JP2008207201A - Method for manufacturing continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing continuously cast slabs, which method can expand an optimum casting condition by improving the efficiency of light rolling reduction in continuously casting molten steel while applying the light rolling reduction to the cast slabs, and can manufacture the cast slabs having the small central segregation and being capable of satisfying the recent severe quality requirement. <P>SOLUTION: When the continuously cast slabs are manufactured, the rolling reduction of the cast slabs 12 is started from the position where the solid phase rate at the central portion of the thickness of the cast slabs 12 is lower than 0.4, and is continued until the position where the solid phase rate at least at the central portion of the thickness of the cast slabs 12 becomes higher than 0.7. In this case, the surface temperature at the middle portion of the long side of the cross section of the cast slabs is kept to be higher than 750°C until the position where the solid phase ratio at the middle portion of the thickness of the cast slabs becomes at least 0.5, and the surface temperature at the middle portion of the long side of the cross section of the cast slabs is kept to be lower than 850°C at the position where the solid phase ratio at the middle portion of the thickness of the cast slabs is higher than 0.5, and the rolling reduction is continued. Thus, the cast slabs having the small central segregation can be manufactured. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a continuous cast slab, and more particularly, to a method for producing a continuous cast slab that can produce a continuous cast slab of light steel with a central segregation.

  In the solidification phenomenon of steel, solute elements such as carbon, phosphorus and sulfur are usually 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 between rolls of a continuous casting machine, the molten steel concentrated by this microsegregation Flows and 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 countermeasures for reducing the center segregation of the slab have been proposed from the casting process to the rolling process.

  Among them, as a means for effectively reducing the center segregation of the slab, a method of reducing the slab at the end of solidification having an unsolidified phase with a reduction amount of about the solidification shrinkage (hereinafter referred to as “lightening” Many have been proposed. In this light reduction technology, the amount of reduction is equal to the amount of shrinkage obtained by adding the amount of solidification shrinkage and heat shrinkage of the slab, that is, depending on the amount of shrinkage of the unsolidified phase at the end of solidification. As the steel moves toward the center of the slab, the slab is gradually reduced at a reduction speed and a reduction amount that considers the reduction efficiency to prevent the formation of voids at the center of the slab and at the same time the flow of the concentrated molten steel This is intended to reduce the center segregation of the slab.

  For example, Patent Document 1 discloses that the slab surface temperature is maintained at 900 ° C. or lower in the region where the solid phase ratio at the center of the slab thickness is from 0.1 to 0.3 to the flow limit solid phase ratio. A method has been proposed in which a slab is continuously lightly reduced at a reduction speed of 5 mm / min to 2.5 mm / min. In Patent Document 2, bulging force is applied to the solidified shell positively directly under the mold to increase the thickness of the unsolidified phase in the slab, and then the slab is lightly reduced to generate center segregation. A method for reducing the above has been proposed.

In Patent Document 3, while the slab is lightly reduced, it is strongly cooled until the slab surface temperature reaches about 500 ° C. until the central part of the slab thickness is completely solidified, and the thermal shrinkage rate of the slab is set to 0.25 to 0.25. By controlling to the range of 1.0 mm / min, a method has been proposed in which the volume of the unsolidified phase is reduced by the thermal contraction of the solidified shell and the flow of the unsolidified phase is suppressed to reduce the central segregation. In Patent Document 4, in the technique of once bulging a slab including an unsolidified portion and then lightly reducing the bulging equivalent, the specific water amount of secondary cooling from immediately below the mold to immediately before the light pressure is reduced per kg of slab. A method has been proposed in which the slab surface temperature is controlled to be less than 1000 ° C. at 1.0 liter (hereinafter referred to as “L / steel kg”) or more and 3.0 L / steel kg or less. Further, Patent Document 5 discloses that the specific water amount of secondary cooling in the secondary cooling zone from directly under the mold to the bending correction point of the slab is 1.0 L / kg or less, and the surface temperature of the slab is 1000 ° C. or more. And the surface temperature of the slab is 1000 ° C. or less with a specific water volume of the secondary cooling zone from the bending correction point of the slab to the position where light reduction starts to 0.2 to 1.5 L / steel kg. There has been proposed a method in which the slab is lightly reduced after cooling so that
JP-A-62-158555 JP-A-62-34461 JP 2001-138021 A JP 11-123513 A JP 2001-62551 A

  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. However, it is considered that the rolling efficiency changes within a range of about 10% to 70% depending on the solidified portion of the slab, that is, the strength and rigidity of the solidified shell.

  If the rolling efficiency is low, even if rolling is applied, the effect of reducing the center segregation cannot be sufficiently expected. 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, Patent Document 1 is a method of continuously reducing the slab while maintaining the slab surface temperature at 900 ° C. or less. Since the surface temperature of the slab is low over the entire range to be applied, the strength of the solidified shell is high, and the effect of the slab pressure reduction is spent on deforming the solidified shell, and the slab where central segregation occurs The rolling efficiency at the center of thickness decreases. In particular, as shown in Patent Document 2 and the like, in the case where the slab is bulged in advance before the light reduction is applied, the surface temperature of the slab is set to 900 ° C. or less in the light reduction application region to thereby reduce the solidified shell. The strength becomes high and the solidified shell remains in a bulging state, so that the light reduction does not work sufficiently and the center segregation is deteriorated. Patent Document 2 does not specifically describe the slab surface temperature during rolling.

  Patent Document 3 is a technique for rapidly cooling the slab surface temperature to about 500 ° C. at the time of light reduction, and in this case as well as Patent Document 1, the effect of slab reduction is spent on deforming the solidified shell. It will be. In Patent Documents 4 and 5, the surface temperature is controlled to 1000 ° C. or lower, but only an upper limit is set, there is no lower limit value, and it is not specific how to control the surface temperature.

  That is, in Patent Documents 1 to 5, it is not clear how the slab surface temperature should be controlled when the slab is lightly reduced, and there is room for improvement.

  The present invention has been made in view of the above circumstances, and its purpose is to reduce the efficiency of light reduction when continuously casting molten steel while applying light reduction to the slab in order to reduce the center segregation of the slab. It is possible to provide a method for producing a continuous cast slab capable of producing a cast slab capable of increasing the optimum conditions and expanding optimum conditions and capable of producing a slab that can cope with severe quality requirements in recent years.

  In order to solve the above problems, the method for producing a continuous cast slab according to the present invention starts the slab reduction from the time when the solid phase ratio at the thickness center of the slab is 0.4 or less, and at least the slab When producing a continuous cast slab by continuously reducing the slab until the solid phase ratio at the center of the thickness reaches 0.7 or more, the solid phase ratio at the center of the slab thickness is at least 0.5. Up to the point in time, the surface temperature of the central part on the long side of the slab is kept at 750 ° C. or higher, the solid phase ratio in the central part of the thickness of the slab exceeds 0.5 and the reduction is continued. The surface temperature of the central part on the long side of the piece is set to 850 ° C. or less, and a slab with a slight center segregation is manufactured.

  According to the present invention, when the slab is lightly reduced, the surface temperature of the slab before and during light reduction is regulated within a predetermined range by controlling the amount of secondary cooling water. Since the effect of suppressing the flow of the concentrated molten steel due to light pressure is exhibited and the bulging between rolls can be reduced, the center segregation of the slab can be greatly reduced. As a result, it is 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, when the solid phase ratio at the center of the slab thickness is 0.4 or less, light reduction of the slab is started, and at least the solid phase ratio at the center of the thickness of the slab becomes 0.7 or more. When the continuous cast slab is manufactured by continuing to lightly reduce the surface temperature of the central part of the long side of the slab to 750 until the solid phase ratio at the thickness center of the slab reaches at least 0.5. Keep above ℃.

  In the case of continuous casting without light reduction, it is generally effective to strongly cool the slab surface from the viewpoint of suppressing center segregation. The reason is that by strongly cooling a continuous cast slab having an unsolidified phase inside, the temperature of the slab surface layer portion is lowered and the strength of the solidified shell of the slab is increased. This is because the bulging of the slab is reduced and the flow of the unsolidified phase due to the bulging between rolls can be suppressed. Further, by performing strong cooling, the volume of the unsolidified portion is reduced by the thermal contraction of the solidified shell itself, and the flow of the unsolidified phase can be suppressed.

  However, in the technique of preventing center segregation by light reduction, it is not always a good idea to strongly cool the slab. When the temperature of the slab decreases, the deformation resistance of the solidified shell increases. As described above, this is effective in preventing bulging between rolls, but it becomes a resistance against light pressure, and even if the surface of the slab is squeezed, the slab becomes a problem due to center segregation. The reduction effect does not reach the center of thickness.

  In order to solve this problem, in the present invention, the surface temperature of the central part on the long side of the slab is kept at 750 ° C. or higher until the solid phase ratio at the thickness center of the slab becomes at least 0.5. As a result, it is possible to prevent a solidified shell having a low temperature and a large deformation resistance from being formed before the light reduction starts. If the deformation resistance is small, it is generally rolled and easily deformed in the casting direction or in the width direction of the slab, but at this point, there is an unsolidified portion at the center of the thickness of the slab, and the casting is cast. The solid phase ratio at the center of one thickness is about 0.5, and the solid phase ratio corresponding to ZST (Zero Strength Temperature; the temperature at which the tensile strength becomes zero), which is generally the maximum temperature at which strength is developed (generally, solid phase) Therefore, most of the reduction applied to the surface of the slab is effectively spent on the reduction of the unsolidified portion. In addition, the period during which the surface temperature of the central part on the long side of the slab is maintained at 750 ° C. or higher is a period in which the solid phase ratio at the center part of the slab is up to about 0.5. Even if bulging between rolls occurs, if the solid phase ratio at the thickness center is about 0.5, the flow of the molten steel is relatively easy and the segregation degree (concentration ratio with respect to the original molten steel concentration) There is also an advantage that it is difficult to cause high center segregation.

  On the other hand, after the solid phase ratio in the center portion of the slab exceeds 0.5, the surface temperature of the central portion on the long side of the slab is set to 850 ° C. or less at the time when light reduction is applied to the slab. This is effective in causing light pressure to effectively act on the uncoagulated portion.

  When the solid phase ratio at the center of the slab increases, not only the solidified shell but also the unsolidified portion comes into contact with the dendrite structure and begins to develop strength. At this time, if the strength and deformation resistance of the solidified shell are not sufficient, the reduction applied to the surface of the slab increases the ratio spent on the deformation of the solidified shell, which reduces the unsolidified portion to reduce central segregation. Will not work effectively. This phenomenon becomes prominent when the solid phase ratio at the thickness center of the slab is about 0.7 or more, but the change in the secondary cooling strength to the slab affects the entire thickness of the slab. Since a certain time lag is required, the secondary cooling strength is strengthened when the solid fraction of the central portion exceeds 0.5 in advance, and the surface temperature of the central portion on the long side of the slab is rapidly reduced to 850 ° C. or less. Control to become. This also prevents the occurrence of bulging between rolls in the vicinity of the final solidified part.

  Although it is difficult to actually confirm the solid phase ratio at the center of the slab, it can be generally estimated by heat transfer calculation or the like.

  Thus, in the present invention, when the unsolidified slab is lightly reduced, the cooling strength of the slab is appropriately determined, so that the flow control effect of the concentrated molten steel due to the light reduction is effective, and the center segregation is effective. Can be reduced. The light reduction of the slab starts from the time when the solid phase ratio at the center portion of the slab thickness is 0.4 or less, and is performed until at least the solid phase ratio at the center portion of the slab thickness becomes 0.7 or more. 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. However, the effect of light reduction cannot be fully exhibited, and the flow of molten steel may occur until the solid phase ratio exceeds 0.7, and the light reduction is stopped earlier than that. If this occurs, the flow of the concentrated molten steel occurs, which causes central segregation, and the effect of light reduction cannot be fully exhibited.

  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 slab continuous casting machine 1 is provided with a mold 5, and a molten steel 11 supplied from a ladle (not shown) is relay-supplied to the mold 5 at a predetermined position above the mold 5. A tundish 2 is installed. 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”). A sliding nozzle 3 for adjusting the flow rate of the molten steel 11 is installed at the bottom of the tundish 2, and an immersion nozzle 4 is installed on the lower surface of the sliding nozzle 3. 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.

  Before and after the casting direction 15 across the solidification completion position 15 of the slab 12, the distance between the opposing guide rolls 7 (referred to as “roll interval”) was set so as to be 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 provided. Here, it is possible to lightly reduce the slab 12 over 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”.

  The molten steel 11 injected into the mold 5 through the immersion nozzle 4 is cooled by the mold 5 to form a solidified shell 13, and a support provided below the mold 5 as a slab 12 having an unsolidified phase 14 inside. While being supported by the roll 6, the guide roll 7, and 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, the thickness of the solidified shell 13 is increased, and the solidification completion position while being lightly reduced by the light pressure lowering zone 16. 15 completes the solidification to the inside. 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 thickness of the solidified shell 13 and the solid phase ratio at the center of the slab thickness are obtained in advance by using heat transfer calculation, etc. The casting conditions such as the slab drawing speed and the amount of secondary cooling water are adjusted so that the solid phase ratio at the center of the slab thickness is 0.4 or less. 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. 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 usually 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 surface temperature of the central part of the long side of the slab 12 is kept at 750 ° C. or more until the solid phase ratio at the center part of the slab thickness becomes at least 0.5, and in addition to that, If the solid phase ratio at the center part of the piece thickness exceeds 0.5, the secondary cooling strength is adjusted so that the surface temperature of the central part on the long side of the slab 12 is 850 ° C. or lower. In this case, since the effect of changing the secondary cooling strength may not immediately appear as the slab surface temperature, the long side of the slab 12 instantly if the solid phase ratio at the center of the slab thickness exceeds 0.5 This does not mean that the surface temperature of the side center is 850 ° C. or lower. It does not matter if there is some time delay. In short, if the solid phase ratio at the center part of the slab thickness exceeds 0.5, the surface temperature of the central part on the long side of the slab 12 during light reduction is set to 850 ° C. or less as quickly as possible. That is.

  By carrying out continuous casting of steel in this way, light reduction effectively and effectively acts on the slab 12, and the flow of the concentrated molten steel accompanying solidification shrinkage is suppressed, and the center of the slab 12a Segregation can be greatly reduced.

  Using a slab continuous casting machine as shown in FIG. 1, casting was performed while changing the secondary cooling strength and the light rolling speed. A test piece was collected from the slab slab, the center segregation of each test piece was investigated, and the influence of the secondary cooling strength on the center segregation was investigated.

  The continuous casting machine used is a vertical bending type slab continuous casting machine that has a vertical part of 2.8 m directly under the mold and the radius of the curved part that follows it is 10 m. It is installed in the range of 16-32m. 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% by mass was cast as a cast piece having a thickness of 250 mm and a width of 2100 mm at a drawing speed of 1.4 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 0.2 to 0.3, and the roll gradient thereafter is set at a casting direction distance of 1 m. 0.9 mm per unit, that is, 1.26 mm / min (= 1.4 × 0.9) when converted to a light rolling speed.

Under such casting conditions, the secondary cooling conditions were variously changed based on the calculated solid phase ratio, and the comparative evaluation of the center segregation investigation results of the slab specimens was performed. Table 1 shows the conditions and the results of investigation of center segregation under each condition. “C / C ₀ ” shown in Table 1 represents the segregation degree of the carbon concentration, “◯” mark in the evaluation column is good, “X” mark is bad, and “Δ” mark is somewhat bad. .

  As shown in Table 1, the center segregation of the slab varies depending on the secondary cooling conditions even when the drawing speed, the amount of light reduction, etc. are the same, specifically, based on the surface temperature of the slab. It was found that central segregation can be discriminated.

It is a side surface schematic diagram of the vertical bending type slab continuous casting machine which implemented the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Support roll 7 Guide roll 8 Pinch roll 9 Conveyance roll 10 Slab cutting machine 11 Molten steel 12 Cast slab 13 Solidified shell 14 Unsolidified phase 15 Solidification completion position 16 Light pressure lower belt

Claims (1)

  Start of slab reduction when the solid phase ratio at the center of the slab thickness is 0.4 or less, and reduce the slab until at least the solid phase ratio at the center of the slab thickness is 0.7 or more. When the continuous cast slab is continuously manufactured, the surface temperature of the central part on the long side of the slab is 750 ° C. or higher until the solid phase ratio at the thickness center of the slab becomes at least 0.5. When the solid phase ratio at the thickness center of the slab exceeds 0.5 and the reduction is continued, the surface temperature of the long side central portion of the slab is set to 850 ° C. or less, and the center segregation is slight. A method for producing a continuous cast slab characterized by producing a continuous slab.

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