FIELD OF THE INVENTION

The present invention concerns a method and a plant for producing flat rolled products such as, for example, but not limited to, metal strip wound to form rolls or coils.

BACKGROUND OF THE INVENTION

Rolling plants are known which have at least one rolling mill disposed in line downstream of a continuous casting machine which produces thin slabs, the so- called “thin slab caster”, for producing metal strip wound to form rolls or coils.

One example of this type of rolling plant is described in the patent applications WO 2011 /141790 and WO 2102/140531 , in the name of the present Applicant, and comprises a casting machine downstream of which a plurality of roughing stands, or roughing mill, and a plurality of finishing stands, or finishing mill, are disposed, between which a rapid heating device is interposed, typically an induction furnace consisting of several inductor modules.

Downstream of the finishing stands there is an outlet table, also called run-out table, provided with cooling showers, and finally two winding reels which wind the strip to produce a coil.

In order for the rolling in the finishing mill to take place in the austenitic range, that is, without phase transformations in the structure of the metal, the strip has to exit the last finishing stand of the rolling mill at a temperature not lower than about 850 °C.

Therefore, the rolling mass flow in the finishing stands has to be set to obtain a strip having said optimum temperature of at least about 850 °C at the outlet of the last stand of the rolling mill.

The mass flow is the product of the thickness of the strip by its speed; in casting the mass flow is typically expressed by the unit of measurement mm*m/min, while in rolling it is typically expressed by the unit of measurement mm*m/s.

WO ’790 also teaches how to design and configure such plants so that they can be made to work in “coil-to-coil” mode, or in “semi-endless” mode, or in “endless” mode. In particular, the mode in which the rolling process is carried out is selected from said three modes based on the quality of steel produced, on the maximum casting speed possible for said quality of steel, on the final thickness of the strip and on the cost of production.

The characteristics of the three rolling modes mentioned above are summarized below.

Endless: the cast slab feeds the rolling mill directly and continuously, so the so- called rolling “mass flow” has to be equal to the casting mass flow. When operating, there are no entries into the rolling stands and, therefore, the endless mode is optimal for producing ultra-thin thicknesses from 0.7 mm to 1.5 mm, since it reduces wear on the rolls and the risk of blockage, allowing stationary rolling. In addition, in the endless mode, apart from the first strip produced, the strip head is not transported between the last finishing stand of the rolling mill and the winding reel. This contributes to increasing the stability of the process. However, the endless mode cannot be used for some grades of steel which have to be cast at very low speeds. Furthermore, since in this mode the rolling mill has to adapt to the casting mass flow, the temperature of the strip at the outlet of the last finishing stand depends on the casting mass flow and on the thermal contribution given by the rapid heating device.

Therefore, when the maximum casting mass flow is defined, the final temperature of the strip is controlled only by acting on the power supplied by the rapid heating device. Moreover, in endless rolling the final speed of the strip is linked to the casting mass flow and to the final thickness of the strip.

Coil-to-coil and semi-endless: there is no continuity between the casting and the rolling mill, and each slab is formed at exit from the casting device by means of shearing by a pendular shear, so that the rolling mass flow is different from the casting mass flow. In coil-to-coil mode, the weight of each individual slab is equivalent to the weight of a single coil, while in semi-endless mode, the weight of each super slab is equivalent to the weight of “n” coils which are defined by the fast shear located in front of the winding reels. These modes allow to produce the entire range of steels castable with a thin slab caster. On the contrary, for each slab (or super slab) rolled there is an entry into the rolling mill, and this complicates the production of strip with a final thickness below 1.5 mm for the coil-to-coil mode, or below 1.2 mm for the semi-endless mode, due to the difficulty of the strip to enter into the last finishing stands because it is very thin. In the coil-to-coil and semi-endless modes, to obtain the correct final temperature of the rolled product it is possible to act on the value of the rolling mass flow, which is normally greater than 2.0 - 3.0 times the casting mass flow. Therefore, once the final thickness of the strip has been defined, the mass flow of the rolling mill can be varied by acting on the rolling speed, so that the rapid heating device is normally kept switched off.

In the coil-to-coil and semi-endless modes, it is necessary to limit the maximum speed of the strip exiting from the last finishing stand in order to prevent the strip head from rising dangerously, on the path that goes from the last finishing stand to the winding reel, because of aerodynamic type effects due to speed.

Typically, the maximum speed of the strip head is limited to about 11 - 12 m/s.

Due to this speed limitation, it may happen that, especially for thin strip, the optimum temperature of at least 850 °C at the outlet of the last finishing stand cannot be reached.

To overcome this, after the head enters the winding reel, the so-called “speedup” of the rolling stands of the finishing mill is carried out, in order to make the strip transit faster and thus reduce temperature losses, allowing the body and tail of the strip to exit from the last rolling stand at the target temperature of at least about 850 °C.

The “speed-up” consists in increasing the rotation speed of the rolls of the stands of the rolling mill, and consequently the rolling speed of the strip, after its head has been wound on the winding reel, up to the speed value at which a rolling mass flow is obtained which is sufficient to obtain said optimum temperature at the outlet of the last finishing stand.

This increase in speed is on average 40% - 50% and, at times, can even reach 80%.

Furthermore, this increase in speed implies that the head of the strip is rolled at a first speed (for example 12 m/s) while the body and tail of the strip are rolled at a second speed (for example 18 m/s), higher than the first speed. This variation in the rolling speed of a single strip generates a transient in the rolling process which disturbs the control of both the geometric parameters of the strip, mainly “crown”, flatness and thickness, and also the winding temperature on the reel.

Therefore, if the head is rolled at the set limit speed of about 12 m/s, when on the contrary the mass flow necessary to obtain the optimal exit temperature entailed a higher one, the head is produced at a lower temperature than the optimal one.

This leads to unwanted phase changes of the steel and implies that the mechanical and geometric properties of the metal strip are not uniform along the length development of the final coil.

In addition, the “speed up” requires equipping the stands of the roughing and finishing rolling mill, and also the winding reels, with large-sized electric motors.

These large-sized electric motors are oversized for the functioning of the plant in endless mode, and therefore, the utilization factor of the plant is reduced against a high initial investment (CAPEX).

Moreover, since with the “speed up” the tail of the strip exits at a higher speed than the head, the dynamic control of the cooling showers is necessary in order to ensure that the strip is wound onto the winding reel at a temperature of about 550 - 600 °C, almost constant along its entire length. This also imposes a considerable length of the run-off table since the speed of the strip requires a longer cooling segment.

Therefore, a first purpose of the present invention is to provide a plant and to perfect an improved method for producing flat rolled products which allow to avoid, or at least greatly reduce, the “speed-up” of the rolling stands, in a plant which can operate in all three of the above operating modes, always keeping the temperature of the strip at the optimum value at the outlet of the last finishing stand of the rolling mill.

Another purpose of the invention is to provide a plant for producing flat rolled products which is more compact, and which has a lower construction cost compared with plants known in the state of the art.

Another purpose of the present invention is to provide a plant and to perfect a method for producing flat rolled products which allow to carry out a rolling process in coil-to-coil, or semi-endless or endless mode, always obtaining a high utilization factor of the plant.

Another purpose of the present invention is to provide a plant and to perfect a method for producing flat rolled products which allow to produce thin and ultrathin metal strip, even when the casting conditions do not guarantee an ideal mass flow for operating in endless mode. Another purpose of the present invention is to provide a plant and to perfect a method for producing flat rolled products which allow to improve the final quality of the strip produced both in terms of sizes and also of mechanical characteristics.

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.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims. The dependent claims describe evolved and perfected aspects with respect to the independent claims.

In accordance with the above purposes, a rolling method according to the present invention to produce a metal strip having a final thickness comprised between 0.6 mm and 25 mm is executed in an operating mode selected from a group comprising coil-to-coil mode, semi-endless mode and endless mode, and in a rolling plant which comprises at least:

- a continuous casting device configured to produce thin slabs, having an initial thickness comprised between 50 mm and 160 mm;

- a furnace configured to maintain the slabs at a certain temperature and/or to heat the slabs;

- at least one roughing stand configured to reduce the thickness of at least one of the slabs in order to produce an intermediate rolled product, and a plurality of finishing stands configured to reduce the thickness of the intermediate rolled product in order to obtain the metal strip;

- a rapid heating device, consisting of selectively activatable elements, which is interposed between the at least one roughing stand and the plurality of finishing stands and is configured to heat the intermediate rolled product;

In accordance with one aspect of the present invention, the rapid heating device is always active at least in the coil-to-coil or semi-endless mode to produce a strip having a final thickness smaller than 4.0 mm, preferably smaller than 2.5 mm, to heat the intermediate rolled product so that the temperature of the metal strip in correspondence with the outlet of the last finishing stand is comprised between 830 °C and 860 °C.

In accordance with another aspect of the present invention, if the operating mode is the coil-to-coil mode or the semi-endless mode, the rolling speed of the metal strip in correspondence with the plurality of finishing stands is lower than or equal to about 12 m/s at exit from the last rolling stand.

In accordance with another aspect of the present invention, the rolling speed in the finishing stands is substantially constant.

In accordance with another aspect of the present invention, for the same final thickness, the rolling speed in the finishing stands is substantially the same between the coil-to-coil mode and the semi-endless mode.

In accordance with another aspect of the present invention, the rolling speed is adjusted to an optimal value corresponding to the speed that minimizes the operating costs of the plant, which are calculated as follows:

COPEX(V _L , SF) = CHI(VL, SF) + CRI(VL, SF) wherein CHI(VL, SF) is the operating cost for the electrical power supply of the rapid heating device and CRI(VL, SF) is the operating cost for the increasing risk of blockage of the metal strip resulting from the increase in the rolling speed and the poor quality of the strip that would be achieved without reaching the optimal temperature comprised between about 830 °C and about 860 °C in correspondence with the outlet of the last finishing stand.

In accordance with another aspect of the present invention, a rolling plant to produce a metal strip having a final thickness comprised between 0.6 mm and 25 mm and configured to operate in an operating mode selected in a group comprising coil-to-coil mode, semi-endless mode and endless mode, comprises at least:

- a continuous casting device configured to produce preferably thin slabs, having an initial thickness comprised between 50 mm and 160 mm;

- a furnace configured to maintain the slabs at a certain temperature and/or to heat the slabs;

- a first selectively activatable cutting device configured to cut the slabs produced by the continuous casting device;

- at least one roughing stand configured to reduce the thickness of at least one of the slabs in order to produce an intermediate rolled product, and a plurality of finishing stands configured to reduce the thickness of the intermediate rolled product in order to obtain the metal strip;

- a rapid heating device, consisting of selectively activatable elements and interposed between the at least one roughing stand and the plurality of finishing stands, which is configured to heat the intermediate rolled product;

- a second selectively activatable cutting device configured to cut the metal strip according to a predefined length;

In accordance with another aspect of the present invention, the plant also comprises a management and control unit configured at least to command the functioning of the cutting devices so as to define the operating mode, and also to always keep the rapid heating device active in the coil-to-coil or semi-endless mode in order to produce a strip having a final thickness smaller than about 4.0 mm, preferably smaller than about 2.5 mm.

In accordance with another aspect of the present invention, the management and control unit is also configured to set the rolling speed of the plurality of finishing stands to a value lower than about 12 m/s in correspondence with the last finishing stand in the coil-to-coil or semi-endless operating mode.

DESCRIPTION OF THE DRAWINGS

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 an embodiment of a plant for producing flat rolled products, in accordance with the present invention;

- fig. 2 is a graph which relates, for a defined mass flow, the final thickness of a flat rolled product and the rolling speed required for it;

- fig. 3 is a graph which relates, for a defined final thickness of a strip produced, the operating costs, relating to a coil produced, in a plant according to the present invention, and the rolling speed;

- fig. 4 is a graph which relates the optimal rolling speed in accordance with the method of the present invention and the final thickness of a flat rolled product.

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

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. DESCRIPTION OF SOME EMBODIMENTS OF THE PRESENT INVENTION

With reference to fig. 1, this shows a plant 10 in accordance with the present invention for rolling flat rolled products, such as, for example, metal strips P having a final thickness SF comprised between 0.6 mm and 25 mm.

The plant 10 is configured to execute a rolling method in an operating mode chosen from “coil-to-coil”, “semi-endless” and “endless”, and it comprises a continuous casting machine 11 having a mold 12 to produce a preferably thin slab, that is, having a thickness comprised between 50 mm and 160 mm.

The plant 10 comprises a first water scaling device 14 and a first cutting device, in this case a pendular shear 15 which, in the coil-to-coil operating mode, cuts slab segments of such length as to obtain a roll, or coil, of a desired weight, for example 25 tons. On the other hand, in the semi-endless operating mode, the pendular shear 15 cuts slab segments (super slab) having a weight between 2 and 5 times greater than the slab of the coil-to-coil mode. In the steady state conditions of the endless operating mode, the pendular shear 15 does not make any cut of the slab coming from the mold 12.

Downstream of the pendular shear 15 there is disposed a tunnel furnace 16 into which the slab, or the slab segments, are introduced to restore or maintain their temperature.

In the example given here, the tunnel furnace 16 comprises a penultimate module 17 of the laterally mobile type with shuttle function to allow the use of a second casting line, parallel to the first, which shares the same rolling mill. This module 17 can also possibly be used to temporarily house a plurality of slab segments, for example in the case of blockages and/or replacement of the rolling cylinders.

Furthermore, the tunnel furnace 16 comprises a last module 18 which can instead have a parking function, in the event of interruption of the line for the same reasons indicated above.

Downstream of the tunnel furnace 16 there are disposed, in sequence, an oxycutting device 19, a second water scaling device 20, a vertical or edging stand 21 and a third scaling device 23, which are of a known type, and which will not be described in detail.

The plant 10 also comprises a rolling mill comprising at least one roughing stand 25 to reduce the thickness of the slab and produce an intermediate rolled product, and a plurality of finishing stands 31 to further reduce the thickness of the intermediate rolled product in order to produce the metal strip P.

In the example solution shown, downstream of the one or more roughing stands 25 there is disposed a crop shear 26 to trim the heads and tails of the intermediate rolled product in order to facilitate its entry into the finishing stands 31.

The plant 10 also comprises a rapid heating device 28 interposed between the roughing stands 25 and the finishing stands 31 and comprising, for example, an induction furnace consisting of selectively activatable elements.

Furthermore, preferably, the rapid heating device 28 is disposed downstream of the crop shear 26.

Downstream of the rapid heating device 28 there are disposed, in succession, a fourth water scaling device 29 and the plurality of finishing stands 31.

The fourth scaling device 29 has the function of cleaning the surface of the intermediate rolled product from the scale which, during use, forms at the outlet of the rapid heating device 28.

Downstream of the finishing stands 31 there is disposed a cooling device to cool the strip P, comprising a plurality of showers 34 at the outlets of which there are disposed in sequence a final cutting device, in this case a flying shear 35, and two winding reels 36, 38.

The flying shear 35 is used only in the “semi-endless” and “endless” operating modes, and it cuts the strip P at a given length in order to obtain the desired final weight of the coil.

In accordance with one aspect of the present invention, the rapid heating device 28 is configured to heat the intermediate rolled product selectively and in an adjustable manner, in the coil-to-coil mode as well as in the semi-endless mode and in the endless mode.

The temperature to which the intermediate rolled product is heated is selected, amongst other parameters, at least as a function of the thickness of the starting slab and the final thickness SF of the strip P, in order to reach the most suitable value for the rolling and to ensure that the strip P has an optimal temperature comprised between about 830 °C and about 860 °C, preferably about 850 °C, at the outlet of the last finishing stand 31.

This allows to reduce the value of the rolling mass flow MFL required in the coil-to-coil or semi-endless operating mode to obtain said optimal temperature at the outlet of the last finishing stand 31.

By way of a purely non-limiting example, we refer to the graph in fig. 2, in which the final thickness SF of the strip P is shown on the abscissa, and the rolling speed in the coil-to-coil or semi-endless operating mode is shown on the ordinate. Curve A represents the trend of the rolling mass flow MFL required in the coil-to- coil or semi-endless mode, as a function of the final thickness SF, without the heat input of the rapid heating device 28, while curve B represents the trend of the rolling mass flow MFL required in the coil-to-coil or semi-endless mode, as a function of the final thickness SF, with the heat input of the rapid heating device 28.

The reduction of the rolling mass flow MFL in the coil-to-coil or semi-endless operating mode reduces the maximum rolling speed required, as a whole, from the plant 10 in order to obtain a strip P having the optimal end-of-rolling temperature. This allows to avoid, or at least greatly reduce, the so-called “speed up” during rolling in the coil-to-coil or semi-endless mode and to equip the rolling stands 25, 31 and the winding reels 36, 38 with electric motors of smaller sizes than known plants, thus reducing the overall cost of the plant 10 (CAPEX).

The plant 10 also comprises a management and control unit 40, of the electronic and programmable type, connected to the roughing stands 25 and to the finishing stands 31 and configured to set the rolling speed VL with which they operate.

In particular, the management and control unit 40 is configured to limit the rolling speed to a maximum value of about 12 m/s in the coil-to-coil and semiendless operating modes.

Furthermore, the management and control unit 40 is also connected to the pendular shear 15 and to the flying shear 35, and is configured to manage their operation so as to define and select the operating mode of the plant 10.

In particular, the management and control unit 40 is configured to drive the pendular shear 15 when it is necessary to cut the slab at exit from the continuous casting machine 11.

Furthermore, the management and control unit 40 is configured to drive the flying shear 35 when it is necessary to cut the strip P before it reaches one or the other winding reel 36, 38.

For example, in the coil-to-coil operating mode, the management and control unit 40 activates the pendular shear 15 to produce slabs having a weight corresponding to that of a coil to be produced, and it keeps the flying shear 35 deactivated.

Or, in the semi-endless operating mode, the management and control unit 40 activates the pendular shear 15 to produce slabs having a weight corresponding to that of “n” coils, and it activates the flying shear 35 to cut the strip P at a predetermined length corresponding to a single coil.

Or again, in the endless operating mode, the management and control unit 40 keeps the pendular shear 15 deactivated and activates the flying shear 35 to cut the strip P to a predetermined length corresponding to a single coil.

The management and control unit 40 is also configured to keep the rapid heating device 28 always active in the coil-to-coil or semi-endless mode, in order to produce a strip having a final thickness SF smaller than about 4.0 mm, preferably smaller than about 2.5 mm, and to modulate the thermal power delivered, and therefore the heating of the intermediate rolled product, for example by activating one or more of the selectively activatable elements which constitute it.

We must specify that in the present description and in the attached claims, the expression “always active” means that the rapid heating device 28 is active when at least one portion of the intermediate rolled product being rolled transits in correspondence with the rapid heating device 28, while it can be inactive, or deactivated, between the passing of one intermediate rolled product and another subsequent one.

Preferably, the rapid heating device 28 is active throughout the entire passing of the intermediate rolled product in correspondence with the rapid heating device 28.

Specifically, the management and control unit 40 is configured to keep active at least one of the selectively activatable elements that constitute the rapid heating device 28, in each operating mode, so as to obtain a strip P at a temperature comprised between about 830 °C and about 860 °C, preferably about 850 °C, in correspondence with the outlet of the last finishing stand 31.

The present invention also concerns a method to produce a strip P which provides to keep the rapid heating device 28 active, even when the rolling occurs in coil-to-coil mode or in semi-endless mode, in order to produce a strip P having a final thickness SF smaller than about 4.0 mm, preferably smaller than about 2.5 mm.

By doing so, for the same final thickness SF, the heat input provided to the intermediate rolled product by the rapid heating device 28 reduces the rolling mass flow MFL required in the coil-to-coil or semi-endless operating mode, allowing at the same time to reach the target temperature of the strip P, comprised between about 830 °C and about 860 °C, preferably of about 850 °C, in correspondence with the exit from the last finishing stand 31.

The reduction of the rolling mass flow MFL required in the coil-to-coil or semiendless operating mode allows to perform the rolling with a reduced rolling speed VL, preferably lower than 12 m/s, and at the same time to reach the optimal temperature of about 850 °C at the outlet of the last finishing stand 31 also for the head of the strip P, so as to prevent, or at least reduce, the so-called “speed up”.

More particularly, in the coil-to-coil mode, the method of the present invention allows, thanks to the activation of the rapid heating device 28, to immediately reach the optimal end-of-rolling temperature of about 850 °C for the head of the strip P without performing the speed up, for final thickness SF values larger than about 2.0 mm, preferably larger than about 1.5 mm, and to limit the speed-up to a value of about 15% - 25% for final thickness SF values comprised between 0.6 mm and 1.5 mm. This modest speed up is conveniently applied in order to simultaneously reduce the heat input required from the heating device 28 while always maintaining the optimal end-of-rolling temperature of about 850 °C constant.

Furthermore, the increase of the temperature of the intermediate rolled product, achieved thanks to the rapid heating device 28, allows the finishing stands 31 to carry out greater reductions in thickness and therefore produce strips P with a thin thickness, even smaller than 1.2 mm, with coil-to-coil mode or semi-endless mode.

This advantageously allows to expand the range of thin thicknesses that can be achieved in the coil-to-coil or semi-endless mode for steels that cannot be cast in endless mode, that is, for those steels with a greater hardness, such as, for example, HSLA type steels.

The reduction of the rolling speed on the one hand also facilitates the entry of the intermediate rolled product into the finishing stands 31, and on the other hand it also facilitates the transfer of the strip P to the winding reels 36, 38.

In addition, for the same final thickness SF, the rolling speed in the finishing stands 31 is substantially the same for each of said operating modes.

The absence of variation of the rolling speed, or at least its reduction, in the coil- to-coil or semi-endless mode allows both to keep the temperature of the strip P 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 final use of the strip P.

Another advantage consists in the fact that performing the rolling at an almost constant rolling speed in the coil-to-coil or semi-endless mode also allows a high degree of control both of the final shape of the strip P, for example of the crown and flatness thereof, which will therefore be advantageously uniform along the entire length of the coil, and also of the mechanical properties of the strip P which will be advantageously constant and uniform along the entire length of the coil.

This last advantage is of considerable importance, above all for the quality production of those steels that cannot be cast in endless mode, that is, for those steels with a greater hardness, such as, for example, HSLA type steels.

Finally, the reduction of the rolling speed allows to reduce the number of showers 34 present in the cooling device, thus allowing to reduce the length thereof by about 20 - 30% compared to the plants of the prior art.

In accordance with another aspect of the present invention, when the rolling occurs in the coil-to-coil or semi-endless mode, the method also provides to set the rolling speed to an optimal value VL-OPT.

In the example given here, the optimal rolling speed VL-OPT is the rolling speed which, having set the final thickness SF, minimizes the function: wherein CHI(VL, SF) is the operating cost for the electrical power supply of the rapid heating device 28, and CRI(VL, SF) is the operating cost, linked to the rolling speed VL, represented by the growing risk of blockage resulting from the increase in the rolling speed and the low quality of the strip P which would be obtained if the optimal end-of-rolling temperature were not reached.

For example, fig. 3 shows a graph in which the rolling speed VL is reported on the abscissa and the value of the function COPEX(VL, SF) is reported on the ordinate, for a certain final thickness SF of the strip P, for example corresponding to 1.2mm. In particular, curve C represents the function CHI(VL, SF), curve D represents the function CRI(VL, SF) and curve E represents the function COPEX(VL, SF).

It is evident that in correspondence with the speed VL-OPT, the function COPEX(VL, SF) assumes its minimum value, in this case around 10 m/s.

The optimal rolling speed VL-OPT is therefore comprised between the rolling speed VLE (for example of about 6 m/s) which would be achieved with the rapid heating device 28 at maximum power (that is, activating all the selectively activable elements and within the limit of the maximum permissible temperature at exit from the rapid heating device 28) and the rolling speed VLC (for example of about 14 m/s) which would be achieved without using the rapid heating device 28, reaching the optimal end-of-rolling temperature for the P strip for both speeds VLE and VLC.

As can be seen, below VLE the costs CRI(VL, SF) increase since, despite the rapid heating device 28 being at maximum power, the strip P is produced with an end- of-rolling temperature lower than the optimal one and therefore with a low quality.

Furthermore, below VLE the costs CHI(VL, SF) also increase, since the strip advances at a reduced speed and therefore, with the same power delivered, the specific consumption per ton produced (kWh/ton) of the rapid heating device 28 increases.

On the other hand, above VLC the costs CHI(VL, SF) are zeroed, since the rapid heating device 28 is inactive, while the costs CRI(VL, SF) increase exponentially since the risk of blockage is ever higher.

In addition, the graph of fig. 4 shows, by way of a non-limiting example, the relation between the final thickness SF of the strip P and the optimal rolling speed VL-OPT in the coil-to-coil or semi-endless mode. In particular, the values of the optimal rolling speed VL-OPT are shown on the ordinate and the values of the final thickness SF are shown on the abscissa.

Specifically, curve F represents the value of the rolling speed required in the coil-to-coil or semi-endless rolling mode to obtain a strip P having a temperature of about 850 °C in correspondence with the last finishing stand 31, without the heat input of the rapid heating device 28. Please note that a rolling speed higher than about 14 m/s, that is, higher than the speed VLC of the graph of fig. 3, would correspond to strips P having a final thickness SF smaller than about 1.2 mm.

In addition, curve G represents the value of the optimal rolling speed VL-OPT required in the coil-to-coil or semi-endless rolling mode to obtain a strip P, having a final thickness SF comprised between about 0.6 mm and about 2.5 mm, having a temperature of about 850 °C in correspondence with the last finishing stand 31, with the heat input of the rapid heating device 28. Please note that in these cases the optimal rolling speed VL-OPT is always lower than the maximum value of 12 m/s set so that the head of the strip P does not rise due to aerodynamic effects.

Furthermore, it is also evident that, for strips P having a final thickness SF smaller than about 2.5 mm, the optimal rolling speed VL-OPT with the heat input of the rapid heating device 28 is always lower than the rolling speed required without using the rapid heating device 28.

Furthermore, as shown by curve H, for strips P having a final thickness SF comprised between about 0.6 mm and about 1.4 mm, a modest speed up can be conveniently carried out, limited to a value of about 15% - 25%, in order to simultaneously reduce the energy consumption required by the rapid heating device 28 while always keeping the optimal end-of-rolling temperature of about 850 °C constant.

For strips P having a final thickness SF larger than 2.5 mm, the optimal rolling speed VL-OPT corresponds to the rolling speed required in the coil-to-coil or semiendless rolling mode to obtain a strip P having a temperature of about 850 °C in correspondence with the last finishing stand 31, without the heat input of the rapid heating device 28.

This allows to identify the optimal rolling speed VL-OPT with which it is possible to roll a strip P having a certain final thickness SF, in coil-to-coil and/or semiendless mode, keeping operating costs to a minimum.

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

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 same claims.