JP2008119726A - Light rolling reduction method for vicinity of solidification completion position in continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light rolling reduction method for the vicinity of the solidification completion position in a continuously cast slab where quality control for segregation or the like and rolling reduction control for an unsolidified part can be securely performed. <P>SOLUTION: In the method for performing light rolling reduction while controlling the rolling draft in a plurality of rolling reduction rolls arranged in the vicinity of the solidification completion position based on the conditions of the unsolidified part and solidified part of a slag cross-section in the vicinity of the solidification completion position, the central solid phase ratio shown by the ratio of the solid phase part to the whole width in the slab width direction at the center in the slab thickness direction is obtained, and, in accordance with the width direction distribution of the central solid phase ratio, the rolling draft is controlled. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for reducing the slab according to the state of solidification in the case where non-uniform solidification occurs in the slab width direction in the vicinity of the solidification completion point of the slab produced by continuous casting.

  In the continuous casting slab manufacturing technology, it is important to grasp the position and state of the solidification completion point of the slab for quality control such as cooling control and segregation and reduction control of the solidification completion point. Usually, solidification start and solidification completion in the slab is the slowest at the center, and the solidification completion point tends to be formed to extend downstream of the continuous cast slab. In the vicinity of the solidification completion point of such a continuous cast slab, the solidification completion point of the full thickness is called a crater end. Further, the mass ratio of the solid phase per unit volume in the slab can be defined as the solid phase ratio.

  10 to 12 are schematic diagrams of three typical shapes of the crater end described above. As shown in FIG. 10, when the vertical cross section of the slab 101 is viewed from above, the solidification completion points 102 on the slab corner side, in other words, on both ends of the slab width direction are solidified portions 103 on the center side of the slab width direction. 11, the solidified portion 105 on the central side of the slab width direction is smooth and close to the full width W at substantially the same position in the slab width direction as shown in FIG. In the case of a wide shape, and as shown in FIG. 12, after the solidified portion 108 wider than the solidified portion 107 on both ends of the slab is formed, the solidified portion 109 about half of the entire width W of the slab is formed on the center side of the slab. There are known three typical shapes in which the solidification completion point of the formation of the shape becomes a tongue-like shape.

These differences in the shape of the crater end are due to the influence of the flow of molten steel inside the mold, the effect of excessive cooling of both ends of the slab due to the influence of the water density from the nozzle in the cooling zone, It is thought that various casting conditions, such as the effects of being deprived, occur in relation to each other.
In the three representative crater end shapes, there is no problem as a good cooling state if the crater end shape shown in FIG. 11 is used. However, if the crater end shape shown in FIG. It is easy for corner cracks to enter both ends. In addition, although the corner break does not occur in the crater end shape shown in FIG. 10, the part of the end of the solidification completion point is a shape that goes in and out of the other solidified portion, so that segregation occurs, porosity occurs, etc. It is said that there are problems in cooling management. Usually, various casting conditions including cooling conditions are controlled so as not to generate such a crater end shape, and these problems are attempted to be solved. In addition to the control of these casting conditions, the crater end is lightly reduced using a reduction roll, and the above-mentioned problem of segregation is addressed, so the situation shown in FIG. 10 or FIG. 12 appears. Even so, light reduction is what you are doing. (Hereafter, the reduction roll used when performing the light reduction may be referred to as a light reduction roll.)
This light reduction method is known as a casting method in which a reduction with a slight plastic deformation is applied in the vicinity of the solidification completion point of the slab in order to prevent internal defects such as segregation.

As described above, the analysis of the crater end shape is an important consideration when trying to produce a continuous cast slab. Conventionally, for example, in the slab thickness direction of radiation having two or more different energy spectra. Based on the slab width direction distribution of the slab, there is provided a technique for obtaining the shape of the slab cross section near the solidification completion point. (See Patent Document 1)
As a continuous casting machine simulation device, the heat transfer coefficient hw from the finite element model Mj of the slab that is continuously guided by the rolls Ri and Ri + 1 and sprayed with cooling water or the like to the atmosphere is the previously calculated finite element. By calculating based on the surface temperature of the slab obtained from the heat transfer coefficient hw from the model Mj-1 and the spray amount distribution which is actual measurement data regarding the spray amount of the cooling water or the like, There is provided a method capable of accurately simulating the thermal behavior of the slab inside. (See Patent Document 2)

Furthermore, in the secondary cooling zone composed of a plurality of cooling zones divided in the casting direction, the steel is cast without spraying the secondary cooling water on the long side surface of the slab in the range of the distance Ln from the short side surface of the slab. In the continuous casting method, the distance (mm) at the n-th cooling zone is Ln, the solidified shell thickness (mm) at the short side of the slab at the entry side of the n-th cooling zone is Tn _in , When the solidified shell thickness (mm) at the short side of the slab at the cooling zone exit side is Tn _out , the equation is (Tn _in + Tn _out ) / 4 ≦ Ln ≦ (Tn _in + Tn _out ) / 2. Techniques for controlling the above are known. (See Patent Document 3)
Japanese Patent Laid-Open No. 10-325714 Japanese Patent Laid-Open No. 04-231158 Japanese Patent Laid-Open No. 2000-15412

However, in the continuous casting method based on the slab width direction distribution of the above-mentioned radiation transmittance, even if the crater end shape can be specified, a casting method and a processing method suitable for the specific crater end shape are provided. It was not a technique that could be used, but it was a technique that could identify the crater end shape by measurement.
In the method of forming the zone in which the secondary cooling water is not sprayed in the above-described secondary cooling zone according to the formula of (Tn _in + Tn _out ) / 4 ≦ Ln ≦ (Tn _in + Tn _out ) / 2, Although it is a technique to control the crater end shape by controlling the cooling state of the water, the cooling in the zone where the cooling water is not sprayed is insufficient, and the crater end shape cannot be sufficiently controlled as shown in FIG. There was a problem of solidification.

  The present invention has been made in view of the background described above, and is a case where the state of the unsolidified portion and the solidified portion on the crater end side of the continuous cast slab, that is, the state in the vicinity of the solidification completion point exhibits an undesirable state as the solidified state. It is an object of the present invention to provide a light reduction method in the vicinity of the solidification completion point of a continuous cast slab that can be lightly reduced without any problem and can reliably perform quality control such as segregation and reduction control in the vicinity of the solidification completion point.

(1) The light reduction method in the vicinity of the solidification completion point of the continuous cast slab of the present invention is such that molten steel is poured into the mold, and the solidified shell formed by cooling the molten steel is continuously drawn below the mold. When the solidification shell surface is cooled in the cooling zone on the downstream side to complete solidification of the slab, multiple pieces are arranged near the solidification completion point based on the state of the unsolidified part and solidification part near the solidification completion point of the slab It is a method of lightly rolling while controlling the rolling amount of the rolling roll, characterized by obtaining the central solid fraction in the slab thickness direction and adjusting the rolling amount according to the width direction distribution of the central solid fraction. And
(2) The light reduction method in the vicinity of the solidification completion point of the continuous cast slab of the present invention is the light reduction method according to (1), in which the amount of heat removed in the mold according to the casting direction distance from the meniscus, In the cross section of the slab perpendicular to the casting direction, based on the amount of heat removed by the cooling water sprayed from the cooling nozzle in the downstream cooling zone to the slab, the amount of heat removed by the continuous casting roll, and the amount of heat removed by radiation of the uncooled part (3) A light reduction method in the vicinity of the solidification completion point of the continuous cast slab of the present invention is characterized in that heat transfer solidification calculation is performed to calculate the central solid fraction in the slab thickness direction in the vicinity of the solidification completion point. In the light reduction method described in (1), the central solid fraction in the slab thickness direction near the solidification completion point of the slab is measured by a non-contact sensor.

(4) The light reduction method in the vicinity of the solidification completion point of the continuous cast slab of the present invention is the light reduction method according to any one of (1) to (3), in the width direction of the continuous cast slab of width W. An arbitrary position is y, an arbitrary position in the casting direction of the continuous cast slab is x, and a central solid fraction in the slab thickness direction at an arbitrary point (x, y) of the continuous cast slab is fs (x , Y), and a function for binarizing a value other than 0 when the central solid phase ratio fs (x, y) is within a predetermined range and binarizing to 0 when the other range is within the predetermined range g ( x (y) is defined, and B (x) obtained by integrating the function indicated by g (x, y) from 0 to W in the width direction of the continuous cast slab is obtained, and the value of B (x) is 0. And when the value of the center solid phase ratio fs (x, W / 2) in the slab thickness direction at the center of the slab width is equal to or greater than a predetermined value, (5) The light reduction method in the vicinity of the solidification completion point of the continuous cast slab according to the present invention is characterized in that the reduction completion point is reduced in the light reduction method according to (4). When controlling the reduction amount of the roll, when a plurality of reduction rolls are arranged in the same reduction segment and the reduction amount is adjusted by the reduction gradient of the segment, the B (x at each reduction roll position in the segment is adjusted. ) And at least one value other than 0, and the center solid phase ratio fs (x, W / 2) in the slab thickness direction at the center of the slab width at each rolling roll position in the segment When at least one of the values is equal to or greater than a predetermined value, the slab is crushed by the segment.

(6) The light reduction method in the vicinity of the solidification completion point of the continuous cast slab of the present invention is the light reduction method according to (5), in which the reduction amount depends on the width direction distribution of the central solid fraction in the slab thickness direction. Is adjusted, the gradient in the x direction of the central solid fraction fs (x, y) is obtained in the width direction (y direction) of the slab at the target roll position (arbitrary x position), and the average value of the values Or find the maximum value,
From the average value of the values obtained for each of the rolling rolls in the target segment arranged in plural for the average value of the gradient obtained in the width direction,
Up to the maximum value of the values determined for each of the rolling rolls in the target segment that is arranged in plural for the maximum value of the gradient determined in the width direction,
Convert the solidification shrinkage of the slab from any value within the range of
The amount of reduction is adjusted with a reduction gradient corresponding to the amount of contraction.
(7) The light reduction method in the vicinity of the solidification completion point of the continuous cast slab of the present invention is the light reduction method described in (5) or (6). When adjusting the amount of reduction in accordance with this, the range below the reduction force of the segment according to the total value of each of the reduction rolls in the segment in the width direction integral value at the reduction roll position of the central solid phase ratio fs (x, y). It is characterized by controlling the rolling force with.

(8) The light reduction method in the vicinity of the solidification completion point of the continuous cast slab according to the present invention is a desired reduction amount according to the method described in (6) when performing the light reduction in the vicinity of the solidification completion point of the continuous cast slab. , And the reduction is performed by setting the amount of reduction in advance.
(9) The light reduction method near the solidification completion point of the continuous cast slab of the present invention is the light reduction method described in (8). The segment is pressed against a device having a function of keeping the gradient constant, and the segment rolling gradient is set to a constant value so as to obtain a desired rolling amount.
(10) The light reduction method in the vicinity of the solidification completion point of the continuous cast slab of the present invention is the light reduction method according to (8) or (9), in which a desired reduction amount is set in advance and the reduction is performed. As described above, the rolling gradient of the segment is adjusted to a constant value by controlling the rolling force so as to obtain a desired rolling amount.
Here, the center solid phase ratio in the slab thickness direction is a solid phase ratio at an arbitrary position in the center of the slab thickness direction. (Hereinafter, the central solid fraction in the slab thickness direction may be simply referred to as “central solid fraction”.)

According to the present invention, since light reduction is performed while controlling the amount of reduction by the reduction roll in accordance with the distribution in the width direction of the central solid phase ratio on the crater end side of the slab, the unsolidified portion and the solidified portion in the vicinity of the solidification completion point are Even if the crater end shape is mixed in various forms on the crater end side, a desirable light reduction can be performed for each part, and a desirable solidification state that can suppress segregation can be managed.
In addition, in order to obtain the central solid fraction from the calculation result of the central solid fraction based on the heat transfer solidification calculation or from the measurement result of the central solid fraction by the sensor, The desired light reduction can be performed for each part.
Furthermore, as specific state control under light pressure, B (x) as an integral value in the slab width direction is obtained from the central solid phase ratio fs (x, y), and the value of B (x) is more than zero. When it is large and the solid phase ratio fs (x, W / 2) at the center of the slab width is equal to or greater than a predetermined value, the vicinity of the solidification completion portion is crushed by the roll, so that it is possible to reliably grasp the lightly crushed state. .
Further, in actual light reduction, since a plurality of reduction rolls are usually arranged in the same reduction segment, the reduction amount is adjusted by the reduction gradient of the segment. When at least one of the values of x) is greater than 0, and when at least one of the values of the central solid fraction fs (x, W / 2) at the center of the slab width is equal to or greater than a predetermined value, Since the vicinity of the solidification completion part is reduced, it is possible to grasp the light reduction state surely.
Further, the average of the values obtained for each of the rolling rolls in the target segment arranged in plural with respect to the average value of the gradient in the x direction of the central solid fraction fs (x, y) at each rolling roll position in the casting direction of the slab. From the value, the solidification shrinkage amount of the slab is converted from any value within the range from the value to the maximum value obtained for each of the reduction rolls in the target segment arranged for the maximum value of the x-direction gradient, Since the amount of reduction by the roll is adjusted with a reduction gradient corresponding to the amount of solidification shrinkage, it is possible to perform light reduction under more preferable conditions according to the shape of the crater end.
Furthermore, in the present invention, when adjusting the amount of reduction, the rolling force of the segment is controlled according to the total value of each of the rolling rolls in the segment of the integrated value in the width direction at the central solid fraction reduction roll position. It is possible to perform light reduction while grasping whether the existing light reduction facility can be operated within the possible range. Also, the required rigidity of the segment can be designed in advance according to the operating conditions.

Next, the present invention can be easily applied in actual operation by obtaining a desired reduction amount by calculation in advance, setting the reduction amount in advance, and performing the reduction.
Also, when setting the amount of rolling reduction in advance, the segment is pressed against a device that has a function of keeping the segment gradient constant, and the segment rolling gradient is set to a constant value so that the desired rolling amount is obtained. The applicability to actual operation is easy, and a certain rolling gradient can be reliably maintained.
Furthermore, when setting the reduction amount in advance, the segment is pressed against a device that has the function of keeping the segment gradient constant, and the segment reduction gradient is set to a constant value so that the desired reduction amount is obtained. However, by controlling the rolling force, the rolling gradient of the segment is adjusted to a constant value. Therefore, the rolling gradient is controlled with a necessary rolling force according to the central solid phase ratio, so that the rolling gradient can be accurately controlled to a constant value. In addition, if the device can be controlled only by the rolling force without pressing the segment against the device having the function of keeping the segment gradient constant, a device having the function of keeping the segment gradient constant is unnecessary, so it is simple. It can be a device.

The best mode of the light reduction method in the vicinity of the solidification completion point of the continuous cast slab according to the present invention will be described below, but the method of the present invention is not limited to the following best mode.
FIG. 1 is a schematic side sectional view at the center position in the slab width direction in a state where a slab is being manufactured in a vertical bending type continuous casting machine to which the present invention is applied.
In FIG. 1, a mold 2 is provided below the tundish 1, an immersion nozzle 3 extending downward is provided at the center of the bottom of the tundish 1, and the molten steel 5 accommodated in the tundish 1 is It is configured so that it can be supplied to the mold 2 through the immersion nozzle 3.

  In the configuration of FIG. 1, a slab guide roll comprising a support roll 6, guide rolls 7, 8 and a drive roll 9 is installed on the lower side of the mold 2. Secondary cooling divided downward into six parts, namely, a first cooling zone 10a, a second cooling zone 10b, a third cooling zone 10c, a fourth cooling zone 10d, a fifth cooling zone 10e, and a sixth cooling zone 10f. A zone 10 is formed. Each of the cooling zones 10a to 10f can cool the slab at a desired water density by, for example, arranging a plurality of spray nozzles (not shown) between the slab guide rolls so as to be positioned above and below the slab. It is configured as follows. These spray nozzles are preferably configured such that the cooling water ejection position can be adjusted as necessary, and can be configured to control the cooling state at both ends in the width direction of the slab.

  For example, as shown in FIG. 12 described above by performing width cutting, which is known as a technique for reducing the water density at both ends of the slab or providing a region where the cooling water is not ejected at both ends of the slab. Although it is desirable to have a configuration that can prevent the solidified portion 108 from being generated, the present invention does not particularly limit the configuration and function of these spray nozzles, and can impart desired cooling conditions to the slab. If there is, the structure of the spray nozzle is not particularly limited.

  The molten steel 5 of the tundish 1 is continuously supplied into the mold 2 through the immersion nozzle 3, and the lower end portion of the immersion nozzle 3 is disposed so as to reach the meniscus 11 of the molten steel in the mold 2. The molten steel 5 supplied into the mold 2 contacts the inner surface of the mold 2 and is cooled to form a solidified layer (solidified shell) 12 on the outer periphery, and the solidified layer 12 is then a support roll 6, a guide roll 7, and a guide roll. 8. It passes through the light pressure lower roll 17 and the drive roll 9 and is continuously pulled out downward. During this drawing, the surface of the solidified layer 12 is gradually cooled in the secondary cooling zone 10, the thickness of the unsolidified portion 13 inside the solidified layer 12 is gradually reduced, and solidification is completed at the crater end 15. It is a slab 16. Hereinafter, in the slab 16 manufactured by the continuous casting machine, a side close to the mold 2 is called an upstream side and a side away from the mold 2 is called a downstream side for convenience.

  In the continuous casting machine shown in FIG. 1, a plurality of light reduction rolls 17 are arranged so as to be located on the upstream side and the downstream side of the generation region of the crater end 15 of the slab 16. These lightly rolling rolls 17 are, for example, a required number of four or more (for example, four in the configuration of FIG. 1), and are configured as one segment with a pair of rolls sandwiched between the upper and lower sides of the slab 16. Even if the crater end 15 of the piece 16 is slightly displaced to the upstream side or the downstream side in FIG. The light pressure lowering rolls 17 may be arranged so that the upper and lower pairs are arranged so as to be close to and away from each other and the slab 16 can be slightly plastically deformed in the thickness direction (vertical direction).

  2 to 4 show an example of a specific configuration of the light reduction device K. For example, the light reduction device K of this example is provided with disk-shaped segment bases 30 and 31 that are vertically paired. The segment bases 30 and 31 are configured to be movable in a direction of approaching and separating by a plurality of hydraulic devices 32. That is, for example, five light pressure lower rolls 17 are supported on the lower surface side of the upper segment base 30 via bearings 33, 33, and on the upper surface side of the lower segment base 31 via bearings 34, 34. For example, five lightly pressed rolls 17 are supported. Also, a plurality of brackets 35 project from both side portions of the segment bases 30 and 31, and a hydraulic device (hydraulic cylinder device) 32 is provided so as to be connected to the brackets 35 of both the segment bases 30 and 31 and pin-coupled. The upper and lower segment bases 30 and 31 can be moved closer to or away from each other by operating each hydraulic device 32 and extending and contracting the cylinder rod 32a.

In the light reduction device K having the configuration shown in FIGS. 2 to 4, the slab around the crater end with a desired pressure while changing the inclination state of the segment base 30 by adjusting the operating force of the hydraulic devices 32, 32. It is comprised so that 16 can be lightly reduced.
FIG. 4 shows the state in which the segment base 30 is tilted by adjusting the operating state of the hydraulic devices 32, 32 arranged on the left and right sides, and the slab is obtained by controlling the hydraulic devices 32, 32 in this way. 16 is configured so that the light reduction force applied to 16 can be adjusted. Of course, it is also possible to release the reduction by the segment by setting the segment bases 30, 31 so that their opposing surfaces are in parallel.

As for an example of the control mode of the light pressure lowering roll 17 of the present embodiment described below, a case where four light pressure lowering rolls 17a, 17b, 17c, 17d are provided on the upstream side of the crater end 15 as shown in FIG. explain.
In this embodiment, a pair of upper and lower light pressure lowering rolls 17a, 17b, 17b, 17c, and 17d arranged so as to sandwich the slab 16 from above and below the crater end 15 are provided. A four-unit light reduction roll device arranged in an arrangement is configured.
In the light reduction device of this form, the positions of the rolls 17a to 17d are controlled so as to obtain an optimum reduction gradient obtained by calculation or the like to be described later, and as a result, the reduction force at each roll is changed optimally. The slab can be controlled lightly with a rolling gradient.
Here, the rolling amount of each of the rolls 17a to 17d may be adjusted individually. However, from the viewpoint of the apparatus configuration, the rolling gradient is regulated by providing the segment bases 30 and 31 with an inclined gradient to regulate the rolling gradient. Realistic.

The crater end shape of the slab that is lightly reduced by the lightly rolling rolls 17a, 17b, 17c, and 17d having the configuration shown in FIG. 5 can be the state shown in FIG. 6 and the state shown in FIG.
The state shown in FIG. 6 is a solidified state that is unlikely to cause center segregation and porosity problems, and is considered to be good. 7 is a state in which solidified portions 25 having a narrow width are present on both ends of the slab width direction, and the state shown in FIG. 7 is a solidification completion point 26 on both ends of the slab width direction. Is in a state of extending in a square shape downstream of the solidification completion point 27 on the slab width direction center side in the casting direction.
The solidification state described above based on FIG. 12 is also conceivable. However, in the solidification state shown in FIG. 12, there is a risk of causing a corner crack. The state shown in FIG. 12 is avoided by using a technique in which the spray nozzle is not cooled, and as a result of this avoidance, the solidification completion points on both ends of the slab are likely to be formed in a square shape on the downstream side. The present invention becomes effective when the state shown in FIG.

However, the crater end shape has various shapes, and not only the state schematically shown in FIG. 7 but also a more complicated shape is conceivable. Therefore, the method described below generates the solidified portion 27 on the center side. In the crater end region, the solidified portion 27 is of course generally applicable when the solidification completion point is formed in an indefinite shape at a position back and forth in the casting direction.
In addition, since general light reduction itself is normally performed at the time of continuous casting, it is not limited to the state shown in FIG. 7, and the operation performed regardless of the crater end shape of FIGS. 10 to 12. Think of it as

In this embodiment, based on the state of the unsolidified portion and the solidified portion in the vicinity of the solidification completion point of the slab as exemplified in FIGS. This is a method of lightly rolling down while controlling the amount, and during casting, the central solid fraction of the slab is obtained by the method described later, and the amount of reduction is adjusted according to the distribution in the width direction of this central solid fraction. It is.
In other words, it is a new focus on the distribution in the width direction of the central solid fraction of the slab. By adjusting the amount of reduction according to the distribution in the width direction of the central solid fraction, the solidification completion point is in an indefinite state. Even if it is formed and has a crater end shape in various forms, a desired light reduction can be performed for each part, and segregation can be suppressed.
The vicinity of the solidification completion point is not particularly defined as long as it is a constant region upstream from the solidification completion point, but the center solidification is such that the effect of suppressing segregation is exhibited when lightly reduced. It is preferable that it is the area | region beyond a phase ratio.
Specifically, a method for obtaining an application range under light pressure near the solidification completion point in accordance with the width direction distribution of the center solid phase ratio of the slab will be described below.
An overall position of the slab 16 is W, an arbitrary position in the width direction of the slab 16 is y, an arbitrary position in the casting direction of the slab 16 is x, and an arbitrary point (x, y of the slab 16 ) Is defined as fs (x, y), and when the central solid phase ratio fs (x, y) is within a predetermined range, a value other than 0 is set. A function that binarizes to 0 in the case other than the range is defined as g (x, y). Here, as a typical example, a case where the central solid phase ratio fs (x, y) is within a predetermined range is set to 0, and a case where the center solid phase ratio fs (x, y) is other than that is binarized to 1. Note that the predetermined range of the center solid phase ratio fs (x, y) is not particularly specified, and may be set as appropriate according to the requirements of the quality of the slab. These relationships can be expressed by the following formula.

Here, as a typical example, a case where the central solid phase ratio fs (x, y) is within a predetermined range is set to 0, and a case where the center solid phase ratio fs (x, y) is other than that is binarized to 1. Note that the predetermined range of the center solid phase ratio fs (x, y) is not particularly specified, and may be set as appropriate according to the requirements of the quality of the slab. Therefore, the central solid phase ratio fs (x, y) can be arbitrarily set from the range of more than 0 and less than 1. However, when the central solid fraction is less than 0.2, the central solid fraction may be 0.2 because the effect of suppressing segregation or eliminating porosity may be ineffective even when lightly reduced. The above is preferable. However, in the case of all solid phases, since the light pressure cannot be reduced, the upper limit of the central solid phase ratio is less than 1.0. Furthermore, if the central solid phase ratio is too large, it may be difficult to lightly reduce the pressure, so the central solid phase ratio is more preferably 0.2 or more and 0.7 or less.
The reason why the central solid phase ratio fs (x, W / 2) at the center of the slab width is set to a predetermined value or more is to determine the vicinity of the solidification completion point. However, this predetermined value is not particularly specified, and may be set as appropriate according to the requirement of the quality of the slab. However, if the center solid phase ratio fs (x, W / 2) at the center of the slab width is less than 0.2, the effect of suppressing segregation may be difficult to be exerted when the pressure is lightly reduced. The above is preferable. However, in the case of all solid phases, since there is almost no effect of suppressing segregation by light pressure, the upper limit of the central solid fraction at the center of the slab width is set to less than 1.0.
Therefore, according to the above equation (3), when B (x)> 0 and the center solid phase ratio fs (x, W / 2) at the center of the slab width at which the vicinity of the solidification completion point can be determined is equal to or greater than a predetermined value. What is necessary is just to carry out light pressure reduction.

Here, when multiple light reduction rolls are placed in the same reduction segment and the amount of reduction is adjusted by the segment's reduction gradient, the reduction of the segment is not adjusted for each light reduction roll. Since the slope is defined and the amount of reduction is controlled by the reduction slope, at least one position in the light reduction roll in the segment, when B (x)> 0, and the light reduction in the segment. When at least one position in the reduction roll has a center solid phase ratio fs (x, W / 2) at the center of the slab width that can determine the vicinity of the solidification completion point is a predetermined value or more, light reduction may be performed. it can.
Incidentally, for a predetermined range of the central solid fraction fs (x, y) when obtaining B (x) and a predetermined value of the central solid fraction fs (x, W / 2) at the center of the slab width, Same as above.

Next, an example of how to obtain the central solid fraction in the slab thickness direction at an arbitrary point (x, y) of the slab 16 as fs (x, y) will be described.
“When calculating fs (x, y) by calculation”
When heat transfer solidification calculation is performed in a cross section (transverse cross section) perpendicular to the casting direction of the slab 16, as a boundary condition of the calculation, the amount of heat removal q to the mold is obtained by the following well-known formula (4). be able to. In the following formula (4), X is a casting direction distance from the meniscus, and α and β are constants.

Next, for heat transfer solidification calculation after the mold 2, the heat transfer coefficient is the heat removal by the cooling water sprayed from the cooling nozzle to the slab 16, the heat removal by the continuous casting roll, and the heat removal by radiation of the uncooled part. Can be given as. Here, the continuous casting roll means a roll disposed after the mold 2.
The calculation of this coefficient can apply the technique currently disclosed by Unexamined-Japanese-Patent No. 4-231158 as an example. For example, the heat transfer rate hw from the finite element model Mj to the atmosphere of the slab 16 guided by the rolls Ri and Ri + 1 and sprayed with cooling water is the heat transfer rate hw from the previously calculated finite element model Mj-1. It can be calculated based on the slab surface temperature obtained more and the spray amount distribution which is actual measurement data regarding the spray amount of the cooling water. As shown in FIG. 9, the slab 16 is cooled by cooling water and air as actual measurement data sprayed onto the elliptical spray region 16a, and the measured data are also used for the spray amount distributions 16b and 16c in the spray region 16a.

  Here, the heat transfer rates hr and ha of heat escaping from the slab 16 to the rolls Ri and Ri + 1 or from the air cooling part AP of the slab 16 to the atmosphere are not affected by the cooling water 16e and depend on the temperature of the atmosphere. Further, the heat transfer rate hm from the molten steel in the unsolidified portion or the slab 16 to the mold 2 is substantially constant without depending on the surface temperature of the slab 16. On the other hand, the heat transfer rate hw of heat escaping from the water cooling part WP of the slab 16a to which the cooling water 16e is sprayed to the atmosphere depends on the cooling water 16e sprayed on the water cooling part WP, the amount of air sprayed, and the surface temperature of the water cooling part WP. .

Furthermore, the heat transfer coefficient hw related to the finite element E corresponding to the position of the water cooling part WP is defined by the following formula, and is expressed in the surface temperature T of the slab for each finite element E, the spray area of the spray nozzle, and the spray amount distribution. The cooling water amount W and the air amount A obtained for each included finite element E are applied.
hw = hw (T, W, A) = α × T ^f × W ^g × A ⁿ
However, in the above formula, α, f, g, and n are coefficients obtained beforehand by experiments or the like for each nozzle Ni, and are recorded in a database or the like together with the spray area and spray amount for each nozzle. Calculate based on that.
By the above calculation, the temperature at any casting direction position x and width direction position y can be obtained.

With the above heat transfer calculation, the “heat transfer solidification calculation” can be performed from the temperature at an arbitrary casting direction position x and width direction position y, and the central solid phase ratio fs (x, y) at the thickness center of the slab is obtained. be able to.
In addition to the technique disclosed in JP-A-4-231158 as the heat transfer solidification calculation, the enthalpy method described in "Introduction to computer heat transfer, solidification analysis, written by Onchu, published by Maruzen Co., Ltd." The equivalent specific heat method may be applied or performed.

"When calculated by actual measurement"
In order to measure the temperature of the crater end of the slab, the measurement method using radiation described in JP-A-10-325714 can be applied.
Briefly described, a radiation transmission measuring device for measuring the transmittance in the thickness direction of a slab of radiation having two or more different energy spectra, and scanning the radiation transmission measuring device in the width direction of the slab And a solidification completion point for obtaining a shape in the vicinity of the solidification completion point of the cross section of the slab based on the distribution in the slab width direction of the transmittance obtained by the radiation transmission measuring device and the width direction scanning device. A shape calculation device in the vicinity, a heat transfer calculation device for obtaining the temperature distribution of the slab based on the molten steel temperature, spray cooling water temperature, cooling water nozzle state and casting speed, and solidification completion of the slab temperature distribution and the cross section A temperature correction calculation device that corrects the temperature distribution based on the shape near the point, and a shape calculation device near the three-dimensional solidification completion point that obtains a shape near the three-dimensional solidification completion point based on the corrected temperature distribution; A detection device that includes used.

By applying the measurement method using radiation described in JP-A-10-325714, the shape of the crater end of the slab near the three-dimensional solidification completion point can be measured. The central solid phase ratio fs (x, y) of can be obtained.
When determining the central solid phase ratio fs (x, y) at the thickness center of the slab, all the shapes in the vicinity of the three-dimensional solidification completion point before and after the crater end are described as described in JP-A-10-325714. If the center solid phase ratio fs (x, y) at the thickness center of the slab is to be estimated without grasping it, the radiation transmittance measuring device is scanned before and after the crater end of the slab to solidify in the slab thickness direction. The value may be used by performing an operation of distinguishing the solidified part and the non-solidified part from the difference in permeability between the solid part and the non-solidified part.

"Control method of light reduction"
As shown in FIG. 5, four light rolling rolls 17 a, 17 b, 17 c, and 17 d are incorporated into the segment bases 20 and 21 that perform light rolling to form one segment. The control method of each light rolling roll will be described. In addition, although the rolling force is applied to the slab as a result by each of the light rolling rolls 17a to 17d, in this example, the individual pressing force of each of the light rolling rolls 17a to 17d is not changed, but the segment base The position control for determining the rolling gradient of each of the light rolling rolls 17a to 17d is performed by the rolling rolling force (clamping force).
The position where the light reduction roll 17a is installed is x1 in the casting direction, the position where the light reduction roll 17b is installed is x2, the position where the light reduction roll 17c is installed is x3, and the light reduction roll 17d is installed. Let x4 be the current position.
B (x) according to the previously defined equation (3) is obtained at the position of each light rolling roll, and when B (x)> 0 at at least one of the light rolling rolls in the segment, and the segment When at least one position in the inner light rolling roll, the center solid phase ratio fs (x, W / 2) at the center of the slab width is a predetermined value or more, the light rolling is performed.
At that time, the maximum value or the average value in the y direction of the gradient in the x direction of the central solid phase ratio fs (x, y) is obtained. That is, fs (x, y) is calculated for each light rolling roll at the positions of x1 to x4 (four light rolling rolls to be rolled down), and ∂ · fs (x, y) / ∂ · x is set in the y direction. Calculate and find the maximum or average value.

Next, using the average value of the values of ∂ · fs (x, y) / ∂ · x obtained for each of the lightly rolling rolls, the value obtained by obtaining these average values is set as the lower limit value.
Further, using the maximum value of the values of ∂ · fs (x, y) / ∂ · x obtained for each lightly-rolled roll, and using these maximum values as upper limit values,
An arbitrary value within the range from the lower limit value to the upper limit value is defined as C.

Here, an arbitrary value within the range from the lower limit value to the upper limit value is not particularly defined, and can be appropriately selected according to requirements from product quality.
For example, since the amount of change in the central solid phase ratio indicates the coagulation contraction state, when lightly reducing the most contracted object, the value of ∂ · fs (x, y) / ∂ · x Is preferably calculated for each roll under light pressure, and the maximum value is preferably C.

On the other hand, when the pressing capability of the light reduction device is taken into consideration, the average value of the values of ∂ · fs (x, y) / ∂ · x is calculated for each light reduction roll, and the average value thereof (light reduction) The rolling rolls may be C at four locations).
Incidentally, although any value within the range from the lower limit value to the upper limit value can be selected, it is practical to use an average value or a maximum value in practice. Therefore, as a combination of the average value and the maximum value, four patterns can be exemplified as typical cases. Specifically, the reduction amount is adjusted according to the distribution in the width direction of the central solid phase ratio. In this case, the gradient in the x direction of the central solid fraction fs (x, y) is obtained in the width direction (y direction) of the slab at the target roll position (arbitrary x position), and the average value or maximum value of the values is obtained. In addition, the average value or the maximum value of the values obtained for each of the light rolling rolls in the target segment arranged in plural,
The solidification shrinkage amount of the slab is converted from the average value of the gradient obtained in the width direction and the average value of the values obtained for each of the light rolling rolls, or the maximum value of the gradient obtained in the width direction and the The solidification shrinkage amount of the slab is converted from the maximum value obtained for each of the light rolling rolls, or the maximum value of the gradient obtained in the width direction and the average value obtained for each of the light rolling rolls, and The solidification shrinkage amount of the slab is converted from the average value of the gradient obtained in the width direction and the maximum value of the value obtained for each of the light rolling rolls. The reduction amount can be adjusted with a reduction gradient corresponding to the amount of contraction.

  Thereafter, the obtained C value is converted into a solidification shrinkage amount, and light reduction is performed with a reduction gradient corresponding to the solidification shrinkage amount. As a conversion method from the C value to the solidification shrinkage, for example, the reduction amount (C × D) of one light reduction roll in the segment can be obtained by multiplying C by a coefficient D. Here, the coefficient D is obtained by calculating in advance the relationship between the solidification shrinkage amount of the slab and the casting direction gradient of fs (x, y), and the like. ).

"Control method of light rolling force"
As described above, it is important to consider the necessary rolling force when adjusting the rolling amount with the rolling gradient according to the solidification shrinkage amount of the slab, in order to determine whether the operation can be continued.
Therefore, the rolling reduction force of the segment necessary for light reduction according to the total value of each of the light rolling rolls in the segment of the integrated value in the width direction at the light rolling roll position of the central solid phase ratio fs (x, y). Focused on grasping.
Specifically, A (x) is defined as the value obtained by integrating the solid phase ratio at the center in the slab thickness direction in the width direction from the central solid phase ratio fs (x, y), and the following (5 ) Calculate based on the formula.

A (x) in the equation (5) represents an integral value of the solid phase ratio in the slab width direction, and the rolling force of the segment necessary for lightly rolling can be obtained using this A (x).
That is, when the integral value of the solid phase ratio is small, a small reduction force is sufficient, but as the integral value of the solid phase ratio increases, a large reduction force is required, and the integral value of the solid phase ratio can be grasped as an index. it can.
Specifically, the present inventor has found that the rolling force F and the coefficient E can be expressed by the following equation (6). This reduction force F becomes the maximum value of the reduction force necessary for light reduction. Therefore, when a device capable of light reduction with this reduction force is used, it can be determined that the light reduction can be continued by controlling the reduction force within the range of the reduction force or less.

In addition, the maximum required rolling force of the segment can be calculated from the total value in the light reduction target segment of the integral value of the solid phase ratio indicated by A (x), and the required rigidity of the segment can be designed.
Furthermore, in the existing light reduction segment, it is possible to examine the possible light reduction range and the limit value of the reduction gradient from the total value.

An example in which the crater end portion of the slab 16 is specifically lightly reduced by the light reduction device K having the above-described configuration will be described with reference to FIG. When the central solid phase ratio in the slab thickness direction at the center of the slab width is used as a management index as in the conventional method, only light reduction is performed from the F1 position to the F2 position. Therefore, a portion where the solidification completion points on both ends in the width direction of the slab extend in a square shape downstream of the solidification portion in the casting direction from the central portion is not lightly reduced. .
On the other hand, the method of the present invention performs desirable light reduction according to the shape of the crater end in consideration of the distribution in the width direction of the central solid fraction, and therefore, from the shape of the crater end in FIG. The light pressure may be reduced from the F1 position to the F4 position. However, since most of F4 is in a solid phase, it cannot actually be lightly reduced, so the F1 position from the F1 position, which is a region in which light pressure can be reduced (for example, the solid phase ratio is 0.7 or less), is changed. By lightly reducing, no significant segregation site is generated when the pressure is reduced from the F1 position to the F3 position. That is, since it can be lightly reduced in a wide range according to the distribution in the width direction of the central solid phase ratio, segregation can be prevented, and a slab of good quality can be manufactured.

Further, as another form of the method for adjusting the amount of reduction during casting, it is possible to obtain an optimum amount of reduction in advance and set the amount of reduction to a constant value before casting.
That is, rather than calculating and controlling from operation data during casting, etc., the desired reduction amount is obtained in advance from the planned operation conditions, etc., and this reduction amount is set to a constant value before casting. In this state, the operation is performed.
Note that the desired amount of reduction may be obtained by converting the central solid phase ratio into the amount of solidification shrinkage by a series of calculations as described above using the planned operating conditions and the like.
In addition, the desired reduction amount may be obtained by using an actual measurement result under the same operation condition as the planned operation condition, or the desired reduction amount may be obtained by an experiment simulating the planned operation condition. Also good.

  Specifically, as a method of performing the reduction by setting a desired reduction amount in advance, the segment is pressed against a device having a function of keeping the segment gradient constant so that the desired reduction amount is obtained. Alternatively, a method of setting the segment rolling gradient to a constant value may be employed. An example of a device having a function of keeping the segment gradient constant is a jack or the like.

In addition, as another method for performing the reduction by setting the desired reduction amount in advance, as a device having a function of keeping the segment gradient constant, for example, while pressing the segment against a jack or the like, the desired reduction amount is reduced. It is also possible to adjust the rolling gradient of the segment to a constant value by controlling the rolling force to a constant value so that the setting becomes a constant value.
However, if a high degree of control is possible, as a device having a function of keeping the segment gradient constant, for example, without pressing the segment against a jack or the like, the segment rolling gradient is adjusted in advance so that the desired rolling amount is obtained. Is set to a constant value, and the reduction force of the segment is controlled so as to maintain this reduction gradient.

  As described above, in the present invention, even if the crater end shape has various forms, desired light reduction is performed for each part. Therefore, there is a possibility that a larger rolling force may be required than before. In that case, for example, the number of light rolling rolls is increased or the rigidity of the segment frame is increased. it can.

FIG. 1 is a schematic side sectional view at the center position in the width direction of a slab in a state where a slab is manufactured for a vertical bending type continuous casting machine to which the present invention is applied. FIG. 2 is a cross-sectional view showing an example of a light reduction device attached to the continuous casting machine. FIG. 3 is a partial plan view of the light reduction device. FIG. 4 is a side view of the light reduction device. FIG. 5 is an explanatory view showing a schematic configuration of a light reduction roll of the light reduction apparatus used in the present invention. FIG. 6 is an explanatory view showing an example of a crater end shape of a slab to which the present invention is applied. FIG. 7 is an explanatory view showing another example of a crater end shape of a slab to which the present invention is applied. FIG. 8 is a diagram for explaining an example of solidification heat transfer calculation of a slab based on a finite element model. FIG. 8A shows the positional relationship between the slab and the nozzle as viewed from the side with respect to the slab casting direction. FIG. 8B is an explanatory view seen from the casting direction. FIG. 9 is an explanatory diagram viewed from above the slab in order to explain an example of solidification heat transfer calculation of the slab based on the finite element model. FIG. 10 shows an example of a crater end shape, and is a cross-sectional view showing a state in which unsolidified portions on both ends of the slab are extended to the downstream side in the casting direction with respect to the solidified portion at the center of the slab. FIG. 11 shows another example of the crater end shape, and shows a state in which the solidified portion at the center in the slab width direction is formed widely. FIG. 12 is a view showing still another example of the crater end shape, in which a solidified portion is formed at the center portion in the slab width direction, and a solidified portion having an oblique gradient is formed on both sides thereof.

Explanation of symbols

2 molds,
5 Molten steel,
11 Meniscus,
12 Solidified layer (solidified shell),
15 Crater end,
16 slab,
17 Light pressure roll,
17a-17d roll under light pressure,
20, 21 segment base,
26 Unsolidified part,
27 Solidification part,
30, 31 segment base,
32 hydraulic device,

Claims (10)

  When the molten steel is poured into the mold, and the solidified shell formed by cooling the molten steel is continuously drawn below the mold, and the solidified shell surface is cooled in the cooling zone downstream of the mold to complete the solidification of the slab. In addition, based on the state of the unsolidified part and the solidified part near the solidification completion point of the slab, a method of lightly rolling down while controlling the reduction amount of the reduction rolls arranged near the solidification completion point, in the slab thickness direction A light reduction method in the vicinity of a solidification completion point of a continuous cast slab, wherein a central solid fraction is obtained and a reduction amount is adjusted in accordance with a distribution in the width direction of the central solid fraction. The amount of heat removed in the mold according to the casting direction distance from the meniscus,
Based on the amount of heat removed by the cooling water sprayed from the cooling nozzle in the cooling zone on the downstream side of the mold to the slab, the amount of heat removed by the continuous casting roll, and the amount of heat removed by radiation of the uncooled part,
The solidification of the continuous cast slab according to claim 1, wherein the heat transfer solidification calculation is performed on a cross section of the slab perpendicular to the casting direction, and the central solid fraction in the slab thickness direction in the vicinity of the solidification completion point is calculated. Light reduction method near the completion point.   2. The method of light reduction near the solidification completion point of a continuous cast slab according to claim 1, wherein the central solid fraction in the slab thickness direction near the solidification completion point of the slab is measured by a non-contact type sensor. .   An arbitrary position in the width direction of the continuous cast slab of width W is y, an arbitrary position in the casting direction of the continuous cast slab is x, and a slab at an arbitrary point (x, y) of the continuous cast slab When the central solid fraction in the thickness direction is defined as fs (x, y), the central solid fraction fs (x, y) is a value other than 0 when the central solid fraction fs (x, y) is within a predetermined range. Is defined as g (x, y), and B (x) obtained by integrating the function indicated by g (x, y) from 0 to W in the width direction of the continuous cast slab is defined as g (x, y). When the value of B (x) is a value other than 0, and the value of the central solid fraction fs (x, W / 2) in the slab thickness direction at the slab width center is equal to or greater than a predetermined value. The light reduction method near the solidification completion point of the continuous cast slab according to any one of claims 1 to 3, wherein the solidification completion point vicinity is reduced by the reduction roll   When controlling the reduction amount of the plurality of reduction rolls arranged, when a plurality of reduction rolls are arranged in the same reduction segment and the reduction amount is adjusted by the reduction gradient of the segments, each reduction roll in the segment The central solid phase ratio fs (in the thickness direction of the slab at the center of the slab width at each rolling roll position in the segment when at least one of the B (x) values at the position is a value other than 0. The light cast near the solidification completion point of the continuous cast slab according to claim 4, wherein when at least one of the values of x, W / 2) is equal to or greater than a predetermined value, the slab is crushed by the segment. Reduction method. When adjusting the amount of reduction according to the distribution in the width direction of the central solid fraction in the slab thickness direction, the gradient in the x direction of the central solid fraction fs (x, y) is determined as the target roll position (arbitrary x position). In the width direction (y direction) of the slab, find the average value or the maximum value of the values,
From the average value of the values obtained for each of the rolling rolls in the target segment arranged in plural for the average value of the gradient obtained in the width direction,
Up to the maximum value of the values determined for each of the rolling rolls in the target segment that is arranged in plural for the maximum value of the gradient determined in the width direction,
Convert the solidification shrinkage of the slab from any value within the range of
6. The light reduction method near the solidification completion point of the continuous cast slab according to claim 5, wherein the reduction amount is adjusted with a reduction gradient corresponding to the shrinkage amount.   When adjusting the reduction amount according to the distribution in the width direction of the central solid fraction in the slab thickness direction, each reduction in the segment of the integral value in the width direction at the reduction roll position of the central solid fraction fs (x, y). The light reduction method near the solidification completion point of the continuous cast slab according to claim 5 or 6, wherein the reduction force is controlled within a range equal to or less than the reduction force of the segment according to the total value in the roll.   When performing light reduction near the solidification completion point of the continuous cast slab, a desired reduction amount is obtained by the method according to claim 6, and the reduction is performed by setting the reduction amount in advance. A light reduction method near the solidification completion point of a continuous cast slab.   As a method of performing a reduction by setting a desired reduction amount in advance, the segment is pressed against a device having a function of keeping the segment gradient constant, and the segment reduction gradient is kept constant so that the desired reduction amount is obtained. The light reduction method near the solidification completion point of the continuous cast slab according to claim 8, wherein the value is set to a value.   The method of performing a reduction by setting a desired reduction amount in advance, and adjusting a reduction gradient of a segment to a constant value by controlling a reduction force so as to obtain a desired reduction amount. A light reduction method in the vicinity of the solidification completion point of the continuous cast slab according to 8 or 9.

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