JP7853618B2 - Plated steel sheet - Google Patents

Description

This invention relates to plated steel sheets.

In recent years, the demand for high-strength steel sheets has been increasing in the automotive and building materials sectors due to the need for weight reduction. High-strength steel sheets often require various processing methods, such as pressing and bending, to achieve desired shapes, and high corrosion resistance is frequently required. Therefore, high-strength steel sheets must possess a combination of strength, workability, and corrosion resistance. To achieve high corrosion resistance, various types of plated steel sheets, using high-strength steel sheets as the base material, are increasingly being used.

When high-strength steel sheets are used as the base material for plating, it is known that hydrogen embrittlement is likely to occur due to hydrogen inevitably penetrating the steel during the plating process. In particular, with molten Zn-Al-Mg plated steel sheets, hydrogen embrittlement can occur even when relatively low-strength steel sheets, such as those with a 590 MPa rating, are used as the base material. Therefore, various countermeasures against hydrogen embrittlement have been proposed.

For example, Patent Document 1 below proposes a technique for reducing the hydrogen concentration in steel by applying strain to a molten Zn-Al-Mg plated steel sheet to introduce cracks into the plating layer, and then performing a baking treatment on the plated steel sheet.

Japanese Patent Publication No. 2018-204065

The present inventors examined the technology proposed in Patent Document 1 and found that in such technology, it is important to strike a balance between achieving the amount of crack introduction necessary to facilitate hydrogen desorption and the reduction in ductility of the steel sheet caused by introducing too many cracks. In order to satisfy this trade-off relationship, Patent Document 1 requires careful attention to the strain applied when introducing cracks to control the amount of cracks introduced, and it was found that there is still room for improvement in terms of hydrogen desorption properties.

This invention has been made in view of the above-mentioned aspects, and the object of this invention is to provide a plated steel sheet that can further improve hydrogen desorption while maintaining corrosion resistance after coating.

The inventors focused on the fact that the Zn-Al-Mg plating disclosed in Patent Document 1 has a relatively hard plating structure. Based on this, the inventors conceived the idea that if a structure that serves as a crack initiation point could be introduced into the plating layer, it would be possible to efficiently generate a more appropriate amount of cracks without paying excessive attention to controlling strain application, thereby enabling further improvement of hydrogen desorption properties.
The gist of the present invention, completed based on this idea, is as follows:

(1) A plated steel sheet having a plating layer on its surface having a chemical composition in which, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, Fe: 0.01 to 15.00%, and optionally further containing one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the remainder being Zn and impurities, wherein the average area ratio of the α-deposited η phase, which is a metallic structure in which the α phase is deposited in the η matrix phase, is 5 to 95% in the surface structure when the surface of the plating layer is viewed in plan, and when a 130 μm × 100 μm region of the surface of the plating layer is observed with an electron microscope, the sum of the lengths of the cracks present in the region is 50 μm or more.
[Element Group A]: One or two elements selected from the group consisting of Si: greater than 0% and 2.00% or less, and Ca: greater than 0% and 2.00% or less. [Element Group B]: One or more elements selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less. [Element Group C]: Cu: greater than 0% and 1. Selected from the group consisting of: 0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.0000% or less. (2) The plated steel sheet according to (1), having a chemical composition containing the above element group A.
(3) The plated steel sheet according to (1), having a chemical composition containing the element group B.
(4) The plated steel sheet according to (1), having a chemical composition containing the element group C.
(5) The plated steel sheet according to (1), having a chemical composition containing the element group D.
(6) The plated steel sheet according to (1), having a chemical composition containing the element group E.
(7) The plated steel sheet according to (1), having a chemical composition containing the element group F.
(8) The plated steel sheet according to (1), having a chemical composition containing the element group G.
(9) The plated steel sheet according to any one of (1) to (8), wherein the plating layer contains 1.00 to 5.00% by mass of Al and 1.00 to 3.00% by mass of Mg.
(10) The plated steel sheet according to (1), wherein the tensile strength of the steel sheet is 980 MPa or more.
(11) The plated steel sheet according to (1), wherein the tensile strength of the steel sheet is 1180 MPa or more.
(12) The plated steel sheet according to (1), wherein the average area ratio of the α precipitated η phase is 5 to 70%.
(13) The plated steel sheet according to (1), wherein the average area ratio of the α precipitated η phase is 5 to 40%.

As described above, according to the present invention, it is possible to further improve the hydrogen desorption properties of Zn-Al-Mg plated steel sheets while maintaining corrosion resistance after coating.

This is an explanatory diagram schematically showing the structure of a plated steel sheet according to an embodiment of the present invention. This is an explanatory diagram schematically showing the structure of a plated steel sheet according to the same embodiment. This is a schematic diagram illustrating the plating layer of the plated steel sheet in the same embodiment. This is a schematic diagram illustrating the plating layer of the plated steel sheet in the same embodiment.

Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, thus omitting redundant explanations.

(Regarding plated steel sheets)
First, the overall configuration of the plated steel sheet according to an embodiment of the present invention will be described with reference to Figures 1A and 1B. Figures 1A and 1B are schematic explanatory diagrams showing an example of a plated steel sheet according to this embodiment.

As schematically shown in Figure 1A, the plated steel sheet 1 according to this embodiment has a base steel sheet 11 and a plating layer 13 located on at least a part of the surface of the steel sheet 11. Furthermore, the plating layer 13 does not have to be present on only one surface of the steel sheet 11 as shown in Figure 1A, but may be present on both surfaces of the steel sheet 11 as schematically shown in Figure 1B.

<Regarding steel plate 11>
The dimensions, composition, structure, and mechanical properties of the steel sheet 11 used as the base material for the plated steel sheet 1 according to this embodiment are not particularly limited. For example, various types of steel sheets can be used depending on the mechanical strength (e.g., tensile strength) required for the plated steel sheet 1. Examples of such steel sheets 11 include steel materials standardized by Japanese Industrial Standards (JIS), such as carbon steel, alloy steel, and high-tensile steel used for general structural and machine structural purposes. Specific examples of such steel materials include cold-rolled steel, hot-rolled steel, hot-rolled steel sheets for automobile structures, hot-rolled high-tensile steel sheets for automobile processing, cold-rolled steel sheets for automobile structures, cold-rolled high-tensile steel sheets for automobile processing, and high-tensile steel generally called hot-stamped steel, which is hardened during hot working. The composition of such steel materials is not particularly limited, but in addition to Fe and C, it may contain one or more of the following: Si, Mn, S, P, Al, N, Cr, Mo, Ni, Cu, Ca, Mg, Ce, Hf, La, Zr, and Sb. One or more of these optional additive elements can be appropriately selected to obtain the desired material strength and formability, and their content can also be appropriately adjusted.

Among the high-strength steels described above, using high-strength steel with a tensile strength of 980 MPa or more (so-called 980 MPa class or higher high-strength steel) is preferable because it is possible to further improve the robustness of the manufactured articles. Here, the tensile strength of the steel plate 11 can be measured by a known method. For example, a test piece can be prepared from a part of the steel plate whose tensile strength is to be measured, as specified in JIS Z 2241:2011, and the tensile strength of the obtained test piece can be measured using the specified method.

Furthermore, the thickness of the steel plate 11 is not particularly limited and can be set appropriately according to the mechanical strength and other requirements for the plated steel plate 1.

<Regarding the plating layer 13>
The plating layer 13 is provided on the surface of the steel sheet 11, as schematically shown in Figures 1A and 1B, and more preferably, it is provided over the entire surface of the steel sheet 11. Below, the chemical composition of such plating layer 13 will be described in detail.

◇Chemical composition of the plating layer 13 The chemical composition of the plating layer 13 according to this embodiment, in one embodiment, contains, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, Fe: 0.01 to 15.00%, with the remainder being Zn and impurities.

Furthermore, according to another embodiment, the chemical composition of the plating layer 13 according to this embodiment contains, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, Fe: 0.01 to 15.00%, and further contains one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the remainder being Zn and impurities. In other words, in the chemical composition of the plating layer 13 according to this embodiment, the content of Al, Mg, and Fe is within the above range, and the total content of Al, Mg, Fe, and element groups A to G is less than 100% by mass, with the remainder being Zn and impurities.

[Element Group A]: One or two elements selected from the group consisting of Si: greater than 0% and 2.00% or less, and Ca: greater than 0% and 2.00% or less. [Element Group B]: One or more elements selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less. [Element Group C]: Cu: greater than 0% and 1.0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1. One or more elements selected from the group consisting of 0% or less [Element Group D]: Sn: greater than 0% and 1.0000% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less. One or more elements selected from the group consisting of 0% or less [Element Group E]: Zr: greater than 0% and 1.0000% or less, Ag: greater than 0% and 1.0000% or less, and Li: greater than 0% and 1.0000% or less. One or more elements selected from the group consisting of 0% or less [Element Group F]: La: greater than 0.5000% or less, Ce: greater than 0% and 0.5000% or less, and Y: greater than 0% and 0.5000% or less. One or more elements selected from the group consisting of 0% or less [Element Group G]: B: greater than 0% and 0.5000% or less.

Thus, the plating layer 13 according to this embodiment has a chemical composition in which, by mass%, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, Fe: 0.01 to 15.00%, and optionally further contains one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the remainder being Zn and impurities.

[Al: 0.50 to 5.00% by mass]
Al is an element necessary for constituting the main phase (Zn-Al-Mg alloy phase) of the plating layer 13 according to this embodiment, and is included in a certain amount or more to ensure corrosion resistance in the weld heat-affected zone and the non-welded areas after welding as a plated steel sheet. If the Al content in the plating layer 13 is less than 0.50 mass%, the corrosion resistance of the weld heat-affected zone and non-welded areas as described above cannot be guaranteed. Therefore, in the plating layer 13 according to this embodiment, the Al content is 0.50 mass or more. Preferably, the Al content is 1.00 mass or more, and more preferably 1.50 mass or more. By having an Al content within the above range, it is possible to guarantee the corrosion resistance of the plated steel sheet 1.

On the other hand, if the Al content in the plating layer 13 exceeds 5.00% by mass, the solidification behavior from the liquid phase to the solid phase of the plating layer changes, making it difficult to crystallize the η phase, and an Al-dominant dendritic structure is formed. When an Al-dominant dendritic structure is formed, it promotes corrosion of the surrounding area, making it impossible to guarantee the corrosion resistance of the plated steel sheet 1. In addition, the lack of η phase reduces the interface between the η phase, which is a source of crack generation, and the eutectic structure, which can lead to a lack of cracks generated during processing. For this reason, in the plating layer 13 according to this embodiment, the Al content is 5.00% by mass or less. Preferably, the Al content is 4.00% by mass or less, and more preferably 3.00% by mass or less.

[Mg: 0.50 to 3.00% by mass]
Mg is an element necessary for constituting the main phase (Zn-Al-Mg alloy phase) of the plating layer 13 according to this embodiment, and is included in a certain amount or more to ensure corrosion resistance in the weld heat-affected zone and corrosion resistance in the non-welded areas of the plated steel sheet. For this reason, the Mg content in the plating layer 13 according to this embodiment is 0.50% by mass or more. Preferably, the Mg content is 1.50% by mass or more, and more preferably 2.00% by mass or more. By keeping the Mg content within the above range, it is possible to ensure the corrosion resistance of the plated steel sheet 1. If the Mg content exceeds 3.00%, it becomes difficult for the α phase to precipitate from the η phase. Since the η phase, in which Al is solid-dissolved, is hard, deformation cannot be concentrated in the η phase during processing, which causes a decrease in the number of cracks. For this reason, the upper limit of the Mg content is 3.00%.

On the other hand, if the Mg content in the plating layer 13 is less than 0.50% by mass, the effect of improving corrosion resistance by modifying corrosion products is insufficient, and therefore the corrosion resistance of the plated steel sheet 1 cannot be guaranteed. Furthermore, if the Mg content is insufficient, the entire plating layer 13 becomes soft, and deformation cannot be concentrated in the η phase during processing, which causes a decrease in the number of cracks. For this reason, in the plating layer 13 according to this embodiment, the Mg content is 0.50% by mass or more. Preferably, the Mg content is 1.50% by mass or more, and more preferably 2.00% by mass or more. By having an Mg content within the above range, it is possible to guarantee the corrosion resistance of the plated steel sheet 1.

[Fe: 0.01 to 15.00% by mass]
The plating layer 13 may contain elements from the base material, the steel sheet 11. In particular, in the hot-dip galvanizing method, elements from the steel sheet 11 are easily mixed into the plating layer 13 due to interdiffusion of elements caused by solid-liquid reactions between the steel sheet 11 and the plating layer 13. Due to this mixing of elements, a certain amount of Fe is contained in the plating layer 13, and this content is generally 0.01% by mass or more. If the above interdiffusion is promoted, the adhesion between the steel sheet 11 and the plating layer 13 is improved. From the viewpoint of improving the adhesion between the steel sheet 11 and the plating layer 13, it is preferable that the Fe content in the plating layer 13 be 0.20% by mass or more.

Furthermore, within a range that does not impair the effects of the present invention, Fe may be intentionally added to the plating bath used when manufacturing the plating layer 13. However, if the Fe content in the plating layer 13 is 15.00% by mass or more, high-melting-point intermetallic compounds of Fe and Al will form in the plating bath, and such high-melting-point intermetallic compounds will adhere to the plating layer as dross, significantly degrading the appearance quality, which is undesirable. From this viewpoint, the Fe content in the plating bath is adjusted so that the Fe content in the plating layer 13 is 15.00% by mass or less. More preferably, the Fe content in the plating layer 13 is 10.00% by mass or less.

In the plating layer 13, the remainder of the Al, Mg, and Fe consists of Zn and impurities.
Zn is an essential element for constituting the main phase (Zn-Al-Mg alloy phase) of the plating layer 13 according to this embodiment, and is an important element for improving the corrosion resistance of the plated steel sheet. Furthermore, by containing Al, Mg, and Fe within the above ranges, and also containing Zn, the corrosion resistance of the plated steel sheet can be ensured.

Next, we will describe in detail the element groups A to E that may be present in the chemical composition of the plating layer 13 according to another embodiment of this product.

Furthermore, in the plating layer 13 according to this embodiment, if at least one of the elements belonging to element group B to element group E below is included, it is preferable that at least one of the elements belonging to element group B to element group E below is included within the following content range, and the total content is 5.0000% by mass or less.

By keeping the total content of elements belonging to element groups B to E to 5.0000% by mass or less, it becomes possible to enjoy the effects exhibited by the addition of each element, as detailed below, without impairing each other. The total content of elements belonging to element groups B to E is preferably 1.0000% by mass or less, and more preferably 0.2000% by mass or less.

◇Element group A
In another embodiment of the plating layer 13 according to this embodiment, the element group A that the plating layer 13 may contain will be described. At least one of the elements of element group A shown below may be included in the plating layer 13 in place of a portion of the remaining Zn.
[Element Group A]: One or two elements selected from the group consisting of Si: greater than 0% and 2.00% or less, and Ca: greater than 0% and 2.00% or less.

[Si: 0 to 2.00% by mass]
Since it is possible that the plating layer 13 according to this embodiment does not contain Si, the lower limit of the Si content is 0% by mass. On the other hand, Si is an element that can suppress the excessive growth of the Fe-Al intermetallic compound phase formed at the interface between the plating layer and the steel sheet, and further improve the adhesion between the plating layer and the steel sheet. When Si is included in the plating layer 13, the Si content is preferably 0.05% by mass or more, and more preferably 0.10% by mass or more, in order to suppress the excessive growth of the Fe-Al intermetallic compound phase.

On the other hand, if the Si content exceeds 2.00% by mass, the Si may excessively form a high-melting-point intermetallic compound phase with Mg, potentially inhibiting the formation of an Al-Mg oxide film that has a Zn evaporation suppression effect. Therefore, it becomes difficult to suppress Zn evaporation when welding such plated steel sheets. For this reason, the Si content in the plating layer 13 is preferably 2.00% by mass. Furthermore, if the Si content in the plating bath used to produce the plating layer 13 is too high, the viscosity of the plating bath may increase unnecessarily, potentially reducing the plating operability. Therefore, the Si content in the plating bath is adjusted from the viewpoint of plating operability, and the Si content in the plating layer 13 is preferably 1.00% by mass or less, and more preferably 0.50% by mass or less.

[Ca: 0 to 2.00% by mass]
Since the plating layer 13 according to this embodiment may not contain Ca, the lower limit of the Ca content is 0% by mass. On the other hand, when Ca is contained in the plating layer 13, it forms an intermetallic compound phase with Al and Zn. Furthermore, when Si is contained together with Ca in the plating layer 13, Ca forms an intermetallic compound phase with Si. These intermetallic compound phases have high melting points and stable structures, making it possible to suppress liquid metal embrittlement cracking (LME) during welding of plated steel sheets. When Ca is contained in the plating layer 13, this LME suppression effect during welding is achieved by setting the Ca content to 0.01% by mass or more. The Ca content in the plating layer 13 is more preferably 0.05% by mass or more.

On the other hand, if the Ca content in the plating layer 13 exceeds 2.00% by mass, the corrosion resistance of the plated steel sheet may decrease. From this viewpoint, the Ca content in the plating layer 13 is 2.00% by mass or less. Preferably, the Ca content in the plating layer 13 is 1.50% by mass or less, and more preferably 1.00% by mass or less.

◇Element group B
Next, in another embodiment of the plating layer 13 according to this embodiment, the group of elements B that the plating layer 13 may contain will be described. At least one of the elements of the group of elements B shown below is an element that can be contained in the plating layer 13 in place of a portion of the remaining Zn.
[Element Group B]: One or more elements selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less.

[Sb: 0 to 0.5000% by mass]
[Pb: 0 to 0.5000% by mass]
[Sr: 0 to 0.5000% by mass]
Since the plating layer 13 according to this embodiment may not contain Sb, Pb, or Sr, the lower limit of the content of these elements is 0% by mass. On the other hand, if at least one of Sb, Pb, or Sr is contained in the plating layer 13, spangles are formed on the surface of the plating layer 13, making it possible to improve the metallic luster. Therefore, from the viewpoint of improving the design of the plated steel sheet, it is preferable that at least one of Sb, Pb, or Sr is contained in the plating layer 13. This design improvement effect is manifested when the content of at least one of Sb, Pb, or Sr is 0.0500% by mass or more. Therefore, when at least one of Sb, Pb, or Sr is contained in the plating layer 13, it is preferable that the content of each of these elements be independently 0.0500% by mass or more.

On the other hand, when forming a plating layer 13 in which the content of any of Sb, Pb, or Sr exceeds 0.5000% by mass, the amount of dross generated in the plating bath used to form the plating layer 13 increases, making it impossible to produce a plated steel sheet with good plating properties. Therefore, the content of Sb, Pb, and Sr in the plating layer 13 is independently 0.5000% by mass or less. Preferably, the content of Sb, Pb, and Sr is independently 0.2000% by mass or less.

◇Element group C
Next, in another embodiment of the plating layer 13 according to this embodiment, the group of elements C that the plating layer 13 may contain will be described. At least one of the elements of the group of elements C shown below is an element that can be contained in the plating layer 13 in place of a portion of the remaining Zn.
[Element Group C]: One or more elements selected from the group consisting of Cu: greater than 0% and 1.0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.0000% or less.

[Cu: 0 to 1.0000% by mass]
[Ti: 0 to 1.0000% by mass]
[Cr: 0 to 1.0000% by mass]
[Nb: 0 to 1.0000% by mass]
[Ni: 0 to 1.0000% by mass]
[Mn: 0 to 1.0000% by mass]
[Co: 0 to 1.0000% by mass]
[V: 0 to 1.0000% by mass]
Since the plating layer 13 according to this embodiment may not contain Cu, Ti, Cr, Nb, Ni, Mn, Co, or V, the lower limit of the content of these elements is 0 mass%. On the other hand, if at least one of Cu, Ti, Cr, Nb, Ni, Mn, Co, or V is contained in the plating layer 13, when such plated steel sheet is welded, these elements are incorporated into the Al-Fe alloy phase produced by welding, making it possible to improve the corrosion resistance of the welded part. This effect of improving the corrosion resistance of the welded part is exhibited when the content of at least one of Cu, Ti, Cr, Nb, Ni, Mn, Co, or V in the plating layer 13 is 0.0050 mass% or more. Therefore, when at least one of Cu, Ti, Cr, Nb, Ni, Mn, Co, or V is contained in the plating layer 13, it is preferable that the content of each of these elements be independently 0.0050 mass% or more.

On the other hand, when forming a plating layer 13 in which the content of any of Cu, Ti, Cr, Nb, Ni, Mn, Co, or V exceeds 1.0000% by mass, these elements form various intermetallic compound phases in the plating bath used to form the plating layer 13, leading to an increase in the viscosity of the plating bath and making it impossible to produce a plated steel sheet with good plating properties. Therefore, the content of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V in the plating layer 13 is independently set to 1.0000% by mass or less. Preferably, the content of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is independently set to 0.2000% by mass or less.

[Mo: 0 to 1.0000% by mass]
Since the plating layer 13 according to this embodiment may not contain Mo, the lower limit of the Mo content is 0% by mass. On the other hand, if Mo is included in the plating layer 13, it is possible to further improve corrosion resistance. This improvement in corrosion resistance is achieved when the Mo content is 0.0100% by mass or more. Therefore, when Mo is included, it is preferable that its content be 0.0100% by mass or more.

On the other hand, forming a plating layer 13 with a Mo content exceeding 1.0000% by mass is undesirable because it causes a large amount of dross to be generated in the plating bath. Therefore, the Mo content is 1.0000% by mass or less. Preferably, the Mo content is 0.0500% by mass or less.

◇Element group D
Next, in another embodiment of the plating layer 13 according to this embodiment, the group of elements D that the plating layer 13 may contain will be described. The elements of the group of elements D shown below are elements that may be contained in the plating layer 13 in place of a portion of the remaining Zn.
[Element Group D]: One or more elements selected from the group consisting of Sn: greater than 0% and 1.0000% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less.

[Sn: 0 to 1.0000% by mass]
[In: 0 to 1.0000% by mass]
[Bi: 0 to 1.0000% by mass]
Since the plating layer 13 according to this embodiment may not contain Sn, In, and Bi, the lower limit of the Sn content is 0% by mass. On the other hand, Sn, In, and Bi are elements that increase the Mg elution rate when the plating layer 13 containing Sn, In, and Bi is placed in a corrosive environment. When the Mg elution rate increases, Mg ions are supplied to the exposed parts of the steel sheet 11, improving corrosion resistance. From this viewpoint, when Sn, In, and Bi are included, it is preferable that the content of Sn, In, and Bi be 0.0050% by mass or more, independently of each other. On the other hand, excessive addition of Sn, In, and Bi may excessively promote the Mg elution rate, potentially reducing the corrosion resistance of the plated steel sheet. Since this increase in the Mg elution rate becomes significant when any of the Sn, In, and Bi content exceeds 1.0000% by mass, the content of Sn, In, and Bi is 1.0000% by mass or less, independently of each other. The content of Sn, In, and Bi is preferably 0.2000% by mass or less, independently of each other.

◇Element group E
Next, in another embodiment of the plating layer 13 according to this embodiment, the group of elements E that the plating layer 13 may contain will be described. At least one of the elements of the group of elements E shown below is an element that can be contained in the plating layer 13 in place of a portion of the remaining Zn.
[Element Group E]: One or more elements selected from the group consisting of Zr: greater than 0% and 1.0000% or less, Ag: greater than 0% and 1.0000% or less, and Li: greater than 0% and 1.0000% or less.

[Zr: 0 to 1.0000% by mass]
[Ag: 0 to 1.0000% by mass]
[Li: 0 to 1.0000% by mass]
Since the plating layer 13 according to this embodiment may not contain Zr, Ag, or Li, the lower limit of the content of these elements is 0% by mass. On the other hand, if at least one of Zr, Ag, or Li is included in the plating layer 13, it is possible to improve the plating operability. This improvement in plating performance is achieved when the content of at least one of Zr, Ag, or Li in the plating layer 13 is 0.0100% by mass or more. Therefore, when at least one of Zr, Ag, or Li is included, it is preferable that the content of each of these elements be independently 0.0100% by mass or more.

On the other hand, when forming a plating layer 13 in which the content of any one of Zr, Ag, or Li exceeds 1.0000% by mass, a large amount of dross tends to be generated in the plating bath used to form the plating layer 13. Therefore, the content of at least one of Zr, Ag, or Li is independently 1.0000% by mass or less. Preferably, the content of at least one of Zr, Ag, or Li is independently 0.1000% by mass or less.

◇Element group F
Next, in another embodiment of the plating layer 13 according to this embodiment, the group of elements F that the plating layer 13 may contain will be described. At least one of the elements in the group of elements F shown below is an element that can be contained in the plating layer 13 in place of a portion of the remaining Zn.
[Element group F]: One or more elements selected from the group consisting of La: greater than 0% and 0.5000% or less, Ce: greater than 0% and 0.5000% or less, and Y: greater than 0% and 0.5000% or less.

[La: 0 to 0.5000% by mass]
[Ce: 0 to 0.5000% by mass]
[Y: 0 to 0.5000% by mass]
In this embodiment, it is possible that the plating layer 13 may not contain La, Ce, and Y; therefore, the lower limit of the content of these elements is 0 mass%. On the other hand, La, Ce, and Y are elements that exhibit almost the same effect as Ca, and further suppress blowhole formation during welding. This is because the atomic radii of each element are close to those of Ca. When these elements are contained in the plating layer 13, they are substituted for Ca. Therefore, these elements are detected at the same position as Ca in EDS. Furthermore, even if these elements become oxides after welding the plated steel sheet, the oxides of these elements are detected at the same position as CaO.

The effect of suppressing blowhole formation during welding is achieved by setting the content of each of these elements to 0.0100% by mass or more, independently. Therefore, when at least one of Zr, Ag, and Li is included, it is preferable that the content of each of these elements be 0.0100% by mass or more, independently. The content of La, Ce, and Y in the plating layer 13 is more preferably 0.0500% by mass or more, independently.

On the other hand, if the La, Ce, and Y content in the plating bath for producing the plating layer 13 is too high, the viscosity of the plating bath may increase excessively, potentially reducing the operability of the plating process. Therefore, from the viewpoint of operability, the La, Ce, and Y content in the plating bath is adjusted so that the content of La, Ce, and Y is independently 0.5000% by mass or less. Preferably, the content of La, Ce, and Y is independently 0.1000% by mass or less.

◇Element group G
Next, in another embodiment of the plating layer 13 according to this embodiment, the group of elements G that the plating layer 13 may contain will be described. The elements of the group of elements G shown below are elements that may be contained in the plating layer 13 in place of a portion of the remaining Zn.
[Element group G]: B: More than 0% and not more than 0.5000%

[B: 0 to 0.5000% by mass]
In this embodiment, it is possible that the plating layer 13 may not contain B, so the lower limit of its content is 0% by mass. On the other hand, when B is included in the plating layer 13, it has the effect of suppressing LME. This is presumed to be because when B is included in the plating layer 13, it combines with at least one of Zn, Al, Mg, and Ca to form various intermetallic compound phases. Furthermore, it is thought that the presence of B in the plating layer 13 causes B to diffuse from the plating layer 13 to the steel sheet 11, and that this has the effect of suppressing LME of the steel sheet 11 through grain boundary strengthening. Moreover, it is presumed that the various intermetallic compounds formed with respect to B have extremely high melting points and therefore also act to suppress Zn evaporation during welding. These improvement effects are achieved by including 0.0500% by mass or more of B. Therefore, when B is included, the B content is preferably 0.0500% by mass or more.

On the other hand, if an excessive amount of B is added to the plating bath in order to include B in the plating layer 13, it causes a rapid increase in the plating melting point, reducing the plating operability and making it impossible to manufacture a plated steel sheet with excellent plating properties. This reduction in plating operability becomes significant when the B content exceeds 0.50% by mass, so the B content is 0.5000% by mass or less. Preferably, the B content is 0.1000% by mass or less.

[Method for measuring chemical components]
The chemical composition of the plating layer 13 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometer) or ICP-MS (Inductively Coupled Plasma Mass Spectrometer). When analyzing chemical components down to 0.1 mass%, ICP-AES should be used, while for analyzing trace amounts of chemical components less than 0.1 mass%, ICP-MS should be used. The plated steel sheet is immersed in a 10% HCl aqueous solution with an inhibitor for about 1 minute to remove the plating layer, and a solution is prepared by dissolving this plating layer. The resulting solution can be analyzed using ICP-AES or ICP-MS to obtain the overall average chemical composition of the plating layer.

◇Regarding the amount of plating layer 13 attached As described above, there is no particular requirement for the amount of plating layer 13 attached, but for example, it is preferable to have about 15 to 250 g/ ^m² per side of the steel sheet. By having the amount of plating layer 13 attached within the above range, the plated steel sheet 1 according to this embodiment can exhibit sufficient corrosion resistance.

The amount of the plating layer 13 is measured as follows. First, a sample measuring 30 mm x 30 mm in plan view is cut from the plated steel sheet, and its mass is measured beforehand. When cutting the sample, the entire thickness is cut out. A tape seal is applied to one side of the sample to prevent the plating layer on that side from dissolving in the next step. Then, the sample is immersed in a 10% HCl aqueous solution with an inhibitor added to pickle and remove the plating layer, and the mass of the sample after pickling is measured. From the change in mass of the sample before and after pickling, it is possible to determine the amount of the plating layer 13 attached to each side.

◇Regarding the Metal Structure of the Plating Layer 13 Next, we will explain the metal structure of the plating layer 13 having the chemical composition described above.
The plating layer 13 according to this embodiment has the above chemical composition. Furthermore, the plating layer 13 is formed by the manufacturing method described in detail below and contains metallic phases and intermetallic compound phases such as α phase, η-Zn phase, MgZn ₂ phase, η/α/MgZn ₂ ternary eutectic phase, and η/MgZn ₂ binary eutectic phase. Depending on the elements that the plating layer 13 may further contain, the plating layer 13 may also contain intermetallic compounds such as Al-Si-Ca phase, Al-Si-Ca-Fe phase, Mg ₂ Si phase, and Mg ₂ Sn phase in addition to the above phases. The plating layer 13 according to this embodiment exhibits excellent corrosion resistance due to having the above metallic structure.

In addition, in the plating layer 13 according to this embodiment, a portion of the α phase and η-Zn phase exist in a specific state, as detailed below, and function as a starting point for crack generation. As a result, in the plating layer 13 according to this embodiment, a more appropriate amount of cracks can be efficiently generated without excessive attention to controlling strain application, enabling further improvement of hydrogen desorption properties in the plated steel sheet 1.

Here, the phase of the plating layer 13 according to this embodiment can be determined by observing the surface of the plating layer 13 with a scanning electron microscope (SEM). That is, the solidification structure of the surface of the plating layer 13 is observed with an SEM, and the phase can be determined from the point analysis results by an SEM-EPMA (Electron Probe Micro Analyzer) and the morphology of the crystalline phase in the backscattered electron image within the observation field. In this case, it is not necessary to perform pretreatment such as polishing before SEM observation. However, if a chemical conversion coating or the like is applied, polishing or the like may be performed to remove the chemical conversion coating or the like. If polishing is performed, the plating layer should be polished so that 80% or more of its thickness remains, and the surface should be mirror-finished before SEM observation to obtain the surface structure.

Next, with reference to Figures 2 and 3, a specific state exhibited by some of the α-phase and η-Zn phase in the plating layer 13 according to this embodiment will be described in detail. Figures 2 and 3 are schematic diagrams illustrating the α-deposited η-phase in the plating layer 13 of the plated steel sheet 1 according to this embodiment.

Let us focus on the surface of the plating layer 13 according to this embodiment (the X-Y plane in Figure 2) when observed with an electron microscope (SEM) from the direction normal to the surface (the positive Z-axis direction in Figure 2).

During such observation, the surface structure of the plating layer 13 of the plated steel sheet 1 according to this embodiment has an α-deposited η phase (reference numeral 101 in Figure 2) as described below, with an average area ratio of 5 to 95%. The remainder of the α-deposited η phase 101 is a hard structure 103 composed of MgZn ₂ phase, η/α/MgZn ₂ ternary eutectic phase, η/MgZn ₂ binary eutectic phase, etc. Since the plating layer 13 according to this embodiment contains 0.50% by mass or more of Mg as part of the chemical composition of the plating, the hard structure 103 described above is inevitably formed from this chemical composition.

As schematically shown in Figure 3, this α-precipitated η phase 101 is a softened metallic structure in which Al, which was supersaturated and solid-dissolved in the matrix phase (hereinafter referred to as "η matrix phase 113") composed of the η-Zn phase, precipitates as α phase 111. When focusing on the backscattered electron image during SEM observation, the α phase 111 exists as a black substance, and the η matrix phase 113 exists as a white substance. Therefore, when observing the α-precipitated η phase 101 using backscattered electron images with SEM, it is recognized as a structure in which black particles are dispersed in a white phase.

The presence of such α-precipitation η phase 101 within the hard structure 103 results in non-uniformity in the hardness of the plating layer 13. When a relatively soft metallic structure is present within a hard metallic structure, and strain is applied to the plating layer 13 due to processing or other treatments, the applied deformation concentrates on the soft metallic structure, the α-precipitation η phase 101. As a result, in the plating layer 13 according to this embodiment, cracks 105 are generated starting from the α-precipitation η phase 101, as schematically shown in Figure 2.

Verification by the inventors revealed that the ends of cracks 105 generated in the hard structure 103 often reached the interface between the plating layer 13 and the steel sheet 11, provided the amount of plating layer 13 was within the range described above. In the plated steel sheet 1 according to this embodiment, hydrogen present in the steel sheet 11 is released to the outside (i.e., the outside air) through such cracks 105. Furthermore, even if the cracks 105 do not reach the interface between the plating layer 13 and the steel sheet 11, the hydrogen in the steel that reaches the ends of the cracks 105 is subsequently released to the outside by traveling along the cracks 105.

Furthermore, in the surface structure of the plating layer 13 according to this embodiment, as described above, the average area ratio of the α-deposited η phase 101 is within the range of 5 to 95%. Because the average area ratio of the α-deposited η phase 101 is within this range, cracks 105 are introduced into the plating layer 13 in an appropriate amount, even within the range of strains applied when processing the plated steel sheet into various shapes. As a result, the plated steel sheet 1 according to this embodiment can further improve hydrogen desorption while maintaining corrosion resistance.

If the average area ratio is less than 5%, the amount of α-precipitation η phase 101 is too small to introduce an appropriate amount of cracks 105, and thus the hydrogen desorption properties of the plated steel sheet 1 cannot be improved. By having an average area ratio of 5% or more, the hydrogen desorption properties of the plated steel sheet 1 can be improved while maintaining corrosion resistance. The average area ratio of α-precipitation η phase 101 is preferably 8% or more, and more preferably 15% or more.

On the other hand, if the average area ratio of the α-precipitation η phase 101 exceeds 95%, the soft α-precipitation η phase occupies most of the plating layer, causing the plating layer as a whole to exhibit ductility, and the α-precipitation η phase ceases to function as a crack initiation site, which is undesirable. By keeping the average area ratio of the α-precipitation η phase 101 at 95% or less, the hydrogen desorption properties of the plated steel sheet 1 can be improved while maintaining corrosion resistance and allowing the α-precipitation η phase 101 to function as a crack initiation site in the plating layer. The average area ratio of the α-precipitation η phase 101 is preferably 70% or less, more preferably 40% or less, and even more preferably 30% or less.

◇Method for calculating the average area ratio of the α-deposited η phase 101 of the plating layer 13 Here, the average area ratio described above is measured as follows.
Specifically, an arbitrary position on the surface of the plating layer 13 is observed using a SEM, and elemental mapping is obtained using SEM-EPMA. The obtained elemental mapping is binarized using the binarization function of a commercially available image analysis application, the region corresponding to the α-deposited η phase 101 is identified, and its area ratio is calculated.

More specifically, a region of 120 μm × 100 μm in plan view (corresponding to approximately 1000x magnification) at an arbitrary position on the surface of the plating layer 13 is observed using a SEM, and point analysis is performed using SEM-EPMA.

Specifically, the above 120 μm × 100 μm region is observed at a magnification of 1000x with an accelerating voltage of 15.0 kV, an irradiation current of 4.999 × ^10⁻⁸ A, and an irradiation time of 50 milliseconds. After obtaining a backscattered electron image of the area of interest under these conditions, point analysis of each metal structure can be performed at three points using the contrast of the backscattered electron image.

In such point analysis, a phase satisfying the following conditions—Al: 20–99 atomic%, Zn: 0.5–80 atomic%, Mg: 0–5 atomic%, and the sum of Al and Zn being 70 atomic% or more—can be identified as the α phase. Furthermore, a phase with a Zn content of 98 atomic% or more and a total content of other elements of 2 atomic% or less is identified as the η-Zn phase.

Furthermore, a phase in which the content of Mg and Zn is 10 atomic percent or more each, and the total content of Mg and Zn is 85 atomic percent or more, is determined to be a hard structure 103 composed of MgZn ₂ phase, η/α/MgZn ₂ ternary eutectic phase, η/MgZn ₂ binary eutectic phase, etc.

While the η-Zn phase does not contain Mg, a hard structure 103 containing Mg exists around the η-Zn. Therefore, by focusing on the distribution of the Mg element, it is possible to identify the contour line representing the boundary between the metal structure containing Mg and the metal structure that does not contain Mg. The metal structure that does not contain Mg, enclosed by the contour line thus identified, can be identified as the η-Zn phase. Here, the η-Zn phase and the surrounding hard structure 103 can be easily distinguished by visual inspection by a person skilled in the art.

During SEM observation, the contour lines of η-Zn are identified within the field of view as described above, and these contour lines are manually drawn using various image analysis applications (e.g., ImageJ). Subsequently, by binarizing the image using the image analysis application, it is easy to determine whether or not black precipitates corresponding to the α phase are present in the η-Zn phase. In this case, the binarization threshold can be set, for example, by setting the Minimum value of Brightness/Contrast to 200.

Next, we determine whether or not the α phase is present within the η-Zn phase identified as described above. As described above, when observed using backscattered electron imaging, the α phase is visible as black precipitates (i.e., black grains), while the η-Zn phase is visible as a white matrix phase. Therefore, the α-precipitated η phase 101 that we are focusing on in this embodiment is observed as a phase in which black grains are dispersed in a white matrix phase, as schematically shown in Figure 3. In Figure 3, the dashed outline corresponds to the outline of the η-Zn phase as described above.

Therefore, after identifying the phase in which black particles are dispersed in a white matrix phase by observation using backscattered electron images as described above, point analysis is performed using SEM-EPMA. Focusing on one of the phases in which black particles are dispersed in a white matrix phase, point analysis is performed using SEM-EPMA on an arbitrary 5 μm × 5 μm area containing both the black particles and the white matrix phase, as schematically shown in Figure 3. In the results of the point analysis, the region in which the Zn content is 90 atomic percent or more and the Al content is in the range of 0.05 to 10.00 atomic percent can be determined to be the α phase 111 in the η matrix phase 113. In this case, the black phase surrounding the area determined to be the α phase can be considered as part of the α phase collectively.

By the method described above, the region of the α-deposited η phase 101 can be identified within a 120 μm × 100 μm area in plan view at any position on the surface of the plating layer 13. Then, the area ratio of the identified region can be calculated using various image analysis applications (e.g., ImageJ). Specifically, the area of the region corresponding to the α-deposited η phase 101 is calculated using an image analysis application, and then the area of the obtained α-deposited η phase 101 is divided by the total area of the field of view to obtain the area ratio of the α-deposited η phase 101 in the field of view of interest.

The above measurement and calculation process is performed at five arbitrary locations, and the average value of the five resulting area ratios is calculated. The average value obtained in this way is taken as the average area ratio of the α-precipitated η phase 101.

Furthermore, when focusing on plated steel sheets 1 that have already been processed into products by welding, bonding, etc., one should focus on an unprocessed flat area, for example, 50 mm or more away from the edges of the product and the weld heat-affected zone or bonding area, and perform the above observation on any 120 μm × 100 μm area. In addition, if various coatings are present on the surface of the plating layer 13, the existing coatings should be removed using a coating remover.

◇Length of cracks 105 in the plating layer 13 Here, focusing on a region that is, for example, 50 mm or more away from the edge of the plated steel sheet 1, when a region having a size of 130 μm × 100 μm in a plan view is observed with an electron microscope (more specifically, SEM) at an arbitrary position on the surface of the plating layer 13 according to this embodiment, the sum of the lengths of the cracks 105 present in the above region is 50 μm or more.

When the total length of the cracks 105 present in the above region is 50 μm or more, the plated steel sheet 1 according to this embodiment exhibits excellent hydrogen desorption properties. The total length of the cracks 105 present in the above region is preferably 150 μm or more, and more preferably 500 μm or more.

On the other hand, the larger the sum of the lengths of the cracks 105, the better. There is no specific upper limit, but in practice, it is around 1000 μm. Even if the sum of the lengths of the cracks 105 becomes large, the corrosion resistance of the plated steel sheet 1 is maintained because the Mg contained in the plating layer 13 exhibits corrosion resistance.

Here, when measuring the sum of the lengths of the cracks 105 described above, observation by SEM can be performed in the same manner as the observation method for the metal structure of the plating layer 13. During such observation, focus on an arbitrary region of 130 μm × 100 μm in size, and measure the sum of the lengths of the cracks 105 present in this region using a length measuring application attached to the SEM. Perform the above measurement process at five arbitrary locations and calculate the average of the five obtained measurements. The average value obtained in this way is taken as the sum of the lengths of the cracks 105 in the 130 μm × 100 μm region.

The plated steel sheet 1 according to this embodiment has been described in detail above with reference to Figures 1A to 3. The plated steel sheet 1 according to this embodiment, as described above, can be suitably used, for example, as a material for automobile undercarriage parts.

Furthermore, the plated steel sheet 1 according to this embodiment may have one or more additional coatings on the above-mentioned plating layer 13. Examples of such coatings include chromate coatings, phosphate coatings, chromate-free coatings, and organic resin coatings.

(Regarding the manufacturing method of plated steel sheets)
Next, we will explain an example of a method for manufacturing plated steel sheets as described above.
The plated steel sheet 1 according to this embodiment is manufactured by using the steel sheet 11 described above as a base material, forming a plating layer 13 on the surface of the steel sheet 11, and then applying strain to the steel sheet 11 on which the plating layer 13 has been formed by various methods.

Here, in addition to the hot-dip galvanizing method, thermal spraying, cold spraying, sputtering, vapor deposition, electroplating, etc., can be applied to form the plating layer 13. However, for forming a plating layer of a thickness commonly used in automobiles and the like, the hot-dip galvanizing method is the most preferable in terms of cost.

The plated steel sheet (steel sheet 11 having a plating layer 13) obtained by the above method is subjected to a specific heat treatment process as described below. This forms an α-deposited η phase 101 in the plating layer 13. Subsequently, by applying strain to the plated steel sheet that has undergone the heat treatment process using various methods, cracks 105 originating from the α-deposited η phase 101 are generated, and the plated steel sheet 1 according to this embodiment can be obtained.

Below, an example of a manufacturing method for obtaining the plated steel sheet 1 according to this embodiment using a hot-dip galvanizing method will be described in detail.
In the manufacturing process of the plated steel sheet 1, first, the steel sheet 11 to be used as the base material is rolled to the desired thickness by the Zenzimir method, then wound into a coil, and placed on the hot-dip galvanizing line.

In the hot-dip galvanizing line, steel sheets are continuously fed through a coil. Subsequently, the steel sheets are subjected to a heat reduction treatment at 800°C in an annealing facility installed on the line, for example, in an environment where oxidation is unlikely to occur, such as an oxygen concentration of 20 ppm or less, under an _N2-5 % _H2 gas atmosphere. After that, they are air-cooled with _N2 gas to approximately 20°C above the temperature of the subsequent plating bath, and then immersed in the plating bath.

Here, a molten plating alloy having the above chemical components is prepared in the plating bath. The temperature of the plating bath is set to be above the melting point of the plating alloy (for example, around 400-500°C).

When preparing the material for the plating alloy, it is preferable to use pure metal (purity of 99% or higher) as the alloying material. First, a predetermined amount of alloying metal is mixed to achieve the composition of the plating layer described above, and then completely melted into an alloy using a high-frequency induction furnace or arc furnace under vacuum or inert gas displacement conditions. Furthermore, the alloy mixed with the predetermined components (composition of the plating layer described above) is melted in the atmosphere, and the resulting molten material is used as the plating bath.

Furthermore, there are no particular restrictions on using pure metals in the production of the plating alloys described above; existing Zn alloys, Mg alloys, and Al alloys may be melted and used. In this case, as long as an alloy with a predetermined composition and low impurity levels is used, there will be no problems.

After immersing the steel plate in the plating bath described above, it is withdrawn at a predetermined speed. At this time, the amount of plating deposited is controlled, for example, by an _N2 wiping gas, so that the formed plating layer 13 is of the desired thickness. Here, conditions other than the bath temperature can be general plating operation conditions, and no special equipment or conditions are required.

Next, the molten plating alloy located on the steel plate is subjected to the following first and second cooling steps to form a plating layer 13 and generate an α-deposited η phase 101 within the plating layer 13. The first and second cooling steps will be described in detail below.

The first cooling step is performed when the temperature of the plated alloy is within the range of the bath temperature or 240°C or higher. In this first cooling step, the plated steel sheet, which is within the above temperature range, is rapidly cooled at an average cooling rate of 15.0°C/second or higher. If the average cooling rate is less than 15.0°C/second, the concentration of Al dissolved in the η phase decreases, resulting in insufficient driving force for the subsequent α phase precipitation in the η phase, making it difficult to form the α-deposited η phase. When the hot-dip plating method is used in the plating process, this first cooling step is performed immediately after the steel sheet is removed from the plating bath. This allows Al to dissolve in the η-Zn phase during solidification.

Here, the average cooling rate is preferably 25.0°C/second or higher. While there is no specific upper limit for the average cooling rate, for example, around 90.0°C/second is a practical upper limit.

Subsequently, a second cooling step is performed when the temperature of the plated alloy (plating layer) is within the range of 70°C to less than 240°C. In this second cooling step, the plated steel sheet, which is within the above temperature range, is slowly cooled at an average cooling rate of 1.0°C/second or less. If the average cooling rate exceeds 1.0°C/second, there is insufficient time for the α phase to precipitate from the η phase, making it difficult to form the α-precipitated η phase. As a result, the Al that was dissolved in the η-Zn phase in the first cooling step precipitates as the α phase, softening the η-Zn phase and forming the α-precipitated η phase 101, while a hard structure 103 comes into existence around the α-precipitated η phase 101. Here, the above average cooling rate is preferably 0.5°C/second or less.

As described above, by undergoing a two-stage cooling process—rapid cooling in the temperature range of 240°C or higher below the bath temperature, and slow cooling in the temperature range of 70°C or higher below 240°C—it becomes possible to form the α-deposited η phase 101 in the plating layer 13 with a desired average area ratio.

Furthermore, there are no specific requirements for the cooling process from 70°C to room temperature; cooling to room temperature using various methods is acceptable.

Here, the interval between the completion of the first cooling step and the start of the second cooling step is preferably 3 seconds or less, and it is preferable to start the second cooling step immediately after the completion of the first cooling step. If the interval between the completion of the first cooling step and the start of the second cooling step exceeds 3 seconds, an unintended cooling process will occur, and the desired α-precipitation η phase 101 cannot be produced.

Furthermore, if either the first or second cooling step described above is not performed, the desired α-deposited η phase 101 cannot be achieved. By performing both the first and second cooling steps described above, the α-deposited η phase 101 can be generated in the plating layer 13 with the desired average area ratio, thereby appropriately introducing a soft structure into the plating layer 13.

Furthermore, if an alloying heat treatment process (for example, a heat treatment process involving heating to a plate temperature of approximately 480-550°C) is performed after the second cooling process described above, which is commonly carried out in the manufacture of alloyed hot-dip galvanized steel sheets, the state of the α-precipitation η phase 101 controlled by the first and second cooling processes will be disrupted, and as a result, the hydrogen desorption properties that are the focus of this embodiment cannot be obtained. From this viewpoint, it is important not to perform a heat treatment process after the second cooling process.

In the cooling process described above, commonly known methods such as _N2 gas cooling, mist cooling, and immersion in water can be applied. In addition to _N2 gas, other gases with high heat removal efficiency, such as He gas and hydrogen gas, may also be used as the cooling gas.

Furthermore, for measuring the temperature of the plating layer, a contact-type thermocouple (K-type) can be used, for example. By attaching the contact-type thermocouple to the base steel plate, the average temperature of the entire plating layer can be constantly monitored. In addition, by mechanically controlling various speeds and thicknesses and standardizing various operating conditions such as the preheating temperature of the steel plate and the temperature of the molten plating bath, it becomes possible to monitor the temperature of the entire plating layer at any given time under these manufacturing conditions with near accuracy. This makes it possible to precisely control the cooling process in the first and second cooling steps. Although not as accurate as the contact-type thermocouple, the surface temperature of the plating layer may also be measured using a non-contact infrared thermometer.

Furthermore, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer may be determined by performing a thermal conduction analysis simulation. Specifically, the surface temperature of the plating layer and the average temperature of the entire plating layer are determined based on various manufacturing conditions such as the preheating temperature of the steel sheet, the temperature of the molten plating bath, the rate at which the steel sheet is removed from the plating bath, the thickness of the steel sheet, the thickness of the plating layer, the amount of heat exchanged between the plating layer and the manufacturing equipment, and the amount of heat dissipated by the plating layer. Then, the relationship between the surface temperature of the plating layer and the average temperature of the entire plating layer can be determined using the obtained results. This makes it possible to estimate the average temperature of the entire plating layer at that time under those manufacturing conditions by actually measuring the surface temperature of the plating layer during the manufacturing of the plated steel sheet. As a result, it becomes possible to precisely control the cooling process in the first and second cooling processes.

Next, strain is applied to the plated steel sheet obtained as described above. This causes cracks to form in the plating layer 13 having the α-deposited η phase 101. The method for applying strain is not particularly limited, and can be performed by bending and stretching deformation treatment using a tension leveler, rolling treatment using a skin pass rolling mill, or cold pressing treatment when processing the plated steel sheet into a desired shape.

Here, it is preferable to impart a strain of 0.2% or more to the plated steel sheet during the strain-imparting treatment described above. By imparting a strain corresponding to such a total elongation, when any 130 μm × 100 μm region on the surface of the plating layer 13 is observed with an electron microscope, the sum of the lengths of cracks present in the region can be set to 50 μm or more. Here, the total elongation R _TOTAL (unit: %) is a value determined by the following formula (101). In the following formula (101), _L0 is the length in the direction of the plate in any plated steel sheet before the strain-imparting treatment is applied (unit: m), and _L1 is the length in the direction of the plate in the plated steel sheet after the strain-imparting treatment is applied to the portion originating from any plated steel sheet after the strain-imparting treatment is applied. More preferably, the total elongation is 0.6% or more.

R _TOTAL (%) = {(L ₁ - L ₀ )/L ₀ }×100 ... (101)

Furthermore, while there is no specific upper limit for the above total growth rate R _TOTAL , approximately 1.5% is considered the practical upper limit.

Furthermore, the process for applying the above-mentioned strain can be performed at any time. It may be performed immediately after the two-stage cooling process, or it may be performed after a certain amount of time has elapsed since the completion of the two-stage cooling process. In the plated steel sheet according to this embodiment, the α-precipitation η phase 101 is generated in the plating layer 13 with a desired average area ratio after the two-stage cooling process. Therefore, after the two-stage cooling process, the process for applying strain can be performed at any time to generate cracks 105 in the plating layer 13.

The above describes in detail an example of a method for manufacturing plated steel sheets according to this embodiment.

Furthermore, in the method for manufacturing plated steel sheets according to this embodiment, after the second cooling step described above, a treatment to form one or more layers of various coatings may be carried out. Examples of such treatments include chromate treatment, phosphate treatment, chromate-free treatment, and organic resin coating formation treatment.

Chromate treatments include electrolytic chromate treatment, which forms a chromate film by electrolysis; reactive chromate treatment, which forms a film by reacting with the material and then washes away excess treatment solution; and coating chromate treatment, which forms a film by applying a treatment solution and drying it without washing with water. Any of these chromate treatments may be used.

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

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

Chromate-free treatment is particularly suitable because it does not burden the environment. Such chromate-free treatments include electrolytic chromate-free treatment, which forms a chromate-free film by electrolysis; reactive chromate-free treatment, which forms a film using a reaction with the material and then washes away excess treatment solution; and coating-type chromate-free treatment, which forms a film by applying a treatment solution and drying it without rinsing with water. Any of these chromate-free treatments may be used.

Furthermore, the organic resin used in the organic resin film formation treatment is not limited to a specific resin; various resins can be used, such as polyester resins, polyurethane resins, epoxy resins, acrylic resins, polyolefin resins, and modified versions of these resins. Here, a modified version refers to a resin obtained by reacting the reactive functional groups contained in the structure of these resins with other compounds (for example, monomers or crosslinking agents) that contain functional groups in their structure that can react with such functional groups.

As the organic resin, one type as described above may be used alone, or two or more types of organic resins (unmodified) may be used in mixture form. Alternatively, one or more types of organic resins obtained by modifying at least one other organic resin in the presence of at least one other organic resin may be used in mixture form. Furthermore, an aqueous organic resin may be used by dissolving or dispersing it in water. In addition, various coloring pigments and rust-preventive pigments may be contained within such organic resin films.

The plated steel sheet according to the present invention will be described in detail below with reference to examples and comparative examples. Note that the following examples are merely examples of plated steel sheets according to the present invention, and the plated steel sheet according to the present invention is not limited to the examples shown below.

For the plating base sheets, cold-rolled steel sheets a to e, each with a thickness of 1.6 mm, were used (all manufactured by Nippon Steel Corporation). The chemical composition of each cold-rolled steel sheet is as follows, with the remainder being Fe and impurities.

Cold rolled steel plate a: 0.04 mass% C - 0.40 mass% Si - 2.20 mass% Mn
b: 0.09 mass% C-0.40 mass% Si-2.20 mass% Mn
c: 0.20 mass% C-0.80 mass% Si-2.20 mass% Mn
d: 0.25 mass% C-0.40 mass% Si-2.40 mass% Mn
e: 0.35 mass% C-0.70 mass% Si-2.40 mass% Mn

For each cold-rolled steel sheet a to e, a JIS No. 13B test specimen was taken from an arbitrary position on the cold-rolled steel sheet in accordance with JIS Z2201:1998, and its tensile strength was measured using a commercially available tensile testing machine. As a result, the tensile strengths of cold-rolled steel sheets a to e were 590 MPa (cold-rolled steel sheet a), 980 MPa (cold-rolled steel sheet b), 1180 MPa (cold-rolled steel sheet c), 1470 MPa (cold-rolled steel sheet d), and 2500 MPa (cold-rolled steel sheet e), respectively.

The above-mentioned plating base sheet was cut to a size of 100 mm x 200 mm, and then plated using a batch-type hot-dip galvanizing test apparatus manufactured in-house. Multiple plated steel sheets with the plating layer composition shown in Table 1 were produced for each level. The plate temperature was measured using a thermocouple spot-welded to the center of the plating base sheet. Before immersion in the plating bath, the surface of the plating base sheet was heat-reduction treated at 800°C in an _N2-5 % _H2 gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less. After the heat-reduction treatment, it was air-cooled with _N2 gas, and after the immersion plate temperature reached the bath temperature + 20°C, it was immersed in a plating bath at the bath temperature shown in Table 1 for approximately 3 seconds.

After immersion in the plating bath, the steel plates were removed at a speed of 20 to 200 mm/second. During removal, the amount of plating deposited was controlled by _N2 wiping gas. After removing the steel plates from the plating bath, they were cooled from the plating bath temperature to room temperature under the conditions shown in Table 1.

From each of the obtained plated steel sheets, a sample measuring 200 mm x 80 mm was taken and rolled using a skin-pass rolling mill to achieve the total elongation shown in Table 1 below, thereby applying strain to the plated layer.

The surface of each plated steel sheet after applying strain was observed using SEM according to the method described above, and the average area ratio of the α-deposited η phase and the sum of the lengths of cracks present in an arbitrary 130 μm × 100 μm region were calculated.

The composition of the plating layer in each plated steel sheet was determined by immersing a 30 mm x 30 mm sample in a 10% HCl aqueous solution with an inhibitor added to pickle and remove the plating layer, and then measuring the elements dissolved in the aqueous solution by ICP analysis.

The hydrogen desorption properties and coating blistering status of the obtained plated steel sheets were evaluated. The evaluation methods are as follows:

<Hydrogen desorption>
Samples measuring 50 mm x 30 mm were taken from each plated steel sheet after straining. Hydrogen was intentionally charged into the obtained samples while the plating layer remained intact. The amount of hydrogen in the sample immediately after hydrogen charging was measured by heating the sample from room temperature to 250°C at a heating rate of 100°C/h in an N2 atmosphere, and analyzing the amount of hydrogen released as the temperature rose using gas chromatography (CHROMATOGRAPH G2800, J-Science Lab Co., Ltd.). After measurement, the samples were left to stand for 24 hours in a constant temperature and humidity chamber at 25°C and 30% RH, and the amount of diffusible hydrogen was measured using the same temperature-induced desorption method.

[Hydrogen Charging Conditions]
Electrolyte: 3.0% NH ₄ SCN + 33.0% NaCl aqueous solution Current density: 1.0 mA/cm ²
Charging time: 18 hours; Liquid temperature: r.t. (approximately 25°C)

[Temperature Desorption Method]
Measurement temperature: normal temperature to 250℃
Heating rate: 100°C/h
Measurement interval: 5 minutes

Hydrogen desorption was evaluated based on the ratio (in %) obtained by dividing the diffusible hydrogen content of the sample after 24 hours of standing by the hydrogen content of the sample immediately after hydrogen charging. The evaluation criteria were as follows, with a score of "A" or higher being considered a pass.
[Evaluation Criteria]
Rating AAA: Less than 5% AA: 5% to 20%
A: More than 20% and less than 50%
B: More than 50%

<Corrosion resistance after painting>
Samples measuring 50 mm x 100 mm were taken from each plated steel sheet, and their corrosion resistance after painting was evaluated. More specifically, the obtained samples were subjected to phosphoric acid Zn treatment (SD5350 system: Nippon Paint Industrial Coating Co., Ltd. standard). Subsequently, electrodeposition coating (PN110 Powernix Gray: Nippon Paint Industrial Coating Co., Ltd. standard) was applied to a thickness of 20 μm, and the samples were baked at a temperature of 150°C for 20 minutes. After baking, a cut reaching the base metal was introduced in the center of the sample. Then, a combined cycle corrosion test in accordance with JASO (M609-91) was performed for 180 cycles, and the width of the coating blister was measured. The obtained coating blister width was evaluated based on the following evaluation criteria. The evaluation criteria are as follows, and a score of "A" or higher was considered a pass. If the score of the coating blister width is A or higher, it can be said that the plated steel sheet in question has excellent corrosion resistance after painting.
[Evaluation Criteria]
Rating AAA: Paint film blistering width 2 mm or less AA: Paint film blistering width exceeding 2 mm and 3 mm or less
A: Paint film blistering width more than 3 mm and less than 4 mm
B: Paint film blistering width exceeding 4mm

As is clear from Table 1 above, the plated steel sheet corresponding to the embodiment of the present invention showed excellent hydrogen desorption properties, while the plated steel sheet corresponding to the comparative example of the present invention showed inferior hydrogen desorption properties.

Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention.

The embodiments disclosed herein are illustrative and not restrictive in all respects. The above embodiments may be omitted, substituted, or modified in various ways without departing from the appended claims, the technical scope of the invention as described below, or the spirit thereof. For example, the constituent elements of the above embodiments can be combined in any way without impairing their effects. Furthermore, such arbitrary combinations will naturally yield the effects and benefits of each constituent element in the combination, as well as other effects and benefits that will be apparent to those skilled in the art from the description herein.

Furthermore, the effects described herein are merely descriptive or illustrative, and not limiting. In other words, the technology according to the present invention may produce other effects that will be apparent to those skilled in the art from the description herein, in addition to or instead of the effects described above.

Furthermore, the following configurations also fall within the technical scope of the present invention.
(1)
On the surface of the steel plate, by mass%,
Al: 0.50-5.00%
Mg: 0.50-3.00%
Fe:0.01~15.00%
The plating layer contains, optionally further containing one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the remainder being Zn and impurities, and having a chemical composition.
In the surface structure when the surface of the aforementioned plating layer is viewed in plan view, the average area ratio of the α-deposited η phase, which is a metallic structure in which the α phase is deposited in the η matrix phase, is 5 to 95%.
A plated steel sheet in which, when a 130 μm × 100 μm region on the surface of the plated layer is observed with an electron microscope, the sum of the lengths of the cracks present in the region is 50 μm or more.
[Element Group A]: One or two elements selected from the group consisting of Si: greater than 0% and 2.00% or less, and Ca: greater than 0% and 2.00% or less. [Element Group B]: One or more elements selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less. [Element Group C]: Cu: greater than 0% and 1.0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1.00% One or more elements selected from the group consisting of 0% or less [Element Group D]: Sn: greater than 0% and 1.0000% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less. One or more elements selected from the group consisting of 0% or less [Element Group E]: Zr: greater than 0% and 1.0000% or less, Ag: greater than 0% and 1.0000% or less, and Li: greater than 0% and 1.0000% or less. One or more elements selected from the group consisting of 0% or less [Element Group F]: La: greater than 0.5000% and 0.5000% or less, Ce: greater than 0% and 0.5000% or less, and Y: greater than 0% and 0.5000% or less. One or more elements selected from the group consisting of 0% or less [Element Group G]: B: greater than 0% and 0.5000% or less (2)
The plated steel sheet according to (1), having a chemical composition containing the aforementioned group of elements A.
(3)
The plated steel sheet according to (1), having a chemical composition containing the aforementioned group of elements B.
(4)
The plated steel sheet according to (1), having a chemical composition containing the aforementioned group of elements C.
(5)
The plated steel sheet according to (1), having a chemical composition containing the aforementioned group of elements D.
(6)
The plated steel sheet according to (1), having a chemical composition containing the aforementioned group of elements E.
(7)
The plated steel sheet according to (1), having a chemical composition containing the aforementioned group of elements F.
(8)
The plated steel sheet according to (1), having a chemical composition containing the aforementioned group of elements G.
(9)
The plated steel sheet according to any one of (1) to (8), wherein the plating layer contains 1.00 to 5.00% by mass of Al and 1.00 to 3.00% by mass of Mg.
(10)
The plated steel sheet according to any one of (1) to (9), wherein the tensile strength of the steel sheet is 980 MPa or more.
(11)
The plated steel sheet according to any one of (1) to (10), wherein the tensile strength of the steel sheet is 1180 MPa or more.
(12)
The plated steel sheet according to any one of (1) to (11), wherein the average area ratio of the α precipitated η phase is 5 to 70%.
(13)
The plated steel sheet according to any one of (1) to (12), wherein the average area ratio of the α precipitated η phase is 5 to 40%.

1 Plated steel sheet 11 Steel sheet 13 Plating layer 101 α-deposited η phase 103 Hard structure 105 Crack 111 α phase 113 η matrix

Claims (13)

On the surface of the steel plate, by mass%,
Al: 0.50-5.00%
Mg: 0.50-3.00%
Fe:0.01~15.00%
The plating layer contains, optionally further containing one or more elements selected from the group consisting of element group A, element group B, element group C, element group D, element group E, element group F, and element group G, with the remainder being Zn and impurities, and having a chemical composition.
In the surface structure when the surface of the aforementioned plating layer is viewed in plan view, the average area ratio of the α-deposited η phase, which is a metallic structure in which the α phase is deposited in the η matrix phase, is 5 to 95%.
A plated steel sheet in which, when a 130 μm × 100 μm region on the surface of the plated layer is observed with an electron microscope, the sum of the lengths of the cracks present in the region is 50 μm or more.
[Element Group A]: One or two elements selected from the group consisting of Si: greater than 0% and 2.00% or less, and Ca: greater than 0% and 2.00% or less. [Element Group B]: One or more elements selected from the group consisting of Sb: greater than 0% and 0.5000% or less, Pb: greater than 0% and 0.5000% or less, and Sr: greater than 0% and 0.5000% or less. [Element Group C]: Cu: greater than 0% and 1.0000% or less, Ti: greater than 0% and 1.0000% or less, Cr: greater than 0% and 1.0000% or less, Nb: greater than 0% and 1.0000% or less, Ni: greater than 0% and 1.0000% or less, Mn: greater than 0% and 1.0000% or less, Mo: greater than 0% and 1.0000% or less, Co: greater than 0% and 1.0000% or less, and V: greater than 0% and 1. One or more elements selected from the group consisting of 0% or less [Element Group D]: Sn: greater than 0% and 1.0000% or less, In: greater than 0% and 1.0000% or less, and Bi: greater than 0% and 1.0000% or less. One or more elements selected from the group consisting of 0% or less [Element Group E]: Zr: greater than 0% and 1.0000% or less, Ag: greater than 0% and 1.0000% or less, and Li: greater than 0% and 1.0000% or less. One or more elements selected from the group consisting of 0% or less [Element Group F]: La: greater than 0.5000% or less, Ce: greater than 0% and 0.5000% or less, and Y: greater than 0% and 0.5000% or less. One or more elements selected from the group consisting of 0% or less [Element Group G]: B: greater than 0% and 0.5000% or less. A plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group A. A plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group B. The plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group C. A plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group D. A plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group E. A plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group F. A plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group G. The plated steel sheet according to any one of claims 1 to 8, wherein the plating layer contains 1.00 to 5.00% by mass of Al and 1.00 to 3.00% by mass of Mg. The plated steel sheet according to claim 1, wherein the tensile strength of the steel sheet is 980 MPa or more. The plated steel sheet according to claim 1, wherein the tensile strength of the steel sheet is 1180 MPa or more. The plated steel sheet according to claim 1, wherein the average area ratio of the α-deposited η phase is 5 to 70%. The plated steel sheet according to claim 1, wherein the average area ratio of the α-deposited η phase is 5 to 40%.