JPH0596351A - Method for continuously casting steel slab by traveling magnetic field and magnetostatic field - Google Patents

Abstract

PURPOSE:To prevent alumina from sticking to an immersion nozzle to prevent inclusions and bubbles from intruding deep into a mold molten steel flow discharged into the mold from the immersion nozzle and to prevent mold powder from entrapping into the molten steel. CONSTITUTION:A traueling magnetic field generator 3 is arranged to width center zone of the back surfaces of the long side walls 1b in the mold and also a magnetostatic field generator 6 is arranged to the whole zone in the width direction at near the molten steel surface of the back surfaces of the long side walls 1b in the mold above this traveling magnetic field generator 3 and further a magnetostatic field generator 7 is arranged below the traveling magnetic field generator 3, too. Braking to the molten steel flow discharged from the straight immersion nozzle 2 is applied by acting the traveling magnetic field crossing orthogonally to the long side walls 1b in the mold and shifting upward and the magnetostatic field is applied to the whole zone in the width direction at near the molten steel surface above the traveling magnetic field to settle down the molten steel surface, and also the static field is applied below the traveling magnetic field to uniformize the descending flow of the molten steel.

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 a steel slab by a progressive magnetic field and a static magnetic field, which makes it possible to improve the surface and internal quality of the steel slab obtained by continuous casting. It is a thing.

[0002]

2. Description of the Related Art Conventionally, in continuous casting of a steel slab such as a slab used for producing a wide steel sheet, a molten steel passage between a tundish containing molten steel and a continuous casting mold is usually made of a refractory material. Immersion nozzles are used. In this immersion nozzle, in particular, during continuous casting of alumina-killed steel, alumina tends to adhere to the inner surface of the nozzle, so that the molten steel flow passage is narrowed with the elapse of casting time, and there is a problem that a desired molten steel flow rate cannot be obtained.

For this reason, normally, an inert gas such as argon was supplied to the nozzle during the supply of the molten steel to cope with this, but when the supply rate of the inert gas is high, the gas is supplied. Could not float to the bath surface in the mold, as shown in Fig. 1 (a),
Since it is trapped in the solidified shell a shown in (b), it may be a defect in the final product, and simply blowing an inert gas is not sufficient to avoid nozzle clogging, and frequent nozzle replacement is required. In particular, in the case of a two-hole nozzle type immersion nozzle 2 in which the tip of the immersion nozzle 2 is provided with a bilaterally symmetrical ejection port 5 as shown in FIGS. There was a problem that the quality was deteriorated due to the asymmetrical blockage on the left and right sides.

As an attempt to solve such a problem,
Attempts have been made to use a nozzle containing CaO, which produces a low melting point compound with alumina, but the effect has not been fully obtained yet. In addition to this, JP-A-60-92064 discloses a method for injecting molten metal that suppresses nozzle clogging by applying a DC magnetic field to the molten metal flow in the nozzle to make the molten metal flow laminar. However, since the molten metal flow flows down deep inside the molten metal crater in the mold, accompanying inclusions may not be able to float and may be trapped in the solidified shell.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to propose a continuous casting method using a progressive magnetic field and a static magnetic field, which solves the above-mentioned problems in continuous casting and can obtain a steel slab with good internal quality. Is the purpose.

[0006]

[Means for solving the problems] As a result of various investigations and examinations on nozzle clogging during continuous casting using aluminum-killed steel deoxidized with Al, which has a carbon concentration of 500 ppm or less, oxygen in molten steel It has been clarified that there is almost no nozzle clogging when the concentration is adjusted to 30 ppm or less, more preferably 20 ppm or less, and a straight nozzle that opens the tip of the nozzle body of the immersion nozzle to serve as a molten steel discharge port is used. Moreover, in such a straight nozzle,
Since the molten steel discharge velocity is toward the outlet side (downward) of the mold, there is a risk that inclusions and gas bubbles in the molten steel will penetrate deep into the crater. It is extremely effective to apply a braking force to the downward discharge flow by arranging a progressive magnetic field generation device that acts on a progressive magnetic field that moves orthogonally to the long side wall of the mold and moves upward of the mold. I got the knowledge of. Furthermore, it was clarified that the static magnetic field can be applied to the molten steel surface in the continuous casting mold to suppress the fluctuation of the molten steel surface and to make the flow on the molten metal surface uniform. At the same time, it was clarified that the downward flow could be homogenized by using the static magnetic field even under the traveling magnetic field, whereby the downward flow was attenuated and the bubbles and inclusions were reduced.

The present invention is based on the above findings, and the gist thereof is as follows. That is, the present invention, molten steel from the tundish, through a straight immersion nozzle that releases the tip of the nozzle body connected to the tundish in a continuous casting mold consisting of a pair of mold short side wall and mold long side wall In continuously casting the steel slab while supplying, while arranging the progressive magnetic field generator in the central region of the back width of the mold long side wall, the mold long side is located at the position of the molten metal above and above the progressive magnetic field generator. The static magnetic field generator is arranged in the entire width of the wall, and in a state where it is positioned in the magnetic pole region of the traveling magnetic field generator in the vicinity of the discharge port of the straight immersion nozzle, the molten steel flow discharged from the straight immersion nozzle and the mold long side wall and A progressive magnetic field that is orthogonal and moves upward is applied to dampen, and a static magnetic field that is orthogonal to the long side wall of the mold is applied at the molten metal surface position above the progressive magnetic field. While calming down the molten steel surface, a static magnetic field orthogonal to the mold long side wall is added at a position below the progressive magnetic field to homogenize the downward flow of the molten steel. This is a continuous casting method, and in the present invention, blowing of an inert gas into the straight immersion nozzle can be omitted when the molten steel oxygen concentration is as low as 20 ppm or less in the molten steel injection process.

1 (a) and 1 (b) show the structure of the main part of a continuous casting apparatus suitable for carrying out the present invention, and the numeral 1 in the figure indicates a pair of short side wall 1a and long side wall 1b. 2 is a straight immersion nozzle connected to a tundish. The straight immersion nozzle 2 has a structure in which the tip of the nozzle body is released to form a molten steel straight discharge port 4. Further, 3 is arranged on the back surface of the long side wall 1b of the continuous casting mold 1 and causes the molten steel flow discharged from the straight immersion nozzle 2 to act on a progressive magnetic field which is orthogonal to the long side wall 1b of the mold and moves upward. It is a traveling magnetic field generator. 6 is arranged on the back surface of the long side wall 1b of the continuous casting mold 1,
This is a static magnetic field generator that creates a static magnetic field that is perpendicular to the long side wall 1b of the mold and flows through the molten metal upper surface and the molten metal lower surface below the molten steel flow discharged from the straight immersion nozzle 2. 7 is also arranged on the back surface of the long side wall 1b of the continuous casting mold 1, and the static molten magnetic field passing through the molten metal flow discharged from the straight immersion nozzle 2 is orthogonal to the long side wall 1b of the mold and passes through the molten metal upper surface and the molten metal lower surface. It is a static magnetic field generator that produces.

[0009]

[Operation] The molten steel discharge port 5 is symmetrical Fig. 4
The two-hole immersion nozzle 2 as shown in (a) and (b) has a structure that prevents the molten steel flow ejected from the nozzle from flowing deep into the crater, so that inclusions and bubbles in the injected molten steel can be prevented. Are not trapped in the solidification shell. However, it has been confirmed that the mold powder is entrained because the flow from the discharge port becomes a reverse flow at the short side. However, in the immersion nozzle having such a structure, as described above, alumina or the like is likely to adhere, particularly in the vicinity of the discharge port, and the nozzle is likely to be clogged.

In the present invention, the dipping nozzle having the structure having the straight discharge port 4 with the tip of the nozzle body opened is used as shown in FIGS. 2 (a) and 2 (b), and the straight dipping nozzle 2 shown in FIG. As shown in a) and (b), the molten steel supplied into the continuous casting mold 1 is heated by the static magnetic field generator 6 while applying braking in the magnetic pole region of the progressive magnetic field generator 3 arranged in the continuous casting mold 1. Since the casting was performed by calming the surface and making the downward flow of the molten steel uniform by the static magnetic field generator 7, there was no problem such as nozzle clogging due to alumina adhesion, and therefore the desired Even if the molten steel is poured into the mold at a high speed, inclusions do not penetrate deep into the molten steel and the rising flow of the molten steel does not involve the powder on the bath surface.

Further, when the oxygen concentration in the molten steel is set to 30 ppm, preferably 20 ppm or less, the amount of alumina decreases, so that even if the blowing of the inert gas into the straight nozzle is omitted, the adhesion of alumina to the discharge port is prevented. It can be significantly reduced.

[0012]

EXAMPLES Next, the present invention will be explained based on examples. Example 1 Using a two-strand continuous casting machine, a low-charcoal aluminum killed steel having an oxygen concentration of 28 to 32 ppm was subjected to a 3-cast continuous casting experiment using the immersion nozzle of the present invention. The casting conditions at this time are shown below.

Casting mold size: thickness direction 200 mm width direction 1500 mm height direction 800 mm Super heat in tundish: about 30 ° C. Casting speed: 1.7 m / min Static magnetic field and progressive magnetic field using straight dipping nozzle on one strand A casting experiment was conducted by applying the above-mentioned method to the other strand, and a casting experiment was conducted using a conventional two-hole type immersion nozzle for comparison with the other strand. The strengths of the static and progressive magnetic fields and their generators are as follows.

Static magnetic field generator: Width direction 1700 mm Height direction 200 mm Maximum magnetic flux density 0.4 T (Tesla) * Uses a generator with similar characteristics for both upper and lower parts. Progressive magnetic field generation: 700mm in width direction 400mm in height direction Maximum magnetic flux density 0.3T (Tesla) Progressive magnetic field velocity 1.5m / sec As a result, the conventional 2-hole type in which 10l / min of gas for preventing nozzle clogging is blown into the nozzle In the continuous casting using the immersion nozzle of, a layer of alumina deposit having a maximum thickness of 10 mm was recognized in the vicinity of the nozzle discharge port, but in the continuous casting according to the present invention, argon gas was not blown into the nozzle. Despite this, the thickness of the alumina adhesion layer was 1.5 mm on average at the opening of the discharge port, which revealed that nozzle clogging was extremely small.

Example 2 A continuous casting experiment was conducted under the same conditions as in Example 1 but without performing gas blowing on both strands. At this time, the casting speed was 1.7 m / min, which was the same as in Example 1. In addition, the experiment was conducted by sufficiently refining the ladle to reduce the oxygen concentration in the molten steel to 15 to 20 ppm.

As a result, in the two-hole type immersion nozzle, the predetermined injection speed could not be achieved near the end of the third charge pouring due to nozzle clogging, and the casting speed was lowered. However, in the casting experiment of the present invention, the injection speed did not decrease, and therefore the casting speed did not decrease. When both nozzles were collected after the experiment was completed and the clogging conditions were compared, the nozzles cast using the method of the present invention still showed an average
Only 1.5 mm or less of alumina was attached. On the other hand, when the conventional two-hole type immersion nozzle was used, it became clear that alumina adhered to the discharge ports, and at the same time, both discharge ports were not uniformly clogged and the discharge flow was uneven. became.

Example 3 A continuous casting experiment was conducted under the same casting conditions as in Example 1. The casting speed was 1.9 m / min, and a static magnetic field was generated in one strand by a straight nozzle at the molten metal surface and the lower portion where molten steel was jetted from the discharge port, and casting was performed without applying a progressive magnetic field. A conventional two-hole type immersion nozzle was used for the other strand. Both were cast using gas blowing.

At this time, the discharge flow from the nozzle is blocked by the static magnetic field generator 7 and becomes a horizontal flow similar to that of the conventional two-hole type immersion nozzle, so that the solidified shell generated on both short side walls is washed. Solidification did not develop, uneven solidification occurred, and the quality of the obtained slab was not good.
On the other hand, in Examples 1 and 2, stable casting is possible even by using the straight nozzle by using the traveling magnetic field.

Example 4 Further, the casting conditions were the same as in Example 1, and one strand was used with a straight nozzle to apply a static magnetic field to the molten metal surface above the mold, and to apply a progressive magnetic field to the nozzle discharge port, Casting was performed by applying a static magnetic field to the lower part of the. A straight nozzle was used for the other strand, and a static magnetic field was applied to the molten metal surface and the lower part of the traveling magnetic field, and the traveling magnetic field was also added to the nozzle discharge port for the experiment. In both cases, the casting speed was 1.7 m / min. No nozzle clogging occurred in both strands, however, in the continuous casting slab in which the static magnetic field was not applied to the molten metal surface, defects due to mold powder partially exist as shown below. It was considered that this was caused by the instability of the molten metal surface due to no static magnetic field being applied.

Example 5 Under the same casting conditions as in Example 1, one strand uses a straight nozzle and does not apply a static magnetic field to the lower portion of the molten steel jetted from the nozzle discharge port, but a traveling magnetic field at the nozzle discharge port. Was added for casting. A straight nozzle was used for the other strand, and a static magnetic field was applied to the molten metal surface and the lower part of the traveling magnetic field. In both cases, the casting speed was 1.7 m / min. No nozzle clogging occurred on both strands.

Further, the results of the examples are shown together in FIG. FIG. 3 shows an average of surface defects per unit area of the cold rolled sheet. In FIG. 3 (a), it is clear that the cold rolled material obtained from the steel slab cast according to the present invention has a very low defect rate. It is considered that this is because the injection flow of the molten steel does not penetrate deep into the crater due to the application of the magnetic field in the continuous casting mold.

The results of the present invention in Example 2 are better than those of Example 1 in Example 1 because the oxygen concentration of the molten steel is low and the inert gas for preventing nozzle clogging, which is the main cause of the blistering defect, is present. This is because the blowing is not performed. In the second embodiment, the cold rolled sheet using the two-hole type immersion nozzle of the conventional example has some good results.
As described above, one of the two holes was clogged up to a state where it was almost completely closed, and uneven flow was generated. Therefore, even in the cold-rolled sheet, the defects varied. Therefore, this cold-rolled sheet cannot be used for high-grade steel.

Further, since the nozzle is clogged, the casting speed is lowered and the productivity is extremely lowered, so that it is difficult to use it as a process. Further, FIG. 3B shows the results of comparing the surface defect rates of cold-rolled sheets as hot-rolled and cold-rolled slabs that were continuously cast in Example 3 in the same manner. From this result, in the comparative example, the defects are slightly reduced as compared with the continuous casting method of the conventional example, but the surface defects are almost the same. It is considered that this is because nozzle clogging occurs as the nozzle of the conventional example casts, and the molten steel causes a drift in the mold. It is considered that a similar flow is generated in the strands cast without applying a traveling magnetic field, but it is considered that there are few surface defects as long as there is no nozzle clogging.

Further, in FIG. 3 (c), the surface defect rate when the steel slab obtained by continuous casting in Example 4 was hot-rolled similarly and then cold-rolled to form a cold-rolled sheet showed that. As a result, it was confirmed that the defect rate was not better than that of the straight nozzles obtained in Examples 1 and 2. Therefore, in order to know the cause of this, a more detailed investigation of defects in the cold-rolled sheet revealed that defects due to the inclusion of mold powder occurred in the portions where the defects increased. This is considered to be the result of casting without applying a static magnetic field to the molten metal surface in the continuous casting mold.

In FIG. 3 (d), the slab produced in Example 5 was hot-rolled in the same manner and then cold-rolled to form a cold-rolled sheet. Indicated. It is clear that the cold rolled sheet obtained by rolling the slab to which the static magnetic field is applied has a lower surface defect occurrence rate. The cold rolled sheet of the slab manufactured by the strand without a static magnetic field at the bottom has a variation in the surface defect occurrence rate compared to the cold rolled sheet of the slab manufactured by other strands is that the molten steel in the mold It is thought that this is because the downflow of is not uniform.

Therefore, the following can be said from these experimental results. By using a progressive magnetic field at the discharge port of the straight nozzle, continuous casting without nozzle clogging can be achieved, which improves productivity and, importantly, because there is no nozzle clogging, uneven flow of the molten steel flow can be achieved. It has become possible to suppress the above, and it has become possible to manufacture a clean slab.

Further, by applying a static magnetic field to the molten metal surface in the continuous casting mold, fluctuations in the molten metal surface can be suppressed, and by applying a static magnetic field to the lower portion of the nozzle outlet, a uniform downward flow of molten steel can be obtained. By doing so, it became possible to manufacture a cleaner steel slab without entrapment of mold powder. Note that the magnetic field plays an important role in the present invention, but in the region of this magnetic field, a better effect can be obtained by performing the following. First with respect to the traveling field, it is to apply below, including the tip of the nozzle. In particular, when there is a gap with the magnetic field at the discharge port at the tip of the nozzle, the discharge flow of molten steel is ejected from the gap between the nozzle and the magnetic field, producing the effect of a conventional two-hole type immersion nozzle. Causes horizontal flow. This molten steel flow collides with the short side wall of the mold and deeply descends along the short side wall to wash the solidified shell on the short side wall side, which is not very good in terms of cleanliness of the cast piece.

Regarding the static magnetic field, it is important to generate the static magnetic field so as to cover the entire molten metal surface in a region including the molten metal molten metal surface portion. For example, if a magnetic field is generated only below the molten metal surface without applying a static magnetic field to the molten metal surface, the flow below the molten metal surface can be damped, but fluctuations in the molten metal surface cannot be suppressed. Therefore, entrainment of mold powder on the molten metal surface occurs due to fluctuations in the molten metal surface.

Further, there is a need to generate a static magnetic field so as to cover the continuous casting mold over the entire surface of the nozzle discharge port where molten steel is jetting. If even a part of this is missing, the flow of molten steel concentrates there, and a uniform downward flow cannot be achieved, and the quality deteriorates.

[0030]

As described above, according to the present invention, continuous casting can be stably performed, and quality and productivity can be improved. In particular, by using a static magnetic field and a progressive magnetic field together, it has become possible to obtain a high quality continuous cast slab that has not been obtained in the past. Also, when the oxygen concentration of the molten steel is low, it was confirmed that continuous casting is possible without blowing gas to prevent nozzle clogging, and at the same time it became possible to eliminate product defects due to gas. .

[Brief description of drawings]

FIG. 1 is a sectional view showing a continuous casting apparatus according to the present invention.

FIG. 2 is a view showing a straight immersion nozzle according to the present invention.

FIG. 3 is a comparative diagram showing the results of Examples with respect to the surface defect occurrence rate (index).

FIG. 4 is a view showing a two-hole type immersion nozzle according to a conventional example.

[Explanation of symbols]

 1 Continuous casting mold 1a Short side wall 1b Long side wall 2 Immersion nozzle 3 Progressive magnetic field generator 4 Straight nozzle discharge port 5 Bilateral symmetrical discharge port 6 Static magnetic field generator 7 Static magnetic field generator

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

Claims (2)

[Claims] 1. Molten steel is supplied from a tundish into a continuous casting mold comprising a pair of short side walls and a long side wall of a pair of molds through a straight dipping nozzle having an open tip of a nozzle body connected to the tundish. While continuously casting the steel slab while placing the progressive magnetic field generator in the central region of the back width of the mold long side wall, the mold long side wall at the position of the molten metal above and above the progressive magnetic field generator The static magnetic field generator is arranged over the entire width of the straight immersion nozzle, and is positioned in the magnetic pole region of the progressive magnetic field generator in the vicinity of the discharge port of the straight immersion nozzle. Then, a progressive magnetic field that moves upward is applied to provide damping, and a static magnetic field perpendicular to the long side wall of the mold is applied at the molten metal surface position above the progressive magnetic field to increase the molten metal surface. A continuous casting method of a steel slab by a progressive magnetic field and a static magnetic field, characterized in that a static magnetic field orthogonal to the long side wall of the mold is applied at a position below the progressing magnetic field to homogenize the downward flow of the molten steel. 2. The method according to claim 1, wherein the immersion gas is not blown into the immersion nozzle by using a molten steel having an oxygen concentration of 20 ppm or less.