Metal treatment – Stock – Magnetic
Reexamination Certificate
2002-03-27
2003-10-21
King, Roy (Department: 1742)
Metal treatment
Stock
Magnetic
Reexamination Certificate
active
06635125
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides a grain-oriented electrical steel sheet containing from 2.0 to 7.0% of Si and excellent in film characteristics and iron loss characteristics. Moreover, the present invention provides a process for producing a grain-oriented electrical steel sheet extremely excellent in film characteristics and excellent in iron loss characteristics by controlling the initial oxide film of a steel strip which has been rapidly heated in the heating stage for decarburization annealing prior to introducing the steel strip into the decarburization annealing furnace. Furthermore, the present invention provides a decarburization annealing facility used in the production process. The present invention relates to the products, the production process and the facility.
BACKGROUND OF THE INVENTION
The magnetic characteristics of grain-oriented electrical steel sheets are generally evaluated for both iron loss and excitation characteristics. Improving the excitation characteristics is effective in downsizing an apparatus of which the designed magnetic flux density is to be increased. On the other hand, decreasing the iron loss is effective in reducing the energy lost as thermal energy and saving power consumption during the use of the steel sheet in electrical appliances. Moreover, aligning the <100> orientation of the grains of the product improves the excitation characteristics and lowers the iron loss. Many investigations have been carried out in this field in recent years, and various products and production technologies have been developed.
For example, Kokoku (Japanese Examined Patent Publication) No. 40-15644 discloses a process for producing a grain-oriented electrical steel sheet for obtaining a high magnetic flux density. In the process, AlN+MnS functions as an inhibitor, and the steel sheet is forcibly rolled with a reduction ratio exceeding 80% in the final cold rolling step. According to the process, the density of the {110}<001> orientation of the secondary recrystallization is high, and a grain-oriented electrical steel sheet having a high magnetic flux density of at least 1.870 T in terms of B
8
can be obtained.
However, although the iron loss can be decreased to some extent by the production process, the macroscopic grain diameter of secondary recrystallized grains is of the order as large as 10 mm. As a result, the eddy-current loss which is a factor influencing the iron loss cannot be decreased, and a superior iron loss has not been obtained.
In contrast to the process mentioned above, Kokoku (Japanese Examined Patent Publication) No. 6-51187 discloses a process for making secondary recrystallized grains smaller to improve the magnetic characteristics. The process comprises ultrarapidly annealing a steel sheet (strip) which has been rolled at an ambient temperature at temperatures of at least 657° C. at a heating rate of at least 140° C./sec, decarburizing the steel sheet, and final annealing the steel sheet at high temperatures so that secondary grain growth takes place, whereby the steel sheet contains secondary grains having a decreased size and has a lasting improved iron loss without a significant change even after stress relieving annealing.
However, it is difficult to obtain an electrical steel sheet exhibiting an iron loss comparable to that of an electromagnetic steel sheet having fine magnetic domains, by merely converting the secondary grains into fine ones by the production process. In particular, in final annealing where the steel sheet is rapidly exposed to high temperatures by rapid heating to form an oxide film having a different composition and to preferentially form fayalite (Fe
2
SiO
4
), coating the steel sheet with MgO does not necessarily result in an excellent formation of forsterite (2MgO•SiO
2
). As a result, there arises the problem that excellent magnetic characteristics cannot be obtained due to an insufficient film tension.
In order to solve such a problem, Kokai (Japanese Unexamined Patent Publication) No. 7-62436 proposes the following method: directly before annealing a steel strip having been rolled to a final sheet thickness or in a heating stage of decarburization annealing, the steel strip is heated to at least 700° C. at a heating rate of at least 100° C./sec in a nonoxidizing atmosphere having a PH
2
O/PH
2
ratio of up to 0.2, and heat treated. Moreover, the patent publication also proposes the use of two pairs of conductor rolls as a concrete example of rapid heating.
However, it has been found that a dense oxide layer is sometimes formed on the steel sheet during rapid heating in such a production method. When such an oxide layer is formed, it becomes a barrier, and influences the decarburization. In particular, decarburization of a magnetic steel sheet having a residual C content of up to 40 ppm becomes difficult. As a result, the magnetic characteristics of the products are deteriorated due to magnetic aging, although an electrical steel sheet having excellent magnetic characteristics can be obtained immediately after the production. Moreover, it becomes impossible to sufficiently decarburize the steel sheet to have a residual C content of up to 20 ppm even by extending the decarburization time.
Furthermore, a grain-oriented electrical steel sheet is generally bent when wound cores are prepared therefrom and incorporated into transformers, etc. Accordingly, the electrical steel sheet is required to have such an excellent film adhesion, particularly at the corner portions having a large curvature, that no peeling of the surface film consisting of a primary film and a secondary film (insulating coating) takes place. In the production process mentioned above, there is still room for improving the film adhesion.
DISCLOSURE OF THE INVENTION
The present invention provides a grain-oriented electrical steel sheet containing from 2.0 to 7.0% of Si and excellent in film characteristics (film adhesion) and magnetic characteristics (iron loss characteristics), a process for producing the same, and a decarburization annealing facility used for the production process.
In order to obtain a grain-oriented electrical steel sheet excellent in both the film characteristics (film adhesion) and the magnetic characteristics (iron loss characteristics), the present inventors carried out many tests wherein a steel strip rolled to have a final product thickness was rapidly heated to at least 800° C. at a heating rate of at least 100° C./sec in the heating stage in the decarburization step.
The tests were carried out using a decarburization annealing facility prepared by altering a conventional decarburization annealing furnace which had already been installed and was generally used for practicing a decarburization annealing step and which had, on the steel strip entry side (usually within 5 m from the steel strip inlet), an exhaust vent to the atmosphere.
That is, the tests were carried out using a decarburization annealing facility, wherein a rapid heating chamber provided with an apparatus for conducting the rapid heating was connectively provided to the entry side of a decarburization annealing furnace having already been installed with or without a throat portion provided between the furnace and the chamber, and the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace were exhausted through the exhaust vent mentioned above.
During conducting the decarburization annealing step using the decarburization annealing facility, investigations were made on the relationships between an atmosphere of the rapid heating chamber (including the throat portion when provided), an atmosphere of the decarburization annealing furnace, a residence time of the steel strip at temperatures of at least 750° C. in the rapid heating chamber (including the throat portion when provided), a film adhesion of the product and iron loss characteristics prior to and subsequent to magnetic aging. As a result, the following discoveries have been made.
1) A product excellent in
Ishii Shinya
Kosuge Kenji
Mochinaga Kishio
Nanba Eiichi
Tachibana Nobuo
Kenyon & Kenyon
King Roy
Nippon Steel Corporation
Wilkins, III Harry D.
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