JPH0577007A - Method for continuously casting steel slab using static magnetic field - Google Patents

Abstract

(57) [Abstract] [Purpose] Prevents nozzle clogging due to adhesion of alumina to the discharge port of the immersion nozzle, and inclusions with molten steel injected into the mold penetrate deep into the crater and are trapped in the solidification shell. Prevent. [Structure] In a continuous casting mold 1, a straight dipping nozzle 2 having an open tip of a nozzle body is dipped and poured, and at the back of the long side 1b of the mold 1, lower parts are formed below and above the lower end of the dipping nozzle 2, respectively. A static magnetic field generator 3 and an upper static magnetic field generator 5 are provided. The downward static molten steel flow from the nozzle 2 is controlled by the lower static magnetic field generator 3, and the fluctuation of the molten steel formed in the meniscus direction is controlled by the lower static magnetic field generator 5.

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 a steel slab using a static magnetic field, which can further improve the surface and internal quality of the steel slab obtained by continuous casting.

[0002]

2. Description of the Related Art In continuous casting of steel slabs such as slabs used for the production of wide steel sheets, a refractory dipping nozzle is usually used as a molten steel flow path between a tundish containing molten steel and a continuous casting mold. Is used. This immersion nozzle has a problem that the molten steel flow passage is narrowed as the casting time elapses because alumina is likely to adhere to the inner surface of the nozzle during continuous casting of aluminum killed steel, so that a desired molten steel flow rate cannot be obtained.

For this reason, normally, an inert gas such as Ar 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. Since it cannot be floated on the bath in the mold and is trapped in the solidified shell a shown in FIGS. 1 (a) and 1 (b), it may be a defect in the final product. However, the effect of avoiding nozzle clogging is not sufficient, and frequent nozzle replacement work is required.
As shown in (a) and (b), a two-hole nozzle type immersion nozzle 6 having symmetrical discharge ports 7 at the tip of the immersion nozzle 2
In the above, there is a problem that the quality is deteriorated due to the left and right asymmetrical closing of the discharge port.

As an attempt to solve such a problem,
Attempts have been made to use a nozzle containing CaO, which forms a compound with a low melting point, and alumina, but no sufficient effect has been obtained.
In addition, Japanese Patent Laid-Open No. 60-92064 discloses a molten metal injection method for suppressing nozzle clogging by applying a DC magnetic field to a molten metal flow in a 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, the accompanying inclusions may not be able to float and may be trapped in the solidified shell.

Further, as a method of preventing inclusions from being included, a static magnetic field generator is installed on the back surface of the mold corresponding to the upper and lower portions of the discharge port of a two-hole nozzle, as disclosed in Japanese Patent Laid-Open No. 2-284750, and the discharge port is Although a method of controlling the molten steel flow from the above has been proposed, as described above, when the nozzle clogging of the left and right discharge ports becomes uneven, there is a risk of degrading the quality of the cast slab.

[0006]

It is proposed to propose a continuous casting method for a steel slab using a static magnetic field, which can solve the above-mentioned problems in continuous casting and obtain a steel slab having good surface and internal quality. It is the purpose of the invention.

[0007]

[Means for Solving the Problems] Various investigations on nozzle clogging during continuous casting using a low carbon aluminum-killed steel deoxidized with Al, which has a carbon concentration of 500 ppm or less,
As a result of repeated studies, when the oxygen concentration in the molten steel was adjusted to 30 ppm or less, more preferably 20 ppm or less, and if a straight nozzle that opened the tip of the nozzle body of the immersion nozzle and used as the molten steel discharge port was used, nozzle clogging was almost eliminated It became clear that there was not. 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.

In order to prevent the intrusion of such inclusions, it is extremely effective to dispose a static magnetic field generator for applying a static magnetic field in the continuous casting mold and apply a braking force to the downward molten steel flow. Due to the damping effect, the fluctuation of the molten meniscus steel caused by the flow of molten steel in the meniscus direction is also
It was found that it is effective to apply a static magnetic field to the meniscus portion for braking.

The present invention has been made based on the above findings, and the gist thereof is as follows.
That is, the present invention, the molten steel contained in the tundish is supplied through a straight immersion nozzle having an open tip of a nozzle body connected to the tundish in a continuous casting mold consisting of a pair of short side mold and long side mold. While continuously casting the steel slab while installing the static magnetic field generator in the upper and lower parts in the continuous casting mold, the downward discharge molten steel flow from the straight immersion nozzle to the static magnetic field by the lower static magnetic field generator. A static magnetic field is used which is characterized by simultaneously acting and braking, and at the same time, damping the fluctuation of the molten steel of meniscus caused by the flow of molten steel in the meniscus direction by the braking effect by the action of the static magnetic field by the static magnetic field generator at the upper part of the mold. It is a continuous casting method for steel slabs, and in the present invention,
If the molten steel oxygen concentration is particularly low, 20 ppm or less during the molten steel injection process, do not blow an inert gas into the straight immersion nozzle.

1 (a) and 1 (b) show the structure of the main part of a continuous casting apparatus suitable for carrying out the present invention. The symbol 1 in the figure indicates a pair of short side walls 1a and long sides. A continuous casting mold composed of a wall 1b, 2 is a straight immersion nozzle connected to a tundish, and this straight immersion nozzle 2 opens a tip portion of a nozzle body to form a straight discharge port 4 for molten steel.
It has a structure.

Reference numeral 3 denotes a lower static magnetic field generator arranged on the back surface of the long side wall 1b of the continuous casting mold 1 for braking the molten steel flow discharged from the straight immersion nozzle 2. Reference numeral 5 is an upper static magnetic field generator that suppresses fluctuations in the molten meniscus steel caused by a flow formed in the meniscus direction by conversely damping the molten steel flow by the static magnetic field generated by the lower static magnetic field generator 3.

[0012]

Operation: The discharge holes for molten steel are symmetrical, as shown in Fig. 4 (a),
The two-hole type immersion nozzle 6 as shown in (b) is designed so that the molten steel flow ejected from the nozzle does not flow deep into the crater and traps inclusions and bubbles in the injected molten steel in the solidification shell. Has a structure that prevents the entrainment of mold powder toward the bath surface in the mold. Further, with such a structure, the molten steel jet from the discharge port 7 is prevented from directly contacting the solidification shell, and the solidification is allowed to proceed uniformly and stably. However, the immersion nozzle with such a structure is
It has been described above that alumina or the like is likely to adhere particularly near the discharge port 7 and the nozzle is likely to be clogged.

In the present invention, the immersion nozzle is a straight immersion nozzle 2 as shown in FIGS. 2 (a) and 2 (b), which has a structure in which the tip of the nozzle body has a straight discharge port 4. From Figures 1 (a) and (b)
As shown in (4), the molten steel supplied into the continuous casting mold 1 is continuously cast while applying braking in the magnetic pole region of the static magnetic field generator 3 arranged in the lower part of the continuous casting mold 1. There is no problem such as nozzle clogging caused by the above.Therefore, even if molten steel is injected into the mold at the desired speed, inclusions do not penetrate deep into the molten steel, and due to its braking effect, molten steel flow in the meniscus direction. Even if occurs, since the molten steel flow is damped by the static magnetic field from the static magnetic field generator 5 installed at the upper part of the continuous casting mold 1, that is, at the position corresponding to the meniscus portion, the mold powder on the steel bath surface may be included. Can be prevented.

[0014]

EXAMPLES Examples of the present invention will be described below. Example-1 C concentration 3 after ladle refining by applying a two-strand continuous casting machine 3
60-450ppm, Al concentration 450-620ppm, Oxygen concentration 27-30ppm
The molten steel of No. 3 was continuously cast for 3 charges (280 t / charge) under the following conditions, and the adhesion state of alumina in the straight immersion nozzle was investigated. When performing continuous casting according to the present invention, the lower static magnetic field generator is arranged so that its upper end is located 100 mm below the lowermost end of the immersion nozzle and 600 mm below the lowermost end of the discharge port. The other upper static magnetic field generator was installed so that its upper end was 100 mm above the molten steel meniscus and its lower end was 200 mm below the meniscus. Of the two strands, one strand used a conventional two-hole type immersion nozzle, the other strand used the straight immersion nozzle of the present invention, and the static magnetic field was applied only to the strand using the straight immersion nozzle.

The casting conditions are as follows. Mold size: 220 mm short side wall, 1600 mm long side casting speed: 1.7 m / min Superheat of molten steel in tundish; 25-30 ℃ Static magnetic field generator dimensions and maximum magnetic flux Upper static magnetic field generator; Width 1700 mm, length 300 mm, approx. 3000 gauss Lower static magnetic field generator; width 1700 mm, length 500 mm, approx. 3000 gauss As a result, a conventional two-hole type immersion nozzle with a nozzle clogging prevention gas of 10 Nl / min injected into the nozzle was used. In continuous casting, a layer of alumina deposits having a maximum thickness of 10 mm was observed in the vicinity of the nozzle discharge port, but in continuous casting according to the present invention, alumina was blown into the nozzle even though Ar gas was not blown. The maximum deposit layer is 2 mm
It was confirmed that the 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, and FeO in the slag on the molten steel bath surface in the ladle was added.
Was added and the ladle refining with FeO concentration of 3% or less was performed to adjust the oxygen concentration in the molten steel to 15 to 18 ppm.
Under the same casting conditions as in No. 1, continuous casting was performed continuously for 3 charges (280 t / charge), and the adhesion state of alumina on the immersion nozzle at that time was investigated. In this example, no gas for preventing nozzle clogging was blown into the immersion nozzle for both strands.

As a result, when the conventional method using the two-hole immersion nozzle was used, the predetermined injection speed could not be achieved due to nozzle clogging at the third charge, and the casting speed was 1.7.
Although it decreased from m / min to 1.2 m / min, in the continuous casting according to the present invention, the casting speed did not decrease, and when the straight dipping nozzle was collected after the casting and the inner surface was observed, it was found to be 1 Only about 2 mm of alumina adhered.

An experiment in which a static magnetic field was not applied and an experiment in which only a static magnetic field generator was applied were performed separately using a straight immersion nozzle. Under the former condition, a molten steel jet having a high temperature discharged from the nozzle tip is generated. As a strong flow flows vertically downward to wash the solidification shell, the progress of solidification at that portion is hindered. Therefore, so-called breakout occurred and casting was impossible. In the latter experiment, the fluctuation of the molten metal level was large and stable operation was impossible. Further, when the slab cast under these conditions was rolled and the surface of the cold-rolled steel sheet was observed, a large amount of entrainment of mold powder was present. On the other hand, in Examples 1 and 2 of the present invention, stable casting was possible as described above by applying the upper and lower static magnetic fields.

The continuous cast slab obtained in Example 1 above was then hot-rolled and cold-rolled into a cold-rolled sheet having a thickness of 0.7 mm. The total incidence of defects and streak defects) was investigated. The result is shown in Figure 3.
Shown in. It can be seen from FIG. 3 that the occurrence rate of surface defects is very small when continuous casting is performed according to the present invention. It is considered that this is because the application of the magnetic field in the continuous casting mold prevented the injection flow of the molten steel from penetrating deep into the crater and suppressed the flow of the molten steel in the meniscus portion. Further, the reason why the result of the conforming example in Example-2 is better than that of the conforming example of Example-1 is considered to be that the oxygen concentration of the molten steel is low and that Ar gas that is the main cause of the blistering defect is not blown. .. It should be noted that although comparatively good results are obtained also in the comparative example in this Example-2, nozzle clogging occurs and a desired casting speed cannot be obtained because the gas for preventing nozzle clogging is not blown into the nozzle. , There is a problem in productivity.

From the above, a cold-rolled steel sheet with extremely few surface defects can be obtained by using a straight immersion nozzle and further applying a static magnetic field generator of two upper and lower stages to perform casting.

[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 surface and 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 since the solidified shell of Ar gas bubbles is not captured, a slab with excellent quality can be obtained. Further, the upper or lower static magnetic field generator may be located at a position including the tip of the nozzle discharge port.

[Brief description of drawings]

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

FIG. 2 is a cross-sectional view showing a straight dipping nozzle suitable for application to the casting method according to the present invention.

FIG. 3 is a bar graph comparing the results of Examples with respect to the surface defect occurrence rate (index).

FIG. 4 is a cross-sectional view showing a conventional bilaterally symmetrical discharge port immersion nozzle.

[Explanation of symbols]

 1 Continuous casting mold 1a Short side wall 1b Long side wall 2 Immersion nozzle 3 Lower static magnetic field generator 4 Straight discharge port 5 Upper static magnetic field generator 6 Left-right symmetrical 2-hole nozzle 7 Right-left symmetrical 2-hole nozzle outlet

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

Claims (2)

[Claims] 1. Molten steel contained in a tundish is supplied into a continuous casting mold composed of a combination of a pair of short-side molds and a pair of long-side molds through a straight immersion nozzle having an open tip of a nozzle body connected to the tundish. While continuously casting the steel slab, a static magnetic field generator is installed in the upper and lower parts of the continuous casting mold, and the downward discharge molten steel flow from the straight immersion nozzle is generated by the static magnetic field generator of the lower part. A continuous casting method for a steel slab using a static magnetic field, characterized in that the flow of molten meniscus steel caused by the braking is stopped by the action of a static magnetic field generated by a static magnetic field generator on the upper part of the mold. 2. The continuous casting method for a steel slab using a static magnetic field according to claim 1, wherein an inert gas is not blown into the immersion nozzle by using a molten steel having an oxygen concentration of 20 ppm or less.