JP2005264264A - Method and apparatus for vacuum refining of molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To apply a vacuum-refining to molten steel with the industrial scale while improving the service life of a refractory by preventing the damage to the refractory at the bottom of a vacuum vessel with the blowing of oxidizing gas from a top-blown lance without sacrificing the refining performance of vacuum-degassing etc., and restraining the sucking-up of slag and further, thermal-spalling crack caused by a large opening hole part in an immersion cylinder. <P>SOLUTION: One pair of immersion cylinders 13 arranged so as to communicate with the lower part of the vacuum vessel 12, is dipped into the molten steel 16 held in a ladle 15 set at the lower part of the immersion cylinder 13 and the vacuum-refining of the molten steel 16 is applied by blowing-stirring gas G into the molten steel 16 in the inner part of the immersion cylinder 13 to generate the circulation flow ascending/descending the inner part of the immersion cylinder 13. The immersion cylinder 13, in which at least the inner surface of the lower end opening hole part in the immersion cylinder 13 is formed as non-circular horizontal sectional shape almost along the outer-edge shape in an area where the circulation flow 20 exists is used to apply the vacuum-refining of the molten steel 16. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a molten steel vacuum refining method and a vacuum refining apparatus, for example, a molten steel vacuum refining method and a vacuum refining apparatus performed while blowing an oxidizing gas onto the surface of the molten steel in contact with a vacuum atmosphere.

  As a vacuum refining method for molten steel, for example, a method of degassing the whole molten steel accommodated in a ladle under a vacuum atmosphere, such as the VOD method, a RH degassing method, a DH degassing method, etc. A method is known in which a dip tube provided in the lower part of a vacuum chamber is immersed in molten steel accommodated in a ladle and vacuum refining is performed with the inside of the vacuum chamber as a vacuum atmosphere.

  Here, in the VOD method, it is difficult to increase the stirring power of the molten steel because there is a danger of overflowing the molten steel because a large free board cannot be secured, and the vacuum refining capacity is limited. In addition, in the DH degassing method, it is necessary to mechanically repeatedly move up and down either a ladle or a vacuum tank with a dip tube attached to the lower part with one dip tube immersed in molten steel. Expenses increase significantly.

  On the other hand, in the RH degassing method schematically showing the principle in FIG. 10, as shown in the figure, a vacuum chamber 3 having two ascending pipes 1 and 2 as dip pipes is provided at the bottom. After the molten steel 5 is supplied to the inside of the vacuum chamber 3 through the rising pipe 1 by being dipped in the molten steel 5 accommodated in the pot 4 and blowing the circulating gas G from the rising pipe 1, the ladle 4 is supplied from the down pipe 2. By returning to, a circulating flow of molten steel is formed between the ladle 4 and the vacuum chamber 3. Thus, in this RH degassing method, a circulating flow of the molten steel 5 can be formed by blowing the circulating gas G, and the ladle 4 or the vacuum chamber 3 is mechanically repeated at high speed as in the DH degassing method described above. Since there is no need to raise or lower, the increase in equipment costs can be suppressed, and it is now widely used.

FIG. 11 is an explanatory view schematically showing a situation in which an oxidizing gas is further blown onto the surface of the molten steel 5 existing inside the vacuum chamber 3 in the RH degassing method. As shown in the figure, in the RH degassing method, the molten steel 5 is heated as an oxygen source for the vacuum decarburization reaction (C + O = CO) in the vacuum chamber 3 or by the reaction heat between Al and oxygen in the molten steel 5. As an oxygen source, an oxidizing gas typified by an oxygen gas O ₂ is also widely sprayed from an upper blowing lance 6 disposed inside the vacuum chamber 3 on the surface of the molten steel 5 inside the vacuum chamber 3. .

  Generally, when performing vacuum degassing such as RH degassing or DH degassing, it is necessary to increase the reaction interface area in order to ensure the degassing reaction rate in a vacuum environment. In order to increase the interface area, it is necessary to secure a sufficient bath surface area inside the vacuum chamber 3. Here, in order to ensure a sufficient bath surface area, it is most advantageous to make the horizontal cross-sectional shape of the bath surface circular. Further, it is most advantageous from the viewpoint of the equipment structure that the shape of the dip tube provided in the lower part of the vacuum chamber 3 is a circle having the same diameter as the vacuum chamber 3. Therefore, in a vacuum degassing apparatus that performs a vacuum degassing method, a circular dip tube having the same diameter as that of the vacuum chamber is attached to a lower portion of a circular vacuum chamber having a bath surface having a circular horizontal cross-sectional shape.

On the other hand, many inventions have been proposed so far for performing vacuum refining by enhancing the degassing ability of the DH degassing method and the RH degassing method which are widely known.
For example, in Patent Document 1, a snorkel having a large inner diameter is immersed in molten steel accommodated in a ladle, and the inside of the snorkel is evacuated to suck up the molten steel. An invention is disclosed in which degassing is performed by blowing argon gas from the entire circumferential surface. In this invention, the molten steel forms an upward flow along the inner peripheral wall surface of the snorkel by the argon gas bubbling that rises along the inner peripheral wall surface of the snorkel, and the downward flow at the central portion of the snorkel corresponds to the increased amount. In order to form, a circulating stream of molten steel is formed inside the snorkel, and vacuum degassing is performed.

  Further, in Patent Document 2, the amount of molten metal to be sucked up is reduced as much as possible at the time of depressurization inside the downwardly opened cylinder forming the main part of the decompression tank, and the molten metal sucked up inside the cylinder On the other hand, the molten metal on the tuyere side is given an upward flow and the molten metal on the facing side is given a downward flow, so that one or more wings are installed so as to be stirred and mixed with the whole molten metal to be treated An invention for refining molten metal under reduced pressure by blowing a gas for stirring and refining into the molten metal from the mouth is disclosed.

  Further, in Patent Document 3, the lower end of the cylindrical container is formed by immersing the lower part of the cylindrical container provided with a vacuum exhaust port in the upper part into the molten steel and sucking the molten steel into the cylindrical container by reducing the pressure inside the cylindrical container. An invention in which an inert gas is blown into the molten steel from the inner surface side in the vicinity and vacuum degassing is performed by forming a circulating flow that raises and lowers the molten steel inside the cylindrical container is disclosed.

On the other hand, the invention described below is also known as an invention in which an oxidizing gas is sprayed onto the surface of molten steel in a vacuum atmosphere using a vacuum chamber.
In Patent Document 4, as shown in FIG. 12 schematically showing the invention described in Patent Document 4, the bath depth and the RH molten steel reflux velocity inside the vacuum chamber 7 having a circular horizontal cross-sectional shape are defined. An invention is disclosed in which vacuum decarburization is performed by blowing oxygen gas from a tuyere 10 provided at the bottom of a ladle 9 to the surface of molten steel 8 inside the vacuum chamber 7. According to the present invention, by setting the bath depth inside the vacuum chamber 7 as large as 0.4 m or more, it is possible to prevent the sprayed oxygen gas from damaging the refractory constituting the bottom of the vacuum chamber 7. It is said.

  Furthermore, in Patent Document 5, as shown in FIG. 13 schematically showing the invention described in Patent Document 5, one immersion having an inner diameter 0.4 to 0.8 times the inner diameter of the ladle 9 An invention in which the tube 7 is dipped in the molten steel 8 accommodated in the ladle 9 and the stirring gas is blown into the molten steel 8 inside the ladle 9 from the lower part (including the bottom) and the oxidizing gas is blown from the upper blowing lance 6. It is disclosed.

Japanese Patent Laid-Open No. 3-6317 JP 52-52109 A Japanese Patent Laid-Open No. 5-271748 JP-A-52-89513 JP-A-61-37912

However, the inventions disclosed in Patent Documents 1 to 3 described above have the common problems listed below.
As a first problem, in the inventions disclosed in Patent Documents 1 to 3, a large amount of slag existing in the upper part of the molten steel accommodated in the ladle at the time of degassing is sucked up from the dip tube, and the sucked slag The refractory constituting the dip tube is damaged early.

  That is, in the inventions disclosed in Patent Documents 1 to 3, since the circular dip tube having the same diameter is attached to the lower part of the circular vacuum tank, the horizontal sectional area of the lower end portion of the dip tube is inevitably vacuum. It becomes as large as the tank, and a large amount of slag existing in the upper part of the molten steel contained in the ladle is unavoidably sucked into the dip tube. In this way, if a large amount of slag is sucked into the inside of the vacuum tank, the sucked slag comes into contact with the refractory on the inner wall of the dip tube or the vacuum tank and reacts with the refractory to lower the melting point. Therefore, the refractory is melted or infiltrated into the surface of the refractory to change the characteristics of the surface of the refractory, thereby increasing the wear rate of the refractory. Therefore, the life of the refractory is reduced, the repair cost of the refractory is increased, and the repair cycle of the refractory is shortened, so that the actual operation time rate of the vacuum degassing device is lowered and the productivity is lowered.

  As a second problem, since the opening at the lower end of the dip tube is large, the temperature of the refractory constituting the inner wall of the dip tube after the degassing process is apt to drop rapidly. Therefore, when it contacts with high-temperature molten steel at the time of siphoning in the next heat, the thermal history (temperature difference) applied to the refractory is increased, and a temperature change and a temperature gradient are greatly generated. For this reason, since the expansion and contraction of the refractory accompanying the temperature change increase, cracks occur in the refractory, and it is easy to cause spall cracking and peeling due to the progress of the crack. Decreases. Accordingly, since the repair cost of the refractory increases and the repair cycle of the refractory is shortened, the actual operation time rate of the vacuum degassing apparatus is lowered and the productivity is lowered.

  As described above, depending on the invention disclosed in Patent Documents 1 to 3, the life of the refractory is reduced, the repair cost of the refractory is increased, and the repair cycle of the refractory is shortened. The operating time rate decreases and productivity also decreases.

On the other hand, the invention disclosed in Patent Document 4 or Patent Document 5 in which an oxidizing gas is blown onto the surface of the molten steel 8 accommodated in the vacuum chamber 7 has the problems described below.
In the invention disclosed in Patent Document 4, although Patent Document 4 has a description that it can be understood that the bath depth of the vacuum chamber 7 can be increased as much as possible, in practice, the brick thickness at the bottom of the vacuum chamber 7 or the molten steel 8 Taking into account the flange thickness and the like of the part to be immersed, it is generally not easy to secure a bath depth of 0.4 m or more in the case of blowing oxygen to the molten steel 8 accommodated in the vacuum chamber 7, Securing 0.5 m, which is desirable in Patent Document 4, cannot be realized in reality in consideration of equipment costs and the like. Thus, since it is difficult to actually carry out the invention disclosed in Patent Document 4, it is a fact that the damage to the refractory constituting the bottom of the vacuum chamber 7 that occurs when oxygen is strongly blown is solved. It is impossible.

  Further, in the invention disclosed in Patent Document 5, since the tank bottom does not exist in the vacuum chamber 7, even if oxygen is strongly blown, the refractory constituting the tank bottom of the vacuum chamber 7 is not damaged. However, in order to ensure that the diameter of the dip tube 7 is as large as 0.4 times the diameter of the ladle 9, a large amount of slag present at the top of the molten steel 8 accommodated in the ladle 9 is sucked into the dip tube 7. Thus, the refractory constituting the dip tube 7 is damaged by the slag sucked up as described above, and the life of the refractory is also reduced.

  As described above, according to any of the inventions disclosed in Patent Documents 1 to 5, the bottom of the vacuum tank by blowing the oxidizing gas from the top blowing lance without sacrificing the refining ability such as vacuum degassing. On the industrial scale while improving the life of the refractory by preventing damage to the refractory, suppressing slag uptake, and further suppressing thermal spall cracking due to the large dip tube opening. It was impossible to vacuum refine the molten steel.

  The purpose of the present invention is to prevent damage to the refractory at the bottom of the vacuum chamber by blowing oxidizing gas from the top blowing lance without sacrificing refining capability such as vacuum degassing, and suppress slag suction Furthermore, by suppressing the heat spall cracking caused by the large opening of the dip tube, the molten steel vacuum refining method can improve the life of the refractory while vacuum refining the molten steel on an industrial scale. And providing an apparatus.

  First, when the present inventors perform degassing treatment of molten steel by attaching a dip tube having a circular horizontal cross-section and the same diameter to the bottom of a vacuum tank having a circular horizontal cross-section, the inside of the vacuum tank In order to prevent the life of the refractory from deteriorating due to a large amount of slag being sucked into the ladle, first install the dip tube at the bottom of the ladle before dipping into the molten steel contained in the ladle. By blowing up the stirring gas from the tuyere into the molten steel, the molten steel on the bath surface located immediately above the tuyere was raised by the upward flow formed by the stirring gas, and the bare surface of the molten steel without slag appeared and appeared. We thought that slag inhalation could be suppressed by immersing the dip tube in the bare surface of the molten steel.

  However, the inner diameter of the vacuum tank used when processing molten steel of a class exceeding 250 tons, that is, the inner diameter of the dip tube is about 2 m, and the diameter is increased by the stirring gas blown from the tuyere provided at the bottom of the ladle. However, it is difficult to form a bare surface of molten steel having a size of about 2 m. Further, in this method, since the slag is suspended in the molten steel by the gas agitation performed to form the bare surface of the molten steel, even if it is possible to form the bare surface of the molten steel of a desired size, There is a high possibility that the turbid slag is sucked into the vacuum chamber together with the molten steel.

  As described above, it is assumed that a dip tube with a circular horizontal cross-sectional shape of the same diameter is attached to the lower part of a vacuum tank with a circular horizontal cross-sectional shape, and degassing of molten steel is performed. It is virtually difficult to do.

  Next, in order to suppress both the suction of the slag into the vacuum chamber and the peeling due to the temperature change of the refractory constituting the inner wall of the dip tube, the inventors have made a horizontal cross section inside the lower end of the dip tube. Considering that it would be effective to reduce the area, we examined the installation of a dip tube having a smaller diameter than that of the vacuum chamber, although it was also circular at the bottom of the circular vacuum chamber. This means that since the horizontal cross-sectional area inside the refractory at the lower end of the dip tube becomes smaller, the smaller the dip tube diameter, the more the slag sucked up from the dip tube immersed in the molten steel. It is possible to reduce, and the refractory constituting the inner wall of the dip tube can be surely suppressed.

  However, since this means reduces the inner diameter of the dip tube having a circular horizontal cross-sectional shape, the existence region of the upflow and the downflow flowing inside the dip tube is inevitably narrowed. For this reason, the upflow and downflow of the molten steel inside the dip tube interfere with each other, and a part of the downflow does not reach the ladle and is directly accompanied by the upflow and led to the vacuum chamber. End up. For this reason, the momentum of the downward flow in the ladle is reduced, and the circulation of the molten steel between the ladle and the vacuum chamber becomes insufficient. For this reason, a degassing speed | rate deteriorates and the component adjustment of molten steel becomes difficult. In particular, when the ratio D / Do between the diameter D of the dip tube and the diameter Do of the ladle becomes less than 0.5, this tendency becomes remarkable.

  Thus, it is certainly superior in terms of securing the life of the refractory by suppressing slag sucking up, by performing vacuum degassing treatment by installing a dip tube having a smaller diameter than this vacuum tank at the bottom of the circular vacuum tank, Since deterioration of refining performance cannot be denied, it cannot be applied to actual operations.

  Therefore, the present inventors are not premised on the conventional technical common sense about the vacuum chamber and the dip tube, but are keenly aware of the need to create a new technical idea that drastically overturns this technical common sense, Furthermore, earnest examination was repeated.

  As a result, as shown in FIG. 1 to be described later, at the lower part of the vacuum tank having a circular horizontal cross-sectional shape, the inner surface of at least the lower end opening of one dip tube is a region where the circulating flow exists in the lower end opening. A dip tube having a non-circular horizontal cross-sectional shape substantially along the outer edge shape is placed in communication with the vacuum chamber, and the molten steel is kept from narrowing the existence region of the upflow and the downflow flowing inside the dip tube. If vacuum refining is performed, damage to the refractory at the bottom of the vacuum tank caused by blowing oxidizing gas from the top blowing lance can be prevented without degrading the refining performance, and slag suction can be suppressed. In addition, it is possible to create a new and important technical idea that heat spall cracking due to the large opening of the dip tube can be suppressed, and that the molten steel can be vacuum refined on an industrial scale while improving the life of the refractory, Main departure It was completed.

  According to the present invention, a single dip tube provided in communication with the lower part of a vacuum tank having a circular horizontal cross-sectional shape is immersed in molten steel accommodated in a ladle disposed below the single dip tube. When a vacuum smelting of molten steel is performed by blowing a stirring gas into the molten steel inside the single dip tube to create a circulating flow that rises and falls inside the single dip tube. The molten steel vacuum refining method is characterized in that at least an inner surface of a lower end opening of a pipe has a non-circular horizontal cross-sectional shape substantially along an outer edge shape of a region where a circulating flow exists in the lower end opening.

  In the vacuum refining method for molten steel according to the present invention, it is desirable that the non-circular horizontal cross-sectional shape is an oval shape obtained by flattening a circular shape in one direction. In this case, the oval shape forms a long diameter portion that forms a long diameter in one direction, and at least one short diameter that is shorter than the long diameter and that extends in at least one direction that intersects the one direction. A shape having a short diameter part is desirable.

Further, in the molten steel vacuum refining method according to the present invention, it is desirable that the flow rate of the stirring gas is 3 NL / min or more and 15 NL / min or less per ton of the processed molten steel.
Further, in these methods of vacuum refining of molten steel according to the present invention, a vertical distance H between the lower end of a lance arranged to be movable up and down in the vacuum chamber and the surface of the molten steel existing in the vacuum chamber. Then, while raising and lowering this lance so that the ratio (H / S) of the short diameter of the dip tube to 2 or more and 8 or less, the oxidizing gas is transferred from this lance to the molten steel contained in the vacuum chamber. It is desirable to spray.

  Furthermore, in these vacuum steel refining methods according to the present invention, the vertical distance H between the lower end of the lance arranged to be movable up and down in the vacuum chamber and the surface of the molten steel existing in the vacuum chamber. Then, while raising and lowering this lance so that the ratio (H / V) to the length V of the inner diameter of the vacuum chamber is 3 or less, oxidizing gas is sprayed from this lance onto the molten steel contained in the vacuum chamber. Is desirable.

  From another point of view, the present invention relates to a single dip pipe that internally generates a circulating flow of molten steel that repeatedly rises and descends, and is arranged in communication with the upper part of the single dip pipe so as to be removed from the circulating flow. A molten steel vacuum smelting apparatus comprising a vacuum tank for performing gas treatment and a stirring gas introducing device for generating a circulating flow by blowing a stirring gas into the molten steel inside the single dip tube. The molten steel vacuum refining apparatus is characterized in that at least an inner surface of a lower end opening of a pipe has a non-circular horizontal cross-sectional shape substantially along an outer edge shape of a region where a circulating flow exists in the lower end opening.

  In the vacuum refining apparatus for molten steel according to the present invention, it is desirable that the non-circular horizontal cross-sectional shape is an oval shape obtained by flattening a circular shape in one direction. In this case, the oval shape forms a long diameter portion that forms a long diameter in one direction, and at least one short diameter that is shorter than the long diameter and that extends in at least one direction that intersects the one direction. A shape having a short diameter portion is desirable.

Further, in the vacuum refining apparatus for molten steel according to the present invention, it is desirable that the ratio (L / S) of the length L of the major axis to the length S of the minor axis is 1.5 or more.
Further, in the molten steel vacuum refining apparatus according to the present invention, it is desirable that the outer edge shape of the lower end opening of the dip tube is surrounded by the outer edge shape of the vacuum chamber in the horizontal section.

Moreover, in these molten steel vacuum refining apparatuses according to the present invention, it is desirable that the stirring gas introducing apparatus is provided in the long diameter portion.
Furthermore, the vacuum refining apparatus for molten steel according to the present invention includes a lance that is disposed so as to be movable up and down inside the vacuum chamber and sprays an oxidizing gas onto the molten steel existing inside the vacuum chamber. desirable.

  According to the method and apparatus for vacuum refining of molten steel according to the present invention, (i) immersion by slag sucked up in order to prevent a large amount of slag present at the top of the molten steel contained in the ladle from being sucked up from the dip tube It is possible to prevent the refractory constituting the pipe from being damaged early, and (ii) it is possible to suppress spall cracking and peeling due to cracks generated in the refractory constituting the inner wall of the dip pipe after the degassing treatment is completed. Further, it is possible to prevent the life of the refractory from being reduced, and (iii) it can be reliably carried out at a low equipment cost.

  Thus, according to the method and apparatus for vacuum refining of molten steel according to the present invention, without sacrificing refining ability such as vacuum degassing, the bottom of the vacuum tank by blowing oxidizing gas from the top blowing lance. On the industrial scale while improving the life of the refractory by preventing damage to the refractory, suppressing slag uptake, and further suppressing thermal spall cracking due to the large dip tube opening. Molten steel can be vacuum refined.

  Further, according to the present invention, by introducing an oxidizing gas into the molten steel accommodated in the vacuum chamber, the reaction efficiency between the oxidizing gas and each element in the molten steel is improved without deteriorating the refractory wear rate. be able to.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the best mode for carrying out a method and apparatus for vacuum refining molten steel according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a perspective view showing the structure of a molten steel vacuum refining apparatus 11 according to the present embodiment in a simplified state with a partial perspective. 2A is a vertical sectional view of the vacuum refining apparatus 11 shown in FIG. 1, and FIG. 2B is a sectional view taken along line AA in FIG.

  As shown in FIGS. 1 and 2, the vacuum refining device 11 of the present embodiment includes a vacuum chamber 12, a dip tube 13, a stirring gas introducing device 14, and a lance 17. Therefore, the vacuum tank 12, the dip tube 13, the stirring gas introducing device 14, and the lance 17 which are components of the vacuum refining device 11 will be sequentially described below.

[Vacuum chamber 12, lance 17]
The vacuum chamber 12 used in the present embodiment is a cylindrical body having a circular horizontal cross-sectional shape, and a refractory brick is mounted on the inner wall. The inner diameter of the vacuum chamber 12 is V.

In the present embodiment, the vacuum chamber 12 itself may be a conventional one, and such a vacuum chamber is well known to those skilled in the art, and thus the description of the vacuum chamber 12 is omitted.
A sealing device 18 is attached to the center of the upper surface 12 a of the vacuum chamber 12, and a lance 17 is provided through the sealing device 18. The lance 17 is provided so as to be movable up and down with respect to the vacuum chamber 12 by a lifting mechanism (not shown). In this Embodiment, the lance 17 is arrange | positioned in order to spray oxidizing gas on the molten steel which exists in the inside of the vacuum chamber 12, and this raises the efficiency of the decarburization process and heat processing which are mentioned later. is there.

As the lance 17 itself, a conventional one may be used. Since such a lance is well known to those skilled in the art, a description of the lance 17 is omitted.
The vacuum chamber 12 and the lance 17 of the present embodiment are configured as described above.

[Immersion tube 13]
One dip tube 13 is attached to the bottom of the vacuum chamber 12. In the present embodiment, the dip tube 13 has a circular shape (indicated by a two-dot chain line in FIG. 1) that is a horizontal cross-sectional shape of the vacuum chamber 12, and at least one flat direction (a direction indicated by a double arrow in the illustrated example). It has an oval horizontal cross-sectional shape obtained by flattening and is connected to the vacuum chamber 12.

  As shown in the enlarged view in FIG. 1, in this embodiment, the dip tube 13 of the vacuum refining device 11 has a pair of semicircular shapes that form a major axis with a length L in the flat direction in a horizontal section. The length of the long diameter portions 13a and 13a is shorter than the length L of the long diameter and at least one direction intersecting the flat direction (in the illustrated example, one direction intersecting the flat direction by 90 °) is S. It has linear short diameter portions 13b and 13b forming a short diameter.

In the present embodiment, it is desirable that the ratio (L / S) between the length L of the major axis and the length S of the minor axis is 1.5 or more. Hereinafter, the reason will be described.
The present inventors use a water model to determine the ratio of the major axis length L to the minor axis length S within a range where the major axis length L of the dip tube 13 does not exceed the inner diameter V of the vacuum chamber 12 ( L / S) was varied and the degassing rate was measured. At this time, the degassing rate in a dip tube (ratio (L / S) = 1) having a circular horizontal cross-sectional shape was set to 1.0, and the degassing rate index was obtained using the relative value. The ratio of the inner diameter D of the dip tube to the inner diameter Do of the ladle when the ratio (L / S) = 1 was 0.45. The resulting degassing rate index is shown graphically in FIG.

  As shown in the graph of FIG. 4, when the ratio (L / S) is less than 1.5, the length L of the major axis of the dip tube 13 is too short, so the upward flow and the downward flow of the molten steel 16 inside the dip tube 13. The circulating flow 20 (shown by the white arrow in FIGS. 1 and 2A) is likely to interfere, and the momentum of the downward flow is reduced to mix the molten steel 16 inside the ladle 15 and the ladle 15 Since the mixing of the molten steel 16 between the vacuum chamber 12 and the vacuum chamber 12 becomes insufficient, the degassing speed is reduced, so that the molten steel inside the vacuum chamber 12 is moved from the lance 17 disposed inside the vacuum chamber 12 as described later. When oxidizing gas is blown into 16, the Al reaction efficiency also decreases.

For the above reason, in this embodiment, the ratio (L / S) of the length L of the major axis to the length S of the minor axis is set to 1.5 or more.
When the ratio (L / S) exceeds 5.0, separation of the upward flow and downward flow of the molten steel 16 inside the dip tube 13 is good, but the short diameter length S of the dip tube 13 is short. Therefore, the basin width of the downward flow is also reduced, the molten steel 16 accommodated in the ladle 15 cannot be sufficiently mixed, the degassing speed is reduced, and the lance 17 disposed in the vacuum chamber 12 to the vacuum chamber 12. When oxidizing gas is blown into the molten steel 16 inside, the Al reaction efficiency is also lowered.

From such a viewpoint, the preferable range of the ratio (L / S) is 1.85 or more and 5.0 or less, and more preferably 2.0 or more and 4.5 or less from the same viewpoint.
Furthermore, in this vacuum refining apparatus 11, as shown in the enlarged view in FIG. 1, the outer edge shape of the dip tube 13 in the horizontal section is surrounded by the outer edge shape of the vacuum chamber 12 in the horizontal section. Thereby, since the horizontal cross-sectional area of the lower end opening part of the dip tube 13 is configured to be smaller than the horizontal cross-sectional area of the lower end opening part of the vacuum chamber 12, in other words, the lower end opening part of the dip pipe 13 that sucks molten steel 16. The area of the slag can be reliably reduced to reduce the amount of slag sucked up.

  As clearly shown in the enlarged view in FIG. 1 and FIG. 2 (b), the shape of the inner surface of the lower end opening of the dip tube 13 of the present embodiment is different from the circular shape in the known dip tube, and this lower end opening. 1 has an oval shape substantially along the outer edge shape of the region where the circulating flow 20 of the molten steel 16 exists (the portion not hatched in the enlarged view of FIG. 1), that is, a non-circular horizontal cross-sectional shape. For this reason, the inner surface of the dip tube 13 of the present embodiment has a non-circular horizontal cross-sectional shape substantially including the outer edge shape of the region where the circulating flow 20 exists, including the inner surface of the lower end opening.

That is, conceptually describing the dip tube 13 of the present embodiment, the circulating flow 20 of the molten steel 16 inside from a dip tube having a known circular horizontal cross-sectional shape indicated by a two-dot chain line in FIG. 1 and its enlarged view. The dead spaces D ₁ and D ₂ (hatched areas shown in FIG. 1 and its enlarged view) that are not interfering with each other are compressed into a noncircular dip tube 13 having an oval horizontal cross-sectional shape. is there.

  The “oval shape” of the dip tube 13 of the present embodiment means a shape obtained by flattening a circle in at least one direction. More specifically, the shape of the molten steel 16 inside the dip tube 13 is described. By having an inner surface shape that substantially conforms to the outer edge shape of the region where the circulating flow 20 exists, it means a shape that does not hinder the formation of the circulating flow 20. The horizontal cross-sectional shape of the inner surface of the dip tube 13 may be such a shape, and such shapes can be considered innumerable, but these are regions where the circulating flow 20 of the molten steel 16 exists in the dip tube 13. As long as it has a shape that does not impede the formation of the circulating flow 20 by having an inner surface shape that substantially conforms to the outer edge shape of the present invention, all are included in the present invention.

  FIG. 3A and FIG. 3B are explanatory diagrams showing an example of “non-circular horizontal inner surface shape” of the dip tube 13 of the present embodiment. The dip tube 13 of the present embodiment has, for example, an oval shape in which a rectangle as shown in FIGS. 2A and 3B is sandwiched between two semicircles from both sides, as shown in FIG. Examples thereof include an elliptical shape as shown, and a horizontal cross-sectional shape such as a combination of these as appropriate. These shapes are merely examples, and it goes without saying that other horizontal cross-sectional shapes may be used.

  The configuration other than that described above for the dip tube 13 may be the same as that conventionally used as this type of dip tube, and it is considered that no special explanation is required for those skilled in the art. Description of is omitted.

The dip tube 13 of the present embodiment is configured as described above.
[Stirring gas introduction device 14]
Further, in this vacuum refining device 11, a stirring gas introduction device 14 is provided on one long diameter portion 13 a of the dip tube 13. By this stirring gas introducing device 14, the stirring gas or the circulating gas G is blown into the immersing tube 13 to form a circulating flow 20 of the molten steel 16. The blowing position of the stirring gas G is not limited to the dip tube 13 as in the present embodiment, and may be a position where the circulating flow 20 of the molten steel 16 can be formed, and is not limited to a specific position. . For example, you may make it blow from the inside of the molten steel 16 accommodated in the ladle 15 or the bottom part of the ladle 15.

  However, when the stirring gas G is blown from the bottom of the ladle 15, bubbles are formed in the molten steel 16 accommodated in the ladle 15, and some of the bubbles are dispersed in the horizontal direction when rising due to buoyancy. Depending on the conditions of 15 and molten steel 16, the bubbles that rise while being dispersed in the horizontal direction may not be able to fully enter the dip tube 13. As a result, the bath surface oscillates on the surface of the molten steel accommodated in the ladle 15, slag particles or molten iron particles scatter, or in the worst case, slag or molten iron overflows from the ladle 15 and operates. Sometimes it is necessary to interrupt.

  Moreover, when stirring gas is blown in from the bottom part of the ladle 15, the ladle 15 which accommodated the molten steel 16 melt | dissolved in the converter leaves the molten steel 16 in contact with the gas blowing tuyere of the bottom part of the ladle 15 And it is conveyed to the position where the vacuum refining process is performed without blowing gas, and the stirring gas is blown for the first time after the gas pipe is connected here. At this time, even if the gas pipe is connected and pressure is applied, the molten steel 16 inserted into the tuyere and solidified adheres to the tuyere and closes the tuyere, and the stirring gas can be introduced into the molten steel 16. Sometimes it becomes impossible to do so. In such a case, the molten steel in the ladle 15 is processed in a backup process that deviates from the originally assembled process without being subjected to vacuum processing. For this reason, as a matter of course, the planned casting is changed, the production process is forced to be reassembled, and a serious problem arises in actual operation that production hindrance occurs.

  For the above reasons, in the present embodiment, the splash or bath is not caused by worrying about the tuyere opening ratio of the ladle 15 and the blown gas G is detached from the surface of the ladle 15 outside the dip tube 13. In order to operate without worrying about surface fluctuations or the like, a stirring gas introducing device 14 was provided below the dip tube 13. As a result, if the blowing gas G is blown from the dip tube 13, all of the blown bubbles rise inside the dip tube 13 and are released from the bath surface of the molten steel 16 inside the vacuum chamber 12 to the vacuum atmosphere. It is possible to reliably prevent the bath surface from oscillating on the surface of the molten steel 16 accommodated in the pan 15 and the scattering of slag particles and molten iron particles.

  As shown in FIG. 2A, when the stirring gas G is blown from the dip tube 13, it is blown so as to have a dense distribution in the major axis direction, rather than being blown uniformly around the entire circumference of the dip tube 13. Specifically, it is desirable that the amount of gas introduced into the left portion of the dip tube 13 in FIG. 2A is larger than the amount of gas introduced into the right portion or vice versa. As a matter of course, the stirring gas may be introduced only from the left part (or conversely from the right part). As a result, the gas can be introduced so that the bubble density on the left side portion is larger than that on the right side portion or vice versa, so that as shown by the arrow in FIG. A large molten steel flow loop can be formed inside in the major axis direction (left-right direction in FIG. 2A). Therefore, an active molten steel circulating flow 20 can be formed between the vacuum chamber 12 and the ladle 15.

  Here, when the vacuum degassing process is performed while blowing the blowing gas G as in the present embodiment, there is an optimum gas flow rate range from the viewpoint of the degassing reaction on the surface of the bubbles of the blowing gas G. FIG. 5 is a graph showing an example of the influence of the stirring gas flow rate on the above-described degassing rate index.

  From the graph shown in FIG. 5, when the stirring gas flow rate is less than 3 NL / min per ton of the processed molten steel, the degassing rate index becomes small. This is considered not only because the circulating flow 20 of the molten steel 16 is weakened and the Al reaction efficiency, which will be described later, is lowered, but also because the number of gas bubbles that are degassing reaction sites is reduced. If the stirring gas flow rate is less than 3 NL / min, the absolute value of the circulating speed of the molten steel 16 becomes small, and the Al reaction efficiency may be reduced. For this reason, it is desirable that the stirring gas flow rate is 3 NL / min or more per ton of the molten steel to be treated.

  On the other hand, even if the stirring gas flow rate exceeds 15 NL / min per ton of the processed molten steel, the effect of improving the Al reaction efficiency is saturated and the degassing speed is not improved. The reason why the dehydrogenation rate does not improve even if the stirring gas flow rate is increased more than 15 NL / min is that the bubble diameter increases due to the coalescence of bubbles and the total reaction interface area does not increase effectively. On the other hand, when the flow rate of the stirring gas is increased excessively, when the stirring gas bubbles break off on the surface of the molten steel 16, the molten iron particles are scattered from the surface of the molten steel 16 and adhere to the lance 17, so-called metal phenomenon. Occurs, the lance 17 is easily damaged, or a load is applied to the vacuum system. When the bare metal adheres to the nozzle at the tip of the lance 17, the substantial nozzle shape changes. Further, when high-temperature molten iron particles adhere, the tip of the lance 17 is thermally deformed. Due to these phenomena, the jet characteristics inherent to the lance 17 cannot be exhibited. Therefore, it is desirable that the reflux gas flow rate be 15 NL / min or less per ton of the processed molten steel.

The vacuum refining apparatus 11 according to the present invention is configured as described above. Next, the situation where the molten steel is refined using the vacuum refining apparatus 11 will be described.
In the present embodiment, at the lower part of the vacuum chamber 12 having a circular horizontal cross-sectional shape, at least the inner surface of the lower end opening is a non-circular horizontal that is substantially along the outer edge shape of the region where the circulating flow 20 exists in the lower end opening. One dip tube 13 having a cross-sectional shape is attached, the dip tube 13 is immersed in molten steel 16 accommodated in a ladle 15, and the stirring gas is immersed from the stirring gas introduction device 14 attached to the lower portion of the dip tube 13. The molten steel 16 is vacuum smelted while forming a circulating flow 20 of the molten steel 16 that rises and falls inside the dip tube 13 by blowing into the molten steel 16 inside the tube 13.

In this embodiment, in order to suck up the molten steel 16 to the vacuum chamber 12 using the dip tube 13 having an oval horizontal cross-sectional shape in this way, the slag existing above the molten steel 16 accommodated in the ladle 15 is used. Is compared with the case where a conventional dip tube having a circular horizontal cross-sectional shape is used, the dead spaces D ₁ and D which are the hatched regions in FIG. 1 and the enlarged view thereof are compared. It can be reduced by the amount flowing in from ₂ .

  Moreover, in this Embodiment, although the short diameter parts 13b and 13b are provided in the dip tube 13, and the horizontal cross-sectional area of the dip tube 13 is set smaller than the horizontal cross-sectional area of the vacuum chamber 12, the long diameter is set in the dip tube 13. Since the portions 13a and 13a are provided, the circulating flow 20 of the molten steel 16 that rises and falls inside the dip tube 13 is provided in the long diameter direction, that is, the flat shape of the dip tube 13 by providing a stirring gas blowing device 14 in the long diameter portion 13a. By making it substantially coincide with the direction, the formation of the circulating flow 20 can be ensured with almost no hindrance. For this reason, it is possible to prevent the upflow and downflow of the molten steel 16 inside the dip tube 13 from interfering with each other, and the upflow and downflow are reliably separated to sufficiently secure the refining ability such as vacuum degassing. be able to.

  In the present embodiment, the vertical distance H between the lower end of the lance 17 that can be moved up and down inside the vacuum chamber 12 and the surface of the molten steel 16 accommodated in the vacuum chamber 12, and the dip tube 13, while raising and lowering the lance 17 so that the ratio (H / S) to the minor axis S is 2 or more and 8 or less, an oxidizing gas such as oxygen is blown from the lance 17 to the molten steel 16 accommodated in the vacuum chamber 12. It is desirable to perform vacuum refining while attaching. The reason for this will be explained.

For one purpose of refining while blowing an oxidizing gas such as oxygen on the surface of the molten steel 16 in a vacuum environment, the carbon in the molten steel 16 reacts with the oxidizing gas to obtain C + (1/2) O ₂ = There is a so-called decarburization process in which carbon in the molten steel 16 is removed by a reaction called CO. When performing this decarburization treatment, it is important for refining to promote the reaction between the oxidizing gas to be sprayed and the carbon in the molten steel 16, that is, to improve the decarburization efficiency. This is because if the decarburization efficiency is low, the decarburization processing time becomes long, leading to a decrease in productivity and an increase in processing cost.

For another purpose of refining by blowing an oxidizing gas such as oxygen to the surface of the molten steel 16 in a vacuum environment, the Al in the molten steel 16 reacts with the oxidizing gas to produce 2Al + (3/2) O. There is a so-called heat treatment in which the temperature of the molten steel 16 is raised by an exothermic reaction of ₂ = Al ₂ O ₃ . When this heat treatment is performed, it is important to enhance the Al reaction efficiency by promoting the reaction between the oxidizing gas to be sprayed and Al in the molten steel 16. When the Al reaction efficiency is low, it reacts with the oxidizing gas sprayed by Mn, Fe, or the like in the molten steel 16 to produce a so-called lower oxide such as MnO or FeO. In order to increase the oxygen potential of the ladle slag, these lower oxides react slowly with Al in the molten steel 16 during or after casting to form Al ₂ O ₃ as inclusions. Therefore, the cleanliness of the steel is reduced. Therefore, it is necessary to increase the Al reaction efficiency.

  On the other hand, when the oxidizing gas is sprayed on the surface of the molten steel 16 in a vacuum environment, if the distance between the oxidizing gas jet and the inner wall of the refractory in the vacuum chamber 12 is too short, damage to the inner wall of the refractory becomes significant. It has been known. For this reason, it is necessary to make the horizontal cross-sectional shape of the vacuum chamber 12 into a circular shape or a shape close to a circular shape rather than a shape in which the distance between the oxidizing gas jet and the inner wall of the refractory is partially reduced like an ellipse.

  In the present embodiment, as described above, the dip tube 13 having a non-circular horizontal cross-sectional shape in which at least the inner surface of the lower end opening substantially conforms to the outer edge shape of the region where the circulating flow 20 exists in the lower end opening is used. Therefore, as an effect of using such a dip tube 13, not only can the amount of slag sucked up be reduced, but a flow loop is easily formed in the major axis direction inside the dip tube 13. And the circulating flow 20 of the molten steel 16 in each of the ladle 15 becomes active, and this improves both the decarburization efficiency or the Al reaction efficiency described above.

  In the present embodiment, as a method for supplying an oxidizing gas such as oxygen to the molten steel 16 inside the vacuum chamber 12, the oxidizing gas is injected into the molten steel 16 from the immersion tuyere on the side surface of the vacuum chamber 12. Method, a method in which oxidizing gas is sprayed onto the surface of the molten steel 16 from the non-immersed tuyere of the side wall of the vacuum chamber 12, and the oxidizing gas is further melted from a nozzle mounted on a lance disposed inside the vacuum chamber 12 so as to be movable up and down. What is necessary is just to employ | adopt the method etc. which spray on the surface of 16 suitably.

  However, the method of spraying the oxidizing gas onto the surface of the molten steel 16 from the nozzle mounted on the lance that can be moved up and down inside the vacuum chamber 12 has the following advantages as compared with other methods.

  That is, when the oxidizing gas is supplied to the molten steel 16 from the side wall tuyere of the vacuum chamber 12, the tuyere is in contact with the molten steel 16 or exists in the very vicinity of the molten steel 16, so that it is not necessary to supply the oxidizing gas. In order to maintain the integrity of the tuyere, it is necessary to flow a large amount of purge gas. When a large amount of purge gas is flowed, it becomes difficult to obtain a high vacuum state necessary for vacuum refining, so that the vacuum refining performance decreases. In addition, as is noticeable in the case of the immersion tuyere, a phenomenon called splash in which the purge gas blows up into the molten steel 16 inside the vacuum chamber 12 and blows up molten iron particles when the bubbles burst on the surface of the molten steel 16 is generated. And the molten iron particles adhere to and grow on the side wall of the vacuum chamber 12, so-called bulging occurs, which causes an operational problem. There are similar problems with non-immersed tuyere, albeit to varying degrees.

  On the other hand, in the method in which the lance 17 that can be moved up and down is arranged inside the vacuum chamber 12 and the oxidizing gas is sprayed, if it is not necessary to spray the oxidizing gas, the lance 17 is lifted upward to generate the minimum necessary purge gas. Since it may be introduced, the above-described problem can be avoided.

  Further, the immersing tube 13 used in the present embodiment is attached to the lower portion of the vacuum chamber 12 having a circular horizontal cross-sectional shape, and the oxidizing gas is supplied from the lance 17 disposed in the vacuum chamber 12 so as to be movable up and down. Since there is no example in the past of spraying the molten steel, the condition of the lance 17 was examined.

First, the effect on the Al reaction efficiency was investigated by variously changing the ratio (H / S) between the height H of the lance 17 and the minor axis S of the dip tube 13. The results are shown graphically in FIG.
As shown in the graph of FIG. 6, when the value of the ratio (H / S) is as small as 2 or less, the height of the lance 17 is small, and the surface of the molten steel 16 is in a state where the oxidizing gas jet has a relatively high dynamic pressure. The reactivity with Al in the molten steel 16 is increased. In this case, since the length S of the minor axis is relatively large, when the upward flow of the molten steel 16 reaching the vacuum chamber 12 shifts to a horizontal flow on the bath surface of the vacuum chamber 12, it is directly below the oxidizing gas jet. Since the molten steel flow 20 supplied to the steel increases, the reactivity between the oxidizing gas jet and Al in the molten steel 16 increases.

  However, as the ratio (H / S) increases, the height of the lance 17 increases and the jet dynamic pressure reaches the surface of the molten steel 16 with a relatively low state, or the minor axis becomes relatively small. Since the supply of the molten steel flow 20 immediately below the jet is lowered, the reactivity between the oxidizing gas and Al in the molten steel 16 is lowered, and when the ratio (H / S) exceeds 8, the reactivity is rapidly lowered. .

  From the above, it can be seen that it is effective to operate at a ratio (H / S) of 8 or less in order to ensure Al reaction efficiency. Further, if the ratio (H / S) is reduced too much, the height of the lance 17 is lowered, a splash due to the upper blowing jet is generated, and the amount of metal attached to the inside of the vacuum chamber 12 is increased. The deformation of the lance 17 occurs due to the bulging of the metal and the radiant heat from the surface of the molten steel 16. Therefore, it is effective to secure a ratio (H / S) of 2 or more.

  At the same time, the relationship between the life index of the refractory at the bottom of the vacuum chamber 12 and the ratio (H / S) was examined. The wear rate of the refractory at the bottom of the vacuum chamber 12 was measured, the calculated life was calculated by dividing the brick thickness at the time of brickworking by the wear rate, and calculated as the bath refractory life index. The tank bottom refractory life index is a relative value when the calculated life at a ratio (H / S) = 2.0 is 1.0. The results are shown graphically in FIG.

  When the value of the ratio (H / S) is as small as about 1, for example, the height H of the lance 17 is low and the horizontal spread of the oxidizing gas jet is small, or the short diameter S of the dip tube 13 is large and wears. Since the existence area of the bottom of the vacuum chamber 12 that may be used is narrow, the life index of the refractory at the bottom of the chamber is maintained at a high level.

  However, when the value of the ratio (H / S) is increased, the height H of the lance 17 is high and the horizontal spread of the oxidizing gas jet is large, or the short diameter S of the dip tube 13 is small and the vacuum chamber 12 The tank bottom area is wide. Accordingly, as the ratio (H / S) increases, the life index of the refractory at the bottom of the tank decreases, and when the value of the ratio (H / S) exceeds 8, it rapidly decreases.

  From the above, operating so that the value of the ratio (H / S) is 2 or more and 8 or less maintains the reaction efficiency of the oxidizing gas at a high level and ensures the lifetime of the bottom refractory of the vacuum chamber 12. It is effective for.

  Further, in the present embodiment, the vertical distance (H) between the lower end of the lance 17 that is disposed so as to be movable up and down inside the vacuum chamber 12 and the surface of the molten steel 16 inside the vacuum chamber 12, and the vacuum While raising and lowering the lance so that the ratio (H / V) to the inner diameter (V) of the tank 12 is 3 or less, it is desirable to spray oxidizing gas from the lance onto the molten steel 16 accommodated in the vacuum tank 12. Hereinafter, the reason will be described.

  As described above, the ratio (H / V) is dominant with respect to the reaction efficiency of the refining performed by blowing the oxidizing gas and the life of the refractory at the bottom of the vacuum chamber 12, but depending on certain conditions, the vacuum It was found that the wear of the refractory on the side wall of the tank 12 was significant. The refractory on the side wall of the vacuum chamber 12 determines the life of the vacuum chamber 12 and is an important factor that affects the productivity and the refractory cost.

  The oxidizing gas blown up causes an exothermic reaction with the molten steel 16 and forms a high temperature region on the surface of the molten steel 16. The stirring gas, the leak gas to the vacuum chamber 12, or the gas produced by the reaction, or the molten steel 16 and the unreacted top blown oxidizing gas are basically discharged from the exhaust system above the vacuum chamber 12, A part of the gas is entrained in the upper blowing gas jet and is blown to the high temperature region on the surface of the molten steel 16 together with the oxidizing gas jet. When these entrained gases pass through the high temperature region, the gas temperature rises, and a part of them is again entrained in the oxidizing gas jet.

  As described above, when the oxidizing gas jet is blown upward from the lance 17, a high temperature gas region is formed in the lower region of the lance 17 of the vacuum chamber 12, and the formed high temperature gas region is formed of the refractory on the side wall of the vacuum chamber 12. Presumed to have a negative effect on lifespan.

  Therefore, the conditions under which these high-temperature gas regions do not adversely affect as much as possible were examined using an index (H / R). H is the height of the lower end of the lance 17 from the surface of the molten steel 16, and R is the distance from the center axis of the lance to the inner wall of the vacuum chamber 12. Using this index (H / R), the life index of the side wall refractory of the vacuum chamber 12 was determined. The wear rate of the side wall refractory in the vacuum chamber 12 was measured, and the calculated life was calculated by dividing the brick thickness at the time of brickwork by the wear rate. The index is a relative value when the calculated life when the ratio (H / V) is 4 is 1.0. V indicates the inner diameter of the vacuum chamber 12. The results are shown graphically in FIG.

  As shown in the graph of FIG. 8, when the value of the ratio (H / V) is as small as about 1.0, for example, the height H of the lance 17 is small and the amount of surrounding ambient gas is small, or the vacuum chamber The ratio of the gas to be exhausted with a large inner diameter V of 12 is high. For this reason, the temperature of the gas in contact with the inner wall of the vacuum chamber 12 is relatively low, and the lifetime of the side wall refractory is maintained at a relatively high level.

  On the other hand, when the value of the ratio (H / V) increases, the height H of the lance 17 is large and the amount of surrounding atmospheric gas is large, or the inner diameter V of the vacuum chamber 12 is small and the exhausted gas The rate is low. Therefore, the temperature of the gas in contact with the inner wall of the vacuum chamber 12 is relatively high, and the lifetime of the side wall refractory is also relatively low. And when the value of ratio (H / V) exceeds 3, the lifetime of a side wall refractory will fall remarkably.

For this reason, in order to ensure the lifetime of the side wall refractory of the vacuum chamber 12, it is desirable to operate so that the value of the ratio (H / V) is 3 or less.
According to this embodiment, since the above-described dip tube 13 in which the area of the lower end opening is reduced is used, the horizontal sectional area of the lower end portion of the dip tube 13 is necessarily smaller than that of the vacuum chamber 12, and The amount of slag that is present in the upper part of the molten steel 16 accommodated in the pan 15 can be reduced in the dip tube 13. For this reason, it is possible to suppress an increase in the wear rate of the refractory, to extend the life of the refractory, to suppress an increase in the repair cost of the refractory, and to extend the repair cycle of the refractory, thereby degassing the vacuum. The actual operation time rate of the equipment can be improved and the productivity can be improved.

  Further, according to the present embodiment, since the above-described dip tube 13 in which the area of the lower end opening is reduced is used, the temperature of the refractory constituting the inner wall of the dip tube after the degassing process is rapidly increased. Reduction can be suppressed. Therefore, the heat history (temperature difference) applied to the refractory when contacting the high-temperature molten steel at the time of sucking in the next heat is reduced, and the temperature change and the temperature gradient are reduced. For this reason, the expansion and contraction of the refractory accompanying the temperature change are reduced, and it becomes difficult to cause cracking of the refractory and to cause spall cracking and peeling due to the progress of the crack, so that the life of the refractory can be extended. Therefore, since the repair cost of the refractory can be suppressed and the repair cycle of the refractory can be extended, the actual operation time rate of the vacuum degassing apparatus can be improved and the productivity can be improved.

  Moreover, since this embodiment can be performed simply by manufacturing the dip tube 13 whose shape has been changed from a cylindrical shape to an oval shape, it can be performed easily and is extremely practical. is there.

  Thus, according to the present embodiment, damage to the refractory at the bottom of the vacuum chamber 12 caused by blowing oxidizing gas from the lance 17 can be prevented without sacrificing refining capability such as vacuum degassing. Further, by suppressing the suction of slag, and further suppressing the thermal spall cracking caused by the large opening at the lower end of the dip tube 13, the life of the refractory and the degassing rate are both improved. Vacuum refining can be performed.

  Moreover, according to this Embodiment, by introduce | transducing oxidizing gas into the molten steel 16 accommodated in the vacuum chamber 12, oxidizing gas and each element in the molten steel 16 are not deteriorated without deteriorating the refractory wear rate. Reaction efficiency can be improved.

Further, the present invention will be specifically described with reference to examples.
240 tons of molten steel blown in a converter are put into a ladle 15 shown in FIG. 1, and the dip tube 13 (long diameter L / short diameter S = 1.8 m, 2.6 m) and dip tube shown in FIG. Vacuum smelting of the molten steel was performed using a vacuum smelting apparatus 11 comprising a vacuum tank 12 provided with a lower part 13.

Above the vacuum chamber 12, an alloy addition port for adding an alloy and a vacuum exhaust duct (both not shown) are provided, and the vacuum exhaust duct is connected to a predetermined vacuum exhaust device. In both the inventive examples and the comparative examples, the stirring gas (or reflux gas) flow rate was 2.0 Nm ³ / min (8.3 Nl / min · ton).

9A and 9B show horizontal cross sections of the lower end openings of the dip tube 13 of the present invention and the dip tube of the comparative example, respectively.
In both the inventive example and the comparative example, twelve stirring gas introducing tuyere were provided on the side surface of the arc portion on the left side of the drawing at a position 0.3 m from the lower end of the dip tube. The tuyere introduction opening angle θ of the left arc portion is a semicircular shape opened 180 degrees in both the present invention example and the comparative example, and the tuyere angle is 16 degrees. The tuyere used a single tube made of stainless steel.

In the comparative example, the ratio D / D ₀ between the inner diameter D of the dip tube and the inner diameter D _{0 of the} ladle is 0.41, and the ratio D / D ₀ is 0.6. The two types were tested. A test using a conventional RH vacuum degassing RH was designated as Comparative Example B.

Under the above premise, in Example 1, the molten steel 16 blown to a carbon concentration of 0.04% by mass in the converter was put into the ladle 15 and subjected to vacuum dehydrogenation treatment. In both the inventive examples and comparative examples A and B, the flow rate of stirring gas (or reflux gas) was 2.0 Nm ³ / min (8.3 Nl / min · ton), and the dehydrogenation rate was measured. The life of the refractory was calculated by dividing the thickness of the refractory in the vacuum chamber 12 by the refractory wear rate, and was expressed as a relative value with the refractory wear rate in Comparative Example B being 1.0. The test results are summarized in Table 1.

  From Table 1, it can be seen that a dehydrogenation rate higher than that of Comparative Examples A and B can be obtained by setting the dip tube 13 to the shape of the present invention. Moreover, it turns out that a dehydrogenation rate can further be improved by making ratio (L / S) into the optimal range.

  Moreover, according to the example of this invention, it turns out that a refractory life index | exponent can also be improved with respect to any of the comparative examples A and B. This is considered to be because the amount of ladle slag brought into the vacuum chamber 12 was reduced by replacing the dip tube of Comparative Example A with the dip tube of the present invention example, and refractory wear due to slag was reduced. The reason why the refractory life index increased compared to Comparative Examples A and B is considered to be that the discharge of slag into the ladle 15 was promoted by using the dip tube 13 of the present invention method.

Under the premise described above, the molten steel 16 blown to a carbon concentration of 0.04% by mass in the converter was taken out into the ladle 15 and vacuum decarburized. In both the inventive example and the comparative example, the flow rate of stirring gas (or reflux gas) was 2.0 Nm ³ / min (8.3 Nl / min · ton), and the decarburization time until the carbon concentration was less than 20 ppm was measured. . The test results are summarized in Table 2.

  From Table 2, it can be seen that the decarburization time can be shortened for both Comparative Examples A and B by making the dip tube 12 the shape of the present invention.

  Under the above-mentioned assumption, the ratio of the major axis length to the minor axis length of the dip tube 13 of the present invention is set to 2.6, and the stirring gas (or reflux gas) flow rate is changed to affect the dehydrogenation rate. The impact was investigated. The test results are summarized in Table 3.

  In Table 3, when the gas flow rate is lowered too much, the dehydrogenation rate is lowered because the molten steel circulation rate is lowered and the bubble generation number itself is reduced and the reaction interface area is lowered. The reason why the dehydrogenation rate does not improve even if the gas flow rate is increased excessively is that the bubble diameter increases due to the coalescence of bubbles and the total reaction interface area does not increase effectively.

In the present embodiment, oxygen gas was sprayed onto the bath surface of the molten steel 16 inside the vacuum chamber 12 from the nozzle at the tip of the lance 17 disposed inside the vacuum chamber 12 under the above-described premise.
In Comparative Example A, the ratio D / D ₀ = 0.42 between the inner diameter D of the dip tube and the ladle inner diameter D ₀ , and stirring was performed from 12 tuyere provided in the range of an opening angle of 180 degrees on the inner wall of the dip tube. Gas was flushed.
The test results are summarized in Table 4.

  As shown in Table 4, it can be seen that the Al reaction efficiency is improved with respect to any of Comparative Examples A and B by making the dip tube 13 into the shape of the present invention.

  It can also be seen that the Al reaction efficiency gradually decreases as the ratio (H / S) value increases, and rapidly decreases when the ratio (H / S) value exceeds 8. This is because as the ratio (H / S) increases, the height H of the lance 17 increases and the jet impinging on the surface of the molten steel 16 weakens.

  Next, the tank bottom refractory life index was compared. The amount of refractory wear at the bottom of the vacuum chamber 12 was divided by the number of charges used, and the relative value with the ratio (H / S) being 2.0 and the bottom refractory life being 1.0 was shown.

It turns out that the example of this invention improves a tank bottom refractory life index | exponent with respect to the comparative example A. FIG. In Comparative Example B, since there is no vacuum tank bottom, the tank bottom refractory life index is blank.
It can be seen that the life of the tank bottom refractory gradually decreases as the ratio (H / S) increases, and rapidly decreases when the ratio (H / S) exceeds 8. This is because the height of the lance 17 is increased and the horizontal spread of the jet is increased, which adversely affects the vacuum chamber bottom region.

The test similar to Example 4 was done and the lifetime of the side wall refractory of the vacuum chamber 12 was investigated. The side wall refractory life index was calculated by dividing the amount of refractory wear on the side wall of the vacuum chamber 12 by the number of charges used, and the relative value assuming that the bottom refractory life at H / V = 4 was 1.0.
Table 5 shows the test results.

  It can be seen that as the ratio value (H / V) increases, the side wall life index gradually decreases, and when (H / V) exceeds 3, it rapidly decreases.

It is a perspective view which shows the structure of the vacuum refining apparatus of the molten steel in embodiment in the state simplified by seeing through a part. 2A is a vertical sectional view of the vacuum refining apparatus shown in FIG. 1, and FIG. 2B is a sectional view taken along line AA in FIG. FIG. 3A and FIG. 3B are explanatory diagrams showing an example of “non-circular horizontal inner surface shape” of the dip tube of the present embodiment. It is a graph which shows the relationship between ratio (L / S) and a degassing rate index | exponent. It is a graph which shows an example of the influence which the stirring gas flow volume has on a degassing speed index | exponent. It is a graph which shows the relationship between ratio (H / S) and Al reaction efficiency. It is a graph which shows the relationship between ratio (H / S) and a tank bottom refractory life index. It is a graph which shows the relationship between ratio (H / V) and a vacuum chamber side wall refractory life index. FIG. 9A and FIG. 9B are explanatory views showing horizontal sections of lower end openings of the dip tube of the present invention example and the dip tube of the comparative example in Example 1. FIG. It is explanatory drawing which shows the principle of RH degassing method typically. It is explanatory drawing which shows typically the condition which sprays oxidizing gas on the surface of the molten steel which exists in the inside of a vacuum chamber in RH degassing method. It is explanatory drawing which shows the invention described in patent document 4 typically. It is explanatory drawing which shows the invention described in patent document 5 typically.

Claims (13)

A single dip tube provided in communication with the lower part of the vacuum tank having a circular horizontal cross-sectional shape is immersed in a molten steel housed in a ladle disposed below the single dip tube, and stirring gas is supplied. When performing vacuum refining of the molten steel by blowing into the molten steel inside the single dip tube and generating a circulating flow that rises and falls inside the single dip tube,
The inner surface of at least the lower end opening of the one dip tube has a noncircular horizontal cross-sectional shape substantially along the outer edge shape of the region where the circulating flow exists in the lower end opening. Refining method.   The method for vacuum refining molten steel according to claim 1, wherein the non-circular horizontal cross-sectional shape is an oval shape obtained by flattening a circular shape in one direction.   The ellipse shape has a long diameter portion that forms a long diameter in one direction, and a short diameter that is shorter than the long diameter and forms at least one short diameter in at least one direction intersecting the one direction. The method for vacuum refining molten steel according to claim 2, wherein the method has a shape having a portion.   The method for vacuum refining of molten steel according to any one of claims 1 to 3, wherein a flow rate of the stirring gas is 3 NL / min or more and 15 NL / min or less per 1 ton of the processed molten steel.   The vertical distance (H) between the lower end of the lance that can be moved up and down inside the vacuum chamber and the surface of the molten steel existing inside the vacuum chamber, and the length of the minor axis of the dip tube ( The oxidizing gas is sprayed from the lance to the molten steel accommodated in the vacuum tank while raising and lowering the lance so that the ratio (H / S) to S) is 2 or more and 8 or less. 4. The method for vacuum refining molten steel described in 4.   The distance (H) in the vertical direction between the lower end of the lance that can be moved up and down in the vacuum chamber and the surface of the molten steel existing in the vacuum chamber, and the length (V) of the inner diameter of the vacuum chamber The oxidizing gas is sprayed from the lance to the molten steel accommodated in the vacuum tank while raising and lowering the lance so that the ratio (H / V) to 3) or less The method for vacuum refining molten steel as described in any one of the above items. A single dip tube that internally generates a circulating flow of molten steel that repeatedly rises and descends, a vacuum tank that is disposed in communication with the upper portion of the single dip tube and performs degassing treatment on the circulating flow, and stirring A molten steel vacuum smelting apparatus comprising a stirring gas introducing device that generates the circulation flow by blowing gas into the molten steel inside the one dip tube,
The inner surface of at least the lower end opening of the one dip tube has a noncircular horizontal cross-sectional shape substantially along the outer edge shape of the region where the circulating flow exists in the lower end opening. Refining equipment.   8. The molten steel vacuum refining apparatus according to claim 7, wherein the non-circular horizontal cross-sectional shape is an oval shape obtained by flattening a circular shape in one direction.   The ellipse shape has a long diameter portion that forms a long diameter in one direction, and a short diameter that is shorter than the long diameter and forms at least one short diameter in at least one direction intersecting the one direction. The apparatus for vacuum refining molten steel according to claim 8, having a shape having a portion.   The molten steel vacuum refining apparatus according to claim 9, wherein a ratio (L / S) of the length (L) of the major axis to the length (S) of the minor axis is 1.5 or more.   11. The molten steel vacuum smelting apparatus according to claim 7, wherein an outer edge shape of the lower end opening of the dip tube is surrounded by an outer edge shape of the vacuum tank in a horizontal section.   The molten steel vacuum refining device according to any one of claims 9 to 11, wherein the stirring gas introducing device is provided in the long diameter portion.   Furthermore, it is arrange | positioned so that raising / lowering is possible inside the said vacuum chamber, The lance for spraying oxidizing gas on the molten steel which exists in the inside of this vacuum chamber is provided in any one of Claim 7-12. The vacuum refining equipment for molten steel described.

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