STRIP OF A COBALT IRON ALLOY, LAMINATED CORE AND METHOD OF PRODUCING A STRIP OF A COBALT IRON ALLOY

Abstract

A method of producing a strip from a CoFe alloy is provided. A slab consisting substantially of 35 wt %≤Co≤55 wt %, 0 wt %≤V≤3 wt %, 0 wt %≤Ni≤2 wt %, 0 wt %≤Nb≤0.50 wt %, 0 wt %≤Zr+Ta≤1.5 wt %, 0 wt %≤Cr≤3 wt %, 0 wt %≤Si≤3 wt %, 0 wt %≤Al≤1 wt %, 0 wt %≤Mn≤1 wt %, 0 wt %≤B≤0.25 wt %, 0 wt %≤C≤0.1 wt %, the remainder being Fe and up to 1 wt % of impurities, is hot rolled and then quenched from a temperature above 700° C. to less than 200° C. The hot rolled strip is cold rolled. The cold rolled strip is stationary annealed to produce an intermediate strip, and the intermediate strip is continuously annealed.

Claims

1. A strip of a CoFe alloy, the strip comprising a composition consisting substantially of 35 wt %≤Co≤55 wt %, 0 wt %≤V≤3 wt %, 0 wt %≤Ni≤2 wt %, 0 wt %≤Nb≤0.50 wt %, 0 wt %≤Zr+Ta≤1.5 wt %, 0 wt %≤Cr≤3 wt %, 0 wt %≤Si≤3 wt %, 0 wt %≤Al≤1 wt %, 0 wt %≤Mn≤1 wt %, 0 wt %≤B≤0.25 wt %, 0 wt %≤C≤0.1 wt %, with a remainder of Fe and up to 1 wt % of impurities, wherein the impurities may have one or more elements from the group consisting of O, N, S, P, Ce, Ti, Mg, Be, Cu, Mo and W, a strip width of a maximum of 0.50 mm, a permanent growth in length after magnetic annealing in the region of 700° C. up to the phase transition α.fwdarw.α+γ of a maximum of 0.08%, an elongation to fracture A≥5% and a yield strength Rp0.2≤1000 MPa, and a tensile strength Rm≥800 MPa.

2. A strip according to claim 1, which furthermore has an inorganic coating.

3. A strip according to claim 2, wherein the inorganic coating comprises MgO.

4. A strip according to claim 1, wherein a difference between the permanent growth in the longitudinal direction and the permanent growth in the transverse direction of the strip is less than 0.08%.

5. A laminated core, comprising a plurality of stacked sheets of a CoFe alloy, the alloy comprises a composition consisting substantially of 35 wt %≤Co≤55 wt %, 0 wt %≤V≤3 wt %, 0 wt %≤Ni≤2 wt %, 0 wt %≤Nb≤0.50 wt %, 0 wt %≤Zr+Ta≤1.5 wt %, 0 wt %≤Cr≤3 wt %, 0 wt %≤Si≤3 wt %, 0 wt %≤Al≤1 wt %, 0 wt %≤Mn≤1 wt %, 0 wt %≤B≤0.25 wt %, 0 wt %≤C≤0.1 wt %, the remainder being Fe as well as up to 1 wt % of impurities, wherein the impurities can have one or more from the group O, N, S, P, Ce, Ti, Mg, Be, Cu, Mo and W, wherein the sheets have a maximum thickness of 0.50 mm, a permanent length growth, after annealing in the region of 700° C., up to the phase transition α.fwdarw.α+γ no more than 0.08%, an elongation to fracture A 5%, a yield strength Rp0.2≤1000 MPa, and a tensile strength Rm≥800 MPa.

6. A laminated core according to claim 5, wherein the sheets have an inorganic coating.

7. A laminated core according to claim 6, wherein the inorganic coating comprises MgO.

8. A laminated core according to claim 5, wherein a difference between the permanent growth in the longitudinal direction and the permanent growth in the transverse direction of the strip is less than 0.08.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Embodiments and examples will now be explained in more detail with reference to the drawings and tables.

[0059] FIG. 1 shows a diagrammatic illustration of the permanent growth by the dotted line for a circular and a rectangular sheet.

[0060] FIG. 2 shows the measured growth longitudinally and transversely to the rolling direction for some examples.

[0061] FIG. 3 shows a diagrammatic illustration of the number of bending cycles N.

[0062] FIG. 4 shows a graphic illustration of the difference between R.sub.m and R.sub.p0.2.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0063] FIG. 1 shows a diagrammatic illustration of a sheet of a soft magnetic CoFe alloy, showing the permanent growth after annealing. The permanent growth is illustrated diagrammatically in FIG. 1 by the dotted line for a circular and a rectangular sheet.

[0064] The soft magnetic cobalt iron alloy (CoFe) can have a composition of 49 wt % Fe, 49 wt % Co and 2% V and contain additives of Ni, Nb, Zr, Ta or B. In the case of a composition of this type, a saturation polarisation of about 2.3 T is attained, while at the same time the electrical resistance remains sufficiently high at 0.4 μ.Math.Ohm.Math.m.

[0065] The CoFe alloy undergoes heat treatment to achieve the magnetic properties, said heat treatment also being known as magnetic annealing or final annealing. This heat treatment takes place above the recrystallisation temperature and below the phase transition α.fwdarw.α+γ, mostly in the region of 700° C. to 900° C. During the subsequent cooling, ordering takes place, i.e. a B2 superstructure is formed. Depending on the composition and the desired combination of strength and magnetic properties, the annealing takes place at different temperatures. For example, VACOFLUX 50 (49% Co, 1.9% V, 0.4% Ni, the remainder being Fe) is annealed in this manner at 820° C., VACOFLUX 48 (49% Co, 1.9% V, the remainder being Fe) at 880° C. and VACODUR 49 (49% Co, 1.9% V, 0.1% Nb, the remainder being Fe) at 880° C., or, depending on the application, at 750° C. also.

[0066] Due to the magnetic annealing and the ordering associated with it, the dimensions of the sheet are altered. Thus, the permanent variation in length dl is to be understood as the “growth in length” which happens when the material of initial length l.sub.0 undergoes typical annealing. Below therefore, “growth in length” will always be understood as the growth dl/l.sub.0 relative to the initial length. This growth in length is in the region of 0.03% to 0.20%.

[0067] The described change in the dimensions by the annealing is undesirable since it directly affects the dimensional accuracy of a single lamella, as well as the finished laminated core. When used as a stator or rotor, this can cause a change in the geometry of the air gap so that the result is impairment of the magnetic circuit.

[0068] A rectangular test piece of a VACOFLUX 50 strip with a strip thickness of, for example, 0.35 mm incurs, during an annealing process of 4 hrs at 820° C., a change in length in the region of 0.1% to 0.2%. During this time, anisotropy of the growth is observed, i.e. in the longitudinal direction the strip undergoes a greater growth than in the transverse direction wherein the longitudinal direction is parallel to the rolling direction of the strip.

[0069] A closer observation shows that this anisotropy of the growth is correlated with the degree of cold forming. With degrees of cold forming >60%, the growth in the longitudinal direction rises significantly, whereas the growth in the transverse direction hardly changes. This anisotropy of the growth reveals a problem, particularly in the case of circular geometries, as would be found, for example, in electrical machines, since a round shape becomes an ellipse, as illustrated in FIG. 1.

[0070] In one embodiment the strip according to the invention is produced as follows: [0071] hot rolling a slab of a CoFe alloy, typically to a thickness of 2.0 mm to 3.5 mm, [0072] quenching from a temperature above 700° C., [0073] cold rolling to a final thickness, which can be, for example, 0.50 mm to 0.050 mm to form a cold rolled strip, [0074] covering the cold rolled strip with an electrically insulating layer, for example, with an inorganic layer based on Mg methylate, [0075] stationary annealing, such as in a hood furnace, followed by [0076] continuous annealing.

[0077] Annealing to adjust the magnetic properties can be carried out after the continuous annealing and the subsequent cooling to room temperature.

[0078] Conventional intermediate steps may be inserted at each stage during the production of the strip, such as shot blasting, pickling, grinding, trimming, stretch-bend-levelling or the attachment of a tie rod.

[0079] The CoFe alloy can be, for example, VACODUR 49 with a composition of 48.65 wt % Co, 1.87 wt % V, 0.10 wt % Nb, 0.17% Ni, 0.03% Mn, 0.02% Si, less than 0.0050% C and the remainder being Fe. As to melting properties, the contents include other elements, each no greater than 0.02 wt %. This embodiment is representative of the class of materials also designated as V-Permendur with the rough constituents 49% Fe 49% Co and 2% V.

[0080] The coating of the cold rolled strip serves to avoid the layers of strip welding together during the subsequent stationary annealing. A coating solution used is an inorganic coating based on Mg methylate which is partly converted during heat treatment into solid Mg oxide. The coating before stationary annealing can be dispensed with if the risk of the welding of the layers of strip is low. In some embodiments the strip is coated at a later time to form the electrical insulation between the stacked sheets of the laminated core.

[0081] The stationary annealing can take place in a hood furnace. This first heat treatment serves to deliberately alleviate the earlier cold forming of the strip, i.e. it removes dislocations and the start of primary recrystallisation. The maximum temperature at which the strip is raised is chosen such that the ordering is at least partially removed and is not allowed to exceed the phase transition α.fwdarw.α+γ. The soaking temperature of the stationary annealing is therefore in the range of 650° C. to 900° C., preferably at 680° C. to 760° C., after which it is cooled to room temperature.

[0082] During stationary annealing, typically the cooling rate is between 50 K/hr and 200 K/hr in the temperature range of 700° C. to 500° C. However, this can vary depending on the furnace used and the loading of the furnace. Generally, rapid cooling is preferable, e.g. using a fan, so that cooling rates are attained in the aforementioned range of at least 300 K/hr.

[0083] This second continuous annealing serves to make the at least partially recrystallised strip ductile by rapid cooling. The maximum temperature of the strip is selected in this process such that the ordering is at least partially broken up and the phase transition α.fwdarw.α+γ is not exceeded. Therefore, the temperature range is 650° C. to 950° C. and, depending on the composition, can vary. Preferably in the case, for example, of VACODUR 49, 760° C. at 2 m/min to 4 m/min is specified to avoid any further recrystallisation of the strip. The cooling rate should reach in the region of 700° C. to 500° C. at a minimum of 1 K/sec, preferably at least 10 K/sec.

[0084] The strip produced in this manner has the following properties: [0085] strip thickness maximum 0.50 mm, preferably lower than 0.35 mm [0086] growth in length of a maximum of 0.08%, preferably a maximum of 0.05%, after magnetic annealing in the region of 700° C. up to the phase transition α.fwdarw.α+γ. [0087] presence of a hard inorganic coating, e.g. in the form of Mg oxide. [0088] elongation to fracture A>5%, preferably A>10%; 0.2%-yield strength Rp.sub.0.2>1000 MPa

[0089] In the following, the invention is explained with the aid of a series of tests with 7 different samples.

TABLE-US-00001 TABLE 1 Classifi- Stationary Continuous # cation annealing annealing annealing 1 Comparison — — 6 hrs 880° C., H.sub.2 2 Comparison 700° C. 2 hrs, — 6 hrs 880° C., H.sub.2 H.sub.2 3 Comparison 750° C. 2 hrs, — 6 hrs 880° C., H.sub.2 H.sub.2 4 Comparison — 820° C. 3 m/min, 6 hrs 880° C., H.sub.2 H.sub.2 5 Comparison — 760° C. 3 m/min, 6 hrs 880° C., H.sub.2 H.sub.2 6 Invention 700° C. 2 hrs, 760° C. 3 m/min, 6 hrs 880° C., H.sub.2 H.sub.2 H.sub.2 7 Invention 750° C. 2 hrs, 760° C. 3 m/min, 6 hrs 880° C., H.sub.2 H.sub.2 H.sub.2

[0090] Table 1 shows an overview of the conducted tests. In it, the embodiment 1 represents the as-rolled comparison state. Examples 2 and 3 were subject just to stationary annealing and examples 4 and 5 just to continuous annealing. These examples 2 to 5 therefore are not a part of the present invention. In a first step, examples 6 and 7 are stationary annealed and in a subsequent second step are continuously annealed. Thus examples 6 and 7 are part of the present invention.

[0091] In the above, a strip of VACODUR 49 is used with a composition of 48.65 wt % Co, 1.87 wt % V, 0.10 wt % Nb, 0.17% Ni, 0.03% Mn, 0.02% Si, less than 0.0050% C and the remainder being Fe. As to melting properties, the contents include other elements, each no greater than 0.02 wt %. This embodiment is representative of the class of materials also designated as V-Permendur with the rough constituents 49% Fe 49% Co and 2% V.

[0092] The embodiment 1 did not undergo any stationary annealing nor any continuous annealing. After being formed by punching, the parts thus obtained were subject to magnetic annealing for 6 hrs at 880° C. The annealing was performed in this case under dry hydrogen with a dew point of −50° C. This annealing was carried out in the same manner as for the variants 2 to 7 described below.

[0093] In contrast, embodiments 2 and 3 were subject to stationary annealing before the magnetic annealing. In doing so, the coil with a weight of typically 50 kg to 1000 kg was annealed in a hood furnace under dry hydrogen. Temperatures of 700° C. or 750° C. with a soaking time of 2 hrs were chosen for both variants. In embodiment 2, this caused the healing or removal of dislocations, while in example 3 this led directly to significant recrystallisation. The strips were coated beforehand with a Mg methylate solution so that the annealing caused no welding together of layers of strip.

[0094] Embodiments 4 and 5 were subject to continuous annealing before the magnetic annealing. In doing so, the uncoated strip was conveyed through a vertical tower furnace with a heating zone of length 3 m. The strip speed was 3 m/min, i.e. the soaking time in the heating zone was 1 minute. The atmosphere used was dry hydrogen, wherein the dew point was below −30° C. Cooling took place by being conveyed through a water-cooled muffle. Here, cooling rates in the strip of about 5 K/sec to 40 K/sec were achieved.

[0095] Embodiments 6 and 7 represent the states according to the invention. In these cases, the stationary annealed strips from embodiments 2 and 3 were then continuously annealed in the tower furnace. The other parameters (strip speed, atmosphere, cooling rate) were analogous to examples 4 and 5.

[0096] In Table 2, mechanical characteristics from the hardness testing (Vickers hardness) and from the tensile test are illustrated. Direct characteristics from the tensile test are the 0.2% yield strength (Rp.sub.0.2) and the tensile strength (R.sub.m) as well as the elongation to fracture A. Here, in each case, tensile tests lengthwise and transverse to the rolling direction were undertaken and tested. The difference R.sub.m−Rp.sub.0.2, which represents a measure for the range of plastic deformability still serves as the calculated characteristic value.

TABLE-US-00002 TABLE 2 Tensile test, lengthwise Tensile test, transverse Stationary Hardness Rp.sub.0.2 R.sub.m R.sub.m- A Rp.sub.0.2 R.sub.m R.sub.m- A # R/E Continuous HV MPa MPa RP.sub.0.2 % MPa MPa Rp.sub.0.2 % 1 R — 390 1168 1298 130 1.4 1257 1404 147 0.7 — 2 R 700° C. 2 hrs 326 727 972 245 7.0 844 942 97 4.2 — 3 R 750° C. 2 hrs 229 402 577 175 4.4 449 535 86 3.3 — 4 R — 261 537 1155 618 20.5 574 975 402 10.2 820° C. 3 m/min 5 R — 346 880 1195 315 9.3 1009 1111 102 3.9 760° C. 3 m/min 6 E 700° C. 2 hrs 324 796 1363 567 17.3 899 1139 241 7.9 760° C. 3 m/min 7 E 750° C. 2 hrs 251 498 1164 667 18.8 539 1085 546 12.7 760° C. 3 m/min

[0097] Table 2 shows mechanical characteristics from hardness tests and tensile tests on the tested examples 1 to 7 for the comparison states (R) and states according to the invention (E).

[0098] In the cold rolled comparison example 1, due to the high deformation from rolling, it has a very high yield strength and tensile strength and the elongation to fracture is very low at less than 2%.

[0099] The comparison examples 2 and 3 show that, due to stationary annealing alone both the yield strength as well as the tensile strength are significantly lowered. Because of the slightly reduced hardness of comparison example 2 of 326 HV it can be seen that the material is not yet completely recrystallised. On the other hand, embodiment 3 with a Vickers hardness of 229 is significantly softer and displays a recrystallised structure, as can be seen also in the microstructure slide.

[0100] Comparison examples 4 and 5 show that, with continuous annealing alone, the yield strength is also severely lowered. Here, compared with comparison example 5, due to the higher annealing temperature of 820° C. against 760° C. at the same throughput speed, comparison example 4 shows a more severe lowering of the yield strength. In contrast, the tensile strength in the longitudinal direction rises to values greater than 1100 MPa, regardless of the choice of annealing temperature. The tensile strength in the transverse direction also rises and reaches values greater than 900 MPa. The elongation to fracture in the transverse direction is markedly different and, in the case of comparison example 5, only attains a value of below 4%.

[0101] Examples 6 and 7 according to the invention show an almost unchanged but slightly elevated yield strength compared with the respective stationary annealed comparison examples 2 and 3. In contrast, the tensile strength in the longitudinal direction rises similarly as in the continuously annealed strips to a value greater than 1100 MPa. In the longitudinal direction, both strips display a very high elongation to fracture of more than 17%. In the transverse direction, it is also very advantageous. Both strips attain values for

[0102] tensile strength greater than 1000 MPa and values for elongation to fracture greater than 7%.

[0103] The increase in the tensile strength in all continuous annealings is attributed to the associated rapid cooling and to the associated degree of ordering.

[0104] In order to adjust a high elongation to fracture it is necessary to establish a region of plastic deformability which is as wide as possible, i.e. to produce a strip with a low yield strength and a high tensile strength.

[0105] As a general principle, the annealing temperature can be raised to set a low yield strength, wherein the two phase region presents a limitation, i.e. during the transition α.fwdarw.α+γ. This temperature is exceeded, a non-magnetic phase is formed which no longer completely changes during cooling and causes a severe weakening of the soft magnetism. In the case of VACODUR 49, for example, this transition is at 880° C., whereas with VACOFLUX 50 it is 820° C. due to the higher Ni content.

[0106] In continuous annealing alone, due to the short dwell time, a significantly higher temperature must be selected so that the structure recrystallises well. Thus, it is not possible to lower Rp.sub.0.2 without damaging the material permanently with regard to its magnetic properties.

[0107] The method according to the invention enables the material to recrystallise completely and a low yield strength to be set in a first step. In a second step, the tensile strength can then be increased to values higher than 1000 MPa by continuous annealing. In doing so, a dwell temperature that is as low as possible and above the ordering temperature is chosen, for example, at 760° C. By using this combination, a high elongation to fracture can be set even with alloys which have a relatively low two phase region due to their composition.

[0108] In Table 3, the permanent growth in length dl/l.sub.0 of the embodiments is quoted. For this, strips measuring 160 mm×20 mm long and transverse to the rolling direction are cut and their initial length l.sub.0 is measured optically. The strips then underwent magnetic annealing (6 hrs 880, H.sub.2), after which they were measured again. The relative growth in length dl/l.sub.0 was then calculated from the measured growth dl.

[0109] Furthermore, the amount of the difference between the characteristic values 1=dl/l.sub.0 (growth in the longitudinal direction) and q=dl/l.sub.0 (growth in transverse direction) was calculated as the measure for the anisotropy of the growth. This quantity is designated here as |l−q|.

TABLE-US-00003 TABLE 3 Growth 1 = dl/l.sub.0 q = dl/l.sub.0 Stationary in % in % # R/E Continuous longitud. transv'se |1 − q| 1 R —   0.138%   0.092% 0.046% — 2 R 700° C. 2 hrs   0.000%   0.004% 0.004% — 3 R 750° C. 2 hrs −0.006% −0.002% 0.004% — 4 R —   0.031%   0.036% 0.005% 820° C. 3 m/min 5 R —   0.028%   0.032% 0.004% 760° C. 3 m/min 6 E 700° C. 2 hrs   0.028%   0.041% 0.013% 760° C. 3 m/min 7 E 750° C. 2 hrs   0.032%   0.034% 0.002% 760° C. 3 m/min

[0110] Table 3 shows the permanent growth in length dl/l.sub.0 after additional annealing for 6 hrs at 880° C. for comparison states (R) and states according to the invention (E).

[0111] The permanent growth in length of the as-rolled comparison example 1 is 0.138% in the longitudinal direction and 0.092% in the transverse direction in the expected range for a 50% CoFe material with significant cold deformation of over 80%. The growth is so large that taking this growth into account is almost not possible, or can be achieved only with great difficulty by a corresponding adaptation of a punching tool. This is made more difficult in particular because the anisotropy |l−q| at 0.046% is very large.

[0112] The permanent growth in length of the stationary annealed material of examples 2 and 3 is the lowest and falls within the range of measurement inaccuracy. The disadvantage of these states is the inadequate ability to be punched.

[0113] The permanent growth in length of purely continuously annealed material of examples 4 and 5 is in the region of 0.03% to 0.04% and is very isotropic. Therefore, it is so low that it can be compensated by a corresponding design of the punching tool.

[0114] The permanent growth in length of the material according to the invention of examples 6 and 7 is practically identical in the region 0.03% to 0.04%. It can be assumed therefore that the absolute amount of the growth is determined by the cooling rate as it continues though. In these tests, the cooling rate was in the range 700° C. to 500° C. at 5 K/sec to 40 K/sec.

[0115] FIG. 2 shows the measured growth longitudinally and transversely to the rolling direction for the tested states (1-7). FIG. 2 shows the longitudinal and transverse growth of the tested states after additional magnetic annealing for 6 hrs at 880° C.

[0116] The plotted dotted line corresponds to an isotropic growth, i.e. an identical value for the longitudinal growth and the transverse growth.

[0117] Also, the cutting and bending properties were calculated, as in Table 4.

TABLE-US-00004 TABLE 4 To and fro bending test Stationary N Assess N Assess # R/E Continuous Cutting long. long. trans trans 1 R — very good 66 + 9 − — 2 R 700° C. 2 hrs very bad 109 ++ 15 − — 3 R 750° C. 2 hrs bad 1 − 7 − — 4 R — very good 70 + 68 + 820° C. 3 m/min 5 R — very good 79 + 38 + 760° C. 3 m/min 6 E 700° C. 2 hrs very good 128 ++ 31 + 760° C. 3 m/min 7 E 750° C. 2 hrs very good 158 ++ 70 + 760° C. 3 m/min

[0118] Table 4 shows the cutting and bending properties of the tested states.

[0119] The cutting property was determined by having the material cut with cuts lengthwise to the rolling direction. An assessment of “very good” corresponds in this case to a smooth cut edge with no tears or other deficiencies. This corresponds to the as-rolled comparison example 1.

[0120] This cutting property is worsened significantly by stationary annealing. Depending on the annealing, the cutting produced was very bad, displaying tears in comparison example 2 extending diagonal to the cutting direction, or at least a poor cutting property, seen as small tears on the cut edge of the comparison example 3.

[0121] The purely continuously annealed comparison examples 4 and 5 display a very good cutting property.

[0122] The examples 6 and 7 according to the invention also display a very good cutting property.

[0123] In addition, the bending property was determined in the alternating bending test under DIN EN ISO 7792. The bending coefficient N was determined as follows: A strip of dimensions 20 mm×20 mm was taken lengthwise to the rolling direction. The bending was then performed perpendicular to the extraction direction so that the strip was bent transverse to the rolling direction for the “lengthwise” bending coefficient. In the same manner, the strip was taken transverse to the rolling direction and was bent lengthwise to the rolling direction for the “transverse” bending coefficient.

[0124] The assessment of the bending coefficient was done according to the following division: fewer than 20 bending cycles (N<20) corresponds to a poor bending property (−), up to 99 bending cycles (20≤N≤99) corresponds to a good bending property (+) and 100 or more bending cycles (N≥100) corresponds to a very good bending property (++). It should be noted that the number of bending cycles is strongly dependant on the thickness of the strip used. With thicker material, generally a smaller bending coefficient can be expected.

[0125] In the lengthwise direction, the as-rolled comparison example 1 displays a good bending property (+), but transversely the bending property is inadequate (−). The stationary annealed comparison examples 2 and 3 show very different results longitudinally (++ or, respectively, −) and transversely a consistently poor bending property (−). The continuously annealed comparison examples 4 and 5 show a consistently good bending property both in the longitudinal as well as in the transverse directions (+).

[0126] The examples 6 and 7 according to the invention display by far the best bending property longitudinally (++) and transversely a good bending property also (+).

[0127] A graphic illustration of the number of bending cycles N can be found in FIG. 3.

[0128] FIG. 4 shows a graphic illustration of the calculated difference R.sub.m−Rp.sub.0.2 derived from Table 2, illustrating a measure for the range of plastic deformability.

[0129] In order to set a high elongation to fracture it is advantageous to set a range of plastic deformability as broad as possible, i.e. to produce a strip with a low yield strength and a high tensile strength. A high tensile strength and low yield strength and therefore a large difference can be attained by the method according to the invention.

[0130] Summarising:

[0131] The stationary annealing alone completely removes the growth in length. The cutting and bending properties in the transverse direction are, however, inadequate for further processing, such as punching.

[0132] Examples which are only continuously annealed have a low and isotropic permanent growth. The bending property is improved significantly.

[0133] The annealed examples according to the invention which are first stationary annealed and then continuously annealed also have a small and isotropic permanent growth. In this case, the bending property in the longitudinal direction is significantly better than in the case of all comparison examples.

[0134] According to the invention, the recrystallisation can be precisely controlled by stationary annealing in the first step. As a result of this, depending on the degree of recrystallisation, a reduction in the yield strength Rp.sub.0.2 can be adjusted with accuracy. The yield strength remains practically unchanged due to the subsequent continuous annealing in the second step, whereas the tensile strength R.sub.m rises again significantly to values greater than 1000 MPa.

[0135] Therefore, the method according to the invention decouples the lowering of the yield strength and the raising of the tensile strength and therefore enables the specific setting of a high elongation to fracture A in a strip of a CoFe alloy. Therefore, this strip can be processed further, for example by punching, in order to form parts for a laminated core which also have a low permanent growth in length after annealing. Thus, a laminated core for a rotor or stator can be produced with more accurate geometry.