JP2024061121A - Surface-treated steel sheet - Google Patents

Abstract

[Problem] To provide a surface-treated steel sheet that allows characters, designs, etc. to be more easily and clearly expressed on the surface of a plating layer. [Solution] A surface-treated steel sheet is used that includes a steel sheet, a hot-dip plating layer formed on the surface of the steel sheet, and a chemical conversion coating layer formed on the surface of the hot-dip plating layer, the hot-dip plating layer containing, in average composition, Al: 0-90 mass %, Mg: 0-10 mass %, with the balance containing Zn and impurities, the hot-dip plating layer is formed with a patterned portion and a non-patterned portion that are arranged to have a predetermined shape, the color difference ΔE between the patterned portion and the non-patterned portion is 3.0 or more, and the film thickness ratio (tP/tB) of the thickness tP of the chemical conversion coating layer on the patterned portion to the thickness tB of the chemical conversion coating layer on the non-patterned portion is 1.5 or more. [Selected Figure] None

Description

The present invention relates to surface-treated steel sheets.

Zn-Al-Mg hot-dip galvanized steel sheets have higher corrosion resistance than hot-dip galvanized steel sheets, and are therefore widely used in a variety of manufacturing industries, including building materials, home appliances, and the automotive sector, and their usage has been increasing in recent years.

By the way, in order to make characters, patterns, designs, etc. appear on the surface of the hot-dip plating layer of a plated steel sheet, the plating layer may be subjected to processes such as printing and painting, thereby making characters, patterns, designs, etc. appear on the surface of the hot-dip plating layer.

However, when processes such as printing and painting are performed on the plating layer, there is a problem that the cost and time required to apply letters, designs, etc. increase. Furthermore, when letters, designs, etc. are displayed on the surface of the plating layer by printing or painting, not only is the metallic luster appearance that is highly popular with consumers lost, but there is also the problem of deterioration of the coating film itself over time and of the adhesion of the coating film over time, resulting in poor durability and the risk of the letters, designs, etc. disappearing over time. Furthermore, when letters, designs, etc. are displayed on the surface of the plating layer by stamping ink onto the surface of the plating layer, although the cost and time are relatively low, there is a concern that the corrosion resistance of the plating layer may be reduced by the ink.

Therefore, the following Patent Document 1 proposes a technology for displaying letters, designs, etc. on the surface of the plating layer of a Zn-Al-Mg hot-dip plated steel sheet.

The Zn-Al-Mg hot-dip plated steel sheet described in Patent Document 1 has a first region and a second region on the surface of the plated layer, and the first region is arranged to have an intentional shape consisting of straight lines and curved lines, thereby allowing characters, designs, etc. to appear on the surface of the plated layer. The Zn-Al-Mg hot-dip plated steel sheet has the advantage that the durability of the design, etc. is excellent, and the corrosion resistance of the plated layer is also excellent. However, there is a demand for characters, designs, etc. that look different from those in Patent Document 1.

Patent No. 6648871

The present invention was made in consideration of the above circumstances, and aims to provide a surface-treated steel sheet that makes it possible to more easily and clearly express letters, designs, etc. on the surface of the plating layer.

In order to solve the above problems, the present invention employs the following configuration.
[1] A steel sheet, a hot-dip galvanized layer formed on a surface of the steel sheet, and a chemical conversion coating layer formed on a surface of the hot-dip galvanized layer,
The hot-dip plating layer contains, in an average composition, Al: 0 to 90 mass%, Mg: 0 to 10 mass%, and the balance contains Zn and impurities,
The hot-dip plating layer has a pattern portion and a non-pattern portion arranged to have a predetermined shape,
a color difference ΔE between the patterned portion and the non-patterned portion is 3.0 or more;
A surface-treated steel sheet, wherein a thickness ratio ( _tP / _tB ) of a thickness _tP of the chemical conversion layer on the patterned portion to a thickness _tB of the chemical conversion layer on the non-patterned portion is 1.5 or more.
[2] The surface-treated steel sheet according to [1], characterized in that the pattern portion is arranged so as to have a shape that is any one of straight line portions, curved line portions, dot portions, figures, numbers, symbols, designs, and letters, or a combination of two or more of these.
[3] The surface-treated steel sheet according to [1], characterized in that the pattern portion is intentionally formed.
[4] The surface-treated steel sheet according to [1], characterized in that the pattern portion is arranged so as to have a shape that is one or a combination of two or more of straight line portions, curved portions, dot portions, figures, numbers, symbols, patterns, or letters, and the pattern portion is intentionally formed.
[5] A steel sheet, a hot-dip galvanized layer formed on a surface of the steel sheet, and a chemical conversion coating layer formed on a surface of the hot-dip galvanized layer,
The hot-dip plating layer contains, in an average composition, Al: 0 to 90 mass%, Mg: 0 to 10 mass%, and the balance contains Zn and impurities,
The chemical conversion layer has thick film portions and non-thick film portions arranged in a predetermined shape,
A surface-treated steel sheet, wherein a thickness ratio ( _tP / _tB ) of a thickness _tP of the thick film portion of the chemical conversion layer to a thickness _tB of the non-thick film portion of the chemical conversion layer is 1.5 or more.
[6] The surface-treated steel sheet according to [5], wherein the thick film portion has a thickness t _P in the range of 0.2 to 25 μm, and the non-thick film portion has a thickness t _B in the range of 0.1 to 15 μm.
[7] The surface-treated steel sheet according to [5], characterized in that the thick film portion is arranged so as to have a shape that is any one of straight line portions, curved portions, dot portions, figures, numbers, symbols, patterns, and letters, or a combination of two or more of these.
[8] The surface-treated steel sheet according to [5], wherein the thick film portion is intentionally formed.
[9] The surface-treated steel sheet according to [5], characterized in that the thick film portion is arranged so as to have a shape that is any one of straight line portions, curved portions, dot portions, figures, numbers, symbols, patterns, and letters, or a combination of two or more of these, and the thick film portion is intentionally formed.
[10] The surface-treated steel sheet according to any one of claims [1] to [9], wherein the coating weight of the hot-dip coating layer is 30 to 600 g/ ^m2 in total on both sides of the steel sheet.
[11] The surface-treated steel sheet according to any one of [1] to [9], wherein the hot-dip coating layer further contains, in an average composition, one or two selected from the group consisting of the following Group A and Group B:
[Group A] Si: 0.0001 to 2 mass%
[Group B] Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C, any one or more of these, in a total content of 0.0001 to 2 mass%
[12] The surface-treated steel sheet according to [11], wherein the hot-dip plating layer has an average composition containing, in mass%, the Group A.
[13] The surface-treated steel sheet according to [11], wherein the hot-dip plating layer has an average composition containing the B group in mass%.

The present invention provides a surface-treated steel sheet that allows characters, designs, etc. to be more easily and clearly expressed on the surface of the plating layer.

FIG. 1A is a cross-sectional schematic diagram of an example of a surface-treated steel sheet according to an embodiment of the present invention. FIG. 1B is a cross-sectional schematic view of another example of a surface-treated steel sheet according to an embodiment of the present invention. FIG. 2 is a process diagram illustrating an example of a method for producing a surface-treated steel sheet according to an embodiment of the present invention. FIG. 3 is a process diagram illustrating an example of a method for producing a surface-treated steel sheet according to an embodiment of the present invention. FIG. 4 is a process diagram illustrating another example of the method for producing a surface-treated steel sheet according to an embodiment of the present invention.

A chemical conversion coating layer may be formed on the plating layer of Zn-based plated steel sheets, such as Zn-Al-Mg-based hot-dip plated steel sheets, for example, to improve paint adhesion and suppress the occurrence of white rust on the surface. Previously, Zn-based plated steel sheets were provided with a chromate-based chemical conversion coating layer, but in recent years, in response to growing awareness of environmental conservation, chromate-free chemical conversion coating layers have been used as the chemical conversion coating layer.

The chemical conversion layer is generally formed to have a uniform thickness over the entire surface of the plating layer. However, when the inventors formed a chemical conversion layer on a plating layer so that some parts were thicker than others, and left it as is for a certain period of time, they found that in the parts where the chemical conversion layer was thicker, the surface of the plating layer had discolored to a color closer to black.

After further intensive research, the inventors discovered that by providing a thick film section of a specified shape in the chemical conversion layer and leaving it for a specified period of time without applying any coloring treatment to the plating layer, it is possible to form an area in the plating layer below the thick film section where discoloration is accelerated compared to the surrounding area. They also discovered that by arbitrarily setting the shape of the thick film section, it is possible to create a desired design or design in the plating layer, which led to the completion of the present invention.

The reason why discoloration of the plating layer is more accelerated in the areas corresponding to the thick film parts of the chemical conversion layer than in the surrounding areas is unclear, but in the thick film parts of the chemical conversion layer, oxygen transmission from the surface of the chemical conversion layer to the surface of the plating layer is inhibited compared to areas other than the thick film parts, which may have acted as an anode region in the corrosion reaction and accelerated discoloration.

Hereinafter, a surface-treated steel sheet according to an embodiment of the present invention will be described.
The surface-treated steel sheet of this embodiment comprises a steel sheet, a hot-dip plated layer formed on the surface of the steel sheet, and a chemical conversion treatment layer formed on the surface of the hot-dip plated layer, the hot-dip plated layer containing, in average composition, Al: 0-90 mass%, Mg: 0-10 mass%, with the balance containing Zn and impurities, the hot-dip plated layer is formed with a patterned portion and a non-patterned portion arranged to have a predetermined shape, the color difference ΔE between the patterned portion and the non-patterned portion is 3.0 or more, and the film thickness ratio (t _P /t _B ) of the thickness t _P of the chemical conversion treatment layer on the patterned portion to the thickness t _B of the chemical conversion treatment layer on the non-patterned portion is 1.5 or more.

Further, as a surface-treated steel sheet for obtaining the above-mentioned surface-treated steel sheet, this embodiment includes a surface-treated steel sheet comprising a steel sheet, a hot-dip galvanized layer formed on the surface of the steel sheet, and a chemical conversion layer formed on the surface of the hot-dip galvanized layer, the hot-dip galvanized layer containing, in average composition, Al: 0-90 mass%, Mg: 0-10 mass%, with the balance containing Zn and impurities, the chemical conversion layer having a thick film portion and a non-thick film portion arranged to have a predetermined shape, and a film thickness ratio (t _P /t _B ) of a thickness t _P of the thick film portion of the chemical conversion layer to a thickness t _B of the non-thick film portion of the chemical conversion layer is 1.5 or more.

There are no particular restrictions on the material of the steel sheet that serves as the base for the plating layer. As will be described in detail later, general steel and other materials can be used without particular restrictions, and Al-killed steel and some high alloy steels can also be used, and there are no particular restrictions on the shape. The plating layer according to this embodiment is formed by applying, for example, the hot-dip plating method described later to the steel sheet.

Next, the chemical components of the plating layer will be described.
The plating layer contains, in average composition, Al: 0-90 mass%, Mg: 0-10 mass%, and the balance Zn and impurities. Preferably, the plating layer contains Al: 0-90 mass%, Mg: 0-10 mass%, and the balance Zn and impurities.

Furthermore, the plating layer may contain one or two types selected from the group consisting of Group A and Group B below.
[Group A] Si: 0.0001 to 2 mass%
[Group B] Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C, any one or more of these, in a total content of 0.0001 to 2 mass%

The Al content is in the range of 0 to 90 mass% in the average composition, preferably 4 to 22 mass%. Al is preferably included to ensure corrosion resistance. If the Al content in the plating layer is 4 mass% or more, the effect of improving corrosion resistance is enhanced. If it is 90 mass% or less, the plating layer can be formed stably. If it exceeds 22 mass%, the effect of improving corrosion resistance becomes saturated. From the viewpoint of corrosion resistance, it is preferably 5 to 20 mass%. More preferably, it is 6 to 19 mass%.

The Mg content is 0-10 mass% in the average composition, preferably 1-10 mass%. Mg is added to improve corrosion resistance. If the Mg content in the coating layer is 1 mass% or more, the effect of improving corrosion resistance is further enhanced. If it exceeds 10 mass%, when the coating layer is formed by hot-dip coating, dross generation in the coating bath becomes significant, making it difficult to stably manufacture coated steel sheets. From the viewpoint of the balance between corrosion resistance and dross generation, it is preferably 1.5-8 mass%. More preferably, it is in the range of 2-6 mass%.

Note that Al and Mg may each be 0%. In other words, the plating layer of the surface-treated steel sheet of this embodiment is not limited to a Zn-Al-Mg plating layer, but may be a Zn-Al plating layer, a zinc plating layer, or a zinc alloy plating layer. The plating layer may also be a hot-dip plating layer formed by a hot-dip plating method.

The plating layer may contain 0.0001 to 2 mass% Si as an element of group A in an average composition. Si is an effective element for improving the adhesion of the plating layer. By containing 0.0001 mass% or more of Si in the plating layer, the effect of improving the adhesion of the plating layer is realized, so it is preferable to contain 0.0001 mass% or more of Si. On the other hand, even if the plating layer contains Si, the effect of improving the plating adhesion is saturated even if the plating layer contains Si at a content of more than 2 mass%, so the Si content is set to 2 mass% or less. From the viewpoint of plating adhesion, the Si content in the plating layer may be 0.0010 to 1 mass%, or may be 0.0100 to 0.8 mass%.

The plating layer may contain, as elements of group B, one or more of the following elements in an average composition: Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, and C, in a total amount of 0.0001 to 2 mass%. By containing these elements, the corrosion resistance of the plating layer can be further improved. REM is one or more of the rare earth elements with atomic numbers 57 to 71 in the periodic table.

The remainder of the chemical composition of the plating layer is zinc and impurities. Some impurities are inevitably contained in zinc and other base metals, while others are contained in the plating bath as the steel dissolves.

The average composition of the plating layer can be measured by the following method. First, the surface coating is removed with a coating remover that does not corrode the plating (e.g., Neo River SP-751 manufactured by Sansai Kako Co., Ltd.), then the plating layer is dissolved with hydrochloric acid containing an inhibitor (e.g., Hibilon manufactured by Sugimura Chemical Industry Co., Ltd.), and the resulting solution is subjected to inductively coupled plasma (ICP) atomic emission spectrometry to determine the average composition.

The chemical conversion layer in this embodiment is a so-called chromate-free chemical conversion layer. The chemical conversion layer is not particularly limited, and may be an inorganic chemical conversion layer, an organic chemical conversion layer, or an organic-inorganic composite chemical conversion layer. In this embodiment, differences in the material of the chemical conversion layer do not particularly affect the formation of the patterned portion and non-patterned portion.

For example, an organic conversion treatment layer may be one that contains 20% or more by mass of resin and 1-20% by mass of silica particles with a particle size of 5-200 nm. Furthermore, the conversion treatment layer may contain a pigment containing Cu, Co or Fe. This conversion treatment layer is a coating obtained by applying an aqueous composition containing resin, silica particles and pigment to the plating layer and drying it.

The resin contained in the organic chemical conversion coating layer may be a common resin, such as polyolefin resin, fluororesin, acrylic resin, urethane resin, polyester resin, epoxy resin, or phenol resin. These resins may be water-soluble resins, or may be resins (water-dispersible resins) that are originally water-insoluble but can be finely dispersed in water, such as emulsions or suspensions. In addition to water-soluble resins, this term also includes resins (water-dispersible resins) that are originally water-insoluble but can be finely dispersed in water, such as emulsions or suspensions. In particular, it is preferable to contain one or more of the resins polyolefin resin, fluororesin, acrylic resin, and phenol resin, as these have excellent weather resistance.

The resin should be contained in the chemical conversion layer at a ratio of 20% by mass or more. By making the resin content 20% by mass or more, the chemical conversion layer itself does not become brittle, and the plating layer can be stably coated. Note that the chemical conversion layer may contain components other than resin, such as Nb compounds and phosphate compounds, in addition to the resin, silica particles, and pigments, and the resin content may be the remainder of these components.

Silica particles are added to improve the corrosion resistance of the chemical conversion layer. The silica particles preferably have an average particle size in the range of 5 to 200 nm. The silica particles should be contained in the chemical conversion layer at a ratio of 1 to 20 mass%. By making the content of silica particles 1 mass% or more, the effect of improving corrosion resistance can be obtained. Furthermore, by making the content of silica particles 20 mass% or less, the chemical conversion layer itself does not become brittle, and the plating layer can be stably coated. Since it is difficult to obtain silica particles with an average particle size of less than 5 nm, and it is practically difficult to prepare and manufacture a chemical conversion layer containing silica particles with an average particle size of less than 5 nm, the lower limit of the average particle size of the silica particles is set to 5 nm or more. Furthermore, if the average particle size of the silica particles exceeds 200 nm, the chemical conversion layer may become cloudy and the appearance of the plating layer may be impaired.

In addition, in order to improve the corrosion resistance of the chemical conversion coating layer, titania particles, alumina particles, zirconia particles, etc. may be contained in addition to silica particles.

The chemical conversion layer may contain a pigment containing Cu, Co or Fe. Examples of the pigment include copper (II) phthalocyanine, cobalt (II) phthalocyanine, copper sulfate, cobalt sulfate, iron sulfate or iron oxide. The content of the pigment in the chemical conversion layer is preferably in the range of 0.1 to 10% by mass.

The chemical conversion layer may further contain either or both of an Nb compound and a phosphate compound. When an Nb compound or a phosphate compound is contained, the corrosion resistance of the Zn-based plating layer is improved.

The total content of Nb compounds and phosphate compounds in the chemical conversion layer should be 0.5 to 30 mass%. If it is 0.5 mass% or more, the corrosion resistance is improved, and if it is 30 mass% or less, the chemical conversion layer does not become brittle and the plating layer can be stably coated.

The chemical conversion coating layer may contain at least one crosslinking agent selected from the group consisting of silane coupling agents, crosslinkable zirconium compounds, and crosslinkable titanium compounds in order to improve adhesion to the plating layer. These may be used alone or in combination of two or more.

The content of at least one crosslinking agent selected from the group consisting of the silane coupling agent, crosslinkable zirconium compound, and crosslinkable titanium compound is preferably 0.1 to 50% by mass relative to 100% by mass of the solid content of the resin. If the content is less than 0.1% by mass, the content is too low and the effect of improving adhesion may not be obtained, and if the content exceeds 50% by mass, the stability of the aqueous composition may decrease.

The chemical conversion layer may further contain at least one crosslinking agent selected from the group consisting of amino resins, polyisocyanate compounds, their blocks, epoxy compounds, and carbodiimide compounds. These crosslinking agents may be used alone or in combination of two or more.

In order to improve the corrosion resistance of the plating layer, it is preferable that the chemical conversion layer contains at least one compound selected from the group consisting of vanadium compounds, tungsten compounds, and molybdenum compounds. These compounds may be used alone or in combination of two or more.

The content of at least one compound selected from the group consisting of vanadium compounds, tungsten compounds, and molybdenum compounds is preferably 0.01 to 20% by mass relative to 100% by mass of the solid content of the resin. If the content is less than 0.01% by mass, the content is too low and the effect of improving corrosion resistance may not be obtained, and if the content exceeds 20% by mass, the chemical conversion coating layer may become brittle and the corrosion resistance may decrease.

Furthermore, the deposition amount of the chemical conversion layer per one side of the plating layer is preferably 0.1 to 15 g/ ^m2 , regardless of the material of the chemical conversion layer. If the deposition amount is 0.1 g/ ^m2 or more, the deposition amount of the chemical conversion layer is sufficient, and the corrosion resistance of the plating layer can be improved. Furthermore, if the deposition amount is 15 g/ ^m2 or less, even if the chemical conversion layer contains a pigment, the reflection of light on the surface of the chemical conversion layer is reduced, and the metallic appearance of the surface of the plating layer can be visually recognized. A more preferable deposition amount is 0.2 to 2 g/ ^m2 .

In addition, examples of inorganic or organic-inorganic composite chemical conversion coating layers include a chemical conversion coating layer containing a zirconium compound (A1) having 10% by mass or more of zirconium and a vanadyl compound (A2) supplied as a salt of vanadyl ions (VO ²⁺ ) having 5% by mass or more of vanadium. The resin content in this chemical conversion coating layer is more than 0 to 10% by mass. The chemical conversion coating layer contains the zirconium compound (A1) and the vanadyl compound (A2) as essential film-forming components. The chemical conversion coating layer containing the zirconium compound (A1) and the vanadyl compound (A2) as film-forming components has excellent barrier properties against corrosive factors such as water and oxygen, i.e., excellent corrosion resistance.

Examples of the zirconium compound (A1) include zirconyl nitrate, zirconyl acetate, zirconyl sulfate, ammonium zirconium carbonate, potassium zirconium carbonate, sodium zirconium carbonate, zirconium acetate, zirconium hydrofluoric acid, and salts thereof. Among these zirconium compounds, zirconium hydrofluoric acid and salts thereof, and zirconium compounds containing zirconium carbonate complex ions are preferred from the viewpoint of corrosion resistance. The zirconium compounds containing zirconium carbonate complex ions are not particularly limited, and examples thereof include ammonium salts, potassium salts, and sodium salts of zirconium carbonate complex ions [Zr(CO ₃ ) ₂ (OH) ₂ ] ^2- or [Zr(CO ₃ ) ₃ (OH)] ^3- .

The vanadyl compound (A2) is a compound supplied as a salt of vanadyl ion (VO ²⁺ ). The vanadyl ion (VO ²⁺ ) is an oxovanadium cation supplied as a salt with an inorganic acid such as hydrochloric acid, nitric acid, phosphoric acid, or sulfuric acid, or an organic acid anion such as formic acid, acetic acid, propionic acid, butyric acid, or oxalic acid.

Examples of vanadyl compounds (A2) include salts of oxovanadium cations with inorganic acid anions such as hydrochloric acid, nitric acid, phosphoric acid, and sulfuric acid, or organic acid anions such as formic acid, acetic acid, propionic acid, butyric acid, and oxalic acid, and chelates of organic acids and vanadyl compounds, such as vanadyl glycolate and vanadyl dehydroascorbate.

The inorganic chemical conversion coating layer may further contain phosphate compounds, silica particles, cobalt compounds, titanium compounds, etc.

Next, the patterned and non-patterned portions of the plating layer, and the thick and non-thick portions of the chemical conversion layer will be described with reference to Figures 1A and 1B.

Figure 1A shows an example of a schematic cross-sectional view of a surface-treated steel sheet according to this embodiment. The surface-treated steel sheet shown in Figure 1A includes a steel sheet 1, a plating layer 2 formed on the surface of the steel sheet 1, and a chemical conversion layer 3 formed on the surface of the plating layer 2. In Figure 1A, the surface of the plating layer 2 is a flat surface.

The chemical conversion layer 3 has non-thick film portions 3b and thick film portions 3a that are thicker than the non-thick film portions 3b. The thick film portions 3a are arranged to have a predetermined shape when the chemical conversion layer 3 is viewed in a plan view. Since the thick film portions 3a and non-thick film portions 3b are present on the flat plating layer 2, the surface of the chemical conversion layer 3 is not flat but has projections and recesses.

The film thickness ratio ( _tP / _tB ) of the thickness _tP of the thick film portion 3a of the chemical conversion layer 3 to the thickness _tB of the non-thick film portion 3b is set to be 1.5 or more. The film thickness ratio ( _tP / _tB ) may be 1.75 or more, or 2 or more. The film thickness ratio ( _tP / _tB ) may be 4 or less, or 3 or less. By setting the film thickness ratio to be 1.5 or more, a discolored area is formed on the surface of the plating layer 2 located under the thick film portion 3a.

The thickness _tP of the thick film portion 3a of the chemical conversion layer 3 may be in the range of 0.2 to 25 μm, 0.3 to 10 μm, or 0.4 to 3 μm. The thickness _tB of the non-thick film portion 3b may be in the range of 0.1 to 15 μm, 0.2 to 6 μm, or 0.2 to 1.5 μm. By having the thicknesses of the thick film portion 3a and the non-thick film portion 3b in the above-mentioned ranges, the original function of the chemical conversion layer, that is, the improvement of the corrosion resistance and the paint adhesion of the plating layer, is achieved, and it becomes possible to form the pattern portion 2a and the non-pattern portion 2b in the plating layer 2.

The thickness _tP of the thick film portion 13a of the chemical conversion layer and the thickness _tB of the non-thick film portion 13b are measured and calculated by the following method. First, a gold vapor deposition layer with a thickness of 50 nm is formed on the surface of a sample cut to a predetermined size from the surface-treated steel sheet of this embodiment. Next, the gold vapor deposition sample is embedded in resin and polished so that the cross section of the sample in the plate thickness direction is exposed. The thickness of the gold vapor deposition layer is negligibly small compared to the chemical conversion layer.

Then, the cross section of the sample is observed at a magnification of 5000 times using a scanning electron microscope to observe the shape of the gold vapor deposition layer. The gold vapor deposition is performed to clarify the boundary between the embedding resin and the chemical conversion layer. That is, since the gold vapor deposition layer adheres following the shape of the surface of the chemical conversion layer, it is possible to confirm the outline of the surface of the chemical conversion layer when the chemical conversion layer is viewed in cross section by confirming the gold vapor deposition layer by SEM observation. This makes it possible to identify the area sandwiched between the gold vapor deposition layer and the plating layer as the chemical conversion layer, and to measure the thickness of the identified chemical conversion layer from the SEM image. In this way, the thickness t _P of the thick film portion 13a and the thickness t _B of the non-thick film portion 13b of the chemical conversion layer are measured, respectively.

A patterned portion 2a and a non-patterned portion 2b are formed on the surface of the plating layer 2. As shown in FIG. 1A, the patterned portion 2a and the non-patterned portion 2b are on the same surface of the plating layer 2. The patterned portion 2a is a strongly discolored area, and the non-patterned portion 2b is a weakly discolored area compared to the patterned portion 2a. The patterned portion 2a is arranged to have a predetermined shape when the plating layer 2 is viewed in a plan view. The patterned portion 2a is formed at a position corresponding to the thick portion 3a of the chemical conversion layer 3. The non-patterned portion 2b is formed at a position corresponding to the non-thick portion 3b of the chemical conversion layer 3.

The weakly discolored areas (non-patterned areas 2b) of the plating layer 2 are areas that have an appearance close to the original appearance of the plating layer. For example, if the plating layer 2 is a Zn-Al-Mg-based hot-dip plating layer, the weakly discolored areas (non-patterned areas 3b) have an appearance close to a matte finish. Also, if the plating layer 2 is a Zn-based hot-dip plating layer, the weakly discolored areas (non-patterned areas 2b) have an appearance close to the appearance unique to a zinc plating layer.

In contrast, the strongly discolored areas (patterned portion 2a) of the plating layer 2 have an appearance that is discolored black compared to the original appearance of the plating layer. Therefore, by forming patterned portion 2a and non-patterned portion 2b in the plating layer 2, it becomes possible to grasp the shape of the blackened patterned portion 2a with the naked eye.

In this embodiment, the color difference ΔE between the strongly discolored area (patterned portion 2a) and the weakly discolored area (non-patterned portion 2b) is set to 3.0 or more. The color difference ΔE may be 5.0 or more, or 7.0 or more. When the color difference ΔE between the patterned portion 2a and the non-patterned portion 2b is 3.0 or more, the patterned portion 2a is clearly distinguishable with the naked eye. There is no need to limit the upper limit of the color difference ΔE, but it may be, for example, 20 or less, 15 or less, or 10 or less.

The color difference ΔE between the patterned portion 2a and the non-patterned portion 2b is evaluated in accordance with JIS K 5600-4-6:1999. Specifically, the color difference is determined from the difference in the L ^* , a ^* , b ^* color space between the patterned portion 2a and the non-patterned portion 2b. When the measurement area is small, a microsurface spectrophotometer (VSS 7700, manufactured by Nippon Denshoku Industries Co., Ltd.) can be used. The color difference (ΔE) is calculated by the following formula.

ΔE = {(L ^* ₂ - L ^* ₁ ) ² + (a ^* ₂ - ^a* ₁ ) ² + (b ^* ₂ - b ^* ₁ ) ² } ^1/2
Here, L ^* ₁ , a ^* ₁ , b ^* ₁ are the measurement values of the color space L ^* a ^* b ^* of the non-patterned portion, and L ^* ₂ , a ^* ₂ , b ^* ₂ are the measurement values of the color space L ^* a ^* b ^* of the patterned portion.

Furthermore, the surface-treated steel sheet of this embodiment is not limited to the form shown in FIG. 1A, but may also have the form shown in FIG. 1B.

The surface-treated steel sheet shown in FIG. 1B includes a steel sheet 1, a plating layer 12 formed on the surface of the steel sheet, and a chemical conversion layer 13 formed on the surface of the plating layer 12. The surface of the plating layer 12 is an uneven surface.

The plating layer 12 has recesses 12a with a relatively thin plating thickness and protrusions 12b with a relatively thick plating thickness. On top of this plating layer 12 is a chemical conversion layer 13. The upper surface 13c of the chemical conversion layer 13 is a flat surface. This results in the chemical conversion layer 13 having non-thick film portions 13b and thick film portions 13a with a greater thickness than the non-thick film portions 13b. The thick film portions 13a are on the recesses 12a of the plating layer 12, and the non-thick film portions 13b are on the protrusions 12b of the plating layer 12. The thick film portions 13a of the chemical conversion layer 13 and the recesses 12a of the plating layer 12 are arranged to have a predetermined shape when the chemical conversion layer 13 is viewed in a plane.

The film thickness ratio ( _tP / _tB ) of the thickness _tP of the thick film portion 13a of the chemical conversion layer 13 to the thickness _tB of the non-thick film portion 13b in Fig. 1B is 1.5 or more, preferably 1.75 or more, and more preferably 2 or more, as in the case of Fig. 1A. The film thickness ratio ( _tP / _tB ) may be 4 or less, or may be 3 or less. The reason for limiting the film thickness ratio ( _tP / _tB ) is the same as in the case of Fig. 1A.

As in the case of Fig. 1A, the thickness _tP of the thick film portion 13a may be in the range of 0.2 to 25 µm, 0.3 to 10 µm, or 0.4 to 3 µm. The thickness _tB of the non-thick film portion 13b may be in the range of 0.1 to 15 µm, 0.2 to 6 µm, or 0.2 to 1.5 µm. The reasons for limiting the thicknesses ( _tP , _tB ) are the same as in the case of Fig. 1A.

The surface of the plating layer 12 has recesses 12a and protrusions 12b. The recesses 12a are regions of strong discoloration and are patterned portions. The protrusions 12b are regions of weak discoloration and are non-patterned portions. In the following description, both the recesses and the patterned portions are indicated by the reference symbol 12a. The protrusions and the non-patterned portions are indicated by the reference symbol 12b. The patterned portions 12a (recesses) are arranged to have a predetermined shape when the plating layer 12 is viewed in plan. The patterned portions 12a are formed at positions corresponding to the thick film portions 13a of the chemical conversion layer 13. The non-patterned portions 12b (protrusions) are formed at positions corresponding to the non-thick film portions 13b of the chemical conversion layer 13.

The weakly discolored areas (non-patterned areas 12b) of the plating layer 12 are areas that have an appearance close to that of the original plating layer. As in the case of Figure 1A, when the plating layer is a Zn-Al-Mg-based hot-dip plating layer, the weakly discolored areas (non-patterned areas 12b) have an appearance close to that of a matte finish. When the plating layer is a Zn-based hot-dip plating layer, the weakly discolored areas (non-patterned areas 12b) have an appearance close to that characteristic of a zinc plating layer.

In contrast, the strongly discolored areas (patterned portion 12a) of the plating layer 12 have an appearance that is discolored black compared to the original appearance of the plating layer. Therefore, by forming patterned portion 12a and non-patterned portion 12b in the plating layer 12, it becomes possible to grasp the shape of the blackened patterned portion 12a with the naked eye.

As in the case of FIG. 1A, the color difference ΔE between the strongly discolored area (patterned portion 12a) and the weakly discolored area (non-patterned portion 12b) is preferably 3.0 or more, may be 5.0 or more, or may be 7.0 or more. The reason for limiting the color difference ΔE is the same as in the case of FIG. 1A.

The patterned portions 2a, 12a in Figs. 1A and 1B are preferably arranged to have a shape that is one or a combination of two or more of the following: straight lines, curved lines, dots, figures, numbers, symbols, patterns, or letters. The non-patterned portions 2b, 12b are areas other than the patterned portions 2a, 12a. The shape of the patterned portions 2a, 12a is acceptable even if some parts are missing, such as missing dots, as long as they can be recognized as a whole. The non-patterned portions 2b, 12b may be shaped to border the boundaries of the patterned portions 2a, 12a.

When the surface of the plating layer 2, 12 has one or a combination of two or more of the following shapes arranged thereon: straight lines, curved lines, dots, figures, numbers, symbols, patterns, or letters, these areas can be patterned areas 2a, 12a, and the other areas can be non-patterned areas 2b, 12b. The boundaries between the patterned areas 2a, 12a and the non-patterned areas 2b, 12b can be seen with the naked eye. The boundaries between the patterned areas 2a, 12a and the non-patterned areas 2b, 12b can also be seen from a magnified image obtained with an optical microscope or a magnifying glass.

The patterned portions 2a, 12a are preferably formed to a size such that the presence of the patterned portions 2a, 12a can be discerned with the naked eye, under a magnifying glass, or under a microscope. The non-patterned portions 2b, 12b are regions that occupy most of the surface of the plating layer 2, 12, and the patterned portions 2a, 12a may be located within the non-patterned portions 2b, 12b.

The patterned parts 2a, 12a are arranged in a predetermined shape in the non-patterned parts 2b, 12b. Specifically, the patterned parts 2a, 12a are arranged in the non-patterned parts 2b so as to have a shape of one of straight lines, curved lines, figures, dots, figures, numbers, symbols, patterns, or letters, or a shape of a combination of two or more of these. By adjusting the shape of the patterned parts 2a, 12a, a shape of one of straight lines, curved lines, figures, dots, figures, numbers, symbols, patterns, or letters, or a shape of a combination of two or more of these, is formed on the surface of the plating layer 2, 12. For example, a character string, a number string, a symbol, a mark, a line diagram, a design, or a combination of these, consisting of the patterned parts 2a, 12a, is formed on the surface of the plating layer. This shape is a shape formed intentionally or artificially by a manufacturing method described later, and is not formed naturally.

The patterned parts 2a, 12a and the non-patterned parts 2b, 12b may be distinguishable with the naked eye, or may be distinguishable under a magnifying glass or a microscope. Distinguishing under a magnifying glass or a microscope means, for example, that the shape formed by the patterned parts 2a, 12a is distinguishable in a visual field of 50x or less. In a visual field of 50x or less, the patterned parts 2a, 12a and the non-patterned parts 2b, 12b are distinguishable due to the difference in their appearance. The patterned parts 2a, 12a and the non-patterned parts 2b, 12b are distinguishable preferably at 20x or less, more preferably at 10x or less, and even more preferably at 5x or less.

The shapes of the thick film portions 3a, 13a and non-thick film portions 3b, 13b of the chemical conversion layers 3, 13 are similar to the shapes of the patterned portions 2a, 12a and non-patterned portions 2b, 12b of the plating layers, so a description is omitted.

The plated steel sheet according to this embodiment may further have a clear coating film on the surface of the chemical conversion layer. However, when forming a clear coating film on the chemical conversion layer, it is preferable to form it after the pattern portion clearly appears on the plating layer, as described below.

Next, a method for producing the hot-dip plated steel sheet according to this embodiment will be described.
To manufacture the plated steel sheet of this embodiment by a hot-dip plating method, the steel sheet is immersed in a hot-dip plating bath with adjusted chemical composition to allow molten metal to adhere to the steel sheet surface. Next, the steel sheet is pulled out of the plating bath, and the amount of adhesion is controlled by gas wiping, after which the molten metal is solidified. After the plating layer is formed in this manner, a chemical conversion layer is formed on the plating layer surface. When forming the chemical conversion layer, a thick film portion and a non-thick film portion are formed. In this manner, a plated steel sheet provided with a chemical conversion layer having a thick film portion and a non-thick film portion is manufactured. Furthermore, by leaving the plated steel sheet for example in a high humidity environment for one month or more, a plated steel sheet having a pattern portion and a non-pattern portion formed in the plating layer is obtained.

The manufacturing method of the hot-dip galvanized steel sheet of this embodiment will be described in more detail below. The following description is of the manufacturing method of the surface-treated steel sheet shown in FIG. 1A.

First, a hot-rolled steel sheet is manufactured, and if necessary, hot-rolled sheet annealing is performed. After pickling, the sheet is cold-rolled to produce a cold-rolled sheet. The cold-rolled sheet is degreased, washed with water, and then annealed (cold-rolled sheet annealing). The annealed cold-rolled sheet is then immersed in a hot-dip plating bath to form a hot-dip plating layer.

Next, the steel sheet is immersed in a hot-dip galvanizing bath. The hot-dip galvanizing bath contains 0-90% by mass of Al, 0-10% by mass of Mg, and the remainder contains Zn and impurities. The hot-dip galvanizing bath may further contain 0.0001-2% by mass of Si. The hot-dip galvanizing bath may further contain 0.0001-2% by mass of one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, and C in total. The average composition of the hot-dip galvanizing layer in this embodiment is almost the same as the composition of the hot-dip galvanizing bath. The hot-dip galvanizing method may be a continuous hot-dip galvanizing method in which the steel sheet is continuously passed through the hot-dip galvanizing bath, or may be a so-called dip galvanizing method in which the steel sheet is immersed in the hot-dip galvanizing bath and then pulled up.

The temperature of the hot-dip galvanizing bath varies depending on the composition, but is preferably in the range of, for example, 400 to 500° C. This is because if the temperature of the hot-dip galvanizing bath is in this range, a desired hot-dip galvanized layer can be formed.
The adhesion weight of the hot-dip coating layer may be adjusted by means of gas wiping or the like on the steel sheet pulled up from the hot-dip coating bath. The adhesion weight of the hot-dip coating layer is preferably adjusted so that the total adhesion weight on both sides of the steel sheet is in the range of 30 to 600 g/ ^m2 . If the adhesion weight is less than 30 g/ ^m2 , the corrosion resistance of the hot-dip coated steel sheet decreases, which is not preferable. If the adhesion weight exceeds 600 g/ ^m2 , the molten metal attached to the steel sheet drips, making it impossible to smooth the surface of the hot-dip coating layer, which is not preferable.

After adjusting the coating weight of the hot-dip plating layer, the steel sheet is cooled. There is no need to limit the cooling conditions.

Next, a chemical conversion layer having a thick film portion and a non-thick film portion is formed on the surface of the hot-dip plating layer. The chemical conversion layer can be obtained, for example, by applying an aqueous composition containing the above-mentioned resin, silica particles, pigment, Nb compound, phosphate compound, and other components to the surface of the plating layer and drying it. In this embodiment, in order to form the thick film portion, for example, the process described below is carried out.

A solvent may be used in the aqueous composition to improve film-forming properties and form a more uniform and smooth coating. The solvent is not particularly limited as long as it is one that is generally used in paints. For example, from the viewpoint of leveling, examples of the solvent include hydrophilic solvents such as alcohols, ketones, esters, and ethers.

2(a), an example of a method for forming the chemical conversion treatment layer is to coat the entire surface of the plating layer 2 with an aqueous composition and then dry it to form a chemical conversion treatment film 3A. When forming the chemical conversion treatment film 3A, it is preferable to adjust the film thickness so that it is within the range of the thickness _tB of the non-thick film portion.

The method of coating the aqueous composition when forming the chemical conversion coating film 3A is not particularly limited, and commonly used methods such as roll coating, air spraying, airless spraying, and immersion can be used as appropriate.

Next, as shown in FIG. 2(b), a chemical conversion film 3B is formed on the chemical conversion film 3A. Note that FIG. 2(b) shows the state in which the chemical conversion film 3B is in the middle of being formed. The chemical conversion film 3B is formed on only a part of the chemical conversion film 3A, not on the entire surface, and in the area where the pattern portion of the plating layer 2 is to be formed. In this manner, the chemical conversion layer 3 is formed.

After the chemical conversion film 3B is formed, the chemical conversion layer 3 has areas where only the chemical conversion film 3A is laminated and areas where both the chemical conversion films 3A and 3B are laminated. As shown in FIG. 2(c), the areas where only the chemical conversion film 3A is laminated become non-thick film portions 3b, and the areas where both the chemical conversion films 3A and 3B are laminated become thick film portions 3a.

When forming the chemical conversion film 3B, the total thickness of the chemical conversion films 3A and 3B is adjusted to be at least 1.5 times the thickness of the chemical conversion film 3A. Furthermore, the method for forming the chemical conversion film 3B may be, for example, an inkjet method in which an aqueous composition is ejected from a nozzle, or a printing method in which the aqueous composition is printed.

The method for forming the chemical conversion layer 3 is not limited to the example shown in FIG. 2, and may be formed, for example, according to the procedure shown in FIG. 3.

As shown in FIG. 3(a), a chemical conversion film 3C is formed on the plating layer 2. The chemical conversion film 3C is formed in the area of the plating layer 2 where the pattern portion is to be formed. The method for forming the chemical conversion film 3C may be, for example, an inkjet method in which an aqueous composition is ejected from a nozzle, or a printing method in which an aqueous composition is printed.

Next, a chemical conversion film 3D is formed so as to cover the plating layer 2 and the chemical conversion film 3C. The method for forming the chemical conversion film 3D can be appropriately selected from roll coating, air spraying, airless spraying, etc. When forming the chemical conversion film 3D, it is preferable to adjust the film thickness so that it is within the range of the thickness _tB of the non-thick film portion, and so that the total film thickness of the chemical conversion films 3C and 3D is 1.5 times or more the thickness of the chemical conversion film 3C.

After the chemical conversion film 3D is formed, the chemical conversion layer 3 has areas where only the chemical conversion film 3D is laminated, and areas where the chemical conversion films 3C and 3D are laminated. As shown in FIG. 3(c), the areas where only the chemical conversion film 3D is laminated become non-thick film portions 3b, and the areas where the chemical conversion films 3C and 3D are laminated become thick film portions 3a.

In Figures 2 and 3, when forming the chemical conversion coatings 3B and 3C, the planar shape of these films is formed to be an intentional shape that is one of straight lines, curved lines, dots, figures, numbers, symbols, patterns, or letters, or a combination of two or more of these.

In order to enhance the hardening of the chemical conversion layer, the plated layer 2, which is the object to be coated, may be heated before forming the chemical conversion films 3A to 3D, or may be thermally dried after forming the chemical conversion layer 3.

In this manner, a plated steel sheet is manufactured that includes a chemical conversion layer 3 having a thick film portion 3a and a non-thick film portion 3b, and a plating layer 2.

Immediately after the formation of the chemical conversion layer 3, the plating layer 2 of the plated steel sheet has no pattern portion. If such a plated steel sheet is left for, for example, one month or more, discoloration of the surface of the plating layer 2 progresses further in the region where the thick film portion 3a of the chemical conversion layer 3 is formed, and the pattern portion 2a of a predetermined shape appears. The atmosphere during the period in which the plated steel sheet is left can be air, and does not need to be a specially controlled atmosphere, but high humidity may be preferable. In addition, the atmospheric temperature during the period does not need to be a specially controlled temperature, and can be either indoor temperature or outdoor temperature. The location where the plated steel sheet is left can be either indoors or outdoors.

In this way, a plated steel sheet is obtained in which the chemical conversion layer 3 has thick film portions 3a and non-thick film portions 3b, and the plating layer 2 has patterned portions 2a and non-patterned portions 2b.

As another example of the method for manufacturing the surface-treated steel sheet of this embodiment, a chemical conversion layer may be formed on a plating layer having recesses and protrusions on the surface, and then the sheet may be left for one month or more.

That is, as shown in FIG. 4(a), a plating layer 12 is formed on a steel sheet 1 by hot-dip plating. The hot-dip plating may be a continuous hot-dip plating method or a dip-dip plating method. The plating bath temperature and the amount of the hot-dip plating layer may be the same as those in FIG. 2 and FIG. 3.

Next, as shown in FIG. 4(b), recesses 2a and protrusions 2b are provided on the surface of the plating layer 12. For example, recesses 2a are provided to form a predetermined pattern, and the areas other than the recesses 2a are made into protrusions 2b. Examples of means for providing recesses 2a and protrusions include a method of rolling and transferring the uneven pattern of the plating layer 12 using a rolling roll having a roll surface, a method of forming recesses by chemical etching, a method of forming recesses by grinding, and the like.

Next, as shown in FIG. 4(c), a chemical conversion layer 13 is formed on the surface of the plating layer 12 provided with the recesses 2a and the protrusions 2b. The chemical conversion layer 13 is obtained by applying an aqueous composition containing components such as the above-mentioned resin, silica particles, pigment, Nb compound, and phosphate compound to the surface of the plating layer 12 and drying it. At this time, the adhesion amount of the chemical conversion layer 13 is adjusted so that the upper surface 13c of the chemical conversion layer 13 is flat. This results in a chemical conversion layer 13 having a thick film portion 13a and a non-thick film portion 13b. The thicknesses t _P and t _B of the thick film portion 13a and the non-thick film portion 13b are adjusted by adjusting the depth from the upper surface of the protrusions 2b in the plating layer 12 to the bottom surface of the recesses 12a, and the thickness of the chemical conversion layer 13 formed on the protrusions 12b of the plating layer 12.

In this manner, a plated steel sheet is manufactured that includes a chemical conversion layer 13 having a thick film portion 13a and a non-thick film portion 13b, and a plating layer 12.

Immediately after the formation of the chemical conversion layer 13, the plating layer 12 of the plated steel sheet has no discolored pattern portion. If such a plated steel sheet is left for, for example, one month or more, discoloration of the surface of the plating layer 12 progresses further in the region where the thick film portion 13a of the chemical conversion layer 13 is formed, and a discolored pattern portion 12a of a predetermined shape appears. The atmosphere and atmospheric temperature during the period when the plated steel sheet is left can be the same as those in Figures 2 and 3.

In this way, a plated steel sheet is obtained in which the chemical conversion layer 13 has thick film portions 13a and non-thick film portions 13b, and the plating layer 12 has patterned portions 12a and non-patterned portions 12b.

The plated steel sheet of this embodiment has patterned and non-patterned parts in the plating layer, which are made up of areas with varying degrees of discoloration, making it easy to distinguish between the patterned and non-patterned parts. The patterned and non-patterned parts are not formed by printing or painting, and therefore are highly durable. In addition, because the patterned and non-patterned parts are not formed by printing or painting, there is no effect on the corrosion resistance of the hot-dip plating layer. Therefore, the hot-dip plated steel sheet of this embodiment has excellent corrosion resistance.

According to this embodiment, a plated steel sheet can be provided in which the pattern portion formed into a predetermined shape has high durability and has favorable plating properties such as corrosion resistance. In particular, in this embodiment, when forming a chemical conversion treatment layer, a pattern portion can be formed on the surface of the plating layer by a relatively simple method in which a thick film portion of a predetermined shape and a non-thick film portion having a thickness smaller than that of the thick film portion are provided and then left as is, and further the shape of the pattern portion can be made intentional or artificial.

According to this embodiment, the pattern portion can be arranged to have a shape that is one or a combination of two or more of the following: straight lines, curved lines, dots, figures, numbers, symbols, patterns, or letters. This allows various designs, trademarks, and other identification marks to be presented on the surface of the plating layer without printing, painting, or grinding, thereby improving the identifiability of the origin of the plated steel sheet and the design quality, etc. Furthermore, the pattern portion can also provide the plated steel sheet with information necessary for process management, inventory management, and the like, or any information desired by consumers. This can also contribute to improving the productivity of plated steel sheets.

Next, an example of the present invention will be described. Cold-rolled steel sheets were annealed, and the annealed cold-rolled sheets were immersed in a hot-dip plating bath to form a plating layer.

Next, a chemical conversion layer was formed on the surface of the plating layer. The chemical conversion layer had thick film parts and non-thick film parts formed in a grid pattern spaced 50 mm apart.

The chemical conversion treatment layer was formed by carrying out a chemical conversion treatment with a chemical conversion treatment agent containing a phthalocyanine pigment. The chemical conversion treatment agent used was an aqueous coating agent that contained polyurethane resin particles and ethylene-unsaturated carboxylic acid copolymer resin particles having silanol groups and alkoxysilyl groups, silicon oxide particles, an organic titanium compound, and Cu phthalocyanine, in which the content of the Cu phthalocyanine pigment was 0.01 to 10 parts by mass per 100 parts by mass of the total of the polyurethane resin particles and the ethylene-unsaturated carboxylic acid copolymer resin particles, and the primary particle diameter of the Cu phthalocyanine was 0.01 to 1.0 μm.

As a comparative example, after the plating layer was formed, a grid pattern with 50 mm intervals was printed on the surface of the plating layer by the inkjet method. Then, a chemical conversion coating layer was formed. In this way, plated steel sheet No. 75 was manufactured.

The plated steel sheets obtained in this manner were left in the air for one month in an indoor storage yard within the steelworks. The temperature was not specifically controlled, but was left to fluctuate according to the season. The environmental temperature fluctuated within the range of 10 to 20°C.

After leaving the plated steel sheets, it was checked whether a patterned portion had appeared in the plating layer, and if a patterned portion had appeared, the color difference ΔE between the patterned portion and the non-patterned portion was measured. The patterned and non-patterned portions were identified by observing the surface of the plating layer with the naked eye. When identification with the naked eye was difficult, a magnifying glass or a magnified image with an optical microscope was used. In cases where it was difficult to distinguish the patterned portion, the area corresponding to the thick film portion of the chemical conversion coating layer was deemed to be the patterned portion.

The color difference ΔE was evaluated in accordance with JIS K 5600-4-6:1999. Specifically, the color difference was calculated from the difference in the L ^* , a ^* , b ^* color space between the patterned portion 2a and the non-patterned portion 2b. The color difference was measured using a microsurface spectrophotometer (VSS 7700, manufactured by Nippon Denshoku Industries Co., Ltd.). The color difference (ΔE) was calculated by the following formula.

ΔE = {(L ^* ₂ - L ^* ₁ ) ² + (a ^* ₂ - ^a* ₁ ) ² + (b ^* ₂ - b ^* ₁ ) ² } ^1/2
Here, L ^* ₁ , a ^* ₁ , b ^* ₁ are the measurement values of the color space L ^* a ^* b ^* of the non-patterned portion, and L ^* ₂ , a ^* ₂ , b ^* ₂ are the measurement values of the color space L ^* a ^* b ^* of the patterned portion.

The thickness _tP of the thick part of the chemical conversion layer and the thickness _tB of the non-thick part were measured by the following method. The surface of a sample cut to a predetermined size from a hot-dip plated steel sheet was gold-vapor-deposited to a thickness of 50 nm, and the gold-vapor-deposited sample was embedded in resin and polished so that the cross section of the sample in the sheet thickness direction was exposed. The cross section of the sample was observed at a magnification of 5000 times using a scanning electron microscope to observe the form of the gold vapor-deposited layer. Then, the region sandwiched between the gold vapor-deposited layer and the plating layer was identified as the chemical conversion layer, and the thicknesses _tP and _tB of the chemical conversion layer were measured from the SEM image.

[Distinguishing property]
After leaving the plated steel sheet for one month, it was visually evaluated according to the following criteria. ⊚ to △ were considered to be acceptable.

⊚: The pattern can be seen from 5 m away.
◯: The pattern portion cannot be seen from 5 m away, but is highly visible from 3 m away.
Δ: The pattern portion cannot be seen from 3 m away, but visibility is high from 1 m away.
×: The pattern portion cannot be seen from 1 m away.

[Corrosion resistance]
The test plates were cut into 150 x 70 mm pieces and subjected to 30 cycles of accelerated corrosion testing (CCT) in accordance with JASO-M609. The occurrence of rust was then examined and evaluated according to the following criteria. A to B were deemed to be acceptable.
⊚: No rust occurs, and both the patterned and non-patterned parts maintain a beautiful design appearance.
◯: No rust was observed, but very slight changes in the design and appearance were observed in the pattern and non-pattern areas.
Δ: The design appearance is slightly impaired, but the patterned and non-patterned areas are visually distinguishable.
×: The appearance quality of the patterned and non-patterned portions is significantly deteriorated and cannot be distinguished by visual inspection.

As shown in Tables 1A to 1C, in the invention examples No. 1 to No. 71, the chemical composition of the plating layer and the film thickness ratio of the chemical conversion coating layer satisfy the ranges of the present invention, so that a patterned portion consisting of a clearly discolored discolored area appears, and the color difference ΔE between the patterned portion and the non-patterned portion is 3.0 or more. This provides excellent distinguishability and corrosion resistance.

As shown in Table 1C, in the comparative example No. 72, the film thickness ratio of the chemical conversion layer did not satisfy the range of the present invention, so the discoloration of the area that would become the patterned part was insufficient, and the color difference ΔE between the patterned part and the non-patterned part was less than 3.0. This reduced the distinguishability.

In the comparative example No. 73, the Al content of the plating layer was excessive, which reduced the adhesion of the plating layer and resulted in insufficient corrosion resistance.

In the comparative example No. 74, the Mg content in the plating layer was excessive, which reduced the adhesion of the plating layer and resulted in insufficient corrosion resistance.

In the comparative example No. 75, the pattern portion was formed on the plating layer by printing, so the durability of the pattern portion was insufficient and the identifiability was reduced.

1...steel plate, 2...plating layer, 2a...patterned portion, 2b...non-patterned portion, 3...chemical conversion layer, 3a...thick film portion, 3b...non-thick film portion.

Claims (13)

The present invention comprises a steel sheet, a hot-dip plating layer formed on a surface of the steel sheet, and a chemical conversion coating layer formed on a surface of the hot-dip plating layer,
The hot-dip plating layer contains, in an average composition, Al: 0 to 90 mass%, Mg: 0 to 10 mass%, and the balance contains Zn and impurities,
The hot-dip plating layer has a pattern portion and a non-pattern portion arranged to have a predetermined shape,
a color difference ΔE between the patterned portion and the non-patterned portion is 3.0 or more;
A surface-treated steel sheet, wherein a thickness ratio ( _tP / _tB ) of a thickness _tP of the chemical conversion layer on the patterned portion to a thickness _tB of the chemical conversion layer on the non-patterned portion is 1.5 or more. The surface-treated steel sheet according to claim 1, characterized in that the pattern portion is arranged to have a shape that is one of straight lines, curved lines, dots, figures, numbers, symbols, patterns, or letters, or a combination of two or more of these. The surface-treated steel sheet according to claim 1, characterized in that the pattern portion is intentionally formed. The surface-treated steel sheet according to claim 1, characterized in that the pattern portion is arranged to have a shape that is one or a combination of two or more of the following: straight lines, curved lines, dots, figures, numbers, symbols, patterns, or letters, and the pattern portion is intentionally formed. The present invention comprises a steel sheet, a hot-dip plating layer formed on a surface of the steel sheet, and a chemical conversion coating layer formed on a surface of the hot-dip plating layer,
The hot-dip plating layer contains, in an average composition, Al: 0 to 90 mass%, Mg: 0 to 10 mass%, and the balance contains Zn and impurities,
The chemical conversion layer has thick film portions and non-thick film portions arranged in a predetermined shape,
A surface-treated steel sheet, wherein a thickness ratio ( _tP / _tB ) of a thickness _tP of the thick film portion of the chemical conversion layer to a thickness _tB of the non-thick film portion of the chemical conversion layer is 1.5 or more. The surface-treated steel sheet according to claim 5, wherein the thick film portion has a thickness t _P in the range of 0.2 to 25 μm, and the non-thick film portion has a thickness t _B in the range of 0.1 to 15 μm. The surface-treated steel sheet according to claim 5, characterized in that the thick film portion is arranged to have a shape that is one of straight lines, curved lines, dots, figures, numbers, symbols, patterns, or letters, or a combination of two or more of these. The surface-treated steel sheet according to claim 5, characterized in that the thick film portion is intentionally formed. The surface-treated steel sheet according to claim 5, characterized in that the thick film portion is arranged to have a shape that is one of straight lines, curved lines, dots, figures, numbers, symbols, patterns, or letters, or a combination of two or more of these, and that the thick film portion is intentionally formed. The surface-treated steel sheet according to any one of claims 1 to 9, wherein the coating weight of the hot-dip plating layer is 30 to 600 g / m ² in total on both sides of the steel sheet. The surface-treated steel sheet according to claim 1 , wherein the hot-dip plating layer further contains, in an average composition, one or two selected from the group consisting of the following Group A and Group B:
[Group A] Si: 0.0001 to 2 mass%
[Group B] Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C, any one or more of these, in a total content of 0.0001 to 2 mass% The surface-treated steel sheet according to claim 11, wherein the hot-dip coating layer has an average composition containing, by mass%, the A group. The surface-treated steel sheet according to claim 11, wherein the hot-dip coating layer has an average composition containing, in mass %, the B group.

Images