Annealing heat treatment of laminations for a laminated core

Abstract

The present invention relates to a method for manufacturing a laminated core (15; 25), such as the core of a transformer or of a rotor (25) or a stator (15) of an electric machine from sheet metal laminations (10; 20) provided with an electrically insulating coating that includes both inorganic and organic components. According to the present invention and before these are incorporated in the laminated core (15;25), the laminations (10; 20) are subjected to an annealing heat treatment, wherein these are heated to above 815 ºC in an atmosphere containing 2 to 21 vol-% oxygen gas with balance inert gas.

Description

[0001] ANNEALING HEAT TREATMENT OF LAMINATIONS FOR A LAMINATED CORE

[0002] The invention relates to an annealing heat treatment of laminations for a layered stack of such laminations, such as is in particular used as a laminated rotor or stator core in an electric machine. The annealing of the laminations is known to improve -in particular as part of the laminated core- the mechanical and electromagnetic properties thereof, such as their magnetic permeability. The intensity of the annealing heat treatment in terms of the temperature and duration can be optimized 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 (below 625 °C; typically between 500-600 °C for electrical steel) results predominantly in stress relief (so-called recovery), whereas at intermediate temperature (between 625-750 °C; typically at about 650 °C) also recrystallisation and initial grain growth occurs and at even higher temperature (above 750 °C up to 1150 °C; typically between 800-900 °C) grain growth continues and homogenization is obtained.

[0003] Thus, for example a work hardening effect of the punching and / or blanking process steps -that is otherwise somewhat detrimental to the said electromagnetic properties-, can be removed in annealing already at a relatively low temperature. However, the said higher temperature is required for maximizing the beneficial effects of annealing in relation to the operating efficiency of the end-product electric machine. Especially in the known automotive application of the electric machine -as traction motor and generator for kinetic (i.e. braking) energy recovery-, such as in battery electric passenger cars, maximizing the operating efficiency thereof is essential for minimizing required battery capacity and weight in relation to driving range.

[0004] The individual rotor or stator laminations of the laminated core are typically produced by means of a blanking process from a sheet or strip of so-called electrical or iron-silicon (Fe / Si) steel starting material and with a thickness that is small compared to their other dimensions, such as in the range between 0.1 and 0.3 mm. It is well known that by mutually electrically insulating the laminations in the laminated core, while minimising their thickness, it is possible to minimise eddy currents in the rotor and stator cores during operation of the electric machine, as is desired to maximise the operating efficiency thereof. In fact, in the art, even thinner laminations are considered for this specific application, such as for example foils of amorphous steel with a thickness of 0.015-0.030 mm.

[0005] It is noted that the process step of lamination blanking is normally preceded by one or more successive steps of punching, whereby holes for accommodating shafts,bolts, magnets, or wire windings and / or for weight reduction or for forming cooling channels are first cut out of the starting material. Of course, other cutting processes than punching / blanking are known in the art as well, such as laser or waterjet cutting.

[0006] The thus produced laminations are mutually stacked in a desired amount to form the laminated core. It is common practice to clamp or bond the laminations of the laminated core together in order to maintain the integrity thereof in the further processing and / or handling thereof to manufacture the electric machine. To this end, it is for example known to apply a weld seam along the height of the laminated core, to provide a physical interlock between adjacent laminations (such as by so-called clinching), or to apply an adhesive, i.e. glue there between.

[0007] Moreover, regarding the mutual electrical insulation of the laminations in the laminated core, it is known to provide these -and in particular the electrical steel starting material thereof-, with an electrically insulating coating. Common coating-types are classified in ASTM A976-03. However, mostly the temperature that can be applied in annealing is limited by such coating, at least without deteriorating the insulation provided thereby or other undesirable side-effect occurring, such as the forming of a residue that prevents the laminations from being operatively bonded together by means of gluing with a typically at least partially organic adhesive.

[0008] Even ASTM C5 and C5-A(S) coatings that are specifically designed and / or specified to withstand high temperatures, typically cannot be annealed above 800-815 °C. These C5-type coatings are inorganic / ceramic substances (such as silicates, phosphates, titanates, chromates, metal oxides or combinations thereof) that are typically mixed with an organic component such as an (acrylic, colophony, isocyanate and / or polyol) resin. Mostly, a thickness of 0.5 to 3 micron is applied for C5-type coatings.

[0009] Although, a development trend towards coating-stability at an increasingly higher temperature exists, such newer C5-type coatings are proprietary and are reportedly still limited to a temperature of 840-850 °C or less, while requiring an inert atmosphere (e.g. N2 gas) or reducing atmosphere (e.g. N2 / H2-mixture) in the annealing furnace.

[0010] Against the above known technical background, the present invention aims to provide for an improved annealing heat treatment of laminations made from insulation coated electrical steel, wherein a complete and rapid recrystallisation and grain growth is realized without functionally deterioration the coating thereof.

[0011] The annealing heat treatment according to the invention entails processing the insulation coated laminations separately (i.e. before stacking) by heating the lamination to a processing temperature between 815 °C and 930 °C and in a (controlled) processing atmosphere containing between 2 and 21 vol.-% oxygen gas with balanceinert gas, such as nitrogen and / or a noble gas. At such high annealing temperature, the duration of the annealing heat treatment can be favorably short, thus limiting, i.e. minimizing the thermal deterioration of the insulation coating, while the oxygen gas is intended to at least partially remove the residue that does form at such temperature. After annealing, ambient or refrigerated air may be blown on the laminations for cooling these.

[0012] Preferably, according to the invention, the annealing temperature is maintained for a processing time sufficient to completely recrystallize the lamination (i.e. through-and-through), or between 15 and 120 seconds depending on the thickness of the lamination and the processing temperature applied. To further limit, i.e. minimizing the required processing time, the processing temperature is preferably set above 850 °C, more preferably is set in the range between 860 °C and 890 °C. Furthermore, the oxygen gas content in the annealing atmosphere preferably has a value in the range between 5 and 15 vol.-%, more preferably in the range between 8 and 12 vol.-%.

[0013] The above annealing heat treatment according to the invention, can in principle be favorably applied to all silicon-containing electrical steel compositions. Nevertheless, the best lamination annealing results are obtained for electrical steel compositions including 2.0 to 4.0 mass-% silicon (Si), 0.5 to 2.5 mass-% aluminum (Al) with Si+AI > 4.0 mass-%, <1.0 mass-% manganese (Mn), <0.005 mass-% carbon (C) mass-% and iron (Fe), specifically with balance iron (Fe) excluding negligible amounts of contaminants. More specifically, the present annealing heat treatment was developed and evaluated for electrical steel with a nominal composition of 3.2 mass-% Si, 1.0 mass-% Al, 0.2 mass-% Mn, 0.003 mass-% C and balance Fe.

[0014] In the following, the invention and its practical implementation are elucidated further with reference to the drawings, whereof:

[0015] - figure 1 provides two typical examples of known laminations of an electric machine; - figure 2 schematically illustrates how the rotor lamination and the stator lamination are integrated in an electric machine;

[0016] - figure 3 is a flow-chart representation of a possible setup of a process chain for manufacturing a laminated core incorporating rotor or stator laminations that is particularly suited for annealing the laminations in accordance with the invention; and - figures 4 and 5 illustrate a detailed implementation of a series of process steps for manufacturing individual laminations that is likewise particularly suited for annealing the laminations in accordance with the invention.

[0017] Figure 1 provides two examples of a lamination 1 that can be suitably produced with the method for manufacturing the laminated core 15; 25 discussed herein (seefigure 2). In the example depicted on the left side of figure 1, the lamination 1 takes the form of a stator ring 10 for an electric machine, in particular of a permanent magnettype synchronous machine. In the illustrated, non-limiting example of the stator lamination 10, it is shown to include several holes 11 inside its circularly-shaped outer contour that are equally spaced along its circumference, which holes 11 for example serve to accommodate assembly bolts or to channel cooling liquid. Moreover, the inner contour of the stator lamination 10 is shaped by a large number of radial-inwardly extending pole teeth 12 with an equal number of radial slots 13 therebetween, which slots 13 serve to accommodate windings of an electrical conductor 14.

[0018] In the other example of figure 1 that is depicted on the right side thereof, the lamination 1 takes the form of a rotor disc 20 for an electric machine. In the illustrated, non-limiting example of the rotor lamination 20, it is shown to include a central hole 21 that defines the inner contour of the rotor lamination 20 and that serves to receive a rotor shaft 24 of the electric machine. Moreover, within its circularly-shaped outer contour the rotor lamination 20 is provided with eight sets of four holes 22 that serve to accommodate permanent magnets 23. These sets of four magnet holes 22 each, are equally spaced along the circumference of the rotor lamination 20, i.e. with two adjacent such sets being arranged at a 45-degree angle relative to each other.

[0019] Figure 2 illustrates how the stator lamination 10 and rotor lamination 20 are integrated in the electric machine. The stator lamination 10 is incorporated in a stator core 15 that is a stack of such stator laminations 10, whereas the rotor lamination 20 is incorporated in a rotor core 25 that is a stack of such rotor laminations 20. The stator core 15 is provided with the conductor windings 14 and the rotor core 25 is provided with the permanent magnets 23 and the rotor shaft 24, whereafter it is rotatably inserted in the stator core 15.

[0020] Figure 3 is a flow-chart representation of a known setup of a process chain for manufacturing the laminated (stator; rotor) core 15; 25. In a first step of the process chain, the lamination 1; 10; 20 is either partially punched or fully blanked from electrical steel. The difference being that in the first case, the lamination 1; 10; 20 remains part of and / or connected to the (electrical steel) starting material (see figure 4), whereas in case of blanking the lamination 1; 10; 20 is fully separated from the starting material in this first process step. In a second process step, the lamination 1; 10; 20 is heat treated, in particular annealed in accordance with the present invention, whether as a separate part or “in-strip”, i.e. as part of the (sheet or strip of) starting material. In a third process step, an organic glue is applied to a main, i.e. top and / or bottom surface of the lamination 1; 10; 20 and finally, in a fourth process step, anumber of such laminations 1; 10; 20 are mutually stacked to from the laminated core 15; 25. Of course, in case of the said partially punched, i.e. in-strip processed lamination 1; 10; 20, this is first fully separated from the starting material before stacking, such as by blanking or laser / waterjet cutting.

[0021] In figures 4 and 5 a possible implementation of the above-mentioned first and second process steps is illustrated in relation to the said partial punching and in-strip further processing thereof. It being noted that, due to the high processing temperature and the -consequently- short processing time of the annealing heat treatment according to the invention, this is particularly suited for such in-strip further processing.

[0022] In figure 4, the basic setup of the lamination cutting process known from WO2023 / 126073 is schematically illustrated in relation to the rotor lamination 20 and in a plan view of a sheet metal strip 30 of starting material, in particular of high-temperature insulation coated, specifically C5-type electrical steel. Specifically in figure 4, a so-called progressive die cutting process is illustrated, with the part or parts of the sheet metal strip 30 that is or that are being cut and removed from that strip 30 in a respective cutting stage l-IV, i.e. whether by punching or blanking, are shaded. Hereby, the sheet metal strip 30 is supplied to and transported between the cutting stages l-l V in the direction of the arrow S, i.e. from the left to the right in figure 4.

[0023] In a first cutting stage I of the progressive die cutting, a set of pilot holes 40 are punched through the sheet metal strip 30 on either side thereof by means of a punch-and-die-pair (not illustrated). These pilot holes 40 serve to receive locating pins that are used to align the sheet metal strip 30 (not illustrated).

[0024] In a second cutting stage II of the known process, further (sets of) holes 21, 22 are punched through the sheet metal strip 30 by means of further punch-and-die-pairs (not illustrated), which further holes 21, 22 correspond to the said central hole 21 and the magnet holes 22 of the rotor lamination 20. Depending on the design complexity of the rotor lamination 20, i.e. of the number and shape of the holes 21, 22 to be formed therein, this second cutting stage II can potentially be subdivided into multiple substages that are carried out in sequence.

[0025] In a third cutting stage III of known process, multiple, mutually spaced elongated holes 23 are punched through the sheet metal strip 30, following the outer contour of the rotor lamination 20, such that the rotor lamination 20 remains an integral part of the sheet metal strip 30. In other words, in this third cutting stage III connecting tabs or bridges 32 remain between the rotor lamination 20 and a frame part 31 of the sheet metal strip 30, which connecting bridges 32 are defined by and between the said elongated holes 23. Depending on the design complexity of the rotor lamination 20,this third cutting stage III can potentially be combined with the (or one of the) cutting (sub)stage(s) II or can itself be subdivided into multiple sub-stages.

[0026] In a fourth cutting stage IV of the known process, the rotor lamination 20 is completely cut loose from the frame part 31 of the steel metal strip 30 by means of a blanking punch-and-die-pair (not illustrated). A number of thus blanked rotor lamination 20 are mutually stacked to form the laminated rotor core 25.

[0027] Between the initial cutting stages l-lll of punching and the final cutting stage IV of blanking of the known lamination cutting process, the then partially cut rotor lamination 20 can be conveniently processed further “in-strip”, i.e. while still connected to the frame part 31 of the sheet metal strip 30. Specifically, in the context of the present invention, such further processing FP entails annealing the rotor lamination 20 in accordance with the invention and possibly also applying glue to the rotor lamination 20 after such annealing.

[0028] Thus, the (lamination) annealing heat treatment according to the invention can be favorably embodied as an integral continuous process, as is further illustrated in figure 5. In this process embodiment, a coil 33 holding a length of the sheet metal strip 30 is reeled-off RO to supply strip 30 the to a progressive die punching machine 50 that carries out the said initial cutting stages l-lll, i.e. the partial punching of the rotor lamination 20. It being noted that such reeling-off RO being stepwise, i.e. intermittently as interrupted by the actual cutting operation(s) of the punching machine 50. The then partially punched rotor lamination 20 is transported together with the frame part 31 of the strip 30 to an annealing furnace 60 for the further processing FP thereof by the annealing heat treatment in accordance with the present invention. Thereafter, the partially punched and annealed rotor lamination 20 is transported together with the frame part 31 of the strip 30 to a blanking machine 70 that carries out the final cutting stages IV, i.e. the cutting loose of the rotor lamination 20.

[0029] The present invention, in addition to the entirety of the preceding description and all details of the accompanying figures, 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 thus represented herein is 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

CLAIMS1. A method for manufacturing steel laminations (1; 10; 20) for a laminated core (15; 25), such as a transformer core or a stator core (15) or a rotor core (25) of an electric motor, wherein the laminations (1; 10; 20) are subjected to a heat treatment process, in particular an annealing process, characterized in that the heat treatment process comprises heating the laminations (1; 10; 20) to a processing temperature between 815 °C and 930 °C in a processing atmosphere containing between 2 and 21 volume-% oxygen gas with balance inert gas, such as nitrogen and / or a noble gas.

2. The method for manufacturing steel laminations (1; 10; 20) according to claim 1, characterized in that the said processing temperature amounts to more than 850 °C and preferably has a value between 860 and 890 °C.

3. The method for manufacturing steel laminations (1; 10; 20) according to claim 1 or 2, characterized in that the said processing atmosphere comprises between 5 and 15 volume-% oxygen gas and preferably comprises between 8 and 12 volume-% oxygen gas.

4. The method for manufacturing steel laminations (1; 10; 20) according to claim 1, 2 or 3, characterized in that the laminations (1; 10; 20) are subjected to the said heat treatment process during a processing time sufficient to completely recrystallize the laminations (1; 10; 20) and preferably between 15 and 120 seconds.

5. The method for manufacturing steel laminations (1; 10; 20) according to a preceding claim, characterized in that the laminations (1; 10; 20) are made of silicon-containing electrical steel with 2.0 to 4.0 mass-% silicon (Si), 0.5 to 2.5 mass-% aluminum (Al), <1.0 mass-% manganese (Mn), <0.005 mass-% carbon (C) mass-%, with balance iron (Fe) and with Si+AI > 4.0 mass-%.

6. The method for manufacturing steel laminations (1; 10; 20) according to a preceding claim, characterized in that the laminations (1; 10; 20) are provided with an electrically insulating coating having an organic component, in particular an ASTM C5-type coating.

7. The method for manufacturing steel laminations (1; 10; 20) according to apreceding claim, characterized in that, after the said heat treatment process is completed, a glue having an organic component is applied to the laminations (1; 10; 20).

8. The method for manufacturing steel laminations (1; 10; 20) according to a preceding claim, characterized in that, before these are subjected to the said heat treatment process, the laminations (1; 10; 20) are partially cut from a sheet metal strip (30), with connecting tabs (32) remaining between the laminations (1; 10; 20) and a frame part (31) of the sheet metal strip (30), and the laminations (1; 10; 20) are subsequently completely cut loose from the sheet metal strip (30), by cutting through the said connecting tabs (32), only after the said heat treatment process is completed.

9. An electric motor with a laminated core (15; 25) comprising a stack of laminations (1; 10; 20) that are manufactured with the method according to a preceding claim.

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