Hot-dip Zn-Al-Mg plated steel sheet
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- JFE STEEL CORP
- Filing Date
- 2024-11-15
- Publication Date
- 2026-06-30
Smart Images
Figure 0007882411000002 
Figure 0007882411000001
Abstract
Description
[Technical Field]
[0001] This invention relates to a hot-dip Zn-Al-Mg plated steel sheet with excellent corrosion resistance and plating adhesion. [Background technology]
[0002] Hot-dip zinc-plated steel sheets have excellent corrosion resistance and have therefore been widely used as rust-preventive steel sheets in fields such as automobiles, electrical equipment, and building materials. Generally, a hot-dip Zn-based plating film consists of an interfacial alloy layer at the interface with the underlying steel sheet and a main layer on top of the interfacial alloy layer. It exhibits superior corrosion resistance compared to cold-rolled or hot-rolled steel sheets, mainly due to the sacrificial corrosion protection ability of the Zn present in the main layer against Fe. Furthermore, when a typical cold-rolled or hot-rolled steel sheet is used as the base steel sheet, the aforementioned interfacial alloy layer contains Fe-Al or Fe-Zn alloys as constituent components, which are formed by the reaction of Fe in the base steel sheet with Zn or Al in the plating bath.
[0003] In recent years, in order to meet the market's need for high corrosion resistance, multi-component alloy plated steel sheets have been developed, such as molten Zn-Al-Mg plated steel sheets, which have Al, Mg, and Si added to Zn as components of the plating. For example, Patent Document 1 discloses a molten Zn-Al-Mg plated steel sheet in which the composition of the plating film consists of Al: 4.0 to 10% by weight, Mg: 1.0 to 4.0% by weight, and the remainder being Zn and unavoidable impurities. Furthermore, Patent Document 2 discloses a molten Zn-Al-Mg plated steel sheet in which the composition of the plating film consists of Al: 2-19% by weight, Mg: 1.0-10% by weight, Si: 0.01-2% by weight, with the remainder being Zn and unavoidable impurities, and the total content of Al and Mg being 20% by mass or less.
[0004] However, in the case of general molten Zn-Al-Mg plated steel sheets, such as those disclosed in Patent Documents 1 and 2, a complex solidification reaction occurs during the film formation process, resulting in a complex and non-uniform structure for the plated film. Due to its non-uniform structure, hot-dip Zn-Al-Mg plated steel sheets had a problem compared to conventional hot-dip Zn plated steel sheets: the adhesion between the base steel sheet and the plating film (hereinafter referred to as "plating adhesion"), in particular, the adhesion between the interfacial alloy layer and the main layer, was unstable and low. [Prior art documents] [Patent Documents]
[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-226865 [Patent Document 2] Japanese Patent Publication No. 2000-104154 [Overview of the project] [Problems that the invention aims to solve]
[0006] In view of these circumstances, the present invention aims to provide a hot-dip Zn-Al-Mg plated steel sheet that achieves a high level of both corrosion resistance and plating adhesion. [Means for solving the problem]
[0007] The inventors, in order to solve the above problems, conducted research and found that it is important not only to control the concentrations of Zn, Al, Mg, and Si in the composition of the plating film of a molten Zn-Al-Mg plated steel sheet, but also to control the structure of the plating film. They found that by forming needle-shaped inorganic compounds on the interfacial alloy layer present at the interface between the plating film and the underlying steel sheet when observing the cross-section of the plating film in the thickness direction, the adhesion between the main layer of the plating film and the interfacial alloy layer can be improved, thereby improving not only corrosion resistance but also plating adhesion.
[0008] This invention is based on the above findings, and its gist is as follows. 1. A hot-dip Zn-Al-Mg plated steel sheet having a plating film consisting of an interfacial alloy layer present at the interface with the base steel sheet and a main layer present on the interfacial alloy layer, The aforementioned plating film has a composition containing Al: 10-22% by mass, Si: 0.01-2% by mass, and Mg: 3-10% by mass, with the remainder being Zn and unavoidable impurities. A molten Zn-Al-Mg plated steel sheet, characterized in that, when the cross-section of the plating film is observed in the thickness direction, needle-shaped inorganic compounds having a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less are formed on the interface alloy layer. 2. The molten Zn-Al-Mg plated steel sheet according to claim 1, characterized in that, when the cross-section of the plating film is observed in the thickness direction, the needle-shaped inorganic compound extends from the surface of the interface alloy layer into the main layer. 3. The hot-dip Zn-Al-Mg plated steel sheet according to 1 or 2 above, characterized in that the needle-shaped inorganic compound contains Si. 4. The hot-dip Zn-Al-Mg plated steel sheet according to item 3, characterized in that the needle-shaped inorganic compound further contains Ni. 5. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2 above, characterized in that the interface alloy layer contains Ni. 6. The hot-dip Zn-Al-Mg plated steel sheet according to 1 or 2 above, characterized in that the plating film further contains a total of 0.1 to 5% by mass of one or more elements selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb, and Bi. [Effects of the Invention]
[0009] According to the present invention, it is possible to provide a hot-dip Zn-Al-Mg plated steel sheet that achieves a high level of both corrosion resistance and plating adhesion. [Brief explanation of the drawing]
[0010] [Figure 1] This figure shows a schematic, enlarged cross-section of the molten Zn-Al-Mg plated steel sheet of this embodiment. [Modes for carrying out the invention]
[0011] (Hot-dip Zn-Al-Mg coated steel sheet) As shown in Fig. 1, the hot-dip Zn-Al-Mg coated steel sheet of the present invention has a coating film 20 on a base steel sheet 10, and the coating film 20 is composed of an interfacial alloy layer 22 existing at the interface with the base steel sheet 10 and a main layer 21 existing on the interfacial alloy layer. And the coating film 20 contains Al: 10 to 22% by mass, Si: 0.01 to 2% by mass, and Mg: 3 to 10% by mass, and the balance consists of Zn and unavoidable impurities. Note that Fig. 1 shows a magnified cross-section of the hot-dip Zn-Al-Mg coated steel sheet of the present embodiment, but the dimensions and shapes of each component are schematically shown for the convenience of explanation and are different from the actual ones.
[0012] Zn, which is the main component of the coating film, imparts sacrificial corrosion protection ability to the coating film and is an element necessary to obtain excellent corrosion resistance. Since the content of Zn is in the coating layer composed of low specific gravity elements such as Al and Mg when considered in terms of atomic composition ratio, it is necessary to mainly use Zn in terms of atomic composition ratio. Therefore, the Zn content in the coating film needs to be 60% by mass or more, and preferably 70% by mass or more. The upper limit of the Zn content is the content of the balance other than the elements and impurities excluding Zn.
[0013] Al in the coating film forms an Al phase in the main layer and is an essential element to obtain excellent corrosion resistance. When the Al content of the coating film exceeds 5% by mass, an Al phase can be formed in the coating film, and the formation amount of the Al phase increases with the increase in the Al content. In order to obtain more stable and excellent corrosion resistance, it is necessary to form an Al phase in the coating film to a certain extent or more, and the Al content in the coating film may be 10% by mass or more. Therefore, the lower limit value of the Al concentration is 10% by mass. On the other hand, when the Al concentration in the coating film increases, the sacrificial corrosion resistance tends to deteriorate. Therefore, the upper limit value of the Al concentration needs to be 22% by mass or less. From the same perspective, the Al content in the plating film is preferably 12 to 20% by mass, more preferably 15 to 19% by mass.
[0014] Also, Si in the plating film is used to suppress abnormal growth of the Fe-Al-based interfacial alloy layer formed mainly at the interface with the base steel plate and to ensure the workability of the plating film. When the base steel plate is immersed in the molten Zn-Al-Mg-based plating bath containing Si, Fe on the surface of the base steel plate reacts with Al and Si in the bath to form an alloying reaction, and an intermetallic compound layer of Fe-Al-based and / or Fe-Al-Si-based is formed at the base steel plate / plating film interface. At this time, since the growth rate of the Fe-Al-Si-based alloy is slower than that of the Fe-Al-based alloy, the higher the ratio of the Fe-Al-Si-based alloy, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the plating film needs to be 0.01% by mass or more. On the other hand, when the Si content in the plating film exceeds 2% by mass, not only does the growth suppression effect of the above-mentioned interfacial alloy layer saturate, but corrosion is promoted due to the presence of excessive Si in the plating film. Therefore, the Si content is 2% by mass or less.
[0015] Furthermore, Mg in the plating film has a function of stabilizing the corrosion products formed during corrosion and is an essential element for obtaining excellent corrosion resistance. The effect of stabilizing this corrosion product can be obtained when the Mg content in the plating film is 3% by mass or more, and preferably 5% by mass or more for a more reliable effect. On the other hand, when the Mg content in the plating film exceeds 10% by mass, the plating film becomes hard and brittle, and the workability deteriorates. Therefore, the upper limit of the Mg content is 10% by mass. From the same perspective, the Mg content in the plating film is preferably 5 to 8% by mass, more preferably 6 to 8% by mass.
[0016] The plating film contains 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 0.1 to 0.5% by mass.
[0017] Furthermore, the plating film preferably contains, if necessary, one or more elements selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb, and Bi, in a total of 0.1 to 5% by mass. These elements have the effect of improving the stability of corrosion products when the plating film corrodes, thereby delaying the progression of corrosion, and also stabilizing the spangle size on the plating surface, thereby improving the surface appearance.
[0018] As shown in Figure 1, the plating film consists of an interface alloy layer 22 present at the interface with the base steel plate 10 and a main layer 21 present on top of the interface alloy layer 22. Note that Figure 1 schematically shows the cross-sections of the base steel plate 10, the main layer 21, and the interface alloy layer 22 for the sake of explanation, and the actual shapes and dimensions may differ from those shown in Figure 1.
[0019] The aforementioned interfacial alloy layer is formed when the underlying steel sheet reacts with bath components such as Zn, Al, Mg, and Si in the plating bath during the plating process, and is generally an Fe-Al and / or Fe-Al-Si intermetallic compound. Furthermore, when using hot-rolled steel sheets or high-tensile steel sheets with low wettability for plating as the base steel sheet, pre-plating of Ni or Fe may be applied to the base steel sheet before the plating process to ensure wettability. In particular, when a base steel sheet with Ni pre-plating is used, an intermetallic compound containing Ni, such as Ni-Al and / or Fe-Ni-Al, is formed as an interfacial alloy layer.
[0020] Furthermore, when the interface alloy layer exists with an average film thickness of 0.1 to 1 μm, a stable main layer can be formed on the interface alloy layer. If the average film thickness is less than 0.1 μm, the interface alloy layer may not form over the entire plating film, meaning that the substrate steel plate and the plating bath may not react, which may prevent stable plating adhesion and film formation. On the other hand, if the average film thickness exceeds 1 μm, the interface alloy layer may crack during processing, causing plating peeling. Therefore, it is preferable that the average film thickness of the interface alloy layer be 0.1 to 1 μm.
[0021] Furthermore, as shown in Figure 1, the main layer undergoes solidification of the plating bath components that were not consumed in the formation of the interface alloy layer 22, thereby forming mainly an Al phase, a Zn phase, and MgZn2.
[0022] The Al phase is a structure necessary to obtain stable and excellent corrosion resistance, and when the cross-section of the plating film is observed in the thickness direction, the area ratio occupied by the Al phase is preferably 30% or more, and more preferably 40% or more.
[0023] Furthermore, when the plating film corrodes, MgZn2 preferentially dissolves in the initial stages, stabilizing the corrosion products that form. Also, because MgZn2 is a hard intermetallic compound, its presence in the main plating layer can improve the scratch resistance of the plating film. To stably obtain all of these effects, it is preferable that the area ratio of MgZn2 in the main layer when observing the cross-section of the plating film in the thickness direction is 10% or more, and more preferably 30% or more. Furthermore, the Zn phase, together with the MgZn2, primarily exhibits sacrificial corrosion protection against Fe, thereby improving the corrosion resistance of the exposed Fe end face. To stably obtain such an effect, it is preferable that, when observing the cross-section of the plating film in the thickness direction, the combined area ratio of the Zn phase and the MgZn2 in the main layer is 30% or more.
[0024] Furthermore, the molten Zn-Al-Mg plated steel sheet of the present invention is characterized in that, when the cross-section of the plating film is observed in the thickness direction, needle-shaped inorganic compounds having a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less are formed on the interface alloy layer. As shown in Figure 1, when the plated film 20 is observed in cross-section in the thickness direction, the needle-shaped inorganic compound 23 is formed on the interface alloy layer 22, which allows an anchoring effect to be exerted between the interface alloy layer 22 and the main layer 21. Therefore, it is possible to obtain excellent corrosion resistance as a molten Zn-Al-Mg plated steel sheet while also achieving a high level of plating adhesion.
[0025] The major axis of the needle-shaped inorganic compound 23 is determined by observing the cross-section of the plating film 20 in the thickness direction over a range of 2 mm or more in a direction parallel to the surface of the underlying steel plate, as shown in Figure 1, and measuring the major axis L of 10 or more randomly selected needle-shaped inorganic compounds 23, and averaging the result. The minor axis of the needle-shaped inorganic compound 23 is determined by measuring the minor axis D of each compound over the same measurement range and number of samples as the major axis, and averaging the result. If the major axis of the needle-shaped inorganic compound 23 is less than 1 μm, a sufficient anchoring effect may not be obtained, and the desired plating adhesion may not be achieved. On the other hand, if the major axis of the needle-shaped inorganic compound 23 is too large, the needle-shaped inorganic compound 23 penetrates deeply into the main layer 21, which may worsen the processability and corrosion resistance of the processed area. For this reason, the major axis of the needle-shaped inorganic compound 23 is preferably 10 μm or less, and more preferably 5 μm or less. Furthermore, the cross-sectional observation of the plating film 20 in the thickness direction is not particularly limited as long as it is possible to observe the presence or absence of the needle-shaped inorganic compound 23 and the major axis L and minor axis D. For example, it can be observed and measured by SEM-EDX (energy-dispersive X-ray analysis using a scanning electron microscope).
[0026] As shown in Figure 1, the aspect ratio of the needle-shaped inorganic compound 23 is the ratio of the minor axis to the major axis (minor axis D / major axis L). If the aspect ratio of the needle-shaped inorganic compound 23 exceeds 0.2, a sufficient anchoring effect may not be obtained, and the desired plating adhesion may not be achieved. On the other hand, if the aspect ratio of the needle-shaped inorganic compound 23 is less than 0.05, the needle-shaped inorganic compound 23 penetrates deeply into the main layer 21, which may worsen the processability and corrosion resistance of the processed part. From a similar viewpoint, the aspect ratio of the needle-shaped inorganic compound 23 is preferably 0.05 to 0.2, and more preferably 0.10 to 0.15. Furthermore, it is preferable that the average aspect ratio (average aspect ratio) obtained by observing the cross-section of the plating film 20 in the thickness direction over a range of 2 mm or more in a direction parallel to the surface of the underlying steel plate and randomly selecting 10 or more of the aspect ratios of the needle-shaped inorganic compounds 23 is 0.2 or less. When the average aspect ratio of the needle-shaped inorganic compounds 23 is 0.2 or less, stable and excellent adhesion can be obtained. On the other hand, if the average aspect ratio of the needle-shaped inorganic compounds 23 is less than 0.05, the needle-shaped inorganic compounds 23 penetrate deeply into the interior of the main layer 21, which may worsen the processability and corrosion resistance of the processed part. From a similar viewpoint, the average aspect ratio of the needle-shaped inorganic compounds 23 is preferably 0.05 to 0.2, and more preferably 0.10 to 0.15.
[0027] Furthermore, as shown in Figure 1, it is preferable that the needle-shaped inorganic compound 23 extends from the surface of the interface alloy layer 22 into the main layer 21 when the plated film 22 is observed in cross-section in the thickness direction. This is because the needle-shaped inorganic compound 23 increases the adhesive strength between the interface alloy layer 22 and the main layer 21, thereby obtaining better plating adhesion. In this invention, it is preferable that when the cross-section of the plating film 22 in the thickness direction is observed, it can be confirmed that the needle-shaped inorganic compound 23 extends from the surface of the interface alloy layer 22 into the main layer 21. However, in reality, it is thought that most of the needle-shaped inorganic compound 23 extends from the surface of the interface alloy layer 22.
[0028] The constituent components of the needle-shaped inorganic compound are not particularly limited as long as they are inorganic compounds having the shape described above, but more specifically, they are preferably Si-based compounds containing Si. Since the aforementioned Si is abundant near the interface alloy layer, it becomes a component of the needle-shaped inorganic compound and easily takes on a shape with a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less. Therefore, plating adhesion can be improved more reliably.
[0029] Furthermore, it is preferable that the components of the needle-shaped inorganic compound further contain Ni in addition to the Si mentioned above. When Ni is included in the interface alloy layer, the needle-shaped inorganic compound extending from the interface alloy layer will contain Ni. By including Ni in the needle-shaped inorganic compound, integration with the interface alloy layer is promoted, and better plating adhesion can be obtained.
[0030] Furthermore, the amount of the plating film applied is 30 to 300 g / m² per side, from the viewpoint of satisfying various characteristics. 2 It is preferable that the amount of the plating film attached 30 g / m 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 300 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 50 to 150 g / m². 2 It is preferable that it be so.
[0031] 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.
[0032] Furthermore, as shown in Figure 1, the molten Zn-Al-Mg plated steel sheet of the present invention has a plating film 20 formed on the base steel sheet 10, but if necessary, an intermediate layer or a coating film can be further formed on the plating film. The type of coating film and the method of forming the coating film are not particularly limited and can be appropriately selected according to the required performance. For example, methods such as roll coater coating, curtain flow coating, and spray coating can be used. 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. Furthermore, the intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the molten Zn-Al-Mg plated steel sheet and the coating film. Examples include a chemical conversion coating and a primer such as an adhesive layer. The chemical conversion coating can be formed, for example, by a chromate treatment or chromium-free chemical conversion treatment, which involves applying a chromate treatment solution or a chromium-free chemical conversion treatment solution and drying it at a steel sheet temperature of 80 to 300°C without washing with water. These chemical conversion coatings may be single-layer or multi-layer, and in the case of multi-layer coatings, multiple chemical conversion treatments may be performed sequentially.
[0033] (Method for manufacturing molten zinc-al-magnesium plated steel sheets) The method for producing a molten Zn-Al-Mg plated steel sheet according to the present invention is not particularly limited. However, the plating film of the molten Zn-Al-Mg plated steel sheet obtained by the present invention is, overall, approximately the same as the composition of the plating bath. Therefore, the present invention includes a step of forming the plating film on the base steel sheet using a plating bath whose composition is controlled to contain Al: 10-22% by mass, Si: 0.01-2% by mass, and Mg: 3-10% by mass, with the remainder being Zn and unavoidable impurities.
[0034] Furthermore, the process for forming the plating film is not particularly limited to the composition of the plating bath described above. For example, the base steel sheet can be manufactured in a continuous hot-dip galvanizing facility by washing, heating, and immersing it in a plating bath. 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 in preventing oxidation of the steel sheet and reducing the trace oxide film present on the surface.
[0035] Furthermore, while the temperature of the plating bath is not particularly limited, it is preferable to set it in the range of (melting point + 20°C) to 550°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 550°C is that if it exceeds 550°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.
[0036] Furthermore, there are no particular limitations on the method for forming the needle-shaped inorganic compound on the interface alloy layer. For example, it can be formed by adding the needle-shaped inorganic compound to the plating bath and performing a hot-dip plating treatment. In this case, it is preferable that the needle-shaped inorganic compound to be added has a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less.
[0037] Furthermore, the base steel sheet constituting the Zn-Al-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. There are also no particular restrictions on the base steel sheet. 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.
[0038] Furthermore, a pre-plated steel sheet may be used as the base steel sheet. The pre-plated steel sheet is plated, for example, by an electrolytic treatment method or a displacement plating method. In the electrolytic treatment method, the base steel sheet is immersed in a sulfuric acid bath or chloride bath containing metal ions of various pre-plating components and subjected to electrolytic treatment. In the displacement plating method, the base steel sheet is immersed in an aqueous solution containing metal ions of various pre-plating components and whose pH has been adjusted with sulfuric acid, and the metal is deposited by displacement. A typical example of a pre-plated steel sheet is a Ni pre-plated steel sheet. Furthermore, when a Ni pre-plated steel sheet is subjected to hot-dip galvanizing treatment in a bath containing the aforementioned needle-shaped inorganic compound, the needle-shaped inorganic compound is more likely to extend from the surface of the interfacial alloy layer into the main plating layer, improving the adhesion of the resulting molten Zn-Al-Mg plated steel sheet, which is particularly preferable.
[0039] Furthermore, in the method for manufacturing molten Zn-Al-Mg plated steel sheets of the present invention, in addition to the plating film formation step and the heating and cooling step after plating film formation described above, it is possible to appropriately carry out steps that are normally used in plated steel sheets. [Examples]
[0040] [Samples 1-3] (Manufacturing Method A:) 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 3 of molten Zn-Al-Mg plated steel sheets were prepared under the conditions shown in Table 1 by performing annealing and plating treatments using a hot-dip plating simulator manufactured by Resca Co., Ltd. Table 1 shows the composition and temperature of the plating bath used in the production of the molten Zn-Al-Mg plated steel sheet, as well as the composition and amount of plating film attached to each sample.
[0041] [Samples 4-9] (Manufacturing Method B:) Using a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method as the base steel sheet, samples 4 to 9 of hot-dip Zn-Al-Mg plated steel sheets were prepared under the conditions shown in Table 1 by performing annealing and hot-dip plating treatments using a hot-dip plating simulator manufactured by Resca Co., Ltd. Table 1 shows the composition and temperature of the plating bath used in the production of the molten Zn-Al-Mg plated steel sheet, as well as the composition and amount of plating film attached to each sample. Furthermore, for samples 4-9, needle-shaped inorganic compounds were added to the plating bath at a concentration of 0.1% of the total weight of the plating bath. The types, average major axis, and aspect ratio of the needle-shaped inorganic compounds are shown in Table 1.
[0042] [Samples 10-14] (Manufacturing Method C:) Using a base steel sheet made of 0.8 mm thick cold-rolled steel sheet manufactured by a conventional method and coated with Ni pre-plating, samples 10 to 14 of molten Zn-Al-Mg plated steel sheets were prepared under the conditions shown in Table 1 by performing annealing and molten plating treatments using a molten plating simulator manufactured by Resca Co., Ltd. For the Ni pre-plating treatment of cold-rolled steel sheets, a plating bath with a concentration of NiSO4·6H2O at 300 g / L, H3BO3 at 40 g / L, and Na2SO4 at 100 g / L, with a pH of 2.7, was used, at a bath temperature of 60°C, and a current density of 50 A / dm². 2 Under these conditions, the amount of Ni attached is 1 g / m 2 It was controlled to achieve this. Table 1 also shows the composition and temperature of the plating bath used in the production of the molten Zn-Al-Mg plated steel sheet, as well as the composition and amount of plating film attached to each sample. Furthermore, for samples 10-14, needle-shaped inorganic compounds were added to the plating bath at a concentration of 0.1% of the total weight of the plating bath. The types, average major diameters, and aspect ratios of the needle-shaped inorganic compounds are shown in Table 1.
[0043] <Rating> Each sample of the obtained molten Zn-Al-Mg plated steel sheet was evaluated as follows. The evaluation results are shown in Table 1.
[0044] (1) Needle-shaped inorganic compounds For each sample of the fabricated molten Zn-Al-Mg plated steel sheet, cross-sectional observation and analysis were performed at a random location using energy-dispersive X-ray spectroscopy (SEM-EDX) with a scanning electron microscope. Table 1 shows the results of measurements and calculations for each sample, regarding the presence or absence of needle-shaped inorganic compounds observed in the cross-section in the thickness direction of the plating film, the components contained in the needle-shaped inorganic compounds, the average size of the needle-shaped inorganic compounds (major axis, aspect ratio), and the presence or absence of needle-shaped inorganic compounds extending from the interfacial alloy layer.
[0045] (2) Corrosion resistance evaluation For each sample of molten Zn-Al-Mg plated steel sheet, after shearing to a size of 70 mm × 120 mm, the area within 10 mm from each edge of the surface to be evaluated, as well as the end face of the sample and the surface not to be evaluated, were sealed with tape, leaving the surface to be evaluated exposed to a size of 50 mm × 100 mm. This exposed surface was used as the evaluation sample. The evaluation samples were subjected to the Japanese Automotive Standards Combined Cycle Test (JASO-CCT). The test started with an accelerated corrosion test in a wet state and continued for 90 cycles. Subsequently, the corrosion loss of each sample was measured according to the methods described in JIS Z 2383 and ISO 8407, and evaluated according to the following criteria. The evaluation results are shown in Table 1. ◎: Corrosion loss of 60g / m 2 below ○: Corrosion reduction is 80 g / m 2 or less ×: Corrosion reduction exceeds 80 g / m 2 exceeds
[0046] (3) Plating adhesion For each sample of the obtained molten Zn - Al - Mg alloy plated steel sheet, after shearing to a size of 70 mm × 100 mm, 180° adhesion bending (0T bending) was performed so that a vertex of 100 mm was obtained. Then, after strongly attaching cellophane (registered trademark) to the outer surface of the bent part after bending, it was peeled off. And the damage form of the plating film was confirmed by observing the outer surface (vertex part) of the bent part with an optical microscope (DSX1000 manufactured by OLYMPUS) under the condition of a magnification of 30 times, and the plating adhesion was evaluated according to the following criteria. The evaluation results are shown in Table 1. ◎: No peeling of the plating film (no occurrence of cracks or only occurrence of cracks) ○: The plating film is slightly peeled (the total diameter of the peeled parts is less than 5 mm) ×: The plating film is clearly peeled (the total diameter of the peeled parts is 5 mm or more)
[0047] [[ID=2l]]
Table 1
[0048] From the results in Table 1, it can be seen that each sample of the present invention example is excellently balanced in corrosion resistance and plating adhesion compared to each sample of the comparative example.
Industrial Applicability
[0049] According to the present invention, a molten Zn - Al - Mg alloy plated steel sheet in which corrosion resistance and plating adhesion are compatible at a high level can be provided. <s'
Explanation of Signs
[0050] 10 Base steel sheet 20 Plating film [[ID=4B]]21 Main layer 22 Interface alloy layer 23 Acicular inorganic compound L Longest axis of needle-shaped inorganic compound D Needle-shaped inorganic compound short diameter
Claims
1. A hot-dip Zn-Al-Mg plated steel sheet having a plating film comprising an interface alloy layer present at the interface with the base steel sheet and a main layer present on the interface alloy layer, The aforementioned plating film has a composition containing Al: 10 to 22% by mass, Si: 0.01 to 2% by mass, and Mg: 3 to 10% by mass, with the remainder being Zn and unavoidable impurities. A molten Zn-Al-Mg plated steel sheet, characterized in that, when the plated film is observed in cross-section in the thickness direction, a needle-shaped inorganic compound having a major axis of 1 μm or more and an aspect ratio (minor axis / major axis) of 0.2 or less is formed on the interface alloy layer, and the needle-shaped inorganic compound extends from the surface of the interface alloy layer into the main layer.
2. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1, characterized in that the needle-shaped inorganic compound contains Si.
3. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that the needle-shaped inorganic compound further contains Ni.
4. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that the interface alloy layer contains Ni.
5. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that the plating film further contains a total of 0.1 to 5% by mass of one or more elements selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Co, Ni, Sr, In, Sn, Sb, Ce, Pb, and Bi.