WO2022128045A1

WO2022128045A1 - Annealing heat treatment

Info

Publication number: WO2022128045A1
Application number: PCT/EP2020/025593
Authority: WO
Inventors: Arjen Brandsma; Gert Jansen; Bram SPIJKERS; Jan-Willem LENDERINK; Oleg Alexandrov
Original assignee: Robert Bosch Gmbh
Priority date: 2020-12-20
Filing date: 2020-12-20
Publication date: 2022-06-23
Also published as: EP4263881A1; 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

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 a transformer core or a rotor or stator assembly 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 in the transformer as the case may be) 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, i.e. so-called normalizing, is applied to improve the mechanical, electrical and/or magnetic properties of the basic material by the recrystallization and (grain size) recovery of its microstructure after it has been cold-rolled to its final thickness. The temperature that is minimally required for recrystallization to occur depends on the composition of the metal. For example, in case of electrical steel, recrystallization occurs at a temperature above 650 degrees Celsius. However, to also effect grain size recovery (i.e. for removing the grain size refinement effect of plastic deformation) within a reasonable process time, a much higher temperature of up to 1000 deg.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, thus, 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 the individual metal parts in thickness direction. In other words, the lamination has a much smaller surface area per unit of weight compared to its constituent metal parts.

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 transformer cores and rotor and stator assemblies of electric 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 then annealed before they are mutually stacked to form the lamination. 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 and/or grain size refinement 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 insight that the recrystallization and/or and the (grain size) recovery 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 to above its recrystallization or recovery temperature, as long as the material thereof that is adjacent to the cut surfaces thereof reaches such annealing temperature. In fact, according to the invention, the recrystallization and/or recovery 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 the above respect, a 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 annealing 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 core or a rotor or stator assembly of an electric motor;

- 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 core or a rotor or stator assembly of an electric motor 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 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 annealing temperature well above the recrystallization temperature of the metal parts 1. Typically a relatively high annealing temperature of up to 1000 deg. C. is applied not only also effect grain size recovery (i.e. for removing the grain size refinement effect of plastic deformation), but also to heat the lamination 3 to the annealing temperature within a reasonable time. After annealing the metal parts 1 are mutually stacked to form the lamination 3. The lamination 3 is processed further after annealing, at least by incorporating it in an end product, such as a 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 a continuous, 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 circumference holes 113, for accommodating permanent magnets. In the electric motor the rotor lamination is inserted in the stator lamination with a preferably small (air) gap 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 particular 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 recrystallization in 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. 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. At other locations OL of the lamination 3 or its constituent parts 1 or 2, 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 annealing temperature.

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 , due to grain size refinement, 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 direction even when (grain size) recovery is to effected. In particular, the cut surface annealing heat treatment according to the invention is carried out at a temperature in the range from 750 to 810 deg. C for between 5 to 15 minutes, preferably at a temperature of about 800 deg.C for about 10 minutes.

Further according to the invention, such grain size refinement 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 grain size refined parts of the metal part 1 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 annealing heat treatment is confined to those former specific areas SA2, whereas these latter specific areas SA2 are left untreated.

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 so- called heat affected zone is normalised therein.

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 through the circumference holes 113, rather than along its inner circumference 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, 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 , specific areas SA1 of its outer circumference 111 and of its circumference holes 113, where the magnetic permeability is preferred to be high, are heated by the laser beam, whereas other areas SA2 thereof, where the magnetic permeability is preferred to be low, are 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 inner circumference 112. Induction coils are preferably also inserted in the circumference holes 113 of the rotor disc 11. If, however, this is not possible because the circumference 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, should be increased to several millimetres to include these holes 113.

The present disclosure, 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, as the case may be, 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 heated to above the recrystallization temperature, in particular to above the grain size recovery temperature, of the basic material (51), characterized in that the metal parts (1 ) are heated to above the said recrystallization or recovery temperature at the location of a part (SA1) or the whole of a cut surface (CS1) thereof, while these remain below the said recrystallization or recovery temperature at the location of another part (OL; CS2; SA2) thereof.

2. The annealing heat treatment according to claim 1 , characterized in that the metal parts (1) are intended for manufacturing a lamination (3) that in its application is exposed to a magnetic field, such as a transformer core or a rotor or stator lamination of an electric motor, wherein the metal parts (1) are heated to above the said recrystallization or recovery 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) are not heated to above the said recrystallization or recovery temperature at the location 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 1 , 2 or 3, characterized in that the metal parts (1) are heated to above the said recrystallization or recovery 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, characterized in that the said part (SA1) or the whole of the cut surface (CS1) of the metal parts (1 ) is heated by blowing hot gas along it.

6. 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 by irradiating it with a laser beam.

7. 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 by placing an induction coil alongside it that is activated by an alternating current..

8. The annealing heat treatment according to a preceding claim, characterized in that the metal parts (1) are processed therein in the form of separate mini-stacks (2) of a 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 processed therein in the form of a lamination (3) of a large number of mutually stacked metal parts (1), such as a transformer core or a rotor or stator lamination of an electric motor.

10. 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, in particular to above the grain size recovery temperature, of the basic material (51), characterized in that the metal parts (1) are processed therein in separate mini-stacks (2) of a smaller number of such metal parts (1) each, which ministacks (2) are each obtained in a multi-layer blanking process from a corresponding number of mutually stacked strips (50) of the basic material (51).