Patent Application
20080257524
The present invention refers to a delivery device and the relative process for casting liquid steel in a twin-roller crystallizer in continuous strip casting
In strip casting by means of twin-roller crystallizers the liquid steel is distributed between the two rollers by means of a delivery device or nozzle and in a very short time, approximately some fraction of a second, it starts to solidify forming two solid skins that, joining together between the two rollers, form the thin strip.
Only a very small amount of steel is contained between the two rollers of the crystallizer, which means that any disturbance in said steel feeding, from the tundish to the crystallizer, is immediately felt at the meniscus creating a turbulence and thus causing problems during the initial phase of solidification, resulting in the formation of defects on the cast strip.
The prior art discloses several types of devices for delivering liquid steel in continuous strip casting by means of a twin-roller crystallizer.
The delivery device described in document U.S. Pat. No. 6,070,647 attempted to improve the flow feeding the liquid steel to the crystallizer.
In order to reduce disturbance of the steel between the rollers, said delivery device, which is partially immersed in the casting pool, is fed through a plurality of holes arranged on the bottom of the tundish. This plurality of holes produces a plurality of free-falling jet streams distributed along the extension of the delivery device. In this manner there is less disturbance at the meniscus region than that one obtained with a single jet stream feeding.
At its turn, the nozzle distributes the liquid steel in the casting pool, contained between the two rollers, through a series of openings arranged along the length of the rollers, said openings being slightly immersed beneath the meniscus.
Disadvantageously, however, the free-falling jet streams from the tundish to the inside of the nozzle cause air to be drawn into the liquid steel creating bubbles that then flow into the casting pool. As they rise, the bubbles explode and generate turbulence at the meniscus region in the form of fluctuations which result in the formation of defects on the surface of the cast strip as the skin is not able to form properly on the rollers; these defects make the product unmarketable.
The need is therefore felt to produce a new delivery device and a casting process capable of overcoming the drawbacks described above.
A first purpose of the present invention is to produce a delivery device capable of containing within minimum values the turbulence at the meniscus so that the latter is substantially flat and of preventing the formation of defects on the cast strip by passing the liquid steel between the two crystallizer rollers.
Another purpose is to implement a casting process that provides for particular expedients for improving the steel feeding from the tundish to the crystallizer.
The present invention, therefore, proposes to achieve the objectives described above by providing a delivery device for delivering liquid steel into a crystallizer with two rollers that, in accordance with that set forth in claim 1, comprises a first vessel suitable for containing liquid steel, a plurality of substantially vertical and aligned ducts provided on the bottom of said vessel and suitable for generating discharge jet streams of the liquid steel, a second longitudinal vessel with an upper part open, suitable for receiving in its inside said jet streams, and comprising side walls and a bottom, characterized in that said vertical ducts extend inside the second longitudinal vessel whereby said discharge jet streams are guided inside said second vessel, and in that each of said side walls is provided with a longitudinal slot, having an extension equal to the one of the crystallizer rollers, suitable for generating a constant and non-turbulent flow in a casting pool comprised between the two rollers.
Said purposes are also achieved with a process for casting liquid steel in a crystallizer with two rollers for producing steel strip, that uses the delivery device described above, said process comprising the following steps:
production of discharge jet streams of the liquid steel from a first vessel into a second longitudinal vessel with the upper part open, said jet streams being guided by a plurality of vertical and aligned ducts that extend into said second longitudinal vessel,
generation of a constant and non-turbulent flow of said liquid steel, from said second vessel towards a casting pool comprised between the two rollers, through a longitudinal slot provided on each of the side walls of the second vessel, so as to obtain a substantially flat meniscus region in said casting pool.
Advantageously the feeding of guided steel jet streams inside the delivery device eliminates the disturbance at the meniscus region deriving from the air bubbles that form when the free-falling jet streams come into contact with the air, as occurs in the known devices and plants. Furthermore the presence of a single large slot in proximity to the bottom of the delivery nozzle, which is completely immersed beneath the meniscus of the casting pool, guarantees an uniform and almost laminar flow entering said casting pool, having a substantially V shape. This solution permits to eliminate the turbulence at the meniscus region in the area of initial solidification and eliminates the possibility of forming defects on the surface of the strip, cast between the two rollers, due to the same turbulence. The dependent claims describe preferred embodiments of the invention.
Further characteristics and advantages of the invention will become clear from the following detailed description of a preferred, but not exclusive, embodiment of a delivery device, that is merely illustrative and not limitative, with the help of the drawings attached hereto, in which:
a shows an enlarged cross-sectional view of some components of the device in
b shows a cross-sectional view of a component of the delivery device according to the invention;
a and 3b show perspective views respectively of a portion of the device in
a shows a section of a plan view of a second component of the device according to the invention;
b shows a cross-section along the line A-A of the second component in
c shows a cross-section along the line B-B of the second component in
With reference to the drawings, a delivery device for producing a thin strip is represented, comprising:
a ladle (not illustrated),
a tundish 3,
a delivery nozzle 1,
a crystallizer 2 with two rollers 4.
The liquid steel is supplied by the ladle to the tundish 3. Advantageously there may be provided an under-tundish 3′, which is much smaller than the tundish. The tundish 3 and under-tundish 3′ are both made of a refractory material and their cascade arrangement advantageously permits to reduce the kinetic energy of the liquid steel entering the crystallizer 2, and thus also permits to limit meniscus disturbance. Since the level or head of the steel in the under-tundish 3′ is very low with respect to the level in the tundish 3, the disturbance of the steel in the casting pool between the rollers 4 is greatly reduced because the feeding kinetic energy is reduced.
In order to reduce further the disturbance of the steel between the two crystallizer rollers 4, the steel is delivered from the under-tundish 3′ through a plurality of substantially vertical calibrated holes or ducts 5, illustrated in
A further advantage is represented by the fact that these jet streams are not free-falling but are appropriately guided until inside the same delivery nozzle, arranged between the two rollers. In such a way the meniscus disturbance due to air bubbles is avoided since these latter are no longer generated. In fact, the discharge of the jet streams into the delivery nozzle 1 advantageously occurs beneath the head, that is below the meniscus 21 inside the delivery nozzle.
a illustrates a portion of a cross-section of the under-tundish 3′ the bottom of which is provided with a protuberance 6 made of refractory material, either machined or appropriately housed on the bottom of said under-tundish, in which the calibrated holes or ducts 5 are produced. Said protuberance 6 preferably has a tapered shape so that it can easily be inserted inside the delivery nozzle 1 below. This does not exclude the fact that said protuberance may also have different shapes, for example that one of a parallelepiped.
The delivery nozzle 1 has the advantageous configuration illustrated in
Advantageously the bottom end of the protuberance 6 or of the holes or ducts 5 is placed at a distance of between just 5 and 40 mm from the upper ends of the flow breaker walls 9, 10, so that there is only a slight turbulence at the meniscus 21 inside the delivery nozzle.
The guided jet streams pass through the holes 5 into the space or central inner chamber 13 of the delivery nozzle 1; when said chamber 13 is full the flow follows the path shown by the arrows 14 and 15 in
The liquid steel flow entering the casting pool enclosed between the two crystallizer rollers 4 is a substantially laminar flow, constant and non-turbulent, and this guarantees a substantially flat profile of the meniscus 22 during casting.
The diagram in
Lastly, as the steel then passes between the rollers 4, the solidifying areas of skin join to obtain a cast strip of a predefined thickness and with no surface defects.
The specific embodiments here described are not limitative and this patent application covers all the alternative embodiments of the invention as set forth in the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2005A002470 | Dec 2005 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2006/069968 | 12/20/2006 | WO | 00 | 6/19/2008 |
