Provided is a ZrO.sub.2—CaO—C based refractory material which is capable of maintaining high adhesion resistance over a long period of time, while exhibiting significant slaking resistance, and suppressing self-fluxing, i.e., exhibiting corrosion-erosion resistance. The refractory material comprises a CaO—ZrO.sub.2 composition containing a CaO component in an amount of 40% by mass to 60% by mass, wherein a mass ratio of the CaO component to a ZrO.sub.2 component is 0.67 to 1.5, and wherein the CaO—ZrO.sub.2 composition includes a eutectic microstructure of CaO crystals and CaZrO.sub.3 crystals, wherein a width of each of the CaO crystals observable in a cross-sectional microstructure is 50 μm or less.

A bubbling plate for a sliding nozzle includes a plate body defining an inner hole therein and a gas-permeable ring installed inside the plate body to define part of the inner hole. The ring has a lower end surface formed with a stepped part, a convex part or a concave part; and the plate body has an upper surface in contact with the lower end surface with the upper surface formed with a stepped part, a concave part or a convex part capable of being opposed to and fittingly engaged with a corresponding part of the ring. The area in which the part of the ring is fittingly engaged with the corresponding part of the plate body has a length of 2 mm or more along a direction of a central axis of the inner hole, and mortar is interveningly provided in at least a longitudinally-extending joint part in the fitting engagement area.

A bubbling plate for a sliding nozzle includes a plate body defining an inner hole therein and a gas-permeable ring installed inside the plate body to define part of the inner hole. The ring has a lower end surface formed with a stepped part, a convex part or a concave part; and the plate body has an upper surface in contact with the lower end surface with the upper surface formed with a stepped part, a concave part or a convex part capable of being opposed to and fittingly engaged with a corresponding part of the ring. The area in which the part of the ring is fittingly engaged with the corresponding part of the plate body has a length of 2 mm or more along a direction of a central axis of the inner hole, and mortar is interveningly provided in at least a longitudinally-extending joint part in the fitting engagement area.

A tundish upper nozzle structure and a continuous casting method make it possible to cause inclusions to float within a tundish. A flange-shaped member having an outside dimension greater than that of an upper end of a tundish upper nozzle is provided along a part or the entirety of the circumference of the upper end of the tundish upper nozzle, and one or more gas discharge holes are provided in one or more of the following surfaces: a lower surface, an outer peripheral surface and a top surface of the flange-shaped member, and a region of an outer peripheral surface of the tundish upper nozzle below the flange-shaped member. A length in the tundish upper nozzle structure is adjusted to cause almost all gas to float upwardly, or to adjust the flow rate of gas flowing downwardly toward the inner bore of the tundish upper nozzle, and the flow rate of gas floating upwardly.

A tundish upper nozzle structure and a continuous casting method make it possible to cause inclusions to float within a tundish. A flange-shaped member having an outside dimension greater than that of an upper end of a tundish upper nozzle is provided along a part or the entirety of the circumference of the upper end of the tundish upper nozzle, and one or more gas discharge holes are provided in one or more of the following surfaces: a lower surface, an outer peripheral surface and a top surface of the flange-shaped member, and a region of an outer peripheral surface of the tundish upper nozzle below the flange-shaped member. A length in the tundish upper nozzle structure is adjusted to cause almost all gas to float upwardly, or to adjust the flow rate of gas flowing downwardly toward the inner bore of the tundish upper nozzle, and the flow rate of gas floating upwardly.

A method for maintaining the optimal argon injection flow rate which will result in production of steel slab of a chosen alloy having optimal cleanliness. The steel is cast using an argon injected slide gate. The selected steel has a known optimal argon injection flow rate Qb* for casting steel of optimal cleanliness. The method involves calculating the present steel pressure and determining the present injection flow rate conductance Gb′ of the argon injected slide gate during either of 1) a steel pressure change event; or 2) an argon flow change event. The measurements are used to calculate present argon pressure required to insure the required injection flow rate of argon into the steel for optimal cleanliness of the cast steel.

A method for maintaining the optimal argon injection flow rate which will result in production of steel slab of a chosen alloy having optimal cleanliness. The steel is cast using an argon injected slide gate. The selected steel has a known optimal argon injection flow rate Qb* for casting steel of optimal cleanliness. The method involves calculating the present steel pressure and determining the present injection flow rate conductance Gb′ of the argon injected slide gate during either of 1) a steel pressure change event; or 2) an argon flow change event. The measurements are used to calculate present argon pressure required to insure the required injection flow rate of argon into the steel for optimal cleanliness of the cast steel.

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: R.sub.G≦4.3×V.sub.L, where R.sub.G is a gas rate defined as a volume ratio of a flow rate Q.sub.G (Nl/s) of injection gas to a flow rate Q.sub.L (l/s) of molten steel flowing through the bore (R.sub.G=(Q.sub.G/Q.sub.L)×100(%)), and V.sub.L is a flow speed of the molten steel at a lower edge of the 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: R.sub.G≦4.3×V.sub.L, where R.sub.G is a gas rate defined as a volume ratio of a flow rate Q.sub.G (Nl/s) of injection gas to a flow rate Q.sub.L (l/s) of molten steel flowing through the bore (R.sub.G=(Q.sub.G/Q.sub.L)×100(%)), and V.sub.L is a flow speed of the molten steel at a lower edge of the upper nozzle.

A casting apparatus for producing a casting by pouring a metal melt into a gas-permeable casting mold by gravity, comprising: a gas-permeable casting mold comprising a cavity including a sprue composed of a tubular portion and a cup portion having a larger diameter than that of the tubular portion to receive the metal melt, a runner constituting a flow path of the metal melt supplied through the sprue, and a product-forming cavity to be filled with the metal melt sent through the runner; a means for pouring the metal melt into the sprue by gravity; a gas-blowing unit comprising a gas-ejecting member to be connected to the sprue; and a mechanism for moving the gas-ejecting member; the gas-ejecting-member-moving mechanism placing the gas-ejecting member at a position just above the tubular portion and not interfering with gravity pouring of the metal melt, and moving it downward for connection to the tubular portion; the gas-blowing unit having blowing a gas to fill the product-forming cavity with the metal melt.