JP5239554B2 - Immersion nozzle for continuous casting of slabs - Google Patents

Description

The present invention relates to an immersion nozzle for supplying molten metal into a mold when continuously casting a molten metal such as steel to a slab .

  In continuous casting of molten metal including molten steel, molten steel is transferred from a ladle to a tundish, which is an intermediate container that receives molten steel, and injected into a mold through an immersion nozzle disposed at the bottom of the tundish. The molten steel injected into the mold is cooled from the part in contact with the mold to start solidification to form a solidified shell, which is sequentially drawn out into the roll band below the mold and solidification proceeds, and finally all solidification is completed. It becomes a slab.

  The immersion nozzle that injects molten steel from the tundish into the mold has a bottomed cylindrical shape, and the lower part of the side wall is immersed in order to pour the molten steel in parallel along the mold width direction, that is, the long side direction of the mold. Two discharge holes symmetrical with respect to the cylindrical axis of the nozzle are provided. In an immersion nozzle having two discharge holes (hereinafter sometimes referred to as a two-hole nozzle), the two discharge holes are arranged in parallel to the mold width direction so as to face the mold short side.

  The molten steel poured into the mold from the tundish through the two-hole nozzle collides with the solidified shell located on the short side of the mold as a discharge flow, and branches into an upward flow and a downward flow. This upward flow promotes the floating removal of inclusions and bubbles within an appropriate range, but if it is too large, it causes fluctuations in the molten metal surface and entrainment of molten powder, which adversely affects operability and slab quality. Further, since the downward flow contributes to the intrusion of inclusions and bubbles, the smaller one is preferable. The flow rates of the upward flow and the downward flow increase as the discharge flow rate of the two-hole nozzle increases, and the flow rate is also affected by the discharge angle of the two-hole nozzle. The shape of the two-hole nozzle and the flow in the mold formed by the two-hole nozzle have the following problems. That is, when the discharge angle of the two-hole nozzle is downward (that is, the depression angle with respect to the horizontal direction), the descending flow velocity increases, so that inclusions and bubbles accompanying the descending flow penetrate deeply and rise up. Instead, it is trapped by the solidified shell, resulting in inclusion defects and bubble defects, leading to deterioration in slab quality. On the other hand, when the discharge angle is upward (that is, the elevation angle direction with respect to the horizontal direction), the rising flow velocity increases, so the molten metal surface is vigorously shaken and the molten powder is caught in the molten steel and solidified. It is trapped by the shell and becomes a powder-type defect, resulting in deterioration of slab quality. When the discharge angle is horizontal, the molten steel discharge flow collides with the solidified shell on the short side without attenuation, so that the solidified shell may be remelted and breakout may occur.

  As long as the casting speed is reduced and the discharge flow rate is suppressed, the possibility of the above-described problem occurring is low. However, when an attempt is made to increase the production volume, the casting speed must be increased, and the discharge flow rate is increased accordingly, resulting in the above-described problems relating to the slab quality. As a technique for solving this problem, an immersion nozzle described in Patent Document 1 in which a slit connected to a discharge hole in parallel with a mold width direction is provided at the bottom of a two-hole nozzle (hereinafter sometimes referred to as a dispersion nozzle). It has been known. Dispersion nozzles can inject molten steel into the mold through the discharge holes and slits, allowing the molten steel to be discharged in all directions of the mold width. The descending flow velocity is smaller than that of the two-hole nozzle, and the depth of inclusion penetration is reduced. Thus, inclusion defects are reduced. In addition, the dispersion nozzle enables the immersion nozzle to be blocked and the powder to be smoothly melted by uniformly discharging molten steel.

JP 61-14051 A

In order to provide a slab of a high quality level while meeting the demand for further increase in production, it is necessary to further reduce the discharge flow rate of the dispersion nozzle. However, in the dispersion nozzle described in Patent Document 1, when the discharge angle is downward, the descending flow rate increases as the casting speed increases, which only deteriorates the slab quality.
Therefore, it is conceivable to further reduce the discharge flow rate of the dispersion nozzle by extending the direction of the opening to the entire mold direction with the discharge angle of the dispersion nozzle facing upward. However, in the dispersion nozzle described in Patent Document 1, simply by setting the discharge angle upward, the discharge flow is separated from the upper end wall of the discharge hole of the dispersion nozzle, and the discharge flow does not flow in the direction of the discharge angle. It has been found that it is very difficult to discharge molten steel uniformly in all directions of the opening.
Therefore, in the dispersion nozzle described in Patent Document 1, since the discharge flow is not uniformly discharged in all directions of the opening portion of the immersion nozzle even when the discharge angle is upward, the discharge flow rate cannot be reduced, and the productivity is improved and the casting is performed. It is not enough to achieve both quality improvements.

  The present invention relates to an immersion nozzle having two discharge holes and a slit connecting the discharge holes, and discharges molten steel uniformly in all directions of the opening portion of the immersion nozzle to reduce the discharge flow rate, thereby including inclusion defects and bubbles. An object of the present invention is to provide an immersion nozzle for continuous casting that can suppress the occurrence of defects and improve the quality of a slab and improve the productivity.

The gist of the present invention is as follows. That is, the present invention provides an immersion nozzle for injecting molten steel into a mold, a pair of discharge holes formed near the lower end of the immersion nozzle and opened symmetrically from the center line of the immersion nozzle toward the mold width direction, and the immersion nozzle An opening slit is formed at the bottom of the nozzle and connects the pair of discharge holes, and the discharge angle θ of the discharge holes is upward, that is, described as an elevation angle (hereinafter referred to as upward) with reference to the horizontal direction. ) SS is defined as the sum of horizontal projection areas on the inner end surfaces in the mold width direction of the opening portion that is in the range of 5 ° to 30 ° and is constituted by the pair of discharge holes and the opening slit. The vertical downward projection area of the hole slit is SB, and the projection area ratio SS / SB is
2.5 ≦ SS / SB ≦ 15
It is the immersion nozzle for continuous casting of the slab characterized by being in the range. The center line of the immersion nozzle refers to a center line extending in the vertical direction through the center of the immersion nozzle. “Opening symmetrically toward the mold width direction” means opening symmetrically in the same direction as the mold width direction.

Furthermore, the inner wall of the immersion nozzle between the upper end of the immersion nozzle and the upper end of the discharge hole has a cylindrical step whose diameter is smaller than the inner diameter of the immersion nozzle that narrows the flow path of the molten steel whose axial center coincides with the immersion nozzle.
1/2 of the difference between the inner diameter of the immersion nozzle and the nozzle inner diameter of the stepped portion is the step height H1 [mm], and the distance from the upper end of the discharge hole to the lower end of the stepped portion is the stepped position H2 [mm]. When the length in the cylindrical axis direction is the step length H3 [mm] and the vertical length of the inclined portion connecting the lower end of the step portion and the nozzle inner wall is the inclination length H4 [mm], the above H1 to H4 are as follows. (1) to (4) may be satisfied.
2 ≦ H1 ≦ 25 (1)
50 ≦ H2 ≦ 500 (2)
20 ≦ H3 ≦ 300 (3)
0 ≦ H4 ≦ H1 (4)

  According to the present invention, even when the casting speed is increased, the molten steel can be uniformly and stably discharged in all directions of the opening portion of the immersion nozzle. It becomes small, the penetration depth of inclusions and bubbles also becomes small, and the penetration of inclusions and bubbles can be suppressed. Furthermore, since the rising flow velocity is also ensured moderately, it is possible to promote the removal of inclusions and bubbles. Therefore, the occurrence of inclusion defects and bubble defects can be suppressed, and the slab quality can be improved.

As described above, in order to improve the slab quality while increasing the casting speed, the molten steel is discharged from the immersion nozzle in a uniform and uniform manner in all directions of the opening, and the discharge flow rate and the descending flow rate are reduced. It is desirable to suppress the intrusion of inclusions and bubbles. Furthermore, in order to promote the floating removal of inclusions and bubbles, it is desirable to increase the ascending flow rate appropriately within a range where the molten metal surface is not vigorously shaken.
However, when the discharge angle is downward with a dispersion nozzle, the descending flow rate is large and the ascending flow rate is small, which is insufficient for the removal of floating of inclusions and bubbles. In addition, although the molten steel flow when the discharge angle is simply upward with the dispersion nozzle is shown in FIG. 2, the molten steel flowing near the nozzle inner wall peels off from the nozzle inner wall when it reaches the upper end of the discharge hole. Molten steel does not flow out in the direction of the discharge angle, and uniform discharge in all directions of the opening is not achieved. Furthermore, since a negative pressure is generated at the upper portion of the discharge hole at this time, the suction of molten steel or molten powder into the immersion nozzle occurs, making the molten steel flow unstable and reducing the quality of the slab.

  As a result of diligent research, the present inventor has determined that even if the discharge angle of the dispersion nozzle is upward, the discharge flow does not peel from the upper end wall of the discharge hole, but flows out in the discharge angle direction, for example, as shown in FIG. It was clarified that the liquid can be discharged stably and uniformly.

  Hereinafter, the contents of the present invention will be described in detail. In addition, the immersion nozzle of this invention is a dispersion | distribution nozzle shown in FIG. 1 and FIG.

The following three points were examined for the dispersion nozzle according to the present invention. The first point is the discharge angle θ of the immersion nozzle 1. The second point is the sum SS of the horizontal projection areas projected on the inner end surfaces in the width direction of the mold 3 (short sides of the mold 3) of the aperture portion of the immersion nozzle 1, and the vertical downward projection area SB of the aperture slit. The projected area ratio SS / SB. In addition, the opening part of the immersion nozzle 1 is formed in the vicinity of the lower end of the immersion nozzle 1, and from the center line C (FIG. 3A) of the immersion nozzle 1 to the width direction of the mold 3 (Y direction in FIG. A pair of discharge holes 9, 9 opened symmetrically toward the Y direction in FIG. 3A and an opening slit formed at the bottom of the immersion nozzle 1 and connecting the pair of discharge holes 9, 9. And the slit 8. The third point is the immersion nozzle that narrows the flow path of the molten steel in the nozzle vertical hole 12 whose axis coincides with the immersion nozzle 1 to the inner wall 10 of the immersion nozzle between the upper end of the immersion nozzle 1 and the upper end of the discharge hole 9. This is the provision of a cylindrical step having a diameter smaller than the inner diameter of 1.

  The influence of the discharge angle θ and the projected area ratio SS / SB on the flow of the molten steel 7 was examined. The behavior of the fluid flowing out from the immersion nozzle 1 into the mold 3 was confirmed by a 1 / 2.5 scale water model experiment. The water model experimental apparatus reproduced the liquid phase part from the meniscus to the equivalent of 2.3 m, assuming a case where a slab having a width equivalent to 1300 mm and a thickness equivalent to 250 mm is cast by an actual continuous casting machine. The amount of water was varied at various amounts corresponding to the actual casting speed. As the immersion nozzle 1, a dispersion nozzle having various discharge angles θ and projected area ratios SS / SB was used. In order to observe the flow state in the mold 3, the flow state was visually confirmed using colored water.

  As a result of the water model experiment, the discharge angle θ is in the range of elevation angle 5 ° to 30 ° with respect to the horizontal direction, and the projection area ratio SS / SB is in the range of 2.5 to 15, as shown in FIG. It became clear that the discharge flow showed the molten steel flow discharged uniformly in all directions of the opening. This is because the fluid flowing in the immersion nozzle 1 is subjected to sufficient pressure loss at the bottom of the immersion nozzle 1, and when there is no pressure loss, a part of the flow rate flowing out from the slit 8 is discharged to the discharge hole 9. This is because the flow rate is distributed and the flow rate from the discharge hole 9 is increased. Further, as the fluid is uniformly discharged in all directions of the mold, the flow velocity of the upward flow 5 (hereinafter referred to as the upward flow velocity) is relatively increased, and the flow velocity of the downward flow 6 (hereinafter referred to as the downward flow velocity) is decreased. became.

  Specifically, when the discharge angle θ is downward from the elevation angle of 5 ° with respect to the horizontal direction, the molten steel does not discharge in all directions of the mold, and the flow is equivalent to that of the conventional dispersion nozzle as shown in FIG. It became a pattern and the descending flow velocity was large. When the discharge angle θ is higher than 30 ° with respect to the horizontal direction, the discharge flow directly collides with the meniscus, so that the liquid level fluctuates greatly. When the projected area ratio SS / SB is less than 2.5, the bottom of the immersion nozzle 1 cannot receive sufficient pressure loss, and the flow rate flowing out from the slit 8 increases, so that a uniform discharge flow can be obtained from the opening. I couldn't. If the projected area ratio SS / SB exceeds 15, the pressure loss received at the bottom of the immersion nozzle 1 becomes too large, the amount of fluid flowing out from the discharge hole 9 becomes very large, and omnidirectional discharge is not achieved, and the descending flow rate and liquid level fluctuation Both became larger.

  Further, the present inventor has intensively studied the influence of the provision of a step in the immersion nozzle 1 shown in FIG. 3 on the molten steel flow in the immersion nozzle 1 and the molten steel flow in the mold 3. The molten steel flow in the immersion nozzle 1 and the molten steel flow in the mold 3 were evaluated by a water model experiment. The apparatus and experimental conditions used for the experiment are the same as those described above. As the immersion nozzle 1, a dispersion nozzle provided with steps of various dimensions was used. Further, the discharge angle θ at this time was set to an elevation angle of 5 ° to 30 ° with respect to the horizontal direction, and the projection area ratio SS / SB was set to a range of 2.5 to 15. In order to observe the flow state in the mold 3, the flow state was visually confirmed using colored water.

As shown in FIG. 3, the level difference inside the immersion nozzle 1 is set to 2 ≦ H1 ≦ 25, where ½ of the difference between the inner diameter of the immersion nozzle 1 and the inner diameter of the step portion 11 is 2 ≦ H1 ≦ 25. The distance from the upper end of the hole 9 to the lower end of the step portion 11 is set to 50 ≦ H2 ≦ 500 in the step position H2 [mm], and the length of the step portion 11 in the nozzle cylindrical axis direction is set to 20 ≦ 20 in the step length H3 [mm]. The vertical length of the inclined portion 13 connecting the lower end of the stepped portion 11 and the nozzle inner wall 10 was set to H3 ≦ 300, and the inclined length H4 [mm] was set to 0 ≦ H4 ≦ H1.
The step portion 11 means a portion having the step height H1, and does not include the inclined portion 13.
As a result of the water model experiment, by providing the step in the immersion nozzle 1, the flow in the immersion nozzle 1 was further stabilized, and the fluid in all directions of the mold was discharged more uniformly. The reason for this will be explained. After the lower end of the step portion 11, the flow velocity of the fluid that passes through the nozzle central portion is larger than that of the fluid that passes through the inner wall surface of the nozzle. The fluid passing through the center of the nozzle reaches the bottom of the nozzle earlier than the fluid passing through the vicinity of the inner wall of the nozzle and receives a sufficient pressure loss there, so that the fluid flow rate is largely distributed to the discharge holes 9. The fluid passing through the inner wall surface of the nozzle is temporarily peeled off from the nozzle wall surface at the lower end of the stepped portion 11 to reduce the flow velocity. Then, the fluid is pushed back by the fluid that has suffered pressure loss, and the fluid fills the discharge hole 9 and flows out into the mold 3. Therefore, the internal flow of the nozzle is further stabilized, and more uniform fluid discharge is achieved in all directions of the opening portion.

  Specifically, when the step height H1 is less than 2 mm, the flow state is the same as that of the dispersion nozzle having no step and the effect of the step is hardly seen. On the other hand, if the step height H1 exceeds 25 mm, a reversal flow toward the upper end of the immersion nozzle 1 occurs at the upper end of the step portion 11, and the flow inside the immersion nozzle 1 tends to become unstable. Further, in this case, since the flow path becomes very narrow in the step portion 11, it becomes difficult to obtain a sufficient flow rate from the discharge hole 9. If the step position H2 is less than 50 mm, the fluid reaches the upper end of the discharge hole 9 immediately after the fluid is peeled off at the lower end of the step portion 11, and therefore the flow rate from the slit 8 increases without flowing out of the discharge hole 9. When the step position H2 exceeds 500 mm, the distance from the lower end of the step portion 11 to the upper end of the discharge hole 9 is sufficiently long, so that the separated flow becomes a flow along the nozzle wall surface again, and the step effect is difficult to obtain. If the step length H3 is less than 20 mm, a uniform flow in the radial direction cannot be obtained at the step portion 11, and a drift tends to occur inside the immersion nozzle 1. If the step length H3 is more than 500 mm, it is not preferable because of restrictions on the nozzle dimensions. An inclination length H4 of less than 0 mm is not preferable in terms of the nozzle structure. Further, when the inclined length H4> the step height H1, the peeled flow becomes a flow along the nozzle wall surface again, and the step effect is difficult to obtain.

  As described above, the discharge angle θ is an elevation angle of 5 ° to 30 ° with respect to the horizontal direction, the projected area ratio SS / SB is 2.5 to 15, the step height H1 is 2 mm to 25 mm, and the step position H2 is 50 mm. 500 mm, step length H3 from 20 mm to 300 mm, and inclined length H4 from 0 mm to step height H1, the molten steel discharge flow is more stable and more uniform in all directions of the opening portion of the immersion nozzle 1. It became possible to discharge.

  Examples of the present invention will be described below.

Example 1
Converter-degassing and adjusting the composition of ultra-low carbon steel with a carbon concentration of 30 ppm or less using a vertical bending slab continuous casting machine at casting speeds of 1.5, 2.0, 2.5 m /
Continuous casting in min. The slab thickness is 240 mm, the slab width is 1000 to 2000 mm, and the ladle capacity is 300 t. As the immersion nozzle, various dispersion nozzles and two-hole nozzles including the present invention dispersion nozzles described in Tables 1 to 3 were used.

  In order to evaluate the fluctuation level of the molten metal surface when using the dispersion nozzle according to the present invention, the molten metal surface fluctuation state of the meniscus on the short side of the mold was confirmed using a molten metal surface level meter at the upper part of the mold.

  In addition, the number of inclusions remaining in the slab was evaluated for evaluation of slab quality. The number of inclusions in the slab is the number of inclusions with a size of 50 μm or more existing in the slab by cutting out a portion of 120 mm (slab ½ thickness) in the thickness direction from the surface of the slab. Was measured. Inclusions having a size of 50 μm or more are sizes that can cause defects in the slab.

  Similarly, in order to evaluate the quality of the slab, the number of bubbles remaining in the slab was evaluated. The number of bubbles in the slab was measured by cutting a 75 mm (slab ¼ thickness) portion in the slab thickness direction from the slab surface in the full width direction and measuring the number of bubbles having a diameter of 0.1 mm or more. Bubbles having a diameter of 0.1 mm or more are sizes that can cause defects in the slab.

  First, the evaluation result of hot water level fluctuation will be described. The results are shown in Tables 1-3. The state of hot water level fluctuation in the dispersion nozzle according to the present invention is the same level of hot water level fluctuation as that of the immersion nozzles (No. 7, 12, 19, 24, 31 and 36) facing downward in the discharge angle, which is a stable condition of the hot water surface. The situation was at a level where there was no problem in operation.

  Next, the result of investigating the number of inclusions in the slab will be described. The results are shown in Tables 1-3. The inclusion defect index in Tables 1-3 is No. The number of inclusions when using the two-hole nozzle having the largest descending flow velocity of 12, 24 and 36 is set as 1, and is relatively displayed for each casting speed. As a result, no. In the slab cast using the dispersion nozzles of 1 to 6, 13 to 18, and 25 to 30, the inclusion defect index was significantly reduced as compared with the two-hole nozzle. Comparative Example No. Even when compared with the dispersion nozzles of 7-11, 19-23 and 31-35, the inclusion defect index decreased in the dispersion nozzle according to the present invention.

  Next, the investigation result of the number of bubbles in the slab will be described. The results are shown in Tables 1-3. The bubble defect index in Tables 1 to 3 is No. 1 for the same reason as above. The number of bubbles at the time of using 12, 24, and 36 two-hole nozzles is assumed to be 1, and is relatively displayed for each casting speed. As a result, no. In the slab cast using the dispersion nozzles of 1 to 6, 13 to 18, and 25 to 30, the bubble defect index was greatly reduced as compared with the two-hole nozzle. Comparative Example No. Even when compared with 7-11, 19-23, and 31-35 dispersion nozzles, the bubble defect index was reduced in the dispersion nozzles according to the present invention.

(Example 2)
Converter-degassing and adjusting the composition of ultra-low carbon steel with a carbon concentration of 30 ppm or less using a vertical bending slab continuous casting machine at casting speeds of 1.5, 2.0, 2.5 m /
Continuous casting in min. The slab thickness is 240 mm, the slab width is 1000 to 2000 mm, and the ladle capacity is 300 t. As the immersion nozzle, various dispersion nozzles and two-hole nozzles including the dispersion nozzles of the present invention described in Tables 4 to 6 were used.

  In order to evaluate the fluctuation level of the molten metal surface when using the dispersion nozzle according to the present invention, the molten metal surface fluctuation state of the meniscus on the short side of the mold was confirmed using a molten metal surface level meter at the upper part of the mold.

  In addition, the number of inclusions remaining in the slab was evaluated for evaluation of slab quality. The number of inclusions in the slab is the number of inclusions with a size of 50 μm or more existing in the slab by cutting out a portion of 120 mm (slab ½ thickness) in the thickness direction from the surface of the slab. Was measured. Inclusions having a size of 50 μm or more are sizes that can cause defects in the slab.

  Similarly, in order to evaluate the quality of the slab, the number of bubbles remaining in the slab was evaluated. The number of bubbles in the slab was measured by cutting a 75 mm (slab ¼ thickness) portion in the slab thickness direction from the slab surface in the full width direction and measuring the number of bubbles having a diameter of 0.1 mm or more. Bubbles having a diameter of 0.1 mm or more are sizes that can cause defects in the slab.

  First, the evaluation result of hot water level fluctuation will be described. The results are shown in Tables 4-6. The state of fluctuation of the molten metal surface in the dispersion nozzle according to the present invention is the same as that of the immersion nozzle (No. 55, 60, 79, 84, 103 and 108) having a downward discharge angle, which is a stable condition of the molten metal surface. The situation was at a level where there was no problem in operation.

  Next, the result of investigating the number of inclusions in the slab will be described. The results are shown in Tables 4-6. The inclusion defect index of Tables 4-6 is No. The number of inclusions when using the two-hole nozzle with the largest descending flow velocity of 60, 84, and 108 is set to 1 and is relatively displayed for each casting speed. As a result, no. In the slab cast using the dispersion nozzles of 37 to 54, 61 to 78, and 85 to 102, the inclusion defect index was significantly reduced as compared with the two-hole nozzle. Comparative Example No. Even when compared with the dispersion nozzles of 55 to 59, 79 to 83, and 103 to 107, the inclusion defect index decreased with the dispersion nozzle of the present invention.

  Next, the investigation result of the number of bubbles in the slab will be described. The results are shown in Tables 4-6. The bubble defect index of Tables 4-6 is No. When the two-hole nozzles 60, 84 and 108 are used, the number of bubbles is set to 1 and is relatively displayed for each casting speed. As a result, no. In the slab cast using the dispersion nozzles of 37 to 54, 61 to 78, and 85 to 102, the bubble defect index was significantly reduced as compared with the two-hole nozzle. Comparative Example No. Even when compared with the dispersion nozzles of 55 to 59, 79 to 83, and 103 to 107, the bubble defect index decreased with the dispersion nozzle of the present invention.

  This is useful for an immersion nozzle that supplies molten metal into a mold when continuously casting a molten metal such as steel.

It is a figure which shows the molten steel flow in the immersion nozzle by this invention, and a casting_mold | template. It is a figure which shows the molten steel flow in the immersion nozzle by a conventional dispersion | distribution nozzle, and a casting_mold | template. It is the figure which showed the immersion nozzle of this invention, (a) And (b) is a longitudinal cross-sectional view which shows the level | step difference installed in the immersion nozzle, (c) is a top view of the immersion nozzle bottom part.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Submerged nozzle 2 Mold powder 3 Mold 4 Meniscus reversal flow 5 Upflow 6 Downflow 7 Molten steel 8 Nozzle bottom slit 9 Discharge hole 10 Immersion nozzle inner wall 11 Step part 12 Nozzle vertical hole 13 Inclined part

Claims (2)

In the immersion nozzle that injects molten steel into the mold,
A pair of discharge holes formed in the vicinity of the lower end of the immersion nozzle and opened symmetrically from the center line of the immersion nozzle toward the mold width direction;
Formed at the bottom of the immersion nozzle, and provided with an aperture slit that connects the pair of ejection holes;
The discharge angle θ of the discharge hole is in the range of an elevation angle of 5 ° to 30 ° with respect to the horizontal direction,
The sum of the horizontal projection areas on the inner end surfaces in the mold width direction of the aperture portion constituted by the pair of ejection holes and the aperture slit is SS, and the vertical downward projection area of the aperture slit is SB. The area ratio SS / SB is
2.5 ≦ SS / SB ≦ 15
An immersion nozzle for continuous casting of a slab characterized by being in the range of The inner wall of the immersion nozzle between the upper end of the immersion nozzle and the upper end of the discharge hole has a cylindrical step whose diameter is smaller than the inner diameter of the immersion nozzle that narrows the flow path of the molten steel whose axial center coincides with the immersion nozzle.
1/2 of the difference between the inner diameter of the immersion nozzle and the nozzle inner diameter of the stepped portion is the step height H1 [mm], and the distance from the upper end of the discharge hole to the lower end of the stepped portion is the stepped position H2 [mm]. When the length in the cylindrical axis direction is the step length H3 [mm] and the vertical length of the inclined portion connecting the lower end of the step portion and the nozzle inner wall is the inclination length H4 [mm], the above H1 to H4 are as follows. The submerged nozzle for continuous casting of a slab according to claim 1, wherein the following formulas (1) to (4) are satisfied.
2 ≦ H1 ≦ 25 (1)
50 ≦ H2 ≦ 500 (2)
20 ≦ H3 ≦ 300 (3)
0 ≦ H4 ≦ H1 (4)

Images