JP2000015404A - Production of continuously cast slab having little inclusion defect - Google Patents

Abstract

PROBLEM TO BE SOLVED: To continuously cast a cast slab excellent in the cleanliness and the surface characteristic by reducing inclusion on the surface layer of solidified shell starting the solidification in a mold and also, dispersing and reducing the inclusion in the inner layer in the continuous casting method of the cast slab. SOLUTION: At the time of producing the cast slab with the continuous casting, the fluidity in the horizontal direction with stirring magnetic field is given to molten steel in the portion corresponding to a meniscus at the upper part in the mold. Further, the brake is given to the molten steel at the lower part from the portion, in which the flow of the molten steel spouted from an immersion nozzle collides against short side surfaces of the mold and becomes a descending flow, by impressing braking magnetic field, satisfying the following inequality: 1700+3000*V(B<2100+3000*V, wherein V is the descending flow speed (m/s) at the short side of the mold just before the electromagnetic braking device and B is the intensity (Gauss) of the impressed braking magnetic field.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for continuously casting slabs, which reduces inclusions in the surface layer of a solidified shell which starts to solidify from the inside of a mold, and disperses and reduces inclusions in the inner layer to improve cleanliness and cleanliness. The present invention relates to a method for continuously casting a cast piece having excellent surface properties.

[0002]

2. Description of the Related Art In ordinary continuous casting, as shown in FIG. 4, a dipping nozzle 2 having two discharge ports 3 generally oriented in a short side direction is arranged at the center of a mold 1, and molten steel is placed in the mold. The discharge flow 6 collides with the short side surface of the mold and reverses in the upward direction to become an upward flow 13, and the other branches into a downward flow 14 in a downward direction. Some of the inclusions held in the molten steel by the discharge flow 6 float on the surface of the molten steel and are removed, while the remainder is carried to the deep part of the molten steel by the downward flow 14 and becomes solidified in the solidification shell 9 during the floating process. Be captured. The surface inclusions trapped in this solidified shell lead to defects in the product called slivers.

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 by the flow, and suppresses the generation of surface flaws due to the inclusion of inclusions on the surface layer. On the other hand, if the flow of the meniscus 8 is too strong,
Defects increase due to the involvement of continuous casting powder. At the center of the slab 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.
Surface flaws occur.

In order to solve such a problem, an electromagnetic braking device for stopping the flow of molten steel is installed below the mold so that the discharge flow 6 from the casting nozzle collides with the short side of the mold and flows along the same. A braking magnetic field (static magnetic field) acts on the descending flow 14 to attenuate it, and an electromagnetic stirrer for stirring the molten steel is installed above the mold, and a stirring magnetic field (moving magnetic field) is applied to the reverse flow 15 of the meniscus 8 part. Many methods and devices have been proposed for producing a slab having few inclusions in both the inside and the surface of the slab and having few inclusions by promoting the flow of the meniscus 8 parts by acting and forcing the flow by stirring. JP-A-5-177317, JP-A-7-1122
No. 4).

[0005]

Although it was not impossible to suppress the generation of inclusions in the slab by the above-mentioned conventional example, for example, the braking force applied by the electromagnetic braking device described in the above-mentioned publication was reduced. If it is too large, the flow of molten steel at the center of the slab stops, and molten steel flows from the short side of the slab to the center of the slab width at the lower part of the electromagnetic braking device, and the phenomenon of inclusion accumulation at the center of the width occurs. As a result, the effect of dispersing inclusions was not obtained. Further, simply applying both electromagnetic agitation and electromagnetic braking and applying an electromagnetic field has not been able to reliably guarantee the desired quality. The reason is that simply operating electromagnetic stirring and electromagnetic braking blindly
The effects of the application of the electromagnetic field have not been properly enjoyed, and it has been difficult to obtain cast pieces of excellent quality unless casting is performed under appropriate conditions.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a continuous cast slab which has the above-described slab surface and internal inclusions uniformly dispersed in the slab width direction and the thickness direction and has few inclusions. Is what you do.

[0007]

The gist of the present invention lies in the following means. When producing a slab by continuous casting, the molten steel in the upper meniscus applicable part in the mold,
In addition to applying a horizontal flow due to the stirring magnetic field, the molten steel discharge flow from the immersion nozzle collides with the short side surface of the mold, and a braking magnetic field that satisfies the following equation (1) is applied below the downward flow. A method for producing a continuous cast slab having few inclusion defects, characterized by applying braking to a steel sheet. 1700 + 3000 * V <B <2100 + 3000 * V (1) where V is the flow velocity of the mold short side descent immediately before the electromagnetic braking device (m / s) B: The strength of the applied braking magnetic field (Gauss)

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention performed electromagnetic stirring of molten steel in the upper part of a mold, and electromagnetically braked molten steel flow discharged from an immersion nozzle in the lower part of the mold.
By changing the setting position of the device a lot, and repeating various experiments, as a result of trial and error, we succeeded in finding the appropriate arrangement position of the electromagnetic braking device installed below the mold, but this electromagnetic braking device It was confirmed that the arrangement position alone did not provide a sufficient effect as intended, and was greatly affected by the strength of the braking magnetic field generated when the electromagnetic braking device was operated.

Therefore, even if the slag generated in the process of smelting molten steel flows into the mold, the present invention provides
An electromagnetic braking device is installed at an appropriate position below the continuous casting mold, and the discharge flow from the immersion nozzle hits the short side of the mold, and an appropriate braking magnetic field is applied to the descending flow flowing along this side to cause the molten steel flow to flow. It brakes and reduces the inclusions that accumulate in the inner layer of the solidified shell and distributes it evenly. By applying a magnetic field, the ascending flow that rises along the short side surface is promoted, the surface layer inclusions are reduced, and the inner layer of the solidified shell and the surface layer are surely obtained with excellent cleanliness. Things.

The electromagnetic stirring method for applying to the molten steel meniscus in the mold had no particular conditions to be specified. However, the flow of molten steel discharged from the immersion nozzle turned into a downward flow on the short side of the mold was not used. As a result of the research conducted by the present inventors, it has been found that there is an appropriate range for the strength of the braking electromagnetic field for braking the. That is, if the strength of the applied magnetic field is not adjusted according to the descending flow velocity of the molten steel flow to apply the electromagnetic braking, the intended braking effect of the molten steel flow cannot be obtained, and the reduction and dispersion of inclusions cannot be achieved. .

FIG. 2 shows a size of 282 mm (thickness) × 1550.
When casting a slab of mm (width), the molten steel amount was 3.7 t / m from the nozzle discharge port (angle: 35 degrees) immersed in the mold.
and the Ar flow rate in the nozzle is 5 Nl / min
Further, the casting speed was set to 1.1 m / min, and the strength of the applied braking magnetic field was increased by an electromagnetic braking device installed 0.5 m below the position where the molten steel flow from the nozzle outlet collided with the short side of the mold. This is a result of investigating a change in the amount of inclusions (inclusion index) in the slab due to the change in the thickness. The descending flow velocity on the short side of the mold was 0.1 m / s.

Similarly, FIG. 3 shows a size of 282 mm (thickness) ×
When casting a slab of 1830 mm (width), the amount of molten steel from the nozzle discharge port (angle of 35 degrees) immersed in the mold was 5.
2 t / min and Ar flow rate in the nozzle was 15
Nl / min, the casting speed was set to 1.3 m / min, and the molten steel flow from the nozzle discharge port was applied and braked by an electromagnetic brake installed 0.5 m below the position where it collides with the short side surface in the mold. It is a result of investigating a change in the amount of inclusions (inclusion index) in a slab due to a change in the strength of a magnetic field. The descending flow velocity on the short side of the mold was 0.23 m /
s. The reason for supplying Ar gas into the nozzle is to prevent inclusions in the molten steel from adhering to the inner surface of the nozzle and obstructing the flow of the molten steel or closing the discharge port. This is a commonly used thing.

Here, the inclusion index on the vertical axis in FIGS. 2 and 3 is a value empirically obtained by analysis by the present inventors based on requests from users for the cleanliness of steel sheets over the past several years. In the case of a general steel sheet, the inclusion index 1 is a value adopted as a passing level of the quality (cleanliness) of the slab. In this figure, the electromagnetic stirring of the upper part in the mold was performed at the same time. As can be seen from FIGS. 2 and 3, if the casting speed is increased, the descending flow velocity of the molten steel immediately before the electromagnetic braking device is also increased. Therefore, unless the strength of the applied braking magnetic field is increased, the inclusions cannot be dispersed and reduced.

This can be expressed by the following mathematical expression (1). 1700 + 3000 * V <B <2100 + 3000 * V (1) where, V: flow rate of molten steel on the short side of the mold immediately before the electromagnetic braking device (m / s) B: strength of applied braking magnetic field (Gauss) In practice, it is impossible to measure the mold short-side descending flow velocity (V), and it is determined by estimation. This descent velocity value is the casting condition (cast slab size, molten steel supply to the mold, etc.)
Will vary. The descending flow velocity value is determined by the discharge flow velocity from the nozzle discharge port, the distance from the nozzle discharge port to the electromagnetic braking device, and the effect of attenuation. The discharge flow rate from the nozzle outlet is proportional to the amount of molten steel supplied, and the path of the molten steel flow is determined by the geometrical arrangement. In addition, the effects of the attenuation include the Ar flow rate in the nozzle and the attenuation after the short side collision.

The equation obtained empirically is as follows:
It can be expressed as an equation. V = 0.45 * Q * exp (−0.041 * R * L1-10.2 * L2) (2) L1 = W / 2 / cos θ L2 = 1−X−W / 2 * tanθ V: descending flow velocity (m / sec) Q: molten steel supply amount (t / min) L1: distance from discharge port to short side collision point (m) L2: distance from short side collision point to electromagnetic braking device (m) R: Ar flow rate in nozzle (Nl / min) W: Casting width (m) X: Distance from meniscus to discharge port (m) θ: Discharge port angle (deg) The strength of the braking magnetic field to be calculated can be calculated, and appropriate casting of the slab can be performed.

If the strength of the braking magnetic field is set in order to suppress the flow of molten steel under the above conditions, it is possible to prevent inclusions from entering the inside of the slab and to reduce the dispersion of the inclusions. As a result, it is possible to obtain a good product with few defects on the slab surface and the inner layer portion, which is the object of the present invention.

[0017]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples to clarify the effects of the present invention. The molten steel used in the examples is a low-carbon aluminum killed steel.
This molten steel was applied to a vertical bending type continuous casting apparatus as shown in FIG. The immersion nozzle 2 has an inner diameter of 95 mm and an outer diameter of 200
It has a discharge port 3 of 35 mm downward from the horizontal and from which the molten steel flow is discharged to the short side of the mold 1. Mold 1
An electromagnetic stirrer 4 for applying a horizontal electromagnetic force by a moving magnetic field to a corresponding portion of the molten steel meniscus 8 therein was provided. The flow velocity at which the molten steel of the mold 1 flows from the discharge port 3 of the immersion nozzle 2 in the mold 1 to the short side of the mold is 0.2 to 0.
It was 5 m / s. The electromagnetic braking device 5 for applying a braking electromagnetic field for braking the descending flow 14 of molten steel is provided at a position 0.1 mm below the portion where the molten steel from the nozzle discharge port 3 collides with the short side surface of the mold.
It was arranged at a position of 5 m. The results of casting the steel under these conditions are shown in Table 1 together with Comparative Examples.

[0018]

[Table 1]

In this embodiment, as described above, the steel is required to have a high degree of cleanliness. Therefore, the target was set at an inclusion index of 0.8 or less, and a braking magnetic field corresponding to the inclusion index was applied to manufacture the steel. As a result, as is clear from Table 1, in this example, for Experiment Nos. 1 to 7, all satisfied the target values, and the inclusions were also uniformly dispersed.
Good slabs were obtained. On the other hand, in Comparative Example 8, the strength of the braking magnetic field was lower than the range of the present invention, and in Comparative Examples 9 and 10, the strength of the braking magnetic field exceeded the range of the present invention, and the target value of the inclusion index of 0.8 was satisfied. Could not. Comparative Examples 11 to
In No. 13, although the strength of the braking magnetic field was within the range of the present invention, the inclusion index was also 0.8 because no stirring magnetic field was applied.
The following could not be secured. Furthermore, all of these comparative examples have poor dispersibility of inclusions, indicating that it is difficult to obtain good cast slabs as in the present invention.

[0020]

According to the present invention, the molten steel injected into the mold is rotated by an electromagnetic stirrer in the vicinity of the meniscus in the horizontal direction along the inner peripheral surface of the solidified shell. By applying a braking magnetic field to the descending flow that has reversed on the short side surface to attenuate the flow, the inclusions can be uniformly dispersed and the inclusions inside the slab and the inclusions on the slab surface can be simultaneously formed. It is possible to produce a slab having excellent surface and internal quality, which has a great effect to contribute to the slab production in continuous casting.

[Brief description of the drawings]

FIG. 1 is a schematic side view for explaining an outline of the present invention.

FIG. 2 is a diagram showing a relationship between a magnitude of a magnetic field applied to molten steel below a mold and an inclusion index (a casting speed of 1.1 m / min).

FIG. 3 is a diagram showing a relationship between the magnitude of a magnetic field applied to molten steel below a mold and an inclusion index (a casting speed of 1.3 m / min).

FIG. 4 is a schematic side view illustrating a flow state of molten steel in a conventional continuous casting mold.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3 Discharge port 4 Electromagnetic stirring device 5 Electromagnetic braking device 6 Discharge flow 7 Stirring flow 8 Meniscus 9 Solidification shell 13 Upflow 14 Downflow 15 Reverse flow

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Masamitsu Wakao 1 Nishinosu, Oji, Oita City, Oita Prefecture Inside Nippon Steel Corporation Oita Works (72) Inventor Kiyoshi Shigematsu 1 Nishinosu, Oita City, Oita City, Oita New Japan Inside Oita Works of Iron Corporation

Claims (1)

[Claims] In producing a slab by continuous casting, a molten steel in a portion corresponding to an upper meniscus in a mold is given a horizontal flow by a stirring magnetic field, and a molten steel discharge flow from an immersion nozzle is applied to a short side surface of the mold. A braking magnetic field that satisfies the following equation (1) is applied below the downward flow,
A method for producing a continuously cast slab having few inclusion defects, wherein braking is applied to molten steel. 1700 + 3000 * V <B <2100 + 3000 * V (1) where V is the flow velocity of the mold short side descent immediately before the electromagnetic braking device (m / s) B: The strength of the applied braking magnetic field (Gauss)