Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2002-07-30
2004-01-13
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S111000
Reexamination Certificate
active
06676771
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a grain-oriented electrical steel sheet that is very superior in both magnetic characteristics and coating characteristics.
2. Description of the Related Art
Grain-oriented electrical steel sheets are soft magnetic materials used as iron core materials for transformers and generators.
Recently, a demand for reducing energy losses generated in electrical equipment has increased from the viewpoint of energy saving. In grain-oriented electrical steel sheets used as iron core materials, correspondingly, more satisfactory magnetic characteristics have been demanded with a stronger demand than in the past.
A grain-oriented electrical steel sheet has a crystal structure in which the <001> direction, i.e., the axis of easy magnetization, is highly aligned in the rolling direction of a steel sheet. Such a texture is formed with secondary recrystallization, which is performed in finish annealing during the process of manufacturing a grain-oriented electrical steel sheet to grow crystal grains preferentially in the (110)[001] orientation, called the Goss orientation, into a big size. Accordingly, the crystal orientation of secondary recrystallization grains greatly affect the magnetic characteristics.
Also, a glass coating called a forsterite coating is present on the surface of base iron of a grain-oriented electrical steel sheet. The forsterite coating serves not only to ensure insulation between steel sheet layers when grain-oriented electrical steel sheets are laminated to form an iron core, etc., but also to apply a tension to the steel sheet for reducing its iron loss.
Grain-oriented electrical steel sheets are sheared and then subjected to strain releasing annealing at around 800° C. for around 3 hours at a user. Therefore, the forsterite coating is required to endure the strain releasing annealing and not peeled off even when subjected to working, such as bending, after strain releasing annealing. This is called bending peel-off resistance after strain releasing annealing.
Such a grain-oriented electrical steel sheet is generally manufactured through the following steps.
First, a steel slab containing Si of not more than about 4.5 mass % is heated and subjected to hot rolling. After annealing a hot-rolled steel sheet as required, the steel sheet is subjected to cold rolling once, or twice or more with intermediate annealing interposed therebetween to obtain a cold-rolled steel sheet having a final thickness. Then, the steel sheet is subjected to continuous annealing in a humid hydrogen atmosphere to develop primary recrystallization. This is hereinafter referred to as “primary-recrystallization continuous annealing”. After applying an annealing separator made primarily of magnesia, the steel sheet is subjected to finishing annealing performed as batch annealing at around 1200° C. for around 5 hours. During the finishing annealing, secondary recrystallization occurs and formation of the forsterite coating progresses.
Related techniques are disclosed in, e.g., U.S. Pat. No. 1,965,559, Japanese Examined Patent Application Publication Nos. 40-15644 and 51-13469, Japanese Unexamined Patent Application Publication Nos.3-122227 and 2001-30201, etc.
From the viewpoint of preventing deterioration of magnetic characteristics with aging, the C content in an electrical steel sheet is preferably kept as low as about 0.005 mass % in the final product. On the other hand, in case that a slab is heated at high temperature to bring an inhibitor component into a solid solution state, C of about 0.01 to 0.1 mass % is preferably present in the slab to suppress grain growth during heating of the slab. Therefore, decarburization annealing is generally performed before finishing annealing in many cases, so that the C content is reduced to a level required for the final product. The conventional decarburization annealing is often performed to serve also as primary recrystallization annealing. Recently, however, a manufacturing method not using an inhibitor component has also been proposed, as will be described later. It is common knowledge that, in such a case, the C content can be reduced even from the initial stage.
In summary, a conventional general process of manufacturing a grain-oriented electrical steel sheet comprises the steps of slab heating—hot rolling—(annealing of hot-rolled steel sheet)—cold rolling—(intermediate annealing—cold rolling)—continuous annealing (primary recrystallization annealing—decarburization annealing)—application of annealing separator—batch annealing (finishing annealing). After the finishing annealing, it is also possible to perform additional steps by applying a treatment solution to form an insulating coating and baking it.
However, the above-described conventional process of manufacturing a grain-oriented electrical steel sheet has a serious difficulty in obtaining both superior magnetic characteristics and superior coating characteristics.
In other words, the problem is that efforts to improve magnetic characteristics deteriorate the coating characteristics, and conversely the efforts to improve coating characteristics deteriorate the magnetic characteristics.
SUMMARY OF THE INVENTION
As stated above, obtaining both superior magnetic characteristics and superior coating characteristics has been very difficult to realize with the conventional manufacturing process, and this has been a limitation in stably manufacturing a grain-oriented electrical steel sheet that is superior in those characteristics, which has been especially demanded by the industry in recent years.
For the purpose of advantageously solving the problems set forth above, it is an object of the present invention to provide a method of manufacturing a grain-oriented electrical steel sheet, which includes a quite novel manufacturing process capable of obtaining both superior magnetic characteristics and superior coating characteristics.
How the present invention has been accomplished is described below in detail.
We have discovered that a difficulty in achieving both superior magnetic characteristics and superior coating characteristics was attributable to the finishing annealing step at a time in which secondary recrystallization was performed and when a forsterite coating was formed at the same time.
In the conventional manufacturing process, secondary recrystallization develops during finishing annealing. The finishing annealing is usually performed in a hydrogen atmosphere at around 1200° C. for around 5 hours. In that process, the gas composition during finishing annealing, the composition and reactivity of the annealing separator, the composition and form of oxides formed on the surface of a steel sheet, etc. greatly affect the crystal orientation of secondary recrystallization grains, i.e., the magnetic characteristics of the steel.
On the other hand, the forsterite coating is also formed during finishing annealing. As with magnetic characteristics, therefore, the gas composition during finishing annealing, the composition and reactivity of the annealing separator, the composition and form of oxides formed on the surface of a steel sheet, etc. are found to greatly affect behaviors in formation of the forsterite coating, i.e., coating characteristics.
However, preferable conditions for the secondary recrystallization and preferable conditions for the formation of the forsterite coating are not easily matched with each other. Even if there are conditions matched with each other, those conditions are satisfied in very narrow ranges. It has been, therefore, very difficult to manufacture a grain-oriented electrical steel sheet that is superior in both magnetic characteristics and coating characteristics with stability from the industrial point of view.
In view of those situations, the inventors have discovered that superior magnetic characteristics and superior coating characteristics can be both obtained by separating finishing annealing, in which the secondary recrystalli
Hayakawa Yasuyuki
Komatsubara Michiro
Kurosawa Mitsumasa
Takashima Minoru
Toge Tetsuo
JFE Steel Corporation
Piper Rudnick LLP
Sheehan John
LandOfFree
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