EP4667626A1

EP4667626A1 - Surface-treated steel sheet and manufacturing method therefor

Info

Publication number: EP4667626A1
Application number: EP24831307.4A
Authority: EP
Inventors: Takashi Ueno; Masanari Tomozawa; Yusuke Nakagawa
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-06-30
Filing date: 2024-01-31
Publication date: 2025-12-24
Also published as: JPWO2025004426A1; CN121399303A; JP7552960B1; KR20250138771A

Abstract

Provided is a surface-treated steel sheet that can be produced without using hexavalent chromium and that also has excellent corrosion resistance in a BPA-free coated part. The surface-treated steel sheet includes a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet. An oxygen-enriched region containing O at an atomic concentration of 10 % or more is present in a vicinity of an interface between the steel sheet and the chromium-containing layer, and the vicinity is a region in which an atomic ratio of Fe to Cr is in a range of 0.7 to 1.3.

Description

TECHNICAL FIELD

The present disclosure relates to a surface-treated steel sheet, and more particularly to a surface-treated steel sheet having excellent corrosion resistance in a BPA (bisphenol A)-free coated part. The surface-treated steel sheet 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

Sn-plated steel sheet (tinplate) and tin-free steel sheet (TFS) have been widely used as materials for various metal cans such as beverage cans, food cans, pail cans, and 18-liter cans.
Tinplate and TFS are used with an organic resin coating such as epoxy-based paint or PET film to accommodate a variety of contents. In the case of applying an organic resin coating to tinplate or TFS, the steel sheet is subjected to electrolysis or immersion treatment in an aqueous solution containing hexavalent Cr to form a chromium oxide layer on the outermost surface. The chromium oxide layer exhibits excellent adhesion to an organic resin coating layer. Therefore, deformation of the organic resin coating layer also follows the steel sheet deformation that accompanies can manufacturing, thereby ensuring corrosion resistance to the various contents even after can manufacturing.
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 (Patent Literature (PTL) 1, 2), and demand exists for replacing epoxy-based paint with BPA-free paint. However, tinplate and TFS used up until now have poorer adhesion to BPA-free paints compared to their adhesion to epoxy-based paints. Hence, deformation of the BPA-free paint cannot follow the steel sheet deformation that accompanies can manufacturing, and sufficient corrosion resistance to various contents after can manufacturing could not be ensured. As a result, the application of BPA-free paints to various metal cans has not progressed.
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.
Examples of known methods of forming surface-treated steel sheets without using hexavalent chromium are the methods proposed in PTL 3 to 6. In these methods, a surface treatment layer is formed by performing electrolysis in an electrolytic solution containing a trivalent chromium compound such as basic chromium sulfate.

CITATION LIST

Patent Literature

PTL 1: JP 2013-144753 A
PTL 2: JP 2008-50486 A
PTL 3: JP 2016-501985 A
PTL 4: JP 2016-505708 A
PTL 5: JP 2020-172700 A
PTL 6: JP 2020-172701 A

SUMMARY

(Technical Problem)

According to the methods proposed in PTL 3 to 6, a surface treatment layer can be formed without using hexavalent chromium. Also according to PTL 3 to 6, the above method can obtain a surface-treated steel sheet with excellent adhesion to epoxy-based paint. According to PTL 3 and 4, a surface-treated steel sheet that exhibits excellent corrosion resistance, even after being coated with an epoxy-based paint and deformed, can be obtained.
However, while the surface-treated steel sheets obtained by the conventional methods proposed in PTL 3 to 6 have excellent adhesion to epoxy-based paints and excellent corrosion resistance in the epoxy-based coated part, the corrosion resistance in the BPA-free coated part is insufficient. As a result, it was not possible to replace conventional epoxy-based paints with BPA-free paints while maintaining corrosion resistance to various contents.
Therefore, there is a demand for a surface-treated steel sheet that can be produced without using hexavalent chromium and that has excellent corrosion resistance in the BPA-free coated part.
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 corrosion resistance in a BPA-free coated part.

(Solution to Problem)

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

(1) In a surface-treated steel sheet having a chromium-containing layer on at least one side, by providing an oxygen-enriched region between the steel sheet and the chromium-containing layer, it is possible to obtain a surface-treated steel sheet having excellent corrosion resistance in the BPA-free coated part.
(2) The aforementioned surface-treated steel sheet can be produced by contacting a steel sheet with an aqueous solution containing sulfate ions, holding in a state in which more than 30.0 g/m² and 60.0 g/m² or less of the aqueous solution is present on the surface of the steel sheet for 0.10 to 20.0 seconds, and then subjecting the steel sheet to cathodic electrolysis in an electrolytic solution containing 0.05 mol/L or more of trivalent chromium ions.

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

1. A surface-treated steel sheet comprising:
- a steel sheet; and
- a chromium-containing layer disposed on at least one surface of the steel sheet, wherein
- an oxygen-enriched region containing O at an atomic concentration of 10 % or more is present in a vicinity of an interface between the steel sheet and the chromium-containing layer, and
- the vicinity is a region in which an atomic ratio of Fe to Cr is in a range of 0.7 to 1.3.
2. The surface-treated steel sheet of 1, wherein the oxygen-enriched region has a coverage of 50 % or more in the vicinity.
3. The surface-treated steel sheet of 1 or 2, wherein the chromium-containing layer has a chromium coating weight of 40.0 mg/m² to 500.0 mg/m² per side.
4. The surface-treated steel sheet of to any one of 1 to 3, wherein the chromium-containing layer has a chromium oxide coating weight of 40.0 mg/m² or less per side.
5. A method of producing a surface-treated steel sheet comprising a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, the method comprising:
- a steel sheet surface conditioning step of bringing the steel sheet into contact with an aqueous solution containing sulfate ions and holding in a state in which more than 30.0 g/m² and 60.0 g/m² or less of the aqueous solution is present on a surface of the steel sheet for 0.10 to 20.0 seconds; and
- a cathodic electrolysis step of subjecting the steel sheet to cathodic electrolysis in an electrolytic solution containing 0.05 mol/L or more of trivalent chromium ions.
6. The method of producing a surface-treated steel sheet of 5, wherein the electrolytic solution is prepared by mixing a trivalent chromium ion source, a carboxylic acid compound, and water, and adjusting the pH to 4.0 to 7.0 and the temperature to 40 °C to 70 °C.

(Advantageous Effect)

According to the present disclosure, a surface-treated steel sheet that uses no hexavalent chromium and that has excellent corrosion resistance in a BPA-free coated part 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 of 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.
The surface-treated steel sheet according to an embodiment of the present disclosure is a surface-treated steel sheet having a chromium-containing layer on at least one surface of the steel sheet. In the present disclosure, it is important that an oxygen-enriched region exists between the chromium-containing layer and the steel sheet. 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 C, Mn, P, S, Si, Cu, Ni, Mo, Al, and inevitable impurities may be contained 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 0.70 %,
P: 0 % to 0.15 %,
S: 0 % to 0.050 %,
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 %,

The thickness of the steel sheet is not particularly limited but is preferably 0.60 mm or less. On the other hand, no lower limit is placed on the thickness, but the thickness is preferably 0.10 mm or more. The term "steel sheet" as used here includes a "steel strip".

[Chromium-containing layer]

The steel sheet has a chromium-containing layer on at least one surface thereof. The components forming the chromium-containing layer are not particularly limited but may include metallic chromium and a chromium compound. The chromium compound is not particularly limited and may include any chromium compound. The chromium compound may include, for example, at least one selected from the group consisting of chromium oxide, chromium carbide, chromium sulfide, chromium nitride, chromium chloride, chromium bromide, and chromium boride. The chromium-containing layer may contain impurities in addition to the metallic chromium and the chromium compound. Examples of the impurities include metal elements such as Ni, Cu, Sn, and Zn that are mixed as impurities in the electrolytic solution, which will be described later. The metal elements are typically thought to be present in the chromium-containing layer in a metallic state but may also be present as compounds.
In one embodiment of the present disclosure, the chromium-containing layer preferably has a total content of elements forming the metallic chromium and the chromium compound of 90 atomic% or more. Here, the total content is the ratio, expressed as a percentage, of the total number of atoms of elements constituting the metallic chromium and chromium compound to the total number of atoms of all elements other than Fe.
The total content can be determined by measuring the contents (atomic%) of the metallic chromium and the elements constituting the chromium compound contained in the chromium-containing layer by X-ray photoelectron spectroscopy (XPS) and adding up the contents. In the content measurement by XPS, the content (atomic ratio) of each element can be calculated by the relative sensitivity coefficient method from the integrated intensity of the peak corresponding to the element.
For example, the content of chromium carbide (Cr₂C₃) can be determined from the integrated intensity of the peak, appearing near 281.0 eV, of the C 1s carbide. For example, when the C content (atomic ratio to the total of all elements other than Fe) calculated from the integrated intensity of the peak is 6 atomic%, the Cr₂C₃ content is 6 × (2 + 3)/3 = 10 atomic%.
For chromium oxide, the Cr₂O₃ content can be determined from the integrated intensity of the peak, appearing near 576.7 eV, of the Cr 2p oxide. The CrO₃ content can also be determined from the integrated intensity of the peak, appearing near 579.2 eV, of the Cr 2p oxide portion.
Similarly, the contents of other chromium compounds can be determined using, for example, the integrated intensities of the peaks listed below.

Chromium sulfide (Cr₂S₃): S 2p sulfide peak appearing near 162.3 eV
Chromium nitride (CrN): N 1s peak appearing near 397.3 eV
Chromium chloride (CrCl₃): Cl 2p peak appearing near 199.8 eV
Chromium bromide (CrBr₃): Br 3d peak appearing near 69.1 eV
Chromium boride (CrB): Br 1s peak appearing near 188.2 eV

On the other hand, the content of metallic chromium is determined by calculating the Cr content from the integrated intensity of the Cr 2p peak appearing near 573.8 eV and subtracting the content of Cr atoms contained as chromium compounds from the chromium content.
The content of metallic chromium obtained by the above method and the content of each element constituting the chromium compound are added together to obtain the total content of the metallic chromium and the elements constituting the chromium compound.
The total content refers to the value at the intermediate position in the thickness of the chromium-containing layer. The intermediate position can be determined by the following procedure. First, the chromium-containing layer is sputtered from the outermost surface, while the Fe content and the total content of the metallic chromium and the elements constituting the chromium compound are measured by the above-described method. The position that is intermediate (1/2) between the position (depth) where the measured total content of the metallic chromium and elements constituting the chromium compound is equal to the Fe content and the outermost surface of the chromium-containing layer is determined as the intermediate position.
For the measurement by XPS, for example, a scanning X-ray photoelectron spectroscopic analyzer PHI X-tool produced by ULVAC-PHI, Inc. can be used. The X-ray source is a monochrome AlKα ray, the voltage is 15 kV, the beam diameter is 100 µm, the take-off angle is 45°, and the sputtering conditions are Ar ions with an accelerating voltage of 1 kV and a sputtering rate of 1.50 nm/min in terms of SiO₂.
The spatial structure of the components constituting the chromium-containing layer is not particularly limited. The components may be, for example, separated as separate layers in the chromium-containing layer or may be mixed throughout the chromium-containing layer. That is, the spatial structure of the components that form the chromium-containing layer can contain either or both of separate and mixed layers.
The coating weight of chromium in the chromium-containing layer is not particularly limited. However, if the coating weight of chromium in the chromium-containing layer is excessive, cohesive failure may occur in the chromium-containing layer during processing of the surface-treated steel sheet. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free coated part, the chromium coating weight of the chromium-containing layer is preferably 500.0 mg/m² or less per side. The chromium coating weight is preferably 450.0 mg/m² or less per side. On the other hand, from the viewpoint of further improving the corrosion resistance of the BPA-free coated part, the chromium coating weight of the chromium-containing layer is preferably 40.0 mg/m² or more per side. The chromium coating weight is more preferably 50.0 mg/m² or more per side. Here, the "chromium coating weight" refers to the total coating weight of chromium present in various forms.
The chromium coating weight can be measured by X-ray fluorescence analysis. More specifically, the chromium coating weight is measured by the following procedure. First, the Cr content (total Cr content) in the surface-treated steel sheet is measured using an X-ray fluorescence device. Next, the Cr content in the steel sheet before the formation of the chromium-containing layer or in the steel sheet after the chromium-containing layer has been removed (original sheet Cr content) is measured using the X-ray fluorescence device. The chromium coating weight of the chromium-containing layer is determined by subtracting the chromium content of the original sheet from the total chromium content. The chromium-containing layer can be removed using, for example, a commercially available hydrochloric acid-based chromium plating remover.

[Chromium oxide coating weight]

Chromium oxide may be present in the chromium-containing layer. The location of the chromium oxide is not particularly limited. The location of O can be confirmed by, for example, composition analysis using energy dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS) included in a scanning electron microscope (SEM) or a transmission electron microscope (TEM), or by three-dimensional composition analysis using a three-dimensional atom probe (3DAP).
The coating weight of chromium oxide in the chromium-containing layer is not particularly limited. However, if the coating weight of chromium oxide in the chromium-containing layer is excessive, cohesive failure may occur with the chromium oxide in the chromium-containing layer as an initiation point when the surface-treated steel sheet is processed, and the corrosion resistance of the BPA-free coated part may deteriorate. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free coated part, the chromium oxide coating weight of the chromium-containing layer is preferably 40.0 mg/m² or less per side. The chromium oxide coating weight is preferably 35.0 mg/m² or less per side. Alternatively, the chromium-containing layer may be completely free of chromium oxide. Therefore, no lower limit is placed on the coating weight of chromium oxide in the chromium-containing layer, and the coating weight may be 0.0 mg/m² per side.
The chromium oxide coating weight can be measured by X-ray fluorescence analysis. More specifically, the chromium oxide coating weight is measured by the following procedure. First, the Cr content (total Cr content) of the surface-treated steel sheet is measured. Next, the surface-treated steel sheet is subjected to an alkali treatment by immersion in 7.5N-NaOH at 90 °C for 10 minutes to remove the chromium oxide. The surface-treated steel sheet after the alkali treatment is thoroughly washed with water, and the Cr content (Cr content after alkali treatment) is measured again using an X-ray fluorescence device. The chromium oxide coating weight in the chromium-containing layer is determined by subtracting the chromium content after the alkali treatment from the total chromium content.
The chromium-containing layer may be amorphous or crystalline. That is, the chromium-containing layer can contain one or both of an amorphous and a crystalline phase. The chromium-containing layer produced by the method described below generally contains an amorphous phase, and may also contain a crystalline phase. The mechanism by which the chromium-containing layer is formed is not clear, but it is believed that partial crystallization occurs when the amorphous phase is formed, resulting in a chromium-containing layer that contains both amorphous and crystalline phases. The area ratio of crystalline regions is not particularly limited, but the area ratio is preferably 30 % or less when the chromium-containing layer is observed from the surface direction. On the other hand, since the crystalline region does not necessarily have to exist, the lower limit of the area ratio of the crystalline region may be 0 %.
The crystalline regions in the chromium-containing layer can be confirmed by removing the base steel sheet portion from the surface-treated steel sheet to prepare a single-layer sample of the chromium-containing layer and then observing the single-layer sample of the chromium-containing layer from the surface side using a TEM or STEM. The method of preparing the single-layer sample of the chromium-containing layer is not particularly limited, but for example, the sample can be prepared by irradiating an ion beam of Ar or the like from the base steel sheet side to subject the steel sheet to ion milling. When preparing a single chromium-containing layer region using an ion beam, the ion beam is irradiated at an accelerating voltage of 5 kV or less onto the base steel sheet at an incidence angle in the range of 1 to 5 degrees, thereby ensuring a field of view of a single chromium layer region of several µm² or more. At this time, the bottom surface of the chromium-containing layer is also milled to some extent, and the thickness of the chromium-containing layer may be reduced, but this does not affect the measurement results of the area ratio of the crystalline region.
The area ratio of the crystalline regions in the chromium-containing layer can be measured by a TEM. Specifically, first, a diffraction pattern of the chromium-containing layer is obtained by selected area diffraction of a TEM. Next, dark-field images are obtained for all the diffraction spots in the diffraction pattern, and the regions that appear with high brightness in the dark-field images are determined to be crystalline regions. The area of the obtained crystalline regions is calculated by image processing and is divided by the area of the chromium-containing layer within a selected area aperture to calculate the area ratio of the crystalline regions. The area ratio can be calculated using image interpretation software such as image-J.
The chromium-containing layer may contain C. No upper limit is placed on the C content in the chromium-containing layer, but the C content is preferably 50 % or less in terms of atomic ratio to Cr. The C content is more preferably 45 % or less in terms of atomic ratio to Cr. The chromium-containing layer need not contain C, and therefore no lower limit is placed on the atomic ratio of C contained in the chromium-containing layer to Cr, and this atomic ratio may be 0 %.
The C content in the chromium-containing layer can be measured by XPS. That is, the C content in the chromium-containing layer can be determined by sputtering to a depth of 0.2 nm or more in terms of SiO₂ from the outermost layer, quantifying the integrated intensities of the narrow spectra of Cr2p and C1s as atomic ratios by the relative sensitivity coefficient method, and calculating the C atomic ratio/Cr atomic ratio. For the measurement by XPS, for example, a scanning X-ray photoelectron spectroscopic analyzer PHI X-tool produced by ULVAC-PHI, Inc. can be used. The X-ray source is a monochrome AlKα ray, the voltage is 15 kV, the beam diameter is 100 µm, the take-off angle is 45°, and the sputtering conditions are Ar ions with an accelerating voltage of 1 kV and a sputtering rate of 1.50 nm/min in terms of SiO₂.
The mechanism by which C is incorporated into the chromium-containing layer is not clear. However, it is thought that when a carboxylic acid compound is contained in the electrolytic solution in the process of forming a chromium-containing layer on a steel sheet, the carboxylic acid compound decomposes and is incorporated into the coating.
The location of C in the chromium-containing layer is not particularly limited. The location of C can be confirmed by, for example, composition analysis using energy dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS) included in a scanning electron microscope (SEM) or a transmission electron microscope (TEM), or by three-dimensional composition analysis using a three-dimensional atom probe (3DAP).
The chromium-containing layer may contain Fe. No upper limit is placed on the Fe content in the chromium-containing layer, but the Fe content is preferably 100 % or less in terms of atomic ratio to Cr. The chromium-containing layer need not contain Fe, and therefore no lower limit is placed on the atomic ratio to Cr, and this atomic ratio may be 0 %. The Fe content in the chromium-containing layer can be measured by XPS, similarly to the C content. The atomic ratio can be calculated using narrow spectra of Cr2p and Fe2p.
The mechanism by which Fe is contained in the chromium-containing layer is not clear. However, it is thought that in the process of forming the chromium-containing layer on the steel sheet, a small amount of Fe contained in the steel sheet dissolves in the electrolytic solution, and the Fe is incorporated into the coating.
In addition to Cr, O, Fe, and C, the chromium-containing layer may contain metal impurities such as K, Na, Mg, and Ca contained in the water; Sn, Ni, Cu, Zn, and the like contained in the aqueous solution; and S, N, Cl, Br, and the like. However, the presence of these elements may reduce the corrosion resistance in the BPA-free coated part. Therefore, the total atomic ratio of elements other than Cr, O, Fe, and C to Cr is preferably 3 % or less. Such elements are more preferably not contained at all (0 %). The contents of the above elements are not particularly limited but can be measured, for example, by XPS in the same manner as the C content.

[Oxygen-enriched region]

In the surface-treated steel sheet of the present disclosure, an oxygen-enriched region containing O at an atomic concentration of 10 % or more is present in the vicinity of the interface between the steel sheet and the chromium-containing layer (hereinafter, the vicinity may be referred to as the region in the vicinity of the interface). Here, the region in the vicinity of the interface refers to a region in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3. In this way, the presence of an oxygen-enriched region in the region in the vicinity of the interface makes it possible to achieve excellent corrosion resistance in the BPA-free coated part. From the viewpoint of further improving the corrosion resistance of the BPA-free coated part, the coverage of the oxygen-enriched region in the region in the vicinity of the interface is preferably 50 % or more, and more preferably 70 % or more. No upper limit is placed on the coverage of the oxygen-enriched region in the region in the vicinity of the interface, and the coverage may be 100 %. In the following description, the oxygen-enriched region refers only to the region present in the region in the vicinity of the interface.
The reason why the corrosion resistance of the BPA-free coated part is improved by providing the oxygen-enriched region as described above will be explained below.
First, a typical chromium-containing layer formed from a hexavalent chromium bath or a trivalent chromium bath is composed of metallic chromium or chromium oxide. Such a surface-treated steel sheet having a chromium-containing layer is generally processed into a can or the like after an organic resin coating is formed on the surface. However, metallic chromium has poor workability, and therefore the chromium-containing layer cannot completely follow the steel sheet deformation that accompanies processing. This results in damage to the organic resin coating present on the chromium-containing layer. As a result, the corrosion resistance after processing decreases.
Therefore, in conventional surface-treated steel sheets, chromium oxide is provided on the uppermost layer to ensure corrosion resistance after processing. In other words, since chromium oxide has excellent adhesion to epoxy-based paints, even if metallic chromium cannot follow the deformation of the base steel sheet, the chromium-containing layer and the epoxy-based paint adhere firmly to each other, and the coating properties of the epoxy-based paint can be maintained even after can production.
However, such conventional surface-treated steel sheets have poor adhesion to BPA-free paints, and therefore have inferior corrosion resistance in processed areas when a BPA-free coating is applied.
In contrast, in the coated steel sheet of the present disclosure, as described above, an oxygen-enriched region is provided in the region in the vicinity of the interface, thereby realizing excellent corrosion resistance in the BPA-free coated part. This is believed to be because the strain introduced into the chromium-containing layer during processing of the surface-treated steel sheet is dispersed, enabling the chromium-containing layer to follow the deformation of the steel sheet, and because the oxygen-enriched region itself exhibits excellent corrosion resistance. Thus, the present disclosure is based on a technical concept that is completely different from conventional technology, namely, improving the deformability of the chromium-containing layer itself and improving the corrosion resistance in the vicinity of the interface, rather than the adhesion to the paint.
In the present disclosure, the region in the vicinity of the interface and the oxygen-enriched region are determined as follows. First, five measurement regions are selected so as to include both the chromium-containing layer and the steel sheet, and the measurement regions are subjected to three-dimensional composition analysis by 3DAP to obtain five ion maps. The ion map is then divided into voxels of 2 nm × 2 nm × 2 nm, and the composition of each voxel is calculated in terms of atomic concentration. The region consisting of voxels in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3 is defined as the region in the vicinity of the interface. Furthermore, among the voxels in the region in the vicinity of the interface, the region in which O is present at an atomic concentration of 10 % or more is defined as an oxygen-enriched region. In this manner, the atomic ratio of Fe to Cr and the atomic concentration of O are measured by three-dimensional composition analysis using a three-dimensional atom probe, thereby determining the region in the vicinity of the interface and the oxygen-enriched region. In defining the region in the vicinity of the interface, the position where the atomic ratio of Fe to Cr is 1.0 is considered to be the interface between the chromium-containing layer and the steel sheet. That is, the region in the vicinity of the interface includes the interface between the chromium-containing layer and the steel sheet.
In the present disclosure, the coverage is defined as follows. First, for each ion map, the percentage is calculated using the formula O/I×100, where I is the number of voxels in the region in the vicinity of the interface and O is the number of voxels in the oxygen-enriched region. The average value of the percentages for each ion map is defined as the coverage of the oxygen-enriched region in the region in the vicinity of the interface. In this manner, the coverage of the oxygen-enriched region is measured by three-dimensional composition analysis using a three-dimensional atom probe.
The oxygen-enriched region may contain C. No upper limit is placed on the C content in the oxygen-enriched region, but the C content is preferably 20 atomic% or less. The oxygen-enriched region need not contain C, and therefore no lower limit is placed on the C content in the oxygen-enriched region, and the C content may be 0 atomic%.
The oxygen-enriched region may contain metals such as Si and Mn as oxides in addition to O, Fe, Cr, and C. However, if elements other than O, Fe, Cr and C are present in a large amount, the corrosion resistance in the BPA-free coated part may decrease. From this viewpoint, the total content of O, Fe, Cr and C in the oxygen-enriched region is preferably 70 atomic% or more. From a similar viewpoint, the oxygen-enriched region preferably contains absolutely no oxides of Si and Mn (0 %). The content of the oxide can be measured by, for example, XPS.
The atomic concentration of each element in the oxygen-enriched region can be measured by three-dimensional composition analysis using 3DAP, in the same manner as when determining the region in the vicinity of the interface and the oxygen-enriched region. More specifically, first, five measurement regions are selected so as to include both the chromium-containing layer and the steel sheet, and the measurement regions are subjected to three-dimensional composition analysis by 3DAP to obtain five ion maps. The ion map is then divided into voxels of 2 nm × 2 nm × 2 nm, and the composition of each voxel is calculated in terms of atomic concentration. Then, for each ion map, the compositions of the voxels in the oxygen-enriched region are averaged to calculate the average composition for each ion map, that is, the average composition for each measurement region. Finally, the average compositions for each measurement region are further averaged to calculate the atomic concentration of each element in the oxygen-enriched region.
The oxygen-enriched region may be amorphous or crystalline. That is, the oxygen-enriched region may contain one or both of an amorphous and a crystalline phase. However, from the viewpoint of improving corrosion resistance, the oxygen-enriched region is preferably amorphous. Whether an oxygen-enriched region is amorphous can be determined, for example, by preparing an observation sample including the oxygen-enriched region and observing the observation sample with a TEM or the like. That is, when a diffraction pattern of an oxygen-enriched region is acquired, the oxygen-enriched region is amorphous if no diffraction spots appear in the diffraction pattern.

[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 according to one embodiment of the present disclosure is a method of producing a surface-treated steel sheet having a chromium-containing layer on at least one surface of the steel sheet, and the method includes a steel sheet surface conditioning process and a cathodic electrolysis process. Each process is described below.

[Steel sheet surface conditioning process]

In the present disclosure, prior to the cathodic electrolysis described below, it is important to carry out a steel sheet surface conditioning process in which the steel sheet is brought into contact with an aqueous solution containing sulfate ions and is held for a predetermined time in a state in which a predetermined amount of the aqueous solution is present on the surface of the steel sheet.
Amount of aqueous solution: over 30.0 g/m² and 60.0 g/m² or less Holding time: 0.10 seconds to 20.0 seconds
In order to form an oxygen-enriched region in the region in the vicinity of the interface in the finally obtained surface-treated steel sheet, it is necessary, in the steel sheet surface conditioning process, to bring the steel sheet into contact with an aqueous solution containing sulfate ions and to hold the steel sheet in a state in which more than 30.0 g/m² and 60.0 g/m² or less of the aqueous solution is present on the surface of the steel sheet for 0.10 seconds or more and 20.0 seconds or less.
The mechanism by which the oxygen-enriched region is formed in the region in the vicinity of the interface by the steel sheet surface conditioning process is not clear but is thought to be as follows. When a steel sheet is brought into contact with an aqueous solution containing sulfate ions, a dissolution reaction of Fe and a decomposition reaction of dissolved oxygen occur on the surface of the steel sheet, causing the pH of the steel sheet surface to rise. In this case, if the amount of the aqueous solution is within the aforementioned range, the aqueous solution on the steel sheet becomes extremely thin, and therefore the amount of dissolved oxygen in the aqueous solution increases. As a result, the above reaction is further promoted. The state in which the aqueous solution is present on the steel sheet is not particularly limited, but from the viewpoint of making the reaction uniform, the aqueous solution is preferably in the form of a liquid film.
At this time, if dissolved Fe ions are present on the steel sheet surface where the pH has increased, the Fe ions are oxidized to Fe oxide, which accumulates on the steel sheet surface in very small amounts. In the subsequent cathodic electrolysis process, the minute amount of deposited Fe oxide is reduced and a chromium-containing layer is formed. In addition, although the mechanism is not clear, the minute amount of Fe oxide deposited by the above method can form a chromium-containing layer on the surface even if the Fe oxide is not completely reduced in the subsequent cathodic electrolysis process. As a result, it is presumed that an oxygen-enriched region is formed in the region in the vicinity of the interface.
From the viewpoint of forming more oxygen-enriched regions and further improving the corrosion resistance of the BPA-free coated part, the amount of the aqueous solution is preferably 32.0 g/m² or more. The amount of the aqueous solution is more preferably 35.0 g/m² or more. From the same viewpoint, the amount of the aqueous solution is preferably 58.0 g/m² or less. The amount of the aqueous solution is more preferably 55.0 g/m² or less.
From the viewpoint of forming more oxygen-enriched regions and further improving the corrosion resistance of the BPA-free coated part, the holding time is preferably 0.2 seconds or more. The holding time is more preferably 0.3 seconds or more. From the same viewpoint, the holding time is preferably 18.0 seconds or less. The holding time is more preferably 15.0 seconds or less.
The amount of the aqueous solution present on the surface of the steel sheet can be measured by a moisture meter using a filter type infrared absorption method. Specifically, the absorbance at the steel sheet 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 is placed on an electronic balance. The aqueous solution is dropped onto the steel sheet using a pipette to form a liquid film over the entire surface of the steel sheet. The weight of the aqueous solution present on the steel sheet is determined from the weight of the steel sheet before and after the aqueous solution is dropped. The resulting weight of the aqueous solution is divided by the area of the steel sheet to determine the amount of aqueous solution per unit area. At the same time, the absorbance on the surface of the steel sheet is measured using a moisture meter based on 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 steel sheet surface is not limited, and any method can be used. Examples of methods that can be used include squeezing the liquid with a wringer roll or wiping.
The composition of the aqueous solution is not particularly limited but is preferably an aqueous sulfuric acid solution, such as dilute sulfuric acid. Here, the aqueous sulfuric acid solution refers to an aqueous solution of sulfuric acid and comprises the case in which components other than sulfuric acid are included.
When an aqueous sulfuric acid solution is used as a pickling solution in the pretreatment step described below, the pickling solution can also be used as the aqueous solution in the steel sheet surface conditioning process. In addition, pickling solutions generally contain pickling inhibitors and pickling accelerators, but these components do not particularly hinder the formation of the oxygen-enriched region. Therefore, even if a pickling inhibitor or a pickling accelerator is added to the pickling solution, the pickling solution can be used as the aqueous solution in the steel sheet surface conditioning process.
No lower limit is placed on the concentration of sulfate ions contained in the aqueous solution, but the concentration is preferably 3 g/L or higher. The concentration is more preferably 5 g/L or higher. No upper limit is placed on the concentration of sulfate ions contained in the aqueous solution, but the concentration is preferably 200 g/L or lower. The concentration is more preferably 150 g/L or lower.
No lower limit is placed on the temperature of the aqueous solution, but the temperature is preferably 10 °C or higher, more preferably 15 °C or higher. No upper limit is placed on the temperature of the aqueous solution, but the temperature is preferably 70 °C or lower. The temperature is more preferably 60 °C or lower.
After the steel sheet surface conditioning process, the steel sheet is preferably washed with water to remove the aqueous solution adhering to the steel sheet.

[Cathodic electrolysis process]

Next, the steel sheet is subjected to cathodic electrolysis in an electrolytic solution containing 0.05 mol/L or more of trivalent chromium ions. The cathodic electrolysis can form a chromium-containing layer on the steel sheet. Any compound can be used as the trivalent chromium ion source as long as the compound can supply trivalent chromium ions. As the trivalent chromium ion source, for example, at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate can be used.
The temperature of the electrolytic solution during the cathodic electrolysis is not particularly limited but is preferably 40 °C or more in order to efficiently form the chromium-containing layer. For the same reason, the temperature of the electrolytic solution is preferably 70 °C or less. From the viewpoint of stably producing the above-described surface-treated steel sheet, it is preferable to monitor the temperature of the electrolytic solution in the cathodic electrolysis process and maintain the temperature of the electrolytic solution in the temperature range of 40 °C to 70 °C.
The pH of the electrolytic solution used in the cathodic electrolysis is not particularly limited but is preferably 4.0 or higher. The pH is more preferably 4.5 or higher. The pH is preferably 7.0 or lower. The pH is more preferably 6.5 or lower. From the viewpoint of stably producing the above-described surface-treated steel sheet, it is preferable to monitor the pH of the electrolytic solution in the cathodic electrolysis process and maintain the pH within the aforementioned pH range.
The current density in the cathodic electrolysis is not particularly limited and may be appropriately adjusted so that the desired surface treatment layer is formed. However, if the current density is excessively high, the burden on the cathodic electrolysis device becomes excessive. Therefore, the current density is preferably 200.0 A/dm² or less. The current density is more preferably 100.0 A/dm² or less. No lower limit is placed on the current density, but if the current density is excessively low, hexavalent Cr may be produced in the electrolytic solution, causing the bath to lose stability. Therefore, the current density is preferably 5.0 A/dm² or more. The current density is more preferably 10.0 A/dm² or more.
The number of times that the steel sheet is subjected to the cathodic electrolysis is not particularly limited and may be any number of times. In other words, cathodic electrolysis can be performed using an electrolysis treatment device having one pass or any number of passes greater than one. For example, it is also preferable to carry out the cathodic electrolysis continuously by subjecting the steel sheet (steel strip) to a plurality of passes during conveying. If the number of cathodic electrolysis treatments (i.e., the number of passes) is increased, a corresponding number of electrolytic cells will be required, so the number of cathodic electrolysis treatments (number of passes) is preferably 20 or less.
The electrolysis time per pass is not particularly limited. However, if the electrolysis time per pass is too long, the conveying speed (line speed) of the steel sheet decreases, resulting in reduced productivity. Therefore, the electrolysis time per pass is preferably 5 seconds or less. The electrolysis time is more preferably 3 seconds or less. There is no particular lower limit to the electrolysis time per pass, but if the electrolysis time is excessively short, it becomes necessary to increase the line speed accordingly, which makes control difficult. Therefore, the electrolysis time per pass is preferably 0.005 seconds or more. The electrolysis time is more preferably 0.01 seconds or more.
The coating weight of chromium in the chromium-containing layer formed by cathodic electrolysis can be controlled by the total electrical charge density, which is expressed as the product of the current density, the electrolysis time, and the number of passes. As described above, if the coating weight of chromium is excessively small, the corrosion resistance of the BPA-free coated part is impaired, whereas if the coating weight of chromium is excessively large, cohesive failure may occur in the chromium-containing layer during processing. Therefore, from the viewpoint of more stably ensuring the corrosion resistance in the BPA-free coated part, it is preferable to control the total electrical charge density so that the coating weight of chromium per side of the steel sheet in the chromium-containing layer is within an appropriate range. However, since the relationship between the coating weight of chromium per side of the steel sheet in the chromium-containing layer and the total electrical charge density varies depending on the configuration of the apparatus used in the cathodic electrolysis process, the actual electrolysis conditions may be adjusted according to the apparatus.
The type of anode used when carrying out cathodic electrolysis is not particularly limited, and any anode can be used. An insoluble anode is preferably used as the anode. As the insoluble anode, it is preferable to use at least one selected from the group consisting of an anode in which Ti is coated with one or both of a platinum group metal and an oxide of a platinum group metal, and a graphite anode. More specifically, examples of the insoluble anode include an anode in which the surface of a Ti substrate is coated with platinum, iridium oxide, or ruthenium oxide.
In the aforementioned cathodic treatment process, the concentration of the electrolytic solution is constantly changing due to the influence of the formation of the chromium-containing layer on the steel sheet, the introduction and removal of the solution, the evaporation of water, and the like. Since the change in the concentration of the electrolytic solution in the cathodic electrolysis process varies depending on the configuration of the equipment and the manufacturing conditions, from the viewpoint of more stable production of surface-treated steel sheets, it is preferable to monitor the concentrations of the components contained in the electrolytic solution in the cathodic electrolysis process and maintain the concentrations within the ranges described below.
After the cathodic electrolysis process, the steel sheet is preferably washed with water at least once. Water washing removes any residual electrolytic 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 provided downstream of an immersion tank for carrying out the immersion treatment, so that the steel sheet after immersion can be continuously immersed in water. Water washing may also be performed by spraying water on the steel sheet after the immersion.
The water used for the washing is not particularly limited, but it is preferable to use at least one of reverse osmosis water (RO water), ion-exchanged water, and distilled water. The electrical conductivity of the water used for the washing is not particularly limited but is preferably 100 µS/m or less. The electrical conductivity is more preferably 50 µS/m or less. The electrical conductivity is even more preferably 30 µS/m or less.
The temperature of water used for the water washing 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 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.
Drying may optionally be performed after the water washing. The drying method is not limited, and ordinary dryers or electric furnace drying methods, for example, can be applied. The temperature during the drying treatment is preferably 100 °C or less from the viewpoint of preventing deterioration of the surface-coating layer. Although no lower limit is placed on the temperature, the lower limit is typically around room temperature.
Prior to the steel sheet surface conditioning process, the steel sheet may be subjected to any pretreatment. The pretreatment preferably includes at least one of degreasing, pickling, and water washing.
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.
By performing pickling, the natural oxide film present on the surface of the steel sheet can be removed, so that the surface can be effectively conditioned in the subsequent steel sheet surface conditioning process. The pickling is not limited and can be performed by any method. After pickling, water washing is preferably performed to remove any pickling solution adhering to the steel sheet surface. When an aqueous solution containing sulfate ions is used as the pickling solution, it is preferable to subject the steel sheet to the steel sheet surface conditioning process as is.
The method of preparing the electrolytic solution used in the cathodic electrolysis process is not particularly limited, but by preparing the electrolytic solution as described below, it is possible to provide the electrolytic solution for the cathodic electrolysis process stably for a longer period of time.

[Preparation of electrolytic solution]

(i) Mixture

When preparing the aforementioned electrolytic solution, it is preferable to first mix a trivalent chromium ion source, a carboxylic acid compound, and water to prepare an aqueous solution.
Any compound can be used as the trivalent chromium ion source as long as the compound can supply trivalent chromium ions. As the trivalent chromium ion source, for example, at least one selected from the group consisting of chromium chloride, chromium sulfate, and chromium nitrate can be used.
The content of the trivalent chromium ion source in the aqueous solution needs to be 0.05 mol/L or more in terms of trivalent chromium ions. The content of the trivalent chromium ion source is preferably 0.08 mol/L or more. The content of the trivalent chromium ion source is more preferably 0.10 mol/L or more. No upper limit is placed on the content of the trivalent chromium ion source, but the content is preferably 1.50 mol/L or less in terms of trivalent chromium ions. The content of the trivalent chromium ion source is more preferably 1.30 mol/L or less. As the trivalent chromium ion source, BluCr^® (BluCr is a registered trademark in Japan, other countries, or both) TFS A from Atotech Corporation can be used.
The carboxylic acid compound is not particularly limited, and any carboxylic acid compound can be used. The carboxylic acid compound may be at least one of a carboxylic acid and a carboxylic acid salt, and is preferably at least one of an aliphatic carboxylic acid and a salt of an aliphatic carboxylic acid. The number of carbon atoms in the aliphatic carboxylic acid is preferably 1 to 10. The number of carbon atoms is more preferably 1 to 5. The number of carbon atoms in the aliphatic carboxylate is preferably 1 to 10. The number of carbon atoms is more preferably 1 to 5. The content of the carboxylic acid compound is not particularly limited but is preferably 0.1 mol/L or more. The content of the carboxylic acid compound is more preferably 0.15 mol/L or more. The content of the carboxylic acid compound is preferably 5.5 mol/L or less. The content of the carboxylic acid compound is more preferably 5.3 mol/L or less. As the carboxylic acid compound, BluCr^® TFS B from Atotech Corporation can be used.
Water can be used as a solvent for preparing the aqueous solution. It is preferable to use at least one of ion-exchanged water and distilled water as the water.
In order to effectively suppress the generation of hexavalent chromium at the anode in the cathodic electrolysis process and to improve the stability of the above-described electrolytic solution, it is preferable that the aqueous solution further contains at least one type of halide ion. The content of the halide ions is not particularly limited but is preferably 0.05 mol/L or more. The content of the halide ions is more preferably 0.10 mol/L or more. The content of the halide ions is preferably 3.0 mol/L or less. The content of the halide ions is more preferably 2.5 mol/L or less. To include the halide ions, BluCr^® TFS C1 and BluCr^® TFS C2 by Atotech can be used.
It is preferable that no hexavalent chromium is added to the above-described aqueous solution. It has been confirmed that, in principle, hexavalent chromium is not formed in the cathodic electrolysis process. Even if a small amount of hexavalent chromium is formed at the anode or the like, it is immediately reduced to trivalent chromium, so the concentration of hexavalent chromium in the electrolytic solution does not increase.
It is preferable that no metal ions other than trivalent chromium ions are intentionally added to the above aqueous solution. The aforementioned metal ions are not limited, but examples thereof include Cu ions, Zn ions, Fe ions, Sn ions, and Ni ions. The content of each of these is preferably 0 mg/L or more and 40 mg/L or less. The content is more preferably 0 mg/L or more and 20 mg/L or less. The content is most preferably 0 mg/L or more and 10 mg/L or less. Of the aforementioned metal ions, Fe ions may dissolve in the above-described electrolytic solution during the cathodic electrolysis process and the immersion process and be co-deposited in the coating, but this does not affect the corrosion resistance in the BPA-free coated part. The Fe ion concentration is preferably in the aforementioned range when the bath is prepared, but it is also preferable to maintain the Fe ion concentration in the electrolytic solution within the aforementioned range during the cathodic electrolysis process and the immersion process. If the Fe ions are controlled within the aforementioned range, the formation of the chromium-containing layer is not inhibited, and a necessary amount of the chromium-containing layer can be formed.

(ii) Adjustment of pH and temperature

Next, it is preferable to adjust the pH of the aqueous solution to 4.0 to 7.0 and to adjust the temperature of the aqueous solution to 40 °C to 70 °C, thereby preparing the electrolytic solution. As described above, in order to stably provide the cathodic electrolysis process for a long period of time, it is preferable not only to simply dissolve the trivalent chromium ion source and the carboxylic acid compound in water but also to appropriately control the pH and temperature as described above.
pH: 4.0 to 7.0
In preparing the electrolytic solution, it is preferable to adjust the pH of the aqueous solution after mixing to 4.0 to 7.0. The pH is more preferably 4.5 or higher. The pH is more preferably 6.5 or lower.
Any reagent can be used to adjust the pH. For example, it is preferable to use hydrochloric acid, sulfuric acid, nitric acid, or the like when lowering the pH, and to use ammonia water or the like when raising the pH.
Temperature: 40 °C to 70 °C
In preparing the electrolytic solution, it is preferable to adjust the temperature of the aqueous solution after mixing to 40 °C to 70 °C. The holding time in the temperature range of 40 °C to 70 °C is not particularly limited.
The electrolytic solution obtained by the above procedure can be stably used in the cathodic electrolysis process for a long period of time. The electrolytic solution prepared by the above procedure can be stored at 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.

(Preparation of electrolytic solution)

First, electrolytic solutions having compositions A to G listed in Table 1 were prepared under the conditions listed in Table 1. That is, each component listed in Table 1 was mixed with water to prepare an aqueous solution, and then the aqueous solution was adjusted to the pH and temperature listed in Table 1. The electrolytic solution G corresponds to the electrolytic solution used in the examples of PTL 4. To increase the pH, ammonia water was used in all cases. To decrease the pH, sulfuric acid was used in electrolytic solutions A, B, and G, hydrochloric acid in electrolytic solutions C and D, and nitric acid in electrolytic solutions E and F.

(Pretreatment of steel sheets)

The steel sheet used was a cold-rolled steel sheet. More specifically, a steel sheet for cans (T4 base sheet) having a thickness of 0.17 mm was used. As pretreatment, the steel sheet was subjected to electrolytic degreasing, water washing, and pickling in this order. The pickling was performed by immersing the steel sheet in an aqueous sulfuric acid solution having the sulfate ion concentration listed in Table 2. The steel sheet after the pickling was subjected to the subsequent steel sheet surface conditioning process without being washed with water.

(Steel sheet surface conditioning process)

Next, the steel sheet after the pickling was subjected to surface conditioning. Specifically, the pickling solution remaining on the surface of the steel sheet was squeezed out with a wringer roll to adjust the coating weight of the pickling solution to the amount listed in Table 2 as "amount of aqueous solution." Thereafter, while maintaining the coating weight, the steel sheet was held for the holding time listed in Table 2 and then washed with water to remove the pickling solution.

(Cathodic electrolysis process)

Next, the steel sheet was subjected to cathodic electrolysis under the conditions listed in Table 2. The electrolytic solution during the cathodic electrolysis was maintained at the pH and temperature listed in Table 1. The current density during the cathodic electrolysis was 40 A/dm², and the electrolysis time and number of passes were appropriately changed. As the anode during the cathodic electrolysis, an insoluble anode having a Ti substrate coated with iridium oxide was used. After the cathodic electrolysis, the substrate was washed with water having an electric conductivity of 100 µS/m or less and then dried at room temperature using a blower.
For each of the obtained surface-treated steel sheets, the chromium coating weight per side of the steel sheet and the chromium oxide coating weight per side of the steel sheet in the chromium-containing layer were measured by the method described above. For each of the obtained surface-treated steel sheets, the region in which the atomic ratio of Fe to Cr was in the range of 0.7 to 1.3 was determined as the vicinity of the interface between the steel sheet and the chromium-containing layer by the method described above. Then, the presence or absence of an oxygen-enriched region was judged by determining, in the vicinity, an oxygen-enriched region containing 10 % or more of O in atomic concentration by the method described above. Furthermore, the coverage of the oxygen-enriched region was measured by the method described above. The measurement results are listed in Table 3.
In all of the Examples, the chromium-containing layer resulting from the cathodic electrolysis contained chromium compounds, such as chromium oxide and chromium carbide, in addition to metallic chromium. The total content of the metallic chromium and the elements constituting the chromium compounds in the chromium-containing layer was 90 mass % or more. It was also confirmed that the oxygen-enriched region did not contain oxides of Si and Mn. It was also confirmed that the oxygen-enriched region was composed of an amorphous material.

(Corrosion resistance in BPA-free coated part)

Next, the corrosion resistance in the BPA-free coated part of each of the obtained surface-treated steel sheets was evaluated according to the procedure described below.
First, the surface of the surface-treated steel sheet was coated with BPA-free paint to prepare a BPA-free prepainted steel sheet. As the BPA-free paint, a polyester-based paint for the inner surface of a can (BPA-free paint) was used. In the coating, the BPA-free paint was applied to the surface of the surface-treated steel sheet, and the sheet was then baked at 180 °C for 10 minutes. The coating weight of the paint was 60 mg/dm².
The obtained BPA-free prepainted steel sheet was provided with cross cuts penetrating through to the base steel sheet, and an Erichsen tester was then used to yield a stretch formation 4 mm in height, centered on the intersection of the cross cuts, to prepare a test piece.
Next, a corrosion resistance test was carried out using the test piece according to the following procedure. First, the test piece was immersed in a Teflon^® (Teflon is a registered trademark in Japan, other countries, or both) container containing a test liquid, and the container was covered with a lid. In this state, the container was subjected to a retort treatment at a temperature of 121 °C for 1 hour. Subsequently, the test piece was removed from the container, washed with water to remove the test liquid, and then dried with a blower.
After drying, the test pieces were subjected to tape peeling twice, and the surfaces of the test pieces were then observed using a microscope or the like. The areas of peeled coating and areas of discoloration such as rust were visually evaluated and rated on a 5-point scale. 1 represented the poorest performance, and 5 represented the best performance. The same evaluation was performed on two samples per level, and the arithmetic mean of the ratings was calculated and used as an index of the corrosion resistance in the BPA-free coated part. In practice, if the rating is equal to or higher than that of the conventional TFS, the sample can be evaluated as having excellent corrosion resistance in the BPA-free coated part, but it is more preferably for the rating to be equal to or greater than that of the conventional TFS and to be 3.0 or more.
In order to simulate the difference in the corrosion environment depending on the contents when the surface-treated steel sheet is used in a can, the above corrosion resistance test was carried out using four test solutions having the following compositions (1) to (4). The evaluation results are listed in Table 4.

(1) Cysteine
- Sodium dihydrogen phosphate: 3.56 g/L
- Disodium hydrogen phosphate dodecahydrate: 14.52 g/L
- L-cysteine hydrochloride monohydrate: 0.5 g/L
(2) Lactic acid
- Lactic acid: 22.5 g/L
(3) Citric acid
- - Citric acid: 19.2 g/L
- - L(+)-ascorbic acid: 3.92 g/L
(4) Salt + acetic acid
- - Salt: 18.7 g/L
- - Acetic acid: 30 g/L

As is clear from the results listed in Table 4, all of the surface-treated steel sheets satisfying the conditions of the present disclosure can be produced without using hexavalent chromium and have excellent corrosion resistance in the BPA-free coated parts, equal to or better than that of the conventional TFS. However, (4) retort treatment in a test solution of salt + acetic acid is an extremely severe corrosion environment, and thus the surface-treated steel sheet that satisfies the conditions of the present disclosure, like the conventional TFS, received a score of less than 3.0. Therefore, particularly when applying the surface-treated steel sheet of the present disclosure to contents that contain acetic acid, it is necessary to take the same precautions as when using conventional TFS, such as double-coating with BPA-free paint and optimizing the retort treatment conditions.

[Table 1]

Table 1

Electrolytic solution		A	B	C	D	E	F	G
Component (mol/L)	Cr(OH)SO₄·Na₂SO₄	-	-	-	-	-	-	0.39
	Cr₂(SO₄)₃	0.1	0.2	-	-	-	-	-
	CrCl₃	-	-	0.2	0.5	-	-	-
	Cr(NO₃)₃	-	-	-	-	0.2	0.5	-
	HCO₂H	4.2	-	0.4	-	4.8	-	-
	NH₄CHO₂	-	0.5	-	3.5	-	0.5	-
	HCO₂K	-	-	-	-	-	-	0.61
	NH₄Cl	1.1	1.4	0.7	-	1.5	-	-
	NH₄Br	-	0.3	0.6	0.4	0.2	1.3	-
	KCl	-	-	-	-	-	-	3.35
	KBr	-	-	-	-	-	-	0.13
pH		5.0	5.7	5.1	4.3	6.8	5.8	2.3
Temperature [°C]		42	50	65	55	55	53	50

[Table2]

Table 2

No.	Production method					Notes
	Steel sheet surface conditioning process			Cathodic electrolysis process
	Sulfate ion concentration [g/L]	Amount of aqueous solution [g/m²]	Holding time [s]	Electrolytic solution	Electrical charge density [C/dm²]
1	10	43.3	1.3	A	80	Example
2	50	54.4	14.2	B	120	Example
3	100	37.8	3.4	C	360	Example
4	130	42.9	6.5	D	60	Example
5	80	39.0	7.8	E	220	Example
6	90	36.5	3.2	F	60	Example
7	65	46.7	10.1	A	200	Example
8	75	45.9	12.3	B	140	Example
9	40	50.2	9.0	C	80	Example
10	35	35.4	1.0	D	200	Example
11	55	38.8	2.4	E	400	Example
12	20	48.9	3.8	F	200	Example
13	15	45.0	12.5	A	160	Example
14	20	49.7	4.3	B	180	Example
15	90	53.2	5.4	C	120	Example
16	100	35.3	3.7	D	80	Example
17	110	41.1	6.7	E	92	Example
18	60	45.3	1.9	F	120	Example
19	40	44.7	11.5	A	56	Example
20	50	48.0	14.3	B	44	Example
21	40	37.6	8.8	C	440	Example
22	30	51.0	8.0	D	500	Example
23	20	35.9	6.1	E	320	Example
24	20	53.8	2.5	F	260	Example
25	100	33.5	4.5	A	100	Example
26	35	57.2	3.5	B	140	Example
27	60	45.1	0.2	C	300	Example
28	45	51.2	17.4	D	120	Example
29	120	59.3	4.5	E	22	Example
30	110	31.3	13.3	F	360	Example
31	80	38.9	19.1	A	400	Example
32	70	36.0	0.1	B	80	Example
33	20	36.7	0.06	C	200	Comparative Example
34	60	44.5	20.6	D	200	Comparative Example
35	50	28.5	12.1	E	220	Comparative Example
36	70	63.0	1.4	F	220	Comparative Example
37	60	68.2	2.2	G	160	Comparative Example
38	TFS (using hexavalent chromium)					Comparative Example

[Table 3]

Table 3

No.	Measurement results				Notes
No.	Chromium coating weight [mg/m²]	Chromium oxide coating weight [mg/m²]	Presence of oxygen-enriched region	Coverage of oxygen-enriched region [%]	Notes
1	82.4	4.5	Yes	100	Example
2	103.5	9.7	Yes	100	Example
3	323.0	2.5	Yes	76	Example
4	60.5	15.6	Yes	80	Example
5	212.7	26.7	Yes	98	Example
6	53.2	6.3	Yes	94	Example
7	180.9	0.2	Yes	100	Example
8	151.2	10.5	Yes	83	Example
9	70.5	32.3	Yes	89	Example
10	170.5	25.4	Yes	72	Example
11	435.9	17.3	Yes	95	Example
12	219.8	8.7	Yes	92	Example
13	143.2	12.1	Yes	100	Example
14	156.4	14.5	Yes	100	Example
15	120.4	2.1	Yes	94	Example
16	78.5	3.4	Yes	80	Example
17	96.4	6.5	Yes	100	Example
18	130.3	9.8	Yes	81	Example
19	46.1	3.5	Yes	90	Example
20	35.8	8.9	Yes	78	Example
21	468.9	6.5	Yes	90	Example
22	503.2	9.0	Yes	91	Example
23	313.5	36.5	Yes	94	Example
24	221.1	41.3	Yes	100	Example
25	121.5	5.3	Yes	62	Example
26	135.6	6.7	Yes	54	Example
27	302.1	19.8	Yes	67	Example
28	145.9	15.4	Yes	58	Example
29	214.5	3.2	Yes	45	Example
30	326.7	0.5	Yes	32	Example
31	380.6	11.3	Yes	36	Example
32	80.4	1.7	Yes	21	Example
33	194.3	16.9	No	-	Comparative Example
34	193.5	15.2	No	-	Comparative Example
35	209.5	15.4	No	-	Comparative Example
36	211.3	4.9	No	-	Comparative Example
37	105.4	4.9	No	-	Comparative Example
38	101.5	12.5	No	-	Comparative Example

[Table 4]

Table 4

No.	Corrosion resistance in BPA-free coated part				Notes
No.	(1) Cysteine	(2) Lactic acid	(3) Citric acid	(4) Salt + acetic acid	Notes
1	5.0	5.0	5.0	2.5	Example
2	5.0	5.0	5.0	2.5	Example
3	5.0	5.0	5.0	2.5	Example
4	5.0	5.0	5.0	2.5	Example
5	5.0	5.0	5.0	2.5	Example
6	5.0	5.0	5.0	2.5	Example
7	5.0	5.0	5.0	2.5	Example
8	5.0	5.0	5.0	2.5	Example
9	5.0	5.0	5.0	2.5	Example
10	5.0	5.0	5.0	2.5	Example
11	5.0	5.0	5.0	2.5	Example
12	5.0	5.0	5.0	2.5	Example
13	5.0	5.0	5.0	2.5	Example
14	5.0	5.0	5.0	2.5	Example
15	5.0	5.0	5.0	2.5	Example
16	5.0	5.0	5.0	2.5	Example
17	5.0	5.0	5.0	2.5	Example
18	5.0	5.0	5.0	2.5	Example
19	5.0	5.0	4.5	2.0	Example
20	5.0	5.0	4.0	2.0	Example
21	5.0	5.0	4.5	2.0	Example
22	5.0	5.0	4.0	2.0	Example
23	5.0	5.0	4.5	2.0	Example
24	5.0	5.0	4.0	2.0	Example
25	5.0	5.0	4.5	2.0	Example
26	5.0	5.0	4.5	2.0	Example
27	5.0	5.0	4.5	2.0	Example
28	5.0	5.0	4.5	2.0	Example
29	5.0	5.0	4.0	2.0	Example
30	5.0	5.0	4.0	2.0	Example
31	5.0	5.0	4.0	2.0	Example
32	5.0	5.0	4.0	2.0	Example
33	1.5	1.5	2.5	0.5	Comparative Example
34	1.5	1.5	2.5	0.5	Comparative Example
35	1.0	1.5	2.0	0.5	Comparative Example
36	1.0	1.0	1.0	0.0	Comparative Example
37	1.5	1.5	1.0	0.5	Comparative Example
38	5.0	5.0	4.0	2.0	Comparative Example

Claims

A surface-treated steel sheet comprising:
a steel sheet; and

a chromium-containing layer disposed on at least one surface of the steel sheet, wherein

an oxygen-enriched region containing O at an atomic concentration of 10 % or more is present in a vicinity of an interface between the steel sheet and the chromium-containing layer, and

the vicinity is a region in which an atomic ratio of Fe to Cr is in a range of 0.7 to 1.3.
The surface-treated steel sheet according to claim 1, wherein the oxygen-enriched region has a coverage of 50 % or more in the vicinity.
The surface-treated steel sheet according to claim 1 or 2, wherein the chromium-containing layer has a chromium coating weight of 40.0 mg/m² to 500.0 mg/m² per side.
The surface-treated steel sheet according to any one of claims 1 to 3, wherein the chromium-containing layer has a chromium oxide coating weight of 40.0 mg/m² or less per side.
A method of producing a surface-treated steel sheet comprising a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet, the method comprising:
a steel sheet surface conditioning step of bringing the steel sheet into contact with an aqueous solution containing sulfate ions and holding in a state in which more than 30.0 g/m² and 60.0 g/m² or less of the aqueous solution is present on a surface of the steel sheet for 0.10 to 20.0 seconds; and

a cathodic electrolysis step of subjecting the steel sheet to cathodic electrolysis in an electrolytic solution containing 0.05 mol/L or more of trivalent chromium ions.
The method of producing a surface-treated steel sheet according to claim 5, wherein the electrolytic solution is prepared by mixing a trivalent chromium ion source, a carboxylic acid compound, and water, and adjusting pH to 4.0 to 7.0 and temperature to 40 °C to 70 °C.