Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-09-13
2002-10-08
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C427S127000, C428S687000, C428S471000, C428S472000, C428S689000, C428S692100, C428S699000, C428S701000, C428S900000, C428S928000
Reexamination Certificate
active
06461741
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a grain-oriented electromagnetic steel sheet with satisfactory coating properties including cohesion and corrosion resistance and a satisfactory permeability, which can carry and insulation coating with high imparted tension and has a low iron loss, and to a process for its manufacture.
2. Description of the Related Art
Grain-oriented electromagnetic steel sheets have their easily magnetized axes aligned in the rolling direction, and are widely used in electrical devices such as transformers and the like because of their low iron . less. It is known that imparting tension in the rolling direction of a grain-oriented electromagnetic steel sheet further reduces the iron loss and, as disclosed in Japanese Unexamined Patent Publications No. 48-39338 and No. 50-79442, there have been proposed methods of improving iron loss by applying and baking a coating solution composed mainly of aluminum phosphate and colloidal silica on the steel sheet surface, and utilizing the thermal expansion difference to impart tension.
Recently, in Japanese Unexamined Patent Publication No. 6-65755, there has been disclosed a method for obtaining a grain-oriented electromagnetic steel sheet that maintains excellent electromagnetic properties, and allows greater tension to be imparted to steel sheets, by forming an insulation coating composed mainly of aluminum borate on the steel sheet surface.
However, while grain-oriented electromagnetic steel sheets with insulation coatings composed mainly of aluminum borate have more excellent tension-imparting effects than grain-oriented electromagnetic steel sheets bearing insulation coatings composed mainly of aluminum phosphate and colloidal silica, it has been shown that the cohesion, corrosion resistance and space factor are inferior.
As concerns the cohesion, Japanese Unexamined Patent Publication No. 7-207953 discloses a method wherein as aluminum borate-based insulation coating is formed on a grain-oriented electromagnetic steel sheet bearing an insulation coating, composed mainly of a forsterite coating, wherein light acid pickling thereof is followed by formation of a coating composed mainly of a phosphate salt or a phosphate salt and colloidal silica to a coverage of 0.5 g/m
2
to 3.0 g/m
2
on one side, and then a coating solution comprising mainly alumina sol and boric acid is coated and baked thereon.
This technique is aimed at forming, with satisfactory cohesion, an insulating coating that imparts a high degree of tension, such as an aluminum borate coating, on a finished annealed coating composed mainly of forsterite; by applying and baking a coating solution composed mainly of a phosphate salt or a phosphate salt and colloidal silica on the forsterite coating whose mechanical strength has been lowered by acid pickling, to thus exhibit an effect as a maintenance material, the cohesion is improved through maintenance of the forsterite coating into which cracks have been introduced by etching. Consequently, since no modifications are made to the conditions for applying and baking the coating solution composed mainly of a phosphate salt or a phosphate salt and colloidal silica, no improvement can be expected in the corrosion resistance or space factor.
As concerns corrosion resistance, Japanese Unexamined Patent Publication No. 9-272982 discloses a technique whereby a steel sheet is provided with a coating g comprising a first layer containing an aluminum borate and a second layer containing aluminum phosphate, formed thereon.
This technique is aimed at imparting tension to the steel sheet by the aluminum borate coating of the first layer, while the main purpose of the second layer is to improve the corrosion resistance. That is, formation of the second layer on the aluminum borate coating that has inferior corrosion resistance is intended to overcome its drawbacks.
However, since the aluminum borate coating includes excess boron oxide, formation of the second layer containing aluminum phosphate by application and baking results in partial dissolution of the aluminum borate coating of the first layer, thus impairing the corrosion resistance; it is therefore quite difficult, from an industrial standpoint, to form an aluminum phosphate coating by application after formation of the aluminum borate coating, while the cost of this sophisticated technique is also high.
When an aluminum borate coating is formed on an aluminum phosphate coating on a common electromagnetic steel sheet without controlling the application and baking conditions, increasing the coverage to improve the corrosion resistance sometimes results in a poorer space factor as well as poorer cohesion, while reducing the coverage can make it impossible to achieve adequate corrosion resistance; according to the disclosure in this patent, the effect of improved corrosion resistance is based on evaluation upon standing for one week in a thermo-hygrostat at 50° C., 91% RH, but since coil transport often involves shipping, evaluation in a thermo-hygrostat is insufficient and the level of corrosion resistance must be evaluated based on a salt spray test.
Japanese Unexamined Patent Publication No. 9-335679 discloses a technique for coating low iron loss grain-oriented electromagnetic steel sheets with an aluminum borate-containing oxide coating, wherein satisfactory smoothness and cohesion are achieved with good workability during transformer manufacture, by specifying the alumina sol particle shapes.
This patent clearly demonstrates that the shape of the alumina sol particles affects the surface condition of the aluminum borate-containing oxide coating, and it provides a process for manufacture of grain-oriented electromagnetic steel sheets having coatings with satisfactory outer appearance, excellent cohesion and a high tension-imparting effect. Specifically, it is intended to improve the coating surface conditions by way of the alumina sol particle shape. However, while the process allows improvement in the space factor, the corrosion resistance is insufficient.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a grain-oriented electromagnetic steel sheet having a coating that maintains a more excellent tension-imparting effect than coatings obtained by the conventional methods, satisfactory cohesion and corrosion resistance that can withstand practical use, as well as a grain-oriented electromagnetic steel sheet having a coating that maintains a satisfactory space factor.
The present invention is based on the discovery that a grain-oriented electromagnetic steel sheet with excellent film surface roughness and film corrosion resistance can be obtained by controlling the baking conditions when applying and baking a coating solution composed mainly of a phosphate salt or a phosphate salt and colloidal silica on a grain-oriented electromagnetic steel sheet, prior to forming an insulating film with an excellent tension effect composed mainly of aluminum borate.
In other words, the present invention encompasses the following construction.
(1) A grain-oriented electromagnetic steel sheet with excellent coating properties, characterized in that the steel sheet surface has an insulation coating comprising a first layer composed mainly of a divalent or trivalent metal hydrogen phosphate salt and silica, and a second layer composed mainly of aluminum borate.
(2) A grain-oriented electromagnetic steel sheet with excellent coating properties according to (1), characterized in that the hydrogen phosphate salt of the first layer is one or a mixture of two or more from among aluminum primary phosphate, magnesium primary phosphate and calcium primary phosphate.
(3) A grain.-oriented electromagnetic steel sheet with excellent coating properties according to (1) or (2), characterized in that the first layer further contains free phosphoric acid.
(4) A grain-oriented electromagnetic steel sheet with excellent coating properties according to any one of (1) to (3) above, characterized in that the first l
Fujii Hiroyasu
Kobayashi Hideyuki
Takeda Kazutoshi
Yamazaki Shuichi
Kenyon & Kenyon
Koehler Robert R.
Nippon Steel Corporation
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