This invention relates to an improved shrouding apparatus for protecting a molten metal pouring stream from atmospheric reoxidation.
In the continuous casting of molten metals such as steel, molten metal from a ladle is teemed into an intermediate pouring vessel called a tundish positioned above a continuous casting mold. The tundish has a pouring nozzle in its bottom wall. When continuously casting billets, a caster will often have as many as six billet strands issuing from six molds, thus the tundish will require six pouring nozzles.
Atmospheric reoxidation of the steel stream flowing between the tundish and the mold will cause the accumulation and entrapment of undesirable oxide inclusions in each cast billet. Inclusions trapped in the billet render the product cleanliness unacceptable for quality steel grades. To alleviate the problem of reoxidation, various types of shrouds have been developed and used in the continuous casting of steels. Bailey British Patent Specification No. 371,880, Lyman U.S. Pat. No. 3,572,422 and Pollard U.S. Pat. No. 3,908,734 teach shrouding of molten metal pouring streams with inert or reducing gas. Some types of shroud are manufactured from refractory materials and termed "refractory pouring tubes" since they project downward from the bottom of the tundish to beneath the surface of the metal in the mold as shown by Mills, et al U.S. Pat. No. 3,517,726. Alternatively, bellows-type shrouds exist which are attached to both the tundish and the mold, affording a completely enclosed pouring chamber which allows vertical oscillation of the mold. However, neither the stream characteristics nor the metal in the mold can be observed through this bellows-type shroud. Additionally, the bellows-type shroud affords no access to the nozzle. Still other shrouds exist which are mounted on the mold and extends upwards to the tundish. One such shroud is a split cylinder, half of which is removable to provide access to the pouring nozzle as shown in Holmes U.S. Pat. No. 3,439,735.
Although a split shroud affords access to the tundish nozzle, most of the prior art shrouds, including the split shrouds are fixed systems which are not readily removable to accomodate other apparatus beneath the pouring stream such as a launder which, when required, diverts the pouring stream away from the mold. Also, the fixed shroud systems do not allow nozzle cleaning by an oxygen torch during casting nor insertion of a chill plug to stop the flow of the molten metal.
The Pollard patent teaches that the shroud tube must be open at both ends, to afford two-directional gas flow and to allow rapid removal of the shroud from the operating position.
Contrary to the teachings of the Pollard patent, we have determined experimentally that a shroud must be tightly held against the tundish or pouring vessel to prevent entrainment of oxygen from air into the shroud and reoxidation of the steel stream. When a shroud is open at the top, hot air rising off the mold is drawn upwardly through the shroud, exiting at the top, reducing the effectiveness of the inert gas introduced to the interior of the shroud and causing considerable reoxidation of the steel in the pouring stream. When the diameter of a shroud closely approximates the diameter of the pouring stream, the quality of the seal between the shroud tube and the pouring vessel is not as critical as it is when the shroud tube has a diameter in excess of three times the diameter of the pouring stream. However, larger diameter shroud tubes on the order of four inch diameter have more desirable operating characteristics than small diameter tubes, say, of 21/2 inch diameter or less. In smaller diameter tubes, splash and spatter from the pouring stream impinges against the inside of the tube causing a buildup of solid steel. In some instances this buildup becomes so severe that it shuts off the pouring stream. In other instances, the buildup is washed out of the tube by action of the pouring stream and into the mold where it can rupture the solidifying shell causing molten steel to "break-out" and necessitating casting of that strand to be terminated. Experimental data shows that the larger diameter tubes require the seal between the shroud tube and the tundish to be as tight as possible to prevent air leakage into the shroud tube from the surrounding atmosphere.
Japanese researchers have determined that the oxygen content of shrouding gas must be maintained at less than 0.8% to prevent the continuous formation of oxide inclusions from reoxidation of the steel stream. In our experimental work, which involved the accurate measurement of shroud and mold environment oxygen concentrations, we determined: first, that a shroud sealed to the tundish has a significantly lower oxygen concentration in the shroud than one with a gap between the top of the shroud and the bottom of the tundish; and second, that as the gap between the bottom of the shroud and the top of the mold is decreased, the oxygen concentration in both the shroud and in the mold decreases significantly.
Therefore, the shroud tube should extend as far as possible downwardly toward the mold, yet allow space between the shroud and mold for viewing the liquid level in the mold. Heretofore, there has been no convenient mechanism for placing a shroud against a tundish and for removing it when necessary in order to divert the pouring stream from the mold. The invented shroud apparatus is readily positionable tightly against the pouring nozzle of a molten metal pouring stream from a bottom-pour vessel, yet is easily and quickly removed to accomodate other apparatus such as a launder beneath the stream.