Process for coiled steel for electrical applications

Abstract

A process for coiled steel. A hot-rolled steel coil is provided, the steel comprising, in alloy, one or more of: Si comprised between 0.5%wt and 6.5%wt, Al comprised between 0.05%wt and 2.0%wt, Mn comprised between 0.02%wt and 1.0%wt, P comprised between 0.01%wt and 0.5%wt, Sb comprised between 0.001%wt and 0.1%wt. A pickling of the steel coil is provided. The steel coil is subjected to at least one first cold rolling and to a subsequent at least one first annealing. A second cold rolling of the steel coil and a second annealing of the steel coil are provided. The steel coil is subjected to roller leveling in an apparatus comprising a plurality of rollers arranged on two parallel rows and acting on both opposite surfaces of the steel coil, providing a bidirectional bending to mitigate a residual curvature of the steel coil and to balance residual elastic tensions along a thickness of the steel coil. An electrical insulator coating is applied to the steel coil.

Description

Technical field

[0001] The present invention refers to a process for coiled steel. In particular, the present invention refers to steel for electrical applications, with low magnetic losses, high magnetic polarisation and low magnetic aging.

[0002] In general, the present invention finds application in the field of ferrous alloy metallurgy and of modifications to the physical structure of ferrous alloys.

[0003] In particular, the present invention relates to general methods or devices for the thermal treatment, and to the modification of the physical properties of iron or steel by deformation, and especially to the modification of the physical properties by deformation combined with or followed by thermal treatment.Prior art

[0004] Soft ferromagnetic materials are a class of steel which is particularly suitable for motor applications, thanks to a very low coercive field and a very narrow hysteresis loop.

[0005] This ferromagnetic steel, also called "electric steel", is used when it is necessary to limit as much as possible the currents needed to produce and control magnetic induction flows, allowing the magnetization of the material with relatively low intensity magnetic fields.

[0006] In addition to rotors and stators of rotating machines, this electric steel is also used for electromagnetic and transformer cores.

[0007] Important features of electric steel are: high permeability; high saturation induction; low hysteresis losses; low eddy current losses.

[0008] Electric steel is primarily made of Fe-Si alloys; chemical elements alloyed with Fe are therefore Si, Al, Mn, whose content is strictly linked to the steel grade and therefore to the target magnetic properties.

[0009] The main impurities for electric steel are: C, N, O, H, S. These elements cause, directly or through the formation of precipitates, a distortion of the crystal lattice which in turn hinders the movement of the edges of the magnetic domains, consequently compromising the magnetic properties of the base material.

[0010] In general, electric steel has micro-areas that spontaneously become magnetize, forming micro-dipoles; these micro-areas within the crystalline grains, in which there is the same direction of magnetization, are called "magnetic domains". Usually, ferromagnetic materials are polycrystalline and each crystal has a preferential magnetization orientation, called "easy magnetization axis"; this direction for Fe-Si magnetic steel is the direction

[100] , i.e. the edge of the cubic unit cell, while the directions

[110] and

[111] have significantly lower magnetic properties.

[0011] The local magnetization state depends on several factors: the orientation of the crystal lattice with respect to the direction of the magnetic field; the size of the grains; the presence of impurities; residual stresses.

[0012] According to the prior art, production processes for coiled steel, in particular electric coiled steel, use three sub-processes subsequent to hot rolling: 1) Pickling: this process consists in removing the oxide scale which covers the coiled steel after the hot rolling process by immersing the coil in tanks containing hydrochloric acid and can optionally also include annealing at a temperature of 700-1000°C (annealing and pickling). The thickness of the hot coil is typically 1.5-3.0 mm. 2) Cold rolling: this process aims to bring the coil to the final thickness which is typically in the range of 0.10 - 0.50mm. 3) Final treatment line: annealing + applicating electrical insulator coating: this process consists of a continuous annealing line in which the coil is heated up to a temperature of 900-1200°C for a holding time of a few minutes. The furnace atmosphere must be strictly non-oxidizing to avoid oxidation of the coil surface.

[0013] The problems of production processes for coiled steel according to the prior art can be summarized in the following points: High sensitivity to the presence of impurities related to the presence of C and N in solid solution. Sensitivity to surface oxidation of the coil during passage through the final treatment line. Critical issues in controlling the correct microstructure in terms of final grain size, magnetic domain configuration and crystallographic textures or orientations. Critical issues in obtaining thin (0.50-0.20 mm) and ultra-thin (<0.20 mm) thicknesses. Summary of the invention

[0014] An object of the present invention is to overcome drawbacks of the prior art.

[0015] A particular object of the present invention is to provide a process for coiled steel, in particular for coiled steel for electrical applications, which is more effective than known solutions.

[0016] A particular object of the present invention is to provide a process for coiled steel which results in low magnetic losses.

[0017] A particular object of the present invention is to provide a process for coiled steel which results in a high magnetic polarisation.

[0018] A particular object of the present invention is to provide a process for coiled steel which results in having low magnetic aging, i.e. a worsening of the material features during operation at temperature, in particular at temperatures of 100-200°C.

[0019] A particular object of the present invention is to provide a process for coiled steel which allows obtaining thin (0.50-0.20 mm) and ultra-thin (<0.20 mm) thicknesses, with improved quality.

[0020] These and other objects are achieved by a process for coiled steel according to the features of the attached claims which form an integral part of the present description.

[0021] An idea underlying the present invention is to provide a process for coiled steel.

[0022] In a process step, it is envisaged: providing a hot-rolled steel coil, the steel comprising, in alloy, one or more of: Si comprised between 0.5%wt and 6.5%wt, Al comprised between 0.05%wt and 2.0%wt, Mn comprised between 0.02%wt and 1.0%wt, P comprised between 0.01%wt and 0.5%wt, Sb comprised between 0.001%wt and 0.1 %wt.

[0023] In a process step, it is envisaged: providing a pickling of the steel coil.

[0024] In a process step, it is envisaged: subjecting the steel coil to at least one first cold rolling and to a subsequent at least one first annealing. The at least one first cold rolling comprises a plurality of rolling passes. An initial pass of the plurality of rolling passes provides a reduction ratio greater than 10%, preferably a reduction ratio greater than 20%. The at least one first cold rolling provides in output a thickness of the steel coil in the range 0.10 mm - 1.0 mm. The at least one first annealing comprises a static annealing, preferably in a bell furnace, of the steel coil. The static annealing provides a heating rate greater than 30°C / h, preferably a heating rate greater than 60°C / h. The static annealing further provides a holding temperature of the steel coil comprised between 600°C and 850°C. The static annealing provides a holding time comprised between 1 hour and 20 hours. The static annealing provides a cooling time greater than 3 hours to bring the steel coil back to a temperature of 50°C.

[0025] In a process step, it is envisaged: providing a second cold rolling of the steel coil and providing a second annealing of the steel coil. The second cold rolling provides a reduction ratio in the range 2% - 20%. The second cold rolling provides preferably a reduction ratio in the range 6% - 10%. The second cold rolling provides a Shear Strain Ratio parameter value greater than 0.1. The second annealing comprises a static annealing, preferably in a bell furnace, of the steel coil. The static annealing provides a heating rate greater than 20°C / h, preferably a heating rate greater than 60°C / h. The static annealing provides a holding temperature of the steel coil at a temperature comprised between 650°C and 850°C. The static annealing provides a holding time comprised between 5 hours and 20 hours. The static annealing provides a cooling time greater than 3 hours to bring the steel coil back to a temperature of 50°C.

[0026] In a process step, it is envisaged: subjecting the steel coil to roller leveling in an apparatus comprising a plurality of rolls arranged on two parallel rows and acting on both opposite surfaces of the steel coil. In the roller leveling, a bidirectional bending is provided to mitigate a residual curvature of the steel coil and to balance residual elastic tensions along a thickness of the steel coil. The roller leveling provides an over stretch decreasing in the steel coil between an input roll and an output roller. The over stretch being less than 5 at the output roll of the apparatus for roller leveling: in particular, the over stretch is less than 1 or less than 0,5 at the output roll of the apparatus for roller leveling. The over stretch is controlled based on a planarity difference detected on a surface of the steel coil between an input and an output of the apparatus for roller leveling.

[0027] In a process step, it is envisaged: applying an electrical insulator coating to the steel coil.

[0028] Advantageously, the process for coiled steel according to the present invention allows obtaining, in a more effective and alternative way compared to the prior art, steel for electrical applications, with low magnetic losses, high magnetic polarisation, low magnetic aging and ultra-thin thicknesses.

[0029] Further features and advantages will become more apparent from the detailed description given herein below of preferred, non-limiting embodiments of the present invention, and from the dependent claims which outline preferred and particularly advantageous embodiments of the invention.Brief description of the drawings

[0030] The invention is illustrated, by way of non-limiting example, with reference to the following Figures: Figure 1 illustrates the steps of the process for coiled steel according to the present invention. Figure 2 illustrates the trend of energy loss due to different over stretch values. Detailed description

[0031] Figure 1 illustrates, in a schematic and conceptual manner, some of the main steps of the process for coiled steel according to the present invention.

[0032] The process for coiled steel is used for the production of steel coils, or bobbins, for electrical applications. Said steel has low magnetic losses, high magnetic polarisation and low magnetic aging; in particular, magnetic aging is a worsening of the magnetic features of the material during the operation at 100-200°C.

[0033] The process comprises a step a), in which a hot-rolled steel coil is provided. The steel comprises, in alloy, one or more of: Si comprised between 0.5%wt and 6.5%wt, Al comprised between 0.05%wt and 2.0%wt, Mn comprised between 0.02%wt and 1.0%wt, P comprised between 0.01%wt and 0.5%wt, Sb comprised between 0.001%wt and 0.1%wt. "%wt" means a percentage content by weight for each element.

[0034] It is therefore provided to carry out a so-called "metallurgical design", with a detailed design of the chemical composition and the hot rolling process, preferably according to metallurgical criteria supported by mathematical modelling for providing the hot-rolled steel coil.

[0035] Preferably, the thickness of the hot-rolled steel coil is comprised between 1.5 and 3.0 mm.

[0036] The process comprises a step b) wherein a pickling of the steel coil is provided.

[0037] The pickling provides removing the surface oxide layer by immersion in tanks containing acid, for example hydrochloric acid.

[0038] Preferably, prior to step b), a preventive annealing of the steel coil can optionally be provided at step b1). In particular, said preventive annealing is provided at a temperature comprised between 700°C and 1000°C, preferably for a time comprised between 60 seconds and 6000 seconds.

[0039] The process comprises a step c) in which the steel coil is subjected to at least one first cold rolling and a subsequent at least one first annealing.

[0040] The at least one first cold rolling comprises a plurality of rolling passes. Preferably, the first cold rolling is performed by a rolling mill of the reversible type, preferably with one or two supporting rollers.

[0041] The initial pass provides a reduction ratio greater than 10%, preferably a reduction ratio greater than 20%.

[0042] To obtain a magnetic polarisation greater than 80% of the magnetic saturation value, it can be advantageous and preferable to set a Shear Strain Ratio parameter value greater than 0.1 in the initial pass. The Shear Strain Ratio is defined as the ratio between: Maximum Shear

[0043] Strain / Von Mises equivalent strain.

[0044] It should be noted that Maximum Shear Strain may not be a quantity measured by a physical sensor, but a value that can be calculated from the strain distribution in the material, particularly in the deformed volume between the rolls during rolling, e.g., by mathematical finite element models (FEM) or strain grids or imaging techniques.

[0045] The Maximum Shear Strain can be defined at the outer surface of the strip of the coiled steel, basically at the contact portion with a roller of the rolling mill. Similarly, Von Mises equivalent strain can also be defined at the outer surface of the strip of the coiled steel, essentially at the contact portion with a roller of the rolling mill. In fact, the Maximum Shear Strain and the Von Mises equivalent strain, and thus also the Shear Strain Ratio, can be considered at the same portion of the strip of the coiled steel during rolling operations.

[0046] The rolling operations applied to the coiled steel during the process of the present invention define strains in accordance with the assumption of plane strain. Under plane strain conditions, considering unidirectional compression and pure thickness reduction, the Von Mises equivalent strain formula ε eq can be reduced as:

[0047] The first cold rolling provides in output a steel coil thickness in the range 0.10 mm - 1.0 mm.

[0048] Preferably, the at least one first cold rolling at step c) provides that the steel coil is at a temperature comprised between 60-200°C.

[0049] The at least one first annealing comprises a static annealing, preferably in a bell furnace, of the steel coil. Preferably, the annealing furnace provides electrically powered heating elements.

[0050] The static annealing provides a heating rate greater than 30°C / h, preferably a heating rate greater than 60°C / h. The static annealing provides a holding temperature of the steel coil comprised between 600°C and 850°C. The static annealing provides a holding time comprised between 1 hour and 20 hours. The static annealing provides a cooling time greater than 3 hours to bring the steel coil back to a temperature of 50°C.

[0051] Preferably, as illustrated by the dotted arrow in Figure 1, step c) can optionally provide sequentially repeating two or more times the first cold rolling and the first annealing, possibly also varying the process parameters thereof between one time and the next one.

[0052] The process comprises a step d) in which a second cold rolling of the steel coil is provided and a second annealing of the steel coil is provided. It must obviously be understood that a "second" rolling or annealing is not necessarily the second one if rolling or annealing has been carried out several times at step c) described above; the term "second" is therefore to be understood in a denotative manner (in order to distinguish it from what is referred to in step c)) but not strictly numerical.

[0053] The second cold rolling provides a reduction ratio in the range 2% - 20%. The second cold rolling preferably applies a Shear Strain Ratio parameter value greater than 0.1; the Shear Strain Ratio is defined as the ratio between: Maximum Shear Strain / Von Mises equivalent strain. This second cold rolling is also defined with the term "temper rolling".

[0054] This second cold rolling preferably provides a reduction lower than in the previous rolling steps.

[0055] The second annealing comprises a static annealing, preferably in a bell furnace, of the steel coil.

[0056] The static annealing provides a heating rate greater than 20°C / h, preferably a heating rate greater than 60°C / h. The static annealing provides a holding temperature of the steel coil at a temperature comprised between 650°C and 850°C. The static annealing provides a holding time comprised between 5 hours and 20 hours. The static annealing provides a cooling time greater than 3 hours to bring the steel coil back to a temperature of 50°C.

[0057] The process comprises a step e) in which the steel coil is subjected to roller leveling in an apparatus comprising a plurality of rollers arranged on two parallel rows and acting on both opposite surfaces of the steel coil. The roller leveling provides a bidirectional bending to mitigate a residual curvature of the steel coil and to balance residual elastic tensions along a thickness of the steel coil.

[0058] The roller leveling provides an over stretch decreasing in the steel coil between an input roll 1 and an output roll 2.

[0059] The over stretch parameter = ε surf / ε elast is locally defined, for each portion of steel coil in contact with each roller, wherein ε surf = strain that the steel coil undergoes on each surface and ε elast = strain that corresponds to the elastic limit of the steel coil material, typically 0.002 i.e. 0.2%.

[0060] Said over stretch is less than 5 at the output roller. In particular the over stretch is less than 1 or less than 0,5 at the output roller. Therefore, at the output roller, the strain, which the steel coil undergoes on each surface, is less than the elastic limit of the steel coil material.

[0061] The over stretch is controlled based on a planarity difference detected on a surface of the steel coil between an input and an output of the apparatus for roller leveling. The planarity can be detected by known techniques, such as non-contact techniques using a plurality of laser beams.

[0062] Providing a control of the over stretch value during the roller leveling allows not to affect the magnetic properties of the material subjected to roller leveling.

[0063] In particular, preferably, the over-stretch is comprised between 0.5 and 10 in the apparatus for roller leveling. In fact, an experimental campaign conducted by the Applicant has shown that, for over stretch values comprised between 0.5 and 10, in the roller leveling apparatus, the extra contribution of the deformation to the magnetic losses is substantially below the 100 J / m3 range, allowing to minimize the magnetic losses due to the overall deformation determined by the roller leveling, as exemplified in Figure 2.

[0064] Preferably, the over stretch is controlled by a mechanical control system acting on a relative position between the steel coil and at least some rolls of the apparatus for roller leveling.

[0065] In a preferred embodiment, the rolls of the apparatus for roller leveling comprise supporting rollers and working rolls, wherein the supporting rollers support specific portions of respective working rollers aiming to correct the planarity of a surface.

[0066] Preferably the apparatus for roller leveling comprises at least four supporting rollers and three working rollers.

[0067] In general, the apparatus for roller leveling can preferably be of a duo type (i.e. without supporting rollers) or a four or six type (4-high mill or 6-high mill). In some cases, the supporting rollers may not be the same as the working rollers, but they have a smaller diameter so that many of them can be provided and the bending of the working rollers can effectively be counteracted locally. In this way, the planarity can be controlled more finely over the strip width.

[0068] The rollers preferably have a diameter comprised between 10mm and 200mm, and a pitch between rolls is given by said diameter increased by an amount comprised between 1mm and 200mm.

[0069] Preferably, step e) provides subjecting the steel coil to roller leveling at a temperature comprised between 0°C and 300°C.

[0070] The process comprises a step f) in which an electrical insulator coating is applied to the steel coil.

[0071] Preferably, an optional step f1) is provided in which the electrical insulator coating is subjected to heating at a temperature comprised between 100°C and 300°C, preferably for a time comprised between 1 second and 200 seconds, to carry out a curing of the electrical insulator coating.

[0072] Steps e) and f) therefore create a final treatment line for the steel coil, providing for a planarity conditioning and an application of the electrical insulator coating.

[0073] Preferably, steps e) and f) and, if provided, step f1), create a continuous process in which the steel coil is passed through a flattener or alternatively a tensile flattener and subsequently through a system for applying a coating with high electrical insulation features (typically of the C5 type) and subsequent thermal treatment.

[0074] Preferably, the process achieves a magnetic polarisation of a steel of the steel coil having a value comprised between 77-90% of the value of the magnetic saturation field in the presence of an external magnetic field of 2500 A / m, wherein said value of the magnetic saturation field is a maximum magnetization of a material having a certain chemical composition.

[0075] The magnetic properties of the non-oriented grain coiled steel depend on the Si and Al content (%wt) and, to a lesser extent, on the content of other chemical elements Mn, P, Sb, others.

[0076] The magnetic properties of the non-oriented grain coiled steel depend on the advantageous orientation of the crystal grains. This means that it is desirable to provide as many grains as possible which are oriented according to the easy magnetization directions <100> / / ND.

[0077] The magnetic properties of the non-oriented grain coiled steel depend on the degree of cleanliness of the steel, provided for example by the absence of oxides and fine precipitates.

[0078] The magnetic properties of the non-oriented grain coiled steel depend on the local planarity of the coil: this feature does not have an effect on the single sheet of material but on the magnetic properties of the package which will represent the stator or rotor of the electric motor, strongly influencing the stacking factor and therefore the percentage of air contained in the rotor / stator.

[0079] In general, soft magnetic materials are a class of steel that is anisotropic in its magnetic properties; this means that the magnetic field which is necessary to magnetize the single crystal of the material (crystal grain) depends on the orientation of the field vector H with respect to the unit cell of the metal.

[0080] In metals with a BCC cubic (body-centered cubic) structure, such as ferromagnetic metals, the direction <001>, i.e. the edge of the cubic lattice, is the easiest direction to be magnetized. The direction <111> is instead the most difficult direction to be magnetized. If the magnetizing field H has a direction with a small misalignment angle with respect to the easy magnetization direction, a significantly lower magnetization is obtained. This important feature requires that in the industrial field the process conditions are controlled in order to obtain the optimal orientations and the products with the best performance. From what has just been exposed, it is clear that the definition of the metallurgical cycle of the non-oriented grain soft magnetic steel requires the detail of how the final microstructure of the coil is obtained and in particular the evolution of the crystallographic textures up to the final selection of the magnetically optimal orientations which occurs during the final annealing of the coil.

[0081] The present invention provides, at steps c) and d), performing static annealing. The process parameters of the static annealing are important for obtaining the final magnetic features of the steel coil.

[0082] In particular, important process parameters of static annealing are: heating rate of the charge, holding temperature, cooling rate of the charge and shielding atmosphere of the charge.

[0083] In particular, the protection atmosphere of the charge can play an extremely important role in the selection of the most suitable crystallographic textures. In basic products, for example, hydrogen can be used. In more sophisticated products, it can be appropriate and advantageous to modify the chemical composition of the furnace atmosphere by acting for example on the dew point.

[0084] The present invention provides, at steps c) and d), performing cold rolling. The process parameters used to control the microstructure of this cold rolling process are: total deformation (reduction ratio), Shear Strain Ratio (ratio between cutting deformation / equivalent deformation), diameter of the working cylinders.

[0085] The present invention provides, at step f), a painting line for the application of an electrical insulator coating. It is useful to provide a device for controlling the local planarity of the steel coil upstream of the coating application line.

[0086] Said control device is preferably provided with a mathematical model consisting of a flattener or alternatively a tensile-flattener. This device has the purpose of introducing an elastic stress field into the material, defining the optimal configuration of the magnetic domains and optimizing their mobility.

[0087] The at least one first cold rolling, at step c), has a dual metallurgical function: the first function is to approach the target geometric features of the product in terms of thickness and planarity; the second purpose is to condition the crystallographic textures in order to obtain, after the final annealing, the orientations (textures) suitable for the magnetic applications. The deformation mode of the at least one first cold rolling conditions the type of deformation (amount of the cutting component of the deformation, and the degree of uniformity on the thickness). It is possible to use a mathematical model, which calculates the friction coefficient of the cold rolling process, for the modulation of the shear strain ratio.

[0088] The static annealing in bell furnaces has an impact on the development of orientations.

[0089] The at least one first annealing at step c) is aimed to obtain the complete recrystallization of the microstructure downstream of the at least one first cold rolling. The result of this at least one first annealing, from the point of view of the orientations, is a matrix consisting of a still high percentage of magnetically non-optimal orientations, wherein the texture with the planes <111> parallel to the coil plane can still be considerable, and of a part characterized by grains with orientations which are advantageous for magnetic applications, i.e. grains with the planes <100> or <110> parallel to the coil plane.

[0090] The at least one second annealing at step d) aims to obtain a selection of the final orientations and to reach the optimal final grain size through the choice of the optimal soaking time. More specifically, the at least one second annealing has the metallurgical function of determining a secondary recrystallization of the microstructure to obtain the growth of grains characterized by very specific orientations, i.e. those for which the planes <100> or <110> are parallel to the steel coil plane.

[0091] To achieve this result, it is necessary that the microstructure of the coil downstream of the at least one first cold rolling and the subsequent at least one first annealing at step c) is suitably conditioned or stabilized. The stabilization of the microstructure consists in ensuring that the grains with magnetically disadvantageous orientations are disadvantaged or even blocked in the growth process that must occur in the secondary recrystallization at the second annealing at step d). It is important to underline that a difference of a few percentage points of the grain boundary energy (typical grain boundary energy values for steel vary between 0.50 and 1.0 J.m-2) is enough to provide the driving force which is enough to obtain the growth of the preferred orientations.

[0092] In the process for coiled steel, the selection of the optimal microstructure is carried out through one or more mechanisms.

[0093] A first mechanism is represented by the effect of the heating rate. This parameter in the static annealing typically varies between 10-30° / h. In the process of the present invention, the heating rate is significantly increased and varies between 30-1200°C / h depending on the magnetic properties that are to be achieved. Therefore, the heating rate must be higher in order to obtain higher magnetic features (high polarisation and low magnetic losses).

[0094] A second mechanism is characterized by imparting to the coil at the second rolling, called step d), a deformation in the range 6%-10%. Since the deformation tends to preferentially concentrate in the grains with the planes <111> with orientation parallel to the coil plane (also indicated as <111>|ND), during the second annealing of the steel coil, these grains cannot grow as they are disadvantaged by having a higher specific energy than in the other orientations. The creation of this initial situation constitutes a key factor in advantaging the growth of the grains with the planes <100> and <110> parallel to the coil plane to the detriment of the grains with orientations <111>|ND. This second mechanism is defined as Strain Induced Grain Boundary Migration (SIGBM). The SIGBM mechanism is therefore advantaged in grains with low Taylor coefficient values. Conversely, the areas characterized by high deformation are associated with high Taylor coefficient values. The orientation <111> has the highest Taylor coefficient among the standard components of the BCC metal textures and is therefore characterized by the highest stored deformation energy. The second recrystallization mechanism is triggered starting from the regions with low stored energy. The growth of grains with the planes <100> and <110> parallel to the coil to the detriment of those with higher stored energy causes the textural component <111>|ND to be rapidly consumed by the other components. In thermodynamic terms, this occurs because the thermodynamic system consisting of the set of crystalline grains tends to equilibrium by minimizing the elastic distortion energy of the system due to the dislocations generated in the deformation process. This second mechanism is effective in selecting the optimal orientations; however, it alone does not allow to obtain an optimal and homogeneous distribution of the grain sizes. The typical structure obtained at the end of the second annealing at step d) has a bimodal distribution of the grains in which the surface grains are typically smaller than those present at the center of the thickness. This characteristic inevitably has negative effects on the magnetic properties, mostly related to the mobility of the magnetic domains, since the microstructure shown in the figure highlights that both the grain size and the disadvantageous crystallographic orientations represent obstacles to their development.

[0095] A third mechanism is represented by the modulation of the friction coefficient in the at least one first cold rolling at step c) in order to obtain a specific Shear Strain Ratio (SSR), which is defined as SSR = Maximum Shear Strain / Von Mises equivalent strain. It was determined from an experimental activity that, in order to optimize the recrystallization textures downstream of the process, it is necessary to exceed a threshold value of the SSR parameter which is greater than 0.1, preferably comprised in the range 0.1-1.0. The SSR parameter is suitably controlled through the friction coefficient between the coil and the working cylinders.

[0096] Following an experimental activity, a parameter Σ was defined with which it is possible to control the cutting component in cold rolling through the suitable definition of the interface conditions between the working cylinder and the steel (friction coefficient). This parameter is defined as: Σ = K 2 S − μ wherein µ is the friction coefficient, K is the compression yielding of the material on the contact arc between roll and steel strip, S is the average separation force on the contact arc between roll and steel strip. The friction coefficient µ in the cold rolling process can be calculated and controlled online by a mathematical model.

[0097] It was established from the experimental activities carried out that the optimal magnetic properties are achieved if the parameter Σ is comprised between 0.15-0.50, in the at least one first cold rolling at step c) and in said second cold rolling at step d).

[0098] In step f) of the process, a final treatment of the magnetic steel coil occurs by applying a (organic, inorganic or mixed) coating in order to provide an excellent electrical insulation between the steel plates of the stator / rotor pack. In particular, the second annealing process is separated from the planarity control process and the application of the insulator coating, in order to be able to individually exploit all the metallurgical mechanisms exposed above for the annealing. This approach therefore allows to exploit in a very controlled manner a greater number of metallurgical mechanisms in order to obtain excellent magnetic properties.

[0099] A fourth mechanism is represented by the planarity control and the related magneto-mechanical effect.

[0100] The planarity in magnetic steel is linked to two fundamental aspects: the first aspect is linked to automatic blanking processes which evidently require very tight tolerances for the coil planarity during processing; the second aspect is linked to the fact that planarity is strictly connected to the stator / rotor stacking factor and therefore ultimately to the magnetic properties of the motor.

[0101] This present process is able to simultaneously improve the coil planarity and the magnetic properties of the products by exploiting a metallurgical contribution represented by the magneto-mechanical effect, that is, the interaction that exists between an elastic tension field in the material and its magnetic properties. It is important to underline that the use of the flattener in conventional continuous lines for the production of the magnetic products is totally absent.

[0102] The innovative process is represented by the introduction of an apparatus for roller leveling at step e) before the insulator coating application line at step f). The apparatus for roller leveling is preferably controlled by a mathematical model that allows to modify the characteristic stress field of the material coming from cold rolling and then further modified during the final annealing in bell furnace.

[0103] The apparatus for roller leveling is designed for the fine control of the planarity through a rebalancing of the elastic residual stresses which cause the planarity defects to be formed. The apparatus for roller leveling preferably comprises a portal structure characterized by a high mechanical rigidity, with a bridge slide interposed between the base and the upper crosspiece., two multi-roll banks with axes orthogonal to the coil axis, which are opposite but staggered in order to carry out the alternating bending action, are arranged between the base and the bridge. Each roll of the flattener is motorized. The diameter of the rolls depends on the operating conditions and geometric characteristics of the coil. Typically, the diameter varies between 20mm and 100mm, but, in general, it can vary between 10mm and 200mm. The upper and lower banks of the rolls are typically tilted in order to generate a progressively decreasing bidirectional bending action. The tensions induced by the rolls of the apparatus for roller leveling are high close to the coil surface and then decrease towards the internal layers until a minimum is reached at the center of the thickness. The action of the apparatus for roller leveling therefore provides that the plasticization condition is reached only in the very superficial layers of the coil (<0.1mm) while the internal layers remain in a state of elastic tension. One of the fundamental parameters of the apparatus for roller leveling is therefore constituted by the "plasticization", defined as the percentage of coil thickness that has undergone plastic deformation (in traction and compression).

[0104] The apparatus for roller leveling in its planarity control action plays an active metallurgical role by introducing a suitable gradient of elastic stresses and a limited range of plastic deformations in the material.

[0105] The experimental activities carried out have shown that the particular stress state which was present in the material, highlighted by its planarity in the input line, is suitably transformed into an elastic stress field which is symmetric and balanced with respect to the coil thickness in order to obtain higher magnetic properties.

[0106] The metallurgical mechanism therefore provides a magneto-mechanical effect, i.e. an interaction between an elastic stress field and a mobility of the edges of the magnetic domains. This interaction is based on the phenomenon of the positive magnetostriction characterizing Fe-Si alloys. Studies conducted on this phenomenon have ascertained that elastic stresses in traction have a positive effect on the properties of peak polarisation and magnetic losses while elastic stresses in compression have a negative effect on both parameters.

[0107] This process allows to ensure an excellent planarity and at the same time to set a suitable elastic stress field in the material before passing in the apparatus for roller leveling before applying the coating. The apparatus for roller leveling in its planarity control action transforms the initial non-symmetric stress field, which is typically unbalanced with respect to the thickness of the coil, into a symmetric stress field with a better stress isotropy in the plane of the coil.

[0108] Considering the description given herein, the person skilled in the art will be able to devise further modifications and variations, in order to satisfy contingent and specific needs.

[0109] The embodiments described herein are therefore to be understood as illustrative and non-limiting examples of the invention.

Claims

1. A process for coiled steel, said process comprising the steps of: a) providing a hot-rolled steel coil, said steel comprising in alloy one or more of: Si comprised between 0.5%wt and 6.5%wt, Al comprised between 0.05%wt and 2.0%wt, Mn comprised between 0.02%wt and 1.0%wt, P comprised between 0.01%wt and 0.5%wt, Sb comprised between 0.001%wt and 0.1 %wt; b) providing a pickling of said steel coil; c) subjecting said steel coil to at least one first cold rolling and to a subsequent at least one first annealing, wherein said at least one first cold rolling comprises a plurality of rolling passes, an initial pass of said plurality of rolling passes providing a reduction ratio greater than 10%, preferably a reduction ratio greater than 20%, said at least one first cold rolling providing in output a thickness of said steel coil in the range 0.10 mm - 1.0 mm, wherein said at least one first annealing comprises a static annealing, preferably in a bell furnace, of said steel coil, wherein said static annealing provides a heating rate greater than 30°C / h, preferably a heating rate greater than 60°C / h, and further a holding temperature of said steel coil comprised between 600°C and 850°C for a holding time comprised between 1 hour and 20 hours, and a cooling time greater than 3 hours to bring said steel coil back to a temperature of 50°C; d) providing a second cold rolling of said steel coil and providing a second annealing of said steel coil, wherein said second cold rolling provides a reduction ratio in the range 2% - 20%, and a Shear Strain Ratio parameter value greater than 0.1, and wherein said second annealing comprises a static annealing, preferably in a bell furnace, of said steel coil, wherein said static annealing provides a heating rate greater than 20°C / h, preferably a heating rate greater than 60°C / h, and further a holding temperature of said steel coil at a temperature comprised between 650°C and 850°C for a holding time comprised between 5 hours and 20 hours, and a cooling time greater than 3 hours to bring said steel coil back to a temperature of 50°C; e) subjecting said steel coil to roller leveling in an apparatus comprising a plurality of rollers arranged on two parallel rows and acting on both of the opposite surfaces of said steel coil, providing a bidirectional bending to mitigate a residual curvature of said steel coil and to balance residual elastic tensions along a thickness of said steel coil, said roller leveling providing an over stretch decreasing in said steel coil between an input roller and an output roller, said over stretch being less than 5 at said output roller, said over stretch being controlled based on a planarity difference detected on a surface of said steel coil between an input and an output of said apparatus for roller leveling; f) applying an electrical insulator coating to said steel coil.

2. The process according to claim 1, wherein said over-stretch is comprised between 0.5 and 10 in said apparatus for roller leveling, said over stretch being controlled by a mechanical control system acting on a relative position between said steel coil and at least some rollers of said plurality of rollers.

3. The process according to claim 1 or 2, wherein said plurality of rollers of said apparatus for roller leveling comprise supporting rollers and working rollers, wherein said supporting rollers support specific portions of respective working rollers aiming to correct a planarity of said steel coil.

4. The process according to claim 3, wherein said plurality of rollers have a diameter comprised between 10mm and 200mm, and wherein a pitch between rollers is given by said diameter increased by a quantity comprised between 1mm and 200mm.

5. The process according to any one of claims 1 to 4, wherein said step e) provides subjecting said steel coil to said roller leveling at a temperature comprised between 0°C and 300°C.

6. The process according to any one of claims 1 to 5, wherein said first cold rolling is carried out by a rolling mill of the reversible type, preferably with one or two rolling supporting cylinders.

7. The process according to any one of claims 1 to 6, wherein in said at least one first cold rolling in said step c) said steel coil is at a temperature comprised between 60-200°C.

8. The process according to any one of claims 1 to 7, wherein in said step a) a thickness of said hot-rolled steel coil is comprised between 1.5 and 3.0 mm.

9. The process according to any one of claims 1 to 8, wherein said step b) further comprises providing a preventive annealing of said steel coil at a temperature comprised between 700°C and 1000°C, preferably for a time comprised between 60 seconds and 6000 seconds.

10. The process according to any one of claims 1 to 9, wherein said step c) further comprises sequentially repeating two or more times said first cold rolling and said first annealing.

11. The process according to any one of claims 1 to 10, wherein said step f) further comprises subjecting said electrical insulator coating to heating at a temperature comprised between 100°C and 300°C, preferably for a time comprised between 1 second and 200 seconds, to carry out a curing of said electrical insulator coating.

12. The process according to any one of claims 1 to 11, wherein in said at least one first cold rolling at step c) and in said second cold rolling at step d), a parameter Σ has a value comprised between 0.15-0.50, wherein Σ = K 2 S − μ where µ is the friction coefficient, K is the compression yielding of the material on the contact arc between roller and steel strip, S is the average separation force on the contact arc between roller and steel strip; to obtain a magnetic polarisation of a steel of said coil with a value comprised between 77-90% of the value of the magnetic saturation field in the presence of an outer magnetic field of 2500 A / m, wherein said value of the magnetic saturation field is a maximum magnetization of a material having a chemical composition.

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