Plated steel sheet

Abstract

[Problem] To further improve hydrogen desorption while maintaining post-coating corrosion resistance. [Solution] A plated steel sheet according to the present invention includes, atop the surface of the steel sheet, a plating layer that includes, by mass, Al: 0.50 to 5.00%, Mg: 0.50 to 3.00%, Fe: 0.01 to 15.00% and that selectively further includes one or two or more types selected from an element group A, an element group B, an element group C, an element group D, an element group E, an element group F, and an element group G, the remainder of the plating layer having a chemical composition comprising Zn and an impurity. In a surface structure when the plating layer is in plan view, a mean area ratio of an α precipitated η phase, which is a metal structure obtained by an α phase being precipitated in an η matrix phase, is 5 to 95%, and when a region of 130 μm × 100 μm of the surface of the plating layer is observed using an electron microscope, the sum total of the length of cracks present in the region is 50 μm or more.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet.

Background Art

[0002] In recent years, in the fields of automobiles and building materials, the need for high-strength steel sheets has been increasing from the viewpoint of weight reduction. For high-strength steel sheets, in order to achieve a desired shape, various processes such as press working and bending are applied, and high corrosion resistance is often required. Therefore, high-strength steel sheets are required to have both strength, workability, and corrosion resistance. In order to achieve high corrosion resistance, various plated steel sheets using high-strength steel sheets as plating base plates have been increasingly used.

[0003] When a high-strength steel sheet is used as a plating base plate, it is known that so-called hydrogen embrittlement is likely to occur due to hydrogen inevitably entering the steel in the plating line. In particular, in the case of a molten Zn-Al-Mg-based plated steel sheet, even when a relatively low-strength steel sheet such as a 590 MPa grade is used as a plating base plate, hydrogen embrittlement may occur. Therefore, various countermeasure methods against hydrogen embrittlement have been proposed conventionally.

[0004] For example, in Patent Document 1 below, a technique is proposed in which a strain is applied to a molten Zn-Al-Mg-based plated steel sheet to introduce cracks into the plating layer, and then such a plated steel sheet is subjected to a baking treatment to reduce the hydrogen concentration in the steel.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] When the present inventors examined the technology proposed in Patent Document 1, they 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 that the amount of cracks introduced be controlled by paying attention to the strain applied when introducing cracks, and it was found that there is still room for improvement in terms of hydrogen desorption properties.

[0007] 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. [Means for solving the problem]

[0008] 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:

[0009] (1) The surface of the steel plate contains, by mass%, Al: 0.50-5.00%, Mg: 0.50-3.00%, and Fe: 0.01-15.00%. anyFurthermore, the plated steel sheet has a plating layer having a chemical composition in which 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 are present, with the remainder being Zn and impurities, and in the surface structure when the surface of the 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%, 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 less than or equal to 2.00%, and Ca: greater than 0% and less than or equal to 2.00%. [Element Group B]: One or more elements selected from the group consisting of Sb: greater than 0% and less than or equal to 0.5000%, Pb: greater than 0% and less than or equal to 0.5000%, and Sr: greater than 0% and less than or equal to 0.5000%. [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. [Element Group D]: One or more elements selected from the group consisting of Sn: greater than 0% and less than or equal to 1.0000%, In: greater than 0% and less than or equal to 1.0000%, and Bi: greater than 0% and less than or equal to 1.0000%. [Element Group E]: One or more elements selected from the group consisting of Zr (greater than 0% and less than or equal to 1.0000%), Ag (greater than 0% and less than or equal to 1.0000%), and Li (greater than 0% and less than or equal to 1.0000%). [Element group F]: One or more elements selected from the group consisting of La: greater than 0% and less than or equal to 0.5000%, Ce: greater than 0% and less than or equal to 0.5000%, and Y: greater than 0% and less than or equal to 0.5000%. [Element group G]:B: More than 0% and less than 0.5000% (2) The plated steel sheet according to (1), having a chemical composition containing the 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 described in (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%. [Effects of the Invention]

[0010] As described above, according to the present invention, it is possible to further improve hydrogen desorption properties in Zn-Al-Mg plated steel sheets while maintaining corrosion resistance after coating. [Brief explanation of the drawing]

[0011] [Figure 1A] This is an explanatory diagram schematically showing the structure of a plated steel sheet according to an embodiment of the present invention. [Figure 1B] This is an explanatory diagram schematically showing the structure of a plated steel sheet according to the same embodiment. [Figure 2] This is a schematic diagram illustrating the plating layer of the plated steel sheet in the same embodiment. [Figure 3]It is a schematic diagram for explaining the plating layer of the plated steel sheet in the same embodiment.

Embodiments of the Invention

[0012] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant description is omitted.

[0013] (Regarding the plated steel sheet) First, with reference to FIGS. 1A and 1B, the overall configuration of the plated steel sheet according to an embodiment of the present invention will be described. FIGS. 1A and 1B are explanatory diagrams schematically showing an example of the plated steel sheet according to the present embodiment.

[0014] As schematically shown in FIG. 1A, the plated steel sheet

[0015] (Regarding the steel sheet 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 that has been 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 adjusted as appropriate.

[0016] Among the high-strength steels described above, using high-strength steel with a tensile strength of 980 MPa or higher (so-called 980 MPa class or higher high-strength steel) is preferable because it can 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.

[0017] Furthermore, the thickness of the steel plate 11 is not particularly limited and can be set appropriately according to the mechanical strength required for the plated steel plate 1.

[0018] <Regarding 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.

[0019] ◇Chemical composition of 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.

[0020] Furthermore, according to another embodiment, the chemical composition of the plating layer 13 according to this embodiment contains, in mass%, Al: 0.50~5.00%, Mg: 0.50~3.00%, Fe: 0.01~15.00%, and also 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.

[0021] [Element Group A]: One or two elements selected from the group consisting of Si: greater than 0% and less than or equal to 2.00%, and Ca: greater than 0% and less than or equal to 2.00%. [Element Group B]: One or more elements selected from the group consisting of Sb: greater than 0% and less than or equal to 0.5000%, Pb: greater than 0% and less than or equal to 0.5000%, and Sr: greater than 0% and less than or equal to 0.5000%. [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. [Element Group D]: One or more elements selected from the group consisting of Sn: greater than 0% and less than or equal to 1.0000%, In: greater than 0% and less than or equal to 1.0000%, and Bi: greater than 0% and less than or equal to 1.0000%. [Element Group E]: One or more elements selected from the group consisting of Zr (greater than 0% and less than or equal to 1.0000%), Ag (greater than 0% and less than or equal to 1.0000%), and Li (greater than 0% and less than or equal to 1.0000%). [Element group F]: One or more elements selected from the group consisting of La: greater than 0% and less than or equal to 0.5000%, Ce: greater than 0% and less than or equal to 0.5000%, and Y: greater than 0% and less than or equal to 0.5000%. [Element group G]:B: More than 0% and less than 0.5000%

[0022] Thus, the plating layer 13 according to this embodiment contains, by mass%, Al: 0.50-5.00%, Mg: 0.50-3.00%, and Fe: 0.01-15.00%. any Furthermore, the plating layer has a chemical composition that 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.

[0023] [Al:0.50~5.00% 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 of the plated steel sheet after welding. 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.

[0024] On the other hand, if the Al content in the plating layer 13 exceeds 5.00 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 mass% or less. Preferably, the Al content is 4.00 mass% or less, and more preferably 3.00 mass% or less.

[0025] [Mg:0.50~3.00% by mass] Mg is an essential element for forming the main phase (Zn-Al-Mg alloy phase) of the plating layer 13 according to this embodiment. It is included in a certain amount to ensure corrosion resistance in the weld heat-affected zone and corrosion resistance in the non-welded areas of the plated steel sheet. Therefore, 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 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%.

[0026] 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 through modification of 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, preventing deformation from concentrating 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.

[0027] [Fe:0.01~15.00% 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.

[0028] Furthermore, within the limits that do 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.

[0029] 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.

[0030] 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 model.

[0031] Furthermore, in the plating layer 13 according to this embodiment, if at least one of the elements belonging to element groups B to E below is included, it is preferable that at least one of the elements belonging to element groups B to E below is included within the following content range, and the total content is 5.0000% by mass or less.

[0032] 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 compromising 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.

[0033] ◇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 less than or equal to 2.00%, and Ca: greater than 0% and less than or equal to 2.00%.

[0034] [Si:0~2.00% by mass] Since the plating layer 13 according to this embodiment may not contain Si, the lower limit of the Si content is 0 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 mass% or more, and more preferably 0.10 mass% or more, in order to suppress the excessive growth of the Fe-Al intermetallic compound phase.

[0035] 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 operability of the plating process. For this reason, the Si content in the plating bath is adjusted from the viewpoint of operability, so 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.

[0036] [Ca:0~2.00% by mass] Since the plating layer 13 according to this embodiment may not contain Ca, the lower limit of the Ca content is 0 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 in the plating layer 13 along with Ca, 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 (LME) cracking 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 mass% or more. The Ca content in the plating layer 13 is more preferably 0.05 mass% or more.

[0037] 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.

[0038] ◇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 may 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 less than or equal to 0.5000%, Pb: greater than 0% and less than or equal to 0.5000%, and Sr: greater than 0% and less than or equal to 0.5000%.

[0039] [Sb:0~0.5000% by mass] [Pb:0~0.5000% by mass] [Sr:0~0.5000% 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.

[0040] On the other hand, if a plating layer 13 is formed 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. For this reason, 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.

[0041] ◇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 may be included 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.

[0042] [Cu:0~1.0000% by mass] [Ti:0~1.0000% by mass] [Cr:0~1.0000% by mass] [Nb:0~1.0000% by mass] [Ni:0~1.0000% by mass] [Mn:0~1.0000% by mass] [Co:0~1.0000% by mass] [V:0~1.0000% 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.

[0043] 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 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 mass% or less. Preferably, the content of Cu, Ti, Cr, Nb, Ni, Mn, Co, and V is independently set to 0.2000 mass% or less.

[0044] [Mo:0~1.0000% 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.

[0045] 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.

[0046] ◇Element group D Next, in another embodiment of the plating layer 13 according to this embodiment, the element group D that the plating layer 13 may contain will be described. The elements of element group 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 less than or equal to 1.0000%, In: greater than 0% and less than or equal to 1.0000%, and Bi: greater than 0% and less than or equal to 1.0000%.

[0047] [Sn:0~1.0000% by mass] [In:0~1.0000% by mass] [Bi:0~1.0000% by mass] Since it is possible that the plating layer 13 according to this embodiment may not contain Sn, In, and Bi, the lower limit of the Sn content is 0 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 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 mass%, the content of Sn, In, and Bi is 1.0000 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.

[0048] ◇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 may 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 less than or equal to 1.0000%), Ag (greater than 0% and less than or equal to 1.0000%), and Li (greater than 0% and less than or equal to 1.0000%).

[0049] [Zr:0~1.0000% by mass] [Ag:0~1.0000% by mass] [Li:0~1.0000% 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.

[0050] 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 is likely to be generated in the plating bath used to form the plating layer 13. For this reason, 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.

[0051] ◇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 less than or equal to 0.5000%, Ce: greater than 0% and less than or equal to 0.5000%, and Y: greater than 0% and less than or equal to 0.5000%.

[0052] [La:0~0.5000% by mass] [Ce:0~0.5000% by mass] [Y:0~0.5000% 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.

[0053] 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.

[0054] 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.

[0055] ◇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 less than 0.5000%

[0056] [B:0~0.5000% 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 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 mass% or more of B. Therefore, when B is included, the B content is preferably 0.0500 mass% or more.

[0057] 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 plated steel sheets 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.

[0058] [Method for measuring chemical components] The chemical composition of the plating layer 13 described above can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry). 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.

[0059] ◇Regarding the amount of plating layer 13 attached The amount of plating layer 13 deposited as described above is not specifically defined, but for example, 15 to 250 g / m² per side of the steel sheet. 2 It is preferable that the amount of plating layer 13 adheres to the extent described above. By keeping the amount of plating layer 13 within the above range, the plated steel sheet 1 according to this embodiment can exhibit sufficient corrosion resistance.

[0060] 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 plating layer 13 attached to each side.

[0061] ◇Regarding the metallic structure of plating layer 13 Next, we will describe the microstructure 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, MgZn2 phase, η / α / MgZn2 ternary eutectic phase, and η / MgZn2 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, Mg2Si phase, and Mg2Sn 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.

[0062] 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 paying excessive attention to controlling the strain application, and further improvement of hydrogen desorption properties in the plated steel sheet 1 becomes possible.

[0063] Here, the type of 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 type of phase can be determined from the point analysis results by SEM-EPMA (Electron Probe Microanalyzer) 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 any 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.

[0064] Next, with reference to Figures 2 and 3, we will describe in detail the specific states exhibited by some of the α-phase and η-Zn phase in the plating layer 13 according to this embodiment. 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.

[0065] Let's focus on the surface of the plating layer 13 according to this embodiment (the XY plane in Figure 2) when observed with an electron microscope (SEM) from the surface normal direction (the positive Z-axis direction in Figure 2).

[0066] 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 MgZn2 phase, η / α / MgZn2 ternary eutectic phase, η / MgZn2 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.

[0067] As schematically shown in Figure 3, this α-precipitated η phase 101 is a metallic structure in which Al, which was supersaturated and dissolved in the matrix phase (hereinafter referred to as "η matrix phase 113") composed of the η-Zn phase, precipitates as α phase 111 and becomes softened. When focusing on the backscattered electron image during SEM observation, α phase 111 exists as a black substance, and η matrix phase 113 exists as a white substance. Therefore, when observing 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.

[0068] The presence of such α-deposited η phase 101 within the hard structure 103 results in non-uniformity in hardness of the plating layer 13. When a relatively soft metallic structure is present within a hard metallic structure, and the plating layer 13 is subjected to stress due to processing or other treatments, the applied deformation concentrates on the soft metallic structure, the α-deposited η phase 101. As a result, in the plating layer 13 according to this embodiment, cracks 105 are generated starting from the α-deposited η phase 101, as schematically shown in Figure 2.

[0069] 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 then released to the outside by traveling along the cracks 105.

[0070] Furthermore, in the surface structure of the plating layer 13 according to this embodiment, the average area ratio of the α-deposited η phase 101 is within the range of 5 to 95%, as described above. Because the average area ratio of the α-deposited η phase 101 is within the range described above, cracks 105 are introduced into the plating layer 13 in an appropriate amount, even within the range of strain that is 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.

[0071] If the average area ratio is less than 5%, the amount of α-precipitated 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-precipitated η phase 101 is preferably 8% or more, and more preferably 15% or more.

[0072] 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.

[0073] ◇Method for calculating the average area ratio of the α-deposited η phase 101 of the plating layer 13 Here, the average area ratio mentioned above is measured as follows. Specifically, an arbitrary location 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.

[0074] 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 scanning electron microscope (SEM), and point analysis is performed using SEM-EPMA.

[0075] Specifically, the above 120 μm × 100 μm region was treated with an acceleration voltage of 15.0 kV and an irradiation current of 4.999 × 10⁻¹⁴. -8A. Observe at a magnification of 1000x with an irradiation time of 50 milliseconds. Under these conditions, after obtaining a backscattered electron image of the area of ​​interest, perform point analysis of each metal structure at three points using the contrast of the backscattered electron image.

[0076] In such point analysis, a phase satisfying the conditions Al: 20-99 atomic%, Zn: 0.5-80 atomic%, and Mg: 0-5 atomic%, and where the sum of Al and Zn is 70 atomic% or more, can be identified as the α phase. Furthermore, a phase with a Zn content of 98 atomic% or more and the total content of other elements being 2 atomic% or less is identified as the η-Zn phase.

[0077] 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 MgZn2 phase, η / α / MgZn2 ternary eutectic phase, η / MgZn2 binary eutectic phase, etc.

[0078] 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 that represents 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.

[0079] 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 in 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.

[0080] 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 of interest 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. Note that in Figure 3, the dashed outline corresponds to the outline of the η-Zn phase as described above.

[0081] Therefore, after identifying the phase in which black particles are dispersed in a white matrix phase by observing the backscattered electron image 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% or more and the Al content is in the range of 0.05 to 10.00 atomic% 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 as a whole.

[0082] By the method described above, the region of 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 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 area of ​​the entire field of view to obtain the area ratio of α-deposited η phase 101 in the field of view of interest.

[0083] The measurement and calculation process described above 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.

[0084] Furthermore, when focusing on plated steel sheets 1 that have already been processed into products by welding or bonding, the observation should be performed 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 bonded area, and the above observation should be performed 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.

[0085] ◇Length of crack 105 in 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 plated layer 13 according to this embodiment, the sum of the lengths of the cracks 105 present in the region is 50 μm or more.

[0086] 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.

[0087] 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.

[0088] Here, when measuring the sum of the lengths of the cracks 105 described above, observation by SEM should 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.

[0089] 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.

[0090] The plated steel sheet 1 according to this embodiment may have one or more additional coatings on the plated layer 13. Examples of such coatings include chromate coatings, phosphate coatings, chromate-free coatings, and organic resin coatings.

[0091] (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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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).

[0097] 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.

[0098] Furthermore, there are no particular restrictions on using pure metals when producing the plating alloys described above; existing Zn alloys, Mg alloys, and Al alloys can 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.

[0099] 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 those of general plating operation, and no special equipment or conditions are required.

[0100] 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.

[0101] 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 causes Al to dissolve in the η-Zn phase during solidification.

[0102] 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, a practical upper limit would be around 90.0°C / second.

[0103] 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 average cooling rate is preferably 0.5°C / second or less.

[0104] As described above, by going through a two-stage cooling process, which involves 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.

[0105] Furthermore, there are no specific requirements for the cooling process from 70°C to room temperature; it is acceptable to cool the device to room temperature using various methods.

[0106] 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.

[0107] 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 a desired average area ratio, thereby appropriately introducing a soft structure into the plating layer 13.

[0108] 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.

[0109] 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 helium gas and hydrogen gas, may also be used as the cooling gas.

[0110] 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. Furthermore, 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.

[0111] 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 speed 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.

[0112] 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 using a tension leveler, rolling using a skin pass rolling mill, or cold pressing when processing the plated steel sheet into a desired shape.

[0113] Here, during the processing for applying the above-mentioned strain, it is preferable to apply a strain to the plated steel sheet with a total elongation of 0.2% or more. By applying a strain corresponding to such a total elongation, when any 130 μm × 100 μm region on the surface of the plated layer 13 is observed with an electron microscope, the sum of the lengths of the cracks present in the region can be made 50 μm or more. Here, the total elongation R TOTAL The value (in %) is determined by the following formula (101). In the following formula (101), L0 is the length in the direction of the sheet metal in an arbitrary sheet metal section X before the treatment for applying strain (in m), and L1 is the length in the direction of the sheet metal in the portion of the sheet metal that originates from the arbitrary sheet metal section X after the treatment for applying strain. The total elongation rate is more preferably 0.6% or more.

[0114] R TOTAL (%)={(L1-L0) / L0}×100 ···(101)

[0115] Furthermore, the above total growth rate R TOTAL While there is no specific upper limit, approximately 1.5% is considered the practical upper limit.

[0116] The above-mentioned process for applying 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 after the two-stage cooling process is completed. In the plated steel sheet according to this embodiment, the α-precipitation η phase 101 is generated in the plating layer 13 at 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.

[0117] The above describes in detail an example of a method for manufacturing plated steel sheets according to this embodiment.

[0118] In the method for manufacturing plated steel sheets according to this embodiment, a further treatment to form one or more layers of various coatings may be performed after the second cooling step described above. Examples of such treatments include chromate treatment, phosphate treatment, chromate-free treatment, and organic resin coating formation treatment.

[0119] 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.

[0120] 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.

[0121] Examples of phosphate treatments include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.

[0122] 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 by utilizing 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 washing with water. Any of these chromate-free treatments may be used.

[0123] Furthermore, the organic resin used in the organic resin film formation treatment is not limited to a specific resin; for example, various resins such as polyester resins, polyurethane resins, epoxy resins, acrylic resins, polyolefin resins, and modified versions of these resins can be used. 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.

[0124] 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 in such organic resin films. [Examples]

[0125] 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 the plated steel sheet according to the present invention, and the plated steel sheet according to the present invention is not limited to the examples shown below.

[0126] Cold-rolled steel sheets a to e, each with a thickness of 1.6 mm, were used as the plating base material (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.

[0127] 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

[0128] 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.

[0129] 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, the sheet 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.

[0130] After immersion in the plating bath, the steel plates were removed at a speed of 20-200 mm / second. During removal, the amount of plating was controlled using 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.

[0131] 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 imposing strain on the plated layer.

[0132] 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.

[0133] The composition of the plating layer in each plated steel sheet was determined by immersing a 30mm x 30mm sample in a 10% HCl aqueous solution with an inhibitor added to pickle and remove the plating layer, and then performing ICP analysis on the elements dissolved in the aqueous solution.

[0134] The hydrogen desorption properties and coating blistering status of the obtained plated steel sheets were evaluated. The evaluation methods are as follows.

[0135] <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). After measurement, the samples were left to stand for 24 hours in a constant temperature and humidity chamber at 25°C and RH30%, and the amount of diffusible hydrogen was measured using the same temperature-induced desorption method.

[0136] [Hydrogen Charging Conditions] Electrolyte: 3.0%NH4SCN+33.0%NaCl aqueous solution Current density: 1.0mA / cm 2 Charging time: 18 hours Liquid temperature: rt (approx. 25℃) [Temperature Desorption Method] Measurement temperature: normal temperature ~ 250℃ Heating rate: 100℃ / h Measurement interval: 5 minutes

[0137] 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% or more and 20% or less A: More than 20% and less than 50% B: More than 50%

[0138] <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 treated with zinc phosphate (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 coating blister width score is A or higher, the plated steel sheet in question can be said to have excellent corrosion resistance after painting. [Evaluation Criteria] Rating AAA: Coating blister width 2mm or less AA: Paint film blistering width more than 2mm and less than 3mm A: Paint film blistering width more than 3mm and less than 4mm B: Paint film blistering width over 4mm

[0139] [Table 1]

[0140] 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.

[0141] 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.

[0142] The embodiments disclosed herein are illustrative and not restrictive in all respects. The embodiments described above may be omitted, replaced, or modified in various ways without departing from the appended claims, the technical scope of the invention as described later, and the spirit thereof. For example, the constituent elements of the embodiments described above can be combined in any way without impairing their effects. Furthermore, such any combination will naturally yield the effects and benefits of each constituent element in the combination, as well as other effects and benefits that will be obvious to those skilled in the art from the description herein.

[0143] 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.

[0144] 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% It contains, any Furthermore, the plating layer has a chemical composition in which 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 are present, with the remainder being Zn and impurities. 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 less than or equal to 2.00%, and Ca: greater than 0% and less than or equal to 2.00%. [Element Group B]: One or more elements selected from the group consisting of Sb: greater than 0% and less than or equal to 0.5000%, Pb: greater than 0% and less than or equal to 0.5000%, and Sr: greater than 0% and less than or equal to 0.5000%. [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. [Element Group D]: One or more elements selected from the group consisting of Sn: greater than 0% and less than or equal to 1.0000%, In: greater than 0% and less than or equal to 1.0000%, and Bi: greater than 0% and less than or equal to 1.0000%. [Element Group E]: One or more elements selected from the group consisting of Zr (greater than 0% and less than or equal to 1.0000%), Ag (greater than 0% and less than or equal to 1.0000%), and Li (greater than 0% and less than or equal to 1.0000%). [Element group F]: One or more elements selected from the group consisting of La: greater than 0% and less than or equal to 0.5000%, Ce: greater than 0% and less than or equal to 0.5000%, and Y: greater than 0% and less than or equal to 0.5000%. [Element group G]:B: More than 0% and less than 0.5000% (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 element group B. (4) The plated steel sheet according to (1), having a chemical composition containing the aforementioned element group 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 element group 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 tensile strength of the steel sheet is 980 MPa or more, as described in any one of (1) to (9). (11) The galvanized steel sheet described in 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%. [Explanation of symbols]

[0145] 1. Plated steel sheet 11 Steel plate 13 Plating layer 101 α precipitated η phase 103 Hard tissue 105 Crack 111 α phase 113 η matrix

Claims

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]: 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. [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. [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. [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. [Element group G]: B: More than 0% and not more than 0.5000%

2. The plated steel sheet according to claim 1, having a chemical composition containing the aforementioned group of elements A.

3. The plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group B.

4. The plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group C.

5. The plated steel sheet according to claim 1, having a chemical composition containing the aforementioned element group D.

6. The plated steel sheet according to claim 1, having a chemical composition containing the aforementioned group of elements E.

7. The plated steel sheet according to claim 1, having a chemical composition containing the aforementioned group of elements F.

8. The plated steel sheet according to claim 1, having a chemical composition containing the aforementioned group of elements G.

9. 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.

10. The plated steel sheet according to claim 1, wherein the tensile strength of the steel sheet is 980 MPa or more.

11. The plated steel sheet according to claim 1, wherein the tensile strength of the steel sheet is 1180 MPa or more.

12. The plated steel sheet according to claim 1, wherein the average area ratio of the α-deposited η phase is 5 to 70%.

13. The plated steel sheet according to claim 1, wherein the average area ratio of the α-deposited η phase is 5 to 40%.

Images