The present invention relates to a rolling mill comprising two or more rolling stands having a specific solution for driving and controlling the rolling rolls.

Rolling mills are used to reduce and/or transform billets into sections or sheets of suitable cross section. The processing method may include several passes of a bar through the same stand or successive passes through different stands.

Consequently, the billet is the starting product and the section or sheet the finished product of this process, via one or more semi-finished products that are outputted from a stand and fed into the next stand, progressively transforming the cross section. To make the process as continuous as possible, reducing down time and the movement of semi-finished products, rolling mills are built with several stands in series. Figure 1 is a schematic view of a rolling mill in the prior art. With reference to this figure, the most common solution currently adopted to actuate a stand is the use of an asynchronous motor 1 that drives the two rolls 3 of the stands via a movement splitter and/or distributor and a reduction gear, both built into the structure 2. Roll spindles 4 with toothed constant- velocity joints or universal joints able to transmit the motion and movement of the rolls towards and away from each other may be placed between the reduction gear and the rolls. The reduction gear is required to provide the necessary torque at a suitable speed. Finer adjustment of the torque or speed of the rolls is provided using feedback based on torque data sampled using transducers and controlling the tension applied to the semi-finished product between successive stands. The tension is controlled by adjusting the power of the motors, which may be adjusted individually. The adjustment system, which is usually electronic, is able to coordinate operation of the different stands that make up the rolling mill, which are therefore joined by a logical link. Different systems with motors of significant size that can also work without

reduction gears are sometimes used in single-stand mills. Two motors may be used, one for each roll of the stand. This type of solution has been used to date in high-power arrangements, usually required for low-precision processes, such as the formation of steel sheet rolled into coils or similar. The size of the motors used in such applications has limited the application thereof in single-stand mills.

The efficiency of the control described above in rolling mills with multiple stands in series is limited for a range of reasons. Primarily, the mechanical nature of the type of motor commonly used for the medium powers commonly required in this type of process presupposes a variability of the speed as a function of torque, the oscillations in which therefore result in the speed diverging from a target value, until the

adjustment system intervenes. Furthermore, the presence of the reduction gear, which includes a large number of mechanisms, having a certain degree of play and elasticity, generates significant inertia that delays the effect of adjustment, with the possible introduction of oscillations around the target value .

The delayed effect of any adjustment intervention generates several problems, in particular if the rolling mill comprises a high number of stands in series. As the rolled material passes through the different stands in the series, the

progressive reduction of the area of the cross section thereof results in a speed increase. The distance between two

successive stands, to enable the flow of rolled material to be controlled, taking due account of the inertia of the control system, must be suitable and increase as the speed of the rolled material increases, assuming significant values, for example even values of around 5 m towards the end of the rolling mill, which increases the space required for a mill. The prior art includes two ways of controlling the flow of the bar being rolled known as tension and loop, depending on the response times of the electro-mechanical system that actuate the rolls of the stands.

In the first stands of a rolling mill, the speed values reached are such as to enable the tension to be controlled. However, at the speeds commonly reached at the final stands in the series, tension control is often inadequate. The problem can be obviated by introducing a loop control system: means for deflecting the bar are provided between two stands, including in consideration of the fact that in said stands the cross section is already small enough to enable significant flexion within the scope of elastic deformation, and optical or other systems check the presence and extent of the loop thus formed to adjust the speed of the stands.

Figure 2 is a schematic view of the layout of a multiple-stand mill .

In rolling mills, or between two successive stands, there are always guides to channel the bar being rolled. For the reasons specified above, these guides are not simply used for safety reasons, but they perform an active role in holding the bar in an acceptable position. These guides must therefore be

dimensioned and designed in consideration of this role, in particular the possibility of the bar being rolled sticking in the guide must be avoided.

The reduction gear also significantly increases the size and cost of the equipment, and considerably increases energy consumption and noise generation.

In current mills, in particular mills with several stands in series, the presence of the splitter/pinion stand, where used and which links the rotation speed of the rolls of a stand, requires the diameter of the rolls to be kept constant within strict tolerances. It is common practice to remove a layer of material from that surface in order to recondition the

original working profile. This means that the diameter of the roll are reduced. This generates the additional limitation of requiring the rolls of a stand to always be worked

simultaneously and generally preventing a single roll from being replaced or a roll from being swapped with a roll from another stand, even if it is compatible.

The aforementioned problems have been addressed by the present invention, by a rolling mill for the production of sections or sheets, having a plurality of rolling stands each having two oppositely arranged rolls, characterized in that the rolls of at least two of said stands are driven by at least one

synchronous motor per stand, said synchronous motor being supplied by means of a frequency changer, in particular an inverter, the speed of the motors of said at least two stands being variable in an individual but coordinated manner.

According to a preferred aspect, each stand has at least two motors, each able to drive one of said rolls.

According to another preferred aspect, both motors are able to drive the rolls directly.

According to another aspect, the motors are permanent-magnet motors, preferably torque motors.

Preferably, the motors are not linked to one another

mechanically .

According to another aspect of the invention, there is a control system enabling the frequency at which the different motors of the stands are powered to be varied individually but appropriately coordinated with each other. Such a control system may include a logic unit able to use data from appropriate sensors or transducers on the stand, or rolling mill such as torque sensors. Preferably, the torque may be detected on the basis of the current drawn by the motor directly by the motor, which can be detected immediately by the control system, the speed depending on the power supply frequency, determined by the control system itself. This enables the tension between two successive stands to be controlled. According to another possible aspect of the invention, there may be sensors able to detect the formation of loops or positional shifts by the bar between two

successive stands.

The invention also relates to a rolling method undertaken in a mill such as the one described above.

The present invention is illustrated in greater detail through the description of a preferred embodiment, provided purely by way of a non-limiting example in consideration of the scope of protection of the claims, with reference to the attached figures, in which:

Figure 1 (already mentioned above) is a schematic front view of a stand in a mill according to the prior art,

Figure 2 (already mentioned above) is a schematic view of a multiple-stand mill according to the prior art,

Figure 3 is a schematic front view of a stand usable in a mill according to the present invention,

Figure 4 is a schematic front view of a stand usable in a mill according to a different aspect of the present invention, Figure 5 is a schematic front view of a stand usable in a mill according to another aspect of the present invention.

With reference to Figure 3, a stand may include two rolls 4, which may be appropriately shaped in consideration of

requirements. The diameter of the rolls may be identical or different, as required. As commonly occurs, the rolls are mounted on suitable supporting elements. The stand may be fitted with all necessary or useful equipment and devices, according to common practice in the technological field in question .

According to a preferred aspect of the invention, the rolls are connected to the motors by the roll spindles 7 and the joints 8, and each roll is connected to one of the two motors. The joints may be of any suitable type, for example constant- velocity joints supporting positional oscillations or

variations of the rolls during operation, as normally occurs in mills according to the prior art.

As already mentioned, the motors used in at least some of the stands are synchronous motors, preferably permanent-magnet motors fitted with inverters able to power the motor with alternating current at a predetermined frequency, and

preferably controlled variable motors. The arrangement of the motors in Figure 3 enables good accessibility to both motors for maintenance to be combined with small overall size. Other arrangements are possible, including arrangements in which some or all of the roll spindles are removed and the motors are assembled directly at the roll ends, possibly on movable supporting elements, to obviate the need for joints, as shown in Figure 5.

According to another preferred aspect, the motors are the motors commonly referred to as "torque motors", motors able to generate high torque at low revolutions. Motors of this type have a structure with a permanent-magnet ring-shaped rotor, with a diameter significantly greater than the rotors of ordinary synchronous or asynchronous electric motors. The high number of magnets or pole shoes associated with the greater diameter enable high torques to be achieved and eliminate the need to run the motor at high revolutions to obtain high torques through a reduction gear. Indeed, motors of this type are currently used in direct-drive equipment, i.e. equipment with no reduction ratio between the number of revolutions of the motor and the number of revolutions of the structure driven by it.

According to a preferred aspect of the invention, the motors are able to transmit motion to the rolls directly, in at least some of the stands of the mill, as shown in the figures.

It is also possible for both rolls to be connected to a single motor of a specific type, by means of a splitter and a

reduction gear or multiplier, as required.

According to another embodiment, some of the stands may be provided with direct-drive motors, while others may be

connected by reduction gears or multipliers, depending on the type of motor and the anticipated speed of the rolls. For example, it is possible for all of the motors of the mill to be synchronous high-torque motors, as described below, and for the rolls of the first stands—where the semi-finished product moves at lower speeds—to be driven directly by the motor, and the rolls of the last stands—where the speed is higher—to be driven by motors via multipliers and, if a single motor per stand is used, splitters.

It should be noted that, when using motors of this type, the greater space inside the rotor may—if desired—be used to house all or part of the joint, connecting the joint appropriately to the rotor on the side opposite the roll driven, thereby providing an additional opportunity to reduce overall size, as shown schematically in Figure 4.

If the build solutions adopted so require, such as in the case of assembly at the roll ends, motors of the type mentioned above—which have the stator inside the ring-shaped rotor instead of an external stator, as is more common—may be used. An advantage of using synchronous machines is the stability of the rotational speed, even in the presence of variations of the torque transmitted by the roll, unlike other types of machine which react both to the variations normally related to the process, such as the insertion phase of the billet or semi-finished product, and to disturbances, with speed

variations that affect the entire machinery, in particular in the case of a rolling mill having several stands in series, as seen above. This enables the machinery to be controlled much more accurately.

The motors may be controlled by varying the power supply frequency and accuracy may be made exclusively dependent on the resolution of the inverter used to power the motor. Where appropriate, the control system may be able to adjust other operating parameters of the motor, such as the voltage or current applied, for example as a function of torque, to optimize electrical operation. Unlike adjustment on an

asynchronous motor, the adjustment varies speed independently of torque. Furthermore, the absence of a reduction gear reduces the time required for a motor speed adjustment to effect the roll.

This characteristic is used, according to the present

invention, in a rolling mill with several stands. Several motors may be controlled in a coordinated manner using a control system, which may comprise a logic unit, preferably programmable, that can receive operating data, in particular torque and speed detectable by the control system itself on the basis of the supply current or, where present, detectable by sensors and/or transducers, and that can act on the motors, in particular via the power inverters. The control unit uses suitable logic, with software based on such logic.

Using this type of control, it is possible—for example—to control the tension much more accurately, even if the semi ^¬ finished product is travelling at significant speed, on the basis of the torque applied to the individual rolls. This makes it possible to reduce the distance between two

successive stands, including between the last stands in the series, including in consideration of the fact that the formation of loops may be reduced or entirely eliminated, potentially entirely eliminating this type of control or using it only to actuate alarms. Additionally or alternatively, the potential maximum speed of the semi-finished product may be increased, making it possible to create rolling mills with more stands than in the prior art, potentially providing the final stands in this series with multipliers, as seen above, and/or increasing the speed from the first stands. Indeed, to adjust the mechanical tension applied to the semi-finished product between two stands, the system may be able to vary the speed of one or more rolls to set the torque transmitted to a target value, for short periods of time or continuously, depending on the response obtained from the torque sensors, thereby preventing unwanted friction, and excessive tensile or compressive stress, preventing or reducing the formation of loops .

The simplicity of the structure, and in particular the

elimination of reduction gears where possible, considerably reduces energy consumption (in some cases by 10%) , the noise generated by the mill and the need for maintenance.

Furthermore, fewer parts require continuous lubrication, generating a significant saving. It should be noted that stands and rolling mills may be built according to the present invention using existing machinery, such as the machinery shown in Figure 1, by simply removing the motors and reduction gears and applying synchronous motors, for example to the roll spindles, replacing the joints where necessary.

Another advantage related to the stable rotation speed and the speed with which a variation therein affects the process is that sensors or transducers, for example torque sensors, may be used to obtain very precise correlations and data on system operation, in particular in transients. As shown, this is very useful for making adjustments based on feedback, to calibrate the control system, to obtain data that may be used for process optimization, and even to design components of the mill such as rolls, or new mills. For this purpose, the sensors and transducers used may be those normally found in the mill to regulate operation, or sensors and/or transducers added for study or calibration, for example during piloted operations. This also facilitates the development of control software able to self-correct on the basis of data detected and/or to adapt automatically to changing conditions (for example one or more rolls being changed either for maintenance or for another type of work) , without the need to reset the system manually, or reducing the number of instructions involved in such an operation.

Another advantageous aspect of the present invention concerns the rolling of sections with variations along the longitudinal cross section thereof. For example, sections with surface grooves or ribs, such as ribbed rebars for reinforced concrete or bars with ribs intended to form a basic thread, or any other type of rib. This type of structure can currently be created using suitably shaped rolls in the last stand before obtaining the finished product, or in a single-stand mill, which limits the depth or height of the rib or requires passes in single high-power stands providing significant deformation in a single pass. The perfect synchronization of the rolls of two successive stands obtained using the present invention makes it possible to create discontinuous structures with passes through successive stands with shaped rolls to obtain structures with ribs or grooves or engravings. This may be used to increase the thickness or depth of the structures, since the synchronization ensures that the ribs and grooves on the rolls of the stands following the first stand match the structure being rolled, without high mechanical stresses generated when shaping the structure entirely in a single stand. It is also possible to roll more complex forms, previously only possible by molding. Furthermore, in stands provided with two motors, the absence of any mechanical link between the two rolls enables the speed thereof to be

controlled individually on the basis of predetermined control schemes and/or control logic that uses operating data

similarly to the manner seen above and appropriately coordinated, but without linking the speed of a roll to the speed of the second roll with a fixed ratio, which is the case if a distributor is used. This helps to avoid errors caused by machining tolerances, for example in the construction of rolls or when restoring the working surface thereof, for example preventing friction between the semi-finished product and the rolls, thereby helping to reduce wear and increase product quality. Furthermore, it is no longer necessary to keep the ratio between the diameters within strict tolerances, thereby avoiding all of the limitations discussed above.

As already mentioned, the present invention is preferably applied in mills in which, on account of the semi-finished product passing through stands arranged in series, it is possible to work with medium or low torques applied to the rolls. A particular aspect of the invention is that motors of the type described above with a rated torque of between 8000 and 15000 Nm are particularly suitable in terms of overall size, and are suited in particular to the solution of each one driving a roll directly using a single motor, in a mill according to the present invention.

According to another aspect of the invention, the motors may be adapted to operate at speeds between 20 and 1500

revolutions /minute .

These values are also the typical torque values applied to rolls in mills having several stands in series, in which the coordinated precise control described above is most useful. According to a particular aspect, the voltages used may be low, for example around 600 V.