Zn-Al-Mg coated steel sheet

Abstract

The present invention provides a Zn-Al-Mg-plated steel sheet having excellent corrosion resistance and paint adhesion after painting. The Zn-Al-Mg-plated steel sheet of the present invention comprises a substrate steel sheet and a coating layer formed on at least one surface of the substrate steel sheet, the coating layer having a composition, by mass%, of 0.5 to 3.5% Al and 0.5 to 3.5% Mg, with the balance being Zn and unavoidable impurities, the surface of the coating layer having a structure including a Zn single-phase structure and a eutectic structure, the eutectic structure including a Zn-Mg-based intermetallic compound phase and a Zn phase, and optionally further including one or both of an Al phase and an Al-Mg-based intermetallic compound phase, and the coating weight of the coating layer is 35 to 80 g / m 2 wherein the average oxide film thickness on the surface of the plating layer is 50 nm or less, and the standard deviation σ of the oxygen intensity on the surface of the plating layer is 0.30 or less when the average value of the oxygen intensity on the surface of the plating layer is 1.

Description

[Technical Field]

[0001] The present invention relates to a Zn-Al-Mg plated steel sheet. [Background technology]

[0002] Traditionally, hot-dip galvanized steel sheets have been used as rust-resistant steel sheets for automobiles due to their excellent sacrificial corrosion protection and good corrosion resistance. However, with the globalization of the automobile market, there is an increasing need for coated steel sheets with higher corrosion resistance due to factors such as improved quality requirements in emerging countries and worsening corrosive environments caused by increasing air pollution. Under these circumstances, the use of Zn-Al-Mg coated steel sheets, which have improved corrosion resistance by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc coating, is expanding. Furthermore, various technologies have been developed to further improve corrosion resistance and achieve both corrosion resistance and other properties by controlling the coating composition and manufacturing method.

[0003] Patent Document 1 discloses a plated steel material with improved wear resistance and white rust resistance by limiting the Mg content in the plating to 0.5 to 14% and the Mg content to 0.5 to 5% in the plating. Patent Document 2 discloses a zinc alloy plated steel material with improved weldability and corrosion resistance in processed parts by limiting the Al content to 0.1 to 5.0% and the Mg content to 0.1 to 5.0% in the plating and having a predetermined structure between the base steel and the plating layer.

[0004] Furthermore, when Zn-Al-Mg plated steel sheets are used in automobiles, they are subjected to chemical conversion treatment and electrodeposition coating.Chemical conversion treatment is usually a phosphoric acid-based chemical conversion treatment, but in recent years, there has been an increase in the number of OEMs using Zr chemical conversion treatment in order to reduce sludge and phosphorus in wastewater. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2019-501296 [Patent Document 2] Special Publication No. 2018-506644 [Patent Document 3] Special Publication No. 2014-162957 Summary of the Invention [Problem to be solved by the invention]

[0006] However, unlike conventional hot-dip galvanized steel sheets, Zn-Al-Mg-plated steel sheets form various eutectic structures with different oxide film thicknesses depending on the coating composition or manufacturing conditions, resulting in an inconsistent structure on the coating surface. When Zr conversion treatment is applied to such surfaces, removal of the oxide film by etching is uneven across the structure, resulting in an inability to form a uniform Zr conversion film on the coating surface, which leads to issues such as reduced corrosion resistance and paint adhesion after electrodeposition coating.

[0007] Patent Document 3 discloses a method for producing coated steel materials in which the uniformity of a Zr chemical conversion coating is improved by adjusting the composition of the Zr chemical conversion treatment bath. However, limiting the composition of the Zr chemical conversion treatment bath leads to reduced robustness and may affect the conversion properties of materials other than Zn-Al-Mg-plated steel sheets, raising concerns about production. Therefore, there has been a demand for a Zn-Al-Mg-plated steel sheet with a uniform oxide coating that can form a uniform Zr chemical conversion coating regardless of the composition of the Zr chemical conversion treatment bath.

[0008] In view of the above problems, an object of the present invention is to provide a Zn-Al-Mg-plated steel sheet in which a thin and uniform oxide film is formed on the surface of a Zn-Al-Mg-plated coating, thereby exhibiting excellent corrosion resistance and coating adhesion after painting. [Means for solving the problem]

[0009] The present inventors conducted extensive research to solve the above-mentioned problems and have now discovered the following: In a Zn-Al-Mg-plated steel sheet, the coating layer has a structure including a Zn-phase single-phase structure and a eutectic structure having a predetermined phase, and further, by setting the coating mass, average oxide film thickness, and standard deviation σ of the oxygen intensity at the surface of the coating layer within predetermined ranges, the corrosion resistance after painting and the paint adhesion can be improved. Furthermore, in the production of the Zn-Al-Mg-plated steel sheet, the above-mentioned Zn-Al-Mg-plated steel sheet can be obtained by cooling in a predetermined temperature range after gas wiping in a predetermined atmosphere at a predetermined cooling rate.

[0010] That is, the gist and configuration of the present invention are as follows.

[0011] [1] A steel sheet having a base steel sheet and a plating layer formed on at least one surface of the base steel sheet, the plating layer has a component composition containing, in mass%, Al: 0.5 to 3.5% and Mg: 0.5 to 3.5%, with the balance being Zn and inevitable impurities; the surface of the plating layer has a structure including a Zn single-phase structure and a eutectic structure, the eutectic structure contains a Zn-Mg-based intermetallic compound phase and a Zn phase, and optionally further contains one or both of an Al phase and an Al-Mg-based intermetallic compound phase, The coating weight of the plating layer is 35 to 80 g / m 2 and the average oxide film thickness on the surface of the plating layer is 50 nm or less, A Zn-Al-Mg plated steel sheet, characterized in that, when the average oxygen intensity on the surface of the plated layer is taken as 1, the standard deviation σ of the oxygen intensity is 0.30 or less.

[0012] [2] The Zn-Al-Mg-plated steel sheet according to [1] above, wherein the plating layer has a composition further containing, by mass%, 1.0% or less in total of one or more elements selected from Si, Fe, Pb, Ti, Ni, Cu, Co, Mn, Cr, Mo, V, Sr, B, Bi, Cd, Sn, and REM. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a Zn-Al-Mg plated steel sheet having excellent corrosion resistance and paint film adhesion after painting. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is an SEM image of the surface of a plating layer in an example (No. 1) according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the Zn-Al-Mg-plated steel sheet according to the present invention will be described. Note that the embodiment described below is an example of the present invention, and the configuration of the present invention is not limited to this specific example.

[0016] [Zn-Al-Mg coated steel sheet] A Zn-Al-Mg-plated steel sheet according to one embodiment of the present invention comprises a base steel sheet and a coating layer formed on at least one surface of the base steel sheet. The coating layer has a composition containing, by mass%, 0.5 to 3.5% Al and 0.5 to 3.5% Mg, with the balance consisting of Zn and unavoidable impurities, and the surface of the coating layer has a structure including a Zn-phase single-phase structure and a eutectic structure. The eutectic structure includes a Zn-Mg-based intermetallic compound phase and a Zn phase, and optionally further includes one or both of an Al phase and an Al-Mg-based intermetallic compound phase, and the coating weight of the coating layer is 35 to 80 g / m 2 The plating layer is characterized in that the average oxide film thickness on the surface of the plating layer is 50 nm or less, and the standard deviation σ of the oxygen intensity on the surface of the plating layer, where the average value of the oxygen intensity on the surface of the plating layer is 1, is 0.30 or less.

[0017] [Substrate steel plate] The type of substrate steel sheet is not particularly limited, and a general hot-rolled steel sheet or cold-rolled steel sheet such as extra-low carbon mild steel can be used. The thickness of the substrate steel sheet is also not particularly limited, but is preferably 0.7 to 2.0 mm. The coating layer may be formed on one side or both sides of the substrate steel sheet.

[0018] [Composition of plating layer] Next, the component composition of the plating layer will be described.

[0019] If the Al content in the coating layer is less than 0.5% by mass, corrosion resistance and coating adhesion after painting cannot be ensured. Furthermore, a large amount of dross is generated in the coating bath due to oxidation of Mg. Therefore, the Al content in the coating layer is set to 0.5% by mass or more, preferably 1.0% by mass or more. On the other hand, if the Al content in the coating layer exceeds 3.5% by mass, an Al phase or an Al-rich Al-Zn phase precipitates as the primary crystal. The Al phase or the Al-rich Al-Zn phase corrodes preferentially in the coating layer, but unlike the Zn-Mg intermetallic compound phase, it does not have the effect of stabilizing corrosion products and improving corrosion resistance, and instead reduces corrosion resistance after painting. Therefore, by setting the Al content in the coating layer to 3.5% by mass or less, a coating structure having a Zn phase as the primary crystal and a eutectic structure, as described below, can be obtained. Therefore, the Al content in the coating layer is set to 3.5% by mass or less, preferably 3.0% by mass or less.

[0020] If the Mg content in the coating layer is less than 0.5% by mass, corrosion resistance after painting cannot be ensured. Furthermore, if the Mg content in the coating layer is 1.0% by mass or more, a Zn-Mg intermetallic compound phase is stably dispersed in the eutectic structure of the coating layer, and corrosion resistance after painting improves with increasing Mg content, which is preferable. Therefore, the Mg content in the coating layer is set to 0.5% by mass or more, and preferably 1.0% by mass or more. On the other hand, if the Mg content in the coating layer is more than 3.5% by mass, a Zn phase or a Zn-Mg intermetallic compound phase precipitates as the primary crystal. Because the Zn-Mg intermetallic compound phase is more easily oxidized than the Zn phase, the formed oxide film tends to become thick, which reduces the paint film adhesion of the electrodeposition coating on a Zr chemical conversion coating substrate. Therefore, the Mg content in the coating layer is set to 3.5% by mass or less, and preferably 2.0% by mass or less.

[0021] The plating layer may contain one or more optional elements selected from Si, Fe, Pb, Ti, Ni, Cu, Co, Mn, Cr, Mo, V, Sr, B, Bi, Cd, Sn, and rare earth metals (REMs) in a total amount of 1.0 mass% or less. If the total content of these elements exceeds 1.0 mass%, it is likely to cause operational problems due to instability of the plating bath composition or quality problems such as impairing the surface appearance. Therefore, when the plating layer contains one or more elements selected from Si, Fe, Pb, Ti, Ni, Cu, Co, Mn, Cr, Mo, V, Sr, B, Bi, Cd, Sn, and REMs, the total content of these elements should be 1.0 mass% or less. The lower limit of the content is not particularly limited and may be 0.0 mass%.

[0022] The remainder of the plating layer composition is made up of Zn and unavoidable impurities.

[0023] [Plating layer structure] Next, the structure of the plating layer will be described. Note that the structure of the plating layer described below is the structure on the surface of the plating layer.

[0024] The coating layer of the present invention has a structure including a Zn single-phase structure and a eutectic structure. The Zn single-phase structure is necessary for the coating layer to exhibit the sacrificial corrosion protection effect of Zn, and the coating layer having a Zn single-phase structure can obtain good corrosion resistance after painting.

[0025] The eutectic structure contained in the coating layer includes a Zn-Mg intermetallic compound phase and a Zn phase, and optionally further includes one or both of an Al phase and an Al-Mg intermetallic compound phase. Here, the Zn-Mg intermetallic compound phase is an intermetallic compound phase containing Zn and Mg as main components, such as MgZn2, MgZn 11 , Mg2Zn3, and MgZn. The Al-Mg intermetallic compound phase is an intermetallic compound phase whose main components are Al and Mg, such as Mg2Al3, MgAl, and Mg 17 Al 12Such eutectic structures corrode preferentially during the corrosion process of the coating layer. Mg eluted from the eutectic structure during corrosion stabilizes the corrosion products, which are mainly oxides or hydroxides of the coating layer, and suppresses corrosion of the entire coating layer. In other words, a coating layer with a eutectic structure can achieve good corrosion resistance after painting.

[0026] The presence or absence of a Zn-phase single-phase structure and a eutectic structure on the coating surface can be confirmed by combining observation of backscattered electron images using a scanning electron microscope (SEM) with composition analysis using an energy dispersive X-ray analyzer (EDX) or analysis using an electron probe microanalyzer (EPMA) attached to the SEM, as follows. Figure 1 shows an SEM image of the coating layer surface of an example (No. 1, described below) according to one embodiment of the present invention. In Figure 1, the smooth, white region is the Zn phase, and only Zn is detected in this region using EDX or EPMA. Therefore, this region is determined to be a Zn-phase single-phase structure 10. In Figure 1, the region with a lamellar structure is the eutectic structure 20, and the white portion within this region is the Zn phase, and the gray portion is either a Zn-Mg intermetallic compound phase, an Al phase, or an Al-Mg intermetallic compound phase. If Zn and Mg are detected in the eutectic structure 20 using EDX or EPMA, the eutectic structure 20 is determined to contain a Zn-Mg intermetallic compound phase and a Zn phase. If Al is further detected in the eutectic structure 20 by EDX or EPMA, the eutectic structure 20 is determined to contain one or both of an Al phase and an Al-Mg based intermetallic compound phase.

[0027] [Plating layer adhesion weight 35-80g / m 2 ] The inventors have found that the phase structure of the coating surface of Zn-Al-Mg-based coatings changes depending on the coating weight. Specifically, it has been confirmed that the Zn single-phase structure tends to grow into an ellipsoidal shape with its short side in the thickness direction of the substrate steel sheet and its long side horizontally to the substrate steel sheet. As the coating weight increases, a eutectic structure is more likely to intervene between the Zn single-phase structure and the coating surface in the short side direction of the Zn single-phase structure, which grows slowly, and the area proportion of the eutectic structure on the coating surface increases. The eutectic structure is easily oxidized because it contains metals less noble than Zn, such as Mg and Al, and an increase in the area proportion of the eutectic structure on the coating surface increases the average oxide film thickness, as described below. When the coating weight of the coating layer is 80 g / m 2 If the coating weight is more than 80g / m, the corrosion resistance after painting will improve, but the average oxide film thickness will be too thick, inhibiting the formation of the Zr conversion coating and preventing paint adhesion. 2 On the other hand, the coating weight of the plating layer is 35g / m 2 If the coating weight is less than 35g / m, the period during which the coating layer exhibits its sacrificial corrosion protection effect will be shortened, and corrosion resistance after painting will not be ensured. 2 That's all.

[0028] The coating weight of the plating layer can be determined in accordance with JIS H 0401: 2021. Specifically, the plating layer is dissolved and removed from the plated steel sheet with hydrochloric acid, and the coating weight can be determined from the difference in weight between the steel sheet before and after dissolution.

[0029] [Average oxide film thickness on the surface of the plating layer is 50 nm or less] If the average oxide film thickness on the surface of the plating layer exceeds 50 nm, the surface is not sufficiently etched during Zr conversion treatment, suppressing the formation of the Zr conversion film and reducing paint film adhesion. Furthermore, depending on the coating weight of the plating layer, corrosion resistance after painting cannot be ensured. Therefore, the average oxide film thickness on the surface of the plating layer should be 50 nm or less. On the other hand, if the average oxide film thickness is extremely thin, it will cause increases in manufacturing and storage costs, so the average oxide film thickness on the surface of the plating layer should preferably be 2 nm or more.

[0030] The average oxide film thickness on the surface of the plating layer can be determined using an X-ray fluorescence analyzer as follows. The tube voltage is set to 30 kV, the current to 100 mA, the collimator diameter to 30 mm, and the analyzer crystal to be TAP. The O-Kα line is measured on the surface of the test material. The detected O-Kα line is measured for both the peak and background intensities, and the net O-Kα line intensity is calculated. In addition to the test material, silicon wafers with silicon oxide films of different thicknesses are measured under the same conditions as the standard samples, and a calibration curve of oxide film thickness versus O-Kα line intensity is created. The standard samples can be approximately 1 / 2, 1, or 2 times 50 nm thick (e.g., 24 nm, 54 nm, and 96 nm). The integration time for each measurement should be 20 seconds or longer. Using the created calibration curve, the measurement results for the test material are calculated as the oxide film thickness equivalent to a silicon oxide film, and the resulting result is the average oxide film thickness.

[0031] [When the average oxygen intensity on the surface of the plating layer is set to 1, the standard deviation σ of the oxygen intensity is 0.30 or less] As a result of extensive research, the inventors have discovered that controlling the average oxide film thickness as described above and reducing the variation in oxide film thickness on the plated surface leads to improved corrosion resistance and paint adhesion after painting. As described above, in Zn-Al-Mg-plated steel sheets manufactured by conventional methods, the difference in oxide film thickness (variation in oxide film thickness) between the Zn-phase single-phase structure and the eutectic structure is large. When such plated steel sheets are subjected to Zr chemical conversion treatment, etching of the oxide film on the eutectic structure takes longer than on the Zn-phase single-phase structure, reducing the reaction time for forming the Zr chemical conversion film, resulting in a thinner Zr chemical conversion film on the eutectic structure than on the Zn-phase single-phase structure. When electrodeposition coating is applied in this state and the corrosion resistance or paint adhesion after painting is evaluated, the eutectic structure with a thin Zr chemical conversion film acts as the weakest point and becomes the starting point for corrosion or paint peeling, resulting in poor results. Therefore, the inventors defined the difference in oxide film thickness between the Zn-phase single-phase structure and the eutectic structure as "the standard deviation σ of the oxygen intensity when the average value of the oxygen intensity on the surface of the coating layer is set to 1," and investigated conditions for improving corrosion resistance and coating adhesion after painting, and discovered that a significant effect is achieved when σ is 0.30 or less. Therefore, when the average value of the oxygen intensity on the surface of the coating layer is set to 1, the standard deviation σ of the oxygen intensity is set to 0.30 or less. On the other hand, there is no particular lower limit for the standard deviation σ, and it is generally 0.10 or more.

[0032] When the average oxygen intensity on the surface of the plating layer is taken as 1, the standard deviation σ of the oxygen intensity can be determined as follows. The measurement device used is an EPMA-1720HT manufactured by Shimadzu Corporation. The acceleration voltage during measurement is 15 kV, the beam current is 80 nA, and the integration time is 40 ms / point. The oxygen intensity in a 180 × 180 μm field of view on the surface of the test material is measured by mapping, and dot data is obtained. The average value and standard deviation of all the obtained dot data are calculated, and the standard deviation σ is determined by dividing the standard deviation by the average value.

[0033] [Manufacturing method for Zn-Al-Mg-based coated steel sheets] Next, a method for producing a Zn-Al-Mg-plated steel sheet according to one embodiment of the present invention will be described. One example of the method for producing a Zn-Al-Mg-plated steel sheet includes the steps of preparing a substrate steel sheet, immersing the substrate steel sheet in a coating bath containing, by mass, 0.5 to 3.5% Al and 0.5 to 3.5% Mg, with the balance being Zn and unavoidable impurities, thereby subjecting the substrate steel sheet to a coating treatment to produce a coated steel sheet, gas wiping the coated steel sheet to adjust the coating weight, and then cooling the coated steel sheet. The cooling step is further characterized in that the cooling from after gas wiping until the surface temperature of the coated steel sheet reaches 400°C is carried out at a cooling rate of 0.05°C / s or more, and the cooling while the surface temperature of the coated steel sheet is between 300°C and 100°C is carried out in a vacuum atmosphere. The cooling from after gas wiping until the surface temperature of the coated steel sheet reaches 400°C is preferably carried out in an N2 atmosphere or an N2 + 5% H2 atmosphere.

[0034] The type and thickness of the substrate steel sheet are as described above.

[0035] The coating bath used in the production of Zn-Al-Mg coated steel sheets has a composition containing, by mass%, 0.5-3.5% Al and 0.5-3.5% Mg, with the remainder consisting of Zn and unavoidable impurities. Furthermore, the coating bath may optionally contain one or more elements selected from the group consisting of Si, Fe, Pb, Ti, Ni, Cu, Co, Mn, Cr, Mo, V, Sr, B, Bi, Cd, Sn, and rare earth metals (REMs) in a total amount of 1.0 mass% or less. The description of each component in the coating bath is the same as the description of each component in the coating layer described above. The composition of the coating bath and the composition of the coating layer are equivalent.

[0036] When the temperature of the plating bath during plating is 410°C or higher, the plating bath dissolves well and unplated areas can be prevented. Therefore, the temperature of the plating bath is preferably 410°C or higher. On the other hand, by setting the temperature of the plating bath to 550°C or lower, volatilization of plating bath components can be prevented and stable operation can be performed. Therefore, the temperature of the plating bath is preferably 550°C or lower.

[0037] After the plating treatment, the plating coating weight is adjusted by gas wiping. The conditions for gas wiping are not particularly limited, and a conventional method can be used.

[0038] After gas wiping, the plated steel sheet is cooled. In the cooling process from after gas wiping until the surface temperature of the plated steel sheet reaches 400°C, the atmosphere in the process is oxygen-free, and is preferably an N2 atmosphere or an N2 + 5% H2 atmosphere. If the atmosphere contains oxygen in this temperature range, oxidation of the plated surface progresses, making it easier for a thick oxide film to form on the surface, thereby deteriorating paint film adhesion. In the present invention, the temperature during cooling is based on the surface temperature of the plated steel sheet.

[0039] Furthermore, in the cooling process from after gas wiping until the surface temperature of the plated steel sheet reaches 400°C, the cooling rate is preferably 0.05°C / s or more. If the cooling rate is less than 0.05°C / s, the line speed becomes extremely slow, and productivity cannot be achieved. On the other hand, the upper limit of the cooling rate in this temperature range is not particularly limited, but it is preferably 100°C / s or less.

[0040] For the same reason, cooling while the surface temperature of the plated steel sheet is 400 to 300°C is also preferably carried out in an N2 atmosphere or an N2+5% H2 atmosphere.

[0041] In the cooling process while the surface temperature of the plated steel sheet is 300 to 100°C, the process is carried out in a vacuum atmosphere. In the present invention, the "vacuum atmosphere" refers to a vacuum atmosphere having a degree of vacuum of 5 x 10 in absolute pressure notation. -2 ~2×10 -1This means that the pressure is 200 Pa. The temperature range around 300°C is the temperature range where Zn-Al-Mg plating becomes solid phase, and it is also a temperature range where the temperature is higher than room temperature and the surface is highly active. Therefore, in this temperature range, an oxide film is easily formed on the surface, and since the formed oxide film remains on the surface even after cooling is complete, it is necessary to carry out cooling in a way that prevents oxidation as much as possible. During the cooling process, the degree of vacuum in this temperature range is 2 x 10 absolute pressure. -1 If the pressure exceeds 2 Pa, the slight amount of oxygen mixed in accelerates oxidation, forming a thick oxide film on the plating surface. In addition, since oxidation occurs more easily in eutectic structures, differences in oxide film thickness between surface structures are more likely to occur. As a result, coating adhesion deteriorates. Therefore, cooling in this temperature range should be 2×10 -1 On the other hand, in the cooling process, the vacuum level in the temperature range is 5×10 Pa or less in absolute pressure notation. -2 If the pressure is less than 5 Pa, expensive equipment such as a turbomolecular pump is required to obtain the required vacuum, which is cost-inefficient. -2 The cooling is carried out in a vacuum atmosphere of at least Pa. In addition to radiation cooling, cooling in a vacuum atmosphere may also be performed by contact cooling, such as by contacting the sample with a metal block with high thermal conductivity.

[0042] For steps and conditions not described in this specification, conventional methods can be used. [Example]

[0043] [Test material preparation] Ultra-low carbon mild steel (0.8 mm thick) was used as the base steel sheet, and the base steel sheet was plated using a plating bath having the same chemical composition as the chemical composition of the plating layer in each example shown in Table 1. The temperature of the plating bath was 460°C. After the plating process, gas wiping and cooling were performed to produce plated steel sheets. Note that the cooling rate was 5°C / s in the cooling process from gas wiping until the surface temperature of the plated steel sheet reached 400°C. Table 1 shows the atmosphere in each temperature range during cooling. Note that in the examples in Table 1 where the atmosphere between 300 and 100°C was designated as "vacuum," the degree of vacuum in the atmosphere was 8 x 10 in absolute pressure notation.-2 Pa, and cooling was carried out by radiative cooling.

[0044] [Table 1] TIFF0007732157000002.tif219170

[0045] For the plated steel sheets of each example obtained, the coating weight of the coating layer, confirmation of the Zn single-phase structure and eutectic structure, average oxide film thickness, and standard deviation σ of oxygen intensity, assuming the average value of oxygen intensity to be 1, were determined using the methods described above, and the results are shown in Table 1. In the Zn single-phase structure section of Table 1, cases in which a Zn single-phase structure was confirmed are marked with "○", and cases in which a Zn single-phase structure was not confirmed are marked with "×". In addition, in the eutectic structure section of Table 1, cases in which a eutectic structure was confirmed are marked with elements detected in the eutectic structure, and cases in which a eutectic structure was not confirmed are marked with "×".

[0046] The resulting plated steel sheets (test materials) of each example were evaluated for corrosion resistance after painting and paint film adhesion as follows.

[0047] [Corrosion resistance after painting] Test pieces measuring 70 mm x 150 mm were cut from the test material and subjected to Zr conversion treatment and electrodeposition coating. Zr conversion treatment was performed under standard conditions using a PLM2100 manufactured by Nihon Parkerizing Co., Ltd. Electrodeposition coating was performed using a GT150V manufactured by Kansai Paint Co., Ltd. to achieve a coating thickness of 10 μm, and the baking conditions were 170°C and held for 20 minutes. The test pieces were then subjected to a corrosion test (SAE-J2334) and the corrosion status after 180 cycles was evaluated.

[0048] After the corrosion test, a cross-cut was made on the test piece, and the maximum blister width on one side from the cross-cut was measured. Each example was judged according to the following criteria, with a "◎" or "○" being considered a pass. The evaluation results are shown in Table 1. ◎: Maximum bulge width on one side <4.0mm ○: 4.0mm≦Maximum bulge width on one side<5.0mm ×: 5.0mm≦Maximum bulge width on one side

[0049] [Paint film adhesion] To evaluate the adhesion of the coating, we evaluated the secondary water-resistant adhesion. 70mm x 150mm test pieces were cut from the test material and subjected to Zr conversion coating and electrodeposition coating under the same conditions as for the evaluation of corrosion resistance after painting. Each test piece was then immersed in pure water at 60°C for 20 days.

[0050] After the test, the test specimen was cross-cut using an NT Cutter S or A type (manufactured by Nippon Transfer Paper Co., Ltd.) to create 100 2mm-wide grids (10 x 10). Then, tape conforming to JIS Z 1522 (Nichiban Co., Ltd. Cellotape CT-12S) was applied to the cross-cut area and rubbed three times to ensure adhesion. The tape was peeled off at an angle of 90° to the surface of the test specimen at a speed of 0.5 m / s or more, and the number of grids that peeled off was counted. Each example was evaluated according to the following criteria, with a "◎" or "○" indicating a pass. The evaluation results are shown in Table 1. ◎: Number of peeled grids is 0-5 ○: The number of peeled grids is 6 to 15 ×: The number of peeled squares is 16 to 100

[0051] The results in Table 1 show that the examples of the present invention have superior corrosion resistance and paint film adhesion after painting compared to the comparative examples. [Industrial Applicability]

[0052] According to the present invention, it is possible to provide a Zn-Al-Mg plated steel sheet having excellent corrosion resistance and paint adhesion after painting, which is applicable to various uses such as automotive steel sheets. [Explanation of symbols]

[0053] 10 Zn phase single phase structure 20 Eutectic structure

Claims

1. A steel sheet having a base steel sheet and a plating layer formed on at least one surface of the base steel sheet, the plating layer has a component composition containing, in mass%, Al: 0.5 to 3.5% and Mg: 0.5 to 3.5%, with the balance being Zn and inevitable impurities; the surface of the plating layer has a structure including a Zn single-phase structure and a eutectic structure, the eutectic structure contains a Zn-Mg-based intermetallic compound phase and a Zn phase, and optionally further contains one or both of an Al phase and an Al-Mg-based intermetallic compound phase; The coating weight of the plating layer is 35 to 80 g / m ２ and the average oxide film thickness on the surface of the plating layer is 50 nm or less; The Zn-Al-Mg plated steel sheet is characterized in that, when the average oxygen intensity on the surface of the plated layer is taken as 1, the standard deviation σ of the oxygen intensity is 0.30 or less.

2. 2. The Zn-Al-Mg plated steel sheet according to claim 1, wherein the plating layer has a composition further containing, by mass%, 1.0% or less in total of one or more elements selected from Si, Fe, Pb, Ti, Ni, Cu, Co, Mn, Cr, Mo, V, Sr, B, Bi, Cd, Sn, and REM.

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