CN102361712A - Immersion nozzle for continuous casting - Google Patents

Abstract

An immersion nozzle for continuous casting is adapted to discharge molten steel into a mold for continuous steel casting and has a tubular nozzle body which is provided with four molten steel discharge ports consisting of left upper, left lower, right upper and right lower ports which open into a lower end portion of the nozzle body to be immersed into the molten steel in the mold. The two left discharge ports and the two right discharge ports have substantially symmetrical shape with respect to an axis of the nozzle. The left discharge ports are opposed to the inner wall of the left minor side of the mold and the right discharge ports are opposed to the inner wall of the right minor side of the mold. The area of the openings of the lower discharge ports is smaller than the area of the openings of the upper discharge ports. The ratio of the area of the openings of the lower discharge ports to the area of a sum of the areas of the openings of the upper and lower discharge ports is not less than 0.2 but not more than 0.4.

Description

Dip nozzles for continuous casting

technical field

The present invention relates to a submerged nozzle for injecting molten steel into a mold in continuous casting of iron and steel.

Background technique

In the mold, the discharge flow of molten steel discharged from the pair of left and right discharge ports of the submerged nozzle collides with the inner wall of the short side of the mold and is divided into an ascending flow that rises along the inner wall of the mold and a downward flow that descends along the inner wall of the mold.

In this case, especially when the discharge flow velocity is high, uneven flow velocity distribution occurs at the upper and lower portions of the discharge port. As a result, in the upflow and downflow, the right and left flow balance is broken, or a strong discharge flow is locally generated, and the flow fluctuates greatly. Such fluctuations are the main causes of poor formation of solidified shells and defects caused by air bubbles and inclusions.

In order to solve this problem, it is conceivable to perform continuous casting in which defects due to air bubbles and inclusions can be prevented by making the flow of molten steel in the mold slow and uniform. Based on such considerations, for example, the following patent documents propose a four-hole submerged nozzle (four-hole nozzle) in which molten steel discharge ports are provided in two layers in the vertical direction.

In Patent Document 1, each discharge port (upper hole, lower hole) provided on the upper and lower sides of the four-hole nozzle has a horizontally long opening shape, and the length of the mold is L, and the throughput in the four-hole nozzle is y4, When the distance from the upper end of the mold to the meniscus is set as Z, the opening distance I between the upper hole and the lower hole is I<L-Z-64y4-370. In this case, high-quality slabs can be obtained without entrainment of molding powder even if the throughput is increased.

In Patent Document 2, the following scheme is proposed. The cross-sectional area of the internal flow path of the four-hole nozzle is reduced in the discharge part, and the inner dimension (sectional area) of the lower hole is smaller than the inner dimension (sectional area) of the upper hole. The generation of extreme upward discharge flow is suppressed inside. This avoids fluctuations in the liquid level and prevents defects such as powder entanglement.

The technique described in Patent Document 1 aims at eliminating the negative pressure at the upper part of the discharge hole which is a problem in the two-hole nozzle. However, under the condition that the upper hole and the lower hole have the same shape as in Patent Document 1, there is a problem that the drift on the lower hole side becomes larger.

In addition, the technology of Patent Document 2 relates to a nozzle having a special shape in which a step portion is formed in an internal flow path. In this technique, as the cross-sectional area of the stepped portion changes, the internal flow becomes unstable, and the flow from the upper hole and the lower hole may fluctuate greatly.

In addition, in the prior art described above, the flow velocity of molten steel in the vicinity of the inner wall of the short side of the mold has not been sufficiently considered, particularly focusing on the flow velocity of molten steel immediately after being discharged from the discharge port.

For example, when the flow velocity of the molten steel impinging on the upper portion where the solidified shell is thin is high, the solidified part melts again due to the impact flow, making the operation unstable. At the same time, when the flow velocity of the downflow is high, air bubbles and inclusions intrude into the deep part of the slab through the downflow, causing defects in quality.

In this way, it may not be sufficient to merely reduce the jet flow width by making the discharge port four holes. Therefore, regarding the control of the jet flow at the upper and lower discharge ports, it is necessary to further study the molten steel flow rate in the vicinity of the inner wall of the short side of the mold.

Patent Document 1: (Japanese) Unexamined Patent Publication No. 2-187240

Patent Document 2: (Japanese) Unexamined Patent Publication No. 2006-198655

Contents of the invention

Therefore, the present invention provides a four-hole nozzle, which solves the problem that in the continuous casting, the discharge port of the existing two-hole nozzle is divided into two. Intrusion of air bubbles and inclusions into the cast slab can be suppressed, and the occurrence of internal defects can be reduced.

The inventors of the present application considered that the four-hole nozzle, in which the discharge port of the conventional two-hole nozzle was simply divided into two, could not sufficiently suppress the intrusion of air bubbles and inclusions into the slab because molten steel could not be obtained. Sufficient deceleration effect of the flow rate to investigate its cause. As a result, it was found that in the case of a four-hole nozzle, it is important to control the flow balance of the discharge streams from the upper hole and the lower hole within a certain range. In addition, the inventors of the present application found that the jets discharged from the upper hole and the lower hole are merged on the way due to the negative pressure to form one jet.

As a result of examining the conditions under which the flow rate distribution of molten steel passing through the upper hole and the lower hole is uniform and the upper and lower jets do not merge, the present invention is constituted as follows.

The submerged nozzle according to the first aspect of the present invention is a submerged nozzle for continuous casting that discharges molten steel into a mold for continuous casting of iron and steel, and has a cylindrical nozzle body. In the middle part, there are four molten steel outlets in the upper row on the left row, the lower row on the left row, the upper row on the right row and the lower row on the right row. The two outlets on the left row and the two outlets on the right row have relative The nozzles have a substantially symmetrical shape to the axis, the left row of outlets faces the inner wall on the left short side of the mold, and the right row of outlets faces the right short side inner wall of the mold. , the opening area of the outlet of the lower layer is smaller than the opening area of the outlet of the upper layer; The ratio is 0.2 or more and 0.4 or less.

In the submerged nozzle according to the second aspect of the present invention, in the first aspect, the distance between the lower end of the upper discharge port and the upper end of the lower discharge port may be within a range of 15 mm to 150 mm.

In the submerged nozzle of the third aspect of the present invention, on the basis of the above-mentioned first or second aspect, the lower hole and the upper hole may be formed as follows, that is, the discharge angle of the discharge port of the upper layer and the discharge port of the lower layer They are all in the range of upward slope 5° to downward slope 45° relative to the horizontal, and the discharge angle of the discharge port of the lower layer is inclined downward by more than 10° based on the discharge angle of the discharge port of the upper layer.

In the submerged nozzle according to a fourth aspect of the present invention, in the first or second aspect, both the discharge port of the upper layer and the discharge port of the lower layer may be approximately rectangular.

By performing continuous casting using the four-hole nozzle of the present invention, the molten steel flow rate near the inner wall of the short side of the mold can be sufficiently controlled, and the intrusion of air bubbles and inclusions into the deep part of the slab through the downward flow can be suppressed, and the occurrence of internal defects can be obtained. Fewer castings.

Description of drawings

FIG. 1 is a diagram showing a cross-sectional shape of a four-hole nozzle according to an embodiment of the present invention;

2A is a schematic cross-sectional view showing the arrangement of the four-hole nozzle in the mold according to the embodiment of the present invention, which is a view observed from a line of sight perpendicular to the long side of the mold;

2B is a schematic cross-sectional view showing the arrangement of the four-hole nozzle in the mold according to the embodiment of the present invention, which is a view observed from the line of sight along the axial direction of the nozzle;

FIG. 3 is a graph showing discharge flow velocity distributions in a two-hole nozzle and a four-hole nozzle obtained from the results of numerical fluid analysis.

FIG. 4 is a graph showing the attenuation effect of the discharge flow velocity in a two-hole nozzle and a four-hole nozzle obtained from the results of numerical fluid analysis.

Fig. 5 is a diagram illustrating the configuration of a discharge port of a four-hole nozzle used in a water model test;

Fig. 6 is a graph representing the descending velocity of each nozzle obtained from the results of the water model experiment;

7 is a graph showing the relationship between the angle difference of the discharge flow from the upper hole and the lower hole and the amount of air bubbles obtained from the results of numerical fluid analysis and water model experiments.

Symbol Description

1: Immersion nozzle body

2: upper hole

3: Bottom hole

D: The distance between the lower end of the upper hole and the upper end of the lower hole

5: Molding

5a: Short side inner wall

5b: Long side inner wall

Detailed ways

FIG. 1 shows the shape of a four-hole nozzle according to an embodiment of the present invention.

In the mold 5 for continuous casting of steel, generally, a rectangular structure is used in plan view. The molten steel is discharged into the mold 5 through the submerged nozzle. The submerged nozzle main body 1 has a pair of left and right discharge ports 2 , 3 . The outlet on one side includes an upper hole 2 and a lower hole 3 . Through these outlets, the molten steel is discharged into the mold 5 in four directions: upper left, lower left, upper right and lower right. The discharge flow of molten steel from the left and right discharge ports collides with the short side inner wall 5 a of the mold 5 and is divided into an ascending flow rising along the inner wall of the mold 5 and a descending flow descending along the inner wall of the mold 5 .

The immersion nozzle main body 1 is formed in a cylindrical shape so that molten steel can pass through from top to bottom, and a discharge port for molten steel is provided at a position where the lower end is immersed in the mold 5 . The discharge port is divided into an upper hole 2 and a lower hole 3 to form upper and lower layers. Two short-side inner walls 5 a of the steel mold 5 are provided at positions facing each other, two on the left and one on the left and right sides of the nozzle axis, and a total of four discharge ports are provided.

The two discharge ports in the left row and the two discharge ports in the right row have substantially symmetrical shapes with respect to the axis of the nozzle. The left and right discharge ports may be mirror-symmetrical to a plane including the nozzle axis, and the left and right discharge ports may be rotationally symmetric with respect to the nozzle axis. The discharge ports of the left row are opposite to the inner wall of the left short side of the casting mold, and the discharge ports of the right row are opposite to the inner wall of the right short side of the casting mold. The opening area of the discharge port of the lower layer is smaller than the opening area of the discharge port of the upper layer. In each of the left and right rows, the ratio of the opening area of the discharge ports of the lower layer to the sum of the opening areas of the discharge ports of the upper and lower layers is 0.2 or more and 0.4 or less.

2A and 2B show the arrangement of the submerged nozzle body 1 in the mold 5 . The vertical wall surfaces constituting the mold 5 are substantially rectangular in plan view, and have a set of short sides and a set of long sides. A pair of left and right discharge ports 2 and 3 of the submerged nozzle main body 1 face and open to short-side inner walls 5 a of the mold 5 .

In general, it is known that the flow velocity of molten steel discharged from a nozzle increases in attenuation as the jet width decreases, and by forming a four-hole nozzle, the jet width of each discharge hole can be reduced. As a result, due to the attenuation effect of the flow velocity, the value of the maximum flow velocity in the jet flow is reduced, and the effect of suppressing the intrusion of air bubbles and inclusions into the inside of the slab is achieved.

However, the inventors of the present application have found that, as a result of continuous casting using a four-hole nozzle, only a shape in which the discharge port of a conventional two-hole nozzle is simply divided into two cannot obtain a sufficient deceleration effect and cannot sufficiently suppress the Bubbles and inclusions invade the interior of the slab.

The inventors of the present application analyzed the reason why a sufficient deceleration effect cannot be obtained as follows. That is, in the case where the shapes of the upper hole and the lower hole are the same, the flow balance through the upper hole and the lower hole is broken due to the pressure difference in the molten steel height direction. In addition, the jets respectively discharged from the upper hole and the lower hole are merged into one jet on the way by the negative pressure generated between these jets. As a result, the jet width becomes wider and the attenuation effect of the flow velocity becomes smaller. The inventors of the present application consider these to be one of the reasons for the insufficient flow rate.

Therefore, in order to obtain the condition that the flow distribution of molten steel passing through the upper hole and the lower hole is uniform and the upper and lower jets do not merge, the respective sizes of the upper hole and the lower hole and the respective molten steel jets of the upper hole and the lower hole The discharge angle is explored.

First, regarding the optimum values of the size of the upper hole and the lower hole, the behavior of melting steel was studied by numerical fluid analysis.

Due to the pressure difference in the direction of the molten steel height, even if the nozzle shape is the same, the flow rate through the capacity of the upper hole and the lower hole is different. Therefore, the aim of this analysis is to optimize the flow distribution of molten steel through the upper hole and the lower hole to reduce the flow velocity of the downflow of molten steel in the vicinity of the inner wall of the short side of the mold. In this regard, numerical fluid analysis was performed on nozzles of various shapes in which the area ratio of the discharge portions of the upper hole and the lower hole was changed, and the influence of the change in the area ratio was investigated.

In this analysis, the nozzle diameter was 160 mm. As discharge ports, the four-hole nozzles 1 to 5 formed with rectangular upper and lower holes having the opening areas shown in Table 1 and the conventional two-hole nozzle were assumed and evaluated respectively. In these evaluations, the flow velocity distribution was obtained under the condition that the maximum flow velocity of molten steel immediately after discharge from the nozzle was 3.4 m/sec, and the flow velocity at a position 800 mm away from the nozzle axis was evaluated.

FIG. 3 shows the flow rate distribution of the jet discharged from each nozzle, and FIG. 4 shows the maximum flow velocity in each nozzle. Fig. 3 is a cross-sectional view of each nozzle along the axis of the nozzle, and points are shown at points where a certain amount or more of jet flow exists.

As shown in Fig. 3, in the four-hole nozzle, the flow distribution is changed by changing the area ratio of the upper hole to the lower hole.

Next, for the nozzles of the plurality of configurations represented in Table 1, the maximum descending flow velocity (m/sec) at each position 800 mm from the axis of the nozzle was obtained. FIG. 4 analyzes the results, and shows the maximum descending flow rate with respect to the area ratio of the openings of the upper hole and the lower hole (hereinafter referred to as the opening area ratio).

As shown in FIG. 4 , when the opening area ratio is in the range of 0.2 to 0.4, the effect that the maximum descending flow rate is low is obtained. In particular, the maximum descending flow rate further decreases in the range where the opening area ratio is 0.25 to 0.375.

Table 1

Next, a water model test (water model test) was conducted to examine the discharge angles of the molten steel jets from the upper hole and the lower hole.

Using a mold of 240×1300×1390mm (thickness, width, depth), a four-hole nozzle with different discharge angles of the upper hole and the lower hole was produced. A water model test in which water was passed through these nozzles was carried out to measure the flow velocity of the discharge flow, fluctuations in the liquid level, and the amount of air bubbles involved.

Fig. 5 summarizes the configuration of the discharge port of the four-hole nozzle used in the water model test. As shown in the figure, the discharge angles of the upper hole are three types: downward slope angle = 15°, horizontal, and upward slope angle = 7°, and the discharge angles of the lower holes are all downward slope angles = 15°. In addition, for comparison, a two-hole nozzle was also produced and tested.

The measurement of the liquid level fluctuation in the water model test was carried out by photographing the meniscus (water surface) with a high-speed camera and measuring the amplitude of the average liquid level fluctuation for 60 seconds. In addition, the measurement of the amount of air bubbles involved was carried out by blowing air from the middle part of the nozzle, taking a picture of the downflow generation position in the mold with a high-speed camera in the center, and measuring the number of bubbles in the screen.

Fig. 6 shows the descending flow velocity (maximum value) at a position 1000 mm from the meniscus after discharge from the nozzle. The case where the descending flow rate is changed by changing the discharge angle of the upper hole is shown, and the nozzle with a discharge angle of 0° (horizontal) shows the minimum value. In addition, regarding the discharge angle, a negative value indicates an upward direction based on the horizontal direction, and a positive value indicates a downward direction based on the horizontal direction.

Table 2 shows the results of the water model test using each nozzle. In addition, the measured value assumes the measurement result of the two-hole nozzle as 100, and normalizes the measurement result of other nozzles, and expresses it.

With regard to the amount of air bubbles entrained by downflow, all four-hole nozzles were moderate or below. However, in the case of the nozzle in which the upper hole is formed at an upwardly inclined discharge angle of 7°, the fluctuation of the liquid level becomes large. In addition, compared with the case where the discharge angles of the upper hole and the lower hole are the same, the nozzle with a difference of 15° has a smaller amount of entrained air bubbles. From this result, it can be seen that by providing a discharge angle difference between the jets of molten steel from the upper hole and the lower hole, the effect of merging of the discharge flow from the upper hole and the discharge flow from the lower hole can be reduced.

Table 2

The above-mentioned present invention is constituted based on the results of further studies based on the above studies using numerical fluid analysis and experiments using a water model.

Hereinafter, each matter constituting the present invention will be further described.

In the present invention, a four-hole nozzle is formed in which two upper and lower discharge ports are provided on both sides of the nozzle. By using a four-hole nozzle, compared with the case of using a two-hole nozzle, the flow of molten steel is slowed down, and it is easy to form a uniform flow.

In the part immersed in the molten steel at the lower part of the nozzle, at the positions facing the inner walls of the two short sides of the mold, there are two upper and lower discharge ports consisting of an upper hole and a lower hole, respectively.

The shape of the discharge port is not particularly limited, and the upper hole and the lower hole are preferably rectangular. In this way, it is possible to further reduce fluctuations in the discharge amounts discharged from the respective discharge ports, thereby serving to form a uniform flow.

Ideally, the area of the opening of the lower hole is smaller than the area of the opening of the upper hole. Preferably, the opening area of the lower hole is 0.2 to 0.4 times the total opening area of the upper hole and the lower hole.

In this way, by making the opening area of the lower hole 0.2 to 0.4 times the total opening area of the upper hole and the lower hole, even if the molten steel has a pressure difference in the height direction, the flow rate of the molten steel passing through the upper hole and the lower hole can be distributed. Uniform. As a result, the discharge flow from the upper hole and the lower hole can be slowed down and uniformed, the molten steel can be discharged into the mold, and the flow velocity of the downward flow of the molten steel near the inner wall of the short side of the mold can be reduced.

The discharge angles (inclination angles of the outlet axes) of the molten steel jets from the lower hole and the upper hole are preferably in the range of 5° upward to 45° downward relative to the horizontal.

If the upward inclination angle of the upper hole and the lower hole exceeds 5°, the powder will be involved due to the fluctuation of the liquid level. In addition, if the downward inclination angle exceeds 45°, air bubbles and inclusions are likely to invade the inside of the slab. In order to more effectively prevent the entry of air bubbles and inclusions, the discharge angle of the upper hole is preferably in the range of 5° upward slope to 15° downward slope.

1 shows a case where the axis of the upper hole 2 is horizontal (inclination angle: α=0°), and the axis of the lower hole 3 is inclined downward relative to the horizontal (inclination angle: β).

In addition, due to the difference in discharge angle between the upper hole and the lower hole, the confluence position of the discharge flow from the upper hole and the discharge flow from the lower hole changes. Therefore, it is preferable to form the axial center angles of the lower hole and the upper hole so that the discharge angle of the lower hole is further inclined downward by 10° or more than the discharge angle of the upper hole.

Fig. 7 shows the relationship between the angle difference of the discharge flow from the upper hole and the lower hole and the amount of air bubbles obtained from the results of numerical fluid analysis and water model test. In Fig. 7, part (a) is a graph showing the relationship between the angle difference between the upper hole and the lower hole and the confluence position (distance from the center of the nozzle) of the discharge flow from the upper hole and the lower hole, and part (b) is a graph showing the relationship between In the graph of the relationship between the merging velocity at the merging position, part (c) is a graph showing the relationship between the jet flow velocity and the amount of air bubbles involved.

As shown in part (a), when the angle difference of the discharge flow is changed to 0 to 22°, the merging position moves to a position away from the discharge port as the angle difference increases. As the merging position moves away from the discharge hole, as shown in part (b), the jet velocity decreases, and with this decrease, as shown in part (c), the amount of entrained air bubbles decreases.

As shown by the arrow mark from part (c) to part (a) of Fig. 7, the discharge angle of the lower hole is set to have an angle difference of 10° or more in a direction inclined downward with respect to the discharge angle of the upper hole. In the case of an angle, the amount of air bubbles involved decreases steadily. When the angle difference is in the range of not less than 10° and not more than 22°, the amount of bubbles is desirably reduced. More preferably, the angle difference is in the range of not less than 15° and not more than 20°. With such a configuration, it is possible to more effectively prevent the merging of the discharge jets of molten steel from the upper hole and the lower hole.

It is more preferable that the distance D between the lower end of the upper hole (upper layer discharge port) and the upper end of the lower hole (lower layer discharge port) is in the range of 15 mm to 150 mm. When the interval is 15 mm or more, the confluence of the upper and lower molten steel flows discharged from the discharge port is more effectively prevented, and the effect of dispersing and discharging the molten steel from both holes is further enhanced. In addition, if the interval between the upper and lower discharge ports is 150 mm or less, even if the molten steel has a pressure difference in the height direction, the flow balance through the upper and lower discharge ports can be more properly maintained.

The present invention is constituted as above. Below, the implementation possibility and effect of the present invention will be further described through examples.

Example

Amount of C: 0.08% by mass of Al-Si killed steel was cast by a vertical bending continuous casting machine.

In production, nozzles having the following configurations were used as examples.

(1) A four-hole nozzle in which the opening area of the lower hole is 37.5% of the total area of the upper hole and the lower hole, and the discharge angles of the upper hole and the lower hole are both inclined downward by 15° (Example 1).

(2) A four-hole nozzle in which the discharge angle of the upper hole is 0° and the discharge angle of the lower hole is inclined downward by 15° (embodiment 2).

In addition, as a comparative example, nozzles having the following configurations were used.

(3) Two-hole nozzle (Comparative Example 1).

(4) A four-hole nozzle in which the opening area of the lower hole and the upper hole are equal, and the discharge angles are both downwardly inclined at 15° (comparative example 2).

Observation and measurement of air bubbles and inclusions in the center of the obtained slab with an optical microscope, the measurement result of the slab obtained using the two-hole nozzle of Comparative Example 1 was set as 100, and the slab obtained using other nozzles The measurement results are indexed (standardized).

Table 3 shows air bubbles and inclusion indices of slabs cast using each nozzle. In the examples of the present invention, compared with the conventional four-hole nozzle of Comparative Example 2, it was possible to suppress entry of air bubbles and inclusions into the slab. In addition, in the case of Example 2 in which the difference between the discharge angles of the upper hole and the lower hole was 10° or more, better results were obtained.

table 3

Industrial availability

Continuous casting using the four-hole nozzle of the present invention can obtain cast slabs with few occurrences of internal defects, so the present invention has great industrial applicability in the field of continuous casting.

This application claims the priority of Japanese Patent Application No. 2009-074687 for which it applied in Japan on March 25, 2009, and uses the content here.

Claims (4)

1. A submerged nozzle for continuous casting that discharges molten steel into a mold for continuous casting of steel, characterized in that it has a cylindrical nozzle body, At the part of the lower end side of the nozzle main body immersed in the molten steel in the mold, there are four molten steel outlets on the left row of the upper layer, the left row of the lower layer, the right row of the upper layer, and the right row of the lower layer. The two discharge ports of the left row and the two discharge ports of the right row have substantially symmetrical shapes with respect to the axis of the nozzle, The discharge port of the left row is opposite to the inner wall of the short side of the left side of the mold, The discharge port of the right row is opposite to the inner wall of the short side on the right side of the mold, The opening area of the discharge port of the lower layer is smaller than the opening area of the discharge port of the upper layer, A ratio of an opening area of the discharge port of the lower layer to a sum of opening areas of the discharge ports of the upper layer and the lower layer is 0.2 or more and 0.4 or less. 2. The submerged nozzle for continuous casting according to claim 1, wherein the distance between the lower end of the discharge port of the upper layer and the upper end of the discharge port of the lower layer is within a range of 15 mm to 150 mm. 3. The submerged nozzle for continuous casting according to claim 1 or 2, wherein the lower hole and the upper hole are formed in such a manner that the discharge angle of the discharge port of the upper layer and the discharge port of the lower layer They are all in the range of upward slope 5° to downward slope 45° relative to the horizontal, and the discharge angle of the discharge port of the lower layer is inclined downward by more than 10° based on the discharge angle of the discharge port of the upper layer. 4. The submerged nozzle for continuous casting according to claim 1 or 2, wherein both the discharge port of the upper layer and the discharge port of the lower layer are substantially rectangular.

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