Zn-al-mg-si-based plated steel sheet

Abstract

Provided is a Zn-Al-Mg-Si coated steel sheet that has excellent chemical convertibility, suppression of under-coat blistering, and dissimilar joined portion corrosion resistance. The Zn-Al-Mg-Si coated steel sheet includes a base steel sheet and a coated layer formed on at least one surface of the base steel sheet. The coated layer has a chemical composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity, and a Zn-Al-MgZn2 ternary eutectic alloy structure. In a reflected electron image at a magnification of 5000 times obtained with a scanning electron microscope at an accelerating voltage of 7.0 kV, a needle-like Mg2Si alloy phase is present in the ternary eutectic alloy structure at a number density of less than 100,000 / mm2, and at an accelerating voltage of 15.0 kV, the needle-like Mg2Si alloy phase is present at a number density of 500 / mm2 or more and a needle-like SiO2 oxide phase is present at a number density of 100,000 / mm2 or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to Zn-Al-Mg-Si coated steel sheets.BACKGROUND

[0002] Zinc coating that inhibits corrosion of iron through sacrificial protection has excellent rust resistance performance and is highly economical, and is widely used to produce steel material having high corrosion resistance. In particular, hot-dip galvanized steel material, in which a coated layer is formed by hot-dip galvanizing steel material, has a simpler production process and is cheaper than electrogalvanized steel material, and therefore demand is increasing across industries such as the automobile industry, the home appliance industry, and the building material industry. However, due to the worsening of air pollution and corrosion environments accompanying industrial sophistication, and the depletion of zinc resources due to a significant increase in zinc usage, there has been a demand for coated steel material that is more corrosion resistant than conventional zinc coated steel material and also requires less zinc.

[0003] Patent Literature (PTL) 1 describes a Zn-Al-Mg coated steel material in which elements such as aluminum (Al) and magnesium (Mg) are added to zinc (Zn) to improve the corrosion resistance of steel material. Further, PTL 2 reports that by adding silicon (Si) to a Zn-Al-Mg coated layer, the corrosion resistance of flat surface portions and end surface portions of a coated steel material is improved compared to that of a conventional Zn-Al-Mg coated steel sheet. Further, PTL 3 reports that by adding Si to a coated layer of a Zn-Al-Mg coated steel sheet and concentrating Si at the interface between the coated layer and the base steel sheet, fracture toughness is imparted to the coated layer and post-working corrosion resistance is improved. Further, PTL 4 reports that forming a resin coating containing fine silica (SiO 2 ) on a surface of a coated layer of a Zn-Al-Mg coated steel sheet strengthens the coating and improves the workability and corrosion resistance of worked portions.CITATION LISTPatent Literature

[0004] PTL 1: JP H04-147955 A PTL 2: JP 2005-082834 A PTL 3: JP 2021-508771 A PTL 4: JP 2004-338397 A SUMMARY(Technical Problem)

[0005] However, further improvements are sought in terms of chemical convertibility of coated steel sheets and in terms of suppression of under-coat blistering from sheet edges. Further, as the use of aluminum alloy sheets and aluminum die castings in automotive bodies is increasing in order to drastically reduce the weight of automobiles, corrosion resistance at dissimilar joined portions between coated steel sheet members and aluminum alloy members has emerged as a new required property, and dissimilar joined portion corrosion resistance when joined with aluminum alloy components is desired.

[0006] In view of the above problems, it would be helpful to provide a Zn-Al-Mg-Si coated steel sheet that has excellent chemical convertibility, sufficiently suppresses under-coat blistering from sheet edges, and has excellent dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy.(Solution to Problem)

[0007] In order to solve the aforementioned problems, the inventors conducted intensive studies and made the following discoveries. In a Zn-Al-Mg-Si coated steel sheet, excellent chemical convertibility is obtainable by suppressing formation of a Mg 2 Si alloy phase in a surface layer of the coated layer. On the other hand, formation of an SiO 2 oxide phase in the surface layer of the coated layer can improve the dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy. Further, by forming the Mg 2 Si alloy phase inside the coated layer, it is possible to suppress under-coat blistering from sheared ends after further coating. In a method of producing such a coated steel sheet, a salt water application process is carried out after a coating treatment is applied to a base steel sheet, thereby decreasing the Mg 2 Si alloy phase in the surface layer of the coated layer and forming a SiO 2 oxide phase instead.

[0008] Primary features of the present disclosure are as follows. [1] A Zn-Al-Mg-Si coated steel sheet comprising a base steel sheet and a coated layer formed on at least one surface of the base steel sheet, wherein the coated layer has a chemical composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity, and a Zn-Al-MgZn 2 ternary eutectic alloy structure, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of less than 100,000 / mm 2< (including 0 / mm 2< ), in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of 500 / mm 2< or more, and in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped SiO 2 oxide phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of 100,000 / mm 2< or more. (Advantageous Effect)

[0009] According to the present disclosure, it is possible to provide a Zn-Al-Mg-Si coated steel sheet which has excellent chemical convertibility, sufficiently suppresses under-coat blistering from sheet edges, and has excellent dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the accompanying drawings: FIG. 1A is a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of a coated layer with an electron beam at an accelerating voltage of 7.0 kV according to an Example of the present disclosure, and FIG. 1B to FIG. 1E illustrate Mg distribution, Si distribution, Al distribution, and Zn distribution, respectively, obtained by composition analysis using EDS; and FIG. 2A is a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of a coated layer with an electron beam at an accelerating voltage of 15.0 kV according to an Example of the present disclosure, and FIG. 2B to FIG. 2F illustrate Mg distribution, Si distribution, Al distribution, Zn distribution, and O distribution, respectively, obtained by composition analysis using EDS. DETAILED DESCRIPTION

[0011] The following is a description of an embodiment of the Zn-Al-Mg-Si coated steel sheet according to the present disclosure. The embodiment described below is an example embodiment of the present disclosure, and does not limit configuration to the specific example described.

[0012] A Zn-Al-Mg-Si coated steel sheet according to an embodiment of the present disclosure includes a base steel sheet and a coated layer formed on at least one surface of the base steel sheet. The coated layer has a chemical composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity, and a Zn-Al-MgZn 2 ternary eutectic alloy structure. Further, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of less than 100,000 / mm 2< (including 0 / mm 2< ). In a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of 500 / mm 2< or more. In a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped SiO 2 oxide phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of 100,000 / mm 2< or more.

[0013] There is no particular limitation on the type of the base steel sheet used according to the present disclosure, and for example, a hot-rolled steel sheet or hot-rolled steel strip that has been pickled and descaled, or a cold-rolled steel sheet or cold-rolled steel strip obtained by cold rolling a hot-rolled steel sheet or hot-rolled steel strip that has been pickled and descaled may be used. There is no particular limitation on the thickness of the base steel sheet. The thickness of the base steel sheet is preferably 0.7 mm or more. The thickness of the base steel sheet is preferably 2.0 mm or less.

[0014] Next, the chemical composition of the Zn-Al-Mg-Si coated layer according to the present disclosure is described.

[0015] When the Mg content in the coated layer is less than 0.50 mass%, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, the number density of the Mg 2 Si alloy phase and the SiO 2 oxide phase is insufficient, and the effects of suppressing under-coat blistering from edges and improving the dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy are insufficient. The Mg content in the coated layer is therefore 0.50 mass% or more. The Mg content in the coated layer is preferably 1.00 mass% or more. On the other hand, when the Mg content in the coated layer exceeds 3.00 mass%, brittle, blocky MgZn 2 phase is formed, which makes the coated layer brittle and decreases adhesion, and workability and corrosion resistance of a worked portion may be degraded. The Mg content in the coated layer is therefore 3.00 mass% or less. The Mg content in the coated layer is preferably 2.50 mass% or less.

[0016] When the content of Al in the coating layer is less than 0.10 mass%, a Zn-Al-MgZn 2 ternary eutectic alloy structure is not formed in the coating layer, and the Mg 2 Si alloy phase is not sufficiently formed, and therefore the effect of suppressing under-coat blistering from edges is decreased. The Al content in the coated layer is therefore 0.10 mass% or more. The Al content in the coated layer is preferably 0.50 mass% or more. On the other hand, when the Al content in the coated layer exceeds 3.00 mass%, peeling of further coating occurs from sheared ends of the coated steel sheet, and suppression of under-coat blistering is insufficient. The Al content in the coated layer is therefore 3.00 mass% or less. The Al content in the coated layer is preferably 2.00 mass% or less.

[0017] When the content of Si in the coated layer is 0.010 mass% or more, Si together with Mg forms an Mg 2 Si alloy phase in the coated layer, which contributes to improving the corrosion resistance of the coated layer. This is believed to be due to the following action. That is, the Mg 2 Si alloy phase is active, and therefore reacts with water in a corrosion environment and decomposes, and sacrificially protects a metallic microstructure containing one or more of a Zn phase, an Al phase, and a MgZn 2 alloy phase in a Zn-Al-MgZn 2 ternary eutectic alloy coating structure described later. At the same time, the resulting Mg corrosion products form a protective film that inhibits further progress of corrosion. Under further coating, progression of galvanic corrosion with Fe occurring at sheared edges of the coated steel sheet is inhibited, and therefore blisters occurring at the sheared edges and becoming evident on the external coating surface are inhibited. In order to obtain the effects described above, the Si content in the coated layer is 0.010 mass% or more. The Si content in the coated layer is preferably 0.050 mass% or more. When the Si content is less than 0.010 mass%, the Mg 2 Si alloy phase is not sufficiently formed in the coated layer, making it difficult to suppress under-coat blistering that occurs from sheared ends. On the other hand, when the Si content in the coated layer exceeds 0.20 mass%, a large amount of Mg 2 Si alloy phase is formed even in the surface layer of the coated layer. The Mg 2 Si alloy phase is easily oxidized to form an oxide layer, which tends to block and inhibit adhesion of chemical conversion treatment agent, and therefore there is concern that the chemical convertibility may decrease. The Si content in the coated layer is therefore 0.20 mass% or less. The Si content in the coated layer is preferably 0.15 mass% or less.

[0018] When the Fe content in the coated layer exceeds 0.30 mass%, the growth of an alloy layer formed at the interface between the base steel sheet and the coated layer increases, resulting in decreased workability. The Fe content in the coated layer is therefore 0.30 mass% or less. The Fe content in the coated layer is preferably 0.20 mass% or less. On the other hand, a lower limit of the Fe content in the coated layer is not particularly limited, and may be 0.00 mass%.

[0019] The balance of the chemical composition of the coated layer other than the above consists of Zn and inevitable impurity.

[0020] The following describes the microstructure of the Zn-Al-Mg-Si coated layer. The coated layer according to the present disclosure has a Zn-Al-MgZn 2 ternary eutectic alloy coating structure, and contains Mg 2 Si alloy phase in the coating structure. The coating structure may further contain Zn phase, Al phase, or MgZn 2 alloy phase.

[0021] When the Mg 2 Si alloy phase of the Zn-Al-Mg-Si coated layer is present in a large amount in the surface layer of the coated layer, there is a concern that the chemical convertibility may be decreased. Therefore, the Mg 2 Si alloy phase is preferably not formed in the surface layer of the coated layer. On the other hand, when formed inside the coated layer, the Mg 2 Si alloy phase suppresses coating corrosion from occurring at sheared ends after further coating, suppresses corrosion progress under the further coating, and suppresses under-coat blistering occurring from the sheared ends. For this reason, the Mg 2 Si alloy phase is preferably not formed in the surface layer of the coated layer, but is preferably formed inside the coated layer.

[0022] According to the present disclosure, the Mg 2 Si alloy phase being "not formed in the surface layer of the coated layer" means that, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is not observed in the Zn-Al-MgZn 2 ternary eutectic alloy structure. When the number density of the Mg 2 Si alloy phase formed in the surface layer of the coated layer is 100,000 / mm 2< or more, there is a concern that the chemical convertibility may decrease. The number density of the Mg 2 Si alloy phase formed in the surface layer of the coated layer is therefore less than 100,000 / mm 2< . The number density of the Mg 2 Si alloy phase formed in the surface layer of the coated layer is more preferably 0 / mm 2< .

[0023] According to the present disclosure, the Mg 2 Si alloy phase being "formed inside the coated layer" means that, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is observed in the Zn-Al-MgZn 2 ternary eutectic alloy structure. When the number density of the Mg 2 Si alloy phase formed inside the coated layer is 500 / mm 2< or more, the effect of suppressing under-coat blistering from sheared ends after further coating is obtainable. The number density of the Mg 2 Si alloy phase formed inside the coated layer is therefore 500 / mm 2< or more. The number density of the Mg 2 Si alloy phase formed inside the coated layer is preferably 1200 / mm 2< or more. The upper limit of the number density of the Mg 2 Si alloy phase formed inside the coated layer is not particularly limited, but the number density is generally 2500 / mm 2< or less.

[0024] Further, when the Mg 2 Si alloy phase inside the coated layer is corroded, the SiO 2 oxide phase is formed inside the coated layer. The SiO 2 oxide phase formed inside the coated layer, when present together with Zn in a corrosion environment, promotes the production of basic zinc chloride, which has a strong protective effect, and contributes to improving corrosion resistance. Further, the SiO 2 oxide phase strengthens the coated layer and inhibits the occurrence of cracks when subjected to external stress and the progression of corrosion after cracks occur, thereby improving the corrosion resistance of worked portions. The SiO 2 oxide phase is therefore preferably formed inside the coated layer.

[0025] According to the present disclosure, the SiO 2 oxide phase being "formed inside the coated layer" means that, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped SiO 2 oxide phase having a major axis length of 0.5 µm or more is observed in the Zn-Al-MgZn 2 ternary eutectic alloy structure. When the number density of the SiO 2 oxide phase formed inside the coated layer is 100,000 / mm 2< or more, corrosion resistance of worked portions is obtainable. The number density of the SiO 2 oxide phase formed inside the coated layer is therefore 100,000 / mm 2< or more. The number density of the SiO 2 oxide phase formed inside the coated layer is preferably 150,000 / mm 2< or more. The upper limit of the number density of the SiO 2 oxide phase formed inside the coated layer is not particularly limited, but the number density is generally 380,000 / mm 2< or less.

[0026] The formation of the SiO 2 oxide phase requires control of the Si content in the coating bath composition, control of reducibility (control of O 2 concentration) when applying a coating treatment to the surface of the base steel sheet, and a post-treatment process after coating treatment. When such controls or post-treatment process are insufficient, the Mg 2 Si alloy phase may be present in a large amount in the surface layer of the coated layer, resulting in decreased chemical convertibility, or the Mg 2 Si alloy phase may be present in a small amount inside the coated layer, resulting in insufficient suppression of blistering from sheared ends after further coating, or furthermore, the SiO 2 oxide phase may not be sufficiently formed inside the coated layer, resulting in decreased corrosion resistance of worked portions.

[0027] FIG. 1A to FIG. 1E illustrate reflected electron images (at 5000 times magnification) and EDS analysis results observed when a surface of the coated layer of the Zn-Al-Mg-Si coated steel sheet produced according to an embodiment of the present disclosure was irradiated with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope. FIG. 1A is a reflected electron image of the Zn-Al-Mg-Si coated layer, and FIG. 1B to FIG. 1E are the Mg distribution, Si distribution, Al distribution, and Zn distribution, respectively, obtained by composition analysis using EDS. As indicated in FIG. 1A to FIG. 1E, the Mg 2 Si alloy phase has a needle-like shape overlapping distributions of Mg and Si, and is found to be in the Zn-Al-MgZn 2 ternary eutectic alloy structure.

[0028] FIG. 2A to FIG. 2F illustrate reflected electron images (at 5000 times magnification) and EDS analysis results observed when a surface of the coated layer of the Zn-Al-Mg-Si coated steel sheet produced according to an embodiment of the present disclosure was irradiated with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope. FIG. 2A is a reflected electron image of the Zn-Al-Mg-Si coated layer, and FIG. 2B to FIG. 2F are the Mg distribution, Si distribution, Al distribution, Zn distribution, and O distribution, respectively, obtained by composition analysis using EDS. As illustrated in FIG. 2A to FIG. 2F, the Si distribution has a needle-like shape, and the O distribution overlaps with the distributions of Mg, Si, and Al. The SiO 2 oxide phase has a needle-like shape with overlapping distributions of Si and O, and is found to be in the Zn-Al-MgZn 2 ternary eutectic alloy structure.[Method of producing Zn-Al-Mg-Si coated steel sheet]

[0029] Next, a method of producing a Zn-Al-Mg-Si coated steel sheet according to an embodiment of the present disclosure is described. The method of producing a Zn-Al-Mg-Si coated steel sheet according to an embodiment of the present disclosure includes: a process of immersing a base steel sheet in a coating bath having a composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity, thereby subjecting the base steel sheet to a coating treatment; after the coating treatment, a process of subjecting the base steel sheet to gas wiping, thereby adjusting the coating weight of a coated layer formed on the base steel sheet; and after the gas wiping, a process of salt water application to a surface of the coated steel sheet. A method of cooling the coated steel sheet after the gas wiping and prior to the salt water application is not particularly limited, and various methods such as air cooling, air-water cooling, and water cooling may be used.

[0030] By carrying out the process of salt water application to the surface of the coated steel sheet, the coated layer is etched and the Mg 2 Si alloy phase present inside the coated layer is oxidized. When the Mg 2 Si alloy phase is oxidized, Mg, which has a lower natural potential in a saltwater environment, is selectively dissolved and eluted from the coated layer. As a result, the remaining Si is oxidized to become an oxide, forming the SiO 2 oxide phase. The SiO 2 oxide phase formed inside the coated layer, when present together with Zn in a corrosion environment, promotes the production of basic zinc chloride, which has a strong protective effect, and contributes to improving corrosion resistance.

[0031] In the method of producing a Zn-Al-Mg-Si coated steel sheet according to the present disclosure, the base steel sheet to be coated is the same base steel sheet as described above.

[0032] When an annealing heat treatment is carried out on a base steel sheet before coating treatment and the H 2 concentration in the furnace atmosphere during annealing is 1 vol% or more, oxides at the base steel sheet surface can be suitably reduced. The H 2 concentration in the furnace atmosphere during annealing is therefore preferably 1 vol% or more. On the other hand, when the H 2 concentration in the furnace atmosphere during annealing is 10 vol% or less, production costs can be suitably suppressed. The H 2 concentration in the furnace atmosphere during annealing is therefore preferably 10 vol% or less. Further, the furnace atmosphere during annealing is preferably a mixed gas containing H 2 with the balance being N 2 . Temperature of the annealing heat treatment is not particularly limited. The temperature of the annealing heat treatment is preferably 600 °C or more. The temperature of the annealing heat treatment is preferably 850 °C or less. Further, duration of the annealing heat treatment is not particularly limited. The duration of the annealing heat treatment is preferably 10 s or longer. The duration of the annealing heat treatment is preferably 60 s or less.

[0033] The coating bath used in the production of a Zn-Al-Mg-Si coated steel sheet according to the present disclosure has a composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity. The explanation for each component in the coating bath is the same as the explanation of each component of the coated layer described above. The chemical composition of the coating bath is equivalent to the chemical composition of the applied coated layer.

[0034] From the viewpoint of corrosion resistance, the coating weight of the coated layer in the coating treatment is preferably 15 g / m 2< or more. The coating weight of the coated layer in the coating treatment is more preferably 30 g / m 2< or more. On the other hand, when the coating weight of the coated layer is 100 g / m 2< or less, production cost can be suitably suppressed, and therefore the coating weight of the coated layer is preferably 100 g / m 2< or less. Further, temperature of the coating bath during the coating treatment is preferably 430 °C or more. The temperature of the coating bath during the coating treatment is preferably 540 °C or less. When the temperature of the coating bath is less than 430 °C, fluidity of the coating bath decreases, which may hinder uniform coating coverage. For this reason, the temperature of the coating bath is preferably 430 °C or more. On the other hand, when the temperature of the coating bath exceeds 540 °C, there is a risk of an increase in oxides at the surface of the coating bath due to oxidation of Mg in the coating bath, and erosion of refractory material in the coating bath by Al and Mg. For these reasons, the temperature of the coating bath is preferably 540 °C or less. The temperature of the coating bath is more preferably 530 °C or less.

[0035] In order to not form the Mg 2 Si alloy phase in the surface layer of the coated layer and to form a large amount of the Mg 2 Si alloy phase inside the coated layer, decreasing the reducing property of the gas used in gas wiping is effective. When H 2 is used in a mixed gas used in gas wiping, the reducing property becomes very high. When the H 2 concentration is 1 vol% or more and the amount of Si in the coating bath is low, then even when the salt water application is carried out, the Mg 2 Si in the coated layer is difficult to oxidize, and a sufficient amount of SiO 2 oxide phase is not formed, making it difficult to obtain the desired corrosion resistance of worked portions. As a result of intensive studies regarding this point, the inventors have found that the higher the O 2 concentration during gas wiping and the lower the reducing property of the gas, the less likely it is that the Mg 2 Si alloy phase will form in the surface layer of the coated layer, and the more likely it is that the Mg 2 Si alloy phase will form inside the coated layer. As a result, it becomes possible to decrease the SiO 2 oxide phase in the surface layer of the coated layer and increase the SiO 2 oxide phase inside the coated layer during corrosion caused by the salt water application. Therefore, it is preferable to use a mixed gas containing O 2 and N 2 , or air, as the gas used in gas wiping. The gas wiping is preferably carried out in an air environment.

[0036] When the gas used in the gas wiping is a mixed gas containing O 2 and N 2 , and the O 2 concentration is 3 vol% or more, then even when the Si content in the coating bath is low, the Mg 2 Si alloy phase is suitably formed inside the coated layer, and the effect of contributing to improvement of corrosion resistance is suitably obtained. The O 2 concentration of the mixed gas used in the gas wiping is therefore preferably 3 vol% or more. On the other hand, when the O 2 concentration is 15 vol% or less, production cost can be suitably suppressed. The O 2 concentration of the mixed gas used in the gas wiping is therefore preferably 15 vol% or less. Further, the mixed gas may contain H 2 . When the H 2 concentration of the mixed gas used in the gas wiping is 7 vol% or more, the formation of the Mg 2 Si alloy phase in the surface layer of the coated layer is suitably suppressed, the Mg 2 Si alloy phase is formed inside the coated layer, and the formation of the SiO 2 oxide phase inside the coated layer is suitably promoted during the salt water application.

[0037] Air may be used for the gas wiping. In this case, too, the Mg 2 Si alloy phase is preferably formed inside the coated layer even when the Si content in the coating bath is low. During the salt water application after the gas wiping with air, the formation of the SiO 2 oxide phase is favorably promoted inside the coated layer, and the effect of contributing to improved corrosion resistance is favorably obtained.

[0038] The method of salt water application to the surface of the coated steel sheet after coating weight adjustment is not particularly limited. A method may be used in which salt water is applied to the surface of the coated steel sheet, left to stand for a defined period of time, and then washed and dried, thereby corroding and oxidizing the coated layer of the coated steel sheet. The salt water used in the salt water application is not particularly limited. From the viewpoint of production cost, an aqueous solution of sodium chloride (NaCl) is preferred. The salt water concentration is not particularly limited. From the viewpoint of production costs, the salt water concentration is preferably 0.5 mass% or more. The salt water concentration is preferably 5.0 mass% or less. Further, the standing time for the salt water application is preferably 0.5 h or more from the viewpoint of sufficiently oxidizing the coated layer. However, when the standing time exceeds 2 h, corrosion of the coated layer progresses greatly, resulting in a decrease in coating weight and deterioration of corrosion resistance. The standing time for the salt water application is preferably 2 h or less. The standing time for the salt water application is more preferably 1 h or less.

[0039] For processes and conditions not described herein, conventional methods may be used.EXAMPLES

[0040] Using an extremely low carbon cold-rolled steel sheet having a thickness of 0.8 mm as a base steel sheet, all of the Zn-Al-Mg-Si coated steel sheets serving as samples were produced using a hot-dip coating apparatus. Each sample was coated using variously changed coating bath compositions, and the coating bath temperature was set to 450 °C, which is 30 °C higher than a solidification start temperature estimated from a Zn-Al-Mg-Si quaternary phase equilibrium diagram. The base steel sheet was heated in an H 2 (5 vol%) + N 2 atmosphere, the temperature of the base steel sheet when immersed in the coating bath was set to the same temperature as the coating bath temperature, and hot-dip coating was carried out in the coating bath for 1 s. Thereafter, the thickness of the coated layer was controlled to 7 µm (equivalent to 50 g / m 2< ) by adjusting the flow rate of the wiping gas. Some of the samples were further subjected to salt water application in which the sample was immersed in an aqueous solution of 0.5 wt% NaCl for 1 h, and then washed and dried. Table 1 lists the coating bath composition, the wiping gas composition, and the coated layer composition. Further, in the entry for salt water application in Table 1, the samples that underwent salt water application are indicated as "Yes", and the samples that did not undergo salt water application are indicated as "No". The wiping was carried out in an air environment.[Table 1]

[0041] Table 1No.Coating bath composition (mass%)Wiping gas (vol%)Salt water applicationCoated layer composition (mass%)Ternary eutectic alloyMg 2 SiSiO 2 EvaluationRemarksZnAlMgSiFeZnAlMgSiFeNumber density observation at 7.0 kV (per mm 2< )Number density observation at 15.0 kV (per mm 2< )Number density observation at 15.0 kV (per mm 2< )Chemical convertibilityBlister width of under-coat blistering from sheared edgeWorked portion dissimilar joined portion corrosion resistance197.251.501.000.050.20N 2 +10%O 2 Yes97.251.501.000.050.20Yes01200200000⊚⊚⊚Example297.951.500.300.050.20N 2 +10%O 2 Yes97.951.500.300.050.20Yes025056000⊚ΔΔComparative Example 398.151.500.100.050.20N 2 +10%O 2 Yes98.151.500.100.050.20Yes050 16000⊚△×Comparative Example 497.651.500.600.050.20N 2 +10%O 2 Yes97.651.500.600.050.20Yes0850120000⊚○○Example597.501.500.750.050.20N 2 +10%O 2 Yes97.501.500.750.050.20Yes01000160000⊚○⊚Example697.401.500.850.050.20N 2 +10%O 2 Yes97.401.500.850.050.20Yes01150176000⊚○⊚Example796.751.501.500.050.20N 2 +10%O 2 Yes96.751.501.500.050.20Yes01350216000⊚⊚⊚Example896.251.502.000.050.20N 2 +10%O 2 Yes96.251.502.000.050.20Yes01500224000⊚⊚⊚Example995.451.502.800.050.20N 2 +10%O 2 Yes95.451.502.800.050.20Yes01550224000⊚⊚⊚Example1095.051.503.200.050.20N 2 +10%O 2 Yes95.051.503.200.050.20Yes01650216000⊚⊚×Comparative Example 1193.251.505.000.050.20N 2 +10%O 2 Yes93.251.505.000.050.20Yes01800232000⊚⊚×Comparative Example 1298.251.500.000.050.20N 2 +10%O 2 Yes98.251.500.000.050.20No0044000⊚×xComparative Example 1397.291.501.000.010.20N 2 +10%O 2 Yes97.291.501.000.010.20Yes045080000⊚ΔΔComparative Example 1497.291.501.000.010.20N 2 +10%O 2 Yes97.291.501.000.010.20Yes0600104000⊚○○Example1597.271.501.000.040.20N 2 +10%O 2 Yes97.271.501.000.040.20Yes0650112000⊚○○Example1697.231.501.000.070.20N 2 +10%O 2 Yes97.231.501.000.070.20Yes120001250248000○⊚⊚Example1797.201.501.000.100.20N 2 +10%O 2 Yes97.201.501.000.100.20Yes400001500264000○⊚⊚Example1897.151.501.000.150.20N 2 +10%O 2 Yes97.151.501.000.150.20Yes640001700296000○⊚⊚Example1997.111.501.000.190.20N 2 +10%O 2 Yes97.111.501.000.190.20Yes960002050344000○⊚⊚Example2097.071.501.000.230.20N 2 +10%O 2 Yes97.071.501.000.230.20Yes1120002400368000Δ⊚⊚Comparative Example 2197.301.501.000.000.20N 2 +10%O 2 Yes97.301.501.000.000.20Yes000⊚×xComparative Example 2297.251.501.000.050.20N 2 +15%O 2 Yes97.251.501.000.050.20Yes01250208000⊚⊚⊚Example2397.251.501.000.050.20N 2 +10%O 2 No97.251.501.000.050.20Yes11200012000 △⊚×Comparative Example 2497.251.501.000.050.20N 2 +15%O 2 No97.251.501.000.050.20Yes10400012500 Δ⊚×Comparative Example 2597.251.501.000.050.20N 2 +7%O 2 Yes97.251.501.000.050.20Yes40001000192000○○⊚Example2697.251.501.000.050.20N 2 +5%O 2 Yes97.251.501.000.050.20Yes8000750164000○○⊚Example2797.251.501.000.050.20N 2 +3%O 2 Yes97.251.501.000.050.20Yes12000550136000○○○Example2897.251.501.000.050.20N 2 +1%O 2 Yes97.251.501.000.050.20Yes1600015088000○ΔΔComparative Example 2997.251.501.000.120.20N 2 +5%O 2 Yes97.251.501.000.120.20Yes16000900216000○○⊚Example3097.281.501.000.020.20N 2 +5%O 2 Yes97.291.501.000.020.20Yes4000500104000○○○Example3197.251.501.000.050.20N 2 No97.251.501.000.050.20Yes24000500 ○××Comparative Example 3297.251.501.000.050.20airNo97.251.501.000.050.20Yes024000 ⊚⊚×Comparative Example 3397.251.501.000.050.20airYes97.251.501.000.050.20Yes02400368000⊚⊚⊚Example3497.251.501.000.050.20N 2 +1%H 2 No97.251.501.000.050.20Yes112000 8 0 △××Comparative Example 3597.251.501.000.050.20N 2 +3%H 2 No97.251.501.000.050.20Yes120000 50 △××Comparative Example 3697.251.501.000.050.20N 2 +5%H 2 No97.251.501.000.050.20Yes208000 0 0 ×××Comparative Example 3797.251.501.000.050.20N 2 +7%H 2 No97.251.501.000.050.20Yes216000 0 0 ×××Comparative Example 3897.111.501.000.190.20N 2 +1%H 2 No97.111.501.000.190.20Yes136000 5600 △○×Comparative Example 3997.111.501.000.190.20N 2 +0.5%H 2 No97.111.501.000.190.20Yes144000 6500 △○×Comparative Example 4097.251.501.000.050.20N 2 +1%H 2 Yes97.251.501.000.050.20Yes56000 10 120000○×○Comparative Example 4197.251.501.000.050.20N 2 +3%H 2 Yes97.251.501.000.050.20Yes640005136000○×○Comparative Example 4297.151.501.000.150.20N 2 +7%H 2 Yes97.151.501.000.150.20Yes960001150208000○○⊚Example4397.151.501.000.150.20N 2 +5%H 2 Yes97.151.501.000.150.20Yes800001000192000○○⊚Example4497.111.501.000.190.20N 2 +3%H 2 Yes97.111.501.000.190.20Yes88000850176000○○⊚Example4597.111.501.000.190.20N 2 +1%H 2 Yes97.111.501.000.190.20Yes72000600120000○○○Example4697.111.501.000.190.20N 2 +0.5%H 2 Yes97.111.501.000.190.20Yes80000700104000○○⊚Example4798.740.011.000.050.20N 2 +10%O 2 Yes98.740.01 1.000.050.20No 0200 72000 ⊚△△Comparative Example 4898.630.121.000.050.20N 2 +10%O 2 Yes98.630.121.000.050.20Yes0550104000⊚○○Example4997.751.001.000.050.20N 2 +10%O 2 Yes97.751.001.000.050.20Yes0900104000⊚○⊚Example5096.752.001.000.050.20N 2 +10%O 2 Yes96.752.001.000.050.20Yes01300208000⊚⊚⊚Example5195.952.801.000.050.20N 2 +10%O 2 Yes95.952.801.000.050.20Yes01650224000⊚⊚⊚Example5295.553.201.000.050.20N 2 +10%O 2 Yes95.553.20 1.000.050.20Yes02350224000⊚△⊚Comparative Example 5393.755.001.000.050.20N 2 +10%O 2 Yes93.755.00 1.000.050.20Yes02400220000⊚×⊚Comparative Example 5496.252.501.000.050.20N 2 +10%O 2 No96.252.50 1.000.050.20Yes112000 1550Q△⊚×Comparative Example 5598.630.121.000.050.20N 2 +5%H 2 Yes98.630.121.000.050.20Yes64000550120000○○○Example5695.952.801.000.050.20N 2 +5%H 2 Yes95.952.801.000.050.20Yes960001650232000○⊚⊚Example

[0042] Each sample of the obtained Zn-Al-Mg-Si coated steel sheet was sheared to a size of 10 mm × 20 mm. The coated layer of the sample after shearing was observed from a surface direction of the coated steel sheet using a scanning electron microscope to confirm the presence or absence of a Zn-Al-MgZn 2 ternary eutectic alloy structure. When the composition analysis by EDS during the observation indicated that there was a structure composed of three phases, Zn phase, Al phase, and MgZn 2 alloy phase, and when the same surface was analyzed by XRD and the Zn-Al-MgZn 2 ternary eutectic alloy structure was present, then "Yes" was recorded. When the composition analysis by EDS during the observation indicated that there was no structure composed of three phases, Zn phase, Al phase, and MgZn 2 alloy phase, and when the same surface was analyzed by XRD and any of the Zn phase, Al phase, and MgZn 2 alloy phase was not confirmed, the Zn-Al-MgZn 2 crystal structure was absent, and "No" was recorded in Table 1.

[0043] The coated layer of the sample after shearing was observed from a surface direction of the coated steel sheet using a scanning electron microscope to observe the state of the Mg 2 Si alloy phase present in the surface layer and inside the coated layer, and number density was evaluated. The number density of the Mg 2 Si alloy phase present in the surface layer and inside the coated layer was evaluated from a reflected electron image (at 5000 times magnification) observed when irradiating the coated layer with an electron beam at an accelerating voltage of 7.0 kV. The number density of the Mg 2 Si alloy phase present inside the coated layer was evaluated from a reflected electron image (at 5000 times magnification) observed when irradiating the coated layer with an electron beam at an accelerating voltage of 15.0 kV.

[0044] The coated layer of the sample after shearing was observed from a surface direction of the coated steel sheet using a scanning electron microscope to observe the state of the SiO 2 oxide phase present inside the coated layer, and number density was evaluated. The SiO 2 oxide phase present inside the coated layer was evaluated from a reflected electron image (at 5000 times magnification) observed when irradiating the coated layer with an electron beam at an accelerating voltage of 15.0 kV.[Chemical convertibility]

[0045] Next, the chemical convertibility of the obtained Zn-Al-Mg-Si coated steel sheets was evaluated. Each sample was sheared to a size of 30 mm × 30 mm, and then phosphating treatment was carried out for 90 s under the same conditions. The surface of the coated steel sheet was observed using a scanning electron microscope, and the state of the phosphate crystals (the degree of exposure of the coated surface under chemical conversion) was observed from the obtained secondary electron image (at 800 times magnification), and evaluated as follows. In Table 1, a case where there was no phosphate crystal scale and no exposed surface of the coated steel sheet is indicated by "⊚", a case where there was very little phosphate crystal scale and very little exposed surface of the coated steel sheet is indicated by "O", a case where there was very little phosphate crystal scale and very little exposed surface of the coated steel sheet is indicated by "△", and a case where there was phosphate crystal scale and exposed plated surface is indicated by "×". An evaluation of ⊚ or O was considered a pass, and an evaluation of △ or × was considered a fail.[Under-coat blistering from sheared edge after further coating]

[0046] Next, suppression of under-coat blistering from sheared edges of the Zn-Al-Mg-Si coated steel sheet after further coating was evaluated. Each sample was sheared to a size of 80 mm × 80 mm, phosphate treated under the same conditions, and then subjected to electrodeposition coating (the coating thickness was constant at 15 µm), and then sheared to a size of 50 mm × 50 mm to prepare sheared end surfaces. Then, the samples were subjected to 45 cycles of corrosion testing according to JASO M609-91, and the maximum blister width of under-coat blistering from a sheared edge was then evaluated. In Table 1, blister width of less than 1.0 mm is indicated as "⊚", blister width of 1.0 mm or more and less than 1.5 mm is indicated as "O", blister width of 1.5 mm or more and less than 2.0 mm is indicated as "△", and blister width of 2.0 mm or more is indicated as "×". An evaluation of ⊚ or O was considered a pass, and an evaluation of △ or × was considered a fail.[Worked portion dissimilar joined portion corrosion resistance]

[0047] Next, the worked portion dissimilar joined portion corrosion resistance of the Zn-Al-Mg-Si coated steel sheets was evaluated. Each sample was sheared to a size of 70 mm × 120 mm, and mechanically joined at two points by mechanical clinching (5 mm diameter) to an Al alloy that had been sheared to a size of 40 mm × 90 mm to provide a worked portion, thereby producing a worked portion dissimilar joined portion sample of a Zn-Al-Mg-Si coated steel sheet and an Al alloy. In this case, the surfaces other than the mating surfaces were phosphate-treated under the same conditions, and then subjected to electrodeposition coating (with a constant coating thickness of 15 µm) to cover them. The worked portion dissimilar joined portion samples were then subjected to a corrosion test according to N-VDA testing. After 12 weeks of the corrosion test, the corrosion loss of the Al alloy was evaluated. The Al corrosion loss was measured in accordance with ISO 8407:2021 by immersing a worked portion dissimilar joined portion sample in a 60 mass% aqueous nitric acid solution at room temperature to remove corrosion products mainly composed of aluminum, and calculating the difference between the weight before the test and the weight after the corrosion products were removed. In Table 1, the corrosion loss was indicated as "⊚" when less than 5 g / m 2< , "O" when 5 g / m 2< or more and less than 10 g / m 2< , "△" when 10 g / m 2< or more and less than 15 g / m 2< , and "×" when 15 g / m 2< or more. An evaluation of ⊚ or O was considered a pass, and an evaluation of △ or × was considered a fail.

[0048] As indicated in Table 1, the Examples having the coating composition specified in the present disclosure and having a number density of Mg 2 Si alloy phase of less than 100,000 / mm 2< when observed at an accelerating voltage of 7.0 kV and 500 / mm 2< or more when observed at an accelerating voltage of 15.0 kV had excellent chemical convertibility, sufficiently suppressed under-coat blistering from a sheared end, and had excellent worked portion dissimilar joined portion corrosion resistance. That is, it was found that the presence of a small amount of Mg 2 Si alloy phase in the surface layer of the coated layer and a large amount inside the coated layer is advantageous for chemical convertibility, suppression of under-coat blistering from sheared ends, and worked portion dissimilar joined portion corrosion resistance. In particular, the Examples for which the number density of SiO 2 was 150,000 / mm 2< or more when observed at an accelerating voltage of 15.0 kV, that is, for which there was a large amount of SiO 2 inside the coated layer, demonstrated more favorable evaluation results. In contrast, the Comparative Examples outside the scope of the present disclosure were inferior to the Examples in at least one of chemical convertibility, suppression under-coat blistering from a sheared end, and worked portion dissimilar joined portion corrosion resistance.INDUSTRIAL APPLICABILITY

[0049] According to the present disclosure, it is possible to provide a Zn-Al-Mg-Si coated steel sheet which has excellent chemical convertibility, sufficiently suppresses under-coat blistering from sheet edges, and has excellent dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy.

Examples

examples

[0040]Using an extremely low carbon cold-rolled steel sheet having a thickness of 0.8 mm as a base steel sheet, all of the Zn-Al-Mg-Si coated steel sheets serving as samples were produced using a hot-dip coating apparatus. Each sample was coated using variously changed coating bath compositions, and the coating bath temperature was set to 450 °C, which is 30 °C higher than a solidification start temperature estimated from a Zn-Al-Mg-Si quaternary phase equilibrium diagram. The base steel sheet was heated in an H 2 (5 vol%) + N 2 atmosphere, the temperature of the base steel sheet when immersed in the coating bath was set to the same temperature as the coating bath temperature, and hot-dip coating was carried out in the coating bath for 1 s. Thereafter, the thickness of the coated layer was controlled to 7 µm (equivalent to 50 g / m 2< ) by adjusting the flow rate of the wiping gas. Some of the samples were further subjected to salt water application in which the sample was immersed ...

TECHNICAL FIELD

[0001] The present disclosure relates to Zn-Al-Mg-Si coated steel sheets.BACKGROUND

[0002] Zinc coating that inhibits corrosion of iron through sacrificial protection has excellent rust resistance performance and is highly economical, and is widely used to produce steel material having high corrosion resistance. In particular, hot-dip galvanized steel material, in which a coated layer is formed by hot-dip galvanizing steel material, has a simpler production process and is cheaper than electrogalvanized steel material, and therefore demand is increasing across industries such as the automobile industry, the home appliance industry, and the building material industry. However, due to the worsening of air pollution and corrosion environments accompanying industrial sophistication, and the depletion of zinc resources due to a significant increase in zinc usage, there has been a demand for coated steel material that is more corrosion resistant than conventional zinc coated steel material and also requires less zinc.

[0003] Patent Literature (PTL) 1 describes a Zn-Al-Mg coated steel material in which elements such as aluminum (Al) and magnesium (Mg) are added to zinc (Zn) to improve the corrosion resistance of steel material. Further, PTL 2 reports that by adding silicon (Si) to a Zn-Al-Mg coated layer, the corrosion resistance of flat surface portions and end surface portions of a coated steel material is improved compared to that of a conventional Zn-Al-Mg coated steel sheet. Further, PTL 3 reports that by adding Si to a coated layer of a Zn-Al-Mg coated steel sheet and concentrating Si at the interface between the coated layer and the base steel sheet, fracture toughness is imparted to the coated layer and post-working corrosion resistance is improved. Further, PTL 4 reports that forming a resin coating containing fine silica (SiO 2 ) on a surface of a coated layer of a Zn-Al-Mg coated steel sheet strengthens the coating and improves the workability and corrosion resistance of worked portions.CITATION LISTPatent Literature

[0004] PTL 1: JP H04-147955 A PTL 2: JP 2005-082834 A PTL 3: JP 2021-508771 A PTL 4: JP 2004-338397 A SUMMARY(Technical Problem)

[0005] However, further improvements are sought in terms of chemical convertibility of coated steel sheets and in terms of suppression of under-coat blistering from sheet edges. Further, as the use of aluminum alloy sheets and aluminum die castings in automotive bodies is increasing in order to drastically reduce the weight of automobiles, corrosion resistance at dissimilar joined portions between coated steel sheet members and aluminum alloy members has emerged as a new required property, and dissimilar joined portion corrosion resistance when joined with aluminum alloy components is desired.

[0006] In view of the above problems, it would be helpful to provide a Zn-Al-Mg-Si coated steel sheet that has excellent chemical convertibility, sufficiently suppresses under-coat blistering from sheet edges, and has excellent dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy.(Solution to Problem)

[0007] In order to solve the aforementioned problems, the inventors conducted intensive studies and made the following discoveries. In a Zn-Al-Mg-Si coated steel sheet, excellent chemical convertibility is obtainable by suppressing formation of a Mg 2 Si alloy phase in a surface layer of the coated layer. On the other hand, formation of an SiO 2 oxide phase in the surface layer of the coated layer can improve the dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy. Further, by forming the Mg 2 Si alloy phase inside the coated layer, it is possible to suppress under-coat blistering from sheared ends after further coating. In a method of producing such a coated steel sheet, a salt water application process is carried out after a coating treatment is applied to a base steel sheet, thereby decreasing the Mg 2 Si alloy phase in the surface layer of the coated layer and forming a SiO 2 oxide phase instead.

[0008] Primary features of the present disclosure are as follows. [1] A Zn-Al-Mg-Si coated steel sheet comprising a base steel sheet and a coated layer formed on at least one surface of the base steel sheet, wherein the coated layer has a chemical composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity, and a Zn-Al-MgZn 2 ternary eutectic alloy structure, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of less than 100,000 / mm 2< (including 0 / mm 2< ), in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of 500 / mm 2< or more, and in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped SiO 2 oxide phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of 100,000 / mm 2< or more. (Advantageous Effect)

[0009] According to the present disclosure, it is possible to provide a Zn-Al-Mg-Si coated steel sheet which has excellent chemical convertibility, sufficiently suppresses under-coat blistering from sheet edges, and has excellent dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the accompanying drawings: FIG. 1A is a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of a coated layer with an electron beam at an accelerating voltage of 7.0 kV according to an Example of the present disclosure, and FIG. 1B to FIG. 1E illustrate Mg distribution, Si distribution, Al distribution, and Zn distribution, respectively, obtained by composition analysis using EDS; and FIG. 2A is a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of a coated layer with an electron beam at an accelerating voltage of 15.0 kV according to an Example of the present disclosure, and FIG. 2B to FIG. 2F illustrate Mg distribution, Si distribution, Al distribution, Zn distribution, and O distribution, respectively, obtained by composition analysis using EDS. DETAILED DESCRIPTION

[0011] The following is a description of an embodiment of the Zn-Al-Mg-Si coated steel sheet according to the present disclosure. The embodiment described below is an example embodiment of the present disclosure, and does not limit configuration to the specific example described.

[0012] A Zn-Al-Mg-Si coated steel sheet according to an embodiment of the present disclosure includes a base steel sheet and a coated layer formed on at least one surface of the base steel sheet. The coated layer has a chemical composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity, and a Zn-Al-MgZn 2 ternary eutectic alloy structure. Further, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of less than 100,000 / mm 2< (including 0 / mm 2< ). In a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of 500 / mm 2< or more. In a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped SiO 2 oxide phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn 2 ternary eutectic alloy structure at a number density of 100,000 / mm 2< or more.

[0013] There is no particular limitation on the type of the base steel sheet used according to the present disclosure, and for example, a hot-rolled steel sheet or hot-rolled steel strip that has been pickled and descaled, or a cold-rolled steel sheet or cold-rolled steel strip obtained by cold rolling a hot-rolled steel sheet or hot-rolled steel strip that has been pickled and descaled may be used. There is no particular limitation on the thickness of the base steel sheet. The thickness of the base steel sheet is preferably 0.7 mm or more. The thickness of the base steel sheet is preferably 2.0 mm or less.

[0014] Next, the chemical composition of the Zn-Al-Mg-Si coated layer according to the present disclosure is described.

[0015] When the Mg content in the coated layer is less than 0.50 mass%, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, the number density of the Mg 2 Si alloy phase and the SiO 2 oxide phase is insufficient, and the effects of suppressing under-coat blistering from edges and improving the dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy are insufficient. The Mg content in the coated layer is therefore 0.50 mass% or more. The Mg content in the coated layer is preferably 1.00 mass% or more. On the other hand, when the Mg content in the coated layer exceeds 3.00 mass%, brittle, blocky MgZn 2 phase is formed, which makes the coated layer brittle and decreases adhesion, and workability and corrosion resistance of a worked portion may be degraded. The Mg content in the coated layer is therefore 3.00 mass% or less. The Mg content in the coated layer is preferably 2.50 mass% or less.

[0016] When the content of Al in the coating layer is less than 0.10 mass%, a Zn-Al-MgZn 2 ternary eutectic alloy structure is not formed in the coating layer, and the Mg 2 Si alloy phase is not sufficiently formed, and therefore the effect of suppressing under-coat blistering from edges is decreased. The Al content in the coated layer is therefore 0.10 mass% or more. The Al content in the coated layer is preferably 0.50 mass% or more. On the other hand, when the Al content in the coated layer exceeds 3.00 mass%, peeling of further coating occurs from sheared ends of the coated steel sheet, and suppression of under-coat blistering is insufficient. The Al content in the coated layer is therefore 3.00 mass% or less. The Al content in the coated layer is preferably 2.00 mass% or less.

[0017] When the content of Si in the coated layer is 0.010 mass% or more, Si together with Mg forms an Mg 2 Si alloy phase in the coated layer, which contributes to improving the corrosion resistance of the coated layer. This is believed to be due to the following action. That is, the Mg 2 Si alloy phase is active, and therefore reacts with water in a corrosion environment and decomposes, and sacrificially protects a metallic microstructure containing one or more of a Zn phase, an Al phase, and a MgZn 2 alloy phase in a Zn-Al-MgZn 2 ternary eutectic alloy coating structure described later. At the same time, the resulting Mg corrosion products form a protective film that inhibits further progress of corrosion. Under further coating, progression of galvanic corrosion with Fe occurring at sheared edges of the coated steel sheet is inhibited, and therefore blisters occurring at the sheared edges and becoming evident on the external coating surface are inhibited. In order to obtain the effects described above, the Si content in the coated layer is 0.010 mass% or more. The Si content in the coated layer is preferably 0.050 mass% or more. When the Si content is less than 0.010 mass%, the Mg 2 Si alloy phase is not sufficiently formed in the coated layer, making it difficult to suppress under-coat blistering that occurs from sheared ends. On the other hand, when the Si content in the coated layer exceeds 0.20 mass%, a large amount of Mg 2 Si alloy phase is formed even in the surface layer of the coated layer. The Mg 2 Si alloy phase is easily oxidized to form an oxide layer, which tends to block and inhibit adhesion of chemical conversion treatment agent, and therefore there is concern that the chemical convertibility may decrease. The Si content in the coated layer is therefore 0.20 mass% or less. The Si content in the coated layer is preferably 0.15 mass% or less.

[0018] When the Fe content in the coated layer exceeds 0.30 mass%, the growth of an alloy layer formed at the interface between the base steel sheet and the coated layer increases, resulting in decreased workability. The Fe content in the coated layer is therefore 0.30 mass% or less. The Fe content in the coated layer is preferably 0.20 mass% or less. On the other hand, a lower limit of the Fe content in the coated layer is not particularly limited, and may be 0.00 mass%.

[0019] The balance of the chemical composition of the coated layer other than the above consists of Zn and inevitable impurity.

[0020] The following describes the microstructure of the Zn-Al-Mg-Si coated layer. The coated layer according to the present disclosure has a Zn-Al-MgZn 2 ternary eutectic alloy coating structure, and contains Mg 2 Si alloy phase in the coating structure. The coating structure may further contain Zn phase, Al phase, or MgZn 2 alloy phase.

[0021] When the Mg 2 Si alloy phase of the Zn-Al-Mg-Si coated layer is present in a large amount in the surface layer of the coated layer, there is a concern that the chemical convertibility may be decreased. Therefore, the Mg 2 Si alloy phase is preferably not formed in the surface layer of the coated layer. On the other hand, when formed inside the coated layer, the Mg 2 Si alloy phase suppresses coating corrosion from occurring at sheared ends after further coating, suppresses corrosion progress under the further coating, and suppresses under-coat blistering occurring from the sheared ends. For this reason, the Mg 2 Si alloy phase is preferably not formed in the surface layer of the coated layer, but is preferably formed inside the coated layer.

[0022] According to the present disclosure, the Mg 2 Si alloy phase being "not formed in the surface layer of the coated layer" means that, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is not observed in the Zn-Al-MgZn 2 ternary eutectic alloy structure. When the number density of the Mg 2 Si alloy phase formed in the surface layer of the coated layer is 100,000 / mm 2< or more, there is a concern that the chemical convertibility may decrease. The number density of the Mg 2 Si alloy phase formed in the surface layer of the coated layer is therefore less than 100,000 / mm 2< . The number density of the Mg 2 Si alloy phase formed in the surface layer of the coated layer is more preferably 0 / mm 2< .

[0023] According to the present disclosure, the Mg 2 Si alloy phase being "formed inside the coated layer" means that, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped Mg 2 Si alloy phase having a major axis length of 0.5 µm or more is observed in the Zn-Al-MgZn 2 ternary eutectic alloy structure. When the number density of the Mg 2 Si alloy phase formed inside the coated layer is 500 / mm 2< or more, the effect of suppressing under-coat blistering from sheared ends after further coating is obtainable. The number density of the Mg 2 Si alloy phase formed inside the coated layer is therefore 500 / mm 2< or more. The number density of the Mg 2 Si alloy phase formed inside the coated layer is preferably 1200 / mm 2< or more. The upper limit of the number density of the Mg 2 Si alloy phase formed inside the coated layer is not particularly limited, but the number density is generally 2500 / mm 2< or less.

[0024] Further, when the Mg 2 Si alloy phase inside the coated layer is corroded, the SiO 2 oxide phase is formed inside the coated layer. The SiO 2 oxide phase formed inside the coated layer, when present together with Zn in a corrosion environment, promotes the production of basic zinc chloride, which has a strong protective effect, and contributes to improving corrosion resistance. Further, the SiO 2 oxide phase strengthens the coated layer and inhibits the occurrence of cracks when subjected to external stress and the progression of corrosion after cracks occur, thereby improving the corrosion resistance of worked portions. The SiO 2 oxide phase is therefore preferably formed inside the coated layer.

[0025] According to the present disclosure, the SiO 2 oxide phase being "formed inside the coated layer" means that, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped SiO 2 oxide phase having a major axis length of 0.5 µm or more is observed in the Zn-Al-MgZn 2 ternary eutectic alloy structure. When the number density of the SiO 2 oxide phase formed inside the coated layer is 100,000 / mm 2< or more, corrosion resistance of worked portions is obtainable. The number density of the SiO 2 oxide phase formed inside the coated layer is therefore 100,000 / mm 2< or more. The number density of the SiO 2 oxide phase formed inside the coated layer is preferably 150,000 / mm 2< or more. The upper limit of the number density of the SiO 2 oxide phase formed inside the coated layer is not particularly limited, but the number density is generally 380,000 / mm 2< or less.

[0026] The formation of the SiO 2 oxide phase requires control of the Si content in the coating bath composition, control of reducibility (control of O 2 concentration) when applying a coating treatment to the surface of the base steel sheet, and a post-treatment process after coating treatment. When such controls or post-treatment process are insufficient, the Mg 2 Si alloy phase may be present in a large amount in the surface layer of the coated layer, resulting in decreased chemical convertibility, or the Mg 2 Si alloy phase may be present in a small amount inside the coated layer, resulting in insufficient suppression of blistering from sheared ends after further coating, or furthermore, the SiO 2 oxide phase may not be sufficiently formed inside the coated layer, resulting in decreased corrosion resistance of worked portions.

[0027] FIG. 1A to FIG. 1E illustrate reflected electron images (at 5000 times magnification) and EDS analysis results observed when a surface of the coated layer of the Zn-Al-Mg-Si coated steel sheet produced according to an embodiment of the present disclosure was irradiated with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope. FIG. 1A is a reflected electron image of the Zn-Al-Mg-Si coated layer, and FIG. 1B to FIG. 1E are the Mg distribution, Si distribution, Al distribution, and Zn distribution, respectively, obtained by composition analysis using EDS. As indicated in FIG. 1A to FIG. 1E, the Mg 2 Si alloy phase has a needle-like shape overlapping distributions of Mg and Si, and is found to be in the Zn-Al-MgZn 2 ternary eutectic alloy structure.

[0028] FIG. 2A to FIG. 2F illustrate reflected electron images (at 5000 times magnification) and EDS analysis results observed when a surface of the coated layer of the Zn-Al-Mg-Si coated steel sheet produced according to an embodiment of the present disclosure was irradiated with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope. FIG. 2A is a reflected electron image of the Zn-Al-Mg-Si coated layer, and FIG. 2B to FIG. 2F are the Mg distribution, Si distribution, Al distribution, Zn distribution, and O distribution, respectively, obtained by composition analysis using EDS. As illustrated in FIG. 2A to FIG. 2F, the Si distribution has a needle-like shape, and the O distribution overlaps with the distributions of Mg, Si, and Al. The SiO 2 oxide phase has a needle-like shape with overlapping distributions of Si and O, and is found to be in the Zn-Al-MgZn 2 ternary eutectic alloy structure.[Method of producing Zn-Al-Mg-Si coated steel sheet]

[0029] Next, a method of producing a Zn-Al-Mg-Si coated steel sheet according to an embodiment of the present disclosure is described. The method of producing a Zn-Al-Mg-Si coated steel sheet according to an embodiment of the present disclosure includes: a process of immersing a base steel sheet in a coating bath having a composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity, thereby subjecting the base steel sheet to a coating treatment; after the coating treatment, a process of subjecting the base steel sheet to gas wiping, thereby adjusting the coating weight of a coated layer formed on the base steel sheet; and after the gas wiping, a process of salt water application to a surface of the coated steel sheet. A method of cooling the coated steel sheet after the gas wiping and prior to the salt water application is not particularly limited, and various methods such as air cooling, air-water cooling, and water cooling may be used.

[0030] By carrying out the process of salt water application to the surface of the coated steel sheet, the coated layer is etched and the Mg 2 Si alloy phase present inside the coated layer is oxidized. When the Mg 2 Si alloy phase is oxidized, Mg, which has a lower natural potential in a saltwater environment, is selectively dissolved and eluted from the coated layer. As a result, the remaining Si is oxidized to become an oxide, forming the SiO 2 oxide phase. The SiO 2 oxide phase formed inside the coated layer, when present together with Zn in a corrosion environment, promotes the production of basic zinc chloride, which has a strong protective effect, and contributes to improving corrosion resistance.

[0031] In the method of producing a Zn-Al-Mg-Si coated steel sheet according to the present disclosure, the base steel sheet to be coated is the same base steel sheet as described above.

[0032] When an annealing heat treatment is carried out on a base steel sheet before coating treatment and the H 2 concentration in the furnace atmosphere during annealing is 1 vol% or more, oxides at the base steel sheet surface can be suitably reduced. The H 2 concentration in the furnace atmosphere during annealing is therefore preferably 1 vol% or more. On the other hand, when the H 2 concentration in the furnace atmosphere during annealing is 10 vol% or less, production costs can be suitably suppressed. The H 2 concentration in the furnace atmosphere during annealing is therefore preferably 10 vol% or less. Further, the furnace atmosphere during annealing is preferably a mixed gas containing H 2 with the balance being N 2 . Temperature of the annealing heat treatment is not particularly limited. The temperature of the annealing heat treatment is preferably 600 °C or more. The temperature of the annealing heat treatment is preferably 850 °C or less. Further, duration of the annealing heat treatment is not particularly limited. The duration of the annealing heat treatment is preferably 10 s or longer. The duration of the annealing heat treatment is preferably 60 s or less.

[0033] The coating bath used in the production of a Zn-Al-Mg-Si coated steel sheet according to the present disclosure has a composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity. The explanation for each component in the coating bath is the same as the explanation of each component of the coated layer described above. The chemical composition of the coating bath is equivalent to the chemical composition of the applied coated layer.

[0034] From the viewpoint of corrosion resistance, the coating weight of the coated layer in the coating treatment is preferably 15 g / m 2< or more. The coating weight of the coated layer in the coating treatment is more preferably 30 g / m 2< or more. On the other hand, when the coating weight of the coated layer is 100 g / m 2< or less, production cost can be suitably suppressed, and therefore the coating weight of the coated layer is preferably 100 g / m 2< or less. Further, temperature of the coating bath during the coating treatment is preferably 430 °C or more. The temperature of the coating bath during the coating treatment is preferably 540 °C or less. When the temperature of the coating bath is less than 430 °C, fluidity of the coating bath decreases, which may hinder uniform coating coverage. For this reason, the temperature of the coating bath is preferably 430 °C or more. On the other hand, when the temperature of the coating bath exceeds 540 °C, there is a risk of an increase in oxides at the surface of the coating bath due to oxidation of Mg in the coating bath, and erosion of refractory material in the coating bath by Al and Mg. For these reasons, the temperature of the coating bath is preferably 540 °C or less. The temperature of the coating bath is more preferably 530 °C or less.

[0035] In order to not form the Mg 2 Si alloy phase in the surface layer of the coated layer and to form a large amount of the Mg 2 Si alloy phase inside the coated layer, decreasing the reducing property of the gas used in gas wiping is effective. When H 2 is used in a mixed gas used in gas wiping, the reducing property becomes very high. When the H 2 concentration is 1 vol% or more and the amount of Si in the coating bath is low, then even when the salt water application is carried out, the Mg 2 Si in the coated layer is difficult to oxidize, and a sufficient amount of SiO 2 oxide phase is not formed, making it difficult to obtain the desired corrosion resistance of worked portions. As a result of intensive studies regarding this point, the inventors have found that the higher the O 2 concentration during gas wiping and the lower the reducing property of the gas, the less likely it is that the Mg 2 Si alloy phase will form in the surface layer of the coated layer, and the more likely it is that the Mg 2 Si alloy phase will form inside the coated layer. As a result, it becomes possible to decrease the SiO 2 oxide phase in the surface layer of the coated layer and increase the SiO 2 oxide phase inside the coated layer during corrosion caused by the salt water application. Therefore, it is preferable to use a mixed gas containing O 2 and N 2 , or air, as the gas used in gas wiping. The gas wiping is preferably carried out in an air environment.

[0036] When the gas used in the gas wiping is a mixed gas containing O 2 and N 2 , and the O 2 concentration is 3 vol% or more, then even when the Si content in the coating bath is low, the Mg 2 Si alloy phase is suitably formed inside the coated layer, and the effect of contributing to improvement of corrosion resistance is suitably obtained. The O 2 concentration of the mixed gas used in the gas wiping is therefore preferably 3 vol% or more. On the other hand, when the O 2 concentration is 15 vol% or less, production cost can be suitably suppressed. The O 2 concentration of the mixed gas used in the gas wiping is therefore preferably 15 vol% or less. Further, the mixed gas may contain H 2 . When the H 2 concentration of the mixed gas used in the gas wiping is 7 vol% or more, the formation of the Mg 2 Si alloy phase in the surface layer of the coated layer is suitably suppressed, the Mg 2 Si alloy phase is formed inside the coated layer, and the formation of the SiO 2 oxide phase inside the coated layer is suitably promoted during the salt water application.

[0037] Air may be used for the gas wiping. In this case, too, the Mg 2 Si alloy phase is preferably formed inside the coated layer even when the Si content in the coating bath is low. During the salt water application after the gas wiping with air, the formation of the SiO 2 oxide phase is favorably promoted inside the coated layer, and the effect of contributing to improved corrosion resistance is favorably obtained.

[0038] The method of salt water application to the surface of the coated steel sheet after coating weight adjustment is not particularly limited. A method may be used in which salt water is applied to the surface of the coated steel sheet, left to stand for a defined period of time, and then washed and dried, thereby corroding and oxidizing the coated layer of the coated steel sheet. The salt water used in the salt water application is not particularly limited. From the viewpoint of production cost, an aqueous solution of sodium chloride (NaCl) is preferred. The salt water concentration is not particularly limited. From the viewpoint of production costs, the salt water concentration is preferably 0.5 mass% or more. The salt water concentration is preferably 5.0 mass% or less. Further, the standing time for the salt water application is preferably 0.5 h or more from the viewpoint of sufficiently oxidizing the coated layer. However, when the standing time exceeds 2 h, corrosion of the coated layer progresses greatly, resulting in a decrease in coating weight and deterioration of corrosion resistance. The standing time for the salt water application is preferably 2 h or less. The standing time for the salt water application is more preferably 1 h or less.

[0039] For processes and conditions not described herein, conventional methods may be used.EXAMPLES

[0040] Using an extremely low carbon cold-rolled steel sheet having a thickness of 0.8 mm as a base steel sheet, all of the Zn-Al-Mg-Si coated steel sheets serving as samples were produced using a hot-dip coating apparatus. Each sample was coated using variously changed coating bath compositions, and the coating bath temperature was set to 450 °C, which is 30 °C higher than a solidification start temperature estimated from a Zn-Al-Mg-Si quaternary phase equilibrium diagram. The base steel sheet was heated in an H 2 (5 vol%) + N 2 atmosphere, the temperature of the base steel sheet when immersed in the coating bath was set to the same temperature as the coating bath temperature, and hot-dip coating was carried out in the coating bath for 1 s. Thereafter, the thickness of the coated layer was controlled to 7 µm (equivalent to 50 g / m 2< ) by adjusting the flow rate of the wiping gas. Some of the samples were further subjected to salt water application in which the sample was immersed in an aqueous solution of 0.5 wt% NaCl for 1 h, and then washed and dried. Table 1 lists the coating bath composition, the wiping gas composition, and the coated layer composition. Further, in the entry for salt water application in Table 1, the samples that underwent salt water application are indicated as "Yes", and the samples that did not undergo salt water application are indicated as "No". The wiping was carried out in an air environment.[Table 1]

[0041] Table 1No.Coating bath composition (mass%)Wiping gas (vol%)Salt water applicationCoated layer composition (mass%)Ternary eutectic alloyMg 2 SiSiO 2 EvaluationRemarksZnAlMgSiFeZnAlMgSiFeNumber density observation at 7.0 kV (per mm 2< )Number density observation at 15.0 kV (per mm 2< )Number density observation at 15.0 kV (per mm 2< )Chemical convertibilityBlister width of under-coat blistering from sheared edgeWorked portion dissimilar joined portion corrosion resistance197.251.501.000.050.20N 2 +10%O 2 Yes97.251.501.000.050.20Yes01200200000⊚⊚⊚Example297.951.500.300.050.20N 2 +10%O 2 Yes97.951.500.300.050.20Yes025056000⊚ΔΔComparative Example 398.151.500.100.050.20N 2 +10%O 2 Yes98.151.500.100.050.20Yes050 16000⊚△×Comparative Example 497.651.500.600.050.20N 2 +10%O 2 Yes97.651.500.600.050.20Yes0850120000⊚○○Example597.501.500.750.050.20N 2 +10%O 2 Yes97.501.500.750.050.20Yes01000160000⊚○⊚Example697.401.500.850.050.20N 2 +10%O 2 Yes97.401.500.850.050.20Yes01150176000⊚○⊚Example796.751.501.500.050.20N 2 +10%O 2 Yes96.751.501.500.050.20Yes01350216000⊚⊚⊚Example896.251.502.000.050.20N 2 +10%O 2 Yes96.251.502.000.050.20Yes01500224000⊚⊚⊚Example995.451.502.800.050.20N 2 +10%O 2 Yes95.451.502.800.050.20Yes01550224000⊚⊚⊚Example1095.051.503.200.050.20N 2 +10%O 2 Yes95.051.503.200.050.20Yes01650216000⊚⊚×Comparative Example 1193.251.505.000.050.20N 2 +10%O 2 Yes93.251.505.000.050.20Yes01800232000⊚⊚×Comparative Example 1298.251.500.000.050.20N 2 +10%O 2 Yes98.251.500.000.050.20No0044000⊚×xComparative Example 1397.291.501.000.010.20N 2 +10%O 2 Yes97.291.501.000.010.20Yes045080000⊚ΔΔComparative Example 1497.291.501.000.010.20N 2 +10%O 2 Yes97.291.501.000.010.20Yes0600104000⊚○○Example1597.271.501.000.040.20N 2 +10%O 2 Yes97.271.501.000.040.20Yes0650112000⊚○○Example1697.231.501.000.070.20N 2 +10%O 2 Yes97.231.501.000.070.20Yes120001250248000○⊚⊚Example1797.201.501.000.100.20N 2 +10%O 2 Yes97.201.501.000.100.20Yes400001500264000○⊚⊚Example1897.151.501.000.150.20N 2 +10%O 2 Yes97.151.501.000.150.20Yes640001700296000○⊚⊚Example1997.111.501.000.190.20N 2 +10%O 2 Yes97.111.501.000.190.20Yes960002050344000○⊚⊚Example2097.071.501.000.230.20N 2 +10%O 2 Yes97.071.501.000.230.20Yes1120002400368000Δ⊚⊚Comparative Example 2197.301.501.000.000.20N 2 +10%O 2 Yes97.301.501.000.000.20Yes000⊚×xComparative Example 2297.251.501.000.050.20N 2 +15%O 2 Yes97.251.501.000.050.20Yes01250208000⊚⊚⊚Example2397.251.501.000.050.20N 2 +10%O 2 No97.251.501.000.050.20Yes11200012000 △⊚×Comparative Example 2497.251.501.000.050.20N 2 +15%O 2 No97.251.501.000.050.20Yes10400012500 Δ⊚×Comparative Example 2597.251.501.000.050.20N 2 +7%O 2 Yes97.251.501.000.050.20Yes40001000192000○○⊚Example2697.251.501.000.050.20N 2 +5%O 2 Yes97.251.501.000.050.20Yes8000750164000○○⊚Example2797.251.501.000.050.20N 2 +3%O 2 Yes97.251.501.000.050.20Yes12000550136000○○○Example2897.251.501.000.050.20N 2 +1%O 2 Yes97.251.501.000.050.20Yes1600015088000○ΔΔComparative Example 2997.251.501.000.120.20N 2 +5%O 2 Yes97.251.501.000.120.20Yes16000900216000○○⊚Example3097.281.501.000.020.20N 2 +5%O 2 Yes97.291.501.000.020.20Yes4000500104000○○○Example3197.251.501.000.050.20N 2 No97.251.501.000.050.20Yes24000500 ○××Comparative Example 3297.251.501.000.050.20airNo97.251.501.000.050.20Yes024000 ⊚⊚×Comparative Example 3397.251.501.000.050.20airYes97.251.501.000.050.20Yes02400368000⊚⊚⊚Example3497.251.501.000.050.20N 2 +1%H 2 No97.251.501.000.050.20Yes112000 8 0 △××Comparative Example 3597.251.501.000.050.20N 2 +3%H 2 No97.251.501.000.050.20Yes120000 50 △××Comparative Example 3697.251.501.000.050.20N 2 +5%H 2 No97.251.501.000.050.20Yes208000 0 0 ×××Comparative Example 3797.251.501.000.050.20N 2 +7%H 2 No97.251.501.000.050.20Yes216000 0 0 ×××Comparative Example 3897.111.501.000.190.20N 2 +1%H 2 No97.111.501.000.190.20Yes136000 5600 △○×Comparative Example 3997.111.501.000.190.20N 2 +0.5%H 2 No97.111.501.000.190.20Yes144000 6500 △○×Comparative Example 4097.251.501.000.050.20N 2 +1%H 2 Yes97.251.501.000.050.20Yes56000 10 120000○×○Comparative Example 4197.251.501.000.050.20N 2 +3%H 2 Yes97.251.501.000.050.20Yes640005136000○×○Comparative Example 4297.151.501.000.150.20N 2 +7%H 2 Yes97.151.501.000.150.20Yes960001150208000○○⊚Example4397.151.501.000.150.20N 2 +5%H 2 Yes97.151.501.000.150.20Yes800001000192000○○⊚Example4497.111.501.000.190.20N 2 +3%H 2 Yes97.111.501.000.190.20Yes88000850176000○○⊚Example4597.111.501.000.190.20N 2 +1%H 2 Yes97.111.501.000.190.20Yes72000600120000○○○Example4697.111.501.000.190.20N 2 +0.5%H 2 Yes97.111.501.000.190.20Yes80000700104000○○⊚Example4798.740.011.000.050.20N 2 +10%O 2 Yes98.740.01 1.000.050.20No 0200 72000 ⊚△△Comparative Example 4898.630.121.000.050.20N 2 +10%O 2 Yes98.630.121.000.050.20Yes0550104000⊚○○Example4997.751.001.000.050.20N 2 +10%O 2 Yes97.751.001.000.050.20Yes0900104000⊚○⊚Example5096.752.001.000.050.20N 2 +10%O 2 Yes96.752.001.000.050.20Yes01300208000⊚⊚⊚Example5195.952.801.000.050.20N 2 +10%O 2 Yes95.952.801.000.050.20Yes01650224000⊚⊚⊚Example5295.553.201.000.050.20N 2 +10%O 2 Yes95.553.20 1.000.050.20Yes02350224000⊚△⊚Comparative Example 5393.755.001.000.050.20N 2 +10%O 2 Yes93.755.00 1.000.050.20Yes02400220000⊚×⊚Comparative Example 5496.252.501.000.050.20N 2 +10%O 2 No96.252.50 1.000.050.20Yes112000 1550Q△⊚×Comparative Example 5598.630.121.000.050.20N 2 +5%H 2 Yes98.630.121.000.050.20Yes64000550120000○○○Example5695.952.801.000.050.20N 2 +5%H 2 Yes95.952.801.000.050.20Yes960001650232000○⊚⊚Example

[0042] Each sample of the obtained Zn-Al-Mg-Si coated steel sheet was sheared to a size of 10 mm × 20 mm. The coated layer of the sample after shearing was observed from a surface direction of the coated steel sheet using a scanning electron microscope to confirm the presence or absence of a Zn-Al-MgZn 2 ternary eutectic alloy structure. When the composition analysis by EDS during the observation indicated that there was a structure composed of three phases, Zn phase, Al phase, and MgZn 2 alloy phase, and when the same surface was analyzed by XRD and the Zn-Al-MgZn 2 ternary eutectic alloy structure was present, then "Yes" was recorded. When the composition analysis by EDS during the observation indicated that there was no structure composed of three phases, Zn phase, Al phase, and MgZn 2 alloy phase, and when the same surface was analyzed by XRD and any of the Zn phase, Al phase, and MgZn 2 alloy phase was not confirmed, the Zn-Al-MgZn 2 crystal structure was absent, and "No" was recorded in Table 1.

[0043] The coated layer of the sample after shearing was observed from a surface direction of the coated steel sheet using a scanning electron microscope to observe the state of the Mg 2 Si alloy phase present in the surface layer and inside the coated layer, and number density was evaluated. The number density of the Mg 2 Si alloy phase present in the surface layer and inside the coated layer was evaluated from a reflected electron image (at 5000 times magnification) observed when irradiating the coated layer with an electron beam at an accelerating voltage of 7.0 kV. The number density of the Mg 2 Si alloy phase present inside the coated layer was evaluated from a reflected electron image (at 5000 times magnification) observed when irradiating the coated layer with an electron beam at an accelerating voltage of 15.0 kV.

[0044] The coated layer of the sample after shearing was observed from a surface direction of the coated steel sheet using a scanning electron microscope to observe the state of the SiO 2 oxide phase present inside the coated layer, and number density was evaluated. The SiO 2 oxide phase present inside the coated layer was evaluated from a reflected electron image (at 5000 times magnification) observed when irradiating the coated layer with an electron beam at an accelerating voltage of 15.0 kV.[Chemical convertibility]

[0045] Next, the chemical convertibility of the obtained Zn-Al-Mg-Si coated steel sheets was evaluated. Each sample was sheared to a size of 30 mm × 30 mm, and then phosphating treatment was carried out for 90 s under the same conditions. The surface of the coated steel sheet was observed using a scanning electron microscope, and the state of the phosphate crystals (the degree of exposure of the coated surface under chemical conversion) was observed from the obtained secondary electron image (at 800 times magnification), and evaluated as follows. In Table 1, a case where there was no phosphate crystal scale and no exposed surface of the coated steel sheet is indicated by "⊚", a case where there was very little phosphate crystal scale and very little exposed surface of the coated steel sheet is indicated by "O", a case where there was very little phosphate crystal scale and very little exposed surface of the coated steel sheet is indicated by "△", and a case where there was phosphate crystal scale and exposed plated surface is indicated by "×". An evaluation of ⊚ or O was considered a pass, and an evaluation of △ or × was considered a fail.[Under-coat blistering from sheared edge after further coating]

[0046] Next, suppression of under-coat blistering from sheared edges of the Zn-Al-Mg-Si coated steel sheet after further coating was evaluated. Each sample was sheared to a size of 80 mm × 80 mm, phosphate treated under the same conditions, and then subjected to electrodeposition coating (the coating thickness was constant at 15 µm), and then sheared to a size of 50 mm × 50 mm to prepare sheared end surfaces. Then, the samples were subjected to 45 cycles of corrosion testing according to JASO M609-91, and the maximum blister width of under-coat blistering from a sheared edge was then evaluated. In Table 1, blister width of less than 1.0 mm is indicated as "⊚", blister width of 1.0 mm or more and less than 1.5 mm is indicated as "O", blister width of 1.5 mm or more and less than 2.0 mm is indicated as "△", and blister width of 2.0 mm or more is indicated as "×". An evaluation of ⊚ or O was considered a pass, and an evaluation of △ or × was considered a fail.[Worked portion dissimilar joined portion corrosion resistance]

[0047] Next, the worked portion dissimilar joined portion corrosion resistance of the Zn-Al-Mg-Si coated steel sheets was evaluated. Each sample was sheared to a size of 70 mm × 120 mm, and mechanically joined at two points by mechanical clinching (5 mm diameter) to an Al alloy that had been sheared to a size of 40 mm × 90 mm to provide a worked portion, thereby producing a worked portion dissimilar joined portion sample of a Zn-Al-Mg-Si coated steel sheet and an Al alloy. In this case, the surfaces other than the mating surfaces were phosphate-treated under the same conditions, and then subjected to electrodeposition coating (with a constant coating thickness of 15 µm) to cover them. The worked portion dissimilar joined portion samples were then subjected to a corrosion test according to N-VDA testing. After 12 weeks of the corrosion test, the corrosion loss of the Al alloy was evaluated. The Al corrosion loss was measured in accordance with ISO 8407:2021 by immersing a worked portion dissimilar joined portion sample in a 60 mass% aqueous nitric acid solution at room temperature to remove corrosion products mainly composed of aluminum, and calculating the difference between the weight before the test and the weight after the corrosion products were removed. In Table 1, the corrosion loss was indicated as "⊚" when less than 5 g / m 2< , "O" when 5 g / m 2< or more and less than 10 g / m 2< , "△" when 10 g / m 2< or more and less than 15 g / m 2< , and "×" when 15 g / m 2< or more. An evaluation of ⊚ or O was considered a pass, and an evaluation of △ or × was considered a fail.

[0048] As indicated in Table 1, the Examples having the coating composition specified in the present disclosure and having a number density of Mg 2 Si alloy phase of less than 100,000 / mm 2< when observed at an accelerating voltage of 7.0 kV and 500 / mm 2< or more when observed at an accelerating voltage of 15.0 kV had excellent chemical convertibility, sufficiently suppressed under-coat blistering from a sheared end, and had excellent worked portion dissimilar joined portion corrosion resistance. That is, it was found that the presence of a small amount of Mg 2 Si alloy phase in the surface layer of the coated layer and a large amount inside the coated layer is advantageous for chemical convertibility, suppression of under-coat blistering from sheared ends, and worked portion dissimilar joined portion corrosion resistance. In particular, the Examples for which the number density of SiO 2 was 150,000 / mm 2< or more when observed at an accelerating voltage of 15.0 kV, that is, for which there was a large amount of SiO 2 inside the coated layer, demonstrated more favorable evaluation results. In contrast, the Comparative Examples outside the scope of the present disclosure were inferior to the Examples in at least one of chemical convertibility, suppression under-coat blistering from a sheared end, and worked portion dissimilar joined portion corrosion resistance.INDUSTRIAL APPLICABILITY

[0049] According to the present disclosure, it is possible to provide a Zn-Al-Mg-Si coated steel sheet which has excellent chemical convertibility, sufficiently suppresses under-coat blistering from sheet edges, and has excellent dissimilar joined portion corrosion resistance at contact areas between worked portions and aluminum alloy.

Examples

examples

[0040]Using an extremely low carbon cold-rolled steel sheet having a thickness of 0.8 mm as a base steel sheet, all of the Zn-Al-Mg-Si coated steel sheets serving as samples were produced using a hot-dip coating apparatus. Each sample was coated using variously changed coating bath compositions, and the coating bath temperature was set to 450 °C, which is 30 °C higher than a solidification start temperature estimated from a Zn-Al-Mg-Si quaternary phase equilibrium diagram. The base steel sheet was heated in an H 2 (5 vol%) + N 2 atmosphere, the temperature of the base steel sheet when immersed in the coating bath was set to the same temperature as the coating bath temperature, and hot-dip coating was carried out in the coating bath for 1 s. Thereafter, the thickness of the coated layer was controlled to 7 µm (equivalent to 50 g / m 2< ) by adjusting the flow rate of the wiping gas. Some of the samples were further subjected to salt water application in which the sample was immersed ...

Claims

1. A Zn-Al-Mg-Si coated steel sheet comprising a base steel sheet and a coated layer formed on at least one surface of the base steel sheet, wherein the coated layer has a chemical composition containing, in mass%, Mg: 0.50 % to 3.00 %, Al: 0.10 % to 3.00 %, Si: 0.010 % to 0.20 %, and Fe: 0.30 % or less, with the balance being Zn and inevitable impurity, and a Zn-Al-MgZn2 ternary eutectic alloy structure, in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 7.0 kV using a scanning electron microscope, needle-shaped Mg2Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn2 ternary eutectic alloy structure at a number density of less than 100,000 / mm2 (including 0 / mm2), in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped Mg2Si alloy phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn2 ternary eutectic alloy structure at a number density of 500 / mm2 or more, and in a reflected electron image at a magnification of 5000 times obtained by irradiating a surface of the coated layer with an electron beam at an accelerating voltage of 15.0 kV using a scanning electron microscope, needle-shaped SiO2 oxide phase having a major axis length of 0.5 µm or more is present in the Zn-Al-MgZn2 ternary eutectic alloy structure at a number density of 100,000 / mm2 or more.

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