Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-02-25
2002-09-17
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S113000
Reexamination Certificate
active
06451128
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a grain oriented electrical steel sheet for use as iron cores of electric apparatuses such as transformers and the like. More specifically, the present invention relates to a method for manufacturing a high magnetic flux density grain oriented electrical steel sheet, in which inhibitors for restraining the growth of primary recrystallization grains are formed after a cold rolling is carried out to the final thickness, thereby making it possible to carry out a low temperature heating.
2. Description of the Prior Art
The grain oriented electrical steel sheet has a (110) [001] texture in the rolling direction. The method for this was first disclosed by N. P. Goss, and since that time, many researchers have made efforts to improve the method and the properties of the steel sheet. The magnetic properties of the grain oriented electrical steel sheet appear in the secondary recrystallization structure which is obtained by inhibiting the growth of the primary recrystallization grains and by selectively growing the (110) [001] crystal grains from among the inhibited crystal grains.
Therefore, if the grain oriented electrical steel sheet having superior magnetic properties is to be manufactured, it is important how the inhibitors are formed, and how the processes of obtaining a stable (110) [001] texture from among the inhibited grains are constituted.
Specifically, the inhibitors are formed by employing fine precipitates and segregated elements. The precipitates should be uniformly distributed in a sufficient amount and proper sizes, so that the growth of the primary recrystallization grains can be inhibited until the formation of the secondary recrystallization grains. Further, the precipitates should not be decomposed by being maintained in a thermally stable state up to the peak temperature immediately before the formation of the secondary recrystallization grains. The currently used inhibitors which satisfy the above conditions are MnS, MnS+AlN, MnS(Se)+Sb.
The technique of manufacturing the electrical steel sheet by using only MnS is disclosed in Japanese Patent Gazette Sho-40-15644. In this technique, a stable secondary recrystallization structure is obtained by carrying out two stages of cold rolling including an intermediate annealing. However, this method cannot obtain a high magnetic flux density, and the manufacturing cost is increased due to the fact that the two stages of cold rolling are carried out.
The typical technique of manufacturing the oriented electrical steel sheet by using MnS+AlN as the inhibitors are described in Japanese Patent Gazette Sho-30-3651. In this method, a single stage of cold rolling is carried out at a reduction rate of 80% or more, thereby obtaining a high magnetic flux density. However, if this method is applied to the industrial field, the manufacturing conditions are too stern, and therefore, the respective process conditions have to be strictly controlled.
Specifically, in this method, a high temperature slab heating, a hot rolling, a precipitation annealing, a cold rolling, a decarburization annealing and a high temperature annealing are carried out.
Here, the high temperature annealing refers to the process of developing the (110)[001] texture by making the secondary recrystallization occur in the final gauged sheet In any method using the inhibitor, an annealing separator is spread on the steel sheet before carrying out the high temperature annealing to prevent the sticking of the sheets, and during the decarburization, the oxide layer of the surface of the steel sheet reacts with the annealing separator to form a glass film, thereby providing an insulating property on the steel sheet. Thus, by the high temperature annealing, the final product of the steel sheet having the (110)[001] texture is provided with an insulating film on its surface.
The typical technique of manufacturing the grain oriented steel sheet by using MnS(Se)+Sb as the inhibitors are disclosed in Japanese Patent Gazette Sho-51-13469. In this method, a high temperature slab heating, a hot rolling, a precipitation annealing, a first cold rolling, an intermediate annealing, a second cold rolling, a decarburization annealing and a high temperature annealing are carried out. In this method, a high magnetic flux density can be obtained. However, two stages of cold rolling are carried out, and Sb or Se which is very expensive is used as the inhibitor. Therefore, the manufacturing cost is increased, and, still more, the production line shows to be toxic to the human body.
Further, in the above methods, the steel slab is heated at a high temperature for a long time to realize solid solutions of MnS or AlN before carrying out the hot rolling. Then during the cooling of the hot rolled sheet, MnS or AlN is formed into precipitates of proper size and distribution, thereby making it possible to use them as the inhibitor.
Specifically, in order to achieve a high magnetic flux density, it is known that a slab-heating has to be carried out up to 1300° C. in the method using MnS as the inhibitor, a slab-heating has to be carried out up to 1350° C. in the method using MnS and AlN as the inhibitor, and a slab-heating has to be carried out up to 1320° C. in the method using MnS(Se)+Sb as the inhibitor. Actually when it is applied to the industrial production, the heating has to be carried out up to 1400° C. to obtain a uniform temperature up to the inner regions of the slab.
In the case where the slab is heated to a high temperature for a long time, the consumed heat amount is large, and therefore, the manufacturing cost is increased. Further, the surface portions of the slab are melted down, with the result that the repair cost for the furnace is increased, and that the life expectancy of the furnace is shortened.
Particularly, if the columnar crystal (the solidified structure) of the slab surface is coarsely developed, then deep lateral cracks are formed during the later hot rolling. As a result, the yield is markedly decreased, and other problems may occur.
In order to solve the above described problems, if the slab-heating temperature is lowered when manufacturing the grain oriented steel sheet, then many advantages can be obtained in the manufacturing cost and the yield.
Therefore recently, research has been briskly carried out on the methods in which MnS requiring a high solid solution temperature is not used. That is, in these methods, the precipitates as the inhibitors are not formed by only the elements added in the steel making process, but the precipitates are formed at a proper stage during the manufacturing process.
The above methods are described in Japanese Patent Gazette Hei-1-230721 and Hei-1-283324 in which nitrogenization treatment is applied.
Belonging to this category, the following can be cited. One is that in which an annealing separator containing a chemical agent capable of nitrogenization is spread on the steel sheet to nitrogenize the steel sheet. Another is that in which a gas capable of nitrogenization is put into the atmospheric gas during the heating stage of the high temperature annealing to nitrogenize the steel sheet. Still another is that in which the steel sheet is nitrogenized within an atmosphere capable of nitrogenization after the decarburization.
Japanese Patent Gazette Hei-2-228425 discloses a method in which precipitates are formed by putting nitrogen into the steel during a nitrogenization process carried out on the hot rolled steel sheet, or on the first cold rolled steel sheet.
Japanese Patent Gazette Hei-2-294428 discloses a method in which nitrogenization and decarburization are simultaneously carried out during a decarburization annealing after the cold rolling. In this method, (Al,Si)N is used as the inhibitor, and due to the nitrogenization which occurs simultaneously with the decarburization, (Al,Si)N are formed mainly on the grain boundaries of the surf
Choi Gyu Seung
Han Chan Hye
Han Kyu Seok
Hong Byung Deuk
Kim Jae Kwan
Combs-Morillo Janelle
Pohang Iron & Steel Co. Ltd.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
Wyszomierski George
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