IRON-SILICON MATERIAL SUITABLE FOR MEDIUM FREQUENCY APPLICATIONS

Description

[0001] The present invention relates to a grain-oriented steel strip and to the use of such a strip in electric transformers, in electric motors or in other electric devices, preferably in devices in which magnetic flux has to be channeled or contained.

[0002] Unless explicitly stated otherwise, in the present text and the claims the content of particular alloy elements is always reported in % by weight (= "wt.-%") or on by weight (= "wt.-ppm").

[0003] The terms "sheet" or "strip" are used in the present text synonymously to indicate a flat steel product which is obtained by a rolling process an which length and width is much greater than its thickness. Thus, all explanations given here with regard to a grain-oriented steel sheet also apply for a grain-oriented steel strip and vice versa.

[0004] Grain-oriented electrical steel ("GOES") is a soft magnetic material which typically exhibits high silicon contents. GOES has a high permeability to the magnetic field and can be magnetized and demagnetized easily.

[0005] Their magnetic properties make sheets or strips made from GOES material especially suited for manufacturing electric transformer cores with a minimum specific loss and a high achievable working induction, for example up to 1.85 T, for a wide range of sheet thicknesses, e.g. 0.23 to 0.35 mm.

[0006] EP 3 395 961 A1, US 6 287 703 B1, US 2003/180553 A1, JP H11 310882 A, US 5 961 744 A, EP 0 577 124 A2 and US 2010/055481 A1 disclose heat treated cold rolled grain oriented steel sheets. According to Wuppermann et al., Electrical Steel, Stahl-Informations-Zentrum, Düsseldorf, Ed. 2005, pages 5 and 6, the "iron crystal axis" is defined as an axis of easy magnetization of the body-centered cubic iron crystal. In GEOS sheets or strips this axis is closely aligned to the rolling direction. This distinct orientation results in excellent magnetic properties of the GOES sheet in the rolling direction as well. Those grains of GEOS sheets or strips which axis is aligned in this way are called "Goss grains". Goss grains provide a strongly anisotropic behavior and reduce the power loss. However, the distinct Goss texture of GEOS hinders the formation of magnetic moments which are oriented out of the plane of the sheet in a direction diverging from the rolling direction. Here, the forming of magnetic moments aligned perpendicular to the direction of rolling turns out to be especially difficult.

[0007] According to N. Chen et al., Acta Materialia 51 (2003), pages 1755 to 1765 and K. Günther et al., Journal of Magnetism and Magnetic Materials 320 (2008), 2411 to 2422, GOES can be manufactured in different ways. An exemplary production route includes in the following manufacturing steps: Producing a steel by using a blast furnace and basic oxygen converter or by using an electric arc furnace - metallurgy refining of the steel melt by using a vacuum degassing vessel - casting the steel melt into an intermediate product, i.e. a common slab, a thin slab or a cast strip - optionally reheating the intermediate product - hot rolling the intermediate product to a hot rolled steel strip - coiling the hot rolled into a coil - coil surface preparation - hot strip annealing and pickling of the hot rolled strip - cold rolling the hot rolled strip in one or more passes to obtain a cold rolled strip with a final thickness - decarburization annealing of the cold rolled strip - optionally surface nitriding of the cold rolled strip - applying a MgO coating to the surface of the cold rolled strip - high temperature box annealing of the MgO coated cold rolled strip to decarburize the cold rolled strip, the cold rolled strip being coiled to coils which for the box annealing are stacked in a hood type furnace - heat flattening and insulation coating of the annealed strip - optionally magnetic domain refining of the strip.

[0008] According to the so called "High Heating" technology, the casting and the high temperature slab reheating is performed at temperatures of up to 1400 °C. Such high temperature casting and reheating results in a well-developed inhibition system which comprises particles of AIN, MnS and other compounds in the iron matrix even before the cold process. The presence of said particles promotes an abnormal grain growth in the steel structure, which has a positive effect on the magnetic properties of the GEOS sheet.

[0009] In the so called "Low Heating" technology the intermediate product is reheated at low temperatures so that no or only a weak inhibition system is formed in the slab before hot rolling. For this reason, in the low heating technology a nitriding treatment of the cold rolled strip surface has to be performed after the decarburization annealing to form an inhibition system which enables a secondary grain growth in the course of the high temperature box annealing of the cold rolled strip.

[0010] The primary recrystallization (PRX) occurring during the decarburization annealing prepares and controls the secondary grain growth. However, this process step is unstable due to the large number of metallurgical phenomena that compete with each other during the decarburization annealing. These phenomena are in particular carbon removal, formation of the oxide layer, primary grain growth. Nevertheless, it is known that decarburization annealing is essential to obtain efficient nitriding, a high-quality insulating glass film, and a sufficient number of Goss nuclei in the matrix. Furthermore, it is known that a dense oxide layer, which occurs during the beginning of decarburization annealing, can promote surface quality but can also act as a barrier to decarburization and nitriding.

[0011] In the "Low Heating" process after the decarburization and nitriding step the steel strip runs through a high temperature annealing cycle either in a batch annealing furnace or a rotary batch annealing furnace. In the course of the high temperature annealing step secondary recrystallization (SRX) occurs and an abnormal grain growth takes place which leads to the Goss texture controlled by the inhibitors previously formed. Furthermore disturbing elements such as sulphur or nitrogen are removed and a glass film, usually containing Mg2SiO4, is formed on the surface of the strip. This glass film acts as an electric insulation coating layer and applies an additional tension on the surface of the strip which contributes to the magnetic properties of the strip.

[0012] Since an iron-silicon material is electrically-conductive, induced currents develop over the sheet thickness under the influence of a magnetic flux variation over the time when such material is used in electrical applications. In the skilled language these currents are called "Eddy currents".

[0013] A significant reduction of the losses induced by eddy currents can be obtained by reducing the thickness and by increasing the electrical resistivity of the material. An increase of the electrical resistivity can be achieved by increasing the content of at least one of the alloying elements of or by adding additional alloying elements to the Fe-Si-material. For example, a 10%-decrease of the thickness of a GEOS strip results in a reduction of approximately 20 % of the Eddy current losses at identical 50 Hz induction levels. Likewise, an 0.5%- increase of the Silicon content results in a reduction of 12% of the Eddy current losses at the identical 50 Hz induction levels.

[0014] Eddy current losses account for about 10 to 25 % of the total specific losses at 50 Hz. However, at medium frequencies, which usually are in the range of 400 Hz to typically 2 kHz, much higher losses occur caused by Eddy current. In practice, these eddy current losses amount to at least 30 % of the total specific losses. For example, at a magnetic flux density of 1.5 T and a frequency of 1 kHz, the share eddy current losses have on the total specific losses is typically 50 %. Here too, a dependency exists between the material thickness, the frequency and the induction values.

[0015] In addition, with medium-frequency magnetization, the movements of the magnetic domains along the hysteresis loop are impeded by the resistance to changes in magnetization-demagnetization. This is due to pinning points, such as non-metallic inclusions or interface roughness between glass film and iron-silicon-steel matrix. Such an interface reduces the part of the magnetic core material that is magnetically active under the influence of a magnetizing field. As a consequence, the magnetic polarization in the magnetically active cross-section of the material increases depending on the desired level of magnetization and thus the specific total losses. If the magnetization is performed at medium-frequency and the penetration depth is reduced by strong eddy currents, the non-magnetically active part additionally reduces the ability to magnetize the material. Therefore, a thin iron-silicon alloy sheet with a minimized interface roughness, preferably an optimally smooth interface, would be a big step towards reducing the total specific losses in applications of the type under consideration here. However, the problem with conventional GOES production is that a lower thickness and higher silicon content make the material more brittle, which not only makes cold rolling more difficult, but also makes it more difficult to achieve stable secondary recrystallization ("SRX"). This is especially true for material with a final thickness of less than 0.22 mm.

[0016] Against the background of the prior art explained above the object has arisen to develop a grain-oriented electrical steel sheet which is particularly suitable for magnetization at frequencies of at least 400 Hz due to its reduced specific losses. The invention is defined in the appended claims.

[0017] The invention solved this problem by means of a grain oriented electrical steel sheet with at least the features specified in claim 1.

[0018] The general idea and advantageous embodiments of the invention are indicated in the dependent claims and explained in detail below.

[0019] A grain-oriented electrical steel according to the invention thus comprises

• a core layer consisting of Fe, Si and optionally further alloying elements, the core layer having two outer surfaces,
• interface layers one of which being present on each outer surface of the core layer, the interface layers being formed by reaction products of at least one of the alloying elements of the core layer, and
• at least one outer layer present on each of the interface layers, the outer layers constituting an electrical insulation media,

wherein according to the invention the thickness of the core layer is at least 25 times greater than the sum of the thicknesses of the outer layers
and

wherein the thicknesses t_il of the interface layers respectively fulfil the following condition (1):

with

t_il

being the thickness of the interface layer present in the respective surface of the core layer, indicated in nm,

t_ol

being the thickness of the respective outer layer present on the respective interface layer, indicated in µm,

t_core

being the thickness of the core layer, indicated in µm,

p

being the electrical resistivity of the core layer, indicated in Ω m,

µ₀

being the magnetic constant 4 × π × 10^-7,

µ_dif

being the differential permeability of the core layer, and

f

being the frequency of the respective current in Hz.

[0020] Usually a current with a frequency f of 1000 Hz is used for measurement purposes here.

[0021] If the condition (1) is fulfilled, the grain-oriented electrical steel sheet according to the present invention shows particularly improved magnetic loss behavior at medium frequencies, i.e. frequencies of 400 Hz to, for example, 2 kHz.

[0022] The interface layer present on each outer surface of the core are formed by reaction products which are the result of a chemical reaction of the alloying elements contained in the steel material of the core layer, which are at least Fe and Si. In the course of the annealing steps a grain-oriented steel strip according to the invention runs through during its production these alloying elements migrate to the outer surface of the core layer and react with the atmosphere used during the respective heat treatment. Accordingly, depending on the kind of atmosphere under which the heat treatment was performed the reaction products forming the interface layer can be oxides, nitrides and/or carbo-nitrides. Most commonly mixed oxides of iron and silica ("Fayalite") form the interface layer.

[0023] The outer layers being applied on each of the interface layers constitute an electrical insulation layer and can be of mineral or organic nature. For example, they may contain silica and aluminum-phosphate chemicals assembled together. As in common applications the insulating outer layer is provided for separating the layers of an electromagnetic converter core or the like.

[0024] Thus, the invention provides a grain-oriented electrical steel sheet comprising a core layer containing at least Fe and Si having two outer surfaces, at least one interface layer present on each outer surface of the core and at least one outer layer present on each interface layer, wherein the thickness of the core layer is at least 25 times higher than the sum of the thicknesses of the outer layers.

[0025] The grain-oriented electrical steel sheet according to the present invention consists at least of iron ("Fe") and silicon ("Si"), wherein the Fe, as in common GEOS materials, accounts for by far the largest share.
1 to 8 % by weight Si can be present in the steel of the core layer of the grain-oriented electrical steel sheet according the invention, Si contain of 1 to 5 % by weight Si being especially effective for practical applications. For example, Si-contents of 2 to 4 % by weight, particularly 2.5 to 3.5% by weight, of the core layer of prove to be especially advantageous with regard to the magnetic properties of a grain-oriented steel sheet according to the invention.

[0026] In addition to Fe and Si the core layer of a grain-oriented steel sheet according to the invention optionally may contain as further alloying elements at least one element of the group "C, Mn, Cu, Cr, Sn, Al, N, Ti, and B", wherein the sum of the contents of these elements in the alloy of the core layers is preferably restricted to 3 % by weight. For example, according to the present invention, the amount of Mn, if present in the grain-oriented electrical steel sheet, may amount to 0.001 to 3.0% Mn, particularly preferably 0.01 to 0.3% by weight Mn. Also, the amount of Cu, if present in the grain-oriented electrical steel sheet can be 0.001 to 3.0 % by weight, particularly 0.01 to 0.3% Cu. Al can be optionally present as well in the grain-oriented electrical steel sheet according to the invention in contents of 0.001 to 2.0 % by weight, particularly 0.01 to 1.0 % by weight. According to the invention, the contents of Cr, Sn, Ti, and 8 which may also be optionally present in the core layer of the grain-oriented electrical steel sheet according to the invention are delimited such that the sum of the contents of these elements is less than 3 % by weight, preferably less than 1 % by weight.

[0027] Thus, a steel alloy which is especially suited for the core layer of a grain oriented steel sheet according to the invention preferably consists of, in % by weight, 2 to 5% Si, 0.01 to 0.3% Mn, 0.01 to 0.3% Cu, 0.01 to 1.0% Al, the reminder being Fe and unavoidable impurities, which content in sum is preferably restricted to less than 0.5 % by weight.

[0028] The sum of the sulfur (S) and selenium contents of the core layer of a grain-oriented steel sheet according to the invention is preferably restricted to less than 0.010 % by weight.

[0029] According to a further preferred embodiment of the invention the Sulfur ("S") content of the core layer of the grain-oriented steel sheet according fulfills at least one of the following provisions:
The S-content is restricted to less than 7 ppm by weight and/or the S-content in the core layer is less than 0.0007 % by weight related to the total amount of Fe- and Si-contents of the core layer.

[0030] Preferably the content of magnesium in the interface layers is lower than 1 % by weight.

[0031] According to a further advantageous embodiment of the present invention, the grain-oriented electrical steel sheet according to the present invention comprises a soft magnetic material.

[0032] As already mentioned above, a grain-oriented electrical steel sheet according to the invention comprises at least a core layer, at least one interface layer present on each outer surface of the core layer and at least one outer layer present being respectively applied on each of the interface layers.

[0033] According to an embodiment of the present invention, no further coating or layer is present on one or both outer layers of the grain-oriented steel.

[0034] However, further coatings may be present on the outer layers to enhance the properties of the grain-oriented electrical steel sheet, if appropriate. Examples for such coatings are disclosed in DE 10 2008 008 781 A, US 3,948,786 A and JP S53-28375 B2. However, the stipulations given by the invention for adjusting the thicknesses of the core layer, the interface layers and the outer layer apply only to the core layer, the interface layers lying directly on the surfaces of the core layer and the outer layers lying directly on the surfaces of the interface layers.

[0035] As already mentioned as well, the thickness of the core layer is at least 25 times higher than the sum of the thicknesses of the outer layers. Accordingly, the thickness t_core of the core layer and the sum Σ t_ol of the thicknesses of the outer layers have to fulfil the following provision (2):

with

t_core being the thickness of the core layer, indicated in µm, and

t_ol being the thickness of one outer layer, indicated in µm.

[0036] Typically, the core layer of the grain-oriented steel according to the present invention has a thickness of 50 to 220 µm, wherein a thickness of the core layer of at least 100 µm turned out to be especially useful for practical applications.

[0037] The interface layer according to the present embodiment mainly differentiates from the core layer by its magnetic characteristics like magnetic permeability.

[0038] The thickness of the interface layers of a steel sheet according to the invention typically amounts to 1 to 500 nm, wherein in practice thicknesses of at least 10 nm are observed. A restriction of the interface layer to a maximum of 100 nm turned out to be especially advantageous with regard to the magnetic properties of the steel sheet according to the invention.

[0039] The grain-oriented steel according to the present invention comprises at least one interface layer present above each outer surface of the core. According to a preferred embodiment of the present invention, the grain-oriented steel according to the present invention comprises a first interface layer present beneath the top outer surface and a second interface layer beneath the bottom outer surface.

[0040] The grain-oriented electrical steel sheet according to the present invention further comprises at least one, preferably exactly one, outer layer present on each interface layer. The sum of the thicknesses of the outer layers is preferably at least 0.1 µm and less than 5 µm, more preferably 0.1 to 2 µm.

[0041] The grain-oriented electrical steel sheet according to the present invention can be manufactured by performing a method which comprises at least the following working steps:

(A) Providing a hot rolled steel strip which is made from a steel which consists of, in % by weight,

and optionally in sum less than 3 % of at least one element of the group "C, Mn, Cu, Cr, Sn, Al, N, Ti, B"

the remainder being Fe and unavoidable impurities;

(B) cold rolling the hot strip of step (A) in at least one pass to obtain a cold strip;

(C) primary recrystallization annealing of the cold strip obtained in step (B) optionally including a nitriding treatment;

(D) performing a secondary recrystallization annealing treatment by heating the strip obtained in step (C) to a temperature OTAG2 with a heating rate of at least 40 K/s to obtain a grain-oriented electrical steel sheet according to the invention, wherein the temperature OTAG2 is set according to the following condition (I):

with

OTAG2:

being the optimum Temperature of Abnormal Grain Growth, indicated in K,

HRSRX

being the respective heating rate for the Secondary Recrystallization Treatment, indicated in K/s,

PGS:

being the Average Grain Size of the grains obtained by the PRX, indicated in µm,

ΔN:

being the Nitriding Degree, indicated in ppm, and calculated as ΔN = [Nitrogen content before the SRX] - [Nitrogen content before PRX] the nitrogen contents indicated in ppm by weight respectively,

DP:

Atmosphere Dew Point during heating rate, indicated in K,

%S:

sum of the S- and Se-content of the core layer, indicated in ppm,

ρHAGB:

High Angle (> 15°) primary Grain Boundary average density, indicated in µm^-1.

[0042] Step (A) of the process comprises providing a hot rolled steel strip which is made from a steel alloyed in accordance with the explanations and provisions given above.

[0043] Methods for the manufacturing of the hot rolled steel strip provided according to working step (A) are known per se to the skilled expert. Step (A) of the process according to the invention comprises a common steelmaking step to produce a steel melt which afterwards is cast into an intermediate product such as slabs, thin slabs or cast strip. The intermediate product obtained in this way is hot rolled to a hot rolled strip which is coiled to a coil and optionally undergoes a hot strip annealing and pickling if appropriate before further manufacturing. The hot rolled strip provided in step (A) of the process according to the invention preferably has a thickness of 0.5 to 3.5 mm, more preferably 1.0 to 3.0 mm. Examples for known methods suited for the production of a hot rolled strip to be provided in working step (A) can be found in DE 197 45 445 C1 and EP 1 752 549 B1.

[0044] After step (A) hot band strips having the above mentioned composition and thickness are obtained. These hot band strips are preferably directly introduced into step (B) of the process.

[0045] In step (B) of the process according to the invention the hot rolled strip is cold rolled in at least one pass to obtain a cold rolled strip. Method for cold rolling a grain oriented steel strip are generally known to the skilled expert as well and, for example, described in WO 2007/014868 A1 and WO 99/19521 A1.

[0046] Typically, the thickness of the cold rolled strip is 0.05 to 2.00 mm, preferably at least 0.10 mm, after the first cold rolling step, wherein after the second cold rolling a maximum thickness of 0.55 mm, preferably of ≤ 0.35 mm at most or of 0.22 mm at most, are especially favorable. Apparatuses in which such cold rolling can be performed are generally known to the skilled expert and, for example, disclosed in WO 2007/014868 A1 and WO 99/19521 A1.

[0047] The cold rolling in step (B) of the process according to the invention is preferably performed in at least two cold rolling steps to obtain a steel strip of minimized thickness. It turned that exactly to cold rolling steps are especially appropriate for the purposes of the invention.

[0048] Two step cold rolling allows the strip to be subjected to a decarburization annealing between the cold rolling steps. Such decarburization can also be performed according to methods known to the skilled expert. Typically, an intermediate annealing is performed in a temperature range of 700 to 950 °C, preferably 800 to 900 °C, under an atmosphere which dew point is set to 10 to 80 °C. Installations with which such annealing can be performed are generally known and disclosed, for example, in WO 2007/014868 A1 and WO 99/19521 A1. The decarburization annealing is preferably performed such that the carbon content of the steel strip is lowered to less than 30 ppm by weight. Accordingly, in a two-step cold rolling with intermediate decarburization annealing the carbon content of the cold rolled strip preferably is less than 30 ppm by weight before the second cold rolling step in working step (B) of the process according to the invention.

[0049] In working step (C) of the process according to the invention an annealing of the cold strip obtained in step (B) is performed to primary recrystallize and optionally nitride treating the cold rolled strip. The nitriding annealing preferably carried out at temperatures in the range of 400 to 950 °C, e.g. 600 to 900 °C. If a nitriding treatment is to be performed the annealing can be carried out under an atmosphere which comprises N₂ or N-comprising compounds, for example NH₃. Annealing and nitriding can be conducted in two separate steps one after the other with the annealing being performed at first. As an alternative simultaneously annealing and nitriding can be performed.

[0050] If nitriding is performed in working step (C) the conditions of the nitriding treatment should be adjusted such that a nitriding degree of up to 300 ppm, preferably 20 to 250 ppm, is achieved. The nitriding degree is calculated as the difference between the nitrogen content of the steel strip before the second recrystalisation annealing (working step (D)) minus the nitrogen content before the primary recrystallization annealing (working step (C)). The nitrogen content can be determined by usual means, such as the 736 analyzer offered by Leco Corporation, St. Joseph, USA.

[0051] It turns out that the average grain size of the structure of the core layer of the strip obtained after step (C) of the process according the invention typically is 5 to 25µm, especially 5 to 20 µm. In addition, the average High Angle primary Grain Boundary density of the strip obtained after step (C) lies in the range of 0.005 to 0.1 µm^-1, especially of 0.01 to 0.09 µm^-1. The Average Primary Grain Size can be determined with methods known to the skilled expert, for example Grain size measured by Electron Backscatter Diffraction ("EBSD") for which the common software OIM Analyses can be used (s. https://en.wikipedia.org/wiki/Electron_backscatter_diffraction; https://www.edax.com/products/ebsd/oim-analysis).

[0052] A pickling step may be performed after the annealing and the optional nitriding in a manner well known to the skilled expert as well. For example, pickling can be performed by using aqueous solutions of acids like phosphoric acid, sulfuric acid and/or hydrochloric acid. The pickling step should preferably be performed after step (C) and before step (D) of the method according to the invention.

[0053] In step (D) of the process according to the invention the cold rolled strip undergoes a secondary recrystallization annealing treatment by heating to a temperature OTAG2 with a heating rate of at least 40 K/s, preferably at least 50 K/s, to obtain the grain-oriented electrical steel sheet. Heating rate of at least 70 K/s, more preferably at least 100 K/s, is especially favorable. The rapid heating can be carried out by any method known to the skilled expert, for example by induction heating, by resistive heating or by conductive heating.

[0054] The respective temperature OTAG2 is calculated in accordance with the provisions already mentioned above and is set to 1420 K at most. Preferably the upper limit of OTAG2 is 1415 K.

[0055] By heating the cold rolled steel strip to the respective temperature OTAG2 with a heating rate of at least 40 K/s a grain-oriented steel sheet is obtained which has a high peak of magnetic polarization for a peak magnetic field strength of 800 Aim and a low specific total loss. Preferably, the Heating Rate to Secondary Recrystallization Treatment is 20 to 800 K/s, more preferably 50 to 750 K/s. The Heating Rate to Secondary Recrystallization Treatment is acquired with methods known to the skilled expert, for example as described in EP 2 486 157.

[0056] The dew point of the atmosphere during heating is preferably set to 223 to 273 K, more preferably 243 to 270 K. The atmosphere dew point can be determined with methods well known to the skilled expert. Instructions for such determination can be found in WO 2007/014868 and WO 99/19521.

[0057] The high angle (> 15°) primary grain boundary average density of the GEOS sheet according to the invention is preferably 0.005 to 0.1 µm^-1, in particular 0.01 to 0.09 µm^-1. The High Angle (> 15°) primary Grain Boundary can be measured as primary grain boundary length per unit area by EBSD analysis (OIM Analysis software). The pHAGB is the average of the values corresponding to a misorientation higher than 15° (>15°).

[0058] In step (D) of the process according to the invention the Secondary Recrystallization takes place which ensures that the grain-oriented steel sheet processed in this way is prepared to reliably develop the optimized properties of a grain-oriented steel sheet according to the invention as outlined above.

[0059] According to a preferred embodiment of the process according to the invention, on the surface of the cold rolled strip that is introduced into step (D) no outer coating is applied. That means that preferably no annealing separator, especially no MgO based coating, is present on the sheet material which is processed according to working step (D). Rather, the outer coating, i.e. the insulation coating preferably containing MgO, should be applied only after working step (D) to contribute to an optimized result of the SRX.

[0060] To finish the production of the grain-oriented steel sheet according to the invention, also the sheet material obtained after working step (D) of the process according to the invention should run through those process steps which in the common production of grain-oriented steel sheets usually are performed after the SRX.

[0061] That is, that the strip or sheet that is obtained after step (D) undergoes a high temperature annealing for which it can be rapidly heated to a soaking temperature of 1423 K or above, wherein soaking temperatures of at least 1523 K are particularly advantageous. The heating and soaking is preferably carried out under a protective gas atmosphere, which, for example, comprises H₂. Particularly preferably, the heating to and soaking at the respective soaking temperature is performed under an atmosphere which comprises 5 to 95 Vol.-% H₂, the reminder being nitrogen or any inert gas or a mix gas, the dew point of the atmosphere being at least 10 °C. The soaking time, during which the high temperature soaking is carried out in this way, can be determined in a common manner which is well known to the expert. By the soaking performed in this way atoms of elements are removed, which would deteriorate the properties of the grain-oriented steel sheet. These elements are in particular N and S.

[0062] After the high temperature annealing the steel strip is cooled down in a common manner, e.g. by natural cooling, down to room temperature.

[0063] In addition, according to a preferred embodiment of the process, the steel strip is cleaned, and optionally pickled. Methods with which the steel strip is pickled are known to the skilled expert. For pickling the steel strip can be treated with an aqueous acidic solution. Suitable acids are for example phosphoric acid, sulfuric acid and/or hydrochloric acid.

[0064] According to the present invention, at the end of process explained above, the grain-oriented electrical steel is presenting a core layer and two interface layers being present on the outer surfaces of the core layer, the chemical composition of the interface layer resulting from the migration of various species from the core layer towards the outer surface and arising from the reaction of the outer surface of the core layer with the various atmospheres and conditions encountered during the successive treatment phases as for example the ones explained in the previous sections of this text. For example the reaction products which are the result of these reactions is a mixed oxide of iron and silica, also called "Fayalite").

[0065] The reaction products being present on the outer surface of the core layer form interface layers between the core layers and the outer layers. The outer layers constitute an electrical insulation media which can be of mineral or organic nature (e.g. it may contain silica and aluminum-phosphate chemicals assembled together) adapted to the necessary separation between layers of an electromagnetic converter core.

[0066] According to the present invention, the grain-oriented electrical steel sheets can be prepared in any format, like steel strips that are provided as coils, or cut steel pieces that are provided by cutting these steel pieces from the steel strips. Methods to provide coils or cut steel pieces are known to the skilled expert.

[0067] The grain-oriented electrical steel sheet according to the present invention shows improved magnetic loss at medium frequencies compared to grain-oriented electrical steel sheets according to the prior art. Accordingly, the product according to the invention is in particular useful for the manufacture of parts for electric transformers, for electric motors or for other electric devices. This is particularly true for electrical applications in which the magnetic flux has to be channeled or contained.

[0068] The grain-oriented electrical steel sheet according to the present invention shows improved magnetic loss at medium frequencies, in particular frequencies of at least 400 Hz and, for example, 3000 Hz or 2000 Hz at most.

[0069] Experiments have been carried out to demonstrate the effect of the invention.

[0070] In these experiments, 19 samples of grain-oriented steel sheets were produced in the manner described above, the core layer of which consisted of a steel with a Si content %Si, the remainder being iron and unavoidable impurities, the impurities including sulfur contents %S.

[0071] For each of the samples 1 to 19 the following parameters and properties are indicated in Table 1:

• the Si content %Si of the core layer;
• the S content S% of the core layer;
• the thickness t_core of the core layer;
• the thickness t_il of the interface layers being present on the surfaces of the core layer, the thicknesses t_il of the interface layers being identical;
• the thickness t_ol of the outer layers being present on the surfaces of the core layer, the thicknesses t_ol of the outer layers being identical;
• the sum t_ol + t_il of the thicknesses t_ol of the interface layer and the thickness t_il of the outer layer being present on each of the surfaces of the core layer, rounded up to one decimal place;
• the quotient t_core/(Σt_ol) formed by the thickness t_core of the core layer and the sum Σt_ol of the thicknesses t_ol of both outer layers of the respective sample;
• the quotient t_ol/t_core formed by the thickness t_ol of one of the outer layers and the thickness t_core of the core layer;
• the values A of the term (t_ol / t_core) × (ρ / µ₀ × µ_dif × π × f)^1/2 × 10^-9, wherein p indicates the electrical resistivity of the respective core layer, µ₀ indicates the magnetic constant 4 × π × 10^-7, µ_dif indicates the differential permeability of the core layer, π indicates the number pi (pi = 3.14159265359) and f indicates the frequency of the respective current in Hz;
• the peak magnetic polarization J800 for a magnetic field strength of 800 Aim at 1 kHz the.

[0072] Furthermore it is indicated in Table 1 if the provision (1) til < A according to the invention is fulfilled (with A = (t_ol / t_core) × (ρ / µ₀ × µ_dif × π × f)^1/2 × 10^-9).

[0073] The provision (2) according to which the thickness t_core of the core layer has to be 25 times greater than the sum of the thickness t_il of the intermediate layers was met by all samples 1 to 19.

[0074] The experiments clearly show that those samples, which fulfill provision (1) of the invention exhibit J800-values which are significantly better than the J800-values of those samples which do not meet the requirements of the invention. For example, the worst J800-value of the samples that comply with the invention (s. sample 3: J800 = 1,87 T) 0,22 T higher than the best J800-value of the samples that do not comply with the invention (s. samples 10 and 17: J800 = 1,65 T).

Table 1

No.	Core layer		tcore	til	tol	til + tol	tcore/(Σtol)	tol/tcore	A = (tol / tcore) × (ρ / µ0 × µdif × π × f)1/2 × 10-9	J800	til < A	Invention ?
	%Si	%S
	[wt.-%]	[wt.-ppm]	[µm]	[nm]	[µm]	[µm]			[m]	[T]
1	2,90	5	50	50	0,3	0,3	83,3	0,005	0,055*10-6	1,90	YES	YES
2	3,25	10	50	50	0,3	0,3	83,3	0,005	0,055*10-6	1,91	YES	YES
3	3,25	6	50	90	0,5	0,8	50,0	0,010	0,110*10-6	1,87	YES	YES
4	2,90	5	100	12	0,3	0,3	166,7	0,003	0,026*10-6	1,92	YES	YES
5	3,25	5	100	20	0,3	0,3	166,7	0,003	0,026*10.6	1,91	YES	YES
6	2,90	5	150	15	0,3	0,3	250,0	0,002	0,021*10-6	1,90	YES	YES
7	2,90	8	150	500	1,0	1,5	75,0	0,007	0,085*10-6	1,40	NO	NO
8	3,10	7	150	250	0,5	0,8	150,0	0,003	0,043*10-6	1,56	NO	NO
9	3,25	4	150	12	0,3	0,3	250,0	0,002	0,021*10-6	1,90	YES	YES
10	3,25	5	150	100	0,5	0,8	150,0	0,003	0,043*10-6	1,65	NO	NO
11	2,90	3	200	5	0,3	0,3	333,3	0,001	0,017*10-6	1,93	YES	YES
12	2,90	11	200	500	1,0	1,5	100,0	0,005	0,068*10-6	1,42	NO	NO
13	3,10	22	200	250	0,5	0,8	200,0	0,003	0,034*10-6	1,50	NO	NO
14	3,25	15	200	500	1,0	1,5	100,0	0,005	0,068*10-6	1,45	NO	NO
15	2,90	5	220	15	0,3	0,3	366,7	0,001	0,016*10-6	1,88	YES	YES
16	2,90	9	220	500	1,0	1,5	110,0	0,005	0,065*10-6	1,58	NO	NO
17	2,90	5	220	250	0,5	0,8	220,0	0,002	0,032*10-6	1,65	NO	NO
18	3,25	5	220	10	0,3	0,3	366,7	0,001	0,016*10-6	1,89	YES	YES
19	3,25	11	220	500	1,0	1,5	110,0	0,005	0,065*10-6	1,50	NO	NO

Claims

1. A grain-oriented electrical steel comprising

- a core layer which has a thickness of 50 µm to 220 µm and two outer surfaces, the core layer consisting of, in % by weight,

Si: 1 to 8 %,

S + Se: < 0.010%,

and optionally in sum less than 3 % of at least one element of the group "C, Mn, Cu, Cr, Sn, Al, N, Ti, B"

the remainder being Fe and unavoidable impurities,

- interface layers one of which being present on each outer surface of the core layer, the interface layers being formed by reaction products of at least one of the alloying elements of the core layer, wherein the reaction products of at least one of the alloying elements of the core layer are mixed oxides of iron and silica, and

- at least one outer layer present on each of the interface layers, the outer layers constituting an electrical insulation media and the sum of the thicknesses of the outer layers being at least 0.1 µm and less than 5 µm,

wherein the thickness of the core layer is at least 25 times greater than the sum of the thicknesses of the outer layers and

wherein the thicknesses t_il of the interface layers respectively fulfil the following provision (1):

with

t_il being the thickness of the interface layer present in the respective surface of the core layer, indicated in nm,

t_ol being the thickness of the respective outer layer present on the respective interface layer, indicated in µm,

t_core being the thickness of the core layer, indicated in µm,

p being the electrical resistivity of the core layer, indicated in Ω m,

µ₀ being the magnetic constant 4 × π × 10^-7,

µ_dif being the differential permeability of the core layer, and

f being the frequency of the respective current in the range of 400 Hz to 3000 Hz.

2. The grain-oriented electrical steel sheet according to claim 1, wherein the content of magnesium in the interface layers is less than 1 % by weight respectively.

3. The grain-oriented electrical steel sheet according to claim 1 or 2, wherein the sum of the thicknesses of the outer layers is less than 2 µm.

4. The grain-oriented electrical steel sheet according to any of the preceding claims, , wherein the core layer contains 1 - 5 % by weight Si.

5. The grain-oriented electrical steel sheet according to any of the preceding claims, wherein the S-content of the core layer amounts to less than 0.0007% to the sum of the Fe- and Si-contents of the core layer.

6. The grain-oriented electrical steel sheet according to any of the preceding claims, wherein at least one further coating is present on at least one outer layer.

7. Use of a grain-oriented steel according to any one of the preceding claims as material for the production of parts for electric transformers, for electric motors or for other electric devices, wherein the frequency of the current which is used for magnetization of the respective part is 400 Hz to 3000 Hz.

8. Use according to claim 7 that in the respective part made form the grain-oriented steel according to any of claims 1 to 6 the magnetic flux has to be channeled or contained.

Ansprüche

1. Ein kornorientiertes Elektroband, welches:

- eine Kernschicht, die eine Dicke von 50 µm bis 220 µm und zwei Grenzflächen besitzt, wobei die Kernschicht in Gew.-% aus:

· Si: 1 bis 8 %,

· S + Se: < 0,010 %,

wahlweise insgesamt weniger als 3 % mindestens eines Elements aus der Gruppe "C, Mn, Cu, Cr, Sn, Al, N, Ti, B" und

dem Rest aus Fe und unvermeidlichen Verunreinigungen besteht,

- Zwischenschichten, wobei eine davon auf jeder Grenzfläche der Kernschicht vorhanden ist, wobei die Zwischenschichten aus Reaktionsprodukten von mindestens einem der Legierungselemente der Kernschicht gebildet sind, wobei die Reaktionsprodukte von dem mindestens einem der Legierungselemente der Kernschicht Mischkristalle aus Eisen und Silicat sind, und

- mindestens eine Außenschicht, die auf jeder der Zwischenschichten vorhanden ist, wobei die Außenschichten ein elektrisches Isolationsmedium bilden und die Summe der Dicken der Außenschichten mindestens 0,1 µm und weniger als 5 µm beträgt, umfasst, wobei

die Dicke der Kernschicht mindestens 25-mal größer als die Summe der Dicken der Außenschichten ist und

die Dicke t_il der Zwischenschichten jeweils folgende Voraussetzung (1):

erfüllt, wobei:

- t_il die in nm angegebene Dicke der Zwischenschicht, die auf der jeweiligen Grenzfläche der Kernschicht vorhanden ist,

- t_ol die in µm angegebene Dicke der jeweiligen Außenschicht, die auf der jeweiligen Zwischenschicht vorhanden ist,

- t_core die in µm angegebene Dicke der Kernschicht,

- ρ den in Ωm angegebenen spezifischen elektrischen Widerstand der Kernschicht,

- µ₀ die magnetische Permeabilität des Vakuums 4 · π · 10^-7,

- µ_dif die Differenzpermeabilität der Kernschicht und

- f die Frequenz des jeweiligen elektrischen Stroms innerhalb eines Bereichs von 400 Hz bis 3000 Hz

bedeutet.

2. Das kornorientierte Elektroband nach Anspruch 1, wobei der Magnesiumgehalt der Zwischenschichten jeweils weniger als 1 Gew.-% beträgt.

3. Das kornorientierte Elektroband nach Anspruch 1 oder 2, wobei die Summe der Dicken der Außenschichten weniger als 2 µm beträgt.

4. Das kornorientierte Elektroband nach einem der vorhergehenden Ansprüche, wobei die Kernschicht 1 bis 5 Gew.-% Si enthält.

5. Das kornorientierte Elektroband nach einem der vorhergehenden Ansprüche, wobei der S-Gehalt der Kernschicht weniger als 0,0007 % der Summe aus Fe- und Si-Gehalt der Kernschicht beträgt.

6. Das kornorientierte Elektroband nach einem der vorhergehenden Ansprüche, wobei mindestens eine weitere Schicht auf mindestens einer Außenschicht vorhanden ist.

7. Verwendung eines kornorientierten Elektrobands nach einem der vorhergehenden Ansprüche als Material für die Herstellung von Teilen für elektrische Transformatoren, Elektromotoren oder andere elektrische Vorrichtungen, wobei die Frequenz des elektrischen Stroms, der zur Magnetisierung des jeweiligen Teils verwendet wird, 400 Hz bis 3000 Hz beträgt.

8. Verwendung nach Anspruch 7, wobei in dem jeweiligen Teil, das aus dem kornorientierten Elektroband nach einem der Ansprüche 1 bis 6 hergestellt worden ist, der Magnetfluss kanalisiert oder eingeschlossen werden muss.

Revendications

1. Acier électrique à grains orientés comprenant

- une couche d'âme qui a une épaisseur de 50 µm à 220 µm et deux surfaces externes, la couche d'âme étant constituée par, en % en poids,

Si: 1 à 8 %,

S + Se < 0,010 %,

et facultativement en totalité moins de 3 % d'au moins un élément du groupe « C, Mn, Cu, Cr, Sn, Al, N, Ti, B »

le reste étant Fe et des impuretés inévitables,

- des couches d'interface dont l'une est présente sur chaque surface externe de la couche d'âme, les couches d'interface étant formées par des produits de réaction d'au moins l'un des éléments d'alliage de la couche d'âme, les produits de réaction d'au moins l'un des éléments d'alliage de la couche d'âme étant des oxydes mixtes de fer et de silice, et

- au moins une couche externe présente sur chacune des couches d'interface, les couches externes constituant un milieu d'isolation électrique et la somme des épaisseurs des couches externes valant au moins 0,1 µm et moins de 5 µm,

l'épaisseur de la couche d'âme étant au moins 25 fois plus grande que la somme des épaisseurs des couches externes et

les épaisseurs t_il des couches d'interface respectant respectivement la disposition (1) suivante :

avec

t_il étant l'épaisseur de la couche d'interface présente dans la surface respective de la couche d'âme, indiquée en nm,

t_ol étant l'épaisseur de la couche externe respective présente sur la couche d'interface respective, indiquée en µm,

t_core étant l'épaisseur de la couche d'âme, indiquée en µm,

ρ étant la résistivité électrique de la couche d'âme, indiquée en Ω m,

µ₀ étant la constante magnétique 4 × π × 10^-7,

µ_dif étant la perméabilité différentielle de la couche d'âme, et

f étant la fréquence du courant respectif dans la plage de 400 Hz à 3000 Hz.

2. Tôle d'acier électrique à grains orientés selon la revendication 1, dans laquelle la teneur de magnésium dans les couches d'interface est inférieure à 1 % en poids respectivement.

3. Tôle d'acier électrique à grains orientés selon la revendication 1 ou 2, dans laquelle la somme des épaisseurs des couches externes est inférieure à 2 µm.

4. Tôle d'acier électrique à grains orientés selon l'une quelconque des revendications précédentes,, dans laquelle la couche d'âme contient 1 à 5 % en poids de Si.

5. Tôle d'acier électrique à grains orientés selon l'une quelconque des revendications précédentes, dans laquelle la teneur en S de la couche d'âme s'élève à moins de 0,0007 % de la somme des teneurs en Fe et en Si de la couche d'âme.

6. Tôle d'acier électrique à grains orientés selon l'une quelconque des revendications précédentes, dans laquelle au moins un revêtement supplémentaire est présent sur au moins une couche externe.

7. Utilisation d'un acier à grains orientés selon l'une quelconque des revendications précédentes en tant que matériau pour la production de pièces pour transformateurs électriques, pour moteurs électriques ou pour d'autres dispositifs électriques, la fréquence du courant qui est utilisé pour la magnétisation de la pièce respective allant de 400 Hz à 3000 Hz.

8. Utilisation selon la revendication 7 où dans la pièce respective fabriquée à partir de l'acier à grains orientés selon l'une quelconque des revendications 1 à 6 le flux magnétique doit être canalisé ou contenu.