JP2006192441A - Method for continuously casting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for continuously casting a steel by which a cast slab excellent in the surface quality without developing a sliver caused by inclusion and blow hole on a coil surface after rolling can be cast. <P>SOLUTION: This casting method, that is, in the method for continuously casting the steel, using an electromagnetic stirring device, the casting is performed so that the minimum distance d (mm) between the outer wall of an immersion nozzle 3 at the immersion portion into molten steel and the solidified shell 5 formed at the long wall side in a mold 1, satisfies the following formula (A). This formula is d=(t-D)/2-18√(L/Vc)≥86√(f). Wherein, t: the length (mm) of the short wall side in the mold, D: the outer diameter (mm) of the immersion nozzle, L: the distance (m) from a meniscus to a portion, at where the distance between the immersion nozzle and the solidified shell minimizes, Vc: the casting speed and f: the frequency (Hz) of the electromagnetic stirring coil current. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a continuous casting method of steel using electromagnetic stirring in a mold, and in particular, by preventing a decrease in molten steel stirring flow velocity and molten steel temperature near the immersion nozzle, a slab excellent in surface quality can be stably obtained. The present invention relates to a continuous casting method of steel that can be manufactured.

  A continuous casting method of steel that performs casting while imparting swirlability to molten steel in a mold using an electromagnetic stirring device has been widely performed, and a technique for obtaining a stable molten steel stirring flow has been studied. For example, Patent Documents 1 and 2 disclose a method of controlling the electromagnetic force online by measuring the height of rise of the molten metal surface and the meniscus flow velocity. Patent Document 3 discloses a method of optimizing the position of the electromagnetic stirring device and the immersion nozzle to suppress interference between the molten steel stirring flow and the molten steel discharge flow from the immersion nozzle. Further, Patent Document 4 discloses a method of forming a horizontal static magnetic field below the submerged nozzle discharge port to reduce the influence of the molten steel discharge flow and to prevent interference with the molten steel stirring flow.

  However, even with these technologies, it is still difficult to generate a uniform molten steel stirring flow by electromagnetic stirring in the mold, and defects caused by inclusions called sliver generated on the coil surface after rolling cannot be zero. There wasn't. As a result of pursuing the cause, the present inventor has found that the immersion nozzle blowing argon is captured by the solidified shell of the slab surface layer at the width direction center portion and the edge portion of the mold, which is a weak stirring portion of electromagnetic stirring. Investigated the cause. In addition, it was estimated that the reason why the central portion in the width direction of the mold becomes the weak stirring portion is that the flow path width is narrowed around the immersion nozzle and the stirring flow rate is lowered.

  As a result of the search, Patent Documents 5 and 6 that mention the distance between the mold and the immersion nozzle were found. Among them, Patent Document 5 describes that the distance between the mold and the immersion nozzle is 20 to 60 mm. However, the invention of Patent Document 5 does not use an electromagnetic stirring device, and the object of the invention is to prevent drift due to non-metallic inclusions attached to the inner wall of the mold and to prevent breakage of the immersion nozzle.

On the other hand, Patent Document 6 defines the dimensions of the mold and the immersion nozzle for the purpose of preventing solidification delay and breakout associated with remelting of the solidified shell due to the discharge flow. However, the invention of Patent Document 6 does not use an electromagnetic stirring device. Thus, there seems to be no literature pursuing the optimum distance between the mold and the immersion nozzle in order to prevent the flow velocity of the molten steel stirring flow near the immersion nozzle generated in the electromagnetic stirring mold.
JP-A-6-63712 JP 2000-94102 A Japanese Patent Laid-Open No. 2001-47201 JP 2004-42062 A JP-A-5-154625 JP 2000-263199 A

  The present invention solves the above-described conventional problems, and prevents a decrease in the flow rate of the molten steel stirring flow in the vicinity of the submerged nozzle generated in the electromagnetic stirring mold. The present invention was made in order to provide a continuous casting method of steel that enables casting of a slab excellent in surface quality while maintaining a proper distance and having no sliver on the coil surface after rolling.

  As a result of repeated researches to solve the above problems, the present inventor has found that the distance between the inner side of the mold short side and the outer wall of the submerged nozzle is determined in order to prevent a decrease in the molten steel stirring flow rate around the submerged nozzle due to electromagnetic stirring. It was found that there is an appropriate value determined by the frequency of the electromagnetic stirring coil current and the casting speed. That is, the thickness of the molten steel that is moved by the electromagnetic stirrer is determined by the frequency of the coil current, and if the distance between the outer wall of the immersion nozzle and the solidified shell is narrow, the swirl flow due to electromagnetic stirring collides with the immersion nozzle, so the stirring flow rate decreases. It will be.

The present invention made on the basis of the above knowledge, in a continuous casting method of steel that performs casting while imparting swirlability to molten steel in a mold by an electromagnetic stirrer, an immersion nozzle outer wall of a portion immersed in the molten steel, a mold length The casting is performed by setting the frequency so that the minimum distance d (mm) between the solidified shell formed on the side and the following equation (A) is satisfied.
d = (t−D) / 2-18√ (L / Vc) ≧ 86√ (f) (A)
Here, t is the length of the mold short side wall (mm), D is the outer diameter of the immersion nozzle (mm), L is the distance (m) from the meniscus to the portion where the distance between the immersion nozzle and the solidified shell is minimized, Vc is the casting speed, and f is the frequency (Hz) of the electromagnetic stirring coil current.
As the mold, it is preferable to use a square or rectangular mold having an electromagnetic stirring device.

  According to the present invention, the molten steel stirring flow velocity around the immersion nozzle by electromagnetic stirring is appropriately maintained by appropriately maintaining the minimum distance d between the outer wall of the immersion nozzle immersed in the molten steel and the solidified shell formed on the long side of the mold. , And the cleaning effect of inclusions captured by the solidified shell around the immersion nozzle can be enhanced. As a result, it has become possible to stably produce a cast slab having excellent surface quality free from defects such as sliver on the coil surface after rolling.

Embodiments of the present invention will be described below.
FIG. 1 is a sectional view of a continuous casting mold, and FIG. 2 is a plan view thereof. In these drawings, reference numeral 1 denotes a mold, and an electromagnetic stirring coil 2 of an electromagnetic stirring device is provided on the long side. 3 is an immersion nozzle provided at the center of the mold 1, and the molten steel discharged from the discharge hole 4 gradually forms from the contact surface of the mold 1 while forming a stirring flow in a certain direction as shown in FIG. 2. It is cooled to form a solidified shell 5 and is continuously cast.

In the present invention, paying attention to the minimum distance d (mm) between the outer wall of the immersion nozzle 3 in the portion immersed in the molten steel and the solidified shell 5 formed on the long side of the mold, the value is expressed by the equation (A). Cast while satisfying.
d = (t−D) / 2-18√ (L / Vc) ≧ 86√ (f) (A)
The symbols used in this equation are as shown in FIG. 1, t is the mold short side wall length (mm), D is the immersion nozzle outer diameter (mm), and L is the minimum distance between the immersion nozzle and the solidified shell. The distance (m) from the meniscus to the part to become, Vc is the casting speed. F is the frequency (Hz) of the electromagnetic stirring coil current.

  In formula (A), 18√ (L / Vc) is a term indicating the thickness of the solidified shell in continuous casting, and (t−D) / 2-18√ (L / Vc) is at a distance L from the meniscus. The distance d between the immersion nozzle 3 and the solidified shell 5 is shown. As a result of repeating many experiments, as shown in the graph of FIG. 3, when the distance d is less than 86√ (f), the swirling flow caused by electromagnetic stirring collides with the immersion nozzle 3 and the flow velocity in the vicinity thereof decreases. The electromagnetic stirring effect cannot be obtained. As a result, the inclusions and the number of bubbles trapped in the slab surface layer increase, and the surface quality of the product coil also deteriorates as shown in the data of the following examples. However, when the distance d is 86√ (f) or more, it is possible to prevent a drop in the molten steel stirring flow rate in the vicinity of the immersion nozzle 3 and stably manufacture a slab having excellent surface quality. As in the present invention, it is considered that there is no conventional example in which the relationship between the frequency f of the electromagnetic stirring coil current and the distance d is clarified.

  In order to obtain good molten steel swirlability by electromagnetic stirring, from equation (A), the outer diameter D of the immersion nozzle 3 is reduced, the short side wall length t of the mold 1 is increased, the casting speed Vc is increased, electromagnetic What is necessary is just to increase the frequency f of the stirring coil current. The enlargement of the short side wall length t of the mold 1 may enlarge only the periphery of the immersion nozzle 3, but the equipment becomes complicated to use the electromagnetic stirrer, so the shape of the mold 1 is square or rectangular. It is preferable that

  Molten steel 270 tons of ultra-low carbon steel was melted in a converter-RH. The molten steel temperature in the tundish was 1560-1580 ° C., the slab width was 1600 mm, and a vertical curve type continuous casting machine was used. t was 200 to 600 mm, D was 100 to 220 mm, L was 0.2 to 0.3 m, Vc was 0.1 to 2.5 m / min, and f was changed between 0.3 to 50 Hz. The slab was hot-rolled, pickled, cold-rolled, and annealed by an ordinary method to form a steel sheet for automobiles. The number of coil surface defects was 1 or less per coil, and 2 or more was marked as x. The results are shown in Table 1. According to this invention, it turns out that the fall of the molten steel stirring flow rate and molten steel temperature of an immersion nozzle vicinity is prevented reliably, and the slab excellent in surface quality can be manufactured stably.

It is sectional drawing of a casting_mold | template. It is a top view of a casting_mold | template. It is a graph which shows the relationship between distance d, frequency, and the number of coil surface ridges.

Explanation of symbols

1 Mold 2 Electromagnetic stirring coil 3 Immersion nozzle 4 Discharge hole 5 Solidified shell

Claims (2)

In a continuous casting method of steel in which casting is performed while imparting swirlability to molten steel in a mold by an electromagnetic stirring device, between the outer wall of the immersion nozzle immersed in the molten steel and the solidified shell formed on the long side of the mold A continuous casting method for steel, wherein casting is performed so that the minimum distance d (mm) satisfies the following expression (A).
d = (t−D) / 2-18√ (L / Vc) ≧ 86√ (f) (A)
Here, t is the length of the mold short side wall (mm), D is the outer diameter of the immersion nozzle (mm), L is the distance (m) from the meniscus to the portion where the distance between the immersion nozzle and the solidified shell is minimized, Vc is the casting speed, and f is the frequency (Hz) of the electromagnetic stirring coil current.   2. The steel continuous casting method according to claim 1, wherein a square or rectangular mold having an electromagnetic stirring device is used.

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