Plant and method for the continuous production of hot-rolled extra-thin steel strip

Abstract

A plant and method for the continuous production of hot-rolled steel strip with a minimum thickness of 0.3 mm, comprising a continuous casting device (1) for thin or medium slabs with a thickness of 40 to 150 mm and a maximum width of at least 2100 mm, followed by a roughing mill (2), a first induction furnace, a water scale removal device, a second induction furnace, a finishing mill, a cooling station, a cutting station, a coiling station, and a system for providing a protective atmosphere containing not more than 3% by volume of oxygen, provided at least from the inlet of the second induction furnace to the third stand of the finishing mill, and further comprising a thermal conditioning and descaling section (4) between the continuous casting device (1) and the roughing mill (2), the thermal conditioning and descaling section (4) comprising, in order, an induction edge heater (4.1), an induction heater (4.2) for the remaining part of the slab surface, and a water scale removal device (5).

Description

Technical Field

[0001] The present invention relates to a plant and a method for continuously manufacturing hot-rolled ultra-thin steel strips with a thickness of up to 0.3 mm, while limiting the amount of scale, so as to be suitable for direct anti-corrosion coating without performing a specific preliminary surface conditioning treatment.

Background Art

[0002] In the steel industry, considering both the rising costs of raw materials and energy used, the improvement of competitiveness required in the global market, and increasingly restrictive regulations regarding pollution, there is a particular need for a method of manufacturing high-quality hot-rolled steel strips that results in lower investment and production costs and even thinner sheet thicknesses. As a result, the final product processing industry can also reduce energy consumption and enhance competitiveness, thus minimizing the adverse impact on the environment.

[0003] The state-of-the-art technology is essentially as described in previous patents such as Patent Document 1, Patent Document 2, and Patent Document 3 by the same inventor. For further details, please refer to them. In practice, the so-called ESP (Endless Strip Production) technology is used, which is based on "casting rolling" that combines continuous casting of thin slabs with liquid core reduction (LCR) in the first roughing stage passing through a rough rolling mill (HRM = High Reduction Mill) that produces an intermediate product called a "transfer bar". The casting is carried out from an ingot mold system based on Patent Document 4, Patent Document 5, and Patent Document 6 by the same inventor, which relates to the geometric profiles of both the horizontal and vertical cross-sections of the ingot mold, as well as the specific shape of the nozzles designed for a high-quality mass flow rate of materials of up to 7 - 8 tons per minute. For further details, please refer to these.

[0004] Considering that the above-mentioned Patent Document 1 also does not have a controlled cooling system necessary for the production of high-quality sheets in the first part of the plant, in order to avoid interruptions in continuous casting and, as a result, interruptions in the production of the line rather than the production of programmed sheets, when a problem occurs in the plant part downstream of the roughing mill, it assumes the possibility of extracting the rough sheet after the first rough rolling stage as an emergency system.

[0005] After the heating stage in the induction furnace and subsequent scale removal, the transfer bar is further processed in the second stage of finish rolling, and the temperature is controlled so that the temperature exceeds about 820 - 850 °C, corresponding to the lower end of the austenite temperature range of most steels, and thus the steel strip is deformed.

[0006] However, the results so far have shown that, although it is optimal with respect to the quality of the steel strip, there is room for improvement in terms of the compactness of the plant, energy savings, and the current minimum steel strip thickness value of 0.6 mm. Also, the reduction of oxide (scale) formation on the steel strip surface is achieved because the residence time at the material temperature is minimized, but it has not been proven that this reduction is sufficient to avoid the pickling stage before the anticorrosion coating is applied due to the induction heating of the transfer bar between the aforementioned roughing and finishing stages.

[0007] To ensure the desired final rolling in the austenite region with higher production flexibility and further reduce scale formation, a plant of the above type is known from Patent Document 7, which also includes a second induction furnace between the scale removal device and the finishing rolling mill. The heating in the second furnace is carried out in a protective atmosphere that substantially consists of an inert gas (nitrogen) with a minimum oxygen presence (about 5% or less) to prevent oxidation of the transfer bar. Another example of induction heating in a protective atmosphere before final rolling can be found in Patent Document 8, which provides only one induction furnace after the scale removal device. Patent Document 9 also provides a reducing atmosphere using hydrogen but does not provide roughing. Patent Document 10, however, does not include an induction furnace after the scale removal device and teaches using combustion gas generated within the plant itself instead of an inert gas as a protective atmosphere to reduce costs. This gas can also be distributed to various parts of the plant before and after the induction furnace (e.g., induction edge heaters, scale removal devices, finishing rolling mills, exit roller conveyors, winders).

[0008] However, none of these prior art documents assume obtaining a sheet thickness below the current limit of 0.6 mm and do not consider the specific problems that occur when below this limit. In fact, none of the plants described in these documents are suitable for this purpose because there are conflicting requirements to maintain the transfer bar at a high temperature at the inlet of the finishing rolling mill to ensure complete austenite rolling of such thin steel strips. Therefore, greater cooling is required, and it is necessary to limit scale formation despite strong heating in terms of both time and temperature.

Prior Art Documents

Patent Documents

[0009] <S

Patent Document 1

Patent Document 2

Patent Document 3

[0010] The object of the present invention is therefore to provide a solution for the continuous production of hot-rolled steel strip, starting from the casting of slabs 40 to 150 mm thick, without passing through intermediate plants for pickling, cold rolling and annealing, with a thickness of up to 0.3 mm and a maximum width of at least 2100 mm, or whatever the maximum width offered for the ingot mould, which limits the amount of scale so that these steel strips are suitable for direct anticorrosion coating (in particular on a galvanising line) without having to undergo specific preliminary surface preparation treatments, in particular on a pickling line. [Means for solving the problem]

[0011] This result is obtained using continuous production techniques (so-called endless), which minimize production times and consumption and, consequently, reduce production costs, in particular by adopting measures such as the following to control the temperature of the material and limit its degradation while avoiding excessive oxidation of the material surface: a) Before entering the heavy rolling mill (HRM), the slab is cleaned from scale. At the exit of the continuous casting (casting machine), in order to enable several roughing passes from a minimum of 3 to a maximum of 5 times, in the order of the slab's travel direction, there is an initial heat adjustment and scale removal section that includes an induction edge heater, an induction heater for the remaining part of the slab surface, and a water scale removal device. b) To prevent the jet of water and steam from the scale removal device from damaging the induction coil of the surface heater, the scale removal device is provided with a horizontally movable shutter at the inlet that directly rests on the edge of the slab. On the other hand, the closing of the upper and lower surfaces of the slab is done by small drive stands, so-called pinch rolls, arranged adjacent to the said shutter on the inlet side of the scale removal device facing the surface heater. c) Considering that the speed of the slab at the exit of the casting machine is low (less than 10 m / min) in order to minimize the time it takes for the slab to pass from the casting machine to the inlet of the heavy rolling mill, and to minimize scale formation and temperature drop, the said first section should be made as compact as possible so that the edge heater, surface heater, scale removal device (including pinch rolls and screening shutter) occupy a space on the order of 3 - 5 meters in length. d) The edge heater is equipped with a handling system that can maintain the efficiency of the heating system constant even when the width of the slab changes, set the optimal width of the edge area to be heated, and take out / lift the induction coil in case "waves" occur in the slab due to defects in the heavy rolling mill. e) Even if the slab exiting the casting machine shows temperature non-uniformity between the two edges, the edge heater can heat the right and left edges of the slab separately to ensure an optimal and uniform profile of the slab entering the heavy rolling mill. f) The scale removal device is designed to have the diameter and discharge pressure of the cooling water nozzles such that the temperature drop at the exit of the scale removal device is limited to less than 10 °C between when the scale removal device is operating and when it is not operating.

[0012] Other advantageous configurations preferably adopted in the present invention to improve the plant and method are as follows. g) Constructing a second water scale removal device, which is arranged between two induction furnaces in front of the finishing rolling mill, has the same structure as the above-mentioned first scale removal device, and includes pinch rolls at both the inlet and outlet to protect the two induction furnaces from jets of water and steam. h) Attaching a nozzle for supplying a protective atmosphere in the finishing rolling mill to a roller with a movable structure of a so-called "looper" arranged between rolling stands, i.e., vertically movable, and equipped with a steel strip tension sensor that can arrange the material in an appropriate loop between the stands so that a speed control system changes the reciprocating speed of the stands to maintain a constant tension in the steel strip. i) Providing a mechanical scale crushing device consisting of at least three rollers, which is arranged immediately before the second water scale removal device, is alternately arranged above and below the supply line of the transfer bar, and is arranged at a height sufficient to cause plastic elongation of its surface. This crushes the hard layer of the scale, facilitating its removal by the subsequent water scale removal device. j) To enable a high temperature (up to 750°C), in any case higher than the transformation point, for winding ultra-thin steel strips, a coiling reel ("up coiler" or "down coiler") is also provided near the last rolling stand, with a short cooling line and a high-speed shearing machine preceding (in addition to a similar final coiler traditionally provided after the shearing machine relative to the normal cooling line) above or below the surface of the outlet roller conveyor. k) Providing first and second mechanical scale removal devices respectively arranged between the cooling line and the shearing machines of the tightly wound coiler and the final coiler, using reverse-rotating polishing brushes or abrasive slurry jets. l) Providing a corrosion prevention coating line immediately after the final coiler so that the coating can be applied without the need to pre-wind the steel strip onto a coiler to form a coil. m) providing a cooling tank in which the coil removed from the coiler can be immersed in water or a slightly oxidizing aqueous solution.

[0013] Further advantages and configurations of the plant and method according to the present invention will become apparent to those skilled in the art from the following detailed and non-limiting description of some of its embodiments, with reference to the accompanying drawings.

Brief Description of the Drawings

[0014]

Figure 1a

Figure 1b

Figure 1c

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Embodiments for Carrying out the Invention

[0015] Referring to FIGS. 1a to 1c, a plant according to the present invention traditionally includes a casting machine 1 for continuously casting thin or medium slabs with a thickness of 40 to 150 mm, followed by a rough rolling mill (HRM) 2, which in the illustrated example is formed by four stands 2.1 to 2.4, but can also be three or five, and a rough rolling mill (HRM) 2 that transforms the slab into a transfer bar with a thickness of 8 mm or less. It can be seen that in experimental tests, by limiting the reduction of the thickness (to 20% or less) at the first roughing stand 2.1, it was shown that the surface stress can be suppressed within the strength limit of the coarse austenite constituting the slab as a casting. Thus, the almost static recrystallization of the surface in the first roughing step can enable a significant subsequent reduction in thickness necessary to obtain a transfer bar suitable for the production of ultra-thin steel strips, especially in the case of steels with microalloying, without defects or cracks.

[0016] After the HRM 2, an emergency system for the production and removal of rough sheets is arranged in case of problems in the downstream part of the HRM of the plant. Such a system includes a pendulum shear 15, a stacker 16 for taking out the sheets, a rotary shear 17, and a loop former 18. The latter two devices have the purpose of releasing the line from the material between the pendulum shear 15 and the first induction furnace 6.1 at an initial defective stage thereafter.

[0017] The first induction furnace 6.1 is the first component of the central heat adjustment and scale removal section 6. In the advancing direction of the transfer bar, in order, as described above, in this case, a mechanical device 7 (optional) for crushing scale of the type formed by five rollers, a water scale removal device 8, and a second induction furnace 6.2 are further included. In this way, the transfer bar is formed by seven stands 3.1 to 3.7 in the illustrated example, but it can also be five or six, and is further heated before entering the adjacent finishing rolling mill 3. Finally, the steel strip is cooled in a controlled manner by the cooling roller conveyor 12, and then followed by a final winding station including a flying shear 10 and at least a pair of single coilers 11.

[0018] As described above, in order to enable a high winding temperature of the ultra-thin steel strip, the plant preferably also includes a tight-wound coiler, which is, in the form of a pair of "carousel" coilers 9, arranged close to the last rolling stand 3.7, i.e., before the aforementioned elements 10 to 12, and is preceded by a short cooling roller conveyor 12' and a high-speed shearing machine 10' similar to the elements 10, 12. The roller conveyor 12' can preferably be made to perform ultra-rapid cooling in order to obtain a scale that can be more easily removed in a subsequent process of applying a protective coating.

[0019] Between each pair of the elements 10, 12 and 10', 12', preferably, respective mechanical scale removal devices 14, 14' of a known type are also arranged. Therefore, without further explanation, this uses a reverse rotation brush or a jet of abrasive slurry for the final surface treatment of the steel strip before it is wound into a coil on the coiler 9 or 11.

[0020] As described above, the plant shown in FIGS. 1a - 1c also includes a system for distributing a protective atmosphere to its specific zones schematically indicated by the thick - lined boxes, which in the illustrated example extends at least from the inlet of the second induction furnace 6.2 to the third stand 3.3 of the finishing rolling mill 3, preferably to the last stand, and even more preferably also into the subsequent cooling and coiling stations. Clearly, as described in the above prior art, it is also possible to consider extending this system to other components of the plant.

[0021] ]>As described above, the first innovative aspect of the present invention is the presence of an initial heat conditioning and scale removal section 4, which is arranged between the outlet of the casting machine 1 and the HRM 2 and is designed to have a length just slightly longer than 3 m in order to minimize the transit time between the two components. The section 4 includes an induction edge heater 4.1, an induction heater 4.2, and a water scale removal device 5, which are shown in more detail in FIGS. 3 - 5.

[0022] More specifically, the edge heater 4.1 is preferably designed to operate with transverse magnetic flux using side coils 4.1a of a "channel" configuration with flux concentrators for the dual purpose of enhancing the efficiency of the heating system and concentrating the magnetic flux in selected areas of the slab to be heated. Furthermore, due to the presence of two frequency converters, one for each coil 4.1a instead of just one converter for the whole device as usually provided, the right and left edges of the slab can be heated separately. From experimental tests carried out by the applicant, it has been found that the width of the heated band preferably needs to reach up to 150 mm from the edge, and the optimal temperature rise of said band is up to 120 °C in order to avoid melting of the scale.

[0023] The edge heater 4.1 is equipped with a handling system that adapts the device to the slab width, sets the width of the area of the edge to be heated, and performs a lateral movement to move (and lift if necessary) the coil 4.1a away from the edge of the slab in case there are "waves" in the slab due to defects in the roughing mill. Such a handling system can be realized, for example, by placing each coil 4.1a on a slide that moves along a lateral guide under the action of an actuator such as an electric motor that drives a screw jack.

[0024] The induction heater 4.2 includes a surface heating coil designed to be integrated with the edge heater 4.1, which can be controlled so that the temperature rise of the slab reaches a value of up to 150 °C, thus preventing the melting of the slab.

[0025] The next scale removal device 5 consists of a pinch roll 5.1 on the side facing the induction heater 4.2 and the actual scale removal device 5.2 on the side facing the HRM2. As shown in FIGS. 4 - 5, in order to avoid the jet of water and steam coming from the scale removal device 5.2 from damaging the induction coil of the heater 4.2, the scale removal device 5.2 is equipped with a laterally movable shutter 20 at the inlet, which is placed directly on the edge of the slab, while the closing of the upper and lower surfaces of the slab is provided by the pinch roll 5.1.

[0026] More specifically, in the embodiment shown in FIG. 5, each shutter 20 is attached to a parallelogram support formed by a pair of parallel arms 21 that pivot between the shutter 20 and the structure of the scale removal device 5.2 and are moved by an actuator 22. Note that in FIG. 5, the shutter 20 is shown in the open position and is partially shown in the closed position 20' where it abuts against the edge of the slab.

[0027] Water scale removal is performed by an upper nozzle row 23 and a lower nozzle row 24 that are arranged horizontally with respect to the slab and the nozzles are inclined so as to jet in a direction opposite to the moving direction of the slab. Upper scroll 25 and lower scroll 26, which are arranged upstream of the nozzles mirror-finish and their openings face the nozzles, recover most of the water via lips that are in contact with the slab, carry it to their ends and discharge it.

[0028] Also, an upper nozzle row 27 and a lower nozzle row 28 that are arranged horizontally with respect to the slab upstream of the scrolls and the nozzles are inclined so as to supply an air jet in the moving direction of the slab remove residual water. The combination of components 5.1, 20, 25, 26, 27, and 28 ensures that the induction coil of heater 4.2 is not damaged by the water used in the scale removal device 5.

[0029] As described above, the scale removal device 5.2 is designed to suppress the temperature drop during operation and non-operation to less than 10°C. For this purpose, the cooling water pressure is less than 150 bar and the diameter of the nozzles is less than 3 mm. Since the water nozzle rows 23, 24 shown in Fig. 5 (the scrolls 25, 26 and the air nozzle rows 27, 28 are omitted) are sized according to the maximum width of the slab, the nozzles that are wider than the slab and outside the slab being processed can be closed with plugs or the jets from the nozzles collide and are "offset". In this case, it should be noted that the upper and lower nozzles need to be arranged in opposite positions, aligned vertically, and have the same inclination angle (for example, 5°).

[0030] The second water scale removal device 8 shown in Fig. 6 has a similar structure to the first water scale removal device 5, but is arranged between the two induction furnaces 6.1 and 6.2 and is substantially double because water and steam must be prevented from escaping from both upstream and downstream. Thus, it includes a first inlet pinch roll 8.1 on the side facing the first induction furnace 6.1, the actual scale removal device 8.2, and a second outlet pinch roll 8.1' on the side facing the second induction furnace 6.2. In this case, the lateral shutter similar to the shutter 20 of the first scale removal device 5 can be omitted because it has to close a lateral passage equal in height to the thickness of the slab coming from the caster 1, i.e., 40 - 150 mm, whereas the thickness of the transfer bar entering the second scale removal device 8 is on the order of 5 - 20 mm, so it should be noted that the possibility of lateral water leakage is much less.

[0031] Furthermore, a second induction furnace 6.2 follows the second water scale removal device 8 and significantly raises the temperature of the transfer bar before final rolling, so scale removal can be carried out powerfully even at the expense of a greater temperature drop. Thus, a first row 33 of upper nozzles with a corresponding row 34 of lower nozzles arranged laterally with respect to the transfer bar and the nozzles inclined to send jets in a direction opposite to the direction of bar movement, as well as an identical second row 33' of upper nozzles with a corresponding row 34' of lower nozzles, are provided. Preferably, the second rows 33', 34' are arranged staggered laterally by half a pitch with respect to the first rows 33, 34, where the pitch is the distance between two nozzles in a row, and the two consecutive rows 33, 33' and 34, 34' completely cover the upper and lower surfaces of the bar respectively, so as to increase the efficiency of the hydraulic scale removal process by eliminating the inefficiencies that appear in the overlapping bands of adjacent nozzles in each row.

[0032] The two rows 33, 33' of the upper nozzles are similarly preceded by the upper scrolls 35, 35'. In this case, however, the upper scrolls 35, 35' are separated from the lips 32, 32', and the lips 32, 32' contact the upper surface of the transfer bar and are movable between the rest position shown in FIG. 6 and the working position where it rotates clockwise and aligns with the scrolls 35, 35'. Further, the first row 37 of the upper nozzles arranged laterally with respect to the transfer bar similarly precedes the first lip 32. In this case, an air jet substantially perpendicular to the upper surface of the bar is sent out, while the same second row 37' of the upper air nozzles 34' is arranged downstream of the second row 33' of the water upper nozzles.

[0033] Since the scale removal device 8 does not need to be as compact in length as the scale removal device 5, the transfer bar can be supported below by the normal conveying rollers 36, 36' that perform the function of closing on the lower side, similar to the lower scroll 26. For this reason, the scale removal device 8 does not include the lower components corresponding to the upper components 32, 32', 37, 37' and only includes the lower water nozzles 34, 34'. Nevertheless, 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 scale removal device

[0034] As described above, since the scale removal device 8 is designed for more powerful scale removal, even if the temperature of the transfer bar may decrease by up to 150 - 200 °C, when using nozzles with a diameter of less than 3 mm, the pressure of the cooling water can be up to 380 bar at most. Clearly, also in the scale removal device 8, the rows 33, 34 and 33', 34' of the water nozzles are sized according to the maximum width of the bar, and the nozzles outside the bar during processing are closed by plugs or by jets that are "offset" by collisions. In this case, the upper and lower nozzles need to be vertically aligned and have the same inclination angle (for example, 5°).

[0035] Referring to FIG. 7 showing the four inductors 40 of the second induction furnace 6.2, it can be seen that the transfer bar is supported by the lower rollers 41 arranged in the space between the inductors 40, and the space is closed at the bottom by the support structure of the rollers 41 and at the top by the removable cover 42. Therefore, it is advantageous to attach the lateral row of nozzles 43 to the cover 42 so as to obtain a series of chambers into which a protective atmosphere can be injected internally by means of the nozzles 43.

[0036] This protective atmosphere can be of various types as long as the oxygen content is very low or zero in order to limit or prevent the surface oxidation of the material. Typically, oxygen is reduced by continuously supplying nitrogen from the nozzles 43 until a low-oxidation atmosphere with an oxygen content of up to 3% by volume is obtained. As another possibility, an atmosphere composed entirely of an inert gas (such as nitrogen, argon, etc.) can be used, or hydrogen can be added to the inert gas up to a maximum of 5% by volume in order to obtain a slightly reducing atmosphere.

[0037] As described above, a similar solution can be considered to obtain a chamber between the stands of the finishing rolling mill 3 by attaching nozzles to the structure of the loopers arranged in the space between the two stands. The first embodiment of this solution is shown in FIGS. 8 and 9, which show the way in which the protective atmosphere supply system has double mirror symmetry with respect to the cutting plane A-A shown in FIG. 8, i.e., the upstream and downstream sides of the looper 51, and the central vertical longitudinal plane Y of the steel strip shown in FIG. 9, i.e., both the right and left sides of the steel strip. In the example shown in these figures, the system is arranged between the first two stands 3.1 and 3.2 of the finishing rolling mill 3, but it is clear that the same system can be arranged between any pair of stands of this rolling mill.

[0038] This system includes a pair of vertical supply ducts 52, 52' respectively attached to the upstream and downstream sides of the structure of the looper 51 on each side of the steel strip. Each of the ducts 52, 52' branches into two rows of substantially horizontal nozzles arranged longitudinally above and below the steel strip and parallel to its edges. More specifically, each of the two rows 53, 53' of upper nozzles extends substantially up to the plane of section A - A passing through the center of the looper 51 towards both stands 3.1, 3.2, while each of the two rows 54, 54' of lower nozzles extends only towards the adjacent stands 3.1, 3.2 respectively. Further, as shown in detail in Figure 9, the nozzles are inclined in a direction towards the surface of the steel strip within a vertical plane.

[0039] To limit the diffusion of the protective atmosphere, the rows of nozzles are preferably enclosed within a chamber formed by a pair of upper flaps 55, 55' and a pair of lower flaps 56, 56', and are shaped such that the steel strip can clearly pass through the chamber. More specifically, as shown in Figure 8, each of the flaps is pivoted at one of its outer ends and can open the containment chamber by a 90° rotation. The closed chamber is shown in thick lines, and the reference numerals 55, 55', 56, and 56' indicate the flaps rotated to the open position.

[0040] A second embodiment of a system similar to the previous one is shown in Figures 10 and 11, showing the same elements as Figures 8 and 9, and thus their reference numerals are not repeated. It only adds at least two parallel rows 57, 57', 58, 58' of lateral nozzles to the outer surfaces of each flap. The protective atmosphere reaches each pair of the rows through the respective supply ducts 50, 50', 59, 59', and the nozzles are directed in a direction substantially perpendicular to the upper and lower surfaces of the steel strip.

[0041] Finally, FIGS. 12 and 13 show a third embodiment of the system, which is actually obtained from the previous one by removing the elements of FIGS. 8 and 9 and maintaining only at least two horizontal parallel rows 63, 63', 64, 64' arranged on respective flaps 65, 65', 66, 66' and supplied through respective ducts 61, 61', 62, 62'. The differences from the similar elements shown in FIGS. 10 and 11 are as follows. · A plurality of nozzles 57, 57', 58, 58' are replaced by a single nozzle of substantially the same width as the steel strip, i.e., a slit. · The nozzles are not directed in a direction substantially perpendicular to the upper and lower surfaces of the steel strip, but are inclined and directed towards each of the adjacent rolling stands 3.1 and 3.2. · The protective atmosphere is supplied to each pair of the horizontal rows 63, 63', 64, 64' through two lateral ducts 61, 61', 62, 62' as in the first embodiment of FIGS. 8 and 9, rather than through a single central duct as in the second embodiment.

[0042] As described above, the above plant can be integrated, as shown in FIG. 2, with a line 13 for the application of a protective coating connected immediately downstream of the final coiler 11, typically a galvanizing line. In this way, the plant can produce both coils of uncoated steel strip wound on coiler 9 or 11 and coils of coated steel strip wound at the last further winding station of line 13.

[0043] Another possible alternative is to carry out the liquid cooling of the coils wound on coiler 9 or 11 in a tank (not shown) containing water or a slightly oxidizing aqueous solution. This allows to obtain scale that can be more easily removed in subsequent processes for applying the protective coating.

[0044] Furthermore, a thermal scanner not shown in the figures is preferably arranged at the outlets of the casting machine 1, HRM 2, the first induction furnace 6.1, the scale removal device 8, the second induction furnace 6.2, the finishing rolling mill 3, and the cooling roller conveyors 12, 12'. These thermal scanners are operably connected to a temperature control and management system that affects the temperature distribution of the steel in the mold by means of an electromagnetic brake (EMBR) (also not shown in the figure) inserted into the mold, thanks to a thermocouple (not shown) inserted into the copper plate of the ingot mold. In fact, the thermal scanner and the thermocouple provide an image of the temperature distribution within the slab and give the control system the function of modifying the operating parameters of the EMBR and the slab cooling system. This control system also naturally acts on all other components that actively affect 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').

[0045] As an example, the following table represents a possible rolling sheet for manufacturing an extra-thin steel strip with a thickness of 0.4 mm at a coiling temperature of 680 °C in the final coiler.

Table 1

[0046] Therefore, the corresponding manufacturing process using the above plant in the most complete embodiment consists of the following series of steps: (a) Continuous casting (1) step of thin or medium slabs (b) Induction heating (4.1) step of the slab edges (c) Induction heating (4.2) step of the remaining part of the slab surface (d) First water scale removal (5.2) step (e) Rough rolling (2) step of 3 to 5 passes to obtain a transfer bar (f) First induction heating (6.1) step of the transfer bar (g) Mechanical crushing (7) step of the scale (h) Second water scale removal (8.2) step (i) The second induction heating (6.2) step of the transfer bar, (j) The finishing rolling (3) step of 5 to 7 passes to obtain a steel strip, (k) The controlled cooling (12; 12’) step of the steel strip, (l) The mechanical scale removal (14; 14’) step, (m) The cutting (10; 10’) of the steel strip and the winding (9; 11) onto a coiler step, or, (n) The step of passing the steel strip directly through the step of applying a protective coating (13) with final winding, At least steps (i) and (j), up to at least the third pass, preferably steps (k) and (m) as well, are carried out in a protective atmosphere which is slightly oxidizing, inert or slightly reducing as described above, in the winding section.

[0047] It is clear that the embodiments of the plant and process according to the invention described and illustrated above are merely examples which can be subject to many modifications. For example, all the rows of nozzles described above and shown in FIGS. 4 to 6 and FIGS. 8 to 11 are formed by a plurality of nozzles arranged at a constant pitch, but it is also possible to provide nozzles with different pitches according to the region and / or to replace all or part of the nozzles by a slit which extends continuously as shown in FIG. 13. Similarly, both the close coiler and the final coiler can be installed as a carousel coiler 9 or a single coiler 11, whereby the plant can include any combination thereof.

[0048] Furthermore, it is clear that for reasons of space and / or cost, the system can be without the containment chambers shown in FIGS. 8 to 13, but this makes it more difficult to control the composition of the atmosphere in the space between the rolling stands. In this case, the rows of lateral nozzles shown in FIGS. 10 to 13 are attached to a simple rotating support which does not form a containment chamber.

Claims

1. 1. A plant for the continuous production of hot-rolled steel strip with a minimum thickness of 0.3 mm, comprising, in sequence along the direction of movement of the material to be treated: An apparatus (1) for continuous casting of thin or medium slabs with a thickness of 40 to 150 mm and a maximum width of at least 2100 mm; a roughing mill (2) including at least three stands; a first induction furnace (6.1); a second water scale removal device (8); a second induction furnace (6.2); a finishing mill (3) including 5 to 7 stands; a cooling station (12); a cutting station (10); a winding station comprising at least one pair of carousel coilers (9) or single coilers (11); a system for supplying a protective atmosphere containing not more than 3% by volume of oxygen at least from the inlet of the second induction furnace (6.2) to the third stand of the finishing mill (3); In the plant, Between the continuous casting device (1) and the roughing mill (2), an induction edge heater (4.1); an induction heater (4.2) for the rest of the slab surface; a first water scale removal device (5); a thermal conditioning and descaling initial section (4) comprising in turn The plant is characterized in that it further comprises, after the roughing mill (2), an emergency system for the production and removal of rough sheets, which comprises, in sequence, a pendulum shear (15), a stacker (16) for the extraction of metal sheets, a rotary shear (17) and a loop maker (18).

2. 2. A plant according to claim 1, characterized in that the first water descaling device (5) comprises a pinch roll (5.1) on the side facing the induction heater (4.2), followed by the actual descaling device (5.2) with an inlet provided with a pair of laterally movable shutters (20) that abut directly against the edge of the slab.

3. 2. A plant according to claim 1, characterized in that the initial thermal conditioning and descaling section (4) has a length of 3 to 5 meters.

4. 4. The plant according to any one of claims 1 to 3, characterized in that the induction edge heater (4.1) is designed to operate with transverse magnetic flux using side coils (4.1a) with a "channel" configuration equipped with flux concentrators.

5. 5. Plant according to any one of claims 1 to 4, characterized in that the induction edge heaters (4.1) are sized to heat the side bands of the slab up to 150 mm from each edge and / or to obtain a temperature rise of up to 120°C in the side bands.

6. 6. Plant according to any one of claims 1 to 5, characterized in that the induction edge heater (4.1) is provided with a handling system which performs lateral movements in order to adapt the induction edge heater (4.1) to the slab width and, if necessary, to rotate and lift the induction coil of the induction edge heater (4.1) from the edge of the slab in order to set the width of the side bands which are heated and moved away.

7. The first water scale removal device (5) comprises: an upper water nozzle row (23) and a lower water nozzle row (24) arranged transversely to the slab and having nozzles inclined to deliver jets in a direction opposite to the direction of movement of the slab; an upper scroll (25) and a lower scroll (26) arranged mirror-like upstream of the row of upper water nozzles (23) and the row of lower water nozzles (24), with their openings facing them, each of the upper scroll (25) and the lower scroll (26) having an end drain for removal of water recovered through a lip in contact with the slab; 7. The plant according to claim 1, further comprising an upper air nozzle row (27) and a lower air nozzle row (28), arranged transversely to the slab upstream of the upper scroll (25) and the lower scroll (26), the nozzles being inclined so as to deliver jets in the direction of movement of the slab.

8. 8. Plant according to any one of claims 1 to 7, characterized in that the second water descaling device (8) arranged between the two induction furnaces (6.1, 6.2) comprises a first pinch roll (8.1) on the side towards the first induction furnace (6.1) and an actual descaling device (8.2) and a second pinch roll (8.1') on the side towards the second induction furnace (6.2).

9. The second water scale removal device (8) comprises: a first (33) and second (33') row of upper water nozzles and a first (34) and second (34') row of lower water nozzles, all of said rows being arranged transversely to the transfer bar and having nozzles inclined to deliver jets in a direction opposite to the direction of movement of said transfer bar, each of said two rows of upper water nozzles being preceded by an upper scroll (35, 35') and a movable lip (32, 32'), said movable lip (32, 32') contacting an upper surface of said transfer bar in an operating position and being aligned with its respective upper scroll (35, 35'); 9. The plant according to claim 8, characterized in that a first (37) and a second (37') row of upper air nozzles are arranged transversely to the transfer bar, the first row (37) being arranged upstream of the movable lip (32) preceding the first row (33) of upper water nozzles, and the second row (37') being arranged downstream of the second row (33') of upper water nozzles.

10. 10. The plant according to claim 1, wherein the system for supplying the protective atmosphere to the finishing mill (3) comprises a pair of supply pipes (52, 52') attached to the structure of the looper (51) on either side of the strip in the space between two finishing stands (3.1, 3.2, ..., 3.7), respectively, on the upstream and downstream sides thereof, and each of these supply pipes (52, 52') branches into two substantially horizontal rows of nozzles arranged above (53, 53') and below (54, 54') the longitudinal direction of the strip, parallel to its edge.

11. 11. Plant according to claim 10, characterized in that the system for supplying the protective atmosphere further comprises at least two parallel horizontal rows (57, 57', 58, 58') of nozzles arranged transversely above and below the steel strip in each of two substantially horizontal rows of nozzles arranged longitudinally above (53, 53') and below (54, 54') the steel strip and parallel to its edges, the protective atmosphere reaching each pair of the parallel horizontal rows (57, 57', 58, 58') through respective supply pipes (50, 50', 59, 59').

12. 10. Plant according to any one of claims 1 to 9, characterized in that the system for supplying the protective atmosphere to the finishing mill (3) comprises at least two pairs (63, 63', 64, 64') of parallel horizontal rows of nozzles arranged transversely above and below the steel strip both upstream and downstream of the looper (51) in the space between two finishing stands (3.1, 3.2, ..., 3.7), the protective atmosphere reaching each pair (63, 63', 64, 64') of parallel horizontal rows through a respective pair of supply pipes (61, 61', 62, 62').

13. 13. Plant according to any one of claims 10 to 12, characterized in that the row of nozzles is enclosed in a chamber, the chamber being formed by a pair of upper flaps (55, 55'; 65, 65') and a pair of lower flaps (56, 56'; 66, 66') shaped to allow the steel strip to pass through the chamber and rotatable about an end pin to open the chamber.

14. 14. Plant according to claim 13 when dependent on claim 11, characterized in that the parallel horizontal rows (57, 57', 58, 58') of nozzles are attached to the pair of upper flaps (55, 55'; 65, 65') and the pair of lower flaps (56, 56'; 66, 66').

15. 14. A plant according to claim 13 when dependent on claim 12, characterized in that at least two pairs (63, 63', 64, 64') of the parallel horizontal rows of nozzles are attached to the pair of upper flaps (55, 55'; 65, 65') and the pair of lower flaps (56, 56'; 66, 66').

16. 16. A plant according to any one of claims 1 to 15, characterized in that the first stand (2.1) of the roughing mill (2) is a stand designed for a slab thickness reduction of not more than 20%.

17. 17. The plant according to any one of claims 1 to 16, further comprising a mechanical scale crushing device (7) between the first induction furnace (6.1) and the second water scale removal device (8), formed by at least three rollers arranged alternately above and below the feed line of the transfer bar and at a height such that they cause plastic stretching of the surface, which leads to the crushing of the rigid scale layer.

18. 18. A plant according to any one of the preceding claims, further comprising, between the finishing mill (3) and the cooling station (12), a further cooling station (12'), a further cutting station (10') and a further coiling station (9; 11) in that order.

19. 19. Plant according to claim 18, characterized in that it further comprises, between each cooling station (12; 12') and each cutting station (10; 10'), a mechanical descaling device (14; 14') using counter-rotating brushes or abrasive slurry jets.

20. 20. A plant according to any one of the preceding claims, characterized in that it also comprises a line (13) for anti-corrosion coating, arranged immediately after the final winding station (9; 11), allowing said coating to be applied to the steel strip without the need for a first coiling.

21. a system for controlling and managing the temperature of the material being processed, said system being operatively connected to an electromagnetic brake inserted in an ingot mould forming part of said continuous casting apparatus (1), to thermocouples inserted in copper plates of said mould, and to thermal scanners located along said plant; 21. Plant according to any one of claims 1 to 20, characterized in that the control system is also operatively connected to all other components of the plant that actively influence the temperature of the material to be processed, both in heating (4.1, 4.2, 6.1, 6.2) and cooling (5.2, 7, 8.2, 12, 12', 14, 14').

22. 3. A plant according to claim 2, characterized in that each of said shutters (20) is mounted on a parallelogram support formed by a pair of parallel arms pivoted between said shutter (20) and the structure of the actual descaling device (5.2) and moved by an actuator.

23. 5. A plant according to claim 4, characterized in that each of the side coils (4.1a) is equipped with its own frequency converter so that the induction edge heater (4.1) can heat the right and left edges of the slab differently.

24. 7. A plant according to claim 6, characterized in that the handling system is operated by placing the induction coil of each of the induction edge heaters (4.1) on a slide which moves along a lateral guide under the action of an actuator.

25. 8. The plant according to claim 7, wherein the upper water nozzle row (23) and the lower water nozzle row (24) are arranged at opposite positions, and the nozzles are aligned at the same inclination angle in the vertical direction.

26. 10. The plant according to claim 9, wherein the upper water nozzle row (33, 33') and the lower water nozzle row (34, 34') are arranged at opposite positions, and the nozzles are aligned at the same inclination angle in the vertical direction.

27. 11. A plant according to claim 10, characterized in that each of the two rows of nozzles arranged substantially horizontally above the longitudinal direction of the steel strip (53, 53') and parallel to its edge extends towards both of the two stands up to a substantially vertical plane that crosses the steel strip and passes through the centre of the looper (51), while each of the two rows of nozzles arranged substantially horizontally below the longitudinal direction of the steel strip (54, 54') and parallel to its edge extends towards only the adjacent stand.

28. The thermal scanner is used to scan the continuous casting device (1), the roughing mill (2), the first induction furnace (6.1), the second water scale removal device (8), and the second induction furnace (6.1).

22. Plant according to claim 21, characterized in that it is arranged at the exit of the finishing mill (3) and of the cooling station (12, 12') of (6.2).

29. 29. A method for the continuous production of hot-rolled steel strip with a minimum thickness of 0.3 mm by means of a plant according to any one of claims 1 to 28, comprising the following sequence of steps: (a) Continuous casting of thin or medium slabs with a thickness of 40 to 150 mm (1) step; (b) Rough rolling (2) step to obtain a transfer bar in 3 to 5 passes; (c) a first induction heating (6.1) step of the transfer bar; (d) Water descaling (8.2) step; (e) a second induction heating step (6.2) of the transfer bar; (f) a finish rolling (3) step to obtain the steel strip in 5 to 7 passes; (g) a step of controlled cooling (12; 12') of the steel strip; (h) cutting the steel strip (10; 10') and winding it into coils (9; 11); Including, wherein at least steps (e) and (f) are carried out in a protective atmosphere that is slightly oxidizing, inert, or slightly reducing, up to at least the third pass; Between steps (a) and (b), the further step of: (a') induction heating of the edges of the slab (4.1); (a'') induction heating of the remaining part of the slab surface (4.2); (a'') Water descaling (5.2) step; A method, characterized in that:

30. 30. A method according to claim 29, characterized in that step (h) is replaced by passing the steel strip directly to a step of applying a protective coating (13), followed by final winding.

31. 31. The method according to claim 29 or 30, characterized in that in step (b) the first pass (2.1) of rough rolling (2) reduces the slab thickness to no more than 20%.

32. 32. The method according to any one of claims 29 to 31, characterized in that between steps (c) and (d) a further step (c') of mechanical crushing (7) of the scales is provided.

33. Method according to any one of claims 29 to 32, characterized in that between steps (g) and (h) a further step (g') of mechanical descaling (14; 14') is provided.

34. 34. Method according to any one of claims 29 to 33, characterized in that between steps (b) and (c) a further step of production and removal of rough sheets (15, 16) is provided in case of problems in the part of the plant downstream of the rough rolling (2).

35. Step (a''') is 150 bar (1.50 x 10 ７ and / or step (d) is carried out at a water pressure of less than 380 bar (3.80 x 10 ７ 35. The method according to any one of claims 29 to 34, characterized in that it is carried out at a water pressure of 1000 Pa.

36. 36. A method according to any one of claims 29 to 35, characterized in that step (e) is carried out at a final temperature that ensures that step (f) is carried out entirely in the austenitic field.

37. 37. A method according to any one of claims 29 to 36, characterised in that step (a') is carried out on a band up to 150 mm from each edge of the slab and / or results in an increase in the temperature of that band up to 120°C.

38. 38. A method according to any one of claims 29 to 37, characterized in that step (h) is followed by a step (i) of liquid cooling the coil in a tank containing water or a slightly oxidizing aqueous solution.

39. 30. The method of claim 29, wherein steps (g) and (h) are also carried out in a protective atmosphere that is slightly oxidizing, inert, or slightly reducing in the winding section.

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