JPH09108797A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make surface flow speed of molten steel in a continuous casting mold proper and to obtain a continuously cast slab having good quality by arranging a shifting magnetic field type electromagnetic coil capable of accelerating and decelerating the spouting flow from an immersion nozzle and a static magnetic field type electromagnetic coil at the position in the range of a specific casting length. SOLUTION: At the time of continuously casting the steel by immersing the nozzle into the molten steel in the mold, casting is executed in such a manner while accelerating or decelerating the molten steel spouting flow from the immersion nozzle 1 with the shifting magnetic field type electromagnetic coils 7, that the electromagnetic coils 8 impressing the static magnetic field are arranged at a position on both sides of the immersion nozzle 1 in the range of 1/6-1/3 of the casting width from the center of the mold in the width direction of the mold and near a meniscus in the mold 2 in the casting direction and impressing the static magnetic field on a part of the meniscus. Therefore, such fluid pattern as to secure the flow rate of the molten steel to some extent and prevent the collision with the flow in the opposite direction and the variation of the flow rate, is achieved, and the fluid of the molten steel in the mold is properly attained and the development of the involving of mold powder, etc., is prevented.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In continuous casting of steel, it is known that the flow of molten steel in a mold has a great influence on the surface and internal quality of a slab. Especially in the meniscus (molten steel surface) in the mold, when the surface velocity of the molten steel is high,
When a vertical vortex is generated, a phenomenon that the mold powder on the molten steel is caught in the molten steel occurs, and the caught powder is trapped by the solidified shell and often becomes a product defect.

On the other hand, under operating conditions in which the molten steel surface flow velocity of the meniscus is calmed down more than necessary, an initial solidified shell develops in the meniscus portion to trap inclusions and bubbles in the molten steel, and to deckle (meniscus). Occurs, and refers to a lump of metal containing a large amount of unmelted powder) is formed, which interferes with the operation. Therefore, it is necessary to control the molten steel surface flow velocity of the meniscus under conditions that do not cause these problems.

However, such a molten steel flow pattern cannot be controlled only by the shape of the immersion nozzle, the immersion depth of the immersion nozzle, etc. Therefore, conventionally, the flow control of the molten steel is performed by utilizing electromagnetic force. There are many proposed methods for performing.

For example, in the technique disclosed in Japanese Patent Laid-Open No. 57-17356, an electromagnet is installed in a mold, and a magnetic field in a direction perpendicular to the molten steel discharge flow from the immersion nozzle is applied to discharge the molten steel. It is to brake the flow. But,
In this method, the discharge flow from the nozzle may be a flow that bypasses the magnetic field band, and the molten steel flow in the mold becomes rather complicated, and it has not been possible to sufficiently improve the slab quality.

The technique disclosed in Japanese Patent Laid-Open No. 2-284750 applies a magnetic field to the entire region of the mold in the width direction. In this case, although the magnetic field detour phenomenon of the discharge flow as described above does not occur, since the braking force by the magnetic field acts on the entire molten steel flow in the mold, it is difficult to control the pattern of the flow in the mold, Depending on the casting conditions, it was not possible to exert a sufficient effect. This is because when the flow of molten steel is suppressed over the entire width in the meniscus, the effect of preventing powder entrapment is recognized, but inclusions such as alumina and bubbles contained in molten steel are trapped in the initial solidified shell. This is because it tends to be easier.

The technique disclosed in Japanese Unexamined Patent Publication No. 5-84550 applies an upward traveling magnetic field to the nozzle discharge flow and a static magnetic field to the meniscus. The static magnetic field is applied across the entire width of the mold. The above-mentioned JP-A-2-284750.
It had the same drawbacks as the technique disclosed in the publication.

In the technology disclosed in Japanese Patent Laid-Open No. 64-2771, a moving magnetic field type magnetic field applying device is arranged opposite to the long side of the mold, and a flow generated by a moving magnetic field is applied to the discharge flow to control the flow. To do. This method is characterized in that not only the discharge flow can be decelerated but also the acceleration can be achieved depending on the casting conditions, and the molten steel flow pattern in the mold can be optimized. However, even with this technique, it is difficult to control the instability of molten steel flow in the mold, and
In order to maintain the ideal flow pattern, it was necessary to use various sensors to detect the flow condition in the mold and control the direction and strength of the magnetic field.

[0009]

As described above, it has been difficult with the conventionally proposed techniques to carry out molten steel flow control in a mold in consideration of differences in casting conditions and various fluctuations.

In particular, many analyzes have been conducted so far on the entrainment of mold powder, and when the molten steel surface flow velocity is high, entrainment of powder into molten steel occurs such that the powder layer is scraped off by the molten steel. It has been clarified by a water model experiment and so on.Therefore, some techniques for controlling the molten steel surface flow velocity within a velocity range in which such entrainment does not occur have been proposed and implemented. It was difficult to completely prevent all defects from occurring.

In addition, if the flow of molten steel is suppressed over the entire surface in the width direction in the meniscus portion, an effect of preventing powder entrainment is recognized, but inclusions such as alumina and bubbles contained in molten steel and bubbles in the initial solidified shell are recognized. Capture is likely to occur. In order to reduce these, molten steel flow is required for the cleaning effect on the front surface of the solidified shell, which has been reported by several studies (eg, Sawada et al .; Materials and Processes, vol.8 (1995)). -344).

From the above, in order to improve the quality of the cast slab, it is necessary to prevent the collision of the flow in the opposite direction and the remarkable fluctuation of the flow velocity while ensuring a certain flow velocity of the molten steel in the meniscus portion in the mold. Achieving a flow pattern is essential.

The present invention optimizes the flow of molten steel in the mold,
It is intended to provide a method for obtaining a slab of good quality by preventing the entrainment of mold powder and the like.

[0014]

A method for continuously casting steel according to the present invention is a method for continuously casting steel by immersing a nozzle in molten steel in a mold, in which molten steel is discharged from an immersion nozzle by a moving magnetic field type electromagnetic coil. While accelerating or decelerating the flow, near the meniscus in the mold in the casting direction, in the range of 1/6 to 1/3 of the casting width length from the center of the mold in the casting width direction, to the positions on both sides with the immersion nozzle in between. An electromagnetic coil for applying a static magnetic field is installed, and casting is performed while applying a static magnetic field to a part of the meniscus.

The reason why the moving magnetic field is applied to the discharge flow of the immersion nozzle is that the flow speed of the discharge flow from the immersion nozzle must be controlled in order to optimize the flow in the mold. This is because it is effective to accelerate and decelerate. On the other hand, in the application of the static magnetic field to the discharge flow,
This is because the flow velocity can be reduced, but if the discharge flow velocity is slow, it cannot be accelerated and controlled as necessary.

The reason for applying a static magnetic field to the meniscus is to suppress the flow velocity only in a specific region in the width direction,
This is because it is effective to apply the static magnetic field to the specific position.

When the inventors measured the molten steel surface flow velocity of the meniscus while controlling the molten steel flow by applying a moving magnetic field to the molten steel discharge flow from the immersion nozzle in the actual machine, there was a distribution of the flow velocity in the mold width direction. It was also found that, depending on the casting conditions, there is not only a forward flow from the left and right short sides toward the immersion nozzle at the center of the mold, but also a backflow from the center of the mold toward the shorter side of the mold.

As a result of studying this point by a water model experiment and numerical analysis based on actual machine data, such a backflow phenomenon occurs when the strength of the moving magnetic field given to the discharge flow is improper and It was found that even when a strong strength was given, it was caused by the influence of argon gas blown into the molten steel to prevent the adhesion of alumina to the nozzle.

That is, the argon gas bubbles in the molten steel affect the molten steel flow when floating in the mold. that time,
Argon gas bubbles mainly float in the region from the vicinity of the immersion nozzle to about 1/4 of the casting width length, thereby forming a flow (backflow) from the center of the mold to the short side in the central region in the mold width direction. In addition, it was a factor causing the temporal fluctuation of the flow velocity.

FIG. 2 is a schematic view showing a molten steel flow pattern in the mold. As shown in FIG. 2, the molten steel flow in the mold is a reverse flow 10 from the center of the mold to the short side and a discharge flow from the immersion nozzle 1 rises to the meniscus near the short side 2 of the mold and rises up to the meniscus and is reversed. There is a forward flow 9 which is a flow from the short side of the mold to the center of the mold, and vertical vortices are generated due to collision between the two depending on the size relationship between the two, and entrainment of mold powder occurs.

In order to prevent this, it is necessary to damp the flow of the two without causing collision or the like.

In this case, regarding the installation position of the electromagnetic coil, the approximate position is determined in consideration of the fact that the position where gas bubbles mainly float is from the vicinity of the immersion nozzle to the vicinity of 1/4 of the casting width length, and the low position is set. The appropriate range was determined by model experiments and numerical analysis using melting point alloys.

As a result, if the installation position of the electromagnetic coil for the static magnetic field is too close to the immersion nozzle, that is, if it is less than 1/6 of the casting width length from the center of the mold, the ascending flow in the vicinity of the immersion nozzle causes a magnetic field band. The effect of the object of the present invention cannot be obtained because of detouring. Also, the electromagnetic coil of the static magnetic field is near the short side,
That is, when it is installed at a position away from the center of the mold by more than 1/3 of the width of the mold, the ascending current on the short side bypasses the magnetic field band.

[0024]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic sectional view of the continuous casting equipment of the present invention. 1 is an immersion nozzle, 2 is a short-sided mold, 3 is a discharge nozzle of the immersion nozzle, 4 is meniscus, 5 is molten steel, 6 is mold powder, 7 is an electromagnetic coil for generating a moving magnetic field, and 8 is an electromagnetic for generating a static magnetic field. It is a coil.

In the present invention, as shown in FIG. 1, the discharge flow 3 from the dipping nozzle 1 is accelerated or decelerated by the electromagnetic coil 7 by the moving magnetic field, and near the meniscus in the mold in the casting direction and from the center of the mold in the width direction of the mold. 1/6 to 1 / of casting width
Electromagnetic coils 8 with a static magnetic field were installed on the meniscus 4 on both sides of the immersion nozzle 1 in the range of 3.

[0027]

【Example】

(Example 1) A continuous casting machine having a casting thickness of 220 mm and a casting width of 1600 mm was used to cast an extremely low carbon steel slab at a casting speed of 2.5 m.
Minted / min. At this time, an electromagnetic coil with a moving magnetic field divided into two in the mold width direction was installed so that the center of the electromagnetic coil in the casting direction was 150 mm below the discharge hole position of the immersion nozzle, and a moving magnetic field was applied to the molten steel discharge flow. Then, reduce the speed and apply a static magnetic field to the electromagnetic coil centered near the meniscus in the mold in the casting direction, 400 mm from the center of the mold in the width direction of the mold, and placed on both sides with the immersion nozzle in between. A static magnetic field was applied to a part of the meniscus.
Argon gas was introduced from the upper part of the immersion nozzle at 9.5 Nl /
min blown.

At this time, when a refractory rod was immersed in molten steel to determine the molten steel surface flow velocity of the meniscus, a flow from the short side toward the center of the mold was observed in the vicinity of the short side of the mold. , 400mm from the center of the mold (casting width 1
/ 4; static magnetic field electromagnetic coil position), the flow velocity was almost 0, and there was almost no change in flow velocity, and it was found that the surface flow velocity was substantially suppressed by the effect of the static magnetic field application in this region.

On the other hand, as a comparative example, casting was performed under the same casting conditions without applying a static magnetic field to the meniscus, and the surface flow velocity was measured by the same method as described above. However, a maximum flow velocity of 30 cm / sec was observed, and backflow often occurred, and the temporal change in flow velocity was severe.

The quality of the slab thus manufactured is
When the cross-section of the slab was observed and the slab was evaluated by surface quality inspection of the thin plate product after rolling, the slab inclusions were obtained by statistically processing the number and size distribution of inclusions in the slab cross-section with an exponential function approximation. The index of the present invention is about 1/4 of that of the comparative example, and the surface defect occurrence rate of the thin plate product is 1.7% in the comparative example, whereas it is 0 in the example of the present invention. It was extremely good at 0.5% or less.

(Example 2) In Example 2, the casting thickness 22
A slab of ultra-low carbon steel was cast at a casting speed of 2.8 m / min by a continuous casting machine having 0 mm and a casting width of 1200 mm. At this time,
The electromagnetic coil with a moving magnetic field divided in two in the mold width direction is placed so that the center of the electromagnetic coil in the casting direction is below the discharge hole position of the immersion nozzle.
Installed so as to be 50 mm, apply a moving magnetic field to the molten steel discharge flow to accelerate and also apply a static magnetic field to the electromagnetic coil. The electromagnetic coil center is near the meniscus in the casting direction, and the mold center in the mold width direction. From 400 mm to 400 mm, the immersion nozzle was placed at both sides to apply a static magnetic field to a part of the meniscus. The amount of argon gas blown from the upper part of the immersion nozzle was 8 Nl / min.

When a rod made of refractory was immersed in molten steel and the molten steel surface flow velocity of the meniscus was determined, a flow from the short side toward the center of the mold was observed in the vicinity of the short side of the mold. At a position 400 mm from the center (the position of the electromagnetic coil of the static magnetic field), the flow velocity is almost 0.
Even at the 0 mm position (1/4 of the casting width), the maximum flow velocity is 5 cm /
It was sec and there was almost no change in the flow velocity.

On the other hand, as a comparative example, casting was carried out under the same casting conditions without applying a static magnetic field to the meniscus, and the surface flow velocity was measured by the same method as described above. However, a maximum flow velocity of 30 cm / sec was observed, and backflow also occurred, which caused a dramatic change in the flow velocity over time.

When the quality of the slab thus produced was evaluated in the same manner as in Example 1, the slab inclusion index shows that the example of the present invention is about 1/4 of the comparative example.
The surface defect occurrence rate of the thin plate product was 1.6% in the comparative example, but was 0.6% or less in the example of the present invention.

(Example 3) In Example 3, the casting thickness 22
A slab of ultra-low carbon steel was cast at a casting speed of 1.6 m / min with a continuous casting machine having a casting width of 0 mm and a casting width of 2100 mm. At this time, the electromagnetic coil of the moving magnetic field divided into two in the mold width direction is placed at the center of the electromagnetic coil in the casting direction 15 below the discharge hole position of the immersion nozzle.
Installed so as to be 0 mm, apply a moving magnetic field to the molten steel discharge flow to decelerate, and place an electromagnetic coil that applies a static magnetic field in the center of the electromagnetic coil near the meniscus in the casting direction,
A static magnetic field was applied to a part of the meniscus by placing the dipping nozzles 350 mm from the center of the mold in the width direction of the mold with the immersion nozzles in between. Argon gas was blown at 8 Nl / min from the upper part of the immersion nozzle.

At this time, when the molten steel surface flow velocity of the meniscus was determined in the same manner as in Examples 1 and 2, a flow from the short side of the mold to the center of the mold was observed in the vicinity of the short side of the mold. At a position 350 mm from the center of the mold (position of the static magnetic field electromagnetic coil), the flow velocity is almost 0, and 5 from the center of the mold.
Even at the position of 25 mm (1/4 of the casting width length), the maximum flow velocity was 5 cm / sec, and there was almost no change in flow velocity.
In this region, it was found that the surface flow was almost suppressed by the effect of the static magnetic field applied.

On the other hand, as a comparative example, casting was carried out under the same casting conditions as above without applying a static magnetic field in the meniscus portion, and the surface velocity was measured by the same method. A maximum flow velocity of 30 cm / sec was observed even at 4 positions, and backflow often occurred, and the change in flow velocity with time was severe.

The quality of the slab thus manufactured was evaluated in the same manner as in Examples 1 and 2, and the slab inclusion index was obtained. The example of the present invention was about 1/4 of the comparative example. The surface defect occurrence rate in the thin plate product is 1.5% in the comparative example, whereas it is 0.4% in the example of the present invention.
The following was extremely good.

[0039]

According to the steel continuous casting method of the present invention, a moving magnetic field type electromagnetic coil capable of accelerating and decelerating the discharge flow from the dipping nozzle, and casting from the center of the mold in the vicinity of the meniscus in the casting direction and in the mold center in the mold width direction. 1/6 to 1/3 of width
By providing an electromagnetic coil with a static magnetic field at a position within the range, the surface velocity of the molten steel in the continuous casting mold can be optimized, and a continuous casting slab with good quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a positional relationship of electromagnetic coils in a mold of the present invention.

FIG. 2 is a schematic diagram showing a molten steel flow pattern in a mold.

[Explanation of symbols]

 1: Immersion nozzle 2: Short side mold 3: Discharge hole 4: Meniscus 5: Molten steel 6: Mold powder 7: Electromagnetic coil for generating moving magnetic field 8: Electromagnetic coil for generating static magnetic field

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Ishii, Marunouchi 1-2-2, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Noriko Kubo 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Date Main Steel Pipe Co., Ltd.

Claims (1)

[Claims] 1. A method of continuously casting steel by immersing a nozzle in molten steel in a mold, accelerating or decelerating the molten steel discharge flow from the immersion nozzle by a moving magnetic field type electromagnetic coil while In the vicinity of the meniscus, in the width direction of the mold from the center of the mold to a range of 1/6 to 1/3 of the casting width, electromagnetic coils for applying a static magnetic field are installed on both sides of the meniscus by sandwiching the immersion nozzle. A continuous casting method for steel, which comprises casting while applying a static magnetic field to part of the steel.