JP2012086260A - Light rolling reduction controlling method of cast slab in continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To establish a pressure of a pinch roll as a proper value based on a load of a roll segment under casting, and thereby to perform light rolling reduction on a proper condition in a method of carrying out light rolling reduction of a cast slab using a light rolling reduction zone of a roll segment system in which a pinch roll has been arranged.SOLUTION: The light rolling reduction controlling method is characterized as follows. A light rolling reduction zone which consists of a roll segment 15 in which a pair of pinch rolls 21 and two or more pairs of reduction rolls 6 have been included at least is used, in carrying out reduction of a cast slab 10 of a solidification last stage by a rolling draft of the amount of solidification shrinkage, a load which works to the roll segment is measured at least two sections of an upstream side section and a downstream side section of the roll segment, a light rolling reduction load which should be loaded to each of rolls arranged at the roll segment based on a measured load is obtained, and a pushing pressure of the pinch roll is established so that a light rolling reduction load which should be loaded to the pinch roll and a load by the pinch roll may be equivalent in light rolling reduction loads which have been obtained.

Description

  The present invention relates to a light reduction control of a slab in continuous casting, in which the slab at the end of solidification is reduced by a reduction amount corresponding to the solidification shrinkage for the purpose of reducing the center segregation generated at the center of the thickness of the continuous casting slab. Regarding the method.

  In the solidification process of steel, solute elements such as carbon, phosphorus and sulfur are concentrated on the unsolidified liquid phase side by redistribution during solidification. This is the microsegregation formed between dendrite trees. A void is formed in the center of the slab thickness due to solidification shrinkage of the slab being cast by the continuous casting machine and bulging of the solidified shell that occurs between the rolls of the continuous casting machine (a phenomenon that expands due to molten steel static pressure). If a negative pressure occurs, the molten steel is sucked into this part, but since there is not a sufficient amount of molten steel in the unsolidified phase at the end of solidification, the molten steel concentrated by the microsegregation flows and casts. It collects and solidifies in the center of the piece. In the segregation spot formed in this way, the concentration of the solute element is much higher than the initial concentration of the molten steel. This is generally called macro-segregation and is called central segregation because of its existence site.

  This center segregation not only makes the quality of steel products non-uniform but also deteriorates. As a technology to improve center segregation, a slab support roll for supporting and guiding the slab when continuously casting steel. A so-called “light reduction method” has been performed in which the roll interval is controlled to impart a reduction amount equivalent to the solidification shrinkage amount to the slab at the end of solidification, thereby improving the central segregation caused by the solidification shrinkage.

  For example, Patent Document 1 discloses an appropriate strain rate that minimizes the flow of molten metal inside a slab from casting conditions in a light reduction casting method in which an unsolidified slab is completely solidified while being lightly reduced by a plurality of pairs of rolls. On the other hand, the roll reaction force received from the slab by each of the multiple pairs of rolls is measured, and the distortion rate of the slab corresponding to each roll reaction force is ascertained, and each roll according to these distortion rates. There has been proposed a light rolling casting method in which the rolling amount is adjusted so that the strain rate of each part of the slab matches the appropriate strain rate.

  In Patent Document 2, a position control cylinder is provided for each roll bearing in the vicinity of the crater end, a reduction amount calculator for controlling the position of the roll is provided, and the rod movement amount of the cylinder is set at least for each roll. A light reduction control method has been proposed in which the actual reduction amount of the slab, the frame deformation amount, and the roll deformation amount are set in total.

  In Patent Document 3, in a method of controlling the amount of reduction of a light reduction roll by a position control cylinder attached to a bearing of the light reduction roll disposed near the end of solidification of the slab, the light reduction roll is attached to the surface of the slab. A light reduction amount control method has been proposed in which a cylinder target position for obtaining a desired reduction amount is set with a cylinder position at the time of contact as a reference point.

JP 63-242452 A Japanese Patent Laid-Open No. 5-8004 JP-A-8-90186

  In recent years, a continuous casting machine is mainly a roll segment type continuous casting machine configured by opposing a frame in which a plurality of slab support rolls are arranged, and a light reduction roll for applying a reduction force to the slab. The roll segment method is also the mainstream. In this case, the light reduction roll is fixed to the roll segment, and the reduction amount cannot be adjusted independently for each light reduction roll within the roll segment. The range where a plurality of lightly-rolled rolls are continuously installed in the casting direction is called a “lightly-rolled belt”. In the light-rolled belt, the roll interval is set so as to become narrower in the downstream direction in the casting direction. This state is referred to as “rolling gradient”.

  Since the reduction force acting on the slab is determined by the reduction gradient in the light reduction belt, when the light reduction belt is a roll segment system, the light reduction is performed in segment units. That is, the light reduction amount in this roll segment is determined by the difference between the most upstream roll interval value and the most downstream roll interval value of the roll segment.

  The light rolling roll is premised on rolling down a slab having an unsolidified phase inside, and the rolling resistance to the light rolling roll in this case is only the short side of the slab that has been solidified in the thickness direction. The load on the light rolling roll is small, and even with a roll segment type light rolling belt, a rolling amount corresponding to the solidification shrinkage can be added to the slab by each light rolling roll. On the other hand, when rolling down the slab after completion of solidification, the entire width of the slab becomes a rolling resistance, and the roll segment type light rolling roll has insufficient rolling force and cannot be rolled down. In order to prevent damage, the frame that holds the lightly rolling roll is opened by hydraulic pressure or a disc spring.

  By the way, in the roll segment type light pressure lowering belt, a pinch roll for pulling out a slab is usually disposed in a roll segment that performs light pressure reduction. The pinch roll is a driving roll, and in order to transmit the driving force to the slab, the pressure for pressing the slab is set independently of the other lightly rolling rolls in the same roll segment.

  When the pressure setting of this pinch roll is not an appropriate value, the light reduction amount in this roll segment does not become the target value, and the center segregation is not improved due to insufficient light reduction amount, or the light reduction amount is excessive. This causes problems such as internal cracks in the slab.

  From the viewpoint of eliminating this problem, when the above prior art is verified, according to Patent Documents 1 to 3, an appropriate light reduction amount can be ensured. However, Patent Documents 1 to 3 describe each light reduction roll in the roll segment. In addition, a continuous caster with a light roll of the general roll segment type that can measure the light reduction load and control the light reduction amount and set the light reduction conditions only in segment units. Is a technique that cannot be applied, and the above problem cannot be solved.

  Furthermore, Patent Document 1 requires a separate investigation of the relationship between the roll reaction force and the appropriate strain rate, and Patent Documents 2 and 3 describe any pressure or light pressure load to eliminate center segregation. It is not disclosed whether it is necessary.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a roll segment in which a pinch roll is disposed for the purpose of reducing central segregation generated at the thickness center of a continuous cast slab. The pinch roll pressure is set to an appropriate value based on the load applied to the roll segment during casting in the light rolling method that uses the light rolling belt of the method to roll the slab at the end of solidification by a rolling amount equivalent to the solidification shrinkage. Thus, it is an object of the present invention to provide a method for controlling light reduction of a slab in continuous casting, which can perform light reduction under appropriate conditions.

  The present inventors have intensively studied and studied to solve the above problems. As a result, the load load at the time of light pressure acting on the roll segment was measured at at least two parts of the roll segment type light pressure lower zone in the casting direction upstream part and the casting direction downstream part, and from this measured load load By finding the light pressure load to be applied to each light pressure roll, and controlling the pressing pressure of the pinch roll so that the calculated light pressure load and the load by the pinch roll are equal, the above problem can be solved I found.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) Using a light reduction belt composed of a roll segment in which at least a pair of pinch rolls and a plurality of pairs of reduction rolls are incorporated, the slab at the end of solidification is reduced by a reduction amount corresponding to the solidification shrinkage amount. The load load acting on the roll segment is measured at at least two locations, the upstream portion and the downstream portion of the roll segment, and the pinch roll and the rolling roll arranged in the roll segment are measured based on the measured load load. The light pressure load to be applied to each roll is determined, and the pressing pressure of the pinch roll is set so that the light pressure load to be applied to the pinch roll and the load by the pinch roll are equal to the calculated light pressure load. A method for controlling the light reduction of a slab in continuous casting, characterized in that
(2) The method of controlling light reduction of a slab in continuous casting according to (1), wherein a load applied to the roll segment is measured at a position of a tie rod of the roll segment.

  According to the present invention, when the slab is lightly reduced using a roll segment type light pressure lower belt in which at least a pair of pinch rolls are arranged, the pressing pressure of the pinch roll can be appropriately set. Therefore, the amount of light reduction by the pinch roll and the amount of light reduction by the other light reduction rolls arranged in the same roll segment are equivalent, and the center segregation is not improved due to insufficient light reduction amount, or light reduction Problems such as the occurrence of internal cracks in the slab due to the excessive amount are solved, and it is possible to stably manufacture a slab with a slight center segregation.

It is a schematic sectional drawing which shows the example of the roll segment which comprises a light pressure lower belt. It is the schematic which shows typically the behavior of a roll segment when the pressure setting of a pinch roll is too high. It is a figure which shows typically the load of each light reduction roll in the case of performing the same amount of light reduction with the light reduction roll in the same roll segment. It is a side schematic diagram of an example of a slab continuous casting machine used when carrying out the present invention.

  Hereinafter, the present invention will be specifically described. First, the background to the present invention will be described.

  The present inventors include a pinch roll when a slab at the end of solidification is lightly reduced with a reduction amount corresponding to a solidification shrinkage amount using a roll segment type light reduction belt incorporating at least a pair of pinch rolls. The appropriate light rolling conditions of each light rolling roll were investigated. FIG. 1 is a schematic cross-sectional view showing an example of a roll segment constituting a light pressure lower belt. FIG. 1 is a diagram showing an example in which eight pairs of cast slab support rolls 6 are arranged in one roll segment 15 as lightly pressed rolls. As one of the slab support rolls 6, a pair of pinch rolls 21 is shown. Is arranged on the most upstream side in the casting direction (left side of the drawing).

  As shown in FIG. 1, the roll segment 15 includes a pair of frames 16 and 16 ′ holding eight pairs of slab support rolls 6 (where a pair is a pinch roll 21). A total of four tie rods 17 (both sides on the upstream side in the casting direction and both sides on the downstream side) are disposed through the frame 16 and the frame 16 ′, and the worm jack 18 installed on the tie rod 17. Is remotely controlled by a motor (not shown) to adjust the distance between the frame 16 and the frame 16 ', that is, the rolling gradient in the roll segment 15 (set to gradually narrow toward the downstream in the casting direction). Adjustment of the roll interval) is performed. During the casting, the worm jack 18 is self-locking and counters the bulging force of the slab 10 having an unsolidified phase, and under the condition that the slab 10 does not exist, that is, the slab 10 on the slab support roll 6. The reduction gradient is adjusted under a condition in which no load from is applied.

  The seven pairs of slab support rolls 6 excluding the pinch rolls 21 are fixed to the frames 16 and 16 ′ via the roll chock 20. On the other hand, the pinch roll 21 is fixed to the frame 16 via the roll chock 20 in the lower surface side frame 16, but in the upper surface side frame 16 ′, the cylinder rod of the hydraulic cylinder 22 arranged fixed to the frame 16 ′. It is held via a roll chock 23 connected to (not shown). That is, the pinch roll 21 on the upper surface side is configured to press the slab 10 with the original pressing force and the amount of reduction by the hydraulic cylinder 22 regardless of the above-described reduction gradient. The pinch roll 21 is configured such that both the upper side pinch roll 21 and the lower side pinch roll 21 are driven by an electric motor (not shown) to pull out the slab 10.

  The pinch roll 21 needs to pull out a dummy bar (not shown) that is thinner than the slab 10 at the start of casting. For this reason, the amount of reduction and the pressing pressure are different from those of the other slab support rolls 6 as described above. Are configured differently. Incidentally, the slab support roll 6 on the upper surface side of the other seven pairs of slab support rolls 6 usually does not contact the dummy bar. FIG. 1 shows an example in which the number of installed slab support rolls 6 is eight pairs, but in the light segment under the roll segment method, the number of installed slab support rolls 6 is not related to this. The reduction gradient is adjusted.

  A plurality of roll segments 15 of this configuration are arranged in the casting direction to form a light pressure lowering belt, the pressing pressure of the pinch roll 21 is changed to various values, the slab 10 is lightly reduced, and the light pressure acting on the tie rod 17 at that time is reduced. The applied load was measured by a load cell 19 disposed on the top of the tie rod 17. The load cells 19 are installed on all four tie rods 17, and the total of the measured values of the two load cells 19 installed on the tie rods 17 on the upstream side in the casting direction is the load on the tie rods 17 on the upstream side. The total of the measurement values obtained by the two load cells 19 installed on the tie rod 17 at the downstream site in the casting direction was used as the load applied to the tie rod 17 at the downstream site. The frame 16 is fixed to the foundation of the continuous casting machine and is configured not to move during casting.

  As a result, when the pressing pressure of the pinch roll 21 is increased during light pressure, the load on the tie rod 17 in the upstream portion in the casting direction becomes relatively larger than the tie rod 17 in the downstream portion in the casting direction. It was confirmed. This is because when the pressing pressure of the pinch roll 21 is increased, the amount of reduction of the slab 10 by the pinch roll 21 is increased, and the reduction reaction force of the roll segment 15 in which the pinch roll 21 is disposed on the most upstream side. This is because it is transmitted more to the tie rod 17 in the upstream portion from the structure. Moreover, as a result of investigating the center segregation of the cast slab 10, the center segregation changed depending on the pressing pressure of the pinch roll 21, and it was confirmed that an appropriate range exists for the pressure setting of the pinch roll 21. As a result of detailed investigation of this phenomenon, it was found that these were due to the following reasons.

  That is, in the light reduction device using the roll segment method, as described above, when the load load exceeds the specified load of the roll segment 15, a disc spring or hydraulic pressure set in the tie rod 17 is installed to protect the equipment. It has a structure that releases the roll under light pressure.

  The pinch roll 21 arranged in the light pressure lower belt also performs light pressure reduction. When the pressure setting of the pinch roll 21 is too high, it is incorporated into the tie rod 17 in the upstream portion as schematically shown in FIG. Due to the disc spring or the hydraulic setting, the upstream side of the frame 16 ′ is lifted, and the rolling gradient of the roll segment 15 increases, and the slab support roll 6 adjacent to the downstream side of the pinch roll 21 and the slab support roll 6 on the downstream side thereof The force becomes smaller, and in some cases, light reduction is impossible. When the reduction force is reduced, the slab 10 is bulged by the molten steel static pressure, and not only the center segregation of the slab 10 can be improved in the light reduction zone, but also the center segregation of the slab 10 is deteriorated. On the other hand, when the pressure setting of the pinch roll 21 is too low, the target light pressure cannot be achieved with the pinch roll 21, which also causes deterioration of the center segregation. FIG. 2 is a schematic view schematically showing the behavior of the roll segment 15 when the pressure setting of the pinch roll 21 is too high.

  Thus, it has been found that there is an appropriate value for the pressure setting of the pinch roll 21 arranged in the light pressure lower belt. In the present invention, the amount of reduction and light reduction, and the reduction and light reduction have the same meaning.

  Considering from the viewpoint of reducing the center segregation of the slab 10, the optimum pressure setting value of the pinch roll 21 arranged in the lightly pressed belt is that the reduction amount of the slab 10 in the pinch roll 21 is incorporated in the same roll segment. The pressure value is the same as the amount of reduction in the other slab support roll 6 (light reduction roll).

  The knowledge that the optimum pressure setting value of the pinch roll 21 can be set from the load applied to the tie rods 17 installed at two locations, the upstream portion and the downstream portion of the roll segment 15 where the pinch roll 21 is arranged. The present inventors obtained.

  That is, the load acting on the tie rods 17 installed at two locations, the upstream portion and the downstream portion of the roll segment 15 where the pinch rolls 21 are arranged, is measured, and each slab support roll 6 (light weight) is measured based on the measured load. The light pressure reduction load to be applied to the pressure reduction roll) is obtained, and the pressing pressure of the pinch roll 21 is set so that the light pressure reduction load to be applied to the pinch roll and the load on the pinch roll 21 coincide with each other. That is, the knowledge that the hydraulic pressure of the hydraulic cylinder 22 should be set was obtained.

A specific pressure setting method for the pinch roll 21 will be described below. For example, the actual load on the tie rod 17 in the upstream portion is P _t , the actual load on the tie rod 17 in the downstream portion is P _b, and the load applied to the eight pairs of slab support rolls 6 of the roll segment 15 is P _1. When to P _8, the balance of forces in the following equation (1) is satisfied. Here, serial numbers of 1 to 8 are assigned to the eight pairs of slab support rolls 6 of the roll segment 15 from the upstream side, and P _i means a load on the i-th slab support roll 6 from the upstream side.

Further, from the balance of moments acting on the roll segment 15, the following equation (2) is also established. However, in the formula (2), L _{1 to} L ₈ and L _t and L _b are distances from the center position of the roll segment 15 to each slab support roll 6 and each tie rod 17 and are exemplified in FIG.

The loads P _{1 to} P ₈ cannot be obtained by only the two relational expressions (1) and (2), but when the same amount of light reduction is performed on each light reduction roll in the same roll segment, As shown in FIG. 3, the load gradually increases with the transition from the upstream side to the downstream side. FIG. 3 is a diagram schematically showing the load of each lightly rolling roll when performing the same amount of light rolling with the light rolling roll in the same roll segment.

Therefore, the loads P _{1 to} P ₈ are expressed by the sum of the reference load P ₀ and the load increment ΔP for each roll as shown in the following expression (3), whereby the light pressure load of each slab support roll 6 can be expressed. Can be sought.

  In other words, the light pressure load to be applied to each slab support roll 6 is obtained using the equations (1), (2), and (3), and the pinch roll 21 should be loaded among the obtained light pressure loads. By setting the pressing pressure of the pinch roll 21 so that the light reduction load and the load of the pinch roll 21 are equal, the reduction amount of the pinch roll 21 and the reduction of the other slab support roll 6 (light reduction roll). The amount was equivalent, and it was possible to perform light reduction under appropriate conditions, and obtained knowledge that the center segregation of the slab 10 could be improved.

  The present invention has been made on the basis of the above findings, and solidifies the slab at the end of solidification by using a light reduction belt composed of a roll segment in which at least a pair of pinch rolls and a plurality of pairs of reduction rolls are incorporated. When rolling down with a rolling amount equivalent to the amount of shrinkage, the load load acting on the roll segment is measured at at least two parts, the upstream part and the downstream part of the roll segment, and the roll is measured based on the measured load load. The light reduction load to be applied to each roll of the pinch roll and the reduction roll arranged in the segment is obtained, and the light reduction load to be applied to the pinch roll in the obtained light reduction load is equal to the load by the pinch roll. The pressing pressure of the pinch roll is set so that

  Hereinafter, a specific implementation method of the present invention will be described with reference to the drawings. FIG. 4 is a schematic side view of an example of a slab continuous casting machine used in carrying out the present invention.

  As shown in FIG. 4, a slab continuous casting machine 1 is provided with a mold 5 for injecting and solidifying molten steel 9 to form an outer shell shape of the slab 10. Is provided with a tundish 2 for relaying and supplying molten steel 9 supplied from a ladle (not shown) to the mold 5. A sliding nozzle 3 for adjusting the flow rate of the molten steel 9 is installed at the bottom of the tundish 2, and an immersion nozzle 4 is installed on the lower surface of the sliding nozzle 3. On the other hand, a plurality of pairs of slab support rolls 6 including a support roll, a guide roll, and a pinch roll are arranged below the mold 5. 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 6 adjacent in the casting direction. The slab 10 is cooled while being drawn out by cooling water sprayed from the nozzle (also referred to as “secondary cooling water”). A plurality of transport rolls 7 for transporting the cast slab 10 are installed on the downstream side of the final slab support roll 6 in the casting direction. A slab cutting machine 8 for cutting a slab 10a having a predetermined length from the slab 10 is disposed.

  On the upstream side of the solidification completion position 13 of the slab 10, an interval between the slab support rolls facing each other with the slab 10 interposed therebetween (this interval is referred to as “roll interval”) sequentially toward the downstream side in the casting direction. A light reduction belt 14 composed of a plurality of pairs of slab support rolls set so as to be narrow, that is, a reduction gradient is set. In the light reduction belt 14, it is possible to perform light reduction on the slab 10 in the entire region or a partially selected region. A spray nozzle for cooling the slab 10 is also disposed between the slab support rolls of the light pressure lower belt 14.

  Normally, the rolling gradient is indicated by the amount of roll interval narrowing per 1 m in the casting direction, that is, “mm / m”, and therefore the rolling speed (mm / min) of the slab 10 in the light rolling zone 14 is The rolling gradient (mm / m) is multiplied by the casting speed (m / min). Further, the slab support roll 6 of the light reduction belt 14 is also referred to as “light reduction roll” or “reduction roll” because it is a roll for applying light reduction.

  In the slab continuous casting machine 1, the light pressure lower belt 14 is configured by connecting two roll segments each having three pairs of light pressure lower rolls in the casting direction. Here, the structure of the roll segment shown in FIG. 4 is the same as that of the roll segment 15 shown in FIG. 1 except that the slab support roll 6 has three pairs. That is, the most upstream pair of slab support rolls 6 among the three pairs of slab support rolls 6 is a pinch roll, and the lower frame (corresponding to the frame 16) and the upper frame (corresponding to the frame 16 '). With a load cell installed on a tie rod that penetrates, a load acting on the roll segment can be measured at two sites, an upstream site and a downstream site of the roll segment. Further, although not shown, the slab support roll 6 other than the light pressure lower belt also has a roll segment structure.

  Since the light pressure lower belt 14 is such a roll segment system, the roll interval of two pairs of light pressure lower rolls other than the pinch rolls is adjusted in a lump. In this case, the amount of movement of the upper frame (corresponding to the frame 16 ') by remote control is measured and controlled by the number of rotations of the worm jack so that the rolling gradient of each roll segment can be known. In FIG. 4, the light pressure lower belt 14 is composed of two roll segments, but it may be one or three or more. Moreover, although the slab support roll 6 arrange | positioned at one roll segment is three pairs, it is not necessary to make it three pairs, and as long as it is two or more pairs, it may be any number.

  In the present invention, the reduction speed of the light reduction belt 14 with respect to the slab 10 at the end of solidification is preferably in the range of 0.6 to 1.5 mm / min. Therefore, the light reduction is performed according to the expected casting speed. A rolling gradient of the belt 14 is set in advance. When the rolling speed is less than 0.6 mm / min, the effect of reducing the center segregation is small. On the other hand, when the rolling speed exceeds 1.5 mm / min, the concentrated molten steel is squeezed out in the direction opposite to the casting direction. This is because negative segregation may be generated at the center of the piece. Further, it is sufficient that the total reduction amount is about 2 to 6 mm.

  Using the slab continuous casting machine 1 configured as described above, the molten steel 9 is poured from the ladle into the tundish 2 to retain a predetermined amount of molten steel 9 in the tundish 2, and then the molten steel 9 retained in the tundish 2. Is injected into the mold 5 through the immersion nozzle 4. The molten steel 9 injected into the mold 5 is cooled by the mold 5 to form a solidified shell 11, the outer shell is the solidified shell 11, and the slab 10 having an unsolidified phase 12 is formed on the slab support roll 6. While being supported, it is continuously pulled out below the mold 5 by a pinch roll. While the slab 10 passes through the slab support roll 6, the slab 10 is cooled by the secondary cooling water in the secondary cooling zone, the thickness of the solidified shell 11 is increased, and the solidification completion position 13 while being lightly reduced by the light lowering zone 14. Complete solidification to the inside. Thereafter, the slab 10 that has been solidified is cut by the slab cutting machine 8 to become a slab 10a.

  In this case, in the steady casting zone, the casting speed and the secondary speed are adjusted so that the solid fraction of the slab center at the light pressure lower roll position located in the light pressure lower belt 14 arranged in the light pressure lower belt 14 is 0.7 or more. Adjust the amount of cooling water. This can be realized by obtaining a casting speed and an amount of secondary cooling water that satisfy the above conditions in advance using a method such as heat transfer solidification calculation. The reason why the slab center solid phase ratio at the light rolling roll position at the most downstream side in the casting direction is 0.7 or more is that the slab 10 having a slab center solid phase ratio of less than 0.7 is lightly pressed. This is because when passing through the band 14, the unsolidified phase 12 easily moves and the center segregation may occur in the range where the solid fraction of the slab center is less than 0.7. The solid phase ratio at the center of the slab thickness can be obtained by heat transfer solidification calculation, and the position where the solid phase ratio at the center of the slab thickness is 1.0 (the temperature at the center of the slab thickness is the solidus temperature). Is the coagulation completion position 13. In FIG. 4, the solidification completion position 13 is on the downstream side of the light pressure lower belt 14, but the solidification completion position 13 may be within the range of the light pressure lower belt 14.

  Further, in order to reduce the center segregation of the slab 10, it is desirable to reduce the slab 10 with the light reduction belt 14 when the slab center solid phase ratio becomes 0.3, which is satisfied. Thus, it is desirable to set the casting direction length of the light pressure lower belt 14. This is because when the solid phase ratio at the center of the slab is less than 0.3, there is a large amount of the unsolidified phase 12 and even if molten steel flow occurs, center segregation does not occur. This is because the center segregation deteriorates. The required length of the light pressure lower belt 14 can also be obtained by heat transfer solidification calculation.

  In this way, when casting the slab 10 with light reduction, the load acting on the roll segment is measured by the load cell installed on the tie rod of the roll segment constituting the light reduction belt 14. Each time the load is measured by the load cell, each light pressure lowering roll including the pinch roll, which is arranged in the light pressure lowering belt 14, using the formula (1), (2) and (3) based on the measured load. The light pressure load to be applied to the pinch roll is determined, and the pressing pressure of the pinch roll is set so that the light pressure load to be applied to the pinch roll is equal to the load by the pinch roll among the calculated light pressure loads.

  The load required for the target light reduction amount changes due to changes in the casting speed during casting and changes in the secondary cooling conditions that accompany this, so the load acting on the roll segment is constantly measured, and the pinch roll is pressed each time. It is preferable to reset the pressure.

  As described above, according to the present invention, when the slab 10 is lightly reduced using the roll segment type light pressure lower belt 14 in which at least a pair of pinch rolls are arranged, the pressing pressure of the pinch roll is set appropriately. Therefore, the amount of reduction by the pinch roll is equal to the amount of reduction by other light reduction rolls arranged in the same roll segment, and light reduction can always be performed under appropriate conditions. Thus, it is possible to stably manufacture the slab 10a having a small center segregation.

  In the above description, the light pressure load to be applied to each light pressure roll is calculated from the load acting on the tie rod. However, the position where the load acting on the roll segment is measured is not limited to the position of the tie rod, and the casting direction If the load is measured at two or more locations, the light reduction load of each light reduction roll can be obtained by the same method as described above. Further, in the above description, the example in which the pinch roll is disposed on the most upstream side of the roll segment has been described, but the present invention can be applied along the above regardless of the position of the pinch roll.

  Using the slab continuous casting machine in which the three roll segments shown in FIG. 1 are arranged in the range of a distance of 20.0 to 26.0 m from the molten steel surface in the mold as a light pressure lower belt, the carbon concentration is set to 0.00. The present invention was applied when casting carbon steel having 05 mass% and manganese concentration of 1.3 mass% into a slab slab having a thickness of 250 mm and a width of 2100 mm.

The roll segment used has eight pairs of lightly pressed rolls (however, the pair on the most upstream side is a pinch roll) arranged at a roll pitch of 250 mm in one roll segment, and the distance L _t = distance L _b = 500 mm The length of the segment in the casting direction was 2.0 m, the reduction speed target was 1.2 mm / min, and the reduction gradient was 1.0 mm / m. The set pressure of the pinch roll was set so that the pressing pressure was 500 kN in the three roll segments in the initial state.

In the example of the present invention, the load acting on the roll segment is measured by the load cell disposed on the tie rod in the upstream portion and the downstream portion of each roll segment, and each time the load is measured, the formulas (1) to (3) are used. The set pressure of the pinch roll was corrected. During casting, the actually measured loads P _t and P _b varied within the range of 1500 to 3000 kN, although depending on the roll segment. On the other hand, in the comparative example, the pressure setting of the pinch roll maintained the initial state.

  As a result, in the example of the present invention, a slab excellent in internal quality was obtained in which the center segregation was very slight throughout the casting. On the other hand, in the comparative example, a deteriorated portion of the center segregation due to the insufficient amount of light reduction, and internal cracks due to excessive reduction amount are also generated, and the slab is inferior in internal quality as compared with the present invention example. was gotten.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Casting piece support roll 7 Conveying roll 8 Casting piece cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified phase 13 Solidification completion position 14 Light pressure lower belt 15 Roll segment 16 Frame 17 Tie rod 18 Worm jack 19 Load cell 20 Roll chock 21 Pinch roll 22 Hydraulic cylinder 23 Roll chock

Claims (2)

  The roll segment is used to reduce the slab at the end of solidification with a reduction amount equivalent to the solidification shrinkage amount, using a light reduction zone composed of a roll segment in which at least a pair of pinch rolls and a plurality of pairs of reduction rolls are incorporated. The load acting on the roll segment is measured at at least two sites, the upstream site and the downstream site, and the rolls of the pinch roll and the rolling roll arranged in the roll segment are measured based on the measured load load. The light pressure load to be loaded is obtained, and the pressing pressure of the pinch roll is set so that the light pressure load to be loaded on the pinch roll in the obtained light pressure load is equal to the load by the pinch roll. A light reduction control method for a slab in continuous casting.   2. The method of controlling a slab light reduction in continuous casting according to claim 1, wherein a load applied to the roll segment is measured at a position of a tie rod of the roll segment.

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