TWI840251B - Molten-coated steel plate - Google Patents

Abstract

This case uses a molten-coated steel plate, which has a molten-coated layer formed on the surface of the steel plate, and a pattern portion and a non-pattern portion are formed in the molten-coated layer. The pattern portion and the non-pattern portion include a first area and a second area defined below, and the absolute value of the difference in area ratio of the first area between the pattern portion and the non-pattern portion is greater than 30%. Furthermore, a cross section parallel to the surface is exposed at any position among the 3t/4 position, the t/2 position, and the t/4 position from the surface of the molten deposited layer, and imaginary grid lines are drawn in each cross section. Among the multiple regions divided by the imaginary grid lines, the region where the ratio of the [Zn phase] area fraction B to the total area fraction A of the [Zn phase] and the [Al/MgZn ₂ /Zn ternary eutectic structure] is 20% or more is defined as the first region, and the region where the ratio is less than 20% is defined as the second region.

Description

Molten-coated steel plate

The present invention relates to a molten-coated steel plate. This case claims priority based on Japanese Special Application No. 2022-094358 filed in Japan on June 10, 2022, and its contents are cited here.

Hot-dip plated steel sheets have excellent corrosion resistance, and Zn-Al-Mg hot-dip plated steel sheets have particularly excellent corrosion resistance. This type of hot-dip plated steel sheet is widely used in various manufacturing industries such as building materials, home appliances, and automobiles, and its usage has continued to increase in recent years.

However, in order to display text, design drawings, etc. on the surface of the molten-coated layer of the molten-coated steel plate, sometimes a printing or painting step is performed on the molten-coated layer to display the text, design drawings, etc. on the surface of the molten-coated layer.

However, if printing or painting is performed on the melt-coated layer, the cost and time required to apply text or design will increase. Furthermore, when printing or painting text or design on the surface of the coated layer, not only will the metallic glossy appearance that is highly supported by consumers be lost, but the durability may deteriorate and the text or design may disappear over time due to the degradation of the coating itself or the adhesion of the coating. In addition, when printing text or design on the surface of the coated layer by embossing ink, although the cost and time can be suppressed, there is a concern that the corrosion resistance of the melt-coated layer will be reduced due to the ink.

As shown in the following patent documents, various technologies have been continuously developed for Zn-Al-Mg based molten-metal plated steel sheets. However, there is still no known technology that can improve the durability when characters or designs are displayed on the surface of the plated layer.

There is a known technology for Zn-Al-Mg based molten-coated steel sheets, the purpose of which is to make the pear-skin coating appearance observed in the Zn-Al-Mg based molten-coated steel sheets more beautiful.

For example, Patent Document 1 describes a Zn-Al-Mg based hot-dip plated steel sheet having a fine texture and a pear-skin appearance with many smooth glossy parts, that is, a good pear-skin appearance with a large number of white parts per unit area and a large area ratio of glossy parts. In addition, Patent Document 1 describes a poor pear-skin state as a state showing the following surface appearance: irregularly shaped white parts and round glossy parts are mixed and scattered on the surface.

In addition, Patent Document 2 describes a Zn-Al-Mg based coated steel plate, wherein in a thickness direction cross section of the coated layer, a portion where Al crystals do not exist between the interface between the coated layer and the base iron and the coated surface layer accounts for 10% to 50% of the length in the width direction of the cross section, thereby improving the coating appearance.

In addition, Patent Document 3 describes a molten galvanized steel plate with excellent formability, wherein the centerline average roughness Ra of the surface of the galvanized steel plate is 0.5~1.5µm, the PPI (the number of peaks with a size of 1.27µm or more contained in 1 inch (2.54cm)) is 150~300, and the Pc (the number of peaks with a size of 0.5µm or more contained in 1cm) is Pc≧PPI/2.54+10.

In addition, Patent Document 4 describes a high corrosion-resistant hot-dip galvanized steel plate, which increases the overall gloss of the coating layer and improves the appearance uniformity by refining the Al/MgZn ₂ /Zn ternary eutectic structure.

However, there is still no known technology that can improve the durability of a coating layer without reducing its corrosion resistance when displaying text or the like on the surface of the coating layer.

Prior art documents Patent documents Patent document 1: Japanese Patent No. 5043234 Patent document 2: Japanese Patent No. 5141899 Patent document 3: Japanese Patent No. 3600804 Patent document 4: International Publication No. 2013/002358

Problem to be solved by the invention This invention is made in view of the above situation, and its problem is to provide a molten-coated steel plate, which can display text or design on the surface of the coating layer, and has excellent durability and corrosion resistance.

Means for Solving the Problem The gist of the present invention is as follows. [1] A molten-coated steel plate, characterized in that: it comprises: a steel plate and a molten-coated layer formed on the surface of the steel plate; the molten-coated layer contains, in average composition, 5 to 22% by mass of Al and 1.0 to 10% by mass of Mg, and the remainder contains Zn and impurities; the molten-coated layer has a pattern portion and a non-pattern portion; the pattern portion and the non-pattern portion each contain one or two of the first region and the second region obtained by the following measurement method; and the absolute value of the difference between the area ratio of the first region in the pattern portion and the area ratio of the first region in the non-pattern portion is 30% or more; [Measurement method] The thickness of the molten deposited layer is t, and a cross section of 1 to 5 mm square parallel to the surface of the molten deposited layer is exposed at any position of 3t/4, t/2 or t/4 from the surface of the molten deposited layer, and imaginary grid lines are drawn at intervals of 0.5 mm in each of the cross sections. Among the multiple regions demarcated by the imaginary grid lines, the region where the ratio (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 ₂ /Zn ternary eutectic structure] is 20% or more is the first region, and the region where the ratio (B/A(%) is less than 20% is the second region. [2] A molten-coated steel plate, characterized in that: it comprises: a steel plate and a molten-coated layer formed on the surface of the steel plate; the molten-coated layer contains, in average composition, 5-22 mass % of Al and 1.0-10 mass % of Mg, with the remainder containing Zn and impurities; the molten-coated layer further contains one or two selected from the group consisting of the following group A and group B; the molten-coated layer has a pattern portion and a non-pattern portion; the pattern portion and the non-pattern portion each contain one or two of a first region and a second region obtained by the following measurement method; and the absolute value of the difference between the area ratio of the first region in the pattern portion and the area ratio of the first region in the non-pattern portion is 30% or more; [Group A] Si: 0.0001~2 mass%; [Group B] Any one of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C or more in total 0.0001~2 mass%; [Measurement method] Let the thickness of the above-mentioned molten deposited layer be t, and expose a 1~5mm square section parallel to the above-mentioned surface at any position of 3t/4 position, t/2 position or t/4 position from the surface of the above-mentioned molten deposited layer, and draw imaginary grid lines at 0.5mm intervals in each of the above-mentioned sections. In the multiple regions divided by the above-mentioned imaginary grid lines, let the [Zn phase] area fraction B relative to [Zn phase] and [Al/MgZn ₂ [3] The hot-dip galvanized steel sheet of [1] or [2], wherein the pattern portion is configured in the following shapes: any one of a straight line portion, a curved line portion, a dot portion, a figure, a number, a symbol or a character, or a combination of two or more of these shapes. [4] The hot-dip galvanized steel sheet of any one of [1] to [3], wherein the adhesion amount of the hot-dip galvanized layer is 30 to 600 g/ ^m2 in total on both sides of the steel sheet. [5] The hot-dip coated steel sheet as described in any one of [2] to [4], wherein the hot-dip coated layer has an average composition containing the aforementioned group A in mass %. [6] The hot-dip coated steel sheet as described in any one of [2] to [5], wherein the hot-dip coated layer has an average composition containing the aforementioned group B in mass %.

Effect of the invention According to the present invention, a molten-coated steel plate can be provided, which can display text or design on the surface of the molten-coated layer, and has excellent durability and corrosion resistance.

Form for carrying out the invention The inventors of this case have carefully investigated the coating of Zn-Al-Mg based molten-coated steel sheet showing a pear-skin appearance. The pear-skin appearance is caused by the mixed existence of a fine metallic luster portion showing a metallic luster and a fine white portion showing a white color. Among them, the coating structure of the metallic luster portion was investigated, and it was found that the [Zn phase] area fraction on the coating surface of the metallic luster portion was smaller than that of the white portion. On the other hand, the coating structure of the white portion was investigated, and it was found that the ratio of [Zn phase] to [Al/MgZn ₂ /Zn ternary eutectic structure] of the white portion was higher than that of the metallic luster portion.

Therefore, it was studied whether the distribution state of the metallic glossy part and the white part in the molten plating layer could be arbitrarily controlled. As a result, it was found that by adjusting the chemical composition of the molten plating layer and arranging the area with lower cleanliness on the surface of the steel plate in a deliberate shape before immersing the steel plate in the molten plating bath, and then performing the molten plating, it is possible to deliberately arrange the area containing more metallic glossy parts on the surface of the molten plating layer, thereby completing the present invention.

The following describes the molten-plated steel plate according to the embodiment of the present invention. As shown in Figures 1 to 3, the molten-coated steel plate of this embodiment comprises: a steel plate 1, and a molten-coated layer 2 formed on the surface of the steel plate 1; the molten-coated layer 2 contains, in average composition, Al: 5 to 22 mass%, Mg: 1 to 10 mass%, and the remainder contains Zn and impurities; the molten-coated layer 2 has a pattern portion 21 and a non-pattern portion 22; the pattern portion 21 and the non-pattern portion 22 each contain one or two regions of the first region A1 and the second region A2 obtained by the following measurement method; and the absolute value of the difference between the area ratio of the first region A1 in the pattern portion 21 and the area ratio of the first region A1 in the non-pattern portion 22 is 30% or more.

The area ratio of the first region A1 in the pattern portion 21 and the area ratio of the first region A1 in the non-pattern portion 22 are determined as follows. Let the thickness of the molten coating 2 be t, and a 1-5 mm square section parallel to the surface 2a of the molten coating 2 is exposed at any position of 3t/4 position, t/2 position or t/4 position from the surface of the molten coating 2. Then, as shown in FIG3, imaginary grid lines are drawn at intervals of 0.5 mm in each cross section, and in the multiple regions divided by the imaginary grid lines, the region where the ratio (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 ₂ /Zn ternary eutectic structure] is 20% or more is the first region A1, and the region where the ratio (B/A(%) is less than 20% is the second region A2. In addition, the exposure surface for measurement shown in FIG3 is a square of 5 mm square. In the exposure surface, the number of regions divided by the imaginary grid lines is 100. When the pattern portion is very small and it is impossible to form an exposure surface of 5 mm square inside the pattern portion, the size of the exposure surface can also be reduced. In this case, a plurality of exposed surfaces are formed so that the total value of the number of areas divided by the imaginary grid lines is 100. For example, when the exposed surface is a square of 1 mm square, the number of areas divided by the imaginary grid lines in one exposed surface is 4. If exposed surfaces of 1 mm square are formed at 25 locations, the total value of the number of areas divided by the imaginary grid lines is 100. In addition, the number of pattern parts may be more than 2. In this case, the exposed surfaces for measurement may be formed in a plurality of pattern parts respectively. When the pattern part is very small and the number of areas divided by the imaginary grid lines cannot be 100, the intervals between the imaginary grid lines may be reduced. For example, the intervals between the imaginary grid lines may be changed to a value greater than 0.2 mm and less than 0.5 mm. By reducing the interval between the imaginary grid lines, the number of areas (i.e., measurement points) demarcated by the imaginary grid lines can be made 100 in a very narrow pattern portion. When a plurality of exposed surfaces are formed in the pattern portion, the distance between the exposed surfaces should be reduced as much as possible. The plurality of exposed surfaces formed in the pattern portion may also be connected. When a plurality of exposed surfaces are formed in the non-pattern portion, the distance between the plurality of exposed surfaces should also be reduced as much as possible, and the plurality of exposed surfaces may also be connected. In addition, when measuring the area ratio of the first area A1 in the pattern portion 21 and the area ratio of the first area A1 in the non-pattern portion 22, the distance between the exposed surface formed in the pattern portion and the exposed surface formed in the non-pattern portion should also be reduced as much as possible.

In the hot-dip coated steel plate of the present embodiment, a cross section of 1 to 5 mm square is exposed at any position of the 3t/4 position, the t/2 position or the t/4 position from the surface of the hot-dip coated layer 2, and when imaginary grid lines are drawn at intervals of 0.5 mm in the cross section, the plurality of regions demarcated by the imaginary grid lines can be classified as either the first region A1 or the second region A2. Either the first region A1 or the second region A2 is determined by the ratio (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 ₂ /Zn ternary eutectic structure].

The first region A1 is a region where the ratio (B/A(%)) is 20% or more. The ratio (B/A(%) of the first region A1 is high, so when observed with the naked eye or under a microscope, the first region is rich in the molten deposited layer 2, and it looks white or a color close to white. On the other hand, the second region A2 is a region where the ratio (B/A(%)) is less than 20%. The ratio (B/A(%) of the second region A2 is low, so when observed with the naked eye or under a microscope, the second region A2 is rich in the molten deposited layer and the first region A1 is reduced, and it looks like a metallic luster. In addition, the first region A1 and the second region A2 are mixed and the area ratio of the first region A1 is 30-70%, and the appearance looks like pear skin.

As described above, depending on the area ratio of the first area A1, the surface 2a of the molten-plated layer 2 may appear white or a color close to white, metallic luster, or pear-skin-like. Here, in order to make it possible to visually recognize text, graphics, lines, dots, etc. on the surface 2a of the molten-plated layer 2, it is sufficient that the pattern portion 21 constituting the text, etc. and the non-pattern portion 22 other than the pattern portion 21 become recognizable. To this end, it is sufficient that the area ratio of the first area A1 in the pattern portion 21 and the area ratio of the first area A1 in the non-pattern portion 22 are different from each other.

Specifically, the difference between the area ratio of the first area A1 in the pattern portion 21 and the area ratio of the first area A1 in the non-pattern portion 22 may be 30% or more in absolute value. In this way, the pattern portion 21 and the non-pattern portion 22 can be distinguished. In addition, when evaluating the difference in area ratio, it is not necessary to evaluate the entire area of the pattern portion 21 and the non-pattern portion 22. As shown in FIG. 3, the area ratio of the first area A1 in the 1-5 mm square surface (exposed surface) for measurement provided inside the pattern portion 21 can be regarded as the area ratio of the first area A1 in the entire pattern portion 21. Similarly, the area ratio of the first area A1 in the 1-5 mm square surface (exposed surface) for measurement provided inside the non-patterned portion 22 can be regarded as the area ratio of the first area A1 in the entire non-patterned portion 22. From the perspective of further improving the visibility of the patterned portion 21, the absolute value of the difference between the area ratio of the first area A1 in the patterned portion 21 and the area ratio of the first area A1 in the non-patterned portion 22 can also be 40% or more, 45% or more, or 50% or more. There is no need to set an upper limit on the absolute value of the difference between the area ratio of the first area A1 in the pattern portion 21 and the area ratio of the first area A1 in the non-pattern portion 22, but for example, the absolute value of the difference between the area ratio of the first area A1 in the pattern portion 21 and the area ratio of the first area A1 in the non-pattern portion 22 may be set to less than 95%, less than 90%, or less than 85%.

For example, when the area ratio of the first region A1 in the pattern portion 21 is 75%, the pattern portion 21 appears white or a color close to white. In addition, when the area ratio of the first region A1 in the non-pattern portion 22 is 45% or less, it appears pear-skin-like or metallic. Furthermore, when the difference in the area ratio of the first region A1 between the pattern portion 21 and the non-pattern portion 22 is 30% or more, the pattern portion 21 and the non-pattern portion 22 can be distinguished due to this difference in appearance.

Furthermore, when the area ratio of the first region A1 of the pattern portion 21 is about 65% and the area ratio of the first region A1 of the non-pattern portion 22 is about 35%, the pattern portion 21 and the non-pattern portion 22 both look like pear skin, but because the area ratio of the first region A1 in the pattern portion 21 is larger, the pattern portion 21 appears whiter than the non-pattern portion 22. Furthermore, when the difference in area ratio of the first region A1 between the pattern portion 21 and the non-pattern portion 22 is 30% or more, the pattern portion 21 and the non-pattern portion 22 can be distinguished due to this difference in appearance.

Furthermore, when the first area A1 of the pattern part 21 is 50%, the pattern part 21 looks like a pear skin. Also, when the area ratio of the first area A1 in the non-pattern part 22 is 20% or less, it looks like a metallic luster. Moreover, when the difference in the area ratio of the first area A1 between the pattern part 21 and the non-pattern part 22 is 30% or more, the pattern part 21 and the non-pattern part 22 can be distinguished due to this difference in appearance.

As described above, if the difference between the area ratio of the first region A1 in the pattern portion 21 and the area ratio of the first region A1 in the non-pattern portion 22 is 30% or more in absolute value, the appearance of the pattern portion 21 and the non-pattern portion 22 will become different, so that the pattern portion 21 can be clearly identified. That is, in the visible light image of the surface 2a of the coating layer 2, the difference in hue, lightness, and chroma between the pattern portion 21 and the non-pattern portion 22 becomes larger, so that the pattern portion 21 and the non-pattern portion 22 can be identified.

On the other hand, if the difference between the area ratio of the first region A1 in the pattern portion 21 and the area ratio of the first region A1 in the non-pattern portion 22 is less than 30% in absolute value, the pattern portion 21 and the non-pattern portion 22 become indistinguishable in appearance, and the pattern portion 21 cannot be clearly identified. That is, in the visible light image of the surface 2a of the coating layer 2, the difference in hue, lightness, and chroma between the pattern portion 21 and the non-pattern portion 22 becomes smaller, and thus the pattern portion 21 and the non-pattern portion 22 cannot be identified.

As shown above, an example of the existence ratio of the first area A1 in the pattern portion 21 and the non-pattern portion 22 is shown. However, if the difference between the area ratio of the first area A1 in the pattern portion 21 and the area ratio of the first area A1 in the non-pattern portion 22 is greater than 30% in absolute value, there is no need to limit the existence ratio of the first area A1 in each of the pattern portion 21 and the non-pattern portion 22.

The material of the steel plate as the base of the molten metal coating layer is not particularly limited. The details will be described later, but as the material, general steel can be used without particular limitation, and aluminum deoxidized steel or a part of high alloy steel can also be applied, and the shape is also not particularly limited. The molten metal coating layer of this embodiment can be formed by applying the molten metal coating method described later to the steel plate.

Next, the chemical composition of the molten metal coating is described. The molten metal coating contains, as an average composition, Al: 5~22 mass%, Mg: 1.0~10 mass%, and contains Zn and impurities as the remainder. It is preferable that the average composition contains, as an average composition, Al: 5~22 mass%, Mg: 1.0~10 mass%, and the remainder is composed of Zn and impurities. In addition, the molten coating layer may also contain one or two selected from the group consisting of the following A group and B group: [A group] Si: 0.0001~2 mass%; [B group] Any one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C, totaling 0.0001~2 mass%.

The Al content is in the range of 5 to 22 mass % in terms of average composition. Al may be contained to ensure corrosion resistance. If the Al content in the molten deposited layer is 5 mass % or more, the effect of improving corrosion resistance will be further enhanced. If the Al content is 22 mass % or less, the deposited layer can be stably formed. When the Al content is greater than 22 mass %, the effect of improving corrosion resistance will be saturated. From the perspective of corrosion resistance, the Al content is preferably 6 mass % or more, 8 mass % or more, or 11 mass % or more. From the perspective of corrosion resistance, the Al content is preferably less than 20 mass %, less than 19 mass % or less than 17 mass %.

The Mg content is in the range of 1.0~10 mass% in terms of average composition. Mg may be contained in order to improve corrosion resistance. If the Mg content in the molten-plated layer is 1.0 mass% or more, the effect of improving corrosion resistance will be further enhanced. If the Mg content is greater than 10 mass%, scum will be significantly generated in the plating bath, and it will be difficult to stably manufacture the molten-plated steel sheet. From the perspective of the balance between corrosion resistance and scum generation, the Mg content should be set to more than 1.5 mass%, more than 2 mass%, or more than 4 mass%. From the perspective of the balance between corrosion resistance and scum generation, the Mg content should be set to less than 8 mass%, less than 7 mass%, or less than 6 mass%.

In addition, the molten plating layer may contain Si in the range of 0.0001 to 2 mass%. Si may be contained because it may improve the adhesion of the molten plating layer. Since the effect of improving adhesion can be exhibited by containing more than 0.0001 mass% of Si, it is preferable to contain more than 0.0001 mass% of Si. On the other hand, even if more than 2 mass% of Si is contained, the effect of improving the adhesion of the plating will still be saturated, so the Si content is set to less than 2 mass%. From the perspective of plating adhesion, the Si content is preferably more than 0.0100 mass%, more than 0.0300 mass%, or more than 0.1000 mass%. The Si content may also be less than 1 mass%, less than 0.9 mass%, or 0.8 mass%.

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

The remainder of the chemical composition of the molten coating is zinc and impurities. The impurities include those that are inevitably contained in the parent metal and those that are contained in the steel as it is melted in the plating bath.

In addition, the average composition of the hot-dip coating can be determined by the following method. First, the surface coating is removed with a coating stripper that does not corrode the coating (e.g., neoever SP-751 manufactured by Sancai Chemical Co., Ltd.), and then the hot-dip coating is dissolved with hydrochloric acid doped with an inhibitor (e.g., HIBIRON manufactured by Sugimura Chemical Co., Ltd.), and the resulting solution is subjected to inductively coupled plasma (ICP) emission spectrometry to obtain the average composition. In addition, when there is no surface coating, the operation of removing the surface coating can be omitted.

Next, the structure of the molten metal coating is described. The molten metal coating containing Al, Mg and Zn includes [Al phase] and [Al/Zn/MgZn ₂ ternary eutectic structure]. The molten metal coating has a morphology in which [Al phase] is included in the raw material of [Al/Zn/MgZn ₂ ternary eutectic structure]. The raw material of [Al/Zn/MgZn ₂ ternary eutectic structure] further includes [MgZn ₂ phase] and [Zn phase]. Moreover, when Si is added, [Mg ₂ Si phase] may also be included in the raw material of [Al/Zn/MgZn ₂ ternary eutectic structure].

Here, the so-called [Al/Zn/MgZn ₂ ternary eutectic structure] refers to the ternary eutectic structure of Al phase, Zn phase and intermetallic compound MgZn ₂ phase. The Al phase forming the ternary eutectic structure is equivalent to the "Al" phase at high temperature in the ternary equilibrium state diagram of Al-Zn-Mg (Al solid solution with solid Zn and containing a small amount of Mg). The Al" phase at high temperature is usually separated into fine Al phase and fine Zn phase at room temperature. In addition, the Zn phase in the ternary eutectic structure is a Zn solid solution in which a small amount of Al and sometimes a small amount of Mg are further dissolved. The _MgZn2 phase in the ternary eutectic structure is an intermetallic compound phase existing in the Zn-Mg binary system equilibrium state diagram near Zn: about 84 mass%. Judging from the state diagram alone, it can be considered that there is no solid solution of other added elements in each phase or even if there is a solid solution, it is extremely small, but its amount cannot be clearly distinguished by general analysis, so in this specification, the ternary eutectic structure composed of the three phases is expressed as [Al/Zn/ _MgZn2 ternary eutectic structure].

In addition, the so-called "Al phase" refers to a phase with clear boundaries and island-like appearance in the raw material of the aforementioned ternary eutectic structure, which is equivalent to the "Al" phase at high temperature in the ternary equilibrium state diagram of Al-Zn-Mg (Al solid solution with solid Zn and containing a small amount of Mg). The Al" phase at high temperature has different Zn or Mg content in solid solution depending on the Al and Mg concentrations in the plating bath. The Al" phase at high temperature is usually separated into fine Al phase and fine Zn phase at room temperature, and the island shape observed at room temperature can be regarded as the shape of the Al" phase at high temperature. From the state diagram alone, it can be considered that there are no other added elements dissolved in the phase or even if there are solid solutions, they are extremely small, but it cannot be clearly distinguished by general analysis. Therefore, in this manual, the phase derived from the Al" phase at high temperature and with the shape of the Al" phase is called [Al phase]. In microscopic observation, [Al phase] can be clearly distinguished from the Al phase that forms the aforementioned ternary eutectic structure.

In addition, the so-called [Zn phase] refers to a phase with a clear boundary and an island-like appearance in the raw material of the aforementioned ternary eutectic structure, and in fact, a small amount of Al and even a small amount of Mg are solid-dissolved. Judging from the state diagram alone, it can be considered that there are no other added elements dissolved in this phase or even if there are solid-dissolved, they are extremely trace. The [Zn phase] is a region with a circular equivalent diameter of more than 2.5µm, and it can be clearly distinguished from the Zn phase that forms the aforementioned ternary eutectic structure in microscopic observation.

Furthermore, the so-called [MgZn ₂ phase] refers to a phase that has a clear boundary and appears to be in an island shape in the raw material of the aforementioned ternary eutectic structure, and in fact, a small amount of Al is also dissolved in it. Judging from the state diagram alone, it can be considered that no other added elements are dissolved in it, or even if there are dissolved elements, they are extremely small. The [MgZn ₂ phase] can be clearly distinguished from the MgZn ₂ phase that forms the aforementioned ternary eutectic structure in microscopic observation. In the coating of the present invention, there may be a situation where the [MgZn ₂ phase] is not included depending on the manufacturing conditions, but under almost all manufacturing conditions, the coating will include this phase.

The so-called [Mg ₂ Si phase] refers to a phase that has a clear boundary and appears to be an island in the solidified structure of the coating layer when Si is added. Judging from the phase diagram alone, it is believed that there is no solid solution of Zn, Al and other added elements, or even if there is a solid solution, it is a very small amount. In microscopic observation, the [Mg ₂ Si phase] can be clearly distinguished in the coating.

Next, the pattern portion, the non-pattern portion, the first region, and the second region on the surface of the molten deposited layer are described.

On the surface of the molten-plated layer of this embodiment, a pattern portion and a non-pattern portion are formed. From the viewpoint of ensuring the aesthetics of the pattern portion, the pattern portion is preferably configured into a predetermined shape. In addition, from the viewpoint of ensuring the visibility of the pattern portion, the larger the size of the pattern portion, the better. The pattern portion preferably has, for example, an artificial shape. The pattern portion is preferably configured into a deliberate shape. The pattern portion is preferably configured into any one of a straight line portion, a curved line portion, a dot portion, a figure, a number, a symbol, or a text, or a shape formed by combining two or more of these. For example, a text string, a number string, a symbol, a mark, a line graph, a design drawing, or a combination thereof, etc. formed by the pattern portion can be displayed on the surface of the molten-plated layer. The straight line portion or the curved line portion in the pattern portion is preferably more than 1 mm in length. Since these shapes are displayed, it can be said that the pattern portion is formed intentionally. The straight line portion or the curved line portion in the pattern portion should preferably have a width that can be visually identified, for example, as described below, and each should preferably be more than 1 mm in length. The dot portion in the pattern portion should preferably have a circle equivalent diameter of more than 1 mm and less than 10 mm, and more preferably a plurality of dot portions should be neatly arranged. Furthermore, when the pattern portion is a figure, number, symbol, pattern or text, such shapes should be visually identified, for example, as described below. Since such a size and shape are displayed, it can be said that the pattern portion is intentionally formed. In addition, the non-pattern portion is an area outside the pattern portion. Even if the shape of the pattern portion is partially missing, such as a dot dropout, it is permissible if it can be identified as a whole. The non-pattern portion can also be a shape such as a border that frames the pattern portion.

When any one of the shapes of straight line, curved line, dot, figure, number, symbol or text or a combination of two or more of these shapes is arranged on the surface of the molten deposited layer, the area can be defined as the pattern part, and the area outside the pattern part can be defined as the non-pattern part. The shape is a shape deliberately or artificially formed by the manufacturing method described below, and is not a naturally formed shape. The boundary between the pattern part and the non-pattern part can be grasped by the naked eye. The boundary between the pattern part and the non-pattern part can also be grasped from the image magnified by an optical microscope or a magnifying glass.

The pattern portion can be formed to a size that allows the presence of the pattern portion to be discerned with the naked eye, under a magnifying glass or under a microscope. In addition, the non-pattern portion is an area that occupies most of the molten-plated layer (molten-plated layer surface). The pattern portion is arranged in the non-pattern portion. Specifically, the pattern portion is arranged in the non-pattern portion in the following shapes: any one of a straight line portion, a curved line portion, a figure, a dot portion, a figure, a number, a symbol or a character, or a shape formed by combining two or more of them. By adjusting the shape of the pattern portion, any one of a straight line portion, a curved line portion, a figure, a dot portion, a figure, a number, a symbol or a character, or a shape formed by combining two or more of them, can be displayed on the surface of the molten-plated layer. For example, a character string, a number string, a symbol, a mark, a line diagram, a design drawing, or a combination thereof, which is composed of a pattern portion, can be displayed on the surface of the molten-plated coating. The shape is a shape formed intentionally or artificially by the manufacturing method described below, and is not a naturally formed shape. If a person familiar with this technology knows the appearance of a normal molten-plated coating, it is easy to distinguish between a pattern portion with an artificial shape and a non-pattern portion. In addition, from the perspective of improving the visibility of the pattern portion, the area ratio of the pattern portion to the surface of the molten-plated coating should be significantly smaller than that of the non-pattern portion. For example, the area ratio of the pattern portion to the surface of the molten-plated coating should be less than 30%, less than 25%, less than 20%, or less than 15%.

As described above, the pattern portion and the non-pattern portion are regions formed on the surface of the molten deposited layer, and each of the pattern portion and the non-pattern portion includes one or two regions of the first region and the second region.

The first region is a region where the ratio (B/A(%)) is 20% or more, so the first region in the molten deposited layer appears white or close to white. On the other hand, the second region is a region where the ratio (B/A(%)) is less than 20%, so the second region in the molten deposited layer appears to have a metallic luster. In addition, the first region and the second region are dispersed and each gathers, and the first region area ratio is 30-70%, and the appearance looks like pear skin.

The first area and the second area are determined as follows. Let the thickness of the molten coating be t, and cut the molten coating at any position of the 3t/4 position, t/2 position or t/4 position from the surface of the molten coating so that an exposed surface 3, 4 or 5 parallel to the surface 2a and 1 to 5 mm square in a top view appears. In this way, an exposed surface (cross section) of 1 to 5 mm square parallel to the surface of the molten coating is formed. Draw imaginary grid lines at intervals of 0.5 mm on each exposed surface. Among the multiple regions divided by the imaginary grid lines, the region where the ratio (B/A(%) of the [Zn phase] area fraction B to the total area fraction A of the [Zn phase] and the [Al/MgZn ₂ /Zn ternary eutectic structure] is 20% or more is the first region, and the region where the ratio (B/A(%) is less than 20% is the second region.

The specific determination method of the first area and the second area is described below. As shown in Figures 1 and 2, the thickness of the molten coating 2 formed on the steel plate 1 is t, and an exposed surface 3, 4 or 5 of 1 to 5 mm square parallel to the surface is formed at any position of the 3t/4 position, t/2 position or t/4 position from the surface 2a of the molten coating 2. In addition, when the pattern part and/or the non-pattern part is so small that a 5 mm square exposed surface cannot be formed inside the pattern part and/or the non-pattern part, the exposed surface shape can also be set to a minimum of 1 mm square. In this case, the area of the measurement area can be ensured by increasing the number of exposed surfaces.

When forming the exposed surfaces 3, 4 or 5, the molten coating is scraped off by means of grinding or argon sputtering. In addition, the exposed surface is preferably made into a mirror surface, and for example, the maximum height Rz of the exposed surface is preferably set to be less than 0.2µm. The exposed surface to be observed may be any of the exposed surfaces at the 3t/4 position, t/2 position or t/4 position from the surface of the molten coating. The exposed surface at the t/2 position is preferably selected. Regarding the B/A ratio obtained in the exposed surface at the t/2 position, there is a high probability that the same value as the B/A ratio will be obtained at other positions.

Next, as shown in Figure 3, imaginary grid lines were drawn at 0.5 mm intervals on the exposed surface of the observation object, and the ratio (B/A(%)) was measured in the multiple areas divided by the imaginary grid lines. The area with a ratio (B/A(%) of 20% or more is the first area, and the area with a ratio (B/A(%) of less than 20% is the second area.

The ratio (B/A (%)) was determined as follows. The coating structure of each region was observed by secondary electron imaging of a scanning electron microscope (SEM) to identify the [Zn phase] and [Al/MgZn ₂ /Zn ternary eutectic structure]. When identifying each phase and structure, the distribution of Zn, Al and Mg was confirmed by elemental analysis using an energy dispersive X-ray elemental analysis device attached to the SEM. Then, the ratio (B/A (%)) of the [Zn phase] area fraction B to the total area fraction A of the [Zn phase] and [Al/MgZn ₂ /Zn ternary eutectic structure] was calculated. [Zn phase] was measured as the [Zn phase] in the region with a circle equivalent diameter of 2.5µm or more. This is used to distinguish the Zn phase from the Zn phase in the [Al/MgZn ₂ /Zn ternary eutectic structure].

The pattern portion includes a plurality of regions divided by imaginary grid lines, and each region is classified as either a first region or a second region. In addition, the non-pattern portion also includes a plurality of regions divided by imaginary grid lines, and each region is classified as either a first region or a second region. That is, the pattern portion may include only either a first region or a second region, or may include both the first region and the second region. Similarly, the non-pattern portion may include only either a first region or a second region, or may include both the first region and the second region.

Here, the area ratio of each of the first region and the second region can be calculated in the pattern portion. Then, when the area ratio of the first region is greater than 70%, the pattern portion looks white or a color close to white. When the area ratio of the first region is greater than 30% and less than 70%, the pattern portion looks like a pear skin. Moreover, when the area ratio of the first region is less than 30%, the pattern portion looks like a metallic luster. The appearance of the pattern portion is related to the area ratio of the first region as described above.

On the other hand, the area ratio of the first region and the second region can also be calculated in the non-pattern portion. The appearance of the non-pattern portion is related to the area ratio of the first region in the same manner as the pattern portion.

Furthermore, when the difference in area ratio between the first region in the pattern portion and the first region in the non-pattern portion is 30% or more in absolute value, the pattern portion and the non-pattern portion can be distinguished. When the difference in area ratio is less than 30%, the difference in area ratio between the first region in the pattern portion and the first region in the non-pattern portion is small, the appearance of the pattern portion and the non-pattern portion is similar, and it is difficult to distinguish the pattern portion. The larger the difference in area ratio, the better, preferably 40% or more, more preferably 60% or more.

The pattern part and the non-pattern part can be identified by naked eyes or visually under a magnifying glass or a microscope. The so-called visual identification under a magnifying glass or a microscope means that the shape formed by the pattern part can be visually identified in a field of view of 50 times or less. The pattern part has a predetermined shape, so as long as the field of view is 50 times or less, the pattern part and the non-pattern part can be identified by the difference in their appearance. The pattern part and the non-pattern part should be identifiable at 20 times or less, more preferably 10 times or less, and more preferably 5 times or less. The molten-coated steel plate of this embodiment can also have a chemical conversion treatment film layer or a coating layer on the surface of the molten-coated layer. Here, the type of chemical conversion treatment film layer or coating layer is not particularly limited, and a known chemical conversion treatment film layer or coating layer can be used.

The above has described the hot-dip galvanized steel sheet of the first embodiment of the present invention. Next, the hot-dip galvanized steel sheet of the second embodiment of the present invention will be described. The second embodiment of the hot-dip coated steel plate comprises: a steel plate and a hot-dip coated layer formed on the surface of the steel plate; the hot-dip coated layer contains, in average composition, 5-22 mass % of Al and 1.0-10 mass % of Mg, and the remainder contains Zn and impurities; the hot-dip coated layer surface includes one or two regions of a first region and a second region obtained by the following measurement method; and the absolute value of the difference between the area ratio of the first region in the first part, i.e., an area of 1.0 mm square or more, and the area ratio of the first region in the second part, i.e., an area of 1.0 mm square or more adjacent to the first part, is 30% or more. [Measurement method] Let the thickness of the molten deposited layer be t, and expose a 1~5mm square section parallel to the surface at any position of 3t/4 position, t/2 position or t/4 position from the surface of the molten deposited layer, and draw imaginary grid lines at 0.5mm intervals in each section. Among the multiple regions divided by the imaginary grid lines, the region where the ratio (B/A(%) of the [Zn phase] area fraction B to the total area fraction A of the [Zn phase] and [Al/ _MgZn2 /Zn ternary eutectic structure] is 20% or more is the first region, and the region where the ratio (B/A(%) is less than 20% is the second region. The composition of the steel plate and the molten coating layer of the second embodiment is the same as that of the first embodiment. In the second embodiment, the absolute value of the difference between the area ratio of the first area in the first part, i.e., the area larger than 1.0 mm square, and the area ratio of the first area in the second part, i.e., the area larger than 1.0 mm square adjacent to the first part, is made 30% or more. Here, the so-called "area larger than 1.0 mm square" refers to an area larger than 1.0 mm square. The area that can completely include a 1.0 mm square square is the "area larger than 1.0 mm square". Based on this feature, the first part and the second part can be clearly identified with the naked eye. The first part and the second part are made into any shape, so that text or design can be displayed on the surface of the coating layer.

Next, the manufacturing method of the melt-plated steel plate of the present embodiment is described. In manufacturing the melt-plated steel plate of the present embodiment by melt plating, the steel plate is immersed in a melt plating bath after adjusting the chemical composition, so that the molten metal adheres to the surface of the steel plate. Then, the steel plate is lifted from the plating bath and the amount of adhesion is controlled by gas wiping, and then the molten metal is solidified. During solidification, although it depends on the composition, [Al phase] is formed first, and then [Al/Zn/MgZn ₂ ternary eutectic structure] is formed as the temperature of the molten metal decreases. In addition, [MgZn ₂ phase] and [Zn phase] are formed in the raw material of [Al/Zn/MgZn ₂ ternary eutectic structure]. Furthermore, when the molten deposited layer contains Si, a [Mg ₂ Si phase] is formed in the raw material of the [Al/Zn/MgZn ₂ ternary eutectic structure].

The inventors of this case learned that if a coarse [Zn phase] is formed during the solidification of the molten coating, the ratio of the [Al phase] or [MgZn ₂ phase] in the molten coating layer will increase relatively. Since these phases will be exposed on the coating surface, the appearance of the molten coating layer surface will appear close to white. It can be inferred that the formation of the [Zn phase] will be affected by the number of Zn nucleation points. In other words, the inventors of this case finally discovered that when there are few Zn nucleation points, the Zn in the liquid phase before the final solidification cannot crystallize in the form of a fine Zn phase in the [Al/Zn/MgZn ₂ ternary eutectic structure], but will crystallize in the form of a coarse [Zn] phase. As a means of reducing the number of Zn nucleation sites, it is conceivable to improve the surface cleanliness of the steel plate as the base plate and to minimize the substances that can become Zn nucleation sites.

On the other hand, when there are many Zn nucleation points, the Zn in the liquid phase before the final solidification will crystallize in the form of a fine Zn phase in the [Al/Zn/MgZn ₂ ternary eutectic structure], and it is not easy to crystallize in the form of a coarse [Zn] phase, and the appearance of the molten plating layer surface will show a metallic luster. As a means of increasing the Zn nucleation points, it is possible to consider arranging the substances that can become Zn nucleation points into a predetermined pattern after improving the surface cleanliness of the steel plate.

The manufacturing method of the molten-coated steel plate of this embodiment is described in more detail below. The molten-coated steel plate of this embodiment is treated to improve the cleanliness of the steel plate surface, and then the area with low cleanliness is arranged into a predetermined pattern. Then, the steel plate is dipped into the molten plating bath and then lifted up, and then cooled to solidify the molten plating layer.

Specifically, first, a hot-rolled steel plate is manufactured, and the hot-rolled plate is annealed as required. After pickling, it is cold-rolled to produce a cold-rolled plate. After the cold-rolled plate is degreased and washed with water, it is annealed (cold-rolled plate annealing), and then the annealed cold-rolled plate is immersed in a molten plating bath to form a molten plating layer. Here, during the period from cold rolling to cold-rolled plate annealing, the steel plate is alkaline electrolytically cleaned to improve the surface cleanliness, and after washing with pure water, it is dried in an inactive gas environment, and then moved to the cold-rolled plate annealing step. Cold-rolled plate annealing is carried out within 10 seconds from the time when the alkaline electrolytic washing is completed. The time point at which the alkaline electrolytic cleaning is completed is the time point at which the substrate is taken out of the last pure water spray washing of the alkaline electrolytic cleaning. The annealing conditions are not particularly limited.

The cleaning solution used for alkaline electrolytic cleaning is preferably an alkaline cleaning solution containing sodium hydroxide or potassium hydroxide. In terms of the procedure of alkaline electrolytic cleaning, the steel plate is immersed in the cleaning solution for immersion cleaning, and then the steel plate is electrolytically cleaned in the cleaning solution. Electrolytic cleaning is preferably alternating electrolytic cleaning. Then, pure water is sprayed on the surface of the steel plate to rinse off the attached cleaning solution. For spray water cleaning, multiple spray nozzles can be arranged along the moving direction of the steel plate, and pure water is sprayed from each nozzle. Pure water with a resistivity of 1MΩ·cm or more is preferred.

From the perspective of suppressing the adhesion of fine suspended particles in the air, it is advisable to dry in an inert gas environment from the time of removal from the spray water washing to the time of annealing, thereby removing the moisture attached to the surface of the steel plate as much as possible.

Alkaline electrolytic cleaning is used to remove organic dirt attached to the surface of the steel plate. Further, after the last spray water washing with pure water and before annealing, it is dried in an inactive gas environment and then annealed to prevent fine suspended particles in the air from being fixed on the cold-rolled plate.

Then, during the period from annealing the cold rolled sheet to being immersed in the molten coating, Zn powder is attached to the annealed cold rolled sheet in a predetermined shape to increase the nucleation points of Zn. Regarding the attachment of Zn powder to the annealed cold rolled sheet, the following method can also be adopted: Zn powder is attached to the roller in a predetermined shape in advance, and the Zn powder is transferred when the annealed cold rolled sheet passes through the roller.

The attached Zn powder will not be completely melted during the molten plating, and will become the nucleation site of Zn during the final solidification of the plating. A part of the Zn powder will maintain the solid state and diffuse into the plating bath. If the Zn powder is attached before cold rolling annealing, Zn will alloy with the steel plate during annealing, which will hinder the formation of the molten plating layer, so it is not good. In addition, if the Zn powder is attached after immersion in the molten plating bath, the attached Zn powder will become the cause of the poor surface appearance of the plating. The attached Zn powder can be Zn powder containing Zn and impurities. The average particle size of the Zn powder can be, for example, in the range of 4~6µm. The amount of Zn powder attached can be, for example, about 1~5g/ ^m2 . If the average particle size and the amount of adhesion are within this range, the Zn powder can function as a Zn nucleation site.

Next, the steel plate is immersed in a molten plating bath. The molten plating bath contains Al: 5~22 mass%, Mg: 1.0~10 mass%, and the remainder contains Zn and impurities. The molten plating bath may further contain Si: 0.0001~2 mass%. Furthermore, the molten plating bath may also contain 0.0001~2 mass% of any one or more of the following elements: Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C. The molten plating method is a continuous molten plating method in which the steel plate is continuously passed through the molten plating bath.

The temperature of the molten plating bath varies depending on the composition, but is preferably within the range of 400-500°C. The reason is that if the temperature of the molten plating bath is within this range, the desired molten plating layer can be formed. In addition, regarding the adhesion amount of the molten plating layer, it can be adjusted by means such as gas wiping on the steel plate lifted from the molten plating bath. The adhesion amount of the molten plating layer is preferably adjusted so that the total adhesion amount on both sides of the steel plate is within the range of 30-600g/ ^m2 . If the adhesion amount is less than 30g/ ^m2 , the corrosion resistance of the molten plating steel plate will decrease, so it is not good. If the adhesion amount is greater than 600 g/m ² , the molten metal adhered to the steel plate will drip, and it will become impossible to make the surface of the molten coating layer smooth, which is not good.

After adjusting the amount of molten metal coating, the steel plate is cooled. The cooling conditions are not particularly limited. The cooling of the molten metal attached to the steel plate is started after the steel plate is lifted from the molten bath. Although it also depends on the composition of the molten bath, for example, [Al phase] will begin to crystallize from around 430°C. Then, [MgZn ₂ ] will begin to crystallize from around 370°C, [Al/Zn/MgZn ₂ ternary eutectic structure] will crystallize from around 340°C, and further [Zn phase] will crystallize, and solidification will be completed.

In the steel plate surface before melt plating, after improving the cleanliness of the entire surface, Zn powder adhesion areas that serve as Zn nucleation points are arranged. In the Zn powder adhesion area, there are abundant Zn nucleation points, so Zn or _MgZn2 as a eutectic structure will crystallize, and then a large amount of [Al/Zn/ _MgZn2 ternary eutectic structure] will be formed. On the other hand, Zn in the liquid phase will decrease, and the formation of coarse [Zn phase] will be suppressed. As a result, in the Zn powder adhesion area, the ratio (B/A(%) of the [Zn phase] area fraction B to the total area fraction A of [Zn phase] and [Al/ _MgZn2 /Zn ternary eutectic structure] becomes lower. On the other hand, in the region where no Zn powder is attached and a relatively high cleanliness state is maintained, the ratio (B/A (%) becomes high.

The hot-dip plated steel plate of this embodiment is made to have an absolute value of the difference between the area ratio of the first area in the patterned portion and the area ratio of the first area in the non-patterned portion in the first area and the second area to be 30% or more, thereby enabling identification of the patterned portion and the non-patterned portion. The formed patterned portion and the non-patterned portion are not formed by printing or painting, so the durability becomes high. Moreover, the patterned portion and the non-patterned portion are not formed by printing or painting, so there is no effect on the corrosion resistance of the hot-dip plated layer. In addition, the patterned portion and the non-patterned portion are not formed by grinding the surface of the hot-dip plated layer, etc. Therefore, the thickness of the molten-coated layer in the pattern portion is not reduced to such an extent that the corrosion resistance is deteriorated compared to the thickness of the molten-coated layer in the non-pattern portion. Therefore, the molten-coated steel sheet of this embodiment is a steel sheet with excellent corrosion resistance.

According to this embodiment, a molten-plated steel sheet having high durability of the pattern portion and suitable plating characteristics such as corrosion resistance can be provided. In particular, in this embodiment, a roller with Zn powder attached to a predetermined shape is pressed on a cold-rolled sheet after annealing to improve cleanliness, and the surface shape of the roller is transferred to the cold-rolled sheet after annealing, so that after molten plating, the pattern portion or non-pattern portion on the surface of the molten-plated layer can be made into a deliberate shape or an artificial shape, and the pattern portion can be arranged in any one of the shapes of a straight line portion, a curved line portion, a dot portion, a figure, a number, a symbol or a character, or a shape formed by combining two or more of these. In this way, various designs, trademarks and other identification marks can be displayed on the surface of the molten-coated layer without printing, painting or grinding, thereby improving the identification and design of the steel plate. In addition, the pattern part can also be used to give the molten-coated steel plate information required for process management or inventory management, or any information required by consumers. This can also help improve the productivity of the molten-coated steel plate.

Implementation Example Next, an implementation example of the present invention is described. The steel plate after cold rolling is subjected to alkaline electrolytic cleaning and washed with ultrapure water, and then moved to the annealing step within 10 seconds in an inert gas environment. The cleaning solution used for alkaline electrolytic cleaning is set to be an alkaline cleaning solution containing sodium hydroxide. As for the procedure of alkaline electrolytic cleaning, the steel plate is immersed in the cleaning solution for immersion cleaning, and then the steel plate is electrolytically cleaned in the cleaning solution. The electrolytic cleaning is set to alternate electrolytic cleaning. Then, it is sprayed with ultrapure water to rinse off the attached cleaning solution. Ultrapure water is set to water with a resistivity of 1MΩ·cm or more. After drying in an inert gas environment, the cold-rolled plate is annealed. The annealing conditions are set to a soaking temperature of 800℃ and a soaking time of 1 minute.

After annealing, Zn powder with an average particle size in the range of 4 to 6µm was attached to a metal plate having a shape on which a grid pattern shown in FIG4 was transferred. The thickness of the wire constituting the grid pattern was set to 10mm. The interval of the center axis of the wire was set to 50mm. Then, the metal plate was pressed against the annealed steel plate to transfer the Zn powder to the surface of the steel plate, thereby locally (grid with 50mm intervals) forming a Zn nucleation site. The amount of Zn powder attached was set to the range of 1 to 5g/ ^m2 . Thereafter, the steel plate was immersed in a molten plating bath and then lifted up. Thereafter, the amount of attachment was adjusted by gas wiping and cooling was performed. In this way, the molten-coated steel plates No. 1 to No. 51 shown in Table 1A to Table 3B were produced.

Among them, Zn powder was not attached to a part of the steel plate. The steel plate without Zn powder was subjected to the same conditions as No.1 to 48 by molten plating bath plating treatment to produce a molten-plated steel plate. A grid pattern with a spacing of 50 mm was printed on the surface of the molten-plated layer of the steel plate by inkjet method. In this way, the Zn-Al-Mg system molten-plated steel plate No.52 was produced.

In addition, the steel plate without Zn powder was subjected to the same conditions as No. 1 to 48 by molten plating bath plating to produce a molten-plated steel plate. After that, the surface of the molten-plated layer was ground to form a grid pattern with a spacing of 50 mm. In this way, the molten-plated steel plate No. 53 was produced.

For the obtained molten-coated steel plate, the area ratios of the first region and the second region included in the pattern part and the non-pattern part are obtained. First, the surface of the molten-coated layer is observed with the naked eye to identify the boundary between the pattern part and the non-pattern part. When it is difficult to identify the boundary with the naked eye, a magnifying glass or an optical microscope is used to magnify the image. In the case where it is difficult to distinguish the boundary, the area corresponding to the square pattern on the roller surface is taken as the pattern part, and the area ratios of the first region and the second region are evaluated.

Next, the area ratio of each region included in the pattern part and the non-pattern part is obtained by the measurement method described below. First, let the thickness of the molten plating layer formed on the steel plate be t, and a 5mm square exposure surface parallel to the surface is formed at the position t/2 from the surface of the molten plating layer. However, if t/2 is greater than 0.2µm, an exposure surface is formed at a position 0.2µm from the surface. At this time, a 5mm square exposure surface completely included in the pattern part (that is, the entire area corresponds to the exposure surface of the pattern part) and a 5mm square exposure surface completely included in the non-pattern part (that is, the entire area corresponds to the exposure surface of the non-pattern part) are formed. When forming the exposure surface, the molten plating layer is scraped off by grinding. In addition, the maximum height Rz of the exposure surface is set to be less than 0.2µm.

Next, for the exposed surface to be observed, firstly, imaginary grid lines were drawn at intervals of 0.5 mm on the surface of the molten deposited layer, and then the ratio (B/A (%)) was measured in multiple areas (0.5 mm square) divided by the imaginary grid lines.

The ratio (B/A (%)) was determined as follows. The coating structure of each region was observed by secondary electron imaging of a scanning electron microscope (SEM) to identify the [Zn phase] and [Al/MgZn ₂ /Zn ternary eutectic structure]. When identifying each phase and structure, elemental analysis was performed using an energy dispersive X-ray elemental analysis device attached to the SEM to confirm the distribution of Zn, Al, and Mg and identify them simultaneously. That is, the region where Zn was mainly detected among Zn, Al, and Mg was identified as the Zn phase, the region where Al was mainly detected was identified as the Al phase, and the region where Zn and Mg were mainly detected was identified as the MgZn ₂ phase. Based on the distribution of each detected phase, it was classified into [Al phase], [MgZn ₂ phase], [Zn phase], and [Al/Zn/MgZn ₂ ternary eutectic structure] according to the above method. Then, in multiple areas (0.5 mm square) divided by the imaginary grid lines, the ratio of the [Zn phase] area fraction B to the total area fraction A of the [Zn phase] and [Al/MgZn ₂ /Zn ternary eutectic structure] (B/A (%)) was calculated. [Zn phase] is measured as the area with a circle equivalent diameter of 2.5µm or more as [Zn phase]. In this way, the Zn phase in the [Al/MgZn ₂ /Zn ternary eutectic structure] and the [Zn phase] are distinguished.

The area where the ratio (B/A(%)) is 20% or more is defined as the first area, and the area where the ratio (B/A(%)) is less than 20% is defined as the second area.

Then, the area ratio of the first region in the pattern portion and the area ratio of the first region in the non-pattern portion are obtained. Furthermore, the difference between the area ratio of the first region in the pattern portion and the area ratio of the first region in the non-pattern portion is obtained.

[Identification] The test panels with square pattern parts were visually evaluated in the initial state just after manufacture and after exposure to the outdoors for 6 months according to the following criteria. A, B and C were considered acceptable in both the initial state and the extended state.

A: The pattern can be seen even from 5m away. B: Although the pattern cannot be seen from 5m away, the visibility is very high from 3m away. C: Although the pattern cannot be seen from 3m away, the visibility is very high from 1m away. D: The pattern cannot be seen from 1m away.

[Corrosion resistance] The test plate was cut into 150×70mm and tested for 30 cycles according to the corrosion promotion test CCT of JASO-M609. The rust condition was then investigated and evaluated according to the following criteria. A, B, and C were considered qualified. A: No rust, and both the pattern and non-pattern parts maintain a beautiful design appearance. B: Although there is no rust, very slight changes in the design appearance can be observed between the pattern and non-pattern parts. C: The design appearance is slightly damaged, but the pattern and non-pattern parts can be visually distinguished. D: The appearance grade of the pattern and non-pattern parts is significantly reduced, and they cannot be visually distinguished.

As shown in the table, the Zn-Al-Mg system molten-coated steel sheets of the present invention examples No. 1 to No. 45 have a molten-coated layer with a chemical composition within the scope of the present invention, and are molten-coated after alkaline electrolytic cleaning, ultrapure water spray cleaning, drying, annealing, and Zn powder attachment. Therefore, a pattern portion and a non-pattern portion are formed in the molten-coated layer, and the absolute value of the difference between the area ratio of the first region in the pattern portion and the area ratio of the first region in the non-pattern portion is 30% or more. Therefore, both the identification and corrosion resistance are excellent.

Since the Al content of the molten-coated steel sheet No. 46 is low, the absolute value of the difference between the area ratio of the first region in the pattern part and the area ratio of the first region in the non-pattern part is less than 30%. Therefore, both the identification and corrosion resistance are poor. Since the Al content of the molten-coated steel sheet No. 47 is too high, the pattern part is thinned due to exposure to the outdoors for 6 months, and the identification is poor. Since the Mg content of the molten-coated steel sheet No. 48 is low, the pattern part is thinned due to exposure to the outdoors for 6 months, and the identification is poor. In addition, the corrosion resistance is also reduced. The molten-coated steel sheet No. 49 has an excessive amount of Mg in the molten-coated layer, so the identification and corrosion resistance are poor. Although the composition of the molten-coated layer of the molten-coated steel sheet No. 50 is appropriate, Zn powder is not attached. Therefore, for the molten-coated steel sheet No. 50, the absolute value of the difference between the area ratio of the first area in the pattern part and the area ratio of the first area in the non-pattern part is less than 30%. Therefore, the identification and corrosion resistance are poor. The composition of the molten-coated layer of the molten-coated steel sheet No. 51 is appropriate, and Zn powder is attached to the surface of the steel sheet before the molten-coating process. However, the surface of the molten-coated steel sheet No. 51 was not fully cleaned before Zn was attached. Therefore, for the molten-coated steel sheet No. 51, the absolute value of the difference between the area ratio of the first region in the pattern part and the area ratio of the first region in the non-pattern part was less than 30%. Therefore, the identification was poor.

No.52, which has a square pattern printed by inkjet printing, has been exposed outdoors for 6 months, so the pattern has become thinner and the recognition has decreased. In addition, No.53, which has a square pattern formed by grinding, has a reduced coating thickness at the grinding site, and the corrosion resistance at the grinding site has decreased.

[Table 1A]

[Table 1B]

[Table 2A]

[Table 2B]

[Table 3A]

[Table 3B]

1: Steel plate 2: Molten plating 2a: Molten plating surface 3: Section at t/4 4: Section at t/2 5: Section at 3t/4 21: Pattern part 22: Non-pattern part A, A': Point A1: First area A2: Second area t: Molten plating thickness

FIG. 1 is a schematic cross-sectional view illustrating a cross section (exposed surface) used to measure the coating structure of the molten coating layer in the Zn-Al-Mg system molten-coated steel plate of the embodiment of the present invention. FIG. 2 is a stereoscopic view illustrating an exposed surface used to measure the coating structure of the molten coating layer in the Zn-Al-Mg system molten-coated steel plate of the embodiment of the present invention. FIG. 3 is a schematic view of the first region and the second region of the Zn-Al-Mg system molten-coated steel plate of the embodiment of the present invention. FIG. 4 is a schematic view of a metal plate having a lattice shape, which is used to transfer Zn powder to the surface of the steel plate of the embodiment.

1: Steel plate 2: Molten plating 2a: Molten plating surface 3: Section at t/4 4: Section at t/2 5: Section at 3t/4 t: Thickness of the melt plating

Claims (6)

A molten-coated steel plate, characterized in that: it comprises: a steel plate and a molten-coated layer formed on the surface of the steel plate; the molten-coated layer contains, in average composition, 5-22 mass % of Al and 1.0-10 mass % of Mg, and the remainder contains Zn and impurities; the molten-coated layer has a pattern portion and a non-pattern portion; the pattern portion and the non-pattern portion each contain one or two of the first region and the second region obtained by the following measurement method; and the absolute value of the difference between the area ratio of the first region in the pattern portion and the area ratio of the first region in the non-pattern portion is 30% or more; [Measurement method] The thickness of the molten deposited layer is t, and a cross section of 1 to 5 mm square parallel to the surface of the molten deposited layer is exposed at any position of 3t/4, t/2 or t/4 from the surface of the molten deposited layer, and imaginary grid lines are drawn at intervals of 0.5 mm in each of the cross sections. Among the multiple regions demarcated by the imaginary grid lines, the region where the ratio (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 ₂ /Zn ternary eutectic structure] is 20% or more is the first region, and the region where the ratio (B/A(%) is less than 20% is the second region. A molten-coated steel plate, characterized in that: it comprises: a steel plate and a molten-coated layer formed on the surface of the steel plate; the molten-coated layer contains, in average composition, 5-22 mass % of Al and 1.0-10 mass % of Mg, and the remainder contains Zn and impurities; the molten-coated layer further contains one or two selected from the group consisting of the following group A and group B; the molten-coated layer has a pattern portion and a non-pattern portion; the pattern portion and the non-pattern portion each contain one or two regions of a first region and a second region obtained by the following measurement method; and the absolute value of the difference between the area ratio of the first region in the pattern portion and the area ratio of the first region in the non-pattern portion is 30% or more; [Group A] Si: 0.0001~2 mass%; [Group B] Any one of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C or more in total 0.0001~2 mass%; [Measurement method] Let the thickness of the above-mentioned molten deposited layer be t, and expose a 1~5mm square section parallel to the above-mentioned surface at any position of 3t/4 position, t/2 position or t/4 position from the surface of the above-mentioned molten deposited layer, and draw imaginary grid lines at 0.5mm intervals in each of the above-mentioned sections. In the multiple regions divided by the above-mentioned imaginary grid lines, let the [Zn phase] area fraction B relative to [Zn phase] and [Al/MgZn ₂ The region where the total area fraction A of the Zn/Zn ternary eutectic structure (B/A(%)) reaches 20% or more is the aforementioned first region, and the region where the ratio (B/A(%)) is less than 20% is the second region. The molten-coated steel plate of claim 1 or claim 2, wherein the pattern portion is configured into the following shapes: any one of a straight line portion, a curved line portion, a dot portion, a figure, a number, a symbol or a character, or a shape formed by combining two or more of the above shapes. The molten-coated steel plate of claim 1 or claim 2, wherein the adhesion amount of the molten-coated layer is 30-600 g/m ² on both sides of the steel plate. The molten-coated steel plate of claim 2, wherein the molten-coated layer has an average composition containing the A group in mass %. The molten-coated steel plate of claim 2, wherein the molten-coated layer has an average composition containing the B group in mass %.