Grain-oriented flat steel product and method for the production thereof

Abstract

The present invention relates to a method for producing a grain-oriented flat steel product, comprising the following steps: a) providing a cold-rolled flat steel product; b) primary recrystallization annealing of the cold-rolled flat steel product with simultaneous decarburization treatment in a humid atmosphere to a carbon content of less than 30 ppm; c) optionally carrying out a nitriding treatment during step b) or subsequently in step c); d) coating the flat steel product obtained in step b) or in optional step c) with a slurry, wherein the slurry consists of water, MgO and optionally one or more further solid(s) and has a mass ratio w1 of water to the total solid(s) in the slurry; e) coiling the coated flat steel product into a coil;(f) batch annealing of the coil at a soaking temperature of at least 1100°C with a dew point dp1 measured between the coil turns at 400°C during heating to soaking temperature and a dew point dp2 measured between the coil turns at 800°C during heating to soaking temperature; (g) applying an insulating coating to the annealed flat steel product;h) stress-relieving the flat steel product provided with the insulation coating to form an insulation layer, i) performing domain refinement by means of a laser with a wavelength WL on at least one side of the flat steel product provided with the insulation layer, wherein the wavelength WL of the laser satisfies the specification of the following formula (I): 573+60×dp1+10×dp2w1 <WL<1080+18×dp1+3×dp2w1 Die Erfindung betrifft ferner ein kornorientiertes Stahlflachprodukt mit minimierten magnetischen Verlustwerten und optimierten magnetostriktiven Eigenschaften.;

Description

[0001] The invention relates to a grain-oriented flat steel product with minimized magnetic loss values ​​and optimized magnetostrictive properties and a method for its production.

[0002] The grain-oriented steel flat products in question here, also known as "HGO material" in technical terms, are steel strips, also known simply as "electrical strips" in technical terms, or steel sheets, also known simply as "electrical sheets" in technical terms. Such flat steel products are used to manufacture parts for electrical applications.

[0003] Grain-oriented electrical steel strip or sheet is particularly suitable for applications where particularly low core loss is paramount and high permeability or polarization requirements are placed. Such requirements are particularly evident in parts for power transformers, distribution transformers, and high-performance small transformers.

[0004] As explained in detail, for example, in EP 1 025 268 B1, in the course of manufacturing flat steel products, a steel which typically contains (in wt. %) 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 optionally 0.010 to 0.3% Cu, up to 0.060% S, up to 0.100% P, up to 0.2% each of As, Sn, Sb, Te, and Bi, the remainder being iron and unavoidable impurities, is first cast into a starting material, such as a slab, thin slab, or cast strip. The starting material is then subjected to an annealing treatment if necessary, before being hot-rolled into a hot strip.

[0005] After coiling and an optional additional annealing, as well as an optional descaling or pickling treatment, the hot strip is then cold rolled in one or more steps. Intermediate annealing may be performed between the cold rolling steps if necessary. During the subsequent decarburization annealing, the carbon content of the cold strip is typically significantly reduced to prevent magnetic aging.

[0006] After decarburization annealing, an annealing separator, typically MgO, is applied to the strip surfaces. The annealing separator prevents the windings of a coil wound from the cold-rolled strip from sticking together during the subsequent high-temperature annealing. During the high-temperature annealing, which is typically carried out in a bell furnace under protective gas, the texture of the cold-rolled strip is created by selective grain growth. Furthermore, a forsterite layer, the so-called "glass film," forms on the strip surfaces. Furthermore, the steel material is purified by diffusion processes that occur during the high-temperature annealing.

[0007] Following high-temperature annealing, the resulting flat steel product is coated with an insulating layer and thermally straightened and stress-relieved in a final annealing step. This final annealing step can be performed before or after the flat steel product produced in the manner described above has been cut into the blanks required for further processing. A final annealing step after the blanks have been cut can relieve the additional stresses created during the cutting process. Flat steel products produced in this way typically have a thickness of 0.15 mm to 0.5 mm.

[0008] The metallurgical properties of the material, the degrees of deformation of the cold rolling processes used to produce the steel flat products, and the parameters of the heat treatment steps are all coordinated to ensure targeted recrystallization processes. These recrystallization processes lead to the material's typical "Goss texture," in which the direction of easiest magnetization lies in the rolling direction of the finished strip. Grain-oriented steel flat products accordingly exhibit strongly anisotropic magnetic behavior.

[0009] There are various methods for improving the core losses of a grain-oriented flat steel product. For example, the orientation sharpness of the Goss texture of the flat steel product can be improved. Further loss reductions can be achieved by reducing the distances between the 180° domain walls. High tensile stresses in the rolling direction, which are transferred to the steel surface via insulating coatings, also contribute to reducing the domain spacing and thus to a reduction in core losses. However, for technical reasons, the required tensile stress values ​​are only feasible to a limited extent.

[0010] A further possibility for reducing losses, proposed, for example, in DE 18 04 208 B1 or EP 0 409 389 A2, consists in creating partial plastic deformations on the surface of the flat steel product. This can be achieved, for example, by mechanically scoring or piercing the surfaces of the respective flat steel product. The significant improvements in magnetic properties achieved in this way are offset by the disadvantage that the mechanical processing of the surface damages the insulating layer applied to the flat steel product. This can, for example, lead to short circuits in the stacked core of the transformer and to local corrosion when producing transformer laminations from such a flat steel product.

[0011] Attempts to utilize the advantages of mechanical scribing or piercing without destroying the insulation have focused on the use of laser sources (EP 0 008 385 B1, EP 0 100 638 B1, EP 1 607 487 A1). Common to these laser-based processes is that a laser beam is focused on the surface of the flat steel product to be treated, generating thermal stress in the base material. This leads to the formation of dislocations at which components of the magnetic flux escape from the surface of the flat steel product. This locally increases the magnetic stray field energy, which is compensated for by the formation of so-called "termination domains," also referred to as "secondary structures" in technical terms. At the same time, a reduction in the main domain spacing occurs.

[0012] Since the abnormal core loss depends on the spacing of the main domains, the losses are minimized by suitable laser treatment. Laser treatment can improve the core loss of a grain-oriented flat steel product with a typical nominal thickness of 0.23 mm by more than 10% compared to the untreated state. The loss improvements depend both on the properties of the base material, such as grain size and texture sharpness, and on the laser parameters, which include the spacing L of the lines along which the laser beams are directed onto the respective flat steel product, the exposure time t dwell , and the specific energy density U s . The coordination of these parameters has a decisive influence on the achieved reduction in core loss.

[0013] In addition to core losses, noise generation also plays a role in transformers. This is based on a physical effect known as magnetostriction.

[0014] Magnetostriction is the change in length of a ferromagnetic material in the direction of its magnetization. Operating a ferromagnetic component, such as a transformer, in an alternating magnetic field shifts the 180° main domains, but this alone does not contribute to magnetostriction. However, magnetostrictive strains exist in the material at the transitions between the 180° main domains and the 90° terminal domains. These strains form a sound source during operation in an alternating magnetic field and are the cause of transformer noise.

[0015] The introduction of additional 90° termination domains, i.e. secondary structures, by laser treatment generally leads to an increase in magnetostriction and thus in noise emissions, particularly during operation of a transformer.

[0016] The requirements for minimizing noise during transformer operation are constantly increasing. This is due, on the one hand, to continuously tightening legal requirements and standards. On the other hand, consumers generally no longer accept electrical devices that produce audible "transformer hum." For example, the acceptance of large transformers near residential areas today depends crucially on the noise emissions generated during their operation.

[0017] A number of laser treatment processes have been proposed that, by selecting suitable process parameters, can achieve both loss reductions and improved magnetostrictive properties (DE 601 12 357 T2 / EP 1154 025 B1, DE 698 35 923 T2 / EP 0 897 016 B1, EP 2 006 397 A1, EP 1 607 487 A1). However, the optimization of the laser treatment parameters has always been undertaken only with a view to improving the remagnetization losses.

[0018] EP 4 261 853 A1 describes a method for producing a grain-oriented steel strip with low magnetostriction. The method involves first approximately determining the difference in curvature between one side and the other of the steel strip as a function of the laser etching power using a formula. Based on this difference, the application quantities of the respective insulating layer for each side are determined and then applied. The difference in curvature between one side and the other of the steel strip, caused by the single-sided laser etching, is reduced by adjusting a voltage difference between the insulating layer on one side and the other of the steel strip by applying different amounts of the insulating layer to the two sides, thereby also reducing the magnetostrictive deviation between the two sides.As a result, a thicker insulation layer is applied to the side not treated by laser etching than to the side treated with the laser.

[0019] A disadvantage of the procedure described in EP 4 261 853 A1 is that the formula used therein for the approximate calculation of the curvature ignores the fact that the forsterite layers also exert tensile stresses on the flat steel product, which lead to curvature and thus influence the overall curvature. Furthermore, determining the curvature of the steel strip to determine the application quantities for the respective insulation layer is industrially complex and cannot be easily integrated into a continuous manufacturing process. In particular, the inventors of EP 4 261 853 A1 failed to recognize that the transmittance of the forsterite layer for the laser wavelength plays a role in simultaneously achieving a desired magnetostriction and improving the core loss of a grain-oriented flat steel product.

[0020] Another disadvantage of this manufacturing process is that the grain-oriented flat steel product exhibits a lower insulating effect in the area of ​​the laser-etched side. This is due to the manufacturing process, which requires laser etching after application of the insulating layer. This at least partially removes the insulating layer on the steel substrate. This leads to a defective local insulating effect and presents the disadvantage for the manufacturer of an electrical machine or transformer that they must be meticulous about ensuring that the laser-treated sides of the sheets do not overlap to avoid a short circuit.

[0021] Furthermore, the removal of the insulation layer on the steel substrate results in the resulting grain-oriented flat steel product having a different surface roughness than conventional grain-oriented flat steel products. This different roughness results in a different slip behavior than conventional grain-oriented flat steel products when manufacturing machines or transformers from these grain-oriented flat steel products, particularly during automated installation work for the grain-oriented flat steel products. This different slip behavior means, among other things, that the manufacturer must work with single-grade materials and is not allowed to mix grain-oriented flat steel products from different manufacturers, as is otherwise generally the case.

[0022] Against the background of the prior art explained above, the object of the invention was to provide a method for producing a grain-oriented flat steel product which has optimized magnetostrictive properties while minimizing remagnetization losses and is optimally suited for the production of parts for transformers without significantly impairing the insulating effect of the grain-oriented flat steel product.

[0023] Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, as is the general inventive concept.

[0024] In accordance with the prior art explained above, a method according to the invention for producing a grain-oriented flat steel product comprises the steps of a) providing a cold-rolled flat steel product which has, in wt.%, the following composition: Si: 2.0 - 4.0 %, C: 0.01 - 0.10 %, Also: 0.01 - 0.065%, N: 0.003 - 0.015, optionally one or more elements selected from the group consisting of Se, Sn, Sb, wherein the individual contents of these elements are up to 0.2%, optionally one or more elements selected from the group consisting of Cr, Cu, Mn, wherein the individual contents of these elements are up to 0.60%, optionally one or more elements selected from the group consisting of As, Bi, B, Co, P, S, Te, Ti, V, Ni, Nb, Mo, wherein the individual contents of these elements are up to 0.05%, the remainder being iron and unavoidable impurities; b) primary recrystallisation annealing of the cold-rolled flat steel product with simultaneous decarburisation treatment in a humid atmosphere to a carbon content of less than 30 ppm; c) Optionally, carrying out a nitriding treatment during step b) or subsequently in step c); d) coating the flat steel product obtained in step b) or in optional step c) with a slurry, wherein the slurry consists of water, MgO and optionally one or more further solid(s) and has a mass ratio w1 of water to total solid(s) in the slurry; e) coiling the coated flat steel product into a coil, f) bell annealing of the coil at a soaking temperature of at least 1100°C with a dew point dp 1, measured between the coil turns at 400°C during heating to the soaking temperature, and a dew point dp 2, measured between the coil turns at 800°C during heating to the soaking temperature, g) applying an insulating coating to the annealed flat steel product, h) stress relieving the flat steel product provided with the insulation coating to form an insulation layer, i) performing domain refinement by means of a laser having a wavelength WL on at least one side of the flat steel product provided with the insulation layer, wherein the wavelength WL of the laser satisfies the specification of the following formula (I): 537+60×dp1+10×dp2w1 <WL<1080+18×dp1+3×dp2w1

[0025] The process according to the invention may comprise further steps which are known to the person skilled in the art and which are usually carried out in the production of grain-oriented flat steel products.

[0026] The invention is based on the finding that optimized magnetostrictive properties with simultaneously minimized core losses of a grain-oriented electrical steel strip according to the invention can be ensured by carefully controlling the conditions in process steps d), f), and i). In particular, minimized core losses of less than 0.75 W / kg, determined according to IEC 60404-3 (2022) at 50 Hz and 1.7 T with a conversion factor of 0.925 according to IEC 60404-8-7 (2020), can be achieved.

[0027] It was found that the level of laser energy input during the domain refinement performed in step i) is influenced by the transmittance of the forsterite layer for the laser wavelength selected for domain refinement. It was discovered that the transmittance of the forsterite layer can be controlled by adjusting the dew point during the bell annealing in step f). The dew point in step f) is further influenced by the water content in the slurry applied in step d).

[0028] The higher the transmittance of the forsterite layer for the laser wavelength selected for domain refinement in step i), the lower the laser energy loss in the forsterite layer. Consequently, domain refinement due to the laser's thermal shock occurs in deeper, less surface-close regions of the grain-oriented flat steel product at the higher transmittance of the forsterite layer achieved by the method according to the invention.

[0029] In order to achieve a transmittance of the forsterite layer for the selected laser wavelength that is sufficient for the purposes of the invention, the condition of formula (I) must be observed in the process according to the invention.

[0030] The inventors have recognized that this can, on the one hand, reduce the magnetization losses of the grain-oriented flat steel product, and, on the other hand, result in a significantly lower increase in magnetostriction due to domain refinement.

[0031] By combining the measures according to the invention in the production of the grain-oriented flat steel product, in particular in steps d), f) and i) of the process according to the invention and maintaining the condition according to formula (I), reliably improved remagnetization losses of the grain-oriented electrical steel strip can be achieved with simultaneously optimized magnetostrictive properties.

[0032] There are no special requirements regarding the method of manufacturing the cold-rolled flat steel product provided according to step a). Thus, the cold-rolled flat steel product provided for the process according to the invention can be manufactured using the measures generally known to those skilled in the art, summarized at the beginning, which are also already sufficiently known from the prior art. This naturally also includes manufacturing processes that are currently unknown. The cold-rolled flat steel product can be manufactured in particular by casting a steel with a suitable alloy to form a starting material, such as a slab, thin slab, or cast strip, which is subjected to an annealing treatment and subsequently hot-rolled into a hot strip. The hot strip can be coiled in a known manner and optionally annealed and subjected to a descaling or pickling treatment.Subsequently, a cold-rolled flat steel product can be produced from the hot strip in one or more steps by cold rolling, with intermediate annealing being carried out between the cold rolling steps if necessary. Methods for cold rolling a grain-oriented steel strip are generally known to those skilled in the art and are described, for example, in WO 2007 / 014868 A1 and WO 99 / 19521 A1. Typically, intermediate annealing is carried out in a temperature range from 700 to 1150 °C, preferably 800 to 1100 °C, in an atmosphere whose dew point is set at 10 to 80 °C. Typical annealing times are 30 s to 900 s. Systems with which such intermediate annealing can be carried out are generally known and are described, for example, in WO 2007 / 014868 A1 and WO 99 / 19521 A1.The thickness of the cold-rolled flat steel product is typically 0.15 to 0.5 mm, particularly preferred is a maximum thickness of 0.35 mm, preferably of at most 0.27 mm or of at most 0.23 mm.

[0033] The cold-rolled flat steel product provided in step a) by the process according to the invention contains 2.0 to 4.0 wt.% silicon (Si). A silicon content of 2.5 to 3.5 wt.% has proven particularly advantageous with regard to the magnetic properties of a grain-oriented flat steel product according to the invention. Si is required to improve the permeability of the grain-oriented flat steel product. A Si content below 2.0 wt.% is not sufficient to achieve high permeability and thus low magnetic reversal loss. Furthermore, a Si content below 2.0 wt.% is disadvantageous for the formation of a forsterite layer because, at such low silicon contents, too little fayalite (FeSiO 4 ) is formed, from which the forsterite layer is formed during batch annealing. If the Si content is above 4.0 wt.%, the processability of the flat steel product deteriorates, i.e.The flat steel product becomes more brittle and exhibits an increased tendency to crack during processing, such as cold rolling. Furthermore, the microstructural transformation from ferrite to austenite (alpha / gamma transformation) is suppressed.

[0034] The amount of carbon (C) in the cold-rolled flat steel product provided in step a) of the process according to the invention is 0.01 to 0.10 wt.%, particularly preferably 0.03 to 0.08 wt.%. C is used to improve the hot-rolled microstructure of the steel by promoting the formation of austenite. Furthermore, C is required during cold rolling to inhibit dislocation movements and thus promote recrystallization. Therefore, the C content should be at least 0.01 wt.%. However, excessive C contents above 0.10 wt.% lead to problems during decarburization annealing, and residual C in the finished grain-oriented flat steel product leads to increased core loss and should therefore preferably be avoided.

[0035] The cold-rolled flat steel product provided in step a) by the process according to the invention contains 0.01 to 0.065 wt.% acid-soluble aluminum (Al sl ). An aluminum content of 0.015 to 0.050 wt.% has proven particularly advantageous with regard to an optimal content and an optimal grain size of inhibitor particles, which inhibit grain growth and lead to favorable grain orientation of the finished grain-oriented flat steel products. Aluminum contents below 0.01 wt.% lead to few inhibitor particles and thus to weak inhibition of grain growth during batch annealing. An excessively high aluminum content of more than 0.065 wt.% leads to coarse inhibitor particles, which also exhibit weak inhibition.

[0036] Nitrogen (N) is present in the cold-rolled steel provided in step a) of the process according to the invention in an amount of 0.003 to 0.015 wt.%, in particular 0.0035 to 0.013 wt.% N. N is required as an inhibitor-forming element, which, together with Al, leads to the formation of AlN. If the N content is below 0.003 wt.%, the inhibition is insufficient. N contents higher than 0.015 wt.% lead to rolling problems and poor surface quality.

[0037] The cold-rolled flat steel product provided in step a) optionally contains one or more elements selected from the group consisting of Se, Sn, and Sb, with the individual contents of these elements being up to 0.2 wt.%. It has proven particularly practical if the individual contents of the aforementioned elements are at least 0.002 wt.%.

[0038] Tin (Sn) improves magnetic quality by stabilizing the formation of oxide layers and the forsterite film (glass film) and can be present in the steel composition at a minimum of 0.002 wt.%. A Sn content of more than 0.2 wt.% reduces oxidation and prevents the formation of a stable forsterite film (glass film).

[0039] Antimony (Sb) can optionally be included in cold-rolled flat steel products as a segregation element that disrupts grain boundary movement. It inhibits grain growth and thus influences recrystallization toward the formation of a desired final Goss texture in the finished grain-oriented flat steel product. The aforementioned positive effects are reliably achieved at a content of 0.002 wt.% or higher. If the Sb content is increased to more than 0.2 wt.%, processability deteriorates and the likelihood of strip tears during rolling increases. Furthermore, the oxidation-inhibiting effect of antimony leads to the formation of an uneven forsterite film. This has a negative impact on the magnetic properties of the grain-oriented flat steel product.

[0040] Selenium (Se) combines with Mn to form MnSe, which acts as a recrystallization inhibitor and supports the formation of the desired Goss texture. This improves the magnetic loss of the grain-oriented flat steel product. A Se content below 0.002 wt.% does not have a positive effect on the formation of MnSe particles. A Se content above 0.2 wt.% results in coarse particles whose inhibitory effect is insufficient to effectively prevent recrystallization.

[0041] Furthermore, the flat steel product provided in step a) may optionally contain one or more elements selected from the group consisting of Cr, Cu, and Mn, with the individual contents of these elements being up to 0.60 wt.%. Preferably, the individual contents of these elements are at least 0.002 wt.%.

[0042] The cold-rolled steel provided in step a) of the process according to the invention preferably contains 0.002 to 0.60 wt.% manganese (Mn). A manganese content of 0.05 to 0.3 wt.%, preferably 0.05 to 0.25 wt.%, has proven particularly advantageous. The addition of at least 0.01 wt.% Mn reduces the core loss by increasing the resistivity of the grain-oriented flat steel products and improves the hot formability of the flat steel product. A Mn content above 0.5 wt.% reduces the magnetic flux density of the grain-oriented flat steel products and should therefore preferably be avoided.

[0043] Like Sn, Cr improves the magnetic quality by further stabilizing the formation of oxide layers and the forsterite layer (glass film). These positive effects are clearly evident at a Cr content of 0.002 wt.% and above. A Cr content of more than 0.60 wt.% reduces the degree of oxidation, preventing the formation of a stable forsterite layer.

[0044] Cu reduces the degree of oxidation and stabilizes secondary recrystallization during batch annealing. Furthermore, Cu forms CuS precipitates, which act as grain growth inhibitors during batch annealing, thereby promoting favorable secondary recrystallization. To effectively improve magnetic quality, the Cu content should be at least 0.002 wt%, while a Cu content exceeding 0.60 wt% impairs hot rollability due to the formation of hard particles.

[0045] Finally, the flat steel product provided in step a) may optionally contain one or more elements selected from the group consisting of As, Bi, B, Co, P, S, Te, Ti, V, Ni, Nb, and Mo, with the individual contents of these elements being up to 0.05 wt.%. The individual contents of these elements are preferably at least 0.0003 wt.%.

[0046] Sulfur (S) is required as a component of the inhibitors MnS and CuS, which promote stable secondary recrystallization with preferential orientation of the crystal grains in {110} <001> This supporting effect becomes noticeable at an S content of at least 0.0003 wt.%. Sulfur contents above 0.05 wt.%, in turn, lead to cleaning during batch annealing taking too long or not being able to take place sufficiently, so that S-containing inhibitor particles remain, which negatively affect the magnetic properties of the grain-oriented flat steel product.

[0047] Titanium (Ti) reacts with nitrogen to form TiN particles. Due to the stability of the particles at high temperatures, TiN primarily acts as an inhibitor for secondary recrystallization during batch annealing. A Ti content below 0.0003 wt.% does not provide the desired inhibition support. A Ti content above 0.05 wt.%, on the other hand, would result in excessively long cleaning during batch annealing, making it uneconomical, and / or leaving too many particles behind after batch annealing, which would in turn worsen the core losses.

[0048] Molybdenum (Mo) suppresses high-temperature corrosion by forming a thin layer of MoSi2 on the surface of the steel strip. This beneficial effect is clearly evident at a Mo content of 0.0003 wt.% or higher. However, if the Mo content exceeds 0.05 wt.%, no further improvement in this effect occurs.

[0049] Phosphorus (P) increases the resistivity of grain-oriented flat steel products, thus reducing core loss. This effect can be reliably achieved in practice at a P content of 0.0003 wt.% or higher. A P content above 0.05 wt.% leads to poor cold rollability.

[0050] Bismuth (Bi) stabilizes precipitates such as MnS or CuS, thus supporting the inhibition effect. This positive effect only reliably occurs at a content of 0.0003 wt.%. A Bi content above 0.05 wt.% impairs the formation of a forsterite film (glass film) and thus the magnetic properties of the finished grain-oriented flat steel product.

[0051] Ni, Co, As, B, Te, V, and Nb support the inhibition effect during secondary recrystallization and thus have a positive influence on core loss. This effect is reliably achieved at a content of 0.0003 wt.% each. Excessive contents of these elements, above 0.05 wt.% each, lead to cleaning problems during batch annealing and, in some cases, to difficulties with cold rolling.

[0052] According to a preferred embodiment of the process according to the invention, in step a) a cold-rolled flat steel product is provided which has, in wt.%, the following composition: Si: 2,0 - 4,0 %, C: 0,01 - 0,10 %, Al sl : 0,01 - 0,065 %, N: 0,003 - 0,015 %, Mn: 0.002 to 0.60%, Cu: 0.002 to 0.60%, P: 0.0003 to 0.05%, optionally one or more elements selected from the group consisting of Se, Sn, Sb, where the individual contents of these elements are up to 0.2%, optionally up to 0.60% Cr, optionally one or more elements selected from the group consisting of As, Bi, B, Co, S, Te, Ti, V, Ni, Nb, Mo, where the individual contents of these elements are up to 0.05%,

[0053] Rest iron and unavoidable impurities.

[0054] Preferably, the cold-rolled flat steel product provided in step a) of the process according to the invention contains a total of not more than 0.5 wt.%, in particular not more than 0.3 wt.%, of unavoidable impurities.

[0055] In step b) of the process according to the invention, a primary recrystallization annealing with simultaneous decarburization treatment (collectively also referred to as "decarburization annealing") of the cold-rolled flat steel product provided in step a) is carried out in a humid atmosphere to a carbon content, determined according to ASTM E 1019:2018, of less than 30 ppm. The decarburization annealing is preferably carried out at temperatures in the range of 600 to 950 °C, particularly preferably 600 to 900 °C. Temperatures in the range of 600 to 950 °C are required to form the desired oxides, in particular fayalite and silicon dioxide, which are required for the formation of the forsterite layer in step f). In this context, it has proven particularly practical if the temperature during decarburization annealing is at least 820 °C. The duration of the decarburization annealing is preferably 30 to 300 s, preferably 70 to 200 s.If the duration is too short, less than 30 seconds, sufficient oxide formation cannot be guaranteed. Durations of more than 300 seconds result in the resulting oxide layer becoming too thick, and all surface-controlled chemical reactions, such as decarburization, nitriding, and denitriding, can no longer proceed in a controlled manner. Decarburization annealing is generally carried out in a high-dew point atmosphere ("humid atmosphere") with a dew point between 40 and 80 °C, preferably between 40 and 65 °C. A dew point of more than 80 °C leads to a high proportion of oxidized iron, which negatively affects the reactivity of the oxide layer. At a dew point below 40 °C, the resulting oxide layer becomes too dense, meaning that all surface-controlled chemical reactions, such as decarburization, nitriding, and denitriding, can no longer proceed in a controlled manner. The atmosphere can consist of 5 to 95 vol.-% H 2 , the remainder being nitrogen or any inert gas or a mixture of nitrogen and one or more inert gases.

[0056] The process according to the invention optionally provides for the performance of a nitriding treatment during the primary recrystallization annealing with simultaneous decarburization treatment in step b) or subsequently in step c). If a nitriding treatment is to be carried out during the decarburization annealing in step b), the decarburization annealing can be carried out in an atmosphere containing N 2 or N-containing compounds, for example NH 3 . Decarburization annealing and nitriding can alternatively be carried out successively in two separate steps, with the decarburization annealing taking place first. A degree of nitriding of at least 150 ppm is preferably set. A degree of nitriding that is too low, below 150 ppm, leads to a weak inhibition system and impairs secondary recrystallization, which is crucial for the magnetic properties of the finished grain-oriented electrical steel sheet.If nitriding is carried out after decarburization annealing in step c) of the process according to the invention, the conditions of the nitriding treatment should also be adjusted so that a nitriding degree of at least 150 ppm is achieved. Typically, an annealing temperature of between 700 and 900 °C is set and a dew point below 40 °C is selected. The annealing time is usually between 10 and 200 s. The N, N 2 and NH 3 contents are selected so that the desired degree of nitriding is ensured. This also applies to nitriding treatment carried out simultaneously with decarburization annealing in step b). The degree of nitriding is calculated from the difference between the nitrogen content of the steel strip before stress relief annealing (step h) minus the nitrogen content before decarburization annealing (step b). The nitrogen content can be determined in a conventional manner, e.g. using a test kit supplied by Leco Corporation, St.Joseph, USA, offered analyzer 736.

[0057] In step d) of the process according to the invention, the flat steel product obtained in step b) or in the optional step c) is coated with a slurry, wherein the slurry consists of water, MgO, and optionally one or more other solids and has a mass ratio w1 of water to total solid(s) in the slurry. Preferably, a mass ratio w1 to the total solid(s) in the slurry is 5 to 11. A mass ratio w1 of water to the total solid(s) in the slurry of less than 5 leads to uneven application of the slurry and increases the risk of clogged plant components required for slurry application. A mass ratio w1 of water to the total solid(s) in the slurry of more than 11 leads to long drying times for the slurry and to excessively high dew points in the bell annealing in step f).Excessively high dew points are disadvantageous because they lead to the formation of oxides with a high degree of oxidation, such as MgSiOs, which in turn prevent uniform forsterite film formation (glass film formation). For the purposes of the invention, MgO is counted as the solids of the slurry. When reference is made below to the total solid(s) of the slurry, this means the content of all solids in the slurry, including MgO. According to a preferred embodiment of the process according to the invention, further solids in addition to MgO in the slurry can be selected from oxides and / or nitrides of at least one element selected from Al, Cr, Fe, Mn, Si, Ti, Mg, Sn, Cu, Zn, Zr and mixed oxides of the aforementioned oxides with Mg. MgO and the optionally present further solids in the slurry act as nucleation sites to facilitate the phase transition from fayalite to forsterite in step f) of the process according to the invention.The slurry may comprise, based on the total solids content of the slurry, at least 70 wt.% MgO, optionally up to 25 wt.% oxides and / or nitrides of at least one element selected from Al, Cr, Fe, Mn, Si, Ti, Mg, Sn, Cu, Sn, Zr, and mixed oxides of said oxides with Mg, and may further contain up to 5 wt.% additives, based on the total solids content of the slurry. These additives may be, for example, elements such as Ca, B and Sr, ammonium chloride or antimony chloride, and other salts such as magnesium sulfate or sodium chloride, the addition of which controls the density of the forsterite layer formed after the bell annealing in step f) and the gas exchange between the annealing atmosphere and the metal during the bell annealing in step f).

[0058] In step e) of the process according to the invention, the flat steel product coated in step d) is coiled into a coil. The coil tension is preferably between 30 and 300 MPa. A coil tension of less than 30 MPa would lead to coil collapse due to excessive free volumes between the coil turns, and the dew point during the batch annealing would be significantly lowered because the entire water content of the slurry would immediately diffuse out of the coil turns. At a coil tension of more than 300 MPa, the dew point would be significantly higher during the batch annealing because the moisture from the water content of the slurry cannot diffuse out of the coil turns.

[0059] The coil obtained in step e) is annealed in step f) in a bell furnace at a soaking temperature of at least 1100°C with a dew point dp 1, measured between the turns of the coil at 400°C during heating to soaking temperature, and a dew point dp 2, measured between the turns of the coil at 800°C during heating to soaking temperature.

[0060] The dew point dp1 at 400 °C is an indicator of the initial oxidation of the surface by water of crystallization. The dew point dp2 at 800 °C, in turn, is crucial for preventing the incorporation of impurities into the olivine.

[0061] Since the temperature in the coil is not homogeneously distributed during step f), thermocouples are preferably distributed over the coil, as in Figure 1 shown. Dew point measuring devices are also installed at the same location in the coil.

[0062] The dew points dp1 and dp2 can be determined individually using each of these dew point measuring devices by extracting the gas from the windings with pipes of various defined diameters (6, 4, 2 and 1 mm) at 400°C and 800°C respectively and extrapolating these actually determined dew point measured values ​​to the value 0 mm. To do this, the dew point measured values ​​obtained for the defined pipe diameters at 400°C and 800°C are entered into a diagram as a function of the respectively defined pipe diameter. The y-axis of the diagram indicates the dew point in °C and the x-axis the diameter of the pipe in mm. The dew point measured values ​​obtained for the respectively defined pipe diameters at 6, 4, 2 and 1 mm and entered into the diagram are extrapolated to the intersection point with the y-axis. By extrapolation, the dew point is obtained at a theoretical pipe diameter of 0 mm, which corresponds to the dew point dp1 or dp2 to be used in formula (I).dp2 corresponds. To calculate the extrapolation for the theoretical pipe diameter of 0 mm, the dew points at the respective pipe diameters 6, 4, 2 and 1 mm must first be entered into a regular data software program such as MS Excel in order to determine the logarithmic trend line. The formula for the logarithmic trend line calculated in MS Excel is of the form y=a.ln(bx)+c. To calculate the dew point at 0 mm, the function must now be shifted, since the logarithm of 0 is undefined and thus no realistic value for the dew point can be obtained. To do this, the formula given by MS Excel is modified in such a way that one obtains: y=a·ln(bx+1)+c, i.e. the term bx in the brackets of the logarithm is supplemented by +1. The dew point at 0 mm therefore approximately corresponds to the c determined from the logarithmic trend line.

[0063] A significant difference between the implementation of the hood annealing in step f) of the method according to the invention and the prior art is that the hood annealing process is usually controlled based on the hood temperature (see, for example, EP 2 963 130 B1). However, due to the large dimensions of a coil and the associated local differences in the coil with regard to temperature and dew point during the hood annealing, precise and accurate regulation and control of the process is not possible. The determination of the dew points and the actual temperature at specific positions in the coil by means of thermocouples installed at these positions in the coil, as in Figure 1 shown, allows a more accurate and precise regulation and control of the process.

[0064] In step f) of the process according to the invention, secondary recrystallization takes place with the formation of a forsterite layer. The coil obtained in step e) can, for example, be heated rapidly in step f) to a maximum soaking temperature of 1150°C or above, with maximum soaking temperatures of at least 1200°C being particularly advantageous. The heating and soaking preferably take place under a protective gas atmosphere comprising, for example, H 2. More preferably, the heating to and the soaking at the respective soaking temperature takes place under an atmosphere containing 5 to 100 vol.% H 2, preferably 50 to 100 vol.% H 2, the remainder nitrogen or any desired inert gas or a mixture of nitrogen and one or more desired inert gases. The soaking time during which the bell annealing is carried out in this way can be determined in a conventional manner well known to those skilled in the art.Soaking times between 10 and 200 hours are common. Soaking times of at least 10 hours ensure that atoms of elements such as S and N are sufficiently removed, the remaining atoms of which would otherwise impair the properties of the grain-oriented electrical steel. A duration of more than 200 hours would be uneconomical; it would lead to a reduction in the olivine formed in the previous heating phase of the bell annealing and the proportion of silicon dioxide formed would increase. This would have a negative impact on the formation of the forsterite layer (the glass layer). The bell annealing in step f) of the process according to the invention can, for example, be carried out for at least 10 hours at a maximum soaking temperature of up to 1247 °C under an atmosphere of at least 50% H 2.

[0065] The batch annealing in step f) of the process according to the invention is preferably carried out for an annealing time of 10-200 hours in a 100% H2 atmosphere. One or more holding stages can be incorporated during heating to maximum temperature in order to equalize the temperature in the coil and avoid temperature gradients. Typical average heating rates to maximum temperature are between 5 K / h and 50 K / h. Heating rates below 5 K / h prove to be negative in terms of productivity, and in addition, the grains can become too large even before secondary recrystallization, thus reducing the driving forces for secondary recrystallization. Average heating rates above 50 K / h can lead to inhomogeneous temperature distribution in the coil and thus to inhomogeneous product properties.

[0066] Cooling after the hood annealing in step f) of the process according to the invention can also be carried out in a controlled manner. Average cooling rates of less than 50 K / h have proven advantageous. Cooling can take place with or without a hood in a controlled atmosphere or in an ambient atmosphere. Average cooling rates above 50 K / h can lead to strain in the material and thus to poorer processability in subsequent processes and increased core losses.

[0067] According to a preferred embodiment of the process according to the invention, the steel strip is cleaned and, if appropriate, pickled after step f) and before step g). Methods for pickling the steel strip are known to those skilled in the art. For pickling, the steel strip can be treated with an aqueous acidic solution. Suitable acids include phosphoric acid, sulfuric acid, and / or hydrochloric acid.

[0068] In step g) of the process according to the invention, an insulating coating is applied to the flat steel product annealed in step f) and optionally subsequently cleaned and optionally pickled. The insulating coating is preferably applied to at least one side of the flat steel product. The process for applying the insulating coating is known to the person skilled in the art and can be found, for example, in EP 2 902 509 B1 and EP2 954 095 A1. To form the insulating coating, an aqueous solution comprising colloidal silica (colloidal silicon dioxide) and at least one phosphate, nitrate and / or oxide containing at least one element selected from Al, Mn, Si, Ti, Mg, Sn and Cr is preferably applied to the surface of the steel sheet. According to a preferred embodiment of the process according to the invention, the insulating coating is applied to both sides of the flat steel product.Particularly preferably, the application weight of the insulation coating is the same on both sides. It goes without saying that, as described above, the same application weight means that the application weight of the insulation coating is the same, subject to unavoidable deviations due to production technology.

[0069] In step h) of the method according to the invention, the flat steel product provided with the insulation coating is stress-relieved to form the insulation layer. This can be carried out at temperatures in the range of 800°C to 950°C for 10 to 600 s. At the same time, the insulation coating applied in step g) is baked to form the insulation layer. An insulation layer applied to a grain-oriented flat steel product has a positive effect on minimizing hysteresis losses. The insulation layer can transfer tensile stresses to the base material, which not only improves the magnetic loss values ​​of the grain-oriented flat steel product, but also reduces magnetostriction and has a positive effect on the noise behavior of the finished transformer. The insulation layer is completely transparent to the laser used in step i), i.e., it has a transmittance of 100%.

[0070] According to step i) of the method according to the invention, domain refinement is carried out on at least one side of the flat steel product provided with the insulation layer by means of a laser with a wavelength WL, wherein the wavelength WL of the laser satisfies the specification of the following formula (I): 537 + 60 × dp 1 + 10 × dp 2 w 1 < WL < 1080 + 18 × dp 1 + 3 × dp 2 w 1

[0071] The selection of a laser with a wavelength WL that satisfies the above-mentioned formula (I) for the domain refinement according to step i) of the method according to the invention results in the laser wavelength being adapted to the transmittance of the forsterite layer obtained in step f) in such a way that a grain-oriented flat steel product according to the invention with optimized magnetostrictive properties and, at the same time, minimized remagnetization losses can be reliably obtained.

[0072] The method for domain refinement by laser beam treatment is known to those skilled in the art and can be found, for example, in EP 2 675 927 A1. For example, during laser treatment, a laser beam emitted by a laser beam source imparts linear deformations into the surface of the flat steel product at a distance, thereby reducing the width of the domains and the losses of the grain-oriented electrical steel sheet. Preferably, the domain refinement in step i) is carried out transversely to the rolling direction and at predefined intervals in the rolling direction.

[0073] The invention further relates to a grain-oriented flat steel product comprising a steel core which has, in % by weight, the following composition: Si: 2.0 - 4.0 %, C: < 30 ppm Al sl : < 30 ppm, N: < 50 ppm, optionally one or more elements selected from the group consisting of Se, Sn, Sb, the individual contents of these elements being up to 0.2%, optionally one or more elements selected from the group consisting of Cr, Cu, Mn, the individual contents of these elements being up to 0.60%, optionally one or more elements selected from the group consisting of As, Bi, B, Co, Te, Ti, V, Ni, Nb, Mo, the individual contents of these elements being up to 0.05%, optional S: < 50 ppm P: 0.0003% to 0.05% Rest iron and unavoidable impurities; and on at least one side of the steel core a forsterite layer in full-surface contact therewith, wherein the side of the forsterite layer facing away from the steel core is in full-surface contact with an insulation layer, wherein at least one side of the grain-oriented flat steel product has been treated with a laser for domain refinement, characterized in that the difference in magnetostriction, measured according to IEC 60404-17 (2021) with a mirror, between one side of the grain-oriented flat steel product and the other side of the grain-oriented flat steel product is less than 1 dB(A) and wherein the grain-oriented flat steel product has a core loss, measured according to IEC 60404-3 (2022) at 50 Hz and 1.7 T and a conversion factor of 0.925 according to IEC 60404-8-7 (2020), of at most 0.75 W / kg.

[0074] According to a particularly preferred embodiment of the grain-oriented flat steel product according to the invention, the steel core has the following composition in wt.%: Si: 2,0 - 4,0 %, C: < 30 ppm, Al sl : < 30 ppm, N: < 50 ppm Mn: 0.002 to 0.60%, Cu: 0.002 to 0.60%, optionally one or more elements selected from the group consisting of Se, Sn, Sb, the individual contents of these elements being up to 0.2%, optionally up to 0.60% Cr, optionally one or more elements selected from the group consisting of As, Bi, B, Co, Te, Ti, V, Ni, Nb, Mo, the individual contents of these elements being up to 0.05%, optional S: < 50 ppm P: 0.0003% to 0.05% Rest iron and unavoidable impurities.

[0075] The chemical composition of the steel core of the finished grain-oriented flat steel product differs from the chemical composition of the cold-rolled flat steel product used in the process according to the invention with regard to the contents of Al sl , C, N and S. For the other elements as well as the maximum content of unavoidable impurities, what was said above in connection with the cold-rolled flat steel product applies accordingly.

[0076] The Al sl content in grain-oriented flat steel products is lower than in cold-rolled flat steel products. After nitriding, Al sl is present in cold-rolled flat steel products primarily as AlN inhibitor particles. During high-temperature annealing, the particles dissolve, Al diffuses to the steel surface, and combines with other elements to form mixed compounds called spinels. These are not acid-soluble, which is why the Al sl content in the steel core of grain-oriented flat steel products is limited to 30 ppm.

[0077] Since the cold-rolled flat steel product is subjected to decarburization annealing to produce the grain-oriented flat steel product, the carbon content decreases and its maximum content is limited to 30 ppm in the steel core of the grain-oriented flat steel product.

[0078] The maximum nitrogen content in the steel core of the grain-oriented steel flat product is limited to 50 ppm, since the cold-rolled steel flat product can be subjected to nitriding to produce the grain-oriented steel flat product, but is subsequently also subjected to purification during batch annealing.

[0079] At the same time, the flat steel product is purified of sulfur during batch annealing, so that the maximum sulfur content in the steel core of the grain-oriented flat steel product is 50 ppm.

[0080] Because the flat steel product is coated with a phosphate-containing insulation layer, the maximum phosphorus content in the grain-oriented flat steel product is higher than in the cold-rolled flat steel product used for production. The P content in the steel core of the grain-oriented flat steel product does not change compared to the cold-rolled flat steel product used for production.

[0081] Domain refinement on both sides can lead to an increase in the average magnetostriction value and possibly also to an increase in the hysteresis loss, and thus to an increase in the total magnetization loss. Therefore, it is preferable to treat only one of the two sides of the grain-oriented flat steel product with a laser for domain refinement. The inventors have recognized that this results in a different magnetostriction between the treated and untreated sides of the grain-oriented flat steel product, whereas in untreated grain-oriented flat steel products, this magnetostriction is almost identical on both sides.

[0082] In the course of further investigations, it was surprisingly found that the difference in magnetostriction between the two sides of a grain-oriented steel flat product has a significant influence on the noise generation when the grain-oriented steel flat product is used in a transformer.

[0083] The magnetostriction difference is the absolute value of the magnetostriction, measured according to IEC 60404-17 (2021) using a mirror, between one side of the grain-oriented steel flat product and the other side of the grain-oriented steel flat product. It goes without saying that this difference can be negative or positive depending on the direction of the value deduction. However, for the purposes of the invention, only the resulting absolute value is considered as such, without the respective sign. Therefore, if the determined magnetostriction difference between the two sides of the grain-oriented steel flat product is "-2," this is considered a magnetostriction difference of "2" according to the invention. The same applies if the determined magnetostriction difference between the two sides of the grain-oriented steel flat product is "+2."

[0084] It was found that the noise level can be positively influenced by keeping the difference in magnetostriction, measured according to IEC 60404-17 (2021) with a mirror, to less than 1 dB(A), i.e. as low as possible.

[0085] This can be achieved, for example, by producing the grain-oriented flat steel product using the method according to the invention. Alternatively, it is also possible to obtain the grain-oriented flat steel product according to the invention by using conventional manufacturing processes for grain-oriented flat steel products and adjusting the magnetostriction difference, measured according to IEC 60404-17 (2021) using a mirror, to a value of less than 1 dB(A) by other suitable measures.

[0086] According to a preferred embodiment of the grain-oriented flat steel product according to the invention, it comprises a steel core having the composition specified above, and a forsterite layer on both sides of the steel core in full-surface contact therewith, wherein the side of the respective forsterite layer facing away from the steel core is in full-surface contact with an insulation layer, wherein at least one side of the grain-oriented flat steel product has been treated with a laser for domain refinement, characterized in that the difference in magnetostriction, measured according to IEC 60404-17 (2021) with a mirror, between one side of the grain-oriented flat steel product and the other side of the grain-oriented flat steel product is less than 1 dB(A), and wherein the grain-oriented flat steel product has a core loss, measured according to IEC 60404-3 (2022) at 50 Hz and 1.7 T and a conversion factor of 0.925 according to IEC 60404-8-7 (2020), of a maximum of 0.75 W / kg. Preferably, the thickness of the two insulation layers is almost identical, i.e., the difference between the layer thicknesses of the insulation layers on both sides of the grain-oriented steel flat product is a maximum of 25%, preferably a maximum of 15%.

[0087] According to a further preferred embodiment of the grain-oriented flat steel product according to the invention, it has a change in the magnetic loss PL for which the following applies: P 1 − P 0 P 0 ⋅ 100 % = P L ≥ 7 % , where P 0 is the core loss after laser treatment, P 1 is the core loss after annealing treatment for 30 min and PL is the change in core loss due to laser treatment, all measured according to IEC 60404-8-7 (2020) at 50 Hz and 1.7 T.

[0088] In order to determine the improvement in core loss due to domain refinement using laser treatment, the core loss must be measured before and after the laser treatment. This is not possible in a continuous furnace for performing steps g) and h) with subsequent laser treatment for domain refinement (step i)), as this would require interrupting the continuous process and cutting the grain-oriented flat steel product. Furthermore, measurements of core loss with a closed yoke, as required by IEC 60404-2 or -3, cannot be performed in a continuous furnace. For this reason, the improvement in core loss due to laser treatment is determined by cutting the grain-oriented flat steel product into samples measuring 610 mm x 100 mm after step i) of the inventive process and subjecting it to an annealing treatment.The samples are annealed for 30 minutes at 850 °C in a 100% N-containing atmosphere and then cooled at approximately 30 K / h. Since the improvements in core loss induced by the laser treatment are not thermally stable, any improvement in core loss induced by the laser treatment is canceled out by the annealing treatment.

[0089] By determining the difference in core losses before and after annealing, the change in core losses PL can be determined using the formula given above. This represents a measure of the improvement in core losses due to laser treatment. An advantage of this method is that it allows comparison with other grain-oriented domain-refined flat steel products.

[0090] According to a further preferred embodiment of the grain-oriented flat steel product according to the invention, its surface profile across the laser treatment area, i.e. in the rolling direction, over a measuring distance of 2000 µm, has on average no depressions deeper than 0.8 µm, preferably deeper than 0.3 µm. Such depressions, which are deeper than 0.8 µm, are usually found on grain-oriented flat steel products of the prior art. The depressions in the surface profile are measured using a Keyence VK-X3000 3D laser scanning microscope across the laser treatment area, i.e. in the rolling direction, over a measuring range of 2000 µm. The difference between the mean height of the non-laser treated, i.e. non-domain refined, area of ​​the sample surface and the mean height of the domain refined area of ​​the sample surface gives the height of the depression created by the laser.

[0091] Finally, the grain-oriented flat steel product according to the invention is further preferably characterized in that the insulating effect of the insulating layer system is maintained by the described laser treatment, i.e. an insulating effect of at least 50 Ωcm 2< , measured according to IEC 60404-11 (2021), is achieved in the area of ​​the laser treatment.

[0092] The invention is explained in more detail with reference to the following figure. Fig. 1: Cross section of a coil with built-in measuring instruments for temperature and dew point.

[0093] In Figure 1is a schematic representation of how the thermocouples 2, 3, 4 for measuring the temperature and the dew point measuring devices 5, 6 and 7 for determining the dew points dp1 and dp2 in step f) of the method according to the invention can be mounted in the coil 1. The temperature is measured, for example, by means of the thermocouples 2, 3, 4, which are mounted centrally above the coil height H and at three points over the coil radius R, evenly distributed at ¼ (see thermocouple 2), ½ (see thermocouple 3), and ¾ (see thermocouple 4) of the coil height H. As soon as the temperature at the respective thermocouple 2, 3, 4 reaches the value 400°C or 800°C, the dew points dp1 or dp2 are measured by suction through pipes of a defined diameter of 1, 2, 4 and 6 mm using the dew point measuring device 5, 6, 7 which is also attached at the same point.dp2 for formula (I), it is sufficient if the measurement is carried out on a thermocouple 2, 3 or 4 using the dew point measuring device 5, 6 or 7 attached at the same location at 400°C or 800°C, respectively. The attachment of several thermocouples and dew point measuring devices, as in . Figure 1 The control system shown here serves primarily to enable more precise and accurate regulation and control of the bell annealing process. For example, holding times at specific temperatures during bell annealing can be adjusted to ensure that the target temperature is reliably reached inside coil 1 for a specific duration.

[0094] The following examples were examined to demonstrate the effect of the invention. Example 1:

[0095] A cold-rolled steel flat product with a thickness of 0.22 mm was provided, having the following composition, in wt.%: 3.3% Si, 0.075% C, 0.12% Mn, 0.05% AlSi, 0.009% N, 0.1% Cu, 0.011% S, 0.03% P, the balance being iron and unavoidable impurities. The cold-rolled steel flat product was subjected to primary recrystallization annealing with simultaneous decarburization treatment at 850°C for 110 s in an atmosphere of 75% H2 and 25% N2 with a dew point of 60°C. Immediately following, a nitriding treatment was carried out in an atmosphere with a dew point of approximately 5 °C and with the addition of NH 3 to the atmosphere containing 75% H 2 and 25% N 2 , to a nitriding degree of 160 ppm. Both sides of the flat steel product were then coated with a slurry of 95 wt.-% MgO and 5% TiO 2 , based on the total solids content of the slurry, and a mass ratio of water to total solids in the slurry of 8 and wound into a coil with a reel tension of 80 MPa.

[0096] The material was then annealed in a bell furnace at 1200 °C for 24 hours. The dew points during heating to 1200 °C were measured at 400 °C and 800 °C using the method described above in the description of step f) of the process according to the invention and in connection with Figure 1The values ​​were determined using the procedure specified and are listed in Table 1. After cooling to room temperature, the coil was unwound, and the annealing residues, such as non-adhering forsterite and other products, were removed with a smooth brush and water. An insulating coating of aluminum phosphate, colloidal silicon dioxide, and chromic acid was applied to both sides of the flat steel product. The insulating coating was stress-relieved at 860 °C and baked to form an insulating layer.

[0097] Subsequently, one-sided domain refinement was performed using a laser with a wavelength WL given in Table 1.

[0098] The carbon content of the steel core of the grain-oriented flat steel product was below 0.003 weight percent. Due to spinel formation at the interface between forsterite and the steel core, the AlSi content in the steel core of the grain-oriented flat steel product drops below 30 ppm. Purification annealing reduced the nitrogen and sulfur contents in the steel core of the grain-oriented flat steel product to below 10 ppm. Example 2:

[0099] A cold-rolled steel flat product with a thickness of 0.22 mm was provided, having the following composition, in wt.%: 3.3% Si, 0.085% C, 0.08% Mn, 0.03% AlSi, 0.009% N, 0.1% Cu, 0.02% S, 0.01% P, the balance being iron and unavoidable impurities. The cold-rolled steel flat product was subjected to a primary recrystallization annealing with simultaneous decarburization treatment at 850°C for 150 s in an atmosphere containing 75% H2 and 25% N2 with a dew point of 60°C. The flat steel product was then coated on both sides with a slurry containing 95% MgO and 5% TiO 2 , based on the total solid content of the slurry and a mass ratio of water to total solids in the slurry of 8, and wound into a coil with a reel tension of 80 MPa.

[0100] The coil was then annealed in a bell furnace at 1200 °C for 24 hours. The dew points during heating to 1200 °C were measured at 400 °C and 800 °C using the method described above in the description of step f) of the process according to the invention and in connection with Figure 1 specified procedure and are shown in Table 1.

[0101] After cooling to room temperature, the coil was unwound, and the annealing residues, such as non-adhering forsterite and other products, were removed with a smooth brush and water. An insulating coating consisting of aluminum phosphate, colloidal silicon dioxide, and chromic acid was applied to both sides of the flat steel product. The insulating coating was stress-relieved at 860 °C and baked to form an insulating layer.

[0102] Subsequently, one-sided domain refinement was performed using a laser with a wavelength WL given in Table 1.

[0103] The carbon content of the steel core of the grain-oriented flat steel product was below 0.003 weight percent. Due to spinel formation at the interface between forsterite and the steel core, the AlSi content in the steel core of the grain-oriented flat steel product drops below 30 ppm. Purification annealing reduced the nitrogen and sulfur contents in the steel core of the grain-oriented flat steel product to below 10 ppm.By adjusting the wavelength of the laser WL according to the invention depending on the water to solid ratio in the slurry as well as on the dew points at 400°C and 800°C during heating to the soaking temperature during batch annealing, a difference in magnetostriction, measured according to IEC 60404-17 (2021) with a mirror, between one side of the grain-oriented steel flat product and the other side of the grain-oriented steel flat product can be set to less than 1 dB(A), while at the same time minimizing the core losses.

[0104] After performing the magnetostriction measurement, the samples were measured in the laser treatment area according to IEC 60404-11 (2021), and the determined resistance value in Ωcm 2< was entered in Table 2. For non-visible domain-refined areas, the area can be determined using a commercially available domain viewer such as the DV 90 from Brockhaus.

[0105] To examine the morphology of the domain-refined regions, a Keyence VK-X3000 3D laser scanning microscope was used. A surface profile was generated perpendicular to the domain-refined region. The difference between the height of the sample surface and the average height of the domain-refined region yields the height of the laser-generated depression. The depressions thus determined are listed in Table 2.

[0106] The improvement in core loss was determined by annealing the samples for 30 minutes at 850 °C in a 100% nitrogen-containing atmosphere, followed by cooling at approximately 30 K / h. Since the laser treatment is thermally unstable, the improvement in core loss is now eliminated. By determining the change in core loss before and after annealing, the improvement in core loss due to the laser treatment can be determined.

[0107] The calculation is carried out as follows: P 1 − P 0 P 0 ⋅ 100 % = P L .

[0108] The core losses were measured according to IEC 60404-8-7 (2020), with a conversion factor of 0.925 at 50 Hz and 1.7 T. The results of the respective difference in core losses before and after the annealing treatment PL were entered in Table 2.

[0109] To determine the layer thickness of the outer insulation layers on both sides of the grain-oriented steel flat product, the samples must first be weighed. To remove the phosphate layer from one side, the side of the sample whose insulation layer is not to be removed must be taped off with acid-resistant adhesive tape. To remove the insulation layer on the non-taped side of the sample, place the sample in 25% NaOH at 60°C for 20 minutes. Rinse the sample first with water, then with ethanol and dry. Remove the adhesive tape from the sample and then clean it of adhesive residue with Ascusol. Then weigh the sample again. From the weight difference, determine the layer thickness of the non-taped side in g / m². To determine the layer thickness on the other side, reverse the procedure. The determined layer thicknesses are summarized in Table 2. Table 1: Ps: specific core loss, Ss: specific apparent power, λ zp [µm / m]: zero to peak value of magnetostriction, λ pp [µm / m]: peak-to-peak value of magnetostriction, LvA [dB(A)]: A-weighted velocity level of magnetostriction, LvA Diff (AB) [dB(A)]: difference of the A-weighted velocity levels of magnetostriction measured on A and B side of the testing sample, all parameters named in the legend to Table 1 were determined at 1.7 T and 50 Hz. λ zp, λ pp and LvA were measured according to IEC 60404-17 (2021) and Ps according to IEC 60404-3 (2022). Nr. Measured side Ps [W / kg] λ zp [µm / m] λ pp [µm / m] LvA [dB(A)] LvA Diff (AB) [dB(A)] WL1 [nm] WL2 [nm] WL [nm] dp1 [°C] dp2 [°C] w1 Production route according to example 1 or 2? Inventive? YES / NO 1 A 0,707 -0,1 0,4 87,3 -2,5 569 1128 1152,3 16 15 8,0 1 NO 2 B 0,710 -1,0 1,0 89,7 3 A 0,754 -0,5 0,5 87,7 -2,2 565 1108 1152,3 10 -7 3,1 1 NO 4 B 0,746 -1,1 1,1 89,9 5 A 0,736 -0,5 0,5 87,5 -2,4 584 1143 1200 24 5 5,5 2 NO 6 B 0,736 -1,1 1,1 90,0 7 A 0,736 -0,6 0,6 87,5 -2,1 708 1313 694,3 41 15 12,0 2 NO 8 B 0,724 -1,0 1,0 89,6 9 A 0,725 -0,3 0,4 87,2 -3,4 559 1105 1152,3 10 -3 3,2 1 NO 10 B 0,721 -1,1 1,1 90,5 11 A 0,743 -0,2 0,3 85,8 -7,3 800 1460 694,3 44 30 11,3 1 NO 12 B 0,739 -1,3 1,7 93,1 13 A 0,738 -0,4 0,5 87,2 -4,3 535 1050 1062 13 -17 2,3 1 NO 14 B 0,742 -1,2 1,3 91,5 15 A 0,731 0,3 0,8 88,9 -4,7 501 1024 1062 -10 -15 5,6 1 NO 16 B 0,730 -1,6 1,7 93,6 17 A 0,723 0,2 0,6 88,6 -2,8 586 1153 1200 13 11 4,1 2 NO 18 B 0,725 -1,3 1,3 91,5 19 A 0,724 -0,5 0,5 88,0 -3,6 572 1128 1152,3 17 5 5,5 2 NO 20 B 0,716 -1,3 1,3 91,6 21 A 0,852 -0,4 0,4 86,0 -5,4 599 1172 1200 22 20 9,7 2 NO 22 B 0,845 -1,2 1,3 91,4 Nr. Measured side Ps [W / kg] λ zp [µm / m] λ pp [µm / m] LvA [dB(A)] LvA Diff (AB) [dB(A)] WL1 [nm] WL2 [nm] WL [nm] dp1 [°C] dp2 [°C] w1 Production route according to example 1 or 2? Inventive? YES / NO 23 A 0,719 -0,5 0,6 87,5 -0,6 634 1217 1152,3 36 19 8,9 1 YES 24 B 0,718 -0,6 0,7 88,1 25 A 0,698 -0,2 0,4 89,3 -0,7 681 1250 1200 45 -8 5,3 1 YES 26 B 0,700 -0,7 0,9 89,9 27 A 0,719 -0,4 0,6 88,9 -0,9 608 1179 1062 28 11 6,1 1 YES 28 B 0,711 -0,9 1,0 89,8 29 A 0,717 -0,8 0,9 90,7 -0,4 657 1254 694,3 16 9 7,2 2 YES 30 B 0,716 -1,1 1,1 91,2 31 A 0,712 -0,5 0,5 89,2 -0,5 598 1165 1030 25 12 9,3 2 YES 32 B 0,709 -0,9 1,0 89,6 Table 2: Layer thickness of side A of the outer insulation layer on side A in g / m 2< . Layer thickness of side B of the outer insulation layer on side B in g / m 2< . Difference between layer thickness A and B measured according to: (|(AB) / AI - 100%). PL : Difference between P 1 and P 0 measured according to description in %, measured at 1.7 T and 50 Hz with a conversion factor of 0.925 according to IEC 60404-8-7 (2022). Deepening of the domain-refined area in µm measured using a Keyence VK-X3000 3D laser scanning microscope. Insulation effect of the domain-refined area measured according to IEC 60404-11 (2021) in Ωcm 2< . Nr. Layer thickness side A [g / m 2< ] Layer thickness side B [g / m 2< ] Difference between layer thickness A and B (|(AB) / A|· 100 %) PL [%] Deepening of the domain-refined area [µm] Insulation effect of the domain-refined region [Ωcm 2< ] 1-2 3.6 3.1 14% 6.0 0.66 > 50 3-4 4.7 4.0 15% 9.3 0.87 < 50 5-6 4.9 3.6 27% 8.8 0.93 > 50 7-8 4.3 3.4 21% 3.4 1.24 < 50 9-10 4.0 3.3 18% 4.8 1.04 < 50 11-12 4.2 3.6 14% 6.3 1.87 < 50 13-14 4.6 3.4 26% 7.4 1.18 < 50 15-16 4.9 5.5 12% 4.4 1.01 < 50 17-18 3.7 3.5 5% 4.8 0.40 > 50 19-20 5.0 4.3 14% 8.6 1.63 < 50 21-22 4.6 4.3 7% 6.8 0.35 > 50 23-24 4.3 3.7 14% 8.0 0.16 > 50 25-26 3.5 4.0 14% 9.0 0.22 > 50 27-28 3.9 4.4 13% 9.1 0.03 > 50 29-30 4.4 4.7 7% 10.5 0.16 > 50 31-32 4.3 4.6 7% 7.2 0.06 > 50

Claims

1. A process for producing a grain-oriented flat steel product, comprising the steps of a) providing a cold-rolled flat steel product having the following composition in wt.%: Si: 2.0 - 4.0 %, C: 0.01 - 0.10 %, Al sl : 0.01 - 0.065 %, N: 0.003 - 0.015, optionally one or more elements selected from the group consisting of Se, Sn, Sb, where the individual contents of these elements are up to 0.2%, optionally one or more elements selected from the group consisting of Cr, Cu, Mn, where the individual contents of these elements are up to 0.60%, optionally one or more elements selected from the group consisting of As, Bi, B, Co, P, S, Te, Ti, V, Ni, Nb, Mo, where the individual contents of these elements are up to 0.05%, the remainder being iron and unavoidable impurities; b) primary recrystallization annealing of the cold-rolled flat steel product with simultaneous decarburization treatment in a humid atmosphere to a carbon content of less than 30 ppm; c) optionally carrying out a nitriding treatment during step b) or subsequently in step c); d) coating the flat steel product obtained in step b) or in optional step c) with a slurry,wherein the slurry consists of water, MgO and optionally one or more further solid(s) and has a mass ratio w1 of water to the total solid(s) in the slurry, e) coiling the coated flat steel product to form a coil, f) batch annealing the coil at a soaking temperature of at least 1100°C with a dew point dp1, measured between the turns of the coil at 400°C during heating to soaking temperature, and a dew point dp2, measured between the turns of the coil at 800°C during heating to soaking temperature, g) applying an insulating coating to the annealed flat steel product, h) stress relieving the flat steel product provided with the insulating coating to form an insulating layer, i) carrying out domain refinement by means of a laser with a wavelength WL on at least one side of the flat steel product with insulation layer,where the wavelength WL of the laser satisfies the requirement of the following formula (I): , 537 + 60 × dp 1 + 10 × dp 2 w 1 < WL < 1080 + 18 × dp 1 + 3 × dp 2 w 1 2. Method according to claim 1, characterized in that the composition of the cold-rolled flat steel product provided in step a) contains 0.002 to 0.60 wt.%, preferably 0.05 to 0.3 wt.% manganese, particularly preferably 0.05 to 0.25 wt.% manganese.

3. Method according to one of claims 1 or 2, characterized in that the composition of the cold-rolled flat steel product provided in step a) contains 0.002 to 0.60 wt.%, preferably 0.05 to 0.30 wt.%, particularly preferably 0.08 to 0.20 wt.%, of Cu.

4. Method according to one of claims 1 to 3, characterized in that the composition of the cold-rolled flat steel product provided in step a) contains 0.0003 to 0.05 wt.%, preferably 0.005 to 0.045 wt.%, particularly preferably 0.008 to 0.04 wt.%, of P.

5. Method according to one of the preceding claims, characterized in thatStep b) is carried out at a dew point of 40 to 80°C.

6. Method according to one of the preceding claims, characterized in that Step b) is carried out with an annealing time of more than 100 s and at a temperature of at least 820°C.

7. Method according to one of the preceding claims, characterized in that the flat steel product is nitrided during step b) or thereafter in step c) and the nitrogen content after step c) is at least 150 ppm.

8. Method according to one of the preceding claims, characterized in that the further solids in the slurry according to step d) are selected, in addition to MgO, from one or more oxides and / or nitrides of at least one element selected from the group consisting of Al, Cr, Fe, Mn, Si, Ti, Mg, Sn, Zr and mixed oxides of said oxides with Mg.

9. Method according to one of the preceding claims, characterized in thatthe reel tension in step e) is 30 - 300 MPa.

10. Method according to one of the preceding claims, characterized in that the bell annealing in step f) is carried out for an annealing time of at least 10 h in a 100% H2 atmosphere.

11. Method according to one of the preceding claims, characterized in that the insulating coating comprises colloidal silicon dioxide, and at least one phosphate, nitrate and / or oxide containing at least one element selected from Al, Mn, Si, Ti, Mg, Sn and Cr.

12. Grain-oriented flat steel product comprising a steel core having, in % by weight, the following composition: Si: 2.0 - 4.0 %, C: < 30 ppm Al sl : < 30 ppm, N: < 50 ppm, optionally one or more elements selected from the group consisting of Se, Sn, Sb, where the individual contents of these elements are up to 0.2%, optionally one or more elements selected from the group consisting of Cr, Cu, Mn, where the individual contents of these elements are up to 0.60%, optionally one or more elements selected from the group consisting of As, Bi, B, Co, Te, Ti, V, Ni, Nb, Mo, where the individual contents of these elements are up to 0.05%, optionally S: < 50 ppm P: 0.0003% to 0.05% The remainder being iron and unavoidable impurities, and a forsterite layer on at least one side of the steel core in full-surface contact therewith, the side of the forsterite layer facing away from the steel core being in full-surface contact with an insulation layer, at least one side of the grain-oriented flat steel product having been treated with a laser for domain refinement, characterized in thatthe difference in magnetostriction, measured according to IEC 60404-17 (2021) using a mirror, between one side of the grain-oriented steel flat product and the other side of the grain-oriented steel flat product is less than 1 dB(A) and wherein the grain-oriented steel flat product has a core loss, measured according to IEC 60404-3 (2022) at 50 Hz and 1.7 T and a conversion factor of 0.925 according to IEC 60404-8-7 (2020), of not more than 0.75 W / kg.

13. Grain-oriented flat steel product according to claim 12, characterized in that this is a change in the core loss P L for which the following applies: P 1 − P 0 P 0 ⋅ 100 % = P L ≥ 7 % , where P0 is the core loss after laser treatment, P1 is the core loss after annealing for 30 min at 850°C and P L represents the change in core loss due to laser treatment, all measured according to IEC 60404-8-7 (2020) at 50 Hz and 1.7 T.

14. Grain-oriented flat steel product according to claim 12 or 13, characterized in that at least one side of the grain-oriented steel flat product that has been laser-treated for domain refinement has no depressions along the areas that are deeper than 0.8 µm, preferably deeper than 0.3 µm, and the locally measured resistance of the insulation layers according to IEC 60404-11 (2021) in the areas that have been laser-treated is at least 50 Ωcm 2 amounts.

15. Grain-oriented flat steel product according to one of claims 12 to 14, characterized in that the difference in the layer thicknesses of the insulation layers on both sides of the grain-oriented flat steel product is not more than 25%.

Images