JP2000271710A - Steel continuous casting method - Google Patents

Abstract

(57) [Problem] To provide a continuous casting method of steel capable of producing a cast piece having few bubble defects and inclusion defects. A casting is performed by applying a horizontal static magnetic field having a magnetic flux density of 0.1 tesla or more to a molten steel flow immediately after exiting from a molten steel discharge hole of a dipping nozzle. A swirling flow or a periodic stirring flow is formed in the upper molten steel in the mold using an electromagnetic stirring device.

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 producing a steel slab which has few bubble defects and inclusion defects under the slab.

[0002]

2. Description of the Related Art In continuous casting of steel, a flow control method in a mold using various electromagnetic forces has been proposed for the purpose of improving the quality of a slab and increasing the casting speed. They are classified into using a moving magnetic field, using a DC magnetic field, and using both. The purpose of using the moving magnetic field is to form a flow in a different pattern from the flow formed by the nozzle discharge flow from the immersion nozzle.

On the other hand, the purpose of using a DC magnetic field is to stabilize the flow in a mold by reducing the flow velocity. In addition, in an example in which the two are combined, the aim is to invert the nozzle discharge flow upward by a DC magnetic field installed at the lower part of the mold or directly below the mold, and to agitate the upper pool by stirring by the moving magnetic field at the upper part of the mold. .

[0004]

In general, Ar, which is an inert gas, is often blown into an immersion nozzle in order to prevent nozzle blockage. In addition, since molten steel contains nonmetallic inclusions, the molten steel discharged from the nozzle is in a mixed phase containing inclusions and Ar bubbles. Appropriate control of such a multiphase fluid is necessary for improving the quality of the slab.

When a direct current magnetic field is provided below the mold or below the mold, the penetration depth of the nozzle discharge flow can be suppressed, but as the bubble diameter and inclusion diameter decrease, the floating speed decreases. Fine bubbles and inclusions are inevitably transported to the lower molten steel pool. This leads to defects inside the slab.

Accordingly, an object of the present invention is to provide a method capable of preventing bubbles and inclusion defects under the surface of a slab while suppressing the penetration depth of fine inclusions and bubbles existing in the nozzle discharge flow. The challenge is to provide

[0007]

According to the present invention, there is provided (1) an electromagnetic stirrer for stirring molten steel in an upper portion of a mold and a direct current magnetic field having a substantially uniform magnetic flux density distribution in a width direction of the mold below the electromagnetic stirrer. In a method of continuously casting steel while blowing Ar gas into an immersion nozzle using a continuous casting mold provided with an electromagnet capable of being applied to the molten steel, the molten steel immediately after leaving the discharge hole of the immersion nozzle has a diameter of 0.1 Tesla or more. A continuous casting method for steel, characterized in that a direct current magnetic field is applied and casting is performed while forming a swirling flow in a horizontal cross section by an electromagnetic stirrer at an upper portion thereof.

[0008] (2) Continuous casting comprising an electromagnetic stirrer for stirring the molten steel in the upper part of the mold and an electromagnet below the electromagnetic stirrer capable of applying a DC magnetic field having a substantially uniform magnetic flux density distribution in the width direction of the mold in the thickness direction. Ar in the immersion nozzle using a casting mold
In the method of continuously casting steel while blowing gas,
A direct current magnetic field of 0.1 Tesla or more is applied to molten steel immediately after exiting from the discharge hole of the immersion nozzle, and casting is performed while forming a periodic stirring flow in a horizontal cross section with an electromagnetic stirring device above the molten steel. Steel continuous casting method.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The present inventors have investigated the jet behavior of liquid metal in a DC magnetic field. In particular, it was found that a flow opposite to the main flow occurred around the jet. The Lorentz force acting to dampen the jet acts, but due to the continuity of the current induced in the pool, the Lorentz force accelerates the fluid around the jet core and the fluid around the core in the opposite direction. Investigation of the relationship between the flow velocity of the flow in the opposite direction and the applied magnetic flux density revealed that the tendency was remarkable when a magnetic field of 0.1 Tesla or more was applied as shown in FIG.

Therefore, a mercury model experimental apparatus having a size of 1/2 of the actual machine was manufactured, and the behavior of bubbles when an electromagnetic force was applied was investigated and analyzed. In this experimental device, an electromagnetic stirrer is provided near the liquid level of the mercury pool corresponding to the upper part of the continuous cast strand pool, and an electromagnet that can apply a DC magnetic field having a uniform magnetic flux density distribution in the width direction below the mercury pool in the thickness direction. Is incorporated. Also, only one of the pool wide surfaces is made of acrylic so that the behavior of bubbles in the pool can be observed.

In the experiment, the distribution of Ar bubbles on the surface of the mercury pool and the distribution of Ar bubbles on the wide surface were investigated by changing the conditions for applying the electromagnetic force in various ways. As a result, it was found that when a DC magnetic field was applied to the nozzle discharge flow, the number of Ar bubbles around the nozzle became larger than when no electromagnetic force was applied.

On the other hand, it was confirmed that the number of bubbles on the wide surface below the pool was smaller than when no electromagnetic force was applied. Furthermore, when a swirling flow is formed in the horizontal cross section of the pool using the electromagnetic stirring coil installed near the liquid surface, it is found that the number of bubbles around the nozzle decreases and the number of Ar bubbles on the wide surface also decreases. Was.

Next, while applying a DC magnetic field to the nozzle discharge flow, the current applied to the electromagnetic stirring coil installed near the surface is changed at a cycle of 3 seconds. When the stirring direction was changed, the number of bubbles adhering to the acrylic wall below the pool could be further reduced.

[0014]

EXAMPLE The present inventors continuously cast low carbon steel using the slab continuous casting apparatus shown in FIG. FIG. 2A is a schematic diagram of a horizontal cross section, and FIG. 2B is a schematic diagram of a vertical cross section. In the drawing, reference numeral 1 denotes an immersion nozzle, and two discharge holes 13 for forming a discharge molten steel flow obliquely downward toward the short side of the slab are provided near the lower end.

Reference numeral 5 denotes a mold for producing a slab having a width of 1250 mm and a thickness of 250 mm. An electromagnetic stirrer 7 for forming a swirling flow in the direction of arrow 10 in the molten steel in the upper part of the mold is arranged at the upper part. An electromagnet 6 that can apply a DC magnetic field having a substantially uniform magnetic flux density distribution in the width direction in the thickness direction to the molten steel flow 2 immediately after exiting from the discharge hole 13 is disposed below the discharge hole 13. In the figure, reference numeral 8 denotes a solidified shell. As other casting conditions, the casting speed was 2 m / min, and Ar gas was 10 liter / min in the nozzle.

As a condition for applying the electromagnetic force, when no electromagnetic force is applied, when a DC magnetic field of 0.1 Tesla is applied to the nozzle discharge flow, a DC magnetic field of 0.1 Tesla is applied to the nozzle discharge flow. When rotating stirring is performed using an electromagnetic stirring coil, a DC magnetic field of 0.1 Tesla is applied to the nozzle discharge flow, and the stirring direction is periodically changed by using the electromagnetic stirring coil and periodically changing the coil current. This is the case.

When casting was performed while applying a DC magnetic field of 0.1 Tesla or more to the nozzle discharge flow, the internal quality of the slab was significantly improved. However, many air bubbles and air bubbles containing inclusions were trapped under the slab under the center of the width of the slab. This seems to be due to the fact that bubbles and inclusions existing in the nozzle discharge flow are transported to the upper pool extremely efficiently by the reverse flow formed around the nozzle discharge flow. However, since it is easy to stagnate between the nozzle and the long side surface, it is considered that bubbles and inclusions were easily captured.

When continuous casting is performed using a mold having a moving magnetic field capable of forming a swirling flow in a horizontal cross section at the top of the mold, the inner quality is the same as that when only a DC magnetic field is applied, and the skin at the center of the width is obtained. No slab was found under the slab, and a slab of good slab quality was obtained both under the skin and inside the slab.

Further, in the case of (1), interference between the stirring flow by the electromagnetic stirring coil and the nozzle discharge flow is likely to occur near the molten metal surface, and a stagnation region is likely to be formed near the short side from the width of 1/4.
In case (1), the stagnation was not formed, so that the surface quality could be further improved as compared with case (2).

FIG. 2 shows an example of a bottomed immersion nozzle having discharge holes 13 on the left and right sides. The present inventors have shown a bell-shaped immersion nozzle in which a slit connecting both discharge holes 13 is further formed at the bottom. The same test as described in 2 was performed. In this bell-shaped immersion nozzle, molten steel is supplied to the left and right discharge holes 13.
, And out of the slits.

Also in this bell-shaped immersion nozzle, the depth of air bubbles can be reduced by applying a DC magnetic field to the molten steel to be discharged. Can greatly reduce the inclusion inclusion defect.

[0022]

According to the present invention, when continuous casting is performed while supplying Ar gas into the immersion nozzle, the invasion of bubbles and inclusions below the pool by the DC magnetic field is suppressed as much as possible, so that the upper portion of the pool is formed. It can be transported efficiently. Furthermore, in the vicinity of the molten metal surface, a stir flow is formed in a horizontal cross section or a vibrating stirrer is applied to generate bubbles and
Inclusions can be prevented from being trapped. Therefore, the quality of the cast slab is good both on the surface and inside.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a magnetic flux density and a flow velocity of a reverse flow.

FIG. 2 is an explanatory view of a continuous casting apparatus used in the embodiment.

[Explanation of symbols]

1: immersion nozzle, 2: molten steel flow, 5: mold,
6: DC magnetic field, 7: electromagnetic stirring device, 8: solidified shell, 9: meniscus, 10: swirling flow of molten steel by electromagnetic stirring device, 11: reverse flow of molten steel, 12: Ar gas bubble, 13: immersion nozzle Molten steel discharge hole.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Takehiko Fuji 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Eiichi Takeuchi 20-1 Shintomi, Futtsu-shi, Chiba New Japan F-term in Technical Development Division, Steel Corporation (reference) 4E004 AA09 HA01 MC00

Claims (2)

[Claims] 1. A continuous casting mold comprising an electromagnetic stirring device for stirring molten steel in an upper portion of a mold and an electromagnet below the electromagnetic stirring device capable of applying a DC magnetic field having a substantially uniform magnetic flux density distribution in a thickness direction. In a method of continuously casting steel while blowing Ar gas into an immersion nozzle using a method, a direct current magnetic field of 0.1 Tesla or more is applied to molten steel immediately after exiting from a discharge hole of the immersion nozzle, and electromagnetic stirring is performed on an upper part of the molten steel. A continuous casting method for steel, wherein casting is performed while forming a swirling flow in a horizontal section by an apparatus. 2. A continuous casting mold comprising an electromagnetic stirrer for stirring molten steel in an upper portion of a mold and an electromagnet below the electromagnetic stirring device capable of applying a DC magnetic field having a substantially uniform magnetic flux density distribution in a thickness direction. In a method of continuously casting steel while blowing Ar gas into an immersion nozzle using a method, a direct current magnetic field of 0.1 Tesla or more is applied to molten steel immediately after exiting from a discharge hole of the immersion nozzle, and electromagnetic stirring is performed on an upper part of the molten steel. A continuous casting method for steel, wherein casting is performed while forming a periodic stirring flow in a horizontal section by an apparatus.