JP2010069499A - Method for producing continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a continuously cast slab, which has less central segregation by reducing adverse influence on the central segregation caused by slab straightening in the straightening zone, thereby enhancing the effect of light rolling reduction even in the case a light rolling reduction zone provided for reducing the central segregation of a slab is superimposed on the range of a straightening zone. <P>SOLUTION: When a continuously cast slab 11 is produced using a continuous casting machine 1 where two or more pairs of roll groups 17 for imparting a rolling draft corresponding to a solidification-shrinkage amount to a slab are installed so as to be superimposed in the range of a straightening zone 16 for straightening a circular slab into the planar one, casting conditions are regulated in such a manner that the solid phase ratio of the central part in the thickness of the slab at the point of time in which the slab passes through the straightening zone reaches ≤0.3 or ≥0.7, and further, the slab is subjected to rolling reduction at a rolling reduction rate of 0.5 to 1.5 mm/min using the two or more pairs of roll groups. <P>COPYRIGHT: (C)2010,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 process of steel, solute elements such as carbon, phosphorus and sulfur are concentrated on the unsolidified liquid phase side by redistribution during solidification. This is the microsegregation formed between dendrite trees. A void is formed in the center of the slab due to solidification shrinkage of the slab being cast by the continuous casting machine and bulging of the solidified shell that occurs between the rolls of the continuous casting machine (hereinafter referred to as “inter-roll bulging”). If a negative pressure is generated, molten steel is sucked into this part, but since there is not a sufficient amount of molten steel in the unsolidified phase at the end of solidification, the molten steel concentrated by the above microsegregation flows, Accumulate and solidify in the center of the slab. In the segregation spot formed in this way, the concentration of the solute element is much higher than the initial concentration of the molten steel. This is generally called macrosegregation, and is called central segregation because of its existence site.

  Central segregation degrades the quality of steel products. For example, in line pipe materials for oil transportation and natural gas transportation, hydrogen-induced cracking (also referred to as “HIC”) occurs from the center segregation due to the action of sour gas. 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 continuous casting process to the rolling process.

  Among them, as a means to reduce the center segregation of the slab effectively and inexpensively, the slab at the end of solidification having an unsolidified phase is gradually reduced by a roll at a reduction speed of about the solidification shrinkage amount in a continuous casting machine. A casting method (hereinafter referred to as “light reduction”) has been proposed (for example, see Patent Document 1).

  This light reduction technology uses multiple pairs of rolls aligned in the casting direction, and gradually reduces the volume of the unsolidified phase by reducing the volume of the slab at a reduction speed and reduction amount commensurate with the amount of solidification shrinkage. This is a technique for preventing the formation of voids or negative pressure parts in the parts and at the same time preventing the flow of the concentrated molten steel formed between the dendrite trees, thereby reducing the center segregation of the slab. Therefore, in the light reduction technology, generally, the complete solidification position of the slab is controlled within the range of the light reduction zone. Here, the light reduction belt is a group of a plurality of pairs of rolls that imparts a reduction in an amount corresponding to the amount of solidification shrinkage to the slab.

On the other hand, Non-Patent Document 1 examines the light reduction technology in detail, and the optimum time when the light reduction is applied to the slab is in the range of 0.3 to 0.7 in the solid phase ratio at the center of the slab. In other words, it is reported that it is not necessary to reduce the solid phase ratio at the center of the slab at less than 0.3 and more than 0.7. This is because even if there is a sufficient amount of molten steel in the unsolidified phase in the range where the solid fraction of the slab center is less than 0.3, and a void or negative pressure part is formed in the slab center, However, since the replenishment of molten steel from the unsolidified phase is sufficiently performed, center segregation does not occur. On the other hand, when the solid phase ratio at the center of the slab exceeds 0.7, the unsolidified molten steel can flow due to the viscosity relationship. However, even if a void or a negative pressure portion is formed at the center of the slab, the center segregation is not formed.
JP 49-121738 A Fukushima et al., Materials and Processes, vol. 1 (1988) p. 202

  In recent years, continuous casting machines for mass production are mainly curved type continuous casting machines and vertical bending type continuous casting machines. Both continuous casting machines are curved in order to finally carry out the slab in the horizontal direction. There is a straightening band for correcting the arc-shaped slab in the portion into a flat-plate slab. Here, the vertical bending type continuous casting machine is a continuous casting machine having a vertical portion of about 2 to 5 m directly below the mold and a curved portion below the vertical portion.

  For steel grades where center segregation is a problem, it has been conventionally corrected from the viewpoint of preventing internal cracks by correcting unsolidified slabs or reducing the bulging between rolls by reducing the static pressure of molten steel. It was common to complete the solidification before reaching the belt. In this case, the light pressure lower zone was naturally installed upstream of the correction zone.

  Recently, however, high-speed casting has been aimed at improving productivity, and the casting speed has been increased even for steel types in which central segregation is a problem. In order to increase the casting speed, dispersion of straightening stress by multipoint straightening and reduction of bulging between rolls by shortening the roll pitch are performed. Due to the higher casting speed, the solidification completion position reaches the downstream side of the correction band, and accordingly the light pressure lower band is also arranged downstream, and as a result, the light pressure lower band may overlap the range of the correction band. It was way.

  The inventors of the present invention have confirmed that, when the lightly pressed zone overlaps the range of the correction zone, under certain conditions, the effect of improving the center segregation is small even if the slab is slightly reduced. This is thought to be due to the flow of concentrated molten steel between dendritic trees caused by straightening stress during the slab correction in the straightening zone, and the flow of this concentrated molten steel has an adverse effect on the central segregation.

  The present invention has been made in view of the above circumstances, and the object of the present invention is to correct even if the light reduction zone provided to reduce the center segregation of the slab overlaps the range of the correction zone. Continuously capable of producing small slabs with central segregation, which can reduce the adverse effects on center segregation by strip slab correction, enhance the effect of light reduction, and cope with severe quality requirements in recent years. It is providing the manufacturing method of a cast slab.

  The method for producing a continuous cast slab according to the first invention for solving the above-mentioned problems is a range of a correction band for correcting an arc-shaped slab into a flat slab, and a solidification shrinkage amount on the slab. When producing a continuous cast slab using a continuous casting machine in which a plurality of pairs of rolls for providing a reduction corresponding to the amount of roll is overlapped, the center part of the slab thickness when passing through the correction strip The casting condition is adjusted so that the solid phase ratio of the slab thickness is 0.3 or less or 0.7 or more, and the solid phase ratio of the slab thickness center portion from the time when the solid phase ratio of the slab thickness center portion is 0.4 or less The slab is crushed at a rolling speed of 0.5 to 1.5 mm / min using the plurality of pairs of roll groups until the rate is 0.7 or more.

  The method for producing a continuous cast slab according to a second invention is the method according to the first invention, wherein the solid phase ratio at the center of the slab thickness when passing through the correction band is 0.3 or less, and the plurality The pair of roll groups is installed from the exit side of the correction band to a range of 5 m or more on the downstream side in the casting direction.

  According to the present invention, the slab is lightly reduced by using a plurality of pairs of rolls for applying a reduction force to the slab in the range of the correction band, that is, a continuous casting machine in which the light reduction belts are overlapped. In the case of casting while adjusting the solid fraction of the center part of the slab thickness at the time of passing through the correction band to 0.3 or less or 0.7 or more, the solid fraction of the center part of the thickness is 0.3 or less. In the case of adjusting to slab thickness, there is a sufficient unsolidified phase at the center of the slab thickness, and the concentrated molten steel between dendrites formed by straightening the slab is diluted by the unsolidified phase, and the center by the concentrated molten steel The effect on segregation can be reduced. On the other hand, when the solid phase ratio is adjusted to 0.7 or more, the concentrated molten steel between dendrites does not flow due to the viscosity relationship even if the slab is corrected. Therefore, in both cases, the negative effect on the center segregation due to slab correction can be prevented, and the effect of light pressure reduction Sufficiently be exhibited, it is possible to manufacture the minor slab of center segregation.

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

  The inventors have a negative influence on the center segregation due to the correction of the slab in the correction band when casting the slab in the continuous casting machine in which the light reduction band overlaps the range of the correction band and casting the slab. For the purpose of reducing the amount, a test was conducted to investigate the relationship between casting conditions and center segregation by changing the casting conditions using a vertical bending type continuous casting machine. Then, the obtained slab was rolled into a thick steel plate, a UOE steel pipe was manufactured from this thick steel plate, and a segregation degree investigation of the slab and a HIC test (hydrogen resistance induced cracking evaluation test) of the UOE steel pipe were performed.

  The straightening belt of the vertical bending type continuous casting machine used has a distance from the molten metal surface in the mold in the range of 19.4 m to 20.6 m, and any position in the range of the distance from the molten metal surface in the mold to 15 m to 32 m. In addition, the continuous casting machine is configured such that a light pressure belt can be installed according to the casting speed. In test casting, the length of the light pressure lower belt is 6 m, and light reduction is performed according to the casting speed so that the rear end portion (downstream side in the casting direction) in the light pressure lower belt is the solidification completion position of the slab. The installation position of the belt was determined. The reduction speed in the light reduction zone was 1.2 mm / min.

  Chemical components are C: 0.05 mass% (hereinafter referred to as “%”), Si: 0.3%, Mn: 1.3%, P: 0.005%, S: 0.005%, Ti : 0.01%, sol.Al: 0.04%, Nb: 0.04%, Cu; 0.15% of molten steel, casting speed (Vc) is 1.05 m / min (level 1), 1 It was cast into a mold having a width of 1950 mm and a thickness of 250 mm at three casting speeds of .25 m / min (level 2) and 1.40 m / min (level 3). The degree of superheated molten steel in the tundish was 38 ° C. The amount of secondary cooling water is 1.5 to 1.8 L / kg in terms of specific water. The specific water amount is a numerical value representing the cooling water amount per 1 kg of cast slab.

  FIG. 1 shows the result of two-dimensional heat transfer solidification calculation for the relationship between the distance from the molten metal surface in the mold and the solid phase rate at the center of the slab thickness when casting is performed under the above casting conditions. In FIG. 1, the light pressure lowering belt is installed in accordance with each casting speed so that the solidification rate 1.0 is the solidification completion position and this position is the rear end of the light pressure lowering belt. As shown in FIG. 1, at level 1 where the casting speed is 1.05 m / min, solidification is completed before reaching the correction zone, and at level 2 where the casting speed is 1.25 m / min, the slab thickness is centered. At the level 3 where the solid phase ratio of the part passes through the correction band at a time of about 0.4 and the casting speed is 1.40 m / min, the solid phase ratio at the center of the slab thickness in the correction band is 0.05 or less. This is a condition. The total amount of reduction in the light pressure zone is 6.9 mm (1.2 x 6 / 1.05) at level 1, 5.8 mm (1.2 x 6 / 1.25) at level 2, and 5.1 mm (1.2 x 6 / 1.40) at level 3. It is.

A sample for segregation inspection was collected from the cast slab and the degree of segregation was investigated. Specifically, a cross-sectional sample in the casting direction was cut out from the ½ and 1/4 positions in the width direction of the slab cast piece, and an analysis sample was collected by slicing 1 mm in the thickness direction of the cast piece from this cross section. This analysis sample was analyzed for carbon by the combustion gas analysis method. As for the segregation degree of the slab, the carbon analysis value at the 1/4 position in the thickness direction of the slab is defined as a reference value (C _O ) without segregation, and the carbon analysis value (C _i ) at each position and the reference value (C _O ). The ratio (C _i / C _O ) was evaluated as the degree of segregation.

Moreover, the HIC test in the UOE steel pipe carried out as an evaluation at the product stage is a NACE solution (5% NaCl + 0.5% CH ₃ COOH hydrogen sulfide saturated solution, pH = 3.7), and the immersion time is 96. The test solution temperature was 25 ° C. for a time. Table 1 shows the casting conditions, the segregation degree of the slab, and the results of the HIC test of the UOE steel pipe.

  As shown in Table 1, even when the rolling speed is the same, level 1 where the solid phase ratio at the center of the slab thickness during correction in the correction band is 1.0 and the maximum solid phase ratio is 0.04. On the other hand, at level 3 where the center segregation was slight, on the other hand, at level 2 where the solid phase ratio at the center of the slab thickness during correction was 0.4, the center segregation deteriorated. That is, it was confirmed that the flow of concentrated molten steel between dendrite trees caused by the correction of the slab has an adverse effect on the center segregation if the solid phase ratio at the center of the slab thickness at the time of correction in the correction band is not appropriate. . From the test casting in which the casting conditions are further subdivided (see [Example 1] described later), the solid phase ratio at the center of the slab thickness at the time of slab correction in the correction band is 0.3 or less or 0.7 or more In some cases, it has been found that central segregation is reduced.

  The present invention has been made on the basis of the above examination results, and produces a continuous cast slab using a continuous casting machine in which a light-reduced belt is placed in the range of a straightening band for correcting the slab. In this case, the casting conditions are adjusted so that the solid phase ratio at the center portion of the slab thickness when passing through the correction zone is 0.3 or less or 0.7 or more, and the solid phase ratio at the center portion of the slab thickness is 0. The slab is squeezed at a reduction speed of 0.5 to 1.5 mm / min in the light reduction zone from a time point of 4 or less to a time point when the solid phase ratio at the center of the slab thickness is 0.7 or more. And

  The slab is lightly pressed at a time when the solid phase ratio at the center of the slab thickness is 0.4 or less and at least until the solid phase ratio at the center of the slab thickness is 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.

  The rolling speed is in the range of 0.5 to 1.5 mm / min. When the rolling speed is less than 0.5 mm / min, the rolling speed is too small with respect to the solidification shrinkage, and the flow of the concentrated molten steel may not be suppressed. On the other hand, the rolling speed is 1.5 mm / min. When exceeding, the reduction speed becomes larger than the amount of solidification shrinkage, and by squeezing out the concentrated molten steel, there is a possibility that negative segregation may be formed in the center part of the slab. 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. 2 is a schematic side view of a vertical bending slab continuous casting machine embodying the present invention.

  As shown in FIG. 2, the slab continuous casting machine 1 is provided with a mold 5 for injecting molten steel 10 to solidify, and a predetermined position above the mold 5 is supplied from a ladle (not shown). A tundish 2 for relaying and supplying molten steel 10 to the mold 5 is installed. On the other hand, below the mold 5, a cooling grid 6 is installed immediately below the mold, and a plurality of pairs of slab support rolls 7 are disposed below the cooling grid 6. The slab support roll 7 includes a pinch roll which is a drive roll for pulling out the slab 11 and a guide roll, but these are shown without distinction in the drawing. A secondary cooling zone in which a spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is arranged is formed in the gap between the slab support rolls 7 adjacent to each other in the casting direction. The slab 11 is cooled while being drawn out by cooling water sprayed from the nozzle (also referred to as “secondary cooling water”). The cooling grid 6 is a slab support device constituted by a plate (not shown) for supporting the slab 11 with a surface and a spray nozzle (not shown) provided in a gap between the plates. .

  A plurality of pairs of slab support rolls 7 disposed at a position about 1 m to 4 m away from the exit of the mold 5 constitute a bending portion 15 in which the support / guide direction of the slab 11 changes its direction from a vertical direction to a bending direction. is doing. That is, the slab 11 on the flat plate drawn out from the mold 5 in the vertical direction is gradually bent into an arc shape by the bending portion 15 and is corrected to a curved portion having a constant radius. Similarly, the plurality of pairs of slab support rolls 7 disposed in the vicinity of the position where the curved portion comes into contact with the horizontal line constitutes a correction band 16 in which the support / guide direction of the slab 11 changes from the curved direction to the horizontal direction. is doing. That is, the arc-shaped slab 11 is gradually bent back on the flat plate by the correction band 16 and corrected to the horizontal portion. In the case of a curved continuous casting machine, the bent portion 15 does not exist, the curved portion is formed from the mold, and only the correction band 16 exists. Further, in FIG. 2, both the bending portion 15 and the correction band 16 are configured by a plurality of pairs of slab support rolls 7, but may be configured by only a pair of slab support rolls 7.

  A sliding nozzle 3 for adjusting the flow rate of the molten steel 10 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. A plurality of transport rolls 8 for transporting the cast slab 11 is installed on the downstream side of the slab support roll 7. Above the transport roll 8, the cast slab 11 to be cast is provided. A slab cutting machine 9 for cutting a slab 11a having a predetermined length is disposed.

  A plurality of slab support rolls, which include the correction band 16, and are set so that the interval between the slab support rolls facing each other (referred to as “roll interval”) is sequentially narrowed toward the downstream in the casting direction. A light pressure lower belt 17 composed of a pair of slab support rolls is installed. In the light reduction belt 17, it is possible to perform light reduction on the slab 11 in the entire region or a partially selected region. A spray nozzle for cooling the slab 11 is also disposed between the slab support rolls of the light pressure lower belt 17. 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 10 injected into the mold 5 through the immersion nozzle 4 is cooled by the mold 5 to form a solidified shell 12, and a cooling provided below the mold 5 as a slab 11 having an unsolidified phase 13 inside. While being supported by the grid 6 and the slab support roll 7, it is continuously pulled out below the mold 5 by the driving force of the pinch roll. The slab 11 is cooled by the secondary cooling water in the secondary cooling zone while passing through the cooling grid 6 and the slab support roll 7, and is lightly reduced by the light pressure lower zone 17 while increasing the thickness of the solidified shell 12. The solidification to the inside is completed at the solidification completion position 14 while gradually reducing the thickness. The slab 11 that has been solidified is cut by the slab cutting machine 9 to become a slab 11a.

  In such a continuous casting operation, the thickness of the solidified shell 12 and the solid phase ratio at the center of the slab thickness are obtained in advance using heat transfer calculation or the like, and the thickness of the slab 11 at the time when it passes through the correction strip 16. The casting conditions such as the thickness of the slab, the casting speed, and the amount of secondary cooling water are adjusted so that the solid phase ratio at the center is 0.3 or less.

  When the slab 11 having the unsolidified phase 13 is straightened, the concentrated molten steel between the dendrite trees flows due to the straightening stress, but when the solid phase ratio at the center of the thickness of the slab 11 is 0.3 or less, the slab 11 When corrective stress is applied to the steel sheet, the concentrated molten steel is not discharged in a large amount, and a large amount of the unsolidified phase 13 is present, so that the discharged concentrated molten steel is diluted. Center segregation is improved by light reduction.

  In this case, it is preferable to arrange the light pressure lower belt 17 by 5 m or more from the exit side of the correction belt 16 to the downstream side. If the light pressure lower belt 17 is not disposed 5 m or more downstream from the outlet side of the correction belt 16, it remains unsolidified on the outlet side of the light pressure lower belt 17, and the center segregation after passing through the light pressure lower belt 17. This is because there is a risk of occurrence. In the light reduction zone 17, it is necessary to start the reduction from the time when the solid phase rate at the center portion of the slab thickness is 0.4 or less and to reduce to the time when the solid phase rate at the center portion of the slab thickness is 0.7 or more. This condition can be satisfied by disposing the light pressure lower belt 17 by 5 m or more from the outlet side of the correction belt 16 to the downstream side.

On the other hand, the thickness of the slab, the casting speed, the amount of secondary cooling water, etc. so that the solid phase ratio at the center of the thickness of the slab 11 when passing through the correction band 16 is 0.7 or more. It is also effective to adjust the casting conditions. This is because when the solid phase ratio at the center of the thickness of the slab 11 exceeds 0.3 and less than 0.7, when the corrective stress is applied to the slab 11, the concentrated molten steel between the dendrite trees flows due to the correction. Although the effect of light pressure is canceled and the center segregation is worsened, the correction stress is given by setting the solid phase ratio at the center of the thickness of the slab 11 at the time of passing through the correction band 16 to 0.7 or more. This is because the concentrated molten steel between dendrite trees does not flow due to viscosity, and does not worsen central segregation. In this case, it is necessary to set the length of the light pressure lower belt 17 so that the solid phase ratio at the center of the slab thickness at the time of entering the light pressure lower belt 17 is 0.4 or less. Specifically, it is sufficient to install a light pressure belt 17 having a length of 5 m or more on the entry side of the correction belt 16.
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 17 may be regarded as an operation.

  Since the reduction speed in the light reduction belt 17 is obtained by the product of the roll gradient and the casting speed of the slab 11, the reduction speed is light so that the reduction speed is a predetermined value within the range of 0.5 to 1.5 mm / min. What is necessary is just to set the roll gradient of the reduction zone 17. For example, when the rolling speed is 1.2 mm / min when the casting speed is 1.5 m / min, the roll gradient is 0.8 mm (0.8 = 1.2 / 1.5) per 1 m in the casting direction distance.

  As described above, the present invention uses a continuous casting machine in which a plurality of pairs of rolls for applying a reduction force to the slab 11, that is, a light reduction belt 17 is overlapped in the range of the correction band 16. Therefore, when casting the slab 11 with light pressure, the solid phase ratio at the center of the slab thickness at the time of passing through the correction band 16 is adjusted to 0.3 or less or 0.7 or more. Is adjusted to 0.3 or less, it is possible to reduce the influence on the central segregation due to the flow of the concentrated molten steel between dendritic trees caused by the correction of the slab 11, while the solid fraction is 0.7 In the case of the above adjustment, even if the slab 11 is corrected, the concentrated molten steel between the dendrite trees does not flow. Therefore, in both cases, the adverse effect on the center segregation due to the slab correction can be prevented, and the light pressure is reduced. The effect can be fully exerted, and a small slab 11a with central segregation is produced. Theft is possible.

  Using a vertical bending type slab continuous casting machine having a light pressure lower belt of 14 m in length before and after the straightening band, the slab is cast while being lightly reduced, and the resulting slab is rolled into a thick steel sheet. A UOE steel pipe was produced from the slab, and the segregation degree investigation of the slab and the HIC test of the UOE steel pipe were conducted.

  Chemical components are C: 0.05%, Si: 0.3%, Mn: 1.3%, P: 0.005%, S: 0.005%, Ti: 0.01%, sol.Al: Mold with 0.04%, Nb: 0.04%, Cu; 0.15% at a casting speed of 1.05-1.60 m / min, width 1950 mm × thickness 250 mm or width 1950 mm × thickness 220 mm Cast into. The molten steel superheat degree in the tundish was 35 to 48 ° C. The amount of secondary cooling water is 1.48 to 1.77 L / kg in terms of specific water.

  Table 2 shows the casting conditions, the segregation degree of the slab, and the results of the HOE test of the UOE steel pipe. The solid phase ratio shown in Table 2 is a value calculated by two-dimensional heat transfer solidification calculation, and the segregation degree of the slab and the HIC test of the UOE steel pipe are the same as those described above.

  In Invention Examples 1 to 3 and Comparative Examples 1 and 2, the casting speed was changed with a slab thickness of 250 mm and a reduction speed of 1.2 mm / min. Changed. As is clear from the results of Invention Examples 1 to 3 and Comparative Examples 1 and 2, by setting the solid phase ratio at the center of the slab thickness in the straightening zone to 0.3 or less or 0.7 or more, The segregation degree of the piece was 1.1 or less, and the HIC test passed without cracks.

  In Invention Example 4 and Comparative Examples 3 and 4, the slab thickness was 220 mm, the casting speed was constant at 1.60 m / min, and the rolling speed was changed. In Comparative Example 3, the rolling speed is smaller than the range of the present invention, and in Comparative Example 4, the rolling speed is larger than the range of the present invention. In Comparative Example 3, central segregation was not reduced, while in Comparative Example 4, negative segregation occurred. In Comparative Example 4, due to negative segregation, a hardness difference occurred at the center of the plate thickness, and cracks occurred in the HIC test due to this hardness difference.

  Thus, in the continuous casting machine in which the light pressure lowering zone is arranged before and after the correction belt, the effect of light pressure reduction can be achieved by appropriately adjusting the solid phase ratio at the center of the slab thickness when passing through the correction belt. Can be effectively produced, and it is possible to produce a slab with a slight center segregation.

It is a figure which shows the relationship between the distance from the hot metal surface in a mold in test levels 1-3, and the solid-phase rate of slab thickness center part. 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 Cooling grid 7 Slab support roll 8 Transport roll 9 Slab cutting machine 10 Molten steel 11 Slab 12 Solidified shell 13 Unsolidified phase 14 Solidification completion position 15 Bending Part 16 Correction belt 17 Light pressure belt

Claims (2)

  A plurality of pairs of rolls for overlapping the amount of reduction corresponding to the amount of solidification shrinkage on the slab were installed in the range of the correction band for correcting the arc-shaped slab into a flat plate-shaped slab. When producing continuous cast slabs using a continuous casting machine, the casting conditions are adjusted so that the solid phase ratio at the center of the slab thickness when passing through the straightening zone is 0.3 or less or 0.7 or more In addition, from the time when the solid phase ratio at the center part of the slab thickness is 0.4 or less to the time when the solid phase ratio at the center part of the slab thickness is 0.7 or more, 0.5 pairs are used. A method for producing a continuous cast slab, wherein the slab is reduced at a reduction speed of ˜1.5 mm / min.   The solid phase ratio of the slab thickness center at the time of passing through the correction band is 0.3 or less, and the plurality of pairs of rolls are in a range of 5 m or more from the exit side of the correction band to the downstream side in the casting direction. The method for producing a continuous cast slab according to claim 1, wherein

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