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

Abstract

To provide a molten Al-Zn-Mg-Si-based plated steel sheet having an excellent surface appearance, and further excellent in slip resistance and corrosion resistance.SOLUTION: A plating layer has a composition containing Al:45-65 mass%, Si:1.2-4 mass%, Mg:1-6 mass% and Sr:0.01-0.2 mass%, and having a residue comprising Zn and inevitable impurities, and a dynamic friction coefficient on the surface of the plating layer is 0.2 or higher.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hot-dip Al-Zn-Mg-Si plated steel sheet having good surface appearance and excellent slip resistance and corrosion resistance, a method for producing the same, and a coated steel sheet and a method for producing the same. [Background technology]

[0002] Hot-dip Al-Zn-plated steel sheets exhibit high corrosion resistance among hot-dip galvanized steel sheets because they combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al. For example, Patent Document 1 discloses a hot-dip Al-Zn-plated steel sheet containing 25 to 75 mass % Al in the coating layer. Due to 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.

[0003] 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 characteristic coating structure creates a complex corrosion 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.

[0004] Furthermore, a technique is known in which Mg is incorporated into the coating layer of a hot-dip Al-Zn coating to further improve corrosion resistance. As a technique relating to a 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 having a coating layer containing an Al-Zn-Si alloy containing Mg, the Al-Zn-Si alloy being an alloy containing 45 to 60 wt% of elemental aluminum, 37 to 46 wt% of elemental zinc, and 1.2 to 2.3 wt% of elemental silicon, and the Mg concentration is 1 to 5 wt%.

[0005] 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 in which a coating layer is formed containing, by mass%, 1 to 15% Mg, 2 to 15% Si, 11 to 25% Zn, and the remainder being Al and unavoidable impurities, and the size of the MgSi phase present in the plated layer is specified to improve the corrosion resistance of the flat plate and edge surfaces.

[0006] 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.

[0007] However, apart from the above-mentioned improvements in corrosion resistance and surface appearance, hot-dip Al-Zn-plated steel sheets have a problem of being slippery (low slip resistance). If the hot-dip Al-Zn-plated steel sheets have low slip resistance, there is a risk that the hot-dip plated steel sheets (steel strips) may slip when wound up or that the steel sheets may collapse when stacked. Therefore, in addition to the above-mentioned improvements in corrosion resistance by adding Mg and surface appearance by adding Sr, improvements in slip resistance have also been desired. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Special Publication No. 46-7161 [Patent Document 2] Patent No. 5020228 [Patent Document 3] Patent No. 5000039 [Patent Document 4] Japanese Patent Application Laid-Open No. 2002-12959 [Patent Document 5] Patent No. 3983932 Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above circumstances, an object of the present invention is to provide a hot-dip Al-Zn-Mg-Si-plated steel sheet and a coated steel sheet having good surface appearance and excellent slip resistance and corrosion resistance, and a method for manufacturing the hot-dip Al-Zn-Mg-Si-plated steel sheet and the coated steel sheet having good surface appearance and excellent slip resistance and corrosion resistance. [Means for solving the problem]

[0010] The present inventors have conducted studies to solve the above-mentioned problems and have found that the reason for the deterioration of the slip resistance of the plating layer surface is that the unevenness on the plating layer surface is reduced, resulting in a small coefficient of dynamic friction. After further intensive research, they have found that by setting the composition of the plating layer within a specific range, it is possible to improve the surface appearance and corrosion resistance, and by controlling the surface condition of the plating layer to increase the coefficient of dynamic friction on the plating layer surface to 0.2 or more, it is possible to improve the slip resistance.

[0011] 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 plated steel sheet, characterized in that 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 remainder consisting of Zn and unavoidable impurities, and the coefficient of dynamic friction on the surface of the plating layer is 0.2 or more.

[0012] 2. The hot-dip Al-Zn-Mg-Si plated steel sheet according to item 1 above, characterized in that the plating 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.

[0013] 3. The hot-dip Al-Zn-Mg-Si plated steel sheet according to 1 or 2 above, characterized in that, when observed in a cross section in the thickness direction of the plated layer, the difference between the average height of the dendrites exposed at the surface of the plated layer and the average height of the interdendrites exposed at the surface of the plated layer is 2 μm or more.

[0014] 4. A coated steel sheet characterized in that a coating film is formed directly or via an intermediate layer on the coating layer of the hot-dip Al-Zn-Mg-Si coated steel sheet according to any one of items 1 to 3 above.

[0015] 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%)

[0016] 6. The method for producing a hot-dip Al-Zn-Mg-Si plated steel sheet according to item 5, characterized in that the plating 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.

[0017] 7. A method for producing a coated steel sheet, comprising a step of forming a coating film 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. [Effects of the Invention]

[0018] The present invention can provide a hot-dip Al-Zn-Mg-Si-plated steel sheet and a coated steel sheet that have good surface appearance and excellent slip resistance and corrosion resistance, as well as a method for producing a hot-dip Al-Zn-Mg-Si-plated steel sheet and a coated steel sheet that have good surface appearance and excellent slip resistance and corrosion resistance. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1(a) is a photograph of a part of a cross section in the thickness direction of the coating layer of the hot-dip Al-Zn-Mg-Si coated steel sheet of 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 the conventional hot-dip Al-Zn-Mg-Si coated steel sheet, observed by SEM-EDX. [Figure 2] 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. [Figure 3] FIG. 10 is a diagram for explaining a method for measuring the distance between dendrite arms. [Figure 4] FIG. 1 is a diagram for explaining the flow of the Japanese Automobile Standards Combined Cycle Test (JASO-CCT). DETAILED DESCRIPTION OF THE INVENTION

[0020] (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-mentioned composition, thereby ensuring good surface appearance and corrosion resistance.

[0021] The Al content in the coating layer is 45 to 65% by mass, preferably 50 to 60% by mass, to balance corrosion resistance and operability. 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 mainly contains Zn in a supersaturated state, and is composed of portions where Al has solidified dendrites (α-Al phase dendritic portions) and the remaining portions between the dendritic portions (interdendritic portions), achieving a structure with excellent corrosion resistance in which the dendritic portions are stacked in the thickness direction of the coating layer. Furthermore, the more α-Al phase dendritic portions are stacked, 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 has a sacrificial corrosion protection effect 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 is reduced and even if the base steel sheet is easily exposed, the coating layer has a sacrificial corrosion protection effect against Fe, and sufficient corrosion resistance is obtained. Therefore, the Al content in the main coating layer is preferably 60% by mass or less.

[0022] 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 substrate steel sheet, and is therefore inevitably contained in the main layer. In the case of the hot-dip coated steel sheet of the present invention, when hot-dip coating 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 substrate 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 when Si is consumed as Mg2Si, the Si content in the plating layer is preferably set to 2 to 4 mass%, and more preferably 2.3 to 3.5 mass%.

[0023] The plating layer contains 1 to 6 mass % of 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, Mg present in the main layer of the plating layer bonds with the above-mentioned Si to form Mg2Si. When the plated steel sheet corrodes, this Mg2Si 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 enables the generation of MgSi, 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.

[0024] 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 a coated steel sheet using a hot-dip Al-Zn coated steel sheet containing Mg in the coating layer, the Mg2Si phase and Mg-Zn compounds (MgZn2, Mg 32 (Al, Zn) 49The corrosion products containing magnesium are very stable, which inhibits corrosion at an early stage, and this prevents large paint blistering caused by selective corrosion of the Zn-rich phase, which is a problem when using conventional coated steel sheets with Al-Zn coated steel sheets as the base. As a result, hot-dip Al-Zn coated steel sheets containing magnesium in the coating layer exhibit excellent corrosion resistance after painting. The magnesium in the coating layer 3 If the Mg content is less than 6% by mass, the amount of Mg dissolved during corrosion will be small, and there is a risk that the corrosion resistance after painting will not improve. If the Mg content in the plating layer exceeds 6% by mass, not only will the effect saturate, but corrosion of Mg compounds will occur severely, and the solubility of the entire plating layer will increase excessively. As a result, even if the corrosion products are stabilized, their dissolution rate will increase, causing large blister widths and potentially deteriorating the corrosion resistance after painting. Therefore, to consistently achieve excellent corrosion resistance after painting, the Mg content in the plating layer should be 6% by mass or less.

[0025] 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.

[0026] The Sr content in the plating layer must be 0.01% by mass or more. This is to obtain the effect of suppressing the occurrence of streak defects as described above. From the same viewpoint, 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 the occurrence of wrinkle defects will saturate, which will be disadvantageous in terms of cost. From the same viewpoint, the Sr content in the plating layer is preferably 0.15% by mass or less.

[0027] Furthermore, since the plating layer can improve the stability of corrosion products and delay the progression of corrosion in the same way as Mg, it is preferable 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%. The reason why the total content of the above-mentioned elements is set to 0.01 to 10 mass% is that a sufficient corrosion-retarding effect can be obtained and the effect does not saturate.

[0028] The coating layer contains components of the base steel sheet that are incorporated into the coating due to a reaction 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 present in the coating 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 coating, it is preferable that the total amount of unavoidable impurities excluding Fe be 1 mass% or less.

[0029] The interfacial alloy layer in the coating layer is an alloy layer present at the interface with the substrate steel sheet, as described above, and is an Fe-Al and / or Fe-Al-Si compound that is inevitably produced by an alloying reaction between Fe on the steel sheet surface and Al or 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. Therefore, in the present invention, it is preferable to make it as thin as possible.

[0030] 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, more preferably 0.30 or more. Any method may be used to measure the coefficient of dynamic friction. For example, the coefficient of dynamic friction is measured by placing the front and back surfaces of the test piece face to face with a pressure of 0.08 kg / cm. 2 The test can be performed by applying a load of 1000 kJ / s and moving one of the test pieces at a speed of 400 mm / min.

[0031] On the other hand, coated steel sheets using a typical Al-Zn-Mg-Si coating have small surface irregularities (a dynamic friction coefficient of less than 0.2), making it difficult to achieve the desired slip resistance. This can lead to slippage of the steel sheets when wound or to collapse of the stacked steel sheets.

[0032] 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 remainder 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.

[0033] 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, because this more reliably allows the dynamic friction coefficient of the surface of the plating layer to be set to 0.2 or more, thereby achieving better slip resistance.

[0034] Here, Fig. 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 on a cross section in the thickness direction of the plating layer, as shown in Fig. 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 on a cross section in the thickness direction of the plating layer, as shown in Fig. 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 determined as 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 portions of the dendrites and the average height of the exposed portions 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.

[0035] Furthermore, when observing the main layer of the plating layer or the interfacial alloy layer with a scanning electron microscope, it is preferable to polish and / or etch the cross section of the plating layer before observation. There are several types of cross section polishing and etching methods, but they are not particularly limited as long as they are methods commonly used for observing the cross section of a plating layer. Furthermore, conditions for observation and analysis with a scanning electron microscope include, for example, an accelerating voltage of 5 to 20 kV and a magnification of approximately 500 to 5,000 times for secondary electron images or backscattered electron images.

[0036] Furthermore, it is preferable that the main layer of the plating layer has α-Al phase dendritic portions, and 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: plating layer thickness (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.

[0037] 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 FIG. 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 FIG. 3, three arms between A and B are selected), and the distance along the arm alignment direction (distance L in FIG. 3) is measured. The measured distance is then divided by the number of dendrite arms (L / 3 in FIG. 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.

[0038] From the viewpoint of achieving both high levels of workability and corrosion resistance, the thickness of the plating layer is preferably 10 to 30 μm, and more preferably 20 to 25 μm. This is because sufficient corrosion resistance can be ensured when the plating layer is 10 μm or more, and sufficient workability can be ensured when the plating layer is 30 μm or less.

[0039] (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.

[0040] 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 coating material 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.

[0041] 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 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 chromium-free chemical conversion treatment in which a chromate treatment solution or a chromium-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 coating films may be single-layer or multi-layer, and in the case of multi-layers, multiple chemical conversion treatments may be performed sequentially.

[0042] (Method for manufacturing hot-dip Al-Zn-Mg-Si coated steel sheets) 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 having 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; 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.

[0043] 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.

[0044] 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.

[0045] The hot-dip galvanized steel sheet obtained by the production method of the present invention has a composition that is substantially the same as that of the coating bath as a whole, and therefore the composition of the main layer can be controlled with high precision by controlling the composition of the coating bath.

[0046] The method for producing a hot-dip Al-Zn-Mg-Si-plated steel sheet of the present invention includes a step of immersing a base steel sheet in the coating bath (hereinafter, sometimes referred to as the "coating bath immersion step"). The coating bath immersion step uses a coating bath having 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 the obtained coated steel sheet to achieve the desired surface appearance and corrosion resistance.

[0047] 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.

[0048] 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 formulas (1) to (3), M Mg is the Mg content (mass%) in the plating bath, and M Si indicates the Si content (mass%) in the plating bath.

[0049] As described above, in order to improve the surface appearance, corrosion resistance, and slip resistance of the plating layer, it is important to increase the dynamic friction coefficient on the surface of the plating layer to 0.2 or more. 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 more, thereby achieving excellent slip resistance.

[0050] 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 in equation (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.

[0051] 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.

[0052] Furthermore, the cooling rate from the temperature T2 (°C) to the temperature T3 (°C) is set to 10 to 70°C / second because, if the cooling rate is less than 10°C / second, 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 / second, 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.

[0053] (Manufacturing method of coated steel sheets) 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.

[0054] The method for forming the coating film is not particularly limited and can be appropriately selected depending on the required performance. Examples include roll coater coating, curtain flow coating, and spray coating. After applying a coating material containing an organic resin, the coating film can be formed by heating and drying the coating material using means such as hot air drying, infrared heating, and induction heating.

[0055] 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. [Example]

[0056] (Samples 1-74) Using a cold-rolled steel sheet having a thickness of 0.5 mm produced by a conventional method as the 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.

[0057] (evaluation) Each sample of the hot-dip plated steel sheet obtained as described above was evaluated as follows. The evaluation results are shown in Table 1.

[0058] (1) Corrosion resistance evaluation Each sample of the hot-dip galvanized steel sheet obtained was subjected to the Japan Automotive Standards Combined Cyclic Test (JASO-CCT). As shown in Figure 4, the JASO-CCT test involves a cycle of salt spray, dry, and wet conditions 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 for red rust occurrence ≥ 600 cycles ○: Number of cycles in which red rust occurs ≥ 400 cycles ×: Number of cycles for red rust occurrence < 400 cycles

[0059] (2) Slip resistance For each sample of hot-dip galvanized steel sheet obtained, test pieces of 60 x 150 mm (lower side) and 25 x 25 mm (upper side) were cut out, and then the front and back sides of both test pieces were placed face to face and subjected to a pressure of 0.04 kg / cm 2 The slip resistance was evaluated by applying a load of 1000 kJ / cm, tilting the test piece, and measuring the tilt angle (slip angle) at which the test piece placed on top began to slip. The sliding angle was then evaluated according to the following criteria. ○: Sliding angle 20° or more ×: Sliding angle less than 20° 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 it can appropriately evaluate slip resistance.

[0060] (3) Surface appearance For each sample (length 650 mm×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 defects were observed on either the front or back surface. ×: Wrinkle defects were observed on at least one of the front and back surfaces.

[0061] [Table 1]

[0062] The results in Table 1 show that the samples of the invention are well-balanced and superior in all respects of corrosion resistance, slip resistance, and surface appearance of the plating layer compared to the samples of the comparative examples. [Industrial Applicability]

[0063] The present invention can provide a hot-dip Al-Zn-Mg-Si-plated steel sheet and a coated steel sheet that have good surface appearance and excellent slip resistance and corrosion resistance, as well as a method for producing a hot-dip Al-Zn-Mg-Si-plated steel sheet and a coated steel sheet that have good surface appearance and excellent slip resistance and corrosion resistance.

Claims

1. the plating layer has a composition containing 45 to 65 mass% Al, 1.2 to 4 mass% Si, 1 to 6 mass% Mg, and 0.05 to 0.2 mass% Sr, with the balance being 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; The front and back of the test piece are placed face to face with a load of 0.08 kg / cm 2 a coefficient of dynamic friction on the surface of the coating layer measured by a method in which a load of 0.2 or more is applied to a test piece and one of the test pieces is moved at a speed of 400 mm / min.

2. 2. The hot-dip Al-Zn-Mg-Si coated steel sheet according to claim 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. 3. A coated steel sheet comprising the hot-dip Al-Zn-Mg-Si coated steel sheet according to claim 1, wherein a coating film is formed on the coating layer of the hot-dip Al-Zn-Mg-Si coated steel sheet directly or via an intermediate layer.

4. 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: T11(℃)=580-4.5M Mg -5.5M Si ・・・(1) <h2 style=";text-align:left;direction:ltr">42 (℃) = 520 = 4.5M<h2 style=";text-align:left;direction:ltr"> Mg <h2 style=";text-align:left;direction:ltr"> -5.5M<h2 style=";text-align:left;direction:ltr"> Si <h2 style=";text-align:left;direction:ltr">・・・(22) <h2 style=";text-align:left;direction:ltr">43(℃)=375-4.5M<h2 style=";text-align:left;direction:ltr"> Mg <h2 style=";text-align:left;direction:ltr"> -5.5M<h2 style=";text-align:left;direction:ltr"> Si <h2 style=";text-align:left;direction:ltr">・・・(3) M Mg : Mg content in the plating bath (mass%), M Si : Si content in the plating bath (mass%)

5. 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.

6. 6. A method for producing a coated steel sheet, 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 production method according to claim 4 or 5.

Images