JP2773154B2 - 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. 7 is a view showing a state of molten steel in a mold in continuous casting of a slab. 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 powder 8 is melted by the heat of the molten steel 8 to form a molten powder 6 on the molten metal surface side of the mold powder 5, and the powdery powder 7 on the atmosphere side covers the surface of the molten steel 8. Melting powder 6
Flows between the solidified shell 9 and the mold 1 and acts as a lubricant. Accordingly, since the molten powder 6 is consumed, the mold powder 5 having a constant thickness is maintained. As shown in FIG. 7, a discharge hole 3 provided at the tip of an immersion nozzle 2 provided vertically in the center of the mold 1 is opened to face 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 molten discharge flow 4 collides with the short side of the mold and divides into two upper and lower flows, a reverse flow 11 and an inflow flow 12, and rises along the solidification shell 9 on the short side of the mold.
Numeral 11 causes the surface wave near the upper short side surface of the mold 1.
FIG. 8 is a schematic diagram of the surface wave motion. Here, the molten surface wave means that, as shown in FIG. 8, the discharge flow from the discharge hole 3 of the immersion nozzle 2 is divided into a reverse flow 11 and an intrusion flow 12, but when the reverse flow 11 reaches the surface of the molten metal. This is the undulation on the surface of the molten metal in the mold. This level wave is measured by the eddy current distance meter 15, and the voltage value is passed through a filter to a high frequency component (here,
After removing 10Hz or more frequency components), measure with a millivolt meter. As shown in FIG. 8, the installation position of the eddy current distance meter 15 is 50 mm from the short side of the mold and 50 mm from the surface of the molten metal. FIG. 9 is a diagram showing the change over time of the molten metal level for about one minute in the prior art. 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 molten steel discharge rate is 3 ton / min or more, since the discharge flow rate of the discharge hole 3 of the immersion nozzle 2 is large, the reversal flow 11 after colliding with the solidified shell 9 is also large,
A large level wave is generated. FIG. 10 is a graph showing the relationship between the maximum wave height of the molten steel surface and the surface defect index of the hot-rolled sheet in the prior art. As is clear from this figure, the occurrence rate of surface defects of the hot-rolled sheet is small when the maximum level wave height is in the range of 4 mm to 8 mm, and there is an optimum range for the maximum level wave height. When the level wave is large, the molten powder 6 is caught and suspended on the molten steel side by the level wave. 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 a part thereof is 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 value of the appropriate range of the maximum surface wave height of 4 mm to 8 mm shown here is a value obtained from the experience of the continuous casting operation, and the shape of the immersion nozzle 2 and the discharge angle of the immersion nozzle 2 fall within this range. ,
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 ordinary, (1) a multi-continuous casting technique in which a single tundish and a submerged nozzle perform continuous ordinary for several charges continuously, (2) a change of a mold width during casting, ( 3) The casting speed changes from low to high, etc., and the operating conditions have changed.
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. Techniques for controlling the wave height of the molten metal surface: (1) Method of applying a brake to the discharge flow using a DC magnetic field (* 1: hereinafter referred to as conventional method 1) Two pairs of DC magnets are installed in the cooling box on the long side of the mold Then, a DC magnetic field is applied to the discharge flow from the immersion nozzle, and the induced current and the DC magnetic field generated in the flowing molten steel,
The flow of the molten steel is controlled by an electromagnetic force generated in a direction opposite to the flow of the molten steel.

(2) Method of applying a DC magnetic field to the surface of the molten metal (* 2: hereinafter referred to as conventional method 2) A DC magnet is arranged at the position of the surface of the molten metal, and the DC magnetic field is applied horizontally to the surface of the molten metal, so that the molten metal in the mold is heated. It controls the surface wave height.

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, in the conventional method 1, the fluid is braked by applying a DC magnetic field at right angles to the discharge flow on the way between the immersion nozzle and the short side surface of the mold. 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, the magnetic field generator needs to be installed at about 100 mm from the upper end of the long side copper plate on the back side of the long side copper plate. In this case, the structure of the cooling water box becomes complicated, and the direction of the cooling groove of the mold copper plate needs to be horizontal, so that the cooling of the copper plate on the long side of the mold becomes insufficient.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a continuous casting method of steel for controlling a wave motion of a molten metal in a mold to produce a product having good surface properties.

[Means for Solving the Problems] The continuous casting method for steel according to the present invention is directed to a continuous casting method for steel in which a direct current magnetic field is applied to a mold portion of a continuous casting of a slab. Immediately above, the other magnetic pole is a DC electromagnet with the back of the mold long side steel plate below the discharge port of the immersion nozzle, and the DC electromagnet is placed on the back of the long side copper plate of the mold so that the polarities of the magnetic poles facing each other across the immersion nozzle are the same. At least one pair is arranged and the level wave of the molten steel in the steel mold is measured, and while the strength of the magnetic field applied from the DC electromagnet is adjusted based on the measured value of the level wave, the direction of the magnetic field is vertical, A magnetic field whose spread is symmetric in the width direction of the slab across the immersion nozzle is applied to the discharge flow of the molten steel discharged from the immersion nozzle. Immersion nozzle To the molten steel discharge flow from Deana, casting while applying a DC magnetic field in a vertical direction. 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 on the fluid in a direction opposite to the fluid direction, so that the fluid motion is hindered. As a result, the discharge flow is decelerated. When the discharge flow slows down,
The flow velocity of the reversal flow after colliding with the shell on the short side of the mold is also reduced, and the surface wave is less likely to occur. In addition, when the one-sided flow phenomenon occurs and one of the discharge flow rates is large, the electromagnetic force increases as the flow rate increases, resulting in a large braking force. As a result, the one-sided flow phenomenon is suppressed. When a DC magnetic field is applied in the vertical direction, the path of the eddy current draws a circuit around the immersion nozzle as shown in FIG. 6 (b). At this time, since the eddy current flows as a part of the circuit through the mold copper plate having a small electric resistance, the electric resistance of the circuit is reduced and the current density can be increased. 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 in a plane parallel to the mold, and a circuit is created in the molten steel with a high electric resistance. Eddy current density is reduced. Therefore, one magnetic pole of the DC magnet was disposed immediately above the upper end of the copper plate on the long side of the mold and the other magnetic pole was disposed on the back surface of the copper plate on the long side of the mold below the immersion nozzle discharge hole so that a DC magnetic field could be applied in the vertical direction. 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 Therefore, an electromagnetic force can be efficiently generated during melting. If a constant magnetic flux density is applied to the discharge flow, it is difficult to control the optimum surface wave due to changes in casting conditions and changes over time in the discharge flow rate due to the adhesion of Al ₂ O ₃ to the inner wall of the immersion nozzle. It is. Therefore, while measuring the level wave, if the value is larger than a predetermined range, a current is supplied to the electromagnet to adjust the level wave so that the level wave always falls within an appropriate range. This can reduce the entrapment of the mold powder due to the surface wave motion.

[Example] First, the concept of the flow of molten steel when an electromagnetic force is applied to the molten steel will be described. FIG. 6 is a view showing the flow of molten steel when an electromagnetic force is applied to the molten steel in the mold.
The figure is a longitudinal sectional view in the mold, and the figure (b) is a sectional view taken along the line AA of the figure (a). 21 is the copper plate on the long side 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 an eddy current (solid arrow), 29 is a braking force (white arrow) 30 is molten steel, 31 is one magnetic pole of the DC magnet, 32 is the other magnetic pole of the DC magnet, and 33 is a discharge hole of the immersion nozzle. In a continuous casting method in which molten steel 30 is injected from a tundish into a mold through an immersion nozzle 22, at least a pair of electromagnets 23 (DC magnet 24 and DC magnet coil
25, and the DC magnet 24 has one magnetic pole 31 (N pole or S pole) positioned directly above the upper end of the casting side copper plate 21 and the other magnetic pole 32 (S pole or N pole). ) Is arranged on the back side of the mold long side surface 21 below the discharge hole 33 of the immersion nozzle, and the opposite magnetic poles (31 and 31 or
32 and 32) with the same polarity, and casting while applying a magnetic field 26 vertically to the discharge flow 27 from the immersion nozzle 22 to control the flow in the discharge flow 27 in the direction opposite to the direction of movement. By generating the motive power 29, the discharge flow 27 can be attenuated.

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

= × = 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 direction of magnetic flux density Component An eddy current flows in the molten steel by the electromotive force, and a braking force acts in a direction opposite to a moving direction of the molten steel due to an interaction between the eddy current and the applied magnetic field.

= − × = −σV _Y · B _Z ² (2) σ: Specific electric resistance of fluid According to the equation (2), the magnitude of the braking force depends 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.

In the absence of a 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, and the discharge flow 27 is applied by applying a DC magnetic field in a vertical direction. The larger the flow velocity, the greater the braking force acting according to the equation (2), so that the discharge flow is made uniform and the one-sided flow phenomenon is reduced. This makes it possible to control the wave height of the molten metal surface within a certain range.

An electromotive force is generated by a discharge flow having conductivity discharged from the immersion nozzle in a horizontal direction and a magnetic field applied vertically thereto, and an eddy current flows in the molten steel based on the electromotive force. The eddy current flows around the immersion nozzle, and a part of its circuit has a long-sided steel plate with a lower electrical resistivity than the molten steel (specific electrical resistance of molten steel: 150 × 10 ^-8 Ωm). Resistance: 2.5 × 10 ⁻⁸ Ωm), the electric resistance of the eddy current circuit becomes small as a whole, and the current density becomes high.
As a result, a large braking force can be generated according to the equation (2).

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

FIG. 1 is a cross-sectional view showing one embodiment of a continuous casting mold for carrying out the method of the present invention.
(B) is a sectional view taken along line AA of (a), and (c) is (a).
It is a perspective view in the BB cross section of the figure. 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, 34 is an eddy current distance meter, and 35 is a copper plate on a short side of a mold.

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). In the DC magnet, one magnetic pole 31 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 was disposed 300 mm below the discharge hole 33 of the immersion nozzle. The cross-sectional dimension of the DC magnet 24 is 100 (H) x 600 (W) mm
And a DC magnet 24 having a distance between the magnetic pole centers of 600 mm was used. The distance between the facing magnetic poles is 260 mm at the top and 360 mm at the bottom.
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. The magnetic flux density obtained with this DC magnet configuration was 2500 gauss at the maximum at the position of the discharge hole 33 of the immersion nozzle.

In order to measure the level wave in the mold, an eddy current meter 34 was attached to a position near the short side steel plate 35 where the level wave generated by the upward reversal flow was most intense. The water level wave data measured by the eddy current distance meter 34 is collected at intervals of Δt seconds, and the water level wave velocity Vl is obtained by the following equation.

Vl = ((Ht + Δt) −Ht) / Δt (3) where, Ht: the next level wave at time t, Ht + Δt: the next level wave at time t + Δt, the level at which the sign of V1 changes storing position H _1, then, it was the molten metal surface wave height H of the absolute value of the difference between the molten metal surface level position of H ₂ when the sign of the Vl changes.

H = | H ₁ −H ₂ | (4) Since it is empirically known that the range of the surface wave height H at which no surface defects occur in the hot-rolled sheet is 3 to 8 mm,
The eddy current distance meter 34 is set so that the wave height H falls within this range.
Is fed back to control the coil current of the DC magnet.

(Example 1) Using the continuous casting mold in which the DC magnet 24 shown in FIG. 1 was installed, when the DC magnet was cast while performing the ON-OFF operation, the height of the level wave height near the copper plate 35 on the short side of the mold was measured. The measurement results are shown below. Casting was performed with a slab having a cross-sectional dimension of 220 mm thickness and 1200 mm width changed at a drawing speed of 1.4 to 2.5 m / min. The casting speed at this time varied between 2.6 and 4.7 ton / min. FIG. 2 is a graph showing the relationship between the height of the surface wave near the copper plate on the short side of the mold, the drawing speed, and the casting time when a constant DC magnetic field strength is applied ON-OFF. -In this figure shows the ON state of the DC magnet. The place without-indicates that the DC magnet is off. The used immersion nozzle is an inverted Y type with a discharge angle of 25 degrees downward. An eddy current meter 34 was installed at a position 40 mm away from the copper plate 35 on the side of the immersion nozzle 22 from the copper plate 35 on the short side of the mold, and a DC magnetic field was applied based on this data while measuring the wave height H of the molten metal surface. By setting the data sampling time interval Δt of the level wave to 5 seconds, the level wave height H
However, when the diameter became 8 mm or more, the coil current of the DC magnet was passed so that the magnetic flux density at the position of the discharge hole 33 of the immersion nozzle became 2000 gauss. When the surface wave height H became 8 mm or less, the coil current of the DC magnet was set to zero. Looking at the change during casting of the surface wave height H during such ON-OFF control and the operating condition of the DC magnet, the surface wave height H is within 8 mm, except immediately after the operation of the DC magnet. Can be suppressed.

Example 2 Using a continuous casting mold provided with the DC magnet 24 shown in FIG. 1, casting was performed while applying a DC magnetic field to the discharge flow 27 of the immersion nozzle. FIG. 3 is a diagram showing the relationship between the level wave height H and the average level wave velocity using the applied magnetic flux density (Gauss) as a parameter. The average level wave velocity (Ve) was determined using analog data recorded on a millivolt meter.
In the range where the average level wave velocity (Ve) is 0.4 mm / sec or more, the level wave height H and the average level wave velocity (Ve) are as follows.
There is an almost linear relationship. Based on the data shown in FIG. 3, the relationship between the magnetic flux density B when the surface wave height H = 6 mm and the average surface wave speed (Ve) was obtained as an experimental formula by multiple regression analysis.

B = α ₀ + α ₁ Ve + α ₂ Ve ² (5) Here, α ₀ , α ₁ , and α ₂ are constants determined by multiple regression analysis. The magnetic flux density B and the coil current of the DC magnet are univocal. When the magnetic flux density B is determined, the coil current of the DC magnet can be determined. The installation position of the eddy current rangefinder and the data sampling time interval were the same as those in Example 1, and the coil current of the DC magnet was passed according to the equation (5). When the level wave height H was 3 mm or less, the coil current of the DC magnet was set to zero. FIG. 4 is a graph showing a casting time, a casting speed, and a wave height of a molten metal surface when a DC magnetic field control is performed. As is apparent from this figure, the height H of the surface wave during steady casting can all be controlled in the range of 3 to 8 mm. FIG. 5 is a graph showing a relationship between a hot-rolled sheet surface defect index and a casting speed by DC magnetic field control. ○ indicates no DC magnetic field, and ● indicates DC magnetic field control. 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 performing the DC magnetic field control, 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, one or more pairs of DC magnets are installed with an immersion nozzle in between, and one magnetic pole is placed directly above or outside the upper end of the copper plate on the long side of the mold. Then, the other magnetic pole is arranged on the back side of the copper plate on the long side of the mold below the discharge hole of the immersion nozzle, the polarities of the magnetic poles facing each other across the mold are generated, a DC magnetic field is generated, and the molten steel from the immersion nozzle is When casting while applying a DC magnetic field perpendicular to the discharge flow, the level wave height is measured by an eddy current distance meter installed above the molten steel level near the short side of the mold, and the level wave is within a predetermined range. As a result, the electromagnetic force was controlled by adjusting the current flowing through the DC magnet. As a result, the surface wave could always be controlled to an optimum range, and a product having good surface properties could be manufactured.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one embodiment of a continuous casting mold for carrying out the method of the present invention, and FIG. 2 is a view showing the vicinity of a short side copper plate of the casting mold when a constant DC magnetic field strength is turned on and off. FIG. 3 is a graph showing the relationship between the height of the surface wave and the drawing speed and the casting time, FIG.
The figure shows the relationship between the surface wave height H and the average surface wave speed using the applied magnetic flux density as a parameter. FIG. 4 shows the casting time, the drawing speed and the surface wave height when DC magnetic field control is performed. FIG. 5 is a graph showing the relationship between the surface defect index of the hot-rolled sheet by DC magnetic field control and the casting speed, and FIG. 6 is the flow of molten steel when an electromagnetic force is applied to the molten steel in the mold. FIG. 7, FIG. 7 is a view showing a molten steel state in a mold in continuous casting of a slab, FIG. 8 is a schematic view of a level wave, and FIG. 9 is a time-dependent change of the level bell for about 1 minute in the prior art. FIG. 10 is a graph showing the relationship between the maximum wave height of the molten steel surface and the surface defect index of the hot-rolled sheet in the prior art. 21 ... Mold long side copper plate, 22 ... Immersion nozzle, 23 ... Electromagnet, 24 ... DC magnet, 25 ... DC magnet coil, 30 ... Metal steel, 31 ... One pole of DC magnet, 32 ... The other magnetic pole of the DC magnet, 33 ... the discharge hole of the immersion nozzle, 34 ... the eddy current distance meter, 35 ... the copper plate on the short side of the mold.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshiyuki Kanao 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Hironori Yamamoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Examiner Hitoshi Amano within Nippon Kokan Co., Ltd. (56) References JP-A-1-83356 (JP, A) JP-A-2-75456 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB Name) B22D 11/00-11/22

Claims (1)

(57) [Claims] In a continuous casting method of a steel in which a direct current magnetic field is applied to a mold portion of a continuous casting of a slab, one magnetic pole is located directly above an upper end of a copper plate on a long side of the mold, and the other magnetic pole is located below a discharge hole of an immersion nozzle. At least one pair of DC electromagnets on the back of the mold long side steel plate are arranged on the back of the mold long side copper plate so that the polarities of the magnetic poles facing each other across the immersion nozzle are the same. While adjusting the strength of the magnetic field applied from the DC electromagnet based on the measured value of the level wave, the direction of the magnetic field is vertical, and the spread of the magnetic field is symmetrical in the width direction of the slab across the immersion nozzle. A continuous casting method for steel, wherein the magnetic field is applied to a discharge flow of molten steel discharged from an immersion nozzle.