Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2002-08-13
2003-07-29
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S048000, C164S437000, C164S492000, C164S505000
Reexamination Certificate
active
06598662
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an apparatus for supplying molten steel used for the continuous casting, and also to a method for continuously casting with the apparatus for supplying molten steel, which is useful to prevent an immersion nozzle clogging and to reduce slab surface defects.
BACKGROUND ART
As a method for continuously producing a slab, a continuous casting method is normally known, in which molten steel stored in a tundish is supplied to a top of a mold via an immersion nozzle disposed at the lower part of the tundish to form a solidified shell in the mold, and then the slab is continuously produced by withdrawing the solidified shell from a bottom of the mold.
When the molten steel, which is deoxidized with Al, is continuously cast, the Al oxide in the molten steel tends to deposit to the inner surface of the immersion nozzle, and therefore the molten steel is hindered to flow in the immersion nozzle. Therefore, when the casting is carried out using an immersion nozzle having more than one port, a symmetric flow of the molten steel in the mold cannot be obtained. In fact, when a bias flow becomes to be great, a fluid flow in the mold tends to an asymmetric flow. Accordingly, when such an asymmetric flow is generated, a mold flux on a meniscus of the molten steel in the mold is liable to be entrapped into the molten steel, and/or Al oxide or the like, deposited to the inner surface of the immersion nozzle, is peeled off and then tends to be entrapped into the molten steel.
The mold flux and Al oxide or the like entrapped in the molten steel are trapped by the solidified shell in the mold, thus slab surface defects such as powder defect and/or slag spot are liable to occur. The defects on the slab surface cause surface defects in the products, when the slab having the slab surface defects is hot-rolled.
When the amount of Al oxide deposited on the inner surface of the immersion nozzle increases, the so-called nozzle clogging takes place, so that it is difficult to continue the casting. The cleaning of the inner surface of the immersion nozzle with oxygen gas may solve the problem of nozzle clogging. Nevertheless, this deteriorates a cleanliness of the steel.
A method for purging an inert gas into molten steel passing through an immersion nozzle in order to avoid the nozzle clogging has been known (“Tetsu To Hagane”, vol. 66, S868, Iron and Steel Institute of Japan), and various methods for preventing nozzle clogging, which are applicable to the casting, have recently been proposed. For instance, a method for purging an inert gas into molten steel passing through an immersion nozzle is proposed in Japanese Patent Application Laid-open No. H4-319055, in which case, the amount (liter (Nl)/min) of the inert gas to be blown into the molten steel is adjusted in accordance with the throughput (t/min) of the molten steel passing through the immersion nozzle.
In Japanese Patent Application Laid-open No. H6-182513, moreover, a method for purging an inert gas into molten steel, wherein an AC or DC current is supplied between a porous refractory material for purging gas on the inner wall of an immersion nozzle and the molten steel passing through the immersion nozzle. In this method, the deposition of Al oxide or the like onto the inner surface of the immersion nozzle is prevented by purging the inert gas into the molten steel. At the same time, by supplying a current between the inner wall of the immersion nozzle and the molten steel, the resulting electromagnetic force applied to the molten steel promotes bubbles of the purged inert gas to remove form the refractory material for the purging gas, and thereby to reduce the size of generated gas bubbles. As a result, the size of the gas bubbles, which are trapped by the solidified shell in the mold, is reduced, thereby enabling the defects due to the gas bubbles in the slab to prevent on the surface of products, which are manufactured by hot-rolling the slab.
However, in the methods proposed in these specifications, it is found that a decrease in the amount of the purged inert gas to prevent the gas bubble trapping by the solidified shell makes it difficult to prevent the Al oxide or the like in the molten steel from depositing onto the inner surface of the immersion nozzle. On the contrary, the suppression of the deposition of the Al oxide or the like in the molten steel onto the inner surface of the immersion nozzle provides an increase in the amount of the purged gas. Thus the bubbles of the inert gas are trapped more extent by the solidified shell, thereby a greater number of the surface defects are generated in the products.
In these conventional methods, therefore, it is impossible to securely prevent the deposition of Al oxide or the like in the molten steel onto the inner surface of the immersion nozzle. Moreover, even if the deposition of Al oxide or the like in the molten steel onto the inner surface of the immersion nozzle is successfully prevented, the defects due to the gas bubbles generates on the surface of the slab, thereby resulting in the generation of the surface defects on the products. From this viewpoint, it is desirable to provide a secure and effective method for preventing the Al oxide or the like in the molten steel from being deposited on the inner surface of an immersion nozzle.
DISCLOSURE OF INVENTION
Accordingly, it is the object of the present invention to provide an apparatus for supplying molten steel, which effectively prevents Al oxide or the like in molten steel from being deposited onto the inner surface of an immersion nozzle, thereby enabling the generation of the slab surface defects due to mold flux, Al oxide or the like to be prevented, and at the same time enabling the surface defects of products produced from the slab to be effectively prevented. It is another object of the present invention to provide a method for continuously casting with the apparatus for supplying the molten steel.
In order to attain the above objects, the present inventors focused on the electrical capillarity and then developed a method for preventing the Al oxide or the like in the molten steel from being deposited on the inner surface of an immersion nozzle by utilizing the electrical capillarity. The electrical capillarity described herein implies a phenomenon in which the interfacial tension between an ion solution and an electrode immersed therein can be changed by the potential applied to the electrode. The present inventors carefully investigated the phenomenon and succeeded in finding the following features [1] to [7]:
[1] An upper nozzle, a flow control mechanism and an immersion nozzle of a continuous casting apparatus are constituted by a refractory material which exhibits either the electronic conductivity or the ion conductivity at a high temperature. As a result, the application of a potential between the molten steel and the refractory material having either the electronic conductivity and/or the ion conductivity at a high temperature during the continuous casting provides the electrical capillarity on the interfacial surface therebetween. This reduces interfacial tension, so that the depositing force of the Al oxide or the like on the surface of the refractory material is reduced, thereby making it difficult to deposit the Al oxide or the like on the surface of the refractory material.
[2] On the basis of the above presumption, an experiment was carried out wherein, employing a crucible in the laboratory use, an electrode and a refractory material rod both having a good electrical conductivity were immersed in molten steel, and a potential was applied between the refractory material rod and the electrode by supplying a current therebetween. In this experiment, it was found that the build up of the Al oxide or the like in the molten steel on the inner surface of the refractory material was reduced even in the case of a small potential, and that, irrespective of the polarity of the applied potential, an increase in the absolute value of the
Hara Masashi
Kato Toru
Kawamoto Masayuki
Murakami Toshihiko
Nishida Norihiro
Clark & Brody
Lin Kuang Y.
Sumitomo Metal Industries Ltd.
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