Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-08-17
2002-04-30
King, Roy (Department: 1742)
Metal treatment
Stock
Ferrous
C148S325000, C148S326000, C420S043000, C420S044000, C420S045000, C420S062000, C420S094000, C420S097000
Reexamination Certificate
active
06379477
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an Fe—Cr—Ni alloy which is required to be nonmagnetic and is used in electron gun electrodes, and specifically relates to an Fe—Cr—Ni alloy for electron gun electrodes and Fe—Cr—Ni alloy sheet for electron gun electrodes made therefrom, with improved press forming properties for drawing.
In general, electron gun electrodes used in color cathode ray tubes and the like are produced by drawing a nonmagnetic Fe—Cr—Ni stainless steel material with a thickness of 0.1 to 0.7 mm into a predetermined shape using press forming. In order to improve the drawing properties, in particular, to facilitate burring (working in which a circular hole is formed and the circumference thereof is cylindrically projected), improvement in degree of rolling reduction and annealing conditions has been proposed in Japanese Patent Application, First Publication, No. 257253/94. Japanese Patent Application, First Publication, No. 205453/96 proposes a method in which press forming properties are improved by limiting center-line mean roughness and the maximum height of surface roughness in press forming using a low viscosity lubricating oil, which is easy to be removed by degreasing and has been used to increase production efficiency. Japanese Patent Application No. 283039/97 demonstrates that burrs remaining in press punching a through hole relates to cracks in burring, and proposes a method in which burring properties are improved by suitable amounts of S being contained to improve punching properties and in which minute amounts of the elements are controlled to improve the drawing properties.
According to the rapid advances for finer and brighter cathode ray tubes for computers in recent years, requirements on focusing characteristics of the electron guns has become more severe. Therefore, the requirements on materials requires not only high precision formability for the large diameter lens electrodes but also good formability for high speed press forming. However, the prior art alloys have not been adequate since cracks occur at drawing surfaces.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made to respond to the above situation. An object of the invention is to provide an Fe—Cr—Ni alloy for electron gun electrodes, having superior drawing properties, which have been more severe in recent years, in particular, which can inhibit the occurrence of cracks in drawing.
The inventors have intensively researched the surface conditions of materials to solve the problems. As a result, the inventors have found that the drawing properties are influenced by the size and the number of groups of inclusions existing in a surface layer of a material. In particular, they have found that in groups of inclusions (including single inclusion) existing in a surface layer, ones with certain size or more influence the occurrence of cracks in drawing, and they have been able to inhibit the occurrence of cracks by reducing these inclusions.
FIG. 1
is a diagram showing the relationship between the number of groups of inclusions existing in a surface layer of an Fe—Cr—Ni alloy with a thickness of 0.6 mm and the incidence of cracks. It should be noted that the incidence of cracks was obtained by sampling 200 pieces at random from 2000 pieces of punched samples for inspection.
That is, the groups of inclusions were classified by the width and the length of the groups of widths of 5 &mgr;m or more and less than 10 &mgr;m and with lengths of 20 &mgr;m or more, with widths of 10 &mgr;m or more and less than 20 &mgr;m and with lengths of 20 &mgr;m or more, and with widths of 20 &mgr;m or more and with lengths of 20 &mgr;m or more, and the number of the groups of inclusions and the incidence of cracks with respect to each classification were plotted in FIG.
1
. It is shown in
FIG. 1
that the groups of inclusions, with widths of 5 &mgr;m or more and less than 20 &mgr;m and with lengths of 20 &mgr;m or more, do not relatively influence the occurrence of cracks in drawing even if the number thereof per unit area increases.
In contrast, in the case of the groups of inclusions, with widths of 10 &mgr;m or more and less than 20 &mgr;m and with lengths of 20 &mgr;m or more, the incidence of cracks exceeds 1% when the number of the groups nearly exceeds 20/mm
2
, and the incidence of cracks rapidly increases as the number of groups increase further. In the case of groups of inclusions, with widths of 20 &mgr;m or more and with lengths of 20 &mgr;m or more, the incidence of cracks exceeds 1% when the number of the groups nearly exceeds 5/mm
2
, and the incidence of cracks rapidly increases as the number of groups further increase. This shows that the occurrence of cracks in drawing can be inhibited by restricting the groups of inclusions such that the groups of lining inclusions, with widths of 10 &mgr;m or more and less than 20 &mgr;m and with lengths of 20 &mgr;m or more, is 20/mm
2
or less, the groups of lining inclusions, with widths of 20 &mgr;m or more and with lengths of 20 &mgr;m or more, is 5/mm
2
or less.
Furthermore, according to the research by the inventors, it has been demonstrated that the incidence of cracks may exceed 1% when the inclusions are Al
2
O
3
or composite inclusions of MnO and SiO
2
even if the number and the size of the groups of inclusions are restricted as above, and that the probability of cracks in drawing changes according to the chemical composition of the inclusions.
The number and the size of groups of inclusions in a surface layer of a material can be measured as follows. First, a surface of a material is specularly polished and then electropolished in phosphoric acid so as to facilitate distinction of inclusions. Then, the optical microscopic image of the surface is scanned by an image analyzer, and the images of inclusions are specified using the difference in the color tone between the inclusions and the matrix of the Fe—Cr—Ni alloy. Then, each image of the inclusions is enlarged 5 &mgr;m in the rolling direction and enlarged 5 &mgr;m in the transverse direction to the rolling direction, and the image is then reduced 5 &mgr;m in the respective directions. By these operations, the inclusions in the image, which exist over short distances, combined with each other into a group. Finally, the width and the length of each group of the inclusions (including single inclusions) are measure by the image analyzer.
The chemical composition of the group of inclusions is obtained by quantitative analysis with an electron beam microanalizer of ten inclusions chosen randomly.
The Fe—Cr—Ni alloy for electron gun electrodes of the invention has been made based on the above knowledge, and is characterized in comprising: 15 to 20% Cr; 9 to 15% Ni; 0.12% or less C; 0.005 to 1.0% Si; 0.005 to 2.5% Mn; 0.03% or less P; 0.0003 to 0.0100% S; 2.0% or less Mo; 0.001 to 0.2% Al; 0.003% or less O; 0.1% or less N; 0.1% or less Ti; 0.1% or less Nb; 0.1% or less V; 0.1% or less Zr; 0.05% or less Ca; 0.02% or less Mg by weight; balance Fe; and inevitable impurities; wherein when the alloy is rolled into a sheet with a thickness in the range of 0.1 to 0.7 mm, the surface portion of the sheet includes groups of lining inclusions, the number of groups with widths of 10 &mgr;m or more and less than 20 &mgr;m and with lengths of 20 &mgr;m or more is 20/mm
2
or less, and the number of groups with widths of 20 &mgr;m or more and with lengths of 20 &mgr;m or more is 5/mm
2
or less.
According to the preferred embodiment of the invention, the above Fe—Cr—Ni alloy for electron gun electrodes may be specified by the chemical composition of inclusions in 40≧SiO
2
≧100, 0≧Al
2
O
3
≧40, and 0≧MnO≧30 by atomic %.
Furthermore, the invention provides an Fe—Cr—Ni alloy sheet for electron gun electrodes obtained by rolling the above Fe—Cr—Ni alloy for electron gun electrodes to a thickness in the range of 0.1 to 0.7 mm.
In the following, the reasons for the above numerical limitations will be explained. In the following explanation, “%” means “weight %”.
(Cr):
Arent Fox Kintner & Plotkin & Kahn, PLLC
King Roy
Nippon Mining & Metals Co., Ltd.
Wilkins, III Harry D.
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