[Name of Document] DESCRIPTION
[Title of the Invention] Sn-BASED PLATED STEEL SHEET
[Technical Field]
[0001]
5 The present invention relates to a Sn-based plated steel sheet.
[Background Art]
[0002]
A tin (Sn) plated steel sheet is well known as "tinplate" and widely used for
purposes of cans such as a beverage can, a food can, and other purposes. This is because
10 Sn is safe for the human body and beautiful metal. The Sn-based plated steel sheet is
mainly manufactured by an electroplating method. This is because the electroplating
method is more advantageous than a hot-dip plating method to control the use amount of
Sn, which is relatively expensive metal, to a minimum amount. After plating or after
being given beautiful metallic luster by a heating and melting treatment after plating, the
15 Sn-based plated steel sheet is often subjected to chromate coating on a Sn-based plating
layer by a chromate treatment (electrolytic treatment, immersion treatment, and the like)
using a solution of hexavalent chromate. Examples of an effect of the chromate coating
include prevention of yellowing of an external appearance owing to suppression of
oxidation of a surface of the Sn-based plating layer, prevention of deterioration in coating
20 film adhesiveness due to cohesive failure of tin oxide when painted for use, improvement
in sulphide stain resistance.
[0003]
On the other hand, recently, it is required that a final product does not contain
hexavalent chromium and the chromate treatment itself is not performed because of an
25 increase in awareness of the environment and safety. However, a Sn-based plated steel
sheet without the chromate coating yellows in the external appearance due to growth of the
2
tin oxide as mentioned above. Therefore, there are some proposed Sn-based plated steel
sheets subjected to a coating treatment in place of the chromate coating.
[0004]
For example, the following Patent Document 1 proposes a Sn-based plated steel
5 sheet in which a coating containing P and Si is formed by a treatment using a solution
containing phosphate ions and a silane coupling agent.
[0005]
The following Patent Document 2 proposes a Sn-based plated steel sheet in which
a coating containing: Al and P; at least one kind selected from Ni, Co, and Cu; and a
10 reaction product with a silane coupling agent is formed by treatment with a solution
containing aluminum phosphate.
[0006]
The following Patent Document 3 proposes a method of manufacturing a
Sn-based plated steel sheet without a chromate coating of plating Zn on a Sn-based plating
15 and then heating the steel sheet until a Zn independent plating layer vanishes.
[0007]
The following Patent Documents 4 and 5 propose a steel sheet for containers with
a chemical conversion coating containing zirconium, phosphoric acid, phenolic resin, and
the like.
20 [0008]
The following Patent Document 6 proposes a Sn-based plated steel sheet with a
Sn-based plating layer and a conversion treatment layer containing tin oxide and tin
phosphate, which is formed by a cathode electrolytic treatment followed by an anode
electrolytic treatment in an aqueous phosphate solution after the formation of the Sn-based
25 plating layer. Patent Document 6 also proposes that alternating electrolysis, in which a
cathode electrolytic treatment and an anode electrolytic treatment are alternated, may be
3
performed when forming the coating.
[0009]
The following Patent Document 7 proposes a Sn-based plated steel sheet with a
coating film containing tin oxide and Zr, Ti, and P.
5 [Prior Art Document]
[Patent Document]
[0010]
Patent Document 1: Japanese Laid-open Patent Publication No. 2004-060052
Patent Document 2: Japanese Laid-open Patent Publication No. 2011-174172
10 Patent Document 3: Japanese Laid-open Patent Publication No. S63-290292
Patent Document 4: Japanese Laid-open Patent Publication No. 2007-284789
Patent Document 5: Japanese Laid-open Patent Publication No. 2010-013728
Patent Document 6: Japanese Laid-open Patent Publication No. 2009-249691
Patent Document 7: International Publication Pamphlet No. WO 2015/001598
15 [Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0011]
The methods proposed in the above Patent Documents 1 to 7 have a problem that
corrosion resistance is slightly inferior to that of chromate coating tinplate, and there has
20 been room for improvement in the corrosion resistance. Therefore, there has been a need
for a Sn-based plated steel sheet with superior corrosion resistance as well as yellowing
resistance, coating film adhesiveness, and sulphide stain resistance.
[0012]
The present invention has been made in consideration of the above problems, and
25 an object thereof is to provide a Sn-based plated steel sheet excellent in corrosion
resistance, yellowing resistance, coating film adhesiveness, and sulphide stain resistance
4
without the use of a chromate coating.
[Means for Solving the Problems]
[0013]
To solve the above problem, the inventors have studied diligently and found that it
5 is possible to achieve a Sn-based plated steel sheet with better corrosion resistance than
before by forming a coating layer containing zirconium oxide on a surface of a Sn-based
plated steel sheet and by setting the distribution of a crystal structure of the zirconium
oxide in the coating layer to a specific state.
The summary of the present invention completed based on the above findings is as
10 follows.
[0014]
(1) A Sn-based plated steel sheet includes: a steel sheet; a Sn-based plating layer located on
at least one surface of the steel sheet; and a coating layer located on the Sn-based plating
layer, wherein: the Sn-based plating layer contains 1.0 g/m2 to 15.0 g/m2 of Sn per side in
15 terms of metal Sn; the coating layer contains zirconium oxide and a content of the
zirconium oxide is 1.0 mg/m2 to 10.0 mg/m2 per side in terms of metal Zr; the zirconium
oxide includes zirconium oxide with an amorphous structure, and a crystalline layer whose
main component is zirconium oxide with a crystalline structure is present on an upper layer
of the zirconium oxide with the amorphous structure.
20 Here, in an electron beam diffraction pattern, the crystalline structure is
determined when a clear diffraction spot is obtained, and the amorphous structure is
determined when a continuous ring-shaped diffraction pattern is obtained instead of the
clear diffraction spot.
(2) The Sn-based plated steel sheet according to (1), wherein the crystalline layer in the
25 coating layer includes an uppermost surface portion of the coating layer, and the number of
detected locations of the crystalline layer is at least one or more in order from the
5
uppermost surface portion in a thickness direction.
Here, the uppermost surface portion means a portion including an uppermost
surface of the coating layer among each of 10 equal portions of the coating layer in the
thickness direction at any position of the coating layer, and the number of detected
5 locations of the crystalline layer means the number of locations determined to be the
crystalline structure among 10 measured locations in the electron beam diffraction pattern
at a center portion of the thickness direction of each portion among 10 equal portions
where the coating layer is divided into 10 equal portions in the thickness direction at any
position of the coating layer.
10 (3) The Sn-based plated steel sheet according to (2), wherein the number of detected
locations of the crystalline layer is five or less, including the uppermost surface portion of
the coating layer and in order from the uppermost surface portion in the thickness
direction.
[Effect of the Invention]
15 [0015]
As explained above, according to the present invention, it is possible to provide a
Sn-based plated steel sheet excellent in corrosion resistance, yellowing resistance, coating
film adhesiveness, and sulphide stain resistance without performing the conventional
chromate treatment.
20 [Embodiments for Carrying out the Invention]
[0016]
Hereinafter, preferable embodiments of the present invention will be explained in
detail.
Note that the term "step" in this specification includes not only an independent
25 step but also a step even if it cannot be discriminated from other steps but if its desired
object can be achieved. The term "steel sheet" in this specification means a base material
6
steel sheet (so-called plating substrate) being an object on which a Sn-based plating layer
and a coating layer are to be formed.
[0017]
Further, the present invention explained below relates to a Sn-based plated steel
5 sheet widely used for purposes of cans such as a food can and a beverage can and other
purposes and a manufacturing method of the Sn-based plated steel sheet. More concretely,
the present invention relates to a Sn-based plated steel sheet more excellent in corrosion
resistance (in more detail, post-coating corrosion resistance), yellowing resistance, coating
film adhesiveness, and sulphide stain resistance without performing the conventional
10 chromate treatment and a manufacturing method of the Sn-based plated steel sheet.
[0018]
Concretely, a Sn-based plated steel sheet according to this embodiment includes: a
steel sheet; a Sn-based plating layer located on at least one surface of the steel sheet; and a
coating layer located on the Sn-based plating layer. Here, the Sn-based plating layer
15 contains 1.0 g/m2 to 15.0 g/m2 of Sn per side in terms of metal Sn. The coating layer
contains zirconium oxide, and a content of the zirconium oxide is 1.0 mg/m2 to 10.0 mg/m2
per side in terms of metal Zr. The zirconium oxide includes zirconium oxide with an
amorphous structure, and a crystalline layer whose main component is zirconium oxide
with a crystalline structure is present on an upper layer of the zirconium oxide with the
20 amorphous structure.
[0019]
Hereinafter, a Sn-based plated steel sheet and a manufacturing method thereof
according to this embodiment will be described in detail.
[0020]
25
Steel sheets are not limited, and any steel sheet commonly used for Sn-based
7
plated steel sheets for containers can be used. Such steel sheets include, for example, low
carbon steel and ultra-low carbon steel. A manufacturing method and material of steel
sheets are also not limited. For example, steel sheets manufactured through processes
such as casting, hot rolling, pickling, cold rolling, annealing, and temper rolling can be
5 used.
[0021]

A Sn-based plating layer is formed on at least one surface of the steel sheet as
described above, and corrosion resistance of the steel sheet is improved by the Sn-based
10 plating layer. The term "Sn-based plating layer" as used herein refers not only to a
Sn-based plating layer with metal Sn alone, but also to a Sn-based plating layer containing
alloys of metal Sn and metal Fe, metal Ni, and at least one of a trace element other than
metal Sn or impurities (for example, Fe and Ni, Ca, Mg, Zn, Pb, Co, and the like).
[0022]
15 The Sn-based plating layer contains 1.0 g/m2 to 15.0 g/m2 of Sn per side in terms
of metal Sn. In other words, a coating weight of the Sn-based plating layer per side is 1.0
g/m2 to 15.0 g/m2 by metal Sn amount (that is, in terms of metal Sn). When the coating
weight of the Sn-based plating layer per side by metal Sn amount is less than 1.0 g/m2, the
corrosion resistance is poor, which is undesirable. The corrosion resistance is excellent
20 when the coating weight of the Sn-based plating layer per side by metal Sn amount is 1.0
g/m2 or more. The coating weight of the Sn-based plating layer per side by metal Sn
amount is preferably 2.0 g/m2 or more, and more preferably 5.0 g/m2 or more. On the
other hand, when the coating weight of the Sn-based plating layer per side by metal Sn
amount exceeds 15.0 g/m2, an effect of metal Sn in improving corrosion resistance is
25 sufficient, and a further increase in the coating weight is not desirable from an economic
standpoint. When the coating weight of the Sn-based plating layer per side by metal Sn
8
amount exceeds 15.0 g/m2, coating film adhesiveness also tends to decrease. The coating
weight of the Sn-based plating layer per side by metal Sn amount is 15.0 g/m2 or less, and
it becomes possible to achieve both excellent corrosion resistance and coating film
adhesiveness while suppressing cost increase. The coating weight of the Sn-based plating
5 layer per side by metal Sn amount is preferably 13.0 g/m2 or less, and more preferably 10.0
g/m2 or less to achieve both excellent corrosion resistance and coating film adhesiveness at
low cost.
[0023]
Here, the metal Sn amount in the Sn-based plating layer (that is, the coating
10 weight of the Sn-based plating layer per side) is, for example, a value measured by an
electrolytic method described in JIS G 3303 or an X-ray fluorescence method.
[0024]
Alternatively, the metal Sn amount in the Sn-based plating layer can also be found,
for example, by the following method. A test piece without a coating layer is prepared.
15 The test piece is immersed in 10% nitric acid to dissolve the Sn-based plating layer, and Sn
in the obtained solution is found by ICP (inductively coupled plasma) emission
spectrometry (using, for example, 799ce manufactured by Agilent Technologies Japan, Ltd,
and Ar as carrier gas). Then, the metal Sn amount can be found based on an intensity
signal obtained by the analysis, a calibration curve created from a solution having a known
20 concentration, and an area where the Sn-based plating layer is formed on the test piece.
[0025]
Alternatively, in the case of a test piece with a coating layer formed thereon, the
metal Sn amount can be found by a calibration curve method using GDS (glow discharge
spectroscopy), and the method is, for example, as follows. A plating sample having a
25 known metal Sn amount (authentic sample) is used to find a relation between the intensity
signal of the metal Sn in the authentic sample and a sputter rate by GDS and create a
9
calibration curve in advance. Based on the calibration curve, the amount of metal Sn can
be found from an intensity signal of a test piece having an unknown metal Sn amount and
the sputter rate. Here, the Sn-based plating layer is defined as a portion from a depth
where an intensity signal of Zr becomes 1/2 of a maximum value of the intensity signal of
5 Zr to a depth where an intensity signal of Fe becomes 1/2 of a maximum value of the
intensity signal of Fe.
[0026]
From the viewpoint of measurement precision and swiftness, the measurement by
the X-ray fluorescence method is preferable in terms of industry.
10 [0027]
A method of applying the Sn-based plating to a surface of the steel sheet is not
limited, but a publicly-known electroplating method is preferable. As the electroplating
method, for example, an electrolytic method using well-known acidic baths such as
sulfuric acid bath, fluoroborate bath, phenolsulfonic acid bath, and methanesulfonic acid
15 bath or alkaline bath can be used. Note that a melting method of applying Sn-based
plating by immersing the steel sheet in molten Sn may be used.
[0028]
Further, after the Sn-based plating, a heating and melting treatment of heating the
steel sheet having the Sn-based plating layer to 231.9°C or higher, which is the melting
20 point of Sn, may be performed. Through the heating and melting treatment, a surface of
the Sn-based plating layer takes a polish and an alloy layer of Sn and Fe is formed between
the Sn-based plating layer and the steel sheet to further improve the corrosion resistance.
[0029]

25 The Sn-based plated steel sheet of this embodiment has a coating layer containing
zirconium oxide on the surface of the Sn-based plating layer formed on a surface of the
10
steel sheet. The zirconium oxide must include zirconium oxide with an amorphous
structure and zirconium oxide with a crystalline structure.
[0030]
The fact that the coating layer contains zirconium oxide with the amorphous
5 structure reduces the number of crystal grain boundaries that serve as permeation paths for
corrosion factors such as oxygen and chloride ions compared to a coating layer containing
only zirconium oxide with the crystalline structure. As a result, corrosion factors are less
likely to reach the Sn surface and the corrosion resistance of the coating layer is improved.
[0031]
10 Here, the structure of zirconium oxide is determined by an electron beam
diffraction pattern using a transmission electron microscope. That is, the crystalline
structure is defined when a clear diffraction spot is obtained in the electron beam
diffraction pattern, and the amorphous structure is defined when no diffraction spot is
obtained and a continuous ring-shaped diffraction pattern is obtained. Concretely, any
15 portion of the Sn-based plated steel sheet is subjected to FIB (focused ion beam) to prepare
a sample for TEM (transmission electron microscope) observation and a crystal structure
can be determined as stated above by examining the diffraction pattern obtained by
electron beam diffraction at any coating position with a beam diameter of 1 nm.
[0032]
20 The zirconium oxide with the amorphous structure in this embodiment is
preferably contained in the coating layer with an amorphous structure ratio of 50% or more.
The definition of the "amorphous structure ratio" in this embodiment is described below
for convenience of explanation. The amorphous structure ratio in the coating layer of
50% or more makes it possible to further improve the corrosion resistance of the coating
25 layer. The amorphous structure ratio in the coating layer is more preferably 60% or more.
An upper limit of the amorphous structure ratio is 90%.
11
[0033]
The amorphous structure ratio defined here is a value calculated from a percentage
of locations where the amorphous structure was obtained in the coating layer. Concretely,
electron beam diffraction patterns are measured at any 10 locations in a thickness direction
5 at any position on a surface of the coating layer. When the continuous ring-shaped
diffraction pattern, rather than the clear diffraction spot, is obtained in these measurement
results, the structure is determined to be the amorphous structure. The amorphous
structure ratio is defined as the percentage of the amorphous structure obtained among a
total of 10 locations measured in this way.
10 Amorphous structure ratio (%) = (Number of locations where amorphous structure
was obtained/10) × 100
[0034]
It is preferable to measure the number of detected locations of the amorphous
structure as described above at any three positions of the coating layer, and it is more
15 preferable to measure at any five positions of the coating layer. A maximum number of
detected locations at each measurement position is defined as the number of detected
locations of the amorphous structure.
[0035]
The coating layer of this embodiment has a crystalline layer whose main
20 component is zirconium oxide with the crystalline structure on an upper layer of the
zirconium oxide with the amorphous structure as described above. This is because when
a Sn-based plated steel sheet is painted for use, the presence of the zirconium oxide with
the crystalline structure on a surface layer side of the Sn-based plated steel sheet is better
for the coating film adhesiveness. The crystal structure of zirconium oxide includes a
25 monoclinic system, but other crystal structures such as a tetragonal crystal and a cubic
crystal may be included. The above "main component is zirconium oxide with the
12
crystalline structure" means that a content of the zirconium oxide with the crystalline
structure is 50 mass% or more in the crystalline layer.
[0036]
A mechanism of better coating film adhesiveness with the zirconium oxide with
5 the crystalline structure than with the zirconium oxide with the amorphous structure on the
surface layer side may be that a contact interface with a coating film is increased due to
microscopic irregularities of a crystal plane, and that reactivity of the crystalline structure
with the coating film is higher because the crystalline structure is more reactive than the
amorphous structure.
10 [0037]
The crystalline layer in the coating layer preferably includes an uppermost surface
portion of the coating layer, and the number of detected locations of the crystalline layer is
at least one or more in order from the uppermost surface portion in a thickness direction.
Here, the above-mentioned uppermost surface portion means a portion including an
15 uppermost surface of the coating layer among each of 10 equal portions of the coating
layer in the thickness direction at any position of the coating layer. In other words, it
means that the zirconium oxide with the crystalline structure is present on the uppermost
surface of the Sn-based plated steel sheet. The number of detected locations of the
crystalline layer means the number of locations determined to be the crystalline structure
20 among 10 measured locations in the electron beam diffraction pattern at a center portion of
the thickness direction of each portion among 10 equal portions where the coating layer is
divided into 10 equal portions in the thickness direction at any position of the coating layer.
The presence of the crystalline layer at the above position makes it possible to achieve
even better coating film adhesiveness.
25 [0038]
The number of detected locations of the crystalline layer is preferably five or less,
13
including the uppermost surface portion of the coating layer and in order from the
uppermost surface portion in the thickness direction. By setting the number of detected
locations to five or less, it is possible to achieve both the corrosion resistance and the
coating film adhesiveness more reliably.
5 [0039]
The number of detected locations of the crystalline layer as described above is
preferably measured at any three positions of the coating layer, and more preferably at any
five positions of the coating layer.
[0040]
10 The zirconium oxide content in the coating layer is 1.0 mg/m2 to 10.0 mg/m2 per
side in terms of metal Zr. When the zirconium oxide content in the coating layer is 1.0
mg/m2 or more per side in terms of metal Zr, a barrier property provided by the zirconium
oxide is sufficient, and sulphide stain resistance for food products or the like containing
amino acids becomes good. The zirconium oxide content in the coating layer per side is
15 preferably 6.0 mg/m2 or more in terms of metal Zr. On the other hand, when the
zirconium oxide content in the coating layer exceeds 10.0 mg/m2 per side in terms of metal
Zr, the coating film adhesiveness tends to decrease due to cohesive failure of the zirconium
oxide itself. When the zirconium oxide content in the coating layer is 10.0 mg/m2 or less
per side in terms of metal Zr, it is possible to maintain the excellent coating film
20 adhesiveness. The zirconium oxide content in the coating layer per side is preferably 8.0
mg/m2 or less in terms of metal Zr.
[0041]
Here, the zirconium oxide content in the coating layer is the content of zirconium
oxide per side. In addition to the zirconium oxide, the coating layer may also contain
25 other elements such as Fe, Ni, Cr, Ca, Na, Mg, Al, and Si. The coating layer may also
contain one or two or more types of tin fluoride and tin oxide, tin phosphate, zirconium
14
phosphate, calcium hydroxide, and calcium, or a composite compound of these elements.
The zirconium oxide content (metal Zr amount) in the coating layer is a value, which is
obtained by immersing the Sn-based plated steel sheet in an acidic solution such as
hydrofluoric acid and sulfuric acid, for example, to dissolve and the resulting dissolved
5 solution is measured by chemical analysis such as the ICP emission spectrometry. The
zirconium oxide content (metal Zr amount) may be determined by X-ray fluorescence
measurement.
[0042]

10 Hereinafter, a forming method of the coating layer containing zirconium oxide is
described.
The coating layer containing zirconium oxide can be formed on a surface of the
Sn-based plating layer by immersing the Sn-based plated steel sheet in an aqueous solution
containing zirconium ions and performing a cathode electrolytic treatment with the
15 Sn-based plated steel sheet as a cathode. The coating layer containing zirconium oxide
can be formed on the Sn-based plated steel sheet owing to forcible movement of charges
by the cathode electrolytic treatment and surface cleaning by generation of hydrogen at an
interface of the steel sheet in conjunction with an adhesion-promoting effect by pH
increase.
20 [0043]
Here, it is necessary to increase a precipitation rate of zirconium oxide on the
Sn-plated surface and to increase a nucleation rate rather than crystal growth to form the
zirconium oxide with the amorphous structure in the coating. For this purpose, after
forming the Sn-based plating on the surface of the steel sheet or after forming the Sn-based
25 plating layer, the steel sheet is subjected to the heating and melting treatment of heating to
231.9°C or higher, which is the melting point of Sn, then immersed in cooling water with
15
hardness WH (calcium concentration (ppm) × 2.5 + magnesium concentration (ppm) × 4.1)
of in a range of 100 ppm or more and 300 ppm or less, and then the Sn-based plated steel
sheet is immersed in an aqueous solution containing zirconium ions, and subjected to the
cathode electrolytic treatment with the Sn-based plated steel sheet as the cathode at a
5 specified current density range.
[0044]
By setting the hardness of the cooling water within the above range, a compound
containing either or both calcium and magnesium adheres to the Sn-based plated surface
and acts as a nucleus during the subsequent zirconium coating precipitation, resulting in
10 fine precipitation of zirconium oxide to form the zirconium oxide with the amorphous
structure. Here, when the hardness WH of the cooling water exceeds 300 ppm, the
compound containing either or both calcium and magnesium adheres and aggregates
excessively on the Sn-based plated surface, resulting in non-uniform and localized
formation and growth of zirconium oxide, and thus the zirconium oxide with the
15 amorphous structure cannot be obtained. The hardness WH of the cooling water is
preferably 250 ppm or less. When the hardness WH of the cooling water is 250 ppm or
less, zirconium oxide is likely to be more uniformly generated. On the other hand, when
the hardness WH of the cooling water is less than 100 ppm, there are few starting points
for nucleation during zirconium oxide precipitation, and the zirconium oxide is formed at
20 non-uniform points on the Sn-based plated surface, resulting in coarse zirconium oxide and
no formation of the zirconium oxide with the amorphous structure. The hardness WH of
the cooling water is preferably 150 ppm or more.
[0045]
An immersion time in the cooling water is preferably 0.5 seconds to 5.0 seconds.
25 When the immersion time in the cooling water is less than 0.5 seconds, adhesion of the
compound containing either or both calcium and magnesium to the Sn-based plated surface
16
becomes insufficient, and the zirconium oxide with the amorphous structure is difficult to
obtain. On the other hand, when the immersion time in the cooling water exceeds 5.0
seconds, the compound containing either or both calcium and magnesium adheres and
aggregates excessively on the Sn-based plated surface, resulting in generation and growth
5 of the zirconium oxide non-uniformly and locally, making it difficult to obtain the
zirconium oxide with the amorphous structure.
[0046]
A temperature of the cooling water is preferably 10°C to 80°C. When the
temperature of the cooling water is less than 10°C, the adhesion of the compound
10 containing either or both calcium and magnesium to the Sn-based plated surface will be
insufficient, and the zirconium oxide with the amorphous structure is difficult to obtain.
On the other hand, when the temperature of the cooling water exceeds 80°C, the compound
containing either or both calcium and magnesium adheres and aggregates excessively on
the Sn-based plated surface, resulting in generation and growth of the zirconium oxide
15 non-uniformly and locally, making it difficult to obtain the zirconium oxide with the
amorphous structure.
[0047]
An interval between an end of the above cooling water immersion treatment and a
start of the subsequent cathode electrolytic treatment is preferably within 10 seconds, and
20 more preferably within five seconds.
[0048]
A current density for the cathode electrolytic treatment is preferably set from 2.0
A/dm2 to 10.0 A/dm2. When the current density is less than 2.0 A/dm2, a forming rate of
zirconium oxide is slow and the zirconium oxide with the amorphous structure is difficult
25 to obtain. This is thought to be because zirconium and oxygen atoms can diffuse
sufficiently to form a stable crystal lattice in a process of forming the zirconium oxide due
17
to a slow precipitation rate of the zirconium oxide resulting from low hydrogen generation
from the surface of the Sn-based plated steel sheet when the current density is less than 2.0
A/dm2. On the other hand, when the current density exceeds 10.0 A/dm2, the hydrogen
generation from the surface of the Sn-based plated steel sheet becomes active and the pH
5 near the surface of the steel sheet becomes high to a bulk of the treatment solution,
resulting in the generation of the zirconium oxide in the treatment solution. The
generated zirconium oxide becomes larger by the time it adheres to the steel sheet surface,
making it difficult to obtain the zirconium oxide with the amorphous structure, and a
thickness of the zirconium coating becomes thicker and an external appearance is inferior.
10 [0049]
To form the zirconium oxide with the crystalline structure on an upper layer of the
zirconium oxide with the amorphous structure, the Sn-based plated steel sheet having the
zirconium oxide with the amorphous structure is formed by cathode electrolysis in the
electrolytic treatment solution containing zirconium ions, and then electrolytic treatment is
15 performed at low current density. Concretely, after forming zirconium with the
amorphous structure by the cathode electrolytic treatment at the current density of 2.0
A/dm2 to 10.0 A/dm2, the cathode electrolytic treatment at the current density of less than
1.0 A/dm2 is performed.
[0050]
20 A concentration of zirconium ions in the cathode electrolytic solution may be
appropriately adjusted according to production facility and production rate (ability). For
example, the zirconium ion concentration is preferably 1000 ppm or more and 4000 ppm
or less. There is no problem if the solution containing zirconium ions contains other
components such as fluorine ions, phosphate ions, ammonium ions, nitrate ions, sulfate
25 ions, and chloride ions. A supply source of zirconium ions in the cathode electrolytic
solution can be a zirconium complex such as H2ZrF6, for example. Zr in the Zr complex
18
as described above is present in the cathode electrolytic solution as Zr4+ due to an increase
in the pH at a cathode electrode interface. Such Zr ions react further in the cathode
electrolytic solution to form the zirconium oxide.
[0051]
5 As a solvent of the cathode electrolytic solution when performing the cathode
electrolytic treatment, for example, water such as distilled water can be used. However,
the solvent is not limited to water such as distilled water but can be appropriately selected
according to the material to be dissolved, the forming method, or the like.
[0052]
10 A solution temperature of the cathode electrolytic solution for the cathode
electrolytic treatment is preferably set to, for example, a range of 5°C to 50°C.
Performing the cathode electrolysis at 50°C or lower enables the formation of a dense and
uniform structure of the coating layer which is formed of extremely fine particles. On the
other hand, when the solution temperature is less than 5°C, forming efficiency of the
15 coating may be low. When the solution temperature exceeds 50°C, the formed coating is
nonuniform, and defects, cracks, microcracks, or the like occur to make the formation of
the dense coating difficult, resulting in causing a starting point of corrosion or the like,
which is not preferable.
[0053]
20 The pH of the cathode electrolytic solution is preferably set to 3.5 to 4.3. When
the pH is less than 3.5, a precipitation efficiency of a Zr coating may deteriorate, whereas
when the pH exceeds 4.3, the zirconium oxide tends to precipitate in the solution, resulting
in the coarse and rough Zr coating.
[0054]
25 For example, nitric acid, ammonia water, or the like may be added to the cathode
electrolytic solution to adjust the pH and to increase the electrolysis efficiency of the
19
cathode electrolytic solution.
[0055]
When forming the coating layer, the time of the cathode electrolytic treatment is
not limited. The time of the cathode electrolytic treatment only needs to be adjusted
5 according to the current density with respect to the targeted zirconium oxide content (metal
Zr amount) in the coating layer. An electricity pattern for the cathode electrolytic
treatment may be continuous or intermittent.
[0056]
The Sn-based plated steel sheet and its manufacturing method of this embodiment
10 have been described above.
[Examples]
[0057]
Next, the Sn-based plated steel sheet and the manufacturing method of the
Sn-based plated steel sheet according to the present invention will be concretely explained
15 while illustrating examples and comparative examples. Note that the following examples
are merely examples of the Sn-based plated steel sheet and the manufacturing method of
the Sn-based plated steel sheet according to the present invention, and the Sn-based plated
steel sheet and the manufacturing method of the Sn-based plated steel sheet according to
the present invention are not limited to the following examples.
20 [0058]

A method of producing a test material will be explained. Note that
later-explained test materials in examples were produced according to the method of
producing the test material.
25 First, a low-carbon cold-rolled steel sheet with a sheet thickness of 0.2 mm was
subjected to electrolytic alkali degreasing, water washing, dilute sulfuric acid immersion
20
pickling, and water washing as pretreatments, then subjected to Sn-based electroplating
using a phenolsulfonic acid bath, and then subjected to a heating and melting treatment.
Through these treatments, the Sn-based plating layers were formed on both surfaces of the
steel sheet that had undergone these treatments. The coating weight of the Sn-based
5 plating layer per side by metal Sn amount was set to about 2.8 g/m2 as a standard. The
coating weight of the Sn-based plating layer was adjusted by changing an energization time.
Some test materials were not subjected to the above heating and melting treatment.
[0059]
Next, the steel sheet on which the Sn-based plating layers were formed was
10 immersed in cooling water with a predetermined hardness for a predetermined time.
Within five seconds thereafter the plated steel sheet that had undergone the immersion
treatment was subjected to the cathode electrolytic treatment in an aqueous solution
containing zirconium fluoride (cathode electrolytic solution) to form a coating layer
containing zirconium oxide on a surface of each Sn-based plating layer. The temperature
15 of the cathode electrolytic solution was set to 35°C and the pH of the cathode electrolytic
solution was adjusted to be 3.0 to 5.0. The current density of the cathode electrolytic
treatment and the treatment time of the cathode electrolytic treatment were adjusted
according to the targeted zirconium oxide content (metal Zr amount) in the coating layer.
When the cathode electrolytic treatments were performed two times, the second cathode
20 electrolytic treatment was performed immediately after the first cathode electrolytic
treatment was completed and the current density setting was changed.
[0060]
The Sn-based plated steel sheets prepared in this way were subjected to various
evaluations as follows.
25 [0061]
[Coating weight of Sn-based plating layer per side (metal Sn amount of Sn-based plating
21
layer)]
The coating weight of the Sn-based plating layer per side (metal Sn amount of the
Sn-based plating layer) was measured as follows. Several test pieces of steel sheet having
the Sn-based plating layers with known metal Sn content were prepared. Next, each test
5 piece was analyzed using an X-ray fluorescence analysis apparatus (ZSX Primus,
manufactured by Rigaku Corporation) to measure X-ray fluorescence intensity derived
from metal Sn in advance from a surface of the Sn-based plating layer of the test piece.
Then, a calibration curve representing a relationship between the measured X-ray
fluorescence intensity and the metal Sn amount was prepared. Then, the coating layer
10 was removed from the Sn-based plated steel sheet to be a measurement object to prepare a
test piece exposing the Sn-based plating layer. The X-ray fluorescence intensity derived
from metal Sn was measured on the surface where the Sn-based plating layer was exposed
using the X-ray fluorescence apparatus. The coating weight of the Sn-based plating layer
per side (that is, the metal Sn content) was calculated by using the obtained X-ray
15 fluorescence intensity and the calibration curve prepared in advance.

Name of Document] What Is Claimed is
[Claim 1]
A Sn-based plated steel sheet comprising:
a steel sheet;
5 a Sn-based plating layer located on at least one surface of the steel sheet; and
a coating layer located on the Sn-based plating layer,
wherein the Sn-based plating layer contains 1.0 g/m2 to 15.0 g/m2 of Sn per side in
terms of metal Sn,
the coating layer contains zirconium oxide, and a content of the zirconium oxide is
10 1.0 mg/m2 to 10.0 mg/m2 per side in terms of metal Zr,
the zirconium oxide includes zirconium oxide with an amorphous structure, and
a crystalline layer whose main component is zirconium oxide with a crystalline
structure is present on an upper layer of the zirconium oxide with the amorphous structure,
wherein the crystalline structure is determined when a clear diffraction spot is
15 obtained in an electron beam diffraction pattern, and the amorphous structure is determined
when a continuous ring-shaped diffraction pattern is obtained instead of the clear
diffraction spot.
[Claim 2]
The Sn-based plated steel sheet according to claim 1,
20 wherein the crystalline layer in the coating layer includes an uppermost surface
portion of the coating layer, and
the number of detected locations of the crystalline layer is at least one or more in
order from the uppermost surface portion in a thickness direction,
wherein the uppermost surface portion means a portion including an uppermost
25 surface of the coating layer among each of 10 equal portions of the coating layer in the
thickness direction at any position of the coating layer, and
32
the number of detected locations of the crystalline layer means the number of
locations determined to be the crystalline structure among 10 measured locations in the
electron beam diffraction pattern at a center portion of the thickness direction of each
portion among 10 equal portions where the coating layer is divided into 10 equal portions
5 in the thickness direction at any position of the coating layer.
[Claim 3]
The Sn-based plated steel sheet according to claim 2,
wherein the number of detected locations of the crystalline layer is five or less,
including the uppermost surface portion of the coating layer and in order from the
10 uppermost surface portion in the thickness direction.
33
[Name of Document] Abstract
To provide a Sn-based plated steel sheet capable of exhibiting superior corrosion
resistance, yellowing resistance, coating film adhesiveness, and sulphide stain resistance
without using a chromate film. A Sn-based plated steel sheet of the present invention
5 includes: a steel sheet; a Sn-based plating layer located on at least one surface of the steel
sheet; and a coating layer located on the Sn-based plating layer, wherein the Sn-based
plating layer contains 1.0 g/m2 to 15.0 g/m2 of Sn per side in terms of metal Sn, the coating
layer contains zirconium oxide, and a content of the zirconium oxide is 1.0 mg/m2 to 10.0
mg/m2 per side in terms of metal Zr, the zirconium oxide includes zirconium oxide with an
10 amorphous structure, and a crystalline layer whose main component is zirconium oxide
with a crystalline structure is present on an upper layer of the zirconium oxide with the
amorphous structure.