U.S. patent application number 11/629931 was filed with the patent office on 2007-11-15 for black colored steel sheet having excellent electromagnetic shielding property, electromagnetic shielding member, and electromagnetic shielding case.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Hiroki Nakamaru, Chiyoko Tada, Rie Umebayashi.
Application Number | 20070264522 11/629931 |
Document ID | / |
Family ID | 36000068 |
Filed Date | 2007-11-15 |
United States Patent
Application |
20070264522 |
Kind Code |
A1 |
Umebayashi; Rie ; et
al. |
November 15, 2007 |
Black Colored Steel Sheet Having Excellent Electromagnetic
Shielding Property, Electromagnetic Shielding Member, and
Electromagnetic Shielding Case
Abstract
A black colored steel sheet comprises a Zn--Ni-plated steel
sheet having a blackening-treated layer and a coating film on the
surface of the Zn--Ni-plated steel sheet. The blackening-treated
layer has a thickness of 0.01 to 1.0 .mu.m. The Zn--Ni-plated steel
sheet having a blackening-treated layer has an arithmetical mean
roughness Ra of 0.7 to 2.0 .mu.m and the number of peaks per inch
(PPI) of at least 180.
Inventors: |
Umebayashi; Rie; (Chiba,
JP) ; Tada; Chiyoko; (Chiba, JP) ; Nakamaru;
Hiroki; (Hiroshima, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 Fifth Avenue
16TH Floor
NEW YORK
NY
10001-7708
US
|
Assignee: |
JFE STEEL CORPORATION
2-3, Uchisaiwai-cho 2-chome Chiyoda-ku
Tokyo
JP
100-0011
|
Family ID: |
36000068 |
Appl. No.: |
11/629931 |
Filed: |
August 24, 2005 |
PCT Filed: |
August 24, 2005 |
PCT NO: |
PCT/JP05/15827 |
371 Date: |
December 18, 2006 |
Current U.S.
Class: |
428/615 |
Current CPC
Class: |
Y10T 428/12493 20150115;
H05K 9/0088 20130101; C23C 2/18 20130101; C23C 22/24 20130101; C25D
5/48 20130101 |
Class at
Publication: |
428/615 |
International
Class: |
C23C 28/00 20060101
C23C028/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2004 |
JP |
2004-252663 |
Feb 23, 2005 |
JP |
2005-046694 |
Claims
1. A black colored steel sheet having an excellent electromagnetic
shielding property, comprising a Zn--Ni-plated steel sheet having a
blackening-treated layer; and a coating film formed on the
Zn--Ni-plated steel sheet, wherein the blackening-treated layer has
a thickness of 0.01 to 1.0 .mu.m; the Zn--Ni-plated steel sheet
having the blackening-treated layer has an arithmetical mean
roughness Ra of 0.7 to 2.0 .mu.m and the number of peaks per inch
(PPI) of at least 180.
2. The black colored steel sheet according to claim 1, wherein the
number of peaks per inch (PPI) is 200 to 400.
3. The black colored steel sheet according to claim 1, wherein the
arithmetical mean roughness Ra, the number of peaks per inch (PPI),
and a coating weight of coating film applied to one side of the
black colored steel sheet satisfy the equation (1):
z.ltoreq.(0.010x-0.0077)y-1.05x+2.16 (1) wherein x denotes the
arithmetical mean roughness Ra (.mu.m), y denotes the number of
peaks per inch (PPI), and Z denotes the coating weight of coating
film applied to one side of the black colored steel sheet
(g/m.sup.2).
4. An electromagnetic shielding member, wherein all or part of the
member is formed of the black colored steel sheet according to
claim 1.
5. An electromagnetic shielding case formed of the black colored
steel sheet according to claim 1.
6. The black colored steel sheet according to claim 2, wherein the
arithmetical mean roughness Ra, the number of peaks per inch (PPI),
and a coating weight of coating film applied to one side of the
black colored steel sheet satisfy the equation (1):
z.ltoreq.(0.010x-0.0077)y-1.05x+2.16 (1) wherein x denotes the
arithmetical mean roughness Ra (.mu.m), y denotes the number of
peaks per inch (PPI), and Z denotes the coating weight of coating
film applied to one side of the black colored steel sheet
(g/m.sup.2).
7. An electromagnetic shielding member, wherein all or part of the
member is formed of the black colored steel sheet according to
claim 2.
8. An electromagnetic shielding member, wherein all or part of the
member is formed of the black colored steel sheet according to
claim 3.
9. An electromagnetic shielding member, wherein all or part of the
member is formed of the black colored steel sheet according to
claim 6.
10. An electromagnetic shielding case formed of the black colored
steel sheet according to claim 2.
11. An electromagnetic shielding case formed of the black colored
steel sheet according to claim 3.
12. An electromagnetic shielding case formed of the black colored
steel sheet according to claim 6.
Description
TECHNICAL FIELD
[0001] The present invention relates to a black colored steel sheet
having excellent electromagnetic shielding property. In particular,
the present invention intends to advantageously improve the
electromagnetic shielding property of a black colored steel sheet,
not to mention a black appearance and corrosion resistance.
[0002] The present invention also relates to an electromagnetic
shielding member and an electromagnetic shielding case, which use
the black colored steel sheet having electromagnetic shielding
property.
BACKGROUND ARTS
[0003] Conventionally, black colored steel sheets having a
blackening-treated surface have widely been used, for example, in
business machines, such as personal computers and copying machines,
household electrical appliances, such as air conditioners,
automobile parts, and interior building materials. In general, the
black colored steel sheets are manufactured by applying a black
paint to a galvanized sheet iron or by performing a blackening
treatment (for example, anodic electrolysis, cathodic electrolysis,
alternating electrolysis, or anodic oxidation) on a plated surface
of a Zn--Ni-plated steel sheet and forming at least one coating
film thereon. In the latter method, the coating film is formed
because of insufficient corrosion resistance of a
blackening-treated sheet.
[0004] However, in the former black colored steel sheet, the
thickness of the coating must be increased to completely hide the
underlayer with the black paint. This causes a problem in terms of
electromagnetic shielding property.
[0005] In the latter black colored steel sheet, an organic coating
film and/or an inorganic coating film is formed on the
blackening-treated layer. This also causes a problem in terms of
electromagnetic shielding property.
[0006] However, in the latter black colored steel sheet, when the
coating film is formed on the blackening-treated layer, use of a
paint composition containing a metal ion, a water-soluble organic
resin, a water-dispersible organic resin, a glycoluril resin, and
an acid can reduce the film thickness, improve the electromagnetic
shielding property, and provide an excellent black appearance and
high corrosion resistance, in spite of a reduced thickness (for
example, Patent Document 1: Japanese Unexamined Patent Application
Publication No. 2004-188976).
[0007] In the surface-treated steel sheet having an organic film
and/or an inorganic film, an appropriate combination of the
center-line average roughness Ra and the thickness of the film can
provide a surface-treated steel sheet exhibiting excellent
electromagnetic shielding property and high corrosion resistance
(for example, Patent Document 2: Japanese Unexamined Patent
Application Publication No. 2004-156081).
[0008] In recent years, the problem of electromagnetic interference
(EMI) has become obvious in electronic equipment and electrical
equipment. An unnecessary electromagnetic wave leaking from one
piece of equipment causes functional interference or malfunction in
another piece of equipment. Furthermore, the electromagnetic wave
may harm human bodies.
[0009] As a countermeasure, a source of noise may be surrounded by
a metal plate (conductor). However, an electromagnetic wave may
leak from a seam or a joint of a case that surrounds the source of
noise. A case formed of surface-treated steel sheets must therefore
exhibit sufficient electromagnetic shielding property at a seam or
a joint. Thus, a large number of conductive regions must be formed
over the entire contact surface of surface-treated steel sheets
that are in contact with each other at a seam or a joint.
[0010] Patent Document 1 proposes a black colored steel sheet
exhibiting an excellent black appearance, high corrosion
resistance, and excellent electromagnetic shielding property. In
Patent Document 1, a coating film formed on a blackening-treated
Zn--Ni-plated steel sheet has a reduced thickness to improve the
electromagnetic shielding property.
[0011] However, Patent Document 1 does not take the surface
roughness of the blackening-treated Zn--Ni-plated steel sheet into
consideration. When the surface roughness changes, therefore, it is
difficult to form a conductive region at a seam or a joint, or the
number of conductive regions decreases, even when the same coating
weight of coating film is deposited. Hence, the electromagnetic
shielding property deteriorates. Furthermore, Patent Document 1
also does not define the thickness of the blackening-treated layer.
An increase in the thickness results in the deterioration of the
electromagnetic shielding property.
[0012] In a method proposed in Patent Document 2, an appropriate
combination of the center-line average roughness Ra and the film
thickness of the coating film provides a surface-treated steel
sheet exhibiting excellent electromagnetic shielding property and
high corrosion resistance. However, this method does not consider
bumps and dips on a surface of a steel sheet, that is, the number
of peaks per inch (PPI). A smaller PPI also results in a decrease
in the number of conductive regions at a seam or a joint, leading
to poor electromagnetic shielding property.
DISCLOSURE OF THE INVENTION
[0013] The present invention solves the problems described above.
It is an object of the present invention to provide a
surface-treated steel sheet that can effectively reduce an
electromagnetic wave leaking from a seam or a joint of a case
formed of the surface-treated steel sheets to exhibit excellent
electromagnetic shielding property and that has an excellent black
appearance and high corrosion resistance.
[0014] It is another object of the present invention to provide an
electromagnetic shielding member formed of the black colored steel
sheet and an electromagnetic shielding case formed of the black
colored steel sheets.
[0015] Heretofore, the surface roughness of a Zn--Ni-plated steel
sheet having a blackening-treated layer has been defined on the
basis of a common roughness, such as an arithmetical mean roughness
Ra, to improve the electromagnetic shielding property. However, as
a result of diligent and repeated investigations to solve the
problems described above, the present inventors have found that it
is insufficient to define the surface roughness of a steel sheet on
the basis of a common roughness, such as an arithmetical mean
roughness, to effectively improve the electromagnetic shielding
property, and that the number of peaks per inch (PPI) is important.
The present inventors also have found that the thickness of a
blackening-treated layer also has an effect on the electromagnetic
shielding property.
[0016] The present invention is based on these findings.
[0017] The present invention provides the following:
[0018] (1) A black colored steel sheet having an excellent
electromagnetic shielding property, comprising a Zn--Ni-plated
steel sheet having a blackening-treated layer; and a coating film
formed on the Zn--Ni-plated steel sheet, wherein
[0019] the blackening-treated layer has a thickness of 0.01 to 1.0
.mu.m;
[0020] the Zn--Ni-plated steel sheet having the blackening-treated
layer has an arithmetical mean roughness Ra of 0.7 to 2.0 .mu.m and
the number of peaks per inch (PPI) of at least 180.
[0021] (2) The black colored steel sheet according to (1), wherein
the number of peaks per inch (PPI) is 200 to 400.
[0022] (3) The black colored steel sheet according to (1) or (2),
wherein the arithmetical mean roughness Ra, the number of peaks per
inch (PPI), and a coating weight of coating film applied to one
side of the black colored steel sheet satisfy the equation (1):
z.ltoreq.(0.010x-0.0077)y-1.05x+2.16 (1)
[0023] wherein x denotes the arithmetical mean roughness Ra
(.mu.m), y denotes the number of peaks per inch (PPI), and Z
denotes the coating weight of coating film applied to one side of
the black colored steel sheet (g/m.sup.2).
[0024] (4) An electromagnetic shielding member, wherein all or part
of the member is formed of the black colored steel sheet according
to any one of (1) to (3); and
[0025] (5) An electromagnetic shielding case formed of the black
colored steel sheet according to any one of (1) to (3).
[0026] The present invention can provide a black colored steel
sheet that can effectively reduce an electromagnetic wave leaking
from a seam or a joint of a case formed of the surface-treated
steel sheets to exhibit excellent electromagnetic shielding
property and that has an excellent black appearance and high
corrosion resistance.
[0027] The present invention can also provide an excellent
electromagnetic shielding member all or part of which is formed of
the black colored steel sheet.
[0028] The present invention can also provide an excellent
electromagnetic shielding case formed of the black colored steel
sheets.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic view illustrating an optimum range of
the PPI (y) and the coating weight of coating film z (g/m.sup.2)
when the Ra (x) is 2.0 .mu.m and the PPI (y) is in the range of 200
to 400.
[0030] FIG. 2 is a schematic view of an apparatus for measuring
leak noise to evaluate the electromagnetic shielding property.
[0031] FIG. 3 is a schematic view of a case to evaluate the
electromagnetic shielding property.
[0032] FIG. 4 is a chart illustrating the relationship between the
electric field strength of leak noise and the frequency in a sample
of Zn--Ni-plated steel sheet having no coating film (Reference
Example).
[0033] FIG. 5 is a chart illustrating the relationship between the
electric field strength of leak noise and the frequency in a sample
according to Example 7.
[0034] FIG. 6 is a chart illustrating the relationship between the
electric field strength of leak noise and the frequency in an open
state where no sample is mounted on an aluminum case in the
apparatus illustrated in FIG. 2.
[0035] FIG. 7 is a chart illustrating the relationship between the
electric field strength of leak noise and the frequency when
extraneous noise is measured in the absence of an electromagnetic
wave and in an open state where no sample is mounted on an aluminum
case in the apparatus illustrated in FIG. 2.
EMBODIMENT FOR CARRYING OUT THE INVENTION
[0036] The present invention will specifically be described
below.
[0037] The following is a reason for limiting the thickness of a
blackening-treated layer formed on a Zn--Ni-plated steel sheet to a
range of 0.01 to 1.0 .mu.m in the present invention.
[0038] Because the blackening-treated layer deposited on a
Zn--Ni-plated steel sheet is formed, for example, by anodic
oxidation, the blackening-treated layer is not a good electric
conductor. When the thickness of the blackening-treated layer is
more than 1.0 .mu.m, therefore, the electromagnetic shielding
property at a seam or a joint of a case deteriorates even in the
absence of a coating film. Hence, the thickness of the
blackening-treated layer is set to 1.0 .mu.m or less. When the
thickness of the blackening-treated layer is less than 0.01 .mu.m,
the black appearance is poor. Hence, the thickness of the
blackening-treated layer is set to at least 0.01 .mu.m.
[0039] The following is a reason for limiting the arithmetical mean
roughness Ra to 0.7 to 2.0 .mu.m and the number of peaks per inch
(PPI) to at least 180 in the surface roughness of the
blackening-treated Zn--Ni-plated steel sheet.
[0040] Existing common blackening-treated Zn--Ni-plated steel
sheets have an Ra in the range of about 0.5 to 1.5 .mu.m and the
number of peaks per inch (PPI) in the range of about 120 to
160.
[0041] A blackening-treated Zn--Ni-plated steel sheet according to
the present invention has a higher Ra, because a coating film
formed on the steel sheet locally has a reduced thickness in order
that a conductive region can be formed on the steel sheet. When the
Ra is less than 0.7 .mu.m, bumps and dips on a surface of the steel
sheet become smaller. It is therefore difficult to form a local
thin film region in the coating film. Hence, the surface roughness
of the blackening-treated Zn--Ni-plated steel sheet is set to 0.7
.mu.m or more as measured as Ra.
[0042] When the Ra is more than 2.0 .mu.m, while the
electromagnetic shielding property is improved, the coating weight
of coating film must be increased to completely cover the surface
of the blackening-treated Zn--Ni-plated steel sheet to exhibit
corrosion resistance. This increases the cost. Hence, the surface
roughness of the blackening-treated Zn--Ni-plated steel sheet is
set to 2.0 .mu.m or less as measured as Ra.
[0043] A blackening-treated Zn--Ni-plated steel sheet according to
the present invention has a higher PPI in order that a large number
of local thin film regions can be formed over the entire surface of
the coating film deposited on the blackening-treated Zn--Ni-plated
steel sheet. When the PPI is less than 180, the number of local
thin film regions is small and therefore sufficient electromagnetic
shielding property is hardly achieved. Hence, the surface roughness
of the blackening-treated Zn--Ni-plated steel sheet is set to at
least 180 as measured as PPI. More preferably, the PPI is at least
200.
[0044] While there is no limitation on the PPI, the PPI more than
400 may lower the corrosion resistance because of a larger number
of local thin film regions. Hence, the PPI is suitably set to 400
or less.
[0045] The thickness of a Zn--Ni-plated layer is suitably set to
about 1 to 5 .mu.m to achieve high corrosion resistance and strong
adhesion.
[0046] The Zn--Ni-plated layer is not necessarily composed of a
blackening-treated layer across the thickness of the layer. As
described above, it is sufficient to form a surface portion having
a thickness of 0.01 to 1.0 .mu.m with a blackening-treated
layer.
[0047] The blackening treatment may be performed by anodic
electrolysis, cathodic electrolysis, alternating electrolysis, or
anodic oxidation in a common treatment liquid.
[0048] As a method for controlling the thickness of a
blackening-treated layer, it is effective to use a method for
controlling treatment conditions, such as the type, the
concentration, and the pH of an oxidizing agent, which serves as a
treatment liquid, the electric current density, the time of
electrolysis, and the Coulomb density.
[0049] Then, at least one coating film is formed on a
blackening-treated layer thus formed, because the
blackening-treated surface has insufficient corrosion
resistance.
[0050] Such a coating film may be formed of any film that can
improve the corrosion resistance after the blackening treatment
without impairment of the black appearance. Among others, a coating
film composed of a chromate film and an organic film or a coating
film composed of an organic film and/or an inorganic film is
advantageous.
[0051] Preferably, the coating weight of coating film is at least
0.6 g/m.sup.2 to achieve sufficient corrosion resistance.
[0052] The upper limit of the coating weight of coating film
depends on the Ra and the PPI of the surface of a Zn--Ni-plated
steel sheet having a blackening-treated layer. When the Ra is 2.0
.mu.m and the PPI is 180, the upper limit is preferably about 2.3
g/m.sup.2 or less per side of the steel sheet to achieve excellent
electromagnetic shielding property.
[0053] When the Ra is 2.0 .mu.m and the PPI is 200, the upper limit
of the coating weight of coating film is preferably about 2.5
g/m.sup.2 or less per side of the steel sheet.
[0054] When the Ra is 2.0 .mu.m and the PPI is 400, the coating
weight of coating film is preferably about 5.0 g/m.sup.2 or less
per side of the steel sheet.
[0055] As for the upper limit of the coating weight of coating
film, the arithmetical mean roughness Ra, the number of peaks per
inch (PPI), and the coating weight of coating film applied to one
side of the black colored steel sheet satisfy the equation (1):
z.ltoreq.(0.010x-0.0077)y-1.05x+2.16 (1)
[0056] wherein x denotes the arithmetical mean roughness Ra
(.mu.m), y denotes the number of peaks per inch (PPI), and Z
denotes the coating weight of coating film applied to one side of
the black colored steel sheet (g/m.sup.2).
[0057] When the coating weight of coating film z satisfies the
equation (1), the coating weight of coating film z is appropriate
for the Ra and the PPI of the Zn--Ni-plated steel sheet to form
sufficient conductive points, thus achieving particularly excellent
electromagnetic shielding property. FIG. 1 illustrates the
relationship between the PPI (y) and the coating weight of coating
film z (g/m.sup.2) when Ra (x)=2.0 .mu.m is substituted into the
equation (1). The coating weight of coating film is preferred in
the range illustrated in FIG. 1.
[0058] An electromagnetic shielding member formed of a black
colored steel sheet and an electromagnetic shielding case formed of
black colored steel sheets are described below.
[0059] The electromagnetic shielding property is particularly
problematic at a juncture of surface-treated steel sheets in a case
formed of the surface-treated steel sheets. Excellent
electromagnetic shielding property can therefore be achieved by
using black colored steel sheets according to the present invention
at least at the juncture. This is an electromagnetic shielding
member in which a black colored steel sheet according to the
present invention is used in all or part of the member.
[0060] Furthermore, the entire case, including the juncture, formed
of black colored steel sheets according to the present invention
can exhibit particularly excellent electromagnetic shielding
property. This is an electromagnetic shielding case entirely formed
of black colored steel sheets according to the present
invention.
[0061] Examples of a method for controlling the surface roughness
of a blackening-treated Zn--Ni-plated steel sheet according to the
present invention include a method for controlling the surface
roughness of a cold-rolled steel sheet for use in the Zn--Ni-plated
steel sheet, a method for controlling the surface roughness of the
Zn--Ni-plated steel sheet, and a method for controlling the surface
roughness of the blackening-treated steel sheet. Examples of the
method for controlling the surface roughness of a cold-rolled steel
sheet for use in the Zn--Ni-plated steel sheet include a method for
tandem-rolling or temper-rolling the cold-rolled steel sheet with a
dull roll. The dull roll is prepared by surface machining, such as
blasting, electrical discharge machining, laser machining, or
etching, of a roll of a tandem rolling mill or a temper rolling
mill. The cold-rolled steel may be processed directly by
blasting.
[0062] When a Zn--Ni-plated layer according to the present
invention is manufactured by electroplating, the plated layer is
formed substantially along bumps and dips on the surface of a steel
sheet. When the blackening treatment is performed by electrolysis,
such as anodic oxidation, a blackening-treated layer is formed
substantially along bumps and dips on the surface of a steel sheet.
Thus, the surface roughness of a blackening-treated Zn--Ni-plated
steel sheet is preferably controlled by a method for controlling
the roughness of the steel sheet before these layers are
formed.
[0063] The surface roughness of a Zn--Ni-plated steel sheet and a
blackening-treated steel sheet can be controlled with a temper
rolling mill, the roughness of which is previously adjusted.
[0064] In this case, the roughness pattern of the temper rolling
mill is not completely transferred to a steel sheet by temper
rolling. The Ra of the steel sheet is in the range of about 40% to
50% of that of the roll surface and the PPI of the steel sheet is
about 80% of that of the roll surface. Thus, to prepare a
blackening-treated Zn--Ni-plated steel sheet having an arithmetical
mean roughness Ra in the range of 0.7 to 2.0 .mu.m and the number
of peaks per inch (PPI) of at least 180, the temper rolling mill
preferably has an Ra in the range of 1.4 to, 5.0 .mu.m and the PPI
of at least 220.
[0065] The electromagnetic shielding property in the present
invention is evaluated by measuring leak noise with an apparatus
illustrated in FIG. 2.
[0066] A 20 MHz clock 4 is placed as a source of noise in a 100
mm.times.100 mm.times.100 mm aluminum case 3 formed of an aluminum
sheet having a thickness of 2 mm. The aluminum case 3 has an 80
mm.times.80 mm opening at the top surface. The opening is
surrounded by a 10 mm frame 5 protruding inside. A 10 mm.times.1 mm
gasket (urethane sponge wrapped in a conductive cloth
(Cu--Ni-plated fiber)) 6 is placed on the frame 5. A 100
mm.times.100 mm sample 1 is brought into contact with the gasket 6
on the top surface of the aluminum case 3 while the surface 2 to be
evaluated faces downward. The sample 1 is under a load of 19.6 N (2
kgf). An electromagnetic wave leaking from a frame-shaped matching
surface between the gasket 6 and the sample 1 is received by a loop
antenna 7 having a diameter of 30 mm, which is disposed 50 mm away
from the frame 5. The electromagnetic wave is amplified by a 25 dB
preamplifier 8 and is analyzed with a spectrum analyzer (Advantest
Corporation, R3162) 9.
[0067] When the present invention was used as a case material, the
electromagnetic shielding property was evaluated with a case
illustrated in FIG. 3.
[0068] A commercially available ATX tower-type PC case (Owltech
Corporation, OWL-PCR7) was used, while an exterior portion 10 and a
lid 11 of the case were formed of a sample. Components that meet
the following specifications were mounted in the case to fabricate
a desktop personal computer. Windows XP was installed to boot the
computer.
[0069] Power supply: a built-in power supply was used without
modification.
[0070] Motherboard: A-Open, AX4SG Max II
[0071] CPU: Intel Corporation, Pentium 4 processor 3 GHz
[0072] DDR memory: generic 250 MB.times.2
[0073] HDD: Hitachi Global Storage Technologies, HDS722516VLAT80
(capacity 160 GB)
[0074] Optical drive: Pioneer Corporation, DVR-A08-J
EXAMPLES
Example 1
[0075] Samples Nos. 1 to 13 in Table 1 were manufactured according
to the steps of an annealed cold-rolled steel sheet.fwdarw.temper
rolling (control of surface roughness).fwdarw.electroplated Zn--Ni
coating.fwdarw.anodic oxidation (blackening
treatment).fwdarw.formation of a coating film, or an annealed
cold-rolled steel sheet.fwdarw.blasting (control of surface
roughness).fwdarw.electroplated Zn--Ni coating.fwdarw.anodic
oxidation (blackening treatment).fwdarw.formation of a coating
film. The samples were different in the thickness of a
blackening-treated layer and the Ra, the PPI, and the coating
weight of coating film after the blackening treatment. The
thickness of a Zn--Ni-plated layer formed by Zn--Ni electroplating
was 2 .mu.m. The thickness of the blackening-treated layer was
controlled by the electrolysis time and pH in the anodic
oxidation.
[0076] A predetermined coating weight of coating film was deposited
by applying a paint composition containing a metal ion, a
water-soluble organic resin, a water-dispersible organic resin, a
glycoluril resin, and an acid, with a bar coater and curing the
paint in an oven so that the temperature of the sheet reached
190.degree. C. in 21 seconds.
[0077] Table 1 illustrates the surface roughness, the thickness of
the blackening-treated layer, the coating weight of coating film,
the corrosion resistance of a flat portion, the black appearance,
and the electromagnetic shielding property of the samples thus
manufactured.
[0078] Their characteristics were evaluated as follows.
<Surface Roughness>
[0079] The blackening-treated samples were analyzed for the surface
roughness with a stylus roughness meter (Tokyo Seimitsu Co., Ltd.)
using a stylus having a tip curvature radius of 1 .mu.m at a
scanning speed of 0.3 mm/s. The cut-off of the arithmetical mean
roughness Ra according to JIS B 0601-1994 was 0.8 mm. Peaks greater
than 0.635 .mu.m were counted to determine the number of peaks per
inch (PPI).
<Thickness of Blackening-Treated Layer>
[0080] Cross sections of the blackening-treated samples were
observed under a transmission electron microscope. The thickness of
a blackening-treated layer was measured at the center of the field
at a magnification of 90,000 and was determined as a mean value of
four fields.
<Coating Weight of Coating Film>
[0081] A mass change of a sample by the formation of a coating film
was converted into mass per unit area.
<Corrosion Resistance of Flat Portion>
[0082] A 50 mm.times.100 mm sample after the formation of a coating
film was sealed at the end faces and was subjected to a neutral
salt spray test (JIS Z 2371-2000) for 48 hours. The percentage of
area in which white rust generated was determined. On the basis of
this percentage, the corrosion resistance of a flat portion was
assessed according to the following criteria.
[0083] Excellent: 5% or less
[0084] Good: more than 5% and 10% or less
[0085] Fair: more than 10% and 20% or less
[0086] Poor: more than 20%
<Black Appearance>
[0087] The color tone (L*) of a sample after the formation of a
coating film was measured with a spectro-photometric color
difference meter (SQ2000, Nippon Denshoku Industries Co., Ltd.) and
was assessed according to the following criteria.
[0088] Good: L* of 25 or less
[0089] Poor: L* of more than 25
<Electromagnetic Shielding Property>
[0090] As for the electromagnetic shielding property, noise leaking
from the joint between the surface to be evaluated and the case was
measured with the spectral analyzer in the apparatus illustrated in
FIG. 2. FIGS. 4 to 7 show the results.
[0091] In the evaluation of the examples and the comparative
examples, a peak value was read at a frequency of 20 Mhz to 1000
Mhz at intervals of 20 Mhz. The peak value was converted into a
noise score (I) by the equation (2).
I=10.times.log(10.sup.0.1d1+10.sup.0.1d2+ . . . +10.sup.0.1dn)
(2)
[0092] n: number of peaks
[0093] d1, d2, . . . dn: peak value
[0094] FIG. 4 illustrates a measurement of a Zn--Ni-plated steel
sheet having no coating film (Reference Example), which probably
exhibits excellent electromagnetic shielding property at a seam or
a joint of a case, as a reference. FIG. 5 illustrates a measurement
according to Example 7. FIG. 6 illustrates a measurement in the
absence of a sample. FIG. 7 illustrates a measurement in the
absence of both a sample and the output of an electromagnetic wave.
FIG. 7 illustrates extraneous noise. Peak values read from FIGS. 4
to 7 were substituted into the equation (2) to calculate Is. Peaks
marked with x in FIGS. 4 to 7 were assigned to extraneous noise
illustrated in FIG. 7 and were excluded from the calculation of the
equation (2).
[0095] The samples according to Examples were evaluated according
to the following criteria:
[0096] (I-Ib)/(Ia-Ib)>0.35: Poor
[0097] 0.35.gtoreq.(I-Ib)/(Ia-Ib)>0.26: Fair
[0098] 0.26.gtoreq.(I-Ib)/(Ia-Ib)>0.20: Good
[0099] 0.20.gtoreq.(I-Ib)/(Ia-Ib)>0.13: Good.sup.+
[0100] 0.13.gtoreq.(I-Ib)/(Ia-Ib).gtoreq.0: Excellent
[0101] wherein a noise score I was calculated from leak noise, a
noise score Ia was calculated from FIG. 6 (no sample, in the
presence of electromagnetic wave output), and a noise score Ib was
calculated from FIG. 7 (no sample, no electromagnetic wave output).
TABLE-US-00001 TABLE 1 Thickness of blackening- Amount of Corrosion
Sample Ra (x) Value of treated layer covering layer resistance of
Black Electromagnetic No. (.mu.m) PPI (y) Eq. (1) (.mu.m) (z)
(g/m.sup.2) flat portion appearance shielding Remarks 1 1.0 205 1.6
0.15 0.6 Good Good Excellent Example 1 2 1.0 205 1.6 0.07 1.3
Excellent Good Excellent Example 2 3 1.0 205 1.6 0.3 1.3 Excellent
Good Excellent Example 3 4 0.9 380 1.7 0.15 1.3 Excellent Good
Excellent Example 4 5 1.0 180 1.5 0.15 1.3 Excellent Good Good+
Example 5 6 1.3 205 1.9 0.15 1.3 Excellent Good Excellent Example 6
7 1.3 260 2.2 0.15 1.3 Excellent Good Excellent Example 7 8 1.3 380
2.8 0.15 1.4 Excellent Good Excellent Example 8 9 1.8 380 4.2 0.15
1.5 Excellent Good Excellent Example 9 10 1.8 380 4.2 0.15 3.0
Excellent Good Excellent Example 10 11 1.0 205 1.6 0.15 1.8
Excellent Good Good Example 11 12 1.3 380 2.8 0.15 3.0 Excellent
Good Good Example 12 13 1.0 120 1.4 0.15 1.3 Good Good Fair
Comparative Example 1 14 1.3 150 1.6 0.15 1.3 Excellent Good Fair
Comparative Example 2 15 1.3 205 1.9 1.1 1.3 Excellent Good Poor
Comparative Example 3 16 1.0 205 1.6 0 0 -- -- Excellent Reference
Example
[0102] As is apparent from Table 1, the samples according to the
present invention, which had a blackening-treated layer having a
thickness in the range of 0.01 to 1.0 .mu.m and had an arithmetical
mean roughness Ra in the range of 0.7 to 2.0 .mu.m and the number
of peaks per inch (PPI) of at least 180 after blackening treatment,
exhibited not only an excellent black appearance and high corrosion
resistance, but also excellent electromagnetic shielding
property.
[0103] When the arithmetical mean roughness Ra (x), the number of
peaks per inch (PPI) (y), and the coating weight of coating film
applied to one side of a steel sheet (z) satisfy the equation (1),
the electromagnetic shielding property was particularly
excellent.
Example 2
[0104] Table 2 illustrates the electromagnetic shielding property
of various samples (Nos. 5, 6, 8, 13, and 15) illustrated in Table
1, when the samples were applied to an exterior portion 10 and a
lid 11 of the tower-type PC case illustrated in FIG. 3.
[0105] The electromagnetic shielding property of the tower-type PC
case was evaluated as follows.
<Electromagnetic Shielding Property of Electronic Equipment and
Electrical Equipment>
[0106] The electromagnetic shielding property of electronic
equipment and electrical equipment including a case formed of a
sample was measured in an open area test site at a distance of 3 m
as described in note 2 of Table 4.6 in technical standards
V-3/2005.04, 4.3 tolerance of radiated emission electric field
strength issued by the Voluntary Control Council for Interference
by Information Technology Equipment (VCCI). The acceptability was
determined by a Class B criterion.
[0107] Pass: satisfy Class B
[0108] Fail: does not satisfy Class B TABLE-US-00002 TABLE 2
Exterior Electromagnetic Case No. portion Lid shielding Remarks 1
No. 5 No. 6 Pass Example 13 2 No. 6 No. 6 Pass Example 14 3 No. 8
No. 6 Pass Example 15 4 No. 13 No. 6 Fail Comparative Example 4 5
No. 15 No. 6 Fail Comparative Example 5
[0109] As is shown in Table 2, when the exterior portion and the
lid of the case were formed of a black colored steel sheet
according to the present invention (cases Nos. 1 to 3), the case
had satisfactory electromagnetic shielding property.
* * * * *