JP2004148343A - Continuous casting method by vertical bending type continuous casting apparatus and cast slab manufactured thereby - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method capable of executing a stable casting operation by a vertical bending type continuous casting apparatus and a cast slab with high surface quality. <P>SOLUTION: The cast slab is cast so that a cooling water volume Q1(L/min) rate per unit of time in the upper surface side of the cast slab from a vertical section on and after a casting mold exit to a curving complete section by the vertical bending type continuous casting apparatus is more than the cooling water volume Q2(L/min) in the lower surface side of the cast slab or so that a cooling water volume density Qd1(L/m2/min) in the upper surface side of the cast slab is more than the cooling water volume density Qd2(L/m2/min) in the lower surface side of the cast slab. In the continuous casting method, the (2)Qd1, Qd2 and the cooling water volume density Qd3 in the short side surface of the cast slab satisfy 0.8≤ Qd1/Qd2≤2.5, and 0.3≤Qd3/ä(Qd1+ Qd2)/2}≤2.5. Further, the continuous cast slab is set ≤0.2% of an average cast slab width between the cast slab width in the upper surface side and the cast slab width in the lower surface side. <P>COPYRIGHT: (C)2004,JPO

Description

【００３８】
表２に、試験条件として、鋼種、鋳片サイズ、鋳造速度および二次冷却条件を、また試験結果として、鋳片形状をまとめて示した。なお、表２中の鋳片形状の測定結果は、各試験番号で鋳造された鋳片について、その長手方向の５〜１０箇所についての測定結果を平均した値である。
【００３９】
【表２】

【００４０】
炭素含有量：０．０４〜０．６０質量％の普通鋼を用い、鋳片厚さ：２１０〜２３０ｍｍ、鋳片幅：８００〜１８５０ｍｍの鋳片を鋳造速度：０．８〜１．６ｍ／ｍｉｎにて鋳造し、その結果を評価した。
【００４１】
試験番号１〜５は、本発明例についての試験であり、試験番号６〜１０は、本発明で規定する範囲を外れる比較例の試験である。
【００４２】
試験番号１〜５は、少なくとも第１発明で規定する条件を満足する冷却条件下にて鋳造されており、その結果、鋳造された鋳片の横断面形状は、逆台形への変形が抑制されており、第３発明で規定する条件を満足している。
【００４３】
特に、第１発明で規定する条件および第２発明で規定する条件の双方を余裕をもって満足する試験番号２および４は、鋳片形状の逆台形への変形が極めて少なく、特に良好な横断面形状を呈した。
【００４４】
これに対して、試験番号６〜１０は、第１発明で規定する条件および第２発明で規定する条件のいずれをも満足しない冷却条件で鋳造されたものであり、その結果、鋳片の横断面形状は、いずれも逆台形への変形が進行しており、第３発明で規定する条件を外れている。
【００４５】
特に、表２中の＊Ｂ式で表される値が第２発明の（１）式で規定する範囲を外れ、しかも、＊Ｃ式の値が（２）式で規定される範囲の上限に位置する試験番号８は、鋳片の横断面形状の逆台形化が極度に進行し、極めて劣った形状となっている。
次に、本発明の連続鋳造方法による鋳片品質の改善効果を確認するため、試験番号４および９に示される条件にて、炭素含有量：０．５質量％の普通鋼を鋳造し、その鋳片から製造した熱延コイルのエッジ近傍におけるスジ模様疵の発生率を調査した。
【００４６】
図２は、コイルエッジ近傍のスジ疵発生率におよぼす本発明法実施の効果を示す図である。なお、スジ疵発生率は、鋳造スラブ数に対するスジ疵発生スラブ数の割合を百分率にて表示したものである。
【００４７】
本発明法を実施することにより、コイルエッジ近傍のスジ疵の発生率は約半減しており、本発明による鋳片品質改善効果が確認された。
【００４８】
【発明の効果】
本発明の方法によれば、鋳片の曲げによる変形を、変形による影響の小さい鋳片の長辺下面側に負担させることにより、曲げ変形にともなう表面品質の悪化やブレークアウトの問題を解消し、安定操業を達成できる。本発明の方法により鋳造された鋳片は、鋳片の横断面形状の逆台形化が抑制され、また、熱延コイルの段階においてもスジ疵を低減できる。
【図面の簡単な説明】
【図１】垂直曲げ型連続鋳造機の概略を模式的に示す図である。
【図２】コイルエッジ近傍のスジ疵発生率におよぼす本発明法の効果を示す図である。
【符号の説明】
１：タンディッシュ、
２：溶鋼、
３：浸漬ノズル、
４：鋳型、
５：垂直部、
６：曲げ部、
７：二次冷却帯、
８：鋳片上面側の冷却帯、
９：鋳片下面側の冷却帯
１０：凝固シェル、
１１：鋳片[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for cooling a slab in continuous casting, and a cast slab, and in particular, for appropriately dispersing distortion during bending deformation of a slab in casting using a vertical bending type continuous casting machine. The present invention relates to a method for cooling a slab and a slab cast by the method.
[0002]
[Prior art]
In continuous casting, as a method of improving the quality by controlling the amount of cooling water in the vertical portion or the bent portion, a method of improving the surface quality by avoiding the brittle temperature range of the slab by controlling the amount of water, and slab There is a method of improving quality by controlling the amount of cooling water in the width direction. Also, in the method of casting a round billet, by increasing the cooling water flow density of the lower half of the slab than the upper half of the slab, to prevent bending of the slab after drawing in the horizontal direction, There is a method for preventing the occurrence of flaws on the inner surface of the pipe after piercing and rolling.
[0003]
Among them, as a method of improving the surface quality by controlling the amount of cooling water to avoid the embrittlement temperature range, for example, there are methods disclosed in Patent Documents 1 to 6 below.
[0004]
Patent Literature 1 discloses that when a slab passes through an initial bending portion when a slab is continuously cast at a casting speed of 1.6 m / min or more using a vertical bending type continuous casting apparatus. , The surface temperature is set to a temperature range exceeding the brittle temperature range (700 to 900 ° C.) of the steel material, and when the slab passes through the straightening section on the downstream side, the surface temperature is set to the brittle temperature. A method for setting the temperature below is disclosed. The method disclosed herein is intended to enable continuous casting without causing surface cracks of the slab even at a high casting speed of 1.6 m / min or more.
[0005]
In Patent Document 2, the surface temperature at the center of the thickness of the short side of the slab becomes 1000 ° C. or less after the bending position of the continuous casting machine, and the surface temperature of the corner portion becomes 750 at the bending position and the correcting position of the continuous casting machine. A method of cooling to a temperature of not less than ° C. is disclosed. Further, the document describes an example of preventing a corner crack of a slab by using a vertical bending type continuous casting machine.
Patent Document 3 discloses that when casting a slab having a width to thickness ratio of 1.8 to 10.5, cooling is started immediately after the mold exit, and the casting direction is reduced to at least 1.5 m in the casting direction. Then, cooling is performed under the condition that the specific water volume of the secondary cooling is 0.4 to 0.75 liter / kg-steel, the slab surface temperature is set to the A3 transformation point or lower, and then the slab is reheated to reduce the slab surface temperature to 850. There is disclosed a method of correcting a bend at a temperature of not less than ° C. According to the method disclosed herein, even if the casting speed or the slab size changes, the occurrence of lateral cracks and lateral cracks on the slab surface can be prevented, and a low-alloy steel slab with good surface quality can be obtained. It is supposed to be.
Also, in Patent Document 4, when casting a slab having a rectangular cross section and a thickness of 80 to 120 mm using a vertical bending type continuous casting machine, the casting is performed from the mold exit side to 3 m in the casting direction. The slab is cooled under the condition of using 50 to 60% of the total water amount used for the secondary cooling of the slab, and the water density per unit area of the long side on both sides of the slab is 300 to 500 liter / m ² min. A method of cooling under the following conditions is disclosed, and it is described that during high-speed casting at a casting speed of 3 to 5 m / min, fluctuations in the molten metal level and the occurrence of vertical cracks on the surface of the slab can be prevented.
[0006]
Patent Document 5 discloses that when casting using a vertical bending type continuous casting machine, the length of the vertical portion below the mold exit side is H, the length from the last guide roll of the vertical portion to the lower end of the vertical portion. Is L, L = 0.5 to 2.0 m, L / H = 0.5 to 0.9, and the amount of secondary cooling water per unit area of the short side surface of the slab at the vertical portion is 20. A continuous casting method of a thin slab cast under a condition of 1 liter / m ² or less is disclosed, and it is described that occurrence of vertical crack breakout at the time of high speed casting can be prevented.
Further, in Patent Document 6, when a slab is manufactured by a vertical bending type or curved type continuous casting machine, primary cooling by a mold is finished at a solidified shell thickness of 10 to 15 mm to perform secondary cooling. Starting, the surface temperature of the entire slab is once reduced to a range of 600 ° C. or more and the Ar3 transformation point or less within 2 minutes at most after leaving the mold, and then the slab surface temperature in the bent portion and the straightening portion is reduced to 850 ° C. A method of performing secondary cooling to improve the surface quality of a slab as described below is disclosed.
[0007]
In the method disclosed in the above-mentioned document, the surface cracks at the bending portion and the straightening portion where the stress is generated are avoided from the embrittlement temperature range so as to suppress the surface crack.
[0008]
However, no consideration is given to surface cracks caused by deformation of the slab due to the stress generated at the bent portion.
[Patent Document 1]
JP-A-6-246411 (Claims, FIG. 6)
[Patent Document 2]
JP-A-10-43850 (Claims, paragraphs [0012] to [0015])
[Patent Document 3]
JP 2001-138019 A (Claims, paragraphs [0035] to [0042])
[Patent Document 4]
JP 2001-191158 A (claims, paragraphs [0011] and [0044])
[Patent Document 5]
JP 2001-96346 A (claims, paragraph [0042])
[Patent Document 6]
JP-A-9-225607 (Claims, paragraph [0063])
[0009]
[Problems to be solved by the invention]
When the slab enters the bent part from the vertical part, the short side undergoes shear deformation in which the lower side is shifted upward with respect to the upper side, the upper side of the long side undergoes tensile deformation in the width direction, and the lower side of the long side is the casting longitudinal direction. Subject to tensile deformation. Among these deformations, the shear deformation on the short side generates a shear-like vertical crack, and the tensile deformation on the long side in the width direction generates an opening due to a vertical crack. To reach a breakout. On the other hand, adverse effects due to tensile deformation in the casting longitudinal direction on the lower side of the long side are relatively small.
[0010]
Here, an object of the present invention is to solve the problem associated with bending deformation and achieve stable operation by concentrating the deformation of the slab due to bending on the lower surface side of the long side of the slab that is relatively less affected by the deformation. An object of the present invention is to provide a continuous casting method that can be performed and a slab having excellent surface quality produced by the method.
[0011]
[Means for Solving the Problems]
The present inventor studied the above-mentioned conventional problems in order to achieve the above-mentioned problems, and obtained the following findings (a) and (b).
(A) In continuous casting using a vertical bending type continuous casting machine, the cooling of the lower side of the long side of the slab from the vertical portion to the bent portion is weakened compared to the cooling of the upper side of the long side, and the lower side of the long side solidifies. By reducing the strength of the shell and concentrating the deformation due to bending, breakout and surface quality defects of the slab due to bending deformation are prevented, and the cross-sectional shape of the slab is also improved.
(B) The short side of the slab with respect to the average value of the cooling water amount density per unit area and unit time on the slab upper surface side and the unit area and cooling water amount density per unit time on the slab lower surface side from the vertical part to the bent part. By adjusting the ratio of the cooling water amount density per unit area and unit time to the predetermined range, breakout and surface quality defects due to bending deformation are prevented, and the cross-sectional shape of the slab is improved. Is done.
[0012]
The present invention has been completed based on the above findings, and the gist of the present invention resides in a continuous casting method shown in the following (1) and (2) and a continuous cast slab shown in (3).
[0013]
(1) In a secondary cooling zone after a mold outlet of a vertical bending type continuous casting machine having a vertical portion and a curved portion, a cooling water amount Q1 per unit time on a slab upper surface side from a vertical portion to a completed bending portion ( Liter / min) is larger than the cooling water amount Q2 (liter / min) per unit time on the slab lower surface side, or the cooling water amount density Qd1 (liter / m ²⁾ per unit area and unit time on the slab upper surface side. / Min) is larger than the cooling water volume density Qd2 (liter / m ² / min) per unit area and unit time on the lower surface side of the slab (hereinafter, referred to as “first invention”).
[0014]
(2) Cooling per unit area and unit time on the upper surface side of the slab from the vertical portion to the completed bending portion in the secondary cooling zone after the exit of the mold in the vertical bending type continuous casting machine having the vertical portion and the curved portion. The water density Qd1 (liter / m ² / min), the cooling water density Qd2 (liter / m ² / min) per unit area and unit time on the lower side of the slab and the unit area and unit time per unit time on one side of the short side of the slab A continuous casting method in which the cooling water volume density Qd3 (liter / m ² / min) satisfies the relationship represented by the following equations (1) and (2) (hereinafter, referred to as “second invention”).
[0015]
0.8 ≦ Qd1 / Qd2 ≦ 2.5 (1)
0.3 ≦ Qd3 / {(Qd1 + Qd2) / 2} ≦ 2.5 (2)
(3) A slab cast by a vertical bending type continuous casting machine in which the slab width W1 on the slab upper surface side and the slab width W2 on the slab lower surface side satisfy the relationship represented by the following expression (3). (Hereinafter, referred to as “third invention”).
(W1−W2) / {(W1 + W2) / 2} ≦ 0.002 (3)
In the present invention, the “secondary cooling zone” refers to a water cooling zone of the slab by spraying after the primary cooling in the mold until the slab is cut.
[0016]
`` The amount of cooling water from the vertical part to the completed bending part '' means when cooling water is supplied over the entire section from the vertical part after the mold outlet to the bending completed part, and from the vertical part after the mold outlet to the bending completed part. And the case where the cooling water is supplied in some sections of the section of the above.
“The cooling water amount density from the vertical portion to the completed bending portion” refers to a value obtained by dividing the above cooling water amount by the surface area of the slab surface in the section where cooling water is actually supplied.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
(A) Functions and Effects of the Present Invention As described above, the present invention reduces the strength of the solidified shell on the long side lower surface side by relatively weakening the cooling on the long side lower surface side from the vertical portion to the bent portion. It is intended to avoid the problems such as breakout of the slab and occurrence of surface quality defects due to the bending deformation by lowering the load and applying the deformation accompanying the bending to the lower side of the slab.
[0018]
As described in the first aspect of the present invention, the above-described operation and effect are as follows: the cooling water amount or the cooling water amount density of the secondary cooling zone from the vertical portion to the completed bending portion is set to the slab upper surface side rather than the slab lower surface side. Is achieved by increasing the
[0019]
Further, as described in the second invention, these actions and effects are as follows: the cooling water flow rate density Qd1 on the slab upper surface side and the cooling on the slab lower surface side of the secondary cooling zone from the vertical portion to the completed bending portion. The ratio of the cooling water amount density Qd2 on the slab short side to the average value of the cooling water amount density Qd1 on the slab upper surface side and the average value of the cooling water amount density Qd2 on the slab lower surface side, respectively, is ( This can also be achieved by controlling the cooling water density so as to satisfy the relationship represented by the expressions (1) and (2).
[0020]
In the secondary cooling zone from the vertical portion to the completed bending portion, the method of the first invention provides an average of a surface on the upper surface side of the slab after bending the slab and a surface on the lower surface side of the slab after bending. This is the simplest method for controlling the ratio of the cooling water amount (liter / min) or the ratio of the average cooling water amount density (liter / m ² / min).
[0021]
In the method of the first invention, in the section from the vertical portion to the completed bending portion, the upper surface side of the slab is better than the lower surface side of the slab, if the average cooling water amount or the average cooling water amount density is larger, In the section, the cooling water amount or the cooling water density on the slab lower surface side may be locally greater than the upper surface side, or the cooling water amount or the cooling water amount density may be the same on the slab lower surface side and the upper surface side. Absent.
[0022]
In the method of the second invention, the average cooling water density on the short side of the slab is controlled in addition to the average cooling water density on the upper and lower surfaces of the long side of the slab, and the solidification on the lower side of the long side is relatively controlled. This is a method in which the strength of the shell is reduced and strain due to deformation of the cast piece is borne on the lower side of the long side.
[0023]
The inventor reduces the difference (W1-W2) between the slab width W1 on the slab upper surface side and the slab width W2 on the slab lower surface side by implementing the method of the first invention or the second invention. I found an improvement effect.
[0024]
In the casting by the vertical bending type continuous casting machine, the slab width W1 on the slab upper surface side is larger than the slab width W2 on the slab lower surface side in bending deformation at the lower portion of the vertical portion. It is transformed into a “trapezoidal shape”, and its shape is maintained in the corrected cast slab.
[0025]
This inverted trapezoidal deformation not only reduces the accuracy of hitting the target specified width at the time of slab production, but also reduces the quality such as the sharp corner of the slab upper surface corner remaining as a streak pattern after rolling. Was a problem. The method of the first invention or the second invention has an effect of suppressing the deformation into the inverted trapezoid.
[0026]
A third invention is a slab obtained by carrying out the first invention or the second invention, in which the deformation into an inverted trapezoid is suppressed, that is, “the slab width W1 on the slab upper surface side and the slab width on the slab lower surface side. "A slab cast using a vertical bending type continuous casting machine, wherein the difference (W1-W2) from the slab width W2 is 0.2% or less of the average value of the two."
(B) Reasons for numerical limitation in the present invention In the first invention, the cooling water amount Q1 on the slab upper surface side is smaller than the cooling water amount Q2 on the lower surface side or the cooling water amount density Qd1 on the slab upper surface side is lower than the cooling water amount Qd1 on the lower surface side. If the cooling water flow rate density is lower than Qd2, the strength of the solidified shell on the lower side of the long side of the slab becomes higher than the strength of the solidified shell on the upper side of the long side, and the object of the invention cannot be achieved. The value of the ratio Q1 / Q2 of Q1 and Q2 or the ratio Qd1 / Qd2 of Qd1 and Qd2 is preferably adjusted to 1.2 to 2.0.
[0027]
In the second invention, when the value of Qd1 / Qd2 is less than 0.8, the strength of the solidified shell on the upper surface of the long side is clearly reduced as compared with the strength of the solidified shell on the lower surface of the long side. No matter how the ratio of the cooling water density Qd3 to (Qd1 + Qd2) / 2 or the value of Qd3 / {(Qd1 + Qd2) / 2} is adjusted, it becomes difficult to concentrate the strain accompanying the bending deformation on the lower surface of the long side.
[0028]
On the other hand, when the value of Qd1 / Qd2 exceeds 2.5 and becomes large, the cooling water density on the long side lower surface side becomes too low, and it becomes difficult to secure the minimum strength of the solidified shell on the lower surface side, and At the same time, the cooling water volume density on the upper surface side of the long side becomes excessive, and other problems such as occurrence of surface cracks due to overcooling may occur. A preferable range of the value of Qd1 / Qd2 is 1.0 to 2.2, and a more preferable range is 1.2 to 2.0.
[0029]
In the second aspect, when the value of Qd3 / {(Qd1 + Qd2) / 2} is less than 0.3, the strength of the solidified shell on the short side of the slab is insufficient, and the shear strain on the short side of the slab during bending deformation is reduced. Concentration increases the risk of causing defects such as cracks and breakouts. When the value of Qd3 / {(Qd1 + Qd2) / 2} is larger than 2.5, the distortion accompanying the bending deformation is hardly distributed to the short side, and therefore, in order to achieve the object of the present invention, It is necessary to make the control accuracy of the value of the ratio Qd1 / Qd2 of the cooling water density on the upper surface side and the lower surface side of the long side extremely high, and it is difficult to cope with various restrictions in actual operation and absorb variations in operating conditions. It becomes. A preferable range of the value of Qd3 / {(Qd1 + Qd2) / 2} is 0.4 to 1.5.
[0030]
In the practice of the present invention, it is necessary to adjust the strength of the secondary cooling so that the slab temperature in the bent portion or the straightening portion where the slab undergoes deformation is changed so as to avoid the embrittlement temperature inherent to the casting material. Needless to say.
[0031]
In the third invention, (W1−W2) is set to 0.2% or less of the average value of W1 and W2 as described above, because the cast slab obtained by implementing the first invention or the second invention is as described above. It is defined from the shape of the cross section.
[0032]
【Example】
In order to confirm the effects of the continuous casting method and the cast slab of the present invention, casting tests were performed with various test conditions changed.
[0033]
FIG. 1 is a view schematically showing an outline of a vertical bending type continuous casting machine used for the test. The molten steel 2 in the tundish 1 is supplied into a mold 4 through an immersion nozzle 3 attached to the bottom of the tundish. The molten steel 2 is primarily cooled by a mold to form a solidified shell 10, which is drawn down under the mold to form a slab 11 while gradually increasing its thickness. After passing through the vertical portion 5, the slab is bent at the bending portion 6, and then straightened to reach the horizontal portion.
[0034]
At this time, the slab is primarily cooled in the mold, and then secondarily cooled by a plurality of water cooling nozzles 12 in the secondary cooling zone 7 including the vertical portion and the bent portion. Reference numeral 8 denotes a cooling zone on the slab upper surface side, and reference numeral 9 denotes a cooling zone on the slab lower surface side.
[0035]
In the bent portion, the curved portion is divided into a plurality of steps from the section where the radius of curvature is infinite, that is, the vertical portion, and the curved portion is changed to a specified radius of curvature. Although not shown, the position at which the slab is bent into the section of the first finite radius of curvature from the infinite radius of curvature is called a bending start point, the radius of curvature reaches the specified radius of curvature, and the radius of curvature changes. The position where it disappears is called the bending completion point.
For the test, a vertical bending type slab continuous casting machine of a multipoint bending method having a vertical portion length of 3 m and a bending radius of 60 to 11.75 m was used.
[0036]
Table 1 shows the steel numbers used in the tests and their chemical compositions.
[0037]
[Table 1]

[0038]
Table 2 shows steel types, slab sizes, casting speeds and secondary cooling conditions as test conditions, and slab shapes as test results. In addition, the measurement result of the slab shape in Table 2 is a value obtained by averaging the measurement results of 5 to 10 locations in the longitudinal direction of the slab cast by each test number.
[0039]
[Table 2]

[0040]
Carbon content: 0.04 to 0.60 mass% of ordinary steel, slab thickness: 210 to 230 mm, slab width: 800 to 1850 mm Casting speed: 0.8 to 1.6 m / min and the result was evaluated.
[0041]
Test Nos. 1 to 5 are tests on the examples of the present invention, and Test Nos. 6 to 10 are tests on comparative examples out of the range defined by the present invention.
[0042]
Test Nos. 1 to 5 are cast under cooling conditions that satisfy at least the conditions specified in the first invention, and as a result, the cross-sectional shape of the cast slab is suppressed from being transformed into an inverted trapezoid. And satisfies the condition defined in the third invention.
[0043]
In particular, in Test Nos. 2 and 4, which satisfy both the conditions specified in the first invention and the conditions specified in the second invention with a margin, the deformation of the slab into an inverted trapezoid is extremely small, and a particularly good cross-sectional shape is obtained. Was presented.
[0044]
In contrast, Test Nos. 6 to 10 were cast under cooling conditions that did not satisfy any of the conditions specified in the first invention and the conditions specified in the second invention. In each of the surface shapes, the deformation into an inverted trapezoid has progressed, which is outside the conditions defined in the third invention.
[0045]
In particular, the value represented by the formula * B in Table 2 is out of the range defined by the formula (1) of the second invention, and the value of the formula * C is set to the upper limit of the range defined by the formula (2). In Test No. 8, which is located, the trapezoidal cross-sectional shape of the slab is extremely trapezoidal and extremely inferior.
Next, in order to confirm the effect of improving the slab quality by the continuous casting method of the present invention, ordinary steel having a carbon content of 0.5% by mass was cast under the conditions shown in Test Nos. 4 and 9. The occurrence rate of streak pattern flaws near the edge of the hot rolled coil manufactured from the slab was investigated.
[0046]
FIG. 2 is a diagram showing the effect of the method of the present invention on the rate of occurrence of streak flaws near the coil edge. The streak flaw occurrence rate is a percentage of the number of streak flaw occurrence slabs to the number of cast slabs.
[0047]
By performing the method of the present invention, the occurrence rate of streak flaws in the vicinity of the coil edge was reduced by about half, and the effect of improving the quality of the slab according to the present invention was confirmed.
[0048]
【The invention's effect】
According to the method of the present invention, the deformation of the slab due to bending is borne on the lower side of the long side of the slab which is less affected by the deformation, thereby solving the problem of surface quality deterioration and breakout due to bending deformation. , Can achieve stable operation. In the cast slab cast by the method of the present invention, the trapezoidal cross-sectional shape of the cast slab is suppressed from being inverted and the streak flaws can be reduced even at the stage of the hot-rolled coil.
[Brief description of the drawings]
FIG. 1 is a view schematically showing an outline of a vertical bending type continuous casting machine.
FIG. 2 is a diagram showing the effect of the method of the present invention on the rate of occurrence of streak flaws near the coil edge.
[Explanation of symbols]
1: Tundish,
2: molten steel,
3: immersion nozzle,
4: mold,
5: vertical part,
6: bending part,
7: Secondary cooling zone,
8: Cooling zone on the slab upper surface side
9: Cooling zone on the lower side of the slab 10: Solidified shell
11: Slab

Claims (3)

In a vertical cooling type continuous casting machine having a vertical portion and a curved portion, in a secondary cooling zone after a mold exit, a cooling water amount Q1 (liter / min) per unit time on a slab upper surface side from a vertical portion to a completed bending portion. ) Is larger than the cooling water amount Q2 (liter / min) per unit time on the slab lower surface side, or the cooling water amount density Qd1 (liter / m ² / min) per unit area and unit time on the slab upper surface side. Is larger than the cooling water amount density Qd2 (liter / m ² / min) per unit area and unit time on the slab lower surface side. In a vertical cooling type continuous casting machine having a vertical portion and a curved portion, in a secondary cooling zone after a mold outlet, a cooling water amount density Qd1 per unit area and unit time on a slab upper surface side from a vertical portion to a completed bending portion. (Liter / m ² / min), density of cooling water per unit area and unit time on the lower surface side of slab Qd2 (liter / m ² / min) and density of cooling water per unit area and unit time on one side of short side of slab A continuous casting method wherein Qd3 (liter / m ² / min) satisfies the relationship represented by the following equations (1) and (2).
0.8 ≦ Qd1 / Qd2 ≦ 2.5 (1)
0.3 ≦ Qd3 / {(Qd1 + Qd2) / 2} ≦ 2.5 (2) The slab width W1 on the slab upper surface side and the slab width W2 on the slab lower surface side satisfy the relationship represented by the following formula (3), and were cast by a vertical bending type continuous casting machine. Slab.
(W1−W2) / {(W1 + W2) / 2} ≦ 0.002 (3)

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