for melting not only ultra-low carbon steel but also high-purity steel, low-hydrogen steel, and the like.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of a vacuum degassing tank and a ladle.

FIG. 2 is a cross-sectional view of the bottom of a conventional vacuum degassing tank in which the internal contour of the reflux pipe is circular.

FIG. 3 is a cross-sectional view of the bottom of the vacuum degassing tank of the present invention, in which the inner contour of the reflux pipe is circular.

FIG. 4 is a cross-sectional view of the bottom of the vacuum degassing tank of the present invention, in which the internal contour of the reflux pipe is oval.

FIG. 5 is a graph showing the relationship between the amount of refluxed gas and the amount of molten steel refluxed in RH vacuum degassing.

FIG. 6 is a graph showing the relationship between decarburization time and molten steel C in RH vacuum degassing.

[Explanation of symbols]

1 Ladle 2 Vacuum degassing tank 3 Bottom of vacuum degassing tank 4 Reflux tube 4-1 Reflux pipe (rising pipe) 4-2 Reflux pipe (downcomer pipe) 5. Monitoring window 6 Exhaust port 7 Molten steel

Claims (4)

[Claims] 1. A reflux type vacuum degassing apparatus for molten steel having a vacuum degassing tank having a riser pipe and a downcomer pipe as reflux pipes, the vacuum degassing tank being horizontal at least at the bottom of the vacuum degassing tank. A vacuum degassing device characterized in that the inner contour shape of the cross section consists of an elongated closed curve in the direction of the interval between the ascending pipe and the descending pipe. 2. The vacuum degassing apparatus according to claim 1, wherein the horizontal cross-sectional inner surface contour of the bottom of the vacuum degassing tank, which is an elongated closed curve, has an aspect ratio in the range of 1.1 to 3.0. 3. The vacuum degassing according to claim 1 or 2, wherein the horizontal cross-sectional inner surface contour shape of each of the rising pipe and the downcomer pipe is an elongated closed curve orthogonal to the direction of the spacing between the rising pipe and the downcomer pipe. Device. 4. A vacuum degassing tank equipped with a riser pipe and a downcomer pipe as a reflux pipe, wherein the shape of the horizontal cross-sectional inner surface of the vacuum degassing tank at least at the bottom of the vacuum degassing tank is the same as that of the riser pipe. Using a vacuum degassing device consisting of an elongated closed curve in the direction of the gap between the riser and the downcomer, reflux gas is blown into the riser to cause the molten steel to flow back into the vacuum degassing tank, thereby creating a vacuum from the riser to the downcomer. A method for producing ultra-low carbon steel characterized by uniformizing the flow velocity distribution of molten steel in a degassing tank.