Title of Invention: Equipment for continuous casting and continuous casting method used for thin slab casting of steel Technical field [0001] 　The present invention relates to a continuous casting facility and a continuous casting method used for thin slab casting of steel. 　The present application claims priority based on Japanese Patent Application No. 2018-109469 filed in Japan on June 7, 2018, the contents of which are incorporated herein by reference. Background technology [0002] 　A thin slab casting method for casting a thin slab (thin slab) having a slab thickness of 40 to 150 mm and further 40 to 100 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 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 and further 100 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, in order to brake the nozzle discharge flow, a method of arranging an electromagnet on the long side of the mold to brake the flow (electromagnetic brake) has also been proposed (see Patent Document 1). [0004] 　On the other hand, in general slab continuous casting, which is not thin slab casting, an in-mold electromagnetic agitator is used for the purpose of homogenizing the temperature of molten steel near the molten metal surface, homogenizing solidification, and preventing inclusions in the solidified shell. It is used. When using an electromagnetic stirrer, it is necessary to stably form a swirling flow of molten steel within the horizontal cross section of the mold. Therefore, conventionally, the positional relationship between the electromagnetic stirrer and the molten metal surface, the positional relationship between the electromagnetic stirrer and the immersion nozzle discharge hole for supplying molten steel from the tundish into the mold, the flow velocity of the molten steel discharged from the nozzle, and the stirring flow velocity have been used. Various techniques are disclosed regarding the relationship between the two. For example, Patent Document 2 discloses a method of installing the immersion nozzle discharge hole at a position where the magnetic flux density in the immersion nozzle discharge hole is 50% or less of the maximum magnetic flux density of the electromagnetic stirrer. [0005] 　Even in thin slab casting, if a swirling flow can be applied in the C cross section near the molten metal surface for the same purpose, the molten steel temperature near the molten metal surface can be made uniform, solidification can be made uniform, and inclusions can be prevented from being captured by the solidification shell. It can be said that this is preferable. However, in thin slab casting, the in-mold electromagnetic agitation used in general slab continuous casting is not used. This is because it is assumed that it is difficult to form a swirling flow because the mold thickness is thin, and sufficient flow is already given to the front surface of the solidification shell due to high-speed casting, and if a swirling flow is given near the molten metal surface. 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: Japanese Patent Application Laid-Open No. 2001-47196 Patent Document 2: Japanese Patent Application Laid-Open No. 2001-47201 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] 　In thin slab casting, since high-speed casting is performed while the slab thickness is thin, an electromagnetic brake is generally used as described above in order to first brake the nozzle discharge flow and stabilize the molten metal level. However, especially in thin slab casting, the gap between the dipping nozzle and the long side of the mold is narrowed, so that the flow of molten steel tends to stagnate in this narrow gap. Even in thin slab casting, it is preferable to secure the flow between the immersion nozzle and the long side of the mold and to make a uniform swirling flow over the entire surface level. In general slab casting, which is not thin slab casting, an electromagnetic stirrer (hereinafter, also referred to as EMS) is installed on the back side of the long side wall of the mold as described above, and the long side walls facing each other are installed. A method of applying a stirring flow so as to form a swirling flow in the horizontal cross section near the meniscus in the mold by applying thrusts in opposite directions is widely used. [0009] 　By applying the above method, it is possible to make the temperature distribution of the molten steel in the vicinity of the molten metal surface in the mold uniform and the thickness of the solidified shell uniform, and in addition, it is possible to prevent inclusions from being trapped in the solidified shell. Therefore, first, even in thin slab casting, it is preferable to form a swirling flow in the horizontal cross section near the meniscus in the mold. Next, since the effect of equalizing the solidification shell thickness increases as the flow velocity of the stirring flow increases, it is preferable to provide a sufficient stirring flow. In particular, in thin slab casting of steel types that are prone to non-uniform solidification due to δ / γ transformation, such as subcapsular steel, the length is long due to the stagnation of molten steel flow in the narrow gap between the immersion nozzle and the long side of the mold. Vertical cracks are likely to occur in the center of the side, and it is important to provide a sufficient stirring flow. [0010] 　When a swirling flow is formed in the mold, as shown in FIG. 2, at the four corners in the mold, the pressure increases at the portion where the stirring flow collides and the molten metal surface rises, and the short side wall side of the mold On the contrary, the phenomenon that the molten metal surface is dented occurs in the central portion in the thickness direction (hereinafter, also referred to as the central portion in the thickness direction). Specifically, as shown in FIG. 2A, by applying a stirring flow so as to swirl in the horizontal cross section by EMS, the molten steel surface 7 rises at the corner and the thickness on the short side wall side. It swells in the center. The powder layer 18 is present on the molten steel surface 7. [0011] 　In particular, focusing on the short side wall where the distance between the corners is short and the slope is large due to the unevenness of the molten metal level, as shown in FIG. 2B, the solidified shell 19 is first formed at the corner and the thickness is increased. In the central part, solidification starts later than in the corner part due to the unevenness of the molten metal level. Therefore, further below the mold, as shown in FIG. 2C, solidification is delayed most at the central portion of the thickness, and the solidification delay portion 20 is formed. [0012] 　The immersion nozzle 2 is provided with a discharge hole 3 directed in the long side direction of the mold 12, and when a molten steel discharge flow (hereinafter, also referred to as a nozzle discharge flow 4) is formed from the discharge hole 3, the thickness of the slab is formed. In the direction, the flow velocity is highest in the central part of the thickness. The nozzle discharge flow 4 collides with the short side solidification shell. The solidification delay due to the nozzle discharge flow colliding with the short-side solidification shell is most noticeable in the central portion of the thickness in the thickness direction of the slab. In particular, in casting of steel types such as subcapsular steel, which are prone to non-uniform solidification due to δ / γ transformation, the central part of the short side thickness is further lifted by the bending moment, which accelerates the solidification delay and also the interface. Tensile stress acts on the surface and cracks are likely to occur under the skin. [0013] 　From the above, as a result of the unevenness of the molten metal level shape formed by the stirring flow by EMS, in addition to the delay in solidification, the nozzle discharge flow collides with each other, resulting in a locally excessive solidification delay portion, and the degree thereof is remarkable. Then, a breakout occurs. Further, such a phenomenon is likely to occur because the narrower the casting width, the shorter the distance between the immersion nozzle and the short side wall. [0014] 　Due to the above situation, in thin slab casting, it is difficult to perform electromagnetic agitation to apply a swirling flow in the mold, and even if it is performed, the solidified shell is made uniform, especially the long side of the subcapsular steel. It was difficult to provide a sufficient stirring flow rate to prevent vertical cracking in the center. [0015] 　The present invention has been made in view of such circumstances, and an object of the present invention is to provide a continuous casting facility and a continuous casting method for steel capable of preventing vertical cracks in the center of the long side of a slab in thin slab casting. Means to solve problems [0016] 　The gist of the present invention is as follows. (1) The first aspect of the present invention is a continuous casting facility used for thin slab casting of steel having a slab thickness of 150 mm or less and a casting width of 2 m or less in a mold, each of which is composed of a copper plate. A mold for molten steel casting having a pair of long side walls and a pair of short side walls arranged to face each other, a dipping nozzle for supplying molten steel into the mold, and the back side of the pair of long side walls. It has an electromagnetic stirrer which is arranged along the long side wall and can apply a swirling flow on the surface of the molten steel in the mold, and satisfies the following equations (1) -a and (1) -b. As described above, the thickness D Cu (mm) of the copper plate on the long side wall, the thickness T (mm) of the slab, the frequency f (Hz) of the electromagnetic stirrer, and the electrical conductivity σ (S / m) of the molten steel. ), And the equipment for continuous casting of steel in which the electrical conductivity σ Cu (S / m) of the copper plate on the long side wall is adjusted. 　　D Cu <√ (2 / σ Cu ωμ) (1) -a 　　√ (1 / 2σωμ)