WO2024111157A1

WO2024111157A1 - Surface-treated steel sheet and method for producing same

Info

Publication number: WO2024111157A1
Application number: PCT/JP2023/025345
Authority: WO
Inventors: 卓嗣植野; 祐介中川
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-11-24
Filing date: 2023-07-07
Publication date: 2024-05-30
Also published as: TW202421847A

Abstract

The present invention provides a surface-treated steel sheet which can be produced without the use of hexavalent chromium, and which has excellent adhesion with BPA-free paint. Provided is a surface-treated steel sheet comprising, 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 a Zr oxide and a Ti oxide, wherein the ethylene glycol contact angle is not greater than 50°, and the sum of the atomic ratios of the K, Na, Mg, and Ca adsorbed on the surface of the steel sheet is not greater than 5.0%.

Description

Surface-treated steel sheet and its manufacturing method

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

Sn-plated steel sheet (tinplate), a type of surface-treated steel sheet, is widely used as a material for various metal cans such as beverage cans, food cans, pail cans, and 18-liter cans because it has excellent corrosion resistance, weldability, and workability, and is easy to manufacture.

　The surface of the surface-treated steel sheets used for these applications is coated with an organic resin coating, such as an epoxy-based paint, to accommodate the various contents. When the organic resin coating is applied, the chromium oxide layer formed on the outermost surface by electrolytically treating or immersing the steel sheet in an aqueous solution containing hexavalent chromium plays an important role. In other words, the chromium oxide layer provides excellent adhesion to the organic resin coating layer, and as a result, corrosion resistance to the various contents is guaranteed (Patent Documents 1 to 5).

On the other hand, it has been suggested that the BPA contained in epoxy-based paints may have harmful effects on humans. For this reason, development of BPA-free paints using polyester-based resins that do not contain BPA is underway (Patent Documents 6 and 7), and there is a demand to replace epoxy-based paints. However, tinplate, which has been used as steel sheet for cans up to now, has poor adhesion to BPA-free paints compared to its adhesion to epoxy-based paints. As a result, sufficient corrosion resistance against various contents cannot be ensured, and the application of BPA-free paints to various metal cans has not progressed.

Furthermore, in recent years, due to growing environmental awareness, there has been a trend toward restricting the use of hexavalent chromium worldwide. As a result, there is a demand for the establishment of manufacturing methods that do not use hexavalent chromium, even in the field of surface-treated steel sheets used for various metal cans.

A method for producing surface-treated steel sheets without using hexavalent chromium is proposed, for example, in Patent Document 8. This method proposes a surface-treated steel sheet in which a coating containing a zirconium compound is formed on the surface of a Sn-plated steel sheet.

Japanese Patent Application Laid-Open No. 58-110695 Japanese Patent Application Laid-Open No. 55-134197 Japanese Patent Application Laid-Open No. 57-035699 Japanese Patent Application Laid-Open No. 11-117085 JP 2007-231394 A JP 2013-144753 A JP 2008-050486 A JP 2018-135569 A

The method proposed in Patent Document 8 makes it possible to form a surface treatment layer without using hexavalent chromium. Furthermore, Patent Document 8 also shows that the method makes it possible to obtain a surface-treated steel sheet that has excellent adhesion to epoxy-based paints.

However, surface-treated steel sheets obtained by the conventional method proposed in Patent Document 8 have excellent adhesion to epoxy-based paints, but poor adhesion to BPA-free paints, and as a result, the corrosion resistance of the BPA-free paint is insufficient. Therefore, it was not possible to replace the BPA-free paint with a BPA-free paint while maintaining corrosion resistance to various contents.

Therefore, there is a demand for surface-treated steel sheets that can be manufactured without using hexavalent chromium and have excellent adhesion to BPA-free paints.

The present invention was made in consideration of the above situation, and its purpose is to provide a surface-treated steel sheet that can be manufactured without using hexavalent chromium and has excellent adhesion to BPA-free paint.

The inventors of the present invention conducted extensive research to achieve the above objective, and as a result, have come to the following findings (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, the contact angle of ethylene glycol and the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface to all elements can be controlled within a specific range, thereby obtaining a surface-treated steel sheet with excellent adhesion to BPA-free paint.

(2) The above-mentioned surface-treated steel sheet can be produced by forming a coating, adjusting the surface under specified conditions, and then performing a final water wash using water whose electrical conductivity is equal to or lower than a specified value.

The present invention was completed based on the above findings. The gist of the present invention is as follows:

1. At least one surface of the steel plate is
A Sn plating layer;
A surface-treated steel sheet having a coating layer containing at least one of Zr oxide and Ti oxide disposed on the Sn plating layer,
The contact angle of ethylene glycol is 50° or less,
A surface-treated steel sheet having a total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface to all elements of 5.0% or less.

2. The surface-treated steel sheet according to claim 1, wherein the Sn plating layer has a Sn coating weight of 0.1 to 20.0 g/ ^m2 per one side of the steel sheet.

3. The surface-treated steel sheet according to 1 or 2 above, wherein the total amount of deposited Zr oxide and Ti oxide in the coating layer is 0.3 to 50.0 mg/ ^m2 per side of the steel sheet in terms of the amount of metallic Zr and the amount of metallic Ti.

4. The surface-treated steel sheet according to any one of the above 1 to 3, wherein the coating layer further contains P, and the P coating amount is 50.0 mg/ ^m2 or less per one side of the steel sheet.

5. The surface-treated steel sheet according to any one of the above 1 to 4, wherein the coating layer further contains Mn, and the Mn coating amount is 50.0 mg/ ^m2 or less per one side of the steel sheet.

6. The surface-treated steel sheet according to any one of claims 1 to 5 above, further comprising a Ni-containing layer disposed under the Sn-plated layer.

7. The surface-treated steel sheet according to the above 6, wherein the Ni-containing layer has a Ni coating amount of 2 mg/m ² to 2000 mg/m ² per one side of the steel sheet.

8. A method for producing a surface-treated steel sheet having a Sn-plated layer on at least one surface of the steel sheet, and a coating layer containing at least one of Zr oxide and Ti oxide disposed on the Sn-plated layer, comprising the steps of:
a coating formation step of treating a surface of a steel sheet having a Sn plating layer on at least one surface thereof 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 step of maintaining the aqueous solution at 1.0 to 30.0 g/ ^m2 on the surface of the coating layer for 0.1 to 20.0 seconds;
and a water washing step of washing the steel sheet after the surface conditioning step at least once with water,
In the water washing step,
A method for producing a surface-treated steel sheet, comprising using water having an electrical conductivity of 100 μS/m or less in at least the final water washing.

9. The method for producing a surface-treated steel sheet described in 8 above, wherein the surface-treated steel sheet further has a Ni-containing layer disposed under the Sn-plated layer.

According to the present invention, it is possible to provide a surface-treated steel sheet that has excellent adhesion to BPA-free paints without using hexavalent chromium. The surface-treated steel sheet of the present invention can be suitably used as a material for containers, etc.

Below, a method for implementing the present invention will be specifically described. Note that the following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited thereto.

In one embodiment of the present invention, the surface-treated steel sheet has a Sn-plated layer on at least one surface of the steel sheet, and a coating layer disposed on the Sn-plated layer, the coating layer containing at least one of Zr oxide and Ti oxide. In the present invention, it is important that the contact angle of ethylene glycol on the surface-treated steel sheet is 50° or less, and that the total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface to all elements is 5.0% or less. Each of the constituent elements of the surface-treated steel sheet is explained below.

[Steel plate]
The steel sheet is not particularly limited and any steel sheet can be used, but it is preferable to use a steel sheet for cans. The steel sheet can be, for example, an ultra-low carbon steel sheet or a low carbon steel sheet. The manufacturing method of the steel sheet is also not particularly limited and any steel sheet manufactured by any method can be used, but usually a cold-rolled steel sheet can be used. The cold-rolled steel sheet can be manufactured by a general manufacturing process that includes, for example, hot rolling, pickling, cold rolling, annealing, and temper rolling.

The composition of the steel plate is not particularly limited, but the steel plate may contain C, Mn, Cr, P, S, Si, Cu, Ni, Mo, Al, and unavoidable impurities within a range that does not impair the effects of the present invention. In this case, for example, a steel plate having a composition specified in ASTM A623M-09 can be suitably used as the steel plate.

In one embodiment of the present invention, in weight percent:
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%,
Sb: 0 to 0.020%,
and the balance being Fe and unavoidable impurities. Of the above-mentioned composition, the lower the content of Si, P, S, Al, and N, the more preferable the components are, and Cu, Ni, Cr, Mo, Ti, Nb, B, Ca, Sn, and Sb are components that may be added as desired.

The lower limit of the thickness of the steel plate is not particularly limited, but it is preferable that the thickness is 0.10 mm or more. The upper limit of the thickness is not particularly limited, but it is preferable that the thickness is 0.60 mm or less. Here, "steel plate" is defined to include "steel strip".

[Sn plating layer]
The Sn plating layer may be provided on at least one surface of the steel sheet, or may be provided on both surfaces. The Sn plating layer may cover at least a portion of the steel sheet, or may cover the entire surface on which the Sn plating layer is provided. The Sn plating layer may be a continuous layer or a discontinuous layer. An example of the discontinuous layer is a layer having an island structure.

The Sn plating layer also includes a portion of the Sn plating layer that has been alloyed. For example, a portion of the Sn plating layer that has been turned into a Sn alloy layer by a heat melting treatment after Sn plating is also included in the Sn plating layer. 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 the Sn by electrical heating or the like after Sn plating, a part of the steel sheet side of the Sn-plated layer can be made into an Fe-Sn alloy layer. Also, by plating a steel sheet having a Ni-containing layer on its surface with Sn, and then heating and melting the Sn by electrical heating or the like, a part of the steel sheet side of the Sn-plated 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 particularly limited and may be any amount. However, from the viewpoint of further improving the appearance and corrosion resistance of the surface-treated steel sheet, it is preferable that the Sn coating weight is 20.0 g/ ^m2 or less per one side of the steel sheet. From the same viewpoint, it is preferable that the Sn coating weight is 0.1 g/ ^m2 or more, and more preferably 0.2 g/ ^m2 or more. Moreover, from the viewpoint of further improving the workability, it is even more preferable that the Sn coating weight is 1.0 g/ ^m2 or more.

The amount of Sn adhesion can be measured by the electrolytic stripping method specified in JIS G 3303.

The formation of the Sn plating layer is not particularly limited, and can be performed by any method, such as electroplating or hot-dip plating. When 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, or a halogen-based Sn plating bath.

After the Sn plating layer is formed, a reflow process may be performed. When performing the reflow process, an alloy layer such as an Fe-Sn alloy layer can be formed underneath the unalloyed Sn plating layer (on the steel sheet side) by heating the Sn plating layer to a temperature equal to or higher than the melting point of Sn (231.9°C). Also, if the reflow process is omitted, a Sn-plated steel sheet having an unalloyed Sn plating layer is obtained.

The surface side of the Sn plating layer may contain Sn oxides, or it may not contain any at all. However, from the viewpoint of improving coating secondary adhesion, which is adhesion to paint in a wet environment, and sulfide staining resistance, it is preferable for the surface side of the Sn plating layer to contain Sn oxides. Sn oxides can also be formed during reflow processing or due to dissolved oxygen contained in the rinsing water after Sn plating, but it is preferable to control the amount of Sn oxides contained in the Sn plating layer by pretreatment, which will be described later.

[Ni-containing layer]
The surface-treated steel sheet may further have an optional Ni-containing layer. For example, the surface-treated steel sheet in one embodiment of the present invention may further have an Ni-containing layer disposed under the Sn-plated layer. In other words, the surface-treated steel sheet in one embodiment of the present invention may be a surface-treated steel sheet having, on at least one surface of the steel sheet, a Ni-containing layer, a Sn-plated layer disposed on the Ni-containing layer, and a coating layer containing at least one of Zr oxide and Ti oxide disposed on the Sn-plated layer.

The Ni-containing layer may be any layer containing nickel, for example, a Ni layer and/or a Ni alloy layer. An example of the Ni layer is a Ni plating layer. An example of the Ni alloy layer is a Ni-Fe alloy layer. By forming a Sn plating layer on the Ni-containing layer and then performing a reflow treatment, an Fe-Sn-Ni alloy layer or an Fe-Sn alloy layer can be formed on the lower layer (on the steel sheet side) of the unalloyed Sn plating layer.

The method for forming the Ni-containing layer is not particularly limited, and any method, such as electroplating, can be used. When forming a Ni-Fe alloy layer as the Ni-containing layer, the Ni layer can be formed on the steel sheet surface by electroplating or other methods, and then annealing to form the Ni-Fe alloy layer.

Although the Ni adhesion amount of the Ni-containing layer is not particularly limited, from the viewpoint of improving resistance to sulfidation blackening, it is preferable that the Ni adhesion amount per one side of the steel sheet is 2 mg/ ^m2 or more. Also, from the viewpoint of cost, it is preferable that the Ni adhesion amount per one side of the steel sheet is 2000 mg/ ^m2 or less.

The Ni adhesion amount of the Ni-containing layer is measured by a calibration curve method using fluorescent X-rays. First, multiple steel plates with known Ni adhesion amounts are prepared, and the fluorescent X-ray intensity derived from Ni is measured for the steel plates in advance. The relationship between the measured fluorescent X-ray intensity and the Ni adhesion amount is linearly approximated to create a calibration curve. Next, the fluorescent X-ray intensity derived from Ni of the surface-treated steel plate is measured, and the Ni adhesion amount of the Ni-containing layer can be obtained using the above-mentioned calibration curve.

[Coating layer]
A coating layer containing at least one of Zr oxide and Ti oxide is present 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 with a BPA-free paint.

The lower limit of the total adhesion amount of Zr oxide and Ti oxide in the coating layer is not particularly limited. However, from the viewpoint of further improving the adhesion to the BPA-free paint, the total adhesion amount of Zr oxide and Ti oxide is preferably 0.3 mg/m ² or more per one side of the steel sheet in terms of the amount of metal Zr and the amount of metal Ti, more preferably 0.4 mg/m ² or more, and even more preferably 0.5 mg/m ² or more. On the other hand, the upper limit of the total adhesion amount of Zr oxide and Ti oxide in the coating layer is also not particularly limited. However, if the total adhesion amount of Zr oxide and Ti oxide is excessively large, the adhesion to the BPA-free paint may be impaired due to cohesive failure of the coating layer. Therefore, from the viewpoint of ensuring adhesion to the BPA-free paint more stably, the total adhesion amount of Zr oxide and Ti oxide is preferably 50.0 mg/m ² or less per one side of the steel sheet in terms of the amount of metal Zr and the amount of metal Ti, more preferably 45.0 mg/m ² or less, and even more preferably 40.0 mg/m ² or less. In calculating the total amount of Zr oxide and Ti oxide adhesion, the amount of Zr oxide adhesion is calculated by converting the amount of metallic Zr, and the amount of Ti oxide adhesion is calculated by converting the amount of metallic Ti.

The amount of Zr oxide attached in the coating layer is measured using a calibration curve method using fluorescent X-rays. First, multiple steel plates with known amounts of attached metal Zr are prepared, and the fluorescent X-ray intensity originating from Zr is measured in advance for the steel plates. A calibration curve is then created by linearly approximating the relationship between the measured fluorescent X-ray intensity and the amount of attached metal Zr. Next, the fluorescent X-ray intensity originating from Zr in the surface-treated steel plate is measured, and the amount of attached Zr oxide in the coating layer can be calculated in terms of metallic Zr using the above-mentioned calibration curve.

The amount of Ti oxide attached in the coating layer is measured by a calibration curve method using fluorescent X-rays. First, multiple steel plates with known amounts of attached metal Ti are prepared, and the fluorescent X-ray intensity derived from Ti for the steel plates is measured in advance. A calibration curve is then created by linearly approximating the relationship between the measured fluorescent X-ray intensity and the amount of attached metal Ti. Next, the fluorescent X-ray intensity derived from Ti in the surface-treated steel plate is measured, and the amount of attached Ti oxide in the coating layer can be calculated in terms of metallic Ti using the above-mentioned calibration curve.

The coating layer may contain P from the viewpoint of further improving adhesion to the BPA-free paint. The upper limit of the amount of P contained in the coating layer is not particularly limited, but since the adhesion to the BPA-free paint may be impaired due to cohesive failure of the coating layer, it is preferable that the amount is 50.0 mg/ ^m2 or less per one side of the steel sheet. The lower limit of the amount of P contained in the coating layer is not particularly limited, and may be, for example, 0.0 mg/ ^m2 , or may not be contained at all.

The amount of P attached in the coating layer is measured using a calibration curve method using fluorescent X-rays. First, multiple steel plates with known amounts of P attached are prepared, and the fluorescent X-ray intensity originating from P is measured in advance for the steel plates. A calibration curve is then created by linearly approximating the relationship between the measured fluorescent X-ray intensity and the amount of P attached. Next, the fluorescent X-ray intensity originating from P in the surface-treated steel plate is measured, and the amount of P attached in the coating layer can be determined using the above-mentioned calibration curve.

The coating layer may contain Mn from the viewpoint of further improving adhesion to the BPA-free paint. The upper limit of the amount of Mn contained in the coating layer is not particularly limited, but since the adhesion to the BPA-free paint may be impaired due to cohesive failure of the coating layer, it is preferable that the amount is 50.0 mg/ ^m2 or less per one side of the steel sheet. The lower limit of the amount of Mn contained in the coating layer is not particularly limited, and may be, for example, 0.0 mg/ ^m2 , or may not be contained at all.

The amount of Mn attached in the coating layer is measured using a calibration curve method using fluorescent X-rays. First, multiple steel plates with known amounts of Mn attached are prepared, and the fluorescent X-ray intensity derived from Mn for the steel plates is measured in advance. A calibration curve is then created by linearly approximating the relationship between the measured fluorescent X-ray intensity and the amount of Mn attached. Next, the fluorescent X-ray intensity derived from Mn in the surface-treated steel plate is measured, and the amount of Mn attached in the coating layer can be determined using the above-mentioned calibration curve.

The coating layer may contain Sn. The upper limit of the Sn content in the coating layer is not particularly limited. The coating layer may not contain Sn, and may have a Sn content of 0.0 mg/ ^m2 .

The coating layer may contain C. There is no particular upper limit to the C content in the coating layer. The coating layer may not contain C, and the C content may be 0.0 mg/ ^m2 .

The coating layer may contain elements other than Zr, Ti, O, Sn, Mn, P, Ni, and C, as well as K, Na, Mg, and Ca, which will be described later. Examples of elements other than the above elements include metal impurities such as Cu, Zn, and Fe contained in the aqueous solution used in the coating formation process, which will be described later, and elements such as S, N, F, Cl, Br, and Si. However, if elements other than Zr, Ti, O, Sn, Mn, P, Ni, C, K, Na, Mg, and Ca are present in excess, adhesion to the BPA-free paint may decrease. 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 in atomic ratio, and more preferably 20% or less. The coating layer may not contain elements other than Zr, Ti, O, Sn, Mn, P, Ni, C, K, Na, Mg, and Ca, and may be 0% in atomic ratio. The content of the above elements can be measured using XPS (X-ray photoelectron spectroscopy).

[Contact angle of ethylene glycol]
In the present invention, it is important that the contact angle of ethylene glycol of the surface-treated steel sheet is 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 strong bond is formed between the polyester resin contained in the BPA-free paint and the surface-treated steel sheet, and as a result, high adhesion to the BPA-free paint can be obtained. From the viewpoint of further improving the adhesion to the BPA-free paint, the contact angle of ethylene glycol is preferably 48° or less, and more preferably 45° or less. The lower limit of the contact angle of ethylene glycol is not particularly limited, and may be 0°. However, from the viewpoint of ease of production, it may be 5° or more, or 8° or more.

Furthermore, the surface of the surface-treated steel sheet of the present invention, i.e., the surface state of the coating layer containing at least one of Zr oxide and Ti oxide, is stable against heat, and the contact angle of ethylene glycol does not change significantly, for example, even after heat treatment equivalent to paint baking. It is presumed that such thermal stability of the surface state also contributes to improved adhesion with BPA-free paint. Therefore, the contact angle of ethylene glycol of the surface-treated steel sheet after heat treatment equivalent to paint is preferably 50° or less, more preferably 48° or less, and even more preferably 45° or less. In addition, the lower limit of the contact angle of ethylene glycol of the surface-treated steel sheet after heat treatment equivalent to paint is not particularly limited and may be 0°, but the contact angle may be 5° or more, or 8° or more. The conditions of the heat treatment equivalent to paint are a maximum temperature of 200°C and a holding time at the maximum temperature of 10 minutes.

The mechanism by which the contact angle of ethylene glycol on a surface-treated steel sheet becomes 50° or less is unclear, but it is thought that the micro-roughness of the surface is adjusted in the surface conditioning process described below, and the surface is modified to have a high affinity with ethylene glycol. If the surface conditioning process described below is not carried out, even if the surface of the surface-treated steel sheet has a high affinity with ethylene glycol immediately after production, the coating layer cannot be fixed in a state of high affinity, and the contact angle of ethylene glycol exceeds 50°.

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 1 second after the drop is calculated by the θ/2 method. More specifically, it can be measured by the method described in the Examples. Here, the surface of the surface-treated steel sheet may be coated with a rust-preventive oil such as CSO (Cottonseed Oil), DOS (Dioctyl Sebacate), or ATBC (Acetyl Tributyl Citrate). If the surface-treated steel sheet is coated with oil, the contact angle measured by the method described in the Examples after the coating-equivalent heat treatment is performed to vaporize the coated oil is regarded as the contact angle of ethylene glycol on the surface-treated steel sheet after coating. As described above, the surface-treated steel sheet of the present invention is stable against heat treatment, so if the contact angle measured after the heat treatment and the atomic ratio of the adsorbed elements described below satisfy the conditions of the present invention, the effect of the present invention is also expected to be achieved for the surface-treated steel sheet before the heat treatment. Note that additive components such as rust inhibitors contained in the oil applied may remain on the surface of the surface-treated steel sheet even after the heat treatment equivalent to painting, but the amount is so small that it does not affect the contact angle of ethylene glycol and the atomic ratio of the adsorbed elements described above.

It has been reported that in surface-treated steel sheets manufactured using conventional hexavalent chromium baths such as those proposed in Patent Documents 1 to 5, the composition of the chromium hydrate oxide layer present on the surface layer significantly affects adhesion to epoxy-based paints in a humid environment. In a humid environment, water that has penetrated the epoxy-based paint film inhibits adhesion at the interface between the epoxy-based paint film and the chromium hydrate oxide layer. For this reason, it was thought that when a large number of hydrophilic OH groups are present in the chromium hydrate oxide layer, the water's expansion and wetting at the interface is promoted, resulting in a decrease in adhesive strength. Therefore, in conventional surface-treated steel sheets, the adhesion to epoxy-based paints in a humid environment was improved by reducing the number of OH groups through the progression of oxidation of the chromium hydrate oxide, i.e., by hydrophobizing the surface.

In contrast, the present invention focuses on ethylene glycol rather than water, and has found that by adjusting the surface to have a high affinity for ethylene glycol, it is possible to ensure strong adhesion with BPA-free paint. Therefore, it can be said that the present invention is based on a technical concept that is completely different from the conventional technology described above. The mechanism by which adhesion to BPA-free paint is improved by adjusting the surface to have a high affinity for ethylene glycol is not clear. However, because ethylene glycol is one of the hydroxyl monomers that is a component of the polyester resin that makes up BPA-free paint, it is presumed that adhesion to BPA-free paint is improved by adjusting the surface to have a high affinity for ethylene glycol.

[Atomic ratio of adsorbed elements]
As described above, the surface-treated steel sheet of the present invention has a contact angle of ethylene glycol of 50° or less, and its 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. The inventors have found that simply making the contact angle of ethylene glycol 50° or less does not provide the original adhesion due to the influence of the adsorbed cations. In the present invention, the adhesion to BPA-free paint can be improved 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, so there is no particular lower limit and it may be 0.0%. The total atomic ratio can be measured by XPS. In the measurement, the atomic ratio of K, Na, Mg, and Ca to all elements can be obtained by the relative sensitivity coefficient method from the integrated intensity of the narrow spectrum of K2p, Na1s, Ca2p, and Mg1s on the outermost surface of the surface-treated steel sheet. More specifically, it can be measured by the method described in the examples. In addition, if the surface-treated steel sheet is oiled, the atomic ratio measured by the method described in the examples after performing the heat treatment equivalent to painting to vaporize the oil applied is regarded as the atomic ratio of the adsorbed elements of the surface-treated steel sheet after oiling.

[Production method]
In a method for producing a surface-treated steel sheet according to one embodiment of the present invention, a surface-treated steel sheet having the above-mentioned properties can be produced by the method described below.

A method for producing a surface-treated steel sheet according to one embodiment of the present invention is a method for producing a surface-treated steel sheet having a Sn-plated layer and a coating layer disposed on the Sn-plated layer on at least one surface of the steel sheet, and includes the following steps (1) to (3). Each step will be described below.
(1) Film formation process (2) Surface preparation process (3) Water washing process

In one embodiment of the present invention, the surface-treated steel sheet may further have a Ni-containing layer disposed under the Sn-plated layer. When manufacturing a surface-treated steel sheet having a Ni-containing layer, a steel sheet having a Ni-containing layer on at least one surface and a Sn-plated layer disposed on the Ni-containing layer may be subjected to a film formation process.

[Film formation process]
In the above-mentioned coating formation step, a surface of a steel sheet having a Sn-plated 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-plated layer. The coating layer formed is a coating layer containing at least one of Zr oxide and Ti oxide.

The treatment with the aqueous solution is not particularly limited and can be carried out by any method. For example, the treatment can be carried out by electrolysis. When the treatment is carried out by electrolysis, it is preferable to subject the steel sheet having the Sn plating layer to cathodic electrolysis in the aqueous solution. For the cathodic electrolysis, conventional equipment used for chromate treatment and the like can be used as is. Therefore, from the viewpoint of reducing equipment costs, it is preferable to form the coating layer by cathodic electrolysis.

The method for preparing the aqueous solution is not particularly limited, but 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. The water can be distilled water or deionized water, but is not limited thereto and any water can be used.

As the Zr-containing compound and the Ti-containing compound, any compound capable of supplying Zr ions and Ti ions, respectively, can be used. As the Zr _- containing compound, for example, _{a Zr salt such as ZrF4 or a Zr complex such as H2ZrF6} _or _K2ZrF6 _is preferably used. The Zr ions become Zr oxides and form a coating as the pH rises on the surface of the cathode. As the Ti-containing compound, for example, _a Ti salt such as _TiF4 or a Ti complex such as _H2TiF6 or _K2TiF6 is preferably used. The Ti ions become Ti oxides and form a coating as the pH rises on _the surface of the cathode.

The aqueous solution may further contain at least one selected from the group consisting of fluoride ions, nitrate ions, ammonium ions, phosphate ions, Mn ions, and sulfate ions. When the aqueous solution contains both nitrate ions and ammonium ions, processing can be carried out in a short time of several to several tens of seconds, which is extremely advantageous from an industrial standpoint. Therefore, it is preferable that the aqueous solution contains both nitrate ions and ammonium ions in addition to at least one of Zr ions and Ti ions. Hereinafter, the unit of ion concentration "ppm" refers to parts per million by mass unless otherwise specified.

When the aqueous solution contains Zr ions, the lower limit of the Zr ion concentration is not particularly limited, but is preferably 100 ppm or more. The upper limit of the Zr ion concentration is also not particularly limited, but is preferably 4000 ppm or less. Similarly, when the aqueous solution contains Ti ions, the lower limit of the Ti ion concentration is not particularly limited, but is preferably 100 ppm or more. The upper limit of the Ti ion concentration is also not particularly limited, but is preferably 4000 ppm or less.

When the aqueous solution contains fluorine ions, the lower limit of the concentration of fluorine ions is not particularly limited, but is preferably 120 ppm or more. The upper limit of the concentration of fluorine ions is also not particularly limited, but is preferably 4000 ppm or less. When the aqueous solution contains phosphate ions, the lower limit of the concentration of phosphate ions is not particularly limited, but is preferably 50 ppm or more. The upper limit of the concentration of phosphate ions is also not particularly limited, but is preferably 5000 ppm or less. When the aqueous solution contains Mn ions, the lower limit of the concentration of Mn ions is not particularly limited, but is preferably 50 ppm or more. The upper limit of the concentration of Mn ions is also not particularly limited, but is preferably 5000 ppm or less. When the aqueous solution contains ammonium ions, the lower limit of the concentration of ammonium ions is not particularly limited, but may be 0 ppm. The upper limit of the concentration of ammonium ions is also not particularly limited, but is preferably 20000 ppm or less. When the aqueous solution contains nitrate ions, the lower limit of the concentration of nitrate ions is not particularly limited, but may be 0 ppm. In addition, the upper limit of the concentration of nitrate ions is not particularly limited, but it is preferably 20,000 ppm or less. When the aqueous solution contains sulfate ions, the lower limit of the concentration of sulfate ions is not particularly limited, and may be 0 ppm. In addition, the upper limit of the concentration of sulfate ions is not particularly limited, but it is preferably 20,000 ppm or less.

The upper limit of the temperature of the aqueous solution when performing the cathodic electrolysis is not particularly limited, but for example, it is preferable to set it to 50°C or less. By performing cathodic electrolysis at 50°C or less, it is possible to generate a dense and uniform coating structure consisting of very fine particles. Furthermore, by setting the temperature of the aqueous solution to 50°C or less, the occurrence of defects, cracks, microcracks, etc. in the coating layer to be formed can be suppressed, and the adhesion with the BPA-free paint can be further improved. Furthermore, the lower limit of the temperature of the aqueous solution when performing the cathodic electrolysis is not particularly limited, but for example, it is preferable to set it to 10°C or more. By setting the temperature of the aqueous solution to 10°C or more, the efficiency of coating generation can be increased. Furthermore, if the temperature of the aqueous solution is set to 10°C or more, cooling of the aqueous solution is not required even when the outside air temperature is high, such as in summer, so it is economical.

The lower limit of the pH of the aqueous solution is not particularly limited, but is preferably 3 or more. If the pH is 3 or more, the production efficiency of Zr oxide or Ti oxide can be further improved. Furthermore, the upper limit of the pH of the aqueous solution is also not particularly limited, but is preferably 5 or less. If the pH is 5 or less, it is possible to prevent a large amount of precipitation from occurring in the aqueous solution, and to improve continuous productivity.

In addition, for the purpose of adjusting the pH and improving the electrolysis efficiency, for example, nitric acid, ammonia water, etc. may be added to the aqueous solution.

The lower limit of the current density during cathodic electrolysis is not particularly limited, but is preferably 0.05 A/dm ² or more, and more preferably 1 A/dm ² or more. If the current density is 0.05 A/dm ² or more, the production efficiency of Zr oxide or Ti oxide is improved. As a result, a coating layer containing more stable Zr oxide or Ti oxide can be produced, and the adhesion with the BPA-free paint can be further improved. In addition, the upper limit of the current density during cathodic electrolysis is not particularly limited, but is preferably 50 A/dm ² or less, and more preferably 10 A/dm ² or less. If the current density is 50 A/dm ² or less, the production efficiency of Zr oxide or Ti oxide can be moderated, and the production of coarse and poorly adhesive Zr oxide or Ti oxide can be suppressed.

The electrolysis time in the above-mentioned cathodic electrolysis treatment is not particularly limited, and can be adjusted appropriately according to the current density so that the above-mentioned Zr and Ti deposition amounts are obtained.

The current pattern in the above cathodic electrolysis treatment may be continuous or intermittent current. Furthermore, the relationship between the aqueous solution and the steel sheet when performing the above cathodic electrolysis is not particularly limited, and they may be relatively stationary or moving, but from the standpoint of promoting the reaction and improving uniformity, it is preferable to perform cathodic electrolysis while moving the steel sheet and the aqueous solution relative to each other. For example, the steel sheet and the aqueous solution can be moved relative to each other by continuously performing cathodic electrolysis while passing the steel sheet through a treatment tank containing an aqueous solution containing at least one of Zr ions or Ti ions.

When performing cathodic electrolysis while moving the steel sheet and the aqueous solution relative to one another, it is preferable to set the relative flow velocity between the aqueous solution and the steel sheet to 50 m/min or more. If the relative flow velocity is 50 m/min or more, the pH of the steel sheet surface where hydrogen is generated as a result of the passage of electricity can be made more uniform, and the generation of coarse Zr oxides or Ti oxides can be effectively suppressed. There is no particular upper limit to the relative flow velocity.

[Surface conditioning process]
Next, the coating layer obtained in the coating formation step is subjected to surface conditioning. Specifically, the aqueous solution is held on the surface of the coating layer in an amount of 1.0 to 30.0 g/ ^m2 or less for 0.1 to 20.0 seconds. By performing surface conditioning under the above conditions, the coating layer can be fixed in a state with high affinity for ethylene glycol.

The mechanism by which the surface conditioning process allows the coating layer to be fixed in a state with high affinity for ethylene glycol is not clear, but it is thought to be as follows. That is, by bringing the coating layer into contact with the aqueous solution, the surface of the coating layer is slightly etched, and fine irregularities are formed on the surface of the coating layer. The effect of these fine irregularities improves the affinity of the coating layer for ethylene glycol. This affinity differs from the affinity caused by the presence of hydrophilic functional groups such as OH groups, and is caused by the physical structure of the surface roughness, and therefore has excellent thermal stability.

The state of the aqueous solution on the surface of the coating layer is not particularly limited, but from the viewpoint of uniformly progressing etching, it is preferable for the aqueous solution to be in the form of a liquid film.

Amount of aqueous solution: 1.0 to 30.0 g/ ^m2
If the amount of the aqueous solution used in the surface preparation is 1.0 g/ ^m2 or less, etching does not proceed sufficiently, and as a result, the contact angle of ethylene glycol becomes larger than 50°. Therefore, the amount of the aqueous solution is 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 more than 30.0 g/ ^m2 , the affinity to ethylene glycol decreases, and as a result, the contact angle of ethylene glycol becomes larger than 50°. Therefore, the amount of the aqueous solution is 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 to 20.0 seconds In the surface preparation, if the holding time is less than 0.1 seconds, etching does not proceed sufficiently, and as a result, the contact angle of ethylene glycol becomes larger than 50°. Therefore, the holding time is set to 0.1 seconds or more, preferably 0.2 seconds or more, and more preferably 0.3 seconds or more. On the other hand, if the holding time exceeds 20.0 seconds, the contact angle of ethylene glycol also becomes larger than 50°. This is considered to be because etching proceeds excessively, and the surface state is not suitable for expressing affinity for ethylene glycol. Therefore, the holding time is set to 20.0 seconds or less, preferably 18.0 seconds or less, and more preferably 15.0 seconds or less.

The amount of the aqueous solution can be measured with a moisture meter using a filter type infrared absorption method. Specifically, the absorbance on the surface is measured with a moisture meter using a filter type infrared absorption method, and the amount of the aqueous solution is calculated from the absorbance using a calibration curve that has been obtained in advance. The calibration curve can be created by the following procedure. First, the steel plate having the coating layer is placed on an electronic balance. The aqueous solution is dropped onto the steel plate having the coating layer with a pipette to form a liquid film on the entire surface of the steel plate having the coating layer. The weight of the aqueous solution present on the steel plate having the coating layer is calculated from the weight of the steel plate having the coating layer before the aqueous solution is dropped and the weight of the steel plate having the coating layer after the aqueous solution is dropped. The amount of the aqueous solution per unit area is calculated by dividing the weight of the aqueous solution obtained by the area of the steel plate having the coating layer. At the same time, the absorbance on the surface of the steel plate having the coating layer is measured with a moisture meter using a filter type infrared absorption method. The above measurements are performed multiple times while changing the amount of aqueous solution, and a calibration curve showing the correlation between the amount of aqueous solution and the absorbance is created. The calibration curve can be a linear approximation of the correlation between the amount of aqueous solution and the absorbance.

The method for adjusting the amount of aqueous solution present on the surface of the coating layer is not particularly 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 squeezing the liquid with a wringer roll or by wiping.

Before the film formation process, the steel sheet having the Sn-plated layer can be pretreated as desired. By carrying out the pretreatment, for example, it is possible to remove the natural oxide film present on the surface of the Sn-plated layer. By removing the natural oxide film, it is possible to adjust the amount of Sn oxide and also to activate the surface.

The method of the pretreatment is not particularly limited and any method can be used, but it is preferable to perform one or both of electrolysis in an alkaline aqueous solution and immersion treatment in an alkaline aqueous solution as the pretreatment. One or both of cathodic electrolysis and anodic electrolysis can be used as the electrolysis. It is preferable to perform any of the following treatments (1) to (4) 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 followed by anodic electrolysis in an alkaline aqueous solution; (4) Anodic electrolysis in an alkaline aqueous solution.

The alkaline aqueous solution may contain one or more electrolytes. The electrolyte is not particularly limited and any electrolyte may be used. For example, carbonate is preferably used as the electrolyte, and sodium bicarbonate is more preferably used. There is no particular limit to the lower limit of the concentration of the alkaline aqueous solution, but it is preferably 1 g/L or more, and more preferably 5 g/L or more. There is also no particular limit to the upper limit of the concentration of the alkaline aqueous solution, but it is preferably 30 g/L or less, and more preferably 20 g/L or less.

The lower limit of the temperature of the alkaline aqueous solution is not particularly limited, but is preferably 10°C or higher, and more preferably 15°C or higher. The upper limit of the temperature of the alkaline aqueous solution is also not particularly limited, but is preferably 70°C or lower, and more preferably 60°C or lower.

In addition, when cathodic electrolysis is performed as the pretreatment, the lower limit of the electricity density in the cathodic electrolysis is not particularly limited, but is preferably 0.5 C/ ^dm2 or more, and more preferably 1.0 C/ ^dm2 or more. On the other hand, the upper limit of the electricity density in the cathodic electrolysis is not particularly limited, but if it is too high, the effect of the pretreatment will be saturated, so the electricity density is preferably 10.0 C/ ^dm2 or less.

When immersion treatment is performed as the pretreatment, the lower limit of the immersion time in the immersion treatment is not particularly limited, but it is preferably 0.1 seconds or more, and more preferably 0.5 seconds or more. On the other hand, the upper limit of the immersion time is also not particularly limited, but since the effect of the pretreatment is saturated if it is too long, the immersion time is preferably 10 seconds or less.

When anodizing is performed as the pretreatment, the lower limit of the electricity density in the anodic electrolysis is not particularly limited, but is preferably 0.5 C/ ^dm2 or more, and more preferably 1.0 C/ ^dm2 or more. On the other hand, the upper limit of the electricity density in the anodic electrolysis is not particularly limited, but if it is too high, the effect of the pretreatment will be saturated, so the electricity density is preferably 10.0 C/ ^dm2 or less.

After carrying out the pretreatment, it is preferable to wash the surface with water to remove any pretreatment liquid adhering to the surface.

In addition, when forming a Sn plating layer on the surface of the base steel sheet, it is preferable to perform a pretreatment on the base steel sheet. Any pretreatment can be performed, but it is preferable to perform at least one of degreasing, pickling, and water washing.

By degreasing, rolling oil, rust-preventive oil, etc. adhering to the steel plate can be removed. The degreasing can be carried out by any method without any particular restrictions. After degreasing, it is preferable to wash the steel plate with water to remove the degreasing solution adhering to the surface of the steel plate.

In addition, pickling can remove the natural oxide film present on the surface of the steel sheet and activate the surface. There are no particular limitations on the pickling method, and any method can be used. After pickling, it is preferable to rinse the steel sheet with water to remove the pickling solution adhering to the surface.

[Water washing process]
Next, the steel sheet after the surface conditioning step is washed with water at least once. By washing with water, the aqueous solution remaining on the surface of the steel sheet can be removed. The washing with water can be carried out by any method without any particular limitation. For example, a washing tank can be provided downstream of a tank for film formation, and the steel sheet after the film formation step can be continuously immersed in water. Alternatively, washing with water can be carried out by spraying water onto the steel sheet after the film formation step.

The number of times that the washing is performed is not particularly limited, and may be one or more than two times. However, in order to avoid an excessive number of washing tanks, it is preferable to limit the number of washings to five or less. Furthermore, when washing is performed two or more times, each washing may be performed using the same method or different methods.

In the present invention, it is important to use water with an electrical conductivity of 100 μS/m or less in at least the final wash in the water washing process. This reduces the amount of K, Na, Mg, and Ca adsorbed to the surface of the coated steel sheet, thereby improving 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 be, for example, reverse osmosis water, ion-exchanged water, or distilled water. The electrical conductivity of the water used for washing can be measured using a conductivity meter.

When washing with water two or more times in the washing process, the above-mentioned effect can be obtained by using water with an electrical conductivity of 100 μS/m or less for the final wash, so any water can be used for the washes other than the final wash. Water with an electrical conductivity of 100 μS/m or less may also be used for the washes other than the final wash. However, from the viewpoint of reducing costs, it is preferable to use water with an electrical conductivity of 100 μS/m or less only for the final wash, and to use ordinary water such as tap water or industrial water for the washes other than the final wash.

From the viewpoint of further reducing the amount of K, Na, Mg, and Ca adsorbed on the surface of the coated steel sheet, the electrical conductivity of the water used in the final water wash is preferably 50 μS/m or less, and more preferably 30 μS/m or less. On the other hand, the lower limit of the electrical conductivity is not particularly limited and may be 0 μS/m. However, from the viewpoint of reducing costs, it is preferable that the electrical conductivity is 1 μS/m or more.

The temperature of the water used in the washing process is not particularly limited and may be any temperature. However, an excessively high temperature places an excessive burden on the washing equipment, so the temperature of the water used in washing is preferably 95°C or less. On the other hand, the lower limit of the temperature of the water used in washing is not particularly limited, but it is preferably 0°C or higher. The temperature of the water used in the washing may be room temperature.

The washing time per washing process is not particularly limited, but from the viewpoint of enhancing the effect of the washing process, it is preferably 0.1 seconds or more, and more preferably 0.2 seconds or more. In addition, the upper limit of the washing time per washing process is also not particularly limited, but when manufacturing on a continuous line, the line speed is reduced, and therefore, productivity is reduced, so it is preferably 10 seconds or less, and more preferably 8 seconds or less.

After the water washing process, drying may be performed as desired. There are no particular limitations on the drying method, and for example, a normal dryer or electric oven drying method can be used. The temperature during the drying process is preferably 100°C or lower. If the temperature is within the above range, deterioration of the surface treatment film can be suppressed. The lower limit is not particularly limited, but is usually around room temperature.

The uses of the surface-treated steel sheet of the present invention are not particularly limited, but it is particularly suitable as a surface-treated steel sheet for containers used in the manufacture of various containers such as food cans, beverage cans, pail cans, and 18-liter cans.

In order to confirm the effects of the present invention, surface-treated steel sheets were manufactured using the procedures described below, and their properties were evaluated. However, the present invention is not limited to these.

(Sn plating)
First, the steel sheet was electrolytically degreased, washed with water, pickled by immersion in dilute sulfuric acid, and washed with water in sequence, and then electroplated with Sn using a phenolsulfonic acid bath to form a Sn plating layer on both sides of the steel sheet. At that time, the electric current time was changed to obtain the Sn adhesion amount of the Sn plating layer as shown in Tables 2 and 3. In some examples, prior to the electroplating with Sn, the steel sheet was electroplated with Ni using a Watts bath to form a Ni plating layer as a Ni-containing layer on both sides of the steel sheet. At that time, the electric current time and the current density were changed to obtain the Ni adhesion amount of the Ni plating layer as shown in Tables 2 and 3. The Sn adhesion amount of the Sn plating layer was measured by the electrolytic stripping method specified in JIS G 3303. The Ni adhesion amount of the Ni plating layer was measured by the above-mentioned fluorescent X-ray calibration curve method.

Furthermore, in some examples, after the Sn plating layer was formed, a reflow treatment was performed. In the reflow treatment, the sample was heated for 5 seconds at a heating rate of 50°C/sec using a direct current heating method, and then immersed in water and quenched.

The steel plate used was a can steel plate (T4 base plate) with a thickness of 0.17 mm.

(Pretreatment for Sn-plated steel sheet)
Thereafter, the obtained Sn-plated steel sheet was subjected to the pretreatment shown in Tables 2 and 3. "C" indicates cathodic electrolysis, "A" indicates anodic electrolysis, "D" indicates immersion, and "C→A" indicates cathodic electrolysis followed by anodic electrolysis. In the cathodic electrolysis, anodic electrolysis, and immersion treatments in the pretreatments, an aqueous sodium bicarbonate solution with a concentration of 10 g/L was used, and the temperature of the aqueous sodium bicarbonate solution was room temperature. The electric charge density in the cathodic electrolysis was 2.0 C/dm ² , and the electric charge density in the anodic electrolysis was 4.0 C/dm ² . The immersion time in the immersion treatment was 1 second. For comparison, no pretreatment was performed in some examples.

(Film forming process)
Next, the surface of the Sn-plated steel sheet was treated with an aqueous solution to form a coating layer on the Sn-plated layer. Specifically, an aqueous solution having the composition shown in Table 1 was used as the aqueous solution, and a cathodic electrolysis treatment was performed in the aqueous solution to form a coating layer. The temperature of the aqueous solution was set to 35° C., and the pH was adjusted to 3 to 5. The Zr deposition amount and the Ti deposition amount were controlled by adjusting the electric charge density. Zirconium fluoride (ZrF ₄ ) was used as the Zr-containing compound, and titanium fluoride (TiF ₄ ) was used as the Ti-containing compound. The aqueous solution was prepared by further using compounds other than the Zr-containing compound and the Ti-containing compound to adjust the concentration of each ion so as to have the composition shown in Table 1.

(Surface adjustment process)
After the above-mentioned film-forming step, surface conditioning was carried out under the conditions shown in Tables 2 and 3. Specifically, at the time when the film-forming step was completed, the steel sheet with 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 amount shown in Tables 2 and 3. The amount of the aqueous solution was measured with a moisture meter using the filter-type infrared absorption method as described above. Thereafter, the solution was held for the holding time shown in Tables 2 and 3. In other words, the aqueous solution used in the surface conditioning step was the same as the aqueous solution used in the above-mentioned film-forming step.

(Water washing process)
Next, the steel sheet after the surface conditioning step was subjected to a water rinsing treatment. The water rinsing treatment was performed 1 to 5 times under the conditions shown in Tables 2 and 3. The water rinsing method and the electrical conductivity of the water used for each treatment were as shown in Tables 2 and 3. In the treatment where the water rinsing method was "immersion", the steel sheet was immersed in water for water rinsing. On the other hand, in the treatment where the water rinsing method was "spraying", the steel sheet was rinsed by spraying water onto it. The electrical conductivity was measured using a conductivity meter.

The amount of Zr oxide, Ti oxide, P, and Mn attached in the coating layer of each of the obtained surface-treated steel sheets was measured. The measurements were performed using the fluorescent X-ray calibration curve method described above. The measurement results are shown in Tables 4 and 5. In Tables 4 and 5, the amount of Zr oxide and Ti oxide attached is shown as the amount of metallic Zr and the amount of metallic Ti, respectively.

The contact angle of ethylene glycol and the atomic ratio of the adsorbed elements were measured for each of the surface-treated steel sheets obtained using the following procedure. The measurement results are shown in Tables 4 and 5.

(Contact angle of ethylene glycol)
The contact angle of ethylene glycol on the obtained surface-treated steel sheet was measured using an automatic contact angle meter CA-VP type manufactured by Kyowa Interface Science Co., Ltd. The surface temperature of the surface-treated steel sheet was set to 20°C ± 1°C, and ethylene glycol used was 20 ± 1°C, special reagent grade ethylene glycol manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. Ethylene glycol was dropped onto the surface of the surface-treated steel sheet in a droplet amount of 2 μl, and the contact angle was measured one second later by the θ/2 method, and the arithmetic mean value of the contact angles of five drops was taken as the contact angle of ethylene glycol.

In addition, 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, for surface-treated steel sheets that met the conditions of the present invention, the contact angle values were essentially the same before and after the heat treatment. In contrast, for surface-treated steel sheets that did not meet the conditions of the present invention, the contact angle values changed significantly due to the 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 measurement. The atomic ratio to all elements detected by the relative sensitivity coefficient method was quantified from the integrated intensity of the narrow spectrum of K2p, Na1s, Ca2p, and Mg1s on the outermost surface of the sample, and (K atomic ratio + Na atomic ratio + Ca atomic ratio + Mg atomic ratio) was calculated. For the XPS measurement, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI was used, and the X-ray source was a monochrome AlKα ray, the voltage was 15 kV, the beam diameter was 100 μmφ, and the take-off angle was 45°.

Furthermore, the adhesion of the obtained surface-treated steel sheets to BPA-free paint was evaluated using the following method. The evaluation results are shown in Tables 4 and 5.

(Preparation of Samples)
A BPA-free painted steel sheet as a sample to be used for evaluating adhesion to a BPA-free paint was prepared by the following procedure.

A polyester-based paint for the inner surface of a can (BPA-free paint) was applied to the surface of the obtained surface-treated steel sheet, and baked at 180° C. for 10 minutes to produce a BPA-free painted steel sheet. The coating weight of the paint was 60 mg/ ^dm2 .

(Adhesion with BPA-free paint)
Two BPA-free painted steel sheets prepared under the same conditions were laminated with the painted surfaces facing each other with a nylon adhesive film sandwiched therebetween, and then bonded together under pressure conditions of 2.94 x ¹⁰⁵ Pa, 190°C temperature, and 30 seconds of pressure bonding time. This was then divided into test pieces with a width of 5 mm. The divided test pieces were immersed for 168 hours in a test liquid at 55°C consisting of a mixed aqueous solution containing 1.5% by mass of citric acid and 1.5% by mass of salt. After immersion, washing and drying, the two steel sheets of the divided test pieces were peeled off with a tensile tester, and the tensile strength when peeled off was measured. The average value of the three test pieces was evaluated according to the following four levels. In practical terms, if the results are 1 to 3, it can be evaluated as having excellent adhesion to the 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 the results shown in Tables 4 and 5, all of the surface-treated steel sheets that met the conditions of the present invention had excellent adhesion to BPA-free paint, even though they were manufactured without using hexavalent chromium.

Claims

At least one surface of the steel plate is
A Sn plating layer;
A surface-treated steel sheet having a coating layer containing at least one of Zr oxide and Ti oxide disposed on the Sn plating layer,
The contact angle of ethylene glycol is 50° or less,
A surface-treated steel sheet having a total atomic ratio of K, Na, Mg, and Ca adsorbed on the surface to all elements of 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 to 20.0 g/ m2 per one side of the steel sheet.
3. The surface-treated steel sheet according to claim 1, wherein the total amount of deposited Zr oxide and Ti oxide in the coating layer is 0.3 to 50.0 mg/ m2 per side of the steel sheet in terms of the amount of metallic Zr and the amount of metallic Ti.
The surface-treated steel sheet according to any one of claims 1 to 3, wherein the coating layer further contains P, and the P coating amount is 50.0 mg/ m2 or less per one 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 the Mn coating amount is 50.0 mg/ m2 or less per one 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-plated layer.
The surface-treated steel sheet according to claim 6, wherein the Ni-containing layer has a Ni deposition amount of 2 mg/m 2 to 2000 mg/m 2 per one side of the steel sheet.
A method for producing a surface-treated steel sheet having a Sn-plated layer on at least one surface of a steel sheet, and a coating layer containing at least one of Zr oxide and Ti oxide disposed on the Sn-plated layer, comprising:
a coating formation step of treating a surface of a steel sheet having a Sn plating layer on at least one surface thereof 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 step of maintaining the aqueous solution at 1.0 to 30.0 g/ m2 on the surface of the coating layer for 0.1 to 20.0 seconds;
and a water washing step of washing the steel sheet after the surface conditioning step at least once with water,
In the water washing step,
A method for producing a surface-treated steel sheet, comprising using water having an electrical conductivity of 100 μS/m or less in at least the final water washing.
The method for producing a surface-treated steel sheet according to claim 8 , wherein the surface-treated steel sheet further comprises a Ni-containing layer disposed under the Sn-plated layer.