Title of the invention: continuous casting method of steel and reduction roll for continuous casting
Technical field
[0001]
　The present invention relates to a method for continuous casting of steel and a reduction roll for continuous casting.
　The present application claims priority based on Japanese Patent Application No. 2018-041620 filed in Japan on March 8, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　When casting slabs, blooms, and other slabs by the continuous casting method, so-called central segregation, in which components such as phosphorus and manganese segregate in the center of the slab, may occur. In addition, a hole called center porosity is generated in the center of the slab.
[0003]
　At the end of solidification during continuous casting, the amount of steel occupying a predetermined volume in the slab becomes insufficient due to solidification shrinkage when the steel solidifies. At the slab site where the unsolidified molten steel can flow, the unsolidified molten steel flows toward the solidification completion point of the final solidified part, and the impurity-concentrated molten steel at the solid-liquid interface accumulates in the final solidified part, which is the central segregation. It causes. Further, at a position where the unsolidified molten steel cannot flow (the solid phase ratio at the center of the slab is 0.8 or more), a gap is generated in the center of the slab, which causes center porosity.
[0004]
　In order to reduce the central segregation, in the region where the center of thickness is the solid-liquid coexistence region and the unsolidified molten steel can flow, the solidified shell is pressed down by the amount commensurate with the amount of solidification shrinkage of the molten steel. It is effective to suppress the flow of molten steel near the solidified part. Further, in order to reduce the center porosity, it is effective to press the center porosity by pressing the slab near the solidification completion position where the unsolidified molten steel cannot flow or after the complete solidification. Based on this idea, a light reduction technique is used in which the slab is reduced by a support roll before and after the completion of solidification at the end of continuous casting.
[0005]
　When trying to reduce the slab before and after the completion of solidification during continuous casting, the deformation resistance due to the reduction is large because the solidification has already been completed and the temperature has dropped on both short sides of the slab. In some cases, the amount of reduction was not obtained. Therefore, instead of using a roll whose roll diameter is constant in the roll width direction (hereinafter referred to as "flat roll"), the roll diameter of the portion corresponding to the central portion of the slab width is large and corresponds to both sides of the slab width. Using a roll whose roll diameter is smaller than that of the central part of the width (hereinafter referred to as "convex roll"), both short sides where the solidification of the slab is completed are not reduced, and the center of the slab width is not reduced. A technology has been developed to control only the part.
[0006]
　In Patent Document 1, a convex crown (flat surface) roll having a width of 200 mm to 240 mm on a convex plane is used, and a reduction of 0.5 mm to 10.0 mm per stage is applied by applying reduction to a slab in an unsolidified state. It is stated that the occurrence of central segregation can be reduced by applying. However, the present invention presupposes that an unsolidified portion remains inside the slab, and the required equipment requirements tend to be too small. Further, since the main purpose is to compensate the central cavity by solidification shrinkage, there is a problem that the reduction application to the central portion of the slab is not sufficiently optimized.
[0007]
　Furthermore, if the amount of light reduction in the unsolidified region is increased, there is a problem of internal cracking and the occurrence of reverse V segregation, so the amount of light reduction must be reduced, which is insufficient to reduce the center porosity. The result is
[0008]
　In Patent Document 2, as a roll reduction method for reducing center porosity, the surface temperature of the slab is 700 ° C. or higher and 1000 ° C. or lower and the internal center of the slab is formed after the slab is completely solidified and before cutting. A continuous casting method is disclosed in which a region where the temperature difference between the surface and the surface is 250 ° C. or more is sandwiched between rotating upper and lower rolls and reduced. At the reduction portion, the inner side is relatively soft with respect to the surface layer side due to the high temperature, and the reduction force applied to the surface of the slab can be transmitted to the inside of the slab. The convex roll used as the reduction roll has a reduction region having a horizontal portion in the center in the width direction and inclined portions connected to the horizontal portion on both sides of the horizontal portion. It is said that the width of the horizontal portion (compression width) is preferably 40% or less of the width of the slab. The amount of reduction is preferably 2% or more of the thickness of the slab.
[0009]
　Patent Document 3 discloses a continuous casting method in which at least one crown roll is provided as a reduction roll to reduce the central portion of a slab and its vicinity. It is said that the slab is pressed down by a crown roll in the area where the solidified shell formation ratio of the slab is 75% or more, and the concentrated molten steel in the unsolidified portion inside the pressed piece is pushed up and removed. The shape of the crown may be any shape that can be reduced in the center portion in the width direction of the slab and its vicinity, and the drawing describes a reduction roll having a shape in which the center portion in the roll width direction bulges outward. There is. The maximum amount of reduction per stage is 3 mm.
Prior art literature
Patent documents
[0010]
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-94154
Patent Document 2: Japanese Patent Application Laid-Open No. 2009-279652
Patent Document 3: Japanese Patent Application Laid-Open No. 60-162560
Outline of the invention
Problems to be solved by the invention
[0011]
　When reducing the slab during continuous casting, especially when reducing the slab after solidification is completed, by using a convex roll instead of a flat roll as the reduction roll, the portion where the reduction resistance at both ends of the slab width is large. Will not be reduced. Therefore, the reduction force of the reduction roll for realizing the reduction can be reduced. However, even if the conventional convex roll is used, if sufficient reduction is performed to reduce the center porosity, the required reduction force becomes excessive, and a large-scale facility expansion is required to secure the reduction force. Further, as a result of rolling down using a convex roll, a dent is formed on the surface of the slab after continuous casting, and this dent portion causes a defect in hot rolling in a subsequent process. There was something.
[0012]
　INDUSTRIAL APPLICABILITY The present invention provides a continuous steel casting method and a method capable of reducing the center porosity of continuously cast slabs without performing a large-scale facility expansion, and at the same time reducing the occurrence of defects in hot rolling in a subsequent process. It is an object of the present invention to provide a rolling roll for continuous casting.
Means to solve problems
[0013]
　That is, the gist of the present invention is as follows.
(1) In the continuous steel casting method according to the first aspect of the present invention, the slab at a position where the central solid phase ratio of the slab is 0.8 or more and includes after complete solidification during continuous casting is performed. A continuous casting method of steel to be reduced by
　at least one pair of reduction rolls , wherein the width of the slab to be cast is W (mm) and the thickness of the slab is t (mm), and at least one of the pair of reduction rolls is used. On the other hand, the outer peripheral shape of the roll in the cross section including the roll rotation axis has a convex shape protruding outward in the region including the center position in the width direction of the slab, and the
　convex shape is the center position in the width direction. Convex shape with a total length of 0.80 × W on both sides in the roll width direction In the specified range, a curved shape that is convex outward and has no corners, or a curve that is convex outward and the length is 0. It is either a combination with a straight line within 25 × W and has no corners, and the
　reduction roll radius at the center position in the width direction is 0 with respect to the reduction roll radius at both ends of the convex shape specified range. It is larger than .005 × t.
(2) In the above (1), the position of the slab in the casting direction to be reduced by the reduction roll may be the position after complete solidification.
(3) In the above (1) or (2), the amount of reduction of the slab by the pair of reduction rolls may be 0.005 × t or more and 15 mm or less at the center position in the width direction.
(4) The reduction roll for continuous casting according to the second aspect of the present invention is for reducing a slab having a slab width: W (mm) and a slab thickness: t (mm) during continuous casting. It ’s a reduction roll,
　The outer peripheral shape of the roll in the cross section including the roll rotation axis has a convex shape protruding outward in the region including the center position in the width direction of the slab, and the
　convex shape is from the center position in the width direction to the roll width direction. Convex shape with a distance of 0.80 x W on both sides of the curve shape that is convex outward and has no corners, or a curve that is convex outward and a straight line with a length of 0.25 x W or less combination is a by shape having no corners, are either,
　to reduction roll radius at both ends of the convex specified range, pressure roll radius at the widthwise center position is greater than 0.005 × t.
(5) In the above (4), the roll outer peripheral shape has straight lines parallel to the roll rotation axis at both ends in the width direction, and has a
　concave curve on the outside that smoothly connects to the straight line. You may be.
Effect of the invention
[0014]
　When rolling down a slab after complete solidification during continuous casting, by using the convex curved roll of the present invention as the rolling roll, sufficient rolling can be performed with a small amount of rolling to reduce center porosity, and slab reduction can be performed. Defects in hot rolling due to the shape can be reduced.
A brief description of the drawing
[0015]
FIG. 1 is a cross-sectional view showing a situation in which a slab is reduced by a reduction roll according to the first embodiment.
FIG. 2 is a partial cross-sectional view of a reduction roll according to the first embodiment.
FIG. 3 is a detailed partial cross-sectional view of a reduction roll according to the first embodiment.
FIG. 4 is a cross-sectional view of a conventional reduction roll.
FIG. 5 is a graph showing the first embodiment, and is a graph showing the distribution in the width direction of the slab surface reduction amount obtained by the deformation analysis of the finite element method.
FIG. 6 is a graph showing the first embodiment, and is a graph showing the distribution in the width direction of the normalized equivalent plastic strain of the thickness center in a slab obtained by the deformation analysis of the finite element method.
FIG. 7 is a detailed partial cross-sectional view of a reduction roll according to a second embodiment.
FIG. 8 is a graph showing the second embodiment, and is a graph showing the distribution in the width direction of the normalized equivalent plastic strain at the center of thickness in a slab obtained by the deformation analysis of the finite element method.
Mode for carrying out the invention
[0016]
　The first embodiment and the second embodiment will be described with reference to FIGS. 1 to 8.
　Bloom continuous casting or billet continuous casting is applied to continuously cast the slab 10 as a material for producing steel products for strips. In the bloom continuous casting, the cross-sectional shape of the cast slab 10 is rectangular, and for example, a slab having a width of 500 mm and a thickness of 300 mm is cast. When casting a slab 10 having a rectangular cross section, the unsolidified portion of the slab 10 is located in the width direction from the center position in the slab width direction at a position immediately before the central portion of the thickness of the slab 10 is completely solidified. There is a total range of "slab width-slab thickness" on both sides, and center porosity also occurs in this area. Therefore, even when the slab 10 is pressed down by using the convex roll 3 as a measure against the center porosity, the convex roll 3 is conventionally used as the convex roll 3 in order to surely reduce the center porosity generation region, as shown in FIG. A roll having a horizontal portion 20 at a center position (hereinafter, may be referred to as a width center position) 13 in the width direction of the slab 10 (not shown) was used. Inclined portions 21 are provided on both sides of the horizontal portion 20 in the width direction, and the joint position between the horizontal portion 20 and the inclined portion 21 constitutes a corner portion 15. The complete solidification is a state in which the solid phase ratio determined by the ratio of solid and liquid reaches 1.0, the liquid phase does not exist, and the temperature is equal to or lower than the solid phase temperature TS. In other words, complete solidification is a state in which the temperature is below TS at any point in the C cross section (cross section perpendicular to the rolling direction). It can be confirmed that the slab is completely solidified by actually measuring the temperature on the surface or inside of the slab at several points and correcting the estimated solid phase ratio calculated from the temperature distribution estimated by the heat transfer calculation. In addition, when a stud is driven into a slab and the stud component diffuses into the remaining liquid phase, the shape of the solidified shell can be estimated and it can be confirmed that it is not completely solidified, and when the stud retains its original shape, it is completely solidified. Can be confirmed as.
[0017]
　In the convex roll 3 for rolling down the slab 10, the present inventor is not a roll forming the horizontal portion 20-corner portion 15-inclined portion 21 as shown in FIG. 4, but the outer circumference of the convex roll 3. By forming the roll outer peripheral shape 11, which is a portion where the surface and the cross section including the roll rotation shaft 12 intersect, into a curved shape that is convex outward and has no corners, as shown in FIGS. 1 to 3. It was conceived that the center porosity of the slab 10 could be surely reduced, the reduction force required for reduction could be reduced, and the occurrence of defects in hot rolling in the subsequent process could be reduced. Hereinafter, the convex roll 3 having the horizontal portion 20-corner portion 15-inclined portion 21 is referred to as "convex disc roll 5", and the convex roll 3 having a curved shape that is convex outward and has no corner portion is referred to as "convex roll 3". It is called "convex curve roll 4". In addition, "having a corner" means that the second derivative of the function that defines the outer peripheral shape of the roll (the rate of change of the slope of the tangent of the function) is the second derivative of the function defined by an arc having a radius of 10 mm. It can be considered that there is a part that is larger than the differential value. "Smooth connection" can be defined as having an inflection point at which the second derivative value of the function defining the outer peripheral shape of the roll is 0, and the second derivative values ​​are continuous before and after the inflection point.
[0018]
　First, by deformation analysis using the finite element method, when the slab 10 being continuously cast is reduced with the same reduction force using each of the convex disk roll 5 and the convex curved roll 4, the slab is reduced. The deformation behavior of how the surface and the central part of the slab thickness are deformed was determined. The slab 10 for continuous casting has a width W of 550 mm and an aspect ratio (width / thickness) of the slab 10 of 1.3. As shown in FIG. 4, the convex disc roll 5 has a horizontal portion 20 having a width of 0.4 × W in the center of the width, and inclined portions 21 having an inclination of 17 ° are provided on both sides of the horizontal portion 20. .. As shown in FIG. 3, the convex curved roll 4 has an arc shape 18 having an arc radius R 1 of 0.8 × W as the roll outer peripheral shape 11 in a cross section passing through the roll rotation shaft 12 . In both convex rolls 3, the roll radius r C at the width center position 13 is 0.8 × W. The convex disc roll 5 is in contact with the slab 10 only by the horizontal portion 20 and the inclined portion 21 up to a reduction amount of 10 mm. The convex curved roll 4 is in contact with the slab 10 only in an arc shape 18 up to a reduction amount of 10 mm. As shown in FIG. 1, of the reduction roll pairs (1 pair of reduction rolls 1 and 2), the reduction roll 2 on the F side (lower side) is a flat roll, and the reduction roll 1 on the L side (upper side), respectively. The convex roll 3 of the above is used.
[0019]
　As the temperature distribution inside the slab at the position where the reduction is performed, the temperature distribution at the position 3 minutes (10 m) after the completely solidified position was set. The width direction range of the final solidified portion is a range of 0.2 × W, and this range is the center porosity generation region. The surface temperature of the slab was 850 ° C, and the temperature at the center of the thickness and the center of the width was 1400 ° C.
[0020]
　For each of the convex disc roll 5 and the convex curved roll 4, a reduction force was applied with a reduction force of 100 tons (980.665 kN), and deformation analysis was performed by the finite element method. As a result of the deformation analysis, the reduction amount (mm) of the slab surface and the plastic strain (normalized equivalent plastic strain) at the center of the thickness of the slab 10 were analyzed. The dimensions in the slab width direction were standardized so that W / 2 was 1 with the center of the width as the origin, and indicated by x.
[0021]
　Equivalent plastic strain is defined by ε B in (Equation 1) from plastic strain in the uniaxial direction (ε 1 p , ε 2 p , ε 3 p ), and the strain in three-dimensional deformation is converted to uniaxial deformation and scalar. It is a quantification. This analysis is based on the idea that the greater the strain, the greater the amount of internal deformation due to reduction, and the greater the porosity reduction effect. Therefore, the reduction efficiency was evaluated by calculating the equivalent plastic strain for each mesh of the analysis model and outputting the amount of deformation at the center of the thickness for each roll shape. Further, the standardized equivalent plastic strain is standardized so that the value of the equivalent plastic strain ε B at the width center position 13 when reduced by using a convex disc roll is 1. .. 　ε B＝ √ [(2/3) {(ε 1 p ) 2＋ (ε 2 p ) 2＋ (ε 3 p ) 2 }] (Equation 1)
[0022]
　FIG. 5 is a graph showing the distribution of the slab surface reduction amount in the width direction obtained by the deformation analysis of the finite element method. As shown in FIG. 5, the surface reduction amount at the width center position 13 is about 4 mm for the convex disc roll 5 and about 9 mm for the convex curved roll 4 even though the same reduction force of 100 tons is applied. It was. On the other hand, as the distance from the width center position 13 increases, the reduction amount of the convex disc roll 5 is constant, whereas the reduction amount of the convex curved roll 4 decreases, and the distance x = 0. The surface reduction amount is the same in the vicinity of 3, and the convex disc roll 5 has a larger surface reduction amount from the outside to x = 0.4 than that. In each of the convex disc roll 5 and the convex curved roll 4, the surface reduction amount that conforms to the outer shape of each roll is realized.
[0023]
　FIG. 6 is a graph showing the distribution in the width direction of the normalized equivalent plastic strain of the thickness center in the slab obtained by the deformation analysis of the finite element method. As shown in FIG. 6, surprisingly, the value of the normalized plastic strain is larger in the convex curved roll 4 than in the convex disc roll 5 over the entire width direction. .. As for the width center position 13, since the surface reduction amount of the convex curved roll 4 is larger, it is expected that the normalized plastic strain at the thickness center portion is also a large value. On the other hand, in the region where the distance x = 0.3 is exceeded from the width center position 13, the convex disc roll 5 is larger in the surface reduction amount, so that the standardized plastic strain in the thickness center is also convex. It is expected that the disc roll 5 will be larger, but contrary to the expectation in the deformation analysis by the finite element method, the convex curved roll 4 has the normalized plastic strain at the center of the thickness up to the end in the width direction. The result was that it would grow.
[0024]
　From the results of the deformation analysis by the finite element method described above, in order to reduce the center porosity by reduction using the convex roll 3 in the actual continuous casting, if the reduction force is the same, the convex disk is used as the reduction roll 1. It was suggested that the improvement effect would be greater when the convex curved roll 4 was used than when the roll 5.
[0025]
　Therefore, in actual continuous casting, the center porosity reduction effect of the slab 10 was compared when each of the convex disc roll 5 and the convex curved roll 4 was used as the reduction roll 1 for continuous casting. The aspect ratio (width / thickness) of the slab 10 to be cast is 1.3. The width of the slab 10 is W (mm). As the reduction roll 1, the convex disc roll 5 has a horizontal portion 20 having a width of 0.4 × W in the center of the width, and inclined portions 21 having an inclination of 17 ° are provided on both sides of the horizontal portion 20. In the convex curved roll 4, the roll outer peripheral shape 11 in the cross section passing through the roll rotation shaft 12 is an arc shape 18 having an arc radius R 1 of 0.8 × W. In both convex rolls 3, the roll radius r C at the width center position 13 is 0.8 × W. Further, in both convex rolls 3, the roll radius r F in the flat portions on both sides of the width is 0.65 × W. In both cases, a flat roll is used for the reduction roll 2 on the F side of the reduction roll pair.
[0026]
　During continuous casting, a reduction force of 100 tons was applied to the reduction roll at a position (10 m) 3 minutes after the final solidification position to reduce the slab 10. The surface shape of the cast slab 10 and the state of center porosity generation at the center of the slab thickness were evaluated.
[0027]
　A dent due to a convex portion of the convex roll 3 was formed on the upper surface side of the slab 10. Comparing the thickness of both ends of the width of the slab 10 with the thickness of the center of the width, the amount of dent by the convex disc roll 5 was about 4 mm, and the amount of dent by the convex curved roll 4 was about 9 mm. The concave shape was a shape that followed the outer shape of the convex roll 3.
[0028]
　The center porosity of the slab 10 was evaluated using the porosity area ratio calculated by the color check of the slab cross section as an index. As a result, the convex disc roll had a porosity area ratio of 3%, and the convex curved roll 4 had a porosity area ratio of 0.3%. The effect of improving center porosity by using the convex curved roll 4 is clear.
[0029]
　As described above, when the slab 10 is reduced by the reduction roll during continuous casting, by using the convex curved roll 4 according to the first embodiment as the reduction roll, the convex disc roll 5 is subjected to the same reduction force. It was clarified that the effect of improving center porosity was superior to that of using. It was also clarified that, when the center porosity improving effect is the same, the convex curved roll 4 can obtain the same effect with a smaller pressing force than the convex disc roll.
[0030]
　Next, the requirements to be satisfied by the convex curved roll 4 which is the reduction roll 1 according to the present embodiment will be described below in the order of the first embodiment and the second embodiment.
[0031]
　The first embodiment will be described with reference to FIGS. 1 to 3. In the reduction roll 1, the roll outer peripheral shape 11 in the cross section passing through the roll rotation shaft 12 has the following shape. First, the roll outer peripheral shape 11 constitutes a convex shape that projects outward in a region including the width direction center position (width center position 13) of the slab 10. The outside is a direction in which the outer circumference of the roll moves away from the roll rotation shaft 12. By constructing such a shape, the roll radius r C becomes maximum at the width center position 13, and when the slab 10 is reduced, the amount of reduction on the slab surface becomes maximum at the width center position 13. Next, the range of a total length of 0.80 × W from the width center position 13 on both sides in the roll width direction is defined as the “convex shape defined range 14”. When the slab 10 is pressed by using the convex roll 3, both ends of the width of the slab 10 have a large deformation resistance, so that the slab 10 is not reduced. If the slab 10 is reduced in the convex shape specified range 14 or a width narrower than this, the reduction force required for reduction can be suppressed low while ensuring the required reduction amount. Therefore, if the convex shape of the reduction roll 1 is determined within the convex shape specified range 14, good reduction can be performed according to the first embodiment. The convex shape within the convex shape defined range 14 is a curved shape that is convex outward and has no corners. The outward convex means that it is convex in the direction away from the roll rotation axis 12. Further, the thickness of the slab 10 to be cast is t (mm), and the roll radius r C at the width center position 13 is relative to the rolling radius r E at both ends of the convex shape specified range 14. Is larger than 0.005 × t. As a result, when the slab 10 is reduced by the reduction roll 1, if the entire convex shape defined range 14 of the reduction roll 1 is reduced to the slab 10, the reduction amount of the slab 10 at the width center position 13 is 0. It can be .005 × t or more. The roll radius r C at the width center position 13 is more preferably 0.010 × t or more.
[0032]
　As the simplest and most effective shape among the convex shapes within the convex shape defined range 14, the arc shape 18 having a single arc radius R 1 can be used as shown in FIG. At this time, the roll outer peripheral shape 11 in the convex shape defined range 14 constitutes a bow shape in which the length portion of the convex shape defined range 14 is the string 31. The length of the convex shape regulation range 14 (the length of the chord 31) is s, the radius of the bow shape is R, and the height of the arc 32 of the bow shape (the reduction roll radius r E and the width center position 13 at both ends of the convex shape regulation range 14). When the difference from the roll radius r C in ) is h, the following relationship is established. Let the central angle of the bow be 2θ.
　　h = R (1-cosθ) (Equation 2)
　　s = 2R · sineθ (Equation 3) From
these equations, the following equations are derived.
　　cos θ = (s 2 -4h 2 ) / (s 2 + 4h 2 ) (Equation 4)
　Therefore, first determine the target s and h, and then determine θ by substituting s and h into the above (Equation 4). Further, R can be determined by substituting θ into (Equation 2) or (Equation 3). For example, when s = 150 mm and h = 9 mm are targeted, R = 316 mm can be derived by substituting into the above equation.
[0033]
　Convex shapes include the arc shape 18 having a single arc radius R 1 , a parabolic shape, an elliptical shape, a hyperbolic shape, and a shape in which arcs having different radii depending on the location are smoothly connected. Can be selected arbitrarily from. In a curved shape having no corners, which constitutes a convex shape, the radius of curvature of the curve is preferably at least 1 × h or more. As a result, the effect of the first embodiment due to the curved shape of the convex shape can be fully exhibited. The minimum radius of curvature of the curve is the same in the second embodiment described later.
[0034]
　The outer peripheral shape 11 of the roll 1 on the outer side of the convex shape defined range 14 and on the end side in the width direction is not particularly specified. Preferably, the outer peripheral shape of the roll 11 is a straight line or a curved shape having no corners. When the roll shape at both ends of the reduction roll 1 in the width direction is a cylindrical shape (Cylindrical configuration) 22 having an outer peripheral surface substantially parallel to the roll rotation axis 12, the roll outer peripheral shape 11 is from the convex shape defined range 14. It is preferable that the shape is a combination of straight lines and curves and has no corners up to the position of the cylindrical shape 22 at both ends in the width direction. In the roll outer peripheral shape 11, the portion transitioning from the position of the cylindrical shape 22 toward the convex shape defined range 14 may be an outwardly concave curve in a direction away from the roll rotation axis 12. As described above, the roll outer peripheral shape 11 has straight lines parallel to the roll rotation axis 12 at both ends in the width direction, and has a concave curve on the outside that smoothly connects to the straight lines.
[0035]
　As shown in FIG. 3, the simplest and most effective shape of the roll outer peripheral shape 11 of the reduction roll 1 is simply the convex shape defined range 14 and the predetermined ranges (radius R 1 range 23) on both sides thereof. It is an arc shape 18 having an arc radius R 1 . Further, for the radius R 2 range 24 on both sides thereof, the arc shape 19 having a single arc radius R 2 and the concave shape is smoothly connected to the outside, and finally smooth to the straight line of the cylindrical shape 22 of the flat roll. A shape that connects to can be adopted. Therefore, since there is no corner portion in any portion of the roll outer peripheral shape 11, the roll reduction amount in the reduction roll 1 increases, and the reduction range in the roll in the width direction exceeds the convex shape specified range 14, and is convex. Even when the reduction is performed from the shape specified range 14 to the portion of the concave curve on the outside immediately before connecting to the cylindrical shape 22 at both ends in the width direction, the corners of any part of the slab surface after the reduction are performed. Can be a smooth surface on which is not formed. Further, even when the reduction is performed until the cylindrical portion 22 of the flat roll comes into contact with the slab 10, any portion of the surface of the slab after the reduction can be made a smooth surface without forming corners. As described above, even if the roll reduction amount is large, it is possible to obtain a smooth surface on which no corners are formed on any portion of the slab surface after reduction. As a result, it is possible to reduce the occurrence of rolling flaws due to the concave shape of the slab 10 generated by rolling with the convex roll 3 in the hot rolling in the subsequent process following the continuous casting. Arc radius R 2Is preferably 5 mm or more, more preferably 10 mm or more, still more preferably 100 mm or more, from the viewpoint of reducing the occurrence of rolling flaws in the slab 10.
[0036]
　In the reduction control device that performs reduction control on the reduction roll 1, if a device that can control the reduction displacement amount to the target displacement amount (a device that can perform reduction displacement control) is used, the reduction amount can be adjusted as described above for the reduction roll 1. It can be controlled to a value of h or less. As a result, the roll surface in contact with the slab 10 during reduction can be contained within the convex shape specified range 14. Since the convex shape specified range 14 is a curved shape having no corners, a dent with a steep change in the angle of the tangent plane is not formed on the surface of the slab after rolling, and a defect occurs during hot rolling in the subsequent process. Does not cause.
[0037]
　On the other hand, when a device that cannot control the reduction displacement is used as the reduction control device, it is preferable to adopt the most concise and effective shape of the roll outer peripheral shape 11 at a position outside the convex shape regulation range 14. The roll outer peripheral shape 11 of the rolling roll has a smooth shape having no corners at any of the convex shape defined range 14 and any portion extending to the cylindrical shape 22 portion on both sides thereof. Therefore, even if the flat rolls at both ends of the width are reduced to contact the slab 10 due to the large reduction force, the angle of the tangential plane that causes scratches on the surface of the slab after reduction is applied. A shape with steep changes is not formed.
　Therefore, it is possible to reduce the center porosity by performing sufficient reduction with a small amount of reduction, and it is possible to reduce defects in hot rolling due to the slab reduction shape.
[0038]
　As a requirement to be satisfied by the convex curved roll 4 which is the reduction roll 1 according to the present embodiment, the second embodiment will be described with reference to FIGS. 7 and 8. In the second embodiment, in the reduction roll 1, the roll outer peripheral shape 11 in the cross section including the roll rotation shaft 12 has the following shape. That is, in the first embodiment, the convex shape within the convex shape defined range 14 is defined as a curved shape that is convex outward and has no corners. On the other hand, in the second embodiment, the convex shape within the convex shape defined range 14 is a combination of an outwardly convex curve 16 and a straight line 17 having a length of 0.25 × W or less, and is a corner portion. It is defined as a shape that does not have. The grounds for this determination will be described below.
[0039]
　The effectiveness of the second embodiment was also confirmed by deformation analysis using the finite element method. As the roll outer peripheral shape 11, as shown in FIG. 7, for the combination of the convex curve 16 and the straight line 17, the convex curve has an arc shape 18 having an arc radius R 1 of 0.8 × W, and the straight line 17 has a width. A straight line portion of an arbitrary length was provided about the center position 13 in parallel with the roll axis, and the arc shape 18 and the straight line 17 were smoothly connected. After setting various lengths of the straight line 17, the reduction force was applied with the reduction force as a weight of 100 tons, and the deformation analysis was performed by the finite element method. As a result of the deformation analysis, the plastic strain (normalized equivalent plastic strain) at the center of the thickness of the slab 10 was analyzed. The result is shown in FIG. The length D of the straight line 17 is indicated by D / W in the figure. The larger the D / W, that is, the longer the length D of the straight line 17, the smaller the standardized plastic strain at the center of the thickness in the entire width direction, but the length D of the straight line 17 is 0.25 × W or less. It was found that within the range of, a value of plastic strain equivalent to normalization better than that of the convex disc roll 5 can be realized. Therefore, such a shape of the reduction roll 1 is set as the second embodiment.
　Therefore, it is possible to reduce the center porosity by performing sufficient reduction with a small amount of reduction, and it is possible to reduce defects in hot rolling due to the slab reduction shape.
[0040]
　The mechanism by which the convex curved roll 4 according to the second embodiment can satisfactorily improve the center porosity even with the same reducing force as compared with the conventional convex disc roll 5 will be examined. The reduction of porosity due to post-solidification reduction is due to the fact that the porosity is crimped by applying strain to the porosity generation region by reduction. As a general rule, the amount of strain applied increases as the amount of reduction increases. In particular, the distortion of the surface portion directly reflects the amount of pushing in the width direction. Therefore, when the convex curve roll 4 and the conventional convex disc roll 5 are compared, the convex disc roll 5 is viewed in the width direction. There are places where the amount of strain applied on the surface of the slab exceeds. On the other hand, as the strain penetrates into the center of thickness, the strain also diffuses in the width direction. Therefore, as for the amount of strain in the central portion in the thickness direction, the convex curved roll 4 capable of gaining a large reduction amount in the curved portion is dominant, so that the convex curved roll 4 is superior in the overall width. It is thought that it became.
[0041]
　In the continuous steel casting method according to the second embodiment, the reduction roll 1 according to the second embodiment is used, and the central solid phase ratio of the slab 10 is 0.8 or more during continuous casting. The slab 10 at a position including after complete solidification is reduced by at least a pair of reduction rolls 1. If the central solid phase ratio of the slab 10 is 0.8 or more, it is a region where the residual molten steel is difficult to flow at the center of the slab thickness. Therefore, even if the slab is reduced, the problem of internal cracking and the reverse V The problem of segregation is unlikely to occur. For at least one of the pair of reduction rolls 1, the reduction roll 1 according to the second embodiment is used. The central solid phase ratio can be defined as the solid phase ratio at the center in the slab thickness direction in the C cross section and at the center in the slab width direction. The central solid phase ratio can be measured by a method of directly measuring the central temperature with a thermocouple, an estimation by heat transfer calculation, an estimation by tacking, or the like.
[0042]
　The position of the slab in the casting direction to be reduced by the reduction roll 1 is more preferably the position after complete solidification. By reducing the slab 10 at the position after complete solidification, the crimping of the center porosity can be eliminated without causing the problem of internal cracking and the problem of reverse V segregation. When the slab 10 after complete solidification is reduced, the optimum range limit of the reduction position on the downstream side of casting is the region where the width center surface temperature is 650 ° C. or higher. This is because if the width center surface temperature is less than 650 ° C., the slab 10 is hardened due to the temperature drop, and it becomes difficult to sufficiently reduce the pressure regardless of the roll shape.
　In determining the reduction position during continuous casting, the temperature of the slab surface during continuous casting is determined for each of the position where the central solid phase ratio is 0.8, the complete solidification position, and the suitable range limit position of the reduction position after complete solidification. It can be determined by combining the measurement and the heat transfer solidification calculation of the slab 10.
Example
[0043]
　In a curved type bloom continuous casting in which a bloom having a slab shape of 550 mm in width and 400 mm in thickness is cast, a test was conducted in which Examples were applied. At a casting speed of 0.4 m / min, the solidification completion position was a position of 20 m in casting length. A pair of reduction rolls 1 in which the F-plane roll is a flat roll and the L-plane roll is a convex roll 3 are prepared, and reduction is performed at a position of 30 m in casting length. The rolling force was 100 tons.
[0044]
　As shown in FIG. 4, the conventional convex disc roll 5 has a horizontal portion 20 having a width center position 13 having a length of 200 mm, and inclined portions 21 having an angle of 17 ° on both sides thereof via the corner portions 15. .. The roll radius of the horizontal portion 20 is 20 mm larger than the roll radius of the flat roll portions at both ends of the width.
[0045]
　As shown in FIG. 3, the convex curved roll 4 of the embodiment includes a convex shape defined range 14 (a range of a total length of 0.80 × W on both sides in the roll width direction from the width center position 13). A roll having an arc shape 18 having a constant radius of 430 mm and having a roll radius r C at the width center position 13 larger than the reduction roll radius r E at both ends of the convex shape defined range 14 was used. The roll radius r C at the width center position 13 is 400 mm. The arc shape 18 within the convex shape defining range 14 continues to the outside of the convex shape defining range 14 (radius R 1 range 23), and then the arc shape 19 (radius R 2 ) having an arc radius R 2 = 100 mm and being concave outward. It is smoothly connected to the range 24), and finally to a flat roll portion having a cylindrical shape 22 having a roll radius r F of 340 mm.
[0046]
　As described above, the center porosity of the slab 10 was evaluated using the porosity area ratio calculated by the color check of the slab cross section as an index. In the conventional example in which the convex disc roll 5 was used as the reduction roll 1, the center porosity area ratio was 3% or more. In the example using the convex curved roll 4, the center porosity area ratio was 0.3%. As described above, the effect of reducing the center porosity of the continuously cast slab according to the present embodiment was confirmed.
[0047]
　The slabs of Examples and Conventional Examples were hot-rolled as a general hot-rolling process. As a result of comparing the product defect rate due to the surface shape of the slab, the product defect rate was about 5% in the slab of the conventional example, but as a result of using the slab 10 of the example, the product defect rate Was reduced to 0.5% or less. As described above, the effect of reducing the flaws in hot rolling according to the present embodiment could be confirmed.
Industrial applicability
[0048]
　The steel continuous casting method and the reduction roll for continuous casting of the present invention can be used for continuous casting of slabs used as materials for various steel products.
Code description
[0049]
　1 Reduction roll
　2 Reduction roll
　3 Convex roll
　4 Convex curved roll
　5 Convex disc roll
10 Slab
11 Roll outer circumference shape
12 Roll rotation axis
13 Width direction center position (width center position)
14 Convex shape specified range
15 Square portion
16 Curve
17 Straight line
18 Arc shape
19 Arc shape
20 Horizontal part
21 Inclined part
22 Cylindrical shape
23 Radius R 1 Range
24 Radius R 2 Range
31 String
32 Arc
　W Cast piece width
　r C　Width Center position reduction roll radius
　r F　Width end Reduction roll radius of
　r E　Convex roll radius at both ends of the specified range
　R 1　Arc radius
　R 2　Arc radius
　h Bow-shaped arc height
　s Bow-shaped string length
　θ Half of the center angle of the
　bow-R Bow-shaped radius
The scope of the claims
[Claim 1]
　A method for continuously casting steel in which, during continuous casting, the slab at a position where the central solid phase ratio of the slab is 0.8 or more and includes after complete solidification is reduced by at least a pair of reduction rolls. The width of the slab to be cast is W (mm), the thickness of the slab is t (mm), and for
　at least one of the pair of reduction rolls, the outer peripheral shape of the roll in the cross section including the roll rotation axis is the casting. It has a convex shape that projects outward in the area including the center position in the width direction of one piece, and the
　convex shape is a convex shape having a total length of 0.80 × W on both sides in the roll width direction from the center position in the width direction. Within the specified shape range, a curved shape that is convex outward and has no corners, or a shape that is a combination of a curved line that is convex outward and a straight line with a length of 0.25 x W or less and has no corners. , A
　method for continuously casting steel, wherein the reduction roll radius at the center position in the width direction is 0.005 × t or more larger than the reduction roll radius at both ends of the convex shape specified range.
[Claim 2]
　The continuous casting method for steel according to claim 1, wherein the position of the slab in the casting direction to be reduced by the reduction roll is the position after complete solidification.
[Claim 3]
　The continuous steel according to claim 1 or 2, wherein the amount of reduction of the slab by the pair of reduction rolls is 0.005 × t or more and 15 mm or less at the center position in the width direction. Casting method.
[Claim 4]
　A reduction roll for reducing a slab having a slab width: W (mm) and a slab thickness: t (mm) during continuous casting, and the
　roll outer peripheral shape in a cross section including a roll rotation axis is described above. It has a convex shape that projects outward in a region including the center position in the width direction of the slab, and
　the convex shape defines a convex shape with a distance of 0.80 × W on both sides in the roll width direction from the center position in the width direction. In the range, a curved shape that is convex outward and has no corners, or a shape that is a combination of a curved line that is convex outward and a straight line with a length of 0.25 × W or less and has no corners. is any,
　to the reduction roll radius at both ends of the convex specified range, pressure roll for continuous casting rolling roll radius at the widthwise center position being greater than 0.005 × t.
[Claim 5]
　The outer peripheral shape of the roll has straight lines parallel to the roll rotation axis at both ends in the width direction, and has a
　concave curve on the outside that smoothly connects to the straight line. The reduction roll for continuous casting according to 4.