Hot-dip galvanized steel sheet and method for manufacturing the same

Abstract

Provided are: a hot-dip galvanized sheet that has a TS of 1180 MPa or greater, is excellent in formability, peel strength of a corona bond, and delayed peeling resistance at welded portions, and with which a component can be manufactured with a high dimensional accuracy; and a method for manufacturing the same. A hot-dip galvanized steel sheet according to the present invention is characterized by including a base steel sheet and a hot-dip galvanized layer formed on a surface thereof, wherein: the base steel sheet has a prescribed component composition and a prescribed steel structure; the amount of C contained in the surface layer of the base steel sheet is 0.150% or less; the value obtained by dividing the amount of C contained in the surface layer of the base steel sheet by the amount of C contained in the base steel sheet as a whole is less than 0.85; the hydrogen content [H] (mass ppm) in steel of hydrogen having a binding energy between the base steel sheet and a hydrogen trap site of 10 to 100 kJ / mol is 0.30 mass ppm or less; and the average Vickers hardness of the surface layer of the base steel sheet is denoted by HvS and satisfies formula (1) below.　(1): [H] ≤ (480 - HvS) / 320

Description

[Technical Field]

[0001] This invention relates to hot-dip galvanized steel sheets and methods for producing the same. [Background technology]

[0002] To achieve both a reduction in CO2 emissions through vehicle weight reduction and an improvement in collision resistance through vehicle weight reduction, efforts are being made to increase the strength of steel sheets used in automobiles. In addition, new legal regulations are being introduced one after another. For this reason, in order to increase the strength of the vehicle body, there is an increasing application of high-strength steel sheets, in particular high-strength steel sheets with a tensile strength (hereinafter simply referred to as TS) of 1180 MPa or higher, for major structural or reinforcing parts that form the frame of the automobile cabin (hereinafter also referred to as automobile frame structural parts, etc.).

[0003] High-strength steel sheets used in automotive structural components and other applications require excellent formability. Furthermore, the formed parts must exhibit superior dimensional accuracy. For example, parts such as crash boxes have punched edges and bent sections, making steel sheets with high elongation flangeability and bendability preferable from a formability standpoint. From a performance standpoint, increasing the yield ratio (YR = yield strength YS / tensile strength TS) of the steel sheet increases the impact absorption energy during collisions. Furthermore, from the perspective of dimensional accuracy, controlling YR within a certain range suppresses springback after steel sheet forming, thereby controlling the dimensional accuracy of the parts. To increase the application rate of high-strength steel sheets in automotive parts, it is desirable to comprehensively satisfy these characteristics.

[0004] Therefore, from the viewpoint of formability, it is preferable to use steel sheets that have high hole-expanding and bendability in addition to ductility for such parts. In addition, from the viewpoint of vehicle body rust prevention performance, hot-dip galvanized steel sheets, which have been treated with hot-dip galvanizing, may be used for steel sheets that serve as materials for structural components of automobiles.

[0005] Furthermore, in recent years, when spot welding high-strength galvanized steel sheets, a problem has been identified where the peel strength of the corona bond (the pressure-welded area between the steel sheets) decreases as the steel sheets become stronger, i.e., the surface layer of the steel sheets hardens. In addition, stress concentration due to the peeling of the corona bond and crack propagation into the nugget (delayed delamination of the weld) due to hydrogen intrusion into the steel under the steel sheet usage environment are also becoming a concern. For this reason, when applying high-strength steel sheets to structural components, there is a demand for high-strength steel sheets with excellent peel strength of the corona bond and resistance to delayed delamination of the weld.

[0006] Regarding technologies related to such hot-dip galvanized steel sheets, high-strength steel sheets with improved bendability due to the formation of a soft layer on the surface have been proposed. For example, Patent Document 1 discloses a high-strength steel sheet having a tensile strength of 1180 MPa or more, a surface soft layer thickness of more than 10 μm, and containing carbides at a predetermined number density in the surface soft layer. Furthermore, Patent Document 2 discloses a hot-dip galvanized steel sheet having a tensile strength of 1180 MPa or more, with improved bendability achieved by controlling the thickness of the surface softening layer and further reducing the amount of hydrogen diffusible in the low-temperature range of hydrogen within the steel. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2021 / 186510 [Patent Document 2] International Publication No. 2022 / 270053 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, in Patent Document 1, it was necessary to control the amount of hydrogen in the steel from the viewpoint of hydrogen embrittlement resistance, and the balance between these and the properties related to the welded joint was not considered, leaving room for improvement. In Patent Document 2, the properties related to the welded joint were not considered, leaving room for improvement.

[0009] In view of the above issues, the present invention aims to provide a hot-dip galvanized steel sheet and a method for manufacturing the same, which have a TS of 1180 MPa or higher, and are excellent in formability, corona bond peel strength, and weld delay peel resistance, and can be manufactured with high dimensional accuracy. In this invention, formability refers to ductility, hole expandability, and bendability. Furthermore, in this invention, the ability to manufacture parts with high dimensional accuracy (high dimensional accuracy during forming) means that the yield ratio (YR) is 65.0% or more and 95.0% or less. [Means for solving the problem]

[0010] As a result of diligent research to solve the above problems, the inventors of this invention have obtained the following findings. (1) The base steel sheet is given a predetermined composition, and its structure is made to contain a certain amount of tempered martensite, as well as optionally containing ferrite and retained austenite. This allows for a YR of 65.0% to 95.0%, while simultaneously achieving high TS and excellent bendability. (2) By having a carbon content of 0.150% or less on the surface of the base steel sheet, and by dividing the carbon content on the surface of the steel sheet by the total carbon content of the steel sheet, a soft surface layer is formed, and excellent peel strength can be obtained in corona bonds. (3) By reducing the amount of hydrogen trapped in specific hydrogen trapping sites in the underlying steel plate to an amount corresponding to the surface hardness of the steel plate, crack propagation into the nugget can be suppressed. This makes it possible to obtain excellent resistance to delayed delamination of the weld.

[0011] In other words, the gist of the present invention is as follows:

[0012] [1] comprising a base steel plate and a hot-dip galvanized layer formed on the surface of the base steel plate, The aforementioned base steel plate is, by mass %, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less comprising a component composition consisting of the balance being Fe and inevitable impurities, at the position of 1 / 4 of the plate thickness of the base steel plate, the area ratio of tempered martensite is 60.0% or more, the area ratio of quenched martensite is 10.0% or less, the area ratio of ferrite is 30.0% or less, and the volume ratio of retained austenite is 10.0% or less having a steel structure, and the C content in the surface layer of the base steel plate is 0.150% or less, the value obtained by dividing the C content in the surface layer of the base steel plate by the total C content of the base steel plate is less than 0.85, the amount of hydrogen [H] (mass ppm) in the steel with respect to hydrogen having a binding energy with the hydrogen trap site of the base steel plate of 10 kJ / mol or more and 100 kJ / mol or less is 0.30 mass ppm or less, and the average Vickers hardness of the surface layer of the base steel plate is Hv S A hot-dip galvanized steel sheet characterized by satisfying the following formula (1). [H] ≤ (480 - Hv S ) / 320 ··· (1)

[0013] [2] The component composition further comprises, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.100% or less, W: 0.100% or less, B: 0.0100% or less, Cr: 1.000% or less, Mo: 1.000% or less, Ni: 1.000% or less, Co: 0.500% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.1000% or less, Te: 0.1000% or less, Hf: 0.10% or less, Bi:0.200% or less The hot-dip galvanized steel sheet according to [1] above, comprising at least one element selected from the group consisting of the following.

[0014] [3] The hot-dip galvanized steel sheet according to [1] or [2] above, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

[0015] [4] A step of heating a steel slab having the component composition described in [1] or [2] above, and subjecting the steel slab to hot rolling at a finish rolling exit temperature of 750°C or more and 1000°C or less to obtain a hot-rolled steel sheet, The process involves winding the hot-rolled steel sheet at a temperature of 300°C to 750°C, Next, the hot-rolled steel sheet is subjected to cold rolling at a rolling ratio of 60.0% or less to obtain a cold-rolled steel sheet. Next, the cold-rolled steel sheet is heated to (Ac3 transformation point - 50)°C or higher in an annealing process, Next, the cold-rolled steel sheet is passed through a snout whose dew point is controlled to be -60°C or higher. Next, the cold-rolled steel sheet is subjected to a hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet. Next, a first cooling step is performed to cool the hot-dip galvanized steel sheet to a cooling stop temperature below the martensitic transformation start temperature, Next, the hot-dip galvanized steel sheet is reheated to a reheating temperature above the cooling stop temperature but below 450°C. Next, the hot-dip galvanized steel sheet is vibrated so that the maximum amplitude is 10 nm or more and 500 μm or less, while being held at the reheating temperature for 2 seconds or more and 600 seconds or less in a vibration holding step, Next, a second cooling step is performed to cool the hot-dip galvanized steel sheet to room temperature, A method for manufacturing a hot-dip galvanized steel sheet, characterized by having the following features.

[0016] [5] The method for manufacturing a hot-dip galvanized steel sheet according to [4] above, wherein in the vibration holding step, sound waves are irradiated onto the hot-dip galvanized steel sheet such that the sound pressure level on the surface of the hot-dip galvanized steel sheet is 50 dB or more, thereby causing the hot-dip galvanized steel sheet to vibrate.

[0017] [6] The method for manufacturing a hot-dip galvanized steel sheet according to [4] or [5] above, wherein in the second cooling step, the cooling rate in the temperature range of 50°C or higher below the martensitic transformation onset temperature is 15.0°C / s or less.

[0018] [7] A method for manufacturing a hot-dip galvanized steel sheet according to any one of the above [4] to [6], wherein the vibration of the hot-dip galvanized steel sheet is continued in the second cooling step.

[0019] [8] A method for manufacturing a hot-dip galvanized steel sheet according to any one of the above [4] to [7], wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet and a method for manufacturing the same, which have a TS of 1180 MPa or higher, and are excellent in formability, corona bond peel strength, and weld delay peel resistance, and can also be used to manufacture parts with high dimensional accuracy. [Brief explanation of the drawing]

[0021] [Figure 1] This is a schematic diagram of a tensile test specimen used to evaluate the peel strength of corona bond in an embodiment of the present invention. [Figure 2] (a) A top view and (b) A side view of a welded joint used in the evaluation of the delayed peeling characteristics of the welded joint in an embodiment of the present invention. [Modes for carrying out the invention]

[0022] The following describes embodiments of the hot-dip galvanized steel sheet and its manufacturing method according to the present invention. Note that the embodiments described below are examples of the present invention and do not limit the configuration of the present invention.

[0023] (base steel plate) First, the component composition of the base steel sheet for the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described. Note that unless otherwise specified, the "%" representing the content of component elements means "mass%".

[0024] [C: 0.030% or more and 0.250% or less] Carbon (C) is one of the important basic components of steel, and in particular in this invention, it is an important element that affects the area ratio of tempered martensite, the area ratio of quenched martensite, the area ratio of ferrite, and the volume ratio of retained austenite. If the C content is less than 0.030%, the area ratios of tempered martensite and quenched martensite decrease, and the area ratio of ferrite increases, making it difficult to achieve the desired TS and YR. Therefore, the C content should be 0.030% or more, preferably 0.080% or more, and more preferably 0.110% or more. On the other hand, if the C content exceeds 0.250%, the tempered martensite becomes brittle, and the desired ductility cannot be obtained. Therefore, the C content should be 0.250% or less, preferably 0.230% or less, and more preferably 0.220% or less.

[0025] [Si:0.01% or more and 2.50% or less] Si is one of the important basic components of steel, and in particular in this invention, it is an element that affects the volume fraction of retained austenite because it suppresses carbide formation during annealing and promotes the formation of retained austenite. Furthermore, by suppressing carbide formation, Si reduces the void formation initiation points during bending, making it an effective element for improving bendability. To obtain these effects, the Si content should be 0.01% or more, preferably 0.20% or more, and more preferably 0.25% or more. On the other hand, if the Si content exceeds 2.50%, the amount of hydrogen in the underlying steel sheet increases due to the increase in the volume fraction of retained austenite, making it difficult to obtain excellent hole-expanding properties and resistance to delayed delamination of welds. Therefore, the Si content should be 2.50% or less, preferably 2.00% or less, and more preferably 1.50% or less.

[0026] [Mn: 0.10% or more and 5.00% or less] Mn is one of the important basic components of steel, and in particular in this invention, it is an important element that affects the area ratio of tempered martensite, the area ratio of ferrite, and the volume ratio of retained austenite. Furthermore, Mn is an important element that improves the hardenability of steel and suppresses ferrite transformation during cooling after annealing, thereby increasing the area ratio of tempered martensite. To obtain these effects, the Mn content should be 0.10% or more, preferably 1.00% or more, and more preferably 2.00% or more. On the other hand, if the Mn content exceeds 5.00%, the amount of retained austenite increases, which not only increases the amount of hydrogen in the steel but also results in excessive hardenability and an increase in the hardness of the steel plate surface. As a result, excellent delayed delamination resistance of the weld cannot be obtained. Therefore, the Mn content should be 5.00% or less, preferably 4.00% or less, and more preferably 3.50% or less.

[0027] [P:0.100% or less] When phosphorus (P) is present in excess, it segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and reduces the ultimate deformability of the steel sheet, thus decreasing its hole-expanding and bendability. Therefore, the P content should be 0.100% or less, preferably 0.070% or less. Although there is no specific lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable that the P content be 0.001% or more.

[0028] [S:0.0200% or less] S exists as a sulfide and reduces the ultimate deformability of steel; therefore, excessive S content reduces hole-expanding and bendability. For this reason, the S content should be 0.0200% or less, preferably 0.0050% or less. Although there is no specific lower limit for the S content, due to production technology constraints, it is preferable that the S content be 0.0001% or more.

[0029] [Al:0.100% or less] If Al is in excess, the A3 transformation point rises, and a large amount of ferrite is included in the microstructure, making it difficult to achieve the desired TS. Therefore, the Al content should be 0.100% or less, preferably 0.050% or less. Although there is no specific lower limit for the Al content, since Al is an element that suppresses carbide formation during continuous annealing and promotes the formation of retained austenite, the Al content should preferably be 0.001% or more.

[0030] [N:0.0100% or less] Since nitrogen (N) exists as a nitride and reduces the ultimate deformability of the steel sheet, excessive N content reduces hole-expanding and bendability. Therefore, the N content should be 0.0100% or less, preferably 0.0050% or less. Although there is no specific lower limit for the N content, due to production technology constraints, the N content should preferably be 0.0005% or more.

[0031] [O:0.0100% or less] O exists as an oxide and reduces the ultimate deformability of steel sheets; therefore, if O is present in excess, hole-expanding properties and bendability decrease. For this reason, the O content should be 0.0100% or less, preferably 0.0050% or less. Although there is no specific lower limit for the O content, due to production technology constraints, it is preferable that the O content be 0.0001% or more.

[0032] [Remaining components] The base steel sheet has a component composition containing the above elements, with the remainder being Fe and unavoidable impurities. Preferably, the base steel sheet has a component composition containing the above elements, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, Sr, Ge, Cs, and As. These impurities are acceptable if their total content is 0.100% or less.

[0033] The composition of the high-strength steel sheet disclosed herein is, in addition to the essential components listed above, further comprising, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.100% or less, W: 0.100% or less, B: 0.0100% or less, Cr: 1.000% or less, Mo: 1.000% or less, Ni: 1.000% or less, Co: 0.500% or less, C It may contain at least one element selected from the group consisting of u: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.1000% or less, Te: 0.1000% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[0034] [Ti:0.200% or less] Ti generates a large amount of coarse precipitates or inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the Ti content exceeds 0.200%, the hole-expanding properties and bendability decrease. For this reason, when Ti is included, the Ti content should be 0.200% or less, preferably 0.100% or less. Although there is no specific lower limit for the Ti content, by setting the Ti content to 0.001% or more, fine carbides, nitrides, or carbonitrides can be formed during hot rolling or continuous annealing, increasing the strength of the steel sheet and allowing for the suitable acquisition of TS. For this reason, the Ti content should preferably be 0.001% or more.

[0035] [Nb:0.200% or less] Nb generates a large amount of coarse precipitates or inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the Nb content exceeds 0.200%, the hole-expanding properties and bendability decrease. For this reason, when Nb is included, the Nb content should be 0.200% or less, preferably 0.100% or less. Although there is no specific lower limit for the Nb content, by setting the Nb content to 0.001% or more, the strength of the steel sheet can be increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, and TS can be suitably obtained. For this reason, the Nb content should be 0.001% or more.

[0036] [V:0.200% or less] V generates a large amount of coarse precipitates or inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the V content exceeds 0.200%, the hole-expanding properties and bendability decrease. For this reason, when V is included, the V content should be 0.200% or less, preferably 0.100% or less. Although there is no specific lower limit for the V content, by setting the V content to 0.001% or more, the strength of the steel sheet can be increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, and TS can be suitably obtained. For this reason, the V content should be 0.001% or more.

[0037] [Ta:0.100% or less] [W: 0.100% or less] If the content of Ta or W exceeds 0.100%, a large amount of coarse precipitates or inclusions are formed, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Ta or W is included, the content of each should be 0.100% or less, preferably 0.080% or less. There is no specific lower limit for the content of Ta or W, but when the content of Ta or W is 0.010% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, and the strength of the steel sheet is favorably obtained. Therefore, it is preferable that the content of Ta or W be 0.010% or more.

[0038] [B:0.0100% or less] When the B content is 0.0100% or less, cracks inside the steel sheet are suitably suppressed during casting or hot rolling, and the ultimate deformability of the steel sheet is not reduced, thus resulting in suitable bendability. Therefore, when B is included, the B content should be 0.0100% or less, and preferably 0.0080% or less. Although there is no particular lower limit for the B content, since B is an element that segregates at the austenite grain boundaries during annealing and improves hardenability, it is preferable that the B content be 0.0003% or more.

[0039] [Cr:1.000% or less] [Mo: 1.000% or less] [Ni: 1.000% or less] If the content of Cr, Mo, or Ni exceeds 1.000%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Cr, Mo, or Ni are included, their respective contents should be 1.000% or less, preferably 0.800% or less. Although there is no specific lower limit for the content of Cr, Mo, or Ni, since these elements improve hardenability, it is preferable that the content of each Cr, Mo, or Ni be 0.010% or more.

[0040] [Co:0.500% or less] If the Co content exceeds 0.500%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Co is included, the Co content should be 0.500% or less, preferably 0.050% or less. Although there is no specific lower limit for the Co content, since Co is an element that improves hardenability, it is preferable that the Co content be 0.001% or more.

[0041] [Cu:1.00% or less] If the Cu content exceeds 1.00%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Cu is included, the Cu content should be 1.00% or less, preferably 0.80% or less. Although there is no specific lower limit for the Cu content, since Cu is an element that improves hardenability, it is preferable that the Cu content be 0.01% or more.

[0042] [Sn:0.200% or less] If the Sn content exceeds 0.200%, cracks will form inside the steel sheet during casting or hot rolling, reducing the steel sheet's ultimate deformability and thus decreasing its hole-expanding and bendability. Therefore, when Sn is included, the Sn content should be 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less. There is no specific lower limit for the Sn content, but since Sn is an element that improves hardenability, it is preferable that the Sn content be 0.001% or more.

[0043] [Sb:0.200% or less] If the Sb content exceeds 0.200%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Sb is included, the Sb content should be 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less. Although there is no specific lower limit for the Sb content, since Sb is an element that controls the surface softening thickness and allows for strength adjustment, it is preferable that the Sb content be 0.001% or more.

[0044] [Ca: 0.0100% or less] [Mg:0.0100% or less] [REM:0.0100% or less] If the content of Ca, Mg, or REM exceeds 0.0100%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Ca, Mg, or REM are present, the content of each should be 0.0100% or less, and preferably 0.0050% or less. Although there is no specific lower limit for the content of Ca, Mg, or REM, these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the steel sheet, so it is preferable that the content of each Ca, Mg, or REM be 0.0005% or more.

[0045] [Zr:0.1000% or less] [Te:0.1000% or less] If the Zr or Te content exceeds 0.1000%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Zr or Te is included, the content of Zr or Te should be 0.1000% or less, and preferably 0.0800% or less. Although there is no specific lower limit for the Zr or Te content, since these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the steel sheet, it is preferable that the content of Zr or Te be 0.0010% or more.

[0046] [Hf:0.10% or less] If the Hf content exceeds 0.10%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Hf is included, the Hf content should be 0.10% or less, preferably 0.08% or less. Although there is no specific lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides or sulfides, etc., and improves the ultimate deformability of the steel sheet, it is preferable that the Hf content be 0.01% or more.

[0047] [Bi:0.200% or less] If the Bi content exceeds 0.200%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet, and thus decreasing its hole-expanding and bendability. Therefore, when Bi is included, the Bi content should be 0.200% or less, preferably 0.100% or less. Although there is no specific lower limit for the Bi content, since Bi is an element that reduces segregation, it is preferable that the Bi content be 0.001% or more.

[0048] Furthermore, regarding the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, if their respective contents are below the preferred lower limit, they may be included as unavoidable impurities, as this will not impair the effects of the present invention.

[0049] Next, we will describe the structural integrity of the base steel plate. The structural integrity of the base steel plate described below refers to the structure at the 1 / 4 thickness position (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the base steel plate).

[0050] [Area ratio of tempered martensite: 60.0% or more] In the steel structure of the base steel sheet, if the area ratio of tempered martensite is 60.0% or more, the desired YR and high bendability can be achieved. Therefore, the area ratio of tempered martensite should be 60.0% or more, preferably 65.0% or more, more preferably 70.0% or more, and even more preferably 75.0% or more. There is no particular upper limit to the area ratio of tempered martensite, but in addition to the viewpoint of obtaining high TS and high bendability, if the area ratio of tempered martensite exceeds 99.0%, ferrite and retained austenite, which are effective in controlling YR, will not be present in the steel structure. Therefore, the area ratio of tempered martensite should be 99.0% or less, more preferably 98.0% or less, and even more preferably 97.0% or less.

[0051] [Area ratio of quenched martensite: 10.0% or less] In the steel structure of the base steel sheet, if the area ratio of quenched martensite exceeds 10.0%, the YS decreases, making it difficult to achieve the desired YR. Therefore, the area ratio of quenched martensite should be 10.0% or less, preferably 8.0% or less, and more preferably 5.0% or less. There is no particular upper limit to the area ratio of quenched martensite, but from the viewpoint of obtaining a high TS, the area ratio of quenched martensite is preferably 0.1% or more.

[0052] [Ferrite area ratio: 30.0% or less] In the steel structure of the base steel plate, if the area ratio of ferrite is 30.0% or less, the desired TS, excellent hole-expanding properties, and bendability can be achieved. Therefore, the area ratio of ferrite should be 30.0% or less, and preferably 25.0% or less. The lower limit of the area ratio of ferrite is not particularly limited, and the area ratio of ferrite may be 0.0%. In this invention, ferrite also includes bainitic ferrite.

[0053] The area ratios of tempered martensite, quenched martensite, and ferrite can be determined as follows: Cut out a sample so that the cross-section (L section) parallel to the rolling direction of the steel sheet becomes the observation surface. Polish the observation surface of the cut-out sample to a mirror finish using diamond paste, and then etch it with 3 vol.% nital to reveal the microstructure. Using a Scanning Electron Microscope (SEM), observe 10 fields of view at a position 1 / 4 of the steel sheet thickness, with an acceleration voltage of 15 kV and a magnification of 2000x to obtain microstructure images. Using the obtained microstructure images, calculate the area ratio of each microstructure (ferrite, tempered martensite, quenched martensite) for 10 fields of view using Media Cybernetics' Image-Pro. The average value of the obtained values ​​is taken as the area ratio of tempered martensite, quenched martensite, or ferrite, respectively. In the above microstructure images, ferrite (including bainitic ferrite) is a flat structure with no carbides in the recessed areas, quenched martensite is a structure with fine irregularities inside the convex areas, and tempered martensite is a structure with fine carbides in the recessed areas.

[0054] [Volume fraction of retained austenite: 10.0% or less] When the volume fraction of retained austenite in the steel structure of the base steel sheet is 10.0% or less, the amount of hydrogen in the steel of the base steel sheet can be reduced, and excellent hole-expanding properties and resistance to delayed delamination of welds can be obtained. Therefore, the volume fraction of retained austenite should be 10.0% or less, preferably 9.0% or less, more preferably 8.0% or less, and even more preferably 7.0% or less. There is no particular lower limit to the volume fraction of retained austenite, and the desired properties can be obtained even if the volume fraction of retained austenite is 0.0%. In this invention, the volume fraction of retained austenite can be considered equivalent to its area fraction.

[0055] The volume fraction of retained austenite is measured as follows: After grinding the sample so that the observation surface is located 1 / 4 of the plate thickness from the surface of the base steel plate, it is further polished by 0.1 mm using chemical polishing. Using an X-ray diffractometer, the integrated reflectance intensity of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron is measured on the observation surface with Co as the Kα source. The volume fraction of austenite can be determined from the intensity ratio of the integrated reflectance intensity from each plane of fcc iron to the integrated reflectance intensity from each plane of bcc iron.

[0056] [Remaining tissue] In the microstructure of the base steel sheet, the area ratio of the remaining microstructure other than tempered martensite, quenched martensite, ferrite, and retained austenite is preferably 5.0% or less. Examples of the remaining microstructure include those known as other steel sheet microstructures, such as pearlite, cementite, and metastable carbides (e.g., epsilon (ε) carbide, eta (η) carbide, chi (χ) carbide). The identification of the remaining microstructure can be performed, for example, by observation using a scanning electron microscope (SEM).

[0057] The area ratio of the remaining tissue can be calculated using the following formula (2). [Area percentage of remaining structure (%)] = 100 - [Area percentage of tempered martensite (%)] - [Area percentage of quenched martensite (%)] - [Area percentage of ferrite (%)] - [Volume percentage of retained austenite (%)] ... (2)

[0058] [C content in the surface layer of the base steel plate: 0.150% or less] By forming a soft layer on the surface of the base steel sheet and reducing the surface hardness, the toughness of the corona bond is improved, and the peel strength of the corona bond is increased. Surface hardness can be reduced by reducing the carbon content in the surface layer, and the lower the carbon content in the surface layer of the base steel sheet, the better. Therefore, the carbon content in the surface layer of the base steel sheet should be 0.150% or less, and preferably 0.130% or less. However, the lower limit of the carbon content in the surface layer of the base steel sheet is not particularly limited, and the problems of the present invention can be solved even if the surface layer of the base steel sheet does not contain carbon, so the carbon content may be as low as 0.000%.

[0059] [Value obtained by dividing the carbon content in the surface layer of the base steel plate by the total carbon content of the base steel plate: less than 0.85] By creating a large gradient between the carbon content on the surface of the base steel plate and the carbon content inside the base steel plate, stress concentration on the surface of the steel plate when the corona bond is peeled off can be mitigated, thereby suppressing crack propagation. Therefore, the value obtained by dividing the carbon content on the surface of the base steel plate by the total carbon content of the base steel plate should be less than 0.85. However, there is no particular limit to the lower limit of the value obtained by dividing the carbon content on the surface of the base steel plate by the total carbon content of the base steel plate, and this value may be 0.00.

[0060] The carbon content in the surface layer of the base steel sheet can be determined as follows: A 20 mm square sample is taken from the base steel sheet to be measured. The sample is measured using discharge emission spectrometry (GDS) with a 4 mmΦ anode, a high-frequency discharge pressure of 300 MPa, a high-frequency output of 35 W, a pulse frequency of 100 Hz, and a measurement time of 0 to 6000 seconds. Here, Zn originating from the plating is detected in the first 5 μm of the surface layer. The carbon content at a position 10 μm further in the thickness direction of the base steel sheet from the depth position where the Zn concentration is 0.0 mass% was defined as the carbon content in the surface layer of the base steel sheet. Note that the total carbon content of the base steel sheet is the carbon content in the component composition of the base steel sheet as described above, and is equivalent to the carbon content of the steel slab used.

[0061] [Hydrogen content in steel [H]: 0.30 mass ppm or less, where the bonding energy with hydrogen trapping sites in the underlying steel plate is between 10 kJ / mol and 100 kJ / mol] By setting the hydrogen content [H] in the steel to 0.30 mass ppm or less, where the binding energy with the hydrogen trapping sites of the base steel plate is between 10 kJ / mol and 100 kJ / mol, good hole-expanding properties can be ensured. Therefore, the hydrogen content [H] in the steel should be 0.30 mass ppm or less, and preferably 0.20 mass ppm or less. The lower limit of the hydrogen content [H] in the steel is not particularly limited, and the problems of this application can be solved even if no hydrogen is contained in the steel, so the hydrogen content [H] in the steel may be 0.00 mass ppm.

[0062] Furthermore, if the binding energy with the hydrogen trapping sites of the underlying steel plate is low, hydrogen is easily diffused away and does not affect the delayed delamination properties of the weld joint; therefore, a binding energy of 15 kJ / mol or higher is preferable. On the other hand, hydrogen with a high binding energy is difficult to delaminate from the trapping sites and similarly does not affect the delayed delamination properties of the weld joint; therefore, a binding energy of 80 kJ / mol or lower is preferable.

[0063] [H content in steel where the bonding energy with hydrogen trapping sites in the underlying steel plate is between 10 kJ / mol and 100 kJ / mol: (480-Hv S ) / 320 or less] The amount of hydrogen in the steel [H] (mass ppm) where the binding energy with the hydrogen trapping sites of the underlying steel plate is between 10 kJ / mol and 100 kJ / mol is such that the average Vickers hardness of the surface layer of the underlying steel plate is Hv STherefore, satisfying the following equation (1) is an important component of the present invention. Delayed delamination of welds is a phenomenon caused by hydrogen in the steel trapped at specific hydrogen trap sites. Therefore, in order to obtain excellent delayed delamination resistance to welds, it is necessary to reduce the amount of hydrogen in the steel [H] such that the binding energy with the hydrogen trap sites is between 10 kJ / mol and 100 kJ / mol. On the other hand, if the surface hardness of the base steel plate decreases, the susceptibility to hydrogen cracking decreases. Also, if the surface of the base steel plate becomes excessively hardened, the susceptibility to hydrogen cracking increases. Therefore, by reducing the amount of hydrogen in the steel according to the surface hardness of the base steel plate, excellent delayed delamination resistance to welds can be obtained. Furthermore, the upper limit of surface hardness differs depending on the TS, and for steel plates with a TS of 1180 MPa or higher, which is the target of the present invention, it is necessary to control the amount of hydrogen in the steel [H] so that it satisfies the following equation (1). [H]≦(480-Hv S ) / 320 ···(1)

[0064] Average Vickers hardness Hv of the surface layer of the underlying steel plate S If the strength of the surface layer of the steel plate decreases excessively, the overall strength of the steel plate will decrease, so it is preferable that the Vickers hardness is 100 or higher. On the other hand, the average Vickers hardness Hv of the surface layer of the base steel plate S If the strength of the steel plate surface layer increases excessively, its bendability deteriorates, so it is preferable that the strength be 550 or less.

[0065] The binding energy of hydrogen to hydrogen trapping sites can be determined as follows: A large plate sample with a length of 500 mm is taken from a steel strip manufactured on a hot-dip galvanizing line. Multiple test pieces with a length of 30 mm and a width of 5 mm are taken from the center of the large plate sample by shearing. The obtained test pieces are immediately immersed in liquid nitrogen after being taken. Next, the hot-dip galvanizing layer of the test pieces is removed with alkali while controlling the temperature of the treatment solution so that the surface temperature of the test pieces is below room temperature. Then, the amount of hydrogen released from the test pieces when they are heated is measured using a temperature rise desorption analysis method. Specifically, the test pieces are heated from room temperature under the conditions of a target temperature of 800°C and heating rates of 50, 100, 150, and 200°C / hr, and then cooled to room temperature. Using the peak temperature T at which the maximum amount of released hydrogen is obtained in the hydrogen release curve from each heating rate, the activation energy Ea for hydrogen to desorb from the hydrogen trapping site is determined from the following equation (3). In equation (3), φ is the heating rate, R is the gas constant (8.31 J / mol·K), and A is a constant. Specifically, for the measurement results at each heating rate, ln(φ / T 2 The relationship between () and 1 / T is plotted, and the slope (Ea / R) is determined by linear approximation to calculate the activation energy Ea. The obtained activation energy Ea is the sum of the activation energy of hydrogen diffusion in the lattice and the binding energy of hydrogen to the hydrogen trap site. Therefore, by subtracting the activation energy of hydrogen diffusion in the lattice of 7.6 kJ / mol from Ea, the binding energy of hydrogen to the hydrogen trap site can be calculated.

number

[0066] The amount of hydrogen in steel [H], where the binding energy with hydrogen trapping sites is between 10 kJ / mol and 100 kJ / mol, can be determined as follows: The sample is continuously heated from room temperature to 300°C at a heating rate of 200°C / h, and then cooled back to room temperature. During heating, the cumulative amount of hydrogen released from the test piece between room temperature and 210°C is measured and defined as the amount of hydrogen in the steel [H].

[0067] The average Vickers hardness Hv of the surface layer of the base steel plate S can be obtained as follows. At a position 5 μm deep in the thickness direction from the surface layer of the base steel plate (the interface between the plating layer and the base steel plate), 20 measurements were taken at a load of 10 g and a load time of 10 seconds. The average value of the measurement results was defined as the average Vickers hardness Hv of the surface layer of the base steel plate S .

[0068] (Hot-dip galvanized layer) Next, the hot-dip galvanized layer of the hot-dip galvanized steel plate will be described. In the present invention, the hot-dip galvanized layer includes an alloyed hot-dip galvanized layer (a plating layer obtained by subjecting the hot-dip galvanized layer to an alloying treatment). Also, the hot-dip galvanized layer is provided on both surfaces of the surface of the base steel plate

[0069] The component composition of the hot-dip galvanized layer is not particularly limited, and any general one may be used. In one example, it contains Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and further contains at least one element selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more and 3.5% by mass or less, with the balance being Zn and unavoidable impurities. When no alloying treatment is performed, the Fe content of the hot-dip galvanized layer may be less than 7% by mass. When an alloying treatment is performed, the Fe content of the hot-dip galvanized layer is preferably 7% by mass or more and 15% by mass or less, and more preferably 8% by mass or more and 13% by mass or less

[0070] The plating adhesion amount per side is not particularly limited, but is preferably 20 - 80 g / m 2 .

[0071] Note that the plate thickness of the hot-dip galvanized steel plate is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less

[0072] (Mechanical properties of the hot-dip galvanized steel plate) Next, the mechanical properties of a hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0073] [Tensile strength (TS): 1180 MPa or higher] [Yield ratio YR: 65.0% or more and 95.0% or less] [EL: 7.0% or higher] Hot-dip galvanized steel sheets have a TS of 1180 MPa or higher, a YR of 65.0% to 95.0%, and an EL of 7.0% or higher. TS, YR, and EL can be determined as follows: A JIS No. 5 test specimen is taken so that the tensile direction is perpendicular to the rolling direction of the hot-dip galvanized steel sheet. In accordance with JIS Z 2241:2011, the crosshead speed is 1.67 × 10⁻⁶. -1 A tensile test is performed under conditions of mm / s, and TS, YS, and EL are measured. From the measured TS and YS, YR is calculated using the following formula (4). YR = YS / TS × 100 ... (4)

[0074] [Hole-expanding properties] Hot-dip galvanized steel sheets exhibit excellent hole-expanding properties. In this invention, excellent hole-expanding properties mean that the hole expansion ratio λ is 30% or more. The hole expansion ratio λ can be determined as follows: A 100mm x 100mm test piece is taken from the hot-dip galvanized steel sheet. A hole-expanding test is performed in accordance with JIS Z 2256 (2010). Specifically, a 10mm diameter hole is punched with a clearance of 12% ± 1%, and a 60° conical punch is pressed into the hole using a die with an inner diameter of 75mm while holding it down with a wrinkle-holding force of 9 tons, and the hole diameter at the crack initiation limit is measured. From the measurement results, D f Let D be the hole diameter at the time of crack initiation (mm) and D0 be the initial hole diameter (mm). The hole expansion ratio λ (%) is calculated using the following formula (5). λ(%)={(D f -D0) / D0} × 100 ... (5)

[0075] [Bendability] Hot-dip galvanized steel sheets exhibit excellent bendability. Bendability can be evaluated as follows: From a hot-dip galvanized steel sheet 24 hours after manufacturing, a strip-shaped test piece with a width of 30 mm and a length of 100 mm is taken so that the direction parallel to the rolling direction is the axis of the bending test. A 90° V bending test is performed in accordance with JIS Z 2248:2022, under conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds. After that, the presence or absence of cracks is determined on the outer side of the bend using a stereomicroscope, and the smallest bending radius at which no cracks occur is defined as the limit bending radius R. If the limit bending radius R / t ≤ 4.00 (t: thickness of the hot-dip galvanized steel sheet) is satisfied in the 90° V bending test, it is evaluated as having excellent bendability.

[0076] [Peel strength of corona bond] Hot-dip galvanized steel sheets exhibit excellent peel strength of corona bonds. The peel strength of corona bonds can be evaluated as follows: Two flat plates measuring 100 mm x 30 mm are taken from the hot-dip galvanized steel sheet. With the bending axis parallel to the short side of the flat plate plate, a 90° bend is performed at a position 30 mm from the edge of the plate with a curvature of 5 mm to create an L-shaped plate. On the L-shaped plate, a square surface of approximately 30 mm on each side is used as the welding surface, and a surface of approximately 65 mm x 30 mm is used as the gripping area for the tensile test. As shown in Figure 1, the two L-shaped plates 10 are placed so that their respective welding surfaces are in contact, and at the center 12 of the welding surface, the weld is welded so that the diameter of the weld is 3 mm to 5 mm on each L-shaped plate 10 to create a tensile test piece 100. After welding is complete, a tensile test is performed before the hydrogen present inside the weld of the tensile test piece 100 escapes, i.e., within 5 minutes after welding. The conditions for the tensile test are the same as for the evaluation of the TS. From the load-displacement graph obtained from the tensile test, the maximum load just before a sharp load drop occurs is determined and defined as the peel strength of the corona bond. Here, a peel strength of 50 N or higher is considered to indicate excellent peel strength of the corona bond. Furthermore, a peel strength of 55 N or higher is preferable for the corona bond of hot-dip galvanized steel sheets.

[0077] [Weld-resistant delayed peeling properties] Hot-dip galvanized steel sheets exhibit excellent resistance to delayed delamination of welds. The delayed delamination of welds can be evaluated as follows. Two 100mm x 30mm samples are taken from the hot-dip galvanized steel sheet. As shown in Figure 2(b), the sample 20, spacer 22, and sample 20 are stacked in that order to form a welded joint 102. Two 1.5mm thick, 30mm square mild steel plates are used as spacers 22, with the spacers 22 sandwiched between the two samples 20, and tack welding is performed. Tack welding is performed at the two tack welds 24 shown in Figure 2(a). The center of the welded joint 102 after tack welding is designated as a resistance spot weld 26, and resistance spot welding is performed. The welded joint 102 after resistance spot welding is left to stand in the air at room temperature (20°C) for 24 hours or more. It is preferable to perform resistance spot welding at room temperature and to keep the welding electrodes (lower electrode and upper electrode) constantly water-cooled. Both the lower and upper electrodes have a tip diameter of 6 mm and a radius of curvature of 40 mm, and can be made of chromium copper DR type electrodes. The pressure applied is controlled by driving the lower and upper electrodes with servo motors, and a single-phase AC current of 50 Hz is supplied when energized.

[0078] The welded joint obtained as described above is processed so that the cross-section of the weld can be observed. The cross-section of the weld is observed using a SEM. Here, if a crack occurs in the nugget at the joint interface, it is determined that delamination of the weld has occurred. The nugget diameter (mm) of the weld is measured, and if the value obtained by dividing the measured nugget diameter by the square root of t (t: thickness of the hot-dip galvanized steel sheet) is 3.5 or more, and no cracks have occurred in the nugget, it is evaluated as having excellent resistance to delayed delamination of the weld.

[0079] (Method of manufacturing hot-dip galvanized steel sheet) Next, a method for manufacturing hot-dip galvanized steel sheets will be described. First, a steel slab is manufactured by melting a steel material having the above-mentioned component composition. The method for melting the steel material is not particularly limited, and known melting methods such as converters or electric furnaces can be used. In addition, it is preferable to manufacture the steel slab (slab) by continuous casting in order to prevent macrosegregation, but it can also be manufactured by ingot casting or thin slab casting. Furthermore, in addition to the conventional method of cooling the steel slab to room temperature and then reheating it, energy-saving processes such as direct rolling, in which the slab is charged into the heating furnace while still hot without cooling to room temperature, or rolled immediately after a short period of heat retention, can also be applied without any problems.

[0080] [Heating temperature of steel slabs] While not particularly limited, the heating temperature of the steel slab is preferably between 1100°C and 1300°C. Precipitates present during the heating stage of the steel slab exist as coarse precipitates in the final resulting base steel sheet and do not contribute to strength; therefore, it is preferable to remelt the Ti and Nb precipitates deposited during casting. For this reason, the heating temperature of the steel slab is preferably 1100°C or higher, and more preferably 1150°C or higher. Furthermore, from the viewpoint of scaling off defects such as bubbles and segregation on the slab surface, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, it is also preferable to heat the steel slab to 1100°C or higher. On the other hand, if the heating temperature of the steel slab exceeds 1300°C, scale loss increases with the increase in oxidation; therefore, the heating temperature of the steel slab is preferably 1300°C or lower, and more preferably 1250°C or lower. Furthermore, while steel slabs are formed into sheet bars by rough rolling under normal conditions, if the heating temperature is set low, it is preferable to heat the sheet bars using a bar heater or the like before finish rolling to prevent problems during hot rolling.

[0081] [Finishing rolling exit temperature for hot rolling: 750°C to 1000°C] Hot-rolled steel sheets are produced by hot-rolling heated steel slabs. Hot-rolling includes rough rolling and finish rolling. If the finish rolling exit temperature exceeds 1000°C, the amount of oxide (scale) formation increases rapidly, the interface between the base metal and the oxide becomes rough, and the surface quality after pickling or cold rolling tends to deteriorate. In addition, if some hot-rolling scale remains after pickling, it adversely affects the bendability. Furthermore, the grain size may become excessively coarse, which may cause surface roughness in pressed products during processing. Therefore, the finish rolling exit temperature should be 1000°C or less, preferably 950°C or less. On the other hand, if the finish rolling exit temperature is less than 750°C, the rolling load increases, and the rolling load becomes large. In addition, the reduction ratio in the unrecrystallized austenite state becomes high, abnormal textures develop, in-plane anisotropy becomes pronounced in the final product, and not only is the uniformity of the material (material stability) impaired, but the bendability itself also decreases. Therefore, the finish rolling exit temperature should be 750°C or higher, preferably 800°C or higher. The finish rolling exit temperature is based on the temperature of the hot-rolled steel sheet surface.

[0082] [Winding temperature for hot-rolled steel sheets: 300°C to 750°C] Next, the hot-rolled steel sheet is wound up after hot rolling. If the winding temperature of the hot-rolled steel sheet exceeds 750°C, scale will form on the surface of the hot-rolled steel sheet, causing excessive surface decarburization during the subsequent heat treatment process, reducing surface hardness, and thus preventing sufficient peel strength from being obtained for the corona bond. Therefore, the winding temperature of the hot-rolled steel sheet should be 750°C or lower, preferably 650°C or lower. On the other hand, if the winding temperature of the hot-rolled steel sheet is below 300°C, the strength of the hot-rolled steel sheet will increase, leading to increased rolling load during cold rolling and the occurrence of defects in sheet shape, thus reducing productivity. Therefore, the winding temperature of the hot-rolled steel sheet should be 300°C or higher, preferably 400°C or higher. The winding temperature is based on the temperature of the surface of the hot-rolled steel sheet.

[0083] Furthermore, rough-rolled sheets may be joined together during hot rolling and finish rolling may be performed continuously. Alternatively, the rough-rolled sheets (sheet bars) may be wound up before finish rolling. In addition, to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Lubricated rolling is preferable from the viewpoint of uniformizing the shape and material of the steel sheets. The coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0084] Next, the hot-rolled sheet is pickled. Pickling removes oxides from the surface of the steel sheet, which is important for ensuring good chemical conversion treatment properties and plating quality in the final hot-dip galvanized steel sheet product. Pickling may be performed once or in multiple stages.

[0085] [Cold rolling ratio: 60.0% or less] Next, the hot-rolled steel sheet is subjected to cold rolling to produce a cold-rolled steel sheet. The rolling method is not particularly limited, but it is preferable to perform cold rolling by multi-pass rolling, such as tandem multi-stand rolling or reverse rolling, which requires two or more passes, in order to uniformly and efficiently introduce strain and obtain a uniform structure. If the rolling ratio of the cold rolling exceeds 60.0%, the grain size of the austenite formed in the subsequent annealing process becomes finer, the stability of the austenite improves, and the amount of retained austenite becomes excessive. Therefore, the rolling ratio of the cold rolling should be 60.0% or less, preferably 55.0% or less, and more preferably 50.0% or less. On the other hand, the lower limit of the rolling ratio of the cold rolling is not particularly limited, but from the viewpoint of promoting recrystallization in the annealing process, the rolling ratio of the cold rolling should be 5.0% or more, and more preferably 10.0% or more.

[0086] [Heating temperature for annealing process: (Ac3 transformation point - 50)°C or higher] Next, the cold-rolled steel sheet is annealed. If the heating temperature in the annealing process is below (Ac3 transformation point - 50)°C, the annealing process will occur in the two-phase region of ferrite and austenite, and since a large amount of ferrite will be contained after annealing, it will be difficult to achieve the desired TS and high bendability. Therefore, the heating temperature in the annealing process should be (Ac3 transformation point - 50)°C or higher, preferably (Ac3 transformation point - 30)°C or higher, and more preferably (Ac3 transformation point - 10)°C or higher. There is no particular upper limit specified for the heating temperature in the annealing process, but if the heating temperature in the annealing process is 1000°C or lower, it is possible to effectively prevent a decrease in surface hardness after annealing and a decrease in TS. Therefore, the heating temperature in the annealing process is preferably 1000°C or lower, and more preferably 980°C or lower. The heating temperature in the annealing process is measured based on the temperature of the surface of the cold-rolled steel sheet.

[0087] The Ac3 transformation point can be determined using the following equation (6). (Ac3 transformation point)=910-203√(%C)+45×(%Si)-30×(%Mn)-20×(%Cu)-15×(%Ni)+11×(%Cr)+32×(%Mo)+104×(%V)+400×(%Ti)+200×(%Al)...(6) Here, (%C), (%Si), (%Mn), (%Ni), (%Cu), (%Ni)(%Cr), (%Mo), (%V), (%Ti), and (%Al) are the mass percentages of the element in the composition of the base steel sheet, and are set to zero if an element is not present.

[0088] While there are no particular limitations on the heat retention time at the aforementioned heating temperature, it is preferable to set it to 10 seconds or more and 600 seconds or less.

[0089] Next, the cold-rolled steel sheet is cooled. The cooling conditions are not particularly limited, and standard methods can be used. The average cooling rate in the temperature range below the annealing temperature and above 500°C is not particularly limited, but from the viewpoint of controlling the area ratio of martensite and ferrite, it is preferable to set it to 5°C / s or more and 50°C / s or less.

[0090] [Dew point inside the snout: -60°C or higher] Next, the cold-rolled steel sheet is passed through the snout. If the dew point in the snout is below -60°C, decarburization from the surface of the cold-rolled steel sheet is suppressed, and a soft layer is not formed on the surface. As a result, the hardness of the surface does not decrease, the peel strength of the corona bond decreases, and the delayed peel resistance of the weld deteriorates. Therefore, the dew point in the snout should be -60°C or higher, and preferably -50°C or higher. There is no specific upper limit for the dew point in the snout, but if the dew point in the snout is 30°C or lower, it is possible to effectively prevent a decrease in surface hardness after annealing and a decrease in TS. Therefore, the dew point in the snout is preferably 30°C or lower, and more preferably 20°C or lower.

[0091] [Hot-dip galvanizing process] Next, the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet. Annealing, cooling, and galvanizing treatment may be carried out continuously on a single line (CGL (Continuous Galvanizing Line)), and an example thereof is described below. After annealing, the cold-rolled steel sheet is cooled to a temperature range of approximately 500°C. Next, the cold-rolled steel sheet is passed through the steel strip exit side of the cooling zone and moved into the hot-dip galvanizing bath via a snout whose leading end is immersed in the hot-dip galvanizing bath, while being further cooled. The time from the end of cooling of the cold-rolled steel sheet until the cold-rolled steel sheet enters the hot-dip galvanizing bath is not particularly limited, but from the viewpoint of controlling the area ratio of martensite and ferrite, it is preferable to set it to 1 s to 300 s. A roll is provided immediately before the connection between the cooling zone and the snout to change the direction of travel of the cold-rolled steel sheet and allow it to enter the snout, and the cold-rolled steel sheet enters the snout after passing through this roll. Next, the cold-rolled steel sheet, guided through a snout to a molten zinc plating bath, is immersed in the bath and subjected to molten zinc plating to become a plated steel sheet.

[0092] In the hot-dip galvanizing process, cold-rolled steel sheets can be immersed in a hot-dip galvanizing bath at a temperature of 480°C to 500°C. The hot-dip galvanizing bath used in the hot-dip galvanizing process preferably has an Al content of 0.10% to 0.23% by mass, with the remainder being Zn and unavoidable impurities.

[0093] The amount of plating deposited during hot-dip galvanizing is 20-80 g / m² per side. 2 (Double-sided plating) is preferred. The amount of plating can be adjusted by performing gas wiping or the like after the hot-dip galvanizing treatment.

[0094] Alternatively, the cold-rolled steel sheet may be subjected to hot-dip galvanizing before the alloying treatment. It is preferable to perform the alloying treatment on the hot-dip galvanized steel sheet after the hot-dip galvanizing treatment and before the vibration holding process described later. When the alloying treatment temperature is 460°C or higher, the Zn-Fe alloying rate is suitable, and productivity can be favorably obtained. Therefore, the alloying treatment temperature is preferably 460°C or higher, and more preferably 470°C or higher. On the other hand, when the alloying treatment temperature is 600°C or lower, the transformation of untransformed austenite to pearlite is favorably suppressed, and a favorable area ratio of martensite can be obtained. Therefore, the alloying treatment temperature is preferably 600°C or lower, and more preferably 560°C or lower. The alloying treatment temperature is based on the temperature of the surface of the hot-dip galvanized steel sheet.

[0095] [First cooling process] Next, a first cooling step is performed to cool the hot-dip galvanized steel sheet to a cooling stop temperature below the martensitic transformation initiation temperature. If the cooling stop temperature exceeds the martensitic transformation initiation temperature, the amount of martensite that transforms will be small, and the amount of martensite that is tempered during subsequent reheating will be small, resulting in an undesirable amount of tempered martensite. Therefore, the cooling stop temperature in the first cooling step should be below the martensitic transformation initiation temperature, preferably (martensitic transformation initiation temperature - 20)°C or lower. There is no specific lower limit for the cooling stop temperature in the first cooling step, but if the cooling stop temperature is 0°C or higher, it is preferable to prevent austenite from undergoing martensitic transformation and to obtain retained austenite that is effective in controlling YR. Therefore, the cooling stop temperature in the first cooling step is preferably 10°C or higher. The cooling stop temperature is based on the temperature of the surface of the hot-dip galvanized steel sheet.

[0096] The martensitic transformation onset temperature (Ms point) can be determined using the following equation (7). (Martensitic transformation onset temperature) = 550 - 350 × (%C) - 40 × (%Mn) - 10 × (%Cu) - 17 × (%Ni) - 20 × (%Cr) - 10 × (%Mo) - 35 × (%V) - 5 × (%W) + 30 × (%Al) ... (7) Here, (%C), (%Si), (%Mn), (%Ni), (%Cu), (%Cr), (%Mo), (%V), (%Ti), (%W), and (%Al) are the mass percentages of the element in the composition of the base steel sheet, and zero is used if an element is not present.

[0097] [Reheat to a reheating temperature within the range of 450°C or below the cooling stop temperature] Next, the hot-dip galvanized steel sheet is reheated to a reheating temperature above the cooling stop temperature but below 450°C. Reheating to a reheating temperature within the range of above the cooling stop temperature but below 450°C after the first cooling step is an important constituent element of the present invention. If reheating is not performed beyond a temperature range below the cooling stop temperature, the tempering of the quenched martensite will not proceed, resulting in the excessive generation of quenched martensite and the inability to obtain the desired YR and bendability. Furthermore, the diffusion of hydrogen in the steel will not be promoted, and the desired amount of hydrogen in the steel will not be reduced, resulting in the inability to obtain good hole-expanding properties. Therefore, the reheating temperature should be above the cooling stop temperature, preferably above the martensitic transformation start temperature + 20°C. On the other hand, if reheating is performed to a temperature range above 450°C, excessive ferrite formation will occur, and the desired TS, hole-expanding properties, and bendability will not be obtained. Therefore, the reheating temperature should be below 450°C, preferably below 430°C. The reheating temperature is based on the temperature of the surface of the hot-dip galvanized steel sheet.

[0098] [Vibration holding process] Next, a vibration holding process is performed on the hot-dip galvanized steel sheet, in which it is vibrated so that the maximum amplitude is between 10 nm and 500 μm, and held at the reheating temperature for between 2 seconds and 600 seconds. In the vibration holding process, the hot-dip galvanized steel sheet is forcibly subjected to micro-vibrations, thereby repeatedly displacing the sheet in the thickness direction, i.e., bending deformation. As a result, instead of a uniform lattice expansion from the center of the thickness of the hot-dip galvanized steel sheet to the surface, the lattice spacing on the surface of the hot-dip galvanized steel sheet expands compared to the center of the thickness. Carbon in the hot-dip galvanized steel sheet and hydrogen trapped at specific sites diffuse toward the surface of the hot-dip galvanized steel sheet, where the lattice spacing is wide and the potential energy is low, and become easier to detach from the surface. As a result, the amount of carbon in the surface layer of the hot-dip galvanized steel sheet decreases, and the amount of hydrogen in the steel is reduced, improving the weld-delayed peel resistance characteristics.

[0099] [Maximum amplitude of hot-dip galvanized steel sheet: 10 nm to 500 μm] If the maximum amplitude of the hot-dip galvanized steel sheet during the vibration holding process is less than 10 nm, the lattice spacing on the surface of the hot-dip galvanized steel sheet does not expand sufficiently, resulting in insufficient promotion of carbon and hydrogen diffusion, and thus no effect of desorption is obtained. Therefore, the maximum amplitude of the hot-dip galvanized steel sheet during the vibration holding process should be 10 nm or more, preferably 20 nm or more, and more preferably 50 nm or more. On the other hand, if the maximum amplitude of the hot-dip galvanized steel sheet during the vibration holding process exceeds 500 μm, the strain on the surface of the hot-dip galvanized steel sheet increases, causing plastic deformation, which in turn traps hydrogen, and thus no effect of desorption is obtained. From this viewpoint, the maximum amplitude of the hot-dip galvanized steel sheet during the vibration holding process should be 500 μm or less, preferably 400 μm or less, and more preferably 300 μm or less. Note that the hot-dip galvanized steel sheet vibrates naturally during its passage process, or vibrates when it receives gas from, for example, a gas wiping device. However, in these vibrations, the maximum amplitude of the hot-dip galvanized steel sheet exceeds at least 0.5 mm, so the effect of desorbing carbon or hydrogen contained in the hot-dip galvanized steel sheet cannot be obtained.

[0100] Furthermore, the maximum amplitude of the hot-dip galvanized steel sheet during the vibration holding process can be measured using a laser displacement meter or laser Doppler vibrometer positioned at a predetermined distance from the surface. This maximum amplitude can also be controlled by irradiating it with sound waves with controlled sound pressure levels or by applying electromagnetic force using an electromagnet with controlled current.

[0101] [Holding time during vibration holding process: 2 seconds or more and 600 seconds or less] If the holding time in the vibration holding process is less than 2 seconds, the tempering of the quenched martensite will not proceed, resulting in excessive production of quenched martensite and failure to obtain the desired YR and bendability. Therefore, the holding time in the vibration holding process should be 2 seconds or more, preferably 5 seconds or more. On the other hand, if the holding time in the vibration holding process exceeds 600 seconds, excessive ferrite formation will occur, and the desired TS, hole-expanding properties, and bendability will not be obtained. Therefore, the holding time in the vibration holding process should be 600 seconds or less, preferably 550 seconds or less. Note that the holding time refers to the time of isothermal holding at the reheating temperature.

[0102] [Sound pressure level on the surface of hot-dip galvanized steel sheet: 50 dB or higher (preferred conditions)] In the vibration holding process, it is preferable to vibrate the hot-dip galvanized steel sheet by irradiating it with sound waves such that the sound pressure level on the surface of the hot-dip galvanized steel sheet is 50 dB or higher. A sound pressure level of 50 dB or higher ensures that the hot-dip galvanized steel sheet is reliably vibrated, effectively promoting the diffusion of carbon and hydrogen. Therefore, a sound pressure level of 50 dB or higher is preferable, 60 dB or higher is more preferable, and 80 dB or higher is even more preferable. On the other hand, considering the performance of a typical sound wave irradiation device, a sound pressure level of 150 dB or lower is preferable, and 140 dB or lower is more preferable.

[0103] The sound pressure on the surface of the steel sheet can be adjusted by adjusting the intensity of the sound waves generated by the sound wave irradiation device and the position of the sound wave irradiation device (i.e., the distance between the sound wave irradiation device and the hot-dip galvanized steel sheet). The "sound pressure level on the surface of the hot-dip galvanized steel sheet" can be measured inline by installing a sound pressure meter near the surface of the hot-dip galvanized steel sheet during transit and directly below the sound wave irradiation device.

[0104] [Vibration application time: 1 second to 3600 seconds (preferred conditions)] In the vibration holding process, the vibration application time is preferably 1 second or more, more preferably 5 seconds or more, and even more preferably 10 seconds or more, from the viewpoint of more sufficiently reducing carbon and hydrogen from the hot-dip galvanized steel sheet. On the other hand, in the vibration holding process, the vibration application time is preferably 3600 seconds or less, more preferably 1800 seconds or less, and even more preferably 900 seconds or less, from the viewpoint of not hindering productivity. In this invention, the vibration application time to the hot-dip galvanized steel sheet means the time during which vibration is applied to each position on the surface of the hot-dip galvanized steel sheet, and if vibration is applied from multiple vibration application devices, it is the cumulative time of those vibrations.

[0105] [Second cooling process] Next, a second cooling step is performed to cool the hot-dip galvanized steel sheet to room temperature. It is preferable to continue vibrating the hot-dip galvanized steel sheet during the second cooling step as well.

[0106] [In the second cooling step, the cooling rate in the temperature range of 50°C or higher below the martensitic transformation initiation temperature is 15.0°C / s or less (preferred conditions)] In the second cooling step, by setting the cooling rate to 15.0°C / s or less in the temperature range of 50°C or above the martensitic transformation initiation temperature, the transformation to the martensitic phase with low solid solubility of carbon and hydrogen can be suitably promoted. Furthermore, the diffusion of carbon and hydrogen can be suitably promoted while generating vibration, i.e., displacement of the hot-dip galvanized steel sheet in the thickness direction. Therefore, the cooling rate is preferably 15.0°C / s or less, more preferably 12.5°C / s or less, and even more preferably 10.0°C / s or less. On the other hand, there is no particular lower limit for the cooling rate, but due to production technology constraints, the cooling rate is preferably 1.0°C / s or more, and more preferably 2.0°C / s or more. The cooling rate is based on the temperature of the hot-dip galvanized steel sheet surface. In addition, gas jet cooling, mist cooling, water cooling, and air cooling can be applied as cooling methods for the hot-dip galvanized steel sheet in the temperature range of 50°C or above the martensitic transformation initiation temperature.

[0107] For processes and conditions not described in this specification, conventional methods may be used. [Examples]

[0108] A steel slab (steel material) having the component composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter, and steel slabs were obtained by continuous casting. The obtained steel slabs were heated to 1250°C and roughly rolled to obtain sheet bars.

[0109] [Table 1] TIFF0007865464000003.tif233110

[0110] The obtained sheet bar was subjected to finish rolling, wound up, and then cooled to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was pickled and then cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.4 mm. Next, the obtained cold-rolled steel sheet was annealed and hot-dip galvanized to obtain a hot-dip galvanized steel sheet having hot-dip galvanized layers on both sides. In examples other than No. 7 and 8, the obtained hot-dip galvanized steel sheet was cooled while vibration was added by irradiating it with sound waves. In No. 7, the obtained hot-dip galvanized steel sheet was cooled while vibration was added by periodically applying electromagnetic force using an electromagnet. In No. 8, the obtained hot-dip galvanized steel sheet was cooled without adding vibration. Table 2 shows the manufacturing conditions for each example. In the plating type column of Table 2, "GI" is written if only hot-dip galvanizing was performed, and "GA" is written if hot-dip galvanizing and alloying were performed. In addition, vibration was added at the same time as the start of cooling of the hot-dip galvanized steel sheet.

[0111] In the GI example, a molten zinc plating bath containing 0.20% by mass of Al, with the remainder being Zn and unavoidable impurities, was used. In the GA example, a molten zinc plating bath containing 0.14% by mass of Al, with the remainder being Zn and unavoidable impurities, was used. The plating bath temperature was 470°C in both cases. The plating deposition amount in the GI example was 45-72 g / m² per side. 2 (Double-sided plating) is recommended, and in the case of GA, it is 45 g / m² per side. 2 The process was limited to (double-sided plating). In the case of GA, the alloying treatment temperature was set to approximately 550°C.

[0112] In the GI example, the composition of the resulting hot-dip galvanized layer was Fe: 0.1-1.0 mass%, Al: 0.2-1.0 mass%, with the remainder being Zn and unavoidable impurities. In the GA example, the composition of the alloyed hot-dip galvanized layer was Fe: 7-15 mass%, Al: 0.1-1.0 mass%, with the remainder being Zn and unavoidable impurities.

[0113] [Table 2] TIFF0007865464000005.tif233151

[0114] The obtained hot-dip galvanized steel sheets were subjected to microstructure identification at a position 1 / 4 of the thickness of the base steel sheet using the method described above. Furthermore, the carbon content in the surface layer of the base steel sheet, the value obtained by dividing the carbon content by the total carbon content of the base steel sheet, the hydrogen content [H] in the steel, and the average Vickers hardness of the surface layer of the base steel sheet were evaluated. The evaluation results are shown in Table 3. The component composition of the base steel sheet of the obtained hot-dip galvanized steel sheets was substantially the same as the component composition of the steel slab stage. That is, all of the conforming steels were within the range of component composition according to the above embodiment, while all of the comparative steels were outside the range of component composition according to the above embodiment.

[0115] Furthermore, using the methods described above, we evaluated TS, YR, EL, hole expansion properties, bendability, peel strength of corona bond, and delayed peel resistance of the welded joint, and the results are shown in Table 3.

[0116] [Table 3] TIFF0007865464000007.tif233142

[0117] As shown in Table 3, the inventive example had a TS of 1180 MPa or higher and was excellent in formability, corona bond peel strength, and weld-delayed peel resistance. On the other hand, the comparative example was inferior in one or more of the following: TS, bendability, corona bond peel strength, and weld-delayed peel resistance. [Industrial applicability]

[0118] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet and a method for manufacturing the same that have a TS of 1180 MPa or higher, and are excellent in formability, corona bond peel strength, and weld delay peel resistance, and that can be manufactured with high dimensional accuracy. By applying the hot-dip galvanized steel sheet obtained by the present invention to, for example, automotive structural members, it is possible to improve fuel efficiency by reducing the weight of the vehicle body, and thus has great industrial value. [Explanation of symbols]

[0119] 100 tensile test specimens 102 Welded joints 10 L-shaped sample 12. Center of the welding surface 20 samples 22 Spacers 24 Temporary weld 26. Resistance spot weld

Claims

1. It comprises a base steel plate and a hot-dip galvanized layer formed on the surface of the base steel plate, The aforementioned base steel plate is, by mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less The component composition contains, with the remainder being Fe and unavoidable impurities, At the position where the thickness of the base steel plate is 1 / 4, The area ratio of tempered martensite is 60.0% or more. The area ratio of quenched martensite is 10.0% or less. The area ratio of ferrite is 30.0% or less, The volume fraction of retained austenite is 10.0% or less. It has a steel structure, The carbon content in the surface layer of the aforementioned base steel plate is 0.150% or less. The value obtained by dividing the carbon content in the surface layer of the base steel plate by the total carbon content of the base steel plate is less than 0.

85. The hydrogen content in the steel [H] (mass ppm) of hydrogen with a binding energy of 10 kJ / mol or more and 100 kJ / mol or less with respect to the hydrogen trap sites of the base steel plate is 0.30 mass ppm or less, and the average Vickers hardness of the surface layer of the base steel plate is Hv Ｓ A hot-dip galvanized steel sheet characterized by satisfying the following formula (1). [H]≦(480-Hv Ｓ ) / 320 ・・・(1)

2. The aforementioned component composition is further expressed in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.100% or less, W: 0.100% or less, B: 0.0100% or less, Cr: 1.000% or less, Mo: 1.000% or less Ni: 1.000% or less, Co: 0.500% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.1000% or less, Te: 0.1000% or less, Hf: 0.10% or less, Bi: 0.200% or less The hot-dip galvanized steel sheet according to claim 1, comprising at least one element selected from the group consisting of the following.

3. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

4. A step of heating a steel slab having the component composition described in claim 1 or 2, and subjecting the steel slab to hot rolling at a finish rolling exit temperature of 750°C or more and 1000°C or less to obtain a hot-rolled steel sheet, The process involves winding the hot-rolled steel sheet at a temperature of 300°C to 750°C, Next, the hot-rolled steel sheet is subjected to cold rolling at a rolling ratio of 60.0% or less to obtain a cold-rolled steel sheet. Next, the cold-rolled steel sheet (Ac ３ An annealing process in which the temperature is heated to above the transformation point (-50°C), Next, the cold-rolled steel sheet is passed through a snout whose dew point is controlled to be -60°C or higher. Next, the cold-rolled steel sheet is subjected to a hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet. Next, a first cooling step is performed to cool the hot-dip galvanized steel sheet to a cooling stop temperature below the martensitic transformation start temperature, Next, the hot-dip galvanized steel sheet is reheated to a reheating temperature above the cooling stop temperature but below 450°C. Next, the hot-dip galvanized steel sheet is vibrated so that the maximum amplitude is 10 nm or more and 500 μm or less, while being held at the reheating temperature for 2 seconds or more and 600 seconds or less in a vibration holding step, Next, a second cooling step is performed to cool the hot-dip galvanized steel sheet to room temperature, A method for manufacturing a hot-dip galvanized steel sheet, characterized by having and producing the hot-dip galvanized steel sheet described in claim 1 or 2.

5. The method for manufacturing a hot-dip galvanized steel sheet according to claim 4, wherein in the vibration holding step, the hot-dip galvanized steel sheet is vibrated by irradiating the hot-dip galvanized steel sheet with sound waves such that the sound pressure level on the surface of the hot-dip galvanized steel sheet is 50 dB or more.

6. The method for manufacturing a hot-dip galvanized steel sheet according to claim 4, wherein in the second cooling step, the cooling rate in the temperature range of 50°C or higher below the martensitic transformation onset temperature is 15.0°C / s or less.

7. The method for manufacturing a hot-dip galvanized steel sheet according to claim 5, wherein in the second cooling step, the cooling rate in the temperature range of 50°C or higher below the martensitic transformation onset temperature is 15.0°C / s or less.

8. The method for manufacturing a hot-dip galvanized steel sheet according to claim 4, wherein the vibration of the hot-dip galvanized steel sheet is continued in the second cooling step.

9. The method for manufacturing a hot-dip galvanized steel sheet according to claim 5, wherein the vibration of the hot-dip galvanized steel sheet is continued in the second cooling step.

10. The method for manufacturing a hot-dip galvanized steel sheet according to claim 6, wherein the vibration of the hot-dip galvanized steel sheet is continued in the second cooling step.

11. The method for manufacturing a hot-dip galvanized steel sheet according to claim 7, wherein the vibration of the hot-dip galvanized steel sheet is continued in the second cooling step.

12. A method for manufacturing a hot-dip galvanized steel sheet according to claim 4, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step.

13. The method for manufacturing a hot-dip galvanized steel sheet according to claim 5, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step.

14. The method for manufacturing a hot-dip galvanized steel sheet according to claim 6, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step.

15. The method for manufacturing a hot-dip galvanized steel sheet according to claim 7, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step.

16. A method for manufacturing a hot-dip galvanized steel sheet according to claim 8, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step.

17. A method for manufacturing a hot-dip galvanized steel sheet according to claim 9, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step.

18. A method for manufacturing a hot-dip galvanized steel sheet according to claim 10, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step.

19. A method for manufacturing a hot-dip galvanized steel sheet according to claim 11, wherein the hot-dip galvanized steel sheet is subjected to an alloying treatment after the hot-dip galvanizing treatment and before the first cooling step.

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