JP2018095942A - Vacuum degassing apparatus and secondary refining apparatus for molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vacuum degassing apparatus and a secondary refining apparatus for molten steel using the same which can complete desulfurization in a short time while performing vacuum degassing treatment under reduced pressure as well as suppressing melting loss of refractories in a vacuum tank even if the desulfurization is carried out.SOLUTION: A vacuum degassing apparatus 21 used for secondary refining of molten steel has: a vacuum chamber 1 (a straight body vacuum chamber) having a single tubular dip tube 9 at the bottom; and a tuyere 2 for blowing gas over the entire circumference of an inner circumference of the tubular dip tube 9. A secondary refining apparatus for molten steel has the vacuum degassing apparatus 21, a ladle 5, and the tuyere 2 provided on the inner periphery of the tubular dip tube 9 as only means for supplying gas into the molten steel. The relationship between the number N of the tuyeres 2 and the diameter D(m) on the inner side of the tubular dip tube satisfies the following expression (1). 4D+2≤N≤12D...(1)SELECTED DRAWING: Figure 4

Description

  The present invention relates to a vacuum degassing apparatus and a secondary refining apparatus for molten steel using the same.

  With the upgrading of steel materials, further reduction in the concentration of impurity elements is required. Impurity elements are removed through the steel refining process. Molten steel that has undergone primary refining by a converter or the like is stored in a ladle, and secondary refining is performed on the molten steel in the ladle. For varieties that particularly reduce the impurity elements, the impurity elements are removed to the extent of the steel standard in the secondary refining process. Therefore, a secondary refining treatment apparatus having a high ability to remove impurity elements is required.

  In the secondary refining process, carbon in the impurity element reacts with C and O in the molten steel under reduced pressure to remove it as CO gas, and hydrogen and nitrogen are removed from the molten steel as hydrogen gas and nitrogen gas, respectively. In order to increase the removal rate, vacuum degassing treatment is required in which the molten steel is exposed to a reduced pressure atmosphere. In addition, strong agitation is also effective.

  Of the impurity elements, sulfur is basically required to be desulfurized by removing it by adsorbing it to the slag added to the surface of the molten steel. In order to increase the removal rate, it is effective to strongly stir the interface between the slag and molten steel and to increase the sulfur absorption capacity of the slag.

  In order to improve the processing efficiency of the secondary refining process, a processing apparatus capable of simultaneously removing these impurity elements is required. However, it is difficult to remove these impurity elements at the same time. This is because when depressurization is performed for vacuum degassing treatment and desulfurization with slag is performed, it is necessary to increase the stirring force in order to improve the removal speed of each, and the slag reacts with the refractory and the refractory This is because the wear rate of the refractory increases, the refractory repair frequency increases, and the processing efficiency of the secondary refining process rather decreases. For this reason, different secondary refining treatment apparatuses are used for vacuum degassing treatment and desulfurization treatment.

  However, conventionally, in addition to the vacuum degassing treatment with a single vacuum degassing treatment device, a method of performing desulfurization treatment using means for blowing powder desulfurization flux instead of slag into molten steel or spraying it on the molten steel surface Development is underway.

  As a desulfurization method using an RH vacuum degassing apparatus, Patent Document 1 discloses a desulfurization flux mainly composed of calcium oxide and aluminum oxide from a lance provided at the top of a vacuum degassing tank, a carrier gas, a fuel gas, and A refining method is proposed in which it is injected together with oxygen gas and sprayed onto the molten steel in the vacuum degassing tank. Patent Document 2 proposes a desulfurization method for molten steel in which a powdered desulfurizing agent is blown into a molten steel together with a carrier gas, and the desulfurizing agent and molten steel thus blown are mixed and desulfurized in a vacuum tank of an RH vacuum degassing apparatus. doing. By using these methods, vacuum degassing treatment and desulfurization treatment can be performed in parallel.

  In Patent Document 3, a cylindrical dip tube consisting of one leg is immersed in molten steel accommodated in a ladle, the dip tube is evacuated, and the molten steel is sucked into the dip tube. A desulfurization method in which an inert gas is blown from a lower portion of a ladle is disclosed. The desulfurization powder can be added from an upper blow lance or injection lance installed in the dip tube, an immersion tuyere inside the dip tube, or an upper blow tuyere. The inert gas is blown from the bottom of the ladle through a blow lance or a porous plug. In Patent Document 4, a large-diameter straight barrel-shaped container is immersed in molten steel in a ladle, the inside of the straight barrel immersion tank is decompressed, a gas body is supplied from the bottom of the steel bath, and the entire circumference of the dip tube A method for producing ultra-low carbon steel is disclosed in which an inert gas is blown from a position 500 to 1500 mm below the surface of the steel bath from a plurality of gas blowing nozzles.

JP 2012-172213 A JP 2002-161310 A Japanese Patent Laid-Open No. 7-278639 Japanese Patent Laid-Open No. 7-54034

  As in Patent Documents 1 to 3, in a desulfurization method in which a powder desulfurizing agent is blown or sprayed onto molten steel, since a long time is required for supplying the powder desulfurizing agent, the processing efficiency of secondary refining is increased. Will fall. In the method of Patent Document 2, in order to suppress the molten steel from being inserted into the tuyere for blowing the desulfurizing agent into the molten steel, it is necessary to keep the gas flowing even during a period in which the desulfurizing agent does not need to be blown. Invite. Moreover, since the wear of the tuyere progresses due to the flux, the repair frequency increases.

  In addition, in the desulfurization method using the RH vacuum degassing apparatus described in Patent Documents 1 and 2, after desulfurization by the reaction between the molten steel and the desulfurization agent, the desulfurization agent containing S is released from the downcomer of the RH vacuum tank. It descends into the molten steel in the ladle and then rises and is absorbed by the ladle slag on the surface of the molten steel in the ladle. Since ladle slag is mainly oxidized slag that flows out of the steel from the converter, the concentration of lower oxides such as iron oxide and manganese oxide in the ladle slag is high, so the sulfur retention capacity of the slag is low. It will be vulcanized. Therefore, in order to keep the S concentration low, it is necessary to perform a reforming process for reducing the lower oxide concentration of the ladle slag.

  In any method of Patent Documents 1 to 3, the desulfurizing agent stays on the surface of the molten steel in the vacuum chamber because the desulfurizing agent is supplied into the molten steel in the vacuum chamber. For this reason, the refractory is melted by the desulfurization agent on the surface of the molten steel, and the life of the refractory is reduced.

  Therefore, in the present invention, desulfurization can be completed in a short time by performing desulfurization in a vacuum tank without supplying a desulfurizing agent by spraying or blowing while performing vacuum degassing treatment under reduced pressure. An object of the present invention is to provide a vacuum degassing apparatus capable of suppressing the melting loss of the refractory in the vacuum tank and a secondary refining apparatus for molten steel using the same.

The present invention has been made to achieve such an object, and the gist thereof resides in the following straight cylinder type vacuum degassing apparatus.
(1) A vacuum degassing apparatus used for secondary refining of molten steel, having a vacuum tank (hereinafter referred to as “straight barrel vacuum tank”) having a single cylindrical dip tube in the lower part, and the cylinder A vacuum degassing apparatus characterized by being provided with tuyere that blows gas over the entire circumference of the inner dip tube.
(2) The vacuum degassing apparatus according to (1) above, wherein the relationship between the number N of tuyere and the inner diameter D of the cylindrical dip tube satisfies the following expression (1).
4D + 2 ≦ N ≦ 12D (1)
D: Cylindrical dip tube inner diameter (m)
N: Number of tuyere (-)
(3) As means for supplying gas into the molten steel in the vacuum degassing apparatus, having the vacuum degassing apparatus described in (1) or (2) above and a ladle for containing molten steel A secondary refining apparatus for molten steel, characterized by having only tuyere for blowing gas provided on the inner periphery of the cylindrical dip tube.
(4) The relationship between the cylindrical dip tube inner diameter D and the ladle inner diameter D _L is:
D / D _L ≧ 0.40 (2)
The secondary refining apparatus for molten steel according to (3) above, characterized in that:

  According to the present invention, it is possible to perform a refining process such as degassing and desulfurization in a short time using a single processing apparatus, and it is also possible to suppress refractory refractory damage in the vacuum chamber. Therefore, the processing efficiency of secondary refining can be remarkably improved.

It is a figure which shows the influence of the number of tuyere on the uniform mixing time. It is a figure which shows the influence of the number of tuyere on the refractory melting rate. It is a figure which shows the influence of the number of tuyere and the inside diameter of a vacuum chamber which has on the uniform mixing time and the refractory melting rate. It is a figure which shows the schematic structural example of the vacuum degassing processing apparatus of this invention which has a straight body type vacuum tank, (A) is a longitudinal cross-sectional view, (B) is a BB arrow plane cross section of (A). FIG.

  As shown in FIG. 4, the present invention is a vacuum degassing apparatus 21 used for secondary refining of molten steel as a vacuum tank 1 having a single cylindrical dip tube 9 in the lower part (hereinafter referred to as “straight cylinder type vacuum tank”). Is used). In many cases, the diameter of the cylindrical dip tube 9 provided in the lower portion of the vacuum chamber 1 and the diameter of the vacuum chamber 1 located above the cylindrical dip tube 9 are substantially the same. Since the entire shape including the cylindrical dip tube 9 has a straight body type, it is called a straight body type vacuum tank. The lower end of the cylindrical dip tube 9 is open. By immersing the cylindrical dip tube 9 of the vacuum tank 1 on the surface of the molten steel in the ladle 5 and depressurizing the inside of the vacuum tank, the surface of the molten steel rises in the cylindrical dip tube 9, and the molten steel in the vacuum tank 1 is removed. Vacuum degassing and desulfurization are performed as targets.

  When slag is formed on the surface of the molten steel in the ladle 5 and the straight body type vacuum tank is immersed in the molten steel, the slag existing immediately below the cylindrical dip tube 9 is taken into the cylindrical dip tube 9. When the pressure in the vacuum chamber 1 is reduced, the slag taken into the cylindrical dip tube 9 rises in the vacuum chamber together with the molten steel. Here, the slag taken into the vacuum chamber 1 is called “vacuum slag 8”, and the slag existing on the surface of the molten steel in the ladle 5 outside the cylindrical dip tube 9 is called “ladder slag 7”. . Since the straight barrel type vacuum tank can increase the inner diameter of the cylindrical dip tube 9, a large amount of slag can be taken in as the slag 8 in the vacuum tank.

  Since the present invention uses the above-described straight barrel type vacuum tank as the vacuum degassing processing device 21, if a lump desulfurizing agent is added into the ladle when steel is removed from the converter to the ladle, By stirring the molten steel, a slag layer can be formed on the surface of the molten steel in the ladle as the added desulfurizing agent as molten slag, and the desulfurizing agent is contained by immersing the straight barrel type vacuum tank on the molten steel surface in the ladle. Molten slag can be taken in as slag 8 in the vacuum chamber. Therefore, as described in Patent Documents 1 to 3, it is not necessary to blow or spray a powder desulfurization agent, so that the desulfurization time can be shortened.

  Next, the problem of the present invention that provides an apparatus capable of suppressing the melting loss of the refractory in the vacuum chamber even after desulfurization is described. In order to solve this problem, the present inventors, when using a straight barrel type vacuum tank, have a slag 8 in the vacuum tank formed on the surface of the molten steel in the vacuum tank, even under strong stirring under reduced pressure. The method of suppressing the melting loss of the refractory in the vacuum chamber was examined by avoiding contact with the refractory in the vacuum chamber 1. And the method of suppressing a contact was devised by providing the layer of the gas bubble injected for stirring between slag and a refractory. For that purpose, the tuyere 2 for blowing the gas for stirring may be installed over the entire circumference inside the cylindrical dip tube of the vacuum chamber.

  Conventionally, in the secondary refining of molten steel using a straight barrel vacuum tank, the gas is blown up from the bottom of the ladle or from the lance immersed in the ladle so as to rise into the dip tube of the vacuum tank. Gas was blown in and the stirring of the molten steel accompanying the gas rise was used. In the present invention, in order to suppress contact between the slag in the vacuum chamber and the refractory, secondary refining is performed only by blowing gas from the tuyere 2 installed over the entire circumference inside the cylindrical dip tube 9 of the vacuum chamber 1. It is necessary to sufficiently stir the necessary molten steel.

  Therefore, only by blowing water from the tuyere 2 installed over the entire circumference inside the cylindrical dip tube 9 of the straight barrel type vacuum tank, first by water model experiment and then by actual secondary refining of molten steel, the molten steel is stirred. We investigated the situation and the state of uniform mixing of the added components. Both the water model apparatus and the actual molten steel apparatus used the vacuum chamber 1 and ladle 5 having the shape shown in FIG.

  First, a water model experiment will be described. As shown in FIG. 4, gas is blown into the liquid from the tuyere installed over the entire circumference inside the cylindrical dip tube 9, a tracer is added into the vacuum tank 1, and the liquid in the vacuum tank 1 and the ladle 5 is added. We confirmed the flow situation. As a result, it was confirmed that the liquid flowed along the streamline 10 as shown in FIG. That is, even though gas was not blown from the bottom of the ladle or gas was not blown by inserting a lance into the liquid in the ladle, only gas was blown from the entire circumference of the cylindrical dip tube 9, It was confirmed that the entire liquid in the vacuum chamber 1 and the ladle 5 was sufficiently stirred. In addition, in order to confirm the floating state of the non-metallic inclusion particles generated in the molten steel, particles lighter than water were added to the liquid in the vacuum chamber 1 and the behavior was confirmed. It was confirmed that most of the particles circulating in the pan 5 were small, and the ratio of the particles that floated and separated on the surface of the liquid existing between the vacuum chamber 1 and the ladle 5 was small. It is considered that the inner diameter of the cylindrical dip tube 9 of the vacuum chamber 1 is not so small compared to the inner diameter of the ladle 5.

  In addition, the number of tuyere 2 provided on the entire circumference inside the cylindrical dip tube 9 was changed, and a water model experiment was conducted on the generation behavior of gas bubbles. As a result, when the number of tuyere 2 is excessively large, gas bubbles blown from adjacent tuyere 2 are combined to cause blow-through, and there is a concern that the stirring force is insufficient. Conversely, if the number of tuyere 2 is too small, the distance between the gas bubbles is too far, causing contact between the slag and the refractory material, causing drifting and insufficient stirring power. That is, there is a proper range of tuyere according to the inner diameter of the cylindrical dip tube 9 of the vacuum chamber 1.

  Next, a description will be given of the results of experiments conducted in actual molten steel secondary refining using a molten steel secondary refining apparatus as shown in FIG. As described above, since it was predicted that there was an appropriate range for the number of tuyere 2 for blowing gas provided all around the inside of the tubular dip tube, this appropriate range was investigated by a test using molten steel.

  A straight barrel type vacuum chamber having inner diameters of 0.8, 1.5, and 2.5 m was immersed in 80, 150, and 250 tons of molten steel that had been decarburized in a converter and put out in a ladle. Inside the cylindrical dip tube 9 of the straight barrel type vacuum tank, tuyere 2 for blowing stirring gas is provided on the entire circumference at a height of 150 to 500 mm from the bottom of the vacuum tank. The number of tuyere was 4 to 36, and each tuyere was evenly arranged on the circumference. After immersion in the vacuum chamber, the inside of the vacuum chamber was decompressed to 133-3990 Pa using a vacuum exhaust device, and stirring gas was introduced through the tuyere in a total of 5.9 to 8.5 NL / min per ton of molten steel.

After confirming the stability of the pressure in the vacuum chamber and the stirring gas flow rate, the uniform mixing time was measured as an index of stirring force. The uniform mixing time was set to time t when Cu was added to the molten steel in the vacuum chamber and then samples were continuously collected from the molten steel surface of the ladle and the Cu concentration converged to the range of the formula (3).
| 1-([Cu] _t − [Cu] ₀ ) / ([Cu] _e − [Cu] ₀ ) | ≦ 0.05 (3)
[Cu] _t : Cu concentration t seconds after Cu addition, [Cu] ₀ : Cu concentration before Cu addition, [Cu] _e : Cu concentration reached

  Furthermore, the degree of erosion due to contact between the refractory and the slag in the vacuum chamber was measured, the thickness of the refractory after the treatment was measured, and the erosion rate per treatment was investigated.

  The relationship between the uniform mixing time and the number of tuyere is shown in FIG. As shown in the figure, it can be seen that there is a range of tuyere numbers in which the uniform mixing time is short for any vacuum chamber inner diameter. This is presumably because if the number of tuyere is excessively small, the distance between the bubbles blown from adjacent tuyere is large, the bubbles are unevenly distributed, and the molten steel drifts. When the number of tuyere is excessively large, it is considered that bubbles blown from adjacent tuyere collide with each other and coalesce and blow-through occurs, making it difficult for the stirring force by the bubbles to be transmitted to the molten steel.

  FIG. 2 shows the relationship between the refractory melting rate and the number of tuyere. As shown in the figure, it can be seen that there is a range of tuyere numbers where the refractory melting rate is small at any inner diameter of the vacuum chamber. This is considered to be because when the number of tuyere is excessively small, the distance between the bubbles blown from the adjacent tuyere is large, and the effect of suppressing contact between the slag and the refractory by the bubbles is reduced. If the number of tuyere is excessively large, bubbles blown from adjacent tuyere collide and coalesce and blow-through occurs. .

From these results, it can be seen that there is an appropriate range of tuyere depending on the inner diameter of the vacuum chamber. FIG. 3 shows the effects of the number of tuyere and the inner diameter of the cylindrical dip tube on the uniform mixing time and the refractory melting rate. Uniform mixing time and refractory erosion rate can be arranged by number of tuyere and inside diameter of vacuum chamber. From this result, it has been found that it is preferable that the relationship between the number N of tuyere and the inner diameter D (m) of the cylindrical dip tube satisfies the following expression (1).
4D + 2 ≦ N ≦ 12D (1)
It should be noted that the angles θ between the adjacent tuyere 2 with respect to the center of the cylindrical dip tube 9 are preferably the same, but it is not essential that they be the same. If the angle θ is within the range of the following formula (4), the effect of the present invention when the number of tuyere is N can be sufficiently exhibited.
360 / N × 0.75 ≦ θ ≦ 360 / N × 1.25 (4)

(1) Apparatus Configuration FIG. 4 is a diagram showing a schematic configuration example of a molten steel secondary refining apparatus using the straight body type vacuum degassing apparatus of the present invention, and FIG. 4 (A) is a longitudinal sectional view. FIG. 4B is a plan cross-sectional view taken along the line BB in FIG. As shown in FIG. 4A, the straight body type vacuum degassing apparatus 21 includes a vacuum chamber 1. The vacuum chamber 1 has a single cylindrical dip tube 9 at the bottom, and the tubular dip tube 9 has a tuyere 2 for blowing a stirring gas. The upper part of the vacuum chamber 1 is connected to the vacuum exhaust device 3 and the alloy addition hole 4. When refining the molten steel, the vacuum chamber 1 is taken into the molten steel 6 in the ladle 5. The surface of the molten steel 6 in the ladle 5 is covered with a ladle slag 7. The slag that was a part of the ladle slag 7 before immersing the vacuum chamber 1 is taken into the vacuum chamber and stays in the upper part of the molten steel to form the in-vacuum slag 8.

  The tuyere 2 for blowing the stirring gas is preferably provided at a height of 150 mm or more from the bottom surface of the vacuum chamber 1. If it is lower than 150 mm, the pipe may be deformed by heat from the molten steel. More desirably, the distance between the tuyere and the surface of the molten steel in the vacuum chamber is preferably set to a height that can be maintained at 0.8 m or more. This is because when the distance between the tuyere and the molten steel surface in the vacuum chamber is less than 0.8 m, the distance from the tuyere to the molten steel surface in the vacuum chamber is shortened, and the time until the bubble rises is shortened. The stirring force by may become weak.

  The inner diameter of the tuyere 2 is desirably 2 to 7 mm. If it is smaller than 2 mm, the pressure loss in the piping may increase when the stirring gas is blown. If it is larger than 7 mm, the penetration of molten steel into the tuyere occurs, which may cause melting of the tuyere.

(2) Treatment method After decarburizing treatment in a converter and steelmaking in the ladle 5, the ladle 5 containing the molten steel 6 is subjected to vacuum degassing treatment having a straight barrel type vacuum tank shown in FIG. It transfers to the arrangement | positioning place of the apparatus 21, and a refining process is started. In addition, the flux (lump form) which has CaO as a main component is added in order to ensure the sulfur retention capability of ladle slag and to suppress contact between the atmosphere and molten steel during steel output. The slag containing this flux is taken into the vacuum chamber when the vacuum chamber 1 is immersed in the molten steel and functions as a desulfurizing agent as the slag 8 in the vacuum chamber. In order to melt the flux and sufficiently function as a desulfurizing agent, it is preferable to feed the flux as early as possible after the start of steel production. Further, a deoxidizing element may be added in order to reduce the concentration of FeO and MnO contained in the converter slag that has flowed into the ladle at the time of steel output. In addition, as for the mass of the ladle slag as the sum total of the thrown-in flux and the converter slag which flowed out, 5-33 kg per ton of molten steel is desirable. When less than 5 kg per ton of molten steel, the area covering the molten steel in the ladle is small, and the contact inhibition effect between the atmosphere and the molten steel may be small. If the amount is more than 33 kg per ton of molten steel, the thickness of the slag increases and may overflow from the ladle.

  After the transfer, the cylindrical dip tube 9 of the vacuum chamber 1 is immersed in the molten steel 6 in the ladle 5. The immersion depth is preferably 250 to 650 mm. If it is less than 250 mm, the ladle slag 7 is sucked into the vacuum chamber during the treatment, a region where the slag does not cover the ladle surface is generated, and nitrogen and oxygen are absorbed from the atmosphere, thereby lowering the treatment efficiency. There is a case to let you. When it is larger than 650 mm, the static pressure of the molten steel at the position of the tuyere 2 becomes large, and the molten steel may easily enter the tuyere.

  After the cylindrical dip tube 9 of the vacuum chamber 1 is immersed in the molten steel 6, the inside of the vacuum chamber 1 is decompressed using the vacuum exhaust device 3. As for the pressure in the vacuum chamber 1 at the time of a molten steel process, 67-13300Pa is desirable. More desirably, it is 67 to 6650 Pa. If it is lower than 67 Pa, the degree of expansion of the stirring gas bubbles in the vicinity of the molten steel surface in the vacuum chamber increases, and a large amount of splash is generated when bubbles are broken, which may adversely affect the operation. When the pressure is higher than 13300 Pa, the degree of expansion of the stirring gas bubbles is reduced, and the stirring force by the bubbles may be weakened. Moreover, when it exceeds 6650 Pa, the degassing speed in a vacuum chamber may become small.

  Stirring gas is blown into the molten steel through the tuyere 2 together with decompression in the vacuum chamber 1 to stir the molten steel. The stirring gas flow rate is desirably 4.5 to 15.0 NL / min in total for all tuyere per ton of molten steel. If it is less than 4.5 NL / min per 1 ton of molten steel, stirring may become too weak. If it exceeds 15.0 NL / min per ton of molten steel, a large amount of splash is generated at the time of bubble breaking, which may adversely affect the operation.

When the straight body type vacuum degassing apparatus of the present invention is used, component adjustment by adding an alloy, vacuum degassing process, desulfurization process, deoxidation process, and O ₂ gas blowing from a lance installed above the molten steel in the vacuum chamber A refining process such as a temperature rise using Al oxidation heat generation by the can be performed.

  As shown in FIG. 4, the secondary refining apparatus 22 for molten steel of the present invention has the vacuum degassing apparatus 21 of the present invention and a ladle 5 for containing molten steel, and is a vacuum degassing apparatus. As a means for supplying gas into the molten steel, only the tuyere 2 for blowing gas provided on the inner periphery of the cylindrical dip tube 9 is provided. Without injecting gas from the bottom of the ladle or injecting gas by inserting a lance into the liquid in the ladle, the molten steel 6 can be obtained only by injecting gas from the tuyere 2 provided around the entire circumference of the cylindrical dip tube. Stir and mix.

  The vacuum degassing apparatus 21 used for the secondary refining apparatus 22 for molten steel of the present invention uses a straight barrel type vacuum tank 1 having a single cylindrical dip tube at the bottom, and the opening of the cylindrical dip tube 9 Since the area becomes large, the slag on the ladle surface can be taken in when immersed in the molten steel 6 in the ladle 5, and the slag taken in forms the slag 8 in the vacuum chamber and is stirred by the vacuum chamber 1. Since lower oxides such as FeO and MnO in the slag are reduced by Al in the molten steel and given a desulfurization ability, the desulfurization treatment can be performed using the slag 8 in the vacuum chamber as a desulfurizing agent. Since the blowing or blowing of the desulfurizing agent as powder can be omitted, it is possible to reduce the desulfurization processing time. In addition to the slag in the vacuum chamber formed by taking in the slag on the ladle surface, a powder desulfurizing agent may be additionally blown or blown.

  In the molten steel secondary refining apparatus 22 of the present invention, the molten steel 6 is agitated and mixed by blowing gas from the tuyere 2 provided on the entire circumference inside the cylindrical dip tube, so that it is formed on the surface of the molten steel in the vacuum chamber 1. Since the gap between the evacuated slag 8 in the vacuum chamber and the inner wall surface of the vacuum chamber 1 is blocked by gas bubbles, the refractory in the vacuum chamber can be prevented from being melted (see FIG. 4).

  Moreover, since the straight cylinder type vacuum tank 1 which has the single cylindrical dip tube 9 in the lower part is used as the vacuum degassing processing apparatus 21, it was caught in the molten steel by stirring on the molten steel surface of the vacuum tank 1. The minute slag 8 in the vacuum chamber moves along the streamline 10 in the ladle 5, and most of the slag 8 circulates in the vacuum chamber 1 again. Therefore, when compared with the secondary refining using the RH vacuum degassing apparatus, the ratio of rising into the ladle 5 and being taken into the ladle slag 7 formed on the molten steel surface between the vacuum chamber 1 and the ladle 5 Less is. The ladle slag 7 contains slag that has flowed out of the converter and contains lower oxides such as FeO and MnO. So-called sulfidation, in which S deviates and returns to the molten steel 6, is likely to occur. As described above, the present invention has an advantage that there is little resulfurization because the ratio of the slag 8 in the vacuum chamber containing S to the ladle slag 7 is small.

Here, the relationship between the inner diameter D of the cylindrical dip tube 9 and the inner diameter D _L of the ladle 5,
D / D _L ≧ 0.40 (2)
Is preferable. Thereby, when the vacuum tank 1 is immersed in the molten steel 6 in the ladle 5, many ladle slags 7 can be taken in, and the slag 8 in a vacuum tank can be formed, and it circulates in the ladle 5 by stirring. The ratio of the vacuum tank slag 8 taken into the ladle slag 7 can be reduced, and the desulfurization capacity can be improved.

  Hereinafter, examples of the present invention will be described. However, the conditions in the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. Is not to be done. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  From 280 to 300 tons of molten steel decarburized in a converter, steel was put into a ladle, and 7.0 kg of massive CaO was added as desulfurized flux at the initial stage of the steel output per ton of molten steel. Each ladle containing the molten steel was transferred to a processing position of a vacuum degassing apparatus 21 having a straight barrel type vacuum chamber 1 as shown in FIG.

The molten steel composition at the start of vacuum degassing treatment is C = 0.04 to 0.50%, Si = 0.04 to 0.10%, Mn = 0.2 to 1.6%, Al = 0.01 to 0.20%, S = 0.0028-0.0032%, N = 0.445-0.0050%, H = 0.00035-0.00037%. Moreover, slag is molten steel per ton 10-20 kg, the major constituents of the slag 1.0 mass ratio of CaO and Al ₂ O ₃ present in the ladle surface prior to immersion of the vacuum chamber 1 1.4, SiO ₂ concentration is 0.5 to 7.0%, FeO concentration is 0.5 to 8.0%, MnO concentration is 0.5 to 8.0%, and MgO concentration is 3.0 to 8.%. 0%.

  A cylindrical dip tube 9 disposed in the lower part of the vacuum tank 1 of the vacuum degassing treatment device 21 is immersed in the molten steel 6 in the ladle 5 by 350 to 500 mm, and the inside of the vacuum tank is decompressed to 133 to 3990 Pa by the vacuum exhaust device 3. From the tuyere 2 arranged over the entire circumference on the inner circumference of the cylindrical dip tube, a total of 5.6 to 7.4 NL / min of stirring gas was introduced per ton of molten steel. The processing time is 20-25 minutes. Samples were taken at the start of the treatment and immediately after the end of the treatment, and the N, S, and H concentrations were analyzed to evaluate the degassing and desulfurization capabilities. The thickness of the refractory on the inner wall of the vacuum chamber was measured to evaluate the wear rate.

  For comparison, processing using an RH vacuum degassing apparatus will also be described. The ladle containing the same molten steel component, slag component and amount as those used in the vacuum degassing treatment apparatus using the straight body type vacuum tank was transferred to the treatment position of the RH vacuum degassing treatment apparatus. The inner diameter of the vacuum chamber was 2.0 m, the inner diameter of the dip tube was 650 mm, and a tuyere was provided in the dip tube to blow reflux gas to a height of 300 mm from the bottom of the riser tube.

  Immerse the RH type vacuum degassing dip tube 350-500mm in the molten steel in the ladle, depressurize the vacuum tank to 133-3990Pa with a vacuum exhaust device, and the total reflux gas per ton of molten steel 5.6-7.4 NL / min was introduced. After confirming the stability of the pressure in the vacuum chamber and the reflux gas flow rate, desulfurization flux was sprayed at a spraying rate of 0.5 kg / min per ton of molten steel from a lance installed above the molten steel in the vacuum chamber. The treatment time was set to 25 to 30 minutes, which is slightly longer than the time when the above-mentioned straight body type vacuum tank was used. Therefore, the total desulfurization flux sprayed was only 7.0 kg per ton of molten steel.

  Table 1 shows the changes in S, N and H concentrations before and after the treatment, and the amount of refractory erosion.

  No. which is an appropriate number of tuyere according to the inside diameter of the vacuum chamber defined in the present invention. In 1-5, the S concentration after treatment was 3-5 ppm, the N concentration was 23-25 ppm, and the H concentration was stably reduced to 0.8-1.2 ppm. Furthermore, the amount of refractory melt was as low as 1.8 to 2.4 mm.

  On the other hand, No. 2 less than the preferred number of tuyere of the present invention. 7-8, S concentration after treatment is 25-26ppm, N concentration is 41-43ppm, H concentration is as high as 2.5-2.6ppm, refractory erosion loss is 7.5-8.9mm, very high It became bigger. No. higher than the preferred number of tuyere of the present invention. In 9-10, the S concentration after treatment was 20-21 ppm, the N concentration was 37 ppm, the H concentration was as high as 2.1-2.2 ppm, and the refractory erosion amount was as large as 3.8-4.1 mm. .

  No. No. processed with an RH vacuum degassing apparatus. 10 to 11 had a high S concentration after treatment of 18 to 19 ppm. In order to obtain a lower S concentration after desulfurization, it is necessary to extend the treatment time and increase the total amount of desulfurization flux sprayed. Further, the N concentration was as high as 38 to 39 ppm, the H concentration was as high as 2.4 ppm, and the refractory erosion amount was as large as 3.5 to 3.7 mm.

DESCRIPTION OF SYMBOLS 1 Vacuum tank 2 Tuyere 3 Vacuum exhaust apparatus 4 Alloy addition hole 5 Ladle 6 Molten steel 7 Ladle slag 8 Slag in vacuum tank 9 Cylindrical dip pipe 10 Streamline 21 Vacuum degassing apparatus 22 Secondary refinement apparatus of molten steel

Claims (4)

  A vacuum degassing apparatus used for secondary refining of molten steel, comprising a vacuum tank having a single cylindrical dip tube (hereinafter referred to as “straight barrel type vacuum tank”) at the bottom, and the cylindrical dip pipe A vacuum degassing apparatus characterized by being provided with tuyere that blows gas over the entire circumference. The vacuum degassing apparatus according to claim 1, wherein the relationship between the number N of tuyere and the inner diameter D of the cylindrical dip tube satisfies the following expression (1).
4D + 2 ≦ N ≦ 12D (1)
D: Cylindrical dip tube inner diameter (m)
N: Number of tuyere (-)   The cylindrical degassing apparatus according to claim 1 or 2, and a ladle for containing molten steel, the cylindrical shape as means for supplying gas into the molten steel in the vacuum degassing apparatus A secondary refining apparatus for molten steel, characterized by having only tuyere for blowing gas provided on the inner periphery of a dip tube. The relationship between the cylindrical dip tube inner diameter D and the ladle inner diameter D _L is:
D / D _L ≧ 0.40 (2)
The secondary refining apparatus for molten steel according to claim 3, wherein:

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