MOLTEN Al-Zn-BASED PLATED SHEET STEEL, SURFACE-TREATED STEEL PLATE, COATED STEEL PLATE, AND METHOD FOR MANUFACTURING MOLTEN Al-Zn-BASED PLATED SHEET STEEL

Abstract

To provide a molten Al-Zn-based plated sheet steel with excellent plating adhesiveness at a bending-back part even when a plating adhesion amount is large.SOLUTION: The present invention relates to a molten Al-Zn-based plated sheet steel, on which a plating layer having a composition containing 40-70 mass% of Al and 0.5-3.0 mass% of Si, and Zn and inevitable impurities at a remainder, is formed. The plating layer has a dendrite gap mainly containing dendrite made of Al primary crystal and Al-Zn eutectic crystal. The thickness X (μm) of the plating layer and the Vickers hardness HV0.01 of the Al primary crystal satisfy the following expressions (1) and (2). HV0.01≤-4X+205...(1) X≥23...(2)SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a hot-dip Al-Zn alloy coated steel sheet having excellent plating adhesion in the springback portion even when the plating adhesion amount is large, and a method for producing the same.

Background Art

[0002] A hot-dip Al-Zn alloy coated steel sheet containing 20 to 95% by mass of Al in the plating layer, typified by a 55% Al-Zn alloy coated steel sheet, has both the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al, and thus is known to exhibit high corrosion resistance among hot-dip galvanized steel sheets. Therefore, hot-dip Al-Zn alloy coated steel sheets are widely used in the building material field such as roofs and walls that are exposed outdoors for a long time, and in the civil engineering and construction fields such as guardrails, wiring pipes, and soundproof walls. In particular, for applications such as factory buildings, hot-dip Al-Zn alloy coated steel sheets are preferably used as long metal walls and roofing materials in the form of corrugated sheets, angle spans, and sinusoidal waves because of their low cost and excellent workability.

[0003] When using a hot-dip Al-Zn alloy coated steel sheet for a long metal roofing material or the like, it may be subjected to severe processing in order to be applied to rain-end portions such as the edge of a wall and the eaves. Specifically, once the steel sheet is subjected to 180-degree close bending so as to overlap the steel sheets, a springback operation of returning a part thereof to the original smooth state may occur. When such springback processing is performed, there is a problem that the plating layer around the springback portion (the portion returned to the smooth state after 180-degree close bending) of the hot-dip Al-Zn alloy coated steel sheet peels off.

[0004] Here, FIG. 1 shows a cross-section of a 55% Al-Zn alloy coated steel sheet subjected to 180-degree close bending observed with an electron microscope, and FIG. 2 shows a cross-section of a 55% Al-Zn alloy coated steel sheet observed with an electron microscope after returning the 180-degree close bending portion to the original smooth state. As can be seen from FIG. 1, in the case of the outermost part where a close contact bending at 180° is applied, the inner side of the plating layer at the bent portion is subjected to compressive strain and bulges in a wavy manner more towards the inner side. And due to the bulge caused by the compressive stress, the plating layer itself peels off from the Al-Fe alloy layer existing at the interface between the plating layer and the steel plate. Therefore, when the bent portion is returned later, as shown in FIG. 2, the plating layer at the bent-back portion is largely peeled off. In addition, the peeling of the plating layer at the bent-back portion tends to occur remarkably when the plating layer is thickened, and the minimum adhesion amount on both sides with a three-point average is 150 g / m 2 For a general plating adhesion amount of about this level (for example, the plating adhesion amount display symbol AZ150 as defined in JIS G 3322), there are few problems with the peeling of the plating layer at the bent-back portion. However, for a minimum adhesion amount on both sides with a three-point average of 170 g / m 2 or more for the purpose of high durability and high corrosion resistance (plating adhesion amount display symbols AZ170, AZ185, AZ200, etc. as defined in JIS G 3322) in the hot-dip Al-Zn plated steel sheet, the peeling of the plating layer at the bent-back portion has become a major problem.

[0005] For this reason, various attempts have been made to improve the bendability and the corrosion resistance of the bent portion of the hot-dip Al-Zn plated steel sheet (see, for example, Patent Documents 1 to 4).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, Patent Document 1 is a technology related to the bend-back resistance of a 55% Al-Zn plated steel sheet, that is, prevention of cracking due to necking of the base steel during repeated bend-back. Patent Document 2 is a technology of suppressing the plating peeling phenomenon by discontinuously forming an Al-Fe alloy layer that causes peeling by nitriding the surface of the base plate before forming the plating layer. Therefore, neither of them suppressed the peeling at the bend-back part of the plating layer. Also, Patent Documents 3 and 4 are technologies that define the Vickers hardness (HV 0.01 ) to 120 or less for the purpose of improving the elongation workability of plating without compression on the outer side of the bent part surface. However, the influence of the plating layer thickness is not considered, and there is a need for further improvement regarding the plating peeling phenomenon at the bend-back processed part of the thick-coated molten Al-Zn plated steel sheet.

[0008] In view of such circumstances, an object of the present invention is to provide a molten Al-Zn plated steel sheet, a surface-treated steel sheet, and a painted steel sheet, which have excellent plating adhesion at the bend-back part even when the plating adhesion amount is large, and a method for manufacturing a molten Al-Zn plated steel sheet.

Means for Solving the Problems

[0009] As a result of studying a molten Al-Zn plated steel sheet having a plating layer formed with a composition containing Al: 40 to 70% by mass and Si: 0.5 to 3.0% by mass, with the balance being Zn and unavoidable impurities, in order to solve the above problems, regarding the relationship between the thickness of the plating layer and the Vickers hardness of the Al primary crystal constituting the dendrite in the plating layer, it was found that by optimizing, even when the plating adhesion amount is large, the plating adhesion at the bend-back part can be enhanced.

[0010] The present invention has been made based on the above findings, and the gist thereof is as follows. 1. A hot-dip Al-Zn plated steel sheet having a plating layer formed with a composition containing 40 to 70 mass% of Al and 0.5 to 3.0 mass% of Si, with the balance being Zn and inevitable impurities, wherein the plating layer has dendrites mainly composed of primary Al crystals and inter-dendrite gaps containing Al-Zn eutectic, wherein the thickness X (μm) of the plating layer and the Vickers hardness HV of the primary Al crystals 0.01 satisfy the following relationships (1) and (2), characterized in that it is a hot-dip Al-Zn plated steel sheet. HV 0.01 ≦ -4X + 205 ··· (1) X ≧ 23 ··· (2)

[0011] 2. In a bending test conforming to JIS G 3322 (2019), a 180-degree close contact bend is performed with a width of 125 mm without sandwiching the plate at the inner interval of the bend. After returning and smoothing the close contact bent portion, an adhesive tape is attached to the surface of the bent return portion that was on the inner side during the close contact bend, and when peeled in a direction perpendicular to the bend line of the bent return portion, the maximum width in the direction perpendicular to the bend line of the peeled plating layer adhered to the adhesive tape is 1 mm or less, characterized in that it is the hot-dip Al-Zn plated steel sheet according to claim 1.

[0012] 3. A surface-treated steel sheet, characterized in that a chemical conversion coating film is formed on the hot-dip Al-Zn plated steel sheet according to the above 1 or 2.

[0013] 4. A painted steel sheet, characterized in that a coating film is formed on the hot-dip Al-Zn plated steel sheet according to the above 1 or 2, directly or through an intermediate layer.

[0014] 5. A step of forming a plating layer with a plating thickness X (μm) per side on a base steel sheet using a plating bath having a composition containing 40 to 70 mass% of Al and 0.5 to 3.0 mass% of Si, with the balance being Zn and inevitable impurities, and a step of reheating the steel sheet on which the plating layer is formed, A method for manufacturing a hot-dip Al-Zn plated steel sheet, characterized in that when the maximum temperature reached by the steel sheet during the reheating is T °C and the cooling time from T °C to 100 °C is y (hr), the following relationships (2), (3) and (4) are satisfied. X≧23 ···(2) 150≦T≦300 ···(3) 0.5≦y≦24 ···(4)

Advantages of the Invention

[0015] According to the present invention, even when the plating adhesion amount is large, it is possible to provide a hot-dip Al-Zn plated steel sheet, a surface-treated steel sheet, and a painted steel sheet, which are excellent in plating adhesion at the bent-back portion, and a method for manufacturing a hot-dip Al-Zn plated steel sheet.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0017] (Hot-dip Al-Zn alloy coated steel sheet) The hot-dip Al-Zn alloy coated steel sheet of the present invention has a coating layer on the surface of the steel sheet. Here, the coating layer has a composition containing 40 to 70% by mass of Al and 0.5 to 3.0% by mass of Si, with the balance being Zn and unavoidable impurities. By having the above-described composition for the coating layer of the hot-dip coated steel sheet, good corrosion resistance can be achieved. The coating layer is composed of an interfacial alloy layer existing on the interface side with the base steel sheet and a main layer existing on the interfacial alloy layer.

[0018] The Al content in the coating layer is 40 to 70% by mass, preferably 50 to 60% by mass, in view of the balance between corrosion resistance and operability. If the Al content in the coating layer is at least 40% by mass, dendritic solidification of primary Al occurs, and it consists of a portion where Al dendritically solidifies (dendrite phase of α-Al phase) and a portion of the remaining interdendritic gaps (interdendritic phase), and a structure in which the dendrite phase is laminated in the film thickness direction of the coating layer can be obtained. By adopting a structure in which the dendritic solidification structure is laminated in the film thickness direction of the coating layer, the corrosion progress path of the coating layer becomes complicated, and the corrosion resistance can be improved. Also, the more dendrites are laminated, the more complicated the corrosion progress path becomes, and it becomes difficult for corrosion to easily reach the base steel sheet, improving the corrosion resistance. On the other hand, when the Al content in the coating layer exceeds 70% by mass, the content of Zn having a sacrificial corrosion prevention effect on Fe decreases, and the corrosion resistance deteriorates. Therefore, the Al content in the coating layer is set to 70% by mass or less. Also, if the Al content in the coating layer is 60% by mass or less, even when the amount of plating adhesion decreases and the base steel sheet is likely to be exposed, it has a sacrificial corrosion prevention effect on Fe, and sufficient corrosion resistance can be obtained. Therefore, it is preferable that the Al content of the main plating layer is 60% by mass or less.

[0019] Si in the coating layer suppresses the growth of the interfacial alloy layer formed at the interface with the base steel sheet, and is added for the purpose of improving the adhesion, corrosion resistance, workability, etc. between the coating layer and the base steel sheet. In the case of the molten Al-Zn-based plated steel sheet of the present invention, when the steel sheet is immersed in an Al-Zn-based plating bath containing Si, Fe on the surface of the steel sheet and Al and Si in the plating bath undergo an alloying reaction, and Fe-Al-based and / or Fe-Al-Si-based intermetallic compounds are formed in a layered manner at the interface between the base steel sheet and the plating layer (an interfacial alloy layer is formed). Since the growth rate of this Fe-Al-Si-based alloy is slower than that of the Fe-Al-based alloy, the higher the ratio of the Fe-Al-Si-based alloy, the more the growth of the entire alloy phase can be suppressed. Therefore, the Si content in the plating layer needs to be 0.5% by mass or more. On the other hand, Si that remains surplus without being consumed in the formation of the interfacial alloy layer precipitates as Si phase in the plating layer. Since the Si phase is electrochemically nobler than the Al primary crystal and the Al-Zn eutectic and acts as a cathode, it has the effect of promoting the corrosion of the plating layer and reducing the corrosion resistance. Specifically, when the Si content in the plating layer exceeds 3.0% by mass, not only does the growth suppression effect of the above-described alloy phase saturate, but also the amount of the Si phase increases and the corrosion is promoted. Therefore, the Si content should be 3.0% by mass or less. From the same viewpoint, it is more preferable that the Si content in the plating layer is 2.5% by mass or less.

[0020] The plating layer contains Zn as a main component thereof. By containing Zn in the plating layer, a sacrificial corrosion prevention effect can be obtained, and it becomes possible to improve the corrosion resistance. Regarding the content of Zn, although there is no particular limitation, when it is 80% by mass or less, it is preferable in that the Al content can be ensured and the corrosion resistance by the above-described dendrite phase and interdendrite phase can be realized.

[0021] The plating layer contains inevitable impurities. Examples of the inevitable impurities include components of the base steel sheet incorporated into the plating during the reaction between the plating bath and the base steel sheet during the plating process, inevitable impurities contained in the ingots used when preparing the plating bath, and inevitable impurities slightly eluted from the equipment in the bath. As the base steel plate components incorporated during the plating, Fe may be contained in an amount of about several percent. Examples of the types of inevitable impurities in the plating bath include, as the base steel plate components, Fe, Mn, P, S, C, Nb, Ti, B, and the like. Further, examples of the impurities in the ingot include Fe, Pb, Sb, Cd, As, Ga, V, and the like. Furthermore, examples of the impurities from the bath equipment include Cr, Ni, W, Co, and the like. Note that for Fe in the plating layer, it is not possible to distinguish and quantify the Fe taken in from the base steel plate and the Fe present in the plating bath. The total content of the inevitable impurities is not particularly limited, but from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, the total amount of inevitable impurities excluding Fe is preferably 1.0 mass% or less. Furthermore, the total content of the inevitable impurities including Fe is not particularly limited, but when contained in excess, it may affect various properties of the plated steel sheet. Therefore, it is preferably 5.0 mass% or less in total, and more preferably 3.0 mass% or less.

[0022] In addition, in the molten Al-Zn-based plated steel sheet of the present invention, in addition to the above-described components, optional additive components can also be contained in an amount of 5 mass% or less. The optional additive components can be appropriately selected according to the performance required for the plating layer. For example, alkaline earth metals such as Ca and Mg, and additive components such as Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B can be mentioned. Regarding these optional additive components, although effects such as further improving the corrosion resistance can be obtained, there is a risk that the workability of the plating layer will deteriorate and the elongation limit of the molten Al-Zn-based plated steel sheet will be worsened. Therefore, the content of the optional addition is preferably 5 mass% or less.

[0023] When the plating layer corrodes, Mg and / or Ca are contained in the corrosion products as Mg and Ca as the optional additive components, and the stability of the corrosion products is improved, resulting in a delay in the progress of corrosion, and thus the effect of improving the corrosion resistance is obtained. The total content of Ca and / or Mg in the plating layer is not particularly limited as long as it is 5% by mass or less, but is preferably 0.01 to 5% by mass. By setting the total content to 0.01% by mass or more, a sufficient corrosion retardation effect can be obtained. On the other hand, by setting the total content to 5% by mass or less, the effect does not saturate, the increase in manufacturing cost can be suppressed, and the composition management of the plating bath can be easily performed.

[0024] Furthermore, the plating layer preferably contains at least Mg as the optional additive component. By the plating layer containing Mg, Mg can be generated together with Si as described above, and thus a corrosion retardation effect can be obtained. Here, the content of Mg in the plating layer is preferably 0.01 to 5% by mass, and more preferably 2 to 4.9% by mass. 2 This is because Si can be generated, and a corrosion retardation effect can be obtained.

[0025] Note that the means for forming the plating layer on the base steel sheet is not particularly limited, and a normal continuous melting plating facility can be used. For example, the base steel sheet is heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere to remove rolling oil and the like adhering to the steel sheet surface simultaneously with annealing, and after reducing and removing the oxide film, it is immersed in a molten zinc plating bath containing Al and Zn at a predetermined concentration through a snout whose lower end is immersed in the plating bath. Then, the steel sheet immersed in the plating bath is pulled up above the plating bath via a sink roll, and then a pressurized gas is injected from a gas wiping nozzle arranged above the plating bath toward the surface of the steel sheet to adjust the plating adhesion amount, and then the plating layer can be formed by cooling with a cooling device. Note that the cooling rate after plating (after the steel sheet is taken out of the plating bath) is not particularly limited, and normal conditions (for example, 12°C / s or more) can be adopted.

[0026] Further, the component composition of the plating layer can be confirmed, for example, by immersing the plating layer in hydrochloric acid or the like to dissolve it and analyzing the solution by ICP emission spectrometry, atomic absorption spectrometry, or the like. This method is merely an example, and any method can be used as long as it can accurately quantify the component composition of the plating layer, and it is not particularly limited.

[0027] In addition, the plating layer of the molten Al-Zn-based plated steel sheet obtained by the present invention is substantially equivalent to the composition of the plating bath as a whole. Therefore, the composition of the plating layer can be accurately controlled by controlling the composition of the plating bath.

[0028] And in the molten Al-Zn-based plated steel sheet of the present invention, the thickness X (μm) of the plating layer and the Vickers hardness HV of the primary Al crystals constituting the dendrites in the plating layer 0.01 satisfy the following relationships (1) and (2). HV 0.01 ≦ -4X + 205 ···(1) X ≧ 23 ···(2)

[0029] Regarding the adhesion of the plating layer at the bending return portion of the above-described molten Al-Zn-based plated steel sheet, it has been found that there is a correlation between the thickness X of the plating layer and the hardness of the plating layer. In the present invention, the Vickers hardness HV of the primary Al crystals, which has a great influence on the hardness of the plating layer 0.01 is appropriately controlled according to the thickness of the plating layer, that is, by satisfying formula (1). When the steel sheet is subjected to 180-degree adhesion bending, on the inner surface side of the bent portion, the reaction force (the reaction force directly related to the peeling of the plating) caused by the plated layer generating compressive strain and trying to bulge more inward can be suppressed. As a result, the molten Al-Zn-based plated steel sheet of the present invention can maintain a high level of plating adhesion at the bending return portion even when the plating deposition amount is large.

[0030] Specifically, in the case of the molten Al-Zn plated steel sheet of the present invention, in the bending test conforming to JIS G 3322 (2019), a 180-degree close contact bending is carried out with a width of 125 mm without sandwiching the plate between the inner intervals of the bending, and after returning and smoothing the close contact bending part, as shown in Fig. 5, an adhesive tape is attached to the surface of the bending return part that was on the inner side during the close contact bending, and then, when peeling in the direction perpendicular to the bending line of the bending return part, it is preferable that the maximum width A in the direction perpendicular to the bending line of the peeled plating layer adhered to the adhesive tape is 1 mm or less, and it is more preferable that there is no peeling of the plating layer. Regarding the range of the adhesive tape to be pasted on the bending return part of the molten Al-Zn plated steel sheet, there is no particular limitation, but from the viewpoint of being able to more reliably confirm the presence or absence of peeling of the plating layer, it is preferable that the width W in the direction along the bending line of the bending return part of the attached adhesive tape is 18 mm or more. Furthermore, it is more preferable to paste a plurality of adhesive tapes in parallel to widen the evaluation range. Thereby, the plating adhesion of the bending return part at a level sufficient for actual use can be obtained.

[0031] In the molten Al-Zn plated steel sheet of the present invention, the thickness X of the plating layer is 23 μm or more (satisfying formula (2): X≥23). Since the peeling of the plating layer around the bending return part of the molten Al-Zn plated steel sheet, which is a problem in the present invention, tends to occur remarkably as the plating layer becomes thicker, when the thickness X of the plating layer is as thick as 23 μm or more, the effect of improving the plating adhesion of the bending return part according to the present invention can be more enjoyed.

[0032] The thickness of the plating layer can be measured, for example, by observing a cross-section of the molten Al-Zn plated steel sheet after embedding it in a normally dried resin, cutting, and polishing, and then observing it with a scanning electron microscope (SEM). At that time, along the width direction (usually about 1,000 mm) of the molten Al-Zn plated steel sheet, for example, sampling from a plurality of points such as about 5 points and calculating the average value can also improve the accuracy. As another method for calculating the thickness of the plating layer, in accordance with the adhesion amount test method described in JIS H 0401, the coating film of a sample (around 70 mm in diameter) with a certain area is removed using a remover or the like, weighed, and then the plating layer is dissolved in a test solution prepared by dissolving 3.5 g of hexamethylenetetramine in 1 liter of 1+1 hydrochloric acid, and weighed again. Thus, the plating adhesion amount per unit area can be calculated, and the thickness of the plating layer can be calculated by dividing that value by the plating layer specific gravity of the hot-dip Al-Zn plated steel sheet, which is 3.7 g / μm·m. 2 In this case as well, for example, by sampling from a plurality of points, such as about three points, the accuracy can be improved.

[0033] Regarding the thickness of the plating layer, in the case of an adhesion amount of AZ170 or more, which is the plating adhesion amount display symbol described in JIS G 3321, it is considered to be a thick coating in the present invention. The average minimum adhesion amount of three points of AZ170 is 170 g / m 2 and at this time, the plating thickness per side is 170 g / m 2 ÷2÷3.7 g / μm·m 2 which results in 23 μm. Similarly, AZ185: 185 g / m 2 ÷2÷3.7 g / μm·m 2 results in approximately 25 μm AZ200: 20 g / m 2 ÷2÷3.7 g / μm·m 2 results in approximately 27 μm Thus, regarding the peeling of the plating layer around the bending return portion on the inner bending side of the hot-dip Al-Zn plated steel sheet, which is an issue in the present invention, since it tends to occur remarkably as the plating layer becomes thicker, it is defined as X≧23 in formula (2).

[0034] Here, regarding the Vickers hardness, the test is carried out with a pressing load of 10 g (HV 0.01 ). For example, for a sample of a molten Al-Zn plated steel sheet, it is embedded in resin, polished so that the cross-section of the sample can be observed, and then, from the average values of the horizontal length and the vertical length of the indentation made using a indenter with a test force of 98.07 mN and a holding time of 10 seconds on the primary Al crystal part of the cross-section with a Vickers testing machine, the Vickers hardness can be calculated.

[0035] The plating layer mainly has dendrites composed of primary Al crystals and inter-dendritic gaps containing Al-Zn eutectics. The primary Al crystals contain a matrix of α-Al phase and precipitates of Zn, and it is preferable that the Zn content in the matrix is 30 mass% or less. In the present invention, the workability is improved by softening the plating layer by reheating the plating layer. This softening occurs due to the crystallization of Zn from the primary Al crystals as described above. At this time, the Zn concentration in the primary Al crystal matrix will decrease compared to before the heat treatment due to the crystallization of Zn, and specifically, it is preferably 30 mass% or less.

[0036] The Vickers hardness HV of the primary Al crystals 0.01 There are no particular limitations on the method for controlling it. By the cooling conditions after molten plating or the reheating treatment of the plating layer described later, the plating layer is softened, and the Vickers hardness HV of the primary Al crystals is controlled so as to satisfy the above formula (1). 0.01 can be controlled.

[0037] Regarding the interfacial alloy layer in the plating layer, it is a layer existing at the interface with the base steel sheet in the plating layer, and is a layered interfacial alloy layer containing Fe, Al, Si, Zn, and inevitable impurities. As described above, the interfacial alloy layer is inevitably formed by the alloying reaction of Fe on the surface of the base steel sheet with Al and Si in the plating bath. Since this interfacial alloy layer is hard and brittle, if it grows thick, it will become the starting point for crack generation during processing. Therefore, it is necessary to make it as thin as possible. For this reason, in the molten Al-Zn-based plated steel sheet of the present invention, the thickness of the interfacial alloy layer needs to be 2 μm or less, and preferably 1 μm or less. When the thickness of the interfacial alloy layer exceeds 2 μm, the plating adhesion of the bent-back portion will be reduced.

[0038] In addition, the interfacial alloy layer is a value obtained by taking the average of the measured values of the average thickness of the interfacial alloy layer present in each field of view when observing the cross section of the plating layer in three or more fields of view using a scanning electron microscope (SEM) or the like. Also, the method for suppressing the thickness of the interfacial alloy layer is not particularly limited. For example, as described above, a method of adjusting the Si content in the plating layer, or a method of adjusting the cooling time when applying a heat history after forming the plating layer, as described later, can be mentioned.

[0039] (Surface-treated steel sheet) Further, the surface-treated steel sheet of the present invention is characterized in that a chemical conversion coating film is formed on the molten Al-Zn-based plated steel sheet of the present invention. In addition, the chemical conversion coating film is not particularly limited and can be appropriately selected according to the required performance. For example, either a chromate chemical conversion coating film or a chromium-free chemical conversion coating film can be used. For example, in the case of a chromium-free chemical conversion coating film, it is a composite of a resin component and an inorganic component, contains a mixture of a urethane resin and an epoxy resin containing a curing agent in an amount of 70 to 90% by mass, and contains 2 to 10% by mass of a phosphorus-based oxide and 8 to 20% by mass of a zirconium oxide as the inorganic component, and the like can be used.

[0040] Also, the adhesion amount of the chemical conversion coating film is preferably 0.025 to 0.5 g / m 2 . If it is less than 0.025 g / m 2 , a decrease in adhesion and corrosion resistance between the underlying aluminum-zinc-based alloy plated steel sheet and the upper primer coating film may occur. If it is 0.5 g / m 2When it exceeds and is subjected to severe bending, the chemical conversion coating is likely to break (peel off), and the corrosion resistance may also decrease.

[0041] The chemical conversion coating is obtained by continuously coating a molten Al-Zn alloy plated steel sheet with a chemical conversion treatment liquid using a roll coater or the like, and then drying it at a maximum plate temperature (Peak Metal Temperature: PMT) of about 60°C to 200°C using hot air, induction heating, or the like.

[0042] (Painted steel sheet) The painted steel sheet of the present invention is characterized in that a coating film is formed directly or via an intermediate layer on the molten Al-Zn alloy plated steel sheet of the present invention. By forming the coating film, aesthetics can be imparted, and various properties such as workability, weather resistance, chemical resistance, stain resistance, water resistance, and corrosion resistance can also be enhanced.

[0043] Note that the type of the coating film and the method of forming the coating film are not particularly limited, and can be appropriately selected according to the required performance. Examples of the paint used for forming the coating film include polyester resin-based paints, silicon polyester resin-based paints, polyurethane resin-based paints, acrylic resin-based paints, fluororesin-based paints, etc. From the viewpoints of corrosion resistance and weather resistance in particular, fluororesin-based paints are preferable.

[0044] The thickness of the coating film is not particularly limited. For example, the thickness of the coating film can be 5 to 30 μm. By setting it to 5 μm or more, the color tone appearance can be stabilized, and by setting it to 30 μm or less, a decrease in workability (crack generation in the coating film) can be more reliably suppressed.

[0045] In addition, in the coating film, according to the purpose and use, in addition to chromate-based compounds, titanium oxide, mica, mica, carbon black, various other coloring pigments, metallic pigments such as aluminum powder and mica, extender pigments composed of carbonates, sulfates, etc., or various fine particles such as silica fine particles, nylon resin beads, acrylic resin beads, curing catalysts such as p-toluenesulfonic acid and dibutyltin dilaurate, waxes and other additives can be blended in appropriate amounts.

[0046] Moreover, there are no particular restrictions on the coating method of the coating composition for forming the coating film, but preferably the coating composition can be applied by methods such as roll coater coating and curtain flow coating. After the coating composition is applied, it is baked by heating means such as hot air heating, infrared heating, induction heating, etc. to obtain a coating film. The baking treatment is usually carried out with the maximum reachable plate temperature being about 180 to 270 °C and within this temperature range for about 30 seconds to 3 minutes.

[0047] Also, 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 coating base chemical conversion treatment film or an adhesive layer can be mentioned. Regarding the coating base chemical conversion treatment film, for example, it can be formed by chromate coating base treatment or chromium-free coating base chemical conversion treatment in which a chromate coating base treatment liquid or a chromium-free coating base chemical conversion treatment liquid is applied and dried without water washing at a steel sheet temperature of 80 to 300 °C. These coating base chemical conversion treatment films may be single-layer or multi-layer, and in the case of multi-layer, a plurality of coating base chemical conversion treatments may be performed sequentially.

[0048] (Method for manufacturing hot-dip Al-Zn alloy coated steel sheet) The method for manufacturing a hot-dip Al-Zn alloy coated steel sheet of the present invention (hereinafter sometimes referred to as "the manufacturing method of the present invention") uses a plating bath having a composition containing 40 to 70% by mass of Al and 0.5 to 3.0% by mass of Si, with the balance being Zn and inevitable impurities, to form a plating layer with a plating thickness X (μm) per side on a base steel sheet, and a step of reheating the steel sheet on which the plating layer has been formed.

[0049] In addition, the plating formation means in the step of forming the plating layer on the base steel sheet is not particularly limited. For example, it can be manufactured by a continuous melting plating facility by cleaning, heating, and immersing the base steel sheet in a plating bath.

[0050] The type of the base steel sheet used in the manufacturing method of the present invention is not particularly limited. For example, a pickled and descaled hot-rolled steel sheet or steel strip, or a cold-rolled steel sheet or steel strip obtained by cold-rolling them can be used. Furthermore, in the present invention, the components in the steel are not particularly limited. For example, those with C: 0.01 to 0.10% by mass can be used. However, steel sheets with C less than 0.01% are not excluded in the present invention. In addition to C, Al, Si, Mn, and P as component elements, it is also possible to contain trace addition elements such as N, S, O, B, V, Nb, Ti, Cu, Mo, Cr, Co, Ni, Ca, Sr, In, Sn, Sb, etc.

[0051] In addition, the method for obtaining the base steel sheet is not particularly limited. For example, in the case of the hot-rolled steel sheet, those that have undergone a hot-rolling process and a pickling process can be used. In the case of the cold-rolled steel sheet, it can be manufactured by further adding a cold-rolling process. Furthermore, it is also possible to go through a recrystallization annealing process or the like before the melting plating process to obtain the characteristics of the steel sheet.

[0052] Regarding the plating bath used when forming the plating layer, as described above, since the composition of the plating layer as a whole is almost the same as the composition of the plating bath, those having a composition containing Al: 40 to 70% by mass and Si: 0.5 to 3.0% by mass, with the balance being Zn and inevitable impurities, are used.

[0053] In the manufacturing method of the present invention, the thickness of the plating layer is controlled so as to satisfy the following formula (2). X ≧ 23 (μm) ···(2) The thickness and deposition amount of the plating layer can be controlled by adjusting the air pressure ejected from the slit nozzles from both sides of the steel sheet and changing the amount of scraping off, in a method called air wiping, when the steel sheet is lifted after being immersed in the molten plating bath in a continuous melting plating facility. The gas used for the air wiping is not particularly limited, and for example, air, nitrogen, etc. are used.

[0054] And, in the manufacturing method of the present invention, it includes a step of reheating the steel sheet on which the plating layer is formed. When the maximum temperature reached by the steel sheet during the reheating is T °C and the cooling time from T °C to 100 °C is y (hr), the following relationships (3) and (4) are satisfied. 150 ≤ T ≤ 300 ···(3) 0.5 ≤ y ≤ 24 ···(4)

[0055] In the above formula (3), the range of the maximum temperature T reached when reheating the steel sheet on which the plating layer is formed is defined. The reason for setting the maximum temperature T to 150 °C or higher is that if it is not at this temperature or higher, sufficient softening of the plating layer does not occur, and thus sufficient bendability of the molten Al-Zn-based plated steel sheet cannot be obtained. From the same perspective, the maximum temperature T is preferably 160 °C or higher. On the other hand, the reason for setting the maximum temperature T to 300 °C or lower is that if it is at this temperature or higher, the thickness of the interfacial alloy layer formed at the interface between the plating and the steel sheet becomes thick, resulting in a decrease in bendability. From the same perspective, the maximum temperature T is preferably 280 °C or lower.

[0056] The above formula (4) is defined in order to maintain the plating adhesion and productivity of the springback portion at a high level in consideration of the thickness T of the formed plating layer and the maximum temperature T reached during reheating. When the cooling time (residence time) y in the above (4) is less than 0.5 hr, the plating layer cannot be sufficiently softened, and good bendability and thus plating adhesion of the springback portion cannot be ensured. On the other hand, when the cooling time (residence time) y exceeds 24 hr, productivity decreases. From the same perspective, the cooling time (residence time) y is preferably 0.5 ≤ y ≤ 12.

[0057] In the manufacturing method of the present invention, although not particularly limited, the average heating rate from room temperature to the maximum reachable temperature T °C when reheating the steel sheet is preferably 3 °C / hr or more, more preferably 4 °C / hr, and even more preferably 5 °C / hr. This is to suppress the excessive lengthening of the residence time in the high-temperature region in order to improve workability. Furthermore, when cooling the reheated steel sheet, the average cooling rate from the maximum reachable temperature T °C to room temperature is preferably 20 °C / hr or less, more preferably 15 °C / hr or less, and even more preferably 10 °C / hr or less. This is to ensure the necessary minimum residence time in the high-temperature region for improving workability.

[0058] In the manufacturing method of the present invention, when forming a chemical conversion coating film on the plating layer, the formation of the chemical conversion coating film may be performed before or after the reheating. However, when forming the chemical conversion coating film before the reheating, it is preferable to reduce the resin component of the chemical conversion coating film, add aggregates, or prevent the chemical conversion coating films from adhering to each other when reheating with a coil.

[0059] In the manufacturing method of the present invention, as described above, it is only necessary to satisfy the conditions in the step of forming the plating layer and the step of reheating the steel sheet on which the plating layer is formed. There are no particular limitations on other manufacturing conditions. They can be appropriately selected from the manufacturing conditions of known molten Al-Zn-based plated steel sheets according to the required performance.

[0060] Furthermore, the manufacturing method of the molten Al-Zn-based plated steel sheet of the present invention described above can further include a step of forming a chemical conversion coating film on the obtained molten Al-Zn-based plated steel sheet. Note that the types and formation methods of the chemical conversion coating film are the same as those described in the surface-treated steel sheet of the present invention.

[0061] Furthermore, a step of forming a coating film directly or via an intermediate layer on the molten Al-Zn-based plated steel sheet obtained by the method for manufacturing a molten Al-Zn-based plated steel sheet of the present invention described above can also be provided. In addition, the types and formation methods of the coating film and the intermediate layer are the same as those described in the coated steel sheet of the present invention.

Examples

[0062] <Samples 1 to 14> (1) Formation and reheating of the plating layer A cold-rolled steel sheet with a thickness of 0.35 mm manufactured by a conventional method was used as a base steel sheet (a steel sheet composed of 0.06% by mass of C, 0.02% by mass of Si, 0.3% by mass of Mn, 0.05% by mass of Al, 0.02% by mass of P, 0.02% by mass of S, 0.01% by mass of N, and the balance being Fe and unavoidable impurities), and annealing treatment, plating treatment, and skin pass treatment were performed in a continuous molten plating facility. In addition, in each sample, the bath temperature of the plating bath during the plating treatment was 590°C, the entry plate temperature was 600°C, and the skin pass reduction rate was 0.5%. Thereafter, heat treatment (reheating treatment) was performed in a batch-type heating furnace under the conditions shown in Table 1. The temperature of the steel sheet during the reheating treatment was measured with a thermocouple attached to the steel sheet. Further, this coil was subjected to a coating base chemical conversion treatment in a continuous coating line, and a primer and a top coat were applied. In addition, for some samples, an elongation rate of 0.1% was imparted to the steel sheet with a tension leveler after coating.

[0063] (2) Thickness of the plating layer and Vickers hardness HV of the primary Al crystal 0.01 Regarding the thickness of the plating layer of each sample, it was measured by cross-sectional observation (X1000) of the plating layer by SEM, and the average value of 10 locations was calculated. Also, the Vickers hardness HV of the primary Al crystal of the dendrite phase in the plating layer of each sample 0.01Regarding the Vickers hardness, each sample was embedded in a room-temperature drying resin, polished, the dendrite phase of the plating layer was selected from the cross-section, and the Vickers hardness of the selected dendrite phase was measured using a microhardness tester (manufactured by Shimadzu Corporation, Shimadzu Microhardness Tester HMV-G21). The measurement method was carried out in accordance with the method specified in JIS Z 2244, and the indentation load was 10 gf.

[0064] (3) Formation of the coating base chemical conversion treatment film, primer, and coating film (3-1) Coating base chemical conversion treatment film Subsequently, a coating base chemical conversion treatment film, a primer, and a coating film were sequentially formed on the obtained molten Al-Zn-based plated steel sheet of each sample. As the resin component in the coating base chemical conversion treatment film, "Yukarezine RE-1050" manufactured by Yoshimura Oil Chemical Co., Ltd., which is an epoxy resin having a bisphenol skeleton, was used so that the solid content ratio in the coating base chemical conversion treatment film was 43% by mass. As the vanadium compound contained in the coating base chemical conversion treatment film, an organic vanadium compound chelated with acetylacetone was 6% by mass, as the zirconium compound contained in the coating base chemical conversion treatment film, ammonium zirconium carbonate was 50% by mass, and as the fluorine compound contained in the coating base chemical conversion treatment film, ammonium fluoride was 1% by mass in terms of fluorine atoms, and each was used at the solid content ratio in the coating base chemical conversion treatment film. These raw materials were mixed to obtain a chemical conversion treatment solution. The pH of the coating base chemical conversion treatment solution was set to 8 - 10. This coating base chemical conversion treatment solution was applied with a roll coater, and then heated in an oven at 200 °C for 2 seconds and then air-dried to form a coating base chemical conversion treatment film with an adhesion amount of 0.1 g / m 2 ². (3-2) Primer The paint for forming the primer was prepared by the following method. As the resin component of the primer, a bisphenol A type epoxy resin (trade name "JER 1009", manufactured by Mitsubishi Chemical Corporation) which is an epoxy resin having a urethane bond was reacted with a blocked polyisocyanate compound (trade name "Desmodur BL-3175", manufactured by Sumika Bayer Urethane Co., Ltd.) at a mass ratio of 85:15, and an n-butylated melamine resin (trade name "Uban 122", manufactured by Mitsui Chemicals, Inc.) which is a melamine resin were blended at a ratio of 9:1 and used. Also, as the vanadium compound of the rust preventive pigment contained in the primer, magnesium vanadate was used, and as the phosphate compound contained in the primer, calcium phosphate was used so that each would be 12% by mass in the coating film. After adding a solvent and a rust preventive pigment to the above resin, 0.3 part of dibutyltin dilaurate (DBTDL) was added as a reaction catalyst and uniformly mixed to obtain a chromium-free primer coating composition. (3-3) Coating film As the coating film, the following paint was used. Melamine-cured polyester paint (black) "Precolor HD-0030HR" (manufactured by AkzoNobel Coating Co., Ltd.) The paint for the above coating film was applied on the formed primer with a roll coater, baked at PMT 260 °C for 40 seconds, water-cooled, and then a coating film was formed so that the film thickness after baking would be 17 μm.

[0065] <Evaluation> About each sample of the coated steel sheet obtained as above, the following evaluation was carried out. The evaluation results are shown in Table 1.

[0066] (1) Bend-back adhesion For each painted steel sheet sample, a bending test compliant with JIS G 3322 (2019) was carried out. After cutting out to a width of 125 mm, 180-degree close contact bending (hereinafter sometimes referred to as "0T bending") was carried out without sandwiching the plate between the inner sides of the bend so that the surface on which the coating film was formed faced inward. Then, a scraper was inserted into the inner surface side of the bent portion to open the close contact portion, and after making it somewhat smooth, it was pressed with a vise to make the bent portion completely smooth, thereby forming a springback portion. Then, a cellophane adhesive tape was attached to the entire surface of the springback portion on the inner surface side and peeled off in a direction perpendicular to the bending line of the springback portion. Then, the width in the direction perpendicular to the bending line of the peeled plating layer adhering to the peeled adhesive tape was measured to obtain the maximum width. This measurement was carried out at 5 points in the coil width direction, and the average value was evaluated according to the following criteria. 〇: No peeling pieces adhered to the tape, or only fine peeling pieces less than 1 mm △: There are peeling pieces of 1 - 2 mm on the tape ×: There are peeling pieces of 3 mm or more on the tape Also, for each sample, the thickness of the plating layer and the Vickers hardness HV of the primary Al crystals in the plating layer 0.01 and a graph showing the relationship with the evaluation of the peeling of the plating layer in the springback portion were created and shown in Figure 4.

[0067] (2) Corrosion resistance of the springback portion For each painted steel sheet sample, as described above, 180-degree close contact bending compliant with JIS G 3322 (2019) was carried out, and after smoothly returning (after forming the springback portion), an outdoor exposure test was conducted in Chuo-ku, Chiba City. After a 4-year and 8-month exposure test, the springback portion was visually observed and evaluated according to the following criteria. The evaluation results are shown in Table 1. (Evaluation criteria) 1 point: Clearly rusty 2 points: Slightly rusty 3 points: No rust

[0068]

Table 1

[0069] From the results in Table 1 and Figure 4, it can be seen that each sample of the inventive example is excellently balanced in both the plating adhesion of the bent-back portion and the corrosion resistance of the bent-back portion, as compared with each sample of the comparative example.

Industrial Applicability

[0070] According to the present invention, even when the plating deposition amount is large, it is possible to provide a hot-dip Al-Zn based plated steel sheet, a surface-treated steel sheet, and a painted steel sheet, which are excellent in plating adhesion of the bent-back portion, and a method for manufacturing a hot-dip Al-Zn based plated steel sheet.

Claims

1. A hot-dip Al-Zn plated steel sheet having a plating layer formed thereon, the plating layer having a composition containing 40 to 70 mass% Al, 0.5 to 3.0 mass% Si, and the balance being Zn and unavoidable impurities, The plating layer has dendrites mainly composed of Al primary crystals and dendritic gaps containing Al-Zn eutectic, The thickness X (μm) of the plating layer and the Vickers hardness HV ０．０１ and (2) satisfy the following relationships (1) and (2): HV ０．０１ ≦-4X+205 ・・・(1) X≧23 ... (2)

2. The hot-dip Al-Zn coated steel sheet according to claim 1, characterized in that in a bending test in accordance with JIS G 3322 (2019), a 180-degree close bending is performed with a width of 125 mm without placing a plate in the inner space of the bend, the close bending portion is returned to be smooth, and then an adhesive tape is applied to the surface of the returned portion that was on the inside during the close bending, and the tape is peeled off in a direction perpendicular to the bend line of the returned portion, and the maximum width of the peeled plating layer adhering to the adhesive tape in the direction perpendicular to the bend line is 1 mm or less.

3. A surface-treated steel sheet, comprising the hot-dip Al-Zn plated steel sheet according to claim 1 or 2, and a chemical conversion coating formed on the hot-dip Al-Zn plated steel sheet.

4. A coated steel sheet comprising the hot-dip Al-Zn plated steel sheet according to claim 1 or 2, and a coating film formed directly or via an intermediate layer on the hot-dip Al-Zn plated steel sheet.

5. forming a plating layer having a plating thickness of X (μm) per side on a base steel sheet using a plating bath having a composition containing 40 to 70 mass% Al, 0.5 to 3.0 mass% Si, and the balance being Zn and unavoidable impurities; and reheating the steel sheet on which the plating layer is formed. a manufacturing method of a hot-dip Al-Zn coated steel sheet, characterized in that, when a maximum temperature reached by the steel sheet during the reheating is T°C and a cooling time from T°C to 100°C is y (hr), the following relationships (2), (3), and (4) are satisfied: X≧23 ... (2) 150≦T≦300 ... (3) 0.5≦y≦24 ... (4)

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