JPH0584551A - Method for continuously casting steel using static magnetic field - Google Patents

Abstract

PURPOSE:To enable continuous casting of a steel excellent not only in internal quality but also in surface characteristic by impressing static magnetic field at near the meniscus in a mold and using the impressions of DC current together with the static magnetic field. CONSTITUTION:The molten steel in a tundish is supplied into the mold 1 for continuous casting through the immersion nozzle 2. A coil 3 for generating the static magnetic field is arranged at just below the mold 1 so as to be possible to impress over the whole width of the cast slab 5. In order to control the molten steel flow at near the meniscus, the static magnetic field generating coil 8 is arranged along long side walls 1b, 1b' in the mold, and by generating the magnetic field crossing at the right angle to the long side walls 1b, 1b' in the mold at near the molten steel surface, speed of the ascent reflecting flow developed after the discharged spouting flow of the nozzle collides to the short side, is relaxed and the involution of the mold powder is prevented.

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 using a static magnetic field, and particularly to a technique effectively applied to further improve the internal quality and surface quality of a continuously cast slab. Is.

[0002]

2. Description of the Related Art In the production of low carbon and ultra low carbon aluminum killed steel as a material for cold-rolled steel sheets, in the case of continuous casting, it is common to use a two-hole type immersion nozzle 2 as shown in FIG. Is. Therefore, if such a 2
When trying to perform high-speed casting with a high throughput per unit time using a hole nozzle, inclusions and bubbles penetrate deeply into the molten steel 6 in the mold, and moreover, powder entrainment on the molten metal surface in the mold increases, There is a problem in that they are trapped by the solidified shell and many product defects (three bars, blisters, etc.) occur.

[0003] Therefore, conventionally, as a technique for preventing the above-mentioned problems, in particular, product defects, ladle refining enhances the cleaning of molten steel, and a large-capacity tundish prevents the inclusion of ladle slag and tundish powder. Methods such as aiming to promote the floating of inclusions in the mold by adopting the vertical part of the mold, and preventing the inclusion of powder and inclusions by improving the shape of the immersion nozzle have been proposed.

However, these known product defect occurrence preventing techniques have not yet reached a method that is sufficiently effective even in a production process that meets the required product quality level (cleanliness) and the required production amount. It was the reality.

In addition, inclusions brought into the mold and mold powder to be taken into the mold, when the throughput per unit time exceeds a certain limit value, it becomes difficult to remove by floating, and therefore, the trapped particles in the steel. There were many results.

On the other hand, conventionally, a method as shown in FIG. 2 has been proposed as a method for overcoming the problems of the prior arts. This improved technique is to install a static magnetic field generating coil 9 in a mold of a slab continuous casting machine, and to apply a static magnetic field in the molten steel in the mold, thereby causing Lorentz generated by an interaction between a current and a magnetic field induced in the molten steel. The molten steel flow is controlled by force, and the jet flow discharged from the dipping nozzle 2 is prevented from entering deeply into the molten steel pool, thereby preventing the entrainment of mold powder and floating the inclusions brought into the molten steel. This is the method proposed in JP-A-62-254955, for example.

That is, this known improvement technique is a method of using a two-hole nozzle as the dipping nozzle, and letting a static magnetic field act in a direction orthogonal to the long side surface 1b of the mold. Certainly, according to this method, it was possible to reduce the amount of inclusions and bubbles trapped in the vicinity of the 1/4 accumulation zone, which would cause blistering defects in a curved continuous casting machine, to some extent.

[0008]

However, also in this technique, the cross-sectional area of the left and right outlets 7 and 7'becomes unbalanced when the casting speed or the width of the cast piece fluctuates or the immersion nozzle is clogged. It cannot be said that the method is effective even in the case where internal drift occurs, and there is a drawback that inclusions and bubbles are rather deeply involved in the molten steel in the mold.

That is, the drawbacks of the prior art are considered as follows. (1) It is not enough to apply the above static magnetic field to the molten steel pool. As shown in Figure 2,
In the circuit of the induced current generated by the interaction between the discharge jet of the immersion nozzle and the static magnetic field, two regions, that is, the region for decelerating the discharge jet velocity and the region for accelerating the discharge jet velocity, are formed. However, there was a limit to the optimum control of molten steel flow in the mold. This will be specifically described below with reference to FIG. That is, due to the interaction between the jet jet v discharged from the immersion nozzle and the static magnetic field B, an induced current I is generated in the main flow portion of the jet jet. Due to the interaction between the induced current I and the static magnetic field B, an electromagnetic force F is generated in the direction opposite to the direction of the jet flow, and the discharge jet flow v is decelerated. On the other hand, on the other hand, in the region on the long side surface of the mold, an electromagnetic force F'is generated due to the interaction between the return current I'of the induced current I and the static magnetic field B, and the nozzle ejection jet flow is accelerated on this side. It is because During this period, the molten steel flow velocity distribution (v) when the static magnetic field B is not applied is shown by a solid line, while the molten steel flow velocity distribution (v ′) when the static magnetic field B is applied is shown by a one-dot chain line. Further, as shown in FIG. 2-a, the static magnetic field B acts like a reflector on the molten steel jet flow from the immersion nozzle, so when the magnetic field arrangement is poor, the static magnetic field B descends at the center of the mold. The flow velocity was increased, which also caused inclusions and bubbles to penetrate deeply into the molten steel.

(2) Generally, as the submerged nozzle ejection port, conventionally, two ejection ports formed horizontally, downward or upward are adopted. With such a nozzle discharge port, as shown in FIG. 3, a stagnation part is formed in the discharge flow at the part where the direction of the molten steel flow path of the discharge port part 7, 7'changes.
There was a problem that alumina adhered and accumulated at that portion, and the nozzle was clogged with the elapse of casting time, so that a desired molten steel flow rate could not be obtained. In order to prevent this, conventionally, during the supply of molten steel, as shown in FIG. 1, an adverse gas such as argon was supplied into the immersion nozzle to cope with the adverse effect. However, even with this method, when the inert gas supply rate is high,
This inert gas could not float on the molten metal surface in the mold and was trapped by the solidified shell, which sometimes became a defect in the final product. In addition, simply blowing an inert gas is not sufficient for avoiding the nozzle clogging, and requires frequent replacement work for nozzle replacement. Especially, a two-hole nozzle with a symmetrical discharge port at the tip of the immersion nozzle is required. In the immersion nozzle of the type, there is also a problem that the left and right of the discharge port are asymmetrically closed and the quality is degraded.

An object of the present invention is to solve the above problems in continuous casting and to propose a continuous casting method capable of obtaining a steel slab having good internal quality and surface quality.

[0012]

As a means for achieving such an object, the present invention applies a static magnetic field in a direction orthogonal to the long side surface of the mold in the vicinity of the mold meniscus in continuous casting of molten metal. On the other hand, below the immersion nozzle discharge port, by applying a static magnetic field in a direction orthogonal to the long side surface of the slab and applying a direct current in a direction orthogonal to the short side surface of the slab, the latter An upward electromagnetic force is generated with respect to molten steel in a slab at a static magnetic field energization position, and by extension, the entire molten steel flow including the meniscus portion is controlled to continuously cast high quality steel.

[0013]

In the process of developing the above-described method of the present invention, the inventors have aimed to prevent uneven flow in the mold due to imbalance of molten steel outflow from the left and right discharge ports of the immersion nozzle due to nozzle clogging. As a result of conducting experiments with variously changing the nozzle shape, the following knowledge was obtained regarding nozzle clogging. That is, if the carbon concentration is 500 ppm or less,
As a result of various investigations and investigations on nozzle clogging during continuous casting using low carbon aluminum killed steel deoxidized with, the oxygen concentration in the molten steel was 30 ppm or less, more preferably
It was clarified that when the straight nozzle was adjusted to 20 ppm or less and the tip of the nozzle body of the dipping nozzle was opened to serve as the molten steel discharge port, there was almost no nozzle clogging even if Ar gas was not blown into the nozzle. Further, in such a straight nozzle, since the molten steel discharge flow is directed to the outlet side (downward) of the mold, inclusions and gas bubbles in the molten steel may penetrate deep into the crater. It was also found that it is effective to prevent the invasion of the steel by applying an upward flow velocity by the above-mentioned static magnetic field energization method and applying a braking force to the downward molten steel flow. The present invention is a technology developed under such knowledge.

FIG. 4 is a diagram for explaining the configuration of the present invention. Reference numeral 1 shown in the figure denotes a pair of short side walls 1a, 1a '.
And a long side wall 1b, 1b ', a continuous casting mold 2, a dipping nozzle 3 for supplying molten steel in the tundish into the continuous casting mold 1, and a nozzle 3 placed directly under the mold 1. The static magnetic field generating coils 4 and 4 provided are energizing rollers. The static magnetic field generated by the magnetic field generating coil 3 is
So that it can be applied over the entire width of. In this FIG.
The direction of the magnetic field B, the direction of the current I, and the direction of the electromagnetic force F in the molten steel 6 are shown by the one-dot chain line, the dotted line, and the two-dot chain line, respectively.

In the above-mentioned structure of the present invention, lower than the level of the immersion nozzle discharge ports 7, 7 '(in the casting direction).
Static magnetic field generating coil 3 and energizing rolls 4 and 4 installed in
Although each ′ has one stage, two or more stages having the same structure may be set in the casting direction.

Further, in the present invention, in order to control the molten steel flow in the vicinity of the meniscus, the static magnetic field generating coil 8 is arranged along the long side walls 1b and 1b 'of the mold at this position (level) to form the molten steel interface. In the vicinity, by generating a magnetic field orthogonal to the mold long side walls 1b, 1b ', the velocity of the rising reversal flow generated after the nozzle discharge jet collides with the short side is moderated, and such a rising reversal flow is generated. We are trying to prevent the entrainment of the mold powder that is the cause.

As described above, according to the present invention, the static magnetic field is applied at a position lower than the immersion nozzle discharge port to reduce the downflow in the slab and prevent the intrusion of inclusions and bubbles. In addition, a static magnetic field is applied in the vicinity of the molten steel surface to suppress reversing upward flow on the short side and level fluctuations in the molten steel scene, and to prevent the entrainment of mold powder.

Next, the embodiment of the present invention shown in FIG.
This is an example when the immersion nozzle 2 is a single hole straight type. When such a single hole nozzle is used, molten steel is discharged straight without injecting an inert gas, so that as shown in FIG. Since there is no stagnation of the molten steel flow, the nozzle clogging is slight compared with the two-hole nozzle.
Furthermore, since it is a straight type, molten steel drift does not occur on the left and right sides of the mold width. For the above reason, when the present nozzle is used, bubble and powdery slab defects are drastically reduced.

The distribution of the magnetic flux density in the width direction of the slab by the magnetic field generating coil 3 for energizing the static magnetic field is not only applied uniformly in the width direction of the slab, but when a two-hole immersion nozzle is used as shown in FIG. , In order to further reduce the downward flow velocity in the short side, the magnetic flux density in the short side direction of the slab is made higher than that in the central part of the slab, or
As shown in FIG. 5, when using a single-hole straight nozzle,
Depending on the flow condition of the molten steel to be controlled, the magnetic flux density in the center of the width direction of the slab below the nozzle outlet is made higher than that of the short side, and the distribution of the magnetic flux density in the width direction of the slab and the applied current By appropriately selecting the value, the effect of improving the quality of the cast piece is further enhanced.

[0020]

[Examples] The examples described below are ultra-low carbon aluminum-killed steel ( C) obtained by subjecting to RH treatment after blowing in a converter.
= 15-25 ppm), under the experimental conditions shown in Table 1, molten steel throughput of 5.2 tons / min, 5 consecutive strands of the same strand
This is a report of continuous casting (280 tons of molten steel per station). As the casting method, the six casting methods shown in Table 2 were adopted, and the slab cast by each casting method was subjected to hot and cold rolling to produce a cold rolled steel sheet having a thickness of 0.4 mm. On the inspection line, the occurrence rates of three bars and blisters due to steelmaking were compared. The results are shown in Table 3.

In the methods (4) and (6) of the present invention, no Ar gas was blown into the immersion nozzle, and the methods (3), (5) of the present invention and the conventional method (1), In (2), from the nozzle on the tundish or the sliding plate,
12 Nl / min. Of Ar gas was blown into the immersion nozzle during casting.

As is clear from Table 3, when the method of the present invention is adopted, the defect occurrence rate in the cold-rolled steel sheet can be greatly reduced as compared with the conventional method. Casting methods 3) and 4) are respectively 5) and
Compared to 6), the defect generation rate was further reduced, which is the effect of applying a static magnetic field near the molten steel surface.

[0023]

[0024]

[0025]

[0026]

As described above, according to the present invention,
It enables continuous casting of not only the internal quality of steel but also excellent surface properties.

[Brief description of drawings]

FIG. 1 is an explanatory view of a conventional continuous casting method.

2A is an explanatory view of a conventional continuous casting method using a static magnetic field, FIG. 2B is a broken view of a BB line, and FIG. 2C is a conceptual view of a BB nozzle jet flow. ..

FIG. 3 is an explanatory diagram showing how a nozzle clogging occurs in a two-hole immersion nozzle.

FIG. 4 is an explanatory view showing an example (two-hole nozzle) of the continuous casting method of the present invention.

FIG. 5 is an explanatory view showing another example (single hole nozzle) of the method of the present invention.

FIG. 6 is a diagram showing a magnetic flux density distribution (static magnetic field (b)) in the width direction of a cast piece when the methods (3) and (4) of the present invention shown in Table 2 are applied.

[Explanation of symbols]

 1 Continuous casting mold 1a, 1a 'Short side mold 1b, 1b' Long side mold 2 Immersion nozzle 3 Static magnetic field generating coil (method of the present invention) 4, 4'Electrical roll 5 Cast piece 6 Molten steel 7, 7'Nozzle outlet 8 Static magnetic field generating coil provided near the meniscus 9 Static magnetic field generating coil (conventional method)

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[Procedure amendment]

[Submission date] October 26, 1992

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Figure 3]

[Fig. 2]

[Figure 4]

[Figure 5]

[Figure 6]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masanori Nara 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Hisao Yamazaki 1 Kawasaki-cho, Chiba-shi Kawasaki Steel Co., Ltd. Technical Research Division

Claims (2)

[Claims] 1. In continuous casting of molten metal, a static magnetic field is applied in the vicinity of the meniscus of the mold in a direction orthogonal to the long side of the mold, while below the discharge port of the immersion nozzle, it is perpendicular to the long side of the slab. A continuous casting method characterized by applying a static magnetic field in a positive direction and applying a direct current in a direction orthogonal to the short side surface of the slab. 2. The continuous casting method according to claim 1, wherein a single hole nozzle having a downward opening is used as the immersion nozzle.