Hot-dip Zn-Al-Mg plated steel sheet and method for manufacturing the same

Abstract

The objective is to provide a hot-dip Zn-Al-Mg plated steel sheet that achieves a high level of both corrosion resistance at the joints and corrosion resistance at the cut edges of the steel sheet. To achieve the above objective, the present invention comprises a base steel sheet 2 and a plating layer 3 formed on at least one surface of the base steel sheet 2, wherein the plating layer 3 has a composition containing Al: 2.0 to 5.0 mass% and Mg: 1.0 to 6.0 mass%, with the remainder being Zn and unavoidable impurities, and the plating layer 3 includes a single-phase Zn region 4 and a eutectic region 5 containing at least Zn and Mg, and the single-phase Zn region 4 has a large-diameter single-phase Zn phase 41 that penetrates from the surface of the plating layer to the surface of the base steel sheet when viewed in the thickness direction cross-section of the plating layer.

Description

[Technical Field]

[0001] This invention relates to a hot-dip Zn-Al-Mg plated steel sheet that exhibits excellent corrosion resistance at both the joint and cut sections of the steel sheet. [Background technology]

[0002] Hot-dip galvanized steel sheets have been widely used as rust-preventive steel sheets in fields such as automobiles, electrical equipment, and building materials because, in addition to the sacrificial corrosion protection provided by zinc to the base metal, the Zn corrosion products are densely formed, resulting in good corrosion resistance. However, given the limited global reserves of zinc and the recent concerns about its potential depletion, there is a growing demand for the development of zinc-reduced plated steel sheets.

[0003] One example of low-Zn-plated steel sheet technology is that it enhances the corrosion resistance of the plating itself, enabling sufficient rust prevention even with a small amount of plating. For example, Patent Documents 1 and 2 propose a technology to improve corrosion resistance by Zn-Al-Mg plating, in which elements such as Al and Mg are added to a zinc plating bath.

[0004] In the case of general molten Zn-Al-Mg plated steel sheets, such as those disclosed in Patent Documents 1 and 2, a complex solidification reaction occurs during the film formation process, resulting in a complex and non-uniform structure for the plated film. This non-uniform structure allows molten Zn-Al-Mg plated steel sheets to exhibit improved corrosion resistance at the joints of the steel sheets compared to conventional molten Zn plated steel sheets. This corrosion resistance at the joints of the steel sheets is for automotive applications, and a higher Al content tends to yield better results. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2010-275632 [Patent Document 2] Special Publication No. 2020-504237 [Overview of the project] [Problems that the invention aims to solve]

[0006] As mentioned above, with hot-dip Zn-Al-Mg plated steel sheets, it is possible to improve the corrosion resistance of the steel sheet joints while reducing the Zn content to some extent by increasing the Al content. However, similar to the corrosion resistance of steel plate joints, there was a problem with the corrosion resistance of steel plate cut sections, which is also a corrosion characteristic evaluation for automobiles, as it tended to deteriorate with increasing aluminum content. Therefore, there was a need to develop a technology that could achieve a high degree of compatibility between corrosion resistance of steel plate joints and corrosion resistance of cut sections, which had been difficult to achieve until now.

[0007] In view of these circumstances, the present invention aims to provide a hot-dip Zn-Al-Mg plated steel sheet and a method for manufacturing the same, which achieve a high level of both corrosion resistance at the joint portion and corrosion resistance at the cut portion of the steel sheet. [Means for solving the problem]

[0008] The present inventors have investigated a hot-dip Zn-Al-Mg plated steel sheet comprising a base steel sheet and a plating layer formed on at least one surface of the base steel sheet, in order to solve the above problems. As a result, they found that it is important to control the concentrations of Al and Mg, as well as the structure of the plating layer. In particular, they found that by defining the proportion of large-diameter single-phase Zn phases that penetrate from the surface of the plating layer to the surface of the base steel sheet when viewed in the thickness direction cross-section of the plating layer, these large-diameter single-phase Zn phases act as obstacles when corrosion progresses due to cutting, thereby slowing down the progression of corrosion. This improves not only the corrosion resistance of the steel sheet joint but also the corrosion resistance of the cut portion.

[0009] This invention is based on the above findings, and its gist is as follows. 1. comprising a base steel plate and a plating layer formed on at least one surface of the base steel plate, The aforementioned plating layer has a composition containing Al: 2.0 to 5.0 mass% and Mg: 1.0 to 6.0 mass%, with the remainder being Zn and unavoidable impurities. The plating layer comprises a single-phase Zn region and a eutectic region containing at least Zn and Mg, and the single-phase Zn region has a large-diameter single-phase Zn phase that penetrates from the surface of the plating layer to the surface of the underlying steel sheet when viewed in the thickness direction cross-section of the plating layer. When viewing the cross-section of the plating layer in the thickness direction, the sum of the lengths Lz occupied by the large-diameter single-phase Zn phase present on Lw for a length range of 500 μm or more along the interface between the plating layer and the underlying steel sheet is given by the following relationship (1): Lz / Lw≧0.1 A hot-dip Zn-Al-Mg plated steel sheet characterized by satisfying the following conditions. 2. The hot-dip Zn-Al-Mg plated steel sheet according to item 1, characterized in that the area ratio occupied by the eutectic region on the surface of the plating layer is 80% or more. 3. A method for manufacturing plated steel sheets as described in 1 or 2 above, A method for manufacturing a molten Zn-Al-Mg plated steel sheet, characterized by including a step of solidifying the plating layer in a primary cooling step after molten plating, followed by heating and holding the steel sheet at a temperature of 380°C or higher for 10 seconds or more. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a hot-dip Zn-Al-Mg plated steel sheet and a method for manufacturing the same, which achieve a high level of both corrosion resistance at the joint portion and corrosion resistance at the cut portion of the steel sheet. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic, enlarged view of a cross-section of a molten Zn-Al-Mg plated steel sheet according to one embodiment of the present invention. [Figure 2]This is a diagram explaining the flow of the primary cooling process, heating and holding process, and secondary cooling process for a method of manufacturing a hot-dip Zn-Al-Mg alloy coated steel sheet according to an embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the hot-dip Zn-Al-Mg alloy coated steel sheet and the method of manufacturing the hot-dip Zn-Al-Mg alloy coated steel sheet of the present invention will be specifically described with reference to the drawings as necessary. Regarding each part constituting the coated steel sheet disclosed in FIG. 1, for the convenience of explanation, it is schematically shown in scales and shapes different from the actual ones.

[0013] (Hot-dip Zn-Al-Mg alloy coated steel sheet) As shown in FIG. 1, the hot-dip Zn-Al-Mg alloy coated steel sheet of the present invention includes a plating layer 3 on a base steel sheet 2. The plating layer 3 has a composition containing 2.0 to 5.0% by mass of Al and 1.0 to 6.0% by mass of Mg, with the balance being Zn and unavoidable impurities.

[0014] Zn, which is the main component of the plating layer, imparts sacrificial corrosion protection ability to the plating layer and is an element necessary to obtain excellent corrosion resistance. Since the plating layer is composed of low specific gravity elements such as Al and Mg when considered in terms of atomic composition ratio, it is also necessary for Zn to be the main component in terms of atomic composition ratio. Therefore, the Zn content in the plating layer is preferably 80.0% by mass or more, and more preferably 85.0% by mass or more. Note that the upper limit of the Zn content is the content of the balance excluding elements and impurities other than Zn.

[0015] Al in the plating layer is an essential element for forming Al regions or Zn-Al-Mg eutectic regions within the plating layer and obtaining excellent corrosion resistance. If the Al content of the plating layer is less than 2.0 mass%, the plating layer will not contain sufficient Zn-Al-Mg eutectic regions, and the desired corrosion resistance of the steel sheet joint cannot be obtained. Therefore, the Al content in the plating layer should be 2.0 mass or more, preferably 3.0 mass or more. On the other hand, if the Al content exceeds 5.0 mass%, the single-phase Zn regions contained in the plating layer will decrease, and the desired plating layer structure and the desired corrosion resistance of the cut portion cannot be obtained. Therefore, the Al content in the plating layer should be 5.0 mass or less.

[0016] Furthermore, the Mg in the plating layer is an essential element for forming a Zn-MgZn2 eutectic or Zn-Al-Mg eutectic in the plating layer and obtaining excellent corrosion resistance. If the Mg content in the plating layer is less than 1.0% by mass, the plating layer will not contain a sufficient Zn-Al-Mg eutectic region, and the desired corrosion resistance of the steel sheet joint and cut portions cannot be obtained. Therefore, the Mg content in the plating layer should be 1.0% by mass or more, preferably 2.0% by mass or more. On the other hand, if the Mg content in the plating layer exceeds 6.0% by mass, Mg-based oxides are significantly generated on the surface of the plating bath during plating formation, impairing productivity. As a countermeasure against the above issue, it is conceivable to increase the amount of Al in the plating layer, but increasing the amount of Al reduces the large-diameter single-phase Zn phase described later, making it impossible to obtain the desired corrosion resistance of the cut portion. Therefore, the Mg content in the plating layer shall be 6.0% by mass or less.

[0017] The plating film contains unavoidable impurities. Of these, the unavoidable impurities include Fe. This Fe is inevitably included in the plating film as a result of the dissolution of steel plates and equipment in the plating bath, and as a result of diffusion from the underlying steel plate during the formation of the interfacial alloy layer. The Fe content in the plating film is usually about 0.1 to 0.5% by mass.

[0018] Furthermore, the plating layer may also contain, if necessary, one or more elements selected from the group consisting of Si, B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb, and Bi, in a total of 0.1 to 5% by mass. These elements can have effects such as improving the stability of corrosion products and delaying the progression of corrosion when the plating layer corrodes, and stabilizing the spangle size on the plating surface and improving the surface appearance.

[0019] Here, as shown in Figure 1, the Zn-Al-Mg plated steel sheet of the present invention has a plating layer 3 that includes a single-phase Zn region 4 and a eutectic region 5 containing at least Zn and Mg, and the single-phase Zn region 4 has a large-diameter single-phase Zn phase 41 that penetrates from the surface of the plating layer 3 to the surface of the base steel sheet 2 when viewed in the thickness direction cross-section of the plating layer 3. The plating layer may also form a single-phase Al region (not shown), but its presence or absence is not limited.

[0020] The crystal structure of the eutectic region is not particularly limited, as long as it consists of a eutectic containing at least Zn and Mg. For example, the eutectic region may mainly consist of a binary eutectic composed of Zn-MgZn2 or a ternary eutectic composed of Zn-Al-MgZn2.

[0021] The single-phase Zn region 4 is a single crystal of Zn, and as shown in Figure 1, when the cross-section in the thickness direction of the plating layer 3 is viewed, it has a large-diameter single-phase Zn phase 41 that penetrates from the surface of the plating layer 3 to the surface of the base steel plate 2. Furthermore, in the present invention, when viewing the cross-section in the thickness direction of the plating layer 3, the sum of the lengths occupied by the large-diameter single-phase Zn phase 41 present on Lw (Lz1 + Lz2 in Figure 1) along the interface between the plating layer 3 and the underlying steel sheet 2, in a length range of 500 μm or more, is given by the following relationship (1): Lz / Lw≧0.1 It satisfies the condition.

[0022] In the plating layer 3, the eutectic region 5 tends to corrode preferentially to the single-phase Zn region 4. Therefore, when the plating layer 3 is damaged, corrosion progresses in the direction of the bulge from the cut (along the interface between the plating layer 3 and the underlying steel plate 2 in Figure 1). However, the large-diameter single-phase Zn phase 41 penetrates from the surface of the plating layer 3 to the surface of the underlying steel plate 2, thus preventing the progression of corrosion in the direction of the bulge from the cut. Therefore, in the present invention, the sum of the lengths Lz (Lz1 + Lz2 in Figure 1) of the large-diameter single-phase Zn phase present on Lw occupies a certain range for a length range of 500 μm or more along the interface Lw between the plating layer 3 and the base steel sheet 2, that is, relationship (1) is satisfied, thereby efficiently suppressing the progression of corrosion in the bulging width direction from the cut portion and achieving excellent corrosion resistance of the cut portion.

[0023] Furthermore, if Lw and Lz as described above do not satisfy relation (1) (L Z / L w In the case of <0.1), the formation of the large-diameter single-phase Zn phase 41 is insufficient, resulting in significant corrosion in the direction of the bulge from the cut portion of the plating layer 3, and the desired corrosion resistance of the cut portion cannot be obtained. From a similar perspective, the Lw and Lz mentioned above are related by (2): Lz / Lw≧0.15 It is preferable that the following conditions be met.

[0024] Here, the length range Lw of 500 μm or more along the interface between the plating layer 3 and the base steel sheet 2, and the sum of the lengths occupied by the large-diameter single-phase Zn phase present on Lw, Lz, can be determined by observing the cross-section in the thickness direction of the plated steel sheet 1 with a scanning electron microscope (SEM) and analyzing the obtained image. Note that the interface between the plating layer 3 and the base steel sheet 2 is not a straight line, but Lw is defined as the length of the line connecting the interfaces at both ends of the analysis range with a straight line. Then, of the large-diameter single-phase Zn phase 41 that reaches the interface and is present within the length range Lw, the lengths Lz1 and Lz2 are measured, and their sum Lz is calculated. Lz is calculated for the length range parallel to Lw. In the present invention, for the above-mentioned Lw, Lz and L Z / L w values, cross-sectional observations of the plating layer 3 are performed for 10 randomly selected fields of view. For each observation field of view, Lw and Lz are determined, and the value of L Z / L w is calculated. The average value of L Z / L w for the obtained 10 fields of view is taken as the value of L Z / L w of the plating steel sheet. Also, for the observation of the cross-section of the plating layer 3, the field of view is selected such that the parallel length L w is 500 μm or more.

[0025] Note that an interfacial alloy layer (not shown) may exist at the interface between the plating layer 3 and the base steel sheet 2. In that case, the large-diameter single-phase Zn phase 41 reaches the interfacial alloy layer in the single-phase Zn region 4. The thickness of the interfacial alloy layer is not particularly limited.

[0026] Also, in the present invention, it is preferable that the area ratio occupied by the eutectic region 5 on the surface of the plating layer 3 is 80% or more. A corrosion product having high protection against corrosion from the surface layer of the plating layer 3 is formed on the crystal region, and it can be stabilized for a longer period. Therefore, when the area ratio occupied by the eutectic region 5 is 80% or more, the corrosion resistance on the surface of the plating layer is improved, and the corrosion resistance of the steel sheet joint portion can be further improved. From the same viewpoint, it is more preferable that the area ratio occupied by the eutectic region 5 on the surface of the plating layer 3 is 85% or more.

[0027] Note that the deposition amount of the plating layer is not particularly limited. For example, from the viewpoint of corrosion resistance, it is preferable that the deposition amount of the plating layer is 10 g / m or more per one side of the steel sheet. On the other hand, from the viewpoint of manufacturing cost, it is preferable that the deposition amount of the plating layer is 100 g / m or less per one side of the steel sheet, more preferably 60 g / m or less per one side of the steel sheet, and still more preferably 40 g / m or less per one side of the steel sheet. 2 2 2 2 ​​​The following is even more preferable. The amount of the plating layer can be determined by dissolving and removing the plating layer from the surface of the plated steel sheet using an acid solution, and then subtracting the weight after removal from the weight of the plated steel sheet before removal. An inhibitor that suppresses the dissolution of the base steel sheet is added to the acid solution. For example, it can be derived by dissolving and peeling off the plating film over a specific area using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013, and then calculating the amount from the difference in steel sheet weight before and after peeling.

[0028] Furthermore, as shown in Figure 1, the molten Zn-Al-Mg plated steel sheet of the present invention has the plating layer 3 formed on the base steel sheet 2, but if necessary, an intermediate layer or a coating film can be further formed on the plating layer 3. The type of coating film and the method of forming the coating film are not particularly limited and can be appropriately selected according to the required performance. For example, methods such as roll coater coating, curtain flow coating, and spray coating can be used. After applying a paint containing an organic resin, it is possible to form a coating film by heating and drying it using means such as hot air drying, infrared heating, or induction heating. Furthermore, the intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the molten Zn-Al-Mg plated steel sheet and the coating film. Examples include a chemical conversion coating and a primer such as an adhesive layer. The chemical conversion coating can be formed, for example, by a chromate treatment or chromium-free chemical conversion treatment, which involves applying a chromate treatment solution or a chromium-free chemical conversion treatment solution and drying it at a steel sheet temperature of 80 to 300°C without washing with water. These chemical conversion coatings may be single-layer or multi-layer, and in the case of multi-layer coatings, multiple chemical conversion treatments may be performed sequentially.

[0029] (Method for manufacturing molten zinc-al-magnesium plated steel sheets) The method for producing a hot-dip Zn-Al-Mg plated steel sheet of the present invention (hereinafter sometimes simply referred to as "method for producing a hot-dip plated steel sheet") is not particularly limited. However, the plating film of the molten Zn-Al-Mg plated steel sheet obtained by the present invention is, overall, approximately the same as the composition of the plating bath. Therefore, the present invention includes a step of forming the plating layer on the base steel sheet using a plating bath whose composition is controlled to contain Al: 2.0 to 5.0 mass% and Mg: 1.0 to 6.0 mass%, with the remainder being Zn and unavoidable impurities.

[0030] Furthermore, the process for forming the plating layer is not particularly limited to the composition of the plating bath described above. For example, the base steel sheet can be manufactured in a continuous hot-dip galvanizing facility by washing, heating, and immersing it in a plating bath. In the heating process of the steel sheet, recrystallization annealing is performed to control the structure of the base steel sheet itself, and heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective in preventing oxidation of the steel sheet and reducing the trace oxide film present on the surface.

[0031] Furthermore, while the temperature of the plating bath is not particularly limited, it is preferable to set it 10°C to 80°C higher than the liquidus temperature of the alloy in the plating bath composition. In order to perform the molten plating process, the bath temperature must be above the solidification point. This is done by setting the bath temperature 10°C to 80°C higher than the liquidus temperature of the alloy in the plating bath composition to prevent solidification due to a localized drop in the bath temperature. The temperature of the base steel plate that enters the plating bath is not particularly limited, but a temperature of approximately the plating bath temperature to the plating bath temperature + 20°C is preferable from the viewpoint of ease of operation. On the other hand, it is preferable that the temperature of the plating bath be 600°C or lower. If the temperature of the plating bath exceeds 600°C, it becomes difficult to rapidly cool the plating layer, and the interfacial alloy layer formed between the plating layer and the base steel sheet may become thicker.

[0032] Furthermore, the present invention's method for manufacturing a hot-dip galvanized steel sheet is characterized by including a step of solidifying the galvanized layer in a primary cooling step after the hot-dip galvanizing treatment, followed by heating and holding the steel sheet at a temperature of 380°C or higher for 10 seconds or more. By including the heating and holding step described above, a large-diameter single-phase Zn phase can be formed that penetrates from the surface of the plating layer to the surface of the underlying steel sheet, and the area ratio of the eutectic region on the surface of the plating layer can also be increased.

[0033] Here, Figure 2 shows an example of the flow from the primary cooling step to the secondary cooling step in the method for manufacturing a hot-dip galvanized steel sheet according to the present invention, with the vertical axis representing the steel sheet temperature (°C) and the horizontal axis representing time (s). In Figure 2, the solid line represents the flow after plating layer formation according to the present invention, while the dashed line represents the conventional flow after plating layer formation.

[0034] ·Primary cooling process As shown in Figure 2, the method for manufacturing a hot-dip galvanized steel sheet of the present invention includes a primary cooling step for solidifying the galvanized layer after immersing the base steel sheet in a galvanizing bath and removing it. The primary cooling step is not particularly limited as long as the plating layer can be solidified, but typically the steel plate is cooled to 380°C or below. There is no particular limit to the lower limit of the primary cooling. Furthermore, the cooling rate and cooling time in the primary cooling process are not limited and can be appropriately selected according to the state of the plating layer.

[0035] ·Heating and holding process In the method for manufacturing a hot-dip galvanized steel sheet of the present invention, as shown in Figure 2, after the primary cooling step, a heating and holding step is performed in which the steel sheet is heated and held at a temperature of 380°C or higher for 10 seconds or more. By providing the heating and holding step, the eutectic region formed in the plating layer melts slightly, promoting the growth of the single-phase Zn region in the direction perpendicular to the surface of the steel sheet, and forming a single-phase Zn region (large-diameter single-phase Zn phase) that does not contain the eutectic region in the thickness direction of the plating layer. As a result, the plating layer structure (Lz / Lw, area ratio of the eutectic region on the plating layer surface, etc.) can be controlled to a desired range.

[0036] The heating temperature in the heating and holding step is set to 380°C or higher, as shown in Figure 2. This is because if the heating temperature is below 380°C, the melting of the eutectic region such as Zn-Al-Mg will be insufficient. From a similar viewpoint, it is preferable that the heating temperature be 390°C or higher. On the other hand, the heating temperature in the heating and holding process is preferably below the liquidus temperature of the alloy in the plating bath composition, as shown in Figure 2. This is because if the steel sheet temperature is above the liquidus temperature of the alloy in the plating bath composition, alloying between the base steel sheet and the plating layer will progress (an interfacial alloy layer will grow), and the desired corrosion resistance cannot be obtained.

[0037] Furthermore, the heating and holding time in the heating and holding step should be 10 seconds or more. This is because if the heating and holding time is less than 10 seconds, the melting of the eutectic region such as Zn-Al-Mg and the growth of the single-phase Zn region in the thickness direction of the plating layer will be insufficient. In addition, by extending the time of the heating and holding step, it is possible to increase the area ratio of the eutectic region on the surface of the plating layer (to 80% or more). Specifically, it is preferable to set the heating and holding time to 15 seconds or more. This is because if the cooling and holding time is 15 seconds or more, the growth of the single-phase Zn region in the thickness direction of the plating layer is promoted, and as a result, more Al and Mg are distributed on the surface of the plating layer, resulting in an increase in the area ratio of the eutectic region on the surface of the plating layer.

[0038] ·Secondary cooling process In the method for manufacturing a hot-dip galvanized steel sheet of the present invention, as shown in Figure 2, a secondary cooling step is provided after the heating and holding step to cool the plated layer again and solidify it. In this cooling step, the cooling rate is not limited and the plated layer is cooled to room temperature.

[0039] Furthermore, in the method for manufacturing molten Zn-Al-Mg plated steel sheets of the present invention, in addition to the plating film formation step and the heating and cooling step after plating film formation described above, it is also possible to appropriately carry out steps that are normally used in the production of plated steel sheets. [Examples]

[0040] To confirm the effects of the present invention, plated steel sheets were fabricated and their properties were evaluated. [Sample preparation of plated steel sheets] A hot-dip Zn-Al-Mg plating layer was formed on the surface of a base steel sheet using the following procedure, and a sample of plated steel sheet was prepared. Specifically, a plating layer was formed on both sides of a 0.8 mm thick steel sheet (ultra-low carbon mild steel) using a continuous hot-dip galvanizing system. In forming the plating layer, the plating bath temperature was set according to the conditions shown in Table 1, and a cooling process and a heating and holding process were also included to cool the plate down to room temperature. After removing the steel plate from the molten plating bath, cooling was performed using nitrogen gas. Each sample of the obtained plated steel sheet was analyzed for its plating composition and structure as follows.

[0041] (i) Component composition of the plating layer, crystalline components of the plating layer Each sample of the obtained plated steel sheet was immersed in an acid solution to dissolve the plating layer, and the component composition of the plating layer was measured by ICP analysis of the treatment solution. The results are shown in Table 1. Next, the crystalline composition of the plating layer was determined by X-ray diffraction. For X-ray diffraction, a SmartLab® manufactured by Rigaku Corporation was used, with the following settings: X-ray used: Cu-Kα, tube voltage: 40kV, tube current: 30mA, scanning speed: 4° / min. (ii) Lz / Lw For each sample of plated steel sheet obtained, Lz and Lw were measured using SEM, and the Lz / Lw ratio was calculated. Specifically, a scanning electron microscope (SEM-EDX) was used to observe the cross-section of a plated steel sheet at a magnification of 500x, and SEM images of 10 randomly selected fields of view were obtained. From the obtained SEM images, the Lz / Lw ratio for each field of view was determined, and the average value of the 10 fields of view was taken as the Lz / Lw value for the plated steel sheet. The obtained results, along with a determination of whether or not they fall within the scope of the present invention, are shown in Table 1. Since the interface between the plating layer and the underlying steel sheet is not a straight line, the length of the line connecting the interfaces at both ends of the analysis range is defined as the interface length, and for the large-diameter single-phase Zn phase that reaches the interface and is within the length range Lw, the respective lengths Lz are defined. n We measured these values ​​and calculated their sum, Lz. (iii) Area ratio of Zn-Al-Mg eutectic region For each sample of plated steel sheet obtained, the area ratio of the Zn-Al-Mg eutectic region on the surface of the plated layer was measured and calculated using SEM. Specifically, the surface of a plated steel sheet was observed at a magnification of 500x using energy-dispersive X-ray spectroscopy (SEM-EDX) with a scanning electron microscope, and SEM images of 10 randomly selected fields were obtained. From the obtained SEM images, the area percentage (%) for each field was calculated, and the average value of the 10 fields was taken as the area percentage (%) of the Zn-Al-Mg eutectic region in the plated steel sheet. The obtained results, along with a determination of whether or not they fall within the scope of the present invention, are shown in Table 1.

[0042] [evaluation] Each sample of the obtained plated steel sheet was evaluated as follows. The evaluation results are shown in Table 1.

[0043] (1) Corrosion resistance of steel plate joints Test specimens were taken from each sample of plated steel sheet, and identical specimens with the same evaluation surface were aligned and spot-welded to create steel sheet joined test specimens. These were then subjected to phosphate-based chemical conversion treatment and electrodeposition coating to create test specimens for corrosion resistance evaluation. The prepared corrosion resistance test specimens were subjected to a corrosion test (SAE-J2334). After 100 cycles of the corrosion test, the corrosion products on the inside of the steel plate joint, where the welded area had been disassembled, were removed, and the depth of the corrosion holes was measured using a laser displacement meter. The maximum corrosion depth was calculated and evaluated according to the following criteria. The evaluation results are shown in Table 1. Rating 3: Maximum corrosion depth is less than 0.5 mm Rating 2: Maximum corrosion depth is 0.5 mm or more, but less than 0.6 mm. Score 1: Maximum corrosion depth is 0.6 mm or more.

[0044] (2) Corrosion resistance of cut sections Test specimens for corrosion resistance evaluation were prepared by taking specimens from plated steel sheets and subjecting them to phosphate-based chemical conversion treatment and electrodeposition coating. Cross-cut scratches (at an angle of 60°) totaling 160mm in length of 80mm each were made in the center of the corrosion resistance evaluation specimens, and then subjected to a corrosion test (SAE-J2334). Based on the maximum bulging width from the cut area after 140 cycles, the corrosion resistance of the cut area was evaluated according to the following criteria. The evaluation results are shown in Table 1. Rating 2: Maximum bulge length is less than 2mm Score 1: Maximum bulge length is 2mm or more. In this context, a score of 2 indicates sufficient corrosion resistance at the cut surface.

[0045] [Table 1]

[0046] The results in Table 1 show that each sample of the present invention exhibits a good balance of superior corrosion resistance in both the steel plate joint and cut sections compared to each sample of the comparative examples. [Industrial applicability]

[0047] According to the present invention, it is possible to provide a hot-dip Zn-Al-Mg plated steel sheet and a method for manufacturing the same, which achieve a high level of both corrosion resistance at the joint portion and corrosion resistance at the cut portion of the steel sheet. [Explanation of symbols]

[0048] 1. Hot-dip Zn-Al-Mg plated steel sheet 2 Base steel plate 3 Plating layer 4 Single-phase Zn region 41 Large-diameter single-phase Zn phase 5 Eutectic region

Claims

1. It comprises a base steel plate and a plating layer formed on at least one surface of the base steel plate, The aforementioned plating layer contains Al: 2.0 to 5.0% by mass and Mg: 1.0 to 6.0% by mass, and as an optional component, it contains one or more selected from the group consisting of Si, B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb, and Bi in a total amount of 5% by mass or less, with the remainder being Zn and unavoidable impurities. The plating layer comprises a single-phase Zn region and a eutectic region containing at least Zn and Mg, and the single-phase Zn region has a large-diameter single-phase Zn phase that penetrates from the surface of the plating layer to the surface of the underlying steel sheet when viewed in the thickness direction cross-section of the plating layer. When viewing the cross-section of the plating layer in the thickness direction, the sum of the lengths Lz occupied by the large-diameter single-phase Zn phase present on Lw for a length range of 500 μm or more along the interface between the plating layer and the underlying steel sheet is given by the following relationship (1): Lz / Lw≧0.1 A hot-dip Zn-Al-Mg plated steel sheet characterized by satisfying the following conditions.

2. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1, characterized in that the area ratio occupied by the eutectic region on the surface of the plating layer is 80% or more.

3. A method for manufacturing a plated steel sheet according to claim 1 or 2, A method for manufacturing a molten Zn-Al-Mg plated steel sheet, characterized by including a step of solidifying the plating layer in a primary cooling step after molten plating, followed by heating and holding the steel sheet at a temperature of 380°C or higher for 10 seconds or more.

Images