Zn-Al-Mg coated steel plate

Abstract

Invention details;

Description

Zn-Al-Mg plated steel sheet

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

[0002] Hot-dip galvanized steel sheets have traditionally been used as rust-resistant steel sheets for automobiles due to their excellent sacrificial corrosion protection and good corrosion resistance. In addition, hot-dip galvanized steel sheets exhibit significantly superior powdering resistance during press working because the coating layer is made of highly ductile pure zinc.

[0003] In recent years, adhesive bonding has become increasingly common in automobile manufacturing processes, replacing conventional spot welding and other methods for joining steel sheets. In this context, hot-dip galvanized steel sheets offer advantages, such as the ability to use stronger adhesives because the shear strength of the coating / steel sheet interface is higher than that of galvannealed steel sheets. Furthermore, when hot-dip galvanized steel sheets are used as exterior panels, they also exhibit better chipping resistance than galvannealed steel sheets. Because of these excellent performance characteristics, hot-dip galvanized steel sheets are increasingly being used as automotive steel sheets.

[0004] On the other hand, hot-dip galvanized steel sheets have the disadvantage that their weldability, particularly their continuous weldability, is inferior to that of alloyed hot-dip galvanized steel sheets. Weldability depends on the coating weight, and is improved by reducing the coating weight, but this also leads to a deterioration in corrosion resistance. Furthermore, hot-dip galvanized steel sheets are composed of a relatively soft Zn phase, which increases the coefficient of friction between the coating layer and the press die, resulting in inferior press formability compared to alloyed hot-dip galvanized steel sheets.

[0005] In order to overcome these problems, proposals have been made for hot-dip galvanized steel sheets, primarily for the purpose of improving corrosion resistance. For example, Patent Documents 1 and 2 describe Zn-Al-Mg-based plated steel sheets that contain 2 to 19 mass% Al, 1 to 10 mass% Mg, and a trace amount of Si and have excellent corrosion resistance. Patent Document 3 describes a "zinc alloy plated steel sheet that is excellent in phosphate treatability and spot weldability" that contains Al and Mg. Patent Document 4 describes a "Mg- and Al-containing hot-dip Zn-plated steel sheet that is excellent in sliding properties."

[0006] JP 2000-104154 A JP 2003-268519 A JP 2018-507321 A JP 2010-106293 A

[0007] However, the above-mentioned conventional technologies have the following problems. Patent Documents 1 and 2 do not fully consider spot weldability intended for use in automobile bodies, nor the sliding properties that affect press formability. Patent Document 3 also considers spot weldability, but does not consider press formability. On the other hand, Patent Document 4 considers sliding properties, but does not consider spot weldability. Steel sheets for automobiles need to simultaneously satisfy multiple required properties, but as described above, no consideration has been given to achieving all of corrosion resistance, sliding properties that affect press formability, and spot weldability.

[0008] In view of the above problems, an object of the present invention is to provide a Zn—Al—Mg-plated steel sheet that ensures excellent corrosion resistance while also achieving excellent sliding properties and spot weldability.

[0009] As a result of extensive research to solve the above-mentioned problems, the inventors have made the following findings. Simply specifying the Al and Mg contents in the coating layer to ensure high corrosion resistance makes it difficult to achieve good sliding properties and spot weldability. Furthermore, the thicker the oxide layer formed on the coating layer surface, the better the sliding properties but the worse the spot weldability. For this reason, it has been difficult to achieve both sliding properties and spot weldability. The inventors have found that by adopting a predetermined coating layer composition and optimizing the cooling conditions after hot-dip coating, it is possible to form an oxide layer predominantly on the eutectic structure on the coating layer surface while preventing the oxide layer from being formed on the Zn-phase single-phase structure. In this case, by suppressing the area fraction of the eutectic structure on the coating layer surface, excellent spot weldability can be ensured. Moreover, because a thick oxide layer is formed on the eutectic structure, the average thickness of the oxide layer can be maintained at a predetermined level even when the area fraction of the eutectic structure is suppressed. Therefore, excellent sliding properties can also be obtained.

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

[0011] [1] A Zn-Al-Mg plated steel sheet comprising: a steel sheet; a plating layer formed on at least one surface of the steel sheet; and an oxide layer formed on a surface of the plating layer; wherein the plating layer has a component composition containing, in mass%, 0.5 to 10.0% Al and 0.5 to 10.0% Mg, with the balance being Zn and unavoidable impurities; the surface of the plating layer has a Zn single-phase structure and a eutectic structure; the eutectic structure contains a Zn-Mg intermetallic compound phase and further contains one or more phases selected from a Zn phase, an Al phase, and an Al-Mg intermetallic compound phase; an area ratio A of the eutectic structure on the surface of the plating layer is less than 60%; and an average thickness B of the oxide layer on the surface of the plating layer is 10 nm or more and 50 nm or less.

[0012] [2] The Zn—Al—Mg-plated steel sheet according to the above [1], wherein the area ratio A (%) of the eutectic structure and the average thickness B (nm) of the oxide layer satisfy B / A≧0.40.

[0013] [3] The Zn-Al-Mg plated steel sheet according to the above [1] or [2], wherein the plating layer has a composition further containing, by mass%, 1% 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.

[0014] The Zn-Al-Mg plated steel sheet of the present invention has excellent corrosion resistance, while also achieving excellent sliding properties and spot weldability.

[0015] 1 is an SEM image of the plating layer surface of Invention Example No. 1. 2 is a diagram showing an outline of a corrosion test piece. 3 is a diagram showing cycle conditions for a corrosion test. 4 is a schematic front view showing a friction coefficient measuring device. 5 is a schematic perspective view showing the shape and dimensions of a bead.

[0016] 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.

[0017] The Zn-Al-Mg-plated steel sheet of the present invention comprises a steel sheet, a plating layer formed on at least one side of the steel sheet, and an oxide layer formed on the surface of the plating layer. The type of steel sheet is not particularly limited, and may be, for example, a general hot-rolled steel sheet or cold-rolled steel sheet such as an extra-low carbon mild steel. The plating layer is a zinc alloy plating layer, and may be formed on one or both sides of the steel sheet.

[0018] If the Al concentration in the coating layer is too low, corrosion resistance and coating adhesion cannot be ensured, and a large amount of dross is generated due to the oxidation of Mg in the coating bath. Therefore, the Al concentration 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 concentration is excessive, an Al phase or an Al-Zn phase may precipitate as a primary crystal. These phases corrode preferentially in the coating layer, but unlike Zn-Mg intermetallic compounds, they do not stabilize corrosion products and improve corrosion resistance; rather, they actually reduce corrosion resistance. Therefore, the Al concentration in the coating layer is set to 10.0 mass% or less. If the Al concentration is 10.0 mass% or less, a primary Al phase or an Al-rich Al-Zn phase does not precipitate, and a coating structure containing a Zn phase and a eutectic structure can be obtained.

[0019] If the Mg concentration in the coating layer is too low, corrosion resistance cannot be ensured. Therefore, the Mg concentration in the coating layer is set to 0.5 mass% or more, preferably 1.0 mass% or more. A Mg concentration of 1.0 mass% or more is preferable because Zn-Mg intermetallic compounds are stably dispersed in the eutectic structure of the coating layer, and corrosion resistance further improves with increasing Mg concentration. On the other hand, if the Mg concentration in the coating layer is excessive, spot weldability deteriorates. Furthermore, workability, particularly the degree of crack occurrence during bending, cannot be maintained at the same level as conventional hot-dip galvanized steel sheets. For this reason, the Mg concentration in the coating layer is set to 10.0 mass% or less, preferably 7.0 mass% or less. A Mg concentration of 7.0 mass% or less is preferable because it does not reduce coating adhesion.

[0020] The plating layer may 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% by mass or less. If the total content of these elements exceeds 1% by mass, the bath composition may become unstable, potentially resulting in operational problems 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% by mass or less. Because these elements are optional, the lower limit of the content of these elements is not particularly limited and may even be 0% by mass.

[0021] The remainder of the composition of the plating layer is made up of Zn and unavoidable impurities.

[0022] Next, the structure of the Zn-Al-Mg-based coating layer will be described. The coating layer of the present invention has a Zn single-phase structure and a eutectic structure. The Zn single-phase structure is necessary for the sacrificial corrosion protection action of Zn to be exerted in the coating layer, thereby achieving good corrosion resistance.

[0023] The eutectic structure contained in the coating layer in the present invention includes a Zn-Mg intermetallic compound phase, and further includes one or more phases selected from a Zn phase, an Al phase, and an Al-Mg intermetallic compound phase. Here, the Zn-Mg intermetallic compound is an intermetallic compound containing Zn and Mg as main components, such as MgZn. ２ , MgZn １１ , Mg ２ Zn ３ , and MgZn. The Al-Mg intermetallic compound is an intermetallic compound whose main components are Al and Mg, for example, Mg ２ Al ３ , MgAl, and Mg １７ Al １２This eutectic structure is the first to start corroding during the corrosion process of the plating layer. The Mg that dissolves during this process stabilizes the corrosion products, which are mainly oxides or hydroxides of the plating layer, and suppresses the subsequent corrosion of the entire plating layer.

[0024] While this eutectic structure exhibits good corrosion resistance, it has been found that a high area ratio of the eutectic structure on the coating layer surface degrades spot weldability. Because the eutectic structure contains easily oxidized components such as Mg and Al, it tends to form a thicker oxide layer than a Zn single-phase structure. As the area ratio of the eutectic structure on the coating layer surface increases, the area ratio of the oxide layer covering the coating layer also increases, resulting in increased electrical resistance at the interface between the electrode and the steel sheet during spot welding and reduced spot weldability. Therefore, in this embodiment, the area ratio of the eutectic structure on the coating layer surface is set to less than 60%. The area ratio of the eutectic structure is preferably 50% or less. Furthermore, if the area ratio of the eutectic structure on the coating layer surface is too low, the average oxide layer thickness cannot be increased to 10 nm or more, resulting in insufficient sliding properties. Therefore, the area ratio of the eutectic structure on the coating layer surface is preferably set to 5% or more, and more preferably 10% or more.

[0025] The single-phase structure and eutectic structure on the coating layer surface can be identified using, for example, a scanning electron microscope (SEM) and its associated energy dispersive X-ray analyzer (EDX) and electron probe microanalyzer (EPMA). For example, in the SEM image of FIG. 1 , the smooth, white region is a Zn-phase single-phase structure, and in this region, only Zn is detected by EDX or EPMA. In the SEM image of FIG. 1 , the region with a lamellar structure is a eutectic structure, 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. In this region with a lamellar structure, Zn and Mg are detected by EDX or EPMA. If the eutectic structure contains either or both of an Al phase and an Al-Mg intermetallic compound phase, Al is also detected.

[0026] The "area ratio of the eutectic structure on the surface of the coating layer" can be determined by obtaining SEM images or EDX or EPMA mapping images in any three different fields of view on the surface of the coating layer of a Zn-Al-Mg-plated steel sheet, binarizing the SEM images or mapping images to identify the eutectic structure region, determining the area ratio of the eutectic structure in each field of view, and calculating the arithmetic mean of the area ratios of the three fields of view. The magnification of the SEM images or mapping images may be appropriately selected depending on the size of the surface structure, but in the present invention, it can be set to 500 to 2000 times. Furthermore, an acceleration voltage of 5 to 20 kV is preferred, so as not to provide information on extremely surface or deep regions.

[0027] The average thickness of the oxide layer on the surface of the plating layer is set to 10 nm or more. Forming a thick oxide layer on the surface of the plating layer can prevent direct contact of the plating layer with the press die and suppress adhesion of the plating layer to the die. Therefore, the friction coefficient can be stabilized at a low value, good sliding properties can be obtained, and press formability is improved. An average oxide layer thickness of 20 nm or more is more effective. This is because, in press forming with long sliding distances, even if the oxide layer on the surface of the plating layer is worn, the oxide layer is likely to remain, preventing a deterioration in sliding properties. Furthermore, the average thickness of the oxide layer is set to 50 nm or less. If the average thickness of the oxide layer exceeds 50 nm, the electrical resistance value at the interface between the electrode and the steel sheet increases during spot welding, and spot weldability is reduced. The average thickness of the oxide layer is preferably 45 nm or less, more preferably 40 nm or less, and even more preferably 35 nm or less.

[0028] The "average thickness of the oxide layer on the surface of the plating layer" is determined using an X-ray fluorescence analyzer. During measurement, the tube voltage is 30 kV, the current is 100 mA, and the analyzing crystal is set to TAP to detect O-Kα radiation. When measuring O-Kα radiation, the intensity at the peak position as well as the background position is measured, and the net intensity of the O-Kα radiation is calculated. The integration time for each is 20 seconds or longer. In addition to the test material (the Zn-Al-Mg-plated steel sheet to be measured), the O-Kα radiation intensity of silicon wafers coated with silicon oxide films of 96 nm, 54 nm, and 24 nm thicknesses is also measured under the same conditions. A calibration curve of oxide film thickness versus O-Kα radiation intensity is then created, and the oxide layer thickness of the test material is calculated as the oxide film thickness in terms of a silicon oxide film. This is the "average oxide layer thickness."

[0029] As described above, when an oxide layer is formed primarily on a eutectic structure, increasing the area fraction of the eutectic structure increases the average thickness of the oxide layer, improving sliding properties but also degrading spot weldability. Therefore, the inventors conducted extensive research into achieving both sliding properties and spot weldability. As a result, they discovered that by forming a thick oxide layer on the eutectic structure while keeping the area fraction of the eutectic structure below 60%, the average thickness of the oxide layer can be increased, thereby achieving both high levels of sliding properties and spot weldability. Specifically, it is desirable for the area fraction A (%) of the eutectic structure and the average thickness B (nm) of the oxide layer to satisfy B / A ≥ 0.40. This B / A is a numerical value representing the local thickness of the oxide layer formed on the eutectic structure. The larger this value, the more stable the spot weldability and sliding properties can be. Since a higher B / A is preferable, there is no particular upper limit. However, in this embodiment, B / A is approximately 1.10 or less.

[0030] The method for making the area ratio of the eutectic structure on the coating layer surface less than 60% is not particularly limited. For example, by providing a holding step for 10 seconds or more in the temperature range of 400°C to 380°C in the cooling step after the hot-dip coating process, the formation and growth of the primary Zn layer can be promoted, and the area ratio of the eutectic structure that starts to solidify thereafter can be reduced. However, if the material is held in this temperature range in a normal air atmosphere as in the past, a thick oxide layer will form on the Zn phase formed as the primary crystal. In particular, if the average thickness of the oxide layer exceeds 50 nm, this will lead to a decrease in spot weldability. Therefore, in order to make the average thickness of the oxide layer 50 nm or less, the atmosphere for the holding step should be less oxidizing than normal air, preferably a non-oxidizing atmosphere, and more preferably a reducing atmosphere. An atmosphere less oxidizing than air is an atmosphere with a higher O content than air. ２ The non-oxidizing atmosphere may be an inert gas atmosphere, such as N ２ The reducing atmosphere is H 2 as in the annealing process before the hot dip coating process. ２ N including ２ The atmosphere is one of them.

[0031] The method for achieving an average oxide layer thickness of 10 nm or more is not particularly limited. For example, a thick oxide layer can be formed on the eutectic structure by performing a holding step in the temperature range of 400°C to 380°C after the hot-dip galvanizing process, followed by a holding step in the temperature range of 160°C to 140°C for 20 seconds or more. This is because, in this temperature range, the oxide layer is formed mainly on the eutectic structure rather than on the Zn phase. By using an oxidizing atmosphere, a thicker oxide layer can be formed without deteriorating spot weldability. The oxidizing atmosphere may be air, but it is preferable to introduce oxygen, water vapor, or the like into the atmosphere during the holding step at 160°C to 140°C to increase the oxidizing property compared to air.

[0032] The amount of plating layer per side is not particularly limited, but is preferably 10 to 300 g / m ２The "adhesion amount of the plating layer per side" is determined in accordance with JIS H 0401:2021 from the difference in weight before and after the plating layer is dissolved and removed with hydrochloric acid.

[0033] <Preparation of test materials> Zn-Al-Mg plated steel sheets with plating layers of different component compositions were prepared by plating ultra-low carbon mild steel. In each example, the coating weight was 50±5 g / m ２ The plating layer of the obtained plated steel sheet was dissolved in hydrochloric acid, and the treatment solution was subjected to ICP analysis to measure the chemical composition of the plating layer. Table 1 shows the chemical composition of the plating layer for each example. In this example, test materials with different average oxide layer thicknesses and eutectic structure area ratios on the plating layer surface were prepared by changing the cooling conditions after plating. Table 1 shows the cooling conditions for each example. The holding times shown in Table 1 indicate the time held in the corresponding temperature range, and the holding time in each temperature range was adjusted by intentionally keeping the steel sheet warm in each temperature range. However, in the temperature range of 400 to 380°C, Nos. 2 and 9 were allowed to cool naturally without heat retention, and therefore passed through the temperature range in approximately 1 second. In the temperature range of 160 to 140°C, Nos. 7 and 9 were allowed to cool naturally without heat retention, and therefore passed through the temperature range in approximately 3 seconds.

[0034] The structure of the coating layer surface of each Zn-Al-Mg coated steel sheet was observed as follows. The coating layer surface was observed using an SEM, and Zn, Al, and Mg were measured using elemental mapping by EDX. Figure 1 shows an SEM image of No. 1, representing an inventive example. In the SEM image of Figure 1, the area with a smooth, white appearance is a Zn-phase single-phase structure 10, in which only Zn was detected by EDX. In the SEM image of Figure 1, the area with a lamellar structure is a eutectic structure 12, in which both Zn and Mg were detected by EDX. In Table 1, a circle next to the Zn-phase single-phase structure and eutectic structure indicates that the structure was confirmed. The eutectic structure contains a Zn-Mg intermetallic compound phase and one or more phases selected from a Zn phase, an Al phase, and an Al-Mg intermetallic compound phase.

[0035] Furthermore, the "area ratio A of the eutectic structure on the plating layer surface" was determined by the method already described and is shown in Table 1. The observation magnification of the SEM was 2000 times, and the acceleration voltage was 15 kV.

[0036] Furthermore, the "average thickness B of the oxide layer on the surface of the plating layer" was determined by the method already described and is shown in Table 1.

[0037] The test materials (Zn-Al-Mg plated steel sheets) obtained as described above were evaluated for corrosion resistance, sliding properties, and spot weldability as follows. Note that, as comparative materials, alloyed hot-dip galvanized steel sheet (No. 41) and hot-dip galvanized steel sheets (Nos. 42 and 43) having the specifications shown in Table 2 were prepared and also subjected to evaluation.

[0038] <Corrosion Resistance> Corrosion tests were conducted to evaluate the perforation corrosion resistance of steel sheet joints, assuming their use as automotive steel sheets. The corrosion test conditions were in accordance with SAE J2334, a corrosion test standard of the Society of Automotive Engineers (SAE). Figure 2 shows a schematic diagram of a corrosion test specimen. In each example, two plated steel sheets having the dimensions shown in Figure 2 were prepared, and spot welding was performed at three locations so that the spot welds 20 were positioned as shown in Figure 2, to produce test specimens simulating the joints of steel sheets in an automobile.

[0039] The obtained test specimens were subjected to a combined cycle test under the cycle conditions shown in Figure 3. After 120 cycles, the welded joints of each sample were disassembled, the corrosion products on the inside of the steel sheet joints were removed, and the depth of corrosion pits was measured with a point micrometer. Evaluation was performed according to the following criteria and is shown in Table 1. Examples with a smaller maximum corrosion depth than that of the comparative material No. 43, which had the highest coating mass, were evaluated as having particularly excellent corrosion resistance and marked with a "◎". Examples with a maximum corrosion depth equal to or greater than that of No. 43 but less than 80% of the average maximum corrosion depth of No. 41 and No. 42 were evaluated as having excellent corrosion resistance and marked with a "○". Examples with a maximum corrosion depth equal to or greater than 80% of the average maximum corrosion depth of No. 41 and No. 42 were evaluated as having substantially the same corrosion resistance as No. 41 and No. 42 and marked with a "-".

[0040] <Sliding Characteristics> To evaluate the sliding characteristics, the friction coefficient of each test material was measured as follows. FIG. 4 shows a schematic front view of the friction coefficient measurement device. As shown in FIG. 4, a friction coefficient measurement sample 40 was fixed to a sample stage 42, which was fixed to the upper surface of a horizontally movable slide table 44. A vertically movable slide table support 48 having a roller 46 in contact with the slide table 44 was provided on the underside of the slide table 44. A first load cell 52 was attached to the slide table support 48 to measure the pressing load N applied by the bead 50 to the friction coefficient measurement sample 40 by pushing up the slide table 44. A second load cell 54 was attached to one end of the slide table 44 to measure the sliding resistance force F caused by moving the slide table 44 horizontally while the pressing force was applied. The test was performed using a press cleaning oil, Pleton® R352L, manufactured by Sugimura Chemical Co., Ltd., applied to the surface of the sample 40 as a lubricant.

[0041] Figure 5 shows a schematic perspective view illustrating the shape and dimensions of the bead used in measuring the friction coefficient. The underside of the bead 50 is pressed against the surface of the sample 40 in Figure 4 and slides. Using this bead, a test was conducted under the following conditions: a pressing load N of 400 kgf, and a sample pull-out speed (horizontal movement speed of the slide table 44) of 20 cm / min. The friction coefficient μ between the test material and the bead was calculated using the formula μ = F / N. Table 1 shows the friction coefficient measurement results for each example. The smaller the friction coefficient, the better the sliding properties can be evaluated.

[0042] <Spot Weldability> To evaluate the spot weldability of each example, spot welding was performed under the welding conditions shown below, and the number of consecutive welds was investigated. The welding current value used in the investigation of consecutive weldability was the average of the current I1 (kA) and the deposition current I2 (kA) at which a nugget diameter expressed as 4√t was obtained, where t (mm) is the plate thickness. The number of consecutive welds was defined as the number of welds until the nugget diameter fell below 4√t. The evaluation was performed according to the following criteria, and the results are shown in Table 1. A sample with a number of consecutive welds equal to or greater than that of No. 41 was evaluated as having particularly excellent spot weldability and indicated by a "◎". A sample with a number of consecutive welds less than that of No. 41 but equal to or greater than the average of the numbers of consecutive welds of No. 41 and No. 42 was evaluated as having excellent spot weldability and indicated by a "○". A sample with a number of consecutive welds less than that of No. 41 but equal to or greater than the average of the numbers of consecutive welds of No. 41 and No. 42 was evaluated as having excellent spot weldability and indicated by a "○". When the number of consecutive welds was 42 or more, the sample was evaluated as having relatively good spot weldability and indicated by a "△"; when the number of consecutive welds was less than No. 42, the sample was evaluated as having poor spot weldability and indicated by an "X".

[0043] (Welding conditions) Electrode: CF type Tip diameter: 4.5 mmφ Tip angle: 120° Outer diameter: 13 mmφ Material: Cu-Cr Welding time: 10 cycles Pressure: 170 kgf Pressure conditions: Before weld: 30 cycles After weld: 7 cycles No up-down slope

[0044] The results in Table 1 show that the examples of the present invention have superior corrosion resistance, sliding properties, and spot weldability compared to the general galvannealed steel sheet or hot-dip galvanized steel sheet used as comparative examples.

[0045]

[0046]

[0047] The Zn—Al—Mg-plated steel sheet of the present invention has excellent corrosion resistance while also achieving excellent sliding properties and spot weldability, and is therefore applicable to a variety of uses, such as automotive steel sheets.

[0048] 10 Zn phase single phase structure 12 Eutectic structure 20 Spot welded portion 40 Friction coefficient measurement sample 42 Sample stage 44 Slide table 46 Roller 48 Slide table support 50 Bead 52 First load cell 54 Second load cell 56 Rail N Pressing load F Sliding resistance force

Claims

DEPCT681. A Zn-Al-Mg coated steel plate consisting of: a steel plate; a coating layer formed on at least one side of the steel plate; and an oxide layer formed on the surface of the coating layer, where the coating layer has a chemical composition, in % by mass, Al: 0.5% or more and 10.0% or less, and Mg: 0.5% or more and 10.0% or less, with a balance of Zn and avoidable impurities; the surface of the coating layer has a single Zn phase microstructure and a eutectic microstructure; the eutectic microstructure contains a Zn-Mg metallic compound phase and, as well as one or more selective phases from the Zn phase, the Al phase, and the Al-Mg metallic compound phase; the area proportion A of the eutectic microstructure on the surface of the coating layer is less than 60%; and the average thickness B of the oxide layer on the surface of the coating layer is 10 nm or more and 50 nm or less.A steel plate coated with Zn-Al-Mg according to claim 1, where the area ratio A, in %, of the eutectic microstructure and the average thickness B, in nm, of the oxide layer correspond to a true B / A ratio greater than or equal to 0.403; A steel plate coated with Zn-Al-Mg according to claim 1 or 2, where the chemical composition of the coating layer shall contain one or more elements selected from the group comprising Si, Fe, Pb, Ti, Ni, Cu, Co, Mn, Cr, Mo, V, Sr, B, Bi, Cd, Sn, and REM, in a total amount of 1% by mass or less;

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