Zn-Al-Mg-BASED PLATED STEEL SHEET

Abstract

Provided is a Zn-Al-Mg-based plated steel sheet having exceptional post-coating corrosion resistance and coating film adhesion. A Zn-Al-Mg-based plated steel sheet according to the present invention is characterized in having a base steel sheet and a plating layer formed on at least one surface of the base steel sheet, the plating layer having a component composition comprising, in terms of mass, 0.5-3.5% of Al and 0.5-3.5% of Mg with the balance being Zn and unavoidable impurities, the surface of the plating layer having a structure that includes a Zn-phase single phase structure and a eutectic structure, the eutectic structure including a Zn-Mg intermetallic compound phase and a Zn phase, and optionally furthermore including one or both of an Al phase and an Al-Mg intermetallic compound phase, the adhesion amount of the plating layer being 35-80 g / m2, the average oxide film thickness on the surface of the plating layer being 50 nm or less, and the standard deviation σ of the oxygen intensity being 0.30 or less, where the average value of the oxygen intensity on the surface of the plating layer is regarded as 1.

Description

Zn-Al-Mg plated steel sheet

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

[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 plated 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 plated steel sheets, which have improved corrosion resistance by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc plating, is expanding. Furthermore, various technologies have been developed to further improve corrosion resistance and achieve both corrosion resistance and other properties by controlling the plating composition and manufacturing method.

[0003] Patent Document 1 discloses a plated steel material with improved wear resistance and white rust resistance by setting the Al content in the plating to 0.5 to 14% and the Mg content to 0.5 to 5% and limiting the Mg content on the plating surface. Patent Document 2 discloses a zinc alloy plated steel material with improved weldability and corrosion resistance in processed parts by setting the Al content in the plating to 0.1 to 5.0% and the Mg content to 0.1 to 5.0% 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. Phosphate-based conversion treatment is usually used for the conversion treatment, but in recent years, there has been an increase in the number of OEMs using Zr conversion treatment in order to reduce sludge and phosphorus in wastewater.

[0005] Special table 2019-501296 Publication Special table 2018-506644 Publication Special table 2014-162957

[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 plating composition, manufacturing conditions, etc., resulting in an uneven structure on the plating surface. When Zr chemical conversion treatment is applied to such surfaces, removal of the oxide film by etching becomes uneven across the structures, and as a result, a uniform Zr chemical conversion film is not formed on the plating surface, leading to problems such as reduced corrosion resistance and paint film 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 a decrease in robustness and may affect the chemical conversion properties of materials other than the Zn-Al-Mg-plated steel sheet, posing production concerns. Therefore, there has been a demand for a Zn-Al-Mg-plated steel sheet having 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 providing excellent corrosion resistance and coating adhesion after painting.

[0009] As a result of intensive research aimed at solving the above-mentioned problems, the present inventors have 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, it is possible to improve the corrosion resistance after painting and the paint adhesion. 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 and at a predetermined cooling rate.

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

[0011] [1] A steel sheet comprising a substrate steel sheet and a coating layer formed on at least one surface of the substrate steel sheet, wherein the coating 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 coating layer has a structure containing 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, and the coating weight of the coating layer is 35 to 80 g / m ２ an average oxide film thickness on the surface of the plating layer is 50 nm or less, and a 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 taken as 1, is 0.30 or less.

[0012] [2] The Zn-Al-Mg plated steel sheet according to the above [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.

[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.

[0014] 1 is a SEM image of the surface of the plating layer in an example (No. 1) according to one embodiment of the present 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-plated 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 inevitable impurities, and the surface of the coating layer has a structure including a Zn 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] [Base steel sheet] The type of base 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. There is also no particular limit to the thickness of the base steel sheet, but it is preferably 0.7 to 2.0 mm. The coating layer may be formed on one side or both sides of the base steel sheet.

[0018] [Composition of Plated Layer] Next, the composition of the plated layer will be described.

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

[0020] If the Mg content in the plating layer is less than 0.5% by mass, corrosion resistance after painting cannot be ensured. Furthermore, if the Mg content in the plating layer is 1.0% by mass or more, the Zn-Mg intermetallic compound phase is stably dispersed in the eutectic structure of the plating layer, and corrosion resistance after painting improves with increasing Mg content, which is preferable. Therefore, the Mg content in the plating 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 plating layer exceeds 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 be thick, which reduces the paint film adhesion of the electrodeposition coating on a Zr-based chemical conversion coating substrate. Therefore, the Mg content in the plating 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 additive elements selected from Si, Fe, Pb, Ti, Ni, Cu, Co, Mn, Cr, Mo, V, Sr, B, Bi, Cd, Sn, and REM (rare earth metals) in a total amount of 1.0 mass% or less. If the total content of these elements exceeds 1.0 mass%, there is a high possibility that operational problems due to instability of the plating bath composition or quality problems such as impairing the surface appearance may occur. Therefore, when one or more selected from Si, Fe, Pb, Ti, Ni, Cu, Co, Mn, Cr, Mo, V, Sr, B, Bi, Cd, Sn, and REM are contained, the total content of these elements should be 1.0 mass% or less. Note that the lower limit of the content is not particularly limited and may be 0.0 mass%.

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

[0023] [Structure of Plated Layer] Next, the structure of the plated layer will be described. Note that the structure of the plated layer described below is the structure on the surface of the plated layer.

[0024] The plating layer in 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 plating layer to exhibit the sacrificial anticorrosion effect of Zn, and the plating layer having the 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 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, for example, MgZn ２ , MgZn １１ , Mg ２ Zn ３ , and MgZn. The Al-Mg intermetallic compound phase is an intermetallic compound phase whose main components are Al and Mg, for example, Mg ２ Al ３ , 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, mainly oxides or hydroxides, of the coating layer, and suppresses corrosion of the entire coating layer. In other words, the coating layer having 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 as follows by combining observation of backscattered electron images using a scanning electron microscope (SEM) with composition analysis using an energy dispersive X-ray analyzer (EDX) attached to the SEM or analysis using an electron probe microanalyzer (EPMA). 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 in this region, only Zn is detected by 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 a 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. When Zn and Mg are detected in the eutectic structure 20 by EDX or EPMA, the eutectic structure 20 is determined to contain a Zn-Mg intermetallic compound phase and a Zn phase. When Al is also 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 intermetallic compound phase.

[0027] [The coating weight of the plating layer is 35 to 80 g / m ２The inventors have found that in Zn-Al-Mg based coatings, the phase structure on the coating surface 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 in the horizontal direction of the substrate steel sheet. When the coating weight increases, a eutectic structure is more likely to be interposed between the Zn single-phase structure and the coating surface in the short side direction of the Zn single-phase structure, which has a slow growth rate, 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, which will be described later. When the coating weight of the coating layer is 80 g / m ２ If the coating weight is more than 80 g / 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 35 g / m or less. ２ If the coating weight is less than 35 g / m, the period during which the plating layer exhibits the 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 chemical conversion treatment, the formation of a Zr chemical conversion film is suppressed, and paint film adhesion is reduced. In addition, 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 is set to 50 nm or less. On the other hand, if the average oxide film thickness is extremely thin, it will cause an increase in manufacturing costs and storage costs, so the average oxide film thickness on the surface of the plating layer is preferably 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 30φ, and the analyzer crystal to TAP. The O-Kα radiation is measured on the surface of the test material. The detected O-Kα radiation is measured at the background position as well as the peak position, and the net intensity of the O-Kα radiation is calculated. In addition to the test material, silicon wafers with silicon oxide films of different thicknesses are measured as standard samples under the same conditions, and a calibration curve of the oxide film thickness and the O-Kα radiation intensity is created. The oxide film thickness of the standard samples can be approximately 1 / 2, 1, or 2 times 50 nm (e.g., 24 nm, 54 nm, and 96 nm). The integration time for each measurement is 20 seconds or longer. Using the created calibration curve, the measurement results for the test material are calculated as the oxide film thickness in terms of silicon oxide film, and the obtained result is the average oxide film thickness.

[0031] [The standard deviation σ of the oxygen intensity, when the average value of the oxygen intensity on the surface of the plating layer is set to 1, is 0.30 or less.] As a result of extensive research, the present inventors have discovered that controlling the average oxide film thickness as described above and reducing the variation in oxide film thickness on the plating 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. As a result, the Zr chemical conversion film on the eutectic structure becomes thinner than that 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, and therefore excellent results are not obtained. 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 plating layer is set to 1," and investigated the 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 plating 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 limitation on the lower limit of 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. An EPMA-1720HT manufactured by Shimadzu Corporation is used as the measuring device. 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 obtained by dividing the standard deviation by the average value.

[0033] [Method for Manufacturing Zn-Al-Mg-Based Plated Steel Sheet] Next, a method for manufacturing a Zn-Al-Mg-based plated steel sheet according to one embodiment of the present invention will be described. One example of the method for manufacturing a Zn-Al-Mg-based plated steel sheet includes the steps of preparing a base steel sheet, immersing the base steel sheet in a plating 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 base steel sheet to a plating treatment to obtain a plated steel sheet, gas wiping the plated steel sheet to adjust the coating weight, and then cooling the plated steel sheet. Furthermore, in the cooling step, the plated steel sheet is cooled at a cooling rate of 0.05°C / s or more from the time after gas wiping until the surface temperature of the plated steel sheet reaches 400°C, and the plated steel sheet is cooled while the surface temperature is between 300°C and 100°C in a vacuum atmosphere. The cooling from the time after gas wiping until the surface temperature of the plated steel sheet reaches 400°C is performed in a N atmosphere. ２ Atmosphere or N ２ +5% H ２ It is preferable to carry out the treatment in an atmosphere.

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

[0035] The plating bath used in the production of Zn-Al-Mg-plated steel sheets 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. Furthermore, the plating 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 plating bath is the same as the description of each component in the plating layer described above. The composition of the plating bath and the composition of the plating layer are equivalent.

[0036] When the temperature of the plating bath during plating is 410°C or higher, the plating bath dissolves well and non-plating 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 the plating bath components can be prevented well 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 an oxygen-free atmosphere, and N ２ Atmosphere or N ２ +5% H ２ It is preferable to carry out the cooling in an atmosphere containing oxygen in the above temperature range. If the atmosphere contains oxygen in the above temperature range, oxidation of the plating surface will progress, and a thick oxide film will be likely to form on the surface, resulting in poor 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 step 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 obtained. 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, the atmosphere in the process is N 2 for cooling the surface temperature of the plated steel sheet from 400 to 300°C. ２ Atmosphere or N ２ +5% H ２ It is preferable to do this in an ambient 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" means a vacuum of 5 x 10 in absolute pressure notation. －２ ~2 x 10 －１This means that the temperature range around 300°C is the temperature range where the Zn-Al-Mg plating becomes solid phase, and is also a temperature range where the surface is highly active, as the temperature is higher than room temperature. Therefore, in this temperature range, an oxide film is likely to form on the surface, and the formed oxide film remains on the surface even after cooling is complete, so it is necessary to perform cooling in a way that minimizes oxidation. In the cooling process, the degree of vacuum in this temperature range is 2 x 10 in absolute pressure notation. －１ If the pressure exceeds 2 Pa, oxidation is accelerated by the small amount of oxygen mixed in, and a thick oxide film is formed on the plating surface. In addition, since oxidation occurs more easily in the eutectic structure, differences in oxide film thickness between surface structures are 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 in absolute pressure notation. －２ If the pressure is less than 5×10 Pa, expensive equipment such as a turbo molecular pump is required to obtain the desired vacuum, which is disadvantageous in terms of cost. －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 bringing the material into contact with a metal block having high thermal conductivity.

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

[0043] [Preparation of test material] Ultra-low carbon mild steel (plate thickness 0.8 mm) was used as a base steel sheet, and the base steel sheet was subjected to a plating process using a plating bath having the same component composition as the component composition of the plating layer of 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 prepare plated steel sheets. Note that in the cooling process from gas wiping until the surface temperature of the plated steel sheet reached 400°C, the cooling rate was 5°C / s. 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 radiation cooling.

[0044]

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

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

[0047] [Corrosion Resistance After Painting] Test pieces measuring 70 mm x 150 mm were cut out from the test material and subjected to Zr conversion treatment and electrodeposition coating. The Zr conversion treatment was performed under standard conditions using a PLM2100 manufactured by Nihon Parkerizing Co., Ltd. The electrodeposition coating was performed using a GT150V manufactured by Kansai Paint Co., Ltd. to 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 state 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 portion was measured. Each example was evaluated according to the following criteria, with "◎" or "◯" being considered pass. The evaluation results are shown in Table 1. ◎: Maximum blister width on one side < 4.0 mm ○: 4.0 mm ≦ Maximum blister width on one side < 5.0 mm ×: 5.0 mm ≦ Maximum blister width on one side

[0049] [Coating Adhesion] Coating adhesion was evaluated by evaluating water-resistant secondary adhesion. 70 mm × 150 mm test pieces were cut out from the test materials, and the test pieces were subjected to Zr conversion coating and electrodeposition coating under the same conditions as those used to evaluate post-painting corrosion resistance. Each test piece was then immersed in pure water at 60°C for 20 days.

[0050] After the test, the test piece was cross-cut using an NT Cutter S or A-type (manufactured by Nippon Transfer Paper Co., Ltd.) so that 100 (10 x 10) 2 mm wide grids were formed. Then, a tape conforming to JIS Z 1522 (Cellotape CT-12S, manufactured by Nichiban Co., Ltd.) was attached to the cross-cut area and rubbed three times to adhere the tape. The tape was peeled off at an angle of 90° to the attached surface of the test piece 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 "◎" or "◯" representing a pass. The evaluation results are shown in Table 1. ◎: Number of peeled grids was 0 to 5. ○: Number of peeled grids was 6 to 15. ×: Number of peeled grids was 16 to 100.

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

[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.

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

Claims

1. A steel sheet having a base steel sheet and a coating layer formed on at least one surface of the base steel sheet, wherein the coating 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 unavoidable impurities, the surface of the coating layer has a structure containing 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, and the coating weight of the coating layer is 35 to 80 g / m ２ an average oxide film thickness on the surface of the plating layer is 50 nm or less, and a 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 taken as 1, is 0.30 or less.

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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