JP3206426B2 - Continuous casting of ultra-low carbon steel - 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 ultra-low carbon steel, which suppresses carburization from mold powder to a slab at the start of casting.

[0002]

2. Description of the Related Art In recent years, the demand for thin plate quality has become strict. Advances in decarburization refining technology using RH vacuum degassing equipment have made it possible to produce ultra-low carbon molten steel quickly and at low cost. ing.

However, at the start of casting of the first charge of continuous casting (hereinafter abbreviated as "start of casting"), the surface of the molten metal is vigorously agitated by the pouring flow from the immersion nozzle, and the work of confirming skinning is performed. Carbon contained in the mold powder is carburized in the slab (called carbon pickup).

[0004] In particular, in ultra-low carbon steel, the standard of carbon concentration is low and the range is narrow. Therefore, when such a carbon pickup occurs, the carbon standard is deviated. This leads to a decrease in the slab allocation rate and an increase in manufacturing costs.

As a technique for suppressing the carbon pickup at the start of casting, Japanese Patent Application Laid-Open Nos. Hei 6-17056 and Hei 4-81251 are disclosed.
Japanese Patent Application Laid-Open Publication No. 2000-209 (Prior Art Document 1) discloses a method of reducing the carbon content in a mold powder to 1.0% by weight (hereinafter referred to as%) or less. By this, at the start of casting, insert the metal rod into the molten steel in the mold to check the solidification state of the molten steel surface, stir the metal rod to re-dissolve the solidified metal, or remove the solidified metal out of the mold (A series of these operations is referred to as a skinning check operation) can significantly reduce the amount of carbon picked up to the bottom slab.

[0006] Japanese Patent Application Laid-Open No. 6-47505 discloses a method in which a solidified plate obtained by solidifying a mold powder containing no carbon into a plate shape is previously put into a mold (referred to as Reference 2). According to this method, when pouring into the mold is started, the solidified plate immediately rises due to buoyancy and covers the molten metal surface, so that radiant heat from the molten steel surface can be cut. For this reason, the molten steel is kept warm and the occurrence of skinning can be suppressed. In addition, even if the work of confirming skinning is performed, carbon is not contained in the powder slag layer after the solidified plate has turned into slag, so that carbon pickup can be prevented.

[0007]

However, even if a mold powder having a carbon content of 1.0% or less proposed in the prior art document 1 is used, the amount of carbon pick-up on the bottom slab is large, and as expected. The cut length of the bottom slab cannot be reduced.

[0008] In addition, the carbon content in the powder is 1.0
% Or less, the slagging speed is increased and the slag is melted in a short time, so that the heat retention performance is deteriorated. For this reason, radiant heat increases from the surface of the molten steel, so that skinning is likely to occur, and work time and load for checking skinning increase.

[0009] Further, in the prior art document 2, it is necessary to prepare for casting in order to put the solidified plate in the mold in advance. In addition, if casting troubles such as nozzle clogging due to low heat occur at the start of casting, the solidified plate is in the way, restricting recovery work, and possibly leading to casting stoppage.

The present invention has been proposed in order to solve the above-mentioned problems of the prior art. When starting casting of ultra-low carbon steel, bottom cast piece cutting is performed using a mold powder capable of suppressing carbon pickup. In addition to reducing the discard length, even if casting troubles such as nozzle clogging occur, the casting operator is not restricted in work, and extremely low that can suppress skinning even if the heat insulation performance of the mold powder deteriorates An object of the present invention is to provide a continuous casting method of carbon steel.

[0011]

According to the first aspect of the present invention, at the start of casting of ultra-low carbon steel, after pouring into a mold from an immersion nozzle, the carbon content is 0.5% by weight or less. The use of an initial mold powder, a skin check operation after that, and use of a mold powder having a carbon content exceeding 1.0% by weight after the skin check operation is completed. It is a continuous casting method for steel.

According to the present invention, after pouring from an immersion nozzle into a mold, an initial mold powder (in the present application, a mold powder used within a certain period at the start of casting is called an initial mold powder, and is used during steady casting. Molten steel surface (bare water)
Therefore, radiant heat from the molten steel surface is prevented, and skinning immediately after the start of pouring is prevented.

[0013] After that, a skinning check operation is performed. Until this operation is completed, the metal rod is inserted into the molten steel by using an initial mold powder having a low carbon content of 0.5% or less. Even if it is agitated, the amount of carbon picked up from the mold powder during the checking operation is suppressed.

After the completion of such a skinning check operation, even if a mold powder having a carbon content of more than 1.0% by weight is used, the above-mentioned metal rod is not inserted into the molten steel or agitated. Carbon pickup on the piece is suppressed.

According to a second aspect of the present invention, a moving magnetic field type electromagnetic stirrer is arranged outside the long side of the mold to generate a moving magnetic field in the short side direction of the mold during the work of confirming skinning, and molten steel discharged from the immersion nozzle is provided. A continuous casting method for ultra-low carbon steel, characterized in that an electromagnetic force acts on the flow to accelerate the flow velocity and increase the upward reversal flow.

The moving magnetic field type electromagnetic stirrer is a linear motor type electromagnetic stirrer (referred to as EMLA) disclosed by the present applicant in JP-A-63-104763.
When the electromagnetic stirrer is activated, a moving magnetic field can be generated in which the magnetic field moves with time from the center of the long side of the mold to the short side of the mold.

For this reason, since the electromagnetic force acts on the molten steel flow discharged from the immersion nozzle in the direction of the short side of the mold and the flow velocity is accelerated, the upward reversal flow after colliding with the short side of the mold increases, and the molten steel flow increases. The amount of heat supplied to the molten metal surface can be efficiently increased.

As a result, the temperature of the molten steel surface may be lowered even when using an initial mold powder having a carbon content of 1.0% or less, which is 0.5% or less, which further deteriorates the heat retaining performance as compared with a mold powder having a carbon content of 1.0% or less. It is possible to suppress skinning. Further, by suppressing the skinning, it is possible to reduce the work time and load for checking the skinning.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a situation of a skin check operation by a casting operator at the start of casting, and FIG. 2 shows time in a mold operation, molten steel surface level, slab withdrawal, electromagnetic force and the like. The course is schematically shown. FIG. 3 schematically shows a change in the molten steel flow when an electromagnetic force is applied to the molten steel flow discharged from the immersion nozzle.

Here, 1 is a mold, 2 is an immersion nozzle,
3 is a mold powder, 4 is a metal rod, 5 is molten steel, 6 is a casting operator, 7 is a solidified shell, a is a direction in which an electromagnetic force acts, b is a molten steel flow when no electromagnetic force acts, and c is an electromagnetic force. And d is an upward reversal flow after the molten steel stream c collides with the short side.

A moving magnetic field type electromagnetic stirrer (not shown) is opposed to the outside of the mold long side in the thickness direction of the mold, and divided at the center (immersion nozzle position) in the long side width direction. A total of four electromagnetic coils were arranged. In this electromagnetic stirring device, the magnetic field is controlled to move from the center of the long side of the mold to the short side of the mold with time. The thickness of each electromagnetic coil is 200 mm in the direction of drawing the slab.
The center of the coil is 200 m from the discharge port of the immersion nozzle 2.
m horizontally below.

The operation procedure at the start of casting will be described below with reference to FIG. First, when the molten steel in the tundish is poured from the immersion nozzle into the mold, the molten steel level gradually rises in the mold, and the molten steel level rises to the upper end of the immersion nozzle discharge port (the distance from the upper end of the mold is 190 mm). ), A predetermined amount of initial mold powder is injected by the casting operator 6.

Thereafter, the molten steel is continuously injected into the mold, and the molten steel level is 140 degrees from the upper end of the mold.
mm, the mold vibrator is activated to start oscillation, and when the molten steel surface level reaches a position 110 mm from the upper end of the mold, slab drawing is started (start of casting).

When the molten steel surface reaches a level (80 mm from the upper end of the mold) during the steady casting after the start of the slab drawing, the casting operator 6 moves to a diameter of 10 to 10 mm as shown in FIG.
A 15 mm iron bar is inserted into the molten steel over the entire area of the mold, and stirred to check whether the molten steel surface is solidified (skinned). At this time, when the steel is skinned, the solidified metal is pushed into the molten steel by a metal rod to be re-melted, or a large solidified metal that requires time for melting is removed from the outside of the mold. After the completion of the skinning check operation, the use of the initial mold powder is stopped, and the use of the mold powder having a carbon content exceeding 1.0% used in steady casting is started.

At this time, when the electromagnetic stirrer is started at the same time as the start of drawing of the slab, the molten steel flow discharged from the immersion nozzle is applied in a horizontal direction (arrow a direction) from the center of the long side of the mold toward the short side. Electromagnetic force acts, the flow velocity is accelerated, and the molten steel flow is upward (from arrow b direction to arrow c direction).
Changes to Therefore, the upward reversal flow (direction of arrow d) after the molten steel flow (direction of arrow c) collides with the short side increases,
Since the amount of heat supply to the molten steel surface is increased, skinning is suppressed even when an initial mold powder of 0.5% or less is used.

Here, the time (predetermined time) during which the electromagnetic force is applied is a value empirically determined by the size of the cast slab to be cast, the casting temperature, the quality of the slab, and the like.
By acting for a short time, the effect of suppressing skinning can be expected.

[0027]

EXAMPLES In order to confirm the effect of the method of the present invention, a test for casting ultra-low carbon steel having a carbon standard of 30 ppm or less was carried out using a continuous slab casting facility.

The casting size is 220 mm thick x 1250 to 1
Two types of 650 mm width, initial mold powder having carbon content of 0.2% and 0.5% (Example) and two types of carbon content of 0.7% and 2.1% (Comparative Example) A total of four types of powders were used, and after completion of the skinning check operation, casting was performed using a mold powder having a carbon content of 2.1%.

Simultaneously with the start of drawing of the slab, the moving magnetic field type electromagnetic stirring device was started, and a magnetic field having a magnetic flux density of 0.12 T was applied until the casting speed was increased to 1.0 m / min. In other conditions, casting was started in the same procedure as the above-described operation procedure in FIG.

FIG. 4 shows the transition of the pickup amount of the carbon concentration from the bottom to the slab at the time of steady casting. As shown, the lower the carbon content, the lower the pickup amount. In particular, by using the initial mold powder of 0.5% or less until the skinning check operation is completed, the pickup of the carbon concentration can be significantly reduced.

By applying an electromagnetic force during this checking operation, the skinning at the beginning of casting can be suppressed, and the working range of the skinning in the mold by a metal rod can be suppressed to 1 m or less by the slab drawing length. .

[0032]

According to the present invention, the use of the initial mold powder having a carbon content of 0.5% or less until the completion of the skinning check operation suppresses the carbon pickup and reduces the cut length of the bottom slab. it can. Even when a mold powder of 0.5% or less is used, the skinning can be suppressed by applying an electromagnetic force, so that the work time and load at the start of casting can be reduced. In addition, since a solid plate or the like is not put into the mold, a smooth recovery operation can be performed even when a casting trouble occurs.

[Brief description of the drawings]

FIG. 1 is a diagram showing a situation of a skinning check operation by a casting operator.

FIG. 2 is a diagram showing a time course of operations in a mold, a molten steel surface level, a slab drawing length, and the like at the start of casting.

FIG. 3 is a diagram schematically showing a change in the molten steel flow when an electromagnetic force is applied to the molten steel flow.

FIG. 4 is a diagram showing a transition of a pickup amount of carbon concentration from the bottom.

[Explanation of symbols]

 Reference Signs List 1 mold 2 immersion nozzle 3 mold powder 5 molten steel 6 worker 7 solidified shell a direction in which electromagnetic force acts b: molten steel flow when electromagnetic force is not applied c: molten steel flow when electromagnetic force is applied d: upward reversal Flow

Continuation of the front page (56) References JP-A-4-105575 (JP, A) JP-A-3-169467 (JP, A) JP-A-3-193243 (JP, A) JP-A-63-174767 (JP) JP-A-5-177317 (JP, A) JP-A-5-76993 (JP, A) JP-A-4-81251 (JP, A) JP-A-6-47505 (JP, A) 63-104763 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/108 B22D 11/00 B22D 11/115

Claims (2)

(57) [Claims] 1. At the start of casting of ultra-low carbon steel, after pouring into a mold from an immersion nozzle, use of an initial mold powder having a carbon content of 0.5% by weight or less is started, and thereafter, A continuous casting method for ultra-low carbon steel, characterized in that a mold powder having a carbon content of more than 1.0% by weight is used after the work of confirming the tension is completed. 2. A moving magnetic field type electromagnetic stirrer is arranged outside the long side of the mold to generate a moving magnetic field in the direction of the short side of the mold during the work of confirming skinning, and an electromagnetic force is applied to the molten steel flow discharged from the immersion nozzle. The continuous casting method for ultra-low carbon steel according to claim 1, wherein the method is used to accelerate the flow velocity to increase the upward reverse flow.