PLANT AND PROCESS FOR THE CONTINUOUS PRODUCTION OF HOT-ROLLED ULTRA-THIN STEEL STRIPS

Abstract

A plant and process for the continuous production of hot-rolled steel strips with a minimum thickness of 0.3 mm are described, comprising a continuous casting device (1) for thin or medium plates with a thickness between 40 and 150 mm and a maximum width of at least 2100 mm followed by a roughing mill (2), a first induction furnace, a water descaler, a second induction furnace, a finishing mill, a cooling station, a cutting station and a winding station, a feeding system of a protective atmosphere containing ≥3% vol. of oxygen being provided at least from the inlet of the second induction furnace to the third box of the finishing mill, and further comprising, between the continuous casting device (1) and the roughing mill (2), an initial heat conditioning and descaling section (4) comprising in sequence an induction edge heater (4.1), an induction heater (4.2) for the rest of the surface of the iron and a water descaling agent (5).

Description

Field of invention. The present invention relates to a plant and process for the continuous production of hot-rolled ultra-thin steel strips up to 0.3 mm thick and with a limited amount of inclusions, so that they are suitable for direct corrosion protection without undergoing specific prior surface treatments. Background of the invention. It is well known that in the steel industry, due to rising raw material and energy costs, increased competitiveness in the global market, and increasingly stringent pollution regulations, there is a particular need for a high-quality hot-rolled steel strip manufacturing method that requires lower investment and production costs, resulting in progressively thinner strip thicknesses. As a result, the finished product processing industry can also become more competitive with reduced energy consumption, thus minimizing its negative environmental impact. The prior art is essentially as described in earlier patents of the same inventor, such as EP1558408, EP1868748, and EP1909979, which are referenced for further details. In practice, the technology used is called ESP (Endless Strip Production), which is based on cast rolling that combines the continuous casting of a thin slab with liquid core reduction (LCR) and a first roughing stage through a high reduction mill (HRM) that produces an intermediate product, the so-called transfer bar. The casting is carried out from an ingot mold system based on patents EP0946316, EP1011896 and EP3154726, also by the same inventor, which is referenced for further details, which refer to the geometric profile of both the horizontal and vertical section of the ingot mold, as well as the particular geometry of the nozzle designed for a high mass flow of material of up to 7-8 ton / min. The aforementioned patent EP 1558408 also contemplates the possibility of extracting rough sheets after the first roughing stage as an emergency system in case of problems in the part of the plant downstream of the roughing mill, to avoid interruption of Γ / ηηιη / ζζηζ / Ε / γίΛΐ the continuous casting and consequently the production of the line, and not for a scheduled production of sheets given the absence in the first portion of the plant of a controlled cooling system necessary for the production of high quality sheets. The transfer bar, after a heating phase in an induction furnace and subsequent descaling, is processed in a second finishing rolling phase to transform it into a strip, controlling its temperature so that at the exit of the finishing rolling it still has a temperature above approximately 820-850°C, which corresponds to the lower end of the austenitic temperature range for most steels. However, while the results to date are excellent in terms of steel strip quality, they have proven improvable in terms of plant compactness, energy savings, and the current minimum strip thickness of 0.6 mm. Furthermore, although reduced oxide formation (scaling) is achieved on the strip surface due to the minimal material exposure time at the temperature, through the aforementioned induction heating of the transfer bar between the roughing and finishing stages, this reduction has not proven sufficient to eliminate the need for pickling before applying the anti-corrosion coating. To ensure the desired final lamination in the austenitic field with greater production flexibility and to further reduce scale formation, a plant of the type described above is known from document US 9108234, which also includes a second induction furnace between the descaler and the finishing mill, heating taking place in said second furnace in a protective atmosphere that prevents oxidation of the transfer bar being composed substantially of inert gas (nitrogen) with a minimal presence of oxygen (about 5% or less).Other examples of induction heating in a protective atmosphere before final rolling are found in US patent 8479550, which, however, provides only an induction furnace after the descaler; US patent 2012 / 043049, which also provides for a reducing atmosphere using hydrogen but without roughing; and DE 19936010, which, however, does not include an induction furnace after the descaler and, for the protective atmosphere, teaches the use of combustion gas produced in the plant itself instead of an inert gas in order to reduce costs, this gas being able to be distributed also in various parts of the plant before and after the induction furnace (e.g., induction edge heater, descaler, finishing mill, outfeed roller conveyor, winder). However, none of these prior art documents foresee obtaining a strip thickness Γ / ηηιη / ζζηζ / E / γίΛΐ below the current limit of 0.6 mm nor does it consider the particular problems that arise when going below this limit. In fact, none of the plants described in these documents are suitable for this purpose due to the conflicting requirements of maintaining a high temperature of the transfer bar at the entrance of the finishing mill to ensure fully austenitic rolling of such a thin strip and therefore subject to greater cooling, and the need to limit scaling despite the strong heating in terms of both time and temperature. Brief description of the invention. Therefore, the purpose of the present invention is to provide a solution for the continuous production of hot-rolled strips with a thickness of up to 0.3 mm and a maximum width of at least 2100 mm, or whatever the maximum foreseeable width of the ingot in mold may be, from the casting of a roughing of a thickness between 40 and 150 mm without passing through intermediate plants of pickling, cold rolling and annealing, and with a limited amount of scale so that these strips are suitable to be coated directly against corrosion (particularly in galvanizing lines) without undergoing specific surface conditioning pretreatments, particularly in pickling lines. This result is achieved using continuous production technology (called endless), which minimizes production time and consumption and, consequently, reduces production costs, in particular by adopting the following measures to control the material's temperature and limit its reduction, avoiding excessive surface oxidation of the material: a) In order to clean the slab of scale before entering the roughing mill (HRM) and to allow a number of roughing passes from a minimum of three to a maximum of five, at the outlet of the continuous casting machine there is an initial heat conditioning and descaling section comprising sequentially, in the direction of advance of the slab, an induction edge heater, an induction heater for the remainder of the slab surface and a water descaler; b) In order to prevent the water and steam jets from the descaling agent from damaging the induction coils of the surface heater, the descaling agent is provided at the inlet with transversely movable shutters that rest directly on the edges of the plate, while the closure on the top and undersides of the plate is provided by a small drive support, called a pressure roller, positioned next to said Γ / ηηιη / ζζηζ / Ε / γίΛΐ shutters on the inlet side of the descaler facing the surface heater; c) given the low speed of the plate at the exit of the foundry, less than 10 m / min, in order to minimize the time it takes for the plate to pass from the foundry to the entrance of the roughing mill, in order to minimize the formation of scale and temperature drops, said initial section must be as compact as possible, so that said edge heaters, surface heaters and descalers, including the latter pressure rollers and screening shutters, occupy a space on the order of 3-5 meters in length; d) The edge heater is equipped with a handling system that allows the heating system's efficiency to remain constant as the plate width varies, to establish the optimal width of the edge area to be heated, and to remove / lift the induction coils in case of waves on the plate due to cobblestones in the roughing mill; e) The edge heater can heat the right and left edges of the plate differently to ensure an optimal and homogeneous profile of the plate entering the roughing mill, even if the plate coming out of the foundry has a lack of temperature homogeneity between the two edges; f) The water softener is designed to have a diameter of the cooling water nozzles and a supply pressure such that the temperature drop at the outlet of the water softener is limited to less than 10°C between the time when the water softener is active and the time when it is inactive. g) Other advantageous arrangements preferably adopted in the present invention to improve the present plant and process are: h) construct the second water dehuller, located between the two induction furnaces before the finishing mill, with a structure similar to the first dehuller mentioned above and including pressure rollers at both the inlet and outlet to protect said two induction furnaces from water and steam jets; i) Mounting the nozzles for feeding the protective atmosphere in the finishing mill on the movable structure of the so-called looper arranged between the rolling boxes, i.e., a roller equipped with a strip tension sensor that can be moved vertically and allows the material to be arranged with a suitable loop between the supports in such a way that the speed control system varies the reciprocal speed of the supports to maintain a constant tension on the strip Γ / ηηιη / ζζηζ / Ε / γίΛΐ j) providing a mechanical descaling device, located immediately before the second water descaler, consisting of at least three rollers arranged alternately above and below the feed line of the transfer bar and at a sufficient height to cause plastic stretching of the surface thereof, which causes a break in the rigid layer of scale and facilitates its removal in the subsequent water descaling; k) In order to allow high temperatures for winding ultra-thin strips, up to 750°C and in any case higher than the transformation points, also provide winders close to the last rolling box, above (up-coilers) or below (down-coilers) the surface of the outgoing roller conveyor, and preceded by a short cooling line and a high-speed shear (in addition to the similar final winders traditionally provided after a normal cooling line and relative shear); 1) provide a first and a second mechanical descaler, located respectively between the cooling line and the shear of the closed winders and the end winders, using abrasive brushes or abrasive mud jets rotating in opposite directions; m) provide a corrosion protection coating line directly after the final winders so that it is possible to apply said coating without the steel strip having to be wound beforehand on a winder to form coils; n) provide a cooling tank in which the coils removed from the winders can be immersed in water or a slightly oxidizing aqueous solution. Brief description of the figures. Other advantages and features of the plant and process according to the present invention will be evident to those skilled in the art from the following detailed and non-limiting description of some of its embodiments with reference to the accompanying figures in which: Figures 1a, 1b, 1c show a schematic view of the plant in a configuration comprising all optional components except the anti-corrosion coating line; Figure 2 is a schematic view showing only the anti-corrosion coating line connected at the end of the plant in Figures 1a-1c; Figure 3 is a side view of the descaling and initial thermal conditioning section; Figure 4 is a schematic vertical cross-sectional view of the descaler in Figure 3; Γ / ηηιη / ζζηζ / Ε / γίΛΐ Figure 5 is a transparent front view showing some components of the descaler from Figure 3; Figure 6 is a schematic vertical section view of the second water softener; Figure 7 is a schematic vertical section view of some components of the second induction furnace preceding the finishing mill; Figure 8 is a schematic vertical section view of a first modality of the protective atmosphere dispensing device placed between two supports of the finishing mill; Figure 9 is a schematic vertical section view along line AA of Figure 8 of a detail of the dispensing device; Figure 10 is a view similar to Figure 8 of a second modality of the protective atmosphere dispensing device; Figure 11 is a schematic vertical section view along line BB of Figure 10 of a detail of the dispensing device; Figure 12 is a view similar to Figure 8 of a third modality of the protective atmosphere dispensing device; and Figure 13 is a schematic vertical section view along line CC of Figure 12 of a detail of the dispensing device. Detailed description of the invention. Referring to Figures 1a-1c, it can be seen that a plant according to the present invention traditionally comprises a casting machine (1) for the continuous casting of fine or medium slabs with a thickness of 40-150 mm, followed by a roughing mill (HRM) (2), in the illustrated example consisting of four stands (2.1-2.4), but it can also be three or five, which transforms the slabs into transfer bars with a thickness <8 mm. Experimental tests have demonstrated how a limited reduction in thickness (<20%) in the first roughing stand (2.1) can contain the surface stresses within the strength limits of the coarse austenite that constitutes the slab as a casting.In this way, the almost static recrystallization of the surface in the first roughing step, particularly for steels with the presence of microalloys, can allow the considerable subsequent thickness reductions necessary to obtain transfer bars suitable for the production of ultra-thin strips to be carried out without defects or cracks. After the HRM (2), an emergency system is provided for the production and disposal of Γ / ηηιη / ζζηζ / E / γίΛΐ raw sheets in case of problems in the part of the plant downstream of the HRM, said system comprises a pendulum shear (15), a stacker (16) for the extraction of sheets, a rotary shear (17) and a loop former (18), these last two devices are intended to free the line of material between the pendulum shear (15) and the next first induction furnace (6.1) in the initial phase of paving. The first induction furnace (6.1) is the first component of the central heat conditioning and descaling section (6), which further comprises, in the direction of the transfer bar's advance, an optional mechanical descaling device (7) of the type described above, consisting in this case of five rollers, a water descaling unit (8), and a second induction furnace (6.2). In this way, the transfer bar is further heated before entering the adjacent finishing mill (3), which in the illustrated example consists of seven supports (3.1-3.7) but could also have five or six. Finally, the strip is cooled in a controlled manner by a cooling roller conveyor (12) followed by a final winding station comprising a flying shear (10) and at least one pair of single winders (11). In order to allow high winding temperatures for ultra-thin strips, as mentioned above, the plant preferably also comprises close winding winders, i.e., preceding the aforementioned elements (10-12), in the form of a pair of carousel winders (9), arranged in proximity to the last rolling stand (3.7) and preceded by a short cooling roller conveyor (12') and a high-speed shear (10') analogous to said elements (10, 12), although the roller conveyor (12') may preferably be made to perform ultra-rapid cooling in order to obtain a scale that is more easily removable in subsequent protective coating application processes. Between each pair of elements (10, 12 and 10', 12'), there is also preferably arranged a respective mechanical descaler (14, 14') of a known type, and therefore not described below, which uses counter-rotating brushes or abrasive slurry jets for a final surface treatment of the strip before it is wound onto the winders (9 or 11). As mentioned above, the plant depicted in Figures 1a-1c also includes a system for dispensing a protective atmosphere in certain areas of the plant, indicated schematically by thick line boxes, which in the illustrated example extend at least from the entrance of the second induction furnace (6.2) to the third box (3.3) of the finishing mill Γ / ηηιη / ζζηζ / Ε / γίΛΐ (3), preferably up to the last box, and even more preferably also in the subsequent cooling and winding stations. Obviously, it would also be possible to consider extending this system to other components of the plant as described in the aforementioned prior art. A first innovative aspect of the present invention, as mentioned above, is the presence of an initial thermal conditioning and descaling section (4) arranged between the outlet of the casting machine (1) and the HRM (2), and designed to have a length of just over three meters to minimize the transit time between these two components. This section (4) comprises an induction edge heater (4.1), an induction heater (4.2), and a water descaling unit (5), which are best illustrated in detail in Figures 3 to 5. More specifically, the edge heater (4.1) is preferably designed to operate with cross-flux using side coils (4.1a) in a channel configuration with flux concentrators, with the dual purpose of increasing the efficiency of the heating system and concentrating the magnetic flux in the selected area of ​​the plate to be heated. Furthermore, it is capable of heating the right and left edges of the plate differently thanks to the presence of two frequency converters, one for each coil (4.1a), instead of a single converter for the entire device as is usually provided. Experimental tests conducted by the applicant indicate that the width of the strip to be heated should preferably reach up to 150 mm from the edge and that the optimal temperature rise in this strip is up to 120°C to prevent scale melting. The edge heater (4.1) is provided with a handling system that performs a transverse movement to adapt the device to the width of the plate, fix the width of the edge area to be heated, and move the coils (4.1a) away from the plate edges (and, if necessary, lift them by rotation) in case of waviness in the plate due to the cobbles in the roughing mill. Such a handling system can be implemented, for example, by placing each coil (4.1a) on a sliding movable along a transverse guide under the action of an actuator such as an electric motor driving a screw jack. The induction heater (4.2) consists of a surface heating coil, designed to be integrated with the edge heater (4.1), which can be controlled so that the temperature increase of the plate reaches values ​​up to a maximum of 150 °C, thus preventing the plate from melting. The downstream descaler (5) consists of the pressure roller (5.1) on the side facing the induction heater (4.2), and the actual descaler (5.2) on the side facing the HRM (2). As shown in Γ / ηηιη / ζζηζ / E / γίΛΐ figures 4-5, to prevent water jets and steam from the descaler (5.2) from damaging the induction coils of the heater (4.2), the descaler (5.2) is provided with transversely movable shutters (20) at the entrance, which rest directly on the edges of the iron, while the closure on the upper and lower faces of the iron is provided by the pressure roller (5.1). More specifically, in the configuration illustrated in Figure 5, each shutter (20) is mounted on a parallelogram support formed by a pair of parallel arms (21) pivoted between the shutter (20) and the descaler structure (5.2) and moved by an actuator (22). Note that in Figure 5 the shutters (20) are shown in an open position and also partially in a closed position (20') resting on the edges of the plate. Water descaling is carried out by means of a row (23) of upper nozzles and a row (24) of lower nozzles arranged transversely to the plate and with the nozzles angled to project a jet in the opposite direction to the direction of the plate's movement. An upper spiral (25) and a lower spiral (26), arranged mirror-image upstream of the nozzles and with their mouths facing the nozzles, collect most of the water through a lip in contact with the plate and direct it to their ends where it is discharged. In addition, a row (27) of upper nozzles and a row (28) of lower nozzles, arranged transversely to the iron upstream of the spirals and angled to project a jet of air in the direction of the iron's movement, remove residual water. The combination of components (5.1, 20, 25, 26, 27, and 28) ensures that the induction coils of the heater (4.2) are not damaged by the water used in the water softener (5). As mentioned previously, the descaling agent (5.2) is designed to limit the temperature drop to less than 10°C between activation and inactivity. To achieve this, the cooling water pressure is less than 150 bar and the nozzle diameter is less than 3 mm. Note that rows 23 and 24 of the water nozzles shown in Figure 5 (where volutes 25 and 26 and rows 27 and 28 of the air nozzles are omitted) are wider than the plate because they are sized for the plate's maximum width. Nozzles outside the plate being processed can be plugged, or their jets will cancel each other out. In this case, the upper and lower nozzles must be arranged in opposite positions, vertically aligned, and have the same angle of inclination (e.g., 5°). The second water softener (8), illustrated in Figure 6, has a similar structure to the first water softener (5), but is substantially double, since it is positioned between the two The induction furnaces (6.1 and 6.2) must prevent water and steam from escaping from both upstream and downstream. It therefore comprises a first inlet pressing roller (8.1), on the side towards the first induction furnace (6.1), the descaling agent itself (8.2), and a second outlet pressing roller (8.1') on the side towards the second induction furnace (6.2). Note that in this case the transverse shutters analogous to the shutters (20) of the first descaler (5) can be omitted since the latter must close a lateral passage of a height equal to the thickness of the plate coming from the foundry machine (1), i.e. 40-150 mm, while the thickness of the transfer bar that enters the second descaler (8) is on the order of 5-20 mm, so the potential lateral leakage of water is much less. Furthermore, since the second descaler (8) is followed by the second induction furnace (6.2), which significantly increases the temperature of the transfer bar before final rolling, the descaling can be more effective, even at the expense of a greater temperature reduction. Therefore, a first row (33) of upper nozzles is provided with a corresponding row (34) of lower nozzles, also arranged transversely to the transfer bar and with the nozzles angled to deliver a jet in a direction opposite to the direction of movement of the bar, as well as an identical second row (33') of upper nozzles with a corresponding row (34') of lower nozzles.Preferably, the second rows (33', 34') are staggered transversely by half a step, where the step is the separation between two nozzles of a row, with respect to the first rows (33, 34) so ​​that the two successive rows (33, 33' and 34, 34') completely cover the upper and lower surface of the bar, respectively, to increase the efficiency of the hydraulic descaling process by eliminating the inefficiencies that manifest themselves in the overlapping strips of adjacent nozzles in each row. The two rows (33, 33') of upper nozzles are likewise preceded by an upper spiral (35, 35') which, however, in this case is separate from the lip (32, 32') that contacts the upper surface of the transfer bar and is movable between a rest position, illustrated in Figure 6, and a working position in which it rotates clockwise and aligns with the volute (35, 35'). Furthermore, the first lip (32) is likewise preceded by a first row (37) of upper nozzles arranged transversely to the transfer bar to deliver an air jet which in this case is substantially perpendicular to the upper surface of the bar, while an identical second row (37') of upper air nozzles is arranged downstream of the second row (33') of upper water nozzles. Since the descaler (8) is not required to be as compact in length as the In the water softener (5), the transfer bar can be supported below by ordinary transport rollers (36, 36') which perform a closing function on the underside similar to that of the lower spiral (26). For this reason, the water softener (8) does not comprise lower components corresponding to the upper components (32, 32', 37, 37') but only the lower water nozzles (34, 34'). However, the combination of the components (8.1, 8.1', 32, 32', 35, 35', 36, 36', 37 and 37') ensures that the induction coils of the furnaces (6.1 and 6.2) are not damaged by the water used in the water softener (8). As mentioned previously, since the softener (8) is designed for more intensive softening, the cooling water pressure can be up to 380 bar, again with nozzles less than 3 mm in diameter, although this can result in a reduction of up to 150-200°C in the temperature of the transfer bar. Evidently, even in the softener (8), the rows (33, 34 and 33', 34') of water nozzles are sized for the maximum width of the bar. Nozzles outside the bar are either closed with plugs or jets that block the flow upon impact, and in this case, the upper and lower nozzles must be vertically aligned and have the same angle of inclination (e.g., 5°). With reference now to Figure 7, which shows four inductors (40) of the second induction furnace (6.2), it can be seen that the transfer bar is supported by lower rollers (41) arranged in the spaces between the inductors (40). These spaces are closed at the bottom by the support structure for these rollers (41) and at the top by removable covers (42). It is therefore advantageous to mount transverse rows of nozzles (43) in these covers (42) to create a series of chambers into which the protective atmosphere can be injected by means of these nozzles (43). This protective atmosphere can be of various types, provided it has a very low or zero oxygen content to limit or prevent surface oxidation of the material. Typically, oxygen is reduced by the continuous supply of nitrogen from the nozzles (43) to a slightly oxidizing atmosphere with a maximum oxygen content of 3% vol. Other possibilities include using an atmosphere composed entirely of inert gas (nitrogen, argon, etc.), or adding hydrogen to the inert gas up to a maximum content of 5% vol. to obtain a slightly reducing atmosphere. As mentioned previously, a similar solution can be considered for obtaining chambers between the supports of the finishing mill (3) by mounting the nozzles on the shuttle structure positioned in the space between two supports. A first embodiment of this solution is illustrated in the Figures 8 and 9 show how the protective atmosphere feeding system has double-mirror symmetry both with respect to the section plane AA indicated in Figure 8, i.e., with respect to the upstream and downstream sides of the looper (51), and with respect to the vertical longitudinal mid-plane Y of the strip indicated in Figure 9, i.e., with respect to the right and left sides of the strip. In the example illustrated in these figures, the system is arranged between the first two supports (3.1 and 3.2) of the finishing mill (3), but it is clear that the same system can be arranged between any pair of supports of this mill. This system comprises, on each side of the strip, a pair of vertical feed conduits (52, 52') mounted on the shuttle structure (51), respectively upstream and downstream. From each of these conduits (52, 52') branch two rows of substantially horizontal nozzles arranged longitudinally above and below the strip and parallel to its edges. More specifically, each of the two upper rows (53, 53') of nozzles extends towards both supports (3.1, 3.2) almost to the section plane AA passing through the center of the shuttle (51), while each of the two lower rows (54, 54') of nozzles extends only towards the adjacent support (3.1, 3.2). Furthermore, as shown in detail in Figure 9, the nozzles are inclined in the vertical plane with an orientation towards the surface of the strip. To limit the dispersion of the protective atmosphere, the nozzle rows are preferably enclosed within a chamber formed by a pair of upper fins (55, 55') and a pair of lower fins (56, 56') which are obviously shaped to allow the passage of the strip through the chamber. More specifically, each fin is pivoted at one of its outer ends to allow the containment chamber to be opened by a 90° turn, as shown in Figure 8, where the closed chamber is represented by a thicker line, while the numerical references (55, 55', 56, and 56') indicate the fins rotated to the open position. A second, analogous version of the system is illustrated in Figures 10 and 11, showing the same elements as in Figures 8 and 9, whose numerical references are therefore not repeated, with the addition on the outer face of each fin of at least two parallel rows (57, 57', 58, 58') of transverse nozzles. The protective atmosphere reaches each pair of rows through a respective supply duct (50, 50', 59, 59'), and the nozzles are oriented in a direction substantially perpendicular to the upper and lower surfaces of the strip. Finally, Figures 12 and 13 illustrate a third modality of the system, which in practice is obtained from the previous one by eliminating the elements of Figures 8 and 9 and retaining only the Γ / ηηιη / ζζηζ / Ε / γίΛΐ minus two parallel transverse rows (63, 63', 64, 64') arranged in respective fins (65, 65', 66, 66') and fed through respective ducts (61, 61', 62, 62'). The differences with respect to the analogous elements shown in figures 10, 11 are as follows: - the multiple nozzles (57, 57', 58, 58') are replaced by a single nozzle substantially the same width as the strip, i.e., a slit; - the nozzles are not oriented in a direction substantially perpendicular to the upper and lower surfaces of the strip, but are oriented with an inclination towards the adjacent rolling box (3.1 and 3.2), respectively; - the protective atmosphere is fed to each pair of transverse rows (63, 63', 64, 64') not through a single central duct, as in the second embodiment, but through two lateral ducts (61, 6Γ, 62, 62') as in the first embodiment of figures 8 and 9. As mentioned above, the plant described above can be integrated with a line (13) for the application of a protective coating, typically a galvanizing line, connected directly downstream of the final winders (11) as shown in Figure 2. In this way, the plant can produce both uncoated strip coils that are wound on the winders (9 or 11), and coated strip coils that are wound on another winding station at the end of the line (13). Another possible alternative is to perform liquid cooling of the coil wound on the winders (9 or 11) in a tank (not shown) containing water or a slightly oxidizing aqueous solution. This results in a scale that is more easily removed during subsequent protective coating application processes. Furthermore, thermal scanners, not shown in the figure, are preferably positioned at the outlet of the casting machine (1), HRM (2), the first induction furnace (6.1), the descaling unit (8), the second induction furnace (6.2), the finishing mill (3), and the cooling roller conveyors (12, 12'). These thermal scanners are operationally connected to a temperature control and management system which, thanks also to the thermocouples (not shown) embedded in the copper plates of the ingot mold, influences the temperature distribution of the steel in the mold by means of an electromagnetic brake (EMBR) embedded in the mold, also not shown. In fact, the thermal scanners and thermocouples provide a picture of the temperature distribution in the slab, giving the control system the ability to take corrective action on the operating parameters of the EMBR and the slab cooling system.Obviously, this control system too. Γ / ηηιη / ζζηζ / Ε / γίΛΐ acts on all other components that actively influence the temperature of the material being processed, both during heating (4.1, 4.2, 6.1, 6.2) and during cooling (5.2, 7, 8.2, 12, 12', 14, 14'). As an example, the following table represents a possible lamination sheet for the production of an ultra-thin strip 0.4 mm thick with a winding temperature in the final winders of 680°C: Γ / ηηιη / ζζηζ / Ε / γίΛΐ Condition 2.1 2.2 2.3 2.4 Inlet temperature [°C] 1250 Inlet velocity [m / s] 0.1 Inlet thickness [mm] 90 40.5 18.2 10 Outlet thickness [mm] 40.5 18.2 10 6.5 Reduction [%] 55 55 45 35 Outlet temperature [°C] 970 Condition 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Inlet temperature [°C] 1150 Inlet velocity [m / s] 1.38 Inlet thickness [mm] 6.5 2.92 1.6 0.97 0.68 0.54 0.46 Outlet thickness [mm] 2.92 1.6 0.97 0.68 0.54 0.46 0.4 Reduction [%] 55 45 40 30 20 15 13 Outlet temperature [°C] 870 Outlet speed [m / s] 22.6 A corresponding production process that uses the plant described above in its most complete form therefore comprises the following sequence of steps: (a) continuous casting of thin or medium slabs (1); (b) induction heating (4.1) of the edges of the iron; (c) induction heating (4.2) of the remainder of the iron surface; (d) first descaling with water (5.2); (e) approximate lamination (2) in 3-5 passes to obtain a transfer bar; (f) first induction heating (6.1) of the transfer bar; (g) mechanical breakage (7) of the scale; (h) second softening with water (8.2); (i) second induction heating (6.2) of the transfer bar; (j) finish rolling (3) in 5-7 passes to obtain the strip; (k) controlled cooling (12; 12') of the strip; (1) mechanical chipping (14; 14'); (m) cutting the strip (10; 10') and winding it on a winder (9; 11); or (n) direct passage of the strip to a stage (13) of applying a protective coating with final winding; where at least phases (i) and (j), at least up to the third step, and preferably also phases (k) and (m), in the winding part, are carried out in a protective, slightly oxidizing, inert or slightly reducing atmosphere as described above. It is clear that the plant and process configurations according to the invention described and illustrated above are only examples and are subject to numerous variations. For example, although all the nozzle rows described above and shown in Figures 4-6 and 8-11 are formed by a plurality of nozzles arranged with a constant pitch, it would also be possible to provide nozzles with different pitches depending on the zones, and / or replace all or part of the nozzles with a continuously extending slit as shown in Figure 13. Similarly, both the near-winders and the end-winders can be implemented as carousel winders (9) or single winders (11), so the plant can comprise any combination thereof. Furthermore, it is clear that for reasons of space and / or cost, the system could do without the containment chambers shown in Figures 8-13, although this would make it more difficult to control the composition of the atmosphere in the space between the rotating supports. In this case, the rows of transverse nozzles shown in Figures 10-13 would be mounted on simple rotating supports that do not form containment chambers.

Claims

1. A plant for the continuous production of hot-rolled steel strips with a minimum thickness of 0.3 mm, including in sequence, along the direction of movement of the material being processed: - a device (1) for the continuous casting of thin or medium plates with a thickness between 40 and 150 mm and a maximum width of at least 2100 mm, - a roughing mill (2) comprising at least three boxes, - a first induction furnace (6.1), - a water softener (8), - a second induction furnace (6.2), - a finishing mill (3) comprising five to seven boxes, - a cooling station (12), - a cutting station (10), and - a winding station with at least a pair of carousel winders (9) or single winders (11), - as well as a system for supplying a protective atmosphere containing <3% vol. of oxygen at least from the inlet of said second induction furnace (6.2) up to the third box of said finishing mill (3), characterized in that it further comprises, between said continuous casting device (1) and said roughing mill (2), an initial section (4) for heat conditioning and descaling that sequentially comprises an induction edge heater (4.1), an induction heater (4.2) for the remainder of the plate surface and a first water descaler (5).

2. The plant according to claim 1, characterized in that said first water descaler (5) comprises a pressure roller (5.1), on the side towards the induction heater (4.2), followed by a current descaler (5.2) which is provided at the inlet with a pair of transversely movable shutters (20) which rest directly on the edges of the plate, each of said shutters (20) being preferably mounted on a parallelogram support formed by a pair of parallel arms (21) pivoted between the shutter (20) and the descaler structure (5.2) and moved by an actuator (22).

3. The plant according to claim 1, characterized in that said initial section (4) of thermal conditioning and descaling has a length of 3-5 meters. Γ / ηηιη / ζζηζ / E / γίΛΐ 4. The plant according to any of the preceding claims, characterized in that the edge heater (4.1) is designed to operate with cross-flow using side coils (4.1a) with a channel configuration with flow concentrators, each of said side coils (4.1a) preferably being equipped with its own frequency converter so that the edge heater (4.1) can heat the right and left edges of the plate differently.

5. The plant according to any of the preceding claims, characterized in that the edge heater (4.1) is dimensioned to heat a side band of the plate up to 150 mm from each edge and / or to obtain a temperature increase in said side band up to 120°C.

6. The plant according to any of the preceding claims, characterized in that the edge heater (4.1) is equipped with a handling system that performs a transverse movement to adapt the edge heater (4.1) to the width of the plate, to fix the width of the side strip to be heated, and to move away and, if necessary, lift by rotation the induction coils from the edges of the plate, said handling system being preferably implemented by placing each induction coil on a movable carriage along a transverse guide under the action of an actuator, preferably an electric motor driving a screw jack.

7. The plant according to any of the preceding claims, characterized in that the first descaling agent (5) comprises: - a row (23) of upper water nozzles and a row (24) of lower water nozzles arranged transversely to the plate and with the nozzles inclined to project a jet in the opposite direction to the movement of the plate, - an upper spiral (25) and a lower spiral (26) arranged specularly upstream of said rows (23, 24) of nozzles and with their openings facing them, each of said spirals (25, 26) being provided with end drains for the removal of the collected water through a lip in contact with the plate, - a row (27) of upper air nozzles and a row (28) of lower air nozzles arranged transversely to the plate upstream of the spirals (25, 26) and with the nozzles inclined to project a jet in the direction of the movement of the plate,- said rows (23, 24) of water nozzles being preferably arranged in opposite positions, with the nozzles aligned vertically and at the same angle of inclination, and Γ / ηηιη / ζζηζ / E / γίΛΐ preferably having a diameter < 3 mm., 8. The plant according to any of the preceding claims, characterized in that the second water dehuller (8) placed between the two induction furnaces (6.1, 6.2) comprises a first pressing roller (8.1), on the side towards the first induction furnace (6.1), a dehuller proper (8.2) and a second pressing roller (8.1') on the side towards the second induction furnace (6.2).

9. The plant according to the preceding claim, characterized in that the second water descaling unit (8) comprises: - a first row (33) and a second row (33') of upper water nozzles and a first row (34) and a second row (34') of lower water nozzles, all said rows being arranged transversely to the transfer bar and with the nozzles inclined to launch a jet in the opposite direction to the direction of movement of the bar, said second rows (33', 34') preferably being staggered transversely by half a step with respect to said first rows (33, 34), - each of the two rows (33, 33') of upper water nozzles is preceded by an upper spiral (35, 35') and a movable lip (32, 32') which in the working position comes into contact with the upper surface of the transfer bar and aligns with the respective roll (35, 35'),- a first row (37) and a second row (37') of upper air nozzles arranged transversely to the transfer bar and with the nozzles preferably perpendicular to the upper surface of the bar, said first row (37) being located upstream of said first movable lip (32) and said second row (37') being located downstream of the second row (33') of upper water nozzles, the rows (33, 33') of upper water nozzles preferably being arranged opposite the rows (34, 34') of lower water nozzles, with the nozzles aligned vertically and at the same angle of inclination, and preferably having a diameter < 3 mm., 10. The plant according to any of the preceding claims, characterized in that the protective atmosphere feeding system to the finishing mill (3) includes on each side of the strip, in the space between two finishing supports (3.1, 3.2, ..., 3.7), a pair of feed tubes (52, 52') mounted on the structure of a looper (51), respectively on the upstream and downstream sides thereof, and from each of these feed tubes (52, 52') branch two substantially horizontal rows of nozzles arranged longitudinally above (53, 53') and below (54, 54') the strip and parallel to its edges, each of the two rows (53, 53') of upper nozzles preferably extending towards both of said two steps (3.1, 3.2,..., 3.7) almost to the vertical plane transverse to the strip and passing through the center of said shuttle (51), while each of the two rows (54, 54') of lower nozzles extends only towards the adjacent support (3.1, 3.2, ..., 3.7), the nozzles are preferably inclined in the vertical plane with an orientation towards the surface of the strip.

11. The plant according to the preceding claim, characterized in that the system for feeding the protective atmosphere further comprises at least two parallel horizontal rows (57, 57', 58, 58') of nozzles arranged transversely above and below the strip in each of said longitudinal rows (53, 53', 54, 54'), the protective atmosphere reaching each pair of transverse rows (57, 57', 58, 58') through a respective feed pipe (50, 50', 59, 59') and the nozzles being preferably oriented in a direction substantially perpendicular to the upper and lower surfaces of the strip.

12. The plant according to any of claims 1 to 9, characterized in that the protective atmosphere feeding system to the finishing mill (3) includes, in the space between two finishing boxes (3.1, 3.2,..., 3.7), at least two pairs of parallel horizontal rows (63, 63', 64, 64') of nozzles arranged transversely above and below the strip both upstream and downstream of a shuttle (51), the protective atmosphere reaching each of said pairs of transverse rows (63, 63', 64, 64') through a respective pair of feed pipes (61, 61', 62, 62'), the nozzles preferably being inclined in the vertical plane with an orientation towards the adjacent finishing support (3.1, 3.2,..., 3.7).

13. The plant according to any of claims 10 to 12, characterized in that the rows of nozzles are enclosed within a chamber formed by a pair of upper fins (55, 55'; 65, 65') and a pair of lower fins (56, 56'; 66, 66') shaped to allow the strip to pass through said chamber and capable of rotating around an end pin to allow the chamber to open.

14. The plant according to the preceding claim when dependent on claim 11 or 12, characterized in that the transverse rows (57, 57', 58, 58'; 63, 63', 64, 64') of nozzles are mounted on the fins (55, 55'; 56, 56'; 65, 65'; 66, 66').

15. The plant according to any of the preceding claims, characterized in that the first box (2.1) of the roughing mill (2) is a box designed for a reduction of the thickness of the roughings < 20%.

16. The plant according to any of the preceding claims, characterized in that it further comprises, after the roughing mill (2), an emergency system for the production and removal of raw sheets which includes in sequence a pendulum shear (15), a stacker (16) for the extraction of sheets, a rotary shear (17) and a loop producer (18).

17. The plant according to any of the preceding claims, characterized in that it further comprises, between the first induction furnace (6.1) and the second water dehuller (8), a mechanical dehulling device (7) formed by at least three rollers arranged alternately above and below the feed line of the transfer bar and at a height such that it causes a plastic stretching of its surface that causes a break in the rigid hull layer.

18. The plant according to any of the preceding claims, characterized in that it further comprises in sequence, between the finishing rolling mill (3) and the cooling station (12), another cooling station (12'), another cutting station (10') and an additional winding station (9; 11), said additional cooling station (12') being preferably capable of ultra-fast cooling.

19. The plant according to the preceding claim, characterized in that it further comprises, between each cooling station (12; 12') and each cutting station (10; 10'), a mechanical descaler (14; 14') using counter-rotating brushes or abrasive mud jets.

20. The plant according to any of the preceding claims, characterized in that it further comprises an anti-corrosion coating line (13) located directly after the final winding station (9; 11) so that it is possible to apply said coating to the strip without having to wind it onto a coil first.

21. The plant according to any of the preceding claims, characterized in that it further comprises a system for controlling and managing the temperature of the material to be processed, operatively connected to an electromagnetic brake inserted in an ingot mold that forms part of the continuous casting device (1), as well as connected to thermocouples inserted in the copper plates of said mold and thermal scanners arranged along the plant, preferably at the outlet of the continuous casting device (1), the roughing mill (2), the first induction furnace (6. (1), the second water descaler (8), the second induction furnace (6.2), the finishing mill (3) and the cooling station (12, 12'), said control system being operatively connected also to all other components of the plant that actively affect the temperature of the material being processed, both in heating (4.1, 4.2, 6.1, 6.2) as in cooling (5.2, 7, 8.2, 12, 12', 14, 14').

22. A process for the continuous production of hot-rolled steel strips with a minimum thickness of 0.3 mm by means of a plant according to any of the preceding claims, comprising the following sequence of steps: a) continuous casting (1) of thin or medium plates with a thickness of 40-150 mm; b) rough rolling (2) to obtain a transfer bar in 3-5 passes; c) first induction heating (6.1) of the transfer bar; d) descaling with water (8.2); e) second induction heating (6.2) of the transfer bar; f) completing the lamination (3) to obtain the strip in 5-7 passes; g) controlled cooling (12; 12') of the strip; and h) cutting (10; 10') of the lyre and winding it (9; 11) into a coil, wherein at least steps (e) and (f), at least up to the third pass, and preferably also steps (g) and (h), in the winding part, are carried out in a slightly oxidizing, inert or slightly reducing protective atmosphere, characterized in that between steps (a) and (b) additional steps are provided of: (a') induction heating (4.1) of the edges of the plate; (a") induction heating (4.2) of the rest of the surface of the plate; (a'") descaling by water (5.2).

23. The process according to the preceding claim, characterized in that step (h) is replaced by the direct passage of the strip to a step of applying a protective coating (13) with subsequent final rolling.

24. The process according to claim 22 or 23, characterized in that in step (b) the first pass (2.1) of the roughing roll (2) results in a reduction of the roughing thickness < 20%.

25. The process according to any of claims 22 to 24, characterized in that between steps (c) and (d) an additional step (c') of mechanical breaking (7) of the scale is provided.

26. The process according to any of claims 22 to 25, characterized in that an additional mechanical descaling step (g') is provided between steps (g) and (h) (14; 14').

27. The process according to any of claims 22 to 26, characterized in that between steps (b) and (c) an additional step of production and removal of rough sheets Γ / ηηιη / ζζηζ / E / γίΛΐ 22 (15, 16) is provided in case of problems in the portion of the plant downstream of the rough rolling (2).

28. The process according to any of claims 22 to 27, characterized in that step (a'”) is performed with a water pressure of less than 150 bar and / or step (d) is performed with a water pressure of up to 380 bar.

29. The process according to any of claims 22 to 28, characterized in that step (e) is carried out with a final temperature such that it is ensured that step (f) is carried out entirely in the austenitic range.

30. The process according to any of claims 22 to 29, characterized in that step (a') is performed on a strip up to 150 mm from each edge of the plate and / or results in a temperature increase in that strip of up to 120°C.

31. The process according to any of claims 22 to 30, characterized in that step (h) is followed by a step (i) of liquid cooling of the coil in a tank containing water or a slightly oxidizing aqueous solution.

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