WO2025204346A1 - Surface-treated steel sheet and method for producing same - Google Patents

Abstract

Provided is a surface-treated steel sheet that can be produced without using hexavalent chromium and that has excellent film corrosion resistance, paint corrosion resistance, film wet adhesiveness, and weldability. The surface-treated steel sheet comprises a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet. The S/L determined by a prescribed method is 0.10 to 0.70. The chromium-containing layer has a C to Cr atomic ratio of 0.2% to 50.0%.

Description

Surface-treated steel sheet and its manufacturing method

The present invention relates to a surface-treated steel sheet, and in particular to a surface-treated steel sheet that has excellent film corrosion resistance, paint corrosion resistance, film wet adhesion, and weldability. 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) has excellent corrosion resistance, weldability, and workability, and is easy to manufacture, so it has been used for over 200 years as a material for various metal cans such as beverage cans, food cans, pail cans, and 18-liter cans.

However, because Sn is an expensive material, tin-free steel sheet (TFS), a surface-treated steel sheet that does not use Sn, has been developed. Tin-free steel sheet is a surface-treated steel sheet in which a metallic chromium layer and a chromium oxide layer are formed on the surface of the steel sheet, and is typically produced by electrolyzing the steel sheet in an electrolyte containing hexavalent chromium (Patent Documents 1 to 3). Due to its excellent corrosion resistance, tin-free steel sheet is now extremely commonly used as a steel sheet for containers, replacing tinplate. However, because typical tin-free steel sheet has an insulating chromium oxide layer on the surface, it has poor weldability.

In response to this, a type of tin-free steel sheet with excellent weldability is known, in which granular protrusions are formed on the surface of the steel sheet by performing anodic electrolysis between multiple cathodic electrolysis treatments during electrochromic chromium plating (Patent Documents 4 and 5).

However, in recent years, growing environmental awareness has led to a trend toward restricting the use of hexavalent chromium worldwide. Therefore, even in the field of surface-treated steel sheets used for containers, etc., there is a demand for the establishment of manufacturing methods that do not use hexavalent chromium.

Methods for forming surface-treated steel sheets without using hexavalent chromium are known, for example, from patent documents 6, 7, and 8. In these methods, a surface treatment layer is formed by electrolysis in an electrolyte containing a trivalent chromium compound such as basic chromium sulfate.

Japanese Unexamined Patent Publication No. 58-110695 Japanese Unexamined Patent Publication No. 55-134197 Japanese Patent Application Publication No. 57-035699 Japanese Unexamined Patent Publication No. 61-213399 Japanese Patent Application Publication No. 186894/1983 Special table 2016-505708 publication Special Publication No. 2015-520794 Patent No. 6593574

The methods proposed in Patent Documents 6, 7, and 8 make it possible to form a surface treatment layer without using hexavalent chromium. Furthermore, Patent Documents 6, 7, and 8 also show that these methods can produce surface-treated steel sheets that exhibit excellent adhesion to resin films in humid environments (hereinafter referred to as "film wet adhesion"), film corrosion resistance, and paint corrosion resistance.

However, although surface-treated steel sheets obtained using conventional methods such as those proposed in Patent Documents 6, 7, and 8 have excellent film wet adhesion, film corrosion resistance, and paint corrosion resistance, they have poor weldability. Therefore, their performance is insufficient to be used as a replacement for surface-treated steel sheets produced using methods that use hexavalent chromium.

Therefore, there is a demand for surface-treated steel sheets that can be manufactured without using hexavalent chromium and that combine film corrosion resistance, paint corrosion resistance, film wet adhesion, and weldability.

The present invention was made in consideration of the above-mentioned circumstances, and its purpose is to provide a surface-treated steel sheet that can be produced without using hexavalent chromium and that has excellent film corrosion resistance, paint corrosion resistance, film wet adhesion, and weldability.

As a result of extensive research into achieving the above objectives, 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 disposed on at least one surface of the steel sheet, parameters relating to the surface properties of the chromium-containing layer determined by a predetermined method and the atomic ratio of C to Cr in the chromium-containing layer are each controlled within specific ranges. This makes it possible to obtain a surface-treated steel sheet with excellent film corrosion resistance, paint corrosion resistance, film wet adhesion, and weldability.

(2) The above-mentioned surface-treated steel sheet can be produced by carrying out cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using an electrolytic solution prepared by a specified method, while controlling the electrical density of anodic electrolysis A1 and cathodic electrolysis C2 within a specific range. Furthermore, by preparing the electrolytic solution by a specified method, it is possible to prevent the hexavalent chromium in the electrolytic solution from increasing during film formation.

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,
a roughness curve of the chromium-containing layer obtained by applying a low-pass filter with a cutoff value _λs of 2 nm and a high-pass filter with a cutoff value _λc of 65 nm to a cross-sectional curve extracted from a cross-sectional image of the chromium-containing layer, wherein S is a total length in the x-axis direction of a region whose height from the mean line of the roughness curve exceeds 1.9 nm and L is an evaluation length of the roughness curve, and S/L is 0.10 or more and 0.70 or less;
The chromium-containing layer has an atomic ratio of C to Cr of 0.2% or more and 50.0% or less.

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

3. The surface-treated steel sheet according to item 1 or 2 above, wherein the chromium oxide deposition amount of the chromium-containing layer is 40.0 mg/ ^m2 or less per side.

4. 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:
an electrolyte solution preparation step of preparing an electrolyte solution containing trivalent chromium ions;
a coating formation step of forming the chromium-containing layer,
In the electrolyte solution preparation step,
a trivalent chromium ion source, a carboxylic acid compound, and water are mixed;
The electrolyte solution is prepared by adjusting the pH to 4.0 to 7.0 and the temperature to 40 to 70°C.
In the film forming step,
The steel sheet is subjected to cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using the electrolytic solution,
The electricity density of the anodic electrolysis treatment A1 is 0.50 C/dm ² or more and 20.00 C/dm ² or less,
The method for producing a surface-treated steel sheet, wherein the electricity density of the cathodic electrolysis treatment C2 is 1.0 C/dm ² or more and less than 50.0 C/dm ² .

According to the present invention, it is possible to provide a surface-treated steel sheet that has excellent film corrosion resistance, paint corrosion resistance, film wet adhesion, and weldability without using hexavalent chromium. The surface-treated steel sheet of the present invention can be suitably used as a material for containers, etc.

FIG. 1 is a diagram showing a roughness curve.

The following provides a specific description of how the present invention can be implemented. Note that the following description is merely an example of a preferred embodiment of the present invention, and the present invention is not limited to this.

In one embodiment of the present invention, the surface-treated steel sheet comprises a steel sheet and a chromium-containing layer disposed on at least one surface of the steel sheet. In the present invention, it is important that the S/L ratio (described below) is 0.10 or more and 0.70 or less, and that the chromium-containing layer has an atomic ratio of C to Cr of 0.2% or more and 50.0% or less. Each of the constituent elements of the surface-treated steel sheet is explained below.

[Steel plate]
The steel sheet is not particularly limited, and any steel sheet can be used. The steel sheet is preferably 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 method for manufacturing the steel sheet is also not particularly limited, and a steel sheet manufactured by any method can be used. Usually, a cold-rolled steel sheet can be used as the steel sheet. 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 chemical composition of the steel plate is not particularly limited, but for example, steel plate having a chemical composition specified in ASTM A623M-09 can be suitably used.

In one embodiment of the present invention, in mass %:
C: 0.0001 to 0.13%,
Si: 0 to 0.020%,
P: 0 to 0.020%,
S: 0 to 0.030%,
Al: 0 to 0.20%, and N: 0 to 0.040%, and optionally further containing, in mass%,
Mn: 0.01-0.60%,
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: 0-0.020%,
Contains at least one selected from the group consisting of Sn: 0 to 0.020% and Sb: 0 to 0.020%,
It is preferable to use a steel sheet having a composition with the balance consisting of Fe and unavoidable impurities. Of the above composition, the lower the content of Si, P, S, Al, and N, the more preferable it is. Mn, Cu, Ni, Cr, Mo, Ti, Nb, B, Ca, Sn, and Sb are components that can be added optionally.

The thickness of the steel plate is not particularly limited, but is preferably 0.60 mm or less. Note that "steel plate" is defined here to include "steel strip." On the other hand, the lower limit of the thickness is not particularly limited, but is preferably 0.10 mm or more.

[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 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. In addition to the metallic chromium and the chromium compound, the chromium-containing layer may also contain impurities. Examples of such 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 exist in the chromium-containing layer in a metallic state, but may also exist 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 the ratio, expressed as a percentage, 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 element constituting the metallic chromium and chromium compound contained in the chromium-containing layer using X-ray photoelectron spectroscopy (XPS) and adding them up. When measuring the content using XPS, the content (atomic ratio) of each element can be calculated using the relative sensitivity factor method from the integrated intensity of the peak corresponding to that element.

For example, the content of chromium carbide ( _Cr2C3 ) can be determined from the integrated intensity of the peak of C 1s carbide appearing around 281.0 _eV . For example, if 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 content of _Cr2C3 is ₆ × (2 + 3) / 3 = 10 atomic %.

For chromium oxide, _{the Cr2O3 content} can be determined from the integrated intensity of the Cr2p oxide peak appearing near 576.7 _eV . Also, the CrO3 content can be determined from the integrated intensity of the Cr2p oxide peak appearing near 579.2 eV.

Similarly, the content of other chromium compounds can be determined using, for example, the integrated intensity of the peaks listed below.
Chromium sulfide (Cr ₂ S ₃ ): S 2p sulfide peak appearing around 162.3 eV Chromium nitride (CrN): N 1S peak appearing around 397.3 eV Chromium chloride (CrCl ₃ ): Cl 2p peak appearing around 199.8 eV Chromium bromide (CrBr ₃ ): Br 3d peak appearing around 69.1 eV Chromium boride (CrB): Br 1s peak appearing around 188.2 eV

Meanwhile, the metallic chromium content can be determined by calculating the Cr content from the integrated intensity of the Cr 2p peak that appears around 573.8 eV, and then subtracting the content of Cr atoms contained as chromium compounds from the chromium content.

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

The total content refers to the value at the half-thickness position of the chromium-containing layer. The half-thickness position can be determined by the following procedure. First, the chromium-containing layer is sputtered from its outermost surface, while the total content of elements constituting metallic chromium and chromium compounds and the Fe content are measured using the method described above. The position (depth) where the measured total content of elements constituting metallic chromium and chromium compounds and the Fe content are equal is determined as the interface between the chromium-containing layer and the steel sheet. The thickness from the outermost surface of the chromium-containing layer to this interface is defined as the thickness of the chromium-containing layer, and its half-thickness position is determined.

The 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 ions 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 that make up the chromium-containing layer is not particularly limited; for example, they may be separated into separate layers within the chromium-containing layer, or they may be mixed throughout the chromium-containing layer. In other words, the spatial structure of the components that make up the chromium-containing layer can include either or both separate layers and mixed layers.

The chromium coating weight of the chromium-containing layer is not particularly limited. However, an excessive chromium coating weight of the chromium-containing layer may impair weldability and cause deterioration of adhesion due to cohesive failure. Therefore, from the viewpoint of more stably ensuring weldability and film wet adhesion, the chromium coating weight of the chromium-containing layer is preferably 500.0 mg/ ^m2 or less per side, and more preferably 450.0 mg/ ^m2 or less. On the other hand, from the viewpoint of further improving the coating corrosion resistance and film corrosion resistance, the chromium coating weight of the chromium-containing layer is preferably 40.0 mg/ ^m2 or more per side, and more preferably 50.0 mg/ ^m2 or more.

The chromium deposition weight is measured using an X-ray fluorescence spectrometer according to the following procedure. First, the Cr amount (total Cr amount) in the surface-treated steel sheet is measured using the X-ray fluorescence spectrometer. Next, the Cr amount (original sheet Cr amount) is measured using the X-ray fluorescence spectrometer on the steel sheet before the chromium-containing layer is formed or on the steel sheet after the chromium-containing layer has been stripped off. The value obtained by subtracting the original sheet Cr amount from the total Cr amount is the Cr deposition weight of the chromium-containing layer. To strip the chromium-containing layer, for example, a commercially available hydrochloric acid-based chromium plating stripper can be used.

[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 chromium oxide deposition amount of the chromium-containing layer is not particularly limited. However, if the chromium oxide deposition amount of the chromium-containing layer is excessive, it may impair weldability and cause deterioration of adhesion due to cohesive failure. Therefore, from the viewpoint of more stably ensuring weldability and film wet adhesion, the chromium oxide deposition amount of the chromium-containing layer is preferably 40.0 mg/ ^m2 or less per side, and more preferably 35.0 mg/ ^m2 or less. On the other hand, the chromium-containing layer may not contain any chromium oxide at all. Therefore, the lower limit of the chromium oxide deposition amount of the chromium-containing layer is not particularly limited, and may be 0.0 mg/ ^m2 per side.

The chromium oxide deposition amount is measured using an X-ray fluorescence analyzer according to 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 by immersing it in 7.5N NaOH at 90°C for 10 minutes to remove the chromium oxide. After the alkali treatment, the surface-treated steel sheet is thoroughly rinsed with water, and the Cr amount (post-alkali-treatment Cr amount) is measured again using the X-ray fluorescence analyzer. The value obtained by subtracting the post-alkali-treatment Cr amount from the total Cr amount is the chromium oxide deposition amount of the chromium-containing layer.

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. Chromium-containing layers manufactured by the method described below generally contain an amorphous phase, and may also contain a crystalline phase. The mechanism by which the chromium-containing layer is formed is unclear, but it is thought that partial crystallization occurs when the amorphous phase is formed, resulting in a chromium-containing layer containing both an amorphous and a crystalline phase. 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. The lower limit of the area ratio of the crystalline region is not particularly limited, and may be 0%.

The crystalline region in the chromium-containing layer can be confirmed by preparing a chromium-containing single-layer sample by etching the substrate steel sheet and observing the sample from the surface side using a TEM or scanning transmission electron microscope (STEM). The method for preparing the chromium-containing single-layer sample is not particularly limited, but it can be prepared, for example, by irradiating the steel sheet with an ion beam such as Ar from the substrate steel sheet side and ion milling the steel sheet. When preparing the chromium-containing single-layer region using an ion beam, a field of view of a chromium single-layer region of several μm2 or ^more can be ensured by irradiating the ion beam with an acceleration voltage of 5 kV or less and an incident angle relative to the substrate steel sheet in the range of 1 to 5 degrees. In this case, the bottom surface of the chromium-containing layer may also be milled to some extent, which may result in a thinner film thickness of the chromium-containing layer, but this does not affect the measurement results of the area ratio of the crystalline region.

The area ratio of crystalline regions in a chromium-containing layer can be measured using a TEM. Specifically, a diffraction pattern of the chromium-containing layer is obtained using selected-area diffraction with a TEM, and dark-field images are obtained at all diffraction spots in the pattern. The areas that appear brightest in the dark-field image are determined to be crystalline regions. The area of the obtained crystalline regions is calculated using image processing, and the area ratio of the crystalline regions is calculated by dividing the area by 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.

[Atomic ratio of C to Cr]
In the present invention, it is important that the chromium-containing layer contains C, and that the atomic ratio of C to Cr in the chromium-containing layer is 0.2% or more and 50.0% or less. When the atomic ratio of C is 0.2% or more and 50.0% or less, the chromium-containing layer is destroyed by volume change during welding pressure application or initial heat input, making it easier to conduct current, thereby lowering the minimum welding current limit, i.e., improving weldability. If the atomic ratio of C is too low, the above-mentioned effect of improving weldability cannot be obtained. Therefore, the atomic ratio of C is set to 0.2% or more. From the viewpoint of ensuring more stable weldability, the atomic ratio of C is preferably set to 0.3% or more. On the other hand, if the atomic ratio of C is excessive, the weld heat-affected zone will excessively harden, causing weld cracking. Therefore, the atomic ratio of C is set to 50.0% or less. From the viewpoint of ensuring more stable weldability, the atomic ratio of C is preferably set to 40.0% or less.

The atomic ratio of C to Cr in the chromium-containing layer is measured using XPS according to the following procedure. First, sputtering is performed from the outermost layer to a depth of at least 0.2 nm in _SiO2 equivalent, and the integrated intensity of the narrow spectrum of Cr2p and C1s is determined. From the integrated intensity, the atomic ratio is quantified using the relative sensitivity factor method, and the C atomic ratio/Cr atomic ratio is calculated. For the XPS measurement, for example, a scanning X-ray photoelectron spectrometer PHI X-tool manufactured by ULVAC-PHI, Inc. The X-ray source may be a monochromatic AlKα ray, with a voltage of 15 kV, a beam diameter of 100 μmφ, and a take-off angle of 45°. The sputtering conditions are Ar ion with an acceleration voltage of 1 kV and a sputtering rate of 1.50 nm/min in _SiO2 equivalent.

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 the chromium-containing layer on the steel sheet, the carboxylic acid compound decomposes and is incorporated into the film.

The chromium-containing layer may contain Fe. There is no particular upper limit to the Fe content in the chromium-containing layer, but it is preferable that the atomic ratio relative to Cr is 100% or less. The chromium-containing layer may not contain Fe, and therefore the lower limit of the atomic ratio relative 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, traces of Fe contained in the steel plate dissolve 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, and Br. However, the presence of these elements may reduce the corrosion resistance of the coating. For this reason, the total atomic ratio of elements other than Cr, O, Fe, and C to Cr is preferably 3% or less, and it is even more preferable that they are completely absent (0%). The content of the above elements is not particularly limited, but can be measured, for example, by XPS, in the same way as the C content.

[S/L]
In the present invention, it is important that the S/L ratio determined by a predetermined method is 0.10 to 0.70. S/L is the ratio of the total length S in the x-axis direction of the region of the roughness curve whose height from the mean line exceeds 1.9 nm to the evaluation length L of the roughness curve. Here, the roughness curve was obtained by applying a low-pass filter with a cutoff value _λs = 2 nm and a high-pass filter with a cutoff value _λc = 65 nm to a cross-sectional curve extracted from a cross-sectional image of the chromium-containing layer.

First, we will explain in detail how to calculate S/L.

First, a cross-sectional image of the chromium-containing layer is obtained. Specifically, a dark-field image is taken using an STEM of a cross section perpendicular to the surface of the coated steel sheet, and this is used as the cross-sectional image. STEM has a high enough spatial resolution for observing the chromium-containing layer, and dark-field observation allows the chromium-containing layer region to be clearly distinguished from the background.

In order to obtain accurate cross-sectional curves and roughness curves, the resolution of the cross-sectional image shall be 0.5 nm or less per pixel. Furthermore, in order to obtain average information about the chromium-containing layer, images shall be taken at a magnification that allows a chromium-containing layer with a length of 150 nm or more to be confirmed, and cross-sectional images shall be taken from at least five randomly selected fields of view.

The cross-sectional image is set so that the left-right direction coincides with or nearly coincides with the longitudinal direction of the chromium-containing layer, and the up-down direction coincides with or nearly coincides with the thickness direction of the chromium-containing layer. If the line connecting the endpoints of the chromium-containing layer is tilted by 5 degrees or more from the left-right direction, the captured cross-sectional image is rotated using image processing. However, if rotation processing is performed, the resolution of the image before rotation must be 0.25 nm or less per pixel.

To avoid unnecessary information reducing the accuracy of cross-sectional curve extraction, the cross-sectional image is trimmed so that it does not include areas more than 100 nm above or below the chromium-containing layer. Furthermore, to reduce the variability in results due to noise in the cross-sectional image, a median filter with a kernel of 3 x 3 or larger is applied to the observed dark-field image to remove noise. However, to avoid underestimating the surface roughness, the length of one side of the kernel is set to 2 nm or less.

Next, a cross-sectional curve is extracted from the cross-sectional image after noise has been removed. The cross-sectional curve is a curve that follows the surface of the chromium-containing layer on the surface side of the surface-treated steel sheet. First, the cross-sectional image is segmented into the chromium-containing layer and base steel sheet regions and other regions (for example, the background, and layers such as paint and film that may be applied to the surface). Segmentation can be performed using any of a method that uses a brightness threshold, a method that uses manual painting, or a method that uses image analysis by machine learning. Then, in the obtained chromium-containing layer and base steel sheet regions, a cross-sectional curve is extracted by connecting the highest points in the thickness direction of the chromium-containing layer.

Next, a low-pass filter with a cutoff value λ _s =2 nm and a high-pass filter with a cutoff value λ _c =65 nm are applied to the profile curve p(x) extracted by the above procedure to obtain a roughness curve r(x).

Specifically, a Gaussian filter, which is a phase compensation filter, is used as the filter applied to the profile curve p(x). The weighting function s _λ (x) of the Gaussian filter is defined by the following equation.
where x is the coordinate, λ is the cutoff value, and a is a constant (a=0.4697). Using the profile curve p(x) and the above weighting function, the roughness curve r(x) is calculated by the following equation.

Figure 1 shows a schematic diagram of the roughness curve r(x).

From the roughness curve r(x), S/L is calculated using the following formula.
_l1 , _l2 , ..., _ln are the lengths in the x-axis direction of each region of the roughness curve whose height from the mean line exceeds 1.9 nm, and S is _the sum of these lengths (S = _Σnln ). The mean line conforms to JIS B0601:2013 and coincides with the x-axis in Figure 1. L is the evaluation length (length in the x-axis direction) of the roughness curve. For example, the roughness curve r(x) in Figure 1 has five regions with a height of 1.9 nm or more. When the lengths of each region in the x-axis direction are _l1 , ₁₂ , ..., _l5 , S = _l1 + _l2 + _l3 + _l4 + _l5 , and S/L = ( _l1 + _l2 + _l3 + _l4 + _l5 )/L. Note that when f = S/L, f can also be expressed as follows:

Here, c=1.9 nm and h is a step function defined by the following equation.
Here, S/L is calculated from cross-sectional images of five or more fields selected at random using the method described above, and the average value is used as S/L.

The cutoff values _λs and _λc are smaller than those commonly used to derive roughness curves, and by using these cutoff values, parameters suitable for expressing fine surface morphology can be obtained. Furthermore, by setting 1.9 nm as the reference height, the state of granular protrusions present on the surface of the chromium-containing layer can be expressed. In the present invention, the S/L obtained using the cutoff value and reference height accurately reflects factors of the surface morphology of the chromium-containing layer that affect weldability and film wet adhesion.

Next, we will explain in detail why the S/L ratio calculated using the above method is set to 0.10 or more and 0.70 or less.

S/L corresponds to the area ratio of granular protrusions of a specified height present on the surface of the chromium-containing layer. If S/L is small, there will be few granular protrusions of sufficient size on the surface of the chromium-containing layer, making it difficult for the metal oxide on the surface to break down when pressure is applied, and no electrical continuity can be obtained as a starting point for welding. Therefore, if S/L is less than 0.10, weldability will deteriorate, especially in welding with high pressure. Therefore, S/L should be 0.10 or greater, preferably 0.15 or greater, and more preferably 0.20 or greater. On the other hand, if S/L exceeds 0.70, the area ratio of the granular protrusions will be too high, resulting in an insufficient anchoring effect and reduced wet film adhesion. Therefore, S/L should be 0.70 or less, preferably 0.60 or less, and more preferably 0.45 or less.

[Manufacturing 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-described properties can be produced by the method described below.

A method for producing a surface-treated steel sheet according to one embodiment of the present invention is a method for producing a surface-treated steel sheet having a chromium-containing layer disposed on at least one surface of the steel sheet, and includes the following steps (1) and (2). Each step will be described below.
(1) An electrolyte preparation step for preparing an electrolyte containing trivalent chromium ions; (2) A film formation step for forming a chromium-containing layer.

[Electrolyte preparation process]
(i) Mixing In the electrolytic solution preparation step, first, a trivalent chromium ion source, a carboxylic acid compound, and water are mixed to prepare an aqueous solution.

Any compound capable of supplying 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 content of the trivalent chromium ion source in the aqueous solution is not particularly limited, but is preferably 3 g/L or more, and more preferably 5 g/L or more, calculated as trivalent chromium ions. The content of the trivalent chromium ion source is preferably 50 g/L or less, and more preferably 40 g/L or less. BluCr (registered trademark) TFS A from Atotech can be used as the trivalent chromium ion source.

Carboxylic acid stabilizes trivalent chromium ions in the electrolyte. Therefore, adding a carboxylic acid compound to the aqueous solution can suppress an increase in the hexavalent chromium concentration during the film formation process, particularly the anodic electrolysis process A1, described below. While carboxylic acid compounds are not typically used in electrolysis processes using hexavalent chromium, the present invention requires the addition of a carboxylic acid compound to the aqueous solution. 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 an aliphatic carboxylic acid salt. The aliphatic carboxylic acid preferably has 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms. The aliphatic carboxylic acid salt preferably has 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms. The content of the carboxylic acid compound is not particularly limited, but is preferably 0.1 mol/L or more, more preferably 0.15 mol/L or more. The content of the carboxylic acid compound is preferably 5.5 mol/L or less, more preferably 5.3 mol/L or less. Atotech's BluCr (registered trademark) TFS B can be used as the carboxylic acid compound.

In the present invention, water is used as the solvent for preparing the electrolyte solution. It is preferable to use highly pure water, such as ion-exchanged water from which cations have been removed in advance using an ion exchange resin or the like, or distilled water. Furthermore, from the perspective of reducing the amounts of K, Na, Mg, and Ca contained in the electrolyte solution, it is preferable to use water with an electrical conductivity of 30 μS/m or less. The lower limit of electrical conductivity is not limited and may be 0 μS/m.

In order to effectively suppress the generation of hexavalent chromium at the electrodes during the film formation process and improve the stability of the above-mentioned electrolyte, it is preferable for the aqueous solution to further contain at least one type of halide ion. The amount of halide ion is not particularly limited, but it is preferably 0.05 mol/L or more, and more preferably 0.10 mol/L or more. The amount of halide ion is preferably 3.0 mol/L or less, and more preferably 2.5 mol/L or less. Atotech's BluCr(R) TFS C1 and BluCr(R) TFS C2 can be used to incorporate the halide ions.

It is preferable not to add hexavalent chromium to the above-mentioned aqueous solution. As will be described later, the trace amounts of hexavalent chromium that form on the electrode or steel sheet surface during the film formation process are reduced to trivalent chromium, so the hexavalent chromium concentration in the electrolyte does not increase.

It is preferable that no metal ions other than trivalent chromium ions are intentionally added to the above aqueous solution. The metal ions are not limited, but examples include Cu ions, Zn ions, Ni ions, Fe ions, and Sn ions, and each concentration 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.

(ii) Adjustment of pH and Temperature Next, the electrolytic solution is prepared 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. In order to produce the above-mentioned surface-treated steel sheet, it is not sufficient to simply dissolve a trivalent chromium ion source and a carboxylic acid compound in water; it is important to appropriately control the pH and temperature as described above.

pH: 4.0-7.0
In the electrolyte preparation step, the pH of the aqueous solution after mixing is adjusted to 4.0 to 7.0. If the pH is less than 4.0 or more than 7.0, the stability of the electrolyte decreases, causing precipitation and preventing the formation of a chromium-containing layer in the film formation step. Furthermore, the hexavalent chromium concentration in the electrolyte increases during electrolysis. The pH is preferably 4.5 or higher. The pH is preferably 6.5 or lower.

Temperature: 40-70℃
In the electrolyte preparation step, the temperature of the aqueous solution after mixing is adjusted to 40 to 70°C. If the temperature is lower than 40°C or higher than 70°C, the stability of the electrolyte decreases, causing precipitation and preventing the formation of a chromium-containing layer in the film formation step. Furthermore, the hexavalent chromium concentration in the electrolyte increases during the electrolytic treatment. The holding time in the temperature range of 40 to 70°C is not particularly limited.

The above procedure allows the electrolyte solution to be obtained and used in the next film formation process. The electrolyte solution produced using the above procedure can be stored at room temperature.

[Film formation process]
In the film forming step, the steel sheet is subjected to cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using the electrolytic solution prepared in the electrolytic solution preparation step. Specifically, the steel sheet is immersed in the electrolytic solution and subjected to each of the electrolytic treatments. This allows the chromium-containing layer to be formed.

<<Cathode electrolysis treatment C1>>
First, the steel sheet is subjected to cathodic electrolysis C1 using the electrolytic solution, which allows a chromium-containing layer to be formed on the steel sheet.

The charge density of the cathodic electrolysis treatment C1 is not particularly limited. However, the amount of chromium deposited in the chromium-containing layer can be controlled by the charge density of the cathodic electrolysis treatment C1. Therefore, the charge density is preferably 5.0 C/dm² or ^more , more preferably 10.0 C/dm² or ^more . For the same reason, the charge density is preferably 200.0 C/dm² ^{or less} , more preferably 180.0 C/dm² or ^less .

The current density and current application time of the cathodic electrolysis treatment C1 are not particularly limited and can be appropriately set to achieve the desired value of the electricity density, which is expressed as the product of the current density (unit: A/dm ² ) and the current application time (unit: sec.) of the electrolysis treatment.

The temperature of the electrolyte when performing cathodic electrolysis C1 is not particularly limited, but in order to efficiently form a chromium-containing layer, it is preferable to set the temperature in the range of 40°C or higher and 70°C or lower. From the perspective of stably producing the above-mentioned surface-treated steel sheet, it is preferable to monitor the temperature of the electrolyte during cathodic electrolysis C1 and maintain it in the above temperature range.

The pH of the electrolyte used in cathodic electrolysis C1 is not particularly limited, but is preferably 4.0 or higher, and more preferably 4.5 or higher. Furthermore, the pH is preferably 7.0 or lower, and more preferably 6.5 or lower. From the perspective of stably producing the above-mentioned surface-treated steel sheet, it is preferable to monitor the pH of the electrolyte during cathodic electrolysis C1 and maintain it within the above pH range.

There are no particular restrictions on the type of electrode used when performing cathodic electrolysis treatment C1, and any electrode can be used. It is preferable to use an insoluble electrode as the electrode. It is preferable to use at least one selected from the group consisting of an electrode 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 electrode as the insoluble electrode. More specifically, examples of the insoluble electrode include an electrode in which platinum, iridium oxide, or ruthenium oxide is coated on the surface of Ti as a substrate.

In cathodic electrolysis C1, the concentration of the electrolyte constantly changes due to factors such as the formation of a chromium-containing layer on the steel sheet, the introduction and removal of the solution, and the evaporation of water. The change in concentration of the electrolyte in cathodic electrolysis C1 varies depending on the equipment configuration and manufacturing conditions. Therefore, 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 cathodic electrolysis C1 and maintain them within the concentration ranges described above.

Anodic electrolysis treatment A1
Next, the steel sheet subjected to cathodic electrolysis C1 is subjected to anodic electrolysis A1 using the electrolytic solution. By performing anodic electrolysis A1, the chromium-containing layer formed by cathodic electrolysis C1 is dissolved, forming a generation site for granular precipitates of metallic chromium and chromium compounds in cathodic electrolysis C2. In the following description, the granular precipitates of metallic chromium and chromium compounds may be simply referred to as granular chromium. Furthermore, when anodic electrolysis A1 is performed, trivalent chromium is oxidized to hexavalent chromium on the steel sheet surface. However, by using the above-mentioned electrolytic solution, the hexavalent chromium is instantly reduced to trivalent chromium, so that in practice, no hexavalent chromium is present in the electrolytic solution. Therefore, it is important to use the electrolytic solution prepared in the above-mentioned electrolytic solution preparation step, particularly to make anodic electrolysis A1 a process that does not use hexavalent chromium.

If the charge density of the anodic electrolysis treatment A1 is less than 0.50 C/ ^dm² , the chromium-containing layer is not sufficiently dissolved, resulting in the formation of granular chromium generation sites, and the subsequent cathodic electrolysis treatment C2 does not sufficiently precipitate granular chromium. As a result, the S/L ratio of the finally obtained surface-treated steel sheet is less than 0.10. Therefore, the charge density is 0.50 C/dm² or ^more , preferably 0.60 C/dm² or ^more , and more preferably 0.70 C/dm² or ^more . On the other hand, if the charge density exceeds 20.00 C/ ^dm² , the oxidation reaction of trivalent chromium on the steel sheet surface proceeds locally, increasing the hexavalent chromium concentration and making the electrolyte unstable. In addition, the amount of chromium oxide deposited may be excessive. Therefore, the charge density is 20.00 C/dm² ^or less, preferably 18.00 C/dm² ^{or less} , and more preferably 16.00 C/dm² or ^less .

The current density and current application time for the anodic electrolysis treatment A1 are not particularly limited and can be set appropriately to achieve the desired electrical charge density.

The temperature of the electrolyte when performing anodic electrolysis treatment A1 is not particularly limited, and the preferred embodiment is the same as that for cathodic electrolysis treatment C1. From the perspective of stably producing the above-mentioned surface-treated steel sheet and more reliably suppressing an increase in the hexavalent chromium concentration, it is preferable to monitor the temperature of the electrolyte during anodic electrolysis treatment A1 and maintain it within the above temperature range.

The pH of the electrolyte when performing anodic electrolysis A1 is not particularly limited, and the preferred embodiment is the same as that for cathodic electrolysis C1. From the perspective of stably producing the above-mentioned surface-treated steel sheet and more reliably suppressing an increase in the hexavalent chromium concentration, it is preferable to monitor the pH of the electrolyte during anodic electrolysis A1 and maintain it within the above pH range.

There are no particular restrictions on the type of electrode used when performing anodic electrolysis A1, and the preferred embodiment is the same as for cathodic electrolysis C1.

From the same perspective as in cathodic electrolysis C1, it is preferable to monitor the concentrations of the components contained in the electrolyte in anodic electrolysis A1 and maintain them within the concentration ranges described above.

<<Cathode Electrolysis Treatment C2>>
Next, the steel sheet that has been subjected to the anodic electrolysis treatment A1 is subjected to cathodic electrolysis treatment C2 using the electrolytic solution described above. By performing the cathodic electrolysis treatment C2, a chromium-containing layer can be formed on the steel sheet, and particulate chromium can be precipitated starting from the above-mentioned generation sites.

If the electricity density of the cathodic electrolytic treatment C2 is less than 1.0 C/ ^dm2 , granular chromium is not sufficiently precipitated, resulting in the aforementioned S/L being less than 0.10 in the finally obtained surface-treated steel sheet. Therefore, the electricity density is 1.0 C/dm2 or ^more , preferably 3.0 C/dm2 or ^more , and more preferably 5.0 C/dm2 or ^more . On the other hand, if the electricity density is 50.0 C/dm2 or ^more , excessive granular chromium is precipitated, resulting in the finally obtained surface-treated steel sheet having the aforementioned S/L exceeding 0.70. Therefore, the electricity density is less than 50.0 C/ ^dm2 , preferably 45.0 C/dm2 or ^less , and more preferably 40.0 C/ ^dm2 or less.

The current density and current application time for the cathodic electrolysis treatment C2 are not particularly limited and can be set appropriately to achieve the desired electrical charge density.

The temperature of the electrolyte when performing cathodic electrolysis treatment C2 is not particularly limited, and the preferred embodiment is the same as that of cathodic electrolysis treatment C1. From the same perspective as for cathodic electrolysis treatment C1, it is preferable to monitor the temperature of the electrolyte during cathodic electrolysis treatment C2 and maintain it within the above temperature range.

The pH of the electrolyte when performing cathodic electrolysis treatment C2 is not particularly limited, and the preferred embodiment is the same as that of cathodic electrolysis treatment C1. From the same perspective as for cathodic electrolysis treatment C1, it is preferable to monitor the pH of the electrolyte in cathodic electrolysis treatment C2 and maintain it within the above pH range.

There are no particular restrictions on the type of electrode used when performing cathodic electrolysis treatment C2, and the preferred embodiment is the same as for cathodic electrolysis treatment C1.

From the same perspective as in cathodic electrolysis treatment C1, it is preferable to monitor the concentrations of the components contained in the electrolyte in cathodic electrolysis treatment C2 and maintain them within the concentration ranges described above.

[Washing with water]
After the film forming step, the surface-treated steel sheet is preferably washed with water at least once, which makes it possible to remove the electrolytic solution remaining on the surface of the steel sheet.

The water washing can be carried out by any method without particular limitations. For example, a water washing tank can be provided downstream of the immersion tank used for the immersion treatment, and the steel sheet can be continuously immersed in water after immersion. Alternatively, the steel sheet can be washed by spraying water onto it after immersion.

The water used for the rinsing 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 rinsing 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 water washing, drying may be carried out as desired. There are no particular limitations on the drying method, and for example, a conventional dryer or electric oven drying method can be used. From the perspective of preventing deterioration of the surface treatment film, it is preferable that the temperature during the drying process be 100°C or less. There is no particular lower limit, but it is usually around room temperature.

[Preprocessing]
Prior to the film forming step, the steel sheet may be optionally subjected to a pretreatment, which is preferably at least one of degreasing, pickling, and water washing.

Degreasing allows the removal of rolling oil, rust-preventive oil, and other substances adhering to the steel sheet. There are no particular restrictions on the degreasing method, and it can be carried out by any method. After degreasing, it is preferable to rinse the steel sheet with water to remove the degreasing treatment liquid adhering to the surface.

By performing pickling, the natural oxide film present on the surface of the steel sheet can be removed, allowing for the effective formation of a chromium-containing layer in the subsequent film formation process. The pickling can be performed by any method without any particular restrictions. After the pickling, it is preferable to rinse the steel sheet with water to remove any pickling solution adhering to the surface.

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.

To confirm the effects of this invention, surface-treated steel sheets were manufactured using the procedure described below, and their properties were evaluated.

(Electrolyte preparation process)
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 6. 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 for electrolyte solutions C and D, and nitric acid for electrolyte solutions E and F to decrease the pH.

(Pretreatment for 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. The steel sheet was pretreated by electrolytic degreasing, water washing, pickling by immersion in dilute sulfuric acid, and water washing, in that order.

(Film forming process)
Next, the steel sheet was subjected to cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order under the conditions shown in Table 2. The electrolytic solution during each electrolysis was maintained at the pH and temperature shown in Table 1. An insoluble electrode in which iridium oxide was coated on a Ti substrate was used as the electrode for each electrolysis treatment. After performing cathodic electrolysis C2, 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. When the above electrolysis treatment was performed using electrolytic solution G, the stability of the electrolytic solution decreased, and a significant chromium-containing layer could not be formed due to the generation of precipitation and hexavalent chromium. Therefore, subsequent measurements and evaluations were not performed.

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 using the methods described above. Furthermore, for each of the obtained surface-treated steel sheets, the S/L ratio and the atomic ratio of C to Cr in the chromium-containing layer were measured using the methods described above. The measurement results are shown in Table 3.

Furthermore, the resulting surface-treated steel sheets were evaluated for film corrosion resistance, paint corrosion resistance, film wet adhesion, and weldability using the following methods. The evaluation results are also shown in Table 3.

(Sample Preparation)
Laminated steel sheets as samples used to evaluate the film corrosion resistance and film wet adhesion were prepared by the following procedure.

Laminated steel sheets were produced by laminating an isophthalic acid copolymerized polyethylene terephthalate film with a stretch ratio of 3.1 x 3.1, a thickness of 25 μm, a copolymerization ratio of 12 mol%, and a melting point of 224°C on both sides of the resulting surface-treated steel sheet. The lamination was carried out under conditions that resulted in a crystallinity of the resin film of 10% or less, specifically, a steel sheet feed speed of 40 m/min, a rubber roll nip length of 17 mm, and a time from pressing to water cooling of 1 sec. The crystallinity of the resin film was determined using the density gradient tube method in accordance with JIS K7112. The nip length refers to the length in the conveying direction of the area where the rubber roll and steel sheet come into contact.

In addition, painted steel plates were prepared as samples to be used to evaluate the paint corrosion resistance using the following procedure.

An epoxy phenol-based paint was applied to the surface of the obtained surface-treated steel sheet, and baked at 210°C for 10 minutes to prepare a painted steel sheet. The coating weight of the paint was 50 mg/ ^dm2 .

(Film corrosion resistance, paint corrosion resistance)
Crosscuts were made on the film surface of the prepared laminated steel sheet and the painted surface of the coated steel sheet using a cutter, reaching the steel substrate (steel sheet). The crosscut laminated steel sheet and painted steel sheet were immersed for 96 hours in a test solution at 55°C consisting of a mixed aqueous solution containing 1.5% by mass of citric acid and 1.5% by mass of salt. After immersion, washing, and drying, cellophane adhesive tape was applied to the film surface of the laminated steel sheet and the painted surface of the coated steel sheet, and then peeled off. Film corrosion resistance was evaluated using the following four criteria: film peel width (total width extending from the cut) was measured at four random locations on the crosscut of the laminated steel sheet, and the average of the four values was calculated and considered to be the corrosion width. Paint corrosion resistance was evaluated using the following four criteria: film peel width (total width extending from the cut) was measured at four random locations on the crosscut of the coated steel sheet, and the average of the four values was calculated and considered to be the corrosion width. Film corrosion resistance and paint corrosion resistance were evaluated using the following four criteria: In practice, a rating of 1 to 3 can be said to be excellent in corrosion resistance.
1: Corrosion width less than 0.3 mm 2: Corrosion width 0.3 mm or more and less than 0.5 mm 3: Corrosion width 0.5 mm or more and less than 1.0 mm 4: Corrosion width 1.0 mm or more

(Wet film adhesion)
The wet adhesion of the film was evaluated by a 180° peel test using the above-mentioned laminated steel sheet in a retort atmosphere at a temperature of 130° C. and a relative humidity of 100%. The specific procedure was as follows.

First, a total of six test pieces were cut from each of the above laminated steel plates: three with the front surface as the target surface and three with the back surface as the target surface. Each test piece measured 30 mm wide and 100 mm long. Next, the film on the target surface was left intact and the steel plate was cut away from the film on the side opposite the target surface 15 mm from the top along the length of each test piece. After cutting, the test piece was fixed in place from the bottom 15 mm along the length of the test piece so that the steel plate was perpendicular to the ground, and a 30 mm wide and 15 mm long section above the cut position was left hanging down, connected by the film on the target surface. A 100 g weight was then attached to the hanging section, 30 mm wide and 15 mm long.

The test specimens in this state were left in a retort atmosphere at a temperature of 130°C and a relative humidity of 100% for 30 minutes, and then exposed to the atmosphere. The length of the film on the target surface peeled from the surface-treated steel sheet was taken as the film peel length, and the average film peel length for six test specimens was calculated for each laminated steel sheet. Using the obtained average film peel length, the film wet adhesion was evaluated using the following four levels. In practice, a rating of 1 to 3 can be said to indicate excellent film wet adhesion.
1: Peeling length less than 20 mm 2: Peeling length 20 mm or more and less than 40 mm 3: Peeling length 40 mm or more and less than 60 mm 4: Peeling length 60 mm or more

(Weldability)
The obtained surface-treated steel sheets were subjected to a heat treatment at 210°C for 10 minutes, simulating a paint baking process, and then two samples were sandwiched between DR-type 1 mass% Cr-Cu electrodes (electrodes processed to have a tip diameter of 2.3 mm and a curvature R of 40 mm), and a current was passed through them under the following conditions.
- Amada Miyachi transistor power supply: MDA-8000A
・Welding head: AH-200
Pressure: 45 kgf
Current application time: 1.6 msec. (slope 0.2 msec.)
・Waveform: Square wave

The optimum current range (= upper limit current - lower limit current) was determined from the lower limit current at which sufficient strength was obtained and the upper limit current at which no expulsion occurred, and was evaluated using the following four levels. In practice, a rating of 1 to 3 can be said to indicate excellent weldability.
1: 0.6 kA or more 2: 0.4 kA or more, less than 0.6 kA 3: 0.2 kA or more, less than 0.4 kA 4: Less than 0.2 kA

As is clear from the results shown in Table 3, all of the surface-treated steel sheets meeting the conditions of the present invention were manufactured without using hexavalent chromium, yet they exhibited excellent film corrosion resistance, paint corrosion resistance, film wet adhesion, and weldability.

Claims (4)

Steel plate and
A surface-treated steel sheet comprising a chromium-containing layer disposed on at least one surface of the steel sheet,
a roughness curve of the chromium-containing layer obtained by applying a low-pass filter with a cutoff value _λs of 2 nm and a high-pass filter with a cutoff value _λc of 65 nm to a cross-sectional curve extracted from a cross-sectional image of the chromium-containing layer, wherein S is a total length in the x-axis direction of a region whose height from the mean line of the roughness curve exceeds 1.9 nm and L is an evaluation length of the roughness curve, and S/L is 0.10 or more and 0.70 or less;
The chromium-containing layer has an atomic ratio of C to Cr of 0.2% or more and 50.0% or less. 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 surface-treated steel sheet according to claim 1 or 2, wherein the chromium-containing layer has a chromium oxide deposition 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,
an electrolyte solution preparation step of preparing an electrolyte solution containing trivalent chromium ions;
a coating formation step of forming the chromium-containing layer,
In the electrolyte solution preparation step,
a trivalent chromium ion source, a carboxylic acid compound, and water are mixed;
The electrolyte solution is prepared by adjusting the pH to 4.0 to 7.0 and the temperature to 40 to 70°C.
In the film forming step,
The steel sheet is subjected to cathodic electrolysis C1, anodic electrolysis A1, and cathodic electrolysis C2 in this order using the electrolytic solution,
The electricity density of the anodic electrolysis treatment A1 is 0.50 C/dm ² or more and 20.00 C/dm ² or less,
The method for producing a surface-treated steel sheet, wherein the electricity density of the cathodic electrolysis treatment C2 is 1.0 C/dm ² or more and less than 50.0 C/dm ² .