Fecov alloy and method for the heat treatment of a fecov alloy

Abstract

The invention relates to an FeCoV alloy which has a composition consisting substantially of 30 wt.% ≤ Co ≤ 55 wt.%, preferably 45 wt.% ≤ Co ≤ 50 wt.%, 0.5 wt.% ≤ V ≤ 2.5 wt.%, 0 wt.% ≤ Ta ≤ 1wt.%, 0 wt.% ≤ Nb ≤ 1 wt.%, up to 1 wt.% impurities, and the remainder being iron, and which has a crystalline structure with an average grain size dk, where 11 µm ≤ dk < 30 µm, and at most 10% of the grains have a grain size greater than 75 µm.

Description

[0001] P13572DE

[0002] 1

[0003] Description

[0004] FeCoV alloy and methods for the heat treatment of an FeCoV alloy

[0005] The present invention relates to an FeCoV alloy and a method for the heat treatment of an FeCoV alloy.

[0006] Soft magnetic materials, such as FeCoV alloys in the form of strips and sheets, are essential components for the construction of electrical machines because they conduct magnetic flux. For example, in a radial flux machine, sheets are stacked on top of each other and joined to form a stator lamination stack. Similarly, soft magnetic sheets are often stacked and joined to form a rotor lamination stack.

[0007] For some applications, such as in aviation or motorsports, soft magnetic materials made of cobalt-iron (CoFe) are used, which exhibit increased saturation polarization. An example of this alloy class is the V-Permendur alloy, also known as V-Permendur, containing approximately 49 wt% Fe, 49 wt% Co, and 2 wt% V. These alloys undergo heat treatment to achieve a suitable microstructure through recrystallization and grain growth, as the soft magnetic properties are largely determined by the microstructure. Only through this heat treatment can a high maximum permeability (pmax) and a low coercivity (H) be obtained. c and high inductions B(H) even at low field strengths H.

[0008] For CoFe sheets with a Co content of 30 to 55 wt.%, it is common practice to perform heat treatment only after forming, i.e., on the already formed part. This heat treatment on the part, often referred to as "final annealing," serves to recrystallize the microstructure and promote subsequent grain growth, and also reduces residual stresses in the material. For good soft magnetic properties, e.g., for a stator in an electric machine, the resulting microstructure should be fully recrystallized and have a sufficiently large grain size to achieve a low Hc and high pmax.

[0009] 2

[0010] For use as a rotor, however, a fine-grained structure is used, which has higher strengths, in particular a high yield strength Rpo.2, but also significantly worse soft magnetic properties due to the many grain boundaries.

[0011] A common heat treatment for CoFe components to achieve optimal magnetic properties, e.g., for use as stator laminations, involves stationary annealing under dry hydrogen with a holding stage in the range of 800 to 940 °C. The maximum temperature depends on the composition and, for vitrified (V) compositions, should not exceed the phase transition T(a^a+y) to prevent precipitates. Typical holding times range from 4 to 10 hours. This is followed by slow cooling to at least 200 °C with cooling rates in the range of 100 to 300 °C / h.

[0012] The object of the present invention is to provide an FeCoV alloy with homogeneous soft magnetic properties and a method for heat-treating an FeCoV alloy to provide homogeneous soft magnetic properties.

[0013] According to the invention, an FeCoV alloy is provided with a composition consisting essentially of 30 wt% < Co < 55 wt%, preferably 45 wt% < Co < 50 wt%, 0.5 wt% < V < 2.5 wt%, 0 wt% < Ta < 1 wt%, 0 wt% < Nb < 1 wt%, up to 1 wt% impurities, and the remainder Fe. Furthermore, the FeCoV alloy has a crystalline microstructure with a mean grain size dk, wherein 11 pm < dk < 30 pm, and a maximum of 10% grains with a grain size greater than 75 pm.

[0014] The FeCoV alloy can be in the form of a sheet or a lamella or a laminated core or a laminated core with the outer contour of a stator tooth or a laminated core with the outer contour of a stator ring or a laminated core with the outer contour of a stator segment with at least two stator teeth P13572DE

[0015] 3

[0016] It is provided and has a recrystallized microstructure in the heat-treated state.

[0017] In some embodiments, the FeCoV alloy contains niobium, where 0.01 wt% < Nb < 0.25 wt%, preferably 0.05 wt% < Nb < 0.15 wt%.

[0018] The grain size of the FeCoV alloy is determined using a line intercept method, as described, for example, in standards ASTM E112 or DIN EN ISO 643. In this method, at least one micrograph of the alloy's surface is analyzed, and at least one line is drawn. The grain boundaries intersected by this line are marked, and the number of intersections with the grain boundaries is counted. The characteristic value, designated as the mean grain size in the tables, corresponds to the mean length of the line intercept segment, i.e., the total length of the measurement line divided by the number of intersections. To ensure sufficient statistical power, a certain number of intersections must be counted, e.g., at least 50. In addition to the mean grain size, the grain size distribution can also be determined from the individual distances between the intersections.

[0019] In one embodiment, five lines are drawn in a micrograph, resulting in a total length of 4383 pm. For example, 163 intersection points were found along this total length. The resulting mean grain size is 27 pm.

[0020] Not only the average grain size influences the soft magnetic properties, but also the proportion of large grains. It is particularly advantageous if a maximum of 10% of the grains have a grain size greater than 75 pm. Such a homogeneous structure ensures uniform flux flow. Furthermore, the parts exhibit a fully recrystallized structure, which enables the desired excellent soft magnetic properties necessary, for example, for use in stators for electrical machines. P13572DE

[0021] 4

[0022] In rotating electrical machines, the homogeneity of the magnetic properties of the sheet metal is important to ensure uniform magnetization. A homogeneous microstructure with few or no unusually large grains is advantageous for achieving uniform magnetization. This effect is even more pronounced in thin sheets, for example, with a maximum thickness of 0.15 mm. If the diameter of individual large grains is already close to the thickness of the sheet, then the preferred orientation of this single crystal dominates the magnetic properties across the cross-section.

[0023] A particularly advantageous microstructure for rotating electrical machines is therefore present when the grain size is as uniform as possible and, moreover, it contains no individual very large grains. This finding contradicts the usual practice of performing the final magnetic annealing for as long as possible and at the highest possible temperature in order to optimize magnetic properties such as coercivity and maximum permeability. At high annealing temperatures, increased grain growth occurs, and in the case of the aforementioned CoFe alloys, secondary recrystallization also takes place.

[0024] The method according to the invention makes it possible to achieve sufficiently good magnetic properties while simultaneously ensuring improved homogeneity of the microstructure. Furthermore, the overall heat treatment time is advantageously shorter, allowing for the production of more parts in the same amount of time or the same number of parts with reduced effort.

[0025] In one embodiment, the maximum proportion of grains with a grain size greater than 75 pm is further limited. The crystalline structure has a maximum of 5% grains with a grain size greater than 75 pm, preferably a maximum of 2.5% grains with a grain size greater than 75 pm, and preferably a maximum of 1% grains with a grain size greater than 75 pm.

[0026] The size and size distribution of the majority grains can be further restricted to increase homogeneity. (EinP13572DE)

[0027] 5

[0028] A quantitative measure of this is the standard deviation of the particle size, which can be calculated from the particle size distribution. In some embodiments, the mean particle size is dk 11 pm < dk < 26 pm with a standard deviation of less than 20 pm, preferably 16 pm < dk < 26 pm with a standard deviation of less than 20 pm.

[0029] In addition to grain size in pm, the grain size index can also be determined according to one of the measurement standards ASTM E112 or DIN EN ISO 643. This is done by comparing the microstructure with standardized image sequence charts. Smaller numbers represent large grains, and larger numbers represent small grains.

[0030] The FeCoV alloy also exhibits good mechanical properties. In one embodiment, the FeCoV alloy has a yield strength Rp0.2 of 250 MPa < Rp0.2 < 400 MPa.

[0031] The FeCoV alloy can also have a maximum permeability pmax of at least 10,000 and a remanence Br of at least 1.0 T.

[0032] In further embodiments, the FeCoV alloy has 250 MPa < Rp0.2 < 350 MPa, preferably 250 MPa < Rp0.2 < 300 MPa, and / or an induction B800 = B(800 A / m) of at least 2.1 T, preferably an induction B300 = B(300 A / m) of at least 1.9 T, preferably an induction B100 = B(100 A / m) of at least 1.2 T.

[0033] A heat treatment process for an FeCoV alloy is provided, which can achieve the crystalline structure and properties described above. The process includes the following:

[0034] At least one part is provided as an FeCoV alloy. The FeCoV alloy consists essentially of 30 wt% < Co < 55 wt%, preferably 45 wt% < Co < 50 wt%, 0.5 wt% < V < 2.5 wt%, 0 wt% < Ta < 1 wt%, 0 wt% < Nb < 1 wt%, up to 1 wt% impurities, and the remainder Fe. At least one part P13572DE

[0035] 6

[0036] The material is heat-treated in a continuous process at a temperature To for a time duration th, where th ≥ 1 h, and then cooled to a temperature Ti with a first cooling rate CR1, where Ti < To, and then from temperature Ti to a temperature T2 with a second cooling rate CR2, where T2 < Ti and CR2 > CR1, where 700 °C < To < T(a^a+y) + 20 °C, 400 °C <= Ti <= 700 °C, 100 °C <= T2 <= 400 °C and 150 K / h < |CR2 - CR11 < 3000 K / h.

[0037] This heat treatment takes place continuously and can therefore be carried out faster than in a stationary furnace. The heat treatment uses a cooling process with two different cooling rates, where the first cooling step after the holding stage is slower than the second cooling step.

[0038] In further embodiments, 250 K / h < |CR2 - CR11 < 2800 K / h, preferably 250 K / h < |CR2 - CR11 < 2500 K / h.

[0039] In one embodiment, the time duration th begins at time ti and ends at time to. The cooling of the portion from To to Ti begins at time to and ends at time t. In one embodiment, CR1 is the average cooling rate within the temperature range (T1-T0) and |(Ti-To) / (ti-to)| 1000 K / h. In another embodiment, 50 K / h < |(Ti-To) / (ti-to)| 600 K / h, preferably 50 K / h < |(Ti-To) / (ti-to)| 300 K / h.

[0040] The cooling of the portion of Ti to T2 begins at time ti and ends at time t2, where CR2 is the mean cooling rate within the temperature range (T2-T1), and KT2— Ti) / (t2-ti)| > 300 K / h.

[0041] In one embodiment, 300 K / h < | (T2— Ti ) / (t2-ti ) | < 3000 K / h, preferably 300 K / h < | (T2— Ti ) / (t2-ti ) | < 1500 K / h.

[0042] The duration of the holding stage th can be 10 seconds < th 30 minutes, preferably 10 seconds < th 10 minutes. P13572DE

[0043] 7

[0044] The temperatures To and Ti can be further defined, namely 850 °C < To^ T(a^a+y) + 20 °C, preferably 850 °C < To^ 940 °C, preferably 850 °C < To^ 920 °C, preferably 850 °C < To< 900 °C, and / or 400 °C <Ti < 500 °C.

[0045] The second cooling rate CR2 can be selected depending on the temperature Ti at which the cooling step with cooling rate CR2 begins. If Ti is lower, CR2 can be higher. For example, if 650 °C < Ti < 700 °C, then CR2 is > 300 K / h; if 550 °C < Ti < 650 °C, then CR2 is > 600 K / h; if 400 °C < Ti < 550 °C, then CR2 is > 1000 K / h; or if 650 °C < Ti < 700 °C, then 300 K / h < CR2 is < 550 K / h; if 550 °C < Ti < 650 °C, then 600 K / h < CR2 is < 950 K / h; and if 400 °C < Ti < 550 °C, then 1000 K / h < CR2 is < 3000 K / h.

[0046] For the continuous heat treatment of at least one part, a continuous furnace with at least two heatable heating zones can be used, through which the part is conveyed. For example, the part can be conveyed through the continuous furnace on a suitable mechanism, such as a conveyor chain.

[0047] The part can be a single sheet, a stack of loose sheets, or a laminated core consisting of a stack of sheets joined together. The laminated core can have the outer shape of a stator segment, such as a stator tooth or stator ring. The part can be in the form of a sheet, a lamella, a laminated core, a laminated core with the outer contour of a stator tooth, a laminated core with the outer contour of a stator ring, or a laminated core with the outer contour of a stator segment with at least two stator teeth.

[0048] In one of the heating zones, the temperature To can be provided, and in the second heating zone, the temperature Ti. P13572DE

[0049] 8

[0050] The continuous furnace can have at least four heating zones, preferably six heating zones, preferably ten heating zones. The cooling rates CR1 and CR2 can be achieved by appropriately adjusting the temperature of the heating zones.

[0051] In one embodiment, the heat treatment is carried out under a hydrogen-containing atmosphere.

[0052] In one embodiment, at least one part is conveyed through the continuous furnace at temperatures above 700 °C in an atmosphere with less than 1 vol. % nitrogen, preferably less than 0.1 vol. % nitrogen.

[0053] In one configuration, the continuous furnace has at least two heating zones and at least two airlocks at the inlet or outlet. This arrangement allows for atmospheric control during heat treatment, for example, reducing the oxygen and / or nitrogen content. This arrangement can be used to ensure the soft magnetic and mechanical properties of the FeCoV alloy.

[0054] The holding time at temperature To and the cooling rate can be adjusted by a combination of the flow rate and the length of the respective heating zones. The cooling rates CR1 and CR2 can be adjusted by appropriately setting the temperature of the respective heating zones, the length of the heating zones, and the flow rate.

[0055] In one embodiment, at least one part is conveyed through the continuous furnace at a throughput speed of 1 cm / min to 20 cm / min, preferably 3 cm / min to 15 cm / min.

[0056] In one production method, the FeCoV alloy is first provided in the form of a strip with a cold-rolled texture, and the part is cut from the strip. For example, the part can be stamped or cut from the strip. P13572DE

[0057] 9

[0058] After heat treatment, in one embodiment the part has a crystalline structure with a mean grain size dk, where 11 pm < dk < 30 pm, and a maximum of 10% of the grains have a grain size greater than 75 pm.

[0059] After heat treatment, in one embodiment the part has a yield strength of 250 MPa < Rpo.2 400 MPa, a maximum permeability pmax of at least 10000 and a remanence Br of min. 1.0 T.

[0060] After heat treatment, in one embodiment the crystalline structure has a maximum of 5% grains with a grain size greater than 75 pm, preferably a maximum of 2.5% grains with a grain size greater than 75 pm, preferably a maximum of 1% grains with a grain size greater than 75 pm.

[0061] In one embodiment, after heat treatment the crystalline structure has a mean grain size dk, wherein 11 pm < dk < 26 pm with a standard deviation of less than 20 pm, preferably 16 pm < dk < 26 pm with a standard deviation of less than 20 pm.

[0062] In one embodiment, after heat treatment the part has an induction B800 = B(800 A / m) of at least 2.1 T, preferably an induction B300 = B(300 A / m) of at least 1.9 T, preferably an induction B100 = B(100 A / m) of at least 1.2 T.

[0063] In some embodiments, the FeCoV alloy contains niobium, where 0.01 wt% < Nb < 0.25 wt%, preferably 0.05 wt% < Nb < 0.15 wt%.

[0064] The embodiments and examples will now be explained with reference to the drawings.

[0065] Fig. 1 shows the target and actual temperature-time curves of a continuous heat treatment according to the invention, which was carried out in a continuous furnace with ten heating zones. P13572DE

[0066] 10

[0067] Fig. 2 shows the temperature-time profile of the comparative annealing tests carried out in a stationary tube furnace.

[0068] Fig. 3 shows micrographs of the microstructure after final annealing for the comparison specimens A1 and A6 and the specimen C3 according to the invention.

[0069] According to the invention, parts made of an FeCoV alloy are annealed in a continuous process in test series C. The continuous furnace can be, for example, a chain furnace, a roller hearth furnace, or a pusher furnace. In test series C, FeCoV parts are heat-treated in a continuous process using a method with two-stage cooling or a shortened annealing time.

[0070] The parts to be annealed were stamped segments with a composition of 49 wt% Fe, 49 wt% Co, and 2 wt% V (VACODUR 49). These segments can, for example, each have the shape of a single tooth of a stator. Complete stators for motor or generator applications can then be assembled from such annealed teeth in a subsequent step. Test rings made of the same material are added to the annealing process for magnetic characterization.

[0071] This document describes the test series C1, C2, and C3, in which the parts are heat-treated in a continuous process and samples C2 and C3 are cooled according to the invention. Furthermore, three comparative samples A1, A2, and A6 are described, in which the parts are heat-treated in a stationary manner.

[0072] Table 1 summarizes the annealing parameters used and the cooling rates determined from the temperature records for the continuous annealing of reference sample C1, the continuous annealing of the inventive samples C2 and C3, and the comparison samples A1, A2, and A6. The temperature To denotes the temperature at the start and end of the holding stage, and the column th = (to - ti) corresponds to the duration of the holding stage. The cooling rate CR1 refers to the first cooling phase, i.e., the time required to cool from To to P13572DE

[0073] Ti, the cooling rate CR2 refers to the second cooling phase, i.e., the time required to cool Ti to T2. The cooling rates CR1 and CR2 were calculated as average cooling as follows: CR1 = | T1 ~ T ° I and CR2 = | T2 ~ T1 1. The

[0074] Columns I to II ^2 and |T2-T11 describe the width of the temperature range for the rapid cooling of Ti to T2. Column |CR2-CR11 describes the difference between the two cooling rates CR2 and CR1 and is only defined for two-stage cooling.

[0075]

[0076] R = Reference, E = According to the invention

[0077] Table 1

[0078] Table 2 shows the induction values ​​B(H) determined from the static new curve at various field strengths H from 100 A / m to 8000 A / m, which were determined on annealed samples C1 , C2 and C3 and comparison samples A1 , A2 and A6 according to the measurement standard IEC 60404-4.

[0079] Table 3 shows the static magnetic properties of maximum permeability Umax, remanence Br and coercive field strength Hc of annealed stamped rings made of VACODUR 49 for the samples C1, C2 and C3 according to the invention and the comparison samples A1, A2 and A6.P13572DE

[0080] 12

[0081]

[0082] Table 2

[0083]

[0084] Table 3

[0085] In an initial comparative test C1, the annealing process was tested in a continuous furnace with four different heating zones over a length of four meters. The furnace was operated with dry hydrogen. The target values ​​for the heating zones were set to achieve a constant holding temperature of approximately 880 °C for about 30 minutes. After passing through the last heating zone, the parts entered a cooling zone, resulting in very rapid cooling starting at 870 °C. The cooling rate CR2 was over 2600 °C / h. This annealing process thus corresponds to a classic setting, as is normally used in continuous furnaces for soft magnetic iron-silicon parts (P13572DE).

[0086] 13

[0087] Alloys are used. The different heating zones primarily serve the purpose of achieving sufficiently high temperature homogeneity in the holding stage.

[0088] The magnetic properties measured on test pieces C1 are insufficient for some applications. For example, the induction B(300 A / m) is only 0.590 T and the maximum permeability pmax is only 1.818, see Tables 2 and 3. The annealing process performed in this way is therefore not suitable for adequately adjusting the magnetic properties of the CoFe parts. One possible cause for this is the very rapid cooling that begins at 870 °C, which likely leads to internal stress in the parts.

[0089] In the inventive example C2, the setpoints of the four heating zones are adjusted such that a holding stage occurs in the first heating zone, followed by slow cooling in the subsequent three heating zones. The cooling zone then leads to very rapid cooling of the parts. Thus, an annealing process with two-stage cooling is achieved; see the annealing parameters in Table 1 for sample C2.

[0090] The specific temperature profile was estimated using the fixed thermocouples in the heating zones as follows: The holding temperature To was 870 °C, and the holding time was 0.1 h. At the end of the heating zones, the temperature of the part was 630 °C, which corresponds to an average cooling rate CR1 in this temperature range of 130 °C / h, i.e., a controlled, slow cooling. In the cooling zone, the sample then cooled to 300 °C with an average cooling rate of 914 °C / h, i.e., this corresponds to rapid cooling.

[0091] The values ​​measured for sample C2 on the annealed reference samples show that this two-stage heat treatment does indeed result in significantly improved soft magnetic properties. For example, a maximum permeability pmax of 10.663 and a remanence Br of 1.26 T are obtained (see Tables 2 and 3).

[0092] 14

[0093] Another process according to the invention, C3, took place in a second continuous furnace, the heated area of ​​which consists of ten heating zones, each approximately one meter long.

[0094] Figure 1 shows a schematic representation of the set target temperature profile of the second continuous furnace in the form of stages (Z1 - Z10), as well as the actual temperature profile of sample C3 measured on the part as a curve. The actual temperature profile was verified using a type K thermal drag element with a diameter of 1 mm. The element was positioned by means of a hole in a stamped stack of 0.20 mm thick CoFe sheets and moved through the furnace. The corresponding temperature was measured every second and recorded.

[0095] In these zones, designated Z1 to Z10, the temperature T(Z) in each heating zone can be individually set. Control is achieved using thermocouples, which are positioned centrally in each heating zone, just above the workpiece carrier. This configuration allows for precise temperature control of the parts by adjusting the temperature in each heating zone in stages, in conjunction with selecting the throughput speed of the workpiece. This is particularly important for achieving a defined, relatively slow cooling rate. In contrast to conventional methods, these heating zones are not only used to homogenize the holding stage but also specifically to establish a temperature gradient at the exit point.

[0096] Table 4 shows the setting parameters for the continuous furnace with ten adjustable heating zones Z1 to Z10, which was used for the annealing process C3 according to the invention. Zone IN designates the inlet zone and zone RC the outlet zone, both of which are not actively heated.

[0097] In the present embodiment, a holding time th = to - ti of up to 1 h and a holding temperature To = 880 °C were specified, using a throughput speed of 5 cm / min and the setting parameters for the heating zones shown in Table 4. The first, controlled cooling phase takes place in zones Z5 to Z10.

[0098] 15

[0099] The temperature is set to decrease from 880 °C to just under 500 °C. With a cooling time of approximately 120 minutes, this results in an initial average cooling rate CR1 of 180 °C / h. Following the heating zone Z10 is an unheated cooling zone RC, where the samples remain under hydrogen purging and can cool rapidly to room temperature (RC = Rapid Cooling). The achieved average cooling rate CR2 from 478 °C to 200 °C was determined to be 1247 °C / h.

[0100]

[0101] Table 4

[0102] Figure 1 shows that the desired holding temperature To of 880 °C was achieved. The actual residence time th in the holding stage is approximately 40 min, which is very short compared to a steady-state annealing process with a typical annealing time of 6 h.

[0103] The actual temperature T rises briefly. pof the part up to 905 °C. The phase transition T(a^a+y) of the alloy, which can be determined e.g. by differential scanning calorimetry (DSC), is at the composition P13572DE

[0104] 16

[0105] The target temperature of VACODUR 49 is approximately 890 °C and should not actually be exceeded. However, exceeding this temperature is not always technically avoidable in continuous furnaces. Such an exceedance of the target temperature can be reduced by adjusting the setting parameters. For the subsequent tests, this is achieved by lowering the target temperatures in zones 2 to 4 by 25 °C.

[0106] Further spot checks indicate that a brief temperature increase of up to 20 °C for a maximum duration of 30 minutes is still acceptable. Exceeding this temperature leads to a significant and irreversible deterioration of the soft magnetic properties. It is particularly advantageous if the phase transition temperature is exceeded for a maximum of 10 minutes.

[0107] Figure 1 shows that the actual cooling in the first cooling phase, i.e., starting from heating zone Z4, runs almost parallel to the desired cooling curve and can be estimated with an average cooling rate CR1 of 200 °C / h. At approximately 500 °C, the end of the last active heating zone Z10 is reached, and the second, much faster cooling phase begins. In this example, the cooling from Ti = 478 °C to T2 = 200 °C took approximately 13 minutes, corresponding to an average cooling rate CR2 of 1274 °C / h. The total annealing time tg, from the start of the annealing process until reaching 200 °C in the cooling phase, was only about 4 hours in this example.

[0108] After successful adjustment of the temperature profile, particularly the cooling profile, the furnace's protective gas atmosphere is switched to hydrogen. Due to the open design of the system, a residual nitrogen content of over 1% by volume is always present in the heating zones.

[0109] To verify the magnetic properties, stamped rings made of 0.35 mm thick VACODUR 49 were passed through the oven under the aforementioned parameters. After heat treatment, the stamped rings were placed in a plastic trough, wound, and tested according to the specifications of the measurement standard IEC 60404-8-6P13572DE.

[0110] 17

[0111] The magnetically characterized components are described. The measured values ​​obtained in this way are summarized as embodiment C3.

[0112] It is evident that sample C3 exhibits significantly better magnetic properties than the reference sample C1, as shown in Tables 2 and 3. Sample C3 achieves an induction B(300 A / m) of 1.996 T and a maximum permeability pmax of 13,520. In particular, all the specifications of the material standard ASTM A801, Alloy Type 1, are met, e.g., an induction B(800 A / m) of over 2.0 T. Therefore, with the two-stage cooling process according to the invention, very good magnetic properties can be achieved in parts made of a CoFe alloy, even in a continuous process.

[0113] A reference sample A1 was produced. For test series A, ring-shaped test specimens with an outer diameter of 38.1 mm and an inner diameter of 31.75 mm were produced from a 0.20 mm thin strip of the alloy VACODUR 49 with the composition Fe 49% Co 1.9% V and 0.1% Nb. In each test, 10 to 15 of these rings were subjected to final annealing in a stationary tube furnace under a dry hydrogen atmosphere. All samples were first heated to a temperature To of 880 °C and held there for a duration of 6 h.

[0114] The subsequent cooling process was varied as follows: The two reference annealing processes, A1 and A2, only involved single-stage cooling. Sample A1 was cooled slowly throughout, starting from the holding stage, which corresponds to the usual heat treatment for CoFe samples.

[0115] Reference annealing A1 was performed with single-stage furnace cooling, resulting in an average cooling rate of 51 °C / h over the temperature range of 880 °C to 200 °C. To illustrate the difference to the two-stage annealing processes according to the invention, the actual cooling rate has been subsequently divided into two sections in Table 1. In the range from 880 °C to 500 °C, this results in an average cooling rate CR1 of 73 °C / h, and in the range from 500 °C to 200 °C, an average cooling rate CR2 of 33 °C / h is obtained. In example A1, the cooling rate CR2 is therefore lower than the cooling rate CR1.

[0116] 18

[0117] Figure 2 shows an example of the temperature-time profile of comparative annealing processes A1, A2, and A6, which were carried out in a stationary tube furnace with the capability of rapid cooling. Here, T denotes the furnace temperature in °C and t the time after the start of the annealing process in hours. The times ti for reaching temperature To, to for the start of the first cooling phase, ti for reaching temperature Ti and the start of the second cooling phase, and t2 for reaching temperature T2 are shown as examples.

[0118] The reference annealing process A1 with slow cooling corresponds to a standard annealing process for CoFe alloys. Very high inductions B(H) are obtained even at low field strengths H, e.g., an induction B(100 A / m) of 1.663 T. Accordingly, the maximum permeability pmax is 18.273. The induction at very high field strengths, e.g., 8 kA / m, is relatively independent of the annealing process and is primarily determined by the composition. For the composition considered here (Fe 49 wt%, Co 1.9%, V 0.1%, Nb), an induction B(8 kA / m) of 2.299 T is obtained.

[0119] Figure 3 shows micrographs of samples A1, A6, and C3 after final annealing. The comparison samples A1 (6 h at 880 °C, long holding time, and slow cooling), A6 (6 h at 880 °C, long holding time, and rapid cooling from 500 °C), and the inventive sample C3 (same holding temperature, 880 °C, shorter holding time of 0.7 h, and two-stage cooling with rapid cooling from 478 °C), which was annealed continuously. The microstructure of the samples can be seen in the micrographs.

[0120] The grain size and grain size distribution were determined using the line section method, with at least 150 sections per sample. The results are summarized in Table 5. Table 5 includes the mean grain size, its standard deviation, and the grain size index according to ASTM E112 or DIN EN ISO 643. In some samples, isolated large grains, e.g., due to secondary crystallization, were also found, exhibiting a grain size significantly larger than the mean grain size. In addition to these large grains, the proportion of grains larger than 38 pm and larger than 75 pm was also determined.

[0121] > >

[0122]

[0123] Table 5

[0124] The microstructure of sample C3, annealed in a continuous process according to the invention, exhibits a fully recrystallized microstructure with a mean grain size of 18 pm. It is therefore somewhat finer-grained than the reference annealing A1, which is due, among other things, to the short holding time. Nevertheless, the grain growth is already sufficient to achieve the very good magnetic properties, e.g., for a stator lamination, with the resulting microstructure.

[0125] In direct comparison to the reference annealing process A1, the magnetic properties of the continuously annealed sample C3 are still somewhat worse. It is expected that a significant improvement in the magnetic properties can be achieved by exclusively using hydrogen in the continuous furnace. Ideally, the hydrogen used should have an inlet dew point below -30 °C, preferably below -40 °C.

Claims

P13572DE 20 Patent claims 1. FeCoV alloy, which a composition consisting essentially of 30 wt% < Co < 55 wt%, preferably 45 wt% < Co < 50 wt%, 0.5 wt% < V < 2.5 wt%, 0 wt% < Ta < 1 wt%, 0 wt% < Nb < 1 wt%, up to 1 wt% impurities, residual Fe, and a crystalline structure with a mean grain size dk, where 11 pm < dk < 30 pm and a maximum of 10% grains have a grain size greater than 75 pm.

2. FeCoV alloy according to claim 1, wherein the crystalline structure comprises a maximum of 5% grains with a grain size greater than 75 pm, preferably a maximum of 2.5% grains with a grain size greater than 75 pm, preferably a maximum of 1% grains with a grain size greater than 75 pm.

3. FeCoV alloy according to claim 1 or claim 2, wherein 11 pm < dk < 26 pm with a standard deviation less than 20 pm, preferably 16 pm < dk < 26 pm with a standard deviation less than 20 pm, preferably less than 15 pm.

4. FeCoV alloy according to any one of claims 1 to 3, wherein this a yield strength of 250 MPa < Rp0.2 < 400 MPa, has a maximum permeability pmax of at least 10,000, and a remanence Br of at least 1.0 T.

5. FeCoV alloy according to claim 4, wherein this 250 MPa < Rp0.2 < 350 MPa, preferably 250 MPa < Rp0.2 < 300 MPa, and / or an induction B800 = B(800 A / m) of at least 2.1 T, preferably an induction B300 = B(300 A / m) of at least 1.9 T, preferably an induction B100 = B(100 A / m) of at least 1.2 T. P13572DE 21 6. FeCoV alloy according to any one of claims 1 to 5, comprising 0.01 wt% < Nb < 0.25 wt%, preferably 0.05 wt% < Nb < 0.15 wt%.

7. FeCoV alloy according to any one of claims 1 to 6, wherein the FeCoV alloy has the form of a sheet or a lamella or a stack of sheets or a stack of sheets with the outer contour of a stator tooth or a stack of sheets with the outer contour of a stator ring or a stack of sheets with the outer contour of a stator segment having at least two stator teeth.

8. Method for the heat treatment of an FeCoV alloy, the method comprising the following: Providing at least a portion of an FeCoV alloy consisting essentially of 30 wt% < Co < 55 wt%, preferably 45 wt% < Co < 50 wt%, 0.5 wt% < V < 2.5 wt%, 0 wt% < Ta < 1 wt%, 0 wt% < Nb < 1 wt%, up to 1 wt% impurities, remainder Fe, Heat treatment of at least one part in a continuous process at a temperature To for a duration th, where th ≥ 1 h, and then cooling of the part to a temperature Ti with a first cooling rate CR1, where Ti < To, and then cooling of the part from temperature Ti to a temperature T2 with a second cooling rate CR2, where T2 < T1 and CR2 > CR1, where 700 °C < To < T(a^a+y) + 20 °C, 400 °C <= Ti <= 700 °C, 100 °C <= T2 <= 400 °C and 150 K / h < |CR2 - CR11 < 3000 K / h.

9. Method according to claim 8, wherein 250 K / h < |CR2 - CR11 < 2800 K / h, preferably 250 K / h < |CR2 - CR11 < 2500 K / h.

10. Method according to claim 8 or claim 9, wherein the time period th begins at a time ti and ends at a time to, and the cooling of the part from To to Ti begins at time to and ends at a time tiP13572DE ends and where CR1 is the mean cooling rate within the temperature range (T1-T0) and |(Ti-To) / (ti-to)| is 1000 K / h.

11. Method according to claim 10, wherein 50 K / h < |(Ti-To) / (ti-to)| 600 K / h, preferably 50 K / h < |(Ti-To) / (ti-to)| 300 K / h.

12. Method according to any one of claims 8 to 11, wherein the cooling of the part from Ti to T2 begins at time ti and ends at time t2, wherein CR2 is the mean cooling rate within the temperature range (T2-T1), and |(T2-Ti) / (t2-ti)| is > 300 K / h.

13. Method according to claim 12, wherein 300 K / h < | (T2— Ti ) / (t2-ti ) | < 3000 K / h, preferably 300 K / h < KT2— Ti) / (t2-ti)| < 1500 K / h.

14. Method according to any one of claims 8 to 13, wherein 10 seconds is less than 10 minutes.

15. Method according to any one of claims 8 to 14, wherein 850 °C < To T(a^a+y) + 20 °C, preferably 850 °C < To^ 940°C, preferably 850 °C < To^ 920°C, preferably 850 °C < To^ 900°C, and / or 400 °C <Ti < 500 °C beträgt.

16. Method according to any one of claims 8 to 15, wherein If 650 °C < Ti < 700 °C then CR2 > 300 K / h, if 550 °C < Ti < 650 °C then CR2 > 600 K / h, if 400 °C < Ti < 550 °C then CR2 > 1 000 K / h.

17. Method according to claim 16, wherein If 650 °C < Ti < 700 °C then 300 K / h < CR2 < 550 K / h, if 550 °C < Ti < 650 °C then 600 K / h < CR2 < 950 K / h, if 400 °C < Ti < 550 °C then 1000 K / h < CR2 < 3000 K / h, P13572DE 23 18. Method according to any one of claims 8 to 17, wherein at least one part is conveyed through the continuous furnace which has at least two heatable heating zones.

19. Method according to claim 18, wherein the temperature To is provided in a first of the heating zones and the temperature Ti is provided in a second of the heating zones.

20. Method according to claim 18 or claim 19, wherein the part is conveyed through the continuous furnace which has at least four heating zones, preferably six heating zones.

21. Method according to one of claims 18 to 20, wherein the part is conveyed through a continuous furnace having at least two heating zones and at least two airlocks at the inlet or outlet.

22. Method according to any one of claims 8 to 21, wherein the heat treatment is carried out under a hydrogen-containing atmosphere.

23. Method according to any one of claims 18 to 22, wherein at least one part is conveyed through the continuous furnace at temperatures above 700 °C in an atmosphere with less than 1 vol. % nitrogen, preferably less than 0.1 vol. % nitrogen.

24. Method according to one of claims 18 to 23, wherein at least one part is conveyed through the continuous furnace at a throughput speed of 1 cm / min to 20 cm / min, preferably 3 cm / min to 15 cm / min.

25. A method according to any one of claims 8 to 24, wherein the FeCoV alloy is first provided in the form of a strip having a cold-rolled texture, and the part is manufactured from the strip. P13572DE 24 26. Method according to any one of claims 8 to 25, wherein the part has the form of a sheet or a lamella or a stack of laminations or a stack of laminations with the outer contour of a stator tooth or a stack of laminations with the outer contour of a stator ring or a stack of laminations with the outer contour of a stator segment having at least two stator teeth.

27. Method according to any one of claims 8 to 26, wherein after heat treatment the part has a crystalline structure with a mean grain size dk, wherein 11 pm < dk < 30 pm, and a maximum of 10% grains with a grain size greater than 75 pm.

28. Method according to any one of claims 8 to 27, wherein after heat treatment the part a yield strength of 250 MPa < Rp0.2 < 400 MPa, has a maximum permeability pmax of at least 10,000, and a remanence Br of at least 1.0 T.

29. Method according to claim 27 or claim 28, wherein after heat treatment the crystalline structure has a maximum of 5% grains with a grain size greater than 75 pm, preferably a maximum of 2.5% grains with a grain size greater than 75 pm, preferably a maximum of 1% grains with a grain size greater than 75 pm.

30. Method according to any one of claims 27 to 29, wherein after heat treatment 11 pm < dk < 26 pm with a standard deviation less than 20 pm, preferably 16 pm < dk < 26 pm with a standard deviation less than 20 pm.

31. A method according to any one of claims 8 to 30, wherein, after heat treatment, the part has an induction B800 = B(800 A / m) of at least 2.1 T, preferably an induction B300 = B(300 A / m) of at least 1.9 T, preferably an induction B100 = B(100 A / m) of at least 1.2 T. P13572DE 25 32. Method according to any one of claims 8 to 31, wherein the FeCoV alloy has 0.01 wt% < Nb < 0.25 wt%, preferably 0.05 wt% < Nb < 0.15 wt%.

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