4f^ [Document Type] Specification [Title of the Invention] CONTINUOUS CASTING APPARATUS FOR STEEL [Technical Field of the Invention] [0001] The present invention relates to a continuous casting apparatus for steel, which supplies molten steel into a casting mold to manufacture a cast piece. [Background Art] [0002] In continuous casting for steel, in order to improve surface properties of a cast piece, hitherto, an electromagnetic stirring device having an electromagnetic coil provided in the vicinity of the upper portion of a casting mold is used to electromagnetically stir molten steel within the casting mold. [0003] For the electromagnetic stirring, for example, the electromagnetic stirring devices are disposed along a pair of long side walls included in the casting mold. In addition, when the molten steel is discharged from a submerged entry nozzle into the casting mold, current is supplied to the electromagnetic stirring devices to apply a thrust to the uppeFpiSt of the molten steel within the casting mold. The molten steel is stirred in a horizontal plane by the thrust such that a swirling flow of the molten steel is formed. By the swirling flow, trapping of inclusions, bubbles, and the like in the vicinity of a meniscus at the upper portion in the casting mold by a solidified shell formed on the side surfaces of the casting mold is suppressed. [0004] However, since the submerged entry nozzle is submerged in the casting mold, regions between the long side walls and the submerged entry nozzle are narrower than the other regions. Therefore, in the regions between the long side walls and the submerged entry nozzle, compared to the other regions, it is difficult for the molten steel to flow. In addition, inclusions and the like are likely to adhere to and deposit on the periphery of the submerged entry nozzle in the casting mold. There may be cases where the adhered matter deposited as such has a thickness of several tens of millimeters. Therefore, the regions between the long side walls and the submerged entry nozzle are narrower than the other regions. In this case, the flow channel for the swirling flow is partially narrowed, and thus it is difficult for the molten steel to flow in the regions between the long side walls and the submerged entry nozzle. [0005] Here, simultaneously with the use of electromagnetic stirring devices described above, instead of a flat shaped casting mold having a flat internal surface in a furnace, using a so-called irregular shaped casting mold in which, as illustrated in FIG. 7, surfaces 104 and 105 in long side walls 101 and 102, which face a submerged entry nozzle 103 are convexly curved toward electromagnetic stirring devices 106 and 107, respectively, is proposed (Patent Document 1). In addition, in FIG. 7, back plates 108 and 109 made of stainless steel, in which flow channels of cooling water (not illustrated) used to cool the long side walls 101 and 102 are provided are disposed between the long side walls 101 and 102 and the electromagnetic stirring devices 106 and 107. [0006] According to the irregular shaped casting mold, since the surfaces 104 and 105 in the long side walls 101 and 102, which face the submerged entry nozzle 103 are convexly curved toward the electromagnetic stirring devices 106 and 107, respectively, the shortest horizontal distances between the submerged entry nozzle 103 and the long • side walls 101 and 102 are longer than those of the parallel casting mold according to the related art. Therefore, the flow channels of swirling flows 110 and 111 can be vddely ensured to that extent, and thus the molten steel easily flows. [Related Art Document] [Patent Document] [0007] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2008-183597 [Summary of the Invention] [Problem to be Solved by the Invention] [0008] However, in the above-described technique according to the related art, in order to convexly curve the surfaces 104 and 105 in the long side walls 101 and 102, which face the submerged entry nozzle 103, the center portions of the long side walls 101 and 102 made of copper are cut. Therefore, the thicknesses of the long side walls 101 and 102 are extremely small in the parts of the curved surfaces 104 and 105. In general, the electromagnetic field made by the electromagnetic stirring devices 106 and 107 is an alternate current magnetic field, and thus the magnetic field is attenuated in conductors. Therefore, in the parts of the curved surfaces 104 and 105, the magnetic field is less attenuated than the other linear parts, and thus the electromagnetic force increases. Accordingly, the flow velocity of the stirring flow in the regions between the curved surfaces 104 and 105 and the submerged entry nozzle 103 becomes faster than that in the other regions. As a result, the flow velocity of stirring flows 110 and 111 becomes partially non-uniform, a flow disturbance or a stagnant zone is generated in regions 112 and 113 on the downstream sides of the stirring flows 110 and 111 in the long side walls 101 and 102, and there is a problem in that inclusions, bubbles, and the like are likely to be trapped by a solidified shell. Therefore, enhancement of steel quality to an expected degree cannot be obtained. [0009] The inventors had fiirther performed examinations, and found that trapping of inclusions by the solidified shell of the long side walls 101 and 102 could not be suppressed by only forming the cvirved surfaces 104 and 105 which allow the stirring flows 110 and 111 to easily flow. That is, it was proved that when the horizontal distance between the curved surfaces 104 and 105 and the submerged entry nozzle 103 is increased, trapping of bubbles can be suppressed. However, in the parts of the curved surfaces 104 and 105, the electromagnetic force is also strengthened, and the flow velocities of the stirring flows that flow in the regions between the curved surfaces 104 and 105 and the submerged entry nozzle 103 become faster than those of the stirring flows that flow in the other regions. Therefore, a flow disturbance or a stagnant zone is generated in the regions 112 and 113 on the downstream sides of the stirring flows 110 and 111, and thus a problem in which inclusions are likely to be trapped by the solidified shell is not solved. [0010] The present invention has been made taking the foregoing points into consideration, and an object thereof, even in an irregular shaped casting mold in a continuous casting apparatus for steel, is to allow the flow velocity of molten steel at the upper portion in the casting mold to be uniform and to allow a horizontal distance between a surface concavely curved in the casting mold and a submerged entry nozzle to be appropriate, thereby enhancing the quality of a cast piece obtained by casting. [Means for Solving the Problem] [0011] In order to accomplish the object, the present invention employed the following measures. (1) That is, according to anaspect of the present invention, a continuous casting apparatus for steel includes: a casting mold for casting a molten steel, which includes a pair of long side walls and a pair of short side walls; a submerged entry nozzle which discharges the molten steel into the casting mold; and an electromagnetic stirring device which is disposed along an external surface of each of the long side walls and stirs an upper part of the molten steel within the casting mold. A curved portion which is convexly curved toward the electromagnetic stirring device in plan view is formed at least at a position where the curved portion faces the submerged entry nozzle on each of the long side walls, and each of the long side walls including the curved portion has a uniform thickness. The shortest horizontal distance between a top which is a most depressed position when an internal surface of the curved portion is seen in plan view and an outer peripheral surface of the submerged entry nozzle is 30 mm to 80 mm in a range from a lower end portion of the elecfromagnetic stirring device to a position higher than an upper end portion of the elecfromagnetic stirring device by 50 mm when viewed along a vertical direction. [0012] (2) In the aspect according to the above (1), an elecfromagnetic brake device — disposed below the elecfromagnetic stirring device may be fiirther included, and the elecfromagnetic brake device applies a direct current magnetic field having a magnetic flux density distribution which is uniform in a casting mold width direction along each of the long side walls in plan view, in a casting mold thickness direction along each of the short side walls. In addition, in the aspect of the above (1), it is more preferable that the shortest horizontal distance be 50 mm to 75 mm in the range from the lower end portion of the electromagnetic stirring device to a position higher than the upper end portion of the electromagnetic stirring device by 50 mm when viewed along the vertical direction. [Effects of the Invention] [0013] According to the aspect described in the above (l),.each of the long side walls . has the curved portion which is convexly curved toward the elecfromagnetic stirring device at least at a position that faces the submerged entry nozzle, and each of the long side walls including the curved portion has a uniform thickness. Therefore, the elecfromagnetic force generated by the elecfromagnetic stirring device is uniform over the curved portion and the other parts. As a result, the flow velocity of the stirring flow becomes uniform. That is, the intensity distribution of the electromagnetic force when each of the long side walls is seen in plan view is the same in the curved portion and the parts other than the curved portion. Therefore, unlike the related art, the electromagnetic force can be prevented from becoming partially stronger at a point corresponding to the curved portion. Therefore, the generation of a flow disturbance or a stagnant zone as in the related art can be suppressed, and easy trapping of bubbles by the solidified shell can be suppressed. [0014] In addition, since the shortest horizontal distance between the top of the curved portion and the submerged entry nozzle is set to be 30 mm to 80 mm in a range from the position of the lower end portion of the electromagnetic stirring device to a position higher than the upper end portion of the electromagnetic stirring device by 50 mm when viewed in the height direction of the continuous casting apparatus, a smooth and uniform flow of the molten steel can be ensured even in the region between the top of the curved portion and the submerged entry nozzle. [0015] That is, according to the knowledge newly obtained by the inventors, when the shortest horizontal distance between the top of the curved portion and the submerged entry nozzle is smaller than 30 mm, it is difficult for the molten steel to flow in the curved regions, and bubbles and the like in the molten steel are likely to be trapped by • the solidified shell. In contrast, when the shortest horizontal distance exceeds 80 mm, it is difficult to ensure a uniform flow of the molten steel in the curved region, and in a region where the flow velocity of the molten steel is slow, inclusions in the molten steel are likely to be trapped by the solidified shell. [0016] In the present invention, based on the knowledge, the shortest horizontal distance between the top of the curved portion and the submerged entry nozzle is set to 30 mm to 80 mm. Therefore, in the curved region between the top of the curved portion and the submerged entry nozzle, a smooth and uniform flow of the stirring flow of the molten steel is ensured, and bubbles in the molten steel can be prevented from being trapped by the solidified shell. [0017] In addition, the range in the height direction in which the shortest horizontal distance between the top of the curved portion and the submerged entry nozzle is set to 30 mm to 80 mm, as such is a range from the lower end portion of the electromagnetic stirring device to a position higher than the upper end portion of the electromagnetic stirring device by 50 mm. This is because although a part of the molten steel that is directly stirred by the electromagnetic force generated by the electromagnetic stirring device is a part from the lower end portion to the upper end portion of the electromagnetic stirring device, in a practical operation, the surface of a meniscus is positioned at a position higher than the upper end portion of the electromagnetic stirring device. In addition, typically, in the case where the surface of the meniscus is positioned at a position higher than the upper end portion of the electromagnetic stirring device, the height is at a position higher than the upper end portion of the electromagnetic stirring device by about 50 mm. Therefore, the range in the height direction in which the shortest horizontal distance between the top of the curved portion and the submerged entry nozzle is set to be 30 mm to 80 mm is from the lower end portion of the electromagnetic stirring device to the position higher than the upper end portion of the electromagnetic stirring device by 50 mm. [0018] In addition, a uniform thickness of the long side wall is referred to as a thickness in which a change in a penetration degree of aii electromagnetic field in the molten steel due to a change in thickness excluding parts where bolt holes, cooling water grooves, and the like are formed is less than 10% which is an error in an acceptable range. This will be described hereinafter. In a case where a magnetic field having a predetermined magnetic flux density is applied to the inside of the casting mold from the outside of the long side wall, the magnetic field intensity induced inside the casting mold has a loss depending on the value of the thickness of the long side wall. That is, when the thickness of the long side wall is changed, the penetration depth of the magnetic field into the casting mold is changed. When the long side wall is thick, it is difficult for the magnetic field to penetrate. Therefore, the magnetic field intensity in the casting mold is changed with the magnitude of the loss. However, the thickness of the long side wall is caused to be uniform so that the change is less than 10% when viewed in the horizontal direction along the wall surface of the long side wall. In addition, the range in the height direction of the uniform thickness of the long side wall may be a range from the lower end portion of the electromagnetic stirring device to a position higher than the upper end portion of the electromagnetic stirring device by 50 mm as described in the effect of the electromagnetic stirring device. In addition, "the uniform thickness of the long side wall" will be further supplementarily described. When the long side wall disposed along the vertical direction is seen in plan view, the relative relationship between the thickness of the part of the curved portion and the thickness of the adjacent part excluding the curved portion is particularly important. That is, "each of the long side walls including the curved portion has a uniform thickness" described in the above (1) means that, in a case where the thickness of the part of the curved portion is tl and the thickness of the adjacent part excluding the curved portion is t2, tl is within ±10% of t2 (0.9xt2