CA2213630A1

CA2213630A1 - Continuous casting plant

Info

Publication number: CA2213630A1
Application number: CA002213630A
Authority: CA
Inventors: Michael David Malecki; Gerald Hohenbichler; Heinrich Thone; Hermann Rametsteiner; Richard Larry Ruediger; Bruno Lindorfer; Reinhold Angerer; Karl Klima; Christian Furst; Herbert Bumberger
Original assignee: Individual
Current assignee: Primetals Technologies Austria GmbH
Priority date: 1995-02-23
Filing date: 1996-02-22
Publication date: 1996-08-29
Also published as: EP0810909A1; KR19980702508A; AT402266B; ATA33195A; WO1996026028A1; BR9607314A; JPH11500359A; MX9706353A

Abstract

A continuous casting plant for continuously casting a thin metal strip (1) has a melt receiving container (4) with a melt opening (5). A casting surface (8) that receives a thin layer of melt (3), forming a strip (1), can be moved past the melt opening (5). The strip (1) is then transferred from the casting surface (8) to a strip supporting device (9). In order to cast metal strips (1) with no risks of cracks, in particular at high casting speeds, the strip supporting device (9) has a substantially plane and horizontal surface (22) for supporting the strip (1). Gas channels (21) that may be connected to a gas feeding device open into surface (22).

Description

~ r 1 Continuous castin~ plant The invention relates to a continuous casting plant for continuously casting a thin metal strip, in particular a steel strip having a thickness of below 20 mm, preferably from 1 to 12 mm, comprising a melt holding vessel provided with a melt orifice past which a casting surface is movable for recëiving a thin layer of melt under the formation of a strand, and comprising a strand supporting device tal~ing over the strand from the casting surface, and further a process for operating a continuous casting plant of this kind.

A continuous casting plant for producing a thin metal strip of the initially described kind is known f.i. from EP-A - 0 526 886. According to this prior art document, the metal melt is deposited either on a casting roller or on a continuous crawler composed of links and after the formation of a strip-like strand shell is passed on from the casting roller or the crawler to a roughly horizontally oriented strand supporting device designed as a continuous conveying belt. The metal strip at the time of being passed onto the transporting means as yet has no upper surface formed by the melt; it has become solidified only at its bottom surface, where it was in contact with the casting roller or the endless belt.

The transporting means is provided with a cooling, so that the metal strip when leaving the transporting means has already reached complete solidification. The transporting means designed as a continuous belt is formed by a mesh or grid, air or water being sprayed or conducted against the strand through the gaps of the mesh respectively grid for intensive cooling of the strand, i.e. of the bottom surface of the metal strip. Between the casting surface and the transporting means, a stationary intermediate support is provided to bridge the gap between the casting surface and the transporting means.

A plant of this kind has the disadvantage that the transporting means formed by a continuous belt has many movable parts that are necessarily disposed in the immediate casting area of the metal strip. In heavy casting duty they are exposed to heavy strain and thus to considerable wear. A further disadvantage is that by necessity there is a relatively wide gap between the casting surface and the transporting means due to the guide rollers of the same, which must be bridged by an intermediate support.

Here, difficulties arise when transporting the metal strip which exhibits only a thin solidified bottom surface and due to the high temperature is still very soft, since the solidified bottom surface of the metal strip due to the frictional forces incurring between this bottom surface and the intermediate support is very susceptible to cracking. These frictional forces are , 2 particularly great if the usually provided refractory graphite plates or plates made from refractory material are arranged for intermediate support.

According to EP-A - O 526 886 the transporting means may also be formed by a stationary table, the metal strip being transported over the table by driven supporting rollers. In practice, however, it is hardly feasible to support the strip on rollers since the supporting rollers due to the slight thickness and due to the low dimensional stability of the metal strip would have to possess very small radii to enable spacing them as closely as possible. Apart from this, there also result gaps between the casting surface and the transporting means and between the driven supporting rollers, which are caused by the radii of the rollers and which would still have to be bridged by intermediate supports, which in turn would entail the danger of cracks forming within the strip.

A continuous casting plant of the initially described kind is also known from EP-A - O 568 211. Here, a stripper is provided immediately adjacent the casting surface, taking over the strip from the casting surface and conveying it to a subsequently arranged conveying table.
Both the stripper and the conveying table are arranged so as to be stationary and oriented in a roughly horizontal direction. Here, again, the hot and as yet very soft strip separating from the casting surface is strongly susceptible to cracking due to the incurring frictional forces, so that sticking of the strip, in particular the beginning of the strip, to the stripper or the conveying table may ensue. This may result in damages to the strip and even in a discontinuation of the casting operation. Bulging of the strip may, however, also occur.

The invention aims at avoiding these disadvantages and difficulties and has as its object to provide a continuous casting plant of the initially described kind and a process for operating the same enabling the casting of metal strips still having a very thin strand shell as it separates from the casting surface, without any danger of cracking and at a high casting output. In particular, casting at high casting speeds is to be enabled. Moreover, the continuous casting plant is to enable particularly efficient heat tr~nsmi.ssion between the partially solidified metal strip and the strand supporting device arranged downstream of the casting surface, so that there will be rapid complete solidification of the metal strip even if casting at a high casting speed.

In accordance with the invention, this object is achieved in that the strand supporting device is provided with an essentially plane and essentially horizontal surface that supports the strand and has gas passage channels running into it that are connectable to a gas transporting means.

_ CA 02213630 1997-08-22 ~ , A process for producing metal foils is known from EP-A - O 6~9 459. Here, molten metal is poured from a nozzle onto a quench roll where the liquid metal immediately solidifies. After the foil has been in contact with a peripheral surface of the quench roll, the foil is separated from the quench roll by tangential blowing of air and by a continuous conveying belt is conveyed away from the quench roll. By sucking of air, the metal foil is sucked to the continuous conveying belt, for which purpose the continuous conveying belt is air permeable.
The foils have a thickness in the range of 25 ,um, and as the liquid metal impinges, amorphous solidification ensues. The solidified metal foil is brittle and has a tendency to fluttering and due to its marginal weight would lift off from the continuous conveying belt and the quench roll immediately, which is avoided by sucking it against the continuous conveying belt and tensioning it with respect to the quench roll.

Advantageously, in accordance with the invention the gas transporting means is constructed as a means imparting a pressure above atmospheric to the gas, such as inert gas or air, that is to be conveyed through the gas passage channels. Thereby it becomes feasible to form a gas cushion between the surface of the strand supporting device and the bottom surface of the metal strip so that an absolutely safe start-up procedure without any danger of the metal strip adhering or failing to lift off and thus without the risk of kansverse strip corrugations or j~mming of the strip is enabled.

In accordance with a preferred embodiment the gas transporting means is constructed as a means imparting a negative pressure to the gas that is to be conveyed through the gas passage channels. Hereby it becomes feasible to slightly suck the metal strip to the upper surface of the strand supporting device during continuous operation, i.e. after start-up, so as to ensure particularly good contact of the metal strip and the strand supporting device and hence ~
particularly efficient cooling. Hereby, casting at high casting rates, that is, at a high casting speed, or casting of slightly thicker metal strips with only a short timespan left until complete solidification is enabled.

The invention renders it feasible to get by with a strand supporting device of very simple construction, namely a strand supporting device constructed as a rigidly arranged plate, hence without any movable parts.

According to a preferred embodiment, the strand supporting device is formed of a highly thermoconductive material, in particular copper or a copper alloy, and suitably is provided with an internal cooling, in particular an internal liquid cooling.

~ CA 02213630 1997-08-22 .- 4 In accordance with a preferred embodiment, the strand supporting device is composed of two components, namely a strand stripping device directed toward the casting surface and directly abutting on the sarne and with a cooling table arranged adjoining the same in the direction of strand withdrawal, with both the strand stripping device and the cooling table being provided with gas passage channels.

To keep down gas consumption and/or in order to get by with a low-capacity gas transporting means, advantageously the strand supporting device is provided with lateral limitations extending in the withdrawal direction of the strand and comprising the longitudinal side edges of the strand. Hereby, lateral gas leakage can be largely prevented when producing a gas cushion between the metal strip and the strand supporting device.

Advantageously, the gas passage channels at their openings running into the surface of the strand supporting device occupy a total cross-sectional area of 0.01 to ~0 %, preferably 0.1 to 5 %, of the one surface of the strand supporting device that supports the strand.

Preferably, at their openings running into the surface of the strand supporting device the gas passage channels each exhibit a cross-sectional area of 1 to 50 mm2, preferably 3 to 20 mrn2.

A particularly advantageous production of a gas cushion is ensured if by their mouths the gas passage channels enclose an acute angle with the surface of the supporting device. It is of advantage if the mouths of the gas passage channels are oriented such that a gas stream is formed that moves essentially in the direction of strand withdrawal. This gas stream forms already at the channel mouths.

Suitably, the cross-sectional areas of the openings the gas passage channels arranged at the beginning of the strand supporting device form on the surface of the strand supporting device exceed those resulting in such portions of the strand supporting device as are arranged downstream in the direction of strand withdrawal, thereby facilitating notably the critical start-up phase, i.e. the start of casting, during which the metal strip is particularly at risk.

In accordance with a preferred embodiment, the strand supporting device is provided with coolant channels arranged transverse to the direction of strand travel and each of at least several of these coolant channels is connected with a coolant feed duct acting as a distributing main and a coolant discharge duct acting as a collecting main, which are arranged beside the strand supporting device.

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S

Herein, advantageously a coolant channel departing from the distributing main is conducted through the strand supporting device several times and subsequently opens into the collecting main.

To fix the location of the point of complete solidification of the metal strip or to influence the metallurgical/technological properties of the metal strip by the changed cooling conditions, advantageously the pressure between the bottom surface of the strand and the strand supporting device is adjusted by ~L,pl~,pliate suction and/or feeding of gas for control purposes.

A process for starting up a continuous casting plant, wherein from a melt holding vessel melt is continuously deposited on a casting surface being moved past a melt orifice of a melt holding vessel and the melt solidifies on the casting surface while continuously forming a strand, wherein the strand, which is possibly still liquid on its side facing away from the casting surface, from the casting surface is conveyed onward onto a strand supporting device and there is supported and cooled, characterized in that between the strand supporting device and the solidified bottom surface of the strand a gas cushion is produced, notably just at the beginning of the strand.

During the operation of the continuous casting plant, wherein from a melt holding vessel melt is continuously deposited on a casting surface being moved past a mouth of the melt holding vessel and the melt solidifies on the casting surface while continuously forming a strand, wherein the strand, which possibly is still liquid on its side facing away from the casting surface, from the casting surface is conveyed onward onto a strand supporting device and there is supported and cooled, there is suitably generated between the strand supporting-device and the solidified bottom surface of the strand a negative pressure and thus a particularly good contact of the bottom surface of the strand with the surface of the strand supporting device.

Advantageously, between the solidified bottom surface of the partially or completely solidified strand and the strand supporting device a gas cushion is created in which a pressure above atmospheric of 0.1 to 20 mbar, preferably of 0.5 to 10 mbar with respect to the air pressure (atmosphere) surrounding the plant exists.

The negative pressure between the solidified bottom surface of the strand and the strand supporting device advantageously amounts to up to 1000 mbar (vacuum), preferably lies in the range between S and 100 mbar.

~ CA 02213630 1997-08-22 .- 6 In the following, the invention is explained in more detail with reference to the drawings, Fig.
1 representing a schematic side view, partially in section, of a continuous casting plant in accordance with the invention and Fig. 2 a detail of Fig. 1 on an enlarged scale. Fig. 3 depicts a cross-section along the line III-III of Fig. 1, likewise on an enlarged scale. In a representation analogous to Fig. 2, Fig. 4 shows a modified embodiment. The Figs. 5 and 6 illustrate in schematic representation firstly (Fig. 5) a partial view of a continuous casting plant and secondly (Fig. 6) a section taken along the line VI-VI of Fig. 5.

For casting a steel strip 1 having a thickness 2 of below 20 mm, a melt 3 is caused to flow onto a casting roller 6 out of a melt holding vessel 4 via a melt orifice 5 of this vessel 4. At the casting roller 6, which is provided with an internal cooling, there initially forms a very thin strand shell 7 which is lifted off the surface 8 of the casting roller 6 and transferred onto a strand supporting device designated generally by 9. The strand supporting device 9 is provided with a wedge-shaped strand skipping device 10, which by its pointed end 11 is directed straight against the casting surface 8.

The strand stripping device adjoins the casting surface in the region proximate to the uppermost generatrix 13 of the casting surface, which is situated in the vertical plane 13 laid through the axis of rotation 12 of the casting roller. Immediately following the strand stripping device 10, a cooling table 14 is provided that complements the skand supporting device 9. Both the surface of the skand stripping device 10 receiving the metal strip 1 with the solidified bottom surface and the cooling table 14 arranged downstream of the strand stripping device are oriented roughly horizontally.

The metal skip l passing onto the cooling table 14 still exhibits a liquid surface 15, yet with the solidified skand shell 7 of the metal skip 1 increasing in thickness as it moves along the cooling table 14, until the metal strip 1 eventually reaches complete solidification at a position 16 ofthe cooling table 14.

The strand stripping device 10 and the cooling table 14 are made from a highly thermoconductive material, preferably copper or a copper alloy, to ensure good heat tr~n~mi.~ion between the metal skip 1 and the strand supporting device 9. Both the strand stripping device 10 and the cooling table 14 are provided with an internal liquid cooling, formed by channels 17 extending in a direction transverse to the longitudinal extent of the strand supporting device 9. The channels 17 are connected to coolant feed and discharge ducts 18, 19 arranged parallel to the longitudinal extent of the strand supporting device 9 and to the side of the same. As can be seen from Figs. 5 and 6, each of several channels 17 is flow-CA 022l3630 l997-08-22 connected with a coolant feed duct 18 and a coolant discharge duct 19, each coolant channel 17 being meanderingly conducted through the strand supporting device 9 several times.
.

Further, both the strand stripping device 10 and the cooling table 14 are provided with gas channels 20 likewise extending transverse to the longitudinal directions of the same, from which there depart gas passage channels 21 having their mouths on the surface 22 of the cooling table 14 or strand stripping device 10.

Neither the cooling channels nor the gas channels have to extend transverse to the longitudinal extent of the strand supporting device 9; they, too, can extend lengthwise or at any desired angle with respect to the longitudinal direction. Via gas ducts 23 arranged in the longitudinal direction of the strand supporting device 9, the gas channels are connectable to a gas kansporting means designed as a device imparting to the gas inside the gas channels 20 both a pressure and a negative pressure. Hereby it becomes feasible to selectively blow gas, such as inert gas or air, between the bottom surface of the metal strip 1 and the surface 22 of the cooling table 14 or the strand stripping device 10 through the gas passage channels 21.

Thereby, friction between the metal strip 1 and the strand supporting device 9 can be reduced to such an extent that the metal strip 1 is prevented from adhering to or rem~ining on the surface of the stripper 10 or of the cooling table 14. This is of particular importance during start-up, when the beginning of the strip of the as yet very soft metal strip 1 is pushed onto the cooling table 14 via the strand stripping device 10. Hence, formation of the gas cushion is effected notably in the start-up phase, i.e. at the start of casting. Herein, the forces acting on the strand supporting device 9 as a consequence of the weight of the metal strip 1 can be reduced or even compen.c~te-l The gas channels suitably are arranged irregularly and nat in orderly rows, with a view to avoiding unwanted m~rkin~, streaks or inhomogeneities etc. on the metal strip 1.

In the edge region of the metal strip, that is, along the longitudinal edges of the strand supporting device 9, preferably strips 24 or similar superstructures are provided in order to achieve a throttling effect, so that gas consumption for producing a gas cushion between the metal strip 1 and the strand supporting device 9 can be kept down.

By producing a negative pressure by sucking off gas (air) through the gas passage channels 21 it is feasible to establish very good contact between the bottom surface of the strip and the surface 22 of the cooling table 14 or of the strand stripping device 10 which after start-up - as the metal strip 1 already exhibits a greater strength - is no longer harmful, so that a particularly good cooling effect is enabled by intensive heat tr~n~mi~sion due to good contact of the metal strip 1 and the cooling table 14 or the strand stripping device 10. Hereby it becomes feasible to cast at high casting rates.

The invention makes it feasible to allow for changes in the casting performance (changes in strip width 2 and/or changes in casting speed) and to continue the casting operation without any difficulties.

In accordance with the embodiment illustrated in Fig. 3, the gas passage channels 21 are arranged obliquely, namely such that an airflow moving in the direction of strand withdrawal (casting direction) 25 forms as the gas passage channels 21 are pressurized.

The invention is not limited to the exemplary embodiment represented in the drawing but can be modified in various respects. For instance it is conceivable to employ a different device, f.i.
a casting belt or the like, instead of the casting roller 6. A strictly horizontal orientation of the strand supporting device 9 is not required; it just has to be ensured that a strip having a desired thickness can be formed. It is conceivable for some of the gas passage channels to be connectable to a separate gas transporting means, so that over the length of the strand supporting device 9 gas can be supplied or sucked off at different intensities. It is also conceivable to apply gas pressure above atmospheric to some of the gas passage channels and, at the same time, negative gas pressure to other gas passage channels.

Claims

Claims:

1. Continuous casting plant for continuously casting a thin metal strip (1), in particular a steel strip having a thickness (2) of below 20 mm, preferably from 1 to 12 mm, comprising a melt holding vessel (4) provided with a melt orifice (5) past which a casting surface (8) is movable for receiving a thin layer of melt (3) under the formation of a strand (1), and comprising a strand supporting device (9) taking over the strand (1) from the casting surface (8), characterized in that the strand supporting device (9) is provided with an essentially plane and essentially horizontal surface (22) that supports the strand (1) and has gas passage channels (21) running into it that are connectable to a gas transporting means.

2. Continuous casting plant according to claim 1, characterized in that the gas transporting means is constructed as a means imparting a pressure above atmospheric to the gas, such as inert gas or air, that is to be conveyed through the gas passage channels (21).

3. Continuous casting plant according to claim 1 or 2, characterized in that the gas transporting means is constructed as a means imparting a negative pressure to the gas that is to be conveyed through the gas passage channels (21).

4. Continuous casting plant according to one or several of claims 1 to 3, characterized in that the strand supporting device (9) is constructed as a fixedly arranged plate (10, 14).

5. Continuous casting plant according to one or several of claims 1 to 4, characterized in that the strand supporting device (9) is formed of a highly thermoconductive material, in particular copper or a copper alloy.

6. Continuous casting plant according to one or several of claims 1 to 5, characterized in that the strand supporting device (9) is provided with an internal cooling (17), in particular an internal liquid cooling.

7. Continuous casting plant according to one or several of claims 1 to 6, characterized in that the strand supporting device (9) is provided with a strand stripping device (10) directed toward the casting surface (8) and directly abutting on the same and with a cooling table (14) arranged adjoining the same in the direction of strand withdrawal (25), with both the strand stripping device (10) and the cooling table (14) being provided with gas passage channels (21).

8. Continuous casting plant according to one or several of claims 1 to 7, characterized in that the strand supporting device (9) is provided with lateral limitations (24) extending in the withdrawal direction of the strand (1) and comprising the longitudinal side edges of the strand.

9. Continuous casting plant according to one or several of claims 1 to 8, characterized in that the gas passage channels (21 ) at their openings running into the surface (22) of the strand supporting device (9) occupy a total cross-sectional area of 0.01 to 20 %, preferably 0.1 to 5 %, of the one surface of the strand supporting device (9) that supports the strand (1).

10. Continuous casting plant according to one or several of claims 1 to 9, characterized in that at their openings running into the surface (22) of the strand supporting device (9) the gas passage channels (21) each exhibit a cross-sectional area of 1 to 50 mm2, preferably 3 to 20 mm2.

11. Continuous casting plant according to one or several of claims 1 to 10, characterized in that by their mouths the gas passage channels (21) enclose an acute angle with the surface (22) of the supporting device (9).

12. Continuous casting plant according to claim 11, characterized in that the mouths of the gas passage channels (21) are oriented such that a gas stream is formed that moves essentially in the direction of strand withdrawal (25).

13. Continuous casting plant according to one or several of claims 1 to 12, characterized in that the cross-sectional areas of the openings the gas passage channels (21) arranged at the beginning of the strand supporting device (9) form on the surface (22) of the strand supporting device (9) exceed those resulting in such portions of the strand supporting device (9) as are arranged downstream in the direction of strand withdrawal (25).

14. Continuous casting plant according to one or several of claims 1 to 13, characterized in that the strand supporting device (9) is provided with coolant channels (17) arranged transverse to the direction of strand travel and each of at least several of these coolant channels is connected with a coolant feed duct (18) acting as a distributing main and a coolant discharge duct (19) acting as a collecting main, which are arranged beside the strand supporting device (9).

15. Continuous casting plant according to claim 14, characterized in that a coolant channel (17) departing from the distributing main (18) is conducted through the strand supporting device (9) several times and subsequently opens into the collecting main (19).

16. Process for operating a continuous casting plant according to one or several of claims 1 to 15, characterized in that the pressure between the bottom surface of the strand and the strand supporting device (9) is adjusted by appropriate suction and/or feeding of gas for control purposes.

17. Process according to claim 16, for starting up a continuous casting plant, wherein from a melt holding vessel (4) melt (3) is continuously deposited on a casting surface (8) being moved past a melt orifice (5) of a melt holding vessel (4) and the melt (3) solidifies on the casting surface (8) while continuously forming a strand (1), wherein the strand (1), which is possibly still liquid on its side facing away from the casting surface (8), from the casting surface (8) is conveyed onward onto a strand supporting device (9) and there is supported and cooled, characterized in that between the strand supporting device (9) and the solidified bottom surface of the strand (1) a gas cushion is produced, notably just at the beginning of the strand.

18. Process according to claim 16, for operating a continuous casting plant, wherein from a melt holding vessel (4) melt (3) is continuously deposited on a casting surface (8) being moved past a mouth (5) of the melt holding vessel (4) and the melt (3) solidifies on the casting surface (8) while continuously forming a strand (1), wherein the strand (1), which possibly is still liquid on its side facing away from the casting surface (8), from the casting surface (8) is conveyed onward onto a strand supporting device (9) and there is supported and cooled, characterized in that between the strand supporting device (9) and the solidified bottom surface of the strand (1) a negative pressure and thus a particularly good contact of the bottom surface of the strand with the surface (22) of the strand supporting device (9) is generated.

19. Process according to one or several of claims 16 to 18, characterized in that between the solidified bottom surface of the partially or completely solidified strand (1) and the strand supporting device (9) a gas cushion is created which exhibits a pressure above atmospheric of 0.1 to 20 mbar, preferably of 0.5 to 10 mbar with respect to the air pressure surrounding the plant.

20. Process according to one or several of claims 16 to 19, characterized in that between the solidified bottom surface of the strand (1) and the strand supporting device (9) a negative pressure of up to 1000 mbar, preferably between 5 and 100 mbar, is created.