Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2001-04-24
2002-11-19
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S121000
Reexamination Certificate
active
06482271
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a grain-oriented electrical steel sheet having magnetic properties improved by irradiation with laser beams.
2. Description of the Related Art
In manufacturing processes of grain-oriented electrical steel sheets, various methods have so far been proposed to fractionate 180° magnetic domains and reduce core loss by inducing mechanical strains at the surface of a steel sheet and forming local closure domains after forming a glass film on the surface of the steel sheet and further applying an insulation coating. Among such methods, the method of irradiating the focused beams of a pulsed YAG laser on the surface of a steel sheet and inducing strains by the evaporation reaction of a film at the irradiated portions, as disclosed in Japanese Unexamined Patent Publication No. S55-18566, is a highly reliable, controllable and excellent method for manufacturing a grain-oriented electrical steel sheet since the method provides a great iron loss improvement effect and is a non-contact processing method.
In such a method, an insulation film on the surface of a steel sheet is destroyed, causing the marks of laser irradiation where the substrate steel is exposed. Therefore, additional coating for rust prevention and insulation is required after the laser irradiation. Then, as further advanced methods, various technologies have been designed to introduce strains while suppressing the damages of a film and are disclosed in U.S. Pat. No. 4,645,547, Japanese Examined Patent Application Nos. S62-49322 and H5-32881 and Japanese Unexamined Patent Publication No. H10-204533, etc.
Further, as a method of laser irradiation, an example of irradiating laser to the locations confronting each other on the both surfaces of a steel sheet is disclosed as one of the embodiments in U.S. Pat. No. 4,645,547. However, this method does not show particularly excellent iron loss improvement compared with a case of the irradiation on only one surface.
The principle of improving iron loss by laser irradiation can be explained as follows. The iron loss of a grain-oriented electrical steel sheet is divided into anomaly eddy current loss and hysteresis loss. When laser is irradiated onto a steel sheet, strains are generated on the surface layer by either evaporation reaction of a film or rapid heating/rapid cooling. Originating in these strains, closure domains are generated having nearly the same width as that of the strains and the 180° magnetic domains are fractionated so as to minimize magnetostatic energy there. As a result, eddy current loss decreases in proportion to the width of the 180° magnetic domains and iron loss decreases accordingly. On the other hand, if strains are introduced, hysteresis loss increases. That is, the reduction of iron loss by laser irradiation is, as shown in the schematic graph of
FIG. 11
, to impose the strains most suitable for minimizing iron loss which is the sum of the reduction of eddy current loss and the increase of hysteresis loss accompanying the increase of the amount of strains.
Therefore, from an ideal viewpoint, it is desirable to lower the eddy current loss sufficiently and, at the same time, to suppress the increase of hysteresis loss to the utmost. The realization of such a grain-oriented electrical steel sheet has been desired.
Further, magnetostriction, which is one of the important parameters of the magnetic properties of a grain-oriented electrical steel sheet, like iron loss, affects noise generation when an electrical steel sheet is used for an iron core of a transformer. When an external magnetic field is imposed, magnetostriction increases since closure domains expand and contract in the direction of the magnetic field. Therefore, though iron loss can be reduced by forming closure domains, there has been a problem that there is a possibility of increasing magnetostriction.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a grain-oriented electrical steel sheet having magnetic properties improved by laser irradiation, the maximum iron loss improvement effect being obtained efficiently, and the increase in magnetostriction being suppressed. Further, another object of the present invention is to provide a grain-oriented electrical steel sheet with excellent magnetic properties wherein the substrate steel is not exposed at the irradiated portions after laser irradiation and an additional coating is not required.
The present invention relates to a grain-oriented electrical steel sheet excellent in magnetic properties, which are improved by irradiating laser beams onto the positions paired on the both surfaces of the steel sheet and forming fine closure domains, characterized in that the width of the closure domains in the rolling direction is 0.3 mm or less and the deviation in the rolling direction between the positions of the paired closure domains on the both surfaces is equal to or smaller than the width of said closure domains in the rolling direction. Further, the present invention relates to a grain-oriented electrical steel sheet excellent in magnetic properties, characterized in that the steel sheet has the marks of laser irradiation on its surface. Yet further, the present invention relates to a grain-oriented electrical steel sheet excellent in magnetic properties, characterized in that the substrate steel is not exposed at the portions of laser irradiation on the surface of the steel sheet.
REFERENCES:
patent: 4645547 (1987-02-01), Krause et al.
patent: 6368424 (2002-04-01), Sakai et al.
patent: 6387522 (2002-05-01), Mogi et al.
patent: 55-18566 (1980-02-01), None
patent: 62-49322 (1987-10-01), None
patent: 5-32881 (1993-05-01), None
patent: 10-204533 (1998-08-01), None
Hamada Naoya
Sakai Tatsuhiko
Nippon Steel Corporation
Yee Deborah
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