Method for manufacturing plated steel sheet for cans

A method of manufacturing a plated steel sheet for cans which must have a good outer appearance for printing and high image clarity. The method comprises forming a plating layer directly on the surface of a steel sheet having an average central surface roughness of 0.10 .mu.m or less, the plating layer being formed by vacuum deposition. The plating layer is a metal or a metal alloy, has a thickness of from 0.01 to 5 .mu.m, and has a high image clarity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for manufacturing a plated steel 
sheet used for printed cans such as food cans and drink cans. 
2. Description of the Related Art 
Cans such as food and drink cans used for canned food or cans used for 
protecting sweet stuffs or dried goods against humidity are made of a 
material such as a tin-plated steel sheet, a chromium-plated steel sheet, 
or an aluminum sheet. Most such cans have printing on their outer surfaces 
to obtain a good outer appearance. In particular, a demand for cans used 
for gifts or souvenirs depends on designs, colors, and color tones of the 
cans. That is, a good outer appearance is important for these cans. 
In order to obtain a good outer appearance, a white coating is formed on 
the surface of a steel sheet to cover a color inherent to a metal sheet so 
that colors can be painted thereon, or printing is performed on the 
surface of a steel sheet using a transparent ink to utilize the color or 
gloss of the metal. In either method, the higher the image clarity of the 
plated steel sheet surface is, the clearer and more beautiful the printed 
colors become. Since low image clarity of a steel sheet allows only dark 
colors, the image clarity on the surface of a steel sheet must be high 
with any method used. 
A conventional plated steel sheet for cans is obtained by forming an 
electroplated layer on the surface of a steel sheet having a surface 
roughness Ra (average central roughness) of 0.12 to 3.0 .mu.m. The image 
clarity of this steel sheet is, however, unsatisfactory. No steel sheet 
capable of solving this problem has been developed. 
As for steel sheets for applications other than cans, e.g., for decoration, 
bright chromium plating, bright nickel plating, and the like have improved 
the image clarity of a steel sheet. In these methods, however, a suitable 
plating condition range is narrow, and complicated treatment steps are 
required. Therefore, these methods cannot be used for can materials which 
must be mass-produced at low cost. Japanese Patent Disclosure (Kokai) No. 
60-67653 discloses a method in which a paint is coated on a steel sheet to 
flatten its surface and then a metal is dry-plated thereon. This method 
is, however, not suited to can materials because electrical welding or 
mass production cannot be performed. 
SUMMARY OF THE INVENTION 
It is, therefore, a first object of the present invention to provide a 
plated steel sheet for cans having high image clarity. 
It is a second object of the present invention to provide a plated steel 
sheet for cans which can be manufactured by simple steps at low cost. 
In order to achieve the above objects of the present invention, there is 
provided a plated steel sheet for cans manufactured by forming plating 
layer by vacuum deposition on the surface of a steel sheet having an 
average central surface roughness of 0.10 .mu.m or less. 
In this specification, the term "average central surface roughness" means, 
as defined in JIS BO601, a value of Ra represented by the following 
equation in units of microns and obtained when a portion of measurement 
length l is extracted from a roughness curve along its central line and 
assuming that a central line of the extracted portion is an X axis and a 
direction of vertical magnification thereof is a Y axis so that the 
roughness curve is represented by y=f(x): 
##EQU1## 
The measurement length is basically three times or more of the cut-off 
value. 
According to this plated steel sheet, the surface roughness of a steel base 
sheet is reduced more and flattened more than that of a conventional steel 
sheet, and a plating layer is formed by vacuum deposition. As a result, a 
plated steel sheet having high image clarity which cannot be obtained by 
conventional methods can be manufactured. Therefore, when printing is 
performed on the surface of this plated steel sheet of the present 
invention, a good outer appearance can be obtained with high image 
clarity. In addition, a method of the present invention can manufacture 
such a plated steel sheet at low cost without complicated manufacturing 
steps as in a conventional method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In consideration of the fact that, recently, cold rolled steel sheets 
having less surface roughness can be manufactured as rolling techniques 
have been developed, the present inventors examined relationships between 
the surface roughness of a steel base sheet and the roughness on a plated 
surface, between roughness and gloss, and between gloss and image clarity. 
As a result of comparing these relationships for base sheets having 
largely different surface roughnesses, it is found that as the average 
central surface roughness Ra of the steel sheet is reduced, both the gloss 
and image clarity are improved. As a result of comparing these 
relationships for base sheets in which Ra falls within a narrow range, it 
is found that Ra or the image clarity differs on surfaces having the same 
brightness. At the same time, a correlation was found between the surface 
roughness of the base sheet and the image clarity of the plated surface. 
Therefore, the present inventors further examined the relationship between 
the type of plating and the image clarity. As a result, it was found that 
a plated steel sheet obtained by vacuum depositing a flat base sheet has 
very high image clarity. This relationship is shown in FIG. 1. The image 
clarity was estimated using a plate having black and white stripes with a 
predetermined interval therebetween reflected by a test plate for 
measuring the image clarity and the reflected black and white stripes were 
observed to check difficulty of discrimination. Assuming that an 
estimation point of a commercially available mirror was 10 and that of a 
cold rolled steel sheet having Ra of 0.2 .mu.m was 1, sensitivity was 
estimated in ten levels in accordance with the difficulty of 
discrimination. That is, higher estimation points represent higher image 
clarity. FIG. 1 shows the relationship between the surface roughness and 
image clarity of a base sheet. In FIG. 1, the broken line represents the 
image clarity on the surface of a cold rolled steel sheet; an alternate 
long and short dashed line, the image clarity on the surface of a 
0.5-.mu.m thick electroplated layer formed on the steel sheet; and a solid 
line, the image clarity on the surface of a 1-.mu.m thick vacuum 
deposition layer formed on the steel sheet. FIG. 1 shows that when the 
surface roughness Ra of the base sheet is reduced so as to be smaller than 
0.1 .mu.m, the image clarity increases rapidly. This phenomenon is more 
significant on the vacuum deposited surface than on the base sheet 
surface. In contrast, no such clear relationship is found on the 
electroplated surface. That is, the image clarity on the plated surface is 
not significantly improved even when Ra of the base sheet is reduced to be 
smaller than 0.1 .mu.m. 
As shown in FIG. 3, a steel sheet of the present invention is obtained by 
forming dry-plated layer 2 on the surface of steel sheet 1. The steel 
sheet is preferably an aluminum-killed steel sheet manufactured by 
FCL-rinsing a steel sheet after cold rolling and then performing refining 
rolling to adjust the shape and roughness of the resultant steel sheet. 
Surface roughness Ra of this steel sheet is 0.1 .mu.m or less. A steel 
sheet having such surface roughness can be easily manufactured by a person 
of ordinary skill in the art. The surface roughness is measured by a 
feeler-type roughness meter. This meter feels the surface of an object to 
be measured in a predetermined direction by a feeler and amplifies the 
vertical movement of the feeler. According to the steel sheet of the 
present invention, a dry-plated surface is formed on the surface of the 
steel sheet by vacuum deposition. The composition of the vacuum deposited 
surface is not limited to Sn, Cr, Ni or Al conventionally used as a plated 
steel sheet for cans or can materials or various metals or alloys used as 
a plating layer in bright electroplating, but may be selected from a wide 
choice of metals or alloys. If the thickness of the vacuum deposition 
plating layer of the present invention is less than 0.01 .mu.m, no 
satisfactory image clarity can be obtained. Although the image clarity 
increases as the thickness of the plating layer is increased, a thickness 
exceeding 5 .mu.m is economically not practical. Therefore, the thickness 
of the plating layer is preferably 0.01 to 5 .mu.m. Forming of the plating 
layer is performed in a vacuum. More specifically, conventionally used 
vacuum deposition methods such as chemical vapor deposition, ion plating, 
vapor deposition and sputtering can be used. 
The present invention will be described in more detail by way of an 
example. 
Steel sheets having various surface roughnesses were subjected to vacuum 
deposition such as vapor deposition, ion plating and electroplating, and 
the image clarity on each obtained plated surface was checked. 
(1) Vapor deposition conditions were as follows: 
vacuum degree; 6.times.10.sup.-6 Torr 
temperature of a steel sheet to be plated; 200.degree. C. 
plating metal evaporating method; 
electron 
beam 
heating 
distance between steel sheet and crucible containing plating metal; 50 cm 
(2) Ion plating conditions were as follows: 
vacuum degree; 6.times.10.sup.-6 Torr 
temperature of steel sheet to be plated; 200.degree. C. 
plating metal evaporating method; 
electron 
beam 
heating 
distance between steel sheet and crucible containing plating metal; 50 cm 
bias voltage; 500 V 
ionizing current; 25 V 
(3) electroplating was Sn plating from a ferrostan bath or Cr plating from 
a chromic acid bath. 
The results are shown in Table 1 and FIG. 2. In FIG. 2, symbols represent 
the types of plating as follows: 
. . . vapor deposition of 1.5 .mu.m of Al; 
. . . ion plating of 0.1 .mu.m of Al; 
.DELTA. . . . vapor deposition of 0.5 .mu.m of Sn; 
. . . vapor deposition of 0.05 .mu.m Cr; 
.circle.. . . electroplating of 0.5 .mu.m of Sn; and 
. . . electroplating of 0.5 .mu.m of Cr. 
As is apparent from FIG. 2, when the surface roughness Ra of the base sheet 
of the steel sheet vacuum deposition plated by vapor deposition or ion 
plating was reduced below 0.1 .mu.m, the image clarity was significantly 
improved. In the steel sheet electroplated by either Sn plating or Cr 
plating, however, the image clarity was not improved as much as in the 
vacuum deposition sheet. Since Ra of a base sheet of a conventional plated 
steel sheet for cans was 0.12 .mu.m to 3.0 .mu.m, its image clarity was at 
most 2. In contrast, in the plated steel sheet of the present invention 
manufactured by performing vacuum deposition of a plating layer on the 
base sheet having a surface roughness Ra of 0.1 .mu.m or less, the image 
clarity was significantly improved to 4 to 6. 
TABLE 1 
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Evalua- 
tion 
Rough- Thick- of image 
ness ness clarity 
of base of plated (average 
sheet Plated layer Plating of 
No. Ra .mu.m 
layer .mu.m method 3 points) 
______________________________________ 
Pre- 1 0.11 Al 1.5 Vacuum 3.3 
sent deposition 
Inven- 
2 0.08 " " Vacuum 4.3 
tion deposition 
.dwnarw. 
3 0.12 " " Vacuum 2.0 
deposition 
4 0.05 " " Vacuum 4.8 
deposition 
5 0.03 " " Vacuum 5.7 
deposition 
6 0.16 " " Vacuum 1.5 
deposition 
7 0.04 " 0.1 Ion plating 
5.2 
8 0.08 " " " 4.0 
9 0.10 " " " 2.3 
10 0.14 " " " 2.0 
11 0.09 " " " 3.5 
12 0.03 Cr 0.05 Vacuum 5.0 
deposition 
13 0.05 " " Vacuum 5.3 
deposition 
14 0.09 " " Vacuum 4.0 
deposition 
15 0.07 " " Vacuum 4.8 
deposition 
16 0.11 " " Vacuum 2.2 
deposition 
17 0.13 " " Vacuum 1.5 
deposition 
18 0.16 " " Vacuum 1.2 
deposition 
19 0.14 Sn 0.5 Vacuum 1.5 
deposition 
20 0.08 " " Vacuum 4.2 
deposition 
21 0.06 " " Vacuum 4.3 
deposition 
22 0.04 " " Vacuum 4.8 
deposition 
Com- 23 0.12 Sn 0.5 electro- 
1.7 
para- plating 
tive 24 0.15 " " electro- 
1.3 
Exam- plating 
ple 25 0.04 " " electro- 
2.5 
.dwnarw. plating 
26 0.11 Cr 0.05 electro- 
1.5 
plating 
27 0.17 " " electro- 
1.3 
plating 
28 0.08 " " electro- 
2.1 
plating 
29 0.03 " " electro- 
2.8 
plating 
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