GB2495465A - A method of processing a soft magnetic Fe-Co-V-Nb/Ta alloy - Google Patents

Abstract

A method of making a soft magnetic alloy by solidifying a melt with a composition which comprises (by weight): 47-50 % Co, 1-3 % V, 0-0.25 % Ni, 0-0.007 % C, 0-0.1 % Mn, 0-0.1 % Si, 0-0.15 % Nb, 0-0.3 % tantalum, where 0.14 ¤ (Ta +2Nb) ¤ 0.3, with the balance being Fe to form a blank. Hot rolling the blank followed by quenching from a temperature above 730 0C, pickled, then cold rolling followed by annealing at a temperature in the range 730-880 0C for 1-6 hours to give a yield strength in the range 200-450 MPa and a coercive field strength in the range 0.3-1.5 A/cm. The blank is hot rolled at a temperature in the range 1100-1300 0C while its thickness is reduced by 90 %. Cold rolling also reduces the bank thickness by 90 %. The alloy is used to make laminated electric motor rotors and stators.

Description

Soft magnetic alloy and method for producing a soft magnetic alloy.

A ferromagnetic material that can be magnetized, but tends not to remain magnetized is described as magnetically soft. When a magnetically soft material is magnetised in a magnetic field and then removed from the magnetic field, it loses most of the magnetism exhibited while in the field. A magnetically soft material preferably displays a low hysteresis loss, high mag-netic permeability and a high magnetic saturation induction.

Magnetically soft materials are used in various static and ro- tating electrical devices, such as motors, generators, alter-nators, transformers and magnetic bearings.

uS 5,501,747 discloses a high strength, soft magnet iron-cobalt-vanadium based alloy which further comprises 0.15 weight percent to 0.5 weight percent niobium and 0.003 weight percent to 0.02 weight percent carbon. This alloy is disclosed as having a combination of yield strength, magnetic properties and electrical properties which enables it to be used for the rotating part, such as a rotor, of a rotating electrical ma-chine. When the alloy is annealed at a temperature of not more than about 740°C for not more than about 4 hours, it has a room temperature yield strength of at least 620 MPa.

However, further soft magnetic alloys having a combination of a high yield strength and suitable magnetic properties for ap-plications such as rotating electrical devices are desirable.

A soft magnetic alloy is provided that consists essentially of 47 weight percent «= Co «= 50 weight percent, 1 weight percent «= V 3 weight percent, 0 weight percent «= Ni «= 0.25 weight per-cent, 0 weight percent «= C «= 0.037 weight percent, 0 weight percent «= Mn «= 0.1 weight percent, 0 weight percent «= Si «= 0.1 weight percent, at least one of niobium and tantalum in amounts of x weight percent of niobium, y weight percent of tantalum, remainder Fe. The nicbium and tantalum contents are within the ranges of 0 weight percent «= x < 0.15 weight per-cent, 0 weight percent «= y «= 0.3 weight percent and 0.14 weight percent «= (y + 2x) «= 0.3 weight percent. The soft mag-netic alloy has been annealed at a temperature in the range of 730°C to 880°C for a time of 1 to 6 hours and comprises a yield strength in the range of 200 MPa to 450 MPa and a coer-

cive field strength of 0.3 to 1.5 A/cm.

The alloy is based on a 49%Co-2%V-Fe-type alloy which further includes niobium and/or tantalum in amounts within the range of 0 weight percent «= x < 0.15 weight percent and 0 weight percent «= y «= 0.3 weight percent, respectively. The total amount of niobium and tantalum is described by (y 4-2x), i.e. the amount of tantalum in weight percent, y, in addition to twice the amount of niobium in weight percent, 2x, and lies within a range of 0.14 weight percent to 0.3 weight percent.

The alloy further includes a maximum carbon content of 0.007 weight percent and optionally Ni up to 0.2 weight percent.

The elements manganese and silicon are also optional and may be added in order to reduce the oxygen content of the alloy.

Oxygen is not intentionally added to the alloy, but may be present as an impurity in amounts up to around 0.009 weight percent. Further impurity elements such as one or more of the elements Cr, Cu, Mo, Al, 5, Ti, Ce, Zr, B, N, Mg, Ca or P may be present in a total amount of not more than 0.5 weight per-cent.

The soft magnetic alloy is also free of Boron. In this con-text, free of Boron includes a boron content of less than 0.0007 weight percent as well as a zero Boron content.

For alloys of the 49%Co-2%V-Fe-type, the annealing temperature is generally observed to have opposing effects on the mechani-cal properties and the magnetic properties. In particular, the yield strength is observed to increase for decreasing anneal-ing temperatures, whilst the magnetic properties are observed to improve by annealing at higher temperatures.

A combination of a niobium content, x, and/or tantalum con-tent, y, with the relationship y + 2x within the range of 0.14 to 0.3 weight percent and a carbon content of less than 0.007 weight percent, or less than 0.C05 weight percent or less than 0.003 weight percent, provides a soft magnetic alloy with a yield strength that can be adjusted as desired over a range of 200 MPa to 450MPa by appropriate selection of the an-nealing conditions. At the same time, soft magnetic properties suitable for soft magnetic parts, such as a rotor or a stator, of a rotating electrical machine can be obtained.

A coercive field strength of 1.5 A/cm may be achieved for an alloy that was annealed at an annealing temperature of 730°C whilst a coercive field strength of 0.3 A/cm may be achieved for an alloy that was annealed at 880°C.

One explanation for this behaviour is that by reducing the carbon content, the formation of Laves phases (Co/Fe, Nb) is favoured while the formation of carbides is reduced, thus ena-bling a suitably high yield strength to be obtained without resulting in a worsening of the magnetic properties to such a degree that they are no longer suitable for use in electric machines.

In a rotating electrical machine, the rotor typically requires a higher yield strength than the stator as the rotor rotates during use and is subjected to centrifugal forces. It may he useful if the yield strength of the material of the rotor is sufficiently high that the rotor remains below its elastic limit despite the centrifugal forces. In contrast, the stator is static and not subjected to centrifugal force so that the stator may have a lower yield strength than that of the rotor.

Usefully, the yield strength and the magnetic properties of the soft magnetic alloy according to the invention can be ad-justed by annealing the parts for the rotor and for the stator at different annealing temperatures so that the same composi- tion can be used for both the rotor and the stator of an elec-trical machine.

In a further embodiment, the total of the niobium and tantalum content is limited to 0.25 so chat 0.14 weight percent «= (y + 2x) «= 0.25 weight percent.

It tantalum is omitted so that y = 0, the niobium content may be 0.07 weight percent «= x C 0.15 weight percent.

If niobium is omitted, so that x 0, the tantalum content may be 0.14 weight percent «= y «= 0.3 weight percent.

Tn a further embodiment, the upper limit of the nickel content is reduced to 0.2 weight percent so that 0 weight percent «= Ni «= 0.20 weight percent.

The maximum amount of carbon may be reduced to 0 weight per-cent «= C «= 0.005 weight percent or to 0 weight percent «= C < 0.003 weight percent. Reducing the carbon content may be use-ful in improving the magnetic properties.

As discussed above, manganese and silicon are optional. In some embodiments the soft magnetic alloy includes manganese and/or silicon within a range of 0 weight percent < Mn «= 0.07 weight percent and/or 0 weight percent < Si «= 0.07 weight per-cent. In further embodiments, 0.07 weight percent < Mn «= 0.1 weight percent and/or 0.07 weight percent C Si «= 0.1 weight percent.

In an embodiment, the soft magnetic alloy comprises a yield strength (0.2% strain), Rp3.2, of between 200 MPa and 450 MPa in an annealed state. The yield strength can be adjusted as desired by adjusting the annealing conditions, in particular, by selecting a suitable annealing temperature.

The soft magnetic alloys having a composition within the ranges given above display a linear dependence of the yield strength with annealing temperature. This feature is not dis-played by commercially available alloys with about 0.05 wt.% Nb and 100 ppm C such as HIPERCO 50. In the following, alloys with about 0.05 wt.% Nb and 100 ppm C are referred to as ref-erence alloys.

In an embodiment, the soft magnetic alloy comprises a yield strength (0.2% strain) that is a linear function of annealing temperature over an annealing temperature range of 740°C to 865°C or 730°C to 900°C.

In an embodiment, in an annealed state, the soft magnetic al-by comprises a yield strength (0.2% strain) that lies within ± 10% of a linear function of yield strength (0.2% strain) against annealing temperature obtained for the alloy.

In an annealed state, the soft magnetic alloy may comprise a resistivity of at least 0.4 4m and/or an induction 3(8 A/rn) of at least 2.12 T. As discussed above, the soft magnetic alloy comprises a combi-nation of mechanical strength and soft magnetic properties that are suitable for the soft magnetic parts of a rotating electrical machine. In an embodiment, the soft magnetic alloy is annealed such that it has, in the annealed state, an induc-tion 5(8 A/m) of at least 2S2 I and a yield strength of at least 370 MPa. This combination of properties is suitable for a rotor of an electric machine.

In a particular embodiment, after annealing at a temperature in the range of 720°C to 900°C, the soft magnetic alloy com-prises a yield strength in the range of 200 MPa and 450 P4Pa, and a power loss density at 2T and 400 Hz of less than 90 W/kg. In further embodiments, for an annealing temperature of 720°C, the power loss density at 21 and 400 Hz is less than 90 W/kg and for an annealing temperature of 900°C is less than 65 W/kg.

A stator for an electric motor and a rotor for an electric mo-tor comprising a soft magnetic alloy according to one of the previously described embodiments is also provided. An electric motor comprising a stator and a rotor each comprising a soft magnetic alloy having a composition according to one of the previously described embodiments is also provided. The rotor and the stator may have the same composition, but differing mechanical properties and magnetic properties. This may be provided by annealing the rotor or parts forming the rotor un-der different annealing conditions compared to the stator or parts forming the stator.

The rotor and/or the stator nay comprise a plurality of plates or layers that are stacked together to form a laminate.

The electric machine may be a motor, a generator, an alterna-tor, or a transformer.

A method for manufacturing a soft magnetic alloy is provided which comprises providing a melt consisting essentially of 47 weight percent «= Co «= 50 weight percent, 1 weight percent «= V «= 3 weight percent, 0 weight percent «= Ni «= 0.25 weight per-cent, 0 weight percent «= C «= 0.007 weight percent, 0 weight percent «= Mn «= 0.1 weight percent, 0 weight percent «= Si «= 0.1 weight percent, at least one of niobium and tantalum in amounts of x weight percent of niobium or y weight percent of tantalum, remainder Fe, wherein 0 weight percent «= x < 0.15 weight percent, 0 weight percent «= y «= 0.3 weight percent and 0.14 weight percent «= (y + 2x) «= 0.3 weight percent. This melt is cooled and solidified to form a blank. The blank is hot rolled, quenched and then cold rolled. Subsequently, at least a portion of the blank is annealed at a temperature in the range of 730°C to 880°C and a yield strength in the range of 200 MPa to 450 MPa and a coercive field strength of 0.3 A/cm to 1.5 A/cm is produced.

After cold rolling, the blank may have the form of a plate or ribbon. Pieces of the blank may be removed by stamping or cut-ting, for example, and the piece or pieces annealed at a suitably selected temperature to obtain the desired mechanical and magnetic properties.

In further embodiments, at least a portion of the blank is an-nealed at a temperature in the range of 740°C to 865°C or in the range of 730°C to 790°C or in the range of 800°C to 880°C.

The higher temperature range of 800°C to 880°C may be used when fabricating a stator from the soft magnetic alloy and the lower temperature range of 730°C to 790°C may be used when fabricating a rotor from the soft magnetic alloy.

In a further embodiment, a thickness reduction in the blank of about 90% is produced by the hot rolling of the blank. This thickness reduction may be selected so as to select the de-sired thickness reduction in the subsequent cold rolling step and the amount of deformation introduced into the soft mag-netic alloy.

The blank may be hot rolled at a temperature in the range of 1100°C to 1300°C. After hot rolling, the blank may be natu-rally cooled. After hot rolling, the strip is quenched from a temperature above 730°C to room temperature or to below room temperature. This may be carried out whilst the strip is cool-ing from the hot rolling temperature. Alternatively, the strip may be cooled to room temperature and afterwards reheated to a temperature above 730°C and quenched to room temperature or to below room temperature.

After hot rolling and before cold rolling, the blank may be cleaned, for example pickled and/or mechanically worked, for example by sand blasting, to clean the surface. This improves the surface finish of the blank after cold rolling and may also aid in improving the magnetic properties of the alloy af-ter annealing.

In an embodiment, a thickness reduction in the blank of 90% is produced by the cold rolling of the blank. After cold rolling, the thickness of the blank may lie in the range of 0.3 mm to 0.4 nun. This thickness is suitable for producing laminated ar-ticles such as laminated rotors and laminated stators for electric machines.

A method for manufacturing a semi-finished part is also pro-vided that comprises performing the method according to one of the previously described embodiments and separating a portion of the blank to produce a semi-finished part.

A laminated article may be formed by assembling a plurality of semi-finished parts comprising a soft magnetic alloy according to one of the embodiments described above.

A rotor for an electric motor may be provided by annealing the soft magnetic alloy or the laminated article according to one of the previously described embodiments at a temperature of 730 to 790°C.

A stator for an electric motor may be provided by annealing the soft magnetic alloy or the laminated article according to one of the previously described embodiments at a temperature of 800°C to 880°C.

Specific examples and embodiments will now be described with reference to the accompanying drawings and table.

Figure 1 illustrates a graph of yield strength Rp0.2 vs. (Ta + 2 x Nb) for a]ow carbon content.

Figure 2 illustrates a graph of yield strength Rp0.2 vs. carbon for a Nb-und Ta-content according to the invention.

Figure 3 illustrates a graph of magnetic induction 3(3 A/cm) vs. (Ta + 2 x Nb) for a low carbon content.

Figure 4 illustrates a graph of magnetic induction 3(3 A/cm) vs. carbon content for a Nb-und Ta-content according to the invention.

Figure 5 illustrates a graph of coercive field strength Nc vs. (Ta + 2 x Nb) for a low carbon content.

Figure 6 illustrates a graph of coercive field strength Nc vs. carbon content for a Nb-und Ta-content ac-cording to the invention.

Figure 7 illustrates a graph of power loss density P(2T; 400Hz) vs. (Ta + 2 x Nb) for a low carbon con- tent -Figure 8 illustrates a graph of power loss density P(2T; 400Hz) vs. Carbon content for alloys having a Nb-und Ta-content according to the invention.

II

Figure 9 illustrates a graph of magnetic induction 8(8 A/cm) vs. (Ta + 2 x Nb) for a low carbon content.

Figure 10 illustrates a graph of magnetic induction 8(8 A/cm) vs. carbon content for alloys having a Nb-und Ta-content according to the invention.

Figure 11 illustrates a graph of magnetic induction 8(80 A/cm) vs. (Ta + 2 x Nb) for low carbon contents.

Figure 12 illustrates a graph of magnetic induction 3(80 A/cm) vs. carbon content for Nb-und Ta-contents according to the invention.

Figure 13 illustrates a graph of power loss density P(2T; 50Hz) vs. (Ta + 2 x Nb) for a low carbon con--tents.

Figure 14 illustrates a graph of power loss density P(2T; 50Hz) vs. carbon content for Nb-und Ta-contents according to the invention.

Figure 15 illustrates a graph of the range of the yield strength vs. Ta and Nb content for C «= 0.0070%.

Figure 16 illustrates a graph of the range of the yield strength vs. carbon content for 0.14 wt. % <= Ta + 2 x Nb <= 0.30 wt. %.

Figure 17 illustrates a graph of power loss density F(2T; 400 Hz) vs. yield strength Rp02.

Figure 18 illustrates a graph of coercive field strength Ho vs. yield strength Rpo2.

Figure 19 illustrates a graph of power loss density P(2T; Hz) vs. yield strength Rp0.2.

Figure 20 illustrates a graph of magnetic induction B(3 A/cm) vs. yield strength Rp0.2.

Figure 21 illustrates a graph of magnetic induction 8(8 A/cm) vs. yield strength Rp02.

Table 1 illustrates a summary of compositions, mechanical properties and magnetic properties of alloys and comparison alloys.

A soft magnetic alloy is provided that consists essentially of 47 weight percent «= Co «= 50 weight percent, 1 weight percent «= V «= 3 weight percent, 0 weight percent «= Ni «= 0.25 weight per-cent, 0 weight percent «= C «= 0.007 weight percent, 0 weight percent «= Mn «= 0.1 weight percent, 0 weight percent «= Si «= 0.1 weight percent, at least one of niobium and tantalum in amounts of x weight percent of niobium, y weight percent of tantalum, remainder Fe. The alloy includes 0 weight percent «= x < 0.15 weight percent, 0 weight percent «= y «= 0.3 weight percent and 0.14 weight percent «= (y + 2x) «= 0.3 weight per- cent. The soft magnetic alloy has been annealed at a tempera-ture in the range of 730°C to 880°C for a time of 1 to 6 hours and comprises a yield strength in the range of 200 MPa to 450 MPa and a coercive field strength of 0.3 A/cm to 1.5 A/cm.

The soft magnetic alloy may be produced by providing a melt consisting essentially of 47 weight percent «= Co «= 50 weight percent, 1 weight percent «= V «= 3 weight percent, 0 weight percent «= Ni «= 0.25 weight percent, 0 weight percent «= C «= 0.007 weight percent, 0 weight percent «= Mn «= 0.1 weight per-cent, 0 weight percent «= Si «= 0.1 weight percent, at least one of niobium and tantalum in amounts of x weight percent of nio-bium or y weight percent of tantalum, remainder Fe, wherein 0 weight percent «= x < 0.15 weight percent, 0 weight percent «= y «= 0.3 weight percent and 0.14 weight percent «= (y + 2x) «= 0.3 weight percent. The melt is then cooled and solidified to form a blank. The blank is then hot rolled, for example at 1200°C, cooled or reheated to 730°C and then quenched to room tempera-ture. The blank is then cold rolled at room temperature to a final thickness of 0.35 mm, for example. Subsequently at least a portion of the blank is annealed at a temperature in the range of 730°C to 880°C to form a semi-finished product com-prising a yield strength in the range of 200 MPa to 450 MPa and a coercive field strength of 0.3 A/cm to 1.5 A/cm.

The annealing temperature is chosen so that it lies between the recrystallization temperature of around 720°C and the phase transformation from the alpha, a, phase to the gamma, y, phase at around 885°C. The annealing temperature is selected within this range so that the semi-finished product has the desired mechanical properties, in particular, the desired yield strength (0.2% strain), Rp02, in combination with the desired magnetic properties, in particular, power loss den-sity.

It is observed that a combination of a niobium and/or tantalum content described by (y + 2x) , whereby y is the tantalum con-tent in weight percent and x is the niobium content in weight percent, within the range of 0.14 to 0.3 weight percent and a carbon content of less than 0.007 weight percent, or less than 0.005 weight percent, or less than 0.003 weight percent, pro-vides a soft magnetic alloy with a yield strength that can be adjusted as desired over a range of 200 MPa to 450°C by appro-priate selection of the annealing temperature. At the same time, soft magnetic properties suitable for soft magnetic parts of rotating electrical machines can be obtained.

Usefully, the yield strength and the magnetic properties can be adjusted so that the same composition can be used for both the rotor and the stator of an electrical machine by annealing the parts for the rotor and for the stator at dIfferent an-nealing temperatures. For example, parts for a rotor may be annealed at 750°C and have a higher yield strength than parts for the stator which are annealed at 870°C. In this example, the stator has significantly better magnetic properties than the rotor.

The composition, annealing conditions and measured mechanical and magnetic properties of sample alloys according to the in-vention and of comparison alloys are summarized in table 1.

In a first set of embodiments, the effect of composition on the mechanical and magnetic properties is investigated. For each sample alloy, an anneal of 750°C for 3 hours and an an-neal at 871°C for 2 hours is illustrated and connected with one another with a dotted line.

In the figures, the relationship of niobium and tantalum (y + 2x) is represented as Ta + 2 x Nb.

Figure 1 illustrates a graph of yield strength Rp02 vs. (Ta + 2 x Nb) for alloys with a low carbon content, in particular a carbon content of less than or equal to 0-007 weight percent and differing (y + 2x) The lowest achieved yield strength and the highest achieved yield strength for each sample alloy increase a similar amount with increasing Nb and Ta content(y + 2x) . The range over which the yield strength may be adjusted remains relatively large. This is useful as a single composition can comprise a larger range of yield strengths by selecting the annealing conditions.

Figure 2 illustrates a graph of yield strength Rpo.2 vs. carbon for a single Nb-und Ta-content (y + 2x) according to the in-vention. The yield strength increases with increasing carbon content. The range over which the yield strength can be ad-justed by selecting the annealing temperature is reduced for increased carbon contents. The carbon content should be kept small in order to be able to adjust the yield strength over a large range.

Figure 3 illustrates a graph of magnetic induction 3(3 A/cm) vs. (Ta + 2 x Nb) for sample alloys with a carbon content of less than or equal to 0.007 weight percent.

The magnetic induction B (3A/cm) decreases with increasing Nb and Ta content, particularly greater than 0.5. However, for the alloys having a Ta and Nb content (y + 2x) according to the invention, i.e. within the range of 3.14 to 0.3 weight percent, the decrease is moderate after an annealing treatment at 871°C for 2 hours.

Figure 4 illustrates a graph cf magnetic induction 3(3 A/cm) vs. carbon content for a Nb-und Ta-content (y + 2x) according to the invention. No significant trend is observed.

Figure 5 illustrates a graph of coercive field strength R vs. (Ta + 2 x Nb) for a carbon content of less than or equal to 0.007 weight percent. The difference in H is smaller for smaller (Ta + 2Nb) contents. The worsening effect on 1-is of the decreased annealing temperature is lower for lower Nb and Ta contents.

Figure 6 illustrates a graph of coercive field strength Hc vs. carbon content for a Nb-und Ta-content (y + 2x) of less than 0.3.

Figure 7 illustrates a graph of power loss density P(2T; 400Hz) vs. (Ta i-2 x Nb) for low carbon contents of less than or equal to 0.307 weight percent. The losses after an anneal-ing treatment of 871°C for 2 hours remain low for Ta and Nb contents (y + 2x) of less than 0.3.

Figure 8 illustrates a graph of power loss density P(2T; 400Hz) vs. Carbon content for alloys having a Nb-und Ta-content according to the invention. A carbon content of lOOppm increases the losses after an annealing treatment of 871°C for 2 hours. The carbon content should be kept low to achieve low losses.

Figure 9 illustrates a graph of magnetic induction 6(8 A/cm) vs. (Ta + 2 x Nb) for a low carbon content of maximum 0.007.

The magnetic induction 3 (BA/cm) decreases with increasing Nb and Ta content. However, for the alloys having a Ta and Nb (y + 2x) of less than around 0.3, the decrease is moderate after an annealing treatment at 871°C for 2 hours.

Figure 10 illustrates a graph of magnetic induction 8(8 A/cm) vs. carbon content for alloys having a Nb-und Ta-content ac-cording to the invention. A higher carbon content leads to a lower magnetic induction.

Figure 11 illustrates a graph of magnetic induction 8(80 A/cm) vs. (Ta + 2 x Nb) for sample alloys with a carbon content of less than or equal to 0.007 weight percent. The magnetic in- duction B (BOA/cm) decreases with increasing Nb and Ta con- tent. However, for the alloys having a *Ta and Nb content (y 4- 2x) of less than around 0.3, the decrease is moderate after an annealing treatment at 871°C for 2 hours.

Figure 12 illustrates a graph of magnetic induction 2(80 A/cm) vs. carbon content for Nb-und Ta-contents according to the invention. No significant effect is observed.

Figure 13 illustrates a graph of power loss density P(2T; 50Hz) vs. (Ta + 2 x Nb) for a carbon contents of less than or equal to 0.007 weight percent. A strong increase in losses is observed for (y + 2x) greater than 0.3.

Figure 14 illustrates a graph of power loss density P(2T; 50Hz) vs. carbon content for Nb-und Ta-contents (y + 2x) ac- cording to the invention. It is observed that alloys with in-creasing carbon contents have increased losses.

Figure 15 illustrates a graph of the range of the yield strength for an alloy annealed at 750°C for 3 hours and at 871°C for 2 hours vs. tho Ta and Nb contont (y + 2x) for C «= 0.0070%: The yield strength remains largely unaffected with increasing Ta and Nb.

Figure 1 illustrates a graph of the range of the yield strength obtained for an alloy annealed at 750°C for 3 hours and at 871°C for 2 hours vs. carbon content for 0.14 wt. % «= Ta ± 2 x Nb «= 0.30 wt. % The range over which the yield strength can be adjusted with a temperature difference of 121°C decreases with increasing carbon content. The largest range of yield strength values is achievable with a carbon content of less than 0.005 weight percent.

In a second set of embodiments, illustrated in Figures 17 to 21, magnetic properties are illustrated as a function of yield strength, Rp02.

In Figures 17 to 21, "A" denotes alloys according to the in-vention, i.e., 0.14 wt. % «= (Ta + 2 x Nb) «= 0.30 wt. %, C «= 0.0070 % and "B" denotes a comparison composition of a refer-ence alloy with (Ta + 2 x Nb) «= 0.12 wt. %, 0.0080 wt. % «= C «= 0.0120 wt. %. These reference alloys have compositions similar to those disclosed in US 3,634,072 and similar to the commer-cially available alloy Hiperco 50.

Figures 17 to 21 illustrate that the highest and lowest values of Rpo*2 are achieved with alloy "A" indicating that the yield strength is adjustable over a wider range than is achievable with the comparison alloy B. Figure 17 illustrates a graph of power loss density P(2T; 400 Hz) vs. yield strength Rpo2. The losses increase with increas-ing Rp0.2 with clearly lower losses for alloys "A".

A soft magnetic alloy with low power loss density is provided by the soft magnetic alloy according to the invention. The al-by can be used as a stator of an electric machine due to the low losses and good magnetic properties.

Figure 18 illustrates a graph of coercive field strength Ho vs. yield strength Rp0.2, Figure 19 illustrates a graph of pow-er loss density P(2T; 50 Hz) vs. yield strength Rp°2, Figure illustrates a graph of magnetic induction B(3 A/cm) vs. yield strength Rp0.2 and Figure 21 illustrates a graph of mag-netic induction 3(8 A/cm) vs. yield strength Rp0.2. Generally, the magnetic properties are observed to worsen with increasing Rp32.

Figures 17 to 21 illustrate that due to the small carbon con-tent of «= 0.007 weight percent and 0.14 weight percent «= (y + 2x) «= 0.3 weight percent of the alloy according to the inven-tion, denoted TVA!T, an extended range of values of the yield strength of around 200 MPa to around 450 MPa can be provided for a single composition compared to the composition B which has a lower value of (Ta + 2 x Nb) «= 0.12 wt. % and a higher carbon content of 0.0080 wt. % «= C «= 0.0120 wt. %.