U.S. patent application number 14/086350 was filed with the patent office on 2014-03-20 for method of producing tinned steel sheets.
This patent application is currently assigned to JFE Steel Corporation. The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Hiroki Iwasa, Norihiko Nakamura, Takeshi Suzuki.
Application Number | 20140079886 14/086350 |
Document ID | / |
Family ID | 41318838 |
Filed Date | 2014-03-20 |
United States Patent
Application |
20140079886 |
Kind Code |
A1 |
Suzuki; Takeshi ; et
al. |
March 20, 2014 |
METHOD OF PRODUCING TINNED STEEL SHEETS
Abstract
A method of producing a tinned steel sheet that includes forming
an Sn-containing plating layer on at least one surface of a steel
sheet with a mass per unit area of Sn is 0.05 to 20 g/m.sup.2;
immersing the steel sheet in a chemical conversion solution
containing 60 g/L or more and 200 g/L or less of aluminum phosphate
monobasic and which has a pH of 1.5 to 2.4 or cathodically
electrolyzing the steel sheet at a current density of 10 A/dm.sup.2
or less in the chemical conversion solution; and drying the steel
sheet to form a chemical conversion coating.
Inventors: |
Suzuki; Takeshi; (Tokyo,
JP) ; Nakamura; Norihiko; (Tokyo, JP) ; Iwasa;
Hiroki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
JFE Steel Corporation
Tokyo
JP
|
Family ID: |
41318838 |
Appl. No.: |
14/086350 |
Filed: |
November 21, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12990839 |
Nov 3, 2010 |
|
|
|
PCT/JP2009/059101 |
May 11, 2009 |
|
|
|
14086350 |
|
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Current U.S.
Class: |
427/383.7 ;
205/188 |
Current CPC
Class: |
C23C 28/322 20130101;
C25D 5/50 20130101; C25D 5/505 20130101; Y10T 428/12722 20150115;
C23C 22/20 20130101; C25D 5/36 20130101; C25D 9/08 20130101; C25D
11/36 20130101; C25D 11/34 20130101; C25D 5/48 20130101; C23C 28/34
20130101; C23C 28/321 20130101 |
Class at
Publication: |
427/383.7 ;
205/188 |
International
Class: |
C23C 28/00 20060101
C23C028/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 12, 2008 |
JP |
2008-124856 |
Apr 22, 2009 |
JP |
2009-103900 |
Claims
1. A method of producing a tinned steel sheet comprising: forming
an Sn-containing plating layer on at least one surface of a steel
sheet with a mass per unit area of Sn is 0.05 to 20 g/m.sup.2;
immersing the steel sheet in a chemical conversion solution
containing 60 g/L or more and 200 g/L or less of aluminum phosphate
monobasic and which has a pH of 1.5 to 2.4 or cathodically
electrolyzing the steel sheet at a current density of 10 A/dm.sup.2
or less in the chemical conversion solution; and drying the steel
sheet to form a chemical conversion coating.
2. The method according to claim 1, wherein the Sn-containing
plating layer is a plating layer consisting of a Sn layer or a
plating layer consisting of an Fe--Sn layer and a Sn layer
deposited thereon.
3. The method according to claim 1, wherein drying is performed at
a temperature of lower than 60.degree. C.
4. The method according to claim 1, wherein cathodic electrolyzing
is performed such that the temperature of the chemical conversion
solution is adjusted to 70.degree. C. or higher.
5. The method according to claim 1, wherein the steel sheet is
subjected to the immersing or the cathodic electrolyzing for less
than 1 second.
Description
TECHNICAL FIELD
[0001] This disclosure relates to tinned steel sheets used for DI
cans, food cans, beverage cans, and other cans. The disclosure
particularly relates to a method of producing a tinned steel sheet
having a chemical conversion coating, disposed thereon, containing
no chromium (Cr); a tinned steel sheet; and a chemical conversion
solution.
BACKGROUND
[0002] Tinned steel sheets referred to as "tinplate" have been
widely used as surface-treated steel sheets for cans. In the tinned
steel sheets, chromate coatings are formed on tin plating layers by
chromating in such a manner that steel sheets are immersed in
aqueous solutions containing hexavalent chromium compound such as
bichromic acid or are electrolyzed in the aqueous solutions. This
is because the formation of the chromate coatings prevents the
surface oxidation of the tin plating layers, which are likely to be
oxidized during long-term storage, to suppress the deterioration of
appearance (yellowing) and also prevents cohesive failure due to
the growth of tin (Sn) oxide coatings to secure the adhesion
(hereinafter simply referred to as "paint adhesion") with organic
resins such as paints in the case of painting the tinned steel
sheets.
[0003] In the light of recent environmental issues, efforts to
restrict the use of Cr are being made in every field. For tinned
steel sheets for cans, several chemical conversion techniques
alternative to chromating have been proposed. For example, Japanese
Examined Patent Application Publication No. 55-24516 discloses a
method for surface-treating a tinned steel sheet. In the method, a
chemical conversion coating is formed in such a manner that the
tinned steel sheet is subjected to direct-current electrolyzing in
a phosphate solution using the tinned steel sheet as a cathode.
Japanese Examined Patent Application Publication No. 58-41352
discloses a chemical conversion solution which contains phosphoric
ions, tin ions, and one or more of a chlorate and a bromate and
which has a pH of 3 to 6. Japanese Unexamined Patent Application
Publication No. 49-28539 discloses a method for surface-treating
tinplate. In that method, one or more of calcium phosphate,
magnesium phosphate, and aluminum phosphate are applied to tinplate
to form a coating with a thickness corresponding to 15
.mu.g/cm.sup.2 or less. Japanese Unexamined Patent Application
Publication No. 2005-29808 discloses a surface-treated steel sheet
for containers. In the surface-treated steel sheet, an iron-nickel
(Fe--Ni) diffusion layer, an Ni--Sn alloy layer, and a non-alloyed
Sn layer are arranged on a surface of a steel sheet in that order
and a phosphoric acid coating having a mass per unit area of 1 to
100 mg/m.sup.2 in terms of phosphorus (P) is disposed on the
non-alloyed Sn layer.
[0004] The chemical conversion coatings disclosed in JP '516, JP
'352, JP '539 and JP '808 are less capable of preventing the
deterioration of appearance and the reduction of paint adhesion due
to the surface oxidation of tin plating layers as compared to
conventional chromate coatings.
[0005] Japanese Unexamined Patent Application Publication No.
2007-239091 discloses a method for producing a tinned steel sheet.
In that method, after a steel sheet is tinned, the tinned steel
sheet is immersed in a chemical conversion solution containing tin
ions and phosphoric ions or cathodically electrolyzed in the
chemical conversion solution and a chemical conversion coating is
then formed by heating the tinned steel sheet to a temperature of
60.degree. C. to 200.degree. C., whereby the deterioration of
appearance and the reduction of paint adhesion due to the surface
oxidation of a tin plating layer can be prevented.
[0006] The chemical conversion coating disclosed in JP '091 has
performance equal to or better than that of conventional chromate
coatings. However, that chemical conversion coating has a problem
that the cost of forming this chemical conversion coating is high
because expensive stannous chloride, stannic chloride, tin sulfate,
or the like is used as a tin ion source to form this chemical
conversion coating and a heating unit used subsequently to chemical
conversion is necessary.
[0007] It could therefore be helpful to provide a method for
producing a tinned steel sheet which is capable of preventing the
deterioration of appearance and the reduction of paint adhesion due
to the surface oxidation of a tin plating layer without using Cr
and which can be subjected to chemical conversion at low cost, a
tinned steel sheet, and a chemical conversion solution.
SUMMARY
[0008] We conducted intensive studies on tinned steel sheets
capable of preventing the deterioration of appearance and the
reduction of paint adhesion due to the surface oxidation of tin
plating layers without using Cr and which can be subjected to
chemical conversion at low cost. As a result, we found that it is
effective that a chemical conversion coating is formed in such a
manner that after an Sn-containing plating layer is formed such
that the mass of Sn per unit area is 0.05 to 20 g/m.sup.2, the
Sn-containing plating layer is immersed in a chemical conversion
solution which contains greater than 18 to 200 g/L or less of
aluminum phosphate monobasic and which has a pH of 1.5 to 2.4 or is
cathodically electrolyzed in the chemical conversion solution.
[0009] We thus provide a method of producing a tinned steel sheet.
The method includes forming an Sn-containing plating layer on at
least one surface of a steel sheet such that the mass per unit area
of Sn is 0.05 to 20 g/m.sup.2, immersing the steel sheet in a
chemical conversion solution which contains greater than 18 to 200
g/L or less of aluminum phosphate monobasic and which has a pH of
1.5 to 2.4 or cathodically electrolyzing the steel sheet at a
current density of 10 A/dm.sup.2 or less in the chemical conversion
solution, and drying the steel sheet to form a chemical con-version
coating.
[0010] The Sn-containing plating layer is preferably a plating
layer consisting of a Sn layer or a plating layer consisting of an
Fe--Sn layer and a Sn layer deposited thereon. Drying is preferably
performed at a temperature of lower than 60.degree. C. Cathodic
electrolyzing is preferably performed in such a manner that the
temperature of the chemical conversion solution is adjusted to
70.degree. C. or higher.
[0011] We also provide a tinned steel sheet produced by the
method.
[0012] In the tinned steel sheet, the chemical conversion coating
preferably has a mass per unit area of 1.5 to 10 mg/m.sup.2 in
terms of P and the mass ratio (Al/P) of Al to P in the chemical
conversion coating is preferably 0.20 to 0.87.
[0013] Furthermore, we provide a chemical conversion solution,
having a pH of 1.5 to 2.4, containing greater than 18 to 200 g/L or
less of aluminum phosphate monobasic.
[0014] The following sheet can be produced: a tinned steel sheet
which is capable of preventing the deterioration of appearance and
the reduction of paint adhesion due to the surface oxidation of a
tin plating layer without using Cr and which can be subjected to
chemical conversion at low cost. A chemical conversion coating of a
tinned steel sheet can be formed at a high line speed of 300
m/minute as is formed by current chromating.
DETAILED DESCRIPTION
(1) Formation of Tin-Containing Plating Layer
[0015] The following layer is formed on at least one surface of a
cold-rolled steel sheet, made of low carbon steel or ultra-low
carbon steel, for general cans: a tin-containing plating layer such
as a plating layer (hereinafter referred to as the "Sn layer")
including a Sn layer; a plating layer (hereinafter referred to as
the "Fe--Sn/Sn layer") having a two-layer structure including an
Fe--Sn layer and a Sn layer deposited thereon; a plating layer
(hereinafter referred to as the "Fe--Sn--Ni/Sn layer") having a
two-layer structure including an Fe--Sn--Ni layer and a Sn layer
deposited thereon; or a plating layer (hereinafter referred to as
the "Fe--Ni/Fe--Sn--Ni/Sn layer") having a three-layer structure
including an Fe--Ni layer, an Fe--Sn--Ni layer, and a Sn layer, the
Fe--Sn--Ni layer and the Sn layer being deposited on the Fe--Sn--Ni
layer in that order.
[0016] In the Sn-containing plating layer, the mass per unit area
of Sn needs to be 0.05 to 20 g/m.sup.2. This is because when the
mass per unit area thereof is less than 0.05 g/m.sup.2 or greater
than 20 g/m.sup.2, the plating layer is likely to have low
corrosion resistance or has an increased thickness to cause an
increase in cost, respectively. The mass per unit area of Sn can be
determined by coulometry or X-ray fluorescence surface analysis.
The Sn-containing plating layer may be a continuous layer or a
discontinuous layer with a dotted pattern.
[0017] The Sn-containing plating layer can be formed by a known
process. The Sn-containing plating layer can be formed by the
following procedure: for example, electroplating is performed using
an ordinary tin phenolsulfonate plating bath, tin methanesulfonate
plating bath, or tin halide plating bath such that the mass per
unit area of Sn is 2.8 g/m.sup.2; a plating layer including an
Fe--Sn layer and a Sn layer is formed in such a manner that
reflowing is performed at a temperature not lower than the melting
point of Sn, that is, a temperature of 231.9.degree. C. or higher;
cathodic electrolyzing is performed in a 10-15 g/L aqueous solution
of sodium carbonate at a current density of 1 to 3 A/dm.sup.2 such
that an Sn oxide coating formed on the surface by reflowing is
removed; and water-washing is then performed.
[0018] A Ni-containing layer which may be included in the
Sn-containing plating layer is formed in such a manner that nickel
plating is performed prior to tin plating and annealing is then
performed as required or reflowing is performed subsequently to tin
plating. Hence, a nickel plating unit and complex steps are
necessary. Therefore, the Ni-containing layer is higher in cost
than Ni-free layers. Thus, the Sn-containing plating layer is
preferably an Ni-free layer such as the Sn layer or the Fe--Sn/Sn
layer.
(2) Formation of Chemical Conversion Coating
[0019] A chemical conversion coating is formed on the Sn-containing
plating layer in such a manner that immersion is performed in a
chemical conversion solution which contains greater than 18 to 200
g/L or less of aluminum phosphate monobasic and which has a pH of
1.5 to 2.4 or cathodic electrolyzing is performed at a current
density of 10 A/dm.sup.2 or less in the chemical conversion
solution and drying is then performed. In this operation, water
washing may be performed prior to drying.
[0020] The reason for using the chemical conversion solution, which
contains greater than 18 to 200 g/L or less of aluminum phosphate
monobasic, is as described below. When the concentration of
aluminum phosphate monobasic is 18 g/L or less, the homogeneous
dispersion of Al in the chemical conversion coating is low and the
local excess in mass per unit area causes the deterioration of
paint adhesion and/or corrosion resistance. When the concentration
thereof is greater than 200 g/L, the stability of the chemical
conversion solution is low and precipitates are formed in the
chemical conversion solution to adhere to a tinned steel sheet,
thereby causing the deterioration of appearance and/or the
reduction of paint adhesion. The reason for limiting the pH of the
chemical conversion solution to the range of 1.5 to 2.4 is as
described below. When the pH thereof is less than 1.5, it is
difficult to deposit a coating and a sufficient mass per unit area
cannot be achieved even if the time for chemical conversion is
significantly increased to several tens of seconds. When the pH
thereof is greater than 2.4, it is difficult to control the mass
per unit area because a precipitation reaction occurs quickly
during cathodic electrolyzing and the mass per unit area varies
significantly with respect to the variation of the current density.
The pH thereof can be adjusted by adding an acid such as phosphoric
acid or sulfuric acid or an alkali such as sodium hydroxide to the
chemical conversion solution. The chemical conversion solution may
further contain an accelerator such as FeCl.sub.2, NiCl.sub.2,
FeSO.sub.4, NiSO.sub.4, sodium chlorate, or a nitrite; an etchant
such as a fluorine ion; and a surfactant such as sodium lauryl
sulfate or acetylene glycol.
[0021] Since current chromating is usually performed at a line
speed of 300 m/minute or more and is extremely high in
productivity, novel chemical conversion alternative to chromating
can be preferably performed at at least the same line speed as that
of current chromating. This is because an increase in time for the
chemical conversion requires an increase in the size of a treatment
tank and/or an increase in the number of tanks and therefore causes
an increase in equipment cost and an increase in maintenance cost.
To perform chemical conversion at a line speed of 300 m/minute or
more without the modification of equipment, the time for the
chemical conversion is preferably 2.0 seconds or less as is taken
for current chromating and more preferably one second or less. To
form the chemical conversion coating, immersion or cathodic
electrolyzing needs to be performed in the chemical conversion
solution. The current density during cathodic electrolyzing needs
to be 10 A/dm.sup.2 or less. This is because when the current
density is greater than 10 A/dm.sup.2, the variation range of the
mass per unit area is large with respect to the variation of the
current density and therefore it is difficult to stably secure the
mass per unit area. Processes such as coating and anodic
electrolyzing can be used to form the chemical conversion coating
in addition to immersion and cathodic electrolyzing. For coating,
uneven surface reactions are likely to occur and therefore uniform
appearance is unlikely to be obtained. For anodic electrolyzing, a
powdery coating is likely to precipitate and therefore the
deterioration of appearance and/or paint adhesion is likely to be
caused. Thus, these processes are inappropriate.
[0022] After immersion or cathodic electrolyzing is performed,
drying is preferably performed at a temperature of lower than
60.degree. C. This is because even if the drying temperature is
lower than 60.degree. C., the growth of the Sn oxide coating can be
securely prevented and therefore no special heating unit is
necessary in a producing method. The reason why the growth of the
Sn oxide coating can be securely prevented at a reduced temperature
of lower than 60.degree. C. is not necessary clear but is probably
that the introduction of an Al component into a coating leads to
the formation of a complex phosphate coating with high barrier
properties. The drying temperature is defined as the maximum
temperature of the steel sheet during drying. The temperature of
the chemical conversion solution is preferably adjusted to
70.degree. C. or higher during cathodic electrolyzing. This is
because when the temperature thereof is 70.degree. C. or higher,
the rate of deposition increases with an increase in temperature
and therefore treatment can be performed at a higher line speed.
However, when the temperature thereof is excessively high, the
evaporation rate of water from the chemical conversion solution is
large and therefore the composition of the chemical conversion
solution varies with time. Thus, the temperature of the chemical
conversion solution is preferably 85.degree. C. or lower.
[0023] The chemical conversion coating, which is formed as
described above, preferably has a mass per unit area of 1.5 to 10
mg/m.sup.2 in terms of P. The mass ratio (Al/P) of Al to P in the
chemical conversion coating is preferably 0.20 to 0.87. This is
because when the mass per unit area in terms of P is less than 1.5
mg/m.sup.2 or the mass ratio (Al/P) is less than 0.20, the effect
of preventing the surface oxidation of the Sn-containing plating
layer is insufficient and the deterioration of appearance and the
reduction of paint adhesion are caused. When the mass per unit area
in terms of P is greater than 10 mg/m.sup.2, cohesive failure
occurs in the chemical conversion coating and therefore the paint
adhesion thereof is likely to be reduced. The upper limit of the
mass ratio (Al/P) is 0.87 and is the maximum stoichiometrically
derived from the case where the coating is entirely made of
aluminum tertiary phosphate. The mass per unit area in terms of P
can be determined by X-ray fluorescence surface analysis. The mass
ratio (Al/P) can be determined in such a manner that the mass per
unit area of P and that of Al are measured by X-ray fluorescence
surface analysis.
[0024] To allow the mass per unit area in terms of P to reach 1.5
to 10 mg/m.sup.2 in a short time, the concentration of aluminum
phosphate monobasic is preferably 60 to 120 g/L. To allow the mass
per unit area in terms of P to reach 1.5 to 10 mg/m.sup.2 at a high
line speed, cathodic electrolyzing is more preferable than
immersion and the pH of the chemical conversion solution is
forcibly increased in such a manner that protons located near the
interface between the surface of a tin containing plating layer and
the chemical conversion solution are consumed by generating gaseous
hydrogen by cathodic electrolyzing.
[0025] The chemical conversion solution does not contain Sn, which
is expensive. Therefore, a method for producing a tinned steel
sheet that can be subjected to chemical conversion at low cost can
be provided. The chemical conversion coating, which contains Al and
P, is unavoidably contaminated with Sn migrating from the
Sn-containing plating layer. In this case, the fact remains that
substantially the same advantages can be obtained.
EXAMPLES
[0026] The following sheets were used as raw materials:
[0027] Steel Sheets A that were low-carbon cold-rolled steel sheets
with a thickness of 0.2 mm. Steel Sheets B that were low-carbon
cold-rolled steel sheets with a thickness of 0.2 mm, both surfaces
of the steel sheets were plated with nickel using a Watts bath to
have a mass per unit area of 100 mg/m.sup.2, and then annealed at
700.degree. C. in an atmosphere containing 10 volume percent
H.sub.2 and 90 volume percent N.sub.2, whereby nickel was
diffused.
[0028] After Sn layers were formed on Steel Sheets A and B using a
commercially available tin-plating bath such that the mass per unit
area of Sn was as shown in Table 2, the Sn layers were reflowed at
a temperature not lower than the melting point of Sn, whereby
Sn-containing plating layers each including an Fe--Sn layer and a
Sn layer were formed on Steel Sheets A and Sn-containing plating
layers each including an Fe--Ni layer, an Fe--Ni--Sn layer, and a
Sn layer were formed on Steel Sheets B. To remove surface Sn oxide
coatings formed by reflowing, cathodic electrolyzing was performed
at a current density of 1 A/dm.sup.2 in a 10 g/L aqueous solution
of sodium carbonate at a bath temperature of 50.degree. C. After
Steel Sheets A and B were washed with water and then each
cathodically electrolyzed at a current density for a time as shown
in Table 1 in a chemical conversion solution having an aluminum
phosphate monobasic amount, pH, and temperature shown in Table 1,
Steel Sheets A and B were washed with water, wrung with wringer
rollers, and then dried at room temperature using an ordinary
blower such that chemical conversion coatings were formed, whereby
Sample Nos. 1 to 25 of tinned steel sheets were produced. In Sample
No. 13, the chemical conversion coatings were formed in such a
manner that immersion was performed at one second in a chemical
conversion solution shown in Table 1 instead of cathodic
electrolyzing. In Sample No. 12, the steel sheet was finally dried
with hot air without using any blower in such a manner that the
steel sheet is heated to 70.degree. C. The pH of each chemical
conversion solution shown in Table 1 was adjusted by the addition
of orthophosphoric acid.
[0029] After each layer and coating were formed, the mass per unit
area of Sn in the Sn-containing plating layers, the mass per unit
area of the chemical conversion coatings in terms of P, the mass
per unit area of the chemical conversion coatings in terms of Al,
and the mass ratio (Al/P) were determined by the above-mentioned
methods. The tinned steel sheets were evaluated for appearance
immediately after production, the amount of the Sn oxide coatings
and appearance after long-term storage, paint adhesion, and
corrosion resistance by methods below. Appearance immediately after
production:
[0030] The appearance of each tinned steel sheet was visually
observed immediately after production and then evaluated in
accordance with standards below. A good appearance was rated as A
or B. [0031] A: a good appearance having no surface powdery
precipitates and a metallic luster. [0032] B: a good appearance
having no surface powdery precipitates and a slightly whitish cast.
[0033] C: an uneven appearance having surface powdery precipitates
locally present and a slightly whitish cast. [0034] D: a whitish
appearance having a large amount of surface powdery precipitates.
Amount of Sn Oxide Coatings and Appearance after Long-Term
Storage:
[0035] Each tinned steel sheet was stored for ten days in an
atmosphere having a temperature of 60.degree. C. and a relative
humidity of 70%, the appearance thereof was visually observed, the
amount of the Sn oxide coatings formed thereon was determined in
such a manner that the Sn oxide coatings were electrolyzed at a
current density of 25 .mu.A/cm.sup.2 in a 1/1000 N HBr electrolytic
solution and the charge required for electrochemical reduction was
determined, and the tinned steel sheet was evaluated in accordance
with standards below. A tinned steel sheet having a small amount of
Sn oxide coatings and a good appearance after long-term storage was
rated as A or B. [0036] A: a reduction charge of less than 2
mC/cm.sup.2 and an excellent appearance (better than a chromated
material). [0037] B: a reduction charge of 2 to less than 3
mC/cm.sup.2 and a good appearance (substantially equal to a
chromated material). [0038] C: a reduction charge of 3 to less than
5 mC/cm.sup.2 and a slightly yellowish appearance. [0039] D: a
reduction charge of 5 mC/cm.sup.2 or more and a clearly yellow
appearance.
Paint Adhesion:
[0040] After an epoxy-phenolic paint was applied to some of the
tinned steel sheets immediately after production to have a mass per
unit area of 50 mg/dm.sup.2, the tinned steel sheet was baked at
210.degree. C. for ten minutes. Two of the coated and baked tinned
steel sheets were stacked such that a nylon adhesive film is
sandwiched between the coated surfaces thereof. After the two
tinned steel sheets were laminated under pressing conditions such
as a pressure of 2.94.times.10.sup.5 Pa, a temperature of
190.degree. C., and a pressing time of 30 seconds, the laminate was
divided into specimens with a width of 5 mm. The specimens were
measured for adhesion strength with a tensile tester and then
evaluated in accordance with standards below. A tinned steel sheet
with good paint adhesion was rated as A or B. The tinned steel
sheets were stored for six months in a room temperature atmosphere
and then evaluated for paint adhesion. [0041] A: 19.6 N (2 kgf) or
more (substantially equal to a chromated material for welded cans).
[0042] B: 3.92 N (0.4 kgf) to less than 19.6 N (substantially equal
to a chromated material for welded cans). [0043] C: 1.96 N (0.2
kgf) to less than 3.92 N. [0044] D: less than 1.96 N (0.2 kgf).
Corrosion Resistance:
[0045] After an epoxy-phenolic paint was applied to each tinned
steel sheet to have a mass per unit area of 50 mg/dm.sup.2, the
tinned steel sheet was baked at 210.degree. C. for ten minutes. The
tinned steel sheet was immersed in a commercially available tomato
juice at 60.degree. C. for ten days and then visually evaluated
whether a coating was stripped off and rust was present. A tinned
steel sheet having good corrosion resistance was rated as A or B.
[0046] A: neither stripped coating nor rust. [0047] B: no stripped
coating and a slight number of rust spots. [0048] C: no stripped
coating and fine rust spots. [0049] D: stripped coating and
rust.
[0050] The results are shown in Table 2. Sample Nos. 1 to 18 of the
tinned steel sheets produced by our method each have a good
appearance immediately after production and after long-term
storage, a small amount of Sn oxide coatings after long-term
storage, excellent paint adhesion, and excellent corrosion
resistance.
TABLE-US-00001 TABLE 1 Cathodic Chemical conversion solutions
electrolyzing Amount of Amount of (immersion) aluminum ortho-
conditions Drying Steel sheets phosphate phosphoric Current
Attained Sample for raw monobasic acid Temperature density Time
temperature Nos. materials (g/L) (g/L) pH (.degree. C.)
(A/dm.sup.2) (s) System (.degree. C.) Remarks 1 A 19 8.5 1.74 70 4
1 Blower Room temperature Inventive example 2 A 19 4.2 1.97 70 4 1
Blower Room temperature Inventive example 3 A 19 3.0 2.08 70 4 1
Blower Room temperature Inventive example 4 A 54 3.0 2.12 80 6 1
Blower Room temperature Inventive example 5 A 19 20.0 1.60 70 4 2
Blower Room temperature Inventive example 6 A 19 8.5 1.74 50 4 1
Blower Room temperature Inventive example 7 A 60 8.5 1.80 50 4 0.5
Blower Room temperature Inventive example 8 A 80 8.5 1.80 50 4 0.5
Blower Room temperature Inventive example 9 A 120 8.5 1.80 50 4 0.5
Blower Room temperature Inventive example 10 A 200 8.5 1.80 50 4
0.5 Blower Room temperature Inventive example 11 A 19 8.5 1.74 70 4
1 Blower Room temperature Inventive example 12 A 60 8.5 1.80 50 4
0.5 Hot air drying 70 Inventive example 13 A 60 8.5 1.80 70
Immersion 0.8 Blower Room temperature Inventive example 14 A 19 8.5
1.74 70 5 1 Blower Room temperature Inventive example 15 B 19 8.5
1.74 70 5 1 Blower Room temperature Inventive example 16 A 19 8.5
1.74 70 3 1 Blower Room temperature Inventive example 17 B 19 8.5
1.74 70 3 1 Blower Room temperature Inventive example 18 A 80 0
1.91 70 4 0.5 Blower Room temperature Inventive example 19 B 2 8.5
1.73 70 4 1 Blower Room temperature Comparative example 20 A 250
8.5 2.00 70 4 2 Blower Room temperature Comparative example 21 A 60
8.5 1.30 85 6 20 Blower Room temperature Comparative example 22 A
60 8.5 2.50 50 4 0.5 Blower Room temperature Comparative example 23
A 10 30.0 1.80 70 4 2 Blower Room temperature Comparative example
24 A * 6.0 2.10 60 6 1 Blower Room temperature Comparative example
25 A 19 8.5 2.08 70 15 1 Blower Room temperature Comparative
example * 2.7 g/L of SnCl.sub.4.cndot.5H.sub.2O
TABLE-US-00002 TABLE 2 Sn- containing Chemical conversion coatings
Amount of Sn plating layers Mass per unit Mass per unit Appearance
oxide coatings Paint adhesion Mass per unit area of in area of in
Mass immediately and appearance Immediately Sample area of Sn terms
of P terms of Al ratio after immediately after After 6 Corrosion
Nos. (g/m.sup.2) (mg/m.sup.2) (mg/m.sup.2) (Al/P) production after
production production months resistance Remarks 1 2.8 3.2 1.7 0.53
A A B B A Inventive example 2 2.8 4.5 2.4 0.53 A A B B A Inventive
example 3 2.8 6.5 3.5 0.54 A A B B A Inventive example 4 2.8 9.5
5.1 0.54 B A B B B Inventive example 5 2.8 1.8 1.0 0.56 A A B B A
Inventive example 6 2.8 2.5 1.4 0.56 A A B B A Inventive example 7
2.8 3.0 1.6 0.53 A A B B A Inventive example 8 2.8 4.0 2.2 0.55 A A
B B A Inventive example 9 2.8 5.0 2.9 0.58 A A B B A Inventive
example 10 2.8 5.1 3.0 0.59 A A B B A Inventive example 11 2.8 3.2
1.7 0.53 A A B B A Inventive example 12 2.8 3.0 1.6 0.53 A A B B A
Inventive example 13 2.8 1.8 1.4 0.78 A A B B A Inventive example
14 1.1 3.3 1.8 0.55 A A B B A Inventive example 15 1.1 3.4 1.8 0.53
A A B B A Inventive example 16 0.1 3.6 1.9 0.53 A A A A B Inventive
example 17 0.1 3.7 2.0 0.54 A A A A B Inventive example 18 2.8 4.1
2.2 0.54 A A B B A Inventive example 19 2.8 2.5 0.5 0.20 A C B C C
Comparative example 20 2.8 11.0 7.6 0.69 D A D D C Comparative
example 21 2.8 1.4 0.7 0.50 A D B D B Comparative example 22 2.8
12.0 6.7 0.56 C A C C C Comparative example 23 2.8 5.4 2.9 0.54 A A
C C C Comparative example 24 2.8 10.8 0.0 0.00 B D B D A
Comparative example 25 2.8 140.0 65.8 0.47 D A D D D Comparative
example
INDUSTRIAL APPLICABILITY
[0051] The following sheet can be produced: a tinned steel sheet
that is capable of preventing the deterioration of appearance and
the reduction of paint adhesion due to the surface oxidation of a
tin plating layer without using Cr, which causes environmental
problems, and that can be subjected to chemical conversion at low
cost. A chemical conversion coating of a tinned steel sheet can be
formed at a high line speed of 300 m/minute as is formed by current
chromating. This is a great contribution to industry.
* * * * *