METHOD FOR USING UPPER NOZZLE

With a view to adding, to an upper nozzle formed with a bore having a shape capable of creating a less energy loss or smooth (constant) molten steel flow to suppress the occurrence of adhesion of inclusions and metals in molten steel, a gas injection function to thereby further suppress the occurrence of the adhesion, the present invention provides a method of using an upper nozzle configured to have a cross-sectional shape of a wall surface defining the bore, taken along an axis of the bore, comprising a curve represented by the following formula: log(r (z))=(1/n)×log((H+L)/(H+z))+log(r (L)) (n=1.5 to 6), where: L is a length of the upper nozzle; H is a calculational hydrostatic head height; and r (z) is an inner radius of the bore at a position downwardly away from an upper edge of the bore by a distance z. The method comprises using the upper nozzle in such a manner as to satisfy the following relationship: RG 4.3×VL, where RG is a gas rate defined as a volume ratio of a flow rate QG (Nl/s) of injection gas to a flow rate QL (l/s) of molten steel flowing through the bore (RG=(QG/QL)×100(%)), and VL is a flow speed of the molten steel at a lower edge of the upper nozzle.