Plated steel

Abstract

The present invention provides a plated steel material having a steel material and a plated layer containing a Zn-Al-Mg alloy layer provided on the surface of the steel material, wherein the plated layer has a prescribed chemical composition (wherein Sn: 0 to 0.20%), after the surface of the Zn-Al-Mg alloy layer is polished to 1 / 2 of the layer thickness, Al crystals are present in a backscattered electron image of the Zn-Al-Mg alloy layer obtained when observed by a scanning electron microscope at a magnification of 100 times, and the average value of the cumulative perimeter of the Al crystals is 88 to 195 mm / mm 2 .

Description

[0001] This application is a divisional application of the Chinese patent application filed on October 16, 2020, with application number 202080076441.3 and title "Coated Steel". Technical Field

[0002] This invention relates to a plated steel material. Background Technology

[0003] For example, in the building materials industry, a wide variety of coated steels are used, most of which are Zn-coated steels. Driven by the requirement for long service life in building materials, research into improving the corrosion resistance of Zn-coated steels began early on, leading to the development of various coated steels. The earliest high-corrosion-resistant coated steel for building materials was Zn-5%Al coated steel (Galfan coated steel), which improved corrosion resistance by adding Al to the Zn-based coating. It is a well-known fact that adding Al to the coating improves corrosion resistance; by adding 5% Al, Al crystals are formed in the coating (specifically, in the Zn phase), thereby improving corrosion resistance. Zn-55%Al-1.6%Si coated steel (Galvalume steel) is also a coated steel with improved corrosion resistance based on essentially the same reasoning.

[0004] Therefore, increasing the Al concentration can essentially improve the corrosion resistance of the planar portion. However, increasing the Al concentration leads to a decrease in the corrosion protection capability of the sacrificial anode.

[0005] Here, the appeal of Zn-plated steel lies in its sacrificial anodic corrosion protection relative to the base steel. That is, at the cut ends of the plated steel, in areas where the coating cracks during processing, and in areas where the base steel is exposed due to coating peeling, the surrounding coating dissolves before the base steel corrodes, and the dissolved components form a protective film. This, to some extent, prevents red rust from the base steel.

[0006] This effect is generally preferred when the Al concentration is low and the Zn concentration is high. Therefore, highly corrosion-resistant coated steels with a relatively low Al concentration of around 5% to 25% have been put into practical use in recent years. In particular, coated steels with a lower Al concentration, containing around 1% to 3% Mg, exhibit superior surface corrosion resistance and sacrificial anode corrosion protection compared to Galfan steel. Therefore, this coated steel has become a popular trend in the market and is now widely recognized.

[0007] As a coated steel containing a certain amount of Al and Mg, for example, the coated steel disclosed in Patent Document 1 has also been developed.

[0008] Specifically, Patent Document 1 discloses a hot-dip Zn-Al-Mg-Si coated steel, wherein the steel surface has a coating containing Al: 5-18% by mass, Mg: 1-10% by mass, Si: 0.01-2% by mass, with the remainder being Zn and unavoidable impurities. The coating thickness is approximately 1 mm. 2 There are more than 200 Al phases.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 2001-355053 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] However, in coated steel containing a certain amount of Al, corrosion of the coating (specifically, the Zn-Al-Mg alloy layer) occurs locally, with a higher tendency to reach the base steel early. As a result, the corrosion resistance of the planar portion sometimes deteriorates, and the deviation in planar corrosion resistance increases. Therefore, there is a current need to seek coated steel with stable and high planar corrosion resistance.

[0014] Furthermore, to improve the corrosion resistance of the sacrificial anode in the coating, it is necessary to include a water-soluble component (hereinafter also referred to as "water-soluble component"). However, the water-soluble component also dissolves in water formed by condensation of atmospheric moisture on the surface of the coated steel. As a result, sometimes the surface of the coated steel turns black even in the early stages after the manufacturing process.

[0015] Therefore, one aspect of the present invention is to provide a coated steel that can simultaneously ensure corrosion resistance of the planar portion and corrosion resistance of the sacrificial anode, while also having high resistance to discoloration.

[0016] Methods for solving problems

[0017] The above-mentioned problems can be solved by the following methods.

[0018] <1> A type of plated steel, comprising a base steel and a coating containing a Zn-Al-Mg alloy layer disposed on the surface of the base steel, wherein...

[0019] The chemical composition of the coating, expressed in % by mass, contains:

[0020] Zn: Over 65.0%

[0021] A1: Above 5.0% to below 25.0%

[0022] Mg: greater than 3.0% to less than 12.5%

[0023] Sn: 0-0.20%

[0024] Bi: 0% to less than 5.0%,

[0025] In: 0% to less than 2.0%

[0026] Ca: 0%–3.0%

[0027] Y: 0%~0.5%

[0028] La: 0% to less than 0.5%

[0029] Ce: 0% to less than 0.5%

[0030] Si: 0% to less than 2.5%

[0031] Cr: 0%–0.25%

[0032] Ti: 0%~0.25%

[0033] Ni: 0%~0.25%

[0034] Co: 0%–0.25%

[0035] V: 0%~0.25%

[0036] Nb: 0%–0.25%

[0037] Cu: 0%–0.25%

[0038] Mn: 0%~0.25%

[0039] Fe: 0%–5.0%

[0040] Sr: 0% to less than 0.5%

[0041] Sb: 0% to less than 0.5%

[0042] Pb: 0% to less than 0.5%

[0043] B: 0% to less than 0.5%, and

[0044] Impurities

[0045] After grinding the surface of the Zn-Al-Mg alloy layer to half its thickness, Al crystals were observed in the backscattered electron image of the Zn-Al-Mg alloy layer under a scanning electron microscope at 100x magnification. The average cumulative perimeter of the Al crystals was 88–195 mm / mm. 2.

[0046] <2> According to the above <1> In the aforementioned plated steel, the Sn content, expressed as a percentage by mass, is 0 to less than 0.10%.

[0047] <3> According to the above <1> or <2> The plated steel, wherein the coating has an Al-Fe alloy layer with a thickness of 0.05 to 5 μm between the base steel and the Zn-Al-Mg alloy layer.

[0048] The effects of the invention

[0049] According to one aspect of the present invention, a plated steel material can be provided that ensures both the corrosion resistance of the planar portion and the corrosion resistance of the sacrificial anode, while also having high resistance to discoloration. Attached Figure Description

[0050] Figure 1 This is a backscattered electron image (magnification 100x) of an SEM showing an example of a Zn-Al-Mg alloy layer on a coated steel material according to the present invention.

[0051] Figure 2 This is a backscattered electron image (500x magnification) of an SEM showing an example of a Zn-Al-Mg alloy layer on a coated steel material according to the present invention.

[0052] Figure 3 This is a backscattered electron image (magnification 10000x) of an SEM showing an example of a Zn-Al-Mg alloy layer on a coated steel material according to the present invention.

[0053] Figure 4 This is an illustration of an example of an image obtained by image processing (binarization) of a backscattered electron image (SEM backscattered electron image) of the Zn-Al-Mg alloy layer of the coated steel of the present invention in a manner that enables the identification of Al crystals. Detailed Implementation

[0054] Hereinafter, an example of the present invention will be described.

[0055] Furthermore, in this invention, the "%" expression for the content of each element in the chemical composition means "mass %".

[0056] The numerical range represented by "~" means that the range includes the values ​​recorded before and after "~" as the lower and upper limits.

[0057] The numerical ranges indicated by “~” with “more than” or “less than” mean that these values ​​are not included as lower or upper limits.

[0058] The content of elements in a chemical composition is sometimes expressed as element concentration (e.g., Zn concentration, Mg concentration, etc.).

[0059] The term "process" not only refers to an independent process, but also includes any process that can achieve its intended purpose, even if it cannot be clearly distinguished from other processes.

[0060] The term "planar corrosion resistance" refers to the resistance of the coating (specifically, the Zn-Al-Mg alloy layer) to corrosion.

[0061] The term "sacrificial anode corrosion protection" refers to the property of inhibiting corrosion of the base steel on exposed parts of the base steel (such as the cut ends of the coated steel, the cracked parts of the coating during processing, and the parts exposed due to coating peeling).

[0062] The term "colorfastness" refers to the property that the surface of the coated steel (i.e., the coating surface) is unlikely to turn black after the steel has been manufactured.

[0063] The plated steel of the present invention is a plated steel having a base steel and a coating containing a Zn-Al-Mg alloy layer disposed on the surface of the base steel.

[0064] Furthermore, the coating of the steel material of the present invention has a specified chemical composition. After grinding the surface of the Zn-Al-Mg alloy layer to 1 / 2 of its thickness, Al crystals are present in the backscattered electron image of the Zn-Al-Mg alloy layer obtained by observation with a scanning electron microscope at 100x magnification. The average cumulative perimeter of the Al crystals is 88-195 mm / mm. 2 .

[0065] The plated steel of the present invention, through the above-described configuration, becomes a plated steel that simultaneously ensures corrosion resistance of the planar portion and sacrificial anode corrosion protection while possessing high resistance to discoloration. The plated steel of the present invention was discovered based on the following insights.

[0066] The inventors analyzed the initial corrosion behavior of coatings containing a Zn-Al-Mg alloy layer. The results revealed that corrosion of the coating (specifically, the Zn-Al-Mg alloy layer) occurs locally in a ant-hole pattern, with preferential corrosion occurring around the Al crystals.

[0067] This can be inferred as follows: Potential difference corrosion occurs between Al crystals with relatively high potential and the surrounding tissues with relatively low potential. Therefore, the larger the contact area between Al crystals and the phases surrounding them, the easier it is for corrosion to occur around the Al crystals, resulting in a deterioration in the corrosion resistance of planar areas and an increase in the deviation of corrosion resistance in planar areas.

[0068] Therefore, in order to minimize the contact area between Al crystals and the phases surrounding them, the inventor devised a method to coarsely precipitate Al crystals by controlling the cooling conditions after immersion in the plating bath during coating manufacturing.

[0069] The results yielded the following insights. As an indicator of Al crystal size, the cumulative perimeter of Al crystals, as determined by image analysis, is closely related to the corrosion resistance of the planar portion. Furthermore, if the average cumulative perimeter of the Al crystals is kept within a specified range, the contact area between the Al crystals and the surrounding phases decreases. Consequently, preferential corrosion around the Al crystals can be suppressed, resulting in stable corrosion resistance of the planar portion. However, if the average cumulative perimeter of the Al crystals is excessively reduced, processability decreases.

[0070] On the other hand, the inventors studied the Sn content to improve the corrosion resistance of sacrificial anodes and obtained the following insights.

[0071] If the coating contains more than 0.20% Sn to improve the corrosion resistance of the sacrificial anode, a water-soluble structure, namely the Mg2Sn phase, will be formed. However, the Mg2Sn phase also dissolves in water formed by condensation of atmospheric moisture on the surface of the coated steel. As a result, the surface of the coated steel sometimes darkens over time after manufacturing.

[0072] Therefore, by suppressing the Sn content to 0–0.20%, the excessive formation of the water-soluble Mg2Sn phase can be inhibited. As a result, after ensuring the corrosion resistance of the sacrificial anode along with the corrosion resistance of the planar portion, the resistance to discoloration is also improved.

[0073] As can be seen from the above, the plated steel of the present invention is a plated steel that ensures both the corrosion resistance of the planar portion and the corrosion resistance of the sacrificial anode, while also having high resistance to discoloration.

[0074] The details of the plated steel material of the present invention will be described below.

[0075] The base steel material to be plated is described.

[0076] There are no particular restrictions on the shape of the base steel. Besides steel plates, other base steel materials that can be formed into steel pipes, civil engineering materials (fences, corrugated pipes, drainage covers, sand-blocking panels, bolts, wire mesh, guardrails, retaining walls, etc.), appliance components (air conditioner outdoor unit frames, etc.), and automotive parts (running components, etc.). Forming processes can utilize various plastic forming methods such as pressure processing, roll forming, and bending.

[0077] There are no particular restrictions on the material of the base steel. For example, it can be any type of base steel, such as ordinary steel, pre-plated steel, Al-killed steel, ultra-low carbon steel, high carbon steel, various high-strength steels, and some high-alloy steels (steels containing reinforcing elements such as Ni and Cr).

[0078] There are no particular restrictions on the manufacturing methods of the base steel and the base steel plate (hot rolling, pickling, cold rolling, etc.).

[0079] Furthermore, as a base steel, it can also be used for hot-rolled steel plates, hot-rolled steel strips, cold-rolled steel plates, and cold-rolled steel strips as described in JIS G 3302 (2010).

[0080] The base steel can also be pre-plated steel. Pre-plated steel can be obtained, for example, through electrolytic treatment or displacement plating. In the electrolytic treatment method, the pre-plated steel is obtained by immersing the base steel in a sulfuric acid or chloride solution containing various pre-plating components of metal ions for electrolytic treatment. In the displacement plating method, the pre-plated steel is obtained by immersing the base steel in an aqueous solution containing various pre-plating components of metal ions, with the pH adjusted using sulfuric acid, causing the metal to be displaced and precipitated.

[0081] As a representative example of pre-plated steel, pre-plated Ni steel can be cited.

[0082] Next, the coating will be explained.

[0083] The coating contains a Zn-Al-Mg alloy layer. In addition to the Zn-Al-Mg alloy layer, the coating may also contain an Al-Fe alloy layer. The Al-Fe alloy layer is located between the base steel and the Zn-Al-Mg alloy layer.

[0084] In other words, the coating can be a single-layer structure of Zn-Al-Mg alloy layer, or a stacked structure containing both Zn-Al-Mg alloy layer and Al-Fe alloy layer. In the case of a stacked structure, it is preferable to designate the Zn-Al-Mg alloy layer as the layer constituting the surface of the coating.

[0085] However, although an oxide film of about 50 nm thick is formed on the surface of the coating, it is relatively thin compared to the overall thickness of the coating and can be regarded as not constituting the main body of the coating.

[0086] Here, the thickness of the Zn-Al-Mg alloy layer is set to be, for example, 2 μm or more and 95 μm or less (preferably 5 μm or more and 75 μm or less).

[0087] On the other hand, the overall coating thickness can be, for example, less than 100 μm. Since the overall coating thickness is influenced by the plating conditions, there are no specific upper or lower limits for its thickness. For example, in typical hot-dip plating, the overall coating thickness is related to the viscosity and specific gravity of the plating bath. Furthermore, the coating weight per unit area can be adjusted according to the drawing speed of the base steel and the strength of the frictional contact. Therefore, the lower limit of the overall coating thickness can be considered to be around 2 μm.

[0088] On the other hand, based on the weight and uniformity of the plated metal, the upper limit of the coating thickness that can be produced by hot-dip plating is approximately 95 μm.

[0089] Since the thickness of the coating can be freely varied according to the drawing speed and friction contact conditions from the plating bath, it is not particularly difficult to manufacture a coating with a thickness of 2 to 95 μm.

[0090] The preferred coating thickness per single side is 20–300 g / m². 2 .

[0091] If the coating adhesion is to be 20g / m 2 The above ensures more reliable corrosion resistance of the planar portion and corrosion protection of the sacrificial anode. On the other hand, if the coating adhesion is maintained at 300 g / m²... 2 The following steps can suppress appearance defects such as coating sagging.

[0092] Next, the Al-Fe alloy layer will be explained.

[0093] The Al-Fe alloy layer forms on the surface of the base steel (specifically, between the base steel and the Zn-Al-Mg alloy layer), and its microstructure is dominated by the Al5Fe phase. The Al-Fe alloy layer is formed through atomic diffusion between the base steel and the plating bath. When hot-dip galvanizing is used as the manufacturing method, the Al-Fe alloy layer easily forms in plating containing Al. Because the plating bath contains a certain concentration of Al, the Al5Fe phase is formed most abundantly. However, atomic diffusion takes time, and there are areas with increased Fe concentration near the base steel. Therefore, the Al-Fe alloy layer sometimes also contains small amounts of AlFe, Al3Fe, and Al5Fe2 phases locally. Furthermore, because the plating bath also contains a certain concentration of Zn, the Al-Fe alloy layer also contains small amounts of Zn.

[0094] Regarding corrosion resistance, there is little difference between the Al5Fe, Al3Fe, AlFe, and Al5Fe2 phases. The corrosion resistance mentioned here refers to the corrosion resistance of the parts not affected by welding.

[0095] Here, when the coating contains Si, Si sometimes readily enters the Al-Fe alloy layer, thus forming an Al-Fe-Si intermetallic compound phase. As an identifying intermetallic compound phase, the AlFeSi phase exists, and as isomers, α, β, q1, and q2-AlFeSi phases also exist. Therefore, these AlFeSi phases can sometimes be detected in Al-Fe alloy layers. Al-Fe alloy layers containing these AlFeSi phases are also referred to as Al-Fe-Si alloy layers.

[0096] Furthermore, since the Al-Fe-Si alloy layer is also thinner than the Zn-Al-Mg alloy layer, it has a smaller impact on the overall corrosion resistance of the coating.

[0097] Furthermore, when using various pre-plated steel materials as the base steel (base steel plate, etc.), the structure of the Al-Fe alloy layer sometimes changes due to variations in the amount of pre-plating. Specifically, there are several possibilities: a pure metal layer used during pre-plating may remain around the Al-Fe alloy layer; an alloy layer may form from intermetallic compound phases (such as Al3Ni phase) formed by the combination of the components of the Zn-Al-Mg alloy layer and the pre-plating components; an Al-Fe alloy layer may form by partially replacing Al atoms and Fe atoms; or an Al-Fe-Si alloy layer may form by partially replacing Al atoms, Fe atoms, and Si atoms. In short, since these alloy layers are also relatively thin compared to the Zn-Al-Mg alloy layer, their impact on the overall corrosion resistance of the coating is relatively small.

[0098] In other words, the so-called Al-Fe alloy layer is an alloy layer that, in addition to the Al5Fe phase as the main component, also contains the aforementioned alloy layers in various other forms.

[0099] Furthermore, when a coating is formed on pre-plated Ni steel among various pre-plated steels, an Al-Ni-Fe alloy layer is formed as an Al-Fe alloy layer. Since the Al-Ni-Fe alloy layer is relatively thin compared to the Zn-Al-Mg alloy layer, it has a smaller impact on the overall corrosion resistance of the coating.

[0100] The thickness of the Al-Fe alloy layer is, for example, greater than 0 μm and less than 5 μm.

[0101] In other words, the Al-Fe alloy layer may not be formed. From the viewpoint of improving the adhesion of the coating (specifically the Zn-Al-Mg alloy layer) and ensuring processability, the thickness of the Al-Fe alloy layer is preferably 0.05 μm or more and 5 μm or less.

[0102] However, if a hot-dip galvanizing method is typically used to form a coating with the chemical composition specified in this invention, an Al-Fe alloy layer of 100 nm or more is usually formed between the base steel and the Zn-Al-Mg alloy layer. There is no particular limitation on the lower limit of the Al-Fe alloy layer thickness, but it has been determined that an Al-Fe alloy layer will inevitably form when forming a hot-dip galvanized coating containing Al. Furthermore, based on experience, a thickness around 100 nm is the thickness that best suppresses the formation of the Al-Fe alloy layer, and can be considered a thickness that sufficiently ensures adhesion between the coating and the base steel. Unless special methods are employed, it is difficult to form an Al-Fe alloy layer thinner than 100 nm in the hot-dip galvanizing method due to the high Al concentration. However, it can be inferred that even if the thickness of the Al-Fe alloy layer is less than 100 nm, and even if an Al-Fe alloy layer does not form, it will not have a significant impact on the coating performance.

[0103] On the other hand, if the thickness of the Al-Fe alloy layer exceeds 5 μm, the Al content of the Zn-Al-Mg alloy layer formed on the Al-Fe alloy layer is insufficient, which tends to lead to extreme deterioration in the adhesion and processability of the coating. Therefore, it is preferable to limit the thickness of the Al-Fe alloy layer to less than 5 μm.

[0104] Furthermore, the growth rate of the Al-Fe alloy layer is closely related to the Al and Sn concentrations. Generally speaking, higher Al and Sn concentrations tend to result in faster growth.

[0105] Since Al-Fe alloy layers are mainly composed of the Al5Fe phase, the chemical composition of Al-Fe alloy layers can be illustrated by containing Fe: 25-35%, Al: 65-75%, Zn: less than 5%, and the remainder being impurities.

[0106] Generally, because the Zn-Al-Mg alloy layer is thicker than the Al-Fe alloy layer, the Al-Fe alloy layer contributes less to the corrosion resistance of the planar portion of the coated steel than the Zn-Al-Mg alloy layer. However, as can be inferred from the compositional analysis results, the Al-Fe alloy layer contains a certain concentration or higher of corrosion-resistant elements, namely Al and Zn. Therefore, the Al-Fe alloy layer provides a certain degree of sacrificial anodic corrosion protection and corrosion barrier effect to the base steel.

[0107] Here, it is difficult to quantitatively determine the individual corrosion resistance contribution of a thin Al-Fe alloy layer. However, for example, when the Al-Fe alloy layer has sufficient thickness, the individual corrosion resistance of the Al-Fe alloy layer can be evaluated by precisely removing the Zn-Al-Mg alloy layer from the coating surface using a milling machine or similar method, followed by a corrosion test. Because the Al-Fe alloy layer contains Al and a small amount of Zn, it produces pitted red rust when present, unlike the widespread red rust that forms when the base steel is exposed without an Al-Fe alloy layer.

[0108] Furthermore, in corrosion tests, if cross-sectional observation of the coating is conducted up to the point where the base steel is about to develop red rust, it can be confirmed that even if the upper Zn-Al-Mg alloy layer dissolves and rusts, only the Al-Fe alloy layer remains, providing corrosion protection to the base steel. This is because, electrochemically, although the Al-Fe alloy layer has a positive potential compared to the Zn-Al-Mg layer, it is at a negative potential compared to the base steel. Therefore, it can be determined that the Al-Fe alloy layer also possesses a certain degree of corrosion resistance.

[0109] From a corrosion perspective, a thicker Al-Fe alloy layer is preferred, as it delays the onset of red rust. However, a thick Al-Fe alloy layer can significantly degrade the workability of the coating, so a thickness below a certain level is preferable. From a workability perspective, the thickness of the Al-Fe alloy layer is preferably 5 μm or less. If the Al-Fe alloy layer thickness is 5 μm or less, the amount of cracking and powdering that originates from the Al-Fe alloy layer, such as in V-bending tests, is reduced. A thickness of 2 μm or less is more preferable.

[0110] Next, the chemical composition of the coating will be explained.

[0111] Regarding the composition of the Zn-Al-Mg alloy layer in the coating, the Zn-Al-Mg alloy layer can roughly maintain the composition ratio of the plating bath. Because the formation of the Al-Fe alloy layer in the hot-dip plating process is completed within the plating bath, the reduction in Al and Zn content in the Zn-Al-Mg alloy layer caused by the formation of the Al-Fe alloy layer is usually very small.

[0112] Furthermore, to achieve stable corrosion resistance on the planar surface, the chemical composition of the coating is as follows.

[0113] In other words, the chemical composition of the coating, expressed as a percentage by mass, is set as follows:

[0114] Zn: Over 65.0%

[0115] A1: Above 5.0% to below 25.0%

[0116] Mg: greater than 3.0% to less than 12.5%

[0117] Sn: 0-0.20%

[0118] Bi: 0% to less than 5.0%,

[0119] In: 0% to less than 2.0%

[0120] Ca: 0%–3.0%

[0121] Y: 0%~0.5%

[0122] La: 0% to less than 0.5%

[0123] Ce: 0% to less than 0.5%

[0124] Si: 0% to less than 2.5%

[0125] Cr: 0%–0.25%

[0126] Ti: 0%~0.25%

[0127] Ni: 0%~0.25%

[0128] Co: 0%–0.25%

[0129] V: 0%~0.25%

[0130] Nb: 0%–0.25%

[0131] Cu: 0%–0.25%

[0132] Mn: 0%~0.25%

[0133] Fe: 0%–5.0%

[0134] Sr: 0% to less than 0.5%

[0135] Sb: 0% to less than 0.5%

[0136] Pb: 0% to less than 0.5%

[0137] B: 0% to less than 0.5%, and

[0138] Impurities.

[0139] In the chemical composition of the coating layer, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb and B are optional components. That is to say, these elements may not be contained in the coating layer either. When these optional components are contained, the content of each element is preferably in the range described below.

[0140] Here, the chemical composition of the coating layer is the average chemical composition of the entire coating layer (when the coating layer is a single-layer structure of a Zn-Al-Mg alloy layer, it is the average chemical composition of the Zn-Al-Mg alloy layer; when the coating layer is a laminated structure of an Al-Fe alloy layer and a Zn-Al-Mg alloy layer, it is the combined average chemical composition of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer).

[0141] Generally, in the hot-dip coating method, since most of the coating formation reaction is completed in the plating bath, the chemical composition of the Zn-Al-Mg alloy layer is roughly the same as that of the plating bath. In addition, in the hot-dip coating method, the Al-Fe alloy layer is instantaneously formed and grows soon after being immersed in the plating bath. Moreover, the formation reaction of the Al-Fe alloy layer is completed in the plating bath, and its thickness is mostly very thin relative to the Zn-Al-Mg alloy layer.

[0142] Therefore, after plating, as long as no special heat treatment such as alloying heat treatment is carried out, the average chemical composition of the entire coating layer is substantially equal to the chemical composition of the Zn-Al-Mg alloy layer, and the composition of the Al-Fe alloy layer can be ignored.

[0143] Hereinafter, each element of the coating layer will be described.

[0144] <Zn: More than 65.0%>

[0145] In addition to obtaining the corrosion resistance of the flat part, Zn is also an element necessary for obtaining sacrificial anode corrosion protection. When considering the Zn concentration in terms of atomic composition ratio, since the coating layer is composed of elements with low specific gravity such as Al and Mg, even in terms of atomic composition ratio, it needs to be set with Zn as the main body.

[0146] Therefore, the Zn concentration is set to be more than 65.0%. The Zn concentration is preferably 70% or more. Furthermore, the upper limit of the Zn concentration is the concentration of the remaining part except for the elements and impurities other than Zn.

[0147] <Al: More than 5.0% to less than 25.0%>

[0148] Al is an element necessary for forming Al crystals and jointly ensuring the corrosion resistance of the flat part and sacrificial anode corrosion protection. Moreover, Al is also an element necessary for improving the adhesion of the coating layer and ensuring the workability. Therefore, the lower limit value of the Al concentration is set to be more than 5.0% (preferably 10.0% or more).

[0149] On the other hand, if the Al concentration increases, there is a tendency for the sacrificial anode corrosion resistance to deteriorate. Therefore, the upper limit value of the Al concentration is set to be less than 25.0% (preferably 23.0% or less).

[0150] <Mg: more than 3.0% to less than 12.5%>

[0151] Mg is an element necessary for ensuring both the corrosion resistance of the flat part and the sacrificial anode corrosion resistance. Therefore, the lower limit value of the Mg concentration is set to be more than 3.0% (preferably more than 5.0%).

[0152] On the other hand, if the Mg concentration increases, there is a tendency for the workability to deteriorate. Therefore, the upper limit of the Mg concentration is set to be less than 12.5% (preferably 10.0% or less).

[0153] <Sn: 0 to 0.20%>

[0154] Sn is an element that forms a water-soluble structure, namely the Mg2Sn phase, and imparts high sacrificial anode corrosion resistance. However, if Sn is contained excessively, a large amount of the water-soluble structure, namely the Mg2Sn phase, is generated, thereby deteriorating the anti-discoloration property. However, from the perspective of improving the sacrificial anode corrosion resistance, it is preferable to contain a certain amount of Sn. Therefore, the upper limit value of the Sn concentration is set to be 0.20% or less (preferably less than 0.10%). Furthermore, the upper limit value of the Sn concentration can also be 0.09% or less, 0.08% or less, 0.07% or less, 0.06% or less, or 0.05% or less.

[0155] On the other hand, from the perspective of improving the anti-discoloration property, it is preferable not to contain Sn. Therefore, the lower limit value of the Sn concentration is set to 0%. However, from the perspective of improving the sacrificial anode corrosion resistance, the lower limit value of the Sn concentration can also be more than 0%, 0.01% or more, 0.02% or more, or 0.03% or more.

[0156] <Bi: 0% to less than 5.0%>

[0157] Bi is an element that contributes to the sacrificial anode corrosion resistance. Therefore, the lower limit value of the Bi concentration is preferably more than 0% (preferably 0.1% or more, more preferably 3.0% or more).

[0158] On the other hand, if the Bi concentration increases, there is a tendency for the corrosion resistance of the flat part to deteriorate. Therefore, the upper limit value of the Bi concentration is set to be less than 5.0% (preferably 4.8% or less).

[0159] <In: 0% to less than 2.0%>

[0160] In is an element that contributes to the corrosion resistance of the sacrificial anode. Therefore, the lower limit value of the In concentration is preferably more than 0% (preferably 0.1% or more, more preferably 1.0% or more).

[0161] On the other hand, if the In concentration increases, there is a tendency for the corrosion resistance of the flat part to deteriorate. Therefore, the upper limit value of the In concentration is set to be less than 2.0% (preferably 1.8% or less).

[0162] <Ca: 0% to 3.0%>

[0163] Ca is an element that can adjust the Mg dissolution amount to optimize the corrosion resistance of the flat part and the corrosion resistance of the sacrificial anode. Therefore, the lower limit value of the Ca concentration is preferably more than 0% (preferably 0.05% or more).

[0164] On the other hand, if the Ca concentration increases, there is a tendency for the corrosion resistance and workability of the flat part to deteriorate. Therefore, the upper limit value of the Ca concentration is set to 3.0% or less (preferably 1.0% or less).

[0165] <Y: 0% to 0.5%>

[0166] Y is an element that contributes to the corrosion resistance of the sacrificial anode. Therefore, the lower limit value of the Y concentration is preferably more than 0% (preferably 0.1% or more).

[0167] On the other hand, if the Y concentration increases, there is a tendency for the corrosion resistance of the flat part to deteriorate. Therefore, the upper limit value of the Y concentration is set to 0.5% or less (preferably 0.3% or less).

[0168] <La and Ce: 0% to less than 0.5%>

[0169] La and Ce are elements that contribute to the corrosion resistance of the sacrificial anode. Therefore, the lower limit values of the La concentration and the Ce concentration are preferably more than 0% (preferably 0.1% or more) respectively.

[0170] On the other hand, if the La concentration and the Ce concentration increase, there is a tendency for the corrosion resistance of the flat part to deteriorate. Therefore, the upper limit values of the La concentration and the Ce concentration are set to less than 0.5% (preferably 0.4% or less) respectively.

[0171] <Si: 0% to less than 2.5%>

[0172] Si is an element that helps to improve the corrosion resistance by suppressing the growth of the Al-Fe alloy layer. Therefore, the Si concentration is preferably more than 0% (preferably 0.05% or more, more preferably 0.1% or more). In particular, when Sn is not contained (that is, when the Sn concentration is 0%), from the viewpoint of ensuring the corrosion resistance of the sacrificial anode, the Si concentration is preferably 0.1% or more (more preferably 0.2% or more).

[0173] On the other hand, if the Si concentration increases, there is a tendency for the corrosion resistance of the planar part, the sacrificial anode corrosion protection property, and the workability to deteriorate. Therefore, the upper limit value of the Si concentration is set to be less than 2.5%. In particular, from the viewpoints of the corrosion resistance of the planar part and the sacrificial anode corrosion protection property, the Si concentration is preferably 2.4% or less, more preferably 1.8% or less, and still more preferably 1.2% or less.

[0174] <Cr, Ti, Ni, Co, V, Nb, Cu, and Mn: 0% to 0.25%>

[0175] Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are elements that contribute to the sacrificial anode corrosion protection property. Therefore, the lower limit values of the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are preferably each more than 0% (preferably 0.05% or more, more preferably 0.1% or more).

[0176] On the other hand, if the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn increase, there is a tendency for the corrosion resistance of the planar part to deteriorate. Therefore, the upper limit values of the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are set to be 0.25% or less, respectively. The upper limit values of the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are preferably 0.22% or less.

[0177] <Fe: 0% to 5.0%>

[0178] When forming a coating layer by the hot-dip plating method, the Zn-Al-Mg alloy layer and the Al-Fe alloy layer may contain a certain Fe concentration.

[0179] It has been confirmed that even if the coating layer (especially the Zn-Al-Mg alloy layer) contains an Fe concentration up to 5.0%, there is no adverse effect on the performance. Since most of the Fe is contained in the Al-Fe alloy layer, if the thickness of this layer is relatively thick, generally the Fe concentration increases.

[0180] <Sr, Sb, Pb, and B: 0% to less than 0.5%>

[0181] Sr, Sb, Pb, and B are elements that contribute to the sacrificial anode corrosion protection property. Therefore, the lower limit values of the concentrations of Sr, Sb, Pb, and B are preferably each more than 0% (preferably 0.05% or more, more preferably 0.1% or more).

[0182] On the other hand, if the concentrations of Sr, Sb, Pb, and B increase, there is a tendency for the corrosion resistance of the planar part to deteriorate. Therefore, the upper limit values of the concentrations of Sr, Sb, Pb, and B are set to be less than 0.5%, respectively.

[0183] <Impurities>

[0184] Impurities are components contained in raw materials or mixed in during the manufacturing process; they refer to components that are not intentionally present. For example, in coatings, components other than Fe are sometimes mixed in as trace amounts as impurities through atomic diffusion between the base steel and the plating bath.

[0185] The chemical composition of the coating was determined using the following method.

[0186] First, the coating is peeled and dissolved using an acid containing a corrosion inhibitor that suppresses corrosion of the base steel, thus obtaining an acid solution. Next, the acid solution is analyzed by ICP to determine the chemical composition of the coating (the chemical composition of the Zn-Al-Mg alloy layer is obtained when the coating is a single-layer structure of a Zn-Al-Mg alloy layer; the combined chemical composition of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer is obtained when the coating is a laminated structure of an Al-Fe alloy layer and a Zn-Al-Mg alloy layer). The type of acid is not particularly limited as long as it can dissolve the coating. Furthermore, the chemical composition is determined as an average chemical composition. Finally, the Zn concentration is calculated by ICP analysis using the formula: Zn concentration = 100% - other element concentrations (%).

[0187] Here, when using pre-plated steel as the base steel, the composition of the pre-plating is also tested.

[0188] For example, when using pre-plated Ni steel, ICP analysis detects not only the Ni in the coating but also the Ni in the pre-plated Ni. Specifically, for example, when using a base steel with a Ni adhesion of 1 g / m³... 2 ~3g / m 2 When using pre-plated Ni steel as the base steel, even if the Ni concentration in the coating is 0%, a Ni concentration of 0.1% to 15% can be detected. On the other hand, when using pre-plated Ni steel as the base steel, a small amount of Ni in the pre-plated Ni layer dissolves in the plating bath during immersion. Therefore, the Ni concentration in the plating bath is 0.02% to 0.03% higher than the Ni concentration in the prepared plating bath. Thus, when using pre-plated Ni steel, the Ni concentration in the coating can be increased by a maximum of 0.03%.

[0189] Therefore, in this invention, when using pre-plated Ni steel, if the Ni concentration detected by ICP analysis exceeds 0.28% (0.25% (the upper limit of Ni concentration in the coating) + 0.03%) but is below 15%, the Ni concentration in the coating is considered to be 0%. The Zn concentration at this time can be calculated using the formula: Zn concentration = 100% - concentration of other elements besides Ni (%).

[0190] On the other hand, when using pre-plated Ni steel, if the Ni concentration detected by ICP analysis exceeds 15%, it is considered that the coating contains Ni at a concentration exceeding 0.25% (the upper limit of Ni concentration in the coating). Furthermore, in this invention, only the composition of the coating is determined using the ICP analysis method, but the Ni concentration in the coating can also be analyzed by using glow discharge emission spectroscopy (quantitative GDS) in conjunction with the ICP analysis method.

[0191] Next, the microstructure of the Zn-Al-Mg alloy layer will be described.

[0192] The Zn-Al-Mg alloy layer contains Al crystals, with an average cumulative perimeter of 88–195 mm / mm. 2 .

[0193] If the average cumulative perimeter of Al crystal is less than 88 mm / mm 2 If the Al crystals are too coarse, the processability will be compromised.

[0194] On the other hand, if the average cumulative perimeter of Al crystals exceeds 195 mm / mm 2 As the Al crystals become finer, the contact area between the Al crystals and the surrounding phases increases. Consequently, the larger the contact area between the Al crystals and the surrounding phases, the easier it is for corrosion to occur around the Al crystals, thus deteriorating the corrosion resistance of the planar portion and increasing the deviation in corrosion resistance of the planar portion.

[0195] Therefore, the average cumulative perimeter of Al crystals was set to 88–195 mm / mm. 2 The lower limit of the average cumulative perimeter of Al crystals is preferably 95 mm / mm. 2 The above is preferred, with 105mm / mm being more ideal. 2 The upper limit of the average cumulative perimeter of the Al crystal is preferably 185 mm / mm. 2 Hereinafter, 170mm / mm is preferred. 2 the following.

[0196] The microstructure of the Zn-Al-Mg alloy layer contains Al crystals. In addition to Al crystals, the microstructure of the Zn-Al-Mg alloy layer may also contain Zn-Al phases.

[0197] Al crystals are equivalent to "α phases in which Zn is dissolved at a concentration of 0-3%". On the other hand, Zn-Al phases are equivalent to "β phases containing more than 70%-85% Zn phase (η phase) and with fine separation between the α phase and the Zn phase (η phase)".

[0198] here, Figures 1-3The image shows an example of a backscattered electron image of a Zn-Al-Mg alloy layer on a polished surface that has been ground to half the thickness of the layer. Figure 1 It is a backscattered electron image from an SEM at 100x magnification. Figure 2 It is a backscattered electron image from an SEM at 500x magnification. Figure 3 It is a backscattered electron image from an SEM at a magnification of 10,000.

[0199] Furthermore, Figures 1-3 In this context, Al represents Al crystals, Zn-Al represents the Zn-Al phase, MgZn2 represents the MgZn2 phase, and Zn-Eu represents the Zn-based eutectic phase.

[0200] In the backscattered electron image of the Zn-Al-Mg alloy layer, the area fraction of each structure is not particularly limited, but from the viewpoint of improving the corrosion resistance of the stable planar portion, the area fraction of Al crystals is preferably 8 to 45%, more preferably 15 to 35%. That is to say, it is preferable that Al crystals exist within the above-mentioned area fraction range.

[0201] Other microstructures besides Al crystals and Zn-Al phases include MgZn2 phase and Zn-based eutectic phases (specifically Zn-Al-MgZn2-Mg2Sn, etc.).

[0202] Here, the methods for determining the average cumulative perimeter of Al crystals and the area fraction of Al crystals are explained.

[0203] The average cumulative perimeter of Al crystals and the area fraction of Al crystals can be determined using backscattered electron images of the Zn-Al-Mg alloy layer obtained by observing it at 100x magnification using a scanning electron microscope after grinding the surface of the Zn-Al-Mg alloy layer to half its thickness. Specifically, the procedure is as follows.

[0204] First, samples are collected from the plated steel to be tested. However, samples are collected from the area without plating defects, outside the punched end face of the plated steel (2 mm from the end face).

[0205] Next, the surface of the coating (specifically, the Zn-Al-Mg alloy layer) of the sample is ground in the thickness direction of the coating (hereinafter also referred to as the "Z-axis direction").

[0206] The Z-axis grinding of the coating surface continued until the Zn-Al-Mg alloy layer was ground to half its thickness. This grinding was performed by dry grinding of the Zn-Al-Mg alloy layer surface using a #1200 grinding wheel, followed by polishing with polishing slurry containing alumina with an average particle size of 3μm, polishing slurry containing alumina with an average particle size of 1μm, and polishing slurry containing colloidal silica, respectively.

[0207] Furthermore, before and after grinding, the Zn strength on the surface of the Zn-Al-Mg alloy layer was measured by XRF (fluorescence X-ray analysis). When the Zn strength after grinding was half of the Zn strength before grinding, it was taken as half the thickness of the Zn-Al-Mg alloy layer.

[0208] Next, the polished surface of the Zn-Al-Mg alloy layer of the sample was observed using a scanning electron microscope (SEM) at a magnification of 100x to obtain a backscattered electron image of the Zn-Al-Mg alloy layer (hereinafter also referred to as "backscattered electron image of SEM"). The SEM observation conditions were set as follows: accelerating voltage: 15kV, irradiation current: 10nA, and field of view size: 1222.2μm × 927.8μm.

[0209] To identify the various phases present in the Zn-Al-Mg alloy layer, a FE-SEM or TEM (transmission electron microscope) equipped with an EDS (energy-dispersive X-ray diffraction) device was used. When using TEM, the polished surface of the Zn-Al-Mg alloy layer of the same sample was processed using FIB (focused ion beam). After FIB processing, an electron diffraction image of the polished surface of the Zn-Al-Mg alloy layer was obtained using TEM. Then, the metals contained in the Zn-Al-Mg alloy layer were identified.

[0210] Next, the identification results of the backscattered electron images from SEM and the electron diffraction images from FE-SEM or TEM are compared. In the SEM backscattered electron images, the various phases present in the Zn-Al-Mg alloy layer are identified. Furthermore, in the identification of the various phases present in the Zn-Al-Mg alloy layer, EDS point analysis is performed, and the results of the EDS point analysis are compared with the identification results of the TEM electron diffraction images. Additionally, an EPMA device can also be used for phase identification.

[0211] Next, in the backscattered electron image of SEM, the brightness, hue, and contrast values ​​of the gray levels of each phase in the Zn-Al-Mg alloy layer were determined. Since these three values ​​reflect the atomic numbers of the elements contained in each phase, phases with smaller atomic numbers and higher Al and Mg contents tend to appear blacker, while phases with higher Zn contents tend to appear whiter.

[0212] Based on the comparison results of the above EDS, image processing (binarization) with color change was performed only within the range of the above three values ​​shown by Al crystals contained in the Zn-Al-Mg alloy layer, in order to match with the backscattered electron image of SEM (e.g., displaying a white image only for a specific phase, thereby calculating the area (number of pixels) of each phase in the field of view, etc., see reference). Figure 4 By performing this image processing, the area fraction of Al crystals in the Zn-Al-Mg alloy layer in the backscattered electron image of SEM is determined.

[0213] Furthermore, Figure 4 This is an example of an image that has undergone image processing (binarization) of a backscattered electron image (SEM backscattered electron image) of a Zn-Al-Mg alloy layer in a way that can identify Al crystals. Figure 4 In the text, Al represents Al crystals.

[0214] Then, the area fraction of Al crystals in the Zn-Al-Mg alloy layer is set as the average of the area fractions of Al crystals obtained through the above operations in three fields of view.

[0215] Furthermore, when it is difficult to identify Al crystals, electron diffraction using TEM or point analysis using EDS should be performed.

[0216] As an example, a method for identifying Al crystals in SEM backscattered electron images (grayscale images stored in 8 bits and displayed in 256 colors) is described, using the binarization function of WinROOF 2015 (image analysis software) manufactured by Mitani Shoji, with two thresholds. Furthermore, in the 8-bit grayscale image, a light intensity of 0 represents black, and a maximum value of 255 represents white. In the previously described SEM backscattered electron images, if the light intensity thresholds are set to 10 and 95, it is determined from the identification results of FE-SEM and TEM that Al crystals can be identified with high precision. Therefore, by processing the image with colors changed within these light intensity ranges of 10 to 95, Al crystals can be identified. Furthermore, binarization processing can also be performed using image analysis software other than WinROOF 2015.

[0217] Next, using the automatic shape feature measurement function of WinROOF2015 (image analysis software) manufactured by Mitani Corporation, the perimeter of the Al crystal identified through the above image processing was accumulated to calculate the cumulative perimeter of the Al crystal. Then, the cumulative perimeter of the Al crystal was divided by the field of view area to calculate the perimeter per unit area (mm). 2 The cumulative perimeter of Al crystals.

[0218] This operation was performed in three fields of view, with each unit area (mm²) 2The arithmetic mean of the cumulative perimeter of Al crystals is taken as the "average of the cumulative perimeter of Al crystals".

[0219] Furthermore, the area fraction of Al crystals can also be determined using the automatic shape feature determination function of WinROOF2015 (image analysis software) manufactured by Mitani Corporation. Specifically, in the backscattered electron image of the Zn-Al-Mg alloy layer mentioned above, this function is used to calculate the area fraction of Al crystals (area fraction relative to the field of view area) identified by binarization. Then, this operation is performed in three fields of view, and their arithmetic mean is taken as the area fraction of Al crystals.

[0220] The thickness of the Al-Fe alloy layer is determined as follows.

[0221] After the sample was embedded in resin and ground, the thickness of the Al-Fe alloy layer was measured at any five points in the backscattered electron image of the coating cross-section (a cross-section of the coating along its thickness direction) using SEM (with a magnification of 5000x and a field of view of 50μm x 200μm). The arithmetic mean of the five measurements was then taken as the thickness of the Al-Fe alloy layer.

[0222] Next, an example of a method for manufacturing the plated steel of the present invention will be described.

[0223] The coated steel of the present invention can be obtained by forming a coating having the chemical composition and metal structure specified above on the surface (i.e., one or both sides) of a base steel material (base steel plate, etc.) by hot-dip galvanizing.

[0224] Specifically, as an example, hot-dip plating is performed under the following conditions.

[0225] First, the plating bath temperature is set to be 20°C above the plating bath melting point. After the base steel is lifted out of the plating bath, the temperature range from the plating bath temperature to the coating solidification start temperature is cooled at an average cooling rate that is faster than the average cooling rate of the temperature range from the coating solidification start temperature to -30°C.

[0226] Next, the temperature range from the coating solidification start temperature to -30°C is cooled at an average cooling rate of less than 12°C / s.

[0227] Next, the temperature range from -30°C to -300°C is cooled at an average cooling rate that is faster than the average cooling rate of the temperature range from -30°C to -300°C.

[0228] In other words, one example of the method for manufacturing coated steel according to the present invention is a method in which the plating bath temperature is set to the melting point of the plating bath +20°C or more, and after the base steel is lifted out of the plating bath, the base steel is hot-dip coated under the following conditions: the average cooling rate in the temperature range from the plating bath temperature to the solidification start temperature of the coating is set as A, the average cooling rate in the temperature range from the solidification start temperature of the coating to -30°C is set as B, and the average cooling rate in the temperature range from -30°C to -300°C is set as C.

[0229] The plating bath temperature is set to 20°C above the melting point of the plating bath, and the base steel is lifted out of the plating bath, thereby generating Al crystals.

[0230] Then, by cooling the Zn-Al-Mg alloy layer at an average cooling rate of 12°C / s or less in the temperature range from the coating solidification start temperature to -30°C, a metallic structure containing Al crystals and with an average cumulative perimeter of the Al crystals within the aforementioned range is formed. This average cooling rate can be achieved, for example, by air cooling with a weak airflow.

[0231] However, from the viewpoint of preventing the coating from winding onto the upper roller, the lower limit of the average cooling rate in the temperature range from the coating solidification start temperature to -30°C is set to 0.5°C / s or more.

[0232] Furthermore, the solidification start temperature of the coating can be determined as follows: A sample is collected from the plating bath. After heating the sample to above the melting point of the plating bath by DSC, the temperature at which the differential thermal peak first appears when cooling at a rate of 10°C / min is taken as the solidification start temperature of the coating.

[0233] In the method for manufacturing coated steel of the present invention, there is no particular limitation on the average cooling rate of the temperature range from the temperature at which the base steel is lifted from the plating bath (i.e., the plating bath temperature) to the temperature at which the coating begins to solidify. However, from the viewpoint of preventing the coating from rolling onto the upper roller or the like and suppressing appearance defects such as ripples, it is preferable to set it to 0.5°C / s to 20°C / s.

[0234] However, the average cooling rate of the temperature range from the plating bath temperature to the coating solidification start temperature is set to be faster than the average cooling rate of the temperature range from the coating solidification start temperature to -30°C. This increases the number of nucleation sites for Al crystals and suppresses excessive coarsening of the Al crystals.

[0235] Furthermore, there is no particular limitation on the average cooling rate in the temperature range from the coating solidification start temperature of -30°C to the coating solidification start temperature of -300°C, but from the viewpoint of preventing the coating from rolling onto the upper roller, it is advisable to set it to 0.5°C / s to 20°C / s.

[0236] However, the average cooling rate of the temperature range from the coating solidification start temperature of -30°C to the coating solidification start temperature of -300°C is set to be faster than the average cooling rate of the temperature range from the coating solidification start temperature to the coating solidification start temperature of -30°C. This suppresses excessive coarsening of Al crystals, thereby ensuring processability.

[0237] Furthermore, the Al-Fe alloy layer formed between the base steel and the Zn-Al-Mg alloy layer rapidly forms and grows within less than one second after immersion in the plating bath. Its growth rate is faster at higher plating bath temperatures and further increases with longer immersion times. However, if the plating bath temperature is below 500°C, growth is almost nonexistent; therefore, it is advisable to shorten the immersion time or immediately transition from solidification to the cooling process.

[0238] Furthermore, regarding coated steel, once the coating solidifies, it can be remelted by reheating, causing all constituent phases to disappear and the material to become a liquid phase. Therefore, even coated steel that has undergone quenching or similar processes can achieve the microstructure control specified in this invention through an appropriate heat treatment process using offline reheating. In this case, the reheating temperature of the coating is preferably set near the melting point of the plating bath, within a temperature range where the Al-Fe alloy layer will not grow excessively.

[0239] The following describes the post-processing of the plated steel that is applicable to the present invention.

[0240] The plated steel of the present invention can also form a film on the plating layer. One or more films can be formed. Examples of types of films directly above the plating layer include chromate films, phosphate films, and chromate-free films. The chromate treatment, phosphate treatment, and chromate-free treatment for forming these films can be performed using known methods.

[0241] As for chromate treatment, there are electrolytic chromate treatments that form a chromate film through electrolysis, reactive chromate treatments that form a film by reacting with the substrate and then rinsing off excess treatment solution, and coating-type chromate treatments that form a film by applying the treatment solution to the object to be coated and drying it without rinsing. Any of these treatments can be used.

[0242] Examples of electrolytic chromate treatments include those using chromic acid, silica sol, resins (acrylic resins, vinyl ester resins, vinyl acetate acrylic emulsions, carboxylated styrene-butadiene latex, diisopropanolamine-modified epoxy resins, etc.), and hard silica.

[0243] Examples of phosphate treatments include zinc phosphate treatment, calcium zinc phosphate treatment, and manganese phosphate treatment.

[0244] Chromate-free treatments are particularly environmentally friendly and have no negative impact on the environment. There are electrolytic chromate-free treatments, which form a chromate-free film through electrolysis; reactive chromate-free treatments, which form a film by reacting with the substrate and then rinsing off excess treatment solution; and coating-type chromate-free treatments, which apply the treatment solution to the workpiece, dry it without rinsing, and thus form a film. Any of these methods can be used.

[0245] Furthermore, one or more layers of organic resin film may be present on the film directly above the coating. The organic resin is not limited to a specific type; examples include polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, or modified forms of these resins. Here, "modified form" refers to a resin obtained by reacting reactive functional groups contained in the structure of these resins with other compounds (monomers or crosslinking agents, etc.) containing functional groups capable of reacting with those functional groups.

[0246] As such an organic resin, one or more unmodified organic resins can be used in combination, or one or more organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin can be used in combination. Additionally, the organic resin film may optionally contain coloring pigments or anti-rust pigments. Aqueous materials obtained by dissolving or dispersing in water can also be used.

[0247] (Example)

[0248] The following describes embodiments of the present invention. However, the conditions in these embodiments are merely examples used to confirm the feasibility and effectiveness of the invention, and the invention is not limited to these single examples. Various conditions can be used to achieve the objectives of the invention as long as they do not depart from its spirit.

[0249] (Example)

[0250] To obtain the coatings with the chemical compositions shown in Tables 1 and 2, the ingots are melted in a vacuum melting furnace using a specified amount of pure metal ingots, and then a plating bath is established in the atmosphere. An intermittent hot-dip galvanizing apparatus is used in the production of the coated steel sheets.

[0251] As the base steel, ordinary hot-rolled carbon steel plates with a thickness of 2.3mm (C concentration < 0.1%) are used, and degreasing and pickling are carried out before the plating process.

[0252] In addition, in several examples, pre-coated Ni steel was used as the base steel, consisting of ordinary hot-rolled carbon steel sheets with a thickness of 2.3 mm that had been pre-coated with Ni. The Ni coating amount was set to 1 g / m. 2 ~3g / m 2 Furthermore, examples of using pre-plated Ni steel as the base steel are recorded as "pre-plated Ni" in the "Base Steel" column of the table, and the Ni concentration in the plating bath is recorded in parentheses in the Ni concentration column.

[0253] Regardless of the sample preparation process, the base steel undergoes the same reduction treatment method up to the immersion in the plating bath. That is, in an environment of N2-H2 (5%) (dew point below -40°C, oxygen concentration below 25ppm), the base steel is heated from room temperature to 800°C by electric heating, held for 60 seconds, cooled to the plating bath temperature +10°C by blowing N2 gas, and then immediately immersed in the plating bath.

[0254] Furthermore, the immersion time in the plating bath for any coated steel sheet is set as shown in the table. The coated steel sheet is manufactured by adjusting the frictional contact pressure of N2 gas to achieve a coating thickness of 30 μm (±1 μm).

[0255] The plating bath temperature is based on the melting point +20°C, and the temperature is further increased by a certain percentage for plating. The immersion time in the plating bath is set to 2 seconds. After the base steel is removed from the plating bath, the coating is obtained by a cooling process in which the average cooling rate of sections 1 to 3 shown in Tables 1 and 2 is set to the conditions shown in Tables 1 and 2.

[0256] • Section 1 Average Cooling Rate: The average cooling rate over the temperature range from the plating bath temperature to the onset temperature of plating solidification.

[0257] • Second section: Average cooling rate: The average cooling rate in the temperature range from the coating solidification start temperature to 30°C below the coating solidification start temperature.

[0258] • Section 3 Average Cooling Rate: The average cooling rate in the temperature range from the coating solidification start temperature -30℃ to the coating solidification start temperature -300℃.

[0259] -Various measurements-

[0260] Samples were cut from the obtained coated steel sheet. Then, the following parameters were determined according to the methods already described.

[0261] • The average cumulative perimeter of Al crystals (referred to as "Al crystal perimeter" in the table).

[0262] • Area fraction of Al crystals

[0263] • Thickness of the Al-Fe alloy layer (wherein, in the example where a pre-plated Ni steel sheet is used as the base steel, the thickness of the Al-Ni-Fe alloy layer is indicated).

[0264] -Corrosion resistance of planar sections-

[0265] To compare the corrosion resistance of the planar sections, a 120-cycle accelerated corrosion test (JASOM 609-91) was conducted on the manufactured samples. White rust was removed by immersion in a 30% chromic acid aqueous solution at room temperature, and the corrosion resistance of the planar sections was evaluated by the reduction in corrosion. The test was performed five times, with an average corrosion reduction of 80 g / m². 2 The following values, where n=5, are rated "A+" if the maximum and minimum values ​​of corrosion reduction are within ±100% of the average value, and the average corrosion reduction is set at 100 g / m². 2 The following values, where n=5, are rated "A" if the maximum and minimum values ​​of corrosion reduction are within ±100% of the average value, and "NG" if they are otherwise.

[0266] -Sacrificial anode corrosion resistance (corrosion resistance of the cut end face)-

[0267] To compare the corrosion resistance of sacrificial anodes (corrosion resistance of the cut end face), samples were cut into 50mm × 100mm pieces, the upper and lower end faces were sealed, and 120 cycles of accelerated corrosion testing (JASO M609-91) were conducted. The average red rust infestation area of ​​the exposed end face on the side was evaluated. A red rust infestation area of ​​less than 50% was rated as "A+", less than 70% was rated as "A", and more than 70% was rated as "NG".

[0268] -Processability-

[0269] To evaluate the processability of the coating, the coated steel sheet was bent into a 90° V-shape, and 24mm wide cellophane tape was pressed onto the valley of the V-shape. The tape was then pulled apart, and the chalking was evaluated by visual inspection. No chalking or peeling powder adhered to the tape was rated "A", slight adhesion was rated "A-", and complete adhesion was rated "NG".

[0270] -Colorfastness-

[0271] To evaluate colorfastness, samples were cut into 50mm × 100mm pieces, and all samples were stacked with the burrs on the end faces facing the same direction. They were then wrapped in waterproof paper, with iron plates placed above and below the packaged samples, and the four corners of the iron plates secured with bolts and nuts. A load of 12 N·m was applied using a torque wrench when tightening the nuts. The samples were then inserted into a constant temperature and humidity bath (EYELA KCL-2000) at 50°C and 80% RH, and the color difference was evaluated after 7 days. Regarding color difference, the L value, a* value, and b* value of the samples were measured using a colorimeter (Konica Minolta Optics CR-400) before and after the test, and the color difference ΔE was investigated.

[0272] Then, a ΔE of 3 or less is rated as “A+”, a ΔE of more than 3 to less than 5 is rated as “A”, and a ΔE of more than 5 is rated as “NG”.

[0273] -Overall Evaluation-

[0274] Examples where all evaluation results for the corrosion resistance of the planar part, the corrosion resistance of the sacrificial anode, the processability evaluation, and the discoloration resistance evaluation are "A", are evaluated as "A", and even if only one example has "NG", it is evaluated as "NG".

[0275] Examples are shown in Tables 1 and 2.

[0276] Table 1-1

[0277]

[0278]

[0279]

[0280]

[0281]

[0282] The results above show that the embodiment of the plated steel of the present invention has stable corrosion resistance of the planar portion compared with the comparative example.

[0283] In particular, it was learned that the colorfastness of the comparative example (Test No. 50) with a Sn concentration exceeding 0.2% deteriorated.

[0284] Furthermore, it was learned that even if the chemical composition of the coating meets the requirements of the present invention, the average cumulative perimeter of the Al crystals in the comparative example (Test No. 71) which did not change the average cooling rate at 15°C / s increased excessively, and stable corrosion resistance of the planar portion was not obtained.

[0285] On the other hand, it was found that the average cooling rate of the second stage was too low (Comparative Example No. 72), the average cooling rate was changed only in the second stage (Test No. 73), and the average cooling rate was not changed at 6℃ / s (Test No. 74). The average cumulative perimeter of the Al crystals was excessively reduced, and the processability deteriorated.

[0286] Furthermore, regarding the Ni concentration of the coating in the examples using pre-plated Ni steel sheets (Experiments No. 41-44), since the Ni concentration detected by ICP analysis was greater than 0.28% and less than 15%, it is equivalent to considering the Ni concentration of the coating as an example of 0%.

[0287] The above description, with reference to the accompanying drawings, details suitable embodiments of the present invention, but the present invention is not limited to such examples. It is obvious that anyone skilled in the art to which this invention pertains can conceive of various modifications or alterations within the scope of the technical concept described in the patent claims, and these should also be understood to fall within the technical scope of this invention.

[0288] The symbols are explained as follows:

[0289] Al: Al crystal

[0290] Zn-Al: Zn-Al phase

[0291] MgZn2:MgZn2 phase

[0292] Zn-Eu: Zn-based eutectic phase

[0293] Furthermore, the entire disclosure of Japanese Patent Application No. 2019-205998 is incorporated herein by reference.

[0294] All documents, patent applications and technical standards described in this specification are incorporated herein by reference to the same extent as those specifically described and individually referenced in this specification.

Claims

1. A plated steel material, comprising a base steel material and a coating containing a Zn-Al-Mg alloy layer disposed on the surface of the base steel material, wherein, The chemical composition of the coating, expressed in % by mass, contains: Zn: Over 65.0% A1: Above 5.0% to below 25.0% Mg: 3.5% to less than 12.5% Sn: 0 to below 0.10% Bi: 0% to less than 5.0%, In: 0% to less than 2.0% Ca: 0%–3.0% Y：0％～0.5％、 La: 0% to less than 0.5% Ce: 0% to less than 0.5% Si: 0% to less than 2.5% Cr：0％～0.25％、 Ti: 0%~0.25% Ni: 0%~0.25% Co: 0%–0.25% V：0％～0.25％、 Nb: 0%–0.25% Cu: 0%–0.25% Mn: 0%~0.25% Fe: 0%–5.0% Sr: 0% to less than 0.5% Sb: 0% to less than 0.5% Pb: 0% to less than 0.5% B: 0% to less than 0.5%, and Impurities After grinding the surface of the Zn-Al-Mg alloy layer to half its thickness, Al crystals were observed in the backscattered electron image of the Zn-Al-Mg alloy layer obtained by scanning electron microscopy at 100x magnification. The average cumulative perimeter of the Al crystals was 88–195 mm / mm. 2 .

2. The plated steel according to claim 1, wherein, The Al content, expressed as a percentage by mass, is 10.0% to less than 25.0%.

3. The plated steel according to claim 1 or 2, wherein, The Mg content, expressed as a percentage by mass, is greater than 5.0% and less than 12.5%.

4. The plated steel according to claim 1 or 2, wherein, The coating has an Al-Fe alloy layer with a thickness of 0.05 to 5 μm between the base steel and the Zn-Al-Mg alloy layer.

5. The plated steel according to claim 3, wherein, The coating has an Al-Fe alloy layer with a thickness of 0.05 to 5 μm between the base steel and the Zn-Al-Mg alloy layer.

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