Hot-dip zn-al-mg-based plated steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide a hot-dip Zn-Al-Mg-based plated steel sheet which achieves, at high levels, both corrosion resistance at steel sheet joining parts and corrosion resistance at cut parts.　In order to achieve said purpose, 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. The plating layer 3 has a composition which contains 2.0-5.0 mass% of Al and 1.0-6.0 mass% of Mg and in which the remaining portion is Zn and unavoidable impurities. The plating layer 3 includes a single-phase Zn region 4 and a eutectic region 5 containing at least Zn and Mg. The single-phase Zn region 4 has, when a cross section thereof in the thickness direction of the plating layer is viewed, a large-diameter single-phase Zn-phase 41 that penetrates the same from the surface of the plating layer to the surface of the base steel sheet.

Description

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

[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.

[0002] Hot-dip galvanized steel sheets have traditionally been widely used as rust-preventive steel sheets in fields such as automobiles, electrical equipment, and building materials because Zn provides sacrificial corrosion protection to the base metal, and the dense formation of Zn corrosion products results in excellent corrosion resistance. However, given the limited global reserves of Zn and the recent concerns about its depletion, there is a growing demand for the development of Zn-reducing galvanized steel sheets.

[0003] One technology for reducing zinc-plated steel sheets is to improve the corrosion resistance of the plating itself, thereby achieving 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 using a Zn-Al-Mg system plating, in which elements such as Al and Mg are added to the 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.

[0005] Japanese Patent Publication No. 2010-275632, Japanese Patent Publication No. 2020-504237

[0006] As mentioned above, with hot-dip Zn-Al-Mg plated steel sheets, increasing the Al content makes it possible to improve the corrosion resistance of the steel sheet joints while reducing the Zn content to some extent. However, there was a problem in that the corrosion resistance of the cut sections of the steel sheets, which is also a corrosion characteristic evaluation for automobiles, tends to deteriorate with increasing Al content. Therefore, there was a need for the development of a technology that could achieve a high degree of compatibility between the corrosion resistance of the steel sheet joints and the corrosion resistance of the 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.

[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] The present invention is based on the above findings, and its gist is as follows: 1. A molten 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, wherein the 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 includes 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 base steel sheet when viewed in the thickness direction cross-section of the plating layer, and when viewed in the thickness direction cross-section of the plating layer, the sum of the lengths occupied by the large-diameter single-phase Zn phase present on Lw for a length range of 500 μm or more along the interface Lw between the plating layer and the base steel sheet satisfies the following relationship (1): Lz / Lw ≥ 0.1. 2. 1. A 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, characterized in that after solidifying the plating layer in a primary cooling step after the hot-dip plating treatment, the method includes a step of heating and holding the steel sheet at a temperature of 380°C or higher for 10 seconds or more.

[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.

[0011] This figure shows a schematic, enlarged cross-section of a molten Zn-Al-Mg plated steel sheet according to one embodiment of the present invention. This figure illustrates the flow of the primary cooling step, heating and holding step, and secondary cooling step in the manufacturing method of a molten Zn-Al-Mg plated steel sheet according to one embodiment of the present invention.

[0012] The embodiments of the molten Zn-Al-Mg plated steel sheet and the method for manufacturing the molten Zn-Al-Mg plated steel sheet of the present invention will be described in detail below, with reference to drawings as necessary. Note that the parts constituting the plated steel sheet disclosed in Figure 1 are schematically represented with different scales and shapes than those of the actual sheet for the sake of explanation.

[0013] (Hot-dip Zn-Al-Mg plated steel sheet) As shown in Figure 1, the hot-dip Zn-Al-Mg plated steel sheet of the present invention comprises a plating layer 3 on a base steel sheet 2. 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.

[0014] The main component of the plating layer, Zn, is an element necessary to impart sacrificial corrosion protection to the plating layer and obtain excellent corrosion resistance. When considering the Zn content in terms of atomic composition ratio, since the plating layer is composed of low-density elements such as Al and Mg, it is necessary for Zn to be the main component in terms of atomic composition ratio as well. For this reason, the Zn content in the plating layer is preferably 80.0% by mass or more, and more preferably 85.0% by mass or more. The upper limit of the Zn content is the content of the remainder of elements other than Zn and impurities.

[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 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. For this reason, the Mg content in the plating layer should be 1.0 mass or more, preferably 2.0 mass or more. On the other hand, if the Mg content in the plating layer exceeds 6.0 mass%, Mg-based oxides will be 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 will reduce the large-diameter single-phase Zn phase described later, making it impossible to obtain the desired corrosion resistance of the cut portions. For this reason, the Mg content in the plating layer should be 6.0 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 sheet 2. In the present invention, when the cross-section in the thickness direction of the plating layer 3 is viewed, the total length Lz (in Figure 1, Lz) of the length range Lw of 500 μm or more along the interface between the plating layer 3 and the base steel sheet 2 is the length occupied by the large-diameter single-phase Zn phase 41 present on Lw. １ +Lz ２ ) satisfies the following relationship (1): Lz / Lw≧0.1.

[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 width from the cut portion (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 width from the cut portion. Accordingly, in the present invention, the total length Lz (in Figure 1, Lz) of the large-diameter single-phase Zn phase present on the Lw length range of 500 μm or more along the interface between the plating layer 3 and the underlying steel plate 2 is defined as Lz. １ +Lz ２)(1) is satisfied, that is, it occupies a certain range, so that the progress of corrosion in the bulge width direction from the cut portion can be efficiently suppressed, and excellent cut portion corrosion resistance can be realized.

[0023] Incidentally, when Lw and Lz described above do not satisfy the relationship (1) (L Ｚ / L ｗ <0.1), the formation of the large-diameter single-phase Zn phase 41 is not sufficient, so that the corrosion in the bulge width direction from the cut portion in the plating layer 3 becomes remarkable, and the desired cut portion corrosion resistance cannot be obtained. From the same viewpoint, it is preferable that Lw and Lz described above satisfy the relationship (2): Lz / Lw≧0.15.

[0024] Here, in the plating layer 3, 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 total length Lz of the lengths occupied by the large-diameter single-phase Zn phase existing on the Lw can be obtained 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. The interface between the plating layer 3 and the base steel sheet 2 is not a straight line, but the length of the line connecting the interfaces at both ends of the analysis range with a straight line is defined as Lw. And for the large-diameter single-phase Zn phase 41 that has reached the interface, those existing within the length range Lw are respectively measured for the length Lz １ , Lz ２ are measured, and their total sum Lz is calculated. Lz calculates the length range parallel to Lw. In the present invention, the values of Lw, Lz, and L Ｚ / L ｗ are obtained by observing the cross section of the plating layer 3 for 10 randomly selected fields of view, obtaining Lw and Lz for each observation field of view, and calculating the value of L Ｚ / L ｗ . 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 plated steel sheet. Further, the observation of the cross section of the plating layer 3 is performed by selecting a field of view such that the parallel length L ｗ is 500 μm or more.

[0025] Furthermore, an interface alloy layer (not shown) may be present 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 extends from the single-phase Zn region 4 to the interface alloy layer. The thickness of the interface alloy layer is not particularly limited.

[0026] Furthermore, in the present invention, it is preferable that the area ratio of the eutectic region 5 on the surface of the plating layer 3 is 80% or more. Highly protective corrosion products are formed on the eutectic region against corrosion from the surface of the plating layer 3, allowing for longer-term stabilization. Therefore, when the area ratio of the eutectic region 5 is 80% or more, the corrosion resistance of the plating layer surface is improved, and the corrosion resistance of the steel plate joint can be further improved. From a similar viewpoint, it is more preferable that the area ratio of the eutectic region 5 on the surface of the plating layer 3 is 85% or more.

[0027] The amount of the plating layer is not particularly limited. For example, from the viewpoint of corrosion resistance, the amount of the plating layer can be 10 g / m² per side of the steel sheet. 2 It is preferable to keep the above amounts. On the other hand, from the viewpoint of manufacturing cost, the amount of the plating layer to adhere is 100 g / m² per side of the steel sheet. 2 Preferably, it should be 60 g / m 2 It is more preferable to use the following, and even more preferably 40g / m² 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 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 of a specific area with a mixture of hydrochloric acid and hexamethylenetetramine as shown in JIS H 0401:2013, and calculating 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 an intermediate layer or a coating film can be further formed on the plating layer 3 as needed. 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 by means of hot air drying, infrared heating, induction heating, etc. Also, 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. For example, a chemical conversion coating film or a primer such as an adhesive layer can be used. The chemical conversion coating film can be formed, for example, by chromate treatment or chromium-free chemical conversion treatment, in which a chromate treatment solution or a chromium-free chemical conversion treatment solution is applied and dried at a steel sheet temperature of 80 to 300°C without washing with water. These chemical conversion coatings can be single-layer or multi-layer; in the case of multi-layer coatings, multiple chemical conversion treatments should be performed sequentially.

[0029] (Method for manufacturing molten Zn-Al-Mg plated steel sheet) The method for manufacturing a molten Zn-Al-Mg plated steel sheet of the present invention (hereinafter sometimes simply referred to as "method for manufacturing a molten galvanized steel sheet") is not particularly limited. However, the plating film of the molten Zn-Al-Mg plated steel sheet obtained by the present invention will be approximately the same as the composition of the plating bath overall. Therefore, the method includes a step of forming the plating layer on a 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] In addition, the process of forming the plating layer is not particularly limited other than the composition of the plating bath described above. For example, it can be manufactured by a continuous melting plating facility by cleaning, heating, and immersing the base steel sheet in the plating bath. In the heating process of the steel sheet, recrystallization annealing or the like is performed for controlling the structure of the base steel sheet itself, and heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective for preventing oxidation of the steel sheet and reducing a trace amount of oxide film present on the surface.

[0031] Also, the bath temperature of the plating bath is not particularly limited, but it is preferably 10°C to 80°C higher than the liquidus temperature of the alloy in the plating bath composition. In order to perform the melting plating treatment, it is necessary to set the bath temperature above the freezing point. By setting the bath temperature 10°C to 80°C higher than the liquidus temperature of the alloy in the plating bath composition, it is to prevent solidification due to local bath temperature drop of the plating bath. The plate temperature of the base steel sheet entering the plating bath is not particularly limited, but is preferably about the plating bath temperature to the plating bath temperature + 20°C from the viewpoint of ease of operation. On the other hand, the bath temperature of the plating bath is preferably 600°C or lower. This is because when the bath temperature of the plating bath exceeds 600°C, it becomes difficult to rapidly cool the plating layer, and there is a possibility that the interfacial alloy layer formed between the plating layer and the base steel sheet becomes thick.

[0032] And, the manufacturing method of the melt-plated steel sheet of the present invention is characterized by including a step of heating and holding at a steel sheet temperature of 380°C or higher for 10 seconds or more after solidifying the plating layer in the primary cooling step after the melt plating treatment. By including the above heating and holding step, a large-diameter single-phase Zn phase penetrating from the surface of the plating layer to the surface of the base steel sheet can be formed, and the area ratio of the eutectic region on the surface of the plating layer can also be increased.

[0033] Here, FIG. 2 shows an example of the flow from the primary cooling step to the secondary cooling step in the manufacturing method of the melt-plated steel sheet of the present invention, where the vertical axis is the steel sheet temperature (°C) and the horizontal axis is the time (s). In FIG. 2, the solid line indicates the flow after forming the plating layer of the present invention, and the broken line indicates the flow after forming the conventional plating layer.

[0034] - Primary Cooling Step The method for manufacturing a hot-dip plated steel sheet of the present invention includes a primary cooling step to solidify the plating layer after immersing the base steel sheet in a plating bath and removing it, as shown in Figure 2. The primary cooling step is not particularly limited as long as the plating layer can be solidified, but usually the steel sheet is cooled to 380°C or below. There is no particular limit to the lower limit of the primary cooling. The cooling rate and cooling time in the primary cooling step 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 plated steel sheet of the present invention, as shown in Figure 2, after the primary cooling process, a heating and holding process 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 process, 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 surface of the plating layer, 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, it is preferable that the heating temperature in the heating and holding step be 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] Also, the heating and holding time in the heating and holding step shall be 10 seconds or more. This is because if the heating and holding time is less than 10 seconds, melting in the eutectic region such as Zn-Al-Mg and growth of the plating layer in the plating layer thickness direction of the single-phase Zn region will be insufficient. Furthermore, by setting the time of the heating and holding step longer, the area ratio of the eutectic region on the plating layer surface can also be increased (to 80% or more). Specifically, it is preferable that the heating and holding time is 15 seconds or more. This is because when the cooling and holding time is 15 seconds or more, growth of the plating layer in the plating layer thickness direction of the single-phase Zn region is promoted, and as a result, a large amount of other Al and Mg is distributed on the plating layer surface, so the area ratio of the eutectic region on the plating layer surface increases.

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

[0039] In addition, in the method for manufacturing a hot-dip Zn-Al-Mg-based galvanized steel sheet of the present invention, in addition to the above-described plating film forming step and the heating and cooling steps after the plating film formation, steps usually employed for ordinary galvanized steel sheets can be appropriately implemented.

[0040] To confirm the effects of the present invention, galvanized steel sheets were produced and their properties were evaluated. [Production of galvanized steel sheet samples] A hot-dip Zn-Al-Mg-based plating layer was formed on the surface of a base steel sheet serving as a base material by the following procedure to obtain a galvanized steel sheet sample. Specifically, plating layers were formed on both sides of a steel sheet (extra-low carbon mild steel) with a thickness of 0.8 mm by a continuous hot-dip plating facility. In forming the plating layer, the plating bath temperature was set under the conditions shown in Table 1, and further a cooling step and a heating and holding step were provided, and cooling was carried out to room temperature. After the steel sheet was pulled out of the hot-dip plating bath, cooling was performed with nitrogen gas. Each sample of the obtained galvanized steel sheet was analyzed for the plating composition and plating structure as follows.

[0041] (i) Component composition of the plating layer and crystalline composition 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 obtained 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, and the analysis was performed 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 the obtained plated steel sheet, Lz and Lw were measured by SEM, and Lz / Lw was calculated. Specifically, the cross-section of the 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, Lz / Lw was determined for each field of view, 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 are shown in Table 1, along with the determination of whether or not they fall within the scope of the present invention. Note that since the interface between the plating layer and the base steel sheet is not a straight line, the length of the line connecting the interfaces at both ends of the analysis range was taken as the interface length, and for the large-diameter single-phase Zn phase that reached the interface and was present within the length range Lw, the respective lengths Lz ｎ The area ratios were measured and their sum Lz was calculated. (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 the 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 of view were obtained. From the obtained SEM images, the area ratio (%) for each field of view was determined, and the average value of the 10 fields of view was taken as the area ratio (%) of the Zn-Al-Mg eutectic region in the plated steel sheet. The obtained results are shown in Table 1, along with a determination of whether or not they fall within the scope of the present invention.

[0042] [Evaluation] The following evaluations were performed on each sample of plated steel sheet obtained. The evaluation results are shown in Table 1.

[0043] (1) Test specimens taken from each sample of corrosion-resistant plated steel sheet were joined together with the same type of sheet and spot-welded to create steel sheet joint 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 evaluation 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 sheet joint were removed by disassembling the welded area, the depth of the corrosion holes was measured with a laser displacement meter, the maximum corrosion depth was calculated, and the evaluation was performed according to the following criteria. The evaluation results are shown in Table 1. Score 3: Maximum corrosion depth less than 0.5 mm Score 2: Maximum corrosion depth 0.5 mm or more and less than 0.6 mm Score 1: Maximum corrosion depth 0.6 mm or more

[0044] (2) Test specimens for evaluating corrosion resistance were prepared by taking test pieces from the cut section of the plated steel sheet and applying phosphate-based chemical conversion treatment and electrodeposition coating. Cross-cut scratches (angle 60°) totaling 160 mm in length of 80 mm each were made in the center of the test specimens for evaluating corrosion resistance, and then subjected to a corrosion test (SAE-J2334). Based on the maximum bulge width from the cut section after 140 cycles, the corrosion resistance of the cut section was evaluated according to the following criteria. The evaluation results are shown in Table 1. Score 2: Maximum bulge width less than 2 mm Score 1: Maximum bulge width 2 mm or more In this case, a score of 2 was considered to indicate sufficient corrosion resistance of the cut section.

[0045]

[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.

[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.

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

Claims

1. A molten 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, wherein the 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 includes 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 base steel sheet when viewed in the thickness direction cross-section of the plating layer, and when viewed in the thickness direction cross-section of the plating layer, 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 base steel sheet satisfies the following relationship (1): Lz / Lw ≥ 0.

1.

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, characterized in that, after solidifying the plating layer in a primary cooling step after hot-dip plating, the method includes a step of heating and holding the steel sheet at a temperature of 380°C or higher for 10 seconds or more.

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