JPWO2014020860A1 - Steel continuous casting method - Google Patents

Abstract

The slab is intentionally bulged, and then the slab having an unsolidified layer is squeezed continuously for continuous casting, without causing the slab to break out and without causing internal cracks in the solidified shell, In addition, the center segregation of the slab and the positive segregation near the center of the thickness are reduced. Until the thickness of the solidified shell 11 of the slab 10 reaches 15 mm, the roll opening of the slab support roll 6 is set to the same value as that immediately below the mold, and then the roll opening is increased stepwise. Is bulged with a total amount of bulging of 3 to 20 mm, and then the roll opening is set constant in the section of 0.5 to 5.0 m on the downstream side in the casting direction. The slab of 2 to 0.9 is squeezed at least once by the squeezing roll 7 under the condition that the product of the squeezing speed and the casting speed is 0.3 to 1.0 mm · m / min 2.

Description

  In the present invention, the slab drawn from the mold is intentionally bulged without breaking out and without causing internal cracks, and then the slab during continuous casting having an unsolidified layer is crushed and cast. The present invention relates to a method for reducing the center segregation of a piece.

  In continuous casting of steel, molten steel is poured into a water-cooled mold and cooled (called “primary cooling”) to form a solidified shell on the inner wall of the mold. And the slab which uses this solidified shell as an outer shell is continuously pulled out downward while being supported by a number of slab support rolls installed below the mold. During this drawing, the surface of the slab is cooled with spray cooling water or the like (referred to as “secondary cooling”) and completely solidified to the center of the thickness of the slab, and then the slab is cut to a predetermined length. Manufactures steel slabs.

  An internal defect called center segregation may occur at the thickness center of the steel slab thus manufactured. This center segregation is caused by concentration of solute components such as carbon (C), sulfur (S), phosphorus (P), and manganese (Mn) in the final solidified portion of the slab, that is, the thickness center portion. . Center segregation of the slab is known to cause a decrease in toughness of the thick steel plate, which is the final product, and hydrogen-induced cracking in large-diameter welded steel pipes that are manufactured by bending after the thick steel plate is bent. .

  The generation mechanism of the center segregation of the slab is considered as follows. That is, as the solidification of the slab progresses, the solute component concentrates between dendritic dendritic trees, which are the solidification structure of the slab, based on the partition law. This is the microsegregation formed between dendrite trees. When a void is formed or a negative pressure is generated at the center of the slab thickness due to shrinkage of the slab during solidification or swelling of the slab called bulging, the molten steel is sucked into this part. However, since there is not a sufficient amount of molten steel in the unsolidified layer at the end of solidification, the molten steel concentrated in the solute component by the above-mentioned microsegregation flows and accumulates in the center of the thickness of the slab, and the accumulated state To solidify. Since the molten steel enriched with the solute component accumulates and solidifies, a concentrated zone of the solute component is formed at the center of the slab thickness. This concentrated zone is central segregation, and is called macro segregation in contrast to the above micro segregation.

  To prevent the center segregation of the slab, the movement of molten steel (concentrated molten steel) that has concentrated solute components and that is present between dendritic trees, and the local accumulation of concentrated molten steel It is effective to prevent this, and several methods for preventing center segregation using these principles have been proposed.

  Among them, in the continuous casting machine, a slab at the end of solidification having an unsolidified layer is subjected to a reduction roll group ("light reduction") with a reduction amount and a reduction speed corresponding to the sum of the solidification shrinkage and the heat shrinkage. A method of casting while being gradually reduced (referred to as “light reduction”) is widely performed (refer to Patent Document 1, for example). Here, the total reduction amount is the reduction amount from the start of reduction to the end of reduction.

  This light reduction method is a technique for preventing central segregation by preventing the movement of concentrated molten steel existing between dendritic trees. However, the reduction force is weak because it is the total reduction amount slightly exceeding the amount of coagulation shrinkage. That is, in the light reduction method, since the reduction force is weak, when the solidification completion position of the slab is not the same position in the slab width direction, the already solidified part becomes the reduction resistance, and the unsolidified part to be reduced It occurs that no rolling force is applied. In such a case, the effect of improving the center segregation is small in the portion where no rolling force is applied. Therefore, there is a limit to the effect of improving the center segregation even with the light reduction method.

  Further, as a method for improving the center segregation of the slab, a method of reducing the slab at the end of solidification by a rolling roll pair with a total reduction amount far larger than the sum of the solidification shrinkage amount and the heat shrinkage amount is also performed. (For example, see Patent Document 2). This method is also called “large reduction” in contrast to the above “light reduction”.

  In order to apply a large reduction to the slab, the short side portions of the slab that have been solidified and located at both ends of the slab must be reduced, and a large reduction force is required. In other words, because a large rolling force is applied, when the large rolling method is applied to an ordinary continuous casting machine, the support frame that supports the pair of rolling rolls is bent and a sufficient rolling effect cannot be obtained. Will occur. Moreover, the operation may be difficult due to equipment troubles such as bending or breaking of the rolling roll. In order to prevent such bending of the support frame and bending of the rolling roll pair, it is necessary to make the continuous casting equipment capable of withstanding a high load. The problem due to this high load occurs in the same way when the total amount of reduction that reduces the short side of the slab is increased in the light reduction method.

  Therefore, several proposals have been made to increase the total amount of reduction in the light reduction method and reduce the load on the continuous casting machine due to reduction, or to reduce the load on the continuous casting machine in the large reduction method. Has been made.

  For example, in Patent Document 3, the slab is intentionally bulged at a position where the solid fraction of the center part of the slab is 0.1 or less, and the thickness of the center part in the width direction of the slab is reduced within the mold. The thickness is 20 to 100 mm thicker than the thickness of the side, and then at least one reduction roll pair immediately before the solidification completion position gives a reduction of 20 mm or more per reduction roll pair to reduce the amount corresponding to the total amount of bulging. However, methods for preventing center segregation have been proposed. Here, the total amount of bulging is the amount of bulging from the start of intentional bulging to the end of intentional bulging.

  In Patent Document 4, the thickness of the center part in the width direction of the slab is equal to 10% to 50% of the thickness of the short side of the slab until the thickness of the unsolidified layer of the slab reaches 30 mm. Bulging only intentionally, and then, by at least one reduction roll pair until the solidification completion position, reduction is applied with a reduction gradient of 80 mm / m or more per slab length to reduce the amount corresponding to the total amount of bulging, and central segregation A method for preventing this problem has been proposed.

  Further, in Patent Document 5, after 3% or more and 25% or less of the thickness of the slab at the start of intentional bulging is positionally bulged, the slab center part solid phase ratio is 0.2 or more and 0.7. A method has been proposed in which center segregation is prevented by reducing an arbitrary position of a slab in the following range by a thickness corresponding to 30% or more and 70% or less of the total amount of bulging by one reduction roll pair.

JP-A-8-132203 JP-A-6-218509 JP-A-9-57410 JP-A-9-206903 JP 2000-288705 A

  In Patent Documents 3 to 5, the slab is squeezed within an amount corresponding to the total amount of bulging or less, so the short side portion of the slab is not squeezed and continuous casting by squeezing is performed. The load on the machine is reduced. However, Patent Documents 3 to 5 have the following problems.

  That is, in Patent Documents 3 to 5, when the slab is intentionally bulged, the thickness of the solidified shell of the slab before bulging is not defined, and therefore when the bulging start time is too early, There is a risk of breakout due to cracking and swelling of the solidified shell. Further, Patent Documents 3 to 5 do not stipulate the bulging speed at which the slab is intentionally bulged. Therefore, if the slab is suddenly bulged, internal cracking occurs in the solidified shell that bulges. If this internal crack is severe, there is even a risk of breakout.

  Furthermore, in Patent Documents 3 to 5, the bulging area and the reduction area are continuously installed in the continuous casting machine, and the shape of the bulged slab is stable because the slab is immediately reduced after bulging. However, depending on the part of the slab, there is a possibility that the center segregation is not improved because the rolling force is not transmitted to the central part of the thickness of the slab. Furthermore, when the reduction time and the total reduction amount are not appropriate, also in Patent Documents 3 to 5, the center segregation of the solute component occurs in the center of the slab thickness, or the solute component near the center of the slab thickness. Or positive segregation.

  The present invention has been made in view of the above circumstances, and the first object is to intentionally bulge the slab drawn from the mold, and then unsolidify the interior to reduce segregation of the slab. In continuous casting of a slab by rolling down a slab having a layer, the slab drawn from the mold is intentionally bulged without causing breakout and without causing internal cracks in the solidified shell of the slab. And it is providing the continuous casting method of steel which can reduce the center segregation of a slab, and the positive segregation in the thickness center part vicinity of a slab.

  The second purpose is to intentionally bulge the slab drawn from the mold, and then to reduce the segregation of the slab by rolling the slab having an unsolidified layer inside with a light reduction belt. In continuous casting of slabs, the total amount of bulging when intentionally bulging can be reduced, and the total amount of bulging during intentional bulging, the timing of reduction under light pressure, the total amount of reduction, the speed of reduction, etc. are optimized. Thus, it is an object of the present invention to provide a continuous casting method of steel that can stably reduce the center segregation of the slab without breaking out the slab.

The gist of the present invention for solving the above problems is as follows.
[1] The roll opening of the slab support rolls arranged in the casting direction is set to be the same as the roll opening just below the mold until the solidified shell thickness of the slab drawn from the continuous casting mold reaches at least 15 mm. ,
Thereafter, the roll opening of the slab support roll is gradually increased toward the downstream side in the casting direction, and the long side surface of the slab is bulged with a bulging total amount of 3 to 20 mm,
After bulging, in the section of 0.5 to 5.0 m downstream in the casting direction, the roll opening of the slab support roll is set constant so that the thickness of the slab does not change,
Subsequently, the product of the rolling speed and the casting speed is 0.3 to 1.0 mm · m / min ^{2 on} the long side surface of the slab having a solid phase ratio of 0.2 to 0.9 at the thickness center of the slab. A steel continuous casting method in which the steel sheet is reduced at least once by a reduction roll under the following conditions.
[2] When bulging the long side surface of the slab, the roll opening degree of the slab support rolls arranged in the casting direction is increased stepwise with a gradient of 4.0 mm or less per 1 m of the casting direction. 1] The continuous casting method of steel described in 1].
[3] The roll opening degree of the plurality of pairs of slab support rolls is gradually decreased toward the downstream side in the casting direction, and the long side surface of the slab having a rectangular cross section drawn out from the continuous casting mold is formed. A rolling force is applied such that the product of the rolling speed and the casting speed is 0.3 to 1.0 mm · m / min ^2, and the width of the slab short side is reduced by this rolling force at the bottom of the mold. 3-20mm smaller than the width,
After reducing the short side width of the slab, the roll opening of the pairs of slab support rolls is increased stepwise toward the downstream side in the casting direction, and the long side surface of the slab is increased by a bulging total amount of 3 to 20 mm. Bulging,
After bulging the long side of the slab, in the light pressure zone where the roll opening of multiple pairs of slab support rolls is gradually reduced toward the downstream side in the casting direction, the solid fraction at the center of the slab thickness is From the time of at least 0.2 to the time of 0.9 or more, a rolling force with a product of the rolling speed and the casting speed of 0.3 to 1.0 mm · m / min ² is applied to the long side surface of the slab. The continuous casting method of steel, in which the long side surface of the slab is squeezed by the squeezing force with a squeezed total amount equivalent to the bulging total amount or a squeezed total amount smaller than the bulging total amount.
[4] The solidification completion position of the slab is detected online using a solidification completion position detector, and the solid phase ratio at the thickness center of the slab is at least 0.2 based on the detected solidification completion position information. From the following time to 0.9 or more, any one or two of the amount of secondary cooling water, the width of the secondary cooling, and the casting speed are set so that the slab is located in the light pressure lower zone. The continuous casting method for steel according to the above [3], wherein the seed or more is adjusted.

  According to the invention of [1] above, since the long side surface of the slab is intentionally bulged after the thickness of the solidified shell of the slab exceeds 15 mm, breakout of the slab can be prevented in advance. In addition, after bulging the slab, the roll opening is set so that the thickness of the slab does not change in the section of 0.5 to 5.0 m downstream in the casting direction. Shape. Since the solidified shell has a flat shape, the reduction efficiency during the subsequent reduction is improved, and it is possible to stably reduce the center segregation of the slab and the positive segregation near the center of the slab thickness. Become.

  In addition, according to the invention [2], the short side width of the slab is made narrower than the mold lower end dimension when the deformation strength of the solidified shell is low, so that the total amount of bulging when intentionally bulging is reduced. Can do. Thereby, the breakout of a slab is prevented and the internal crack of a slab is suppressed. Moreover, since the product of the time of light reduction and a reduction speed, and a casting speed is prescribed | regulated in the case of light reduction, the center segregation of a slab can be reduced stably. Further, in the light reduction belt, the short side portions at both ends of the slab are not reduced, and the slab can be reduced with a small load, and the load on the equipment constituting the light reduction belt is reduced. In addition, since the short side of the slab is not rolled down, the rolling force is transmitted to the inside of the slab even in the unsteady casting zone at the beginning or end of the continuous casting operation where the short side of the slab is likely to be particularly cold. The center segregation of the slab in the steady casting region can be greatly improved as compared with the conventional case. As a matter of course, in the steady casting region, the center segregation of the slab can be improved in the same manner or more than the conventional one.

FIG. 1 is a schematic sectional view of a slab continuous casting machine used in the first embodiment of the present invention. FIG. 2 is a schematic side view of a vertical bending slab continuous casting machine used in the second embodiment of the present invention. FIG. 3 is a diagram showing an example of a roll opening profile of a slab support roll according to the second embodiment. FIG. 4 is a diagram showing a sampling position of a cross section sample in Example 1 and an analysis position of manganese (Mn) by EPMA.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. First, the first embodiment of the present invention will be described. FIG. 1 is a schematic sectional view of a slab continuous casting machine used in the first embodiment of the present invention.

  As shown in FIG. 1, a molten steel 9 is injected into the slab continuous casting machine 1, and a mold 5 is installed for cooling and solidifying the molten steel 9 to form the outer shell shape of the slab 10. . A tundish 2 for relaying and supplying molten steel 9 supplied from a ladle (not shown) to the mold 5 is installed at a predetermined position above the mold 5. A sliding nozzle 3 for adjusting the flow rate of the molten steel 9 is installed on 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, below the mold 5, a plurality of pairs of slab support rolls including a guide roll 6, a reduction roll 7 and a pinch roll 8 are arranged. A spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is disposed in the gap between the slab support rolls adjacent to each other in the casting direction, thereby forming a secondary cooling zone. The slab 10 is configured to be cooled by cooling water (also referred to as “secondary cooling water”) sprayed from a spray nozzle in the secondary cooling zone while being drawn.

  The molten steel 9 injected from the tundish 2 into the mold 5 is cooled by the mold 5 to form a solidified shell 11. The slab 10 having the solidified shell 11 as an outer shell and the inside as an unsolidified layer 12 is continuously supported below the mold 5 by the pinch roll 8 while being supported by the guide roll 6 and being reduced by the reduction roll 7. Pulled out. Meanwhile, the slab 10 is cooled by the secondary cooling water in the secondary cooling zone, and the thickness of the solidified shell 11 is increased.

  Until the slab 10 passes through the mold 5 and the thickness of the solidified shell 11 of the slab 10 reaches at least 15 mm, the roll opening of the guide roll 6 is set to be the same as the roll opening just below the mold. The thickness of the solidified shell 11 is obtained by a two-dimensional heat transfer solidification calculation considering the casting conditions, a sensor for measuring the thickness of the solidified shell from the transmission time of ultrasonic waves that pass through the slab, or the like. When the thickness of the solidified shell 11 is different between the long side surface and the short side surface of the slab 10 due to casting conditions, the thinner solidified shell 11 is the target.

  After the thickness of the solidified shell 11 of the slab 10 exceeds 15 mm, the roll opening degree of the guide rolls 6 arranged in the casting direction is increased stepwise toward the downstream side in the casting direction, and the long side surface of the slab 10 Is intentionally bulged in the slab thickness direction with a total bulging amount of 3 to 20 mm. In this case, since the length in the casting direction of the continuous casting machine is limited, it is preferable to start intentional bulging until the thickness of the solidified shell 11 reaches 30 mm. Here, the roll opening is a gap distance between slab support rolls facing each other with the slab 10 interposed therebetween, and is also called a roll interval. The total amount of bulging is the amount of bulging of the slab 10 from the start of intentional bulging to the end of intentional bulging.

  If the slab 10 is intentionally bulged in a state where the thickness of the solidified shell of the slab 10 is 15 mm or less, the strength of the solidified shell 11 is not sufficient, and a crack or crack occurs in the solidified shell 11 due to stress due to bulging. There is also a risk that breakout may occur due to cracks or cracks in the solidified shell 11. In contrast, in the first embodiment of the present invention, since the bulging is performed after the thickness of the solidified shell 11 exceeds 15 mm, the strength of the solidified shell 11 is ensured, and cracks and cracks in the solidified shell 11 are obviated. Can be prevented. Thereby, the breakout resulting from the crack of the solidified shell 11 and a crack is naturally avoided.

  By increasing the roll opening degree of the guide roll 6 stepwise toward the downstream side in the casting direction, the long side surface of the slab 10 is formed by the molten steel static pressure of the unsolidified layer 12 existing inside the guide roll 6. Bulging following the roll opening. On the other hand, the short side surface of the slab 10 is narrower than the long side surface of the slab and has a low temperature and high rigidity at the corner (the intersection of the long side surface of the slab and the short side surface of the slab). Since it is supported, even if the roll opening degree of the guide roll 6 is increased stepwise, the shape of the slab short side surface hardly changes. Similarly, the slab short side surface side of the long side surface of the slab 10 does not change. On the long side surface of the slab 10, bulging occurs in a range from the portion having the unsolidified layer 12 inside to the center of the long side surface, which is farther from the short side surface of the slab.

  When bulging occurs on the long side surface of the slab 10, the short side surface side of the long side surface of the slab 10 does not change, so the short side surface side of the long side surface of the slab 10 contacts the guide roll 6. It will be in a state that does not. When the rate of increase in the thickness of the solidified shell 11, that is, the rate of increase in the slab thickness by bulging is faster than the rate of solidification, the thickness of the unsolidified layer 12 increases by bulging.

  In this case, in order to reduce the stress acting on the solidified shell 11 due to bulging, that is, in order to prevent the internal crack of the solidified shell 11 and the breakout due to the crack or crack of the solidified shell 11, the guide roll It is preferable to gradually increase the roll opening degree 6 with a gradient of 4.0 mm or less per m in the casting direction, desirably 1.0 mm or less per m in the casting direction toward the downstream side in the casting direction. This is because when the bulging is performed, when the roll opening degree of the guide roll exceeds 4.0 mm per 1 m in the casting direction (hereinafter referred to as “4.0 mm / m”), the gradient is too large and the slab 10 is internally cracked. However, if it is 4.0 mm / m or less, internal cracking is prevented.

After bulging the slab 10, the roll opening of the guide roll 6 is kept constant so that the thickness of the bulged slab 10 does not change in the section of 0.5 to 5.0 m toward the downstream side in the casting direction. Set. Thereafter, the slab 10 having a solid phase ratio of 0.2 to 0.9 at the center of the slab thickness is such that the product of the rolling speed and the casting speed is 0.3 to 1.0 mm · m / min ^2. Then, the slab is squeezed at least once or multiple times in the slab thickness direction by the squeezing roll 7. In this case, the thickness of the slab 10 after being reduced by the reduction roll 7 is equal to or greater than the thickness of the slab before bulging. That is, no reduction is performed to reduce the short side surface of the slab 10.

  In the first embodiment of the present invention, after the slab 10 is bulged, the thickness of the bulged slab 10 is not changed in the section of 0.5 to 5.0 m toward the downstream side in the casting direction. Since the roll opening degree of the guide roll 6 is set to be constant, the thickness of the solidified shell 11 in the bulged portion of the slab 10 is increased, the solidified shell 11 becomes a flat shape, and the reduction efficiency during the subsequent reduction is reduced. improves.

  After bulging, the length of the section in which the roll opening of the guide roll 6 is set to be constant is 0.5 m or more, so that the growth of the solidified shell thickness of the bulged portion of the slab 10 is promoted, and the solidified shell thickness Becomes a flat shape. This makes it possible to perform uniform reduction in the slab width direction during subsequent reduction. When the length of the section is less than 0.5 m, it is too short to obtain this effect. On the other hand, after the bulging, the length of the section where the roll opening degree of the guide roll 6 is set to be constant is 5 m or less, so the thickness of the solidified shell 11 in the bulged portion of the slab 10 does not become excessively thick. Thereby, the rolling efficiency at the time of rolling performed after that improves. If the length of the section exceeds 5 m, the thickness of the solidified shell 11 is excessively increased and the rolling efficiency is lowered.

  In the first embodiment of the present invention, the reason why the intentional total bulging amount is in the range of 3 to 20 mm is as follows. The thickness of the slab slab is generally 200 to 300 mm. Therefore, when the slab bulged over 20 mm is squeezed, an excessive reduction equipment is required, and the equipment cost becomes expensive, and the internal cracks. The amount of generation increases. In order to prevent this, the upper limit of the total amount of intentional bulging is set to 20 mm. On the other hand, when the intentional total bulging amount is less than 3 mm, the thickness that can be reduced is small, and the effect of discharging the molten steel concentrated in the solute component to the upstream side in the casting direction is small. That is, the effect of improving the center segregation of the slab 10 is small. Therefore, in order to obtain the effect of improving the center segregation of the slab 10, the intentional total bulging amount is set to 3 mm or more.

Here, the intentional total bulging amount is specifically the slab thickness at the position of the most downstream guide roll 6 where the roll opening degree of the guide roll 6 is set to be the same as the roll opening degree immediately below the mold ( D ₀ ) and a difference (= D ₁ −D ₀ ) between the slab thickness (D ₁ ) at the position of the guide roll 6 closest to the most upstream reduction roll 7.

  In the first embodiment of the present invention, after bulging the slab 10, the slab 10 having a solid phase ratio of 0.2 to 0.9 at the center of the slab thickness is once or less by the rolling roll 7. Decrease several times. That is, the slab 10 having a solid phase ratio of 0.2 to 0.9 at the center of the slab thickness is squeezed at least once by the squeezing roll 7 (two reductions are performed in FIG. 1). Here, the solid phase rate is defined as a solid phase rate of 0 before the start of solidification and a solid phase rate of 1.0 at the time of completion of solidification. It can be calculated by dimensional heat transfer solidification calculation.

  When the slab 10 having a solid phase ratio of less than 0.2 at the center of the slab thickness is squeezed and not squeezed thereafter, the unsolidified layer 12 of the slab 10 is thick at the squeeze position immediately after the reduction. Central segregation occurs with the progress of the subsequent solidification. On the other hand, if the slab 10 is not reduced until the solid phase ratio at the center of the slab thickness reaches 0.9 and the slab 10 at which the solid phase ratio at the center of the slab thickness exceeds 0.9, the thickness of the solidified shell 11 is reduced. However, the rolling force is not sufficiently transmitted to the center part of the slab thickness, and the molten steel concentrated in the solute component is difficult to be discharged. As a result, the effect of improving the center segregation is reduced. Furthermore, when the slab 10 whose slab thickness center part has a solid phase ratio exceeding 0.9 is reduced, positive segregation of solute components occurs in the vicinity of the thickness center part of the slab 10.

  In the first embodiment of the present invention, the slab 10 having a solid phase ratio in the center of the slab thickness within the range of 0.2 to 0.9 is squeezed by the squeezing roll 7, so the above problem does not occur. The center segregation of the slab 10 can be stably prevented.

In the first embodiment of the present invention, the slab 10 is reduced in the thickness direction by the reduction roll 7 in a range where the product of the reduction speed and the casting speed is 0.3 to 1.0 mm · m / min ² .

If the product of the reduction speed and the casting speed is less than 0.3 mm · m / min ² , the unsolidified layer 12 of the slab 10 at the reduction position after reduction is thick, and the dendritic tree in which the solute component is concentrated. The molten steel in between is not fully discharged from the dendrite trees. Thereby, center segregation occurs after reduction. On the other hand, when the product of the rolling speed and the casting speed exceeds 1.0 mm · m / min ² , almost all of the molten steel concentrated in the solute component between dendrites is squeezed out and discharged upstream in the casting direction. The However, since the unsolidified layer 12 is thin, the squeezed concentrated molten steel is trapped by the solidified shells 11 on both sides in the casting direction and slightly upstream of the slab thickness direction from the reduction position. Thereby, the positive segregation of the solute component occurs in the vicinity of the thickness center portion of the slab 10.

In the first embodiment of the present invention, since the reduction is performed in a range where the product of the reduction speed and the casting speed is 0.3 to 1.0 mm · m / min ² , the above problem does not occur, and Center segregation and positive segregation near the thickness center can be stably prevented.

  The effect of the reduction on the center segregation of the slab 10 and the prevention of the occurrence of positive segregation near the center of the thickness is also influenced by the solidified structure of the slab 10. In other words, when the solidification structure at the center of the slab thickness is equiaxed, there is concentrated molten steel that causes semi-macro segregation between the equiaxed crystals, and the reduction force is less likely to be transmitted to the center of the slab thickness. Less effective. Therefore, it is preferable to set the casting conditions so that the solidified structure of the slab 10 has a columnar crystal structure.

  As described above, according to the first embodiment of the present invention, since the slab 10 is bulged after the thickness of the solidified shell of the slab 10 exceeds 15 mm, the breakout of the slab 10 is prevented in advance. can do. In addition, after the slab 10 is bulged, the roll opening is set so that the thickness of the slab 10 does not change in the section of 0.5 to 5.0 m downstream in the casting direction, so that the solidified shell 11 is flat. Shape. Since the solidified shell 11 has a flat shape, the reduction efficiency during the subsequent reduction is improved, and the center segregation of the slab 10 and the positive segregation near the center of the thickness of the slab 10 are stably reduced. Is possible.

  Next, a second embodiment of the present invention will be described. FIG. 2 is a schematic side view of a vertical bending slab continuous casting machine used in the second embodiment of the present invention.

  As shown in FIG. 2, molten steel 31 is injected into the slab continuous casting machine 21, and this molten steel 31 is cooled and solidified, and a mold for forming an outer shell shape of a slab 32 having a rectangular cross section. 25 is installed. A tundish 22 for relaying and supplying molten steel 31 supplied from a ladle (not shown) to the mold 25 is installed at a predetermined position above the mold 25. A sliding nozzle 23 for adjusting the flow rate of the molten steel 31 is installed on the bottom of the tundish 22, and an immersion nozzle 24 is installed on the lower surface of the sliding nozzle 23.

  On the other hand, below the mold 25, a plurality of pairs of slab support rolls including a support roll 26, a guide roll 27 and a pinch roll 28 are arranged. Among these, the pinch roll 28 is a drive roll for pulling out the slab 32 at the same time as supporting the slab 32. A spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is disposed in the gap between the slab support rolls adjacent to each other in the casting direction, thereby forming a secondary cooling zone. The slab 32 is cooled by the secondary cooling water sprayed from the spray nozzle of the secondary cooling zone while being drawn out.

  A plurality of transport rolls 29 for transporting the cast slab 32 are installed on the downstream side of the slab support roll. A slab cutting machine 30 for cutting a slab 32 a having a predetermined length from the slab 32 to be cast is disposed above the transport roll 29.

  Light pressure lower belts 36 are installed on the upstream side and the downstream side in the casting direction across the solidification completion position 35 of the slab 32. The light pressure lower belt 36 is composed of a plurality of pairs of guide rolls which are set so that the roll opening degree with the opposing guide roll 27 is sequentially narrowed toward the downstream side in the casting direction, that is, a roll gradient is applied. Has been. In the light reduction belt 36, it is possible to perform light reduction on the slab 32 in the entire region or a partially selected region. A spray nozzle for cooling the slab 32 is also disposed between the guide rolls of the light pressure lower belt 36. In FIG. 2, only the guide roll 27 is disposed in the light pressure lower belt 36, but a pinch roll 28 may be disposed. The slab support roll disposed in the light reduction belt 36 is also referred to as a “reduction roll”.

  The slab support roll disposed between the lower end of the mold 25 and the liquidus crater end position of the slab 32 constitutes a short side narrowing reduction belt 37 and an intentional bulging belt 38. In the short side narrowing reduction belt 37, each casting is performed such that the roll opening becomes narrower sequentially for each roll or every several rolls toward the downstream side in the casting direction until the reduction amount of the roll opening reaches a predetermined value. A single support roll is set. In the intentional bulging zone 38, each slab support roll is formed so that the roll opening gradually increases every roll or every several rolls toward the downstream side in the casting direction until the enlargement amount of the roll opening reaches a predetermined value. Is set. Here, the intentional bulging band 38 is installed on the downstream side of the short side narrowing reduction band 37.

  The slab support roll installed on the downstream side of the intentional bulging zone 38 is narrowed to such an extent that the roll opening degree corresponds to a constant value or the amount of shrinkage accompanying the temperature drop of the slab 32, and the downstream light pressure lower zone 36. Connected to.

  In the second embodiment of the present invention, the reason why the short side narrowing reduction band 37 and the intentional bulging band 38 are arranged between the lower end of the mold 25 and the liquidus crater end position of the slab 32 is as follows. It is as follows. That is, on the upstream side in the casting direction from the liquid phase crater end position of the slab 32, the center part of the slab thickness is all in the liquid phase, and the solidified shell 33 of the slab 32 has a high temperature and a low deformation resistance. Therefore, the width of the short side of the slab can be easily reduced by reducing the width. In addition, when the slab 32 is intentionally bulged, if the bulging is performed at a time when the unsolidified layer 34 present in the slab 32 is small, the center segregation is worsened. However, when bulging is performed upstream of the liquid phase crater end position of the slab 32 in the casting direction, the molten steel having an initial concentration in which the solute elements are not concentrated is abundant in the slab at this point. And this molten steel flows easily. Even if this molten steel flows, segregation does not occur. Therefore, bulging at this point does not cause central segregation.

The liquidus of the slab 32 is a solidification start temperature determined by the chemical component of the slab 32, and can be obtained from the following equation (1), for example.
TL = 1536- (78 × [% C] + 7.6 × [% Si] + 4.9 × [% Mn] + 34.4 × [% P] + 38 × [% S] + 4.7 × [% Cu] + 3.1 × [ % Ni] + 1.3 × [% Cr] + 3.6 × [% Al]) ... (1)
However, in the formula (1), TL is the liquidus temperature (° C.), [% C] is the carbon concentration (mass%) of the molten steel, [% Si] is the silicon concentration (mass%) of the molten steel, and [% Mn]. Is the manganese concentration (mass%) of the molten steel, [% P] is the phosphorus concentration (mass%) of the molten steel, [% S] is the sulfur concentration (mass%) of the molten steel, and [% Cu] is the copper concentration (mass%) of the molten steel. ), [% Ni] is the nickel concentration (mass%) of the molten steel, [% Cr] is the chromium concentration (mass%) of the molten steel, and [% Al] is the aluminum concentration (mass%) of the molten steel.

  The liquidus crater end position of the slab 32 can be obtained by comparing the temperature gradient inside the slab obtained by two-dimensional heat transfer solidification calculation with the liquidus temperature determined by the equation (1). The liquid phase crater end position can also be obtained by driving a metal pin having a known melting point into the center of the thickness of the slab 32 during casting and examining the molten state of the metal pin.

  As described above, the liquidus crater end position of the slab 32 can be accurately obtained by two-dimensional heat transfer solidification calculation. However, assuming that the distance from the molten steel surface in the mold to the entry side of the light pressure lower zone 36 is L, the distance from the molten steel surface in the mold if the solidification completion position 35 is a casting condition existing in the light pressure lower band 36. It is clear from the calculation result of two-dimensional heat transfer solidification that the range of L × 2/3 is upstream of the liquidus crater end position. Therefore, for example, as shown in FIG. 2, a short side narrowing reduction band 37 and an intentional bulging band 38 may be disposed on the upper side of the secondary cooling band.

  The short side narrowing reduction belt 37 and the intentional bulging belt 38 do not require a special mechanism and are configured only by adjusting the roll opening, so that the liquidus crater of the slab 32 from the lower end of the mold 25 is formed. As long as it is within the range of the end position, it can be installed at any position. However, it is necessary to install the crushing band 37 with a narrower side width on the upstream side of the intentional bulging band 38 in the casting direction.

  In FIG. 3, the example of the profile of the roll opening degree of the slab support roll in 2nd Embodiment is shown. In the example shown in FIG. 3, a slab having a thickness of 250 mm at the lower end of the mold is squeezed to 245.2 mm by a short side narrowing reduction belt 37, that is, a slab short side width (cast (Same meaning as the thickness of the slab) is narrowed to 245.2 mm, and then the long side surface of the slab is bulged with the intentional bulging band 38 to set the thickness of the central part of the slab long side surface to 254.4 mm (total bulging amount = 9 2 mm), and thereafter, rolling is performed (total reduction amount = 9.0 mm) until the thickness of the center part of the long side surface of the slab becomes 245.4 mm with the light reduction belt 36. The total reduction amount is the reduction amount of the slab 32 from the start of reduction to the end of reduction.

  That is, in the short side narrowing reduction belt 37, the roll opening of the slab support roll is gradually narrowed toward the downstream side in the casting direction, so that the short side of the slab 32 is reduced and the thickness of the slab short side is reduced. Becomes thinner. That is, the slab short side width is narrowed. Further, in the intentional bulging band 38, the long side surface except for the vicinity of the short side of the slab 32 is formed by the unsolidified layer 34 by gradually increasing the roll opening degree of the slab support roll toward the downstream side in the casting direction. It is intentionally bulged by the molten steel static pressure following the slab support roll. Since the vicinity of the short side of the long side surface of the slab is held by the short side surface of the slab that has been solidified, the thickness when the intentional bulging is started is maintained. Therefore, as for the slab 32, only the bulging part of the slab long side surface by intentional bulging contacts the slab support roll. Further, in the light reduction belt 36, only the bulged portion of the long side surface of the slab is reduced.

  In the second embodiment of the present invention, in the short side width narrowing reduction band 37, the total amount of reduction for narrowing the short side width is 3 mm or more and 20 mm or less. By reducing the total amount of reduction for narrowing the short side to 3 mm or more and 20 mm or less, it is not necessary to increase the total amount of intentional bulging thereafter, and surface cracks on the short side surface of the slab can be prevented. it can. If the total amount of reduction is less than 3 mm, the amount of narrowing of the short side of the slab is small, and it is necessary to increase the total amount of intentional bulging thereafter. In this case, the slab 32 may be internally cracked due to excessive bulging. On the other hand, if the total amount of reduction for narrowing the short side exceeds 20 mm, the compressive strain on the short side surface of the slab increases, and surface cracks may occur on the short side surface of the slab, causing breakout. .

Moreover, in the short side width narrowing reduction belt 37, it is necessary to apply a reduction force in which the product of the reduction speed and the casting speed is in the range of 0.3 to 1.0 mm · m / min ² . If the product of the rolling speed and the casting speed is less than 0.3 mm · m / min ² , the distance until the predetermined amount of rolling is completed becomes long, and the length of the short side narrowing rolling belt 37 becomes long. Therefore, the installation length of the intentional bulging band 38 after that cannot be secured. On the other hand, when the value of the product exceeds 1.0 mm · m / min ² , a rapid reduction occurs, and the slab support roll is subjected to a load higher than the load resistance, causing not only equipment damage, but also in the slab 32. Induces cracking.

  In addition, the rolling speed in the short side width narrowing rolling belt 37 includes a roll gradient (mm / m) when the roll opening degree of the slab support roll is sequentially narrowed toward the downstream side in the casting direction, and a casting speed (m / m). min). The reduction speed in the light pressure lower belt 36 is also expressed by the product of the roll gradient (mm / m) of the light pressure lower belt 36 and the casting speed (m / min).

  In the intentional bulging belt 38, the total amount of bulging is 3 to 20 mm. If the total amount of bulging is less than 3 mm, assuming that the short side portion of the slab 32 is not squeezed by the next light reduction zone 36, the amount that can be reduced in the light reduction zone 36 is small and the center segregation with respect to the slab 32 There is a possibility that the improvement effect is insufficient. Therefore, in order to obtain the effect of improving the center segregation of the slab 32, the intentional total bulging amount is set to 3 mm or more. On the other hand, if the total amount of bulging intentionally exceeds 20 mm, there is a risk of causing internal cracks in the slab 32 due to distortion caused by bulging. Therefore, in order to prevent the internal crack of the slab 32, the intentional total bulging amount is set to 20 mm or less.

  In addition, in the intentional bulging zone 38, the amount of expansion of the roll opening per roll is preferably 1.5 mm or less. This is to prevent the occurrence of cracks at the boundary position between the bulging part and the non-bulging part on the long side surface of the slab.

  In the light reduction zone 36, the reduction starts at the time when the solid phase ratio at the center portion of the slab thickness is 0.2 or less, and the reduction is continued until the solid phase ratio at the center portion of the slab thickness becomes 0.9 or more. . This is because, even if the reduction starts after the solid phase ratio at the center of the slab thickness exceeds 0.2, the flow of the concentrated molten steel may occur before that, causing the center segregation. The effect of light pressure cannot be obtained sufficiently. In addition, the flow of molten steel may occur until the solid phase ratio reaches 0.9. If the reduction is stopped earlier than that, the flow of concentrated molten steel occurs, which causes the center segregation. Is generated, and the effect of light pressure cannot be sufficiently obtained. At least, the center segregation of the slab 32 can be reliably prevented by lightly reducing the solid phase ratio at the center of the slab thickness from 0.2 to 0.9.

  The solid phase ratio at the center portion of the slab thickness can be obtained by two-dimensional heat transfer solidification calculation as in the case of obtaining the liquidus crater end position. 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. Therefore, the position where the solid phase ratio at the center of the slab thickness is 1.0 is the solidification completion position 35 (solid line crater end position), and the liquidus crater end position is the solid phase at the center of the slab thickness. This corresponds to the most downstream position where the rate is zero.

  Further, the solid phase ratio at the center portion of the slab thickness can also be obtained by a solidification completion position detecting device using a transverse wave ultrasonic wave or a longitudinal wave ultrasonic wave that can detect the solidification completion position 35 online. This solidification completion position detecting device transmits transverse wave ultrasonic waves or longitudinal wave ultrasonic waves to the slab 32, and detects the solidification completion position 35 online based on the propagation time of these ultrasonic waves in the slab 32. is there.

  Specifically, an accurate position of the solidification completion position 35 is obtained by the solidification completion position detection device, and the slab thickness center is obtained by using a method such as two-dimensional heat transfer solidification calculation with the obtained solidification completion position 35 as a reference. This is a method of obtaining the solid phase ratio in the casting direction of the part. The solid phase rate at the center of the slab thickness can also be obtained by a solidification completion position detection device that detects the solidification completion position 35 by utilizing the property that shear wave ultrasonic waves do not pass through the liquid phase. That is, based on the fact that the solidification completion position 35 coincides with the installation position of the transverse ultrasonic sensor, the solidification completion position 35 is used as a reference, and a method such as two-dimensional heat transfer solidification calculation is used in combination with the center of the slab thickness. This is a method of obtaining the solid phase ratio in the casting direction.

Therefore, in the vicinity of the exit side of the light pressure lower belt 36 of the slab continuous casting machine 21 for carrying out the second embodiment of the present invention, as shown in FIG. It is preferable that the transmission sensor 39 and the ultrasonic reception sensor 40 are arranged. As another constituent device, the solidification completion position detection device is a coagulator for the slab 32 using a calculation formula based on a transmission unit that transmits a signal to the ultrasonic transmission sensor 39 or a reception signal received by the ultrasonic reception sensor 40. Although it comprises a coagulation completion position calculation unit for obtaining the completion position 35, they are omitted in FIG. As a solidification completion position detecting device using ultrasonic waves, a device of a method for obtaining the solidification completion position 35 from the propagation time of the slab 32 of the transverse wave ultrasonic wave or the longitudinal wave ultrasonic wave, or the transverse wave ultrasonic wave does not pass through the liquid phase. There is an apparatus of a method for obtaining the solidification completion position 35 by utilizing this, and any solidification completion position detection device may be used as long as the solidification completion position 35 can be obtained. In this embodiment, in order to prevent the center segregation of the slab 32 and the positive segregation in the vicinity of the thickness center portion, the product of the rolling speed and the casting speed in the light rolling belt 36 is 0.3 to 1.0 mm · m / m. A rolling force that falls within the range of min ² is applied to the slab 32. When the product of the rolling speed and the casting speed in the light rolling zone 36 is less than 0.3 mm · m / min ² , the rolling amount per unit time is small relative to the solidification shrinkage, and the effect of reducing the center segregation due to the light rolling is sufficient. Not. On the other hand, if the product of the rolling speed and casting speed in the light rolling zone 36 exceeds 1.0 mm · m / min ² , the rolling amount per unit time becomes too large, and the unsolidified molten steel is squeezed upstream. There is a possibility that negative segregation (a state where the solute concentration is lower than the surroundings) is generated in the center part of the slab.

  Molten steel 31 injected from the tundish 22 through the immersion nozzle 24 into the mold 25 is cooled by the mold 25 to form a solidified shell 33. The slab 32 having the solidified shell 33 as an outer shell and having an unsolidified layer 34 inside is supported by a support roll 26, a guide roll 27, and a pinch roll 28 provided below the mold 25, and below the mold 25. It is pulled out continuously. The slab 32 is cooled by the secondary cooling water in the secondary cooling zone while passing through these slab support rolls, and the thickness of the solidified shell 33 is increased. The slab 32 reduces the thickness of the slab in the short side narrowing reduction band 37, while the intentional bulging band 38 increases the thickness of the portion excluding the short side end of the slab long side surface. Further, the light pressure lower belt 36 completes the solidification to the inside at the solidification completion position 35 while being lightly reduced. The slab 32 after completion of solidification is cut by the slab cutting machine 30 to become a slab 32a.

  In the second embodiment of the present invention, the total reduction amount in the light reduction belt 36 is adjusted to be equal to or smaller than the total bulging amount in the intentional bulging belt 38. Further, the solid phase ratio at the center of the slab thickness when entering the light pressure lower belt 36 is 0.2 or less, and the solid phase ratio at the center of the slab thickness at the time of exiting from the light pressure lower belt 36 is 0. Adjust one or more of the secondary cooling water amount, the secondary cooling width cut, and the casting speed so as to be 9 or more. The control of the solid fraction at the center of the slab thickness should be obtained in advance by determining the thickness of the solidified shell 33 under various casting conditions and the solid fraction at the center of the slab thickness using a two-dimensional heat transfer solidification calculation or the like. This can be done easily. Further, the solid phase ratio at the center portion of the slab thickness can also be easily controlled by obtaining the solid phase ratio at the center portion of the slab thickness online using the solidification completion position detecting device. Here, “secondary cooling width cutting” is to stop spraying of cooling water to both ends of the long side surface of the slab. By performing the width cutting of the secondary cooling, the secondary cooling is weakened, and generally, the solidification completion position 35 is extended downstream in the casting direction.

  As described above, according to the second embodiment of the present invention, at the stage where the deformation strength of the solidified shell 33 is low, the long side surface of the slab 32 is once crushed to reduce the short side width of the slab 32 to the mold lower end dimension. Therefore, the total amount of bulging during intentional bulging can be suppressed to a small value, thereby preventing internal cracks in the slab 32. Further, during light reduction, since the time of light reduction and the product of the reduction speed and the casting speed are defined, the center segregation of the slab 32 can be stably reduced.

  The continuous casting machine shown in FIG. 2 is a vertical bending type continuous casting machine. However, the continuous casting machine shown in FIG. 2 may be a curved continuous casting machine or a vertical continuous casting machine. Embodiments can be applied. The effect of light reduction on the center segregation of the slab 32 is also affected by the solidified structure of the slab 32. Specifically, when the solidification structure of the center portion of the slab 32 is an equiaxed crystal, concentrated molten steel exists that causes semi-macro segregation between the equiaxed crystals, and the center segregation is improved by light reduction. The effect is reduced. Therefore, in the second embodiment of the present invention, it is preferable to set the casting conditions so that the solidified structure at the center of the thickness of the slab 32 has a columnar crystal structure.

Using the slab continuous casting machine shown in FIG. 1, a test applying the first embodiment of the present invention was performed, in which the slab was intentionally bulged and then reduced. The width of the slab was 2100 mm, and the thickness of the slab was 250 mm, the thickness immediately before the start of bulging (D ₀ ). The casting speed was 0.85 to 1.2 m / min, and the secondary cooling specific water amount of the slab was 1.0 to 2.0 liters / steel-kg. The total amount of intentional bulging was set to 3.0 mm to 21.0 mm. The thickness of the solidified shell immediately before bulging, the gradient of the roll opening during bulging, the casting direction length of the section where the slab thickness after bulging is constant (the section where the roll opening of the guide roll is set constant), and The total amount of reduction was tested with various changes. The cast steel type is a steel type for thick steel plates having a carbon concentration of 0.05 to 0.08 mass%.

  In each test, a full-width test piece having a length of 1000 mm in the casting direction was taken from a cast piece corresponding to the steady casting state, and one cross-sectional sample having a thickness of 50 mm was cut out from the full-width test piece. Manganese (Mn) was analyzed using EPMA (Electron Probe Micro Analyzer) in the thickness direction of the slab at the center of the width of the cross section sample. FIG. 4 is a diagram showing a sampling position of a cross-sectional sample and an analysis position of manganese by EPMA. According to manganese analysis by EPMA, the center segregation at the center of the slab thickness and the positive segregation near the center of the slab thickness (segregation where the concentration of the solute component is higher than the initial concentration) and negative segregation (the concentration of the solute component is the initial concentration) Lower segregation).

  Value obtained by dividing the manganese analysis value measured at each position of the cross-sectional sample by the manganese concentration of the analytical sample collected from the molten steel in the ladle (manganese concentration (mass%) at each position of the cross-sectional sample / inside the ladle The manganese concentration (mass%) of the analytical sample collected from the molten steel was determined as the segregation degree, and the distribution of the segregation degree in the slab thickness direction was investigated. Of the center segregation and the positive segregation in the vicinity of the center of the slab thickness, a high value was defined as the segregation degree of the slab. In addition, the presence or absence of internal cracks was investigated from a sulfur print test of a cross-sectional sample.

  Table 1 shows test conditions and survey results. In the remarks column of Table 1, tests conducted under conditions within the scope of the first embodiment of the present invention are displayed as “examples of the present invention”, and “other examples” are displayed as “comparative examples”.

  In Test Nos. 1 to 6, which are examples of the present invention, center segregation and positive segregation near the center of the slab thickness did not occur, and no internal cracks occurred. Test Nos. 1 to 6 are the solidified shell thickness immediately before bulging, the total amount of bulging, the length in the casting direction of the section where the slab thickness after bulging is constant, the product of the rolling speed and casting speed, the center of the slab thickness at the time of rolling All five conditions of the solid phase ratio of the part are within the range of the first embodiment of the present invention, and the gradient of the roll opening degree during bulging is also within the preferred range of the first embodiment of the present invention. By being within.

  In test number 7, the gradient of the roll opening degree during bulging was outside the preferred range of the first embodiment of the present invention, and a slight internal crack occurred. However, the thickness of the solidified shell immediately before bulging was as thick as 25 mm, and the internal crack was slight. Other conditions were within the range of the first embodiment of the present invention, and no center segregation and positive segregation near the center of the slab thickness occurred.

  In test number 8, the length in the casting direction of the section in which the slab thickness after bulging is constant is outside the range of the first embodiment of the present invention, and the segregation degree is 1.090 near the center of the slab thickness. Positive segregation occurred. This is because a uniform reduction in the slab width direction could not be performed because the length in the casting direction of the section in which the slab thickness after bulging was constant was too short.

  In Test No. 9, the thickness of the solidified shell immediately before bulging and the length in the casting direction of the section in which the slab thickness after bulging is constant are outside the range of the first embodiment of the present invention, and internal cracks occur. In addition, positive segregation with a segregation degree of 1.085 occurred in the vicinity of the center of the slab thickness. The occurrence of internal cracks is due to the fact that the thickness of the solidified shell immediately before bulging is out of the range of the first embodiment of the present invention. Moreover, the occurrence of positive segregation is due to the fact that the length in the casting direction of the section in which the thickness of the slab after bulging is constant is too short, and uniform reduction in the slab width direction cannot be performed.

  Test No. 10 shows that the thickness of the solidified shell immediately before bulging is outside the range of the first embodiment of the present invention, and the gradient of the roll opening during bulging is within the preferred range of the first embodiment of the present invention. Since it was outside, internal cracks occurred in the slab. However, since other conditions were within the range of the first embodiment of the present invention, the center segregation and the positive segregation near the center of the slab thickness did not occur.

  In Test No. 11, the total amount of bulging exceeded 20 mm, which is the range of the first embodiment of the present invention, and therefore internal cracks occurred in the slab. Further, since the product of the reduction speed and the casting speed is smaller than the range of the first embodiment of the present invention, the reduction effect cannot be obtained, and the segregation degree is 1.158 in the vicinity of the center portion of the slab thickness. Segregation occurred.

  Test number 12 is a test that was reduced when the solid phase ratio at the center of the slab thickness was 0.95. Since the reduction time is outside the range of the first embodiment of the present invention, the effect of reduction was not obtained. As a result, positive segregation with a segregation degree of 1.198 occurred in the vicinity of the center of the slab thickness. However, the thickness of the solidified shell and the total amount of bulging immediately before bulging are within the range of the first embodiment of the present invention, and the gradient of the roll opening degree during bulging is also within the preferred range of the first embodiment of the present invention. Because it was inside, no internal cracks occurred.

  Using the slab continuous casting machine having the configuration shown in FIG. 2, a test for casting by applying the second embodiment of the present invention was performed (examples of the present invention: test numbers 21 to 25). The width of the slab was 2100 mm, the thickness of the slab was 250 mm, and the slab slab for a thick steel plate having a carbon content of 0.05 to 0.08% by mass was cast. The casting speed was 0.85 to 1.42 m / min, and the secondary cooling specific water amount of the slab was 1 to 2 liters / steel-kg.

  The total reduction amount in the short side narrowing reduction zone is 3.0 to 20.0 mm, the total bulging amount in the intentional bulging zone is 3.0 to 20.0 mm, and the total reduction amount in the light reduction zone is the bulging total amount. Equivalent or less. The solid phase ratio at the center of the slab thickness at the end of light pressure was 0.9 or more.

  Further, for comparison, any one of the total amount of reduction in the narrow side narrowing reduction zone, the total amount of bulging in the intentional bulging zone, the product of the reduction rate and casting speed in the light reduction zone, and the disclosure period of reduction in the light reduction zone 1 One test was performed that falls outside the scope of the present invention (Comparative Example: Test Nos. 26-30).

  In each casting test, a full-width test piece having a casting direction length of 1000 mm was taken from a slab corresponding to a steady casting state, and a cross-sectional sample having a thickness of 50 mm was cut out from the full-width test piece. Manganese was analyzed using EPMA in the thickness direction of the slab at the center of the width of the surface sample. According to manganese analysis by EPMA, the center segregation at the center of the slab thickness and the positive segregation near the center of the slab thickness (segregation where the concentration of the solute component is higher than the initial concentration) and negative segregation (the concentration of the solute component is the initial concentration) Lower segregation).

A value obtained by dividing the manganese analysis value (mass% Mn) measured at each position of the cross section sample by the manganese concentration (mass% Mn ₀ ) of the analysis sample collected from the molten steel in the ladle before casting (mass% Mn / Mass% Mn ₀ ) was defined as the degree of segregation, and the distribution of the degree of segregation in the slab thickness direction was investigated. Of the center segregation and the positive segregation in the vicinity of the center of the slab thickness, a high value was defined as the segregation degree of the slab. In addition, the presence or absence of internal cracks was investigated from a sulfur print test of a cross-sectional sample. Table 2 shows test conditions and survey results.

  In test numbers 21 to 25, which are examples of the present invention, the center segregation at the center of the slab thickness and the positive segregation near the center of the slab thickness did not occur, and no internal cracks occurred.

  Test No. 26 is a test in which the total amount of reduction in the short side width narrowing reduction zone was 23.0 mm outside the range of the second embodiment of the present invention. Other conditions were within the range of the second embodiment of the present invention, but the total amount of reduction in the short side narrowing reduction zone was too large, and internal cracks occurred in the slab.

  Test No. 27 is a test in which the total amount of intentional bulging was 30.0 mm outside the range of the second embodiment of the present invention. Other conditions were within the range of the second embodiment of the present invention, but the total amount of bulging was too large and internal cracks occurred in the slab.

  Test number 28 is a test in which the product of the reduction speed and the casting speed at the time of light reduction is 0.2, which is outside the range of the second embodiment of the present invention. Although no internal cracks occurred in the slab, the degree of manganese segregation was 1.080, and center segregation was confirmed due to insufficient light pressure.

  Test No. 29 is a test in which the total amount of reduction in the short side width narrowing reduction zone is 2.0 mm outside the range of the second embodiment of the present invention. Other conditions were performed within the scope of the second embodiment of the present invention. As a result, no internal crack was generated, but a central segregation with a manganese segregation degree of 1.040 was confirmed although it was slight. This is presumed that the effect of reducing the thickness of the short side was reduced and the effect of light reduction was reduced because the total amount of reduction in the reduction width of the short side width was small.

  Test No. 30 is an example in which light reduction was performed after the solid phase ratio at the center of the slab became 1.0 (complete solidification) outside the range of the second embodiment of the present invention. Even if the pressure is lightly reduced after completion of solidification, there is no effect, and central segregation with a manganese segregation degree of 1.080 was observed.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Guide roll 7 Reduction roll 8 Pinch roll 9 Molten steel 10 Slab 11 Solidified shell 12 Unsolidified layer 21 Slab continuous casting machine 22 Tundish 23 Sliding nozzle 24 Immersion Nozzle 25 Mold 26 Support roll 27 Guide roll 28 Pinch roll 29 Conveying roll 30 Slab cutting machine 31 Molten steel 32 Cast slab 33 Solidified shell 34 Unsolidified layer 35 Solidification completion position 36 Light pressure lowering band 37 Short side narrowing pressure lowering band 38 Intention Bulging belt 39 Ultrasonic transmission sensor 40 Ultrasonic reception sensor

Claims (4)

Until the solidified shell thickness of the slab drawn from the continuous casting mold reaches at least 15 mm, the roll opening of the slab support roll arranged in the casting direction is set to be the same as the roll opening directly under the mold,
Thereafter, the roll opening of the slab support roll is gradually increased toward the downstream side in the casting direction, and the long side surface of the slab is bulged with a bulging total amount of 3 to 20 mm,
After bulging, in the section of 0.5 to 5.0 m downstream in the casting direction, the roll opening of the slab support roll is set constant so that the thickness of the slab does not change,
Subsequently, the product of the rolling speed and the casting speed is 0.3 to 1.0 mm · m / min ^{2 on} the long side surface of the slab having a solid phase ratio of 0.2 to 0.9 at the thickness center of the slab. A steel continuous casting method in which the steel sheet is reduced at least once by a reduction roll under the following conditions.   The bulge opening of the slab support roll arranged in the casting direction is gradually increased at a gradient of 4.0 mm or less per 1 m of the casting direction when bulging the long side surface of the slab. Steel continuous casting method. The roll opening rate of the slab support rolls in pairs is gradually reduced toward the downstream side in the casting direction, and the rolling speed is reduced on the long side surface of the slab having a rectangular cross section drawn out from the continuous casting mold. And the casting speed is 0.3 to 1.0 mm · m / min ^2, and the rolling force causes the width of the short side of the slab to be smaller than the width of the short side of the slab at the lower end of the mold. 3-20mm smaller,
After reducing the short side width of the slab, the roll opening of the pairs of slab support rolls is increased stepwise toward the downstream side in the casting direction, and the long side surface of the slab is increased by a bulging total amount of 3 to 20 mm. Bulging,
After bulging the long side of the slab, in the light pressure zone where the roll opening of multiple pairs of slab support rolls is gradually reduced toward the downstream side in the casting direction, the solid fraction at the center of the slab thickness is From the time of at least 0.2 to the time of 0.9 or more, a rolling force with a product of the rolling speed and the casting speed of 0.3 to 1.0 mm · m / min ² is applied to the long side surface of the slab. The continuous casting method of steel, in which the long side surface of the slab is squeezed by the squeezing force with a squeezed total amount equivalent to the bulging total amount or a squeezed total amount smaller than the bulging total amount.   The solidification completion position of the slab is detected online using a solidification completion position detector, and the solid phase ratio at the thickness center of the slab is at least 0.2 or less based on the detected solidification completion position information From time to 0.9 or more, at least one or more of the amount of secondary cooling water, the width of the secondary cooling, and the casting speed are set so that the slab is located in the light pressure lower zone. The continuous casting method of steel according to claim 3, wherein the adjustment is performed.

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