Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
1999-01-19
2001-11-13
Le, Hoa T. (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S403000, C428S404000, C501S099000, C501S132000, C501S133000, C501S154000
Reexamination Certificate
active
06316106
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a filler sand filled in a ladle tap hole valve, such as a sliding nozzle or a rotary nozzle, which is used in tapping molten steel from a steelmaking ladle etc.
BACKGROUND INFORMATION
A ladle for receiving molten steel is used in a ladle refining process or a continuous casting process carried out following a converter refining process, and a ladle tap hole valve (sliding nozzle or rotary nozzle) is arranged at the bottom of the ladle for tapping molten steel. In the ladle provided with such a ladle tap hole valve, to prevent molten steel from solidifying within a nozzle of the apparatus, the nozzle is charged with a refractory filler sand before receiving molten steel, and after molten steel is poured into the ladle, the nozzle is opened, whereby the filler sand falls freely, creating an opening by itself, or a free opening, through which the molten steel flows down.
Conventionally as such filler sand, silica sand (SiO
2
:90 to 99%) is generally used. The purity of SiO
2
is adjusted as needed depending on use to prevent sintering (Unexamined Japanese Patent Piblication (KOKAI) No. 64-48662), or conversely, orthoclase (K
2
O.Al
2
O
3
.6SiO
2
) is added to cause sintering, thereby forming a viscous film in a region which comes into contact with molten steel to prevent penetration of the molten steel.
In the former case, however, although the filler sand can be prevented from sintering, penetration of molten steel cannot be effectively prevented, and thus no great improvement in the free opening ratio of the ladle can be expected. In the latter case, on the other hand, the filler sand can be used satisfactorily in ordinary operation, but in cases where molten steel needs to be processed at high temperature for a long time in ladle refining, etc. to produce high-grade steel, sintering of the filler sand itself progresses to such an extent that an unyielding film is formed, with the result that the free opening very often fails to be created. If non-free opening is created, it is necessary that oxygen be blown from below with a long nozzle detached, to forcibly make an opening. However, contact of molten steel with air adversely affects the quality of the resulting steel, and thus the grade down of steel or scrap is produced, causing a great deal of damage.
To solve the problem, attempts have recently been made to admix the filler sand with flake graphite or earthy graphite, taking account of properties of graphite, that is, the property of inhibiting sintering and the property of being less wettable by molten steel. However, segregation is caused by a phenomenon occurring before graphite is put to use and is contained in a hopper, paper bag or container bag, such as by a difference in specific gravity or good sliding property of graphite, and thus expected results are not achieved yet in practice. Attempts have also been made to use pitch, but the use of pitch is not preferred because it has a 30 to 70% content of volatiles, gas is produced during use, and because segregation occurs.
There has also been proposed to add 0.05 to 5.0 wt % of carbon black to a filler sand such as silica sand, MgO clinker or zircon sand (Unexamined Japanese Patent Publication No. 4-84664). Carbon black has a high percentage of residue, has a small content of volatiles, and is excellent in preventing sintering and preventing penetration of molten steel, compared with the blending material such as flake or earthy graphite, pitch, etc. Also, since carbon black has a large specific surface, it shows excellent dispersion when added to the filler sand and can prevent segregation. Further, carbon black is excellent in adhesion to silica sand. Filler sand admixed with carbon black is therefore regarded as a potential material having excellent properties required of the filler sand, such as the property of preventing sintering and penetration of molten steel.
However, although the filler sand disclosed in Unexamined Japanese Patent Publication No. 4-84664 is effective in some degree, the free opening ratio during a high tapping temperature and long lead time process involving ladle refining is not of a satisfactory level, and thus there is a demand for a filler sand which ensures a high free opening ratio even under such severe conditions.
As such filler sand, chromite sand having a higher melting point than silica sand is also used. However, chromite sand becomes sintered when molten steel is tapped, and the opening often fails to be created; therefore, chromite sand is seldom used singly and is used in combination with silica sand.
Even such a filler sand having chromite sand mixed with silica sand does not ensure a satisfactory free opening ratio in a high tapping temperature and long lead time process involving ladle refining. Also, the filler sand is liable to be sintered to the surface of a well block during such a high tapping temperature and long lead time process, and accordingly, the well block needs to be cleaned with oxygen with increased frequency, possibly shortening the life of the well block and lowering the yield because of residual steel in the ladle.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a filler sand for a ladle tap hole valve which filler sand ensures a high free opening ratio even during a high tapping temperature and long lead time process involving ladle refining without shortening the life of a well block or entailing reduction in the yield.
According to a first aspect of the present invention, there is provided a filler sand for a ladle tap hole valve which contains 70 to 90 wt % of chromite sand and 10 to 30 wt % of silica sand and is blended externally with 0.05 to 5 wt % of carbon black calculated based on a total amount of the chromite sand and the silica sand.
The above filler sand is preferably blended with 0.05 to 1 wt % of carbon black calculated based on the total amount of the chromite sand and the silica sand. Also, preferably, 95% or more of the chromite sand consists of particles having particle diameters falling within a range of 150 to 850 &mgr;m, 60% or more of the chromite sand consists of particles having particle diameters falling within a range of 200 to 425 &mgr;m, 95% or more of the silica sand consists of particles having particle diameters falling within a range of 200 to 850 &mgr;m, and 60% or more of the silica sand consists of particles having particle diameters falling within a range of 300 to 600 &mgr;m. Further, in the above filler sand, the silica sand preferably has a of particle diameter coefficient of 1.4 or less. Preferably, moreover, the chromite sand contains substantially no particles having particle diameters smaller than 53 &mgr;m and substantially no particles having particle diameters exceeding 850 &mgr;m. Still preferably, the silica sand contains substantially no particles having particle diameters smaller than 106 &mgr;m and substantially no particles having particle diameters exceeding 1180 &mgr;m. Further, the carbon black is preferably blended in such a manner that it is coated on the silica sand.
According to a second aspect of the present invention, there is provided a filler sand for a ladle tap hole valve which contains 70 to 90 wt % of chromite sand and 10 to 30 wt % of silica sand, wherein 99% or more of the chromite sand consists of particles having particle diameters falling within a range of 150 to 850 &mgr;m, 95% or more of the chromite sand consists of particles having particle diameters falling within a range of 200 to 600 &mgr;m, 95% or more of the silica sand consists of particles having particle diameters falling within a range of 150 to 850 &mgr;m, and 80% or more of the silica sand consists of particles having particle diameters falling within a range of 200 to 600 &mgr;m.
The filler sand according to the second aspect of the present invention is preferably blended externally with 0.05 to 5 wt % of carbon black calculated based on a total amount of the chromite sand and the silica sand. A still prefer
Akai Shin-ichi
Ishii Takeshi
Nakashima Hirohisa
Shirayama Akira
Takasugi Hideto
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Le Hoa T.
NKK Corporation
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