JP7552960B1 - Surface-treated steel sheet and its manufacturing method - Google Patents

Abstract

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

Description

The present invention relates to a surface-treated steel sheet, and in particular to a surface-treated steel sheet having excellent corrosion resistance in a BPA (bisphenol A)-free painted area. 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) 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 organic resin coatings such as epoxy paints and PET films to accommodate a variety of contents. When applying an organic resin coating to tinplate or TFS, the steel plate is electrolytically treated or immersed in an aqueous solution containing hexavalent Cr to form a chromium oxide layer on the outermost surface, which provides excellent adhesion to the organic resin coating layer. As a result, the deformation of the organic resin coating layer follows the deformation of the steel plate that accompanies can manufacturing, ensuring corrosion resistance to a variety of contents even after can manufacturing.

On the other hand, it has been suggested that the BPA contained in epoxy-based paints may have harmful effects on humans. Therefore, the development of BPA-free paints using polyester-based resins that do not contain BPA is underway (Patent Documents 1 and 2), and there is a demand to replace epoxy-based paints with BPA-free paints. However, compared to their adhesion to epoxy-based paints, conventional tinplate and TFS have poor adhesion to BPA-free paints, so the deformation of the BPA-free paint cannot follow the deformation of the steel plate associated with can manufacturing, and corrosion resistance against various contents after can manufacturing cannot be sufficiently ensured. Therefore, the current situation is that 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.

Methods proposed in Patent Documents 3 to 6, for example, are known as methods for forming surface-treated steel sheets without using hexavalent chromium. In these methods, a surface treatment layer is formed by electrolytic treatment in an electrolyte solution containing a trivalent chromium compound such as basic chromium sulfate.

JP 2013-144753 A JP 2008-50486 A Special table 2016-501985 publication Special table 2016-505708 publication JP 2020-172700 A JP 2020-172701 A

According to the methods proposed in Patent Documents 3 to 6, a surface treatment layer can be formed without using hexavalent chromium. According to Patent Documents 3 to 6, the above methods can provide a surface-treated steel sheet that has excellent adhesion to an epoxy-based paint. According to Patent Documents 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 provided.

However, surface-treated steel sheets obtained by conventional methods such as those proposed in Patent Documents 3 to 6 have excellent adhesion to epoxy-based paints and excellent corrosion resistance in the epoxy-based paint-processed areas, but the corrosion resistance in the BPA-free paint-processed areas is insufficient. Therefore, it has not been possible to replace conventional epoxy-based paints with BPA-free paints 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 corrosion resistance in BPA-free painted areas.

The present invention has been made in consideration of the above-mentioned circumstances, and its object is to provide a surface-treated steel sheet that can be produced without using hexavalent chromium and has excellent corrosion resistance in the BPA-free painted area.

As a result of intensive research into achieving the above-mentioned objective, the inventors of the present invention have come to the following findings (1) and (2).

(1) In a surface-treated steel sheet having a chromium-containing layer on at least one side, by creating an oxygen-enriched area between the steel sheet and the chromium-containing layer, a surface-treated steel sheet with excellent corrosion resistance in the BPA-free painted area can be obtained.

(2) The above-mentioned surface-treated steel sheet can be produced by contacting a steel sheet with an aqueous solution containing sulfate ions, holding the aqueous solution on the surface of the steel sheet in a state in which more than 30.0 g/ ^m2 and not more than 60.0 g/ ^m2 is present 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 invention was completed based on the above findings. The gist of the present invention is as follows:

1. A steel plate;
A surface-treated steel sheet comprising 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 the vicinity of the interface between the steel sheet and the chromium-containing layer,
The vicinity is a region in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3.

2. The surface-treated steel sheet described in 1, wherein the coverage rate of the oxygen-enriched region in the vicinity is 50% or more.

3. The surface-treated steel sheet according to 1 or 2 above, wherein the chromium-containing layer has a chromium coating amount of 40.0 to 500.0 mg/ ^m2 per side.

4. The surface-treated steel sheet according to any one of 1 to 3 above, wherein the chromium-containing layer has a chromium oxide deposition amount of 40.0 mg/m2 ^or less per side.

5. A method for 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, comprising the steps of:
a steel sheet surface conditioning step of contacting the steel sheet with an aqueous solution containing sulfate ions and holding the aqueous solution on the surface of the steel sheet in a state in which the aqueous solution is present in an amount of more than 30.0 g/ ^m2 and not more than 60.0 g/ ^m2 for 0.10 to 20.0 seconds;
and a cathodic electrolysis step of cathodic electrolysis of the steel sheet in an electrolytic solution containing 0.05 mol/L or more of trivalent chromium ions.

6. A method for producing a surface-treated steel sheet as described in 5 above, wherein the electrolytic solution is prepared by mixing a trivalent chromium ion source, a carboxylic acid compound, and water, adjusting the pH to 4.0 to 7.0, and adjusting the temperature to 40 to 70°C.

According to the present invention, it is possible to provide a surface-treated steel sheet having excellent corrosion resistance in the BPA-free painted area without using hexavalent chromium. The surface-treated steel sheet of the present invention can be suitably used as a material for containers, etc.

The following is a detailed description of a method for implementing the present invention. Note that the following description is merely an example of a preferred embodiment of the present invention, and the present invention is not limited thereto.

The surface-treated steel sheet in one embodiment of the present invention is a surface-treated steel sheet having a chromium-containing layer on at least one side of the steel sheet. In the present invention, it is important that an oxygen-enriched region exists between the chromium-containing layer and the steel sheet. 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 sheet is not particularly limited, but may contain C, Mn, 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, the steel sheet may preferably have a composition as specified in ASTM A623M-09, for example.

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 0.70%,
P: 0 to 0.15%,
S: 0 to 0.050%,
Al: 0-0.20%,
N: 0 to 0.040%,
Cu: 0 to 0.20%,
Ni: 0 to 0.15%,
Cr: 0 to 0.10%,
Mo: 0-0.05%,
Ti: 0 to 0.020%,
Nb: 0 to 0.020%,
B: 0 to 0.020%,
Ca: 0-0.020%,
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 thickness of the steel plate is not particularly limited, but is preferably 0.60 mm or less. On the other hand, the lower limit of the thickness is not particularly limited, but is preferably 0.10 mm or more. Here, "steel plate" is defined to include "steel strip".

[Chromium-containing layer]
A chromium-containing layer is present on at least one surface of the steel sheet. The components constituting the chromium-containing layer are not particularly limited, but may include metallic chromium and a chromium compound. The chromium compound may include any chromium compound without being particularly limited. 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 described below. The metal elements are typically considered 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 invention, the chromium-containing layer preferably has a total content of elements constituting metallic chromium and chromium compounds of 90 atomic % or more. Here, the total content is expressed as a percentage of the ratio of the total atomic number of elements constituting metallic chromium and chromium compounds to the total atomic number of all elements other than Fe.

The total content can be determined by measuring the content (atomic %) of each of the elements constituting the chromium compound and the metallic chromium contained in the chromium-containing layer by X-ray photoelectron spectroscopy (XPS) and adding them up. In measuring the content 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 each element.

For example, the content of chromium carbide ( _{Cr2C3 )} can be determined from the integrated intensity of the C 1s carbide peak appearing near 281.0 eV. 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 x (2 ₊ 3) / 3 = 10 atomic %.

For chromium oxide, the _Cr2O3 content can be determined from the integrated intensity of the Cr 2p _oxide peak appearing near 576.7 eV. Also, the _CrO3 content can be determined from the integrated intensity of the Cr 2p oxide peak appearing near 579.2 eV.

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 around 162.3 _eV Chromium nitride (CrN): N 1s peak appearing around 397.3 eV Chromium chloride ( _CrCl3 ): Cl 2p peak appearing around 199.8 eV Chromium bromide ( _CrBr3 ): Br 3d peak appearing around 69.1 eV Chromium boride (CrB): Br 1s peak appearing around 188.2 eV

On the other hand, the metallic chromium content is determined by calculating the Cr content from the integrated intensity of the Cr 2p peak appearing around 573.8 eV and subtracting the content of Cr atoms contained as chromium compounds from the chromium content.

By adding the content of metallic chromium obtained by the above method and the content of each element that constitutes the chromium compound, the total content of metallic chromium and the elements that constitute the chromium compound can be calculated.

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 its outermost surface, and the total content of the elements constituting the metallic chromium and the chromium compound and the Fe content are measured by the above-mentioned method. Then, the intermediate position is determined to be the position (depth) halfway between the outermost surface of the chromium-containing layer and the position where the measured total content of the elements constituting the metallic chromium and the chromium compound and the Fe content are equal.

The above-mentioned XPS measurement can be performed using, for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI, Inc. The X-ray source is a monochrome AlKα ray, the voltage is 15 kV, the beam diameter is 100 μmφ, the take-off angle is 45°, and the sputtering conditions are Ar ion with an acceleration 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, and may be, for example, separated as separate layers in the chromium-containing layer, or may be mixed throughout the chromium-containing layer. In other words, the spatial structure of the components constituting the chromium-containing layer may contain one or both of separate layers and mixed layers.

The chromium deposition amount of the chromium-containing layer is not particularly limited. However, if the chromium deposition amount of the chromium-containing layer is excessive, cohesive failure may occur in the chromium-containing layer when the surface-treated steel sheet is processed. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free painted part, the chromium deposition amount of the chromium-containing layer is preferably 500.0 mg/m ² or less per side, and more preferably 450.0 mg/m ² or less. On the other hand, from the viewpoint of further improving the corrosion resistance of the BPA-free painted part, the chromium deposition amount of the chromium-containing layer is preferably 40.0 mg/m ² or more per side, and more preferably 50.0 mg/m ² or more. Here, the "chromium deposition amount" refers to the total deposition amount of chromium present in various forms.

The amount of chromium attached can be measured by X-ray fluorescence analysis. More specifically, the amount of chromium attached is measured by the following procedure. First, the amount of Cr (total Cr amount) in the surface-treated steel sheet is measured using an X-ray fluorescence device. Next, the amount of Cr (original sheet Cr amount) in the steel sheet before the chromium-containing layer is formed or in the steel sheet after the chromium-containing layer is peeled off is measured using an X-ray fluorescence device. The value obtained by subtracting the original sheet Cr amount from the total Cr amount is the amount of chromium attached in the chromium-containing layer. For example, a commercially available hydrochloric acid-based chromium plating stripper can be used to strip the chromium-containing layer.

[Chromium oxide deposition amount]
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) attached to 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 amount of chromium oxide attached to the chromium-containing layer is not particularly limited. However, if the amount of chromium oxide attached to the chromium-containing layer is excessive, cohesive failure may occur starting from the Cr oxide in the chromium-containing layer when the surface-treated steel sheet is processed, and the corrosion resistance of the BPA-free painted part may deteriorate. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free painted part, the amount of chromium oxide attached to the chromium-containing layer is preferably 40.0 mg/m ² or less per side, more preferably 35.0 mg/m ² or less. On the other hand, the chromium-containing layer may not contain chromium oxide at all. Therefore, the lower limit of the amount of chromium oxide attached to the chromium-containing layer is not particularly limited, and may be 0.0 mg/m ² per side.

The amount of chromium oxide attached can be measured by X-ray fluorescence analysis. More specifically, the amount of chromium oxide attached is measured by the following procedure. First, the Cr amount (total Cr amount) of the surface-treated steel sheet is measured. Next, the surface-treated steel sheet is subjected to an alkali treatment in which it is immersed in 7.5N-NaOH at 90°C for 10 minutes to remove the chromium oxide. After the surface-treated steel sheet is thoroughly washed with water after the alkali treatment, the Cr amount (amount of Cr after alkali treatment) is measured again using an X-ray fluorescence device. The value obtained by subtracting the amount of Cr after alkali treatment from the total Cr amount is determined as the amount of chromium oxide attached in the chromium-containing layer.

The chromium-containing layer may be amorphous or crystalline. That is, the chromium-containing layer may contain one or both of the amorphous and crystalline phases. The chromium-containing layer manufactured by the method described below generally contains an amorphous phase, and may also contain a crystalline phase. The mechanism of formation of the chromium-containing layer is not clear, but it is believed that the chromium-containing layer contains both the amorphous and crystalline phases as a result of partial crystallization progressing when the amorphous phase is formed. The area ratio of the crystalline region is not particularly limited, but it is preferably 30% or less when the chromium-containing layer is observed from the surface direction. On the other hand, since the crystalline region may not exist, the lower limit of the area ratio of the crystalline region may be 0%.

The crystalline region in the chromium-containing layer can be confirmed by removing the base steel sheet from the surface-treated steel sheet to prepare a chromium-containing layer single layer sample, and observing the chromium-containing layer single layer sample from the surface side with a TEM or STEM. The method for preparing the chromium-containing layer single layer sample is not particularly limited, but for example, it can be prepared by irradiating an ion beam such as Ar from the base steel sheet side and ion milling the steel sheet. When preparing the chromium-containing layer single layer region with an ion beam, the ion beam is irradiated with an acceleration voltage of 5 kV or less and an incidence angle in the range of 1 degree to 5 degrees relative to the base steel sheet, thereby ensuring a field of view of the chromium single 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 film thickness of the chromium-containing layer may become thin, but this does not affect the measurement result of the area ratio of the crystalline region.

The area ratio of the crystalline regions in the chromium-containing layer can be measured by TEM. Specifically, a diffraction pattern of the chromium-containing layer is first obtained by selected area diffraction of the TEM. Then, 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 image are defined as crystalline regions. The area of the obtained crystalline regions is calculated by image processing, and the area ratio of the crystalline regions is calculated by dividing the area of the chromium-containing layer within the selected area aperture. Image analysis software such as Image-J can be used to calculate the area ratio.

The chromium-containing layer may contain C. The upper limit of the C content in the chromium-containing layer is not particularly limited, but the atomic ratio to Cr is preferably 50% or less, and more preferably 45% or less. The chromium-containing layer may not contain C, and therefore the lower limit of the atomic ratio of C to Cr contained in the chromium-containing layer is not particularly limited and 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 measured by sputtering from the outermost layer to a depth of 0.2 nm or more in terms of _SiO2 , quantifying the integrated intensity of the narrow spectrum of Cr2p and C1s as an atomic ratio by the relative sensitivity coefficient method, and calculating the C atomic ratio/Cr atomic ratio. For the measurement of the XPS, for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI, Inc. The X-ray source is a monochrome AlKα ray, the voltage is 15 kV, the beam diameter is 100 μmφ, and the take-off angle is 45°, and the sputtering conditions are Ar ion with an acceleration 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 unclear, but it is thought that if a carboxylic acid compound is contained in the electrolyte during the process of forming a chromium-containing layer on the steel sheet, the carboxylic acid compound decomposes and is incorporated into the film.

The location of C in the chromium-containing layer is not particularly limited. The location of C can be confirmed, for example, by composition analysis using energy dispersive X-ray spectroscopy (EDS) or wavelength dispersive X-ray spectroscopy (WDS) attached to 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. The upper limit of the Fe content in the chromium-containing layer is not particularly limited, but it is preferable that the atomic ratio to Cr is 100% or less. The chromium-containing layer may not contain Fe, and therefore the lower limit of the atomic ratio to Cr is not particularly limited and may be 0%. The Fe content in the chromium-containing layer can be measured by XPS, as with the C content. The atomic ratio can be calculated using narrow spectra of Cr2p and Fe2p.

The mechanism by which Fe is incorporated into the chromium-containing layer is not clear, but it is thought that during the process of forming the chromium-containing layer on the steel plate, a small amount of Fe contained in the steel plate dissolves in the electrolyte, 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, and Zn contained in the aqueous solution, as well as S, N, Cl, Br, etc. However, the presence of these elements may reduce the corrosion resistance of the BPA-free painted part. Therefore, the total of elements other than Cr, O, Fe, and C is preferably 3% or less in atomic ratio to Cr, and more preferably none (0%). The content of the above elements is not particularly limited, but can be measured by XPS in the same way as the content of C, for example.

[Oxygen enriched region]
In the surface-treated steel sheet of the present invention, an oxygen-enriched region containing O at an atomic concentration of 10% or more exists in the vicinity of the interface between the steel sheet and the chromium-containing layer (hereinafter, the vicinity may be referred to as the interface vicinity region). Here, the interface vicinity region 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 the oxygen-enriched region in the interface vicinity region can realize excellent corrosion resistance of the BPA-free paint-processed part. From the viewpoint of further improving the corrosion resistance of the BPA-free paint-processed part, the coverage of the oxygen-enriched region in the interface vicinity region is preferably 50% or more, more preferably 70% or more. The upper limit of the coverage of the oxygen-enriched region in the interface vicinity region is not particularly limited and may be 100%. In the following description, the oxygen-enriched region refers only to the region existing in the interface vicinity region.

The reason why the corrosion resistance of the BPA-free painted parts is improved by providing an oxygen-enriched area as described above is 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. Surface-treated steel sheets with such a chromium-containing layer are generally processed into cans and the like after an organic resin coating is formed on the surface. However, because metallic chromium has poor workability, the chromium-containing layer cannot completely follow the deformation of the steel sheet that accompanies processing, resulting in damage to the organic resin coating present on the chromium-containing layer. As a result, corrosion resistance after processing is reduced.

Therefore, in conventional surface-treated steel sheets, chromium oxide was provided as the top layer to ensure corrosion resistance after processing. In other words, because chromium oxide has excellent adhesion to epoxy-based paints, even if the metallic chromium cannot follow the deformation of the base steel sheet, the chromium-containing layer and the epoxy-based paint adhere firmly, and the coating properties of the epoxy-based paint can be maintained even after can making.

However, such conventional surface-treated steel sheets had poor adhesion to BPA-free paint, and therefore had poor corrosion resistance in processed areas when BPA-free paint was applied.

In contrast, the surface-treated steel sheet of the present invention, as described above, provides an oxygen-enriched region in the region near the interface, thereby realizing excellent corrosion resistance in the BPA-free paint-processed area. This is believed to be because the strain introduced into the chromium-containing layer during processing of the surface-treated steel sheet is dispersed, allowing the chromium-containing layer to follow the deformation of the steel sheet, and the oxygen-enriched region itself exhibits excellent corrosion resistance. Thus, the present invention is based on a technical concept that is completely different from the conventional one, which is to improve the deformability of the chromium-containing layer itself and the corrosion resistance near the interface, rather than the adhesion to the paint.

In the present invention, the interface vicinity region 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 five ion maps are obtained by performing three-dimensional composition analysis by 3DAP on the measurement regions. Next, the ion map is divided into voxels of 2 nm x 2 nm x 2 nm, and the composition of each voxel is calculated in atomic concentration. The interface vicinity region is a region consisting of voxels in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3. In addition, the oxygen-enriched region is a region consisting of voxels in the interface vicinity region in which O is 10% or more in atomic concentration. In this way, the interface vicinity region and the oxygen-enriched region are determined by measuring the atomic ratio of Fe to Cr and the atomic concentration of O by three-dimensional composition analysis using a three-dimensional atom probe. In defining the interface vicinity region, the position where the atomic ratio of Fe to Cr is 1.0 is the interface between the chromium-containing layer and the steel sheet. That is, the near-interface region includes the interface between the chromium-containing layer and the steel sheet.

In addition, in the present invention, the coverage is defined as follows. First, for each ion map, the number of voxels in the near-interface region is defined as I, the number of voxels in the oxygen-enriched region is defined as O, and the percentage is calculated from the formula O/I x 100. The average value of the percentages for each ion map is then defined as the coverage of the oxygen-enriched region in the near-interface region. In this way, 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. The upper limit of the C content in the oxygen-enriched region is not particularly limited, but is preferably 20 atomic % or less. The oxygen-enriched region may not contain C, and therefore the lower limit of the C contained in the oxygen-enriched region is not particularly limited and may be 0 atomic %.

In addition to O, Fe, Cr, and C, the oxygen-enriched region may contain metals such as Si and Mn as oxides. However, if there are many elements other than O, Fe, Cr, and C, the corrosion resistance of the BPA-free painted part may decrease. From this perspective, it is preferable that the total content of O, Fe, Cr, and C in the oxygen-enriched region is 70 atomic % or more. From the same perspective, it is preferable that the oxygen-enriched region does not contain any oxides of Si and Mn (0%). The content of the above oxides can be measured, for example, by XPS.

The atomic concentration of each element in the oxygen-enriched region can be measured by three-dimensional composition analysis using 3DAP, as in the case of determining the interface region and the oxygen-enriched region. More specifically, first, five measurement regions are selected to include both the chromium-containing layer and the steel plate, and five ion maps are obtained by performing three-dimensional composition analysis using 3DAP on the measurement regions. Next, the ion map is divided into voxels of 2 nm x 2 nm x 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, i.e., the average composition for each measurement region. Finally, the atomic concentration of each element in the oxygen-enriched region can be calculated by further averaging the average composition for each measurement region.

The oxygen-enriched region may be amorphous or crystalline. That is, the oxygen-enriched region may contain one or both of the amorphous and crystalline phases. However, from the viewpoint of improving corrosion resistance, it is preferable that the oxygen-enriched region is amorphous. Whether the oxygen-enriched region is amorphous or not 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 the oxygen-enriched region is obtained, if no diffraction spots appear in the diffraction pattern, the oxygen-enriched region is amorphous.

[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.

The 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 chromium-containing layer on at least one side of the steel sheet, and includes a steel sheet surface adjustment process and a cathodic electrolytic treatment process. Each process will be described below.

[Steel plate surface conditioning process]
In the present invention, prior to the cathodic electrolysis described below, a steel sheet surface conditioning step is performed in which the steel sheet is contacted with an aqueous solution containing sulfate ions and 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. It is important to carry out the process.

Amount of aqueous solution: more than 30.0 g/ ^m2 and not more than 60.0 g/ ^m2 Holding time: 0.10 to 20.0 seconds In order to cause an oxygen-enriched region to exist in the near-interface region in the finally obtained surface-treated steel sheet, it is necessary to contact the steel sheet with an aqueous solution containing sulfate ions in the steel sheet surface conditioning step and hold the aqueous solution on the surface of the steel sheet in an amount of more than 30.0 g/ ^m2 and not more than 60.0 g/ ^m2 for 0.10 seconds or more and not more than 20.0 seconds.

The mechanism by which the oxygen-enriched region is formed in the region near the interface by the steel sheet surface conditioning process is not clear, but it is thought to be as follows. When the 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, and the pH of the steel sheet surface increases. In this case, if the amount of the aqueous solution is within the above range, the thickness of the aqueous solution on the steel sheet becomes very thin, so that the amount of dissolved oxygen in the aqueous solution increases. As a result, the above reaction is further promoted. The state of the aqueous solution on the steel sheet is not particularly limited, but from the viewpoint of making the reaction uniform, it is preferable that the aqueous solution be 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 risen, the Fe ions are oxidized to Fe oxide, which accumulates on the steel sheet surface in very small amounts. In the subsequent cathodic electrolytic treatment process, the accumulated trace amount of Fe oxide is reduced and a chromium-containing layer is formed. Although the mechanism is not clear, the Fe oxide accumulated in small amounts by the above method can form a chromium-containing layer on top of it even if it is not completely reduced in the subsequent cathodic electrolytic treatment process. As a result, it is presumed that an oxygen-enriched region is formed in the region near the interface.

From the viewpoint of forming more oxygen-enriched regions and further improving the corrosion resistance of the BPA-free painted part, the amount of the aqueous solution is preferably 32.0 g/m2 or ^more , and 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 , and 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 painted part, the holding time is preferably 0.2 seconds or more, and more preferably 0.3 seconds or more. From the same viewpoint, the holding time is preferably 18.0 seconds or less, and 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 on the surface of the steel sheet is measured by 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, 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 on the entire surface of the steel sheet. The weight of the aqueous solution present on the steel sheet is calculated from the weight of the steel sheet before the aqueous solution is dropped and the weight of the steel sheet 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 dividing it by the area of the steel sheet. At the same time, the absorbance on the surface of the steel sheet is measured by a moisture meter using a filter-type infrared absorption method. The above measurements are performed multiple times while changing the amount of the aqueous solution, and a calibration curve showing the correlation between the amount of the aqueous solution and the absorbance is created. As the calibration curve, a linear approximation of the correlation between the amount of the aqueous solution and the absorbance can be used.

There are no particular limitations on the method for adjusting the amount of aqueous solution present on the steel sheet surface, and any method can be used. For example, a method of squeezing the liquid with a wringer roll or a method such as wiping can be used.

The composition of the aqueous solution is not particularly limited, but it is preferably an aqueous sulfuric acid solution such as dilute sulfuric acid. Here, the aqueous sulfuric acid solution means an aqueous solution of sulfuric acid, and includes cases where components other than sulfuric acid are included.

When an aqueous sulfuric acid solution is used as the 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 step. Pickling solutions generally contain pickling inhibitors and pickling accelerators, but these components do not particularly hinder the formation of oxygen-enriched regions. Therefore, even if a pickling inhibitor or pickling accelerator is added to the pickling solution, the pickling solution can be used as the aqueous solution in the steel sheet surface conditioning step.

The lower limit of the concentration of sulfate ions contained in the aqueous solution is not particularly limited, but is preferably 3 g/L or more, and more preferably 5 g/L or more. The upper limit of the concentration of sulfate ions contained in the aqueous solution is not particularly limited, but is preferably 200 g/L or less, and more preferably 150 g/L or less.

The lower limit of the temperature of the 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 aqueous solution is not particularly limited, but is preferably 70°C or lower, and more preferably 60°C or lower.

After the steel sheet surface conditioning process, it is preferable to rinse the steel sheet with water to remove the aqueous solution adhering to the steel sheet.

[Cathodic electrolysis treatment 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 that can supply trivalent chromium ions can be used 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 as the trivalent chromium ion source.

The temperature of the electrolyte when performing the cathodic electrolytic treatment is not particularly limited, but is preferably 40°C or higher in order to efficiently form a chromium-containing layer. For the same reason, it is preferable to set the temperature of the electrolyte to 70°C or lower. From the viewpoint of stably producing the above-mentioned surface-treated steel sheet, it is preferable to monitor the temperature of the electrolyte in the cathodic electrolytic treatment process and maintain the temperature of the electrolyte in the temperature range of 40 to 70°C.

The pH of the electrolyte when performing the cathodic electrolysis is not particularly limited, but is preferably 4.0 or more, and more preferably 4.5 or more. The pH is preferably 7.0 or less, and more preferably 6.5 or less. From the viewpoint of stably producing the above-mentioned surface-treated steel sheet, it is preferable to monitor the pH of the electrolyte in the cathodic electrolysis process and maintain it within the above pH range.

The current density in the cathodic electrolysis is not particularly limited, and may be appropriately adjusted so that a desired surface treatment layer is formed. However, if the current density is excessively high, the burden on the cathodic electrolysis treatment device becomes excessive. Therefore, the current density is preferably 200.0 A/dm ² or less, and more preferably 100.0 A/dm ² or less. In addition, although there is no particular limit to the lower limit of the current density, if the current density is excessively low, hexavalent Cr may be generated in the electrolytic solution, which may cause the stability of the bath to be lost. Therefore, the current density is preferably 5.0 A/dm ² or more, and more preferably 10.0 A/dm ² or more.

The number of times that the steel sheet is subjected to cathodic electrolysis is not particularly limited and can be any number of times. In other words, cathodic electrolysis can be performed using an electrolytic treatment device having any number of passes, such as one or more. For example, it is also preferable to perform cathodic electrolysis continuously by passing the steel sheet (steel strip) through multiple passes while transporting it. Note that, since increasing the number of cathodic electrolysis treatments (i.e., the number of passes) requires a corresponding number of electrolytic cells, it is preferable to set the number of cathodic electrolysis treatments (number of passes) to 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, and productivity decreases. Therefore, the electrolysis time per pass is preferably 5 seconds or less, and more preferably 3 seconds or less. There is no particular limit to the lower limit of the electrolysis time per pass, but if the electrolysis time is too short, it becomes necessary to increase the line speed accordingly, making control difficult. Therefore, the electrolysis time per pass is preferably 0.005 seconds or more, and more preferably 0.01 seconds or more.

The amount of chromium deposited in the chromium-containing layer formed by cathodic electrolysis can be controlled by the total electric charge density, which is the product of the current density, the electrolysis time, and the number of passes. As described above, if the amount of chromium deposited is too small, the corrosion resistance of the BPA-free painted part is impaired, and if the amount of chromium deposited is too large, cohesive failure may occur in the chromium-containing layer during processing. Therefore, from the viewpoint of more stably ensuring the corrosion resistance of the BPA-free painted part, it is preferable to control the total electric charge density so that the amount of chromium deposited per side of the steel sheet in the chromium-containing layer is within an appropriate range. However, since the relationship between the amount of chromium deposited per side of the steel sheet in the chromium-containing layer and the total electric charge density varies depending on the configuration of the device used in the cathodic electrolysis process, the actual electrolysis conditions can be adjusted according to the device.

The type of anode used when performing cathodic electrolysis is not particularly limited, and any anode can be used. It is preferable to use an insoluble anode as the 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 as the insoluble anode. More specifically, examples of the insoluble anode include an anode in which the surface of Ti as a substrate is coated with platinum, iridium oxide, or ruthenium oxide.

In the above-mentioned cathodic treatment process, the concentration of the electrolyte constantly changes due to the formation of a chromium-containing layer on the steel sheet, the introduction and removal of the liquid, the evaporation of water, etc. The change in concentration of the electrolyte in the cathodic electrolytic treatment process varies depending on the configuration of the equipment and the manufacturing conditions, so from the perspective of more stable production of surface-treated steel sheets, it is preferable to monitor the concentrations of the components contained in the electrolyte in the cathodic electrolytic treatment process and maintain them within the concentration ranges described below.

It is preferable to rinse the steel sheet at least once after the cathodic electrolytic treatment process. By rinsing with water, the electrolyte remaining on the surface of the steel sheet can be removed.

The water washing can be carried out by any method without any particular limitation. For example, a water washing tank can be provided downstream of the immersion tank for carrying out the immersion treatment, and the steel sheet after immersion can be continuously immersed in water. Water washing can also be carried out by spraying water onto the steel sheet after 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 it is preferably 100 μS/m or less, more preferably 50 μS/m or less, and even more preferably 30 μS/m or less.

The temperature of the water used for the washing 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 for washing is preferably 95°C or less. On the other hand, the lower limit of the temperature of the water used for washing is not particularly limited, but it is preferably 0°C or higher. The temperature of the water used for the washing may be room temperature.

After the above water washing, 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 applied. From the viewpoint of suppressing deterioration of the surface treatment film, it is preferable that the temperature during the drying process be 100°C or less. The lower limit is not particularly limited, but is usually around room temperature.

Prior to the steel sheet surface conditioning process, the steel sheet may be pretreated as desired. The pretreatment is preferably at least one of degreasing, pickling, and water washing.

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

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 adjusted in the subsequent steel sheet surface adjustment process. The pickling can be performed by any method without any particular limitation. After the pickling, it is preferable to rinse the steel sheet with water to remove the pickling solution adhering to the surface. When an aqueous solution containing sulfate ions is used as the pickling solution, it is preferable to directly subject the steel sheet to the steel sheet surface adjustment process.

There is no particular limitation on the method for preparing the electrolyte used in the cathode electrolysis process, but by preparing the electrolyte as described below, it can be used in the cathode electrolysis process stably for a longer period of time.

[Preparation of electrolyte solution]
(i) Mixing When preparing the above-mentioned electrolytic solution, it is preferable to first mix a trivalent chromium ion source, a carboxylic acid compound, and water to prepare an aqueous solution.

As the trivalent chromium ion source, any compound that can supply trivalent chromium ions can be used. 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 must be 0.05 mol/L or more in terms of trivalent chromium ions, preferably 0.08 mol/L or more, and more preferably 0.10 mol/L or more. The upper limit of the content of the trivalent chromium ion source is not particularly limited, but is preferably 1.50 mol/L or less in terms of trivalent chromium ions, and more preferably 1.30 mol/L or less. As the trivalent chromium ion source, BluCr (registered trademark) TFS A from Atotech 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 carboxylate, and is preferably at least one of an aliphatic carboxylic acid and a salt of an aliphatic carboxylic acid. The carbon number of the aliphatic carboxylic acid is preferably 1 to 10, and more preferably 1 to 5. The carbon number of the aliphatic carboxylate is preferably 1 to 10, and 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, and more preferably 0.15 mol/L or more. The content of the carboxylic acid compound is preferably 5.5 mol/L or less, and more preferably 5.3 mol/L or less. As the carboxylic acid compound, BluCr (registered trademark) TFS B from Atotech 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 electrolytic treatment process and to improve the stability of the above-mentioned electrolyte, it is preferable to further contain at least one type of halide ion in the aqueous solution. The content of the halide ion is not particularly limited, but is preferably 0.05 mol/L or more, and more preferably 0.10 mol/L or more. The content of the halide ion is preferably 3.0 mol/L or less, and more preferably 2.5 mol/L or less. To contain the halide ion, Atotech's BluCr (registered trademark) TFS C1 and BluCr (registered trademark) TFS C2 can be used.

It is preferable not to add hexavalent chromium to the above-mentioned aqueous solution. It has been confirmed that hexavalent chromium is not formed in principle in the cathodic electrolytic treatment process, but even if a small amount of hexavalent chromium is formed at the anode, it is immediately reduced to trivalent chromium, so the concentration of hexavalent chromium in the electrolyte does not increase.

It is preferable that the above-mentioned aqueous solution does not intentionally add metal ions other than trivalent chromium ions. The above-mentioned metal ions are not limited, but include Cu ions, Zn ions, Fe ions, Sn ions, Ni ions, etc., and each is preferably 0 mg/L to 40 mg/L, more preferably 0 mg/L to 20 mg/L, and most preferably 0 mg/L to 10 mg/L. Of the above-mentioned metal ions, Fe ions may dissolve in the above-mentioned electrolyte in the cathodic electrolytic treatment process and the immersion process and co-deposit in the film, but do not affect the corrosion resistance of the BPA-free painted part. It is preferable that the Fe ion concentration is in the above range when the bath is made, but it is also preferable to maintain the Fe ion concentration in the electrolyte in the above range in the cathodic electrolytic treatment process and the immersion process. If the Fe ions are controlled within the above range, the formation of the chromium-containing layer is not inhibited, and the required amount of the chromium-containing layer can be formed.

(ii) Adjustment of pH and temperature Next, it is preferable to prepare the electrolytic solution by adjusting the pH of the aqueous solution to 4.0 to 7.0 and adjusting the temperature of the aqueous solution to 40 to 70° C. As described above, in order to stably provide the aqueous solution for the cathodic electrolytic treatment step 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: 4.0-7.0
In preparing the electrolytic solution, it is preferable to adjust the pH of the aqueous solution after mixing to 4.0 to 7.0. It is more preferable to adjust the pH to 4.5 or more. It is more preferable to set it to 6.5 or less.

Any reagent can be used to adjust the pH. For example, it is preferable to use hydrochloric acid, sulfuric acid, nitric acid, etc. to lower the pH, and ammonia water, etc. to raise the pH.

Temperature: 40-70°C
In preparing the electrolytic solution, it is preferable to adjust the temperature of the aqueous solution after mixing to 40 to 70° C. The time for which the aqueous solution is kept in the temperature range of 40 to 70° C. is not particularly limited.

The electrolyte obtained by the above procedure can be stably used for the cathodic electrolysis process for a long period of time. The electrolyte produced by the above procedure can be stored at 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 procedure described below and their properties were evaluated.

(Preparation of Electrolyte)
First, electrolyte solutions having compositions A to G shown in Table 1 were prepared under the conditions shown in Table 1. That is, each component shown in Table 1 was mixed with water to prepare an aqueous solution, and then the aqueous solution was adjusted to the pH and temperature shown in Table 1. Note that electrolyte solution G corresponds to the electrolyte solution used in the examples of Patent Document 4. Ammonia water was used to increase the pH in all cases, and sulfuric acid was used for electrolyte solutions A, B, and G, hydrochloric acid was used for electrolyte solutions C and D, and nitric acid was used for electrolyte solutions E and F to decrease the pH.

(Pretreatment of steel sheets)
A cold-rolled steel sheet was used as the steel sheet. More specifically, a can steel sheet (T4 base sheet) having a sheet thickness of 0.17 mm was used. The steel sheet was subjected to electrolytic degreasing, water washing, and pickling in this order as pretreatment. For the pickling, an aqueous sulfuric acid solution having a sulfate ion concentration shown in Table 2 was used, and the steel sheet was immersed in the aqueous solution to perform pickling. The steel sheet after the pickling was subjected to the subsequent steel sheet surface conditioning step without being washed with water.

(Steel plate 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 remove the pickling solution. The amount of adhesion was adjusted to the amount shown as "amount of aqueous solution" in Table 2. Thereafter, the plate was held for the holding time shown in Table 2 while maintaining the amount of adhesion, and then washed with water to obtain the above pickling solution. The treatment solution was removed.

(Cathodic electrolytic treatment process)
Next, the steel sheet was subjected to cathodic electrolysis under the conditions shown in Table 2. The electrolytic solution during cathodic electrolysis was maintained at the pH and temperature shown in Table 1. The current density during cathodic electrolysis was 40 A/ ^dm2 , and the electrolysis time and number of passes were appropriately changed. As the anode during cathodic electrolysis, an insoluble anode formed by coating iridium oxide on a Ti substrate was used. After cathodic electrolysis, the steel sheet was washed with water having an electrical conductivity of 100 μS/m or less, and dried at room temperature using a blower.

For each of the obtained surface-treated steel sheets, the amount of chromium deposited per side of the chromium-containing layer and the amount of chromium oxide deposited per side of the steel sheet were measured by the method described above. Furthermore, for each of the obtained surface-treated steel sheets, the area 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 the oxygen-enriched area was determined by determining, by the method described above, an oxygen-enriched area in the vicinity where O was contained at an atomic concentration of 10% or more. Furthermore, the coverage of the oxygen-enriched area was measured by the method described above. The measurement results are shown in Table 3.

In each of the examples, the chromium-containing layer obtained by cathodic electrolysis contained chromium compounds such as chromium oxide and chromium carbide in addition to metallic chromium. The total content of the elements constituting the metallic chromium and 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 of BPA-free painted parts)
Next, the corrosion resistance of the BPA-free painted portion of each of the obtained surface-treated steel sheets was evaluated according to the procedure described below.

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

A cross cut was made in the obtained BPA-free painted steel sheet, penetrating all the way to the base steel sheet, and then an Erichsen testing machine was used to form a protrusion 4 mm high centered on the intersection of the cross cut to prepare a test specimen.

Next, a corrosion resistance test was conducted using the test specimens according to the following procedure. First, the test specimens were immersed in a Teflon (registered trademark) container containing test liquid and covered with a lid. In this state, they were subjected to a retort treatment at a temperature of 121°C for one hour. The test specimens were then removed from the container, washed with water to remove the test liquid, and then dried with a blower.

After drying, the test specimens were subjected to tape peeling twice, and then the surfaces of the test specimens were observed using a microscope or similar tool. The areas of peeled paint and areas of discoloration such as rust were visually evaluated and scored on a 5-point scale, with 1 being the poorest and 5 being the best performance. The same evaluation was performed on two samples per level, and the arithmetic mean of the scores was calculated and used as an index of the corrosion resistance of the BPA-free painted parts. In practice, a score equal to or higher than that of conventional TFS can be evaluated as excellent in corrosion resistance of the BPA-free painted parts, but a score equal to or higher than that of conventional TFS and a score of 3.0 or higher is more preferable.

In order to simulate the difference in the corrosive 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 shown 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: 22.5g/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: 30g/L

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

Claims (7)

A steel plate,
A surface-treated steel sheet comprising 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 the vicinity of the interface between the steel sheet and the chromium-containing layer,
The vicinity is a region in which the atomic ratio of Fe to Cr is in the range of 0.7 to 1.3. The surface-treated steel sheet according to claim 1, wherein the coverage of the oxygen-enriched region in the vicinity is 50% or more. The surface-treated steel sheet according to claim 1 , wherein the chromium-containing layer has a chromium coating amount of 40.0 to 500.0 mg/ ^m2 per side. The chromium coating amount of the chromium-containing layer is 40.0 to 500.0 mg/m per side. ² The surface-treated steel sheet according to claim 2 . The surface-treated steel sheet according to any one of claims 1 to 4 , wherein the chromium-containing layer has a chromium oxide coating amount of 40.0 mg/ ^m2 or less per side. A method for 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 the steps of:
a steel sheet surface conditioning step of contacting the steel sheet with an aqueous solution containing sulfate ions and holding the aqueous solution on the surface of the steel sheet in a state in which the aqueous solution is present in an amount of more than 30.0 g/ ^m2 and not more than 60.0 g/ ^m2 for 0.10 to 20.0 seconds;
and a cathodic electrolysis step of cathodic electrolysis of the steel sheet in an electrolytic solution containing 0.05 mol/L or more of trivalent chromium ions. The method for producing a surface-treated steel sheet according to claim 6 , wherein the electrolytic solution is prepared by mixing a trivalent chromium ion source, a carboxylic acid compound, and water, adjusting the pH to 4.0 to 7.0, and adjusting the temperature to 40 to 70° C.