JPH0275456A - Method for continuously casting steel - Google Patents

Abstract

PURPOSE:To uniformize discharged flow and to obtain hot-rolled plate having good surface characteristic by casting while impressing a DC magnetic field toward vertical direction to the discharged flow of molten steel from a submerged nozzle. CONSTITUTION:Each of one pair of electromagnets 23 (composing of DC magnets 24 and DC magnet coils 25) is arranged to back of mold long side copper plate 21 at front and rear faces sandwiching the submerged nozzle 22, respectively. The magnetic pole 21 at one side of the DC magnets is set just above the upper end of the mold long side copper plate 21 and the magnetic pole 32 at the other side of the DC magnets is set below the discharged hole 33 of the submerged nozzle. Polarity of the pole 31 or 32 in the DC magnets 24 is selected so as to become facing to the same pole while sandwiching the submerged nozzle 22 and the magnetic field direction is made to vertical direction, that is, parallel with the submerged nozzle 22. By this method, the discharged flow is uniformized and the waving height of the molten metal surface can be controlled in the fixed range and the hot-rolled plate having good surface characteristic can be obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention attenuates the flow velocity of the discharge flow from the submerged nozzle in the mold, and at the same time suppresses one-sided flow, thereby controlling the height of the molten metal surface wave in the mold and improving the quality of the flow. This invention relates to a continuous steel casting method for manufacturing products with surface texture.

[Prior Art] FIG. 6 is a diagram showing the main part of the molten metal surface of a mold in a continuous slab casting machine. The conventional technology will be explained with reference to this figure. On the surface of the molten steel 8 in the mold 1, there is mold powder 5, which has the functions of preventing oxidation and keeping the molten steel 8 warm, lubricating between the solidified shell 9 and the mold 1, and adsorbing nonmetallic inclusions. The surface side of the mold powder 5 is in a molten state due to the heat of the molten steel 8, and the atmosphere side of the mold powder 5 is turned into powder 7, which covers the surface of the molten steel 8. The molten powder 6 flows between the solidified shell 9 and the mold 1 and acts as a lubricant.Therefore, the molten powder 6 is consumed, so in order to maintain a constant thickness of the mold powder 5, the amount of consumption of the molten powder 6 is reduced. will be replenished accordingly. As shown in FIG. 6, a discharge hole 3 provided at the tip of a submerged nozzle 2 vertically provided in the middle of the mold 1 opens opposite to the short side of the mold 1. Molten steel is discharged into the mold from this discharge hole 3. The discharge stream 4 of molten steel is injected into the mold in a V-shape in the direction of the short side of the mold. Discharge amount of this molten steel 4
collides with the short side and splits into two upper and lower flows, a reversed flow 11 and an intrusion flow 12. The reversed flow 11, which rises along the solidified shell 9 on the short side, flows into the hot water near the upper short side of the mold 1. Causes surface waves. FIG. 7 is a schematic diagram of the surface wave motion. Here, the molten metal surface wave refers to the discharge flow from the discharge hole 3 of the immersion nozzle 2 as shown in FIG. ripples the surface of the molten metal. This hot water surface wave is measured by an eddy current distance meter 15, and the voltage value is passed through a filter for high frequency components (
Here, after removing frequency components of 10 Hz or higher, measurements were taken with a millivolt meter. The installation position of this eddy current distance meter 15 is 50 m from the short side and 5 m from the hot water surface, as shown in Fig. 7.
It is 0m5. FIG. 8 is a diagram showing changes in the hot water level over time for about 1 minute. Measure the maximum wave level for 1 minute,
Data processing was performed using the maximum value as the maximum water surface wave height h. The upward arrow indicates an upward direction, and the upward arrow indicates a downward direction.Especially in high-speed casting where the discharge rate of molten steel is 3 ton/sin or more, the discharge flow rate from the discharge hole 3 of the immersion nozzle 2 is high, so that the solidified shell The reversed flow 11 after colliding with the hot water 9 also generates large surface waves. FIG. 9 is a graph showing the relationship between the maximum surface wave height and the hot-rolled sheet surface defect index. As is clear from this figure, the maximum surface wave height is 4mm to 8mm.
There is an optimum range for the maximum surface wave height in which the incidence of surface defects on hot-rolled sheets is small. When the molten metal surface wave motion is large, the molten powder 6 is drawn into the molten steel side by the molten metal wave motion and becomes suspended. The molten powder 6 caught in the molten steel floats to the surface due to the 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 waves are small, there is less supply of new molten steel to the molten steel surface.
It is thought that the meltability of the mold powder 5 is also poor, and therefore the ability of the molten powder 6 to dissolve and adsorb inclusions in the molten steel is poor, and the inclusions are captured in the solidified shell 9 and become internal defects in the slab. The appropriate range of the maximum surface wave height shown here is 4
The values 1 to 8 are values obtained from experience in continuous casting operations, and the shape of the immersion nozzle 2, the discharge angle of the immersion nozzle 2, the clogging of the immersion nozzle 2, and the mold 1 are adjusted to fall within this range. The width, etc., were regulated.

However, in order to improve the productivity of recent continuous casting machines, (1) multi-continuous casting technology that performs continuous casting for several charges with one tundish and immersion nozzle, (2) changing the mold width during casting, (3) Operating conditions have changed, such as casting speed changing from low to high speed. As a result, operating conditions were created that could not be satisfied with the shape and discharge angle of the discharge hole of the submerged nozzle that were suitable for the initial operating conditions, and the height of the molten metal surface wave could no longer be controlled within the optimal range. As a technology to control the height of surface waves, (1) a method of applying a brake to the discharge flow using a DC magnetic field (
*1: In the method (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, and a DC magnetic field is applied to the flow discharged from the immersed nozzle to generate a DC magnetic field in the flowing molten steel. The flow of molten steel is controlled by electromagnetic force generated in the opposite direction to the flow of molten steel using an induced current and a DC magnetic field.

(2) A method of applying a DC magnetic field to the hot water level (*2: hereinafter referred to as conventional method 2), in which a DC magnetic field is placed at the hot water level,
By applying a DC magnetic field horizontally to the hot water surface, the height of the hot water surface waves within the magnetic field is controlled.

Example (*l) Nagai et al. = 68. Iron and Steel (1982), 5
270 Suzuki et al.: 68. Iron and Steel (1982), 592
(*2) Small ring et al. 2. Iron and Steel (1986), 97
1g [Problems to be Solved by the Invention] The generation of surface waves in the mold is caused by the discharge flow discharged from the immersed nozzle colliding with the solidified shell and splitting into an upward reversal flow and a downward intrusion flow. Among these, the kinetic energy of the upward reverse flow causes the hot water surface to vibrate, resulting in hot water surface waves.

However, in conventional method 1, a direct current magnetic field is applied perpendicularly to the discharge flow between the immersion nozzle and the short side surface to brake the fluid, but the discharge flow after being discharged from the immersion nozzle is diffused. In order to do this, it is necessary to apply a DC magnetic field over a wide range.

This increases the size of the equipment and increases the cost. In addition, with this method, an eddy current circuit is created in the molten steel due to the interaction between the discharge flow and the applied DC magnetic field, so the current density cannot be increased.Therefore, in order to generate a large braking force, the magnetic flux density must be increased. There is a problem that this increases the equipment cost.

In conventional method 2, since a direct current magnetic field is directly applied to the molten metal surface wave, it is easiest to control the wave, but the most intense position of the molten metal surface wave is within 100 mm from the short side of the mold. Therefore, it is sufficient to apply a DC magnetic field to this position, and therefore the magnetic field generator needs to be installed on the back side of the copper plate on the long side of the mold at a distance of approximately 100 cm from the upper end of the copper plate on the long side of the mold. In this case, the cooling water box needs to be extensively remodeled, and the direction of the cooling grooves in the mold copper plate must also be oriented horizontally, resulting in insufficient cooling of the long side copper plate of the mold.

This invention was made in view of the circumstances, and
By reducing the surface fluctuations in the mold to prevent powder from being entrained, and by reducing the depth of penetration of inclusions,
The object of the present invention is to provide a method for continuous casting of steel that allows inclusions to float and produces products with good surface properties.

[Means for solving the problem] The continuous steel casting method of the present invention involves
At least a pair of DC magnets are installed, one magnetic pole of the DC magnet is placed directly 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 long side of the mold below the discharge hole of the immersion nozzle. It is characterized in that a direct current magnetic field is generated by making the polarities of opposing magnetic poles the same across the nozzle, and casting is performed while applying the direct current magnetic field perpendicularly to the discharge flow from the submerged nozzle.

[Operation] This 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 a conductive fluid flows in a magnetic field, an electromotive force is generated within the fluid due to Fleming's right-hand rule, and an eddy current flows. The interaction between this eddy current and the applied magnetic field causes an electromagnetic force (Fleming's left-hand rule) to act in a direction opposite to the direction of the fluid, thereby hindering the movement of the fluid. As a result, the discharge flow slows down. When the discharge flow decelerates, the flow velocity of the reversed flow after colliding with the short side shell also decreases, and surface waves become less likely to occur. Furthermore, when a one-sided flow phenomenon occurs, the electromagnetic force generated is greater in the direction where the discharge flow velocity is higher. As a result,
One-sided flow phenomenon is suppressed. When a DC magnetic field is applied vertically, the eddy current bus draws a circuit around the submerged nozzle as shown in Figure 5. At this time, a molded copper plate (copper electrical resistivity 2. 5×10 −8 Ω·m), the electrical resistance of the circuit decreases and the current density increases. As a result, the generated electromagnetic force increases and can be efficiently generated. When a DC magnetic field is applied in the horizontal direction (same as the thickness direction of the slab), the generated eddy current creates a circuit in the molten steel (electrical resistivity of molten steel is 150 x 10-'Ω・m) in a plane parallel to the mold copper plate. Therefore, the electrical resistance of the circuit increases and the eddy current density decreases. Therefore, in order to apply a DC magnetic field in the vertical direction, one magnetic pole of the DC magnet was placed directly above the upper end of the copper plate on the long side of the mold, and the other magnetic pole was placed on the rear surface of the copper plate on the long side of the mold below the immersion nozzle discharge hole.

[Example] First, the concept of flow of molten steel when electromagnetic force is applied to molten steel will be explained. Figure 5 is a diagram showing the flow of molten steel when an electromagnetic force is applied to the molten steel in the mold, (a) is a longitudinal cross-sectional view inside the mold, and (b) is a plane A-A in Figure (a). FIG.

21 is a mold long side copper plate, 22 is an immersion nozzle, 23 is an electromagnet, 24 is a DC magnet, 25 is a DC magnet coil, 26 is a magnetic field (■ mark or dotted arrow), 27 is a discharge flow (black arrow), 2
8 is eddy current (solid arrow), 2 is braking force (white arrow) 30
31 is the molten steel, 31 is one magnetic pole of the DC magnet, 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 injected from a tundish into a mold through an immersion nozzle 22, at least one pair of electromagnets 23 (DC magnets 24
and a DC magnet coil 25, and one magnetic pole 31 (N pole or S pole) of the DC magnet 24 is installed.
A pole) is placed directly above the upper end of the mold long side copper plate 21, and the other magnetic pole 32 (S pole or N pole) of the DC magnet is placed on the back side of the mold long side surface 21 below the discharge hole 33 of the immersion nozzle. By making the polarities of opposing magnetic poles (31 or 32) the same across 22 and applying the magnetic field 26 perpendicularly to the discharge flow 27 from the submerged nozzle 22, the inside of the discharge flow 27 is Braking force 2 in the opposite direction to the direction of motion
9, the discharge flow 27 can be attenuated.

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

E=VXB=Vy HBz --(1)V:
Velocity of molten steel (m/air) B: Magnetic flux density Bz: Vertical component of magnetic flux density Eddy current molecules flow in the molten steel due to this electromotive force, and the interaction between the eddy current I and the magnetic flux density causes the direction of movement of the molten steel to change. Braking force F acts in the opposite direction.

F= IxB=-σVy-Bz" ・=-(2)σ
: Electrical resistivity of fluid (Ω・m) According to equation (2), the magnitude of braking force depends on VY and 1322.

In continuous casting of molten steel, in the case of low-speed casting, (7) is small, so the braking force F acting on the molten steel is small, but as the casting speed becomes higher, VY becomes larger, so the braking force F becomes larger.

In the absence of a DC magnetic field, the discharge flow 27 discharged from the immersion nozzle 22 tends to flow preferentially from one discharge hole 33, resulting in a one-sided flow phenomenon.By applying a DC magnetic field in the vertical direction, the discharge flow velocity can be reduced. In the faster direction, a larger braking force acts according to equation (2), so the discharge flow is made uniform and the one-sided flow phenomenon is alleviated. By doing this, the maximum surface wave height can be controlled within a certain range.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a cross-sectional view of a continuous casting mold according to an embodiment of the present invention, (a) is a side cross-sectional view, (b) is a cross-sectional view taken along line A-A in (a) of FIG. C) is B in <a) in Figure 1
-B is a perspective view of the 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, and 25 is a DC magnet coil.

30 is molten steel, 31 is one magnetic pole of a DC magnet, 32 is the other magnetic pole of the DC magnet, and 33 is a discharge hole of an immersion nozzle.

A pair of electromagnets 23 (consisting of a DC magnet 24 and a DC magnet coil 25) are placed behind the copper plate 21 on the long side of the mold on the front and rear sides, sandwiching the immersion nozzle 22 attached to the lower part of the tundish (not shown). has been done. One magnetic pole 31 of the DC magnet is placed directly 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 is placed directly above the discharge hole 33 of the immersion nozzle.
It was placed at a position 250+sm below. The cross-sectional dimensions of the DC magnet 24 are 100 (H) x 600 (W) am. The polarity of the magnetic poles (31 or 32) of the DC magnet 24 was selected so that they were opposite to each other with the immersion nozzle 22 in between, thereby making the magnetic field direction vertical, that is, parallel to the immersion nozzle 22. Can be done.

(Example 1) Using continuous casting molds each equipped with a pair of electromagnets 23 as shown in FIG. Thickness, 1
Pulling out slabs with cross-sectional dimensions of 200mg + width from 0.7 to
Castings were carried out with variations in the range of 2.7 m/am. The casting speed at this time was 1, 4 to 2, 7 tons/win.
changed between. FIG. 2 is a graph showing the relationship between the maximum surface wave height near the copper plate on the short side of the mold and the drawing speed or casting speed when a DC magnetic field is applied and when it is not applied. The horizontal axis of this figure shows the relationship between the drawing speed and the casting speed, where ◯ indicates no magnetic field and ◯ indicates that there is a magnetic field. The magnetic flux density of the DC magnetic field is 2
It was adjusted in the range of 000 to 2500 Gauss. The maximum surface wave height when a magnetic field is applied is considerably smaller than the maximum surface wave height when no magnetic field is applied.
At casting speeds below 2.5 ton/win, the maximum surface wave height is suppressed;
Even at an injection rate of n or more, the maximum surface wave height can be suppressed to 8 m+a or less.

(Example 2) Casting was carried out using continuous casting molds each equipped with a pair of electromagnets 23 shown in FIG. 1 while applying a DC magnetic field to the flow discharged from the immersion nozzle. The conditions for applying the DC magnetic field were determined based on the results shown in Example 1. That is, 3. Qton/■
The magnetic flux density of the DC magnetic field was uniformly set to 2000 Gauss under the casting conditions of a casting speed of 1.5 in. or more. 220 Hooroku thickness, 12
A slab with a cross-sectional dimension of 0.00 mm lid width was cast. FIG. 3 is a graph showing temporal changes in pouring time and drawing speed, and in casting time and maximum surface wave height in the presence and absence of a DC magnetic field. Immediately after the start of casting, it was not possible to measure the maximum molten metal surface wave height because it required setting and adjustment of the eddy current distance meter. The wave height value can be controlled within an appropriate range over almost the entire casting area.

゛ When replacing the pot, the water surface wave motion is quiet because the discharge flow rate is slow, 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 casting speed depending on the presence or absence of a DC magnetic field.

O means there is no magnetic field, and * means there is a magnetic field. The direct current magnetic field is applied when the casting speed is 3. The surface defect index used when the casting speed is 0 ton/win or more is the index value obtained by dividing the number of scratches by the observed area. As is clear from this figure, by applying a DC magnetic field, the hot-rolled sheet surface defect index is significantly reduced during high-speed casting.

[Effects of the Invention] As explained above, in the continuous steel casting method according to the present invention, at least one pair of DC magnets are installed with the immersion nozzle in between, and one magnetic pole is placed above the upper end of the copper plate on the longer 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 polarity of the magnetic poles facing each other across the mold is made the same to generate a DC magnetic field, which increases the discharge flow of molten steel from the immersion nozzle. Since casting is performed while applying a DC magnetic field perpendicular to the molten metal, a large braking force is applied to the faster discharge flow rate.-The discharge flow is made uniform, so the height of the surface wave can be controlled within a certain range. , a hot rolled sheet having good surface properties can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of a continuous casting mold according to an embodiment of the present invention, and Fig. 2 shows the maximum surface 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. Figure 3 is a graph showing the relationship between casting time and drawing speed, as well as casting time and maximum surface wave height in the presence and absence of a DC magnetic field. Figure 5 is a graph showing the relationship between hot rolled sheet surface defect index and casting speed, Figure 5 is a diagram showing the flow of molten steel when electromagnetic force is applied to the molten steel in the mold, and Figure 6 is a graph showing the relationship between the hot rolled sheet surface defect index and casting speed. A diagram showing the main parts of the molten metal surface of the mold of the casting machine, Figure 7 is a schematic diagram of the molten metal surface wave, Figure 8 is a diagram showing the change in the molten metal level over time for about 1 minute, and Figure 9 is the maximum molten metal level. FIG. 2 is a graph showing the relationship between wave height and 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...molten steel, 31...one magnetic pole of the DC magnet, 32...other magnetic pole of the DC magnet, 33.・・・
Discharge hole of immersion nozzle.

Claims (1)

[Claims] In a continuous steel casting method in which molten steel is cast into a mold from a tundish through an immersion nozzle, at least a pair of DC magnets are installed across the immersion nozzle, and one magnetic pole of the DC magnet is connected to the upper end of the copper plate on the longer side of the mold. Directly above, 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 with the immersion nozzle in between, the opposing magnetic poles have the same polarity, and a DC magnetic field is generated. A continuous steel casting method characterized by casting while applying a direct current magnetic field perpendicular to the discharge flow of molten steel.