JPH06605A - Controller for flow of molten steel in continuous casting mold - Google Patents

Abstract

PURPOSE:To produce a cast slab excellent in surface characteristic by impressing electromagnetic coils for controlling flow velocity at a meniscus in a mold in a specific condition. CONSTITUTION:The electromagnetic coils 7-1, 7-2 for electromagnetic-stirring molten steel discharged from an immersion nozzle inserted into the continuous casting mold 1 are provided on each longitudinal side in the 1/2 zone of the mold. By independently controlling the magnetic field impressing direction of each electromagnetic coil, the stirring pattern is made to be selectable and the flow velocity of the molten steel at the meniscus is made to be 10-60cm/sec. To the flow velocity at the meniscus, e.g. a thermalloy-made cylinder is inserted into the molten steel flow and the resistant force caused by the flow is measured by a strain gage. By impressing the shifting magnetic field having 50-40000 of coil pitch L (mm) X magnetic field frequency (f) (herz) and giving the proper fluctuation of molten steel surface, the cast slab excellent in surface characteristic is obtd.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In continuous casting, it is generally practiced to electromagnetically stir the unsolidified portion of the slab to reduce segregation inside the slab and obtain a good slab. For example, in Japanese Examined Patent Publication No. 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 electromagnetic stirrer installed on the long side is used to melt the molten steel in the mold. 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 ports 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 the continuous casting mold is an important factor that affects the quality of the cast slab. The present invention provides a molten steel flow control device in a continuous casting mold for controlling the meniscus flow velocity in the mold to obtain a slab having excellent surface properties.

[0005]

According to the present invention, an electromagnetic coil divided into two or more parts in the width direction of a continuous casting mold is provided such that the center of the electromagnetic coil in the casting direction is located near the meniscus. The molten steel can be controlled by independently controlling the magnetic field application direction of each electromagnetic coil so that the stirring pattern oriented toward the center of the mold can be selected on each long side of the mold in the two zones, and a moving magnetic field satisfying the following formula is applied. A flow control device for molten steel in a continuous casting mold, characterized in that a control system for obtaining a meniscus flow velocity of 10 to 60 cm / sec is connected to the electromagnetic coil. Note 50 ≦ L × f ≦ 40000 However, L: coil pitch (mm) f: magnetic field frequency (Hz) Further, in the present invention, a stirring pattern can be selected from the center of the mold to the short side.

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 dipping nozzle 2 attached to a tundish (not shown) is inserted. The discharge holes provided in the lower part of the immersion nozzle 2 are opposed to each other in the direction of the short 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 in the direction of the short side, hits the short side and is divided into upper and lower parts, and upward. The heading molten steel flow becomes a discharge reversal flow a, forming a meniscus flow 6. On the other hand, molten steel flow b directed downward
Becomes a downflow.

According to the present invention, the opposite long side surfaces of the mold 1-
Agitating electromagnetic coils 7-1 and 7-2, which are divided into two or more parts in the mold width direction, are provided on the outside of the parts 1 and 1-2 to generate a moving magnetic field. In addition, a switching device for changing the direction of the moving magnetic field and a current controller 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 electromagnetic coil.

According to the experiments 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, the present invention is provided so that the center of the electromagnetic coil in the casting direction is located near the meniscus, but preferably within 300 mm directly below the meniscus.

In order to wash away the surface layer of the solidified shell and remove inclusions and segregation, a certain molten steel discharge flow rate is necessary. Furthermore, it is necessary to control the meniscus flow in order to prevent inclusions from being trapped in the meniscus. That is, FIG. 4 shows the molten steel discharge flow for each distance from the meniscus. That is, the critical point can be 300 mm from the meniscus. Therefore, the center of the electromagnetic coil is installed in a range up to 300 mm directly below the meniscus where only the meniscus flow exists, and only the meniscus flow is controlled.

In the present invention, the meniscus flow velocity is 10 cm.
When it is less than / sec or more than 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. Here, the relationship between the meniscus flow velocity and the magnetic field moving velocity will be described. That is, 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) Equations (1) and (2) ) Or equation (3) is solved simultaneously to solve 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. When expressed by the following equation, Vp is as in the equation (6) (C ₇ = 1 / order). 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 included in the change range of C ₆ and C ₈ . Actually, the operation was performed in the range of 0 to 2500 mA. Here, the proper value range (Vp of the meniscus flow velocity Vp
(7) is obtained from ( _min , Vp _max ) and the equation (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, equation (8) can be obtained regardless of the order of V = f (Vp). In FIG. 5, the horizontal axis is L × f and the vertical axis is Vp on the assumption that the linear equation is used.
0 ≦ L × f ≦ 40000 (L: coil pitch (mm),
f: magnetic field frequency (Hz)) makes it possible to properly control the meniscus flow velocity.

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 resistance can be determined in advance by drawing a calibration curve using a weight and using a regression equation. The control unit 10 shown in FIG. 2 can control the direction and strength of the moving magnetic field for each of the electromagnetic coils 7-1, 7-2, ... To select the stirring pattern shown in FIG. 6 or 8.

[0016]

【Example】

Example 1 Continuous casting was performed using the stirring pattern shown in FIG. 6 under the mold conditions and electromagnetic stirring conditions shown in Table 1, and the occurrence rate of surface defects was examined. The result is shown in FIG. 7. (A) is a conventional example,
(B) shows an example of the present invention.

[0017]

[Table 1]

[0018]

[Table 2]

By the stirring pattern for decelerating the discharge reversal flow, the meniscus flow velocity was controlled within the range of 10 to 30 cm / sec, and the occurrence rate of surface defects was lowered.

Example 2 The occurrence rate of surface defects was examined by continuous casting using the stirring pattern shown in FIG. 8 under the mold conditions and coil specifications shown in Example 1. The result is shown in FIG. (A) is a conventional example,
(B) shows an example of the present invention. By the stirring pattern that accelerates the discharge reversal flow, the meniscus flow velocity was controlled in the range of 40 to 60 cm / sec, and the occurrence rate of surface defects decreased.

[0021]

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

[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 is a graph showing the presence of a molten steel discharge flow per unit volume and a meniscus.

FIG. 5 is a graph of magnetic field moving speed and L × f.

FIG. 6 is a stirring pattern of the present invention.

7A and 7B are graphs of the surface defect occurrence rate and the meniscus flow velocity.

FIG. 8 is another stirring pattern of the present invention.

9A and 9B are graphs of surface defect occurrence rate and meniscus flow velocity.

Claims (2)

[Claims] 1. An electromagnetic coil divided into two or more sections in the width direction of a continuous casting mold is provided such that the center of the electromagnetic coil in the casting direction is located near the meniscus, and each length of the mold is a half region of the mold size. On the side, it is possible to independently control the magnetic field application direction of each electromagnetic coil so that the stirring pattern directed to the center of the mold can be selected, and a moving magnetic field satisfying the following formula is applied,
A flow control device for molten steel in a continuous casting mold, wherein a control system for obtaining a meniscus flow velocity of 10 to 60 cm / sec in molten steel is connected to the electromagnetic coil. Note 50 ≦ L × f ≦ 40000 However, L: Coil pitch (mm) f: Magnetic field frequency (Hz) 2. The flow control device for molten steel in a continuous casting mold according to claim 1, wherein a stirring pattern directed from the center of the mold to the short side can be selected.