A cleaning apparatus for metal strips and process thereof

Abstract

A cleaning apparatus for cleaning a cold-rolled metal strip advancing along at least one plane, said apparatus comprising - at least one first laser cleaning device (40) for cleaning a first strip surface; - at least one second laser cleaning device (40') for cleaning a second strip surface, opposite said first surface; - a casing (22) containing inside at least partially said at least one first laser cleaning device (40) and said at least one second laser cleaning device (40'), said casing (22) being configured to be crossed by the strip; - at least one suction and purification system (32, 33, 34) for sucking up and purifying fumes produced inside said casing (22) during the cleaning of the strip by means of laser.

Description

[0001] A CLEANING APPARATUS FOR METAL STRIPS AND PROCESS THEREOF

[0002] ***********

[0003] Field of the invention

[0004] The present invention relates to the field of continuous plants for treating ferrous or non-ferrous metal strips arranged downstream a cold-rolling mill. More specifically, the invention relates to a cleaning apparatus for upper and lower surfaces of the strip and process thereof, said apparatus being arranged upstream a heating / annealing oven or a coating section, such as, for example painting. Background art

[0005] As is known, ferrous and non-ferrous metal strips undergo the cold-rolling process, allowing obtaining the thickness required by the end user of the aforesaid strip.

[0006] During the cold-rolling process, a fluid is normally used for lubricating the rolling compartment and removing the heat produced by the plastic deformation process. The type of fluid is defined by the features of the metal strip and by the surface finishing desired to be obtained after the cold-rolling process. By way of nonlimiting example, emulsions of water and rolling oil, or pure rolling oil, can be used for ferrous strips; whereas for non-ferrous strips, e.g., made of aluminum, emulsions of water and rolling oil, or kerosene with opportune additives, are used. In all of the above cases, after the cold rolling, the strip is uniformly covered, on both the upper and lower surfaces, with a micrometric layer of rolling oil residues and metal residues of the same rolled metal as a result of sliding phenomena and contact between the metal strip and the rolling cylinders.

[0007] These residues must be removed from both strip surfaces before each annealing process and / or coating process to avoid potential problems of different surface quality and / or poor coating adherence.

[0008] In the prior art, the most commonly used technology for obtaining said result is cleaning with an alkaline solution having strong bases, such as potassium or sodium hydroxide. It is a physical chemical cleaning process where the rolling oil present on the surface is removed by contact with the aqueous solution with dissolved hydroxide. Moreover, the cleaning action is improved by the use of abrasive brushes acting mechanically on the surfaces to be cleaned. In plants where high cleaning efficiency is required, an electrolytic cleaning cell is used in addition to the above, where the conduction effect of the current increases the surface cleaning capacity by virtue of the disassociated ions.

[0009] These surface cleaning methods include a number of undesired aspects, such as for example:

[0010] - High water consumption as a result of the need to replace the evaporated or removed solution to maintain the efficiency of the cleaning process, and as a result of the need to remove the hydroxide residues from the surface to avoid strip surface quality damage;

[0011] - High thermal energy consumption, in the form of steam or hot water, to keep the process baths hot, since the temperature increases the efficiency of the process;

[0012] - Need to treat the wastewater to eliminate the pollutants present in the water, such as chemical products, oil and iron powder;

[0013] - Need to suck up the toxic vapors produced by the process, which must be purified through opportune washing column before introduction in the environment, with further water consumption which must be periodically sent to the wastewater treatment plant;

[0014] - Potential negative impact on the environment relating to spillages or leaks caused by faulty tanks or recirculation tubes;

[0015] - Potential risk for the environment connected to the storage of considerable amounts of harmful toxic products in the event of unforeseeable disasters, such as earthquakes, floods or fires;

[0016] - Potential risk for operators caused by the caustic products used in the cleaning section in the event of contact, inhalation or swallowing of the chemical solution or the vapors produced therefrom.

[0017] Considering the above, the need for an innovative revision of cold-rolled metal strip cleaning apparatuses in metal strip coating / annealing lines is apparent in order to overcome the aforesaid drawbacks.

[0018] Summary of the invention

[0019] It is an object of the present invention to produce a ferrous or non-ferrous metal strip continuous cleaning apparatus capable of reducing to the minimum the environmental impact until it is almost eliminated. In particular, this means not using harmful toxic products, not producing wastewater, minimizing the negative effects of the fumes emitted, simultaneously reducing the flow rate and removing all safety risks relating to the use of harmful toxic products for the environment and employees following the process.

[0020] It is a further object of the invention to produce a more efficient cleaning apparatus, which allows accelerating the outflow of the fumes, produced by the interaction of the laser beam with the dirt present on the metal strip surfaces, from the inner volume of the closed casing crossed by the metal strip, preventing the dwelling of said fumes inside the casing from soiling the clean parts of the metal strip surfaces.

[0021] It is another object of the invention to produce an apparatus that eliminates the consumption of thermal energy, simultaneously minimizing the consumption of electricity.

[0022] It is another object of the invention to produce a highly efficient and flexible cleaning process capable of quickly adapting to the different conditions of the strip and to the process transients.

[0023] It is another object of the invention to obtain a surface cleaning level comparable with, or better than the one usually obtained with the current physical chemical cleaning process.

[0024] The present invention achieves at least one of said objects, and other objects, which will be apparent in light of the present description, by means of a cleaning apparatus for cleaning a cold-rolled metal strip advancing along at least one plane, said apparatus comprising

[0025] - at least one first laser cleaning device for cleaning a first strip surface;

[0026] - at least one second laser cleaning device for cleaning a second strip surface, opposite said first surface;

[0027] - a casing containing at least partially therein said at least one first laser cleaning device and said at least one second laser cleaning device, said casing being configured to be crossed by the strip;

[0028] - at least one suction and purification system for sucking up and purifying fumes produced inside said casing during the cleaning of the strip by means of laser; characterized in that there is provided inside the casing at least one injection device configured to inject jets of air or inert gas so as to convey said fumes toward said at least one suction and purification system.

[0029] A further aspect of the invention relates to a cleaning process for cleaning a metal strip by means of the aforesaid apparatus, comprising the following steps:

[0030] - providing a cold-rolled metal strip advancing along at least one plane and crossing an inner volume of the casing;

[0031] - cleaning a first strip surface by means of the at least one first laser cleaning device;

[0032] - cleaning a second strip surface, opposite said first surface, by means of the at least one second laser cleaning device;

[0033] - sucking up and purifying the fumes produced inside said casing, during the cleaning of the strip, by means of the at least one suction and purification system; characterized in that there is provided, inside the casing, an injection of jets of air or inert gas, by means of at least one injection device, for conveying said fumes toward said at least one suction and purification system.

[0034] Further advantages of some embodiments of the invention are:

[0035] - An elevated production flexibility regardless of the horizontal or vertical configuration of the cleaning apparatus;

[0036] - Possibility of rapidly adapting the process conditions to the different conditions of the surfaces to be treated, such as different oils or a different metal of the surface to be treated, e.g., when passing from steel to aluminum or vice versa;

[0037] - Possibility of obtaining improved cleaning levels with the combined use of several laser devices for cleaning a same surface;

[0038] - Easy modification of the process cleaning parameters as the strip inlet conditions vary, such as width or speed; in fact, the response time goes from minutes / hours for the chemical processes to milliseconds for the Laser.

[0039] Further features and advantages of the invention will become more apparent in light of the detailed description of exemplary but non-exclusive embodiments.

[0040] The dependent claims describe particular embodiments of the invention. Brief description of the figures

[0041] In the description of the invention reference is made to the accompanying drawings, provided by way of non-limiting example, in which:

[0042] Figure 1 depicts a diagram of a first embodiment of an apparatus according to the invention;

[0043] Figure 2 depicts a diagram of a second embodiment of the apparatus according to the invention;

[0044] Figure 3 depicts a diagram of a third embodiment of the apparatus according to the invention;

[0045] Figure 4 depicts a diagram of a first variant of part of the apparatus according to the invention;

[0046] Figure 5 depicts a diagram of a second variant of said part of the apparatus according to the invention;

[0047] Figure 6 depicts a diagram of a third variant of said part of the apparatus according to the invention;

[0048] Figure 7 depicts a first part of an optical diagram of the apparatus according to the invention;

[0049] Figure 8 depicts a second part of an optical diagram of the apparatus according to the invention;

[0050] Figure 9 depicts a definition of the control axes of the laser beam footprint on the strip according to the apparatus according to the invention;

[0051] Figure 10 depicts the value of the footprint surface in relation to the power of a laser source for two different irradiance values.

[0052] The same reference numerals and letters in the drawings identify the same elements or components.

[0053] Description of exemplary embodiments of the invention

[0054] With reference to the Figures, there are illustrated examples of a cleaning apparatus for cleaning a continuous cold-rolled metal strip advancing along at least one plane.

[0055] As is known, the metal strip is a product that has a dimension, i.e. , the thickness, considerably smaller than the other two dimensions, i.e., the length and the width.

[0056] The two main surfaces of the metal strip have cold-rolling residue products, which form a continuous layer on both the upper and lower surfaces, said layer consisting of rolling oil and metal powder produced by contact, and in particular by sliding, between the metal strip and the working cylinders of the cold-rolling stands.

[0057] In all embodiments of the invention, the cleaning apparatus comprises

[0058] - at least one first laser cleaning device 40 for cleaning a first surface of strip 23;

[0059] - at least one second laser cleaning device 40’ for cleaning a second surface of strip 23, opposite said first surface;

[0060] - a casing 22 containing inside at least partially the at least one first laser cleaning device 40 and the at least one second laser cleaning device 40’, said casing 22 being configured to be crossed by the strip and to separate the laser radiation zone from the external environment;

[0061] - at least one suction and purification system 32, 33, 34 for sucking and purifying the fumes produced inside the casing 22, during the cleaning of the strip, by means of laser.

[0062] For example, the at least one first laser cleaning device 40 faces toward a first surface of strip 23, while the at least one second laser cleaning device 40’ faces toward a second surface of strip 23, opposite the first surface.

[0063] Preferably, as illustrated in Figures 1-3, the casing 22 is a closed casing, optionally a closed container, e.g. in the shape of a parallelepiped, provided with two openings or slits dimensioned for the entering and exiting of the strip from said casing, respectively, and having the task of separating the laser radiation zone from the external environment.

[0064] Advantageously, there is provided inside the casing 22 at least one injection device 36, 37 configured to inject jets of air or inert gas so as to convey the fumes toward the at least one suction and purification system 32, 33, 34. This allows accelerating the outflowing of the fumes from the inner volume of the casing 22, preventing the residence of said fumes inside the casing from dirtying the clean parts of the metal strip surfaces.

[0065] By way of example, there is provided only one suction and purification system comprising

[0066] - a suction duct 32, arranged outside the casing 22 and communicating with the inner volume thereof;

[0067] - a suction fan 33 arranged along the suction duct 32; - a possible filtering and purification unit 34 arranged downstream of said suction fan 33.

[0068] A chimney 35 is provided downstream of said filtering and purification unit 34 for the emission into the atmosphere of the filtered and purified fumes.

[0069] For example, said at least one injection device 36, 37 can comprise one or more injection nozzles adapted to be directed for conveying the fumes, produced by the interaction of the laser beam with the dirt present on the strip surfaces 23, towards the suction duct 32.

[0070] Preferably, said at least one injection device 36, 37 is positioned so as to inject jets of air or inert gas along a direction transversal to the advancing direction of the metal strip, e.g. along the width of the metal strip, so as to convey the fumes toward an inner wall of the casing 22 onto which the at least one suction and purification system 32, 33, 34 is arranged.

[0071] For example, a first injection device 36 and a second injection device 37 can be provided, e.g., each comprising one or more injection nozzles, configured to inject jets of air or inert gas along said direction transversal to the advancing direction of the metal strip.

[0072] The filtering and purification unit 34 can comprise, for example, a filter box comprising an inlet for the air contaminated by said fumes; filtering units; an outlet for the clean air; and preferably further comprising a filter regeneration system, a powder collection hopper and a powder discharge pan.

[0073] However, the option of providing two or more suction and purification systems, cooperating with the inner volume of the casing 22, is not excluded.

[0074] In a preferred variant, the at least one first laser cleaning device 40 and the at least one second laser cleaning device 40’ comprise:

[0075] - a respective laser source 25, 25’ arranged outside the casing 22;

[0076] - a respective first optical group 26, 30 arranged inside the casing 22, for collimating and focusing a laser beam coming from the corresponding laser source;

[0077] - a respective second optical group 27, 31 arranged inside the casing 22 for scanning a corresponding strip surface by means of the laser beam coming from the corresponding first optical group 26, 30. The scanning movement by means of the laser beam occurs, back and forth, along a direction transverse, preferably perpendicular, to a strip advancing direction and at an opportune speed, so as to clean the strip surface along the width thereof.

[0078] Preferably, each first optical group 26, 30 comprises, in sequence, or consists of a collimator mirror 43 and a focuser mirror 44, while each second optical group 27, 31 comprises, in sequence, or consists of a rotating polygonal mirror 46 and a parabolic mirror 47 (Figures 7-8).

[0079] Each laser source 25, 25’ is connected to the respective first optical group 26, 30, in particular it is connected to the respective collimator mirror 43 by means of a corresponding optical fiber connection 29, 29’.

[0080] The collimator mirror 43 allows collimating the laser beam.

[0081] The focuser mirror 44 allows deviating and focusing the laser beam, with opportunely modified geometrical beam dimensions, towards the rotating polygonal mirror 46.

[0082] The second optical group or optical scanning group is preferably a laser scanning head with a polygonal motor using a rotating polygonal mirror 46 for scanning a laser beam on a surface. The rotating polygonal mirror 46 rotates at high speed, reflecting the laser beam onto the surface, forming a diagram of charged areas corresponding to the image.

[0083] In particular, upon rotation, the rotating polygonal mirror 46 deviates the laser beam onto the parabolic mirror 47 forming the scan of the laser beam on the surface of the strip 23 to be cleaned. In fact, the parabolic mirror 47 is designed to reflect and focus the laser beam at 90 degrees or any other convenient angle.

[0084] In a preferred variant, illustrated diagrammatically in Figures 7-8, the collimator mirror 43 and the focuser mirror 44 are arranged inside a first container 48, while the rotating polygonal mirror 46 and parabolic mirror 47 are arranged inside a second container 49. This configuration allows opportune treatment and / or modifying of the laser beam footprint, which is caused to move on the surface of the metal strip.

[0085] The focalized laser beam 45, exiting the focuser mirror 44, exits the container 48 and enters the container 49 reaching the rotating polygonal mirror 46. Optionally, in all embodiments of the invention, at least one drying and purification device 60, arranged in proximity of the optical groups 26, 27, 30, 31 can be provided for drying and purifying the air so as to keep both the first optical groups 26, 30 and the second optical groups 27, 31 clean. Drying the air allows keeping the optical groups cool and possibly slightly overpressure, to prevent the humidity or powder from negatively impacting the surface quality of the optics.

[0086] This drying and purification device 60 can comprise, for example, filtering and dehumidification elements for compressed air, produced by a compressor included in said device that pushes air, which circulates around the optical groups, toward said filtering and dehumidification elements.

[0087] By way of example, Figure 1 diagrammatically illustrates two drying and purification devices 60 inside the casing 22. A first drying and purification device is arranged in proximity of the optical groups 30, 31 ; while a second drying and purification device is arranged in proximity of the optical groups 26, 27.

[0088] In all variants of the invention, there are provided, inside the casing 22, at least two motorized rollers 28, 28’, 38 for continuously advancing the strip. These motorized rollers can be deflector rollers and / or pass line rollers, the latter being useful when it is necessary to minimize the deviation of the strip from the pass line in proximity of the laser cleaning devices, keeping a flat surface for the strip and devoid of vibrations, on which the laser beam moves transversely with respect to the travel direction, performing the surface cleaning. In particular, the pass line rollers are configured to keep the strip at a predetermined distance from the corresponding laser cleaning device.

[0089] If contact is provided with the laser beam, said rollers are preferably coated with tungsten carbide or chromium carbide depending on whether the temperature reached on the surface thereof is lower or higher than 300°C.

[0090] In a first embodiment of the apparatus, illustrated in Figure 1 , said at least two motorized rollers comprise at least:

[0091] - a first pair of pass line rollers 28 arranged along a first advancing plane for the strip, and between which there is arranged the at least one first laser cleaning device 40 for cleaning an first strip surface facing upwards; - a deflector roller 38 for deviating the strip from said first advancing plane to a second advancing plane, positioned above said first advancing plane, so that the second strip surface is facing upwards;

[0092] - and a second pair of pass line rollers 28’ arranged along said second advancing plane and between which there is arranged the at least one second laser cleaning device 40’ for cleaning the second strip surface.

[0093] According to the example in Figure 1 , the first optical group 26 and the second optical group 27 of the first laser cleaning device 40 are arranged above the first advancing plane and below the second advancing plane; while the first optical group 30 and the second optical group 31 of the second laser cleaning device 40’ are arranged above the second advancing plane.

[0094] In this case, the strip 23 preferably enters the casing 22 at a lower zone thereof and exits said casing 22 at an upper zone thereof.

[0095] Downstream of the second pair of pass line rollers 28’ there is provided at least one further deflector roller 38’ for deviating the strip towards the outlet opening 2 of the casing 22.

[0096] Advantageously, the two pairs of pass line rollers 28, 28’ delimit two cleaning positions, the first for the upper surface and the second for the lower surface of the strip, so that after each laser cleaning device the respective strip surface is clean with only a minimum residue of polluting elements, such as oil and powder. In particular, after cleaning the upper surface, the strip 23 is deviated by the deflector roller 38 so as to bring the lower surface of the strip facing upwards. In this way, both laser cleaning devices 40, 40’ can be mounted above the surface to be cleaned with the relative process and accessibility advantages.

[0097] Alternatively, in a variant not illustrated in which the strip 23 enters the casing 22 at an upper zone thereof and exits said casing 22 at a lower zone thereof, the second advancing plane is positioned below the first advancing plane. In this case, the first optical group 26 and the second optical group 27 of the first laser cleaning device 40 are arranged above the first advancing plane; while the first optical group 30 and the second optical group 31 of the second laser cleaning device 40’ are arranged above the second advancing plane and below the first advancing plane. In a second embodiment of the apparatus, illustrated in Figure 2, said at least two motorized rollers comprise, in sequence, at least:

[0098] - a first pair of pass line rollers 28 arranged along a first strip advancing plane and between which there is arranged the at least one second laser cleaning device 40’ for cleaning the second strip surface facing downwards, i.e. the lower surface;

[0099] - and a second pair of pass line rollers 28’ immediately downstream of the first pair and arranged along a second advancing plane, above or below said first advancing plane, and between said pass line rollers 28’ there is arranged the at least one first laser cleaning device 40 for cleaning the first strip surface facing upwards, i.e. the upper surface.

[0100] According to the example in Figure 2, the first optical group 30 and the second optical group 31 of the second laser cleaning device 40’ are arranged below the first advancing plane, while the first optical group 26 and the second optical group 27 of the first laser cleaning device 40 are arranged above the second advancing plane; or vice versa.

[0101] Again in this case, the two pairs of pass line rollers 28, 28’ delimit two cleaning positions, the first for the lower surface and the second for the upper surface of the strip, or vice versa, so that after each laser cleaning device the respective strip surface is clean, with only minimum residue of polluting elements, such as oil and powder. In particular after cleaning a first surface, the strip 23 is slightly deviated by means of the cooperation between the last pass line roller 28 of the first pair and the first pass line roller 28’ of the second pair, proximal to each other, for passing from the first advancing plane to the second advancing plane, preferably parallel to the first plane. In this case, the strip continues advancing inside the casing 22 without the surfaces thereof being overturned.

[0102] In a third embodiment of the apparatus, illustrated in Figure 3, said at least two motorized rollers comprise two deflector rollers 38, preferably only two deflector rollers, configured to deviate the strip 23 from a first advancing plane, preferably horizontal, to a second advancing plane transversal to the first advancing plane, said second plane preferably being vertical, and then to a third advancing plane transversal to the second advancing plane, preferably horizontal. Advantageously, the at least one first laser cleaning device 40 and the at least one second laser cleaning device 40’ are arranged on opposite sides of the second advancing plane so that the cleaning of the two strip surfaces can also be carried out at the same time.

[0103] In this case, the pair of deflector rollers 38 delimits a region with both cleaning positions, the first for the lower surface and the second for the upper surface of the strip, so that after each laser cleaning device the respective strip surface is clean, with only minimum residue of polluting elements, such as oil and powder. In particular the two deflector rollers 38 have the aim of keeping the strip surfaces flat and devoid of vibrations.

[0104] In the examples in Figures 1 -3 there are provided only one first laser cleaning device 40 and only one second laser cleaning device 40’.

[0105] However, in all of the above-described embodiments, there can be provided at least two first laser cleaning devices 40, 41 for cleaning the first surface of strip 23 and / or at least two second laser cleaning devices for cleaning the second surface of strip 23, arranged along a respective strip advancing plane.

[0106] In the example in Figure 4, two first laser cleaning devices 40, 41 are illustrated arranged one after another along a strip advancing direction for carrying out successive cleaning treatments on the first surface of strip 23. Likewise, additionally or alternatively, there can be provided two second laser cleaning devices (not shown) arranged one after another along the strip advancing direction for carrying out successive cleaning treatments on the second surface of strip 23, opposite the first surface.

[0107] For example, the first laser cleaning devices 40, 41 are arranged on a first side at the first surface of strip 23, while the second laser cleaning devices are arranged on a second side at the second surface of strip 23.

[0108] In this variant, the layer of polluting elements is partially removed by the first laser cleaning device 40 and cleaning is completed by the second laser cleaning device 41 . After the first laser cleaning device 40, the strip 23 surface is in a state of intermediate cleaning, with residue still present on the surface. This residue is completely eliminated by the second laser cleaning device 41 (Figure 4). In the example in Figure 5, three first laser cleaning devices 40, 41 , 42 are shown arranged one after another along a strip advancing direction for performing successive cleaning treatments on the first surface of strip 23. Likewise, additionally or alternatively, there can be provided three second laser cleaning devices (not illustrated) arranged one after another along the strip advancing direction for carrying out successive cleaning treatments on the second surface of strip 23, opposite the first surface.

[0109] For example, the first laser cleaning devices 40, 41 , 42 are arranged on a first side at the first surface of strip 23, while the second laser cleaning devices are arranged on a second side at the second surface of strip 23.

[0110] In this variant, the layer of polluting elements is partially removed by the first laser cleaning device 40 and by the second laser cleaning device 41 , and cleaning is completed by the third laser cleaning device 42. After the first laser cleaning device 40, the surface of the strip 23 is in a state of intermediate cleaning, with residue still present on the surface. This residue is partially eliminated by the second laser cleaning device 41 and then completely eliminated by the third laser cleaning device 42.

[0111] Whereas, in the example in Figure 6, two first laser cleaning devices 40, 41 are illustrated, arranged side-by-side along a transversal direction, preferably perpendicular, to the strip advancing direction, so as to clean together a respective portion of the first strip surface along the width thereof, preferably a respective half of said portion along the strip width.

[0112] Likewise, additionally or alternatively, there can be provided two second laser cleaning devices (not illustrated) arranged side-by-side along said transversal direction so as to clean together a respective portion of the second strip surface, opposite the first strip surface, along the width thereof, preferably a respective half of said portion along the strip width.

[0113] This solution is particularly usable with strips having an elevated width.

[0114] For example, the first laser cleaning devices 40, 41 are arranged on a first side at the first surface of strip 23, while the second laser cleaning devices are arranged on a second side at the second surface of strip 23. Advantageously, the cleaning apparatus of the invention can be arranged in a hot- dip galvanizing line (HDGL), or in a continuous annealing line (CAL), in particular upstream of the annealing oven, and preferably downstream of a skin pass rolling stand.

[0115] Alternatively, this apparatus can be provided in a painting line, upstream of the painting section.

[0116] A cleaning process is described below for cleaning a metal strip by means of an apparatus according to any one of the preceding embodiments, comprising the following steps:

[0117] - providing a cold-rolled metal strip 23 advancing along at least one plane and crossing an inner volume of the casing 22;

[0118] - cleaning a first strip surface by means of the at least one first laser cleaning device 40;

[0119] - cleaning a second strip surface, opposite said first surface, by means of the at least one second laser cleaning device 40’;

[0120] - sucking and purifying the fumes, produced inside said casing 22 during the cleaning of the strip, by means of the at least one suction and purification system 32, 33, 34.

[0121] Advantageously, there is provided, inside the casing 22, an injection of jets of air or inert gas by means of at least one injection device 36, 37 for conveying the fumes toward said at least one suction and purification system 32, 33, 34.

[0122] In a variant of the process, the fumes are sucked up along the suction duct 32, externally connected to the casing 22 by means of the suction fan 33, and then filtered and purified by means of a filtering and purification unit 34.

[0123] In a preferred variant of the process of the invention, in the steps of cleaning the first strip surface and the second strip surface, a laser beam coming from a corresponding laser source 25, 25’, arranged outside the casing 22, is collimated and focused by means of a respective first optical group 26, 30, arranged inside said casing 22, and it is then used for scanning the corresponding strip surface along a direction transversal to an advancing direction of the strip, by means of a respective second optical group 27, 31 arranged inside said casing 22. The process of cleaning ferrous or non-ferrous metal strips can preferably be controlled by at least some of the following parameters:

[0124] - P = power of the laser beam emitted by the source 25, 25’ (kW);

[0125] - 1 = irradiance, i.e. , the power per surface unit (MW / cm2);

[0126] - 1 = dwell time (s), i.e., the dwelling time of the laser beam on the strip surface;

[0127] - F = fluence i.e., the energy absorbed per surface unit (J / cm2);

[0128] - v = strip advancing speed (m / s), on which plant productivity depends;

[0129] - %OL = overlapping percentage of the laser beam for two successive passages along the direction transversal to the strip advancing direction.

[0130] In particular, the fluence is the parameter that best defines the level of cleaning. If the fluence were too low, the strip would not be well cleaned; whereas, if the fluence were too high, the strip could be perforated.

[0131] The first optical groups 26, 30 and the second optical groups 27, 31 allow controlling the footprint 50 of the laser beam impacting the surface of the strip to be cleaned. Said footprint 50 moves along a direction Y, preferably orthogonal to the strip advancing direction X (Figure 9).

[0132] The shape of the laser beam footprint 50 on the strip can therefore be opportunely modified and it can be, for example, circular, elliptical with greater axis along axis Y, elliptical with greater axis along axis X.

[0133] The dimensions of the laser beam footprint 50 along axis Y and axis X are W and L, respectively, expressed in cm. It is possible to calculate the area A of the footprint 50 on the strip 23 from these dimensions. The dimension L is preferably always greater than, or equal to the dimension W, thus making an elliptical or circular surface of the footprint 50, respectively.

[0134] The ratio between the laser beam power and the area of the laser beam footprint 50 on the strip allows calculating the irradiance value according to the following formula: l= P / (A*1000) (MW / cm2).

[0135] Thus, by knowing the irradiance and laser beam power it is possible to obtain the area of the footprint 50:

[0136] A = P / (l*1000) (cm2). Figure 10 shows the trend of the surface of footprint 50 as the power varies for two extreme cases of irradiance.

[0137] Instead, the dwell time is calculated with the following formula: t= W / 1000 / vs (s) in which W is the length in mm of the footprint 50 along the axis Y and vsis the laser scanning speed along the axis Y expressed in m / s.

[0138] Fluence is calculated as the multiplication of irradiance by dwell time, as given by the following formula:

[0139] F = l*t*1000000 (J / cm2).

[0140] Instead, the percentage of overlapping of the laser beam for two successive treatments is given by the distance Sn covered by the strip during a rotation of the rotating polygonal mirror 46 with respect to the length L of the footprint 50 along axis X.

[0141] If said distance were:

[0142] - Sn > L, it would give a strip stretch having a length equal to Sn-L not treated by the laser beam, and there would be no overlapping between the two successive passages;

[0143] - Sn = L, the distance covered by the strip would coincide with the length L of the footprint 50, and there would be an overlapping equal to 100%;

[0144] - preferably Sn < L, the distance covered by the strip would be less than the length L of the footprint 50, and so there would be overlapping of less than 100% increasing the cleaning effect on the strip surface.

[0145] The laser cleaning of the strip surfaces, with the presence of oil and powder residues from the cold-rolling, is obtained by the simultaneous effect of two phenomena occurring on the product surface, specifically:

[0146] 1 ) the ablative effect whereby the energy transferred from the laser beam onto the strip surface causes the oil covering the strip surface to evaporate;

[0147] 2) vibration linked to the shock wave caused by the sudden increase in temperature as a result of the passage of the laser beam, and the relative thermal dilation and subsequent contraction, causing every element not integral with the base material of the strip to detach. Specific ranges of values of some laser cleaning process parameters have been carefully selected from the above considerations, allowing obtaining optimal cleaning of the strip, simultaneously preventing damage to the surface, which can occur when the fluence applied is too high, with a concurrent relatively high dwell time.

[0148] Advantageously, the fluence F is comprised in a range from 0.2 to 5.5 J / cm2.

[0149] Preferably, the power P of the laser beam emitted by the laser source 25, 25’ is comprised in a range from 5 to 60 kW; the dwell time of the laser beam hitting the strip surface is comprised in a range from 40 to 500 ns; the irradiance I of the laser beam is comprised in a range from 2 to 15 MW / cm2; the strip advancing speed is comprised in a range from 0.2 to 8 m / s.

[0150] The overlapping percentage of the laser beam between two successive passages along said transversal direction can be comprised in a range from 30 to 90%.

[0151] Preferably, it is attempted to obtain an irradiance I between 4 and 10 MW / cm2and a dwell time between 70 and 200 ns, resulting in a fluence of about 0.2 to 2 J / cm2. The above parameters are also selected as a function of the width of the strip and the conditions of the presence of the polluting elements on the surface.

[0152] The width of the strip to be cleaned is preferably comprised in a range from 500 to 2500 mm.

[0153] Rolling oil in a range comprised from 10 to 100 mg / m2and metal powder in a range comprised from 10 to 300 mg / m2are generally found on each surface of the steel strip, after cold rolling.

[0154] In a first variant of the process of the invention, in order to achieve optimal cleaning with low productivity, it is possible to work with a fluence inversely proportionate to the speed, preferably from 0.8 to 3 J / cm2, obtained by means of high laser beam power kept constant, preferably in a range from 20 to 60 kW; a strip advancing speed preferably in a range from 0.5 to 2.5 m / s; and a dwell time inversely proportionate to the strip speed, preferably in a range from 40 to 500 ns. In a second variant of the process of the invention, in order to achieve optimal cleaning with high productivity, but significant electricity consumption, it is possible to work with a fluence having an intermediate value, preferably from 1 to 2.5 J / cm2, obtained by means of high laser beam power, preferably in a range from 20 to 60 kW, kept constant; a high strip advancing speed, preferably in a range from 2 to 5 m / s; and an intermediate dwell time, preferably in a range from 70 to 200 ns.

[0155] In a third variant of the process of the invention, in order to achieve acceptable cleaning with low productivity, but considerable energy saving, it is possible to work with a low fluence, kept constant, preferably from 0.3 to 2 J / cm2, obtained by means of a low laser beam power, preferably in a range from 5 to 40 kW; a low strip advancing speed, preferably in a range from 0.5 to 2 m / s; and a constant dwell time, preferably in a range from 60 to 200 ns.

[0156] In a fourth variant of the process of the invention, as the strip advancing speed varies, in order to keep the fluence constant, the laser beam footprint surface is kept constant on the strip, while the irradiance and, therefore, the laser beam power are varied in a directly proportionate manner.

[0157] In a fifth variant of the process of the invention, as the strip advancing speed varies, in order to keep the fluence constant, the laser beam power and the laser beam dwell time are kept constant, while the laser beam footprint surface on the strip varies in an inversely proportionate manner.

Claims

CLAIMS1 . A cleaning apparatus for cleaning a cold-rolled metal strip advancing along at least one plane, said apparatus comprising- at least one first laser cleaning device (40) for cleaning a first strip surface;- at least one second laser cleaning device (40’) for cleaning a second strip surface, opposite said first surface;- a casing (22) containing inside at least partially said at least one first laser cleaning device (40) and said at least one second laser cleaning device (40’), said casing (22) being configured to be crossed by the strip;- at least one suction and purification system (32, 33, 34) for sucking and purifying fumes produced inside said casing (22) during the cleaning of the strip by means of laser; characterized in that there is provided inside the casing (22) at least one injection device (36, 37) configured to inject jets of air or inert gas so as to convey said fumes toward said at least one suction and purification system (32, 33, 34).

2. An apparatus according to claim 1 , wherein said at least one suction and purification system (32, 33, 34) comprises a suction duct (32), externally connected to the casing (22), and a suction fan (33), arranged along said suction duct (32) and connected with a possible filtering and purification unit (34).

3. An apparatus according to any one of the preceding claims, wherein both said at least one first laser cleaning device (40) and said at least one second laser cleaning device (40’) comprise- a respective laser source (25, 25’) arranged outside the casing (22);- a respective first optical group (26, 30) arranged inside said casing (22), for collimating and focusing a laser beam coming from the corresponding laser source;- a respective second optical group (27, 31 ) arranged inside said casing (22), for scanning a corresponding strip surface by means of the laser beam coming from the corresponding first optical group (26, 30).

4. An apparatus according to claim 3, wherein each first optical group (26, 30) comprises, or consists of, in sequence, a collimator mirror (43) and a focuser mirror (44), preferably arranged inside a first container (48);wherein each second optical group (27, 31 ) comprises, or consists of, in sequence, a polygonal rotating mirror (46) and a parabolic mirror (47), preferably arranged inside a second container (49).

5. An apparatus according to claim 3 or 4, wherein each laser source (25, 25’) is connected to the respective first optical group (26, 30) by means of an optical fiber connection (29, 29’).

6. An apparatus according to any one of claims 3 to 5, wherein there is provided at least one drying and purification device (60) arranged in proximity of the first optical groups (26, 30) and the second optical groups (27, 31 ), for drying and purifying the air so as to keep the first optical groups and the second optical groups clean.

7. An apparatus according to any one of the preceding claims, wherein there are provided, inside the casing (22), at least two motorized rollers (28, 38, 28’) for continuously advancing the strip.

8. An apparatus according to claim 7, wherein said at least two motorized rollers at least comprise:- a first pair of pass line rollers (28) arranged along a first advancing plane for the strip and between which there is arranged the at least one first laser cleaning device (40) for cleaning the first strip surface facing upwards;- a deflector roller (38) for deviating the strip from said first advancing plane to a second advancing plane, placed above or below said first advancing plane, so that the second strip surface is facing upwards;- and a second pair of pass line rollers (28’) arranged along said second advancing plane and between which there is arranged the at least one second laser cleaning device (40’) for cleaning the second strip surface.

9. An apparatus according to claim 7, wherein said at least two motorized rollers at least comprise:- a first pair of pass line rollers (28) arranged along a first advancing plane for the strip and between which there is arranged the at least one second laser cleaning device (40’) for cleaning the second strip surface facing downwards;- and a second pair of pass line rollers (28’) arranged along a second advancing plane, arranged above or below said first advancing plane, and between whichthere is arranged the at least one first laser cleaning device (40) for cleaning the first strip surface facing upwards.

10. An apparatus according to claim 7, wherein said at least two motorized rollers comprise two deflector rollers (38), preferably only two deflector rollers, configured to deviate the strip from a first advancing plane, preferably horizontal, to a second advancing plane, transversal to said first advancing plane, said second plane preferably being vertical, and then to a third advancing plane, preferably horizontal; and wherein said at least one first laser cleaning device (40) and said at least one second laser cleaning device (40’) are arranged on opposite sides of said second advancing plane.11 . An apparatus according to any one of the preceding claims, wherein there are provided at least two first laser cleaning devices (40, 41 ) and / or at least two second laser cleaning devices, arranged along a respective strip advancing plane; preferably, wherein said at least two first laser cleaning devices (40, 41) and / or said at least two second laser cleaning devices are arranged one after the other, respectively, along a strip advancing direction for carrying out successive cleaning treatments, or they are arranged one next to the other, respectively, along a direction transversal, preferably perpendicular, to said strip advancing direction so as to clean a respective surface portion of the strip along the width thereof.

12. An apparatus according to any one of the preceding claims, wherein the casing (22) is a closed casing provided with two openings or slits, dimensioned for the entering and exiting, respectively, of the metal strip from said casing (22); preferably wherein said at least one injection device (36, 37) is positioned so as to inject jets of air or inert gas along a direction transversal to the advancing direction of the metal strip, preferably along the width of the metal strip so as to convey the fumes toward an inner wall of the casing (22) onto which the at least one suction and purification system (32, 33, 34) is arranged.

13. A system for producing a continuous metal strip comprising, in sequence, a cold-rolling mill, a cleaning apparatus according to any one of claims 1 to 12, and a heating or annealing oven and / or a coating section.

14. A cleaning process for cleaning a metal strip, by means of an apparatus according to any one of the preceding claims, comprising the following steps:- providing a cold-rolled metal strip advancing along at least one plane and crossing an inner volume of the casing (22);- cleaning a first strip surface by means of the at least one first laser cleaning device (40);- cleaning a second strip surface, opposite said first surface, by means of the at least one second laser cleaning device (40’);- sucking and purifying the fumes produced inside said casing (22), during the cleaning of the strip, by means of the at least one suction and purification system (32, 33, 34); characterized in that there is provided, inside the casing (22), an injection of jets of air or inert gas, by means of at least one injection device (36, 37), for conveying said fumes toward said at least one suction and purification system (32, 33, 34).

15. A process according to claim 14, wherein said fumes are sucked along a suction duct (32), externally connected to the casing (22), by means of a suction fan (33), and filtered and purified by means of a filtering and purification unit (34).

16. A process according to claim 14 or 15, wherein, in the steps of cleaning said first strip surface and said second strip surface, a laser beam coming from a corresponding laser source (25, 25’), arranged outside the casing (22), is collimated and focused by means of a respective first optical group (26, 30), arranged inside said casing (22), and then it is used for scanning the corresponding strip surface along a direction transversal to a strip advancing direction, by means of a respective second optical group (27, 31 ) arranged inside said casing (22).

17. A process according to claim 16, wherein the irradiance I of the laser beam is comprised in a range from 2 to 15 MW / cm2, preferably from 4 to 10 MW / cm2; the dwell time of the laser beam on the strip surface is comprised in a range from 40 to 500 ns, preferably from 60 to 200 ns; and the fluence F of the laser beam is comprised in a range from 0.2 to 5.5 J / cm2, preferably from 0.2 to 3 J / cm2

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