STEEL CONTINUOUS CASTING METHOD

Abstract

Provided is a steel continuous casting method that can identify the final solidification position accurately and inexpensively. A steel continuous casting method comprises: increasing a gap between the facing cast steel support rolls toward a downstream side in a casting direction of the cast steel, in a section; applying rolling reduction to the cast steel by a roll segment having a plurality of pairs of cast steel support rolls whose rolling reduction amounts are controlled by hydraulic cylinders, at least in a predetermined range; measuring a pressure difference between a plurality of hydraulic cylinders and estimating a final solidification position of the cast steel based on the pressure difference; and controlling a rolling reduction amount of the cast steel so as to satisfy predetermined formulas in a roll segment estimated to be the final solidification position and a roll segment immediately preceding the roll segment.

Claims

1. A steel continuous casting method of casting cast steel withdrawn from a mold while supporting the cast steel by a plurality of pairs of cast steel support rolls, cast steel support rolls of each of the plurality of pairs facing each other with the cast steel therebetween, the steel continuous casting method comprising: increasing a gap between the facing cast steel support rolls toward a downstream side in a casting direction of the cast steel, in a section; applying rolling reduction to the cast steel by a roll segment having a plurality of pairs of cast steel support rolls whose rolling reduction amounts are controlled by hydraulic cylinders, at least in a range from a position where a solid phase rate of a thickness central part of the cast steel is 0.2 to a position where the solid phase rate is a critical solid fraction of fluid flow in the casting direction; measuring a pressure difference between a plurality of hydraulic cylinders and estimating a final solidification position of the cast steel based on the pressure difference; and controlling a rolling reduction amount of the cast steel so as to satisfy the following formulas (1) and (2) in a roll segment estimated to be the final solidification position and a roll segment immediately preceding the roll segment: $\begin{array}{cc}0.5<VZ<3.& \left(1\right)\end{array}$ $\begin{array}{cc}0.5R<\mathrm{Db}<2R& \left(2\right)\end{array}$ where Vis a withdrawal speed of the cast steel in m/min, Z is a rolling reduction gradient in mm/m, Db is a bulging amount of the cast steel in mm, and R is a total rolling reduction amount of the cast steel in mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the accompanying drawings:

[0019] FIG. 1 is a diagram illustrating an example of the schematic structure of a slab continuous casting machine;

[0020] FIG. 2 is a schematic diagram illustrating an example of a roll segment included in a light rolling reduction zone in the slab continuous casting machine, as seen from the side of the continuous casting machine;

[0021] FIG. 3 is a schematic diagram illustrating the roll segment illustrated in FIG. 2 in a cross section orthogonal to the casting direction of cast steel; and

[0022] FIG. 4 is a graph illustrating the pressure difference between hydraulic cylinders against the casting length in each roll segment.

DETAILED DESCRIPTION

[0023] Embodiments of the present disclosure will be described below. The present disclosure is not limited to these embodiments.

[0024] A steel production method according to an embodiment of the present disclosure is a steel continuous casting method of casting cast steel withdrawn from a mold while supporting the cast steel by a plurality of pairs of cast steel support rolls, cast steel support rolls of each of the plurality of pairs facing each other with the cast steel therebetween, the steel continuous casting method comprising: increasing a gap between the facing cast steel support rolls toward a downstream side in a casting direction of the cast steel, in a section; applying rolling reduction to the cast steel by a roll segment having a plurality of pairs of cast steel support rolls whose rolling reduction amounts are controlled by hydraulic cylinders, at least in a range from a position where a solid phase rate of a thickness central part of the cast steel is 0.2 to a position where the solid phase rate is a critical solid fraction of fluid flow in the casting direction; measuring a pressure difference between a plurality of hydraulic cylinders and estimating a final solidification position of the cast steel based on the pressure difference; and controlling a rolling reduction amount of the cast steel so as to satisfy the following formulas (1) and (2) in a roll segment estimated to be the final solidification position and a roll segment immediately preceding the roll segment:

[00002]$\begin{array}{cc}0.5<VZ<3.& \left(1\right)\end{array}$$\begin{array}{cc}0.5R<\mathrm{Db}<2R& \left(2\right)\end{array}$ [0025] where Vis a withdrawal speed of the cast steel in m/min, Z is a rolling reduction gradient in mm/m, Db is a bulging amount of the cast steel in mm, and R is a total rolling reduction amount of the cast steel in mm.

[0026] FIG. 1 is a diagram illustrating the schematic structure of a slab continuous casting machine used when carrying out the steel continuous casting method according to the embodiment. As illustrated in FIG. 1, in one example, a mold 5 in which molten steel 9 is poured and solidified to form the outer shell shape of cast steel (slab) 10 is installed in a slab continuous casting machine 1. A tundish 2 for relaying the molten steel 9 supplied from a ladle (not illustrated) to the mold 5 is installed at a certain position above the mold 5. A sliding nozzle 3 for adjusting the flow rate of the molten steel 9 is installed at the bottom of the tundish 2. An immersion nozzle 4 is installed on the underside of the sliding nozzle 3. A plurality of pairs of cast steel support rolls 6 composed of support rolls, guide rolls, and pinch rolls are arranged below the mold 5. A secondary cooling zone is formed by arranging spray nozzles such as water spray nozzles or air mist spray nozzles (not illustrated) in the gaps between the cast steel support rolls 6 adjacent in the casting direction. A coolant (also referred to as secondary coolant) sprayed from the spray nozzles in the secondary cooling zone cools the cast steel 10 that is being withdrawn. A plurality of conveyance rolls 7 for conveying the cast steel 10 are installed on the downstream side of the cast steel support rolls 6 at the end in the casting direction. A cast steel cutter 8 for cutting cast steel 10a of a certain length from the cast steel 10 is located above the conveyance rolls 7.

[0027] A light rolling reduction zone 14 composed of a plurality of pairs of cast steel support rolls is installed on both the upstream and downstream sides of the solidification completion position 13 of the cast steel 10 in the casting direction. In the light rolling reduction zone 14, the distance (referred to as roll gap) between the cast steel support rolls facing each other with the cast steel 10 therebetween gradually narrows toward the downstream side in the casting direction. In other words, a rolling reduction gradient (i.e. the state of the roll gap set to gradually narrow toward the downstream side in the casting direction) is set in the light rolling reduction zone 14. The cast steel 10 can be subjected to light rolling reduction in the entire region or a selected region of the light rolling reduction zone 14. Spray nozzles for cooling the cast steel 10 are arranged between the cast steel support rolls in the light rolling reduction zone 14. The cast steel support rolls 6 arranged in the light rolling reduction zone 14 are also called rolling reduction rolls.

[0028] In the slab continuous casting machine 1 illustrated in FIG. 1, three roll segments each of which is composed of three pairs of cast steel support rolls 6 are connected in the casting direction. In the present disclosure, however, the light rolling reduction zone 14 need not necessarily be composed of three roll segments, and the number of roll segments constituting the light rolling reduction zone 14 may be one, two, or four or more. Moreover, although each roll segment is composed of three pairs of cast steel support rolls 6 in this embodiment, the number of cast steel support rolls 6 constituting one roll segment is not limited.

[0029] FIGS. 2 and 3 illustrate an example of a roll segment included in the light rolling reduction zone 14. FIGS. 2 and 3 illustrate an example in which five pairs of cast steel support rolls 6 as rolling reduction rolls are arranged in one roll segment 15. FIG. 2 is a schematic diagram of the roll segment 15 as seen from the side of the continuous casting machine. FIG. 3 is a schematic diagram of the roll segment 15 as seen in a cross section orthogonal to the casting direction of cast steel.

[0030] As illustrated in FIGS. 2 and 3, in one example, the roll segment 15 includes a pair of frames 16 and 16 that hold five pairs of cast steel support rolls 6 via roll chocks 21. A total of four support columns 17 (both sides on the upstream side and both sides on the downstream side) are arranged to pass through the frames 16 and 16. Driving worm jacks 19 installed on the support columns 17 using motors 20 can adjust the distance between the frames 16 and 16, that is, the rolling reduction gradient in the roll segment 15. The upper frame 16 holds the cast steel support rolls 6 via roll chocks 21 connected to cylinder rods (not illustrated) of hydraulic cylinders 22 fixed to the frame 16.

[0031] In the slab continuous casting machine 1, the molten steel 9 poured into the mold 5 from the tundish 2 through the immersion nozzle 4 is cooled in the mold 5 to form a solidified shell 11. The molten steel 9 is then continuously withdrawn below the mold 5 while being supported by the cast steel support rolls 6 located below the mold 5, as the cast steel 10 having an unsolidified layer 12 inside. While passing through the cast steel support rolls 6, the cast steel 10 is cooled by the secondary coolant in the secondary cooling zone, as a result of which the thickness of the solidified shell 11 increases. Moreover, in the light rolling reduction zone 14, the cast steel 10 is subjected to rolling reduction and is completely solidified to the inside at the solidification completion position 13. The cast steel 10 after the solidification completion is cut by the cast steel cutter 8 into the cast steel 10a.

[0032] In the steel casting method according to this embodiment, the cast steel withdrawn from the mold 5 is cast while being supported by the plurality of pairs of cast steel support rolls 6, each pair of cast steel support rolls 6 facing each other with the cast steel therebetween. The size of the cast steel produced is not limited. The thickness of the cast steel is preferably 200 mm or more. The thickness of the cast steel is preferably 600 mm or less. The width of the cast steel is preferably 1000 mm or more. The width of the cast steel is preferably 2500 mm or less. If the thickness and width of the cast steel are within these ranges, the rolling reduction gradient is better.

[0033] The gap between the facing cast steel support rolls 6 is increased toward the downstream side in the casting direction, in some section. By increasing the gap between the cast steel support rolls 6 toward the downstream side in the casting direction, the rectangular cast steel having the unsolidified layer inside can be bulged on its wide faces. In one example, the cast steel can be bulged on its wide faces by 0.1% to 10% of the thickness of the cast steel in the mold. This can prevent the cast steel from being under excessive load during the subsequent light rolling reduction. If the bulging amount is 10% or less, it is possible to prevent excessive strain from being applied to the solidification interface of the slab, so that internal cracking can be suppressed more appropriately. The range where bulging is performed is preferably set between the lower end of the mold 5 and the liquidus crater end position of the cast steel 10. This is because, on the upstream side of the liquidus crater end position of the cast steel 10 in the casting direction, the thickness central part of the cast steel is entirely the unsolidified layer 12 (liquid phase) and the solidified shell 11 of the cast steel 10 is high in temperature and low in deformation resistance and can be bulged easily. If the cast steel 10 is bulged when the amount of the unsolidified layer 12 inside the cast steel 10 is small, central segregation worsens. In contrast, if the cast steel 10 is bulged on the upstream side of the liquidus crater end position of the cast steel 10 in the casting direction, molten steel of the initial concentration in which solute elements are not concentrated is abundantly present inside the cast steel at this stage and this molten steel flows easily. Flow of such molten steel does not cause central segregation. Thus, bulging on the upstream side of the liquidus crater end position in the casting direction does not cause central segregation.

[0034] Rolling reduction is applied to the cast steel by a roll segment having a plurality of pairs of cast steel support rolls whose rolling reduction amounts are controlled by hydraulic cylinders, at least in the range from the position where the solid phase rate of the thickness central part of the cast steel is 0.2 to the position where the solid phase rate is the critical solid fraction of fluid flow in the casting direction of the cast steel. Rolling reduction is started at least from the position where the solid phase rate of the thickness central part of the cast steel is 0.2. Rolling reduction may be started before the solid phase rate of the thickness central part of the cast steel is 0.2. For example, rolling reduction may be started from the position where the solid phase rate of the thickness central part of the cast steel is 0. Herein, the solid phase rate is 0 before the solidification start and 1.0 at the solidification completion. The solid phase rate of the central part is calculated from the estimated temperature of the central part by two-dimensional heat transfer solidification calculation. The critical solid fraction is typically 0.7 to 0.8, and can be derived by a known method.

[0035] The part of the cast steel to which rolling reduction is applied is not limited, but it is preferable to apply rolling reduction over the entire width of the cast steel. For example, in the case of measuring the reaction force in the width central part as in the method using a load measurement device described in PTL 1, the measured reaction force corresponds to the load difference between the ferrostatic pressure in the unsolidified part and the rolling reduction load in the solidified part in the width central part. Therefore, if rolling reduction is applied to the cast steel in a state where such a rolling reduction gradient is set that balances the rolling reduction load in the solidified part with the ferrostatic pressure, it may not be possible to determine whether the cast steel is solidified. If rolling reduction is applied over the entire width of the cast steel, on the other hand, the pressure difference corresponding to the reaction force received over the entire width of the cast steel is measured, so that the final solidification position can be estimated more accurately with there being no need to consider the balance between the ferrostatic pressure and the rolling reduction load in the solidified part.

[0036] In the steel casting method according to this embodiment, the pressure difference between a plurality of hydraulic cylinders is measured, the final solidification position of the cast steel is estimated based on the pressure difference, and the rolling reduction amount of the cast steel is controlled to satisfy the following formulas (1) and (2) in the roll segment estimated to be the final solidification position and the roll segment immediately preceding the roll segment:

[00003]$\begin{array}{cc}0.5<VZ<3.& \left(1\right)\end{array}$$\begin{array}{cc}0.5R<\mathrm{Db}<2R& \left(2\right)\end{array}$ [0037] where V is the withdrawal speed of the cast steel (m/min), Z is the rolling reduction gradient (mm/m), Db is the bulging amount of the cast steel (mm), and R is the total rolling reduction amount of the cast steel (mm).

[0038] By performing rolling reduction so as to satisfy the formulas (1) and (2), it is possible to effectively prevent central segregation and porosity without internal cracking.

[0039] In the formula (1), VZ means the rolling reduction speed of the cast steel (mm/min). As a result of VZ being more than 0.50, the flow of concentrated molten steel can be appropriately suppressed and V segregation can be appropriately prevented. As a result of VZ being less than 3.00, excessive pushing of concentrated molten steel due to excessive rolling reduction can be appropriately prevented and inverse V segregation can be appropriately prevented. VZ is preferably 0.70 or more. VZ is preferably 1.20 or less.

[0040] In the formula (2), as a result of Db being more than 0.5R, the effect of preventing the rolling reduction reaction force of the cast steel from being excessive is achieved. As a result of Db being less than 2R, the effect of preventing the rolling reduction reaction force of the cast steel from being excessive and also preventing internal cracking of the cast steel due to excessive bulging is achieved. Db is preferably 0.7R or more. Db is preferably 1.8R or less.

[0041] Here, pinch rolls are rotated to withdraw the cast steel. The withdrawal speed V of the cast steel (m/min) means the cast steel withdrawal speed calculated based on the rotation speed of the pinch rolls.

[0042] The rolling reduction gradient Z (mm/m) means the narrowing amount of the roll gap per 1 m in the casting direction between the roll segment estimated to be the final solidification position and the roll segment immediately preceding the roll segment.

[0043] The bulging amount Db of the cast steel (mm) means the difference between the roll gap at the lower end of the mold and the roll gap of the segment before light rolling reduction starts.

[0044] The total rolling reduction amount R of the cast steel (mm) means the total rolling reduction in casting.

[0045] As mentioned above, in the case of measurement using a load measurement device, in a high temperature environment during casting, the load measurement device is under heavy load and is highly likely to fail. When the load measurement device fails, the segment needs to be replaced, which is costly. According to the present disclosure, the final solidification position of the cast steel is estimated by measuring the pressure difference between a plurality of hydraulic cylinders, so that the final solidification position can be identified easily and rolling reduction can be applied at an appropriate position. Here, the pressure difference between the hydraulic cylinders is (the head pressure on the entry side of the roll segment)(the head pressure on the exit side of the roll segment). By measuring the respective pressures of the hydraulic cylinder 22a on the entry side of the roll segment and the hydraulic cylinder 22b on the exit side of the roll segment, the respective loads exerted on the support column 17a on the entry side of the roll segment and the support column 17b on the exit side of the roll segment can be calculated. There are two types of pressure of a hydraulic cylinder: head pressure and rod pressure. The frame 16 is pressed by the hydraulic cylinder with the rod pressure set constant (for example, 20 MPa). Responsively, the head pressure is balanced to control the positions of the cast steel support rolls 6 constant. When the frame receives a reaction force from the cast steel, the head pressure is decreased to balance the sum of the reaction force from the cast steel and the head pressure with the rod pressure. That is, the rod pressure=the head pressure+the reaction force from the cast steel. Thus, when the reaction force from the cast steel is greater, the head pressure is lower. When solidified cast steel after the final solidification position in a roll segment is subjected to rolling reduction, the reaction force from the roll segment is greater than when cast steel having an unsolidified part is subjected to rolling reduction. Meanwhile, in a roll segment corresponding to the final solidification position, the load is small on the segment entry side and large on the segment exit side. Hence, by identifying a roll segment where the pressure difference between the hydraulic cylinder located on the segment entry side and the hydraulic cylinder located on the segment exit side is greater than or equal to a predetermined value, the final solidification position of the cast steel can be estimated. The pressure difference between the hydraulic cylinder located on the segment entry side and the hydraulic cylinder located on the segment exit side is not limited, but is preferably 2 MPa or more.

[0046] FIG. 4 is a graph illustrating the pressure difference between hydraulic cylinders against the casting length in each roll segment. In FIG. 4, segments 1, 2, and 3 are data for roll segments located at the most downstream, the middle, and the most upstream, respectively. As can be seen from the drawing, when the roll segment is located more downstream, the pressure difference between the hydraulic cylinder located on the segment entry side and the hydraulic cylinder located on the segment exit side is greater. The pressure difference between the hydraulic cylinders increases from the point when the bottom part of the cast steel enters the roll segment and then decreases from the point when the bottom part of the cast steel leaves the roll segment, but the pressure difference remains large in the case where the final solidification position is in the roll segment. In the example in FIG. 4, in each of the most upstream segment 3 and the middle segment 2, the pressure difference decreases to less than 2 MPa after the bottom part passes through the segment. In the most downstream segment 1, on the other hand, the pressure difference remains 2 MPa or more even after the bottom part passes through the segment. The segment 1 can therefore be estimated to be the final solidification position of the cast steel.

[0047] In the case where a load measurement device is used to estimate the final solidification position, it is necessary to install a load measurement device for each rolling reduction roll. This requires a large number of sensors in order to estimate the final solidification position from a wide range in the casting direction. According to the present disclosure, the final solidification position is estimated by calculating the pressure difference between hydraulic cylinders for each roll segment. Hence, it is easy to estimate the final solidification position from a wide range even if the final solidification position varies greatly due to changes in the thickness of cast steel, the withdrawal speed, or the steel type.

[0048] Casting conditions other than those described above may be in accordance with conventional methods.

[0049] This steel continuous casting method can reduce central segregation of cast steel. In a cross section orthogonal to the withdrawal direction of cast steel, the carbon concentration is analyzed at equal intervals in the thickness direction of the cast steel to determine C.sub.max/C.sub.0 as the central segregation degree, where C.sub.max is the maximum carbon concentration in the thickness direction and C.sub.0 is the carbon concentration analyzed for molten steel collected from the tundish during casting. A central segregation degree closer to 1.000 indicates better cast steel with less central segregation. With this steel continuous casting method, cast steel whose C.sub.max/C.sub.0 is less than 1.100 can be produced. No lower limit is placed on the central segregation degree, but the central segregation degree is typically 1.000 or more. This steel continuous casting method can also suppress porosity and internal cracking of cast steel.

EXAMPLES

[0050] The presently disclosed techniques will be described in more detail below by way of examples.

[0051] The slab continuous casting machine used in the test is the same as the slab continuous casting machine 1 illustrated in FIG. 1. Low carbon aluminum killed steel was cast using this slab continuous casting machine. The thickness of cast steel at the lower end of the mold was 250 mm, 400 mm, or 600 mm. The width of cast steel was 2200 mm in each test. Table 1 shows the casting conditions and the results of investigating the central segregation degree in the cast steel, whether porosity occurred, and whether internal cracking occurred. Table 1 also shows the casting conditions and investigation results of the tests conducted as comparative examples. In Table 1, the rolling reduction start position, the rolling reduction end position, the position where the solid phase rate of the thickness central part of the cast steel is 0.2, and the final solidification position are expressed in terms of the distance from the meniscus in the mold. In Table 1, in each test in which rolling reduction range control is indicated as Performed, since, in the initially set light rolling reduction range, the rolling reduction start position and rolling reduction end position were outside the appropriate range, the rolling reduction range was adjusted based on the final solidification position measured during casting.

TABLE-US-00001 TABLE 1 Position Cast of solid Final Rolling Rolling Rolling steel phase solidifi- reduction reduction Rolling Withdrawal reduction thick- rate cation start end reduction speed gradient ness of 0.2 position position position range V Z No. (mm) (m) (m) (m) (m) control (m/min) (mm/m) 1 Example 250 24.0 26.2 20 28 Not 1.20 1.2 performed 2 22.5 25.8 20 28 Not 1.10 1.1 performed 3 18.0 22.1 16 24 Performed 0.80 1.5 4 24.0 26.0 20 28 Not 1.00 0.6 performed 5 25.0 27.7 24 30 Not 1.00 2.9 performed 6 Comparative 16.0 19.8 18 26 Not 0.70 1.1 Example performed 7 23.8 26.1 20 28 Not 1.10 0.4 performed 8 24.0 26.5 20 28 Not 1.40 2.8 performed 9 Example 400 30.0 33.4 28 34 Not 0.60 2.0 performed 10 28.2 31.4 26 32 Not 0.70 2.4 performed 11 31.0 34.2 30 36 Performed 0.60 2.2 12 30.0 33.8 28 34 Not 0.52 1.0 performed 13 28.5 30.0 28 32 Not 0.60 4.8 performed 14 Comparative 30.2 33.6 26 32 Not 0.65 2.5 Example performed 15 26.2 28.9 24 30 Not 0.70 0.6 performed 16 27.2 30.0 24 30 Not 0.60 2.0 performed 17 Example 600 32.0 36.5 30 38 Not 0.42 3.0 performed 18 34.0 37.2 32 40 Not 0.55 3.4 performed 19 33.4 36.4 32 40 Performed 0.60 2.5 20 34.2 36.8 32 38 Not 0.50 1.1 performed 21 34.5 37.2 34 38 Not 0.58 5.0 performed 22 Comparative 29.0 32.1 30 38 Not 0.45 2.4 Example performed 23 32.2 36.8 30 38 Not 0.60 5.8 performed 24 34.5 37.7 32 40 Not 0.65 1.5 performed Bulging Total amount rolling 0.1-10% reduction Bulging Central of cast amount amount segregation steel V R Db degree Internal thickness No. Z (mm) 0.5 R 2 R (mm) (C.sub.max/C.sub.0) Porosity cracking (mm) 1 1.44 9.6 4.8 19.2 5.0 1.058 Not Not 0.25-25 occurred occurred 2 1.21 8.8 4.4 17.6 10.0 1.049 Not Not 0.25-25 occurred occurred 3 1.20 12.0 6.0 24.0 8.0 1.060 Not Not 0.25-25 occurred occurred 4 0.55 4.4 2.2 8.8 4.0 1.096 Not Not 0.25-25 occurred occurred 5 2.90 17.4 8.7 34.8 10.0 1.064 Not Not 0.25-25 occurred occurred 6 0.77 8.8 4.4 17.6 5.0 1.129 Occurred Not 0.25-25 occurred 7 0.44 3.2 1.6 6.4 4.0 1.105 Not Not 0.25-25 occurred occurred 8 3.92 22.4 11.2 44.8 10.0 1.108 Not Occurred 0.25-25 occurred 9 1.20 12.0 6.0 24.0 8.0 1.062 Not Not 0.4-40 occurred occurred 10 1.68 14.4 7.2 28.8 8.0 1.055 Not Not 0.4-40 occurred occurred 11 1.32 13.2 6.6 26.4 9.0 1.058 Not Not 0.4-40 occurred occurred 12 0.52 6.0 3.0 12.0 8.0 1.089 Not Not 0.4-40 occurred occurred 13 2.88 19.2 9.6 38.4 15.0 1.072 Not Not 0.4-40 occurred occurred 14 1.63 15.0 7.5 30.0 10.0 1.130 Occurred Not 0.4-40 occurred 15 0.42 3.6 1.8 7.2 5.0 1.108 Occurred Not 0.4-40 occurred 16 1.20 12.0 6.0 24.0 5.0 1.120 Occurred Not 0.4-40 occurred 17 1.26 24.0 12.0 48.0 18.0 1.048 Not Not 0.6-60 occurred occurred 18 1.87 27.2 13.6 54.4 20.0 1.059 Not Not 0.6-60 occurred occurred 19 1.50 20.0 10.0 40.0 15.0 1.055 Not Not 0.6-60 occurred occurred 20 0.55 6.6 3.3 13.2 9.0 1.088 Not Not 0.6-60 occurred occurred 21 2.90 20.0 10.0 40.0 15.0 1.069 Not Not 0.6-60 occurred occurred 22 1.08 19.2 9.6 38.4 5.0 1.135 Occurred Not 0.6-60 occurred 23 3.48 46.4 23.2 92.8 25.0 1.102 Not Occurred 0.6-60 occurred 24 0.98 12.0 6.0 24.0 30.0 1.243 Occurred Not 0.6-60 occurred

[0052] The central segregation degree of the cast steel used in the evaluation test was measured by the following method: In a cross section orthogonal to the withdrawal direction of the cast steel, the carbon concentration was analyzed at equal intervals in the thickness direction of the cast steel to determine C.sub.max/C.sub.0 as the central segregation degree, where C.sub.max is the maximum carbon concentration in the thickness direction and C.sub.0 is the carbon concentration analyzed for molten steel collected from the tundish during casting. A central segregation degree closer to 1.000 indicates better cast steel with less central segregation. Here, cast steel with a central segregation degree of 1.100 or more was determined to have a poor central segregation degree.

[0053] Whether porosity and internal cracking occurred in the cast steel was determined through microscopic observation of the thickness central part of the cast steel in a cross section orthogonal to the withdrawal direction of the cast steel.

[0054] The cast steel withdrawal speed for each cast steel thickness was set so that at least the cast steel in the section from the position where the solid phase rate of the thickness central part of the cast steel is 0.2 to the position where the solid phase rate is the critical solid fraction of fluid flow would be located in the light rolling reduction zone. In addition, in Test Nos. 1 to 5, 9 to 13, and 17 to 21, the rolling reduction gradient and the bulging amount were set to satisfy the foregoing formulas (1) and (2) in the roll segment estimated to be the final solidification position and the roll segment immediately preceding the roll segment. As the final solidification position, the roll segment in which the pressure difference between the hydraulic cylinder located on the segment entry side and the hydraulic cylinder located on the segment exit side was 2 MPa or more was identified. In Test Nos. 6, 14, and 22 of Comparative Examples, either the rolling reduction start position or the rolling reduction end position was outside the appropriate range. In Test Nos. 7, 8, 15, and 23, VZ was outside the range of the formula (1). In Test Nos. 16, 22, and 24, the total rolling reduction amount and the bulging amount were outside the range of the formula (2).

[0055] As is clear from the central segregation degree shown in Table 1, in Test Nos. 1 to 5, 9 to 13, and 17 to 21 of Examples, the central segregation degree was less than 1.100, which was good. Moreover, neither porosity nor internal cracking was observed in the cast steel.

[0056] In Test Nos. 6, 14, and 22 of Comparative Examples, either the rolling reduction start position or the rolling reduction end position was outside the appropriate range, so that the central segregation degree was 1.100 or more and porosity was observed inside the cast steel. In Test Nos. 7 and 15 of Comparative Examples, the rolling reduction gradient was insufficient and the central segregation degree was 1.100 or more. Further, in Test No. 15, porosity was observed inside the cast steel. In Test Nos. 8 and 23, the rolling reduction gradient was excessively large, so that the central segregation degree was 1.100 or more and internal cracking occurred in the cast steel. In Test Nos. 16, 22, and 24, the total rolling reduction amount and the bulging amount were outside the range of the formula (2), so that the central segregation degree was 1.100 or more and porosity was observed inside the cast steel.

REFERENCE SIGNS LIST

[0057] 1 slab continuous casting machine [0058] 2 tundish [0059] 3 sliding nozzle [0060] 4 immersion nozzle [0061] 5 mold [0062] 6 cast steel support roll [0063] 7 conveyance roll [0064] 8 cast steel cutter [0065] 9 molten steel [0066] 10 cast steel [0067] 11 solidified shell [0068] 12 unsolidified layer [0069] 13 solidification completion position [0070] 14 light rolling reduction zone [0071] 15 roll segment [0072] 16 frame [0073] 17 support column [0074] 19 worm jack [0075] 20 motor [0076] 21 roll chock [0077] 22 hydraulic cylinder