US20180237876A1

US20180237876A1 - Method for producing a grain-oriented electrical steel strip and grain-oriented electrical steel strip

Info

Publication number: US20180237876A1
Application number: US15/754,143
Authority: US
Inventors: Christian Hecht; Ludger Lahn; Carsten Schepers
Original assignee: ThyssenKrupp Electrical Steel GmbH
Current assignee: ThyssenKrupp Electrical Steel GmbH
Priority date: 2015-08-28
Filing date: 2016-08-29
Publication date: 2018-08-23
Also published as: DE102015114358B4; DE102015114358A1; RU2018110082A3; CN107922987A; RU2719864C2; ES2781335T3; BR112018003100A2; EP3341500A1; CN107922987B; PL3341500T3; RU2018110082A; PT3341500T; WO2017037019A1; MX2018002064A; KR20180057632A; EP3341500B1; JP2018532041A

Abstract

A method is used to produce a grain-oriented electrical steel strip having optimized magnetic properties. The method may utilize a steel comprising in percent by weight 2.0-4.0% Si, 0.010-0.100% C, at most 0.065% Al, at most 0.02% N, and optionally further constituents, the balance being iron and unavoidable impurities. The steel may be processed to give a cold strip, which may then be subjected to oxidation/primary recrystallization annealing. The resultant cold strip may have an oxide layer to which an annealing separator layer is applied. In a subsequent high-temperature anneal, a forsterite layer forms therefrom, and an insulation layer may be applied thereto prior to a final anneal. After the oxidation/primary recrystallization anneal, a spectrum of the oxide layer obtained is determined by way of FTIR, and an area (Fe2SiO4) is determined for a peak representing Fe2SiO4 molecules present in the oxide layer and is at 980 cm−1, and an area (αSiO2) for a peak representing αSiO2 molecules present in the oxide layer and is at 1250 cm−1. Then the composition of the steel, parameters for the cold rolling, parameters for the oxidation/primary recrystallization anneal, or parameters for a hot strip anneal are adjusted such that 0.5×area (Fe2SiO4)≤area (αSiO2)≤2×area (Fe2SiO4).

Description

The invention relates to a method of producing a grain-oriented electrical steel strip and to a grain-oriented electrical steel strip.
When the present application refers to “electrical steel strips”, this means electrical steel sheets and electrical steel strips produced by rolling steels of suitable composition, and circuit boards or blanks that have been divided therefrom, which are intended for the production of parts for electrical engineering applications.
Grain-oriented electrical steel strips of the type in question here are especially suitable for uses in which the emphasis is on a particularly low cyclic magnetization loss and high demands are made on permeability or polarization. Such demands exist especially in the case of parts for power transformers, distribution transformers and higher-quality small transformers.
As elucidated specifically, for example, in EP 1 025 268 B1, in the course of the production of electrical steel strips, generally a steel comprising (in % by weight) typically 2.5% to 4.0% Si, 0.010% to 0.100% C, up to 0.150% Mn, up to 0.065% Al and up to 0.0150% N, and in each case optionally 0.010% to 0.3% Cu, to 0.060% S, to 0.100% P, and to in each case 0.2% As, Sn, Sb, Te and Bi, the balance being iron and unavoidable impurities, is first cast to give a preliminary material, such as a slab, thin slab or a cast strip. The preliminary material is then, if required, subjected to an annealing treatment and then hot-rolled to give a hot strip.
The resultant hot strip is coiled to give a coil and can then, if required, be subjected to annealing and to a likewise optionally executed descaling or pickling treatment. Then a cold strip is rolled from the hot strip in one or more stages, with performance of intermediate annealing if required between the cold rolling steps in a multistage cold rolling operation effected in multiple steps.
The resultant cold strip then typically undergoes a decarburization anneal, in order to minimize the carbon content of the cold strip for avoidance of magnetic aging.
After the decarburization anneal, an annealing separator is applied to the strip surfaces, which typically comprises MgO. The annealing separator prevents the windings of a coil wound from the cold strip from being welded to one another in a subsequently conducted high-temperature anneal.
During the high-temperature anneal, which is typically conducted in a bell furnace under protective gas, a microstructure texture that makes a significant contribution to the magnetic properties forms in the cold strip as a result of selective grain growth.
At the same time, a forsterite layer forms on the strip surfaces during the high-temperature anneal, often also referred to in the technical literature as “glass film”.
In addition, the steel material is cleaned by diffusion processes that proceed during the high-temperature anneal. Subsequent to the high-temperature anneal, the flat steel product having the forsterite layer which is obtained in this way is coated with an insulation layer, thermally aligned and subjected to stress-relief annealing in a concluding “final anneal”. This final anneal can be effected before or after the finishing of the flat steel product produced in the manner described above to give the blanks required for further processing. By means of a final anneal which is conducted after the blanks have been divided off, the additional stresses that have arisen in the course of the dividing operation can be dissipated. Electrical steel strips produced in such a way generally have a thickness of 0.15 mm to 0.5 mm.
As further elucidated in WO 03/000951 A1, it is likewise prior art that the domain structure can additionally be improved by the application of an insulation layer which exerts a permanent tensile stress on the sheet substrate, and additionally also that by a treatment in which lines of local stresses are generated transverse or oblique to the rolling direction in the flat steel product, the magnetic properties of grain-oriented electrical steel strips can be further improved. Surface structures of this kind can be generated, for example, by local mechanical deformations (EP 0 409 389 A2), laser or electron beam treatments (EP 0 008 385 B1; EP 0 100 638 B1; EP 0 571 705 A2) or etching of trenches (EP 0 539 236 B1).
For example, it is additionally known from EP 0 225 619 B1 that the forsterite film also has an important influence on essential use properties of electrical steel strips. For example, the losses, the noise characteristics in the transformer or else the bond strength of the insulation are affected by the forsterite film between magnetically active base material and insulation layer.
Therefore, the following demands are made in practice on the forsterite film:

- It is to assure the bond strength of the insulation film on the base material by optimal interdigitation with the steel substrate.
- It is to exert a tensile stress on the base material, by means of which the magnetic properties of the electrical steel strip are significantly improved.
- It is to ensure a homogeneous dark gray color of the surfaces of the finished material.

Means of optimizing the effect of the forsterite film by chemical additions to the annealing separator applied to the cold-rolled flat steel product prior to the high-temperature anneal are elucidated, for example, in WO 95/25820 A1.
It is likewise known that the properties and effect of the forsterite film are also affected by the process steps that the steel substrate undergoes in the production of grain-oriented electrical steel strips prior to the application of the annealing separator. An indicator here is the composition of the oxide layer present on the steel substrate prior to the bell anneal, which leads to reproducible glass films in the case of a high-temperature anneal executed according to the prior art with subsequent relaxation annealing and optional additionally conducted laser treatment.
In the consideration of the relationship between the condition of the steel substrate in the production of grain-oriented electrical steel strips prior to the application of the annealing separator and the high-temperature anneal and the resultant properties of the forsterite film obtained, the main emphasis in the literature is typically on the interdigitation of the forsterite with the steel substrate, since the adhesion of the composite composed of forsterite film and insulation coating which is formed in the subsequent steps is significantly dependent thereon.
For example, JP 2004/191217 A1 has proposed improving the bond strength of the insulation layer by optimizing the uppermost oxide layer by means of examinations on the basis of Fourier transform infrared spectrometry “FTIR”. For this purpose, an infrared beam is guided onto the surface at a defined angle and the directed reflection is measured. Since multiple reflections occur within the material, depending on the angle of incidence, it is possible to measure only the uppermost portion of the oxide layer. Therefore, this method can permit only conclusions about the bond strength; it is not possible to use it to determine other properties, for example the later tensile strength.
Therefore, the already published JP 2004/191217 A1 or other technical publications, for example the article “Rapid quantitative analysis of fayalite and silica formed during decarburization of electrical steel” by Jung et al., published in Surface Interface Analysis 2012, 44, 270-275, or the article “Characterization of chemical information and morphology for in-depth oxide layers in decarburized electrical steel with glow discharge sputtering”, likewise by Jung et al., published in Surface Interface Analysis 2013, 45, 1119-1128, discuss a combination with invasive techniques, for example rf-GDOES or else wet chemistry. However, these methods do not permit any conclusions as to the molecular composition of the oxide layer or the forsterite layer produced thereon, since the removal of the surface disrupts the molecular composition. Even a combination with microscopic methods as proposed in the second article by Jung et al. cited above does not result in any conclusions that would allow the oxide layer to be described in such a way as to permit direct conclusions to be drawn for practical use.
Against the background of the prior art, the problem addressed was that of specifying a method of producing grain-oriented electrical steel strips with which the surface constitution of the respective flat steel product can be adjusted in a controlled manner prior to the application of the annealing separator such that a forsterite film with optimal effect in terms of the magnetic properties of the electrical steel strip to be produced is obtained.
The invention has solved this problem by following the procedure of the method specified in claim 1 in the production of grain-oriented electrical steel strips.
Advantageous configurations of the invention are specified in the dependent claims and are elucidated specifically hereinafter, as is the general concept of the invention.
According to the invention, in the production of grain-oriented electrical steel strips, the steps that are typically envisaged for this purpose in the prior art are implemented.
These include

- a) smelting a steel melt having (in % by weight) 2.0-4.0% Si, 0.010-0.100% C, up to 0.065% Al and up to 0.020% N, and in each case optionally up to 0.5% Cu, up to 0.060% S and likewise optionally in each case up to 0.3% Cr, Mn, Ni, Mo, P, As, Sn, Sb, Se, Te, B or Bi, the balance being iron and unavoidable impurities;
- b) casting the steel melt to give a preliminary material, such as a slab, thin slab or a cast strip;
- c) hot rolling the preliminary material to give a hot strip;
- d) coiling the hot strip to give a coil;
- e) optionally annealing the hot strip;
- f) cold rolling the hot strip in one or more cold rolling steps to give a cold strip;
- g) oxidation/primary recrystallization annealing of the cold strip, where this step optionally comprises a nitriding treatment and where the cold strip after the oxidation/primary recrystallization anneal has an oxide layer on its surface;
- h) applying an annealing separator layer to the surface of the cold strip having the oxide layer;
- i) high-temperature annealing of the cold strip coated with the annealing separator to form a forsterite layer on the surface of the calcined cold strip;
- j) applying an insulation layer to the surface of the cold strip having the forsterite layer;
- k) finally annealing the cold strip;
- l) optionally laser-treating the cold strip.

It will be apparent that the method of the invention may comprise further steps which are conducted in the conventional production of electrical steel strips in order to achieve optimized magnetic properties or properties that are important for practical use. These include, for example, reheating of the precursor obtained after the casting of the steel, descaling of the hot strip prior to the cold rolling or, in the case of the multistage performance of cold rolling, intermediate annealing conducted in a conventional manner between the cold rolling stages in each case.
Crucial factors here in deciding whether, in the production of electrical steel strips, a surface constitution of the respective flat steel product prior to the application of the annealing separator which enables the reliable formation of a forsterite film with optimal effect in terms of the magnetic properties of the electrical strip to be produced is obtained are, in accordance with the invention,

- a spectrum of the oxide layer present on the surface of the cold strip is recorded after step g) by means of diffuse reflectance Fourier transformation infrared spectroscopy,
- the area “area(Fe₂SiO₄)” is determined by integration for the peak present in the spectrum obtained at 980 cm⁻¹, which represents the Fe₂SiO₄molecules, also called “fayalite molecules”, present in the oxide layer,
- the area “area(αSiO₂)” is determined by integration for the peak present in the spectrum obtained at 1250 cm⁻¹, which represents the αSiO₂molecules present in the oxide layer
  and
- the composition of the steel (step a)) or the parameters for the optionally conducted hot strip anneal (step e)), for the cold rolling (step f)) or for the oxidation/primary recrystallization anneal (step g)) are adjusted such that the area(αSiO₂) and the area(Fe₂SiO₄) satisfy the condition

0.5×area(Fe₂SiO₄)≤area(αSiO₂)≤2×area(Fe₂SiO₄).
The invention proceeds here from the finding that, firstly, the bond strength of the forsterite film on the steel substrate is controlled solely by the uppermost atomic layers of the oxide layer, whereas the stress transmitted to the base material can be modified only within certain limits. In order, however, to further increase the tensile stress, it is necessary, for example, in the case of an annealing separator consisting essentially of MgO, to change the morphological arrangement of the magnesium atoms in the matrix of the SiO₄tetrahedra within the forsterite film formed from the annealing separator.
For this purpose, according to the findings of the invention, it is necessary not just to control the molecular composition of the near-surface layer and the atomic composition of the oxide layer, but also to molecularly characterize and influence the entire oxide layer in a controlled manner.
In order to assure this, in accordance with the invention, the oxide layer is characterized by means of “diffuse reflectance Fourier transformation infrared spectroscopy”, also referred to as “DRIFT method” for short. In the DRIFT method, an IR light beam is directed onto the sample surface by means of concave mirrors and the reflected light is also detected by means of concave mirrors (see Beasley et al., “Comparison of transmission FTIR, ATR and DRIFT spectra”, Journal of Archeological Science, Vol. 46, June 2014, pages 16-22). This enables the evaluation of deeper-lying oxide layers and hence a deeper analysis of the molecular components in the oxide layer. On the basis of the result of the DRIFT analysis, the process parameters in the subsequent processing of the flat steel products are then adjusted such that an oxide layer favorable for the formation of an optimally adhering forsterite film that simultaneously exerts optimally high tensile stresses is formed on the steel substrate.
The analysis of the DRIFT spectrum of the oxide layer present on the surface of the flat steel product after the cold rolling should be checked continuously in order firstly to detect the quality of the oxide film across the entire surface of the flat steel product in question for each batch of electrical steel strips. Secondly, the information derived in accordance with the invention from the DRIFT spectrum allows optimization of the results in the production of subsequent batches of electrical steel strips. If the DRIFT spectrum shows that the ratio of the proportions of α-SiO₂and fayalite (Fe₂SiO₄) molecules does not meet the specifications of the invention, for this purpose, the process steps in the method of the invention that have been implemented up to the application of the annealing separator are adjusted. In other words, the steel analysis, the parameters for the hot strip anneal, the parameters for the cold rolling and the parameters for the oxidation/primary recrystallization anneal are adjusted such that the condition set in accordance with the invention for the molecular proportions that show in the DRIFT spectrum
0.5×area(Fe₂SiO₄)≤area(αSiO₂)≤2×area(Fe₂SiO₄).
is satisfied.
The oxidation/primary recrystallization anneal can be combined in a manner known in practice with a decarburization anneal, in which the carbon content of the steel substrate is minimized, and a nitriding treatment which is likewise optionally conducted in a manner known per se, which has the aim of increasing the nitrogen content of the steel substrate.
The area(αSiO₂) and area(Fe₂SiO₄) can be determined here for the peaks representing the proportion of the αSiO₂and Fe₂SiO₄molecules in a manner known per se (see Foley, “Equations for chromatographic peak modeling and calculation of peak area”, Analytical Chemistry, Vol. 59, Aug. 1, 1987, pages 1984-1985) as the area enclosed by the respective peak and its baseline, the start and end of the baseline being determined by the two foot points F1, F1′; F2, F2′ of the respective peak, i.e. the points where the line of the spectrum gives way to the respective peak (see FIG. 1).
Typically, in the method of the invention, the cold rolling (step f)) is conducted in at least three cold rolling steps, typically with an intermediate anneal between the cold rolling steps in a manner known per se, in order to eliminate the cold solidifications that arise in each preceding cold rolling step and to assure rollability for the subsequent rolling step. In the practical implementation of the method of the invention, the hot strip is likewise optionally subjected to a hot strip anneal in a manner which is likewise known, in order to assure optimal cold rollability.
The character of the oxide layer on the cold strip obtained after the cold rolling can then be influenced via the steel composition smelted in step a) and the adjustment of the parameters for the optional hot strip anneal, for the cold rolling and for the oxidation/primary recrystallization anneal, taking account of the inventive measure that follows in each case, where the measures in question can be utilized in combination with one another or alternatively to one another:

- In the case that a hot strip anneal (step e)) is conducted, an index kH is determined by the formula

kH=T_max/(8×DP_max+10×K)

- with T_max: maximum temperature, reported in ° C., in the hot strip anneal,
  - DP_max: maximum dew point, reported in ° C., achieved in the atmosphere under which the hot strip anneal takes place,
  - K: cooling rate, reported in ° C./s, in the cooling within the temperature range of 700-400° C. conducted after the hot strip anneal.
- For the cold rolling (step f)) an index kC is determined by the formula

kC=T_ob/(2×Ab)

- with T_ob: average surface temperature, reported in ° C., during the last three cold rolling passes,
  - Ab: total decrease in the thickness of the cold strip, reported in %, achieved over the last three cold rolling passes.

For the oxidation/primary recrystallization anneal (step g)) an index kOx is determined by the formula
kOx=T_ox/(5×DP_ox)

- with T_ox: maximum temperature, reported in ° C., achieved during the oxidation/primary recrystallization anneal with optional nitriding component,
- DP_ox: maximum dew point, reported in ° C., achieved in the atmosphere under which the oxidation/primary recrystallization anneal takes place.

Then the parameters T_max, DP_max, K, T_ob, Ab, T_oxand DP_oxfor steps a), f), g) of the process of the invention and the composition of the steel substrate processed in accordance with the invention are adjusted such that the indices kH, kC and kOx satisfy the conditions
% Sn/% Cu≤kC≤3×(% Sn/% Cu+% Cr+kH) (1)
¼×(kH+kC+% Sn/% Cu)≤kOx≤2×(kH+kC+% Sn/% Cu+% Cr) (2)
and, if a hot strip anneal is conducted,
γ₁₁₅₀/100×3≤kH≤γ₁₁₅₀/100×15 (3)
where
γ₁₁₅₀=694×% C−23×% Si+64.8
and with % C the carbon content of the steel melt, with % Sn the Sn content of the steel melt, with % Cu the copper content of the steel melt and with % Cr the chromium content of the steel melt, each reported in % by weight.
The parameter γ₁₁₅₀is the percentage alpha/gamma conversion, which is elucidated in detail in EP0600181.
It has been found that, surprisingly, an oxide layer produced in accordance with the invention enhances the diffusion of nitrogen into the steel base material when the steel substrate processed in accordance with the invention is subjected to a nitriding process as described, for example, in EP 0 950 120 A1.
Suitable annealing separators for the purposes of the invention are especially those conventional annealing separators which consist predominantly, i.e. typically to an extent of at least 85% by weight, of MgO.
In principle, it is conceivable to execute the high-temperature anneal in a continuous run. However, a particularly advantageous high-temperature annealing method in relation to the desired optimization of the magnetic properties and the practical utility of electrical steel strips produced in accordance with the invention has been found to be a high-temperature anneal (step i)) conducted in the form of a bell anneal. The temperatures for the high-temperature anneal are typically in the temperature range of 1000-1250° C. known per se for this purpose.
In accordance with the above elucidations, a grain-oriented electrical steel strip of the invention comprises a cold-rolled steel substrate consisting of a steel comprising (in % by weight) 2.0-4.0% Si, up to 0.100% C, up to 0.065% Al and up to 0.020% N, and in each case optionally up to 0.5% Cu, up to 0.060% S and likewise optionally in each case up to 0.3% Cr, Mn, Ni, Mo, P, As, Sn, Sb, Se, Te, B or Bi, the balance being iron and unavoidable impurities, wherein a forsterite film present on said steel substrate features a higher peak at the wavenumber of 977 cm⁻¹than at the wavenumber of 984 cm⁻¹in a spectrum recorded by means of diffuse reflectance Fourier transformation infrared spectroscopy. An electrical steel strip of this kind can especially be produced by employing the method of the invention.
The carbon content of the electrical steel strip having the characteristics of the invention is typically at least 0.01% by weight, but may also be lower as a result of the process steps implemented in the course of its production, especially in the case of corresponding performance of the optional decarburization anneal.

The invention is elucidated in detail hereinafter by working examples. The figures show:

FIG. 1 DRIFT spectra of oxide layers present on an inventive sample and a noninventive sample;

FIG. 2 DRIFT spectra of forsterite films present on an inventive sample and a noninventive sample.

Melts A-F with the compositions specified in table 1 have been smelted and cast to give a 65 mm strand, from which thin slabs have been divided as intermediate product.
In 28 experiments, cold strips for the production of grain-oriented electrical steel strips have been produced from the thin slabs in the manner described hereinafter.
After reheating to a reheating temperature of typically 1170° C., the thin slabs have been hot-rolled to give a hot strip having a thickness of typically 2.3 mm, which has then been coiled to give a coil. The coiling temperature was typically 540° C.
Subsequently, the respective hot strip has been subjected to a hot strip anneal in which it has been through-heated in each case at a maximum temperature T_maxunder an atmosphere having a maximum dew point Dp_max, and after which it has been cooled to room temperature in each case with a cooling rate K.
The hot strips have subsequently been cold-rolled in five passes to give a cold strip in each case. The mean surface temperature T_obof the cold strip during the last three cold rolling passes and the total decrease in thickness Ab achieved over the last three cold rolling passes have been determined here.
The cold strips obtained after the cold rolling have been subjected to a combined annealing treatment in which a decarburization under an atmosphere with a maximum dew point Dp_decand a maximum annealing temperature T_dec, an oxidation/primary recrystallization at a maximum annealing temperature T_oxand a maximum dew point Dp_ox, and in some selected samples a nitriding treatment at a maximum temperature T_nitunder an atmosphere with a maximum dew point Dp_nithave been conducted.
An oxide layer was present on each of the cold strips thus obtained, for each of which a DRIFT spectrum has been recorded.
In addition, for the samples, the indices kH, kC and kOx and the areas “area(Fe₂SiO₄)” and “area(αSiO₂)” that are taken up by the peaks attributed to the Fe₂SiO₄molecules and the αSiO₂molecules at the wavenumber 980 cm⁻¹(Fe₂SiO₄molecules) and 1250 cm⁻¹(αSiO₂molecules), and the ratio “area(Fe₂SiO₄)/area(αSiO₂)” have been determined. Samples for which the area(Fe₂SiO₄)/area(αSiO₂) ratio is in the range of 0.5-2 meet the requirements of the invention.
Subsequently, an annealing separator that consisted to an extent of 90% of MgO has been applied to the cold strips.
The cold strips that have thus been coated have been subjected to a high-temperature anneal conducted as a bell anneal, the maximum temperature of which was 1200° C. First of all, in a manner known per se, the cold strips have been kept here under an atmosphere consisting to an extent of 75% by volume of hydrogen and of 25% by volume of nitrogen, and finally in a cleaning phase under an atmosphere consisting to an extent of 100% by volume of hydrogen.
After the cooling, a forsterite film was present on each of the samples, of which a DRIFT spectrum has again been recorded in each case. In the DRIFT spectra of the forsterite film recorded for the 28 samples, the peak heights at the wavenumbers of 977 cm⁻¹and 984 cm⁻¹have been determined and compared to one another. Samples for which the peak height at the wavenumber of 977 cm⁻¹is smaller than at the wavenumber of 984 cm⁻¹are not in accordance with the invention.
In table 2, for examples 1-28 of the alloys A-F specified in table 1 of which the steel substrate of the respective sample consisted, the temperature T_maxand the maximum dew point Dp_maxfor the hot strip anneal, the cooling rate K in the subsequent cooling, the mean surface temperature T_obof the cold strip during the last three cold rolling passes and the total decrease in thickness Ab achieved over the last three cold rolling passes, the maximum dew point Dp_decand the maximum annealing temperature T_decfor the decarburizing anneal, the maximum annealing temperature T_oxand the maximum dew point Dp_oxof the oxidation/primary recrystallization anneal, the maximum temperature T_nitand the maximum dew point Dp_nitof the optionally conducted nitriding treatment are specified.
In table 3, for examples 1-28, the indices kH, kC and kOx and the values of “area(Fe₂SiO₄)”, “area(αSiO₂)”, the ratio F/αS, i.e. the ratio “area(Fe₂SiO₄)”/“area(αSiO₂)”, the peak heights at the wavenumbers 977 cm⁻¹and 984 cm⁻¹, the difference 977-984, i.e. the “peak height at wavenumber 977 cm⁻¹”−“peak height at wavenumber 984 cm⁻¹” are reported. In addition, table 3 emphasizes which of examples 1-28 are inventive and which are not.
The tensile stress exerted on the respective steel substrate by the forsterite film obtained on samples 1-28 was typically 6 MPa for the inventive samples.
All measurements on samples 1-28 were conducted with conventional analysis units. A Bruker Tensor 27 from Bruker Corporation was used for the FT-IR measurements, which were conducted either in the state after the decarburizing anneal or after the bell anneal, and a “Praying Mantis” cell, which is manufactured by HARRICK SCIENTIFIC PRODUCTS INC., for the DRIFT measurements.
FIG. 1 shows for the inventive sample 11 as a solid line and for the non-inventive sample 18 as a dotted line the DRIFT spectra determined prior to the application of the annealing separator present oxide layer.
FIG. 2 shows for the inventive sample 11 as a solid line and for the non-inventive sample 18 as a dotted line the DRIFT spectra determined after the high-temperature anneal present forsterite layer.

TABLE 1

Figures in % by weight, balance: Fe and impurities

	Melt	C	Si	Cr	Cu	Sn

A	0.04	3.07	0.069	0.18	0.061
B	0.04	3.12	0.023	0.068	0.079
C	0.04	3.252	0.039	0.071	0.116
D	0.04	3.16	0.014	0.069	0.081
E	0.04	3.21	0.11	0.013	0.048
F	0.04	3.19	0.055	0.006	0.05

TABLE 2

		T_max	Dp_max	K¹	Ab	T_ob	T_dec	Dp_dec	T_nit	Dp_nit	T_ox
Sample	Melt	° C.	° C.	° C. s⁻¹	%	° C.	° C.	° C.	° C.	° C.	° C.

1	A	950	25	20	80	200	860	67	—	—	860
2	A	1130	20	20	70	250	820	24	—	—	820
3	A	1130	20	20	60	180	840	76	860	45	860
4	B	1130	15	60	65	300	830	34	900	45	900
5	B	950	5	60	70	200	880	48	—	—	880
6	B	1050	20	60	80	225	820	12	900	45	900
7	B	1100	15	40	85	225	840	69	820	45	840
8	B	1100	20	40	75	225	840	44	—	—	840
9	C	1100	25	40	70	200	850	31	—	—	850
10	C	950	20	20	73	225	820	22	—	—	820
11	C	1130	20	80	84	200	860	63	860	45	860
12	C	950	20	20	76	180	820	48	900	45	900
13	D	950	15	20	83	250	860	34	—	—	860
14	E	1130	25	60	85	200	830	36	800	45	830
15	A	950	20	40	75	230	860	15	—	—	860
16	F	950	5	60	70	200	880	48	—	—	880
17	E	950	20	50	84	190	860	35	860	45	860
18	E	1050	40	60	67	150	850	64	—	—	850
19	F	1050	40	70	87	220	860	78	—	—	860
20	A	1050	5	20	80	200	870	22	—	—	870
21	C	1100	5	30	77	230	830	40	900	45	900
22	F	1100	15	50	70	250	810	64	820	45	820
23	A	1100	20	60	75	300	850	20	—	—	850
24	F	1100	15	50	70	180	830	29	920	45	920
25	F	1100	40	40	75	190	820	35	770	−5	820
26	C	1100	20	20	65	200	860	48	—	—	860
27	F	950	20	40	74	180	830	64	820	45	830
28	C	1130	20	20	73	240	880	16	900	45	900

TABLE 3

								Peak	Peak
	Dp_ox				Area of	Area of		height	height	Difference
Sample	° C.	kH	kC	kOx	αSiO₂	Fe₂SiO₄	F/αS	984	977	977 − 984	*)

1	67	2.38	3.75	2.57	0.169	0.233	1.378	0.364	0.492	0.127	Y
2	24	3.14	5.36	6.83	0.02	0.039	1.998	0.244	0.26	0.016	Y
3	76	3.14	4.50	2.26	0.134	0.19	1.419	0.371	0.462	0.09	Y
4	45	1.57	6.92	4.00	0.064	0.117	1.826	0.336	0.492	0.156	Y
5	48	1.48	4.29	3.67	0.029	0.047	1.644	0.365	0.369	0.004	Y
6	45	1.38	4.22	4.00	0.061	0.052	0.865	0.278	0.308	0.029	Y
7	69	2.12	3.97	2.43	0.062	0.055	0.896	0.319	0.474	0.155	Y
8	44	1.96	4.50	3.82	0.049	0.097	1.983	0.34	0.469	0.129	Y
9	31	1.83	4.29	5.48	0.041	0.055	1.352	0.355	0.55	0.195	Y
10	22	2.64	4.62	7.45	0.012	0.007	0.585	0.214	0.388	0.174	Y
11	63	1.18	3.57	2.73	0.142	0.112	0.789	0.38	0.388	0.008	Y
12	48	2.64	3.55	3.75	0.159	0.09	0.567	0.379	0.395	0.016	Y
13	34	2.97	4.52	5.06	0.035	0.055	1.578	0.233	0.249	0.017	Y
14	45	1.41	3.53	3.69	0.124	0.241	1.947	0.304	0.396	0.092	Y
15	15	1.70	4.60	11.47	0.084	0.003	0.038	0.328	0.314	−0.014	N
16	48	1.48	4.29	3.67	0.05	0.013	0.25	0.304	0.234	−0.07	N
17	45	1.44	3.39	3.82	0.087	0.007	0.082	0.208	0.15	−0.058	N
18	64	1.14	3.36	2.66	0.154	0.051	0.331	0.381	0.377	−0.004	N
19	78	1.03	3.79	2.21	0.091	0.033	0.36	0.282	0.225	−0.057	N
20	22	4.38	3.75	7.91	0.093	0.029	0.314	0.339	0.157	−0.182	N
21	45	3.24	4.48	4.00	0.006	0.016	2.575	0.365	0.178	−0.187	N
22	64	1.77	5.36	2.56	0.188	0.463	2.457	0.33	0.23	−0.1	N
23	20	1.45	6.00	8.50	0.136	0.329	2.415	0.228	0.217	−0.011	N
24	45	1.77	3.86	4.09	0.113	0.246	2.176	0.385	0.382	−0.003	N
25	35	1.53	3.80	4.69	0.082	0.235	2.856	0.396	0.375	−0.021	N
26	48	3.06	4.62	3.58	0.063	0.18	2.857	0.297	0.177	−0.12	N
27	64	1.70	3.65	2.59	0.05	0.143	2.835	0.314	0.145	−0.169	N
28	45	3.14	4.93	4.00	0.05	0.102	2.024	0.281	0.265	−0.016	N

*) Inventive? (Y = YES; N = NO)

Claims

1.-10. (canceled)

11. A method of producing a grain-oriented electrical steel strip, the method comprising:

smelting a steel melt comprising 2.0-4.0% by weight Si, 0.010-0.100% by weight C, up to 0.065% by weight Al, up to 0.02% by weight N, iron, and unavoidable impurities;

casting the steel melt to give a preliminary material;

hot rolling the preliminary material to give a hot strip;

coiling the hot strip to give a coil;

cold rolling the hot strip to give a cold strip;

oxidation/primary recrystallization annealing the cold strip, wherein a surface of the cold strip includes an oxide layer after the oxidation/primary recrystallization annealing;

recording a spectrum of the oxide layer by way of diffuse reflectance Fourier transformation infrared spectroscopy;

determining an area (Fe₂SiO₄) by integrating for a peak present in the spectrum obtained at 980 cm⁻¹, which represents Fe₂SiO₄molecules present in the oxide layer;

determining an area (αSiO₂) by integrating for a peak present in the spectrum obtained at 1250 cm⁻¹, which represents αSiO₂molecules present in the oxide layer;

adjusting a composition of the steel melt, parameters of the cold rolling of the hot strip, or parameters of the oxidation/primary recrystallization annealing so that 0.5×the area (Fe₂SiO₄)≤the area (αSiO₂)≤2×the area (Fe₂SiO₄);

applying an annealing separator layer to the surface of the cold strip that includes the oxide layer;

high-temperature annealing the cold strip coated with the annealing separator layer to form a forsterite layer on the surface of the calcined cold strip;

applying an insulation layer to the surface of the cold strip having the forsterite layer; and

annealing the cold strip.

12. The method of claim 11 wherein the steel melt further comprises:

up to 0.5% by weight Cu;

up to 0.060% by weight S; and

up to 0.3% by weight Cr, Mn, Ni, Mo, P, As, Sn, Sb, Se, Te, B or Bi.

13. The method of claim 11 wherein the oxidation/primary recrystallization annealing comprises at least one of a decarburizing treatment or a nitriding treatment.

14. The method of claim 11 comprising laser-treating the cold strip after the cold strip is annealed.

15. The method of claim 11 wherein the cold rolling of the hot strip to give the cold strip is performed in at least three cold rolling steps.

16. The method of claim 11 comprising annealing the hot strip after the coiling of the hot strip.

17. The method of claim 16 wherein the adjustment comprises adjusting the composition of the steel melt, the parameters of the cold rolling of the hot strip, the parameters of the oxidation/primary recrystallization annealing, or parameters of the annealing of the hot strip so that 0.5×the area (Fe₂SiO₄)≤the area (αSiO₂)≤2×the area (Fe₂SiO₄).

18. The method of claim 11 wherein the steel melt further comprises up to 0.5% by weight Cu; up to 0.060% by weight S; and up to 0.3% by weight Cr, Mn, Ni, Mo, P, As, Sn, Sb, Se, Te, B or Bi; wherein the cold rolling of the hot strip to give the cold strip is performed in at least three cold rolling steps, the method further comprising:

annealing the hot strip after the coiling of the hot strip;

determining an index kC for the cold rolling wherein kC=T_ob/(2×Ab), wherein T_obis an average surface temperature in degrees Celsius during a last three passes of the at least three cold rolling steps, wherein Ab is a total percent decrease in a thickness of the cold strip achieved over the last three cold rolling passes;

determining an index kOx for the oxidation/primary recrystallization annealing wherein kOx=T_ox/(5×DP_ox), wherein T_oxis a maximum temperature in degrees Celsius achieved during the oxidation/primary recrystallization annealing, wherein DP_oxis a maximum dew point in degrees Celsius achieved in an atmosphere under which the oxidation/primary recrystallization annealing occurs;

determining an index kH for the annealing of the hot strip wherein kH=T_max/(8×DP_max+10×K), wherein T_maxis a maximum temperature in degrees Celsius in the annealing of the hot strip, wherein DP_maxis a maximum dew point in degrees Celsius achieved in an atmosphere in which the annealing of the hot strip occurs, wherein K is a cooling rate in degrees Celsius per second while cooling within a temperature range of 700° C.-400° C. after the hot strip annealing; and

adjusting T_max, DP_max, K, T_ob, Ab, T_oxand DP_oxsuch that

percent by weight Sn/percent by weight Cu≤kC≤3×(percent by weight Sn/percent by weight Cu+percent by weight Cr+kH), (1)

¼×(kH+kC+percent by weight Sn/percent by weight Cu)≤kOx≤2×(kH₊ kC+percent by weight Sn/percent by weight Cu+percent by weight Cr), and (2)

γ₁₁₅₀/100×3≤kH≤γ₁₁₅₀/100×15, wherein γ₁₁₅₀=694×percent by weight C−23×percent by weight Si+64.8, (3)

wherein the percentages by weight in the adjustment of T_max, DP_max, K, T_ob, Ab, T_oxand DP_oxare with respect to the steel melt.

19. The method of claim 11 wherein the annealing separator layer comprises predominantly MgO.

20. The method of claim 11 wherein the high-temperature annealing is a bell anneal.

21. The method of claim 20 wherein a temperature in the high-temperature annealing is more than 1150° C.

22. A grain-oriented electrical steel strip comprising:

a forsterite film disposed on a cold-rolled steel substrate comprised of a steel comprising:

2.0-4.0% by weight Si,

up to 0.100% by weight C,

up to 0.065% by weight Al,

up to 0.020% by weight N,

iron, and

unavoidable impurities,

wherein the forsterite film has a higher peak at a wavenumber of 977 cm⁻¹than at a wavenumber of 984 cm⁻¹in a spectrum recorded by way of diffuse reflectance Fourier transformation infrared spectroscopy.

23. The grain-oriented electrical steel strip of claim 22 comprising:

up to 0.5% by weigh Cu;

up to 0.060% by weight S; and

up to 0.3% by weight Cr, Mn, Ni, Mo, P, As, Sn, Sb, Se, Te, B or Bi.

24. The grain-oriented electrical steel strip of claim 23 comprising a carbon content of at least 0.010% by weight.

25. The grain-oriented electrical steel strip of claim 23 produced by the method of claim 11.