Steel sheets coated with Al-Zn-Mg-Si-Sr by hot-dip immersion, using the same manufacturing method.

Abstract

The purpose of the present invention is to provide a hot-dip Al-Zn-Mg-Si-Sr-plated steel sheet having a good surface appearance and excellent corrosion resistance in a flat sheet section and a processed section.　In order to achieve the abovementioned purpose, the present invention is characterized in that a plating layer thereof has a composition containing 40-70 mass% Al, 0.6-5 mass% Si, 0.1-10 mass% Mg, and 0.001-1.0 mass% Sr, the remainder comprising Zn and unavoidable impurities, the plating layer comprising an interfacial alloy layer which is present at an interface with a base steel sheet, and a main layer which is present on the alloy layer, and, among Mg2Si observed in a thickness-directed cross section of the plating layer, the area ratio of Mg2Si present in a range of 50% of the thickness from the surface of the main layer being 50% or greater, and the area ratio of Mg2Si extending from the surface of the main layer to the interfacial alloy layer being 50% or less.

Description

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

[0001] The present invention relates to a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet and a method for producing the same, which have good surface appearance and excellent corrosion resistance in the flat plate portion and processed portion.

[0002] Hot-dip Al-Zn plated steel sheets exhibit high corrosion resistance among hot-dip galvanized steel sheets because they combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al. For example, Patent Document 1 discloses a hot-dip Al-Zn plated steel sheet containing 25 to 75% by mass of Al in the plating layer. Due to its excellent corrosion resistance, demand for hot-dip Al-Zn plated steel sheets has been growing in recent years, mainly in the building materials sector such as roofs and walls that are exposed to the outdoors for long periods, and in the civil engineering and construction sectors such as guardrails, wiring and piping, and soundproof walls.

[0003] The plating layer of a hot-dip Al-Zn plated steel sheet consists of a main layer and an interfacial alloy layer present at the interface between the substrate steel sheet and the main layer. The main layer is composed of a portion where Al containing mainly Zn has solidified in dendrites (the dendrite portion of the α-Al phase) and the remaining portion of the dendrite gaps (interdendrites) where Zn is the main component, and has a structure in which multiple α-Al phases are stacked in the direction of the plating layer thickness. Due to this characteristic film structure, the corrosion progression path from the surface becomes complex, making it difficult for corrosion to easily reach the substrate steel sheet, and hot-dip Al-Zn plated steel sheets can achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same plating layer thickness.

[0004] Furthermore, there is a known technology that aims to further improve corrosion resistance by incorporating Mg into the plating layer of molten Al-Zn plating. As a technology relating to molten Al-Zn plated steel sheets containing Mg (molten Al-Zn-Mg-Si plated steel sheets), for example, Patent Document 2 discloses an Al-Zn-Mg-Si plated steel sheet in which the plating layer contains an Al-Zn-Si alloy containing Mg, the Al-Zn-Si alloy is an alloy containing 45 to 60% by weight of elemental aluminum, 37 to 46% by weight of elemental zinc, and 1.2 to 2.3% by weight of elemental silicon, and the concentration of Mg is 1 to 5% by weight.

[0005] However, while the hot-dip Al-Zn-Mg plated steel sheet containing Mg disclosed in Reference Document 2 has excellent corrosion resistance, it is prone to developing wrinkle-like defects (hereinafter referred to as "wrinkle-like defects") ranging in length from several millimeters to several hundred millimeters, caused by the oxide layer formed on the surface of the plating layer, which impairs the appearance of the plating layer surface. For this reason, for example, Patent Document 3 discloses a technique for improving the surface appearance of a hot-dip Al-Zn-Mg plated steel sheet by incorporating Sr into the plating layer. Furthermore, Patent Document 4 discloses a technique for improving the processability of a hot-dip Al-Zn-Mg plated steel sheet by incorporating Sr into the plating layer.

[0006] Japanese Patent Publication No. 46-7161 Patent No. 5020228 Patent No. 3983932 Patent No. 6368730

[0007] The molten Al-Zn-Mg plated steel sheets described in Patent Documents 3 and 4 above contain Sr in the plating layer, which suppresses the occurrence of wrinkle-like defects and improves surface appearance. However, in the Sr-containing molten Al-Zn-Mg plated steel sheets of Reference Documents 3 and 4, the inclusion of Sr causes Mg near the surface of the plating layer to... ２ The Si content decreased, which could result in a decrease in corrosion resistance. In addition, in the hot-dip Al-Zn plated steel sheets disclosed in Patent Documents 2 and 3, Mg was generated in the plating layer. ２ Although Si improves corrosion resistance, there was a problem in that the plating layer cracked when bending was performed, resulting in poor corrosion resistance in the processed area.

[0008] In view of these circumstances, the present invention aims to provide a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet having good surface appearance and excellent corrosion resistance in the flat plate portion and processed portion, as well as a method for manufacturing a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet having excellent corrosion resistance in the flat plate portion and processed portion.

[0009] The inventors of this invention conducted studies to solve the above problems and found that the corrosion resistance improvement effect due to Mg content is due to the Mg present in the plating layer during corrosion of the plating layer.２ Since it is expressed by the preferential dissolution of Si and the concentration of dissolved Mg in the corrosion products formed on the surface of the plating layer, Mg present near the surface of the plating layer ２ Attention was paid to the fact that Si is more important. As a result of further intensive research, Mg contained in the main layer constituting the plating layer (hereinafter, sometimes referred to as "plating main layer" or "main layer"). ２ Even when Sr is contained in the plating layer by collecting most of Si among Mg and Si near the surface of the plating main layer, sufficient Mg exists on the surface of the plating layer ２ Si exists, and it was found that not only can the generation of wrinkle-like defects be suppressed, but excellent corrosion resistance can also be realized. Further, the present inventors have found that Mg in the plating main layer ２ Although Si has an effect of improving corrosion resistance, it becomes a propagation path of cracks generated in the interfacial alloy layer during bending to the surface of the plating main layer, reducing the workability, so that the desired corrosion resistance of the processed part cannot be obtained. Attention was paid to this. And by reducing the amount of Mg and Si that reaches from the interfacial alloy layer to the surface of the plating main layer, cracks generated starting from the interfacial alloy layer during processing of the steel sheet can be suppressed from penetrating the plating main layer and propagating to the surface of the plating main layer. Therefore, it was found that the corrosion resistance of the processed part can also be improved. ２ The present invention has been made based on the above findings, and the gist thereof is as follows. 1. The plating layer contains Al: 40 to 70% by mass, Si: 0.6 to 5% by mass, Mg: 0.1 to 10% by mass, and Sr: 0.001 to 1.0% by mass, and the balance consists of Zn and inevitable impurities. The plating layer consists of an interfacial alloy layer present at the interface with the base steel sheet and a main layer present on the alloy layer. In the cross section in the thickness direction of the plating layer, Mg

[0010] The present invention has been made based on the above findings, and the gist thereof is as follows. 1. The plating layer contains Al: 40 to 70% by mass, Si: 0.6 to 5% by mass, Mg: 0.1 to 10% by mass, and Sr: 0.001 to 1.0% by mass, and the balance consists of Zn and inevitable impurities. The plating layer consists of an interfacial alloy layer present at the interface with the base steel sheet and a main layer present on the alloy layer. In the cross section in the thickness direction of the plating layer, Mg ２ Among Si, Mg existing within a thickness range of up to 50% from the surface of the main layer ２ The area ratio of Si is 50% or more, and Mg continuously present in the interdendritic part from the interfacial alloy layer to the surface of the main layer (hereinafter, sometimes referred to as "extending"). ２A hot-dip Al-Zn-Mg-Si-Sr plated steel sheet characterized by having an area ratio of Si of 50% or less.

[0011] 2. Mg observed in the cross-section in the thickness direction of the plating layer ２ Of the Si, Mg present within a thickness range of up to 50% from the surface of the main layer ２ The area ratio of Si is 60% or more, and Mg extends from the surface of the main layer to the interface alloy layer. ２ The hot-dip Al-Zn-Mg-Si-Sr plated steel sheet according to claim 1, characterized in that the area ratio of Si is 50% or less.

[0012] 3. Mg observed in the cross-section in the thickness direction of the plating layer ２ Of the Si, Mg present within a thickness range of up to 50% from the surface of the main layer ２ The area ratio of Si is 60% or more, and Mg extends from the surface of the main layer to the interface alloy layer. ２ The hot-dip Al-Zn-Mg-Si-Sr plated steel sheet according to claim 2, characterized in that the area ratio of Si is 40% or less.

[0013] 4. The Si phase observed in the cross-section in the thickness direction of the plating layer is the same as the Mg phase observed in the cross-section in the thickness direction of the plating layer. ２ A hot-dip Al-Zn-Mg-Si-Sr plated steel sheet according to any one of the above 1 to 3, characterized in that the ratio of the area ratio of the Si phase to the total area ratio of Si and the Si phase is 30% or less.

[0014] 5. A hot-dip Al-Zn-Mg-Si-Sr plated steel sheet according to any one of items 1 to 4, characterized in that the main layer has a dendrite portion of the α-Al phase, and the average distance between dendrite arms of the dendrite portion and the thickness of the plating layer satisfy the following formula (1): t / d ≥ 1.5 ... (1) t: thickness of the plating layer (μm), d: average distance between dendrite arms (μm)

[0015] 6. A method for producing a molten Al-Zn-Mg-Si-Sri plated steel sheet, characterized in that when applying molten plating to a steel sheet using a plating bath having a composition containing Al: 40-70% by mass, Si: 0.6-5% by mass, Mg: 0.1-10% by mass, and Sr: 0.001-1.0% by mass, with the remainder being Zn and unavoidable impurities, and having a bath temperature of 585°C or less, the temperature of the steel sheet when it enters the plating bath (entry plate temperature) is 20°C more than the temperature of the plating bath (plating bath temperature + 20°C) or less.

[0016] 7. The method for manufacturing a molten Al-Zn-Mg-Si-Sr plated steel sheet according to 6, characterized in that the temperature at which the steel sheet enters the plate is less than or equal to the temperature of the plating bath.

[0017] 8. A method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet according to 6 or 7, characterized in that, after applying molten plating to the steel sheet, the steel sheet is cooled at an average cooling rate of 10°C / s or more until the sheet temperature reaches a temperature obtained by subtracting 150°C from the temperature of the plating bath (plating bath temperature - 150°C).

[0018] The present invention provides a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet having good surface appearance and excellent corrosion resistance in the flat plate portion and processed portion, as well as a method for manufacturing a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet having good surface appearance and excellent corrosion resistance in the flat plate portion and processed portion.

[0019] (a) is a diagram showing the state of a molten Al-Zn-Mg plated steel sheet before and after corrosion, and (b) is a diagram showing the state of a molten Al-Zn plated steel sheet before and after corrosion. (a) shows the state of each element of the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention as shown by energy-dispersive X-ray spectroscopy (SEM-EDX) using a scanning electron microscope, and (b) shows a part of the molten Al-Zn-Mg-Si-Sr plated steel sheet shown in (a), with regard to Mg ２ This diagram illustrates a method for observing Si and the Si phase. The Mg observed in the cross-section in the thickness direction of the plating layer. ２ Of the Si, Mg present within a thickness range of up to 50% from the surface of the main layer ２This is a diagram illustrating a method for calculating the area ratio of Si. It shows the Mg observed in a cross-section in the thickness direction of the plating layer. ２ Of the Si, Mg extends from the surface of the main layer to the interface alloy layer. ２ This diagram illustrates how to calculate the area ratio of Si. This diagram illustrates how to measure the distance between dendrite arms. This diagram illustrates the flow of the Japanese Automotive Standards Combined Cycle Test (JASO-CCT).

[0020] (Hot-dip Al-Zn-Mg-Si-Sr plated steel sheet) The hot-dip Al-Zn-Mg-Si-Sr plated steel sheet that is the subject of the present invention has a plating layer on the surface of the steel sheet, and the plating layer consists of an interface alloy layer present at the interface with the base steel sheet and a main layer present on the alloy layer. Furthermore, the plating layer has a composition containing Al: 40 to 70 mass%, Si: 0.6 to 5 mass%, Mg: 0.1 to 10 mass%, and Sr: 0.001 to 1.0 mass%, with the remainder being Zn and unavoidable impurities.

[0021] The Al content in the plating layer is set to 40 to 70% by mass, preferably 45 to 65% by mass, considering the balance between corrosion resistance and operational aspects. If the Al content of the main layer of the plating layer is 40% by mass or more, good corrosion resistance can be ensured. The main layer mainly contains supersaturated Zn and consists of a portion where Al has solidified in dendrites (the dendritic portion of the α-Al phase) and the remaining portion of the dendrite gaps (the interdendrite portion), and a structure with excellent corrosion resistance can be realized in which the dendrite portions are stacked in the direction of the plating layer thickness. Furthermore, the more of these α-Al phase dendrite portions 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. From a similar viewpoint, it is preferable that the Al content in the plating layer be 45% by mass or more. On the other hand, if the Al content in the plating layer exceeds 70% by mass, the amount of Zn, which has a sacrificial corrosion protection effect against Fe, decreases, and the corrosion resistance deteriorates. For this reason, the Al content in the plating layer should be 70% by mass or less. Furthermore, if the Al content in the plating layer is 65% by mass or less, even if the amount of plating deposited is reduced and the underlying steel plate is more likely to be exposed, it will still have a sacrificial corrosion protection effect against Fe, and sufficient corrosion resistance can be obtained. For this reason, it is preferable that the Al content of the main plating layer be 65% by mass or less.

[0022] The Si in the plating layer is added to the plating bath to improve corrosion resistance and workability, with the aim of suppressing the growth of the interfacial alloy layer formed at the interface with the substrate steel sheet, and is therefore inevitably contained in the main layer. In the case of the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, when molten plating is performed with Si contained in the plating bath, the substrate steel sheet is immersed in the plating bath, and at the same time, the Fe on the surface of the steel sheet reacts with the Al and Si in the bath to form an alloy, generating an alloy consisting of Fe-Al and / or Fe-Al-Si compounds. The formation of this Fe-Al-Si interfacial alloy layer can suppress the growth of the interfacial alloy layer. Furthermore, when the Si content in the plating layer is 0.6% by mass or more, the growth of the interfacial alloy layer can be sufficiently suppressed. On the other hand, when the Si content in the plating layer exceeds 5%, the workability of the plating layer decreases, and Si phases that become cathode sites are more likely to precipitate. The precipitation of this Si phase can be suppressed by increasing the Mg content and establishing a certain relationship between the Si content and the Mg content, as will be described later, but in that case, the manufacturing cost will increase, and Mg ２ A high Si content leads to a decrease in workability and makes it more difficult to control the composition of the plating bath. For this reason, the Si content in the plating layer should be 5% or less. Furthermore, it is possible to more reliably suppress the growth of the interfacial alloy layer and the precipitation of the Si phase, and Mg ２ Considering the need to address situations where Si is consumed as Si, it is preferable to have a Si content in the plating layer of more than 2.3% to 3.5%.

[0023] The plating layer contains 0.1 to 10% by mass of Mg. When the main layer of the plating layer corrodes, Mg is included in the corrosion products, improving the stability of the corrosion products and delaying the progression of corrosion, resulting in improved corrosion resistance. More specifically, the Mg present in the main layer of the plating layer combines with the Si mentioned above, and Mg ２ It generates Si. This Mg ２As shown in Figure 1(a), when the plated steel sheet corrodes, Si dissolves in the initial stages, resulting in Mg being included in the corrosion products. The Mg contained in these corrosion products has the effect of densifying them, improving the stability of the corrosion products and their barrier properties against external corrosion factors. On the other hand, if the plating layer does not contain Mg, as shown in Figure 1(b), no Mg will be included in the corrosion products, and the desired corrosion resistance cannot be obtained. Here, the reason why the Mg content of the plating layer is set to 0.1% by mass or more is that when the plating layer contains Si within the concentration range described above, setting the Mg concentration to 0.1% by mass or more allows Mg to be incorporated. ２ This is because it becomes possible to generate Si, thereby obtaining a corrosion delay effect. From a similar viewpoint, the Mg content of the plating layer is preferably 1% by mass or more, and more preferably 3% by mass or more. On the other hand, the reason for setting the Mg content of the plating layer to 10% by mass or less is that if the Mg content of the plating layer exceeds 10%, in addition to saturation of the corrosion resistance improvement effect, manufacturing costs increase and it becomes difficult to control the composition of the plating bath. From a similar viewpoint, the Mg content of the plating layer is preferably 6% by mass or less.

[0024] Furthermore, by setting the Mg content in the plating layer to 1% by mass or more, it is possible to improve corrosion resistance after painting. When the plating layer of a conventional hot-dip Al-Zn plated steel sheet that does not contain Mg is exposed to the atmosphere, dense and stable Al forms around the α-Al phase. ２ O ３ An oxide film is immediately formed, and due to the protective effect of this oxide film, the solubility of the α-Al phase is significantly lower than that of the Zn-rich phase in the interdendrite. As a result, in painted steel sheets using conventional Al-Zn plated steel sheets as a base, if the coating is damaged, selective corrosion of the Zn-rich phase occurs at the coating / plating interface starting from the damaged area, progressing deep into the sound coating and causing large blistering of the coating, resulting in poor post-coating corrosion resistance. Therefore, from the viewpoint of obtaining excellent post-coating corrosion resistance, it is preferable to have a Mg content of 1% by mass or more in the plating layer, and more preferably 3% by mass or more. On the other hand, in the case of painted steel sheets using molten Al-Zn plated steel sheets containing Mg in the plating layer, Mg precipitates in the interdendrite.２ Si phase and Mg-Zn compound (MgZn ２ Mg ３２ (Al, Zn) ４９ (etc.) dissolves in the initial stages of corrosion, and Mg is incorporated into the corrosion products. Corrosion products containing Mg are very stable, and this suppresses corrosion in the initial stages, thereby suppressing large paint film blistering due to selective corrosion of the Zn-rich phase, which is a problem in painted steel sheets using conventional Al-Zn plated steel sheets as a base. As a result, molten Al-Zn plated steel sheets with Mg contained in the plating layer exhibit excellent post-painting corrosion resistance. If the Mg content in the plating layer is less than 1% by mass, the amount of Mg that dissolves during corrosion is small, and there is a risk that the post-painting corrosion resistance will not improve. Furthermore, if the Mg content in the plating layer exceeds 10% by mass, not only does the effect saturate, but corrosion of the Mg compound occurs severely, and the solubility of the entire plating layer increases excessively. As a result, even if the corrosion products are stabilized, the dissolution rate increases, which can cause large blistering and deterioration of post-painting corrosion resistance. For this reason, in order to stably obtain excellent post-painting corrosion resistance, it is preferable to set the Mg content in the plating layer to 10% by mass or less.

[0025] Furthermore, the plating layer contains 0.001 to 1.0 mass% of Sr. By including Sr in the plating layer, the occurrence of wrinkle-like defects can be suppressed, and the surface appearance of the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention can be improved. The wrinkle-like defects are defects that form wrinkle-like irregularities on the surface of the plating layer and are observed as whitish streaks on the surface of the plating layer. Such streak-like defects are more likely to occur when a large amount of Mg is added to the plating layer. Therefore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, by including Sr in the plating layer, Sr is preferentially oxidized over Mg on the surface of the plating layer, and the oxidation reaction of Mg is suppressed, thereby making it possible to suppress the occurrence of the streak-like defects.

[0026] The Sr content in the plating layer must be 0.001% by mass or more. This is to obtain the effect of suppressing the occurrence of the streak-like defects described above. From a similar viewpoint, the Sr content in the plating layer is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, and particularly preferably 0.05% by mass or more. On the other hand, the Sr content in the plating layer must be 1.0% by mass or less. This is because if the Sr content becomes too high, the effect of suppressing the occurrence of streak-like defects will saturate, which is disadvantageous in terms of cost. From a similar viewpoint, the Sr content in the plating layer is preferably 0.7% by mass or less, more preferably 0.5% by mass or less, and particularly preferably 0.3% by mass or less.

[0027] The plating layer contains components of the base steel sheet incorporated into the plating through a reaction between the plating bath and the base steel sheet during the plating process, as well as unavoidable impurities contained in the ingot used when preparing the plating bath. The base steel sheet components incorporated into the plating may include several percent of Fe. Examples of unavoidable impurities in the plating bath include Fe, Mn, P, S, C, Nb, Ti, and B as base steel sheet components, and Fe, Pb, Sb, Cd, As, Ga, and V as impurities in the ingot. It is not possible to distinguish between Fe incorporated from the base steel sheet and Fe present in the plating bath when quantifying the Fe in the plating layer. While there are no particular limitations on the total content of these unavoidable impurities, from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, it is preferable that the total amount of unavoidable impurities excluding Fe be 1% by mass or less. Furthermore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the plating layer may further contain at least one element selected from Cr, Ni, Co, Mn, Ca, V, Ti, B, Mo, Sn, Zr, Li, Ag, etc., which are known as stabilizing elements for corrosion products in Zn-Al plateds, in a content of less than 1% of each element. If the content of each of these elements is less than 1%, the effects disclosed in the present invention will not be hindered, and further improvement in corrosion resistance will be possible due to the corrosion product stabilizing effect.

[0028] The interface alloy layer is a layer present at the interface between the plating layer and the underlying steel sheet. As mentioned above, it is an Fe-Al and / or Fe-Al-Si compound that is inevitably formed by the alloying reaction between Fe on the steel sheet surface and Al or Si in the bath. Because this interface alloy layer is hard and brittle, if it grows too thick, it can become a starting point for crack formation during processing; therefore, it is preferable to make it thin.

[0029] Furthermore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, Mg is observed in the cross-section in the thickness direction of the plating layer. ２ Of the Si, Mg present within a thickness range of up to 50% from the surface of the main layer ２ The area ratio of Si is 50% or more, and Mg extends from the surface of the main layer to the interface alloy layer. ２ A key feature is that the area ratio of Si is 50% or less. This allows for a good surface appearance and also improves the corrosion resistance of the flat plate portion and the processed portion.

[0030] In the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, Mg is observed in the cross-section in the thickness direction of the plating layer. ２ Of the Si, Mg present within a thickness range of up to 50% from the surface of the main layer ２ The area ratio of Si shall be 50% or more. Mg ２ By containing Si, when the plating layer corrodes, this Mg ２ The preferential dissolution of Si and the concentration of dissolved Mg in the corrosion products formed on the surface of the plating layer make it possible to exhibit excellent corrosion resistance. However, if Sr is included in the plating layer for the purpose of suppressing wrinkle-like defects, etc., sufficient Mg is present on the surface of the main layer of the plating layer. ２ The absence of Si made it difficult to concentrate Mg in the corrosion products, which led to a problem of reduced corrosion resistance. Therefore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the Mg contained in the main layer of the plating layer ２ By concentrating the majority of the Si (specifically, 50% or more) near the surface of the main plating layer, even when Sr is included in the plating layer, sufficient Mg is present on the surface of the main layer.２ Since Si can be present, a sufficient amount of Mg can be present during corrosion. ２ By dissolving Si and concentrating Mg in the corrosion products, excellent corrosion resistance can be achieved while suppressing the occurrence of wrinkle-like defects.

[0031] In other words, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, Mg is present in a thickness range from the surface of the main layer up to 50%. ２ Because the area proportion of Si is large (more than 50%), Mg on the surface of the main layer during corrosion ２ Much of the Si dissolves, and the concentration of Mg in the corrosion products formed on the plated surface after corrosion increases. Therefore, the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention can achieve excellent corrosion resistance while suppressing the occurrence of wrinkle-like defects. On the other hand, in conventional molten Al-Zn-Mg-Si-Sr plated steel sheets, Mg present in the thickness range from the surface of the main layer up to 50% ２ Because the amount of Si is low (less than 50%), Mg dissolves during corrosion. ２ The amount of Si is insufficient, resulting in a relatively low Mg concentration in the corrosion products on the plated surface after corrosion. Therefore, while conventional molten Al-Zn-Mg-Si-Sr plated steel sheets can suppress the occurrence of wrinkle-like defects, their corrosion resistance is reduced.

[0032] Furthermore, the above-mentioned Mg ２ Area ratio of Si (Mg present within a thickness range of 50% from the surface of the main layer) ２ The area ratio of Si can be derived, for example, using energy-dispersive X-ray spectroscopy (SEM-EDX) with a scanning electron microscope. For example, as shown in Figure 2(a), after obtaining the cross-sectional state in the thickness direction of the plating layer, mapping is performed for Mg and Si respectively, as shown in Figure 2(b) (Mg is shown in red and Si in blue). Then, among the mapped Mg and Si, the portion where they overlap at the same position (the portion shown in purple in Figure 2(b)) is determined to be Mg ２ It can be Si. Here, the obtained Mg ２ As for Si, as shown in Figure 3, Mg relative to the total area of ​​the main layer of the plating layer ２The area fraction (A%) of Si is measured. Then, the main layer is divided in half in the thickness direction, and the Mg present in the thickness range from the surface to 50% of the total area of ​​the main layer is measured. ２ The area percentage (B%) of Si is measured. Then, the Mg in the entire main layer of the plating layer is measured. ２ The amount of Mg present in the thickness range from the surface to 50% of the main layer relative to the area ratio of Si (A%) ２ By calculating the area fraction of Si (B%) ((B%) / (A%) × 100%), the amount of Mg present in the observation field can be determined. ２ Of the Si, Mg present within a thickness range of up to 50% from the surface of the main layer ２ The area percentage (X%) occupied by Si can be obtained.

[0033] Furthermore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, Mg extends from the surface of the main layer to the interface alloy layer. ２ By reducing the area ratio of Si to 50% or less, the corrosion resistance of the processed area can also be greatly improved. Mg contained in the main plating layer ２ Although Si has the effect of improving corrosion resistance, as mentioned above, it becomes a propagation path for cracks that occur in the interfacial alloy layer, reducing workability. Therefore, conventional hot-dip Al-Zn-Mg plated steel sheets could not obtain sufficient workability and, consequently, corrosion resistance of the processed part. For this reason, in the hot-dip Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, Mg reaches from the interfacial alloy layer to the surface of the main plated layer. ２ Reduce the amount of Si (Mg ２ By limiting the area ratio of Si to 50% or less, even if cracks occur in the hard interfacial alloy layer during processing of the steel sheet, most of these cracks propagate only near the interfacial alloy layer and do not reach the main layer surface of the plating layer. As a result, good workability and corrosion resistance after processing can be ensured. On the other hand, in conventional hot-dip Al-Zn-Mg plated steel sheets, if cracks occur starting from the interfacial alloy layer, most of them reach the surface of the main plating layer, making it impossible to obtain sufficient corrosion resistance in the processed area.

[0034] Also, the above-mentioned Mg ２Area ratio of Si (Mg extending from the surface of the main layer to the interface alloy layer) ２ The derivation of the area ratio of Si can also be performed, for example, by energy-dispersive X-ray spectroscopy (SEM-EDX) using a scanning electron microscope. As described above, after obtaining the cross-sectional state in the thickness direction of the plating layer (Figure 2(a)), mapping is performed for Mg and Si respectively (Figure 2(b)). Then, among the mapped Mg and Si, the portion where they overlap at the same position is Mg ２ It can be Si (Figure 2(b)). Here, the obtained Mg ２ As for Si, as shown in Figure 4, Mg relative to the total area of ​​the main layer of the plating layer ２ The area fraction (A%) of Si is measured. Then, the amount of Mg present in the observation field is measured. ２ From among the Si particles, Mg extends from the surface of the main layer to the interface alloy layer. ２ The area ratio (C%) of Si particles (particles indicated by arrows in Figure 4) relative to the entire main layer of the plating layer is measured. Then, the Mg in the entire main layer of the plating layer is measured. ２ The area ratio (A%) of Si is relative to the amount of Mg extending from the surface of the main layer to the interface alloy layer in the entire main layer of the plating layer. ２ By calculating the area fraction of Si (C%) ((C%) / (A%) × 100%), the amount of Mg present in the observation field can be determined. ２ Among the Si particles, Mg extends from the surface of the main layer to the interface alloy layer. ２ The area percentage (Y%) of Si can be obtained.

[0035] Furthermore, in the present invention, the Mg ２ Regarding the point that Si "extends from the surface of the main layer to the interface alloy layer", the Mg ２ Not only does Si extend from the surface of the main layer to reach (come into contact with) the interface alloy layer, but also the Mg ２ When the distance between the lower end of Si and the upper end of the interface alloy layer is very small and substantially reaches the interface layer (for example, when observed in a cross-section in the thickness direction of the plating layer, the Mg ２This also includes the case where the distance between the lower end of Si and the upper end of the interface alloy layer is 1.0 μm or less. In this case as well, if cracks occur starting from the interface alloy layer, many of them will reach the surface of the main plating layer.

[0036] Mg observed in the cross-section in the thickness direction of the plating layer ２ Regarding Si, from the viewpoint of achieving better corrosion resistance, Mg present in the thickness range up to 50% from the surface of the main layer ２ It is preferable that the area ratio of Si is 60% or more. Also, Mg observed in the cross-section in the thickness direction of the plating layer ２ Regarding Si, from the viewpoint of achieving better corrosion resistance of the processed part, Mg extends from the surface of the main layer to the interface alloy layer. ２ It is preferable that the area ratio of Si is 40% or less. Furthermore, Mg observed in the cross-section in the thickness direction of the plating layer ２ Regarding Si, from the viewpoint of achieving better corrosion resistance and corrosion resistance of the processed part, Mg present in the thickness range up to 50% from the surface of the main layer ２ The area ratio of Si is 60% or more, and Mg extends from the surface of the main layer to the interface alloy layer. ２ The area ratio of Si extends from the surface of the main layer to the interface alloy layer, with Mg ２ It is more preferable that the area ratio of Si is 40% or less.

[0037] Here, the Mg observed in the cross-section in the thickness direction of the plating layer described above ２ The area ratio of Si is determined by Mg observed in 10 randomly selected cross-sections of the plating layer. ２ This is the average area ratio of Si.

[0038] Furthermore, since the plating layer contains Si as a component, as mentioned above, depending on the composition of Si and Mg in the plating layer, a Si phase may be formed in the plating layer. However, from the viewpoint of further improving corrosion resistance and workability, it is preferable to suppress the formation of the Si phase as much as possible. In particular, in the present invention, Mg improves corrosion resistance. ２It has been found that the content ratio of Si, which becomes a cathode site during the corrosion of Si and the plating layer and deteriorates the corrosion resistance, is important. That is, the essence of the present invention is that even if the absolute amount of Mg2Si that improves the corrosion resistance is large, if the amount of Si phase that deteriorates the corrosion resistance is large, good corrosion resistance cannot be ensured. Therefore, it is to control the ratio to a certain value or less.

[0039] Therefore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, Mg observed in the cross-section in the thickness direction of the plating layer, measured by the method shown below ２ The area ratio of the Si phase (area ratio of Si phase / Mg) observed in the cross-section in the thickness direction of the plating layer, with respect to the total area ratio of Si and Si phase ２ Si and the total area ratio of Si phase) is preferably 30% or less, and more preferably 10% or less. Regarding the method for deriving the area ratio of the Si phase, for example, similar to Mg described above ２ Si, it can be carried out by energy dispersive X-ray spectroscopy (SEM-EDX) using a scanning electron microscope. As described above, after obtaining the cross-sectional state in the thickness direction of the plating layer (Fig. 2(a)), mapping is performed for each of Mg and Si (Fig. 2(b)). Then, among the mapped Mg and Si, the portion shown in blue in Fig. 2(b) where Mg does not exist at the position where Si exists can be regarded as the Si phase. The area ratio (D%) of the Si phase can be calculated from the ratio of the total area of this blue portion in the observed field of view to the area of the plating layer. Furthermore, the area ratio of the Si phase (area ratio of Si phase / Mg) observed in the cross-section in the thickness direction of the plating layer, with respect to the total area ratio of Si and Si phase ２ Si and the total area ratio of Si phase) can be calculated as (D% / (A% + D%) × 100%). Here, regarding the area ratio of the Si phase to the total area of Mg ２ Si and Si phase observed in the cross-section in the thickness direction of the plating layer described above, it is the average of the area ratios of the Si phase observed in 10 randomly selected cross-sections of the plating layer.

[0040] ​​Also, the area ratio of the Si phase (the area ratio of the Si phase in the observation field of view: D%) observed in the cross-section in the thickness direction of the plating layer is preferably 10% or less, and more preferably 3% or less. Here, the area ratio of the Si phase observed in the cross-section in the thickness direction of the above-described plating layer is the average of the area ratios of the Si phase observed in 10 randomly selected cross-sections of the plating layer. <{

[0041] Further, from the viewpoint of further enhancing the initial corrosion resistance of the plated steel sheet, the area ratio of the Si phase (the area ratio of the Si phase in the observation field of view) observed on the surface of the plating layer is preferably 1% or less, and more preferably 0.5% or less. Here, regarding the method for deriving the area ratio of the Si phase on the surface of the plating layer, similar to the case of observing the cross-section, it can be carried out by using energy dispersive X-ray spectroscopy (SEM-EDX) with a scanning electron microscope. The method for obtaining the area ratio can be carried out according to the cross-section observation method, and can be the average of the area ratios of the Si phase observed on 10 randomly selected surfaces of the plating layer.

[0042] Furthermore, from the viewpoint of similarly enhancing the initial corrosion resistance, Mg ２ The area ratio of the Si phase (the area of the Si phase / Mg ２ The total area of Si and the Si phase) observed in the cross-section in the thickness direction of the plating layer is preferably 20% or less, and more preferably 10% or less. The actual observation method and the method for obtaining the area ratio are in accordance with the cross-section observation method described above.

[0043] When observing the main layer of the plating layer or the interface alloy layer using a scanning electron microscope, the cross-section or surface of the plating layer is polished and / or etched before observation. There are several methods for polishing or etching the cross-section or surface, but any method commonly used when observing the cross-section or surface of a plating layer is not particularly limited. Furthermore, the observation conditions for the scanning electron microscope can be clearly observed if, for example, the acceleration voltage is 5 to 20 kV and the magnification is 500 to 5000 times for secondary electron beam images or backscattered electron images. Similarly, when mapping with EDX, the area ratio can be determined by analyzing at the same magnification.

[0044] Furthermore, the main layer of the plating layer preferably has α-Al phase dendrite portions, and the average distance between dendrite arms of the dendrite portions and the thickness of the plating layer preferably satisfy the following equation (1): t / d ≥ 1.5 ... (1) t: thickness of the plating layer (μm), d: average distance between dendrite arms (μm) By satisfying the above equation (1), the arms of the dendrite portions made of the α-Al phase can be made relatively smaller, and the path of the interdendritic, which is preferentially corroded, can be made longer, thereby further improving corrosion resistance.

[0045] The dendrite arm distance in the dendrite portion refers to the center distance between adjacent dendrite arms (dendrite arm spacing). In this invention, for example, as shown in Figure 5, the surface of the main layer of the plating layer is observed under magnification using a scanning electron microscope (SEM) or the like (for example, observed at 200x magnification), and the spacing of the second widest dendrite arms (secondary dendrite arms) in a randomly selected field of view is measured as follows. A section in which three or more secondary dendrite arms are aligned is selected (in Figure 5, three arms between A and B are selected), and the distance (in Figure 5, distance L) is measured along the direction in which the arms are aligned. Then, the measured distance is divided by the number of dendrite arms (in Figure 5, L / 3) to calculate the dendrite arm distance. The dendrite arm distance is measured at three or more locations within one field of view, and the average of the dendrite arm distances obtained from each measurement is calculated to determine the average dendrite arm distance.

[0046] Furthermore, the thickness of the plating layer is preferably 10 to 30 μm, and more preferably 20 to 25 μm, from the viewpoint of achieving a high level of both processability and corrosion resistance. This is because sufficient corrosion resistance can be ensured when the plating layer is 10 μm or thicker, and sufficient processability can be ensured when the plating layer is 30 μm or less.

[0047] Furthermore, the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention may also be a surface-treated steel sheet further comprising a chemical conversion coating and / or paint coating on its surface.

[0048] (Method for manufacturing molten Al-Zn-Mg-Si-Sr plated steel sheet) Next, the method for manufacturing molten Al-Zn-Mg-Si-Sr plated steel sheet according to the present invention will be described. The method for manufacturing molten Al-Zn-Mg-Si-Sr plated steel sheet according to the present invention is characterized by using a plating bath having a composition containing Al: 40 to 70% by mass, Si: 0.6 to 5% by mass, Mg: 0.1 to 10% by mass, and Sr: 0.001 to 1.0% by mass, with the remainder being Zn and unavoidable impurities, and having a bath temperature of 585°C or less, and when applying molten plating to the steel sheet, the temperature of the steel sheet when it enters the plating bath (entry plate temperature) is set to a temperature of 20°C added to the bath temperature of the plating bath (plating bath temperature + 20°C) or less. The molten Al-Zn-Mg-Si-Sr plated steel sheet obtained by the above manufacturing method has a good surface appearance and also has excellent corrosion resistance in the flat plate portion and the processed portion.

[0049] The method for manufacturing a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet according to the present invention is not particularly limited, but a continuous hot-dip plating facility is usually used from the viewpoint of manufacturing efficiency and quality stability.

[0050] Furthermore, there are no particular limitations on the type of base steel sheet used in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention. For example, a hot-rolled steel sheet or strip that has been pickled and descaled, or a cold-rolled steel sheet or strip obtained by cold-rolling them, can be used. Also, there are no particular limitations on the conditions of the pretreatment and annealing processes, and any method can be adopted.

[0051] In the method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the plating bath has a composition containing Al: 40-70% by mass, Si: 0.6-5% by mass, Mg: 0.1-10% by mass, and Sr: 0.001-1.0% by mass, with the remainder being Zn and unavoidable impurities. This makes it possible to obtain a molten Al-Zn-Mg-Si-Sr plated steel sheet with a desired composition. The types, contents, and functions of each element contained in the plating bath are explained in the above-described section on the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention.

[0052] Furthermore, the molten Al-Zn-Mg-Si-Sr plated steel sheet obtained by the manufacturing method of the present invention has an overall composition that is almost the same as that of the plating bath. Therefore, the composition of the main layer can be controlled with high precision by controlling the composition of the plating bath.

[0053] Furthermore, in the method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, it is preferable that the content of Mg and Si in the plating bath satisfies the following formula (2). Ｍｇ / (M Ｓｉ -0.6)≧1.0...(2) M Ｍｇ : Mg content (mass%), M Ｓｉ : Si content (mass%) When the Mg and Si content in the plating bath satisfies the above relationship, the formation of the plated layer has suppressed Si phase generation (for example, the area ratio of the Si phase observed in the cross-section in the thickness direction of the plated layer is 10% or less, and the area ratio of the Si phase observed on the surface of the plated layer is 1% or less), making it possible to further improve workability and corrosion resistance. From a similar viewpoint, M Ｍｇ / (M Ｓｉ It is more preferable that -0.6) be 2.0 or greater, and even more preferable that it be 3.0 or greater.

[0054] In the method for manufacturing a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the bath temperature of the plating bath is preferably 585°C or lower, and more preferably 580°C or lower. This bath temperature setting allows large amounts of Mg to reach from the surface of the main layer of the plating layer to the interfacial alloy layer. ２ The amount of Si can be reduced. It also has the effect of suppressing the growth of the interfacial alloy layer. The reason why lowering the bath temperature is effective is because Mg ２ This is because it becomes possible to shorten the time the steel sheet stays in the high-temperature region where Si and the interface alloy layer grow. When the bath temperature of the plating bath exceeds 585°C, even if the temperature of the steel sheet at the time of entry into the plating bath is optimized, large amounts of Mg that reach from the surface of the main layer to the interface alloy layer may still be present. ２ The high amount of Si and the resulting thicker interfacial alloy layer make it impossible to obtain the desired machinability and corrosion resistance of the machined area.

[0055] Furthermore, in the method for manufacturing a molten Al-Zn-Mg-Si-Sr plated steel sheet according to the present invention, the steel sheet temperature at the time of entry into the plating bath (entry sheet temperature) is set to be 20°C higher than the temperature of the plating bath (plating bath temperature + 20°C). This prevents large Mg particles from reaching from the surface of the main layer of the plating layer to the interfacial alloy layer. ２ The amount of Si can be reduced. By reducing the entry plate temperature, a large amount of Mg can be reduced. ２ The reason why the amount of Si can be reduced is similar to the effect of lowering the bath temperature, Mg ２ This is because it is possible to shorten the time the steel sheet stays in the high-temperature region where the Si and interfacial alloy layers grow. From a similar viewpoint, the entry temperature of the steel sheet is preferably 10°C higher than the temperature of the plating bath (plating bath temperature + 10°C), and preferably lower than or equal to the temperature of the plating bath.

[0056] Furthermore, in the method for manufacturing a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, it is preferable to cool the steel sheet at an average cooling rate of 10°C / s or more after molten plating, until the sheet temperature reaches a temperature obtained by subtracting 150°C from the temperature of the plating bath (plating bath temperature - 150°C). Mg formed in the plating layer described above ２ It is known that Si is easily formed in the temperature range up to 150°C below the plating bath temperature (plating bath temperature - 150°C), and by increasing the cooling rate in that temperature range to an average of 10°C / sec or more, Mg ２ To suppress the growth of Si and to prevent large Mg particles from reaching from the surface of the main layer of the plating layer to the interfacial alloy layer. ２ The amount of Si can be reduced. Furthermore, by increasing the cooling rate of the steel sheet after hot-dip plating, the growth of the interface alloy layer can be suppressed, resulting in excellent corrosion resistance of the processed part. The average cooling rate is determined by finding the time it takes for the steel sheet to reach a temperature 150°C lower than the plating bath temperature, and then dividing 150°C by that time.

[0057] Furthermore, by increasing the cooling rate in the aforementioned temperature range to an average of 10°C / sec or more, the Mg present near the surface of the main plating layer can be removed. ２It becomes possible to increase the amount of Si to more than 50% of the total. The reason for this is as follows: The interface alloy layer is formed by a solid-liquid reaction in the plating bath (reaction of Al in the bath with the steel sheet), and Si is incorporated into the interface alloy layer during this reaction. Therefore, the Si concentration near the interface alloy layer when the interdendritic solidifies is lower than the average Si concentration of the main plating layer, and Mg ２ The distribution of Si is higher on the surface side of the main plating layer compared to the interface alloy layer side. On the other hand, during the cooling process after plating, the plated steel sheet is cooled from the surface, so the solidification of interdendritic material occurs from the surface. Mg in Zn ２ The solubility of Si decreases with decreasing temperature, and at the solid-liquid interface, Mg and Si are discharged into the liquid phase and concentrated in the interdendritic portion of the liquid. This phenomenon becomes more pronounced as the temperature approaches equilibrium, i.e., as the cooling rate slows down. Therefore, when the cooling rate is slow, Mg ２ The distribution of Si tends to be concentrated near the interface alloy layer. In other words, by increasing the cooling rate (to an average of 10°C / sec or more), the aforementioned Mg ２ Regarding the distribution of Si, the uneven distribution near the interface alloy layer is suppressed, and the original Mg ２ The distribution of Si can be maintained. That is, Mg in the plating layer ２ This ensures that the distribution of Si is greater on the surface side of the main layer than near the interface alloy layer. From a similar viewpoint, it is more preferable to cool the steel sheet after hot-dip plating at an average cooling rate of 20°C / sec or higher, even more preferable at an average cooling rate of 30°C / sec or higher, and particularly preferable at an average cooling rate of 40°C / sec or higher.

[0058] In addition, in the manufacturing method of the present invention, there are no particular limitations other than the bath temperature and entry plate temperature during hot-dip plating, and the cooling conditions after hot-dip plating, and a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet can be manufactured according to a conventional method.

[0059] The molten Al-Zn-Mg-Si-Sr plated steel sheet obtained by the manufacturing method of the present invention can be further subjected to a chemical conversion treatment film on its surface (chemical conversion treatment step) or a coating film can be further formed in a separate coating facility (coating film formation step). The chemical conversion treatment film can be formed, for example, by a chromate treatment or chromate-free chemical conversion treatment in which a chromate treatment solution or a chromate-free chemical conversion treatment solution is applied and dried at a steel sheet temperature of 80 to 300°C without washing with water. These chemical conversion treatment films may be single-layer or multi-layer, and in the case of multi-layer films, multiple chemical conversion treatments can be performed sequentially. As for the coating film, 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 by means of hot air drying, infrared heating, induction heating, etc.

[0060] (Samples 1-25) Using 0.5 mm thick cold-rolled steel sheets manufactured by conventional methods as the base sheet, hot-dip Al-Zn plated steel sheets of samples 1-25 were manufactured in a continuous hot-dip galvanizing facility. The composition of the plating bath used in the manufacturing process was approximately the same as that of the plating layer for each sample shown in Table 1. The bath temperature of the plating bath, the temperature at which the steel sheet entered the bath, and the cooling rate to a temperature obtained by subtracting 150°C from the bath temperature are shown in Table 1. Subsequently, each sample of the obtained hot-dip Al-Zn plated steel sheet was observed in cross-section at a random location using energy-dispersive X-ray spectroscopy (SEM-EDX) with a scanning electron microscope. Then, for each sample, the conditions of the formed plating layer and the manufacturing conditions of the plating were measured or calculated and are shown in Table 1.

[0061] (Evaluation) The following evaluations were performed on each sample of the molten Al-Zn plated steel sheet obtained as described above. The evaluation results are shown in Table 1.

[0062] (1) Surface Appearance For each sample of molten Al-Zn plated steel sheet, the surface of the plating layer (both sides of each sample) was visually observed within an observation field of view of approximately 1000-1600 mm (steel sheet width x 1000 mm length). The observation results were then evaluated according to the following criteria: ○: No wrinkle-like defects were observed on either the front or back surface. ×: Wrinkle-like defects were observed on at least one of the front or back surface.

[0063] (2) Corrosion resistance evaluation (Corrosion resistance of flat plate portion) For each sample of hot-dip Al-Zn plated steel sheet, a combined cycle test (JASO-CCT) of the Japanese Automotive Standards was performed. As shown in Figure 6, the JASO-CCT is a test in which salt spray, drying, and wetting are considered as one cycle under specific conditions. The number of cycles until red rust occurred in each sample was measured and evaluated according to the following criteria: ○: Number of cycles until red rust occurred ≥ 400 cycles △: 300 cycles ≤ Number of cycles until red rust occurred < 400 cycles ×: Number of cycles until red rust occurred < 300 cycles (3) Corrosion resistance evaluation of bent portion (Corrosion resistance of processed portion) For each sample of hot-dip Al-Zn-Mg-Si-Sr plated steel sheet, three plates of the same thickness were sandwiched inside and bent at 180° (3T bend), and then a combined cycle test (JASO-CCT) of the Japanese Automotive Standards was performed on the outside of the bend. As shown in Figure 6, the JASO-CCT test is a test in which salt spray, drying, and wetting constitute one cycle under specific conditions. The number of cycles until red rust appeared on the processed part of each sample was measured and evaluated according to the following criteria: ○: Number of cycles until red rust appeared ≥ 400 cycles △: 300 cycles ≤ Number of cycles until red rust appeared < 400 cycles ×: Number of cycles until red rust appeared < 300 cycles

[0064]

[0065] Table 1 shows that each sample of the present invention is superior to each sample of the comparative examples in terms of surface appearance, corrosion resistance, and corrosion resistance of the processed part, all in a well-balanced manner.

[0066] According to the present invention, it is possible to provide a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet having good surface appearance and excellent corrosion resistance in the flat plate portion and processed portion, as well as a method for manufacturing a hot-dip Al-Zn-Mg-Si-Sr plated steel sheet having good surface appearance and excellent corrosion resistance in the flat plate portion and processed portion.

Claims

DEPCT651. A flat, hot-dip coated steel plate with an Al-Zn-Mg-Si-Sr alloy layer composed of a hot-dip coating layer containing 40% to 70% by mass of Al, 0.6% to 5% by mass of Si, 0.1% to 10% by mass of Mg, and 0.001% to 1.0% by mass of Sr, with an unavoidable balance of Zn and impurities. The hot-dip coating layer is composed of an interface alloy layer present at the interface with the base steel plate, and the primary layer is present on the alloy layer. For Mg2Si observed in the cross-section in the thickness direction of the hot-dip coating layer, the ratio of the area of ​​Mg2Si existing within the range from the surface of the primary layer to 50% of the thickness of the primary layer is 50% or more, and the ratio of the area of ​​Mg2Si extending from the surface of the primary layer to the interface alloy layer is 50% or less.Hot-dip Al-Zn-Mg-Si-Sr coated steel plates according to claim 1, whereby for Mg2Si observed in the cross-section in the thickness direction of the hot-dip coating layer, the ratio of the area of ​​Mg2Si existing within the range from the surface of the primary layer to 50% of the thickness of the primary layer is 60% or more, and the ratio of the area of ​​Mg2Si extending from the surface of the primary layer to the interface alloy layer is 50% or less.

3. Hot-dip Al-Zn-Mg-Si-Sr coated steel plates. Under claim 1 or 2, where for Mg2Si observed in the cross-section in the thickness direction of the hot-dip coating layer, the ratio of the area of ​​Mg2Si existing within the range from the surface of the primary layer to 50% of the thickness of the primary layer is 60% or more, and the ratio of the area of ​​Mg2Si extending from the surface of the primary layer to the interface alloy layer is 40% or less.

4. Steel plates hot-dip coated with Al-Zn-Mg-Si-Sr under one of claims 1 to 3.Where the ratio of the region of the Si phase observed in the cross-section in the thickness direction of the hot-dip coating layer to the ratio of the total region of Mg2Si and Si phase observed in the cross-section in the thickness direction of the hot-dip coating layer is 30% or less.

5. Steel plate hot-dip coated with Al-Zn-Mg-Si-Sr according to one of the claims 1 to 4, where the principal layer has an alpha-Al phase in the dendritic region, and the average distance between the dendritic arms of the dendritic region and the The thickness of the hot-dip coated steel layer which satisfies the following expression(1),t / d ≥ 1.5(1), where t is the thickness of the hot-dip coated layer in micrometers, and d is the average distance between the dendritic arms in micrometers.6.Method of the production of Al-Zn-Mg-Si-Sr hot-dip coated steel plates, which is composed of the use of a molten bath containing a composition of 40% by mass to 70% by mass of Al, 0.6% by mass to 5% by mass of Si, 0.1% by mass to 10% by mass of Mg, and 0.001% by mass to 1.0% by mass of Sr, with an equilibrium value of Zn and unavoidable impurities, and a bath temperature of 585°C or lower to perform hot-dip coating on steel plates, where the temperature of the steel plate at the time of molten bath entry, which is the plate temperature at the entry, is equal to or 20°C lower than the filling temperature to the molten bath temperature, which is the molten bath temperature +20°C.

7. Method of producing hot-dip coated steel plates with Al-Zn-Mg-Si-Sr according to claim 6, where the plate temperature at the time of molten bath entry, which is the molten bath temperature +20°C.

8. The method of producing Al-Zn-Mg-Si-Sr hot-dip coated steel plates according to claim 6 or 7, where after hot-dip coating on the steel plate, the steel plate is cooled at an average cooling rate of 10 °C / s or higher until the plate temperature reaches a temperature of minus 150 °C from the molten bath temperature, which is the molten bath temperature of -150 °C.