JP4549112B2 - Continuous casting method - Google Patents

Description

  The present invention relates to a method for continuously casting molten steel in a continuous casting apparatus.

In a continuous casting apparatus that continuously casts a steel ingot such as a slab, molten steel in the tundish is injected into a mold via an immersion nozzle body provided at the bottom of the tundish. In the mold, the molten steel is discharged from a discharge hole provided at the tip of the immersion nozzle body, collides with the short side and the long side of the rectangular mold in plan view, and then flows along the upper and lower edges.
Conventionally, it has been known that depending on the location where the discharged molten steel collides and the flow condition after the collision, the solidification delay of the shell occurs or, in the worst case, causes breakout.

In order to avoid the above problem, in Patent Document 1, in a mold for a slab having a large length ratio between the long side and the short side, the discharge direction of the discharge molten steel and the center of the immersion nozzle body are lowered to the short side wall surface of the mold. By making the angle θ formed with the perpendicular to the range of 5 ° to 15 °, it is possible to prevent the discharged molten steel from directly hitting the short side and to cause the flow of the discharged molten steel in the vicinity of the short side. Techniques for preventing defects such as solidification delay have been disclosed.
JP 2000-263199 A (2nd to 3rd pages, FIG. 1)

However, when continuously casting a steel type having a large shrinkage rate during solidification, it has been found from the actual results of the field that another problem arises by using the technique described in Patent Document 1.
For example, in a slab mold having a long side of about 150 cm and a short side of about 25 cm, the angle between the diagonal of the mold and the long side in plan view is tan ⁻¹ (25/150) ≈9.4 °, When the discharge angle θ of the discharged molten steel is 5 ° to 15 °, the discharged molten steel flow directly hits the vicinity of the intersection between the long side and the short side of the mold from the immersion nozzle body.
The molten steel in contact with the mold intersection is in contact with the long and short sides of the mold and loses a lot of heat, so the cooling proceeds more rapidly than the other parts, and the shell shrinks due to this cooling. Also occurs. In the case of a steel type having a large shrinkage rate at the time of solidification, the amount of shell shrinkage at the intersection is relatively large, so that the solidified shell near the intersection is pulled toward the intersection, and as shown in SC of FIG. Get away from the wall.

Since the shell peeling part is not in contact with the mold, the amount of heat dissipation is very small, and if the situation where high temperature discharged molten steel from the immersion nozzle collides continues, a solidification delay or breakout of the peeling part will occur. It can also be a cause.
Then, in view of the said problem, this invention aims at providing the continuous casting method which does not produce the solidification defect near the cross | intersection part of a casting_mold | template.

In order to achieve the above object, the present invention takes the following technical means.
That is, the technical means for solving the problem in the present invention is that an immersion nozzle body in which discharge holes are provided toward the short side is inserted into a rectangular mold having a short side and a long side in plan view. This is a continuous casting apparatus in which the flow rate of the molten steel in the immersion nozzle body is variable by moving the slide plate provided on the base end side of the immersion nozzle body toward the long side of the mold, and the carbon amount is 0 In the continuous casting method of continuously casting 0.08 to 0.18% molten steel, the molten steel flow is prevented so that the molten steel flow discharged from the discharge hole does not hit the vicinity of the intersection of the long side and the short side of the mold. It is characterized by controlling the discharge direction of the liquid.

According to this technical means, during continuous casting of a steel type (medium carbon steel, carbon content 0.08 to 0.18%) having a large shrinkage during cooling, a molten steel flow discharged from the immersion nozzle body, that is, discharged molten steel It becomes possible to avoid the flow from colliding with the shell peeling portion generated at the intersecting portion of the mold, and it becomes possible to avoid defects such as solidification delay of the peeling portion and eliminate the cause of breakout. .
In addition, it is good to control the discharge direction of the said molten steel by changing the flow velocity of the molten steel in an immersion nozzle body.

In order to find this, the applicant of the present invention made a rectangular mold with a short side and a long side made of transparent plastic as an experimental model of slab casting, inserted an immersion nozzle body from above, and assumed it as molten steel. A water flow observation experiment was conducted.
As shown in FIG. 2, a discharge hole is formed on the tip side of the immersion nozzle body toward the short side of the mold, and the base end side of the immersion nozzle body enters and exits perpendicularly to the axial direction of the immersion nozzle body. A slide plate for adjusting the flow rate and flow rate of the molten steel (nozzle molten steel) in the nozzle is provided. In order to increase the flow rate of the nozzle molten steel, the slide plate is moved to the positive side of the Y axis, and to decrease the flow rate, the slide plate is moved to the negative side of the Y axis.

As a result of numerous experiments, the nozzle molten steel flow that has passed through the opening formed by the slide plate collides with the bottom of the immersion nozzle body and becomes a vortex flow. By this vortex flow, the discharge direction of the discharge molten steel flow is the X axis in plan view. It turned out that it was not parallel but inclined to the long side opposite to the opening in the immersion nozzle. It was found that when the opening degree is large, the inclination angle is large, and when the opening degree is small, ejection is performed substantially parallel to the short side, that is, the X axis.
Therefore, it is possible to control the discharge direction of the molten molten steel by changing the flow rate of the molten molten steel, that is, by moving the slide plate of the immersion nozzle body.

  Further, the long side of the mold has a length more than twice the short side, and the change of the flow rate of the molten steel is performed by changing the casting speed of the slab or the inner cross-sectional area of the immersion nozzle body. And it is good to set so that the said casting speed and the inner side cross-sectional area of an immersion nozzle body may satisfy | fill following Formula.

In other words, in actual operation of continuous casting equipment, it is usual to change the casting speed of the slab depending on the steel type and required product characteristics, and the flow rate of the nozzle molten steel is changed by moving the slide plate in conjunction with the casting speed. I am doing so. Further, when the casting lot changes, the immersion nozzle body is also replaced. When a different type of immersion nozzle body is used, the flow velocity of the nozzle molten steel also changes with the change of the inner cross-sectional area of the nozzle.
Thus, since the flow rate of the nozzle molten steel has a predetermined relationship with the casting speed or the submerged nozzle body cross-sectional area, the nozzle molten steel flow rate can be changed by changing the parameter of the casting speed or the submerged nozzle body cross-sectional area. As a result, the discharge direction can be controlled so that the discharge molten steel flow does not hit the vicinity of the intersecting portion of the mold.

Under this technical idea, the applicant of the present application conducted a continuous casting experiment under various conditions, and obtained a result as shown in FIG.
In FIG. 4, the horizontal axis is the average flow velocity of the nozzle molten steel, and includes the casting speed and the submerged nozzle cross-sectional area as control parameters. The vertical axis represents the corner angle (angle between the mold diagonal and the X axis) of the mold used when the experiment was performed. By setting the corner angle and the discharge angle of the discharged molten steel to be different from each other, the discharged molten steel flow can be prevented from hitting the vicinity of the intersection, and the occurrence of solidification defects in the shell can be prevented.

From experimental data under various conditions, it was found that the solidification delay of the shell occurred in the region between the solid line graph and the broken line graph. Therefore, it is preferable to set the casting speed or the cross-sectional area of the immersion nozzle so as to satisfy conditions other than the region.
The formula (1) expresses that the casting speed or the inner cross-sectional area of the immersion nozzle body is outside the above-mentioned region is the formula (1), and the casting speed or the immersion nozzle body of the continuous casting apparatus that satisfies this formula By setting it as an inner side cross-sectional area, it can avoid that a molten steel flow collides with a shell peeling part, and it becomes possible to avoid defects, such as a solidification delay of this peeling part.

Note that the corner angle θ satisfies the relationship of θ ≦ tan ⁻¹ (1/2) ≈26.6 ° because the long side of the mold has a length that is at least twice as long as the short side. It has become a thing.
As a technical means for solving the most preferable problem in the present invention, for rectangular slab casting having a short side and a long side, and the long side having a length more than twice as long as the short side. An immersion nozzle body in which discharge holes are provided toward the short side is inserted into the mold, and a slide plate provided on the base end side of the immersion nozzle body moves to the long side of the mold. In the continuous casting apparatus in which the flow rate of molten steel in the immersion nozzle body is variable, and continuously casting molten steel having a carbon content of 0.08 to 0.18%, molten steel discharged from the discharge holes In order to prevent the flow from hitting the vicinity of the intersection between the long side and the short side of the mold, the immersion nozzle body is changed by changing the casting speed of the slab or the inner cross-sectional area of the immersion nozzle body within the range satisfying the formula (1). Change the flow rate of the molten steel in the It may control the discharge direction of the flow.

  According to the present invention, solidification defects in the vicinity of the intersecting portion of the mold do not occur in slab continuous casting.

Embodiments of a continuous casting method according to the present invention will be described below with reference to the drawings.
As shown in FIG. 1, a continuous casting apparatus 1 that employs the present casting method includes a tundish 3 that temporarily stores molten steel 2, a mold 4, and a cooling stand 5 that cools a slab 9 that comes out of the mold 4, It has a plurality of support rolls 6 that support and move the slab 9.
The molten steel 2 carried by the ladle 7 is poured into the tundish 3 disposed below, and injected into the mold 4 while controlling the flow rate by a plurality of immersion nozzles 8 provided at the bottom of the tundish 3. It has become so. In the mold 4, the molten steel 2 is cooled (primary cooling), and only the surface portion thereof becomes a solidified slab 9 that is drawn out from the lower part of the mold 4.

The slab 9 pulled out in the vertical direction is gradually curved in the horizontal direction while being supported by the support roll 6, and the slab 9 that has become horizontal has a predetermined length by a gas cutting machine (not shown) provided on the downstream side. Divided into slab pieces. The cross section of this slab piece is substantially rectangular, and consists of narrow surfaces on both sides in the width direction and wide surfaces on both sides in the plate thickness direction.
In the mold 4 of the continuous casting apparatus 1, the present invention prevents the flow of the molten steel 2 a discharged from the discharge hole 15 from hitting the vicinity of the intersection 10 between the long side 24 and the short side 25 of the mold 4. The discharge direction of the flow of the molten steel 2a is controlled, and the flow of the molten steel 2a is prevented from colliding with the shell peeling portion SC generated at the intersecting portion 10 of the mold 4, thereby solidifying the shell peeling portion SC. It avoids defects such as delays.

In the following description, the molten steel flowing in the immersion nozzle 14 is referred to as “nozzle molten steel 2b”, and the molten steel discharged from the discharge hole 15 at the tip of the immersion nozzle is referred to as “discharge molten steel 2a”. The molten steel stored in the mold 4 and the tundish 3 is simply referred to as “molten steel 2”.
As shown in FIG. 3, the mold 4 is a substantially rectangular shape having a short side 25 and a long side 24 in a plan view, and the long side 24 has a length that is at least twice as long as the short side 25. It is for slab casting. The long side 24 of the mold 4 forms a wide surface of the slab 9 and the short side 25 forms a narrow slab surface.

The tip of the immersion nozzle 8 is inserted at a substantially central position in plan view from above the mold 4.
As shown in FIG. 2, one or more immersion nozzles 8 are provided on the bottom surface of the tundish 3, an injection hole 11 provided in the bottom of the tundish, a cylindrical insert nozzle 12 following the injection hole 11, and the insert nozzle 12 is composed of a cylindrical chute nozzle 13 following the nozzle 12 and a long cylindrical immersion nozzle 14 following the chute nozzle 13.
A pair of discharge holes 15, 15 facing substantially opposite in the horizontal direction are formed on the cylindrical side wall at the tip of the immersion nozzle 8, and “the total area of the discharge holes 15 ≧ the cross-sectional area S inside the immersion nozzle 14”. It has become. A dam portion 16 for adjusting the flow rate of the molten steel 2 flowing out from the tundish 3 is provided between the base end side of the immersion nozzle 8, specifically between the insert nozzle 12 and the chute nozzle 13.

  The dam portion 16 includes an upper plate 18 and a lower plate 19 each having a substantially circular through hole 17 penetrating vertically, and a slide plate that is slidable in the horizontal direction between the upper plate 18 and the lower plate 19. 20 is provided. The slide plate 20 is formed with a circular hole 21 having substantially the same shape as the through hole 17, and the nozzle molten steel is formed through an opening 22 formed by overlapping the circular hole 21 and the through hole 17. The flow rate of 2b can be controlled. The slide plate 20 slides in parallel with the long side 24 side of the mold 4, that is, the Y axis in FIG. 3, and the opening of the weir 16 is a position biased toward the long side 24 b side. It is something that exists. Each component of the immersion nozzle 8 described above is formed of a heat-resistant brick or a heat-resistant sintered body in order to withstand the high-temperature molten steel 2.

The molten steel 2 stored in the tundish 3 flows down from the bottom injection hole 11 by the head pressure in the order of insert nozzle 12 → weir 16 → chute nozzle 13 → immersion nozzle 14, and discharge holes 15 and 15 at the tip of the immersion nozzle. Into the mold 4.
Since the inner sectional area S of the immersion nozzle 14 is constant, the slide plate 20 controls the flow rate of the nozzle molten steel 2b.
As shown in FIGS. 2 and 3, the behavior of the discharged molten steel 2a discharged from the discharge hole 15 of the immersion nozzle 14 is clarified from a number of water experiment models performed by the present applicant.

That is, the nozzle molten steel 2b flow (falling flow) that has passed through the weir portion 16 collides with the tip in the immersion nozzle 14 to form a vortex flow, and the discharge direction of the discharge molten steel 2a flow is opposite to the opening 22 by this vortex flow. It becomes the drift which inclines to the long side 24a side. When the opening degree is large, in other words, when the nozzle molten steel has a large flow velocity, the discharge angle is large, and when the opening degree is small, that is, when the nozzle molten steel has a small flow velocity, the discharge is performed substantially parallel to the short side 25 side, that is, the X axis. Become.
In the case of this embodiment, the molten steel 2 supplied to the tundish 3 from the ladle 7 is called medium carbon steel, and the carbon amount [C] in the molten steel 2 is 0.08 to 0.18%. The molten steel 2 having such a component has a very large shrinkage rate when cooled compared to other component steels.

  When the molten steel 2 is primarily cooled in the mold 4, the molten steel 2 is in contact with the two sides of the long side 24 and the short side 25 of the mold 4 at the intersecting portion 10 of the mold 4, and much heat is taken away. Cooling proceeds more rapidly than the other portions, and shell shrinkage accompanying cooling occurs. At this time, since the shrinkage amount of the intersecting portion 10 is relatively large, the shell 23 solidified in the vicinity of the intersecting portion 10 is pulled to the intersecting portion 10 side, and the shell peeling portion SC is formed as shown in FIG. . Since the shell peeling portion SC is peeled from the peripheral wall of the mold 4 by about 1 to 2 mm, and the flux added to the gap enters, the heat dissipation amount toward the mold 4 is very large. It is in a very small state. Therefore, the shell peeling portion SC is slower in solidification than other portions, and the thickness of the shell 23 is thin.

On the other hand, as described above, the molten molten steel 2a has a drift, and if this drift continues to collide with the shell peeling part SC, the solidification delay of the shell peeling part SC may occur or it may cause a breakout. .
Therefore, in the present embodiment, the flow rate of the molten molten steel 2b is changed using the slab casting speed Vc and the cross sectional area S of the immersion nozzle 14 as control parameters so that the discharged molten steel 2a does not hit the vicinity of the intersection 10 of the mold 4. In order to avoid the occurrence of defects such as solidification delay.
Regarding the relationship between the occurrence of such solidification delay and the relationship between the casting speed Vc and the discharge direction of the discharged molten steel 2a, the applicant of the present application conducts casting experiments under a number of conditions.

FIG. 4 summarizes the experimental results. The horizontal axis represents the average flow velocity of the nozzle molten steel 2b, and includes the casting speed Vc and the cross-sectional area S of the immersion nozzle 14 which are control parameters. The vertical axis represents the corner angle of the mold 4 (angle between the mold 4 diagonal line and the X axis) used when the experiment was performed. By setting the corner angle and the discharge angle of the discharged molten steel 2a to be different from each other, the flow of the discharged molten steel 2a can be prevented from hitting the vicinity of the intersecting portion 10 and the occurrence of solidification defects in the shell 23 can be prevented.
From experimental data under various conditions, it became clear that the solidification delay of the shell 23 (） in the figure) occurred in the region sandwiched between the solid line graph and the broken line graph. Therefore, the casting speed Vc or the immersion nozzle cross-sectional area S may be set so as to satisfy conditions other than the region.

Expression (1) is a mathematical expression that the control parameter Vc or S is outside the region where solidification delay occurs. By satisfying this expression, the discharge molten steel 2a flow becomes the shell peeling portion. Collision with the SC can be avoided, and defects such as a solidification delay of the peeling portion SC can be avoided.
The corner angle θ is such that θ ≦ tan ⁻¹ (1/2) ≈26.6 ° because the long side 24 of the mold 4 has a length more than twice the short side 25. It satisfies the relationship.

  In addition, the above-mentioned determination of solidification delay is considered to cause a breakout at the solidification delay degree ≧ 50% (▲ in the figure), and stable casting if the solidification delay degree ≦ 30% (◇ in the figure). I can do it. The solidification state can be known by the negative segregation line (white band) generated when there is a flow in the molten steel, and the degree of solidification delay is that of the shell observed from the white band at a position of about 10 mm from the surface. It is defined by the formula (2) based on the thinnest δb among the thicknesses and the central thickness δa of each of the wide surface and the narrow surface, and is an index representing how much solidification is delayed.

When the casting speed Vc and the immersion nozzle cross-sectional area S, which are control parameters, satisfy the above-described formula (1), for example, as shown in FIG. The 2a flow avoids the intersection 10 and comes into contact with the short side 25 of the mold 4. On the contrary, the casting speed Vc = 1.6, and the discharged molten steel 2a flow hits the long side 24.
The present invention is not name limited to the above embodiment.

Also, the upper portion of the mold 4, when the electromagnetic stirring means for stirring the molten steel 2 in the mold 4 by an electromagnetic force is also installed, the technical idea of the present invention is applicable. As the electromagnetic stirring means, a pair of induction type linear motors installed so as to be opposed to the long side of the mold 4 is preferable, and when the linear motors are disposed above the discharge holes 15 and 15 of the immersion nozzle body 8. Good.

It is the schematic of a continuous casting apparatus. It is right side sectional drawing of a casting_mold | template. It is a top view of a casting_mold | template. It is the figure which showed the relationship between the molten steel flow velocity in an immersion nozzle, and solidification delay generation | occurrence | production. It is a corner sectional view of the molten steel being cooled in the mold.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Continuous casting apparatus 2 Molten steel 2a Discharge molten steel 2b Nozzle molten steel 4 Mold 8 Immersion nozzle body 15 Discharge hole

Claims (1)

A slab casting mold having a short side and a long side, the long side having a length that is at least twice as long as the short side, and having a discharge hole provided toward the short side. A continuous casting machine in which the nozzle body is inserted and the flow rate of the molten steel in the immersion nozzle body is variable by moving the slide plate provided on the base end side of the immersion nozzle body to the long side of the mold. In a continuous casting method for continuously casting molten steel having a carbon content of 0.08 to 0.18%,
In order to prevent the molten steel flow discharged from the discharge holes from hitting the vicinity of the intersection between the long side and the short side of the mold, the casting speed of the slab or the inner cross-sectional area of the submerged nozzle body is within a range satisfying the formula (1). The continuous casting method characterized by changing the flow rate of the molten steel in the immersion nozzle body by changing and controlling the discharge direction of the molten steel flow.

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