JP2733991B2 - Steel continuous casting method - Google Patents

Description

The present invention attenuates the flow velocity of the discharge flow from a submerged nozzle in a mold, suppresses one-sided flow, controls the wave height of the molten metal in the mold, and improves the quality. The present invention relates to a method for continuously casting steel for producing a product having a surface property.

[Prior Art] FIG. 6 is a view showing a state of molten steel in a mold in a continuous slab casting machine. The prior art will be described with reference to FIG. On the surface of the molten steel 8 in the mold 1, there is a mold powder 5 having a role of preventing oxidation of the molten steel 8 and keeping it warm, lubricating between the solidified shell 9 and the mold 1, adsorbing nonmetallic inclusions, and the like. The molten metal surface of the mold powder 5 is melted by the heat of the molten steel 8 to form a molten powder 6, and the air side of the mold powder 5 becomes a powdery powder 7 and covers the surface of the molten steel 8. The molten powder 6 comprises a solidified shell 9 and a mold 1
And flows in between to serve as a lubricant. Therefore, since the molten powder 6 is consumed, the mold powder 5 having a constant thickness is maintained, so that the molten powder 6 is replenished in proportion to the consumed amount of the molten powder 6. As shown in FIG. 6, a discharge hole 3 provided at the tip of an immersion nozzle 2 provided vertically in the center of the mold 1
Are open facing the short side of the mold 1. Molten steel is discharged from the discharge holes 3 into the mold. The molten steel discharge stream 4 is injected obliquely downward in the mold toward the short side of the mold. The discharge flow 4 of the molten steel collides with the short side, and the upper and lower two flows,
The reversing flow 11, which is divided into the reversing flow 11 and the inflowing flow 12 and rises along the solidified shell 9 on the short side surface, causes the surface wave near the upper short side surface of the mold 1. FIG. 7 is a schematic view of the surface wave motion. This molten surface wave means that the discharge flow from the discharge hole 3 of the immersion nozzle 2 is divided into a reverse flow 11 and an inflow flow 12 as shown in FIG. 7, but the reverse flow 11 reaches the surface of the molten metal and Make the surface of the melt ruffled. This level wave was measured by an eddy current distance meter 15, and its voltage value was measured by a millivolt meter after removing a high frequency component (here, a frequency component of 10 Hz or more) through a filter. As shown in FIG. 7, the installation position of the eddy current distance meter 15 is 50 mm from the short side surface and 50 mm from the molten metal surface.
FIG. 8 is a diagram showing the change over time of the level of the molten metal for about 1 minute. The maximum wave level for one minute was measured, and the maximum value was used as the maximum wave height h to perform data processing. The upward arrow indicates the upward direction, and the downward arrow indicates the downward direction. In particular, in high-speed casting in which the discharge rate of molten steel is 3 ton / min or more, since the discharge flow velocity of the discharge hole 3 of the immersion nozzle 2 is large, the reversal flow 11 after collision with the solidified shell 9 is large, and a large surface wave is generated. . FIG. 9 is a graph showing the relationship between the maximum surface wave height and the surface defect index of the hot-rolled sheet. As is clear from this figure, when the maximum level wave height is in the range of 4 mm to 8 mm, the incidence of surface defects on the hot-rolled sheet is small, and there is an optimum range for the maximum level wave height. When the level wave of the molten metal is large, the molten powder 6 is caught and suspended in the molten steel by the wave of the molten metal. The molten powder 6 entrained in the molten steel floats due to a difference in specific gravity between the molten steel and the molten powder 6, but is partially captured by the solidified shell 9. On the other hand, when the surface wave is small, the supply of new molten steel to the molten steel surface is small, so that the melting property of the mold powder 5 is poor. Therefore, it is considered that the inclusion and dissolution of inclusions in the molten steel to the molten powder 6 is poor, and the inclusions are trapped by the solidified shell 9 and become internal defects of the slab. The appropriate range of the maximum surface wave height shown here is a value of 4 mm to 8 mm obtained from the experience of the continuous casting operation, and the shape of the immersion nozzle 2 and the discharge of the immersion nozzle 2 fall within this range. The angle, the area of the discharge hole 3 of the immersion nozzle 2, the width of the mold 1, and the like were regulated.

However, in order to improve the productivity of recent continuous casting machines, (1) a multi-continuous casting technique in which a single tundish and an immersion nozzle perform continuous casting for several charges continuously, and (2) a change in mold width during casting. (3) Operating conditions have changed, for example, the casting speed has changed from low to high. As a result, operating conditions that cannot be satisfied with the shape and the discharge angle of the immersion nozzle of the immersion nozzle suitable for the initial operating conditions are generated, and the level of the surface wave cannot be controlled to the optimum range. As a technology to control the wave height of the molten metal surface, (1) a method of applying a brake to the discharge flow by a DC magnetic field (* 1: hereinafter referred to as conventional method 1), and two pairs of DC magnets in a cooling box on the long side of the mold A DC magnetic field acts on the discharge flow from the immersion nozzle, and the induced current and the DC magnetic field generated in the flowing molten steel cause the flow of the molten steel by the electromagnetic force generated in the opposite direction to the flow of the molten steel. Is controlled.

(2) In the method of applying a DC magnetic field to the molten metal surface position (* 2: hereinafter referred to as conventional method 2), the DC magnetic field is arranged at the molten metal surface position and the DC magnetic field is applied horizontally to the molten metal surface, so that the inside of the magnetic field is reduced. To control the wave height of the molten metal surface.

Example (* 1) Nagai et al .: 68, Iron and Steel (1982), S270 Suzuki et al .: 68, Iron and Steel (1982), S92 (* 2) Kozuka et al .: 72, Iron and Steel (1986), S718 [Invention Problems to be Solved] When the surface wave in the mold is generated, the discharge flow discharged from the immersion nozzle collides with the solidification shell and is divided into an upward reverse flow and a downward inflow flow. Among them, the kinetic energy of the upward reversal flow causes the surface of the metal to vibrate, so that a surface wave of the surface of the metal occurs.

However, the conventional method 1 is a method in which a DC magnetic field is applied at right angles to the discharge flow on the way between the immersion nozzle and the short side surface to brake the fluid, but the discharge flow discharged from the immersion nozzle diffuses. Therefore, it is necessary to apply a DC magnetic field to a wide range. For this reason, the equipment becomes large and the cost increases. Further, in this method, an eddy current circuit generated by the interaction between the discharge flow and the applied DC magnetic field can be formed in the molten steel, so that the current density cannot be increased. Therefore, in order to generate a large braking force, it is necessary to increase the magnetic flux density, which causes a problem that the equipment cost increases.

In the conventional method 2, since the direct current magnetic field is applied directly to the level wave, the control of the level wave is easiest, but the position where the level wave is most intense is within a range of 100 mm from the short side surface of the mold. Therefore, it is sufficient to apply a DC magnetic field to this position. Therefore, it is necessary to install the magnetic field generating device on the back surface of the copper plate on the long side of the mold and about 100 mm from the upper end of the copper plate on the long side of the mold. In this case, a large-scale modification of the cooling water box is necessary, and the direction of the cooling groove of the mold copper plate also needs to be horizontal, so that the cooling of the mold long side copper plate becomes insufficient.

The present invention has been made in view of the above circumstances, and reduces the wave motion of the molten metal in the mold to prevent the entrainment of the powder and the shallow penetration depth of the inclusion, thereby reducing the floating of the inclusion. It is an object of the present invention to provide a continuous casting method of steel for producing a product having good surface properties.

[Means for Solving the Problems] In the continuous casting method for steel according to the present invention, at least one pair of DC magnets is installed on the back of a copper plate on the long side of a mold with an immersion nozzle interposed therebetween, and one magnetic pole of the DC magnet is used as a mold. Directly above the upper end of the long side copper plate, the other magnetic pole is placed on the back side of the long side copper plate of the mold below the discharge hole of the immersion nozzle, and the characteristics of the magnetic poles facing each other across the immersion nozzle are made identical to generate a DC magnetic field The casting is performed while a DC magnetic field is applied vertically to the molten steel discharge flow from the immersion nozzle.

[Operation] The present invention performs casting while applying a DC magnetic field in the vertical direction to the molten steel discharge flow from the immersion nozzle discharge hole in the continuous casting mold. When molten steel, which is a conductive fluid, flows in a magnetic field, an electromotive force is generated in the fluid by Fleming's right-hand rule, and an eddy current flows. Due to the interaction between the eddy current and the applied magnetic field, an electromagnetic force (Fleming's left-hand rule) acts in a direction opposite to the fluid direction, so that the fluid is prevented from moving. As a result, the discharge flow is decelerated. When the discharge flow is decelerated, the flow velocity of the reversal flow after colliding with the short-side surface shell also decreases, and the surface wave wave is less likely to occur. Also, when the one-sided flow phenomenon occurs,
The higher the discharge flow rate, the greater the generated electromagnetic force. As a result, the one-sided flow phenomenon is suppressed. When a DC magnetic field is applied vertically, the path of the eddy current draws a circuit around the immersion nozzle as shown in FIG. At this time, as a part of the eddy current circuit, a current flows through a mold copper plate having a small electric resistance (copper electric resistivity of 2.5 × 10 ⁻⁸ Ω · m), so that the electric resistance of the circuit decreases and the current density increases. . As a result, the generated electromagnetic force increases, and the electromagnetic force can be generated efficiently. When a DC magnetic field is applied in the horizontal direction (same as the slab thickness direction), the generated eddy current is generated after melting with high electric resistance on the surface parallel to the mold copper plate (electrical resistivity after melting).
Since a circuit is formed within 150 × 10 ⁻⁸ Ω · m), the electrical resistance of the circuit increases and the eddy current density decreases. Therefore, one magnetic pole of the DC magnetic field was disposed immediately above the upper end of the long copper piece of the mold and the other magnetic pole was disposed on the back of the long side copper sheet of the mold below the discharge hole of the immersion nozzle so that the DC magnetic field could be applied in the vertical direction.
In addition, by placing one magnetic pole directly above the copper plate on the long side of the mold, most of the magnetic flux connecting this magnetic pole and the magnetic pole provided below the discharge port of the immersion nozzle passes through the molten steel without passing through the copper plate on the long side of the mold. As a result, the electromagnetic force can be efficiently generated in the molten steel.

[Example] First, the concept of the flow of molten steel when an electromagnetic force is applied to the molten steel will be described. FIG. 5 is a diagram showing the flow of molten steel when an electromagnetic force is applied to the molten steel in the mold.
FIG. 2 is a longitudinal sectional view in the mold, and FIG. 2B is a sectional view taken along the line AA in FIG. 21 is the long side copper plate of the mold, 22 is the immersion nozzle, 23
Is an electromagnet, 24 is a DC magnet, 25 is a DC magnet coil, 26 is a magnetic field (marked or dotted arrow), 27 is a discharge flow (black arrow), 28
Is the eddy current (solid arrow), 29 is the braking force (white arrow) 30, molten steel 30, 31 is one magnetic pole of the DC magnetic field, 32 is the other magnetic pole of the DC magnet, and 33 is the discharge hole of the immersion nozzle. In a continuous casting method in which molten steel 30 is poured from a tundish into a mold through an immersion nozzle 22, at least one pair or more of opposed electromagnets 23 (consisting of a DC magnet 24 and a DC magnet coil 25) are sandwiched by the immersion nozzle 22. Install the DC magnet 24
One of the magnetic poles 31 (N pole or S pole)
The other magnetic pole 32 (S-pole or N-pole) of the DC magnet is directly above the upper end of
The casting is performed while applying the magnetic field 26 vertically to the discharge flow 27 from the immersion nozzle 22 with the same polarity of the magnetic poles (31 or 32) facing each other with the immersion nozzle 22 interposed therebetween. Thus, the discharge flow 27 can be attenuated by generating a braking force 29 in the discharge flow 27 in a direction opposite to the direction of movement.

When a DC magnetic field is applied to the flowing molten steel 30, an electromotive force is generated by the following equation.

E = × = V _Y · B _Z …… (1): Speed of molten steel (m / sec): Magnetic flux density V _Y : Component of velocity of molten steel in mold width direction (m / sec) B _Z : Vertical of magnetic flux density Directional component Due to this electromotive force, eddy current flows in the molten steel and a braking force acts in a direction opposite to the direction of movement of the molten steel due to the interaction between the eddy current and the magnetic flux density.

_{= - × = -σV Y · B} Z 2 ...... (2) σ: the electrical resistivity of the fluid (Ω · m) (2) equation, the magnitude of the braking force is dependent on V _Y and B _Z ^2.

In continuous casting of molten steel, for the case of low-speed casting V _Y is small, although the braking force acting on the molten steel is small, enough to be faster casting, the braking force since V _Y increases increases.

When there is no DC magnetic field, the discharge flow 27 discharged from the immersion nozzle 22 tends to cause a one-flow phenomenon that preferentially flows out of one of the discharge holes 33.By applying a DC magnetic field in the vertical direction, the discharge flow rate is reduced. In the faster one, a larger braking force acts according to the equation (2), so that the discharge flow is made uniform,
The one-sided flow phenomenon is mitigated. By doing so, the maximum level wave height can be controlled within a certain range.

Hereinafter, an embodiment of the present invention will be specifically described with reference to the accompanying drawings.

FIG. 1 is a sectional view of a continuous casting mold according to one embodiment of the present invention, in which (a) is a side sectional view, (b) is a sectional view taken along line AA of (a) of FIG. c) is B in FIG. 1 (a).
It is a perspective view of -B side. 21 is a copper plate on the long side of the mold, 22 is an immersion nozzle, 23 is an electromagnet, 24 is a DC magnet, 25 is a DC magnet coil, 30 is molten steel, 31 is one magnetic pole of the DC magnet, 32 is the other magnetic pole of the DC magnet, 33 Is a discharge hole of the immersion nozzle.

A pair of electromagnets 23 (a DC magnet 24 and a DC magnet coil) facing each other behind a long side copper plate 21 on the front and rear sides of a dipping nozzle 22 attached to the lower part of a tundish (not shown)
Consists of 25). One magnetic pole 31 of the DC magnet was disposed immediately above the upper end of the copper plate 21 on the long side of the mold, and the other magnetic pole 32 of the DC magnet was disposed 250 mm below the discharge hole 33 of the immersion nozzle. The cross-sectional dimension of DC magnet 24 is 100 (H) x 600
(W) mm. The polarity of the magnetic pole (31 or 32) of the DC magnet 24 was selected so as to be of the same polarity across the immersion nozzle 22. By doing so, the direction of the magnetic field can be made vertical, that is, parallel to the immersion nozzle 22.

(Example 1) Using a continuous casting mold in which a pair of opposed electromagnets 23 shown in FIG. 1 are installed, a measurement result of the level wave height in the vicinity of the short side copper plate 34 of the casting mold is shown below. 220mm thickness, 1200m
Casting was performed with a slab having a cross-sectional dimension of m width changed at a drawing speed of 0.7 to 2.7 m / min. The casting speed at this time is 1.4
It varied between ~ 5.0ton / min. FIG. 2 is a graph showing the relationship between the maximum wave height in the vicinity of the copper plate on the short side of the mold and the drawing speed or the casting speed when a DC magnetic field is not applied. The horizontal axis in this figure shows the relationship between the drawing speed and the casting speed. ○ indicates no magnetic field, and ● indicates a magnetic field. The magnetic flux density of the DC magnetic field was adjusted in the range of 2000 to 2500 Gauss. The maximum level wave height when a magnetic field is applied is considerably smaller than the maximum level wave height when a magnetic field is not applied. At a casting speed of 2.5 ton / min or less, the maximum level wave height was suppressed. On the other hand, at a casting speed of 2.5 ton / min or more, the maximum level wave height could be suppressed to 8 mm or less.

(Example 2) Using a continuous casting mold provided with a pair of opposing electromagnets 23 shown in Fig. 1, casting was performed while applying a DC magnetic field to the discharge flow of the immersion nozzle. The conditions for applying the DC magnetic field were determined from the results shown in Example 1. That is, the magnetic flux density of the DC magnetic field is uniformly 20 at the casting condition of the casting speed of 3.0 ton / min or more.
Set to 00 Gauss. A slab with a cross-sectional dimension of 220 mm thickness and 1200 mm width was cast. FIG. 3 is a graph showing a change with time of the casting time and the drawing speed, and the casting time and the maximum surface wave height in the presence or absence of a DC magnetic field. Immediately after the start of casting, measurement was not possible because the setting and adjustment of the eddy current distance meter for measuring the maximum height of the surface wave were not possible. The value of the wave height can be controlled in an appropriate range over almost the entire casting area.

In addition, when the pot is replaced, the discharge flow speed is low, so that the surface wave is quiet, and there is no need to apply a DC magnetic field to apply a braking force to the discharge flow.

FIG. 4 is a graph showing the relationship between the hot-rolled sheet surface defect index and the casting speed depending on the presence or absence of a DC magnetic field. ○ indicates no magnetic field, and ● indicates a magnetic field. The DC magnetic field was applied when the casting speed was 3.0 ton / min or more. The surface defect index is an index obtained by dividing the number of scabs by the observation area. As is apparent from this figure, by applying a DC magnetic field, the hot-rolled sheet surface defect index is significantly reduced in high-speed casting.

[Effects of the Invention] As described above, in the continuous casting method of steel according to the present invention, at least one pair of DC magnets is installed with the immersion nozzle interposed therebetween, and one magnetic pole is placed above the upper end of the copper plate on the long side of the mold. The other magnetic pole is placed on the back side of the copper plate on the long side of the mold below the discharge hole of the immersion nozzle, and the poles of the magnetic poles facing each other are sandwiched between the molds to generate the DC magnetic field, thereby discharging the molten steel from the immersion nozzle. Casting while applying a DC magnetic field vertically to the flow,
Since a large braking force acts on the one with a higher discharge flow velocity, the discharge flow is made uniform, so that the surface wave height can be controlled within a certain range, so that a hot rolled sheet having good surface properties can be obtained. it can.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a continuous casting mold according to one embodiment of the present invention, and FIG. 2 is a diagram showing the maximum wave height and drawing speed or casting speed near the copper plate on the short side of the mold with and without applying a DC magnetic field. FIG. 3 is a graph showing the relationship between the casting time and the drawing speed, and FIG. 3 is a graph showing the change over time between the casting time and the maximum surface wave height in the presence or absence of the DC magnetic field, and FIG. 4 is the presence or absence of the DC magnetic field. FIG. 5 is a graph showing the relationship between the surface defect index of a hot-rolled sheet and the casting speed, FIG. 5 is a view showing the flow of molten steel when an electromagnetic force is applied to molten steel in a mold, and FIG. 6 is continuous casting of a slab. Figure 7 shows the molten steel state in the mold of the machine, FIG. 7 is a schematic view of the level wave, FIG. 8 is a view showing the change of the level of the level for about 1 minute, and FIG. 9 is the maximum level wave height. It is a graph which shows the relationship of a hot-rolled sheet surface defect index. 21 ... Mold long side copper plate, 22 ... Immersion nozzle, 23 ... Electromagnet, 24 ... DC magnet, 25 ... DC magnet coil, 30 ... Metal steel, 31 ... One magnetic pole of DC magnet, 32 ... The other magnetic pole of the DC magnet, 33 ... the discharge hole of the immersion nozzle.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Yoshiyuki Kanao, Inventor 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Norio Blue 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan (72) Inventor Hironori Yamamoto 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd.Examiner Hitoshi Amano

Claims (1)

(57) [Claims] 1. A continuous casting method of steel in which molten steel is cast into a mold from a tundish through an immersion nozzle. One magnetic pole is placed just above the upper end of the copper plate on the long side of the mold, and the other magnetic pole is placed on the back side of the copper plate on the long side of the mold below the discharge hole of the immersion nozzle, and the characteristics of the magnetic poles facing each other across the immersion nozzle are made the same. A continuous casting method for steel, comprising: generating a direct-current magnetic field by applying a direct-current magnetic field to a discharge flow of molten steel from an immersion nozzle.