In a process for continuous casting of aluminum-killed steel, when molten steel is deoxidized by the addition of aluminum, Al2O3-based inclusions formed in the molten steel adhere and build up onto an inner wall surface of a casting nozzle to cause a nozzle clogging phenomenon where an inner bore diameter of the nozzle is gradually reduced.
This nozzle clogging phenomenon leads directly to shortening of a usable life of the casting nozzle. Moreover, in consequence of the nozzle clogging phenomenon, molten steel flows into a mold at an uneven flow rate to cause drift in the mold, to cause a casting defect due to inclusion of mold powder and bubbles (so-called blowholes). These adverse affects obstruct the continuous casting operation and lead to deterioration in steel quality.
As measures against the nozzle clogging phenomenon, various proposals, such as the use of a material and structure intended to prevent adhesion of Al2O3, have been made.
As to the use of a material intended to prevent adhesion of Al2O3, there has been known a technique of inducing a reaction between CaO in a ZrO2—CaO—C refractory material and Al2O3-based inclusions in molten steel to form a low-melting-point material, and melting a surface of the low-melting-point material in such a manner as to flow down together with the molten steel to obtain an anti-adhesion effect, as disclosed, for example, in the following Patent Publication 1. However, in cases where the cleanness of molten steel is relatively low, this technique cannot achieve a sufficient anti-adhesion effect, because a compound, such as CaO·6Al2O3, with a higher melting point than that of molten steel, is formed concurrently with the formation of the low-melting-point material.
As to the structure intended to prevent adhesion of Al2O3, the following Patent Publication 2 proposes an immersion nozzle for continuous casting. The immersion nozzle comprises a spiral swirl vane disposed in an inner hole thereof and designed to generate a swirling flow in molten steel passing through the inner hole, and a position and dimensions of the spiral swirl vane are adjusted to prevent adhesion of Al2O3-based inclusions to the spiral swirl vane. In this immersion nozzle, due to a swirling flow generated by the spiral swirl vane, low-density Al2O3-based inclusions and bubbles are apt to be concentrated in the center of molten steel flow on a downstream side relative to the spiral swirl vane. Thus, the molten steel flow adjacent to an inner wall surface is liable to stagnate in a region of the inner hole ranging from the spiral swirl vane to a position of a molten steel level (secondary meniscus) to cause a problem about increase in adhesion of Al2O3 to the inner wall in this region.                [Patent Publication 1] Japanese Patent Publication No. 07-34978        [Patent Publication 2] Japanese Patent Laid-Open Publication No. 2003-33846        