Title of Invention

PROCESS FOR CONTINUOUSLY CASTING A THIN STRIP AS WELL AS ARRANGEMENT FOR CARRYING OUT THE PROCESS

Abstract

The invention relates to a method for the continuous casting of a thin strip (1) using the two-roll method. According to said method molten metal (7) is cast into a casting slit (3), which is formed by two casting rolls (2) and corresponds to the thickness of the strip (1) to be cast, resulting in the formation of a molten bath (6). The surfaces (11) of the casting rolls (2) located above the molten bath (6) are rinsed with an inert gas or an inert gas mixture in accordance with the state of the surfaces (11) of the casting rolls (2). To avoid local thermal deformations, the surfaces (11) of the casting rolls (2) are observed along their entire length so as to detect local variations in their state. When local variations in state are detected, the gas rinsing of the surfaces (11) of the casting rolls (2) is carried out such that it differs locally in accordance with local variations observed along the entire length of the casting rolls (2).

Full Text

FORM 2 THE PATENTS ACT,1970 [39 OF 1970] COMPLETE SPECIFICATION [See Section 10] "PROCESS FOR CONTINUOUSLY CASTING A THIN STRIP AND AN APPARATUS FOR THE SAME" VOEST-ALPINE INDUSTRIEANLAGENBAU GmbH, an Austrian company, of Turmstrasse 44, A-4020 Linz, Austria, and ACCIAI SPECIALI TERNI S.p.A., an Italian company, of Viale B Brin, 218, I-05100 Terni, Italy, The following specification particularly describes the nature of the invention and the manner in which it is to be performed:- The invention relates to a process for continuously casting a thin strip, in particular a steel strip, preferably having a thickness of less than 10 mm, in a two-roll process, wherein metal melt is cast into a casting gap formed by two casting rolls in the thickness of the strip to be cast while forming a melt bath and the surfaces of the casting rolls above the melt bath are swept with an inert gas or an inert gas mixture as a function of the condition of the surfaces of the casting rolls, as well as to an arrangement for carrying out the process. When casting a thin strip in the two-roll process, the cross section of the strip is determined by the section of the casting rolls in the hot state. It is essential that the hot section exactly corresponds to the desired strip cross section, since the strip section can no longer be changed after the casting process, i.e. not even by means of a rolling process. The hot section of the casting rolls deviates considerably from the cold section due to the periodically occurring very high thermal loads exerted on the surfaces of the casting rolls. Thermal cambering will be caused, which, however, may be compensated for at least partially by concave rough-grinding of the casting rolls. Since the thermal load exerted on the casting rolls in the casting process is, however, influenced by a plurality of parameters and, in addition, a strip caster should encompass a wide operating range (e.g. a casting speed range between 0.2 and 2.5 m/s, a" strip thickness range between 1 and 10 mm, different rolling forces occurring on the casting rolls, different temperatures of the metal melt to be cast, different melt quantities such as, e.g., different steel grades etc.), sufficient pre-profiling of the casting rolls by rough-grinding is not feasible. Rather, it is necessary to effect an on-line adjustment of the casting roll surfaces for adaptation to different operating points. Such an on-line adjustment as described in the introduction is known, for instance, from AU-A- 50 340/_9.6.. There, the surfaces of the casting rolls are observed by sensors coupled to a computer. The computer controls a gas feed to the casting rolls, wherein two different gases, i.e. nitrogen and argon, are fed to the casting rolls, and hence to the melt bath in different partial amounts depending on the condition of the surfaces of the casting rolls, in oder to influence the heat transfer just above the bath level of the melt bath. The mixed gas thus formed is fed to the surfaces of the casting rolls in a manner distributed over the total longitudinal extent of the same. This is to avoid thermal cambering of the casting rolls and to safeguard a uniform thickness of the strip produced. As an alternative, another suggestion is to measure the thickness of the strip distributed over the width of the strip so as to be able to detect deviations from a rectangular cross section of the strip and compensate for the same by appropriate mixing ratios of the gases fed to the casting roll surfaces. As already mentioned, the heat transfer between the casting rolls and the metal melt may be decisively influenced by the different gas compositions, thus bringing about changes in the geometries of the casting rolls. Internal research work in the field of two-roll casting has revealed that a satisfactory product cannot be obtained despite the above-described measures. There was observed the phenomenon that a roughness present as uniformly as possible over the total surface of the casting rolls is not maintained due to thermal deformation of the casting rolls and due to a slightly uneven solidification of the metal melt on the surface of the casting rolls despite the supply of specifically adjusted gas mixtures, but that circumferentially oriented smooth sites not extending over the total longitudinal extent of the casting rolls occur. Thus, brighter, smoother sites are, for instance, formed on the circumference of the casting rolls. Since such smooth sites, due to their reduced roughness, cause a more rapid solidification of the metal melt and hence a better contact within the casting gap, the so-called "kissing point", which, in turn, induces higher local specific rolling forces, the smoothness of the casting rolls in these areas which are already smoother is" intensified. This causes a building-up process and hence an ever increasing deterioration of the strip quality, which cannot be obviated by the above-" described measures, i.e. a change in the mixing ratio } of the gas fed near the bath level. The invention aims at avoiding these disadvantages and difficulties and has as its object to provide a process, as well as an arrangement for carrying out the process, of the initially defined kind, which allow for the production of a strip having an ideal cross section even with strongly varying operating states. The occurrence of thermal deformations of the casting rolls due to local smooth sites is to be avoided, in particular. In accordance with the invention, this object is achieved in that gas sweeping of the surfaces of the casting rolls is carried out over the longitudinal extent of the casting rolls in a locally different nanner. A preferred embodiment is characterized in that the surfaces of the casting rolls are observed over their Longitudinal extent with respect to locally different ronditions and in that gas sweeping of the surfaces of the casting rolls is carried out as a function of what has been observed. Preferably, locally different gas sweeping is carried out with locally different gas compositions. Locally different gas sweeping may, however, also be carried out with locally different gas amounts and/or with locally different gas pressures. Preferably, locally different surface roughness conditions of the casting rolls are observed. According to another embodiment, locally different surface reflection property conditions of the casting rolls are observed. It is, however, also possible to observe locally different discolorations of the surfaces of the casting rolls. Simple realization of the process is feasible if the surfaces of the casting rolls in the direction of their longitudinal extent are divided into consecutively arranged zones and each zone is observed with respect to the condition of the surfaces, and locally different gas sweeping is effected in zones, i.e. by gas sweeping that is uniform and constant within each zone, wherein at least three adjacently located zones and up to 40 adjacently located zones are preferably formed. A preferred embodiment is characterized in that the observation of the surfaces of the casting rolls is carried out by receiving electromagnetic waves emitted and/or reflected from the surfaces, in particular in the range of visible • light and/or in the range of heat radiation. According to another embodiment of the invention, the observation of the surfaces of the casting rolls is effected indirectly by observing the cast strip over its width after the emergence of the strip from the casting gap, wherein, expediently, at least one surface of the strip is observed over its width immediately after the emergence of the strip from the casting gap and wherein, preferably, electromagnetic waves emitted and/or reflected from the surface of the strip, in particular in the range of visible light and/or in the range of heat radiation, are received. Preferably, gas sweeping is carried out at a pressure on the gas outlet openings of at least 1.05 to a maximum of 2 bar and, preferably, at least 1.5 bar, wherein gas sweeping is expediently carried out at a gas outlet speed on the gas outlet openings of at least 0.2 m/s and, preferably, at least 1.5 m/s. An arrangement for continuously casting a thin strip by applying the process, comprising a continuous casting mould formed by two casting rolls defining a casting gap, wherein the width of the casting gap corresponds to the thickness of the strip to be cast and a melt bath receptacle covered by a lid is formed between the casting rolls above the casting gap, a gas feeding device feeding an inert gas to the casting rolls and having at least one gas outlet opening just above the melt bath present between the casting rolls, a device for observing the surfaces of the casting rolls and a control unit for influencing the gas feed to the casting rolls as a function of the condition of the casting roll surfaces is chaYactferized in that several gas feeding devices are provided, wherein each gas feeding device is associated with a partial surface area of a casting roll and each partial surface area is feedable with gas by means of the associated gas feeding device as a function of an observed value allocated to said partial surface area surface area [sic] by the control unit. to said partial Preferably, each gas feeding device comprises several closely adjacent gas outlet openings. A preferred embodiment is characterized in that the gas feeding devices are connected to two or more gas reservoirs each containing a different gas via gas ducts equipped with throttle or shut-off members, wherein the gas ducts of each gas feeding device open into a mixing device, preferably a mixing chamber, associated with the gas feeding device and from which in each case at least one gas feeding duct leads to the gas outlet opening(s) associated with the gas feeding device. Expediently, the devices for observing the surfaces of the casting rolls are formed by sensors directed towards the surfaces of the casting rolls. For a particularly thorough observation of the surfaces of the casting rolls, a profile sensor is provided as the sensor for each of the casting rolls for the purpose of an integral observation of the surfaces of the casting rolls over their longitudinal extent, preferably over their total longitudinal extent. It is also possible to observe the surfaces of the casting rolls indirectly, i.e. via the cast strip, wherein the devices for observing the surfaces of the casting rolls are formed by sensors directed towards at least one of the surfaces of the cast strip. According to another preferred embodiment, two or more, preferably at least three, devices for observing the surfaces of the casting rolls are distributed over the longitudinal extent of the casting rolls, each of said devices being separately coupled with a respective gas feeding device via a control unit. Preferably, the axes of the gas outlet openings are oriented in the circumferential direction towards the surfaces of the casting rolls within a range of between +60° and -60° and, preferably, between +20° and -30°. A preferred embodiment is characterized in that the surfaces of the casting rolls have a roughness of more than 4 μm and, preferably, more than 8 urn. According to a further preferred embodiment, the surfaces of the casting rolls are provided with dimples whose depths are between 10 and 100 μm and whose diameters are between 0.2 and 1.0 mm, dimples advantageously contacting one another, preferably 5 to 20% of the dimples. Good gas sweeping is ensured if more than 20% of the dimples contact one another. In the following text, the invention is explained in more detail by way of two exemplary embodiments schematically illustrated in the drawing. Fig. 1 shows a side view of an arrangement according to the invention for continuously casting a thin strip according to a first embodiment. Fig. 2 illustrates a detail from this Fig. 1, and Fig. 3 is a top view in the direction of the arrow III of Fig. 1. Fig. 4 is a diagram illustrating the gas sweeping of individual circumferential zones. A continuous casting mould formed by two casting rolls 2 arranged adjacent and parallel to one another serves to cast a thin strip 1, in particular a steel strip having a thickness of between 1 and 10 mm. The casting rolls 2 form a casting gap 3, the so-called "kissing point", on which the strip 1 emerges from the continuous casting mould. Above the casting gap 3, there is formed a space 4 which is upwardly screened by a cover plate 5 forming a cover and which serves to receive a melt bath 6. The metal melt 7 is supplied via an opening 8 of the cover, through which an immersed, tube projects into the melt bath 6 as far as to below the bath level 9. The casting rolls 2 are provided with an internal cooling not illustrated. Laterally of the casting rolls 2, side plates 10 are provided for sealing the space 4 receiving the melt bath 6. A strand shell 12 forms on the surfaces 11 of the casting rolls 2, said strand shells being united to form a strip 1 in the casting gap 3, i.e. on the kissing point. For the optimum formation of a strip 1 with an approximately uniform thickness - preferably with a slight curvature conforming to standards - the presence in the casting gap 3 of a specific rolling force distribution in rectangular form is essential. The cover plate 5 is arranged in such a manner that a gap 13 of slight width is provided between the cover plate and the surfaces 11 of the casting rolls 2, which gap is externally sealed relative to the surfaces 11 of the two casting rolls 2 by means of an optionally resilient sealing lip 14, a labyrinth seal, etc. in order to prevent air from getting in. The edge of the cover plate 5 that is directed towards the casting rolls 2 is adapted in each case to the surfaces 11 of the casting rolls 2 so as to form a gap 13 having an approximately constant width. Inert gas is fed via this gap 13 by means of gas feeding ducts 15 fastened to the cover plate 5 by means of quick couplings 16, one quick coupling 16 advantageously being provided for two or more gas feeding ducts 15 at a time. What is important is a tight and precise connection, which may also be in the form of a butt joint, since the gas pressures in the individual gas feeding ducts 15 need not be identical. Bores 17 (which could also be slits) are provided in the cover plate as am extension of the gas feeding ducts 15 and, via a gas outlet opening 18, open into the gap 13 between the cover plate and the respective casting roll 2. These bores 17 may also open out at the lower end of the gap 13 in the already horizontal edge region of the cover plate 5. The diameters or gap widths of the gas outlet openings 18 are smaller than 5 mm and, preferably, smaller than 3 mm. The surfaces 11 of the casting rolls 2 are swept with an inert gas as a function of their condition, to which end the surfaces 11 of the casting rolls 2 are provided by means of [sic] a device 19 or observing them. According to the exemplary embodiment illustrated, a profile sensor 19} is directed in each case towards a surface 11 of a casting roll 2, measuring a temperature profile integrally over the longitudinal extent of each casting roll 2. The profile sensor 19 is coupled with a computer and control unit 20. in such a manner that temperature values or temperature mean values may each be allocated to adjacently located partial surface areas a, b, c, ..., i.e. individual adjacent circumferential zones a, b, c, ... distributed over the longitudinal extent of the casting rolls 2. The profile sensor 19 could also be replaced with a radiation sensor for detecting smooth sites on the surfaces 11 of the casting rolls 2. In order to be able to influence, by means of inert gas, individual zones of the adjacently located circumferential zones a, b, c, ... of each casting roll 2 separately and independently of one another, a plurality of gas feeding devices 21 is provided, according to the exemplary embodiment illustrated, each gas feeding device 21 being allocated to a circumferential zone a, b, c, ... of a casting roll 2. Compressed gas reservoirs 22 for different gases are provided for gas sweeping; for instance, three compressed gas reservoirs 22 according to the exemplary embodiment illustrated, wherein each of the compressed gas reservoirs 22 is filled with a specific gas, e.g. one with nitrogen, one with argon and one with helium. From each of these compressed gas reservoirs 22, gas ducts 24 lead to a mixing chamber 23 associated with in each case one of the circumferential zones a, b, c, ..., wherein a specific gas composition formed from one or more of the gases contained in the compressed gas reservoirs 22 may be set in each of the mixing chambers 23 by means of throttle and shut-off members 25 installed in the gas ducts 24. These throttle and shut-off members 25 are coupled with the controller 20 and are activated by the same such that a specific gas composition in accordance with the temperature profile present over the longitudinal extent of each casting roll 2 may be set for each mixing chamber 23 and hence for each of the circumferential zones a, b, c, .... The set values to be selected are determined by the controller 20 on the basis of the temperature profiles detected by the respective sensor 19. A gas feeding duct 15 leads from each of the mixing chambers 23 to a gas outlet opening 18 provided on the edge of the cover plate 5, whereby the surfaces 11 of the casting rolls 2 may each be acted upon by different gas compositions, i.e. locally different gas mixtures -viewed in the longitudinal direction of the casting rolls 2 - in a circumferential-zone-related manner. It is also possible to combine several adjacently located gas outlet openings 18 (e.g., in the form of bores) to form a group and to feed them from a single gas feeding duct 15, whereby wider circumferential zones a, b, c, ... are formed, i.e. larger surface areas of the surfaces 11 are each supplied with a gas mixture. Hence it results that a gas feeding device for feeding gas to a circumferential zone a, b, c, ... is formed from gas ducts 24 (their number corresponding to the number of compressed gas reservoirs 22), throttle and shut-off members 25, a mixing chamber 23, a gas feeding duct 15 and at least one gas outlet opening 18. The incoming gas should have impact pressures of at least 1.05 bar and, preferably, more than 1.5 bar up to 2 bar, wherein the axes of the gas outlet openings 18 may be substantially perpendicular to the casting roll surface, yet are inclined in, or opposite to, the direction of movement of the roll surface, to be precise in the range of + 60°. The choice of the widths of the circumferential zones a, b, c, ... depends on the possible susceptibility to failures of the casting process, which, in turn, is largely a function of the process parameters. According to another embodiment of the invention, the surfaces 11 of the casting rolls 2 are not directly observed, but a conclusion is drawn as to the conditions of the surfaces 11 of the casting rolls 2 from a direct observation of one of the surfaces 26, or both of the surfaces 26, of the strip 1. Consequently, the sensors 19 in this embodiment are directed towards the surfaces 26 of the strip 1, i.e. as immediately as possible after the emergence of the strip 1 from the casting gap 3, as indicated in Fig. 1 by dot-and-dash lines. The invention is not limited to the exemplary embodiments depicted in the drawing, but may be modified in various respects. It is, for instance, possible to achieve the object underlying the invention by observing the local surface roughness of the casting rolls 2 instead of measuring the locally occurring temperature on the casting roll surfaces 11. Conclusions may also be drawn from observing the surface reflection properties of the casting rolls 2, or of the strip 1, by means of image recognition systems, or locally different discolorations of the surfaces of the casting rolls 2 may be observed and used for selecting the gas composition to be swept towards the circumferential zones. The surfaces 11 of the casting rolls 2 may also be influenced by additionally adjusting locally different gas amounts and/or locally different gas pressures instead of the local variation of the gas composition. Fig. 4 represents schematically in diagram form the different feeds of different gas compositions A, B, C, ... to circumferential zones a, b, c, .... The individual adjacently arranged circumferential zones a, b, c, ... are plotted on the abscissa of the diagram. In sum, they correspond to the length of a casting roll 11. In the direction of the ordinate, the temperature values allocated to the individual circumferential zones a, b, c, ... are plotted, a temperature profile according to line 27 resulting from a very fine measurement. In addition, gas quantity values with which the individual circumferential zones a, b, c, ... are swept per time unit are plotted in the ordinate direction. References A, B, C, ... relate to different gas compositions such as may be formed by mixing the different gases contained in the compressed gas. reservoirs 22. It is apparent that each temperature mean value of a circumferential zone a, b, c, ... (the mean values being indicated by broken lines) is allocated a defined gas composition and a defined gas amount to act on the circumferential zones a, b, c, The invention is based on the idea that local influencing of a partial surface of the overall surface 11 of a casting roll 2 is possible by means of locally differently fed gas mixtures or gas amounts when feeding these gas mixtures just above the melt bath level 9. By way of experiments, it has been shown that different gas mixtures inducing different solidification rates may be introduced even into closely adjacent regions, i.e. even into directly adjacent regions of the melt bath level 9 while, nevertheless, it is possible to exert different influences on adjacently located surface zones or circumferential zones a, b, c, ... of the casting rolls 2, thereby preventing the surfaces 11 of the casting rolls 2 from becoming non-uniform. As a result, the surfaces 11 of the casting rolls 2 will reguire repair or replacement only after considerably longer casting sequences or substantially higher product tonnages than has been the case until now. By way of an experiment it has been shown that the solidification speed may be kept lower by up to 30% when using 100% argon than with the use of 100% helium. Thus, it was found that zones on the surfaces 11 of the casting rolls 2 exhibiting red-brownish discolorations or stains could be removed again by increasing the supply of helium, which considerably increases the local solidification rate; the red-brown coloration fades or disappears. Furthermore, it was found that in regions of glossy stains the solidification rate can be reduced by increasing the argon feed, thereby causing the glossy stains to disappear again. In general, varying casting roll surface conditions over the longitudinal extent of the casting rolls 2 are eliminated by the process according to the invention and the scattering range of the surface quality differences during, or on account of, the casting procedure does not increase, but heat transfer in the event of local changes to the surfaces is influenced by a change in the locally applied gas mixture in such a manner that these changes to the surface do not increase, but decline again. By surface quality, its roughness, optical reflection properties, discolorations, stains, or the presence of striae or dimples, for example, are to be understood. In accordance with the invention, the solidification structure, in particular the central globulitic-dentric solidification structure, of the strip 1 produced will become more uniform over the total width, on the one hand, and reconditioning (rendering the surfaces 11 of the casting rolls 2 uniform) will be required only after a larger number of casts, on the other hand. Thus, not only the service life of the surface layer, but also, in particular, the service life of the casting rolls 2 as a whole will be markedly increased. WE CLAIM: 1 Process for continuously casting a thin strip (1), in particular a steel strip, preferably having a thickness of less than 10 mm, in a two-roll process, wherein metal melt (7) is cast into a casting gap (3) formed by two casting rolls (2) in the thickness of the strip (1) to be cast while forming a melt bath (6) and the surfaces (11) of the casting rolls (2) above the melt bath (6) are swept with an inert gas Or an inert gas mixture, characterized in that gas sweeping of the surfaces (11) of the casting rolls (2) is carried out over the longitudinal extent of the casting rolls (2) in a locally different manner, wherein the surfaces (11) of the casting rolls (2) are observed over their longitudinal extent with respect to locally different conditions and in that gas sweeping of the surfaces (11) of the casting rolls (2) is carried out as a function of what has been observed. 2 Process as claimed in claim 1, wherein locally different gas sweeping is carried out with locally different gas compositions. 3 Process as claimed in one or more of claim 1 or 2, wherein locally different gas sweeping is carried out with locally different gas amounts. 4 Process as claimed in one or more of claims 1 to 3, wherein locally different gas sweeping is carried out with locally different gas pressures. 5 Process as claimed in one or more of claims 1 to 4, wherein locally different surface roughness conditions of the casting rolls (2) are observed. 6 Process as claimed in one or more of claims 1 to 5, wherein locally different surface reflection property conditions of the casting rolls (2) are observed. 7 Process as claimed in one or more of claims 1 to 6, wherein locally different discolorations of the surfaces (11) of the casting rolls (2) are observed. 8 Process as claimed in one or more of claims 1 to 7, wherein the surfaces (11) of the casting rolls (2) in the direction of their longitudinal extent are divided into consecutively arranged zones (a, b, c, ...) and in that each zone (a, b, c, ...) is observed with respect to the condition of the surfaces (11), and in that locally different gas sweeping is effected in zones, i.e. by gas sweeping that is uniform and constant within each zone (a, b, c, ...). 9 Process as claimed in claim 8, wherein at least three adjacently located zones (a, b, c, ...) are formed. 10 Process as claimed in claim 8, wherein up to 40 adjacently located zones (a, b, c, ...) are formed. 11 Process as claimed in one or more of claims 1 to 10, wherein the observation of the surfaces (11) of the casting rolls (2) is carried out by receiving electromagnetic waves emitted and/or reflected from the surfaces (11), in particular in the range of visible light and/or in the range of heat radiation. 12 Process as claimed in one or more of claims 1 to 11, wherein the observation of the surfaces (11) of the casting rolls (2) is effected indirectly by observing the cast strip (1) over its width after the emergence of the strip (1) from the casting gap (3). 13 Process as claimed in claim 12, wherein at least one surface (26) of. the strip (1) is observed over its width immediately after the emergence of the strip (1) from the casting gap (3), wherein electromagnetic waves emitted and/or reflected from the surface (26) of the strip (1), in particular in the range of visible light and/or in the range of heat radiation, are received. 14 Process as claimed in one or more of claims 1 to 13, wherein gas sweeping is carried out at a pressure on the gas outlet openings (18) of at least 1.05 to a maximum of 2 bar and, preferably, at least 1.5 bar. 15 Process as claimed in one or more of claims 1 to 14, wherein gas sweeping is carried out at a gas outlet speed on the gas outlet openings (18) of at least 0.2 m/s and, preferably, at least 1.5 m/s. 16 Apparatus for continuously casting a thin strip (1) by applying the process according to one or more of claims 1 to 16, comprising a continuous casting mould formed by two casting rolls (2) defining a casting gap (3), wherein the width of the casting gap (3) corresponds to the thickness of the strip (1) to be cast and a melt bath receptacle (4) covered by a lid (5) is formed between the casting rolls (2) above the casting gap (3), a gas feeding device (21) feeding an inert gas to the casting rolls (2) and having at least one gas outlet opening (18) just above the melt bath (6) present between the casting rolls (2), and a control unit (20) for influencing the gas feed to the casting rolls (2), wherein several gas feeding devices (21) are provided, wherein each gas feeding device (21) is associated with a partial surface area (a, b, c, ...) of a casting roll (2) and each partial surface area (a, b, c, ...) is feedable with gas by means of the associated gas feeding device (21) as a function of a value allocated to said partial surface area (a, b, c, ...), and a device (19) for observing partial surface areas (a, b, c, ...) of the surfaces (11) of the casting rolls (2) is provided, which is coupled with the control unit (20). 17 Apparatus as claimed in claim 16, wherein each gas feeding device (21) comprises several closely adjacent gas outlet openings (18). 18 Apparatus as claimed in one or more of claims 16 or 17, wherein the gas feeding devices (21) are connected to two or more gas reservoirs (22) each containing a different gas via gas ducts (24) equipped with throttle or shut-off members (25), wherein the gas ducts (24) of each gas feeding device (21) open into a mixing device (23), preferably a mixing chamber (23), associated with the gas feeding device (21) and from which at least one gas feeding duct in each case leads to the gas outlet opening(s) (18) associated with the gas feeding device (21). 19 Apparatus as claimed in one or more of claims 16 to 18, wherein the devices for observing the surfaces (11) of the casting rolls (2) a.re formed by sensors (19) directed towards the surfaces (11) of the casting rolls (2). 20 Apparatus as claimed in claim 19, wherein a profile sensor (19) is provided as the sensors for each of the casting rolls (2) for the purpose of an integral observation of the surfaces (11) of the casting rolls (2) over their longitudinal extent, preferably over their total longitudinal extent. 21 Apparatus as claimed in one or more of claims 16 to 20, wherein the devices for observing the surfaces (11) of the casting rolls (2) are formed by sensors (19) directed towards at least one of the surfaces (26) of the cast strip (1). 22 Apparatus as claimed in one or more of claims 16 to 21, wherein two or more, preferably at least three, devices (19) for observing the surfaces (11) of the casting rolls (2) are distributed over the longitudinal extent of the casting rolls (2), each of said devices being separately coupled with a respective gas feeding device (21) via a control unit (20). 23 Apparatus as claimed in one or more of claims 16 to 22, wherein the axes of the gas outlet openings (18) are oriented in the circumferential direction towards the surfaces (11) of the casting rolls (2) at an angle a with a range of between +60° and -60" and, preferably, between +20° and -30°. 24 Apparatus as claimed in one or more of claims 16 to 23, wherein the surfaces (11) of the casting rolls (2) have a roughness of more than 4 μm and, preferably, more than 8 μm. 25 Apparatus as claimed in one or more of claims 16 to 24, wherein the surfaces (11) of the casting rolls (2) are provided with dimples whose depths are between 10 and 100 μm and whose diameters are between 0.2 and 1.0 mm. 26 Apparatus as claimed in claim 25, wherein dimples contact one another, preferably 5 to 20% of the dimples. 27 Apparatus as claimed in claim 26, wherein more than 20% of the dimples contact one another. Dated this 18th day of August, 2000. [Dr. Ramesh "Kumar Mehta] of Remfry & Sagar Attorney for the Applicants

Documents:

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in-pct-2000-00310-mum-abstract(7-6-2004).doc

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in-pct-2000-00310-mum-claims(granted)-(7-6-2004).doc

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in-pct-2000-00310-mum-form 1(18-8-2000).pdf

in-pct-2000-00310-mum-form 13(31-7-2007).pdf

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in-pct-2000-00310-mum-form 2(granted)-(7-6-2004).doc

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in-pct-2000-00310-mum-form 3(11-2-2004).pdf

in-pct-2000-00310-mum-form 3(18-8-2000).pdf

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in-pct-2000-00310-mum-form 5(18-8-2000).pdf

in-pct-2000-00310-mum-form-pct-ipea-409(26-2-1999).pdf

in-pct-2000-00310-mum-form-pct-isa-210(26-2-1999).pdf

in-pct-2000-00310-mum-other documents(18-12-2002).pdf

in-pct-2000-00310-mum-petition under rule 137(7-6-2004).pdf

in-pct-2000-00310-mum-petition under rule 138(7-6-2004).pdf

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in-pct-2000-00310-mum-power of authority(7-6-2004).pdf