JPH1015648A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize the flow of molten steel in a mold and to improve the quality of a cast slab by using the mold having a specific rectangular cross sectional cavity and setting an immersion nozzle into a specific range from the inner wall of a short side. SOLUTION: The mold having the cavity of the rectangular cross section of >=0.8m width, is used and the immersion nozzle for continuous casting is set so as to shift from the center in the width direction of the mold, and a spouting hole is formed on the peripheral surface of this immersion nozzle so as to direct to the short side at the other side. The immersion nozzle 1 is set in the range within 300mm from the inside of the short side at the one side of the mold and an electromagnetic coil is set at long sides of the mold so as to position the range of the center of the electromagnetic coil in the casting direction from the spouting hole height to the height level of an intersection point of the center line of the spouting hole and the inner wall of the short side at the other side. A shifting magnetic field shifting the magnetic field to the width direction of the mold, is formed by conducting the current to this electromagnetic coil, and the electromagnetic force induced with this magnetic field, is acted to the molten steel spouting flow to control the spouting flow.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting steel to improve the quality of the surface and inside of a slab by optimizing the flow of molten steel in a mold.

[0002]

2. Description of the Related Art In a conventional continuous casting method of slab, molten steel is poured into a continuous casting mold from a tundish for temporarily storing molten steel received from a pot. At this time, as shown in FIG. 4, a refractory nozzle 1 whose lower end is immersed in molten steel 5 in a mold is usually used for pouring. The immersion nozzle 1 has two or more molten steel discharge holes 3 that open symmetrically toward the inner wall on the short side of the mold, and is installed substantially at the center of the mold 2.

The reason for disposing the immersion nozzle 1 at such a location is to prevent the molten steel discharge flow from penetrating deep inside the slab, and to allow the molten steel to flow almost equally to the left and right.
This is to prevent the cast slab from being deflected.

However, the flow of molten steel in the immersion nozzle 1 depends on the way of narrowing the sliding gate to increase or decrease the amount of molten metal, the adhesion of alumina to the nozzle inner wall, and the argon blown to prevent the adhesion of alumina. It fluctuates variously under the influence of gas. Accordingly, the amount and speed of the molten steel discharged from the discharge holes 3 are not necessarily equal between the left and right discharge holes 3. When the left and right discharge flows are unbalanced as described above, the molten steel flow in the mold is caused to be non-uniform (drift) on the left and right sides, and a vertical vortex is generated in the molten steel meniscus portion (the molten steel surface in the mold). May be involved in molten steel.

In addition, due to its structure, the immersion nozzle 1 needs to be accurately set at the center of the mold. Nozzle 1 is mold 2
If it is shifted from the center of the molten steel, even if the molten steel discharge amount from the left and right discharge holes 3 is equal, the molten steel surface flow velocity in the mold, particularly in the meniscus part important for powder entrainment, differs between the left and right, This may cause eddy currents and the like, causing the mold powder 6 to be involved.

To cope with these problems, Japanese Patent Laid-Open Publication No. Hei 3 (1999) discloses a method in which an immersion nozzle is installed near the short side of a mold and molten steel is poured toward a wide area in the mold.
No. 138053.

[0007]

However, in the above-mentioned conventional method, it is difficult to completely prevent the instability of the flow of molten steel in the mold, and the quality of all the slabs to be produced is at a level that can be sufficiently satisfied. Can not be improved until.

That is, in the method of disposing the nozzle near the short side and pouring molten steel toward a wide area in the mold, as shown in FIG. Although the normal drift between the mold left and right, which occurs when pouring the molten steel, can be suppressed, it is difficult to control and maintain the molten steel flow in an appropriate state. The reason is that when the conditions such as the mold width and casting speed change, the molten steel flow changes significantly in the mold accordingly, and even under the same casting conditions, due to the adhesion of alumina to the nozzle inner wall, etc.
This is because the discharge flow rate and the discharge angle of the molten steel change.

Further, in the conventional method, since the trajectory of the molten steel itself is left-right asymmetric, the development of the solidified shell and the distribution of inclusions are left-right asymmetric. For example, if the development of solidified shell in the mold has unevenness in the slab width direction, it will not be resolved even by secondary cooling etc., and will affect the shape of the final solidified part,
It has an adverse effect on quality, such as segregation. Also, if the thickness and temperature of the solidified shell differ in the slab width direction, distortion occurs,
It also causes cracks and other defects.

Further, even if the immersion nozzle is installed eccentrically on the short side, if the position is close to the center of the mold, the area on the side opposite to the side where the nozzle has the discharge holes is too wide, and the area is Molten steel tends to stagnate, causing a drop in molten steel temperature and poor melting of mold powder. Further, the flow of the molten steel surface becomes almost completely unidirectional, and a vertical vortex which causes powder entrainment is likely to be generated near the downstream side of the nozzle. This phenomenon is particularly remarkable when the flow velocity is large.

The present invention has been made in view of such circumstances, and provides a continuous casting method of steel capable of stabilizing the control of the flow of molten steel in a mold and realizing an improvement in slab quality. The purpose is to do.

[0012]

The continuous casting method for steel according to the present invention uses a mold having a cavity having a rectangular cross section having a width of 0.8 m or more, and the immersion nozzle for continuous casting is shifted from the center in the mold width direction. A continuous casting method of steel in which a discharge hole is formed on the peripheral surface of the immersion nozzle so as to face the short side on the other side, and molten steel is discharged into the cavity of the mold through the discharge hole. Is arranged in a region up to 300 mm from the short side inner wall of one side of the mold, and the electromagnetic coil is arranged such that the center of the electromagnetic coil in the casting direction is at the discharge line center line and the other side from the height level of the discharge hole. It is arranged on the long side of the mold so as to be located in the region up to the height level of the intersection with the inner wall of the short side, and a current is supplied to this electromagnetic coil to form a moving magnetic field in which the magnetic field moves in the width direction of the mold. The induced electromagnetic force is applied to the molten steel discharge flow. Characterized by braking the discharge flow by.

[0013] When a moving magnetic field is applied to the molten steel flow in the cavity under the installation conditions of the immersion nozzle and the electromagnetic coil, the discharge flow from the immersion nozzle can be decelerated or accelerated as necessary. Can control the flow of molten steel.

In the method of the present invention, as shown in FIG. 1, only one discharge hole 3 is formed in the nozzle 1, and the installation position of the nozzle 1 is not at the center of the mold 2 but at the left or right of the mold 2. An electromagnetic coil is disposed on the long side of the mold in order to control the flow of molten steel discharged from the nozzle 1 from the short side on one side to 300 mm.

Note that the installation position of the nozzle 1 is 30
If it is closer to the center than 0 mm, the area between the nozzle and the short side becomes a flow stagnation area, and even if the flow is controlled by electromagnetic force, the heat supply to the meniscus will be insufficient, and trapping of inclusions will occur. It will be easier.

Here, as for the installation position of the electromagnetic coil, it is necessary to select a position suitable for controlling the flow of molten steel coming out of the discharge hole. That is, as shown in FIG. 5, it is most effective to arrange the electromagnetic coil 7 at a position where the obliquely downward discharge flow from the nozzle 1 crosses the magnetic field. In other words, to control the molten steel discharge flow to an optimal flow pattern regardless of the casting conditions such as the discharge flow rate and the mold size, it is necessary to use a moving magnetic field that moves in the mold width direction. It is required that the installation location of the electromagnetic coil be an appropriate position. By applying such a moving magnetic field, the discharge flow from the immersion nozzle is further urged in the longitudinal direction of the mold, the flow of molten steel in the meniscus portion is improved, and the generation of a vertical vortex is prevented.

As a result of repeated studies by the inventors through model experiments and numerical analysis, as shown in FIG. 2, the center of the electromagnetic coil in the casting direction is shifted from the height level of the discharge hole 3 to the center line of the discharge hole. It has been found that it is most effective to arrange the mold on the long side of the mold so as to be located in a region up to the height level of the intersection with the inner wall of the other short side.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Various embodiments of the present invention will be described below with reference to the accompanying drawings.

(Example 1) A mold size is 220 mm × 16.
Using a 00 mm continuous casting machine, test casting of a slab of extremely low carbon steel was performed while changing the installation position of the immersion nozzle 1 and the like.
The casting speed was 2 m / min. The cast slab was paid out for quality evaluation, and a part of the slab was subjected to test rolling to evaluate the quality of the product. The tundish is moved at regular time intervals, and the center axis of the immersion nozzle 1 is set to 200 mm, 300 mm, 400 mm, 5 mm from one short side of the mold 2.
Various castings were performed so as to be at positions of 00 mm, 600 mm, and 800 mm (center in the width direction). In addition, only one discharge hole 3 was formed in the nozzle 1 so as to be directed to the short side on one side.

The center of the electromagnetic coil 7 in the casting direction is about 150 m from the discharge hole 3 of the immersion nozzle 1.
m so that a moving magnetic field can be applied to the discharged molten steel flow.

The quality of the slab produced in this way is
The evaluation as shown in FIG. 6 was obtained by evaluating inclusions at the slab stage for each casting condition and evaluating the quality of the thin plate after rolling. FIG. 6 is a graph in which the horizontal axis represents the distance (mm) from the short side of the immersion nozzle, and the vertical axis represents the quality evaluation index of inclusions in steel. Evaluation was made by observing the cross section at the slab stage and inspecting the surface quality of the steel sheet product after rolling. In the figure, black circles show the results of the example where the magnetic field was applied, and white circles show the results of the comparative example where no magnetic field was applied.

Here, the quality evaluation index of inclusions in steel is
The distribution of the number and size of inclusions appearing in a certain region of the slab cross section is statistically processed by exponential function approximation, and is numerically obtained by point conversion. The smaller the index is, the better the quality is. The larger the index is, the worse the quality is.

As is apparent from the figure, when the moving magnetic field is applied, when the immersion nozzle 1 is arranged in a region from the inner wall of the short side to 300 mm, the quality evaluation index of inclusions in steel is 1.0.
It turned out to be below. As a result, the rate of occurrence of surface defects in a thin steel sheet product was 0.9% or less in the method of the present embodiment, whereas the rate of the conventional method was 1.7% in the conventional method.

In the method of the above embodiment, if it becomes difficult to continue casting due to such blockage or wear of the nozzle, as shown in FIG.
A new nozzle 1a is installed near the short side on the opposite side to the position, and casting can be continued using the nozzle 1a. By performing the replacement of the immersion nozzle in this manner, continuous continuous casting (continuous casting) can be performed without interrupting or stopping continuous casting.
In this case, the new nozzle 1a is
It may be installed on the same tundish as above, or may be installed on another tundish.

When the installation position of the immersion nozzle is shifted to the vicinity of the short side on the opposite side, when the moving magnetic field type magnetic field is applied, the moving direction of the magnetic field is reversed. , Can respond.

Further, in the conventional method, in order to control the flow of each of the right and left sides of the mold, a plurality of electromagnetic coils divided into two or more in the width direction of the mold are often required. According to the method of the present invention, the flow can be basically controlled by a single electromagnetic coil, which is advantageous in terms of capital investment.

[0027]

According to the present invention, it is possible to stabilize the flow of molten steel in a mold, especially the flow of molten steel on a molten metal surface, and no longitudinal vortex is generated, so that mold powder and the like are not involved. And a continuous cast slab of good quality can be obtained even in a steel type such as an ultra-low carbon aluminum killed steel.

In the conventional method, the continuous casting is continued because the nozzle is gradually closed due to the adhesion of alumina to the inner wall of the nozzle as the casting proceeds, or the nozzle is worn out by reaction with mold powder or the like. Became impossible. In that case, continuous casting had to be interrupted or stopped, and the nozzle had to be replaced, etc.
The use of the method of the present invention facilitates continuous casting over a long period of time.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an outline of a continuous casting mold used in a steel continuous casting method according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing a trajectory of a molten steel discharge flow when molten steel is continuously cast in a mold using the method of the present embodiment.

FIG. 3 is a schematic diagram illustrating a case where continuous casting is continuously performed by replacing an immersion nozzle in the method of the present embodiment.

FIG. 4 is a schematic view showing an outline of a mold and a dipping nozzle used in a conventional continuous casting method.

FIG. 5 is a schematic diagram showing the installation position of an immersion nozzle and an electromagnetic coil with respect to a mold.

FIG. 6 is a graph showing the effect of the present invention.

[Explanation of symbols]

1, 1a: immersion nozzle, 2: mold, 3, 3a: discharge hole,
4: molten steel surface (meniscus), 5: molten steel, 6: mold powder, 7: electromagnetic coil, 8: locus of molten steel.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Noriko Kubo 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd.

Claims (1)

[Claims] 1. A mold having a cavity having a rectangular cross section having a width of 0.8 m or more is used, and an immersion nozzle for continuous casting is displaced from a center in a width direction of the mold, and a short side on the other side is provided on a peripheral surface of the immersion nozzle. A continuous casting method of steel in which molten steel is discharged into a cavity of a mold through the discharge hole by forming a discharge hole so as to face the die, and an immersion nozzle is provided in an area up to 300 mm from a short side inner wall on one side of the mold. And the electromagnetic coil is positioned in a region where the center of the electromagnetic coil in the casting direction is from the height level of the discharge hole to the height level of the intersection of the discharge hole center line and the other short side inner wall. The electromagnetic coil is energized to form a moving magnetic field in which a magnetic field moves in the width direction of the mold, and the electromagnetic force induced by this magnetic field is applied to the molten steel discharge flow to discharge. Characterized by damping the flow Continuous casting method of steel.