The present invention relates to a method for continuous casting of steel.
　The present application claims priority based on Japanese Patent Application No. 2018-231136 filed in Japan on December 10, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　In recent years, in steel materials such as thick steel sheets, many low alloy steels containing alloying elements such as Ti, Nb, Ni, and Cu have been produced in order to improve mechanical properties. However, with the addition of these alloying elements, surface crack defects occur in the slabs produced in continuous casting, which poses a problem in terms of operation and product quality. The surface crack here means a general term for crack forms such as lateral cracks that are not in the casting direction.
[0003]
　As a method for preventing surface cracking of a slab containing an alloying element in continuous casting, for example, there is a method as disclosed in Patent Document 1. The method disclosed in Patent Document 1 increases the average water volume density of the water-cooled nozzle directly under the mold and sprays cooling water onto the slab at a predetermined collision pressure to cast while peeling the powder adhering to the slab surface. the surface temperature of the piece a 3 stably cooled to the transformation temperature or less, then performing recuperation of the slab, the casting surface temperature of the slab at the bending portion or straightening unit as a temperature higher than the brittle temperature range It is something to do.
[0004]
　It is known that the surface cracks generated after the secondary cooling zone of continuous casting are cracks along the old austenite grain boundaries on the surface layer of the slab. This crack occurs when stress is concentrated on the austenite grain boundaries embrittled by precipitation of AlN, NbC, etc., and the film-like ferrite formed along the old austenite grain boundaries. The form of the crack depends on the direction of the applied stress, and the lateral crack is caused by the tensile stress in the casting direction. In particular, cracks are likely to occur in the temperature range near the phase transformation region from austenite to ferrite. Therefore, as disclosed in Patent Document 1, mechanical stress avoids the surface temperature in the bending or straightening band applied to the slab surface from the temperature range (embrittlement temperature range) in which ductility decreases, and cracks occur. A method of suppressing the occurrence is taken.
Prior art literature
Patent documents
[0005]
Patent Document 1: Japanese Patent Application Laid-Open No. 2018-099704
Outline of the invention
Problems to be solved by the invention
[0006]
　In recent years, as the number of alloy steel types to which various elements have been added has increased in order to improve mechanical properties, the number of steel types with high slab surface cracking sensitivity has increased. It is not possible to prevent the occurrence of cracks on one side. As described above, in the conventional continuous casting method of steel, there is room for improvement in preventing cracks on the surface of the slab while ensuring the desired cooling capacity.
[0007]
　The present invention has been made in view of the above circumstances, and the microstructure of the surface layer of the slab can be controlled, cracks on the surface of the slab due to non-uniform secondary cooling can be suppressed, and strain at the bent portion is caused. It is an object of the present invention to provide a continuous casting method of steel capable of suppressing cracking on the surface of a slab.
Means to solve problems
[0008]
(1) In the continuous steel casting method according to one aspect of the present invention, a vertical portion for pulling out a slab downward from a mold in the vertical direction and a bent portion for bending the slab pulled out from the vertical portion are provided. A method of continuously casting steel using a vertical bending type continuous casting apparatus including a first cooling zone including a roll and a cooling spray nozzle in the vertical portion, wherein the steel is continuously cast in the first cooling zone. The air-water ratio A 1 / R 1 , which is the ratio of the amount of air A 1 (L / min) to the amount of water R 1 (L / min) per cooling spray nozzle , is set to 10 or more, and from the cooling spray nozzle. The collision pressure of the cooling water that collides with the surface of the slab is 12 gf / cm 2 or more, and the cooling water density W 1 (L / min / m 2 ) in the first cooling zone and the slab are the first. The cooling strength W 1 × t 1 , which is defined as the product of the time t 1 (min) for passing through the cooling zone, is set to 350 or more, and the casting from the time of passing through the first cooling zone to the time when the bent portion is reached. The reheat time t 2 of one piece is set to 0.5 min or more.
[0009]
(2) In the steel continuous casting method according to (1) above, in the first cooling zone, the amount of water R 1 (L / min) per cooling spray nozzle is 20 L / min or more and 50 L / min or more. It may be min or less.
[0010]
(3) In the steel continuous casting method according to (1) or (2) above, the cooling water density W 1 (L / min / m 2 ) is set to 500 L / min / m in the first cooling zone. It may be 2 or more and 2000 L / min / m 2 or less.
[0011]
(4) In the steel continuous casting method according to any one of (1) to (3) above, the vertical bending type continuous casting apparatus is used between the first cooling zone and the bent portion. May be provided with a second cooling zone, and in the second cooling zone, the cooling water density W 2 (L / min / m 2 ) is set to 0 L / min / m 2 or more and 50 L / min / m 2 or less. By doing so, the surface of the slab may be reheated.
[0012]
(5) In the steel continuous casting method according to any one of (1) to (4) above, the surface of the slab is reheated after passing through the first cooling zone, and the slab is formed. The temperature of the surface of the slab may be set to a temperature of 3 or more points of Ac at the time of reaching the bent portion .
[0013]
(6) In the steel continuous casting method according to any one of (1) to (5) above, the roll may be a split roll.
The invention's effect
[0014]
　In the continuous steel casting method of the present invention, the slab is cooled by a mist spray having a high air-water ratio and a high collision in a first cooling zone provided in a vertical portion. By using a mist spray with a high air-water ratio and a high collision pressure, the mold powder on the surface of the slab can be peeled off, the generation of accumulated water between the rolls can be suppressed, and the slab is uniformly secondary cooled. It is thought that it can be done.
[0015]
　Further, in the continuous steel casting method of the present invention, the cooling strength in the first cooling zone is increased to a predetermined value or more. It is considered that the microstructure of the slab surface layer can be controlled more appropriately by setting the cooling strength to a predetermined value or higher.
[0016]
　Further, in the continuous steel casting method of the present invention, the reheating time from cooling by the first cooling zone to reaching the bent portion is set to a predetermined time or longer, and the slab surface can be appropriately reheated. .. As a result, a fine structure can be generated on the surface of the slab, and surface cracking of the slab at the bent portion can be suppressed.
[0017]
　As described above, according to the continuous steel casting method of the present invention, the microstructure of the surface layer of the slab can be controlled, cracks on the surface of the slab due to non-uniform secondary cooling can be suppressed, and strain at the bent portion is caused. Cracking on the surface of the slab can be suppressed.
A brief description of the drawing
[0018]
FIG. 1 is a schematic view for explaining a method for continuously casting steel of the present invention.
FIG. 2 is an enlarged and schematic view of a part of the first cooling zone 21 of FIG.
[0019]
　Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the present specification and the drawings, elements having substantially the same functional configuration are designated by the same reference numerals, so that duplicate description will be omitted.
[0020]
　In the present specification, the numerical range represented by using "-" means a range including the numerical values ​​before and after "-" as the lower limit value and the upper limit value. In the present specification, the term "process" is used not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes. included. Further, it is obvious that each element of the following embodiment can be combined with each other.
Mode for carrying out the invention
[0021]
　The steel continuous casting method of the present invention will be described with reference to FIG. FIG. 1 is a diagram schematically showing the positional relationship between the mold 10, the vertical portion 20, the bent portion 30, and the like in the vertical bending type continuous casting apparatus 100. In FIG. 1A, the cooling spray nozzle and the like are omitted for the sake of clarity. FIG. 2 is an enlarged and schematic view of a part of the first cooling zone 21 of the vertical portion 20, and schematically shows the positional relationship between the roll 21a and the cooling spray nozzle 21b. Depending on the conditions such as the amount of cooling water, as shown in FIG. 2, the cooling water discharged from the cooling spray nozzle 21b collects between the slab 1 and the roll 21a and remains as water W.
[0022]
　The steel continuous casting method of the present embodiment includes a vertical portion 20 that pulls out the slab 1 downward from the mold 10 in the vertical direction, and a bending portion 30 that bends the slab 1 drawn from the vertical portion 20, and also has a vertical portion. A method of continuously casting steel using a vertical bending type continuous casting apparatus 100 including a first cooling zone 21 including a roll 21a and a cooling spray nozzle 21b in the first cooling zone 21. , The air-water ratio A 1 / R 1 , which is the ratio of the amount of air A 1 (L / min) to the amount of water R 1 (L / min) per cooling spray nozzle 21b , is set to 10 or more, and the cooling spray nozzle The collision pressure of the cooling water colliding with the surface of the slab 1 from 21b is set to 12 gf / cm 2 or more, and the cooling water density W 1 (L / min / m 2 ) in the first cooling zone 21 and the slab 1 are the first. The cooling intensity W 1 × t 1 defined as the product of the time t 1 (min) passing through the cooling zone 21 of 1 is set to 350 or more, and the period from passing through the first cooling zone 21 to reaching the bent portion 30 is reached. The reheating time t 2 of the slab 1 is set to 0.5 min or more.
[0023]
(Structure of Continuous Casting Device 100)
　The continuous casting method according to the present embodiment can be preferably used for a known vertical bending type continuous casting device. The mold 10 has a cross-sectional shape corresponding to the shape of the slab 1 to be cast. A vertical portion 20 is provided directly below the mold 10, and a bent portion 30 is provided directly below the vertical portion 20.
[0024]
　The height of the vertical portion 20 (distance from directly below the mold 10 to the bent portion 30) can be, for example, 0.5 m or more and 3.0 m or less. A first cooling zone 21 is provided at least on the upper side of the vertical portion 20. The first cooling zone 21 includes a roll 21a and a cooling spray nozzle 21b. In the first cooling zone 21, the number of rolls 21a that support one side of the slab 1 is not limited to the five shown in FIG. For example, the number may be 1 or more and 7 or less. More preferably, the number is 6 or less on the one side (12 or less in total on the one side and the other side). That is, the number of cooling stages in the first cooling zone is not limited to the five stages shown in FIG. 1, and is preferably 6 stages or less.
[0025]
　In the first cooling zone 21, the roll pitch (P in FIG. 2) between the rolls 21a adjacent to each other in the casting direction can be, for example, 50 mm or more and 300 mm or less, and the distance between the rolls (I in FIG. 2). Can be, for example, 10 mm or more and 100 mm or less. In the first cooling zone 21, a cooling spray nozzle 21b is provided between the mold 10 and the roll 21a directly under the mold and / or between the rolls 21a adjacent to each other in the casting direction, and casting is performed from the cooling spray nozzle 21b. Cooling water is sprayed onto the surface of the piece 1. The number of cooling spray nozzles 21b between each roll 21a is, for example, one in the casting direction and at least one in the slab width direction.
[0026]
　The vertical portion 20 may include a second cooling zone 22 between the first cooling zone 21 and the bent portion 30 (immediately below the first cooling zone 21). In the second cooling zone 22, the number of rolls 22a supporting one side of the slab 1 can be, for example, 0 or more and 10 or less. In the second cooling zone 22, a cooling spray nozzle (not shown) may be arranged between the rolls 21a and the rolls 22a adjacent to each other in the casting direction or between the rolls 22a. In this case, each roll 22a The number of cooling spray nozzles in between can be, for example, one in the casting direction and at least one in the slab width direction.
[0027]
　The roll 21a may be a split roll. The divided roll means a roll in which the roll surface is divided into two or more in the direction along the axis of the roll. The roll surface may be divided into three surfaces, four surfaces, or five or more surfaces. The divided roll has a shaft portion having a diameter smaller than that of the roll surface between the plurality of divided roll surfaces. When the roll 21a is not a split roll, both ends of the roll are supported by bearings, but in the case of a split roll, the shafts between the roll surfaces are supported by bearings.
　The vicinity of the end of the slab 1 is more likely to be cooled than the central portion of the slab 1 in the width direction in which accumulated water is likely to be generated, and the temperature difference in the width direction of the slab 1 caused by this causes the end of the slab 1 to be cooled. Surface cracks tend to occur in the vicinity. By using the roll 21a as a split roll, accumulated water is discharged from the shaft portion between the plurality of roll surfaces, the temperature difference in the width direction of the slab 1 is alleviated, and surface cracking of the slab is suppressed. can do. Further, by supporting the roll not only at both ends of the roll 21a but also at the shaft portion in the middle of the roll, bending of the roll can be suppressed even when the roll diameter is small.
[0028]
　As for the roll 22a, a split roll may be adopted for the same reason as the roll 21a as described above.
[0029]
　The slab 1 that has passed through the vertical portion 20 is conveyed in the horizontal direction after being bent and straightened at the bent portion 30. The "bent portion" referred to in the present application means a portion where the casting direction of the slab 1 changes from the vertical direction to the horizontal direction. Since the bent portion 30 may have the same configuration as that conventionally known, detailed description thereof will be omitted here.
[0030]
(Air-water ratio in the first cooling zone 21) In
　order to increase the collision pressure of the cooling water from the cooling spray nozzle 21b, the amount of cooling water is increased or the amount of air is increased while the amount of cooling water is secured. Is effective. Here, when the amount of cooling water is simply increased, accumulated water in the roll 21a is likely to be generated. In order to increase the collision pressure of the cooling water while suppressing the accumulated water, it is preferable to increase the ratio of the amount of air to the amount of cooling water (air-water ratio). From this point of view, in the continuous steel casting method of the present embodiment, in the first cooling zone 21, the amount of air A 1 (L / min ) with respect to the amount of water R 1 (L / min) per cooling spray nozzle 21b. ), Which is the ratio of air-water ratio A 1 / R 1, is set to 10 or more. The upper limit of the air-water ratio is not particularly limited, but is preferably 100 or less from the viewpoint of spray stability. More preferably, it is 50 or less.
[0031]
(Water R in a first cooling zone 21 1 )
　water R of cooling spray nozzles 21b 1 may be adjusted in consideration of the impact pressure and cooling intensity to be described later. In particular, in the continuous steel casting method of the present embodiment, in the first cooling zone 21, the amount of water R 1 (L / min) per cooling spray nozzle 21b is set to 20 L / min or more and 50 L / min or less. Is preferable. As a result, the collision pressure of the spray can be increased more easily while suppressing the generation of accumulated water more easily.
[0032]
(Collision pressure of cooling water in the first cooling zone 21) The
　present inventor has a cooling capacity (heat transfer coefficient) of a spray when cooling a high-temperature slab (for example, 950 ° C. or higher) with a mist spray. We found that it has a good correlation with the collision pressure. This is because the heat transfer resistance of the boiling film acts predominantly in the heat transfer of the slab surface in the transition boiling region, so that the boiling film is physically pushed away as the collision pressure increases, resulting in heat. This is because the transfer coefficient increases. In addition, when the collision pressure exceeds a certain level, the mold powder adhering to the surface of the slab is peeled off, and temperature unevenness in the width direction due to spray cooling can be reduced. From this point of view, in the continuous steel casting method of the present embodiment, the collision pressure of the cooling water colliding with the surface of the slab 1 from the cooling spray nozzle 21b is set to 12 gf / cm 2 or more in the first cooling zone 21. .. It is preferably 13 gf / cm 2 or more, more preferably 15 gf / cm 2 or more, and even more preferably 17 gf / cm 2 or more. On the other hand, if the collision pressure is too large, the solidified shell of the slab 1 is partially dented, and cooling water is blown upward from between the roll 21a and the slab 1, which may cause a breakout. From this point of view, in the continuous steel casting method of the present embodiment, it is preferable that the collision pressure of the cooling water colliding with the surface of the slab 1 from the cooling spray nozzle 21b is 50 gf / cm 2 or less. More preferably 40 gf / cm 2 or less, more preferably 30 gf / cm 2It is as follows.
[0033]
　The collision pressure of the cooling water that collides with the surface of the slab 1 can be estimated, for example, by a method of measuring offline using a pressure receiving sensor or by the following simple formula 1.
[0034]
[Number 1]

[0035]
　In the above formula 1, Pc [gf / cm 2 ]: collision pressure, W [L / min / m 2 ]: water volume density, Va [m / s]: pressure air discharge flow rate (air flow rate [Nm 3 / s] / air Orifice area [m 2 ]), H [m]: injection distance, A / R [−]: air-water ratio (volume ratio of air to water).
[0036]
(Cooling Strength in First Cooling Zone 21)
　According to a new finding of the present inventor, by increasing the cooling strength (W 1 × t 1 ) in the first cooling zone 21 , a fine structure is formed on the surface layer of the slab. It can be generated and the occurrence of cracks can be suppressed. By increasing the cooling intensity in the first cooling zone 21, the surface of the slab can be appropriately and quickly cooled to a temperature of Ar 3 points or less, and the fine structure of the surface of the slab can be more easily controlled. It is thought that this is the reason. From this point of view, in the continuous steel casting method of the present embodiment, the cooling water density W 1 (L / min / m 2 ) in the first cooling zone 21 and the slab 1 pass through the first cooling zone 21. Let the cooling intensity W 1 × t 1 , which is defined as the product of the time t 1 (min), be 350 or more. The upper limit of the cooling intensity is not particularly limited, but is preferably 1500 or less, for example. More preferably, it is 1200 or less.
[0037]
　The “cooling water density W 1 ” refers to the amount (L) of cooling water injected per unit time (min) per unit area (m 2 ) of the slab surface . The "cooling water density W 1 " is, for example, "the amount of water R 1 (L / min) per cooling spray nozzle 21b is set to the roll pitch P (m) in the casting direction and the spray injection width in the slab width direction. It can be defined as "divided by the product of m)".
[0038]
　The cooling water density W 1 may be adjusted in consideration of the above-mentioned air-water ratio, collision pressure, and the like. Here, in the first cooling zone 21, the vicinity of the corner to be cooled two-dimensionally tends to be supercooled, and especially when the amount of water is high, accumulated water in the roll is likely to be generated, so that the secondary surface of the slab is secondary. Cooling may be uneven. On the other hand, if the amount of water is too low, it becomes difficult to achieve the above-mentioned collision pressure and the like. In this respect, in the continuous steel casting method of the present embodiment, the cooling water density W 1 (L / min / m 2 ) is set to 500 L / min / m 2 or more and 2000 L / min / m 2 in the first cooling zone 21. The following is preferable. The lower limit is more preferably 600 L / min / m 2 or more, and the upper limit is more preferably 1750 L / min / m 2 or less.
[0039]
(Reheat after passing through the first cooling zone 21) In
　the continuous steel casting method of the present embodiment, the surface of the slab 1 is reheated after passing through the first cooling zone 21, and the slab 1 is bent. When it reaches 30, it is preferable that the temperature of the surface of the slab 1 is set to a temperature of 3 points or more of Ac . In order to achieve this more easily, in the continuous casting method of steel in this embodiment, recuperation time t of the slab 1 to reach the bending portion 30 from the first cooling zone 21 after passing 2 0. 5 min or more. By setting the recovery time t 2 to 0.5 min or more, the surface of the slab cooled to a temperature of Ar 3 points or less in the first cooling zone 21 becomes Ac 3 points or more due to the sensible heat inside the slab . It is reheated to the temperature, the surface layer of the slab becomes stable, and the γ grain boundary becomes an indistinct microstructure. The upper limit of the recovery time t 2 is not particularly limited, but is preferably 2.0 min or less, and more preferably 1.75 min or less.
[0040]
(Other) In
　the continuous steel casting method of the present embodiment, even if the vertical bending type continuous casting apparatus 100 includes a second cooling zone 22 between the first cooling zone 21 and the bent portion 30. good. Here, in the continuous steel casting method of the present embodiment, the surface of the slab is cooled to a temperature of Ar 3 points or less in the first cooling zone 21, and then the secondary cooling is adjusted to Ac 3 points or more. It is good to reheat to the temperature. In this case, it is necessary to pass through the first cooling zone 21 with sufficient sensible heat inside the slab and complete the reheat to 3 points of Ac by the bent portion 30 to which mechanical strain is applied. .. Therefore, in the second cooling zone 22, it is necessary to lower the cooling water density as compared with the first cooling zone 21. Specifically, in the second cooling zone 22, the surface of the slab 1 is set by setting the cooling water density W 2 (L / min / m 2 ) to 0 L / min / m 2 or more and 50 L / min / m 2 or less. It is preferable to reheat.
[0041]
　In the present application, A temperature at which the transformation from the body-centered cubic lattice (bcc ferrite phase) to face-centered cubic lattice of austenitic (fcc) 3 in point, A at the time of cooling 3 the temperature at which transformation (ferrite transformation) Ar 3 point, a in heating 3 the temperature at which transformation (austenite transformation) Ac 3 to as points.
[0042]
　As described above, in the continuous steel casting method of the present embodiment, a mist having a high air-water ratio and a high collision pressure in the first cooling zone 21 provided on the upper side of the vertical portion 20 which is the secondary cooling zone. By cooling the slab 1 by spraying, it is possible to control the microstructure of the slab surface layer and prevent the slab surface cracking due to the non-uniform secondary cooling. Here, when steel is continuously cast by the vertical bending type continuous casting apparatus 100, it is preferable to strongly cool the steel directly under the mold 10 to cool at least 2 mm from the surface of the slab to a temperature of 3 Ar points or less. After that, by reheating the slab surface to a temperature of 3 points or more by the time it reaches the bent portion 30, cracks on the slab surface can be suppressed more appropriately.
[0043]
　The cooling spray nozzle 21b installed in the first cooling zone 21 needs to be designed so that a large flow rate mist spray nozzle and stable spray can be obtained even at a high air-water ratio. Further, in order to secure the collision pressure, it is desirable that the distance from the slab 1 is small. Specifically, the distance (spray height) from the surface of the slab 1 to the cooling spray nozzle 21b is preferably 50 mm or more and 150 mm or less. If it is 50 mm or less, the distance between the cooling spray nozzle 21b and the slab 1 is short, the risk of nozzle clogging increases, and there is a risk of adversely affecting equipment maintenance such as spray check.
[0044]
　In the continuous steel casting method of the present embodiment, conditions other than the above are not particularly limited. The target steel type is not particularly limited. From the viewpoint of obtaining a more remarkable effect, it is preferable to target a low alloy steel containing at least one alloying element of Ti, Nb, Ni and Cu. As for the casting speed, it is possible to handle both low speed and high speed. Preferably, the casting speed Vc is 500 mm / min or more and 3000 mm / min or less. In the continuous casting method of the present embodiment, the casting conditions after the bent portion 30 may be the same as those in the conventional method. According to the continuous steel casting method of the present embodiment, for example, a slab can be manufactured.
[0045]
　According to another embodiment of the present invention, there is provided a continuous steel casting apparatus that employs each configuration of the above-described embodiment.
[0046]
　As described above, in the continuous steel casting method of the present invention, the slab is cooled by a mist spray having a high air-water ratio and a high collision in the first cooling zone 21 provided on the upper side of the vertical portion 20. By increasing the cooling intensity in the first cooling zone 21 to a predetermined value or more, and further setting the reheating time of the slab 1 from cooling by the first cooling zone 21 to reaching the bent portion to a predetermined value or more. The microstructure of the surface layer of the slab can be controlled, cracks on the surface of the slab due to non-uniform secondary cooling can be suppressed, and cracks on the surface of the slab due to strain in the bent portion can be suppressed.
Example
[0047]
　Hereinafter, the method for continuously casting steel of the present invention will be described in more detail with reference to Examples.
[0048]
1. 1. Experimental conditions A
　slab having a width of 2200 mm and a thickness of 300 mm was produced using a vertical bending type continuous casting apparatus. The steel type was a low alloy steel having the composition (mass%) shown in Table 1 and having high crack susceptibility.
　The Ac three- point temperatures of steel types A and B are 898 ° C and 872 ° C, respectively.
[0049]
[table 1]

[0050]
　In the secondary cooling zone of the continuous casting equipment, 15 mist spray nozzles are installed in each of the 5 stages of rolls from directly under the mold to the 1st to 6th stages every 150 mm in the width direction, and the amount of cooling water in each stage is independent. And made it controllable. This cooling zone was referred to as a "first cooling zone", and experiments were conducted by appropriately changing the amount of water and the amount of air. In addition, the experiment was conducted by appropriately changing the shape of the roll of the first cooling zone. The "split roll 1" is a split roll having one bearing portion having a size of 100 mm in the width direction, and the "split roll 2" is a split roll having two bearing portions having a size of 100 mm in the width direction. Yes, a single roll is a roll in which the entire width of the slab and the roll come into contact with each other without a split portion.
[0051]
　In the cooling zone (second cooling zone) from directly below the first cooling zone to the bent portion , the product of the average water volume density W 2 and the transit time t 2 is 0 to 50 (L / m 2 ). After passing through the first cooling zone, the slab was reheated before reaching the bent portion.
[0052]
　Table 2 below shows other casting conditions.
[0053]
[Table 2]

[0054]
2. Evaluation conditions
　Regarding the state of surface cracking of the slab, two full-width samples with a length of 100 mm in the casting direction were cut out in the casting direction in the stationary part of each casting condition, the surface of the slab was acid-cleaned, and the observed slab surface was 5 mm or more. The total number of surface cracks of length was evaluated as the "number of cracks". In addition, five samples for microscopic observation having a width of 30 mm and a width of 50 mm were cut out from the surface layer of the sample in the width direction, and the cast structure was also observed. The stationary portion means a portion of the slab drawn out at the target casting speed.
[0055]
　Table 3 below shows the details of the casting conditions and the evaluation results of the number of cracks in Examples and Comparative Examples.
[0056]
[Table 3]

[0057]
　As is clear from the results shown in Table 3, in Examples 1 to 6, there was no surface crack as described above, and in Examples 7 to 10, only shallow surface cracks were observed, and there was no problem. Further, when the cross section of the surface layer was subjected to nital etching and observed with an optical microscope, it was confirmed that a structure composed of fine ferrite pearlite having a size of 50 μm or less at least 2 mm from the surface was uniformly formed in the width direction.
[0058]
　In Examples 1 to 6, it is considered that the powder adhering to the surface of the slab could be peeled off and the cooling with reduced accumulated water could be performed in the first cooling zone directly under the mold. The surface layer of the slab can be stably cooled to a temperature of Ar 3 points or less even in the slab width direction, and then the temperature of the slab surface is restored to the temperature of Ac 3 points or more by the time it reaches the bent portion. It is probable that the tissue could be heated and the structure was hard to break.
[0059]
　In Examples 7 to 10, the fine structure of the surface layer had some unevenness, and it was considered that the surface layer was affected by the accumulated water, which was considered to be the cause of the shallow cracks.
[0060]
　In any of Examples 1 to 10, it was confirmed that there was no powder or scale adhering to the surface of the slab, and these could be peeled off by sufficient collision pressure.
[0061]
　On the other hand, in Comparative Example 1, the cooling strength (W 1 × t 1 ) is insufficient, and the position where the fine structure of the surface layer is 1 mm or less (the position where the length of the structure in the thickness direction of the slab is 1 mm or less). ), Many surface cracks occurred.
[0062]
　In Comparative Example 2, the cooling strength (W 1 × t 1 ) was sufficient, but the reheating time (t 2 ) was short, so that the slab was strained at the bent portion before the fine structure was formed on the surface of the slab. , It is probable that many surface cracks occurred. In particular, remarkable cracks were observed near the corners that were cooled two-dimensionally.
[0063]
　In Comparative Example 3, the cooling intensity (W 1 × t 1 ) was sufficient, but the brackish water ratio (A 1 / R 1 ) was small, and it is probable that the discharge of accumulated water deteriorated. As a result, many cracks were generated unevenly in the width direction.
[0064]
　In Comparative Examples 4 and 5, the collision pressure was insufficient, and many non-uniform cracks occurred due to uneven cooling. Adhering powder and scale were also confirmed from the surface layer sample, and it was found that sufficient collision pressure was not applied to peel them off.
[0065]
　From the above results, in order to prevent slab surface cracking that occurs when steel is continuously cast using a vertical bending type continuous casting device, the slab cooling conditions in the secondary cooling zone are as follows. It can be said that it is effective to do.
(1) In the first cooling zone provided on the upper side of the vertical portion, the ratio of the amount of air A 1 (L / min) to the amount of water R 1 (L / min) per cooling spray nozzle. The water ratio A 1 / R 1 is 10 or more. (2) In the first cooling zone, the collision pressure of the cooling water colliding with the surface of the slab from the cooling spray nozzle is set to 12 gf / cm 2 or more. (3) Cooling intensity defined as the product of the cooling water density W 1 (L / min / m 2 ) in the first cooling zone and the time t 1 (min) for the slab to pass through the first cooling zone. Let W 1 × t 1 be 350 or more. (4) The reheat time t 2 of the slab from passing through the first cooling zone to reaching the bent portion is set to 0.5 min or more.

Industrial applicability
[0066]
　INDUSTRIAL APPLICABILITY The present invention is a continuous steel casting method capable of controlling the microstructure of the slab surface layer, suppressing slab surface cracking due to secondary cooling non-uniformity, and suppressing slab surface cracking due to strain at a bent portion. It has high industrial applicability because it can provide.
Code description
[0067]
1 Shard
10 Mold
20 Vertical part
21 First cooling zone
21a Roll
21b Cooling spray nozzle
22 Second cooling zone
22a Roll
30 Bending part
100 Continuous casting equipment
The scope of the claims
[Claim 1]
　A first cooling zone including a vertical portion for drawing a slab downward from a mold in a vertical direction and a bending portion for bending the slab drawn from the vertical portion, and the vertical portion includes a roll and a cooling spray nozzle. This is a method of continuously casting steel using a vertical bending type continuous casting apparatus provided with the
　above, with respect to the amount of water R 1 (L / min) per cooling spray nozzle in the first cooling zone. The air- water ratio A 1 / R 1 , which is the ratio of the amount of air A 1 (L / min), is set to 10 or more, and the collision pressure of the cooling water colliding with the surface of the slab from the cooling spray nozzle is 12 gf / cm. 2 and above, 　the cooling water density W in the first cooling zone 1 (L / min / m 2 and), the time t in which the slab passes through the first cooling zone 1 defined as the product of the (min) The cooling strength W 1 × t 1 is set to 350 or more, and the reheating 　time t 2 of the slab from passing through the first cooling zone to reaching the bent portion is set to 0.5 min or more.

A method for continuous casting of steel.
[Claim 2]
The continuous steel according to claim 1, 　wherein in the first cooling zone, the amount of water R 1 (L / min) per cooling spray nozzle is 20 L / min or more and 50 L / min or less.
Casting method.
[Claim 3]
　According to claim 1 or 2, the cooling water density W 1 (L / min / m 2 ) is set to 500 L / min / m 2 or more and 2000 L / min / m 2 or less in the first cooling zone.
The method for continuous casting of steel according to the description.
[Claim 4]
　The vertical bending type continuous casting apparatus includes a second cooling zone between the first cooling zone and the bent portion, and in the second
　cooling zone, the cooling water density W 2 (L / min /). The steel according to any one of claims 1 to 3 , wherein the surface of the slab is reheated by setting m 2 ) to 0 L / min / m 2 or more and 50 L / min / m 2 or less.
Continuous casting method.
[Claim 5]
　The first cooling zone is recuperation surface of the slab after passing, the temperature of the surface of the slab Ac when the slab reaches the bent portion 3 and a temperature of more points
, characterized in that The method for continuously casting steel according to any one of claims 1 to 4.
[Claim 6]
　The
method for continuously casting steel according to any one of claims 1 to 5, wherein the roll is a split roll .