Continuous casting of molten steel is carried out, for example, by pouring molten steel, received from a ladle in a tundish, through a molten steel pouring nozzle secured to the bottom wall of the tundish, into a vertical mold arranged below the molten steel pouring nozzle to form a cast steel strand, and continuously withdrawing the thus formed cast steel strand into a long strand.
As the above-mentioned molten steel pouring nozzle, a nozzle comprising an alumina-graphite refractory is widely used in general.
However, the molten steel pouring nozzle comprising an alumina-graphite refractory has the following problems:
When casting an aluminum-killed molten steel, aluminum added as a deoxidizer reacts with oxygen present in molten steel to produce non-metallic inclusions such as .alpha.-alumina. The thus produced non-metallic inclusions such as .alpha.-alumina deposit and accumulate onto the surface of the bore of the molten steel pouring nozzle, through which molten steel flows, to clog up the bore, thus making it difficult to achieve stable casting. In addition, the non-metallic inclusions such as .alpha.-alumina thus accumulated onto the surface of the bore are peeled off or fall down, and are entangled into the cast steel strand, thus degrading the quality of the cast steel strand.
For the purpose of preventing the above-mentioned reduction or clogging of the bore of the molten steel pouring nozzle caused by the non-metallic inclusions such as .alpha.-alumina, there is proposed a method which comprises ejecting an inert gas from the surface of the bore of the molten steel pouring nozzle toward molten steel flowing through the bore, to prevent the non-metallic inclusions such as .alpha.-alumina present in molten steel from depositing and accumulating onto the surface of the bore (hereinafter referred to as the "prior art 1").
However, the above-mentioned method of the prior art 1 has the following problems:
A larger amount of the ejected inert gas causes entanglement of bubbles produced by the inert gas into the cast steel strand, resulting in the production of defects such as pinholes in a steel product after the completion of rolling. This problem is particularly serious in casting of molten steel for a high-quality thin steel sheet. A smaller amount of the ejected inert gas causes, on the other hand, deposition and accumulation of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore of the molten steel pouring nozzle, thus causing reduction or clogging of the bore. In the casting of molten steel for a long period of time, a stable control of the amount of ejected inert gas from the surface of the bore of the molten steel pouring nozzle becomes gradually more difficult, accordingly as the structure of the refractory forming a molten steel pouring nozzle degrades. As a result, the non-metallic inclusions such as .alpha.-alumina deposit and accumulate onto the surface of the bore of the molten steel pouring nozzle, thus causing reduction or clogging of the bore. Furthermore, in the casting of molten steel for a long period of time, a local erosion of the surface of the bore of the molten steel pouring nozzle is considerably accelerated by the ejected inert gas. This makes it impossible to continue the ejection of the inert gas and may cause rapid clogging of the bore.
With a view to solving the above-mentioned problems without employing a mechanical means such as the ejection of an inert gas, a molten steel pouring nozzle comprising a boron nitride refractory is usually used (hereinafter referred to as the "prior art 2").
However, the above-mentioned molten steel pouring nozzle of the prior art 2 has the following problems:
The molten steel pouring nozzle comprising a boron nitride refractory has a very high manufacturing cost. In addition, the function of this nozzle for inhibiting deposition of the non-metallic inclusions such as .alpha.-alumina onto the surface of the bore is not sufficient. Furthermore, boron nitride has a high thermal expansion coefficient. When, therefore, integrally manufacturing a molten steel pouring nozzle by the use of an alumina-graphite refractory for an outer portion of the molten steel pouring nozzle, and a boron nitride refractory for an inner portion of the molten steel pouring nozzle, which inner portion forms the bore, the connection is poor in the interface between the alumina-graphite refractory forming the outer portion of the molten steel pouring nozzle and the boron nitride refractory forming the inner portion of the molten steel pouring nozzle, so that peeloff of the boron nitride refractory from the alumina-graphite refractory during casting of molten steel may make it difficult to accomplish casting.
For the purpose of preventing clogging of the bore of the molten steel pouring nozzle, furthermore, there is disclosed in Japanese Patent Provisional Publication No. 57-71,860 a molten steel pouring nozzle formed with a refractory consisting essentially of:
graphite:from 10 to 50 wt. %, PA1 calcium oxide:from 20 to 75 wt. %, PA1 zirconia clinker comprising cubic zirconia and calcium zirconate:from 40 to 85 wt. %, PA1 graphite:from 10 to 30 wt. %; PA1 silica:from 1 to 15 wt. %, PA1 magnesia:from 1 to 15 wt. %.
and
the balance being a refractory aggregate.
The above-mentioned refractory aggregate may additionally contain metallic aluminum within the range of from 1 to 15 weight parts relative to 100 weight parts of the refractory aggregate (hereinafter referred to as the "prior art 3").
However, the above-mentioned molten steel pouring nozzle of the prior art 3 has the following problems:
It is true that calcium oxide (CaO) rapidly reacts with the non-metallic inclusions such as .alpha.-alumina produced through reaction of aluminum added as a deoxidizer with oxygen present in molten steel, to produce low-melting-point compounds. Therefore, calcium oxide has a function of preventing the non-metallic inclusions such as .alpha.-alumina from depositing and accumulating onto the surface of the bore. However, calcium oxide, when present alone, violently reacts with water or moisture in the air even at a room temperature to produce calcium hydroxide (Ca(OH).sub.2), which is easily disintegrated and tends to become powdery, thus easily causing degradation of the structure of the molten steel pouring nozzle. Great care is therefore necessary for storing the molten steel pouring nozzle. In addition, because of the high thermal expansion coefficient of calcium oxide, a considerable thermal stress is produced in the interior of the molten steel pouring nozzle when calcium oxide is present alone and subjected to heating which causes a non-uniform temperature distribution, thus resulting in a lower thermal shock resistance of the molten steel pouring nozzle.
For the problems as described above, it is difficult to use a molten metal pouring nozzle made of a refractory, in which calcium oxide is present alone, for a long period of time for the continuous casting of molten steel.
Under such circumstances, there is a strong demand for the development of a molten steel pouring nozzle which permits prevention of reduction or clogging of the bore of the nozzle and degradation of the structure of the refractory forming the nozzle, economically and for a long period of time without employing a mechanical means such as the ejection of an inert gas, but such a molten steel pouring nozzle has not as yet been proposed.