Hot-dip galvanized steel sheet and method for manufacturing 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

Hot-dip galvanized steel sheet and its manufacturing method

[0001] The present invention relates to a hot-dip galvanized steel sheet and a method for producing the same.

[0002] CO2 emissions from lighter vehicles ２

[0003] With the aim of achieving both reduced emissions and improved crashworthiness by reducing the vehicle body weight, efforts are being made to increase the strength of automotive steel sheets. Furthermore, new legal regulations are being introduced one after another. Therefore, in order to increase the vehicle body strength, there has been an increase in the number of applications of high-strength steel sheets, particularly high-strength steel sheets with a tensile strength (hereinafter simply referred to as TS) of 1180 MPa or more, for the main structural or reinforcing parts that form the framework of the automobile cabin (hereinafter also referred to as automotive framework structural parts, etc.).

[0003] High-strength steel sheets used in automotive structural components, etc., are required to have excellent formability. Furthermore, the formed parts are required to have excellent dimensional accuracy. For example, parts such as crash boxes have punched edges and bent sections, so from the standpoint of formability, steel sheets with high stretch flangeability and bendability are suitable. Furthermore, from the standpoint of part performance, increasing the yield ratio (YR = yield strength YS / tensile strength TS) of the steel sheet can increase the impact absorption energy during a collision. Furthermore, from the standpoint of part dimensional accuracy, controlling the YR within a certain range can suppress springback after steel sheet forming and control the dimensional accuracy of the part. To increase the application rate of high-strength steel sheets to automotive parts, it is necessary to comprehensively satisfy these properties.

[0004] Therefore, from the viewpoint of formability, it is preferable to apply a steel sheet having high hole expandability and bendability in addition to ductility to such parts. In addition, from the viewpoint of corrosion prevention performance of the vehicle body, a hot-dip galvanized steel sheet that has been subjected to hot-dip galvanization may be applied to a steel sheet that is used as a material for an automobile frame structural part or the like.

[0005] Furthermore, in recent years, when spot welding high-strength galvanized steel sheets, the corona bond, which is the pressure joint between the steel sheets, has been found to have a reduced peel strength due to the increased strength of the steel sheets, i.e., the increased hardness of the steel sheet surface. Furthermore, stress concentration due to corona bond peeling and crack propagation to the nugget (delayed peeling at the weld) caused by hydrogen penetration in the steel sheet under the operating environment of the steel sheet have also become a problem. Therefore, when applying high-strength steel sheets to structural components, there is a demand for high-strength steel sheets that have excellent corona bond peel strength and resistance to delayed peeling at the weld.

[0006] As a technology relating to such hot-dip galvanized steel sheets, high-strength steel sheets having improved bendability by forming 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 soft surface layer having a thickness of more than 10 μm, and containing carbides at a predetermined number density in the soft surface layer. Furthermore, Patent Document 2 discloses a hot-dip galvanized steel sheet having a tensile strength of 1180 MPa or more, and having improved bendability by controlling the thickness of the soft surface layer and reducing the amount of low-temperature diffusible hydrogen in the steel.

[0007] International Publication No. 2021 / 186510 International Publication No. 2022 / 270053

[0008] However, in Patent Document 1, it is necessary to control the amount of hydrogen in the steel from the viewpoint of hydrogen embrittlement resistance, and the compatibility with the properties of the welded portion is not considered, so there is room for improvement. In Patent Document 2, the properties of the welded portion are not considered, so there is room for improvement.

[0009] In view of the above problems, the present invention aims to provide a hot-dip galvanized steel sheet having a TS of 1180 MPa or more, excellent formability, corona bond peel strength, and weld delayed peel resistance, and capable of producing parts with high dimensional accuracy, and a method for producing the same. In the present invention, formability refers to ductility, hole expandability, and bendability. In the present invention, being able to produce parts with high dimensional accuracy (high dimensional accuracy during forming) means having a yield ratio (YR) of 65.0% or more and 95.0% or less.

[0010] The present inventors conducted extensive research to solve the above problems and have made the following discoveries. (1) The substrate steel sheet has a predetermined chemical composition, a certain amount of tempered martensite, and optionally, ferrite and retained austenite. This allows for a YR of 65.0% or more and 95.0% or less, and achieves both high TS and excellent bendability. (2) By ensuring that the carbon content in the surface layer of the substrate steel sheet is 0.150% or less and that the value obtained by dividing the carbon content in the surface layer of the steel sheet by the carbon content of the entire steel sheet is less than 0.85, a soft surface layer is formed, resulting in excellent peel strength in corona bonding. (3) By reducing the amount of hydrogen trapped in specific hydrogen trapping sites in the substrate steel sheet to an amount corresponding to the hardness of the surface layer of the steel sheet, crack propagation to the nugget can be suppressed. This allows for excellent resistance to delayed peeling of the weld.

[0011] That is, the gist and configuration of the present invention are as follows.

[0012] [1] A steel sheet having a substrate steel sheet and a hot-dip galvanized layer formed on a surface of the substrate steel sheet, wherein the substrate steel sheet has a chemical composition containing, in 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, with the balance being Fe and unavoidable impurities, and wherein at a 1 / 4 position of the thickness of the substrate steel sheet, an area fraction of tempered martensite is 60.0% or more, an area fraction of quenched martensite is 10.0% or less, an area fraction of ferrite is 30.0% or less, and a volume fraction of retained austenite is 10.0% or less. and a steel structure comprising: a surface layer of the substrate steel sheet having a C content of 0.150% or less; a value obtained by dividing the C content of the surface layer of the substrate steel sheet by the total C content of the substrate steel sheet is less than 0.85; an amount of hydrogen in steel [H] (ppm by mass) having a binding energy of 10 kJ / mol or more and 100 kJ / mol or less with hydrogen trapping sites of the substrate steel sheet is 0.30 ppm by mass or less; and an average Vickers hardness of the surface layer of the substrate steel sheet is Hv Ｓ The hot-dip galvanized steel sheet is characterized in that it satisfies the following formula (1): [H]≦(480−Hv Ｓ ) / 320 ... (1)

[0013] [2] The composition further includes, 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, The hot-dip galvanized steel sheet according to the above [1], containing at least one element selected from the group consisting of Hf: 0.10% or less, and Bi: 0.200% or less.

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

[0015] [4] A process of heating a steel slab having the component composition described in [1] or [2] above, and hot-rolling the steel slab at a finish rolling delivery temperature of 750°C or more and 1000°C or less to obtain a hot-rolled steel sheet; A process of coiling the hot-rolled steel sheet at 300°C or more and 750°C or less; Then, a process of cold-rolling the hot-rolled steel sheet at a rolling reduction rate of 60.0% or less to obtain a cold-rolled steel sheet; And then, a process of rolling the cold-rolled steel sheet into a rolled steel sheet (Ac ３a step of passing the cold-rolled steel sheet through a snout in which the dew point is controlled to be -60°C or higher; a step of hot-dip galvanizing the cold-rolled steel sheet to obtain a hot-dip galvanized steel sheet; a first cooling step of cooling the hot-dip galvanized steel sheet to a cooling stop temperature that is equal to or lower than a martensitic transformation start temperature; a step of reheating the hot-dip galvanized steel sheet to a reheating temperature that is higher than the cooling stop temperature and equal to or lower than 450°C; a vibration holding step of holding the hot-dip galvanized steel sheet at the reheating temperature for 2 seconds to 600 seconds while vibrating the hot-dip galvanized steel sheet so that the maximum amplitude is 10 nm to 500 μm; and a second cooling step of cooling the hot-dip galvanized steel sheet to room temperature.

[0016] [5] The method for producing a hot-dip galvanized steel sheet according to the above [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 so that a sound pressure level on the surface of the hot-dip galvanized steel sheet satisfies 50 dB or more.

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

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

[0019] [8] The method for producing a hot-dip galvanized steel sheet according to any one of [4] to [7] above, 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.

[0020] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet having a TS of 1180 MPa or more, excellent formability, corona bond peel strength, and resistance to delayed peeling of welded joints, and capable of manufacturing parts with high dimensional accuracy, and a method for manufacturing the same.

[0021] 1A and 1B are schematic diagrams of a tensile test piece used in evaluating the peel strength of a corona bond in an example of the present invention, and (a) a top view and (b) a side view of a welded joint used in evaluating the resistance to delayed peeling of a welded joint in an example of the present invention.

[0022] Hereinafter, embodiments of the hot-dip galvanized steel sheet and the manufacturing method thereof according to the present invention will be described. Note that the embodiment described below is an example of the present invention, and the configuration of the present invention is not limited to this specific example.

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

[0024] [C: 0.030% or more and 0.250% or less] C is one of the important basic components of steel. In the present invention, C is an important element that affects the area fraction of tempered martensite, the area fraction of quenched martensite, the area fraction of ferrite, and the volume fraction of retained austenite. If the C content is less than 0.030%, the area fractions of tempered martensite and quenched martensite decrease, and the area fraction of ferrite increases, making it difficult to achieve the desired TS and YR. Therefore, the C content is set to 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 embrittled, and the desired ductility cannot be obtained. Therefore, the C content is set to 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. In particular, in the present invention, Si suppresses carbide formation during annealing and promotes the formation of retained austenite, thereby affecting the volume fraction of retained austenite. Furthermore, by suppressing carbide formation, Si reduces the number of void initiation points during bending, making it an effective element for improving bendability. To achieve these effects, the Si content is set to 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 increase in the volume fraction of retained austenite increases the amount of hydrogen in the steel of the base steel sheet, making it difficult to achieve excellent hole expandability and weld delayed disbonding resistance. Therefore, the Si content is set to 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. In particular, in the present invention, Mn is an important element that affects the area fraction of tempered martensite, the area fraction of ferrite, and the volume fraction 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 fraction of tempered martensite. To achieve these effects, the Mn content is set to 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, increasing the amount of hydrogen in the steel and also resulting in excessive hardenability and increased hardness of the steel sheet surface. As a result, excellent resistance to delayed disbonding of welded joints cannot be obtained. Therefore, the Mn content is set to 5.00% or less, preferably 4.00% or less, and more preferably 3.50% or less.

[0027] [P: 0.100% or less] Excessive P segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, resulting in reduced hole expandability and bendability. Therefore, the P content is set to 0.100% or less, and preferably 0.070% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, the P content is preferably set to 0.001% or more.

[0028] [S: 0.0200% or less] S exists as sulfide and reduces the ultimate deformability of steel, so if S is contained in excess, hole expandability and bendability will decrease. Therefore, the S content is 0.0200% or less, and preferably 0.0050% or less. Note that there is no particular lower limit for the S content, but due to production technology constraints, the S content is preferably 0.0001% or more.

[0029] [Al: 0.100% or less] When Al is excessive, A ３ The transformation point rises and a large amount of ferrite is contained in the microstructure, making it difficult to achieve the desired TS. Therefore, the Al content is set to 0.100% or less, and preferably 0.050% or less. Although there is no particular lower limit for the Al content, since Al is an element that suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite, the Al content is preferably set to 0.001% or more.

[0030] [N: 0.0100% or less] N exists as a nitride and reduces the ultimate deformability of the steel sheet. Therefore, if N is contained in excess, the hole expandability and bendability will decrease. Therefore, the N content is set to 0.0100% or less, and preferably 0.0050% or less. Note that there is no particular lower limit for the N content, but due to constraints on production technology, the N content is preferably set to 0.0005% or more.

[0031] [O: 0.0100% or less] O exists as an oxide and reduces the ultimate deformability of the steel sheet, so if O is contained in excess, the hole expandability and bendability will decrease. Therefore, the O content is set to 0.0100% or less, and preferably 0.0050% or less. Note that there is no particular lower limit for the O content, but due to constraints on production technology, the O content is preferably set to 0.0001% or more.

[0032] [Remaining Elements] The base steel sheet has a composition containing the above elements, with the balance including Fe and unavoidable impurities. Preferably, the base steel sheet has a composition containing the above elements, with the balance consisting of Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, Sr, Ge, Cs, and As. These impurities are permitted to be contained in a total content of 0.100% or less.

[0033] The chemical composition of the high-strength steel plate of the present disclosure, in addition to the above-mentioned essential components, further includes, 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, etc., which reduces the ultimate deformability of the steel sheet. Therefore, if the Ti content exceeds 0.200%, the hole expandability and bendability are reduced. Therefore, when Ti is contained, the Ti content is 0.200% or less, and preferably 0.100% or less. Note that there is no particular lower limit for the Ti content, but by setting the Ti content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, which can increase the strength of the steel sheet and favorably obtain TS. Therefore, the Ti content is preferably 0.001% or more.

[0035] [Nb: 0.200% or less] Nb generates large amounts of coarse precipitates or inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the Nb content exceeds 0.200%, the hole expandability and bendability are reduced. Therefore, when Nb is contained, the Nb content is 0.200% or less, and preferably 0.100% or less. Although there is no particular lower limit for the Nb content, by setting the Nb content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet and favorably achieving TS. Therefore, the Nb content is preferably 0.001% or more.

[0036] [V: 0.200% or less] V generates large amounts of coarse precipitates or inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the V content exceeds 0.200%, the hole expandability and bendability are reduced. Therefore, when V is contained, the V content is 0.200% or less, and preferably 0.100% or less. Note that there is no particular lower limit for the V content, but by setting the V content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet and favorably obtaining TS. Therefore, the V content is preferably 0.001% or more.

[0037] [Ta: 0.100% or less] [W: 0.100% or less] When the Ta or W content exceeds 0.100%, a large amount of coarse precipitates or inclusions is generated, reducing the ultimate deformability of the steel sheet, resulting in reduced hole expandability and bendability. Therefore, when Ta or W is contained, the respective contents are set to 0.100% or less, preferably 0.080% or less. Note that there is no particular lower limit for the Ta or W content, but when the Ta or W content 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 suitably obtained. Therefore, the Ta or W content is preferably set to 0.010% or more.

[0038] [B: 0.0100% or less] When the B content is 0.0100% or less, cracking inside the steel sheet is suitably suppressed during casting or hot rolling, and the ultimate deformability of the steel sheet is not reduced, thereby achieving suitable bendability. Therefore, when B is contained, the B content is 0.0100% or less, and preferably 0.0080% or less. Note that there is no particular lower limit for the B content, but since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is preferably 0.0003% or more.

[0039] [Cr: 1.000% or less] [Mo: 1.000% or less] [Ni: 1.000% or less] When the content of Cr, Mo, or Ni exceeds 1.000%, coarse precipitates or inclusions increase, reducing the ultimate deformability of the steel sheet, thereby reducing the hole expandability and bendability. Therefore, when Cr, Mo, or Ni is contained, the content of each is 1.000% or less, preferably 0.800% or less. Note that although the lower limit of the content of Cr, Mo, or Ni is not particularly specified, since these elements are elements that improve hardenability, the content of Cr, Mo, or Ni is preferably 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, resulting in reduced hole expandability and bendability. Therefore, when Co is contained, the Co content is 0.500% or less, preferably 0.050% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, the Co content is preferably 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, resulting in reduced hole expandability and bendability. Therefore, when Cu is contained, the Cu content is 1.00% or less, preferably 0.80% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is preferably 0.01% or more.

[0042] [Sn: 0.200% or less] If the Sn content exceeds 0.200%, cracks are generated inside the steel sheet during casting or hot rolling, reducing the ultimate deformability of the steel sheet, resulting in reduced hole expandability and bendability. Therefore, when Sn is contained, the Sn content is 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less. Although the lower limit of the Sn content is not particularly specified, since Sn is an element that improves hardenability, the Sn content is preferably 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, resulting in reduced hole expandability and bendability. Therefore, when Sb is contained, the Sb content is set to 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less. Although the lower limit of the Sb content is not particularly specified, since Sb is an element that controls the surface softening thickness and enables strength adjustment, the Sb content is preferably set to 0.001% or more.

[0044] [Ca: 0.0100% or less] [Mg: 0.0100% or less] [REM: 0.0100% or less] When the content of Ca, Mg, or REM exceeds 0.0100%, coarse precipitates or inclusions increase, reducing the ultimate deformability of the steel sheet, resulting in reduced hole expandability and bendability. Therefore, when Ca, Mg, or REM is contained, the content of Ca, Mg, or REM is each 0.0100% or less, preferably 0.0050% or less. Note that although there is no particular 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 the content of Ca, Mg, or REM is preferably 0.0005% or more.

[0045] [Zr: 0.1000% or less] [Te: 0.1000% or less] When the Zr or Te content exceeds 0.1000%, coarse precipitates or inclusions increase, reducing the ultimate deformability of the steel sheet, resulting in reduced hole expandability and bendability. Therefore, when Zr or Te is contained, the Zr or Te content is set to 0.1000% or less, and preferably 0.0800% or less. Although the lower limit of the Zr or Te content is not particularly specified, since these elements spheroidize the shape of nitrides or sulfides and improve the ultimate deformability of the steel sheet, it is preferable that the Zr or Te content 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, resulting in reduced hole expandability and bendability. Therefore, when Hf is contained, the Hf content is set to 0.10% or less, preferably 0.08% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides or sulfides and improves the ultimate deformability of the steel sheet, the Hf content is preferably set to 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, resulting in reduced hole expandability and bendability. Therefore, when Bi is contained, the Bi content is set to 0.200% or less, preferably 0.100% or less. Although the lower limit of the Bi content is not particularly specified, since Bi is an element that reduces segregation, the Bi content is preferably set to 0.001% or more.

[0048] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferred lower limit, the effect of the present invention is not impaired, and therefore, these elements may be contained as inevitable impurities.

[0049] Next, the steel structure of the substrate steel sheet will be described. Note that the steel structure of the substrate steel sheet described below is that at a 1 / 4 position in the plate thickness direction of the substrate steel sheet (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the substrate steel sheet).

[0050] [Area Fraction of Tempered Martensite: 60.0% or More] When the area fraction of tempered martensite in the steel structure of the base steel sheet is 60.0% or more, the desired YR and high bendability can be achieved. Therefore, the area fraction of tempered martensite is 60.0% or more, preferably 65.0% or more, more preferably 70.0% or more, and even more preferably 75.0% or more. Note that the upper limit of the area fraction of tempered martensite is not particularly limited. However, in addition to obtaining high TS and high bendability, if the area fraction of tempered martensite exceeds 99.0%, ferrite and retained austenite, which are effective for controlling YR, will not be present in the steel structure. Therefore, the area fraction of tempered martensite is preferably 99.0% or less, more preferably 98.0% or less, and even more preferably 97.0% or less.

[0051] [Area Fraction of Quenched Martensite: 10.0% or Less] If the area fraction of quenched martensite in the steel structure of the base steel sheet exceeds 10.0%, the YS decreases, making it difficult to achieve the desired YR. Therefore, the area fraction of quenched martensite is set to 10.0% or less, preferably 8.0% or less, and more preferably 5.0% or less. Note that there is no particular upper limit for the area fraction of quenched martensite, but from the viewpoint of obtaining a high TS, the area fraction of quenched martensite is preferably 0.1% or more.

[0052] [Area fraction of ferrite: 30.0% or less] When the area fraction of ferrite in the steel structure of the base steel sheet is 30.0% or less, the desired TS, excellent hole expandability, and bendability can be achieved. Therefore, the area fraction of ferrite is 30.0% or less, and preferably 25.0% or less. The lower limit of the area fraction of ferrite is not particularly limited, and the area fraction of ferrite may be 0.0%. In the present invention, ferrite also includes bainitic ferrite.

[0053] The area ratios of tempered martensite, quenched martensite, and ferrite can be determined as follows. A sample is cut out so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate serves as the observation surface. The observation surface of the cut sample is mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Using an SEM (Scanning Electron Microscope), the observation position is set to 1 / 4 of the plate thickness of the steel plate, and 10 fields of view are observed at an acceleration voltage of 15 kV and a magnification of 2000 times to obtain a structure image. Using the obtained structure image, the area ratio of each structure (ferrite, tempered martensite, quenched martensite) is calculated for 10 fields of view using Image-Pro from Media Cybernetics. The average of the obtained values ​​is taken as the area ratio of tempered martensite, quenched martensite, or ferrite, respectively. In the above structural images, ferrite (including bainitic ferrite) is a recessed structure that is flat and does not contain carbides, quenched martensite is a protruding structure with fine irregularities inside, and tempered martensite is a recessed structure that contains fine carbides.

[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 expandability and resistance to delayed disbonding of welded joints can be obtained. Therefore, the volume fraction of retained austenite is set to 10.0% or less, preferably 9.0% or less, more preferably 8.0% or less, and even more preferably 7.0% or less. Note that there is no particular restriction on the lower limit of the volume fraction of retained austenite, and desired properties can be obtained even if the volume fraction of retained austenite is 0.0%. Note that, in the present invention, the volume fraction of retained austenite can be considered to be the same as 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 at a position 1 / 4 of the sheet thickness from the surface layer of the substrate steel sheet, the sample is further polished by 0.1 mm by chemical polishing. Using an X-ray diffractometer, the integrated reflection intensities of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron are measured for the observation surface using a Co Kα ray source. The volume fraction of austenite can be determined from the intensity ratio of the integrated reflection intensity from each plane of the fcc iron to the integrated reflection intensity from each plane of the bcc iron.

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

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

[0058] [C Content in Surface Layer of Substrate Steel Sheet: 0.150% or Less] Forming a soft layer in the surface layer of the substrate steel sheet and reducing the surface layer hardness improves the toughness of the corona bond and increases the peel strength of the corona bond. The surface layer hardness can be reduced by reducing the C content in the surface layer, and the lower the C content in the surface layer of the substrate steel sheet, the better. Therefore, the C content in the surface layer of the substrate steel sheet is set to 0.150% or less, and preferably 0.130% or less. Note that there is no particular restriction on the lower limit of the C content in the surface layer of the substrate steel sheet, and since the problem of the present invention can be solved even if the surface layer of the substrate steel sheet does not contain C, the C content may be 0.000%.

[0059] [Value obtained by dividing the C content in the surface layer of the substrate steel sheet by the C content in the entire substrate steel sheet: less than 0.85] By forming a large gradient between the C content in the surface layer of the substrate steel sheet and the C content inside the substrate steel sheet, stress concentration on the steel sheet surface layer when the corona bond is peeled can be alleviated, and crack propagation can be suppressed. Therefore, the value obtained by dividing the C content in the surface layer of the substrate steel sheet by the C content in the entire substrate steel sheet is set to less than 0.85. Note that there is no particular restriction on the lower limit of the value obtained by dividing the C content in the surface layer of the substrate steel sheet by the C content in the entire substrate steel sheet, and this value may be 0.00.

[0060] The C content in the surface layer of the substrate steel sheet can be determined as follows. A 20 mm square sample is collected from the substrate steel sheet to be measured. The sample is measured using discharge optical emission spectroscopy (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 derived from the plating is detected approximately 5 μm from the surface layer. The C content at a position 10 μm further in the thickness direction of the substrate steel sheet from the depth position where the Zn concentration is 0.0 mass% is defined as the C content in the surface layer of the substrate steel sheet. The C content of the entire substrate steel sheet is the C content in the chemical composition of the substrate steel sheet described above, and is equivalent to the C content of the steel slab used.

[0061] [Hydrogen content [H] in steel where hydrogen has a bond energy of 10 kJ / mol to 100 kJ / mol inclusive with hydrogen trapping sites in the base steel sheet: 0.30 mass ppm or less] By setting the hydrogen content [H] in steel where hydrogen has a bond energy of 10 kJ / mol to 100 kJ / mol inclusive with hydrogen trapping sites in the base steel sheet to 0.30 mass ppm or less, good hole expandability can be ensured. Therefore, the hydrogen content [H] in steel is set to 0.30 mass ppm or less, and preferably 0.20 mass ppm or less. Note that the lower limit of the hydrogen content [H] in steel is not particularly limited, and since the problem of the present application can be solved even if hydrogen is not contained in the steel, the hydrogen content [H] in steel may be 0.00 mass ppm.

[0062] Furthermore, when the bond energy with the hydrogen trapping site of the base steel sheet is low, hydrogen is easily desorbed by diffusion and does not affect the delayed disbonding resistance of the weld, so the bond energy is preferably 15 kJ / mol or more.On the other hand, hydrogen with high bond energy is less likely to desorb from the trapping site and similarly does not affect the delayed disbonding resistance of the weld, so the bond energy is preferably 80 kJ / mol or less.

[0063] [Hydrogen content in steel [H] of hydrogen having a binding energy of 10 kJ / mol or more and 100 kJ / mol or less with respect to the hydrogen trapping site of the base steel sheet: (480-Hv Ｓ ) / 320 or less] The hydrogen content [H] (mass ppm) in the steel of hydrogen having a binding energy of 10 kJ / mol or more and 100 kJ / mol or less with the hydrogen trapping site of the substrate steel sheet is such that the average Vickers hardness of the surface layer of the substrate steel sheet is Hv Ｓ In the present invention, it is an important component that the following formula (1) is satisfied. Delayed peeling in welds is a phenomenon that occurs due to hydrogen in steel trapped at specific hydrogen trapping sites. Therefore, in order to obtain excellent resistance to delayed peeling in welds, it is necessary to reduce the amount of hydrogen in steel [H], where the binding energy with hydrogen trapping sites is 10 kJ / mol or more and 100 kJ / mol or less. On the other hand, a decrease in the surface hardness of the substrate steel sheet reduces hydrogen cracking susceptibility. Moreover, excessive hardening of the surface layer of the substrate steel sheet increases hydrogen cracking susceptibility. Therefore, excellent resistance to delayed peeling in welds can be obtained by reducing the amount of hydrogen in steel according to the surface hardness of the substrate steel sheet. Furthermore, the upper limit of surface hardness varies depending on the tensile strength (TS). For steel sheets with a TS of 1180 MPa or more, which are the subject of the present invention, the amount of hydrogen in steel [H] must be controlled to satisfy the following formula (1): [H]≦(480−Hv Ｓ ) / 320 ... (1)

[0064] Average Vickers hardness Hv of the surface layer of the base steel sheet Ｓ If the strength of the surface layer of the steel sheet is excessively reduced, the strength of the entire steel sheet will be reduced, so it is preferable that the average Vickers hardness Hv of the surface layer of the base steel sheet is 100 or more. ＳIf the strength of the surface layer of the steel sheet increases excessively, the bendability deteriorates, so it is preferably 550 or less.

[0065] The binding energy of hydrogen to hydrogen trapping sites can be determined as follows. A large plate sample, 500 mm long, is taken from a steel strip produced on a hot-dip galvanizing line. Multiple test pieces, 30 mm long and 5 mm wide, are taken from the center of the large plate sample by shearing. The obtained test pieces are immediately immersed in liquid nitrogen after taking. Next, the hot-dip galvanized layer of the test piece is alkali-removed while controlling the temperature of the treatment solution so that the surface temperature of the test piece is below room temperature. Next, the amount of hydrogen released from the test piece when heated is measured by thermal desorption analysis. Specifically, the test piece is heated from room temperature to a target temperature of 800°C at heating rates of 50, 100, 150, and 200°C / hr, and then cooled to room temperature. The activation energy Ea for hydrogen desorption from the hydrogen trapping sites is calculated from the following equation (3) using the peak temperature T at which the maximum amount of released hydrogen is obtained from the hydrogen desorption curve obtained at each heating rate. 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 1 / T and 1 / T is plotted, and a linear approximation is performed to determine the slope (Ea / R) 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 with the hydrogen trap site. Therefore, the binding energy of hydrogen with the hydrogen trap site can be calculated by subtracting the activation energy of hydrogen diffusion in the lattice, 7.6 kJ / mol, from Ea.

[0066] The amount of hydrogen in steel [H], which is hydrogen having a binding energy of 10 kJ / mol or more and 100 kJ / mol or less with respect to hydrogen trap sites, can be determined as follows: A sample is continuously heated from room temperature to 300°C at a temperature increase rate of 200°C / h, and then cooled to room temperature. During heating, the cumulative amount of hydrogen released from the test piece during the temperature change from room temperature to 210°C is measured and used as the amount of hydrogen in steel [H].

[0067] Average Vickers hardness Hv of the surface layer of the base steel sheet Ｓ The hardness of the surface layer of the substrate steel sheet (the interface between the coating layer and the substrate steel sheet) was measured at 20 points 5 μm in the thickness direction, with a load of 10 g and a load time of 10 seconds. The average of the measurement results was taken as the average Vickers hardness Hv of the surface layer of the substrate steel sheet. Ｓ It was decided.

[0068] (Hot-dip galvanized layer) Next, the hot-dip galvanized layer of the hot-dip galvanized steel sheet 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 a hot-dip galvanized layer to an alloying treatment). The hot-dip galvanized layer is provided on both surfaces of the surface of the base steel sheet.

[0069] The composition of the hot-dip galvanized layer is not particularly limited and may be any common composition. For example, the hot-dip galvanized layer may contain 20% by mass or less of Fe, 0.001% by mass or more and 1.0% by mass or less of Al, and a total of 0.0% by mass or more and 3.5% by mass or less of 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, with the balance consisting of Zn and unavoidable impurities. If alloying treatment is not performed, the Fe content of the hot-dip galvanized layer may be less than 7% by mass. If 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, more preferably 8% by mass or more and 13% by mass or less.

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

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

[0072] (Mechanical Properties of Hot-Dip Galvanized Steel Sheet) Next, the mechanical properties of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0073] [Tensile strength (TS): 1180 MPa or more] [Yield ratio YR: 65.0% or more and 95.0% or less] [EL: 7.0% or more] The hot-dip galvanized steel sheet has a TS of 1180 MPa or more, a YR of 65.0% or more and 95.0% or less, and an EL of 7.0% or more. TS, YR, and EL can be determined as follows. A JIS No. 5 test piece 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, a crosshead speed of 1.67 × 10 －１ A tensile test is carried out under the condition of 1 / 3000 mm / s to measure TS, YS, and EL. From the measured TS and YS, YR is calculated using the following formula (4): YR = YS / TS × 100 (4)

[0074] [Hole Expandability] Hot-dip galvanized steel sheets have excellent hole expandability. In the present invention, excellent hole expandability means that the hole expandability ratio λ is 30% or more. The hole expandability ratio λ can be determined as follows. A 100 mm x 100 mm 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 hole with a diameter of 10 mm 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 75 mm and a blank holding force of 9 tons to measure the hole diameter at the crack initiation limit. From the measurement results, D ｆ is the hole diameter (mm) when the crack occurs, D ０ is the initial hole diameter (mm), and the hole expansion ratio λ (%) is calculated using the following formula (5): λ (%) = {(D ｆ -D ０ ) / D ０} × 100 ... (5)

[0075] [Bendability] Hot-dip galvanized steel sheets have excellent bendability. Bendability can be evaluated as follows. A strip-shaped test piece, 30 mm wide and 100 mm long, is taken from a hot-dip galvanized steel sheet 24 hours after production, so that the axial direction of the bending test is parallel to the rolling direction. A 90° V-bend test is performed in accordance with JIS Z 2248:2022 under conditions of an indentation load of 100 kN and a holding time of 5 seconds. Thereafter, the presence or absence of cracks is determined on the outside of the bent portion using a stereomicroscope, and the smallest bending radius at which no cracks occur is defined as the limit bending radius R. A steel sheet is evaluated as having excellent bendability when it satisfies the condition of limit bending R / t≦4.00 (t: thickness of the hot-dip galvanized steel sheet) in a 90° V-bend.

[0076] [Corona Bond Peel Strength] Hot-dip galvanized steel sheets have excellent corona bond peel strength. Corona bond peel strength can be evaluated as follows. Two 100 mm × 30 mm flat samples are taken from the hot-dip galvanized steel sheet. The flat samples are bent 90° with a curvature of 5 mm at a position 30 mm from the end of the sample, with the bending axis parallel to the short side, to prepare an L-shaped sample. In the L-shaped sample, a square surface approximately 30 mm square serves as the welding surface, and a surface approximately 65 mm × 30 mm serves as the gripping portion for the tensile test. As shown in Figure 1, two L-shaped samples 10 are arranged so that their welding surfaces are in contact, and are welded at the center 12 of the welding surface so that the welded portion has a diameter of 3 mm to 5 mm on each L-shaped sample 10, to prepare a tensile test piece 100. After welding, a tensile test is performed before hydrogen present in the welded portion of the tensile test piece 100 is released, i.e., within 5 minutes after welding. The conditions for the tensile test are the same as those for the TS evaluation. From the load-displacement graph obtained from the tensile test, the maximum load immediately before a sudden load drop occurs is determined and used as the corona bond peel strength. Here, a corona bond peel strength of 50 N or more is evaluated as excellent. Furthermore, the corona bond peel strength of hot-dip galvanized steel sheet is preferably 55 N or more.

[0077] [Weld Delayed Peeling Resistance] Hot-dip galvanized steel sheets have excellent weld delayed peeling resistance. The weld delayed peeling resistance can be evaluated as follows. Two 100 mm × 30 mm samples are taken from the hot-dip galvanized steel sheet. As shown in FIG. 2( b), a welded joint 102 is formed by stacking a sample 20, a spacer 22, and another sample 20 in this order. Two 1.5 mm thick, 30 mm square mild steel sheets are used as the spacers 22. The spacers 22 are sandwiched at both ends of the samples 20, and tack welding is performed. Tack welding is performed at two tack welds 24 shown in FIG. 2( a). Resistance spot welding is performed on the center of the tack-welded welded joint 102, forming a resistance spot weld 26. The welded joint 102 after resistance spot welding is left to stand in the air at room temperature (20°C) for at least 24 hours. Resistance spot welding is preferably performed at room temperature, with the welding electrodes (lower and upper electrodes) always water-cooled. The lower and upper electrodes each have a tip diameter of 6 mm and a curvature radius of 40 mm, and can be DR-type electrodes made of chromium copper. The lower and upper electrodes are driven by a servo motor to control the welding pressure, and single-phase AC with a frequency of 50 Hz is supplied during current application.

[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 an SEM. If cracks occur in the nugget at the joint interface, it is determined that delamination has occurred in the weld. 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 occur in the nugget, the weld is evaluated as having excellent delayed delamination resistance.

[0079] (Method for manufacturing hot-dip galvanized steel sheet) Next, a method for manufacturing hot-dip galvanized steel sheet will be described. First, a steel slab is manufactured by melting a steel material having the above-described chemical composition. The method for melting the steel material is not particularly limited, and known melting methods such as a converter or an electric furnace can be used. Furthermore, the steel slab is preferably manufactured by a continuous casting method in order to prevent macrosegregation, but it can also be manufactured by an ingot casting method, a thin slab casting method, or the like. Furthermore, in addition to the conventional method in which the steel slab is once cooled to room temperature after production and then reheated, energy-saving processes such as direct rolling, in which the hot slab is charged into a heating furnace without being cooled to room temperature, or is immediately rolled after a short period of heat retention, can also be applied without any problems.

[0080] [Heating Temperature of Steel Slab] Although not particularly limited, the heating temperature of the steel slab is preferably 1100°C or higher and 1300°C or lower. Precipitates present during the heating stage of the steel slab exist as coarse precipitates in the final base steel sheet and do not contribute to strength. Therefore, it is preferable to redissolve the Ti and Nb-based precipitates precipitated during casting. Therefore, the heating temperature of the steel slab is preferably 1100°C or higher, more preferably 1150°C or higher. Furthermore, from the viewpoint of scaling off defects such as bubbles and segregations on the surface of the slab, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, the heating temperature of the steel slab is preferably 1100°C or higher. On the other hand, if the heating temperature of the steel slab exceeds 1300°C, the amount of oxidation increases, resulting in increased scale loss. Therefore, the heating temperature of the steel slab is preferably 1300°C or lower, more preferably 1250°C or lower. Furthermore, steel slabs are made into sheet bars by rough rolling under normal conditions, but when the heating temperature is set low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[0081] [Finishing Rolling Outlet Temperature in Hot Rolling: 750°C or More and 1000°C or Less] The heated steel slab is hot-rolled to produce a hot-rolled steel sheet. Hot rolling includes rough rolling and finishing rolling. When the finishing rolling outlet temperature exceeds 1000°C, the amount of oxide (scale) generated increases rapidly, the interface between the base steel and the oxide becomes rough, and the surface quality after pickling or cold rolling tends to deteriorate. In addition, the presence of residual hot-rolling scale after pickling has an adverse effect on bendability. Furthermore, the crystal grain size becomes excessively coarse, which may cause surface roughness of the pressed product during processing. Therefore, the finishing rolling outlet temperature is set to 1000°C or less, and preferably 950°C or less. On the other hand, when the finishing rolling outlet temperature is less than 750°C, the rolling load increases, resulting in a large rolling load. Furthermore, the rolling reduction rate increases when the austenite is in an unrecrystallized state, an abnormal texture develops, and the in-plane anisotropy in the final product becomes significant, which not only impairs the uniformity of the material (material stability) but also reduces the bendability itself. Therefore, the finish rolling outlet temperature is set to 750°C or higher, and preferably 800°C or higher. The finish rolling outlet temperature is based on the temperature of the hot-rolled steel sheet surface.

[0082] [Coiling temperature of hot-rolled steel sheet: 300°C or higher, 750°C or lower] Next, the hot-rolled steel sheet is coiled after hot rolling. If the coiling temperature of the hot-rolled steel sheet exceeds 750°C, scale is formed on the surface layer of the hot-rolled steel sheet, causing excessive surface decarburization during the subsequent heat treatment process, reducing the surface hardness and making it difficult to obtain sufficient corona bond peel strength. Therefore, the coiling temperature of the hot-rolled steel sheet is set to 750°C or lower, and preferably 650°C or lower. On the other hand, if the coiling temperature of the hot-rolled steel sheet is lower than 300°C, the strength of the hot-rolled steel sheet increases, which increases the rolling load in cold rolling and causes defects in the sheet shape, thereby reducing productivity. Therefore, the coiling temperature of the hot-rolled steel sheet is set to 300°C or higher, and preferably 400°C or higher. The coiling temperature is based on the temperature of the surface of the hot-rolled steel sheet.

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

[0084] Next, the hot-rolled sheet is subjected to pickling. Pickling can remove oxides from the steel sheet surface, and is therefore important for ensuring good chemical conversion treatability and plating quality in the final hot-dip galvanized steel sheet product. Pickling may be performed once or multiple times.

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

[0086] [Heating temperature in annealing process: (Ac ３ Transformation point -50) ° C or higher] Next, the cold-rolled steel sheet is annealed. ３ If the heating temperature is less than the transformation point -50°C, the annealing treatment will be performed 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. ３ Transformation point -50) ° C or higher, (Ac ３Transformation point -30) ° C or higher is preferred, (Ac ３ It is more preferable that the temperature is equal to or higher than the transformation point -10°C. Although the upper limit of the heating temperature in the annealing step is not particularly specified, if the heating temperature in the annealing step is 1000°C or lower, it is possible to suitably prevent a decrease in the surface hardness after annealing and a decrease in TS. Therefore, the heating temperature in the annealing step is preferably 1000°C or lower, and more preferably 980°C or lower. The heating temperature in the annealing step is measured based on the temperature of the surface of the cold-rolled steel sheet.

[0087] In addition, Ac ３ The transformation point can be calculated using the following formula (6): ３ 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 contents (mass%) of elements in the chemical composition of the base steel sheet, respectively, and are set to zero if the element is not contained.

[0088] The time for which the heating temperature is maintained is not particularly limited, but is preferably 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 a conventional method can be used. The average cooling rate in the temperature range of 500°C or more below the annealing temperature is not particularly limited, but from the viewpoint of controlling the area ratio of martensite and ferrite, it is preferably 5°C / s or more and 50°C / s or less.

[0090] [Dew point in snout: −60°C or higher] Next, the cold-rolled steel sheet is passed through the snout. If the dew point in the snout is lower than −60°C, decarburization from the surface layer of the cold-rolled steel sheet is suppressed and a soft layer is not formed in the surface layer, so the hardness of the surface layer does not decrease, the peel strength of the corona bond decreases, and the delayed peel resistance of the welded portion deteriorates. Therefore, the dew point in the snout is set to −60°C or higher, and −50°C or higher is preferable. Note that there is no particular 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 suitably prevent a decrease in the surface layer 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 treatment step] Next, the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet. The annealing, cooling, and plating treatment may be performed continuously in one line (CGL (Continuous Galvanizing Line)), an example of which is described below. After annealing, the cold-rolled steel sheet is cooled to a temperature range of about 500°C. Next, the cold-rolled steel sheet is passed through the steel strip outlet side of the cooling zone, and further cooled while being moved into the hot-dip galvanizing bath via a snout whose tip is immersed in the hot-dip galvanizing bath. 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 is preferably 1 s or more and 300 s or less from the viewpoint of controlling the area ratios of martensite and ferrite. 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 cause it to enter the snout, and the cold-rolled steel sheet passes through the roll before entering the snout. Next, the cold-rolled steel sheet is guided to the hot-dip galvanizing bath via the snout and immersed in the hot-dip galvanizing bath to be subjected to hot-dip galvanizing treatment, thereby producing a plated steel sheet.

[0092] In the hot-dip galvanizing treatment, the cold-rolled steel sheet can be immersed in a hot-dip galvanizing bath at 480° C. or more and 500° C. or less. The hot-dip galvanizing bath used in the hot-dip galvanizing treatment preferably contains 0.10 mass % or more and 0.23 mass % or less of Al, with the balance being Zn and unavoidable impurities.

[0093] The coating weight in the hot dip galvanizing process is 20 to 80 g / m per side. ２ (Double-sided plating) is preferred. The coating weight 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 a hot-dip galvanizing treatment and then to an alloying treatment. It is preferable to subject the hot-dip galvanized steel sheet to an alloying treatment after the hot-dip galvanizing treatment and before the vibration holding step described below. When the alloying treatment temperature is 460°C or higher, the Zn—Fe alloying rate is favorable, and productivity is 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 the area ratio of martensite is favorably 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 surface temperature of the hot-dip galvanized steel sheet.

[0095] [First Cooling Step] Next, the hot-dip galvanized steel sheet is cooled to a cooling stop temperature below the martensitic transformation start temperature in the first cooling step. If the cooling stop temperature exceeds the martensitic transformation start temperature, the amount of martensite transformed is small, and the amount of martensite tempered by subsequent reheating is small, resulting in an inability to obtain the desired amount of tempered martensite. Therefore, the cooling stop temperature in the first cooling step is set to the martensitic transformation start temperature or lower, preferably (martensitic transformation start temperature - 20) °C or lower. While there is no particular lower limit for the cooling stop temperature in the first cooling step, a cooling stop temperature of 0 °C or higher effectively prevents austenite from transforming into martensite and effectively obtains retained austenite, which is effective for 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 surface temperature of the hot-dip galvanized steel sheet.

[0096] The martensitic transformation start temperature (Ms point) can be determined using the following formula (7): (Martensitic transformation start temperature)=550−350×(%C)−40×(%Mn)−10×(%Cu)−17×(%Ni)−20×(%Cr)−10×(%Mo)−35×(%V)−5×(%W)+30×(%Al) (7) where (%C), (%Si), (%Mn), (%Ni), (%Cu), (%Cr), (%Mo), (%V), (%Ti), (%W), and (%Al) are the contents (mass%) of elements in the chemical composition of the base steel sheet, respectively, and are set to zero if the element is not contained.

[0097] [Reheating to a Reheating Temperature in a Range Exceeding the Cooling Stop Temperature but Not Exceeding 450°C] Next, the hot-dip galvanized steel sheet is reheated to a reheating temperature in a range exceeding the cooling stop temperature but not exceeding 450°C. Reheating to a reheating temperature in a range exceeding the cooling stop temperature but not exceeding 450°C after the first cooling step is an important inventive feature of the present invention. If the steel is reheated only to a temperature range below the cooling stop temperature, tempering of the quenched martensite does not proceed, resulting in excessive production of quenched martensite, and the desired YR and bendability are not obtained. Furthermore, the diffusion of hydrogen in the steel is not promoted, and the desired hydrogen content in the steel is not reduced, resulting in poor hole expandability. Therefore, the reheating temperature is preferably above the cooling stop temperature and at least 20°C above the martensitic transformation start temperature. On the other hand, if the steel is reheated to a temperature range exceeding 450°C, excessive ferrite is produced, and the desired TS, hole expandability, and bendability are not obtained. Therefore, the reheating temperature is set to 450°C or less, preferably 430°C or less. Note that the reheating temperature is based on the surface temperature of the hot-dip galvanized steel sheet.

[0098] [Vibration Holding Step] Next, a vibration holding step is performed in which the hot-dip galvanized steel sheet is vibrated to a maximum amplitude of 10 nm to 500 μm and held at the reheating temperature for 2 seconds to 600 seconds. In the vibration holding step, the hot-dip galvanized steel sheet is forcibly vibrated slightly, thereby repeatedly displacing the hot-dip galvanized steel sheet in the thickness direction, i.e., bending deformation. This results in an expansion of the lattice spacing at the surface compared to the center of the thickness of the hot-dip galvanized steel sheet, rather than a uniform expansion of the lattice spacing from the center to the surface. 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 they are easily desorbed from the surface. As a result, the carbon content in the surface layer of the hot-dip galvanized steel sheet is reduced, and hydrogen in the steel is reduced, thereby improving the delayed peeling resistance of the weld.

[0099] [Maximum Amplitude of Hot-Dip Galvanized Steel Sheet: 10 nm or More and 500 μm or Less] If the maximum amplitude of the hot-dip galvanized steel sheet in the vibration holding step is less than 10 nm, the lattice spacing on the surface of the hot-dip galvanized steel sheet is not sufficiently expanded, the diffusion of carbon and hydrogen is not sufficiently promoted, and the effect of desorbing them is not obtained. Therefore, the maximum amplitude of the hot-dip galvanized steel sheet in the vibration holding step is set to 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 in the vibration holding step exceeds 500 μm, the strain on the surface of the hot-dip galvanized steel sheet increases, causing plastic deformation and resulting in hydrogen trapping, thereby not achieving the effect of desorbing hydrogen. From this perspective, the maximum amplitude of the hot-dip galvanized steel sheet in the vibration holding step is set to 500 μm or less, preferably 400 μm or less, and more preferably 300 μm or less. Note that the hot-dip galvanized steel sheet naturally vibrates during the sheet threading process or vibrates when exposed to gas, for example, from a gas wiping device. However, in these vibrations, the maximum amplitude of the hot-dip galvanized steel sheet exceeds at least 0.5 mm, and therefore the effect of desorbing carbon or hydrogen contained in the hot-dip galvanized steel sheet cannot be obtained.

[0100] The maximum amplitude of the hot-dip galvanized steel sheet in the vibration holding step can be measured by a laser displacement meter or a laser Doppler vibrometer arranged at a predetermined distance from the surface. The maximum amplitude can also be controlled by irradiating sound waves with a controlled sound pressure level or by applying electromagnetic force using an electromagnet with a controlled current.

[0101] [Holding time of vibration holding step: 2 seconds or more and 600 seconds or less] If the holding time in the vibration holding step is less than 2 seconds, the tempering of the quenched martensite does not proceed, an excessive amount of quenched martensite is generated, and the desired YR and bendability cannot be obtained. Therefore, the holding time in the vibration holding step is set to 2 seconds or more, and preferably 5 seconds or more. On the other hand, if the holding time in the vibration holding step exceeds 600 seconds, excessive ferrite is generated, and the desired TS, hole expandability, and bendability cannot be obtained. Therefore, the holding time in the vibration holding step is set to 600 seconds or less, and preferably 550 seconds or less. Note that the holding time is the time during which the steel is isothermally held at the reheating temperature.

[0102] [Sound pressure level on the surface of the hot-dip galvanized steel sheet: 50 dB or more (preferred condition)] In the vibration holding step, it is preferable to vibrate the hot-dip galvanized steel sheet by irradiating it with sound waves so that the sound pressure level on the surface of the hot-dip galvanized steel sheet is 50 dB or more. A sound pressure level of 50 dB or more ensures that the hot-dip galvanized steel sheet is vibrated, thereby favorably promoting the diffusion of carbon and hydrogen. Therefore, the sound pressure level is preferably 50 dB or more, more preferably 60 dB or more, and even more preferably 80 dB or more. Meanwhile, considering the performance of a typical sound wave irradiation device, the sound pressure level is preferably 150 dB or less, more preferably 140 dB or less.

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

[0104] [Vibration application time: 1 second or more and 3600 seconds or less (preferred conditions)] The vibration application time in the vibration holding step 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, the vibration application time in the vibration holding step 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 impairing productivity. In the present invention, the vibration application time for 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 when vibration is applied from multiple vibration applying devices, it means the cumulative time of all of the vibrations.

[0105] [Second Cooling Step] Next, the 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 even during the second cooling step.

[0106] [Cooling rate in the second cooling step in the temperature range of 50°C below the martensitic transformation start temperature: 15.0°C / s or less (preferred condition)] By setting the cooling rate in the second cooling step to 15.0°C / s or less in the temperature range of 50°C below the martensitic transformation start temperature, the transformation to the martensitic phase, which has low amounts of dissolved carbon and hydrogen, can be favorably promoted. Furthermore, the diffusion of carbon and hydrogen can be favorably promoted while generating vibration, i.e., displacement in the thickness direction of the hot-dip galvanized steel sheet. 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 constraints on production technology, the cooling rate is preferably 1.0°C / s or more, more preferably 2.0°C / s or more. The cooling rate is based on the surface temperature of the hot-dip galvanized steel sheet. In addition, as a method for cooling the hot-dip galvanized steel sheet in a temperature range of 50° C. or more below the martensitic transformation start temperature, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be applied.

[0107] For steps and conditions not described in this specification, conventional methods can be used.

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

[0109]

[0110] The obtained sheet bar was subjected to finish rolling, coiled, and 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 sheet thickness of 1.4 mm. The obtained cold-rolled steel sheet was then annealed and hot-dip galvanized to obtain a hot-dip galvanized steel sheet having a hot-dip galvanized layer on both sides. In Examples other than Nos. 7 and 8, the obtained hot-dip galvanized steel sheet was cooled while applying vibration by irradiating it with sound waves. In No. 7, the obtained hot-dip galvanized steel sheet was cooled while applying vibration by periodically applying electromagnetic force from an electromagnet. In No. 8, the obtained hot-dip galvanized steel sheet was cooled without applying vibration. Table 2 shows the manufacturing conditions for each Example. In the column for coating type in Table 2, "GI" is entered when only hot-dip galvanizing treatment was performed, and "GA" is entered when both hot-dip galvanizing treatment and alloying treatment were performed. The vibration was applied simultaneously with the start of cooling of the hot-dip galvanized steel sheet.

[0111] In the GI example, a hot-dip galvanizing bath containing 0.20 mass% Al, with the balance being Zn and unavoidable impurities, was used as the plating bath. In the GA example, a hot-dip galvanizing bath containing 0.14 mass% Al, with the balance being Zn and unavoidable impurities, was used. The plating bath temperature was 470°C in both cases. The coating weight in the GI example was 45 to 72 g / m per side. ２ (Both sides plating) and in the case of GA, 45 g / m per side ２ In the example of GA, the alloying treatment temperature was set to about 550°C.

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

[0113]

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

[0115] Furthermore, TS, YR, EL, hole expandability, bendability, corona bond peel strength and weld delayed peel resistance were evaluated by the above-mentioned methods, and the results are shown in Table 3.

[0116]

[0117] As shown in Table 3, the inventive examples had a TS of 1180 MPa or more and were excellent in formability, corona bond peel strength, and weld delayed peel resistance. On the other hand, the comparative examples were inferior in one or more of TS, bendability, corona bond peel strength, and weld delayed peel resistance.

[0118] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet having a TS of 1180 MPa or more, excellent formability, corona bond peel strength, and resistance to delayed peeling of welded joints, and capable of producing parts with high dimensional accuracy, and a method for producing the same. The hot-dip galvanized steel sheet obtained by the present invention can be applied to, for example, automotive structural members, thereby reducing the weight of the vehicle body and improving fuel efficiency, and is therefore of great industrial value.

[0119] 100 Tensile test piece 102 Welded joint 10 L-shaped sample 12 Center of welded surface 20 Sample 22 Spacer 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 the following features.

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. The 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. The 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. The 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. The 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. The 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.