JP3096879B2 - Continuous casting method for slabs with excellent surface and internal quality - 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 slabs having excellent surface and internal quality by controlling the flow of molten steel in a mold in continuous casting of steel.

[0002]

2. Description of the Related Art In continuous casting, as shown in FIG. 2, an immersion nozzle 10 having two discharge ports 11 which are generally directed in a short side direction is disposed at the center of a mold 1 and molten steel is introduced into the mold. The discharge flow 12 collides with the short side and branches into an ascending flow 13 and a descending flow 14.

[0003] Inclusions and bubbles contained in the molten steel are partly levitated to the surface of the molten steel and discharged, while the remainder is carried to the deep part of the molten steel by the downward flow 14, and the solidified shell 9 is raised during the floating process. And remain in the slab, resulting in internal defects and blisters in the product.

On the other hand, in the upward flow 13, in the vicinity of the meniscus 8, a reverse flow 15 is generated from the short sides on both sides toward the nozzle. The presence of such a flow in the vicinity of the meniscus 8 provides a cleaning effect of inclusions and bubbles due to the flow, and suppresses generation of surface flaws due to trapping of inclusions and bubbles in the surface layer.

On the other hand, if the flow of the meniscus 8 is too strong, the defects increase due to the involvement of the continuous casting powder. In addition, since the surface of the molten metal is disturbed, hot water wrinkles and oscillation mark disturbances occur, and this phenomenon degrades the surface quality of a product particularly in stainless steel.

At the center of the width, the flow velocity of the reverse flow near the meniscus 8 becomes slow, so that the cleaning effect cannot be obtained, and inclusions and bubbles are trapped. In this part, vertical cracks are easily generated.

In order to solve such a problem, as disclosed in Japanese Patent Application Laid-Open No. 4-147754, an electromagnetic brake is installed on a slab indicating roll section below a mold to discharge a discharge flow from a casting nozzle. Collides with the short side of the mold and applies a static magnetic field to the descending flow flowing along it to attenuate it, and installs an electromagnetic stirring coil in the mold to apply a moving magnetic field to the reverse flow of the meniscus, and A method has been proposed in which the flow of meniscus is promoted by a forced flow to produce a slab which has no inclusions or bubbles in the inside and the surface of the slab and has few defects.

[0008]

However, in the invention disclosed in Japanese Patent Application Laid-Open No. 4-147754, the reversal flow 15 in the nozzle direction due to the discharge flow 12 from the immersion nozzle 10 is not attenuated as shown in FIG. And the molten steel flow at 8 parts of the mold meniscus is a flow in which this flow and the stirring flow 16 by the electromagnetic stirring device 4 are superimposed. , The reverse flow 15 by the discharge flow 12 and the stirring flow 1
There are places where the direction of 6 becomes the same direction and where it becomes the opposite direction. The flow is accelerated when the two directions are in the same direction, but is attenuated in the opposite direction, and the flow velocity required to achieve the purpose cannot be obtained. Is not completely gone.

Further, since there is a portion where the flow of the meniscus 9 becomes excessively strong, a defect increases due to entrainment of the continuous casting powder. Also, in this portion, the surface of the product is distorted due to the disturbed surface of the molten metal, and the quality of the surface of the product is deteriorated, particularly in stainless steel.

[0010] Further, in order to provide the stirring flow 16 that overcomes the attenuation due to the reverse flow 15, a large stirring force is required, and equipment costs and energy costs are increased.

The present invention reduces the above-described inclusions and bubbles inside the slab and the inclusions and bubbles on the slab surface, hot lines, and vertical cracks, and produces a slab excellent in surface and internal quality. A casting method is provided.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a continuous casting method in which molten steel is injected into a mold from an immersion nozzle. An electromagnetic stirrer is installed to cause the molten steel in the mold near the meniscus to generate a flow that rotates horizontally along the inner peripheral surface of the solidified shell, and a static magnetic field is applied below the nozzle outlet to attenuate the discharge flow. This is a method for continuously casting slabs having excellent surface and internal quality.

[0013]

As a result of various experiments, the inventor has found that the flow in the nozzle direction near the meniscus due to the discharge flow from the immersion nozzle is caused by the discharge flow colliding with the short side of the mold and becoming a reverse flow. It is possible to prevent the flow toward the nozzle in the vicinity of the meniscus by discharging molten steel downward so that the discharge flow does not collide with the short side of the mold. I found that I can do it.

According to the present invention, as shown in FIG. 1, an immersion nozzle 2 provided with a discharge port 3 at the tip of the nozzle is installed, whereby the discharge flow 6 discharged from the discharge port 3 of the immersion nozzle 2 flows downward. Since the flow does not collide with the short side, a strong flow toward the nozzle at the meniscus 8 is not generated as in the case of discharging in the lateral direction.

When a stirring flow is generated by the electromagnetic stirring device 4 in this state, only the stirring flow 7 due to the electromagnetic stirring is generated in the vicinity of the meniscus 8, and the discharge flow 6 and the stirring flow 7 are generated.
And uniform flow over the inner circumference of the mold 1 can be obtained.

By these means, it is possible to uniformly flow the inner surface of the solidified shell 9 near the meniscus 8. This makes it possible to produce a slab having few cracks and inclusions on the surface. In addition, the fluctuation of the molten metal level is small, a cast slab having good surface properties without hot water wrinkles and the like is obtained, and the surface quality of the product is also improved.

On the other hand, when the molten steel is discharged downward, the downward flow becomes stronger as compared with the case where the molten steel is discharged laterally, so that the inclusions brought in from the nozzle 2 do not float up to the deep portion of the continuous casting machine. Intrusion occurs and is trapped in the slab at the inclusion accumulation part generated in the curved part in the continuous casting machine having the curved part, and the occurrence of internal defects of the product due to inclusions inside the slab increases. .

In order to prevent this, it is necessary to make the downward flow velocity at the inclusion accumulation section the same as that in the case of horizontal discharge. Therefore, a magnet 5 is installed below the nozzle discharge port, and the discharge flow 6 is braked by applying a static magnetic field, whereby the descending flow velocity can be attenuated. Is almost the same as

As described above, the electromagnet 5 disposed below the discharge port 3 is injected while being injected by the immersion nozzle 2 having the discharge port 3 at the bottom.
By applying a static magnetic field, the discharge flow 6 is attenuated, and the electromagnetic stirrer 4 gives the stirring flow 7 in the vicinity of the meniscus 8 to perform casting, so that both the surface and the inside have good surface properties with few inclusions and bubbles. It is possible to manufacture simple cast pieces.

The cross-sectional shape of the nozzle may be any shape such as a circle, an ellipse, and a rectangle. If the space between the mold wall and the outer wall of the nozzle is too narrow, the flow resistance increases, and the required rotational flow is obtained by electromagnetic stirring. I can't. In addition, it is desirable to have an interval of 35 mm or more, since surface cracks and double skin are caused by unevenness of molten steel near the nozzle.

In this method, since there is no flow interference, a rotating flow can be obtained efficiently, and the stirring power of the electromagnetic stirring required to obtain the same flow rate can be small, and the equipment cost and energy cost can be reduced. There are advantages.

[0022]

EXAMPLE As an example, as shown in FIG. 1, low carbon aluminum killed steel was 1200 mm wide and 24 mm thick in a continuous casting mold 1 by a dipping nozzle 2 having a discharge port 3 of 90 mm diameter at the bottom.
Casting was performed at 5 mm, a casting speed of 1.3 and 2.0 m / min. In this case, the distance from the meniscus 8 to the discharge port 3 of the immersion nozzle 2 was set to 200 mm. Further, an electromagnet 5 is installed immediately below the mold to reduce the magnetic flux density to 1000 to 5000 Ga.
uss. An electromagnetic stirrer 4 is installed in the mold 1, and the stirring thrust is 20 to 500 KVA, the frequency is 1 to 5 Hz.
It was used under the condition of 50 mmFe.

The results are shown in Table 1 together with the quality of the slab, and it can be seen that the occurrence of surface defects, internal defects, longitudinal cracks, and hot water wrinkles of the slab are reduced with a lower stirring thrust than the comparative example described later. confirmed.

[0024]

[Table 1]

As a comparative example, as shown in FIG. 3, a low carbon aluminum killed steel having a width of 1200
mm, thickness 245mm, casting speed 1.3 and 2.0m
/ Min. Immersion nozzle 10 from meniscus 8
The distance to the discharge port 11 was 200 mm.

The electromagnet 5 was placed directly below the mold 1, and the magnetic flux density was 1000 to 5000 Gauss. An electromagnetic stirrer 18 is installed in the mold 1 and has a frequency of 500 kVA and a frequency of 1 kV.
５5 Hz and a stirring thrust of 40-80 mmFe were used.

The results are shown in Table 1 together with the quality of the slab. The internal defects were reduced as compared with the case where the electromagnet and the electromagnetic stirring were not used, but the surface defects of the slab occurred. Further, when the thrust was increased, a defect due to powder entrainment occurred.
In addition, occurrence of hot water wrinkles was observed near the short side.

[0028]

As described above, according to the present invention, molten steel is poured downward, the molten steel near the meniscus is rotated horizontally along the inner peripheral surface of the solidified shell by an electromagnetic stirrer, and the nozzle discharge port is provided. By applying a static magnetic field below the surface of the slab to attenuate the discharge flow, inclusions and bubbles inside the slab and inclusions, bubbles, hot lines and vertical cracks on the slab surface can be reduced at the same time. And a slab excellent in internal quality can be manufactured. In addition, a rotating flow can be obtained with a small facility cost and an energy cost, and the facility cost and the operating cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic side sectional view showing a positional relationship between an electromagnetic stirrer and a static magnetic field generator suitable for carrying out the present invention.

FIG. 2 is a schematic side sectional view illustrating a flow state of molten steel in a conventional continuous casting mold.

FIG. 3 is a schematic side sectional view illustrating a flow state of molten steel in a conventional continuous casting equipped with an electromagnetic stirrer and a static magnetic field generator.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 mold 2 immersion nozzle 3 discharge port 4 electromagnetic stirring device 5 static magnetic field generator 6 discharge flow 7 stirring flow 8 meniscus 9 solidification shell 10 immersion nozzle 11 discharge port 12 discharge flow 13 ascending flow 14 descending flow 15 inversion flow 16 stirring flow

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-5-77007 (JP, A) JP-A-5-293620 (JP, A) JP-A-5-245601 (JP, A) JP-A-5-245601 84550 (JP, A) JP-A-7-112248 (JP, A) JP-A-7-108356 (JP, A) JP-A-7-9098 (JP, A) JP-A-6-226409 (JP, A) JP-A-6-190520 (JP, A) JP-A-5-104218 (JP, A) JP-A-4-147754 (JP, A) JP-A-63-119959 (JP, A) JP-A-62-130752 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/04 311 B22D 11/10 330 B22D 11/11 B22D 11/115

Claims (1)

(57) [Claims] 1. In continuous casting in which molten steel is injected into a mold from an immersion nozzle, a discharge port is provided at the bottom of the nozzle to inject molten steel downward, and an electromagnetic stirrer is installed outside the mold to form molten steel in the mold near the meniscus. The surface and the internal quality are excellent in that a flow that rotates in the horizontal direction along the inner peripheral surface of the solidified shell is generated, and the discharge flow is attenuated by applying a static magnetic field below the nozzle discharge port. A continuous casting method for slabs.