JP7475162B2 - Coated steel sheet and method for producing coated steel sheet - Google Patents

Description

The present invention relates to a coated steel sheet that has good corrosion resistance, as well as excellent pressure mark resistance and surface appearance of the plating layer, and a method for manufacturing the same.

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

The coating layer of hot-dip Al-Zn coated steel sheets consists of a main layer and an alloy layer that exists at the interface between the base steel sheet and the main layer. The main layer is mainly composed of a portion where Al is supersaturated with Zn and solidified into dendrites (the α-Al phase dendrite portion) and the remaining portion between the dendrites (interdendrite), and has a structure in which the α-Al phase is laminated in the thickness direction of the coating layer. This characteristic coating structure makes the corrosion progression path from the surface complex, making it difficult for corrosion to easily reach the base steel sheet, and hot-dip Al-Zn coated steel sheets can achieve superior corrosion resistance compared to hot-dip galvanized steel sheets with the same coating layer thickness.

In addition, a technology is known that aims to further improve corrosion resistance by including Mg in the coating layer of hot-dip Al-Zn coating. As a technology related to hot-dip Al-Zn coated steel sheet containing Mg (hot-dip Al-Zn-Mg-Si coated steel sheet), for example, Patent Document 2 discloses an Al-Zn-Mg-Si coated steel sheet containing an Al-Zn-Si alloy containing Mg in the coating layer, the Al-Zn-Si alloy being an alloy containing 45 to 60% by weight of elemental aluminum, 37 to 46% by weight of elemental zinc, and 1.2 to 2.3% by weight of elemental silicon, and the concentration of the Mg is 1 to 5% by weight.

Furthermore, as a technology for incorporating Mg in a coating layer, Patent Document 3 discloses a hot-dip Al-Zn coated steel sheet in which a certain amount of Mg and Ca is incorporated in the coating layer, thereby aiming to improve corrosion resistance and protective effect after the base steel sheet is exposed.
Furthermore, Patent Document 4 discloses an Al-based plated steel sheet which forms a coating layer containing, by mass%, 1-15% Mg, 2-15% Si, 11-25% Zn, with the remainder being Al and unavoidable impurities, and which improves the corrosion resistance of the flat plate and end faces by specifying the size of the _Mg2Si phase present in the plated layer.

However, the hot-dip Al-Zn coated steel sheets disclosed in the cited documents 1 and 2 have excellent corrosion resistance, but have a problem in that wrinkle-like defects (hereinafter referred to as "wrinkle defects") caused by an oxide layer formed on the surface of the coating layer are likely to occur, which impairs the appearance of the coating layer surface.
Therefore, for example, Patent Document 5 discloses a technique for improving the surface appearance of hot-dip Al-Zn-Mg plated steel sheets by incorporating Sr into the plated layer.
Furthermore, Patent Document 6 discloses a technique for improving the workability of hot-dip Al-Zn-Mg plated steel sheet by incorporating Sr into the plated layer.
Furthermore, Patent Document 7 discloses a technology in which at least 250 ppm of Sr is added to a plating bath containing an Al-Zn-Si-Mg alloy to reduce the content of _Mg2Si particles in the plating layer, thereby improving the surface appearance.

Japanese Patent Publication No. 46-7161 Patent Publication No. 5020228 Patent No. 5000039 JP 2002-12959 A Patent Publication No. 3983932 Patent No. 6368730 JP 2011-514934 A

As described above, the hot-dip Al-Zn-Mg-Si plated steel sheets of Patent Documents 3 and 4 enable improvement in corrosion resistance, while the Sr-containing hot-dip Al-Zn-Mg-Si plated steel sheets of Patent Documents 5 to 7 enable improvement in surface appearance.
However, for coated steel sheets in which a coating film is formed on a plated steel sheet, in addition to the above-mentioned improvements in corrosion resistance and surface appearance of the plating layer, there has also been a demand for improvements in suppressing the occurrence of pressure marks (pressure mark resistance).

Here, the pressure mark defect refers to spot-like gloss unevenness that occurs mainly in the center of the width direction of the steel sheet when the front and back surfaces of the steel sheet are pressed together when the coated steel sheet (coated steel strip) is wound into a coil, and part of the coating film formed by the pressure is plastically deformed.
In the techniques disclosed in Patent Documents 5 to 7, the surface appearance of the plating layer is improved by the inclusion of Sr, and therefore the occurrence of pressure marks can be suppressed to some extent. However, the suppression effect cannot be said to be sufficient by simply adding Sr, and further improvement in pressure mark resistance has been desired.

In view of the above circumstances, the present invention aims to provide a coated steel sheet having good corrosion resistance, excellent pressure mark resistance and excellent surface appearance of the plating layer, and a method for manufacturing a coated steel sheet having good corrosion resistance, excellent pressure mark resistance and excellent surface appearance of the plating layer.

The inventors conducted research to solve the above problems with coated steel sheets in which a coating film is formed directly or via an intermediate layer on the coating layer of hot-dip galvanized steel sheets, and discovered that by setting the composition of the coating layer of hot-dip galvanized steel sheets to a specific range and optimizing the thickness and standard deviation of the thickness, it is possible to improve pressure mark resistance while maintaining good corrosion resistance.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A coated steel sheet having a coating film formed directly or via an intermediate layer on a coating layer of a hot-dip galvanized steel sheet, the coating layer having a composition containing 40-70 mass% Al, 1.2-4 mass% Si, 1-6 mass% Mg, 0.001-0.2 mass% Sr, with the balance being Zn and unavoidable impurities, the coating layer having a thickness of 15-30 μm with a standard deviation of 5 μm or less, and the coating film having a thickness of 20 μm or less.

2. The coated steel sheet according to 1, characterized in that the plating layer further contains 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 mass%.

3. A method for producing a coated steel sheet, comprising the steps of immersing a base steel sheet in a coating bath containing 45-65 mass% Al, 1.2-4 mass% Si, 1-6 mass% Mg, with the balance being Zn and unavoidable impurities, and forming a coating film having a thickness of 20 μm or less on the obtained hot-dip coated steel sheet directly or via an intermediate layer, wherein the bath temperature of the coating bath satisfies the following formula (1):
Plating bath temperature (℃) ≦600 - 4.5M _Mg - 5.5M _Si ... (1)
M _Mg : Mg content in the plating bath (mass%), M _Si : Si content in the plating bath (mass%)

4. The method for producing a coated steel sheet according to 3, wherein the plating bath further contains 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 mass%.

The present invention provides a coated steel sheet that has good corrosion resistance, excellent pressure mark resistance, and excellent surface appearance of the plating layer, as well as a method for manufacturing a coated steel sheet that has good corrosion resistance, excellent pressure mark resistance, and excellent surface appearance of the plating layer.

FIG. 2 is a diagram comparing the variation in thickness of the plating layer of the coated steel sheet of the present invention with the variation in thickness of the plating layer of a conventional coated steel sheet. FIG. 13 is a diagram for explaining a method for measuring the distance between dendrite arms. This is a diagram to explain the flow of the Japanese Automotive Standard Combined Cycle Test (JASO-CCT).

(painted steel plate)
The coated steel sheet of the present invention is a coated steel sheet in which a paint film is formed directly or via an intermediate layer on the plating layer of a hot-dip galvanized steel sheet.
In the present invention, the coating layer is a hot-dip Al-Zn-Mg-Si coated steel sheet having a composition containing 40 to 70 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, 0.001 to 0.2 mass% Sr, with the balance being Zn and unavoidable impurities. The coating layer of the hot-dip coated steel sheet has the above-mentioned composition, which ensures the desired corrosion resistance and improves pressure mark resistance.

The Al content in the plating layer is 40-70% by mass, preferably 45-65% by mass, in consideration of the balance between corrosion resistance and operational aspects. If the Al content of the main layer of the plating layer is at least 40% by mass, dendritic solidification of Al occurs sufficiently. As a result, the main layer mainly contains Zn in a supersaturated state, and a structure excellent in corrosion resistance can be realized in which the main layer is composed of a portion where Al has been solidified into dendrites (dendrite portion of α-Al phase) and the remaining portion between dendrites (interdendritic portion), and the dendrite portion is stacked in the thickness direction of the plating layer. In addition, the more the α-Al phase dendrite portion is stacked, the more complex the corrosion progression path becomes, and the more difficult it becomes for corrosion to reach the base steel sheet, improving the corrosion resistance. On the other hand, if the Al content in the plating layer exceeds 70% by mass, the content of Zn, which has a sacrificial corrosion protection effect against Fe, decreases, and the corrosion resistance deteriorates. For this reason, the Al content in the plating layer is set to 70% by mass or less. Furthermore, if the Al content in the plating layer is 65% by mass or less, even if the coating weight is reduced and the base steel sheet is easily exposed, the coating layer will have a sacrificial corrosion protection effect against Fe, and sufficient corrosion resistance will be obtained. Therefore, it is preferable that the Al content of the plating main layer be 65% by mass or less.

The Si in the coating layer is added to the coating bath for the purpose of suppressing the growth of an interfacial alloy layer formed at the interface with the base steel sheet, and for the purpose of improving corrosion resistance and workability, and is inevitably contained in the main layer. In the case of the hot-dip coated steel sheet used in the coated steel sheet of the present invention, when the hot-dip coating process is performed with Si contained in the coating bath, Fe on the steel sheet surface reacts with Al and Si in the bath to form an alloy consisting of Fe-Al and/or Fe-Al-Si compounds as soon as the base steel sheet is immersed in the coating bath. The growth of the interfacial alloy layer can be suppressed by the formation of this Fe-Al-Si interfacial alloy layer. And, when the Si content in the coating layer is 1.2 mass% or more, the growth of the interfacial alloy layer can be sufficiently suppressed. On the other hand, when the Si content in the coating layer exceeds 4 mass%, the coating layer is easily precipitated with a Si phase that reduces workability and becomes a cathode site. The precipitation of the Si phase can be suppressed by increasing the Mg content, but in that case, the manufacturing cost increases, the workability decreases due to the large amount of Mg ₂ Si, and the composition control of the plating bath becomes more difficult. For this reason, the Si content in the plating layer is set to 4 mass% or less. Furthermore, considering that the growth of the interface alloy layer and the precipitation of the Si phase can be more reliably suppressed, and that it is possible to deal with the case where Si is consumed as Mg ₂ Si, it is preferable that the Si content in the plating layer is set to 1.2 to 3 mass%.

The plating layer contains 1 to 6 mass % Mg. When the main layer of the plating layer corrodes, Mg is contained in the corrosion product, which improves the stability of the corrosion product and retards the progress of corrosion, resulting in an effect of improving corrosion resistance. More specifically, Mg present in the main layer of the plating layer bonds with the above-mentioned Si to generate Mg ₂ Si. When the plated steel sheet corrodes, this Mg ₂ Si dissolves early, so Mg is contained in the corrosion product. The Mg contained in the corrosion product has the effect of densifying the corrosion product, and can improve the stability of the corrosion product and the barrier property against external corrosion factors.
Here, the reason why the Mg content of the plating layer is set to 1 mass% or more is that when the plating layer contains Si in the above-mentioned concentration range, by setting the Mg concentration to 1 mass% or more, Mg ₂ Si can be generated, and a corrosion retardation effect can be obtained. From the same viewpoint, the Mg content of the plating layer is preferably 2.5 mass% or more, and more preferably 3 mass% or more. On the other hand, the reason why the Mg content of the plating layer is set to 6 mass% or less is that when the Mg content of the plating layer exceeds 6%, the effect of improving the corrosion resistance is saturated, and in addition, the manufacturing cost increases and the composition control of the plating bath becomes difficult. From the same viewpoint, the Mg content of the plating layer is preferably 5 mass% or less.

In addition, by making the Mg content in the coating layer 3% by mass or more, it is possible to improve the corrosion resistance after painting. When the coating layer of a conventional hot-dip Al-Zn-based coated steel sheet not containing Mg is exposed to the air, a dense and stable oxide film of Al ₂ O ₃ is immediately formed around the α-Al phase, and the solubility of the α-Al phase becomes much lower than that of the Zn-rich phase in the interdendrite due to the protective effect of this oxide film. As a result, in the case of a coated steel sheet using a conventional Al-Zn-based coated steel sheet as a base, when damage occurs in the coating, selective corrosion of the Zn-rich phase occurs at the coating/coating interface starting from the scratch, and progresses deep into the coating sound area to cause large coating blistering, resulting in poor corrosion resistance after painting. Therefore, from the viewpoint of obtaining excellent corrosion resistance after painting, it is preferable to make the Mg content in the coating layer 3% by mass or more.
On the other hand, in the case of a coated steel sheet using a hot-dip Al-Zn-plated steel sheet containing Mg in the coating layer, the Mg ₂ Si phase and Mg-Zn compounds (MgZn ₂ , Mg ₃₂ (Al,Zn) ₄₉ , etc.) precipitated in the interdendrite dissolve in the early stage of corrosion, and Mg is taken into the corrosion product. The corrosion product containing Mg is very stable, which suppresses corrosion in the early stage, and therefore it is possible to suppress large paint film blistering due to selective corrosion of the Zn-rich phase, which is a problem in the case of coated steel sheets using conventional Al-Zn-plated steel sheets as a base. As a result, the hot-dip Al-Zn-plated steel sheet containing Mg in the coating layer exhibits excellent corrosion resistance after coating. If the Mg content in the coating layer is less than 3 mass%, the amount of Mg dissolved during corrosion is small, and there is a risk that the corrosion resistance after coating will not improve. If the Mg content in the plating layer exceeds 6% by mass, not only will the effect saturate, but corrosion of the Mg compounds will occur vigorously, and the solubility of the entire plating layer will increase excessively, so that even if the corrosion products are stabilized, their dissolution rate will increase, causing a large blister width and possibly deteriorating the corrosion resistance after painting. Therefore, in order to stably obtain excellent corrosion resistance after painting, the Mg content in the plating layer is set to 6% by mass or less.

The plating layer contains 0.001 to 1.0 mass % of Sr. By containing Sr in the plating layer, the occurrence of wrinkle defects can be suppressed and the surface appearance of the hot-dip plated steel sheet can be improved.
The wrinkle-like defect is a wrinkle-like uneven defect formed on the surface of the coating layer, and is observed as a whitish streak on the surface of the coating layer. Such a wrinkle-like defect is likely to occur when a large amount of Mg is added to the coating layer. Therefore, in the hot-dip plated steel sheet, by including Sr in the coating layer, Sr is oxidized preferentially over Mg in the surface layer of the coating layer, and the oxidation reaction of Mg is suppressed, thereby making it possible to suppress the occurrence of the streak-like defect.

The Sr content in the plating layer must be 0.001% by mass or more. This is to obtain the effect of suppressing the occurrence of wrinkle defects as described above. From the same viewpoint, the Sr content in the plating layer is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, and particularly preferably 0.05% by mass or more. On the other hand, the Sr content in the plating layer must be 0.2% by mass or less. If the Sr content is too high, the effect of suppressing the occurrence of streak defects will saturate, which will be disadvantageous in terms of cost.

The plating layer preferably further contains 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 mass%, because these elements, like Mg, improve the stability of corrosion products and retard the progression of corrosion. The reason why the total content of the above-mentioned elements is set at 0.01 to 10 mass% is that a sufficient corrosion retardation effect can be obtained and the effect does not saturate.

The plating layer contains components of the base steel sheet that are incorporated into the plating due to the reaction between the plating bath and the base steel sheet during plating processing, and unavoidable impurities in the plating bath. The base steel sheet components that are incorporated into the plating may contain up to about 2% Fe. Examples of the types of unavoidable impurities in the plating bath include Fe, Cu, Zr, etc. Fe in the plating layer cannot be quantified separately as being incorporated from the base steel sheet and being in the plating bath. There are no particular limitations on the total content of unavoidable impurities, but from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, it is preferable that the total amount of unavoidable impurities excluding Fe is 1 mass% or less.

The interfacial alloy layer is the layer of the plating layer that exists at the interface with the base steel sheet, and as described above, is an Fe-Al and/or Fe-Al-Si compound that is inevitably produced by an alloying reaction between the Fe on the steel sheet surface and the Al and Si in the bath. This interfacial alloy layer is hard and brittle, and if it grows too thick it will become the starting point for cracks during processing, so it is preferable to make it as thin as possible.

In the coated steel sheet of the present invention, the plating layer has a thickness of 15 to 30 μm and a standard deviation of the thickness of 5 μm or less.
In the present invention, the thickness of the plating layer and the standard deviation of the thickness are optimized, thereby making it possible to improve pressure mark resistance while maintaining good corrosion resistance.

In the present invention, the above pressure marks are caused by large unevenness in the plating layer formed under the coating, i.e., large variations in thickness, which results in unevenness on the surface of the coating, which results in differences in the pressure applied to the surface of the coating when the steel sheet is wound into a coil, and the occurrence of pressure marks is more noticeable when the coating is thin. Therefore, in the present invention, by suppressing the standard deviation of the thickness of the plating layer to 5 μm or less when the thickness of the plating layer is set to 15 to 30 μm, the surface appearance of the plating layer is improved and the unevenness on the surface of the coating caused by the variation in thickness of the plating layer can be reduced, making it possible to effectively suppress the occurrence of pressure marks.

Here, Fig. 1 is a diagram comparing the standard deviation of the plating thickness of the Al-Zn-Mg-Si plating layer (thickness 20 μm) in the coated steel sheet of the present invention with the standard deviation of the plating thickness of a general Al-Zn-Mg-Si plating layer (thickness 20 μm). As can be seen from Fig. 1, the standard deviation of the plating thickness of the Al-Zn-Mg-Si plating layer in the coated steel sheet of the present invention is smaller (1/3 or less) than the standard deviation of the plating thickness of a general Al-Zn-Mg-Si plating layer.
As a result, in the case of coated steel sheets, the unevenness of the coating surface caused by the variation in the thickness of the plating layer can be reduced, and the occurrence of pressure marks can be effectively suppressed. On the other hand, in the case of coated steel sheets using a general Al-Zn-Mg-Si-based plating layer, the unevenness of the coating surface caused by the variation in the thickness of the plating layer becomes large, and a difference in pressure is generated on the coating surface, which is expected to result in the occurrence of pressure marks.

The thickness of the plating layer must be 15 to 30 μm, but this is the average thickness of the plating layer. In the present invention, the cross section of the plating layer in the thickness direction is observed at five randomly selected points of the plating layer, the average thickness of the plating layer is calculated for each observation field, and the average value of the five points can be regarded as the thickness of the plating layer. Here, the thickness of the plating layer refers to the combined thickness of the main layer and the alloy layer of the plating layer.
In addition, regarding the standard deviation of the thickness of the plating layer, the cross section of the plating layer in the thickness direction is observed at five randomly selected points on the plating layer, and the standard deviation of the thickness of the plating layer within a length range of 5 mm in the observation field of view is defined as the standard deviation of the thickness of the plating layer in the present invention.

The cross-section of the plating layer in the thickness direction can be observed, for example, by energy dispersive X-ray spectroscopy using a scanning electron microscope (SEM-EDX).
For example, when identifying Mg ₂ Si in the plating layer, after obtaining the cross-sectional state of the plating layer in the thickness direction, mapping is performed for each of Mg and Si, and then the portion of the mapped Mg and Si that overlaps at the same position can be identified as Mg ₂ Si.

In addition, when observing the main layer of the plating layer or the interfacial alloy layer with a scanning electron microscope, the cross section of the plating layer is polished and/or etched before observation. There are several types of methods for polishing and etching the cross section, but they are not particularly limited as long as they are methods generally used for observing the cross section of a plating layer. In addition, the conditions for observation and analysis with a scanning electron microscope are, for example, an accelerating voltage of 5 to 20 kV and a magnification of about 500 to 5000 times for secondary electron images or backscattered electron images.
As for observation conditions for the scanning transmission electron microscope (STEM-EDX), for example, a cross-sectional sample of the FIB-processed plating layer can be clearly observed and analyzed by a magnification of about 1,000 to 50,000 under conditions of an acceleration voltage of 20 kV.

It is also preferable that the main layer of the plating layer has a dendritic portion of an α-Al phase, and that the average dendrite arm distance of the dendritic portion and the thickness of the plating layer satisfy the following formula (1).
t/d≧1.5 (1)
t: thickness of plating layer (μm), d: average distance between dendrite arms (μm)
By satisfying the above formula (1), the arms of the dendrite portion made of the above-mentioned α-Al phase can be made relatively small, and the path of the interdendrites that are preferentially corroded can be secured to be long, thereby further improving corrosion resistance.

The dendrite arm distance in the dendrite portion means the center distance between adjacent dendrite arms (dendrite arm spacing). In the present invention, for example, as shown in FIG. 3, the surface of the polished and/or etched main layer of the plating layer is observed under magnification (for example, at 200 times) using a scanning electron microscope (SEM) or the like, and the distance between the second widest dendrite arms (secondary dendrite arms) in a randomly selected field of view is measured as follows. A portion where three or more secondary dendrite arms are aligned is selected (in FIG. 3, three arms between A and B are selected), and the distance (distance L in FIG. 3) is measured along the direction in which the arms are aligned. The measured distance is then divided by the number of dendrite arms (L/3 in FIG. 3) to calculate the dendrite arm distance. The dendrite arm distance is measured at three or more points in one field of view, and the average of the dendrite arm distances obtained is calculated to be the average dendrite arm distance.

The thickness of the plating layer must be 15 to 30 μm, and is preferably 20 to 25 μm, from the viewpoint of achieving both high levels of workability and corrosion resistance. This is because sufficient corrosion resistance can be ensured when the plating layer is 15 μm or more, and sufficient workability can be ensured when the plating layer is 30 μm or less.

The coated steel sheet of the present invention has a coating film formed on the plating layer directly or via an intermediate layer, and the coating film has a thickness of 20 μm or less.
The reason why the thickness of the coating film is set to 20 μm or less is to contribute to thinner steel sheets and lower costs.

The type of coating film and the method for forming the coating film are not particularly limited and can be appropriately selected depending on the required performance. Examples of methods include roll coater coating, curtain flow coating, and spray coating. After applying a paint containing an organic resin, the paint can be heated and dried by means of hot air drying, infrared heating, induction heating, etc. to form a coating film.

The intermediate layer is not particularly limited as long as it is a layer formed between the plating layer of the hot-dip plated steel sheet and the coating film. Examples include a chemical conversion coating film and a primer such as an adhesive layer. The chemical conversion coating film can be formed, for example, by a chromate treatment or a chromate-free chemical conversion treatment in which a chromate treatment liquid or a chromium-free chemical conversion coating liquid is applied, and then dried without rinsing with water until the steel sheet temperature reaches 80 to 300°C. These chemical conversion coating films may be single-layered or multi-layered, and in the case of multi-layered films, multiple chemical conversion treatments may be performed sequentially.

(Manufacturing method of coated steel sheet)
Next, the method for producing the coated steel sheet of the present invention will be described.
The method for producing a coated steel sheet of the present invention comprises the steps of: immersing a base steel sheet in a coating bath containing 40 to 70 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.001 to 0.2 mass% Sr, with the balance being Zn and unavoidable impurities; and forming a coating film having a thickness of 20 μm or less on the obtained hot-dip coated steel sheet, directly or via an intermediate layer.
The bath temperature of the plating bath satisfies the following formula (1):
Plating bath temperature (℃) ≦600 - 4.5M _Mg - 5.5M _Si ... (1)
The coated steel sheet obtained by the above-mentioned production method has good corrosion resistance, and is excellent in pressure mark resistance and surface appearance of the plating layer.

In the method for producing a coated steel sheet of the present invention, although there is no particular limitation, a continuous hot-dip galvanizing facility is usually adopted from the viewpoints of production efficiency and quality stability.
The type of base steel sheet used for the hot-dip Al-Zn-Mg-Si-plated steel sheet of the present invention is not particularly limited. For example, a hot-rolled steel sheet or strip that has been pickled and descaled, or a cold-rolled steel sheet or strip obtained by cold rolling the hot-rolled steel sheet or strip may be used. The conditions of the pretreatment step and the annealing step are also not particularly limited, and any method may be used.

In the method for producing a coated steel sheet of the present invention, the coating bath has a composition containing 40 to 70 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.001 to 0.2 mass% Sr, with the balance being Zn and unavoidable impurities.
In this way, a hot-dip Al-Zn-Mg-Si-plated steel sheet having a desired composition can be obtained. The types, contents and functions of the elements contained in the plating bath are described in the above-mentioned coated steel sheet of the present invention.

The hot-dip galvanized steel sheet obtained by the manufacturing method of the present invention has a composition that is almost equivalent to that of the plating bath as a whole. Therefore, the composition of the main layer can be controlled with high precision by controlling the plating bath composition.

The method for producing a coated steel sheet of the present invention includes a step of immersing a base steel sheet in the coating bath, and is characterized in that the bath temperature of the coating bath satisfies the following formula (1).
Plating bath temperature (℃) ≦600 - 4.5M _Mg - 5.5M _Si ... (1)

As described above, in order to improve the pressure mark resistance and surface appearance of the plating layer of a coated steel sheet, it is important to suppress the unevenness of the plating layer formed under the coating. Therefore, in the manufacturing method of the present invention, the composition of the plating bath used in hot dip plating is optimized, and the bath temperature satisfies the above formula (1), thereby eliminating wrinkles in the plating layer. Furthermore, the time required for solidification is shortened, and the variation in plating thickness is reduced, making it possible to suppress unevenness on the plating layer surface. As a result, even if a thin coating is formed on the plating layer, the occurrence of pressure marks can be effectively suppressed. In addition, since the plating bath contains Mg, it is also possible to improve corrosion resistance.

Furthermore, in the manufacturing method of the present invention, after the steel sheet is subjected to hot-dip plating, it is preferable to cool the steel sheet at an average cooling rate of 10 to 50°C/sec to a temperature obtained by subtracting 50°C from the bath temperature of the plating bath (plating bath temperature -50°C). It is known that Mg ₂ Si formed in the above-mentioned plating layer is likely to be formed in a temperature range up to a temperature obtained by subtracting 50°C from the bath temperature of the plating bath (plating bath temperature -50°C), and by increasing the cooling rate in this temperature range to an average of 10°C/sec or more, the formed Mg ₂ Si is dispersed and the plating structure is refined, thereby reducing the variation in plating thickness. From the same viewpoint, the cooling of the steel sheet after the hot-dip plating is more preferably performed at an average cooling rate of 10°C/sec or more, and even more preferably at an average cooling rate of 20°C/sec or more.
In the manufacturing method of the present invention, the hot-dip galvanized steel sheet is usually subjected to temper rolling for the purpose of adjusting the steel sheet properties and smoothing the surface before being wound into a coil in a continuous hot-dip galvanizing facility.

The manufacturing method of the present invention further includes a step of forming a coating film having a thickness of 20 μm or less on the obtained hot-dip plated steel sheet directly or via an intermediate layer.
The method for forming the coating film is not particularly limited and can be appropriately selected depending on the required performance. For example, the coating film can be formed by a method such as roll coater coating, curtain flow coating, spray coating, etc. After coating the coating material containing an organic resin, the coating film can be formed by heating and drying the coating film by means of hot air drying, infrared heating, induction heating, etc.

The intermediate layer is not particularly limited as long as it is a layer formed between the coating layer 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 coated steel sheet of the present invention.

(Samples 1 to 41)
Cold-rolled steel sheets having a thickness of 0.5 mm produced by a conventional method were used as the base steel sheet, and hot-dip plated steel sheets having a thickness of 20 to 23 μm were produced in a continuous hot-dip plating facility to produce samples 1 to 41. The composition of the plating bath used in the production was almost the same as the composition of the plating layer of each sample shown in Table 1, and the measured bath temperature of the plating bath, the calculated value of the right side of equation (1) (600 - 4.5M _Mg - 5.5M _Si ), and the standard deviation of the thickness of the obtained plating layer are also shown in Table 1.
Then, a chromate-free chemical treatment of 130±50 mg/ ^m2 , an epoxy-containing polyester-based undercoat coating of 4±1 μm, and a polyester-based topcoat coating of 16±2 μm were applied to the hot-dip plated steel sheet samples 1 to 41, producing coated steel sheet samples 1 to 41 with a total coating thickness of 20 μm.

(evaluation)
The following evaluations were carried out on the samples of hot-dip galvanized steel sheets and coatings obtained as described above. The evaluation results are shown in Table 1.

(1) Corrosion resistance evaluation Each sample of the obtained hot-dip galvanized steel sheets was subjected to a combined cycle test (JASO-CCT) of the Japanese Automotive Standards. As shown in Figure 3, the JASO-CCT is a test in which salt spray, drying, and wetting are cycled under specific conditions.
The number of cycles until red rust appeared on each sample was measured and evaluated according to the following criteria.
◎: Number of cycles at which red rust appears ≧600 cycles ○: Number of cycles at which red rust appears ≧400 cycles △: Number of cycles at which red rust appears ≦300 cycles <400 cycles ×: Number of cycles at which red rust appears <300 cycles

(2) Pressure Mark Resistance For each of the obtained coated steel plate samples (length 650 mm×width 914 mm), the surface of the coating was visually observed.
The observation results were evaluated according to the following criteria.
○: No pressure marks were observed ×: Pressure marks were observed

(3) Surface Appearance of Plated Layer For each sample (length 650 mm x width 914 mm) of the obtained hot-dip plated steel sheet, the surface of the plated layer (both sides of each sample) was visually observed.
The observation results were evaluated according to the following criteria.
◯: No wrinkle-like defects were observed on either the front or back surface. ×: Wrinkle-like defects were observed on at least one of the front and back surfaces.

The results in Table 1 show that the samples of the present invention are well-balanced and superior in terms of corrosion resistance, pressure mark resistance, and surface appearance of the plating layer compared to the samples of the comparative examples.

The present invention provides a coated steel sheet that has good corrosion resistance, excellent pressure mark resistance, and excellent surface appearance of the plating layer, as well as a method for manufacturing a coated steel sheet that has good corrosion resistance, excellent pressure mark resistance, and excellent surface appearance of the plating layer.

Claims (4)

A coated steel sheet in which a coating film is formed directly or via an intermediate layer on a coating layer of a hot-dip galvanized steel sheet,
The plating layer has a composition containing 40 to 70 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.05 to 0.2 mass% Sr, with the remainder being Zn and unavoidable impurities;
The coated steel sheet, wherein the plating layer has a thickness of 15 to 30 μm with a standard deviation of 5 μm or less, and the coating film has a thickness of 20 μm or less. The coated steel sheet according to claim 1, characterized in that the plating layer further contains 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 mass%. a step of immersing a base steel sheet in a coating bath containing 40 to 70 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, 0.05 to 0.2 mass% Sr, with the balance being Zn and unavoidable impurities , to form a coating layer having a thickness of 15 to 30 μm ;
and forming a coating film having a thickness of 20 μm or less on the obtained hot-dip plated steel sheet directly or via an intermediate layer,
The bath temperature of the plating bath satisfies the following formula (1),
A method for producing a coated steel sheet, characterized in that after the steel sheet is hot-dip coated, the steel sheet is cooled at an average cooling rate of 10 to 50°C/sec until the temperature of the coating bath is reduced by 50°C (coating bath temperature - 50°C).
Plating bath temperature (℃) ≦600 - 4.5M _Mg - 5.5M _Si ... (1)
M _Mg : Mg content in the plating bath (mass%), M _Si : Si content in the plating bath (mass%) The method for producing coated steel sheet according to claim 3, characterized in that the plating bath further contains 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 mass%.

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