Heat treatment method for FeCoV alloy parts

Abstract

Provided is a heat treatment method for a component made of a FeCoV alloy. [Solution] At least one component made of a FeCoV alloy is provided, wherein the FeCoV alloy comprises, by mass%, 30 to 55% inclusive of Co, 0.5 to 2.5% inclusive of V, 0 to 1% inclusive of Ta, 0 to 1% inclusive of Nb, up to 1% of impurities as impurities, with the balance being Fe, and the component is held at a temperature T0 for a holding time t h (t h ≦1 hour), then cooled to a temperature T1 at a first cooling rate CR1 (T1<T0), and further cooled from the temperature T1 to a temperature T2 at a second cooling rate CR2 (T2<T1). In this heat treatment method, 800°C≦T0≦T (α→α+γ) +20°C, 400°C≦T1≦700°C, 100°C≦T2≦400°C, 50K / h≦CR1≦300K / h, and CR2>250K / h and CR2>CR1.

Description

[Technical Field]

[0001] The present invention relates to a method for heat-treating parts made of FeCoV alloy.

[0002] As conductors of magnetic flux, soft magnetic materials such as FeCoV alloys in the form of strips and laminates manufactured from such strips are important elements in the construction of electrical machinery. For example, in radial flux type electric motors, laminated stator cores are formed by stacking and bonding laminated plates together. In the case of rotors, laminated rotor cores are often formed by similarly stacking and bonding soft magnetic laminated plates.

[0003] For example, in some applications in the aerospace and motorsport fields, soft magnetic materials made of cobalt-iron (CoFe) with high saturation polarization are used. An example of this type of alloy is so-called V-Permendur, which contains approximately 49 mass% Fe, 49 mass% Co, and 2 mass% V. Since the soft magnetic properties of these materials are mainly determined by their microstructure, heat treatment is performed to form a suitable structure through recrystallization and grain growth. Such heat treatment results in a high maximum magnetic permeability μ max This results in a low coercivity Hc and a high magnetic flux density B(H) at a low magnetic field strength H.

[0004] In the case of CoFe laminates with a Co content of 30% to 55% by mass, heat treatment is usually performed after molding, i.e., heat treatment of the molded parts. This heat treatment (often called "final annealing") promotes recrystallization of the structure and subsequent grain growth, as well as reducing residual stress in the material. For example, to obtain good soft magnetic properties in an electromechanical stator, the resulting structure must be completely recrystallized, with low coercivity Hc and high maximum permeability μ. max It is desirable to have a grain size sufficient to obtain [the desired result]. In contrast, the fine-grained structure obtained for rotor applications has high strength, particularly high yield strength R, due to the abundance of grain boundaries. p0.2 While exhibiting [certain characteristics], its soft magnetic properties are significantly inferior.

[0005] For example, standard heat treatment for CoFe parts intended to obtain optimal magnetic properties for use as laminated sheets for stators is stationary annealing in dry hydrogen, which includes a holding step in the range of about 800°C to 940°C. The maximum temperature depends on the composition. For a composition containing V, to avoid precipitation, the transformation temperature T (α→α+γ) is preferably not exceeded. A typical holding time is about 4 hours to 10 hours, after which the material is slowly cooled to at least 200°C at a cooling rate of about 100°C / h to 300°C / h.

[0006] An object of the present invention is to provide a heat treatment method for parts made of FeCoV alloy for obtaining uniform soft magnetic properties.

[0007] According to the present invention, a heat treatment method for a part made of at least one FeCoV alloy includes the following. That is, at least one part made of a FeCoV alloy substantially consisting of Co: 30 mass% to 55 mass%, preferably 45 mass% to 50 mass%, V: 0.5 mass% to 2.5 mass%, Ta: 0 mass% to 1 mass%, Nb: 0 mass% to 1 mass%, impurities: maximum 1 mass%, and the balance Fe is prepared, and the at least one part is held at a temperature T0 for a holding time t h (where t h ≦1 hour), heat-treated, then cooled to a temperature T1 at a first cooling rate CR1 (T1<T0), and then cooled to a temperature T2 at a second cooling rate CR2 (T2<T1). Here, 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.

[0008] In the present heat treatment, the holding time t h at the maximum temperature T0 is less than 1 hour, and the cooling rate CR2 from T1 to T2 is faster than the cooling rate CR1 from T0 to T1, so the overall treatment time is short. In addition, this heat treatment provides a uniform structure.

[0009] In addition to the average grain size, the proportion of large-diameter grains has been found to affect the soft magnetic properties. Furthermore, it has been confirmed that the variability in the measured soft magnetic properties is related to the proportion of large-diameter grains.

[0010] Uniformity of the magnetic properties of laminates in rotating electrical machinery is crucial for ensuring uniform magnetization. A uniform structure with few or no crystal grains larger than average is advantageous for ensuring uniform magnetization. This effect is even more pronounced in thin laminates, for example, with a maximum thickness of 0.15 mm. When the diameter of individual large crystal grains approaches the thickness of the strip, the preferred orientation of that single crystal will dominate the magnetic properties in the cross-section.

[0011] Therefore, a microstructure with as uniform grain size as possible and free from extremely large grains is particularly advantageous for rotating electromachines. This finding contradicts conventional methods of performing final annealing at the highest possible temperature and for the longest possible time to optimize magnetic properties such as coercivity and maximum permeability. High annealing temperatures lead to increased grain growth and, in the CoFe alloys discussed herein, cause secondary recrystallization.

[0012] The method according to the present invention makes it possible to obtain sufficiently good magnetic properties while simultaneously improving the uniformity of the structure. Furthermore, since the total duration of the heat treatment is advantageously shortened, it becomes possible to manufacture more parts in the same amount of time, or to manufacture the same quantity of parts in a shorter amount of time.

[0013] In one embodiment, at least one of the components is subjected to static heat treatment in a furnace.

[0014] In a stationary furnace, the cooling of components, and consequently the cooling rate, can be controlled by directing a gas or airflow outwards from the annealing area using a fan for external cooling, or by actively cooling the process gas using a heat exchanger.

[0015] In one embodiment, the first cooling rate CR1 is achieved by generating a gas flow directed to the outside of the annealing region by a fan, and the second cooling rate CR2 is achieved by active gas cooling of the process gas using a heat exchanger at a temperature below 500°C. The furnace may be a retort furnace.

[0016] In one embodiment, the heat treatment has a total duration t G , and t G is measured from room temperature through T0, T1 and T2 to the final temperature, the final temperature is 200°C, and t G is less than 12 hours, preferably less than 10 hours, more preferably less than 6 hours, and even more preferably less than 4 hours.

[0017] In one embodiment, 50K / h≦CR1≦250K / h and CR2>300K / h, preferably 50K / h≦CR1≦200K / h and CR2>300K / h.

[0018] The difference between the cooling rates CR2 and CR1 can be further defined. In one embodiment, 50K / h≦|CR2-CR1|≦3000K / h, preferably 150K / h≦|CR2-CR1|≦3000K / h, more preferably 250K / h≦|CR2-CR1|≦2800K / h, and even more preferably 250K / h≦|CR2-CR1|≦2500K / h.

[0019] In one embodiment, the holding time t h starts at time point t i and ends at time point t0, cooling of the part from T0 to T1 starts at time point t0 and ends at time point t1, CR1 is the average cooling rate in the temperature range (T1-T0), and 50K / h≦|(T1-T0) / (t1-t0)|≦200K / h.

[0020] In one embodiment, cooling of the component from T1 to T2 starts at time point t1 and ends at time point t2, CR2 is an average cooling rate in the temperature range (T2-T1), and 300K / h<|(T2-T1) / (t2-t1)|≦3000K / h, preferably 300K / h<|(T2-T1) / (t2-t1)|≦500K / h.

[0021] In still another embodiment, the holding time t h is defined in more detail, and may be not less than 10 seconds and not more than 45 minutes, not less than 10 seconds and not more than 30 minutes, preferably not less than 10 seconds and not more than 15 minutes.

[0022] In still another embodiment, the temperature T0 is defined in more detail, and is not lower than 820°C, preferably not lower than 850°C, more preferably not lower than 880°C, and / or T0 is not higher than 940°C, preferably not higher than 920°C, more preferably not higher than 900°C. In still another embodiment, the temperature T1 is defined in more detail, and is not lower than 500°C and not higher than 700°C, preferably not lower than 550°C and not higher than 700°C, more preferably not lower than 600°C and not higher than 700°C.

[0023] In still another embodiment, first, the FeCoV alloy is provided in the form of a strip having a cold-rolled structure. A component is separated from the strip by, for example, punching, cutting or laser cutting. The component may have an outer shape corresponding to a desired outer shape of, for example, a stator or a part of a stator (e.g., a tooth or a ring).

[0024] A plurality of these separated components can be stacked to form a laminated core. In one embodiment, the laminated core is manufactured by in-die stacking. The laminated core may correspond to an outer shape of a stator tooth, an outer shape of a stator ring, or an outer shape of a stator segment having at least two stator teeth. In some embodiments, the laminated core is heat-treated.

[0025] The parts heat-treated by this method can take the form of a laminate, a sheet, a laminated core, a laminated core having the shape of stator teeth, a laminated core having the shape of a stator ring, or a laminated core having the shape of a stator segment having at least two stator teeth.

[0026] This method is particularly suitable for the manufacture of soft magnetic stators or parts thereof.

[0027] In some embodiments, after heat treatment, the component has an average grain size d k The material has a crystalline structure with grain sizes between 11 μm and 30 μm, and a maximum of 10% of the grains having a grain size exceeding 75 μm. Such a uniform structure ensures uniform magnetic properties in plane. Furthermore, the component has a completely recrystallized, uniform structure, achieving excellent soft magnetic properties required for use, for example, in stators of electromechanical devices.

[0028] In one embodiment, after heat treatment, the component has a yield strength Rp0.2 greater than 250 MPa and 400 MPa or less, and a maximum magnetic permeability μ max The magnetic field density (B) is at least 10,000, and the residual magnetic flux density (Br) is at least 1.0T.

[0029] In yet another embodiment, after heat treatment, the crystal structure contains up to 5%, preferably up to 2.5%, and more preferably up to 1%, crystal grains with a grain size exceeding 75 μm.

[0030] In yet another embodiment, after heat treatment, the crystal structure contains up to 30%, preferably up to 25%, more preferably up to 15%, and still more preferably up to 10% of crystal grains with a grain size greater than 38 μm.

[0031] In yet another embodiment, after heat treatment, the average crystal grain size d is 11 μm or more and less than 26 μm. k It has the following characteristics, and its standard deviation is less than 20 μm, preferably 16 μm or more and less than 26 μm, with a standard deviation of less than 20 μm.

[0032] In yet another embodiment, after heat treatment, the component has a magnetic flux density B800=B(800A / m) of at least 2.1T, preferably a magnetic flux density B300=B(300A / m) of at least 1.9T, and preferably a magnetic flux density B100=B(100A / m) of at least 1.2T.

[0033] The composition of the FeCoV alloy can be defined in more detail. In one embodiment, Nb is between 0.01% by mass and 0.25% by mass. In another embodiment, Nb is between 0.05% by mass and 0.15% by mass. [Brief explanation of the drawing]

[0034] Embodiments of the present invention will be described in more detail below with reference to the drawings and examples. [Figure 1] Figure 1 illustrates the temperature-time curves for two samples A2 and A6, and a comparative annealed sample A1, which were annealed and then rapidly cooled in a static tubular furnace according to the present invention. [Figure 2] Figure 2 shows the semi-logarithmic representation of the static initial curves B(H) for annealed samples A1 to A6. [Figure 3] Figure 3 shows the semi-logarithmic representation of the static permeability μ(H) of annealed samples A1 to A6. [Figure 4] Figure 4 shows micrographs of sample A6 and comparative sample A1 according to the present invention. [Figure 5] Figure 5 shows the temperature-time curves for two samples B1 and B2 and a comparative annealed sample A1 according to the present invention, which were annealed in a stationary bell furnace equipped with active cooling by a heat exchanger. [Figure 6-1] Figure 6-1 shows micrographs of samples B1 to B4 and comparative sample A1 according to the present invention. [Figure 6-2] Figure 6-2 shows a micrograph of sample B5 according to the present invention.

[0035] In the first test series A, the samples were subjected to static annealing with very rapid cooling.

[0036] In test series A, ring-shaped specimens with an outer diameter of 38.1 mm and an inner diameter of 31.75 mm were fabricated from a 0.2 mm thick strip of VACODUR49 alloy (composition: Fe 49 mass%, Co 49 mass%, V 1.9 mass%, Nb 0.1 mass%). In each test, 10 to 15 of these rings were final annealed in a static tubular furnace in a dry hydrogen atmosphere. All specimens were first heated to a temperature T0 = 880°C and held at this temperature for a holding time t h It was retained.

[0037] Figure 1 illustrates the temperature-time curves for annealed samples A2 and A6, as well as comparative annealed sample A1, which were rapidly cooled in a stationary tubular furnace. Here, T represents the furnace temperature in °C, and t represents the time (in hours) after the start of annealing. The time t at which temperature T0 is reached is also shown. i The time t0 at which the first cooling stage begins, the time t1 at which the second cooling stage begins after reaching temperature T1, and the time t2 at which temperature T2 is reached are also shown exemplarily. Sample A1 represents a conventional heat treatment process, where the sample is slowly cooled to approximately 200°C, and serves as a reference or comparative sample. Sample A2 represents a heat treatment where the sample is rapidly cooled from a holding temperature T0, and Sample A6 represents a heat treatment with a two-stage cooling process, where the cooling rate from temperature T1 is greater than the cooling rate from T0 to T1.

[0038] Cooling was performed by moving the sample boat to a water-cooled cooling zone. On the other hand, the four samples A3, A4, A5, and A6 underwent a two-stage cooling process: first, they were slowly cooled from T0 to temperatures T1 of 700°C, 600°C, 550°C, and 500°C respectively at a first cooling rate CR1, and then cooled more rapidly at a second cooling rate CR2 until the temperature T2 reached 200°C to 300°C. Cooling was performed by moving the sample boat to a water-cooled cooling zone.

[0039] Table 1 shows the parameters of the heat treatment performed in test series A. Since no rapid cooling was performed for annealing process A1 (i.e., T2=T1), only a single cooling rate CR1 is shown. In this case, the average cooling rate CR1 over the entire temperature range up to 200°C is approximately 51°C / h. [Table 1]

[0040] For annealed sample A2, which was rapidly cooled from a holding temperature T0, there is no gradual cooling (i.e., T1=T0), so only a single cooling rate CR2 is shown. In this case, the average cooling rate CR2 in the temperature range from 880°C to 300°C is 2235°C / h.

[0041] The four samples A3, A4, A5, and A6 were also rapidly heated and then held at 880°C for 6 hours. Cooling was carried out in two stages. The first slow cooling stage was performed to temperatures T1 of 700°C, 600°C, 550°C, and 500°C, respectively. This slow cooling stage was carried out at a cooling rate CR1 from 95°C / h for sample A6 to 144°C / h for sample A3. From temperature T1, the ring-shaped samples were cooled very rapidly to at least temperature T2 (200°C to 300°C). As a result, very high cooling rates CR2 were obtained, ranging from 747°C / h for sample A6 to 2541°C / h for sample A3.

[0042] Tables 2 and 3 show the measured magnetic properties, namely magnetic flux density B and maximum permeability μ. max The residual magnetic flux density Br and coercivity Hc are shown. Figures 2 and 3 show the static initial curves for B(H) and μ(H), respectively. [Table 2] [Table 3]

[0043] Comparative annealing process A1 with slow cooling corresponds to a standard annealing process for CoFe alloys. In this process, a very high magnetic flux density B(H) is obtained even at low magnetic field strength H, for example, B(100 A / m) is 1.663 T. Correspondingly, the maximum permeability μ max The value is 18,273. The magnetic flux density at very high magnetic field strengths, such as 8 kA / m, is relatively unaffected by annealing and is mainly determined by the composition. In this study, with the composition (Fe 49 mass%, Co 49 mass%, V 1.9 mass%, Nb 0.1 mass%), B(8 kA / m) is 2.299 T. On the other hand, in annealing process A2, where the temperature was cooled very rapidly from the holding temperature, the characteristic values ​​decreased significantly; for example, B(100A / m) was 1.001T, μ max The result is 8,645. Similar magnetic properties are obtained in annealing processes A3 and A4, where very rapid cooling is initiated from 700°C and 600°C, respectively.

[0044] The reason why the soft magnetic properties of samples A1 to A4 are not optimal may be due to internal stresses generated during the very rapid cooling process. This type of stress is particularly important in Fe-Co alloys with a Co content of 30% to 55% by mass, as magnetostriction can reach up to 70 ppm in these alloys. As a result, even slight internal stresses in the material cause significant stress anisotropy, which degrades the soft magnetic properties.

[0045] In contrast, in annealing processes A5 and A6 according to the present invention, very rapid cooling does not begin until the temperature reaches 550°C and 500°C, respectively, thus obtaining magnetic properties equivalent to or better than those of comparative annealing process A1. For example, sample A6 obtained a very high B(300A / m) = 2.067T, and samples A5 and A6 obtained a very low coercivity Hc of approximately 36A / m. Therefore, two-stage cooling makes it possible to obtain a material state with excellent magnetic properties while significantly shortening the annealing time. In static annealing, the total annealing time largely depends on the slow cooling process, so this is an important economic advantage.

[0046] Samples A7 and A8 were annealed for a short holding time (0.5 hours). However, while sample A7 was slow-cooled (furnace-cooled), sample A8 was also rapidly cooled at 500°C. This rapid cooling improved the magnetic properties of the sample (similar to that of sample B1 in the second test series B).

[0047] Figure 4 shows micrographs of the samples from test series A. The measured grain sizes are shown in Table 4. In all cases (A1-A6), a completely recrystallized ferrite structure was obtained, with an average grain size ranging from 32 μm to 76 μm, which corresponds to grain size numbers 7.0-4.5 according to ASTM E112. Such a structure is typical for applications such as stator materials, where the large grains reduce coercivity Hc and maximum permeability μ max This will improve. [Table 4]

[0048] In the second test series B, the holding time t at temperature T0 was h and the total duration of the heat treatment t G The time required was shortened, and the effect of short-term holding combined with two-stage cooling in a static furnace was further investigated.

[0049] In test series B1 to B9, components made of CoFe alloy VACODUR49 and ring-shaped samples for magnetic property evaluation were annealed in a stationary furnace equipped with an active cooling mechanism. Examples B10 and B11 were prepared using alloys that did not contain Nb, while Example B12 had a higher Nb content (0.2 mass%). Examples B13 to B15 were prepared using alloys that contained 0.1 mass% Nb but had a lower V content (1.1 mass%). These compositions are shown in Table 9.

[0050] In this test, cooling was performed using a combination of a fan to effectively cool the outside of the annealing chamber and a heat exchanger to actively cool the process gas. This technique makes it possible to set a very precisely controlled cooling rate throughout the entire annealing process on an industrial scale. Figure 5 shows the temperature-time curves of samples B1 and B2 in a stationary furnace equipped with a fan and heat exchanger, and comparative sample A1. Table 5 shows the heat treatment conditions used in test series B. Sample B2 according to the present invention has a holding time of less than 1 hour at T0, and a second cooling rate CR2 exceeding 300 K / h. [Table 5]

[0051] In comparative annealing process B1, after a holding step at 880°C, the part was controlled-cooled to 500°C at a moderate cooling rate of approximately 150°C / h in the first cooling stage. Subsequently, from 500°C, the cooling rate was significantly increased to 363°C / h in the second cooling stage. The magnetic properties obtained in this way were comparable to those of the reference annealing process A1.

[0052] Surprisingly, however, cooling in the range of 500°C to 200°C required only about 1.4 hours, compared to approximately 13 hours for the standard annealing process A1, confirming that the total duration of annealing could be significantly reduced.

[0053] In sample B2 according to the present invention, the annealing holding time is significantly reduced while maintaining the two-stage cooling characteristics, i.e., the holding time of 6 hours in the reference example is reduced to t h Instead, a very short holding time of 0.2 hours was selected. The purpose of the holding process is to ensure sufficient grain growth and thereby obtain good soft magnetic properties. Therefore, a decrease in soft magnetic properties is expected if the holding time is short. Since the holding time generally recommended for annealing stator laminates in this type of alloy is about 4 to 10 hours, the heat treatment performed on sample B2 represents a significant reduction in holding time.

[0054] Table 6 shows the magnetic properties of samples B1 to B15, namely magnetic flux density B and maximum permeability μ. max The residual magnetic flux density (Br) and coercivity (Hc) are shown. For all samples, the magnetic flux density values ​​have been fully measured. Measurement points that were not measured are indicated with a dash (―). [Table 6]

[0055] Surprisingly, the magnetic properties measured in samples B2 and B5 according to the present invention, which have a very short holding time at temperature T0, are as good as those of comparative sample A1. In particular, their magnetic flux density B(H) is at a very high level. Therefore, it is possible to shorten the cooling process and significantly shorten the holding process while maintaining sufficiently good magnetic properties.

[0056] Despite a very short annealing time, sample B8, which was held at 800°C for a short time and rapidly cooled from 500°C, also exhibits good magnetic properties, similar to sample B7, which was held at 780°C for 4 hours and then slowly cooled.

[0057] Even in sample B15, which was made from an alloy with a low V content (1.1 mass%) and heat-treated at a high annealing temperature of 920°C, a very high magnetic flux density of B(100A / m) = 1.930T was obtained by combining a very short annealing time of 0.5 hours with rapid cooling from 500°C. This value is even higher than that of comparative sample B13, which had a long holding time of 6 hours, and sample B14, which had a short holding time but slow cooling.

[0058] Figure 5 shows the total duration of Examples B1 and B2 according to the present invention compared with comparative annealed sample A1, which had a longer holding time and was slowly cooled throughout. Examples B1 and B2 have shorter total durations than comparative sample A1. Here, the total duration t GThis is defined as the period from the start of annealing until the temperature reaches 200°C during cooling. In particular, the total duration of annealing for sample B2 was only about 4.2 hours, while for comparative sample A1 it exceeded 20 hours, which is significantly longer. By using this method, it is possible to anneal more parts in the same amount of time, or to increase the number of annealed parts by operating the same furnace at a higher frequency.

[0059] Figure 5 further shows that in comparative annealed sample A1, the first cooling stage is faster than the second cooling stage, meaning the cooling rate decreases as annealing progresses. In contrast, in example B2 according to the present invention, the cooling rate CR2 in the subsequent second cooling stage is higher than the cooling rate CR1 in the first cooling stage.

[0060] Further advantages of samples B1 and B2 are evident from the comparison of their mechanical properties shown in Table 7. Faster cooling allows for increased yield strength R of the material. p0.2 The yield strength increased slightly, from 233 MPa for comparative annealed sample A1 to 256 MPa for annealed sample B1, which underwent two-stage cooling. Furthermore, the mechanical properties were further improved by shortening the holding time, and the yield strength R of annealed sample B2 increased. p0.2 The strength reaches 275 MPa. In this sample, the elongation at break improves to 11.2%, and the hardness is clearly increased. [Table 7]

[0061] Samples B3 to B5 supplement the example where the starting temperature T1 for faster cooling is 625°C.

[0062] For samples B6 to B9, the holding temperature varied in the range of 750°C to 820°C, and the holding time varied in the range of 0.3 hours to 4 hours. At 750°C (sample B6), recrystallization was insufficient with short holding times, resulting in very poor magnetic properties and incomplete recrystallization of the microstructure. Samples B7 and B9 are comparative examples held at 780°C and 820°C for 4 hours, respectively, while sample B8 is an example of the present invention, showing the lower limit temperature (800°C) in short-time annealing, with a holding time of 0.75 hours.

[0063] Samples B10 and B11 were prepared from alloys without Nb addition. Their alloy compositions are shown as mass percentages in Table 9.

[0064] Because this grain-refining element is absent, even with short annealing times (e.g., 0.5 hours at 880°C) at very high annealing temperatures, the formation of coarse grains cannot be avoided. In this case, it is necessary to further shorten the holding time or further lower the annealing temperature. In the method of the present invention, alloys with added Nb are advantageous.

[0065] Sample B12 exhibits the effect of a high Nb content (0.2 mass%). In this case, a fine-grained structure is obtained even with a relatively long holding time (4 hours, 820°C), but the magnetic properties are still insufficient (e.g., B(300A / m) = 0.793T).

[0066] In samples B13 to B15, alloys with a low V content (1.1 mass%) while containing 0.1 mass% Nb are used. Due to the high phase transformation temperature of α→α+γ, annealing at 920°C is possible. At such high temperatures, excellent soft magnetic properties can be obtained even with short annealing time (0.5 hours). However, this is not sufficient to satisfy the condition of a fine-grained structure without coarse grains. Therefore, at temperatures above 900°C, the holding time should be less than 0.5 hours, preferably a maximum of 15 minutes, depending on the Nb content.

[0067] Figure 6 shows a micrograph of the sample, and Table 8 summarizes the measured grain size. The microstructure of sample B2, which had an average grain size of 27 μm (KW 7.5) due to the shortened retention time, is only slightly finer than that of comparative example A1, which had an average grain size of 45-64 μm (KW 6.0-5.0). Therefore, sufficient recrystallization and good grain growth were achieved despite the shortened retention time. Another advantage is that the microstructure is significantly more uniform than the comparative example, particularly with fewer secondary recrystallized grains. [Table 8] [Table 9]

[0068] In summary, the test series A (A1-A6), annealed using a static process, demonstrates that very rapid cooling is only effective if it is not initiated until a predetermined temperature T1, lower than the holding temperature T0, is reached. That is, a two-stage cooling process is required, consisting of a first stage with a low cooling rate and a second stage with a high cooling rate. This method allows for the acquisition of exceptionally good soft magnetic properties in the annealed samples.

[0069] Even more surprisingly, test series B (B1-B15) demonstrates a significant reduction in holding time without substantially degrading the soft magnetic properties. This is combined with faster cooling than conventional techniques, which further shortens the overall duration and also yields a slight improvement in mechanical properties.

Claims

1. A method for heat-treating a part made of FeCoV alloy, The present invention provides to prepare at least one component made of an FeCoV alloy consisting of substantially 30 mass% ≤ Co ≤ 55 mass%, preferably 45 mass% ≤ Co ≤ 50 mass%, 0.5 mass% ≤ V ≤ 2.5 mass%, 0 mass% ≤ Ta ≤ 1 mass%, 0 mass% ≤ Nb ≤ 1 mass%, a maximum of 1 mass% of impurities, and the remainder being Fe. heat-treating the at least one component at a temperature T ０ for a holding time t ｈ (where t ｈ ≦ 1 hour), subsequently cooling the component at a first cooling rate CR1 to a temperature T １ (where T １ < T ０ ), and thereafter cooling the component at a second cooling rate CR2 from the temperature T １ to a temperature T ２ (where T ２ < T １ ), 800℃≦T ０ ≦T （α→α＋γ） +20°C, 400°C ≤ T １ ≤700℃, 100℃ ≤T ２ A method where the temperature is ≤400°C, 50K / h ≤CR1 ≤300K / h, and CR2 > 250K / h and CR2 > CR1.

2. A method according to claim 1, wherein at least one component is subjected to static heat treatment in a furnace.

3. A method according to claim 1 or claim 2, wherein at least one component is heat-treated in a hydrogen-containing atmosphere.

4. A method according to claim 2 or 3, wherein the first cooling rate and / or the second cooling rate are set by active gas cooling.

5. A method according to claim 4, wherein the first cooling rate is set by a fan located outside the furnace for generating a gas flow directed toward the outer surface of the furnace, and / or the second cooling rate is set by a heat exchanger at a temperature below 500°C.

6. A method according to any one of claims 1 to 5, wherein the heat treatment is performed for a total duration t Ｇ It has, Ｇ From room temperature to T ０ , T １ , and T ２ The temperature is measured through this process until the final temperature (which is 200°C) is reached. Ｇ A method in which the duration is less than 12 hours, preferably less than 10 hours, preferably less than 6 hours, and preferably less than 4 hours.

7. A method according to any one of claims 1 to 6, wherein 50K / h ≤ CR1 ≤ 250K / h and CR2 > 300K / h, preferably 50K / h ≤ CR1 ≤ 200K / h and CR2 > 300K / h.

8. A method according to any one of claims 1 to 7, 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, and preferably 250 K / h ≤ |CR2-CR1| ≤ 2500 K / h.

9. A method according to any one of claims 1 to 8, wherein the holding time t ｈ at point t ｉ Starting at point t ０ The process ends with the T of the aforementioned part. ０ From T １ Cooling to point t ０ Starting at point t １ The process ends there, and CR1 is within the temperature range (T １ -T ０ This is the average cooling rate within (T), and 50K / h ≤ |(T) １ -T ０ ) / (t １ -t ０ A method where ) | ≤ 200 kJ / h.

10. A method according to any one of claims 1 to 9, wherein the part T １ From T ２ Cooling to point t １ Starting at point t ２ The process ends, and CR2 is within the temperature range (T ２ -T １ This is the average cooling rate within the ) and 300K / h < | (T ２ -T １ ) / (t ２ -t １ )|≧3000K / h, preferably 300K / h <|(T ２ -T １ ) / (t ２ -t １ A method where ) | ≤ 500 K / h.

11. A method according to any one of claims 1 to 10, wherein 10 seconds ≤ t ｈ ≦45 minutes, 10 seconds≦t ｈ ≤30 minutes, preferably 10 seconds ≤t ｈ A method that takes ≤ 15 minutes.

12. A method according to any one of claims 1 to 11, wherein 820°C ≤ T ０ Preferably 850°C ≤ T ０ Preferably 880°C ≤ T ０ , and / or T ０ ≤940℃, preferably T ０ ≤920℃, preferably T ０ ≤900°C and / or 500°C ≤ T １ ≤700℃, preferably 550℃ ≤T １ ≤700°C, preferably 600°C ≤T １ A method where the temperature is ≤700℃.

13. A method according to any one of claims 1 to 12, wherein the FeCoV alloy is initially provided in the form of a strip having a cold-rolled texture, and the parts are manufactured from the strip.

14. A method according to any one of claims 1 to 13, wherein the component has the form of a laminate, a sheet, or a laminated core, a laminated core having the shape of stator teeth, a laminated core having the shape of a stator ring, or a laminated core having the shape of a stator segment having at least two stator teeth.

15. A method according to any one of claims 1 to 14, wherein the part after heat treatment has an average grain size d ｋ 11 μm ≤ d ｋ A method having a crystal structure in which the particle size is <30 μm and the proportion of crystal grains with a particle size greater than 75 μm is up to 10%.

16. A method according to any one of claims 1 to 15, wherein the heat-treated component has a yield strength of 250 MPa < Rp0.2 ≤ 400 MPa and a maximum permeability of a minimum of 10,000 μm ｍａｘ A method comprising a minimum residual magnetic flux density Br of 1.0 T.

17. A method according to claim 15 or claim 16, wherein the crystalline structure after the heat treatment contains up to 5%, preferably up to 2.5%, and preferably up to 1%, crystalline grains having a particle size greater than 75 μm.

18. A method according to any one of claims 15 to 17, wherein the crystalline structure after the heat treatment contains up to 30%, preferably up to 25%, preferably up to 15%, and preferably up to 10%, crystalline grains having a particle size greater than 38 μm.

19. A method according to any one of claims 15 to 18, wherein after the heat treatment, 11 μm ≤ d ｋ <26 μm (standard deviation less than 20 μm), preferably 16 μm ≤ d ｋ A method where the particle size is <26 μm (standard deviation less than 20 μm).

20. A method according to any one of claims 1 to 19, wherein the heat-treated component has a magnetic flux density B800 = B (800 A / m) of at least 2.1 T, preferably B300 = B (300 A / m) of at least 1.9 T, and preferably B100 = B (100 A / m) of at least 1.2 T.

21. A method according to any one of claims 1 to 10, wherein the FeCoV alloy comprises 0.05 mass% ≤ Nb ≤ 0.15 mass%.

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