JP5920083B2 - Continuous casting method for steel slabs - Google Patents

Description

  The present invention relates to a method for continuously casting a steel slab for producing a steel slab having a slight center segregation.

  In the solidification process of steel, solute elements such as carbon, phosphorus, sulfur and manganese are concentrated on the unsolidified liquid phase side by redistribution during solidification. This is the microsegregation formed between dendrite trees. Thickness center of slab due to solidification shrinkage and heat shrinkage of slab being cast by continuous casting machine, bulging of solidified shell generated between rolls of continuous casting machine When voids are formed or negative pressure is generated, molten steel is sucked into this part, but since there is not a sufficient amount of molten steel in the unsolidified layer at the end of solidification, the molten steel concentrated by the above microsegregation Flows and accumulates in the center of the slab and solidifies. 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.

  The center segregation of the slab deteriorates the quality of the steel product. For example, in line pipe materials for oil transportation and natural gas transportation, hydrogen-induced cracking (also referred to as “HIC”) occurs from the center segregation due to the action of sour gas. In a line pipe material used in a sour gas environment, when MnS or Nb carbide is generated in the central segregation part, hydrogen that has penetrated into the steel due to a corrosion reaction diffuses and accumulates around the MnS or Nb carbide. Cracks occur due to internal pressure. Furthermore, since the center segregation portion has a high hardness, cracks propagate. This is hydrogen-induced cracking.

  Thus, since center segregation deteriorates the quality of steel products, many measures for reducing center segregation of slabs have been proposed from the continuous casting process to the rolling process.

  Among them, as a means for effectively reducing the center segregation of the slab, in the continuous casting machine, the slab at the end of solidification having an unsolidified layer is equivalent to the sum of the solidification shrinkage and the heat shrinkage. A method of casting while gradually reducing with a slab support roll at a reduction amount and a reduction speed is employed (see, for example, Patent Document 1 and Patent Document 2). The above technique of gradually reducing the slab during continuous casting with a reduction amount and a reduction speed corresponding to the sum of the solidification shrinkage amount and the heat shrinkage amount is called “light reduction” or “light reduction method”. Yes. This light reduction technology uses multiple pairs of slab support rolls aligned in the casting direction, and gradually reduces the slab with a light reduction amount commensurate with the sum of the solidification shrinkage and heat shrinkage of the slab. Reduces the volume of the solidified layer and prevents the formation of voids or negative pressure at the center of the slab, and at the same time prevents the flow of concentrated molten steel formed between dendritic trees, thereby reducing the center segregation of the slab. It is a technology.

  By the way, a continuous casting machine in recent years is mainly a roll segment type continuous casting machine in which a plurality of roll segments configured by reversing a frame in which a plurality of slab support rolls are arranged in the casting direction, Along with this, a roll group for applying a light rolling force to the slab (hereinafter referred to as “light rolling belt”) is also mainly composed of a plurality of roll segments. In this case, the light pressure lower belt is formed by adjusting the roll interval of the opposed slab support rolls to be larger on the entry side than on the exit side of the roll segment. In the light reduction belt, the state of the roll interval set so as to become gradually narrower toward the downstream in the casting direction is referred to as a “rolling gradient”. Since the reduction force acting on the slab is determined by the reduction gradient in the light reduction zone, in the case of the light reduction zone of the roll segment structure, the light reduction is performed in units of roll segments. Moreover, since the slab support roll of a light reduction belt | band | zone is a roll for performing a light reduction, it is also called the "light reduction roll" or the "reduction roll".

  The light reduction roll for improving center segregation is based on the premise that the slab having an unsolidified layer is drawn inside. In this case, the reduction resistance to the light reduction roll is the shortness of the slab that has been solidified in the thickness direction. It is only on the side, and the load on the lightly-rolled roll is small, and even if the lightly-rolled belt has a roll segment structure, a light-rolled amount equivalent to the sum of the amount of solidification shrinkage and the amount of heat shrinkage is applied to each light-rolled roll. Can be added to the slab. On the other hand, when rolling down the slab after completion of solidification, the entire width of the slab becomes the rolling resistance, and the roll segment structure light rolling roll has insufficient rolling force and cannot be rolled down. In order to prevent damage to the roll segment, the frame is opened by hydraulic pressure or a disc spring.

  Conventionally, in the light reduction technology, generally, the solidification completion position of the slab (also referred to as “crater end position”) is controlled within the range of the light reduction belt. However, this light reduction technique has the following problems.

  That is, in the method of controlling the solidification completion position within the range of the light pressure lower belt, the solidification completion position is actually determined on the cast piece at the end of solidification depending on the position in the casting direction in the roll segment forming the light pressure lower belt. There arises a problem that the amount of light reduction applied to the material changes. In other words, when the solidification completion position exists on the exit side of the roll segment that forms the light reduction zone, the unsolidified layer occupies most of the thickness center of the slab in the roll segment. The light pressure can be reduced according to the set value or close to the set value.

  On the other hand, when the solidification completion position exists on the entry side of the roll segment forming the light reduction belt, since most of the slab in the roll segment has been solidified, the slab is lightly reduced. As a result, an excessive load is applied to the roll segment, and when the load exceeds the load resistance, the frame is opened by hydraulic pressure or a disc spring. When the frame is opened by the oil pressure or disc spring to prevent overload, the roll interval of the opposing light reduction rolls becomes larger than the set value, and it becomes impossible to secure a predetermined reduction gradient. The interval and the roll interval on the exit side become the same, and it may be impossible to perform light reduction.

  There is an optimum range for the amount of light reduction for reducing the center segregation (see, for example, Patent Document 3). When the frame is opened and the amount of light reduction is insufficient, the flow of the concentrated molten steel occurs, resulting in a V-shape. Segregation occurs or central segregation worsens.

  Further, in actual continuous casting operation, the water temperature of the secondary cooling water, the molten steel temperature, and the slab drawing speed change, so that the solidification completion position of the slab varies in the casting direction. Accordingly, even if the light pressure lower belt is fixed at a predetermined position and the amount of secondary cooling water and the slab extraction are made the same to reduce the center segregation, it is applied to the slab as the solidification completion position changes. There is a possibility that the amount of light reduction greatly varies in the casting direction of the slab, and there is a possibility that center segregation is not prevented in all slabs in the casting direction.

JP-A-8-132203 JP-A-8-192256 JP-A-62-275556

  The level of quality requirements for continuous cast slabs has increased, and there has been a demand for slabs with less central segregation than ever before. Therefore, as a countermeasure against center segregation, the roll segment that forms the light reduction zone is under light reduction that is considered to be optimal for the slab, but as described above, in practice, the solidification completion position varies in the casting direction. As a result, the amount of light reduction applied to the slab is greatly deviated from the optimum value.

  Moreover, since the steel grade which requires light reduction is increasing, the casting ratio which implements light reduction is increasing more than before. Under light pressure, more load is applied to the roll segment than when light reduction is not performed, and even if the target site for light reduction includes the solidification completed part, until the load load from the slab exceeds the load resistance The roll segment is not opened, and the life of the roll segment forming the lightly pressed belt is shortened by such repeated load from the slab.

  The present invention has been made in view of the above circumstances, and its object is to provide a light reduction amount close to a set value to a slab even if the solidification completion position of the slab fluctuates during casting. It is possible to provide a continuous casting method of a steel slab that can improve the life of a roll segment that forms a lightly pressed belt.

The gist of the present invention for solving the above problems is as follows.
[1] The sum of solidification shrinkage and heat shrinkage of a slab at the end of solidification using a light reduction belt composed of one or more roll segments incorporating a plurality of light reduction rolls. In the continuous casting method for steel slabs, which is continuously cast while being reduced by a light reduction amount equivalent to, a disc spring for preventing overloading of the roll segment is provided on the roll segment forming the light reduction zone. A load applied to each tie rod, when the disc spring starts to change, is applied when the solid fraction of the slab thickness center at the roll segment inlet of the slab to be lightly reduced is 0.8. A continuous casting method for a steel slab, characterized in that the slab is lightly reduced after setting the load to 100 to 105% of the load applied to each tie rod when the slab is reduced at a set reduction speed.
[2] The set value of the disc spring installed on the downstream side of the roll segment is set larger than the set value of the disc spring installed on the upstream side of the roll segment. The continuous casting method for steel slabs according to [1] above.

  According to the present invention, when continuous casting is carried out while lightly lowering a slab using a lightly-lowered belt having a roll segment structure, the load of the disc spring installed on the roll segment forming the lightly-lowered belt is applied to this roll. Since continuous casting is performed by setting the solid phase ratio at the center of the slab thickness at the segment inlet to 100 to 105% of the load applied when the slab thickness is 0.8, the solid at the center of the slab thickness at the roll segment inlet is set. A slab having a phase ratio of less than 0.8 is subjected to a light reduction amount set in the roll segment or a light reduction amount approximate to the set light reduction amount. On the other hand, a roll that does not require light reduction The slab where the solid phase ratio at the center of the slab thickness at the segment entrance is 0.8 or more is not subjected to light reduction by the contraction of the disc spring, and as a result, the center segregation of the slab is reduced, Roll segment forming a light pressure zone Long life of the and is realized.

It is a schematic sectional drawing which shows an example of the roll segment which comprises a light pressure lower belt. It is the side schematic diagram of the slab continuous casting machine used when implementing the present invention. It is a figure which compares and shows the investigation result of the center segregation (segregation object component; manganese) of slab by the example of this invention, and a comparative example. The results of measuring the amount of change of the disc spring in the light pressure lower belt roll segment one downstream from the roll segment where the solid phase ratio at the center of the slab thickness is 0.8 existed. FIG. It is a figure which shows the casting charge until replacement | exchange of the roll segment which forms a light pressure lower belt in contrast with the example of this invention, and a prior art example.

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

  In the case where the slab at the end of solidification is lightly squeezed with a reduction amount corresponding to the sum of the amount of solidification shrinkage and the amount of heat shrinkage using the roll-structured light reduction zone, In order to improve the service life of the roll segment that forms the light reduction zone at the same time as making sure that the target reduction amount is slightly reduced, we investigated the light reduction behavior of the slab in the light reduction zone of the roll segment structure. 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 view showing an example in which five pairs of cast slab support rolls 6 are arranged in one roll segment 15 as light pressure rolls.

  As shown in FIG. 1, the roll segment 15 is composed of a pair of frames 16 and a frame 16 ′ holding five pairs of slab support rolls 6 via a roll chock 21. A total of four tie rods 17 (both upstream and downstream sides) are disposed through the frame 16 ', and the worm jack 19 installed on the tie rod 17 is remotely driven by a motor 20. Thus, the adjustment of the interval between the frame 16 and the frame 16 ′, that is, the adjustment of the rolling gradient in the roll segment 15 (the state of the roll interval set so as to become gradually narrower toward the downstream in the casting direction) is performed. ing. During casting, the worm jack 19 is self-locking and counters the bulging force of the slab 10 having an unsolidified layer, and under the condition that the slab 10 does not exist, that is, from the slab 10 to the slab support roll 6. The reduction gradient is adjusted under the condition that no load is applied. FIG. 1 shows an example in which the number of installed slab support rolls 6 is five, but in the light rolling belt of the roll segment structure, the rolling gradient is not related to the number of slab support rolls 6 installed. Adjustments are made.

  The tie rod 17 is provided with a disc spring 18 between the frame 16 ′ and the worm jack 19. The disc spring 18 is not configured by a single disc spring, but is configured by stacking a plurality of disc springs (the greater the number of disc springs, the higher the rigidity). The disc spring 18 has a certain thickness without contracting when a load load greater than a predetermined load does not act on the disc spring 18, but contracts when a certain predetermined load load is applied. Then, after a certain predetermined load is exceeded, it is configured to contract in proportion to the load. That is, when solidification of the slab 10 is completed, an excessive load is applied to the roll segment 15 by lightly reducing the solidified slab 10, and when such an excessive load is applied. In other words, the disc spring 18 contracts to open the frame 16 ′ so that an excessive load is not applied to the roll segment 15.

  A plurality of roll segments 15 having this structure are arranged in the casting direction to form a light pressure lowering belt, and the solidification completion position of the slab 10 is changed by changing the slab drawing speed and the amount of secondary cooling water to lightly lower the slab 10. Then, the displacement (change amount from the set position) in the rolling direction at the upstream end portion in the casting direction and the downstream end portion in the casting direction of the frame 16 ′ was measured. The frame 16 on the lower surface side is a continuous casting machine that is fixed to the foundation of the continuous casting machine and is configured not to move during casting.

  As a result, the displacement in the rolling direction of the frame 16 'tends to be larger on the downstream side in the casting direction than on the upstream side in the casting direction. In particular, in the roll segment where the solidification completion position exists, the displacement on the downstream side is less. It turns out that it grows. In addition, as a result of investigating the center segregation of the slab, if the displacement of the frame 16 'on the upstream side and the displacement of the frame 16' on the downstream side are equal in the vicinity of zero, the center segregation is minor. I understood. Further, it was found that when the load load at which the disc spring 18 starts to contract is set high, the displacement of the frame 16 'is reduced, but the wear and damage of the bearing of the roll chock 21 and the slab support roll 6 are increased.

  By the way, in continuous casting of steel, in the range where the solid phase rate at the thickness center portion of the slab 10 is greater than or equal to the flow limit solid phase rate, a gap is formed in the thickness center portion of the slab 10 due to bulging or negative pressure. It is known that the unsolidified layer cannot move even if sag occurs, and the center segregation of the slab 10 does not worsen further (for example, see Patent Document 3). That is, in the light reduction method for reducing the center segregation of the slab 10, it is known that it is not necessary to lightly reduce the solid phase ratio in the thickness center portion of the slab 10 beyond the flow limit solid phase ratio. Yes.

  Conventionally, the solidification completion position is controlled within the range of the light pressure lower belt because the solidification completion position changes even during casting, so only the range above the flow limit solid phase ratio can be excluded from the light pressure lower belt range. This is based on the difficulty, that is, based on reliably controlling the range of the slab 10 below the flow limit solid phase ratio within the range of the light pressure zone. Incidentally, the flow limit solid phase ratio of the steel slab is generally known to be 0.7 to 0.9. However, the present inventors have established a flow limit solid phase in order to reliably prevent center segregation. The rate was 0.8. Here, the solid phase ratio is defined as the solid phase ratio = 0 before the start of solidification and the solid phase ratio = 1.0 when the solidification is completed.

  From the above-mentioned measurement results of the displacement of the frame 16 'during casting, and from the two points that the solid phase ratio at the center of the slab thickness is not less than the flow limit solid phase ratio, there is no need for light reduction. The load load (hereinafter also referred to as “limit load load”) at which the Belleville spring 18 of the roll segment 15 forming the slab is changed has a solid phase ratio of 0.8 at the center of the thickness of the slab 10 at the entrance of the roll segment 15. At this time, when the slab 10 is lightly reduced at the set reduction speed, a value corresponding to the load applied from the slab 10 is set so that the solid fraction at the center of the slab thickness is less than 0.8. A slab of less than the limit solid phase ratio is subjected to a predetermined amount of light reduction, and the slab exceeding the limit solid phase ratio is not subjected to excessive light reduction by opening the frame 16 '. 10 center segregation reduction and roll segments that form a lightly pressed belt 5 was obtained knowledge that it is possible to achieve both life improvement. It should be noted that the load applied to the roll segment 15 forming the light reduction zone is determined by the size of the slab 10, the reduction speed, and the ratio of the liquid phase of the slab.

  The present invention is based on the above knowledge, and the continuous casting method of a steel slab according to the present invention is a light rolling belt composed of one or two or more roll segments each incorporating a plurality of light rolling rolls. In a continuous casting method for steel slabs, a roll that forms a lightly-reduced zone is used to continuously cast a slab at the end of solidification with a light-reduction amount corresponding to the sum of the solidification shrinkage amount and the heat shrinkage amount. The segment is provided with a disc spring for preventing overloading of the roll segment on each tie rod of the roll segment. When the slab thickness center part solid phase ratio is 0.8 at 100% to 105% of the load applied to each tie rod when the slab is reduced at the set reduction speed, Lightly reduce the piece It is characterized in.

  Hereinafter, a specific implementation method of the present invention will be described with reference to the drawings. FIG. 2 is a schematic side view of a slab continuous casting machine used in carrying out the present invention.

  As shown in FIG. 2, 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 a slab 10, and a predetermined position above the mold 5. 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 downstream side and upstream side of the solidification completion position 13 of the slab 10, the interval between the slab support rolls facing each other with the slab 10 interposed therebetween (this interval is referred to as “roll interval”) downstream in the casting direction. A light reduction belt 14 composed of a plurality of pairs of slab support rolls, which is set so as to become narrower toward the surface, that is, has a reduction gradient, is provided. 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”. 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 slab drawing speed (m / min).

  In the slab continuous casting machine 1, the light pressure lower belt 14 is configured by connecting three roll segments each including three pairs of light pressure lower rolls in the casting direction. Here, the structure of the roll segment 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 roll segment rolling gradient can be adjusted remotely by a worm jack installed on a tie rod that passes through the lower frame (corresponding to the frame 16) and the upper frame (corresponding to the frame 16 '). 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 has such a roll segment structure, the roll intervals of the three pairs of light pressure lower rolls arranged in the respective roll segments are collectively adjusted. 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. 2, the light pressure lower belt 14 is composed of three roll segments, but it may be one, two, or even four or more. . The slab support rolls arranged in one roll segment are three pairs, but need not be three pairs, and may be any number as long as there are two or more pairs.

  The limit load load of the disc springs installed in each roll segment forming the light pressure lower belt is preliminarily determined by using this roll segment for the slab 10 whose slab thickness central portion solid phase ratio is 0.8 at the entrance of the roll segment. It is set to 100 to 105% of the load applied from the slab 10 when lightly reduced.

  As a method for obtaining the load applied from the slab 10 when the slab 10 having a solid phase ratio of 0.8 at the inlet of the roll segment is lightly reduced by this roll segment, A method of calculating by a numerical simulation using a solid phase ratio, a planned reduction speed, and a size of a cast slab to be cast, a method of actually measuring with a strain gauge, and the like can be used. When actually measuring the load, the solid phase ratio at the center of the slab thickness at the entrance of the roll segment that forms the light reduction zone is calculated from the heat transfer solidification calculation or the propagation time of the ultrasonic slab that passes through the slab. Determine the load applied to the roll segment when the solid phase ratio at the center of the slab thickness at the entrance of the roll segment is 0.8 by attaching a measuring device such as a strain gauge to the roll segment frame 16 'or the bearing of the roll chock 21. By obtaining, it can be measured.

  In this way, the load when the slab thickness center part solid phase ratio at the entrance of the roll segment is 0.8 is determined in advance for each light reduction and slab size, and before casting, the casting conditions are Accordingly, the limit load load of the disc spring is set to 100 to 105% of the load load obtained in advance. The limit load of the disc spring can be adjusted by changing the material of the disc spring and the number of disc springs to be installed.

  Even if the light reduction amount, the solidification completion position of the slab, and the slab size are the same, some errors may occur in the load applied to the roll segment due to the difference in temperature distribution in the slab thickness direction. If the error is not taken into account, the disc spring may contract and the set light reduction may not be performed even though there is a slab to be lightly reduced in the roll segment. Therefore, in the present invention, in consideration of the error, 105% of the load applied when the solid phase ratio at the center of the slab thickness at the entrance of the roll segment is 0.8 is set as the upper limit. Increasing this error is not preferable because it shortens the life of the roll segment.

  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 0.4 to 1.5 mm / min. Therefore, depending on the expected slab drawing speed. A rolling gradient of the light rolling belt 14 is set in advance. When the rolling speed is less than 0.4 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 light reduction is about 2 to 6 mm.

  Using the slab continuous casting machine 1 configured as described above, the present invention is carried out as follows.

  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 poured 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 layer 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 region, the slab drawing speed and the secondary cooling are performed so that the position where the solid phase ratio at the center of the slab thickness becomes 0.8 is not downstream of the light pressure lower belt 14 in the casting direction. Adjust the amount of water. When the position where the solid phase ratio at the center of the slab thickness becomes 0.8 passes through the light pressure lower zone 14, the unsolidified layer 12 moves in the range where the solid phase ratio at the center of the slab thickness is less than 0.8. This is because segregation may occur. In order to ensure that the position where the solid phase ratio at the center of the slab thickness is 0.8 is not downstream of the light pressure lower belt 14 in the casting direction, the solidification completion position 13 is within the range of the light pressure lower belt 14. It is preferable to control so that. This can be realized by obtaining the slab drawing speed and the amount of secondary cooling water that satisfy the above conditions in advance using a method such as heat transfer solidification calculation.

  Further, in order to reduce the center segregation of the slab 10, it is necessary to reduce the slab 10 with the light reduction belt 14 at least when the slab center solid phase ratio becomes 0.4. It is necessary to set the casting direction length of the light pressure lower belt 14 so as to satisfy the above. This is because when the solid phase ratio at the center of the slab is less than 0.4, even if there are many unsolidified layers 12 and 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.

  By casting in this way, the slab 10 having a solid phase ratio of less than 0.8 at the center of the slab thickness at the roll segment inlet has a light reduction amount or a set light reduction amount set in the roll segment. On the other hand, a slab 10 having a solid phase ratio of 0.8 or more at the center of the slab thickness at the roll segment inlet, in which a light reduction amount similar to is applied, is unnecessary. As a result of the constriction of the disc spring, light reduction is not performed, and as a result, reduction of the center segregation of the slab 10 and extension of the life of the roll segment forming the light reduction belt 14 are achieved.

  A test was performed in which the low carbon aluminum killed steel was continuously cast into a size having a slab thickness of 250 mm and a slab width of 2000 mm. The roll segments forming the lightly pressed belt are No. 8 segments (No. 8 seg) from 21 to 23 m from the meniscus (molten steel surface position in the mold), No. 9 segments (No. 9 seg) from 23 to 25 m from the meniscus, No. 10 segment (No. 10 seg) of 25 to 27 m from the meniscus, and No. 11 segment (No. 11 seg) of 27 to 29 m from the meniscus. The specific water amount of the secondary cooling was 1.3 to 1.6 L / kg-steel, and the slab drawing speed was 1.2 to 1.4 m / min.

  The amount of light reduction required to prevent the flow at the end of coagulation is calculated by heat transfer coagulation calculation in advance, and the rolling gradient of No. 8 segment to No. 11 segment is set to 0.35 to 0.35 for both the present invention example and the comparative example. It was set to 0.45 mm / m (0.7 to 0.9 mm per roll segment).

  Table 1 shows the rolling gradient, slab drawing speed, and rolling speed set in each test. In addition, when the solid phase ratio at the center of the slab thickness at the roll segment inlet is 0.8, the load applied to each tie rod of the roll segment is calculated by numerical simulation, and the disc spring Table 1 also shows the set load (limit load load) at the time of shrinkage and the ratio of the set load (limit load load) to the calculated load obtained by calculation. Note that the set load (limit load load) when the disc spring contracts does not differ between the roll segments, and is the same in the No. 8 to 11 segments.

  As shown in Table 1, in Examples 1 to 5 of the present invention, both the disc spring installed on the upstream tie rod of the roll segment and the disc spring installed on the downstream tie rod are within the range of 100 to 105% within the scope of the present invention. Set to. On the other hand, in Comparative Examples 1 to 3, both the disc spring installed on the upstream tie rod of the roll segment and the disc spring installed on the downstream tie rod are set to less than 100%, which is outside the scope of the present invention. 4 and 5, only the disc spring installed on the upstream tie rod of the roll segment is set within the range of 100 to 105% within the scope of the present invention, and the disc spring installed on the downstream tie rod of the roll segment is of the present invention. It was set to less than 100%, which is outside the range.

  During continuous casting, longitudinal wave ultrasonic waves are transmitted through the slab through the ultrasonic transceiver installed in the No. 11 segment, and the propagation time of this longitudinal wave ultrasonic wave in the slab is measured. Based on this propagation time Thus, the solidification completion position of the slab (the position where the solid phase ratio at the center of the slab thickness reached 1.0) was detected. Furthermore, the distance from the solidification completion position obtained based on the propagation time of the longitudinal wave ultrasonic wave to the position where the solid phase ratio at the center of the slab thickness is 0.8 is calculated by heat transfer solidification, and the slab thickness The roll segment in which the position where the solid phase ratio in the center part becomes 0.8 was identified. Table 2 shows the distance from the meniscus to the position where the solid phase ratio at the center of the slab thickness becomes 0.8, and the roll segment where the position where the solid phase ratio at the center of the slab thickness becomes 0.8 exists. Show. The roll segment having a position where the solid phase ratio at the center of the slab thickness is 0.8 is within the range from the exit side of the No. 9 segment to the exit side of the No. 10 segment according to the slab drawing speed became.

  Further, in the No. 8 segment to the No. 11 segment, the change amount (shrinkage amount) of the disc spring installed on the tie rod on the downstream side of each roll segment was measured with an overcurrent distance meter. Table 2 shows the presence or absence of contraction of the disc spring of the downstream tie rod in the roll segment where the solid phase ratio at the center of the slab thickness is 0.8. In Inventive Examples 1 to 5, the disc spring did not contract no matter where in the roll segment the solid phase ratio at the center of the slab thickness was 0.8. On the other hand, in Comparative Example 1, the position where the solid phase ratio at the center of the slab thickness becomes 0.8 was located on the exit side of the No. 10 segment, so the disc spring did not contract. In ~ 5, it was confirmed that the disc spring contracted and the frame opened at the downstream portion of the roll segment.

  Also, during casting, the displacement of the frame in the rolling direction is measured by a laser distance meter at the upstream end and the downstream end of the upper frame (corresponding to the frame 16 ') of each roll segment. The rolling reduction was calculated, and the rolling speed was calculated from the determined rolling gradient. Table 2 shows the rolling speed measured in the roll segment where the solid phase ratio at the center of the slab thickness is 0.8. In Inventive Examples 1 to 5, it was confirmed that the light reduction was performed at the reduction speed close to the set value or according to the set value. On the other hand, in Comparative Example, light reduction was performed at a reduction speed close to the set value in Comparative Example 1 in which no contraction of the disc spring was observed, but in Comparative Examples 2 to 5, the reduction was greatly different from the set value. It was speed.

  FIG. 3 shows the results of the investigation of the center segregation of the slab (segregation target component: manganese) in the present invention example and the comparative example. This center segregation is a value obtained by analyzing the manganese at the center of the slab cross section by EPMA and dividing the manganese concentration obtained by EPMA by the manganese concentration of the sample taken from the molten steel. The horizontal axis of FIG. 3 shows the ratio of the rolling speed actually applied to the slab shown in Table 2 with respect to the rolling speed set in Table 1. In the comparative example, with respect to the set reduction speed, there was a case where light reduction was possible and a case where light reduction was not possible, and as a result, the degree of center segregation varied. As shown, or because the light reduction was performed at a reduction speed close to the set value, the center segregation was good.

  A test was performed in which the low carbon aluminum killed steel was continuously cast into a size having a slab thickness of 250 mm and a slab width of 2000 mm. The roll segments forming the light pressure zone are: No. 8 segment from 21 to 23 m from the meniscus, No. 9 segment from 23 to 25 m from the meniscus, No. 10 segment from 25 to 27 m from the meniscus, and No. 10 from 27 to 29 m from the meniscus. .11 segments. The specific water amount of the secondary cooling was 1.3 to 1.6 L / kg-steel, and the slab drawing speed was 1.2 to 1.4 m / min.

  The amount of light reduction required to prevent the flow at the end of coagulation is calculated by heat transfer coagulation calculation in advance, and the rolling gradient of No. 8 segment to No. 11 segment is set to 0.35 to 0.35 for both the present invention example and the comparative example. It was set to 0.45 mm / m (0.7 to 0.9 mm per roll segment).

  In the present invention example, the set load (limit load load) when the disc spring installed on the tie rod of each roll segment contracts is calculated by numerical simulation, and the center of the slab thickness at the roll segment inlet is obtained. 100% to 105% of the load applied to each tie rod of the roll segment when the solid phase ratio is 0.8, while in the comparative example, the disc spring installed on the tie rod of each roll segment contracts The load applied to each tie rod of the roll segment when the solid phase ratio at the center of the slab thickness at the roll segment inlet is 0.8, which is calculated by numerical simulation to calculate the set load (limit load load) Of 105% and 120% or less.

  Under such casting conditions, the present invention example and the comparative example were each continuously cast by 10 charges.

  In FIG. 4, the amount of change (shrinkage) of the disc spring in the light pressure belt roll segment one downstream from the roll segment where the solid phase ratio at the center of the slab thickness is 0.8 is measured. Results are shown. As shown in FIG. 4, the disc spring is not changed in the comparative example, and the slab whose solid fraction at the center of the slab thickness exceeds 0.8 is lightly reduced. It was found that an excessive load was applied. On the other hand, in the example of the present invention, the disc spring is contracted, and in the light pressure lower belt roll segment that is one downstream from the roll segment where the solid phase ratio at the center portion of the slab thickness is 0.8, the cast spring is cast. The piece was not lightly pressed, and it was confirmed that an excessive load was prevented from being applied to this roll segment. In the comparative example of FIG. 4, the value is such that the disc spring is slightly changed. However, this has a “back” in the roll segment itself, and even if the disc spring is not contracted, it is always 0. This is because a backlash of about 2 mm is measured.

  After that, as a result of continuous casting by applying the present invention, as shown in FIG. 5, by applying the example of the present invention, the average casting charge until the replacement of the roll segment forming the light pressure lower belt exceeds 4000 charges, In the past, the roll segment was replaced with a charge of about 2,000 charges, whereas the life of the roll segment could be greatly extended.

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 Cast piece cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified layer 13 Solidification completion position 14 Light pressure lower belt 15 Roll segment 16 Frame 17 Tie rod 18 Disc spring 19 Worm jack 20 Motor 21 Roll chock

Claims (1)

Equivalent to the sum of the amount of solidification shrinkage and the amount of heat shrinkage of the slab at the end of solidification using a light pressure lower belt composed of one or two or more roll segments incorporating a plurality of light reduction rolls. In the continuous casting method for steel slabs, in which continuous casting is performed while reducing with a light reduction amount,
In the roll segment constituting the light pressure lower belt, a disc spring for preventing overload of the roll segment is installed in each tie rod of the roll segment, and the load load at which the disc spring starts changing is lightly reduced. slab 100 to 105% of the load thickness center solid phase ratio is loaded into the tie rods upon reduction at a reduction rate set the slab when it is 0.8 in the roll segment inlet of the slab And set the set value of the load of the disc spring larger than the set value of the disc spring installed on the upstream side of the roll segment than the set value of the disc spring installed on the upstream side of the roll segment. The slab thickness center part at the roll segment inlet is not less than 0.8, and the slab that does not need to be lightly pressed is set not to be lightly pressed . said the slab Characterized by soft reduction at a reduction zone, the continuous casting method of steel slabs.

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