TW202526056A - Hot dip plated Zn-Al-Mg steel plate - Google Patents

Abstract

The object is to provide a hot-dip plated Zn-Al-Mg steel plate which has both high corrosion resistance and coating adhesion. In order to achieve the above object, the present invention is a hot-dip plated Zn-Al-Mg steel plate having a coating film (20). The coating film (20) is composed of an interface alloy layer (22) existing at the interface with the base steel plate (10) and a main layer (21) existing on the interface alloy layer (22). The coating film (20) is characterized in that the coating film (20) has a lower The composition comprises Al: 10-22 mass%, Si: 0.01-2 mass% and Mg: 3-10 mass%, with the remainder being Zn and inevitable impurities. When observing the cross section in the thickness direction of the coating film (20), a needle-shaped inorganic compound (23) having a length of 1 μm or more and an aspect ratio (minor diameter/long diameter) of 0.2 or less is formed on the interface alloy layer (22).

Description

Hot dip plated Zn-Al-Mg steel plate

The present invention relates to a hot-dip plated Zn-Al-Mg steel sheet with excellent corrosion resistance and coating adhesion.

Hot-dip zinc-coated steel has been widely used as rust-proof steel in the automotive, electrical, and building materials industries due to its excellent corrosion resistance. Typically, a hot-dip zinc-coated coating consists of an interface alloy layer at the interface with the base steel sheet and a main layer located above the interface alloy layer. The zinc, primarily present in the main layer, sacrifices its corrosion resistance to the iron present in the coating, resulting in superior corrosion resistance compared to cold-rolled or hot-rolled steel. Furthermore, when a conventional cold-rolled steel plate or hot-rolled steel plate is used as the base steel plate, the interface alloy layer comprises an Fe-Al alloy or Fe-Zn alloy formed by the reaction between the Fe of the base steel plate and the Zn or Al components of the coating bath.

In recent years, to meet market demand for high corrosion resistance, multi-element alloy-coated steel sheets have been developed, such as hot-dip Zn-Al-Mg steel sheets, which contain Al, Mg, and Si as coating components in addition to Zn. For example, Patent Document 1 discloses a hot-dip Zn-Al-Mg steel sheet whose coating composition is Al: 4.0-10% by weight, Mg: 1.0-4.0% by weight, with the remainder consisting of Zn and unavoidable impurities. Patent Document 2 discloses a hot-dip Zn-Al-Mg steel plate, wherein the coating composition is Al: 2-19 wt%, Mg: 1.0-10 wt%, Si: 0.01-2 wt%, with the remainder consisting of Zn and unavoidable impurities, with the combined Al and Mg contents being set to 20 wt% or less.

However, as disclosed in Patent Documents 1 and 2, in conventional hot-dip Zn-Al-Mg steels, complex solidification reactions occur during film formation, resulting in a complex and non-uniform coating structure. Due to this non-uniform structure, hot-dip Zn-Al-Mg steels suffer from poor adhesion between the base steel and the coating (hereinafter referred to as "coating adhesion"), particularly between the interface alloy layer and the main layer, compared to conventional hot-dip Zn-based steels. Prior Art Patent

Patent Document 1: Japanese Patent Application Publication No. 10-226865 Patent Document 2: Japanese Patent Application Publication No. 2000-104154

Problem to be Solved by the Invention In light of these circumstances, the present invention aims to provide a hot-dip Zn-Al-Mg-based steel sheet that achieves both high corrosion resistance and high plating adhesion. Means for Solving the Problem

The inventors of the present invention conducted intensive research to address the aforementioned issues, focusing on the composition of the coating film for hot-dip Zn-Al-Mg steel sheets. The key is to control not only the concentrations of Zn, Al, Mg, and Si, but also the structure of the coating film. They discovered that, when observing a cross-section of the coating film in the thickness direction, the formation of needle-shaped inorganic compounds on the interfacial alloy layer at the interface between the coating film and the base steel sheet improves the adhesion between the main layer of the coating film and the interfacial alloy layer. This improves not only corrosion resistance but also coating adhesion.

This invention was completed based on the above knowledge and insights, and its gist is as follows. 1. A hot-dip Zn-Al-Mg steel plate having a coating film, wherein the coating film comprises an interface alloy layer at the interface with the base steel plate and a main layer located above the interface alloy layer. The coating film comprises: The coating film has a composition comprising 10-22% by mass of Al, 0.01-2% by mass of Si, and 3-10% by mass of Mg, with the remainder consisting of Zn and unavoidable impurities. When observed in a cross-section through the thickness direction of the coating film, a needle-shaped inorganic compound having a length of 1 μm or greater and an aspect ratio (minor diameter/major diameter) of 0.2 or less is formed on the interface alloy layer. 2. The hot-dip Zn-Al-Mg steel plate according to 1 above, wherein, when observed in a cross-section in the thickness direction of the coating, 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 steel plate according to 1 or 2 above, wherein the needle-shaped inorganic compound contains Si. 4. The hot-dip Zn-Al-Mg steel plate according to 3 above, wherein the needle-shaped inorganic compound further contains Ni. 5. The hot-dip Zn-Al-Mg steel plate according to 1 or 2 above, wherein the interface alloy layer contains Ni. 6. The hot-dip Zn-Al-Mg-based steel sheet according to 1 or 2 above, wherein the coating further contains 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

According to the present invention, a hot-dip plated Zn-Al-Mg steel sheet can be provided, which achieves both high corrosion resistance and high plating adhesion.

Embodiments of the Invention (Hot-Dip Zn-Al-Mg Steel Plate) As shown in Figure 1, the hot-dip Zn-Al-Mg steel plate of the present invention comprises a coating 20 formed on a base steel plate 10. The coating 20 comprises an interface alloy layer 22 at the interface with the base steel plate 10 and a main layer 21 formed on the interface alloy layer. The coating 20 has a composition comprising 10-22% by mass of Al, 0.01-2% by mass of Si, and 3-10% by mass of Mg, with the remainder consisting of Zn and unavoidable impurities. Furthermore, Figure 1 is an enlarged, schematic illustration of a cross-section of a hot-dip plated Zn-Al-Mg steel plate according to this embodiment. However, the dimensions and shapes of the various components are shown schematically for ease of explanation and differ from the actual dimensions.

Zn, the main component of the coating, is an essential element for imparting corrosion resistance to the coating, achieving excellent corrosion resistance. Considering the Zn content in terms of atomic composition, since the coating is composed of elements with low specific gravity, such as Al and Mg, the Zn content must be primarily Zn. Therefore, the Zn content in the coating must be 60% by mass or higher, preferably 70% by mass or higher. Furthermore, the upper limit of the Zn content is the remaining content excluding elements other than Zn and impurities.

Al in the coating is essential for forming an Al phase in the main layer and achieving excellent corrosion resistance. If the Al content of the coating exceeds 5% by mass, an Al phase may form in the coating, and the amount of Al phase formed increases with increasing Al content. To achieve more stable and excellent corrosion resistance, a certain level of Al phase must be formed in the coating. Setting the Al content in the coating to 10% by mass or higher suffices. Therefore, the lower limit of Al concentration is set to 10% by mass. On the other hand, increasing the Al concentration in the coating tends to compromise corrosion resistance. Therefore, the upper limit of Al concentration should be set to 22% by mass or lower. Based on the same viewpoint, the Al content in the aforementioned coating film is preferably 12-20% by mass, more preferably 15-19% by mass.

Furthermore, the Si in the aforementioned coating is primarily used to suppress the abnormal growth of the Fe-Al interfacial alloy layer formed at the interface with the base steel plate, thereby ensuring the workability of the coating. If the base steel plate is immersed in the aforementioned Si-containing hot-dip Zn-Al-Mg bath, the Fe on the surface of the base steel plate undergoes an alloying reaction with the Al or Si in the bath, forming a Fe-Al and/or Fe-Al-Si intermetallic compound layer at the base steel plate/coating interface. At this time, since the Fe-Al-Si alloy grows slower than the Fe-Al alloy, the higher the ratio of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer can be suppressed. Therefore, the Si content in the aforementioned coating must be set to 0.01% by mass or more. On the other hand, if the Si content in the coating film exceeds 2% by mass, not only will the effect of suppressing the growth of the interface alloy layer be saturated, but the presence of excess Si phase in the coating film will promote corrosion. Therefore, the Si content is set to 2% by mass or less.

Furthermore, Mg in the coating stabilizes corrosion products formed during corrosion and is an essential element for achieving excellent corrosion resistance. To achieve this corrosion product stabilization effect, the Mg content in the coating must be 3% by mass or greater, and for more reliable results, it is preferably 5% by mass or greater. On the other hand, if the Mg content in the coating exceeds 10% by mass, the coating becomes harder and more brittle, impairing workability. Therefore, the upper limit of the Mg content is set at 10% by mass. Based on the same considerations, the Mg content in the coating is preferably 5% to 8% by mass, and more preferably 6% to 8% by mass.

Furthermore, the coating film contains unavoidable impurities. Among these unavoidable impurities is Fe. This Fe is inevitably contained in the coating film due to elution from the steel sheet or bath equipment into the coating bath, and is also inevitably contained in the coating film due to diffusion from the base steel sheet during formation of the interface alloy layer. The Fe content in the coating film is typically approximately 0.1 to 0.5% by mass.

The coating film may further contain, if necessary, 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. These elements can improve the stability of corrosion products during corrosion of the coating film, thereby delaying the progression of corrosion, and can also stabilize the size of zinc spangles on the coating surface, thereby improving the surface appearance.

Furthermore, as shown in Figure 1 , the coating film is composed of an interface alloy layer 22 at the interface with the base steel plate 10 and a main layer 21 located above the interface alloy layer 22. Furthermore, Figure 1 schematically illustrates the cross-sections of the base steel plate 10, the main layer 21, and the interface alloy layer 22 for ease of explanation; the actual shapes and dimensions may differ from those shown in Figure 1 .

The aforementioned interface alloy layer is formed during the plating process by the reaction of the base steel sheet with bath components such as Zn, Al, Mg, and Si in the plating bath. It is generally an Fe-Al and/or Fe-Al-Si intermetallic compound. In addition, when using hot-rolled steel or high-tension steel with low wettability as the base steel sheet, the base steel sheet is sometimes pre-coated with Ni or Fe to ensure wettability before the plating process. In particular, when the base steel sheet is pre-coated with Ni, a Ni-Al and/or Fe-Ni-Al intermetallic compound containing Ni is formed as the interface alloy layer.

Furthermore, when the interface alloy layer has an average thickness of 0.1 to 1 μm, a stable main layer can be formed on the interface alloy layer. If the average thickness is less than 0.1 μm, the interface alloy layer will not form throughout the entire coating film, meaning that the base steel plate and the coating bath will not react, resulting in unstable coating adhesion and film formation. On the other hand, if the average thickness exceeds 1 μm, there is a risk of cracking the interface alloy layer during processing, causing the coating to peel off. Therefore, the average thickness of the interface alloy layer is preferably 0.1 to 1 μm.

Furthermore, in the main layer, as shown in FIG. 1 , the coating bath components that are not consumed in the formation of the interface alloy layer 22 solidify to mainly form an Al phase, a Zn phase, and MgZn ₂ .

The Al phase is a necessary structure for obtaining stable and excellent corrosion resistance. When observing a cross section of the coating film in the thickness direction, the Al phase occupies an area ratio of preferably 30% or more, more preferably 40% or more.

Furthermore, the _MgZn2 has the function of preferentially dissolving in the initial stages of corrosion of the coating, stabilizing the resulting corrosion products. Furthermore, because _MgZn2 is a hard intermetallic compound, its presence in the main coating layer improves the scratch resistance of the coating. To stably achieve both of these effects, the area ratio of the _MgZn2 in the main layer, when observed in a cross-section through the thickness of the coating, is preferably 10% or more, and more preferably 30% or more. Furthermore, the Zn phase primarily works together with the _MgZn2 to provide sacrificial corrosion protection against Fe, improving the corrosion resistance of the end faces where Fe is exposed. In order to stably obtain such an effect, when observing a cross section in the thickness direction of the coating film, the area ratio of the Zn phase and the _MgZn2 in the main layer is preferably 30% or more in total.

Furthermore, the hot-dip Zn-Al-Mg-based steel sheet of the present invention is characterized by the formation of needle-shaped inorganic compounds having a length of 1 μm or greater and an aspect ratio (minor diameter/major diameter) of 0.2 or less on the interface alloy layer, when viewed in a cross-section through the thickness of the coating film. As shown in Figure 1, when viewed in a cross-section through the thickness of the coating film 20, the formation of needle-shaped inorganic compounds 23 on the interface alloy layer 22 creates an anchoring effect between the interface alloy layer 22 and the main layer 21. This allows the hot-dip Zn-Al-Mg-based steel sheet to achieve both excellent corrosion resistance and high-level coating adhesion.

The major diameter of the needle-shaped inorganic compound 23 is determined by measuring the major diameters (L) of at least ten randomly selected needle-shaped inorganic compounds 23 in a cross-section parallel to the surface of the base steel plate over a 2 mm or greater range. The average value is then taken. The minor diameter (D) of the needle-shaped inorganic compound 23 is determined by measuring the minor diameters (D) over the same measurement range and number as the major diameter. If the major diameter of the needle-shaped inorganic compound 23 is less than 1 μm, a sufficient anchoring effect may not be achieved, and the desired coating adhesion may not be achieved. On the other hand, if the length of the needle-shaped inorganic compound 23 is too large, the needle-shaped inorganic compound 23 may penetrate deep into the main layer 21, potentially deteriorating processability or corrosion resistance of the processed portion. Therefore, the length of the needle-shaped inorganic compound 23 is preferably 10 μm or less, and more preferably 5 μm or less. The method for observing a cross section of the coating film 20 in the thickness direction is not particularly limited, as long as the presence or absence of the needle-shaped inorganic compound 23 and the length L and length D of the needle-shaped inorganic compound 23 can be determined. For example, observation and measurement can be performed using SEM-EDX (scanning electron microscope energy dispersive X-ray analysis).

The aspect ratio of the needle-shaped inorganic compound 23 is the ratio of the minor diameter to the major diameter (minor diameter D/major diameter L) of the needle-shaped inorganic compound 23, as shown in Figure 1. If the aspect ratio of the needle-shaped inorganic compound 23 exceeds 0.2, a sufficient anchoring effect may not be achieved, 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 may penetrate deeply into the main layer 21, potentially deteriorating workability or corrosion resistance of the processed portion. Based on the same viewpoint, the aspect ratio of the needle-shaped inorganic compound 23 is preferably 0.05 to 0.2, more preferably 0.10 to 0.15. Furthermore, when observing a cross-section of the coating film 20 in the thickness direction over a range of 2 mm or more parallel to the surface of the base steel plate, the average aspect ratio of ten or more randomly selected needle-shaped inorganic compounds 23 is preferably 0.2 or less (the average aspect ratio). When the average aspect ratio of the needle-shaped inorganic compound 23 is 0.2 or less, stable and excellent adhesion is achieved. On the other hand, if the average aspect ratio of the needle-shaped inorganic compound 23 is less than 0.05, the needle-shaped inorganic compound 23 may penetrate deeply into the main layer 21, potentially deteriorating processability or corrosion resistance of the processed portion. Based on the same viewpoint, the average 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, as shown in Figure 1 , the needle-shaped inorganic compound 23 preferably extends from the surface of the interface alloy layer 22 into the main layer 21 when observing a cross-section of the coating film 22 in the thickness direction. This is because the needle-shaped inorganic compound 23 further enhances the adhesion between the interface alloy layer 22 and the main layer 21, thereby achieving superior coating adhesion. Also, the present invention stipulates that it is preferred that the needle-shaped inorganic compound 23 be visible extending from the surface of the interface alloy layer 22 into the main layer 21 when observing a cross-section of the coating film 22 in the thickness direction. However, in practice, it is believed that the majority of the needle-shaped inorganic compound 23 extends from the surface of the interface alloy layer 22.

The components of the needle-shaped inorganic compound are not particularly limited as long as they possess the aforementioned shape. More specifically, Si-based compounds containing Si are preferred. Si, being present primarily near the interface alloy layer, is a component of the needle-shaped inorganic compound, making it easy to achieve a shape with a length of 1 μm or greater and an aspect ratio (minor diameter/long diameter) of 0.2 or less. Consequently, plating adhesion can be more reliably improved.

Furthermore, the needle-shaped inorganic compound preferably contains nickel in addition to the aforementioned Si. When the nickel is contained in the interface alloy layer, the needle-shaped inorganic compound extending from the interface alloy layer also contains nickel. The inclusion of nickel in the needle-shaped inorganic compound integrates the compound with the interface alloy layer, resulting in even better plating adhesion.

Furthermore, from the perspective of satisfying various properties, the coating weight is preferably 30 to 300 g/ ^m² per side. This is because a coating weight of 30 g/ ^m² or greater provides sufficient corrosion resistance even for applications requiring long-term corrosion resistance, such as building materials. Furthermore, a coating weight of 300 g/ ^m² or less achieves excellent corrosion resistance while suppressing the occurrence of coating cracking during processing. Based on the same perspective, the coating weight is more preferably 50 to 150 g/ ^m² .

The amount of coating film adhered can be determined, for example, using the method specified in JIS H 0401:2013, which involves dissolving and peeling a specific area of the coating film in a mixture of hydrochloric acid and hexamethylenetetramine, and calculating the difference in weight between the steel plate before and after peeling. In this method, the coating film adhered per side can be determined by sealing the non-coated surface with tape, then performing the dissolution.

As shown in Figure 1, the hot-dip Zn-Al-Mg-based steel sheet of the present invention has a coating film 20 formed on a base steel sheet 10. Alternatively, an intermediate layer or coating may be formed on the coating film as needed. The type of coating or the method for forming the coating is not particularly limited and can be appropriately selected based on the desired performance. Examples include roll coating, curtain coating, and spray coating. After applying the coating containing an organic resin, the coating film can be dried by hot air drying, infrared heating, induction heating, or other methods. The intermediate layer is not particularly limited as long as it is formed between the hot-dip Zn-Al-Mg plated steel sheet and the coating. Examples include a chemical conversion coating or a primer such as an adhesive layer. The chemical conversion coating can be formed, for example, by applying a chromate treatment solution or a chromium-free chemical conversion treatment solution, then drying the steel sheet at a temperature of 80-300°C without rinsing. These chemical conversion coatings can be single or multiple layers. In the case of multiple layers, multiple chemical conversion treatments can be performed sequentially.

(Method for Manufacturing Hot-Dip Zn-Al-Mg Steel Sheet) The method for manufacturing the hot-dip Zn-Al-Mg steel sheet of the present invention is not particularly limited. However, the coating film of the hot-dip Zn-Al-Mg steel sheet obtained by the present invention has a composition substantially identical to that of the coating bath. Therefore, the method includes forming the coating film on a base steel sheet using a coating bath. The coating bath composition is controlled to contain 10-22% Al by mass, 0.01-2% Si by mass, and 3-10% Mg by mass, with the remainder consisting of Zn and unavoidable impurities.

The process for forming the aforementioned coating film is not particularly limited, other than the composition of the coating bath. For example, the base steel sheet can be manufactured using a continuous hot-dip coating system, where the base steel sheet is cleaned, heated, and then immersed in a coating bath. During the steel heating step, recrystallization annealing is performed to control the base steel sheet's structure. Furthermore, heating in a reducing atmosphere, such as a nitrogen-hydrogen atmosphere, is effective to prevent oxidation and reduce trace oxide films on the surface.

The bath temperature of the coating bath is not particularly limited, but is preferably within the range of (melting point + 20°C) to 550°C. The lower limit of the bath temperature is set at the melting point + 20°C because hot dip coating requires the bath temperature to be above the solidification point. Setting the bath temperature at the melting point + 20°C prevents solidification caused by localized temperature drops in the coating bath. On the other hand, the upper limit of the bath temperature is set at 550°C because temperatures exceeding 550°C make it difficult to rapidly cool the coating, potentially thickening the interface alloy layer formed between the coating and the steel plate.

Furthermore, there are no particular limitations on the method for forming the needle-shaped inorganic compound on the interface alloy layer. For example, the needle-shaped inorganic compound can be added to a plating bath and then hot-dip plated. In this case, the needle-shaped inorganic compound preferably has a length of 1 μm or greater and an aspect ratio (minor diameter/major diameter) of 0.2 or less.

The base steel sheet constituting the Zn-Al-Mg-based plated steel sheet of the present invention is not particularly limited. Cold-rolled or hot-rolled steel sheets can be used, depending on the required performance or specifications. Furthermore, the base steel sheet is not particularly limited. Furthermore, the method for obtaining the base steel sheet is not particularly limited. For example, in the case of hot-rolled steel sheet, a steel sheet that has undergone hot rolling and pickling steps can be used, while in the case of cold-rolled steel sheet, a cold-rolling step can be added. Furthermore, to enhance the steel sheet's properties, a recrystallization annealing step or the like may be performed before the hot-dip plating step.

Alternatively, a pre-coated steel sheet can be used as the base steel sheet. The pre-coated steel sheet is coated, for example, by electrolytic treatment or displacement plating. In the electrolytic treatment method, the base steel sheet is immersed in a sulfuric acid bath or chloride bath containing metal ions of the various pre-coating components, and then electrolytic treatment is performed. In the displacement plating method, the base steel sheet is immersed in an aqueous solution containing metal ions of the various pre-coating components and having its pH adjusted with sulfuric acid, and the precipitated metal is displaced. A representative example of pre-coated steel sheet is pre-coated nickel steel sheet. Furthermore, when hot-dip plating is performed on pre-Ni-plated steel sheets in a bath containing the aforementioned needle-shaped inorganic compound, the needle-shaped inorganic compound readily extends from the surface of the interface alloy layer into the main coating layer, resulting in improved adhesion of the resulting hot-dip Zn-Al-Mg-based steel sheet.

Furthermore, in the method for producing hot-dip Zn-Al-Mg-based steel sheets of the present invention, in addition to the aforementioned steps of forming the coating and the subsequent heating and cooling steps, conventional steps employed in coating steel sheets may also be appropriately implemented. Examples

[Samples 1-3] (Production Method A:) Using a conventionally cold-rolled steel plate with a thickness of 0.8 mm as the base steel plate, annealing and plating were performed in a RHESCA hot-dip plating simulator. Samples 1-3 were produced under the conditions shown in Table 1. Table 1 also shows the composition and bath temperature of the hot-dip Zn-Al-Mg steel plate, as well as the composition and adhesion of the coating film for each sample.

[Samples 4-9] (Method B:) Using a conventionally cold-rolled steel plate with a thickness of 0.8 mm as the base steel plate, annealing and hot-dip plating were performed in a RHESCA hot-dip plating simulator. Samples 4-9 were produced under the conditions shown in Table 1. Table 1 shows the composition and bath temperature of the hot-dip Zn-Al-Mg steel plate, as well as the composition and adhesion weight of the coating film for each sample. For Samples 4-9, the needle-shaped inorganic compound was added to the plating bath at a concentration of 0.1% relative to the total weight of the plating bath. Table 1 shows the types, average lengths, and aspect ratios of needle-shaped inorganic compounds.

[Samples 10-14] (Production Method C:) Using a conventionally cold-rolled steel plate with a thickness of 0.8 mm and pre-plated with Ni as the base steel plate, annealing and hot-dip plating were performed in a hot-dip plating simulator manufactured by RHESCA Co., Ltd., to produce Samples 10-14 of hot-dip plated Zn-Al-Mg steel plates under the conditions shown in Table 1. Furthermore, the cold-rolled steel sheets were pre-nickel-plated using a _plating bath containing 300 g/L _NiSO₄ · _6H₂O , 40 g/L _H₃BO₃ , ₁₀₀ g/L _Na₂SO₄ , and a pH of 2.7. Under the conditions of a bath temperature of 60°C and a current density of 50 A/ ^dm₂ , the Ni deposition amount was controlled to 1 g/ ^m₂ . Table 1 shows the composition and bath temperature of the plating bath used in the production of hot-dip Zn-Al-Mg steel sheets, as well as the composition and deposition amount of the coating films for each sample. Furthermore, for samples 10-14, the needle-shaped inorganic compound was added to the plating bath at a concentration of 0.1% relative to the total weight of the plating bath. Table 1 shows the types, average lengths, and aspect ratios of needle-shaped inorganic compounds.

<Evaluation> The following evaluations were performed on each sample of the resulting hot-dip Zn-Al-Mg steel plate. The evaluation results are shown in Table 1.

(1) Needle-shaped inorganic compounds For each sample of the hot-dip-coated Zn-Al-Mg steel plate, a cross-section was observed and analyzed at any location using a scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX). For each sample, the presence or absence of needle-shaped inorganic compounds observed in the cross-section in the thickness direction of the coating film, the content of the needle-shaped inorganic compounds, the average size of the needle-shaped inorganic compounds (aspect ratio, aspect ratio), and the presence or absence of needle-shaped inorganic compounds extending from the interface alloy layer are shown in Table 1.

(2) Corrosion resistance evaluation For each sample of hot-dip Zn-Al-Mg steel plate, cut it into a size of 70mm×120mm, and seal the area 10mm away from each edge of the evaluation target surface and the end face of the sample with the evaluation non-target surface with tape, leaving the evaluation target surface exposed in a size of 50mm×100mm as the evaluation sample. The evaluation samples were subjected to the Japanese automobile standard composite cycle test (JASO-CCT). The test is a corrosion promotion test starting from wetness and continuing until 90 cycles. Afterwards, the corrosion reduction of each sample was measured using the method described in JIS Z 2383 and ISO8407, and evaluated using the following standards. The evaluation results are shown in Table 1. ◎: Corrosion loss is 60g/ ^m2 or less ○: Corrosion loss is 80g/ ^m2 or less ×: Corrosion loss exceeds 80g/ ^m2

(3) Adhesion of the coating Each sample of the hot-dip-coated Zn-Al-Mg steel sheet was cut into a size of 70 mm × 100 mm and then subjected to a 180° adhesive bend (0T bend) to obtain a 100 mm apex. Cellotape (registered trademark) was then firmly attached to the outside of the bent portion and then torn off. The outside of the bent portion (apex) was then observed using an optical microscope (DSX1000 manufactured by OLYMPUS) at a magnification of 30 times to confirm the damage morphology of the coating film. The adhesion of the coating was evaluated using the following criteria. The evaluation results are shown in Table 1. ◎: No peeling of the coating (no cracking or only cracking) ○: Slight peeling of the coating (the total diameter of the peeled area is less than 5mm) ×: Significant peeling of the coating (the total diameter of the peeled area is 5mm or more)

The results in Table 1 show that compared to the samples in the comparative examples, the samples in the present invention exhibit a well-balanced and superior corrosion resistance and coating adhesion. Possible Industrial Applications

According to the present invention, a hot-dip plated Zn-Al-Mg steel sheet can be provided, which achieves both high corrosion resistance and high plating adhesion.

10: Base steel plate 20: Coating film 21: Main layer 22: Interface alloy layer 23: Needle-shaped inorganic compound L: Longest diameter of the needle-shaped inorganic compound D: Shortest diameter of the needle-shaped inorganic compound

FIG1 is an enlarged schematic diagram showing a cross section of a hot-dip plated Zn-Al-Mg steel sheet according to this embodiment.

10: Base steel plate

20: Coated film

21: Main Floor

22: Interface alloy layer

23: Needle-shaped inorganic compounds

D: Short path of needle-shaped inorganic compounds

L: Length of needle-shaped inorganic compound

Claims (6)

A hot-dip Zn-Al-Mg steel plate having a coating film. The coating film comprises an interface alloy layer at the interface with the base steel plate and a main layer located above the interface alloy layer. The coating film has a composition comprising 10-22% by mass of Al, 0.01-2% by mass of Si, and 3-10% by mass of Mg, with the remainder consisting of Zn and unavoidable impurities. When observed in a cross-section through the thickness of the coating film, needle-shaped inorganic compounds having a length of 1 μm or greater and an aspect ratio (minor diameter/major diameter) of 0.2 or less are formed on the interface alloy layer. As for the hot-dip plated Zn-Al-Mg steel plate of claim 1, when observing the cross section of the coating film in the thickness direction, the needle-shaped inorganic compound extends from the surface of the interface alloy layer into the main layer. In the hot-dip plated Zn-Al-Mg steel sheet of claim 1 or 2, the needle-shaped inorganic compound contains Si. The hot-dip plated Zn-Al-Mg steel sheet of claim 3, wherein the needle-shaped inorganic compound further contains Ni. The hot-dip plated Zn-Al-Mg steel sheet of claim 1 or 2, wherein the interface alloy layer contains Ni. The hot-dip plated Zn-Al-Mg steel sheet of claim 1 or 2, wherein the coating further contains 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.