Process to produce non-grain oriented steel metal strip, metal strip so obtained and plant

Abstract

A process for manufacturing non-grain oriented steel metal sheet with a thickness between 0.1 and 0.5 mm, adapted to be intended for ferromagnetic cores of electrical machines, is described. A plant (100) for manufacturing such metal sheet and such non-grain oriented steel metal sheet are also described.

Description

[0001] PROCESS TO PRODUCE NON-GRAIN ORIENTED STEEL METAL STRIP, METAL STRIP SO OBTAINED AND PLANT

[0002] The present invention relates to a process for manufacturing non-grain oriented steel metal sheet with a thickness between 0.1 and 0.5 mm, adapted to be intended for ferromagnetic cores of electrical machines . The present invention also relates to a non-grain oriented steel metal sheet thus obtained and to a plant for manufacturing such metal sheet .

[0003] The present invention finds advantageous employment in the field of rotating electrical machines .

[0004] The functional properties of non-grain oriented magnetic sheets (NGO-ES) are mainly regulated by the chemical composition of the alloy, by the thickness of the produced strips and by the microstructure thereof, with particular regard to the size of the crystalline grains forming the metallic matrix and their crystallographic orientation .

[0005] The products available on the market are classified on the basis of their magnetic properties, mainly defined in the standards UNI EN 10106 and UNI EN 10303. Such magnetic properties are associated with a crystalline structure of the components characterized by average crystalline grain sizes typically not greater than 0.25 mm and with crystallographic texture as isotropic as possible in the plane of the rolled product in order to ensure similar magnetic behaviour for the various angles of application of the magnetic field during operation of the electrical machines .

[0006] The main qualifying magnetic features are the magnetic losses measured at specific working induction andmagnetization frequency conditions and the polarization levels achievable at specific values of applied magnetic field according to the standards IEC 60404-2 and IEC 60404-3 .

[0007] The existing technologies for manufacturing non-grain oriented magnetic sheets are multiple and exploit, for the reduction of magnetic losses, some strategies such as the reduction of the final thickness of the rolled products and / or the increase of the electrical resistivity of the metallic alloy by addition of elements such as Si, Al, Mn, etc .

[0008] Another metallurgical feature exploited for improving the magnetic quality of the products is the maximum possible reduction of non-metallic second phases present in the metallic matrix of the steel, such as sulphides, nitrides, carbides and oxides . The most modern production practices therefore provide the manufacture of steel with very low contents of sulphur, nitrogen, carbon and oxygen already in the step of solidifying the molten steel during slab casting .

[0009] The non-grain oriented magnetic sheets, typically of Fe-Si alloy with silicon contents varying between 0.1% and 3.5% by weight, are manufactured from a molten alloy solidified in slabs, which are hot-rolled in order to obtain rolled strips that are subsequently subj ected to cold-rolling up to the final application thickness . Except for strip casting solidification processes, long described in international scientific and patent literature but for which no industrial reference application is known to date, the slabs for manufacturing non-grain oriented sheet are produced by continuous solidification at variable thicknesses depending on thetechnology used, within the range between 20 mm and 300 mm. The hot rolling of the slabs is carried out in operating modes sometimes very different depending on the technology and the plant used, however within a temperature range between 1300°C and 700°C, in order to obtain hot-rolled products with thickness typically in the range of 1.8 - 2.5 mm, although in patent literature manufacturing methods with very different hot-strip thicknesses are proposed.

[0010] The magnetic losses of NGO-ES products, with particular regard to those used in rotating electrical machines operating also at high electrical supply frequency, are significantly improved if relatively high amounts of elements that increase the electrical resistivity of the metallic matrix of the sheet are included in the alloy composition, such as for example silicon and aluminium, as well as if the sheets are made of very thin thicknesses, for example with thickness lower than 0.35 mm.

[0011] The products for final use, typically offered on the market, have thickness varying between 1 mm and 0.35 mm for conventional applications and at lower thickness, down to 0.2 mm or lower for special and high-frequency applications .

[0012] In particular, there is an established tendency to produce hot-rolled strips with increasingly thinner thickness in order to reduce as much as possible the cold-rolling rate applied. The claimed advantages are a cost reduction and yield improvement connected with less onerous cold-rolling, and in parallel an improvement of the magnetic quality associated with an improvement of the crystallographic textures obtainable in the finishedproducts .

[0013] A method for manufacturing non-grain oriented magnetic sheet from hot-rolled strips with a thickness lower than 1.5 mm is known, which provides a limited content of silicon and aluminium ( [ %Si ] +2 [ %A1 ] <1 . 8 ) , while also providing the possibility of adding other alloying elements such as P, Sn, Sb, Zr, V, Ti, N, Ni, Co, Nb, B up to an overall value not higher than 1.5% . According to such method, disadvantageously, the obtainable magnetic losses are poor compared with the range of high-quality products currently required by the market .

[0014] In US 2005 / 0067053 Al a procedure for manufacturing hot strip suitable for manufacturing non-grain oriented magnetic sheet is described, with a maximum hot-strip thickness of 1.8 mm or 1.2 mm. The Si and Al content that can be adopted is in this case higher than in the previous case, but still limited to [ %Si ] +2 [ %A1 ] <5% . In the document describing a specific crystallographic texture of the hot-rolled matrix, no specific advantages obtainable in terms of high magnetic features of the products which can be made by such procedure are reported .

[0015] As an alternative to the preparation of reduced-thickness hot strips, methods have been proposed which exploit, on the contrary, the adoption of high cold reduction rates claiming positive effects thereof on the final quality of the products, connected with the control of microstructural homogeneity after recrystallization during the continuous annealing of the products at final thickness, as in the case of US 10337080, where a method for manufacturing non-grain oriented magnetic sheet from hot-rolled strips with a thickness between 2.5 and 12 mmis proposed, characterized by a Si content between 1.8 and 6.0%, Al between 0.2 and 4%, Mn between 0.2 and 3%, S between 0.0005 and 0.01%, N between 0.001 and 0.01%, C between 0.001 and 0.01%, with Mn / S > 100 and Al / N > 200, and characterized by the adoption of an overall cold-rolling deformation higher than 80%, divided into two reductions with intermediate annealing.

[0016] In the industrial manufacture of product grades with the highest magnetic quality, the action of increasing the content of alloying elements and producing sheet with very thin thickness is accompanied by important limitations in terms of workability, production capacity, productivity and physical yield. The main problems derive from the increase of material brittleness, the absolute increase of cold-rolling deformation to be applied and the increase of the surf ace / volume ratio of the strips in production.

[0017] A further critical issue related to the need to modify the alloy composition in order to increase the electrical resistivity and to reduce the product thicknesses in order to reduce the losses at working frequencies higher than 50-60 Hz is related to the simultaneously associated worsening of Magnetic Induction (e . g. B5000) . This is due to several factors among which the main ones are : ( 1 ) the reduction of the magnetic saturation induction of the material with the increase of the concentration of non-f erromagnetic alloying elements, (2 ) the reduction of crystallographic texture components favourable to magnetization on the plane of the rolled products which occurs when the cold reduction degree increases (in particular when it exceeds 85%) , and (3) the effect of the structure of magnetic closure domainson the surface, relatively more significant in proportion to the increase of the Surf ace / Volume ratio of the new products .

[0018] A further problem in the industrial production of NGO-ES product grades with excellent magnetic features is the difficulty of obtaining, for all produced strips, the same microstructural features of the finished products, with particular regard to the average crystalline grain size and its constancy in the various portions of the metallic matrix constituting the strip as a whole . It is known that the crystalline grain size is one of the most important parameters for controlling the magnetic and mechanical features of the above-mentioned NGO-ES product grades and that, particularly in the case of applications in electrical motors operating at variable working frequencies up to high frequencies ( for example for electrical traction motors of electrical vehicles) , manufacturers are often required to design and make specific grades depending on the type of vehicle (and characteristic driving cycle) to be satisfied (small cars, medium cars, large cars, trucks, etc . ) , and this is achieved by very precisely obtaining the final grain size of the product . Currently NGO-ES grades are produced aiming to limit as much as possible the content of particles of non-metallic second phases such as sulphides and nitrides because their presence, with specific size distribution and volumetric fraction, determines a deterioration of the magnetic losses . Nevertheless, the minimization of second-phase particles is complex and difficult to achieve in the primary step of preparing NGO-ES steels, since it requires sophisticated equipment, highly controlledprocedures, use of expensive raw materials and very long treatment times of the molten alloy, with an increase in manufacturing costs . For these reasons, the current typical production of NGO-ES is still characterized by the presence of fractions of non-metallic second phases which, although limited in volumetric fraction, are able to interfere with the growth kinetics of the crystalline grain (inhibition of grain boundary movement) and to prevent reliable control of the grain size in the finished products .

[0019] The task underlying the present invention consists in providing a process and a plant for manufacturing nongrain oriented steel metal sheet, and such non-grain oriented steel metal sheet, which allow the improvement of the quality of the products, with particular regard to magnetic induction and increase of manufacturing yield .

[0020] A further obj ect of the present invention is to provide innovative solutions for conditioning the surface of the hot-rolled strip upstream of cold-rolling, aimed at improving the efficiency of the mechanical removal of scale, reducing or eliminating the step of shot peening / sandblasting of the oxidized surfaces with consequent improvement of the surface roughness, reduction of energy consumption and of the reagents of the subsequent pickling section .

[0021] The above task, as well as the mentioned purposes and others that will appear more clearly hereinafter, are achieved by a process as recited in claim 1, by a metal sheet as recited in claim 16 and by a plant as recited in claim 17 .

[0022] Other features are provided in the dependent claims .The features and advantages of the process, of the sheet and of the plant according to the present invention will become more apparent from the following description, provided by way of example and not of limitation, referred to the attached schematic drawings in which: - figures la and lb show two different variants of a plant according to a first exemplary embodiment;

[0023] - figures 2a and 2b show two different variants of a plant according to a second embodiment according to the present invention;

[0024] - figures 3a and 3b show two different variants of a plant according to a third embodiment according to the present invention;

[0025] - figures 4a and 4b show two different variants of a plant according to a fourth embodiment according to the present invention;

[0026] - figure 5 shows a plant according to a fifth embodiment according to the present invention;

[0027] - figures 6a and 6b show two different variants of a plant according to a sixth embodiment according to the present invention;

[0028] figure 7 shows a plant according to a seventh embodiment according to the present invention;

[0029] figure 8 shows a plant according to an eighth embodiment according to the present invention;

[0030] - figure 9 shows a graph in which the resistivity values are reported on the abscissa and the P10@400Hz values are reported on the ordinate, i . e . the magnetic losses measured at an induction of 1.0 Tesla and 400 Hz, for the samples made according to Example 1 (both according to the invention and outside the invention) .

[0031] A process for manufacturing non-grain oriented steelmetal sheet intended for manufacturing ferromagnetic cores of electrical machines, having a final thickness between 0.1 and 0.5 mm, is described below.

[0032] The described process refers in particular to the steps of transforming strips with a well-defined alloy composition, produced by hot rolling at temperatures higher than 700 °C from steel solidified at thickness between 0.5 and 2.5 mm, i . e . from hot-deformed strips, independently of the adopted solidification and continuous casting technology (e . g. conventional continuous casting, thin slab, strip casting...) .

[0033] Preferably, the thickness of the strip is between 0.7 and 2 . 4 mm.

[0034] Such process limits the negative effects of cold-rolling and in particular allows to obtain a less brittle material .

[0035] The process according to the present invention comprises a first step of providing a strip with a thickness between 0.5 and 2.5 mm of a solidified iron-based alloy containing the following elements within the compositional limits described below by weight :

[0036] Silicon (Si) greater than 0% and lower than 6%, preferably between 2.0% and 4.5%,

[0037] Carbon (C) greater than 0% and lower than 0.007%, Aluminium (Al) greater than 0% and lower than 2%, Manganese (Mn) greater than 0% and lower than 3%, preferably lower than 2%,

[0038] Copper (Cu) greater than 0% and lower than 2%, preferably lower than 1%,

[0039] Nitrogen (N) between 0.0005 and 0.010%,

[0040] Sulphur (S) between 0.0005 and 0.020%,

[0041] Titanium (Ti) between 0.0005 and 0.01%Boron (B) greater than 0% and lower than 0.0020%; wherein the concentrations of Mn, Cu, Al, Ti, N, S, B and C are such as to simultaneously satisfy the following relations :

[0042] a) (Mn+Cu) / (S) > 100

[0043] b) (Ti + B+ Al* 10A-3 ) / N=0 . 5-4 . 5

[0044] c) (S+N) / (Ti+B) =0 . 7-10 .

[0045] The concentrations of the single elements inserted in relations a) -c) are expressed in PPM.

[0046] In the alloy composition described and claimed above, the following specific elements can also be contained within the concentration limits described below:

[0047] Chromium (Cr) lower than 6%

[0048] Nickel (Ni) lower than 2%

[0049] Cobalt (Co) lower than 1%

[0050] Molybdenum (Mo) lower than 1%

[0051] Selenium (Se) lower than 0.02%

[0052] Bismuth (Bi) lower than 0.02%

[0053] Antimony (Sb) lower than 0.1%

[0054] Tin (Sn) lower than 0.1%

[0055] Phosphorus (P) lower than 0.1%

[0056] Arsenic (As) lower than 0.05%

[0057] Vanadium (V) lower than 0.01%

[0058] Niobium (Nb) lower than 0.01%

[0059] Tungsten (W) lower than 0.01%

[0060] Zirconium (Zr) lower than 0.01%

[0061] Magnesium (Mg) lower than 0.005%

[0062] Calcium (Ca) lower than 0.005% .

[0063] The remainder of the composition is iron and unavoidable impurities .

[0064] The strip is produced by hot rolling.

[0065] The process comprises a step of submitting the strip toan initial annealing treatment in controlled atmosphere non- oxi di zing .

[0066] The process comprises a step of submitting the strip to a surface scale and oxide removal treatment .

[0067] The step of submitting the strip to a surface scale and oxide removal treatment comprises a sub-step of pickling the strip .

[0068] The process according to the present invention comprises at least one step among descaling and pre-rolling the strip .

[0069] The step of descaling in particular provides cracking, breaking and at least partial removal of the scale . In particular, the step of descaling is carried out by means of at least one among a "Scale Breaker" machine 22 adapted to impose an elongation of the strip between 0.05% and 1%, one or more laser sources 23 and one or more abrasive brushes 24. The step of descaling is carried out previously to the step of pickling. The step of descaling is a sub-step of submitting the strip to a surface scale and oxide removal treatment, as is also the step of pickling, as reported above .

[0070] In detail, the surface scale and oxide removal treatment can be of mechanical type, aimed at the partial or total removal of the surface scale (given by oxides of Fe and Si and others) , or of chemical type, aimed at the complete removal of the residual scale and of the oxides present below the scale itself and also trapped as inclusions in the most superficial part of the metallic substrate .

[0071] The present invention in particular provides to carry out the surface scale and oxide removal treatment by applying one or more treatments of mechanical typefollowed by one of chemical type .

[0072] Pickling falls among the treatments of chemical type, i . e . aimed at the removal of residual oxides and of those present in the superficial region of the metallic substrate .

[0073] In substance, the scale is a layer of surface oxides and it can be removed both by purely mechanical methods and by exclusively chemical means .

[0074] In particular, with the term descaling reference will hereinafter be made to the mechanical-type scale removal treatment .

[0075] Therefore, the descaling treatments by means of a "Scale Breaker" machine, laser or abrasive brushes fall among the treatments of mechanical type .

[0076] The descaling step, if carried out by means of abrasive brushes 24, is referred to as brushing.

[0077] When the process comprises a step of descaling, followed by pickling, the descaling, among which there is also laser descaling, can therefore be referred to as cracking and partial removal of the scale; pickling instead has the function of completing the scale removal action more deeply up to the removal of all oxides, including those penetrated into the surface of the metallic substrate . The step of pre-rolling is carried out at a temperature between 70°C and 300°C with thickness reduction between 10% and 50% . The step of pre-rolling is carried out previously to the step of cold-rolling.

[0078] The step of pre-rolling can be carried out in addition or as an alternative to the step of descaling. In particular, the step of pre-rolling operates a reduction of thickness of the hot-rolled strip and, under the conditions according to the present process, performs apartial cracking and reduction of the thickness of the scale, thus also performing a descaling function, applying non-negligible elongations higher than those typical of the "Scale Breaker" machine, elongations generated by an appropriate combination of pressure / compression stresses (referred to as separation force of the working rolls) and tension applied to the strip .

[0079] In pre-rolling, the percentage ratio between rolling tension and roll separating force (rolling force expressed as compression force) is between 0.5% and 10%, expressing the rolling tension and force with the same unit of measurement (typically tonnes or kilonewton) , preferably between 2% and 7% .

[0080] The value of 0.5% is the minimum safety value necessary to have a significant effect on the scale breaking and at the same time useful to avoid the risk of imprinting the fragmented scale, with the consequence of not obtaining the desired roughness . Conversely, the value of 10% allows to obtain the desired effect without incurring problems of quality and continuity of rolling. The indicated ranges depend both on the features of the rolling plant (mainly roll size) and on the features of the treated material (strip thickness and yield strength of the material) .

[0081] Still with reference to pre-rolling, the elongation is related (according to the principle of conservation of the strip volume before and after rolling) with the thickness reduction rate applied, which in the present case, as specified above, is between 10% and 50% .

[0082] This means that if the pre-rolling reduction is 40% (or 50%) , the relative elongation of the strip will resultin 67% (or, respectively, 100%) .

[0083] In other words, the step of pre-rolling can be an alternative to the step of descaling, since it also partially breaks the scale . Preferably, when carried out as an alternative to the step of descaling, the step of pre-rolling is carried out previously to the step of pickling, i . e . previously to the step of submitting the strip to a scale and oxide reduction treatment .

[0084] When carried out in addition to the step of descaling, the step of pre-rolling can be carried out previously or subsequently to the step of submitting the strip to a scale and oxide reduction treatment, as reported below. The initial annealing treatment can be carried out previously or subsequently to the scale and oxide removal treatment .

[0085] The process comprises a step of cold-rolling the strip that had been submitted to the initial annealing and scale and oxide removal treatment until thickness between 0.1 and 0.5 mm. In other words, the step of coldrolling occurs in any case subsequently to the initial annealing treatment and to the scale and oxide removal treatment, independently of the order in which these are carried out . The step of cold-rolling can occur in a single stage or in a double stage, in such latter case being also provided a further intermediate annealing. According to such embodiment, the step of cold-rolling comprises in the following order : a first sub-step of cold-rolling, a step of submitting the strip to an intermediate annealing treatment and a second sub-step of cold-rolling.

[0086] The process comprises a step of submitting the cold-rolled strip to a final continuous annealing treatmentat a temperature between 800°C and 1150°C for a time not lower than 5 seconds in controlled atmosphere with dew point lower than 10 °C, measured in the treatment furnace by an in-line analyser .

[0087] The process according to the present invention preferably applies to strips obtained by hot rolling at a temperature higher than 700 °C applying a thickness reduction between 30% and 99% .

[0088] Before the step of providing a strip, the process can comprise a step of casting into a slab or ingot an ironbased alloy having the chemical composition defined above .

[0089] Previously to the step of providing a strip and subsequently to the step of casting, the process can comprise a step of hot rolling the slab or ingot at a temperature higher than 700 °C until obtaining a strip with a thickness between 0.5 and 2.5 mm, applying a thickness reduction between 30% and 99% .

[0090] According to embodiments of the present invention, the process comprises the step of pre-rolling and the step of descaling.

[0091] A high thickness reduction obtained by cold-rolling, particularly if higher than 85%, induces not only strain hardening phenomena, high brittleness and risk of breakage and tearing with consequent production losses, but also and above all a decrease of the presence of crystallographic texture components favourable to magnetization on the plane of the rolled product and therefore worsening of the magnetic induction B and of the magnetic losses P . Therefore, preferably, independently of the order in which the annealing treatment and the other scale and oxide removaltreatments are carried out, pre-rolling is carried out before cold-rolling.

[0092] Preferably, the step of pre-rolling is carried out previously to the step of submitting the strip to an initial annealing treatment and previously to the step of submitting the strip to a surface scale and oxide removal treatment, as visible for example in figures 2a-2b and 7 .

[0093] Alternatively, the step of pre-rolling can be carried out subsequently to the step of submitting the strip to a surface scale and oxide removal treatment and previously to the step of submitting the strip to an initial annealing treatment, as for example represented in figure 5.

[0094] Or the step of pre-rolling can be carried out subsequently to the step of submitting the strip to an initial annealing treatment and subsequently to the step of submitting the strip to a surface scale and oxide removal treatment, as for example represented in figure 8 .

[0095] The initial annealing can be continuous or static .

[0096] The step of submitting the strip to an initial annealing treatment can be carried out subsequently to the step of submitting the strip to a scale and oxide removal treatment in the case in which the annealing is carried out in a static furnace .

[0097] In the case of static annealing, preferably, the step of submitting the strip to an initial annealing treatment is carried out at a temperature between 700 °C and 870°C for a time between 1 and 60 hours in an atmosphere containing nitrogen, argon, helium, hydrogen or mixtures thereof .Alternatively, the step of submitting the strip to an initial annealing treatment is carried out continuously at a temperature between 750°C and 1100°C for a time not lower than 10 seconds, in an atmosphere containing nitrogen, argon, helium, hydrogen or mixtures thereof . In a preferred case, the step of submitting the strip to an initial annealing treatment is carried out subsequently to the step of pre-rolling the strip .

[0098] Still preferably, the process comprises a step of controlling the temperature of the strip at a temperature higher than 65°C, preferably between 65°C and 150°C . Such step is carried out previously to the step of submitting the strip to a surface scale and oxide removal treatment .

[0099] Such step of controlling the temperature is also carried out previously to the step of cold-rolling up to a thickness reduction not higher than 85% and to the step of pre-rolling with reduction of 10-50%, if present . In this way the strip is subj ected to the various steps after it has been heated.

[0100] In particular, the step of descaling or pre-rolling is carried out immediately subsequently to the step of controlling the temperature . In other words, the strip entering the descaling or pre-rolling preferably has a temperature between 65°C and 150°C . When both are present, the step of controlling the temperature is carried out immediately previously to the step of descaling. Preferably, the step of controlling the temperature occurs immediately previously to the step of descaling with Scale Breaker, if present .

[0101] Advantageously, the temperature range between 65-150°C combined with the descaling or the pre-rolling and withpickling according to the present invention is optimal for obtaining a strip with a roughness Ra equal to or lower than 4 exiting the pickling. This allows to obtain a final finished product with low roughness .

[0102] In particular, according to the present invention, it is possible to obtain strips with low average roughness Ra after cold-rolling, reducing the use of the shot peening machine compared with what is known or eliminating it completely, by combining one or more surface scale and oxide reduction treatment of mechanical type with a treatment of chemical type carried out subsequently thereto .

[0103] Preferably, the strip exiting the cold-rolling treatment is characterized by a roughness Ra < 0.25 pm, corresponding to the final roughness .

[0104] The roughness Ra is measured according to ISO21920 standard .

[0105] Still preferably, the step of controlling the temperature is carried out subsequently to the initial annealing when in-line .

[0106] According to a preferred embodiment, the "Scale Breaker" 22 comprises rolls having a diameter between 40 and 180 mm, preferably between 70 and 100 mm.

[0107] Preferably, the one or more laser sources 23, if provided, are of the pulsed type .

[0108] Preferably, the brushing step is carried out wet .

[0109] Optionally, the step of submitting the strip to a surface scale and oxide removal treatment comprises a further sub-step of shot peening or sandblasting preferably carried out respectively with a shot peening machine or blasting machine 25, using abrasive shots (also called Shot) or grits (also called Grit) having various sizes,preferably having Shot size between 0.18 and 0.71 mm, i . e . equal to S70 or S110 or S170 and preferably S110 (according to ASTM J444 standard or similar standards) , or Grit size between 0.125 and 0.71 mm, i . e . equal to G120 or G80 or G50 and preferably G80 (according to ASTM J444 standard or similar standards) . The steps of shot peening or sandblasting are also types of mechanical scale removal treatment and fall within the descaling step .

[0110] According to the present invention, shot peening or sandblasting, if present, are operated at reduced proj ection velocity; in particular preferably the shot peening machine or blasting machine 25 are operated at a proj ection velocity of the shots (or grit) lower than 80 m / s . Preferably, the proj ection velocity is lower than 75 m / s in the case of shots or grit having smaller granulometry such as Shot S70 (or Grit G120) , or lower than 65 m / s in the case of shots or grit having larger granulometry such as Shot S110 (or Grit G80) , or lower than 55 m / s in the case of shots or grit having even larger granulometry such as Shot S170 (or Grit G50) . In this way it is possible to maintain good values of surface roughness of the strip Ra, in particular Ra d 4 pm, exiting the step of submitting the strip to a scale and oxide reduction treatment and in particular after pickling .

[0111] Still preferably, both the sub-step of descaling by means of a "Scale Breaker" machine and the brushing sub-step, if both are provided, are carried out previously to the sub-step of pickling, where the sub-step of brushing is preferably intermediate between the sub-step of descaling and the sub-step of pickling.Preferably, the sub-step of sandblasting or shot peening, if provided, is carried out previously to the sub-step of pickling and subsequently to the step of descaling by means of "Scale Breaker" or laser .

[0112] Essentially, the mechanical-type scale removal treatments are always carried out previously to the chemical-type treatment according to the present invention .

[0113] More preferably, both the sub-step of descaling preferably by means of "Scale Breaker" and the sub-step of shot peening or sandblasting, if both are provided, are carried out previously to the sub-step of pickling, wherein the sub-step of sandblasting or shot peening is preferably intermediate between the sub-step of descaling by means of "Scale Breaker" and the sub-step of pickling.

[0114] Preferred embodiments of the process according to the present invention are shown in figures 2a-7.

[0115] According to preferred embodiments of the present invention, the sub-step of pickling is carried out by means of one or more tanks 21 containing a solution of hydrochloric or hydrofluoric acid at a temperature between 45°C and 85°C . Optionally, such sub-step is also carried out by means of one or more tanks 21 containing sulphuric acid at a temperature between 80°C and 98 °C located upstream of the tanks 21 containing hydrochloric or hydrofluoric acid. In such second case, a rinse being provided between the sulphuric acid tank 21 and the subsequent hydrochloric or hydrofluoric acid tanks . In other words, pickling can be carried out in hydrochloric or hydrofluoric acid or by means of sulphuric acid followed by hydrochloric or hydrofluoricacid .

[0116] In particular, in the case where the sub-step of pickling provides the use of hydrochloric acid, the acid concentration in the tank 21 is between 180 and 200 g / 1 ( free and combined total acid) while the iron concentration is between 30 and 130 g / 1. In such case the temperature is preferably between 60°C and 85°C . Conversely, in the case where the sub-step of pickling provides the use of hydrofluoric acid, the acid concentration in the tank 21 is between 10 and 40 g / 1 ( free acid) and the temperatures are preferably between 45°C and 65°C .

[0117] When present, the concentration of sulphuric acid in the tank 21 is between 100 and 250 g / 1 and preferably between 200 and 250 g / 1 and the temperature is between 80 °C and 98 °C .

[0118] Another obj ect of the present invention is a non-grain oriented steel metal sheet, adapted to be intended for ferromagnetic cores of rotating electrical machines . The metal sheet is obtainable by means of the process described above . The metal sheet has a thickness between 0.1-0. 5 mm and a magnetic polarisability B5000 > 1.749- ( 0.002p) , where B5000, expressed in Tesla, is the induction measured at a magnetic field of 5000 A / m and p is the electrical resistivity, expressed in pQ -cm, according to the following equation: p=9+11.25 • (%Si) +6.25 • (%Mn) +11.52 • (%Al) +2.5

[0119] • (%Cu) +14 • (%) P+5.35 • (%Sn) .

[0120] The sheet according to the present invention preferably has a chemical weight composition defined as follows :

[0121] Silicon (Si) greater than 0% and lower than 6%, preferably between 2.0% and 4.5%,Carbon (C) greater than 0% and lower than 0.007%, Aluminium (Al) greater than 0% and lower than 2%, Manganese (Mn) greater than 0% and lower than 3%, preferably lower than 2%,

[0122] Copper (Cu) greater than 0% and lower than 2%, preferably lower than 1%,

[0123] Nitrogen (N) between 0.0005 and 0.010%,

[0124] Sulphur (S) between 0.0005 and 0.020%,

[0125] Titanium (Ti) between 0.0005 and 0.01%,

[0126] Boron (B) greater than 0% and lower than 0.0020%, where the concentrations of Mn, Cu, Al, Ti, N, S, B and C are such as to simultaneously satisfy the following relations :

[0127] a) (Mn+Cu) / (S) > 100

[0128] b) (Ti + B+ Al* 10A-3 ) / N=0 . 5-4 . 5

[0129] c) (S+N) / (Ti+B) =0 . 7-10 .

[0130] Such alloy can also contain the following specific elements within the concentration limits by weight described below:

[0131] Chromium (Cr) lower than 6%

[0132] Nickel (Ni) lower than 2%

[0133] Cobalt (Co) lower than 1%

[0134] Molybdenum (Mo) lower than 1%

[0135] Selenium (Se) lower than 0.02%

[0136] Bismuth (Bi) lower than 0.02%

[0137] Antimony (Sb) lower than 0.1%

[0138] Tin (Sn) lower than 0.1%

[0139] Phosphorus (P) lower than 0.1%

[0140] Arsenic (As) lower than 0.05%

[0141] Vanadium (V) lower than 0.01%

[0142] Niobium (Nb) lower than 0.01%

[0143] Tungsten (W) lower than 0.01%Zirconium (Zr) lower than 0.01%

[0144] Magnesium (Mg) lower than 0.005%

[0145] Calcium (Ca) lower than 0.005% .

[0146] The remainder of the composition is iron and unavoidable impurities .

[0147] The magnetic sheet is characterized by a roughness Rad 0.25 pm.

[0148] The roughness Ra is measured according to ISO21920 standard .

[0149] The non-grain oriented magnetic sheets (NGO-ES) obtained with the process described in particular exhibit excellent magnetic features, with particular regard to the magnetic polarisability feature .

[0150] The greatest advantages, in terms of final properties and processability compared with the current state of the art, are obtainable in the case of final grades having very thin thicknesses (< 0.50 mm, and in particular < 0.35 mm) .

[0151] Finally, the present invention also relates to a plant 100 for manufacturing a non-grain oriented steel metal sheet as described above .

[0152] The plant 100 according to the present invention allows to manufacture non-grain oriented steel metal sheet with a thickness between 0.1 and 0.5 mm, adapted to be intended for ferromagnetic cores of electrical machines . The plant 100 is adapted to manufacture such steel metal sheet from a strip with a thickness between 0.5 and 2.5 mm, preferably between 0.7 and 2.4 mm, of an iron-based alloy having the following chemical weight composition:

[0153] Silicon (Si) greater than 0% and lower than 6%, preferably between 2.0% and 4.5%,

[0154] Carbon (C) greater than 0% and lower than 0.007%,Aluminium (Al) greater than 0% and lower than 2%, Manganese (Mn) greater than 0% and lower than 3%, preferably lower than 2%,

[0155] Copper (Cu) greater than 0% and lower than 2%, preferably lower than 1%,

[0156] Nitrogen (N) between 0.0005 and 0.010%,

[0157] Sulphur (S) between 0.0005 and 0.020%,

[0158] Titanium (Ti) between 0.0005 and 0.01%,

[0159] Boron (B) greater than 0% and lower than 0.0020%; wherein the concentrations of Mn, Cu, Al, Ti, N, S, B and C are such as to simultaneously satisfy the following relations :

[0160] a) (Mn+Cu) / (S) > 100

[0161] b) (Ti + B+ Al* 10A-3 ) / N=0 . 5-4 . 5

[0162] c) (S+N) / (Ti+B) =0 . 7-10 .

[0163] In the alloy composition described and claimed above the following specific elements can also be present within the concentration limits described below:

[0164] Chromium (Cr) lower than 6%

[0165] Nickel (Ni) lower than 2%

[0166] Cobalt (Co) lower than 1%

[0167] Molybdenum (Mo) lower than 1%

[0168] Selenium (Se) lower than 0.02%

[0169] Bismuth (Bi) lower than 0.02%

[0170] Antimony (Sb) lower than 0.1%

[0171] Tin (Sn) lower than 0.1%

[0172] Phosphorus (P) lower than 0.1%

[0173] Arsenic (As) lower than 0.05%

[0174] Vanadium (V) lower than 0.01%

[0175] Niobium (Nb) lower than 0.01%

[0176] Tungsten (W) lower than 0.01%

[0177] Zirconium (Zr) lower than 0.01%Magnesium (Mg) lower than 0.005%

[0178] Calcium (Ca) lower than 0.005%,

[0179] and the remainder iron and unavoidable impurities .

[0180] The plant 100 comprises an initial annealing treatment station 10, preferably in controlled atmosphere nonoxidizing .

[0181] The plant 100 comprises a scale and oxide removal station 20 .

[0182] The scale and oxide removal station 20 can be upstream or downstream of the initial annealing treatment station 10 .

[0183] The plant 100 comprises a cold-rolling station 30 located downstream of the initial annealing treatment station 10 and of the scale and oxide removal station 20. The coldrolling station 30 can comprise an intermediate annealing station in the case where cold-rolling is performed in double stage .

[0184] The plant 100 comprises a final annealing station 40 configured to heat the cold-rolled strip continuously at a temperature between 800 °C and 1150°C for a time not lower than 5 seconds in controlled atmosphere with dew point lower than 10°C, measured by an in-line analyser . In particular, the final annealing station 40 is located downstream of the cold-rolling station 30.

[0185] In particular, the scale and oxide removal station 20 comprises one or more pickling tanks 21.

[0186] According to the present invention, the plant 100 comprises at least one among a descaling station 70 and a pre-rolling station 50 located upstream of the coldrolling station 30.

[0187] The descaling station 70, if present, is part of the scale and oxide removal station 20; in other words, thescale and oxide removal station 20 comprises one or more pickling tanks 21 and can further comprise the descaling station 70.

[0188] The descaling station 70 comprises one or more among: a "Scale Breaker" machine 22 adapted to impose an elongation of the strip between 0.05% and 1%, one or more laser sources 23 and one or more abrasive brushes 24. The one or more pickling tanks 21 are located downstream of the descaling station 70 if present . If the pre-rolling station is present as an alternative to the descaling station 70, preferably one or more pickling tanks 21 are located downstream of the pre-rolling station 50.

[0189] Optionally, the plant 100 comprises the pre-rolling station 50 and the descaling station 70.

[0190] If both the pre-rolling station 50 and the descaling station 70 are present, the pre-rolling station 50 can be positioned upstream or downstream of the scale and oxide removal treatment station 20.

[0191] Preferably, the strip processed in the plant 100 is obtained by hot rolling a slab or ingot at a temperature higher than 700°C applying a thickness reduction between 30% and 99% .

[0192] In particular, in one embodiment the plant 100 can be preceded by a casting station to cast into a slab or ingot an iron-based alloy having the chemical composition defined above, and by a hot-rolling station configured to roll the slab or ingot at a temperature higher than 700 °C until obtaining a strip with a thickness between 0.5 and 2.5 mm, applying a thickness reduction between 30% and 99% .

[0193] Preferably, the pre-rolling station 50 can be locatedupstream of the initial annealing station 10 and upstream of the scale and oxide removal station 20, as for example represented in figures 2a and 2b and 7.

[0194] Alternatively, the pre-rolling station 50 can be located downstream of the scale and oxide removal station 20 and upstream of the initial annealing station 10, as for example represented in figure 5.

[0195] Alternatively, the pre-rolling station 50 can be located downstream of the initial annealing station 10 and downstream of the scale and oxide removal station 20, as for example represented in figure 8.

[0196] Still preferably, the pre-rolling station 50 is located in-line with the scale and oxide removal station 20. The cold-rolling station 30 can be located in-line with the pre-rolling station 50 and with the scale and oxide removal station 20 or in a separate line .

[0197] The initial annealing station can comprise (separate) static furnaces or one or more furnaces in-line with the pre-rolling station 50 and with the scale and oxide removal station 20.

[0198] Preferably, the static furnace is configured to heat at a temperature between 700 °C and 870°C for a time between 1 and 60 hours in an atmosphere containing nitrogen, argon, helium, hydrogen or mixtures thereof .

[0199] Preferably, the plant 100 comprises a pyrometer 60 configured to allow measure and control of the predetermined temperature of the strip, preferably higher than 65°C .

[0200] Preferably, the pyrometer 60 is located upstream of the scale and oxide removal station 20. The pyrometer 60 can also be located upstream of the pre-rolling station, if provided, and upstream of the cold-rolling station 30.Preferably, the in-line furnaces are configured to heat at a temperature between 750°C and 1100°C for a time not lower than 10 seconds, in an atmosphere containing nitrogen, argon, helium, hydrogen or mixtures thereof . The pyrometer 60 is preferably located downstream of the initial annealing station 10 when this comprises an inline furnace .

[0201] Preferably, the pyrometer 60 is located immediately upstream of the descaling station 70 or of the prerolling station 50 if present as an alternative to the descaling station 70.

[0202] If both are present, the pyrometer 60 is preferably located immediately upstream of the descaling station 70 .

[0203] Preferably, the pyrometer 60 is located upstream of the "Scale Breaker" machine 22 if present .

[0204] The one or more laser sources 23 are preferably of the pulsed type .

[0205] Preferably, the strip exiting the cold-rolling station 30 is characterized by a roughness Rad0.25 pm.

[0206] The abrasive brushes 24 are preferably adapted to brush the strip wet .

[0207] Optionally, the scale and oxide removal station 20 comprises at least one among a shot peening machine or blasting machine 25 using respectively abrasive shots (also called Shot) or grits (also called Grit) having various sizes, preferably having Shot size equal to S170 or S110 or S70 and more preferably S110 (according to ASTM J444 or similar standards) or Grit size preferably equal to G50 or G80 or G120 and more preferably G80 (according to ASTM J444 or similar standards) , configured to shot peen or sandblast the strip . Theabrasive brushes 24 or the shots or grit, if present, are preferably used upstream of the pickling tank (s) 21. According to the present invention, the shot peening machine or blasting machine 25, if present, are operated at reduced proj ection velocity; in particular preferably the shot peening machine or blasting machine 25 are operated at a proj ection velocity of the shots (or grit) lower than 80 m / s . Preferably, the proj ection velocity is lower than 75 m / s in the case of shots or grit having smaller granulometry such as Shot S70 (or Grit G120) , or lower than 65 m / s in the case of shots or grit having larger granulometry such as Shot S110 (or Grit G80) , or lower than 55 m / s in the case of shots or grit having even larger granulometry such as Shot S170 (or Grit G50) . With reference to what has been reported above, the scale and oxide removal station 20 according to the present invention comprises a mechanical station, adapted to partially or totally remove the surface scale, and a chemical station, adapted to completely remove the residual scale and the oxides present below the scale itself and also trapped as inclusions in the most superficial part of the metallic substrate, located downstream of the mechanical station.

[0208] As mentioned above, the above-cited "Scale Breaker" machine 22, laser source 23 and abrasive brushes 24 remove the scale mechanically and therefore are all part of the mechanical station, if present, together with the shot peening machine or blasting machine 25, if present . The above-mentioned one or more pickling tanks 21 remove the oxides and fall within the chemical station.

[0209] The laser sources 23 and / or the "Scale Breaker" machine 22, if present, are upstream of the pickling tank (s) 21.If both the laser sources 23 and / or the "Scale Breaker" machine and the abrasive brushes 24 are present, the abrasive brushes 24 are preferably interposed between the laser sources 23 and / or the "Scale Breaker" machine 22 and the one or more pickling tanks 21.

[0210] The blasting machine 25, or the shot peening machine, can be coupled with a "Scale Breaker" machine 22 and / or with one or more laser sources 23, and in such case is preferably interposed between the latter and the one or more pickling tanks 21. The blasting machine 25, or the shot peening machine, can possibly also be coupled with a pre-rolling station 50.

[0211] Preferably, the scale and oxide removal station 20 comprises a pickling tank 21 containing a solution of hydrochloric or hydrofluoric acid at a temperature between 45°C and 85°C .

[0212] Optionally, the plant 100 comprises one or more pickling tanks 21 containing sulphuric acid at a temperature between 80°C and 98 °C located upstream of the pickling tank 21 containing hydrofluoric or hydrochloric acid. In such second case, a rinse being provided between the sulphuric acid and hydrochloric or hydrofluoric acid tanks 21 .

[0213] In particular, in the case where one or more pickling tanks 21 containing hydrochloric acid are present, the acid concentration in the tank is between 180 and 200 g / 1 ( free and combined total acid) while the iron concentration is between 30 and 130 g / 1. In such case the temperature is preferably between 60°C and 85°C . Conversely, in the case where one or more hydrofluoric acid pickling tanks 21 are present, the acid concentration in the tank 21 is between 10 and 40 g / 1( free acid) and the temperatures are preferably between 45°C and 65°C .

[0214] Conversely, in the case in which sulphuric acid pickling tanks 21 are also present, the acid concentration is between 100 and 250 g / 1 and preferably between 200 and 250 g / 1 and the temperature is between 80°C and 98 °C . With reference to the chemical composition of the alloy indicated above, the following is specified.

[0215] Silicon is a substitutional element of the iron atoms and increases the electrical resistivity of the metallic matrix and reduces magnetic losses particularly in the case of high working frequencies (>50 Hz) . Silicon contents lower than 2% by weight do not allow the desired electrical resistivity to be obtained, while contents higher than 4.5% produce an increase in alloy brittleness not compatible with the industrial workability of the material with the proposed process .

[0216] Carbon at contents higher than 0.01% produces carbides and carbo-nitrides (in association with nitrogen) that worsen the final magnetic features of the product and in some steps of the process can deteriorate the workability of the semi-finished products . Contents higher than 0.007% moreover produce a deterioration of the magnetic loss features once inserted into electrical machines, due to a phenomenon known as "magnetic ageing" . Carbon contents lower than 0.0005% are compatible with the present invention but, when industrially achievable, significantly increase manufacturing costs without obtaining significant advantages in the features obtainable in the finished products .

[0217] Nitrogen, in the presence of aluminium and titanium, tends to form aluminium and titanium nitrides and carbo-nitrides (in association with carbon) preferentially with respect to the formation of silicon nitrides . Aluminium and titanium nitrides contribute to regulating the growth of the crystalline grain in the metallic matrix in various steps of the process . Values lower than 0.0005% are very difficult to achieve, with an increase in manufacturing costs, without generating significant advantages in the product properties, while values higher than 0.01% are detrimental to the final magnetic properties of the products . Within the manufacturing process according to the invention, the nitrogen concentration range defined above produces the best results when the concentrations of Al, Ti, C and S meet relations 2 and 3 described above .

[0218] Sulphur is a substitutional element of the iron atoms which, in the presence of manganese and copper, forms manganese and copper sulphides . As in the case of nitrogen, such precipitates contribute to regulating the growth of the crystalline grain in the metallic matrix in the various steps of the process . Values lower than 0.0005% are difficult to make, and achieving lower concentrations does not justify, in terms of quality and process control, the associated increase in manufacturing costs . Values higher than 0.020% are critically detrimental to the final magnetic properties of the products . Within the manufacturing process claimed, the sulphur concentration range identified as useful produces the best results when the concentrations of Mn, Cu, Ti, C and S meet relations 1 and 3 described above .

[0219] Aluminium is a substitutional element of the iron atoms and, like silicon, is useful for increasing theelectrical resistivity of the metallic alloy. It regulates the precipitation and the size of nitride precipitates and is therefore useful for regulating the parallel processes of recrystallization and grain growth. Contents higher than 2% critically increase the brittleness of the material, create process continuity problems during the industrial continuous casting step, and excessively reduce the magnetic saturation induction of the finished products (in association with the silicon content) .

[0220] Manganese is a substitutional element of the iron atoms and, like silicon and aluminium, contributes to increasing the electrical resistivity of the metallic alloy. Furthermore, it is useful for controlling the precipitation of sulphides . Contents higher than 3% are disadvantageous for the functional features of the finished products, particularly due to the potential activation of phase transformations during the thermomechanical treatments of production and the associated excessive reduction of the saturation magnetic induction, as well as due to uncontrolled variations in the microstructural evolution.

[0221] Copper is a substitutional element of the iron atoms and, like silicon, aluminium and manganese, contributes to increasing the electrical resistivity of the metallic alloy. In association with manganese, it is useful for controlling sulphide precipitation. Contents higher than 1% lead to surface defects in the starting hot strips which are detrimental to the process and to the final quality of the products, without providing significant improvements in the control of the microstructure .

[0222] Titanium is an element that forms particles of non-metallic second phases in association with C, S and N. Experimental evidence from laboratory experiments indicates that the useful range for the use of titanium in the present invention is titanium between 0.0005% and 0.01% . It was not possible to test values lower than 0.0005%, while values higher than 0.01% were found to be detrimental to the final product quality.

[0223] Boron is an element that forms particles of non-metallic second phases in association with C, S and N, useful for controlling the microstructure and the mechanical features of the steel . Contents higher than 0.0020% critically increase the brittleness of the material and adversely affect the control of the desired surface morphology .

[0224] According to the present invention, once a hot strip with the chemical composition and the features described has been obtained, it must be subjected to an annealing treatment . Such treatment can be carried out either by adopting industrial solutions of static annealing with coiled coil in bell furnaces or by adopting continuous annealing solutions for the strips .

[0225] The annealed hot-rolled strip must then subsequently undergo a surface conditioning treatment in order to remove the scale and oxides by descaling followed by pickling in sequence .

[0226] For pickling, hydrochloric acid solutions can be used with a total acid concentration ( free and combined) between 180 and 200 g / 1 and iron concentration between 30 and 130 g / 1, at a temperature between 60°C and 85°C . Alternatively, hydrofluoric acid solutions can be used having a free acid concentration between 10 and 40 g / 1 at a temperature of 45-65°C . Alternatively, the picklingsequence is based on a first immersion in sulphuric acid with a concentration between 100 and 250 g / 1 and preferably between 200 and 250 g / 1 and temperature between 80°C and 98 °C followed by a second immersion in hydrochloric or hydrofluoric acid at the concentrations and temperatures indicated above .

[0227] The strip is then cold-rolled to final thickness in one or more thickness reduction states, possibly separated by intermediate annealing .

[0228] The claimed process therefore provides for continuous annealing of the cold-rolled strip at final thickness, characterized by a residence of the strip at a temperature between 800°C and 1150°C for a time not lower than 5 seconds, and in controlled atmosphere with dew point PDR<10°C . The annealing must generate recrystallization of the metallic matrix . The strip, exiting the cooling zone, can optionally be subj ected, continuously in the same treatment line, to deposition of an insulating surface coating .

[0229] The inventors have verified that by adopting the chemical compositions and the process conditions described, recrystallization of the cold strips during continuous annealing of the same at final thickness is particularly homogeneous through the thickness and the fluctuations of the average crystalline grain size at the end of the treatment result, after initial calibration, reliably controlled with constant values proportional to the thermal treatment conditions in terms of time (line velocity) and thermal cycle ( furnace zone settings) . The following examples illustrate some technical and qualitative aspects of the process described, but are in no way to be considered limiting with respect to thefeatures and purposes of the present invention.

[0230] EXAMPLES

[0231] Example 1

[0232] A series of ingots of approximately 100 kg each were produced on a pilot scale, with different alloy compositions both according to the present invention and outside the specifications of the present invention (the concentration of the elements is expressed in % by weight in Table 1 below) .

[0233] The produced ingots followed the following transformation cycle to the finished product :

[0234] - Geometrically conditioning the cast ingots in order to obtain pieces representative of the body of the solidified materials with a thickness of 4 cm, identical for all the ingots ;

[0235] - Heating the produced pieces using static annealing furnaces with holding at a temperature of 1080°C for 40 minutes ;

[0236] - Hot rolling in a pilot plant until reaching a final strip thickness of about 2.5 mm, with total deformation operated for all samples within a temperature range between 1050°C and 750°C;

[0237] Annealing samples of the produced hot strips, simulating in laboratory a continuous annealing of strips at a treatment temperature between 950-980°C for a residence time of 40 seconds ;

[0238] - Conditioning the surface of the rolled pieces, carried out in laboratory by means of sandblasting and hydrochloric pickling in sequence;- Cold-rolling in a double stage with thickness reduction from 2.5 mm to 0. 65 mm by means of the first rolling, annealing in laboratory furnaces to simulate a continuous annealing at a temperature of 1000°C for a residence time of 30 seconds in a dry N2 / H2 atmosphere, and second cold-rolling up to the final thickness of 0.26 mm; the double-stage rolling with intermediate annealing promotes microstructures favourable for magnetic losses and at the same time promotes the achievement of optimal surface roughness of the final product ;

[0239] - Heat treatment of the various produced samples to simulate a continuous annealing at 1030 °C for a time of 30 seconds in a hydrogen (H2) atmosphere and pdr<10°C .

[0240] All the produced samples were sampled and conditioned to a size of 320 mm x 50 mm, with the long side parallel to the rolling direction with which they were produced. The samples were then stacked and compressed between two nickel plates and treated at a temperature of 780°C for 30 minutes and slowly cooled to room temperature in order to restore a flat geometry and relieve residual stresses after the last metallurgical annealing undergone .

[0241] Finally, the samples ( 4 for each studied condition) were conditioned by mechanical cutting on all four sides of each sample to the size of 280 mm x 30 mm and magnetically characterized by measure on an Epstein frame .

[0242] The properties measured for the samples thus obtained are summarized in Table 1. In particular, B5000 and P10@400 Hz were measured and the electrical resistivity value was calculated as a function of the alloy composition (expressed in pQ -cm) , according to thefollowing reference equation :

[0243] p=9+11.25 • (%Si) +6.25 • (%Mn) +11.52 • (%Al) +2.5 • (%Cu) +14 • (%) P+5 . 35 • ( %Sn) .

[0244] The samples reported in the table below, in the cases where the chemical compositions are in accordance with the present invention, are marked as comparison samples (COMP) .

[0245] The samples marked with "NO" instead have a composition not in accordance with the present invention.

[0246] TABLE 1

[0247]

[0248] It can be observed that all the steel sheets produced with the composition according to the invention and with double-stage cold-rolling produced B5000 induction features significantly higher than 1. 6 Tesla and magnetic losses measured at an induction of 1.0 Tesla and 400 Hz inversely proportional to the electrical resistivity of the material (see the graph of figure 9) ,which allows to obtain loss values P10@400Hz with a nominal thickness of 0.27 mm lower than 14 W / kg. All the measured samples had a thickness of 0.26 mm (± 0.005 mm) .

[0249] Example 2

[0250] A series of samples produced from ingot no . 5 having the chemical composition reported in Table 1 with a thickness of 4 cm were taken and processed by hot rolling with the same sequence and conditions of Example 1.

[0251] The hot-rolled strips were then processed with different surface conditioning parameters according to different innovative descaling sequences followed by pickling until complete removal of the visible scale and measure of pickling time and weight losses in order to compare the different sequences .

[0252] Sandblasting was "calibrated" so as to reproduce a grade SA 2 (according to ISO 8501-1 standard) and removal of approximately 40-50% by weight of the scale; in some cases, the descaling power (peripheral velocity at turbine outlet reduced by approximately 40% with respect to the maximum) was reduced, in other cases it was eliminated .

[0253] For some samples, a pre-rolling of the hot strip (with scale on the surface) was performed with a reduction of approximately 30% on a pilot Duo reversible rolling mill and the effects on descaling and subsequent pickling and magnetic properties were subsequently verified.

[0254] Some samples were processed by a "Scale Breaker" . The "Scale Breaker" is a device for cracking the scale by means of successive bending operations under tension. In the specific case of the examples according to thepresent invention, considering the brittleness characteristics of steels with high silicon content, a "Scale Breaker" with rollers having diameter preferably between 70 and 100 mm was studied, with temperature selected above the ductile-brittle transition point of the material (in the specific case higher than 80°C) and reduced inlet tension, in the specific case of the examples set at 15 N / mm2. Under these conditions no brittle fractures or crack propagation occurred in all the tested samples .

[0255] Furthermore, some samples were subj ected to abrasive brushing by means of brushes having grit 120 operating at 1000 rpm with strip passage velocity of 60 m / min. All the samples thus produced were then processed up to the finished product, adopting the same process conditions and laboratory measure tests described in Example 1, and in particular a final thickness after cold-rolling of 0.26 mm.

[0256] In a series of tests, a sequence similar to that described in Example 1 was adopted but with reversed order between pickling and annealing as described in detail below.

[0257] The adopted experimental conditions and the characterizations of semi-finished and finished products are reported in Tables 2 and 3.

[0258] Table 2 reports comparison tests and results (COMP) related to :

[0259] sample 5.1 made according to the process of figure la (in-line initial annealing and pickling) ; sample 5.2 made according to the process of figure lb (in-line initial annealing, sandblasting andpickling) .

[0260] For both samples, the starting hot-rolled thickness was approximately 1. 6 mm.

[0261] TABLE 2

[0262]

[0263] Table 3 reports tests and results related to :

[0264] sample 5.3 made according to the process of figure 2a (pre-rolling, in-line initial annealing, sandblasting with grit proj ection velocity of 55 m / s and pickling) ;

[0265] samples 5.4 and 5.5 made according to the process of figure 2b (pre-rolling, in-line initial annealing, descaling with "Scale Breaker", brushing and pickling) ;

[0266] sample 5. 6 made according to the process of figure 3a (in-line initial annealing, descaling with "Scale Breaker", sandblasting and pickling) ;

[0267] sample 5.7 made according to the process of figure 3b (in-line initial annealing, descaling with "Scale Breaker", brushing and pickling) ;

[0268] sample 5.8 made according to the process of figure 4a (in-line initial annealing, descaling with laser and pickling) ;

[0269] sample 5.9 made according to the process of figure 4b (in-line initial annealing, descaling with laser, brushing and pickling) ;sample 5.10 made according to the process of figure 5 (descaling with "Scale Breaker", pickling, prerolling and static not-in-line initial annealing, subsequently to the pre-rolling) .

[0270] TABLE 3

[0271]

[0272] The roughness reported in Table 3 (Hot Band Roughness) is intended downstream of the mechanical scale removal treatments and the subsequent pickling.

[0273] (* ) : quality index from 1 (low) to 5 (excellent) . The highest values refer to high stripping quality, low roughness Ra combined with shorter pickling time and lower pickling losses .

[0274] (** ) Sandblasting (Shot S110) : St : standard sandblasting parameters; Low: impact velocity reduced by 35% .

[0275] (*** ) after the pre-rolling performed at cold conditions followed by annealing, a reduction of the scale weight per unit surface was observed.

[0276] (**** ) ; the samples of series 5.10 were subj ected, after hot rolling, to preheating at a temperature higher than70°C .

[0277] From Table 3 it can be observed that all the steel sheets produced according to the invention exhibit B5000 induction features higher than 1. 6 Tesla and excellent magnetic losses measured at an induction of 1.0 Tesla and 400 Hz significantly lower than 13 W / Kg.

[0278] In the case of sample 5.10, the 30% pre-rolling was carried out after pickling, maintaining the samples at a temperature higher than 70°C in order to simulate in laboratory the same process performed on a Deca-Treno or Deca-Tandem line (sometimes abbreviated as PLTCM -Pickling Line & Tandem Cold Mill) , where the strip exiting the pickling section is maintained at temperature by appropriately regulating the washing and drying temperature or by heating furnaces located between the pickling outlet and the inlet of the rolling section. At the outlet of the Deca-Treno the coil can be annealed by means of bell furnaces or in a continuous line, and subsequently rolled to final thickness and annealed at final thickness .

[0279] Example 3

[0280] A series of samples were taken from ingot no . 14 having the chemical composition reported in Table 1, which complies with the specifications claimed in the present patent . A test campaign was then carried out from sampled pieces of material with a thickness of 40 mm by actuating the following metallurgical operations :

[0281] - Heating the 40 mm-thick pieces in static annealing furnaces with holding time at a temperature of 1100 °Cfor 30 minutes;

[0282] - Hot rolling in a pilot plant until reaching a final strip thickness of approximately 2.1 mm, with total deformation operated for all samples within a temperature range between 1050 °C and 750°C .

[0283] Annealing of samples of the produced hot strips simulating in laboratory a static annealing, i . e . conducted simulating that on a coiled strip, at 800°C for a time of 4 hours in an atmosphere containing nitrogen;

[0284] - Cooling the pieces down to 100 °C and conditioning the surface of the hot-rolled pieces operated according to the following treatment sequence :

[0285] - descaling with "Scale Breaker Multiroll"

[0286] - abrasive brushing (grit 80 operating at 1000 rpm with strip passage velocity of 60 m / min) with weight losses of 5.3 g / m2;

[0287] - pickling with hydrochloric acid at 80°C, time 14 s, with weight losses of 15.8 g / m2;

[0288] - Cold-rolling performed up to intermediate thickness simulating a process operated in an off-line reversible rolling mill (downstream of the descaling and pickling line) or in a "Tandem Mill" within the descaling and pickling line, operated with inlet temperature of 80°C . The samples thus prepared were then annealed at intermediate thickness at 980 °C for a time of 30 seconds in a dry atmosphere of mixed N2 / H2 (N225% / H275%) and subsequently rolled to final thickness according to the rolling reduction scheme reported in Table 4.

[0289] TABLE 4Intermediate final

[0290] thickness thickness

[0291] Mm mm

[0292] 0 . 60 0.26

[0293] 0 . 50 0.24

[0294]

[0295] The final cold-rolled strips were then annealed in laboratory in an atmosphere of 100% dry hydrogen (d.p . < -40°C) , simulating a continuous annealing at 1025°C for a time of 30 seconds and subsequently rapidly cooled in a controlled atmosphere of dry H2 .

[0296] All the produced materials were then sampled and conditioned to a size of 320 mm x 50 mm, with the long side parallel to the rolling direction with which they were produced. The samples were then packed and compressed between two nickel plates and all treated at a temperature of 780 °C for 30 minutes and slowly cooled down to room temperature, in order to restore a flat geometry and relieve residual stresses after the last metallurgical annealing undergone .

[0297] Finally, the samples ( 8 for each studied condition) were conditioned by mechanical cutting on all four sides of each sample to the size of 280 x 30 mm and magnetically characterized by measure on an Epstein frame .

[0298] TABLE 5

[0299] Test Produced Carbon Roughness Average GS Factor K B5000 P10@400Hz nominal Ra GS (d) distribution (o / d)

[0300] thickness St. Dev. (0)

[0301] N Lim PPm Lim Lim Lim Tesla W / Kg 3 0.26 20 0.17 95 48 0.51 1.69 12.5 4 0.24 21 0.19 100 52 0.52 1.68 12.2

[0302]

[0303] The innovative procedures according to Example 3 (samples 3 and 4 in Table 5) can be actuated according to the schematizations illustrated in figures 6a, in which the cold-rolling / annealing is carried out off-line in double stage, or in figure 6b, with the first coldrolling stage carried out in-line .

[0304] The magnetic results obtained are excellent both in terms of magnetic induction and magnetic losses .

[0305] Example 4

[0306] A series of samples were taken from ingot no . 14 having the composition reported in Table 1, which complies with the specifications claimed in the present patent . A testing campaign was therefore conducted from sampled pieces of material with a thickness of 40 mm by actuating the following metallurgical operations :

[0307] - Heating the 40 mm-thick pieces in static annealing furnaces with a holding time at a temperature of 1100 °C for 30 minutes;

[0308] - Hot rolling in a pilot plant until reaching a final strip thickness of approximately 2.5 mm, with total deformation applied for all samples within a temperature range between 1050°C and 750 °C .

[0309] - Cooling the pieces and pre-rolling the pieces from 2.5 mm to 1.5 mm for all samples (approximately 35%) ;

[0310] Annealing the produced samples simulating in laboratory a continuous annealing of strips at a treatment temperature of 1000°C for a residence time of 30 seconds;

[0311] - Conditioning the surface of the rolled pieces operatedaccording to the following sequence of treatments :

[0312] - descaling with "Scale Breaker Multiroll"

[0313] - abrasive brushing (grit 80 operating at 1000 rpm with strip passage velocity of 60 m / min) with weight losses of 5.3 g / m2;

[0314] - pickling with hydrochloric acid at 80°C, time 13 s, with weight losses of 15.3 g / m2.

[0315] The samples thus prepared were then cold-rolled in double stage with intermediate annealing at 980°C for a time of 30 seconds in a dry atmosphere of mixed N2 / H2 (N225% / H2?5%) , adopting the rolling reduction scheme reported in Table 6.

[0316] TABLE 6

[0317] Intermediate Final

[0318] thickness thickness

[0319] mm mm

[0320] 0.70 0.34

[0321] 0.60 0.29

[0322] 0.50 0.26

[0323] 0.50 0.26

[0324] 0.50 0.25

[0325] 0.40 0.20

[0326]

[0327] The final cold-rolled strips were then annealed in laboratory in an atmosphere of 100% dry hydrogen (d.p . < -40°C) , simulating a continuous annealing at 1025°C for a time of 30 seconds and then rapidly cooled in a controlled atmosphere of dry H2 .

[0328] All the produced materials were then sampled and conditioned to a size of 320 mm x 50 mm, with the long side parallel to the rolling direction with which theywere produced. The samples were then packed and compressed between two nickel plates and all treated at a temperature of 780 °C for 30 minutes and slowly cooled down to room temperature, in order to restore a flat geometry and relieve residual stresses after the last metallurgical annealing undergone .

[0329] Finally, the samples ( 8 for each studied condition) were conditioned by mechanical cutting on all four sides of each sample to the size of 280 x 30 mm and magnetically characterized by measure on an Epstein frame .

[0330] The test conditions and the results of the characterizations carried out are reported in Table 7.

[0331] TABLE 7

[0332] Test Produced Carbon Roughness Ra Average GS B5000 P10@400Hz nominal thickness (d)

[0333] N Lim PPm Lim Lim Tesla W / Kg

[0334] 1 0.34 30 0.20 85 1.67 15.1

[0335] 2 0.29 18 0.18 88 1.69 14.5

[0336] 3 0.26 20 0.17 95 1.69 12.2

[0337] 4 0.26 21 0.19 102 1.68 12.4

[0338] 5 0.25 15 0.16 90 1.70 11.1

[0339] 6 0.20 12 0.14 81 1.71 10.2

[0340]

[0341] The procedure according to Example 4 (samples 1 to 6 in Table 7 ) can be actuated according to the schematizations represented in figures 2b, in which the cold-rolling / annealing is carried out off-line in a double stage, or in figure 7, with the first cold-rolling stage carried out in-line .

[0342] Table 7 shows excellent functional magnetic features of the materials produced by the present invention, inparticular by adopting the described conditions for surface conditioning in the various steps of the claimed process, correlated with the roughness of the final sheets produced at decreasing thicknesses .

[0343] The process, the sheet, and the plant thus conceived are susceptible to numerous modifications and variants, all falling within the scope of the inventive concept; furthermore, all details are replaceable by technically equivalent elements . In practice, the materials used, as long as compatible with the specific use, as well as the contingent dimensions and shapes, can be any according to the technical requirements .

Claims

CLAIMS1. Process for manufacturing non-grain oriented steel metal sheet with a thickness between 0.1 and 0.5 mm, adapted to be intended for ferromagnetic cores of electrical machines; said process comprising the steps of :- providing a strip produced by hot rolling with a thickness between 0.5 and 2.5 mm of a solidified ironbased alloy having the following chemical weight composition :Silicon (Si) greater than 0% and lower than 6%, preferably between 2.0% and 4.5%,Carbon (C) greater than 0% and lower than 0.007%, Aluminium (Al) greater than 0% and lower than 2%, Manganese (Mn) greater than 0% and lower than 3%, preferably lower than 2%,Copper (Cu) greater than 0% and lower than 2%, preferably lower than 1%,Nitrogen (N) between 0.0005 and 0.010%,Sulphur (S) between 0.0005 and 0.020%,Titanium (Ti) between 0.0005 and 0.01%,Boron (B) greater than 0% and lower than 0.0020%; wherein the concentrations of Mn, Cu, Al, Ti, N, S, B and C are such as to simultaneously satisfy the following relations :a) (Mn+Cu) / (S) > 100b) (Ti+ B+ Al* 10A-3 ) / N=0 . 5-4 . 5c) (S+N) / (Ti+B) =0 . 7-10;optionally the alloy also contains the following specific elements within the concentration limits described below:Chromium (Cr) lower than 6%Nickel (Ni) lower than 2%Cobalt (Co) lower than 1%Molybdenum (Mo) lower than 1%Selenium (Se) lower than 0, 02%Bismuth (Bi) lower than 0, 02%Antimony (Sb) lower than 0, 1%Tin (Sn) lower than 0, 1%Phosphorus (P) lower than 0, 1%Arsenic (As) lower than 0, 05%Vanadium (V) lower than 0, 01%Niobium (Nb) lower than 0, 01%Tungsten (W) lower than 0, 01%Zirconium (Zr) lower than 0, 01%Magnesium (Mg) lower than 0, 005%Calcium (Ca) lower than 0, 005% ;and the remainder iron and unavoidable impurities;- subj ecting the strip to an initial annealing treatment in controlled atmosphere non-oxidizing;- submitting the strip to a surface scale and oxide removal treatment;cold-rolling the strip submitted to the initial annealing and scale and oxide removal treatment until thickness between 0.1 and 0.5 mm;- submitting the cold-rolled strip to a final continuous annealing treatment at a temperature between 800°C and 1150°C for a time not lower than 5 seconds in a controlled atmosphere with a dew point below 10°C; wherein said step of subj ecting the strip to a surface scale and oxide removal treatment comprises a sub-step of pickling;said process comprising at least one step among:- descaling carried out by at least one of a "ScaleBreaker" (22 ) machine adapted to impose an elongation of the strip between 0.05% and 1% and one or more laser sources (23) and one or more abrasive brushes (24 ) , said descaling step being a sub-step of submitting the strip to a surface scale and oxide removal treatment and said descaling step being carried out previously to said step of pickling; and- pre-rolling the strip at a temperature between 70° and 300°C with a thickness reduction between 10% and 50%, said pre-rolling step being carried out prior to the step of cold rolling .

2. Process according to the preceding claim, comprising the step of pre-rolling and the step of descaling.

3. Process according to claim 1 or 2, wherein said process comprises said step of pre-rolling; wherein the step of pre-rolling is carried out prior to the step of submitting the strip to an initial annealing treatment and prior to the step of submitting the strip to a surface scale and oxide removal treatment .

4. Process according to claim 1 or 2, wherein said process comprises said step of pre-rolling; wherein the step of pre-rolling is carried out subsequent to the step of submitting the strip to a surface scale and oxide removal treatment and prior to the step of submitting the strip to an initial annealing treatment .

5. Process according to claim 1 or 2, wherein said process comprises said step of pre-rolling; wherein the step of pre-rolling is carried out subsequent to the step of submitting the strip to an initial annealing treatment and subsequent to the step of submitting the strip to a surface scale and oxide removal treatment .

6. Process according to any one of preceding claims,wherein said step of submitting the strip to an initial annealing treatment is carried out at a temperature between 700°C and 870°C for a time between 1 and 60 hours in an atmosphere containing nitrogen, argon, helium, hydrogen or mixtures thereof .

7. Process according to any one of claims 1-5, wherein said step of submitting the strip to an initial annealing treatment is carried out continuously at a temperature between 750°C and 1100°C for a time not lower than 10 seconds, in an atmosphere containing nitrogen, argon, helium, hydrogen or mixtures thereof .

8. Process according to any one of the preceding claims, wherein said strip produced by hot rolling has thickness between 0.7 and 2.4 mm.

9. Process according to any one of the preceding claims, comprising a further step of controlling the temperature of the strip at a temperature above 65°C, preferably between 65°C and 150 °C, carried out prior to said step of submitting the strip to a surface scale and oxide removal treatment .

10. Process according to any one of the preceding claims, wherein said process comprises said step of descaling carried out by means of one or more laser source (23) , wherein said one or more laser source (23) are selected of the pulsed type .

11. Process according to any one of the preceding claims, wherein said step of submitting the strip to a surface scale and oxide removal treatment comprises at least one sub-step of shot peening or sandblasting with a shot peening machine or blasting machine (25) operated at reduced proj ection velocity using abrasive shot or grit preferably having Shot size equal to S170 or S110 or S70according ASTM J444 or Grit size equal to G50 or G80 or G120 according ASTM J444.

12. Process according to any one of the preceding claims, wherein the sub-step of pickling is carried out by means of one or more tanks containing a hydrochloric or hydrofluoric acid solution at a temperature between 45° and 85°C, optionally said sub-step being carried out by means of one or more tanks (21 ) containing sulphuric acid at a temperature between 80° and 98 °C located upstream of said tanks (21 ) containing hydrochloric or hydrofluoric acid, a rinse being provided between said tanks (21 ) of sulphuric acid and hydrochloric or hydrofluoric acid being provided a rinsing.

13. Process according to any one of the preceding claims, wherein said strip is obtained by hot-rolling at a temperature of more than 700°C while applying a thickness reduction between 30% and 99% .

14. Process according to any one of the preceding claims, wherein the strip after the step of cold-rolling is characterised by a roughness Rad0.25 pm.

15. Process according to any one of the preceding claims, wherein said process comprises said step of pre-rolling; wherein the step of pre-rolling is made of maintaining a percentage ratio between rolling tension and rolling force between 0.5% and 10% .

16. Non-grain oriented steel metal sheet, intended for ferromagnetic cores of rotating electrical machines, which can be manufactured by a process according to any one of the preceding claims, having a thickness between 0.1-0.50 mm and a magnetic polarisability B5000>1.749- ( 0.002p) , where B5000, expressed in Tesla, is the induction measured at a magnetic field of 5000 A / m andp is the electrical resistivity, expressed in pQ -cm, according to the following equation: p=9+11.25 • (%Si) +6.25 • (%Mn) +11.52 • (%Al) +2.5• ( %Cu) +14 • ( % ) P+5 . 35 • ( %Sn) , characterized by a roughness Rad 0.25 pm.

17. Plant ( 100) for manufacturing non-grain oriented steel metal sheet with a thickness between 0.1 and 0.5 mm, intended for ferromagnetic cores of electrical machines; said plant ( 100) being adapted to manufacture said steel metal sheet from a strip having a thickness between 0.5 and 2.5 mm; said strip being produced by hot rolling of an iron-based alloy having the following chemical weight composition :Silicon (Si) greater than 0% and lower than 6%, preferably between 2.0% and 4.5%,Carbon (C) greater than 0% and lower than 0.007%, Aluminium (Al) greater than 0% and lower than 2%, Manganese (Mn) greater than 0% and lower than 3%, preferably lower than 2%,Copper (Cu) greater than 0% and lower than 2%, preferably lower than 1%,Nitrogen (N) between 0.0005 and 0.010%,Sulphur (S) between 0.0005 and 0.020%,Titanium (Ti) between 0.0005 and 0.01%,Boron (B) greater than 0% and lower than 0.0020%; wherein the concentrations of Mn, Cu, Al, Ti, N, S, B and C are such as to fulfil the following relationships :a) (Mn+Cu) / (S) > 100b) (Ti+ B+ Al* 10A-3 ) / N=0 . 5-4 . 5c) (S+N) / (Ti+B) =0 . 7-10;optionally, the alloy also contains the followingspecific elements within the concentration limits described below:Chromium (Cr) lower than 6%Nickel (Ni) lower than 2%Cobalt (Co) lower than 1%Molybdenum (Mo) lower than 1%Selenium (Se) lower than 0.02%Bismuth (Bi) lower than 0.02%Antimony (Sb) lower than 0.1%Tin (Sn) lower than 0.1%Phosphorus (P) lower than 0.1%Arsenic (As) lower than 0.05%Vanadium (V) lower than 0.01%Niobium (Nb) lower than 0.01%Tungsten (W) lower than 0.01%Zirconium (Zr) lower than 0.01%Magnesium (Mg) lower than 0.005%Calcium (Ca) lower than 0.005%;and the remainder iron and unavoidable impurities; said plant ( 100) comprising :- an initial annealing station ( 10) ;- a scale and oxide removal station (20) ;- a cold rolling station (30) located downstream of the initial annealing station ( 10) and of the scale and oxide removal station (20) adapted to reduce the thickness of the strip up to a thickness between 0.1 and 0.5 mm;- a final annealing station (40) configured to heat the cold rolled strip continuously to a temperature between 800°C and 1150 °C for a time not lower than 5 seconds in a controlled atmosphere with a dew point lower than 10°C, said final annealing station (40) being located downstream of said cold rolling station (30) ;wherein the scale and oxide removal station (20) comprises one or more pickling tanks (21 ) ;wherein said plant ( 100) comprises at least one among: - a descaling station (70) comprising at least one among: a "Scale Breaker" machine (22 ) adapted to impose an elongation of the strip between 0.05% and 1%, one or more laser source (23) and one or more abrasive brushes (24 ) ; said scale and oxide removal station (20) comprising said descaling station (70) ; said one or more pickling tanks (21 ) being downstream of said descaling station (70) ; and- a pre-rolling station (50) positioned upstream of the cold-rolling station (30) .

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