Hot-dip galvanized steel sheet and its manufacturing method

Abstract

The present invention provides a hot-dip galvanized steel sheet having a TS of 1700 MPa or more and a YS of 1250 MPa or more, and excellent formability, corona bond peel strength, and weld delayed peel resistance, and a method for manufacturing the same. The hot-dip galvanized steel sheet of the present invention comprises a substrate steel sheet and a hot-dip galvanized layer, the substrate steel sheet having a predetermined chemical composition and a predetermined steel structure, the C content in the surface layer of the substrate steel sheet is 0.250% or less, 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 less than 0.85, the amount of hydrogen in the steel [H] (ppm by mass) having a binding energy with hydrogen trap sites in the substrate steel sheet of 10 kJ / mol to 100 kJ / mol is 0.30 ppm by mass or less, and the average Vickers hardness of the surface layer of the substrate steel sheet is Hv S The present invention is characterized in that the following formula (1) is satisfied: [H]≦(580-Hv S ) / 320 ···(1)

Description

[Technical Field]

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

[0002] To achieve both CO2 emissions reduction through vehicle weight reduction and improved crashworthiness through vehicle weight reduction, the application of high-tensile steel with a tensile strength (TS) of 1470 MPa or higher to body frame components is increasing year by year. Furthermore, the rapid global increase in electric vehicles (EVs) requires protection of high-voltage components such as batteries and motors from collisions. Therefore, the use of steel sheets with a TS of 1700 MPa or higher, which is higher than conventional steel sheets, is increasing. In particular, electric vehicle batteries tend to have a larger protective area compared to gasoline-powered vehicle fuel tanks and a narrower crush zone compared to gasoline-powered vehicles. Therefore, there is a demand for high-strength steel sheets that can provide high impact absorption performance with a smaller stroke. To address this need, from the perspective of crashworthiness, increasing the yield strength (YS) can increase the impact energy absorbed during a collision.

[0003] Furthermore, among the structural components of automobiles, for example, crash boxes have bending parts. Therefore, from the viewpoint of formability, it is preferable to apply a steel sheet having high bendability to such components. In addition, from the viewpoint of the rust prevention performance of the vehicle body, a hot-dip galvanized steel sheet may be applied to the steel sheet used as the material for the structural components of automobiles.

[0004] 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 sheet, 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 are also becoming 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.

[0005] As a technology relating to such hot-dip galvanized steel sheets, high-strength steel sheets in which a soft surface layer is formed to improve bendability 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, in which the thickness of the soft surface layer is controlled, and in which the amount of low-temperature diffusible hydrogen in the steel is reduced to improve bendability. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2021 / 186510 [Patent Document 2] International Publication No. 2022 / 270053 Summary of the Invention [Problem to be solved by the invention]

[0007] 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 there is no consideration of balancing the properties of the welded portion, leaving room for improvement. Furthermore, to ensure dimensional accuracy, it is necessary to control TS and YS, but there is no disclosure about YS in Patent Document 1, leaving room for improvement. In Patent Document 2, there is no consideration of the properties of the welded portion, leaving room for improvement.

[0008] In view of the above problems, the present invention aims to provide a hot-dip galvanized steel sheet having a TS of 1700 MPa or more and a YS of 1250 MPa or more, and also excellent formability, corona bond peel strength, and weld delayed peeling resistance, as well as a method for manufacturing the same. In the present invention, formability refers to hole expandability and bendability. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems and have come to the following findings. (1) The base steel sheet has a predetermined chemical composition, a steel structure containing a certain amount of tempered martensite, and a microstructure containing ferrite and / or retained austenite. This allows for a YS of 1250 MPa or more, and also achieves both high tensile strength and excellent bendability. (2) By making the C content in the surface layer of the base steel sheet 0.250% or less and the value obtained by dividing the C content in the surface layer of the steel sheet by the C content of the entire steel sheet less than 0.85, a soft surface layer is formed, and excellent peel strength is obtained in the corona bond. (3) By reducing the amount of hydrogen trapped in specific hydrogen trapping sites in the base steel, high hole expandability can be achieved. Furthermore, by reducing the amount to match the hardness of the steel surface, crack propagation to the nugget can be suppressed, resulting in excellent resistance to delayed disbonding of the weld.

[0010] That is, the gist and configuration of the present invention are as follows.

[0011] [1] A steel sheet having a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, The base steel sheet comprises, in mass%, C: Over 0.250% and 0.400% 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 and the remainder being Fe and unavoidable impurities; At a 1 / 4 position of the 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 Volume fraction of retained austenite is 10.0% or less and a steel structure in which The C content in the surface layer of the substrate steel sheet is 0.250% or less, The value obtained by dividing the C content in the surface layer of the substrate steel sheet by the C content of the entire substrate steel sheet is less than 0.85, The hydrogen content [H] (ppm by mass) in the steel of hydrogen having a binding energy with the hydrogen trapping site of the substrate steel sheet of 10 kJ / mol or more and 100 kJ / mol or less is 0.30 ppm by mass or less, and the average Vickers hardness of the surface layer of the substrate steel sheet is Hv S A hot-dip galvanized steel sheet characterized by satisfying the following formula (1): [H]≦(580-Hv S ) / 320 ···(1)

[0012] [2] The component 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, Hf: 0.100% or less, and Bi:0.200% or less The hot-dip galvanized steel sheet according to the above [1], containing at least one element selected from the group consisting of:

[0013] [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.

[0014] [4] A step 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 step of coiling the hot-rolled steel sheet at 300°C or higher and 750°C or lower; Next, a step 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; Next, an annealing step of heating the cold-rolled steel sheet to (Ac3 transformation point - 50) ° C. or higher; Next, a step of passing the cold-rolled steel sheet through a snout in which a dew point is controlled to -60°C or higher; Next, a step of subjecting the cold-rolled steel sheet to a hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet; Next, a first cooling step of cooling the hot-dip galvanized steel sheet to a cooling stop temperature that is equal to or lower than the martensitic transformation start temperature; Next, reheating the hot-dip galvanized steel sheet to a reheating temperature that is higher than the cooling stop temperature and not higher than 450°C; Next, 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; Next, a second cooling step of cooling the hot-dip galvanized steel sheet to room temperature; A method for producing a hot-dip galvanized steel sheet, comprising:

[0015] [5] The method for producing a hot-dip galvanized steel sheet according to [4] above, 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 the sound pressure level on the surface of the hot-dip galvanized steel sheet satisfies 50 dB or more.

[0016] [6] The method for producing a hot-dip galvanized steel sheet according to [4] or [5] above, 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.

[0017] [7] The method for producing 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 as well.

[0018] [8] The method for producing 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 vibration holding step. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet having a TS of 1700 MPa or more and a YS of 1250 MPa or more, and also having excellent formability, corona bond peel strength, and resistance to delayed peeling of welded joints, as well as a method for manufacturing the same. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 2 is a schematic diagram of a tensile test piece used in evaluating the peel strength of a corona bond in the examples of the present invention. [Figure 2] 1A and 1B are a top view and a side view, respectively, of a welded joint used in an evaluation of delayed peeling resistance in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] 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.

[0022] (base steel plate) 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 "% by mass" unless otherwise specified.

[0023] [C: Over 0.250% and up to 0.400%] Carbon (C) is one of the important basic components of steel, and in the present invention, it is an important element that affects tensile strength (TS), bendability, and weld corona bond peel strength. If the C content is 0.250% or less, the hardness of quenched martensite and tempered martensite decreases, making it difficult to achieve the desired TS. Therefore, the C content should be greater than 0.250%, preferably 0.255% or more, and more preferably 0.260% or more. On the other hand, if the C content exceeds 0.400%, the quenched martensite and tempered martensite become embrittled, making it difficult to achieve the desired bendability. Furthermore, as the quenched martensite and tempered martensite harden, the hardness of the surface layer of the base steel sheet increases, making it difficult to achieve the desired weld corona bond peel strength. Therefore, the C content should be 0.400% or less, preferably 0.380% or less, and more preferably 0.370% or less.

[0024] [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 sites 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 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.

[0025] [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, it 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%, not only the amount of retained austenite increases, which increases the amount of hydrogen in the steel, but also the hardenability becomes excessive and the hardness of the steel sheet surface increases. 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.

[0026] [P:0.100% or less] If P is excessive, it 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.

[0027] [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 set to 0.0200% or less, and preferably 0.0050% or less. There is no particular lower limit for the S content, but due to constraints on production technology, the S content is preferably set to 0.0001% or more.

[0028] [Al:0.100% or less] If Al is excessive, the A3 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.

[0029] [N:0.0100% or less] N exists as a nitride and reduces the ultimate deformability of the steel sheet, so 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. 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.

[0030] [O:0.0100% or less] O exists as an oxide and reduces the ultimate deformability of the steel sheet, so if it contains too much O, the hole expandability and bendability will decrease. Therefore, the O content is set to 0.0100% or less, and preferably 0.0050% or less. 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.

[0031] [Remaining components] The base steel sheet has a composition containing the above elements with the balance being Fe and unavoidable impurities. Furthermore, the base steel sheet preferably has a composition containing the above elements with the balance being Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, Sr, Ge, Cs, and As. The total content of these impurities is permitted to be 0.100% or less.

[0032] The chemical composition of the high-strength steel sheet of the present disclosure further contains, in addition to the above-mentioned essential components, the following, 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 It may contain at least one element selected from the group consisting of: Sn: 1.00% 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: 0.100% or less, and Bi: 0.200% or less.

[0033] [Ti:0.200% or less] Ti generates large amounts of coarse precipitates or inclusions, reducing the ultimate deformability of the steel sheet, and therefore, if the Ti content exceeds 0.200%, the hole expandability and bendability decrease. Therefore, when Ti is contained, the Ti content is set to 0.200% or less, and preferably 0.100% or less. While there is no particular lower limit for the Ti content, by setting the Ti content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, which increases the strength of the steel sheet and allows for a favorable TS. Therefore, the Ti content is preferably set to 0.001% or more.

[0034] [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 decrease. Therefore, when Nb is contained, the Nb content is set to 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 achieving a favorable TS. Therefore, the Nb content is preferably set to 0.001% or more.

[0035] [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 decrease. Therefore, when V is contained, the V content is set to 0.200% or less, and preferably 0.100% or less. Although there is no particular lower limit for the V content, 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 achieving a favorable TS. Therefore, the V content is preferably set to 0.001% or more.

[0036] [Ta:0.100% or less] [W: 0.100% or less] If 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. Although there is no particular lower limit for the Ta or W content, if the Ta or W content is 0.010% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby favorably achieving the strength of the steel sheet. Therefore, the Ta or W content is preferably set to 0.010% or more.

[0037] [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 set to 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 set to 0.0003% or more.

[0038] [Cr:1.000% or less] [Mo: 1.000% or less] [Ni: 1.000% or less] If the Cr, Mo, or Ni content exceeds 1.000%, the amount of coarse precipitates or inclusions increases, reducing the ultimate deformability of the steel sheet and resulting in reduced hole expandability and bendability. Therefore, when Cr, Mo, or Ni is contained, the respective contents are set to 1.000% or less, and preferably 0.800% or less. Although there is no particular lower limit for the Cr, Mo, or Ni content, since these elements improve hardenability, the Cr, Mo, or Ni content is preferably set to 0.010% or more.

[0039] [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 set to 0.500% or less, and 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 set to 0.001% or more.

[0040] [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 set to 1.00% or less, and 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 set to 0.01% or more.

[0041] [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 set to 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability, the Sn content is preferably set to 0.001% or more.

[0042] [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 there is no particular lower limit for the Sb content, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is preferably set to 0.001% or more.

[0043] [Ca: 0.0100% or less] [Mg:0.0100% or less] [REM:0.0100% or less] If the Ca, Mg, or REM content exceeds 0.0100%, 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 Ca, Mg, or REM is contained, the Ca, Mg, or REM content is set to 0.0100% or less, and preferably 0.0050% or less. Although there is no particular lower limit for the Ca, Mg, or REM content, these elements spheroidize the shape of nitrides or sulfides, improving the ultimate deformability of the steel sheet, so the Ca, Mg, or REM content is preferably set to 0.0005% or more.

[0044] [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, 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 there is no particular lower limit for the Zr or Te content, since these elements spheroidize the shape of nitrides or sulfides and improve the ultimate deformability of the steel sheet, the Zr or Te content is preferably set to 0.0010% or more.

[0045] [Hf:0.100% or less] If the Hf content exceeds 0.100%, 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 0.100% or less, and preferably 0.080% 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 0.010% or more.

[0046] [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, and preferably 0.100% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is preferably set to 0.001% or more.

[0047] 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 effects of the present invention are not impaired, and therefore these elements may be contained as unavoidable impurities.

[0048] 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 the 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).

[0049] [Area ratio 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 YS and high bendability can be achieved. Therefore, the area fraction of tempered martensite is set to 60.0% or more, preferably 65.0% or more, more preferably 70.0% or more, and even more preferably 75.0% or more. While there are no particular limitations on the upper limit of the area fraction of tempered martensite, in addition to the viewpoint of 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 YS, 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.

[0050] [Area ratio 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%, it becomes difficult to achieve the desired YS. 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. 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.

[0051] [Ferrite area ratio: 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 set to 30.0% or less, and preferably 25.0% or less. There is no particular lower limit for the area fraction of ferrite, and the area fraction of ferrite may be 0.0%. In the present invention, ferrite also includes bainitic ferrite.

[0052] The area fractions of tempered martensite, quenched martensite, and ferrite can be determined as follows. A sample is cut out so that the observation surface is the thickness cross section (L cross section) parallel to the rolling direction of the steel plate. The observation surface of the cut out sample is mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Using a scanning electron microscope (SEM), the observation position is set to a position ¼ of the steel plate thickness, and 10 fields of view are observed at an acceleration voltage of 15 kV and a magnification of 2000x to obtain structure images. Using the obtained structure images, the area fractions of each structure (ferrite, tempered martensite, quenched martensite) are calculated for each of the 10 fields of view using Media Cybernetics' Image-Pro. The average of the obtained values ​​is taken as the area fraction 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, while quenched martensite is a protruding structure with fine irregularities inside, and tempered martensite is a recessed structure that contains fine carbides.

[0053] [Volume fraction of retained austenite: 10.0% or less] When the volume fraction of retained austenite in the steel structure of the substrate steel sheet is 10.0% or less, the amount of hydrogen in the steel of the substrate 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. 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%. In the present invention, the volume fraction of retained austenite can be considered to be the same as its area fraction.

[0054] 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 sheet thickness from the surface layer of the substrate steel sheet, the sample is further polished by 0.1 mm using 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 radiation source. The volume fraction of austenite can be calculated 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.

[0055] [Remaining tissue] 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 an SEM.

[0056] The area ratio of the remaining structure can be calculated by the following formula (2). [Area fraction of residual structure (%)] = 100 - [Area fraction of tempered martensite (%)] - [Area fraction of quenched martensite (%)] - [Area fraction of ferrite (%)] - [Volume fraction of retained austenite (%)] (2)

[0057] [C content in the surface layer of the base steel sheet: 0.250% or less] Forming a soft layer on the surface of the substrate steel sheet and reducing the surface layer hardness improves the toughness of the corona bond and increases the corona bond peel strength. The surface layer hardness can be reduced by reducing the C content in the surface layer. Therefore, the C content in the surface layer of the substrate steel sheet is set to 0.250% or less, and preferably 0.230% or less. There is no particular lower limit for the C content in the surface layer of the substrate steel sheet, but if the surface layer is too soft, it may be difficult to achieve the desired TS, so the C content is preferably set to 0.010% or more.

[0058] [C content in the surface layer of the substrate steel sheet divided by the total C content of the 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, it is possible to alleviate the stress concentration on the steel sheet surface layer when the corona bond is peeled off, and to suppress crack propagation. 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. 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.

[0059] The carbon 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 mm diameter 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 in the surface layer approximately 5 μm deep. The carbon content at a position 10 μm further in the thickness direction of the substrate steel sheet from the depth where the Zn concentration is 0.0 mass% was defined as the carbon content in the surface layer of the substrate steel sheet. The carbon content of the entire substrate steel sheet is the carbon content in the chemical composition of the substrate steel sheet described above, and is equivalent to the carbon content of the steel slab used.

[0060] [Hydrogen content in steel with a binding energy of 10 kJ / mol or more and 100 kJ / mol or less with respect to hydrogen trapping sites in the base steel sheet [H]: 0.30 mass ppm or less] By setting the hydrogen content [H] in the steel, which has a bond energy of 10 kJ / mol to 100 kJ / mol with the hydrogen trapping site of the base steel sheet, to 0.30 ppm by mass or less, good hole expandability can be ensured. Therefore, the hydrogen content [H] in the steel is set to 0.30 ppm by mass or less, and preferably 0.20 ppm by mass or less. Note that the lower limit of the hydrogen content [H] in the 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 the steel may be 0.00 ppm by mass.

[0061] 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 welded parts, 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 welded parts, so the bond energy is preferably 80 kJ / mol or less.

[0062] [Hydrogen content in steel with binding energy of 10 kJ / mol or more and 100 kJ / mol or less with hydrogen trapping sites of the base steel sheet [H]: (580-Hv S ) / 320 or less] The hydrogen content in 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 trapping site of the substrate steel sheet is determined by the average Vickers hardness of the surface layer of the substrate steel sheet. SIn the present invention, satisfying the following formula (1) is an important component. Delayed peeling in welds is a phenomenon that occurs due to hydrogen in steel trapped at specific hydrogen trapping sites. Therefore, 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 TS. For steel sheets with a TS of 1700 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]≦(580-Hv S ) / 320 ···(1)

[0063] Average Vickers hardness Hv of the surface layer of the substrate 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. S If the strength of the surface layer of the steel sheet increases excessively, the bendability deteriorates, so it is preferably 550 or less.

[0064] 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, each 30 mm long and 5 mm wide, are sheared from the center of the large plate sample. The resulting test pieces are immediately immersed in liquid nitrogen after collection. The hot-dip galvanized layer of the test pieces is then alkali-removed while controlling the temperature of the treatment solution so that the surface temperature of the test pieces is below room temperature. The amount of hydrogen released from the test pieces when heated is then measured using thermal desorption analysis. Specifically, the test pieces are 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 peak temperature T at which the maximum amount of released hydrogen is obtained from the hydrogen release curves obtained at each heating rate is used to determine the activation energy Ea for hydrogen desorption from the hydrogen trapping sites using 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 ) vs. 1 / T, and perform a linear approximation to find the slope (Ea / R) to calculate the activation energy Ea. The resulting activation energy Ea is the sum of the activation energy for hydrogen diffusion in the lattice and the binding energy of hydrogen to the hydrogen trap site. Therefore, by subtracting the activation energy for hydrogen diffusion in the lattice (7.6 kJ / mol) from Ea, the binding energy of hydrogen to the hydrogen trap site can be calculated.

number

[0065] The amount of hydrogen in steel [H], which is hydrogen with a binding energy of 10 kJ / mol to 100 kJ / mol with hydrogen trap sites, can be determined as follows: The sample is heated continuously from room temperature to 300°C at a heating rate of 200°C / h, and then cooled to room temperature. The cumulative amount of hydrogen released from the test piece during heating from room temperature to 210°C is measured and used as the amount of hydrogen in steel [H].

[0066] Average Vickers hardness Hv of the surface layer of the substrate 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.

[0067] (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.

[0068] 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 to 1.0% by mass of Al, and a total of 0.0% by mass to 3.5% by mass 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 remainder 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 to 15% by mass, more preferably 8% by mass to 13% by mass.

[0069] The plating weight per side is not particularly limited, but is preferably 20 to 80 g / m 2 is preferred.

[0070] 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.

[0071] (Mechanical properties of hot-dip galvanized steel sheets) Next, the mechanical properties of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0072] [Tensile strength (TS): 1700 MPa or more] Hot-dip galvanized steel sheets have a tensile strength of 1700 MPa or more. TS can be determined as follows: JIS No. 5 test pieces are 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 mm / s, and TS is measured.

[0073] [Yield strength (YS): 1250 MPa or more] The hot-dip galvanized steel sheet has a YS of 1250 MPa or more, which can be determined by the above-mentioned tensile test.

[0074] [Hole expansion] 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 × 100 mm test piece is taken from the hot-dip galvanized steel sheet. A hole expandability test is performed in accordance with JIS Z 2256 (2010). Specifically, a 10 mm 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 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, and D0 is the initial hole diameter (mm), and the hole expansion ratio λ (%) is calculated using the following formula (4). λ(%)={(D f -D0) / D0}×100 (4)

[0075] [Bendability] Hot-dip galvanized steel sheets have excellent bendability. Bendability can be evaluated as follows. 24 hours after production, strip-shaped test pieces 30 mm wide and 100 mm long are taken from the hot-dip galvanized steel sheet 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, with an indentation load of 100 kN and a holding time of 5 seconds. The presence or absence of cracks is then determined on the outside of the bent portion using a stereomicroscope, and the minimum 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 the limit bending radius R / t in a 90° V-bend is ≦4.50 (t: thickness of the hot-dip galvanized steel sheet).

[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 specimens are taken from the hot-dip galvanized steel sheet. The flat specimens are bent 90° with a curvature of 5 mm at a position 30 mm from the end of the specimen, with the bending axis parallel to the short side. An L-shaped specimen is prepared by bending the specimen at a 90° angle with a curvature of 5 mm. The L-shaped specimen has a square surface approximately 30 mm square as the welding surface, and a surface approximately 65 mm × 30 mm as the gripping portion for the tensile test. As shown in Figure 1, two L-shaped specimens 10 are arranged with their welding surfaces in contact. The specimens are welded at the center 12 of the welding surface so that the welds have a diameter of 3 mm to 5 mm on each L-shaped specimen 10, producing a tensile test specimen 100. After welding, a tensile test is performed before hydrogen present inside the welds of the tensile test specimen 100 is released, i.e., within 5 minutes after welding. The tensile test conditions 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] [Welded part delayed peeling resistance] Hot-dip galvanized steel sheets have excellent resistance to delayed peeling at welds. The resistance to delayed peeling at welds can be evaluated as follows. Two 100 mm × 30 mm samples were taken from the hot-dip galvanized steel sheet. As shown in Figure 2(b), a welded joint 102 was formed by stacking a sample 20, a spacer 22, and another sample 20 in that order. Two 1.5 mm thick, 30 mm square mild steel sheets were used as the spacers 22. The spacers 22 were sandwiched at both ends of the samples 20, and tack welding was performed. Tack welding was performed at two tack welds 24 shown in Figure 2(a). The center of the tack-welded welded joint 102 was designated as a resistance spot weld 26, and resistance spot welding was performed. After resistance spot welding, the welded joint 102 was left in the air at room temperature (20°C) for at least 24 hours. Resistance spot welding was preferably performed at room temperature, with the welding electrodes (lower and upper electrodes) always water-cooled. The lower and upper electrodes are both DR-type electrodes made of chromium copper, with a tip diameter (tip diameter) of 6 mm and a curvature radius of 40 mm. The pressure is controlled by driving the lower and upper electrodes with a servo motor, and a single-phase alternating current with a frequency of 50 Hz is supplied when electricity is applied.

[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 of manufacturing hot-dip galvanized steel sheets) Next, a method for manufacturing hot-dip galvanized steel sheets will be described. First, a steel slab is produced by melting a steel material having the above-mentioned chemical 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. Furthermore, the steel slab is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method or a thin slab casting method. Furthermore, in addition to the conventional method in which the steel slab is 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 temperature at the delivery side of hot rolling: 750°C to 1000°C] The heated steel slab is hot-rolled to produce a hot-rolled steel sheet. Hot rolling includes rough rolling and finish rolling. If the finish-rolling exit 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. Furthermore, if hot-rolling scale remains in some areas after pickling, it adversely affects bendability. Furthermore, excessive coarsening of the grain size may result in surface roughness of the pressed product during processing. Therefore, the finish-rolling exit temperature is set to 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, resulting in a large rolling load. Furthermore, the rolling reduction rate increases when austenite is not recrystallized, leading to the development of an abnormal texture and significant in-plane anisotropy in the final product. This not only impairs the uniformity (material stability) of the material, but also reduces the bendability itself. Therefore, the finish rolling exit temperature is set to 750° C. or higher, and preferably 800° C. or higher. The finish rolling exit temperature is based on the temperature of the surface of the hot-rolled steel sheet.

[0082] [Coiling temperature of hot-rolled steel sheet: 300℃ to 750℃] 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 will form on the surface of the hot-rolled steel sheet, causing excessive surface decarburization during the subsequent heat treatment process and reducing the surface hardness, resulting in insufficient corona bond peel strength. Therefore, the coiling temperature of the hot-rolled steel sheet is set to 750°C or less, and preferably 650°C or less. On the other hand, if the coiling temperature of the hot-rolled steel sheet is less than 300°C, the strength of the hot-rolled steel sheet will increase, which will increase the rolling load in cold rolling and cause defects in the sheet shape, resulting in reduced productivity. Therefore, the coiling temperature of the hot-rolled steel sheet is set to 300°C or more, and preferably 400°C or more. The coiling temperature is based on the temperature of the surface of the hot-rolled steel sheet.

[0083] During hot rolling, the rough-rolled sheets may be joined together and continuously subjected to finish rolling. The rough-rolled sheet (sheet bar) may also be wound up once before finish rolling. To reduce the rolling load during hot rolling, part or all of the finish rolling may be performed as lubricated rolling. Lubricated rolling is also preferred from the viewpoint of uniformity of the steel sheet shape and material properties. 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 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 cold-rolled 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 for uniform and efficient introduction of strain and results in a uniform structure. If the cold-rolling reduction ratio exceeds 60.0%, the grain size of austenite formed in the subsequent annealing step 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 step, the cold-rolling reduction ratio is preferably 5.0% or more, and more preferably 10.0% or more.

[0086] [Annealing process heating temperature: (Ac3 transformation point - 50)℃ or higher] Next, the cold-rolled steel sheet is annealed. If the heating temperature in the annealing step is lower than (Ac3 transformation point - 50)°C, the annealing treatment will be performed in the two-phase region of ferrite and austenite, and a large amount of ferrite will be contained after annealing, making it difficult to achieve the desired TS, high hole expandability, and high bendability. Therefore, the heating temperature in the annealing step is set to (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. Note that there is no particular upper limit for the heating temperature in the annealing step, but if the heating temperature in the annealing step is 1000°C or lower, it is possible to effectively 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. Note that the heating temperature in the annealing step is measured based on the temperature of the surface of the cold-rolled steel sheet.

[0087] The Ac3 transformation point can be calculated using the following formula (5). (Ac3 transformation point)=910-203√(%C)+45×(%Si)-30×(%Mn)-20×(%Cu)-15×(%Ni)+11×(%Cr)+32×(%Mo)+104×(%V)+400×(%Ti)+200×(%Al)...(5) 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, 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 threaded 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 weld 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 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 a hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet. The annealing, cooling, and galvanizing treatments may be performed continuously in a single 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 leading end is immersed in the hot-dip galvanizing bath. The time from the end of cooling of the cold-rolled steel sheet to its entry into 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. Note that a roll for changing the traveling direction of the cold-rolled steel sheet to enter the snout is provided immediately before the connection between the cooling zone and the snout, and the cold-rolled steel sheet enters the snout after passing through this roll. The cold-rolled steel sheet is then guided to a hot-dip galvanizing bath through 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 a temperature of 480° C. to 500° C. The hot-dip galvanizing bath used in the hot-dip galvanizing treatment preferably contains 0.10 mass % to 0.23 mass % Al, with the balance being Zn and unavoidable impurities.

[0093] The coating weight in hot dip galvanizing 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 process] Next, a first cooling step is performed in which the hot-dip galvanized steel sheet is cooled to a cooling stop temperature below the martensitic transformation start temperature. 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, making it impossible 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. Note that there is no particular 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 possible to effectively prevent austenite from transforming into martensite and to effectively obtain retained austenite, which is effective for controlling YS. Therefore, the cooling stop temperature in the first cooling step is preferably 10°C or higher. Note that 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 calculated using the following formula (6). (Martensitic transformation start temperature) = 550 - 350 × (%C) - 40 × (%Mn) - 10 × (%Cu) - 17 × (%Ni) - 20 × (%Cr) - 10 × (%Mo) - 35 × (%V) - 5 × (%W) + 30 × (%Al) (6) Here, (%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, and are set to zero if the element is not contained.

[0097] [Reheating to a reheating temperature within the range of 450°C above the cooling stop temperature] Next, the hot-dip galvanized steel sheet is reheated to a reheating temperature above the cooling stop temperature but not exceeding 450°C. Reheating to a reheating temperature above 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 sheet is reheated only to a temperature range below the cooling stop temperature, tempering of the quenched martensite does not proceed, resulting in excessive formation of quenched martensite, and the desired YS 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 sheet is reheated to a temperature range above 450°C, excessive ferrite is formed, and the desired TS, hole expandability, and bendability are not obtained. Therefore, the reheating temperature is set to 450°C or lower, preferably 430°C or lower. Note that the reheating temperature is based on the surface temperature of the hot-dip galvanized steel sheet.

[0098] [Vibration holding process] Next, a vibration holding process is performed in which the hot-dip galvanized steel sheet is vibrated to a maximum amplitude of 10 nm to 500 μm while being held at the reheating temperature for 2 to 600 seconds. In the vibration holding process, the hot-dip galvanized steel sheet is forcibly vibrated slightly, causing repeated displacement in the thickness direction, i.e., bending deformation, to the hot-dip galvanized steel sheet. 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 to 500 μm] 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 achieved. 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 due 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] [Vibration holding time: 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 formed, and the desired YS 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 formed, 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 quenched martensite is isothermally held at the reheating temperature.

[0102] [Sound pressure level on the surface of hot-dip galvanized steel sheet: 50 dB or more (optimal conditions)] In the vibration holding step, the hot-dip galvanized steel sheet is preferably vibrated 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 reliably, 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 sonic 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 by 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 addition time: 1 second to 3600 seconds (optimal 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 is the cumulative time of all of the vibrations.

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

[0106] [In the second cooling step, the cooling rate in the temperature range of 50°C below the martensitic transformation start temperature: 15.0°C / s or less (optimal 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 more below the martensitic transformation start temperature, the transformation to the martensite phase, which has low amounts of carbon and hydrogen dissolved therein, 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. This reduces the C content in the surface layer and the amount of hydrogen in the steel, thereby improving the hole expandability and corona bond peel strength. 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. While there is no particular lower limit for the cooling rate, due to constraints on production technology, 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 surface temperature of the hot-dip galvanized steel sheet. In addition, methods of cooling the hot-dip galvanized steel sheet in the temperature range of 50° C. or more below the martensitic transformation start temperature include gas jet cooling, mist cooling, water cooling, and air cooling.

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

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

[0109] [Table 1] TIFF0007786653000003.tif233103

[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 being subjected to vibration by irradiating it with sound waves. In No. 7, the obtained hot-dip galvanized steel sheet was cooled while being subjected to vibration by periodically applying electromagnetic force from an electromagnet. In No. 8, the obtained hot-dip galvanized steel sheet was cooled without being subjected to 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. 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 2 In the case 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 0.1-1.0 mass% Fe, 0.2-1.0 mass% Al, and the balance being Zn and unavoidable impurities.In the GA example, the composition of the alloyed hot-dip galvanized layer was 7-15 mass% Fe, 0.1-1.0 mass% Al, and the balance being Zn and unavoidable impurities.

[0113] [Table 2] TIFF0007786653000005.tif233159

[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. Table 3 shows the evaluation results. 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, YS, hole expandability λ, bendability, corona bond peel strength and weld delayed peel resistance were evaluated using the methods described above, and the results are shown in Table 3.

[0116] [Table 3] TIFF0007786653000007.tif233124

[0117] As shown in Table 3, the inventive examples had TS of 1700 MPa or more and YS of 1250 MPa or more, and were excellent in hole expandability, bendability, corona bond peel strength, and weld delayed peel resistance. On the other hand, the comparative examples were inferior in at least one of 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 having a TS of 1700 MPa or more and a YS of 1250 MPa or more, and excellent formability, corona bond peel strength, and resistance to delayed peeling of welded joints, as well as 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 economy, and is therefore of great industrial value. [Explanation of symbols]

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

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: Over 0.250% and under 0.400% 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 0.250% 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]≦(580-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: 0.100% 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.

Images