JP2888312B2 - Continuous casting method of steel slab by 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 steel slabs by a static magnetic field, which can further improve the surface and internal quality of steel slabs obtained by continuous casting.

[0002]

2. Description of the Related Art In the continuous casting of steel slabs such as slabs used for producing wide steel sheets, a refractory immersion nozzle, usually made of refractory, is used as a molten steel flow path between a tundish containing molten steel and a continuous casting mold. Is used. In this immersion nozzle, alumina tends to adhere to the inner surface of the nozzle particularly during continuous casting of aluminum-killed steel, so that the flow path of the molten steel is narrowed as the casting time elapses, 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 Ar 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. Since it cannot be floated on the bath in the mold and is trapped in the solidified shell a shown in FIGS. 1A and 1B, the final product may be defective. However, the effect of avoiding nozzle clogging is not sufficient, and frequent replacement work of nozzle replacement is required.
(A) and (b), a two-hole nozzle type immersion nozzle 5 having a symmetrical discharge port 6 at the tip of the immersion nozzle 5
, There is a problem that the quality is deteriorated due to asymmetric left and right closing of the discharge port.

Attempts to solve such problems include:
Attempts have been made to use a nozzle containing alumina and CaO, which produces a compound with a low melting point, but no satisfactory effect has been obtained.
In addition, Japanese Patent Application Laid-Open No. 60-92064 discloses a method for injecting molten metal that suppresses nozzle blockage 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 flows down deep into the molten metal crater in the mold, the accompanying inclusions cannot 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 of a steel slab by a static magnetic field which can solve the above-mentioned problems in the continuous casting and can obtain a steel slab having good surface and internal quality. Is the purpose.

[0006]

Various investigations have been made on nozzle clogging during continuous casting using low carbon aluminum killed steel mainly deoxidized with Al, which has a carbon concentration of 500 ppm or less.
As a result of repeated studies, it was found that adjusting the oxygen concentration in the molten steel to 30 ppm or less, more preferably 20 ppm or less, and using a straight nozzle that opened the tip of the nozzle body of the immersion nozzle and used it as the discharge port of the molten steel almost caused nozzle clogging. It became clear that there was none. Further, in such a straight nozzle, since the discharge flow of the molten steel goes toward the outlet side (downward) of the mold, inclusions and gas bubbles in the molten steel may penetrate deep into the crater.

In order to prevent such inclusions from entering, a static magnetic field generator for applying a static magnetic field is arranged in the continuous casting mold to brake the flow of molten steel discharged from the straight immersion nozzle. It has been found that it is effective to apply braking to the generated molten steel flow in the meniscus direction to improve the quality in the slab. The present invention has been made based on the above findings, and the gist thereof is as follows.

That is, according to the present invention, a molten steel accommodated in a tundish is placed in a continuous casting mold comprising a combination of a pair of short-side molds and a pair of long-side molds. In continuously casting a steel slab while feeding through an immersion nozzle, a T-shaped static magnetic field such that a static magnetic field is generated in the continuous casting mold at the entire width of the meniscus portion and the center of the width including the discharge portion of the immersion nozzle. The generator is arranged on the back side of the long side mold, and the molten steel from the straight immersion nozzle is discharged into the static magnetic field in the center of the width, and the discharge flow is braked by the T-shaped static magnetic field generator. At the same time, a continuous casting method of a steel slab by a static magnetic field characterized by applying a braking to the flow in the meniscus direction generated by the braking, in the present invention,
If the oxygen concentration of molten steel is particularly low at 20 ppm or less in the process of pouring molten steel, do not blow inert gas into the straight immersion nozzle.

FIGS. 1 (a) and 1 (b) show the structure of a 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. A continuous casting mold 2 composed of a wall 1b is a straight immersion nozzle 2 connected to the tundish. The straight immersion nozzle 2 opens the tip of the nozzle body and opens a straight discharge port 4 of molten steel.
It has a structure.

A T-shaped member 3 is disposed on the back side of the long side wall 1b of the continuous casting mold 1 to apply braking to the molten steel flow discharged from the straight immersion nozzle and simultaneously apply braking to the resulting flow in the meniscus direction. Type static magnetic field generator.

[0011]

[Function] FIG. 5 (a), where the discharge port of molten steel is symmetrical.
The two-hole immersion nozzle 5 as shown in (b) 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 are not trapped in the solidified shell. Is designed so that the mold powder is not entrained toward the bath surface in the mold. Further, by adopting such a structure, the molten steel jet from the discharge port 6 is prevented from directly contacting the solidified shell, and solidification can proceed uniformly and stably. However, the immersion nozzle having such a structure
In particular, as described above, alumina and the like tend to adhere to the vicinity of the discharge port 6 and clog the nozzle.

In the present invention, a straight immersion nozzle 2 as shown in FIGS. 2 (a) and 2 (b) having a straight discharge port 4 having an open end at the tip of the nozzle body is used as the immersion nozzle. Fig. 1 (a), (b)
Since the molten steel supplied into the continuous casting mold 1 is braked in the magnetic pole region of the T-shaped static magnetic field generator 3 arranged in the continuous casting mold 1 as shown in FIG. There is no problem such as nozzle clogging caused by the adhesion of the molten steel.Therefore, even if molten steel is injected into the mold at a desired speed, inclusions may penetrate deep into the molten steel or an upward flow may occur in the mold. There is no entrainment of mold powder on the meniscus molten steel.

[0013]

Embodiments of the present invention will be described below. Example 1 C concentration after ladle refining using a two-strand continuous caster 3
60-450ppm, Al concentration 450-620ppm, Oxygen concentration 27-30ppm
Was continuously cast for 3 charges (280 t / charge) under the following conditions, and the adhesion of alumina in the straight immersion nozzle was investigated.

In performing the continuous casting according to the present invention, the T-shaped static magnetic field generator 3 was arranged in a dimensional relationship as shown in FIG. Of the two strands, a conventional two-hole immersion nozzle is used for one strand, and a straight immersion nozzle is used in the present invention for the other strand, and the T-shaped static nozzle is used only for the strand using the straight immersion nozzle. A static magnetic field by a magnetic field generator was applied.

The casting conditions are as follows. Mold size; short side 220mm, long side 1600mm Casting speed; 1.7m / min Superheat degree of molten steel in tundish; 25-30 ° C Maximum magnetic flux of static magnetic field generator; 3000 gauss As a result, nozzle clogging of 10Nl / min in nozzle In continuous casting using a conventional two-hole immersion nozzle into which a gas for prevention was blown, a layer of alumina deposits with a maximum thickness of 10 mm was observed near the nozzle discharge port, but in the continuous casting according to the present invention, Indicates that the alumina deposit layer is 2 mm at maximum even though Ar gas was not injected into the nozzle.
It was confirmed that nozzle clogging was extremely small.

Example 2 Molten steel in the ladle (same composition as in Example 1) Al powder was added to the slag on the bath surface to add FeO in the slag on the molten steel bath surface in the ladle.
After reducing the iron content and performing ladle refining to reduce the FeO concentration to 3% or less to bring the oxygen concentration in the molten steel to 15 to 18 ppm,
Under the same casting conditions as in 1, continuous casting was carried out continuously for 3 charges (280 t / charge), and the adhesion of alumina to the immersion nozzle at that time was investigated. In this example, no gas for preventing nozzle clogging was blown into the immersion nozzle in both strands.

As a result, when the conventional method using a two-hole immersion nozzle is followed, a predetermined injection speed cannot be achieved due to nozzle clogging at the third charge, and the casting speed is reduced to 1.7.
m / min to 1.2 m / min, but in the continuous casting according to the present invention, the casting speed did not decrease. After the completion of casting, the straight immersion nozzle was recovered and its inner surface was observed. Only alumina of about 2 mm was attached.

Although an experiment was conducted separately using a straight immersion nozzle and applying no static magnetic field, under these conditions, a high-temperature molten steel jet discharged from the nozzle tip flows vertically downward to form a solidified shell. The washing hinders the solidification progress of that part. Therefore, a so-called breakout occurred, and casting was impossible. On the other hand, in Examples 1 and 2 of the present invention, stable casting was possible by applying a static magnetic field as described above.

The continuously cast slab obtained in the above Example-1 was then hot-rolled and cold-rolled to form a cold-rolled sheet having a thickness of 0.7 mm. (The sum of defects and streak defects) was investigated. The result is shown in FIG.
Shown in FIG. 4 shows that the rate of occurrence of surface defects is very low when continuous casting is performed according to the present invention. It is considered that the reason for this is that the application of the static magnetic field in the continuous casting mold does not allow the injection flow of the molten steel to penetrate deep into the crater, and that the flow of the molten steel of the meniscus is suppressed and the mold powder is not wound. Also, the result of the conforming example in the embodiment-2 is the same as that of the embodiment-1
What is better than the conforming example is the low oxygen concentration of the molten steel. It is also considered that Ar gas, which is the main cause of blistering defects, was not blown. Although a comparatively good result is obtained in the comparative example in Example-2, since a gas for preventing nozzle clogging is not blown into the nozzle,
A desired casting speed cannot be obtained due to nozzle clogging, and there is a problem in productivity.

From the above, it was confirmed that a cold-rolled steel sheet having extremely few surface defects can be obtained by the method of the present invention.

[0021]

As described above, according to the present invention,
Despite the use of a straight immersion nozzle, stable continuous casting is possible, and a slab with good internal quality can be obtained. In particular, when the oxygen concentration of the molten steel is as low as 20 ppm or less, it is not necessary to blow Ar gas for preventing nozzle clogging, and there is no trapped solidified shell of Ar gas bubbles, so that a slab of excellent quality can be obtained.

[Brief description of the drawings]

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

FIG. 2 is a sectional view showing a straight immersion nozzle suitable for being applied to a casting method according to the present invention.

FIG. 3 is a sectional view showing an arrangement of a continuous casting apparatus according to an embodiment of the present invention together with dimensions.

FIG. 4 is a bar graph comparing the results of Examples with respect to the incidence (index) of surface defects.

FIG. 5 is a cross-sectional view showing a conventional symmetric discharge port immersion nozzle.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Continuous casting mold 1a Short side wall 1b Long side wall 2 Immersion nozzle 3 Static magnetic field generator 4 Straight discharge port 5 Left-right symmetric 2-hole nozzle 6 Left-right symmetric 2-hole nozzle discharge port

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masanori Nara 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corporation Technology Research Division (56) References JP-A-5-77007 (JP, A) JP JP-A-4-361858 (JP, A) JP-A-58-55157 (JP, A) JP-A-4-197553 (JP, A) JP-A-63-154246 (JP, A) JP-A-62-130752 (JP) JP-A-2-284750 (JP, A) JP-A-5-77006 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/10 350 B22D 11/10 330 B22D 27/02

Claims (2)

(57) [Claims] 1. A molten steel contained in a tundish is supplied through a straight immersion nozzle having a tip end of a nozzle body connected to the tundish into a continuous casting mold comprising a combination of a pair of short-side molds and a pair of long-side molds. In the continuous casting of steel slabs, a T-shaped static magnetic field generator which generates a static magnetic field in the entire width of the meniscus portion and the center of the width including the discharge portion of the immersion nozzle in the continuous casting mold is lengthened. The molten steel from the straight immersion nozzle is discharged into the static magnetic field at the center of the width, and the T flow is applied to the discharge flow.
A continuous casting method of a steel slab by a static magnetic field, wherein braking is applied to a flow in a meniscus direction caused by the braking at the same time as braking is applied by a V-shaped static magnetic field generator. 2. The continuous casting method of a steel slab by a static magnetic field according to claim 1, wherein inert gas is not blown into the straight immersion nozzle using molten steel having an oxygen concentration of 20 ppm or less.