JP5045408B2 - Manufacturing method of continuous cast slab - Google Patents

Description

  The present invention relates to a method for producing a continuous cast slab. More specifically, the center segregation at the center of the slab thickness is slight, and the non-oxide-based non-oxide near the slab thickness on the upper surface side of the slab. The present invention relates to a continuous casting slab manufacturing method capable of manufacturing a steel slab having few metal inclusions.

  In the solidification phenomenon of steel, solute elements such as carbon, phosphorus and sulfur are concentrated on the unsolidified liquid phase side by redistribution during solidification. This is the microsegregation formed between dendrite trees. When a void is formed in the center of the slab or negative pressure is generated due to solidification shrinkage when the slab solidifies or bulging that occurs between rolls of a continuous casting machine (referred to as "bulging between rolls") The molten steel concentrated by this microsegregation flows, accumulates in the center of the slab and solidifies. The segregation spot formed in this way has a much higher concentration than the initial concentration of molten steel. This is generally called macrosegregation, and is called central segregation because of its existence site.

  Such central segregation generally degrades the quality of steel products. For example, in a line pipe material for oil / natural gas transportation, hydrogen-induced cracking occurs starting from central segregation due to the action of sour gas. Further, in deep drawn materials used for beverage can products, anisotropy may appear in workability due to segregation of components.

  Therefore, many measures for reducing the center segregation of the slab have been proposed from the casting process to the rolling process. Among them, as a means to reduce the center segregation of the slab at low cost and effectively, a method of gradually reducing the slab at the end of solidification with a reduction amount corresponding to the solidification shrinkage amount of the slab in a continuous casting machine ( Hereinafter, this is referred to as “light reduction”). This light reduction technology reduces the volume of the unsolidified phase by gradually reducing the slab so that the reduction amount is equivalent to the shrinkage amount obtained by adding the solidification shrinkage amount and the heat shrinkage amount of the slab, The object is to prevent the formation of voids at the center of the slab and at the same time prevent the flow of concentrated molten steel, thereby reducing the center segregation of the slab.

  As this light reduction technique, for example, in Patent Document 1, in applying light reduction to a slab, a slab surface temperature is controlled to 800 to 900 ° C., and a strain of 0.3 to 0.5% per minute is controlled. It has been proposed to apply a speed reduction force to the slab. Further, in Patent Document 2, the slab is temporarily bulged within a predetermined range between the position corresponding to the liquid phase crater end and the position corresponding to the solid phase crater end of the slab, and this bulging is lightly reduced. It has been proposed that the slab surface temperature in the range from directly under the mold to the slab correction point is controlled to 1000 ° C. or more, and the slab surface temperature from the correction point to the reduction start position is controlled to 1000 ° C. or less. Yes.

  In the light rolling technique, the rolling force is applied to the solidification interface at the center of the slab by rolling down the surface of the slab having a thickness of 200 mm or more. The ratio of the rolling force acting on the solidification interface to the applied rolling force, depending on the solidified part of the slab, that is, the strength and rigidity of the solidified shell (this ratio is called "rolling efficiency") Varies in the range of approximately 10% to 70%. In particular, the influence of the solidified shell on the short side of the slab is large.

  If the rolling efficiency is low because the surface temperature of the slab is low, the effect of reducing the center segregation cannot be sufficiently expected even if a rolling force is applied. In addition, increasing the amount of reduction in order to compensate for this in a state where the reduction efficiency is low places a load on the structure such as the slab support roll and segments of the continuous casting machine, This will cause a reduction in the service life of equipment such as roll bearings. In other words, in the light reduction technique, it is important to reduce the slab under conditions that increase the reduction efficiency.

  From this point of view, when the above prior art is verified, the above-mentioned Patent Document 1 says that the slab surface temperature is controlled to 800 to 900 ° C., but the slab surface temperature is low, particularly in the vicinity of the lower limit of 800 ° C. Since the surface temperature of the slab is low, the strength of the solidified shell is high, and the effect of slab pressure reduction is spent on deforming the solidified shell, and the center of the slab thickness where the central segregation occurs The rolling efficiency at the part is lowered. Moreover, although patent document 2 is supposed to control the slab surface temperature at the time of light pressure to 1000 degrees C or less, there is no lower limit only by setting the upper limit, and it is not concrete how to control.

  That is, in Patent Documents 1 and 2, it is not clear how the slab surface temperature should be controlled when the slab is lightly reduced, and there is room for improvement.

  Incidentally, oxide-based nonmetallic inclusions (hereinafter simply referred to as “inclusions”) contained in the slab also cause the hydrogen-induced cracking. That is, hydrogen concentrates around the inclusions, and hydrogen-induced cracking occurs starting from the inclusions. In a vertical bending type continuous casting machine (also referred to as a “vertical progressive bending type continuous casting machine”) having a vertical portion of about 1 to 5 m directly under the mold and thereafter having a bending portion and a bending back portion, Due to the characteristics of the shape, inclusions often accumulate in the vicinity of ¼ position of the slab thickness on the upper surface side of the slab (hereinafter referred to as “1/4 thickness position”), and hydrogen-induced cracking at this part Is likely to be a problem.

  Conventionally, as a measure to float and separate the inclusions in the slab, a DC magnetic field is applied to the slab in the thickness direction of the slab in the secondary cooling zone directly below the mold to suppress the flow of the molten steel and the molten steel is cast. In the secondary cooling zone, a method for suppressing penetration of the unsolidified phase into the depth (for example, see Patent Document 3) and an electromagnetic stirrer for rotating the molten steel in the slab width direction are installed in the secondary cooling zone. There has been proposed a method (see, for example, Patent Document 4) in which molten steel is stirred in the width direction and the molten steel is prevented from penetrating deeply into the slab unsolidified phase.

However, when installing a static magnetic field generator or electromagnetic stirrer, equipment costs are required, and when installing a magnetic field generator in the secondary cooling zone, an electromagnetic coil is placed inside the slab support roll. It is necessary to take measures such as installing a coil or a coil having a special shape such as a flat shape, and an increase in equipment cost is inevitable. Therefore, there has been a demand for means that can reduce inclusions near the 1/4 thickness position without using a magnetic field generator.
JP-A-63-252654 JP 2001-62551 A Japanese Patent Laid-Open No. 61-1459 Japanese Patent Laid-Open No. 61-140356

  The present invention has been made in view of the above circumstances. The object of the present invention is that the center segregation at the center portion of the slab thickness is slight, and inclusions in the vicinity of the ¼ thickness position on the upper surface side of the slab are present. It is an object of the present invention to provide a continuous casting slab manufacturing method capable of stably manufacturing a small number of steel slabs.

  The method for producing a continuous cast slab according to the first invention for solving the above-mentioned problem uses a vertical bending type continuous caster provided with a light reduction belt comprising a plurality of pairs of reduction rolls, and the thickness center portion of the slab From the time when the solid phase ratio of the steel sheet is 0.4 or less, the slab starts to be reduced at a reduction speed within the range of 0.6 to 1.5 mm / min in the light reduction zone, and a reduction force is applied to the slab. However, when producing a continuous cast slab of steel by completing solidification within the range of the light pressure lower zone, the slab is cooled until the surface temperature of the long side surface of the slab becomes 800 ° C. or less in the vertical portion immediately below the mold, After the bent portion that follows the vertical portion, reheating is performed to ensure the surface temperature of the long side surface of the slab at 900 ° C. or higher, and the surface temperature of the long side surface of the slab remains at 900 ° C. or higher in the light pressure zone. It is characterized by applying a rolling force.

  The method for producing a continuous cast slab according to the second invention is the method according to the first invention, wherein the surface temperature of the long side surface of the slab after the bent portion is 900 ° C. or more in the slab thickness direction of the guide roll group. The slab is bulged by widening the interval, and after bulging, a rolling force is applied to the slab at the light reduction zone.

  According to the present invention, since the surface temperature of the long side surface of the slab is cooled to 800 ° C. or less in the vertical portion immediately below the mold of the vertical bending die continuous casting machine, the cooling rate of the solidified shell to be generated is increased, thereby The dendritic solidification structure is refined, and inclusions trapped in the dendritic solidification structure are reduced, and the size of the inclusions trapped is reduced. Inclusions near the ¼ thickness position on the upper surface side of the slab are removed. Can be reduced. Moreover, after cooling to 800 degrees C or less once, it reheats after the bending part following a perpendicular part, the surface temperature of the slab long side surface shall be 900 degreeC or more, and the surface temperature of the slab long side surface was kept at 900 degreeC or more Since the rolling force is applied to the slab in a state, the deformation resistance of the solidified shell is low, the rolling force acts efficiently on the solid interface, and the center segregation of the slab can be greatly reduced. This makes it possible to stably manufacture a slab that can cope with recent severe quality requirements.

  Hereinafter, the present invention will be specifically described.

  In the present invention, a vertical bending type continuous casting machine having a light rolling belt composed of a plurality of pairs of rolling rolls, having a vertical portion of about 1 to 5 m directly under the mold, and thereafter having a bending portion and a bending back portion. When the molten steel is continuously cast, the long side surface of the slab is cooled with secondary cooling water in the vertical portion immediately below the mold until the surface temperature becomes 800 ° C. or less over the entire width direction of the slab long side surface. As a result, the cooling rate of the solidified shell generated is increased, the dendrite solidified structure (also referred to as “dendritic crystal”) is refined, and the trunk of the adjacent dendrite tree (also referred to as “primary arm”) and the trunk at the solidification interface. (Referred to also as “primary dendrite arm spacing”).

  Inclusions and bubbles suspended in the molten steel that have penetrated deep into the unsolidified phase of the slab with the discharge flow from the immersion nozzle change the ease of trapping in the solidified shell according to the primary dendrite arm spacing. To do. This is because the primary dendrite arm spacing is typically in the range of one hundred to several hundred microns, which roughly matches the upper limit of inclusions and bubble sizes that can penetrate deep into the slab unsolidified phase, Inclusions and bubbles are likely to be trapped in the gap between the trunks of adjacent dendrite tree branches, and accordingly, the way in which inclusions and bubbles are trapped varies depending on the primary dendrite arm spacing. That is, the narrower the primary dendrite arm interval, the fewer inclusions and bubbles that are trapped.

  Inclusions and bubbles trapped in the slab naturally have a greater negative effect on steel quality as the size increases. Therefore, by cooling the long side surface of the slab in the vertical portion to 800 ° C. or less, narrowing the primary dendrite arm interval of the dendrite tree to be generated is captured in the vicinity of the ¼ thickness position on the upper surface side of the slab. There are few inclusions and bubbles, which is extremely effective in producing a high quality slab. In addition, the time when the solidified shell thickness becomes about 1/4 thickness corresponds to the position where the bent portion moves from the vertical portion. In other words, inclusions and bubbles trapped around the position moving from the vertical portion to the bent portion form an inclusion accumulation band near the ¼ thickness position on the upper surface side of the slab. In addition, inclusions are often attached to the bubbles, and trapping the bubbles can be regarded as equivalent to trapping inclusions. This bubble is Ar gas blown into the molten steel flowing down the immersion nozzle.

  On the other hand, when the temperature of the solidified shell (the average temperature in the thickness direction of the solidified shell) is low, the deformation resistance of the solidified shell on the short side of the slab is high, and the reduction efficiency when the slab is reduced in the light reduction zone decreases. The effect of reducing segregation cannot be obtained. Further, in order to ensure a sufficient amount of reduction for segregation reduction in a state where the reduction efficiency is low, an excessive load is applied to a structure such as a slab support roll and a segment of a continuous casting machine, This may cause equipment malfunctions and reduce the life of equipment such as roll bearings.

  Therefore, in the present invention, the slab cooled at the vertical part until the surface temperature of the long side surface of the slab becomes 800 ° C. or lower is reheated by weakening the cooling with the secondary cooling water after the bent part, The surface temperature of the side surface is maintained at 900 ° C. or higher, and a rolling force is applied to the slab at the light reduction zone while the surface temperature of the long side surface of the slab remains at 900 ° C. or higher. By increasing the temperature of the slab, the deformation resistance of the solidified shell is lowered, the rolling efficiency is improved, and the center segregation is reduced.

  For the purpose of increasing the efficiency of light reduction by making the solidification completion position uniform in the width direction of the slab or avoiding deformation resistance of the slab short side solidification shell when applying light reduction. In some cases, bulging is performed intentionally and then light reduction is performed. Also in this case, if the temperature of the slab solidified shell remains low, it is difficult to obtain the desired bulging amount, and the effect of bulging cannot be sufficiently obtained.

  In the present invention, the long side surface of the slab is once cooled to 800 ° C. or lower, and then reheated to 900 ° C. or higher. Therefore, even when the slab is intentionally bulged once, a desired bulging amount can be easily obtained. Can do. That is, as a preferred embodiment of the present invention, a technique for intentionally bulging the slab once can be employed.

  In the present invention, as described above, the surface temperature of the long side surface of the slab is controlled to a predetermined value when the unsolidified slab is lightly reduced by the vertical bending die continuous casting machine, so that the trapping of inclusions is suppressed. In addition, it is possible to reduce inclusions near the ¼ thickness position on the upper surface side of the slab, and at the same time, the effect of suppressing the flow of the concentrated molten steel due to light pressure effectively acts, effectively reducing the center segregation. Can do.

  In the present invention, the time when light reduction starts is when the solid phase ratio at the center of the thickness of the slab is 0.4 or less, and it is necessary to complete solidification within the range of the light reduction zone. Therefore, in order to satisfy these conditions, the slab drawing speed or the amount of secondary cooling water is adjusted based on heat transfer calculation or the like. This is because, even if light reduction starts after the solid phase ratio at the center of the slab thickness exceeds 0.4, the flow of concentrated molten steel may occur before that, which causes center segregation. Therefore, if the effect of light reduction cannot be fully exhibited, and the solidification completion position extends downstream beyond the light pressure reduction zone, the reduction force does not work and the effect of improving center segregation is not achieved. It is because it cannot be obtained. The reduction speed in the light reduction zone is in the range of 0.6 to 1.5 mm / min. When the rolling speed is less than 0.6 mm / min, the effect of reducing the center segregation is small. On the other hand, when the rolling speed exceeds 1.5 mm / min, the concentrated molten steel is squeezed out in the direction opposite to the casting direction. This is because negative segregation may be generated at the center of the piece. Further, it is sufficient that the total reduction amount is about 2 to 6 mm.

  Next, a specific implementation method of the present invention will be described with reference to the drawings. FIG. 1 is a schematic side view of a vertical bending slab continuous casting machine embodying the present invention.

  As shown in FIG. 1, a vertical bending slab continuous casting machine 1 is provided with a mold 5 for cooling and solidifying molten steel 11 to form an outer shell shape of a slab 12. A tundish 2 for relaying and supplying molten steel 11 supplied from a ladle (not shown) to the mold 5 is installed at a predetermined position. On the other hand, below the mold 5, a plurality of pairs of slab support rolls including a support roll 6, a guide roll 7 and a pinch roll 8 are arranged. Among these, the pinch roll 8 is a drive roll for drawing the slab 12 at the same time as supporting the slab 12. 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 in the gap between the slab support rolls adjacent in the casting direction is configured. The slab 12 is cooled while being drawn out by cooling water sprayed from (also referred to as “secondary cooling water”). This secondary cooling zone is divided into several zones in the casting direction so that the amount of secondary cooling water can be adjusted individually in each zone. A sliding nozzle 3 for adjusting the flow rate of the molten steel 11 injected from the tundish 2 into the mold 5 is installed at the bottom of the tundish 2, and the molten steel 11 is injected into the mold 5 at the lower surface of the sliding nozzle 3. An immersion nozzle 4 made of a refractory material is installed. Further, on the downstream side of the slab support roll, a plurality of transport rolls 9 for transporting the cast slab 12 are installed, and above the transport roll 9, from the cast slab 12 to be cast. A slab cutting machine 10 for cutting a slab 12a having a predetermined length is disposed.

  A plurality of pairs of guide rolls 7 arranged at a position about 1 to 5 m away from the exit of the mold 5 constitutes a bending portion 17 in which the support / guide direction of the slab 12 changes from the vertical direction to the bending direction. Yes. Similarly, the plurality of pairs of guide rolls 7 arranged in the vicinity of the position where the curved portion contacts the horizontal line constitutes a bending return portion 18 in which the support / guide direction of the slab 12 changes from the curved direction to the horizontal direction. ing. In addition, in FIG. 1, although the bending part 17 and the bending return part 18 are comprised with several pairs of guide rolls 7, you may comprise only with a pair of guide rolls.

  Before and after the solidification completion position 15 of the slab 12 in the casting direction, an interval between the opposing guide rolls 7 and the guide rolls 7 (referred to as “roll interval”) is gradually narrowed toward the downstream in the casting direction. A light pressure lower belt 16 composed of a plurality of pairs of guide rolls is set. In the light reduction belt 16, it is possible to perform light reduction on the slab 12 in the entire region or a partially selected region. A spray nozzle for cooling the slab 12 is also disposed between the guide rolls of the light pressure lower belt 16. A state in which the roll interval is set so as to become narrower toward the downstream in the casting direction is also referred to as “roll gradient”.

  FIG. 2 schematically shows the setting of the roll interval of the guide roll 7 from directly under the mold 5 to the end of the vertical bending slab continuous casting machine 1 in this case. As shown in FIG. 2, the roll interval starts to increase immediately below the bent portion 17 and gradually increases to reach the maximum value. The slab 12 is bulged by the guide roll group in which the roll interval is increased. The roll interval is set so as to be gradually narrowed toward the downstream in the casting direction so that the roll is lightly reduced at a predetermined reduction speed in the light reduction belt 16 after the bending return portion 18 has passed with the maximum value. In the present invention, bulging the slab 12 is not an indispensable condition. If the slab 12 is not bulged, the roll interval from the mold outlet to the light pressure lower belt inlet is constant, and the light pressure lower belt 16 faces the downstream in the casting direction. Can be set so as to narrow sequentially.

  In the vertical bending slab continuous casting machine 1 configured as described above, the slab 12a is manufactured as follows.

  Molten steel 11 is injected into the mold 5 through the immersion nozzle 4. The molten steel 11 injected into the mold 5 is cooled by the mold 5 to form a solidified shell 13, and as a cast piece 12 having an unsolidified phase 14 therein, a support roll 6 and a guide roll 7 provided below the mold 5. And while being supported by the pinch roll 8, it is continuously pulled out below the mold 5 by the driving force of the pinch roll 8. While the slab 12 passes through these slab support rolls, the slab 12 is cooled by the secondary cooling water in the secondary cooling zone and gradually bulged from directly below the bending portion 17 while increasing the thickness of the solidified shell 13. While the thickness is increased, while the light pressure lower belt 16 is lightly reduced and gradually decreases in thickness, solidification to the center within the light pressure lower belt 16 is completed. A position at which the solidification to the center is completed is a solidification completion position 15. Thereafter, the slab 12 is cut by the slab cutting machine 10 to become a slab 12a.

  Under various casting conditions in such a continuous casting operation, the surface temperature of the long side surface of the slab, the thickness of the solidified shell 13 and the solid phase ratio at the center of the slab thickness are obtained in advance using heat transfer calculation and the like. The surface temperature of the long side surface of the slab becomes 800 ° C. or lower before reaching the bent portion 17, and after reaching the bent portion 17, the surface temperature of the long side surface of the slab becomes 900 ° C. or higher. The solid phase ratio at the center of the slab thickness at the time of passing through the light pressure lower belt 16 while maintaining the temperature at 900 ° C. or higher and entering the light pressure lower belt 16 is 0.4 or less. The casting conditions such as the slab drawing speed and the amount of secondary cooling water are adjusted so that solidification to the center of the slab is completed within the range of the band 16. The solid phase ratio at the center of the slab thickness at the start of light reduction may be any amount as long as it is 0.4 or less. The start point of bulging the slab 12 is after the surface temperature of the long side surface of the slab becomes 900 ° C. or higher. In addition, when the installation range of the light reduction belt is long in the casting direction, and there are a roll group that applies light reduction and a roll group that does not apply light reduction, the roll group that actually applies light reduction. Only the above-mentioned light pressure lower belt 16 may be regarded as an operation.

  In the secondary cooling zone, a secondary cooling zone is set for each single or a plurality of roll segments, and the amount of secondary cooling water can be determined for each cooling zone. By using this function, the amount of secondary cooling water is such that the surface temperature of the long side surface of the slab becomes 800 ° C. or lower before reaching the bending portion 17, and the surface temperature of the long side surface of the slab becomes 900 ° C. or higher after the bending portion 17. Adjust. In this case, it is desirable to control the surface temperature of the short side surface of the slab 12 to be equal to that of the long side surface.

  Since the reduction speed in the light reduction belt 16 is obtained by the product of the roll gradient and the drawing speed of the slab 12, the reduction speed is light so that the reduction speed becomes a predetermined value within the range of 0.6 to 1.5 mm / min. What is necessary is just to set the roll gradient of the reduction zone 16. For example, when the slab drawing speed is 1.5 m / min and the rolling speed is 1.2 mm / min, the roll gradient is 0.8 mm (0.8 = 1.2 / 1.5 / m in the casting direction distance). )

  By performing continuous casting of steel in this way, inclusions near the 1/4 thickness position on the upper surface side of the slab 12a can be reduced, and light reduction is effective and effective for the slab 12 The flow of the concentrated molten steel accompanying solidification shrinkage or the like is suppressed, and the center segregation of the slab 12a can be greatly reduced.

  Using a vertical bending slab continuous casting machine as shown in FIG. 1, casting was performed while changing the secondary cooling strength. Specimens are taken from the slab slab, and the inclusions and central segregation near the 1/4 thickness position of each test specimen are investigated, and the secondary cooling strength exerted on the inclusions and central segregation near the 1/4 thickness position. The effect of was investigated.

  The continuous casting machine used is a vertical bending type slab continuous casting machine having a vertical portion of 2.8 m directly under the mold and a radius of the curved portion that follows that is 10 m. It is installed in the range of ~ 32m. Within the range of the light reduction zone, the position, range, and reduction amount (reduction speed) for light reduction can be set according to the casting conditions. Using this continuous casting machine, steel for a sour line pipe having a carbon content of 0.04 to 0.05 mass% was cast as a slab having a thickness of 250 mm and a width of 2000 mm at a drawing speed of 1.5 m / min.

  The heat transfer calculation simulating this casting was performed to estimate the solid phase ratio at the center of the slab thickness under each cooling condition. In the lightly reduced belt, the slab is only supported without light reduction until the calculated solid fraction in the center of the slab thickness direction becomes 0.3 to 0.4, and the roll gradient thereafter is 1 m in the casting direction distance. 0.9 mm per unit, that is, 1.35 mm / min (1.35 = 1.5 × 0.9) when converted to a light rolling speed.

  Under such casting conditions, the secondary cooling conditions are variously changed based on the calculated solid phase ratio, and the comparative evaluation of the investigation results of inclusions and central segregation near the 1/4 thickness position of the slab specimens. Went. Inclusions in the vicinity of the 1/4 thickness position of the test piece are C cross sections (transverse) in the slab width direction taken from three locations in the slab width direction (1/4 width, 1/2 width, 3/4 width). Surface) The sample was subjected to ultrasonic flaw detection (flaw detection depth: 1 mm), and even if there was even one inclusion having a size of 100 microns or more, the sample was rejected. The center segregation of the slab was evaluated based on the segregation degree of the carbon concentration in the slab, and the segregation degree of the carbon concentration was 1.20 or less. The degree of segregation of carbon is a value obtained by dividing the carbon concentration value at the center portion of the slab by the carbon concentration value at the slab bulk portion (for example, 1/4 thickness position). Table 1 shows the surface temperature of the long side surface of the slab and the investigation results of inclusions and central segregation near the 1/4 thickness position. In the quality evaluation column shown in Table 1, “◯” indicates pass, and “x” indicates failure.

  As shown in Table 1, it was confirmed that inclusions near the 1/4 thickness position on the upper surface of the slab can be reduced by cooling the surface temperature of the long side surface of the slab to 800 ° C. or lower at the vertical portion directly below the mold. . Further, it has been found that the center segregation of the slab varies depending on the secondary cooling conditions even when the slab drawing speed, the amount of reduction, etc. are the same, and the center segregation can be determined based on the surface temperature of the slab.

It is a side surface schematic diagram of the vertical bending type slab continuous casting machine which implemented the present invention. It is a figure which shows typically the setting of the roll space | interval of a guide roll.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vertical bending type slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Support roll 7 Guide roll 8 Pinch roll 9 Conveyance roll 10 Cast piece cutting machine 11 Molten steel 12 Cast piece 13 Solidified shell 14 Unsolidified phase 15 Solidified Completion position 16 Light pressure lower belt 17 Bending part 18 Bending part

Claims (2)

  Using a vertical bending type continuous casting machine equipped with a light reduction belt composed of a plurality of pairs of reduction rolls, the solid pressure ratio at the center of the thickness of the slab is 0.4 or less in the light reduction zone from the point of time of 0.4 or less. When producing a continuous cast slab of steel by starting to reduce the slab at a reduction speed in the range of 1.5 mm / min and completing solidification within the light reduction zone while applying a reduction force to the slab. In addition, the slab is cooled until the surface temperature of the long side surface of the slab becomes 800 ° C. or less at the vertical part immediately below the mold, and the surface temperature of the long side surface of the slab is set to 900 ° C. by reheating after the bent part following the vertical part. A method for producing a continuous cast slab, characterized in that a pressing force is applied to the slab at the above-described lightly-reduced band while the surface temperature of the slab long side surface is maintained at 900 ° C. or higher.   In the state where the surface temperature of the long side surface of the slab after the bent portion is 900 ° C. or higher, the slab is bulged by widening the gap in the slab thickness direction of the guide roll group, and then casted in the lightly pressed belt. 2. The method for producing a continuous cast slab according to claim 1, wherein a rolling force is applied to the piece.

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