WO2024074465A1

WO2024074465A1 - Method for producing a cofe alloy for a laminated core

Info

Publication number: WO2024074465A1
Application number: PCT/EP2023/077264
Authority: WO
Inventors: Johannes Tenbrink; Niklas Volbers; Jan Frederik Fohr; Maximilian Staab
Original assignee: Vacuumschmelze Gmbh & Co Kg
Priority date: 2022-10-04
Filing date: 2023-10-02
Publication date: 2024-04-11
Also published as: DE102022125560A1

Abstract

A first embodiment relates to a method for producing a CoFe alloy for a laminated core. According to the method, a stack of a plurality of laminations made of a CoFe alloy with 5 to 55 wt.% Co is thermally treated at a temperature (T1) in a background magnetic field for a holding time (t1) in a final annealing process, T1 ranging from 700 °C to 960 °C, preferably 720 °C to 950 °C, preferably 760 °C to 920 °C. The stack is then cooled to a temperature of less than 300°C, preferably less than 200°C, and is then thermally treated at least temporarily in an externally applied magnetic field at a temperature (T2) for a holding time (t2) in a magnetic annealing process, T2 ranging from 500 °C to T1, preferably from 650 °C to T1, preferably from 700 °C to T1. The stack is then cooled to a temperature of less than 300 °C, preferably less than 200 °C. The externally applied magnetic field is greater than the background magnetic field.

Description

Process for producing a CoFe alloy for a laminated core

The invention relates to a method for producing a CoFe alloy for a laminated core.

Electrical machines can be operated as motors or generators and in most designs have a stator made of a soft magnetic material. When electrical machines are in operation, the direction and strength of the magnetic flux in the rotor and stator are subject to periodic changes. This alternating magnetic field induces eddy currents in a circular direction, i.e. also in the direction of the longitudinal axis of the rotor and stator, which dissipate part of the energy supplied to the system as heat, so that this part no longer contributes to the performance of mechanical work. In order to keep eddy currents and the associated losses low, a high electrical resistance of the rotor and stator in the axial direction (direction of the longitudinal axis of the rotor and stator) is desirable. This is typically achieved by a layered structure.

The rotor and/or the stator of an electrical machine can have a large number of identical individual sheets (also called "laminations" or "layers") that are stacked on top of one another and electrically insulated from one another. Such a structure is called a laminated core (also called a "package"). The eddy current losses are proportional to the square of the sheet thickness of the individual layers. Consequently: the thinner the layers, the lower the eddy current losses and the higher the efficiency.

It is desirable that the stator and rotor have a high power density so that the electrical machine has a small construction volume and high performance. The fill factor of a laminated core describes the proportion of magnetically conductive material within the laminated core, which results from the total volume minus the volume of insulation layers, adhesive layers, air gaps and other layers with poor magnetic conductivity. The fill factor depends on, among other things, the power and torque density achievable with the drive, so that a high fill factor is desirable.

Furthermore, a high magnetic conductivity (permeability) of the material and the ability to carry as high a flux density (induction) as possible are advantageous in order to increase the performance of the stator. In applications where the highest possible power density is necessary or desired, cobalt-iron (CoFe) alloys are used. Commercially available CoFe alloys typically have a composition of 49% Fe, 49% Co and 2% V by weight.

The task is therefore to create a process with which a laminated core with a high power density can be produced.

According to the invention, in a first embodiment, a method for producing a CoFe alloy for a laminated core is provided. A stack of a plurality of sheets made of a CoFe alloy with 5 to 55% by weight of Co is provided. The stack is heat treated in a final annealing at a temperature T1 in a background magnetic field for a holding time t1, wherein T1 is between 700°C and 960°C, preferably 720°C to 950°C, preferably 760°C to 920°C. The stack is then cooled to a temperature of less than 300°C, preferably less than 200°C. The stack is then heat-treated in a magnetic field annealing process, at least temporarily, in an externally applied magnetic field at a temperature T2 for a holding time t2, where T2 is between 500°C and T1, preferably between 650°C and T1, preferably between 700°C and T1, and is then cooled to a temperature less than 300°C, preferably less than 200°C. The externally applied magnetic field is greater than the background magnetic field.

Even if no magnetic field is deliberately applied, a background magnetic field is present, since the earth itself generates a magnetic field with a field strength of approximately 0.05 mT. There may also be interfering magnetic fields, which are generated, for example, by the electrically conductive heating elements of a furnace. At the same time, a metal hood covering the stack can serve as a shield. Consequently, a background magnetic field is created, the value of which is determined from these and other factors. The background magnetic field is at most 1 kA/m, preferably less than 0.5 kA/m, preferably less than 0.1 kA/m.

In the magnetic field annealing, a further magnetic field is actively applied in addition to the background magnetic field, so that the total magnetic field in the magnetic field annealing is greater than the background magnetic field that is inherently present in the final annealing. The external magnetic field that is actively applied is thus greater than 0.1 kA/m. In some embodiments, the external magnetic field is set so that a field strength of greater than 0.5 A/cm, preferably greater than 1 A/cm, particularly preferably greater than 3 A/cm is present in the soft magnetic material to be annealed.

Heat treatment at elevated temperatures in an externally applied magnetic field provides sheets or a sheet package made of a CoFe alloy with improved soft magnetic properties.

The sheets each have a first main surface and a second main surface opposite the first main surface. The first main surface of a first sheet may be stacked on the second main surface of a second sheet in a stacking direction perpendicular to the first main surface to form the stack. In some embodiments, the sheets are loose in the stack. After magnetic field annealing, the sheets are processed into a laminated core. In other embodiments, the sheets of the stack are fastened together, for example with one or more welds or points, and form a laminated core. The laminated core is subjected to the final annealing and then to a separate magnetic field annealing.

The CoFe alloy contains between 5 and 55% Co by weight. The CoFe alloy can be a 50% CoFe alloy, the basic composition of which is approximately 49% Co by weight, approximately 49% Fe by weight and approximately 2% V by weight. Well-known trade names are VACOFLUX 48, VACOFLUX 50, VACODUR 50, VACODUR 49, VACODUR S Plus, HIPERCO 50, HIPERCO 50A, HIPERCO 50HS AFK 502 and AFK 502R. In addition to the main alloying elements Co, Fe and V are partly contained in these commercially available alloys as grain-refining additives such as Nb, Ta, Zr, which limit grain growth and thus enable increased strength.

CoFe alloys are subjected to a so-called final annealing process to improve their soft magnetic properties. The invention is based on the finding that these CoFe alloys have soft magnetic properties that are largely isotropic in the plane of the sheet and that the soft magnetic properties could be further improved for use in electrical machines if a magnetically preferred direction could be created.

The laminations of the stack or lamination packet may comprise a full section of a stator or a part of a stator such as a stator ring or individual teeth. The stack or lamination packet may comprise the shape of a stator or a segment of a stator, such as a lateral part, i.e. building blocks stacked on top of each other to form the part of a stator, or a part of a stator such as a stator ring or individual teeth (a so-called T-tooth), or a stator ring with individual teeth inserted into the stator ring.

For the production of laminated cores, it would be desirable if the individual parts had improved soft magnetic properties, such as increased permeability, in the direction of their magnetic stress. In the case of individual teeth, this would be the radial direction, i.e. the longitudinal axis of the tooth, which in the application points towards the center of the laminated core. In the case of a stator ring, this would be the circumferential direction for the ring.

The individual segments of a motor or generator laminated core should therefore ideally be manufactured in such a way that improved properties such as increased permeability and thus reduced magnetic resistance are present in the direction of their main magnetic stress. Improved properties can be seen in comparison to an isotropic sheet. According to the invention, these anisotropic soft magnetic properties are achieved by suitable magnetic field annealing, whereby these improved magnetic properties can be produced in the desired direction.

In some embodiments, after magnetic field annealing, the CoFe alloy has a remanence ratio Br/Bs of greater than or equal to 0.8 with a remanence Br and a magnetic saturation Bs measured at 160 A/cm and/or a maximum permeability p _max of more than 25,000, preferably more than 30,000, particularly preferably more than 40,000.

A distinction can be made between two types of technical implementation of magnetic field annealing. If the magnetic field is parallel to the later direction of the flux density in the component, this is referred to as a longitudinal magnetic field. In so-called longitudinal field annealing, the magnetic field can be specified by current-carrying lines in the annealing furnace. If an external uniaxial field is applied in a magnetic field furnace by coil systems, which is parallel to the later direction of the flux density in the component of the parts annealed in it, this can also be referred to as longitudinal field annealing.

A longitudinal field annealing differs from a so-called transverse field annealing, in which, for example, toroidal cores are strung on a rod and the magnetic field is in the axial ring direction or transverse to the band direction of the toroidal cores. This arrangement can be used to produce toroidal cores with a flat, linear "F" loop.

In the case of annealing a complete stator in the form of a stack of loose sheets or a laminated core, the desired preferential direction can be generated by suitable annealing in a longitudinal magnetic field, ie the external field is generated by energizing electrical conductors running perpendicular to the sheet plane, which generate a magnetic field parallel to the later magnetic stress direction of the parts, ie the main surface of the sheet. In the case of annealing individual stator segments, however, an external magnetic field can be used, the source of which is located outside the annealing chamber of the furnace. For example, the source can be a coil system that is located in the wall or outside the wall of a furnace. The magnetic field source and/or the segment or segments are oriented in such a way that the circumferential direction of the stator segments is as parallel as possible to the course of the applied magnetic field. In this case, there is no ideally tempered preferred direction, since it cannot run almost parallel to the circumferential direction. Nevertheless, an improvement in the properties is achieved compared to an isotropic sheet.

In the case of annealing individual stator segments, the stator segments can be assembled to form a stator ring before annealing and then the complete stator ring thus assembled can be subjected to annealing in the longitudinal field in order to achieve the desired preferred direction along the circumference.

If the stator ring and individual teeth of a stator are manufactured separately, longitudinal field annealing can be used for the stator ring, in which the necessary longitudinal magnetic field is generated by current-carrying conductors. In this case, the individual teeth can be annealed in an external magnetic field.

When annealing stator segments in an external magnetic field, one or more flux guides can be used. The flux guide or flux guides are arranged in such a way that the flux impressed into the stator segment by the flux guides corresponds as closely as possible to the direction of flow in the stator composed of the individual segments in the subsequent motor application.

In magnetic field annealing, the external magnetic field can be applied only partially or temporarily or for the entire duration of the heat treatment.

In a first process, the magnetic field annealing is carried out in a separate process after the normal final annealing, which serves to adjust the structure in the cooling phase. For example, the final annealing can be carried out in a conventional furnace without a magnetic field source and the stack is transported to another furnace equipped with a magnetic field source.

The final annealing is typically carried out at temperatures T1 of over 700°C or greater than 720°C, preferably between 750°C and 960°C. The maximum possible final annealing temperature is limited by the temperature at which the phase transition a/a+y takes place. The good soft magnetic properties are only achieved by annealing in the remote a range. The temperature T1 can therefore be between 700°C and T _a /a+ _Y. The temperature at which the phase transition a/a+y takes place depends on the composition of the CoFe alloy. The holding time t1 of the final annealing can, for example, be between 0.5 and 10 hours. The cooling rate of T1 can be between 10 K/h and 1000 K/h, preferably between 30 K/h and 300 K/h. In magnetic field annealing, a maximum temperature is used that is below the temperature of the final annealing.

In some embodiments, the external magnetic field is only applied during the cooling of the magnetic field annealing to a temperature of less than 300°C, preferably less than 200°C. The external magnetic field is thus not applied during the heating and during the holding time t2 of the magnetic field annealing.

In some embodiments, the external magnetic field is applied during the holding time t2 and during the subsequent cooling to a temperature of less than 300°C, preferably less than 200°C. The magnetic field can be applied over the entire holding time t2 and entire cooling of T2 to a temperature of less than 300°C, preferably less than 200°C.

In some embodiments, the external magnetic field is applied during less than 50% of the holding time t2 and during the subsequent entire cooling at a temperature less than 300°C, preferably less than 200°C. In some embodiments, the magnetic field is only applied in the second half of the holding time t2 and during the subsequent entire cooling to a temperature of less than 300°C, preferably less than 200°C.

The holding time t2 of the magnetic field annealing can be between 1 minute and 10 hours. The cooling rate of T2 can be between 10 K/h and 1000 K/h, preferably between 30 K/h and 300 K/h.

The final annealing and the magnetic field annealing can be carried out separately. For example, in some embodiments the final annealing and the magnetic field annealing are carried out in different furnaces.

The final annealing can be carried out under a protective gas atmosphere or a reducing atmosphere and/or the magnetic field annealing in a vacuum or under a protective gas atmosphere. In a two-stage process with separate final and magnetic field annealings, the atmosphere in the final annealing and magnetic field annealing can be different. Pure nitrogen or argon can be used as a protective atmosphere. A hydrogen-containing atmosphere can be used for a reducing atmosphere. The hydrogen-containing atmosphere can be pure hydrogen or nitrogen and/or argon with an admixture of hydrogen. In one embodiment, the protective gas atmosphere or reducing atmosphere is dry. In some embodiments, the hydrogen-containing atmosphere has an initial dew point of less than -40°C.

In an alternative method, the magnetic field annealing is integrated into the final annealing so that the stack is not first cooled from T 1 to a temperature lower than 300°C and then heated up again to a temperature T2. In particular, the external magnetic field is applied at least temporarily during the cooling of the stack from the holding temperature T1 of the final annealing, with the heating up and the holding time t1 being carried out in the background magnetic field. In the alternative method for producing a laminated core, a stack of a plurality of sheets of a CoFe alloy with 5 to 55 wt.% Co is provided. The stack is heat treated in a final annealing at a temperature T1 in the background magnetic field for a holding time t1, where T1 is between 700°C and 960°C, preferably 720°C to 950°C, preferably 760°C to 920°C, followed by cooling to a temperature less than 200°C. At least during cooling, an external magnetic field is applied, the external magnetic field being greater than the background magnetic field.

The final annealing may be carried out under a protective gas atmosphere or a reducing atmosphere. The protective gas atmosphere may be pure nitrogen or argon. A hydrogen-containing atmosphere may be used for a reducing atmosphere. The hydrogen-containing atmosphere may be pure hydrogen or nitrogen and/or argon with an admixture of hydrogen. In one embodiment, the protective gas atmosphere or reducing atmosphere may be dry. In some embodiments, the hydrogen-containing atmosphere has an initial dew point of less than -40°C.

In both processes, i.e. single-stage and two-stage, the holding time t1 can be between 0.5 and 10 hours and/or cooling rates between 10 K/h and 1000 K/h, preferably between 30 K/h and 300 K/h, can be used during cooling.

In some embodiments, the external magnetic field is applied with respect to the stack such that a preferred magnetic direction is created in the laminations. For example, the external magnetic field may be approximately parallel to the main surfaces of the laminations. In some embodiments, the external magnetic field is approximately parallel to the desired preferred direction.

The source of the external magnetic field, which can be applied in a targeted manner, can be arranged within the annealing chamber of the furnace. In some embodiments, at least one conductor, for example an electrically conductive wire or a cable through which a current can flow, is arranged around the stack. For example, the wire can be wound around the stack, wherein multiple windings may be used to increase the magnetic field strength. To apply the external magnetic field, a voltage is applied to the conductor, causing current to flow through the conductor and creating a magnetic field around the conductor. The magnetic field thus created thus flows through the stack as the conductor is arranged around the stack. To turn off the magnetic field, the voltage is removed from the conductor so that no magnetic field is anymore generated. Typically, multiple conductors or multiple windings of one or more conductors are used. This arrangement can be used with ring-shaped objects such as a stator ring or a cylindrical stator, with the conductor arranged in the central opening and wrapped around the outside of the ring-shaped object.

In some embodiments, the conductor has the form of copper rods that are connected to each other at the top and bottom of the annealing frame, thus forming an electrical circuit that encloses the part to be annealed like a winding.

The current flowing through the conductor can be adjusted to generate the desired magnetic field. In some embodiments, the current per conductor is greater than 5 A, preferably greater than 50 A, preferably greater than 100 A.

In some embodiments, the magnetic field annealing is carried out in a magnetic field furnace which has an excitation magnetic field of greater than 50 kA/m, preferably greater than 100 kA/m, preferably between 100 kA/m and 300 kA/m. In a magnetic field furnace, the conductors for generating the magnetic field are typically arranged outside the annealing chamber or in the walls of the annealing chamber.

In some embodiments, the external magnetic field is adjusted such that a field strength of greater than 0.5 A/cm, preferably greater than 1 A/cm, particularly preferably greater than 3 A/cm is present in the CoFe alloy.

In some embodiments, to amplify the magnetic field in the sheets, at least one flux guide piece is provided on the stack and/or on the stack and/or arranged under the stack. The flux guide piece can comprise a soft magnetic alloy, for example a CoFe alloy.

In some embodiments, the flux piece is not integrated with the sheets and the stack, but is designed as a separate part, so that an air gap is formed between them. The aim is for the air gap to be as small as possible so that the effect of the flux piece is more effective. In some embodiments, the air gap is at most 0.1 mm, preferably at most 0.05 mm, particularly preferably at most 0.01 mm.

In some embodiments, the laminations are loose in the stack and are secured to form a lamination stack after magnetic field annealing. In some embodiments, the laminations of the stack are secured to form a lamination stack and the lamination stack is subjected to the final annealing and the subsequent magnetic field annealing if a separate process is used for the magnetic field annealing.

In some embodiments, a return ring is provided which forms a magnetic circuit together with the stack or laminated core. This increases the magnetic field in the stack or laminated core. In some embodiments, the return ring is a part of the respective sheet or laminated core that is removed after the magnetic field annealing. The return ring can be removed by means of laser cutting or erosion.

In some embodiments, the laminated core has the shape of a stator with a stator ring and stator teeth. In these embodiments, the yoke has the shape of an inner yoke ring that extends between the inner ends of the stator teeth.

In some embodiments, the inner return ring has a width that is at least half the width of the stator teeth.

In some embodiments, the return is not integrated into the laminated core, but is a separate part, so that an air gap between the laminated core and the return path. The aim is for the air gap to be as small as possible so that the effect of the return path is more effective. In some embodiments, the air gap is at most 0.1 mm, preferably at most 0.05 mm, particularly preferably at most 0.01 mm.

The laminated core can, for example, have a ring shape or an elongated shape. For example, an annular laminated core can be a stator ring and an elongated laminated core can be a stator tooth. The laminated core can thus be a segment of a stator.

In some embodiments, the laminated core is assembled from several parts or segments after magnetic field annealing.

In some embodiments, the laminated core is segmented into parts before the final annealing and the parts are heat treated in the final annealing and in the magnetic field annealing in an externally applied magnetic field. This embodiment has the advantage that the parts can be subjected to different magnetic field annealings. For example, the magnetic field can be aligned parallel to the length of the stator teeth in a magnetic field annealing and parallel to the circumferential direction of the stator ring in another magnetic field annealing. The magnetic properties of the parts can thus be optimized separately. Furthermore, the final annealing can also serve to heal damage caused by segmentation.

Alternatively, in some embodiments, the laminated core is segmented into parts only after the final annealing and the parts are heat-treated in the magnetic field annealing in an externally applied magnetic field. This process also makes it possible to optimize the magnetic properties of the parts in a targeted and separate manner.

In some embodiments, the parts are heat treated together with flux guide pieces in the magnetic field annealing in an externally applied magnetic field. In some embodiments, the flux piece is not integrated with the sheets or the stack, but is a separate part, so that an air gap is formed between them. The aim is for the air gap to be as small as possible so that the effect of the flux piece is more effective. In some embodiments, the air gap is at most 0.1 mm, preferably at most 0.05 mm, particularly preferably at most 0.01 mm.

In some embodiments, the laminations are loose in the stack. In a loose stack, an integrated yoke may also be provided that is part of the respective laminations. In some embodiments, the respective laminations have the shape of a stator with a stator ring and stator teeth. In these embodiments, the laminations may have the yoke in the form of an inner yoke ring that extends between the inner ends of the stator teeth. In some embodiments, the inner yoke ring of the respective laminations has a width that is at least half the width of the stator teeth. In some embodiments, the yoke ring is a part of the lamination that is removed after magnetic field annealing. The yoke ring can be removed by laser cutting or eroding or punching.

In some embodiments, the return path is a separate part that is arranged at, on or under the stack. An air gap is typically formed between the stack and the return path, which should be kept as small as possible. In some embodiments, the air gap is at most 0.1 mm, preferably at most 0.05 mm, particularly preferably at most 0.01 mm.

In some embodiments, the respective laminations have a ring shape or an elongated shape. For example, an annular lamination can have the shape of a stator ring and an elongated lamination can have the shape of a stator tooth. The lamination can thus be part of a segment of a stator.

In some embodiments, after magnetic field annealing and possible removal of the return ring, the sheets are assembled and secured together to form a sheet stack. This can be done by gluing and/or welding. The CoFe alloy may have various compositions. In some embodiments, the CoFe alloy may be a commercially available alloy such as HIPERCO or PERMENDUR. In some embodiments, the CoFe alloy has

35 to 55 wt% Co and up to 2.5 wt% V, balance Fe and unavoidable impurities, or

45 wt% < Co < 52 wt%, 45 wt% < Fe < 52 wt%, 0.5 wt% < V < 2.5 wt%, balance Fe and unavoidable impurities, or

35 wt% < Co < 55 wt%, preferably 45 wt% < Co < 52 wt%, 0 wt% < Ni < 0.5 wt%, 0.5 wt% < V < 2.5 wt%, balance Fe and unavoidable impurities, or

35 wt% < Co < 55 wt%, 0 wt% < V < 2.5 wt%, 0 wt% < (Ta + 2Nb) < 1 wt%, 0 wt% < Zr < 1.5 wt%, 0 wt% < Ni < 5 wt%, 0 wt% < C < 0.5 wt%, 0 wt% < Cr < 1 wt%, 0 wt% < Mn < 1 wt%, 0 wt% < Si < 1 wt%, 0 wt% < AI < 1 wt%, 0 wt% < B < 0.01 wt%, balance Fe and unavoidable impurities, or

5 to 25 wt% Co, 0.3 to 5.0 wt% V, 0 wt% < Si < 3 wt%, 0 wt% < Cr < 3 wt%, 0 wt% < Mn < 3 wt%, 0 wt% < Al < 3 wt%, balance Fe and unavoidable impurities, or

47 wt% < Co < 50 wt%, 1 wt% < V < 3 wt%, 0 wt% < Ni < 0.2 wt%, 0.08 wt% < Nb < 0.12 wt%, 0 wt% < C < 0.007 wt%, 0 wt% < Mn < 0.5 wt%, 0 wt% < Si < 0.1 wt%, balance Fe and unavoidable impurities.

The sheets can be produced using metallurgical processes. In some embodiments, a melt of the FeCo alloy with the desired composition is provided and cast under vacuum to form an ingot after subsequent solidification. The ingot is hot rolled into the slab and the slab into a hot rolled strip, followed by Quenching the hot-rolled strip from a temperature above 700°C to a temperature below 200°C. The cooled hot-rolled strip is cold-rolled to a cold-rolled strip and the sheets are formed from the cold-rolled strip. The sheets can therefore have a cold-rolled texture before the final annealing.

The sheets can be formed from the strip by means of, for example, cutting, punching, laser cutting, eroding or cutting to length. The sheets can have a thickness db, where 0.01 mm < db < 0.35 mm, preferably 0.01 mm < db < 0.2 mm, preferably 0.01 mm < db < 0.1 mm, preferably 0.01 mm < db < 0.06 mm.

In some embodiments, the sheets are coated with a solution with Mg-containing methylate or Mg-containing propylate or Zr-containing methylate or Zr-containing propylate or with a boehmite-containing suspension, which are converted to MgO or ZrO2 or Al2O3 during the heat treatment. Typically, the strip from which the sheets are formed is coated with the solution and the sheets are formed with the coating from the coated strip. The coating can have a thickness d _s , where 0.01 pm < d _s 1 pm, 0.01 pm < d _s 1 pm, preferably 0.01 pm < d _s 0.5 pm, preferably 0.01 pm < d _s 0.2 pm. In the case of the coating containing AI2O3, the thickness d _s can also be larger, where 0.1 pm < d _s 6 pm, preferably 0.5 pm < d _s 4 pm.

In some embodiments, the sheets are coated with a boehmite-containing suspension, which converts to AI2O3 during heat treatment.

Design examples and examples are now explained in more detail using the drawings.

Figure 1 shows a perspective view of a stack of laminations, each having the shape of a stator. Figure 2 shows diagrams of three heat treatments.

Figure 3 shows a diagram of a setup for heat treatment of a stack in a longitudinal magnetic field.

Figure 4 shows a hysteresis loop for a comparative example which is finally annealed in the background magnetic field and for an example according to the invention in which a magnetic field is applied during the cooling phase of the final annealing.

Figure 5 shows a diagram of measured permeabilities for sheets that are finally annealed in different longitudinal magnetic fields and for comparison sheets that are finally annealed without an additional applied magnetic field.

Figure 6 shows a graph of the maximum permeabilities and coercive field strengths as a function of the longitudinal field.

Figure 7 shows a graph of the course of the induction B3 at a field strength of 3 A/cm, the remanence Br and the remanence ratio Br/Bs as a function of the longitudinal field.

Figure 8 shows a graph of the remanence ratio Br/B160 for examples cooled in different magnetic fields.

Figure 9 shows the magnetic properties He, B3, pmax and Br after post-annealing.

Figure 10 shows a graph of the decay of pmax and Br/B3 after post-annealing at different temperatures.

Figure 11 shows a graph of the permeabilities of the annealings with magnetic field and their reference without magnetic field. Figure 12A shows a representation of a stator with a return ring.

Figure 12B shows another representation of the stator with the return ring.

Figure 12C shows a representation of the stator after removal of the

return ring.

Figure 12D shows a representation of a stator with a separate flux guide.

Figure 13 shows a stator tooth with a flux conductor.

Figure 14 shows the distribution of flux density in the stator tooth with and without flux guide.

Figure 15 shows different manufacturing processes by which a laminated core or a part of a laminated core can be manufactured.

Figure 1 shows a perspective view of a stack 10 made of a plurality of sheets 11 comprising a soft magnetic CoFe alloy. The Co-Fe alloy can contain between 5 and 55% cobalt by weight. In one embodiment, the CoFe alloy contains approximately 49% Co, approximately 49% Fe and approximately 2% V by weight. In addition to the main alloying elements Co, Fe and V, these commercially available alloys sometimes contain grain-refining additives such as Nb, Ta and Zr, which limit grain growth and thus enable increased strength. As a CoFe alloy, a CoFe alloy with one of the trade names VACOFLUX 48, VACOFLUX 50, VACODUR 50, VACODUR 49, VACODUR S Plus, HIPERCO 50, HIPERCO 50A, HIPERCO 50HS, AFK 502 and AFK 502R can be used.

In the embodiment of Figure 1, the respective laminations have the shape of a stator 12 with a stator ring 13 and a plurality of stator teeth 14 extending from the inside of the stator ring 13 in the direction of the axis 15 of the stator. In other embodiments, the laminations 11 may have a rectangular or square shape, or the shape of a part of a stator, for example a ring shape for the stator ring or a T-shape or an I-shape for a stator tooth.

The respective sheets 11 have a first main surface 16, a second main surface 17 arranged opposite the first main surface 16, and a thickness db, wherein 0.01 mm < db < 0.35 mm, preferably, 0.01 mm < db < 0.2 mm, preferably 0.01 mm < db < 0.1 mm, preferably 0.01 mm < db < 0.06 mm. The second main surface 17 of a first sheet 11 is arranged on the first main surface 16 of a second sheet 11 to build up the stack in a stacking direction 18 extending perpendicular to the main surfaces 16 and 17.

One or both of the main surfaces 16, 17 of the respective sheets 11 can be completely or partially coated with an electrically insulating coating 19, which serves as an annealing separator and as electrical insulation between the sheets 11 in the finished laminated core. The coating 19 can comprise a ceramic such as Al2O3, MgO or ZrO2 after the final annealing.

In the embodiment of Figure 1, the laminations 11 of the stack 10 are loose and not connected to one another. In other embodiments, the laminations are connected to one another, for example by means of one or more weld seams, and form a laminated core or a segment of a laminated core, for example a stator ring or stator tooth.

The stack 10 is subjected to a heat treatment to adjust the soft magnetic properties of the Co-Fe alloy.

Figure 2 shows diagrams of three examples of possible heat treatments for a CoFe alloy with 5 to 55 wt% Co. These heat treatments can be used to produce a laminated core, a stack of CoFe alloy sheets, or segments and parts of a stator.

Figure 2A shows an embodiment of a heat treatment 30 which has two separate stages, namely a final annealing 31 which is followed by a separate Magnetic field annealing 32 is followed. In the final annealing 31, the stack is heated and held at a temperature T1 for a time t1 and then cooled to a temperature of less than 300°C. The stack can be cooled to room temperature and then stored. After this final annealing, the magnetic field annealing 32 is carried out. This can be carried out at a later time after the stack has been stored at room temperature. In the magnetic field annealing, the stack is heated and held at a temperature T2 for a time t2 and then cooled to a temperature of less than 300°C. An external magnetic field is applied at least partly during the magnetic field annealing.

The period of time during which the external magnetic field is applied is shown by the black bar 33 in Figure 2. Outside this range, the heat treatment is carried out in the background magnetic field. The background magnetic field is typically at most 1 kA/m, preferably less than 0.5 kA/m, preferably less than 0.1 kA/m. The external magnetic field that is actively applied is thus greater than the background magnetic field, i.e. greater than 0.1 kA/m. In some embodiments, the external magnetic field is set so that a field strength of greater than 0.5 A/cm, preferably greater than 1 A/cm, particularly preferably greater than 3 A/cm is present at the stack.

In the embodiment of Figure 2A, the holding time t2 is short, for example half an hour, and the magnetic field is applied after the temperature T2 has been reached and maintained until a temperature of approximately 200°C is reached due to the subsequent cooling from the holding temperature T2. Thereafter, the external magnetic field can be switched off and further cooling to low temperatures can be carried out in the background magnetic field.

Figure 2B also discloses a two-stage heat treatment 30 with a conclusion 31 and subsequent separate magnetic field annealing 32. This heat treatment 30 differs from the heat treatment 30 of Figure 2A by the duration of the holding time t2, which is longer, for example 6 hours. In this embodiment, the magnetic field is applied during the entire holding time t2 as well as during cooling to a temperature of approximately 200°C. In In other embodiments not shown, the magnetic field is only applied during cooling from the temperature T2, ie after the expiry of the time period t2, or is only applied during the holding time, so that a first period of the holding time t2 is carried out in the background of the magnetic field and a subsequent period of the holding time t2 and the cooling are carried out in the externally applied magnetic field.

The use of two separate annealings, which can be similar to the examples in Figures 2A and 2B for the final annealing and subsequent magnetic field annealing, has the advantage that different process parameters can be optimized and selected for the respective heat treatment. One example is the cooling rate, which should be sufficiently high for the final annealing to avoid a lingering period in the precipitation area of the y2 phase, and which should be sufficiently low for the magnetic field annealing to take into account the establishment of a short-range order. Different furnace types can be used, which offers technical advantages in terms of temperature accuracy, for example, but also logistical advantages. Different annealing atmospheres can be used, e.g. hydrogen for the final annealing and vacuum or protective gas atmosphere for the magnetic field annealing.

Figure 2C shows a heat treatment 30' according to a further embodiment in which the magnetic field annealing is integrated in the conclusion 31. The heat treatment 30' is thus one-stage. In this embodiment, the stack is heated to a temperature T1 in the background magnetic field, wherein the temperature T1 is maintained for a holding time t1. After the holding time t1, the stack is cooled to a temperature less than 300°C. An external magnetic field is applied at least during the cooling, as schematically shown by the bar 33 in Figure 2C. In this example, the magnetic field is switched on during the second half of the holding time t1 and remains switched on during the cooling down to a temperature of approximately 250°C. Thereafter, the external magnetic field can be switched off and further cooling to low temperatures in the background magnetic field can be carried out. In other embodiments not shown, the magnetic field can be applied earlier, for example during the holding time t1. The cooling rate for the final annealing and the separate magnetic field annealing can be between 10 K/h and 1000 K/h, for example between 30 K/h and 300 K/h, and the cooling rate during cooling can vary within this range. Normally the cooling rate becomes slower with decreasing temperature, as shown in Figure 2.

In all heat treatments, the magnetic field can be applied in relation to the orientation of the stack 10 so as to generate a preferred magnetic direction in the laminations 11. In particular, the magnetic field can be applied so as to run approximately parallel to the main surfaces 16, 17 of the laminations 11. Furthermore, the external magnetic field can run approximately parallel to the desired preferred direction in the laminations 11. For example, with respect to the stator of Figure 1, the magnetic field can be applied so as to run along the length of the stator teeth 14.

In a further embodiment in which the laminations 11 of the stack 10 comprise part of a stator, for example a stator tooth, the magnetic field can run not only parallel to the main surfaces, but also parallel to the longitudinal direction of the stator tooth. In an embodiment in which the laminations have the shape of a stator ring, the magnetic field can be applied or the laminations can be oriented with respect to the magnetic field in such a way that the magnetic field runs in the circumferential direction of the ring and parallel to the main surfaces.

The externally applied magnetic field can be generated in different ways. For example, the stack can be arranged in a so-called magnetic field furnace, which has a switchable magnetic field source. The switchable magnetic field source can be a coil system arranged in or outside the walls of the furnace. The stack can be arranged in the annealing chamber of the furnace with respect to the magnetic field source of the furnace so that the magnetic field runs in the desired preferred direction of the part. Alternatively, one or more electrically conductive conductors can be wound around the stack, this structure can be arranged in a furnace and a current flowing through the conductor can be generated. the magnetic field. This arrangement can be called a longitudinal magnetic field (LF).

If the stator ring and individual teeth are manufactured separately, longitudinal field annealing is provided for the stator ring, in which the necessary longitudinal magnetic field is generated during annealing using current-carrying conductors. For the individual teeth, however, annealing treatment in a magnetic field furnace is provided. The individual teeth are oriented in the magnetic field in such a way that the direction of magnetization impressed by this field corresponds as closely as possible to the direction of magnetization in the later application.

Figure 3 shows a representation of a structure 40 for heat treating a stack 41 in a longitudinal magnetic field. In this embodiment, the sheets each have a ring shape with a central opening 43, so that the stack 41 of loose sheets is tubular. In this embodiment, six turns of a conductor 42 are wound around the stack 41. The conductor 42 extends through the opening 43 and then alongside the edge of the stack 41 to form the six turns. The conductor 42 can be a copper conductor. The copper lines are electrically insulated from each other and from the furnace walls by ceramic tubes and connected to an amplifier (not shown) for supplying DC power.

The external magnetic field is generated by energizing electrical conductors 42 running perpendicular to the sheet plane, which generate a magnetic field parallel to the later magnetic stress direction of the parts, i.e. the main surface of the sheets, whereby a longitudinal magnetic field is generated.

Examples

Annealing tests were carried out with CoFe alloy rings in a longitudinal magnetic field. For this purpose, punched rings 038.1 mm x 031.75 mm made of VACODUR 49 were wound in a ceramic trough with bare copper wire 02 mm. To ensure electrical insulation of the cables from each other and from the furnace, ceramic I-tubes with 03 mm inner holes were threaded onto the copper wire, as shown in Figure 3. Two series of measurements are carried out. In a first series of measurements, a heat treatment is carried out according to Figure 2C and a magnetic field is actively applied in the cooling phase of a final annealing at 880°C for 6 hours. In a second series of measurements, a two-step process is investigated in which a final annealing at 800°C for 6 hours is carried out separately from a subsequent magnetic field annealing after the heat treatment according to Figures 2A and 2B. An external magnetic field is applied at least partially during the magnetic field annealing. The final annealing takes place in the background magnetic field.

First series of measurements

In a first series of measurements, the external magnetic field is applied during the cooling phase of the final annealing (6h at 880°C) and varied by varying the applied current. The current intensities used, I = 0 / 0.5 / 1 / 2.5 / 5 / 10 / 20 A, correspond to magnetic fields at the location of the rings of H = 0 / 0.27 / 0.5 / 1.37 / 2.74 / 5.47 / 10.9 A/cm. A new sample is wound and annealed for each current.

Figure 4 shows a hysteresis loop for a comparative example of a sheet made of VACODUR 49, which is final annealed without an additionally applied magnetic field, because H(LF) = 0 A/cm, and only in the background magnetic field at 800°C for 6 hours, and an example according to the invention in which a magnetic field H(LF) of 10.9 A/cm is applied during the cooling phase of the final annealing. Figure 4 shows a rectangular Z-loop for the example according to the invention, which is cooled in the longitudinal magnetic field, while the comparative example, which is subjected to final annealing without an additional externally applied magnetic field and only in the background magnetic field, has a round R-loop. This comparison shows that the use of the additional externally applied magnetic field during the cooling phase of the final annealing leads to anisotropic magnetic properties and thus a preferred direction of the magnetic properties in the sheets. Figure 5 shows a diagram of measured permeabilities (p) for the sheets final annealed in different longitudinal magnetic fields and for a comparison sheet final annealed without additional applied magnetic field. The highest applied field of 10.9 A/cm produces a maximum permeability of over 50,000, which is 2.5 times higher than the comparison example without externally applied magnetic field, which has a maximum permeability of 20,000.

Figure 6 shows a graph of the maximum permeability (pmax) and coercive field strengths as a function of the longitudinal field. For these examples, the maximum permeability (pmax) increases and the coercive field strength (Hc) decreases with increasing magnetic field strength.

Figure 7 shows a graph of the course of the inductions at a field strength of 3 A/cm (B3), the remanence (Br) and the remanence ratio (Br(Bs)) as a function of the longitudinal field.

Figure 8 shows a graph of the remanence ratio Br/B160 for examples cooled in different magnetic fields. Figure 8 shows an increase in the remanence ratio with increasing longitudinal field. An isotropic domain distribution (perfectly round R-loop, no anisotropy) would produce a remanence ratio of 0.637, shown with the dashed line in Figure 8.

Table 1 shows magnetic characteristics of the samples after final annealing with applied longitudinal magnetic field HLF during cooling, whereby the longitudinal magnetic field HLF is generated with a current IF in the conductor, and Table 2 shows the activity of the longitudinal magnetic field LF in the cooling phase of the final annealing.

Table 1

Table 2

It is also investigated to what extent the improved magnetic properties are maintained during operation of the laminated core when the laminated core is exposed to elevated temperatures. This is investigated by means of post-annealing as mentioned herein. These post-annealing processes are not part of the manufacturing process, but are merely used to determine the temperature stability of the magnetic properties.

Figure 9 shows the magnetic properties He, B3, pmax and Br after post-annealing the I = 20A annealed sample at a temperature between 200°C and 880°C in 50°C steps. Post-annealing was carried out for 1 hour in each case. For B3 and Br, a local minimum can be seen in the temperature range between 600 and 650°C. Figure 10 shows a graph of the decrease of p _ma x and Br/B3 with increasing afterglow temperature.

These results show that the improvement in magnetic properties compared to the comparative example, which is final annealed without additional magnetic field, is maintained at elevated temperatures of up to approximately 300°C. Thus, the sheets are suitable for applications where they are exposed to elevated temperatures.

Second series of measurements

In this series of measurements, the final magnetic annealing at 880°C for 6 hours in the background magnetic field and the magnetic field annealing at 600°C are carried out separately.

After the final annealing in the background magnetic field, one of two different magnetic field anneals is carried out, one with a half-hour hold time, where the magnetic field is either actively applied 15 minutes before the start of cooling and during cooling to below 200°, and one with an eight-hour hold time at 800°C, where the magnetic field is applied during the entire hold time and during cooling to below 200°C, corresponding to the heat treatment of Figures 2A and 2B, respectively. For both anneals, reference samples that were not exposed to the field are also included.

Figure 11 shows a graph of the permeabilities of the annealings with magnetic field and their reference without magnetic field and Table 3 shows magnetic values of the separate final annealing and magnetic field annealing from Figure 11 . The two sample numbers 2002506 and 2002522 from Table 1 are added for reference.

Table 3

Figure 11 shows that the 8h magnetic field annealing at 600°C produces poorer magnetic properties than the 0.5h at 600°C. This is probably because in the 50% CoFe alloys between 500°C and 650°C there is a precipitation range of a V-rich phase, which is also called the y2 phase. On the one hand, this range should be passed through quickly after the final annealing in order to avoid the magnetically disadvantageous precipitations. On the other hand, the cooling rate should be high enough to adjust the zero crossing of the magnetocrystalline anisotropy constant K1, which depends on both the Co content and the state of order. Typical cooling rates after the final annealing are in the range 100-200°C/h. With magnetic field annealing, it can be advantageous to choose a different cooling rate in order to adjust the short-range order. To set different cooling rates, it may therefore be advantageous to carry out magnetic final annealing and magnetic field annealing separately with different temperature profiles.

To increase the magnetic field on the stack or on the sheets, a flux conductor and/or a return element can be used.

Figures 12A and 12B show a schematic perspective view and a plan view of a stator 50 with a stator ring 51 and stator teeth 52, the further comprises an integrated magnetic return inner ring 53 extending between the inner ends of the stator teeth 52. Figure 12A further shows the current-carrying conductors 54 arranged in the gaps of the stator. The magnetic return inner ring 53 is removed after the magnetic field annealing, for example by means of eroding or laser cutting, in order to produce the stator 50 with stator ring 51 and stator teeth 52, as shown in Figure 12C.

Another possibility to generate a sufficient flux density in the individual tooth despite the limitation of the external magnetic field strength is to use an additional soft magnetic piece 55, which can be referred to as a "flux conductor". It concentrates additional magnetic air flow and directs it into the individual tooth. The flux conductor 55 can take on various forms. An example is shown in Figure 12D, in which the flux conductor 55 has the form of an inner return ring 53, which is separated from the adjacent stator teeth 52 by an air gap 56. Analogously, an outer return ring could be used as a flux conductor for a stator designed as an external rotor.

Another example is shown in Figure 13, in which the flux guide 60 has the shape of an elongated cuboid that is flush with the single tooth 61: In other embodiments, a separate flux guide can be used that is arranged as close as possible to the stack so that an air gap between the flux guide and the stack remains small.

Figure 14 shows the distribution of the flux density in the stator tooth with and without a flux conductor over the height, the middle height is marked by the black vertical line. It is possible that the flux density that can be achieved in the tooth is not distributed evenly over the tooth height, as shown by curve 70. On the other hand, when using a flux conductor, even the areas that are more difficult to saturate are completely magnetized, as shown by curve 71. Using such a flux conductor, it is therefore possible to anneal individual teeth in an external magnetic field in such a way that the setting of a preferred magnetic direction is more effective. Preferably, there is as little air gap as possible between the flux conductor and the stator tooth to be annealed. To avoid welding of the parts, one or both parts can have an annealing-resistant coating or the Parts should be separated from each other using an annealing separator such as ceramic annealing powder applied before annealing.

Figure 15 shows six different manufacturing paths 100 by which a laminated core or a part of a laminated core can be manufactured.

The first four steps are performed for all six different manufacturing routes. In box 101, a strip of a CoFe alloy according to one of the compositions described herein is provided. This strip can be produced by metallurgical processes. For example, a melt of the FeCo alloy with the desired composition is provided and cast under vacuum to form an ingot after subsequent solidification. The ingot is hot rolled into the slab and the slab into a hot rolled strip, followed by quenching the hot rolled strip from a temperature above 700°C to a temperature below 200°C. The cooled hot rolled strip is cold rolled into a cold rolled strip.

Then, in box 102, the strip is coated with an electrically insulating layer or a layer that can form an electrically insulating layer after heat treatment. Then, a plurality of sheets are formed from the coated strip and stacked and connected in box 103 to form a laminated core. The sheets can be stacked to form a laminated core, for example by punching and secured to one another via a heat-resistant connection or connections, for example at least one laser weld seam or laser weld points. Then, in box 104, the laminated core is subjected to final annealing according to one of the embodiments described herein. The laminated core can have different shapes. The subsequent steps of the manufacturing process, including the magnetic field annealing, are adapted depending on this.

In the first production process (a), the laminated core in box 105 has the shape of a stator. In general, the laminated core is produced as a single cut and is in its final shape during the final annealing. Then, in box 106, the Laminated core in final contour or the stator annealed in a longitudinal magnetic field.

For example, the structure of Figure 3 can be used.

In the second manufacturing path (b), the laminated core has an integrated return path in box 107. In this embodiment, the laminated core has the shape of a stator with an inner return path ring that extends between the inner ends of the stator teeth. In box 108, the stator with the inner return path ring is annealed in the longitudinal magnetic field. For example, the structure of Figure 12A can be used. Then, in box 109, the inner return path ring is removed, for example by laser cutting.

In the third manufacturing path (c), the laminated core has the shape of a stator ring. The stator ring is annealed in the longitudinal magnetic field. For example, the structure of Figure 3 can be used. In the fourth manufacturing path (d) of the box 112, the laminated core has the shape of a stator tooth, which can have an I-shape. Typically, several stator teeth are provided and finally annealed. The stator teeth are subjected to magnetic field annealing in a magnetic field furnace. The magnetic field can run along the main surface and longitudinal direction of the respective stator teeth.

The third and fourth manufacturing paths can then be combined by assembling the stator ring of the third manufacturing path with the stator teeth of the fourth manufacturing path to form a stator in box 111.

In the fifth production path (e), one or more stator teeth with a T-shape are annealed in the magnetic field furnace in box 113. These stator teeth can be subjected to magnetic field annealing with one or more flux guide pieces. These flux guide pieces can be separate parts or connected to the respective stator tooth or integrated into the respective stator tooth. In these embodiments, after magnetic field annealing, the flux guide piece is removed from the stator tooth, for example by laser cutting. The T-shaped stator teeth are then joined to form a stator in box 114. In the sixth production path (f), the laminated core or laminated cores have the shape of a stator tooth with a T shape. These stator teeth are first joined to form a stator in box 115. The assembled stator is then annealed in a longitudinal magnetic field in box 116. For example, the structure of Figure 3 can be used. Optionally, an inner return part can be used which extends between the inner ends of the stator teeth, as shown in Fig. 12D. However, the return part is not materially connected here, but is attached to the T-teeth with as small an air gap as possible. The production paths shown in Fig. 15 can also be modified such that the packaging described in box 103 is limited to the stacking of loose sheets and the materially bonding of the sheets to form a laminated core only takes place after the final annealing or after the magnetic field annealing.

Claims

Patent claims

1 . A method for producing a CoFe alloy for a laminated core, the method comprising:

Providing a stack of a plurality of sheets made of a CoFe alloy with 5 to 55% by weight of Co, in a final annealing heat treating the stack at a temperature T1 in the background magnetic field for a holding time t1, wherein T1 is between 700°C and 960°C, preferably 720°C to 950°C, preferably 760°C to 920°C, and followed by cooling to a temperature less than 300°C, preferably less than 200°C, and then in a magnetic field annealing heat treating the stack at least temporarily in an externally applied magnetic field at a temperature T2 for a holding time t2, wherein T2 is between 500°C and T1, preferably between 650°C and T1, preferably between 700°C and T1, followed by cooling to a temperature less than 300°C, preferably less than 200°C, wherein the externally applied magnetic field is larger than the background magnetic field.

2. The method according to claim 1, wherein the final annealing and the magnetic field annealing are carried out in different furnaces.

3. A method according to claim 1 or claim 2, wherein the external magnetic field is applied only during cooling to a temperature less than 300°C, preferably less than 200°C.

4. Method according to one of the preceding claims, wherein the external magnetic field is applied during the holding time t2 and during the subsequent cooling at a temperature of less than 300°C, preferably less than 200°C.

5. Method according to one of the preceding claims, wherein the external magnetic field is applied during less than 50% of the holding time t2 and during the subsequent cooling to a temperature of less than 300°C, preferably less than 200°C. Method according to one of the preceding claims, wherein the magnetic field is only applied in the second half of the holding time t2 and during the subsequent cooling to a temperature of less than 300°C, preferably less than 200°C. Method according to one of the preceding claims, wherein t2 is between 1 minute and 10 hours. Method for producing a laminated core, the method comprising the following:

Providing a stack of a plurality of sheets made of a CoFe alloy with 5 to 55 weight % Co, in a final annealing heat treating the laminated core at a temperature T1 in the background magnetic field for a holding time t1, wherein T1 is between 700°C and 960°C, preferably 720°C to 950°C, preferably 760°C to 920°C, followed by cooling to a temperature less than 200°C, wherein at least during the cooling an external magnetic field is applied at least temporarily, wherein the external magnetic field is greater than the background magnetic field. Method according to one of the preceding claims, wherein t1 is between 0.5 and 10 hours and/or cooling rates between 10 K/h and 1000 K/h, preferably between 30 K/h and 300 K/h are used during the cooling. A method according to any preceding claim, wherein the external magnetic field is applied with respect to the stack to produce a preferred magnetic direction in the sheets. A method according to any preceding claim, wherein the external magnetic field is approximately parallel to the main surfaces of the sheets. 12. Method according to one of the preceding claims, wherein the external magnetic field is approximately parallel to the desired preferred direction.

13. A method according to any one of the preceding claims, wherein at least one conductor through which a current can flow is arranged around the stack and a voltage is applied to the conductor to apply the external magnetic field, whereby current flows through the conductor.

14. The method according to claim 13, wherein the current per conductor is greater than 5 A, preferably greater than 50 A, preferably greater than 100 A.

15. The method according to any one of claims 1 to 12, wherein the magnetic field annealing is carried out in a magnetic field furnace having an excitation magnetic field of greater than 50 kA/m, preferably greater than 100 kA/m, preferably between 100 kA/m and 300 kA/m.

16. Method according to one of the preceding claims, wherein at least one flux guide piece is arranged on the stack and/or on the stack and/or under the stack to amplify the magnetic field in the sheets.

17. Method according to one of the preceding claims, wherein the external magnetic field is adjusted such that a field strength of greater than 0.5 A/cm, preferably greater than 1 A/cm, particularly preferably greater than 3 A/cm is present in the CoFe alloy.

18 Method according to one of the preceding claims, wherein after the magnetic field annealing the CoFe alloy has a remanence ratio Br/Bs of greater than or equal to 0.8 with a remanence Br and a magnetic saturation Bs measured at 160 A/cm and/or a maximum permeability pmax of more than 25,000, preferably more than 30,000, particularly preferably more than

40,000.

19. Method according to one of the preceding claims, wherein the sheets of the stack are fastened to form a sheet stack and the sheet stack is subjected to final annealing.

20. Method according to one of the preceding claims, wherein a return path is provided which together with the laminated core forms a magnetic circuit.

21. The method of claim 20, wherein the laminated core has the shape of a stator with a stator ring and stator teeth, wherein the yoke has the form of an inner yoke ring extending between the inner ends of the stator teeth.

22. The method of claim 21, wherein the inner return ring has a width that is at least half the width of the stator teeth.

23. A method according to claim 21 or claim 22, wherein the return ring is a part of the laminated core which is removed after the magnetic field annealing.

24. The method according to claim 23, wherein the return ring is removed by means of laser cutting or erosion.

25. The method according to claim 21 or claim 22, wherein an air gap is formed between the laminated core and the return path, wherein the air gap is at most 0.1 mm, preferably at most 0.05 mm, particularly preferably at most 0.01 mm.

26. The method according to any one of claims 19 to 25, wherein the laminated core has a ring shape or an elongated shape.

27. The method according to claim 26, wherein after the magnetic field annealing the laminated core is assembled from several parts.

28. Method according to one of claims 11 to 27, wherein the laminated core is segmented into parts before or after the final annealing and the parts are heat treated in the magnetic field annealing in the externally applied magnetic field.

29. The method according to claim 28, wherein the parts are heat treated together with flux guide pieces in the magnetic field annealing in the externally applied magnetic field.

30. Method according to one of claims 1 to 18, wherein the sheets are loose in the stack.

31. A method according to claim 30, wherein a return path is provided which forms a magnetic circuit together with at least one of the sheets.

32. The method of claim 31, wherein the respective laminations have the shape of a stator with a stator ring and stator teeth, wherein the yoke has the form of an inner yoke ring extending between the inner ends of the stator teeth.

33. The method of claim 32, wherein the inner return ring has a width that is at least half the width of the stator teeth.

34. A method according to claim 32 or claim 33, wherein the return ring is a part of the sheet which is removed after the magnetic field annealing.

35. The method according to claim 34, wherein the return ring is removed by means of laser cutting or erosion.

36. The method according to claim 31, wherein an air gap is formed between the stack and the return path, wherein the air gap is at most 0.1 mm, preferably at most 0.05 mm, particularly preferably at most 0.01 mm. Method according to one of claims 30 to 36, wherein the sheets have a ring shape or an elongated shape. Method according to claim 37, wherein after the magnetic field annealing the sheets are assembled together to form a laminated core. Method according to one of the preceding claims, wherein the CoFe alloy

35 to 55% by weight Co and up to 2.5% by weight V, balance Fe and unavoidable impurities, or

35 weight-% < Co < 55 weight-%, 0 weight-% < V < 2.5 weight-%, 0 weight-% < (Ta + 2 Nb) < 1 weight-%, 0 weight-% < Zr < 1.5 weight-%, 0 weight-% < Ni < 5 weight-%, 0 weight-% < C < 0.5 weight-%, 0 weight-% < Cr < 1 weight-%, 0 weight-% < Mn < 1 weight-%, 0 weight-% < Si < 1 weight-%, 0 weight-% < AI < 1 weight-%, 0 weight-% < B < 0.01 weight-%, balance Fe and unavoidable impurities, or