Grain-oriented electrical steel strip and method for producing such an electrical steel strip

Abstract

The present invention relates to a grain-oriented electrical steel strip having a magnesium silicate layer on at least one surface, wherein at least one peak in the range of 960 to 975 cm-1 (A) and at least one peak in the range of 976 to 990 cm-1 are present in a DRIFT spectrum of the magnesium silicate layer, the peak height of the at least one peak in the range (A) being higher than the peak height of the at least one peak in the range (B), a method for the production thereof under particular atmospheric conditions in the annealing step and the use of such an electrical steel strip.

Description

[0001] The present invention relates to a grain-oriented electrical steel strip having a magnesium silicate layer on at least one surface, wherein in a DRIFT spectrum of the magnesium silicate layer at least one peak is present in the range 960 to 975 cm-1 (A) and at least one peak is present in the range 976 to 990 cm-1 (B), wherein the peak height of the at least one peak in the range (A) is higher than the peak height of the at least one peak in the range (B), a method for its production under certain atmospheric conditions in the annealing step and the use of such an electrical steel strip.

[0002] The invention makes it possible to optimize the nitrogen level during a bell annealing process carried out in the course of the method according to the invention.

[0003] Processes for producing grain-oriented electrical steel strips and corresponding electrical steel strips are already known from the prior art.

[0004] EP 1 025 268 B1 discloses a method for producing grain-oriented electrical steel sheet, wherein, after a hot rolling step and a cold rolling step, the sheet is subjected to recrystallization and decarburization annealing, then provided with an annealing separator, and finally subjected to final annealing. The establishment of a specific annealing atmosphere is not disclosed in this document.

[0005] WO 2017 / 037019 A1 also discloses a process for producing grain-oriented electrical steel strip. This process comprises an annealing step after cold rolling the electrical steel strip, during which an oxide layer forms on the surface. This oxide layer is examined using FTIR spectroscopy. The FTIR analysis then determines the area ratio between Fe2SiO4 and αSiO2 in the layer. In the annealing step disclosed in this document, a specific ratio of the maximum temperature reached during the annealing step to the dew point of the atmosphere during the annealing step is to be maintained.

[0006] EP 1752548 A1 and EP 1752549 A1 disclose processes for producing grain-oriented electrical steel strip. These processes also each include an annealing step in which the cold-rolled electrical steel strip is subjected to a recrystallization and decarburization annealing, preferably in an ammonia atmosphere.

[0007] EP 0 699 771 A1, CN 102 758 127 B, and EP 0 947 597 A2 disclose processes for producing grain-oriented electrical steel strip, each of which includes an annealing step. None of these documents discloses adjusting the annealing conditions such that the high-temperature annealing results in an N content of < 14 / 27 x Al sl in the electrical steel strip.

[0008] In the production of grain-oriented electrical steel, controlling the nitride phase during high annealing is important, as it can interact with the annealing atmosphere even at temperatures below the secondary crystallization temperature. A nitride phase that is set too low results in a low inhibition effect and, consequently, higher core losses. A nitride phase that is set too high can both shift the secondary crystallization temperature above the inhibitor dissolution temperature and create an inhomogeneous forsterite layer, which can lead to insufficient interlocking and thus reduced bond strength.

[0009] The object of the present invention is therefore to provide a grain-oriented electrical steel strip that exhibits an optimized combination of low core losses and good adhesion of the forsterite layer to the surface. Furthermore, it is an object of the present invention to provide a method for producing a corresponding grain-oriented electrical steel strip.

[0010] These objects are achieved by a grain-oriented electrical steel strip having the features specified in claim 7.

[0011] The objects are further achieved by a method for producing a grain-oriented electrical steel strip, in which at least the steps specified in claim 1 are completed.+

[0012] According to the invention, "electrical steel strip" refers to an electrical steel sheet produced by rolling steel of a suitable composition, as well as blanks cut from it, intended for the manufacture of parts for electrical engineering applications. Grain-oriented electrical steel strips of the type in question are particularly suitable for applications where particularly low core loss is paramount and high demands are placed on permeability or polarization. Such requirements exist in particular for parts for power transformers, distribution transformers, high-quality small transformers, or in rotating electrical machines.

[0013] The process according to the invention is described in detail below. In addition to those explicitly described here, the process according to the invention can comprise further work steps that are carried out in the conventional production of electrical steel strip in order to achieve optimized magnetic properties or properties important for practical use. These include, for example, reheating the precursor product obtained after casting the steel, descaling the hot strip before cold rolling, or, in the case of multi-stage cold rolling, intermediate annealing carried out in the conventional manner between the cold rolling stages.

[0014] Step (A) of the process according to the invention comprises melting a steel melt which (in each case in wt.%) 2.0 to 4.0 Si, 0.010 to 0.100 C, 0.0200 to 0.0400 Alsi and 0.0020 to 0.02 N, and optionally up to 0.5 Cu, up to 0.060 S and also optionally up to 0.3 Cr, Mn, Ni, Mo, P, As, Sn, Sb, Se, Te, B or Bi, balance iron and unavoidable impurities contains.

[0015] Preferably, in step (A) of the process according to the invention, a steel melt is melted which (each in wt. %) optionally contains 0.005 to 0.5 Cu, 0.005 to 0.060 S and also optionally 0 to 0.3 Cr, Mn, Ni, Mo, P, As, Sn, Sb, Se, Te, B or Bi, the remainder being iron and unavoidable impurities.

[0016] Step (A) of the process according to the invention can be carried out by any method known to those skilled in the art. Preferably, a steel melt of known composition is melted for this purpose. This melt is then subjected to secondary metallurgical treatment. This treatment preferably initially takes place in a vacuum system in order to adjust the chemical composition of the steel within the required narrow analytical ranges and preferably to achieve low hydrogen contents of a maximum of 10 ppm in order to minimize the risk of strand breakouts during casting of the steel melt.

[0017] Following treatment in the vacuum system, it is advisable to use the furnace in a ladle furnace to ensure that the temperature required for casting is maintained in the event of delays in casting and to prevent the immersion tube nozzles in the mold from clogging during thin slab continuous casting, thus preventing the casting from being aborted, by conditioning the slag there.

[0018] According to the invention, in step (A), it would also be possible to initially use a ladle furnace for slag conditioning, followed by treatment in a vacuum system to adjust the chemical composition of the molten steel within narrow analytical limits. However, this combination has the disadvantage that, in the event of pouring delays, the temperature of the melt drops so low that the molten steel can no longer be poured.

[0019] It is also possible to use only the ladle furnace in step (A). However, this has the disadvantage that the analytical accuracy is not as good as with treatment in a vacuum system, and high hydrogen contents can occur in the casting melt, posing the risk of strand breakouts.

[0020] According to the invention, it is further preferred to use only the vacuum system in step (A). However, this entails the risk that, in the event of pouring delays, the temperature of the melt will drop so far that the molten steel can no longer be poured. Furthermore, there is a risk that the submerged nozzles will become clogged during the sequence, thus requiring the sequence to be aborted.

[0021] In step (B) of the process according to the invention, the steel melt produced in step (A) is then cast into a precursor material. According to the invention, this precursor material can be, for example, a slab, a thin slab, or a cast strip. For this purpose, a strand is preferably first produced from the melt by casting.

[0022] For the production of thin slabs, it is preferable to cast a strand having a thickness of, for example, 25 mm to 150 mm.

[0023] According to the invention, the aim is to avoid the formation of nitride precipitates before and during hot rolling as much as possible in order to be able to make extensive use of the possibility of controlled generation of such precipitates during the cooling of the hot strip. To support this, according to an advantageous embodiment of the invention, it is preferred to carry out an inline thickness reduction of the strand cast from the melt, but still with a liquid core.

[0024] Known methods for thickness reduction include "Liquid Core Reduction," hereinafter "LCR," and "Soft Reduction," hereinafter "SR." These options for reducing the thickness of a cast strand can be used alone or in combination. With LCR, the strand thickness is reduced while the core of the strand is molten, just below the mold. State-of-the-art LCR is primarily used in thin-slab continuous casting plants to achieve lower final hot strip thicknesses, particularly for high-strength steels. In addition, LCR can be used to reduce pass reductions and rolling forces in the rolling stands of a hot strip mill, successfully reducing work roll wear and scale porosity in the hot strip, and improving strip runability. According to the invention, the thickness reduction achieved by LCR is preferably in the range of 5 mm to 30 mm.

[0025] SR refers to the targeted thickness reduction of the strand in the tip of the slurry near final solidification. The aim of SR is to reduce central segregation and core porosity. The reduction achievable through SR, particularly in silicon central segregation in the subsequently hot-rolled precursors, enables a uniformity of the chemical composition across the strip thickness, which is beneficial for the magnetic values. Good results from SR are obtained when the thickness reduction achieved through the application of SR is, for example, 0.5 to 5 mm. The following specification can serve as a guide for the time at which SR is applied in connection with the continuous casting carried out according to the invention: the beginning of the SR zone at a degree of solidification fs of 0.2 and the end of the SR zone at fs equal to 0.7 to 0.8.

[0026] According to the invention, thin slabs are preferably produced in step (B) of the process according to the invention. In thin-slab continuous casting plants, the strand, which usually emerges vertically from the casting mold, is bent at lower points and guided in a horizontal direction. In an advantageous embodiment of the invention, the strand cast from the melt is bent and straightened at a temperature of 700 to 1000°C, preferably 850 to 950°C. This prevents cracks on the surface of the thin slabs separated from the strand, which can otherwise occur, particularly as a result of edge cracks in the strand. In the aforementioned temperature range, the steel used according to the invention exhibits good ductility on the strand surface or in the edge region, so that it can easily follow the deformations occurring during bending and straightening. Thin slabs are cut from the strand thus cast in a conventional manner.

[0027] Step (C) of the process according to the invention comprises hot rolling the starting material from step (B), in particular the thin slabs produced, into a hot strip. For this purpose, the starting material obtained in step (B), in particular a thin slab, is preferably heated in a furnace to the appropriate starting hot rolling temperature and then fed to the hot rolling stage. The temperature at which the starting material, in particular a thin slab, enters the furnace is preferably above 650°C. The residence time in the furnace should be less than 60 minutes to avoid sticky scale.

[0028] Preferably, hot rolling in step (C) is carried out following the first forming pass in the two-phase region (α / γ). This measure also aims to reduce the formation of nitride precipitations during hot rolling as much as possible in order to be able to control these precipitations specifically via the cooling conditions on the run-out roller table behind the last rolling stand of the hot strip mill. To ensure this, according to the invention, hot rolling is preferably carried out at temperatures at which austenitic and ferritic components are mixed in the microstructure of the hot strip. Typical temperatures at which this is the case for the steel alloys used according to the invention are above approximately 800 °C, in particular in the range from 850 to 1150 °C. In the γ-phase, the AlN is kept in solution at these temperatures. Another positive aspect of hot rolling in the two-phase mixed region is the grain refinement effect.The conversion of austenite to ferrite following the hot rolling passes results in a finer-grained and more homogeneous hot strip structure, which has a positive effect on the magnetic properties of the final product. The avoidance of nitride precipitations during hot rolling is further supported, for example, by achieving a degree of deformation of at least 40% in the first forming pass, so that only relatively small pass reductions are necessary in the last stands to achieve the desired final strip thickness. In this respect, the total degree of deformation achieved over the first two forming passes is preferably above 60%, whereby in a further advantageous embodiment of the invention, a degree of deformation of more than 40% is achieved in the first stand of the finishing train, and the pass reduction in the second stand of the finishing train is more than 30%.

[0029] The preferred use of high pass reductions (degrees of deformation) in the first two stands achieves the necessary transformation of the coarse-grained solidification microstructure into a fine rolled microstructure, which is a prerequisite for good magnetic properties of the final product. Accordingly, the pass reduction in the last stand should preferably be limited to a maximum of 30%, particularly preferably less than 20%. For an optimal hot rolling result with regard to the desired properties, it is also advantageous if the pass reduction in the penultimate stand of the finishing train is less than 25%.

[0030] To avoid a coarse, uneven structure or coarse precipitations on the hot strip, which would adversely affect the magnetic properties of the final product, early cooling of the hot strip after the last rolling stand of the finishing train is advantageous. According to a practical embodiment of the invention, it is therefore preferable to begin water cooling within a maximum of five seconds after leaving the last rolling stand. The goal is to keep the pause times as short as possible, for example, one second or less.

[0031] The cooling of the hot strip can be carried out by methods known to those skilled in the art, for example by water and / or air.

[0032] Step (D) of the process according to the invention comprises coiling the hot strip obtained in step (C) into a coil. Corresponding processes for coiling hot strip are known per se to those skilled in the art.

[0033] The coiling temperature should preferably be in the range of 500 to 780 °C. Higher temperatures would lead to undesirable coarse precipitates and impair pickling properties. For higher coiling temperatures above 700 °C, a so-called short-distance coiler is preferably used, which is located directly adjacent to the compact cooling zone.

[0034] Within the limits specified by the invention, the process according to the invention is preferably carried out in the production of the hot strip such that the resulting hot strip contains sulfide and / or nitride precipitates with an average particle diameter of less than 150 nm and an average density of at least 0.05 µm-2. A hot strip of this type offers optimal conditions for the effective control of grain growth during the subsequent process steps. The thickness of the hot strip obtained according to the invention is preferably 1.5 to 3.5 mm, particularly preferably 2 to 2.7 mm.

[0035] In the optional step (E) of the process according to the invention, the hot strip thus produced can optionally be annealed after coiling or before cold rolling in order to further optimize the microstructure.

[0036] In step (F), the hot strip is rolled into a cold strip in one or more cold rolling steps. In multi-stage cold rolling, intermediate annealing may be performed between the cold rolling steps if necessary. The thickness of the cold strip obtained according to the invention is preferably 0.10 to 0.35 mm, particularly preferably 0.15 to 0.23 mm.

[0037] Step (G) of the process according to the invention comprises annealing the cold strip at a temperature of 700 to 950°C, preferably 800 to 900°C. Step (G) of the process according to the invention can, in principle, be carried out in any device known to the person skilled in the art.

[0038] For the process according to the invention, it is essential that step (G) is carried out in such a way that a dew point DP according to the following equation (1) Alsl / N Schmelze * 10 < DP < Alsl / N Schmelze * 30 is present, and thus the nitrogen concentration N in the material after step (G) is adjusted so that the following equation (2) 2 / 3 * Alsl * 14 / 27 − 0,7 * DP < N < Alsl * 14 / 27 + N Schmelze is fulfilled, whereby AlslAluminium content (acid soluble) in the melt in ppm, N(melt)Nitrogen content in the melt in ppm, DPTew point in °C and NNitrogen content in the material after step (G) in ppm.

[0039] According to the invention, the annealing in step (G) should take place in an atmosphere in which the dew point DP, expressed in °C, corresponds to the above-mentioned equation (1). Here, Alsl denotes the aluminum content in the melt used in step (A) of the process according to the invention. The unit of Alsl is ppm. According to the invention, Alsl is 200 to 400 ppm. N(melt) denotes the nitrogen content in the melt used in step (A) of the process according to the invention. The unit of N(melt) is ppm. According to the invention, N(melt) is 20 to 200 ppm, preferably 40 to 150 ppm, very particularly preferably 60 to 120 ppm.

[0040] In the process according to the invention, the dew point DP is 20 to 90 °C, particularly preferably 30 to 70 °C.

[0041] The dew point DP set in step results in the nitrogen concentration N in the material after step (G) according to equation (2) 2 / 3 * Alsl * 14 / 27 − 0,7 * DP < N < Alsl * 14 / 27 + N Schmelze

[0042] Here, ASI and N(melt) have the meanings given above. N represents the nitrogen content in the material after step (G) in ppm. The unit of N is ppm.

[0043] The nitrogen content in the material after step (G) is N 70 to 180 ppm, particularly preferably 90 to 160 ppm.

[0044] Step (G) of the process according to the invention is preferably carried out in a nitrogen-containing atmosphere. It is known to those skilled in the art that step (G) is preferably carried out in the presence of ammonia, nitrogen, and / or hydrogen. Corresponding amounts are known per se to those skilled in the art. The ammonia-containing atmosphere can be present throughout the entire furnace section, or preferably only in the last furnace section.

[0045] By carrying out step (G) of the process according to the invention under specific conditions with respect to the dew point of the furnace atmosphere and the nitrogen concentration in the material after step (G), it is possible according to the invention to provide a grain-oriented electrical steel strip which is characterized by particularly low magnetic reversal losses and particularly good adhesion of the forsterite layer (magnesium silicate layer) formed in step (I) of the process according to the invention. By setting the conditions in step (G), a specific nitrogen content is obtained in the material after step (G), the relationship being given by formulas (I) and (II). This in turn results in a specific, particularly advantageous nitrogen content of the material in step (I) of the process according to the invention at a temperature of 700 to 900°C. This is preferably less than 14 / 27*Alsl of the melt.

[0046] In particular, a grain-oriented electrical steel strip is obtained which has a magnesium silicate layer on at least one surface which, in a DRIFT spectrum, exhibits at least one peak in the range from 960 to 975 cm-1 (A) and at least one peak in the range from 976 to 990 cm-1. According to the invention, it has been shown that the ratio of the peaks to one another is influenced by the denitrification and nitriding processes during step (I), wherein, in the embodiment according to the invention, the peak height of the at least one peak in the range (A) is higher than the peak height of the at least one peak in the range (B).

[0047] In step (H) of the process according to the invention, an annealing separator layer is applied to at least one surface of the cold strip from step (G); preferably, such an annealing separator layer is applied to both surfaces of the cold strip from step (G).

[0048] The annealing separator is typically MgO. The annealing separator prevents the windings of a coil wound from cold-rolled strip from fusing together during subsequent high-temperature annealing.

[0049] Step (I) of the inventive process comprises high-temperature annealing of the cold-rolled strip coated with the annealing separator, forming a forsterite layer on the surface of the annealed cold-rolled strip. Step (I) of the inventive process is typically carried out in a bell furnace under protective gas, preferably at a temperature of 650 to 950 °C. This creates a microstructure in the cold-rolled strip through selective grain growth, which contributes significantly to the magnetic properties. At the same time, a forsterite layer, often referred to as a "glass film" in the technical literature, forms on the strip surfaces during the high-temperature annealing. Furthermore, the steel material is purified by diffusion processes occurring during the high-temperature annealing.

[0050] According to the prior art, the nitrogen content of the material after step (G) is adjusted to 200 ppm, for example. However, according to the invention, this nitrogen value is only an auxiliary value; rather, according to the invention, the nitrogen value is controlled in the temperature range from 700 to 900 °C in step (I) and adjusted so that it is below 14 / 27*Alsl.

[0051] It has been shown according to the invention that by controlling the dew point and the nitrogen content N in the material after step (G) of the inventive process using the two formulas (1) and (2), a grain-oriented electrical steel strip is accessible, which results in a specific nitrogen content in the material in step (I) at a temperature of 700 to 900 °C and thus results in a particularly advantageous configuration of the glass film (forsterite layer) located on the surface. This advantageous nature of the glass film is in turn demonstrated by the inventive occurrence of specific peaks in the DRIFT spectrum.

[0052] The optional step (J) of the process according to the invention comprises the application of an insulating layer to the surface of the cold-rolled strip containing the forsterite layer. Suitable materials that can act as an insulating layer include, for example, phosphates, silicates, or mixtures thereof. Corresponding processes are known per se to those skilled in the art.

[0053] Furthermore, the cold-rolled strip obtained from step (J) of the process according to the invention can be thermally straightened. Corresponding methods for this are also known to those skilled in the art.

[0054] The optional step (K) of the inventive process comprises a final annealing of the cold-rolled strip produced according to the invention. In the so-called "stress relief annealing," the cold-rolled strip is stress-relieved. This final annealing 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, which is performed after the blanks have been cut, can relieve the additional stresses created during the cutting process.

[0055] The grain-oriented electrical steel produced according to the invention is characterized by low core losses and, due to the high interlocking of the forsterite layer with the surface of the electrical steel, good adhesion of the forsterite layer. The present invention therefore also relates to a grain-oriented electrical steel produced by the process according to the invention.

[0056] The present invention also relates to grain-oriented electrical steel strip having a magnesium silicate layer on at least one surface, wherein in a DRIFT spectrum of the magnesium silicate layer, at least one peak is present in the range from 960 to 975 cm-1 (A) and at least one peak is present in the range from 976 to 990 cm-1 (B), wherein the peak height of the at least one peak in the range (A) is higher than the peak height of the at least one peak in the range (B). The electrical steel strip according to the invention is obtained by the process according to the invention.

[0057] The so-called DRIFT method (diffuse reflection Fourier transform infrared spectroscopy) is well known to those skilled in the art. In the DRIFT method, an IR light beam is directed onto the sample surface using concave mirrors, and the reflected light is also detected using concave mirrors (see, for example, Beasley et al., "Comparison of transmission FTIR, ATR and DRIFT spectroscopy," Journal of Archaeological Science, Vol. 46, June 2014, pages 16 to 2). This enables the evaluation of deeper oxide layers and thus a more in-depth analysis of the molecular components in the oxide layer.

[0058] The electrical steel strip according to the invention is characterized in that it has a magnesium silicate layer, i.e., a so-called forsterite layer, which has at least two peaks in a DRIFT spectrum of this layer: firstly, at least one peak in a wavenumber range from 960 to 975 cm-1 (region (A)), and secondly, at least one peak in a wavenumber range from 976 to 990 cm-1 (region (B)). Furthermore, the peak height of the at least one peak in region (A) is higher than the peak height of the at least one peak in region (B). In the context of the present invention, "higher" means that the at least one peak in region (A) is at least 5%, preferably at least 30%, higher than the at least one peak in region (B).

[0059] According to the invention, the ratio of the peak height of the at least one peak in the region (A) to the peak height of the at least one peak in the region (B) is preferably greater than 1.0, preferably greater than or equal to 1.05, particularly preferably greater than or equal to 1.3.

[0060] The present invention further preferably relates to the electrical steel strip according to the invention, wherein an annealing separator layer is present on at least one surface, preferably such an annealing separator layer is present on both surfaces.

[0061] The forsterite layer of the electrical steel strip according to the invention, which has at least the two characteristic peaks mentioned, exhibits advantageous, particularly strong interlocking with the surface of the electrical steel strip. Furthermore, the electrical steel strip according to the invention exhibits particularly low remagnetization losses.

[0062] The electrical steel strip according to the invention is particularly suitable for use in power transformers, distribution transformers, small transformers or in rotating electrical machines.

[0063] The present invention therefore further relates to the use of the electrical steel strip according to the invention in power transformers, distribution transformers, small transformers or in rotating electrical machines. Examples

[0064] The following embodiments serve to explain the invention in more detail.

[0065] Grain-oriented electrical steel strips 1 to 14 according to the invention were produced by the steps of melting a steel melt containing the alloying elements specified in Table 1, casting the steel melt into a thin slab, hot rolling the thin slab at a temperature of 1050 °C into a hot strip, coiling the hot strip at a temperature of 700 °C into a coil, cold rolling the hot strip in 5 steps to a thickness between 0.23 and 0.3 mm, and annealing the cold strip at a temperature of 700 to 900 °C according to the process parameters specified in Table 2.

[0066] The surfaces of the resulting grain-oriented electrical steel strips were measured using DRIFT spectroscopy. The results for the peaks at 977 and 985 ppm are shown in Table 2. Furthermore, the polarizability was determined at 1.7 Tesla. These results are also shown in Table 2. Comparative Examples V15 to V28 were prepared for comparison. These differ from the inventive examples in that equations (1) and (2) are not satisfied. Industrial applicability

[0067] The electrical steel strip according to the invention can be used in power transformers, distribution transformers, small transformers or in rotating electrical machines. Table 1: Compositions of the melts used, Rest Fe and unavoidable impurities Nr. Si Mn C Cu S Cr P Al sl N [%] [ppm] 1 3,13 0,08 346 152 42 1314 668 263 119 2 3,10 0,27 472 184 119 646 680 272 91 3 2,97 0,12 387 76 78 367 530 292 95 4 3,32 0,31 344 107 36 2274 563 317 79 5 3,16 0,17 444 231 120 1346 555 269 89 6 3,30 0,23 335 149 40 685 308 282 126 7 3,12 0,09 329 86 35 1114 358 276 117 8 3,04 0,09 420 217 98 1711 501 287 73 9 3,12 0,34 336 241 80 1767 772 297 107 10 3,36 0,07 482 212 72 2262 428 297 116 11 3,31 0,20 312 86 93 1502 781 266 108 12 3,28 0,32 462 85 92 1611 496 294 75 13 3,02 0,09 399 224 69 629 553 309 81 14 3,47 0,34 377 178 43 2284 402 284 108 V15 3,34 0,08 375 228 67 1826 673 278 113 V16 3,42 0,08 425 189 54 1817 775 282 117 V17 3,07 0,09 387 244 54 2067 436 302 127 V18 3,04 0,22 492 135 46 1829 336 290 100 V19 3,25 0,16 375 116 56 1581 743 284 113 V20 3,04 0,24 324 169 94 1283 554 305 124 V21 3,44 0,06 449 81 55 1355 473 294 76 V22 3,07 0,31 382 70 69 1625 719 285 94 V23 2,91 0,23 429 167 104 1854 718 261 97 V24 3,36 0,33 497 226 77 2140 387 275 118 V25 3,30 0,21 387 52 68 967 610 263 117 V26 2,97 0,12 433 188 43 819 633 313 120 V27 3,05 0,14 353 194 104 668 489 298 128 V28 3,21 0,15 448 228 84 610 346 274 110 Table 2: Results Nr. Al sl N (melt) DP N in the material after step (G) N in the material in step (I) at 700 - 900 °C Height of the peak at 985 cm -1*< Height of the peak at 977 cm -1*< Peak height at 977 cm -1*< - Peak height at 985 cm -1*< P1,7 Adhesive strength [ppm] [°C] [ppm] [W / kg at 50Hz] [mm] 1 263 119 37 151 136 0,318 0,322 0,003 0,72 7 2 272 91 74 206 141 0,393 0,395 0,002 0,70 4 3 292 95 54 171 139 0,291 0,334 0,044 0,72 2 4 317 79 113 162 133 0,326 0,347 0,022 0,68 5 5 269 89 69 193 96 0,264 0,463 0,200 0,74 6 6 282 126 57 216 142 0,225 0,353 0,128 0,75 3 7 276 117 39 155 118 0,392 0,429 0,037 0,73 3 8 287 73 92 129 143 0,358 0,446 0,088 0,85 4 9 297 107 70 133 118 0,321 0,406 0,084 0,83 5 10 297 116 59 135 126 0,339 0,479 0,141 0,84 7 11 266 108 32 155 93 0,298 0,389 0,091 0,95 2 12 294 75 42 207 119 0,342 0,388 0,046 0,98 4 13 309 81 60 186 155 0,348 0,409 0,061 0,94 5 14 284 108 45 197 131 0,352 0,528 0,175 0,67 4 V15 278 113 31 273 55 0,381 0,276 -0,105 0,85 15 V16 282 117 62 277 77 0,231 0,134 -0,097 0,86 9 V17 302 127 32 327 59 0,312 0,232 -0,079 0,87 11 V18 290 100 77 120 158 0,328 0,241 -0,087 0,86 23 V19 284 113 67 128 180 0,235 0,165 -0,070 0,83 18 V20 305 124 63 142 93 0,340 0,220 -0,120 0,81 11 Table 2: Results Nr. Al sl N (melt) DP N in the material after step (G) N in the material in step (I) at 700 - 900 °C Height of the peak at 985 cm -1*< Height of the peak at 977 cm -1*< Peak height at 977 cm -1*< - Peak height at 985 cm -1*< P1,7 Adhesive strength [ppm] [°C] [ppm] [W / kg at 50Hz] [mm] V21 294 76 79 125 96 0,334 0,230 -0,104 0,82 17 V22 285 94 46 152 63 0,252 0,134 -0,118 0,97 14 V23 261 97 50 256 152 0,350 0,181 -0,170 0,94 9 V24 275 118 48 286 185 0,271 0,130 -0,141 0,96 9 V25 263 117 54 270 61 0,366 0,186 -0,180 1,10 30 V26 313 120 60 287 83 0,277 0,147 -0,130 1,06 26 V27 298 128 58 325 186 0,339 0,290 -0,049 1,12 14 V28 274 110 46 272 149 0,334 0,306 -0,028 1,34 16 V Comparison example * unitless

Claims

1. Method for producing a grain-oriented electrical steel strip, comprising at least the following steps: (A) melting a steel melt containing (in each case in wt.%) 2.0 to 4.0 Si, 0.010 to 0.100 C, 0.0200 to 0.0400 AIsl (where AIsl = acid-soluble AI) and 0.0020 up to 0.02 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 remainder being iron and unavoidable impurities, (B) casting the steel melt to form a preliminary material, for example to form a slab, thin slab or a cast strip, (C) hot rolling the preliminary material to form a hot strip, (D) coiling the hot strip to form a coil, (E) optionally annealing the hot strip (F) cold rolling the hot strip in one or more cold rolling steps to form a cold strip, (G) annealing the cold strip from step (F) at a temperature of 700 to 950°C, (H) applying an annealing separator layer to at least one surface of the cold strip from step (G), (I) high-temperature annealing of the cold strip coated with the annealing separator to form a forsterite layer on the surface of the annealed cold strip, (J) optionally applying an insulating layer onto the surface of the cold strip having the forsterite layer, and (K) optionally final annealing the cold strip, characterized in that step (G) is carried out in an atmosphere of which the dew point DP is 20°C to 90°C and satisfies the following equation (1) Alsl / N melt * 10 < DP < Alsl / N melt * 30 so that after step (G) the material has a nitrogen content N of 70 to 180 ppm, which satisfies the following equation (2) 2 / 3 * Alsl * 14 / 27 − 0.7 * DP < N < Alsl * 14 / 27 + N melt and during the high-temperature annealing according to step (I) in the temperature range of 700 - 900°C, the nitrogen content of the electrical steel strip is controlled such that an N content in the electrical steel strip is less than 14 / 27 x Alsl, where Alsl denotes aluminum content (acid-soluble) in the melt in ppm, N(melt) denotes nitrogen content in the melt in ppm, DP denotes dew point in °C and N denotes nitrogen content in the material after step (G) in ppm.

2. Method according to claim 1, characterized in that the dew point DP is 30 to 70°C.

3. Method according to claim 1 or claim 2, characterized in that the aluminum content Alsl is between 240 and 340 ppm.

4. Method according to any of claims 1 to 3, characterized in that the N(melt) is 40 to 150 ppm.

5. Method according to claim 4, characterized in that N(melt) is 60 to 120 ppm.

6. Method according to any of claims 1 to 5, characterized in that after step (G) the nitrogen content in the material is 90 to 160 ppm.

7. Grain-oriented electrical steel strip obtainable by the method according to any of claims 1 to 6, having a magnesium silicate layer on at least one surface, characterized in that in a DRIFT spectrum of the magnesium silicate layer, at least one peak in the range 960 to 975 cm-1 (A) and at least one peak in the range 976 to 990 cm-1 (B) are present, the peak height of the at least one peak in the range (A) being higher than the peak height of the at least one peak in the range (B).

8. Electrical steel strip according to claim 7, characterized in that the ratio of the peak height of the at least one peak in the range (A) to the peak height of the at least one peak in the range (B) is greater than 1.05.

9. Electrical steel strip according to claim 7 or claim 8, characterized in that an annealing separator layer is present on at least one surface.

10. Use of a grain-oriented electrical steel strip according to any of claims 7 to 9 for the production of parts for power transformers, distribution transformers, small transformers or rotating electrical machines.

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