Plant and method for producing flat rolled products

Abstract

Rolling plant (10) and method for producing a final strip (P) starting from a conventional slab (50) or from a thin slab (65), comprising at least a first heating device (16) configured to heat at least the conventional slab (50), or the thin slab (65), to a certain first temperature; at least one roughing stand (23) configured to subject the conventional slab (50), or the thin slab (65), to at least one rolling pass in order to obtain an intermediate rolled product (51); and a continuous rolling train (25) disposed operatively in-line with the roughing stand (23) and comprising at least one pre-finishing stand (26) to reduce the thickness of the intermediate rolled product (51) and obtain a pre-finished rolled product (52), and a plurality of finishing stands (31) to reduce the thickness of the pre-finished rolled product (52) and obtain the final strip (P); the first heating device (16) comprising at least a first heating tunnel (18) at least for the thin slab (65) and disposed upstream of the roughing stand (23), the plant (10) comprising a second heating device (20) interposed between the roughing stand (23) and the rolling train (25) and provided with a second heating tunnel (19), and a rapid heating device consisting of selectively activatable elements is interposed between said at least one pre- finishing stand (26) and said plurality of finishing stands (31).

Description

[0001] “PLANT AND METHOD FOR PRODUCING FLAT ROLLED PRODUCTS”

[0002] FIELD OF THE INVENTION

[0003] The present invention concerns a plant and a method for producing flat rolled products such as, as a non-limiting example, strips of steel wound in the form of coils.

[0004] BACKGROUND OF THE INVENTION

[0005] There are known rolling plants called Hot Strip Mills, referred to more simply below with the acronym HSM, which are designed to operate in coil-to-coil mode, in which the rolled product is obtained starting from individual conventional slabs, for example with a thickness comprised between about 200 mm and about 300 mm, until a strip with a minimum final thickness of up to 1.2 mm is obtained.

[0006] An example of such known plants 100 is shown schematically in fig. 1.

[0007] The known plant 100 comprises one or more gas-fired heating furnaces 600, for example of the type known in the field with the term “walking beam”, configured to receive at least one slab 500, supplied even at ambient temperature, and heat it to a certain furnace exit temperature. Advantageously, at exit from the gas-fired heating furnace 600 the slab 500 has a temperature comprised between about 1200 °C and about 1250 °C.

[0008] The slabs 500 are, as a rule, fed, according to desired feeding sequences and timings, from a warehouse 400 disposed substantially in-line with, and upstream of, the gas-fired heating furnace 600.

[0009] Inside the walking beam furnace the slabs advance transversely with respect to their longitudinal axis, so that when they are unloaded onto the roller conveyor for rolling, this axis is already aligned with the rolling axis.

[0010] Downstream of the gas-fired heating furnace 600 there is disposed at least one roughing stand 300, normally of the reversible type, which is configured to subject the slab 500 to a certain number of passes and reduce its thickness until an intermediate rolled product is obtained, for example with a thickness comprised between about 30 mm and about 80 mm. As an indication only, at the end of the desired roughing passes, the intermediate rolled product has a temperature ranging between about 1020 °C and about 1120 °C.

[0011] Downstream of the reversible roughing stand 300 there is disposed at least one continuous rolling train 200, or finishing train, which normally consists of six or seven finishing stands disposed in succession to each other, all at the same distance.

[0012] The continuous rolling train 200 is configured to progressively reduce the thickness of the intermediate rolled product, in order to obtain the final strip, with a minimum thickness of about 1.2 mm.

[0013] From here the final strip is cooled by means of specific cooling devices 800 and subsequently wound into coils.

[0014] This type of solution, if for example applied to the production of a strip having a minimum final thickness of 1.2 mm, requires an increase of about 40% in the rolling speed after the head of the strip has been fed onto the down coiler, in order to make the strip transit more rapidly and reduce temperature losses, thus guaranteeing a minimum temperature of 830°C at exit from the last stand of the train 200, as schematized in the graph of fig. 2, since the only active thermal input takes place in the heating furnace 600 upstream of the line.

[0015] However, wanting to achieve rolled products with rolling thicknesses of less than 1.2 mm, despite using a speed-up of even 60% or more, with conventional HSM plants it is not possible to guarantee that the desired minimum temperature of 830°C required at exit from the last stand will be maintained, because the temperature loss of the product being rolled is excessive, resulting in unwanted phase changes of the steel during rolling which negatively affect the quality of the final product.

[0016] As can be seen from the same graph of fig. 2, with a known HSM plant 100 there is always a temperature difference between the head and tail of the metal product during the aforementioned process steps, to the detriment of a high rolling uniformity of the metal product itself, which can affect the quality of the strip produced.

[0017] These known HSM plants, although particularly advantageous with regard to the production of steels with particular characteristics (for example stainless steels and grain-oriented and non-oriented magnetic steels), have a specific layout that provides to feed only conventional slabs which are purchased and / or produced offline and stored in a yard.

[0018] A disadvantage of known plants is therefore caused by the specificity of the feed required for rolling, which substantially prevents the possibility of expanding the production mix to obtain thin and ultra-thin thicknesses, and those grades of steel that are normally produced starting from thin slabs rolled in-line with the casting.

[0019] Existing walking beam furnaces are designed to heat conventional slabs with a thickness for example of 250 mm and a length of 10-12 meters to produce a coil weighing about 30 tons. To produce a coil of the same weight with a thin slab, for example 120 mm thick, the slab would have to be about 24 meters long. Walking beam furnaces of this size do not exist in the state of the art, since they would be extremely expensive and would have excessively high consumptions. In addition, a thin slab heated in the WB furnace would undergo thermal deformations due to the reduced thickness, which would make it unsuitable for rolling. Therefore, due to the above problems, an HSM plant of the state of the art can only roll conventional slabs.

[0020] US 1 1,097,323 describes a combined continuous casting and rolling plant in which a continuous casting machine feeds a first heating tunnel furnace located upstream of a first, a second and a third continuous rolling train. Rapid heating systems can be present between the first and the second, and between the second and the third continuous rolling train. The absence of selectively reversible stands makes this plant unsuitable for operating alternately on slabs obtained from continuous casting and on slabs obtained from off-line warehouses.

[0021] US 2010 / 0212856 describes a casting and rolling plant similar to that of US’323, in which a continuous casting machine sequentially feeds slabs to at least one tunnel furnace followed by an induction furnace and by a continuous rolling train. Also in this case, the stands are not configured to be selectively reversible so as to adapt to a feed coming from continuous casting or from an off-line warehouse. Furthermore, US ’856 provides a tunnel furnace only between the casting and the first roughing train, and not between the roughing stand and the continuous rolling train. EP 2 767 600, CN 112 024 595 and EP 3 238 844 describe other examples of integrated casting and rolling plants.

[0022] One purpose of the present invention is to provide a plant for producing flat rolled products and to implement a method which allow to retrofit, renew or expand a known and existing HSM plant, in order to allow a feed both with conventional slabs as well as with thin slabs coming directly from casting, that is, to provide two types of slabs, conventional and thin, to be fed to a same rolling line. Another purpose of the invention is to provide a plant with the characteristics as above ex novo, and possibly in subsequent steps.

[0023] Another purpose of the invention is to provide a single plant that allows to produce the entire possible range of hot strips, both in terms of thickness range as well as steel grades. Another purpose of the invention is to provide a plant with a high flexibility of use that optimizes production costs.

[0024] Another purpose of the present invention is to provide a plant and perfect a method for producing rolled products that allow to achieve a high quality of the finished product, both in terms of thickness size uniformity and also in terms of microstructural uniformity.

[0025] The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

[0026] SUMMARY OF THE INVENTION The present invention is set forth and characterized in the independent claims.

[0027] The dependent claims set forth evolutionary and improvement aspects in relation to the independent claims.

[0028] In accordance with the above purposes, a rolling plant, in which the present invention is applied, for producing a steel strip starting from a conventional slab fed from a storage warehouse or, and alternatively, starting from a thin slab obtained directly from continuous casting, comprises:

[0029] - at least a first heating device configured to heat at least the conventional slab, or the thin slab, to a certain first temperature;

[0030] - at least one roughing stand configured to subject the conventional slab, or the thin slab, to at least one rolling pass in order to obtain an intermediate rolled product;

[0031] - a continuous rolling train disposed operatively in-line with the roughing stand and configured to reduce the thickness of the intermediate rolled product, until the final strip with a certain final thickness is obtained. Therefore, the plant is configured as a new generation rolling plant capable of rolling both conventional slabs with a thickness comprised between 200 and 350 mm, purchased and / or produced off-line and coming from a storage warehouse, as well as thin slabs with a thickness comprised between 110 and 140 mm coming from a continuous casting machine disposed in line with the rolling train.

[0032] Advantageously, with a single plant built according to the invention, it is possible to cover the entire hot production of coils, both in terms of thickness range as well as steel grades, in particular:

[0033] - ultra- thin thicknesses up to 0.6 mm with thin slab feed in endless mode; - thin thicknesses up to 0.9 - 1.0 mm both with thin slab feed in endless mode and also with conventional slab feed in coil-to-coil mode;

[0034] - high thicknesses up to 32 mm with conventional slab feed in coil-to-coil mode;

[0035] - “sophisticated” steels such as for example stainless steels or grain-oriented and non-oriented magnetic steels, high-strength Silicon Steel and Dual Phase steels and, more generally, third-generation multi-phase steels, with conventional slab feed.

[0036] In accordance with one aspect of the present invention, the first heating device comprises at least a first heating tunnel furnace disposed upstream of the roughing stand and suitable to carry out an active heating of at least the thin slab, so that the exit temperature is in a range comprised between 1130 °C and 1200 °C, preferably between 1150 °C and 1180 °C, before it enters the roughing stand.

[0037] In the remainder of the description, “active heating” means that the furnace is equipped with heating systems, such as electrical resistors or inductors or gas-fired burners, or combinations thereof, which generate heat to raise the tunnel’s internal temperature, and therefore that of the metallurgical product in transit. This heating is precisely controlled by control systems to reach the desired temperatures and / or to maintain a uniform and stable temperature along the entire length of the tunnel. Therefore, the term active heating can be understood to mean both that the temperature of the metallurgical product is raised up to a predetermined value, and also that the temperature of such product is kept substantially constant at a desired value by the active and controlled supply of heat into the furnace.

[0038] In an advantageous embodiment, the first heating device also comprises a walking beam heating furnace of a known type, configured to heat conventional slabs from ambient temperature to an optimum rolling temperature in a range comprised between 1180 °C and 1300 °C. Conventional slabs that exit from the walking beam furnace are then transferred to the rolling axis, by means of a translatable roller conveyor, immediately upstream of the roughing stand. Advantageously, with thin slab feed the roughing stand is configured to operate unidirectionally, while with conventional slab feed the roughing stand can operate reversibly to perform several consecutive rolling passes.

[0039] In accordance with another aspect of the present invention, a second heating device is provided, which is interposed between the roughing stand and the rolling train. This second heating device is provided with a second tunnel furnace which can be suitable to carry out an active heating of the intermediate rolled product, for example by means of gas-fired burners, up to a temperature comprised between 1050 °C and 1120 °C, in particular when feeding thin slabs coming from the continuous casting. When, on the other hand, conventional slabs are fed, the second tunnel furnace can carry out an active heating so as to homogenize the temperature of the slab between head and tail, that is, bring both the tail and also the head of the intermediate rolled product to a substantially uniform temperature after the reversible rolling in the roughing train. The active heating of the conventional slabs in the second tunnel furnace is advantageous to achieve a higher geometric and microstructural quality of the product, and it is provided in particular for the production of thin thicknesses or for the production of sophisticated steels.

[0040] In accordance with a variant, when conventional slabs are fed, the second tunnel furnace does not carry out an active heating and acts as a passive tunnel, for example by powering off the burners or other heating devices, in order to contain heat losses through the insulation that characterizes the furnace.

[0041] In accordance with another aspect of the present invention, the finishing rolling train is divided between at least one pre-finishing stand and a plurality of finishing stands.

[0042] In particular, the at least one pre-finishing stand is disposed at a minimum distance from the reversible roughing stand, advantageously such that the intermediate rolled product is not operatively engaged with both types of stands simultaneously. Furthermore, the at least one pre-finishing stand is able to reduce the thickness of the intermediate rolled product until a pre-finished rolled product is obtained, for example with a thickness comprised between about 8 mm and about 50 mm.

[0043] The finishing stands, on the other hand, are configured to reduce the thickness of the pre-finished rolled product so as to obtain the final strip of the desired thickness.

[0044] In accordance with another aspect of the present invention, a rapid heating device is provided, interposed between the at least one pre-finishing stand and the plurality of finishing stands and comprising, for example, an induction furnace consisting of elements or modules which are able to be selectively activated, even independently of each other. The rapid heating device is configured to heat the prefinished rolled product selectively and in an adjustable manner, before it enters the finishing stands.

[0045] The temperature to which the pre-finished rolled product is heated is selected, among other parameters, at least as a function of its thickness and its speed, and of the final thickness of the final strip, to ensure that the latter has an optimal temperature of at least 830 °C at exit from the finishing unit, and in particular at exit from the last finishing stand.

[0046] By way of example only, the temperature to which the pre-finished rolled product is heated, that is, the temperature it reaches at exit from the rapid heating device, reaches a value comprised, advantageously, between about 1000 °C and about 1100 °C, or in any case a temperature such that, also as a function of the operating and product parameters, the temperature of the final strip at exit from the last finishing stand is higher than at least 830 °C.

[0047] When rolling conventional slabs, this allows to reduce the value of the rolling mass flow required, to achieve the above-mentioned optimum temperature of at least 830 °C, for example comprised between 830 °C and 900 °C, at exit from the last stand of the finishing unit.

[0048] The reduction of the rolling mass flow allows both to carry out the rolling with a reduced rolling speed, preferably lower than 12 m / s, and at the same time to also reach the optimum temperature of at least 830 °C at exit from the continuous rolling train, even for the tail of the final strip, eliminating the need for the “speedup” of the finishing train’s stands as a tool for reaching the target temperature.

[0049] Advantageously, in the absence of speed-up the rolling speed in the finishing stands is substantially constant and allows both to keep the temperature of the final strip constant between its head and tail, and also to choose the most suitable temperature control (for example thermomechanical treatment) as a function of the steel grade and the use of the final strip. Another advantage of not carrying out the speed-up when feeding with conventional slabs is that this allows to have a high level of control both of the strip’s final shape, for example its crown and flatness, which will therefore be advantageously uniform along the entire length of the coil, and also of the final strip’s mechanical properties, which will be advantageously constant and uniform along the entire length of the coil.

[0050] This last advantage, which cannot be achieved with the plants of the state of the art, is of considerable importance, especially for quality productions such as, for example, strips intended for molding.

[0051] When rolling thin slabs, on the other hand, the rapid heating device allows to carry out endless rolling. With coil-to-coil rolling, this device can be powered off or taken off-line.

[0052] The plant according to the present invention allows to produce hot strips in the entire range of thicknesses and steel grades required by the market, with a productivity that can reach up to 3.5 - 4.0 million tons / year. Furthermore, since the plant according to the present invention is configured to effectively manage both conventional slabs and also thin slabs, it can be advantageously fed in a double alternative feeding configuration, that is, from a conventional slab storage warehouse located off-line, or from a thin slab continuous casting machine disposed in-line with the rolling train. This advantage given by the solution according to the present invention also allows to provide a progressive management of any investments relating to the type of feed of slabs to be sent to rolling. For example, in a first step, an investment can be provided to install a rolling train fed only with conventional slabs supplied by a storage warehouse, providing a first heating device consisting substantially of a heating furnace, for example of the walking beam type and disposed operatively off-line upstream of the roughing stand; a transfer device, for example a roller conveyor, picks up the conventional slabs at exit from the heating furnace in order to selectively feed them to the roughing stand and then to the stands of the rolling train located further downstream.

[0053] Subsequently, with the solution according to the present invention, it is possible to install, in-line with the rolling train, a casting machine for thin slabs, traditionally provided with an ingot mold fed by a ladle, through a tundish. In this solution, a cutting device can also be provided to cut the thin slabs cast to a predetermined length, before they enter the first heating device. In particular, the latter also comprises the first heating tunnel, which the thin slabs enter and exit at the rolling temperature, before being rolled in the roughing stand.

[0054] According to another aspect of the present invention, the continuous rolling train comprises between one and three pre-finishing stands and between four and seven finishing stands.

[0055] According to another aspect of the present invention, the plant comprises at least first descaling means interposed between the heating furnace and the at least one roughing stand, advantageously of the reversible type, second descaling means interposed between the reversible roughing stand and a first of the pre-finishing stands, and advantageously third descaling means interposed between the rapid heating device and a first of the finishing stands.

[0056] According to another aspect of the present invention, the at least one reversible roughing stand is in turn equipped with descaling means mounted on board, and forming an integral part of, the stand itself and advantageously disposed both on the entry side and also on the exit side of the stand.

[0057] According to another aspect of the present invention, the plant comprises a cutting machine interposed between the pre-finishing stands and the rapid heating device to perform the head and tail cropping of the pre-finished rolled product. As mentioned, in this segment of the plant the thickness of the pre-finished rolled product is already reduced sufficiently for a small-sized cutting machine to be provided, for example of the type known as Crop Shear, which has low production and management costs compared to conventional shears disposed upstream of the entire rolling finishing train.

[0058] According to another aspect of the present invention, the plant comprises a second cutting machine, for example a rapid shear, disposed downstream of the last finishing stand of the rolling train so as to allow for endless rolling when feeding thin slabs from the casting line. According to another aspect of the present invention, the plant comprises a second continuous casting machine, located off-line and parallel to the first casting machine, to produce the conventional slabs. In this way, instead of being purchased externally, the conventional slabs are produced within the same plant, exploiting the capacity of the steelworks at times when the first continuous casting machine for the thin slabs is not in use.

[0059] The present invention also concerns a rolling method for producing a final strip starting from a conventional slab selectively fed from a storage warehouse, or starting from a thin slab produced directly by a casting machine through continuous casting, in a rolling plant of the type described heretofore.

[0060] According to one aspect of the present invention, the method provides at least one step of active heating at least of the thin slab, so as to bring it to a rolling temperature, by means of at least a first heating tunnel disposed upstream of the roughing stand and forming part of a first heating device. In accordance with another aspect of the present invention, a second active heating of the intermediate rolled product can be provided, carried out by means of a second heating tunnel of a second heating device, interposed between the roughing stand and the rolling train, when feeding thin slabs or when feeding conventional slabs and the intention is to advantageously carry out a homogenization of the temperature of the intermediate rolled product, that is, bring both the tail and also the head thereof to a substantially uniform temperature. Always in the case of conventional slabs, the second tunnel may not carry out any active heating and may act only as an insulated path to contain the metallurgical product’s heat losses, therefore have a “passive” use. In accordance with another aspect of the present invention, there is provided a pre-fmishing rolling of the intermediate rolled product, by means of at least one pre-finishing stand of the rolling train, so as to reduce the thickness of the intermediate rolled product and obtain a pre-finished rolled product; and at least one finishing rolling of the pre-finished rolled product, by means of a plurality of finishing stands, so as to reduce the thickness of the pre-finished rolled product and obtain the strip with the desired final thickness.

[0061] Furthermore, the method according to the present invention provides at least one step of heating the pre-finished rolled product, by means of a rapid heating device consisting of selectively activatable elements or modules and interposed between the at least one pre-finishing stand and the plurality of finishing stands, so that the temperature of the final strip, in correspondence with the exit of the last finishing stand, is higher than at least 830 °C, even in the case of thin thicknesses. In some embodiments, a temperature measurement system, preferably infrared, is provided before the last two induction modules so that the latter can give the pre-finished product any missing temperature. They are therefore not made to work at maximum power, as the preceding modules, but are kept powered off or active with reduced power so that they have a margin that is suitable to integrate any thermal deficiencies before the product enters the finishing stands (they have a “trimming” function).

[0062] In accordance with another aspect of the present invention, the rolling plant provides to produce strips with a final thickness comprised between about 0.6 mm and about 32 mm, which can be wound on the reels, for productions of up to 3.5 - 4.0 million tons per year.

[0063] DESCRIPTION OF THE DRAWINGS

[0064] These and other aspects, characteristics and advantages of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein: - fig. 1 is a schematic representation of a Quali-HSM plant for producing flat rolled products from conventional slabs, in accordance with the state of the art;

[0065] - fig. 2 is a schematic representation of the slabs’ temperature trend during the succession of rolling steps actuated by the plant of fig. 1 ;

[0066] - fig. 3 is a schematic representation of a plant for producing flat rolled products from conventional slabs, or from thin slabs, in accordance with the present invention;

[0067] - fig. 3 bis is a duplicate of fig. 3;

[0068] - fig. 4 is a schematic representation of the conventional slabs’ temperature trend during a first operating mode of rolling steps actuated by the plant of fig. 3; - fig. 5 is a schematic representation of the conventional slabs’ temperature trend during a second operating mode of rolling steps actuated by the plant of fig. 3;

[0069] - fig. 6 is a schematic representation of the conventional slabs’ speed trend during the rolling steps actuated by the plant of fig. 3; - fig. 7 is a schematic representation of the thin slabs’ temperature trend during the succession of rolling steps actuated by the plant of fig. 3bis;

[0070] - fig. 8 is a schematic representation of the thin slabs’ speed trend during the rolling steps actuated by the plant of fig. 3bis; and

[0071] - fig. 9 is a schematic plan view of the plant of fig. 3.

[0072] We must clarify that the phraseology and terminology used in the present description, as well as the figures in the attached drawings also in relation as to how described, have the sole function of better illustrating and explaining the present invention, their purpose being to provide a non-limiting example of the invention itself, since the scope of protection is defined by the claims.

[0073] To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can be conveniently combined or incorporated into other embodiments without further clarifications.

[0074] DESCRIPTION OF AN EMBODIMENT OF THE PRESENT INVENTION

[0075] With reference to fig. 3, this shows a plant 10 in accordance with the present invention for producing a flat rolled product, for example a final strip P, with a thickness comprised between about 0.9 mm and about 32 mm wound to form a coil, starting from conventional slabs 50 having a starting thickness comprised between about 200 mm and about 350 mm.

[0076] In the same way, the plant 10 according to the present invention is also suitable to roll thin slabs 65 having a starting thickness comprised between about 110 mm and about 140 mm, coming from a casting machine 60 of a substantially traditional type, for producing a flat rolled product, for example a final strip P, with a thickness comprised between about 0.6 mm and about 26 mm wound to form a coil.

[0077] The plant 10 according to the present invention comprises, downstream of the casting machine 60, a first heating device 16 which comprises at least a first tunnel furnace 18 suitable to carry out an active heating of at least the thin slab 65, so as to bring it to an exit temperature in a range comprised between 1130 °C and 1200 °C, preferably between 1150 °C and 1180 °C.

[0078] In an advantageous embodiment, the first heating device 16 comprises a walking beam heating furnace 41 of a known type configured to heat the conventional slabs 50, so as to bring them to an optimal rolling temperature in a range comprised between 1180 °C and 1300 °C.

[0079] Downstream of the first heating device 16 there are disposed a vertical or edging stand 21 and a roughing stand 23, the latter able to be selectively configured to be reversible and subject the conventional slab 50 to a certain number of passes, or to be unidirectional and subject the thin slab 65 to a single pass, and reduce their thickness until an intermediate rolled product 51 is obtained. The latter has, in an advantageous embodiment, a thickness comprised between about 30 mm and 80 mm.

[0080] It is not excluded, in other embodiments, that two roughing stands 23 can be provided.

[0081] According to another aspect of the present invention, the at least one roughing stand 23 is in turn equipped with descaling means mounted on-board, and forming an integral part of, the stand itself, which are disposed both on the entry side and also on the exit side of the stand (not shown in the drawings).

[0082] A second heating device 20, a second descaling device 24 and a continuous rolling train 25 are disposed, in succession, downstream of the reversible roughing stand 23.

[0083] The second heating device 20 mainly comprises a second tunnel furnace 19 which can operate with active heating both in the case of feed with thin slabs 65 and also in the case of feed with conventional slabs 50 and, in the latter case, the aim is to advantageously achieve the thermal homogenization of the intermediate rolled product 51 after the reversible rolling in the roughing stand 23, that is, effectively make its temperature uniform between the head portion and the tail portion before it enters the finishing train 25, as shown in fig. 5, to the advantage of the size and microstructural quality of the finished rolled product.

[0084] In particular, the continuous rolling train 25 consists of two macro rolling units, one pre-finishing unit comprising, in the solution shown purely by way of example, two pre-fmishing stands 26, and one finishing unit comprising a plurality of finishing stands 31 , in the example case shown five.

[0085] The continuous rolling train 25 is configured to progressively reduce the thickness of the intermediate rolled product 51 in order to obtain the final strip P with a minimum thickness of about 0.8 mm. In some embodiments, not shown in the drawings, the plant 10 can also comprise a vertical or edging stand 21, both downstream of the reversible roughing stand 23 and also upstream of the continuous rolling train 25.

[0086] In general, the number of pre-finishing stands 26 of the rolling train 25 is comprised between one and three, while the number of finishing stands 31 is comprised between four and seven, and their number and disposition is chosen as a function of the steel grades, the use of the finished product and the minimum and maximum thicknesses that the final strip P assumes during rolling.

[0087] In the solution according to the present invention, two pre- finishing stands 26 are provided, spaced apart from the remaining finishing stands 31 of the rolling train 25, so that a pre-finished rolled product 52 having a thickness comprised between about 8 mm and about 50 mm exits from the pre-finishing stands 26.

[0088] In addition, the pre-finishing stands 26 are disposed at a certain distance D from the roughing stand 23 so that, in the event of conventional slab 50 feed, during the reversible roughing rolling the intermediate rolled product 51 is never operatively engaged with both types of stands simultaneously.

[0089] In the example solution shown, downstream of the pre-finishing stands 26 there is disposed a flying shear 27, of the crop shear type, to crop the heads and tails of the pre-finished rolled product 52 during the discontinuous rolling (coil-to-coil), in order to facilitate its entry into the finishing stands 31 and to reduce the chances of cobble, especially for the production of final strips having a thickness of less than 3.0 mm.

[0090] The plant 10 according to the present invention also comprises a rapid heating device 28 interposed between the pre-finishing stands 26 and the finishing stands 31 and provided, for example, with an induction furnace disposed downstream of the flying shear 27 and consisting of elements or modules which are able to be selectively activated, even independently of each other.

[0091] The rapid heating device 28 is configured to heat, selectively and in an adjustable manner, the pre-finished rolled product 52 before it enters the finishing stands 31.

[0092] The temperature to which the pre-finished rolled product 52 is heated is selected, among other parameters, at least as a function of its thickness and speed, and of the final thickness of the final strip P, so that the latter has an optimal temperature of at least 830 °C at exit from the continuous rolling train 25, and in particular at exit from the last finishing stand.

[0093] By way of example only, the temperature to which the pre-finished rolled product 52 is heated, that is, the temperature that it assumes at exit from the rapid heating device 28, reaches a value advantageously comprised between about 1000 °C and about 1100 °C.

[0094] In the case of conventional slab 50 feed, the heating provided by the rapid heating device 28 is variable, heating the head of the pre-finished rolled product 52 to a certain temperature and then increasing, substantially linearly, the heating provided thereto so that the body and tail of the final strip P can also exit from the last finishing stand 31 of the continuous rolling train 25 at the optimal temperature of at least 830 °C.

[0095] Advantageously, downstream of the rapid heating device 28 and upstream of the finishing stands 31 there is also disposed a third water descaling device 29 having the function of further cleaning the surface of the pre-finished rolled product of scale, before it enters the finishing stands 31.

[0096] Downstream of the finishing stands 31 there is disposed a cooling device 33, for example of the type provided with a plurality of showers which are able to be selectively activated even independently of each other in order to cool the strip P. In the solution shown, downstream of the cooling device 31 there is provided a rapid shear 35 provided to divide the strip P into desired portions, in particular in the case of endless rolling.

[0097] In addition, at least two down coilers 36, 38 are disposed at exit from the rapid shear 35 to wind the strip P into coils and for its subsequent storage and shipment. The plant 10 also comprises a first heating device 16, which provides at least one heating furnace 41, for example of the type known in the sector with the term walking beam, disposed in parallel and off-line with respect to the rolling axis and configured to receive and heat to a certain temperature at least one conventional slab 50 supplied by a warehouse 40, even starting from ambient temperature. Advantageously, at exit from the gas-fired heating furnace 41 the conventional slab

[0098] 50 has a temperature comprised between about 1180 °C and about 1300 °C.

[0099] Downstream of the heating furnace 41 there are disposed, in sequence, a first water descaling device 42 (optional) and a mobile roller conveyor 43 able to receive the conventional slab 50 at exit from the furnace 41 at the end of the heating step. The roller conveyor 43 (fig. 6) can be selectively translated in a direction transverse to the rolling axis, to allow the introduction of the conventional slabs 50 on the rolling line according to desired feeding sequences and timings. When the roller conveyor 43 moves in rolling axis, it takes the place of the last mobile section 18a of the tunnel furnace 18, also called shuttle, which translates laterally moving off-line, as shown with the dotted line in fig. 3. The roller conveyor 43 remains in rolling axis until the rolling in the roughing stand 23 is completed, after which it returns to pick up another conventional slab which is released from the furnace 41. This cycle is repeated until the feed mode switches to thin slab 65 feed. In this case, the shuttle 18a again aligns with the tunnel furnace 18 and the roller conveyor 43 remains parked off-line.

[0100] By way of example only, the casting machine 60 provided to feed the thin slabs 65 is disposed in-line with the rolling axis and comprises an ingot mold 63, which comprises a crystallizer which can be of the type with flat and parallel faces.

[0101] The casting machine 60 can also be provided with devices for feeding the liquid metal, which in this case comprise a ladle 61 and a tundish 62.

[0102] The casting machine 60 can comprise liquid core pre-rolling devices, not shown in the drawings, configured to reduce the thickness of the thin slab 65, at exit from the ingot mold 63.

[0103] According to some embodiments, the casting machine 60 comprises a cutting device 64, such as for example a pendulum shear, to cut the cast thin slabs 65 to a predetermined length.

[0104] In the first tunnel furnace 18 and in the second tunnel furnace 19 the active heating can be carried out by means of conventional natural gas (methane) burners or by means of hydro-methane (hydrogen-methane mixture) burners or, in order to zero the CO2 emissions and to drastically reduce those of NOx, by means of 100% hydrogen burners or by means of electric heating with thermistors. Other embodiments to carry out this active heating provide mixed solutions that combine one or more of the above- described solutions.

[0105] When the plant 10 is fed using the thin slabs 65 supplied by the casting machine 60, the first tunnel furnace 18 also allows to generate an accumulation buffer for the thin slabs 65, with which to manage any interruptions in the rolling process, for programmed change of the rolling rolls or for cobbles or small accidents, without particular repercussions on the casting which can therefore continue to operate for a certain amount of time, also called “buffer time”.

[0106] In particular, the buffer capacity of the first tunnel furnace 18 prevents casting interruptions during short scheduled maintenance interventions on the parts located downstream of the casting machine 60, or in the case of small accidents during rolling.

[0107] The present invention also concerns a method for producing a strip P, wound to form a coil, starting from slabs 50 having a starting thickness comprised between about 200 mm and about 350 mm, or from thin slabs 65 having a starting thickness comprised between about 110 mm and about 140 mm coming directly from the casting machine 60.

[0108] The solution of the plant 10 according to the present invention described heretofore can provide both a feed using a thin slab 65, directly from a casting machine 60 disposed in-line with the rolling train, and also a lateral feed using conventional slabs 50 coming from a storage warehouse 40. From an operational point of view, the roughing stand 23 operates a unidirectional rolling, that is, with a single rolling pass, with regard to the thin slab 65, or a reversible rolling, that is, with an odd number of passes from 3 onward, with regard to the conventional slab 50.

[0109] As a result, a possible change of feed provides a programmed set-up of the roughing stand 23.

[0110] Depending on whether the feed occurs with conventional slabs 50 or thin slabs 65, the plant 10 described heretofore can provide different operating conditions, in particular with regard to the heating or temperature maintenance carried out by the second heating device 20.

[0111] With the feed of conventional slabs 50 in coil-to-coil mode, there can be provided a passive operating condition of the second tunnel 19, that is, with the burners or other heating devices powered off (fig. 4), or an active operating condition of the second tunnel 19, that is, with the burners or other heating devices powered on (fig. 5) to equalize the temperature between the head and tail especially, but not exclusively, if producing thin thicknesses and / or sophisticated steels. As can be seen in the graph of fig. 6, under both these operating conditions the speeds of the head and tail are managed in the same way.

[0112] In the case of a conventional slab 50, the difference in temperature between head and tail of the intermediate rolled product is due to the rolling passes in the roughing stand 23, which is used in reversible configuration.

[0113] In the case of a thin slab 65, on the other hand, as can be seen from the graph of fig. 7, referring to fig. 3 bis, there is no appreciable temperature difference between the head and tail of the intermediate rolled product after the roughing rolling, since this occurs only in the unidirectional sense and the thermal drop is substantially uniform. The second tunnel furnace 19, as the first tunnel 18, still performs active heating to compensate for the thermal losses that would be greater in the case of thin slabs. The graph of fig. 7 shows, in particular, the temperature trend in the endless rolling mode, that is, without a break in continuity, and the heating that is carried out by the rapid heating device before the finishing rolling is evident. In this case, the rolling speeds (fig. 8) are lower than the speeds shown in the previous fig. 6, since in endless mode the rolling train is connected in direct engagement with the continuous casting machine which has a lower process speed, to which the rolling train has to adjust.

[0114] It is not excluded that with the solution according to the present invention there can be provided an initial investment relating, for example, to a feed with only a conventional slab 50 coming from a warehouse 40 (purchased and / or selfproduced) and, subsequently, a second investment to complete the plant 10 with the additional feed of thin slabs 65 directly from the casting machine 60. The advantage of this aspect of the present invention lies in investment planning for manufacturers, while still guaranteeing uniformity in the quality of the strip P produced.

[0115] It is clear that modifications and / or additions of parts may be made to the plant 10 and to the method for producing flat rolled products as described heretofore, without thereby departing from the field and scope of the present invention, as defined by the claims.

[0116] It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art will be able to achieve other equivalent forms of method and plant 10 for producing flat rolled products, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

[0117] In the following claims, the sole purpose of the references in brackets is to facilitate their reading and they must not be considered as restrictive factors with regard to the field of protection defined by the claims.

Claims

CLAIMS1. Rolling plant (10) for producing a strip (P) starting from a conventional slab (50) selectively supplied from a storage warehouse (40) disposed off-line, or starting from a thin slab (65) supplied by a continuous casting machine (60) disposed in-line, comprising:- at least a first heating device (16) configured to heat said conventional slab (50), or said thin slab (65), to a certain first temperature;- at least one roughing stand (23) selectively configured to be reversible and to subject said conventional slab (50), or said thin slab (65), to one or more rolling passes in order to obtain an intermediate rolled product (51);- a continuous rolling train (25) disposed in-line with said roughing stand (23) and comprising between 1 and 3 pre- finishing stands (26) able to reduce the thickness of said intermediate rolled product (51) in order to obtain a pre-finished rolled product (52), and between 4 and 7 finishing stands (31) able to reduce the thickness of said pre-finished rolled product (52) in order to obtain said final strip (P);- wherein said first heating device ( 16) comprises at least a first tunnel furnace ( 18) for the active heating of at least said thin slab (65) and disposed in-line and upstream of said roughing stand (23);- wherein a second heating device (20) is interposed between said roughing stand (23) and said rolling train (25) and is provided with a second heating tunnel (19); and- wherein a rapid heating device (28), consisting of selectively activatable elements, is interposed between said at least one pre-finishing stand (26) and said plurality of finishing stands (31).

2. Plant (10) as in claim 1, characterized in that said first heating device (16) comprises a heating furnace (41) configured to heat said conventional slabs (50) and disposed operatively off-line and upstream with respect to said roughing stand (23).

3. Plant (10) as in claim 1, characterized in that said pre-finishing stand (26) is disposed at a minimum distance (D) from said roughing stand (23) so that the intermediate rolled product (51) is not operatively engaged with both said prefinishing (26) and roughing (23) stands simultaneously.

4. Plant (10) as in one or the other of the previous claims, characterized in thatsaid rapid heating device (28) is configured to heat said pre-finished rolled product (52), in order to reach a certain temperature of said final strip (P) in correspondence with the exit of the last finishing stand (31) of said rolling train (25).

5. Plant (10) as in one or the other of the previous claims, characterized in that it comprises at least first descaling means (20) interposed between said first heating device (16) and said roughing stand (23), second descaling means (24) interposed between said roughing stand (23) and a first of said pre-finishing stands (26), and third descaling means (29) interposed between said rapid heating device (28) and a first of said finishing stands (31).

6. Plant (10) as in one or the other of the previous claims, characterized in that it comprises a first cutting machine (27) interposed between said pre-finishing stand (26) and said rapid heating device (28).

7. Plant (10) as in one or the other of the previous claims, characterized in that it comprises a second cutting machine (35) disposed downstream of the last finishing stand (31) of said rolling train (25).

8. Plant (10) as in claim 2, characterized in that it comprises a transfer device (43) interposed between said first heating furnace (41) and said roughing stand (23) to selectively feed said conventional slabs (50) to said roughing stand (23).

9. Rolling method, for producing a final strip (P) starting from a conventional slab (50) selectively supplied from an off-line storage warehouse (40), or starting from a thin slab (65) supplied by an in-line casting machine (60), in a rolling plant (10) in which:- at least a first heating device (16) is configured to heat at least said conventional slab (50), or said thin slab (65), to a certain first temperature;- at least one roughing stand (23) is configured to be selectively reversible and to subject said conventional slab (50), or said thin slab (65), to one or more rolling passes in order to obtain an intermediate rolled product (51);- a continuous rolling train (25) is disposed operatively in-line with said at least one roughing stand (23) and comprises at least one pre-finishing stand (26) able to reduce the thickness of said intermediate rolled product (51) in order to obtain a pre-finished rolled product (52), and a plurality of finishing stands (31) able to reduce the thickness of said pre-finished rolled product (52) in order to obtain said final strip (P);- said first heating device (16) comprises at least a first heating tunnel (18) at least for said thin slab (65) and disposed in-line and upstream of said roughing stand (23);- at least a second heating device (20) is interposed between said roughing stand (23) and said rolling train (25) and is provided with a second heating tunnel (19); and- wherein a rapid heating device (28), consisting of selectively activatable elements, is interposed between said at least one pre-finishing stand (26) and said plurality of finishing stands (31).

11. Rolling method as in claim 9, characterized in that said first heating device(16) comprises a heating furnace (41) which heats said conventional slabs (50), and in that said second tunnel (19) is suitable to perform a thermal homogenization of said intermediate rolled product (51) obtained from said conventional slabs (50).

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