Surface-Treated Steel Sheet and Production Method Therefor

The present disclosure relates to a surface-treated steel sheet, and in particular to a surface-treated steel sheet having excellent corrosion resistance at bisphenol A (BPA)-free painted worked part. The surface-treated steel sheet according to the present disclosure is suitable for use for containers such as cans. The present disclosure also relates to a production method for the surface-treated steel sheet.

Sn coated steel sheets (tinplate) and tin-free steel sheets (TFS) have been widely used as materials for various metal cans such as beverage cans, food cans, pails, and 18-liter cans.

Tinplate and TFS are used with organic resin coatings such as epoxy-based paint and PET films in order to accommodate various contents. When applying an organic resin coating to tinplate or TFS, a Cr (chromium) oxide layer formed on the outermost surface of the steel sheet by subjecting the steel sheet to electrolysis treatment or immersion treatment in an aqueous solution containing hexavalent Cr exhibits excellent adhesion to the organic resin coating layer. Thus, the deformation of the organic resin coating layer follows the deformation of the steel sheet during can production, as a result of which corrosion resistance to various contents is ensured after can production.

Meanwhile, given the indication that BPA contained in epoxy-based paint may be harmful to humans, development of BPA-free paint using polyester-based resin not containing BPA is underway (PTL 1 and PTL 2), and there is a demand to replace epoxy-based paint with BPA-free paint. However, tinplate and TFS that have been used have poor adhesion to BPA-free paint compared to epoxy-based paint. Accordingly, the deformation of the BPA-free paint cannot follow the deformation of the steel sheet during can production, and sufficient corrosion resistance to various contents cannot be ensured after can production. Hence, the application of BPA-free paint to various metal cans has not progressed.

In recent years, growing environmental awareness has accelerated the worldwide trend toward restricting the use of hexavalent Cr. In the field of surface-treated steel sheets used for various metal cans, too, there is a need to establish a production method that does not use hexavalent chromium.

Examples of known methods of forming a surface-treated steel sheet without using hexavalent chromium include the methods proposed in PTL 3 to PTL 6. These methods form a surface-treatment layer by performing electrolysis treatment in an electrolytic solution containing a trivalent chromium compound such as basic chromium sulfate.

With the methods proposed in PTL 3 to PTL 6, a surface-treatment layer can be formed without using hexavalent chromium. According to PTL 3 to PTL 6, a surface-treated steel sheet with excellent adhesion to epoxy-based paint can be obtained by the methods. Moreover, according to PTL 3 and PTL 4, a surface-treated steel sheet that exhibits excellent corrosion resistance even after being painted (coated) with epoxy-based paint and deformed can be obtained.

However, while the surface-treated steel sheet obtained by each of the conventional methods proposed in PTL 3 to PTL 6 has excellent adhesion to epoxy-based paint and has excellent corrosion resistance at epoxy-based painted worked part, its corrosion resistance at BPA-free painted worked part is insufficient. This makes it impossible to replace conventional epoxy-based paint with BPA-free paint while maintaining corrosion resistance to various contents.

There is thus a demand for a surface-treated steel sheet that can be produced without using hexavalent chromium and has excellent corrosion resistance at BPA-free painted worked part.

It could therefore be helpful to provide a surface-treated steel sheet that can be produced without using hexavalent chromium and has excellent corrosion resistance at BPA-free painted worked part.

Upon careful examination, we discovered the following (1) and (2).

The present disclosure is based on these discoveries. We thus provide the following.

It is thus possible to provide a surface-treated steel sheet having excellent corrosion resistance at BPA-free painted worked part without using hexavalent chromium. The surface-treated steel sheet is suitable for use as a material for containers and the like.

A method for carrying out the present disclosure will be described in detail below. The following description shows an example of a preferred embodiment of the present disclosure, and the present disclosure is not limited to such.

A surface-treated steel sheet in one embodiment of the present disclosure is a surface-treated steel sheet comprising: a steel sheet; and a chromium-containing layer disposed on a surface of the steel sheet on at least one side. In the present disclosure, it is important that, when the chromium-containing layer is observed from the surface direction, linear regions in which an element smaller in atomic number than chromium is concentrated are present and the number of the linear regions is 5 or more per 100 nm. Each of the components in the surface-treated steel sheet will be described below.

The steel sheet is not limited and any steel sheet may be used. The steel sheet is preferably a steel sheet for cans. As the steel sheet, for example, an ultra low carbon steel sheet or a low carbon steel sheet may be used. The production method for the steel sheet is not limited, and a steel sheet produced by any method may be used. Typically, a cold-rolled steel sheet may be used as the steel sheet. The cold-rolled steel sheet can be produced, for example, by a typical production process that includes hot rolling, pickling, cold rolling, annealing, and temper rolling.

The chemical composition of the steel sheet is not limited, and may contain C, Mn, P, S, Si, Cu, Ni, Mo, Al, and inevitable impurities within such ranges that do not undermine the effects according to the present disclosure. In this case, for example, a steel sheet having a chemical composition specified in ASTM A623M-09 can be suitably used as the steel sheet.

In one embodiment of the present disclosure, it is preferable to use a steel sheet having a chemical composition containing, in mass %,

The sheet thickness of the steel sheet is not limited, but is preferably 0.60 mm or less. No lower limit is placed on the sheet thickness, but the sheet thickness is preferably 0.10 mm or more. Herein, the term “steel sheet” is defined to include “steel strip”.

The chromium-containing layer is present on at least one side of the steel sheet. The components constituting the chromium-containing layer are not limited, but may include metallic chromium and one or more chromium compounds. The one or more chromium compounds are not limited, and any chromium compound(s) may be contained. As the chromium compound(s), 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 may be contained. The chromium-containing layer may also contain impurities in addition to metallic chromium and chromium compounds. Examples of the impurities include metallic elements such as Ni, Cu, Sn, and Zn that are mixed in the below-described electrolytic solution as impurities. The metallic elements are considered to typically exist in the chromium-containing layer in a metallic state, but may exist as compounds.

In the chromium-containing layer in one embodiment of the present disclosure, the total content of metallic chromium and elements constituting chromium compounds is preferably 90 at % (atomic %) or more. Herein, the total content is the ratio of the total atomic number of metallic chromium and elements constituting chromium compounds to the total atomic number of all elements other than Fe, expressed as a percentage.

The total content can be determined by measuring the content (at %) of each of the metallic chromium and elements constituting chromium compounds contained in the chromium-containing layer by X-ray photoelectron spectroscopy (XPS) and adding them up. In the measurement of the content of each element by XPS, the content (atomic ratio) of the element can be calculated from the integrated intensity of the peak corresponding to the element by the relative sensibility coefficient method.

For example, the content of chromium carbide (CrC) can be determined from the integrated intensity of the C 1s carbide peak that appears 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 at %, the CrCcontent is 6×(2+3)/3=10 at %.

For chromium oxide, the CrOcontent can be determined from the integrated intensity of the Cr 2p oxide peak that appears around 576.7 eV. The CrOcontent can be determined from the integrated intensity of the Cr 2p oxide peak that appears around 579.2 eV.

Likewise, the contents of other chromium compounds can be determined using the integrated intensities of the following peaks, for example.

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

Adding the content of metallic chromium and the contents of elements constituting chromium compounds obtained by this method can yield the total content of metallic chromium and elements constituting chromium compounds.

The total content refers to the value at the 1/2 position (i.e. position of 1/2) of the thickness of the chromium-containing layer. The 1/2 position can be determined by the following procedure. First, while sputtering the chromium-containing layer from its outermost surface, the total content of metallic chromium and elements constituting chromium compounds and the Fe content are measured by the foregoing method. The position (depth) at which the measured total content of metallic chromium and elements constituting chromium compounds and Fe content are equal is taken to be the interface between the chromium-containing layer and the steel sheet. The thickness from the outermost surface of the chromium-containing layer to the interface is taken to be the thickness of the chromium-containing layer, and the 1/2 position of the thickness is determined.

A scanning X-ray photoelectron spectrometer PHI X-tool produced by ULVAC-PHI, Inc. can be used for the measurement by XPS, for example. For example, the X-ray source is a monochromatic AlKα ray, the voltage is 15 kV, the beam diameter is 100 μmϕ, the extraction 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 SiO.

The spatial structure of the components constituting the chromium-containing layer is not limited. For example, the components may be separated as separate layers within 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 a mixed layer.

The chromium coating weight of the chromium-containing layer is not limited. If the chromium coating weight of the chromium-containing layer is excessively high, however, cohesive fracture may occur in the chromium-containing layer when working the surface-treated steel sheet. Therefore, from the viewpoint of more stably ensuring corrosion resistance at BPA-free painted worked part, the chromium coating weight of the chromium-containing layer per one side is preferably 500.0 mg/mor less and more preferably 450.0 mg/mor less. From the viewpoint of further improving corrosion resistance at BPA-free painted worked part, the chromium coating weight of the chromium-containing layer per one side is preferably 40.0 mg/mor more and more preferably 50.0 mg/mor more. Herein, the term “chromium coating weight” refers to the total coating weight of chromium present in various forms.

The chromium coating weight can be measured by X-ray fluorescence analysis. More specifically, the chromium coating weight is measured by the following procedure. First, the Cr amount (total Cr amount) in the surface-treated steel sheet is measured using an X-ray fluorescence instrument. Next, the Cr amount (blank sheet Cr amount) in the steel sheet on which the chromium-containing layer has not been formed yet or the steel sheet from which the chromium-containing layer has been peeled off is measured using the X-ray fluorescence instrument. The value obtained by subtracting the blank sheet Cr amount from the total Cr amount is taken to be the chromium coating weight of the chromium-containing layer. For example, a commercially available chromium coating separating agent such as a hydrochloric acid-based agent may be used to peel off the chromium-containing layer.

Chromium oxide may be present in the chromium-containing layer. The location of chromium oxide is not limited, and chromium oxide may be present in the form of O concentrated in the below-described linear regions. The location of O can be determined, 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 transmission electron microscope (TEM), or by three-dimensional composition analysis using a three-dimensional atom probe (3DAP).

The chromium oxide coating weight of the chromium-containing layer is not limited. If the chromium oxide coating weight of the chromium-containing layer is excessively high, however, cohesive fracture may occur starting from Cr oxide in the chromium-containing layer when working the surface-treated steel sheet, causing degradation in corrosion resistance at BPA-free painted worked part. Therefore, from the viewpoint of more stably ensuring corrosion resistance at BPA-free painted worked part, the chromium oxide coating weight of the chromium-containing layer per one side is preferably 40.0 mg/mor less and more preferably 35.0 mg/mor less. The chromium-containing layer may contain no chromium oxide. Thus, no lower limit is placed on the chromium oxide coating weight of the chromium-containing layer, and the chromium oxide coating weight of the chromium-containing layer per one side may be 0.0 mg/m.

The chromium oxide coating weight can be measured by X-ray fluorescence analysis. More specifically, the chromium oxide coating weight is measured by the following procedure. First, the Cr amount (total Cr amount) in the surface-treated steel sheet is measured. Next, the surface-treated steel sheet is subjected to alkali treatment of immersing in 7.5N—NaOH at 90° C. for 10 minutes to remove chromium oxide. The surface-treated steel sheet after the alkali treatment is thoroughly washed with water, and then the Cr amount (the Cr amount after alkali treatment) is measured again using an X-ray fluorescence instrument. The value obtained by subtracting the Cr amount after alkali treatment from the total Cr amount is taken to be the chromium oxide coating weight of the chromium-containing layer.

The chromium-containing layer may be amorphous or crystalline. In other words, the chromium-containing layer may contain one or both of amorphous and crystalline phases. The chromium-containing layer produced by the below-described method usually contains amorphous phase, and may also contain crystalline phase. The mechanism by which the chromium-containing layer is formed is not clear, but it is presumed that, during formation of amorphous phase, the amorphous phase is partially crystallized and as a result a chromium-containing layer containing both amorphous and crystalline phases is formed. The area ratio of the crystalline region is not limited, but is preferably 30% or less when the chromium-containing layer is observed from the surface direction. Since the crystalline region may not be present, the lower limit of the area ratio of the crystalline region may be 0%.

The crystalline region in the chromium-containing layer can be determined by removing the base steel sheet from the surface-treated steel sheet to prepare a single-layer sample of the chromium-containing layer and observing the single-layer sample of the chromium-containing layer from the surface side using a TEM or STEM. The method of preparing the single-layer sample of the chromium-containing layer is not limited. For example, the single-layer sample of the chromium-containing layer can be prepared by applying an ion beam of Ar or the like from the base steel sheet side and ion milling the steel sheet. In the case of preparing the single-layer region of the chromium-containing layer with an ion beam, the ion beam is applied with an acceleration voltage of 5 kV or less and an incidence angle of 1 degree to 5 degrees relative to the base steel sheet, thereby ensuring an observation field of a single chromium layer region of several μmor more. Here, the bottom of the chromium-containing layer is also milled to some extent and as a result the film thickness of the chromium-containing layer decreases in some cases. This, however, does not affect the measurement result of the crystalline region.

The area ratio of the crystalline region in the chromium-containing layer can be measured using a TEM. Specifically, first, a diffraction pattern of the chromium-containing layer is obtained by selected area diffraction of the TEM. Next, a dark field image is obtained for all diffraction spots in the diffraction pattern, and a region that appears with high brightness in the dark field image is taken to be the crystalline region. The area of the obtained crystalline region is calculated by image processing, and the calculated area is divided by the area of the chromium-containing layer in the selected area aperture to calculate the area ratio of the crystalline region. For example, image analysis software such as Image-J may be used to calculate the area ratio.

The chromium-containing layer may contain C. No upper limit is placed on the C content in the chromium-containing layer, but the atomic ratio of C to Cr is preferably 50% or less and more preferably 45% or less. The chromium-containing layer may not contain C. Thus, no lower limit is placed on the atomic ratio of C to Cr in the chromium-containing layer, and the lower limit may be 0%.

The C content in the chromium-containing layer can be measured by XPS. In detail, for the C content in the chromium-containing layer, sputtering is performed from the outermost layer to a depth of 0.2 nm or more in terms of SiO, the respective atomic ratios are quantified by the relative sensibility coefficient method from the integrated intensities of the narrow spectra of Crp and C1s, and (the C atomic ratio)/(the Cr atomic ratio) is calculated. A scanning X-ray photoelectron spectrometer PHI X-tool produced by ULVAC-PHI, Inc. can be used for the measurement by XPS, for example. For example, the X-ray source is a monochromatic AlKα ray, the voltage is 15 kV, the beam diameter is 100 μmϕ, the extraction 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 SiO.

The mechanism by which C is contained in the chromium-containing layer is not clear, but it is presumed that, in the process of forming the chromium-containing layer on the steel sheet, if the electrolytic solution contains a carboxylic acid compound, the carboxylic acid compound decomposes and is incorporated into the layer.

The location of C in the chromium-containing layer is not limited, and C may be present in the form of being concentrated in the below-described linear regions. The location of C can be determined, 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 transmission electron microscope (TEM), or by three-dimensional composition analysis using a three-dimensional atom probe (3DAP).

The chromium-containing layer may contain Fe. No upper limit is placed on the Fe content in the chromium-containing layer, but the atomic ratio of Fe to Cr is preferably 100% or less. The chromium-containing layer may not contain Fe. Thus, no lower limit is placed on the atomic ratio of Fe to Cr, and the lower limit 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 the narrow spectra of Cr2p and Fe2p.

The mechanism by which Fe is contained in the chromium-containing layer is not clear, but it is presumed that, in the process of forming the chromium-containing layer on the steel sheet, Fe contained in the steel sheet dissolves in a small amount into the electrolytic solution and is incorporated into the layer.

The chromium-containing layer may contain metal impurities such as K, Na, Mg, and Ca contained in water and Sn, Ni, Cu, and Zn contained in the aqueous solution, and S, N, Cl, Br, etc., besides Cr, O, Fe, and C. However, the presence of these elements may cause a decrease in corrosion resistance at BPA-free painted worked part. Therefore, the total content of elements other than Cr, O, Fe, and C is preferably 3% or less and more preferably 0% (i.e. the other elements are not contained at all) in terms of atomic ratio to Cr. For example, the content of these elements can be measured by XPS as with the C content, without being limited thereto.

When the chromium-containing layer in the surface-treated steel sheet according to the present disclosure is observed from the surface direction, linear regions in which an element smaller in atomic number than chromium is concentrated are present, and the number of the linear regions is 5.0 or more per 100 nm. As a result of the number of the linear regions being 5.0 or more per 100 nm, excellent corrosion resistance at BPA-free painted worked part can be achieved. From the viewpoint of further improving corrosion resistance at BPA-free painted worked part, the number of the linear regions is preferably 7.0 or more per 100 nm and more preferably 10.0 or more per 100 nm. No upper limit is placed on the number of the linear regions, and the number of the linear regions may be, for example, 50.0 or less per 100 nm, 45.0 or less per 100 nm, or 40.0 or less per 100 nm.

The reason why the provision of the linear regions as described above improves corrosion resistance at BPA-free painted worked part is as follows.

A typical chromium-containing layer formed from a hexavalent Cr bath or trivalent Cr bath is composed of metallic chromium and chromium oxide. A surface-treated steel sheet including such a chromium-containing layer is usually worked into cans or the like after an organic resin coating is formed on its surface. However, since metallic chromium has poor workability, the chromium-containing layer cannot fully follow the deformation of the steel sheet during working, and consequently the organic resin coating on the chromium-containing layer is damaged. This causes a decrease in post-working corrosion resistance.

In view of this, in conventional surface-treated steel sheets, chromium oxide is provided in the top layer to ensure post-working corrosion resistance. Since chromium oxide has excellent adhesion to epoxy-based paint, even though metallic chromium cannot follow the deformation of the base steel sheet, the chromium-containing layer and the epoxy-based paint firmly adhere to each other and the coating properties of the epoxy-based paint can be maintained even after can production.