JP6331810B2 - Metal continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method of metal, and more particularly to a continuous casting method for stabilizing the flow of molten metal in a mold.

  Usually, in continuous casting of metal, molten metal is supplied from a tundish into a mold through a single immersion nozzle (hereinafter also simply referred to as “nozzle”), and the molten metal is solidified in the mold. The During the continuous casting operation, the nozzle discharge hole is immersed in the molten metal in the mold.

  The immersion nozzle is provided with a sliding gate device (hereinafter also referred to as “SG device”) for adjusting the flow rate of the molten metal. The SG device includes a fixed plate that is fixedly arranged and a sliding plate that slides relative to the fixed plate. A through hole is formed in each of the fixed plate and the sliding plate. When the sliding plate slides with respect to the fixed plate, the area of the portion where the through hole formed in the fixed plate and the through hole formed in the sliding plate overlap is changed. Since the molten metal flows through a portion where these through-holes overlap, when the area of the overlapping portion (cross-sectional area of the flow path of the molten metal) changes, the flow rate of the molten metal flowing through this portion changes. Therefore, the flow rate of the molten metal supplied from the tundish to the mold can be controlled by changing the position of the sliding plate with respect to the fixed plate.

  If bubbles or non-metallic inclusions are introduced into a slab such as a slab slab (plate-shaped slab) manufactured by continuous casting, it causes surface defects in a product obtained from the slab. For this reason, conventionally, reduction of these bubbles and non-metallic inclusions has been demanded. When the molten metal flow is unstable at the molten metal meniscus in the mold, bubbles and non-metallic inclusions are easily introduced into the slab. For this reason, stabilization of the flow of the molten metal in this meniscus part is calculated | required.

  The “meniscus” means the surface of the molten metal in the mold, and the “meniscus portion” means a portion near the meniscus including the meniscus in the molten metal. That is, the “meniscus portion” includes a range from about 0 to 50 mm in the depth direction of the molten metal from the meniscus.

  On the other hand, in the continuous casting operation, solid matter may adhere to the inner wall of the nozzle and nozzle clogging may occur. In this case, due to this nozzle clogging, the flow of molten metal discharged from the nozzle becomes uneven, making it difficult to form a stable molten metal flow in the mold. This situation not only lowers the casting speed but also deteriorates the quality of the slab. The reason why it is difficult to stabilize the flow of the molten metal in the mold is that it is difficult to stably discharge the molten metal from the nozzle into the mold.

  In Patent Documents 1 and 2, for the purpose of stabilizing the discharge of molten metal from the nozzle into the mold and the flow of molten metal in the mold, a swirl flow imparting mechanism is installed in the tundish. Has been proposed.

  As such a swirl flow imparting mechanism, Patent Document 1 discloses a hollow refractory structure in which side holes are formed. The refractory structure is disposed above the nozzle in the tundish. As the molten metal flows into the refractory structure through the side holes, a swirling flow of the molten metal is formed. In Patent Document 1, an opening is provided in the upper end portion of the refractory structure, and a refractory stopper rod extending from the upper part of the tundish to the vicinity of the bottom of the refractory structure is disposed through the opening. Is also disclosed. By arranging the stopper rod, the slag on the molten metal surface can be prevented from being caught in the molten metal by the swirling flow.

  Patent Document 2 discloses that the refractory structure is a double cylinder structure based on the configuration disclosed in Patent Document 1. According to this configuration, it is supposed that it is possible to prevent the slag from being caught by the swirling flow provided by the refractory structure without providing the stopper rod.

  In any of the above techniques, the molten metal flows through the immersion nozzle while maintaining the swirling flow formed in the tundish, and is discharged into the mold. This stabilizes the flow of the molten metal in the mold and suppresses the introduction of non-metallic inclusions such as slag into the slab. Therefore, it is possible to achieve stable continuous casting operation and quality improvement of the slab.

Japanese Patent No. 4419934 Japanese Patent No. 4670762

  However, when the throughput (slab manufacturing speed) decreases, the molten metal flow path is narrowed by the SG device in order to reduce the flow rate of the molten metal supplied into the mold. Specifically, the sliding plate protrudes so as to block a part of the through hole of the fixed plate. In this case, since the flow of the molten metal is inhibited by the sliding plate, the swirl flow formed by the refractory structure is attenuated before being supplied into the mold. For this reason, it has been difficult to stabilize the flow of the molten metal in the mold during operation at a low throughput.

  An object of the present invention is to provide a metal continuous casting method capable of stabilizing the flow of molten metal in a mold not only at the time of operation at a high throughput but also at the time of operation at a low throughput. And

This invention is made | formed in view of such a subject, and makes a summary the continuous casting method of following (1).
(1) A metal continuous casting method using a continuous casting apparatus configured to supply molten metal from a discharge hole of the immersion nozzle into the mold through the immersion nozzle from the tundish,
Above the immersion nozzle in the tundish, a refractory structure having a double structure including an outer cylinder and an inner cylinder is provided,
The outer cylinder has a cylindrical shape, a conical shape or a truncated cone shape, and a side hole is formed,
The lower end of the outer cylinder is in contact with the bottom of the tundish, and there is a gap of 10 to 300 mm in the vertical direction between the lower end of the inner cylinder and the bottom of the tundish or the upper end of the immersion nozzle. And
The side hole of the outer cylinder has the center of the outlet opening of the side hole at the intersection of an imaginary line extending radially from the axis of the refractory structure and the inner surface of the outer cylinder,
In the exit opening, a central axis of the side hole is inclined with respect to the imaginary line in a horizontal plane,
The immersion nozzle is provided with a sliding gate that adjusts the flow rate of the molten metal flowing from the tundish to the mold by changing the opening that is a value corresponding to the cross-sectional area of the flow path of the molten metal. And
An electromagnetic stirrer is disposed on the side of the mold,
The continuous casting method is
The molten metal outside the refractory structure in the tundish is caused to flow into the refractory structure through the side holes, thereby allowing the molten metal to move around the axis of the outer cylinder. Providing a swirling flow of
When the opening degree of the sliding gate device is 70 to 90%, the molten metal contained in the height position of the discharge hole of the immersion nozzle is horizontally moved in the mold using the electromagnetic stirring device. A continuous casting method including a step of stirring and suppressing a fluctuation in a molten metal surface height of the molten metal in the mold to 2 mm or less.

  According to this continuous casting method, by appropriately applying electromagnetic stirring, the flow of molten metal in the mold can be stabilized during operation at a low throughput. For this reason, it becomes difficult to introduce bubbles and non-metallic inclusions into the slab, and the quality of the product obtained from the slab can be increased.

FIG. 1A is a cross-sectional view of a continuous casting apparatus that can be used in a continuous casting method according to an embodiment of the present invention, and shows a state when operating at a high throughput. FIG. 1B is a cross-sectional view of a continuous casting apparatus that can be used in a continuous casting method according to an embodiment of the present invention, and shows a state when operating at a low throughput. 1C is a cross-sectional view taken along the line AA in FIG. 1A. FIG. 2 is a diagram showing the flow of molten steel in the mold. FIG. 3 is a diagram showing the relationship between the opening degree of the sliding gate device and the fluctuation width of the height of the meniscus for each of the continuous casting apparatuses having various configurations. FIG. 4 is a diagram showing the relationship between the fluctuation width of the meniscus height and the product defect evaluation index of the slab.

  For the sliding gate device, the “opening” is a value corresponding to the cross-sectional area of the flow path of the molten metal, and is determined by the position of the sliding plate with respect to the fixed plate. The opening degree is 100% of the value when the overlap between the through hole of the fixed plate and the through hole of the sliding plate is maximized (for example, when the sliding plate is at the forward limit position of its operating range). The value when the through hole of the fixed plate and the through hole of the sliding plate do not overlap (for example, corresponding to when the sliding plate is at the retreat limit position of its operating range) is 0%. . When the position of the sliding plate with respect to the fixed plate is between the above-described positions, a value obtained by proportionally distributing the percentage at the position is defined as the opening degree. When the sliding plate is operated via the cylinder, the opening degree may be defined by the position of the cylinder.

  “Throughput” is the production rate of the slab, and is determined by (horizontal cross-sectional area of the mold) × (drawing speed of the slab) × (specific gravity of the slab). The horizontal cross-sectional area of the mold is determined by the size of the slab to be cast, and the specific gravity is determined by the molten metal species. Therefore, in normal continuous casting operations, the throughput is controlled by the slab drawing speed.

  “Low throughput” means that the opening of the sliding gate device is 80% or less. “High throughput” means that the opening of the sliding gate device is larger than 80%. The flow rate of the molten metal corresponding to the opening degree of 80% is, for example, 4.1 t / min.

  As described above, the present invention is a continuous metal casting method using a continuous casting apparatus configured to supply molten metal from a tundish through an immersion nozzle to a mold through a discharge hole of the immersion nozzle. . A refractory structure having a double structure including an outer cylinder and an inner cylinder is provided above the immersion nozzle in the tundish. The outer cylinder has a cylindrical shape, a conical shape, or a truncated cone shape, and has a side hole. The lower end of the outer cylinder is in contact with the bottom of the tundish, and there is a gap of 10 to 300 mm in the vertical direction between the lower end of the inner cylinder and the bottom of the tundish or the upper end of the immersion nozzle. . The side hole of the outer cylinder has the center of the outlet opening of the side hole at the intersection of an imaginary line extending radially from the axis of the refractory structure and the inner surface of the outer cylinder. In the side opening, the central axis of the side hole is inclined with respect to the imaginary line in a horizontal plane. The immersion nozzle is provided with a sliding gate device that adjusts the flow rate of the molten metal flowing from the tundish to the mold by changing the opening that is a value corresponding to the cross-sectional area of the flow path of the molten metal. ing. An electromagnetic stirrer is disposed on the side of the mold. In this continuous casting method, the molten metal outside the refractory structure in the tundish is caused to flow into the refractory structure through the side holes, to the molten metal, The step of applying a swirling flow around the axis of the outer cylinder and the opening degree of the sliding gate device is 70 to 90%, and are included in the height position of the discharge hole of the immersion nozzle in the mold. A continuous casting method including a step of stirring the molten metal in the horizontal direction using the electromagnetic stirrer and suppressing a fluctuation in the molten metal surface height of the molten metal in the mold to 2 mm or less.

Hereinafter, the present invention will be described by taking the case where the molten metal is molten steel as an example.
1A and 1B are cross sections of a continuous casting apparatus that can be used in a continuous casting method according to an embodiment of the present invention. FIG. 1A shows a state operating at a high throughput, and FIG. 1B shows a state operating at a low throughput.

  The continuous casting apparatus 1 includes a tundish 2, a mold 3 disposed below the tundish 2, and an immersion nozzle 4 used to transfer molten steel in the tundish 2 into the mold 3. .

  A refractory structure 5 is provided at the bottom of the tundish 2 and above the nozzle 4. The immersion nozzle 4 passes through the bottom of the tundish 2. The refractory structure 5 is disposed above the immersion nozzle 4.

  The refractory structure 5 has a double structure including an outer cylinder 5A and an inner cylinder 5B. The outer cylinder 5A has a cylindrical shape, a conical shape, or a truncated cone shape, and the inner cylinder 5B is disposed inside the outer cylinder 5A and coaxially with the outer cylinder 5A. The axis of the refractory structure 5 is substantially along the vertical direction. The outer cylinder 5A and the inner cylinder 5B are connected to each other at the upper part of the refractory structure 5. The outer cylinder 5A is disposed such that the lower end thereof is in contact with the bottom of the tundish. Regarding the axial direction of the refractory structure 5, the inner cylinder 5B is shorter than the outer cylinder 5A. With respect to the vertical direction (height direction), there is a gap of 10 to 300 mm between the lower end of the inner cylinder 5 </ b> B and the bottom of the tundish 2 or the upper end of the immersion nozzle 4.

  The outer cylinder 5A has a plurality of side holes 5h around its axis. 1C is a cross-sectional view taken along the line AA in FIG. 1A. In this embodiment, 8 side holes 5h are formed in the outer cylinder 5A. Each side hole 5h is an exit side opening of the side hole 5h (an opening inside the outer cylinder 5A) at the intersection of virtual lines X1 to X8 extending radially from the axis of the refractory structure 5 and the inner surface of the outer cylinder 5A. The center axis Y1 to Y8 of the side hole 5h is inclined by θ1 ° with respect to the imaginary line X1 to X8 in the horizontal plane. Thereby, when the molten steel M in the tundish 2 flows into the refractory structure 5 through the side holes 5h, a swirling flow is generated in the molten steel M.

  When the inclination angle θ1 is smaller than 15 °, the strength of the swirling flow to be applied (the magnitude of the velocity component of the molten steel M around the axis of the outer cylinder 5A) is insufficient. On the other hand, when the inclination angle θ1 is larger than 80 °, the thickness of the outer cylinder 5A is reduced, which causes a problem in strength. Accordingly, θ1 is preferably set in the range of 15 ° to 80 °.

  Referring to FIGS. 1A and 1B, the immersion nozzle 4 has a cylindrical shape, and an upper nozzle 4A fitted in a hole penetrating the bottom of the tundish 2 in the thickness direction, and an upper nozzle 4A below the upper nozzle 4A. The lower nozzle 4B arrange | positioned coaxially with the nozzle 4A is provided. The axis of the immersion nozzle 4 is substantially along the vertical direction. The refractory structure 5 is arranged coaxially with the immersion nozzle 4.

  Between the upper nozzle 4A and the lower nozzle 4B, a sliding gate device 6 for adjusting the flow rate of the molten steel M flowing through the immersion nozzle 4 is provided. The sliding gate device 6 includes two fixed plates 6A and a sliding plate 6B disposed between the fixed plates 6A. The fixed plate 6A is fixed to the immersion nozzle 4, and the sliding plate 6B is slidable with respect to the fixed plate 6A.

  A through hole is formed in each of the fixed plate 6A and the sliding plate 6B. As the sliding plate 6B slides with respect to the fixed plate 6A, the area of the overlapping portion of the through hole formed in the fixed plate 6A and the through hole formed in the sliding plate 6B changes. The opening degree of the sliding gate device 6 is changed.

  The lower end of the lower nozzle 4B is closed. A pair of discharge holes 4h is formed in the side wall near the lower end of the lower nozzle 4B. The pair of discharge holes 4h are formed on opposite sides with respect to the axis of the lower nozzle 4B. Molten steel M in the immersion nozzle 4 is discharged into the mold 3 through the discharge holes 4h. During operation of the continuous casting apparatus 1, the discharge hole 4 h of the immersion nozzle 4 is immersed in the molten steel M in the mold 3.

  An electromagnetic stirrer is provided on the side of the mold 3. The core 7 of the electromagnetic stirring device is arranged in a height range including the meniscus Mm portion and the discharge hole 4h formed in the lower nozzle 4B in the vertical direction.

  Next, a continuous casting method according to an embodiment of the present invention using the continuous casting apparatus 1 will be described.

  First, molten steel M is supplied into the tundish 2. Molten steel M flows into the refractory structure 5 through the side holes 5h. At this time, the swirling flow around the axis of the outer cylinder 5A is given to the molten steel M. The molten steel M flows into the immersion nozzle 4.

  By providing the refractory structure 5 with the inner cylinder 5 </ b> B, vortexes caused by the swirling flow are suppressed from being generated on the surface of the molten steel M. Therefore, when non-metal such as slag is floating on the surface of the molten steel M in the tundish 2, such non-metal is caught in the molten steel M, and the non-metallic inclusions are obtained in the slab obtained by this continuous casting. Can be prevented from being introduced. In order to sufficiently exhibit such an effect, it is preferable that the upper end of the refractory structure 5 is higher than the surface of the molten steel M in the tundish 2 as shown in FIGS. 1A and 1B. However, even when the upper end of the refractory structure 5 is lower than the surface of the molten steel M in the tundish 2, the non-metal is caught in the molten steel M as compared with the case where the inner cylinder 5B is not provided. Can be suppressed.

  Hereinafter, the length in the vertical direction of the gap between the bottom of the tundish 2 or the upper end of the immersion nozzle 4 and the lower end of the inner cylinder 5B is referred to as “inner cylinder lower gap length”. As described above, the inner cylinder lower gap length is 10 to 300 mm. When the inner cylinder bottom gap length is less than 10 mm, the passage of the molten steel M when the molten steel M provided with the swirling flow flows into the immersion nozzle 4 becomes too narrow, and the possibility that the passage is blocked increases. Further, when the surface height of the molten steel M in the tundish 2 becomes lower than the lower end of the inner cylinder 5B, the vortex suppressing action of the inner cylinder 5B is lost. For this reason, if the gap length under the inner cylinder exceeds 300 mm, a swirling flow is generated under the condition that the surface height of the molten steel M in the tundish 2 is lowered at the start of casting and immediately before the end of casting. The vortex formation that accompanies it can no longer be suppressed. In order to easily obtain the effect of setting the inner cylinder lower gap height to 10 to 300 mm, the inner cylinder lower gap height is preferably based on the bottom of the tundish 2.

  The molten steel M flows downward in the immersion nozzle 4 and flows into the mold 3 through the discharge holes 4h. In FIG. 1A and FIG. 1B, the direction in which the molten steel M flows is indicated by arrows.

  During operation at a high throughput (see FIG. 1A), since the opening of the sliding gate device 6 is increased, the flow of the molten steel M is hardly obstructed by the sliding gate device 6, and the strength of the swirling flow The molten steel M reaches the lower end portion of the immersion nozzle 4 without being attenuated.

  At this time, the molten steel M to which the swirl flow is applied descends while being given a force laterally (in a direction away from the axis of the immersion nozzle 4) by centrifugal force, so that the molten steel M is evenly discharged from the plurality of discharge holes 4h. Is done.

  FIG. 2 is a diagram showing the flow of molten steel in the mold. In the following description, it is assumed that the mold 3 has a rectangular cross section. In the following description, the “short side surface” is a surface that is the inner surface of the mold 3 and includes the short side in the cross section, and the “long side surface” is the inner surface of the mold 3 and is the long length in the cross section. A surface that includes an edge.

  A portion of the molten steel M discharged obliquely downward from the discharge hole 4h of the immersion nozzle 4 rises after colliding with the short side surface of the mold 3, and in the vicinity of the meniscus Mm, from the short side surface side of the mold 3 3 flows to the center side in the width direction (generation of reverse flow).

  When a sufficiently strong swirl flow is formed in the immersion nozzle 4, the flow rate of the molten steel M discharged from each of the plurality of discharge holes 4h is uniform as compared with the case where no swirl flow is involved. Thereby, a part of the molten steel M rises at a large flow velocity in the vicinity of the side wall of the mold 3 and flows at a large flow velocity even at the meniscus Mm portion. For this reason, the fluctuation range of the height of the meniscus Mm of the molten steel M in the mold 3 is suppressed.

  The fluctuation range of the height of the meniscus Mm is a difference between the control target value and the actually measured value with respect to the height position. The measured values are, for example, all the position data of the meniscus when measured at a detection frequency of once every 500 msec using an eddy current sensor (a sensor that senses the magnetic field lines generated by the high-frequency coil and reflected by the meniscus surface). Is the average value.

  Further, when an electromagnetic force is applied to the molten steel M by a core 7 (see FIGS. 1A and 1B) provided in the vicinity of the mold 3, and a force that causes the molten steel M to flow clockwise as viewed from above, a meniscus is applied. In the Mm part, the flow of the molten steel M is accelerated in the part where the direction of the reversal flow and the direction of the force by the electromagnetic force are the same, and conversely, in the part where these directions are opposite to each other, the flow of the molten steel M is decelerated. Or the molten steel M will stay.

  Of the defects on the slab surface, it is necessary to obtain a stable flow velocity distribution in the meniscus Mm portion in order to suppress the occurrence of defects due to the incorporation of bubbles and inclusions during casting. For that purpose, the mutual influence of the stirring flow by electromagnetic stirring and the discharge flow from the immersion nozzle 4 must be considered.

  As the mutual influence between the stirring flow and the discharge flow, for example, when the flow velocity of the molten steel M is excessively accelerated, the powder for lubricating between the mold 3 and the solidified shell is placed on the meniscus Mm. If present, the quality of the slab may deteriorate due to the entrainment of the powder. Conversely, in the portion where the flow of the molten steel M is slowed or the molten steel M stays, bubbles and / or inclusions are easily trapped in the solidified shell, and the surface quality of the slab may deteriorate.

  Therefore, in order to reduce the influence of the discharge flow of the molten steel M from the discharge hole 4h on the meniscus Mm portion, electromagnetic stirring is applied at the height position of the discharge hole 4h. That is, in addition to the positive flow application to the meniscus Mm portion by electromagnetic stirring, the flow of the molten steel M is stabilized by electromagnetic stirring against the discharge flow from the discharge holes 4h. Specifically, with respect to the discharge flow directed from the discharge hole 4h of the immersion nozzle 4 toward the short side surface of the mold 3, it is swung horizontally (within a horizontal plane) by electromagnetic stirring at the height position of the discharge hole 4h. By imparting a flow velocity to the surface, the flow that collides with the short side surface is suppressed, and the flow velocity of the reversal flow that moves upward after colliding with the short side surface is suppressed. Thereby, the adverse effect of the reverse flow on the meniscus Mm portion can be minimized. The fluctuation range of the height of the meniscus Mm is suppressed to 2 mm or less.

  During operation at a low throughput (see FIG. 1B), since the opening of the sliding gate device 6 is reduced, the sliding plate 6B becomes an obstacle against the flow of the molten steel M. For this reason, the swirling flow of the molten steel M is attenuated. In this case, sufficient centrifugal force is not applied to the molten steel M below the sliding plate 6B in the immersion nozzle 4, and the molten steel M is less likely to be discharged uniformly from the plurality of discharge holes 4h. The molten steel M may be sucked into the immersion nozzle 4 through the discharge holes 4h. In the methods of Patent Documents 1 and 2, such a problem when operating at a low throughput cannot be solved, and drift may occur.

  In the continuous casting method according to an embodiment of the present invention, an electromagnetic force is applied to the molten steel M in the mold 3 by the core 7 of the electromagnetic stirrer, and a flow of the molten steel M equivalent to that at the time of operation at a high throughput occurs. Like that. When the flow rate of the molten steel M discharged from the discharge hole 4h is not sufficiently large, an electromagnetic force is applied so as to increase the flow rate. By such an operation, the fluctuation range of the height of the meniscus Mm is suppressed to 2 mm or less even during operation at a low throughput.

  Therefore, according to this method, the flow of the molten steel M in the mold 3 can be stabilized not only at the time of operation at a high throughput but also at the time of operation at a low throughput. Air bubbles and non-metallic inclusions are hardly introduced into the slab obtained by solidification.

  FIG. 3 is a diagram showing the relationship between the opening of the sliding gate device (SG device) and the fluctuation width of the meniscus height (water surface fluctuation width) for each of the continuous casting apparatuses having various configurations. The management target range of the fluctuation range of the meniscus height is 2 mm or less.

  The meaning of the legend in FIG. 3 is as follows. What is described as "Comparison" is that a continuous casting apparatus is not provided with a refractory structure (thus, no forced swirling flow is given to the molten steel flowing from the tundish to the mold) and in the mold It shows that electromagnetic stirring was not performed on the molten steel (hereinafter referred to as “casting method A”). What is described as “a refractory structure” was provided with a refractory structure in a continuous casting apparatus to give a swirling flow to the molten steel, but no electromagnetic stirring was performed (hereinafter referred to as “casting method B”). ) Casting method B corresponds to the one in which the invention of Patent Document 2 is implemented. What was described as “a refractory structure + electromagnetic stirrer” was provided with a refractory structure in a continuous casting apparatus to give a swirl flow to the molten steel, and the magnetic stirrer was performed so as to satisfy the requirements of the present invention. (Hereinafter referred to as “casting method C”). Casting method C corresponds to the method of the present invention.

  In the casting method A, in many cases, the fluctuation width of the height of the meniscus greatly exceeds 2 mm and reaches a maximum of about 4 mm.

  In the casting method B, the fluctuation width of the meniscus height is reduced as compared with the casting method A. This is considered to be because the flow of molten steel at the meniscus portion could be stabilized by generating a swirl flow with the refractory structure, as compared with the case where such a swirl flow is not generated. However, in casting method B, the smaller the opening of the sliding gate device, the larger the fluctuation range of the meniscus height. When the opening degree is 80% or less, the fluctuation width of the meniscus height may exceed 2 mm. Become more. This is presumably because the swirling flow is attenuated by the sliding gate device (sliding plate).

  In the casting method C, the variation width of the meniscus height is 2 mm or less regardless of the opening degree of the sliding gate device. The reason why the fluctuation range of the meniscus height can be reduced even when the opening of the sliding gate device is small is that the flow rate is given to the molten steel by electromagnetic stirring so as to compensate for the attenuation of the swirling flow by the sliding gate device. it is conceivable that.

  FIG. 4 is a diagram showing the relationship between the fluctuation width of the meniscus height and the product defect evaluation index of the slab. The “product wrinkle evaluation index” is an index for quantitatively evaluating the amount and type of wrinkles generated on the product surface. For high-end products, this index needs to be zero. As is clear from FIG. 4, when the variation width of the meniscus height exceeds 2 mm, the product 疵 evaluation index rarely becomes 0, whereas the variation width of the meniscus height is 2 mm or less. In this case, the product defect evaluation index is stably 0.

  In order to confirm the effect of the present invention, a continuous casting test was performed, and the obtained slab was evaluated. A general slab slab continuous casting apparatus was used for the test. This continuous casting apparatus has a structure similar to that shown in FIGS. 1A to 1C, and the vertical portion located immediately below the mold is 3.0 m, and the bending portion has a radius of 10.5 m. It was a mold continuous casting machine.

  Table 1 shows normal use conditions of the continuous casting apparatus used.

  Electromagnetic stirring was applied to the molten steel in the mold. The applied intensity of electromagnetic stirring is defined by the frequency of the alternating current. If the frequency is too low, the penetration depth of the magnetic flux into the mold copper plate becomes deep, the facing coils interfere with each other, and the molten steel in the mold cannot be efficiently stirred. For this reason, 0.5 Hz or more is appropriate as an applied frequency condition that can secure 50% or more of the magnetic flux in the center in the thickness direction of the mold (thickness: 200 to 300 mm) of the continuous casting apparatus used in this test. Further, when the frequency is increased, there is no adverse effect on stirring the molten steel in the mold. However, although it becomes an excessive facility, the stirring effect is saturated, so 5 Hz or less is appropriate.

  In the test described below, in order to apply electromagnetic stirring simultaneously to the molten steel at the height position of the discharge hole of the immersion nozzle and the meniscus portion, as an electromagnetic stirring device, continuous in a height range including the height position of these molten steel. The one having one core was used. The length of the core was 500 mm with respect to the vertical direction. However, even if electromagnetic stirring is simultaneously applied to the molten steel and the meniscus portion at the height position of the discharge hole of the immersion nozzle using, for example, an electromagnetic stirring device having a core divided in the vertical direction, the same applies. It is obvious that the effect of can be obtained.

  In the tundish, a refractory structure for providing a swirling flow to the molten steel was provided. This refractory structure has an outer cylinder and an inner cylinder that are cylindrical. The outer cylinder and the inner cylinder were arranged coaxially. Therefore, the refractory structure as a whole had a double structure having axial symmetry. The outer cylinder and the inner cylinder are oriented such that their axes are along the vertical direction.

  The outer cylinder had an inner diameter of 520 mm, an outer diameter of 600 mm, and a height of 240 mm. The inner cylinder had an inner diameter of 142 mm, an outer diameter of 160 mm, and a height of 30 mm. The lower end of the outer cylinder was in contact with the bottom of the tundish, and the inner cylinder lower gap length was 240 mm.

  On the side wall of the outer cylinder, five side holes were formed around the axis of the outer cylinder at intervals of 72 °, ie, equiangular intervals. The side hole was formed in the position whose height from the bottom face of an outer cylinder is 120-210 mm. The height of the side hole is 90 mm, the width of the side hole is 80 mm, and the imaginary line extending radially from the axis of the refractory structure and the central axis of the side hole in the horizontal plane at the outlet side opening It was inclined at an angle of 35 °.

  Using this apparatus, the inner space has a width of 950 to 1625 mm from a tundish through a submerged steel with a C content of 0.001 to 0.003 mass% and a superheat degree ΔT of 20 to 35 ° C. The slab cast was cast into a mold having a thickness of 270 mm and a copper plate height of 900 mm, and the slab drawing speed was 1.0 to 1.4 m / min. From these casting conditions, the throughput (t / min) was calculated according to the above definition.

  Table 2 shows test conditions and evaluation results.

  As shown in Table 2, conditions were changed for the C content of molten steel, the throughput, the opening of the sliding gate device, the degree of superheat of the molten steel, the position of the core of the electromagnetic stirrer, and whether or not electromagnetic stirring was applied. In any test, a refractory structure was provided in the tundish, and the conditions were not changed. When the opening degree of the sliding gate device is 80% or less, the swirl flow formed by the refractory structure is easily attenuated. Therefore, in order to clarify the effect of the present invention, the opening degree of the sliding gate device is 80%. % Or less.

  In Examples 1 and 2 and Comparative Examples 1 and 2, with respect to the vertical direction, the height from the lower end of the core to the center of the immersion nozzle was 230 mm, and the height from the center of the immersion nozzle to the upper end of the core was 220 mm. It was. That is, in these examples and comparative examples, the center of the immersion nozzle was substantially at the center of the core in the vertical direction. In Examples 3 to 6 and Comparative Example 3, with respect to the vertical direction, the height from the lower end of the core to the center of the immersion nozzle was 200 mm, and the height from the center of the immersion nozzle to the upper end of the core was 250 mm. That is, in these examples and comparative examples, the center of the immersion nozzle was in a position lower than the center of the core in the vertical direction. In either case, the core was at a height position that could also act on the meniscus portion of the molten steel.

  The relative position of the core of the electromagnetic stirrer with respect to the mold does not change, whereas the relative position of the tundish and the immersion nozzle changes depending on the casting operating conditions. The distance in the vertical direction (hereinafter referred to as “core height”) from the center of the discharge hole of the immersion nozzle shown in Table 2 is a representative value, and has a value different from these values during continuous casting. There was a thing. In Comparative Examples 1 to 3, electromagnetic stirring was not performed. For this reason, in Table 2, parentheses are added to the core height values of Comparative Examples 1 to 3.

  In Examples 1 to 6, the meniscus height variation width (hereinafter referred to as “water surface variation width”) in the mold was 2 mm or less, and the product wrinkle evaluation index was 0 in all cases. On the other hand, in Comparative Examples 1 to 3, the hot-water surface fluctuation widths were all greater than 2 mm, and the product wrinkle evaluation index was not 0.

  From the above results, when the opening of the sliding gate device is 80% or less, simply installing a refractory structure in the tundish and giving a swirling flow to the molten steel will reduce the fluctuation level of the molten metal surface to 2 mm or less. It cannot be suppressed, and it can be seen that the fluctuation width of the molten metal surface can be suppressed to 2 mm or less by using electromagnetic stirring together. It has been clarified that the continuous casting method of the present invention can stabilize the flow of molten steel in the mold and improve the product quality.

  With the above continuous casting apparatus, continuous casting was performed under high throughput conditions where the throughput was 4.3 t / min or more and 4.8 t / min or less. This throughput corresponds to a range in which the opening of the sliding gate device is 80% or more and 90% or less. In this case, the product evaluation index could be set to 0 without performing electromagnetic stirring on the molten steel in the mold.

1: continuous casting device 2: tundish 3: mold, 4: immersion nozzle,
5: refractory structure, 5A: outer cylinder, 5B: inner cylinder, 5h: side hole 6: sliding gate device, 7: core, M: molten steel

Claims (1)

A continuous metal casting method using a continuous casting apparatus configured to supply molten metal from a discharge hole of the immersion nozzle into a mold through an immersion nozzle from a tundish,
Above the immersion nozzle in the tundish, a refractory structure having a double structure including an outer cylinder and an inner cylinder is provided,
The outer cylinder has a cylindrical shape, a conical shape or a truncated cone shape, and a side hole is formed,
The lower end of the outer cylinder is in contact with the bottom of the tundish, and there is a gap of 10 to 300 mm in the vertical direction between the lower end of the inner cylinder and the bottom of the tundish or the upper end of the immersion nozzle. And
The side hole of the outer cylinder has the center of the outlet opening of the side hole at the intersection of an imaginary line extending radially from the axis of the refractory structure and the inner surface of the outer cylinder,
In the exit opening, a central axis of the side hole is inclined with respect to the imaginary line in a horizontal plane,
The immersion nozzle is provided with a sliding gate device that adjusts the flow rate of the molten metal flowing from the tundish to the mold by changing the opening that is a value corresponding to the cross-sectional area of the flow path of the molten metal. And
An electromagnetic stirrer is disposed on the side of the mold,
The continuous casting method is
The molten metal outside the refractory structure in the tundish is caused to flow into the refractory structure through the side holes, thereby allowing the molten metal to move around the axis of the outer cylinder. Providing a swirling flow of
When the opening degree of the sliding gate device is 70 to 90%, the molten metal contained in the height position of the discharge hole of the immersion nozzle is horizontally moved in the mold using the electromagnetic stirring device. And a step of suppressing the fluctuation of the molten metal surface height of the molten metal in the mold measured by using an eddy current sensor to 2 mm or less.

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