Hot-Dip Plated Steel Sheet

The present invention relates to a hot-dip plated steel sheet.

Priority is claimed on Japanese Patent Application No. 2022-094358, filed Jun. 10, 2022, the content of which is incorporated herein by reference.

Hot-dip plated steel sheets have excellent corrosion resistance, and among them, a Zn—Al—Mg-based hot-dip plated steel sheet has particularly excellent corrosion resistance. Such a hot-dip plated steel sheet is widely used in various manufacturing industries such as building materials, home electric appliances, and automobile fields, and the use amount thereof has increased in recent years.

Incidentally, there is a case where characters, design drawings, and the like are displayed on a surface of a hot-dip plated layer of a hot-dip plated steel sheet by subjecting the hot-dip plated layer to a step such as printing or coating for the purpose of displaying characters, design drawings, and the like on the surface of the hot-dip plated layer.

However, when steps such as printing and coating are performed on the bot-dip plated layer, there is a problem in that the cost and time for applying characters, designs, and the like increase. Furthermore, when characters, designs, and the like are displayed on the surface of the plated layer by printing or coating, not only is the metallic gloss external appearance favored by consumers lost, but durability is also poor due to problems of degradation of the coating film itself with time and degradation of adhesion of the coating film with time, and characters, designs, and the like may disappear over time. Furthermore, when characters, designs, and the like are displayed on the surface of the plated layer by stamping the ink, cost and time are relatively suppressed, but corrosion resistance of the hot-dip plated layer may be reduced by the ink.

As shown in the following Patent Documents, various technical developments have been made in the field of Zn—Al—Mg-based hot-dip plated steel sheets, but there is no known technique for improving the durability when characters, designs, and the like are displayed on the surface of a plated layer.

Regarding Zn—Al—Mg-based hot-dip plated steel sheets, there are prior art techniques aimed at improving the satin-like plating appearance of these Zn—Al—Mg-based hot-dip plated steel sheets to make them more beautiful.

For example, Patent Document 1 describes a Zn—Al—Mg-based hot-dip plated steel sheet having a satin-like external appearance with fine texture and many smooth glossy portions, that is, a Zn—Al—Mg-based hot-dip plated steel sheet having a good satin-like external appearance in which the number of white portions per unit area is large and the ratio of the area of the glossy portions is large. In addition, Patent Document 1 describes that an unfavorable satin state is a state where a surface appearance in which indefinite white portions and circular glossy portions are mixed and scattered on the surface is exhibited.

In addition, Patent Document 2 describes a Zn—Al—Mg-based plated steel sheet in which, in a cross section of a plated layer in a thickness direction, a part where Al crystals are not present between an interface between the plated layer and a base metal and a plated surface layer occupies 10% to 50% of a length of the cross section in a width direction, thereby improving plating appearance.

Further, Patent Document 3 describes a hot-dip galvanized steel sheet having excellent formability, in which a center line average roughness Ra of a surface of the plated steel sheet is 0.5 to 1.5 μm, a PPI (the number of peaks having a size of 1.27 μm or more included per 1 inch (2.54 cm)) is 150 to 300, and Pc (the number of peaks having a size of 0.5 μm or more included per 1 cm) is Pc≥PPI/2.54+10.

Furthermore, Patent Document 4 describes a highly corrosion-resistant hot-dip galvanized steel sheet in which a ternary eutectic structure of Al/MgZn/Zn is refined to increase glossiness of a plated layer as a whole and improve external appearance evenness.

However, a technique for improving the durability and preventing the corrosion resistance from being lowered when characters or the like are displayed on the surface of the plated layer is not conventionally known.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-dip plated steel sheet which can display characters, designs, and the like on the surface of a plated layer, has excellent durability thereof, and also has excellent corrosion resistance.

The gist of the present invention is as follows.

[1] A hot-dip plated steel sheet, including:

Assuming that a thickness of the hot-dip plated layer is t, a cross section of 1 to 5 mm square parallel to a surface of the hot-dip plated layer is exposed at any position of 3t/4 position, t/2 position, or t/4 position from the surface of the hot-dip plated layer, virtual lattice lines are drawn at intervals of 0.5 mm on each of the cross sections, a region in which a proportion (B/A (%)) of an area fraction B of a [Zn phase] to a total area fraction A of the [Zn phase] and an [Al/MgZn/Zn ternary eutectic structure] is 20% or more in each of a plurality of regions partitioned by the virtual lattice lines is defined as the first region, and a region in which the proportion (B/A (%)) is less than 20% is defined as the second region.

[2] A hot-dip plated steel sheet, including:

Assuming that a thickness of the hot-dip plated layer is t, a cross section of 1 to 5 mm square parallel to a surface of the hot-dip plated layer is exposed at any position of 3t/4 position, t/2 position, or t/4 position from the surface of the hot-dip plated layer, virtual lattice lines are drawn at intervals of 0.5 mm on each of the cross sections, a region in which a proportion (B/A (%)) of an area fraction B of a [Zn phase] to a total area fraction A of the [Zn phase] and an [Al/MgZn/Zn ternary eutectic structure] is 20% or more in each of a plurality of regions partitioned by the virtual lattice lines is defined as the first region, and a region in which the proportion (B/A (%)) is less than 20% is defined as the second region.

[3] The hot-dip plated steel sheet described in [1] or [2], in which the pattern portion is disposed to have a shape of any one of a straight line portion, a curve portion, a dot portion, a figure, a number, a symbol, and a character, or a combination of two or more thereof.

[4] The hot-dip plated steel sheet described in any one of [1] to [3], in which an adhesion amount of the hot-dip plated layer is 30 to 600 g/min total on both surfaces of the steel sheet.

[5] The hot-dip plated steel sheet described in any one of [2] to [4], in which the hot-dip plated layer has an average composition containing the group A in terms of mass %.

[6] The hot-dip plated steel sheet described in any one of [2] to [5], in which the hot-dip plated layer has an average composition containing the group B in terms of mass %.

According to the present invention, it is possible to provide a hot-dip plated steel sheet which can present characters, designs, and the like on the surface of a hot-dip plated layer, which has excellent durability thereof, and which also has excellent corrosion resistance.

The present inventors investigated in detail a plated layer of a Zn—Al—Mg-based hot-dip plated steel sheet having a satin-like external appearance. The satin-like external appearance is provided by a mixture of a fine metallic glossy part exhibiting metallic gloss and a fine white part exhibiting white. Among them, when the microstructure of the plated layer in the metallic glossy part was examined, it was found that the area fraction of the [Zn phase] on the surface of the plated layer was smaller than that in the white part. On the other hand, when the microstructure of the plated layer in the white part was examined, it was found that the ratio of the [Zn phase] to the [Al/MgZn/Zn ternary eutectic structure] was higher than that in the metallic glossy part.

Therefore, the present inventors studied whether it is possible to control the distribution state of the metallic glossy part and the white part in the hot-dip plated layer, and found that a region including a relatively large number of metallic glossy parts can be intentionally disposed on the surface of the hot-dip plated layer by adjusting the chemical composition of the hot-dip plated layer and performing hot-dip plating after disposing a region having relatively low cleanliness on the sheet surface into an intentional shape before immersing the steel sheet in a hot-dip plating bath, thereby completing the present invention.

Hereinafter, the hot-dip plated steel sheet according to an embodiment of the present invention will be described.

As shown in, the hot-dip plated steel sheet of the present embodiment is a hot-dip plated steel sheet including: a steel sheet; and a hot-dip plated layerformed on a surface of the steel sheet, the hot-dip plated layer, in terms of average composition, contains 5 to 22 mass % of Al and 1 to 10 mass % of Mg with the remainder including Zn and impurities, the hot-dip plated layerincludes a pattern portionand a non-pattern portion, the pattern portionand the non-pattern portioneach includes one or two of a first region Aand a second region Aobtained by the following measurement method, and the absolute value of the difference between the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionis 30% or more.

The method for measuring the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionis as follows. Assuming that the thickness of the hot-dip plated layeris t, a cross section of 1 to 5 mm square parallel to a surfaceof the hot-dip plated layeris exposed at any position of 3t/4 position, t/2 position, or t/4 position from the surface of hot-dip plated layer. Then, as shown in, virtual lattice lines are drawn at intervals of 0.5 mm in each cross section, and in each of a plurality of regions partitioned by the virtual lattice lines, a region in which a proportion (B/A (%)) of an area fraction B of the [Zn phase] to a total area fraction A of the [Zn phase] and the [Al/MgZn/Zn ternary eutectic structure] is 20% or more is defined as the first region A, and a region in which the proportion (B/A (%)) is less than 20% is defined as the second region A.

The exposed surface for measurement shown inis a square of 5 mm square. In the exposed surface, the number of regions partitioned by the virtual lattice line is 100. When the pattern portion is small and an exposed surface of 5 mm square cannot be formed inside the pattern portion, the size of the exposed surface may be reduced. In this case, a plurality of exposed surfaces are formed, whereby the total value of the number of regions partitioned by the virtual lattice line is set to 100. For example, when the exposed surface is a square of 1 mm square, the number of regions partitioned by the virtual lattice line in one exposed surface is four. When the exposed surface of 1 mm square is formed at 25 locations, the total value of the number of regions partitioned by the virtual lattice line is 100.

Note that the number of pattern portions may be two or more. In this case, the exposed surface for measurement may be formed on each of the plurality of pattern portions.

When the pattern portion is extremely narrow and the number of regions partitioned by the virtual lattice lines cannot be set to 100, the interval between the virtual lattice lines may be narrowed. For example, the interval between the virtual lattice lines may be changed to a value of 0.2 mm or more and less than 0.5 mm. By narrowing the interval between the virtual lattice lines, the number of regions (that is, measurement points) partitioned by the virtual lattice lines can be set to 100 inside the extremely narrow pattern portion.

When a plurality of exposed surfaces are formed inside the pattern portion, the distance between the exposed surfaces is reduced as much as possible. A plurality of exposed surfaces formed inside the pattern portion may be in contact with each other. Even when a plurality of exposed surfaces are formed inside the non-pattern portion, the distance between the plurality of exposed surfaces is preferably reduced as much as possible, and the plurality of exposed surfaces may be in contact with each other.

In addition, when the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionare measured, it is preferable to reduce the distance between the exposed surface formed inside the pattern portion and the exposed surface formed inside the non-pattern portion as much as possible,

In the hot-dip plated steel sheet of the present embodiment, when a cross section of 1 to 5 mm square is exposed at any position of 3t/4 position, t/2 position, or t/4 position from the surface of the hot-dip plated layer, and virtual lattice lines are drawn at intervals of 0.5 mm in the cross section, a plurality of regions partitioned by the virtual lattice lines are divided into either the first region Aor the second region A, Which region is divided into the first region Aand the second region Ais determined according to the proportion (B/A (%)) of the area fraction B of the [Zn phase] to the total area fraction A of the [Zn phase] and the [Al/MgZn/Zn ternary eutectic structure],

The first region Ais a region where the proportion (B/A (%)) is 20% or more. Since the proportion (B/A (%) of the first region Ais high, a location including a large number of the first regions in the hot-dip plated layerlooks white or nearly white when observed with the naked eye or under a microscope. On the other hand, the second region Ais a region in which the proportion (B/A (%)) is less than 20%. Since the proportion (B/A (%) of the second region Ais low, a location where a large number of the second regions Ais included and the number of first regions Ais reduced in the hot-dip plated layer looks like the location has a metallic gloss when observed with the naked eye or under a microscope. Further, the external appearance of a location where the first region Aand the second region Aare mixed and the area fraction of the first region Ais 30 to 70% looks satin-like.

As described above, depending on the area fraction of the first region A, the surfaceof the hot-dip plated layerlooks white or nearly white, metallic glossy, or satin-like. Here, in order to make characters, figures, lines, dots, and the like visible on the surfaceof the hot-dip plated layer, it is only required that the pattern portionsconstituting these characters and the like and the other non-pattern portionscan be identified. For this purpose, the area percentage of the first region Ain the pattern portionand the area percentage of the first region Ain the non-pattern portionmay be different.

Specifically, the difference between the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionis preferably 30% or more in absolute value. Thus, the pattern portionand the non-pattern portioncan be identified from each other. When the difference in area fraction is evaluated, it is not necessary to evaluate the entire region of the pattern portionand the non-pattern portion. As shown in, the area fraction of the first region Ain a 1 to 5 mm square surface (exposed surface) for measurement provided inside the pattern portioncan be regarded as the area fraction of the first region Ain the entire pattern portion. Similarly, the area fraction of the first region Ain a 1 to 5 mm square surface (exposed surface) for measurement provided inside the non-pattern portioncan be regarded as the area fraction of the first region Ain the entire non-pattern portion.

From the viewpoint of further improving the visibility of the pattern portion, the absolute value of the difference between the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionmay be 40% or more, 45% or more, or 50% or more. Although it is not necessary to provide an upper limit of the absolute value of the difference between the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portion, for example, the absolute value of the difference between the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionmay be 95% or less, 90% or less, or 85% or less.

For example, when the area percentage of the first region Ain the pattern portionis 75%, the pattern portionlooks white or nearly white. In addition, when the area percentage of the first region Ain the non-pattern portionis 45% or less, the first region Alooks satin-like or metallic glossy. When the difference in area fraction between the first regions Ain the pattern portionand the non-pattern portionis 30% or more, the pattern portionand the non-pattern portioncan be identified from each other due to such a difference in external appearance.

When the area percentage of the first region Aof the pattern portionis approximately 65% and the area fraction of the first region Aof the non-pattern portionis approximately 35%, although both the pattern portionand the non-pattern portionlook satin-like, since the area fraction of the first region Ain the pattern portionis large, the pattern portionhas a whiter appearance than the non-pattern portion. When the difference in area fraction between the first regions Ain the pattern portionand the non-pattern portionis 30% or more, the pattern portionand the non-pattern portioncan be identified from each other due to such a difference in external appearance.

Furthermore, when the first region Aof the pattern portionis 50%, the pattern portionlooks satin-like. In addition, when the area fraction of the first region Ain the non-pattern portionis 20% or less, the first region Alooks satin-like or metallic glossy. When the difference in area fraction between the first regions Ain the pattern portionand the non-pattern portionis 30% or more, the pattern portionand the non-pattern portioncan be identified from each other due to such a difference in external appearance.

As described above, when the difference between the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionis 30% or more in absolute value, external appearances of the pattern portionand the non-pattern portionbecome different, and thus the pattern portioncan be clearly identified. That is, in the visible-light image of the surfaceof the plated layer, the difference in hue, luminosity, saturation, and the like between the pattern portionand the non-pattern portionincreases, and thus the pattern portionand the non-pattern portioncan be identified.

On the other hand, when the difference between the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionis less than 30% in absolute value, there is no difference in external appearance between the pattern portionand the non-pattern portion, and the pattern portioncannot be clearly identified. That is, in the visible-light image of the surfaceof the plated layer, the difference in hue, luminosity, saturation, and the like between the pattern portionand the non-pattern portiondecreases, and thus the pattern portionand the non-pattern portioncannot be identified.

As described above, an example of an abundance ratio of the first region Ain the pattern portionand the non-pattern portionhas been described. However, it is sufficient that the difference between the area fraction of the first region Ain the pattern portionand the area fraction of the first region Ain the non-pattern portionis 30% or more in absolute value, and it is not necessary to limit an existence ratio of the first region Ain each of the pattern portionand the non-pattern portion.

The material of the steel sheet as a base of the hot-dip plated layer is not particularly limited. Although details will be described later, general steel or the like can be used as a material without particular limitation, Al-killed steel or some high alloy steel can also be applied, and the shape is also not particularly limited. The hot-dip plated layer according to the present embodiment is formed by applying a hot-dip plating method described later to the steel sheet.

Next, the chemical composition of the hot-dip plated layer will be described.