JPH05329598A - Method for controlling molten steel flow in continuous casting mold - Google Patents

Abstract

PURPOSE:To improve the surface characteristic of a cast slab by arranging an immersion nozzle having a specific form, impressing a specific moving magnetic field to molten steel with coils divided in the width direction of a mold and controlling meniscus flow velocity. CONSTITUTION:The immersion nozzle 2 is made to be upward at 45 downward at 180 deg. of the angle of discharging holes and arranged in the range of 50-400mm the dipping depth. At the time of pouring the molten steel 3 into the mold from a tundish, 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 direction to the upper part becomes discharged turnover flow (a) to form the meniscus flow 6. On the other hand, the molten steel flow (b) directed to the lower part becomes descending flow. At this time, by impressing the moving magnetic field satisfying the inequality to the molten steel 3, the meniscus flow velocity having 10-60cm/sec is obtd. That is, by controlling the meniscus flow formed with the discharged turnover flow (a) in the fixed range while securing the hitting intensity of the molten steel 3 poured from the immersion nozzle 2 to solidified shell 4, the excellent surface characteristic of the cast slab is obtd.

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]

SUMMARY OF THE INVENTION The gist of the present invention is that when a nozzle is immersed in molten steel of a continuous casting mold for continuous casting, the nozzle discharge hole angle is 45 ° upward to 180 ° downward.
Nozzle No. is provided in a range of immersion depth of 50 mm to 400 mm, and 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,
A method for controlling molten steel flow in a continuous casting mold, which comprises applying a meniscus flow velocity of 10 cm / sec to 60 cm / sec to molten steel to form the desired stirring pattern shown in FIG. 4 on the molten steel. 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 continuous casting mold according to the present invention so as to be easily seen. 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, and the discharge flow 5 discharged from the immersion nozzle is directed toward the short side, hits the short side, is divided into upper and lower parts, and the molten steel flow directed upward is The discharge reverse flow a becomes the meniscus flow 6. 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 phase sequence switching device that changes the direction of the moving magnetic field and a current controller are provided in the control chamber 10 away from the mold, and they 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)

If equations (1) and (2) or (3) are combined and solved for Vp, 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 and the magnetic field frequency of the mold electromagnetic stirrer 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 required 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, when the meniscus flow velocity is less than 10 cm / sec and more than 60 cm / sec, the surface defect occurrence rate of the obtained cast piece is high. Therefore, the present invention provides a meniscus flow velocity of 10
It is controlled within the range of up 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 flow resistance F 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.

In the present invention, the nozzle immersion depth is in the range of 50 mm to 400 mm from the bath surface. That is, as shown in FIG. 7, if it is less than 50 mm, an extreme waviness phenomenon W is observed on the surface of the molten steel, while if it exceeds 400 mm, no new molten steel is supplied to the surface of the molten steel and the molten steel state Z is dead, which is not preferable.

Further, the nozzle discharge hole angle is 45 ° upward to 180 ° downward. That is, as shown in FIG.
At 135 ° and upward of 135 °
Is not preferred. According to experiments, the number of discharge holes provided in the immersion nozzle is not particularly limited in the present invention. The shape of the nozzle discharge hole may be round, oval, rectangular or square.

The stirring pattern in the present invention is shown in FIG. (A) is directed to the short side only to the left half of the mold, (b) is directed to the center of the mold from the short side only to the left half of the mold, and (c) is reversed on the long side only to the left half of the mold. And (d) are directed in opposite directions on the long side.

The control unit 10 shown in FIG.
1, 7-2 ... The moving magnetic field direction and strength are 50 ≦ L
The desired stirring pattern shown in FIG. 4 can be selected according to the steel type and the slab shape by controlling in the range of xf ≦ 40,000.

[0021]

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.

[0022]

[Table 1]

Table 2 shows the coil specifications and capabilities at this time.

[0024]

[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.

[0026]

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.

4 (a) to (d) 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 showing the relationship between the EMS controllable range of the meniscus flow velocity and the nozzle immersion depth.

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

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, the nozzle discharge hole angle is upward 4 °.
5 ° ~ 180 ° downward nozzle, immersion depth 50mm ~
The moving magnetic field determined by the following formula is applied to the molten steel by the coil divided into two or more parts in the width direction of the mold in the range of 400 mm, and the meniscus flow velocity of 10 cm / sec to 60 cm / sec is applied to the molten steel. A method for controlling molten steel flow in a continuous casting mold, comprising 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)