JP7617415B2 - Continuous casting method for carbon steel slabs - Google Patents

Description

The present invention relates to a method for continuously casting carbon steel slabs having a rectangular cross section.

In a continuous casting machine that continuously casts carbon steel slabs (hereinafter referred to as slabs) having a rectangular cross section, molten steel temporarily stored in a tundish is poured from above into a mold through an immersion nozzle, where the slab, whose outer circumferential surface has been cooled and solidified, is withdrawn from the bottom end of the mold, thereby continuously casting the slab. The part of the slab whose outer circumferential surface has solidified is called the solidified shell. The slab withdrawn from the mold is transported while being cooled in the continuous casting machine, and is cut to the desired size at the end of the continuous casting machine.
The submerged nozzle is formed with a pair of discharge holes on both sides of the long side of the mold, each opening toward the short side walls, and is configured to discharge molten steel toward the pair of short side walls.

In addition, the slab that is withdrawn from below the mold is curved after being drawn downward and then drawn horizontally. The slab is configured to completely solidify in the horizontal band along which the continuous cast slab is drawn horizontally.
Here, the slab that is pulled out of the mold is not completely solidified when it leaves the mold and has an unsolidified part inside. Therefore, there is a risk of the slab deforming in a bulging manner due to the static pressure of the molten steel in the mold. For this reason, the horizontal strip is configured to reduce the slab by a pair of reduction rolls, one above the other.

In the above-mentioned continuous casting machine, if inclusions accumulate inside the submerged nozzle and cause clogging, it becomes impossible to stably supply molten steel, and casting cannot be performed stably for a long period of time. Therefore, in order to prevent clogging of the submerged nozzle, a non-oxidizing gas such as argon gas is conventionally blown into the molten steel being supplied into the mold.
In addition, casting powder is supplied above the molten steel in the mold to maintain the temperature of the molten steel and to ensure lubrication between the solidified shell and the long and short side walls.
At this time, if bubbles of the argon gas blown in and the casting powder are trapped in the solidified shell growing in the mold, surface defects will occur in the cast piece.

In addition, air bubbles, casting powder, inclusions, etc. may be mixed into the mold at a lower position by the discharge flow from the submerged entry nozzle. In this case, these air bubbles, casting powder, inclusions, etc. are captured at a position where the solidified shell has fully grown, and become a cause of internal defects of the cast slab.
As a means for preventing the above-mentioned surface defects and internal defects, for example, Patent Documents 1 to 3 propose a continuous casting machine equipped with an electromagnetic stirring device above the mold and an electromagnetic brake device below the mold.

The electromagnetic stirrer applies an electromagnetic force to the molten steel in the mold, thereby imparting a swirling flow to the molten steel in the mold.
When the molten steel in the mold has a swirling flow, the molten steel near the interface with the solidified shell has a velocity boundary layer, and the flow velocity becomes slower as it approaches the interface with the solidified shell. Therefore, the bubbles and casting powder present in this velocity boundary layer are subjected to a force in a direction away from the interface with the solidified shell, and have a velocity component in a direction away from the interface with the solidified shell. The faster the flow velocity of the molten steel, the larger the velocity component of the bubbles and casting powder moving away from the interface with the solidified shell. Based on this principle, in a continuous casting machine equipped with an electromagnetic stirrer, it is possible to prevent the bubbles and casting powder in the molten steel in the mold from being captured by the solidified shell, and improve the surface quality.

The electromagnetic brake device applies a braking force to the discharge flow of molten steel ejected from the submerged entry nozzle.
Here, the discharge flow from the submerged nozzle collides with the short side wall of the mold, forming an upward ascending flow and a downward descending flow. The electromagnetic brake device weakens the momentum of the discharge flow, thereby weakening the momentum of the ascending flow and suppressing fluctuations in the molten steel surface. In addition, the force of the discharge flow colliding with the solidified shell is also weakened, which has the effect of suppressing breakouts caused by remelting of the solidified shell. Thus, electromagnetic brake devices are often used for high-speed stable casting. Furthermore, the electromagnetic brake device suppresses the flow rate of the descending flow formed by the discharge flow, promoting the floating and separation of impurities in the molten steel, making it possible to improve the internal quality of the cast slab.

Japanese Patent Application Publication No. 11-156502 Japanese Patent Application Publication No. 05-154623 JP 2020-078815 A

Meanwhile, mass production of general-purpose carbon steel for home appliances, building materials, automobiles, etc. is required, and it is necessary to efficiently produce cast pieces in continuous casting. Increasing the molten steel throughput is considered as a means for improving production efficiency. Here, the molten steel throughput is the mass of cast pieces cast per unit time, and the molten steel throughput (ton/min) in ton/min is calculated by mold thickness (mm) × mold width (mm) × casting speed (mm/min) × molten steel density (7.0 × 10 ^-6 kg/mm ³ )/1000 (kg/ton). Therefore, in order to increase the molten steel throughput, for example, the casting speed may be increased. The casting speed is defined as the length of the cast pieces cast per unit time.

On the other hand, carbon steel with a carbon content of 0.07% to 0.3% by mass falls within the range in which a peritectic reaction occurs, in which the δ phase reacts with the liquid phase to produce the γ phase during the cooling process from molten steel. In particular, carbon steel with a carbon content of 0.1% to 0.18% by mass is called hypoperitectic steel, and the transformation from the δ phase to the γ phase occurs at a relatively high temperature, so shell deformation due to strain during transformation becomes significant, promoting slight uneven solidification and leading to cracks on the surface of the cast slab, or shell deformation caused by uneven solidification can cause the solidified shell to detach from the mold, resulting in insufficient heat removal and the occurrence of remelting defects.

When such steel types are cast at a high throughput of, for example, 3 tons/min or more, the flow rate of molten steel discharged from the discharge hole of the submerged nozzle increases, and collision with the solidified shell causes uneven solidification to become even more pronounced, resulting in frequent remelting defects, or, even if remelting does not occur, a shell of uneven thickness is formed, which causes frequent surface cracks. Furthermore, in high throughput areas, the reverse upward flow toward the molten metal surface after the discharge flow collides with the short side also increases, and this upward flow causes significant fluctuations in the molten metal surface and temperature. Fluctuations in the molten metal surface and temperature cause large variations in the solidification start point in the mold, leading to uneven solidification that causes surface cracks and remelting of the shell.

Furthermore, depending on the destination of the carbon steel product, cracks may occur during product molding due to a split in the center of the thickness of the slab. Splits are internal defects that occur in the final solidification part at the center of the thickness of the slab, and are a type of central segregation that occurs linearly along the width direction of the slab. This defect is observed as a streak pattern when observing the cross section of the slab, and if the degree is severe, it may open up even at the slab stage. Splits remain as central segregation even after the rolling process, and for example, when the reduction rate of the product's drawing process increases, cracks occur due to a decrease in ductility of the segregated part, which becomes a problem. This split is particularly likely to occur in the high throughput range of 3 ton/min or more, when the slab temperature rises or the solidified shell thickness becomes thin, which increases the amount of bulging and worsens the central segregation. Splits are also likely to occur when there is a large deviation in the final solidification position in the width direction of the slab. This deviation in the final solidification position in the width direction tends to increase under high throughput conditions.

In the above-mentioned Patent Documents 1 and 2, the static magnetic field of the electromagnetic brake device is one-pole in both cases, and when the braking force due to the static magnetic field is increased, an excessive braking force acts on the discharge hole, generating an upward flow in the vicinity of the nozzle, which causes large fluctuations in the molten metal level in the high throughput region, and there is a risk of surface cracks occurring.
In addition, in Patent Document 3, the molten steel flow can be controlled by the alternating magnetic field of an electromagnetic stirring device and the multiple divided static magnetic fields of an electromagnetic braking device, and therefore it is possible to suppress surface cracks and remelting defects by optimizing the conditions of these electromagnetic tools. However, there is a risk of cracking into two pieces when casting under high throughput conditions.

The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a method for manufacturing carbon steel slabs that can suppress the occurrence of surface defects and splitting of the slab even when the slab is cast under high throughput conditions, and that can stably and continuously cast steel slabs with excellent surface and internal quality.

In order to solve the above problems, the present inventors have conducted extensive research as described below.
One possible measure to suppress the increase in the discharge flow rate that accompanies an increase in throughput is to increase the discharge hole area of the submerged nozzle to reduce the linear flow rate. On the other hand, if the discharge hole area is increased, the throughput becomes smaller relative to the discharge hole area in the process of increasing the throughput, which may cause the discharge hole pressure to become unstable, resulting in a biased discharge flow or negative pressure at the discharge hole, resulting in a deterioration in quality. Furthermore, a large opening area may reduce the nozzle strength, which places restrictions on the nozzle design.

Therefore, in order to reduce the deterioration of surface quality due to surface cracks in slabs at molten steel throughputs of 3.00 tons/min or more and 6.50 tons/min or less, we considered using electromagnetic stirring and electromagnetic brakes in combination to suppress the discharge flow and fluctuations in the molten metal surface in the mold. In addition, we considered preventing the occurrence of splits in the center of the thickness of the slab by appropriately setting the roll spacing in the thickness direction of the slab in the horizontal band of the continuous casting machine.

The method for continuous casting of carbon steel slabs according to the present invention is a method for continuous casting of carbon steel slabs using a continuous casting machine having a curved band for curving a carbon steel slab having a rectangular cross section drawn from below a mold, and a horizontal band for transporting the carbon steel slab in a horizontal direction, the carbon steel having a chemical composition of C: 0.07% by mass to 0.3% by mass, Mn: 0.01% by mass to 1.5% by mass, Si: 0.001% by mass to 0.3% by mass, P: 0.07% by mass to 0.02% by mass to 0.3% by mass, S: 0.02% by mass to 0.3% by mass, the balance being Fe and impurities, the molten steel throughput is within the range of 3.00 ton/min to 6.50 ton/min, and the upper long side surface of the mold is An alternating magnetic field is applied to the molten steel in the mold using an electromagnetic stirring core installed on the outside, and a static magnetic field is applied to the molten steel using an electromagnetic brake core installed on the outside of the long side surface of the lower part of the mold and having multiple and different magnetic poles facing the long side surface, the magnetic flux density of the alternating magnetic field by the electromagnetic stirring core is 0.02 T to 0.15 T, and the magnetic flux density of the static magnetic field by the electromagnetic brake core is 0.15 T to 0.40 T, and in the horizontal band, the carbon steel cast piece is reduced by a pair of upper and lower reduction rolls, and the reduction taper amount in the horizontal band is within the range of 0.10 mm/m to 0.80 mm/m.

In the method for continuous casting of carbon steel slabs having this configuration, a static magnetic field is applied to the molten steel using an electromagnetic brake core that is installed on the outside of the long side face of the lower part of the mold and has a plurality of different magnetic poles facing the long side face, and the magnetic flux density of the AC magnetic field generated by the electromagnetic stirring core is set to 0.02 T or more and 0.15 T or less, and the magnetic flux density of the static magnetic field generated by the electromagnetic brake core is set to 0.15 T or more and 0.40 T or less. Therefore, it is possible to appropriately control the flow of molten steel in the mold and to suppress surface cracks and remelting defects of the carbon steel slab.
Furthermore, since the rolling taper amount in the horizontal band is within the range of 0.10 mm/m or more and 0.80 mm/m or less, even if the magnetic flux density of the static magnetic field generated by the electromagnetic brake core is set to 0.40 T or less, bulging deformation in the horizontal band of the carbon steel cast piece can be suppressed and the occurrence of splitting into two pieces can be sufficiently suppressed.

The present invention provides a method for producing steel slabs that can suppress the occurrence of surface defects and splitting of the slab even when the slab is cast under high throughput conditions, and can stably and continuously cast steel slabs with excellent surface and internal quality.

1 is a schematic explanatory diagram showing an example of a continuous casting machine for carrying out a method for continuously casting a carbon steel slab according to an embodiment of the present invention. 2 is a cross-sectional view of the mold equipment in the continuous casting machine shown in FIG. 1 in the YZ plane. 3 is a cross-sectional view of the mold facility taken along the line AA in FIG. 2. FIG. 4 is a cross-sectional view of the mold facility taken along the line BB of FIG. 3 . 4 is a cross-sectional view of the mold facility taken along the line CC shown in FIG. 3. FIG. 4 is an explanatory diagram showing an electromagnetic force acting on a discharge flow of molten steel by an electromagnetic brake device. FIG. 2 is an explanatory diagram of a roll segment device installed on a horizontal band.

Hereinafter, a method for continuously casting a carbon steel slab according to an embodiment of the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiment.
In addition, in each drawing shown in this specification, the size of some of the components may be exaggerated for the purpose of explanation. The relative sizes of the components shown in each drawing do not necessarily accurately represent the size relationships between the actual components.

FIG. 1 shows an example of a continuous casting machine 1 for carrying out the method for continuously casting a carbon steel slab according to the present embodiment.
1, a continuous casting machine 1 according to this embodiment is an apparatus for continuously casting molten steel 2 using a continuous casting mold 110 to produce a slab 3 having a rectangular cross section. The continuous casting machine 1 includes the mold 110, a ladle 4, a tundish 5, a submerged nozzle 6, a secondary cooling device 7, and a slab cutter 8.

The ladle 4 is a movable container for transporting the molten steel 2 from the outside to the tundish 5. The ladle 4 is disposed above the tundish 5, and the molten steel 2 in the ladle 4 is supplied to the tundish 5.
The tundish 5 is disposed above the mold 110, stores the molten steel 2, and removes inclusions from the molten steel 2.
The submerged nozzle 6 extends downward from the lower end of the tundish 5 toward the mold 110, and its tip is immersed in the molten steel 2 in the mold 110. The submerged nozzle 6 continuously supplies the molten steel 2 from which inclusions have been removed in the tundish 5 into the mold 110.

The mold 110 is a rectangular cylinder corresponding to the width and thickness of the slab 3, and is assembled, for example, by sandwiching a pair of short-side mold plates between a pair of long-side mold plates. The long-side mold plates and short-side mold plates (hereinafter sometimes collectively referred to as mold plates) are, for example, water-cooled copper plates with water channels through which cooling water flows. The mold 110 cools the molten steel 2 that comes into contact with the mold plates to produce the slab 3. As the slab 3 moves downward in the mold 110, the solidification of the internal unsolidified portion 3b progresses, and the thickness of the solidified shell 3a of the outer shell gradually increases. The slab 3 including the solidified shell 3a and unsolidified portion 3b is pulled out from the lower end of the mold 110. The mold 110 may be vibrated.

In the following description, the up-down direction (i.e., the direction in which the cast piece 3 is pulled out of the mold 110) is also referred to as the Z-axis direction. The Z-axis direction is also referred to as the vertical direction. The two directions that are perpendicular to each other in a plane (horizontal plane) perpendicular to the Z-axis direction are also referred to as the X-axis direction and the Y-axis direction, respectively. The X-axis direction is defined as the direction parallel to the long side of the mold 110 in the horizontal plane (i.e., the mold width direction or mold long side direction), and the Y-axis direction is defined as the direction parallel to the short side of the mold 110 in the horizontal plane (i.e., the mold thickness direction or mold short side direction). The direction parallel to the X-Y plane is also referred to as the horizontal direction.

In FIG. 1, an electromagnetic force generating device is not shown to avoid cluttering the drawing, but in this embodiment, an electromagnetic force generating device is installed on the outer surface of the long side mold plate of the mold 110. Continuous casting is performed while driving the electromagnetic force generating device. The electromagnetic force generating device includes an electromagnetic stirring device and an electromagnetic brake device. In this embodiment, continuous casting is performed while driving the electromagnetic force generating device, which enables casting at a higher speed while ensuring the quality of the cast piece. The configuration of the electromagnetic force generating device will be described later with reference to FIG. 2 and FIG. 3.

The secondary cooling device 7 is provided in the secondary cooling zone 9 below the mold 110, and supports and transports the slab 3 pulled out from the lower end of the mold 110 while cooling it. This secondary cooling device 7 has multiple pairs of support rolls (e.g., support rolls 11, pinch rolls 12, guide rolls 13, reduction rolls 31) arranged on both sides of the thickness of the slab 3, and multiple spray nozzles (not shown) that spray cooling water onto the slab 3.

The support rolls provided in the secondary cooling device 7 are arranged in pairs on both sides of the thickness direction of the slab 3, and function as a support and transport means that supports and transports the slab 3. By supporting the slab 3 from both sides in the thickness direction with the support rolls, breakout and bulging of the slab 3 during solidification in the secondary cooling zone 9 can be prevented.

The support rolls 11, pinch rolls 12, guide rolls 13, and reduction rolls 31 form a transport path (pass line) for the cast slab 3 in the secondary cooling zone 9. As shown in FIG. 1, this pass line is vertical just below the mold 110, then curves in a curved shape, and finally becomes horizontal. In the secondary cooling zone 9, the vertical part of the pass line is called the vertical zone 9A, the curved part is called the curved zone 9B, and the horizontal part is called the horizontal zone 9C. A continuous casting machine 1 having such a pass line is called a vertical bending type continuous casting machine 1. Note that the present invention is not limited to the vertical bending type continuous casting machine 1 shown in FIG. 1, but can also be applied to various other continuous casting machines such as a curved type.

The support rolls 11 are non-driven rolls provided in the vertical band 9A directly below the mold 110, and support the slab 3 immediately after it has been pulled out of the mold 110. The slab 3 immediately after it has been pulled out of the mold 110 has a thin solidified shell 3a, and so must be supported at a relatively short interval (roll pitch) to prevent breakout and bulging. For this reason, it is desirable to use small-diameter rolls as the support rolls 11, which allow the roll pitch to be shortened. In the example shown in FIG. 1, three pairs of support rolls 11 made of small-diameter rolls are provided on both sides of the slab 3 in the vertical band 9A, with a relatively narrow roll pitch.

The pinch rolls 12 are driven rolls that are rotated by a driving means such as a motor, and have the function of pulling the slab 3 out of the mold 110. The pinch rolls 12 are arranged at appropriate positions in the vertical band 9A, the curved band 9B, and the horizontal band 9C. The slab 3 is pulled out of the mold 110 by the force transmitted from the pinch rolls 12, and is transported along the pass line. The arrangement of the pinch rolls 12 is not limited to the example shown in FIG. 1, and the arrangement position may be set arbitrarily.

The guide rolls 13 are non-driven rolls provided on the curved band 9B, and support and guide the slab 3 along the pass line. The guide rolls 13 may be arranged with different roll diameters and roll pitches depending on their position on the pass line and whether they are provided on the F surface (Fixed surface, the surface on the lower left in FIG. 1) or the L surface (Loose surface, the surface on the upper right in FIG. 1) of the slab 3.

The reduction rolls 31 are rolls provided in the horizontal band 9C, which support and guide the slab 3 along the pass line and reduce the slab 3 in the thickness direction.
The roll segment device 30 including the reduction rolls 31 arranged in the horizontal band 9C will be described later with reference to FIG.

The billet cutter 8 is located at the end of the horizontal band 9C of the pass line, and cuts the billet 3 transported along the pass line to a specified length. The cut billet 3 in the form of a thick plate is transported to the equipment for the next process by the table rolls 15.

The overall configuration of the continuous casting machine 1 according to this embodiment has been described above with reference to FIG. 1. In this embodiment, an electromagnetic force generating device having the configuration described below is installed in the mold 110, and continuous casting is performed using this electromagnetic force generating device. The configuration of the continuous casting machine 1 other than the electromagnetic force generating device may be the same as that of a general conventional continuous casting machine. Therefore, the configuration of the continuous casting machine 1 is not limited to that shown in the figure, and any configuration may be used as the continuous casting machine 1.

Next, the configuration of the electromagnetic force generating device installed for the above-mentioned casting mold 110 will be described in detail with reference to Figures 2 to 5. Figures 2 to 5 are diagrams showing an example of the configuration of the casting mold equipment 10 according to this embodiment.

Figure 2 is a cross-sectional view of the mold equipment 10 according to this embodiment in the Y-Z plane. Figure 3 is a cross-sectional view of the mold equipment 10 taken along the A-A section shown in Figure 2. Figure 4 is a cross-sectional view of the mold equipment 10 taken along the B-B section shown in Figure 3. Figure 5 is a cross-sectional view of the mold equipment 10 taken along the C-C section shown in Figure 3. Note that since the mold equipment 10 has a symmetrical configuration with respect to the center of the mold 110 in the Y-axis direction, Figures 2, 4, and 5 only show the portion corresponding to one of the long side mold plates 111. To facilitate understanding, Figures 2, 4, and 5 also show the molten steel 2 in the mold 110.

Referring to Figures 2 to 5, the casting equipment 10 according to this embodiment is configured by installing two water boxes 130, 140 and an electromagnetic force generating device 170 on the outer surface (i.e., the outside of the long side surface) of the long side mold plate 111 of the casting mold 110 via a backup plate 121.

As described above, the mold 110 is assembled by sandwiching a pair of short side mold plates 112 between a pair of long side mold plates 111. The mold plates 111 and 112 are made of copper plates. However, this embodiment is not limited to this example, and the mold plates 111 and 112 may be made of various materials that are generally used as molds for continuous casting machines.

Here, in this embodiment, the subject is continuous casting of a slab 3 of carbon steel with a carbon content of 0.07% by mass or more and 0.3% by mass or less, and the slab size is, for example, about 800 to 2300 mm in width (i.e., length in the X-axis direction), or about 1000 to 1800 mm, and about 150 to 300 mm in thickness (i.e., length in the Y-axis direction), or about 200 to 270 mm. In other words, the mold plates 111 and 112 also have a size corresponding to the slab size. That is, the long side mold plate 111 has a width in the X-axis direction that is at least longer than the width of the slab 3 (e.g., 800 to 2300 mm), and the short side mold plate 112 has a width in the Y-axis direction that is approximately the same as the thickness of the slab 3 (e.g., 200 to 300 mm). Of course, the slab size is not limited to this example.

In this embodiment, in order to more effectively obtain the effect of improving the quality of the slab 3 by the electromagnetic force generator 170, it is preferable to configure the mold 110 so that the length in the Z-axis direction is as long as possible. It is generally known that as solidification of the molten steel 2 progresses in the mold 110, the slab 3 may move away from the inner wall of the mold 110 due to solidification shrinkage, resulting in insufficient cooling of the slab 3. For this reason, the length of the mold 110 is limited to a maximum of about 1000 mm from the surface of the molten steel.
In this embodiment, taking such circumstances into consideration, it is preferable to make the length of the mold plates 111, 112 in the Z-axis direction sufficiently longer than 1000 mm so that the length from the molten steel surface to the lower end of the mold plates 111, 112 is approximately 1000 mm.

The backup plates 121, 122 are made of, for example, stainless steel, and are provided to cover the outer surfaces of the mold plates 111, 112 in order to reinforce the mold plates 111, 112 of the mold 110. For the sake of distinction, the backup plate 121 provided on the outer surface of the long side mold plate 111 will be referred to as the long side backup plate 121, and the backup plate 122 provided on the outer surface of the short side mold plate 112 will be referred to as the short side backup plate 122.

Since the electromagnetic force generator 170 applies electromagnetic force to the molten steel 2 in the mold 110 via the long side backup plate 121, at least the long side backup plate 121 may be made of a non-magnetic material (e.g., non-magnetic stainless steel, etc.). However, in order to ensure the magnetic flux density of the electromagnetic brake device 160, magnetic soft iron 124 is embedded in the portion of the long side backup plate 121 that faces the end portion 164 of the iron core 162 (hereinafter also referred to as the electromagnetic brake core 162) of the electromagnetic brake device 160 described below.

The long side backup plate 121 is further provided with a pair of backup plates 123 extending in a direction perpendicular to the long side backup plate 121 (i.e., in the Y-axis direction). As shown in Figs. 3 to 5, an electromagnetic force generator 170 is installed between the pair of backup plates 123. In this way, the backup plate 123 can determine the width (i.e., length in the X-axis direction) of the electromagnetic force generator 170 and the installation position in the X-axis direction. In other words, the installation position of the backup plate 123 is determined so that the electromagnetic force generator 170 can apply electromagnetic force to a desired range of the molten steel 2 in the mold 110. Hereinafter, for the sake of distinction, the backup plate 123 is also referred to as the width direction backup plate 123. The width direction backup plate 123 is also formed of, for example, stainless steel, like the backup plates 121 and 122.

The water boxes 130 and 140 store cooling water for cooling the mold 110. In this embodiment, as shown in the figure, one water box 130 is installed in an area a predetermined distance from the upper end of the long side mold plate 111, and the other water box 140 is installed in an area a predetermined distance from the lower end of the long side mold plate 111. By providing the water boxes 130 and 140 at the upper and lower parts of the mold 110, respectively, it is possible to secure a space between the water boxes 130 and 140 to install the electromagnetic force generator 170. Hereinafter, for the sake of distinction, the water box 130 provided at the upper part of the long side mold plate 111 is also referred to as the upper water box 130, and the water box 140 provided at the lower part of the long side mold plate 111 is also referred to as the lower water box 140.

A water channel (not shown) through which cooling water passes is formed inside the long side mold plate 111 or between the long side mold plate 111 and the long side backup plate 121. The water channel extends to the water boxes 130, 140. A pump (not shown) causes cooling water to flow through the water channel from one water box 130, 140 to the other water box 130, 140 (for example, from the lower water box 140 to the upper water box 130). This cools the long side mold plate 111, and the molten steel 2 inside the mold 110 is cooled through the long side mold plate 111. Although not shown, a water box and a water channel are also provided for the short side mold plate 112, and the short side mold plate 112 is cooled by the flow of cooling water.

The electromagnetic force generating device 170 includes an electromagnetic stirring device 150 and an electromagnetic brake device 160. As shown in the figure, the electromagnetic stirring device 150 and the electromagnetic brake device 160 are installed in the space between the water boxes 130 and 140. In this space, the electromagnetic stirring device 150 is installed at the top, and the electromagnetic brake device 160 is installed at the bottom. In other words, the electromagnetic stirring device 150 is installed on the outside of the long side surface of the upper part of the mold, and the electromagnetic brake device 160 is installed on the outside of the long side surface of the lower part of the mold.

The electromagnetic stirring device 150 applies an alternating magnetic field to the molten steel 2 in the mold 110, thereby applying an electromagnetic force to the molten steel 2. The electromagnetic stirring device 150 is driven to apply an electromagnetic force to the molten steel 2 in the width direction (i.e., the X-axis direction) of the long side mold plate 111 on which it is installed. In FIG. 4, the direction of the electromagnetic force applied to the molten steel 2 by the electromagnetic stirring device 150 is shown by a bold arrow as a simulation. Here, the electromagnetic stirring device 150 provided on the long side mold plate 111 (i.e., the long side mold plate 111 facing the long side mold plate 111 shown in the figure), which is not shown in the figure, is driven to apply an electromagnetic force in the width direction of the long side mold plate 111 on which it is installed, in the opposite direction to the direction shown in the figure.

In this way, the pair of electromagnetic stirring devices 150 are driven to generate a stirring flow (swirling flow) in a horizontal plane. The electromagnetic stirring device 150 generates such a stirring flow, which suppresses fluctuations in the molten metal surface. This suppresses entrainment and insufficient inflow of casting powder. Furthermore, since the molten steel 2 at the solidified shell interface flows, a cleaning effect is obtained in which the capture of bubbles and inclusions in the solidified shell 3a is suppressed. This improves the surface quality of the cast slab 3.

The detailed configuration of the electromagnetic stirring device 150 will be described. The electromagnetic stirring device 150 is composed of a case 151, an iron core 152 (hereinafter also referred to as the electromagnetic stirring core 152) stored in the case 151, and a plurality of coils 153 formed by winding a conductor around the electromagnetic stirring core 152.

The case 151 is a hollow member having a substantially rectangular parallelepiped shape. The size of the case 151 can be appropriately determined so that the electromagnetic stirring device 150 can apply electromagnetic force to a desired range of the molten steel 2, that is, so that the coil 153 provided inside can be positioned at an appropriate position relative to the molten steel 2. For example, the width W4 of the case 151 in the X-axis direction, that is, the width W4 of the electromagnetic stirring device 150 in the X-axis direction, is preferably determined to be larger than the width of the slab 3 so that electromagnetic force can be applied to the molten steel 2 in the mold 110 at any position in the X-axis direction. For example, W4 is about 1800 mm to 2500 mm. In addition, in the electromagnetic stirring device 150, since the electromagnetic force is applied to the molten steel 2 from the coil 153 through the side wall of the case 151, it is preferable to use a non-magnetic member that can ensure strength, such as non-magnetic stainless steel or FRP (Fiber Reinforced Plastics), as the material of the case 151.

The electromagnetic stirring core 152 is a solid member having a roughly rectangular parallelepiped shape, and is installed in the case 151 so that its longitudinal direction is roughly parallel to the width direction (i.e., the X-axis direction) of the long side mold plate 111. The electromagnetic stirring core 152 is formed, for example, by laminating electromagnetic steel sheets.

A coil 153 is formed by winding a conductor around the electromagnetic stirring core 152 with the X-axis direction as the central axis. The conductor is, for example, made of copper and has a cross section of 10 mm x 10 mm and an internal cooling water channel with a diameter of about 5 mm. When a current is applied, the conductor is cooled using the cooling water channel. The surface of the conductor is insulated with insulating paper or the like, and it is possible to wind it in layers. For example, one coil 153 is formed by winding the conductor in about 2 to 4 layers. Coils 153 with similar configurations are arranged in parallel at a predetermined interval in the X-axis direction.

An AC power supply (not shown) is connected to each of the coils 153. The AC power supply applies an AC magnetic field to the molten steel 2 in the mold from the electromagnetic stirring core 152. Specifically, by applying a current to the coils 153 so that the phases of the currents in the adjacent coils 153 are appropriately shifted, an electromagnetic force that generates a stirring flow in the molten steel 2 can be applied.
The driving of the AC power supply can be appropriately controlled by a control device (not shown) consisting of a processor or the like operating in accordance with a predetermined program. The control device appropriately controls the amount of current applied to each of the coils 153, the timing of applying the current to each of the coils 153, and the like, and can control the strength of the electromagnetic force applied to the molten steel 2. As the method of driving the AC power supply can be any of various known methods used in general electromagnetic stirring devices, a detailed description thereof will be omitted here.

The width W1 of the electromagnetic stirring core 152 in the X-axis direction can be appropriately determined so that the electromagnetic stirring device 150 can apply electromagnetic force to a desired range of the molten steel 2, i.e., so that the coil 153 can be positioned appropriately relative to the molten steel 2. For example, W1 is about 1800 mm.

The electromagnetic brake device 160 applies an electromagnetic force to the molten steel 2 in the mold 110 by applying a static magnetic field to the molten steel 2. Here, FIG. 6 is a diagram for explaining the direction of the electromagnetic force applied to the molten steel 2 by the electromagnetic brake device 160. FIG. 6 shows a schematic cross section in the X-Z plane of the configuration in the vicinity of the mold 110. Also, FIG. 6 shows the positions of the electromagnetic stirring core 152 and the end 164 of the electromagnetic brake core 162 described later by dashed lines.

As shown in FIG. 6, the submerged nozzle 6 may be provided with a pair of discharge holes at positions facing the narrow side mold plate 112. The molten steel 2 is discharged from these discharge holes into the mold 110. The discharge flow of the molten steel 2 advances toward the narrow side of the mold 110 and collides with the solidified shell 3a formed on the narrow side. The discharge flow then forms an upward flow (reverse discharge flow) in the upward direction (i.e., in the direction in which the molten steel surface is present). The discharge flow that collides with the solidified shell 3a may also form a downward flow in the downward direction (i.e., in the direction in which the cast piece is withdrawn). The electromagnetic brake device 160 is driven to apply an electromagnetic force to the molten steel 2 in a direction that suppresses the flow (discharge flow) of the molten steel 2 from the discharge holes of the submerged nozzle 6. In FIG. 6, the direction of the discharge flow is shown as a simulated thin arrow, and the direction of the electromagnetic force applied to the molten steel 2 by the electromagnetic brake device 160 is shown as a simulated thick arrow.

The electromagnetic brake device 160 generates an electromagnetic force in a direction that suppresses such a discharge flow, thereby softening the impact when the discharge flow collides with the solidified shell 3a. This suppresses remelting of the solidified shell 3a, and ultimately the occurrence of surface defects and breakouts. Furthermore, the momentum of the reverse discharge flow caused by the discharge flow is weakened, suppressing fluctuations in the molten steel 2 surface. This suppresses entrainment and insufficient inflow of casting powder. Furthermore, the downward flow that occurs when the discharge flow collides with the solidified shell 3a is also suppressed, which can promote floating and separation of bubbles and inclusions. As a result, the quality of the cast piece 3 is improved.

The detailed configuration of the electromagnetic brake device 160 will be described. The electromagnetic brake device 160 is composed of a case 161, an electromagnetic brake core 162, a part of which is stored inside the case 161, and a plurality of coils 163 formed by winding a conductor around a portion of the electromagnetic brake core 162 inside the case 161.

The case 161 is a hollow member having a substantially rectangular parallelepiped shape. The size of the case 161 can be appropriately determined so that the electromagnetic brake device 160 can apply electromagnetic force to a desired range of the molten steel 2, that is, so that the coil 163 provided inside can be positioned at an appropriate position relative to the molten steel 2. For example, the width W4 of the case 161 in the X-axis direction, i.e., the width W4 of the electromagnetic brake device 160 in the X-axis direction, is determined to be larger than the width of the slab 3 so that electromagnetic force can be applied to the molten steel 2 in the mold 110 at a desired position in the X-axis direction. In the illustrated example, the width W4 of the case 161 is substantially the same as the width W4 of the case 151. However, this embodiment is not limited to such an example, and the width of the electromagnetic stirring device 150 and the width of the electromagnetic brake device 160 may be different.

In addition, in the electromagnetic brake device 160, electromagnetic force is applied to the molten steel 2 from the coil 163 through the side wall of the case 161, so the case 161, like the case 151, is made of a material that is nonmagnetic and has sufficient strength, such as nonmagnetic stainless steel or FRP.

The electromagnetic brake core 162 is a solid member having an approximately rectangular parallelepiped shape, and is composed of a pair of end portions 164 on which the coil 163 is provided, and a connecting portion 165, which is also a solid member having an approximately rectangular parallelepiped shape and connects the pair of end portions 164. The electromagnetic brake core 162 is configured such that the pair of end portions 164 are provided so as to protrude from the connecting portion 165 in the Y-axis direction toward the long side mold plate 111. The pair of end portions 164 can be provided at positions where it is desired to apply an electromagnetic force to the molten steel 2, that is, positions where the discharge flows from the pair of discharge holes of the submerged nozzle 6 each pass through an area where a magnetic field is applied by the coil 163 (see also FIG. 6). The electromagnetic brake core 162 is formed, for example, by stacking electromagnetic steel sheets.

A coil 163 is formed by winding a conductor around the end 164 of the electromagnetic brake core 162 with the Y-axis direction as the central axis. The structure of the coil 163 is similar to the coil 153 of the electromagnetic stirring device 150 described above. For each end 164, multiple coils 163 are provided in parallel at a predetermined interval in the Y-axis direction.

A DC power source (not shown) is connected to each of the coils 163. By applying a DC current to each coil 163 by the DC power source, an electromagnetic force that weakens the momentum of the discharge flow can be applied to the molten steel 2. That is, each end 164 becomes a magnetic pole, one end 164 becomes a north pole, and the other end 164 becomes a south pole. Therefore, two different magnetic poles (a total of two, a north pole and a south pole in this example) face the long side surface. Furthermore, the submerged nozzle 6 is arranged at a position facing the space 164a between the two magnetic poles (see FIG. 6). In addition, a similar electromagnetic brake core 162 is arranged on the other long side, so that a total of two pairs of magnetic poles are arranged. In addition, the drive of the DC power source can be appropriately controlled by a control device (not shown) consisting of a processor or the like operating according to a predetermined program. The control device appropriately controls the amount of current applied to each coil 163, and the strength of the electromagnetic force applied to the molten steel 2 can be controlled. The DC power supply can be driven by any of a variety of known methods used in typical electromagnetic brake devices, so detailed explanations will be omitted here.

The width W0 of the electromagnetic brake core 162 in the X-axis direction, the width W2 of the ends 164 in the X-axis direction, and the distance W3 between the ends 164 in the X-axis direction can be appropriately determined so that the electromagnetic stirring device 150 can apply electromagnetic force to a desired range of the molten steel 2, i.e., so that the coil 163 can be positioned appropriately relative to the molten steel 2. For example, W0 may be approximately 1600 mm, W2 may be approximately 500 mm, and W3 may be approximately 350 mm.

Here, for example, the electromagnetic brake device disclosed in Patent Documents 1 and 2 has a single magnetic pole. The single magnetic pole is provided on the outside of the long side surface of the mold, and the magnetic pole extends across both ends in the width direction (i.e., the length direction of the long side surface). The magnetic field generated from such a magnetic pole has a characteristic that the magnetic flux density is maximum at the center part of the magnetic pole in the width direction. Therefore, when the magnetic flux density of the magnetic field generated from the electromagnetic brake device is increased, the magnetic flux density near the submerged nozzle becomes extremely high. For this reason, the braking force due to the static magnetic field becomes excessive near the discharge hole of the submerged nozzle, and the discharge flow tends to become an upward flow near the nozzle without spreading in the width direction. Such an upward flow causes large fluctuations in the molten metal surface. And such fluctuations in the molten metal surface can cause the entrainment of casting powder. Note that even if the electromagnetic brake device disclosed in Patent Documents 1 and 2 is used in combination with the electromagnetic stirring device 150 according to this embodiment, such problems are not sufficiently solved. The upward flow generated by the electromagnetic brake device disclosed in Patent Documents 1 and 2 is too strong, so even if electromagnetic stirring is performed using the electromagnetic stirring device 150, the effect of the electromagnetic stirring is canceled out by the large fluctuations in the molten metal surface caused by the upward flow.

In contrast, in this embodiment, the electromagnetic brake device 160 is configured to have two ends 164, i.e., two magnetic poles, as described above. Furthermore, the submerged nozzle 6 is arranged at a position facing the space 164a between the two magnetic poles. With this configuration, for example, when the electromagnetic brake device 160 is driven, these two magnetic poles function as N and S poles, respectively, and the application of current to the coil 163 can be controlled by the above-mentioned control device so that the magnetic flux density in the region approximately near the center in the width direction (i.e., the X-axis direction) of the mold 110 is lower than the magnetic flux density in other regions. Therefore, the braking force due to the static magnetic field can be reduced near the discharge hole of the submerged nozzle 6, so that the generation of an excessive upward flow can be suppressed. As a result, the effect of electromagnetic stirring is less likely to be impaired by the electromagnetic brake, and the effect of the electromagnetic brake and the effect of the electromagnetic stirring can be further enhanced. Therefore, it is possible to respond to a wider range of casting conditions.

In the illustrated configuration example, the electromagnetic brake device 160 is configured to have two magnetic poles, but the present embodiment is not limited to this example. The electromagnetic brake device 160 may be configured to have three or more end portions 164 and to have three or more different types of magnetic poles. In this case, the amount of current applied to the coil 163 of each end portion 164 is appropriately adjusted, respectively, thereby making it possible to control in more detail the application of the electromagnetic force to the molten steel 2 related to the electromagnetic brake.
That is, the number of magnetic poles may be appropriately adjusted depending on the level of the upward flow generated near the submerged nozzle 6, and there is no particular upper limit. Even if there are three or more magnetic poles, it is preferable to place the submerged nozzle 6 at a position facing the space between the magnetic poles. In this case, the magnetic poles include at least one N pole and one S pole.

The installation position and size of the electromagnetic force generating device are not particularly limited, and for example, the installation position disclosed in Patent Document 3 may be applied to this embodiment. For example, the installation position and size of the electromagnetic force generating device may be set so that the height H1 (the length in the Z-axis direction from the upper end to the lower end of the electromagnetic stirring core 152) (mm) and the height H2 (the length in the Z-axis direction from the upper end to the lower end of the electromagnetic brake core 162) (mm) satisfy the following formula (1) and further satisfy formula (2) or (3).
(1) Formula: H1+H2≦500mm
(2) Formula: 0.80<H1/H2<2.33
(3) Formula: 1.00<H1/H2<2.00

Next, the roll segment device 30 including the reduction rolls 31 disposed in the horizontal band 9C will be described with reference to FIG.
This roll segment device 30 has an upper frame 33 arranged on the upper surface side of the cast 3 and having a plurality of reduction rolls 31 arranged at intervals F in the drawing direction of the cast 3, a lower frame 34 arranged on the lower surface side of the cast 3 and having a plurality of reduction rolls 31 arranged at intervals F in the drawing direction F, and a plurality of support members 35, 36 connecting the upper frame 33 and the lower frame 34.
In addition, in the horizontal band 9C, a plurality of the above-mentioned roll segment devices 30 are arranged along the drawing direction F of the cast slab 3.

Here, the roll gap of the reduction rolls 31 in the thickness direction of the slab is configured to become gradually smaller from the entry side to the exit side of the horizontal band 9C.
The reduction taper amount (mm/m) is determined by the following formula (4) based on the roll spacing D1 (mm) in the thickness direction of the slab of the segment roll located on the entry side of the horizontal band 9C, the roll spacing D2 (mm) in the thickness direction of the slab of the segment roll located on the exit side of the horizontal band 9C, the distance L1 (m) from the meniscus of the segment roll located on the entry side of the horizontal band 9C, and the distance L2 (m) from the meniscus of the segment roll located on the exit side of the horizontal band 9C.
(4) Formula: reduction taper amount = (D1 - D2) / (L2 - L1)
In this embodiment, the amount of reduction taper is set within the range of 0.10 mm/m to 0.80 mm/m.
The roll gap in the thickness direction was set for each horizontal strip segment, and a constant reduction taper was applied so that the thickness became smaller along the casting direction. The roll gap in the thickness direction may be decreased for each segment, or may be decreased for each roll.

Next, a method for continuously casting a carbon steel slab using the above-mentioned continuous casting machine 1 will be described. In the method for continuously casting a carbon steel slab according to this embodiment, a carbon steel slab 3 having a chemical composition of C: 0.07% by mass to 0.3% by mass, Mn: 0.01% by mass to 1.5% by mass, Si: 0.001% by mass to 0.3% by mass, P: 0.07% by mass, S: 0.02% by mass or less, the balance being Fe and impurities, is continuously cast at a molten steel throughput of 3.00 tons/min to 6.50 tons/min. The carbon steel slab 3 may further contain other elements depending on the application of the carbon steel, etc.

Here, the mass % of each component means the mass % with respect to the mass of the carbon steel slab 3 (more strictly, the total mass of the sample of the carbon steel slab 3). The impurities include unavoidable impurities, that is, components that are inevitably mixed into the molten steel 2 during the production process of the molten steel 2. The composition of the carbon steel slab 3 may be measured by a general analytical method. For example, the steel components may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C may be measured using a combustion-infrared absorption method.
The reasons for specifying the contents of each element in carbon steel as described above are as follows.

(C: 0.07% by mass or more and 0.3% by mass or less)
In order to ensure the strength of carbon steel, the C content is set to 0.07 mass% or more. On the other hand, if the C content is high, the formability of carbon steel decreases, so the C content is set to 0.3 mass% or less. do.

(Mn: 0.01% by mass or more and 1.5% by mass or less)
Since Mn has the effect of strengthening carbon steel, Mn is added in an amount of 0.01 mass% or more depending on the required strength of the carbon steel. On the other hand, if the Mn content is too high, the formability decreases, so the Mn content is 1.5 mass % or less.

(Si: 0.001% by mass or more and 0.3% by mass or less)
Since Si has the effect of strengthening carbon steel, Si is added in an amount of 0.002 mass% or more depending on the required strength of the carbon steel. On the other hand, if the Si content is too high, the formability decreases, so the Si content is reduced by 0.3% by mass.

(P: 0.07% by mass or less)
Since P has the effect of embrittling carbon steel, its content is limited to 0.07 mass % or less.

(S: 0.02% by mass or less)
S has the effect of embrittling carbon steel, so its content is limited to 0.02 mass % or less.

Next, the reason for specifying the throughput will be explained below. The throughput (ton/min) is calculated by mold thickness (mm)×mold width (mm)×casting speed (mm/min)×molten steel density (7.0×10 ⁻⁶ kg/mm ³ )/1000 (kg/ton). The impact strength of the molten steel flow against the short side of the slab and the fluctuation of the molten steel level are greatly dependent on the molten steel flow rate from the discharge hole of the submerged nozzle 6, and since this is proportional to the throughput, it is appropriate to use this indicator.
If the throughput is less than 3.00 ton/min, the flow rate of the molten steel from the discharge hole of the submerged nozzle 6 is sufficiently small, and good quality slabs can be cast without applying electromagnetic force. On the other hand, in the range where the throughput exceeds 6.50 ton/min, the flow rate of the molten steel is extremely high, and goes beyond the control range of the electromagnetic force equipment, making it difficult to stably obtain good quality slabs (current technology requires very expensive separate equipment, which is not worth the cost).

When the carbon steel slab having the above-mentioned chemical composition is continuously cast under high throughput conditions of 3.00 tons/min to 6.50 tons/min, the molten steel flow in the mold 110 is controlled by the electromagnetic stirring device 150 and the electromagnetic brake device 160 .
Unless otherwise specified, the unit "T" for the electromagnetic stirring strength and electromagnetic braking strength described below means Tesla.
In this embodiment, the electromagnetic stirring strength and the electromagnetic brake strength were determined by measuring the relationship between the application conditions (current, frequency) and the magnetic field strength at the center of the electromagnetic stirring core and the electromagnetic brake core when they were operated in a mold in which no molten steel was present in a cold state. Note that, since the electromagnetic stirring strength is an AC magnetic field, the maximum value of the time change in magnetic flux density was taken as the magnetic field strength.

The magnetic flux density of the AC magnetic field generated by the electromagnetic stirring device 150 (i.e., the electromagnetic stirring core 152) (hereinafter, also simply referred to as "electromagnetic stirring intensity") is set to 0.02T or more and 0.15T or less.
When the electromagnetic stirring strength is less than 0.02 T, the molten steel temperature is not sufficiently uniformized by stirring, and the cast piece is likely to crack due to non-uniform solidification. On the other hand, when the electromagnetic stirring strength is more than 0.15 T, the stirring flow becomes excessive, which causes the molten steel surface to fluctuate significantly, and the cast piece is likely to crack due to non-uniform solidification. For this reason, the electromagnetic stirring strength is set to 0.02 T or more and 0.15 T or less.
In order to more stably suppress the occurrence of cracks on the surface of the cast slab, the electromagnetic stirring strength is preferably set to 0.05 T or more. Also, the electromagnetic stirring strength is preferably set to 0.10 T or less.

On the other hand, the magnetic flux density of the static magnetic field generated by the electromagnetic brake device 160 (i.e., the electromagnetic brake core 162) (hereinafter, also simply referred to as "electromagnetic brake strength") is set to 0.15T or more and 0.40T or less.
If the electromagnetic brake strength is less than 0.15T, the discharge flow is not sufficiently braked, and the rising flow generated at the short side causes fluctuations in the molten steel level, deteriorating the surface quality. Furthermore, if the degree of this fluctuation is poor, remelting defects occur. On the other hand, if the electromagnetic brake strength is more than 0.40T, the molten steel concentrates near the nozzle, causing large fluctuations in the molten steel level and resulting in surface cracks. For this reason, the electromagnetic brake strength is set to 0.15T or more and 0.40T or less.
In order to more stably suppress the occurrence of cracks on the surface of the cast slab, the electromagnetic brake strength is preferably set to 0.2 T or more.

In addition, when a conventional one-pole (single-pole) electromagnetic brake core is installed, when the electromagnetic brake strength is 0.2 T or more, the braking force near the submerged nozzle 6 becomes excessively large, causing large fluctuations in the molten metal surface. Therefore, when the electromagnetic brake device 160 has multiple magnetic poles as in this embodiment, it is possible to improve the quality of the slab over a wider range of magnetic flux density than with a conventional single-pole electromagnetic brake device.

Next, the relationship between the electromagnetic stirring strength and the electromagnetic brake strength and the occurrence of lamination at the center of the thickness of the carbon steel slab 3 will be described.
The splitting into two pieces is promoted by an increase in the deviation of the final solidification position in the width direction of the slab 3. On the other hand, when the flow in the mold 110, particularly the molten steel flow from the discharge hole of the submerged nozzle 6, is strong, a deviation of the final solidification position in the width direction is formed. Here, by making the electromagnetic brake strength larger than 0.40 T, the deviation of the final solidification position in the width direction is sufficiently small, and the splitting into two pieces can be prevented. On the other hand, the electromagnetic stirring strength had almost no effect on the deviation of the final solidification position. Incidentally, as described above, in order to improve the surface quality of the slab 3, the electromagnetic brake strength needs to be 0.15 T or more and 0.40 T or less, and it is found that it is difficult to improve both the surface quality and the splitting into two pieces by controlling the electromagnetic brake strength alone.

In this embodiment, the horizontal band 9C is configured to reduce the slab 3 using a pair of upper and lower reduction rolls, and the reduction taper amount in the horizontal band 9C is set within the range of 0.10 mm/m or more and 0.80 mm/m or less.
If the reduction taper amount is less than 0.10 mm/m, the slab 3 cannot be sufficiently reduced, causing splitting into two pieces. On the other hand, if the reduction taper amount is more than 0.80 mm/m, the reduction force and the drawing force become excessively large, requiring a large investment in equipment.
Therefore, the appropriate taper amount for the reduction in the horizontal band 9C is 0.10 mm/m or more and 0.80 mm/m or less. By setting the taper amount for the reduction in the horizontal band 9C in this range, the occurrence of splitting in two can be suppressed even if the electromagnetic brake strength is set to 0.15 T or more and 0.40 T or less, which ensures surface quality. On the other hand, if the electromagnetic brake strength is less than 0.15 T, in addition to the deterioration of surface quality, the deviation of the final solidification position becomes large, so that even if the taper amount for the reduction is 0.10 mm/m or more and 0.80 mm/m or less, unacceptable splitting may occur.
In order to more stably suppress splitting into two pieces, the amount of reduction taper in the horizontal band 9C is preferably 0.30 mm/m or more.

According to the method for continuous casting of carbon steel slabs of this embodiment configured as described above, a static magnetic field is applied to the molten steel 2 by using the electromagnetic brake core 162 which is installed on the outside of the long side face of the lower part of the mold 110 and has a plurality of different magnetic poles facing the long side face, and the magnetic flux density of the AC magnetic field generated by the electromagnetic stirring core 152 is set to 0.02 T or more and 0.15 T or less, and the magnetic flux density of the static magnetic field generated by the electromagnetic brake core 162 is set to 0.15 T or more and 0.40 T or less. Therefore, the flow of molten steel in the mold 110 can be appropriately controlled and surface cracks and remelting defects in the slab 3 can be suppressed.
Furthermore, since the rolling taper amount in the horizontal band 9C is within the range of 0.10 mm/m or more and 0.80 mm/m or less, even if the magnetic flux density of the static magnetic field generated by the electromagnetic brake core 162 is set to 0.40 T or less, bulging deformation in the horizontal band 9C of the cast piece 3 can be suppressed and the occurrence of splitting into two pieces can be sufficiently suppressed.

The method for continuous casting of carbon steel slabs according to an embodiment of the present invention has been specifically described above. However, the present invention is not limited thereto and can be appropriately modified without departing from the technical spirit of the invention.
For example, in the present embodiment, the continuous casting machine shown in FIG. 1 is used, but the present invention is not limited to this, and continuous casting machines of other configurations may be used.

In this example, an actual machine test was carried out on a continuous casting method for carbon steel slabs. In this actual machine test, the electromagnetic force generator according to the present embodiment described with reference to Figures 2 to 5 was installed in a continuous casting machine actually used in operation. In Comparative Example 2, an electromagnetic brake core with a single pole was used.
The mold was made of copper and water-cooled (water-cooled copper mold), and had a rectangular cross section and a length of 900 mm. The continuous casting machine was of the vertical bending type.

Then, continuous casting of carbon steel slabs was performed by changing various parameters such as molten steel throughput, electromagnetic stirring strength, and electromagnetic brake strength. Here, the mold was a copper water-cooled type (water-cooled copper mold) with a rectangular cross section and a height (length from the upper end to the lower end of the mold) of 900 mm. The type of continuous casting machine was a vertical bending type.
Also, molten steel having a C content of 0.11 mass%, a Si content of 0.05 mass%, a Mn content of 1.0 mass%, a P content of 0.03 mass%, a S content of 0.002 mass%, and the balance being Fe and impurities was used.
The specific water amount for secondary cooling was 1.5 to 2.5 L/kg-steel. The specific gravity for calculating the molten steel throughput was 7.0×10 ⁻⁶ kg/mm ^3. The amount of argon gas blown from the submerged nozzle was 7 NL/min.

The upper end of the electromagnetic stirrer core was 100 mm from the upper end of the mold, and the height (casting direction distance) H1 from the upper end to the lower end of the electromagnetic stirrer core was 250 mm. The upper end of the electromagnetic brake core was 500 mm from the upper end of the mold, and the height H2 from the upper end to the lower end of the electromagnetic brake core was 200 mm. The mold was vibrated up and down with a constant period and amplitude.
A mist spray of mixed air and water was used as a secondary cooling spray for the slab in the continuous casting machine. The horizontal band was located 22 to 42 m from the meniscus. The roll spacing of the reduction rolls in the thickness direction in the horizontal band was set for each roll segment of the horizontal band, and a constant reduction taper was applied so that the thickness became smaller along the casting direction.

After casting, the surface of the slab cut with a torch was visually observed, and the presence or absence of surface cracks and remelting defects was recorded. Regarding surface cracks, the following evaluations were given: no cracks were given an "A", cracks less than 10 mm long that would not become rolling defects were given a "B", defects between 10 mm and 50 mm long that required maintenance were given a "C", and defects 10 mm or longer that could not be maintained were given a "D".
Furthermore, the cross section of the slab after torch cutting was observed to check for the presence or absence of cracks in two. If there were no cracks in two, they were rated as "A." If there were very slight cracks of less than 50 mm in length that would not cause a problem during product molding, they were rated as "B." If there were problematic cracks in length of 50 mm or more, they were rated as "D."
A comprehensive evaluation was performed by combining these. A sample that satisfied all of the following criteria was rated as "passed": no remelting defects, surface cracking rated as A or B, and bifurcation rated as A or B. A sample that did not satisfy any of the following criteria was rated as "failed."

(Example 1)
Example 1 is the basis for the following examples and comparative examples. In Example 1, the thickness and width of the slab were 250 mm and 1100 mm, which are typical sizes for continuous casters for thin plates. The casting speed was 1.6 m/min. The steel throughput was 3.08 ton/min.
The electromagnetic stirring strength and electromagnetic brake strength were set based on a flow analysis performed in advance, and the electromagnetic stirring strength was set to 0.02 T and the electromagnetic brake strength to 0.20 T as conditions for suppressing the discharge flow. Electromagnetic stirring was performed by applying an AC magnetic field to the molten steel, and electromagnetic braking was performed by applying a static magnetic field to the molten steel. The number of electromagnetic brake cores (i.e., the number of magnetic poles facing the long side faces) was set to two (two per long side face, i.e., two pairs on both sides) to reduce the static magnetic field strength near the discharge hole. In other words, a magnetic pole pair of N and S poles was arranged on each of the long side faces. The rolling taper amount in the horizontal band was set to 0.15 mm/m.
The operating conditions of Example 1 were within the range of the present embodiment, and no remelting defects, surface cracks, or splitting occurred, resulting in a good overall evaluation of passing.

(Example 2)
In Example 2, the casting speed was increased compared to Example 1, and the electromagnetic stirring and electromagnetic brake strength conditions were the same. The rolling taper amount was also set to 0.15 mm/m, the same as in Example 1.
In Example 2, the molten metal level tends to fluctuate greatly due to the increased throughput, but there was no remelting defect, surface crack, or splitting, and the overall evaluation was pass.

(Example 3)
In Example 3, the width of the slab was increased compared to Example 1. The electromagnetic stirring and electromagnetic brake strengths were both increased within the ranges of the invention in accordance with the increase in throughput. The reduction taper was set to 0.15 mm/m, the same as in Example 1.
Inventive Example 3 also had no remelting defects, surface cracks, or splitting into two, and was therefore generally acceptable.

(Example 4)
In Example 4, the width of the slab and the casting speed were increased compared to Example 1. With the increase in throughput, the electromagnetic stirring strength and the electromagnetic brake strength were both further increased within the range of the present invention. The reduction taper amount was set to 0.15 mm/m, the same as Example 1.
In Example 4, the throughput was high, but there was no occurrence of remelting defects or surface cracks. On the other hand, regarding the splitting in the center, due to the increase in the amount of inter-roll bulging caused by the increase in casting speed, very slight splitting was observed, but it was not to the extent that it affected the product, and the overall evaluation was acceptable.

(Examples 5 and 6)
In Examples 5 and 6, the width of the slab was increased and the casting speed was increased compared to Example 1. The molten steel throughput was 6.35 ton/min, and the electromagnetic stirring strength and the electromagnetic brake strength were increased within the range of the invention. The reduction taper was set to 0.15 mm/m, the same as Example 1.
In Examples 5 and 6, the throughput was high, but no remelting defects or surface cracks occurred. As with Example 4, very slight cracks were observed, but the overall evaluation was acceptable.

(Example 7)
In Example 7, the slab width was made larger and the casting speed was made higher than those in Example 1. The molten steel throughput was 6.35 ton/min, and the electromagnetic stirring strength and electromagnetic brake strength were the same as those in Example 1. The reduction taper amount was set to 0.15 mm/m, the same as in Example 1.
In Example 7, no remelting defects were observed, but very slight surface cracks that would not become defects during rolling were observed. As with Example 4, very slight splitting was observed, but the overall evaluation was acceptable.

(Example 8)
In Example 8, the casting speed, electromagnetic stirring strength, and electromagnetic brake strength were the same as those in Example 6, but the rolling taper was increased to 0.30 mm/m.
In Example 8, remelting defects and surface cracks did not occur, similar to Example 6. On the other hand, the two-piece crack disappeared because the reduction taper amount was increased. The overall evaluation was pass.

(Example 9)
In Example 9, the casting speed, electromagnetic stirring strength, and electromagnetic brake strength were the same as those in Example 11, but the rolling taper amount was reduced to 0.10 mm/m, within the range of the invention.
In Example 9, remelting defects and surface cracks did not occur, similar to Example 1. Regarding splitting, very slight splitting was observed due to the effect of reducing the amount of reduction taper, but it did not affect the product, and the overall evaluation was acceptable.

(Example 10)
In Example 10, the casting speed, electromagnetic stirring strength, and electromagnetic brake strength were the same as those in Example 1, but the rolling taper amount was increased to 0.80 mm/m, within the range of the invention.
In Example 10, remelting defects and surface cracks did not occur, similar to Example 1. Splitting into two also did not occur, similar to Example 1, and the overall evaluation was acceptable.

(Comparative Example 1)
In Comparative Example 1, only the electromagnetic stirring core was installed, and no electromagnetic brake was applied. The rolling taper amount was set to 0.15 mm/m.
In this Comparative Example 1, remelting of the solidified shell occurred due to the discharge flow. Surface cracks also occurred. Although splitting into two pieces did not occur, the overall evaluation was unacceptable.

(Comparative Example 2)
In Comparative Example 2, although an electromagnetic stirring core and an electromagnetic brake core were provided, the electromagnetic brake core was a pair and the static magnetic field in the slab width direction was unipolar. The rolling taper amount was set to 0.15 mm/m.
In this Comparative Example 2, the magnetic field in the width center portion was larger than in the results for the two pairs of electromagnetic brake cores of the present invention, so an upward flow occurred near the nozzle, causing fluctuations in the molten metal surface, and surface cracks requiring repair were observed in this vicinity. On the other hand, no remelting defects or splitting occurred, but the overall evaluation was unsuccessful.

(Comparative Example 3)
In Comparative Example 3, two electromagnetic stirring cores and two electromagnetic brake cores were installed, and the electromagnetic stirring strength was set to 0.01 T, which is outside the range of the present invention. The electromagnetic brake strength was set to 0.20 T, and the rolling taper amount was set to 0.15 mm/m.
In this Comparative Example 3, the electromagnetic stirring strength was low, the stirring force was insufficient, and a sufficient stirring flow was not provided to suppress fluctuations in the molten metal surface and uneven temperature, and deep surface cracks that could not be repaired due to uneven solidification occurred. Although there was no remelting defect or splitting, the overall evaluation was unacceptable.

(Comparative Example 4)
In Comparative Example 4, two electromagnetic stirring cores and two electromagnetic brake cores were installed, and the electromagnetic stirring strength was set to 0.18 T, which is outside the range of the present invention. The electromagnetic brake strength was set to 0.20 T, and the rolling taper amount was set to 0.15 mm/m.
In this Comparative Example 4, the fluctuation of the molten metal surface due to the electromagnetic stirring became large, and deep surface cracks that could not be repaired occurred especially near the corners where the flow rate of the electromagnetic stirring became high. Although there was no remelting defect or splitting, the overall evaluation was unacceptable.

(Comparative Example 5)
In Comparative Example 5, two electromagnetic stirring cores and two electromagnetic brake cores were installed, and the electromagnetic brake strength was set to 0.10 T, which is outside the range of the present invention. The electromagnetic stirring strength was set to 0.05 T, and the rolling taper amount was set to 0.15 mm/m.
In this Comparative Example 5, remelting defects occurred and surface cracks that required repair occurred. Furthermore, although the rolling taper amount was sufficient, the electromagnetic brake strength was 0.10 T, which was a condition that increased the deviation of the final solidification position, so that two-piece cracks occurred and the overall evaluation was unacceptable.

(Comparative Example 6)
In Comparative Example 6, two electromagnetic stirring cores and two electromagnetic brake cores were installed, and the electromagnetic brake strength was set to 0.50 T, which is outside the range of the present invention. The electromagnetic stirring strength was set to 0.05 T, and the rolling taper amount was set to 0.15 mm/m.
In this Comparative Example 6, since the electromagnetic brake strength was large, the molten steel accumulated near the nozzle, which caused large fluctuations in the molten steel level at the width center of the slab, and surface cracks were observed in the vicinity of the width center as in Comparative Example 2. No remelting defects or splitting occurred, but the overall evaluation was unacceptable.

(Comparative Example 7)
In Comparative Example 7, two electromagnetic stirring cores and two electromagnetic brake cores were installed, and the electromagnetic stirring strength and electromagnetic brake strength were the same as those in Invention Example 1. On the other hand, the rolling taper amount was set to 0.05 mm/m, which is smaller than the range of the invention.
In this Comparative Example 7, although there were no remelting defects or surface cracks, the insufficient reduction taper did not completely prevent the splitting caused by bulging, and splitting occurred to a degree that affected the product. Therefore, the overall evaluation was unsuccessful.

(Comparative Example 8)
In Comparative Example 8, two electromagnetic stirring cores and two electromagnetic brake cores were installed, the electromagnetic stirring strength was the same as in Invention Example 1, and the electromagnetic brake strength was set to 0.45 T, which is larger than the range of the invention. The rolling taper amount was set to 0.05 mm/m, which is smaller than the range of the invention.
In this Comparative Example 8, no remelting defects occurred, but because the strength of the electromagnetic brake was large, the molten metal level fluctuated greatly near the nozzle, and surface cracks were observed here and there as in Comparative Example 2. As for the splitting, although the reduction taper amount was small, the electromagnetic brake was sufficiently large and the deviation of the final solidification position in the width direction was small, so no splitting that would affect the product occurred. The overall evaluation was unsuccessful.

(Comparative Example 9)
In Comparative Example 9, two electromagnetic stirring cores and two electromagnetic brake cores were installed, the electromagnetic stirring strength was the same as in Invention Example 1, and the electromagnetic brake strength was set to 0.10 T, which is smaller than the range of the invention. The rolling taper amount was set to 0.05 mm/m, which is smaller than the range of the invention.
In this Comparative Example 9, although no remelting defects occurred, an upward flow occurred on the short side due to insufficient electromagnetic brake strength, which resulted in surface cracks. Furthermore, because the reduction taper amount was insufficient, two-piece cracks occurred, resulting in an overall failure.

The results are shown in Table 1. Inventive Examples 1 to 10, the quality of the carbon steel slabs was good, but in Comparative Examples 1 to 9, problems were observed in the quality of the carbon steel slabs.
From the above experimental results, it has been found that the method for continuous casting of carbon steel slabs according to this embodiment makes it possible to stably cast high-quality slabs even when continuously casting carbon steel slabs at a high throughput.

1 Continuous casting machine 2 Molten steel 3 Cast piece (carbon steel cast piece)
9C Horizontal belt 10 Mold equipment 30 Roll segment device 31 Reduction roll 110 Mold 150 Electromagnetic stirring device 152 Electromagnetic stirring core 160 Electromagnetic brake device 162 Electromagnetic brake core 170 Electromagnetic force generating device

Claims (1)

A method for continuously casting a carbon steel slab using a continuous casting machine having a curved band for curving a carbon steel slab having a rectangular cross section drawn out from below a mold, and a horizontal band for transporting the carbon steel slab in a horizontal direction, comprising:
The carbon steel has a chemical composition of C: 0.07% by mass or more and 0.3% by mass or less, Mn: 0.01% by mass or more and 1.5% by mass or less, Si: 0.001% by mass or more and 0.3% by mass or less, P: 0.07% by mass or less, S: 0.02% by mass or less, and the balance being Fe and impurities,
The molten steel throughput is within a range of 3.00 tons/min to 6.50 tons/min,
Applying an AC magnetic field to the molten steel in the mold using an electromagnetic stirring core installed on the outside of the long side surface of the upper part of the mold, and applying a static magnetic field to the molten steel using an electromagnetic brake core installed on the outside of the long side surface of the lower part of the mold and having multiple and different magnetic poles facing the long side surface;
The magnetic flux density of the AC magnetic field generated by the electromagnetic stirring core is set to 0.02 T or more and 0.15 T or less, and the magnetic flux density of the static magnetic field generated by the electromagnetic brake core is set to 0.15 T or more and 0.40 T or less,
a pair of upper and lower reduction rolls are used to reduce the carbon steel slab in the horizontal band, and a reduction taper amount in the horizontal band is set within a range of 0.10 mm/m or more and 0.80 mm/m or less.

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