DESCRIPTION

ROLLING METHOD AND PLANT

Technical field

The present invention relates to a rolling method according to the characteristics of the pre-characterizing part of claim 1.

The present invention relates to a rolling mill according to the characteristics of the pre- characterizing part of claim 19.

Definitions

In the present description and in the appended claims the following terms must be understood according to the definitions given in the following.

In the present description by the term "metallic product" one means a generic product of metallic material to be processed in a hot rolling mill or being processed inside a hot rolling mill to obtain a finished product.

As it is meant in the present description, the metallic product to be processed can indifferently be a bloom, a billet or a slab.

As it is meant in the present description, the finished product, that is to say, the product coming out of the rolling mill, can indifferently be a long finished product or a flat finished product.

By long product or long finished product one means a product whose development in length has much greater dimensions with respect to the section size of the product itself measured on a section taken on an orthogonal plane with respect to the straight line defining its development in length. Although in the terminology usually employed in the specific sector of rolling a distinction is made between "bar", "wire", "rod" depending on the diameter, or, in general, on section size of the product of metallic material, in the present description the term "long product" is meant to comprise all the products usually identified by the terms "bar", "wire", "rod".

By flat product or flat finished product one means a product having at least two reciprocally opposite flat surfaces, such as metal sheets and strips. In general, for the purposes of the present description, by finished product one means to comprise all the metallic products obtained through a hot rolling process which can be wound in the form of bobbins or coils, or cut with a set length, packed, and made available for final use or for following working processes.

By the expression "profile" of a finished product one indicates the shape of the product along one of its sections orthogonal to its development in length. By the expression "profile" one means to indicate all the possible shapes of the rolled products, such as circular, oval, elliptical, quadrangular, square, hexagonal, flat, strip or sheet, "L", "C", "H" shapes, etc. It will be evident, in the light of the following description, that the present invention is applicable to a profile corresponding to a generic section, with minimum corrections, which will be obvious to a person skilled in the art. The term "finished product" is meant to also comprise different section shapes as in the mentioned examples or other shapes suitable to be obtained by rolling.

By the expression "rolling" one means hot rolling processes.

By the expression "rolled product" or "rolled metallic product" one means a product resulting from a hot rolling process.

The expressions roller/rollers and/or cylinder/cylinders must be understood as substantially equivalent, as cylindrical-shaped rotating elements intended for the mechanical working of the rolled product, which mechanically work the rolled product. The rolled product is fed through a pair of rollers or rotating cylinders to be subjected to the mechanical deformation intended to progressively reduce the thickness of the rolled product by means of the successive passage in one or more rolling stations. By the expression rollers or cylinders one means to comprise both rollers and cylinders having an essentially flat shape in which the rolling of flat products occurs, both rollers and cylinders in which a first cylinder is provided with at least one first semi-channel and a second cylinder is provided with at least one second semi-channel, the side-by-side arrangement of the cylinders implying the side-by-side arrangement of the at least one first semi-channel with the at least one second semi-channel whose assembly constitutes at least one rolling channel intended for the passage of the rolled product to be processed between said cylinders.

Prior art

In the field of rolling mills, the conventional rolling method provides that the product to be rolled is brought to a temperature between 1050 and 1150°C before entering the rolling mill. The section of the product to be rolled is made as uniform as possible along its section in terms of temperature and of microstructure. Reheating and/or maintenance furnaces are used to make the temperature as uniform as possible along the entire section of the product to be subjected to the hot rolling process.

Patent Application EP 0 761 327 in the name of Danieli & C. Officine Meccaniche describes a continuous casting method of long products such as billets or blooms, in which the solidification of the cast product is completed in a position downstream of the outlet of the mould and in which the cast product comes out of the continuous casting machine at a speed of at least 4 m/min, the cast product being transferred into the horizontal segment with at least 12% of the section consisting of the liquid core without being cut to size, the liquid core being solidified before the cast product is sent to a temperature maintenance and pre-heating system. The cast product is sent to a roll train without discontinuity and/or interruption of the process, the casting speed being set at a value at least higher than the critical speed of the rollers of the roll train. Before the roll train the plant comprises a fast heating system for the equalization of the temperature of the product before it enters the rolling mill. Therefore, the method comprises a phase in which the temperature of the core and the temperature of the surface of the product are made uniform and homogenous. In particular, the heating and/or equalization system comprises an induction furnace in which the temperature of the cast product is considerably increased. The induction furnace features operating parameters, such as power, operating frequency and length, which are such as to ensure the reaching of a homogenous and uniform temperature in any type of product and in any working condition which may occur. The reaching of an optimally homogenous and uniform temperature in the whole cross section of the product allows to solve problems of elongation, bending and deformation which may occur during rolling due to a prominent lack of temperature homogeneity. The temperature of the surface of the cast product is brought to a high value and it is thus necessary to determine an optimal distance between the outlet of the induction furnace and the inlet of the first rolling stand so that in this segment of optimal distance the core of the product can have the time to be further heated while the surface of the product can have the time to be cooled. In this way the product enters the first rolling stand with a substantially uniform and homogenous temperature at a value which can be determined on the basis of the optimal rolling parameters.

Patent Application US 2011 272116 in the name of Danieli & C. Officine Meccaniche describes a rolling method in a rolling line of slabs with a thickness from 30 mm to 140 mm wherein the rolling process allows to obtain a strip with a thickness ranging from 0.7 mm to 20. The plant comprises a continuous casting machine, a tunnel furnace for the maintenance/equalization and possible heating of the product downstream of the continuous casting machine and upstream of a roll train. The plant comprises a fast induction heating unit, with heating elements which are selectively activable and which are interposed between a roughing section of the roll train and a finishing section of the roll train. The fast heating unit is configured in its heating and sizing parameters in such a way that the cast slab reaches the last rolling stand of the finishing section of the roll train with a temperature not lower than 830-850°C. According to the invention, the positioning of the fast heating unit, for example the inductor, inside the rolling line, is determined in such a way as to optimize the use of energy to heat the product and taking into consideration the maximum heating capacity of the specific fast heating unit. Therefore, the invention allows to identify the best position of the fast heating unit inside the roll train depending on the range of initial and final thicknesses of the product and depending on the feeding speed of the strip. The aim of the fast heating unit consists in bringing the temperature of the sheet to the value, which is most suitable for rolling. Patent Application EP 1 187 687 in the name of Danieli Technology Inc. describes an integrated plant for the production of long metallic products, such as bars, in which the product is initially cast in a continuous casting machine and sent to a rolling mill, which is arranged in line with the continuous casting machine. Between the continuous casting machine and the rolling mill there are a tunnel furnace for the heating, equalization and/or maintenance of the product at a pre-set rolling temperature and descaling unit, which removes the scale from the product before the product enters the rolling mill. The tunnel furnace has a length of about 100 m and is designed to realize a gradual heating profile of the bar in such a way as to heat the bar. The aim of the tunnel furnace consists in equalizing the temperature of the bar to bring it to a correct rolling temperature before it enters the rolling mill. The tunnel furnace can be heated by means of open flame burners, radiant pipes, induction heaters, or a combination thereof. In the space between the continuous casting machine and the rolling mill an equalization of the temperature of the product to be rolled is performed in such a way as to obtain the temperature necessary for the rolling process. The temperature of the product obtained by means of the tunnel furnace is maintained for a time sufficient to ensure an improvement of quality from the metallurgic point of view, but not too long to prevent an excessive decarburization of the product inside the oven. Thanks to temperature equalization, the solution allows to prevent an excessive reduction in temperature before the entrance in the tunnel furnace, it thus not being necessary to increase the temperature of the surface of the product up to a value which is much higher than that of the internal temperature. Patent Application EP1909980 in the name of Arvedi Giovanni describes a manufacturing process of long steel products which begins with a casting phase of a product in a continuous casting machine with the reduction of the liquid core, followed by a heating phase by means of an induction heater to then proceed with a rolling phase without interruption of the process, performing one single process which begins in the continuous casting machine and which ends at the outlet of the rolling mill. The blooms or billets subjected to this process have an initial thickness between 120 and 400 mm and have an average temperature which in the cross section is higher than the surface temperature, the temperature of the core or internal central region of the bloom or billet being 100°C higher with respect to the temperature of the surface of the bloom or billet, which is of about 1200°C. By means of the induction furnace, the average temperature of the product is higher than the surface temperature with a temperature at least 100°C higher in the core of the product with respect to the external surface, where the temperature is of about 1200°C or lower.

Patent Application WO 00/71272 in the name of Danieli Technology Inc. describes a solution which aims at solving the problems of the solution described in the prior Patent Application EP 0 761 327 and in particular which aims at solving drawbacks due to the presence of an induction furnace before the rolling plant to increase the surface temperature of the billet and then carry out a temperature equalization phase. In the solution described in WO 00/71272 a tunnel furnace is used in which a gradual heating of the billet is carried out. The tunnel furnace has the function of heating and equalizing the temperature of the billet to bring it to the correct rolling temperature before it enters the rolling mill. The tunnel furnace is placed before a roughing section of the rolling mill and operates in order to prevent a rise in the temperature of the surface of the product up to a value much higher than that of the internal temperature of the product, thus avoiding waiting for a successive equalization process as on the other hand occurs in the solution described in the prior Patent Application EP 0 761 327.

Patent application DE 10 2006 002505 describes a rolling method of products in the form of strips in which a uniform heating is to be achieved in correspondence of the exit of a finishing rolling mill in order to address the fact that the side edges of the entering material are typically exposed as soon as the material comes out of the casting mould and in this respect act as heat radiating surfaces and the fact that the entire amount of cooling water that is used for example during a descaling phase on the broad surface of the entering material runs down along the edges so that the edges experience an additional strong cooling. Therefore, DE 10 2006 002505 discloses that before the entry in the finishing mill, the entering material is typically cooled more at its edges with respect to its centre while it is desired that, as far as possible, the material is uniformly heated throughout - also transversely with respect to the direction of the material flow - no later than when it leaves the finishing mill. In order to achieve this aim the solution as in DE 10 2006 002505 provides the positioning of the heating devices of the material within the finishing mill with an orientation directed towards the edges of the strip and proposes that they are controlled by means of a computerised model in such a way that the overcooled edges of the entering material are heated more with respect to the centre of the material in such a way that the material is uniformly heated, even in the transverse direction, no later than when it leaves the finishing mill. In DE 10 2006 002505 it is further stated that the process has the advantage that the temperature level and the temperature variations of the material inside the finishing mill are kept small having as a result only moderate temperature increases and overall relatively low temperatures in the entire finishing mill. In DE 10 2006 002505 it is further stated that the induction heaters are used in order to maintain a homogenous temperature in the product because in this way one is able to maintain the temperature range in the finishing mill at a constant level so that, therefore, an almost-isothermal rolling of the material occurs.

Patent application DE 10 2011 006357 describes a solution of a rolling mill of products in the form of strips in which the use of a loop lifter is provided in which the loop lifter is placed within the volume defined by a rolling stand. Furthermore, patent application DE 10 2011 006357 describes the possibility of using induction heating means between two consecutive rolling stands to adjust the rolling temperature.

Patent application EP 1 452 247 describes a processing method of a metallic product which occurs downstream with respect to a rolling mill of strips, the processing method providing consecutive uniform heating phases of the product and cooling of the latter with interposed rectification phases of the product itself.

Problems of the prior art

The prior art solutions have the drawback of rolling the product at a temperature as homogenous and uniform as possible between the surface and the core in such a way that the product has a temperature as homogenous and uniform as possible along the section of the product when it enters one of the rolling stations of the rolling mill. The solution disclosed in EP 1 909 980, on the other hand, provides that the temperature of the core is raised to a temperature at least 100°C higher than the surface temperature. Such types of solutions have limits concerning the rolling stress which is high with the consequence that other correlated aspects have negative or undesired characteristics such as the presence of a high force of separation between the rolling cylinders exerted by the product, limits concerning the admissible angle of entrance for the product entering the rolls, limits concerning the useful life of the rolling cylinders, as well as considerable limits linked to the surface quality of the rolled product.

Furthermore, the rolling cylinders are subject, as a consequence, to a fast progressive wear of the rolling channels, which implies the frequent replacement of the rollers themselves in order to maintain suitable quality and size standards of the finished product coming out of the rolling mill.

Furthermore, due to the great rolling stress present, it is also necessary to provide specific limits with reference to the mechanical reduction ratios which one can use.

Furthermore, due to the great rolling stress present, it is also necessary to provide the resort to motors and drives with powers suitable to overcome such rolling stress.

Furthermore, the prior art solutions in which the product to be rolled is heated until the moment in which it enters the rolling mill have the drawback that, as the product is processed, its processability decreases with a consequent increase in rolling stress and the obtainment of high specific pressure values on the rollers. This further implies limits concerning the angle of entrance in the rolling stations.

Aim of the invention

The aim of the present invention consists in providing a hot rolling method and plant in which the rolling stress present during the phases of mechanical working of the material is reduced. Concept of the invention

The aim is achieved by the characteristics of the main claim. The sub-claims represent advantageous solutions.

Advantageous effects of the invention

The solution according to the present invention, by the considerable creative contribution the effect of which constitutes an immediate and important technical progress, has various advantages.

By means of the solution according to the invention it is possible to achieve an effective reduction in the rolling stress present during the phases of mechanical working of the material between the rolling stations present in the rolling mill.

Furthermore, by means of the solution according to the invention it is possible to advantageously resort to driving motors of the rollers having lower powers, with benefits both in terms of costs of the plant and in terms of production costs of the final material. By means of the solution according to the invention it is possible to achieve an extension of the useful life of the rolling cylinders, which are advantageously subjected to less wear, as a consequence enabling longer production cycles without the need to interrupt production to adjust the rolling cylinders in order to compensate for their wear, to check the rolling cylinders, to replace the rolling cylinders. The extension of the useful life of the rolling cylinders also has a considerable beneficial effect on the management costs of the rolling mill and, as a consequence, also on the costs of the finished product of the rolling process.

Furthermore, by means of the method according to the invention and of the related plant it is also possible to obtain products having better mechanical characteristics.

Description of the drawings

In the following a solution is described with reference to the enclosed drawings, which are to be considered as a non-exhaustive example of the present invention in which: Fig. 1 shows a schematic view of a steel production plant comprising a rolling mill made in accordance with the present invention. Fig. 2 shows a schematic view of a rolling mill made in accordance with the present invention.

Fig. 3 and Fig. 4 schematically show two possible illustrative sections of the product entering the rolling mill made in accordance with the present invention in the case of long products.

Fig. 5 schematically show a possible illustrative section of the product entering the rolling mill made in accordance with the present invention in the case of long products.

Fig. 6 schematically shows the temperature profile of the surface zone and of the core zone of the product entering the rolling mill made in accordance with the present invention.

Fig. 7 schematically shows the temperature profile of the surface zone and of the core zone of the product inside the rolling mill made in accordance with the present invention, that is to say, after the first rolling station.

Fig. 8 schematically shows the phase of reduction of the section of the rolled material carried out by means of the rollers of a rolling station.

Fig. 9 schematically shows the trend of the specific rolling pressure depending on the temperature of the product being rolled.

Fig. 10 schematically shows the trend of the rolling speed factor depending on the temperature of the product being rolled, said trend being represented in different curves which represent different rotation speeds of the rollers.

Fig. 11 schematically shows the trend of the force of separation between the rollers depending on the temperature of the product being rolled.

Fig. 12 schematically shows the trend of the force of separation between the rollers depending on the temperature of the product being rolled, said trend being represented in different curves which represent different rotation speeds of the rollers.

Fig. 13 schematically shows the trend of the coefficient of friction on the rollers depending on the temperature of the product being rolled.

Fig. 14 schematically shows the phase of reduction of the section of the rolled material carried out by means of the rollers of a rolling station and its effect on the internal structure of the rolled material.

Description of the invention

With reference to the Figures (Fig. 1 , Fig. 2) the present invention relates to a hot rolling method and related hot rolling plant or rolling mill (1) for long products or for flat products. In general the rolling mill (1) can be:

- a rolling plant (Fig. 2) in which the product (25) to be rolled is introduced in the rolling mill (1) through a heating furnace which heats the product (25) starting from a condition of the product which is at room temperature;

or

- a rolling plant (Fig. 1) which is part of an integrated steel production plant (4) which comprises the rolling mill (1) itself directly connected to a casting machine (2), possibly with the interposition of an interconnection section (3) which is between the casting machine (2) and the rolling mill (1), wherein the product (25) to be rolled is introduced in the rolling mill (1) directly from the casting machine (2) already in a condition of high temperature corresponding to the temperature of exit from the casting machine (2).

In the casting machine (2) the liquid steel contained (Fig. 1) in a ladle (5) is distributed in one or more moulds (7) by means of a distribution tundish (6). By means of an extraction unit (8) the partially solidified steel is extracted from the mould (7) and, in the case of vertical casting machines, it passes through a straightening unit (9) to be then conveyed by transport means (10) towards cooling plates (not shown) or to be sent towards the entry of the rolling mill (1).

The method according to the invention is used to roll in a continuous and/or semi- continuous way long products (Fig. 3, Fig. 4) such as bars and/or sections or flat products (Fig. 5) such as sheets and/or strips with an extremely high temperature of the surface zone (M) with respect to the temperature of the core zone (N), therefore in conditions of complete non-homogeneity along the section of the product (25).

It is necessary to point out that, in general, there exists a technical prejudice against the fact of rolling a product (25) in conditions of temperature non-homogeneity between the surface zone (M) and the core zone (N) on the basis of the fact that the quality of the finished product would be affected, in addition to increasing the risk of having non- homogenous elongations with the risk that the product being rolled may be subject to cobbles with the line consequently getting stuck. For this reason, although some prior art solutions provide the use of heating means, in particular induction heating means, to increase the temperature of the product before it enters the rolling mill, the same solutions also provide that downstream of the heating means, before the successive rolling station, a certain minimum distance is provided so as to allow the product to return to an essential temperature homogeneity condition between the surface zone (M) and the core zone (N). This is confirmed by the prior art documents and by the graphs showing the trend of the temperatures, in which it is indicated that the product being rolled is subjected to a localised increase in the surface temperature but it is essential to determine an optimal distance between the exit of the induction heating means and the entry of the successive rolling station so that in this distance the core zone of the product can have the time to be further heated while the surface zone of the product can have the time to be cooled to obtain again a temperature homogeneity condition along the section of the product being rolled.

Some solutions have suggested to heat more the core zone of the product with respect to the surface zone, but, in this case too, the technical prejudice against operating on the basis of the method according to the present invention still remains.

Therefore, the solution according to the present invention aims at overcoming such technical prejudices providing a solution in which the product (25) is rolled in rolling conditions in which the temperature of the surface zone (M) is greater with respect to the temperature of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27), in particular a first rolling cylinder (26) and a second rolling cylinder (27). In this way, as will be explained in the following of the present description, the specific rolling pressures on the surface and the related rolling stress are reduced. Therefore, in the method according to the invention one induces on the product (25) a localised overheating of the surface zone (M) to reduce the hot rolling stress, said surface overheating being applied in a portion of the rolling mill between at least two phases of mechanical working of the product operated by means of the rolling stations (14, 17, 20, 23) of the rolling mill (1).

The method according to the invention is used for long products and for flat products and allows to obtain an increase in the operating and energy efficiency of the plant.

It is provided (Fig. 1 , Fig. 2) to use at least first heating means (15) which are configured and structured in such a way as to overheat the surface zone (M) of the product being rolled in a heating position which is located after the first rolling station (14) of the rolling mill (1), the first heating means (15) being placed at a first distance (X1) from the second rolling station (17) such that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the second rolling station (17). In particular, the temperature TM of the surface zone (M) of the product (25) is greater by a quantity DT with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the second rolling station (17).

In general it is provided to roll the product (25) in rolling conditions in which the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) by a quantity DT between 200 degrees centigrade and 400 degrees centigrade, preferably by a quantity between 250 degrees centigrade and 350 degrees centigrade. In other words, the process according to the invention can provide at least one heating phase or the rolling mill can comprise at least one heating means (11 , 15, 18, 21 ) configured and structured in such a way that the difference between the temperature TM of the surface zone (M) of the product (25) and the temperature TN of the core zone (N) is equal to a quantity DT between 200 degrees centigrade and 400 degrees centigrade, even more preferably in such a way that the difference between the temperature TM of the surface zone (M) of the product (25) and the temperature TN of the core zone (N) is equal to a quantity DT between 250 degrees centigrade and 350 degrees centigrade.

Preferably it is provided (Fig. 1 , Fig. 2):

- to use at least first heating means (15) which are configured and structured in such a way as to overheat only the surface zone (M) of the product (25) being rolled in a heating position which is located after a first rolling station (14), the first heating means (15) being placed at a first distance (X1) from a second rolling station (17) such that the temperature TM of the surface zone (M) of the product (25) is greater by a quantity DT with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the second rolling station (17) or in correspondence of the entry (117) of the second rolling station (17);

and

- to use at least second heating means (18) which are configured and structured in such a way as to overheat only the surface zone (M) of the product being rolled in a heating position which is located after the second rolling station (17), the second heating means (18) being placed at a second distance (X2) from a third rolling station (20) such that the temperature TM of the surface zone (M) of the product (25) is greater by a quantity DT with respect to the temperature TN of the core zdne (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the third rolling station (20) or in correspondence of the entry (I20) of the third rolling station (20).

In other words, the hot rolling process according to the invention can comprise more than one in-line heating phase of the product (25) by means of a series of in-line heating means (11 , 15, 18, 21), each of said heating phases being a heating phase in which the surface zone (M) of the product (25) is overheated with respect to the core zone (N) of the product (25) by a quantity such that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of the rolling station (14, 17, 20, 23) successive to each of the respective heating means (11 , ^" 15, 18, 21) of the described series of heating means (11 , 15, 18, 21). For example, the process comprises:

- at least one first phase of said in-line heating phases of the product (25) by first heating means (15) which are configured and structured in such a way as to overheat only the surface zone (M) of the product (25) being rolled in a heating position which is located after a first rolling station (14), the first heating means (15) being placed at a first distance (X1) from a second rolling station (17) such that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of the second rolling station (17) or in correspondence of an entry (117) of the second rolling station (17);

and

- at least one second phase of said in-line heating phases of the product (25) by second heating means (18) which are configured and structured in such a way as to overheat only the surface zone (M) of the product (25) being rolled in a heating position which is located after the second rolling station (17), the second heating means (18) being placed at a second distance (X2) from a third rolling station (20) such that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of the third rolling station (20) or in correspondence of an entry (I20) of the third rolling station (20).

Even more preferably it is provided (Fig. 1 , Fig. 2):

- to use at least first heating means (15) which are configured and structured in such a way as to overheat only the surface zone (M) of the product (25) being rolled in a heating position which is located after a first rolling station (14), the first heating means (15) being placed at a first distance (X1) from a second rolling station (17) such that the temperature TM of the surface zone (M) of the product (25) is greater by a quantity DT with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the second rolling station (17) or in correspondence of the entry (117) of the second rolling station (17);

and

- to use at least second heating means (18) which are configured and structured in such a way as to overheat only the surface zone (M) of the product being rolled in a heating position which is located after the second rolling station (17), the second heating means

(18) being placed at a second distance (X2) from a third rolling station (20) such that the temperature TM of the surface zone (M) of the product (25) is greater by a quantity DT with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the third rolling station (20) or in correspondence of the entry (120) of the third rolling station (20);

and

- to use at least further heating means (19) which are configured and structured in such a way as to overheat only the surface zone (M) of the product being rolled in a heating position which is located after an upstream rolling station (20), the further heating means

(19) being placed at a distance (X) from a downstream rolling station (23) such that the temperature TM of the surface zone (M) of the product (25) is greater by a quantity DT with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the downstream rolling station (23) or in correspondence of the entry (I20) of the downstream rolling station (23).

In the present description by the expression "first rolling station" one does not necessarily mean the first rolling station which the product (25) reaches during the advancement through the rolling mill (1), but the expression "first" is used to distinguish one of the rolling stations present in the plant with respect to the others.

In the present description by the expression "second rolling station" one does not necessarily mean the second rolling station which the product (25) reaches during the advancement through the rolling mill (1), but the expression "second" is used to distinguish one of the rolling stations present in the plant with respect to the others and in particular with respect to the previously indicated first rolling station.

Therefore, in general, in the hot rolling process according to the invention, at least one of the in-line heating phases of the product (25) occurs by means of heating means (11 , 15, 18, 21) which are placed at a distance (X) from one of the rolling stations (14, 17, 20, 23) which is a downstream rolling station (23) placed immediately downstream with respect to the heating means (1 1 , 15, 18, 21) themselves. The distance (X) is such that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of the corresponding downstream rolling station (23).

The rolling mill (1 ) can be preceded by an interconnection section (3) which constitutes an interconnection interface with other devices, such as reheating furnaces, descalers, casting machines (2). In the interconnection section (3) there can also be third heating means (11) which are configured and structured in such a way as to overheat only the surface zone (M) of the product (25) entering the rolling mill in a heating position which is located before the rolling mill (1), the third heating means (11) being placed at a third distance (X3) from the first rolling station (14) such that the temperature TM of the surface zone (M) of the product (25) is greater by a quantity DT with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the first rolling station (14) or in correspondence of the entry (I20) of the first rolling station (14). In that case the hot rolling process according to the invention can provide at least one in-line heating phase of the product (25) by the third heating means (11) which are placed at a third distance (X3) from an initial entrance rolling station of the rolling mill (1). The third distance (X3) will be such that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the initial entrance rolling station of the rolling mill (1 ) or in correspondence of an entry of the initial entrance rolling station of the rolling mill (1).

The heating means (11 , 15, 18, 21) are preferably configured and structured to apply on the product (25) a localised overheating in the surface zone (M) up to (Fig. 3, Fig. 4, Fig. 5) a maximum depth (S) of 10 mm from the surface of the product (25), leaving essentially unchanged the temperature of the core zone (N) of the product (25). It is provided to reach temperatures which are much higher with respect to the temperatures in use in the conventional rolling processes during the rolling process between the rolling stations. The temperature of the surface zone (M) is brought (Fig. 6, Fig. 7) to a temperature essentially between 1250 and 1400°C maintaining a temperature of the core zone (N) essentially between 1000°C and 1050°C. In other words at least one of the heating means (11 , 15, 18, 21 ) can be configured and structured in such a way that the process provides that at least one of the heating phases is such that the temperature of the surface zone (M) is brought to a temperature essentially between 1250 and 1400°C maintaining a temperature of the core zone (N) essentially between 1000°C and 1050°C when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of the rolling station (14, 17, 20, 23) successive to the heating means (11 , 15, 18, 21) themselves.

The distance (X) between the heating means (11 , 15, 18, 21) and the downstream rolling station (23) placed immediately downstream with respect to the heating means (11 , 15, 18, 21) themselves can be calculated as a function of a desired minimum difference between the temperature TM of the surface zone (M) of the product (25) and the temperature TN of the core zone (N) when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of said downstream rolling station (23). The heating means (11 , 15, 18, 21 ) are preferably multi-turn inductors. Each of the heating means (1 1 , 15, 18, 21) is situated at a distance X approximately between 1 and 4 metres from the successive rolling station with respect to the rolling direction (24), preferably at a distance X approximately between 1 and 3 metres from the successive rolling station with respect to the rolling direction (24), even more preferably at a distance X approximately between 1 and 2.5 metres from the successive rolling station with respect to the rolling direction (24).

For example it can be provided that the heating means (11 , 15, 18, 21) can be situated:

- at a smaller distance X in the rolling stations in which the product (25) moves at a lower speed, that is to say, those in which the product (25) has a greater section;

- at a medium distance X in the rolling stations in which the product (25) moves at a medium speed, that is to say, those in which the product (25) has an intermediate section between the entrance section and the final section of exit from the rolling mill;

- at a greater distance X in the rolling stations in which the product (25) moves at a high speed, that is to say, those in which the product (25) has a small section close to the final section of exit from the rolling mill.

As an example only, in the rolling mill (1) there are:

- first heating means (15) which are placed at a first distance (X1) from the second rolling station (17) wherein the first distance (X1) is approximately between 1 and 4 metres from the entry (117) of the second rolling station (17);

and/or

- second heating means (18) which are placed at a second distance (X2) which is approximately between 1 and 3 metres from the entry (I20) of the third rolling station

(20);

and/or

- further heating means (19) which are placed at a distance (X) from a downstream rolling station (23) wherein the distance (X) is approximately between 1 and 2.5 metres from the entry (I20) of the downstream rolling station (23).

In correspondence of the heating means (11 , 15, 18, 21) some respective temperature detection means (12, 16, 19, 22) are installed to monitor the trend of the temperature of the surface zone (M). For example one can provide:

- first temperature detection means (16) in correspondence of first heating means (15);

- second temperature detection means (19) in correspondence of second heating means (18);

- third temperature detection means (12) in correspondence of third heating means (11 );

- further temperature detection means (22) in correspondence of further heating means (21).

Preferably for at least one of the heating means (11 , 15, 18, 21) two temperature detection means (12, 16, 19, 22) are used of which one is installed before the heating means (1 1 , 15, 18, 21) with respect to the rolling direction (24) and the other is installed after the heating means (11 , 15, 18, 21) with respect to the rolling direction (24).

As a consequence, the hot rolling process according to the invention can provide measuring phases of the temperature of the product (25) and adjustment phases of the heating means (11 , 15, 18, 21), the adjustment phases being configured in such a way as to control the heating means (1 1 , 15, 18, 21) depending on the temperature measured in the measuring phases by means of the temperature detection means (12, 16, 19, 22). The heating means (11 , 15, 18, 21) will be adjusted in such a way that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of the rolling station (14, 17, 20, 23) successive with respect to the in-line heating means (11 , 15, 18, 21) and with respect to the rolling direction (24). The heating means (11, 15, 18, 21) are thus interfaced and synchronized, through the temperature detection means (12, 16, 19, 22) with the measurement of the speed of advancement of the product (25), in such a way as to ensure a constant temperature non-homogeneity between the surface zone (M) and the core zone (N) at the entry of the respective rolling station (14, 17, 20, 23).

Through the localised surface overheating it is possible to reduce the conventional rolling stress and the related limits of the rolling plants of the prior art. The product (25) entering the rolling mill (1) can have a section between 2000 and 10000 mm ² .

The heating means (11 , 15, 18, 21) will be applicable before the entry into the rolling mill (1) and also in further successive positions of application with respect to the rolling direction (24), in said successive positions of application the section of the product (25) being rolled being progressively reduced by the cylinders (26, 27) of the rolling stations (14, 17, 20, 23).

As previously explained, in general, the distance (X) of each of the heating means (11 , 15, 18, 21) from the respective rolling station (14, 17, 20, 23) successive with respect to the rolling direction (24) can be different depending on the rolling speed and on the section of the product (25) being rolled in correspondence of the zone of application of the heating means (11 , 15, 18, 21 ) themselves.

The localised overheating of the surface zone (M) of the product (25) operated by means of the heating means (1 1 , 15, 18, 21 ) affects (Fig. 3, Fig. 4, Fig. 5) a depth (S) between 1 and 10 mm. The depth (S) of the surface zone (M) of the product (25) which is overheated can be adjusted through the supply frequency of the inductor exploiting the skin effect principle.

The plastic deformation at a high temperature, for example between 1250°C and 1400°C, activates a dynamic re-crystallization process on the surface, which on the other hand does not occur in conventional rolling, favouring the formation of a fine-grained microstructure. This effect cannot be obtained with the prior art solutions which, on the other hand, provide that the product (25) being rolled is characterised by a temperature as homogeneous as possible between the core zone (N) and the surface zone (M) or, in some cases, provide that the product (25) being rolled is characterised by a temperature of the core zone (N) greater than the temperature of the surface zone (M).

The core zone (N) of the product (25) being rolled, on the other hand, is not overheated (Fig. 6, Fig. 7) by the induction heating means and is thus rolled at an essentially constant rolling temperature or anyway between 1000 and 1100°C, to maintain high toughness and mechanical resistance characteristics in particular in the core zone (N). With reference to the entrance zone (Fig. 1 , Fig. 2, Fig. 6) of the rolling mill (1) and to the trend of the temperature of the surface zone (M) of the product (25) being rolled, in a position which is located before the third heating means (11), the surface zone (M) underwent a greater cooling with respect to the cooling of the core zone (N) and, therefore, the temperature TN of the core zone (N) is greater than the temperature TM of the surface zone (M). By effect of the third heating means (11) the temperature of the surface zone (M) is raised until bringing it to a temperature greater than the temperature of the core zone (N), approximately with a temperature rise DT equal to the difference between the temperature TN of the core zone (N) and the temperature TM of the surface zone (M) wherein DT is between 200°C and 400°C, preferably between 250°C and 350°C. The third heating means (11) are thus configured and structured in such a way as to overheat only the surface zone (M) of the product being rolled in a heating position which is located at a third distance (X3) from the first rolling station (14) such that the temperature TM of the surface zone (M) of the product (25) is greater by a quantity DT with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between a pair of rolling cylinders (26, 27) of the first rolling station (14) or in correspondence of the entry (114) of the first rolling station (14).

After the first rolling station (14) the process can be repeated (Fig. 7) in various other points of the roll train maintaining along the entire rolling process a marked difference in temperature between the core zone (N) and the surface zone (M) wherein the surface zone (M) has a temperature greater by a quantity DT with respect to the core zone (N). Therefore, the process according to the invention which provides the rolling of the product (25) maintaining, nearly along the entire rolling process, a temperature TM of the surface zone (M) greater by a quantity DT with respect to the temperature TN of the core zone (N), offers the following advantages:

- reduction of the rolling stress;

- reduction of the wear of the rolling cylinders (26, 27); - improvement of the surface quality of the finished product coming out of the rolling mill (1 ). With particular reference to the last point, the process according to the invention allows to reduce the occurrence of surface defects of the finished product (25), therefore with less production waste.

The phases of entrance of the product (25) at the entries (117, I20, I23) of the rolling stations (14, 17, 20, 23) are further favoured as well.

Furthermore, the process according to the invention allows to overcome the following limits present in the prior art solutions:

- limit concerning the section reduction operated in each rolling station;

- limit concerning the angle of entrance of the product (25) being rolled; :

- limit concerning the useful life of the rolling cylinders (26, 27).

The reduction of the rolling stress, meant as a reduction of the stress of separation between the cylinders (26, 27) of the rolling station, is directly proportional to the specific i rolling pressure. ^'

The force of separation between the cylinders can be considered as follows:

F= (Bm x (R x VH»/1000 x ( Pv x Fn x g) [ ton]

Wherein (Fig.8):

- D indicates the diameter of the rolling cylinders (26, 27), expressed in mm;

- Bm indicates the average width of the product (25) being rolled which depends on the sizing, expressed in mm;

- H1 indicates the height of the product (25) being rolled in correspondence of the entry (114, 117, I20, I23) of the rolling station (14, 17, 20, 23), expressed in mm;

- H2 indicates the height of the product (25) being rolled in correspondence of the exit of : the rolling station (14, 17, 20, 23), expressed in mm;

- VH indicates the difference H1-H2, expressed in mm;

- R indicates the working radius calculated as (D-H1) / 2, expressed in mm;

- Pv indicates the specific rolling pressure, which depends on the reduction ratio and on the temperature; expressed in kg/mm ² ;

- Fn indicates the speed factor which depends on the rolling speed of the product (25) and on temperature;

- g indicates the shape factor of the product (25) being rolled which depends on the shape and sizing.

The force of separation between the rollers depends on the temperature through two variables, namely the specific rolling pressure (Pv) and the speed factor (Fn).

The specific rolling pressure (Pv) has a decreasing trend (Fig. 9) with respect to the increases in the temperature of the product (25) being rolled, keeping the other parameters (D, Bm, R, VH, g) constant.

The speed factor (Fn) has a growing trend (Fig. 10) with respect to the increases in the temperature of the product (25) being rolled, keeping the other parameters (D, Bm, R, VH, g) constant. The speed factor (Fn) can be kept constant at low (5 rpm) rolling speeds of the product (25), thus not having a significant influence on the force of separation (F) between the rolling cylinders. As a consequence, at low rolling speeds, such as 5 rpm, which is a condition occurring at the entry (114) of the first rolling station (14), the force of separation (F) between the rolling cylinders depends on the temperature only through the specific rolling pressure (Pv) and, as can be seen from the corresponding trend (Fig. 9), upon increase in the rolling temperature of the product (25) there is a decrease in the specific rolling pressure (Pv) which results in a decrease in the force of separation (F) between the rolling cylinders. The force of separation (F) between the cylinders will thus have (Fig. 1 1) a decreasing trend upon increase in the surface temperature of the product (25) being rolled.

At rolling speeds of the product (25) higher than 5 rpm the speed factor (Fn), on the other hand (Fig. 10), contributes to increasing the specific rolling pressure (Pv) thus reducing the effect of reduction of the force of separation (F) between the rolling cylinders due to the effect of the specific rolling pressure (Pv) which in its turn is caused by the temperature change induced on the product (25) being rolled. Therefore, along the rolling mill (1), downstream of the first rolling station (14), the force of separation (F) between the rolling cylinders will depend, upon change in the temperature of the product (25) being rolled, both on the specific rolling pressure (Pv) which tends to reduce the force of separation (F) between the rolling cylinders upon increase in the temperature of the product (25) being rolled, and on the speed factor (Fn) which tends to increase the force of separation (F) between the rolling cylinders upon increase in the temperature of the product (25) being rolled. The contribution of the specific rolling pressure (Pv) prevails on the contribution of the speed factor (Fn). As a consequence, the process according to the invention, which provides the localised overheating of the surface zone (M) of the product (25) being rolled, reduces the force of separation (F) between the rolling cylinders and, as a result, reduces (Fig. 12) the rolling stress.

A further parameter (Fig. 13), which has to be taken into account, is the coefficient of friction (Ca), which depends on the temperature of the surface zone (M) of the product (25) being rolled. Upon increase in the temperature of the surface zone (M) of the product (25), the coefficient of friction (Ca) varies according to the formula:

Ca = 1.05 0.0005 T 0.056 V

wherein

T indicates the temperature expressed in °C;

V indicates the rolling speed of the product (25) being rolled expressed in m/s.

By means of the process according to the invention, in which the rolling of the product (25) occurs maintaining, nearly along the entire rolling process or at least along a portion of the rolling mill (1), a temperature TM of the surface zone (M) greater by a quantity DT with respect to the temperature TN of the core zone (N), therefore, one obtains a reduction of the coefficient of friction (Ca) increasing the useful life of the rolling cylinders. In other words it is provided that in a preferred embodiment of the process according to the invention at least one part of the series of phases of mechanical working of the product (25) consists of successive phases of mechanical working which occur one after the other in reciprocally contiguous rolling stations (14, 17, 20, 23) along a portion of the rolling mill (1) wherein each of the phases of mechanical working is carried out in a condition in which the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) of the product (25) in such a way that in this portion of rolling mill (1) all the phases of mechanical working occur in a condition in which the product (25) has the temperature TM of the surface zone (M) of the product (25) greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between the rolling cylinders (26, 27).

In that case it is provided that the rolling mill (1) is provided with more than one of the described in-line heating means (11 , 15, 18, 21). If needed, there will be means constituting a series of in-line heating means (11 , 15, 18, 21), each of said heating means (11 , 15, 18, 21 ) being configured and structured to overheat the surface zone (M) of the product (25) with respect to the core zone (N) of the product (25) in such a way that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N). Preferably, therefore, at least one portion of the rolling mill (1) comprises a series of reciprocally contiguous rolling stations (14, 17, 20, 23) along said portion of rolling mill (1). The series of in-line heating means (11 , 15, 18, 21) will be arranged according to a configuration such that in this portion of rolling mill (1) the heating means (11 , 15, 18, 21) are configured and structured to overheat the surface zone (M) with respect to the core zone (N) of the product (25) in such a way that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) in each of the rolling stations (14, 17, 20, 23) of said portion of rolling mill (1).

As far as the microstructure of the product (25) is concerned, the surface zone (M) subjected to overheating between 1250 and 1400°C has a favourable trend for dynamic re-crystallization, thus generating a finer surface grain in a re-crystallization zone (13) with respect to the core zone (N), improving the surface quality of the product (Fig.14). In practice, in the overheated product which enters the rolling station one can have a lower temperature of the skin Tp with respect to what is indicated by effect of the heat loss in the air and a surface temperature TM intended as sub-skin temperature which is greater with respect to the temperature TN of the core zone (N).

Therefore, the process according to the invention allows to carry out phases of mechanical working of the product in which the surface zone (M) subjected to overheating has a dynamic re-crystallization with the generation of a finer surface grain in a re-crystallization zone (13) with respect to the core zone (N), improving the surface qualities of the product (Fig.14) in such a way that, in general, the rolling process according to the invention can provide a dynamic re-crystallization phase with the generation of a finer surface grain in a re-crystallization zone (13) with respect to the grain of the core zone (N).

In conclusion, the present invention relates (Fig. 1 , Fig. 2) to a hot rolling process of a metallic product (25) wherein the process comprises a series of phases of mechanical working of the product (25) by means of rolling cylinders (26, 27) of corresponding rolling stations (14, 17, 20, 23) of a rolling mill (1) through which the product (25) is guided. The rolling process further comprises one or more in-line heating phases of the product (25) by means of in-line heating means in which at least one of the in-line heating phases of the product (25) by means of the in-line heating means (11 , 15, 18, 21) is a heating phase in which a surface zone (M) of the product (25) is overheated with respect to a core zone (N) of the product (25) by a quantity such that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of the rolling station (14, 17, 20, 23) successive with respect to the in- line heating means (1 1 , 15, 18, 21) and with respect to a rolling direction (24).

The rolling process can comprise control phases of the induction heating means (11 , 15, 18, 21) selected from control phases of the operating frequency of the induction heating means (11 , 15, 18, 21) according to operating modes in which the operating frequency is controlled in such a way as to obtain a penetration depth (S) of heating induced currents between 0 and 20 mm, preferably between 0 and 15 mm, even more preferably between 0 and 10 mm, control phases of the power of the heating means (11 , 15, 18, 21), a combination of said control phases of the operating frequency of the heating means (1 1 , 15, 18, 21) and of said control phases of the power of the heating means (11 , 15, 18, 21). One can provide processes in which the penetration depth (S) is between 0 and 20 mm. One can provide processes in which the penetration depth (S) is between 0 and 15 mm. One can provide processes in which the penetration depth (S) is between 0 and 10 mm. One can provide processes in which the penetration depth (S) is adjusted in each of the heating means depending on the section size of the product (25).

More generally, the hot rolling process can comprise adjustment phases selected from adjustment phases consisting of control phases of the operating frequency of the induction heating means (1 1 , 15, 18, 21), adjustment phases consisting of control phases of the power of the heating means (11 , 15, 18, 21), adjustment phases consisting of a combination of control phases of the operating frequency of the induction heating means (11 , 15, 18, 21) and control phases of the power of the heating means (11 , 15, 18, 21). The operated adjustment can depend on one or more of temperature of the product (25) measured by means of temperature detection means (12, 16, 19, 22), section size of the product (25), section shape of the product (25), rolling speed of the product (25), material constituting the product (25).

The adjustment phases' can further comprise a control phase of the distance (X) of at least one of the heating means (11 , 15, 18, 21) with respect to the corresponding downstream rolling station (23) which is placed immediately downstream, the control of said distance (X) being carried out depending on one or more of the following operating parameters:

- temperature of the product (25) measured by means of the temperature detection means (12, 16, 19, 22);

- section size of the product (25); - section shape of the product (25);

- rolling speed of the product (25);

- material constituting the product (25).

In that case it is provided that at least one of the heating means (11, 15, 18, 21) is mobile along at least one movement portion according to a movement direction which is parallel with respect to the rolling direction (24). Therefore, there will be adjustment means of the distance (X) in the form of means of reciprocal approach or spacing apart between at least one of the heating means (11 , 15, 18, 21) and the corresponding downstream rolling station (23) which is placed immediately downstream with respect to said heating means (1 1 , 15, 18, 21). For example it can be provided that the means of approach or spacing apart are made in the form of a trolley supporting the heating means (1 1 , 15, 18, 21) or in the form of a fixed guide along which the heating means (11 , 15, 18, 21) are mobile according to a movement direction which is parallel with respect to the rolling direction (24).

Furthermore, the present invention relates (Fig. 1 , Fig. 2) to a hot rolling mill (1) comprising rolling stations (14, 17, 20, 23) provided with rolling cylinders (26, 27) of a metallic product (25), the rolling mill (1) being provided with guiding means of the product (25) through a series of rolling stations (14, 17, 20, 23) which are placed one after the other according to a rolling direction (24). The rolling mill (1) further comprises in-line heating means wherein at least some of the in-line heating means (11 , 15, 18, 21 ) are placed at a distance (X) from one of the rolling stations (14, 17, 20, 23) which is a downstream rolling station (23) placed immediately downstream with respect to the heating means (11 , 15, 18, 21) themselves. The distance (X) is such that the temperature TM of the surface zone (M) of the product (25) is greater with respect to the temperature TN of the core zone (N) of the product (25) when the product (25) is in an engagement condition between the rolling cylinders (26, 27) of the downstream rolling station (23).

The rolling mill (1) can operate according to a hot rolling process having the previously described characteristics and operating phases.

With respect to the mentioned prior art documents, the solution described in DE 10 2006 002505 is an example of prior art solutions which are intended to be overcome by the application under examination introducing a new concept of rolling with an over- temperature condition of the edge of the material with respect to the core. Indeed, in paragraph §0016 DE 10 2006 002505 states that the side edges of the entering material are typically exposed as soon as the material comes out of the casting mould and in this respect act as heat radiating surfaces. Moreover, the entire amount of cooling water that is used for example during a descaling phase on the broad surface of the entering material runs down along the edges, so that the edges experience an additional strong cooling. As a result, before the entry of the finishing mill, the entering material is therefore typically cooled more at its edges with respect to its centre. However, it is desired that, as far as possible, the material is uniformly heated throughout - also transversely with respect to the direction of the material flow - no later than when it leaves the finishing mill. In order to achieve this aim DE 10 2006 002505 proposes the positioning of the material heating means within the finishing mill with an orientation directed towards the edges of the strip and proposes that they are controlled by means of a computerised model in such a way that the overcooled edges of the entering material are heated more with respect to the centre of the material in such a way that the material is uniformly heated, even in the transverse direction, no later than when it leaves the finishing mill. As a consequence, the purpose of DE 10 2006 002505 consists in creating a temporary non-uniformity of the temperature on the edges of a strip in such a way as to ensure, on the other hand, the uniformity of the temperature of the product due to the following distribution of heat, contrarily with respect to the solution of the present application which explicitly claims a technical solution in which the aim is to overheat the external part of the material and maintain said overheating of the external part with respect to the core, above all in the engagement position on the rolling cylinders, this solution being neither described nor suggested by DE 10 2006 002505 which has, rather, a completely opposite aim. This is well explained in paragraph §0007 of DE 10 2006 002505 wherein it is stated that the claimed process has the advantage that the temperature level and the temperature variations of the material inside the finishing mill are kept small. The heating method of DE 10 2006 002505 results in only moderate temperature increases and overall relatively low temperatures in the entire finishing mill. This is exactly the contrary with respect the solution of the present application as it can be easily understood also by the comparison between figure 4 of DE 10 2006 002505, showing the claimed inventive method whose aim is temperature constancy, and figure 7 of the present application which shows the clear and considerable difference in temperature between the surface and the core of the product induced by the application of the claimed method before the entry in each rolling stand of the rolling mill. Furthermore, in paragraph §0017 of DE 10 2006 002505 it is stated that induction heaters are used because with them it is possible to obtain a more homogenous temperature distribution. That is to say, the inductors of DE 10 2006 002505 are used in order to maintain a homogenous temperature in the product contrarily to the solution of the present application in which a method of use of the inductor is claimed which is exactly the opposite because the inductors are intentionally controlled in order to ensure, instead, a non-homogenous temperature distribution between the surface and the core of the product when it is in the engagement condition with the rolling cylinders downstream with respect to the rolling direction. This is also confirmed in DE 10 2006 002505, paragraphs §0037 and §0038 in which it is stated that, referring to figure 4, in this way one is able to keep the temperature range in the finishing mill at a constant level and an almost isothermal rolling of the material occurs, which is contrary to the aim of the method of the present application. Finally, it is observed that DE 10 2006 002505 refers to a heating of a strip and its aim does not consist in heating the core of the product (i.e. its internal part) to a lower temperature with respect to its external surface but its aim consists in heating in a more intensive way only the edges or corners of the product in the form of a strip with respect to the central flat zone of the product itself. In the present application, on the contrary, the purpose is to heat not only edges of a strip but the overall external surface of the product which contains a core.

Patent application DE 10 2011 006357 describes a solution of a rolling mill for products in the form of strips in which the use of a loop lifter roller is provided which is placed within the volume defined by a rolling stand. Furthermore, patent application DE 10 201 1 006357 describes the possibility of using induction heating means between two consecutive rolling stands to adjust the rolling temperature. Nothing in the text of the description of DE 10 2011 006357 could be interpreted to mean that the induction heating means are used in a different way with respect to the known solutions, i.e. for the purpose of maintaining a temperature uniformity of the product. In DE 10 2011 006357, therefore, there is no suggestion to use induction heating means which are controlled in such a way as to obtain a non-homogeneous temperature between the surface and the core of the product when it is in an engagement condition with the rolling stand which is located downstream with respect to the induction heating means.

The solution described in EP 1 452 247 relates, as explicitly indicated in paragraphs §0050 and §0058, to a treatment which occurs after a rolling mill and in which the aim is, again, to obtain a uniform heating. Therefore, referring both to the positioning of the system described in EP 1 452 247 which is not located within or before a rolling line, and to its purpose which is not to obtain a non-homogeneous temperature, the solution described in EP 1 452 247 cannot be considered as anticipating or suggesting the solution described in the present application.

The description of the present invention has been made with reference to the enclosed figures in a preferred embodiment, but it is evident that many possible changes, modifications and variants will be immediately clear to those skilled in the art in the light of the previous description. Thus, it must be underlined that the invention is not limited to the previous description, but it includes all the changes, modifications and variants in accordance with the appended claims.

Nomenclature used With reference to the identification numbers in the enclosed figures, the following nomenclature has been used:

1. Rolling mill

2. Casting machine

3. Interconnection section

4. Steel production plant

5. Ladle

6. Tundish

7. Mould

8. Extraction unit

9. Straightening unit

10. Transport means

11. Third heating means

12. Third temperature detection means

13. Re-crystallization zone

14. First rolling station

15. First heating means

16. First temperature detection means

17. Second rolling station

18. Second heating means

19. Second temperature detection means

20. Third rolling station

21. Further heating means

22. Further temperature detection means

23. Downstream rolling station

24. Rolling direction

25. Product

26. First cylinder 27. Second cylinder

N. Core zone

M. Surface zone

S. Depth

A. Second distance

K. First distance

P. Specific rolling pressure

D. Diameter

H1. Dimension of the product at the entry

H2. Dimension of the product at the exit

114. Entry of the first rolling station

117. Entry of the second rolling station

I20. Entry of the further upstream rolling station I23. Entry of the further downstream rolling station