Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2002-02-21
2003-12-23
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S112000, C148S110000
Reexamination Certificate
active
06666929
ABSTRACT:
FIELD OF THE INVENTION
The present invention refers to a process for the improvement of magnetic characteristics in grain oriented electrical silicon steel sheets by laser scribing, and more particularly it refers to a radiation process of the steel sheet after final annealing, in order to improve its induction characteristics, losses and magnetostriction with respect to the non-treated steel sheet.
STATE OF THE ART
Grain oriented electrical silicon steel sheets are mainly used in the manufacturing of cores for transformers; in this use, one of the most studied magnetic characteristics of the material, particularly after the oil crisis of the Seventies and more recently according to the increasing interest in energetic saving, is the one related to the so-called core losses, or losses, that is to say related to the quantity of energy lost during the working of the transformer. Losses are expressed in watt by kg of weight of the core and depend on various factors and particularly by the movement of the magnetic domain walls, defined as areas within the material wherein electrons responsible for the ferromagnetism have parallel spins and therefore a magnetic moment which is not null. In silicon magnetic steel sheets, the magnetic moment within a single domain is oriented according to the directions of easy magnetization, that is to say according to the crystallographic directions <100>. Domain walls are areas among adjacent domains through which the magnetic moment rotates and they are characterized by the value of this rotation, therefore we are talking about 180° and non-180° walls (in this material 90°).
In the demagnetized status, the vectorial sum of the magnetic moments is zero, below an external applied field, initial magnetization essentially takes place by movement of the domain walls, the ones being favorably oriented with respect to the applied field, take less energy and grow at the expenses of the other domains by lateral movement of the walls at 180°. A higher mobility of the walls makes magnetization easier and therefore the movement of the walls needs less energy. Energetic consumption associated with the movement of the walls at 180° is due to the electromotive forces generated by the movements of the walls opposing to this movement.
It has been found out that such component of the losses is proportional to the ratio between the distance among 180° walls and the thickness of the steel sheet. Moreover, it has been found out that losses also depend on the size of the steel sheet grains and on the orientation of the crystalline lattice of the grain with respect to the steel sheet surface.
Therefore the most obvious and immediate choice consists in the fact of having high-oriented grain silicon steel sheets having given grain sizes and a low thickness.
Efforts which have been made up to now have produced excellent results, which cannot be further improved in a considerable way from the manufacturing point of view; particularly, it has been found out that the optimum size of the grains is around 4 mm, while as far as the thickness of the steel sheet is concerned, going below some values is unsuitable both for the cost of these processes and because the ratio (called “space factor”) between the volume of the steel sheet and the one of the needed insulating coatings decreases too much, therefore a considerable part of the core would be occupied by the insulating coating.
Therefore, other factors influencing the losses of the core have been taken into consideration and particularly the ones related to magnetic domain sizes.
First, it has been found out that applying a tension to the steel sheet an anisotropy is induced in the steel sheet plane which, in the presence of the typical structure of these materials (Goss texture) increases the magnetization energetic difference between the crystallographic direction <100> parallel to the rolling direction and the direction <011> perpendicular to the rolling direction. Consequently, the balance between the magnetostatics energy and the one of the domain walls shifts in favor of the wall energy, thus causing the formation of a high number of walls becoming thinner and closer. In such a way, a considerable decrease of the eddy current contribution to the total value of the losses is obtained. Therefore, tensioning insulating coatings have been developed able to obtain such improvements.
However, since 1924, the opportunity that such tensions can be obtained also by creating localized compression microstresses has been also suggested. From this point of view, it has been proposed to submit the steel sheet to shot-peening or mechanical scribing with drills, blades or rolls equipped with relieves. These methods, even though effective and capable of giving heat treating-resistant improvements at high temperatures, have the drawbacks to be of difficult application from the manufacturing point of view and of destroying the steel sheet insulating coating thus exposing the same to fast oxidation therefore requiring another insulating coating and forming burs or relieves of the metals at the edges of the scribing or impressions, thus decreasing the space value of the core and making in the same short circuits more frequent.
A further step has been the one related to the treatment of the steel sheet surface with concentrated energetic pulses in the form of laser beams, electron beams, plasma and the like.
An article produced to “1986 ASM Material Week Conference” of Oct. 4-9, 1986 in Orlando, Fla. by J. W. Schoen and A. L. von Hollen having title “Domain refinement of oriented electrical steel: from early beginning to an emerged technology” clearly describes all the issues recalling the first experiences on the subject; particularly referring to laser scribing treatment.
On this matter, FIGS.
7
and
9
—and their relevant discussion in the text—it is stated that the improvement obtained by refining magnetic domains can be connected to the magnetostriction condition after laser treatment as the magnetostriction change represents a quantitative measure of the non-180° walls proportion introduced in areas submitted to laser treatment; the best refining domain results are obtained with a magnetostriction increase. This situation can also be found in other documents; for instance in European Patent n. 87587, priority Jan. 25
th
1980, a radiation process of electromagnetic steel sheet with laser beam. The invention consists in applying a liquid coating agent to the steel sheet after laser treatment and in annealing this coating at a temperature not higher than 600° C. This temperature limit is due to the fact that improvements on losses due to the laser-type treatments completely disappear at temperatures higher than 500-600° C. This patent states that the laser treatment effects are used not only in order to reduce losses, but also to improve magnetostriction; however, on this aspect, no convincing demonstrations of the results obtained are given; in fact, Table 1—the only one in which magnetostriction evaluations are given—shows that measures concerning magnetostriction are expressed as size variations under a mechanical load of 17 kg. Therefore, in this regard, it must be pointed out that, as it is well known, a mechanical traction improves the magnetostriction. Moreover, from the data reported, it can be noticed that the results according to the invention are lower, as long as magnetostriction is concerned, than the ones which can be obtained by simply applying the final insulating coating, without laser treatment. Therefore, the only advantage of the laser treatment is to improve the value of the total core losses.
European Patent Application n. 611.829 filed on Aug. 24
th
1994, refers to the electronic beam treatment of an electrical oriented grain steel sheet surface, in order to obtain a product (core of transformer) having improved characteristics in shape and acoustic emissions. The invention consists in the fact that the steel sheet, provided with final insulating coating, is radiated with an electron beam sent
Acciai Speciali Terni S.p.A.
Morgan & Finnegan , LLP
Sheehan John
LandOfFree
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