WO2019221193A1

WO2019221193A1 - Plated steel material

Info

Publication number: WO2019221193A1
Application number: PCT/JP2019/019359
Authority: WO
Inventors: 完齊藤; 高橋　武寛; 石塚　清和
Original assignee: 日本製鉄株式会社
Priority date: 2018-05-16
Filing date: 2019-05-15
Publication date: 2019-11-21
Also published as: KR20200130850A; TW201947061A; JPWO2019221193A1; KR102425278B1; JP6687175B1; TWI686510B; CN111989420B; CN111989420A

Abstract

To provide a plated steel material having stable and high planar section corrosion resistance, and a method for manufacturing the same. A plated steel material and a method for manufacturing the same, the plated steel material having a steel material and a plating layer including a Zn-Al-Mg alloy layer disposed on the surface of the steel material, the plating layer having a predetermined chemical composition, Al crystals being present in a backscattered electron image of the Zn-Al-Mg alloy layer obtained when the surface of the Zn-Al-Mg alloy layer is observed at a magnification of 100x by a scanning electron microscope after being polished to a depth of 1/2 the layer thickness of the Zn-Al-Mg alloy layer, and the average value of the total circumference length of the Al crystals being 88-195 mm/mm².

Description

Plated steel

This disclosure relates to plated steel materials.

For example, in the field of building materials, a wide variety of plated steel materials are used. Most of them are Zn-plated steel materials. In view of the need to extend the life of building materials, research on increasing the corrosion resistance of Zn-plated steel materials has been conducted for a long time, and various plated steel materials have been developed. The first highly corrosion-resistant plated steel material for building materials is Zn-5% Al-plated steel material (galfan-plated steel material) in which Al is added to a Zn-based plated layer to improve corrosion resistance. It is a well-known fact that Al is added to the plating layer to improve the corrosion resistance. When 5% Al is added, an Al crystal is formed in the plating layer (specifically, the Zn phase), and the corrosion resistance is improved. Zn-55% Al-1.6% Si plated steel (galvalume steel) is also basically a plated steel with improved corrosion resistance for the same reason.
Therefore, the planar portion corrosion resistance basically improves as the Al concentration increases. However, an increase in the Al concentration causes a decrease in sacrificial anticorrosive ability.

Here, the attraction of Zn-based plated steel is its sacrificial anti-corrosion effect on the base steel. In other words, at the cut end surface of plated steel material, plated layer cracked part during processing, and exposed surface of the base steel material that appears due to peeling of the plated layer, the surrounding plating layer is eluted before corrosion of the base steel material, and the plating elution components are protected Form a film. Thereby, it is possible to prevent red rust from the base steel material to some extent.

For this action, it is generally preferable that the Al concentration is low and the Zn concentration is high. Accordingly, high corrosion-resistant plated steel materials in which the Al concentration is suppressed to a relatively low concentration of about 5% to 25% have been put into practical use in recent years. In particular, a plated steel material that keeps the Al concentration low and contains about 1 to 3% Mg has better planar part corrosion resistance and sacrificial corrosion resistance than galfan-plated steel material. For this reason, it has become a market trend as a plated steel material and is widely known in the market today.

For example, a plated steel material disclosed in Patent Document 1 has been developed as a plated steel material containing a certain amount of Al and Mg.

Specifically, Patent Document 1 includes, on the surface of a steel material, Al: 5 to 18% by mass, Mg: 1 to 10% by mass, Si: 0.01 to 2% by mass, the balance Zn and inevitable impurities. A molten Zn—Al—Mg—Si plated steel material having 200 or more Al phases per 1 mm ^{2 on the} surface of a plated steel material having a plated layer is disclosed.

JP 2001-355053 A

However, in a plated steel material containing a certain amount of Al concentration, corrosion of the plating layer (specifically, a Zn—Al—Mg alloy layer) proceeds locally, and tends to reach the base steel material at an early stage. As a result, the planar portion corrosion resistance may deteriorate, and the variation in the planar portion corrosion resistance may increase. Therefore, the present condition is that the plated steel material which has the stable high plane part corrosion resistance is calculated | required.

Therefore, an object of one aspect of the present disclosure is to provide a plated steel material having stable and high flat surface corrosion resistance.

The above problem can be solved by the following means. That is,

<1>
A plated steel material comprising: a base steel material; and a plating layer including a Zn—Al—Mg alloy layer disposed on a surface of the base steel material,
The plating layer is mass%,
Zn: more than 65.0%,
Al: more than 5.0% to less than 25.0%,
Mg: more than 3.0% to less than 12.5%,
Sn: 0.1% to 20.0%,
Bi: 0% to less than 5.0%,
In: 0% to less than 2.0%,
Ca: 0% to 3.00%,
Y: 0% to 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% to less than 0.25%,
Ti: 0% to less than 0.25%,
Ni: 0% to less than 0.25%,
Co: 0% to less than 0.25%,
V: 0% to less than 0.25%
Nb: 0% to less than 0.25%,
Cu: 0% to less than 0.25%,
Mn: 0% to less than 0.25%,
Fe: 0% to 5.0%,
Sr: 0% to less than 0.5%,
Sb: 0% to less than 0.5%,
Pb: 0% to less than 0.5%,
B: having a chemical composition consisting of 0% to less than 0.5% and impurities,
In the backscattered electron image of the Zn—Al—Mg alloy layer, obtained by polishing the surface of the Zn—Al—Mg alloy layer to ½ of the layer thickness and then observing it at a magnification of 100 times with a scanning electron microscope, A plated steel material in which an Al crystal is present and an average value of the total peripheral length of the Al crystal is 88 to 195 mm / mm ² .
<2>
The plated steel material according to <1>, wherein the plated layer has an Al—Fe alloy layer having a thickness of 0.05 to 5 μm between the base steel material and the Zn—Al—Mg alloy layer.

According to one aspect of the present disclosure, it is possible to provide a plated steel material having stable and high flat surface corrosion resistance.

2 is an SEM reflected electron image (magnification 100 times) showing an example of a Zn—Al—Mg alloy layer of the plated steel material of the present disclosure. 2 is an SEM reflected electron image (magnification 500 times) showing an example of a Zn—Al—Mg alloy layer of the plated steel material of the present disclosure. 2 is an SEM reflected electron image (magnification of 10,000 times) showing an example of a Zn—Al—Mg alloy layer of the plated steel material of the present disclosure. It is a figure which shows an example of the image which image-processed (binarized) the reflected electron image (SEM reflected electron image) of the Zn-Al-Mg alloy layer of the plated steel material of this indication so that an Al crystal could be identified.

Hereinafter, an example of the present disclosure will be described.
In addition, in this indication, "%" display of content of each element of chemical composition means "mass%".
A numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
The numerical range in the case where “over” or “less than” is added to the numerical values described before and after “to” means a range not including these numerical values as the lower limit value or the upper limit value.
The element content of the chemical composition may be expressed as an element concentration (for example, Zn concentration, Mg concentration, etc.).
The term “process” is not limited to an independent process, and is included in this term if the intended purpose of the process is achieved even when it cannot be clearly distinguished from other processes.
“Plane surface corrosion resistance” refers to the property of a plating layer (specifically, a Zn—Al—Mg alloy layer) that is not easily corroded.
“Sacrificial corrosion resistance” refers to the corrosion of the base steel material at the exposed part of the base steel material (for example, the cut end face part of the plated steel material, the cracked part of the plated layer during processing, and the part where the base steel material is exposed due to peeling of the plated layer). Inhibiting properties.

The plated steel material of the present disclosure is a plated steel material having a base steel material and a plating layer that is disposed on the surface of the base steel material and includes a Zn—Al—Mg alloy layer.
In the plated steel material of the present disclosure, the plating layer has a predetermined chemical composition, and after polishing the surface of the Zn—Al—Mg alloy layer to ½ of the layer thickness, the magnification is 100 times by a scanning electron microscope. In the backscattered electron image of the Zn—Al—Mg alloy layer obtained when observed, Al crystals are present, and the average value of the total peripheral length of the Al crystals is 88 to 195 mm / mm ² .

The plated steel material of the present disclosure becomes a plated steel material having stable high flat surface corrosion resistance due to the above configuration. The plated steel material of this indication was discovered by the following knowledge.

The inventors analyzed the initial corrosion behavior of the plating layer containing the Zn—Al—Mg alloy layer. As a result, it was found that the corrosion of the plating layer (specifically, the Zn—Al—Mg alloy layer) locally progressed in the form of a ant nest, and the periphery of the Al crystal was preferentially corroded.
This is estimated as follows. In comparison, potentiometric corrosion occurs between the Al crystal having a high potential and the surrounding structure having a low potential. Therefore, the larger the contact area between the Al crystal and the surrounding phase of the Al crystal, the more easily the corrosion around the Al crystal occurs, the flat portion corrosion resistance deteriorates, and the variation in the flat portion corrosion resistance also increases.

In order to reduce the contact area between the Al crystal and the surrounding phase of the Al crystal as much as possible, the inventors control the cooling conditions after immersion of the plating bath during the production of the plating layer to deposit the Al crystal coarsely. I was inspired by that.
As a result, the following was found. As an index of the size of the Al crystal, the total circumference of the Al crystal by image analysis and the corrosion resistance of the plane portion correlate well. Then, when the average value of the cumulative peripheral length of the Al crystal is set within a predetermined range, the contact area between the Al crystal and the surrounding phase of the Al crystal is reduced. As a result, corrosion around the preferential Al crystal is suppressed, and stable flat surface corrosion resistance is obtained. However, if the average value of the cumulative peripheral length of Al crystals is excessively lowered, the workability is lowered.

From the above, it has been found that the plated steel material of the present disclosure is a plated steel material having stable and high flat surface corrosion resistance.

Hereinafter, the details of the plated steel material of the present disclosure will be described.

The base steel material to be plated will be described.
The shape of the base steel material is not particularly limited. The base steel material is steel plate, steel pipe, civil engineering construction material (fence fence, corrugated pipe, drainage ditch cover, flying sand prevention plate, bolt, wire mesh, guardrail, water blocking wall Etc.), home appliance members (such as a casing of an outdoor unit of an air conditioner), and automobile parts (such as suspension members). For the forming process, for example, various plastic working methods such as press working, roll forming, and bending can be used.

There is no restriction | limiting in particular in the material of a base steel material. The base steel is, for example, general steel, pre-plated steel, Al killed steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, some high alloy steels (such as steel containing strengthening elements such as Ni and Cr), etc. Various base steel materials can be applied.
The base steel material is not particularly limited with respect to conditions such as a manufacturing method of the base steel material and a manufacturing method of the base steel plate (hot rolling method, pickling method, cold rolling method, etc.).
In addition, as a base steel material, the hot-rolled steel plate, hot-rolled steel strip, cold-rolled steel plate, and cold-rolled steel strip described in JIS G 3302 (2010) are also applicable.

The base steel material may be a pre-plated pre-plated steel material. The pre-plated steel material is obtained by, for example, an electrolytic treatment method or a displacement plating method. In the electrolytic treatment method, a pre-plated steel material is obtained by immersing the base steel material in a sulfuric acid bath or a chloride bath containing metal ions of various pre-plating components and subjecting it to an electrolytic treatment. In the displacement plating method, a base steel material is immersed in an aqueous solution containing metal ions of various pre-plating components and adjusted in pH with sulfuric acid to displace and deposit the metal, thereby obtaining a pre-plated steel material.
A typical example of the pre-plated steel material is Ni pre-plated steel material.

Next, the plating layer will be described.
The plating layer includes a Zn—Al—Mg alloy layer. The plating layer may include an Al—Fe alloy layer in addition to the Zn—Al—Mg alloy layer. The Al—Fe alloy layer is provided between the base steel material and the Zn—Al—Mg alloy layer.

That is, the plating layer may have a single-layer structure of a Zn—Al—Mg alloy layer or a laminated structure including a Zn—Al—Mg alloy layer and an Al—Fe alloy layer. In the case of a laminated structure, the Zn—Al—Mg alloy layer is preferably a layer constituting the surface of the plating layer.
However, although an oxide film of a plating layer constituent element is formed on the surface of the plating layer by about 50 nm, it is considered that the thickness of the plating layer is small with respect to the entire thickness of the plating layer and does not constitute the main body of the plating layer.

Here, the thickness of the Zn—Al—Mg alloy layer is, for example, 2 μm to 95 μm (preferably 5 μm to 75 μm).

On the other hand, the thickness of the entire plating layer is, for example, about 100 μm or less. Since the thickness of the entire plating layer depends on the plating conditions, the upper limit and the lower limit of the thickness of the entire plating layer are not particularly limited. For example, the thickness of the entire plating layer is related to the viscosity and specific gravity of the plating bath in a normal hot dipping method. Further, the basis weight is adjusted by the drawing speed of the base steel material and the strength of wiping. Therefore, it may be considered that the lower limit of the thickness of the entire plating layer is about 2 μm.
On the other hand, the upper limit of the thickness of the plating layer that can be produced by the hot dipping method due to the weight and uniformity of the plated metal is approximately 95 μm.
Since the thickness of the plating layer can be freely changed according to the drawing speed from the plating bath and the wiping conditions, formation of the plating layer having a thickness of 2 to 95 μm is not particularly difficult to produce.

Next, the Al—Fe alloy layer will be described.

The Al—Fe alloy layer is formed on the surface of the base steel material (specifically, between the base steel material and the Zn—Al—Mg alloy layer), and an Al ₅ Fe phase is a main phase layer as a structure. The Al—Fe alloy layer is formed by mutual atomic diffusion of the base steel material and the plating bath. When the hot dipping method is used as a manufacturing method, an Al—Fe alloy layer is easily formed in a plating layer containing an Al element. Since Al of a certain concentration or more is contained in the plating bath, the Al ₅ Fe phase is most formed. However, atomic diffusion takes time, and there is a portion where the Fe concentration is high in a portion close to the base steel material. Therefore, the Al—Fe alloy layer may partially contain a small amount of an AlFe phase, an Al ₃ Fe phase, an Al ₅ Fe ₂ phase, or the like. In addition, since a certain concentration of Zn is contained in the plating bath, the Al—Fe alloy layer contains a small amount of Zn.

In terms of corrosion resistance, there is no significant difference in any of the Al ₅ Fe phase, Al ₃ Fe phase, AlFe phase, and Al ₅ Fe ₂ phase. The term “corrosion resistance” as used herein refers to corrosion resistance at a portion not affected by welding.

Here, when Si is contained in the plating layer, Si is particularly likely to be taken into the Al—Fe alloy layer and may become an Al—Fe—Si intermetallic compound phase. The identified intermetallic compound phase includes an AlFeSi phase, and isomers include α, β, q1, q2-AlFeSi phase, and the like. Therefore, the Al—Fe alloy layer may detect these AlFeSi phases. These Al—Fe alloy layers containing the AlFeSi phase and the like are also referred to as Al—Fe—Si alloy layers.
Since the Al—Fe—Si alloy layer is smaller in thickness than the Zn—Al—Mg alloy layer, the influence on the corrosion resistance of the entire plating layer is small.

When various pre-plated steel materials are used for the base steel material (base steel plate, etc.), the structure of the Al—Fe alloy layer may change depending on the amount of pre-plating. Specifically, when the pure metal layer used for the pre-plating remains around the Al—Fe alloy layer, an intermetallic compound phase in which the constituent components of the Zn—Al—Mg alloy layer and the pre-plating component are combined (for example, Al ₃ Ni phase, etc.) forms an alloy layer, Al—Fe alloy layer in which some Al atoms and Fe atoms are substituted, or some Al atoms, Fe atoms, and Si atoms are substituted. In some cases, an Al—Fe—Si alloy layer is formed. In any case, since these alloy layers have a smaller thickness than the Zn—Al—Mg alloy layer, the influence on the corrosion resistance of the entire plating layer is small.

In other words, the Al—Fe alloy layer is a layer that includes the alloy layers of the various aspects described above in addition to the alloy layer mainly composed of the Al ₅ Fe phase.

In addition, when a plating layer is formed on a Ni pre-plated steel material among various pre-plated steel materials, an Al—Ni—Fe alloy layer is formed as the Al—Fe alloy layer. Since the thickness of the Al—Ni—Fe alloy layer is smaller than that of the Zn—Al—Mg alloy layer, the influence on the corrosion resistance of the entire plating layer is small.

The thickness of the Al—Fe alloy layer is, for example, not less than 0 μm and not more than 5 μm.
That is, the Al—Fe alloy layer may not be formed. The thickness of the Al—Fe alloy layer is preferably 0.05 μm or more and 5 μm or less from the viewpoint of improving the adhesion of the plating layer (specifically, the Zn—Al—Mg alloy layer) and ensuring the workability.

However, usually, when a plating layer having a chemical composition specified in the present disclosure is formed by a hot dipping method, an Al—Fe alloy layer of 100 nm or more may be formed between the base steel material and the Zn—Al—Mg alloy layer. Many. The lower limit of the thickness of the Al—Fe alloy layer is not particularly limited, and it has been found that an Al—Fe alloy layer is inevitably formed when forming a hot-dip plated layer containing Al. . Further, it is empirically determined that the thickness of about 100 nm is the thickness when the formation of the Al—Fe alloy layer is most suppressed, and the thickness sufficiently secures the adhesion between the plating layer and the base steel material. Since the Al concentration is high unless special measures are taken, it is difficult to form an Al—Fe alloy layer thinner than 100 nm by the hot dipping method. However, even if the thickness of the Al—Fe alloy layer is less than 100 nm, and even if the Al—Fe alloy layer is not formed, it is estimated that the plating performance is not greatly affected.

On the other hand, when the thickness of the Al—Fe alloy layer exceeds 5 μm, the Al component of the Zn—Al—Mg alloy layer formed on the Al—Fe alloy layer is insufficient, and the adhesion and workability of the plating layer are further reduced. Tend to be extremely worse. Therefore, the thickness of the Al—Fe alloy layer is preferably limited to 5 μm or less.
The Al—Fe alloy layer is also closely related to the Al concentration and the Sn concentration. Generally, the higher the Al concentration and the Sn concentration, the higher the growth rate.

Since the Al—Fe alloy layer is often mainly composed of an Al ₅ Fe phase, the chemical composition of the Al—Fe alloy layer is as follows: Fe: 25 to 35%, Al: 65 to 75%, Zn: 5% or less, And the balance: a composition containing impurities can be exemplified.

Usually, since the thickness of the Zn—Al—Mg alloy layer is usually larger than that of the Al—Fe alloy layer, the contribution of the Al—Fe alloy layer to the planar portion corrosion resistance as a plated steel material is Zn— Small compared to the Al—Mg alloy layer. However, the Al—Fe alloy layer contains a certain concentration or more of Al and Zn, which are corrosion resistant elements, as estimated from the component analysis results. Therefore, the Al—Fe alloy layer has a certain degree of sacrificial anticorrosive ability and corrosion barrier effect with respect to the base steel material.

Here, it is difficult to confirm the single corrosion resistance contribution of the thin Al—Fe alloy layer by quantitative measurement. However, for example, when the Al-Fe alloy layer has a sufficient thickness, the Zn-Al-Mg alloy layer on the Al-Fe alloy layer is precisely removed by cutting from the surface of the plating layer by end milling, etc. By applying the above, it is possible to evaluate the single corrosion resistance of the Al—Fe alloy layer. Since the Al-Fe alloy layer contains an Al component and a small amount of Zn component, when it has an Al-Fe alloy layer, red rust is generated in the form of dots, and there is no Al-Fe alloy layer, and the base steel material is exposed. Like time, the entire surface does not become red rust.

Also, during the corrosion test, when the cross section of the plated layer up to just before the occurrence of red rust on the base steel material was observed, only the Al—Fe alloy layer remained even if the upper Zn—Al—Mg alloy layer was eluted and rusted. It can be confirmed that the base steel material is anticorrosive. This is because the Al—Fe alloy layer is electrochemically more noble than the Zn—Al—Mg layer, but is more base than the base steel material. From these, it can be determined that the Al—Fe alloy layer also has a certain corrosion resistance.

From the viewpoint of corrosion, the thicker the Al—Fe alloy layer is, the more preferable it is to delay the red rust occurrence time. However, since a thick Al—Fe alloy layer causes a significant deterioration in plating processability, the thickness is preferably equal to or less than a certain thickness. From the viewpoint of workability, the thickness of the Al—Fe alloy layer is preferably 5 μm or less. When the thickness of the Al—Fe alloy layer is 5 μm or less, the amount of cracks and powdering generated from the plated Al—Fe alloy layer is reduced by a V-bending test or the like. The thickness of the Al—Fe alloy layer is more preferably 2 μm or less.

Next, the chemical composition of the plating layer will be described.
As for the component composition of the Zn—Al—Mg alloy layer contained in the plating layer, the component composition ratio of the plating bath is almost maintained even in the Zn—Al—Mg alloy layer. In the hot dipping method, since the formation of the Al—Fe alloy layer is completed in the plating bath, the reduction of the Al component and Zn component of the Zn—Al—Mg alloy layer by the formation of the Al—Fe alloy layer is usually There are few.

And the chemical composition of the plating layer is as follows in order to realize stable corrosion resistance on the flat surface.

In other words, the chemical composition of the plating layer is mass%,
Zn: more than 65.0%,
Al: more than 5.0% to less than 25.0%,
Mg: more than 3.0% to less than 12.5%,
Sn: 0.1% to 20.0%,
Bi: 0% to less than 5.0%,
In: 0% to less than 2.0%,
Ca: 0% to 3.00%,
Y: 0% to 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% to less than 0.25%,
Ti: 0% to less than 0.25%,
Ni: 0% to less than 0.25%,
Co: 0% to less than 0.25%,
V: 0% to less than 0.25%
Nb: 0% to less than 0.25%,
Cu: 0% to less than 0.25%,
Mn: 0% to less than 0.25%,
Fe: 0% to 5.0%,
Sr: 0% to less than 0.5%,
Sb: 0% to less than 0.5%,
Pb: 0% to less than 0.5%,
B: A chemical composition comprising 0% to less than 0.5% and impurities.

In the chemical composition of the plating layer, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb, and B are optional. It is an ingredient. That is, these elements may not be included in the plating layer. When these optional components are included, the content of the optional elements is preferably in the range described below.

Here, the chemical composition of this plating layer is the average chemical composition of the entire plating layer (in the case where the plating layer has a single layer structure of a Zn—Al—Mg alloy layer, the average chemical composition of the Zn—Al—Mg alloy layer, the plating layer) In the case of a laminated structure of an Al—Fe alloy layer and a Zn—Al—Mg alloy layer, the total average chemical composition of the Al—Fe alloy layer and the Zn—Al—Mg alloy layer).

Usually, in the hot dipping method, the chemical composition of the Zn—Al—Mg alloy layer is almost equal to the chemical composition of the plating bath because the formation reaction of the plating layer is almost completed in the plating bath. In the hot dipping method, the Al—Fe alloy layer is instantly formed and grown immediately after immersion in the plating bath. The formation reaction of the Al—Fe alloy layer is completed in the plating bath, and the thickness thereof is often sufficiently smaller than that of the Zn—Al—Mg alloy layer.
Therefore, the average chemical composition of the entire plated layer is substantially the same as the chemical composition of the Zn—Al—Mg alloy layer unless special heat treatment such as heat alloying treatment is performed after plating. Ingredients can be ignored.

Hereinafter, each element of the plating layer will be described.

<Zn: more than 65.0%>
Zn is an element necessary for obtaining sacrificial corrosion resistance in addition to planar surface corrosion resistance. When the Zn concentration is considered in terms of the atomic composition ratio, it is a plating layer composed of an element having a low specific gravity such as Al or Mg. Therefore, the atomic composition ratio needs to be mainly Zn.
Therefore, the Zn concentration is over 65.0%. The Zn concentration is preferably 70% or more. Note that the upper limit of the Zn concentration is the concentration which is the remainder other than the elements and impurities except Zn.

<Al: more than 5.0% to less than 25.0%>
Al is an essential element for forming an Al crystal and ensuring both the flat surface corrosion resistance and the sacrificial corrosion resistance. And Al is an essential element in order to improve the adhesiveness of a plating layer and to ensure workability. Therefore, the lower limit of the Al concentration is more than 5.0% (preferably 10.0% or more).
On the other hand, when the Al concentration increases, the sacrificial corrosion resistance tends to deteriorate. Therefore, the upper limit value of the Al concentration is less than 25.0% (preferably 23.0% or less).

<Mg: more than 3.0% to less than 12.5%>
Mg is an essential element for ensuring both the corrosion resistance and the sacrificial corrosion resistance of the planar portion. Therefore, the lower limit of the Mg concentration is over 3.0% (preferably over 5.0%).
On the other hand, when the Mg concentration increases, the workability tends to deteriorate. Therefore, it is less than 12.5% (preferably 10.0% or less).

<Sn: 0.1% to 20.0%>
Sn is an essential element that imparts high sacrificial corrosion resistance. Therefore, the lower limit value of the Sn concentration is set to 0.1% or more (preferably 0.2% or more).
On the other hand, when the Sn concentration increases, the planar portion corrosion resistance tends to deteriorate. Therefore, the upper limit value of the Sn concentration is 20.0% or less (preferably 5.0% or less).

<Bi: 0% to less than 5.0%>
Bi is an element contributing to sacrificial corrosion resistance. Therefore, the lower limit of the Bi concentration is preferably more than 0% (preferably 0.1% or more, more preferably 3.0% or more).
On the other hand, when the Bi concentration increases, the flat surface corrosion resistance tends to deteriorate. Therefore, the upper limit of Bi concentration is less than 5.0% (preferably 4.8% or less).

<In: 0% to less than 2.0%>
In is an element that contributes to sacrificial corrosion resistance. Therefore, the lower limit of the In concentration is preferably more than 0% (preferably 0.1% or more, more preferably 1.0% or more).
On the other hand, when the In concentration increases, the flat surface corrosion resistance tends to deteriorate. Therefore, the upper limit of In concentration is set to less than 2.0% (preferably 1.8% or less).

<Ca: 0% to 3.00%>
Ca is an element capable of adjusting the optimum Mg elution amount for imparting planar portion corrosion resistance and sacrificial corrosion resistance. Therefore, the lower limit value of the Ca concentration is preferably more than 0% (preferably 0.05% or more).
On the other hand, when the Ca concentration increases, the flat surface corrosion resistance and workability tend to deteriorate. Therefore, the upper limit value of the Ca concentration is 3.00% or less (preferably 1.00% or less).

<Y: 0% to 0.5%>
Y is an element contributing to sacrificial corrosion resistance. Therefore, the lower limit of the Y concentration is preferably more than 0% (preferably 0.1% or more).
On the other hand, when the Y concentration increases, the flat surface corrosion resistance tends to deteriorate. Therefore, the upper limit of the Y concentration is 0.5% or less (preferably 0.3% or less).

<La and Ce: 0% to less than 0.5%>
La and Ce are elements that contribute to sacrificial corrosion resistance. Therefore, the lower limits of the La concentration and the Ce concentration are each preferably more than 0% (preferably 0.1% or more).
On the other hand, when the La concentration and the Ce concentration increase, the flat surface corrosion resistance tends to deteriorate. Therefore, the upper limits of the La concentration and the Ce concentration are each less than 0.5% (preferably 0.4% or less).

<Si: 0% to less than 2.5%>
Si is an element that contributes to improving 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).
On the other hand, when the Si concentration is increased, the flat portion corrosion resistance, sacrificial corrosion resistance, and workability tend to deteriorate. Therefore, the upper limit value of the Si concentration is less than 2.5%. In particular, from the viewpoint of the corrosion resistance and the sacrificial corrosion resistance, the Si concentration is preferably 2.4% or less, more preferably 1.8% or less, and further preferably 1.2% or less.

<Cr, Ti, Ni, Co, V, Nb, Cu and Mn: 0% to less than 0.25%>
Cr, Ti, Ni, Co, V, Nb, Cu and Mn are elements that contribute to sacrificial corrosion resistance. Therefore, the lower limit values of the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn each preferably exceed 0% (preferably 0.05% or more, more preferably 0.1% or more).
On the other hand, when the concentration of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn increases, the planar portion corrosion resistance tends to deteriorate. Therefore, the upper limit values of the concentrations of Cr, Ti, Ni, Co, V, Nb, Cu, and Mn are each less than 0.25%. The upper limit of the concentration of Cr, Ti, Ni, Co, V, Nb, Cu and Mn is preferably 0.22% or less.

<Fe: 0% to 5.0%>
When the plating layer is formed by the hot dipping method, the Zn—Al—Mg alloy layer and the Al—Fe alloy layer contain a certain Fe concentration.
It has been confirmed that when the Fe concentration is up to 5.0%, the performance is not adversely affected even if it is contained in the plating layer (in particular, the Zn—Al—Mg alloy layer). Since much of Fe is often contained in the Al—Fe alloy layer, the Fe concentration generally increases as the thickness of this layer increases.

<Sr, Sb, Pb and B: 0% to less than 0.5%>
Sr, Sb, Pb and B are elements that contribute to sacrificial corrosion resistance. Therefore, the lower limit values of the concentrations of Sr, Sb, Pb and B are each preferably greater than 0% (preferably 0.05% or more, more preferably 0.1% or more).
On the other hand, when the concentrations of Sr, Sb, Pb and B are increased, the flat surface corrosion resistance tends to deteriorate. Therefore, the upper limit values of the concentrations of Sr, Sb, Pb, and B are each less than 0.5%.

<Impurity>
An impurity refers to a component contained in a raw material or a component mixed in a manufacturing process and not intentionally included. For example, a component other than Fe may be mixed in the plating layer as impurities due to mutual atomic diffusion between the base steel material and the plating bath.

The chemical component of the plating layer is measured by the following method.
First, an acid solution is obtained in which the plating layer is peeled and dissolved with an acid containing an inhibitor that suppresses corrosion of the base steel material. Next, by measuring the obtained acid solution by ICP analysis, the chemical composition of the plating layer (in the case where the plating layer is a single layer structure of a Zn—Al—Mg alloy layer, the chemical composition of the Zn—Al—Mg alloy layer) In the case where the plating layer has a laminated structure of an Al—Fe alloy layer and a Zn—Al—Mg alloy layer, the total chemical composition of the Al—Fe alloy layer and the Zn—Al—Mg alloy layer can be obtained. The acid species is not particularly limited as long as it is an acid that can dissolve the plating layer. The chemical composition is measured as an average chemical composition.

Next, the metal structure of the Zn—Al—Mg alloy layer will be described.

The metal structure of the Zn—Al—Mg alloy layer has Al crystals, and the average value of the total perimeter of Al crystals is 88 to 195 mm / mm ² .

If the average value of the total peripheral length of the Al crystal is less than 88 mm / mm ² , the Al crystal is excessively coarsened and the workability is deteriorated.
On the other hand, if the average value of the cumulative perimeter length of the Al crystal exceeds 195 mm / mm ² , the Al crystal is refined, and the contact area between the Al crystal and the surrounding phase of the Al crystal increases. As a result, the larger the contact area between the Al crystal and the surrounding phase of the Al crystal, the easier the corrosion around the Al crystal occurs and the flat surface corrosion resistance deteriorates, and the variation in the flat surface corrosion resistance also increases.
Therefore, the average value of the total peripheral length of Al crystals is set to 88 to 195 mm / mm ² . The lower limit value of the average value of the cumulative peripheral length of the Al crystal is preferably 95 mm / mm ² or more, more preferably 105 mm / mm ² or more. The upper limit of the average value of the cumulative peripheral length of the Al crystal is preferably 185 mm / mm ² or less, more preferably 170 mm / mm ² or less.

The metal structure of the Zn—Al—Mg alloy layer has Al crystals. The metal structure of the Zn—Al—Mg alloy layer may have a Zn—Al phase in addition to the Al crystal.

The Al crystal corresponds to “α phase in which Zn having a concentration of 0 to 3% is dissolved”. On the other hand, the Zn—Al phase corresponds to “β phase containing 70% to 85% Zn phase (η phase) and finely separating α phase and Zn phase (η phase)”.

Here, FIGS. 1 to 3 show examples of SEM reflected electron images of the Zn—Al—Mg alloy layer on the polished surface obtained by polishing the surface of the Zn—Al—Mg alloy layer to ½ of the layer thickness. FIG. 1 is a reflected electron image of an SEM at a magnification of 100, FIG. 2 at a magnification of 500, and FIG. 3 at a magnification of 10,000.
1 to 3, Al represents an Al crystal, Zn—Al represents a Zn—Al phase, MgZn ₂ represents an MgZn ₂ phase, and Zn—Eu represents a Zn-based eutectic phase.

In the reflected electron image of the Zn—Al—Mg alloy layer, the area fraction of each structure is not particularly limited, but the area fraction of the Al crystal is preferably 8 to 45% from the viewpoint of stable improvement of the corrosion resistance of the plane portion. 15 to 35% is more preferable. That is, the Al crystal is preferably present in the range of the area fraction.

Examples of the remaining structure other than the Al crystal and the Zn—Al phase include an MgZn ₂ phase and a Zn-based eutectic phase (specifically, Zn—Al—MgZn ₂ —Mg ₂ Sn).

Here, a method for measuring the average value of the total perimeter of Al crystals and the area fraction of Al crystals will be described.

The average value of the total perimeter of Al crystals and the area fraction of Al crystals were determined by polishing the surface of the Zn—Al—Mg alloy layer to half the layer thickness and then using a scanning electron microscope at a magnification of 100 times. It is measured using the backscattered electron image of the Zn—Al—Mg alloy layer obtained when observed. Specifically, it is as follows.

First, a sample is taken from the plated steel material to be measured. However, the sample is collected from a place where there is no defective portion of the plating layer other than the vicinity of the punched end surface portion (2 mm from the end surface) of the plated steel material.

Next, the surface of the sample plating layer (specifically, the Zn—Al—Mg alloy layer) is polished in the thickness direction of the plating layer (hereinafter also referred to as “Z-axis direction”).
Polishing of the surface of the plating layer in the Z-axis direction is performed by polishing the surface of the Zn—Al—Mg alloy layer to ½ of the layer thickness. In this polishing, the surface of the Zn—Al—Mg alloy layer is dry-polished with a # 1200 polishing sheet, and then a finishing solution containing alumina with an average particle size of 3 μm, a finishing solution containing alumina with an average particle size of 1 μm, and colloidal Each of the finishing liquids containing silica is finished and polished in this order.
Before and after polishing, the Zn intensity on the surface of the Zn—Al—Mg alloy layer was measured by XRF (fluorescence X-ray analysis), and the Zn intensity after polishing became 1/2 of the Zn intensity before polishing. The thickness of the Zn—Al—Mg alloy layer is ½.

Next, the polished surface of the sample Zn—Al—Mg alloy layer was observed with a scanning electron microscope (SEM) at a magnification of 100 times, and a reflected electron image of the Zn—Al—Mg alloy layer (hereinafter referred to as “SEM reflected electron image”). Is also called). The SEM observation conditions are an acceleration voltage: 15 kV, an irradiation current: 10 nA, and a field size: 1222.2 μm × 927.8 μm.

In order to identify each phase in the Zn—Al—Mg alloy layer, an FE-SEM or TEM (transmission electron microscope) equipped with an EDS (energy dispersive X-ray analyzer) is used. When TEM is used, FIB (focused ion beam) processing is performed on the polished surface of the Zn—Al—Mg alloy layer of the sample to be measured. After FIB processing, a TEM electron diffraction image of the polished surface of the Zn—Al—Mg alloy layer is obtained. Then, the metal contained in the Zn—Al—Mg alloy layer is identified.

Next, the reflected electron image of the SEM and the identification result of the electron diffraction image of the FE-SEM or TEM are compared, and each phase of the Zn—Al—Mg alloy layer is identified in the reflected electron image of the SEM. In identification of each phase in the Zn—Al—Mg alloy layer, EDS point analysis may be performed, and the result of EDS point analysis and the result of identification of the electron diffraction image of the TEM may be collated. Note that an EPMA apparatus may be used for identification of each phase.

Next, in the reflected electron image of SEM, the three values of gray scale brightness, hue and contrast value indicated by each phase in the Zn—Al—Mg alloy layer are determined. Since the three values of brightness, hue, and contrast value indicated by each phase reflect the atomic number of the element contained in each phase, usually the amount of Al with a small atomic number, the phase with a large content of Mg, black The phase with a large amount of Zn tends to exhibit a white color.

From the EDS collation result, image processing (binarization) that changes the color only in the above three value range indicated by the Al crystal contained in the Zn—Al—Mg alloy layer so as to match the SEM reflected electron image. (For example, only a specific phase is displayed as a white image, and the area (number of pixels) of each phase in the visual field is calculated. See FIG. 4). By performing this image processing, the area fraction of the Al crystal in the Zn—Al—Mg alloy layer in the SEM reflected electron image is obtained.
FIG. 4 is an example of an image obtained by performing image processing (binarization) so that an Al crystal can identify a reflected electron image (SEM reflected electron image) of a Zn—Al—Mg alloy layer. In FIG. 4, Al indicates an Al crystal.

The area fraction of the Al crystal in the Zn—Al—Mg alloy layer is the average value of the area fraction of the Al crystal obtained by the above operation in three fields of view.
If it is difficult to discriminate Al crystals, electron beam diffraction by TEM or EDS point analysis is performed.

As an example, using the binary processing function with two thresholds of WinROOF2015 (image analysis software) manufactured by Mitani Corporation, the Al crystal in the reflected electron image of SEM (grayscale image stored in 8 bits, 256 colors display) Describe how to identify. In a gray scale image stored in 8 bits, when the luminous intensity is 0, it represents black, and when the luminous intensity is 255, it represents white. In the case of the reflected electron image of the SEM described above, it has been found from the identification results by FE-SEM and TEM that the Al crystal can be identified with high accuracy when the light intensity threshold is set to 10 and 95. Therefore, the image is processed so that the color range of these luminosities of 10 to 95 changes, and the Al crystal is identified. The binarization process may use image analysis software other than WinROOF2015.

Next, by using the automatic shape feature measurement function of WinROOF2015 (image analysis software) manufactured by Mitani Corporation, the perimeter length of the Al crystal identified by the image processing is accumulated to obtain the total perimeter length of the Al crystal. Then, the Al crystal cumulative perimeter is divided by the visual field area to calculate the Al crystal cumulative perimeter per unit area (mm ² ).
This operation is performed in three fields of view, and the arithmetic average of the Al crystal cumulative perimeter per unit area (mm ² ) is defined as “the average value of the Al crystal total perimeter”.

Also, the area fraction of Al crystal can be obtained by using the automatic shape feature measurement function of WinROOF2015 (image analysis software) manufactured by Mitani Corporation. Specifically, the area fraction (area fraction with respect to the visual field area) of the Al crystal identified by binarization in the reflected electron image of the Zn—Al—Mg alloy layer is calculated using this function. And this operation is implemented by 3 visual fields and the calculated average is made into the area fraction of an Al crystal.

The thickness of the Al—Fe alloy layer is measured as follows.
SEM reflected electron image of the cross section of the plating layer (cut surface along the thickness direction of the plating layer) after embedding the resin with a resin (however, the magnification is 5000 times, the size of the field of view is 50 μm long × 200 μm wide, In the field of view in which the Al—Fe alloy layer is visually recognized), the thickness is measured at any five locations of the identified Al—Fe alloy layer. And the arithmetic average of five places is made into the thickness of an interface alloy layer.

Next, an example of a method for producing a plated steel material according to the present disclosure will be described.

The plated steel material of the present disclosure can be obtained by forming a plating layer having the above predetermined chemical composition and metal structure on the surface (that is, one surface or both surfaces) of a base steel material (base steel plate or the like) by a hot dipping method.

Specifically, as an example, the hot dipping process is performed under the following conditions.
First, the temperature of the plating bath is set to the melting point of the plating bath + 20 ° C. or higher, and after pulling up the base steel material from the plating bath, the temperature range from the plating bath temperature to the plating solidification start temperature, from the plating solidification start temperature to the plating solidification start temperature −30 ° C. Cooling at an average cooling rate greater than the average cooling rate in the temperature range.
Next, the temperature range from the plating solidification start temperature to the plating solidification start temperature −30 ° C. is cooled at an average cooling rate of 12 ° C./s or less.
Next, the temperature range from the plating solidification start temperature −30 ° C. to 300 ° C. is cooled at an average cooling rate larger than the average cooling rate in the temperature range from the plating solidification start temperature to the plating solidification start temperature −30 ° C.

In other words, an example of a method for producing a plated steel material according to the present disclosure is that the plating bath temperature is set to the melting point of the plating bath + 20 ° C. or higher, the base steel material is pulled up from the plating bath, and then the average cooling in the temperature range from the plating bath temperature to the plating solidification start temperature is performed. Assuming that the rate is A, the average cooling rate in the temperature range from the plating solidification start temperature to the plating solidification start temperature -30 ° C is B, and the average cooling rate from the plating solidification start temperature -30 ° C to 300 ° C is C, A> The base steel material is subjected to a hot dipping process under the three-stage cooling conditions of B, B ≦ 12 ° C./s, and C> B.

Al crystal is produced by setting the plating bath temperature to the melting point of the plating bath + 20 ° C. or higher and pulling up the base steel material from the plating bath.
Then, by cooling the temperature range from the plating solidification start temperature to the plating solidification start temperature −30 ° C. at an average cooling rate of 12 ° C./s or less, Al crystals exist in the Zn—Al—Mg alloy layer, and Al crystals A metal structure is formed in which the average value of the cumulative perimeter lengths is in the above range. The cooling at the average cooling rate is performed by, for example, air cooling in which the atmosphere is blown with a weak wind.
However, from the viewpoint of preventing plating winding on the top roll or the like, the lower limit value of the average cooling rate in the temperature range from the plating solidification start temperature to the plating solidification start temperature −30 ° C. is 0.5 ° C./s or more.

The plating solidification start temperature can be measured by the following method. A temperature at which a suggested heat peak appears first when the sample is taken from the plating bath and heated by DSC to the melting point of the plating bath + 20 ° C. or higher and cooled at 10 ° C./min is the plating solidification start temperature.

In the method for producing a plated steel material according to the present disclosure, the average cooling rate in the temperature range from the temperature at which the base steel material is pulled up from the plating bath (that is, the plating bath temperature) to the plating solidification start temperature is not particularly limited. From the viewpoints of preventing plating winding on the surface and suppressing appearance defects such as wind ripples, the temperature is preferably set to 0.5 ° C./s to 20 ° C./s.
However, the average cooling rate in the temperature range from the plating bath temperature to the plating solidification start temperature is higher than the average cooling rate in the temperature range from the plating solidification start temperature to the plating solidification start temperature −30 ° C. Thereby, the nucleation site of Al crystal can be increased, and excessive coarsening of Al crystal can be suppressed.

Further, the average cooling rate in the temperature range from the plating solidification start temperature of −30 ° C. to 300 ° C. is not particularly limited, but from the viewpoint of preventing plating winding on the top roll or the like, 0.5 ° C./s to 20 ° C. It is good to set it as deg.
However, the average cooling rate in the temperature range from the plating solidification start temperature −30 ° C. to 300 ° C. is higher than the average cooling rate in the temperature range from the plating solidification start temperature to the plating solidification start temperature −30 ° C. . Thereby, excessive coarsening of the Al crystal can be suppressed and workability can be ensured.

Note that the Al—Fe alloy layer formed between the base steel and the base steel material is rapidly formed and grown immediately after plating immersion in a time of less than 1 second. The growth rate is higher when the plating bath temperature is higher, and is further increased when the immersion time in the plating bath is longer. However, when the temperature of the plating bath is less than 500 ° C., it hardly grows. Therefore, it is better to reduce the immersion time or shift to the cooling process immediately after solidification.

In addition, regarding the plated steel material, if it is solidified once and then reheated to remelt the plating layer, all the constituent phases disappear and become a liquid phase state. Therefore, for example, even in a plated steel material that has been subjected to rapid cooling or the like, it is also possible to perform the structure control defined in the present disclosure in a process of reheating offline and performing an appropriate heat treatment. In this case, it is preferable that the reheating temperature of the plating layer is in the vicinity of the temperature just above the melting point of the plating bath so that the Al—Fe alloy layer does not grow excessively.

Hereinafter, post-processing applicable to the plated steel material of the present disclosure will be described.

In the plated steel material of the present disclosure, a film may be formed on the plating layer. The coating can form one layer or two or more layers. Examples of the type of film directly above the plating layer include a chromate film, a phosphate film, and a chromate-free film. The chromate treatment, phosphate treatment, and chromate-free treatment for forming these films can be performed by known methods.

For chromate treatment, electrolytic chromate treatment, which forms a chromate film by electrolysis, a film is formed by utilizing the reaction with the material, and then the reactive chromate treatment, in which excess treatment liquid is washed away, is applied to the substrate. There is a coating-type chromate treatment in which a film is formed by drying without washing with water. Any processing may be adopted.

Electrolytic chromate treatment includes electrolytic chromate treatment using chromic acid, silica sol, resin (acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resin, etc.), and hard silica. It can be illustrated.

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

Chromate-free treatment is particularly suitable because it has no environmental impact. For chromate-free treatment, electrolytic chromate-free treatment that forms a chromate-free coating by electrolysis, a reaction-type chromate-free treatment that removes excess treatment liquid, and a treatment solution that forms a film using the reaction with the material. There is a coating-type chromate-free treatment in which a film is formed by applying to an object and drying without washing with water. Any processing may be adopted.

Further, one or more organic resin films may be provided on the film immediately above the plating layer. The organic resin is not limited to a specific type, and examples thereof include polyester resins, polyurethane resins, epoxy resins, acrylic resins, polyolefin resins, and modified products of these resins. Here, the modified product is obtained by reacting a reactive functional group contained in the structure of these resins with another compound (such as a monomer or a crosslinking agent) containing a functional group capable of reacting with the functional group in the structure. Refers to resin.

As such an organic resin, one or two or more organic resins (unmodified) may be mixed and used, or in the presence of at least one organic resin, at least one other One or two or more organic resins obtained by modifying the organic resin may be used. The organic resin film may contain an arbitrary colored pigment or rust preventive pigment. What was made water-based by melt | dissolving or disperse | distributing in water can also be used.

An embodiment of the present disclosure will be described, but the conditions in the embodiment are one condition example adopted to confirm the feasibility and effect of the present disclosure, and the present disclosure is limited to this one condition example. It is not a thing. The present disclosure can adopt various conditions as long as the object of the present disclosure is achieved without departing from the gist of the present disclosure.

(Example)
A predetermined amount of pure metal ingot was used to melt the ingot in a vacuum melting furnace so that a plating layer having the chemical composition shown in Tables 1 and 2 was obtained, and then a plating bath was built in the atmosphere. A batch-type hot dipping apparatus was used for producing the plated steel sheet.
As the base steel material, a 2.3 mm general hot-rolled carbon steel plate (C concentration <0.1%) was used, and degreasing and pickling were performed immediately before the plating step.
In some examples, a Ni pre-plated steel sheet obtained by performing Ni pre-plating on a 2.3 mm general hot-rolled carbon steel sheet was used as the base steel material. The Ni adhesion amount was 2 g / m ² . In addition, the example which used Ni pre-plated steel plate as a base steel material was described as "Ni pre-plating" in the column of "base steel material" in a table | surface.

In any sample preparation, the base steel material was subjected to an equivalent reduction treatment method until the plating bath was immersed. That is, the green body steel _{_{N 2 -H 2 (5%)}} ( a dew point of -40 ℃ or less, an oxygen concentration less than 25 ppm) environment, the temperature was raised in electrically heated to 800 ° C. from room temperature, after holding 60 sec, _{N 2} It was cooled to the plating bath temperature + 10 ° C. by gas blowing and immediately immersed in the plating bath.
In any of the plated steel sheets, the immersion time in the plating bath was the time shown in the table. N2 gas wiping pressure was adjusted, and a plated steel sheet was prepared so that the plating thickness was 30 μm (± 1 μm).

The plating bath temperature was based on a melting point of + 20 ° C., and the plating was performed by raising the temperature further at some levels. The plating bath immersion time was 2 seconds. After the base steel material was pulled out of the plating bath, a plating layer was obtained by a cooling process in which the following average cooling rates in the first to third stages shown in Tables 1 and 2 were set as shown in Tables 1 and 2.
・ First stage average cooling rate: Average cooling rate in the temperature range from plating bath temperature to plating solidification start temperature ・ Second stage average cooling rate: Average cooling in the temperature range from plating solidification start temperature to plating solidification start temperature −30 ° C Speed ・ 3rd stage average cooling rate: Plating solidification start temperature-Temperature range average cooling rate from -30 ℃ to 300 ℃

-Various measurements-
A sample was cut out from the obtained plated steel sheet. The following items were measured according to the method described above.
・ Average value of the total perimeter of Al crystal (indicated as “Al crystal perimeter” in the table)
-Area fraction of Al crystal-Thickness of Al-Fe alloy layer (however, in the example using Ni pre-plated steel sheet as the base steel material, the thickness of the Al-Ni-Fe alloy layer is shown)

-Corrosion resistance of flat surface-
In order to compare stable flat surface corrosion resistance, the manufactured sample was subjected to 120 cycles of corrosion acceleration test (JASO M609-91) and immersed in 30% chromic acid aqueous solution at room temperature to remove white rust. Corrosion resistance was evaluated. The test was performed 5 times, and the average corrosion weight loss was 80 g / m ² or less, and the maximum value and the minimum value of corrosion weight loss when n = 5 were within ± 100% of the average value. The case where the corrosion weight loss is 100 g / m ² or less and the maximum value and the minimum value of the corrosion weight loss during n = 5 are within ± 100% of the average value, and “NG” evaluation is given otherwise. .

-Sacrificial anti-corrosion (corrosion resistance at the edge of the cut part)
In order to compare the sacrificial corrosion resistance (corrosion resistance at the edge of the cut part), the specimen was cut into a shear of 50 mm × 100 mm, the upper and lower ends were sealed, and subjected to a corrosion acceleration test (JASO M609-91) for 120 cycles. The average value of the area ratio of red rust occurrence was evaluated. A red rust occurrence area ratio of 50% or less was evaluated as “A +”, 70% or less as “A”, and more than 70% as “NG”.

-Processability-
In order to evaluate the workability of the plating layer, the plated steel sheet was bent by 90 ° V, and a cellophane tape having a width of 24 mm was pressed against the valley of the V-bending and pulled apart, and the powdering was visually evaluated. The case where powdering peeling powder did not adhere to the tape was evaluated as “A”, the case where it slightly adhered was evaluated as “A-”, and the case where it was adhered as “NG”.

-Comprehensive evaluation-
Examples where the evaluation results of the corrosion resistance evaluation, sacrificial anticorrosion property and workability evaluation are all “A”, “A +” or “A−” as “A”, and at least “NG” is evaluated as “NG” did.

Examples are listed in Tables 1 and 2.

From the said result, it turns out that the Example applicable to the plated steel material of this indication has the stable plane part corrosion resistance compared with a comparative example.
In particular, even if the chemical composition of the plating layer of the present disclosure is satisfied, the average value of the cumulative perimeter of the Al crystal is excessively large in the comparative example (test No. 70) in which the average cooling rate is not changed at 15 ° C./s. It can be seen that stable flat surface corrosion resistance is not obtained.
On the other hand, a comparative example (Comparative Example No. 71) in which the average cooling rate of the second stage is excessively low, a comparative example (Test No. 72) in which the average cooling rate was changed only in two stages, and the average cooling rate at 6 ° C / s It can be seen that the comparative example (Test No. 73) which is not changed has an average value of the total peripheral length of Al crystals that is excessively small and the workability is deteriorated.

The preferred embodiments of the present disclosure have been described in detail above with reference to the accompanying drawings, but the present disclosure is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present disclosure belongs can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present disclosure.

The description of the symbols is as follows.
Al Al crystal Zn-Al Zn-Al phase MgZn ₂ MgZn ₂ phase Zn-Eu Zn-based eutectic phase

The entire disclosure of Japanese Patent Application No. 2018-094481 is incorporated herein by reference.
All documents, patent applications, and technical standards mentioned in this specification are to the same extent as if each individual document, patent application, and technical standard were specifically and individually described to be incorporated by reference, Incorporated herein by reference.

Claims

A plated steel material comprising: a base steel material; and a plating layer including a Zn—Al—Mg alloy layer disposed on a surface of the base steel material,
The plating layer is mass%,
Zn: more than 65.0%,
Al: more than 5.0% to less than 25.0%,
Mg: more than 3.0% to less than 12.5%,
Sn: 0.1% to 20.0%,
Bi: 0% to less than 5.0%,
In: 0% to less than 2.0%,
Ca: 0% to 3.00%,
Y: 0% to 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% to less than 0.25%,
Ti: 0% to less than 0.25%,
Ni: 0% to less than 0.25%,
Co: 0% to less than 0.25%,
V: 0% to less than 0.25%
Nb: 0% to less than 0.25%,
Cu: 0% to less than 0.25%,
Mn: 0% to less than 0.25%,
Fe: 0% to 5.0%,
Sr: 0% to less than 0.5%,
Sb: 0% to less than 0.5%,
Pb: 0% to less than 0.5%,
B: having a chemical composition consisting of 0% to less than 0.5% and impurities,
In the backscattered electron image of the Zn—Al—Mg alloy layer, obtained by polishing the surface of the Zn—Al—Mg alloy layer to ½ of the layer thickness and then observing it at a magnification of 100 times with a scanning electron microscope, A plated steel material in which an Al crystal is present and an average value of the total peripheral length of the Al crystal is 88 to 195 mm / mm 2 .
2. The plated steel material according to claim 1, wherein the plated layer has an Al—Fe alloy layer having a thickness of 0.05 to 5 μm between the base steel material and the Zn—Al—Mg alloy layer.