EP4263881A1

EP4263881A1 - Annealing heat treatment for a blanked metal part or a lamination of blanked metal parts

Info

Publication number: EP4263881A1
Application number: EP21839334.6A
Authority: EP
Inventors: Arjen Brandsma; Gert Jansen; Bram SPIJKERS; Jan-Willem LENDERINK; Oleg Alexandrov
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2020-12-20
Filing date: 2021-12-20
Publication date: 2023-10-25
Also published as: WO2022128045A1; CN116670306A; WO2022128162A1

Abstract

The invention relates to a heat treatment for the annealing of blanked metal parts (1; 10; 11), in particular of metal parts (1; 10; 11) obtained in a multi-layer blanking process from a multi-layered basic material (51) with a number of mutually stacked individual layers (50). According to the invention, the annealing heat treatment is limited to the cut surfaces (CS1) of the metal parts (1; 10; 11). In particular, the annealing heat treatment is limited to specific areas (SA1) of the cut surfaces (CS1) of the metal parts (1; 10; 11) where a functional magnetic field is present during use of the metal parts (1) in a lamination (3) of an electrical device.

Description

ANNEALING HEAT TREATMENT FOR A BLANKED METAL PART OR A LAMINATION

OF BLANKED METAL PARTS

The present disclosure relates to the annealing heat treatment of blanked metal parts, in particular of a lamination of such blanked metal parts, such as an electric transformer core or a rotor or stator core of an electric motor. Such laminations are well known and are produced by stacking, i.e. layering multiple, relatively thin metal parts that have been cut from a strip or sheet of basic material in a blanking process. By minimising the thickness of the individual metal parts in the lamination, it is possible to minimise eddy current losses in the electric motor or the electric transformer during operation.

In the known blanking process at least the outer contour of the metal part is cut by pressing a correspondingly shaped blanking punch against and through the basic material, which basic material is clamped between a blanking die and a blank holder of a blanking device. The blanking die and the blank holder thereto define a respective cavity that is shaped to accommodate the blanking punch. An edge of the blanking die defining the contour of the cavity thereof, carves into and finally completely cuts through the basic material, as such basic material is progressively pressed into the cavity by the movement of the blanking punch relative to the blanking die. The metal part may define cut holes that are either punched in the metal part either prior to the blanking thereof, or simultaneous therewith. The metal parts of the lamination may be blanked individually from a single strip of basic material, or several thereof simultaneously from a layered basic material in a multi-layer (fine) blanking process, such as is known from WO2019/086146 A1 that also mentions the annealing heat treatment of the metal parts.

The known annealing heat treatment is applied to improve the mechanical, electrical and/or magnetic properties of the basic material. The intensity of such annealing heat treatment in terms of the temperature and duration can be optimised according to need, as for instance described in relation to rotor and stator cores in US-10199910 B2 and JP- 5228379 B. In this respect it is known that the annealing at relatively low temperature results predominantly in stress relief (so-called recovery), whereas at intermediates temperatures also recrystallisation and grain growth occurs and at even higher temperatures grain refinement and homogenisation is obtained as a result of the phase transformation from ferrite to austenite at such higher temperatures and back again during the gradual cooling to ambient (so-called normalizing). The aforementioned annealing intensity also depend on the composition of the metal. For example, in case of electrical steel, the said stress-relief annealing is typically carried at a temperature below 650 °C, typically between 500 °C and 600 °C, recrystallization annealing is typically carried out at a temperature between 625 °C and 750 °C, e.g. at about 650 °C, and for normalizing annealing a temperature above 800 °C is applicable. Nevertheless, to complete the normalizing annealing of electrical steel within a reasonable process time, a much higher temperature of 850 °C up to 1000 °C is typically applied.

According to WO2019/086146 A1, the annealing heat treatment is carried out in relation to the blanked metal parts, i.e. after, rather than before blanking. In particular according to WO2019/086146 A1 , the work hardened basic material after rolling with a relatively high modulus of elasticity and low ductility is more favourable for the multi-layer blanking process, as compared to annealed condition thereof, by limiting plastic deformation during blanking and providing a more consistent blanking result between the simultaneously blanked metal parts.

A drawback of this latter annealing heat treatment is that the many constituent parts of the lamination are handled and heat treated individually, requiring relatively complex equipment with relatively large capacity. These requirements can, of course, be mitigated by assembling the lamination prior to the annealing heat treatment. However, in this case more time and energy is required for the annealing heat treatment. After all, the lamination extends substantially in all three dimensions, whereas its constituent metal parts are thin relative to their other two dimensions, allowing heat energy to quickly penetrate these constituent parts in thickness direction. In other words, compared to its constituent metal part, the lamination has a much smaller surface area per unit of weight.

The present invention aims to mitigate such drawbacks of the known annealing heat treatment, in particular in relation to laminations composed of metal parts obtained with the multi-layer blanking process, more in particular in relation to laminations used in electric devices, such as transformers and motors.

According to the present invention and specifically in case of the multi-layer blanking process, the simultaneously blanked metal parts are held together after blanking as a set of mutually stacked metal parts, denoted mini-stack hereinafter. Hereto, such blanked mini-stacks are handled as a whole, rather than the metal parts thereof individually. In this case, the number of blanked metal parts in each mini-stack corresponds to the number of stacked strips that constitutes the layered basic material. Preferably, the metal parts in each mini-stack are mutually connected, e.g. are glued together or are mechanically interlocked such as by means of a clinching process, more preferably such mutually connection is realised before the multi-layer blanking process in relation to the strips of basic material of the layered basic material.

These mini-stacks are annealed before they are mutually stacked, i.e. laminated. In this way, the relatively energy intensive process of annealing the complete lamination is avoided, while the number of parts to be handled and heat treated is reduced relative to annealing the metal parts individually. Thus, such mini-stack annealing provides a favourable optimum between, on the one hand, annealing equipment utilisation and annealing process time and energy consumption, on the other hand.

The invention further relies on the following considerations:

1) in blanking and punching a work hardening of the metal part occurs mainly, i.e. is largest, at the cut surfaces and extends from such cut surface into the material of the metal part over a limited depth only;

2) in the typical application of the lamination in an electric device, magnetic field hysteresis losses likewise occur mainly, i.e. are highest, at such cut surfaces of the metal part and extend into the metal part over a limited depth only.

Based on these considerations, the present invention additionally provides the separate insight that the stress-relief, recrystallization and/or normalizing annealing that is realised in the annealing heat treatment, can be favourably limited to, or at least concentrated at the cut surfaces of the metal parts. In other words, according to the invention, it is not necessary in annealing after blanking to heat the complete lamination, nor even the complete metal part, as long as the material thereof that is adjacent to the cut surfaces thereof reaches the desired annealing temperature. In fact, according to the invention, the stress-relief, recrystallization and/or normalizing annealing can even be favourably limited to, or at least concentrated at the parts of the cut surfaces where a functional magnetic field is present in use, meaning a magnetic field contributing to the operation and/or function of the electric device in question. In this latter case, however, normalizing annealing is preferred over stress-relief or recrystallisation annealing to maximise magnetic performance.

In the above respect, an annealing depth of 0.5 mm below the cut surfaces, i.e. a surface layer thickness of 0.5 mm typically suffices according to the invention, with 0.1 mm as a practical minimum value and with around 0.3 mm as a broadly applicable value.

With the novel, cut surface annealing heat treatment according to the invention, a similar (electro-)magnetic and mechanical performance of the lamination can be realised as with conventional, i.e. through-and-through annealing. However, since in this novel heat treatment only a relatively thin (surface) layer at the cut surfaces thereof is heated above the normalizing temperature of the basic material, it is considerably faster and, moreover, allows for favourable heating techniques to be applied.

In the following, the present invention and its practical implementation is elucidated further with reference to the drawings, whereof:

- figure 1 is a flow-chart representation of the known process chain for manufacturing a lamination of metal parts such as a transformer or electric motor rotor/stator cores;

- figures 2 and 3 illustrate two examples of such metal parts, namely a stator ring for a stator lamination of an electric motor and a rotor disc for a rotor lamination thereof;

- figures 4A to 4F schematically illustrate an example of a blanking process for forming metal parts for the lamination in a cross-section of a multi-layer fine blanking device;

- figure 5 is a flow-chart representation of a novel process chain for manufacturing a lamination of metal parts such as a transformer or electric motor rotor/stator cores in accordance with the present invention; and

- figure 6 indicates the cut surfaces of the stator ring and of the rotor disc in a top view thereof representing such part not only individually, but also as applied in a so-called mini-stack or a lamination thereof.

In figure 1 , the known process chain for manufacturing a lamination 3 of metal parts 1 is summarized. Firstly, strips 50 of basic material 51 are prepared by suitable and generally known process steps, such as melting, mixing/alloying, slab casting, re- melting/refining, hot and cold rolling, slitting/cutting, annealing etc. Optionally, a number of such trips 50 are mutually stacked into a layered basic material 51.

The basic material 51 (whether layered or not) is fed to a blanking device 90 that cuts the metal parts 1 out of the basic material 51, either individually from a single strip 50 of basic material 51 or in the form of a mini-stack 2 of such metal parts 1 that is blanked from a layered basic material 51 composed of several such strips 50 in a multi-layer blanking process (see figures 4A-4F). After blanking, the individual metal parts 1 are subjected to the heat treatment of normalizing annealing to improve the mechanical, electrical and/or magnetic properties thereof. Hereto, the metal parts 1 are typically placed in an oven filled with hot gas (such as air or nitrogen) at a specified temperature well above the ferrite-to-austenite phase transition temperature of the metal parts 1. Typically a relatively high temperature of up to 1000 deg. C. is applied in annealing to heat the lamination 3 to above the normalizing temperature within a reasonable time. After normalizing annealing the metal parts 1 are mutually stacked to form the lamination 3. The lamination 3 is processed further after normalizing annealing, at least by incorporating it in an end product, such as an electric transformer or an electric motor.

Figures 2 and 3 provide examples of metal parts 1 that are used to form the lamination 3. In the example of figure 2, the metal part 1 takes the form of a stator ring 10 for an electric motor. In the electric motor a number of such stator rings 10 are stacked and clamped or interconnected in axial direction to form a stator lamination. In the presently illustrated, non-limiting, example of the stator ring 10, it is shown with an effectively circular outer circumference 101 and with a series of radially oriented slots 102 along its inner circumference. These slots 102 serve to accommodate copper wire or copper bars that extend in axial direction through the whole of the stator lamination. In the example of figure 3, the metal part 1 takes the form of a rotor disc 11 of an electric motor. In the electric motor a number of such rotor discs 11 are stacked and clamped or interconnected in axial direction to form a rotor lamination. In the presently illustrated, non-limiting, example of the rotor disc 11, it is shown to with a continuous, circular outer circumference 111, a central hole 112 for accommodating a rotor shaft and with a number of holes 113 for accommodating permanent magnets. These latter magnet holes 113 each have two longer sides and two shorter sides, i.e. have a somewhat rectangular shape. The short sides of the magnet holes 113 are in relatively close proximity with (the short side of) other magnet holes 113 or with the outer circumference 111 of the rotor disc 11. In the electric motor, the rotor lamination is inserted in and is rotatable relative to the stator lamination with a preferably small (air) gap provided there between.

The figures 4A-4F illustrate a known embodiment of the blanking process step for cutting the metal parts 1 of the lamination 3, such as the stator rings 10 and the rotor discs 11, out of the strip 50 of basic material 51. This particular embodiment of the blanking process is referred to as the multi-layer fine blanking process.

The figures 4A-4F each represent a simplified cross-section of a blanking device 90 that is used to simultaneously, i.e. in a single stroke of the blanking device 90, cut-out a number of such metal parts 1 from a layered basic material 51 comprising two or more (i.e. four in the example of figures 4A-4F) of mutually stacked strips 50 of basic material 51. The blanking device 90 includes four tool parts, namely a blanking punch 30, a counter punch 40, a blank holder 70 and a blanking die 80. The blank holder 70 and the blanking die 80 each define a respective cavity 71 , 81 , wherein the blanking punch 30 and the counter punch 40 are contained, which cavities 71, 81 are shaped to correspond to the metal part 1 , i.e. to the 2D contour thereof. This particular type of blanking process/blanking device 90 using a counter punch 40 is referred to in the art as fine blanking.

In figure 4A, the blanking device 90 is shown in a first open state, wherein the blanking punch 30 is fully retracted into the blank holder 70, the counter punch 40 is fully retracted into the blanking die 80 and wherein the blank holder 70 and the blanking die 80 are separated from one another, at least sufficiently for allowing the layered basic material 51 to be inserted and/or advanced relative to the blanking device 90, as schematically indicated by the dashed arrow.

In figure 4B the blanking device 90 is shown after the blank holder 70 and the blanking die 80 have been moved towards each other to clamp the layered basic material 51 between them.

In figure 4C the blanking device 90 is shown after the blanking punch 30 and the counter punch 40 have been moved towards each other to also clamp the layered basic material 51 between them.

In figures 4D and 4E the actual cutting out a number of the metal parts 1 , as determined by the number of strips 50 of basic material of the layered basic material 51, by the forced relative movement of the combination of the blanking punch 30 and the counter punch 40 relative to the blanking die 80, is schematically illustrated. In figure 4D the blanking device 90 is shown during the actual cutting and in figure 4E the blanking device 90 is shown after the metal parts 1 are cut completely, i.e. after these have been severed from the layered basic material 51, and are still held between the blanking punch 30 and the counter punch 40 inside the said cavity 81 of the blanking die 80.

In figure 4F the blanking device 90 is shown in a second open state, wherein the blanking punch 30 is fully retracted into the blank holder 70, the layer basic material is lifted of the blanking die 80 and wherein the counter punch 40 protrudes from the blanking die 80 after pushing the metal parts 1 upwards out of the cavity 81 of the blanking die 80 to allow the extraction thereof from the blanking device 90. After such extraction, the blanking device 90 returns to its first open state shown in figure 4A etc.

During and after the said extraction thereof, a set of metal parts 1 is preferably kept together in the mini-stack 2 thereof that is obtained with the multi-layer blanking process and the lamination 3 is formed by mutually stacking these mini-stacks 2.

According to the present invention, the known process chain for manufacturing the lamination 3 and, in particular, the process step of annealing therein, can be improved upon in terms of process efficiency.

A first embodiment of the present invention is illustrated in figure 5 in a flow-chart representation of a novel process chain. The novel process chain specifically includes the multi-layer blanking process and is set apart from the known process chain by the process step of annealing the mini-stacks 2 of metal parts 1 obtained in a multi-layer blanking process before these are mutually stacked to form the lamination 3. In this case, the relatively slow heat treatment of lamination annealing (i.e. of annealing the lamination 3 as a whole) can be favourably avoided. Also, by annealing the mini-stacks 2 of metal parts 1 rather than the metal parts 1 individually, more efficient use can be made of annealing equipment such as the annealing oven. Thus, mini-stack annealing provides a favourable optimum between, on the one hand, annealing equipment utilisation and annealing process time on the other hand. Additionally or alternatively to such first embodiment, the annealing heat treatment as such (i.e. irrespective of whether it is carried out in relation to the complete lamination 3, in relation to the metal parts 1 individually or in relation to the mini-stack 2 of metal parts 1 obtained with the multi-layer blanking process) can be improved upon as well according to the invention. In this respect, the normalizing annealing is concentrated at the surfaces of the lamination 3 or its constituent parts 1 or 2 that have been cut in the basic material 51 in blanking and/or punching.

This latter, second embodiment of the present invention relies on the insight that in blanking and punching a work hardening of the metal part 1 occurs mainly, i.e. is largest, at the cut surfaces thereof and extends from such cut surface into the material of the metal part 1 over a limited depth only. In this respect, a depth of about 0.3 mm thickness was found to be generally applicable in practice. According to the invention, this limited depth or layer thickness favourably allows a relatively low annealing temperature and a surprisingly short annealing process duration even when normalizing annealing is to be realised therein. In particular, the annealing heat treatment according to the invention is carried out at a temperature between 750 to 850 °C for between 5 to 15 minutes.

In particular -as illustrated in figure 6 in relation to the stator ring 10 and the rotor disc 11-, these are heat treated specifically at those cut surfaces CS1 , or even only at a specific area SA1 thereof, where a functional magnetic field will be present in use. In this respect it is noted that, in case of the rotor disc 11, such specific area SA1 carrying a functional magnetic field in use is adjacent to the longer sides of the magnet holes 113. In case of the stator ring 10, its inner circumference with the radially oriented slots 102 carries the magnetic field in use.

At other locations OL of the lamination 3 or its constituent parts 1 or 2, possibly including other cut surfaces CS2 or another specific area SA2 of the first-mentioned cut surfaces CS1 where no (functional) magnetic field is present in use, the temperature can favourably remain below the normalizing temperature. It is noted that, in case of the rotor disc 11 , such latter specific area SA2 is adjacent to the shorter sides of the magnet holes 113 thereof. Moreover, the circumference of the inner hole 102 of the rotor disc 11 and the outer circumference 101 of the stator ring 10 do not carry such magnetic field in use.

Still, in case of the rotor disc 11, the material adjacent to the shorter sides of the magnet holes 113, i.e. the rotor material making-up the relatively narrow bridges between the shorter sides of two adjacent magnet holes 113 or between such shorter side of the magnet holes 113 and the outer circumference of the rotor disc 11, is preferably recrystalised annealed to locally improve the fatigue strength of the rotor disc 11.

Thus, according to the invention, such work hardening at the cut surfaces CS1 , CS2 can even be beneficial, at least in a certain specific area SA2 of the metal part 1. After all, the work-hardened parts of the metal part 1 not only improve (fatigue) strength but also have relative low magnetic permeability that resists the penetration of the magnetic field lines/magnetic flux in a specific area SA2 where indeed no magnetic field is desired. Thus, one and the same cut surface CS1 can include both specific areas SA1 where the magnetic permeability is preferred to be high and specific areas SA2 where the magnetic permeability is preferred to be low, such as in the known rotor disc 11. Thus, ideally, the normalizing annealing heat treatment is confined to those former specific areas SA1, whereas these latter specific areas SA2 are recrystalized annealed or even only stressrelief annealed.

In a first elaboration of this second embodiment of the present invention (not illustrated) that is particularly suited for the stator ring 10 and the rotor disc 11 illustrated in figures 2 and 3, such surface layer annealing is carried out in the annealing oven as is customary, however, with the added feature that hot gas is forced to flow (i.e. is blown) in the thickness direction of the stator ring 10 or the rotor disc 11. In particular, in case of the stator ring 10, the hot gas is specifically blown along its inner circumference and through the radially oriented slots 102, rather than along its outer circumference 101, whereas in case of the rotor disc 11, the hot gas is specifically blown along its outer circumference 111 and/or through the magnet holes 113, rather than along its central hole 112.

In a second elaboration of this second embodiment of the present invention (not illustrated) that is particularly suited for either the metal parts 1 individually or the ministacks 2 of metal parts 1 obtained with the multi-layer blanking process, such surface layer annealing is carried out by irradiating and heating part or parts of the outer and/or inner contours of the metal parts 1 by one or more laser beams. This second elaboration of the second embodiment is particularly suited for heat treating only certain specific areas SA1 of a cut surface CS1 , while leaving other specific areas SA2 of that cut surface CS1 untreated. In particular, in case of the stator ring 10, the whole of its inner circumference with the radially oriented slots 102 is heated by the laser beam, while its outer circumference 101 is left untreated. In case of the rotor disc 11, at least the said specific areas SA1 of its outer circumference 111 (where the magnetic permeability is preferred to be high), are heated by the laser beam, whereas for example the circumference of its central hole 112 is left untreated.

In a third elaboration of this second embodiment of the present invention (not illustrated) that is particularly suited for either the complete lamination 3 or the mini-stack 2 of metal parts 1 obtained with the multi-layer blanking process, the surface layer annealing is carried out by inductively heating the outer and/or inner contours of the metal parts 1 by one or more induction coils that are energized with an alternating current. The frequency of such coil current to a large extend determines the penetration depth of the induction heating that can thus be favourably set to correspond to the desired depth of, for instance, 0.3 mm. In this respect a frequency of -1 ,000 Hertz is required, which relatively high frequency also enables a favourably quick heating to the required annealing temperature. In particular case of the stator ring 10, an induction coil is placed inside its inner circumference with the radially oriented slots 102, but not around its outer circumference 101 , whereas in case of the rotor disc 11 , an induction coil is placed around its outer circumference 111, but not inside its central hole 112. Induction coils are preferably also inserted in the magnet holes 113 of the rotor disc 11. If, however, this is not possible because the magnet holes 113 are too small to accommodate an induction coil, the depth of the heat penetration from the outer circumference 111 of the rotor disc 11 by the said indication coil placed around it, can be increased to several millimetres to include these holes 113.

Yet further according to the invention and in particular if the metal parts 1 of the lamination 3 are welded together, as is typically the case, the annealing heat treatment according to the invention is carried out in relation to the lamination 3 rather than in relation to the metal parts 1 or to the mini-stack 2 of metal parts 1 , such that also the weld seam and the so-called heat affected zone thereof is annealed therein. In figure 6 such welds W are schematically indicated as black dots in relation to the stator ring 10 and the rotor disc 11. These welds W extend along the height of the lamination 3, i.e. in the (combined) thickness direction of the metal parts 1 therein. These welds are typically located where these do neither interfere with the function of the lamination, e.g. do not interfere with the said functional magnetic field, nor are these highly stressed during use. Therefore, stress-relief annealing or possibly recrystallisation annealing typically suffices for the welds W.

The present invention, in addition to the entirety of the preceding description and all details of the accompanying drawings, also concerns and includes all the features of the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as non-binding examples of the respective features. The claimed features can be applied separately in a given product or a given process, but it is also possible to apply any combination of two or more of such features therein.

The invention(s) represented by the present disclosure is (are) not limited to the embodiments and/or the examples that are explicitly mentioned herein, but also encompasses amendments, modifications and practical applications thereof that lie within reach of the person skilled in the relevant art.

Claims

1. An annealing heat treatment of metal parts (1) that are cut, in particular blanked, from a basic material (51), wherein the metal parts (1) are temporarily heated to above the stress-relief temperature of the basic material (51), in particular to above the recrystallization temperature thereof, more in particular to above the ferrite-to-austenite phase transformation temperature thereof, characterized in that the metal parts (1) are heated to above the said stress-relief temperature, in particular to above the recrystallization temperature, more in particular to above the said ferrite-to-austenite phase transformation temperature at the location of a part (SA1) or the whole of a cut surface (CS1) thereof, while these remain below such annealing temperature at the location of another part (OL; CS2; SA2; W) thereof.

2. The annealing heat treatment according to claim 1 , wherein the metal parts (1) are intended for a lamination (3) that in its application is exposed to a magnetic field, such as an electric transformer core or a rotor or stator core of an electric motor, characterized in that the metal parts (1) are heated to above the said recrystallization temperature, preferably to above the said ferrite-to-austenite phase transformation temperature at the location of a part (SA1) or the whole of a cut surface (CS1) thereof where these are exposed to a functional magnetic field in the said application of the lamination (3).

3. The annealing heat treatment according to claim 2, characterized in that the metal parts (1) remain below the said recrystallization temperature, respectively below the said ferrite-to-austenite phase transformation temperature at the location (OL; CS2; SA2; W) where these are not exposed to a functional magnetic field in the said application of the lamination (3).

4. The annealing heat treatment according to claim 2 or 3, characterized in that the metal parts (1) are heated to above the said recrystallization temperature, respectively the said ferrite-to-austenite phase transformation temperature at the location of the said part (SA1) or the whole of the cut surface (CS1) thereof in a surface layer having a thickness of between 0.1 and 0.5 mm and preferably of about 0.3 mm.

5. The annealing heat treatment according to a preceding claim, wherein the metal parts (1) are stator rings (10) intended for an electric motor stator core lamination (3), each stator ring (10) having an inner circumference with radially oriented slots (102), characterized in that, the whole of the inner circumference with radially oriented slots (102) of the stator rings (10) is heated to above the said recrystallization temperature, preferably to above the said ferrite-to-austenite phase transformation temperature.

6. The annealing heat treatment according to a preceding claim, wherein the metal parts (1) rotor discs (11) intended for an electric motor rotor core lamination (3), each rotor disc (11) having magnet holes (113) with longer sides and shorter sides for accommodating permanent magnets, which magnet holes (113) are located near an outer circumference (111) of the rotor disc (11), characterized in that, the parts (SA2) of the rotor discs (11) located between the shorter sides of its magnet holes (113) and/or between such shorter sides and its outer circumference (111) are heated to above the said recrystallization temperature, but not to above the said ferrite-to-austenite phase transformation temperature.

7. The annealing heat treatment according to claim 6, characterized in that other parts (SA1) of the rotor discs (11) located between the said longer sides of its magnet holes (113) and its outer circumference (111) are heated to above the said ferrite-to-austenite phase transformation temperature.

8. The annealing heat treatment according to a preceding claim, characterized in that the metal parts (1) are thus annealed in the form of separate mini-stacks (2) of a small number of such metal parts (1) each, which mini-stacks (2) are each obtained in a multilayer blanking process from a corresponding number of mutually stacked strips (50) of the basic material (51).

9. The annealing heat treatment according to one of the claims 1 to 7, characterized in that the metal parts (1) are thus annealed in the form of a lamination (3) of a large number of mutually stacked metal parts (1), such as an electric transformer core or a rotor or stator core of an electric motor.

10. The annealing heat treatment according to claim 9, characterized in that the mutually stacked metal parts (1) of the lamination (3) are fixed together by one or more welds (W), characterized in that the welds (W) of the lamination (3) are heated to above the said stress-relief temperature, but not to above the said ferrite-to-austenite phase transformation temperature, preferably not to above the said recrystallization recrystallisation temperature.

11. The annealing heat treatment according to a preceding claim, characterized in that the said part (SA1) or the whole of the cut surface (CS1) of the metal parts (1) is heated therein by forcing hot gas along it.

12. The annealing heat treatment according to a preceding claim, characterized in that the said part (SA1) or the whole of the cut surface (CS1) of the metal parts (1) is heated therein by irradiating it with a laser beam.

13. The annealing heat treatment according to a preceding claim, characterized in that the said part (SA1) or the whole of the cut surface (CS1) of the metal parts (1) is heated therein by placing an induction coil alongside it that is activated by an alternating current.

14. The annealing heat treatment according to a preceding claim, characterized in that both a stator core lamination (3) and a rotor core lamination (3) of an electric motor are processed therein, whereof the stator core lamination (3) is composed of a stack of stator rings (10) that are fixed together in the lamination (3) by one or more welds (W), each stator ring (10) having an inner circumference with radially oriented slots (102), and whereof the rotor core lamination (3) is composed of a stack of rotor discs (11) that are likewise fixed together in the lamination (3) by one or more welds (W), each rotor disc

(I I) having magnet holes (113) with longer sides and shorter sides for accommodating permanent magnets, which magnet holes (113) are located near an outer circumference

(I I I) of the rotor disc (11), wherein:

- the whole of the inner circumference of the stator rings (10) with the radially oriented slots (102) is heated to above the said ferrite-to-austenite phase transformation temperature;

- parts (SA2) of the rotor discs (11) located between the shorter sides of its magnet holes (113) and/or between such shorter sides and its outer circumference (111) are heated to above the said recrystallization temperature, but not to above the said ferrite-to-austenite phase transformation temperature, whereas other parts (SA1) of its outer circumference (111) are preferably heated to above the said ferrite-to-austenite phase transformation temperature; and wherein:

- the welds (W) of both the stator core and rotor core laminations (3) are heated to above the said stress-relief temperature, but not to above the said ferrite-to-austenite phase transformation temperature, preferably not to above the said recrystallization temperature.

15. An annealing heat treatment of metal parts (1) intended for a lamination (3) of a large number of such metal parts (1), wherein the metal parts (1) are heated to above the recrystallization temperature of the basic material (51), in particular to above the ferrite-to- austenite phase transformation temperature thereof, characterized in that the metal parts

(1) are subjected to the annealing heat treatment in the form of mini-stacks (2) of a small number of such metal parts (1) each, which mini-stacks (2) are each obtained in a multilayer blanking process from a corresponding number of mutually stacked strips (50) of the basic material (51).