TWI882305B - Immersion nozzle for continuous casting and continuous steel casting method - Google Patents

Abstract

The present invention provides an immersion nozzle and a method for manufacturing steel that can improve the quality of castings. The bottomed cylindrical continuous casting immersion nozzle has two or more pairs of discharge holes that are axially symmetrical with respect to the axis of the nozzle at the portion where the molten steel in the continuous casting mold is immersed. The inner diameter of the lower part of the molten steel flow path in the straight body is the same as the inner diameter of the upper part or a reduced diameter. The opening area of the upper discharge hole is set to S3, and the opening area of the lower discharge hole is set to S4. Then, the ratio of the inner cross-sectional area S1 of the upper straight body to the single-side total opening area of the discharge hole is between 0.30 and 0.5. The ratio of the inner cross-sectional area S2 of the lower straight body to the single-side total opening area of the above-mentioned discharge hole is in the range of 0.10 to 0.40, and satisfies the relationship of 0.20≦(S2/S4)≦(S1/S3)≦1.0. The discharge angle of the discharge hole is based on the horizontal direction, with upward as positive, and is in the range of +20° to -50°. The discharge angle of the discharge hole on the lower side is based on the discharge angle of the discharge hole on the upper side, and is downward in the range of 20° to 55°.

Description

Immersion nozzle for continuous casting and continuous steel casting method

The present invention relates to an immersion nozzle for injecting molten steel into a casting mold during continuous casting of molten steel and a continuous casting method of steel using the immersion nozzle. More specifically, the present invention relates to an immersion nozzle for continuous casting and a continuous casting method of steel that can suppress both the inert gas bubbles blown into the immersion nozzle from being captured in a solidified shell and the casting mold powder from being carried into the solidified shell.

The following two factors can be cited as the criteria for judging the quality of steel castings produced by continuous casting machines. The first is that the amount of inert gas bubbles such as argon blown into the molten steel flowing down the immersion nozzle is captured in the casting. The second is that the amount of mold powder added to the curved liquid surface (molten steel liquid surface in the mold) is captured in the casting. Since the inert gas bubbles and mold powder captured by the casting will become surface defects of steel products, it is very important to reduce this phenomenon.

When a continuous casting machine is used to cast steel billets, generally speaking, the immersion nozzle used is an immersion nozzle having discharge holes facing the short sides of the left and right molds. Therefore, the discharge flow of molten steel discharged from the discharge hole of the immersion nozzle collides with the solidified shell on the short side of the mold (solidified shell on the short side of the casting sheet), and after the collision, it branches up and down. One of the branched flows is the flow toward the bottom of the mold (hereinafter referred to as "branched descending flow"), and the other becomes the flow toward the curved liquid surface at the top (hereinafter referred to as "branched ascending flow").

Among them, the divergent upflow toward the curved liquid surface forms an upflow along the short side solidified shell of the steel billet casting (hereinafter referred to as "short side upflow"). Moreover, the short side upflow has a greater impact on the flow rate of the molten steel in the curved liquid surface. That is, the faster the divergent upflow is, the faster the short side upflow is. As the flow rate of the short side upflow increases, the flow rate of the molten steel in the curved liquid surface becomes faster. As a result, the frequency of the casting powder on the curved liquid surface being sandwiched to the solidified shell increases. That is, in order to reduce the defects of the casting powder, how to slow down the divergent upflow after the collision with the solidified shell of the short side of the casting becomes a problem.

On the other hand, the branched downflow toward the bottom of the casting mold forms a downflow along the short side solidified shell of the steel billet casting (hereinafter referred to as "short side downflow"). Moreover, the short side downflow can reach the depth of the unsolidified layer. In this case, the faster the branched downflow is, the faster the short side downflow is, and the penetration depth of the short side downflow into the unsolidified layer increases.

In order to prevent the aluminum oxide from adhering to the inner wall of the immersion nozzle, the inert gas such as argon blown into the molten steel flowing down the immersion nozzle becomes bubbles and is discharged from the discharge hole of the immersion nozzle into the molten steel in the casting mold. A part of the inert gas bubbles discharged into the molten steel in the casting mold infiltrates to the bottom of the casting mold together with the branched downflow. The faster the branched downflow is, the faster the short side downflow is. As a result, the penetration depth of the inert gas bubbles into the unsolidified layer becomes deeper, and the amount of inert gas bubbles captured in the casting increases. That is, in order to reduce the amount of inert gas bubbles captured in the casting, how to decelerate the branched downflow after the short side solidified shell of the casting hits becomes a problem.

Several countermeasures have been proposed to address these problems. One of the countermeasures is a technique for continuous casting using an immersion nozzle having a plurality of discharge holes above and below the left and right sides of the immersion portion of the immersion nozzle in the molten steel.

For example, Patent Document 1 discloses an immersion nozzle for continuous casting, wherein the immersion portion of the immersion nozzle in molten steel has a plurality of upper and lower discharge holes formed in such a manner that the opening area of the discharge hole arranged on the lower side is smaller than the opening area of the discharge hole arranged on the upper side, and the inner diameter of the molten steel flow path inside the immersion nozzle is smaller in the range where the discharge holes are arranged than in the upper portion where no discharge holes are arranged.

According to Patent Document 1, the discharge flow from the discharge hole can be made slow and uniform, thereby weakening the short side downflow and reducing the amount of inert gas bubbles or non-metallic inclusions captured in the casting.

Patent document 2 discloses an immersion nozzle for continuous casting, which has four discharge holes, namely, a left upper section, a left lower section, a right upper section, and a right lower section, at the immersion portion of the immersion nozzle in molten steel. The discharge holes are formed in the following manner: the opening area of the discharge hole in the lower section is smaller than the opening area of the discharge hole in the upper section, and the ratio of the opening area of the discharge hole in the lower section to the total opening area of the discharge holes in the upper section and the lower section is greater than 0.2 and less than 0.4, and the discharge angle of the discharge hole in the lower section is more than 10° downward based on the discharge angle of the discharge hole in the upper section.

According to Patent Document 2, the flow rate of molten steel near the inner wall of the solidified shell side of the short side of the casting can be fully controlled, thereby preventing inert gas bubbles or non-metallic inclusions from flowing down the short side and penetrating into the deep part of the casting, thereby obtaining a casting with fewer internal defects.

Patent document 3 discloses an immersion nozzle, which has two or more pairs of discharge holes that are symmetrical with respect to the axis of the immersion nozzle at the part where the molten steel in the casting mold is immersed, and the opening area of the discharge hole located at the lower side is equal to or larger than the opening area of the discharge hole located at the upper side, and the discharge angle of each discharge hole is symmetrical with respect to the horizontal. The upper limit is 15° upward, and the lower limit is 50° relative to the horizontal. Among the two discharge holes located in the upper and lower positions of the lead, the discharge angle of the discharge hole located on the lower side is larger than the discharge angle of the discharge hole located on the upper side, and the difference between the discharge angle of the discharge hole located on the lower side and the discharge angle of the discharge hole located on the upper side is greater than 20° and less than 55°. [Prior technical literature] [Patent literature]

Patent document 1: Japanese Patent Publication No. 2006-198655 Patent document 2: International Publication No. 2010/109887 Patent document 3: Japanese Patent Publication No. 2019-63851

(Invent the problem you want to solve)

However, the above-mentioned prior art has the following problems. In the technology described in Patent Document 1, the preferred range of the discharge angle of the discharge hole is 10° upward to 45° downward. However, in the embodiment of Patent Document 1, the discharge angle of the discharge hole arranged on the lower side is the same as the discharge angle of the discharge hole arranged on the upper side. That is, there is no difference between the discharge angle of the discharge hole arranged on the lower side and the discharge angle of the discharge hole arranged on the upper side. Therefore, the discharge flows discharged from the upper and lower discharge holes merge with each other, and it is difficult to obtain an ideal attenuation effect of the discharge flow.

In the technology described in Patent Document 2, since the opening area of the lower discharge hole is smaller than the opening area of the upper discharge hole, and the difference between the discharge angle of the lower discharge hole and the discharge angle of the upper discharge hole is smaller, the flow rate of the molten steel in the curved liquid surface becomes faster, and the risk of powder trapping in the casting mold is higher.

Although the technology described in Patent Document 3 describes the angle of the discharge hole, there is no specific description of the relationship between the cross-sectional area of the straight body of the immersion nozzle and the discharge hole area. The detailed conditions are unclear, and it is difficult to imagine that it can achieve actual improvement effects.

The present invention is completed in view of the above situation, and its purpose is to provide an immersion nozzle for continuous casting, which can improve the quality of the casting when the molten steel is injected into the continuous casting mold during the continuous casting of steel. That is, its purpose is to stably suppress the inert gas bubbles such as argon blown into the molten steel flowing down in the immersion nozzle from being captured in the casting, and to stably suppress the casting mold powder added to the curved liquid surface from being captured in the casting. In addition, its purpose is to propose a continuous casting method for steel using the immersion nozzle. (Technical means for solving the problem)

The continuous casting immersion nozzle of the present invention for solving the above-mentioned problem is a bottomed cylindrical continuous casting immersion nozzle for injecting molten steel into a continuous casting mold; and its characteristics are: at the portion where the molten steel is immersed in the continuous casting mold, there are two or more pairs of discharge holes that are axially symmetrical with respect to the axis of the immersion nozzle, and the molten steel flow path in the straight body of the immersion nozzle is from the upper side of the discharge hole The range from the upper end to the bottom of the above-mentioned immersion nozzle has the same inner diameter as other parts or a diameter that is reduced compared to other parts, the single-side opening area of the upper discharge hole is set as S3, the single-side opening area of the lower discharge hole is set as S4, and the inner cross-sectional area (S1) of the straight body portion of the range from the upper end of the above-mentioned immersion nozzle to the upper end of the above-mentioned upper discharge hole is equal to the single-side total opening area of the above-mentioned discharge hole. The ratio of the inner cross-sectional area (S3+S4) of the straight body portion from the upper end of the upper discharge hole to the bottom of the immersion nozzle to the total opening area (S3+S4) on one side of the discharge hole is in the range of 0.10 to 0.40. The inner cross-sectional area (S1, S2) of the straight body portion of the immersion nozzle and the total opening area (S3+S4) on one side of the discharge hole are in the range of 0.30 to 0.50. The opening area (S3, S4) satisfies the relationship of 0.20≦(S2/S4)≦(S1/S3)≦1.0. The discharge angle of each discharge hole is based on the horizontal plane and upward as positive, and is in the range of +20° to -50°. The discharge angle of the discharge hole on the lower side of the lead vertical direction is based on the discharge angle of the discharge hole on the upper side of the lead vertical direction, and the lead vertical direction is downward in the range of 20° to 55°.

Furthermore, in the immersion nozzle for continuous casting of the present invention, the two discharge holes located in the upper and lower positions in the vertical direction of the lead are oriented in different directions in the horizontal plane, and at least one pair of the discharge holes is oriented in a direction parallel to the long side of the casting mold. This structure can become a better solution.

Furthermore, the continuous steel casting method of the present invention is characterized in that, using the above-mentioned immersion nozzle, casting powder is added to the surface of the molten steel in the continuous casting mold, and an inert gas is blown into the molten steel flowing down in the molten steel flow path of the above-mentioned immersion nozzle, and the molten steel in the intermediate ladle is injected into the above-mentioned casting mold through the above-mentioned immersion nozzle.

Furthermore, in the continuous steel casting method of the present invention, (a) a DC magnetic field generating device provided on the back side of the continuous casting mold applies a DC static magnetic field to the molten steel in the casting mold from the DC magnetic field generating device provided on the upper side of the discharge hole at the uppermost part of the lead vertical direction of the immersion nozzle and the lower side of the discharge hole at the lowermost part of the lead vertical direction, while (b) the DC static magnetic field is applied to the molten steel in the casting mold through the immersion nozzle. (a) injecting the molten steel in the tundish into the above-mentioned casting mold; (b) applying an AC moving magnetic field to the molten steel in the above-mentioned casting mold from an AC magnetic field generating device installed on the back of the above-mentioned casting mold, and injecting the molten steel in the tundish into the above-mentioned casting mold through the above-mentioned immersion nozzle; such a structure can become a better solution. (Compared with the effect of the previous technology)

By using the immersion nozzle of the present invention, the discharge flow rate of the molten steel discharged from the upper and lower discharge holes of the immersion nozzle can be appropriately maintained, and the discharge flows discharged from the upper and lower discharge holes will not merge and collide with the solidified shell of the short side of the casting. Thereby, the discharge flow from the discharge hole located on the upper side collides with the solidified shell of the short side of the casting, and the discharge flow from the discharge hole located on the lower side collides with the solidified shell of the short side of the casting. The respective flow rates are attenuated. As a result, the short-side downflow that affects the capture of inert gas bubbles in the casting is mainly composed of the branched downflow after the discharge flow discharged from the discharge hole on the lowermost side in the vertical direction of the lead collides with the solidified shell on the short side of the casting. On the other hand, the short-side upflow that affects the flow rate of the molten steel in the curved liquid surface that determines the casting powder banding is mainly composed of the branched upflow after the discharge flow discharged from the uppermost discharge hole collides with the solidified shell on the short side of the casting. In this way, both the short-side upflow and the short-side downflow can be decelerated, thereby achieving stable suppression of the casting powder banding and the capture of inert gas bubbles in the casting. Therefore, when the immersion nozzle of the present invention is used to inject molten steel into a casting mold for continuous casting during continuous steel casting, the quality of the casting sheet can be improved.

The following is a detailed description of the embodiments of the present invention. Furthermore, each figure is a schematic diagram, which may be different from the actual object. In addition, the following embodiments are exemplary devices or methods used to embody the technical concept of the present invention, and they are not necessarily configured as follows. That is, the technical concept of the present invention can be modified in various ways within the technical scope described in the scope of the patent application.

The present invention is described in detail below. The cross-section of a steel sheet is such that the sheet width is particularly large relative to the sheet thickness, and steel sheets of various sheet widths are required. Generally speaking, the sheet width/sheet thickness is mostly in the range of about 4 to 12. Therefore, in a continuous casting machine for manufacturing steel sheets, a continuous casting mold corresponding to the cross-sectional size of the steel sheet to be cast is used. In order to adjust the inner space of the rectangular casting mold, the continuous casting mold has a pair of opposite long sides and a pair of opposite short sides of the casting mold, and the short sides of the casting mold are configured to move inside the long sides of the casting mold.

As an immersion nozzle for injecting molten steel into the continuous casting mold, an immersion nozzle having one or more discharge holes opposite to a pair of opposite short sides of the mold is used. Molten steel is injected from each discharge hole toward each short side of the mold. Therefore, the discharge flow of the molten steel discharged from the discharge hole collides with the solidified shell on the short side of the mold, that is, the solidified shell on the short side of the casting, and diverges up and down after the collision. One of them becomes a flow toward the bottom of the mold, that is, a diverged descending flow. In addition, the other becomes a flow toward the curved liquid surface at the top, that is, a diverged ascending flow. The diverged ascending flow toward the curved liquid surface forms an ascending flow along the short side solidified shell of the steel billet casting, that is, a short side side ascending flow. The divergent downflow toward the bottom of the casting mold forms a downflow along the short side solidification shell of the steel billet casting, that is, the short side downflow.

The inventors of the present invention have studied an immersion nozzle that can decelerate both the short side downflow and the short side upflow in such a continuous steel billet casting machine. As a result, it is found that the immersion nozzle of the following shape is the best. That is, the immersion nozzle of the present embodiment is a bottomed cylindrical refractory that injects molten steel into a continuous casting mold. Moreover, at the portion where the molten steel is immersed in the continuous casting mold, there are two or more pairs of discharge holes that are symmetrical with respect to the axis of the immersion nozzle. The molten steel flow path in the straight body of the immersion nozzle has the same inner diameter as other parts or a diameter that is reduced compared to other parts in the range from the upper end of the upper discharge hole to the bottom of the immersion nozzle. In addition, the cross-sectional area S1 of the upper straight body of the immersion nozzle, the cross-sectional area S2 of the straight body with the discharge hole, the opening area S3 of the discharge hole arranged on the upper side, and the opening area S4 of the discharge hole arranged on the lower side satisfy the relationship of S1/(S3+S4)=0.30～0.50, S2/(S3+S4)=0.10～0.40. In addition, the relationship of 0.20≦(S2/S4)≦(S1/S3)≦1.0 is satisfied. Furthermore, regarding the discharge angle of each discharge hole, the discharge holes arranged up and down are located in the range of +20°～-50°. Moreover, the discharge angle of the discharge hole on the lower side of the lead vertical direction is based on the discharge angle of the discharge hole on the upper side of the lead vertical direction. When it is in the range of 20°～55°, the lead vertical direction is downward. Here, the so-called "discharge angle of the discharge hole" refers to the angle formed by the center axis of the discharge hole and the horizontal plane, with upward being positive.

The immersion nozzle of this embodiment has two or more pairs of discharge holes that are axially symmetrical with respect to the axis of the immersion nozzle at the part where the molten steel in the continuous casting mold is immersed. The reason is that by having two or more pairs of discharge holes that are axially symmetrical, the discharge flow discharged from the discharge hole is dispersed and the flow rate of the discharge flow is slowed down. As a result, the branched downflow and the branched upflow formed after the discharge flow collides with the solidified shell of the short side of the casting are decelerated.

Furthermore, the molten steel flow path in the straight body of the immersion nozzle has the same inner diameter as other parts or a diameter that is reduced compared to other parts in the range from the upper end of the upper discharge hole to the bottom of the immersion nozzle. In addition, the cross-sectional area S1 of the upper straight body of the immersion nozzle, the cross-sectional area S2 of the straight body with the discharge hole, the opening area S3 of the discharge hole arranged on the upper side, and the opening area S4 of the discharge hole arranged on the lower side satisfy the relationship of S1/(S3+S4)=0.30～0.50, S2/(S3+S4)=0.10～0.40. In addition, the relationship of 0.20≦(S2/S4)≦(S1/S3)≦1.0 is also satisfied. This can alleviate the phenomenon that the flow rate of the discharge flow discharged from the discharge hole on the lower side of the lead vertical direction increases due to the pressure difference. In addition, by distributing the discharge flow from the discharge holes arranged above and below, it can further suppress the short side downflow. When this condition is not met, there is a risk that the flow balance between the discharge holes will change, causing the aluminum oxide attached to the inner wall of the immersion nozzle to block the molten steel flow path.

In addition, regarding the discharge angle of each discharge hole, the discharge holes arranged up and down are located in the range of +20° to -50°. The reason is that if the discharge angle of each discharge hole is directed upwards compared to the horizontal upward angle of 20°, there is a risk that the discharge flow from the discharge hole set at the uppermost side in the lead vertical direction will not collide with the solidified shell of the short side of the casting, and will flow directly to the curved liquid surface without deceleration. On the other hand, the reason is that if the discharge angle of each discharge hole is directed downwards compared to the horizontal downward angle of 50°, there is a risk that the position where the discharge flow from the discharge hole set at the lowermost side in the lead vertical direction collides with the solidified shell of the short side of the casting will become a deeper position than the lower end of the casting mold, and the downward flow on the short side will not decelerate. Therefore, the discharge angle of the discharge hole is set within the range of +20° to -50°.

Furthermore, of the two discharge holes located in the vertical positional relationship of the lead, the discharge angle of the discharge hole located on the lower side is larger than the discharge angle of the discharge hole located on the upper side, and the difference between the discharge angle of the discharge hole located on the lower side and the discharge angle of the discharge hole located on the upper side is set to be greater than 20° and less than 55°. The reason for such setting is to prevent the discharge flow from the discharge hole located on the upper side from merging with the discharge flow from the discharge hole located on the lower side before colliding with the solidified shell of the short side of the casting. If the difference in the discharge angle is less than 20°, there is a risk that the discharge flows from the two discharge holes will merge. On the other hand, if the difference in discharge angle exceeds 55°, the discharge flow from the discharge hole located on the lower side may collide with the solidified shell of the short side of the casting below the lower end of the mold, and the penetration depth of the short side downflow may become deeper. Alternatively, the discharge flow from the discharge hole located on the upper side may collide with the solidified shell of the short side of the casting just below the curved liquid surface, and the short side upflow may become faster.

Furthermore, in the immersion nozzle of the present embodiment, it is preferred that the two discharge holes located in the vertical position relationship of the lead face different directions in the horizontal plane, and at least one pair of the discharge holes face a direction parallel to the long side of the casting mold. If the discharge flow from the upper discharge hole and the discharge flow from the lower discharge hole are in different directions in the horizontal plane, the discharge flows interfere with each other and the risk of merging is reduced, and the dispersion effect of the discharge flow is improved, which is preferred. The maximum angle of different directions in the horizontal plane is less than 90°. The reason is that if the discharge flow collides with the solidified shell on the long side of the casting mold, the thickness balance of the solidified shell is destroyed, which may have an adverse effect on the quality of the casting. In particular, the flow of molten steel in the mold can be properly controlled by orienting the outlet hole with a larger discharge flow rate among the two outlet holes located in a vertical position relationship with respect to the lead in a direction parallel to the long side surface of the mold.

FIG1 shows an immersion nozzle for continuous casting of an embodiment of the present invention. FIG1(a) is a longitudinal sectional view when cut at a plane including the central axis C of the immersion nozzle 1 and the central axis of the discharge hole 2, and FIG1(b) is a three-dimensional view observed from an oblique upper side opposite to the discharge hole 2. Furthermore, the immersion nozzle 1 of the embodiment shown in FIG1 is an immersion nozzle 1 having two pairs of discharge holes 2 axially symmetrical with respect to the axis C of the immersion nozzle 1 above and below the lead vertical direction of the portion immersed in the molten steel.

In FIG. 1 , symbol 1 is an immersion nozzle, 2 is a discharge hole, 3 is a discharge hole located on the upper side in the vertical direction of the lead (hereinafter referred to as “upper discharge hole”), 4 is a discharge hole located on the lower side in the vertical direction of the lead (hereinafter referred to as “lower discharge hole”), 5 is a steel liquid flow path provided inside the immersion nozzle 1, 6 is the bottom of the immersion nozzle 1, 7 is the upper end position of the immersion nozzle 1, 8 is the upper end position of the upper discharge hole 3, α is the discharge angle of the upper discharge hole 3, β is the discharge angle of the upper discharge hole 3, is the discharge angle of the lower discharge hole 4, S1 is the area of the molten steel flow path from the upper end position 7 of the immersion nozzle 1 to the upper end position 8 of the upper discharge hole 3 (the inner cross-sectional area of the straight body), S2 is the area of the molten steel flow path from the upper end position 8 of the upper discharge hole 3 to the bottom 6 of the immersion nozzle 1 (the inner cross-sectional area of the straight body), S3 is the single-side opening area of the upper discharge hole 3, and S4 is the single-side opening area of the lower discharge hole 4. In the immersion nozzle 1 shown in FIG. 1 , since the discharge holes 2 are arranged in two pairs in the vertical direction of the lead, the upper discharge hole 3 corresponds to the discharge hole 2 arranged at the uppermost side in the vertical direction of the lead, and the lower discharge hole 4 corresponds to the discharge hole 2 arranged at the lowermost side in the vertical direction of the lead. In the example of FIG. 1 , the upper discharge hole 3 and the lower discharge hole 4 are arranged in the same direction in the horizontal plane.

The immersion nozzle 1 of the present embodiment is configured as described above. Hereinafter, a continuous steel casting method using the immersion nozzle 1 of the present embodiment configured as described above will be described.

An immersion nozzle 1 is provided at the bottom of the tundish, and the tundish is provided above the continuous casting mold in such a manner that the immersion nozzle 1 is located approximately at the center of the casting mold space. Molten steel is injected into the tundish from a steel ladle containing molten steel in a refining furnace such as a converter, and molten steel is injected into the continuous casting mold from the tundish through the immersion nozzle 1. Mold powder is added to the curved liquid surface, i.e., the liquid surface of the molten steel in the casting mold, to coat the surface of the molten steel in the casting mold. Inert gas such as argon or nitrogen is blown into the molten steel flowing down the molten steel flow path 5 of the immersion nozzle 1 through a sliding nozzle or an upper nozzle.

FIG2 schematically shows the results of the investigation of the flow in the casting mold in the water model experiment simulating the flow of molten steel in the casting mold using the immersion nozzle 1 of the present embodiment having two pairs of discharge holes 2 arranged vertically in the lead direction. The flow of molten steel in the casting mold is described below based on FIG2. FIG2 shows only one half of the casting mold for continuous casting, and the other half also shows the same shape with the central axis C as the symmetry axis.

Symbol 9 in FIG. 2 denotes the short side of the mold, 10 denotes the liquid surface in the mold (equivalent to the curved liquid surface), 11 denotes the discharge flow from the upper discharge hole 3, 12 denotes the discharge flow from the lower discharge hole 4, 13 denotes the divergent upward flow formed by the divergence of the discharge flow 11 from the upper discharge hole 3, 14 denotes the divergent downward flow formed by the divergence of the discharge flow 11 from the upper discharge hole 3, 15 denotes the divergent upward flow formed by the divergence of the discharge flow 12 from the lower discharge hole 4, 16 denotes the divergent downward flow formed by the divergence of the discharge flow 12 from the lower discharge hole 4, 17 denotes the short side upward flow flowing along the short side 9 of the casting, 18 denotes the short side downward flow flowing along the short side 9 of the casting, and 19 denotes the curved liquid surface flow. In FIG. 2 , the same parts as those in FIG. 1 are indicated by the same symbols, and their description is omitted. In addition, the symbol 20 in FIG. 2 is a velocity meter sensor for measuring the flow rate of the curved liquid surface flow 19 in the water model experiment, and 21 is a velocity meter sensor for measuring the flow rate of the short side downflow 18. In FIG. 2 , the discharge flow 11 and the discharge flow 12 are shown as straight lines, but the discharge flow 11 and the discharge flow 12 actually change up and down as time passes, while flowing toward the short side 9 of the mold.

As shown in FIG. 2 , the discharge flow 11 discharged from the upper discharge hole 3 and the discharge flow 12 discharged from the lower discharge hole 4 do not merge in the range up to the collision with the mold short side 9, but collide with the mold short side 9. After colliding with the mold short side 9, the discharge flow 11 is branched into a branched upward flow 13 and a branched downward flow 14, and the discharge flow 12 is branched into a branched upward flow 15 and a branched downward flow 16. In the immersion nozzle 1 of this embodiment, the discharge flow is divided into two, the discharge flow 11 and the discharge flow 12. When the amount of molten steel injected into the casting mold is the same, that is, when the casting sheet drawing speed is the same, compared with the diverging upward flow and diverging downward flow when the discharge hole 2 is a pair (only one on one side), in this embodiment, the flow speed of each of the diverging upward flow 13, the diverging downward flow 14, the diverging upward flow 15, and the diverging downward flow 16 is slowed down. Furthermore, the diverging downward flow 14 and the diverging upward flow 15 are flows toward opposite sides, and the diverging downward flow 14 and the diverging upward flow 15 collide and interfere with each other and then decelerate.

As a result, the short side upflow 17 that affects the curved liquid surface flow 19 that determines the casting powder band is almost not affected by the branched upflow 15, but is mainly determined by the flow rate of the branched upflow 13. Since the speed of the branched upflow 13 is reduced compared to the speed of the branched upflow when the discharge holes 2 are a pair, the short side upflow 17 is decelerated, thereby reducing the flow rate of the curved liquid surface flow 19.

Similarly, the short side downflow 18 that affects the capture of gas bubbles in the casting is almost unaffected by the branch downflow 14, but is mainly determined by the flow rate of the branch downflow 16. Since the branch downflow 16 is slower than the branch downflow when the discharge holes 2 are a pair, the flow rate of the short side downflow 18 is also reduced.

That is, by continuously casting molten steel using the immersion nozzle 1 of this embodiment, the curved liquid surface flow 19 that affects the entrainment of the mold powder and the short side downflow 18 that affects the capture of gas bubbles in the casting are both decelerated. As a result, both the entrainment of the mold powder and the capture of inert gas bubbles in the casting can be stably suppressed.

In the above embodiment, one pair of discharge holes 2 are used on the upper side and one pair on the lower side. Alternatively, two or more pairs of discharge holes 2 may be used on either the upper or lower side at the same position in the lead vertical direction and discharge in different directions in the horizontal direction. In this case, the opening cross-sectional area of the discharge holes 2 is set to be the total opening area on one side relative to the central axis.

As described above, according to this embodiment, the short side upflow 17 that affects the flow rate of the molten steel in the curved liquid surface that determines the casting powder banding and the short side downflow 18 that affects the capture of the inert gas bubbles in the casting sheet can be decelerated, thereby achieving stable suppression of the casting powder banding and the capture of the inert gas bubbles in the casting sheet. [Example]

The present invention is constructed as described above. The implementation possibility and effect of the present invention are further described below by way of an embodiment. In an actual steel billet continuous casting machine, the impregnation nozzle 1 of the present embodiment is used to implement a continuous casting operation (invention example). The cross-sectional dimensions of the continuously cast steel billet casting sheet are 220 to 260 mm in thickness and 1000 to 2200 mm in width. Argon is used as the inert gas blown into the impregnation nozzle 1. On the surface of the molten steel in the casting mold, the optimal casting mold powder is added according to the casting sheet drawing speed and the type of steel. For comparison, an immersion nozzle having a pair of discharge holes on the left and right sides of the immersion nozzle and an immersion nozzle having two pairs of discharge holes on the left and right sides of the immersion nozzle, but the arrangement conditions of the two pairs of discharge holes are outside the scope of the present invention, were also used.

The continuous casting molds used are the following three types of continuous casting molds, namely: a mold without a magnetic field generating device; a mold with a DC magnetic field generating device, wherein the DC magnetic field generating device is on the back side of the continuous casting mold, on the upper side of the upper discharge hole and on the lower side of the lower discharge hole, respectively, in one section and two sections in total, and is arranged opposite to each other across the long side of the mold; a mold with an AC magnetic field generating device, wherein the AC magnetic field generating device is arranged opposite to each other across the long side of the mold on the back side of the continuous casting mold.

In the continuous casting operation using the continuous casting mold provided with the DC magnetic field generating device, the DC static magnetic field is applied to the molten steel in the mold from the upper and lower DC magnetic field generating devices, respectively, on the upper side of the upper discharge hole and on the lower side of the lower discharge hole. In addition, in the continuous casting operation using the continuous casting mold provided with the AC magnetic field generating device, the AC moving magnetic field is applied to the molten steel in the continuous casting mold from the AC magnetic field generating device, and the molten steel in the mold is continuously cast while swirling in the horizontal direction in the curved liquid surface.

The steel sheet cast by the continuous steel casting machine was hot rolled into a hot rolled steel plate, and the surface defects caused by arsenic bubbles and mold powder in the hot rolled steel plate were studied. Based on the surface defects, the arsenic bubbles and mold powder remaining in the casting were evaluated. That is, the lower the defect index of the steel product, the less arsenic bubbles and mold powder remaining in the casting.

Table 1 shows the conditions of the immersion nozzle, and Table 2 shows the operating conditions and operating results. By designing the immersion nozzle within the scope of the present invention, the defect mixing rate of the steel product can be reduced.

[Table 1] No. Extrusion Angle Area ratio Remarks α β S1/(S3＋S4) S2/(S3＋S4) S1/S3 S2/S4 ° ° - - - - 1 +10 -25 0.3 0.1 0.1 0.1 Invention Example 2 +5 -30 0.3 0.1 0.1 0.1 Invention Example 3 0 -35 0.3 0.1 0.1 0.1 Invention Example 4 +10 -25 0.4 0.2 0.3 0.2 Invention Example 5 +5 -30 0.4 0.2 0.3 0.2 Invention Example 6 0 -35 0.4 0.2 0.3 0.2 Invention Example 7 +10 -25 0.5 0.3 0.5 0.4 Invention Example 8 +5 -30 0.5 0.3 0.5 0.4 Invention Example 9 0 -35 0.5 0.3 0.5 0.4 Invention Example 10 +10 -25 0.5 0.1 0.7 0.5 Invention Example 11 +5 -30 0.5 0.1 0.7 0.5 Invention Example 12 0 -35 0.5 0.1 0.7 0.5 Invention Example 13 +10 -25 0.3 0.2 1.0 0.8 Invention Example 14 +5 -30 0.3 0.2 1.0 0.8 Invention Example 15 0 -35 0.3 0.2 1.0 0.8 Invention Example 16 +10 -25 0.4 0.3 0.3 0.1 Invention Example 17 +5 -30 0.4 0.3 0.3 0.1 Invention Example 18 0 -35 0.4 0.3 0.3 0.1 Invention Example 19 0 -15 0.3 0.2 0.1 0.1 Comparative example 20 0 -5 0.3 0.2 0.1 0.1 Comparative example twenty one +10 -50 0.3 0.2 0.1 0.1 Comparative example twenty two +10 -50 0.4 0.3 0.3 0.2 Comparative example twenty three - -15 - 0.4 - 0.4 Comparative example twenty four - -25 - 0.4 - 0.4 Comparative example 25 - -45 - 0.5 - 0.4 Comparative example 26 +10 -25 0.5 0.5 1 0.9 Comparative example 27 +5 -30 0.6 0.4 1.1 0.9 Comparative example 28 +10 -25 0.4 0.07 0.8 0.2 Comparative example 29 +5 -30 1.6 1.1 3.2 2.2 Comparative example 30 0 -35 0.4 0.5 0.8 1 Comparative example

[Table 2] No. Liquid steel flow rate DC electromagnetic field AC moving magnetic field Steel product defect index Remarks ton/min Is there any imposition Is there any imposition - 1 4.2 have without 0.25 Invention Example 2 4.2 have without 0.23 Invention Example 3 4.2 have without 0.21 Invention Example 4 5.4 have without 0.18 Invention Example 5 5.4 have without 0.15 Invention Example 6 5.4 have without 0.16 Invention Example 7 5.0 have without 0.19 Invention Example 8 5.0 have without 0.18 Invention Example 9 5.0 have without 0.13 Invention Example 10 4.2 without have 0.17 Invention Example 11 4.2 without have 0.12 Invention Example 12 4.2 without have 0.15 Invention Example 13 5.4 without have 0.14 Invention Example 14 5.4 without have 0.19 Invention Example 15 5.4 without have 0.20 Invention Example 16 5.0 without have 0.21 Invention Example 17 5.0 without have 0.23 Invention Example 18 5.0 without have 0.22 Invention Example 19 4.2 without without 0.32 Comparative example 20 4.2 without without 0.35 Comparative example twenty one 4.2 without without 0.37 Comparative example twenty two 4.2 without without 0.33 Comparative example twenty three 4.2 without without 0.35 Comparative example twenty four 4.2 without without 0.38 Comparative example 25 4.2 without without 0.40 Comparative example 26 5.0 without without 0.40 Comparative example 27 5.0 without without 0.40 Comparative example 28 4.2 without without 0.32 Comparative example 29 4.2 without without 0.37 Comparative example 30 4.2 without without 0.39 Comparative example

1: Immersion nozzle 2: Discharge hole 3: Upper discharge hole 4: Lower discharge hole 5: Liquid steel flow path 6: Bottom of immersion nozzle 7: Upper end position of immersion nozzle 8: Upper end position of upper discharge hole 9: Short side of casting mold 10: Liquid surface in casting mold 11: Discharge flow from upper discharge hole 12: Discharge flow from lower discharge hole 13: Branched upflow 14: Branched downflow 15: Branched upflow 16: Branched downflow 17: Short side upflow 18: Short side downflow 19: Curved liquid surface flow 20: Velocity meter sensor 21: Velocity meter sensor α, β: discharge angle S1: (the range from the upper end of the immersion nozzle to the upper end of the upper discharge hole) the inner cross-sectional area of the straight body S2: (the range from the upper end of the upper discharge hole to the bottom of the immersion nozzle) the inner cross-sectional area of the straight body S3: (the single side of the upper discharge hole) the opening area S4: (the single side of the lower discharge hole) the opening area C: center axis (axis center)

FIG. 1 (a) is a longitudinal sectional view of an immersion nozzle of one embodiment of the present invention, and (b) is a three-dimensional view observed from the upper part of the discharge direction. FIG. 2 is a diagram schematically showing the results of the investigation of the flow in the casting mold in the water model experiment simulating the flow of molten steel in the casting mold using the immersion nozzle of the above embodiment.

1: Dip nozzle

2: Discharge hole

3: Upper discharge hole

4: Lower side discharge hole

5: Liquid steel flow path

6: Soak the bottom of the nozzle

7: Upper end position of the immersion nozzle

8: Upper end position of the upper discharge hole

α,β: discharge angle

S1: (the area from the upper end of the immersion nozzle to the upper end of the upper discharge hole) the inner cross-sectional area of the straight body

S2: (the area from the upper end of the upper discharge hole to the bottom of the immersion nozzle) the inner cross-sectional area of the straight body

S3: (single side of the upper discharge hole) opening area

S4: (single side of the lower discharge hole) opening area

Claims (6)

An immersion nozzle for continuous casting, which is a bottomed cylindrical continuous casting nozzle for injecting molten steel into a casting mold for continuous casting, and has two or more pairs of discharge holes axially symmetrical with respect to the axis of the immersion nozzle at the portion where the molten steel is immersed in the above-mentioned casting mold, and the molten steel flow path in the straight body of the immersion nozzle has the same inner diameter as other portions or a diameter reduced compared to other portions in the range from the upper end of the upper discharge hole to the bottom of the above-mentioned immersion nozzle, The single-side opening area of the upper discharge hole is set as S3, and the single-side opening area of the lower discharge hole is set as S4. The ratio of the inner cross-sectional area (S1) of the straight body from the upper end of the above-mentioned immersion nozzle to the upper end of the above-mentioned upper discharge hole to the single-side total opening area (S3+S4) of the above-mentioned discharge hole is in the range of 0.30 to 0.50. The ratio of the inner cross-sectional area (S2) of the straight body from the upper end of the above-mentioned upper discharge hole to the bottom of the above-mentioned immersion nozzle to the single-side total opening area (S3+S4) of the above-mentioned discharge hole is in the range of 0.10 to 0.40. The cross-sectional area (S1, S2) of the straight body of the above-mentioned immersion nozzle and the single-side opening area (S3, S4) of the discharge hole satisfy the relationship of 0.20≦(S2/S4)≦(S1/S3)≦1.0. The discharge angle of the discharge hole is positive with the horizontal plane as the reference, and the discharge angle of the discharge hole located at the uppermost side of the lead vertical direction is less than +20°, and the discharge angle of the discharge hole located at the lowermost side of the lead vertical direction is more than -50°. Among the two discharge holes located in the upper and lower positions of the lead vertical direction, the discharge angle of the discharge hole located at the lower side is larger than the discharge angle of the discharge hole located at the upper side. When the discharge angle of the discharge hole on the lower side of the lead vertical direction is within the range of 20° to 55° based on the discharge angle of the discharge hole on the upper side of the lead vertical direction, the lead vertical direction is downward. As for the immersion nozzle for continuous casting as claimed in claim 1, the two above-mentioned discharge holes located in the upper and lower positions in the vertical direction of the lead face different directions in the horizontal plane, at least one pair of the above-mentioned discharge holes face a direction parallel to the long side of the above-mentioned casting mold, and the other above-mentioned discharge holes do not face the long side of the casting mold. A continuous steel casting method, which uses an immersion nozzle for continuous casting as in claim 1 or 2, adds mold powder to the surface of molten steel in a continuous casting mold, blows inert gas into molten steel flowing down in a molten steel flow path of the immersion nozzle, and injects molten steel in a tundish into the casting mold through the immersion nozzle. The continuous casting method of steel as claimed in claim 3 is characterized in that a DC magnetic field generating device disposed on the back side of the continuous casting mold applies a DC static magnetic field to the molten steel in the mold from the upper side of the discharge hole at the uppermost part of the lead vertical direction of the immersion nozzle and the lower side of the discharge hole at the lowermost part of the lead vertical direction, and at the same time, the molten steel in the intermediate ladle is injected into the mold through the immersion nozzle. The continuous casting method of steel as claimed in claim 3 is characterized in that an AC moving magnetic field is applied to the molten steel in the above-mentioned casting mold from an AC magnetic field generating device installed on the back side of the above-mentioned continuous casting mold, while the molten steel in the casting mold at the curved liquid surface position is swirled in the horizontal direction, and the molten steel in the intermediate ladle is injected into the above-mentioned casting mold through the above-mentioned immersion nozzle. The continuous casting method of steel as claimed in claim 4 is characterized in that an AC moving magnetic field is applied to the molten steel in the above-mentioned casting mold from an AC magnetic field generating device installed on the back side of the above-mentioned continuous casting mold, while the molten steel in the casting mold at the curved liquid surface position is swirled in the horizontal direction, and the molten steel in the intermediate ladle is injected into the above-mentioned casting mold through the above-mentioned immersion nozzle.