JP5745750B2 - Drawing method of continuous cast slab after completion of casting - Google Patents

Description

  The present invention relates to a method for drawing a continuous cast slab after completion of casting in a continuous casting machine. More specifically, the present invention relates to a method for preventing leakage steel from the top slab in a secondary cooling zone of the continuous caster to prevent the steel from being cast. The present invention relates to a method for extracting a continuous cast slab after the end of casting, which enables the slab to be stably extracted from a continuous casting machine.

  In a continuous casting machine for steel, a slab being cast is supported by a large number of slab support rolls and pulled out below the mold. Among these slab support rolls, several are rotated by an electric motor while being pressed against the slab by hydraulic pressure, and the slab is pulled out at a predetermined speed. The slab support roll pressed against the slab by this hydraulic pressure and rotated by an electric motor is called a drive roll (also referred to as “pinch roll”). The remaining slab support rolls other than the drive rolls are generally called guide rolls (those with a small diameter directly under the mold may be called “support rolls”), and are fixedly placed at predetermined positions on the continuous casting machine. And rotating in contact with the surface of the slab to be pulled out. Usually, in the range where the slab support roll is installed, a spray nozzle for spraying spray water or air mist toward the slab surface is installed, and this portion is called a secondary cooling zone.

  At the end of casting, usually, when the remaining molten steel in the tundish reaches a predetermined amount, the slab drawing speed is decreased sequentially, and then the slab is filled when the molten steel is injected from the tundish into the mold. After temporarily stopping the drawing, the coolant is immersed in the molten steel in the mold, or the cooling water is injected to forcibly solidify the surface of the final casting part and solidify the surface of the final casting part. The drawing of the slab is resumed. The restarted slab drawing is also referred to as “re-drawing” in order to distinguish it from drawing during continuous casting. In addition, a slab having the final cast portion that has been forcibly solidified in this manner is called the “top slab”. Iron has a low thermal conductivity, and even when the final casting is forcibly cooled, unsolidified molten steel remains inside the top slab.

  In recent years, from the viewpoint of productivity improvement, in continuous casting machines, not only the slab drawing speed during steady casting has been increased, but also the slab redrawing speed after completion of casting has been increased. However, if the redrawing speed is increased, the inside of the top slab is in an unsolidified state, so that the angle of the long side surface of the top end of the top slab with respect to the vertical direction increases, in other words, For example, in the secondary cooling zone where the top end of the top slab is almost horizontal, there is a problem that the inner solidified molten steel leaks from the final casting part of the top slab that should have solidified the surface. . This leakage of unsolidified molten steel varies depending on the amount of leakage, but if molten steel adheres to the slab support roll or spray nozzle, it may take several hours to remove it, improving the productivity of continuous casting machines. It becomes an obstacle.

  Accordingly, several proposals have been made to prevent leakage of molten steel from the top slab. For example, in Patent Document 1, after completing the injection of molten steel into the mold, the speed is reduced to a drawing speed of 0.2 m / min or less while decelerating, and a slab having a length corresponding to the pitch between the rolls is drawn. A method has been proposed in which the solidified molten steel inside the slab overflows into the mold, the solidified shell is formed again in the mold by the overflowed solidified molten steel, and a container for the unsolidified molten steel is formed by the solidified shell. Yes.

  Also, in Patent Document 2, when the injection of molten steel from the tundish into the mold is completed, the slab drawing speed is first lowered, and then the slab is formed in the thickness direction by a reduction segment disposed directly below or below the mold. By forcibly crushing the slab, the unsolidified molten steel overflows upward from the top slab, and the overflowed molten steel is brought into contact with the mold wall. A solidified shell having solidified molten steel was formed again, and the slab that formed the solidified shell was not removed from the mold, and the slab was crushed so that the squeezed segment was restored to come into contact with the slab. A method of reducing the level of molten steel inside the solidified shell by returning the slab to its original shape, and thus forming a space for storing unsolidified molten steel inside the regenerated solidified shell. It is. According to Patent Document 1 and Patent Document 2, leakage steel from the top slab can be prevented without immersing and introducing a coolant into the mold.

  Furthermore, in Patent Document 3, the injection of molten steel into the mold is completed while maintaining the normal slab drawing speed, and the slab is bulged by expanding the roll interval of the unsolidified portion of the slab, A method has been proposed in which the bulged portion is squeezed by a subsequent roll so that the completely solidified portion has a predetermined slab thickness. According to Patent Document 3, leakage of steel from the top slab can be prevented without reducing the slab drawing speed and without immersing / injecting a coolant into the mold.

JP-A-6-226414 JP-A-9-122845 JP-A-10-244347

  When molten steel leaks from the top slab, the unsolidified molten steel leaked from the top slab does not immediately solidify but flows according to its viscosity. In the methods of Patent Documents 1 to 3, a space more than the capacity of the leaked molten steel is formed in advance at the upper end of the top slab, and if the unsolidified molten steel leaks, the molten steel is removed in this space. It is a technology to prevent leakage of molten steel to the reservoir and continuous casting machine. If a leak of molten steel occurs in the area where the angle between the long side surface of the top end of the top slab and the vertical direction is small, in other words, near the mold, the leaked molten steel accumulates in the space, and enters the continuous casting machine. Although leakage of molten steel can be prevented, leakage of molten steel occurs under the secondary cooling zone where the long side surface of the top end of the top slab is nearly horizontal, such as when the redrawing speed is increased. When generated, the molten steel flows out of the space, and steel leakage to the continuous casting machine occurs.

  That is, in the methods of Patent Documents 1 to 3, the long side surface of the top end of the top slab is close to the horizontal state as in the case where the redrawing speed of the slab is increased. There is a problem that steel cannot be prevented.

  The present invention has been made in view of the above circumstances. The purpose of the present invention is to increase the drawing speed when drawing the slab in the continuous casting machine downward after the casting of the molten steel into the mold is completed. As in the case of the casting, it is possible to prevent the leakage steel from the final casting portion of the top slab in the secondary cooling zone region where the inclination angle with the horizontal line of the long side surface of the top slab tip is small. It is to provide a method for drawing a continuous cast slab after completion.

  A method for drawing a continuous cast slab after completion of casting according to the first aspect of the invention for solving the above-described problem is the method of drawing a slab in a continuous caster with a plurality of drive rolls after injection of molten steel into a mold is completed. When pulling out from a continuous casting machine while being supported by a plurality of guide rolls, at least the driving roll installed in the straightening band for correcting the arc-shaped cast piece into a flat-shaped cast piece is the tip of the topmost cast piece Before the final casting portion reaches the installation position of the drive roll, the pressing pressure applied to the slab in order to grip the slab is released.

  According to a second aspect of the present invention, there is provided a method for drawing a continuous cast slab after completion of casting. In the first invention, the pressing pressure is released before the distance between the final casting portion and each drive roll is less than 4 m. It is characterized by doing.

  According to the present invention, when pulling out the slab after the end of casting, at least the driving roll arranged in the correction band, before the final casting portion of the top end of the top slab reaches each driving roll, Because the pressing pressure of each drive roll is released, the slab is not subjected to the reduction force by the drive roll, and as a result, the unsolidified molten steel inside the slab is squeezed out by the reduction force by the drive roll. Conventionally, steel leakage from the top slab, which has occurred in the secondary cooling zone where the inclination angle of the long side surface of the top slab tip with respect to the horizontal line is small, is prevented in advance, and the continuous casting machine It is possible to improve the productivity of conventional products.

It is a side surface schematic diagram of the vertical bending type slab continuous casting machine used when implementing the present invention.

  Hereinafter, the present invention will be described in detail. The present inventors, when pulling out the slab after completing the injection of molten steel into the mold from the continuous casting machine, the top end of the top slab becomes almost horizontal, in other words, the top slab. We investigated how to prevent leakage steel from the final top casting of the top slab in the secondary cooling zone region where the inclination angle of the long side surface of the front end (upper end) with respect to the horizontal line becomes small.

  In the present invention, the “final casting portion” is a portion corresponding to the molten steel surface in the mold at the completion of pouring of molten steel from the tundish into the mold. Normally, at the end of casting, when the molten steel has been injected from the tundish into the mold, the drawing of the slab is once stopped, and a coolant composed of an iron plate or the like is immersed in the molten steel in the mold. This is a portion that is subjected to treatment such as being forcibly cooled by being cooled, or forcibly cooled by being poured into the molten steel surface in the mold. However, as in Patent Documents 1 to 3, even when the coolant is not charged and the cooling water is not injected and the slab drawing speed is not reduced, the time when the molten steel is completely injected from the tundish into the mold. As long as it is a portion corresponding to the molten steel surface in the mold in the present invention, in the present invention, it is defined as a “cast final portion”. The “top slab” in the present invention is a slab having this final casting portion at the upper end.

  Usually, in a continuous casting machine for steel, a slab support roll is arranged from directly under a mold to the end of the machine. Continuous casting in which a so-called cooling grid composed of a wear plate that supports the slab with a “surface” and a water spray nozzle installed in the gap between the wear plates is arranged within a range of about 1 to 2 m from directly under the mold. Although there is a machine, even in such a continuous casting machine, a slab support roll is arranged in a range from directly below the cooling grid to the machine end.

  The slab support roll applies a rolling force to the slab by hydraulic pressure to grip the slab, and at the same time, rotates the slab at a predetermined number of rotations by an electric motor to pull the slab at a predetermined speed downstream in the casting direction. And a guide roll that rotates in contact with the surface of the slab that is fixedly placed at a predetermined position of the continuous casting machine and pulled out. Of these, the drive roll is configured to retreat to the same position as the guide roll with respect to the slab when the pressing force to the slab is released. That is, the drive roll whose pressing force is released is configured to have a function equivalent to that of the guide roll with respect to the slab.

  The present inventors used an actual continuous casting machine for the purpose of preventing leakage steel from the final top casting part at the lower part of the secondary cooling zone where the top part of the top casting is close to a horizontal state. Then, a two-level test was performed to change the pressing pressure of the drive roll when the cast piece after the completion of casting was pulled out. Level 1 is a method in which all the driving rolls are pressed against the slab at a predetermined pressing pressure until the top slab is completely passed, and the slab is pulled out. Level 2 is the casting of the top end of the top slab. When the final part reaches each drive roll to a distance of 1 m, the slab is pulled out by sequentially releasing the pressing pressure of the driving roll that has been pressed with a predetermined pressing pressure until then. In both Level 1 and Level 2, when the injection of molten steel from the tundish to the mold was completed, the drawing of the slab was temporarily stopped (several seconds to tens of seconds), and cooling water was poured into the molten steel in the mold and forcedly cooled. .

  As a result, at level 1, leakage steel frequently occurred from the final casting portion of the top end of the top slab. On the other hand, at level 2, the leaked steel decreased significantly compared to level 1, but it was not completely absent. Moreover, in both levels, the leakage steel generation position is the lower cooling zone region where the inclination angle with respect to the horizontal line of the long side surface at the top end position of the top slab is 30 ° or less. There was no leakage of steel in the horizontal part after the straightening zone.

  From this result, it was estimated that the cause of leakage steel from the final casting part of the top slab at the lower part of the secondary cooling zone where the tip part of the top slab is almost horizontal is as follows.

  That is, when the slab is pulled out while the drive roll is pressed, unsolidified molten steel remains inside the top slab, but the top slab has a molten steel pressure lower than that during steady casting. For this reason, the top slab continues to be deformed by the pressing pressure of the drive roll, and the unsolidified molten steel inside is squeezed to the upstream side in the casting direction. After that, it was thought that leakage steel would be generated by cracking in the final casting part of the top slab without being able to withstand the pressure of the squeezed unsolidified molten steel, and the unsolidified molten steel flowing out from there. In this case, when the angle formed with the vertical direction of the long side surface at the top end position of the top slab is small, in other words, when the final casting part of the top slab directly below the mold is almost horizontal, the final casting It is thought that the unsolidified molten steel that has flowed out of the crack of the part solidifies in the state of adhering to the final part of the casting, but if the angle made with the vertical direction of the long side surface at the top position of the top slab is large, in other words, When the final casting part of the top slab below the secondary cooling zone is greatly inclined with respect to the horizon, the unsolidified molten steel that has flowed out of the crack in the final casting part flows out of the final casting part, and the slab of the continuous casting machine It is thought that it adheres and solidifies on the support roll and spray nozzle.

  Moreover, the reason why leakage steel does not occur in the horizontal part of continuous casting is that the solidification of the unsolidified molten steel is completed by the time the final casting part of the top slab tip reaches the horizontal part, or Even if the unsolidified molten steel is squeezed out, it is considered that the solidification thickness of the final casting portion of the top slab is thicker than it can withstand the pressure.

  In this way, it has been estimated that the main cause of leakage steel is that the top slab is continuously reduced by the drive roll, so a test was conducted to change the timing for releasing the pressing pressure of the drive roll. .

  In the test, the timing to release the pressing pressure of the drive roll was changed to 4 levels when the distance between the casting final part of the top slab tip and each drive roll reached 2m, 3m, 4m and 6m. Then, the occurrence of leakage steel was investigated. As a result, the earlier the timing of releasing the pressing pressure of the driving roll, the less frequent the occurrence of leaking steel, and the opening between the final casting portion of the top end of the top slab and each driving roll was released under the condition of 4 m or more. In some cases, it was found that there was no leakage of steel. Further, in the continuous casting machine, since it is necessary to insert the dummy bar from the mold, the driving rolls are respectively installed in the bending portion, the correction band, and the horizontal portion. The drive rolls arranged on the machine end side of the horizontal part are not involved in the leakage steel, and the drive rolls arranged in the straightening band, the curved part near the correction band and the horizontal part are involved in the leakage steel. It was confirmed.

  The present invention has been made based on these experimental results, and after the molten steel has been injected into the mold, the slab in the continuous casting machine is continuously supported while being supported by a plurality of drive rolls and a plurality of guide rolls. At the time of drawing from the casting machine, at least the drive roll installed in the straightening strip for correcting the arc-shaped slab into a flat-shaped slab, the final casting part of the top slab tip is the part of the drive roll. Before reaching the installation position, the pressing pressure applied to the slab for holding the slab is released. In this case, it is preferable to release the pressing pressure before the distance between the final casting portion and each drive roll becomes less than 4 m.

  Hereinafter, a specific implementation method of the present invention will be described with reference to the drawings. FIG. 1 is a schematic side view of a vertical bending type slab continuous casting machine used in carrying out the present invention.

  As shown in FIG. 1, a vertical bending type slab continuous casting machine 1 is provided with a mold 5 for injecting and solidifying molten steel 11 to form an outer shell shape of a slab 12. A tundish 2 for relaying and supplying the molten steel 11 supplied from a ladle (not shown) to the mold 5 via the immersion nozzle 4 is installed at a predetermined upper position. On the other hand, below the mold 5, a plurality of pairs of slab support rolls including a support roll 6, a guide roll 7 and a drive roll 8 are arranged. In this slab continuous casting machine 1, as shown by 8a, 8b, 8c, 8d, 8e, and 8d from the upstream side in the casting direction, drive rolls 8 are arranged at five locations in the casting direction.

  A plurality of pairs of slab support rolls disposed at a position about 1 m to 4 m away from the exit of the mold 5 constitutes a bending band 15 in which the support / guide direction of the slab 12 changes from a vertical direction to a bending direction. ing. That is, the flat slab 12 drawn from the mold 5 in the vertical direction is gradually bent into an arc shape by the bending band 15 and is bent and corrected to a curved portion having a constant radius. Similarly, the plurality of pairs of support rolls arranged in the vicinity of the position where the curved portion comes into contact with the horizontal line constitutes a correction band 16 in which the support / guide direction of the slab 12 changes from the curved direction to the horizontal direction. . That is, the arc-shaped slab 12 is gradually bent back to a flat shape by the correction band 16 and bent back to the horizontal portion. In FIG. 1, both the bending band 15 and the correction band 16 are configured by a plurality of pairs of slab support rolls, but may be configured by only a pair of slab support rolls. The correction belt 16 of the present invention includes a case where correction is performed with a pair of slab support rolls.

  Among the drive rolls 8 described above, the drive roll 8a is at the upper part of the curved part, the drive roll 8b is at the substantially central part of the curved part, the drive roll 8c is at the correction band 16, and the drive roll 8d is at the upstream side of the horizontal part. The drive roll 8e is disposed on the downstream side of the horizontal portion, that is, on the machine end.

  A secondary cooling zone in which a spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is arranged is formed in the gap between the slab support rolls adjacent in the casting direction. The slab 12 is cooled while being drawn out by cooling water (also referred to as “secondary cooling water”) sprayed from the spray nozzle. This secondary cooling zone is divided into several zones in the casting direction so that the amount of secondary cooling water can be adjusted individually in each zone.

  A sliding nozzle 3 for adjusting the flow rate of the molten steel 11 injected from the tundish 2 into the mold 5 is installed at the bottom of the tundish 2 and is casted downstream of the slab support roll. A plurality of transport rolls 9 for transporting the slab 12 are installed, and a gas for cutting a slab 12 a having a predetermined length from the cast slab 12 to be cast is disposed above the transport roll 9. A cutting machine 10 is arranged.

  The molten steel 11 injected from the tundish 2 into the mold 5 through the immersion nozzle 4 is cooled by the mold 5 to form a solidified shell 13, and a slab support is formed as a slab 12 having an unsolidified molten steel layer 14 inside. While being supported by the roll, it is continuously pulled out downward. While the slab 12 passes through the slab support roll, it is cooled in the secondary cooling zone to increase the thickness of the solidified shell 13 and eventually complete the solidification to the center. The slab 12 after completion of solidification is cut by the gas cutter 10 to become a slab 12a.

  The vertical bending slab continuous casting machine 1 having such a configuration is used to carry out the present invention as follows.

  When injection of the molten steel 11 from the ladle to the tundish 2 is completed and the molten steel 11 staying in the tundish 2 is reduced, the drawing speed of the slab 12 is stepwise or continuous compared with that during steady casting. Reduce to. And when injection | pouring of the molten steel 11 from the tundish 2 to the casting_mold | template 5 is completed, while drawing | extracting the slab 12 is once stopped, the tundish 2 is raised and the immersion nozzle 4 is pulled up from molten steel in a casting_mold | template. Thereafter, cooling water is supplied to the molten steel surface in the mold from the upper opening of the mold 5 to cool and solidify the molten steel surface in the mold. In this case, it is possible to forcibly promote solidification by immersing and introducing a coolant made of an iron plate or the like into the molten steel in the mold. Next, the drawing of the slab 12 is resumed. After the resumption of drawing, the drawing speed is increased stepwise or continuously to increase to a predetermined speed and maintained.

  In the present invention, the portion of the molten steel surface in the mold when the injection of the molten steel 11 from the tundish 2 into the mold 5 is completed is referred to as a “cast final part”, and the cast piece 12 having this final cast part at the upper end. Is called the “top slab”. It should be noted that when the molten steel 11 has been injected from the tundish 2 into the mold 5, the present invention can be applied even if the drawing speed is not reduced and the steady casting is being performed. From the viewpoint of preventing steel leakage from the portion, it is preferable to stop the drawing once and forcibly cool the molten steel surface in the mold with cooling water or the like.

  After the resumption of drawing, each drive roll 8 preferably has a casting final part of the top slab and each driving roll 8 before the final casting part of the top end of the top slab reaches the respective driving roll 8. The pressure applied to the slab 12 is released before the distance to the slab becomes less than 4 m. In this case, the drive rolls 8 grouped into the five locations in the casting direction described above may be opened simultaneously or individually. However, if the pressing pressure of all the drive rolls 8 is released at the same time, the slab 12 cannot be pulled out, so the distance between the final casting part of the top slab and each of the drive rolls 8 is 4 m or more. It is preferable to open sequentially at a time of 7 m or less. Further, if the pressing pressure of all the drive rolls 8 is released, there is a possibility that the slab 12 cannot be pulled out, so that it does not affect the leakage steel installed on the machine end side of the horizontal portion of the continuous casting machine. It is preferable that the drive roll 8e continues to press the slab 12. However, if the slab 12 can be pulled out, the pressing pressure may be released also in the drive roll 8e. Furthermore, since the drive roll 8a arranged in the curved portion does not affect the leakage steel, the pressing pressure may or may not be released, and the timing may be any time when it is released.

  That is, what is necessary is just to release the pressing pressure of the slab 12 about the correction | amendment strip | belt 16 which influences leakage steel, and the drive roll 8 near the correction strip | belt 16. FIG. In this case, the vicinity of the straightening band 16 varies depending on the specifications of the continuous casting machine (curvature radius, etc.), but if it is within a range of about 5 m from both ends of the straightening band 16 to the upstream and downstream sides in the casting direction. Good. The top end of the top slab is often uneven in slab shape, and the drive roll 8 may be wrinkled due to this, so the occurrence of this wrinkle is caused by releasing the pressing pressure. Can also be prevented. From this point of view, it is preferable to release the pressing pressure before the final casting portion of the top slab arrives even if it is the drive roll 8 that does not affect leakage steel, as long as it does not hinder the drawing of the slab 12. . Further, the position of the final casting portion at the tip of the top slab is measured by a measure roll (not shown) for measuring the casting length installed in the continuous casting machine.

  As described above, according to the present invention, when pulling out the slab 12 after the completion of casting, at least the driving roll 8 disposed in the correction band 16 has the final casting part at the top end of the top slab corrected. Before reaching the respective drive rolls 8 disposed in the belt 16, the pressing pressure of the respective drive rolls 8 is released, so that the slab 12 does not act on the slab 12, and as a result, Leakage from the top slab, which has occurred in the secondary cooling zone where the top end of the top slab is nearly horizontal, is prevented in advance.

  The present invention was applied to a two-strand vertical bending slab continuous casting machine having a slab thickness of 250 mm, a vertical part length of 2.5 m, a curved part radius of 8 m, and a machine length of 42 m. Each drive roll was made to release slab pressing pressure sequentially when the distance from the last casting part of the top slab reached 4 m from each drive roll (example of the present invention). However, the driving roll arranged on the machine end side of the horizontal portion of the continuous casting machine maintained the pressing pressure. For comparison, a test was also conducted to release the slab pressing pressure when the final casting portion of the top slab reached each drive roll (Comparative Example).

  As a result, in the comparative example, steel leakage occurred at a frequency of about twice / month, but in the present invention example, there was no steel leakage in the past nine months. Moreover, the torque current value of the motor of the drive roll was not changed between the examples of the present invention and the comparative example, and it was confirmed that no problem occurred in the slab drawing even when the present invention was applied.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Support roll 7 Guide roll 8 Drive roll 9 Transport roll 10 Gas cutting machine 11 Molten steel 12 Slab 13 Solidified shell 14 Unsolidified molten steel layer 15 Bending band 16 Correction band

Claims (3)

After the molten steel is poured into the mold, the slab in the continuous casting machine is supported by multiple drive rolls and multiple guide rolls while being drawn from the continuous casting machine. The drive roll installed in the straightening belt for correcting the slab of the slab before the distance between the casting final part of the top slab tip and the installation position of each drive roll becomes less than 4 m. The continuous casting slab after the end of casting, wherein the pressing pressure applied to the slab for gripping is released so that the drive roll retracts to the same position as the guide roll with respect to the slab Drawing method. 2. The after-casting according to claim 1, wherein the pressing pressure is released before the distance between the final casting portion and each drive roll becomes less than 4 m and before the distance becomes less than 4 m. Drawing method of continuous cast slab. The method for drawing a continuous cast slab after completion of casting according to claim 1 or 2, wherein the slab drawing is temporarily stopped after injection of molten steel into the mold is completed.

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