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

Abstract

The objective is to provide a hot-dip Al-Zn plated steel sheet and a method for manufacturing the same, which are stable and have excellent bendability and corrosion resistance of the bent portion. [Solution] In order to achieve the above objective, the present invention provides a hot-dip Al-Zn plated steel sheet comprising: a plating film having a composition containing Al: 45-65 mass% and Si: 1.0-3.0 mass% with the remainder being Zn, Fe and unavoidable impurities; and an interface alloy layer formed on the interface side of the plating film with the underlying steel sheet, the interface alloy layer containing Fe, Al, Si, Zn and unavoidable impurities, wherein the plating film has dendrites mainly consisting of primary Al crystals and dendrite gaps containing Al-Zn eutectic crystals, the primary Al crystals contain an α-Al phase matrix and Zn precipitates, the Zn content in the matrix is ​​30 mass% or less, and the thickness of the interface alloy layer is 1 μm or less.

Description

Technical Field

[0001] The present invention relates to a hot-dip Al-Zn alloy coated steel sheet excellent in bending workability and corrosion resistance of the bent portion, and a method for producing the same.

Background Art

[0002] Hot-dip Al-Zn alloy coated steel sheets are known to exhibit high corrosion resistance among hot-dip zinc coated steel sheets because the sacrificial anticorrosion property of Zn and the high corrosion resistance of Al can coexist. Therefore, hot-dip Al-Zn alloy coated steel sheets are widely used in the building materials field such as roofs and walls exposed outdoors for a long time, and in the civil engineering and construction fields such as guardrails, wiring pipes, and soundproof walls. In particular, due to the increasing demand for materials with excellent corrosion resistance and maintenance-free materials in more severe usage environments such as acid rain caused by air pollution, spraying of snow-melting agents for preventing road freezing in snow-covered areas, and coastal area development, the demand for hot-dip Al-Zn alloy coated steel sheets has been increasing in recent years.

[0003] Here, the coating film of the hot-dip Al-Zn alloy coated steel sheet is composed of a main layer and an interfacial alloy layer present at the interface between the base steel sheet and the main layer. The main layer is mainly composed of a portion in which Zn is supersaturated and Al dendrites are solidified (dendrite portion of α-Al phase) and a portion of the remaining interdendritic gaps (interdendrite). The α-Al phase has a structure in which a plurality of layers are laminated in the film thickness direction of the coating film. Due to such a characteristic coating film structure, the corrosion progress path from the surface becomes complicated, so that corrosion is less likely to reach the base steel sheet easily. The hot-dip Al-Zn alloy coated steel sheet can achieve more excellent corrosion resistance compared to a hot-dip zinc coated steel sheet having the same coating film thickness.

[0004] However, while hot-dip Al-Zn plated steel sheets have excellent corrosion resistance, they have the drawback of having a harder plating film and inferior bendability compared to hot-dip galvanized steel sheets. As a result, when the steel sheets are bent, cracks tend to form in the plating film at the tip of the bent section. These cracks not only damage the appearance, but if the cracks reach partway through the plating film, the thickness of the plating in that area will decrease, or the cracks will penetrate the plating film and expose the underlying steel sheet. This causes the excellent corrosion resistance inherent in hot-dip Al-Zn plated steel sheets to be significantly reduced in the bent section.

[0005] For this reason, various attempts have been made to improve the bendability of hot-dip Al-Zn plated steel sheets. For example, one technique involves applying a predetermined thermal history to a molten Al-Zn plated steel sheet after plating to improve its bendability (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 3654521 [Patent Document 2] Japanese Patent Publication No. 2013-245355 [Overview of the project] [Problems that the invention aims to solve]

[0007] In techniques such as those described in Patent Documents 1 and 2, which involve subjecting molten Al-Zn plated steel sheets to thermal history, the plating film can be softened, and a certain degree of improvement in bendability has been achieved. However, the improved bendability achieved by the technologies described in Patent Documents 1 and 2 was insufficient when subjected to more severe bending processes. Considering its application to various building components, further improvements in bendability and corrosion resistance of the processed parts were desired. Furthermore, the development of technologies that could more reliably (stably) improve bendability and corrosion resistance of the processed parts was also desired.

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

[0009] The present inventors investigated a hot-dip Al-Zn plated steel sheet having a plating film containing Al: 45-65% by mass and Si: 1.0-3.0% by mass, with the remainder being Zn, Fe, and unavoidable impurities, in order to solve the above problems. As a result, they found that the plating film has dendrites mainly consisting of primary Al crystals and dendrite gaps containing Al-Zn eutectic crystals. Focusing on the fact that in the primary Al crystals, when the Zn precipitates scattered within the α-Al phase matrix are fine and less than 100 nm in size, it affects the hardening of the dendrites, they found that by keeping the Zn content in the matrix low, it is possible to soften the dendrites while suppressing the fineness and increase of the Zn precipitates mentioned above, thereby stably achieving excellent bendability and corrosion resistance of the processed part. Furthermore, focusing on the fact that the Zn precipitates in the primary Al crystals mentioned above are closely related to the thermal history conditions after the formation of the plating film, we found that by optimizing the maximum temperature reached, as well as the heating time and cooling time, it is possible to keep the Zn content in the matrix low and obtain a hot-dip Al-Zn plated steel sheet with excellent bendability and corrosion resistance of the processed parts. Furthermore, we focused on the fact that in Al-Zn eutectic structures, when the Al and Zn parts are arranged alternately in stripes (hereinafter referred to as "striped structure") and the period of these stripes is 2 μm or less, it reduces the bendability of molten Al-Zn plated steel sheets. We also found that by eliminating the striped structure, excellent bendability and corrosion resistance of the processed area can be achieved. In addition, we discovered that bending processability can be further improved by keeping the thickness of the highly hard interfacial alloy layer to 1 μm or less.

[0010] In this invention, "excellent bendability" refers to practically sufficient bendability, and when evaluated using "T-bending," at least "6T no crack," preferably "4T no crack," is required. "T-bending" is a 180° bending test performed with a steel plate of the same thickness sandwiched between the materials. For example, for "6T bending," six plates of the same thickness are sandwiched inside the target material and then bent 180°. "No crack" means that, for example, no cracks are observed when the outer tip of the bent section is observed with a magnifying glass at 10x magnification. The bending test is a bending test in accordance with the plating adhesion test described in JIS G 3321 (2019). Incidentally, the bendability of typical hot-dip Al-Zn plated steel sheets is generally "12T no crack" or better, although this depends on the plating film conditions. Even at "10T bend," it often does not result in "no cracks."

[0011] This invention is based on the above findings, and its gist is as follows. 1. A hot-dip Al-Zn plated steel sheet comprising a plating film having a composition containing Al: 45-65% by mass and Si: 1.0-3.0% by mass, with the remainder being Zn, Fe and unavoidable impurities, and an interface alloy layer formed on the interface side between the plating film and the underlying steel sheet, containing Fe, Al, Si, Zn and unavoidable impurities, The aforementioned plating film has dendrites mainly composed of primary Al crystals and interdendritic gaps containing Al-Zn eutectic crystals. The primary Al crystals contain a matrix of α-Al phase and precipitates of Zn, and the Zn content in the matrix is 30% by mass or less. A molten Al-Zn plated steel sheet, characterized in that the thickness of the interfacial alloy layer is 1 μm or less.

[0012] 2. The molten Al-Zn plated steel sheet according to 1 above, characterized in that the average maximum diameter of the Zn precipitates in the primary Al crystals is 100 nm or more.

[0013] 3. The molten Al-Zn plated steel sheet according to 1 or 2 above, characterized in that the Al-Zn eutectic in the dendrite gaps does not have a striped structure with a period of 2 μm or less.

[0014] 4. A method for manufacturing the molten Al-Zn plated steel sheet according to any one of 1 to 3 above, A step of forming a plating film on a base steel sheet, the plating film having a composition containing 45 to 65% by mass of Al and 1.0 to 3.0% by mass of Si, and the balance being composed of Zn, Fe, and inevitable impurities; After forming the plating film, a step of applying a thermal history to the steel sheet such that the maximum reached temperature is 150°C or higher and 277°C or lower. In the step of applying the thermal history, the cooling time from the maximum reached temperature to 150°C is less than 2 hours, and the cooling time from 150°C to room temperature is 3 hours or more. A method for manufacturing a molten Al-Zn plated steel sheet, characterized by this.

Effects of the Invention

[0015] According to the present invention, it is possible to stably provide a molten Al-Zn plated steel sheet excellent in bending workability and corrosion resistance of the bent portion, and a method for manufacturing the same.

Brief Description of the Drawings

[0016] [Figure 1] It is an Al-Zn binary system equilibrium diagram. [Figure 2]Samples of the molten Al-Zn electroplated steel sheets of Comparative Example 1 and Inventive Example 14 are respectively shown with the Zn content in the matrix of the α-Al phase, the average value of the maximum diameter of the Zn precipitates in the Al primary crystal, and a photograph of the cross-section of the Al primary crystal observed. [Figure 3] Photographs of the cross-sections of the electroplated coatings observed for samples of the molten Al-Zn electroplated steel sheets of Comparative Example 1, Inventive Example 14, and Comparative Example 22 are respectively shown. [Figure 4] Graphs showing the evaluation results of the corrosion resistance of the bent portions and photographs of the 1T bent portions observed for samples of the molten Al-Zn electroplated steel sheets of Comparative Example 1 and Inventive Example 14 are respectively shown.

Embodiments for Carrying Out the Invention

[0017] (Molten Al-Zn Electroplated Steel Sheet) The molten Al-Zn electroplated steel sheet of the present invention has an electroplated coating on the steel sheet surface. And the electroplated coating contains Al: 45 to 65% by mass and Si: 1.0 to 3.0% by mass, and the balance consists essentially of Zn, Fe, and inevitable impurities. By having the above-described composition, the electroplated coating of the molten electroplated steel sheet can achieve good corrosion resistance. Incidentally, the electroplated coating consists of an interfacial alloy layer present on the interface side with the base steel sheet and a main layer present on top of the interfacial alloy layer.

[0018] The Al content in the electroplated coating is 45 to 65% by mass, preferably 50 to 60% by mass, in view of the balance between corrosion resistance and operation. If the Al content in the electroplated coating is at least 45% by mass, dendrite solidification of the Al primary crystal occurs, and a structure in which the dendrite solidification structure is laminated in the film thickness direction of the electroplated coating can be obtained. By adopting the structure in which the dendrite solidification structure is laminated in the film thickness direction of the electroplated coating, the corrosion progression path of the electroplated coating becomes complicated, and the corrosion resistance can be improved. Also, the more dendrites are laminated, the more complicated the corrosion progression path becomes, and it becomes difficult for corrosion to easily reach the base steel sheet, thereby improving the corrosion resistance. On the other hand, if the Al content in the plating film exceeds 65% by mass, most of the Zn present in the dendrites is incorporated into the structure where it is solid-solved in the primary Al crystal, making it impossible to suppress the dissolution reaction of the primary Al crystal during corrosion progression, and thus the corrosion resistance deteriorates.

[0019] The Si in the plating film is added to suppress the growth of the interfacial alloy layer that forms at the interface with the underlying steel sheet, and to prevent deterioration of the adhesion between the plating film and the underlying steel sheet. In the case of the molten Al-Zn plated steel sheet of the present invention, when the steel sheet is immersed in an Al-Zn plating bath containing Si, the Fe on the surface of the steel sheet reacts with the Al and Si in the plating bath to form an alloy, and Fe-Al and / or Fe-Al-Si intermetallic compounds are formed in layers at the interface between the underlying steel sheet and the plating film (an interfacial alloy layer is formed). At this time, the Fe-Al-Si alloy grows more slowly than the Fe-Al alloy, so the higher the proportion of the Fe-Al-Si alloy, the more the growth of the entire alloy phase can be suppressed. For this reason, the Si content in the plating film must be 1.0% by mass or more. On the other hand, excess Si that is not consumed in the formation of the interfacial alloy layer precipitates as a Si phase in the plating film. However, the Si phase is electrochemically nobler than the primary Al or Al-Zn eutectic and acts as a cathode, thus promoting corrosion of the plating film and reducing its corrosion resistance. Specifically, if the Si content in the plating film exceeds 3.0 mass%, not only does the aforementioned effect of inhibiting the growth of the alloy phase saturate, but the amount of Si phase increases and corrosion is promoted. Therefore, the Si content should be kept below 3.0 mass. From a similar viewpoint, it is more preferable that the Si content in the plating film be 2.5% by mass or less.

[0020] The plating film contains Zn, Fe, and unavoidable impurities. Of these components, Fe is inevitably included in the plating film, either because steel plates and equipment in the plating bath dissolve into the plating bath, or because it is supplied by diffusion from the underlying steel plate during the formation of the interfacial alloy layer. It is not possible to distinguish and quantify the Fe in the plating film from that incorporated from the underlying steel plate and that dissolved from the plating bath. The Fe content in the plating film is usually around 0.3 to 2.0 mass%. In addition to Fe, other unavoidable impurities include Cr, Ni, Cu, and the like. The total content of Fe and the unavoidable impurities is not particularly limited, but if present in excess, it may affect various properties of the plated steel sheet. Therefore, it is preferable that the total content be 5.0% by mass or less, and more preferably 3.0% by mass or less.

[0021] Furthermore, in the molten Al-Zn plated steel sheet of the present invention, the plated film may further contain one or more elements selected from Mg, Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb, and B in a total amount of 0.01 to 10% by mass, as this improves the stability of corrosion products and delays the progression of corrosion. The reason for setting the total content of the above-mentioned components to 0.01 to 10% by mass is that a sufficient corrosion delaying effect can be obtained without the effect becoming saturated.

[0022] Furthermore, the amount of the plating film applied is 45 to 120 g / m² per side, from the viewpoint of satisfying various characteristics. 2 Preferably, the amount of the plating film is 45 g / m². 2 In the above case, sufficient corrosion resistance can be obtained even for applications requiring long-term corrosion resistance, such as building materials, and the amount of the plating film attached is 120 g / m². 2 In the following cases, it is possible to achieve excellent corrosion resistance while suppressing the occurrence of plating cracks during processing. From a similar perspective, the amount of the plating film attached is 45-100 g / m². 2 It is preferable that it be so.

[0023] Here, the amount of plating film attached can be derived, for example, by dissolving and peeling the plating film from a specific area using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401:2013, and calculating the amount from the difference in steel plate weight before and after peeling. To obtain the amount of plating attached to one side using this method, the plating surface of the non-target side can be sealed with tape to prevent exposure before carrying out the dissolution described above.

[0024] Furthermore, the component composition of the plating film can be determined, for example, by immersing the plating film in hydrochloric acid or the like to dissolve it, and then confirming the solution using ICP emission spectrometry or atomic absorption spectrometry. This method is merely one example, and any method that can accurately quantify the component composition of the plating film is acceptable and is not particularly limited.

[0025] Furthermore, the plating film of the molten Al-Zn plated steel sheet obtained according to the present invention has a composition that is approximately the same as that of the plating bath. Therefore, the composition of the plating film can be precisely controlled by controlling the composition of the plating bath.

[0026] Furthermore, the interface alloy layer in the plating film is a layer present at the interface between the plating film and the underlying steel sheet, and is a layered interface alloy layer containing Fe, Al, Si, Zn, and unavoidable impurities. As described above, the interface alloy layer is inevitably formed by an alloying reaction between Fe on the surface of the underlying steel sheet and Al and Si in the plating 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 needs to be as thin as possible. For this reason, in the hot-dip Al-Zn plated steel sheet of the present invention, the thickness of the interface alloy layer must be 1 μm or less, and preferably 0.8 μm or less. If the thickness of the interface alloy layer exceeds 1 μm, the bendability will decrease.

[0027] The interface alloy layer is defined as the average of the measured values ​​of the average thickness of the interface alloy layer present in each field of view when the cross-section of the plating film is observed in three or more fields of view using a scanning electron microscope (SEM) or the like. Furthermore, there are no particular limitations on the method for reducing the thickness of the interface alloy layer. For example, as mentioned above, this could involve adjusting the Si content in the plating film, or, as will be described later, adjusting the cooling time when applying a thermal history after the plating film has been formed.

[0028] In the molten Al-Zn plated steel sheet of the present invention, the plating film has dendrites mainly composed of primary Al crystals and dendritic gaps containing Al-Zn eutectic crystals. Furthermore, the molten Al-Zn plated steel sheet of the present invention is characterized in that the primary Al crystal has Zn precipitates in the matrix of the α-Al phase, and the Zn content in the matrix is ​​30% by mass or less.

[0029] When Zn remains supersaturated (Zn content exceeds 30% by mass) in the matrix of the α-Al phase and solidifies, the hardness increases due to solid solution strengthening of Zn, reducing elongation and decreasing bendability. Therefore, in this invention, by limiting the Zn content in the matrix to 30% by mass or less, solid solution strengthening of the Al primary crystal is suppressed, improving the bendability of the molten Al-Zn plated steel sheet and, consequently, the corrosion resistance of the processed part. Furthermore, since the decrease in bendability due to precipitation strengthening tends to be more pronounced the finer the Zn precipitates are, limiting the Zn content in the matrix of the α-Al phase to 30% by mass or less can also promote the growth of Zn precipitates. Note that the Zn content in the matrix refers to the Zn content contained in the matrix and does not include the content of precipitated or separated Zn (Zn precipitates). From a similar viewpoint, the Zn content in the matrix is ​​preferably 25% by mass or less, and more preferably 20% by mass or less.

[0030] The aforementioned Zn precipitates are granular precipitates mainly composed of Zn. However, in this invention, the spatial resolution of the ultra-low accelerating voltage scanning electron microscope (Ultra Low Accelerating Voltage Scanning Electron Microscope, hereinafter referred to as "ultra-low accelerating SEM") used for observation is approximately 30 nm. Since Zn precipitates smaller than this cannot be observed, precipitates with a diameter of 30 nm or more are considered to be Zn precipitates.

[0031] Regarding the primary Al crystal, when Zn precipitates are scattered within the α-Al phase matrix, as described above, the bendability tends to decrease due to precipitation strengthening, and this tendency becomes more pronounced the finer the precipitates are. Therefore, growing the Zn precipitates to a larger size is advantageous for bendability. Specifically, it is preferable that the average maximum diameter of the Zn precipitates in the primary Al crystal is 100 nm or more. The average of the maximum diameter of the Zn precipitates is, for example, the average of the longest diameters of 10 Zn-based precipitates in each field of view when observing Al primary crystals in three or more fields of view using an extremely low-acceleration SEM (acceleration voltage of 3kV, magnification of 20,000 times or more), and then taking the average of these measurements.

[0032] Furthermore, the plating film has dendrite gaps containing an Al-Zn eutectic. In addition to the Al-Zn eutectic, these dendrite gaps may also contain elemental Si phases. The Al-Zn eutectic constituting the dendritic gap consists of Al and Zn portions. When the Al-Zn eutectic is heated to 277°C or higher, the solid solubility of Zn in the Al portion increases, causing the Zn portion to almost completely dissolve, resulting in an Al portion containing a more supersaturated amount of Zn. Subsequently, when the Al-Zn eutectic is cooled, it changes back to an Al-Zn eutectic at temperatures below 277°C, but at this point, the Al-Zn eutectic has a striped structure in which the Al and Zn portions are arranged alternately in stripes.

[0033] As a result of our research, although the mechanism is not yet clear, we have found that this striped structure of the Al-Zn eutectic reduces the bendability of molten Al-Zn plated steel sheets, and that the reduction in bendability is particularly pronounced when the period of the stripes in the striped structure is small, less than 2 μm. Therefore, from the viewpoint of further improving the bendability and corrosion resistance of the processed portion of the molten Al-Zn plated steel sheet of the present invention, it is preferable that the Al-Zn eutectic in the dendrite gaps does not contain a stripe-like structure with a period of 2 μm or less. The lower limit of the stripe period of the stripe-like structure is not particularly limited. However, due to the performance of the measuring device described later, it is difficult to confirm the presence of a stripe-like structure with a period of less than 30 nm. Therefore, in the present invention, a stripe-like structure is considered to have a stripe period of 30 nm or more.

[0034] The striped structure of the Al-Zn eutectic described above can be measured using an extremely low-acceleration SEM (acceleration voltage 3kV), similar to the Zn precipitates in the primary Al crystal. While the striped structure of the Al-Zn eutectic, with a stripe period of 2μm or less, could not be detected with an SEM using a high acceleration voltage, such as 15kV or higher, the present invention makes it possible to confirm its presence or absence by observing it using an extremely low-acceleration SEM. Na Furthermore, since the Zn precipitates and the striped structure of the Al-Zn eutectic are both finer than those formed when thermal history is applied, their presence or absence was not considered in observations using, for example, an accelerating voltage of 15kV.

[0035] Furthermore, the method for controlling the Zn content in the matrix, the maximum diameter of the Zn precipitates, and the presence or absence of a striped structure with a period of 2 μm or less, as described above, is not particularly limited and can be appropriately controlled by optimizing the manufacturing conditions, etc. For example, as will be described later, by determining the composition of the plating bath and optimizing the thermal history conditions after the formation of the plating film, it is possible to control the Zn content in the matrix, the maximum diameter of the Zn precipitates, and the presence or absence of a stripe-like structure with a period of 2 μm or less.

[0036] Furthermore, the molten Al-Zn plated steel sheet of the present invention can have a coating film formed on the plated film either directly or via an intermediate layer. The type of coating and the method of forming the coating are not particularly limited and can be appropriately selected according to the required performance. For example, methods such as roll coater coating, curtain flow coating, and spray coating can be used. After applying a paint containing an organic resin, it is possible to form a coating by heating and drying it using means such as hot air drying, infrared heating, or induction heating.

[0037] Furthermore, the intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the hot-dip galvanized steel sheet and the coating film. Examples include a chemical conversion coating and a primer such as an adhesive layer. The chemical conversion coating can be formed, for example, by a chromate treatment or chromium-free chemical conversion treatment, which involves applying a chromate treatment solution or a chromium-free chemical conversion treatment solution and drying it at a steel sheet temperature of 80 to 300°C without washing with water. These chemical conversion coatings may be single-layer or multi-layer, and in the case of multi-layer coatings, multiple chemical conversion treatments may be performed sequentially.

[0038] (Method for manufacturing hot-dip Al-Zn plated steel sheet) The present invention provides a method for manufacturing a molten Al-Zn plated steel sheet, comprising the steps of forming a plating film on a base steel sheet and, after the formation of the plating film, subjecting the steel sheet to a thermal history.

[0039] The method for forming the plating film on the base steel plate is not particularly limited. For example, it can be produced by washing, heating, and immersing the base steel plate in a plating bath using a continuous hot-dip galvanizing facility. In the heating process of the base steel sheet, recrystallization annealing is performed to control the structure of the base steel sheet itself, and heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective in preventing oxidation of the steel sheet and reducing the trace amount of oxide film present on the surface.

[0040] Furthermore, there are no particular limitations on the type of base steel sheet or its composition. Cold-rolled steel sheets, hot-rolled steel sheets, etc., can be used as appropriate depending on the required performance and specifications. For example, steel sheets with a carbon content of 0.01 to 0.10% by mass can be used. However, steel sheets with less than 0.01% carbon are not excluded in this invention. In addition, steel sheets containing trace elements such as N, S, O, B, V, Nb, Ti, Cu, Mo, Cr, Co, Ni, Ca, Sr, In, Sn, Sb, etc., in addition to C, Al, Si, Mn, and P as constituent elements are also within the scope of this invention. Furthermore, there are no particular limitations on the method for obtaining the base steel sheet. For example, in the case of hot-rolled steel sheets, those that have undergone a hot-rolling process and a pickling process can be used, and in the case of cold-rolled steel sheets, a cold-rolling process can be added to the manufacturing process. In addition, it is possible to go through a recrystallization annealing process or the like before the hot-dip galvanizing process in order to obtain the properties of the steel sheet.

[0041] As mentioned above, the plating bath used to form the aforementioned plating film is one in which the overall composition of the plating film is approximately the same as that of the plating bath. Therefore, a plating bath containing Al: 45-65% by mass and Si: 1.0-3.0% by mass, with the remainder being substantially Zn, Fe, and unavoidable impurities, is used.

[0042] Furthermore, while the temperature of the plating bath is not particularly limited, it is preferable to set it in the range of (melting point + 20°C) to 650°C. The reason the lower limit of the plating bath temperature is set to the melting point + 20°C is that in order to perform molten plating, it is necessary to raise the bath temperature above the solidification point, and setting it to the melting point + 20°C prevents solidification due to a localized drop in the bath temperature of the plating bath. On the other hand, the reason the upper limit of the bath temperature is set to 650°C is that if it exceeds 650°C, rapid cooling of the plating film becomes difficult, and there is a risk that the interfacial alloy layer formed at the interface between the plating film and the underlying steel plate will become thicker.

[0043] Furthermore, the temperature of the base steel plate that enters the plating bath (entry plate temperature) is not particularly limited. For example, from the viewpoint of ensuring plating characteristics and preventing changes in the bath temperature in the continuous hot-dip galvanizing operation, it is preferable to control the temperature of the plating bath to within ±20°C.

[0044] Furthermore, it is preferable that the immersion time of the base steel plate in the plating bath be 0.5 seconds or longer. If the immersion time is less than 0.5 seconds, there is a risk that a sufficient plating film may not be formed on the surface of the base steel plate. Although there is no particular upper limit to the immersion time, it is preferable to keep it within 8 seconds, as a longer immersion time may result in a thicker interfacial alloy layer formed between the plating film and the steel plate.

[0045] Furthermore, the present invention provides a method for manufacturing a molten Al-Zn plated steel sheet, characterized in that, in the step of imparting the thermal history, the maximum temperature reached is 150°C or higher and 277°C or lower, the cooling time from the maximum temperature to 150°C is less than 2 hours, and the cooling time from 150°C to room temperature is 3 hours or more. By applying such a thermal history, it is possible to stably obtain hot-dip Al-Zn plated steel sheets with excellent bendability and corrosion resistance of the bent parts.

[0046] The reason why the maximum temperature reached when applying the aforementioned thermal history is set to 150°C or higher and 277°C or lower is that if the maximum temperature reached is below 150°C, the diffusion of Zn slows down, and the solid solution strengthening and precipitation strengthening in the Al primary crystal cannot be sufficiently eliminated. Furthermore, the striped structure in the Al-Zn eutectic remains, making it impossible to obtain sufficient bendability for the molten Al-Zn plated steel sheet. On the other hand, if the maximum temperature reached exceeds 277°C, the solid solution strengthening and precipitation strengthening in the Al primary crystal are eliminated, and the striped structure in the Al-Zn eutectic also decomposes. However, when the Al-Zn eutectic cools and passes through 277°C, the striped structure is regenerated in the Al-Zn eutectic, leading to a deterioration in the bendability of the molten Al-Zn plated steel sheet. From a similar viewpoint, the maximum temperature reached when imparting the thermal history is preferably 170°C to 250°C, and more preferably 190°C to 230°C.

[0047] Furthermore, in the process of imparting the thermal history, the cooling time from the highest temperature reached to 150°C is set to less than 2 hours in order to suppress the growth of the interface alloy layer, thereby improving bendability, and to minimize the change in the structure of the plating film achieved in the heating stage during the cooling stage, thereby maintaining the elimination of the solid solution strengthening and precipitation strengthening mentioned above, and suppressing the occurrence of a striped structure. From a similar viewpoint, it is preferable that the cooling time from the highest temperature reached to 150°C is 1 hour or less.

[0048] Furthermore, the reason why the cooling time from 150°C to room temperature is set to 3 hours or more in the process of imparting the thermal history is to ensure the temperature and time at which Zn diffuses in the primary Al crystal, to keep the Zn content in the matrix at 30% by mass or less, and to ensure that the average maximum diameter of the Zn precipitates is 100 nm or more, thereby sufficiently eliminating solid solution strengthening and precipitation strengthening in the primary Al crystal. Note that "room temperature" refers to room temperature, which is assumed to be around 25°C. Furthermore, from the viewpoint of manufacturing efficiency, the cooling time from 150°C to room temperature is preferably within 10 hours.

[0049] Here, Figure 1 shows the Al-Zn binary equilibrium phase diagram. In a typical hot-dip galvanizing process, the cooling after plating is rapid, so the Zn is not released from the dendrites in time to solidify, and the matrix solidifies with Zn in a supersaturated state (over 30% by mass). As a result, solid solution strengthening occurs due to the supersaturated Zn in the α-Al phase (matrix) of the primary Al crystal, resulting in increased hardness, reduced elongation, and decreased bendability. When the plated film is heated after formation, supersaturated Zn precipitates in the α-Al phase, reducing the Zn solid solubility. Subsequent cooling causes solidification, separating the α-Al phase matrix from the Zn precipitate. It can be seen that by controlling the Zn content in the matrix to 30% by mass or less, the solid solubility strengthening of the Al primary crystal is eliminated. Furthermore, the Al-Zn eutectic consists of Al and Zn parts. When heated above 277°C, the solid solubility of Zn in the Al part increases, causing the Zn part to almost completely dissolve, resulting in an Al part with a more supersaturated Zn content. Upon cooling after heating, it reverts back to an Al-Zn eutectic below 277°C, but this Al-Zn eutectic exhibits a striped structure in which Al and Zn parts are arranged alternately in stripes.

[0050] In addition, in the method for manufacturing a molten Al-Zn plated steel sheet of the present invention, steps other than the steps for forming the plating film and the steps for imparting a thermal history are not particularly limited, and any steps can be appropriately performed depending on the performance required for the molten Al-Zn plated steel sheet.

[0051] Furthermore, the method may further include a step of forming a coating film directly or via an intermediate layer on the molten Al-Zn plated steel sheet obtained by the method for manufacturing a molten Al-Zn plated steel sheet according to the present invention described above.

[0052] The method for forming the coating film is not particularly limited and can be appropriately selected according to the required performance. Examples of coating methods include roll coater coating, curtain flow coating, and spray coating. After applying a paint containing an organic resin, it is possible to form a coating film by heating and drying it using means such as hot air drying, infrared heating, or induction heating.

[0053] Furthermore, the intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the hot-dip galvanized steel sheet and the coating film. The type and method of forming the intermediate layer are the same as those described in the hot-dip Al-Zn galvanized steel sheet of the present invention. [Examples]

[0054] <Samples 1-30> (1) Manufacturing of hot-dip Al-Zn plated steel sheets Using a cold-rolled steel sheet with a thickness of 0.35 mm manufactured by a conventional method as the base steel sheet, hot-dip Al-Zn plated steel sheets A to C, with the plating film composition and adhesion amount shown in Table 1, were produced by annealing and plating treatments in a continuous hot-dip galvanizing facility. The composition of the plating bath used in the production of the hot-dip galvanized steel sheets was based on a composition (Plating A) consisting of Al: 55% by mass, Si: 1.6% by mass, Fe: 0.4% by mass, with the remainder being substantially Zn and unavoidable impurities. Compositions with varying amounts of each component were used (Plating B, Plating C). Furthermore, the plating bath temperature was set to 590°C in all cases, and the temperature of the base steel plate before plating was controlled to be the same as the plating bath temperature. In addition, the amount of plating film deposited was 85±10 g / m² per side in all cases. 2 It was controlled to achieve this.

[0055] (2) Assignment of thermal history The obtained molten Al-Zn plated steel sheets were subjected to thermal history under the conditions shown in Table 2 to obtain molten Al-Zn plated steel sheets for each sample.

[0056] (3) Confirmation of the amount and composition of the plating film From each sample of molten Al-Zn plated steel sheet, a 100 mm diameter piece was punched out, the non-measurement surface was sealed with tape, and the plating was dissolved and removed using a mixture of hydrochloric acid and hexamethylenetetramine as specified in JIS H 0401 (2013). The amount of plating film attached was calculated from the difference in mass of the sample before and after removal. Subsequently, the stripping solution was filtered, and the filtrate and solid components were analyzed separately. Specifically, the filtrate was analyzed by ICP emission spectroscopy to quantify components other than insoluble Si. The solid components were dried and ashed in a 650°C heating furnace, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was then dissolved in hydrochloric acid, and the insoluble Si was quantified by ICP emission spectroscopy analysis of the solution. The Si concentration in the plating film was calculated by adding the insoluble Si concentration obtained from solid component analysis to the soluble Si concentration obtained from filtrate analysis. The composition and deposition amounts of the obtained plating films A to C are shown in Table 1.

[0057] [Table 1]

[0058] <Rating> Each sample of the molten Al-Zn plated steel sheet obtained as described above was evaluated as follows. The evaluation results are shown in Table 1.

[0059] (1) Conditions for the plating film For each sample of molten Al-Zn plated steel sheet, the cross-section of the plating film was observed using an ultra-low acceleration SEM and analyzed by energy-dispersive X-ray spectroscopy (EDX). The observation and analysis conditions for the above-mentioned plating film were as follows: Zeiss ULTRA55 (ultra-low acceleration SEM) and Oxford Instruments Ultim Extreme (EDX), with an acceleration voltage of 3kV, observation magnifications of 3000x and 20000x, and point analysis of predetermined locations. The average maximum diameter of Zn-dominant precipitates present in the primary Al crystal was obtained by observing three fields of view at 20,000x magnification, extracting 10 Zn-dominant precipitates in descending order of size from the primary Al crystal in each field of view, measuring their major axes, and calculating the average. The minimum period of the striped structure was determined by observing three fields of view at 20,000x magnification, measuring the stripe periods of the present striped structures, and selecting the smallest of these as the minimum period. Table 2 shows the conditions for the obtained plating film (Zn concentration in the matrix, average maximum diameter of Zn precipitates, presence or absence of stripe-like structure in the Al-Zn eutectic and minimum period, and thickness of the interfacial alloy layer). Furthermore, Figure 2 shows photographs of the molten Al-Zn plated steel sheets of Sample 1 and Sample 14, showing the Zn-dominant precipitates present in the primary Al crystal. Furthermore, Figure 3 shows photographs of the striped structure of the Al-Zn eutectic in the molten Al-Zn plated steel sheets of Sample 1, Sample 14, and Sample 22.

[0060] (2) Bendability For each sample of molten Al-Zn plated steel sheet, a "T-bend" bending test (a bending test compliant with the plating adhesion test described in JIS G 3321 (2019)) was performed while decreasing the bending T by 2T in the range of 10T to 0T, and the limit of bending T at which "no cracks" were observed with a magnifying glass at 10x magnification was confirmed. The results are shown in Table 2. "T-bending" refers to a 180° bending test performed with the thickness of a steel plate sandwiched between two pieces of material. "No cracks" in the observation refers to a state where no cracks are observed at all when the outer tip of the bent section is observed with a magnifying glass at 10x magnification. Furthermore, the "limit of bending T" is the smallest T among the T-bending tests that showed no cracks. For example, if no cracks were observed in a 6T bend but cracks were observed in a 4T bend, the limit of bending T would be "6T".

[0061] (3) Corrosion resistance of the bent part Hot-dip Al-Zn plated steel sheets of Sample 1 and Sample 14 were subjected to outdoor exposure tests in Chuo-ku, Chiba City, after being bent to a T-shape in the range of 0T to 9T. After 4 years and 8 months of exposure testing, the bent sections were visually inspected and evaluated according to the following criteria. The evaluation results are shown in Figure 4. (Evaluation Criteria) 1 point: Clearly present with red rust. Points 2: Slight red rust present. 3 points: No red rust

[0062] [Table 2]

[0063] From the results in Table 2 and Figure 4, it can be seen that each sample of the present invention is superior to each sample of the comparative example in terms of both bendability and corrosion resistance of the processed part, with a good balance. [Industrial applicability]

[0064] According to the present invention, it is possible to provide a hot-dip Al-Zn plated steel sheet and a method for manufacturing the same that are stable and have excellent bendability and corrosion resistance of the bent portion.

Claims

1. A hot-dip Al-Zn plated steel sheet comprising a plating film having a composition containing Al: 45 to 65 mass% and Si: 1.0 to 3.0 mass% with the remainder being Zn, Fe and unavoidable impurities, and an interface alloy layer formed on the interface side between the plating film and the underlying steel sheet, containing Fe, Al, Si, Zn and unavoidable impurities, The aforementioned plating film has dendrites mainly composed of primary Al crystals and interdendritic gaps containing Al-Zn eutectic crystals. The primary Al crystal comprises an α-Al phase matrix and Zn precipitates, wherein the Zn content in the matrix is ​​30% by mass or less. A hot-dip Al-Zn plated steel sheet characterized by having a thickness of 1 μm or less of the aforementioned interface alloy layer.

2. The hot-dip Al-Zn plated steel sheet according to claim 1, characterized in that the average of the maximum diameters of the Zn precipitates in the primary Al crystal is 100 nm or more.

3. The hot-dip Al-Zn plated steel sheet according to claim 1 or 2, characterized in that the Al-Zn eutectic in the dendrite gaps does not have a stripe-like structure with a period of 2 μm or less.

4. A method for manufacturing a hot-dip Al-Zn plated steel sheet according to claim 1 or 2, A step of forming a plating film on a base steel sheet having a composition containing Al: 45-65% by mass and Si: 1.0-3.0% by mass, with the remainder consisting of Zn, Fe and unavoidable impurities, The process includes, after the formation of the aforementioned plating film, a step of subjecting the steel sheet to a thermal history such that the maximum temperature reached is between 150°C and 277°C. A method for manufacturing a hot-dip Al-Zn plated steel sheet, characterized in that, in the step of imparting the thermal history, the cooling time from the highest temperature reached to 150°C is less than 2 hours, and the cooling time from 150°C to room temperature is 3 hours or more.

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