ELECTRO SHEET WITH STRUCTURED SURFACE FOR DOMAIN REFINERATION

Description

[0001] The present invention relates to a method for producing an electrical steel sheet. Furthermore, the invention relates to an electrical steel sheet produced by said method.

[0002] Electrical machines consist of variously arranged windings through which electric current flows. The resulting magnetic flux is guided in a targeted manner within a magnetic circuit, also known as a magnetic or iron core. This core is made of materials that can conduct magnetic flux well, such as laminated electrical steel. The layering serves to prevent unwanted eddy currents.

[0003] Standard cores are made from stamped individual sheets, which were previously insulated by layers of paper glued to one side, but in more modern forms by chemically applied phosphating layers. The sheet thickness for normal applications in electrical machines is generally between 0.35 mm and 0.50 mm. For electrical transformers of higher frequencies or particularly low-loss devices, thinner, grain-oriented sheets with a thickness of 0.35 mm are used. Cut-core and toroidal cores are often wound from even thinner, similarly insulated strips. So-called NO20 sheets with a thickness of 0.20 mm are also already being discussed.

[0004] During operation of magnetic cores, the changing polarity of the magnetic field in the core causes losses called iron losses or core losses. These are the sum of hysteresis losses, eddy current losses, excess or additional losses, and residual losses.

[0005] Excess or additional losses, also known as anomalous eddy current losses, are attributed to the energy required to magnetize magnetic elements due to the displacement of the Bloch walls. For high inductions, they are negligible compared to hysteresis and eddy current losses; for low inductions, they can play a significant role.

[0006] It is known that anomalous eddy current losses can be reduced by structuring the surfaces of electrical steel sheets. The surface must be structured in such a way that domain refinement, i.e., a beneficial influence on the magnetized domains, also known as Weiss domains, in the electrical steel sheet is achieved.

[0007] An exemplary surface structuring for domain refinement is disclosed in European patent application EP 0 869 190 A1. Here, electrical steel sheets are characterized with a large number of grooves on the surface, which have a groove width of 50 µm to 1000 µm and a groove depth of 10 µm to 50 µm.

[0008] It is also known to create surface structures by scratching, etching or using a laser.

[0009] All the processes and structures mentioned have so far only been applied to conventional rolled electrical steel sheets (see, for example, US 4 085 603 A).

[0010] Screen printing and stencil printing represent new processes for the production of electrical steel sheets for electrical machines. In these processes, a printing paste is first created from metal powders. This is then processed into a thick film, also known as a green body, using screen or stencil printing techniques. The resulting green body is then converted into a metallic sheet through thermal treatment.

[0011] Another process potentially suitable for producing electrical steel is tape casting. The tape casting process involves dispersing a finely ground powder in a solvent mixture with suitable dispersants, adding organic binders and plasticizers, and processing it into a viscous casting slip. The material is formed using a doctor blade, applying the doctor blade method. During the drying process, the solvent is completely removed, resulting in a flexible, cuttable, and punchable green film. The organic components contained in the film are completely removed during the debinding and sintering processes. The height of the cast film can be influenced by the slip rheology and the doctor blade gap.

[0012] The anomalous eddy current losses mentioned above can also occur in electrical steel sheets that are not conventionally rolled but are produced using a printing or foil casting process.

[0013] The task is therefore to reduce the anomalous eddy current losses in electrical sheets produced in this way.

[0014] The invention is based on the features of the independent claims. Advantageous developments and refinements are the subject of the dependent claims. Further features, possible applications, and advantages of the invention will become apparent from the following description and the figures.

[0015] The invention relates to a method for producing an electrical sheet comprising the following steps: a) providing a green body on a substrate, b) structuring at least one surface of the green body, and c) thermally treating the green body.

[0016] Domain refinement to reduce anomalous eddy current losses in electrical steel sheets is currently only known for rolled electrical steel sheets. In this case, the surface of the finished electrical steel sheet is structured by scribing, etching, or laser treatment.

[0017] An obvious solution to the problem of reducing anomalous eddy current losses in newly manufactured, non-rolled electrical steel sheets would be to apply the above-mentioned and known surface structuring techniques to printed or foil-drawn electrical steel sheets. Specifically, this would involve structuring the surface of a finished printed or foil-drawn electrical steel sheet by scribing, etching, or laser treatment.

[0018] This would actually result in a reduction of the anomalous eddy current losses in the electrical steel sheet.

[0019] However, the present invention takes a completely different approach, which not only differs from the obvious solution described, but also has decisive advantages over it.

[0020] In the process according to the invention, the surface of the green compact is structured, not the surface of the finished electrical steel sheet. After the printing paste or casting slurry has dried and a green compact is formed, the surface of the green compact is structured. Only then does the thermal treatment of the green compact take place.

[0021] The surface structuring applied to the green body is largely preserved by the thermal treatment. It is preserved at least to the extent that it can fulfill its purpose of domain refinement.

[0022] An important advantage of the process according to the invention is that damage to the crystal structure in the green compact, which usually occurs during surface structuring, can be healed by the subsequent thermal treatment of the green compact. No separate process step is required for this, since the thermal treatment is necessary for debinding and sintering the green compact anyway.

[0023] As a result, the clever sequence of process steps (production of the green body - structuring of the surface - thermal treatment of the structured green body) makes it possible to achieve a significant reduction of anomalous eddy current losses in a printed or foil-drawn electrical steel sheet with relatively little additional effort.

[0024] The procedural steps a) to c) must necessarily be carried out in the specified order.

[0025] To avoid any misunderstandings, the following clarification regarding the term "electrical sheet" used in this patent application follows: For the purposes of this patent application, "electrical sheets" refers not only to rolled sheets as known from the prior art, but also to formed bodies produced using printing techniques or foil drawing, which exhibit the function and properties of conventional electrical sheets. The printed or drawn electrical sheets can also be referred to as "material layers"; this term should be considered a synonym for "electrical sheets." In technical circles, electrical sheets are also referred to as "magnetic sheets" or, depending on their intended use, as dynamo, motor, or transformer sheets.

[0026] A sintered electrical steel sheet, i.e. an electrical steel sheet produced, for example, by means of a printing technique or foil drawing including a thermal treatment, can be clearly distinguished in its microstructure from an electrical steel sheet produced by other processes, for example rolling.

[0027] A "green compact" refers to a blank that has not yet been sintered. It can also be referred to as a green body or, in the case of foil drawing, as a green foil.

[0028] The green body provided in step a) can be obtained in various ways.

[0029] On the one hand, a printing paste can be applied to a substrate using screen or stencil printing and then dried.

[0030] Screen printing is a printing process in which a printing paste is printed using a squeegee through a screen, e.g. a fine-mesh fabric, onto the material to be printed, in this case the substrate. In those areas of the fabric where no printing paste is to be printed according to the print image, the mesh openings of the fabric are made opaque with a stencil. The fabric supports the plastic stencil, for which the entire surface of the stretched fabric is coated with a photopolymer and exposed to the motif to be printed via a positive film. The photopolymer hardens in the areas not to be printed, and the unexposed material is washed out. During the printing process, the printing paste only seeps through the fabric where it has been washed out.

[0031] When stencil printing without a supporting screen, the stencil itself must be sufficiently strong and can be made of steel, for example, and stretched directly into the frame. However, the possible print images are limited with stencil printing.

[0032] The substrate on which the printing paste is printed can be self-supporting, such as a plate. Alternatively, flexible substrates, such as films, are also possible.

[0033] The printing paste is usually based on a metal powder. Alternatively, a casting slip can be applied to a substrate, which is often designed as a film, and then dried.

[0034] The casting slip comprises, for example, a finely ground powder in a solvent mixture. Suitable dispersants (mixtures of substances to accelerate or facilitate dispersion) are advantageously added to disperse the powder in the solvent. Optionally, the casting slip also contains organic binders and / or plasticizers to achieve the desired viscosity of the casting slip.

[0035] The green body is shaped using a doctor blade. This advantageously uses the doctor blade process, which has been used for decades in rotary and gravure printing.

[0036] During the drying process, the solvent is completely removed and a flexible, cuttable and punchable green film is created.

[0037] The method according to the invention for producing an electrical steel sheet therefore relates specifically to the production of printed, in particular screen- or stencil-printed, or drawn, in particular foil-drawn, electrical steel sheets.

[0038] Structuring the surface of the green body can be done in various ways and with various means. A non-exhaustive list of the different ways to structure the surface includes: Structuring of the surface by additive manufacturing processes; Structuring of the surface by producing the green body on a structured substrate; Structuring of the surface by embossing the structure; Structuring of the surface by local heating of the surface.

[0039] Additive manufacturing processes ( englisch: Additive Manufacturing ) ,Also known as 3D printing, these are manufacturing processes in which material is applied layer by layer to create three-dimensional objects. The layered construction of the object is computer-controlled, using one or more materials according to specified dimensions and shapes.

[0040] The additive structuring of the surface of the green body is achieved by adding elevations to the surface of the green body.

[0041] On the one hand, a material identical to the material of the green compact can be used. Alternatively, a material that is decidedly different from the material of the green compact can be used.

[0042] For the additive structuring of the surface, a ferromagnetic or ferrimagnetic material, in particular a soft magnetic ferrimagnetic material, can advantageously be used. Soft magnetic ferrites have very low electrical conductivity and can therefore have an insulating effect. Examples of soft magnetic ferrites that can be used include manganese-zinc ferrites (MnZn) with the composition Mn a Zn (1-a) Fe 2 O 4 or nickel-zinc ferrites (NiZn) with the composition Ni a Zn (1-a) Fe 2 O 4.

[0043] The structuring of the surface of the green compact by manufacturing the green compact on a structured substrate is achieved by providing the substrate with a surface profile on the side facing the green compact. This results in the green compact being structured on the side facing the substrate in a manner complementary to the surface profile of the substrate.

[0044] If the green compact is produced using a screen printing process, for example, the underside of the green compact is structured by permanently applying a pattern, such as grooves, to the top surface of the substrate. When the printing paste is then pressed through the screen, not only is the surface of the substrate generally covered with the printing paste, but the grooves in the substrate are also specifically filled with the printing paste. After the printing paste dries, a green compact is created with a series of elevations on its underside that complementarily replicate the grooves on the top surface of the substrate.

[0045] Analogously, it is also possible, for example, to structure the film, which is transferred to the dried casting slip, i.e. to the green body produced by means of the film drawing process.

[0046] The surface is structured using an embossing process after the green body has at least sufficiently dried. For this purpose, an embossing stamp is pressed onto the surface to be structured. Considerable force can be applied. Embossing processes include cold, warm, and hot stamping.

[0047] Finally, the structuring of the surface of the green body can also be achieved by locally heating the green body partially using electromagnetic radiation.

[0048] One advantage of this process is that it is contactless. Another advantage is that, in principle, very fine structures can be created on the surface of the green body.

[0049] The local heating of the green body by means of electromagnetic radiation includes the evaporation and compaction of material near the surface of the green body.

[0050] Heating can be achieved using a laser, such as a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. Alternatively, the green body can also be heated using microwaves, i.e., electromagnetic radiation in the range of 1 mm to 300 mm.

[0051] Laser vaporization, also known as laser ablation, is the removal of material from a surface by bombarding it with pulsed laser radiation. The high-power-density laser radiation used in this process leads to rapid heating and the formation of a plasma on the surface. With laser pulses in the nanosecond range, the laser energy heats the surface (in the sense of thermal motion of the atoms) during the laser pulse. Since heat conduction only allows for slow energy transfer into the volume, the radiated energy is concentrated on a very thin layer (approximately 1 µm with a 10 ns pulse length). As a result, the surface reaches very high temperatures, and the material evaporates suddenly.

[0052] The surface structure, for example grooves, which was applied to the green body by laser ablation, is retained even after the thermal treatment at least to such an extent that a domain refinement of the electrical steel sheet is achieved.

[0053] When densifying the near-surface material of the green compact through local heating, however, the treated material is not evaporated but merely "compressed." The effect of this densification is that the sintering behavior is modified in the subsequent thermal treatment of the green compact, resulting in a structuring of the surface.

[0054] If the green compact is produced by applying a casting slip to the substrate, another option for structuring the surface of the green compact is the use of a focused spray jet. Depending on the casting slip used, the spray jet is designed to spray water or a solvent onto the surface of the green compact in a focused manner. This allows the surface of the green compact, particularly the organic binders present there, to be selectively dissolved, and material to be flushed out. Here, too, the resulting structuring is retained after thermal treatment, at least to the extent that domain refinement of the electrical steel sheet is achieved.

[0055] Finally, the invention also relates to an electrical steel sheet produced by one of the aforementioned methods.

[0056] Such electrical sheets can advantageously be used in an electrical machine. This includes rotating electrical machines, in particular electric motors and electric generators for energy conversion, as well as stationary electrical machines, in particular transformers for converting an input AC voltage into an output AC voltage.

[0057] The invention will now be described by way of example with reference to the attached drawings, in which Fig. 1 shows a first embodiment of a method for surface structuring a green compact, Fig. 2 shows a second embodiment of a method for surface structuring a green compact, Fig. 3 shows a third embodiment of a method for surface structuring a green compact, and Fig. 4 shows a fourth embodiment of a method for surface structuring a green compact.

[0058] The Fig. 1 schematically illustrates a green compact 10, one surface of which has been structured using an additive manufacturing process. Specifically, several elevations 20 were applied layer by layer to the surface of the green compact 10. Additive manufacturing processes known to those skilled in the art are used for this purpose. After the elevations 20 have been formed, the green compact 10 is subjected to a thermal treatment, which both debinding and sinters the green compact 10 and cures the crystal structure of the elevations 20.

[0059] In concrete terms, Fig. 1 In the example shown, the green body 10 consists of a different material than the elevations 20 applied in layers. However, it may alternatively be advantageous to choose the same material for the elevations 20 as for the green body 10 itself.

[0060] The Fig. 2 schematically illustrates a green body 10, one surface of which has been structured by producing the green body 10 on a structured substrate 30.

[0061] The substrate 30 has a first side 31 and an opposite second side 32. The first side 31 is intended for the green compact 10 or its starting material to be applied to it. In the case of a printing process for producing the green compact 10, the first side 31 of the substrate 30 is therefore intended for the printing paste to be applied to it. In the case of film drawing for producing the green compact 10, the first side 31 of the substrate 30 is intended for the casting slip to be applied to it.

[0062] The substrate 30 is essentially planar, ie it has a high level of planarity. However, it has several elevations 20 on the first side 31. If the material forming the green compact 10 (e.g. the printing paste or the casting slip) is then applied to the substrate 30, the side of the green compact 10 facing the substrate 30, which is in Fig. 2 as the second side 12, a structuring complementary to the structuring of the first side 31 of the substrate 30. In the specific example of elevations 20 on the substrate 32, the green compact thus has corresponding grooves 21. The side opposite the second side 12 of the green compact, which is referenced as the first side 11, is, however, essentially planar, i.e., unstructured.

[0063] The Fig. 3 schematically illustrates a green body 10, one surface of which is structured by an embossing process.

[0064] For this purpose, the green compact 10 should be dried to such an extent that the imprint remains permanently after the removal of the stamping die. On the other hand, the green compact must still be malleable; however, this is usually achieved by applying a sufficiently large force to the workpiece, i.e., the green compact (think, for example, of the minting of coins). The stamping die 40 is pressed onto the surface of the green compact 10 to be structured, which, in the specific embodiment, Fig. 3 has several grooves 21. A complementary structuring is achieved, here corresponding elevations 20 on the green body 10, by pressing the areas between the grooves 21 together or away. After removal of the embossing die 40 (see the direction of movement 41 of the departing embossing die 40 in Fig. 3 ) the structuring of the green body 10 is permanently preserved.

[0065] The Fig. 4 schematically illustrates a green body 10, one surface of which is processed by several lasers 50.

[0066] The lasers 50 emit electromagnetic radiation 51 with high power density. These are pulsed lasers 50 with laser pulses in the nanosecond range. This leads to rapid heating and the formation of plasma in the affected areas. Due to the very high temperatures, a sudden evaporation of material occurs. This results in the Fig. 4 visible grooves 21. The grooves 21, which were applied to the green body 10 by means of laser ablation, are retained even after the thermal treatment at least to such an extent that a domain refinement of the electrical sheet is achieved.

Claims

1. Method for producing an electrical steel sheet, wherein the method comprises the following steps: a) providing a green body (10) on a substrate (30), b) structuring at least one surface of the green body (10), and c) thermally treating the green body (10) characterised in that the provision of the green body (10) comprises the following steps: applying a printing paste onto the substrate (30) by means of screen printing or stencil printing; and drying the printing paste situated on the substrate (30) to obtain the green body (10); or applying a castable slurry to the substrate (30), which is configured, in particular, as a tape, and drying the castable slurry situated on the tape to obtain the green body (10).

2. Method according to claim 1, wherein the structuring of the surface of the green body (10) takes place by means of additive structuring of the surface by means of an additive production method.

3. Method according to claim 2, wherein the additive structuring of the surface takes place by adding elevations (20) with a material that is identical to the material of the green body (10).

4. Method according to claim 2, wherein the additive structuring of the surface takes place by adding elevations (20) with a material that is different from the material of the green body (10).

5. Method according to one of claims 2 to 4, wherein the additive structuring of the surface takes place with a ferromagnetic or ferrimagnetic material, in particular, a soft ferrimagnetic material.

6. Method according to claim 1, wherein the structuring of the surface of the green body (10) is achieved in that the substrate (30) has a surface profile on the side facing toward the green body (10) so that the green body (10) is structured complementary to the surface profile of the substrate (30) on the side facing toward the substrate (30).

7. Method according to claim 1, wherein the surface structure of the green body (10) is stamped before its thermal treatment.

8. Method according to claim 1, wherein the structuring of the surface is achieved in that the green body (10) is locally partially heated by means of electromagnetic radiation.

9. Method according to claim 8, wherein the heating is realised by means of a laser (50).

10. Method according to claim 8, wherein the electromagnetic radiation lies in the microwave range.