Metal treatment – Stock – Magnetic
Reexamination Certificate
2002-01-24
2003-08-05
Sheehan, John P. (Department: 1742)
Metal treatment
Stock
Magnetic
Reexamination Certificate
active
06602357
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a grain oriented electrical steel sheet and a production method for the same. More particularly, the invention is intended to obtain a product of a grain oriented electrical steel sheet subjected to domain refining (or subdividing) treatment and having high magnetic flux density, the product having lower iron loss than conventional values.
2. Description of the Related Art
Grain oriented electrical steel sheets are primarily employed as materials for laminated cores and coiled cores of transformers. For the purpose of reducing the power transmission and distribution cost, such a grain oriented electrical steel sheet is required to minimize energy loss caused upon power conversion, called “iron loss.”
Iron loss is expressed by the sum of hysteresis loss and eddy current loss. One technique for reducing hysteresis loss is to align a <001> axis of an iron crystal, which is a relatively easily magnetizable axis, with the rolling direction. Thus, it is known that permeability is increased and iron loss is reduced by orienting the crystal structure of iron in a {110}<001> direction, called Goss orientation, at a higher concentration.
Such a crystal structure oriented in the Goss direction at a higher concentration is generally obtained by utilizing a phenomenon called secondary recrystallization. In other words, a desired structure can be produced through preferential growth of crystal grains only in the Goss direction, which is developed by utilizing abnormal grain growth with very high direction selectivity during the thermal growth process of primary recrystallized grains. On that occasion, control of two factors, i.e., direction selectivity and growth rate of abnormal grains, is important in achieving a secondary recrystallized structure oriented in the Goss direction at a higher concentration.
To that end, it is usually pursued to develop a primary recrystallized structure before secondary recrystallization in the form of a particular texture, and to form a precipitate dispersed phase, which is called an inhibitor, in a uniform and appropriate size for selectively inhibiting growth of primary recrystallized grains.
As one technique regarding an inhibitor, Japanese Examined Patent Application Publication No. 46-23820 discloses a technique for forming a composite precipitate phase of MnSe or MnS and AlN to act as a strong inhibitor. It has, however, been confirmed that even when a crystal structure oriented in the Goss direction at a higher concentration is obtained by the disclosed technique, iron loss of a product is not always reduced. The reason is that the sizes of secondary recrystallized grains are increased as the concentration in the Goss direction increases, and at the same time the so-called &bgr; angle between the grain direction [001] and the rolled surface comes closer to 0°, whereby the width of a 180°-magnetic domain is widened and eddy current loss is increased.
Various techniques have recently been proposed to reduce the magnetic domain width by an artificial method and to lower the eddy current loss. Those techniques include, for example, methods of irradiating a laser beam (Japanese Examined Patent Application Publication No. 57-2252) and a plasma flame (Japanese Unexamined Patent Application Publication No. 62-96617) in a direction substantially perpendicular to the rolling direction of a steel sheet.
With those proposed methods, the so-called “stress-pattern” magnetic domains, which have a linear or linearly continued shape, are generated in an irradiated area by introducing thermal strains in the surface of a steel sheet. Inside such a magnetic domain, since magnetization occurs in the [100] and [010] directions, the width of a 180°-magnetic domain is reduced due to the effect of magneto-static energy caused by magnetic poles, which are generated in the boundary between the 180°-magnetic domain in the [100] direction and a stress pattern.
Also, in consideration of the fact that strain removing annealing is essential in coiled cores and the like, various methods utilizing grooves formed in a steel sheet have been developed as domain refining techniques which are endurable against the strain removing annealing. One of those techniques is represented, for example, by a method of locally forming a groove in a steel sheet after final finishing annealing so that a magnetic domain is refined due to the diamagnetic field effect produced by the formed groove. One example of methods for forming such a groove comprises the steps of locally removing an insulation coating and a primer coating by mechanical machining or irradiation of a laser beam, and then performing electrolyte etching (see Japanese Unexamined Patent Application Publication No. 63-76819).
Further, Japanese Examined Patent Application Publication No. 62-53579 discloses a method for refining a magnetic domain, wherein a steel sheet is subjected to strain removing annealing after impressing it with a gear-shaped roll, thereby achieving both groove formation and recrystallization. Additionally, Japanese Unexamined Patent Application Publication No. 59-197520 discloses a method for forming grooves in a steel sheet before finishing annealing.
It is known that, by applying those domain refining methods to a conventional grain-oriented electrical steel sheet, iron loss of a grain oriented electrical steel sheet having high magnetic flux density and having coarse crystal grains can effectively be reduced, and a value of iron loss is reduced as magnetic flux density B
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increases. However, materials exhibiting even lower iron loss are demanded under recent energy situations, whereas a drastic improvement in iron loss is difficult to achieve with conventional domain refining techniques.
On the other hand, as one of technique for improving magnetic characteristics of a grain oriented electrical steel sheet based on steel composition, it is proposed to add Cr in steel.
Aiming at reducing iron loss of a grain oriented electrical steel sheet, for example, Japanese Unexamined Patent Application Publication No. 10-259424 discloses a method of adding a predetermined amount of silicon, chromium, manganese, etc. in a hot rolled sheet to increase electrical resistivity of the hot rolled sheet to a value not lower than 45 &mgr;&OHgr;.cm, thereby reducing eddy current loss. Although the disclosed method proposes addition of Cr for increasing volume resistivity of a base material, it has not yet succeeded in realizing a high concentration of the grain direction and achieving a steel sheet with low iron loss, which has recently been demanded.
Also, Japanese Examined Patent Application Publication Nos. 62-54846 and 63-1371 and Japanese Unexamined Patent Application Publication Nos. 61-190017 and 2-228425 disclose techniques for adding Cr in a steel slab for the purpose of preventing deterioration of magnetic characteristics due to a change in the amount of acid-soluble Al.
Further, Japanese Unexamined Patent Application Publication Nos. 2-228425 and 5-78743, etc. disclose techniques for improving magnetic flux density with a combination of slab reheating at low temperature not higher than 1300° C. and nitriding after decarburization annealing, wherein Cr is contained in a steel slab to widen the range of Al content in which high magnetic flux density is obtained. Japanese Unexamined Patent Application Publication No. 11-217631 also discloses a technique wherein Cr is contained in a steel slab, which is subjected to slab reheating at low temperature and nitriding. However, this Publication aims to prevent deterioration in formation of a forsterite coating.
Moreover, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 10-46297, Cr is contained in a product by a similar method, i.e., a combination of slab reheating at low temperature and nitriding with NH
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gas after decarburization annealing. A primary action of added Cr is to develop a satisfactory inter
Kurosawa Mitsumasa
Nakanishi Tadashi
Senda Kunihiro
Takamiya Toshito
Toda Hiroaki
Kawasaki Steel Corporation
Piper Rudnick LLP
Sheehan John P.
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