Patent Application
20100221140
The present invention relates to a low carbon hot rolled steel strip having such features that it can replace, in producing cut lamination packs, such as stators and rotors of electric motors, the cold rolled strips till now used for these utilizations.
For example WO2004/013365 and EP1411138 disclose non-oriented grain magnetic strips provided with special chemical-physical features that, upon cold rolling and annealing treatment, render the same suitable to be used for producing, after cutting, lamination packs such as stators and rotors of electric motors.
It is also known that cold rolling involves a cycle of operations which is rather burdensome when considering the required costs and time. These strips of known type are further characterized by a relatively high content of silicon and by a structure with not particularly fine grains. It is known in fact that the steel strip that is commonly used in the technique for the above-mentioned utilizations generally shows a silicon content >0.5%, having a structure with ferritic grains not particularly fine and usually even lower than grade 7 of the ASTM standard, in order to enhance its magnetic permeability.
It is an object of the present invention to provide a low carbon hot rolled steel strip having a reduced silicon content and thickness comprised between 0.65 and 1.5 mm, which shows without subsequent cold rolling or additional treatments, particular metallurgical and geometrical features, as well as relating to planarity and hardness, which may render the same particularly, although not exclusively, suitable to the production of lamination that, upon cutting can form the multilayer packs suitable for the above-mentioned utilizations.
The strip according to the present invention is preferably, although not exclusively, manufactured by means of in-line systems of the “thin-slab” type, like the one described in the international publication WO 2004/026497 in the name of the same applicant, as schematically represented in
The mean thickness is preferably of 0.65-1.0 mm with strict tolerances of ±0.05 mm, whereas the parallelism is preferably even less than 0.01 mm. Upon possible pickling and skinpassing operations, the hardness of the strip according to the present invention can reach values of HRB 55/70 or HV110/140.
The particular roughness ≧1.3 μm of the strip is helpful to prevent the cut pieces from closely joining together when packed to form a multilayer, thanks to the air being present in the gaps caused by the roughness, and in general the above-described features make this type of hot rolled strip particularly suitable to a fine cutting without any need of having to trim and straighten the cut pieces, thereby rendering them ready for the subsequent packing steps, which in general are carried out in-line and automatically, thus eliminating the trimming and straightening operations which are required in the traditional systems.
These and other objects, advantages and features of the steel strip according to the invention will become clearer from the following detailed description with reference to the annexed drawings in which:
As already stated above, the hot rolled steel strip according to the present invention can replace, without annealing treatment, the cold rolled strips for producing, upon cutting, lamination packs of magnetic sheets. The thickness of said steel strip is of 0.65-1.5 mm, preferably 0.65-1.0 mm with strict tolerances of ±0.05 mm and parallelism rate <0.02, preferably 0.01 mm. While the magnetic strip according to the prior art is characterized by a silicon content >0.5% and a ferritic grain with fineness lower than grade 7 of ASTM E 112 standard to enhance the magnetic permeability, the strip according to the invention, in spite of the very low silicon content (<0.03%) and the grain fineness higher than grade 9 of the above-mentioned standard, shows magnetic features comparable with those of non-oriented grain silicon-based strips being hot rolled and subsequently annealed to increase the size of the ferritic grain. This appears to be due to the substantial uniformity of the ferritic grain, wherein the 70% of the grains show a fineness grade comprised between levels 9 and 12 of the above-mentioned ASTM standard, thus rendering particularly permeable the magnetism of the same strip. Although the grain size plays a basic role concerning the magnetic permeability of the steel, experimental tests have in fact shown that in this respect the feature of the grain uniformity is also very important, irrespective of its size.
With reference to
The feature of uniformity of the ferritic grain, which is fine and particularly homogeneous, descends also in particular from the microphotograph magnified one thousand times as represented in
Coming now to another feature of the strip according to the invention, i.e. the little height of the cutting burrs, the upper limit of which as requested on the market is of 0.04 mm, the graph of
In order to determine the features of planarity and parallelism of the steel strip, in relation with the product of the intended use, i.e. lamination packs of magnetic sheets, in particular but not exclusively for producing stators and rotors of electric motors, reference is usually made to a packing factor which is defined as a ratio between the weight of a multilayer packet of regular shape (P) and that of a solid steel block having the same size (P′). Obviously the highest value of packing factor that is possible to reach is equal to 1, as can be seen with reference to
In the flow-chart of
That the strip according to the invention is a valid alternative solution to the cold rolled silicon-based strips with non-oriented grains, when the applications do not require particular limits of the magnetic features, has been proved by means of experimental tests which have given the results listed in the following table 1. It will be noted that these experimental tests have been carried out on multi-layer packs obtained from a strip of the present invention, in other words hot rolled without additional treatments, which have been compared with similar packs obtained from a strip of the prior art, that has been cold rolled, annealed and skinpassed (1%).
Tesla, measured with intensity of magnetic field H alternate at 50 Hz, of 2500, 5000, 10000 A/m, respectively.
Through observations of the results listed in the table it can be noted that the performances of the hot rolled strip according to the invention are fully comparable, under the aspect of the quality, with those of a strip according to the prior art further subjected to cold rolling, annealing and skinpassing treatment. The values of magnetic permeability which have been found are in fact fairly similar (highest difference: 0.6% at B10000), while the magnetic losses are even lower with the strip of the invention.
It is also clear that the manufacturing of the steel according to the invention is more economical with respect to that of steel according to the prior art both for addition of less quantities of silicon and for the elimination of the cold rolling and annealing steps, as already remarked above. This saving can reach a value corresponding to about an amount of 15% of the total manufacturing costs.
Another advantage of the steel according to the invention is that of avoiding the critical state of the traditional non-oriented grain silicon steel, the slabs of which must be heated at temperatures higher (by about 200° C.) than requested by the other steels which do not include silicon and must be cooled more slowly with a controlled process before the subsequent rolling step to avoid cracks on the slab itself. Finally it is given in the following a typical example of chemical analysis of the strip according to the invention, when bearing in mind that it is not the case of a binding composition, except for the lower carbon and silicon content, as already mentioned before: C≦0.06%, Mn 0.10÷0.20%, Si<0.03%, P≦0,010%, S≦0.005%, Cr≦0.10%, Ni≦0.12%, Mo≦0.03%, Al 0.030±0.050%.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IT2006/000045 | 1/26/2006 | WO | 00 | 7/21/2008 |
