Title of the invention: In-mold flow control device and in-mold flow control method in thin slab casting Technical field [0001] 　The present invention relates to an in-mold flow control device and an in-mold flow control method in thin slab casting of steel. 　This application claims priority based on Japanese Patent Application No. 2018-109150 filed in Japan on June 7, 2018 and Japanese Patent Application No. 2018-211091 filed in Japan on November 9, 2018. And the contents are used here. Background technology [0002] 　A thin slab casting method for casting a thin slab (thin slab) having a slab thickness of 40 to 150 mm is known. The cast thin slab is heated and then rolled in a small-scale rolling mill having 4 to 7 steps. As the continuous casting mold used for thin slab casting, a method using a funnel mold (funnel mold) and a method using a rectangular parallel mold are adopted. In the funnel-shaped mold, the opening at the lower end of the mold (the part filled with molten steel and solidified shell) is rectangular, and for the opening at the meniscus of the mold, the opening width at the short side is the short side width at the lower end of the mold. This is a mold having a funnel shape in which the opening width of the portion where the immersion nozzle is inserted is widened and the surface shape of the opening gradually narrows below the lower end of the immersion nozzle. In continuous casting of thin slabs, it is necessary to secure productivity by high-speed casting, and industrially, high-speed casting of 5 to 6 m / min and a maximum of 10 m / min is possible (see Non-Patent Document 1). ). [0003] 　In thin slab casting, as described above, the casting thickness is generally as thin as 150 mm or less, while the casting width is about 1.5 m and the aspect ratio is high. And since the casting speed is as high as 5 m / min, the throughput is also high. In addition, a funnel-shaped mold is often used to facilitate pouring of molten steel into the mold, further complicating the flow in the mold. Therefore, it is common to reduce the nozzle discharge flow velocity by flattening the nozzle shape, making the nozzle discharge hole porous, and dividing the discharge flow (see Patent Document 1). Further, in order to brake each of a plurality of nozzle discharge flows, a method of arranging a plurality of electromagnets on the long side of the mold to brake the flow has also been proposed (see Patent Documents 2 and 3). [0004] 　The dipping nozzle used in ordinary continuous casting, which is not thin slab casting, has a bottomed cylindrical shape and has discharge holes on both side surfaces of the dipping portion. On the other hand, a nozzle having a slit that opens downward to the outside is known at the bottom of the immersion nozzle (see Patent Documents 4 and 5). The slit opens by connecting the bottom of the cylinder and the bottoms of the left and right discharge holes. Since the molten metal flowing out into the mold through the immersion nozzle flows out from the slits in addition to the left and right discharge holes, the flow velocity of the molten metal flowing out from the discharge holes can be relatively reduced. However, in normal continuous casting, which is not thin slab casting, as a result of blowing Ar gas into the molten metal passing through the immersion nozzle for the purpose of preventing clogging of the immersion nozzle, bubbles blown downward along with the nozzle discharge flow from the slit. As it rises upward, it boils around the nozzle and cannot be used well. [0005] 　Further, in normal slab continuous casting which is not thin slab casting, electromagnetic stirring in a mold is used to form a swirling flow in a horizontal cross section. On the other hand, in thin slab casting, such in-mold electromagnetic agitation is not used. This is because the thickness of the mold is thin, so it is assumed that it is difficult to form a swirling flow. This is probably due to the fact that the flow in the mold became complicated and was considered unfavorable. Prior art literature Patent documents [0006] Patent Document 1: US Pat. No. 6,152,336 Patent Document 2: Japanese Patent Application Laid-Open No. 2001-47196 Patent Document 3: US Pat. No. 9,352,386 Patent Document 4: Japanese Patent Application Laid-Open No. 2001-205396 Patent Document 5: Japan Japanese Patent Application Laid-Open No. 2007-105769 Non-patent literature [0007] Non-Patent Document 1: 5th Edition Steel Handbook Volume 1 Ironmaking and Steelmaking, pp. 454-456 Non-Patent Document 2: "Iron and Steel" by Shinobu Okano 61 (1975), pp. 2892 Outline of the invention Problems to be solved by the invention [0008] 　As described above, in thin slab casting, a method is proposed in which the nozzle discharge hole is made porous and the discharge flow is divided to reduce the nozzle discharge flow rate, and a plurality of electromagnets are arranged on the long side of the mold to brake the flow. Has been done. However, when the nozzle discharge flow is divided, it cannot be said that a constant flow pattern is formed because it is a turbulent flow. Further, when a plurality of electromagnets are provided to form a magnetic field, the magnetic field is lowered at the end of the electromagnet, and the magnetic field distribution becomes non-uniform. As a result, it is difficult to stably reduce the flow distribution because the fluid easily slips through the part where the magnetic field is weak. Therefore, in thin slab casting, it cannot be said that how to form the nozzle discharge flow has not been solved yet. [0009] 　Therefore, according to the present invention, in thin slab casting of steel, by stably controlling the flow in the mold and effectively supplying heat to the meniscus in the mold, casting of a slab excellent in both surface and internal component positions. It is an object of the present invention to provide an in-mold flow control device and an in-mold flow control method. Means to solve problems [0010] 　The gist of the present invention is as follows. (1) The first aspect of the present invention is an in-mold flow control device used for thin slab casting of steel having a meniscus portion having a short side thickness of 150 mm or less and a casting width of 2 m or less, in 　the total width in the mold width direction. , A DC magnetic field generating unit having a core for applying a DC magnetic field in the mold thickness direction, discharge holes formed on both side surfaces in the mold width direction, and opening to the outside in connection with the bottoms of these discharge holes. An immersion nozzle having a slit formed at the bottom thereof is provided, and the discharge hole and the slit exist in a DC magnetic field band which is a height region in which the core of the DC magnetic field generating unit exists, and the DC magnetic field is present. The magnetic flux density B (T) of the band and the distance L (m) from the lower end of the immersion nozzle to the lower end of the core satisfy the following equations (1) and (2) . This is an in-mold flow control device for thin slab casting. 　　0.35T ≤ B ≤ 1.0T ... (1) Equation 　　L ≥ 0.06m ... (2) [0011] (2) In the in-mold flow control device according to (1) above, the discharge hole diameter d (mm) of the discharge hole, which is the equivalent circle diameter of the same cross-sectional area as the total cross-sectional area of ​​the portion opened on the side surface of the immersion nozzle. , The slit thickness δ (mm) of the slit and the inner diameter D (mm) of the immersion nozzle may satisfy the following equations (3) and (4). 　　D / 8≤δ≤D / 3 ... (3) Equation 　　δ≤d≤2 / 3 x D ... (4) Equation (3) Flow control in the mold according to (1) or (2) above. In the device, the discharge hole may be formed so that the discharge flow is perpendicular to the axial direction of the immersion nozzle. (4) The in-mold flow control device according to any one of (1) to (3) above may further have an electromagnetic stirring unit capable of applying a swirling flow on the surface of the molten steel in the mold. Good. (5) In the in-mold flow control device according to (4) above, the thickness D Cu (mm) of the copper plate constituting the long side wall of the mold , the thickness T (mm) of the slab, and the frequency of the electromagnetic stirring unit. f (Hz) and the electric conductivity σ Cu (S / m) of the copper plate may be adjusted so as to satisfy the following equations (7A) and (7B). 　　D Cu <√ (2 / (σ Cu ωμ)) ・ ・ ・ (7A) equation 　　√ (1 / (2σωμ))