WO2023020945A1

WO2023020945A1 - Alloy and method for producing a nanocrystalline metal strip

Info

Publication number: WO2023020945A1
Application number: PCT/EP2022/072603
Authority: WO
Inventors: Christian Polak
Original assignee: Vacuumschmelze Gmbh & Co. Kg
Priority date: 2021-08-17
Filing date: 2022-08-11
Publication date: 2023-02-23
Also published as: DE102021121345A1

Abstract

An alloy is provided, which is described by the formula Co_{100-a-b-c-d-x-y-z}Fe_aCu_bM_cT_dSi_xB_yZ_z, where M is one or more of the group of elements Nb, Mo and Ta, T is one or more of the group of elements Mn, V, Cr and Ni, Z is one or more of the group of elements C, P and Ge, a, b, c, d, x, y and z are specified in at.%, and a, b, c, d, x, y and z meet the following conditions: 1.5 < a < 15; 0.1 < b < 1.5; 1 ≤ c < 5; 0 ≤ d < 5; 12 < x < 18; 5 < y < 8; 0 ≤ z < 2; there is up to 1 at.%, preferably up to 0.5 at.%, of impurities.

Description

The present invention relates to an alloy, in particular an amorphous Co-based alloy, and a nanocrystalline Co-based alloy and a method for producing a nanocrystalline metal band, in particular a nanocrystalline metal band from a Co-based alloy. Due to their high magnetic flux density, high permeability, low core losses and the associated low energy consumption, soft-magnetic, metallic materials are used in electronic components in different areas. Typical applications are, for example, in the area of current transformers, transformers and chokes. Co-base amorphous alloys can be used for these applications, which are manufactured in ribbon form by rapid solidification technology. They are characterized by outstandingly good soft magnetic properties such as extremely low coercive field strength _c and negligible magnetostriction λ _s . High permeabilities in the range of up to several 100,000 are also achieved. In order to convert the amorphous strip into a nanocrystalline strip, the metal strip is continuously post-treated, for example heat-treated in the run under tensile stress. A suitable method and conveyor system is disclosed in DE 102015 102765 A1 and in WO2103/156010 A1. There is a need for optimized electronic devices with higher efficiency or lower energy consumption, with higher operating frequencies and smaller construction volumes. To achieve these goals, improved soft magnetic alloys that can be reliably manufactured are desirable. The object is therefore to provide a soft-magnetic alloy that enables higher operating frequencies in electronic devices and that can be produced reliably. According to the invention there is provided an alloy described by the formula Co _100-abcdxyz Fe _a Cu _b M _c T _d Si _x B _y Z _z where M is one or more of the group of elements Nb, Mo and Ta and T is one or more is the group of elements Mn, V, Cr and Ni and Z is one or more of the group of elements C, P and Ge, where a, b, c, d, x, y, z are given in atomic % , and a, b, c, d, w, y, z satisfy the following conditions: 1.5 < a < 15 0.1 < b < 1.5 1 ≤ c < 5 0 ≤ d < 5 12 <x<185<y<80≦z<2 Up to 1 at.%, preferably up to 0.5 at.% of impurities can be present in the alloy, these not being covered by the formula . This alloy is a Co-based alloy that can be produced by rapid solidification technology and thus in the form of a ribbon with a small thickness. The alloy in the form of a ribbon can be heat treated in-line under tension to produce a nanocrystalline ribbon with a low permeability. Since the alloy according to the invention can be produced as a strip with a small thickness and low permeability, the alloy is suitable for ensuring higher efficiency and lower energy consumption at higher operating frequencies and lower structural volume in their applications such as toroidal cores and the operating frequency in electronic devices to increase. Furthermore, the tensile stress at which the low permeability is achieved can be kept low. The risk of cracks is thus reduced, so that the tape can be reliably manufactured on an industrial scale. The alloy according to the invention has a value dH _k /dσ (tensile stress sensitivity) of greater than 1.0 A/cm/MPa, preferably greater than 1.5 A/cm/MPa. The tensile stress sensitivity dH _k /dσ describes the response behavior of an alloy to the development of an induced anisotropy to an applied tensile stress. The term describes the change in the induced anisotropy field H _k with the tensile stress σ applied during the heat treatment. The greater the value of the tensile stress sensitivity dH _k /dσ of an alloy, the less tensile force or tensile stress has to be applied to achieve the desired permeability level during continuous heat treatment. Only a tensile stress difference Δσ of less than 200 MPa, preferably less than 100 MPa, is required due to the high value of the tensile stress sensitivity in the alloy according to the invention in order to form a permeability of μ=60. Consequently, the heat treatment of the alloy according to the invention can be carried out as a continuous heat treatment under tensile stress, with the necessary tensile stress being minimized in terms of alloy technology via the high sensitivity to tensile stress dH _k /dσ. This minimizes breaks and tears and ensures the economy of the entire process. The economical production of low-permeability strip material, for example with a permeability of less than 200, with soft magnetic properties that are suitable for the production of toroidal tape cores, is created, which is used, for example, in interphase transformers, isolation transformers, fly-back transformers, storage chokes and PFC chokes are used. The strong frequency dependence of the permeability of the inventive alloy is used herein to increase the frequency of operation of these applications. The invention is based on the following findings. According to the eddy current theory, the intrinsic permeability (µ _i ) remains constant until the eddy current limit frequency f _g is reached. Within this range, there is a constant permeability for the dimensioning of a component. After that, the permeability drops drastically. So with constant permeability properties can not here more to be expected. For the high permeabilities in the range of up to several 100,000, the achievable frequency is therefore limited to eddy current limit frequency values below 100kHz. In order to optimize components by increasing the working frequency, the eddy current limit frequency for the materials used must be increased. From the calculation formula (1) for the eddy current limit frequency f _g it can be seen that there are several possibilities for this.

(1) where ρel. the specific electrical resistance, µ ₀ , µ _DC and d the thickness of the material. In principle, the specific electrical resistance ρ _el. could be increased by appropriate alloy design. However, the use of non-metallic additives, ie metal odes such as B, Si, P and C, is limited to approx. 25 at% if the material is produced as an amorphous ribbon using a rapid solidification process. On the other hand, the eddy current limit frequency f _g can be shifted towards higher values very efficiently (d ² ) by reducing the thickness d of the tape used for a toroidal core. With regard to magnetic applications, the strip thickness for amorphous strips from a rapid solidification process can be varied in a range from 10 µm to 25 µm - i.e. by a good factor of 2. However, the most efficient way to drastically increase the eddy current limit frequency f _g to higher values is to change the intrinsic permeability (µ _i ) of the soft-magnetic strip material used. Since the permeability can in principle be varied by several orders of magnitude, eddy current cutoff frequency values in the two-digit MHz range are possible. To do this, however, the permeability of the soft magnetic material must be reduced to values below 100. For the nanocrystalline Fe-based and amorphous Co-based alloys, the permeability must therefore be reduced by several orders of magnitude from the current permeability level of around 100,000 to values below 100. FIG. 1a shows the achievable permeability μ for different methods of introducing a transverse field anisotropy, ie the anisotropy transverse to the longitudinal axis of the strip from which the toroidal strip core was wound, as a function of the saturation induction B _s for different alloy systems and material states. In a first approach, the anisotropy is introduced by a stationary heat treatment in a magnetic field perpendicular to the longitudinal axis of the strip from which the toroidal core was wound (marked with – “Core: Transverse Field Annealing”). In this case, for the group of amorphous Co-based alloys, the achievable permeability can be set in a range from 200,000 to just under 1000 by varying the composition and by varying the heat treatment conditions. Extremely low permeability below 100 cannot be achieved in this system. The same applies to the group of nanocrystalline Fe-based alloys. The standard VITROPERM® alloy with the composition Fe-Cu ₁ Nb ₃ Si _15.5 B _6.5 (at%) shows a permeability variation due to different heat treatment conditions in the range from 200000 to 25000. Other VITROPERM®-like alloys, all of them 1.2T to 1.3T saturation induction and in which the development of a higher anisotropy was favored by foreign elements such as Ni and Co, show permeabilities in the range of 12000 to 2500. Thus, neither with the Co-based alloys nor with The extremely low permeability range below 100 can be achieved with the nanocrystalline Fe-based alloys. Another approach to introduce a high transverse anisotropy into a soft magnetic material relates to heat treatment under a tensile stress applied along the strip axis (marked with – “Strip: Stress Annealing”). With this method, in which a so-called tensile or strain-induced anisotropy is introduced into the tape, the permeability can be reduced by orders of magnitude in most cases. For nanocrystalline Fe-based alloys, the induced anisotropy can be specifically increased with this method up to the range of the magnetocrystalline anisotropy of approx. 8 kJ/m ³ . That's what you get with these nanocrystalline Fe-based alloys in the saturation magnetization range B _s approx. 1.2T to 1.3T permeabilities of 2000 to 200 under production conditions, as well as permeabilities of up to 100 under laboratory conditions. Due to the lower tensile stress sensitivity of these alloys, comparatively high tensile stresses are required, which leads to frequent tears, so that even with the greatest effort and the best conditions (laboratory conditions) for relevant production lengths, a permeability level of 100 cannot be achieved . FIG. 1a also shows another material class, the nanocrystalline Co-based alloys with a saturation magnetization in the range from 0.8T to 1.0T. The process of heat treatment under tensile stress along the strip axis leads to an enormously high tensile stress-induced anisotropy and a very low permeability far below 100 in these nanocrystalline Co-based alloys. These alloys cannot be reliably produced on an industrial scale. FIG. 1b shows, for example, the 60 Hz permeability μ of toroidal strip cores for nanocrystalline Fe-based and nanocrystalline Co-based alloys, which were produced using the heat treatment process under tensile stress along the strip axis, as a function of the tensile stress which is necessary to achieve the desired permeability level. Normally, there is a more or less linear, positive relationship between the tensile stress-induced anisotropy and the applied tensile stress in this method. Therefore, lower permeability is achieved with higher tensile stresses. FIG. 1b also shows an assignment to various electronic component applications. DC-tolerant current transformers are used in the 60Hz permeability range of 3000 to 1000, flyback transformers in the permeability range of 400 to 100, for example, and storage chokes and PFC chokes in the lowest permeability range below 100. According to FIG. 1b, very low permeability with moderately applied tensile stress can therefore only be achieved with nanocrystalline Co-based alloys. The Curie temperature of amorphous Co-based materials is typically below 400°C and the soft magnetic properties degrade approaching and above this temperature. Starting with Fe-based nanocrystalline alloys, which already have a Curie temperature of around 600°C, the addition of cobalt increases the Curie temperature of the amorphous phase and thus extends the operating or application temperature range. Optimized nanocrystalline ribbon cores open up - with high saturation magnetization and at the same time very precisely adjustable permeability - a comparatively large permeability range. This makes them usable for a wide variety of applications. For storage chokes, this also makes permeability values above and below approx. 100 accessible, which opens up new possibilities for realizing chokes with a comparatively lower number of turns in order to reduce copper losses. The permeability range from several hundred to a few 1000 is interesting for highly linear, DC-tolerant current transformers, since the strips heat-treated under tensile stress have an almost constant permeability up to saturation (µ(H)=constant) and this property now, independent of the modulation can be obtained for the entire core. As already explained, a very low permeability of soft-magnetic toroidal tape cores can only be achieved with nanocrystalline Fe-based or Co-based alloys, whereby the tape material required for the toroidal tape core is laid in a stretched form and under applied tensile stress along the Tape axis is heat treated to convert the amorphous alloy into the nanocrystalline state. A toroidal tape core has a wound metallic and soft magnetic tape with the minimum possible tape thickness. In both cases, the ribbon undergoes a transformation from the initially amorphous state to the nanocrystalline state during the process. Among other things, the nanocrystalline state differs from the amorphous state in that it is significantly more brittle. In order to set tensile stresses in the metallic strips, S-roller systems or modified conveyor systems, as in DE 10, are used, for example 2015102 765 A1. However, the introduction of tensile stress leads to a significant increase in tears during this heat treatment process. After the transition to the nanocrystalline state, thermal relaxation occurs in the strip material, which leads to embrittlement of the material as a whole or to brittle spots along the metallic strip. The nanocrystalline state is therefore a glassy, brittle state compared to the amorphous state. The material can no longer be folded or cut. If the bending is too great or if the tensile force is not applied parallel to the axis, the belt breaks into small fragments. The nano-crystalline strip material can only be exposed to very high tensile stresses if the force is applied precisely to the degree along the longitudinal axis of the strip. Many years of tests have shown that only a bending load leads to fractures and tears in the strip material, but a pure tensile load along the strip axis is relatively uncritical. However, deflections and bending of the material are necessary for a continuous heat treatment process under applied tensile stress. Furthermore, production-related deviations from axis-parallel force introduction and imperfect straight running of the strip material are unavoidable. Therefore, there will always be breaks and tears in the strip material in production. A continuous heat treatment under tension on metallic strips only leads to an economical process if the number of tears of the strip under tension, in the stretched state between the conveyor rollers and in the area of the conveyor rollers, can be minimized. The modulus of elasticity for these alloys in the nanocrystalline state is around 200GPa. Thus, the maximum tensile stresses of less than 600 MPa applied during heat treatment would only lead to minimal stretching in the elastic area of the material and can thus be ruled out as the cause of the material tearing off or breaking. According to the invention, the surface and edge structure of the strip material produced via a rapid solidification process is regarded as the cause of breakage and tears in the nanocrystalline state. In the rapid solidification technology required to produce amorphous foils, a glass-forming metal alloy is melted in a crucible that typically consists essentially of oxidic ceramics and/or graphite. Depending on the reactivity of the melt, the melting process can take place in air, under vacuum or in an inert gas such as argon. After the alloy has been melted to temperatures well above the liquidus point, the melt is transported to a casting table and sprayed through a casting nozzle, which usually has a slit-shaped outlet opening, onto a rotating copper alloy wheel. For this purpose, the casting nozzle is brought very close to the surface of the rotating wheel, typically a copper wheel, at a distance of about 50 – 500 µm. The melt, which passes through the nozzle outlet and hits the moving copper surface, solidifies there at a cooling rate of around 10 ⁶ K/s. The rotating movement of the copper wheel transports the solidified melt away as a continuous strip of film, loosens it from the chill roller and wound up on a winding device as a continuous strip of film or strip. Wear of the casting wheel surface during the casting process leads to an increased roughness of the wheel surface. This leads to the formation of cavities or similar structures, which on the one hand transport process gas under the melt gob or lead to larger gas bubbles in the contact area of the melt gob to the casting wheel. When the melt solidifies, these gas bubbles are frozen in the amorphous strip and can lead to hole-like defects in the strip. On the other hand, the increased wear of the casting wheel leads to unevenness and increased roughness on both surfaces of the solidified strip via an embossing effect. Thus, in the case of metallic amorphous foils, which were produced via a rapid solidification process, one must generally always reckon with the surface and edge structures described above. In particular, this also applies to all strips used in the above-mentioned heat treatment processes under applied tensile stress along the strip axis. The tapes will always have imperfections, which can then be regarded as the cause of breakage and tearing. As shown in FIGS. 1 and 2, modern, optimized applications require extremely low-permeability, soft-magnetic tape-shaped materials for the production of toroidal cores, which are combined with nanocrystalline Fe-based or nanocrystalline Co-based alloys via a heat treatment process applied tensile stress. The problem here is that high tensile stresses are required for low permeability, while the material in the nanocrystalline state is very brittle and increasingly breaks and tears at high tensile stresses during bending and deflection. Under production conditions, this leads to a process that is no longer economical with excessive material and personnel costs and ultimately to excessively high production costs. According to the invention, this problem is solved by striving to minimize the necessary tensile stress. The lower the necessary tensile stress to achieve the desired permeability level can be kept during the continuous heat treatment process, the less the nanocrystalline tape is mechanically stressed and the fewer breaks or tears will occur. A continuously running process can thus be achieved. As a result, the profitability of the process should increase or the personnel expenses and product costs should be minimized. According to the invention, an alloy system is thus provided in which maximum anisotropy can be induced with minimal tensile stress, but also covers a wide anisotropy range or permeability range in order to meet the requirements of different component applications. Of course, the good castability of the amorphous starting material and the soft magnetic behavior of the alloy such as the lowest coercive field H _c , minimum magnetostriction λ _s , minimum remanence ratio B _r /B _max and the lowest non-linearity of the hysteresis loop nlin after the heat treatment must of course be maintained as boundary conditions. The non-linearity of the hysteresis loop nlin describes the linearity of the central part of the hysteresis loop that lies between the anisotropy field strength points that mark the transition to saturation. A linear part of this central part of the hysteresis loop is defined herein by a non-linearity factor nlin or NL, where the non-linearity factor NL is given by the formula

can be calculated and described. Here, δBauf and δBab denote the standard deviation of the magnetic polarization from a regression line through the ascending or descending branch of the hysteresis loop between polarization values of ±75% of the saturation polarization B _s . The loop is therefore the more linear the smaller NL is. The alloys according to the invention have an NL value of less than 0.8%. Soft-magnetic Co-base alloys are provided according to the invention, which can be produced as an amorphous strip via a rapid solidification process and can then be converted into an amorphous-nanocrystalline mixed state via a heat treatment with minimal, axially applied tensile stress and then a desired low-permeability and be in an appropriately soft magnetic state. The strip-shaped material obtained from this process is then processed into toroidal tape cores and used in transformers or wire coils. In some exemplary embodiments, the composition is further defined, with the iron content a being 4.0≦a≦15.0, preferably 4.6≦a≦14.6.0 and/or the copper content being 0.1<b<0. 9, preferably 0.7 < b < 0.9. In some embodiments, the element M is exclusively Nb and 2≦c≦4, preferably 2≦c≦3. In some embodiments, the element T is exclusively Mn and 0≦d<≦2.5. In some exemplary embodiments, the content of metalloids or glass-forming elements is further defined. In some exemplary embodiments, 14≦x≦16 applies to the silicon content and/or 5≦y≦7 to the boron content. In some exemplary embodiments, the sum of metalloids is defined more precisely, where 20≦(x+y+z)≦25. The alloy can be provided in the form of a strip and has a thickness of 10 μm to 25 μm, preferably 12 μm to 20 μm and/or a width of 2 mm to 300 mm, preferably 40 mm to 300 mm, and/or a continuous length of at least 2 km, or at least 8 km, as a cast strip. In some exemplary embodiments, after heat treatment, the alloy has a nanocrystalline structure in which at least 90% by volume of the grains have an average size of less than 25 nm. In the nanocrystalline state, the alloy can have a permeability of 20 to 130, preferably a maximum of 100, preferably a maximum of 60 and/or a saturation magnetization B _s ^nano of =0.75 to 1.05T and/or a saturation magnetostriction Iλ _s ^nano I of <8 ppm, preferably <2 ppm and/or a remanence ratio Br/B _max <0.1, preferably <0.05 and/or a coercivity (f=60Hz) H _c of <2A/cm, preferably <1.5A/cm and/or a ratio of H _c to the induced one Anisotropy field H _k , H _c /H _k <0.05, preferably _{Ht c} /H _k <0.01 and/or a non-linearity of the hysteresis loop of <1%, preferably <0.5%. In the nanocrystalline state, the alloy can have a tensile stress sensitivity dH _k /dσ greater than 1.0 A/cm/MPa, preferably greater than 1.5 A/cm/MPa, where H _k denotes the induced anisotropy field and σ denotes tensile stress. This tensile stress sensitivity has the advantage that a lower permeability, preferably from 20 to 130, preferably at most 100, preferably at most 60 using a small tensile stress in a continuous heat treatment in order to ^{avoid cracks in the strip during the heat treatment process.} In some embodiments, the alloy is at least 80% by volume amorphous. This alloy can be in the cast condition and can serve as a starting alloy for the production of a nanocrystalline alloy. A method of making a nanocrystalline metal ribbon is also provided. An amorphous metal strip made from an alloy according to one of the preceding exemplary embodiments is provided and heat-treated in the run under tensile stress σ from 1 MPa to 300 MPa at a temperature T _a , where 450° C.≦T _a ≦750° C. to produce a nanocrystalline metal ribbon in which at least 90% by volume of the grains have an average size of less than 25 nm. In some implementations, the tension is varied during the on-line heat treatment. Thus, the uniformity of the magnetic properties over the length of the metal strip can be improved. In some embodiments, the nanocrystalline metal strip has a tensile stress sensitivity dH _k /dσ greater than 1.0 A/cm/MPa, preferably greater than 1.5 A/cm/MPa. This can be achieved through appropriate compositional selection and used to achieve lower permeability at lower tensile stress. In some exemplary embodiments, the nanocrystalline metal strip has a permeability of 20 to 130, preferably a maximum of 100, preferably a maximum of 60 and/or a saturation magnetization B _s ^nano of =0.75 to 1.05T and/or a saturation magnetostriction Iλ _s ^nano I of <8 ppm after the heat treatment , preferably <2ppm and/or a remanence ratio Br/B _max <0.1, preferably <0.05 and/or a coercivity (f=60Hz) H _c of <2A/cm, preferably <1.5A/cm and/or a ratio of H _c to the induced anisotropy field H _k , H _c /H _k , Hc/H _k <0.05, preferably H _c /H _k <0.01 and/or a non-linearity of the hysteresis loop of <1%, preferably <0.5%. In some implementations, the metal strip is pulled through a continuous furnace with a heating zone with a length of 30 cm to 3 m at a speed s, so that the film stays in a temperature zone of the continuous furnace with the temperature T _a between 2 seconds and 10 minutes lies. In some implementation forms, after the continuous heat treatment, the metal strip is wound into a toroidal strip-wound core. After the heat treatment, the metal strip can be wound directly on a take-up spool to form a toroidal tape core. The amorphous metal strip can be coated with an electrical insulation layer before the heat treatment. This is advantageous if, after continuous heat treatment, the metal strip is wound directly onto a take-up spool, which is arranged downstream of the furnace in which the continuous heat treatment is carried out. In some implementations, a desired value of the permeability μ and/or the anisotropy field H _k and/or a magnetic strip cross section A _Fe and a permitted deviation range are measured inline during the process and in the throughput by continuously measuring the magnetic properties of the metal strip as it leaves the continuous furnace be measured while the metal strip is no longer under tension. If deviations from the permitted deviation ranges of the magnetic properties are determined, the tensile stress σ on the metal strip is readjusted accordingly to the measured Bringing values of magnetic properties back within the allowed deviation ranges. In some implementations, a predetermined active material cross-section Core-A _Fe is provided for the manufactured toroidal core. In some embodiments, in order to provide a predetermined active material cross-section of a toroidal strip core Core-A _Fe , the magnetic strip cross-section A _Fe on the metal strip is determined locally and continuously as it leaves the continuous furnace, the number of strip layers of the toroidal strip core is based on the determined values of the magnetic Strip cross-section A _Fe calculated and the toroidal tape core wound with the calculated number of tape layers. The amorphous metal ribbon can be made with a rapid solidification technology wherein a melt is poured onto a moving outer surface of a moving heat sink, the melt solidifies on the outer surface and the amorphous metal ribbon is formed. In some implementations, the outer surface of the heatsink is continuously machined to smooth the outer surface of the heatsink while the melt is poured onto the moving outer surface of the heatsink. The surface treatment agents can be used to treat the outer surface by removing or forming. As the forming surface treatment means, there may be provided a rolling device which is pressed onto the outer surface of the casting wheel while the casting wheel is rotating. In this context, “reformed” and “transforming” are understood to denote the redistribution of material. The removal of material from the outer surface, as can be done with a brush, is not the purpose of using the rolling device. arise therefore no chips, almost no abrasion and dust, which could negatively affect the manufacturing process of the metal strip. As the abrasive surface treatment means, a polishing device that is pressed onto the outer surface of the casting wheel while the casting wheel rotates, and/or a grinding device that is pressed onto the outer surface of the casting wheel while the casting wheel rotates, and/or one or more brushes - ten to be pressed onto the outer surface of the casting wheel while the casting wheel rotates can be provided. The brushes can also have a cleaning effect and neither wear away nor reshape the outer surface itself. In one embodiment, the surface treatment agent is pressed against the outer surface of the casting wheel so that it continuously smooths the outer surface of the casting wheel while the melt is poured onto the outer surface of the casting wheel. This embodiment can be used for the rolling device. The alloy according to one of the preceding exemplary embodiments can be used in a DC-tolerant current converter or an interphase transformer or an isolation transformer or a fly-back transformer or a fly-back converter or a storage choke or a PFC choke for industrial and automotive automotive applications or an electronic control unit such as a DC/DC converter or a storage choke or a storage transformer or a filter choke with low-permeability core materials or an inductive energy store. In these applications, a high operating frequency is advantageous and can be achieved with the lower permeability alloy described herein. A toroidal strip core comprising a wound strip of an alloy according to any one of having previous embodiments is also provided. The toroidal core can also include an electrical insulation coating. Exemplary embodiments will now be explained with reference to the drawings. FIG. 1a shows the achievable permeability μ for different methods of introducing a transverse field anisotropy as a function of the saturation induction _Bs for different alloy systems and material states. FIG. 1b shows the 60Hz permeability μ of toroidal cores for nanocrystalline Fe-based and for nanocrystalline Co-based alloys. FIG. 2 shows a schematic representation of a device for post-treating a strip made of a Co-based alloy. Figure 3 shows the value of the tensile stress sensitivity dH _k /dσ for different nanocrystalline Fe-based and nanocrystalline Co-based alloys. Figure 4 shows the necessary tensile stress to reach the permeability level µ = 60 for different nanocrystalline Fe- Base and nano-crystalline Co-base alloys. FIG. 5 shows an example of a flat hysteresis loop with high linearity properties and definitions. Figure 6 shows the course of the anisotropy field H _k after a continuous heat treatment under a constant tensile stress of 60 MPa as a function of the heat treatment temperature T _a for alloys from Table 1. FIG. 7 shows the XRD spectrum (Cu-K _a ) on a sample after heat treatment at T _a =590° C. for alloy no. 21 (Co ₆₅ Fe _9.6 Cu _0.8 Nb _2.6 Si _15.5 B _6.6 ). FIG. 8 shows the course of the coercive field H _c after heat treatment at temperatures in the range from 400° C. to 800° C. for the alloys listed in Table 1. FIG. 9 shows the functional relationship between H _c and the heat treatment temperature T _a . FIG. 10 shows the course of the remanence ratio B _r /B _max after the heat treatment. FIG. 11 shows individual magnetostriction measured values on samples of different nanocrystalline Fe-based alloys and nanocrystalline Co-based alloys according to the invention. FIG. 12 shows the changes in magnetization and magnetostriction in the amorphous state and in the nanocrystalline state when the Co content is increased from 0 to 70at%. Figure 13 shows the anisotropy field H _k of the flat hysteresis loop after heat treatment at the optimum heat treatment temperature T _a as a function of the applied tensile stress for Fe-base and Co-base alloys from Table 1. Figure 14 shows measurements of the hysteresis loop on heat-treated samples, in which different tensile stress-induced anisotropies were introduced. Figure 15 shows measurements of the hysteresis loop on heat treated samples in which a different tensile stress-induced anisotropy was introduced. A Co-based alloy amorphous ribbon is produced by rapid solidification technology. The Co-based alloy is melted in a crucible, which typically essentially consists of oxidic ceramics and/or graphite. Depending on the reactivity of the melt, the melting process can take place under air, in a vacuum or in an inert gas such as argon. After the alloy has been melted to temperatures well above the liquidus point, the melt is transported to a casting table and sprayed through a casting nozzle, which usually has a slit-shaped outlet opening, onto a rotating wheel made of a copper alloy. For this purpose, the casting nozzle is brought very close to the surface of the rotating wheel, typically a copper wheel, at a distance of about 50 – 500 µm. The melt, which passes through the nozzle outlet and hits the moving copper surface, solidifies there at a cooling rate of about 10 ⁶ K/s. The rotating movement of the copper wheel transports the solidified melt away as a continuous strip of foil, loosens it from the chill roll and winds it up as a continuous strip on a winding device. The surface of the casting wheel is smoothed during the casting process to reduce wear of the casting wheel surface during the casting process, which would lead to increased roughness of the wheel surface. According to the invention, the Co-base alloy has a composition described by the formula Co _100-abcdxyz _Fea _Cub _Mc _Td _Six _By _Zz , where M is one or more of the group of elements Nb, Mo and Ta and T is one or more of the group of elements Mn, V, Cr and Ni and Z is one or more of the group of elements C, P and Ge, where a, b, c, d, x, y, z in Atomic % are given and a, b, c, d, w, y, z satisfy the following conditions: 1.5 < a < 15 0.1 < b < 1.5 1 ≤ c < 5 0 ≤ d < 5 12 < x < 18 5 < y < 8 0 ≤ z < 2 Up to 1 at%, preferred up to 0.5 at% of impurities may be present, which are not covered by the formula. The strip is produced in an amorphous state due to the high cooling rate and can be at least 90% by volume amorphous. The amorphous ribbon is then heat treated to produce a nanocrystalline state in which at least 90% by volume of the grains have an average size less than 25 nm. FIG. 2 shows a schematic representation of a device 20 for heat-treating a strip 21 made of a Co-based alloy under tensile stress. The device 20 has a continuous furnace 22 with a temperature zone 23, this temperature zone 23 being set in such a way that the temperature in the furnace in this zone is within +/- 5° C. of the tempering temperature Ta. The apparatus 20 further includes a spool 24 at the beginning of the furnace 22 on which the amorphous state alloy ribbon 21 is wound and a take-up spool 26 at the end of the furnace 22 on which the heat treated nanocrystalline ribbon 27 is taken up . The strip 21 is pulled from the spool 24 through the continuous furnace 22 to the take-up spool 26 at a speed s. In some embodiments, tape 21 is wound on take-up reel 26 into a magnetic core. The tension can be applied to the belt 21 by means of a tensioning arrangement 27 with a pair of rollers 28 at the beginning of the oven 22 and a tensioning roller arrangement 29 with a pair of rollers 30 at the end of the oven 22, so that the belt 21 is continuously conveyed through the oven 22 under a predetermined tension becomes. The tension roller assembly 29 has a single drive roller 31 and a freely rotating pinch roller 21 and may have a braking function. The tensioning arrangement 27 has a single drive roller 33 and a freely rotating pressure roller 34 . The drive roller 31 or 33 of each pair of S-rollers 28 or 30 is driven by a motor with a gear. While the pair of rollers 28 of the tensioning arrangement 27 at the beginning of the oven 22 exerts a braking function with a braking force F _B on the strip 21 to be post-treated, the pair of rollers 30 of the tensioning roller arrangement 29 at the end of the oven 22 generates a driving force FD which occurs in a start-up or acceleration phase at the beginning of the promotion is greater than during the after-treatment phase, in which the belt 21 passes the pairs of rollers 28 and 30 at a constant speed in the direction of passage under a tensile force F _Z =F _D =F _B . In an area between the two pairs of rollers 28 and 30, the metal strip 21 is thus exposed to an adjustable tensile stress σ along the strip axis. In the areas outside of the S-roller pairs 28 and 28 there is almost no or a significantly low tensile stress in the band 21 . A conveyor system 5 is thus provided with an S-roller system, in which a continuous post-treatment of strips 21 under tensile stresses is possible. Co-based alloys with a significantly higher anisotropy can be induced by heat treatment under tensile stress along the strip axis. The strength of the induced anisotropy K _u and thus the achievable permeability on the band 21 or on a toroidal tape core made from it depends directly on the tensile stress σ in the band 21 . The amorphous ribbon 21 is placed under tensile stress along the ribbon axis and transitions into the nanocrystalline state as it passes through the furnace 22 . The heat treatment under tensile stress leads to a high tensile stress-induced anisotropy with a magnetically easy plane transverse to the longitudinal direction of the strip and thus to low magnetic permeability behavior. The now nanocrystalline strip 21 is then immediately processed further in a winding process to form a toroidal strip core. However, the winding process to form a toroidal tape core can also take place in a separate process. The production of nanocrystalline toroidal tape cores therefore consists of two process parts. The first process is a heat treatment on the inserted strip of tape and determines the magnetic properties in the material. On the input side, an amorphous strip 21 made of a Co-based alloy, which has been cut to the final width and wound into a coil and has the potential to be converted into the nanocrystalline state, is used. The strip 21 is unwound from this coil 24 and drawn through a tubular heat treatment furnace 22 in a stretched form. With the help of a rocker arm that can be subjected to variable loads, the strip of tape is subjected to tensile stress along the axis of the tape. At annealing temperatures _Ta , above the crystallization temperature, the initially amorphous material in the heat treatment zone Ta of furnace 22 transitions to a nanocrystalline state. A typical device is a continuous furnace 22 with a furnace temperature profile with 5 heating zones, in which temperatures of up to 800° C. are possible. The length of the WB furnace can be 30 cm to 40 cm for laboratory furnaces and up to 3 m for production furnaces. An anisotropy is induced in the band 21 via the applied tensile stress, so that the soft-magnetic band 21 that is running out has a pronounced, flat hysteresis loop with a defined permeability μ, the permeability being measured along the band axis. The achievable permeability level µ and the induced anisotropy K _u are proportional to the applied tensile stress in the strip. The tape strip, which is no longer under tensile stress, is then guided through a measuring system 35, which determines in real time all the measured variables relevant to magnetic characterization, such as magnetic saturation flux, magnetic tape cross-sectional area, anisotropy field, permeability, coercive field, remanence ratio, losses, etc The continuously running process and the permanent measurement of the magnet properties after the annealing process now allow the overall process to be controlled, ie set to the desired target variable, which is shown schematically in FIG. How As stated above, a corresponding anisotropy K _u is induced in the material passing through for a given, constant tensile stress in the strip. The induced anisotropy K _u and thus the permeability µ= B _s ² /(2 µ ₀ K _u ) can be kept constant over the entire strip length with the help of tension control, i.e. a force along the strip axis that can be variably adjusted in the process . This represents the first major advantage of the method. To implement this, the force in strip 21 must be varied in small steps around a target tensile stress value in order to compensate for local influences such as strip thickness fluctuations, contact temperature differences and slight deviations in throughput speed. Another advantage is that the induced anisotropy K _u can be adjusted in a very wide range. Depending on the selected alloy and assuming that the tensile force in the band 21 can be changed over a wide area, permeabilities μ in the range from 2000 to below 100 are achieved. The combination of the two advantages, ie both approaching a desired anisotropy and then keeping it constant, is advantageous. For example, it is not sufficient to apply only a high tensile force to the strip 21 in order to achieve a low permeability—the target permeability would then only be set exactly for a short length section of the starting material. The reason for this is the relatively large variation in strip thickness of amorphous strips that are produced using a rapid solidification process. This strip thickness variation can be up to 10% over the length of the casting wheel circumference. It must therefore be possible to carry out additional, extremely fine and non-jerky traction force adjustments. The information about the necessary traction force variations are made available by the measuring system 35. In the second process step, the strip material with preset magnetic properties is further processed into toroidal strip cores with a defined inductance. The technology described above provides soft-magnetic strip material of a specific permeability level with extremely low µ-deviations over the entire strip length in a continuous process. In order to produce toroidal cores with a defined inductance, it is advantageous to keep the active material cross-section (core A _Fe ) on the core constant in addition to the permeability. The information required for this (magnetic strip cross-section, local A _Fe of the strip) is also determined by the measuring system35. Knowing the local A _Fe on the strip 21 passing through can thus be used to calculate the number of strip layers which are necessary in order to achieve the specified active material cross section on the core (core A _Fe ). A variation in the strip thickness, caused for example by the rapid solidification process itself, can thus be compensated for and the deviations in the active material cross section from core to core are thus minimized. If the two sub-processes are now combined, the result is an overall process that leads to toroidal cores with a very precisely adjusted permeability value µ and a very precisely adjusted active material cross-section (core A _Fe ). The averaging process when winding the tape 21 into a toroidal core also has a very positive effect on maintaining close tolerances. The positive and negative deviations from the target values compensate each other when winding up over several meters. Due to the active, continuous control process, very low sample scattering with regard to μ and core A _Fe is therefore expected even for large numbers of cores. The response behavior of an alloy to develop an induced anisotropy to an applied tensile stress is described with the value tensile stress sensitivity dH _k /dσ. The term tensile stress sensitivity dH _k /dσ describes the change in the induced anisotropy field H _k with the tensile stress σ applied during heat treatment. The greater the value of the tensile stress sensitivity dH _k /dσ of an alloy, the less tensile force or tensile stress has to be applied to achieve the desired permeability level during continuous heat treatment. Therefore, the tensile stress sensitivity dH _k /dσ was measured for different alloys and alloy systems. Typical heat treatment conditions are a tensile stress σ of 1 MPa to 300 MPa at a temperature T _a , where 450° C.≦T _a ≦750° C., in order to produce a nano-crystalline metal ribbon. The continuous furnace 22 can have a heating zone have a length of 30 cm to 3 m and the metal strip 21 has a speed s when passing through the continuous furnace 22, so that the strip stays in the temperature zone of the continuous furnace with the temperature T _a between 2 seconds and 10 minutes. FIG. 3 shows the result of the investigations. The value of the tensile stress sensitivity dH _k /dσ for different nanocrystalline Fe-based and nanocrystalline Co-based alloys, but in particular for the alloy system Co _x Fe _74.6-x Cu _0.8 Nb _2.6 Si _15.5 B _6.5 ( at%), where x was varied in the range 0 to 80%. The interpretation of the value dH _k /dσ is as follows: For example, the value dH _k /dσ for a pure nanocrystalline Fe-based alloy, i.e. with x = 0%, is approx. 0.2 A/cm/MPa. This means that a tensile stress difference of ∆σ = 5MPa is required to induce an anisotropy field strength of (a) 1A/cm. However, it is desirable to have to apply as little tensile stress difference as possible in order to induce an anisotropy field strength of (a) 1A/cm. However, if one considers nanocrystalline Co-based alloys from the range according to the invention, such as the alloy with x=67.5%, then a tensile stress sensitivity dH _k /dσ of approximately 2.3 to 2.4 A/cm/MPa results so you only need about 0.4MPa to induce an anisotropy field strength of (a) 1A/cm. This corresponds to an improvement by a factor of 12. In Figure 4, for different nanocrystalline Fe-based and nanocrystalline Co-based alloys, but in particular for the alloy system Co _x Fe _74.6-x Cu _0.8 Nb _2.6 Si _15.5 B _6.5 (at %), where x was varied in the range 0 to 80%, the necessary tensile stress to reach the permeability level µ = 60 is shown. While tensile stresses of 1000 MPa or more are necessary for nanocrystalline Fe-based alloys in order to achieve a permeability of μ=60, tensile stresses below 50 MPa are entirely sufficient for Co-based alloys according to the invention. Thus, the alloys according to the invention are distinguished not only by their chemical composition and compliance with certain soft-magnetic properties, but also by the property of having a tensile stress sensitivity dH _k /dσ above a certain minimum level. Only these alloys then remain below a certain limit tensile stress, in order to achieve µ=60, for example. FIG. 5 shows an example of a flat hysteresis loop with high linearity properties and the terms remanence ratio B _r /B _max , coercive field strength H _c , anisotropy field H _k , and permeability μ are explained. A measure of the linearity of the hysteresis loop is described by the non-linearity ratio nlin, which is calculated from the formula given. δB _up and δB _down designate the standard deviation of the magnetization from a regression line through the ascending and descending branch of the hysteresis loop between magnetization values of ±75% of the saturation magnetization B _s . The loop is therefore the more linear the smaller the value nlin is. In the following, a composition for an alloy is determined with which a lower permeability can be achieved at a tensile stress which is not too high so that cracks can be avoided and with which the economics of the process can be increased. Table 1 shows a summary of the results of all alloy variants examined. The nominal composition of the alloys in at%, the density and the saturation magnetization B _s in the amorphous and in the nanocrystalline state are given. All the alloys specified were produced as amorphous ribbons with a thickness of approx. 20 μm using a rapid solidification process. Table 1 also shows the crystallization temperatures (o-onset, p-peak) of the first T _x1 and the second crystallization phase T _x2 , determined from a DSC measurement at a heating rate of 10K/min. Table 1 also lists the alloys according to the invention. Table 2 shows the chemical analysis (GDOES method) of the amorphous ribbons produced, first in percent by weight (wt%) and then converted into atomic percent (at%). The comparison of the analyzed at% values with the nominal composition in at% suggests that the target alloy compositions were met with an accuracy of better than 1%. In order to determine the tensile stress sensitivity (dH _k /dσ) of the alloys, the Co-based alloys shown in Table 1 are subjected to continuous annealing under constant tensile stress (60 MPa) with constantly increasing heat treatment temperature in the range from 400°C to 800°C C performed. However, the increase in the heat treatment temperature (approx. 1°C to 2°C per minute) was so small that the strip passed through the furnace (length approx. 40cm) at a speed of 1.6m/min at practically every temperature in the range of 400°C to 800°C was in thermal equilibrium. The magnetic properties, saturation flux F _s , anisotropy field H _k , coercive field H _c , remanence ratio B _r /B _max and non-linearity nlin are measured in a measuring system as part of continuous annealing after leaving the tube furnace on the strip. FIG. 6 shows the course of the anisotropy field H _k after a continuous heat treatment under a constant tensile stress of 60 MPa as a function of the heat treatment temperature T _a for alloys from Table 1. Both inventive alloys and other comparative examples are shown. One sequence can be seen in the response behavior to an applied tensile stress in relation to the development of anisotropy. While the nanocrystalline Fe-based alloys such as VP800® and VP712® have very low anisotropy field strengths and when alloying 3at% to 9at% Co the anisotropy field drops even further, from 60at% Co the result is always higher Anisotropy field strengths with constant tensile stress. From the knowledge of the course of the anisotropy field strength as a function of the heat treatment temperature, the optimal heat treatment temperature for each alloy was determined, at which the applied tensile stress was then varied in order to finally determine the tensile stress sensitivity dH _k /dσ for each alloy determine. For example, an optimum heat treatment temperature T _a of 590° C. was specified for alloy No. 21 (Co ₆₅ Fe _9.6 Cu _0.8 Nb _2.6 Si _15.5 B _6.6 ). The nanocrystalline state of the Co-based alloy was examined on strip samples at different heat treatment temperatures T _a . Figure 7 shows the XRD Spectrum (Cu-K _a ) on a sample after heat treatment at T _a = 590°C for alloy no. 21 (Co ₆₅ Fe _9.6 Cu _0.8 Nb _2.6 Si _15.5 B _6.6 ). The spectrum shows clear crystalline (bcc) structures, which clearly indicate a nanocrystalline state. The evaluation of the "peak" widths resulted in a nanocrystalline grain size of approx. 14nm to 18nm. Figures 8, 9 and 10 complete the graph of magnetic parameters for the heat treatments at temperatures ranging from 400°C to 800°C for the alloys listed in Table 1. FIG. 8 shows the course of the coercive field H _c after the heat treatment. After the transition to the nanocrystalline state, the coercive field generally remains low; only after a temperature in the vicinity of the second crystallization peak has been exceeded and fewer soft-magnetic phases have formed does H _c rise steeply. In FIG. 8, the soft magnetic area of interest is marked with a rectangle. This section of the functional relationship between H _c and the heat treatment temperature T _a is shown in FIG. 9 as a detailed image. At these heat treatment temperatures, the alloys according to the invention have a coercive field H _c in the range of 1 A/cm. FIG. 10 shows the course of the remanence ratio B _r /B _max after the heat treatment. After the transition to the nanocrystalline state, the remanence ratio B _r /B _max for the selected alloys decreases from the maximum value of 1 towards zero. This shows that the hysteresis loop changes from an initially “round” shape to a “flat” hysteresis loop with high induced anisotropy. The optimum heat treatment range can again be determined from FIG. In the optimum heat treatment range, there are always “flat” hysteresis loops with a very low remanence ratio B _r /B _max . Another important parameter that describes the hysteresis, the non-linearity nlin, can also be measured on the strip as part of the continuous annealing after leaving the tube furnace and in a similar way to H _c and B _r /B _max as a function of the Heat treatment temperature T _a are shown. The last parameter relevant for an optimal soft magnetic state is the saturation magnetostriction λ _s in the nanocrystalline state of the respective alloys. The magnetostriction λ _s cannot be determined "online" during the continuous annealing process, which is why there is no continuous measurement data for this variable over the entire heat treatment range. However, FIG. 11 shows individual magnetostriction measured values both on samples of different nanocrystalline Fe-based alloys and nanocrystalline Co-based alloys according to the invention, which were taken from the continuous continuous annealing process at a constant tensile stress of 60 MPa at the respective heat treatment temperatures T _a . The left part of FIG. 11 shows the magnetostriction values at the amorphous initial state of the alloys—for Fe-based alloys the magnetostriction λ _s is +20 ppm, for Co-based alloys in the range below +7 ppm. In order to achieve a usable soft magnetic state for optimized applications, the magnetostriction after heat treatment must be as low as possible. FIG. 11 shows that this goal is achieved for Fe-based alloys and also for the nanocrystalline Co-based alloys according to the invention. In order to work out such nanocrystalline Co-based alloys from all possible alloy ranges, which require the highest tensile stress sensitivity dH _k /dσ or which require the smallest tensile stress difference ∆σ in order to develop a permeability of µ = 60 and at the same time to meet all the conditions for optimal soft-magnetic cal behavior, the behavior of the saturation magnetization B _s and the saturation magnetostriction λ _s as a function of the Co content (Co _x ) in the system Co _x Fe _74.6-x Cu _0.8 Nb _2.6 Si _15.5 B _6.6 will now be discussed in more detail. FIG. 12 shows the changes in magnetization (symbol: rectangle) and magnetostriction (symbol: circle) in the amorphous state (closed symbols: ■, ●) and in the nanocrystalline state (open symbols: □, O) for samples from the Alloy system when increasing the Co content from 0 to 70 at%. The detailed image in FIG. 12 shows the values for magnetization and magnetostriction for the alloy range according to the invention from x=60 atom % to x=70 atom %. For alloys from this range, it could be shown that the highest tensile stress sensitivity dH _k /dσ and the smallest tensile stress difference Δσ can be achieved, with the saturation magnetization B _s in the nanocrystalline state being in the range from 0.75T to 1.05T, and a saturation magnetostriction λ _s in the nanocrystalline state is in the range Iλ _s ^nano I <8 ppm, ^preferably <2 ppm, as can be seen from the detailed image of FIG. Depending on the alloy, the magnetostriction λ _s shows a zero crossing and has both positive and negative values. The following explains how the parameter tensile stress sensitivity dH _k /dσ was determined. The previously shown FIGS. 6, 8, 9, 10 and 11 give an overview of how the magnetic material properties develop with the heat treatment temperature T _a . It is therefore possible to define an optimum heat treatment temperature T _a for each alloy, at which further targeted continuous heat treatment is carried out at a constant temperature T _a . With this continuous heat treatment, however, the applied tensile stress σ is increased discretely, with the process then running again long enough at constant tensile stress after each increase in the tensile stress until a state of equilibrium is established. In this state or on these samples, all magnetic parameters such as saturation flux Φ _s , saturation magnetization B _s , saturation magnetostriction λ _s , anisotropy field H _k , coercive field H _c , remanence ratio B _r /B _max and non-linearity nlin are then determined. Figure 13 shows the anisotropy field H _k of the flat hysteresis loop after heat treatment at the optimum heat treatment temperature T _a as a function of the applied tensile stress σ for Fe-based and Co-based alloys from Table 1. From the linearization of the measured values recorded for the anisotropy field H _k and for the tensile stress σ of the individual alloys the tensile stress sensitivity dH _k /dσ was determined. Very low gradients dH _k /dσ result for Fe-based alloys with small Co additions of up to 9at%, whereas very high gradients _dHk /dσ are observed in the alloy range according to the invention with 60 atom% to 70 atom% Co . Table 3 shows a summary of all relevant magnetic properties for the alloys according to the invention and, for comparison purposes, the magnetic properties in the amorphous and for the nanocrystalline state for sought-after Fe-based alloys. Table 3 shows that with the alloys according to the invention, a tensile stress sensitivity dH _k /dσ of over 3 A/cm/MPa can be achieved in the best case, or that only about 30 MPa tensile stress difference is needed to set a permeability level of µ = 60 is required. In order for the alloy to be suitable for increasing the working frequency, not only the tensile stress sensitivity dH _k /dσ is maximized, but also soft magnetic properties are kept within the desired limits. The desired properties are a remanence ratio B _r /B _max <0.1, preferably <0.05, a coercive field strength (f=60 Hz) H _c <2 A/cm, preferably <1.5 A/cm and a ratio H _c /H _k < 0.05, preferably H _c /H _k < 0.01. In addition to Table 3, Tables 4 to 12 show the detailed measured values of the anisotropy field H _k , coercive field H _c , remanence ratio B _r /B _max , non-linearity nlin and the achievable permeability µ as a function of the heat treatment applied tensile stress σ. For comparison purposes, Tables 4 to 7 show results for nanocrystalline Fe-based alloys with Co additions of up to 9 at%, while Tables 8 to 12 contain the results for the alloys according to the invention. Some of the examples of nanocrystalline Co-based alloys do not show the defined soft magnetic properties in the nanocrystalline state after heat treatment under tensile stress, although the tensile stress sensitivity dH _k /dσ is relatively high, see alloys 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 24, 25 and 26 of Table 1. This means that only certain alloy systems can fulfill the combination of maximized tensile stress sensitivity dH _k /dσ and specially defined soft magnetic properties. An example of an alloy in which the defined soft-magnetic properties are not maintained is the alloy from Table 1 with No. 1 with the chemical composition Co _75.9 Fe _2.3 Mn _2.3 Nb ₄ Si ₂ B _13.5 . Figure 14 shows hysteresis loop measurements on heat treated samples of this alloy where different tensile stress induced anisotropies were determined using the above described continuous annealing process were introduced. As with the alloys according to the invention, very low permeabilities in the range from .mu.=130 to .mu.=22 are also achieved with this alloy with an increase in the tensile stress .sigma. during the heat treatment, which suggests a high sensitivity to tensile stress. However, the calculation of the tensile stress sensitivity dH _k /dσ results in a value of 0.97 to 1.08A/cm/MPa and is therefore below or at the limit of the required limit of 1.0A/cm/MPa. Furthermore, the soft magnetic properties such as H _c and B _r /B _max of these samples have to be evaluated. The detailed image in FIG. 14 shows a section around the origin of the hysteresis. One can clearly see the increased coercive field H _c and the increase in the remanence ratio B _r /B _max , especially with low tensile stress-induced anisotropy. The precise measurements revealed a remanence ratio _Br / _Bmax = 0.05, a coercive field _Hc = 4.9A/cm and a ratio _Hc / _Hk = 0.07. However, these values do not correspond to the soft magnetic properties defined above. In contrast, alloy No. 2 from Table 1 with the chemical composition Co ₆₆ Fe _8.3 Cu _0.6 Nb _2.6 Si ₁₆ B _6.5 can be used as an example according to the invention. Figure 15 shows hysteresis loop measurements on heat treated samples of this alloy in which a different tensile stress-induced anisotropy was introduced using the continuous annealing process described above. By varying the tensile stress applied during the heat treatment process in the range from 20MPa to 300MPa, permeabilities µ in the range from 110 to 10 are achieved with simultaneously optimized soft magnetic properties such as high linearity of the hysteresis loop (nlin), low coercive field _Hc and low Remanence ratio B _r /B _max in all permeability ranges. The calculation of the tensile stress sensitivity dH _k /dσ results in a value of 2.83A/cm/MPa and is thus far above the required limit of 1.0A/cm/MPa. One can clearly see the low coercive field H _c and the low remanence ratio B _r /B _max in the entire range of tensile stress-induced anisotropy. The precise measurements revealed a remanence ratio B _r /B _max < 0.01, a coercive field H _c < 1.5A/cm and a ratio H _c /H _k < 0.01, as required for the soft magnetic properties defined above. Accordingly, the Co-base alloy of _the present invention has a composition described by the formula Co _100-abcdxyz _Fea _Cub Mc _Td _Six _By _Zz where M is one or more of the group of elements Nb, Mo and Ta and T is one or more of the group of elements Mn, V, Cr and Ni and Z is one or more of the group of elements C, P and Ge, where a, b, c, d, x, y, z are given in atomic %, and a, b, c, d, w, y, z satisfy the following conditions: 1.5 < a < 15 0.1 < b < 1.5 1 ≤ c < 5 0 ≤ d < 5 12 < x < 18 5 < y < 8 0 ≤ z < 2 Up to 1 at.%, preferably up to 0.5 at.% of impurities may be present, which are not covered by the formula are.

Claims

Claims 1. Alloy described by the formula Co _100-abcdxyz Fe _a Cu _b M _c T _d Si _x B _y Z _z where M is one or more of the group of elements Nb, Mo and Ta and T is one or more of Group of elements Mn, V, Cr and Ni and Z is one or more of the group of elements C, P and Ge, where a, b, c, d, x, y, z are given in atomic %, and a , b, c, d, w, y, z satisfy the following conditions: 1.5 < a < 15 0.1 < b < 1.5 1 ≤ c < 5 0 ≤ d < 5 12 < x < 18 5<y<8 0≦z<2 and up to 1 at.%, preferably up to 0.5 at.%, of impurities.

2. The alloy of claim 1, wherein 4.0≦a≦15.0, preferably 4.6≦a≦14.6.

3. An alloy according to claim 1 or claim 2, wherein 0.1<b<0.9, preferably 0.7<b<0.9.

4. Alloy according to any one of the preceding claims, wherein M is Nb and 2≦c≦4, preferably 2≦c≦3. 5. Alloy according to any one of the preceding claims, wherein T is Mn and 0 ≤ d < ≤ 2,

5 applies.

6. Alloy according to one of the preceding claims, where 14≦x≦16 and/or 5≦y≦7 applies.

7. Alloy according to any one of the preceding claims, wherein 20 ≤ (x + y + z) ≤ 25.

8. Alloy according to one of the preceding claims, which has a thickness of 10 μm to 25 μm, preferably 12 μm to 20 μm and/or a width of 2 mm to 300 mm, preferably 40 mm to 300 mm and/or in the cast state has a continuous length of at least 2 km or at least 8 km.

9. The alloy as claimed in any of claims 1 to 8, which, after heat treatment, has a nanocrystalline structure in which at least 90% by volume of the grains have an average size of less than 25 nm.

10. Alloy according to claim 9, which in the nanocrystalline state has a permeability of 20 to 130, preferably a maximum of 100, preferably a maximum of 60 and/or a saturation magnetization B _s ^nano of = 0.75 to 1.05T and/or a saturation magnetostriction Iλ _s ^nano I of <8ppm, preferably <2ppm and/or a remanence ratio B _r /B _max <0.1, preferably <0.05 and/or a coercivity (f=60Hz) H _c of <2A/cm, preferably <1.5A/cm and/or a ratio of H _c to the induced anisotropy field H _k , H _c /H _k , <0.05, preferably H _c /H _k <0.01 and/or a non-linearity of the hysteresis loop of <1%, preferably <0.5%.

11. Alloy according to claim 9 or claim 10, which in the nanocrystalline state has a tensile stress sensitivity dH _k /dσ of greater than 1.0 A/cm/MPa, preferably greater than 1.5 A/cm/MPa, where H _k is the induced anisotropy field and σ denotes tensile stress.

12. An alloy according to any one of claims 1 to 8 which is at least 80% by volume amorphous.

13. A method for producing a nanocrystalline metal strip, providing an amorphous metal strip from an alloy according to claim 12, heat treatment of the amorphous metal strip in the run under tensile stress σ from 1 MPa to 300 MPa at a temperature T _a , where 450 ° C ≤ T _a ≦750° C. in order to produce a nanocrystalline metal strip in which at least 90% by volume of the grains have an average size of less than 25 nm.

14. The method of claim 13, wherein the tension is varied during the on-line heat treatment.

15. The method according to claim 13 or claim 14, wherein the nanocrystalline metal strip has a tensile stress sensitivity dH _k /dσ of greater than 1.0 A/cm/MPa, preferably greater than 1.5 A/cm/MPa.

16. The method according to any one of claims 13 to 15, wherein the nanocrystalline metal strip has a permeability of 20 to 130, preferably at most 100, preferably at most 60 and/or a saturation magnetization B _s ^nano of = 0.75 to 1.05T and/or a saturation magnetostriction Iλs ^nano I of <8ppm, preferably <2ppm and/or a remanence ratio Br/B _max <0.1, preferably <0.05 and/or a coercive field strength (f=60Hz) _Hc of <2A/cm, preferably <1.5A/ cm and/or a ratio of H _c to the induced anisotropy field H _k , H _c /H _k , H _c /H _k <0.05, preferably H _c /H _k <0.01 and/or a non-linearity of the hysteresis loop of <1 %, preferably <0.5%.

17. The method according to any one of claims 13 to 18, wherein the metal strip is drawn at a speed s in the run through a continuous furnace with a heating zone with a length of 30 cm to 3 m, so that the metal strip stays in a temperature zone of the continuous furnace with the temperature T _a is between 2 seconds and 10 minutes.

18. The method according to any one of claims 13 to 17, wherein after the heat treatment in the run, the metal strip is wound into a toroidal core.

19. The method according to any one of claims 13 to 18, characterized in that a desired value of the permeability µ and/or the anisotropy field H _k and/or a magnetic strip cross section A _Fe and a permitted deviation range of each of these values are predetermined, by continuously measuring the magnetic properties of the metal strip as it leaves the continuous furnace while the metal strip is no longer under tensile stress, and if deviations from the permitted deviation ranges of the magnetic properties are detected, the tensile stress σ on the metal strip is readjusted accordingly in order to obtain the measured values of the bring magnetic properties back within the allowed deviation ranges.

20. The method according to any one of claims 13 to 19, wherein, in order to provide a predetermined active material cross section of a toroidal strip core Core-A _Fe , when leaving the continuous furnace the magnetic strip cross section A _Fe on the metal strip is determined locally and continuously, the number of strip layers of the The toroidal tape core is calculated on the basis of the determined values of the magnetic tape cross section A _Fe and the toroidal tape core is wound with the calculated number of tape layers.

21. The method according to any one of claims 13 to 20, further comprising: Manufacture of the amorphous metal ribbon with a rapid solidification technology wherein a melt is poured onto a moving outer surface of a moving heat sink, the melt solidifies on the outer surface and the amorphous metal ribbon is formed.

22. The method of claim 21, wherein the outer surface of the heat sink is continuously machined to smooth the outer surface of the heat sink while the melt is poured onto the moving outer surface of the heat sink.

23. Use of the alloy according to one of claims 1 to 11 in a DC-tolerant current transformer or an interphase transformer or an isolation transformer or a fly-back transformer or a fly-back converter or a storage choke or a PFC choke for industrial Industrial and automotive applications or an electronic control unit such as a DC/DC converter or a storage choke or a storage transformer or a filter choke with low-permeability core materials or an inductive energy store.

24. A toroidal tape core comprising a wound tape of an alloy according to any one of claims 1 to 11.

25. The toroidal core of claim 24, further comprising an electrical insulation coating.