US20070264522A1

US20070264522A1 - Black Colored Steel Sheet Having Excellent Electromagnetic Shielding Property, Electromagnetic Shielding Member, and Electromagnetic Shielding Case

Info

Publication number: US20070264522A1
Application number: US11/629,931
Authority: US
Inventors: Rie Umebayashi; Chiyoko Tada; Hiroki Nakamaru
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2004-08-31
Filing date: 2005-08-24
Publication date: 2007-11-15
Also published as: WO2006025418A1; KR20070037506A

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 μm. The Zn—Ni-plated steel sheet having a blackening-treated layer has an arithmetical mean roughness Ra of 0.7 to 2.0 μm and the number of peaks per inch (PPI) of at least 180.

Description

TECHNICAL FIELD

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.
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

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.
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.
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.
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).
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).
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.
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.
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.
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.
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

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.
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.
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.
The present invention is based on these findings.
The present invention provides the following:
(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 μm;
the Zn—Ni-plated steel sheet having the blackening-treated layer has an arithmetical mean roughness Ra of 0.7 to 2.0 μm and the number of peaks per inch (PPI) of at least 180.
(2) The black colored steel sheet according to (1), wherein the number of peaks per inch (PPI) is 200 to 400.
(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≦(0.010x−0.0077)y−1.05x+2.16 (1)
wherein x denotes the arithmetical mean roughness Ra (μm), 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/m²).
(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
(5) An electromagnetic shielding case formed of the black colored steel sheet according to any one of (1) to (3).
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.
The present invention can also provide an excellent electromagnetic shielding member all or part of which is formed of the black colored steel sheet.
The present invention can also provide an excellent electromagnetic shielding case formed of the black colored steel sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an optimum range of the PPI (y) and the coating weight of coating film z (g/m²) when the Ra (x) is 2.0 μm and the PPI (y) is in the range of 200 to 400.
FIG. 2 is a schematic view of an apparatus for measuring leak noise to evaluate the electromagnetic shielding property.
FIG. 3 is a schematic view of a case to evaluate the electromagnetic shielding property.
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).
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.
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.
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

The present invention will specifically be described below.
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 μm in the present invention.
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 μm, 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 μm or less. When the thickness of the blackening-treated layer is less than 0.01 μm, the black appearance is poor. Hence, the thickness of the blackening-treated layer is set to at least 0.01 μm.
The following is a reason for limiting the arithmetical mean roughness Ra to 0.7 to 2.0 μm 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.
Existing common blackening-treated Zn—Ni-plated steel sheets have an Ra in the range of about 0.5 to 1.5 μm and the number of peaks per inch (PPI) in the range of about 120 to 160.
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 μm, 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 μm or more as measured as Ra.
When the Ra is more than 2.0 μm, 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 μm or less as measured as Ra.
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.
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.
The thickness of a Zn—Ni-plated layer is suitably set to about 1 to 5 μm to achieve high corrosion resistance and strong adhesion.
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 μm with a blackening-treated layer.
The blackening treatment may be performed by anodic electrolysis, cathodic electrolysis, alternating electrolysis, or anodic oxidation in a common treatment liquid.
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.
Then, at least one coating film is formed on a blackening-treated layer thus formed, because the blackening-treated surface has insufficient corrosion resistance.
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.
Preferably, the coating weight of coating film is at least 0.6 g/m²to achieve sufficient corrosion resistance.
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 μm and the PPI is 180, the upper limit is preferably about 2.3 g/m²or less per side of the steel sheet to achieve excellent electromagnetic shielding property.
When the Ra is 2.0 μm and the PPI is 200, the upper limit of the coating weight of coating film is preferably about 2.5 g/m²or less per side of the steel sheet.
When the Ra is 2.0 μm and the PPI is 400, the coating weight of coating film is preferably about 5.0 g/m²or less per side of the steel sheet.
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≦(0.010x−0.0077)y−1.05x+2.16 (1)
wherein x denotes the arithmetical mean roughness Ra (μm), 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/m²).
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/m²) when Ra (x)=2.0 μm is substituted into the equation (1). The coating weight of coating film is preferred in the range illustrated in FIG. 1.
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.
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.
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.
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.
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.
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.
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 μm 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 μm and the PPI of at least 220.
The electromagnetic shielding property in the present invention is evaluated by measuring leak noise with an apparatus illustrated in FIG. 2.
A 20 MHz clock 4 is placed as a source of noise in a 100 mm×100 mm×100 mm aluminum case 3 formed of an aluminum sheet having a thickness of 2 mm. The aluminum case 3 has an 80 mm×80 mm opening at the top surface. The opening is surrounded by a 10 mm frame 5 protruding inside. A 10 mm×1 mm gasket (urethane sponge wrapped in a conductive cloth (Cu—Ni-plated fiber)) 6 is placed on the frame 5. A 100 mm×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.
When the present invention was used as a case material, the electromagnetic shielding property was evaluated with a case illustrated in FIG. 3.
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.
Power supply: a built-in power supply was used without modification.
Motherboard: A-Open, AX4SG Max II
CPU: Intel Corporation, Pentium 4 processor 3 GHz
DDR memory: generic 250 MB×2
HDD: Hitachi Global Storage Technologies, HDS722516VLAT80 (capacity 160 GB)
Optical drive: Pioneer Corporation, DVR-A08-J

EXAMPLES

Example 1

Samples Nos. 1 to 13 in Table 1 were manufactured according to the steps of an annealed cold-rolled steel sheet→temper rolling (control of surface roughness)→electroplated Zn—Ni coating→anodic oxidation (blackening treatment)→formation of a coating film, or an annealed cold-rolled steel sheet→blasting (control of surface roughness)→electroplated Zn—Ni coating→anodic oxidation (blackening treatment)→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 μm. The thickness of the blackening-treated layer was controlled by the electrolysis time and pH in the anodic oxidation.
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° C. in 21 seconds.
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.
Their characteristics were evaluated as follows.
<Surface Roughness>
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 μm 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 μm were counted to determine the number of peaks per inch (PPI).
<Thickness of Blackening-Treated Layer>
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>
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>
A 50 mm×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.
Excellent: 5% or less
Good: more than 5% and 10% or less
Fair: more than 10% and 20% or less
Poor: more than 20%
<Black Appearance>
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.
Good: L* of 25 or less
Poor: L* of more than 25
<Electromagnetic Shielding Property>
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.
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×log(10^0.1d1+10^0.1d2+ . . . +10^0.1dn) (2)
n: number of peaks
d1, d2, . . . dn: peak value
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).
The samples according to Examples were evaluated according to the following criteria:
(I−Ib)/(Ia−Ib)>0.35: Poor
0.35≧(I−Ib)/(Ia−Ib)>0.26: Fair
0.26≧(I−Ib)/(Ia−Ib)>0.20: Good
0.20≧(I−Ib)/(Ia−Ib)>0.13: Good⁺
0.13≧(I−Ib)/(Ia−Ib)≧0: Excellent

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 1


				Thickness of
				blackening-	Amount of	Corrosion
Sample	Ra (x)		Value of	treated layer	covering layer	resistance of	Black	Electromagnetic
No.	(μm)	PPI (y)	Eq. (1)	(μm)	(z) (g/m²)	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

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 μm and had an arithmetical mean roughness Ra in the range of 0.7 to 2.0 μm 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.
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

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.
The electromagnetic shielding property of the tower-type PC case was evaluated as follows.
<Electromagnetic Shielding Property of Electronic Equipment and Electrical Equipment>
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.
Pass: satisfy Class B

Fail: does not satisfy Class B

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

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.

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 μm;

the Zn—Ni-plated steel sheet having the blackening-treated layer has an arithmetical mean roughness Ra of 0.7 to 2.0 μm 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≦(0.010x−0.0077)y−1.05x+2.16 (1)

wherein x denotes the arithmetical mean roughness Ra (μm), 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/m²).

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≦(0.010x−0.0077)y−1.05x+2.16 (1)

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.