JP3595464B2 - Immersion nozzle for continuous casting and continuous casting method for steel - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a continuous casting immersion nozzle and a method for continuously casting steel.
[0002]
[Prior art]
FIG. 7 is a schematic diagram showing a conventional immersion nozzle and a flow of molten steel flowing out. The upper end of the immersion nozzle 2 is attached to a tundish (not shown), and the lower portion is used by immersing it in molten steel 1 in a continuous casting mold. The immersion nozzle 2 has a bottomed cylindrical shape, and a right discharge hole 3 and a left discharge hole 4 are arranged at symmetrical positions below the cylindrical side wall. The molten steel 7 supplied into the immersion nozzle 2 from a tundish is provided. Flows out of the right discharge hole 3 and the left discharge hole 4 as a molten steel flow 6. The molten steel flow 6 collides with the side wall of the mold and is divided into an upward flow 8 and a downward flow 9. As the casting speed increases, the flow velocity of the molten steel flow 6 flowing out of the right discharge hole 3 and the left discharge hole 4 increases. In this case, however, the flow velocity of the upward flow becomes excessive, and the molten steel meniscus 10 in the mold is strongly rocked. As a result, the casting surface of the slab is impaired, and at the same time, the powder 11 on the molten steel in the mold is involved, resulting in inclusion defect of the slab. When the flow rate of the molten steel flow 6 is high, the flow velocity of the descending flow is also excessive, and the descending flow penetrates deeply into the molten steel in the mold. Due to the inclusion of inclusions, inclusions and inert gas bubbles also penetrate deeply into the molten steel, and are trapped by the solidified shell during penetration or floating, causing inclusion defects and bubble defects in the slab.
[0003]
In order to solve such a problem, Japanese Patent Application Laid-Open No. Sho 62-296944 discloses an immersion nozzle shown in FIG. One slit 5 is provided so that one end of the immersion nozzle reaches the right discharge hole 3 and the other end reaches the left discharge hole 4. When this immersion nozzle is used, since the molten steel flows out of the slit 5 as well as the right discharge hole 3 and the left discharge hole 4, the flow velocity of the molten steel flow 6 from the right discharge hole 3 and the left discharge hole 4 decreases. Therefore, the swing of the molten steel meniscus 10 in the mold is reduced, and the inclusions and inert gas bubbles are prevented from deeply penetrating into the molten steel in the mold. For this reason, the immersion nozzle of FIG. 8 is suitable for producing a cast piece having a good casting surface and having few inclusion defects and bubble defects.
[0004]
[Problems to be solved by the invention]
However, according to the findings of the present inventors, in the immersion nozzle shown in FIG. 8, when the short side length S of the slit 5 is narrow, the problem is that the slit 5 becomes narrow or easily closed during casting. is there. Conversely, if the short side length S of the slit 5 is large, the downward flow coming out of the slit will penetrate deeply into the molten steel. As a result, there is a problem that inclusions and inert gas bubbles also penetrate deeply into the molten steel, causing inclusion defects and bubble defects in the cast slab.
[0005]
Further, in the immersion nozzle in which the right discharge hole 3 and the left discharge hole 4 are present, a so-called drift phenomenon occurs, in which the balance of the amount of molten steel flowing out of the left and right discharge holes is lost depending on the shape of the nozzle. Oscillation of the molten steel meniscus 10 in the mold becomes large and the casting surface of the slab is impaired, and at the same time, inclusions and bubbles of inert gas penetrate deeply into the molten steel, causing inclusion defects and bubble defects in the slab. There is a problem that causes.
[0006]
The present invention provides an immersion nozzle that can sufficiently prevent the slit 5 from being closed during casting, and can also prevent inclusion defects and air bubble defects in the slab, and can also simultaneously prevent the drift phenomenon. It is an issue.
[0007]
[Means for Solving the Problems]
The present invention
(1) An immersion nozzle for pouring a molten metal into a mold, a left discharge hole and a right discharge hole are disposed at lower portions of side walls of the immersion nozzle at positions symmetrical with respect to the center of the nozzle cross section. In a continuous casting immersion nozzle provided with one slit reaching one end to the right discharge hole and the other end to the left discharge hole, the slit long side length W [mm] and the slit short side length S [mm] Is 3 ≦ W / S ≦ 10
A continuous casting immersion nozzle characterized by satisfying the following relationship:
(2) An immersion nozzle for casting a molten metal into a mold, wherein a left discharge hole and a right discharge hole are disposed at symmetrical positions with respect to the center of the nozzle cross section at a lower portion of a side wall of the immersion nozzle. In a continuous casting immersion nozzle in which one end reaches the right discharge hole and the other end reaches the left discharge hole, the slit short side length S [mm] and the discharge hole diameter d [mm] are 0. .1 ≦ S / d ≦ 0.5
The immersion nozzle for continuous casting according to claim 1, wherein the following relationship is satisfied.
(3) The immersion nozzle for continuous casting according to ( 1) or (2) , wherein a gas blowing hole is provided in the immersion nozzle.
(4) A continuous casting method for steel, comprising casting using the continuous casting immersion nozzle according to any one of (1) to (3) .
(5) A continuous casting method for steel, characterized in that an inert gas is blown into the immersion nozzle for casting using the immersion nozzle for continuous casting according to (3) .
[0008]
The shape of the discharge hole may be elliptical or the like in addition to the circle, and the equivalent diameter of the discharge hole is the diameter of a circle having the same area as the area of the discharge hole.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is an explanatory view of the immersion nozzle of the present invention, and FIG. 1 (a) is a view showing a cross section of a lower part of the immersion nozzle. FIG. 1B is an explanatory diagram of a vertical section taken along a vertical plane parallel to the slit 5. FIG. 1C is an explanatory view of a vertical section taken along a vertical plane perpendicular to the slit 5. In the figure, d is the discharge hole diameter [mm], W is the length of the slit long side [mm], and S is the length of the slit short side [mm].
[0010]
The present inventors performed continuous casting using the conventional type of immersion nozzle shown in FIG. 8, and observed the bottom after the casting was completed. As a result, the slit 5 was completely closed. In the immersion nozzle used at this time, the slit long side length W was 110 [mm], the slit short side length S was 10 [mm], and W / S = 11.0.
[0011]
Based on this finding, the present inventors made a test nozzle having the same shape as that of FIG. 8 using transparent plastic, immersed it in a water tank, introduced an inert gas from above, The behavior of the bubbles was investigated. According to the water model test, bubbles of the inert gas mainly flowed out of the right discharge hole 3 and the left discharge hole 4, and almost no inert gas bubbles flowed out of the slit 5.
[0012]
From these results, for example, if the shape of the immersion nozzle is changed to increase the amount of bubbles of the inert gas discharged from the slit 5, it is considered that the clogging of the slit 5 is also resolved. The test nozzle shown in FIG. 1 was manufactured, and the same water model test as described above was performed. At this time, the test was performed by changing the slit long side length W [mm] and the slit short side length S [mm] variously. FIG. 2 shows the relationship between the ratio (W / S) of the slit long side length W [mm] and the slit short side length S [mm] to the number of bubbles discharged from the slit 5. When the W / S is 10 or less, the number of bubbles discharged from the slit 5 increases, and it is considered that closing the slit 5 can be prevented by setting the W / S within this range.
[0013]
Based on the results of the water model test, the inventors of the present invention used an immersion nozzle having a shape shown in FIG. 1 and a slit long side length W of 130 to 250 [mm] and a slit short side length S of 5 to 65 mm. [Mm], W / S was changed from 2 to 20 in each case, and 10 pieces were produced, and 5 [L / min] of argon gas was used as an inert gas in an immersion nozzle in continuous casting of normal molten steel. 1250t of ultra-low carbon steel having a carbon concentration of 30 ppm was cast. After completion of the casting, the obstruction state of the slit 5 of the immersion nozzle was observed. FIG. 3 shows the relationship between the ratio (W / S) of the slit long side length W [mm] and the slit short side length S [mm] in the immersion nozzle and the number of closed nozzles. As a result, in the immersion nozzle having a W / S of greater than 10, the closing ratio of the slit 5 was 50% or more, and the slit 5 was partially closed for the rest. On the other hand, in the immersion nozzle having W / S of 10 or less, the closing ratio of the slit 5 was less than 50%, and in the immersion nozzle having W / S of 7 or less, there was no blocked nozzle.
[0014]
Next, the slab obtained by casting was cut into a length of 8500 mm to obtain a slab of one coil unit. The slab was hot-rolled and cold-rolled according to a conventional method, and the number of surface defects of the cold-rolled steel sheet was measured. FIG. 4 shows the relationship between the ratio (W / S) of the slit long side length W and the slit short side length S (W / S) and the number of surface defects. As a result, in an immersion nozzle having a W / S of less than 4, the number of defects considered to be caused by the downward flow from the slit penetrating deeply into the molten steel tends to increase. And the number of defects increased rapidly. In the immersion nozzle having a W / S of more than 10, the number of defects considered to be caused by the blockage of the slit was increased.
[0015]
From these facts, it is necessary that the ratio (W / S) of the slit long side length W to the slit short side length S satisfies the relationship of 3 ≦ W / S ≦ 10, and preferably 4 ≦ W / S It is necessary to satisfy the relationship of S ≦ 7.
[0016]
Next, the present inventors made an immersion nozzle having the shape shown in FIG. 1 from a transparent plastic, immersed it in a water tank, supplied water to the inside, and set the discharge hole diameter d [mm] and the slit short side. By changing the length S [mm], the flow velocity of the water flowing out of the right discharge hole 3 and the left discharge hole 4 at that time was measured, and the drift phenomenon was evaluated. The drift phenomenon is based on the absolute value of the difference between the flow velocity of the water flowing out of the right discharge hole 3 and the flow velocity of the water flowing out of the left discharge hole 4. Was evaluated by the value (deviation index) divided by the sum of the flow velocities. FIG. 5 shows the relationship between the ratio (S / d) of the length S of the short side of the slit to the diameter d of the discharge hole and the drift index. It can be seen that the drift phenomenon can be suppressed by setting S / d to 0.1 or more and 0.5 or less.
[0017]
Based on the results of the water model test, the present inventors estimated that the slit short side length S was 15 to 30 [mm], the W / S was 5, and the immersion nozzle having the shape shown in FIG. / D was changed from 0.05 to 0.8, respectively, and used while blowing argon gas at a rate of 5 [L / min] as an inert gas into an immersion nozzle in normal continuous casting of molten steel. Then, 1250t of ultra-low carbon steel having a carbon concentration of 30 ppm was cast. The obtained slab was hot-rolled and cold-rolled by a conventional method, and the number of surface defects of the cold-rolled steel sheet was measured. FIG. 6 shows the relationship between the ratio (S / d) of the slit short side length S to the ejection hole diameter d (S / d) and the number of surface defects. As a result, the number of defects considered to be caused by the drift phenomenon decreases when S / d is 0.1 or more and 0.5 or less, but further decreases when S / d is 0.2 or more and 0.4 or less. I was Therefore, it is necessary that the slit short side length S [mm] and the discharge hole diameter d [mm] satisfy the relationship of 0.1 ≦ S / d ≦ 0.5, and preferably 0.2 ≦ S / d. It is necessary to satisfy the relationship of ≦ 0.4.
[0018]
【The invention's effect】
The immersion nozzle of the present invention, by determining the discharge hole diameter, slit long side length, slit short side length to an appropriate level, it is possible to sufficiently prevent the slit from being blocked during casting, and Continuous casting can be performed while reducing surface defects due to the drift phenomenon.
[Brief description of the drawings]
1A and 1B are explanatory views of an example of an immersion nozzle according to the present invention, in which FIG. 1A is an explanatory view of a cross section under a immersion nozzle, and FIG. FIG. 4C is an explanatory view of a vertical section taken along a vertical plane roll perpendicular to the slit 5.
FIG. 2 is a view showing a relationship between a ratio (W / S) of a slit long side length W and a slit short side length S (W / S) and the number of bubbles discharged from a slit 5;
FIG. 3 is a diagram showing the relationship between the type of immersion nozzle and the number of slits closed.
FIG. 4 is a view showing the relationship between the ratio (W / S) of the slit long side length W and the slit short side length S (W / S) and the number of surface defects.
FIG. 5 is a diagram showing a relationship between a ratio (S / d) of a slit short side length S to a discharge hole diameter d and a drift index.
FIG. 6 is a view showing the relationship between the ratio (S / d) of the length S of the short side of the slit to the diameter d of the ejection hole and the number of surface defects.
FIG. 7 is a schematic view showing a conventional immersion nozzle and a flow of molten steel flowing out.
FIG. 8 is a view showing an example of a conventional immersion nozzle having left and right discharge holes at the bottom and a slit at the bottom.
[Explanation of symbols]
1: molten steel, 2: immersion nozzle, 3: right discharge hole, 4: left discharge hole, 5: slit, 6: molten steel flow from the discharge hole, 7: molten steel in the immersion nozzle, 8: ascending flow near the side wall, 9: Downflow near the side wall, 10: Meniscus of molten steel in the mold, 11: Cast powder, 12: Submerged nozzle bottom

Claims (5)

An immersion nozzle for pouring the molten metal into a mold, a left discharge hole and a right discharge hole are disposed at symmetric positions with respect to the center of the nozzle cross section at the lower portion of the side wall of the immersion nozzle, and one end is provided at the bottom. In the continuous casting immersion nozzle in which the right discharge hole and the other end have one slit reaching the left discharge hole, the slit long side length W [mm] and the slit short side length S [mm] are 3 ≦. W / S ≦ 10
An immersion nozzle for continuous casting characterized by satisfying the following relationship: An immersion nozzle for pouring the molten metal into a mold, a left discharge hole and a right discharge hole are disposed at symmetric positions with respect to the center of the nozzle cross section at the lower portion of the side wall of the immersion nozzle, and one end is provided at the bottom. In a continuous casting immersion nozzle in which one slit reaching the right discharge hole and the other end reaching the left discharge hole, the slit short side length S [mm] and the discharge hole diameter d [mm] are 0.1 ≦. S / d ≦ 0.5
The immersion nozzle for continuous casting according to claim 1, wherein the following relationship is satisfied. Immersion nozzle according to claim 1 or 2, characterized in that a gas blowing holes into the immersion nozzle. A continuous casting method for steel, comprising casting using the continuous casting immersion nozzle according to any one of claims 1 to 3 . 4. A continuous casting method for steel, wherein an inert gas is blown into the immersion nozzle for casting using the immersion nozzle for continuous casting according to claim 3 .

Images