JPH11320052A - Method for controlling fluid of molten steel in continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control method of the fluid of molten steel, with which impurity at the loose side of a slab with a bending type continuous caster is reduced. SOLUTION: An electromagnetic induction stirring device is disposed at an interval between (Lo +H/2) to (2m-H/2), (wherein, LO is the distance from a meniscus of the molten steel to a position, at which the solidified interface in the loose side becomes perpendicular, and H is the height of the electromagnetic induction stirring device). Circular flow in the horizontal direction along the solidified interface is formed in the molten steel with the electromagnetic induction stirring device, and the intensity of the circular flow is changed over into strong flow and weak flow in the following frequency (f): 1 sec<=f<=(10<3> .W/ F<1/2> ) sec. (wherein, W is casting width and F is the thrust of the electromagnetic induction stirring device). Further, instead of change-over into the strong flow and the weak flow, the circular flow can be changed over from one direction to the reverse direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION In the continuous casting of a curved mold or a bending type, after casting molten steel in a vertical direction, a slab is moved laterally. Hereinafter, these are collectively referred to as curved continuous casting. The present invention relates to a continuous casting method for producing a slab by such curved continuous casting.

[0002]

2. Description of the Related Art FIG. 3 is a schematic view of a conventional curved continuous casting, in which (A) is a longitudinal sectional view, and (B) and (C) are II sectional views. In the figure, 1 is a mold, 2 is molten steel, 3 is a solidified shell,
4 is an immersion nozzle, 5-1 and 5-2 are electromagnetic induction stirrers,
6 is a flux, and 11 is an interface between the solidified shell and the molten steel.

In the molten steel 2 supplied from the immersion nozzle 4, slag inclusions in a previous process, for example, a tundish, are apt to be mixed in, and a flux 6 covering the molten steel is liable to be involved at the time of injection. Since an inert gas is often blown into the immersion nozzle 4 during casting, the molten steel 2 may contain gas bubbles. These inclusions, flux and gas bubbles (abbreviated as impurities in the present specification) are involved in the molten steel injection flow 2 ′ supplied from the immersion nozzle and infiltrate into the molten steel.

The impurities infiltrated into the molten steel float in the molten steel 2 because they are lighter than the molten steel, reach the flux 6 and are absorbed by the flux and removed from the molten steel. 7A and 8 in FIG. 3A are examples of impurities. The impurities 8 existing at the solidified shell interface on the fixed side in FIG. 3A are removed by floating in the molten steel 2 because there is nothing that hinders the floating. However, the impurities 7 present at the solidified shell interface on the loose side are prevented from rising by the solidified shell 3 projecting upward. As a result, there is a problem in that the impurities 7 are not removed and are caught by the solidified shell, resulting in defects of the slab after the solidification is completed.

[0005] Although not limited to curved continuous casting, a linear motor type electromagnetic induction stirrer for generating a moving magnetic field of a linear motor type is provided so as to sandwich the long side surface of the slab being cast. 2. Description of the Related Art A technique for forming a steady swirling flow in which the strength and direction of a flow do not always change in molten steel is known. 3 in FIG.
-1 and 5-2 are examples of this electromagnetic induction stirrer. The electromagnetic induction stirrer 5-1 generates a moving magnetic field to generate 5
The −1 side molten steel is moved in the direction of arrow 9-1, and the electromagnetic induction stirrer 5-2 forms a moving magnetic field having a moving direction opposite to that of the moving magnetic field formed by 5-1. The molten steel is moved in the direction of arrow 9-2. As a result, a steady swirling flow, for example, counterclockwise, is formed in the molten steel 2 as shown in the figure.

The present inventors have proposed an electromagnetic induction stirrer 5-1.
A steady swirling flow of arrows 9-1 and 9-2 is formed in the unsolidified molten steel 2 by 5-2, and the impurities 7 on the loose side in FIG. 3 are transferred to the fixed side by this swirling flow and floated and removed. An attempt was made. However, according to the knowledge of the present inventors, this method using a steady swirling flow causes a problem that the impurities 7 are locally accumulated without turning to the fixed side, and a portion where the impurities 7 are accumulated is generated in a slab. I do. Although the reason for this is not clear, the steady swirling flow has a locally high flow velocity portion and a stagnant portion indicated by 7 'in FIG. 3C, for example, and the impurity 7 does not move to the fixed side. It is conceived that this is to accumulate in the stagnant place 7 '.

Although different from slab continuous casting, Japanese Patent Application Laid-Open
No. 44858 discloses a method for improving a final solidified portion accompanied by porosity at the center of a slab when a cylindrical or prismatic billet is manufactured by continuous casting.
It describes that an electromagnetic induction stirrer is arranged near the final solidification part of 7 m, and a swirling flow whose direction is reversed is used. However, since this method is a method for improving the final solidified portion, the place where the electromagnetic induction stirrer is provided is located below the mold near the final solidified portion.
From the meniscus of the molten steel in the mold.
m above the continuous casting device.

[0008]

SUMMARY OF THE INVENTION The present invention reduces the impurities in the slab on the loose side and reduces the occurrence of the accumulation of such defects when the slab is manufactured by the curved continuous casting machine. It is an object of the present invention to provide a continuous casting method for molten steel capable of performing the above-mentioned tasks.

[0009]

Means for Solving the Problems and Embodiments of the Invention
FIG. 1 is a schematic explanatory view of an example of the present invention, and FIG.
(B), (C), and (D) are cross-sectional views. The present invention relates to continuous slab casting by a curved continuous casting machine. In the present invention, a pair of electromagnetic induction stirrers arranged so as to sandwich the long side surface of the slab at a height where the distance to the meniscus 10 of the molten steel is L in the following formula (1) is used. That is, the electromagnetic induction stirrer is disposed at a position where the distance L between the meniscus of the molten steel and the center of the height of the electromagnetic induction stirrer is within the range of the following expression (1).

L ₀ + (H / 2) <L <2- (H / 2) (1) Above point P in FIG. 1 (A), the interface 11 between the loose-side solidified shell and the molten steel Is inclined upward and downward below the point P, and downward at the point P.
In the equation (1), L ₀ is the distance from the meniscus 10 to the point P. The point P is grasped in association with the casting conditions by calculation or by measuring the top of the broken-out slab. In the equation (1), H is the height of each of the electromagnetic induction stirrers 5-1 and 5-2.

Above the point P, even if impurities are present in the vicinity of the interface 11 of the solidified shell, the interface 11 is widened, so that the rising and floating is not hindered, the floating is removed and the solidified shell is not caught by the solidified shell. . For this reason, the electromagnetic induction stirrer is disposed below L ₀ + (H / 2). Further, impurities that are involved in quality control and are involved in the molten steel injection flow 2 ′ infiltrate into the molten steel up to 2 m below the meniscus, but do not infiltrate deeper. For this reason, the electromagnetic induction stirrer is disposed above 2- (H / 2).

In the present invention, swirling flows 9-1 and 9-2 which flow horizontally along the interface with the solidified shell are formed in the unsolidified molten steel 2 inside the solidified shell 3. This swirling flow is energized so that the moving direction of the moving magnetic field formed by the electromagnetic induction stirrer 5-2 is opposite to the moving direction of the moving magnetic field formed by the electromagnetic induction stirrer 5-1. Obtained by

Further, in the present invention, the strength of the swirling flow is determined by comparing the strength of the swirling flow with the strong flows 9-1 and 9-2 shown in FIG. 1B and the weak flow 9--9 shown in FIG.
It is periodically switched to 1 ', 9-2'. This switching is performed by changing the current value flowing through the electromagnetic induction stirrers 5-1 and 5-2,
This can be performed by periodically switching between a high current value (ampere) and a low current value (ampere).

Further, in the present invention, the period f (second) for switching between the strong flow and the weak flow is set within the range of the following equation (2).

1 second ≦ f ≦ (10 ³ · W / ΔF) seconds (2) When the stirring period f is less than 1 second, the turning movement of the molten steel 2 follows the change in the strength of the moving magnetic field. Otherwise, the flow of molten steel is less effective because it cannot be changed as desired. The present inventors performed electromagnetic induction stirring while changing the cycle f, and examined the produced slabs to determine whether or not the accumulation of impurities occurred. However, when the cycle was (10 ³ · W / √F) seconds or more, It has been found that if the time is too long, accumulation of impurities occurs in the slab. The reason for this is not clear, but if the period is too long, exceeding (10 ³ · W / √F), 9-1 and 9-2 will also become 9-1 'and 9-
It is thought that 2 'flows like a steady flow and a place where stagnation occurs in the flow of molten steel, and impurities accumulate in this portion.

Further, according to the findings of the present inventors, for example, the non-swirl flow of 9-1 and 9-2 in FIG.
2, the turning direction is opposite to that of FIG.
When the direction is periodically switched, the remarkable effects can be obtained in the reduction of impurities and the prevention of generation of an impurity accumulation portion. That is, in the present invention, the electromagnetic induction stirrer is arranged as in the following formula (3), which is the same as the formula (1), and the following formula (4), which is the same as the formula (2)
This is a method in which the direction of the swirling flow is periodically switched between left turning and right turning in the cycle of the formula. In addition, the reason for specifying as in Expressions (3) and (4) is the same as the reason for specifying in Expressions (1) and (2).

L ₀ + H / 2 <L <2-H / 2 (3) where L: distance between the meniscus of the molten steel and the center of the height of the electromagnetic induction stirrer (m) L ₀ : the molten steel Distance from meniscus to position where interface between solidified shell and molten steel is vertical (m) H: Height of electromagnetic induction stirrer (m) 1 sec ≤ f ≤ (10 ³ · W / √F) sec ... … (4) where f: cycle (seconds) of the swirl flow in one direction and the swirl flow in the opposite direction W: casting width (m) F: thrust of the electromagnetic induction stirrer (N / m ² )

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors of the present invention used a curved continuous casting machine having a bending radius of 10.5 m and a sheet width of 1000 mm and a sheet thickness of 2 mm.
The invention was tested when continuously casting a 50 mm low carbon steel slab at a casting speed of 1.3 m / min. The immersion nozzle is an immersion nozzle having a discharge hole with a discharge angle of 25 degrees downward,
Ar gas for preventing nozzle blockage was blown into the inside at a rate of 5 L / min. The electromagnetic induction stirrer has a width of 1500 mm and a height H of 250 mm, and a half of the height of the device is arranged at a position 1500 mm below the molten steel meniscus.

The present inventors assumed that the conventional method in which no electromagnetic induction stirring was performed during the first casting period corresponding to about one-fourth of the total casting amount from the start of casting, and about one-fourth to about one-half In the second casting period, a steady swirling flow shown in FIG. 3B is formed at 10 kN / m ² and agitated, and about 1/2 to about 3/4 is cast. ) And (C) are switched at a cycle of 3 seconds. Note thrust of 9-1 and 9-2 of the third molding stage (B) is at 10kN / m ² (C) of 9-1 ', 9-2' thrust is 5 kN / m ^2. In the fourth casting period from the third casting period to the end of casting, (B) and (D) of FIG.
The present invention has been implemented in which switching is performed in a second cycle. Incidentally, 9-1 and 9-2 of (B) and 9-1 "and 9-2" of (D) in the fourth casting period.
Are 10 kN / m ² .

After completion of the casting, a sample was taken from the slab at each casting period, and was 20 mm to 40 m below the long side surface on the loose side.
The abundance of impurities in the portion m was evaluated. FIG. 2 shows the result. The width end in the figure is a position 40 mm from the short side of the slab, and the center of the width is the center of the slab width.

As shown in FIG. 2, during the first casting period in which electromagnetic induction stirring is not performed, the evaluation of impurities is generally high and the amount of impurities is large. The second casting stage, which forms a conventional steady swirl flow,
The impurity score at the center of the width and one of the width ends is low and good, but the impurity score at the width end downstream of the swirling flow is extremely high, and impurities are accumulated in this portion. In the third casting period and the fourth casting period of the present invention, the scores of impurities are remarkably lower than those in the first casting period, and there is no accumulation of impurities.

Although the reason why the amount of impurities is small and there is no accumulation of impurities in the third casting period and the fourth casting period of the present invention is not clear, the location of stagnation generated by, for example, a strong swirling flow of the present invention is the following weak point. It is presumed that the stagnation does not occur in the swirling flow, and the impurities collected in the stagnation of the strong swirling flow are washed away by the next weak swirling flow, thereby preventing the accumulation of impurities. Similarly, for example, impurities collected in the stagnation generated by the clockwise swirling flow are washed away by the next counterclockwise swirling flow, and thus it is presumed that the accumulation of impurities is prevented. For the above reason, in the present invention, the molten steel flows evenly, and this flow causes the impurity 7 in FIG. 3 to move to the position of the impurity 8 in FIG. It is thought that it was separated from the surface.

[0023]

According to the present invention, when a slab is manufactured by a curved continuous casting machine, impurities on the loose side of the slab can be reduced. In addition, it is possible to effectively prevent generation of an impurity accumulation portion.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a method for controlling molten steel flow according to the present invention.

FIG. 2 is a schematic explanatory view of a conventional curved continuous casting.

FIG. 3 is an explanatory diagram of an effect of the present invention.

[Explanation of symbols]

1: Mold, 2: Molten steel, 2 ': Injection flow of molten steel, 3:
Solidified shell, 4: immersion nozzle, 5 (5-1, 5-
2): electromagnetic induction stirrer, 6: flux, 7: impurity, 8: impurity, 9: (9-1, 9-2, 9-)
1 ′, 9-2 ′, 9-1 ″, 9-2 ″): moving direction and flow strength of molten steel, 10: meniscus of molten steel, 1
1: Interface between solidified shell and molten steel.

Continuation of the front page (72) Inventor Takahiro Isono 1 Fujimachi, Hirohata-ku, Himeji-shi, Hyogo Nippon Steel Corporation Hirohata Works

Claims (2)

[Claims] In a slab continuous casting by a curved continuous casting machine, a pair of electromagnetic inductions arranged so as to sandwich a long side surface of a slab at a height where the meniscus of molten steel becomes L in the following formula (1). Using a stirrer, a swirl flow that flows horizontally along the interface with the solidified shell is formed in the molten steel inside the solidified shell, and the strength of the swirl flow is set to a strong flow and a weak flow at a period f of the following equation (2). A method for controlling molten steel flow in continuous casting, characterized by switching to flow. L ₀ + H / 2 <L <2-H / 2 (1) where L: distance from the meniscus of molten steel to the center of the height of the electromagnetic induction stirrer (m) L ₀ : loose from meniscus of molten steel Distance to the position where the interface between the solidified shell on the side and the molten steel is vertical (m) H: Device height of the electromagnetic induction stirrer (m) 1 second ≤ f ≤ (10 ³ · W / √F) seconds ... … (2) where f: cycle of strong swirling flow and weak swirling flow (second) W: casting width (m) F: thrust of electromagnetic induction stirrer (N / m ² ) In a continuous slab casting by a curved continuous casting machine, a pair of electromagnetic inductions arranged so as to sandwich the long side surface of the slab at a height where the meniscus of the molten steel becomes L in the following formula (3). Using a stirrer, a swirling flow that flows horizontally along the interface with the solidified shell is formed in the molten steel inside the solidified shell, and the swirling direction of the swirling flow is switched at a period f in the following equation (4). A method for controlling the flow of molten steel in continuous casting. L ₀ + H / 2 <L <2-H / 2 (3) where L: distance (m) from the meniscus of the molten steel to the center of the height of the electromagnetic induction stirrer L ₀ : from the meniscus of the molten steel Distance to the position where the interface between the solidified shell and molten steel is vertical (m) H: Height of the electromagnetic induction stirrer (m) 1 sec ≤ f ≤ (10 ³ · W / √F) sec ... (4) where f: cycle (seconds) of the swirl flow in one direction and the swirl flow in the reverse direction (second) W: casting width (m) F: thrust of electromagnetic induction stirrer (N / m ² )