JP7724051B2 - Hot-dip Al-Zn-Mg-Si plated steel sheet and its manufacturing method, and coated steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a hot-dip Al-Zn-Mg-Si plated steel sheet and its manufacturing method, as well as a coated steel sheet and its manufacturing method, which have good surface appearance and excellent slip resistance and corrosion resistance.

Hot-dip Al-Zn coated steel sheets combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al, making them highly corrosion-resistant even among hot-dip galvanized steel sheets. For example, Patent Document 1 discloses hot-dip Al-Zn coated steel sheets containing 25 to 75 mass% Al in the coating layer. Due to their excellent corrosion resistance, demand for hot-dip Al-Zn coated steel sheets has been growing in recent years, particularly 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, including guardrails, wiring and piping, and soundproof walls.

The coating layer of hot-dip Al-Zn coated steel sheet consists of a main layer and an alloy layer present at the interface between the base steel sheet and the main layer. The main layer is primarily composed of areas where Al is supersaturated with Zn and solidified as dendrites (the α-Al phase dendritic areas) and the remaining areas between the dendrites (interdendrites), with the α-Al phase structure consisting of multiple layers stacked in the thickness direction of the coating layer. This unique film structure creates a complex corrosion progression path from the surface, making it difficult for corrosion to reach the base steel sheet. This allows hot-dip Al-Zn coated steel sheet to achieve superior corrosion resistance compared to hot-dip galvanized steel sheet with the same coating layer thickness.

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

Furthermore, as a technology for incorporating Mg into a coating layer, Patent Document 3 discloses a hot-dip Al-Zn coated steel sheet in which certain amounts of Mg and Ca are incorporated into the coating layer, thereby improving corrosion resistance and protecting the substrate steel sheet after it is exposed.
Furthermore, Patent Document 4 discloses an Al-based plated steel sheet that contains, by mass%, 1 to 15% Mg, 2 to 15% Si, 11 to 25% Zn, with the remainder being Al and unavoidable impurities, and that forms a coating layer comprising the Mg ₂ Si phase present in the plated layer by specifying the size of the Mg 2 Si phase present in the plated layer, thereby improving the corrosion resistance of the flat plate and end faces.

However, although the hot-dip Al-Zn-plated steel sheets disclosed in Patent Documents 1 and 2 have excellent corrosion resistance, they have a problem in that wrinkle-like defects (hereinafter referred to as "wrinkle-like defects") caused by an oxide layer formed on the surface of the plating layer are likely to occur, which impairs the appearance of the plating 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.

However, apart from the improvements in corrosion resistance and surface appearance mentioned above, hot-dip Al-Zn coated steel sheets have the problem of being slippery (low slip resistance). If the slip resistance of hot-dip Al-Zn coated steel sheets is low, there is a risk that the steel sheets (steel strips) will slip when wound after hot-dip coating, or that the steel sheets will collapse when stacked. Therefore, in addition to the improvement in corrosion resistance achieved by adding Mg and the improvement in surface appearance achieved by adding Sr, there was a need to improve slip resistance as well.

Special Publication No. 46-7161 Patent No. 5020228 Patent No. 5000039 Japanese Patent Application Laid-Open No. 2002-12959 Patent No. 3983932

In light of these circumstances, the present invention aims to provide hot-dip Al-Zn-Mg-Si plated steel sheets and painted steel sheets that have good surface appearance and excellent slip resistance and corrosion resistance, as well as methods for manufacturing hot-dip Al-Zn-Mg-Si plated steel sheets and painted steel sheets that have good surface appearance and excellent slip resistance and corrosion resistance.

The inventors conducted research to solve the above-mentioned problems and discovered that the reason for the reduced slip resistance of the plating layer surface was that the surface irregularities of the plating layer were reduced, resulting in a lower coefficient of dynamic friction. After further intensive research, they discovered that by setting the composition of the plating layer within a specific range, it is possible to improve surface appearance and corrosion resistance, and by controlling the surface condition of the plating layer and increasing the coefficient of dynamic friction of the plating layer surface to 0.2 or more, it is possible to improve slip resistance.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A hot-dip Al-Zn-Mg-Si coated steel sheet, characterized in that the coating layer has a composition containing 45 to 65 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.01 to 0.2 mass% Sr, with the balance consisting of Zn and unavoidable impurities, and the coefficient of dynamic friction on the surface of the coating layer is 0.2 or more.

2. The hot-dip Al-Zn-Mg-Si coated steel sheet described in paragraph 1, wherein the coating layer further contains 0.01 to 10 mass% in total of one or more elements selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb, and B.

3. The hot-dip Al-Zn-Mg-Si coated steel sheet according to paragraphs 1 or 2, characterized in that, when observed in a cross section in the thickness direction of the coating layer, the difference between the average height of the dendrites exposed on the surface of the coating layer and the average height of the interdendrites exposed on the surface of the coating layer is 2 μm or more.

4. A coated steel sheet characterized by having a coating film formed directly or via an intermediate layer on the coating layer of the hot-dip Al-Zn-Mg-Si coated steel sheet described in any one of items 1 to 3.

5. Immersing a substrate steel sheet in a coating bath containing 45 to 65 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.01 to 0.2 mass% Sr, with the balance being Zn and unavoidable impurities;
cooling the steel sheet after immersion in the coating bath from temperature T1 shown in formula (1) to temperature T2 shown in formula (2) at a cooling rate of 10 to 50°C/sec, and then cooling from temperature T2 to temperature T3 shown in formula (3) at a cooling rate of 10 to 70°C/sec;
A method for producing a hot-dip Al-Zn-Mg-Si plated steel sheet, comprising:
T1 (℃) = 580-4.5M _Mg -5.5M _Si ... (1)
T2 (℃) = 520-4.5M _Mg -5.5M _Si ... (2)
T3 (℃) = 375-4.5M _Mg -5.5M _Si ... (3)
M _Mg : Mg content in the plating bath (mass%), M _Si : Si content in the plating bath (mass%)

6. The method for producing hot-dip Al-Zn-Mg-Si coated steel sheet described in 5 above, wherein the coating bath further contains 0.01 to 10 mass% in total of one or more elements selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb, and B.

7. A method for producing a coated steel sheet, comprising a step of forming a coating film, either directly or via an intermediate layer, on a hot-dip Al-Zn-Mg-Si-plated steel sheet obtained by the production method described in 5 or 6 above.

The present invention provides hot-dip Al-Zn-Mg-Si plated steel sheets and painted steel sheets that have good surface appearance and excellent slip resistance and corrosion resistance, as well as methods for manufacturing hot-dip Al-Zn-Mg-Si plated steel sheets and painted steel sheets that have good surface appearance and excellent slip resistance and corrosion resistance.

FIG. 1(a) is a photograph of a part of a cross section in the thickness direction of the coating layer of a hot-dip Al-Zn-Mg-Si coated steel sheet according to the present invention, observed by SEM-EDX; and FIG. 1(b) is a photograph of a part of a cross section in the thickness direction of the coating layer of a conventional hot-dip Al-Zn-Mg-Si coated steel sheet, observed by SEM-EDX. 1 is a diagram schematically illustrating the cross-sectional state of a coating layer in the thickness direction of a hot-dip Al-Zn-Mg-Si coated steel sheet according to the present invention. FIG. 10 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 Automobile Standards Combined Cycle Test (JASO-CCT).

(Hot-dip Al-Zn-Mg-Si coated steel sheet)
The hot-dip Al-Zn-Mg-Si-plated steel sheet of the present invention has a plating layer on the steel sheet surface. The plating layer comprises an interfacial alloy layer present at the interface with the base steel sheet and a main layer (hereinafter, also referred to as the "plating main layer" or "main layer") present on the alloy layer. The plating layer has a composition containing 45 to 65 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.01 to 0.2 mass% Sr, with the balance consisting of Zn and unavoidable impurities. The plating layer of the hot-dip plated steel sheet has the above-described composition, ensuring good surface appearance and corrosion resistance.

The Al content in the coating layer is 45 to 65% by mass, preferably 50 to 60% by mass, to balance corrosion resistance and operational efficiency. When the Al content in the main layer of the coating layer is at least 45% by mass, sufficient dendritic solidification of Al occurs. As a result, the main layer contains primarily Zn in a supersaturated state, consisting of dendritic solidification of Al (α-Al phase dendritic portions) and the remaining interdendritic portions (interdendritic portions), resulting in a structure with excellent corrosion resistance in which the dendritic portions are layered in the thickness direction of the coating layer. Furthermore, the more dendritic portions of the α-Al phase are layered, the more complex the corrosion progression path becomes, making it more difficult for corrosion to reach the substrate steel sheet, thereby improving corrosion resistance. On the other hand, if the Al content in the coating layer exceeds 65% by mass, the content of Zn, which acts as a sacrificial protector against Fe, decreases, resulting in a deterioration of corrosion resistance. For this reason, the Al content in the coating layer is set to 65% by mass or less. Furthermore, if the Al content in the coating layer is 60% by mass or less, the coating weight will be reduced and even if the base steel sheet becomes more likely to be exposed, the coating layer will still have a sacrificial corrosion protection effect against Fe, providing sufficient corrosion resistance. Therefore, it is preferable that the Al content in the main coating layer be 60% by mass or less.

The Si in the coating layer is added to the coating bath to improve corrosion resistance and workability and to suppress the growth of an interfacial alloy layer formed at the interface with the base steel sheet, and is therefore inevitably contained in the main layer. In the case of the hot-dip galvanized steel sheet of the present invention, when hot-dip galvanizing is performed with Si added to 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 formation of this Fe-Al-Si interfacial alloy layer can suppress the growth of the interfacial alloy layer. Furthermore, when the Si content in the coating 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 in the coating layer exceeds 4% by mass, the coating layer is prone to precipitation of Si phases that reduce workability and serve as cathode sites. The precipitation of this Si phase can be suppressed by increasing the Mg content, but this increases production costs, reduces workability due to the increased amount of _Mg2Si , and makes it more difficult to control the composition of the plating bath. For this reason, the Si content in the plating layer is set to 4 mass% or less. Furthermore, considering that the growth of the interfacial alloy layer and the precipitation of the Si phase can be more reliably suppressed and that it can be handled in the event that Si is consumed as _Mg2Si , the Si content in the plating layer is preferably 2 to 4 mass%, and more preferably 2.3 to 3.5 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, improving the stability of the corrosion product and slowing the progression of corrosion, resulting in improved corrosion resistance. More specifically, the Mg present in the main layer of the plating layer bonds with the above-mentioned Si to form Mg ₂ Si. When the plated steel sheet corrodes, this Mg ₂ Si dissolves initially, so Mg is contained in the corrosion product. The Mg contained in the corrosion product has the effect of densifying the corrosion product, improving the stability of the corrosion product and its barrier properties against external corrosion factors.
The Mg content of the plating layer is set to 1% by mass or more because, when the plating layer contains Si within the above-mentioned concentration range, a Mg concentration of 1% by mass or more allows the formation of _Mg2Si , thereby achieving a corrosion retardation effect. From the same perspective, 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 Mg content of the plating layer is set to 6% by mass or less because, if the Mg content of the plating layer exceeds 6%, the corrosion resistance improvement effect saturates, production costs increase, and control of the plating bath composition becomes difficult. From the same perspective, the Mg content of the plating layer is preferably 5% by mass or less.

Furthermore, by increasing the Mg content in the coating layer to 3% by mass or more, it is possible to _improve corrosion resistance after painting. When the coating layer of a conventional hot-dip Al-Zn-plated steel sheet, which does not contain Mg, is exposed to the atmosphere, a dense and stable _Al2O3 oxide film quickly forms around the α-Al phase. The protective effect of this oxide film makes the α-Al phase much less soluble than the Zn-rich phase in the interdendrites. As a result, when a coated steel sheet using a conventional Al-Zn-plated steel sheet as a base sheet is damaged, selective corrosion of the Zn-rich phase begins at the coating/coating interface, progressing deep into the intact coating and causing significant blistering, resulting in poor corrosion resistance after painting. Therefore, from the perspective of achieving excellent corrosion resistance after painting, it is preferable that the Mg content in the coating layer be 3% by mass or more.
On the other hand, in the case of painted steel sheets using hot-dip Al-Zn-plated steel sheets containing Mg in the coating layer, the Mg ₂ Si phase and Mg-Zn compounds (e.g., MgZn ₂ , Mg ₃₂ (Al,Zn) ₄₉ ) precipitated in interdendrites dissolve in the early stages of corrosion, resulting in the incorporation of Mg into the corrosion products. The corrosion products containing Mg are highly stable, thereby suppressing corrosion in the early stages. This prevents significant paint blistering due to selective corrosion of the Zn-rich phase, a problem that occurs with painted steel sheets using conventional Al-Zn-plated steel sheets as a base. As a result, hot-dip Al-Zn-plated steel sheets containing Mg in the coating layer exhibit excellent corrosion resistance after painting. If the Mg content in the coating layer is less than 3 % by mass, the amount of Mg dissolved during corrosion is small, potentially resulting in poor corrosion resistance after painting. 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 violently, causing the solubility of the entire plating layer to increase excessively. As a result, even if the corrosion products are stabilized, their dissolution rate will increase, causing large blister widths and the risk of deteriorating 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 0.01 to 0.2 mass % of Sr. The inclusion of Sr in the plating layer can suppress the occurrence of wrinkle defects and improve the surface appearance of the hot-dip plated steel sheet.
The wrinkle defects are wrinkle-like irregularities formed on the surface of the coating layer, and are observed as whitish streaks on the surface of the coating layer. Such wrinkle defects are more likely to occur when a large amount of Mg is added to the coating layer. Therefore, in the hot-dip galvanized steel sheet, by adding Sr to 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 wrinkle defects.

The Sr content in the plating layer must be 0.01% by mass or more. This is to achieve the effect of suppressing the occurrence of streak defects mentioned above. From the same perspective, the Sr content in the plating layer is 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 wrinkle defects will saturate, which will be disadvantageous in terms of cost. From the same perspective, the Sr content in the plating layer is preferably 0.15% by mass or less.

Furthermore, the plating layer preferably further contains a total of 0.01 to 10 mass% of one or more elements selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb, and B, which, like the above-mentioned Mg, improve the stability of corrosion products and delay the progression of corrosion. The reason for specifying a total content of the above elements as 0.01 to 10 mass% is that a sufficient corrosion-retarding effect can be obtained and the effect does not saturate.

The coating layer contains components of the base steel sheet that are incorporated into the coating through reactions between the coating bath and the base steel sheet during the coating process, as well as unavoidable impurities in the coating bath. The base steel sheet components that are incorporated into the coating may contain several percent of Fe. Examples of unavoidable impurities in the coating bath include Fe, Cu, and Zr. It is not possible to quantify the Fe in the coating layer separately, distinguishing between that incorporated from the base steel sheet and that in the coating bath. There are no particular restrictions on the total content of unavoidable impurities, but from the perspective of maintaining the corrosion resistance and uniform solubility of the coating, it is preferable that the total amount of unavoidable impurities excluding Fe be 1% by mass or less.

The interfacial alloy layer in the coating layer is an alloy layer that exists at the interface with the base steel sheet, as described above, and is an Fe-Al and/or Fe-Al-Si compound that is inevitably produced when the Fe on the steel sheet surface reacts with the Al and Si in the bath to form an alloy. Because this interfacial alloy layer is hard and brittle, if it grows too thick it can become the starting point for cracks during processing. Therefore, in the present invention, it is preferable to make it as thin as possible.

The hot-dip Al-Zn-Mg-Si plated steel sheet of the present invention is characterized in that the coefficient of dynamic friction on the surface of the plated layer is 0.2 or more.
By increasing the number of dendrites protruding from the surface of the plating layer, unevenness is formed on the surface of the plating layer, which increases the dynamic friction coefficient on the surface of the plating layer to 0.2 or more, thereby improving the slip resistance of the surface of the plating layer.
From the same viewpoint, the coefficient of dynamic friction on the surface of the plating layer is preferably 0.25 or more, and more preferably 0.30 or more. Note that any method may be used to measure the coefficient of dynamic friction, but for example, it can be measured by a method in which the front and back surfaces of a test piece are faced to each other, a load of 0.08 kg/ ^cm2 is applied, and one of the test pieces is moved at a speed of 400 mm/min.

On the other hand, with coated steel sheets using a typical Al-Zn-Mg-Si coating, the surface irregularities of the coating are small (the dynamic friction coefficient is less than 0.2), making it impossible to achieve the desired slip resistance, which can lead to slippage of the steel sheets when wound or to collapse of the stacked steel sheets.

1(a) and 1(b) are photographs of the cross-sections of the coating layers of an example of a hot-dip Al-Zn-Mg-Si coated steel sheet according to the present invention and an example of a hot-dip Al-Zn-Mg-Si coated steel sheet according to the prior art (Al: 55 mass%, Si: 2.5 mass%, Mg: 4.5 mass%, Sr: 0.10 mass%, the balance being Zn and unavoidable impurities), observed by energy dispersive X-ray spectroscopy using a scanning electron microscope (SEM-EDX).
As can be seen from Figure 1(a), in the hot-dip Al-Zn-Mg-Si coated steel sheet within the scope of the present invention, the dendrites and interdendrites are exposed at different surface positions, resulting in the formation of irregularities on the surface of the coating layer. This allows a dynamic friction coefficient of 0.2 or more to be obtained, realizing excellent slip resistance. On the other hand, in the hot-dip Al-Zn-Mg-Si coated steel sheet of the prior art shown in Figure 1(b), the difference in surface positions between the dendrites and interdendrites is small, and almost no irregularities are formed, meaning that the desired slip resistance cannot be achieved.

Furthermore, when observing a cross section of the plating layer in the thickness direction, the difference between the average height of the dendrites exposed on the surface of the plating layer and the average height of the interdendrites exposed on the surface of the plating layer is preferably 2 μm or more, and more preferably 3 μm or more. This is because it is possible to more reliably set the dynamic friction coefficient of the surface of the plating layer to 0.2 or more, thereby achieving better slip resistance.

Here, Figure 2 is a schematic diagram showing a portion of a cross section of a hot-dip Al-Zn-Mg-Si-plated steel sheet according to the present invention. The average height of the dendrites exposed on the surface of the plating layer is the average height H1 of the dendrites exposed on the surface of the plating layer when observed in a cross section in the thickness direction of the plating layer, as shown in Figure 2. The average height of the interdendrites exposed on the surface of the plating layer is the average height H2 of the interdendrites exposed on the surface of the plating layer when observed in a cross section in the thickness direction of the plating layer, as shown in Figure 2. The difference between the average height of the dendrites and the average height of the interdendrites is the difference between H1 and H2.
The average height H1 of the dendrites exposed on the surface of the plating layer and the average height H2 of the interdendrites exposed on the surface of the plating layer only need to be the average for the entire plating layer. For example, the cross section of the plating layer in the thickness direction can be observed at five randomly selected locations on the plating layer, and the average height of the exposed parts of the dendrites and the average height of the exposed parts of the interdendrites in the plating layer within a length range of 5 mm in the observation field can be determined as the average height H1 of the dendrites and the average height H2 of the interdendrites, respectively.

When observing the main layer of the plating layer or the interfacial alloy layer using a scanning electron microscope, it is preferable to polish and/or etch the cross section of the plating layer before observation. There are several methods for polishing and etching the cross section, but they are not particularly limited as long as they are methods commonly used for observing the cross section of a plating layer. Observation and analysis using a scanning electron microscope can be performed, for example, at an acceleration voltage of 5 to 20 kV and a magnification of approximately 500 to 5,000 times for secondary electron images or backscattered electron images.

It is also preferable that the main layer of the plating layer has α-Al phase dendritic portions, and that the average dendrite arm distance of the dendritic portions 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 consisting of the above-mentioned α-Al phase can be made relatively small, and the path of the interdendrites that corrode preferentially can be secured to be long, thereby further improving corrosion resistance.

The inter-dendrite arm distance in the dendrite portion refers to the center-to-center distance between adjacent dendrite arms (dendrite arm spacing). In the present invention, for example, as shown in Figure 3, the polished and/or etched surface of the main plating layer is observed under magnification (e.g., 200x magnification) using a scanning electron microscope (SEM) or the like, and the spacing of 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 Figure 3, three arms between A and B are selected), and the distance along the arm alignment (distance L in Figure 3) is measured. The measured distance is then divided by the number of dendrite arms (L/3 in Figure 3) to calculate the inter-dendrite arm distance. The inter-dendrite arm distance is measured at three or more locations in one field of view, and the average of the inter-dendrite arm distances obtained is calculated to obtain the average inter-dendrite arm distance.

In order to achieve a high level of both workability and corrosion resistance, the thickness of the plating layer is preferably 10 to 30 μm, and more preferably 20 to 25 μm. A plating layer thickness of 10 μm or more ensures sufficient corrosion resistance, while a plating layer thickness of 30 μm or less ensures sufficient workability.

(painted steel plate)
The coated steel sheet of the present invention is characterized in that a coating film is formed directly or via an intermediate layer on the plating layer of the above-mentioned hot-dip Al-Zn-Mg-Si plated steel sheet of the present invention.
By using the hot dip Al-Zn-Mg-Si plated steel sheet of the present invention, it is possible to obtain a good surface appearance of the plated layer, and also to improve slip resistance and corrosion resistance.

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

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. Examples include a chemical conversion coating and a primer such as an adhesive layer. The chemical conversion coating can be formed, for example, by a chromate treatment or chrome-free chemical conversion treatment in which a chromate treatment solution or chrome-free chemical conversion coating solution is applied and then dried at a steel sheet temperature of 80 to 300°C without rinsing with water. These chemical conversion coatings may be single-layer or multi-layer; in the case of multi-layers, multiple chemical conversion treatments can be performed sequentially.

(Method for manufacturing hot-dip Al-Zn-Mg-Si coated steel sheet)
Next, the method for producing the hot-dip Al-Zn-Mg-Si plated steel sheet of the present invention will be described.
The method for producing a hot-dip Al-Zn-Mg-Si coated steel sheet of the present invention includes the steps of: immersing a substrate steel sheet in a coating bath containing 45 to 65 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.01 to 0.2 mass% Sr, with the balance being Zn and unavoidable impurities;
cooling the steel sheet after immersion in the coating bath from temperature T1 shown in formula (1) to temperature T2 shown in formula (2) at a cooling rate of 10 to 50°C/sec, and then cooling from temperature T2 to temperature T3 shown in formula (3) at a cooling rate of 10 to 70°C/sec;
The present invention is characterized by comprising:
T1 (℃) = 580-4.5M _Mg -5.5M _Si ... (1)
T2 (℃) = 520-4.5M _Mg -5.5M _Si ... (2)
T3 (℃) = 375-4.5M _Mg -5.5M _Si ... (3)
M _Mg : Mg content in the plating bath (mass%), M _Si : Si content in the plating bath (mass%)
The hot-dip Al-Zn-Mg-Si plated steel sheet obtained by the above-described production method has good corrosion resistance, and is excellent in slip resistance and corrosion resistance.

In the method for producing a hot-dip Al-Zn-Mg-Si-plated steel sheet of the present invention, although not particularly limited, a continuous hot-dip plating facility is usually adopted from the viewpoint of production efficiency and quality stability.
The type of substrate steel sheet used in 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 such a hot-rolled steel sheet or strip, can be used. The conditions for the pretreatment step and the annealing step are also not particularly limited, and any method can be used.

In the method for producing a hot-dip Al-Zn-Mg-Si coated steel sheet of the present invention, the coating bath has a composition containing 45 to 65 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.01 to 0.2 mass% Sr, with the balance being Zn and unavoidable impurities.
This allows a hot-dip Al-Zn-Mg-Si-plated steel sheet of the desired composition to be obtained. The types, contents, and functions of the elements contained in the coating bath are explained in the description of the hot-dip Al-Zn-Mg-Si-plated steel sheet of the present invention described above.

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

The method for producing hot-dip Al-Zn-Mg-Si-plated steel sheet of the present invention includes a step of immersing a base steel sheet in the plating bath (hereinafter, sometimes referred to as the "plating bath immersion step"). The plating bath immersion step uses a plating bath containing 45-65% by mass of Al, 1.2-4% by mass of Si, 1-6% by mass of Mg, and 0.01-0.2% by mass of Sr, with the balance consisting of Zn and unavoidable impurities. This allows the resulting plated steel sheet to achieve the desired surface appearance and corrosion resistance.

In the plating bath immersion step, conditions other than the above-mentioned plating bath composition are not particularly limited, and ordinary hot-dip plating conditions in continuous hot-dip plating equipment can be changed as appropriate.
For example, the temperature of the coating bath may be set to 560 to 610° C. The time for immersing the base steel sheet in the coating bath may be set to 1 to 5 seconds.

The method for producing a hot-dip Al-Zn-Mg-Si-plated steel sheet of the present invention includes, after the coating bath immersion step, a step of cooling the steel sheet after immersion in the coating bath (immediately after coming out of the coating bath) from temperature T1 to temperature T2 at a cooling rate of 10 to 50°C/sec, and then cooling from temperature T2 to temperature T3 at a cooling rate of 10 to 70°C/sec, where temperatures T1, T2, and T3 are expressed by the following equations (1), (2), and (3).
T1 (℃) = 580-4.5M _Mg -5.5M _Si ... (1)
T2 (℃) = 520-4.5M _Mg -5.5M _Si ... (2)
T3 (℃) = 375-4.5M _Mg -5.5M _Si ... (3)
In the formulas (1) to (3), M _Mg represents the Mg content (mass%) in the plating bath, and M _Si represents the Si content (mass%) in the plating bath.

As mentioned above, in order to improve the surface appearance and corrosion resistance of the plating layer as well as its slip resistance, it is important to increase the dynamic friction coefficient on the surface of the plating layer to 0.2 or greater. More specifically, it is effective to increase the difference in average height between the dendrites exposed at the surface of the plating layer and the interdendrites exposed at the surface of the plating layer. Therefore, in the manufacturing method of the present invention, by optimizing the cooling start and end temperatures for solidifying the plating layer after the plating bath immersion step, as well as the cooling rate within that temperature range, it is possible to increase the dynamic friction coefficient on the surface of the plating layer to 0.2 or greater, thereby achieving excellent slip resistance.

Here, the cooling start temperature T1 (°C) of the steel sheet is set to the condition shown in formula (1) because it is a temperature at which the coating bath has not yet solidified.
Furthermore, the temperature T2 (°C) at which the cooling from the temperature T1 (°C) ends and the next cooling starts is set as a condition by the formula (2) in order to set the temperature at which only the dendritic portion of the plating bath solidifies and the interdendritic portion has not yet solidified.
Furthermore, the temperature T3 at which cooling from the temperature T2 (°C) is completed is set to the condition shown in formula (3) in order to set the temperature at which solidification of the plating bath is completed.

The cooling rate from temperature T1 (°C) to temperature T2 (°C) is set to 10 to 50°C/second because, if the cooling rate is less than 10°C/second, the growth rate of the dendrites exposed from the surface of the plating layer is slow, resulting in a smooth surface of the plating layer; on the other hand, if the cooling rate exceeds 50°C/second, the dendrites exposed from the surface of the plating layer become too small, reducing the unevenness of the plating layer surface and making it impossible to obtain a dynamic friction coefficient of 0.2 or more.
From the same viewpoint, the cooling rate from the temperature T1 (° C.) to the temperature T2 (° C.) is preferably 20 to 40° C./second.

Furthermore, the cooling rate from temperature T2 (°C) to temperature T3 (°C) is set to 10 to 70°C/sec because, if the cooling rate is less than 10°C/sec, the interdendrites exposed from the surface of the plating layer grow slowly, resulting in a smooth surface of the plating layer; on the other hand, if the cooling rate exceeds 70°C/sec, the interdendrites exposed from the surface of the plating layer solidify quickly, reducing the unevenness of the plating layer surface and making it impossible to obtain a dynamic friction coefficient of 0.2 or more.
From the same viewpoint, the cooling rate from the temperature T2 (° C.) to the temperature T3 (° C.) is preferably 20 to 50° C./second.
In the production method of the present invention, when a coated steel sheet is produced in which a coating film is formed on the coating layer of a hot-dip Al-Zn-Mg-Si based hot-dip coated steel sheet, temper rolling is usually carried out for the purposes of adjusting the steel sheet properties and smoothing the surface before the hot-dip coated steel sheet is wound into a coil in a continuous hot-dip coating facility.

(Manufacturing method of coated steel sheet)
The method for producing a coated steel sheet of the present invention is characterized by comprising a step of forming a coating film directly or via an intermediate layer on the hot-dip Al-Zn-Mg-Si-plated steel sheet obtained by the above-mentioned method for producing a hot-dip Al-Zn-Mg-Si-plated steel sheet of the present invention.
By using the hot dip Al-Zn-Mg-Si plated steel sheet of the present invention, it is possible to obtain a good surface appearance of the plated layer, and also to improve slip resistance and corrosion resistance.

The method for forming the coating film is not particularly limited and can be selected appropriately depending on the required performance. Examples include roll coater coating, curtain flow coating, and spray coating. After applying a paint containing an organic resin, the coating film can be formed by heating and drying using means such as hot air drying, infrared heating, and induction heating.

Furthermore, there are no particular limitations on the intermediate layer, 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 74)
Using a cold-rolled steel sheet having a thickness of 0.5 mm produced by a conventional method as a base steel sheet, hot-dip coated steel sheet samples 1 to 74 were produced in a continuous hot-dip coating facility. The composition of the coating bath used in the production was approximately the same as the composition of the coating layer of each sample shown in Table 1. Table 1 also shows the temperatures T1, T2, and T3 after immersion in the coating bath, the cooling rate from T1 to T2, the cooling rate from T2 to T3, the dynamic friction coefficient on the surface of the obtained coating layer, and the difference between the average height H1 of the dendrites exposed on the surface of the coating layer and the average height H2 of the interdendrites.

(evaluation)
Each sample of the hot-dip galvanized steel sheet obtained as described above was evaluated as follows. The evaluation results are shown in Table 1.

(1) Evaluation of corrosion resistance Each sample of hot-dip galvanized steel sheet was subjected to the Japan Automotive Standards Combined Cyclic Test (JASO-CCT). As shown in Figure 4, the JASO-CCT is a test in which salt spray, dry and wet conditions 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 in which red rust appears ≧600 cycles ○: Number of cycles in which red rust appears ≧400 cycles ×: Number of cycles in which red rust appears <400 cycles

(2) Slip resistance Test pieces of 60 × 150 mm (lower side) and 25 × 25 mm (upper side) were cut out from each sample of the obtained hot-dip plated steel sheet, and the front and back surfaces of both test pieces were placed face to face, a load of 0.04 kg/ ^cm2 was applied, and the test piece was tilted and the tilt angle (slip angle) at which the test piece placed on top began to slide was measured to evaluate slip resistance.
The sliding angle was then evaluated according to the following criteria.
○: Slip angle of 20° or more ×: Slip angle of less than 20° Note that the above-mentioned method for evaluating slip resistance is merely an example, and it is not necessary to use the same method. Any method may be used as long as slip resistance can be appropriately evaluated.

(3) Surface Appearance For each sample (length 650 mm x width 914 mm) of the obtained hot-dip plated steel sheet, the surface of the plating 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, slip resistance, and surface appearance of the plating layer compared to the samples of the comparative examples.

The present invention provides hot-dip Al-Zn-Mg-Si plated steel sheets and painted steel sheets that have good surface appearance and excellent slip resistance and corrosion resistance, as well as methods for manufacturing hot-dip Al-Zn-Mg-Si plated steel sheets and painted steel sheets that have good surface appearance and excellent slip resistance and corrosion resistance.

Claims (6)

the plating layer has a composition containing Al: 45 to 65 mass%, Si: 1.2 to 4 mass%, Mg: 1 to 6 mass%, Sr: 0.05 to 0.2 mass%, and the balance consisting of Zn and unavoidable impurities;
When observed in a cross section in the thickness direction of the plating layer, the difference between the average height of dendrites exposed on the surface of the plating layer and the average height of interdendrites exposed on the surface of the plating layer is 2 μm or more;
A hot-dip Al-Zn-Mg- ^Si coated steel sheet, characterized in that the coefficient of dynamic friction on the surface of the coating layer is 0.2 or more, as measured by a method in which a test piece is placed with its front and back surfaces facing each other, a load of 0.08 kg/cm2 is applied, and one of the test pieces is moved at a speed of 400 mm/min. The hot-dip Al-Zn-Mg-Si coated steel sheet according to claim 1, characterized in that the coating layer further contains one or more elements selected from the group consisting of Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb, and B in a total amount of 0.01 to 10 mass%. A coated steel sheet characterized by having a coating film formed directly or via an intermediate layer on the coating layer of the hot-dip Al-Zn-Mg-Si coated steel sheet according to claim 1 or 2. immersing a substrate steel sheet in a coating bath containing 45 to 65 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.05 to 0.2 mass% Sr, with the balance being Zn and unavoidable impurities;
cooling the steel sheet after immersion in the coating bath from a temperature T1 shown in formula (1) to a temperature T2 shown in formula (2) at a cooling rate of 20 to 40°C/sec, and then cooling from the temperature T2 to a temperature T3 shown in formula (3) at a cooling rate of 20 to 50°C/sec;
A method for producing a hot-dip Al-Zn-Mg-Si plated steel sheet, comprising:
T1 (℃) = 580-4.5M _Mg -5.5M _Si ... (1)
T2 (℃) = 520-4.5M _Mg -5.5M _Si ... (2)
T3 (℃) = 375-4.5M _Mg -5.5M _Si ... (3)
M _Mg : Mg content in the plating bath (mass%), M _Si : Si content in the plating bath (mass%) 5. The method for producing a hot-dip Al-Zn-Mg-Si-plated steel sheet according to claim 4 , wherein the coating bath further contains 0.01 to 10 mass% in total of one or more elements selected from Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb, and B. A method for producing coated steel sheet, comprising the step of forming a coating film directly or via an intermediate layer on the hot-dip Al-Zn-Mg-Si-plated steel sheet obtained by the production method described in claim 4 or 5.