EP4365319A1

EP4365319A1 - Grain-oriented electrical steel strip and method for its production

Info

Publication number: EP4365319A1
Application number: EP23207333.8A
Authority: EP
Inventors: Christian Hecht; Paul Koop; Ludger Lahn; Matthias Schick; Mustafa Seckin Aydin; Guy Ligi; Jonathan Blaszkowski; Sophie Merling; Tobias LEWE
Original assignee: ThyssenKrupp Electrical Steel GmbH
Current assignee: ThyssenKrupp Electrical Steel GmbH
Priority date: 2022-11-03
Filing date: 2023-11-02
Publication date: 2024-05-08

Abstract

The invention relates to a grain-oriented electrical steel sheet having a peak magnetic polarization of ≥ 1.3 T at an external field of 100 A/m and an excitation of 1000 Hz and comprising: a cold-rolled steel core layer consisting of Fe, Si and optionally further alloying elements, the steel core layer having two outer surfaces, a forsterite layer on at least one of the two outer surfaces of the cold-rolled steel core layer wherein the grain-oriented electrical steel sheet has a bending radius of 9 mm or less, determined using a taper mandrel bending device and bending a specimen of the grain oriented electrical steel continuously 180° around a taper mandrel with a taper base of 30 mm and a taper tip of 5 mm, the bending radius being the radius at which visible cracks appear in the grain-oriented electrical steel sheet. The invention further relates to a method of producing a grain-oriented electrical steel sheet of the invention, to laminated stacks of grain-oriented electrical steel sheets, wherein the stack comprises at least two grain-oriented electrical steel sheets according to the invention laminated together with a resin and to the use of the grain-oriented electrical steel sheet of the invention as material for the production of parts for electric motors, for electric transformers or for other electric devices.

Description

The present invention relates to a grain-oriented electrical steel strip, to a method for producing a grain-oriented electrical steel strip, to a laminated stack of at least two grain-oriented electrical steel strips of the invention and to the use of such a grain-oriented electrical steel strip or laminated stack thereof as material for the production of parts for electric motors, for electric transformers or for other electric devices and. In particular, the present invention relates to a grain-oriented electrical steel strip with improved adhesion of a forsterite layer, formed thereon and excellent electromagnetic properties.
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 ppm by weight (= "wt.-ppm").
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 and 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.
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.
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. 0.10 to 0.35 mm.
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 the following manufacturing steps:

a) Producing a steel by using a blast furnace and basic oxygen converter or by using an electric arc furnace;
b) metallurgy refining of the steel melt by using a vacuum degassing vessel;
c) casting the steel melt into an intermediate product, i.e. a common slab, a thin slab or a cast strip;
d) optionally reheating the intermediate product;
e) hot rolling the intermediate product to a hot rolled steel strip;
f) coiling the hot rolled steel strip into a coil;
g) coil surface preparation;
h) hot strip annealing and pickling of the hot rolled strip;
i) cold rolling the hot rolled strip in one or more passes to obtain a cold rolled strip with a final thickness;
j) decarburization annealing of the cold rolled strip;
k) optionally surface nitriding of the cold rolled strip;
l) applying a MgO coating to the surface of the cold rolled strip;
m) 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;
n) heat flattening and insulation coating of the annealed strip;
o) optionally magnetic domain refining of the strip.

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 AlN, 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 GOES sheet.
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.
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 forsterite 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.
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 forsterite layer, also often called "glass film" is formed on the surface of the strip. This forsterite layer 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.
In a further process step, as known for example from DE 22 47 269 C3 , a solution based on magnesium phosphate or aluminum phosphate or mixtures of both with various additives, such as chromium compounds and Si oxide, can be applied to the forsterite layer and baked at temperatures above 350 °C. The layer system thus formed on the electrical steel forms an insulating layer, which transfers additional tensile stresses to the steel material that have a favorable effect on the electromagnetic properties of the electrical steel or sheet.
To ensure that these tensile stresses are transmitted reliably under harsh operating conditions over a long period of use, excellent adhesion of the forsterite layer to the cold-rolled steel material of the electrical steel strip must be ensured. For example, it must be ensured that the forsterite layer adheres firmly to the steel substrate even when the electrical steel coated with it is wound into a coil or cut to form blanks or other sheet parts required for further processing.
In case that the grain-oriented electrical steel strip is used to produce laminated stacks of grain-oriented electrical steel strips for use in electric motors, in particular for use in stator and rotor teeth for axial flux motors, the interface between the forsterite layer and the steel substrate is usually the weak point of the laminated stack under load. In these applications, stacks of the GOES sheets are usually laminated together via a backlack type resin or an epoxy-based resin as described in DE 10 2015 012172 A1 . Therefore, for the production of laminated stacks of grain-oriented electrical steel strips for use in electric motors the adherence properties of the forsterite layer to the base steel of the GOES are of primary importance.
The high-temperature annealing step during which the forsterite layer is formed usually takes 6 - 7 days and requires considerable energy input. Only after this long annealing period has elapsed it becomes possible to determine with conventional production methods whether the forsterite layer has formed properly or whether it is only insufficiently adhering to the steel substrate. Interventions in the production process to eliminate a defective formation of the forsterite layer can therefore, only be carried out after a considerable delay. As production continues during this time, larger quantities of likewise defective electrical steel strip may be produced until the cause of the defect has been eliminated.
On the other hand, it is known that anchorages, such as non-metallic inclusions or interface roughness, between the forsterite layer and the iron-silicon-steel matrix improve the adhesion of the forsterite layer to the steel base material of the electrical steel strip. However, these anchorages have the drawback that they penetrate the magnetic active part of the grain-oriented electrical steel sheet and act as a pinning point for the Bloch wall movement and thereby deteriorate the overall magnetic properties, especially the polarization properties of the grain-oriented electrical steel sheet at a given external field.
Therefore, an improvement in adhesion of the forsterite layer to the base steel strip is usually accompanied by a deterioration of the magnetic properties of the grain-oriented electrical steel strip.
Against the background of the prior art explained above the object of the present invention was to provide a grain-oriented electrical steel strip, in particular for use in electric motors, in electric transformers or in other electric devices, with improved adhesion of the forsterite layer to the base steel without negatively affecting its electromagnetic properties.
The invention solved this problem by means of a grain-oriented electrical steel sheet with at least the features specified in claim 1, a laminated stack thereof with at least the features specified in claim 8 and a process for producing the grain-oriented electrical steel sheet with at least the features specified in claim 11.
The general idea and advantageous embodiments of the invention are indicated in the dependent claims and explained in detail below.
A grain-oriented electrical steel sheet according to the invention has a peak magnetic polarization of ≥ 1.3 T at an external field of 100 A/m and an excitation of 1000 Hz and comprises

a cold-rolled steel core layer consisting of Fe, Si and optionally further alloying elements, the steel core layer having two outer surfaces,
a forsterite layer on at least one of the two outer surfaces of the cold-rolled steel core layer,

The grain-oriented electrical steel sheet according to the present invention comprises a cold-rolled steel core layer consisting at least of iron ("Fe") and silicon ("Si"), wherein the Fe, as in common GOES 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. A silicon content 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 have proven to be especially advantageous with regard to the magnetic properties of a grain-oriented steel sheet according to the invention. A Si content of at least 2% by weight has proven to be particularly beneficial to ensure a high permeability and thereby an improved iron loss of the grain-oriented steel sheet. To improve the workability of the steel sheet the Si content is preferably 4% by weight or less.
In addition to Fe and Si the core layer of the grain-oriented steel sheet according to the invention optionally may contain further alloying elements. Such further elements may include at least one element of the group "C, Mn, Cu, Cr, Sn, Al, N, Ti, S, Se, Mo 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.
According to the present invention, the amount of C, if present in the grain-oriented electrical steel sheet, may amount to 0.0001 to 0.01% C, particularly preferably 0.0001 to 0.005% C. A C content of more than 0.005% leads to a significant deterioration of the iron loss of the grain-oriented electrical steel sheet and should therefore be avoided.
For example, according to the present invention, the amount of Cr, if present in the grain-oriented electrical steel sheet, may amount to 0.001 to 3.0% Cr, particularly preferably 0.01 to 0.50% by weight Cr. Cr improves the magnetic properties of the grain-oriented steel sheet by stabilizing the mixed oxide and glass film formation. In order to securely achieve this effect, the amount of Cr should be at least 0.001% by weight. Particularly good results are achieved in case the Cr content is limited to 0.50% by weight.
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 0.50% Cu. Cu decreases the degree of oxidation and stabilizes the secondary recrystallization during bell anneal. To effectively improve the magnetic quality of the grain-oriented steel sheet, the content of Cu should be at least 0.001% by weight, whereas when the Cu content is higher than 3.0%by weight, the hot rollability is decreased due to the formation of hard particles.
Sn can be optionally present as well in the grain-oriented electrical steel sheet according to the invention in contents of 0.01 to 0.5% by weight, particularly 0.02 to 0.30% by weight. Sn improves magnetic quality by stabilizing the formation of the oxide layers and the glass film. This effect is achieved by adding at least 0.01% by weight Sn to the steel composition. A Sn content above 0.5% by weight shows negative influences on the oxidation process and as a result a stable glass film (forsterite film) cannot be formed.
Furthermore, the amount of Mn, if present in the grain-oriented electrical steel sheet can be 0.001 to 3.0% by weight, particularly 0.01 to 0.50% Mn. Mn increases the specific resistivity of the steel, which effectively decreases the iron loss. This can be achieved by addition of an amount of at least 0.001% by weight Mn. In case the amount of Mn exceeds 3.0% by weight the magnetic flux density of the steel is unfavorably reduced.
According to the invention, the contents of Al, N, Ti, S, Se, Mo and B, 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.
Thus, a steel alloy before hot rolling which is especially suited to serve as basis for the core layer of a grain-oriented steel sheet according to the invention preferably comprises, in % by weight, 0.01 to 0.10% C, 2 to 5% Si, 0.01 to 0.5% Mn, 0.01 to 0.5% Cr, 0.01 to 0.5% Cu, 0.01 to 0,3% Sn, optionally in sum less than 3%, preferably less than 1%, of one or more elements selected from Al, N, Ti, S, Se, Mo and B, the remainder being Fe and unavoidable impurities. The sum of the content of unavoidable impurities is preferably restricted to less than 0.5% by weight.
The sum of the sulfur (S) and selenium (Se) 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.
According to a further preferred embodiment of the invention the sulfur ("S") content of the core layer of the grain-oriented steel sheet according to the invention is restricted to less than 50 ppm by weight, preferably to less than 30 ppm by weight.
The steel alloy before hot rolling, according to the present invention, may also further contain Al and N, which can be used for the control of the inhibition system. The amount of Al in the steel alloy can be 0.001 to 1.0% by weight, particularly 0.001 to 0.10% Al.
According to the present invention, N can be part of the steel alloy and be 0.001 to 0.80% by weight, particularly 0.001 to 0.10% N. As N deteriorates the final properties of the grain-oriented electrical steel sheet, during the optional step k) the amount of N is reduced, so that the grain-oriented electrical steel can contain 0.0001 to 0.010% N, preferably 0.0001 to 0.005% N.
The grain-oriented steel according to the present invention comprises at least one forsterite layer present above one or both outer surfaces of the cold-rolled steel core layer.
A forsterite layer as used herein is a layer that comprises mainly forsterite and may further comprise oxides and mixed oxides of iron, aluminum, titanium, magnesium and/or silica. For example, the forsterite layer may comprise at least 50%, preferably at least 75%, more preferably at least 90%, forsterite. The remainder of the forsterite layer may be comprised of iron oxides, silica oxides, aluminum oxides, titanium oxides, magnesium oxides and/ or mixed oxides of iron, aluminum, titanium, magnesium and/or silica.
According to an embodiment of the present invention, on the surface of the at least one forsterite layer present on at least one of the surfaces of the cold-rolled steel core layer an insulating layer may further be present. Such insulating layers are known to the skilled person and may be formed, e.g., from a solution based on magnesium phosphate or aluminum phosphate or a mixture thereof with various additives, such as chromium compounds and silicon dioxide.
Apart from the coatings mentioned above further coatings may be present on the insulating 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 .
Typically, the cold rolled steel core layer of the grain-oriented steel according to the present invention has a thickness of 160 to 380 µm, wherein a thickness of the core layer of at least 180 to 300 µm turned out to be especially useful for practical applications in rotating machines.
The thickness of the forsterite layer of a steel sheet according to the invention typically amounts to 0.5 to 3 µm, wherein in practice thicknesses of at least 1 to 2 µm are observed. A restriction of the forsterite layer to a maximum of 3 µm turned out to be especially advantageous with regard to the iron filling factor of the steel sheet according to the invention.
The grain-oriented electrical steel sheet according to the present invention further may comprise at least one, preferably exactly one, insulating layer present on each forsterite layer. The sum of the thicknesses of the insulating layers is preferably at least 0.1 µm and less than 5 µm, more preferably 0.1 to 2.5 µm.
Preferably, the grain-oriented steel according to the present invention comprises anchorages protruding from the forsterite layer into the cold rolled steel core layer, wherein the anchorages have a depth of ≥ 0.5 µm and the mean number of anchorages is 1 to 8 per 10 µm length in rolling direction on at least one of the surfaces of the cold-rolled steel core layer.
An anchorage as used herein is a part of the forsterite layer that protrudes into the cold rolled steel base layer with a depth of ≥ 0.5 µm. Such anchorages can be determined by scanning electron microscopy (SEM). In particular, the determination of the anchorages is carried out by using a SEM-micrograph of the grain-oriented electrical steel sheet and placing a horizontal line, in the area between the forsterite layer and the cold-rolled steel core layer. By using the EDX unit of the SEM it is possible to determine exactly the elemental composition along the horizontal line in the SEM-micrograph. The horizontal line is moved down in the micrograph towards the cold-rolled steel core layer and the point at which the amount of iron in the elemental composition of the horizontal line is 10% is defined as the end of the forsterite layer. This horizontal line is used as zero point from which the depth of the anchorages protruding into the cold-rolled steel base layer is determined. Thereby, parts of the forsterite layer protruding from this line into the cold-rolled steel core layer having a depth of less than 0.5 µm are seen as surface roughness.
The grain-oriented electrical steel sheet according to the invention has a bending radius of 9 mm or less determined using a taper mandrel bending device and bending a specimen of the grain oriented electrical steel continuously 180° around a taper mandrel with a taper base of 30 mm and a taper tip of 5 mm, the bending radius being the radius at which visible cracks appear in the grain-oriented electrical steel sheet. The aforementioned method of determining the bending radius corresponds to DIN 6860:2006-06. A low bending radius can be equated to a good adhesion of the forsterite layer. In other words, the better the adhesion of the forsterite layer, the lower the bending angle determined according to the method described above.
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 comprising Fe, Si and optionally further alloying elements;
b) cold rolling the hot strip of step a) by single-stage rolling in at least three passes to obtain a cold strip or by multi-stage rolling in at least three passes with a temperature T_CR during the third pass in a single-stage rolling or during the last pass prior to an intermediate annealing in a multi-stage rolling,
c) primary recrystallization annealing of the cold rolled strip obtained in step b), optionally including a nitriding treatment, at an annealing temperature between 600 to 950 °C for the duration t_A in a high dew-point atmosphere DP,
d) coating the cold-rolled steel strip obtained in step c) with an annealing separator comprising MgO;
e) performing a high temperature annealing treatment to obtain a grain-oriented electrical steel sheet according to the invention,
f) optionally applying an insulating layer on at least one side of the GOES,
g) optionally performing of a domain refinement transverse to the rolling direction,
- wherein the cold-rolling temperature T_CR in step b) during the third pass in a single-stage rolling or during the last pass prior to an intermediate annealing in a multi-stage rolling and the duration of the annealing t_A and the dew point of the atmosphere DP during primary recrystallization annealing in step c) fulfil the following condition (I): $2.45 < (3.4 \times T_{CR} - (1.2 \times DP - 5.8 \times t_{A})) / 500 < 2.75$
- wherein
  
  TCR
  
  is the cold-rolling temperature during the third pass in a single-stage rolling or the cold rolling temperature during the last pass prior to an intermediate annealing in a multi-stage rolling according to step (B) above, indicated in °C,
  
  DP
  
  is the dew point of the atmosphere during the primary recrystallization annealing, indicated in °C,
  
  tA
  
  is the duration of the primary recrystallization annealing, indicated in s.

The method of the invention may comprise further steps that are known to the skilled person and that are usually performed when producing grain-oriented electrical steel sheets.
The invention is based here on the realization that the formation of an optimally adhering forsterite layer on the steel substrate of a grain-oriented electrical steel strip according to the invention can be ensured by fulfilling the condition (I) above during cold rolling of the hot strip and primary recrystallization annealing of the cold-rolled steel strip, which forms the cold-rolled electrical steel core layer of the grain-oriented electrical steel strip according to the invention and is subsequently coated with the forsterite layer.
In this context, the invention makes use of the fact that on each outer surface of the steel core mixed oxides, in particular of at least iron, silica and/or aluminum, are formed during cold-rolling between consecutive rolling passes with or without intermediate annealing as a result of a chemical reaction of the alloying elements contained in the steel material of the core layer with the atmosphere.
It has been found that the accumulations of these mixed oxides on the steel core layer surface contribute to the formation of the forsterite layer and its adhesion to the steel core layer. In particular, it was found that these oxides once formed are rolled into the steel substrate during the next cold rolling pass and show a positive influence on the formation of anchorages protruding from the forsterite layer into the steel core layer as the rolled-in oxides appear to provide good starting anchor points for the formation of these anchorages.
It was further observed that with increasing temperature of the steel sheet during the cold-rolling passes the type of oxides formed on the surface changes and their ability to be optimally rolled into the steel core decreases. Thereby insufficient starting anchor points are formed and an optimal formation of anchorages protruding from the forsterite layer into the steel core layer cannot be obtained.
In order to allow for an optimal formation of anchorages protruding from the forsterite layer into steel core layer of the grain-oriented electrical steel sheet it was found that the negative influences mentioned above may be compensated by carefully adjusting the dew point of the atmosphere and the annealing time during primary recrystallization annealing depending on the temperature of the third pass during single-stage rolling or the temperature of the last pass prior to an intermediate annealing during multi-stage rolling.
The combination of measures according to the invention in the production of the cold-rolled steel strip, especially in steps b) and c) of the method of the invention, makes it possible to reliably achieve optimized bonding strength of the forsterite layer while at the same time achieving excellent magnetic properties of the grain-oriented electrical steel sheet.
Step a) of the method comprises providing a hot rolled steel strip, which is made from a steel comprising Fe, Si and optionally further alloying elements. In particular, the steel may be alloyed in accordance with the explanations and provisions given above with regard to the cold-rolled steel core layer of the grain-oriented electrical steel sheet of the invention, which are equally applicable to the hot-rolled steel strip provided in the method of the invention.
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 1.5 to 3.5 mm, more preferably 1.8 to 2.7 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 .
After step a) hot rolled steel strips having the above-mentioned composition and thickness are obtained. These hot rolled steel strips are preferably directly introduced into step b) of the process. Alternatively, the hot rolled steep strip may be annealed after step a) and before step b).
In step b) of the method according to the invention, the hot rolled strip is cold rolled in at least three passes with a temperature T_CR during the third pass in a single-stage rolling or during the last pass prior to an intermediate annealing in a multi-stage rolling, to obtain a cold rolled strip. As used herein "single stage rolling" is cold-rolling in at least three passes, wherein no intermediate annealing takes place between individual passes. As used herein "multi-stage rolling" is cold rolling in at least three passes, wherein the hot-rolled strip is cold rolled to an intermediate thickness, an intermediate annealing takes place, and after the intermediate annealing the strip is further cold-rolled to the final thickness. Methods 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 . Typically, an intermediate annealing is performed in a temperature range of 700 to 1150 °C, preferably 800 to 1100 °C, under an atmosphere which dew point is set to 10 to 80 °C. Typical annealing times are 30s to 900 s. 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 temperature T_CR as used herein is the temperature of the strip determined during the third pass of single stage rolling or during the last pass of multi-stage rolling prior to intermediate annealing, which is significantly higher than the temperature of the strip during the first pass of single stage rolling or of multi-stage rolling, e.g., at least 50°C, preferably at least 100°C, most preferably at least 150°C, higher. Preferably, the temperature T_CR is between 150 and 450°C, more preferably between 200 and 270°C.
Typically, the thickness of the cold rolled strip is 0.05 to 0.5 mm, preferably a maximum thickness of 0.35 mm, preferably of ≤ 0.27 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 .
In working step c) of the process according to the invention primary recrystallization annealing of the cold rolled strip obtained in step b), optionally including a nitriding treatment, at an annealing temperature between 600 to 950 °C for the duration t_A in a high dew-point atmosphere DP takes place. The primary recrystallization annealing is preferably carried out at temperatures in the range of 600 to 900 °C. The duration t_A of the primary recrystallization annealing is preferably 30 to 300 s. A high dew point atmosphere as used herein is an atmosphere with a dew point between 40 and 80°C, preferably between 40 and 65°C. A dew point of more than 80°C cannot be set in industrial settings, while with a dew point below 40°C the resulting oxide layer becomes too dense and all surface-controlled chemical reactions, e.g. decarburization, nitriding, de-nitriding, can no longer take place as desired. The atmosphere may comprise 5 to 95 Vol.-% Hz, the reminder being nitrogen or any inert gas or a mix gas.
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 annealing and nitriding can be performed simultaneously.
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 850 ppm, preferably up to 400 ppm, more preferably 20 to 400 ppm, is achieved. The nitriding degree is calculated as the difference between the nitrogen content of the steel strip before the second recrystallization annealing (working step e)) 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.
In step d) of the method according to the invention the cold-rolled steel strip obtained in step c) is coated with an annealing separator comprising MgO and optionally oxides and mixed oxides of iron, aluminum, titanium, magnesium and/or silica. The annealing separator comprising MgO and optionally oxides and mixed oxides of iron, aluminum, titanium, magnesium and/or silica applied to the cold-rolled steel strip to produce the forsterite layer during annealing in step e) in a manner known per se can consist of at least 70% by weight MgO, optionally up to 25% by weight of oxides and mixed oxides of iron, aluminum, titanium, magnesium and/or silica and can further contain up to 5% by weight additives, based on the total dry weight of the annealing separator. These additives may be, for example, elements like Ca, B and Sr, ammonium chloride or antimony chloride, and other salts like magnesium sulfate or sodium chloride, the addition of which controls the density of the subsequent forsterite layer and the gas exchange between the annealing atmosphere during high-temperature annealing and the metal.
In step e) of the process according to the invention the cold rolled strip obtained in step c) and coated with the annealing separator in step d) undergoes a high temperature annealing treatment during which the forsterite layer is formed and secondary recrystallization occurs. This high temperature annealing treatment can also be carried out in a manner known per se. For this purpose, the cold-rolled steel strip obtained after step c) and coated with the annealing separator in step d) can be wound into a coil and kept in a bell furnace for 10 - 200 hours at a temperature of 730 - 1325°C under an atmosphere consisting of at least 50% H₂. For example, the strip or sheet that is obtained after step d) can be rapidly heated to a soaking temperature of 1150°C or above, wherein soaking temperatures of at least 1200°C are particularly advantageous. The heating and soaking is preferably carried out under a protective gas atmosphere, which, for example, comprises Hz. Particularly preferably, the heating to and soaking at the respective soaking temperature is performed under an atmosphere which comprises 5 to 95 Vol.-% Hz, the reminder being nitrogen or any inert gas or a mix gas, the local 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 electrical steel sheet. These elements are in particular N and S.
After the high temperature annealing the steel strip is cooled down in a common manner, e.g. by natural cooling, down to room temperature.
In step e) 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.
In addition, according to a preferred embodiment of the method of the invention, after step e) 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.
In optional step f) of the method of the invention an insulating layer is applied on at least one side of the GOES. The method for applying the insulating layer is known to the artisan and can be found in e.g. EP 2 902 509 B1 and EP2 954 095 A1 . An insulation coating applied to a grain oriented electrical steel product has a positive effect on minimization of the hysteresis losses. The insulation coating can transfer tensile stresses to the base material, which not only improves the magnetic loss values of the grain oriented electrical steel product but also reduces the magnetostriction, thereby having in turn a positive effect on the noise behavior of the finished transformer. Formation of the insulation coating involves applying an aqueous solution of metallic phosphate containing colloidal silica and optional chromium compounds onto the surface of the steel sheet and baking the same at temperatures in the range of 800 °C to 950 °C for 10 to 600 s. The phosphate coating is usually applied on top of the forsterite layer. However, it is also possible to apply the insulation directly onto the steel surface with no forsterite layer in between.
According to optional step g) of the method of the invention a domain refinement transverse to the rolling direction is performed. The method for domain refinement by laser or electron beam treatment is known to the artisan and can be found in e.g. EP 2 675 927 A1 . E.g. in the course of laser treatment, linear deformations, which are arranged with a spacing, are formed into the surface of the flat steel product by means of a laser beam emitted by a laser beam source, thereby decreasing the length of the domains and reducing the losses of the grain-oriented electrical steel sheet.
According to the present invention, at the end of the method explained above, the grain-oriented electrical steel is presenting a cold-rolled steel core layer and at least one forsterite layer being present on at least one of the outer surfaces of the cold-rolled steel core layer and anchorages protruding from the forsterite layer into the cold rolled steel core layer, wherein the anchorages have a depth of ≥ 0.5 µm and the mean number of anchorages is 1 to 8 per 10 µm length in rolling direction on at least one of the surfaces of the cold-rolled steel core layer.
Such a grain-oriented electrical steel sheet has a peak magnetic polarization of ≥ 1.3 T at an external field of 100 A/m and an excitation of 1000 Hz. For the purpose of the invention the magnetic polarization is determined according to IEC 60404-3. 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.
The grain-oriented electrical steel sheet according to the present invention shows improved adhesion of the forsterite layer and at the same time excellent magnetic properties in comparison to grain-oriented electrical steel sheets according to the prior art.
Accordingly, the grain-oriented electrical steel sheet according to the invention is in particular useful for the manufacture of parts for electric transformers, for electric motors or for other electric devices.
A preferred use of the grain-oriented electrical steel sheet of the present invention is as material for the manufacture of stator and/or rotor core for axial flux motors.
To manufacture such stator or rotor core stacks of the grain-oriented electrical steel sheet are typically formed by laminating the GOES sheets together with a resin, for example, an epoxy-based resin, e.g., as described in paragraph [0008] of DE 10 2015 012172 A1 , or a backlack type resin.
A preferred epoxy-based resin comprises 60 parts by weight of an epoxy resin, 0.5 to 15 parts by weight of a latent curing agent and 1 to 15 parts by weight of a latent accelerator, based on the total dry weight of the epoxy-based resin. The components in this preferred epoxy-based resin are present as solids in the indicated parts by weight based on the total dry weight of the epoxy-based resin. This dry coating mixture is dispersed and/or dissolved in a suitable medium before being applied to the GOES sheets.
In particular, the dry coating mixture comprising the specified components is preferably provided as a dispersion of the above-mentioned composition in a dispersion medium. Preferably the dispersion medium is water.
The epoxy resin present in the epoxy-based resin may comprise one or more epoxy resins having more than one epoxy group. Preferably at least one epoxy resin in the epoxy-based resin has a softening point of greater than 50°C.
The epoxy resin in the epoxy-based resin can be selected from aliphatic, cycloaliphatic and/or aromatic epoxy resins.
Preferably, the epoxy-based resin comprises 1 to 10 parts by weight of the latent curing agent, particularly preferably 2 to 5 parts by weight of the latent curing agent, based on the total dry weight of the epoxy-based resin. The latent curing agent preferably undergoes curing reactions with the epoxy resin of the epoxy-based resin at temperatures in the range from 80°C to 200°C.
The term latent curing agent refers to a substance which serves to cure the epoxy resin, but which must be activated for curing, in particular by supplying chemical and/or thermal energy. The latent curing agent is added to the epoxy-based resin for example, as a solid in powder form. Preferably, the latent curing agent contains a dicyandiamide, an imidazole, a BF³ amine complex or a combination thereof.
Preferably the epoxy-based resin comprises 1 to 10 parts by weight of a latent accelerator, preferably 1 to 5 parts by weight of a latent accelerator, more preferably 1 to 4 parts by weight of a latent accelerator, based on the total dry weight of the epoxy-based resin.
The term latent accelerator refers to a substance that accelerates the curing of the epoxy resin by the latent curing agent. The attribute latent also refers, in the context of the accelerator, to the fact that the accelerator must also be activated beforehand by chemical and/or thermal energy in order to perform its function. The latent accelerator is added to the epoxy-based resin, for example, as a solid in powder form. Preferably, the latent accelerator contains a urea derivative and/or imidazole.
The epoxy-based resin may further contain other components, such as anticorrosion additives, insulation additives, colorants and/or fillers.
Preferably the epoxy-based resin is applied to the GOES sheet on one side or on both sides. If an epoxy-based resin is applied on both sides of the GOES sheet, the thickness of the coating can be the same, but different thicknesses can also be provided.
The preferred thickness of the epoxy-based resin, i.e. the thickness of the coating on one side of the GOES sheet in the case of a single-sided application of the epoxy-based resin or the total thickness of the epoxy-based resin coating on both sides of the GOES sheet in the case of a double-sided epoxy-based resin coating, is between 1 µm and 20 µm, preferably between 2 µm and 10 µm. A total thickness of the epoxy-based resin coating between 4 and 8 µm is particularly preferred.
For the lamination of the GOES sheets temperatures below 230°C are usually used. The optimal duration of the lamination depends on the lamination temperature used and on the thickness of the stack to be laminated and is typically adapted depending on the stack geometry and the type of heating used. The exact conditions for the manufacturing of the resin coating depend on the resin type used and can be found in the respective data sheets of the resin manufacturer.
Usually the optimal duration for the lamination increases with decreasing lamination temperature and/or increasing laminated stack thickness. For example, lamination of a thin stack of GOES at a lamination temperature of 200°C using a lamination time of 2 minutes may be sufficient, while for laminating a thicker stack of GOES a lower temperature of 180°C for a duration of 1 hour or at 140°C for a duration of 2 hours may be preferable. The temperatures mentioned for the lamination are not the furnace temperatures but the core temperatures of the stack and the holding time is the time that elapses between reaching the core temperature and removal from the furnace.
The lamination is usually carried out using a pressure between 150 to 300 N/cm³.
Another aspect of the present invention is a laminated stack of grain-oriented electrical steel sheets, wherein the stack comprises at least two grain-oriented electrical steel sheets according to the invention laminated together with a resin.
Typically, a laminated stack of grain-oriented electrical steel sheets comprises at least 2 or at least 3 grain-oriented electrical steel sheets according to the invention laminated together with a resin.
The resin for laminating the grain-oriented electrical steel sheets together is not particularly limited. Any resin known to the skilled person for this purpose can be used. Preferably, the resin for laminating the grain-oriented electrical steel sheets together is selected from a backlack type resin,an epoxy-based resin, e.g., as described in paragraph [0008] of DE 10 2015 012172 A1 or as described in detail above. These resin types are known to the skilled person and are described, e.g., in DE 10 2015 012172 A1 .
Laminated stacks according to the invention preferably have a peel strength of ≥ 13 N/cm determined according to ISO 11339-2010-06. Such peel strength has been found to be particularly beneficial for the practical application of these laminated stacks as a material for the production of stator or rotor core in axial flux motors. The laminated stacks show good adhesion and magnetic properties. In particular, it was found that in laminated stacks according to the invention the weak point lies no longer between the forsterite layer and the grain-oriented steel sheet but is transferred to the resin, which is favorable as such stacks fulfill all demands of the targeted applications.
The invention is explained further using the following Figure.

Fig. 1: Schematic representation of a SEM micrograph of an embodiment of a grain-oriented electrical steel sheet according to the present invention

Figure 1 shows a schematic representation of a SEM micrograph of an embodiment of a grain-oriented electrical steel sheet according to the present invention. The grain-oriented electrical steel sheet 1 comprises a cold-rolled steel sheet core layer 2 and a forsterite layer 3 as well as an insulating layer 7. Anchorages 4 protrude from the forsterite layer 3 into the cold-rolled steel sheet core layer 2. The anchorages have a depth of ≥ 0.5 µm starting from a horizontal line 5 in the micrograph, wherein the elemental composition of the horizontal line determined by EDX comprises an amount of iron that is 10% by weight. As the micrograph represents a two-dimensional section of the grain-oriented electrical steel sheet some of the anchorages 4 are visible as inclusions 6 in the cold-rolled steel sheet core. However, in the three-dimensional grain-oriented electrical steel sheet these inclusions 6 are in fact connected to the forsterite layer 3, i.e. the connection to the forsterite layer lies outside the two-dimensional plane of the micrograph.
Experiments have been carried out to demonstrate the effect of the invention.
In these experiments, 17 samples of grain-oriented electrical steel sheets were produced in the manner described above, the core layer of which consisted of a steel with a composition as shown in Table 1, the samples having the approximate size of 300 mm in rolling direction and 60 mm in transversal direction.
The bending radius of these samples was determined using a taper mandrel bending device and bending a specimen of the grain oriented electrical steel continuously 180° around a taper mandrel with a taper base of 30 mm and a taper tip of 5 mm, the bending radius being the radius at which visible cracks appear in the grain-oriented electrical steel sheet.
Laminated stacks of these grain-oriented electrical steel sheets were produced by coating specimen of GOES with a Backlack like type resin and additional laminating of two specimen to produce a small stack for testing. The peel strength of these laminated stacks was determined according to ISO 11339- 2016-06.
For each of the samples 1 to 17 the following parameters and properties are indicated in Table 2:

the temperature T_CR in °C during the third pass in a single-stage rolling or during the last pass prior to an intermediate annealing in a multi-stage rolling in step b) of the method of the invention;
the dew point DP in °C during step c) of the method of the invention;
the duration of the annealing t_A in s during step c) of the method of the invention;
the result when calculating the factor (3.4 x T_CR - (1.2 x DP - 5.8 × t_A))/500;
the peak magnetic polarization J₁₀₀ for a magnetic field strength of 100 AIm at 1000 Hz;
the peel strength in N/cm determined according to ISO 11339- 2016-06 on laminated stacks of samples 1 to 17 prepared as mentioned above;
the bending radius in mm determined as described above.

Furthermore, it is indicated in Table 2 if the condition (I) 2.45 < (3.4 x T_CR - (1.2 x DP - 5.8 × t_A))/500 < 2.75 is fulfilled.
It can be seen from Table 2 that Examples 4, 6, 9-13 and 17 are Comparative Examples.

The experiments clearly show that those Examples, which fulfill condition (I) of the method of the invention exhibit good adhesion of the forsterite layer as exemplified by a low bending radius, while at the same time having J₁₀₀-values, which are at least comparable or better than the J₁₀₀-values of the Comparative Examples, which do not meet the requirements of the invention. In addition, laminated stacks made of grain-oriented electrical steel sheets according to the invention show improved peel strength determined according to ISO 11339- 2016-06 compared to laminated stacks made of grain-oriented steel sheets not according to the invention.

Table 1: Figures in % by weight, balance: Fe and impurities

Steel	C	Si	Mn	Cr	Cu	Sn
A	0,0007	3,03	0,113	0,061	0,195	0,102
B	0,0009	3,43	0,026	0,012	0,089	0,076
C	0,0010	3,21	0,051	0,121	0,149	0,018
D	0,0014	3,19	0,203	0,039	0,056	0,099

Table 2

No.	Steel type	T_CR	DP	t_A	(3.4 × T_CR-(1.2xDP-5.8 × t_A))/500	Peel strength	Bending radius	J₁₀₀ at 1000 Hz	Condition (I) met?	Invention ?
No.	Steel type	[°C]	[°C]	[s]		[N/cm]	[mm]	[T]	Condition (I) met?	Invention ?
1	A	180	45	120	2.51	16.3	7.2	1.35	YES	YES
2	A	180	50	125	2.55	13.5	7.0	1.34	YES	YES
3	A	180	60	130	2.59	15.1	6.1	1.32	YES	YES
4	A	190	45	160	3.04	15.0	8.0	1.25	NO	NO
5	A	190	50	115	2.51	13.1	7.5	1.34	YES	YES
6	A	190	60	60	1.84	3.0	14.0	1.35	NO	NO
7	A	200	45	115	2.59	16.8	5.8	1.38	YES	YES
8	A	200	50	120	2.63	14.8	8.0	1.31	YES	YES
9	A	200	60	90	2.26	11.2	10.0	1.29	NO	NO
10	A	210	45	60	2.02	8.4	14.0	1.24	NO	NO
11	A	210	50	90	2.35	9.8	11.1	1.25	NO	NO
12	A	210	60	80	2.21	11.5	9.8	1.27	NO	NO
13	A	220	45	80	2.32	6.3	14.0	1.35	NO	NO
14	A	220	50	100	2.54	16.8	6.3	1.34	YES	YES
15	A	220	60	100	2.51	14.3	7.5	1.32	YES	YES
16	A	230	45	110	2.73	14.9	7.3	1.31	YES	YES
17	A	230	50	60	2.14	8.2	14.0	1.24	NO	NO
18	B	180	45	120	2.51	14,9	6,3	1.34	YES	YES
19	B	180	50	80	2.03	10.2	13.1	1.28	NO	NO
20	B	180	65	140	2.69	13.8	6.8	1.32	YES	YES
21	B	190	45	130	2.69	14.1	8.2	1.31	YES	YES
22	B	190	50	140	2.80	12.4	8.0	1.31	NO	NO
23	B	190	65	120	2.53	15.2	7.3	1.32	YES	YES
24	B	200	45	110	2.53	13.2	7.2	1.35	YES	YES
25	B	200	50	110	2.52	13.0	6.5	1.34	YES	YES
26	B	200	65	115	2.54	13.3	7.0	1.36	YES	YES
27	B	220	45	80	2.32	10.5	8.8	1.27	NO	NO
28	B	220	50	100	2.54	14.7	7.0	1.32	YES	YES
29	B	220	65	130	2.85	9.6	13.7	1.26	NO	NO
30	B	240	45	80	2.45	15.5	6.3	1.32	YES	YES
31	B	240	50	90	2.56	16.1	5.9	1.35	YES	YES
32	B	240	65	105	2.69	14.8	6.4	1.34	YES	YES
33	B	250	45	105	2.81	12.4	12.2	1.24	NO	NO
34	B	250	65	95	2.65	15.3	8.4	1.32	YES	YES
35	C	180	40	120	2.52	15.2	7.1	1.35	YES	YES
36	C	180	55	110	2.37	11.8	9.5	1.28	NO	NO
37	C	180	70	135	2.62	13.4	6.8	1.32	YES	YES
38	C	190	40	120	2.59	13.9	7.3	1.33	YES	YES
39	C	190	55	110	2.44	10.2	10.6	1.29	NO	NO
40	C	190	70	135	2.69	14.6	8.1	1.36	YES	YES
41	C	200	40	100	2.42	12.7	13.0	1.30	NO	NO
42	C	200	55	115	2.56	13.5	8.0	1.30	YES	YES
43	C	200	70	130	2.70	15.7	7.7	1.33	YES	YES
44	C	220	40	80	2.33	8.8	11.2	1.26	NO	NO
45	C	220	55	120	2.76	13.1	12.8	1.25	NO	NO
46	C	220	70	120	2.72	14.8	7.5	1.35	YES	YES
47	C	240	40	75	2.41	9.2	10.5	1.28	NO	NO
48	C	240	55	95	2.60	15.4	6.4	1.32	YES	YES
49	C	240	70	110	2.74	16.2	5.8	1.33	YES	YES
50	C	260	40	80	2.60	13.6	8.3	1.30	YES	YES
51	C	260	55	100	2.80	9.4	13.5	1.29	NO	NO
52	D	180	45	120	2.51	15.4	7.1	1.35	YES	YES
53	D	180	50	110	2.38	8.9	11.6	1.27	NO	NO
54	D	180	60	100	2.24	13.6	10.2	1.29	NO	NO
55	D	190	45	95	2.29	10.9	13.8	1.25	NO	NO
56	D	190	50	120	2.56	14.0	6.7	1.33	YES	YES
57	D	190	60	140	2.77	12.9	11.3	1.28	NO	NO
58	D	200	45	120	2.64	13.8	7.3	1.31	YES	YES
59	D	200	50	110	2.52	13.4	8.8	1.31	YES	YES
60	D	200	60	130	2.72	14.5	6.7	1.33	YES	YES
61	D	210	45	80	2.25	10.7	12.0	1.28	NO	NO
62	D	210	50	130	2.82	12.5	8.9	1.30	NO	NO
63	D	210	60	120	2.68	14.3	8.2	1.32	YES	YES
64	D	220	45	90	2.43	9.7	14.0	1.26	NO	NO
65	D	220	50	100	2.54	13.9	8.4	1.32	YES	YES
66	D	220	60	110	2.63	15.8	7.1	1.34	YES	YES
67	D	230	45	110	2.73	14.6	7.5	1.34	YES	YES
68	D	230	50	90	2.49	16.6	6.2	1.36	YES	YES

Claims

A grain-oriented electrical steel sheet having a peak magnetic polarization of ≥ 1.3 T at an external field of 100 A/m and an excitation of 1000 Hz and comprising:
- a cold-rolled steel core layer consisting of Fe, Si and optionally further alloying elements, the steel core layer having two outer surfaces,

- a forsterite layer on at least one of the two outer surfaces of the cold-rolled steel core layer,
wherein the grain-oriented electrical steel sheet has a bending radius of 9 mm or less, determined using a taper mandrel bending device and bending a specimen of the grain oriented electrical steel continuously 180° around a taper mandrel with a taper base of 30 mm and a taper tip of 5 mm, the bending radius being the radius at which visible cracks appear in the grain-oriented electrical steel sheet.
The grain-oriented electrical steel sheet according to claim 1, wherein 1 to 8% by weight Si is present in the cold-rolled steel core layer.
The grain-oriented electrical steel sheet according to claim 1 or 2, wherein further alloying elements are present in the cold-rolled steel core layer and selected from at least one element of the group consisting of C, Mn, Cu, Cr, Sn, Al, N, Ti, S, Se, Mo and B, wherein the sum of the contents of these elements in the alloy of the cold-rolled steel core layers is restricted to 3% by weight
The grain-oriented electrical steel sheet according to one of the preceding claims, wherein the cold-rolled-steel core layer comprises 0.01 to 0.3% by weight of Sn.
The grain-oriented electrical steel sheet according to any of the preceding claims, wherein the cold-rolled steel core layer consists of, in % by weight, 0.0001 to 0.005% C, 2 to 5% Si, 0.01 to 0.5% Mn, 0.01 to 0.5% Cr, 0.01 to 0.5% Cu, 0.01 to 0,2% Sn, the remainder being Fe and unavoidable impurities.
The grain-oriented electrical steel sheet according to any of the preceding claims, wherein at least one insulating layer is present on the at least one forsterite layer.
The grain-oriented electrical steel sheet according to any of the preceding claims, wherein the grain-oriented electrical steel sheet further comprises anchorages protruding from the forsterite layer into the cold rolled steel core layer, wherein the anchorages have a depth of ≥ 0.5 µm and the mean number of anchorages is 1 to 8 per 10 µm length in rolling direction on at least one of the surfaces of the cold-rolled steel core layer.
Laminated stack of grain-oriented electrical steel sheets, wherein the stack comprises at least two grain-oriented electrical steel sheets according to claims 1 to 7 laminated together with a resin.
Laminated stack of claim 8, wherein the resin is selected from a backlack type resin or an epoxy-based resin.
Laminated stack of any of the preceding claims, wherein the laminated stack has a peel strength of ≥ 13 N/cm determined according to ISO 11339- 2016-06.
Method of producing a grain-oriented electrical steel sheet according to any one of claims 1 to 7, comprising at least the following working steps:
a) Providing a hot rolled steel strip, which is made from a steel comprising Fe, Si and optionally further alloying elements;

b) cold rolling the hot strip of step a) by single-stage rolling in at least three passes to obtain a cold strip or by multi-stage rolling in at least three passes with a temperature T_CR during the third pass in a single-stage rolling or during the last pass prior to an intermediate annealing in a multi-stage rolling,

c) primary recrystallization annealing of the cold rolled strip obtained in step b), optionally including a nitriding treatment, at an annealing temperature between 600 to 950 °C for the duration t_A in a high dew-point atmosphere DP,

d) coating the cold-rolled steel strip obtained in step c) with an annealing separator comprising MgO;

e) performing a high temperature annealing treatment to obtain a grain-oriented electrical steel sheet according to the invention,
wherein the cold-rolling temperature T_CR in step b) during the third pass in a single-stage rolling or during the last pass prior to an intermediate annealing in a multi-stage rolling before intermediate annealing and the duration of the annealing t_A and the dew point of the atmosphere DP during primary recrystallization annealing in step c) fulfil the following condition (I): $2.45 < (3.4 \times T_{CR} - (1.2 \times DP - 5.8 \times t_{A})) / 500 < 2.75$

wherein
T_CR is the cold-rolling temperature during the third pass in a single-stage rolling or the cold rolling temperature during the last pass prior to an intermediate annealing in a multi-stage rolling according to step (b) above, indicated in °C,

DP is the dew point of the atmosphere during the annealing, indicated in °C,

t_A is the duration of the annealing, indicated in s.
Method according to claim 11, wherein the cold-rolling temperature T_CR in step b) during the third pass in a single-stage rolling or during the last pass prior to an intermediate annealing in a multi-stage rolling is 150 to 450°C.
Method according to claim 11 or 12, wherein the duration of the annealing t_A in step c) is 30 to 300 s.
Method according to any of the preceding claims, wherein the dew point of the atmosphere during the annealing in step c) is between 40 and 80°C.
Use of a grain-oriented steel sheet according to any one of claims 1 to 7 or of a laminated stack according to any one of claims 8 to 10 as material for the production of parts for electric motors, for electric transformers or for other electric devices, preferably as material for the production of stator or rotor teeth in axial flux motors.