USE OF A Cr-Al STEEL IN LAMINATED MAGNET CORES

Abstract: Use, in laminated cores for electrical apparatuses and machines, which cores are to conduct electromagnetically induced magnetic alternating fluxes, of a thin plate of a ferritic stainless steel which besides Fe contains in weight-%, 10-30 Cr, 4-12 Al and Si from traces and up to max 0.7 Si.

Description: USE OF A Cr-Al STEEL IN LAMINATED MAGNET CORES TECHNICAL FIELD The invention relates to a new use of a ferritic stainless steel in laminated cores for electrical machines and apparatuses, which cores are to conduct electromagnetically induced magnetic alternating fluxes under high frequency. The ferritic stainless steel which is to be used and laminated in the cores is in the form of a thin plate. Most typical applications of the invention in electric apparatuses having laminated cores, are transformers, rotating electrical machines and ballasts. PRIOR ART A magnetic core used in applications with alternating magnetic fields is always constructed in laminated form by stacking of cut or punched thin sheet, or in some cases, wound strip. Typical thicknesses are 0.50 mm and 0.35 mm, but even thinner gauges are sometimes used. The reason for laminating is to reduce the eddy currents induced in the core and to reduce the associated losses. To accomplish this purpose, the development has been towards constantly thinner sheets. In order to reduce the eddy current losses, it is common practice to increase the specific electrical resistance of the sheet by the iron being alloyed mainly by silicon. The use of silicon however results in a decrease in the ductility of the alloy, which limits the amount of silicon that may be added, whilst retaining the ability for the sheet material to be rolled to the desired thickness. In order to increase the ductility of the sheet material, it is known from EP 0 915 179-A2 to alloy the material by chromium, which makes it possible to considerably increase the amount of added silicon. Chromium both improves the ductility and, just as silicon, improves the resistivity. In the indicated publication it is thus reported that a steel containing 18.3 % Cr and 6.4 % Si exhibited a specific resistivity of 133 μΩ cm and that the material exhibited a ductility sufficient at least to enable hot rolling. This material also contains a small amount of aluminium. It is further known fromEP 0408 281-B1 that a stainless steel with up to as much as 35 % Cr and also containing 0.03- 1.89 % Al can be used for electromagnetic components, which material besides good electromagnetic properties also has good corrosion resistance and can be cold worked, i.e. has good ductility. EP 0 601 854- A2 reports properties of a number of electromagnetic stainless steels, one of them being a steel which contains 18.0 % Cr and 2.9 % Al and another being a steel which contains 10.5 % Cr and 6.8 % Al. The former has an electric resistivity of 119 μΩ cm and the latter an electric resistivity of 115 μΩ cm. JP 57-192246 presents a steel which contains 25 Cr and 4 Al and which has an electric resistivity of 125 μΩ cm. As far as known to the applicant, these reported results have however not been implemented in a product which can be used in practice and/or which is available on the market and which forms a solution to high performance requirements. There is still scope therefore for improvements in the present area. BRIEF ACCOUNT OF THE INVENTION A material adapted to be used as a magnetic core in electrical apparatuses and machines should generally satisfy the following criteria: - the material should exhibit low specific magnetic resistance in order to allow a high magnetic flux density, i.e. a high saturation for the magnetic flux density, normally implying a high iron content in the material, - the material should exhibit a high electrical resistivity to suppress the formation of eddy currents in the core. An electrical resistivity above 100 μΩ cm, preferably at least 120 μΩ cm, even more preferred above 125 μΩ cm and suitably at least 130 μΩ cm, is desirable for the material to be used at a magnetic alternating field having a frequency of 400 Hz or higher, - the material should exhibit a ductility good enough to enable rolling of the material to strips or sheets having a thickness down to at least 0.05 mm and preferably down to 0.02 mm, - the material should give a low power loss also at very high frequencies. Moreover, it is an advantage if the material has a good corrosion resistance. It is also a requirement that the manufacturing costs are not unacceptably high. According to the present invention, the above requirements can be achieved by the use of a steel which not necessarily contains any considerable amount of Si (Si may be allowed from contamination content and up to max 0.7 Si according to a widest aspect of the invention), but which contains 10-30 Cr and 4-12 Al. Contents mentioned throughout this text are all in weight-%. Additional characteristics and aspects of the invention are clear from the subsequent, dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS In the drawing figure, there is shown a surface in a coordinate system in the form of a %Al/%Cr diagram. The frame of this surface and the area within the frame defines steels having Al and Cr contents that in said steels yield a resistivity above 125 μΩ cm. STUDIED STEELS The chemical compositions of the studied steels are presented in the table below. Of these steels, no. 1-6 are steels according to prior art. The electric resistivity is also presented in Table 1. According to prior art, a maximum resistivity 125 μΩ cm is achieved for steel no. 6, which contains 4% Al and 25% Cr. Steel no. 7 has a composition according to the invention. The composition presented in Table 1 is the nominal composition. The carbon content was max about 0.01% after decarburisation. The properties of the steel have been studied and these studies are presented in Example 1 and 2 below. Steels no. 8-16 are represented in the table by the nominal compositions of the steels and the calculated electric resistivities. These steels too have a carbon content after decarburisation less than 0.02% or max 0.01%. Table 1 Chemical composition in weight-% and electric resistivity for studied steels EXAMPLE 1 Steel no. 7 having the nominal composition according to Table 1 was rolled to a thickness of 0.05 mm. The resistivity was measured to 138 μΩ cm. The thin strip was annealed at 800°C for 20 min in an atmosphere of nitrogen with 3% hydrogen and a dew point of 40°C. The power loss for this material was measured to be 12.5 W/kg at 10 kHz, Bpk = 0.1 T. (Bpk = Peak Magnetic Flux Density. T = Tesla.) EXAMPLE 2 The same thin strip, thickness 0.05 mm, as in Example 1, having a resistivity of 138 μΩ cm was annealed in a decarburising atmosphere, whereby the carbon content was lowered to max about 0.01%. The measured power loss after annealing and decarburisation was in this case measured to be 11.5 W/kg at 10 kHz, Bpk = 0.1 T. In the diagram of the figure, the position of the studied steels in the coordinate system is shown. The points for the different steels are identified by the numbers of the steels according to the table and the electric resistivity of the respective steel is shown within parenthesis. The corner points A-H for the figure in the diagram have the following coordinates: The steels according to the invention, having a resistivity above 125 μΩ cm, are positioned within the frame of the figure having said corner coordinates.

Claims: CLAIMS 1. Use, in laminated cores for electrical apparatuses and machines, which cores are to conduct electromagnetically induced magnetic alternating fluxes, of a thin plate of a ferritic stainless steel which besides Fe contains in weight-%, 10-30 Cr, 4-12 Al and Si from traces and up to max 0.7 Si. 2. Use according to claim 1 in cores which are to conduct electromagnetically induced magnetic alternating fluxes having a frequency above 50 Hz, preferably above 400 Hz. 3. Use according to claim 1 or 2 of a thin sheet having a specific electric resistivity of at least 100 μΩ cm, preferably at least 120 μΩ cm. 4. Use according to any one of claims 1-3 of a steel which contains 15-25 Cr. 5. Use according to any one of claims 1-3 of a steel which contains more than 5 Al. 6. Use according to any one of claims 1-4 of a steel which contains 15-25 Cr and 4.5-7 Al. 7. Use according to claim 6 of a steel which contains 18-24 Cr and 4.5-6 Al. 8. Use according to any one of claims 1-4 of a steel which contains 18-22 Cr. 9. Use according to claim 8 of a steel which contains 5.2-5.8 Al. 10. Use according to any one of claims.1-4 of a steel which contains 15-21 Cr and 7- 12 Al. 11. Use according to claim 10 of a steel which contains 19-21 Cr and 8-12 Al. 12. Use according to any one of claims 1-11 of a steel having a specific electric resistivity greater than 125 μΩ cm, which steel contains Al and Cr in contents not exceeding the frame of the area A, B, C, D, E, F, G, H, A in the coordinate system in the enclosed figure, which corner points A-H have the following coordinates: 13. Use according to any one of claims 1-12 of a steel which besides Fe, Cr, Al and optionally Si has a total content of max 2% other elements. 14. Use according to claim 13, in which said other elements with a total content of max 2% comprise up to max 0.1 C and a total content of max 0.5 of Ce + La + Y + Hf + Sc + and optionally other rare earth metals.

Inventors: WEST ROGER, WEST, ROGER
Applicants: SURAHAMMARS BRUK AKTIEBOLAG, WEST ROGER, WEST, ROGER
Application year: 2001
Main classification: H01F1
IPC codes: C22C38, H01F1, H02K1