Hot-dip Al-Zn-Si-Mg plated steel sheet and method for manufacturing the same

Abstract

To provide a hot-dip Al-Zn-Si-Mg coated steel sheet having stable and excellent corrosion resistance. [Solution] In order to achieve the above-mentioned object, the present invention provides a hot-dip Al-Zn-Si-Mg plated steel sheet having a plating film, characterized in that the plating film has a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the remainder consisting of Zn and unavoidable impurities, and the Ni content of the unavoidable impurities is 0.010 mass% or less relative to the total mass of the plating film.

Description

[Technical Field]

[0001] This invention relates to a hot-dip Al-Zn-Si-Mg plated steel sheet having stable and excellent corrosion resistance, and a method for producing the same. [Background technology]

[0002] Hot-dip galvanized steel sheets, such as the 55% Al-Zn type, are known to exhibit high corrosion resistance among hot-dip galvanized steel sheets because they combine the sacrificial corrosion protection of zinc with the high corrosion resistance of aluminum. For this reason, due to their excellent corrosion resistance, hot-dip galvanized steel sheets are mainly used in building materials such as roofs and walls that are exposed to the outdoors for long periods, as well as in civil engineering and construction fields such as guardrails, wiring and piping, and sound barriers. In particular, the demand for materials with superior corrosion resistance and maintenance-free materials in harsher operating environments, such as acid rain caused by air pollution, the application of de-icing agents to prevent road freezing in snowy areas, and coastal development, has been increasing in recent years.

[0003] The plating film of hot-dip Al-Zn plated steel sheets is characterized by a structure in which multiple α-Al phases are stacked in the direction of the plating film thickness, consisting of a portion where Al containing supersaturated Zn has solidified in a dendrite-like manner (α-Al phase) and a Zn-Al eutectic structure present in the interdendrite gaps. Due to this characteristic film structure, the corrosion progression path from the surface becomes complex, making it difficult for corrosion to progress easily. As a result, hot-dip Al-Zn plated steel sheets are known to achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same plating film thickness.

[0004] Attempts have been made to further extend the lifespan of such molten Al-Zn plated steel sheets, and molten Al-Zn-Si-Mg plated steel sheets with added Mg have been put into practical use. As an example of such a hot-dip Al-Zn-Si-Mg plated steel sheet, Patent Document 1 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which the plating film contains an Al-Zn-Si alloy containing Mg, the Al-Zn-Si alloy is an alloy containing 45-60% by weight of elemental aluminum, 37-46% by weight of elemental zinc, and 1.2-2.3% by weight of Si, and the concentration of Mg is 1-5% by weight. Furthermore, Patent Document 2 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet that aims to improve corrosion resistance and enhance the protective effect after the underlying steel sheet is exposed by containing 2-10% Mg and 0.01-10% Ca in the plating film. Furthermore, Patent Document 3 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet that improves the corrosion resistance of flat plates and end faces by forming a coating layer containing Mg: 1-15%, Si: 2-15%, and Zn: 11-25% by mass, with the remainder being Al and unavoidable impurities, and by making the size of intermetallic compounds such as Mg2Si phase and MgZn2 phase present in the plating film 10 μm or less.

[0005] As mentioned above, hot-dip molten Al-Zn plated steel sheets have a beautiful appearance with a white metallic spangle pattern, and are often used without painting. Therefore, there is a strong demand for a high-quality appearance. Consequently, technologies to improve the appearance of hot-dip molten Al-Zn plated steel sheets have been developed. For example, Patent Document 4 discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which wrinkle-like unevenness defects are suppressed by containing 0.01 to 10% Sr in the plating film. Furthermore, Patent Document 5 also discloses a hot-dip Al-Zn-Si-Mg plated steel sheet in which mottled defects are suppressed by containing 500 to 3000 ppm of Sr in the plating film. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 5020228 [Patent Document 2] Patent No. 5000039 [Patent Document 3] Japanese Patent Publication No. 2002-12959 [Patent Document 4] Patent No. 3983932 [Patent Document 5] Special Publication No. 2011-514934 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the technique of incorporating Mg into the plating film, as disclosed in Patent Documents 1 to 3, does not necessarily guarantee an improvement in corrosion resistance. In the molten Al-Zn-Si-Mg plated steel sheets disclosed in Patent Documents 1 to 3, corrosion resistance is improved solely by including Mg in the plating components. However, the influence of components other than the four elements mentioned above (Al, Zn, Si, Mg) and the characteristics of the metallic phase and intermetallic compound phase constituting the plating film are not considered, making it impossible to uniformly discuss the superiority or inferiority of corrosion resistance. Therefore, even when molten Al-Zn-Si-Mg plated steel sheets are manufactured using a plating bath composition with equivalent content of the four elements mentioned above, corrosion acceleration tests show variability in corrosion resistance, and there is a problem in that they are not necessarily superior to Al-Zn plated steel sheets without added Mg. Similarly, in improving the appearance of the plating, simply adding Sr to the plating film does not necessarily eliminate wrinkle-like uneven defects, and the molten Al-Zn-Si-Mg plated steel sheets disclosed in Patent Documents 4 and 5 sometimes failed to achieve both corrosion resistance and appearance. In addition, because Mg is an element that oxidizes easily, the Mg contained in the plating bath can generate oxides (top dross) near the bath surface, and in the case of molten plating, over time, Fe-Al compounds containing iron (bottom dross) may be generated, which are unevenly distributed in the bath or at the bottom of the plating bath. These drosses can adhere to the surface of the plating film, causing convex defects and potentially impairing the appearance of the plating film surface. Furthermore, it is known that when steel plates are plated using a molten Al-Zn-Si bath to which Mg has been added, in addition to the α-Al phase, Mg2Si phase, MgZn2 phase, and Si phase precipitate in the plating film. However, the influence of the amount and relative abundance of each phase on corrosion resistance had not been clarified.

[0008] In view of these circumstances, the present invention aims to provide a hot-dip Al-Zn-Si-Mg plated steel sheet having stable and excellent corrosion resistance, and a method for manufacturing the same. [Means for solving the problem]

[0009] As a result of their investigations to solve the above problems, the inventors focused on the fact that, in order to control the composition of the plating film of a molten Al-Zn-Si-Mg plated steel sheet, it is important not only to control the concentrations of Al, Zn, Si, and Mg, but also to control the concentration of elements included as impurities. They found that by appropriately controlling the Ni content, the deterioration of corrosion resistance can be effectively suppressed, and furthermore, by appropriately controlling the size and distribution of Ni-based compounds present as impurities in the plating film, the deterioration of corrosion resistance can be suppressed even more effectively. Furthermore, regarding the Mg2Si, MgZn2, and Si phases formed in the plating film of molten Al-Zn-Si-Mg plated steel sheets, the amount of deposition increases or decreases depending on the balance of each component in the plating film and the conditions for the formation of the plating film, changing their relative abundance. Depending on the balance of the composition, one of the phases may not precipitate at all. It was investigated that the corrosion resistance of molten Al-Zn-Si-Mg plated steel sheets changes depending on the relative abundance of these phases, and that corrosion resistance stably improves, especially when the MgZn2 phase is more abundant than the Mg2Si and Si phases. However, it is known that it is extremely difficult to distinguish between these Mg2Si, MgZn2, and Si phases using general methods, such as observing the plating film from the surface or cross-section using a scanning electron microscope to obtain secondary electron images or backscattered electron images. While it is possible to obtain microscopic information by observing with a transmission electron microscope, it was not possible to grasp the relative abundance of the Mg2Si, MgZn2, and Si phases, which affect macroscopic information such as corrosion resistance and appearance. Therefore, the inventors continued their diligent research and, focusing on X-ray diffraction, discovered that the phase ratios can be quantitatively defined by utilizing the intensity ratios of specific diffraction peaks of the Mg2Si phase, MgZn2 phase, and Si phase. Furthermore, they found that when the Mg2Si phase and MgZn2 phase satisfy specific ratios in the plating film, stable and excellent corrosion resistance can be achieved, and dross generation can be suppressed, ensuring a good surface appearance. In addition, the inventors have found that by controlling the Ni content and film structure in the plating film as described above, and by controlling the Sr concentration in the plating bath, it is possible to reliably suppress the occurrence of wrinkle-like unevenness defects and obtain a plated steel sheet with excellent surface appearance.

[0010] This invention is based on the above findings, and its gist is as follows. 1. A hot-dip Al-Zn-Si-Mg plated steel sheet having a plating film, The aforementioned plating film has a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities. The hot-dip Al-Zn-Si-Mg plated steel sheet is characterized in that the Ni content in the inevitable impurities is 0.010% by mass or less based on the total mass of the plating film.

[0011] 2. The hot-dip Al-Zn-Si-Mg plated steel sheet according to 1 above, wherein the plating film contains a Ni-based compound, and the major axis of the Ni-based compound is 4.0 μm or less. 3. The hot-dip Al-Zn-Si-Mg plated steel sheet according to 1 or 2 above, wherein the plating film contains a Ni-based compound, and the number of the Ni-based compounds present in the direction parallel to the surface of the base steel sheet is 5 or less per mm. 4. The hot-dip Al-Zn-Si-Mg plated steel sheet according to 1 above, wherein the plating film does not contain a Ni-based compound.

[0012] 5. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of 1 to 4 above, wherein the diffraction intensities of Mg2Si and MgZn2 in the plating film by X-ray diffraction method satisfy the following relationship (1). Mg2Si (111) / MgZn2(100)≦2.0 ···(1) Mg2Si (111): The diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Mg2Si <00​​​​​​​​​​​​​

[0015] 8. A hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of 1 to 7, characterized in that the Al content in the plating film is 50 to 60% by mass.

[0016] 9. A hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of 1 to 8, characterized in that the Si content in the plating film is 1.0 to 3.0% by mass.

[0017] 10. A hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of 1 to 9, characterized in that the Mg content in the plating film is 1.0 to 5.0% by mass.

[0018] 11. A method for manufacturing a hot-dip Al-Zn-Si-Mg plated steel sheet having a plating film, The formation of the aforementioned plating film is carried out using Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass. The process comprises a hot-dip plating treatment step in which a base steel plate is immersed in a plating bath having a composition in which mass% contains Zn and the remainder consists of Zn and unavoidable impurities. A method for producing a molten Al-Zn-Si-Mg plated steel sheet, characterized by controlling the Ni content in the unavoidable impurities of the plating bath to 0.010% by mass or less relative to the total mass of the plating bath. 12. The method for producing a molten Al-Zn-Si-Mg plated steel sheet according to 11, characterized in that the plating bath further contains Sr: 0.01 to 1.0% by mass. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a hot-dip Al-Zn-Si-Mg plated steel sheet that has stable and excellent corrosion resistance. [Brief explanation of the drawing]

[0020] [Figure 1] This diagram illustrates the flow of the combined cycle test (JASO-CCT) according to Japanese automotive standards. [Modes for carrying out the invention]

[0021] (Hot-dip Al-Zn-Si-Mg plated steel sheet) The molten Al-Zn-Si-Mg plated steel sheet of the present invention has a plating film on the surface of the steel sheet. The plating film has a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities.

[0022] The Al content in the plating film is 45 to 65% by mass, preferably 50 to 60% by mass, considering the balance between corrosion resistance and operational aspects. This is because if the Al content in the plating film is at least 45% by mass, Al dendrite solidification occurs, and a plating film structure mainly consisting of a dendrite solidified α-Al phase can be obtained. When this dendrite solidified structure is stacked in the direction of the thickness of the plating film, the corrosion progression path becomes complex, and the corrosion resistance of the plating film itself is improved. Furthermore, the more of these α-Al phase dendrites that are stacked, the more complex the corrosion progression path becomes, making it more difficult for corrosion to easily reach the underlying steel plate, thus improving corrosion resistance. For this reason, it is preferable to have an Al content of 50% by mass or more. On the other hand, if the Al content in the plating film exceeds 65% by mass, the structure changes to one in which most of the Zn is solid-dissolved in α-Al, the dissolution reaction of the α-Al phase cannot be suppressed, and the corrosion resistance of the Al-Zn-Si-Mg plating deteriorates. Therefore, the Al content in the plating film must be 65% by mass or less, preferably 60% by mass or less.

[0023] The Si in the aforementioned plating film is added primarily to suppress the growth of Fe-Al and / or Fe-Al-Si interfacial alloy layers that form at the interface with the underlying steel sheet, thereby preventing deterioration of the adhesion between the plating film and the steel sheet. In fact, when a steel sheet is immersed in an Al-Zn plating bath containing Si, Fe on the surface of the steel sheet reacts with Al and Si in the bath to form an alloy, and Fe-Al and / or Fe-Al-Si intermetallic compound layers are formed at the underlying steel sheet / plating film interface. However, since the Fe-Al-Si alloy grows more slowly than the Fe-Al alloy, the higher the proportion of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer is suppressed. For this reason, the Si content in the plating film must be 1.0% by mass or more. On the other hand, if the Si content in the plating film exceeds 4.0% by mass, not only does the aforementioned effect of suppressing the growth of the interfacial alloy layer saturate, but the presence of an excess Si phase in the plating film promotes corrosion, so the Si content should be 4.0% or less. Furthermore, the Si content in the plating film is preferably 3.0% or less, from the viewpoint of suppressing the presence of an excess Si phase. In addition, from the viewpoint of easily satisfying the relationship (1) described below in relation to the Mg content, it is preferable that the Si content be 1.0 to 3.0% by mass.

[0024] The aforementioned plating film contains 1.0 to 10.0% Mg. By including Mg in the plating film, the aforementioned Si can be present in the form of an intermetallic compound of the Mg2Si phase, thereby suppressing the acceleration of corrosion. Furthermore, when Mg is included in the plating film, an intermetallic compound, the MgZn2 phase, is also formed in the plating film, resulting in an improved corrosion resistance. If the Mg content in the plating film is less than 1.0% by mass, sufficient corrosion resistance cannot be ensured because Mg is used for solid solution into the α-Al phase, which is the main phase, rather than for the formation of the intermetallic compounds (Mg2Si, MgZn2). On the other hand, if the Mg content in the plating film is too high, the effect of improving corrosion resistance saturates, and the processability decreases due to the weakening of the α-Al phase; therefore, the content should be 10.0% or less. Moreover, from the viewpoint of suppressing dross generation during plating formation and facilitating plating bath management, the Mg content in the plating film is preferably 5.0% by mass or less. Furthermore, in relation to the Si content, from the viewpoint of easily satisfying the relationship (1) described below, it is preferable to set the Mg content to 3.0 mass%, and from the viewpoint of compatibility with dross suppression, it is more preferable to set the Mg content to 3.0 to 5.0 mass%.

[0025] Furthermore, the plating film contains Zn and unavoidable impurities. Of these, the unavoidable impurities include Fe. This Fe is inevitably included in the plating film as a result of the elution of steel plates and equipment in the plating bath, and as a result of diffusion from the underlying steel plate during the formation of the interfacial alloy layer. The Fe content in the plating film is usually about 0.3 to 2.0 mass%. Other unavoidable impurities include Cr, Ni, and Cu. These components inevitably become present in the plating film when the underlying steel plate or stainless steel bath equipment dissolves into the plating bath, when they are present as impurities in the metal ingots that serve as the raw materials for the plating bath, and when the plated steel sheets are manufactured using pots and bath equipment that were used in the production of plated steel sheets that had these components intentionally added.

[0026] Furthermore, the molten Al-Zn-Si-Mg plated steel sheet of the present invention is characterized in that the Ni content in the unavoidable impurities is 0.010% by mass or less relative to the total mass of the plating film. Since Ni contained in the plating film may degrade the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet, the degradation of corrosion resistance can be suppressed by appropriately controlling the content of Al, Zn, Si, and Mg in the plating film as described above, and further suppressing the Ni content as an unavoidable impurity. From a similar viewpoint, it is preferable that the Ni content in the unavoidable impurities be 0.005% by mass or less relative to the total mass of the plating film.

[0027] Furthermore, if Ni is included in the aforementioned unavoidable impurities, Ni-based compounds may be present as impurities in the plating film of the molten Al-Zn-Si-Mg plated steel sheet. Here, the Ni-based compounds mainly refer to binary intermetallic compounds such as Ni-Al compounds, and ternary intermetallic compounds such as Ni-Al-Fe compounds. Examples of Ni-Al compounds include intermetallic compounds such as NiAl3, and examples of Ni-Al-Fe compounds include intermetallic compounds such as (Ni,Fe)Al3 in which some of the Ni in NiAl3 is replaced with Fe, but the compound is not limited to these. The presence of Ni-based compounds in the plating film can be confirmed, for example, by using a scanning electron microscope to observe the plating film from the surface or cross-section using secondary electron images or backscattered electron images, and then analyzing it by energy-dispersive X-ray spectroscopy (EDS). For example, by selecting 5 to 10 locations on a 100 μm cross-section of the plating, observing and performing elemental mapping analysis on each location at an accelerating voltage of 5 kV or less, and then performing point analysis on the areas where Ni is detected, the composition of the Ni-based content can be confirmed. This method is merely one example, and any method that can confirm the presence of Ni-based compounds is acceptable and is not particularly limited.

[0028] Furthermore, if the plating film contains a Ni-based compound, it is preferable that the major axis of the Ni-based compound is 4.0 μm or less. Ni-based compounds present in the aforementioned plating film can function as cathodes in corrosive environments, forming local galvanic cells with the surrounding solidified tissue, which can lead to a deterioration in corrosion resistance. In particular, if coarse Ni-based compounds are present in the aforementioned plating film, the molten Al-Zn-Si-Mg plating... The corrosion resistance of the plated steel sheet may be significantly reduced. Therefore, in order to obtain a hot-dip Al-Zn-Si-Mg plated steel sheet with superior corrosion resistance, it is effective to control the size of the Ni-based compounds contained as impurities in the plating film to a small size. Specifically, it is preferable to set the major axis of the Ni-based compounds to 4.0 μm or less, more preferably to 3.0 μm or less, and even more preferably to 2.0 μm or less. The major axis of the Ni-based compound can be measured, for example, by using a scanning electron microscope to observe the plating film in cross-section using a backscattered electron image, confirming that it is a Ni-based compound using EDS, and then observing a magnified backscattered electron image of the observation field containing the Ni-based compound. The major axis of the Ni-based compound is defined as the maximum major axis of the Ni-based compound confirmed within the observation field of the plating film.

[0029] Furthermore, when the plating film contains a Ni-based compound, it is also effective to reduce the amount of the Ni-based compound present, which acts as the starting point for corrosion, from the viewpoint of obtaining more stable and high corrosion resistance. Specifically, it is preferable to have 5 or fewer Ni-based compound particles in the plating film in the direction parallel to the surface of the underlying steel sheet, more preferably 2 or fewer, and most preferably 0 or none (absent). Therefore, by suppressing the amount of Ni-containing compounds present in the plating film, the deterioration of the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet can be more reliably suppressed. In order to obtain such a film structure (a film structure that does not contain Ni-based compounds), it is important to reduce the Ni content in the unavoidable impurities, specifically, to set the Ni content to 0.005% by mass or less relative to the total mass of the plating film. Furthermore, the number of Ni-based compound particles can be determined, for example, by using a scanning electron microscope to continuously observe a cross-section parallel to the surface of the underlying steel sheet with a plating film using backscattered electron images for a length of 1 mm or more, and then dividing the number of Ni-based compounds confirmed by EDS by the measured length (mm) to calculate the number of Ni-based compounds present within a 1 mm length range.

[0030] Furthermore, while there are no particular limitations on the total content of unavoidable impurities in the plating film, it is preferable to keep the total content at 5.0% by mass or less, as excessive impurities may affect various properties of the plated steel sheet.

[0031] From the viewpoint of being able to more stably improve corrosion resistance while controlling the concentrations of Al, Zn, Si, Mg, and Ni as an unavoidable impurity as described above, it is preferable that the diffraction intensities of Mg2Si and MgZn2 in the plating film, measured by X-ray diffraction, satisfy the following relationship (1). Mg2Si (111) / MgZn2(100)≦2.0 (1) Mg2Si (111): Diffraction intensity of the (111) plane (interplanar spacing d=0.3668 nm) of Mg2Si, MgZn2(100): Diffraction intensity of the (100) plane (interplanar spacing d=0.4510 nm) of MgZn2

[0032] As described above, in the molten Al-Zn-Si-Mg plated steel sheet of the present invention, it is important to control the proportion of intermetallic compounds such as Mg2Si and MgZn2 that are formed in the plating film due to the presence of Mg to a specific ratio. The effects of these on corrosion resistance are currently under investigation and many aspects remain unclear, but the following mechanism is hypothesized.

[0033] When a molten Al-Zn-Si-Mg plated steel sheet is exposed to a corrosive environment, the intermetallic compounds dissolve preferentially over the α-Al phase, resulting in a Mg-rich environment near the formed corrosion products. In such a Mg-rich environment, the formed corrosion products are less likely to decompose, and as a result, the protective effect of the plating film is presumed to be enhanced. Furthermore, since the protective effect of MgZn2 is greater than that of Mg2Si, it is considered effective to increase the proportion of MgZn2 in the intermetallic compounds present in the plating film.

[0034] Furthermore, it is preferable that the ratio of Mg2Si to MgZn2 in the plating film satisfies the relationship (1): Mg2Si (111) / MgZn2(100) ≤ 2.0, using the diffraction peak intensity obtained by X-ray diffraction. If the ratio of Mg2Si and MgZn2 in the plating film does not satisfy relationship (1), that is, if Mg2Si (111) / MgZn2(100) > 2.0, then there is a large amount of Mg2Si in the intermetallic compounds present in the plating film, making it difficult to obtain the aforementioned Mg-rich environment near the corrosion products, and thus the protective effect of the plating film may not be improved.

[0035] Here, in relation (1) above, Mg2Si (111) is the diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Mg2Si, and MgZn2 (100) is the diffraction intensity of the (100) plane (interplanar spacing d = 0.4510 nm) of MgZn2. As a method for measuring Mg2Si (111) and MgZn2 (100) by the aforementioned X-ray diffraction, a portion of the plating film is mechanically scraped off, and X-ray diffraction is performed on the powdered material (powder X-ray diffraction measurement method). For the measurement of diffraction intensity, the diffraction peak intensity of Mg2Si corresponding to the interplanar spacing d = 0.3668 nm and the diffraction peak intensity of MgZn2 corresponding to the interplanar spacing d = 0.4510 nm are measured, and the ratio of these can be calculated to obtain Mg2Si (111) / MgZn2 (100). Furthermore, the amount of plating film required for powder X-ray diffraction measurement (the amount of plating film to be removed) is 0.1g or more, and preferably 0.3g or more, from the viewpoint of accurately measuring Mg2Si (111) and MgZn2 (100). In addition, when removing the plating film, steel sheet components other than the plating film may be included in the powder, but these intermetallic compound phases are contained only in the plating film and do not affect the peak intensity mentioned above. Moreover, the reason for performing X-ray diffraction on the plating film in powder form is that if X-ray diffraction is performed on the plating film formed on the plated steel sheet, it is difficult to calculate the correct phase ratio due to the influence of the surface orientation of the solidification structure of the plating film.

[0036] Furthermore, in the molten Al-Zn-Si-Mg plated steel sheet of the present invention, it is preferable that the diffraction intensity of Si in the plating film, measured by X-ray diffraction, satisfies the following relationship (2), in order to control the concentrations of Al, Zn, Si, Mg, and Ni as an unavoidable impurity as described above, and to improve corrosion resistance more stably. Si (111) = 0 ···(2) Si (111): Diffraction intensity of the (111) plane (interplanar spacing d = 0.3135 nm) of Si Generally, in the dissolution reaction of Al alloys in aqueous solutions, the presence of the Si phase as a cathode site is known to promote the dissolution of the surrounding α-Al phase. Therefore, reducing the amount of Si phase is effective in suppressing the dissolution of the α-Al phase, and among these methods, creating a film without any Si phase (making the diffraction peak intensity of Si(111) zero), as shown in relation (2), is the best for stabilizing corrosion resistance. Furthermore, the method for measuring the diffraction peak intensity of the (111) plane of Si by X-ray diffraction can be the same as the method for measuring Mg2Si (111) and MgZn2 (100) described above.

[0037] Here, the method for satisfying the above-mentioned relationships (1) and (2) is not particularly limited. For example, in order to satisfy relationships (1) and (2), the relative abundance of Mg2Si, MgZn2, and Si (diffraction intensities of Mg2Si (111), MgZn2 (100), and Si (111)) can be controlled by adjusting the balance of the Si content, Mg content, and Al content in the plating film. The balance of the Si content, Mg content, and Al content in the plating film does not necessarily satisfy relationships (1) and (2) by setting the content ratio to a constant ratio; for example, it may be necessary to change the content ratio of Mg and Al depending on the Si content (mass%). In addition to adjusting the balance of Si, Mg, and Al content in the plating film, the diffraction intensities of Mg2Si(111), MgZn2(100), and Si(111) can be controlled to satisfy relationships (1) and (2) by adjusting the conditions during plating film formation (for example, cooling conditions after plating).

[0038] Furthermore, in the molten Al-Zn-Si-Mg steel sheet of the present invention, it is preferable that the plating film contains 0.01 to 1.0 mass% of Sr. By including Sr in the plating film, the occurrence of surface defects such as wrinkle-like unevenness defects can be more reliably suppressed, and a good surface appearance can be achieved. The aforementioned wrinkle-like defects are defects that appear as wrinkle-like irregularities on the surface of the plating film, and are observed as whitish streaks on the surface of the plating film. Such wrinkle-like defects are more likely to occur when a large amount of Mg is added to the plating film. Therefore, in the hot-dip galvanized steel sheet, by including Sr in the plating film, Sr is preferentially oxidized over Mg on the surface layer of the plating film, thereby suppressing the oxidation reaction of Mg and making it possible to suppress the occurrence of the aforementioned wrinkle-like defects.

[0039] Furthermore, in the molten Al-Zn-Si-Mg steel sheet of the present invention, it is preferable that the ratio of Mg2Si and MgZn2 in the plating film satisfies relation (1) and that the plating film contains 0.01 to 1.0 mass% of Sr. This is because the effect of improving the surface appearance due to Sr described above can be enjoyed to a greater extent. Although the reason for this is not clear, it is presumed that if the amount of Mg2Si in the plating film is high, oxidation of the plating surface layer is not easily suppressed in the first place, which affects the effect of improving the appearance when Sr is added. If the Sr content in the plating film is less than 0.01 mass%, it is difficult to obtain the effect of suppressing the occurrence of the wrinkle-like defects described above, and if the Sr content in the plating film exceeds 1.0 mass%, Sr may be excessively incorporated into the interface alloy layer, which may affect the plating adhesion and other aspects more than the effect of improving the appearance. Therefore, it is preferable that the Sr content in the plating film be 0.01 to 1.0 mass%.

[0040] Furthermore, the plating film, like the Mg mentioned above, can improve the stability of corrosion products and slow down the progression of corrosion. Therefore, it is preferable that it further contains one or more elements selected from Cr, Mn, V, Mo, Ti, Ca, Co, Sb, and B in a total amount of 0.01 to 10% by mass. The reason for setting the total content of the above-mentioned components to 0.01 to 10% by mass is that a sufficient corrosion delaying effect can be obtained without the effect becoming saturated.

[0041] Furthermore, the amount of the plating film applied is 45 to 120 g / m² per side, from the viewpoint of satisfying various characteristics. 2 Preferably, the amount of the plating film is 45 g / m². 2 In the above case, sufficient corrosion resistance can be obtained even for applications requiring long-term corrosion resistance, such as building materials, and the amount of the plating film attached is 120 g / m². 2 In the following cases, excellent corrosion resistance can be achieved while suppressing the occurrence of plating cracks during processing. From a similar viewpoint, the amount of plating film attached is 45-100 g / m². 2 It is preferable that it be so.

[0042] The amount of plating film attached can be determined, for example, by dissolving and peeling the plating film from a specific area using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013, and calculating the amount from the difference in steel plate weight before and after peeling. To determine the amount of plating attached to one side using this method, the plated surface of the non-target side can be sealed with tape to prevent exposure, and then the aforementioned dissolution process can be carried out.

[0043] Furthermore, the component composition of the plating film can be determined in the same way as the Ni content described above. This can be done by immersing the plating film in hydrochloric acid or the like to dissolve it, and then examining the solution using ICP emission spectrometry or atomic absorption spectrometry. This method is merely one example, and any method that can accurately quantify the component composition of the plating film is acceptable and is not particularly limited.

[0044] Furthermore, the plating film of the molten Al-Zn-Si-Mg plated steel sheet obtained according to the present invention has a composition that is approximately the same as that of the plating bath. Therefore, the composition of the plating film can be precisely controlled by controlling the composition of the plating bath.

[0045] Furthermore, the base steel sheet constituting the molten Al-Zn-Si-Mg plated steel sheet of the present invention is not particularly limited, and cold-rolled steel sheets, hot-rolled steel sheets, etc., can be used as appropriate depending on the required performance and specifications.

[0046] Furthermore, there are no particular limitations on the method for obtaining the base steel sheet. For example, in the case of hot-rolled steel sheets, those that have undergone a hot-rolling process and a pickling process can be used, and in the case of cold-rolled steel sheets, a cold-rolling process can be added to the manufacturing process. In addition, it is possible to go through a recrystallization annealing process or the like before the hot-dip galvanizing process in order to obtain the properties of the steel sheet.

[0047] (Method for manufacturing hot-dip Al-Zn-Si-Mg plated steel sheet) The present invention relates to a method for producing a molten Al-Zn-Si-Mg plated steel sheet, which comprises a molten Al-Zn-Si-Mg plated steel sheet having a plating film, wherein the formation of the plating film includes a molten plating step of immersing a base steel sheet in a plating bath having a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities. The aforementioned hot-dip galvanizing process is not particularly limited, except for the conditions of the galvanizing bath described later. For example, the base steel sheet can be manufactured by washing, heating, and immersing it in the galvanizing bath using a continuous hot-dip galvanizing system. In the heating process of the steel sheet, recrystallization annealing is performed to control the structure of the base steel sheet itself, and heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective to prevent oxidation of the steel sheet and to reduce the trace oxide film present on the surface.

[0048] Furthermore, as mentioned above, since the composition of the plated film is approximately the same as that of the plating bath, a plating bath containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities, can be used for the plating bath used in the molten plating process.

[0049] Furthermore, the present invention's method for manufacturing a molten Al-Zn-Si-Mg plated steel sheet is characterized by controlling the Ni content in the unavoidable impurities of the plating bath to 0.010% by mass or less relative to the total mass of the plating bath. As mentioned above, Ni contained in the plating film can degrade the corrosion resistance of the molten Al-Zn-Si-Mg plated steel sheet. Therefore, by appropriately controlling the content of Al, Zn, Si, and Mg in the plating bath and further suppressing the Ni content as an unavoidable impurity, the degradation of corrosion resistance can be suppressed. Furthermore, the Ni content in the plating bath as an unavoidable impurity must be controlled to 0.010% by mass or less relative to the total mass of the plating bath, and preferably to 0.005% by mass or less. If the Ni content in the plating bath exceeds 0.005% by mass, the corrosion resistance of the manufactured molten Al-Zn-Si-Mg plated steel sheet may deteriorate, and if it exceeds 0.010%, a significant deterioration in corrosion resistance may occur. There is no lower limit for the Ni content that adversely affects corrosion resistance.

[0050] The means for reducing the Ni content in the plating bath are not particularly limited. For example, since it is effective to suppress the elution of stainless steel bath equipment into the plating bath, it is preferable to treat the surface of the bath equipment with a thermal spray coating or the like. This is because the formation of the thermal spray coating or the like can impart corrosion resistance to the plating bath to the bath equipment, thereby suppressing the elution of the bath equipment into the plating bath. The type of thermal spray coating is not particularly limited, but a coating with heat resistance and corrosion resistance such as WC-based or MoB-based coatings can be selected. Furthermore, it is more effective to use bath equipment made of heat-resistant material that does not contain Ni. In this case, even if the bath equipment elutes, an increase in Ni content can be prevented.

[0051] Furthermore, as another means of reducing the Ni content in the plating bath, it is preferable to use a metal ingot with a low Ni content among impurities as the raw material for the plating bath. Furthermore, the pots and bath equipment used in the production of plated steel sheets with intentionally added Ni were molten aluminum. -It is also effective not to use it in the manufacture of Zn-Si-Mg plated steel sheets. This is because it prevents Ni-containing metal lumps adhering to the pot and the equipment in the plating bath from dissolving and mixing into the plating bath.

[0052] Furthermore, while the temperature of the plating bath is not particularly limited, it is preferable to set it in the temperature range of (melting point + 20°C) to 650°C. The reason the lower limit of the bath temperature is set to the melting point + 20°C is that in order to perform the molten plating process, it is necessary to raise the bath temperature above the solidification point, and setting it to the melting point + 20°C prevents solidification due to a localized drop in the bath temperature of the plating bath. On the other hand, the reason the upper limit of the bath temperature is set to 650°C is that if it exceeds 650°C, rapid cooling of the plating film becomes difficult, and there is a risk that the interfacial alloy layer formed between the plating film and the steel sheet will become thicker.

[0053] Furthermore, while there are no particular limitations on the temperature of the base steel plate that enters the plating bath (entry plate temperature), it is preferable to control it to within ±20°C of the temperature of the plating bath from the viewpoint of ensuring plating characteristics and preventing changes in the bath temperature during continuous hot-dip galvanizing operations.

[0054] Furthermore, the immersion time of the base steel plate in the plating bath is preferably 0.5 seconds or more. This is because if it is less than 0.5 seconds, there is a risk that a sufficient plating film may not be formed on the surface of the base steel plate. There is no particular upper limit to the immersion time, but since a longer immersion time may result in a thicker interfacial alloy layer formed between the plating film and the steel plate, it is more preferable to keep it within 8 seconds.

[0055] Furthermore, depending on the required performance, a coating can be formed on the hot-dip Al-Zn-Si-Mg plated steel sheet either directly or via an intermediate layer.

[0056] The method for forming the coating film is not particularly limited and can be appropriately selected according to the required performance. Examples of coating methods include roll coater coating, curtain flow coating, and spray coating. After applying a paint containing an organic resin, it is possible to form a coating film by heating and drying it using means such as hot air drying, infrared heating, or induction heating.

[0057] Furthermore, the intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the hot-dip galvanized steel sheet and the coating film. [Examples]

[0058] (Samples 1-62) Using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method as the base steel sheet, samples 1 to 62 of hot-dip galvanized steel sheets were prepared under the conditions shown in Table 1 by performing annealing and galvanizing treatments using a hot-dip galvanizing simulator manufactured by Resca Co., Ltd. The composition of the plating bath used in the production of the hot-dip galvanized steel sheets was varied within the range of Al: 5-75 mass%, Si: 0.0-4.5 mass%, Mg: 0-10 mass%, and Ni: 0.000-0.025 mass%, so that the composition of the plating film for each sample shown in Tables 2 and 3 would match. The bath temperature of the plating bath was set to 450°C for Al: 5 mass%, 480°C for Al: 15 mass%, 590°C for Al: 30-60 mass%, and 630°C for Al: over 60 mass%, so that the plate temperature of the base steel sheet upon plating penetration was the same as the plating bath temperature. Furthermore, for Al: 30-60 mass%, the plating process was carried out under conditions where the plate temperature was cooled to the 520-500°C temperature range in 3 seconds. Furthermore, the amount of plating film adhering to each side was 85±5 g / m² for samples 1-59. 2 In sample 60, the g / m² was 50±5 g / m² per side. 2 In sample 61, the density was 100±5 g / m² per side. 2 In sample 62, the g / m² was 125±5 g / m² per side. 2 It was controlled to achieve this.

[0059] (evaluation) Each sample of the hot-dip galvanized steel sheet obtained as described above was evaluated as follows. The evaluation results are shown in Table 1.

[0060] (1) Plating film (composition, amount of coating, Ni-based compound, X-ray diffraction intensity) For each plated sample, a 100 mm diameter hole was punched out, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013. The amount of plating film attached was calculated from the difference in mass of the sample before and after removal. The calculated amount of plating film attached is shown in Table 1. Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, the filtrate was analyzed by ICP emission spectroscopy to quantify components other than insoluble Si. Furthermore, the solid components were dried and ashed in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was then dissolved in hydrochloric acid, and the insoluble Si was quantified by ICP emission spectroscopy analysis of the solution. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from solid content analysis to the soluble Si concentration obtained from filtrate analysis. The resulting composition of the plating film is shown in Table 1. Furthermore, after shearing each sample to a size of 15 mm × 15 mm and embedding it in conductive resin so that the cross-section of the steel plate could be observed, mechanical polishing was performed. Then, using a scanning electron microscope (Carl Zeiss ULTRA55), continuous backscattered electron images were taken at a width of 100 μm under an acceleration voltage of 3 kV for a continuous cross-section of an arbitrarily selected plating film with a length of 2 mm or more in the direction parallel to the surface of the underlying steel plate. Furthermore, elemental mapping analysis (Al, Zn, Si, Mg, Fe, Sr, and Ni) of each cross-section was performed using an energy-dispersive X-ray spectrometer (Oxford Instruments Ultim Extreme) under an acceleration voltage of 3 kV within the same apparatus. For areas where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer under an acceleration voltage of 3 kV, and the substance was identified from the semi-quantitative values ​​of the obtained components. The major axis was measured for all Ni-based compounds confirmed in the observation field, and the largest major axis was determined. Furthermore, the number of Ni-based compound particles present in the observed continuous cross-section was counted and divided by the observed cross-sectional length (mm) to calculate the number of Ni-based compound particles per mm parallel to the surface of the underlying steel plate (particles / mm). For areas where high Ni intensity was detected in this analysis, point analysis was performed using the same spectrometer under an acceleration voltage of 3kV, and the substance was identified from the semi-quantitative values ​​of the obtained components. The analysis results are shown in Table 1. In addition, for each sample, after shearing to a size of 100 mm × 100 mm, the plating film on the evaluation symmetry plane was mechanically shaved until the base steel plate appeared. After thoroughly mixing the obtained powder, 0.3 g was taken out, and using an X-ray diffractometer ("SmartLab" manufactured by Rigaku Corporation), the X-ray used was Cu-Kα (wavelength = 1.54178 Å), removal of Kβ line: Ni filter, tube voltage: 40 kV, tube current: 30 mA, scanning speed: 4° / min, sampling interval: 0.020°, divergence slit: 2 / 3°, solar slit: 5°, detector: high-speed one-dimensional detector (D / teX Ultra), qualitative analysis of the above powder was performed. The intensity obtained by subtracting the base intensity from each peak intensity was taken as each diffraction intensity (cps), and the diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Mg2Si, the diffraction intensity of the (100) plane (interplanar spacing d = 0.4510 nm) of MgZn2, and the diffraction intensity of the (111) plane (interplanar spacing d = 0.3135 nm) of Si were measured. The measurement results are shown in Table 1.

[0061] (2) Corrosion resistance evaluation For each sample of the obtained hot-dip galvanized steel plate, after shearing to a size of 120 mm × 120 mm, the range of 10 mm from each edge of the evaluation target surface and the end face of the sample and the non-evaluation target surface were sealed with tape, and the one with the evaluation target surface exposed in a size of 100 mm × 100 mm was used as an evaluation sample. Note that 3 identical evaluation samples were prepared. For the 3 evaluation samples prepared as described above, a corrosion promotion test was carried out for all of them in the cycle shown in Figure 1. The corrosion promotion test started from wetting and was carried out until 300 cycles. After that, the corrosion weight loss of each sample was measured by the methods described in JIS Z 2383 and ISO8407 and evaluated according to the following criteria. The evaluation results are shown in Table 1. ◎: The corrosion weight loss of all 3 samples is all 45 g / m 2 Below ○: The corrosion weight loss of all 3 samples is all 95 g / m 2 Below ×: The corrosion weight loss of one or more samples exceeds 95 g / m 2 Exceed

[0062] (3) Surface appearance For each sample of the obtained hot-dip galvanized steel sheet, the surface of the galvanized film was observed visually. The observation results were then evaluated according to the following criteria. The evaluation results are shown in Table 1. ◎: No wrinkle-like defects were observed at all. ○: Wrinkle-like defects were observed only within a 50mm radius from the edge. ×: Wrinkle-like defects were observed outside the 50mm range from the edge.

[0063] (4) Processability Each sample of the obtained hot-dip galvanized steel sheet was sheared to a size of 70 mm x 150 mm, and then bent 180° (8T bend) with eight sheets of the same thickness sandwiched inside. After bending, cellophane tape (registered trademark) was firmly attached to the outer surface of the bent section and then peeled off. The surface condition of the galvanized film on the outer surface of the bent section and the presence or absence of adhesion (peeling) of the galvanized film on the surface of the tape used were visually observed, and the processability was evaluated according to the following criteria. The evaluation results are shown in Table 1. ○: No cracks or peeling are observed in the plating film. △: Cracks are present in the plating film, but no peeling is observed. ×: Both cracks and peeling are observed in the plating film.

[0064] (5) Bath stability During the production of each hot-dip galvanized steel sheet sample, the condition of the plating bath surface was visually inspected and compared with the plating bath surface used when producing hot-dip Al-Zn plated steel sheets (a bath surface free of Mg-containing oxides). The evaluation was performed according to the following criteria, and the evaluation results are shown in Table 1. ○: Similar to a molten Al-Zn plating bath (55% by mass Al - remainder Zn - 1.6% by mass bath) △: Contains more white oxides compared to the molten Al-Zn plating bath (55% by mass Al - remainder Zn - 1.6% by mass bath). ×: Formation of black oxides is observed in the plating bath.

[0065] [Table 1]

[0066] The results in Table 1 show that each sample of the present invention is superior to each sample of the comparative examples in terms of corrosion resistance, surface appearance, processability, and bath stability, all in a well-balanced manner. [Industrial applicability]

[0067] According to the present invention, it is possible to provide a hot-dip Al-Zn-Si-Mg plated steel sheet having stable and excellent corrosion resistance, and a method for manufacturing the same.

Claims

1. A hot-dip Al-Zn-Si-Mg plated steel sheet having a plating film, The aforementioned plating film has a composition containing Al: 45-65% by mass, Si: 1.0-4.0% by mass, and Mg: 1.0-10.0% by mass, with the remainder being Zn and unavoidable impurities. The Ni content in the aforementioned unavoidable impurities is 0.010% by mass or less relative to the total mass of the plating film. The aforementioned plating film contains a Ni-based compound, and the number of Ni-based compounds present in a direction parallel to the surface of the underlying steel sheet is 5 or less per mm. Mg in the aforementioned plating film 2 Si and MgZn 2 A hot-dip Al-Zn-Si-Mg plated steel sheet characterized in that the diffraction intensity obtained by X-ray diffraction satisfies the following relationship (1). Mg 2 Si(111) / MgZn 2 (100)≦2.0 ・・・(1) Mg 2 Si (111): Mg 2 Diffraction intensity of the (111) plane (interplanar spacing d = 0.3668 nm) of Si, MgZn 2 (100): MgZn 2 Diffraction intensity of the (100) plane (interplanar spacing d = 0.4510 nm)

2. The hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 1, characterized in that the major axis of the Ni-based compound is 4.0 μm or less.

3. The molten Al-Zn-Si-Mg plated steel sheet according to claim 1 or 2, characterized in that the diffraction intensity of Si in the plating film by X-ray diffraction satisfies the following relationship (2). Si (111)=0...(2) Si (111): Diffraction intensity of the (111) plane of Si (interplanar spacing d = 0.3135 nm)

4. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 3, characterized in that the plating film further contains Sr: 0.01 to 1.0% by mass.

5. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 4, characterized in that the Al content in the plating film is 50 to 60% by mass.

6. A hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 5, characterized in that the Si content in the plating film is 1.0 to 3.0% by mass.

7. The hot-dip Al-Zn-Si-Mg plated steel sheet according to any one of claims 1 to 6, characterized in that the Mg content in the plating film is 1.0 to 5.0% by mass.

Images