Zn-Al-Mg alloy plated steel sheet and its manufacturing method

Abstract

To provide a Zn-Al-Mg based alloy plated steel plate having excellent flat plate part corrosion resistance, processed part corrosion resistance, slidability and plating peeling resistance.SOLUTION: A Zn-Al-Mg based alloy plated steel plate has, on at least one surface of a steel plate, a Zn-Al-Mg based alloy plated layer. A ternary eutectic structure of Zn / Al / MgZn2 present on the plated layer is 80% or more in an area ratio to the whole surface of the plated layer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a Zn-Al-Mg alloy-plated steel sheet, and in particular to a Zn-Al-Mg alloy-plated steel sheet excellent in corrosion resistance, sliding properties and resistance to plating peeling, which is used in fields such as automobiles, construction, civil engineering and home appliances, and a method for producing the same. [Background technology]

[0002] Zn-Al alloy-coated steel sheets have superior corrosion resistance compared to zinc plating and have traditionally been widely used in fields such as automobiles, construction, civil engineering, and home appliances. Typical examples of Zn-Al alloy-coated steel sheets include hot-dip galvanized steel sheets with an Al content of 0.30 mass% or less in the coating layer, hot-dip zinc-5% aluminum alloy-coated steel sheets with an Al content of approximately 5 mass%, and hot-dip 55% aluminum-zinc alloy-coated steel sheets with an Al content of approximately 55 mass%. In recent years, however, magnesium has been added to Zn-Al alloy coatings to improve corrosion resistance, and Zn-Al-Mg alloy-coated steel sheets with various Al and Mg contents and metallographic structures have been developed.

[0003] Research into Zn-Al-Mg alloy coatings is being conducted with a focus on the composition and structure of the coating layer, with the aim of further improving corrosion resistance. Patent Document 1 discloses a hot-dip Zn-Al-Mg coated steel sheet with good corrosion resistance and surface appearance, in which the hot-dip coating layer is made of 4.0 to 10.0 wt% Al, 1.0 to 4.0 wt% Mg, 0.002 to 0.1 wt% Ti, 0.001 to 0.045 wt% B, with the remainder being Zn and unavoidable impurities, and has a metal structure in which a primary Al phase is mixed into a matrix of an Al / Zn / Zn2Mg ternary eutectic structure. Patent Document 2 discloses a Zn-Al-Mg alloy-plated steel sheet with excellent formability, corrosion resistance, and adhesion durability, which includes a hot-dip coating having a crystalline structure containing 2.0 to 7.0 mass% Al, 0.5 to 3.5 mass% Mg, and the balance being Zn and unavoidable impurities, and which contains 60% or more by volume of a Zn-Al-Mg ternary eutectic structure. Patent Document 3 discloses a zinc alloy-plated steel sheet with excellent phosphate treatability and spot weldability, which includes a zinc alloy plating layer with 0.5 to 2.8 mass% Al, 0.5 to 2.8 mass% Mg, and the balance being Zn and unavoidable impurities, and which has a surface structure containing 40% or less of a Zn single-phase structure and 60% or more of a Zn-Al-Mg intermetallic compound, in terms of area ratio. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-306357 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-241962 [Patent Document 3] Special Publication No. 2018-507321 Summary of the Invention [Problem to be solved by the invention]

[0005] In fields such as automobiles, architecture, civil engineering, and home appliances, Zn-Al-Mg alloy-plated steel sheets are often used after processing, and therefore, in addition to corrosion resistance in flat areas, they are required to have corrosion resistance in processed areas, sliding properties, and plating peeling resistance. Plating peeling resistance refers to the ability to suppress powdering, in which the plating peels off in powder form, and flaking, in which the plating peels off in thin flakes, during sliding. However, none of the Zn-Al-Mg alloy-plated steel sheets described in Patent Documents 1, 2, and 3 have been examined in terms of sliding properties and plating peeling resistance, and it is not clear what plating composition and plating structure provide all of the corrosion resistance in flat areas, corrosion resistance in processed areas, sliding properties, and plating peeling resistance.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a Zn-Al-Mg alloy-plated steel sheet having excellent corrosion resistance in a flat portion, corrosion resistance in a worked portion, sliding properties, and resistance to plating peeling, and a method for producing the same. [Means for solving the problem]

[0007] As a result of investigations conducted by the present inventors to achieve the above object, the present inventors have obtained the following findings. (1) When the area ratio of the Zn / Al / MgZn2 ternary eutectic structure present on the surface of the coating layer of a Zn-Al-Mg alloy-coated steel sheet to the entire surface of the coating layer is 80% or more, the sheet exhibits excellent corrosion resistance in flat areas, corrosion resistance in worked areas, sliding properties, and resistance to coating peeling. (2) When the area ratio of the MgZn2 phase contained in the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the Zn / Al / MgZn2 ternary eutectic structure is 35% or more, the alloy exhibits better flat-plate corrosion resistance, processed-part corrosion resistance, sliding properties, and plating peeling resistance. (3) If the average thickness of the Zn / Al / MgZn2 ternary eutectic structure in the thickness direction cross section is 0.10 μm or more, the corrosion resistance of the flat plate portion, the corrosion resistance of the processed portion, the sliding properties, and the resistance to plating peeling are further improved. (4) A preferred method for making the Zn / Al / MgZn2 ternary eutectic structure exist in an area ratio of 80% or more to the entire surface of the Zn-Al-Mg-based alloy plating layer is to form an Mg or Mg-Zn layer on the plating layer of a Zn-Al-based alloy-plated steel sheet and then perform heat treatment by heating. (5) The coating weight of the Mg or Mg-Zn layer on the Zn-Al alloy plating layer is 0.1 g / m per side of the steel sheet. 2 and then heat treating the Zn-Al alloy plating layer at a temperature equal to or higher than the melting point thereof, followed by cooling at an average cooling rate of 10°C / sec or less, makes it easier to have a ternary eutectic structure of Zn / Al / MgZn2 on the surface of the plating layer, with the area ratio of the ternary eutectic structure being 80% or more of the entire surface of the plating layer, which is preferable.

[0008] The present invention was made based on the above findings, and the gist of the present invention is as follows. [1] A Zn-Al-Mg alloy-plated steel sheet having a Zn-Al-Mg alloy plating layer on at least one surface of the steel sheet, Zn-Al-Mg alloy-plated steel sheet, characterized in that the area ratio of a ternary eutectic structure of Zn / Al / MgZn2 present on the surface of the Zn-Al-Mg alloy plating layer to the entire surface of the plating layer is 80% or more. [2] The Zn-Al-Mg alloy-plated steel sheet according to [1], characterized in that the area ratio of the MgZn2 phase contained in the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the Zn / Al / MgZn2 ternary eutectic structure is 35% or more. [3] The Zn-Al-Mg alloy-plated steel sheet according to [1] or [2], characterized in that the average thickness of the Zn / Al / MgZn2 ternary eutectic structure in the thickness direction cross section is 0.10 μm or more. [4] The Zn-Al-Mg alloy plated steel sheet according to any one of [1] to [3], wherein the Zn-Al-Mg alloy plated layer contains 1.0 to 15 mass % of Al. [5] The Zn-Al-Mg alloy plated steel sheet according to [4], wherein the Zn-Al-Mg alloy plated layer further contains 0.005 to 0.1 mass % of Ni. [6] A Zn-Al-based alloy plated steel sheet having a Zn-Al-based alloy plated layer, on which an Mg or Mg-Zn layer is formed, and then subjected to a heat treatment at a heating temperature equal to or higher than the melting point of the Zn-Al-based alloy plated layer, a method for producing a Zn-Al-Mg-based alloy-plated steel sheet, characterized by cooling the steel sheet from the heating temperature to a temperature range of the melting point of the Zn-Al-based alloy plating layer minus 10°C at an average cooling rate of 10°C / second or less. [7] The coating amount of the Mg or Mg-Zn layer is 0.1 g / m per side of the steel sheet. 2 The method for producing a Zn-Al-Mg alloy plated steel sheet according to [6], characterized in that: [8] The method for producing a Zn-Al-Mg alloy plated steel sheet according to [6] or [7], wherein the Zn-Al alloy plated layer contains 1.0 to 15 mass % of Al. [9] The method for producing a Zn-Al-Mg alloy plated steel sheet according to [8], wherein the Zn-Al alloy plated layer further contains 0.005 to 0.1 mass % of Ni. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a Zn-Al-Mg alloy-plated steel sheet that has excellent corrosion resistance in flat portions, corrosion resistance in processed portions, sliding properties, and resistance to coating peeling. The Zn-Al-Mg alloy-plated steel sheet of the present invention is suitable for a variety of applications, including automobiles, home appliances, and building materials. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description in any way.

[0011] The coating weight of the Zn-Al-Mg alloy plating layer of the present invention is 10 g / m per side in order to obtain excellent corrosion resistance in the flat plate portion and the processed portion. 2 More preferably, 60 g / m 2 That's all.

[0012] The Zn-Al-Mg alloy plating layer of the present invention is characterized in that it comprises Zn, Al, Mg, and inevitable impurities, and further has a ternary eutectic structure of Zn / Al / MgZn2 present on the surface of the plating layer, with an area ratio of 80% or more of the entire surface of the plating layer. The reasons for this limitation are explained below.

[0013] The inventors have discovered that when the Zn / Al / MgZn2 ternary eutectic structure accounts for 80% or more of the entire surface area of ​​the coating layer, the corrosion resistance of the flat portion and the corrosion resistance of the processed portion are significantly improved. Note that the corrosion resistance of the flat portion refers to the corrosion resistance of the flat portion, excluding the edge. In a corrosive environment, Mg in the Zn / Al / MgZn2 ternary eutectic structure is preferentially dissolved and incorporated into basic zinc chloride, a highly protective corrosion product, thereby stabilizing the resulting corrosion product. Furthermore, in a corrosive environment, the dissolved Mg exists as Mg(OH)2 and Mg ions, and their coexistence provides a pH buffering effect against both acids and alkalis, thereby delaying corrosion. Meanwhile, Al forms a barrier layer consisting of a sparingly soluble passive film, thereby delaying corrosion. These effects improve the corrosion resistance of the flat portion. On the other hand, when coated steel sheets are subjected to processes such as bending or bulging, cracks may occur in the hard alloy phases present in the coating layer, reaching the steel substrate and causing early corrosion of the steel substrate. However, in the Zn-Al-Mg-based alloy coating layer of the present invention, the hard phase, MgZn2, is present in a fine structure within the ternary eutectic structure, and the ternary eutectic structure containing the MgZn2 phase is present in the outermost layer, so that fine cracks only occur in the surface layer. Therefore, cracks reaching the steel substrate can be suppressed, and Mg in the Zn / Al / MgZn2 ternary eutectic dissolves in the fine cracks, resulting in excellent corrosion resistance in the processed area due to the action of Mg. For these reasons, in the Zn-Al-Mg-based alloy coated steel sheets of the present invention, the area ratio of the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the coating layer is set to 80% or more, and more preferably 90% or more.

[0014] The inventors also found that when the area ratio of the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the coating layer is 80% or more, the sliding properties and coating peeling resistance are significantly improved. In the Zn / Al / MgZn2 ternary eutectic structure of the present invention, the Al phase and the MgZn2 phase are uniformly and finely distributed within the ternary eutectic structure. Therefore, when the area ratio of the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the coating layer is 80% or more, a dense oxide layer containing Al and Mg is formed over almost the entire outermost layer of the coating layer. When the coating layer is subjected to sliding with a mold, contact occurs between the metal and the oxide with low cohesion rather than between metals with high cohesion, resulting in a lower coefficient of friction. Furthermore, after sliding, Al- and Mg-based oxides are transferred to the mold, smoothing the mold and further reducing the coefficient of friction and improving sliding properties. On the other hand, when the surface of the coating layer is subjected to sliding with a mold or other device, shear stress is generated and propagates within the coating layer, causing powdering and flaking. Therefore, it is believed that when the coefficient of friction is reduced as described above, the resistance to plating peeling also improves.

[0015] Furthermore, the inventors have found that when the MgZn2 phase in the Zn / Al / MgZn2 ternary eutectic structure on the surface of the coating layer accounts for 35% or more of the area of ​​the entire surface of the Zn / Al / MgZn2 ternary eutectic structure, the corrosion resistance of the flat portion, the corrosion resistance of the processed portion, the sliding properties, and the resistance to coating peeling are further improved.

[0016] If the MgZn2 phase in the Zn / Al / MgZn2 ternary eutectic structure accounts for 35% or more of the area of ​​the entire surface of the Zn / Al / MgZn2 ternary eutectic structure, the amount of Mg eluted into a corrosive environment increases in both the flat and processed portions, thereby enhancing the effect of Mg, and further improving the corrosion resistance of the flat portion and the processed portion. The area ratio of the MgZn2 phase in the Zn / Al / MgZn2 ternary eutectic structure is preferably 50% or more.

[0017] Furthermore, when the MgZn2 phase in the Zn / Al / MgZn2 ternary eutectic structure accounts for 35% or more of the area of ​​the entire surface of the Zn / Al / MgZn2 ternary eutectic structure, the surface hardness of the coating layer increases, resulting in a lower coefficient of friction and further improved sliding properties and coating peeling resistance.

[0018] Since the Zn / Al / MgZn2 ternary eutectic structure on the surface of the coating layer is a fine structure, when calculating the area ratio of the MgZn2 phase, it is preferable to obtain an SEM image of the surface at a magnification of 10,000x or more and calculate the area ratio using image analysis software.

[0019] Furthermore, the inventors have found that when the Zn / Al / MgZn2 ternary eutectic structure present on the surface of the coating layer has an average thickness of 0.10 μm or more in the cross section in the thickness direction, the corrosion resistance of the flat portion, the corrosion resistance of the processed portion, the sliding properties, and the resistance to coating peeling are further improved.

[0020] When the Zn / Al / MgZn2 ternary eutectic structure has an average thickness of 0.10 μm or more in the thickness direction cross section, the corrosion retardation effect of Mg is continuously exerted in the surface layer of the coating layer, thereby improving corrosion resistance. The average thickness is preferably 0.50 μm or more.

[0021] Furthermore, if the Zn / Al / MgZn2 ternary eutectic structure has an average thickness of 0.10 μm or more in the cross section in the thickness direction, even if the Al and Mg-based oxides present in the surface layer of the coating layer are damaged during sliding, new Al and Mg-based oxides are generated, reducing the coefficient of friction even over long or repeated sliding distances, and demonstrating excellent sliding properties and resistance to coating peeling.

[0022] The Al content in the coating layer of the Zn-Al-Mg alloy-plated steel sheet of the present invention is preferably 1.0 to 15 mass%. When the Al content is 1.0 mass% or more, the amount of Al oxide formed in the outermost layer of the coating layer increases, making it easier to further improve the sliding properties and coating peeling resistance. The Al content is preferably 4.0 mass% or more. On the other hand, when the Al content is 15 mass% or less, an increase in the proportion of Zn / Al binary eutectic on the surface of the coating layer can be suppressed, making it easier to ensure the area ratio of the ternary eutectic structure of Zn / Al / MgZn2, making it easier to further improve the corrosion resistance, sliding properties, and coating peeling resistance. The Al content is preferably 10 mass% or less.

[0023] The Zn-Al-Mg alloy-plated steel sheet may contain Ni to improve its resistance to blackening. When Ni is contained, the Ni content is set to 0.005 to 0.1 mass%. If the Ni content is less than 0.005 mass%, it is difficult to obtain excellent resistance to blackening. On the other hand, if the Ni content exceeds 0.1 mass%, as will be described later, Al-Mg dross containing Ni is generated in the plating bath during the production of the Zn-Al alloy-plated steel sheet before the formation of the Mg or Mg-Zn layer, and the adhesion of the dross may impair the appearance of the plating, which is undesirable.

[0024] The Zn-Al-Mg alloy-plated steel sheet of the present invention is preferably produced by forming an Mg or Mg-Zn layer on a Zn-Al alloy-plated layer of a Zn-Al alloy-plated steel sheet, heating the steel sheet to a temperature equal to or higher than the melting point of the Zn-Al alloy-plated layer, and then cooling the steel sheet at an average cooling rate of 10°C / sec or less.

[0025] The steel sheet of the present invention is not particularly limited, and cold-rolled steel sheets or hot-rolled steel sheets can be used.

[0026] The Zn-Al alloy plating layer before forming the Mg or Mg-Zn layer preferably contains 1.0 to 15 mass% Al and 0 to 10 mass% Mg. The balance other than the above components preferably consists of Zn and unavoidable impurities. The plating layer of the Zn-Al alloy plated steel sheet contains a Zn phase and a Zn / Al eutectic. When the plating layer of the Zn-Al alloy plated steel sheet contains Mg, the plating layer contains a Zn phase, a Zn / Al eutectic, and a Zn / Al / Zn2Mg ternary eutectic.

[0027] The Al content of the Zn-Al alloy plating layer before forming the Mg or Mg-Zn layer is preferably 1.0 to 15 mass%. The reasons for limiting the upper and lower limits of the Al content are as described above. Furthermore, if the Al content exceeds 15 mass%, top dross mainly composed of Al is likely to be generated in the plating bath, which may impair the plating appearance. Therefore, the upper limit is preferably 15 mass%.

[0028] The Mg content of the Zn-Al alloy plating layer is preferably 0 to 10% by mass. If the Mg content exceeds 10% by mass, significant dross is generated, which may deteriorate the plating appearance, so the upper limit is preferably 10% by mass.

[0029] The Zn-Al alloy plating layer may contain Ni to improve its resistance to blackening. When Ni is contained, it is set to 0.005 to 0.1 mass %. The reason for setting the upper and lower limits of the Ni content is as described above.

[0030] In the present invention, when forming an Mg or Mg-Zn layer on a Zn-Al based alloy plating layer, any method can be selected, such as vacuum deposition, thermal evaporation, electromagnetic levitation induction heating evaporation, sputtering, electron beam evaporation, ion plating, or electromagnetic levitation physical vapor deposition.

[0031] In the present invention, when forming an Mg layer on a Zn-Al alloy plating layer, the coating weight per side is 0.1 to 10 g / m 2 The coating weight is preferably 0.1 g / m 2If the content is 0.5 g / m or more, it is easy to make the ternary eutectic structure of Zn / Al / MgZn2 on the surface of the coating layer 80% or more in area ratio. 2 That's all.

[0032] When forming an Mg-Zn layer on the plating layer, the Mg content in the Mg-Zn layer is preferably 10 mass% or more so that the composition can be easily controlled. 2 As described above, it is preferable to determine the deposition amount of the Mg-Zn layer depending on the Mg composition.

[0033] The Zn-Al-Mg alloy-plated steel sheet of the present invention is produced by forming an Mg or Mg-Zn layer on a Zn-Al alloy plating layer, heating the steel sheet to a temperature equal to or higher than the melting point of the Zn-Al alloy plating layer, and then cooling the steel sheet. The melting point (°C) of the Zn-Al alloy plating layer is preferably determined from a Zn-Al-Mg phase diagram. After heating the steel sheet to a temperature (°C) equal to or higher than the melting point of the Zn-Al alloy plating layer to melt the Zn-Al alloy plating layer and the Mg or Mg-Zn layer as a whole, the steel sheet is cooled at a relatively slow average cooling rate of 10°C / sec or less within a temperature range from the heating temperature to (the melting point of the Zn-Al alloy plating layer minus 10°C). This allows primary crystals to slowly grow and enlarge within the coating, while the unsolidified molten liquid flows to the surface of the coating layer and finally solidifies as a ternary eutectic of Zn / Al / MgZn2, resulting in the formation of a coating structure with a high area ratio of the ternary eutectic structure of Zn / Al / MgZn2 on the surface of the coating layer. The average cooling rate is preferably 5°C / sec or less. By forming an Mg or Mg-Zn layer, the Mg concentration in the surface layer of the coating layer is increased when the entire coating layer is melted, so that the area ratio of the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the coating layer is 80% or more. This results in a coating structure with a higher area ratio of the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the coating layer compared to when produced by hot-dip coating. Furthermore, by forming an Mg or Mg-Zn layer, the area ratio of the MgZn2 phase in the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the Zn / Al / MgZn2 ternary eutectic structure can be 35% or more.

[0034] Generally, the melting point of Zn-Al alloy plating having a composition of Al: 1.0 to 15 mass % and Mg: 0 to 10 mass % is 340°C to 420°C.

[0035] The heating method for heat treatment is not particularly limited, and furnace and electrical heating can be used. However, furnace heating is preferred because it is effective in fully melting the coating layer and vapor-deposited film. For furnace heating, the heat treatment time is not particularly limited. However, taking into account the time it takes for the steel sheet to reach the set temperature and be maintained at the specified temperature, it is preferable to set the time from loading into the furnace to removing it to 15 minutes or more. Prolonged heating promotes the formation of Fe2Al5 and other elements between the base steel sheet and the coating layer, reducing the area ratio of the Zn / Al / MgZn2 ternary eutectic structure on the surface of the coating layer. Therefore, the upper limit of heating time at the heating temperature is set to 1 hour. When electrical heating is used, it is preferable to set the ultimate temperature higher than when using furnace heating in order to fully melt the coating layer and Mg or Mg-Zn layer. Heating can also be performed in an inert gas atmosphere, such as N2 or Ar, to suppress oxidation of Mg and Al in the coating layer. The cooling method in the heat treatment is not particularly limited, but air cooling, mist cooling, gas cooling, etc. can be applied. [Example]

[0036] The present invention will be described in detail below with reference to examples. The following examples do not limit the present invention, and appropriate modifications within the scope of the gist and configuration are considered to be included in the scope of the present invention.

[0037] A hot-dip Zn-Al alloy plating layer (except No. 12, which is a hot-dip Zn-Al alloy plating layer not containing Al) was formed on the surface of a steel sheet so as to have the composition and coating weight shown in Table 1. A Mg or Mg-Zn layer was formed by vacuum deposition in the amount shown in Table 1, and the sheet was heated in a heating furnace at the heating temperature and time shown in Table 1, followed by cooling at a predetermined cooling rate to produce a Zn-Al-Mg alloy-plated steel sheet (except No. 12, which is a Zn-Mg alloy-plated steel sheet not containing Al). The coating weight in Table 1 indicates the coating weight per side of the steel sheet. The average cooling rate in Table 1 indicates the average cooling rate in the temperature range from the heating temperature to (the melting point of the Zn-Al alloy plating layer minus 10°C).

[0038] Ten 20mm x 20mm samples were taken from random locations on the coated steel sheets, and the Zn-Mg intermetallic compound phase was identified using thin-film X-ray diffraction. Secondary electron images (SEM images) of each sample surface were then taken at 1000x magnification using a scanning electron microscope (SEM) at an accelerating voltage of 15kV over a 125µm x 95µm field of view. SEM-EDX area analysis of the same area was performed to identify the Zn / Al / MgZn2 ternary eutectic structure within the SEM images. Image analysis software was then used to calculate the area fraction of the Zn / Al / MgZn2 ternary eutectic structure (area fraction relative to the entire surface of the coating layer). The average value of the 3 fields of view x 10 samples was used as the area fraction of the Zn / Al / MgZn2 ternary eutectic structure. In addition, SEM images of a 13 μm x 10 μm field of view were obtained at an acceleration voltage of 15 kV and a magnification of 10,000 times, and SEM-EDX surface analysis was performed on the same area to calculate the area ratio of the MgZn2 phase in the Zn / Al / MgZn2 ternary eutectic structure to the entire surface of the Zn / Al / MgZn2 ternary eutectic structure, and the average value of 3 fields of view x 10 samples was taken as the area ratio of MgZn2.

[0039] Next, the 10 samples that had had their surfaces observed were subjected to resin cross-section embedding and polishing, and 50μm-wide SEM images were taken at an acceleration voltage of 15kV and a magnification of 2500x. SEM-EDX surface analysis was also performed on the same area to identify the Zn / Al / MgZn2 ternary eutectic structure in the SEM image, and the thickness of the Zn / Al / MgZn2 ternary eutectic structure (thickness in the cross section in the thickness direction of the plating layer) was measured at 10 points, and the average value of 3 fields of view x 10 samples was taken as the average thickness of the Zn / Al / MgZn2 ternary eutectic structure.

[0040] (1) Corrosion resistance of flat plate A sample measuring 150 mm x 50 mm was cut out from the prepared plated steel sheet, and the edge and back surfaces were sealed with tape. After that, a salt spray test was carried out in accordance with JIS Z 2371 standard, and the time until red rust appeared on the surface was evaluated according to the following criteria. <Judgment criteria> ◎: Red rust formation time is 4000 hours or more ○: Red rust formation time is between 3,500 and 4,000 hours 〇-: Red rust formation time is between 3000 and 3500 hours △: Red rust formation time: 2000 hours or more but less than 3000 hours ×: Red rust occurrence time is less than 2000 hours

[0041] (2) Corrosion resistance of processed parts Samples measuring 150 mm x 50 mm were cut out from the prepared plated steel sheets, bent at 90°, and the edges and back surface were sealed with tape. After that, a salt spray test was carried out for 2000 hours in accordance with JIS Z 2371 standard, and corrosion resistance was evaluated based on the time it took for red rust to appear on the bent parts. <Judgment criteria> ◎: Red rust formation time is 3000 hours or more ○: Red rust formation time is between 2500 hours and 3000 hours 〇-: Red rust formation time is between 2000 and 2500 hours △: Red rust formation time: 1000 hours or more but less than 2000 hours ×: Red rust occurrence time is less than 1000 hours

[0042] (3) Sliding properties A lubricated sample was attached, and the die was pressed against the test piece with a constant load N. The test piece was then pulled out at a constant speed under the conditions shown below, measuring the pull-out load F, and the coefficient of friction was calculated from F / N to evaluate the sliding properties. The lubricant used was a commercially available general-purpose cleaning oil with a viscosity of 2.0 cSt at 40°C. The friction coefficient was measured three times consecutively on the same sample, and the average of the second and third friction coefficients was calculated. Mold shape: Bead (tip 4.5R) Mold material: SKD11 (JIS) Contact area: 3×10mm Sliding length: 100 mm Surface pressure: 130.4 MPa Sliding speed: 1m / min <Judgment criteria> ◎: Friction coefficient less than 0.1 〇: 0.1 or more and less than 0.2 〇-: 0.2 or more and less than 0.25 △: 0.25 or more and less than 0.3 ×: 0.3 or more

[0043] (4) Resistance to plating peeling After peeling off the plating layer from the non-sliding surface, a sliding test was conducted under the above conditions, and the weight change before and after sliding was determined. Furthermore, tape peeling was performed on the sliding section after sliding, and the weight change before and after peeling was determined. The total weight change before and after sliding and before and after tape peeling was taken as the amount of plating peeling, and resistance to plating peeling was evaluated. The test was conducted three times using different samples, and the average value was calculated. <Judgment criteria> ◎: Plating peeling amount 0.5g / m 2 less than 〇:0.5g / m 2 More than 1.0g / m 2 less than 〇-:1.0g / m 2 More than 1.5g / m 2 less than △: 1.5g / m 2 More than 2.0g / m 2 less than ×:2.0g / m 2 End

[0044] (5) Resistance to blackening A 50 x 50 mm sample was cut from the prepared plated steel sheet and left to stand for 24 hours in a thermo-hygrostat controlled at a temperature of 80°C and a relative humidity of 98%. The change in lightness (L value) after the test (ΔL = L value after test - L value before test) was calculated. The evaluation criteria were as follows: The L value was measured using an SR2000 manufactured by Nippon Denshoku Industries Co., Ltd. in SCI mode (specular reflection included). <Judgment criteria> ○: -10<△L and uniform appearance without unevenness ○-: -14<△L≦-10, and uniform appearance without unevenness △: -14<△L≦-10, and slight unevenness in appearance ×: △L≦-14, or noticeable uneven appearance

[0045] (6) Appearance The appearance of the 230 x 350 mm samples was evaluated by visually counting the number of defects such as unplated areas, peeled plating, and cracks. <Judgment criteria> ○: No defects ○-: 1 to 2 defects △: 3 to 10 defects ×: 11 or more defects

[0046] [Table 1]

[0047] When the Zn / Al / MgZn2 ternary eutectic structure is present at a surface area ratio of 80% or more on the surface of a Zn-Al-Mg alloy coating layer, excellent flat corrosion resistance, processed corrosion resistance, sliding properties, and plating peeling resistance are exhibited. Furthermore, when the MgZn2 phase in the Zn / Al / MgZn2 ternary eutectic structure accounts for 35% or more of the area of ​​the entire ternary eutectic structure, the flat corrosion resistance, processed corrosion resistance, sliding properties, and plating peeling resistance are further improved. Furthermore, when the Zn / Al / MgZn2 ternary eutectic structure has an average thickness of 0.10 μm or more in the thickness direction cross section, the flat corrosion resistance, processed corrosion resistance, sliding properties, and plating peeling resistance are further improved. [Industrial Applicability]

[0048] The Zn-Al-Mg plated steel sheet of the present invention has excellent corrosion resistance in flat parts, corrosion resistance in worked parts, sliding properties, and resistance to plating peeling, and is applicable to a wide range of fields such as automobiles, electrical machinery, and building materials.

Claims

1. A Zn-Al-Mg alloy-plated steel sheet having a Zn-Al-Mg alloy plating layer on at least one surface of the steel sheet, Zn / Al / MgZn present on the surface of the Zn-Al-Mg alloy plating layer ２ the area ratio of the ternary eutectic structure of the plating layer to the entire surface is 80% or more, The Zn / Al / MgZn ２ MgZn contained in the ternary eutectic structure ２ The Zn / Al / MgZn phase ２ The area ratio of the ternary eutectic structure to the entire surface is 35% or more, The Zn—Al—Mg alloy plated steel sheet is characterized in that the Zn—Al—Mg alloy plated layer contains 0.01 to 0.1 mass % of Ni.

2. The Zn / Al / MgZn ２ 2. The Zn-Al-Mg alloy plated steel sheet according to claim 1, wherein the ternary eutectic structure has an average thickness of 0.10 μm or more in a cross section in the thickness direction.

3. 3. The Zn-Al-Mg alloy plated steel sheet according to claim 1, wherein the Zn-Al-Mg alloy plated layer contains 1.0 to 15 mass % of Al.

4. A steel sheet having a Zn-Al-Mg alloy plating layer on at least one surface, wherein the area ratio of the Zn / Al / MgZn 2 ternary eutectic structure present on the surface of the plating layer to the entire surface of the plating layer is 80% or more; a method for producing a Zn-Al-Mg based alloy plated steel sheet, in which an area ratio of an MgZn 2 phase contained in the Zn / Al / MgZn 2 ternary eutectic structure to an entire surface of the Zn / Al / MgZn 2 ternary eutectic structure is 35% or more, A Zn-Al-based alloy plated steel sheet has a Zn-Al-based alloy plated layer, and an Mg or Mg-Zn layer is formed on the Zn-Al-based alloy plated layer, followed by heat treatment at a heating temperature equal to or higher than the melting point of the Zn-Al-based alloy plated layer, a method for producing a Zn-Al-Mg-based alloy-plated steel sheet, characterized in that cooling is performed in a temperature range from the heating temperature to a temperature 10°C below the melting point of the Zn-Al-based alloy plating layer at an average cooling rate of 10°C / second or less.

5. The amount of the Mg or Mg-Zn layer deposited is 0.1 g / m per one side of the steel sheet. ２ The method for producing a Zn-Al-Mg alloy plated steel sheet according to claim 4, characterized in that

6. 6. The method for producing a Zn—Al—Mg alloy plated steel sheet according to claim 4, wherein the Zn—Al alloy plated layer contains 1.0 to 15 mass % of Al.

7. 7. The method for producing a Zn—Al—Mg alloy plated steel sheet according to claim 6, wherein the Zn—Al alloy plated layer further contains 0.005 to 0.1 mass % of Ni.

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