JP2020044556A - Rolling reduction method of continuous casting - Google Patents

Abstract

To provide a rolling reduction method of continuous casting capable of reducing the central segregation and center porosities of a continuously cast slab, and capable of preventing flaw generation in hot rolling without performing large scale facility reinforcement.SOLUTION: A rolling reduction method of continuous casting of subjecting a slab to rolling reduction during continuous casting has a process where, in a region having a center solid phase rate from 0.8 to the completion on solidification (high solid phase region 61), two or more rolling reduction roll pairs 40 are continuously arranged and a slab 10 is subjected to rolling reduction, and regarding at least either the rolling reduction roll composing the rolling reduction roll pairs 40, a roll outer circumferential shape in cross sections including a roll rotary shaft constitutes a convex shape having no bulging corner parts at the outside including the width direction central position (width central position) of the slab.SELECTED DRAWING: Figure 9

Description

  The present invention relates to a rolling reduction method in continuous casting, which aims at efficiently and drastically reducing center segregation and porosity defects in a continuous casting process.

  When casting a slab or a bloom such as a bloom by the continuous casting method, so-called center segregation in which components such as phosphorus and manganese segregate in the center of the slab may occur. Further, voids called center porosity are generated in the center of the slab.

  In the last stage of solidification during continuous casting, the amount of steel occupying a predetermined volume in a slab becomes insufficient due to solidification shrinkage when steel is solidified. In the slab portion where the unsolidified molten steel can flow, the unsolidified molten steel flows toward the solidification completion point of the final solidification part, and the impurity-enriched molten steel at the solid-liquid interface accumulates in the final solidification part, which is the center segregation. Cause. In addition, at a position where the unsolidified molten steel cannot flow (the slab center solid phase ratio is 0.8 or more), voids are formed in the center of the slab, causing center porosity.

  In order to reduce center segregation, in the region where the thickness center is the solid-liquid coexistence region and the unsolidified molten steel can flow, the solidification shell is reduced by an amount corresponding to the solidification shrinkage of the molten steel, thereby achieving the final solidification. It is effective to suppress the flow of molten steel near the part. In order to reduce the center porosity, it is effective to crimp the center porosity near the solidification completion position where the unsolidified molten steel cannot flow or the slab after complete solidification. Based on such a concept, a light reduction technique of reducing a slab by a support roll before and after the completion of solidification at the end of continuous casting is used.

  In continuous casting, center segregation can be reduced by applying an appropriate reduction to compensate for solidification shrinkage as described above. In an actual machine, a light rolling reduction technique of applying an appropriate reduction of about 0.8 to 1.2 mm / min in a low solid phase fraction region having a central solid phase fraction of 0.8 or less is widely applied.

  Patent Document 1 discloses a continuous slab casting method characterized in that a reduction ratio is set to 0.36 to 0.72 mm / min, and the reduction is performed to a portion where a central solid phase ratio is equal to or more than a flow limit solid phase ratio. Has been introduced. The rolling gradient was not changed even at a portion higher than the flow limit solid fraction (the central solid fraction was 0.8 or more).

  Patent Literature 2 discloses a continuous casting method in which a steel slab continuous cast piece is drawn while being lowered between at least one pair of opposing rolls, and a position where a solid phase ratio of the cast slab central portion is 0.1 to 0.4. In the region from to an arbitrary position within the range of 0.8 to 0.9, the slab is pressed down so as to compensate for the total solidification shrinkage, and the solid phase ratio from the above-mentioned arbitrary position to solidification is completed. The region is a range in which a reduction gradient (% / m) indicating a ratio (%) of a reduction amount to a slab thickness per drawing direction length (unit: m) of a slab is defined by a formula based on a C concentration of steel. There has been proposed a continuous casting method for reducing the pressure so as to satisfy the following.

  When trying to reduce the slab before and after solidification is completed during continuous casting, the deformation resistance associated with the reduction is large because both short sides of the slab have already been solidified and the temperature has been reduced. In some cases, it was not possible to obtain the amount of reduction. Therefore, instead of using a roll in which the roll diameter is constant in the roll width direction (hereinafter referred to as a "flat roll"), the roll diameter at the portion corresponding to the central portion of the slab width is large, and the roll diameter is corresponding to both sides of the slab width. The roll diameter of the part to be rolled is smaller than that of the center of the width (hereinafter referred to as “convex roll”), and both short sides where the solidification of the slab is completed are not reduced. A technology to reduce only the part was developed.

  Patent Literature 3 discloses a roll rolling method for reducing center porosity, in which after the slab is completely solidified and before cutting, the surface temperature of the slab is 700 ° C. or more and 1000 ° C. or less, and the inner center of the slab is 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 pressed down. At the rolling portion, the inner side is relatively softer than the surface layer due to the high temperature, and the rolling force applied to the surface of the slab can be transmitted to the inside of the slab. A convex roll used as a roll has a horizontal portion at the center in the width direction, and a roll-out protruding region having an inclined portion connected to the horizontal portion on both sides of the horizontal portion. It is stated that the width of the horizontal portion (reduction width) is preferably 40% or less of the slab width. The reduction amount is preferably 2% or more of the thickness of the slab.

  Patent Document 4 discloses a continuous casting method in which at least one crown roll is provided as a reduction roll, and a central portion of a slab and a vicinity thereof are reduced. It is stated that the slab is rolled down with a crown roll in an area where the rate of formation of the solidified shell of the slab is equal to or greater than 75%, and the concentrated molten steel in the unsolidified portion inside the pressed slab is pushed up and eliminated. The shape of the crown may be any shape as long as the central portion in the slab width direction and the vicinity thereof can be reduced, and the drawing shows a reduction roll having a shape in which the central portion in the roll width direction bulges outward. I have. The amount of reduction per stage is set to a maximum of 3 mm.

JP-A-06-297125 JP-A-11-77269 JP 2009-279652 A JP-A-60-162560

  When rolling down the slab during continuous casting, especially when rolling down the slab with a large amount of reduction in the region of high solid fraction of 0.8 or more, use a flat roll as the rolling roll. In some cases, a large rolling capacity is required. On the other hand, if a convex roll is used instead of a flat roll as the reduction roll, the reduction of the reduction roll for realizing the reduction can be achieved by not reducing the portion where the reduction resistance at both ends of the slab width is large. Power can be reduced. However, even if a conventional convex roll is used, if sufficient rolling is performed in the region of high solid fraction in order to reduce center segregation and center porosity, the required rolling force becomes excessive, and Large-scale equipment reinforcement is required. Also, as a result of rolling down using a convex roll, a pit is formed on the surface of the cast slab after continuous casting, and this dent causes a flaw in hot rolling in a later step. There was something.

  The present invention can reduce the center segregation and center porosity of a continuous cast slab without performing large-scale equipment enhancement, and can also prevent the occurrence of flaws in hot rolling in a subsequent step, and can be used for continuous casting. It is intended to provide a rolling method.

That is, the gist of the present invention is as follows.
(1) A method of rolling down a slab during continuous casting,
In a region where the central solid phase ratio is from 0.8 to the completion of solidification (hereinafter referred to as a “high solid phase ratio region”), two or more pairs of reduction rolls are continuously arranged to reduce the slab,
With respect to at least one of the rolls forming the roll pair, the outer peripheral shape of the roll in a cross section including the roll rotation axis is a region including the center position in the width direction of the slab (hereinafter referred to as “width center position”). A continuous casting with a convex shape having no protruding corners on the outside.
(2) The slab width to be cast is W (mm), the slab thickness is t (mm), and the convex shape has a total length of 0.80 × W on both sides in the roll width direction from the width center position. In a range (hereinafter referred to as a “convex shape defining range”), a curved shape that is outwardly convex and has no corners, or a combination of an outwardly convex curve and a straight line having a length of 0.25 × W or less Wherein the rolling roll radius at the center of the width is 0.005 × t or more larger than the rolling roll radius at both ends of the convex shape defining range. The rolling method for continuous casting according to (1).
(3) The rolling method for continuous casting according to the above (1) or (2), wherein the rolling gradient in the high solid fraction region is 4.0 mm / min or more.
(4) The rolling method for continuous casting as described in any one of (1) to (3) above, wherein three or more pairs of the rolling rolls are continuously arranged to reduce the casting slab.
(5) In a region where the central solid phase ratio is from 0.3 to 0.75 (hereinafter referred to as a “low solid phase ratio region”), the reduction gradient is 0.8 to 1.2 mm / min. The method according to any one of (1) to (4) above, wherein a reduction is performed with a reduction gradient of 0.8 mm / min or more in a region between the low solid fraction ratio region and the high solid fraction ratio region. Rolling method of continuous casting.

  In the present invention, when performing continuous casting, the central solid phase ratio in the high solid phase ratio region from 0.8 to the solidification completion, by using the convex curve roll of the present invention as a roll, a predetermined rolling force with a small rolling force. Rolling is performed with a rolling gradient, so that center segregation and center porosity of the slab can be drastically reduced, and flaws in hot rolling caused by the slab rolling shape can be reduced.

It is a figure showing the situation where a slab is reduced by a reduction roll of the present invention, (A) is a sectional view, and (B) is a partial sectional view of a reduction roll. It is a fragmentary sectional view of a reduction roll of the present invention. It is sectional drawing of the conventional reduction roll. It is a figure which shows the slab surface reduction amount calculated | required by the deformation analysis of the finite element method. It is a figure which shows the plastic strain of the slab thickness center part calculated | required by the deformation analysis of the finite element method. It is a figure which shows about 2nd Embodiment, (A) is sectional drawing of a reduction roll, (B) is a figure which shows the plastic distortion of the slab thickness center part calculated | required by the deformation analysis of the finite element method. It is a figure which shows the calculation method of the amount of reduction by a reduction roll. It is a figure which shows the relationship between the reduction gradient at the center solid-phase ratio of 0.8 or more, and the maximum Mn segregation degree in the slab center part. It is a figure which shows the arrangement | positioning of the reduction roll pair in a high solid phase ratio area | region, (A) is a case where it has one pair, (B) has two pairs, and (C) has three reduction roll pairs. It is a figure which shows the situation which performs light reduction in a low solid phase rate area | region in addition to the reduction using the reduction roll pair in a high solid phase rate area | region.

  A description will be given of a change in the center solid fraction of the slab during continuous casting. From the point where the liquidus lines on the upper and lower sides of the slab meet at the center of the slab thickness (solidification start position), the central solid fraction becomes greater than 0, and the central solid fraction increases toward the downstream side. . The center solid fraction is 0 upstream of the solidification start position. Then, solidification is completed at a point where the solidus lines on the upper surface side and the lower surface side of the slab meet at the center of the slab thickness, and the center solidus ratio becomes 1.0. This point is also referred to as “solidification completion position”. Downstream of the solidification completion position, the central solid phase ratio remains at 1.0. Hereinafter, for convenience, the solidification completion position may be referred to as “the position where the central solid phase ratio is 1.0”. Further, the center solid fraction may be indicated as fs.

Regarding the center solid phase ratio at each position in the casting direction during casting, the temperature T _C at the center in the thickness direction of the slab during continuous casting was obtained by one-dimensional heat transfer solidification calculation, and then the liquidus temperature T _L , It can be calculated by the following equation (1) using the solidus temperature T _S. In the heat transfer / solidification calculation, an enthalpy method, an equivalent specific heat method, or the like can be used. When T _C > T _L , the central solid fraction is 0, and when T _S > T _C , the central solid fraction is 1.0.
Center solid phase ratio _{= (T L -T C) /} (T L -T S) (1)

  As described above, in continuous casting, center segregation is reduced by applying an appropriate reduction to compensate for solidification shrinkage. In an actual machine, light reduction is performed in a low solid fraction ratio region having a central solid fraction of 0.8 or less, and in such a range of the central solid fraction, the light reduction amount for compensating coagulation shrinkage is 0.1 mm. It is about 8 to 1.2 mm / min.

In the present invention, even in the high solid fraction ratio region where the central solid fraction is 0.8 or more and 1.0 or less, if proper reduction is performed, the central segregation and center porosity of the slab may be further improved. Inspired. Therefore, confirmation was made by experiments using a laboratory continuous casting apparatus. The experimental equipment is equipped with a mechanism that can simulate multi-stage roll reduction by actual machine segments. While directly measuring the internal temperature of the slab during solidification, the roll reduction with an arbitrary gradient can be achieved with a center solid fraction of 0.3 to solidification completed. Can be added continuously. The slab is rolled down in the high solid phase ratio region in the section where the slab center solid phase ratio is from 0.8 to 1.0, and the rolling gradient (the amount of rolling per hour (mm / min)) is varied when rolling down. The effect on the maximum Mn segregation degree at the center of the slab thickness was evaluated. In the evaluation of Mn segregation, a Mn concentration mapping analysis was performed on a sample of a cross section in the casting direction in the center part of the steady part under rolling with a beam diameter of 50 μm by EPMA. In the mapping data, a line having a width of 2 mm including the worst part of segregation was set, and the value obtained by dividing the peak value C of the concentration by the average concentration C ₀ in the measurement visual field was defined as the maximum Mn segregation degree C / C ₀ .

  FIG. 8 shows the evaluation results using the laboratory continuous casting apparatus. When the slab center solid phase ratio is reduced in the high solid phase ratio region in the section from 0.8 to 1.0, and the reduction gradient in the reduction is increased, the maximum Mn segregation degree in the slab thickness center decreases. You can see that it goes. When the reduction gradient in the section where the slab center solid phase ratio is 0.8 to 1.0 is 3.0 mm / min, the maximum Mn segregation degree is 1.2, and the reduction gradient is 4.5 mm / min. The maximum degree of Mn segregation when it became 1.45. Further, the maximum Mn segregation degree in the section is 5.5 mm / min and the maximum Mn segregation degree is 1.10. Further, when the reduction gradient is 6.0 mm / min, the maximum Mn segregation degree can be greatly reduced to 1.08. all right.

  When rolling down a slab during continuous casting, especially when rolling down a slab after solidification is completed, especially in a high solid phase ratio region where the central solid phase ratio is 0.8 or more, a flat roll is used as a rolling roll instead of a flat roll. By using, the rolling reduction of the rolling roll for realizing the rolling reduction can be reduced by not rolling down the portion where the rolling resistance at both ends of the slab width is large. However, even if a conventional convex roll is used, it is necessary to improve the center segregation by rolling down with a large rolling gradient in the high solid fraction area or to perform sufficient rolling down to realize center porosity reduction. The rolling force becomes excessive, and large-scale equipment reinforcement is required to secure the rolling force. Also, as a result of rolling down using a convex roll, a pit is formed on the surface of the cast slab after continuous casting, and this dent causes a flaw in hot rolling in a later step. There was something.

  Therefore, it is possible to reduce the center segregation and center porosity of the continuous cast slab without performing large-scale equipment reinforcement, and also to prevent the occurrence of flaws in the hot rolling in the subsequent process. A roll was devised, and a rolling reduction method for continuous casting using the rolling roll was completed.

The rolling roll used in the present invention will be described with reference to FIGS.
In order to continuously cast a raw slab for producing a steel product for a strip, bloom continuous casting or billet continuous casting is applied. In bloom continuous casting, a cast slab has a rectangular cross-sectional shape, for example, a slab having a width of 500 mm and a thickness of 300 mm is cast. When casting a slab having such a rectangular cross section, at a position immediately before the thickness center portion of the slab is completely solidified, the unsolidified portion of the slab is in total from the slab width direction center position to both sides in the width direction. It covers the range of "slab width-slab thickness", and center segregation and center porosity also occur in this region. Therefore, even when the slab is rolled down using a convex roll as a countermeasure against center segregation and center porosity, as shown in FIG. A roll having a horizontal portion 20 at the center position in the width direction of the slab (width center position 13) has been used. Inclined portions 21 are provided on both sides in the width direction of the horizontal portion 20, and the joint position between the horizontal portion 20 and the inclined portion 21 forms a corner portion 15.

  The inventor of the present invention has found that, in the convex roll 3 for rolling down the cast slab, not the roll forming the horizontal portion 20-the corner portion 15-the inclined portion 21 as shown in FIG. As shown in FIGS. 1 and 2, the shape 11 has a convex shape having no outwardly protruding corners, thereby reliably reducing the center porosity of the slab and reducing the rolling force required for rolling. Furthermore, they conceived that the generation of flaws in the hot rolling in the subsequent step could be reduced. Hereinafter, the convex roll 3 having the horizontal portion 20-the corner portion 15-the inclined portion 21 is referred to as a "convex disc roll 5", and the convex roll 3 forming a convex shape having no protruding corner portion is referred to as a "convex curve". Called "Roll 4".

First, by the deformation analysis using the finite element method, when the slab during continuous casting was rolled down with the same rolling force using each of the convex disk roll 5 and the convex curved roll 4, the slab surface was reduced. And the deformation behavior of the slab thickness center portion were determined. The slab to be continuously cast has a width W of 550 mm, and the slab 10 has an aspect ratio (width / thickness) of 1.3. As shown in FIG. 3, the convex disc roll 5 has a horizontal portion 20 having a width of 0.4 × W at the center of the width, and has inclined portions 21 with an inclination of 17 ° on both sides of the horizontal portion 20. . As shown in FIG. 2, in the convex curved roll 4, the roll outer peripheral shape 11 in a cross section passing through the roll rotation shaft 12 is an arc shape 18 having an arc radius R ₁ of 0.8 × W. Each of the convex rolls 3 has a roll radius r _{C at the} width center position 13 of 0.8 × W. The convex disc roll 5 is in contact with the slab only at 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 at only the 18 arc-shaped portions up to a reduction of 10 mm. As shown in FIG. 1, of the pair of pressing rolls, the pressing roll 2 on the F side (lower side) is a flat roll, and the respective convex rolls 3 are used as the pressing rolls 1 on the L side (upper side).

  As the temperature distribution inside the slab at the position where the rolling was performed, the temperature distribution at the center solid fraction of 0.8, 0.9, and 1.0 was set. The range in the width direction of the final solidified portion is a range of 0.2 × W, and this range is a center segregation / center porosity generation region. At each center solid phase ratio, the slab surface temperature was 1054 ° C., 1041 ° C., 1026 ° C., and the temperature at the center of the thickness and the center of the width was 1434 ° C., 1421 ° C., 1406 ° C.

  With respect to each of the convex disk roll 5 and the convex curved roll 4, a rolling force was applied with a rolling force of 100 ton F, and deformation analysis was performed by the finite element method. As a result of the deformation analysis, the amount of reduction (mm) on the slab surface and the plastic strain (standardized equivalent plastic strain) at the center of the thickness of the slab were analyzed. The dimension in the slab width direction was standardized so that W / 2 was 1, with the center at the center of the width, and indicated by x.

The equivalent plastic strain is defined as ε ^B in equation (2) from the plastic strain in the uniaxial direction (ε ₁ ^p , ε ₂ ^p , ε ₃ ^p ). It is quantified. This analysis is based on the idea that the greater the strain, the greater the amount of internal deformation due to the reduction, and the greater the porosity reduction effect. Therefore, the equivalent plastic strain was calculated for each mesh of the analysis model, and the amount of deformation at the center of the thickness was output for each roll shape to evaluate the rolling reduction efficiency. Further, the normalized equivalent plastic strain is the equivalent plastic strain ε ^B , which is normalized such that the value of the equivalent plastic strain 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 }] (2)

  FIG. 4 shows the width direction distribution of the surface reduction amount. Although the same rolling force of 100 tons F was applied, the surface rolling amount at the width center position 13 was about 4 mm for the convex disk roll 5 and about 9 mm for the convex curved roll 4. On the other hand, as the distance from the width center position 13 increases, the amount of reduction of the convex disk roll 5 is constant, whereas the amount of reduction of the convex curved roll 4 decreases, and the distance x = 0. In the vicinity of 3, the surface reduction amount becomes the same, and the surface disk reduction amount of the convex disk roll 5 becomes larger from the outside up to x = 0.4. Each of the convex disk roll 5 and the convex curved roll 4 achieves a surface reduction amount according to the outer shape of each roll.

  FIG. 5 shows a widthwise distribution of normalized equivalent plastic strain at the center of the thickness. Surprisingly, the value of the normalized equivalent plastic strain is larger in the convex curved roll 4 than in the convex disc roll 5 over the entire region in the width direction. As for the width center position 13, since the convex curl roll 4 has a larger surface reduction amount, it is expected that the normalized equivalent plastic strain at the center of the thickness also has a large value. On the other hand, in the region exceeding the distance x = 0.3 from the width center position 13, the convex disc roll 5 is larger in the surface reduction amount. Although the roll 5 is expected to be larger, contrary to the prediction in the deformation analysis by the finite element method, the standardized equivalent plastic strain in the center of the thickness is larger in the convex curved roll 4 up to the end in the width direction. Was the result.

  From the results of the above-mentioned deformation analysis by the finite element method, in realizing continuous reduction of center segregation and center porosity by reduction using the convex roll 3 in continuous casting, if the same reduction force is used, a convex roll is used as a reduction roll. It was suggested that the improvement effect would be greater when the convex curve roll 4 was used than when the mold disk roll 5 was used.

Here, the definition of the average reduction amount when the slab is reduced using the flat roll and various convex rolls will be described with reference to FIG. FIG. 7A shows the case of the convex curved roll 4, FIG. 7B shows the case of the convex disk roll 5, and FIG. 7C shows the case of the flat roll 6. In each case, the hatched portion is the pressing portion 25, and the area of the pressing portion 25 is S. In the reduction by the flat roll 6 shown in FIG. 7C, the entire width of the slab 10 is reduced, and the average reduction amount d is the reduction amount as illustrated. Regarding the convex curved roll 4 in FIG. 7A and the convex disk roll 5 in FIG. 7B, the pressing portion 25 is only a part of the width W of the slab 10, and the width of the pressing portion is w, and The rolling reduction d is d = S / w (3)
Can be calculated as

Then, in actual continuous casting, a comparison was made of the effect of reducing the center porosity of the cast piece 10 when each of the convex disc roll 5 and the convex curved roll 4 was used as the pressing roll 1. The width W of the slab to be cast is 530 mm and the thickness is 400 mm. The convex disc roll 5 has a horizontal portion 20 having a width of 0.4 × W at the center of the width as a pressing roll, and a 17 ° inclined portion 21 is provided on both sides of the horizontal portion 20. In the convex curved roll 4, the roll outer peripheral shape 11 in a cross section passing through the roll rotation shaft 12 is an arc shape 18 having an arc radius R ₁ of 0.8 × W. Each of the convex rolls 3 has a roll radius r _{C at the} width center position 13 of 400 mm. Further, in each of the convex rolls, the roll radius r _{F at} the flat portions on both sides of the width is 0.65 × W. In both cases, a flat roll is used as the roll 2 on the F side of the roll pair.

  During continuous casting, a reduction force of 54 ton F was applied to the reduction roll in the high solid phase ratio region in which the center solid phase ratio was 0.8 to 1.0 to reduce the slab. The surface shape of the cast slab and the occurrence of center porosity at the center of the slab thickness were evaluated.

  On the upper surface side of the slab, a dent caused by the convex portion of the convex roll was formed. Each of the concave shapes had a shape similar to the outer shape of the convex roll. When the average rolling reduction d was calculated, d = 3.0 mm for the convex disc roll and d = 5.6 mm for the convex curve roll.

  The center porosity of the slab was evaluated using the porosity area ratio calculated by color check of the slab cross section as an index. As a result, the porosity area ratio of the convex disk roll was 3%, and the porosity area ratio of the convex curve crawl was 0.3%. The effect of improving the center porosity by using the convex curved roll is apparent.

  As described above, when the slab is reduced by the rolling roll during continuous casting, by using the convex curved roll 4 of the present invention as the rolling roll, compared with the case of using the convex disc roll 5 at the same rolling force. It was found that the center porosity improvement effect was excellent. In addition, when the center porosity improvement effect was made the same, it was also clarified that the convex curved roll 4 can achieve the same effect with a smaller rolling force than the convex disc roll.

  Further, using the same continuous casting apparatus as used for confirming the effect of improving the center porosity, the effect of the reduction in the high solid fraction region on the center segregation of the slab was evaluated. The same convex disc roll 5 and convex curve roll 4 as described above were used as the rolling roll in the high solid fraction region. In the high solid phase ratio region where the center solid phase ratio was 0.8 to 1.0, comparison was made between the case where one pair of pressing roll pairs was provided and the case where two pairs were provided. The center segregation was evaluated based on the maximum Mn segregation degree at the center of the slab thickness. The method of evaluating the maximum Mn segregation degree is the same as the test using the laboratory continuous casting apparatus. As a result, the maximum Mn segregation degree in the case where one pair of reduction rolls using the convex disk roll 5 was provided was 1.20, whereas the maximum reduction Mn segregation degree using the convex curve roll 4 was one pair. The maximum Mn segregation degree when provided was reduced to 1.15, and the maximum Mn segregation degree when two pairs of reduction rolls using the convex curved roll 4 were provided was reduced to 1.10.

  Next, preferred requirements that the convex curved roll 4 which is the rolling roll of the present invention should have are described below in the order of the first embodiment and the second embodiment.

The first embodiment of the present invention will be described with reference to FIGS. In the pressing roll 1, a roll outer peripheral shape 11 in a cross section passing through the roll rotating shaft 12 has the following shape. First, the roll outer peripheral shape 11 has a convex shape that protrudes outward in a region including the width direction center position (width center position 13) of the slab. The outside is a direction in which the outer periphery of the roll moves away from the roll rotation shaft 12. By configuring such a shape, the roll radius r _C is maximum at the width center position 13, and the reduction amount of the slab surface when the slab 10 is reduced is maximized at the width center position 13. Next, a range of a total length of 0.80 × W on both sides in the roll width direction from the width center position 13 is defined as a “convex shape defining range 14”. When the slab is rolled down using the convex roll 3, the width of the slab 10 is characterized by a large deformation resistance, so that the slab 10 is not rolled down. If the slab 10 is rolled down in the convex shape defining range 14 or a width smaller than this, the rolling force required for rolling down can be suppressed while securing a necessary rolling reduction. Therefore, if the convex shape of the rolling roll 1 is determined within the convex shape defining range 14, the preferable rolling of the present invention can be performed. The convex shape within the convex shape defining range 14 is a convex shape having no overhanging corner portion on the outside. The term “outwardly projecting” means outwardly projecting, that is, projecting in a direction away from the roll rotation shaft 12. Further, it is preferable that the thickness of the cast piece to be cast is t (mm), and the roll radius r _{C at the} width center position is 0.005 × t or more larger than the rolling roll radius r _{E at} both ends of the convex shape defining range 14. Accordingly, when the slab is reduced by the reduction roll, if the entire convex shape defining range 14 of the reduction roll is configured to reduce the slab, the reduction amount of the slab at the width center position is set to 0.005 × t or more. can do.

Most concise a manner effective shape of the convex shape of the raised shape determining range 14, as shown in FIG. 2, it can be an arc shape 18 having a single arc radius R _1. At this time, the roll outer peripheral shape 11 in the convex shape defining range 14 forms an arcuate shape in which the length of the convex shape defining range 14 is a chord 31. The length of the convex shape defining range 14 (the length of the chord 31) is s, the radius of the bow is R, and the height of the arc 32 of the bow (the roll radius r _{E at} both ends of the convex defining range and the roll radius at the width center position). The following relationship holds when h is the difference from r _C ). The central angle of the bow is 2θ.
h = R (1-cos θ) (4)
s = 2R · sin θ (5)
From these equations, the following equations are derived.
cos θ = (s ² −4h ² ) / (s ² + 4h ² ) (6)
Therefore, first, target s and h are determined, θ is determined by substituting s and h into the above equation (6), and further, R is determined by substituting θ into the equation (4) or (5). be able to. For example, when s = 150 mm and h = 9 mm are targeted, R = 316 mm can be derived by substituting into the above equation.

The convex shape of the raised shape determining range 14, the other arc 18 having the above-mentioned single arc radius R _1, parabolic, elliptical, hyperbolic, where shape radius is smoothly connect different arcs and the like Can be arbitrarily selected. In a curved shape having no corners and constituting a convex shape, it is preferable that the radius of curvature of the curve is at least 1 × h or more. Thereby, the effect of the present invention due to the convex shape being a curve can be sufficiently exhibited. The minimum radius of curvature of the curve is the same in a second embodiment described later.

  The outer peripheral shape 11 of the roll on the width direction end side outside the convex shape defining range 14 of the pressing roll is not particularly defined. Preferably, the outer peripheral shape is a straight line or a curved line having no corners. When the roll shape at both ends in the width direction is the cylindrical shape 22, the outer circumferential shape of the roll is a combination of a smooth straight line and a curve from the convex shape defining range 14 to the position of the cylindrical shape 22 and has a corner. It is preferable that the shape is not formed. Immediately before connecting to the cylindrical shape 22, it is preferable to make the curve concave outward.

As shown in FIG. 2, the simplest and most effective roll outer peripheral shape of the pressing roll is a single convex shape defined range 14 and a predetermined range (radius R ₁ range 23) on both sides outside the specified range. An arc shape 18 having an arc radius R ₁ and a radius R ₂ range 24 on both sides thereof are smoothly connected to an arc shape 19 having a single arc radius R ₂ and a concave shape on the outside. A shape that smoothly connects to the straight line of the cylindrical shape 22 of the flat roll can be adopted. Since there is no corner at any part of the outer peripheral shape of the roll, the roll reduction amount by the reduction roll increases, and the reduction range by the roll in the width direction exceeds the convex defined range 14, and the cylindrical shape of the flat roll Even in the case where the rolling is performed until the 22 parts come into contact with the slab 10, any portion of the slab surface after the rolling can be a smooth surface with no corners formed. As a result, in the subsequent hot rolling following the continuous casting, it is possible to prevent the occurrence of rolling flaws due to the concave shape of the slab generated by rolling with the convex roll.

  In a reduction control device that performs reduction control with a reduction roll, if a device capable of controlling a reduction amount to a target displacement amount (a device capable of controlling a reduction displacement) is used, the reduction amount is equal to or less than the above h of the reduction roll. Can be controlled. As a result, the roll surface that comes into contact with the slab at the time of rolling down can be kept within the specified convex shape range 14. Within the convex shape defined range, since it is a curved shape having no corners, no steep dent is formed on the slab surface after rolling down, and it does not cause defects during hot rolling in the subsequent step. .

  On the other hand, in the case of using a device that cannot perform the rolling displacement control as the rolling reduction device, it is preferable to adopt the simplest and most effective roll outer peripheral shape at a position outside the convex shape defining range 14. The roll outer peripheral shape of the rolling roll is a smooth shape having no corners in the convex shape defining range 14 and any portions on both sides extending to the cylindrical shape 22 portion. Therefore, even if the rolling is performed such that the flat roll portions at both ends of the width come into contact with the slab due to a large rolling force, a sharp shape is formed on the slab surface after the rolling to cause a flaw. Never.

  A second embodiment of the present invention will be described with reference to FIG. 6 as a requirement that the convex curved roll 4 that is the pressing roll of the present invention should have. In the second embodiment, the rolling roll has the following shape in the roll outer peripheral shape in a cross section including the roll rotation axis. That is, in the first embodiment, the convex shape within the convex shape defining range is defined as a curved shape that is outwardly convex and has no corners. On the other hand, in the second embodiment, the convex shape in the convex shape defining range is a combination of an outwardly convex curve 16 and a straight line 17 having a length of 0.25 × W or less, and Defined as having no shape. Hereinafter, the grounds thus determined will be described.

The effectiveness of the second embodiment was also confirmed by deformation analysis using the finite element method. As shown in FIG. 6A, as the roll outer peripheral shape 11, as shown in FIG. 6A, for the combination of the convex curve 16 and the straight line 17, the convex curve is an arc shape 18 having an arc radius R ₁ of 0.8 × W, and a straight line 17. A straight portion having an arbitrary length was provided in parallel with the roll axis around the width center position 13, and the arc shape 18 and the straight line 17 were smoothly connected. After setting the length of the straight line 17 variously, a rolling force was applied with a rolling force of 100 tons F, and deformation analysis was performed by the finite element method. As a result of the deformation analysis, the plastic strain (standardized equivalent plastic strain) at the center of the thickness of the slab was analyzed. FIG. 6B shows the result. The length D of the straight line 17 is represented by D / W in the figure. As D / W increases, that is, as the length D of the straight line 17 increases, the normalized equivalent plastic strain at the center of the thickness decreases 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, a value of the normalized equivalent plastic strain better than that of the convex disc roll 5 can be realized. Therefore, the second embodiment is also determined as a preferable shape of the pressing roll of the present invention.

  The mechanism by which the convex curved roll 4 can improve the center porosity satisfactorily with the same rolling force as compared with the conventional convex disc roll 5 will be examined. The porosity reduction by the post-solidification reduction is due to the fact that the porosity generation region is strained by the reduction and the porosity is pressed. In principle, the amount of strain applied increases as the amount of reduction increases. In particular, since the distortion of the surface portion directly reflects the amount of indentation in the width direction, when the convex curved roll 4 and the conventional convex disk roll 5 are compared, when viewed in the width direction, the convex disk roll 5 There are places where the amount of strain applied on the slab surface exceeds the amount of strain applied. On the other hand, as the strain permeates the center of the thickness, the strain also diffuses in the width direction. For this reason, the amount of distortion in the center in the thickness direction is an analysis result that the convex curve roll is superior in the entire width because the convex curve roll capable of obtaining a large amount of reduction in the curved part is superior. Conceivable.

  In the rolling method of the continuous casting of the present invention, using a convex curved roll as a rolling roll, during continuous casting, the central solid phase ratio of the slab is in the high solid fraction region of 0.8 to 1.0, at least This is a continuous casting rolling method in which rolling is performed by two rolling roll pairs. For at least one of the rolls used in each roll pair, a convex curved roll is used.

  In determining the rolling position during continuous casting, the position where the center solid phase ratio is 0.8, for each of the fully solidified positions, combine the temperature measurement of the slab surface during continuous casting and the heat transfer solidification calculation of the slab Can be determined by:

  The number of the reduction roll pairs 40 for performing the reduction in the high solid fraction region 61 having the center solid fraction of 0.8 to 1.0 is at least two. It is difficult to exhibit the effects of the present invention when only one pair of the reduction rolls 40 is used. More preferably, the number of the roll pairs 40 in the region is three or more (see FIG. 9).

  It is preferable that the reduction gradient of the reduction performed in the region where the center solid phase ratio is 0.8 to 1.0 (high solid phase ratio region 61) is 4 mm / min or more. If it is 4 mm / min or more, center segregation and center porosity can be favorably reduced. Further, it is preferable that the rolling gradient in the high solid fraction region 61 be 10 mm / min or less. This is because it has been confirmed by an experimental apparatus that cracks do not occur if the speed is 10 mm / min or less.

  Preferred conditions for reducing the slab in the region where the center solid fraction is 0.8 or less and the solid fraction is low will be described (see FIG. 10). As conventionally known, it is known that the center segregation of a slab is reduced by reducing the slab in view of solidification shrinkage in a region where the solid fraction is low. In the range of the central solid phase ratio in the region where the solid phase ratio is low, the light reduction amount for compensating for solidification shrinkage is about 0.8 to 1.2 mm / min. Also in the present invention, in the region where the center solid phase ratio is from 0.3 to (upper limit) 0.75 (low solid phase ratio region 62), the reduction gradient is 0.8 to 1.2 mm / min. Thereby, the center segregation of the slab can be kept low. The lower limit of the central solid phase ratio was determined from the fact that light reduction is a general lower limit of the effective solid phase ratio range. On the other hand, when the center solid phase ratio exceeds 0.75, the upper limit of the rolling gradient is relaxed. Therefore, the upper limit center solid phase ratio in the low solid phase ratio region is set to 0.75. The range of the reduction gradient in the low solid phase ratio region conforms to a general light reduction appropriate gradient which is regarded as coagulation shrinkage.

  In a region between the low solid fraction ratio region 62 and the high solid fraction ratio region 61 (a region where the central solid fraction is between 0.75 and 0.8, hereinafter referred to as a “transition solid fraction ratio region 63”), a reduction occurs. The reduction may be performed with a gradient of 0.8 mm / min or more (see FIG. 10). The upper limit of the rolling gradient of the transition solid phase ratio region 63 is preferably 10 mm / min or less, like the high solid phase ratio region 61. That is, in the transition solid phase fraction region 63, the rolling gradient may be the same as that of the low solid fraction region 62, or may be the same as that of the high solid fraction region 61. It may be a transition region that sequentially transitions from (light pressure reduction) to a reduction gradient (high pressure) in the high solid fraction ratio region 61.

  Regarding the diameter of the reduction roll in the high solid fraction ratio region where the central solid phase ratio is 0.8 to 1.0, when a convex curved roll is used as the reduction roll, the roll diameter of the thickest part of the roll is 380 mm or more. If so, it has been confirmed that no internal cracks occur.

  The present invention was applied to a curved bloom continuous casting in which a bloom having a width of 530 mm and a thickness of 400 mm was cast. The continuous casting apparatus used corresponds to a general continuous casting apparatus having a light reduction function. The casting speed was 1.2 m / min, and it was confirmed that the distance from the center solid phase ratio of 0.8 to the solidification completion position was 1.2 m in casting length.

Regarding the roll arrangement of the continuous casting apparatus, as shown in FIG. 10, the slab is supported by a normal support roll 7 (280 mm in diameter) on the side of the upstream side 52 where the solid phase ratio is low. Light reduction can be performed by sequentially reducing the interval.
Further, in a region where the solid phase ratio of the downstream side 53 is 0.8-1.0 (high solid phase ratio region 61), the reduction roll pair using the reduction roll 2 on the F side and the reduction roll 1 on the L side is used. 40 is arranged and rolling is performed. In the pressing roll pair 40, the pressing rolls 2 on the F-face side are all flat rolls. On the other hand, as the pressing roll 1 on the L surface side of the pressing roll pair, a convex disc roll and a flat roll were employed in addition to the convex curved roll of the present invention.

  FIG. 9 shows the arrangement of the pressing roll pairs 40 in the high solid fraction region. 9 (A), one pair of reduction rolls 40, (B) two pairs of reduction rolls 40, and (C) three pairs of reduction rolls 40 are arranged. When the number of the roll pairs 40 is three, as shown in FIG. 9C, a first roll pair 41, a second roll pair 42, and a third roll pair 43 are arranged from the upstream side 52. Rolling-down conditions were set for the region where the central solid phase ratio was 0.3 to 0.8 and the region where the central solid phase ratio was 0.8 to 1.0.

Further, in any case where the number of the reduction roll pairs is one to three, the support roll 7U immediately before the first upstream reduction roll pair 41 has a slab center solid phase ratio of 0.8 or less, and The roll pair 40 is arranged in a range where the slab center solid phase ratio is 0.8 or more and the solidification is completed (the slab center solid phase ratio is 1.0). The “cumulative average reduction amount (mm)” shown in Table 1 is based on the thickness of the slab (the distance between the upper and lower rolls of the support roll 7U) at the support roll 7U exit side immediately before the high solid phase ratio region. For each pair of rolling rolls in the region, the average rolling reduction d is calculated based on the above equation (3), and this is defined as the cumulative average rolling reduction. When a flat roll is used as the reduction roll, the difference between the roll interval of the support roll 7U and the roll interval of the pair of reduction rolls is the cumulative average reduction amount. Further, the rolling gradient (mm / min) shown in Table 1 is a value obtained by dividing the difference between the average rolling amount on the entrance side and the exit side on the high solid fraction area by the high solid fraction passing time. Specifically, the high average reduction rate of the solid phase ratio region entry side is zero, leaving the average reduction rate in side accumulated average rolling reduction d _T of the final reduction roll pair corresponds. The passage time in the high solid fraction region is a value obtained by dividing the length L of the high solid fraction region (1.2 m in the embodiment) by the casting speed (1.2 m / min in the embodiment). In the example, as a result, the reduction gradient (mm / min) is a numerical value equal to the cumulative average reduction amount (mm) of the final reduction roll pair.

  As shown in FIG. 3, the convex disk roll 5 (indicated as “disk” in Table 1) has a horizontal portion 20 having a length of 200 mm at a width center position 13 and an angle 17 through a corner 15 on both sides thereof. ° inclined portion 21. The roll radius (shown in Table 1) of the horizontal portion 20 is larger by 20 mm than the roll radius of the flat roll portion at both ends of the width.

As shown in FIG. 2, as the convex curved roll 4 (shown as “convex curve” in Table 1), a convex shape defining range 14 (total length 0.80 × from both sides in the roll width direction from the width center position). (Range of W), a roll having a radius of 430 mm and a constant radius of 430 mm, and a roll radius r _{C at the} width center position 13 being 60 mm larger than a rolling roll radius r _{E at} both ends of the convex shape defining range 14 was used. The roll radius r _{C at the} width center position 13 is as shown in Table 1. 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 thereafter, an arc shape 19 (radius R ₂ ) that is outwardly concave with an arc radius R ₂ = 100 mm. 24), and finally to a flat roll having a cylindrical shape 22.

  In the region where the central solid phase ratio is up to 0.8, a light reduction gradient of 0.8 to 1.2 mm / min which is generally used is adopted as the light reduction condition. In this central solid phase ratio region, coagulation shrinkage can be compensated by adopting 0.8 to 1.2 mm / min. The light pressure reduction in this region uses the support roll 7, which is generally used as described above, and the roll diameter is 280 mm.

  With respect to the rolling conditions of the region having a central solid fraction of 0.8 or more and 1.0 or less (high solid fraction region 61), the rolling gradient that is reduced by the rolling roll pair is variously changed, and the rolling rolls (2, 3) are reduced. As shown in Table 1, the roll diameter was in the order of 280 mm, 380 mm, 400 mm, and 450 mm. The number of the reduction roll pairs 40 was investigated at three levels: one pair, two pairs, and three pairs.

  Regarding the slab quality, the maximum Mn segregation degree (center segregation) at the center of the slab thickness, the center porosity, and the evaluation of surface cracks and internal cracks were evaluated. The method of evaluating the maximum Mn segregation degree is the same as the test using the laboratory continuous casting apparatus.

  As described above, the center porosity of the slab was evaluated using the porosity area ratio calculated by color check of the slab section as an index.

  Regarding surface cracks, the inferior surface of the slab was ground and dyed penetration inspection (PT inspection) evaluated the presence or absence of surface cracks (lateral cracks). If there was no color development, "-" (none) was used. The internal cracks were evaluated based on the presence or absence of visible cracks by observing the etched portion at the center portion of the constant rolling portion and the cross section in the casting direction. If the total length of the visible internal cracks is 5 mm or more, it is determined that the internal crack is “present”. If the total length of the visible internal cracks is 5 mm or less, the internal crack is “somewhat”. Otherwise, “−” (none) And

  Table 1 shows the results of the examples. The number of the reduction roll pairs is as follows. Nos. 1 to 8, 17, 20, and 21 are three pairs. Nos. 9 to 16, 18 are two pairs. 19, 22 to 29 are one pair. No. of the present invention. 4 and No. 8, no. 12 and No. 16, No. 25 and no. 29 is the same embodiment.

  No. of the present invention. 4 (No. 8), No. 12 (No. 16), No. As for the rolls 20 and 21, the number of roll pairs is 2 or 3, and a convex curved roll is used as the roll. Good results are obtained in both the maximum Mn segregation degree and the porosity area ratio. Was completed. No surface cracks or internal cracks were observed.

  No. Nos. 1 to 4 (3 pairs of reduction rolls), Nos. 9 to 12 (reducing roll pair: 2 pairs); In each of 22 to 25 (a pair of rolling rolls), when a flat roll, a convex disc roll, and a convex curved roll are used as the rolling rolls, the rolling load of the rolling rolls is the same, and the rolling is performed. In any case, the cumulative average reduction amount and reduction gradient realize the largest reduction amount and reduction gradient when two or more pairs of convex curved rolls are used, and as a result, both the maximum Mn segregation degree and the porosity area ratio are favorable. The result is.

  No. Nos. 5 to 8 (3 pairs of reduction rolls), No. 13-16 (2 pairs of reduction rolls), In the case of using flat rolls, convex disc rolls, and convex curved rolls as the reduction rolls, the reduction loads are selected so that the cumulative average reduction amounts of the respective reduction rolls become the same. Then, the reduction is performed. It can be seen that the use of a convex curved roll as the rolling roll can reduce the rolling load in realizing the same rolling amount. Further, in spite of the fact that the cumulative average reduction amount and the reduction gradient were the same, in the examples of the present invention using two or more pairs of convex curved rolls, both the maximum Mn segregation degree and the porosity area ratio were favorable. In Comparative Example No. using a flat roll of 280 mmφ. For 5, 13, and 26, the occurrence of internal cracks was observed. Comparative Example No. 1 using a convex disc roll. For 7, 15, and 28, occurrence of surface cracks was observed.

  The slabs of the present invention and the conventional example were subjected to hot rolling as a general hot rolling process. As a result of comparing the product defect rate due to the surface shape of the cast piece, the product defect rate was about 5% in the conventional cast piece, but the product defective rate was reduced as a result of using the cast piece of the present invention. Reduced to 0.5% or less.

DESCRIPTION OF SYMBOLS 1 Reduction roll 2 Reduction roll 3 Convex roll 4 Convex curve roll 5 Convex disk roll 6 Flat roll 7 Support roll 10 Cast piece 11 Roll outer peripheral shape 12 Roll rotation shaft 13 Width center position 14 Convex shape prescribed range 15 Corner 16 Curve 17 Straight line 18 Arc shape 19 Arc shape 20 Horizontal portion 21 Inclined portion 22 Cylindrical shape 23 Radius R ₁ range 24 Radius R ₂ range 25 Compressed lower part 31 String 32 Arc 40 Compressed roll pair 41 First reduced roll pair 42 Second reduced roll Pair 43 Third reduction roll pair 51 Casting direction 52 Upstream side 53 Downstream side 61 High solid phase ratio region 62 Low solid phase ratio region 63 Transition solid phase ratio region W Slab width r _C Roll reduction radius at center of width _C r _F width a reduction roll radius r _E convex shape defining a range across the pressure roll radius R ₁ arc radius R ₂ arc radius h arcuate height s bow chord of the arc of the edge length θ arcuate Half of central angle R Radius of arcuate shape w Width of reduction section S Reduction section area d Average reduction amount d _T Cumulative average reduction amount L Length of high solid fraction area

Claims (5)

A method of rolling down a slab during continuous casting,
In a region where the central solid phase ratio is from 0.8 to the completion of solidification (hereinafter referred to as a “high solid phase ratio region”), two or more pairs of reduction rolls are continuously arranged to reduce the slab,
With respect to at least one of the rolls forming the roll pair, the outer peripheral shape of the roll in a cross section including the roll rotation axis is a region including the center position in the width direction of the slab (hereinafter referred to as “width center position”). A continuous casting with a convex shape having no protruding corners on the outside.   The width of the slab to be cast is W (mm), the thickness of the slab is t (mm), and the convex shape has a total length of 0.80 × W on both sides in the roll width direction from the width center position (hereinafter, referred to as the width). In the “convex shape defining range”), a curved shape that is outwardly convex and has no corners, or a combination of an outwardly convex curve and a straight line having a length of 0.25 × W or less The shape having no corners, wherein the roll radius at the center of the width is 0.005 × t or more larger than the roll radius at both ends of the convex shape defining range. The method for rolling down the continuous casting as described above.   The rolling reduction method for continuous casting according to claim 1 or 2, wherein a reduction gradient in the high solid fraction region is 4.0 mm / min or more.   4. The method according to claim 1, wherein three or more pairs of the reduction rolls are continuously arranged to reduce the casting slab. 5.   In a region where the central solid phase ratio is from 0.3 to 0.75 (hereinafter referred to as a “low solid phase ratio region”), the rolling reduction is performed at a rolling gradient of 0.8 to 1.2 mm / min. 5. The continuous casting according to claim 1, wherein a reduction gradient is 0.8 mm / min or more in a region between the rate region and the high solid phase ratio region. 6. Reduction method.

Images