Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2002-12-05
2004-07-06
Sheehan, John P. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S306000, C148S307000, C148S308000, C148S120000, C148S121000, C148S122000, C427S127000, C427S567000
Reexamination Certificate
active
06758915
ABSTRACT:
TECHNICAL FIELD
The present invention relates to grain-oriented magnetic steel sheets having extremely low iron loss which are suitable for use as iron core materials for transformers and power generators, and to methods for producing the same.
BACKGROUND ART
It is well known that grain-oriented magnetic steel sheets which contain Si and whose crystal orientations are (110) [001] and (100) [001] have excellent soft magnetic properties. Therefore, they are widely used as iron core materials for various types of electrical apparatuses used in the commercial frequency band. In general, in such a grain-oriented magnetic steel sheet, the iron loss (W
17/50
), which is observed when magnetized at 1.7 T at a frequency of 50 Hz, must be low.
Core loss consists of eddy current loss and hysteresis loss. Various methods are known in order to effectively decrease eddy current loss. Examples thereof include a method of increasing electrical resistance by adding Si, a method of decreasing steel sheet thickness, and a method of decreasing grain size. On the other hand, in order to effectively decrease hysteresis loss, a method of aligning crystal orientation is known.
However, the method of increasing electrical resistance by adding Si has limitations. This is because the saturation magnetic flux density decreases if an excessively large amount of Si is added. The decrease in the saturation magnetic flux density results in an increase in the size of iron cores, which is undesirable.
The method of decreasing steel sheet thickness also has limitations. This is because the rolling loads greatly increase, resulting in an extreme increase in manufacturing costs.
With respect to the method of aligning crystal orientation, products which have a magnetic flux density (B
8
) that is close to the saturation value, such as 1.96 T or 1.97 T, have already been obtained. Therefore, there is little room for a further decrease in hysteresis loss.
Recently, techniques for reducing iron loss by artificially decreasing magnetic domain widths have been developed. Examples thereof include a method of locally introducing strain by applying plasma jet or laser beams to the surfaces of steel sheets and a method in which grooves are formed in the surfaces of steel sheets. Although a considerable iron loss reduction effect has been achieved by such magnetic domain-refining techniques, the effect has also reached its limit.
As the other techniques for reducing iron loss, methods related to the surface properties of magnetic steel sheets have also been disclosed.
Japanese Examined Patent Application Publication No. 52-24499 discloses a method in which the roughness at the interface between a metal surface of a steel sheet and a surface of a non-metallic coating film is decreased. Japanese Examined Patent Application Publication Nos. 7-9041, 5-87597, and 6-37694 disclose methods of performing so-called “crystal orientation-intensifying treatment” in which crystals with specific crystal orientations are left on metal surfaces.
In order to reduce iron loss by such methods, strong tension must be applied to a steel sheet. Therefore, it was required to form a tensile coating film on the surface of the steel sheet. If no tensile coating film was provided, since the surface of the steel sheet was smooth, an increase in magnetic domain width was accelerated, resulting in a large increase in iron loss.
In order to overcome this problem, the Japanese Examined Patent Application Publication No. 52-24499 teaches a method in which the surface of the steel sheet is subjected to chemical polishing or electrolytic polishing to form a specular surface, and the surface of the steel sheet is further thinly metal-plated. The method aims to suppress an increase in iron loss by preventing the oxidation of the surface of the steel sheet and by preventing the deterioratation of the surface smoothness of the steel sheet during coating and baking of the insulating coating film.
However, when metal plating had tension, the insulating coating film was likely to be peeled off by baking treatment. Even if the insulating coating film avoided being peeled off, since the insulating coating film was an ordinary phosphate non-tensile insulating coating film, reduction in iron loss was small.
When metal plating did not have a tensile effect, the iron loss reduction was very small. Moreover, even if a phosphate non-tensile insulating coating film was intended to be formed, the adhesion of the coating film was not satisfactory, and it was not possible to reduce iron loss.
Japanese Unexamined Patent Application Publication No. 62-103374 discloses a method in which a mixed ultrathin layer of the matrix and a variety of oxides, borides, silicides, phosphides, or sulfides is formed on a steel sheet surface smoothed by polishing, and a baked insulating coating film is further formed thereon. This method provides excellent adhesion between the steel sheet and the insulation film. However, since the mixed ultrathin layer of the matrix is present, the iron loss reduction effect due to mirror finishing of the steel sheet surface disappears, resulting in a small reduction in iron loss.
Japanese Unexamined Patent Application Publication No. 2-243770 discloses a method in which a ceramic coating film is formed by a sol-gel process. However, in this method, since adhesion with the steel sheet is inferior, it is not possible to provide a satisfactory tension-applying effect to the steel sheet, resulting in a small reduction in iron loss.
Japanese Examined Patent Application Publication No. 56-4150 discloses a method in which a steel sheet surface is smoothed by chemical polishing or electrolytic polishing so as to have a centerline mean roughness (Ra) of 0.4 &mgr;m or less, and a ceramic thin film is further formed thereon. However, since the ceramic thin film having satisfactory adhesion is formed by vacuum deposition, a large apparatus are required. The deposition rate is also low, and this method is not suitable for industrial production.
Japanese Unexamined Patent Application Publication Nos. 3-47957, 3-294465, 3-294466, 3-294467, 3-294468, 3-294469, and 3-294470 disclose methods in which coating films of oxides or silicides are formed by low pressure plasma spraying on the surfaces of smoothed matrices or the surfaces of metal-plated films formed thereon. Japanese Unexamined Patent Application Publication No. 10-245667 discloses a method in which tensile coating films composed of oxides, nitrides, or carbides are formed by plasma spraying. In these methods, although industrially acceptable deposition rates are ensured, since film formation is performed by droplet deposition, it is not possible to form dense films. Moreover, the coating films have rough surfaces, and peeling occurs easily by friction. Furthermore, adhesion between the surfaces of the steel sheets or plated surfaces and the oxide or silicide coating films is not satisfactory. Therefore, a reduction in iron loss is insufficient. Similarly to vacuum deposition, large-scale pressure reducing apparatuses are required.
As described above, in recent techniques for reducing iron loss of grain-oriented magnetic steel sheets, it is absolutely necessary to form a tensile coating film on the steel sheet surface after smoothing the steel sheet surface during finishing annealing or in the subsequent treatment and/or after crystal orientation-intensifying treatment is performed. However, since the tensile coating film applies strong tension to the steel sheet surface, strong shearing stress is applied to the interface between the steel sheet surface and the tensile coating film, and the coating film is easily peeled off. As a result, tension application cannot be achieved, resulting in an insufficient reduction in iron loss.
Accordingly, various ideas have been adopted in order to secure adhesion of tensile coating films. However, when the adhesion was satisfactory, the iron loss reduction effect by smoothing of the steel sheet surface was lost.
Furthermore, when crystal orientation-intensifyi
Komatsubara Michiro
Kurosawa Mitsumasa
Muraki Mineo
Takashima Minoru
Yamaguchi Hiroi
JFE Steel Corporation
Sheehan John P.
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