Metal treatment – Stock – Magnetic
Reexamination Certificate
2001-01-26
2002-12-31
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C148S120000
Reexamination Certificate
active
06500278
ABSTRACT:
TECHNICAL FIELD
This invention relates to a hot rolled electromagnetic steel sheet, and more particularly to a pure iron based hot rolled electromagnetic steel sheet having excellent magnetic properties by aligning <100> axis in a direction perpendicular to a sheet surface at as-rolled state in a high density and an excellent corrosion resistance and a method of producing the same.
BACKGROUND ART
Silicon steel sheets having excellent electromagnetic properties are used in a core for a transformer or a generator from the old time. As such a silicon steel sheet, there are two kinds of a unidirectional silicon steel sheet utilizing a secondary recrystallization to develop {110}<001> oriented grains or so-called Goss oriented grains, and a non-directional silicon steel sheet developing crystal grains with {100} face parallel to a sheet surface. Among them, the non-directional silicon steel sheets have particularly good properties when magnetic field is applied to various directions in the sheet surface and are frequently used in the generator, electric motor and the like.
In case of producing the non-directional silicon steel sheet used for such applications, it has hitherto been required to conduct decarburization annealing in a controlled atmosphere, cross rolling changing a rolling direction during the cold rolling or the like for gathering {100} face parallel to the sheet surface in a higher density.
For example, JP-A-1-108345 relating to silicon steel containing Si: 0.2~6.5 wt % or JP-A-4-224624 relating to steel containing Al+Si: 0.2~6.5 wt % discloses a technique that the steel is cold-rolled and annealed in a weak decarburizing atmosphere, for example, under vacuum of not more than 0.1 torr or in an atmosphere having a dew point of not more than 0° C. and composed of one or more of H
2
, He, Ne, Nr, Ar, Xe, Rn and N
2
to form &agr;-single phase region in a zone corresponding to a depth of 5~50 &mgr;m from the sheet surface and then annealed in a strong decarburizing atmosphere, for example, H
2
having a dew point of not less than −20° C. or a gas obtained by adding an inert gas or CO, CO2 to H
2
having a dew point of not less than −20° C. at 650~900° C. for 5~20 minutes to grow the &agr;-single phase region formed on the surface layer portion into the inside in the thickness direction to thereby improve the magnetic properties.
Thus, complicated steps inclusive the decarburization annealing have be required in addition to the hot rolling—cold rolling steps for gathering the {100} face parallel to the sheet surface in a high density from the old time. And also, the conventional electromagnetic steel sheets including 3% Si steel are low in the corrosion resistance, so that an insulating film having an excellent corrosion resistance is applied onto a final product, which is a factor raising the product cost.
However, it is recently demanded to have high performances in a cheaper cost with the popularization of electrical goods, which is impossible to cope with the aforementioned conventional technique. Although it is considered to more simplify the production steps for satisfying the above demand, the conventional technique is difficult to enhance the gathering of {100} orientation parallel to the sheet surface as hot-rolled.
It is, therefore, an object of the invention to propose a hot rolled electromagnetic steel sheet having improved magnetic properties and corrosion resistance by gathering the {100} orientation parallel to the sheet surface at a time of completing hot rolling and a method of producing the same.
DISCLOSURE OF INVENTION
The inventors have made various studies for solving the above problems in the hot rolled electromagnetic steel sheet and found that the formation of {100} orientation parallel to sheet surface, i.e. <100>//ND orientation of the steel sheet (direction perpendicular to sheet surface) is promoted by highly purifying steel to form a pure iron based component composition and rationalizing hot rolling conditions (particularly rolling reduction at given temperature region, friction coefficient) and cooling rate at &agr;-zone after hot rolling, and as a result the invention has been accomplished.
That is, the invention is a hot rolled electromagnetic steel sheet consisting of a super-high purity iron comprising Fe: not less than 99.95 mass %, C+N+S: not more than 10 mass ppm, O: not more than 50 mass ppm and the remainder being inevitable impurity, and having excellent magnetic properties and corrosion resistance.
As a method of producing the above hot rolled electromagnetic steel sheet, the invention also proposes a method of producing a hot rolled electromagnetic steel sheet having excellent magnetic properties and corrosion resistance, characterized in that a super-high purity iron comprising Fe: not less than 99.95 mass %, C+N+S: not more than 10 mass ppm, O: not more than 50 mass ppm and the remainder being inevitable impurity is heated to &ggr;-zone and subjected in this &ggr;-zone to hot rolling at a total rolling reduction of not less than 50% and under condition that at least one pass is a friction coefficient between roll and rolling material of not more than 0.3 and thereafter cooled at an average cooling rate of 0.5~150° C. over Ar
3
transformation point ~300° C.
As a preferable method, the invention proposes a method of producing a hot rolled electromagnetic steel sheet having excellent magnetic properties and corrosion resistance, characterized in that a super-high purity iron comprising Fe: not less than 99.95 mass %, C+N+S: not more than 10 mass ppm, O: not more than 50 mass ppm and the remainder being inevitable impurity is heated to &ggr;-zone and subjected in this &ggr;-zone to hot rolling at a total rolling reduction of not less than 50% and under condition that at least one pass is a friction coefficient between roll and rolling material of not more than 0.3 and a strain rate of not less than 150 l/second and thereafter cooled at an average cooling rate of 0.5~150° C. over Ar
3
transformation point ~300° C.
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the invention will be described below.
At first, the reason on the limitation of chemical composition in the pure iron based electromagnetic steel sheet according to the invention is described.
Fe: Not Less than 99.95 mass %
A raw material of high purity Fe is hot rolled in &ggr;-zone and then cooled in &agr;-zone, during which <100>//ND oriented grains grow. The purity of Fe is particularly important in the invention. When the purity is less than 99.95 mass %, the <100>//ND oriented grains hardly grow in the cooling. Therefore, Fe is not less than 99.95 mass %, preferably not less than 99.98 mass %.
C+N+S: Not More than 10 Mass ppm, O: Not More than 50 mass ppm
These gas components in the pure iron form carbide, oxide and the like with metallic elements (Al, Ti, Nb, Mn and the like) contained at extremely slight amounts of few~few tens mass ppm in the pure iron to obstruct occurrence and growth of nucleus for <100>//ND oriented grains. And also, the corrosion of pure iron based material is mainly caused by starting from C, N, S segregated in a grain boundary or oxides existing in the grain boundary or in the grains to create rust.
Such a bad influence of C, N, S and O appears even when C+N+S exceeds 10 mass ppm or even when O exceeds 50 mass ppm, so that it is necessary to satisfy C+N+S: not more than 10 mass ppm and O: not more than 50 mass ppm together. Moreover, preferable content ranges are C+N+S: not more than 5 mass ppm and O: not more than 20 mass ppm.
Then, production conditions of the pure iron based electromagnetic steel sheet according to the invention are described.
Hot Rolling
When the raw material of pure iron based steel having the above component composition is hot rolled in &agr;-zone, crystal grains are fined and <100>//ND oriented grains do not qui
Greenblum & Bernstein P.L.C.
Japan Science and Technology Corporation
Sheehan John
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