Method for heat treating a part made of an FeCoV alloy

Abstract

A method for heat treatment of a part made of an FeCoV alloy that consists substantially 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. % of impurities and the remainder of Fe, and being heat-treated at a temperature T0 for a period th, where th≤1 h, then cooled to a temperature T1 at a first cooling rate CR1, where T1<T0, and then cooled from temperature T1 to a temperature T2 at a second cooling rate CR2, where T2<T1, 800° C.≤T0≤T(α→α+γ)+20° C., 400° C.≤T1≤700° C., 100° C.≤T2≤400° C., 50 K / h≤CR1≤300 K / h and CR2>250 K / h and CR2>CR1.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This US Non-Provisional patent application relies on and claims priority to German Patent Application No. 102025107750.9, filed on Feb. 28, 2025, the entire contents of which are incorporated herein by reference.BACKGROUND OF THE INVENTION

[0002] The present invention relates to a method for heat treating a part made of an FeCoV alloy.

[0003] As conductors of magnetic flux, soft-magnetic materials, such as FeCoV alloys in the form of bands and laminations produced from such bands, are a significant component in the construction of electric machines. In cases such as radial flux machines, for example, laminations are stacked one on top of another and attached to form a laminated stator core. In the case of rotors, too, soft-magnetic laminations are also often stacked and attached to form a laminated rotor core.

[0004] Some applications, in the aviation and motor racing industries, for example, use soft-magnetic materials made of cobalt-iron (CoFe) that exhibit increased saturation polarisation. An example of this class of alloys are those known as V-permendur, which comprise approx. 49 wt. % Fe, 49 wt. % Co and 2 wt. % V. Since their soft-magnetic properties are substantially determined by their microstructures, these alloys are heat-treated to produce a suitable structure through recrystallisation and grain growth. It is by means of such heat treatment that high maximum permeability μmax, low coercive field strength He and high inductions B(H) are achieved at low field strengths H.

[0005] In the case of CoFe laminations with a Co content of 30 to 55 wt. % it is usual to carry out heat treatment after forming, i.e. to heat-treat the formed part. The heat treatment of the part, often referred to as “final annealing”, serves to recrystallise the structure and then promote grain growth, and also results in a reduction in residual stress in the material. In order to achieve good soft-magnetic properties in a stator for an electric machine, for example, the resulting structure should be fully recrystallised and exhibit a grain size sufficient to obtain a low Hc and a high μmax. By contrast, the fine-grained structure produced for rotor applications exhibits higher strengths, in particular a high yield strength Rp0.2 due to the many grain boundaries, but also significantly poorer soft-magnetic properties.

[0006] The standard heat treatment for parts made of CoFe and designed to obtain optimum magnetic properties for use as stator laminations, for example, involves stationary annealing in dry hydrogen with a dwell stage in the region of 800 to 940° C. The maximum temperature is dependent on composition. In the case of V-containing compositions it should not exceed the phase transition temperature T(α→α+γ) if precipitations are to be avoided. Typical dwell times are in the region of 4 to 10 hours and are followed by slow cooling to at least 200° C. at cooling rates in the region of 100 to 300° C. / h.SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide a method for the heat treatment of a part made of an FeCoV alloy in order to achieve homogenous soft-magnetic properties.

[0008] According to the invention, a method for the heat treatment of at least one part made of an FeCoV alloy comprises the following. There is provided at least one part made of an FeCoV alloy consisting substantially 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. % of impurities and the remainder Fe. The at least one part is heat-treated at a temperature T0 for a period th, where th≤1 h, and then cooled at a first cooling rate CR1 to a temperature T1, where T1<T0, and then at a second cooling rate CR2 from temperature T1 to a temperature T2, where T2<T1, 800° C.≤T0≤T(α→α+γ)+20° C., 400° C.≤T1≤700° C., 100° C.≤T2≤400° C., 50 K / h≤CR1≤300 K / h and CR2>300 K / h and CR2>CR1.

[0009] The total duration of this heat treatment is short since the dwell time th at the highest temperature T0 is less than one hour and the cooling rate CR2 from T1 to T2 is faster than the cooling rate CR1 from T0 to T1. This heat treatment also produces a homogenous structure.

[0010] It has been established that, in addition to mean grain size, the percentage of large grains also has an impact on soft-magnetic properties. It has also been established that variations in measured soft-magnetic properties are connected to the percentage of large grains.

[0011] The homogeneity of the magnetic properties of laminations in rotating electric machines is important in ensuring uniform magnetisation. A homogenous structure with no or few above-average sized grains is advantageous in ensuring uniform magnetisation. This impact is felt even more strongly in thin laminations, e.g. those with a maximum thickness of 0.15 mm. If the diameter of individual large grains approaches the thickness of the band, the preferred direction of this one crystal dominates the magnetic properties in cross section.

[0012] As a result, a structure in which grain size is as uniform as possible and, in addition, contains no very large grains is particularly advantageous for rotating electric machines. This finding is at odds with the standard process which involves performing magnetic final annealing at the highest possible temperature for as long as possible in order to optimise magnetic values such as coercive field strength and maximum permeability. High annealing temperatures result in increased grain growth and, in the CoFe alloys referred to here, in secondary recrystallisation.

[0013] The method according to the invention permits the obtention of sufficiently good magnetic properties while at the same time ensuring improved homogeneity of structure. Moreover, the total duration of the heat treatment is advantageously shorter, thereby enabling more parts to be produced in the same time or the same number of parts to be produced in a shorter time.

[0014] In an embodiment, the at least one part undergoes stationary heat treatment in a furnace.

[0015] The cooling and so the cooling rate of the parts in a stationary furnace may be set either by externally cooling the annealing area by means of fans with a flow of gas or air being directed towards the outside of the annealing area, or actively by cooling the process gas in a heat exchanger.

[0016] In an embodiment, the first cooling rate CR1 is achieved by externally cooling the annealing area by means of fans, and the second cooling rate CR2 to below 500° C. is achieved by means of active gas cooling of the process gas using a heat exchanger. The furnace may be a retort furnace.

[0017] In an embodiment, the heat treatment has a total duration to, where to is measured from room temperature via T0, T1 and T2 to a final temperature, this final temperature being 200° C. and to being less than 12 hours, preferably less than 10 hours, preferably less than 6 hours, preferably less than 4 hours.

[0018] In an embodiment, 50 K / h≤CR1≤250 K / h and CR2>300 K / h, preferably 50 K / h≤CR1≤200 K / h and CR2>300 K / h.

[0019] The difference between cooling rates CR2 and CR1 can be defined further. In an embodiment, 50 K / h≤|CR2−CR1|≤3000 K / h, preferably 150 K / h≤|CR2−CR1|≤3000 K / h, preferably 250 K / h≤|CR2−CR1|≤2800 K / h, preferably 250 K / h≤|CR2−CR1|≤2500 K / h.

[0020] In an embodiment, period th starts at point ti and ends at a point to, while the cooling of the part from T0 to T1 starts at point to and ends at a point t1, CR1 being the mean cooling rate within the temperature range (T1-T0), and 50K / h≤|(T1-T0) / (t1-t0)|≤200 K / h.

[0021] In an embodiment, the cooling of the part from T1 to T2 starts at point t1 and ends at a point t2, where CR2 is the mean cooling rate within the temperature range (T2-T1), and 300 K / h<|(T2-T1) / (t2-t1)|≤3000 K / h, preferably 300 K / h<|(T2-T1) / (t2-t1)|≤500 K / h.

[0022] In further embodiments, the dwell time th is defined more precisely and may be 10 seconds≤th≤45 minutes, 10 seconds≤th≤30 minutes, preferably 10 seconds≤th≤15 minutes.

[0023] In further embodiments, the temperature T0 is further defined and may be 820° C.≤T0, preferably 850° C.≤T0, preferably 880° C.≤T0 and / or T0≤940° C., preferably T0≤920° C., preferably T0≤900° C. In further embodiments, the temperature T1 is further defined and may be 500° C.≤T1≤700° C., preferably 550° C.≤T1≤700° C., preferably 600° C.≤T1≤700° C.

[0024] In further embodiments, first the FeCoV alloy is provided in the form of a band with a cold-rolled texture. The part is separated from, e.g. punched, cut or laser cut out of, this band. The part may have an outer contour that corresponds to the desired outer contour of a stator or a part of a stator such as a tooth or a ring, for example.

[0025] A plurality of these separated parts may be stacked to form a laminated core. In an embodiment, in-die stacking is used in order to produce a laminated core. The laminated core may correspond to the outer contour of a stator tooth, of a laminated core having the outer contour of a stator ring or of a laminated core having the outer contour of a stator segment with at least two stator teeth. In some embodiments the laminated core is heat-treated.

[0026] The part heat-treated using the method may take the form of a lamination, of a sheet, of a laminated core, of a laminated core having the outer contour of a stator tooth, of a laminated core with the outer contour of a stator ring or of a laminated core with the outer contour of a stator segment having at least two stator teeth.

[0027] The method is particularly suited to the production of soft-magnetic stators or parts of soft-magnetic stators.

[0028] In some embodiments, following the heat treatment the part has a crystalline structure with a mean grain size dk, where 11 μm≤dk<30 μm, and a maximum of 10% of grains with a grain size of greater than 75 μm. A homogenous structure of this type ensures laterally uniform magnetics. The parts also have a fully recrystallised homogenous structure that permits the desired very good soft-magnetic properties required for use in stators for electric machines, for example.

[0029] In an embodiment, following the heat treatment the part has a yield strength of 250 MPa<Rp0.2≤400 MPa, a maximum permeability μmax of min. 10,000 and a remanence Br of min. 1.0 T.

[0030] In further embodiments, following the heat treatment the crystalline structure contains a maximum of 5% of grains with a grain size of greater than 75 μm, preferably a maximum of 2.5% of grains with a grain size of greater than 75 μm, preferably a maximum of 1% of grains with a grain size of greater than 75 μm.

[0031] In further embodiments, following the heat treatment the crystalline structure contains a maximum of 30% of grains, preferably a maximum of 25% of grains, preferably a maximum of 15% of grains, preferably a maximum of 10% of grains with a grain size of greater than 38 μm.

[0032] In further embodiments, following the heat treatment 11 μm≤dk<26 μm with a standard deviation of less than 20 μm, preferably 16 μm≤dk<26 μm with a standard deviation of less than 20 μm.

[0033] In further embodiments, after the 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.

[0034] The composition of the FeCoV alloy may be defined more closely. In an embodiment, 0.01 wt. %≤Nb≤0.25 wt. %. In another embodiment 0.05 wt. %≤Nb≤0.15 wt. %.

[0035] Exemplary embodiments are explained in greater detail below with reference to the drawings and examples.BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows by way of example temperature-time curves for two samples A2, A6 according to the invention that were annealed in a stationary tube furnace followed by rapid cooling, and for a comparison annealing sample A1.

[0037] FIG. 2 shows a semilogarithmic representation of the static initial curves B(H) of annealed samples A1 to A6.

[0038] FIG. 3 shows a semilogarithmic representation of the static permeability μ(H) of annealed samples A1 to A6.

[0039] FIG. 4 shows micrographs of sample A6 according to the invention and of comparison sample A1.

[0040] FIG. 5 shows temperature-time curves for the two samples B1 and B2 according to the invention that were annealed in a stationary bell-type furnace with active cooling using heat exchangers, and for comparison annealing sample A1.

[0041] FIG. 6 shows micrographs for the samples B1 to B5 according to the invention and for the comparison sample A1.DESCRIPTION OF EMBODIMENT(S) OF THE INVENTION

[0042] In a first series of tests A, samples were subjected to stationary annealing with very rapid cooling.

[0043] For test series A, annular test samples of external diameter 38.1 mm and internal diameter 31.75 mm were produced from a 0.2 mm-thick band of the alloy VACODUR 49, which has the composition Fe 49 wt. % Co 1.9% w / v and 0.1 wt. % Nb. In each test, 10 to 15 of these rings were subjected to final annealing in a dry hydrogenous atmosphere in a stationary tube furnace. All samples were first heated to a temperature T0 of 880° C. and maintained at this temperature for a period th.

[0044] FIG. 1 shows by way of example temperature-time curves for two annealing samples A2 and A6 that were annealed in a stationary tube furnace with rapid cooling and for a comparison annealing sample A1. T denotes the temperature of the furnace in ° C.; t denotes time after the start of annealing in hours. Also indicated by way of example are the points ti at which temperature T0 is achieved, to at which the first cooling phase starts, t1 at which temperature T1 is achieved and the second cooling phase starts, and t2 at which temperature T2 is achieved. Sample A1 represents a conventional heat treatment process and was cooled slowly to a temperature of approx. 200° C. A1 constitutes a reference or comparison sample. Sample A2 is heat-treated with rapid cooling from dwell time T0, and sample A6 is heat-treated with two-stage cooling, the cooling rate from temperature T1 being greater than the cooling rate from T0 to T1.

[0045] Cooling was carried out by moving the sample boat into a water-cooled cooling zone. By contrast, the four samples A3, A4, A5, A6 were subjected to two-stage cooling, i.e. they were first cooled slowly at cooling rate CR1 from T0 to temperatures T1 of 700° C., 600° C., 550° C. and 500° C. respectively and then subjected to faster cooling at cooling rate CR2, which was maintained at least until a temperature T2 of 200° C. to 300° C. was reached. Cooling was carried out by moving the sample boat into a water-cooled cooling zone.

[0046] Table 1 shows the parameters of the heat treatments carried out on test series A. A single cooling rate CR1 is given for annealing process A1 since no rapid cooling took place, i.e. T2=T1. This cooling rate CR1 averages out at just 51° C. / h for the entire range to 200° C.TABLE 1T0thCR1T1CR2T2|T1 − T2||CR2 − CR1|SampleR / Ein ° C.in hin ° C. / hin ° C.in ° C. / hin ° C.in ° C.in ° C. / hA1R8805.951200—2000—A2R8805.9—8802235300580—A3R8805.914470025413004002397A4R8805.911860016043003001486A5R8805.110355023162403102213A6R8805.995500747250250652A7R8800.594500213007373A8E8800.593500138630012931293R = reference,E = according to the invention

[0047] Only one cooling rate CR2 is given for annealing sample A2 with rapid cooling from T0 since no slow cooling takes place, i.e. T1=T0. A mean cooling rate CR2 of 2235° C. / h is achieved over the temperature range from 880° C. to 300° C.

[0048] The four samples A3, A4, A5 and A6 were also heated rapidly and maintained at 880° C. for 6 hours. The cooling process was carried out in two stages. The first slow-cooling stage took place at temperatures T1 of 700° C., 600° C., 550° C. and 500° C. respectively. This slow cooling phase was carried out at cooling rates CR1 from 95° C. / h for sample A6 to 144° C. / h for sample A3. From temperature T1 the annular samples were then subjected to very rapid cooling to at least T2, where T2 was between 300° C. and 200° C. This resulted in very high cooling rates CR2 with values of between 747° C. / h for sample A6 and 2541° C. / h for sample A3.

[0049] Tables 2 and 3 provide measured magnetic properties: induction B, maximum permeability μmax, remanence Br and coercive field strength Hc. FIGS. 2 and 3 show static initial curves for B(H) and μ(H).TABLE 2B(100 A / m)B(160 A / m)B(300 A / m)B(800 A / m)B(1, 6 kA / m)B(4 kA / m)B(8 kA / m)Samplein Tin Tin Tin Tin Tin Tin TA11.6631.8292.0112.2022.2562.2862.299A21.0011.5921.8622.0632.1672.2422.259A30.9291.5161.8332.0482.1582.2432.266A40.9451.5631.8562.0662.1742.2482.270A51.5571.7651.9412.1212.2052.2592.275A61.8111.9392.0672.1942.2132.2712.286A71.6941.8512.0132.1852.2372.2682.283A81.7731.9192.0472.1792.2322.2662.283TABLE 3BrHcSampleμmaxin Tin A / mA118,2731.5440.6A28,6450.6446.5A37,9130.6554.1A48,1820.6753.9A513,9401.2936.4A621,6151.6336.3A718,3421.6347.5A819,2851.5739.1The reference annealing process A1 with slow cooling corresponds to a standard annealing process for CoFe alloys. It results in very high inductions B(H) even at low field strengths H, e.g. an induction B(100 A / m) of 1.663 T. Accordingly, the maximum permeability μmax is 18,273. Induction at very high field strengths, e.g. 8 kA / m, is relatively independent of annealing and is determined primarily by composition. With the composition under investigation here of Fe 49 wt. % Co 1.9% V 0.1% Nb the induction B(8 kA / m) is 2.299 T. Annealing process A2 with very rapid cooling from the dwell temperature produces significantly poorer characteristic values, i.e. B(100 A / m) of 1.001 T and μmax of 8,645 T. Similar magnetic values are obtained for both annealing processes A3 and A4 in which very rapid cooling was started at 700° C. and 600° C. respectively.

[0051] The non-optimum soft-magnetic properties of samples A1 to A4 may be due to internal stresses occurring during the very rapid cooling process. Stresses of this type are particularly relevant in Fe—Co systems with high Co contents of between 30 and 55 wt. % (weight percent) since the magnetostriction of these alloys may be as high as 70 ppm in some cases. As a result, even small internal stresses in the material may lead to clear stress anisotropy, which in turn impairs soft-magnetic properties.

[0052] By contrast, annealing processes A5 and A6 according to the invention, which are not subjected to very rapid cooling until 550° C. and 500° C. respectively, exhibit magnetic values that are similar to or even better than those of the reference annealing process A1. This is shown, for example, in the form of the very high induction B(300 A / m) of 2.067 T for sample A6 or of the very low coercive field strength Hc of approx. 36 A / m for samples A5 and A6. The two-stage cooling is therefore successful in both obtaining a magnetically advantageous material state and significantly reducing annealing time. Since total annealing time in stationary annealing is very heavily influenced by the slow cooling process, this represents an important commercial advantage.

[0053] Samples A7 and A8 were annealed with a short dwell time (0.5 h). However, sample A7 was cooled slowly (furnace cooled), while sample A8 was also subjected to rapid cooling at 500° C. The magnetics of this sample were improved by the rapid cooling (e.g. as also for sample B1 in the second series of tests B).

[0054] FIG. 4 shows the micrographs for the samples in test series A. The grain sizes measured are presented in Table 4. In all cases (A1 to A6), there is a fully recrystallised ferritic structure with mean grain sizes of between 32 μm and 76 μm, corresponding to a grain size number as per ASTM E112 of 7.0 to 4.5. This represents a typical structure of the type used for stators, for example, since the large grains result in a reduction in He and an increase in μmax.TABLE 4MeanMax.grainGraingrainsizesizesized(avg)STABWnumberd(max)PercentagePercentageSamplein μmin μmKW(avg)in μmd >= 38 μmd >= 75 μmhomogenous?A135255, 812037%8.6%NoA233236, 512628%6.8%NoA330216, 811526%3.5%NoA442305, 914941%12.8%NoA53321714229%5.4%NoA634236, 512235%5.5%NoA720147, 98311%0.7%YesA820138839.3% 0.0%Yes

[0055] In the second test series B, the dwell time th at T0 and the total duration to of heat treatment were reduced and, in addition, a shortened dwell time th at T0 was examined in conjunction with two-stage cooling in a stationary furnace.

[0056] In test series B1 to B9 parts made of the CoFe alloy VACODUR 49 together with annular samples for magnetic characterisation were annealed in a stationary furnace with active cooling. Examples B10 and B11 were produced using an alloy with no Nb additions. Example B12 has a higher Nb content (0.2 wt. %). Examples B13 to B15 were produced using an alloy containing 0.1 wt. % Nb but a lower V content (1.1 wt. %). These compositions are detailed in Table 9.

[0057] Here, cooling was carried out by a fan in order to effectively cool the outside of the annealing chamber in conjunction with a heat exchanger for active cooling of the process gas. With this technology it is possible to set very controlled cooling rates over the entire annealing process on an industrial scale.

[0058] FIG. 5 shows temperature-time curves for samples B1 and B2 in a stationary furnace with fans and heat exchangers, and a comparison sample A1. Table 5 shows the parameters for the heat treatments used on test series B. Sample B2 according to the invention has a dwell time at TO of less than 1 hour and a second cooling rate CR2 of greater than 300K / h.TABLE 5T0thCR1T1CR2T2|CR2 − CR1 |SampleR / EAdd.in ° C.in hin ° C. / hin ° C.in ° C. / hin ° C.in ° C. / hA1R18805.9735003320040B1R18805.9156500363200207B2E18800.2190500338200148B3R18806.0149625402300253B4R18806.01526251473005B5E18800.5149625439300290B6R17500.3149500393300244B7R178041425007820064B8E18000.7567500420300353B9R182041495007820071B10R2880101425007820064B11R28800.51425007820064B12R384041505008020070B13R492061505001502000B14R49200.51515001502001B15E49200.5151500328200177R = reference,E = according to the invention

[0059] For the comparison annealing process B1, following a dwell stage at 880° C. the parts were cooled in a controlled manner at a moderate cooling rate of approx. 150° C. / h to a temperature of 500° C. in a first cooling phase. From 500° C. the cooling rate was then increased significantly to 363° C. / h in a second cooling phase. The magnetics achieved in this manner are comparable to the values for reference annealing process A1.

[0060] Surprisingly, however, it was established that total annealing time can be substantially reduced since cooling in the 500° C. to 200° C. range lasts only approx. 1.4 hours as opposed to taking around 13 h in the case of reference annealing process A1.

[0061] In a sample B2 according to the invention the annealing dwell time was significantly reduced while maintaining two-stage cooling characteristics, i.e. a very short dwell time of 0.2 h was selected in place of a dwell time th of 6 h as in the reference example. The purpose of the dwell stage is to ensure sufficient grain growth and thus achieve good soft-magnetic properties. It can therefore be expected for a short dwell time to result in a deterioration in soft-magnetic properties. The generally recommended dwell times for this alloy class for the annealing of stator laminations is in the region of 4 h to 10 h, meaning that the heat treatment carried out on sample B2 represents a significant reduction in dwell time.

[0062] Table 6 shows the magnetic properties B(induction), μmax (maximum permeability), Br (remanence) and He (coercive field strength) of samples B1 to B15. All induction values were fully determined for all samples. Non-measured points are indicated by a dash.TABLE 6B(100B(300B(800B(1, 6B(8HcA / m)A / m)A / m)kA / m)kA / m)BrinSampleR / Ein Tin Tin Tin Tin Tμmaxin TA / mA1R1.6632.0112.2022.2562.29918,2731.5440.6B1R1.7872.0522.1852.2372.28520,7381.6237.0B2E1.7762.0472.1792.2352.28518,0271.6242.3B3R1.7672.0462.1822.2382.29117,5821.5138.1B4R1.7482.0442.1872.2412.29119,7241.5436.5B5E1.7302.0352.1742.2332.28915,6281.5345.5B6R0.1771.4401.9882.1252.2673,8231.675223.0B7R—2.0162.1682.2322.28213,8061.69066.7B8E1.5791.9912.1692.2332.29113,5441.65070.1B9R—2.0452.1862.2442.29315,7081.67052.0B10R1.8262.0872.2242.271—26,0601.66527.7B11R—2.0502.2002.260—17,183—41.0B12R—0.7932.1032.221———79.3B13R1.8702.0962.2392.3012.35234,1401.65524.5B14R1.8952.1102.2392.2982.35227,0211.74730.2B15E1.9302.1202.2422.2992.35127,5131.82030.6R = reference,E = according to the invention

[0063] Surprisingly, the magnetic properties measured on samples B2 and B5 according to the invention, which have a very short dwell time at temperature T0, are similarly good to those of reference sample A1. In particular, their induction values B(H) are at a very high level. As a result, it is possible to both shorten the cooling phase and significantly reduce the dwell stage while maintaining sufficiently good magnetic quality.

[0064] Despite a very short annealing time, sample B8, which has a very short dwell time at 800° C. and rapid cooling from 500° C., also exhibits similarly good magnetic properties to sample B7, which was annealed with a 4 h dwell time at 780° C. and then cooled slowly.

[0065] For sample B15, which was made from an alloy with a low V content of 1.1 wt. % and heat-treated at a high annealing temperature of 920° C., the very short annealing time of 0.5 h in combination with rapid cooling from 500° C. also results in a very high induction value B(100 A / m) of 1.930 T. This is even higher than the value for the reference sample B13, which has a long dwell time of 6 h, and of sample 14, which has a short annealing time but slow cooling.

[0066] FIG. 5 shows the total annealing time for examples B1 and B2 according to the invention as compared to the reference annealing sample A1, which has a longer dwell stage and slow cooling throughout. Examples B1 and B2 have a shorter total annealing time than the reference annealing sample A1. Here, total annealing time tg is defined as the period from the start of annealing to the point during cooling at which a temperature of 200° C. is achieved. In particular, the annealing carried out on B2 results in a total annealing time of only 4.2 h. In contrast, this time for the reference annealing sample A1 is over 20 h, i.e. substantially longer. Using this method it is possible either to anneal the same number of parts in a shorter time or to run the same furnace more often and so increase the number of parts annealed in the same length of time.

[0067] FIG. 5 also shows that the first cooling phase for the reference annealing sample A1 is quicker than the second cooling phase, i.e. the speed of cooling slows down as annealing progresses. In contrast, in examples B2 according to the invention the cooling speed CR2 of the later, second cooling phase is higher than the cooling speed CR1 of the first cooling phase.

[0068] A further advantage of samples B1 and B2 is revealed by a comparison of the mechanical properties set out in Table 7. More rapid cooling causes a slight increase in the yield strength Rp0.2 of the material, i.e. from 233 MPa for the reference annealed sample A1 to 256 MPa in the case of annealed sample B1 with two-stage cooling. The reduction in the dwell time leads to a further improvement in mechanics, with a yield strength Rp0.2 of 275 MPa being achieved for annealing sample B2. This sample also very clearly shows an improvement in elongation at break to 11.2% and increased hardness.TABLE 7Rp0.2RmASampleR / Ein MPain MPain %HVA1R2335887.6195B1R2596597.8206B2E27581711.2217B3R2465757.3198B4R2375226.4193B5E2776568.1203B6R535111313.96267B7R37889011.1208B8E35191012.26214B9R3227548.8200B12R3297148.6227R = reference,E = according to the invention

[0069] Samples B3 to B5 supplement the existing examples with a T1 temperature (start of faster cooling) of 625° C.

[0070] In samples B6 to B9 dwell temperatures vary from 750° C. to 820° C., and dwell times vary from 0.3 to 4 hours. At 750° C. (sample B6) the short dwell time is insufficient to recrystallise the structure, resulting in very poor magnetics and an incompletely recrystallised structure. Examples B7 and B9 are reference examples with a longer dwell time of 4 h at 780° C. and 820° C. respectively, while B8 is the example according to the invention that marks the lower temperature (800° C.) for short-time annealing, the dwell time being 0.75 h.

[0071] The samples in examples B10 and B11 are produced from an alloy with no Nb additions. The compositions of the alloys are presented as percentages by weight in Table 9.

[0072] Due to the absence of this grain-refining addition, it is impossible at very high annealing temperatures to avoid the formation of large grains even with short annealing times (0.5 h 880° C.). In these cases, either the dwell time must be reduced still further or the annealing temperature lowered still further. An alloy with a Nb addition is advantageous for the method according to the invention.

[0073] Example B12 shows the effect of a high Nb content (0.2 wt. %). It results in a fine-grain structure despite a longer dwell time (4 h 820° C.), though the magnetic characteristics are nevertheless insufficient (e.g. B3=0.793 T).

[0074] Examples B13 to B15 involved an alloy containing 0.1 wt. % Nb but a lower V content (1.1 wt. %). Due to the higher position of the phase transition α→α+γ annealing at 920° C. is possible. At such a high temperature it is possible to create excellent soft-magnetic properties even with short-time annealing (0.5 h). This is not, however, sufficient to meet the addition criterion of the fine-grained structure without coarse grains. At temperatures above 900° C., the dwell time should therefore be below 0.5 h dependent on the Nb content, preferably max. 15 minutes.

[0075] FIG. 6 shows micrographs of the samples, and Table 8 presents an overview of the grain sizes measured. The structure of sample B2, which has a mean grain size of 27 μm (KW 7.5) due to the shortened dwell time, is only slightly more fine-grained than the reference example A1, which has a mean grain size of 45 to 64 μm (KW 6.0 to 5.0). It was therefore possible, despite the reduced dwell time, to achieve a fully recrystallised structure with good grain growth. Another positive is the fact that the structure is substantially more homogenous than in the reference example; in particular, fewer secondary crystallised grains are present.TABLE 8MaximumMean grainGrain sizegrain sizesize d(avg)STABWnumberd(max)SampleR / Ein μmin μmKW(avg)in μmhomogenous?A1R35255.8120NoB1R27217.1121NoB2E2012868YesB3R25197.3151NoB4R25187.4157NoB5E17108.573YesB8E104, 41031.4YesB10R107—3.5>200NoB11R45—6>200NoB13R36266.3152NoB14R35246.3139NoB15E41245.8132NoR = reference,E = according to the inventionTABLE 9Com-CoVNbpositionFewt. %wt. %wt. %1Remainder491.90.12Remainder491.903Remainder491.90.24Remainder491.10.1In summary, test series A (A1 to A6), which was annealed using a stationary process, shows that very rapid cooling results in good soft-magnetic values only if it does not start until a given temperature T1 below the dwell temperature T0. This results in two-stage cooling with a first phase at a lower cooling rate and a second phase at a higher cooling rate. This method can be used to achieve very good soft-magnetic properties in annealed samples.

[0077] Surprisingly, test series B(B1 to B15) shows that dwell time can be reduced significantly with no substantial deterioration in soft-magnetic properties. This was combined with cooling quicker than that described in the prior art, which on one hand has a positive effect on, i.e. further shortens, total annealing time, and secondly also leads to a slight improvement in mechanical properties.

Examples

example b12

[0073]Example B12 shows the effect of a high Nb content (0.2 wt. %). It results in a fine-grain structure despite a longer dwell time (4 h 820° C.), though the magnetic characteristics are nevertheless insufficient (e.g. B3=0.793 T).

[0074]Examples B13 to B15 involved an alloy containing 0.1 wt. % Nb but a lower V content (1.1 wt. %). Due to the higher position of the phase transition α→α+γ annealing at 920° C. is possible. At such a high temperature it is possible to create excellent soft-magnetic properties even with short-time annealing (0.5 h). This is not, however, sufficient to meet the addition criterion of the fine-grained structure without coarse grains. At temperatures above 900° C., the dwell time should therefore be below 0.5 h dependent on the Nb content, preferably max. 15 minutes.

[0075]FIG. 6 shows micrographs of the samples, and Table 8 presents an overview of the grain sizes measured. The structure of sample B2, which has a mean grain size of 27 μm (KW 7.5) due to th...

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This US Non-Provisional patent application relies on and claims priority to German Patent Application No. 102025107750.9, filed on Feb. 28, 2025, the entire contents of which are incorporated herein by reference.BACKGROUND OF THE INVENTION

[0002] The present invention relates to a method for heat treating a part made of an FeCoV alloy.

[0003] As conductors of magnetic flux, soft-magnetic materials, such as FeCoV alloys in the form of bands and laminations produced from such bands, are a significant component in the construction of electric machines. In cases such as radial flux machines, for example, laminations are stacked one on top of another and attached to form a laminated stator core. In the case of rotors, too, soft-magnetic laminations are also often stacked and attached to form a laminated rotor core.

[0004] Some applications, in the aviation and motor racing industries, for example, use soft-magnetic materials made of cobalt-iron (CoFe) that exhibit increased saturation polarisation. An example of this class of alloys are those known as V-permendur, which comprise approx. 49 wt. % Fe, 49 wt. % Co and 2 wt. % V. Since their soft-magnetic properties are substantially determined by their microstructures, these alloys are heat-treated to produce a suitable structure through recrystallisation and grain growth. It is by means of such heat treatment that high maximum permeability μmax, low coercive field strength He and high inductions B(H) are achieved at low field strengths H.

[0005] In the case of CoFe laminations with a Co content of 30 to 55 wt. % it is usual to carry out heat treatment after forming, i.e. to heat-treat the formed part. The heat treatment of the part, often referred to as “final annealing”, serves to recrystallise the structure and then promote grain growth, and also results in a reduction in residual stress in the material. In order to achieve good soft-magnetic properties in a stator for an electric machine, for example, the resulting structure should be fully recrystallised and exhibit a grain size sufficient to obtain a low Hc and a high μmax. By contrast, the fine-grained structure produced for rotor applications exhibits higher strengths, in particular a high yield strength Rp0.2 due to the many grain boundaries, but also significantly poorer soft-magnetic properties.

[0006] The standard heat treatment for parts made of CoFe and designed to obtain optimum magnetic properties for use as stator laminations, for example, involves stationary annealing in dry hydrogen with a dwell stage in the region of 800 to 940° C. The maximum temperature is dependent on composition. In the case of V-containing compositions it should not exceed the phase transition temperature T(α→α+γ) if precipitations are to be avoided. Typical dwell times are in the region of 4 to 10 hours and are followed by slow cooling to at least 200° C. at cooling rates in the region of 100 to 300° C. / h.SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide a method for the heat treatment of a part made of an FeCoV alloy in order to achieve homogenous soft-magnetic properties.

[0008] According to the invention, a method for the heat treatment of at least one part made of an FeCoV alloy comprises the following. There is provided at least one part made of an FeCoV alloy consisting substantially 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. % of impurities and the remainder Fe. The at least one part is heat-treated at a temperature T0 for a period th, where th≤1 h, and then cooled at a first cooling rate CR1 to a temperature T1, where T1<T0, and then at a second cooling rate CR2 from temperature T1 to a temperature T2, where T2<T1, 800° C.≤T0≤T(α→α+γ)+20° C., 400° C.≤T1≤700° C., 100° C.≤T2≤400° C., 50 K / h≤CR1≤300 K / h and CR2>300 K / h and CR2>CR1.

[0009] The total duration of this heat treatment is short since the dwell time th at the highest temperature T0 is less than one hour and the cooling rate CR2 from T1 to T2 is faster than the cooling rate CR1 from T0 to T1. This heat treatment also produces a homogenous structure.

[0010] It has been established that, in addition to mean grain size, the percentage of large grains also has an impact on soft-magnetic properties. It has also been established that variations in measured soft-magnetic properties are connected to the percentage of large grains.

[0011] The homogeneity of the magnetic properties of laminations in rotating electric machines is important in ensuring uniform magnetisation. A homogenous structure with no or few above-average sized grains is advantageous in ensuring uniform magnetisation. This impact is felt even more strongly in thin laminations, e.g. those with a maximum thickness of 0.15 mm. If the diameter of individual large grains approaches the thickness of the band, the preferred direction of this one crystal dominates the magnetic properties in cross section.

[0012] As a result, a structure in which grain size is as uniform as possible and, in addition, contains no very large grains is particularly advantageous for rotating electric machines. This finding is at odds with the standard process which involves performing magnetic final annealing at the highest possible temperature for as long as possible in order to optimise magnetic values such as coercive field strength and maximum permeability. High annealing temperatures result in increased grain growth and, in the CoFe alloys referred to here, in secondary recrystallisation.

[0013] The method according to the invention permits the obtention of sufficiently good magnetic properties while at the same time ensuring improved homogeneity of structure. Moreover, the total duration of the heat treatment is advantageously shorter, thereby enabling more parts to be produced in the same time or the same number of parts to be produced in a shorter time.

[0014] In an embodiment, the at least one part undergoes stationary heat treatment in a furnace.

[0015] The cooling and so the cooling rate of the parts in a stationary furnace may be set either by externally cooling the annealing area by means of fans with a flow of gas or air being directed towards the outside of the annealing area, or actively by cooling the process gas in a heat exchanger.

[0016] In an embodiment, the first cooling rate CR1 is achieved by externally cooling the annealing area by means of fans, and the second cooling rate CR2 to below 500° C. is achieved by means of active gas cooling of the process gas using a heat exchanger. The furnace may be a retort furnace.

[0017] In an embodiment, the heat treatment has a total duration to, where to is measured from room temperature via T0, T1 and T2 to a final temperature, this final temperature being 200° C. and to being less than 12 hours, preferably less than 10 hours, preferably less than 6 hours, preferably less than 4 hours.

[0018] In an embodiment, 50 K / h≤CR1≤250 K / h and CR2>300 K / h, preferably 50 K / h≤CR1≤200 K / h and CR2>300 K / h.

[0019] The difference between cooling rates CR2 and CR1 can be defined further. In an embodiment, 50 K / h≤|CR2−CR1|≤3000 K / h, preferably 150 K / h≤|CR2−CR1|≤3000 K / h, preferably 250 K / h≤|CR2−CR1|≤2800 K / h, preferably 250 K / h≤|CR2−CR1|≤2500 K / h.

[0020] In an embodiment, period th starts at point ti and ends at a point to, while the cooling of the part from T0 to T1 starts at point to and ends at a point t1, CR1 being the mean cooling rate within the temperature range (T1-T0), and 50K / h≤|(T1-T0) / (t1-t0)|≤200 K / h.

[0021] In an embodiment, the cooling of the part from T1 to T2 starts at point t1 and ends at a point t2, where CR2 is the mean cooling rate within the temperature range (T2-T1), and 300 K / h<|(T2-T1) / (t2-t1)|≤3000 K / h, preferably 300 K / h<|(T2-T1) / (t2-t1)|≤500 K / h.

[0022] In further embodiments, the dwell time th is defined more precisely and may be 10 seconds≤th≤45 minutes, 10 seconds≤th≤30 minutes, preferably 10 seconds≤th≤15 minutes.

[0023] In further embodiments, the temperature T0 is further defined and may be 820° C.≤T0, preferably 850° C.≤T0, preferably 880° C.≤T0 and / or T0≤940° C., preferably T0≤920° C., preferably T0≤900° C. In further embodiments, the temperature T1 is further defined and may be 500° C.≤T1≤700° C., preferably 550° C.≤T1≤700° C., preferably 600° C.≤T1≤700° C.

[0024] In further embodiments, first the FeCoV alloy is provided in the form of a band with a cold-rolled texture. The part is separated from, e.g. punched, cut or laser cut out of, this band. The part may have an outer contour that corresponds to the desired outer contour of a stator or a part of a stator such as a tooth or a ring, for example.

[0025] A plurality of these separated parts may be stacked to form a laminated core. In an embodiment, in-die stacking is used in order to produce a laminated core. The laminated core may correspond to the outer contour of a stator tooth, of a laminated core having the outer contour of a stator ring or of a laminated core having the outer contour of a stator segment with at least two stator teeth. In some embodiments the laminated core is heat-treated.

[0026] The part heat-treated using the method may take the form of a lamination, of a sheet, of a laminated core, of a laminated core having the outer contour of a stator tooth, of a laminated core with the outer contour of a stator ring or of a laminated core with the outer contour of a stator segment having at least two stator teeth.

[0027] The method is particularly suited to the production of soft-magnetic stators or parts of soft-magnetic stators.

[0028] In some embodiments, following the heat treatment the part has a crystalline structure with a mean grain size dk, where 11 μm≤dk<30 μm, and a maximum of 10% of grains with a grain size of greater than 75 μm. A homogenous structure of this type ensures laterally uniform magnetics. The parts also have a fully recrystallised homogenous structure that permits the desired very good soft-magnetic properties required for use in stators for electric machines, for example.

[0029] In an embodiment, following the heat treatment the part has a yield strength of 250 MPa<Rp0.2≤400 MPa, a maximum permeability μmax of min. 10,000 and a remanence Br of min. 1.0 T.

[0030] In further embodiments, following the heat treatment the crystalline structure contains a maximum of 5% of grains with a grain size of greater than 75 μm, preferably a maximum of 2.5% of grains with a grain size of greater than 75 μm, preferably a maximum of 1% of grains with a grain size of greater than 75 μm.

[0031] In further embodiments, following the heat treatment the crystalline structure contains a maximum of 30% of grains, preferably a maximum of 25% of grains, preferably a maximum of 15% of grains, preferably a maximum of 10% of grains with a grain size of greater than 38 μm.

[0032] In further embodiments, following the heat treatment 11 μm≤dk<26 μm with a standard deviation of less than 20 μm, preferably 16 μm≤dk<26 μm with a standard deviation of less than 20 μm.

[0033] In further embodiments, after the 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.

[0034] The composition of the FeCoV alloy may be defined more closely. In an embodiment, 0.01 wt. %≤Nb≤0.25 wt. %. In another embodiment 0.05 wt. %≤Nb≤0.15 wt. %.

[0035] Exemplary embodiments are explained in greater detail below with reference to the drawings and examples.BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows by way of example temperature-time curves for two samples A2, A6 according to the invention that were annealed in a stationary tube furnace followed by rapid cooling, and for a comparison annealing sample A1.

[0037] FIG. 2 shows a semilogarithmic representation of the static initial curves B(H) of annealed samples A1 to A6.

[0038] FIG. 3 shows a semilogarithmic representation of the static permeability μ(H) of annealed samples A1 to A6.

[0039] FIG. 4 shows micrographs of sample A6 according to the invention and of comparison sample A1.

[0040] FIG. 5 shows temperature-time curves for the two samples B1 and B2 according to the invention that were annealed in a stationary bell-type furnace with active cooling using heat exchangers, and for comparison annealing sample A1.

[0041] FIG. 6 shows micrographs for the samples B1 to B5 according to the invention and for the comparison sample A1.DESCRIPTION OF EMBODIMENT(S) OF THE INVENTION

[0042] In a first series of tests A, samples were subjected to stationary annealing with very rapid cooling.

[0043] For test series A, annular test samples of external diameter 38.1 mm and internal diameter 31.75 mm were produced from a 0.2 mm-thick band of the alloy VACODUR 49, which has the composition Fe 49 wt. % Co 1.9% w / v and 0.1 wt. % Nb. In each test, 10 to 15 of these rings were subjected to final annealing in a dry hydrogenous atmosphere in a stationary tube furnace. All samples were first heated to a temperature T0 of 880° C. and maintained at this temperature for a period th.

[0044] FIG. 1 shows by way of example temperature-time curves for two annealing samples A2 and A6 that were annealed in a stationary tube furnace with rapid cooling and for a comparison annealing sample A1. T denotes the temperature of the furnace in ° C.; t denotes time after the start of annealing in hours. Also indicated by way of example are the points ti at which temperature T0 is achieved, to at which the first cooling phase starts, t1 at which temperature T1 is achieved and the second cooling phase starts, and t2 at which temperature T2 is achieved. Sample A1 represents a conventional heat treatment process and was cooled slowly to a temperature of approx. 200° C. A1 constitutes a reference or comparison sample. Sample A2 is heat-treated with rapid cooling from dwell time T0, and sample A6 is heat-treated with two-stage cooling, the cooling rate from temperature T1 being greater than the cooling rate from T0 to T1.

[0045] Cooling was carried out by moving the sample boat into a water-cooled cooling zone. By contrast, the four samples A3, A4, A5, A6 were subjected to two-stage cooling, i.e. they were first cooled slowly at cooling rate CR1 from T0 to temperatures T1 of 700° C., 600° C., 550° C. and 500° C. respectively and then subjected to faster cooling at cooling rate CR2, which was maintained at least until a temperature T2 of 200° C. to 300° C. was reached. Cooling was carried out by moving the sample boat into a water-cooled cooling zone.

[0046] Table 1 shows the parameters of the heat treatments carried out on test series A. A single cooling rate CR1 is given for annealing process A1 since no rapid cooling took place, i.e. T2=T1. This cooling rate CR1 averages out at just 51° C. / h for the entire range to 200° C.TABLE 1T0thCR1T1CR2T2|T1 − T2||CR2 − CR1|SampleR / Ein ° C.in hin ° C. / hin ° C.in ° C. / hin ° C.in ° C.in ° C. / hA1R8805.951200—2000—A2R8805.9—8802235300580—A3R8805.914470025413004002397A4R8805.911860016043003001486A5R8805.110355023162403102213A6R8805.995500747250250652A7R8800.594500213007373A8E8800.593500138630012931293R = reference,E = according to the invention

[0047] Only one cooling rate CR2 is given for annealing sample A2 with rapid cooling from T0 since no slow cooling takes place, i.e. T1=T0. A mean cooling rate CR2 of 2235° C. / h is achieved over the temperature range from 880° C. to 300° C.

[0048] The four samples A3, A4, A5 and A6 were also heated rapidly and maintained at 880° C. for 6 hours. The cooling process was carried out in two stages. The first slow-cooling stage took place at temperatures T1 of 700° C., 600° C., 550° C. and 500° C. respectively. This slow cooling phase was carried out at cooling rates CR1 from 95° C. / h for sample A6 to 144° C. / h for sample A3. From temperature T1 the annular samples were then subjected to very rapid cooling to at least T2, where T2 was between 300° C. and 200° C. This resulted in very high cooling rates CR2 with values of between 747° C. / h for sample A6 and 2541° C. / h for sample A3.

[0049] Tables 2 and 3 provide measured magnetic properties: induction B, maximum permeability μmax, remanence Br and coercive field strength Hc. FIGS. 2 and 3 show static initial curves for B(H) and μ(H).TABLE 2B(100 A / m)B(160 A / m)B(300 A / m)B(800 A / m)B(1, 6 kA / m)B(4 kA / m)B(8 kA / m)Samplein Tin Tin Tin Tin Tin Tin TA11.6631.8292.0112.2022.2562.2862.299A21.0011.5921.8622.0632.1672.2422.259A30.9291.5161.8332.0482.1582.2432.266A40.9451.5631.8562.0662.1742.2482.270A51.5571.7651.9412.1212.2052.2592.275A61.8111.9392.0672.1942.2132.2712.286A71.6941.8512.0132.1852.2372.2682.283A81.7731.9192.0472.1792.2322.2662.283TABLE 3BrHcSampleμmaxin Tin A / mA118,2731.5440.6A28,6450.6446.5A37,9130.6554.1A48,1820.6753.9A513,9401.2936.4A621,6151.6336.3A718,3421.6347.5A819,2851.5739.1The reference annealing process A1 with slow cooling corresponds to a standard annealing process for CoFe alloys. It results in very high inductions B(H) even at low field strengths H, e.g. an induction B(100 A / m) of 1.663 T. Accordingly, the maximum permeability μmax is 18,273. Induction at very high field strengths, e.g. 8 kA / m, is relatively independent of annealing and is determined primarily by composition. With the composition under investigation here of Fe 49 wt. % Co 1.9% V 0.1% Nb the induction B(8 kA / m) is 2.299 T. Annealing process A2 with very rapid cooling from the dwell temperature produces significantly poorer characteristic values, i.e. B(100 A / m) of 1.001 T and μmax of 8,645 T. Similar magnetic values are obtained for both annealing processes A3 and A4 in which very rapid cooling was started at 700° C. and 600° C. respectively.

[0051] The non-optimum soft-magnetic properties of samples A1 to A4 may be due to internal stresses occurring during the very rapid cooling process. Stresses of this type are particularly relevant in Fe—Co systems with high Co contents of between 30 and 55 wt. % (weight percent) since the magnetostriction of these alloys may be as high as 70 ppm in some cases. As a result, even small internal stresses in the material may lead to clear stress anisotropy, which in turn impairs soft-magnetic properties.

[0052] By contrast, annealing processes A5 and A6 according to the invention, which are not subjected to very rapid cooling until 550° C. and 500° C. respectively, exhibit magnetic values that are similar to or even better than those of the reference annealing process A1. This is shown, for example, in the form of the very high induction B(300 A / m) of 2.067 T for sample A6 or of the very low coercive field strength Hc of approx. 36 A / m for samples A5 and A6. The two-stage cooling is therefore successful in both obtaining a magnetically advantageous material state and significantly reducing annealing time. Since total annealing time in stationary annealing is very heavily influenced by the slow cooling process, this represents an important commercial advantage.

[0053] Samples A7 and A8 were annealed with a short dwell time (0.5 h). However, sample A7 was cooled slowly (furnace cooled), while sample A8 was also subjected to rapid cooling at 500° C. The magnetics of this sample were improved by the rapid cooling (e.g. as also for sample B1 in the second series of tests B).

[0054] FIG. 4 shows the micrographs for the samples in test series A. The grain sizes measured are presented in Table 4. In all cases (A1 to A6), there is a fully recrystallised ferritic structure with mean grain sizes of between 32 μm and 76 μm, corresponding to a grain size number as per ASTM E112 of 7.0 to 4.5. This represents a typical structure of the type used for stators, for example, since the large grains result in a reduction in He and an increase in μmax.TABLE 4MeanMax.grainGraingrainsizesizesized(avg)STABWnumberd(max)PercentagePercentageSamplein μmin μmKW(avg)in μmd >= 38 μmd >= 75 μmhomogenous?A135255, 812037%8.6%NoA233236, 512628%6.8%NoA330216, 811526%3.5%NoA442305, 914941%12.8%NoA53321714229%5.4%NoA634236, 512235%5.5%NoA720147, 98311%0.7%YesA820138839.3% 0.0%Yes

[0055] In the second test series B, the dwell time th at T0 and the total duration to of heat treatment were reduced and, in addition, a shortened dwell time th at T0 was examined in conjunction with two-stage cooling in a stationary furnace.

[0056] In test series B1 to B9 parts made of the CoFe alloy VACODUR 49 together with annular samples for magnetic characterisation were annealed in a stationary furnace with active cooling. Examples B10 and B11 were produced using an alloy with no Nb additions. Example B12 has a higher Nb content (0.2 wt. %). Examples B13 to B15 were produced using an alloy containing 0.1 wt. % Nb but a lower V content (1.1 wt. %). These compositions are detailed in Table 9.

[0057] Here, cooling was carried out by a fan in order to effectively cool the outside of the annealing chamber in conjunction with a heat exchanger for active cooling of the process gas. With this technology it is possible to set very controlled cooling rates over the entire annealing process on an industrial scale.

[0058] FIG. 5 shows temperature-time curves for samples B1 and B2 in a stationary furnace with fans and heat exchangers, and a comparison sample A1. Table 5 shows the parameters for the heat treatments used on test series B. Sample B2 according to the invention has a dwell time at TO of less than 1 hour and a second cooling rate CR2 of greater than 300K / h.TABLE 5T0thCR1T1CR2T2|CR2 − CR1 |SampleR / EAdd.in ° C.in hin ° C. / hin ° C.in ° C. / hin ° C.in ° C. / hA1R18805.9735003320040B1R18805.9156500363200207B2E18800.2190500338200148B3R18806.0149625402300253B4R18806.01526251473005B5E18800.5149625439300290B6R17500.3149500393300244B7R178041425007820064B8E18000.7567500420300353B9R182041495007820071B10R2880101425007820064B11R28800.51425007820064B12R384041505008020070B13R492061505001502000B14R49200.51515001502001B15E49200.5151500328200177R = reference,E = according to the invention

[0059] For the comparison annealing process B1, following a dwell stage at 880° C. the parts were cooled in a controlled manner at a moderate cooling rate of approx. 150° C. / h to a temperature of 500° C. in a first cooling phase. From 500° C. the cooling rate was then increased significantly to 363° C. / h in a second cooling phase. The magnetics achieved in this manner are comparable to the values for reference annealing process A1.

[0060] Surprisingly, however, it was established that total annealing time can be substantially reduced since cooling in the 500° C. to 200° C. range lasts only approx. 1.4 hours as opposed to taking around 13 h in the case of reference annealing process A1.

[0061] In a sample B2 according to the invention the annealing dwell time was significantly reduced while maintaining two-stage cooling characteristics, i.e. a very short dwell time of 0.2 h was selected in place of a dwell time th of 6 h as in the reference example. The purpose of the dwell stage is to ensure sufficient grain growth and thus achieve good soft-magnetic properties. It can therefore be expected for a short dwell time to result in a deterioration in soft-magnetic properties. The generally recommended dwell times for this alloy class for the annealing of stator laminations is in the region of 4 h to 10 h, meaning that the heat treatment carried out on sample B2 represents a significant reduction in dwell time.

[0062] Table 6 shows the magnetic properties B(induction), μmax (maximum permeability), Br (remanence) and He (coercive field strength) of samples B1 to B15. All induction values were fully determined for all samples. Non-measured points are indicated by a dash.TABLE 6B(100B(300B(800B(1, 6B(8HcA / m)A / m)A / m)kA / m)kA / m)BrinSampleR / Ein Tin Tin Tin Tin Tμmaxin TA / mA1R1.6632.0112.2022.2562.29918,2731.5440.6B1R1.7872.0522.1852.2372.28520,7381.6237.0B2E1.7762.0472.1792.2352.28518,0271.6242.3B3R1.7672.0462.1822.2382.29117,5821.5138.1B4R1.7482.0442.1872.2412.29119,7241.5436.5B5E1.7302.0352.1742.2332.28915,6281.5345.5B6R0.1771.4401.9882.1252.2673,8231.675223.0B7R—2.0162.1682.2322.28213,8061.69066.7B8E1.5791.9912.1692.2332.29113,5441.65070.1B9R—2.0452.1862.2442.29315,7081.67052.0B10R1.8262.0872.2242.271—26,0601.66527.7B11R—2.0502.2002.260—17,183—41.0B12R—0.7932.1032.221———79.3B13R1.8702.0962.2392.3012.35234,1401.65524.5B14R1.8952.1102.2392.2982.35227,0211.74730.2B15E1.9302.1202.2422.2992.35127,5131.82030.6R = reference,E = according to the invention

[0063] Surprisingly, the magnetic properties measured on samples B2 and B5 according to the invention, which have a very short dwell time at temperature T0, are similarly good to those of reference sample A1. In particular, their induction values B(H) are at a very high level. As a result, it is possible to both shorten the cooling phase and significantly reduce the dwell stage while maintaining sufficiently good magnetic quality.

[0064] Despite a very short annealing time, sample B8, which has a very short dwell time at 800° C. and rapid cooling from 500° C., also exhibits similarly good magnetic properties to sample B7, which was annealed with a 4 h dwell time at 780° C. and then cooled slowly.

[0065] For sample B15, which was made from an alloy with a low V content of 1.1 wt. % and heat-treated at a high annealing temperature of 920° C., the very short annealing time of 0.5 h in combination with rapid cooling from 500° C. also results in a very high induction value B(100 A / m) of 1.930 T. This is even higher than the value for the reference sample B13, which has a long dwell time of 6 h, and of sample 14, which has a short annealing time but slow cooling.

[0066] FIG. 5 shows the total annealing time for examples B1 and B2 according to the invention as compared to the reference annealing sample A1, which has a longer dwell stage and slow cooling throughout. Examples B1 and B2 have a shorter total annealing time than the reference annealing sample A1. Here, total annealing time tg is defined as the period from the start of annealing to the point during cooling at which a temperature of 200° C. is achieved. In particular, the annealing carried out on B2 results in a total annealing time of only 4.2 h. In contrast, this time for the reference annealing sample A1 is over 20 h, i.e. substantially longer. Using this method it is possible either to anneal the same number of parts in a shorter time or to run the same furnace more often and so increase the number of parts annealed in the same length of time.

[0067] FIG. 5 also shows that the first cooling phase for the reference annealing sample A1 is quicker than the second cooling phase, i.e. the speed of cooling slows down as annealing progresses. In contrast, in examples B2 according to the invention the cooling speed CR2 of the later, second cooling phase is higher than the cooling speed CR1 of the first cooling phase.

[0068] A further advantage of samples B1 and B2 is revealed by a comparison of the mechanical properties set out in Table 7. More rapid cooling causes a slight increase in the yield strength Rp0.2 of the material, i.e. from 233 MPa for the reference annealed sample A1 to 256 MPa in the case of annealed sample B1 with two-stage cooling. The reduction in the dwell time leads to a further improvement in mechanics, with a yield strength Rp0.2 of 275 MPa being achieved for annealing sample B2. This sample also very clearly shows an improvement in elongation at break to 11.2% and increased hardness.TABLE 7Rp0.2RmASampleR / Ein MPain MPain %HVA1R2335887.6195B1R2596597.8206B2E27581711.2217B3R2465757.3198B4R2375226.4193B5E2776568.1203B6R535111313.96267B7R37889011.1208B8E35191012.26214B9R3227548.8200B12R3297148.6227R = reference,E = according to the invention

[0069] Samples B3 to B5 supplement the existing examples with a T1 temperature (start of faster cooling) of 625° C.

[0070] In samples B6 to B9 dwell temperatures vary from 750° C. to 820° C., and dwell times vary from 0.3 to 4 hours. At 750° C. (sample B6) the short dwell time is insufficient to recrystallise the structure, resulting in very poor magnetics and an incompletely recrystallised structure. Examples B7 and B9 are reference examples with a longer dwell time of 4 h at 780° C. and 820° C. respectively, while B8 is the example according to the invention that marks the lower temperature (800° C.) for short-time annealing, the dwell time being 0.75 h.

[0071] The samples in examples B10 and B11 are produced from an alloy with no Nb additions. The compositions of the alloys are presented as percentages by weight in Table 9.

[0072] Due to the absence of this grain-refining addition, it is impossible at very high annealing temperatures to avoid the formation of large grains even with short annealing times (0.5 h 880° C.). In these cases, either the dwell time must be reduced still further or the annealing temperature lowered still further. An alloy with a Nb addition is advantageous for the method according to the invention.

[0073] Example B12 shows the effect of a high Nb content (0.2 wt. %). It results in a fine-grain structure despite a longer dwell time (4 h 820° C.), though the magnetic characteristics are nevertheless insufficient (e.g. B3=0.793 T).

[0074] Examples B13 to B15 involved an alloy containing 0.1 wt. % Nb but a lower V content (1.1 wt. %). Due to the higher position of the phase transition α→α+γ annealing at 920° C. is possible. At such a high temperature it is possible to create excellent soft-magnetic properties even with short-time annealing (0.5 h). This is not, however, sufficient to meet the addition criterion of the fine-grained structure without coarse grains. At temperatures above 900° C., the dwell time should therefore be below 0.5 h dependent on the Nb content, preferably max. 15 minutes.

[0075] FIG. 6 shows micrographs of the samples, and Table 8 presents an overview of the grain sizes measured. The structure of sample B2, which has a mean grain size of 27 μm (KW 7.5) due to the shortened dwell time, is only slightly more fine-grained than the reference example A1, which has a mean grain size of 45 to 64 μm (KW 6.0 to 5.0). It was therefore possible, despite the reduced dwell time, to achieve a fully recrystallised structure with good grain growth. Another positive is the fact that the structure is substantially more homogenous than in the reference example; in particular, fewer secondary crystallised grains are present.TABLE 8MaximumMean grainGrain sizegrain sizesize d(avg)STABWnumberd(max)SampleR / Ein μmin μmKW(avg)in μmhomogenous?A1R35255.8120NoB1R27217.1121NoB2E2012868YesB3R25197.3151NoB4R25187.4157NoB5E17108.573YesB8E104, 41031.4YesB10R107—3.5>200NoB11R45—6>200NoB13R36266.3152NoB14R35246.3139NoB15E41245.8132NoR = reference,E = according to the inventionTABLE 9Com-CoVNbpositionFewt. %wt. %wt. %1Remainder491.90.12Remainder491.903Remainder491.90.24Remainder491.10.1In summary, test series A (A1 to A6), which was annealed using a stationary process, shows that very rapid cooling results in good soft-magnetic values only if it does not start until a given temperature T1 below the dwell temperature T0. This results in two-stage cooling with a first phase at a lower cooling rate and a second phase at a higher cooling rate. This method can be used to achieve very good soft-magnetic properties in annealed samples.

[0077] Surprisingly, test series B(B1 to B15) shows that dwell time can be reduced significantly with no substantial deterioration in soft-magnetic properties. This was combined with cooling quicker than that described in the prior art, which on one hand has a positive effect on, i.e. further shortens, total annealing time, and secondly also leads to a slight improvement in mechanical properties.

Examples

example b12

[0073]Example B12 shows the effect of a high Nb content (0.2 wt. %). It results in a fine-grain structure despite a longer dwell time (4 h 820° C.), though the magnetic characteristics are nevertheless insufficient (e.g. B3=0.793 T).

[0074]Examples B13 to B15 involved an alloy containing 0.1 wt. % Nb but a lower V content (1.1 wt. %). Due to the higher position of the phase transition α→α+γ annealing at 920° C. is possible. At such a high temperature it is possible to create excellent soft-magnetic properties even with short-time annealing (0.5 h). This is not, however, sufficient to meet the addition criterion of the fine-grained structure without coarse grains. At temperatures above 900° C., the dwell time should therefore be below 0.5 h dependent on the Nb content, preferably max. 15 minutes.

[0075]FIG. 6 shows micrographs of the samples, and Table 8 presents an overview of the grain sizes measured. The structure of sample B2, which has a mean grain size of 27 μm (KW 7.5) due to th...

Claims

1. A method for heat treating a part made of a FeCoV alloy, the method comprising:providing at least one part made of an FeCoV alloy consisting substantially 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. % of impurities and the remainder Fe; andheat treating this at least one part at a temperature T0 for a period th, where th≤1 h, followed by the cooling of the part to a temperature T1 at a first cooling rate CR1, where T1<T0, and then the cooling of the part from temperature T1 to a temperature T2 at a second cooling rate CR2, where T2<T1, 800° C.≤T0≤T(α→α+γ)+20° C., 400° C.≤T1≤700° C., 100° C.≤T2≤400° C., 50 K / h≤CR1≤300 K / h and CR2>250 K / h and CR2>CR1.

2. A method in accordance with claim 1, wherein the at least one part undergoes a stationary heat treatment in a furnace.

3. A method in accordance with claim 1, wherein the at least one part is heat-treated in a hydrogenous atmosphere.

4. A method in accordance with claim 2, wherein the first cooling rate and / or the second cooling rate is set by means of active gas cooling.

5. A method in accordance with claim 4, wherein the first cooling rate is set by means of fans in order to produce a gas flow outside the furnace that is directed towards an exterior side of the furnace, and / or the second cooling rate is set at below 500° C. by means of heat exchangers.

6. A method according to claim 1, wherein the heat treatment has a total duration to, to being measured from room temperature through T0, T1 and T2 to a final temperature, this final temperature being 200° C., to being less than 12 hours, preferably less than 10 hours, preferably less than 6 hours, preferably less than 4 hours.

7. A method according to claim 1, wherein 50 K / h≤CR1≤250 K / h and CR2>300 K / h, preferably 50 K / h≤CR1≤200 K / h and CR2>300 K / h.

8. A method according to claim 1, wherein 50 K / h≤|CR2-CR1|≤3000 K / h, preferably 150 K / h≤|CR2-CR1|≤3000 K / h, preferably 250 K / h≤|CR2-CR1|≤2800 K / h, preferably 250 K / h≤|CR2-CR1|≤2500 K / h.

9. A method according to claim 1, wherein period th starts at a point ti and ending at a point to, cooling of the part from T0 to T1 starts at point to and ends at point t1, CR1 being the mean cooling rate within the temperature range (T1-T0) and 50 K / h<|(T1-T0) / (t1-t0)|≤200 K / h.

10. A method according to claim 1, wherein the cooling of the part from T1 to T2 starts at point ti and ending at point t2, CR2 being the mean cooling rate within the temperature range (T2-T1), and 300 K / h<|(T2-T1) / (t2-t1)|≥3000 K / h, preferably 300K / h<|(T2-T1) / (t2-t1)|≤500 K / h.

11. A method according to claim 1, where in 10 seconds≤th≤45 minutes, 10 seconds≤th≤30 minutes, preferably 10 seconds≤th≤15 minutes.

12. A method according to claim 1, wherein 820° C.≤T0, preferably 850° C.≤T0, preferably 880° C.≤T0, and / or where T0≤940° C., preferably T0≤920° C., preferably T0≤900° C., and / or 500° C.≤T1≤700° C., preferably 550° C.≤T1≤700° C., preferably 600° C.≤T1≤700° C.

13. A method according to claim 1, wherein the FeCoV alloy is initially provided in the form of a band that has a cold-rolled texture, and the part is being produced from the band.

14. A method according to claim 1, wherein the part has the form of a lamination, of a sheet or laminated core, of a laminated core having the outer contour of a stator tooth, of a laminated core having the outer contour of a stator ring or of a laminated core having the outer contour of a stator segment having at least two stator teeth.

15. A method according to claim 1, wherein following the heat treatment the part has a crystalline structure with a mean grain size dk, where 11 μm≤dk<30 μm, and a maximum of 10% of grains with a grain size greater than 75 μm.

16. A method according to claim 1, wherein following the heat treatment the part comprises:a yield strength of 250 MPa<Rp0.2≤400 MPa,a maximum permeability of μmax of min. 10,000, anda remanence Br of min. 1.0 T.

17. A method in accordance with claim 15, wherein following the heat treatment the crystalline structure comprises a maximum of 5% of grains with a grain size of greater than 75 μm, preferably a maximum of 2.5% of grains with a grain size of greater than 75 μm, preferably a maximum of 1% of grains with a grain size of greater than 75 μm.

18. A method according to claim 15, wherein following the heat treatment the crystalline structure comprises a maximum of 30% of grains, preferably a maximum of 25%, preferably a maximum of 15%, preferably a maximum of 10% with a grain size of greater than 38 μm.

19. A method according to claim 15, wherein following the heat treatment 11 μm≤dk<26 μm with a standard deviation of less than 20 μm, preferably 16 μm≤dk<26 μm with a standard deviation of less than 20 μm.

20. A method according to claim 1, wherein following the 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.

21. A method according to claim 1, wherein the FeCoV alloy comprises 0.05 wt. %≤Nb≤0.15 wt. %.

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