Title of invention: Method for producing molten steel Technical field [0001] 　The present invention relates to a method for producing molten steel, which produces molten steel by reducing and melting reduced iron (DRI) produced by pre-reducing iron oxide (iron ore, etc.) in a melting furnace. Background technology [0002] 　Conventionally, it costs a lot to construct a new blast furnace, so in countries where natural gas is produced, for example, by the Midrex method, iron oxide such as agglomerated iron ore such as pellets is reduced in a shaft furnace to make metal. The mainstream process is to produce reduced iron (DRI) with a conversion rate of 90% or more, melt the DRI in an electric furnace, and directly produce molten steel. [0003] 　In addition, a reduced iron production process that uses a coal material such as coal as a reducing agent instead of natural gas has also been developed and put into practical use. In this reduced iron production process, a method of heating and reducing calcined pellets of iron ore together with coal powder with a rotary kiln (SL / RN method), or a method of mixing carbonaceous material and powdered iron oxide to agglomerate and rotary hearth There is a method (RHF method) for producing reduced iron by heating and reducing the above. With these methods, it is difficult to produce a DRI having a high metallization rate as compared with the shaft furnace method, and the metallization rate is generally as high as about 85%. Therefore, when using this DRI, it is necessary to melt the metallic iron in a melting furnace such as an electric furnace and also reduce the remaining iron oxide. [0004] 　In Patent Document 1, a DRI having a metallized iron content of 60% or more is produced by the RHF method, and then molten iron having a carbon content of 1.5 to 4.5% by mass is produced in an arc heating type melting furnace. A method of desulfurizing, dephosphorizing, and decarburizing in another melting furnace after discharging the molten iron to the outside of the furnace is described. In this method, a carbonaceous material is added to the melting furnace in order to reduce the remaining iron oxide. However, in this method, the heat loss becomes large by transferring the molten iron to another furnace. Further, in order to secure a heat source, a carbon dioxide material is further added to decarburize molten iron having a high carbon content to produce molten steel, so that the amount of CO 2 generated increases. Further, Patent Document 2 discloses a technique for dissolving an iron-based raw material while supplying a hydrocarbon gas. However, this method is costly because it is premised on the use of hydrocarbon gas. Prior art literature Patent documents [0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2001-515138 Patent Document 2: Japanese Patent Application Laid-Open No. 2016-108575 Outline of the invention Problems to be solved by the invention [0006] 　In view of the above-mentioned problems, the present invention has high productivity, low heat loss, and low CO 2 generation when melting and reducing DRI having a particularly low metallization rate in a melting furnace such as an electric furnace. It is an object of the present invention to provide the manufacturing method of. Means to solve problems [0007] 　In the present invention, in order to dissolve and reduce DRI having a low metallization rate to produce molten steel, a part of the molten steel is left in the furnace and used as a seed bath for the next channel. However, if the seed water remains as molten steel, the dissolution and reduction of DRI is delayed. Therefore, before supplying DRI, only the carbon source is first supplied to the seed water to increase the C concentration of the seed water. As will be described later, the C concentration is preferably 0.5% by mass or more and 1.5% by mass or less. [0008] 　The present invention is as follows. (1) The first step of adding a carbon source to the molten steel left in the electric furnace as a seed hot water at the time of steel ejection of the previous channel to obtain 　carbon-containing molten iron, and DRI to the carbon-containing molten iron produced in the first step. The second step of adding and reducing the solution, 　the third step of adding a deoxidizer and performing the desulfurization treatment, and 　the fourth step of discharging the desulfurized slag produced by the desulfurization treatment of the third step. If, 　then, a fifth step of performing blowing in decarburization oxygen, 　the fifth and the sixth step of discharging the decarburization slag generated in the decarburization step, 　the decarburization slag in the sixth step A method for producing molten steel , which comprises a seventh step of ejecting steel while leaving the seed water of the next channel after discharging the slag. (2) a furnace diameter of the electric furnace when the D (m), the seventh seed leaves in step hot water W (t) is × D 0.3 2