JPH05329595A - Method for controlling molten steel flow in continuous casting mold - Google Patents

Abstract

PURPOSE:To produce a cast slab having excellent surface characteristic by impressing a specific moving magnetic field with coils divided into two or more in the width direction of a mold, accelerating/decelerating discharged turnover flow and controlling meniscus flow velocity. CONSTITUTION:Molten steel 3 is poured into the mold from a tundish through an immersion nozzle 2 and them the discharging flow 5 discharged from the immersion nozzle 2 is directed to short side direction and hit to the short side and separated into the upper and the lower parts and the molten steel; flow directed to the upper part becomes the discharged turnover flow (a) to form the meniscus flow 6. By impressing the moving magnetic field satisfying the inequality to the molten steel, the meniscus flow velocity having 10-60m/sec is obtd. That is, it is very effective to the improvement of the surface characteristic of the cast slab that the flow velocity of the meniscus flow 6 formed with the discharged turnover flow (a) is controlled in the fixed range by accelerating/ decelerating while securing the hitting intensity of the molten steel 3 poured from the immersion nozzle 2 to solidified shell 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling molten steel flow in a continuous casting mold.

[0002]

2. Description of the Related Art In continuous casting, it is common practice to electromagnetically stir the unsolidified portion of the slab to reduce segregation inside the slab and obtain a good slab. For example, in JP-B-64-10305, two electromagnetic stirrers are installed facing each other near the meniscus on the long side of at least one side of the mold, and the molten steel in the mold is melted by the electromagnetic stirrer installed on the long side. It is disclosed that a slab of good quality is produced by imparting a flow toward the center in the width direction to reduce the depth of penetration of the molten steel flow from the immersion nozzle into the molten steel in the mold.

Further, in Japanese Unexamined Patent Publication No. 64-2771, a moving magnetic field is applied in accordance with the strength of the molten steel discharge flow from the left and right discharge holes of the immersion nozzle to realize an appropriate level fluctuation of the molten metal surface, which is abnormal. It is disclosed to prevent surface defects due to mold powder entrainment and surface cracking of a cast piece due to fluctuations in the molten metal surface.

[0004]

The flow of molten steel in a continuous casting mold is an important factor that affects the quality of cast slabs. The present invention provides a method for controlling molten steel flow in a continuous casting mold for controlling the meniscus flow velocity in the mold to obtain a slab having excellent surface properties.

[0005]

The gist of the present invention is determined by the following formula by a coil divided into two or more parts in the width direction of the mold when the nozzle is immersed in the molten steel of the continuous casting mold for continuous casting. Applying a moving magnetic field to molten steel,
Controlling the molten steel flow in a continuous casting mold, which accelerates and decelerates the discharge reversal flow to impart a meniscus flow velocity of 10 cm / sec to 60 cm / sec to molten steel to form the desired stirring pattern shown in FIG. Is the way. 50 ≦ L × f ≦ 40000 However, L: Coil pitch (mm) f: Magnetic field frequency (Hz)

The present invention will be described in detail below. FIG. 1 is a partially cutaway view showing a main part of a mold for continuous casting according to the present invention. The mold is composed of long-side mold copper plates 1-1 and 1-2 and short-side mold copper plates 1-3 and 1-4, and the lower part of the immersion nozzle 2 attached to a tundish (not shown) is inserted. The discharge holes provided in the lower portion of the immersion nozzle 2 are opposed to each other in the direction of the shorter side of the mold and are opened one by one on both sides of the immersion nozzle, but there is no particular limitation. Molten steel 3 is injected into the mold from the tundish through this immersion nozzle, but the discharge flow 5 discharged from the immersion nozzle is directed toward the short side, hits the short side and is divided into upper and lower parts, and the molten steel flow directed upward is Reverse discharge flow a
And a meniscus flow 6 is formed. On the other hand, the molten steel flow b directed downward becomes a downward flow.

According to the present invention, the long side surfaces of the mold facing each other 1-
Agitating coils 7-1 and 7-2, which are divided into two or more sections in the mold width direction, are provided outside the elements 1 and 1-2 to generate a moving magnetic field. In addition, a switch and a current controller that change the direction of the moving magnetic field are provided in the control chamber 10 away from the mold, and are connected to an AC power source. L in FIG. 3 is the pole pitch of the coil.

According to the experiments conducted by the present inventors, it is possible to control the meniscus flow formed by the discharge reversal flow within a certain range while ensuring the collision strength of the molten steel poured from the immersion nozzle to the solidified shell. We have found that it is extremely effective in improving the surface properties of cast slabs. That is, in the present invention, 50 ≦ L × f
A moving magnetic field satisfying ≤40,000 (coil pitch: Lmm, magnetic field frequency: fHz) is applied to molten steel to obtain a meniscus flow velocity of 10 cm / sec to 60 cm / sec, for the following reason. ..

That is, when a moving magnetic field is applied to the molten steel in the mold, the magnetic field moving speed V is expressed by equation (1). V = C ₁ × L × f + C ₂ (1) (L: coil pole pitch, f: magnetic field frequency, C ₁ ,
C _2: adjustment factors) In addition, in order to determine the magnetic field moving speed by the meniscus flow speed Vp, the magnetic field moving velocity V is a function of Vp. At this time, the function is considered by the linear expression (2) or the quadratic expression (3).

V = f (Vp) = C ₃ × Vp + C ₄ …………… (2) = C ₃ × Vp ² + C ₄ × Vp + C ₅ …………… (3)

When the equations (1) and (2) or (3) are combined and solved for Vp, the equation (4) or (5) is obtained. Vp = C ₆ × L × f + C ₇ ……………… (4) = C ₆ × L ^0.5 × f ^0.5 + C ₇ ……………… (5) Moreover, V = f (Vp) is increased. Considering the case of the following formula, Vp
Is generally expressed by the equation (6) (C ₇ = 1 / degree). Vp = C ₆ × L ^C7 × f ^C7 + C ₈ (6) At this time, the fluctuation of the coil current I, which affects Vp similarly to L and f, is the change of C ₆ and C ₈ . Included in the range. Actually, the operation was performed in the range of 0 to 2500 mA.

Here, the formula (7) is obtained from the proper value range (Vp _min , Vp _max ) of the meniscus flow velocity Vp and the formula (6). Vp _min ≤ C ₆ × L ^C7 × f ^C7 + C ₈ ≤Vp _max (7) When this is modified, the formula (8) is obtained. C ₉ ≦ L × f ≦ C ₁₀ …………………… (8)

From the above derivation, it is clear that the equation (8) can be obtained regardless of the order of V = f (Vp). Therefore, C ₉ and C
_In order to obtain ₁₀ , the horizontal axis is L × f and the vertical axis is Vp as shown in FIG.
This is shown on the assumption that

According to FIG. 5, the product L × f of the coil pitch of the mold electromagnetic stirring device and the magnetic field frequency is 50 ≦ L × f ≦ 40.
000 (L: coil pitch (mm), f: magnetic field frequency (H
z)) makes it possible to properly control the meniscus flow velocity.

According to experiments, a certain molten steel discharge flow rate is necessary to wash away the surface layer of the solidified shell and remove inclusions and segregation. Furthermore, it is necessary to control the meniscus flow in order to prevent inclusions from being trapped in the meniscus. That is, FIG. 6 shows the molten steel discharge flow by distance from the meniscus. That is, the critical point can be 300 mm from the meniscus. Therefore, the center of the coil is set in a range from the meniscus where only the meniscus flow exists to 300 mm below, and only the meniscus flow is controlled.

In the present invention, the meniscus flow velocity is 10 cm / s.
When it is less than ec and exceeds 60 cm / sec, the surface defect occurrence rate of the obtained slab is high. Therefore, the present invention controls the meniscus flow velocity within the range of 10 to 60 cm / sec. For this reason, the center of the electromagnetic stirring coil must be installed 300 mm directly below the meniscus in the casting direction.

The meniscus flow velocity is measured by, for example, inserting a thermorey cylinder into the molten steel flow and measuring the resistance force F due to the flow with a strain gauge. The strain and the resistance can be determined in advance by drawing a calibration curve using a weight and using a regression equation. The number of discharge holes provided in the immersion nozzle is not particularly limited. That is, FIG.
Is a plot of the EMS controllable range of the meniscus flow velocity for each nozzle discharge hole.
You can select the timely.

Further, FIG. 8 is a plot of the shape of the nozzle discharge hole (however, the hole area is the same) in the EMS controllable range of the meniscus flow velocity.
Either a rectangle or a square can be used.

The stirring pattern in the present invention is shown in FIG. In (a), the stirring direction 8 is directed from the mold center to the short side, and (b) is directed from the short side to the mold center. The control unit 10 shown in FIG.
7-2 ... The moving magnetic field direction and strength are separately set to 50 ≦ L × f
The desired stirring pattern shown in FIG. 4 can be selected according to the steel type and slab shape by controlling in the range of ≦ 40000.

[0020]

EXAMPLE Continuous casting was performed under the molding conditions and electromagnetic stirring conditions shown in Table 1 to examine the occurrence rate of surface defects. FIG. 9 shows this together with a comparative example.

[0021]

[Table 1]

Table 2 shows the coil specifications and capabilities at this time. The stirring pattern is formed by applying a moving magnetic field to the discharge reversal flow to accelerate or decelerate the meniscus flow velocity.

[0023]

[Table 2]

According to the present invention, as shown in FIG.
The occurrence rate of surface defects was 5% or less within the range of the meniscus flow rate of 10 to 60 cm / sec.

[0025]

According to the present invention, since the meniscus flow velocity of the molten steel in the mold of continuous casting is controlled, it is possible to obtain a slab having excellent surface properties.

[Brief description of drawings]

FIG. 1 is a partially cutaway explanatory view of the present invention.

FIG. 2 is a partial perspective view of the present invention.

FIG. 3 is a partial plan view of the present invention.

FIG. 4 (a) and FIG. 4 (b) are agitation patterns of the present invention.

FIG. 5 is a chart showing the relationship between meniscus flow velocity and L × f.

FIG. 6 is a chart of a relationship between a molten steel discharge flow per unit volume and a distance from a meniscus.

FIG. 7 is a chart of the relationship between the EMS controllable range of meniscus flow velocity and the number of nozzle ejection holes.

FIG. 8 is a chart showing the relationship between the EMS controllable range of the meniscus flow velocity and the shape of a nozzle ejection hole.

9A and 9B are charts showing the relationship between meniscus flow velocity and surface defect occurrence rate.

Claims (1)

[Claims] 1. When a nozzle is immersed in molten steel of a continuous casting mold for continuous casting, a moving magnetic field determined by the following formula is applied to the molten steel by a coil divided into two or more parts in the mold width direction and discharged. Reverse flow accelerates and decelerates 10 cm
Apply a meniscus flow velocity of / sec to 60 cm / sec to molten steel,
A method for controlling molten steel flow in a continuous casting mold, which comprises forming a desired stirring pattern shown in FIG. 4 in molten steel. 50 ≦ L × f ≦ 40000 However, L: Coil pitch (mm) f: Magnetic field frequency (Hz)