Method for controlling amount of silicon added to ductile cast iron, method for casting ductile cast iron, and cast product

A method for controlling an amount of silicon added to ductile cast iron, comprising:

• smelting ductile cast iron using scrap steel as a raw material, wherein after the scrap steel is melted into molten iron, a copper alloy is first added so that the molten iron has a copper equivalent of 0.8% to 1.0%, and then ferrosilicon is added so that a content of silicon added to the molten iron satisfies a following formula (I):
• Siadded=1.3CuE+10P+4S−Sioriginal formula (I),
• in formula I, Siadded is the content of silicon added to the molten iron, and CuE is the copper equivalent in the molten iron;
• and P, S, and Sioriginal are a phosphorus content, a sulfur content, and a silicon content in the molten iron before the ferrosilicon is added, respectively;
• wherein the copper equivalent is controlled by formula (II):
• CuE=[WCu]newly added+[(WMn−0.45%)/2.5+1.1WCr+(WMo−0.15%)/2+0.1WNi+12WSn+WCu+4Wv]original formula (II),
• in formula (II) CuE is the copper equivalent, [WCu]newly added is a content of copper added to the molten iron, and WMn, WCr, WMo, WNi, WSn, WCu, and Wv are a manganese content, a chromium content, a molybdenum content, a nickel content, a tin content, a copper content, and a vanadium content originally present in the molten iron, respectively,
• wherein a mixture of microcrystalline graphite powder and silicon carbide crystal is added in duration that scrap steel is being melted into the molten iron, and
• wherein the copper alloy is added within 10 to 15 minutes after the mixture of microcrystalline graphite powder and silicon carbide crystal is added, so that the molten iron has the copper equivalent of 0.8% to 1.0%.