JP2020143369A - Coated steel sheet, and manufacturing method of coated steel sheet - Google Patents

Abstract

To provide a coated steel sheet excellent in corrosion resistance, pressure mark resistance, and surface appearance of a plating layer.SOLUTION: A coated steel sheet has a coating film formed directly or through an intermediate layer on a plating layer of a hot-dip plating steel sheet. The plating layer has a composition of Al of 40-70 mass%, Si of 1.2-4 mass%, Mg of 1-6 mass%, Sr of 0.001-0.2 mass%, and a balance of Zn and inevitable impurities. The plating layer has a thickness of 15-30 μm and a standard deviation of the thickness of 5 μm or less. The coating film has a thickness of 20 μm or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a coated steel sheet having good corrosion resistance, excellent pressure mark resistance and surface appearance of a plating layer, and a method for producing the same.

The hot-dip Al-Zn-based plated steel sheet exhibits both high sacrificial corrosion resistance of Zn and high corrosion resistance of Al, and therefore exhibits high corrosion resistance among hot-dip galvanized steel sheets. For example, Patent Document 1 discloses a hot-dip Al-Zn-based galvanized steel sheet containing 25 to 75% by mass of Al in the plating layer. Due to its excellent corrosion resistance, hot-dip Al-Zn plated steel sheets have been in increasing demand in recent years, mainly in the fields of building materials such as roofs and walls that are exposed outdoors for a long period of time, and in the fields of civil engineering and construction such as guardrails, wiring pipes, and soundproof walls. ing.

The plating layer of the molten Al-Zn-based plated steel sheet consists of a main layer and an alloy layer existing at the interface between the base steel plate and the main layer, and the main layer mainly contains Zn in a hypersaturation and dendrite solidifies Al (α). It is composed of a dendrite portion of the -Al phase) and a portion of the remaining dendrite gap (interdendrite), and has a structure in which a plurality of α-Al phases are laminated in the film thickness direction of the plating layer. Due to such a characteristic film structure, the corrosion progress path from the surface is complicated, so that corrosion does not easily reach the underlying steel sheet, and the molten Al-Zn-based plated steel sheet has the same plating layer thickness of molten zinc. Excellent corrosion resistance can be achieved compared to plated steel sheets.

Further, a technique is known for the purpose of further improving the corrosion resistance by containing Mg in the plating layer of the hot-dip Al-Zn-based plating. As a technique relating to a molten Al-Zn-based plated steel sheet containing Mg (molten Al-Zn-Mg-Si-based plated steel sheet), for example, Patent Document 2 includes an Al-Zn-Si alloy containing Mg in a plating layer. The Al-Zn-Si alloy is an alloy containing 45 to 60% by weight of elemental aluminum, 37 to 46% by weight of elemental zinc and 1.2 to 2.3% by weight of elemental silicon, and the Mg concentration is 1 to 5%. Al-Zn-Mg-Si based plated steel sheets, which are% by weight, are disclosed.

Further, as a technique for containing Mg in the plating layer, Patent Document 3 aims to enhance the corrosion resistance and the protective action after the base steel sheet is exposed by containing a certain amount of Mg and Ca in the plating layer. The hot-dip Al-Zn-based plated steel sheet is disclosed.
Further, in Patent Document 4, Mg: 1 to 15%, Si: 2 to 15%, Zn: 11 to 25% are contained in mass%, and the balance forms a coating layer composed of Al and unavoidable impurities. , Al-based plated steel sheets with improved corrosion resistance of flat plates and end faces are disclosed by defining the size of Mg ₂ Si phase present in the plated layer.

However, although the hot-dip Al-Zn-based plated steel sheets disclosed in References 1 and 2 have excellent corrosion resistance, wrinkle-like defects caused by the oxide layer formed on the surface of the plated layer (hereinafter, "" There is a problem that "wrinkle-like defects") are likely to occur and the appearance of the surface of the plating layer is impaired.
Therefore, for example, Patent Document 5 discloses a technique for improving the surface appearance of a hot-dip Al-Zn-Mg-based plated steel sheet by incorporating Sr in the plating layer.
Further, Patent Document 6 discloses a technique for improving workability of a molten Al-Zn-Mg-based plated steel sheet by containing Sr in the plating layer.
Further, in Patent Document 7, by adding at least 250 ppm of Sr to a plating bath containing an Al-Zn-Si-Mg alloy, the content of Mg ₂ Si particles in the plating layer is reduced, resulting in surface appearance. The technology for improving the above is disclosed.

Special Publication No. 46-7161 Japanese Patent No. 5020228 Japanese Patent No. 5000039 Japanese Unexamined Patent Publication No. 2002-12959 Japanese Patent No. 3983932 Japanese Patent No. 6368730 Special Table 2011-514934

As described above, the hot-dip Al-Zn-Mg-Si-based plated steel sheets of Patent Documents 3 and 4 can improve the corrosion resistance, and the hot-dip Al-Zn-Mg-Si-based steel sheets containing Sr of Patent Documents 5 to 7 can be improved. The surface appearance of the plated steel sheet can be improved.
However, for a coated steel sheet in which a coating film is formed on a plated steel sheet, in addition to the above-mentioned improvement in corrosion resistance and surface appearance of the plated layer, it is also desired to suppress the occurrence of pressure marks (pressure mark resistance). It was rare.

Here, the pressure mark defect is a result of pressure contact between the front surface and the back surface of the coated steel sheet (coated steel strip) when the coated steel sheet (painted steel strip) is wound around the coil, and a part of the coating film formed by the pressure is plastically deformed. , It is a spot-like gloss unevenness that occurs around the center of the steel sheet in the width direction.
In the techniques disclosed in Patent Documents 5 to 7, since the surface appearance of the plating layer is improved by the inclusion of Sr, the generation of pressure marks can be suppressed to some extent, but the suppression effect is achieved by simply adding Sr. It cannot be said that it is sufficient, and further improvement in pressure mark resistance has been desired.

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

The present inventors have studied a coated steel sheet in which a coating film is formed directly or via an intermediate layer on the plating layer of the hot-dip galvanized steel sheet in order to solve the above problems. It has been found that the pressure mark resistance can be improved while having good corrosion resistance by setting the composition of the plating layer to a specific range and optimizing the thickness and the standard deviation of the thickness.

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

2. 2. The plating layer further contains one or more selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb and B in a total of 0.01 to 10% by mass. The coated steel sheet according to 1 above.

3. 3. A step of immersing the base steel sheet in a plating bath containing Al: 45 to 65% by mass, Si: 1.2 to 4% by mass and Mg: 1 to 6% by mass, and the balance is Zn and unavoidable impurities. A step of forming a coating film having a thickness of 20 μm or less directly or via an intermediate layer on the obtained hot-dip galvanized steel sheet is provided, and the bath temperature of the plating bath is the following formula (1). A method for manufacturing a coated steel sheet, which is characterized by being satisfied.
Plating bath temperature (° C) ≤ 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 plating bath further contains one or more selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb and B in a total of 0.01 to 10% by mass. The method for producing a coated steel sheet according to 3 above.

According to the present invention, a coated steel sheet having good corrosion resistance and excellent pressure mark resistance and surface appearance of the plating layer, and a coated steel sheet having good corrosion resistance, pressure mark resistance and surface appearance of the plating layer. It is possible to provide an excellent method for manufacturing a coated steel sheet.

It is a figure which compared the variation in the thickness of the plating layer of the coated steel sheet of the present invention with the variation in the thickness of the plating layer of the conventional coated steel sheet. It is a figure for demonstrating the measuring method of the distance between dendrite arms. It is a figure for demonstrating the flow of the combined cycle test (JASO-CCT) of the Japanese Automotive Standards Organization.

(Painted steel plate)
The coated steel sheet of the present invention is a coated steel sheet in which a coating film is formed directly or via an intermediate layer on the plating layer of the hot-dip galvanized steel sheet.
In the present invention, the plating layer contains Al: 40 to 70% by mass, Si: 1.2 to 4% by mass, Mg: 1 to 6% by mass and Sr: 0.001 to 0.2% by mass, and the balance is Zn and A hot-dip Al-Zn-Mg-Si-based galvanized steel sheet having a composition composed of unavoidable impurities. When the plating layer of the hot-dip galvanized steel sheet has the above-mentioned composition, desired corrosion resistance can be ensured and pressure mark resistance can be improved.

The Al content in the plating layer is 40 to 70% by mass, preferably 45 to 65% by mass, in view of the balance between corrosion resistance and operational aspects. When the Al content of the main layer of the plating layer is at least 40% by mass, dendrite solidification of Al sufficiently occurs. As a result, the main layer mainly contains Zn in supersaturation, and is composed of a portion where Al is dendrite solidified (a dendrite portion of the α-Al phase) and a portion of the remaining dendrite gap (interdendrite portion), and the dendrite portion is plated. It is possible to realize a structure having excellent corrosion resistance laminated in the film thickness direction of the layers. Further, the more the dendrite portions of the α-Al phase are laminated, the more complicated the corrosion progress path becomes, and the more difficult it is for corrosion to reach the underlying steel sheet, so that the corrosion resistance is improved. On the other hand, when the Al content in the plating layer exceeds 70% by mass, the content of Zn having a sacrificial anticorrosion action against Fe decreases, and the corrosion resistance deteriorates. Therefore, the Al content in the plating layer is 70% by mass or less. Further, when the Al content in the plating layer is 65% by mass or less, the amount of adhesion of the plating is reduced, and even when the underlying steel sheet is easily exposed, it has a sacrificial anticorrosion effect on Fe, which is sufficient. Excellent corrosion resistance can be obtained. Therefore, the Al content of the plating main layer is preferably 65% by mass or less.

Si in the plating layer is added to the plating bath for the purpose of improving corrosion resistance and workability for the purpose of suppressing the growth of the interfacial alloy layer formed at the interface with the underlying steel plate, and is inevitably added to the main layer. It is contained. In the case of the hot-dip galvanized steel sheet used in the coated steel sheet of the present invention, when Si is contained in the plating bath and the hot-dip galvanizing treatment is performed, the underlying steel sheet is immersed in the plating bath and at the same time, Fe on the surface of the steel sheet and the bath Al and Si undergo an alloying reaction to produce an alloy composed of Fe-Al and / or Fe-Al-Si compounds. By forming the Fe-Al-Si based interfacial alloy layer, the growth of the interfacial alloy layer can be suppressed. When the Si content in the plating layer is 1.2% by mass or more, the growth of the interfacial alloy layer can be sufficiently suppressed. On the other hand, when the Si content of the plating layer exceeds 4% by mass, the processability is lowered in the plating layer, and the Si phase serving as the cathode site is likely to be precipitated. This precipitation of the Si phase can be suppressed by increasing the Mg content, but in that case, it causes an increase in manufacturing cost and a decrease in processability due to an increase in the amount of Mg ₂ Si, and also in the plating bath. It makes composition control more difficult. Therefore, the Si content in the plating layer is set to 4% by mass or less. Furthermore, considering that the growth of the interfacial alloy layer and the precipitation of the Si phase can be suppressed more reliably and that Si can be dealt with when Si is consumed as Mg ₂ Si, the Si content in the plating layer is determined. It is preferably 1.2 to 3% by mass.

The plating layer contains 1 to 6% by mass of Mg. When the main layer of the plating layer is corroded, Mg is contained in the corrosion product, the stability of the corrosion product is improved, the progress of corrosion is delayed, and as a result, the corrosion resistance is improved. .. More specifically, Mg present in the main layer of the plating layer combines with the above-mentioned Si to form Mg ₂ Si. This Mg ₂ Si dissolves in the early stage when the plated steel sheet is corroded, so Mg is contained in the corrosion product. Mg contained in this corrosion product has an effect of densifying the corrosion product, and can improve the stability of the corrosion product and the barrier property against foreign corrosion factors.
Here, the reason the Mg content of the plating layer is 1 mass% or more, the plating layer is, when the content of the Si in the concentration range described above, by 1 mass% or more of Mg concentration, Mg ₂ This is because Si can be generated and a corrosion delay effect can be obtained. From the same viewpoint, the Mg content of the plating layer is preferably 2.5% by mass or more, and more preferably 3% by mass or more. On the other hand, the reason why the Mg content of the plating layer is 6% by mass or less is that when the Mg content of the plating layer exceeds 6%, in addition to the saturation of the corrosion resistance improving effect, the manufacturing cost increases and the plating This is because it becomes difficult to control the composition of the bath. From the same viewpoint, the Mg content of the plating layer is preferably 5% by mass or less.

Further, by setting the Mg content in the plating layer to 3% by mass or more, it is possible to improve the corrosion resistance after painting. When the plating layer of a conventional molten Al-Zn galvanized steel sheet that does not contain Mg comes into contact with the atmosphere, a dense and stable oxide film of Al ₂ O ₃ is immediately formed around the α-Al phase, and protection by this oxide film is formed. Due to the action, the solubility of the α-Al phase is much lower than that of the Zn-rich phase in the interdendrite. As a result, in the case of a coated steel sheet using a conventional Al-Zn-based plated steel sheet as a base, when the coating film is damaged, the Zn-rich phase is selectively corroded at the coating film / plating interface starting from the scratched portion and coated. Corrosion resistance is inferior after painting because it progresses toward the depths of the healthy part and causes large swelling of the coating film. Therefore, from the viewpoint of obtaining excellent post-coating corrosion resistance, it is preferable that the Mg content in the plating layer is 3% by mass or more.
On the other hand, in the case of a coated steel sheet using a molten Al-Zn-based plated steel sheet containing Mg in the plating layer, the Mg ₂ Si phase and Mg-Zn compound (MgZn ₂ , Mg ₃₂ , Mg ₃₂ (Al,) precipitated in the interdent light Zn) ₄₉ etc.) dissolves in the early stage of corrosion, and Mg is incorporated into the corrosion product. The Mg-containing corrosion products are very stable, which suppresses corrosion in the early stages, which is a problem for coated steel sheets using conventional Al-Zn galvanized steel sheets as a base. It is possible to suppress large swelling of the coating film due to selective corrosion. As a result, the molten Al-Zn-based plated steel sheet containing Mg in the plating layer exhibits excellent post-coating corrosion resistance. When the amount of Mg in the plating layer is less than 3% by mass, the amount of Mg that dissolves during corrosion is small, and the corrosion resistance after painting may not be improved. When the Mg content in the plating layer exceeds 6% by mass, not only the effect is saturated, but also the Mg compound is severely corroded, and the solubility of the entire plating layer is excessively increased, resulting in corrosion. Even if the product is stabilized, its dissolution rate is increased, so that a large swelling width is generated and the corrosion resistance after coating may be deteriorated. Therefore, in order to stably obtain excellent post-coating corrosion resistance, the Mg content in the plating layer is set to 6% by mass or less.

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

The Sr content in the plating layer needs to be 0.001% by mass or more. This is to obtain the effect of suppressing the occurrence of the above-mentioned wrinkle-like defects. 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 needs to be 0.2% by mass or less. This is because if the Sr content is too large, the effect of suppressing the occurrence of streaky defects is saturated, which is disadvantageous in terms of cost.

Further, since the plating layer can improve the stability of the corrosion product and delay the progress of corrosion in the same manner as the above-mentioned Mg, Cr and Mn in total of 0.01 to 10% by mass. , V, Mo, Ti, Ca, Ni, Co, Sb and B are preferably selected from one or more. The reason why the total content of the above-mentioned components is set to 0.01 to 10% by mass is that a sufficient corrosion delay effect can be obtained and the effect is not saturated.

The plating layer contains a base steel plate component incorporated during plating by the reaction between the plating bath and the base steel plate during the plating process, and unavoidable impurities in the plating bath. Fe may be contained in a maximum of about 2% as the base steel sheet component incorporated in the plating. Examples of the types of unavoidable impurities in the plating bath include Fe, Cu, Zr and the like. The Fe in the plating layer cannot be quantified separately from the one taken in from the base steel plate and the one in the plating bath. The total content of unavoidable impurities is not particularly limited, but from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, the total amount of unavoidable impurities excluding Fe is preferably 1% by mass or less.

The interfacial alloy layer is a layer existing at the interface with the underlying steel sheet among the plating layers, and as described above, Fe on the surface of the steel sheet and Al or Si in the bath inevitably undergo an alloying reaction. Fe-Al-based and / or Fe-Al-Si-based compounds produced. Since this interfacial alloy layer is hard and brittle, if it grows thick, it becomes a starting point of crack generation during processing, so it is preferable to make it as thin as possible.

The coated steel sheet of the present invention is characterized in that the plating layer has a thickness of 15 to 30 μm and a standard deviation of the thickness is 5 μm or less.
In the present invention, the thickness of the plating layer and the standard deviation of the thickness can be optimized to improve the pressure mark resistance while having good corrosion resistance.

In the present invention, the pressure mark is a result of the unevenness formed on the surface of the coating film due to the large unevenness of the plating layer formed under the coating film, that is, the large variation in thickness. Pay attention to the fact that when the steel sheet is wound around a coil, a difference in pressure is applied to the surface of the coating film, which causes pressure marks, and that the pressure marks are noticeably generated when the coating film is thin. did. Therefore, in the present invention, when the thickness of the plating layer is set to 15 to 30 μm, the standard deviation of the thickness is suppressed to 5 μm or less to improve the surface appearance of the plating layer and the thickness of the plating layer. Since the unevenness on the surface of the coating film due to the deviation can be reduced, the occurrence of pressure marks can be effectively suppressed.

Here, FIG. 1 shows the standard deviation of the plating thickness of the Al-Zn-Mg-Si-based plating layer (thickness 20 μm) in the coated steel sheet of the present invention and the general Al-Zn-Mg-Si-based plating layer. It is the figure which compared with the standard deviation of the plating thickness of (thickness 20 μm). As can be seen from FIG. 1, the standard deviation of the plating thickness of the Al-Zn-Mg-Si-based plating layer in the coated steel sheet of the present invention is the plating thickness of the general Al-Zn-Mg-Si-based plating layer. It can be seen that it is suppressed to be smaller (less than 1/3) than the standard deviation.
As a result, in the coated steel sheet, the unevenness on the surface of the coating film due to the variation in the thickness of the plating layer can be reduced, so that the occurrence of pressure marks can be effectively suppressed. On the other hand, in a coated steel sheet using a general Al-Zn-Mg-Si plating layer, the unevenness of the coating film surface due to the variation in the thickness of the plating layer becomes large, and the pressure applied to the coating film surface is increased. It is expected that pressure marks will occur as a result of the difference.

The thickness of the plating layer needs to be 15 to 30 μm, which 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 locations of the plating layer, the average thickness of the plating layer is calculated for each observation field, and the average value of the five locations is calculated. It can be the thickness of the plating layer. Here, the thickness of the plating layer is the total thickness of the main layer and the alloy layer of the plating layer.
Also, regarding the standard deviation of the thickness of the plating layer, the cross section of the plating layer in the thickness direction was observed at five randomly selected locations of the plating layer, and within a length range of 5 mm in the observation field of view. The standard deviation of the thickness of the plating layer in the above is taken as the standard deviation of the thickness of the plating layer in the present invention.

The cross-sectional observation of the plating layer in the thickness direction can be performed by, for example, energy dispersive X-ray spectroscopy (SEM-EDX) using a scanning electron microscope.
For example, when identifying Mg ₂ Si in the plating layer, after acquiring the cross-sectional state of the plating layer in the thickness direction, mapping is performed for each of Mg and Si, and then of the mapped Mg and Si. , The part where these overlap at the same position can be called Mg ₂ Si.

When observing the main layer of the plating layer and the interfacial alloy layer with a scanning electron microscope, the cross section of the plating layer is polished and / or etched before observing. There are several types of cross-section polishing methods and etching methods, but the method is not particularly limited as long as it is a method generally used for observing the cross-section of the plating layer. The conditions for observation and analysis with a scanning electron microscope can be, for example, an accelerating voltage of 5 to 20 kV and a magnification of about 500 to 5000 times for a secondary electron image or a backscattered electron image.
As an observation condition with the scanning transmission electron microscope (STEM-EDX), for example, if the cross-sectional sample of the FIB-processed plating layer has a magnification of about 1000 to 50,000 under the condition of an accelerating voltage of 20 kV, the plating is performed. It is possible to clearly observe and analyze the layers.

Further, the main layer of the plating layer has an α-Al phase dendrite portion, and the average distance between the dendrite arms of the dendrite portion and the thickness of the plating layer satisfy the following formula (1). Is preferable.
t / d ≧ 1.5 ・ ・ ・ (1)
t: Thickness of plating layer (μm), d: Average distance between dendrite arms (μm)
By satisfying the above equation (1), the arm of the dendrite portion composed of the above-mentioned α-Al phase can be made relatively small, and by securing a long path for the dendrite that is preferentially corroded, the corrosion resistance is further improved. Can be improved.

The distance between the dendrite arms of 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 plating layer main layer is magnified and observed (for example, observed at 200 times) using a scanning electron microscope (SEM) or the like. The distance between the second widest dendrite arms (secondary dendrite arms) in a randomly selected field of view is measured as follows. Select the part where three or more secondary dendrite arms are aligned (in FIG. 3, three are selected between A and B), and the distance along the direction in which the arms are aligned (FIG. 3). Now, the distance L) is measured. After that, the measured distance is divided by the number of dendrite arms (L / 3 in FIG. 3) to calculate the distance between the dendrite arms. The distance between the dendrite arms is measured at three or more points in one field of view, and the average of the obtained distances between the dendrite arms is calculated as the average distance between the dendrite arms.

The thickness of the plating layer needs to be 15 to 30 μm, preferably 20 to 25 μm, from the viewpoint of achieving both workability and corrosion resistance at a high level. 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.

In the coated steel sheet of the present invention, a coating film is formed on the plating layer directly or via an intermediate layer, and the thickness of the coating film is 20 μm or less.
The thickness of the coating film is set to 20 μm or less in order to contribute to thinning and cost reduction of the steel sheet.

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

Further, the intermediate layer is not particularly limited as long as it is a layer formed between the plating layer of the hot-dip galvanized steel sheet and the coating film. For example, a primer such as a chemical conversion treatment film or an adhesive layer can be mentioned. The chemical conversion treatment film is formed by, for example, a chromate treatment or a chromate-free chemical conversion treatment in which a chromate treatment liquid or a chromium-free chemical conversion treatment liquid is applied and a drying treatment is performed at a steel sheet temperature of 80 to 300 ° C. without washing with water. It is possible. These chemical conversion treatment coatings may be single-layered or multi-layered, and in the case of multiple layers, a plurality of chemical conversion treatments may be sequentially performed.

(Manufacturing method of painted steel sheet)
Next, the method for manufacturing the coated steel sheet of the present invention will be described.
The method for producing a coated steel sheet of the present invention contains Al: 40 to 70% by mass, Si: 1.2 to 4% by mass, Mg: 1 to 6% by mass, and Sr: 0.001 to 0.2% by mass, and the balance is Zn and A step of immersing the base steel sheet in a plating bath having a composition consisting of unavoidable impurities, and a step of forming a coating film having a thickness of 20 μm or less on the obtained hot-dip galvanized steel sheet directly or via an intermediate layer. With,
The bath temperature of the plating bath is characterized by satisfying the following formula (1).
Plating bath temperature (° C) ≤ 600-4.5M _Mg -5.5M _Si ... (1)
The coated steel sheet obtained by the above-mentioned manufacturing method has good corrosion resistance, pressure mark resistance, and surface appearance of the plating layer.

The method for producing a coated steel sheet of the present invention is not particularly limited, but a continuous hot-dip galvanizing facility is usually adopted from the viewpoint of production efficiency and quality stability.
The type of base steel sheet used for the molten Al-Zn-Mg-Si-based plated steel sheet of the present invention is not particularly limited. For example, a hot-rolled steel sheet or steel strip obtained by pickling and descaling, or a cold-rolled steel plate or steel strip obtained by cold-rolling them can be used. Further, the conditions of the pretreatment step and the annealing step are not particularly limited, and any method can be adopted.

In the method for producing a coated steel sheet of the present invention, the plating bath contains Al: 40 to 70% by mass, Si: 1.2 to 4% by mass, Mg: 1 to 6% by mass, and Sr: 0.001 to 0.2% by mass. The balance has a composition consisting of Zn and unavoidable impurities.
Thereby, a molten Al-Zn-Mg-Si-based plated steel sheet having a desired composition can be obtained. The type, content, and action of each element 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 production method of the present invention has almost the same composition as the plating bath as a whole. Therefore, the composition of the main layer can be controlled with high accuracy by controlling the composition of the plating bath.

The method for producing a coated steel sheet of the present invention includes a step of immersing a base steel sheet in the plating bath, and the bath temperature of the plating bath satisfies the following formula (1).
Plating bath temperature (° C) ≤ 600-4.5M _Mg -5.5M _Si ... (1)

As described above, in order to improve the pressure mark resistance of the coated steel sheet and the surface appearance of the plating layer, it is important to suppress the unevenness of the plating layer formed under the coating film. Therefore, in the production method of the present invention, wrinkles in the plating layer can be eliminated by optimizing the composition of the plating bath used for hot-dip galvanizing and satisfying the above formula (1) with respect to the bath temperature. Since the time required for solidification can be shortened and the variation in plating thickness can be reduced, it is possible to suppress unevenness on the surface of the plating layer. As a result, even when a thin coating film is formed on the plating layer, the generation of pressure marks can be effectively suppressed. Further, since the plating bath contains Mg, it is possible to improve the corrosion resistance.

Further, in the production method of the present invention, after hot-dip galvanizing the steel sheet, the temperature obtained by subtracting 50 ° C from the bath temperature of the plating bath at an average cooling rate of 10 to 50 ° C / sec (plating bath temperature −50 ° C. ), It is preferable to cool the steel sheet. It is known that Mg ₂ Si formed in the above-mentioned plating layer is likely to be generated 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). By increasing the cooling rate at 10 ° C / sec or more on average, it is possible to reduce the variation in plating thickness by dispersing the produced Mg ₂ Si and making the plating structure finer. From the same viewpoint, the cooling of the steel sheet after the hot dip galvanizing is more preferably performed at an average cooling rate of 10 ° C./sec or more, and further preferably performed at an average cooling rate of 20 ° C./sec or more.
In the manufacturing method of the present invention, before the hot-dip galvanized steel sheet is wound around a coil by a continuous hot-dip galvanizing facility, temper rolling is usually performed for the purpose of adjusting the steel sheet material and smoothing the surface. Will be done.

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 galvanized 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, a forming method such as roll coater coating, curtain flow coating, or spray coating can be mentioned. After applying a paint containing an organic resin, it is possible to form a coating film by heating and drying by means such as hot air drying, infrared heating, and induction heating.

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

(Samples 1-41)
Using a cold-rolled steel sheet having a thickness of 0.5 mm produced by a conventional method as a base steel sheet, samples 1 to 41 of hot-dip galvanized steel sheets plated with a thickness of 20 to 23 μm were produced in a continuous hot-dip galvanizing facility. The composition of the plating bath used for production is almost the same as the composition of the plating layer of each sample shown in Table 1, the measured bath temperature of the plating bath, and the right side (600-600-) of the calculated formula (1). Table 1 shows the value of 4.5M _Mg −5.5M _Si ) and the standard deviation of the film thickness of the obtained plating layer.
Then, on samples 1-41 of the hot-dip plated steel sheet, chromate-free chemical conversion treatment was applied at 130 ± 50 mg / m ² , an epoxy-containing polyester-based undercoat film was applied at 4 ± 1 μm, and a polyester-based topcoat film was applied at 16 ± 2 μm. Painted steel sheet samples 1-41 having a total coating film thickness of 20 μm were produced.

(Evaluation)
The following evaluations were performed on each of the hot-dip galvanized steel sheets and coating samples 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 sheet was subjected to the Japanese Automotive Standards Organization composite cycle test (JASO-CCT). As shown in FIG. 3, JASO-CCT is a test in which salt spray, drying and wetting are set as one cycle under specific conditions.
The number of cycles until red rust occurred in each sample was measured and evaluated according to the following criteria.
⊚: Number of red rust generation cycles ≥ 600 cycles ○: Number of red rust generation cycles ≥ 400 cycles Δ: 300 cycles ≤ Number of red rust generation cycles <400 cycles ×: Number of red rust generation cycles <300 cycles

(2) Pressure mark resistance The surface of the coating film was visually observed for each sample (length 650 mm × width 914 mm) of the obtained coated steel sheet.
Then, the observation results were evaluated according to the following criteria.
◯: No pressure mark was observed ×: Pressure mark was observed

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

From the results in Table 1, it can be seen that each sample of the example of the present invention is superior to each sample of the comparative example in a well-balanced manner in terms of corrosion resistance, pressure mark resistance, and surface appearance of the plating layer.

According to the present invention, a coated steel sheet having good corrosion resistance and excellent pressure mark resistance and surface appearance of the plating layer, and a coated steel sheet having good corrosion resistance, pressure mark resistance and surface appearance of the plating layer. It is possible to provide an excellent method for manufacturing a coated steel sheet.

Claims (4)

A coated steel sheet in which a coating film is formed directly or via an intermediate layer on the plating layer of a hot-dip galvanized steel sheet.
The plating layer contains Al: 40 to 70% by mass, Si: 1.2 to 4% by mass, Mg: 1 to 6% by mass and Sr: 0.001 to 0.2% by mass, and the balance is composed of Zn and unavoidable impurities. Have,
A coated steel sheet having a thickness of 15 to 30 μm, a standard deviation of the thickness of 5 μm or less, and a coating film having a thickness of 20 μm or less. The plating layer further contains one or more selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb and B in a total of 0.01 to 10% by mass. The coated steel sheet according to claim 1. For plating baths containing Al: 40 to 70% by mass, Si: 1.2 to 4% by mass, Mg: 1 to 6% by mass and Sr: 0.001 to 0.2% by mass, and the balance consisting of Zn and unavoidable impurities. , The process of immersing the base steel plate,
A step of forming a coating film having a thickness of 20 μm or less directly or via an intermediate layer on the obtained hot-dip galvanized steel sheet is provided.
A method for producing a coated steel sheet, wherein the bath temperature of the plating bath satisfies the following formula (1).
Plating bath temperature (° C) ≤ 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 plating bath further contains one or more selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb and B in a total of 0.01 to 10% by mass. The method for producing a coated steel sheet according to claim 3, wherein the coated steel sheet is produced.