WO2024219123A1 - Hot-dip plated steel material - Google Patents

Abstract

This hot-dip plated steel material comprises a plating layer having the chemical composition of more than 10.0% and less than 45.0% of Al, 4.0 to 15.0% of Mg, 0.01 to 2.0% of Si, and at least one of 0.03 to 5.0% of Cu and 0.03 to 6.0% of Ag with the balance of Zn and impurities. In the element distribution profile in GDS analysis from the plating layer toward the steel material, CAZ_max, the maximum value of CAZ, and CAZ_min, the minimum value of CAZ, satisfy the following formulas (1) to (3), when the ratio of the total concentration of the Cu concentration and the Ag concentration to the Zn concentration in the internal region of the plating layer is defined as CAZ. (1): CAZ_max/CAZ_min≤1.20; (2): 0.0005≤CAZ_min; (3): CAZ_max≤0.1000

Description

Hot-dip galvanized steel

The present invention relates to a hot-dip galvanized steel material.
This application claims priority based on Japanese Patent Application No. 2023-067061, filed on April 17, 2023, the contents of which are incorporated herein by reference.

When using steel for a long period of time, it is preferable to apply some kind of rust-proofing treatment to the steel to make it resistant to corrosion. Hot-dip galvanized plating is used as an inexpensive method of rust-proofing steel in a variety of fields where rust prevention is required, such as civil engineering, construction, and automotive.

In terms of corrosion protection of the plating layer, one important issue is the suppression of the amount of white rust that occurs. When a Zn-based plating layer corrodes, it reacts with oxygen in the air to produce corrosion products. Since white rust in the plating layer can easily lead to deterioration of the appearance, it is preferable to prevent it from occurring as much as possible. Furthermore, once a corroded area increases in surface area, the corroded area becomes the starting point for further corrosion, and the corrosion progresses rapidly thereafter. In general, Zn-based plating layers have a sacrificial corrosion protection effect, so even if scratches that reach the base steel and corrosion of the plating layer occur to a certain extent, this does not cause any major problems for the material. However, as mentioned above, the issue of deterioration of appearance due to white rust has been confirmed.

On the other hand, in contrast to such Zn-based plating layers, plating layers containing elements with a higher ionization tendency than Fe, such as Sn, Ni, and Cr, are called barrier-type coatings, and since they do not have a sacrificial corrosion protection effect against Fe, any scratches or other damage can easily become fatal material defects. However, such barrier-type coatings can maintain their metallic luster for a long period of time, and because this metallic luster is maintained even for a certain period after manufacture, they can maintain an appearance quality that gives little sign of use. Therefore, it is believed that by adjusting the ionization tendency of Zn-based plating layers to bring them closer to the potential of Fe, it is possible to reduce the amount of white rust that occurs and improve the appearance of corrosion.

For example, Patent Documents 1 and 2 describe Zn-Al-Mg-based plating layers that have been used in recent years as highly corrosion-resistant plating. These Zn-Al-Mg-based plating layers improve their design and corrosion resistance through structural control, and also disclose techniques for improving corrosion resistance by adding elements to the plating layer or actively forming corrosion products.

International Publication No. 2019/230894 International Publication No. 2018/139619

However, in the case of conventional Zn-Al-Mg-based plating layers as described in Patent Documents 1 and 2, the method of adding elements to the final plating layer and the method of controlling its ionization tendency have not been fully considered, and the suppression of white rust that occurs in the early stages of corrosion has been insufficient in terms of improving appearance and suppressing the initiation of corrosion.

The problem that one embodiment of the present invention aims to solve is to provide a hot-dip galvanized steel material that can suppress the white rust that occurs in the early stages of corrosion.

In order to solve the above problems, each aspect of the present invention employs the following configuration.
[1] A hot-dip plated steel material according to one aspect of the present invention is a hot-dip plated steel material having a steel material and a plating layer disposed on a surface of the steel material,
The plating layer comprises, in mass %,
Al: more than 10.0% and less than 45.0%;
Mg: 4.0% or more, 15.0% or less,
Si: 0.01% or more, 2.0% or less,
At least one of Cu of 0.03% or more and 5.0% or less and Ag of 0.03% or more and 6.0% or less;
and further comprising
Sn: 0% or more, 0.7% or less,
Bi: 0% or more, 0.3% or less,
In: 0% or more, 0.3% or less,
Ca: 0% or more, 0.6% or less,
Y: 0% or more, 0.3% or less,
La: 0% or more, 0.3% or less,
Ce: 0% or more, 0.3% or less,
Sr: 0% or more, 0.3% or less,
Li: 0% or more, 0.3% or less,
Ni: 0% or more, 1.0% or less,
Cr: 0% or more, 0.5% or less,
Mo: 0% or more, 0.3% or less,
Sb: 0% or more, 0.25% or less,
Pb: 0% or more, 0.25% or less,
B: 0% or more, 0.5% or less,
P: 0% or more, 0.5% or less,
Ti: 0% or more, 0.25% or less,
Co: 0% or more, 0.25% or less,
V: 0% or more, 0.25% or less,
Nb: 0% or more, 0.25% or less,
Mn: 0% or more, 0.25% or less,
Zr: 0% or more, 0.25% or less,
W: 0% or more, 0.25% or less,
Fe: 0% or more, 5.0% or less,
The balance includes Zn and impurities,
The total content of Cu and Ag is 0.03% or more and 6.0% or less.
In an element distribution profile obtained by quantitatively analyzing the plating layer from the surface toward the steel material by glow discharge optical emission spectrometry, when the thickness of the plating layer is t and the ratio of the total concentration of Cu concentration and Ag concentration to the Zn concentration in an internal region between a 1/3t position and a 2/3t position with respect to the surface of the plating layer is CAZ, a maximum value CAZ _max of the CAZ and a minimum value CAZ _min of the CAZ satisfy the following formulas (1) to (3):
CAZ _max /CAZ _min ≦1.20…(1)
0.0005≦CAZ _min …(2)
CAZ _max ≦0.1000…(3)
[2] In the hot-dip plated steel material described in [1] above, in a cross section along a thickness direction of the plating layer, an area ratio of a β phase is 3% or more, and each concentration of Al, Zn, Cu and Ag in the β phase may satisfy the following formula (4).
0.85≦[Al]/([Zn]+[Cu]+[Ag])≦1.15…(4)
Here, [Al], [Zn], [Cu] and [Ag] in formula (4) are quantitative analysis values (at %) in the β phase by energy dispersive X-ray analysis.
[3] The hot-dip plated steel material according to the above [1] or [2] may contain, as intermetallic compound particles, 10 or more (Zn+Al)-(Cu+Ag) compounds having an average grain size of 1 μm or more within an area of 10,000 μm2 in a cross section along the thickness direction of the plating ^layer .

According to one embodiment of the present invention, it is possible to provide a plated steel material that can suppress the white rust that occurs in the early stages of corrosion.

FIG. 2 is a graph showing an example of an element distribution profile, showing the results of a GDS analysis performed on a coating layer of a hot-dip coated steel material according to an embodiment of the present invention.

Hereinafter, a hot-dip plated steel material according to one embodiment of the present invention will be described.
In this specification, the "%" designation for the content of each element in the chemical composition of the plating layer means "mass %" unless otherwise specified.
Furthermore, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. When the numerical values before and after "to" are followed by "more than" or "less than," the numerical range does not include these numerical values as the lower or upper limit.

In this specification, "corrosion resistance" refers to the property of the plating layer itself being resistant to corrosion. A Zn-based plating layer has a sacrificial corrosion protection effect on steel material. Therefore, in the process of corrosion of a plated steel sheet, first, the plating layer corrodes and turns into white rust before the steel material corrodes, the white-rusted plating layer disappears, and then the steel material corrodes and red rust appears.
In addition, "sacrificial corrosion protection" as used in this specification refers to the property of suppressing corrosion of steel at exposed portions of the steel (for example, cut end surfaces of plated steel or portions where the steel is exposed due to cracking of the hot-dip plating layer during processing).

First, the results of the inventors' research into means for suppressing the occurrence of white rust will be explained. Specifically, the inventors have intensively researched means for suppressing the occurrence of white rust in the early stages of corrosion on Zn-Al-Mg-plated steel materials.

First, the corrosion morphology of the Zn-Al-Mg plating layer was investigated. As a result, it was found that corrosion of the Zn-Al-Mg plating layer progresses from the Zn-Al (β) phase. Furthermore, it was found that the reason for this is that this Zn-Al (β) phase shows the electrochemically most base corrosion potential in the plating layer (-1.3 V vs. Ag/AgCl in a 1M NaCl aqueous solution).

Furthermore, it was confirmed that when corrosion progresses in the Zn-Al-Mg-based plating layer, the MgZn ₂ phase corrodes after the Zn-Al (β) phase. The corrosion potential of the MgZn ₂ phase was (-1.1 V vs. Ag/AgCl in a 1M NaCl aqueous solution). The β phase is distributed throughout the plating layer, and increasing the potential of the Zn-Al (β) phase, which is a site susceptible to corrosion, is effective in improving the appearance and corrosion resistance.

In order to increase the potential of the β phase, it is preferable for an electrochemically noble element to be contained in the Zn-Al (β) phase. Elements that are effective in increasing the potential of the β phase include Cu and Ag. This is because Cu and Ag have atomic radii close to those of Al and Zn, respectively, and exhibit substitutional solid solutions, making them easily mixed with each other.

The following two-stage plating method is an example of a suitable method for incorporating Cu or Ag into the Zn-Al (β) phase in the plating layer.

The base plate is first electroplated with Zn, and then electroplated with Cu or Ag. The Cu or Ag may be an alloy of these. The base plate plated with Zn, Cu or Ag is then heated to diffuse the Cu and/or Ag into the Zn plating layer to form a base plate. The base plate is then immersed in a Zn-Al-Mg hot-dip plating bath (which may contain Cu and Ag) to form a hot-dip plating layer. In this specification, this method of forming a predetermined pre-plating layer on the base plate and then applying Zn-Al-Mg hot-dip plating in this order is referred to as the "two-stage plating method."

In addition, during plating solidification, the Zn-Al (β) phase is formed and grows in a specific temperature range. Therefore, in such a temperature range, by reducing the cooling rate, the Zn-Al (β) phase can be sufficiently formed, and the Zn-Cu phase that has formed on the plating substrate surface can be dissolved and incorporated into the Zn-Al (β) phase.

In this way, in the plating layer containing Cu and Ag formed, the Zn atom positions are almost in the same positions as the Cu and Ag atoms, and these atoms are different from the Al component distribution in the plating layer.

In addition, in the plating layer according to this embodiment, Cu and Ag, which efficiently exhibit a noble potential, are contained in the Zn-Al (β) phase, which is the most susceptible portion to corrosion, and therefore the corrosion potential of the β phase is increased. This makes the potential distribution of the entire plating layer almost uniform with the MgZn ₂ phase. As a result, the potential difference in a specific portion is reduced, and the amount of white rust generated in the early stages of corrosion in the plated steel material is reduced. Furthermore, even if white rust is generated, it is generated uniformly, making it possible to make the generation of white rust itself less noticeable.

[Hot-dip plated steel]
The hot-dip plated steel material according to this embodiment will be described in detail below.
The hot-dip plated steel material of the present embodiment is a hot-dip plated steel material having a steel material and a plating layer disposed on the surface of the steel material. The average chemical composition of the plating layer is, in mass%,
Al: more than 10.0% and less than 45.0%;
Mg: 4.0% or more, 15.0% or less,
Si: 0.01% or more, 2.0% or less,
At least one of Cu of 0.03% or more and 5.0% or less and Ag of 0.03% or more and 6.0% or less;
and further comprising
Sn: 0% or more, 0.7% or less,
Bi: 0% or more, 0.3% or less,
In: 0% or more, 0.3% or less,
Ca: 0% or more, 0.6% or less,
Y: 0% or more, 0.3% or less,
La: 0% or more, 0.3% or less,
Ce: 0% or more, 0.3% or less,
Sr: 0% or more, 0.3% or less,
Li: 0% or more, 0.3% or less,
Ni: 0% or more, 1.0% or less,
Cr: 0% or more, 0.5% or less,
Mo: 0% or more, 0.3% or less,
Sb: 0% or more, 0.25% or less,
Pb: 0% or more, 0.25% or less,
B: 0% or more, 0.5% or less,
P: 0% or more, 0.5% or less,
Ti: 0% or more, 0.25% or less,
Co: 0% or more, 0.25% or less,
V: 0% or more, 0.25% or less,
Nb: 0% or more, 0.25% or less,
Mn: 0% or more, 0.25% or less,
Zr: 0% or more, 0.25% or less,
W: 0% or more, 0.25% or less,
Fe: 0% or more, 5.0% or less,
The balance includes Zn and impurities,
The total content of Cu and Ag satisfies 0.03% or more and 6.0% or less.

(Steel)
First, the steel material (original sheet) to be plated will be described.
The steel material is, for example, mainly a steel plate, but the size is not particularly limited. The steel plate may be any steel plate that can be applied to a normal hot-dip galvanizing process. Specifically, this applies to steel plates that can be applied to a process in which the steel plate is immersed in molten metal and solidified, such as a continuous hot-dip galvanizing line (CGL). The size of the steel plate may be, for example, a plate thickness of 10 mm or less and a plate width of 2000 mm or less, but the size of the steel plate is not limited to this.

The quality of the steel material is not particularly limited. Examples of applicable steel materials include general steel, pre-plated steel thinly plated with various metals, Al-killed steel, ultra-low carbon steel, high carbon steel, various high tensile steels, some high alloy steels (steels containing corrosion-resistant strengthening elements such as Ni and Cr), steel for bolts, and steel wire for bridge cables. More specifically, the steel material may be, for example, a hot-rolled steel plate defined in JIS G 3131 (2018), a cold-rolled steel plate defined in JIS G 3141 (2017), a general structural rolled steel material corresponding to the so-called SS material, a so-called general steel included in the hot-rolled steel plate shown in JIS G 3193 (2019), JIS H 8641 (2021), JIS G 3302 (2019), 3303 (2017), 3313 (2017), 3314 (2019), 3315 (2017), 3316 (2016), 3317 (2017), 3318 (2017), 3319 (2017), 3320 (2017), 3321 (2017), 3322 (2017), 3323 (2017), 3324 (2017), 3325 (2017), 3326 (2017), 3327 (2017), 3328 (2017), 3329 (2017), 3330 (2017), 3331 (2017), 3332 (2017), 3333 (2017), 3334 (2017), 3335 (2017), 3336 (2017), 3337 (2017), 3338 (2017), 3339 (2017), 334 ... Pre-plated steels thinly plated with various metals as described in JIS G 317 (2019) and 3321 (2019), rolled steel materials for building structures as described in JIS G 3136 (2012), Al-killed steels, extra-low carbon steels, high carbon steels as described in JIS G 3126 (2015), various high tensile steels as described in JIS G 3113 (2018), 3134 (2018), and 3135 (2018), and some high alloy steels (steels containing corrosion-resistant strengthening elements such as Ni and Cr, etc.) are applicable.

Furthermore, the manufacturing process for steel materials includes common processes such as iron and steel making using blast furnaces or electric furnaces, hot rolling, pickling, cold rolling, and heat treatment.

(Plating layer)
Next, the plating layer provided on the steel material will be described.
The plating layer according to the present embodiment includes a Zn-Al-Mg alloy layer. When alloy elements such as Al and Mg are contained in the Zn phase, the corrosion resistance is improved. Therefore, in the case of a plating layer including such a Zn phase, even if it is a thin film (for example, about half the thickness of a normal Zn plating layer), it can exhibit the same corrosion resistance as a normal Zn plating layer. Similarly, even if the plating layer according to the present embodiment is a thin film, it can ensure the same or higher corrosion resistance as a conventional Zn plating layer.

The Zn-Al-Mg alloy layer is made of a Zn-Al-Mg alloy. A Zn-Al-Mg alloy refers to a ternary alloy containing Zn, Al, and Mg.

The plating layer may also include an Al-Fe interfacial alloy layer (however, the thickness is less than 5 μm). The Al-Fe interfacial alloy layer is an interfacial alloy layer between the steel material and the Zn-Al-Mg alloy layer, and is in contact with the surface of the steel material. In other words, the plating layer of this embodiment may have a single-layer structure composed of a Zn-Al-Mg alloy layer, or may have a laminated structure including a Zn-Al-Mg alloy layer and an Al-Fe interfacial alloy layer. In the case of a laminated structure, it is preferable that the Zn-Al-Mg alloy layer is a layer that constitutes the surface of the plating layer.

The Al-Fe interface alloy layer does not have a significant effect on corrosion resistance, but it does affect the adhesion of the plating layer during processing of hot-dip plated steel and the workability (presence or absence of cracks). In particular, the Al-Fe interface alloy layer may affect powdering resistance, which indicates the degree of peeling of the plating layer during processing. Usually, the thinner the Al-Fe interface alloy layer, the fewer the starting points for crack generation in the plating layer during processing, and the better the powdering resistance. For this reason, in hot-dip plated steel that may be subjected to high processing when used as a component, etc., it is preferable that the thickness of the Al-Fe interface alloy layer is as thin as possible. Specifically, the thickness of the intermetallic compound that constitutes the Al-Fe interface alloy layer is less than 5 μm. This thickness is preferably 2 μm or less, more preferably 1 μm or less, and even more preferably 0.5 μm or less. It may be 0.3 μm or less. This makes it possible to suppress the generation of cracks during processing and further improve powdering resistance. Furthermore, the ratio of the thickness of the Al-Fe interfacial alloy layer to the thickness of the plating layer is, on average, less than 10%, and more preferably, less than 5%.

The Al-Fe-based interface alloy layer is formed on the surface of the steel material, specifically, between the steel material and the Zn-Al-Mg-based alloy layer. The Al-Fe-based interface alloy layer is a layer in which the Al ₅ Fe ₂ phase is the main phase in the structure. The Al-Fe-based interface alloy layer is formed by mutual atomic diffusion between the base steel (steel sheet) and the plating bath. When a continuous hot-dip plating method is used as the manufacturing method, the Al-Fe-based interface alloy layer is likely to be formed in the plating layer containing the Al element. In this embodiment, since the plating bath contains Al at a certain concentration or more, the Al ₅ Fe ₂ phase is formed most frequently in the Al-Fe-based interface alloy layer. However, since atomic diffusion takes time, the Fe concentration in the Al-Fe-based interface alloy layer is not uniform, and the Fe concentration may be high in the part close to the base steel. Therefore, the Al-Fe-based interface alloy layer may partially contain small amounts of the AlFe phase, the Al ₃ Fe phase, the Al ₅ Fe ₂ phase, etc. In addition, since the plating bath contains a certain concentration of Zn, the Al-Fe-based interface alloy layer may also contain a small amount of Zn. In addition, the Al-Fe-based interface alloy layer may also contain a small amount of Si, which tends to accumulate at the interface.

In this embodiment, the plating layer contains Si. A portion of the Si is incorporated into the Al-Fe-based interface alloy layer to form an Al-Fe-Si intermetallic compound phase. The intermetallic compound phase identified is the AlFeSi phase. Isomers of the AlFeSi phase include the α phase, β phase, q1 phase, q2 phase, and the like. Therefore, these AlFeSi phases and the like may be detected in the Al-Fe-based interface alloy layer. An Al-Fe-based interface alloy layer that contains these AlFeSi phases and the like is also referred to as an Al-Fe-Si alloy layer.

The upper and lower limits of the thickness of the entire plating layer are not particularly limited. The thickness is also affected by the withdrawal speed of the steel from the plating bath and the wiping conditions. That is, in the case of continuous hot-dip plating, the thickness of the entire plating layer is affected by the viscosity and specific gravity of the plating bath. The maximum thickness of the plating layer formed by the continuous hot-dip plating method is often 100 μm or less. Therefore, the plating thickness of the hot-dip plated steel of this embodiment may be, for example, 100 μm or less.

Next, the average chemical composition of the plating layer will be explained. When the plating layer has a single-layer structure of a Zn-Al-Mg alloy layer, the average chemical composition of the entire plating layer is the average chemical composition of the Zn-Al-Mg alloy layer. When the plating layer has a layered structure consisting of an Al-Fe interface alloy layer and a Zn-Al-Mg alloy layer, the average chemical composition of the entire plating layer is the average chemical composition of the Al-Fe interface alloy layer and the Zn-Al-Mg alloy layer combined.

In the plating layer of this embodiment, the thickness of the Al-Fe interfacial alloy layer is preferably 10% or less of the total thickness of the plating layer. In this way, when the thickness of the Al-Fe interfacial alloy layer is sufficiently small compared to the total thickness of the plating layer, the Fe concentration of the plating layer is often within 5%. Therefore, the average chemical composition of the plating layer is generally approximately the same as the components of the Zn-Al-Mg alloy layer. Furthermore, traces of the original plating material are unlikely to remain as chemical components of the plating layer. Therefore, the average chemical composition of the plating layer is approximately the same as the components of the plating bath used in production.

Al: more than 10.0% and less than 45.0%;
Al is an element that mainly constitutes the plating layer. If the Al content is 10% or less, a sufficient amount of Zn-Al phase may not be secured. Therefore, the Al content is more than 10%. On the other hand, if the Al content is 45.0% or more, the plating layer will be mainly composed of Al-Zn (α) phase, and Al-Zn (β) phase will not be formed. Therefore, the upper limit of the Al content is less than 45%.

Mg: 4.0% or more, 15.0% or less Like Zn, Mg is an element constituting the main part of the plating layer. Mg is an important element for improving the sacrificial corrosion protection in the plated steel sheet according to the present embodiment. If the Mg content in the plating layer is less than 4.0%, the effect of improving the sacrificial corrosion protection is not clear compared to the case where Mg is not contained. Therefore, the Mg content is set to 4.0% or more. On the other hand, if Mg is added excessively to a Zn-Al-Mg plating bath, a rapid oxidation reaction occurs on the bath surface of the plating bath, and plating cannot be performed stably. Therefore, in order to perform stable plating and ensure good manufacturability, the Mg content in the plating layer is set to 15% or less.

Si: 0.01% or more, 2.0% or less Si suppresses the Al-Fe reaction, thereby suppressing the formation of an Al-Fe-based interface alloy layer. In addition, Si is incorporated into a part of the Al-Fe-based interface alloy layer to form an Al-Fe-Si compound. If Si is not contained, the Al-Fe reaction becomes active, the thickness of the Al-Fe alloy layer becomes thick, powdering occurs during processing, and corrosion resistance is significantly impaired. On the other hand, if the Si content is 0.01% or more, the growth rate of the thickness of the interface alloy layer becomes slow. However, if the Si content is 2.0% or more, a large amount of intermetallic compound having a composition of Mg2Si is formed by bonding with Mg, and the viscosity of the plating bath becomes extremely high, so that the amount of molten metal attached to the steel material is reduced when the steel material is pulled out of the plating bath, and the thickness of the plating layer becomes extremely thin. In addition, the plating appearance is significantly deteriorated. For this reason, the upper limit of the Si content is set to 2.0% or less. The preferred range is 0.10 to 0.40%, and more preferably 0.20 to 0.30%. If the Si content is 2.0% or less, almost no Mg2Si is formed.

Cu and Ag have atomic radii close to those of Zn and Al, so they are easily substituted and mixed with these elements in the plating layer. When Cu or Ag is contained in the plating layer, the potential of the Zn-Al (β) phase increases, improving corrosion resistance. Cu and Ag have almost the same effect. In order to change the potential and change the behavior of the initial white rust generation, it is effective to contain at least one of Cu and Ag in the plating layer at 0.03% or more. Preferably, the total concentration of Cu and Ag in the plating layer is 0.4% or more. On the other hand, if the content of each of Cu and Ag is excessive, electrically noble parts may be generated by Cu and Ag that cannot be contained in the β phase, which may promote corrosion. Therefore, the upper limit of the Cu content is 5.0% or less, and the upper limit of the Ag content is 6.0% or less. In addition, when both Cu and Ag are contained, the upper limit of the total content of Cu and Ag is 6%. When the total Cu and Ag content reaches 6.0%, the Cu and Ag that cannot be contained in the β phase generate electrically noble parts, promoting corrosion.

Sn: 0% or more, 0.7% or less Bi: 0% or more, 0.3% or less In: 0% or more, 0.3% or less Sn, Bi, and In are elements that promote the softening of the plating layer by being contained in the plating layer. Since Sn, Bi, and In are elements that can be contained arbitrarily, the content of each is 0% or more. When Sn is contained, Mg ₉ Sn ₅ tends to be formed in the plating layer. Bi forms Mg ₃ Bi ₂ , and In forms Mg ₃ In, etc. These elements are softer than the MgZn ₂ phase, have good workability, and are elements that can clearly confirm the improvement of workability by being contained in the plating layer. In addition, these elements have a very base electrochemical property, so they have a high sacrificial anticorrosion effect. By containing at least one of Sn, Bi, and In, the effect of improving the corrosion resistance of the processed part can be obtained.

Ca: 0% or more, 0.6% or less Y: 0% or more, 0.3% or less La: 0% or more, 0.3% or less Ce: 0% or more, 0.3% or less Sr: 0% or more, 0.3% or less Li: 0% or more, 0.3% or less Ni: 0% or more, 1.0% or less Cr: 0% or more, 0.5% or less Mo: 0% or more, 0.3% or less Sb : 0% or more, 0.25% or less Pb: 0% or more, 0.25% or less B: 0% or more, 0.5% or less P: 0% or more, 0.5% or less Ti: 0% or more, 0 .25% or less Co: 0% or more, 0.25% or less Nb: 0% or more, 0.25% or less Mn: 0% or more, 0.25% or less Zr: 0% or more, 0.25% or lessW 0% or more, 0.25% or less Ca, Y, La, Ce, Sr, Li, Ni, Cr, Mo, Sb, Pb, B, P, Ti, Co, V, Nb, Mn, Zr, W All of these elements form intermetallic compounds with Si, Zn, Al, etc. However, when the contents of these elements are within the above ranges, they do not affect the initial corrosion of the plating layer. If an element is contained in excess, a potential difference occurs in the plating layer, which may result in a large amount of initial white rust. Therefore, if an element is contained, it is preferable that the amount is within the above range.

Fe: 0% or more, 5.0% or less Since the hot-dip plated steel material of this embodiment is manufactured by a continuous hot-dip plating method, Fe may diffuse from the plated base material to the plated layer during manufacturing. As described above, in this embodiment, the Al concentration of the plated layer is high, and an Al-Fe-based interface alloy layer may be formed, but its thickness is thin. As a result, the plated layer may contain Fe up to a maximum of 5.0%, but as long as the Fe concentration is limited to 5.0% or less, there is no effect on the frequency of cracks in the plated layer. Therefore, the Fe content is set to 0 to 5.0%. The Fe content may be more than 0%.

Balance: Zn and impurities The balance preferably contains Zn. Since the hot-dip plated steel material of the present embodiment is a highly versatile Zn-based plated steel material, the element constituting the main phase of the plated layer is Zn.

Impurities refer to components contained in raw materials or components mixed in during the manufacturing process, but not intentionally included. For example, trace amounts of components other than Fe may be mixed into the plating layer as impurities due to atomic diffusion between the steel (base steel) and the plating bath. Also, since metals with a purity of 3N are usually used to manufacture plating alloys, the concentration of impurities may be approximately 0.03% or less in total.

To identify the average chemical composition of the plating layer, the plating layer is stripped and dissolved using an acid containing an inhibitor that suppresses corrosion of the base steel (steel material) to obtain an acid solution. The resulting acid solution is then measured using ICP emission spectroscopy or ICP-MS to obtain the chemical composition. There are no particular limitations on the type of acid, so long as it is an acid that can dissolve the plating layer. If the area and weight are measured before and after stripping, the plating coverage (g/ ^m2 ) can also be obtained at the same time.

Next, the distribution of components of the plating layer with respect to the depth will be described.
Glow discharge optical emission spectrometry (GDS) is used to grasp the component distribution in the depth direction of the plating layer. On the other hand, GDS is effective for grasping the components in a relatively wide range of diameter of several mm or more, and the interface position varies by definition depending on the sputtering speed, etc. In this embodiment, the surface of the plating layer is set as the starting point (zero point), and the interface position of the plating layer is defined as the position where the Fe concentration shows a quantitative analysis value of 5 mass% according to the Fe concentration distribution. Details of the GDS analysis conditions, etc. will be described later.

In some cases, FeAl-based compounds may occur in the interface alloy layer, but in this case the interface is located on the interface alloy layer. The above definition allows us to understand the components focusing only on the plating layer portion of the Zn-Al-Mg alloy layer, excluding the interface alloy layer. Furthermore, if the thickness of the plating layer is t, the center of the plating layer can be considered to be 1/3t to 2/3t of the plating layer thickness t. In other words, the range of 1/3t to 2/3t is the main part of the plating layer. An overview is shown in Figure 1.

In the plating layer of this embodiment, the Cu and Ag elements mostly bond with or replace Zn in the depth direction, so they have a similar distribution. On the other hand, for example, in the case of a plating layer formed by adding Cu and Ag elements to a plating bath, the distribution of Cu and Ag elements will be close to that of Al, and there may be cases where no correlation with Zn is observed.

The corrosion potential of the plating layer increases when electrochemically noble Cu or Ag occupies the electrochemically less noble Zn site. In other words, the inventors have found that when Cu or Ag occupies the same site as Zn, it can be considered that Cu or Ag is contained in the Zn-Al (β) phase or is substituting for Zn in the Zn-Al (β) phase, and that the more constant the proportion (degree) of inclusion or substitution, the higher the potential of the entire plating layer.

The degree of substitution of Zn with Cu and/or Ag (CAZ) can be expressed by the following formula (5) in the GDS component analysis.

CAZ=[Cu+Ag]/[Zn]...(5)
In the formula (5), [Cu], [Ag], and [Zn] are quantitative component analysis values (mass%) in the depth direction in GDS.

"CAZ" is the ratio of the total concentration of Cu and Ag to the Zn concentration in the internal region between the 1/3t position and the 2/3t position on the surface of the plating layer. The larger this CAZ value is, the more substitution has progressed.

In addition, in a plating layer, when the maximum CAZ value CAZ _max and the minimum CAZ value CAZ _min satisfy the following formula (1), an increase in the corrosion potential of the plating layer occurs throughout the plating layer, and the amount of white rust generated on the plated steel sheet tends to be reduced. This is because Cu and/or Ag are uniformly distributed in the plating layer. CAZ _max /CAZ _min is preferably 1.1 or less.

CAZ _max /CAZ _min ≦1.20…(1)

In order to sufficiently increase the potential, it is necessary to satisfy the following formula (2). On the other hand, an excessive increase in corrosion potential will promote corrosion. Therefore, CAZ _max needs to satisfy the following formula (3). More preferably, 0.01≦CAZ _min , CAZ _max ≦0.08. This allows the corrosion potential to be approximately the same as that of the MgZn ₂ phase present in large amounts in the plating layer, reducing the occurrence of white rust due to corrosion and improving the appearance.

0.0005≦CAZ _min …(2)
CAZ _max ≦0.1000…(3)

Increasing the potential of the Zn-Al phase (β phase) is effective in preventing deterioration of appearance due to the formation of white rust in the plating layer. In addition, increasing the potential to reduce the potential difference with the surrounding intermetallic compounds and metal phases, as well as increasing the electrical resistance of the β phase itself, slows the progression of corrosion and improves corrosion resistance. This tendency can be evaluated by the corrosion current density, and can be achieved by keeping the replacement of Al and Zn in the β phase constant. As the β phase has a size of approximately 5 μm or more in the plating layer, its presence can be easily confirmed with SEM. Quantitative analysis values for that area can also be obtained using EDS, EPMA, etc.

The ratio of Zn to Al in the β phase (Zn:Al) is approximately 1:1, and within this range, the β phase can exist with excellent corrosion resistance. In addition, it is preferable that the following formula (4) is satisfied at any point or in any region within the β phase. By satisfying the following formula (4), the electrical resistance of the β phase increases, and corrosion resistance can be further improved.

0.85≦[Al]/([Zn]+[Cu]+[Ag])≦1.15 (4)
Here, [Al], [Zn], [Cu] and [Ag] in formula (4) are quantitative analysis values (at %) in the β phase by energy dispersive X-ray analysis.

Furthermore, the corrosion current density depends on the area fraction of the β phase. Therefore, in any SEM cross section of the plating layer, the area fraction of the β phase is preferably 3% or more. The area fraction of the β phase in the plating layer can be controlled by the manufacturing method. The area fraction of the β phase is more preferably 5% or more, and further preferably, the Cu+Ag concentration in the plating layer is 0.4% or more and the area fraction of the β phase is 10% or more.
Here, the "β phase" referred to in this specification can also be said to be an intermetallic compound region composed of at least one of an Al-Zn-Cu compound, an Al-Zn-Ag compound, and an Al-Zn-Cu-Ag compound.

In order to properly supply Cu and/or Ag to the β phase in the plating layer, it is preferable to provide them from a pre-plating layer formed on the original plate by the method described below. The Zn, Cu, and Ag in the pre-plating layer are thermally diffused during pre-annealing to form a Cu-Ag-Zn diffusion plating layer (an aggregate of Cu, Ag-Zn intermetallic compounds). However, this Cu-Ag-Zn diffusion plating layer disappears during hot-dip plating. However, if a small amount of (Zn+Al)-(Cu+Ag) compounds are ultimately present in the plating layer, the potential of the plating layer can be increased.

The (Zn+Al)-(Cu+Ag) compound is a compound in which small amounts of Cu, Ag, and Al are dissolved in Zn, and has a potential of -1.2V (vs. Ag/AgCl 1M NaCl). Therefore, by leaving the (Zn+Al)-(Cu+Ag) compound in the final plating layer, the potential of the plating layer can be further increased. The (Zn+Al)-(Cu+Ag) compound can be confirmed within the plating layer using SEM, and quantitative analysis values can be confirmed using point analysis in the same manner as above.

In order to obtain a (Zn+Al)-(Cu+Ag) compound exhibiting an appropriate potential, it is preferable that the component ratio is Zn+Al:Cu+Ag=4:1 to 9:1. The (Zn+Al)-(Cu+Ag) compound has an equivalent circular diameter of 1 μm or more in any cross section along the thickness direction of the plating layer, and the number of (Zn+Al)-(Cu+Ag) compounds is preferably 10 or more, more preferably 20 or more ^per 10,000 μm2. The number of (Zn+Al)-(Cu+Ag) compounds tends to depend on the amount of Cu, Ag, and Zn attached to the original sheet, i.e., the amount of the pre-plating layer attached.

Next, we will show an example of how to analyze a plating layer.

A glow discharge optical emission spectrometer (GDS) is preferably used for the component analysis method in the depth direction inside the plating layer. The inventors use a LECO Japan 850A as a glow discharge optical emission spectrometer, but the measurement device is not limited to this. When performing a depth direction analysis, it is preferable to perform the analysis while performing Ar sputtering, and the analysis conditions are argon pressure: 0.27 MPa, output power: 30 W, output voltage: 1000 V, and discharge area: within a circular area with a diameter of 4 mm. The measurement is performed from the surface of the plating layer toward the depth direction until the Fe concentration reaches 100% (reaching the base steel). Therefore, the analysis range of the depth direction analysis by GDS is the range from the plating surface to the Zn-Al-Mg plating layer, the interface alloy layer (Al-Fe alloy layer), and a part of the steel material. After the GDS analysis, the sputter depth of the cross section is measured using a Surfcom 130A manufactured by Tokyo Seimitsu Co., Ltd. GDS analysis provides an element distribution profile in the depth direction of the plating layer. The element distribution profile shows the distribution of the content of each element in the depth direction, assuming that the total amount of detected elements is 100%.

The cross-section observation using an SEM may be carried out in the following procedure.
First, a sample for observation measuring approximately 20 x 20 mm square is cut out from the plated steel sheet so that the cross section of the plating layer is exposed, and this sample is embedded in resin. Next, the observation surface is mirror-polished to observe the cross section of the plating layer. It is advisable to vapor-deposit Au on the cross section of the plating layer before performing the cross-sectional observation. In order to eliminate bias in the selection of the field of view of the plating layer, at least three observation samples are taken from one plated steel sheet, and at least 30 locations are observed randomly so that the field of view is approximately 500 to 2000 times larger, and each phase is identified and the area fraction is measured.

When determining the area fraction of the β phase, the field of view is first specified, and then the field of view is specified to include the region where the component range of the β phase is close to 0.85≦[Al]/([Zn]+[Cu]+[Ag])≦1.15 by point analysis using EDS. If the β phase is found, a quantitative analysis element mapping image of the entire plating layer is taken. The same component range is identified from the Zn and Al in the mapping image taken using the image analysis software "ImageJ" and binarization is performed. The area fraction of the β phase in the plating layer is measured from the obtained binarized area.

Similarly, for Zn-Cu compounds, Zn, Cu and equivalent circle diameter are measured. The distribution of the number of (Zn+Al)-(Cu+Ag) compounds can be obtained using functions attached to known image analysis software such as ImageJ. After observing each field of view, the number distribution is confirmed for each sample until the total area (equivalent to the number of pixels) of the plating layer reaches 10,000 ^μm2 . It is preferable to confirm the number of (Zn+Al)-(Cu+Ag) compounds from at least three samples.

[Method of manufacturing hot-dip plated steel material]
Next, a preferred method for producing the hot-dip plated steel material of this embodiment will be described. In the production method according to this embodiment, Cu and Ag behave almost similarly, so that hereinafter, they will not be distinguished from each other and Cu will be used as a representative. In other words, the description of Cu in the following description may be read as "Ag" or "Cu and Ag".

As a method for forming the above-mentioned plating layer, it is possible to simply add Cu directly to a Zn-Al-Mg plating bath and plate it. However, with this method, there is a concern that Cu is not efficiently contained in the β phase due to the formation of intermetallic compounds such as _CuAl2 and the reaction between the base iron, which is the base material, and Cu. Therefore, as an example of a suitable manufacturing method for producing the hot-dip plated steel material of this embodiment, a method of supplying Cu to the plating layer from a pre-plating layer provided on a plated original sheet will be described below.

First, a base plate such as a cold-rolled steel sheet or a hot-rolled steel sheet is electroplated with Zn to form a pre-Zn plating layer (hereinafter simply referred to as a Zn plating layer or a Zn layer). The amount of adhesion of the Zn plating layer is preferably equal to or greater than the amount of Cu in the final plating layer. The pre-plating method may be electroplating, displacement plating, vapor deposition, etc.

There are no particular restrictions on Zn plating on Fe, as long as it is performed under conditions that normally produce zinc plating, such as a cyanide bath, zincate bath, zinc chloride bath, or zinc sulfate bath.

Then, Cu is plated on the Zn layer to form a pre-Cu plating layer. In this embodiment, it is effective to form a pre-Cu plating layer on the pre-Zn plating layer after forming the pre-Cu plating layer. If the pre-Cu plating layer and the pre-Zn plating layer are formed in this order, the Cu in the pre-Cu plating layer may diffuse to the original sheet side, and the desired plating layer may not be obtained. For this reason, it is preferable to form the pre-Zn plating layer and the pre-Cu plating layer in this order. For Cu plating, copper sulfate, copper cyanide, copper pyrophosphate, alkanol baths, etc. can be used, and there are no particular restrictions on the plating bath. Ag can be silver cyanide, and Cu can also be electrolessly plated.

To increase the number of (Zn+Al)-(Cu+Ag) compounds, it is preferable that the deposition weight of the pre-Cu plating layer exceeds 1/1000 of the final plating deposition weight.

Then, the plated original sheet is heated to 450-600°C. This heating may also serve as annealing of the original sheet (hereinafter, this heating may be referred to as pre-annealing). This heating causes Zn to melt and react with Cu to form an alloy, forming a Zn-Cu plated diffusion layer. If the heating temperature is less than 450°C, Cu may not be efficiently diffused into the plated layer. On the other hand, if the heating temperature exceeds 600°C, this is not preferred as it may cause the pre-Zn plated layer to evaporate and form a reaction layer of Fe and Zn. The pre-annealing temperature range is preferably 450-600°C.

Next, the original sheet on which the Zn-Cu plating diffusion layer is formed is immersed in a Zn-Al-Mg plating bath and then pulled up. At this time, the bath temperature of the Zn-Al-Mg plating bath is preferably 450 to 600°C. When the original sheet on which the Zn-Cu plating diffusion layer is formed is immersed in a Zn-Al-Mg plating bath and pulled up, the higher the bath temperature, the easier it is for the Zn-Cu plating layer to dissolve and for Cu to be finely dispersed in the plating layer. Therefore, the bath temperature is preferably 450°C or higher. More preferably, it is 470°C or higher, even more preferably 500°C or higher, and even more preferably 550°C or higher. On the other hand, if the bath temperature is excessively high, Zn in the plating bath evaporates and the bath balance is easily lost. Therefore, the bath temperature is preferably 600°C or lower. More preferably, it is 580°C or lower.
In this embodiment, it is preferable to control the temperature when the plated original sheet is pulled up. In other words, by performing appropriate temperature control and cooling control after immersion, Cu is finely dispersed in the plating bath, and Cu is contained in the Zn-Al(β) phase during the solidification process of the plating layer.

The Zn-Al (β) phase is formed after the Al (α) phase precipitates and grows from the molten plating state. After the Al (α) phase has grown sufficiently, the Zn-Al (β) phase precipitates and grows to surround it. Since the Zn-Cu fine compounds taken in from the original plate do not dissolve in the solid Al (α) phase, many of the Zn-Cu fine compounds can be left intact.

The temperature range of 550 to 450 ° C is a temperature range in which only the α phase is formed, and the β phase is not formed. If the average cooling rate in this temperature range is more than 10 ° C, the growth of the α phase may be inhibited. If the α phase does not grow sufficiently in the temperature range of 550 to 450 ° C, the subsequent formation of the β phase may be insufficient. In addition, if the average cooling rate in the temperature range of 550 to 450 ° C is too large, the amount of β phase cannot be sufficiently secured, and therefore Zn-Cu cannot be sufficiently incorporated into the β phase, and as a result, CAZ _max /CAZ _min may increase. Therefore, the average cooling rate in the temperature range of 550 to 450 ° C is set to 10 ° C / sec or less.

The average cooling rate in the temperature range of 450 to 350°C is not particularly limited. However, a lower average cooling rate between 450°C and 350°C can increase the amount of β phase. Therefore, the average cooling rate between 450°C and 350°C is preferably 8°C/sec or less.

The average cooling rate in the temperature range below 350°C does not affect the formation of the β phase. In addition, the cooling conditions in the temperature range below 350°C are not particularly limited, as they do not affect the potential or corrosion current density.

Next, we will explain how to evaluate the performance of hot-dip galvanized steel.

(Corrosion Potential)
The corrosion potential of the plating layer can be measured using a general device configuration including an electrochemical cell, a reference electrode, a salt bridge, a potentiostat (a constant potential electric field device), and the like.
One example of the evaluation method is a method using an Ag/AgCl type reference electrode, a 1M NaCl aqueous solution as the measurement solution, and further performing the evaluation under conditions such as degassing and constant room temperature. When the surface of a plated steel sheet is thoroughly washed and then immersed in the measurement solution, the corrosion potential immediately after immersion may be significantly negative, but the potential stabilizes after immersion in the solution for about 1800 seconds. This potential is the corrosion potential of the plating layer surface.

When the corrosion potential approaches -1.1 V, there is less variation in the potential of the plating layer, which is preferable, and the amount of white rust generated in the early stages of corrosion can be minimized. Note that the early stages of corrosion refer to the tendency of the white rust area ratio 24 hours after the salt spray test (SST) specified in JIS Z 2371 (2015).

<Criteria>
E: The corrosion potential is -1.3V or less, and the white rust area ratio of the surface to be evaluated in the corrosion test is 25% or more.
D: The corrosion potential is -1.3 to -1.25 V, and the white rust area ratio of the surface to be evaluated in the corrosion test is 20 to 25% or more.
C: The corrosion potential is -1.25 to -1.2 V, and the white rust area ratio on the evaluation surface of the corrosion test is 15 to 20% or more.
B: The corrosion potential is -1.25 to -1.2 V, and the white rust area ratio on the evaluation surface of the corrosion test is 10 to 15% or more.
A: The corrosion potential is -1.2 to -1.1 V, and the white rust area rate on the evaluation surface of the corrosion test is less than 10%.

(Corrosion Current Density)
A polarization curve is drawn by changing the potential in the positive and negative directions, and the corrosion potential is obtained by Tafel extrapolation. The corrosion current density depends on the corrosion rate. This can also be estimated from the corrosion weight loss in a salt spray test. That is, since only the β phase corrodes in the early stage, the corrosion weight loss after 120 hours of SST is measured. The corrosion weight loss can be measured by immersing the steel sheet before and after the corrosion test in 30% chromic acid for 5 minutes, and the amount of corrosion of the β phase can be estimated.

<Criteria>
E: The obtained corrosion current density log|i| (A/cm ² ) is −4 or more, and the corrosion weight loss is 25 g/m ² or more.
D: The obtained corrosion current density log|i| (A/cm ² ) is −4.5 to −4, and the corrosion weight loss is 20 g/m ² or more.
C: The obtained corrosion current density log|i| (A/cm ² ) is −5 to −4.5, and the corrosion weight loss is 15 g/m ² or more.
B: The obtained corrosion current density log|i| (A/cm ² ) is −5.5 to −5, and the corrosion weight loss is 10 g/m ² or more.
A: The obtained corrosion current density log|i| (A/cm ² ) is −6 to −5.5, and the corrosion weight loss is 5 g/m ² or more.
S: The obtained corrosion current density log|i| (A/cm ² ) is less than −6, and the corrosion weight loss is less than 5 g/m ² .

After the plating layer is formed, various chemical treatments and painting processes may be performed.

In the hot-dip plated steel material of this embodiment, a coating may be formed on the plating layer. The coating may be formed in one layer or in two or more layers. Types of coatings that may be formed directly on the plating layer include, for example, chromate coatings, phosphate coatings, and chromate-free coatings. These coatings may be formed by known methods such as chromate treatment, phosphate treatment, and chromate-free treatment.

There are three types of chromate treatment: electrolytic chromate treatment, which forms a chromate film by electrolysis; reactive chromate treatment, which uses a reaction with the material to form a film and then washes away excess treatment liquid; and paint-type chromate treatment, which applies the treatment liquid to the substrate and dries it without rinsing to form a film. Any of these treatments may be used.

Examples of electrolytic chromate treatments include those using chromic acid, silica sol, resin (phosphoric acid, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine modified epoxy resin, etc.), and hard silica.

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

Chromate-free treatments are particularly suitable as they place no burden on the environment. There are electrolytic chromate-free treatments that form a chromate-free film by electrolysis, reactive chromate-free treatments that form a film by utilizing a reaction with the material and then wash away excess treatment liquid, and coating-type chromate-free treatments that apply a treatment liquid to the substrate and dry it without rinsing to form a film. Any of these treatments may be used.

Furthermore, one or more layers of an organic resin film may be provided on the film directly on the plating layer. The organic resin is not limited to a specific type, and examples include polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, and modified products of these resins. Here, the modified product refers to a resin in which a reactive functional group contained in the structure of these resins has been reacted with another compound (monomer, crosslinking agent, etc.) that contains a functional group in its structure that can react with the functional group.

As such organic resins, one or more organic resins (unmodified) may 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 may be used in combination. The organic resin film may also contain any coloring pigment or rust-preventive pigment. Water-based resins that have been dissolved or dispersed in water may also be used.

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

First, a cold-rolled steel sheet (corresponding to SPCC as defined in JIS G 3141 (2017)) measuring 100 mm x 200 mm and having a thickness of 0.8 mm was prepared as the base sheet for plating. A Zn plating layer was formed by attaching a specified amount of Zn to the surface of this cold-rolled steel sheet using the zinc plating bath shown below.

Zinc plating conditions: zinc chloride: 50 g/L, ammonium chloride: 200 g/L, pH=5.5, bath temperature: 30° C., current density: 2 A/dm ² .

Next, a Cu plating layer, an Ag plating layer, or a plating layer containing Cu and Ag was formed on the plated original sheet on which the Zn plating layer was formed, using the following copper plating bath and the following silver plating bath. When forming a plating layer containing Cu and Ag, the plated original sheet was immersed in the copper plating bath and then the silver plating bath. The copper plating bath was agitated with air. The pH of the silver plating bath was adjusted with H ₄ P ₂ O ₇ and KOH.

Copper plating conditions: copper pyrophosphate: 80 g/L, potassium pyrophosphate: 290 g/L, ammonia water: 3 mg/L, potassium nitrate: 10 g/L, anode: oxygen-free high-purity copper, current density: 3 A/dm ² .

Silver plating conditions: KAg(CN)2: 40 g/L (Ag equivalent), K ₄ P ₂ O ₇ : 150 g/L, EDTA (tetrapotassium salt): 5 g/L, smoothing material HS II (selenium, mercapto compound): 0.5 mL/L, pH = 8-9, current density: 40 A/dm ² , bath temperature: 40°C, anode: Pt/Ti electrode.

After forming a Cu plating layer or an Ag plating layer, or a plating layer containing Cu and Ag on the original plate, the original plate was heated for 0.5 to 3 minutes at the pre-annealing temperature shown in Tables 1A to 1C to form a plating diffusion layer and obtain a plating substrate. The composition of the plating diffusion layer is shown in Tables 1A to 1C.

The obtained plated substrate was subjected to hot-dip plating in a hot-dip plating simulator.
First, alloys having the plating bath components shown in Tables 1A to 1C were prepared by a vacuum melting method, and plating baths were prepared in a completely oxygen-free, nitrogen-substituted atmosphere ( _O2 concentration less than 5 ppm).
Next, one point of the plated original sheet (the back side of the center of the evaluation surface) was attached to a K thermocouple by spot welding, and the temperature history until the completion of plating solidification was grasped. The plated steel sheet was heated to a predetermined temperature in a H ₂ (25%)-N ₂ atmosphere. The plating bath temperature was set to 550-600°C, and the plated substrate was immersed at a dipping speed of 600 mm/sec. After stopping in the bath for 3 seconds, the plated substrate was lifted up at a speed of 600 mm/sec.
Immediately after pulling up, the deposition amount was adjusted to 135 to 140 g/ ^m2 using _N2 wiping gas, and then air-cooled at the average cooling rates shown in Tables 1A to 1C by blowing _N2 gas with a controlled flow rate in an oxygen-free, nitrogen-substituted atmosphere.
Through the above steps, a plated steel sheet was obtained.

Next, evaluation samples were cut out from each type of plated steel sheet. Samples for GDS analysis and SEM observation were cut out at 30 mm square positions on the opposite side of the thermocouple position. Corrosion samples were taken from the center of the plated steel sheets, measuring 100 x 50 mm.

The cut samples were evaluated by electrochemical tests and corrosion tests (SST). The composition of Fe in the plating layer is not shown in the table, but it was in the range of 0 to 5%.
The evaluation results, the composition of the plating layer, the GDS analysis results, and each component in the β-phase region are shown in Tables 2A to 2C. Note that underlines in each table indicate that the results are outside the scope of the present invention, that the manufacturing conditions are not preferable, or that the characteristic values are unfavorable. Also, (*) in Tables 2A to 2C indicates the number of (Zn+Al)-(Cu+Ag) compounds with an equivalent circle diameter of 1 μm or more ^per 10,000 μm2 in any cross section along the thickness direction of the plating layer.

The above-mentioned aspects of the present invention make it possible to provide hot-dip plated steel material that can suppress the occurrence of white rust in the early stages of corrosion.

Claims (3)

Steel and
A plating layer disposed on a surface of the steel material;
A hot-dip galvanized steel material having the following properties:
The plating layer comprises, in mass %,
Al: more than 10.0% and less than 45.0%;
Mg: 4.0% or more, 15.0% or less,
Si: 0.01% or more, 2.0% or less,
At least one of Cu of 0.03% or more and 5.0% or less and Ag of 0.03% or more and 6.0% or less;
and further comprising
Sn: 0% or more, 0.7% or less,
Bi: 0% or more, 0.3% or less,
In: 0% or more, 0.3% or less,
Ca: 0% or more, 0.6% or less,
Y: 0% or more, 0.3% or less,
La: 0% or more, 0.3% or less,
Ce: 0% or more, 0.3% or less,
Sr: 0% or more, 0.3% or less,
Li: 0% or more, 0.3% or less,
Ni: 0% or more, 1.0% or less,
Cr: 0% or more, 0.5% or less,
Mo: 0% or more, 0.3% or less,
Sb: 0% or more, 0.25% or less,
Pb: 0% or more, 0.25% or less,
B: 0% or more, 0.5% or less,
P: 0% or more, 0.5% or less,
Ti: 0% or more, 0.25% or less,
Co: 0% or more, 0.25% or less,
V: 0% or more, 0.25% or less,
Nb: 0% or more, 0.25% or less,
Mn: 0% or more, 0.25% or less,
Zr: 0% or more, 0.25% or less,
W: 0% or more, 0.25% or less,
Fe: 0% or more, 5.0% or less,
The balance includes Zn and impurities,
The total content of Cu and Ag is 0.03% or more and 6.0% or less.
In an element distribution profile obtained by quantitatively analyzing a region from a surface of the plating layer toward the steel material by glow discharge optical emission spectrometry, a maximum value CAZ _max of the CAZ and a minimum value CAZ _min of the CAZ satisfy the following formulas (1) to (3):
CAZ _max /CAZ _min ≦1.20…(1)
0.0005≦CAZ _min …(2)
CAZ _max ≦0.1000…(3) In a cross section along the thickness direction of the plating layer, the area ratio of the β phase is 3% or more,
The hot-dip plated steel material according to claim 1, characterized in that the concentrations of Al, Zn, Cu and Al in the β phase satisfy the following formula (4):
0.85≦[Al]/([Zn]+[Cu]+[Ag])≦1.15…(4)
Here, [Al], [Zn], [Cu] and [Ag] in formula (4) are quantitative analysis values (at %) in the β phase by energy dispersive X-ray analysis. The hot-dip plated steel material according to claim ^{1 or 2} , characterized in that 10 or more (Zn+Al)-(Cu+Ag) compounds having an average crystal grain size of 1 μm or more are contained as intermetallic compound particles within an area of 10,000 μm2 in a cross section along a thickness direction of the plating layer.