Painted steel sheet and its manufacturing method

Abstract

The objective is to provide a painted steel sheet with excellent bendability and corrosion resistance of the bent portion, and a method for manufacturing the same. [Solution] In order to achieve the above objective, the present invention provides a molten Al-Zn plated steel sheet comprising a plating film having a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass, with the remainder being Zn, Fe, and unavoidable impurities, and an interface alloy layer containing Fe, Al, Si, Zn, and unavoidable impurities formed on the interface side between the plating film and the underlying steel sheet, wherein a resin coating layer is formed on the plating film via a chemical conversion treatment layer and a primer layer, and in the cross-section of the plating film, when line analysis is performed at five or more locations along the thickness direction of the plating film at a pitch of 2-5 μm from 1 μm below the outer surface to 1 μm before the interface alloy layer, with a length of 100 μm or more in a direction parallel to the plating film, the ratio of the minimum and maximum values ​​of the Mg detection intensity obtained in each line analysis is 1.5 or less, and the thickness of the interface alloy layer is 1 μm or less.

Description

[Technical Field]

[0001] This invention relates to a painted steel sheet with excellent bendability and corrosion resistance of the bent portion, and a method for manufacturing the same. [Background technology]

[0002] Hot-dip Al-Zn plated steel sheets, used as a base for painted steel sheets, are known to 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 this reason, painted steel sheets are widely used in building materials such as roofs and walls that are exposed to the outdoors for long periods, as well as in civil engineering and construction fields such as guardrails, wiring and piping, and sound barriers. In particular, the demand for materials with superior corrosion resistance and maintenance-free materials in harsher operating environments, such as those involving acid rain due to air pollution, the application of de-icing agents to prevent road freezing in snowy areas, and coastal development, has been increasing in recent years. As a result, the demand for painted steel sheets using hot-dip Al-Zn plated steel sheets as a base has been growing.

[0003] Here, the plating film of the hot-dip Al-Zn plated steel sheet, which is the base for the painted steel sheet, consists 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 mainly contains supersaturated Zn and is composed of a portion where Al has solidified in dendrites (the dendrite portion of the α-Al phase) and the remaining portion of the dendrite gaps (interdendrites). The α-Al phase has a structure in which multiple layers are stacked in the direction of the thickness of the plating film. Due to this characteristic film structure, the corrosion progression path from the surface becomes complex, making it difficult for corrosion to easily reach the base steel sheet. As a result, the hot-dip Al-Zn plated steel sheet can achieve superior corrosion resistance compared to a hot-dip galvanized steel sheet with the same plating 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).

[0006] Furthermore, Patent Document 3 discloses a technology that improves corrosion resistance by adding 0.3-2% Mg to a molten Al-Zn plated steel sheet and appropriately controlling the process of forming the plating layer, thereby achieving a state in which Mg2Si particles are not present in the surface region of the plating film. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 3654521 [Patent Document 2] Japanese Patent Publication No. 2013-245355 [Patent Document 3] Patent No. 6980831 [Overview of the project] [Problems that the invention aims to solve]

[0008] 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.

[0009] Furthermore, the technology described in Patent Document 3 had a problem in that the addition of Mg caused a decrease in bendability, resulting in more cracks occurring in the bent portion compared to the case where Mg was not included in the plating film, and the corrosion resistance of the bent portion was inferior to that of the unprocessed portion. For this reason, there was a pressing need to improve the corrosion resistance of the bent portion to a level comparable to that of the unprocessed portion.

[0010] In view of these circumstances, the present invention aims to provide a painted steel sheet and a method for manufacturing the same, which exhibits stable bendability and excellent corrosion resistance of the bent portion, and whose bent portion has corrosion resistance comparable to that of the unprocessed portion. [Means for solving the problem]

[0011] The present inventors have investigated a coated steel sheet having a plating film having a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass, with the remainder being Zn, Fe, and unavoidable impurities, in order to solve the above problems. As a result, they have found that by keeping the Zn content in the matrix of the α-Al phase in the dendrites made of primary Al crystals low in the plating film, it is possible to soften the dendrites while suppressing the fineness and increase of the Zn precipitates mentioned above, and to achieve stable and excellent bendability and corrosion resistance of the processed parts. Furthermore, by depositing Mg2Si in the plating film on the outer surface while also having it present finely and homogeneously inside the plating film, the bendability of the plating film can be improved, and corrosion resistance in the bent parts can be achieved at a level comparable to that of the unprocessed parts. Furthermore, focusing on the fact that the Zn precipitates in the primary Al crystal and the state of Mg2Si in the plating film are closely related to the thermal history conditions after the plating film is formed, we found that by optimizing the heating time to the maximum temperature and the cooling time in the thermal history after the plating film is formed, the Zn content in the matrix can be kept low, and Mg2Si can be present in a fine and homogeneous manner in the cross-section of the plating film. As a result, it is possible to obtain a coated steel sheet with excellent bendability and corrosion resistance of the processed part. 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 painted steel sheets. We also found that by eliminating the striped structure, excellent bendability and corrosion resistance of the processed area can be achieved.

[0012] 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 painted steel sheets is generally "12T no crack" or better, although this also depends on the plating film conditions. Even with a "10T bend," it often does not result in "no cracks." Furthermore, the bendability of painted steel sheets is determined by which of the two films has inferior bendability. If the bendability of the paint film alone is equal to or better than that of the plating film, then the bendability will be almost the same as that of unpainted hot-dip Al-Zn plated steel sheets.

[0013] This invention is based on the above findings, and its gist is as follows. 1. A painted steel sheet having a resin coating layer formed on a molten Al-Zn plated steel sheet via a chemical conversion treatment layer and a primer layer, comprising a plating film having a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass, with the remainder being Zn, Fe, and unavoidable impurities, and an interface alloy layer containing Fe, Al, Si, Zn, and unavoidable impurities formed on the interface side between the plating film and the underlying steel sheet, wherein a resin coating layer is formed on the plating film via a chemical conversion treatment layer and a primer layer. 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. Mg2Si is present on the outer surface of the aforementioned plating film. In the cross-section of the plating film, when performing line analysis with a length of 100 μm or more in a direction parallel to the plating film at pitches within a range of 2 to 5 μm at five or more positions along the thickness direction of the plating film from 1 μm below the outer surface to 1 μm before the interface alloy layer, the ratio of the minimum value to the maximum value of the Mg detection intensity obtained in each line analysis is within 1.5. A coated steel sheet, characterized in that the thickness of the interface alloy layer is 1 μm or less.

[0014] 2. The coated steel sheet according to 1, characterized in that the average of the maximum diameters of the precipitates of Zn in the primary Al crystals is 100 nm or more.

[0015] 3. The coated steel sheet according to 1 or 2, characterized in that the Al-Zn eutectic in the dendrite gap does not have a striped structure with a period of 2 μm or less.

[0016] 4. A method for manufacturing the coated steel sheet according to any one of 1 to 3, forming a plating film on a base steel sheet, the plating film having a composition containing 45 to 65% by mass of Al, 1.0 to 3.0% by mass of Si, and 0.01 to 10% by mass of Mg, and the balance being composed of Zn, Fe, and unavoidable impurities; after forming the plating film, applying a heat history to the steel sheet such that the maximum temperature reached is 150°C or more and 277°C or less, in the step of forming the plating film, immersing the base steel sheet in a plating bath and controlling the average cooling rate of the steel sheet during cooling from the plating bath temperature to 300°C after taking it out of the bath to be 20°C / second or more, in the step of applying the heat history, controlling the heat history applied to the steel sheet such that the temperature rising time from room temperature to the maximum temperature reached is 10 minutes or less, the cooling time from the maximum 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.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a coated steel sheet excellent in bending workability and corrosion resistance of the bent portion, and a method for manufacturing the same. [Brief explanation of the drawing]

[0018] [Figure 1] This is an Al-Zn binary equilibrium phase diagram. [Figure 2] This shows the Zn content in the α-Al phase matrix, the average maximum diameter of Zn-containing precipitates in the Al primary crystal, and a photograph of the cross-section of the Al primary crystal for a molten Al-Zn plated steel sheet. [Figure 3] This shows a representative example of observing the cross-section of the plating film on a hot-dip Al-Zn plated steel sheet. [Figure 4] This diagram illustrates the state of a plated film formed on a base steel plate when line analysis is performed at six points along the thickness direction of the plated film, at regular intervals, with a length of 100 μm or more. [Modes for carrying out the invention]

[0019] <Painted steel sheet> The painted steel sheet of the present invention is a painted steel sheet in which a resin coating layer is formed on a hot-dip Al-Zn plated steel sheet via a chemical conversion treatment layer and a primer layer.

[0020] (Hot-dip Al-Zn plated steel sheet) The hot-dip Al-Zn plated steel sheet, which is the base material for the painted steel sheet of the present invention, has a plating film on the surface of the steel sheet. The plating film has a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass, with the remainder being substantially Zn, Fe, and unavoidable impurities. The plating film of the molten Al-Zn plated steel sheet having the above composition enables good corrosion resistance. The plating film consists of an interface alloy layer located on the interface side with the underlying steel sheet and a main layer located on top of the interface alloy layer.

[0021] The Al content in the aforementioned plating film is set to 45-65% by mass, preferably 50-60% by mass, in order to balance corrosion resistance and operational surface. If the Al content in the plating film is at least 45% by mass, dendrite solidification of the primary Al crystal occurs, and a structure can be obtained in which the dendrite solidification structure is stacked in the direction of the thickness of the plating film. By adopting a structure in which the dendrite solidification structure is stacked in the direction of the thickness of the plating film, the corrosion progression path of the plating film becomes more complex, and corrosion resistance can be improved. Furthermore, the more dendrites 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. 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.

[0022] 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 present invention, when a 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). However, since the Fe-Al-Si alloy grows more slowly than the Fe-Al alloy, the higher the proportion of the Fe-Al-Si alloy, the more the growth of the entire alloy phase can be suppressed. For this reason, the Si content in the plating film must be 1.0% by mass or more.

[0023] 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.

[0024] The Mg in the aforementioned plating film is added not only to the unprocessed parts but also to the bent parts to improve corrosion resistance. In the coated steel sheet of the present invention, the presence of Mg in the plating film generates Mg2Si within the plating film. When the plated steel sheet is exposed to an external corrosive environment, the Mg2Si first dissolves, generating Mg-containing corrosion products that cover the surface of the plating film, thereby improving corrosion resistance. Therefore, this effect becomes more pronounced when Mg2Si is present on the outer surface of the plating film. Therefore, the Mg content in the plating film must be 0.01% by mass or more and 10.0% by mass or less, and preferably 0.3% by mass or more and 3.0% by mass or less.

[0025] 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.

[0026] Furthermore, in the coated steel sheet of the present invention, the plating film may further contain one or more elements selected from 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.

[0027] Here, 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 by 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.

[0028] Furthermore, the hot-dip Al-Zn plated steel sheet, which serves as the base for the painted steel sheet obtained according to the present invention, has a plating film composition that is approximately the same as the overall composition of the plating bath. Therefore, the composition of the plating film can be precisely controlled by controlling the composition of the plating bath.

[0029] Furthermore, the interface alloy layer in the plating film is a layer located on the interface side with 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 painted 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.

[0030] 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 (hereinafter referred to as "SEM"). 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.

[0031] In the painted 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 painted steel sheet of the present invention is characterized in that the primary Al crystal has Zn precipitates in the α-Al phase matrix, and the Zn content in the matrix is ​​30% by mass or less.

[0032] When Zn solidifies in a supersaturated state (with a Zn content exceeding 30% by mass) within the α-Al phase matrix, the hardness increases due to solid solution strengthening of Zn, leading to reduced elongation and decreased 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 painted steel sheet and, consequently, the corrosion resistance of the processed area. 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 α-Al phase matrix to 30% by mass or less can also promote the growth of Zn precipitates. The Zn content in the matrix refers to the amount of Zn contained in the matrix, and does not include the amount of precipitated or separated Zn (in the 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.

[0033] 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.

[0034] 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. Furthermore, the Zn content in the matrix can be obtained, for example, by point analysis or surface analysis of the matrix while avoiding Zn precipitates using an energy-dispersive X-ray spectrometer (hereinafter referred to as "EDX") attached to an ultra-low acceleration SEM (acceleration voltage 3kV, magnification of 20,000 times or more).

[0035] Here, Figure 2 shows the Zn content in the α-Al phase matrix, the average maximum diameter of Zn-containing precipitates in the Al primary crystal, and a photograph of the cross-section of the Al primary crystal for a molten Al-Zn plated steel sheet. It can be seen that the presence or absence and maximum diameter of Zn-containing precipitates can be determined by observation with an extremely low-acceleration SEM.

[0036] 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.

[0037] 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 painted 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 painted 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.

[0038] 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, the Zn precipitates and the striped structure of the Al-Zn eutectic are both finer than those formed when thermal history is applied; therefore, for example, the presence or absence of these was not considered in conventional observations using an accelerating voltage of 15kV.

[0039] Here, Figure 3 shows a photograph of a cross-section of a hot-dip Al-Zn plated steel sheet observed using an ultra-low acceleration SEM (acceleration voltage 3kV). In the photograph on the left (showing a striped structure), a striped structure with a stripe period of approximately 0.1 μm can be clearly observed.

[0040] 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.

[0041] Furthermore, in the painted steel sheet of the present invention, as described above, Mg2Si is present on the outer surface of the plating film, and when line analysis is performed on five or more points along the thickness direction of the plating film at a pitch of 2 to 5 μm from 1 μm below the outer surface to 1 μm before the interfacial alloy layer, with a length of 100 μm or more in a direction parallel to the plating film, the ratio of the minimum and maximum values ​​of the Mg detection intensity obtained in each line analysis is 1.5 or less. As described above, the Mg2Si in the plating film generates Mg-containing corrosion products when exposed to a corrosive environment, thereby improving corrosion resistance. This effect is more pronounced when Mg2Si is present on the outer surface of the plating film; therefore, the present invention requires the presence of Mg2Si on the outer surface of the plating film. The outer surface of the plating film refers to the side of the plating film opposite to the underlying steel sheet (the upper surface of the plating film).

[0042] Furthermore, Mg2Si present not only on the outer surface of the plating film but also internally is exposed on the fracture surface when cracks occur due to bending, providing the same effect as when present on the outer surface of the plating film. Therefore, improved corrosion resistance can be expected not only in the unbent parts (non-processed parts) but also in the bent parts. The effect of improving the corrosion resistance of the processed area is more pronounced when Mg2Si is present in a fine and uniform manner within the plating film. If the Mg2Si in the plating film is not uniformly distributed but unevenly distributed, the amount of Mg2Si exposed on the fracture surface will differ depending on the location of the crack, and consequently, the effect of improving the corrosion resistance of the processed area will also become unstable. In this case, even if the Mg2Si content in the plating film is within a certain range, it goes without saying that if the size of each Mg2Si particle is small and there are many of them, the distribution of Mg2Si will be more uniform, and a stable effect of improving the corrosion resistance of the processed area will be obtained.

[0043] Therefore, in order to demonstrate that Mg2Si is finely and uniformly distributed, the present invention specifies that the Mg detection intensity when line analysis is performed on a length of 100 μm or more in a direction parallel to the plating film in the cross-section of the plating film is within a certain range in the thickness direction of the plating film. Specifically, when line analysis is performed on five or more locations at a pitch of 2 to 5 μm along the thickness direction of the plating film, for a length of 100 μm or more in a direction parallel to the plating film, the ratio of the minimum and maximum values ​​of the Mg detection intensity obtained in each line analysis is within 1.5. If the ratio of the minimum to maximum Mg detection intensity exceeds 1.5, the Mg2Si present inside the plating film is not homogeneous or has variations in particle size, making it impossible to achieve stable corrosion resistance in the processed area. From a similar viewpoint, the ratio of the minimum to maximum Mg detection intensity is preferably 1.3 or less, and more preferably 1.2 or less. The linear analysis of the aforementioned plating film is performed at a pitch of 2 to 5 μm along the thickness direction. This is because a range of 5 μm or less along the thickness direction of the plating film allows for a reliable understanding of the variations in the particle size and distribution of Mg2Si. The reason for limiting the range to 2 μm or more is... If the thickness is less than 2 μm, the width in the thickness direction of the plating film becomes small, making it difficult to fully grasp the variation in the particle size and distribution of Mg2Si. When observing a cross-section along the stacking direction of the plating film, this corresponds to the vertical direction. Furthermore, for the linear analysis of the plating film, the analysis is performed in a direction parallel to the plating film with a length of 100 μm or more. This is because, by limiting the analysis to a range of 5 μm or less along the plating film, it is possible to reliably grasp the variations in the particle size and distribution state of Mg2Si. The direction parallel to the plating film is the direction in which the plating film extends, and when observing a cross-section along the lamination direction of the plating film, this corresponds to the left-right direction.

[0044] Here, Figure 4 schematically shows the state when line analysis is performed on the plating film 20 formed on the base steel plate 10 at six locations with a length of 100 μm or more at a constant pitch along the thickness direction of the plating film 20, in order to explain the line analysis in the direction parallel to the plating film described above. From Figure 4, it can be seen that by measuring the Mg detection intensity under the above conditions, the presence of Mg2Si can be determined in the thickness direction of the plating film 20 and in the direction parallel to the plating film 20 (extension direction). Note that in Figure 4, for the sake of explanation, the shape and thickness of the plating film 20 and the interface alloy layer 21, as well as the length and pitch of the line analysis, are shown schematically and do not differ from the actual configuration. The pitch along the thickness direction of the plating film does not need to be at a constant interval as long as it is in the range of 2 to 5 μm; measurements can be taken with varying pitches, such as 3 μm, 3 μm, 4 μm, 4 μm, 5 μm, 5 μm, etc. Furthermore, regarding the pitch along the thickness direction of the plating film, if the amount of plating film attached is large (when the plating film is thick), it is preferable to perform line analysis at more than 5 locations (for example, 10 locations, 15 locations, etc.), while if the amount of plating film attached is small (when the plating film is thin), line analysis at 5 locations is sufficient.

[0045] Furthermore, the presence or absence of Mg2Si on the outer surface of the plating film, and the ratio of the detection intensity of Mg by line analysis of the plating film, can be confirmed by SEM and the attached EDX, similar to the interface alloy layer described above. The presence or absence of Mg2Si on the outer surface of the plating film can be confirmed by the presence or absence of locations where both Mg and Si are detected. The uniformity of the distribution of Mg2Si present inside the plating film can be evaluated by the ratio of the detection intensity of Mg in the thickness direction of the plating film. Specifically, this can be confirmed by performing line analysis on the cross-section of the plating film at five or more locations with a pitch of 2 to 5 μm or less in the thickness direction, from 1 μm below the outer surface to 1 μm before the interfacial alloy layer, and a length of 100 μm or more in a direction parallel to the plating film, and calculating the ratio of the minimum and maximum detection intensity of Mg at each analysis location. The analysis method described here is merely an example, and other analysis methods can be used if the detection intensity of Mg (amount of Mg2Si present) in the direction along the plating film can be obtained.

[0046] 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.

[0047] 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.

[0048] (Chemical treatment layer, primer layer) As described above, the painted steel sheet of the present invention has a chemical conversion treatment layer and a primer layer formed on the hot-dip Al-Zn plated steel sheet. Furthermore, the chemical conversion treatment layer and primer layer are not particularly limited as long as they are layers formed between the plating film of the hot-dip galvanized steel sheet and the coating film, and can be appropriately selected according to the required performance. Furthermore, the chemical conversion treatment layer and primer layer are not particularly limited as long as they are layers formed between the plating film of the hot-dip galvanized steel sheet and the coating film. The chemical conversion treatment layer can be formed, for example, by chromate treatment or chromate-free chemical conversion treatment, which involves applying a chromate treatment solution or a chromate-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 treatment films may be single layers or multiple layers, and in the case of multiple layers, multiple chemical conversion treatments may be performed sequentially.

[0049] For example, the chemical treatment layer is not particularly limited and can be either chromate treatment or chromate-free chemical treatment. For the aforementioned chromate-free chemical conversion treatment, a composite of resin and inorganic components can be used, which contains 30-50% by mass of epoxy resin having a bisphenol skeleton, and the inorganic components include 2-10% by mass of vanadium oxide, 40-60% by mass of zirconium oxide, and 0.5-5% by mass of a fluorine compound.

[0050] The preferred amount of the chemical treatment layer to adhere is 0.025 to 0.5 g / m². 2 It is 0.025 g / m 2 Below this level, reduced adhesion to the underlying aluminum-zinc alloy plated steel sheet and the upper primer coating, as well as reduced corrosion resistance, may occur. 0.5 g / m 2 If the temperature exceeds a certain limit, the chemical treatment layer becomes more susceptible to damage (peeling) when subjected to severe bending processes, which can lead to a decrease in corrosion resistance.

[0051] Furthermore, the chemical conversion treatment layer can be obtained, for example, by continuously coating an aluminum-zinc alloy plated steel sheet with a chemical conversion treatment solution using a roll coater or the like, and then drying it using hot air or induction heating at a maximum plate temperature of about 60°C to 200°C. The chemical treatment layer may be a single layer or multiple layers; in the case of multiple layers, multiple chemical treatments may be performed sequentially.

[0052] The primer layer is not particularly limited; for example, either a chromate primer or a chromate-free primer may be used. For example, the chromate-free primer layer may be a composite of a resin component and an inorganic component, wherein the resin component contains a total of 40 to 88% by mass of epoxy resin (e) and melamine resin (m) having urethane bonds, the blending ratio of (e) to (m) is in the range of 97:3 to 60:40 by mass, and it contains 4 to 20% by mass of a vanadium compound, 4 to 20% by mass of a phosphoric acid compound, and 4 to 20% by mass of magnesium oxide.

[0053] The preferred thickness of the primer coating is 3 μm or more. If it is thinner than this, it may lead to a decrease in corrosion resistance and a decrease in adhesion with the chemical conversion treatment layer and the topcoat coating. The resin composition for the primer coating film may, if necessary, contain various known components commonly used in the paint industry. Specifically, these include various surface modifiers such as leveling agents and defoaming agents, various additives such as dispersants, anti-settling agents, ultraviolet absorbers, light stabilizers, silane coupling agents, and titanate coupling agents, various pigments such as coloring pigments, extender pigments, and heat-shielding pigments, glossing agents, curing catalysts, and organic solvents.

[0054] There are no particular restrictions on the method of applying the paint composition for forming the primer coating film, but it is preferable to apply the paint composition by methods such as roll coater coating or curtain flow coating. After applying the paint composition, the primer coating is baked using heating methods such as hot air heating, infrared heating, or induction heating to obtain the primer coating. The baking process is usually carried out at a temperature of approximately 180-270°C for about 20 seconds to 2 minutes.

[0055] (Resin coating layer) In the painted steel sheet of the present invention, a resin coating layer is formed on the primer layer with a topcoat paint. By applying the topcoat paint, an aesthetic appearance can be enhanced, and various properties such as workability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance, which are required for painted Al-Zn alloy plated steel sheets, can be improved.

[0056] Examples of topcoat paints used for the resin coating layer in this embodiment include polyester resin-based paints, silicone polyester resin-based paints, polyurethane resin-based paints, acrylic resin-based paints, and fluororesin-based paints, but fluororesin-based paints are particularly preferred in terms of corrosion resistance and weather resistance.

[0057] Furthermore, the thickness of the resin coating layer is preferably 5 to 30 μm. If the thickness is less than 5 μm, it may be difficult to stabilize the color and appearance, and if the thickness exceeds 30 μm, it may lead to a decrease in processability (crack formation in the topcoat).

[0058] Furthermore, depending on the purpose and application, the topcoat paint used for the resin coating layer may contain appropriate amounts of additives other than chromate compounds, such as titanium dioxide, red iron oxide, mica, carbon black, and other various coloring pigments, metallic pigments such as aluminum powder and mica, extender pigments consisting of carbonates and sulfates, or various fine particles such as silica fine particles, nylon resin beads, and acrylic resin beads, curing catalysts such as p-toluenesulfonic acid and dibutyltin dilaurate, waxes, and other additives.

[0059] There are no particular restrictions on the method of applying the topcoat paint to form the aforementioned resin coating layer, but it is preferable to apply the topcoat paint by methods such as roll coater coating or curtain flow coating. After applying the topcoat paint, the resin coating layer can be formed by baking it using heating means such as hot air heating, infrared heating, or induction heating. In this baking process, for example, the maximum plate temperature to be reached is about 180 to 270°C, and the process is carried out at this temperature range for about 30 seconds to 3 minutes.

[0060] <Manufacturing method for painted steel sheets> The present invention provides a method for manufacturing painted steel sheets, 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.

[0061] 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.

[0062] 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.

[0063] 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, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass is used, with the remainder being substantially Zn, Fe, and unavoidable impurities.

[0064] 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.

[0065] Furthermore, there are no particular limitations on the temperature of the base steel plate that enters the plating bath (entry plate temperature). 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.

[0066] 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.

[0067] Furthermore, in the method for manufacturing painted steel sheets according to the present invention, in the step of forming the plating film, the base steel sheet is immersed in the plating bath, and the average cooling rate during the cooling from the plating bath temperature to 300°C after being removed from the bath is controlled to 20°C / second or more. The reason why the average cooling rate of the steel sheet from the plating bath temperature to 300°C after being removed from the plating bath is set to 20°C / second or more is that by shortening the process in which the plating bath (liquid) adhering to the steel sheet solidifies into a plating film (solid), the deposition of Mg as Mg2Si in the plating film is suppressed, and the state in which Mg remains uniformly and supersaturatedly dissolved in the plating film is maintained. This maintains the driving force for the deposition of Mg2Si in the plating film during the subsequent process of applying heat history, enabling uniform deposition of Mg2Si into the plating film. As a result, the obtained molten Al-Zn plated steel sheet has Mg2Si present on the outer surface of the plating film, as well as finely and uniformly present inside the plating film, improving bendability and corrosion resistance of the bent parts.

[0068] Furthermore, in the method for manufacturing painted steel sheets of the present invention, in the step of imparting the thermal history, the heating time of the steel sheet on which the plating film is formed from room temperature to the maximum temperature reached is controlled to be 10 minutes or less, the cooling time from the maximum temperature reached to 150°C is controlled to be less than 2 hours, and the cooling time from 150°C to room temperature is controlled to be 3 hours or more. By subjecting the steel sheet to such a thermal history, it is possible to obtain a painted steel sheet with stable bendability and excellent corrosion resistance at the bent portion.

[0069] In the heating step of the process that imparts the aforementioned thermal history, the heating time from room temperature to the maximum temperature reached is set to 10 minutes or less in order to suppress the deposition and further growth of Mg, which is supersaturated and solid-dissolved in the plating film, as Mg2Si at this stage. Furthermore, in the subsequent cooling step, the growth of Zn precipitates and the interfacial alloy layer that promotes the growth of Mg2Si can be suppressed, thereby improving the bendability. Here, the growth of Mg2Si refers to the aggregation of Mg2Si particles, which reduces their number and increases their particle size, resulting in a non-uniform distribution of Mg2Si in the plating film. The term "room temperature" refers to room temperature, with approximately 25°C in mind.

[0070] Furthermore, the reason why the cooling time from the highest temperature reached to 150°C in the cooling step of the process of imparting the thermal history is set to less than 2 hours is to suppress the deposition and growth of Mg2Si and the growth (thickening) of the interface alloy layer, thereby minimizing the change in the structure of the plating film achieved in the heating step during the cooling step, allowing Mg2Si to be exposed on the outer surface of the plating film and maintaining a state in which Mg2Si is present finely and uniformly within the plating film. In addition, it is also possible to maintain the elimination of solid solution strengthening and precipitation strengthening of the Zn portion in the Al-Zn eutectic in the plating film, and to suppress 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 less than one hour.

[0071] Furthermore, the reason why the cooling time from 150°C to room temperature is set to 3 hours or more during the cooling step of the process that imparts the thermal history is to ensure the temperature and time at which Zn and Mg diffuse in the Al primary 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 Al primary 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.

[0072] Furthermore, in the method for manufacturing a painted steel sheet of the present invention, in the step of imparting the thermal history to the molten Al-Zn plated steel sheet which is the base material, the maximum temperature reached is 150°C or more and 277°C or less, the cooling time from the maximum 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. By applying such a thermal history, the bendability of painted steel sheets and the corrosion resistance of the bent parts can be improved.

[0073] 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, making it impossible to sufficiently eliminate solid solution strengthening and precipitation strengthening in the primary Al crystal. 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, when the maximum temperature reached exceeds 277°C, the solid solution strengthening and precipitation strengthening of the Zn portion in the primary Al crystal are eliminated, and the striped structure in the Al-Zn eutectic also decomposes. However, when the temperature is cooled 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 aforementioned thermal history is preferably 170°C to 250°C, and more preferably 190°C to 230°C.

[0074] 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.

[0075] Furthermore, in the method for manufacturing a coated steel sheet according to 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 hot-dip Al-Zn plated steel sheet.

[0076] The present invention further includes a step of forming a resin coating layer on the plating film of a molten Al-Zn plated steel sheet obtained by the above-described manufacturing method, via a chemical conversion treatment layer and a primer layer.

[0077] 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.

[0078] Furthermore, the chemical treatment layer and primer layer are not particularly limited as long as they are layers formed between the plating film of the hot-dip galvanized steel sheet and the coating film. The types and formation methods of the chemical treatment layer and primer layer are the same as those described in the painted steel sheet of the present invention. [Examples]

[0079] (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 H, 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 Mg content were then used (Platings B to H). 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 75-90 g / m² per side in all cases. 2 It was controlled to achieve this.

[0080] (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 No. 1 to 22.

[0081] (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 compositions and coating amounts of the obtained plating films A to H are shown in Table 1.

[0082] (4) Manufacture of coated steel sheet On the molten Al-Zn-based plated steel sheet obtained as described above, a chemical conversion treatment layer, a primer layer, and a resin coating layer were formed under the following conditions to obtain a coated steel sheet. (i) As the resin component in the chemical conversion treatment film, "Yukarezine RE-1050" manufactured by Yoshimura Oil Chemical Co., Ltd., which is an epoxy resin having a bisphenol skeleton, was prepared so that the solid content ratio in the chemical conversion treatment film was 43% by mass and used. The vanadium compound contained in the chemical conversion coating film was 6% by mass of an organic vanadium compound chelated with acetylacetone, the zirconium compound was 50% by mass of ammonium zirconium carbonate, and the fluorine compound was 1% by mass in terms of fluorine atoms of ammonium fluoride. Each was prepared so as to be the solid content ratio in the chemical conversion treatment film and used. These raw materials were mixed to prepare a chemical conversion treatment solution (pH: 8 to 10). After applying this chemical conversion treatment solution with a roll coater, it was heated in an oven at 200 °C for 2 seconds and then air-dried, whereby a chemical conversion treatment film with an adhesion amount of 0.1 g / m 2 was formed. (ii) The paint for forming the primer coating film was prepared by the following method. As the resin component of the primer coating film, a bisphenol A type epoxy resin (trade name "JER 1009", manufactured by Mitsubishi Chemical Corporation), which is an epoxy resin having a urethane bond, was reacted with a blocked polyisocyanate compound (trade name "Desmodur BL-3175", manufactured by Sumika Bayer Urethane Co., Ltd.) at a mass ratio of 85:15; and; an n-butylated melamine resin (trade name "Uban 122", manufactured by Mitsui Chemicals, Inc.), which is a melamine resin, was used in a ratio of 9:1. As the vanadium compound of the rust preventive pigment contained in the chemical conversion coating film, magnesium vanadate was used, and as the phosphate compound, calcium phosphate was prepared so as to be 12% by mass in each coating film and used. After adding a solvent and a rust-preventive pigment to the above resin component, 0.3 parts of dibutyltin dilaurine (DBTDL) was added as a reaction catalyst and mixed uniformly to prepare a chromium-free paint composition. This chromium-free paint composition was applied using a roll coater, baked at a maximum plate temperature of 210°C for 30 seconds, and then water-cooled to form a primer coating with a post-baking thickness of 10 μm. (iii) The resin coating layer was formed using melamine-cured polyester paint (black) "Precolor HD-0030HR" (manufactured by AkzoNobel Coatings Co., Ltd.) as the topcoat paint. The topcoat paint described above was applied to the undercoat film using a roll coater, baked at a maximum plate temperature of 260°C for 40 seconds, and then water-cooled to form a resin coating layer with a post-baking thickness of 17 μm.

[0083] [Table 1]

[0084] <Rating> The following evaluations were performed on each sample of the painted steel sheet obtained above, and the hot-dip Al-Zn plated steel sheet used as its base. The evaluation results are shown in Table 1.

[0085] (1) Evaluation of the plating film Each sample of molten Al-Zn plated steel sheet was observed using SEM and analyzed by EDX. The presence or absence of Mg2Si on the outer surface of the plating film was confirmed by observing and analyzing the surface of the plating film and checking for the presence or absence of areas where both Mg and Si were detected. The distribution of Mg2Si within the plating film was evaluated by performing linear analysis on a cross-section of the plating film. Linear analysis was performed at seven locations in the thickness direction, from 1 μm below the outer surface to 1 μm before the interfacial alloy layer, at intervals of 2 to 5 μm. Linear analysis was also performed on 100 μm lengths parallel to the plating film. The ratio of the minimum to maximum detection intensity of Mg at each analysis location was calculated. Furthermore, the cross-section of the plating film was observed using an ultra-low acceleration SEM and analyzed by EDX. The observation and analysis conditions for the above-mentioned plating film were as follows: using a Zeiss ULTRA55 (ultra-low acceleration SEM) and an Oxford Instruments Ultim Extreme (EDX), with an acceleration voltage of 3kV, observation magnifications of 3000x and 20000x, and point analysis at 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 of the obtained plating film (Zn concentration in the matrix, average value of the maximum diameter of Zn precipitates, presence or absence of stripe-like structure of Al-Zn eutectic and minimum period, and thickness of the interface alloy layer). Figure 2 shows a photograph of the Zn precipitate present in the primary Al crystal. Furthermore, Figure 3 shows photographs observing the presence or absence of a striped structure in the Al-Zn eutectic.

[0086] (2) Bendability For each sample of painted 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 1T in the range of 15T 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 5T bend but cracks were observed in a 4T bend, the limit of bending T would be "5T".

[0087] (3) Corrosion resistance of the bent part Each sample of painted steel sheet was subjected to an outdoor exposure test in Chuo Ward, Chiba City, after being bent to a 3T. After 7 years and 2 months of exposure, the bent section was visually inspected and evaluated according to the following criteria. The evaluation results are shown in Table 2. (Evaluation Criteria) 1 point: Clearly present with red rust. Points 2: Slight red rust present. 3 points: No red rust

[0088] [Table 2]

[0089] The results in Table 2 show that each sample of the present invention exhibits superior corrosion resistance in the bent portion compared to each sample of the comparative example. [Industrial applicability]

[0090] According to the present invention, it is possible to provide a painted steel sheet with excellent bendability and corrosion resistance of the bent portion, and a method for manufacturing the same. [Explanation of symbols]

[0091] 10 Base steel plate 20 Plating film 21 Interfacial alloy layer

Claims

1. A coated steel sheet having a resin coating layer formed on a molten Al-Zn plated steel sheet via a chemical conversion treatment layer and a primer layer, comprising a plating film having a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% by mass, with the remainder being Zn, Fe, and unavoidable impurities, and an interface alloy layer containing Fe, Al, Si, Zn, and unavoidable impurities formed on the interface side between the plating film and the underlying steel sheet, wherein a resin coating layer is formed on the plating film via a chemical conversion treatment layer and a primer layer, 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. On the outer surface of the aforementioned plating film, Mg 2 Si exists, In the cross-section of the plating film, when line analysis is performed at five or more locations along the thickness direction of the plating film at a pitch of 2 to 5 μm, from 1 μm below the outer surface to 1 μm before the interface alloy layer, and with a length of 100 μm or more in a direction parallel to the plating film, the ratio of the minimum and maximum values ​​of the Mg detection intensity obtained in each line analysis is within 1.

5. A painted steel sheet characterized in that the thickness of the interface alloy layer is 1 μm or less.

2. The painted steel sheet according to claim 1, characterized in that the average of the maximum diameters of the Zn precipitates in the Al primary crystal (when observing the Al primary crystal with an extremely low-acceleration SEM (acceleration voltage of 3kV, magnification of 20,000 times or more) in three or more fields of view, the maximum diameters of Zn-based precipitates present in each field of view are measured at 10 points in descending order of size, and the average of these measured values) is 100 nm or more.

3. The painted steel sheet according to claim 1, 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 painted steel sheet according to claims 1 to 3, A step of forming a plating film on a base steel sheet having a composition containing Al: 45-65% by mass, Si: 1.0-3.0% by mass, and Mg: 0.01-10% 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. In the process of forming the aforementioned plating film, the average cooling rate of the steel plate during the cooling period from the plating bath temperature to 300°C after immersing the base steel plate in the plating bath and removing it from the bath is controlled to 20°C / second or more. A method for manufacturing a painted steel sheet, characterized in that, in the step of imparting the thermal history to the steel sheet, the heating time from room temperature to the maximum temperature reached is controlled to be 10 minutes or less, the cooling time from the maximum 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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