EP4621095A1

EP4621095A1 - Surface-treated steel sheet and method for producing same

Info

Publication number: EP4621095A1
Application number: EP23894173.6A
Authority: EP
Inventors: Takashi Ueno; Yusuke Nakagawa
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2022-11-24
Filing date: 2023-07-07
Publication date: 2025-09-24
Also published as: JPWO2024111157A1; KR20250065872A; JP7401039B1; MX2025005778A; CN120153125A

Abstract

Provided is a surface-treated steel sheet that can be produced without the use of hexavalent chromium and that has excellent adhesion to BPA-free paint. A surface-treated steel sheet having, on at least one surface of a steel sheet, a Sn plating layer and a coating layer disposed on the Sn plating layer and containing at least one of Zr oxide and Ti oxide, wherein a contact angle of ethylene glycol is 50° or less, and a total atomic ratio of K, Na, Mg, and Ca adsorbed on a surface of the steel sheet to all elements is 5.0 % or less.

Description

TECHNICAL FIELD

The present disclosure relates to surface-treated steel sheets, and in particular to surface-treated steel sheets with excellent adhesion to BPA (bisphenol A)-free paint. The surface-treated steel sheets of the present disclosure can be suitably used in containers such as cans. The present disclosure also relates to a method of producing the surface-treated steel sheet.

BACKGROUND

A Sn plating steel sheet (tinplate), one type of surface-treated steel sheet, is widely used as material for various metal cans such as beverage cans, food cans, pails, and 18-liter cans because of its excellent corrosion resistance, weldability, workability, and ease of production.
Surface-treated steel sheets used for these applications are coated with an organic resin coating, such as epoxy-based paint, on the surface of the steel sheet to accommodate various contents. When applying an organic resin coating, a Cr oxide layer formed on the top surface by electrolytic or immersion treatment of the steel sheet in an aqueous solution containing hexavalent chromium plays an important role. In other words, excellent adhesion to the organic resin coating layer is achieved by the aforementioned Cr oxide layer, and as a result, corrosion resistance with respect to various contents is ensured (Patent Literature (PTL) 1 to 5).
On the other hand, it has been suggested that the BPA included in epoxy-based paint may have harmful effects on humans. Therefore, BPA-free paint using polyester-based resins that do not contain BPA are being developed (PTL 6, 7), and demand exists for replacing epoxy-based paint. However, tinplate, which has been used as steel sheets for cans, has poor adhesion to BPA-free paint compared to adhesion to epoxy-based paint. Therefore, the application of BPA-free paint to various metal cans has not progressed due to insufficient corrosion resistance to various contents.
Furthermore, in recent years, increasing environmental awareness has led to a worldwide trend toward regulating the use of hexavalent chromium. Therefore, demand exists for establishing a production method that does not use hexavalent chromium in the field of surface-treated steel sheets used for various metal cans.
An example of a known method of producing surface-treated steel sheets without using hexavalent chromium is the method proposed in PTL 8. In this method, a surface-treated steel sheet with a coating containing a zirconium compound on the surface of a Sn plating steel sheet is proposed.

CITATION LIST

Patent Literature

PTL 1: PTL 1: JP S58-110695 A
PTL 2: JP S55-134197 A
PTL 3: JP S57-035699 A
PTL 4: JP H11-117085 A
PTL 5: JP 2007-231394 A
PTL 6: JP 2013-144753 A
PTL 7: JP 2008-050486 A
PTL 8: JP 2018-135569 A

SUMMARY

(Technical Problem)

According to the method proposed in PTL 8, a surface treatment layer can be formed without using hexavalent chromium. Also according to PTL 8, the above method can obtain a surface-treated steel sheet with excellent adhesion to epoxy-based paint.
However, while having excellent adhesion to epoxy-based paint, the surface-treated steel sheet obtained by the conventional method as proposed in PTL 8 has poor adhesion to BPA-free paint, resulting in insufficient BPA-free paint corrosion resistance. Replacement with BPA-free paint while ensuring corrosion resistance to various contents has therefore not been possible.
Demand thus exists for surface-treated steel sheets that can be produced without the use of hexavalent chromium and that have excellent adhesion to BPA-free paint.
It is an aim of the present disclosure, conceived in light of the above circumstances, to provide a surface-treated steel sheet that can be produced without using hexavalent chromium and that has excellent adhesion to BPA-free paint.

(Solution to Problem)

As a result of intensive studies to achieve to above aim, we made the following discoveries (1) and (2).

(1) In a surface-treated steel sheet having a coating layer containing at least one of Zr oxide and Ti oxide on a Sn plating layer, controlling each of the contact angle of ethylene glycol and the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface of the steel sheet to all elements each to be within a specific range makes it possible to obtain a surface-treated steel sheet with excellent adhesion to BPA-free paint.
(2) The aforementioned surface-treated steel sheet can be produced by performing surface conditioning under a predetermined set of conditions after coating formation, followed by a final water washing using water whose electrical conductivity is equal to or less than a predetermined value.

The present disclosure has been made based on these discoveries. We provide the following.

1. A surface-treated steel sheet including, on at least one side of a steel sheet,
- a Sn plating layer; and
- a coating layer disposed on the Sn plating layer and containing at least one of Zr oxide and Ti oxide, wherein
- a contact angle of ethylene glycol is 50° or less, and
- a total atomic ratio of K, Na, Mg, and Ca adsorbed on a surface of the steel sheet to all elements is 5.0 % or less.
2. The surface-treated steel sheet according to 1, wherein the Sn plating layer has a Sn coating weight of 0.1 g/m² to 20.0 g/m² per side of the steel sheet.
3. The surface-treated steel sheet according to 1 or 2, wherein a total coating weight of Zr oxide and Ti oxide in the coating layer is 0.3 mg/m² to 50.0 mg/m² per side of the steel sheet in terms of amount of metal Zr and amount of metal Ti.
4. The surface-treated steel sheet according to any one of 1 to 3, wherein the coating layer further contains P, and a P coating weight is 50.0 mg/m² or less per side of the steel sheet.
5. The surface-treated steel sheet according to any one of 1 to 4, wherein the coating layer further contains Mn, and a Mn coating weight is 50.0 mg/m² or less per side of the steel sheet.
6. The surface-treated steel sheet according to any one of 1 to 5, further including a Ni-containing layer disposed below the Sn plating layer.
7. The surface-treated steel sheet according to 6, wherein the Ni-containing layer has a Ni coating weight of 2 mg/m² to 2000 mg/m² per side of the steel sheet.
8. A method of producing a surface-treated steel sheet including, on at least one side of a steel sheet, a Sn plating layer, and a coating layer disposed on the Sn plating layer and containing at least one of Zr oxide and Ti oxide, the method including:
- a coating formation process of treating a surface of a steel sheet having a Sn plating layer on at least one side with an aqueous solution containing at least one of Zr ions and Ti ions to form the coating layer on the Sn plating layer;
- a surface conditioning process of holding the aqueous solution at 1.0 g/m² to 30.0 g/m² on a surface of the coating layer for 0.1 seconds to 20.0 seconds; and
- a water washing process of subjecting the steel sheet after the surface conditioning process to water washing at least once, wherein
- in the water washing process,
  water with an electrical conductivity of 100 µS/m or less is used at least in the last water washing.
9. The method of producing a surface-treated steel sheet according to 8, wherein the surface-treated steel sheet further includes a Ni-containing layer disposed below the Sn plating layer.

(Advantageous Effect)

According to the present disclosure, a surface-treated steel sheet that uses no hexavalent chromium and that has excellent adhesion to BPA-free paint can be provided. The surface-treated steel sheet of the present disclosure can be suitably used as a material for containers and the like.

DETAILED DESCRIPTION

A method for carrying out the presently disclosed techniques will be described in detail below. The following description merely presents examples of preferred embodiments of the present disclosure, and the present disclosure is not limited to these embodiments.
A surface-treated steel sheet in an embodiment of the present disclosure includes, on at least one side of the steel sheet, a Sn plating layer and a coating layer disposed on the Sn plating layer, the coating layer containing at least one of Zr oxide and Ti oxide. In the present disclosure, it is important that the contact angle of ethylene glycol be 50° or less, and that the total atomic ratio of K, Na, Mg, and Ca adsorbed on a surface of the steel sheet to all elements be 5.0 % or less. The following is a description of each of the above constituent elements of the surface-treated steel sheet.

[Steel sheet]

Any steel sheet can be used as the above steel sheet without any particular limitation, but a steel sheet for cans is preferred. For example, an ultra low carbon steel sheet or low carbon steel sheet can be used as the steel sheet. The method of producing the steel sheet is not limited, and a steel sheet produced by any method may be used, but it typically suffices to use a cold-rolled steel sheet. The cold-rolled steel sheet can be produced by general production processes, for example, hot rolling, pickling, cold rolling, annealing, and temper rolling.
The chemical composition of the steel sheet is not limited, but the steel sheet may contain C, Mn, Cr, P, S, Si, Cu, Ni, Mo, Al, and unavoidable impurities to the extent that the effects of the scope of the present disclosure are not impaired. In this case, a steel sheet with the chemical composition specified in ASTM A623M-09, for example, can be suitably used as the steel sheet.
In one embodiment of the present disclosure, a steel sheet having a chemical composition containing, in mass%,

C: 0.0001 % to 0.13 %,
Si: 0 % to 0.020 %,
Mn: 0.01 % to 0.60 %,
P: 0 % to 0.020 %,
S: 0 % to 0.030 %,
Al: 0 % to 0.20 %,
N: 0 % to 0.040 %,
Cu: 0 % to 0.20 %,
Ni: 0 % to 0.15 %,
Cr: 0 % to 0.10 %,
Mo: 0 % to 0.05 %,
Ti: 0 % to 0.020 %,
Nb: 0 % to 0.020 %,
B: 0 % to 0.020 %,
Ca: 0 % to 0.020 %,
Sn: 0 % to 0.020 %, and
Sb: 0 % to 0.020 %,
with the balance being Fe and inevitable impurities, is preferably used. In the aforementioned chemical composition, Si, P, S, Al, and N are components whose content is more preferable the lower it is, and Cu, Ni, Cr, Mo, Ti, Nb, B, Ca, Sn, and Sb are components that can optionally be added.

No lower limit is placed on the thickness of the steel sheet, but the thickness is preferably 0.10 mm or more. No upper limit is placed on the thickness of the steel sheet either, but the thickness is preferably 0.60 mm or less. The term "steel sheet" as used here includes a "steel strip".

[Sn plating layer]

The Sn plating layer need only be provided on at least one side of the steel sheet but may be provided on both sides. The Sn plating layer need only cover at least a portion of the steel sheet but may cover the entire side on which the Sn plating layer is provided. The Sn plating layer may be a continuous layer or a discontinuous layer. The discontinuous layer is, for example, a layer with an island-like structure.
The Sn plating layer includes the case of a portion of the Sn plating layer being alloyed. For example, the Sn plating layer includes the case in which a portion of the Sn plating layer is a Sn alloy layer due to hot melt treatment after Sn plating. Examples of the Sn alloy layer include an Fe-Sn alloy layer and an Fe-Sn-Ni alloy layer.
For example, by heating and melting Sn by electrical resistance heating or the like after Sn plating, a portion of the steel sheet side of the Sn plating layer can be made into an Fe-Sn alloy layer. In addition, by performing Sn plating on a steel sheet having a Ni-containing layer on the surface and further heating and melting the Sn by electrical resistance heating or the like, a portion of the steel sheet side of the Sn plating layer can be made into either or both of an Fe-Sn-Ni alloy layer and an Fe-Sn alloy layer.
The Sn coating weight in the Sn plating layer is not limited and can be any amount. However, from the perspective of further improving the appearance and corrosion resistance of the surface-treated steel sheet, the Sn coating weight is preferably set to 20.0 g/m² or less per side of the steel sheet. From the same perspective, the Sn coating weight is preferably set to 0.1 g/m² or more and more preferably to 0.2 g/m² or more. From the perspective of further improving workability, the Sn coating weight is even more preferably set to 1.0 g/m² or more.
The Sn coating weight can be measured by the electrolytic stripping method specified in JIS G 3303.
The Sn plating layer can be formed by any method without limitation, including methods such as electroplating and hot dip coating. In the case of forming the Sn plating layer by electroplating, any plating bath can be used. Examples of plating baths that can be used include a phenolsulfonic acid Sn plating bath, a methanesulfonic acid Sn plating bath, and a halogen-based Sn plating bath.
After forming the Sn plating layer, reflow treatment may be performed. In the case of reflow treatment, the Sn plating layer is heated to a temperature at or above the melting point of Sn (231.9 °C) to form an alloy layer such as an Fe-Sn alloy layer on the bottom layer (steel sheet side) of the unalloyed Sn plating layer. If the reflow treatment is omitted, a Sn plating steel sheet with an unalloyed Sn plating layer is obtained.
The surface side of the Sn plating layer may contain Sn oxide or may contain no Sn oxide at all. However, from the perspective of improving paint secondary adhesion (coating secondary adhesion), which is the adhesion to paint in a wet environment, and improving sulfide staining resistance, the surface side of the Sn plating layer preferably does contain Sn oxide. Although Sn oxide can be formed by reflow treatment or by dissolved oxygen contained in water used in water washing after Sn plating, the amount of Sn oxide contained in the Sn plating layer is preferably controlled by the below-described pretreatment or the like.

[Ni-containing layer]

The surface-treated steel sheet can further optionally include a Ni-containing layer. For example, the surface-treated steel sheet according to an embodiment of the present disclosure can further include a Ni-containing layer disposed below the Sn plating layer. In other words, a surface-treated steel sheet in an embodiment of the present disclosure includes, on at least one side of the steel sheet, a Ni-containing layer, a Sn plating layer disposed on the Ni-containing layer, and a coating layer disposed on the Sn plating layer and containing at least one of Zr oxide and Ti oxide.
As the Ni-containing layer, any layer that contains nickel can be used. For example, one or both of a Ni layer and a Ni alloy layer can be used. The Ni layer is, for example, a Ni plating layer. The Ni alloy layer is, for example, a Ni-Fe alloy layer. By forming the Sn plating layer on the Ni-containing layer and then performing reflow treatment, an Fe-Sn-Ni alloy layer, an Fe-Sn alloy layer, or the like can be formed on the bottom layer (steel sheet side) of the unalloyed Sn plating layer.
The method of forming the Ni-containing layer is not limited, and any method, such as electroplating, can be used. In the case of forming a Ni-Fe alloy layer as a Ni-containing layer, the Ni-Fe alloy layer can be formed by forming a Ni layer on the steel sheet surface, by electroplating or another such method, and then annealing.
No limit is placed on the Ni coating weight of the Ni-containing layer, but from the perspective of improving sulfide staining resistance, the Ni coating weight is preferably set to 2 mg/m² or more per side of the steel sheet. From a cost perspective, the Ni coating weight per side of the steel sheet is preferably set to 2000 mg/m² or less.
The Ni coating weight of the Ni-containing layer is measured by a calibration curve method using X-ray fluorescence. First, a plurality of steel sheets with known Ni coating weight are prepared, the X-ray fluorescence intensity derived from Ni is measured for the plates in advance, and the relationship between the measured X-ray fluorescence intensity and the Ni coating weight is linearly approximated to yield a calibration curve. The X-ray fluorescence intensity derived from Ni in the surface-treated steel sheet can then be measured, and the above-described calibration curve can be used to determine the Ni coating weight of the Ni-containing layer.

[Coating layer]

A coating layer containing at least one of Zr oxide and Ti oxide exists on the Sn plating layer. The inclusion of at least one of Zr oxide and Ti oxide in the coating layer is necessary to obtain excellent adhesion to BPA-free paint.
No lower limit is placed on the total coating weight of Zr oxide and Ti oxide in the coating layer. However, from the perspective of further improving adhesion to BPA-free paint, the total coating weight of the Zr oxide and Ti oxide is preferably 0.3 mg/m² or more, more preferably 0.4 mg/m² or more, and even more preferably 0.5 mg/m² or more, per side of the steel sheet in terms of the amount of metal Zr and amount of metal Ti. No upper limit is placed on the total coating weight of Zr oxide and Ti oxide in the coating layer either. However, if the total coating weight of Zr oxide and Ti oxide is excessively high, the adhesion to the BPA-free paint may be compromised due to cohesion failure of the coating layer. Therefore, from the perspective of ensuring more stable adhesion to BPA-free paint, the total coating weight of the Zr oxide and Ti oxide is preferably 50.0 mg/m² or less, more preferably 45.0 mg/m² or less, and even more preferably 40.0 mg/m² or less, per side of the steel sheet in terms of the amount of metal Zr and amount of metal Ti. In calculating the total coating weight of Zr oxide and Ti oxide, the value yielded by conversion to the amount of metal Zr is used as the coating weight of Zr oxide, and the value yielded by conversion to the amount of metal Ti is used as the coating weight of Ti oxide.
The coating weight of Zr oxide in the coating layer is measured by a calibration curve method using X-ray fluorescence. First, a plurality of steel sheets with known Zr coating weight are prepared, the X-ray fluorescence intensity derived from Zr is measured for the plates in advance, and the relationship between the measured X-ray fluorescence intensity and the coating weight as metal Zr is linearly approximated to yield a calibration curve. The X-ray fluorescence intensity derived from Zr in the surface-treated steel sheet can then be measured, and the above-described calibration curve can be used to determine the coating weight of Zr oxide in the coating layer in terms of metal Zr.
The coating weight of Ti oxide in the coating layer is also measured by a calibration curve method using X-ray fluorescence. First, a plurality of steel sheets with known Ti coating weight are prepared, the X-ray fluorescence intensity derived from Ti is measured for the plates in advance, and the relationship between the measured X-ray fluorescence intensity and the coating weight as metal Ti is linearly approximated to yield a calibration curve. The X-ray fluorescence intensity derived from Ti in the surface-treated steel sheet can then be measured, and the above-described calibration curve can be used to determine the coating weight of Ti oxide in the coating layer in terms of metal Ti.
The coating layer may contain P from the perspective of further improving adhesion to BPA-free paint. Although no upper limit is placed on the coating weight of P contained in the coating layer, the coating weight of P is preferably 50.0 mg/m² or less per side of the steel sheet, since adhesion to BPA-free paint may be impaired due to cohesion failure of the coating layer. No lower limit is placed on the coating weight of P in the coating layer, and the coating weight of P may, for example, be 0.0 mg/m², i.e., no P whatsoever may be contained.
The coating weight of P in the coating layer is measured by a calibration curve method using X-ray fluorescence. First, a plurality of steel sheets with known P coating weight are prepared, the X-ray fluorescence intensity derived from P is measured for the plates in advance, and the relationship between the measured X-ray fluorescence intensity and the P coating weight is linearly approximated to yield a calibration curve. The X-ray fluorescence intensity derived from P in the surface-treated steel sheet can then be measured, and the above-described calibration curve can be used to determine the coating weight of P in the coating layer.
The coating layer may contain Mn from the perspective of further improving adhesion to BPA-free paint. Although no upper limit is placed on the coating weight of Mn contained in the coating layer, the coating weight of Mn is preferably 50.0 mg/m² or less per side of the steel sheet, since adhesion to BPA-free paint may be impaired due to cohesion failure of the coating layer. No lower limit is placed on the coating weight of Mn in the coating layer, and the coating weight of Mn may, for example, be 0.0 mg/m², i.e., no Mn whatsoever may be contained.
The coating weight of Mn in the coating layer is measured by a calibration curve method using X-ray fluorescence. First, a plurality of steel sheets with known Mn coating weight are prepared, the X-ray fluorescence intensity derived from Mn is measured for the plates in advance, and the relationship between the measured X-ray fluorescence intensity and the Mn coating weight is linearly approximated to yield a calibration curve. The X-ray fluorescence intensity derived from Mn in the surface-treated steel sheet can then be measured, and the above-described calibration curve can be used to determine the coating weight of Mn in the coating layer.
The aforementioned coating layer may contain Sn. No upper limit is placed on the Sn content in the coating layer. The coating layer need not contain Sn, i.e., the content may be 0.0 mg/m².
The aforementioned coating layer may contain C. No upper limit is placed on the C content in the coating layer. The coating layer need not contain C, i.e., the content may be 0.0 mg/m².
The aforementioned coating layer may contain elements other than Zr, Ti, O, Sn, Mn, P, Ni, and C, along with the below-described K, Na, Mg, and Ca. Elements other than those described above include metallic impurities such as Cu, Zn, and Fe, and elements such as S, N, F, Cl, Br, and Si, contained in the aqueous solution used in the coating formation process described below. However, an excessive presence of elements other than Zr, Ti, O, Sn, Mn, P, Ni, C, K, Na, Mg, and Ca may reduce adhesion to BPA-free paint. Therefore, the total content of elements other than Zr, Ti, O, Sn, Mn, P, Ni, C, K, Na, Mg, and Ca in the coating layer is preferably 30 % or less, and more preferably 20 % or less, in atomic ratio. The coating layer need not contain elements other than Zr, Ti, O, Sn, Mn, P, Ni, C, K, Na, Mg, and Ca, i.e., the content may be 0 % in atomic ratio. The content of the above elements can be measured by XPS (X-ray photoelectron spectroscopy).

[Contact angle of ethylene glycol]

In the present disclosure, it is important that the contact angle of ethylene glycol on the surface-treated steel sheet be 50° or less. By controlling the surface of the surface-treated steel sheet so that the contact angle of ethylene glycol is 50° or less, a firm bond is formed between the polyester resin in the BPA-free paint and the surface-treated steel sheet, resulting in high adhesion to the BPA-free paint. From the perspective of further improving adhesion to BPA-free paint, the contact angle of ethylene glycol is preferably set to 48° or less, and even more preferably to 45° or less. No lower limit is placed on the contact angle of ethylene glycol, and the contact angle may be 0°, because a lower contact angle is preferable from the perspective of improving adhesion to BPA-free paint. However, from the perspective of ease of production and the like, the contact angle of ethylene glycol may be 5° or more, or 8° or more.
Furthermore, the surface of the surface-treated steel sheet in the present disclosure, i.e., the surface of the coating layer containing at least one of Zr oxide and Ti oxide, is stable with regard to heat. For example, the contact angle of ethylene glycol does not change significantly after heat treatment equivalent to paint baking. It is assumed that such thermal stability of the surface state also contributes to improved adhesion to BPA-free paint. Therefore, the contact angle of ethylene glycol on the surface-treated steel sheet after heat treatment equivalent to painting is also preferably 50° or less, more preferably 48° or less, and even more preferably 45° or less. No lower limit is placed on the contact angle of ethylene glycol on the surface-treated steel sheet after heat treatment equivalent to painting, and the contact angle may be 0°, but the contact angle may be 5° or more, or 8° or more. The conditions of the heat treatment equivalent to painting are set to a maximum temperature of 200 °C and a holding time at the maximum temperature of 10 minutes.
Although the mechanism by which the contact angle of ethylene glycol on the surface-treated steel sheets becomes 50° or less is not clear, it is believed that the surface is modified to have a high affinity for ethylene glycol by adjustment of the surface micro-roughness in the surface conditioning process described below. In a case in which the surface conditioning process described below is not performed, the aforementioned coating layer cannot be fixed in a state with the aforementioned high affinity, and the contact angle of ethylene glycol exceeds 50°, even if the surface of the surface-treated steel sheet has high affinity for ethylene glycol immediately after production.
The contact angle of ethylene glycol can be measured by the θ/2 method. In the measurement, the temperature of the surface-treated steel sheet to be measured is set to 20 °C, and ethylene glycol at a temperature of 20 °C is dropped onto the surface of the surface-treated steel sheet. The contact angle after 1 second from dropping is calculated by the 0/2 method. More specifically, measurement can be made by the method described in the Examples. Here, the surface of the surface-treated steel sheet may be coated with an anti-rust oil such as CSO (Cottonseed Oil), DOS (Dioctyl Sebacate), and ATBC (Acetyl Tributyl Citrate). In a case in which the surface-treated steel sheet is coated with oil, the contact angle measured by the method described in the Examples after vaporizing the coated oil by the heat treatment equivalent to painting is taken as the contact angle of ethylene glycol of the surface-treated steel sheet after painting with oil. As described above, the surface-treated steel sheet of the present disclosure is stable with respect to heat treatment. Therefore, if the contact angle measured after the aforementioned heat treatment and the atomic ratio of the adsorbed elements described below satisfy the conditions of the present disclosure, the surface-treated steel sheet before the aforementioned heat treatment is also considered to achieve the effects of the present disclosure. Although additives such as rust inhibitors contained in the painted oil may remain on the surface of the surface-treated steel sheet after heat treatment equivalent to painting, the amount thereof is so small that it does not affect the above-described contact angle of ethylene glycol and atomic ratio of adsorbed elements.
In a surface-treated steel sheet produced using a conventional hexavalent chromium bath as proposed in PTL 1 to 5, it has been reported that the composition of the chromium hydrated oxide layer on the surface layer significantly affects the adhesion to epoxy-based paint in wet environments. In wet environments, water that has penetrated the epoxy-based paint inhibits adhesion at the interface between the epoxy-based paint and the chromium hydrated oxide layer. It was thus believed that when numerous OH groups, which are hydrophilic, are present in the chromium hydrated oxide layer, expansion of moisture from water at the interface is promoted, resulting in reduced adhesive strength. In a conventional surface-treated steel sheet, reducing the number of OH groups through the progression of oxidation of chromium hydrated oxide, i.e., making the surface hydrophobic, therefore improves adhesion to epoxy-based paint in wet environments.
In contrast, the present disclosure focuses on ethylene glycol instead of water, and we discovered that by adjusting the surface to have a high affinity for ethylene glycol, firm adhesion to BPA-free paint can be ensured. It can thus be said that the present disclosure is based on a technical concept that is completely different from the conventional technology described above. The mechanism for improving adhesion to BPA-free paint by adjusting the surface to have a high affinity for ethylene glycol is not clear. However, since ethylene glycol is one of the hydroxyl monomers that is a component of the polyester resin that constitutes BPA-free paint, it is assumed that adjusting the surface to have a high affinity for ethylene glycol improves the adhesion to BPA-free paint.

[Atomic ratio of adsorbed elements]

As described above, the contact angle of ethylene glycol on the surface-treated steel sheet of the present disclosure is 50° or less, and the surface is chemically active. Therefore, cations of elements such as K, Na, Mg, and Ca are easily adsorbed on the surface of the surface-treated steel sheet. We discovered that simply setting the contact angle of ethylene glycol to 50° or less does not achieve the intended adhesion, due to the effect of the adsorbed cations. The adhesion to BPA-free paint can be improved in the present disclosure by reducing the amount of the cations adsorbed on the surface of the surface-treated steel sheet.
Specifically, the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface of the surface-treated steel sheet to all elements is 5.0 % or less, preferably 3.0 % or less, and more preferably 1.0 % or less. The lower the total atomic ratio, the better; hence, no lower limit is established, and the total atomic ratio may be 0.0 %. The total atomic ratio can be measured by XPS. In the measurement, it suffices to determine the atomic ratios of K, Na, Mg, and Ca to all elements from the integrated intensity of the narrow spectra of K2p, Na1s, Ca2p, and Mg1s at the top surface of the surface-treated steel sheet, using the relative sensitivity factor method. More specifically, measurement can be made by the method described in the Examples. In a case in which the surface-treated steel sheet is painted with oil, the atomic ratio measured by the method described in the Examples after vaporizing the painted oil by the heat treatment equivalent to painting is taken as the atomic ratio of the elements adsorbed to the surface-treated steel sheet after painting with oil.

[Production method]

In the method of producing a surface-treated steel sheet in an embodiment of the present disclosure, a surface-treated steel sheet with the aforementioned characteristics can be produced by the method described below.
A method of producing a surface-treated steel sheet in an embodiment of the present disclosure is a method of producing a surface-treated steel sheet that includes, on at least one side of the steel sheet, a Sn plating layer and a coating layer disposed on the Sn plating layer, and the method includes the following processes (1) to (3). Each process is described below.

(1) Coating formation process
(2) Surface conditioning process
(3) Water washing process

The surface-treated steel sheet in an embodiment of the present disclosure can further include a Ni-containing layer disposed below the Sn plating layer. In the case of producing a surface-treated steel sheet with a Ni-containing layer, it suffices to subject a steel sheet having a Ni-containing layer on at least one side and a Sn plating layer disposed on the Ni-containing layer to the coating formation process.

[Coating formation process]

In the aforementioned coating formation process, the surface of a steel sheet having a Sn plating layer on at least one side is treated with an aqueous solution containing at least one of Zr ions and Ti ions to form a coating layer on the Sn plating layer. The formed coating layer is a coating layer containing at least one of Zr oxide and Ti oxide.
The treatment with an aqueous solution is not limited and may be performed by any method. The treatment can, for example, be performed by electrolysis. In a case in which the treatment is performed by electrolysis, the steel sheet with the Sn plating layer is preferably subjected to cathodic electrolysis in the aqueous solution. Conventional equipment used for chromating treatment or the like can be used as is for the cathodic electrolysis. Therefore, from the perspective of equipment cost reduction, the coating layer is preferably formed by cathodic electrolysis.
The method of preparing the aqueous solution is not limited. For example, the aqueous solution can be prepared by dissolving one or both of a Zr-containing compound as a Zr ion source and a Ti-containing compound as a Ti ion source in water. Distilled water or deionized water can be used as the water, but these examples are not limiting, and any water can be used.
Any compounds that can provide Zr ions and Ti ions can be used as the Zr-containing compound and Ti-containing compound, respectively. For example, Zr salts such as ZrF₄ or Zr complexes such as H₂ZrF₆ and K₂ZrF₆ are preferably used as the Zr-containing compound. As the pH rises on the cathode surface, Zr ions become Zr oxide and form a coating. For example, Ti salts such as TiF₄ or Ti complexes such as H₂TiF₆ and K₂TiF₆ are preferably used as the Ti-containing compound. As the pH rises on the cathode surface, Ti ions become Ti oxide and form a coating.
The aqueous solution may further contain at least one selected from the group consisting of fluorine ions, nitrate ions, ammonium ions, phosphate ions, Mn ions, and sulfate ions. In a case in which the aqueous solution contains both nitrate ions and ammonium ions, the treatment can be performed in a short time, from several seconds to several tens of seconds, which is extremely advantageous from an industrial standpoint. The aqueous solution therefore preferably contains both nitrate ions and ammonium ions in addition to at least one of Zr ions and Ti ions. Hereafter, the unit of ionic concentration "ppm" refers to parts per million unless otherwise specified.
In a case in which the aqueous solution contains Zr ions, no lower limit is placed on the concentration of the Zr ions, but the concentration is preferably set to 100 ppm or more. No upper limit is placed on the concentration of the Zr ions either, but the concentration is preferably set to 4000 ppm or less. Similarly, in a case in which the aqueous solution contains Ti ions, no lower limit is placed on the concentration of the Ti ions, but the concentration is preferably set to 100 ppm or more. No upper limit is placed on the concentration of the Ti ions either, but the concentration is preferably set to 4000 ppm or less.
In a case in which the aqueous solution contains fluorine ions, no lower limit is placed on the concentration of the fluorine ions, but the concentration is preferably set to 120 ppm or more. No upper limit is placed on the concentration of the fluorine ions either, but the concentration is preferably set to 4000 ppm or less. In a case in which the aqueous solution contains phosphate ions, no lower limit is placed on the concentration of the phosphate ions, but the concentration is preferably set to 50 ppm or more. No upper limit is placed on the concentration of the phosphate ions either, but the concentration is preferably set to 5000 ppm or less. In a case in which the aqueous solution contains Mn ions, no lower limit is placed on the concentration of the Mn ions, but the concentration is preferably set to 50 ppm or more. No upper limit is placed on the concentration of the Mn ions either, but the concentration is preferably set to 5000 ppm or less. In a case in which the aqueous solution contains ammonium ions, no lower limit is placed on the concentration of the ammonium ions, and the concentration may be 0 ppm. No upper limit is placed on the concentration of the ammonium ions either, but the concentration is preferably set to 20000 ppm or less. In a case in which the aqueous solution contains nitrate ions, no lower limit is placed on the concentration of the nitrate ions, and the concentration may be 0 ppm. No upper limit is placed on the concentration of the nitrate ions either, but the concentration is preferably set to 20000 ppm or less. In a case in which the aqueous solution contains sulfate ions, no lower limit is placed on the concentration of the sulfate ions, and the concentration may be 0 ppm. No upper limit is placed on the concentration of the sulfate ions either, but the concentration is preferably set to 20000 ppm or less.
No upper limit is placed on the temperature of the aqueous solution during cathodic electrolysis, but the temperature is preferably set to 50 °C or lower, for example. Cathodic electrolysis at temperatures of 50 °C or lower enables the formation of a dense, uniform coating microstructure constituted by very fine particles. By setting the temperature of the aqueous solution to 50 °C or lower, the occurrence of defects, cracks, microcracks, and the like in the coating layer to be formed can be suppressed, and the adhesion to BPA-free paint can be further improved. No lower limit is placed on the temperature of the aqueous solution during cathodic electrolysis either, but the temperature is preferably set to 10 °C or higher, for example. By setting the temperature of the aqueous solution to 10 °C or higher, the efficiency of coating formation can be increased. In addition, if the temperature of the aqueous solution is 10 °C or higher, cooling of the solution is not necessary even when the outside temperature is high, such as during the summer, which is economical.
No lower limit is placed on the pH of the aqueous solution, but the pH is preferably set to 3 or higher. If the pH is 3 or higher, the formation efficiency of Zr oxide or Ti oxide can be further improved. No upper limit is placed on the pH of the aqueous solution either, but the pH is preferably set to 5 or less. A pH of 5 or less prevents the formation of large amounts of precipitation in the aqueous solution and can achieve good continuous productivity.
Nitric acid, ammonia water, or the like, for example, may be added to the aqueous solution for the purpose of adjusting pH and improving electrolytic efficiency.
No lower limit is placed on the current density for cathodic electrolysis, but the current density is, for example, preferably set to 0.05 A/dm² or higher, more preferably 1 A/dm² or higher. If the current density is 0.05 A/dm² or higher, the formation efficiency of Zr oxide or Ti oxide is improved. As a result, a more stable coating layer containing Zr oxide or Ti oxide can be generated, further improving adhesion to BPA-free paint. No upper limit is placed on the current density during cathodic electrolysis, but the current density is, for example, preferably set to 50 A/dm² or less, more preferably 10 A/dm² or less. If the current density is 50 A/dm² or less, the efficiency of generating Zr oxides or Ti oxides can be made appropriate, and the generation of coarse Zr oxide or Ti oxide with poor adhesion can be suppressed.
No limit is placed on the electrolysis time in the aforementioned cathodic electrolysis, and it suffices to adjust the electrolysis time according to the current density to obtain the Zr coating weight and Ti coating weight described above.
The current pattern in the aforementioned cathodic electrolysis may be continuous current passage or intermittent current passage. The relationship between the aqueous solution and the steel sheet during the aforementioned cathodic electrolysis is not limited, and the solution may be relatively stationary or moving. However, from the perspective of promoting the reaction and improving uniformity, cathodic electrolysis is preferably conducted while the steel sheet and the aqueous solution are moved relative to each other. For example, cathodic electrolysis can be performed continuously while passing a steel sheet through a treatment tank housing an aqueous solution containing at least one of Zr ions and Ti ions, so that the steel sheet and the aqueous solution are moved relative to each other.
When cathodic electrolysis is performed while moving the steel sheet and the aqueous solution relative to each other, the relative flow speed between the aqueous solution and the steel sheet is preferably 50 m/min or more. If the relative flow speed is 50 m/min or more, the pH of the steel sheet surface where hydrogen is generated together with current passage can be made more uniform, effectively suppressing the formation of coarse Zr oxide or Ti oxide. No upper limit is placed on the relative flow speed.

[Surface conditioning process]

Next, surface conditioning is performed on the coating layer obtained in the coating formation process. Specifically, the aqueous solution is held at 1.0 g/m² to 30.0 g/m² on a surface of the coating layer for 0.1 seconds to 20.0 seconds. By performing surface conditioning under the above conditions, the coating layer can be fixed in a state with a high affinity for ethylene glycol.
The mechanism by which the surface conditioning process can fix the coating layer with a high affinity for ethylene glycol is not clear but is thought to be as follows. By bringing the coating layer into contact with the aqueous solution, the surface of the coating layer is slightly etched, forming minute irregularities on the surface of the coating layer. The action of these minute irregularities improves the affinity of the coating layer for ethylene glycol. This affinity is different from the affinity resulting from the presence of hydrophilic functional groups, such as OH groups, and is due to the physical structure provided by the surface roughness, which also has excellent stability with respect to heat.
Although the state of existence of the aqueous solution on the surface of the coating layer is not limited, the aqueous solution is preferably in the form of a liquid film from the perspective of ensuring uniform progress of etching.

- Amount of aqueous solution: 1.0 g/m² to 30.0 g/m²

If the amount of aqueous solution used for surface conditioning is 1.0 g/m² or less, etching does not proceed sufficiently, resulting in a contact angle of ethylene glycol that is greater than 50°. The amount of the aqueous solution is therefore set to 1.0 g/m² or more, preferably 2.0 g/m² or more, and more preferably 3.0 g/m² or more. On the other hand, if the amount of the aqueous solution is greater than 30.0 g/m², the affinity for ethylene glycol is reduced, resulting in a contact angle of ethylene glycol that is greater than 50°. The amount of the aqueous solution is therefore set to 30.0 g/m² or less, preferably 28.0 g/m² or less, and more preferably 25.0 g/m² or less.

- Holding time: 0.1 seconds to 20.0 seconds

If the holding time in the surface conditioning is less than 0.1 seconds, etching does not proceed sufficiently, resulting in a contact angle of ethylene glycol that is greater than 50°. The holding time is therefore set to 0.1 seconds or longer, preferably 0.2 seconds or longer, and more preferably 0.3 seconds or longer. On the other hand, the contact angle of ethylene glycol is also greater than 50° in a case in which the holding time exceeds 20.0 seconds. This is thought to be due to excessive etching, leading to deviation from surface conditions suitable for the development of affinity for ethylene glycol. The holding time is therefore set to 20.0 seconds or shorter, preferably 18.0 seconds or shorter, and more preferably 15.0 seconds or shorter.
The amount of the aforementioned aqueous solution can be measured by a moisture meter using a filter-type infrared absorption method. Specifically, the absorbance at the surface is measured by a moisture meter using a filter-type infrared absorption method, and the amount of aqueous solution is determined from the absorbance using a previously determined calibration curve. The calibration curve can be prepared by the following procedures. First, a steel sheet with the above coating layer is placed on an electronic balance. The aqueous solution is dropped by pipette onto the steel sheet having the coating layer to form a liquid film over the entire surface of the steel sheet having the coating layer. The weight of the aqueous solution present on the steel sheet having the coating layer is determined from the weight of the steel sheet having the coating layer before the aqueous solution is dropped and the weight of the steel sheet having the coating layer after the aqueous solution is dropped. The resulting weight of the aqueous solution is divided by the area of the steel sheet having the coating layer to obtain the amount of aqueous solution per unit area. At the same time, the absorbance on the surface of the steel sheet having the coating layer is measured by a moisture meter using a filter-type infrared absorption method. The above measurements are performed multiple times while varying the amount of aqueous solution, and a calibration curve representing the correlation between the amount of aqueous solution and absorbance is created. A linear approximation of the correlation between the amount of aqueous solution and absorbance can be used as the calibration curve.
The method of adjusting the amount of aqueous solution present on the surface of the coating layer is not limited, and any method can be used. For example, the amount of the aqueous solution on the surface of the steel sheet can be adjusted by wringing out the solution with a wringer roll or by wiping.
Prior to the coating formation process, the steel sheet having a Sn plating layer can optionally be pretreated. The pretreatment can, for example, remove a natural oxide film present on the surface of the Sn plating layer. By removal of the natural oxide film, the amount of Sn oxide can be adjusted and the surface can be activated.
Although the method of the pretreatment is not limited and any method can be used, one or both of electrolytic treatment in an alkaline aqueous solution and immersion treatment in an alkaline aqueous solution are preferably performed as the pretreatment. One or both of cathodic electrolysis and anodic electrolysis can be used as the electrolysis. Any of treatments (1) to (4) below is preferably performed as the pretreatment.

(1) Cathodic electrolysis in an alkaline aqueous solution
(2) Immersion treatment in an alkaline aqueous solution
(3) Cathodic electrolysis in an alkaline aqueous solution and subsequent anodic electrolysis in an alkaline aqueous solution
(4) Anodic electrolysis in an alkaline aqueous solution

The alkaline aqueous solution may contain one or more optional electrolytes. Any electrolyte may be used without limitation. Carbonate, for example, is preferably used as the electrolyte, and sodium bicarbonate is more preferably used. No lower limit is placed on the concentration of the alkaline aqueous solution, but the concentration is preferably set to 1 g/L or higher, more preferably 5 g/L or higher. No upper limit is placed on the concentration of the alkaline aqueous solution either, but the concentration is preferably set to 30 g/L or lower, more preferably 20 g/L or lower.
No lower limit is placed on the temperature of the alkaline aqueous solution, but the temperature is preferably set to 10 °C or higher, more preferably 15 °C or higher. No upper limit is placed on the temperature of the alkaline aqueous solution either, but the temperature is preferably set to 70 °C or lower, more preferably 60 °C or lower.
In the case of performing cathodic electrolysis as the pretreatment, no lower limit is placed on the electrolytic density in the cathodic electrolysis, but the electrolytic density is preferably set to 0.5 C/dm² or higher, more preferably 1.0 C/dm² or higher. No upper limit is placed on the electrolytic density of the cathodic electrolysis either, but the electrolytic density is preferably set to 10.0 C/dm² or less because an excessively high electrolytic density will saturate the effect of the pretreatment.
In the case of performing immersion treatment as the pretreatment, no lower limit is placed on the immersion time in the immersion treatment, but the immersion time is preferably set to 0.1 seconds or longer, more preferably 0.5 seconds or longer. No upper limit is placed on the immersion time either, but the immersion time is preferably set to 10 seconds or shorter, since an excessively long immersion time will saturate the effect of the pretreatment.
In the case of performing anodic electrolysis as the pretreatment, no lower limit is placed on the electrolytic density in the anodic electrolysis, but the electrolytic density is preferably set to 0.5 C/dm² or higher, more preferably 1.0 C/dm² or higher. No upper limit is placed on the electrolytic density of the anodic electrolysis either, but the electrolytic density is preferably set to 10.0 C/dm² or less because an excessively high electrolytic density will saturate the effect of the pretreatment.
After the pretreatment, water washing is preferably performed from the perspective of removing any pretreatment solution adhering to the surface.
When forming the Sn plating layer on the surface of the base steel sheet, pretreatment is preferably performed on the base steel sheet. Although any of the above pretreatments can be performed, at least one of degreasing, pickling, and water washing is preferably performed.
Degreasing removes rolling oil, anti-rust oil, and the like from the steel sheet. The degreasing is not limited and can be performed by any method. After degreasing, water washing is preferably performed to remove any degreasing treatment solution adhering to the steel sheet surface.
Pickling removes the natural oxide film present on the surface of the steel sheet and activates the surface. The pickling is not limited and can be performed by any method. After pickling, water washing is preferably performed to remove any pickling treatment solution adhering to the steel sheet surface.

[Water washing process]

Next, the steel sheet after the surface conditioning process is subjected to water washing at least once. Water washing removes any residual aqueous solution from the surface of the steel sheet. The water washing is not limited and may be performed by any method. For example, a water washing tank can be installed downstream from the tank for coating formation, and the steel sheet after the coating formation process can be continuously immersed in water. Water washing may also be performed by spraying water on the steel sheet after the coating formation process.
The number of times the water washing is performed is not limited and may be one or more. However, to avoid an excessively large number of water washing tanks, the number of times water washing is performed is preferably five or less. In the case of performing water washing treatment two or more times, each water washing may be performed in the same or a different manner.
In the present disclosure, it is important to use water with an electrical conductivity of 100 µS/m or less at least in the last water washing of the water washing process. This reduces the amount of K, Na, Mg, and Ca adsorbed on the surface of the surface-treated steel sheet and thereby improves adhesion. Water with an electrical conductivity of 100 µS/m or less can be produced by any method. The water with an electrical conductivity of 100 µS/m or less may, for example, be reverse osmosis water, ion-exchanged water, or distilled water. The electrical conductivity of the water used for water washing can be measured using a conductivity meter.
When water washing is performed two or more times in the water washing process, any water can be used for the water washing other than the last water washing, since the above-described effect can be obtained by using water with an electrical conductivity of 100 µS/m or less for the last water washing. Water with an electrical conductivity of 100 µS/m or less may also be used for water washing other than the last water washing. However, from the perspective of cost reduction, water with an electrical conductivity of 100 µS/m or less is preferably used only for the last water washing, with normal water such as tap water or industrial water being used for water washing other than the last water washing.
From the perspective of further reducing the amount of K, Na, Mg, and Ca adsorbed on the surface of surface-treated steel sheet, the electrical conductivity of the water used for the last water washing is preferably set to 50 µS/m or less, more preferably 30 µS/m or less. On the other hand, no lower limit is placed on the electrical conductivity, and the electrical conductivity may be 0 µS/m. However, from the perspective of cost reduction, the electrical conductivity is preferably set to 1 µS/m or more.
The temperature of water used for the water washing treatment is not limited and may be any temperature. However, since excessively high temperatures place an excessive burden on the water washing equipment, the temperature of the water used for water washing is preferably set to 95 °C or lower. No lower limit is placed on the temperature of water used for water washing either, but the temperature is preferably 0 °C or higher. The temperature of the water used in the water washing may be room temperature.
No limit is placed on the water washing time per water washing treatment, but from the perspective of increasing the effectiveness of the water washing treatment, the water washing time is preferably 0.1 seconds or longer, more preferably 0.2 seconds or longer. No upper limit is placed on the water washing time per water washing treatment either, but in the case of production on a continuous line, the water washing time is preferably 10 seconds or less, more preferably 8 seconds or less, because a longer water washing time reduces the line speed and lowers productivity.
Drying may optionally be performed after the water washing process. The drying method is not limited, and ordinary dryers or electric furnace drying methods, for example, can be applied. The temperature during the drying process is preferably 100 °C or lower. Within the aforementioned range, the transformation of the surface-treatment coating can be suppressed. Although no lower limit is placed on the temperature, the lower limit is typically around room temperature.
Applications of the surface-treated steel sheet of the present disclosure are not limited, but the surface-treated steel sheet is particularly suitable as a surface-treated steel sheet for containers used in the production of various types of containers, such as food cans, beverage cans, pails, and 18-liter cans, for example.

EXAMPLES

To determine the effects of the present disclosure, surface-treated steel sheets were produced by the following procedures, and their properties were evaluated. The present disclosure is not, however, limited to the following examples.

(Sn plating)

First, steel sheets were sequentially subjected to electrolytic degreasing, water washing, pickling by immersion in dilute sulfuric acid, and water washing, followed by Sn electroplating using a phenol sulfonic acid bath to form an Sn plating layer on both sides of the steel sheets. The Sn coating weight of the Sn plating layer was set at this time to the values illustrated in Tables 2 and 3 by changing the current passage time. For some of the Examples, prior to the Sn electroplating, the steel sheets were subjected to Ni electroplating using a Watts bath to form a Ni plating layer as a Ni-containing layer on both sides of the steel sheets. The Ni coating weight of the Ni plating layer was set at this time to the values illustrated in Tables 2 and 3 by changing the current passage time and the current density. The Sn coating weight of the Sn plating layer was measured by the electrolytic stripping method specified in JIS G 3303. The Ni coating weight of the Ni plating layer was measured by the above-described calibration curve method using X-ray fluorescence.
Furthermore, some of the Examples were subjected to reflow treatment after the Sn plating layer was formed. In the reflow treatment, the steel sheet was heated at a heating rate of 50 °C/sec by a direct current heating method for 5 seconds and then introduced into water for rapid cooling.
A steel sheet for cans (T4 blank sheet) with a thickness of 0.17 mm was used as the steel sheet.

(Pretreatment for Sn plating steel sheet)

The resulting Sn plating steel sheets were then subjected to the pretreatment indicated in Tables 2 and 3. "C" indicates cathodic electrolysis, "A" indicates anodic electrolysis, "D" indicates immersion treatment, and "C→A" indicates further anodic electrolysis after cathodic electrolysis. A sodium bicarbonate solution with a concentration of 10 g/L was used for the cathodic electrolysis, anodic electrolysis, and soaking treatment in the above pretreatment, and the temperature of the aqueous sodium bicarbonate solution was room temperature. The electrolytic density during the cathodic electrolysis was 2.0 C/dm² and the anodic electrolysis was 4.0 C/dm². The immersion time during the immersion treatment was 1 second. For comparison, no pretreatment was performed on some of the Examples.

(Coating formation process)

Next, the surface of the Sn plating steel sheet was treated with an aqueous solution to form a coating layer on the Sn plating layer. Specifically, an aqueous solution with the composition illustrated in Table 1 was used as the aqueous solution, and the coating layer was formed by performing cathodic electrolysis in the aqueous solution. The temperature of the aqueous solution was set to 35 °C, and the pH was adjusted to be between 3 and 5. The Zr coating weight and Ti coating weight were controlled by adjusting the electrical density. Zirconium fluoride (ZrF₄) was used as the Zr-containing compound and titanium fluoride (TiF₄) as the Ti-containing compound. The aqueous solution was prepared by adjusting the concentration of each ion through use of additional compounds other than the Zr-containing compound and the Ti-containing compound for the aqueous solution to have the compositions illustrated in Table 1.

(Surface conditioning process)

After the coating formation process, surface conditioning was performed under the set of conditions illustrated in Tables 2 and 3. Specifically, at the end of the coating formation process, the steel sheet having the aqueous solution adhering to its surface was squeezed with a wringer roll to adjust the amount of aqueous solution present on the surface of the coating layer to the amounts listed in Tables 2 and 3. The amount of aqueous solution was measured by a moisture meter using a filter-type infrared absorption method, as described above. The steel sheets were then held for the holding time illustrated in Tables 2 and 3. In other words, the aqueous solution used in the surface conditioning process is the same as that used in the aforementioned coating formation process.

(Water washing process)

Next, water washing treatment was applied to the steel sheets after the aforementioned surface preparation process. The water washing treatment was performed 1 to 5 times under the set of conditions illustrated in Tables 2 and 3. The method of each water washing and the electrical conductivity of the water used are illustrated in Tables 2 and 3. For the times when the water washing method was "immersion", water washing was performed by immersing the steel sheet in water. For the times when the water washing method was "spray", on the other hand, water washing was performed by spraying water onto the steel sheet. The electrical conductivity was measured using a conductivity meter.
The coating weight of Zr oxide, the coating weight of Ti oxide, the P coating weight, and the Mn coating weight in the coating layer were measured for each of the obtained surface-treated steel sheets. The measurements were performed by the above-described calibration curve method using X-ray fluorescence. The measurement results are listed in Tables 4 and 5. In Tables 4 and 5, the coating weight for Zr oxide and Ti oxide are listed as the amount of metal Zr and amount of metal Ti, respectively.
The contact angle of ethylene glycol and the atomic ratios of adsorbed elements were measured by the following procedures for each of the obtained surface-treated steel sheets. The measurement results are listed in Tables 4 and 5.

(Contact angle of ethylene glycol)

The contact angle of ethylene glycol on the obtained surface-treated steel sheets was measured using an automatic contact angle meter, model CA-VP, produced by Kyowa Interface Science Co., Ltd. The surface temperature of the surface-treated steel sheet was 20 °C ± 1 °C. A special reagent grade ethylene glycol, produced by FUJIFILM Wako Pure Chemical Corporation, at 20 °C ± 1 °C was used as the ethylene glycol. A 2 µl drop of ethylene glycol was dropped onto the surface of the surface-treated steel sheet, and the contact angle was measured 1 second later by the 0/2 method. The arithmetic mean value of the contact angles of 5 drops was used as the contact angle of the ethylene glycol.
To confirm the change in contact angle due to heat, the contact angle was also measured after the surface-treated steel sheet was subjected to heat treatment at 200 °C for 10 minutes. The measurement conditions were the same as above. As a result, the contact angle values were substantially the same before and after heat treatment for the surface-treated steel sheets meeting the conditions of the present disclosure. In contrast, some of the surface-treated steel sheets that did not meet the conditions of the present disclosure exhibited significant changes in contact angle values due to heat treatment.

(Atomic ratio of adsorbed elements)

The total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface of the surface-treated steel sheet to all elements was measured by XPS. No sputtering was performed in the measurements. From the integrated intensity of the narrow spectra of K2p, Nals, Ca2p, and Mgls at the top surface of the sample, the detected atomic ratios to all elements were quantified using the relative sensitivity factor method and calculated as (K atomic ratio + Na atomic ratio + Ca atomic ratio + Mg atomic ratio). For the XPS measurement, the scanning X-ray photoelectron spectrometer PHI X-tool, produced by ULVAC-PHI, was used, with the X-ray source being a monochrome AlKα beam, the voltage being 15 kV, the beam diameter being 100 µm ϕ, and the take-off angle being 45°.
Furthermore, the obtained surface-treated steel sheets were evaluated for adhesion to BPA-free paint by the following method. The evaluation results are listed in Tables 4 and 5.

(Sample preparation)

BPA-free prepainted steel sheets as samples used to evaluate adhesion to a BPA-free paint were prepared according to the following procedure.
The surface of the obtained surface-treated steel sheets was painted with a polyester-based paint for can inner surfaces (BPA-free paint) and baked at 180 °C for 10 minutes to produce BPA-free prepainted steel sheets. The coating weight of the paint was 60 mg/dm².

(Adhesion to BPA-free paint)

Two BPA-free prepainted steel sheets made under the same conditions were stacked so that the coated surfaces faced each other with a nylon adhesive film therebetween and were then pressure bonded under a set of conditions including a pressure of 2.94 × 10⁵ Pa, a temperature of 190 °C, and a pressure bonding time of 30 seconds. The bonded steel sheets were then divided into 5 mm wide test pieces. The divided test pieces were immersed for 168 hours in a 55 °C test solution consisting of a mixed aqueous solution containing 1.5 mass% citric acid and 1.5 mass% common salt. After immersion and subsequent washing and drying, the two steel sheets of the divided test pieces were pulled apart in a tensile tester, and the tensile strength at the time of separation was measured. The average of three test pieces was evaluated at the following four levels. For practical purposes, a result of 1 to 3 can be evaluated as excellent adhesion to BPA-free paint.

1: 2.5 kgf or more
2: 2.0 kgf or more but less than 2.5 kgf
3: 1.5 kgf or more but less than 2.0 kgf
4: Less than 1.5 kgf

As is clear from Tables 4 and 5, all of the surface-treated steel sheets that met the conditions of the present disclosure had excellent adhesion to BPA-free paint, despite being produced without hexavalent chromium.

[Table 1]

Table 1

Aqueous solution	Composition (ppm)
Aqueous solution	Zr⁴⁺	Ti⁴⁺	Mn⁴⁺	PO₄ ^3-	F	NO₃ ^3-	NH₄ ⁺
A	3000	-	-	-	4000	-	-
B	1500	-	-	-	2000	3000	2000
C	2000	-	-	950	2000	1600	1000
D	2000	-	-	950	2000	7000	2500
E	2000	2000	-	950	2000	7000	2500
F	-	1500	-	-	2000	3000	2000
G	-	2000	-	950	2000	1600	1000
H	-	2000	-	950	2000	7000	2500
I	2000	2000	2000	950	2000	7000	2500

[Table 4]

Table 4

No.	Measurement results							Evaluation	Notes
	Coating layer					Contact angle of ethylene glycol [°]	Atomic ratio of adsorbed elements [%]	Adhesion to BPA-free paint
	Zr oxide and Ti oxide			P coating weight [mg/m²]	Mn coating weight [mg/m²]
	Zr coating weight [mg/m²]	Ti coating weight [mg/m²]	Total [mg/m²]	P coating weight [mg/m²]	Mn coating weight [mg/m²]
1	9.3	0.0	9.3	0.0	0.0	22.3	0.0	1	Example
2	4.5	0.0	4.5	0.0	0.0	43.2	0.0	1	Example
3	12.3	0.0	12.3	11.6	0.0	33.6	0.0	1	Example
4	8.3	0.0	8.3	10.9	0.0	16.2	0.0	1	Example
5	15.2	6.8	22.0	7.3	0.0	27.5	0.0	1	Example
6	0.0	15.6	15.6	0.0	0.0	29.1	0.5	1	Example
7	0.0	5.6	5.6	16.5	0.0	39.8	0.0	1	Example
8	0.0	11.3	11.3	48.3	0.0	21.3	0.0	1	Example
9	8.4	2.5	10.9	5.3	15.1	10.6	0.1	1	Example
10	1.3	0.0	1.3	0.0	0.0	15.3	0.0	1	Example
11	8.2	0.0	8.2	0.0	0.0	29.3	0.0	1	Example
12	29.3	0.0	29.3	12.5	0.0	36.3	0.0	1	Example
13	38.6	0.0	38.6	2.1	0.0	8.5	0.1	1	Example
14	16.2	8.3	24.5	9.9	0.0	12.6	0.0	1	Example
15	0.0	22.5	22.5	0.0	0.0	32.8	0.0	1	Example
16	0.0	33.5	33.5	25.6	0.0	14.3	0.8	1	Example
17	0.0	39.4	39.4	14.2	0.0	12.1	0.0	1	Example
18	7.2	0.5	7.7	10.6	8.1	8.6	0.0	1	Example
19	12.6	0.0	12.6	0.0	0.0	18.4	0.6	1	Example
20	21.3	0.0	21.3	0.0	0.0	15.3	0.0	1	Example
21	30.8	0.0	30.8	30.7	0.0	11.7	0.0	1	Example
22	19.2	0.0	19.2	18.2	0.0	31.6	0.0	1	Example
23	20.1	12.5	32.6	4.3	0.0	41.2	0.0	1	Example
24	0.0	13.5	13.5	0.0	0.0	13.6	0.4	1	Example
25	0.0	27.0	27.0	45.6	0.0	34.2	0.0	1	Example
26	0.0	18.6	18.6	37.6	0.0	12.2	0.1	1	Example
27	3.5	19.2	22.7	21.3	47.1	40.8	0.2	1	Example
28	12.1	0.0	12.1	0.0	0.0	39.1	0.0	1	Example
29	19.3	0.0	19.3	0.0	0.0	13.5	0.0	1	Example
30	5.2	0.0	5.2	31.5	0.0	26.3	0.0	1	Example
31	3.6	0.0	3.6	15.6	0.0	15.4	0.0	1	Example
32	5.6	2.8	8.4	17.3	0.0	22.1	0.0	1	Example
33	0.0	1.9	1.9	0.0	0.0	11.6	0.0	1	Example
34	0.0	3.8	3.8	12.6	0.0	13.2	0.0	1	Example
35	0.0	12.9	12.9	0.3	0.0	17.5	0.0	1	Example
36	11.3	22.1	33.4	3.5	22.3	16.8	0.3	1	Example
37	30.5	0.0	30.5	0.0	0.0	13.8	0.1	1	Example
38	15.2	0.0	15.2	0.0	0.0	15.9	0.5	1	Example
39	11.3	0.0	11.3	4.2	0.0	26.3	0.4	1	Example

[Table 5]

Table 5

No.	Measurement results							Evaluation	Notes
	Coating layer					Contact angle of ethylene glycol [°]	Atomic ratio of adsorbed clements [%]	Adhesion to BPA-free paint
	Zr oxide and Ti oxide			P coating weight [mg/m²]	Mn coating weight [mg/m²]
	Zr coating weight [mg/m²]	Ti coating weight [mg/m²]	Total [mg/m²]	P coating weight [mg/m²]	Mn coating weight [mg/m²]
40	13.5	0.0	13.5	2.8	0.0	27.5	0.0	1	Example
41	19.2	9.2	28.4	8.4	0.0	42.1	0.0	1	Example
42	0.0	11.0	11.0	0.0	0.0	33.4	0.0	1	Example
43	10.1	0.0	10.1	4.2	0.0	16.2	0.0	1	Example
44	15.3	0.0	15.3	2.8	0.0	28.3	0.0	1	Example
45	16.2	8.3	24.5	8.4	0.0	41.5	0.1	1	Example
46	0.0	10.5	10.5	0.0	0.0	37.3	0.0	1	Example
47	0.0	48.3	48.3	10.6	0.0	27.1	0.1	2	Example
48	29.8	26.5	56.3	1.8	6.3	24.3	0.0	3	Example
49	0.3	0.0	0.3	0.0	0.0	13.4	0.0	2	Example
50	0.2	0.0	0.2	31.1	0.0	16.2	0.1	3	Example
51	6.5	0.0	6.5	8.2	0.0	47.1	0.0	2	Example
52	0.8	36.9	37.7	25.0	0.0	49.3	0.0	3	Example
53	0.0	14.1	14.1	0.0	0.0	51.6	0.0	4	Comparative example
54	0.0	10.8	10.8	41.3	0.0	45.3	0.2	2	Example
55	0.0	3.9	3.9	12.5	0.0	48.2	0.0	3	Example
56	25.3	10.6	35.9	11.3	1.4	69.3	0.2	4	Comparative example
57	7.2	0.0	7.2	0.0	0.0	45.6	0.0	2	Example
58	14.0	0.0	14.0	0.0	0.0	48.5	0.0	3	Example
59	11.9	0.0	11.9	3.8	0.0	71.3	0.0	4	Comparative example
60	13.5	0.0	13.5	2.5	0.0	47.0	0.0	2	Example
61	7.2	3.1	10.3	6.9	0.0	48.3	0.0	3	Example
62	0.0	4.5	4.5	0.0	0.0	59.3	0.0	4	Comparative example
63	0.0	26.3	26.3	3.4	0.0	43.2	1.3	2	Example
64	0.0	17.2	17.2	1.8	0.0	13.6	3.5	3	Example
65	20.3	2.5	22.8	2.5	3.6	32.4	5.4	4	Comparative example
66	15.7	0.0	15.7	0.0	0.0	16.3	2.2	2	Example
67	13.1	0.0	13.1	0.0	0.0	25.7	4.3	3	Example
68	9.3	0.0	9.3	12.1	0.0	16.6	6.7	4	Comparative example
69	10.1	0.0	10.1	1.3	0.0	34.9	1.5	2	Example
70	5.2	34.5	39.7	8.5	0.0	13.9	3.1	3	Example
71	0.0	38.8	38.8	0.0	0.0	21.7	7.1	4	Comparative example
72	0.0	25.1	25.1	4.5	0.0	25.4	2.2	2	Example
73	0.0	13.3	13.3	8.2	0.0	21.3	4.1	3	Example
74	1.2	5.3	6.5	7.3	24.2	17.4	5.2	4	Comparative example
75	11.3	0.0	11.3	0.0	0.0	38.6	1.9	2	Example
76	8.6	0.0	8.6	0.0	0.0	13.2	4.2	3	Example
77	9.2	0.0	9.2	12.1	0.0	11.9	6.0	4	Comparative example

Claims

A surface-treated steel sheet comprising, on at least one side of a steel sheet,
a Sn plating layer; and

a coating layer disposed on the Sn plating layer and containing at least one of Zr oxide and Ti oxide, wherein

a contact angle of ethylene glycol is 50° or less, and

a total atomic ratio of K, Na, Mg, and Ca adsorbed on a surface of the steel sheet to all elements is 5.0 % or less.
The surface-treated steel sheet according to claim 1, wherein the Sn plating layer has a Sn coating weight of 0.1 g/m² to 20.0 g/m² per side of the steel sheet.
The surface-treated steel sheet according to claim 1 or 2, wherein a total coating weight of Zr oxide and Ti oxide in the coating layer is 0.3 mg/m² to 50.0 mg/m² per side of the steel sheet in terms of amount of metal Zr and amount of metal Ti.
The surface-treated steel sheet according to any one of claims 1 to 3, wherein the coating layer further contains P, and a P coating weight is 50.0 mg/m² or less per side of the steel sheet.
The surface-treated steel sheet according to any one of claims 1 to 4, wherein the coating layer further contains Mn, and a Mn coating weight is 50.0 mg/m² or less per side of the steel sheet.
The surface-treated steel sheet according to any one of claims 1 to 5, further comprising a Ni-containing layer disposed below the Sn plating layer.
The surface-treated steel sheet according to claim 6, wherein the Ni-containing layer has a Ni coating weight of 2 mg/m² to 2000 mg/m² per side of the steel sheet.
A method of producing a surface-treated steel sheet including, on at least one side of a steel sheet, a Sn plating layer, and a coating layer disposed on the Sn plating layer and containing at least one of Zr oxide and Ti oxide, the method comprising:
a coating formation process of treating a surface of a steel sheet having a Sn plating layer on at least one side with an aqueous solution containing at least one of Zr ions and Ti ions to form the coating layer on the Sn plating layer;

a surface conditioning process of holding the aqueous solution at 1.0 g/m² to 30.0 g/m² on a surface of the coating layer for 0.1 seconds to 20.0 seconds; and

a water washing process of subjecting the steel sheet after the surface conditioning process to water washing at least once, wherein

in the water washing process,
water with an electrical conductivity of 100 µS/m or less is used at least in the last water washing.
The method of producing a surface-treated steel sheet according to claim 8, wherein the surface-treated steel sheet further includes a Ni-containing layer disposed below the Sn plating layer.