JP2010240686A - Method of controlling flow of molten steel in casting mold - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of controlling flow of molten steel in a casting mold, a method by which continuous casting is achieved without causing internal defect or surface grade deterioration of a cast slab, even in casting under high throughput conditions. <P>SOLUTION: In the method of controlling flow of molten steel in a casting mold, molten steel is poured in a casting mold 1 from a discharge port 3 of an immersion nozzle 2, with the molten steel turned by an electromagnetic stirring device 5 installed above the discharge port 3, and with the molten steel braked by an electromagnetic braking device 4 in the casting mold under the discharge port. The molten steel is controlled in a manner satisfying a relational expression of U-&alpha;VB<SP>2</SP>¾sin&theta;¾&ge;0.2 (provided &alpha; is a coefficient of 2.1 to 4.3), where V (m/s) is a discharge flow rate of molten steel from the discharge port 3, U (m/s) is a shell front surface flow rate by electromagnetic stirring, B (T) is a magnetic flux density at the discharge port position, and &theta; (&deg;) is a discharge port angle to a horizontal surface. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method for controlling the flow of molten steel in a mold in a steel continuous casting facility equipped with an electromagnetic stirring device and an electromagnetic brake device.

  In continuous casting of steel, molten steel is poured into a continuous casting mold from a tundish via an immersion nozzle and cooled by contact with the mold wall to form a solidified shell. It is performed by a method of continuously pulling downward at a speed. In the immersion nozzle, a discharge port is formed in the vicinity of the lower end portion and is directed downward from the horizontal plane. Molten steel is discharged from the discharge port into the mold.

  As shown in Patent Document 1, the immersion nozzle is generally installed at the center of the mold, and the discharge ports are generally formed in a pair on the left and right sides in the longitudinal direction of the mold. The molten steel discharged obliquely downward from these discharge ports collides with the inner surface of the solidified shell and is branched into an upward flow called reverse flow and a downward flow called osmotic flow.

However, when casting is performed under a high throughput condition exceeding 0.5 m ³ / min, the upward reversal flow becomes strong because the discharge speed of the molten steel from the discharge port of the immersion nozzle increases, and the mold powder on the surface of the melt It becomes easy to entrain air bubbles in molten steel. If the encased mold powder or air bubbles are trapped on the surface of the slab, it will cause a surface layer defect, and if the encapsulated mold powder is brought into the mold, it will cause internal defects. As shown in Patent Document 1.

  Therefore, in order to prevent the occurrence of surface layer defects, an electromagnetic stirring device is provided at the upper part of the mold to rotate the molten steel inside the mold, and an electromagnetic brake device is provided at the lower part of the mold to prevent the occurrence of internal defects. Applying a static magnetic field to brake molten steel is performed (Patent Document 2). This is shown in FIG. In FIG. 1, 1 is a mold, 2 is an immersion nozzle, 3 is a discharge port, and 4 is an electromagnetic brake device.

  However, when the osmotic flow of molten steel is strongly braked by the electromagnetic brake device 4, the downward flow is restricted, so that a counter flow 12 opposite to the main flow is generated around the discharge flow 11 as shown in FIG. 1. . And it became clear that when this counter flow 12 and the reversal flow 13 interfere with the swirl flow 15 by electromagnetic stirring, the swirl flow is locally reduced in flow velocity, and the surface quality of the slab at that portion deteriorates. Conversely, if the braking of the molten steel by the electromagnetic brake device 4 is weakened, the surface layer defects described above are likely to occur.

JP 2003-33847 A JP 2003-117636 A

The present invention solves the above-mentioned conventional problems, and continuous casting is possible without causing internal defects or surface quality of the slab even when casting is performed under high throughput conditions exceeding 0.5 m ³ / min. An object of the present invention is to provide a method for controlling the flow of molten steel in a mold.

The present invention made to solve the above problems is to inject molten steel into the mold from the discharge port of the immersion nozzle, swirl the molten steel by an electromagnetic stirrer installed above the discharge port, and below the discharge port. In the molten steel flow control method in which molten steel is braked by an electromagnetic brake device in the mold of the molten steel, the discharge velocity V (m / s) of the molten steel from the discharge port and the flow velocity U (m / s) of the shell front by electromagnetic stirring The magnetic flux density B (T) at the discharge port position and the discharge port angle θ (°) with respect to the horizontal plane are U−αVB ² | sin θ | ≧ 0.2 (where α is a coefficient of 2.1 to 4.3). Control is performed so as to satisfy the following relational expression.

  As in claim 2, it is preferable that the relational expression is satisfied by adjusting the discharge port angle θ of the immersion nozzle. As in claim 3, it is preferable that the magnetic flux density B at the discharge port position is 0.1 to 0.5T.

According to the present invention, the discharge flow velocity V (m / s) of the molten steel from the discharge port, the shell front surface flow rate U (m / s) by electromagnetic stirring, the magnetic flux density B (T) at the discharge port position, and the horizontal plane By controlling the discharge port angle θ (°) within an appropriate range, the upward flow caused by the counterflow 12 is suppressed even when casting is performed under high throughput conditions exceeding 0.5 m ³ / min. As a result, the surface defects of the slab can be greatly reduced. Moreover, since the molten steel is braked appropriately by the electromagnetic brake device, internal defects of the slab can also be suppressed.

It is sectional drawing explaining the problem of a prior art. It is an image figure of the molten steel flow in the casting_mold | template in a general continuous casting equipment of steel. It is an image figure of the molten steel flow in the casting_mold | template in the conventional case with large discharge port angle (theta). It is an image figure of the molten steel flow in a casting_mold | template when discharge port angle (theta) is made small. It is a graph which shows the relationship between a flow index and the number of surface defects. It is a graph which shows the relationship between discharge port angle (theta) and a flow index.

Preferred embodiments of the present invention are shown below.
FIG. 2 is a plan view and a cross-sectional view showing a general steel continuous casting equipment. As shown in FIG. 1, 1 is a mold for continuous casting, 2 is an immersion nozzle disposed in the center of the mold 1, 3 is a pair of left and right discharge ports formed in the lower part of the immersion nozzle 2, and 4 is an electromagnetic brake device. The electromagnetic brake device 4 is installed below the discharge port 3 of the immersion nozzle 2 and applies a static magnetic field to brake the molten steel. Further, an electromagnetic stirring device 5 is installed above the discharge port 3 of the immersion nozzle 2 to form a moving magnetic field and swirl the molten steel inside the mold 1 as shown in FIG. Note that both the electromagnetic brake device 4 and the electromagnetic stirring device 5 have been widely used in the past and can be controlled freely.

  As shown in FIG. 2, the molten steel discharge flow 11 discharged into the mold 1 from the discharge port 3 of the immersion nozzle 2 collides with the inner surface of the solidified shell 6, an upward flow called a reverse flow 13, an osmotic flow 14, It is branched into a downward flow called. The osmotic flow 14 is attenuated by being braked by the electromagnetic brake device 4. Further, the counter flow 12 in the opposite direction to the main flow of the discharge flow 11 is generated, but when the throughput is small, the counter flow 12 is also weak and does not hinder the swirling flow 15 by the electromagnetic stirring device 5, and there is no big problem.

However, when casting is performed under a high throughput condition exceeding 0.5 m ³ / min, if the braking of the osmotic flow 14 by the electromagnetic brake device 4 is strengthened to prevent the occurrence of internal defects, the counter flow 12 becomes strong, As described above, the local flow velocity lowers due to interference with the swirling flow 15 by the electromagnetic stirring device 5 and the surface quality of the slab deteriorates at that portion.

Therefore, in the present invention, the discharge velocity V (m / s) of molten steel from the discharge port, the shell front surface flow rate U (m / s) by electromagnetic stirring, the magnetic flux density B (T) at the discharge port position, and the discharge to the horizontal plane. The exit angle θ (°) is controlled within an appropriate range. Specifically, control is performed so that these variables satisfy the relationship of U−αVB ² | sinθ | ≧ 0.2. This relational expression means that a value (referred to as a flow index) obtained by subtracting the flow velocity decrease due to the counterflow 12 or the like from the shell front surface flow velocity U is larger than 0.2 m / sec. Here, the magnetic flux density (T: Tesla) at the discharge port position is usually set to 0.1 to 0.5 T in order to obtain an appropriate electromagnetic brake effect. Moreover, (alpha) is a coefficient of 2.1-4.3, and the value is mainly set according to casting cross-sectional size. The reason why the absolute value of sin θ is adopted is that the case where θ is upward, that is, takes a negative value, is taken into consideration.

  In order to satisfy the above relationship, any variable may be changed, but in practice, it is effective to change the discharge port angle θ with respect to the horizontal plane of the immersion nozzle 2 so as to be close to the horizontal. Actually, the discharge port angle θ is adjusted by replacing the immersion nozzle 2 in accordance with the discharge port angle θ. FIG. 3 shows an image of molten steel flow in the mold in the conventional case where the discharge port angle θ is large, and FIG. 4 shows an image of molten steel flow in the mold when the discharge port angle θ is reduced. In the case of FIG. 3, a strong counterflow 12 is generated and interferes with the swirl flow 15, but in the case of FIG. 4, the counterflow 12 is weak.

  In order to confirm the effect of the present invention, continuous casting was performed by varying the discharge flow velocity V, the magnetic flux density B, and the discharge port angle θ, and the evaluation was performed by counting the number of surface inclusions in the cast slab. . The results are summarized in Table 1. Vc is a casting speed. In this experiment, the width of the slab was 1.6 m, the thickness was 0.20 to 0.30 m, and the shell front surface flow velocity U by electromagnetic stirring was a constant value of 0.4 m / sec.

As shown in Table 1, when U-αVB ² | sinθ | ≧ 0.2 was satisfied, the surface inclusion index of the cast slab was zero. Here, the surface inclusion index is a value in terms of surface wrinkles (ke / cm ² ) slab. FIG. 5 shows a graph of this situation. FIG. 6 shows the relationship between the discharge port angle θ and the flow index. According to FIG. 6, a preferable result is obtained when the discharge port angle θ is smaller than 30 °.

  As described above, according to the method for controlling the flow of molten steel in the mold according to the present invention, continuous casting is possible without incurring internal defects or surface quality of the slab even when casting is performed under high throughput conditions. .

DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3 Discharge port 4 Electromagnetic brake device 5 Electromagnetic stirrer 6 Solidified shell 11 Discharge flow 12 Counterflow 13 Reverse flow 14 Osmotic flow 15 Swirling flow

Claims (3)

The molten steel is injected into the mold from the discharge port of the immersion nozzle, and the molten steel is swirled by the electromagnetic stirring device installed above the discharge port, and the molten steel is braked by the electromagnetic brake device in the mold below the discharge port. In the mold flow control method, the molten steel discharge flow velocity V (m / s) from the discharge port, the shell front flow velocity U (m / s) by electromagnetic stirring, the magnetic flux density B (T) at the discharge port position, and the horizontal plane The method of controlling the flow of molten steel in a mold is controlled so that the discharge port angle θ (°) with respect to the above satisfies the following relational expression:
U−αVB ² | sin θ | ≧ 0.2 (where α is a coefficient of 2.1 to 4.3)   2. The molten steel in-mold flow control method according to claim 1, wherein the relational expression is satisfied by adjusting a discharge port angle θ of the immersion nozzle.   The method for controlling the flow of molten steel in a mold according to claim 1, wherein the magnetic flux density B at the discharge port position is 0.1 to 0.5T.

Images