Generally, in order to fabricate a cast piece (which is the general term for slab, billet, bloom, beam blank and the like) in a continuous casting machine, molten steel in a liquid state supplied from a ladle passes through a mold via a tundish that stores the molten steel, and then a solidified shell in a solid state is formed by means of a cooling operation in the mold. While the solidified shell obtained by cooling the molten steel is guided by a guide rolls installed below it, the solidified shell is solidified by a secondary cooling water sprayed by spray nozzles, thereby becoming a cast piece in a complete solid state.
During the continuous casting work of steel, mold flux as a subsidiary material as well as molten steel is input into the mold together when the molten steel is supplied into the mold. The mold flux is generally input in a solid state, such as powder or granule, and is melted by heat generated in the molten steel supplied into the mold, thereby controlling heat transfer between the molten steel and the mold and improving the lubricating ability.
As shown in FIG. 1, the mold flux input into the mold in the shape of powder or granule is melted on an upper surface of the molten steel 12 to form a liquid layer 21, a sintering layer (or semisolid layer) 23 and a powder layer 25 in order from the molten steel surface. The liquid layer 21 is substantially transparent, so that a radiant wave with a wavelength of 500 to 4,000 nm emitted from the molten steel can be easily transmitted through the liquid layer 21. On the other hand, the sintering and powder layers 23 and 25 are optically opaque, thereby blocking a radiant wave and thus preventing a rapid decrease of temperature of the molten steel surface.
However, after the conventional mold flux in the shape of powder or granule is melted by the heat of the molten steel, the liquid layer 21 flows between the mold 10 and the solidification layer 11, thereby being solidified on an inner wall surface of the mold 10 to form a solid slag film 27 and also forming a liquid slag film on the molten steel side to control heat transfer between the molten steel and the mold and improve the lubricating ability.
At this time, at the point where the molten slag begins to flow between the solid slag film 27 and the solidified shell 11, the mold flux adhering to the mold is formed to protrude to the inside of the mold. This portion is referred to as a slag bear 29. The slag bear 29 prevents the molten slag from being introduced between the mold flux film 27 and the solidified shell 11.
This slag bear 29 restricts consumption of mold flux per unit area of a cast piece. Generally, the consumption of mold flux decreases as a casting speed increases, so that the lubricating ability between the cast piece and the mold is deteriorated to thereby increase frequency of occurrence of break-out. In addition, since the thickness of the liquid layer of mold flux becomes irregular due to the slag bear 29, the shape of the solidified shell 11 in the mold 10 becomes irregular, thereby causing surface cracks, which is also more serious as a casting speed is increased.
In this regards, Korean laid-open Patent Publication No. 1998-038065 and U.S. Pat. No. 5,577,545 disclose a method for restraining growth of the slag bear by lowering the melting speed of mold flux by coating mold flux with graphite or fine carbon black. However, this method cannot prevent a slag bear fundamentally. In addition, when the melting speed of mold flux is low, the mold flux in an un-molten state is introduced between the solidified shell and the mold, which causes irregularity of the solidification and also increases break-out defects.
In order to solve the above problem, Japanese Laid-open Patent Publication No. 1989-202349, 1993-023802, 1993-146855, 1994-007907, 1994-007908, 1994-047511, 1994-079419, 1994-154977 and 1994-226111 disclose a method for melting mold flux at the outside of a mold and then injecting it through a molten steel surface. However, the aforementioned documents suggest that the mold flux in a molten state is limitedly used only in an initial casting process, and then, once the casting work reaches a normal state, mold flux in the shape of powder is used to return to the conventional operation. As mentioned above, since the mold flux in a molten state is substantially transparent in a wavelength of 500 to 4,000 nm, a radiant wave emitted from the molten steel may easily pass through the mold flux, so that the surface of the molten steel cannot be kept at a set temperature due to the increased radiant heat transfer. Accordingly, if the casting is progressed for a certain time, the surface of the molten steel may be solidified, which would be an obstacle in performing the continuous casting process.
In addition, paper has been used to supply the mold flux in a molten state into the mold. However, the paper has a limit in supplying the mold flux in a molten state throughout the entire period of the continuous casting process.