JP2856960B2 - Continuous casting method of steel slab by traveling magnetic field and static magnetic field - Google Patents

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 means of a traveling 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. Things.

[0002]

2. Description of the Related Art Conventionally, in continuous casting of steel slabs such as slabs used for manufacturing wide steel sheets, a flow path of molten steel between a tundish containing molten steel and a continuous casting mold is usually made of refractory. An immersion nozzle is used. In this immersion nozzle, alumina tends to adhere to the inner surface of the nozzle particularly during continuous casting of alumina-killed steel, so that the flow path of the molten steel is narrowed with the elapse of the casting time, and there has been a problem that a desired flow rate of the molten steel cannot be obtained.

For this reason, an inert gas such as argon is usually supplied into the nozzle during the supply of molten steel to cope with this. However, when the supply rate of the inert gas is high, the gas is not supplied. Is unable to float on the bath surface in the mold, as shown in FIG.
(B) Since it is trapped in the solidified shell a shown in FIG. 2B, a defect may be caused in the final product, and simply blowing an inert gas is not enough to avoid the nozzle clogging, and the nozzle is frequently replaced. In particular, as shown in FIGS. 4 (a) and 4 (b), in the immersion nozzle 2 of a two-hole nozzle type having a symmetrical discharge port 5 at the tip of the immersion nozzle 2 as shown in FIGS. There is a problem that the quality is deteriorated due to the asymmetrical blockage of the right and left.

Attempts to solve such problems include:
Attempts have been made to use nozzles containing CaO, which produces a compound with a low melting point with alumina, but this has not been effective yet. In addition, Japanese Patent Application Laid-Open No. 60-92064 discloses a molten metal injection method that suppresses nozzle blockage by applying a DC magnetic field to a molten metal flow in a nozzle to laminarize the molten metal flow. However, since the molten metal flow flows deep into the molten metal crater in the mold, the 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 solve the above-mentioned problems in continuous casting and to propose a continuous casting method using a traveling magnetic field and a static magnetic field capable of obtaining a steel slab having good internal quality. Is the purpose.

[0006]

[Means to solve the problem] As a result of repeated investigations and examinations on nozzle clogging during continuous casting using aluminum killed steel mainly deoxidized with Al, which has a carbon concentration of 500 ppm or less, oxygen in molten steel was found. When the concentration was adjusted to 30 ppm or less, more preferably 20 ppm or less, and a straight nozzle was used as a discharge port of molten steel by opening the tip of the nozzle body of the immersion nozzle, it became clear that there was almost no nozzle clogging. In such a straight nozzle,
Since the discharge velocity of molten steel goes toward the exit side (downward) of the mold, inclusions and gas bubbles in the molten steel may penetrate deep into the crater. However, continuous casting is required to prevent the penetration of inclusions. It is extremely effective to apply a braking to a downwardly directed discharge flow by arranging a traveling magnetic field generator that applies a traveling magnetic field that moves upwardly of the mold, perpendicular to the long side wall of the mold, on the mold. Was obtained. Furthermore, it was clarified that the variation of the molten metal surface can be suppressed by applying a static magnetic field to the molten metal surface in the continuous casting mold, and the flow on the molten metal surface can be made uniform. At the same time, it was clarified that the downflow can be made uniform by using the static magnetic field even below the traveling magnetic field, whereby the downflow is attenuated and bubbles and inclusions are reduced.

[0007] The present invention is based on the above findings, and the gist thereof is as follows. That is, the present invention provides a method of melting molten steel from a tundish through a straight immersion nozzle in which a tip of a nozzle body connected to the tundish is opened in a continuous casting mold composed of a pair of a short side wall and a long side wall of a pair of molds. In continuously casting the steel slab while supplying, a traveling magnetic field generator is arranged in the central region of the back width of the long side wall of the mold, and the long side of the mold is located at the position of the molten metal surface above the traveling magnetic field generator and at the position below. A static magnetic field generator is arranged on the entire width of the wall, and a long side wall of a mold is formed in the molten steel flow discharged from the straight immersion nozzle in a state where the static magnetic field generator is positioned in the magnetic pole region of the traveling magnetic field generator near the discharge port of the straight immersion nozzle. A braking force is applied by applying a traveling magnetic field that moves perpendicularly and moves upward, and a static magnetic field that is perpendicular to the long side wall of the mold is applied at the position of the molten metal surface above the traveling magnetic field. While the molten metal surface is settled, a static magnetic field perpendicular to the long side wall of the mold is applied at a position below the traveling magnetic field to equalize the downward flow of the molten steel. This is a continuous casting method. In the present invention, in the process of pouring molten steel, when the oxygen concentration of molten steel is particularly low at 20 ppm or less, it is possible to omit blowing an inert gas into the straight immersion nozzle.

FIGS. 1 (a) and 1 (b) show the structure of the main part of a continuous casting apparatus suitable for use in the present invention. In FIG. 1, reference numeral 1 denotes a pair of a short side wall 1a and a long side wall 1b. Is a straight immersion nozzle connected to a tundish, and the straight immersion nozzle 2 has a structure in which a tip end portion of the nozzle body is opened to form a straight discharge port 4 of molten steel. Numeral 3 is disposed on the back side of the long side wall 1b of the continuous casting mold 1, and applies a traveling magnetic field which moves perpendicularly to the long side wall 1b and moves upward to the molten steel flow discharged from the straight immersion nozzle 2. It is a traveling magnetic field generator. 6 is disposed on the back of the long side wall 1b of the continuous casting mold 1,
This is a static magnetic field generator that generates a static magnetic field that is orthogonal to the long side wall 1b of the mold and passes through the upper and lower surfaces of the molten metal in the molten steel flow discharged from the straight immersion nozzle 2. 7 is also disposed on the back side of the long side wall 1b of the continuous casting mold 1, and a static magnetic field which is perpendicular to the long side wall 1b of the molten metal flow from the straight immersion nozzle 2 and passes through the upper and lower surfaces of the molten metal. Is a static magnetic field generator.

[0009]

[Operation] Fig. 4 shows the molten steel outlet 5 becomes symmetrical.
The two-hole immersion nozzle 2 as shown in (a) and (b) has a structure that prevents the molten steel flow spouted from the nozzle from flowing deep into the crater, so that inclusions and air bubbles in the injected molten steel are prevented. Are not trapped in the solidified shell. However, since the flow from the discharge port is a reverse flow at the short side, it has been confirmed that entrainment of the mold powder occurs. However, in the immersion nozzle having such a structure, as described above, alumina or the like easily adheres particularly in the vicinity of the discharge port, and the nozzle is easily clogged.

In the present invention, a straight immersion nozzle 2 as shown in FIGS. 2A and 2B having a straight discharge port 4 in which the tip of the nozzle body is opened is used as the immersion nozzle, and FIG. As shown in a) and (b), the molten steel supplied into the continuous casting mold 1 is braked in the magnetic pole region of the traveling magnetic field generator 3 arranged in the continuous casting mold 1 while the molten steel is heated by the static magnetic field generator 6. Since the casting is performed by calming the surface and making the descending flow of the molten steel uniform by the static magnetic field generator 7, there is no problem that the nozzle is clogged due to the adhesion of alumina. Even if molten steel is injected into the mold at a high speed, inclusions do not penetrate deep into the molten steel, and the upward flow of the molten steel does not involve powder in 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 accordingly. Therefore, even if the injection of the inert gas into the straight nozzle is omitted, the adhesion of the alumina to the discharge port is prevented. It can be significantly reduced.

[0012]

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

The size of the casting mold: 200 mm in the thickness direction, 1500 mm in the width direction, 1500 mm in the height direction, 800 mm in height direction Superheat in a tundish: about 30 ° C. Casting speed: 1.7 m / min Static and traveling magnetic fields using a straight immersion nozzle on one strand And a casting experiment was performed on the other strand using a conventional two-hole immersion nozzle for comparison. The strengths of the static magnetic field and the traveling magnetic field and the generators are as follows.

Static magnetic field generator: 1700 mm in width direction, 200 mm in height direction, maximum magnetic flux density 0.4 T (tesla) * A generator with the same characteristics is used for both the upper and lower parts. Generating a traveling magnetic field: 700 mm in the width direction, 400 mm in the height direction Maximum magnetic flux density 0.3 T (tesla) Traveling magnetic field speed 1.5 m / sec As a result, a conventional 2-hole type with 10 l / min of nozzle blocking gas injected into the nozzle In the continuous casting using the immersion nozzle, a layer of alumina deposits with a maximum thickness of 10 mm was observed near the nozzle outlet, but in the continuous casting according to the present invention, argon gas was not blown into the nozzle. Nevertheless, the thickness of the alumina adhesion layer was 1.5 mm on average at the opening of the discharge port, and it became clear that nozzle clogging was extremely small.

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

As a result, in the two-hole immersion nozzle, a predetermined injection speed could not be achieved due to nozzle clogging near the end of the third charge pouring, and the casting speed was reduced. However, in the casting experiment of the present invention, the pouring speed did not decrease, and thus the casting speed did not decrease. 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 also showed an average
Only alumina of 1.5 mm or less was adhered. On the other hand, when the conventional two-hole immersion nozzle was used, it was evident that alumina adhered to the discharge port, and at the same time the clogging was not uniform at both discharge ports, resulting in an uneven discharge flow. 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 on one of the strands by a straight nozzle at the molten metal surface and at the lower part where the molten steel was jetted from the discharge port, and the casting was performed without applying a traveling magnetic field. A conventional two-hole immersion nozzle was used for the other strand. Both were cast using gas blowing.

At this time, the discharge flow from the nozzle is stopped by the static magnetic field generator 7 and becomes a horizontal flow similar to that of the conventional two-hole immersion nozzle, in order to wash the solidified shell generated on both short side walls. 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, even when a straight nozzle is used, stable casting is possible by using a traveling magnetic field.

Example 4 The casting conditions were the same as in Example 1, and one of the strands did not apply a static magnetic field to the surface of the molten metal above the mold using a straight nozzle, but applied a traveling magnetic field to the nozzle discharge port. Casting was performed by applying a static magnetic field to the lower part of. For the other strand, an experiment was performed using a straight nozzle by applying a static magnetic field to the molten metal surface and below the traveling magnetic field, and also applying a traveling magnetic field to the nozzle discharge port. In both cases, the casting speed was 1.7 m / min. No nozzle clogging occurred in both strands. However, continuous cast slabs in which a static magnetic field was not applied to the molten metal surface partially had defects due to mold powder as shown below. This was thought to have occurred because the molten steel surface became unstable due to the absence of a static magnetic field.

Example 5 The casting conditions were the same as in Example 1, and one of the strands did not apply a static magnetic field to the lower part where molten steel was jetted from the nozzle discharge port using a straight nozzle. And casting was performed. For the other strand, an experiment was performed using a straight nozzle by applying a static magnetic field to the molten metal surface and below the traveling magnetic field, and also applying a traveling magnetic field to the nozzle discharge port. In both cases, the casting speed was 1.7 m / min. No nozzle clogging occurred in both strands.

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

Further, the results of the present invention in Example 2 are better than those of the present invention in Example 1 because the oxygen concentration of the molten steel is low and the inert gas for preventing nozzle clogging which is a main cause of blistering defects. This is because no blowing was performed. In Example 2, although some good results were obtained for the cold rolled sheet using the conventional two-hole immersion nozzle,
As described above, of the two holes, one of the two holes was almost completely closed and clogged, causing a drift, and therefore, the defects were varied even in the cold rolled sheet. Therefore, this cold-rolled sheet was in a state where it could not be used for high-grade steel.

Further, since the nozzle is clogged, the casting speed is reduced, and the productivity is extremely reduced, so that it is difficult to use it as a process. FIG. 3 (b) shows the results of comparing the surface defect rates of the slabs subjected to continuous casting in Example 3 by hot and cold rolling in the same manner as cold rolled sheets. From this result, in the comparative example, the number of defects is slightly reduced as compared with the conventional continuous casting method, but the surface defects are almost the same. This is presumably because nozzle clogging occurs as the conventional nozzle is cast, and molten steel is caused to drift in the mold. It is considered that the same flow is generated in the strand cast without applying the traveling magnetic field, but it is considered that there is no nozzle clogging and there are few surface defects.

Further, in FIG. 3 (c), the surface defect rate when the steel slab obtained by continuous casting in Example 4 was similarly hot-rolled and then cold-rolled into a cold-rolled sheet. showed that. As a result, it was confirmed that the defect rate of the straight nozzle obtained in Examples 1 and 2 was not better. Therefore, in order to find out the cause, a further detailed examination of the defects of the cold rolled sheet revealed that defects due to entrainment of the 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.

FIG. 3D shows the rate of occurrence of surface defects in the entire width of the cold-rolled sheet when the slab manufactured in Example 5 is hot-rolled and then cold-rolled into a cold-rolled sheet. Indicated. It is clear that the rate of occurrence of surface defects is lower in a cold-rolled sheet obtained by rolling a slab to which a static magnetic field is applied. The difference in the incidence of surface defects between the cold rolled sheets of slabs manufactured with strands without a static magnetic field at the bottom compared to the cold rolled sheets of slabs manufactured with other strands was caused by molten steel in the mold. It is considered that the descending flow of is not uniform.

Therefore, the following can be said from the results of these experiments. By using a traveling magnetic field at the outlet of the straight nozzle, continuous casting without nozzle clogging can be achieved, thereby improving productivity, and more importantly, drifting molten steel flow due to no nozzle clogging , And a clean slab can be manufactured.

Further, by applying a static magnetic field to the molten metal surface in the continuous casting mold, the fluctuation of the molten metal surface can be suppressed, and by applying a static magnetic field to the lower portion of the nozzle discharge port, a uniform downward flow of molten steel can be obtained. By doing so, a cleaner steel slab without mold powder entrainment can be manufactured. Although the magnetic field plays an important role in the present invention, a better effect can be obtained in the region of the magnetic field by performing the following. First, regarding the traveling magnetic field, it includes the tip of the nozzle and applies below it. In particular, when there is a gap between the nozzle and the magnetic field at the discharge port at the tip of the nozzle, the discharge flow of the molten steel is ejected from the gap between the nozzle and the magnetic field, producing an effect similar to that of a conventional two-hole immersion nozzle. Causes a horizontal flow. This flow of molten steel collides with the short side wall of the mold and descends deeply along the short side wall, thereby washing the solidified shell on the short side wall side, which is not very good in terms of cleanliness of the slab.

As for the static magnetic field, it is important to generate a static magnetic field so as to cover the entire surface of the molten steel in a region including the molten steel surface. For example, when a magnetic field is generated only at the lower part of the molten steel surface without applying a static magnetic field to the molten steel surface, it is possible to brake the flow under the molten steel surface, but it is not possible to suppress the fluctuation of the molten steel surface. Therefore, mold powder entrainment of the molten metal surface due to the fluctuation of the molten metal surface occurs.

Further, there is a need to generate a static magnetic field so as to cover the entire continuous casting mold at a lower portion of the nozzle discharge port where the molten steel is jetted. If any of these are missing, the flow of molten steel concentrates there, and a uniform downward flow cannot be achieved, resulting in poor quality.

[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 traveling magnetic field together, it has become possible to obtain a high-quality continuous cast slab that could not be obtained conventionally. Also, when the oxygen concentration of the molten steel was low, it was confirmed that continuous casting was possible without blowing gas for preventing nozzle clogging, and it was possible to eliminate product defects due to gas at the same time. .

[Brief description of the 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 comparison diagram showing the results of Examples with respect to the incidence rate (index) of surface defects.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Continuous casting mold 1a Short side wall 1b Long side wall 2 Immersion nozzle 3 Traveling magnetic field generator 4 Straight nozzle discharge port 5 Symmetric discharge port 6 Static magnetic field generator 7 Static magnetic field generator

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hisao Yamazaki 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corp. Technical Research Division (56) References JP-A-2-284750 (JP, A) JP-A 2-89544 (JP, A) JP-A-1-266949 (JP, A) JP-A-64-66055 (JP, A) JP-A-56-1660862 (JP, A) JP-A-63-154246 (JP, A) A) JP-A-63-119959 (JP, A) JP-A-62-130752 (JP, A) JP-A-5-96345 (JP, A) JP-A-5-77009 (JP, A) JP-A-5 -77006 (JP, A) JP-A-4-313447 (JP, A) JP-A-2-284749 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/10 350 B22D 11/10 330 B22D 11/04 311 B22D 27/02

Claims (2)

(57) [Claims] 1. A molten steel is supplied from a tundish into a continuous casting mold composed of a pair of a short side wall and a long side wall of a pair of molds through a straight immersion nozzle having a tip end of a nozzle body connected to the tundish. In continuously casting a steel slab while moving, a traveling magnetic field generator is arranged in the center of the back width of the long side wall of the mold, and the long side wall of the mold is positioned above and below the traveling magnetic field generator. A static magnetic field generator is disposed over the entire width of the straight immersion nozzle, and the molten steel flow discharged from the straight immersion nozzle is perpendicular to the long side wall of the mold while being positioned in the magnetic pole region of the traveling magnetic field generator near the discharge port of the straight immersion nozzle. Then, a braking force is applied by applying a traveling magnetic field moving upward, and a static magnetic field perpendicular to the long side wall of the mold is applied at a position of the molten metal surface above the traveling magnetic field to apply a braking force. A continuous casting method for a steel slab using a traveling magnetic field and a static magnetic field, wherein a static magnetic field perpendicular to the long side wall of the mold is applied at a position below the traveling magnetic field to make the downflow of molten steel uniform. 2. The method according to claim 1, wherein inert gas is not blown into the immersion nozzle by using molten steel having an oxygen concentration of 20 ppm or less.