Zinc-plated steel sheet and method of manufacturing the same

Abstract

The present invention provides a zinc-plated steel sheet with excellent delayed fracture resistance of the elongated flange processing section and a method for manufacturing the same. The zinc-plated steel sheet of the present invention is a zinc-plated steel sheet having a base steel sheet and a zinc-plated layer formed on the surface of the base steel sheet, wherein the base steel sheet has a predetermined compositional composition, has a predetermined structure at a position 1 / 4 of the thickness of the base steel sheet, has a thickness of a carbon concentration reduction portion in the surface layer of the base steel sheet of 1.0 μm or more and 150.0 μm or less, has a thickness of a bainite precipitation region immediately below the carbon concentration reduction portion of 10 μm or more, has a total coverage rate of residual austenite and fresh martensite in the old austenite grain boundary immediately below the carbon concentration reduction portion of 30.0% or more and 60.0% or less, has a carbon concentration in the residual austenite covering the old austenite grain boundary of less than 0.60 mass%, and has a tensile strength of 1470 MPa or more.

Description

Technology Field

[0001] The present invention relates to a zinc-plated steel sheet and a method for manufacturing the same. Background Technology

[0002] Recently, improving vehicle fuel efficiency has become an important task for the automotive industry in order to reduce CO2 emissions from the perspective of preserving the global environment. While lightweighting the vehicle body is effective for improving fuel efficiency, it is necessary to maintain the strength of the vehicle body. Therefore, lightweighting of the vehicle body has been pursued by maintaining strength through increasing the strength of steel sheets used as materials for automotive parts, while simplifying the structure to reduce the number of parts.

[0003] However, when a high-strength steel sheet with a TS of 1470 MPa or higher is formed into a part by cold pressing or bending, there is a risk of delayed fracture occurring due to an increase in residual stress within the part and deterioration of the steel sheet's delayed fracture resistance. Here, delayed fracture refers to a phenomenon in which, when a part formed after being placed in a hydrogen intrusion environment, hydrogen penetrates into the steel sheet constituting the part, causing microcracks to form by reducing interatomic bonding strength or causing local deformation, and the steel sheet is fractured as the microcracks propagate.

[0004] In this regard, Patent Document 1 describes a plated steel sheet with excellent mechanical properties, which reduces the amount of hydrogen intrusion during manufacturing, and also has excellent hydrogen embrittlement resistance and plating adhesion, and a method for manufacturing the same. Additionally, Patent Document 2 describes an ultra-high strength thin steel sheet with excellent hydrogen embrittlement resistance by controlling the composition of the steel sheet and the residual austenite, and a method for manufacturing the same. Prior art literature

[0005] International Publication No. 2019 / 212047, Japanese Patent Publication No. 2007-197819 The problem to be solved

[0006] However, in Patent Documents 1 and 2, the delayed fracture characteristics of stretch-flanged portions, which are subjected to particularly harsh processing, were not considered, and as a result of the inventors' investigation, it was found that there is room for improvement. In addition, in Patent Document 2, it is recommended that the carbon concentration in the residual austenite be 0.8 mass% or higher, but as a result of the inventors' investigation, it was found that there is room for improvement in hole expansion formability.

[0007] In light of the above problem, the present invention aims to provide a zinc-plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0% or more, a hole expansion rate (λ) of 25% or more, and excellent delayed fracture resistance of the elongated flange processing part, and a method for manufacturing the same. means of solving the problem

[0008] The inventors, as a result of careful consideration to solve the above problem, have obtained the following understanding.

[0009] (1) At the 1 / 4 thickness of the base steel plate, the total area ratio of tempered martensite and fresh martensite is 70.0% or more, the area ratio of bainite is 10.0% or less, the thickness of the carbon concentration reduction portion in the surface layer of the base steel plate is 150.0 μm or less, and the total coverage ratio of residual austenite and fresh martensite in the old austenite grain boundary immediately below the carbon concentration reduction portion is 30.0% or more, thereby realizing a TS of 1470 MPa or more.

[0010] (2) Excellent hole expansion can be achieved by setting the volume ratio of retained austenite to 6.0% or more and 15.0% or less, and also by setting the total coverage ratio of retained austenite and fresh martensite at the old austenite grain boundary immediately below the carbon concentration reduction section to 60.0% or less, and the carbon concentration in the retained austenite covering the old austenite grain boundary to less than 0.60 mass%.

[0011] (3) By making the thickness of the bainite precipitation region immediately below the surface carbon concentration reduction region 10㎛ or more, the extended flange processing region can achieve excellent delayed fracture resistance.

[0012] (4) When manufacturing a zinc-plated steel sheet, a steel slab having a predetermined compositional composition can be used, and by controlling the holding time or cooling time and cooling rate in the holding process or each cooling process after the annealing process and after the plating process, a zinc-plated steel sheet having a structure satisfying (1) to (3) can be obtained.

[0013] In other words, the gist of the present invention is as follows.

[0014] [1] A zinc-plated steel plate having a lower steel plate and a zinc-plated layer formed on the surface of the lower steel plate,

[0015] The above lower steel plate,

[0016] In mass %,

[0017] C: 0.180% or more and 0.250% or less,

[0018] Si: 0.800% or more and 1.550% or less,

[0019] Mn: 2,400% or more, 3,200% or less,

[0020] P: 0.100% or less,

[0021] S: 0.0200% or less,

[0022] Al: 1.000% or less,

[0023] N: 0.0100% or less and,

[0024] O: 0.0100% or less

[0025] A composition containing, wherein the remainder consists of Fe and unavoidable impurities, and

[0026] At a depth from the surface of the above lower steel plate at a position of 1 / 4 of the plate thickness,

[0027] The sum of the area ratios of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less,

[0028] The volume percentage of retained austenite is 6.0% or more and 15.0% or less,

[0029] The area ratio of bainite is 10.0% or less and,

[0030] Tissue with an area ratio of 10.0% or less

[0031] having,

[0032] The thickness of the carbon concentration reduction portion in the surface layer of the above-mentioned lower steel plate is 1.0㎛ or more and 150.0㎛ or less, and

[0033] The thickness of the bainite precipitation region immediately below the carbon concentration reduction section is 10㎛ or more, and

[0034] Immediately below the above carbon concentration reduction section, the sum of the coverage rates of residual austenite and fresh martensite at the prior austenite grain boundary is 30.0% or more and 60.0% or less, and

[0035] Immediately below the above carbon concentration reduction portion, the carbon concentration in the residual austenite covering the prior austenite grain boundaries is less than 0.60 mass%, and

[0036] 1470 MPa or higher tensile strength

[0037] A zinc-plated steel sheet characterized by the following.

[0038] [2] A zinc-plated steel sheet as described in [1], having a yield strength of 1000 MPa or more.

[0039] [3] The above composition of components is additionally, in mass%,

[0040] Ti: 0.200% or less,

[0041] Nb: 0.200% or less,

[0042] V: 0.200% or less,

[0043] Ta: 0.10% or less,

[0044] W: 0.10% or less,

[0045] B: 0.0100% or less,

[0046] Cr: 1.00% or less,

[0047] Mo: 1.00% or less,

[0048] Ni: 1.00% or less,

[0049] Co: 0.010% or less,

[0050] Cu: 1.00% or less,

[0051] Sn: 0.200% or less,

[0052] Sb: 0.200% or less,

[0053] Ca: 0.0100% or less,

[0054] Mg: 0.0100% or less,

[0055] REM: 0.0100% or less,

[0056] Zr: 0.100% or less,

[0057] Zn: 0.100% or less,

[0058] Pb: 0.100% or less,

[0059] Te: 0.100% or less,

[0060] Se: 0.020% or less,

[0061] Ga: 0.020% or less,

[0062] Ge: 0.020% or less,

[0063] Sr: 0.020% or less,

[0064] Hf: 0.10% or less and,

[0065] Bi: 0.200% or less

[0066] A zinc-plated steel sheet described in [1] or [2] containing at least one type selected from the group consisting of

[0067] [4] A zinc-plated steel sheet described in any one of [1] to [3], wherein the zinc-plated layer is an electro-galvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer.

[0068] [5] Mass %,

[0069] C: 0.180% or more and 0.250% or less,

[0070] Si: 0.800% or more and 1.550% or less,

[0071] Mn: 2,400% or more, 3,200% or less,

[0072] P: 0.100% or less,

[0073] S: 0.0200% or less,

[0074] Al: 1.000% or less,

[0075] N: 0.0100% or less and,

[0076] O: 0.0100% or less

[0077] A process for obtaining a hot-rolled steel sheet by performing hot rolling on a steel slab having a compositional composition containing and the remainder consisting of Fe and unavoidable impurities, and

[0078] A process of obtaining a cold-rolled steel sheet by performing cold rolling on the above hot-rolled steel sheet, and

[0079] An annealing process in which the above cold-rolled steel sheet is maintained for at least 10 seconds at an annealing temperature of Ac3(°C) or higher, defined by the following equation (1), in an atmosphere with a dew point of -25°C or higher and 35°C or lower; and

[0080] Next, a first cooling process in which the above cold-rolled steel sheet is cooled to a temperature T1 of T-20(℃) or higher and T+20(℃) or lower at a cooling rate of 7℃ / s or higher using T defined by the following equation (2), and

[0081] Subsequently, a process of maintaining the cold-rolled steel sheet at T1(℃) for a time of 30s or more and 200s or less, and

[0082] Subsequently, a second cooling process for cooling the cold-rolled steel sheet to a temperature T2 of less than T-20(℃), and

[0083] Subsequently, a process of obtaining a plated steel sheet by performing a zinc-based plating treatment on the above cold-rolled steel sheet, and

[0084] Next, a third cooling process in which the plated steel sheet is cooled to a temperature T3 of Ms-200(℃) or higher and Ms-80(℃) or lower at a cooling rate of 5℃ / s or higher using Ms defined by the following equation (3), and

[0085] Subsequently, a fourth cooling process for cooling the plated steel sheet to a cooling stop temperature T4 of 100°C or higher and less than T3°C at a cooling rate of 3.0°C / s or less, and

[0086] Subsequently, a tempering process in which the plated steel sheet is maintained at a tempering temperature exceeding T4 (°C) and not exceeding 350°C for 10 seconds or more and 1000 seconds or less.

[0087] having,

[0088] A method for manufacturing a zinc-plated steel sheet, wherein between the zinc-plated treatment and the third cooling process, the total residence time of the galvanized steel sheet, in which the temperature of the galvanized steel sheet is in a temperature range of Bf (°C) or higher and T - 20 (°C) or lower, is 30 s or less, using Bf and T defined by the following equation (4).

[0089] Ac3 (℃) = 881 - 205.7

[0090] T (℃) = 743 - 90

[0091] Ms = 539 - 423

[0092] Bf(℃)=710-270×[%C]-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo]···(4)

[0093] Here, [%X] represents the content (mass%) of element X in the above component composition, and is set to 0 if the above component composition does not contain element X.

[0094] [6] A method for manufacturing a zinc-plated steel sheet as described in [5], wherein, in the above tempering process, at least 6 s after the start of holding at the tempering temperature and at least 10 s before the end of holding at the tempering temperature, a tension with an average load tension of 2.0 kgf / mm² or more is applied, and at least one bending and unbending is performed.

[0095] [7] The above composition of components is additionally, in mass %,

[0096] Ti: 0.200% or less,

[0097] Nb: 0.200% or less,

[0098] V: 0.200% or less,

[0099] Ta: 0.10% or less,

[0100] W: 0.10% or less,

[0101] B: 0.0100% or less,

[0102] Cr: 1.00% or less,

[0103] Mo: 1.00% or less,

[0104] Ni: 1.00% or less,

[0105] Co: 0.010% or less,

[0106] Cu: 1.00% or less,

[0107] Sn: 0.200% or less,

[0108] Sb: 0.200% or less,

[0109] Ca: 0.0100% or less,

[0110] Mg: 0.0100% or less,

[0111] REM: 0.0100% or less,

[0112] Zr: 0.100% or less,

[0113] Zn: 0.100% or less,

[0114] Pb: 0.100% or less,

[0115] Te: 0.100% or less,

[0116] Se: 0.020% or less,

[0117] Ga: 0.020% or less,

[0118] Ge: 0.020% or less,

[0119] Sr: 0.020% or less,

[0120] Hf: 0.10% or less and,

[0121] Bi: 0.200% or less

[0122] A method for manufacturing a zinc-plated steel sheet as described in [5] or [6], containing at least one type selected from the group consisting of

[0123] [8] A method for manufacturing a zinc-plated steel sheet as described in any one of [5] to [7], wherein the zinc-plating treatment is an electro-galvanizing treatment, a hot-dip galvanizing treatment, or a hot-dip galvanizing treatment followed by an alloying treatment. Effects of the invention

[0124] According to the present invention, a zinc-plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0% or more, a hole expansion rate (λ) of 25% or more, and excellent delayed fracture resistance of the elongated flange processing part, and a method for manufacturing the same can be provided. Brief explanation of the drawing

[0125] Figure 1 is a graph showing the relationship between temperature and time in a method for manufacturing an alloyed hot-dip galvanized steel sheet according to one embodiment of the present invention. Specific details for implementing the invention

[0126] (Form for carrying out the invention)

[0127] Hereinafter, embodiments of a zinc-plated steel sheet and a method for manufacturing the same according to the present invention will be described. Furthermore, the embodiments described below are examples of embodying the present invention, and the configuration of the present invention is not limited to these specific examples.

[0128] (Composition of the lower steel plate)

[0129] A zinc-plated steel sheet according to one embodiment of the present invention comprises a base steel sheet and a zinc-plated layer formed on the surface of the base steel sheet. The base steel sheet has a composition in mass% comprising C: 0.180% or more and 0.250% or less, Si: 0.800% or more and 1.550% or less, Mn: 2.400% or more and 3.200% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities. The basic components of the base steel sheet are described below. Furthermore, in the following description, "%" indicating the content of the constituent elements of the base steel sheet means "mass%" unless specifically stated otherwise.

[0130] [C: 0.180% or more, 0.250% or less]

[0131] Carbon is one of the important basic components of the base steel sheet, and in particular in the present invention, it is an important element that affects the sum of the area percentages of tempered martensite and fresh martensite and the area percentage of bainite. If the carbon content is less than 0.180%, the sum of the area percentages of tempered martensite and fresh martensite decreases, and the area percentage of bainite increases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the carbon content should be 0.180% or higher, preferably 0.200% or higher, and more preferably 0.210% or higher. On the other hand, if the carbon content exceeds 0.250%, the tempered martensite and fresh martensite become embrittled, and the delayed fracture resistance of the elongated flange processing section deteriorates. Therefore, the carbon content should be 0.250% or lower, preferably 0.240% or lower.

[0132] [Si: 0.800% or more, 1.550% or less]

[0133] Si is one of the important basic components of the underlayer steel sheet and is a critical element that affects the volume fraction of TS and retained austenite. When the Si content is less than 0.800%, the strength of tempered martensite and fresh martensite decreases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the Si content should be 0.800% or higher, preferably 0.850% or higher, and more preferably 0.900% or higher. On the other hand, if the Si content exceeds 1.550%, the retained austenite increases excessively, leading to a decrease in hole expandability. Therefore, the Si content should be 1.550% or lower, preferably 1.500% or lower, and more preferably 1.400% or lower.

[0134] [Mn: 2.400% or more, 3.200% or less]

[0135] Mn is one of the important basic components of the underlayer steel sheet and is a key element that affects the sum of the area percentages of tempered martensite and fresh martensite, as well as the area percentage of bainite. If the Mn content is less than 2.400%, the sum of the area percentages of tempered martensite and fresh martensite decreases, and the area percentage of bainite increases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the Mn content should be 2.400% or higher, preferably 2.500% or higher, and more preferably 2.600% or higher. On the other hand, if the Mn content exceeds 3.200%, the tempered martensite and fresh martensite become embrittled, and the resistance to delayed fracture in the elongated flange processing section deteriorates. Therefore, the Mn content should be 3.200% or lower, preferably 3.100% or lower, and more preferably 3.000% or lower.

[0136] [P: 0.100% or less]

[0137] Since P segregates at the austenite grain boundaries and embrittles the grain boundaries, it causes the underlay steel sheet to become embrittled; therefore, if the P content exceeds 0.100%, the resistance to delayed fracture of the elongated flange processing section deteriorates. Accordingly, the P content should be 0.100% or less, and preferably 0.070% or less. On the other hand, although a lower limit for the P content is not specifically specified, since P is a solid solution strengthening element and can increase the strength of the underlay steel sheet, it is preferable that the P content be 0.001% or more.

[0138] [S: 0.0200% or less]

[0139] Since S exists as a sulfide and embrittles the lower steel plate, if the S content exceeds 0.0200%, the resistance to delayed fracture of the stretch flange processing part deteriorates. Therefore, the S content should be 0.0200% or less, and preferably 0.0050% or less. Meanwhile, although a lower limit for the S content is not specifically specified, due to production technical constraints, it is preferable that the S content be 0.0001% or more.

[0140] [Al: 1.000% or less]

[0141] Since Al exists as an oxide and embrittles the underlayer steel sheet, if the Al content exceeds 1.000%, the resistance to delayed fracture in the stretch flange processing section deteriorates. Therefore, the Al content should be 1.000% or less, and preferably 0.500% or less. Meanwhile, although there is no specific lower limit for the Al content, since Al suppresses the formation of carbides during continuous annealing and promotes the formation of residual austenite, it is preferable that the Al content be 0.001% or more.

[0142] [N: 0.0100% or less]

[0143] Since N exists as a nitride and embrittles the lower steel plate, if the N content exceeds 0.0100%, the resistance to delayed fracture of the stretched flange processing section deteriorates. Therefore, the N content should be 0.0100% or less, and preferably 0.0050% or less. Meanwhile, although a lower limit for the N content is not specifically specified, due to production technical constraints, it is preferable that the N content be 0.0001% or more.

[0144] [O: 0.0100% or less]

[0145] Since O exists as an oxide and embrittles the underlayer steel plate, if the O content exceeds 0.0100%, the resistance to delayed fracture of the stretched flange processing section deteriorates. Therefore, the O content should be 0.0100% or less, and preferably 0.0050% or less. Meanwhile, although a lower limit for the O content is not specifically specified, due to production technical constraints, it is preferable that the O content be 0.0001% or more.

[0146] (Arbitrary components of the lower steel plate)

[0147] In addition to the aforementioned basic components, the underlayer steel plate further comprises, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% 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: 0.100% or less, Zn: 0.100% or less, Pb: 0.100% or less, Te: 0.100% or less, Se: It may contain at least one selected from the group consisting of 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[0148] [Ti: 0.200% or less]

[0149] [Nb: 0.200% or less]

[0150] [V : 0.200% or less]

[0151] If the content of Ti, Nb, and V is 0.200% or less, a large amount of coarse precipitates or inclusions are not generated, and the ultimate deformability of the base steel sheet is not reduced, so λ does not decrease and bendability does not decrease. Therefore, when one or more of Ti, Nb, and V are contained, their content should be 0.200% or less, and it is preferable that it be 0.100% or less. Meanwhile, although the lower limit of the content of Ti, Nb, and V is not specifically specified, since these elements increase the strength of the base steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, it is preferable that the content of Ti, Nb, and V be 0.001% or more each.

[0152] [Ta: 0.10% or less]

[0153] [W : 0.10% or less]

[0154] If the content of Ta and W is 0.10% or less each, large amounts of coarse precipitates or inclusions are not generated, and the underlayer steel sheet is not embrittled, so the delayed fracture resistance of the elongated flange processing section is not reduced. Therefore, when one or more of Ta and W are contained, their content should be 0.10% or less each, and it is preferable that it be 0.08% or less. Meanwhile, although the lower limit of the content of Ta and W is not specifically specified, since these elements increase the strength of the underlayer steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, it is preferable that the content of Ta and W be 0.01% or more each.

[0155] [B: 0.0100% or less]

[0156] If the content of B is 0.0100% or less, it does not create cracks inside the steel sheet during casting or hot rolling, and thus does not embrittle the lower steel sheet, so the delayed fracture resistance of the elongated flange processing part does not deteriorate. Therefore, when B is included, its content should be 0.0100% or less, and preferably 0.0080% or less. Meanwhile, although the lower limit of the B content is not specifically specified, since B is an element that segregates at the austenite grain boundaries during annealing and improves hardenability, it is preferable that the B content be 0.0003% or more.

[0157] [Cr: 1.00% or less]

[0158] [Mo: 1.00% or less]

[0159] [Ni: 1.00% or less]

[0160] If the content of Cr, Mo, and Ni is 1.00% or less, coarse precipitates or inclusions do not increase, and thus the delayed fracture resistance of the elongated flange processing section does not deteriorate, as this prevents embrittlement of the lower steel plate. Therefore, when one or more of Cr, Mo, and Ni are contained, their content should be 1.00% or less, and it is preferable that the content be 0.80% or less. Meanwhile, although the lower limit of the content of Cr, Mo, and Ni is not specifically specified, it is preferable that the content of Cr, Mo, and Ni be 0.01% or more each, as these elements improve quenchability.

[0161] [Co: 0.010% or less]

[0162] If the content of Co is 0.010% or less, coarse precipitates or inclusions do not increase and do not embrittle the lower steel plate, so the delayed fracture resistance of the stretch flange processing section does not deteriorate. Therefore, when Co is included, the content should be 0.010% or less, and preferably 0.008% or less. On the other hand, although a lower limit for the content of Co is not specifically specified, it is preferable that the content of Co be 0.001% or more, as Co improves quenchability.

[0163] [Cu: 1.00% or less]

[0164] If the content of Cu is 1.00% or less, coarse precipitates or inclusions do not increase, and thus the delayed fracture resistance of the elongated flange processing section does not deteriorate, as it does not embrittle the lower steel plate. Therefore, when Cu is included, its content should be 1.00% or less, and preferably 0.80% or less. On the other hand, although a lower limit for the content of Cu is not specifically specified, it is preferable that the content of Cu be 0.01% or more, as Cu improves quenchability.

[0165] [Sn: 0.200% or less]

[0166] If the Sn content is 0.200% or less, it does not create cracks inside the steel sheet during casting or hot rolling, and thus does not embrittle the lower steel sheet, so the delayed fracture resistance of the elongated flange processing part does not deteriorate. Therefore, when Sn is included, its content should be 0.200% or less, and preferably 0.100% or less. On the other hand, although a lower limit for the Sn content is not specifically specified, it is preferable that the Sn content be 0.001% or more, as Sn improves quenchability.

[0167] [Sb: 0.200% or less]

[0168] If the Sb content is 0.200% or less, coarse precipitates or inclusions do not increase, and thus the delayed fracture resistance of the elongated flange processing section does not deteriorate, as it does not embrittle the lower steel plate. Therefore, when Sb is included, its content should be 0.200% or less, and preferably 0.100% or less. Meanwhile, although the lower limit of the Sb content is not specifically specified, it is preferable that the Sb content be 0.001% or more, as Sb enables strength adjustment by controlling the surface softening thickness.

[0169] [Ca : 0.0100% or less]

[0170] [Mg : 0.0100% or less]

[0171] [REM: 0.0100% or less]

[0172] If the content of Ca, Mg, and REM is 0.0100% or less, coarse precipitates or inclusions do not increase, and thus the underlying steel sheet is not embrittled, which is why the delayed fracture resistance of the elongated flange processing section does not deteriorate. Therefore, when one or more of Ca, Mg, and REM are contained, their content should be 0.0100% or less, and it is preferable that it be 0.0050% or less. Meanwhile, although the lower limit of the content of Ca, Mg, and REM is not specifically specified, since these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformation capacity of the underlying steel sheet, it is preferable that the content of Ca, Mg, and REM be 0.0005% or more.

[0173] [Zr: 0.100% or less]

[0174] [Zn: 0.100% or less]

[0175] [Pb: 0.100% or less]

[0176] [Te: 0.100% or less]

[0177] If the content of Zr, Zn, Pb, and Te is 0.100% or less, coarse precipitates or inclusions do not increase, and thus the delayed fracture resistance of the elongated flange processing section does not deteriorate, as this prevents embrittlement of the underlayment steel sheet. Therefore, when one or more of Zr, Zn, Pb, and Te are contained, their content should be 0.100% or less, and it is preferable that it be 0.080% or less. Meanwhile, although the lower limit of the content of Zr, Zn, Pb, and Te is not specifically specified, it is preferable that the content of Zr, Zn, Pb, and Te be 0.001% or more each, as these elements spheroidize the shape of nitrides or sulfides, thereby improving the ultimate deformation capacity of the underlayment steel sheet.

[0178] [Se: 0.020% or less]

[0179] [Ga: 0.020% or less]

[0180] [Ge: 0.020% or less]

[0181] [Sr: 0.020% or less]

[0182] If the content of Se, Ga, Ge, and Sr is 0.020% or less, coarse precipitates or inclusions do not increase, and thus the delayed fracture resistance of the elongated flange processing section does not deteriorate, as this prevents embrittlement of the underlayment steel sheet. Therefore, if one or more of Se, Ga, Ge, and Sr are contained, their content shall be 0.020% or less. Meanwhile, although the lower limit of the content of Se, Ga, Ge, and Sr is not specifically specified, it is preferable that the content of Se, Ga, Ge, and Sr be 0.001% or more, as these elements spheroidize the shape of nitrides or sulfides, thereby improving the ultimate deformation capacity of the underlayment steel sheet.

[0183] [Hf: 0.10% or less]

[0184] If the Hf content is 0.10% or less, coarse precipitates or inclusions do not increase, and thus the delayed fracture resistance of the elongated flange processing section does not deteriorate, as it does not embrittle the underlayment steel sheet. Therefore, when Hf is included, its content should be 0.10% or less, and preferably 0.08% or less. Meanwhile, although the lower limit of the Hf content is not specifically specified, it is preferable that the Hf content be 0.01% or more, as Hf spheroidizes the shape of nitrides or sulfides, thereby improving the ultimate deformation capacity of the underlayment steel sheet.

[0185] [Bi: 0.200% or less]

[0186] If the Bi content is 0.200% or less, coarse precipitates or inclusions do not increase, and thus the delayed fracture resistance of the elongated flange processing section does not deteriorate, as it does not embrittle the lower steel plate. Therefore, when Bi is included, its content should be 0.200% or less, and preferably 0.100% or less. On the other hand, although a lower limit for the Bi content is not specifically specified, it is preferable that the Bi content be 0.001% or more, as Bi reduces segregation.

[0187] In addition, regarding the above Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Zn, Pb, Te, Se, Ga, Ge, Sr, Hf, and Bi, if the content of each is below their respective preferred lower limit values, it does not impair the effects of the present invention. Therefore, each content may be included below their respective preferred lower limit values, and in such cases, they are treated as unavoidable impurities. A steel plate under a substrate according to one embodiment of the present invention contains the above basic components, and the remainder consists of Fe (iron) and unavoidable impurities. Here, it is preferable that the steel plate under a substrate according to one embodiment of the present invention contains only the above basic components and the remainder, and the remainder consists of Fe (iron) and unavoidable impurities.

[0188] (Tissue of the lower steel plate)

[0189] Next, the structure of the underlayment steel plate is described. The underlayment steel plate has a structure in which, at a depth from the surface of the underlayment steel plate at 1 / 4 of the plate thickness, the sum of the area percentages of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less, the volume percentage of retained austenite is 6.0% or more and 15.0% or less, the area percentage of bainite is 10.0% or less, and the area percentage of the remainder structure is 10.0% or less. In addition, the depth from the surface of the lower steel plate is at a position that is 1 / 4 of the plate thickness, the thickness of the carbon concentration reduction portion in the surface layer of the lower steel plate is 1.0 μm or more and 150.0 μm or less, the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion is 10 μm or more, the sum of the coverage rates of the retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction portion is 30.0% or more and 60.0% or less, and the carbon concentration in the retained austenite covering the prior austenite grain boundary immediately below the carbon concentration reduction portion is less than 0.60 mass%.

[0190] [The sum of the area ratios of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less]

[0191] By making the total area ratio of tempered martensite and fresh martensite 70.0% or more, it becomes possible to achieve a TS of 1470 MPa or more. On the other hand, if the total area ratio of tempered martensite and fresh martensite exceeds 94.0%, it becomes difficult to achieve excellent ductility. Therefore, the total area ratio of tempered martensite and fresh martensite is set to be 70.0% or more and 94.0% or less.

[0192] In particular, it is desirable that the area ratio of tempered martensite be 80.0% or higher, as this allows for a suitable TS to be obtained. Meanwhile, there is no specific upper limit on the area ratio of tempered martensite, but the area ratio of tempered martensite is generally 94.0% or lower. Additionally, it is desirable that the area ratio of fresh martensite be 10.0% or lower, as this allows for a suitable λ to be obtained. Meanwhile, there is no specific lower limit on the area ratio of fresh martensite, but the area ratio of fresh martensite is generally 1.0% or higher.

[0193] The sum of the area percentages of tempered martensite and fresh martensite can be calculated as follows. After polishing the L cross-section of the base steel plate, it is etched with 3 vol.% Nital, and a position at 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) is observed in 10 fields using an SEM at a magnification of 3000x. In the observed image, the area percentage of fresh martensite can be calculated by subtracting the volume percentage of retained austenite, calculated by the method described below, from the area percentage of the microstructure having a smooth surface. Additionally, fresh martensite is a convex portion having a width of 50 nm or more. In this way, the area percentage of tempered martensite can be calculated from the microstructure image observed. Tempered martensite is a structure having a substructure (lath boundaries, block boundaries) and also a structure in which carbides precipitate with multiple variants. In addition, since the volume fraction of retained austenite is almost equal to the area fraction, it is treated as equivalent to the area fraction in the present invention.

[0194] [Volume fraction of retained austenite is 6.0% or more and 15.0% or less]

[0195] When the volume fraction of retained austenite is less than 6.0%, it becomes difficult to achieve excellent ductility. Therefore, the volume fraction of retained austenite is set to 6.0% or more, preferably 6.5% or more, and more preferably 7.0% or more. On the other hand, when the volume fraction of retained austenite exceeds 15.0%, it becomes difficult to achieve excellent hole expandability. Therefore, the volume fraction of retained austenite is set to 15.0% or less, preferably 13.0% or less, and more preferably 11.0% or less.

[0196] The volume fraction of retained austenite can be obtained as follows. The underlayer steel plate is polished to a position 0.1 mm thicker than the position at 1 / 4 of the plate thickness. For a plane at the position at 1 / 4 of the plate thickness, which is further polished by 0.1 mm by chemical polishing, the ratio of the integrated intensity of the diffraction peaks of the {200}, {220}, {311} planes of fcc iron and the {200}, {211}, {220} planes of bcc iron is measured using CoKα rays with an X-ray diffraction device. By averaging the nine obtained integrated intensity ratios, the volume fraction of retained austenite can be obtained.

[0197] [Area ratio of bainite is 10.0% or less]

[0198] If the area ratio of bainite exceeds 10.0%, it becomes difficult to achieve a TS of 1470 MPa or higher. Therefore, the area ratio of bainite is set to 10.0% or less. Meanwhile, the lower limit of the area ratio of bainite is not specifically limited, and the area ratio of bainite may be 0.0%.

[0199] The area percentage of bainite can be determined as follows. After polishing the L cross-section of the base steel plate, it is etched with 3 vol.% Nital, and a position at 1 / 4 of the plate thickness is observed in 10 fields of view using an SEM at a magnification of 3000x. In the observed microstructure image, bainite is a concave structure with a flat interior. The area percentage of bainite is calculated for each field of view, and the average of these values ​​is taken as the area percentage of bainite.

[0200] [Area ratio of residual tissue is 10.0% or less]

[0201] The steel structure of the present invention may include carbides such as pearlite and cementite, or other structures known as steel sheet structures, as the remainder structure. If the area ratio of the remainder structure is 10.0% or less, the effects of the present invention are not compromised. Therefore, the area ratio of the remainder structure is set to 10.0% or less. Meanwhile, the lower limit of the area ratio of the remainder structure is not particularly limited, and the area ratio of the remainder structure may be 0.0%.

[0202] The area percentage of the residual structure can be determined as follows. After polishing the L cross-section of the base steel plate, it is etched with 3 vol.% Nital, and a position at 1 / 4 of the plate thickness is observed in 10 fields using an SEM at a magnification of 3000x. In the observed microstructure image, the area percentage of the residual structure is defined as the value obtained by subtracting the area percentages of tempered martensite, fresh martensite, ferrite, bainitic ferrite, and retained austenite from 100.0%. The area percentage of the residual structure is calculated for each field, and the average value of these values ​​is taken as the area percentage of the residual structure.

[0203] [Thickness of the carbon concentration reduction portion in the surface layer is 1.0㎛ or more and 150.0㎛ or less]

[0204] If the thickness of the carbon concentration reduction portion in the surface layer is less than 1.0 μm, it is not possible to form a region with a high bainite precipitation initiation temperature in the surface layer, and since the bainite area ratio or the thickness of the bainite precipitation region described later cannot be controlled, it becomes difficult to realize excellent delayed fracture resistance characteristics in the elongated flange processing portion. Therefore, the thickness of the carbon concentration reduction portion in the surface layer should be 1.0 μm or more, and preferably 3.0 μm or more. On the other hand, if the thickness of the carbon concentration reduction portion in the surface layer exceeds 150.0 μm, it becomes difficult to realize a TS of 1470 MPa or higher. Therefore, the thickness of the carbon concentration reduction portion in the surface layer should be 150.0 μm or less, and preferably 50.0 μm or less.

[0205] The thickness of the carbon concentration reduction zone in the surface layer can be determined as follows. A test specimen with a length of 20 mm and a width of 20 mm is taken from a zinc-plated steel sheet. The carbon intensity of the test specimen is measured in the depth direction from the surface of the steel sheet by Glow Discharge Spectroscopy (GDS). From the measurement results, the region where the carbon intensity is less than or equal to (C intensity when the GDS measurement value becomes a constant value × 0.8) is defined as the carbon concentration reduction zone in the surface layer, and the thickness of said region is defined as the thickness of the carbon concentration reduction zone. Additionally, the GDS device is a GD-PROFILER 2 manufactured by Horiba Seisakusho, and the measurement conditions can be determined as follows.

[0206] · Power supply: High frequency

[0207] · Anode diameter: 4 mm

[0208] · Measurement Mode: Constant Power 35W

[0209] · Carrier gas: Argon

[0210] · Discharge section gas pressure: 300 Pa

[0211] · Pulse frequency: 100 Hz

[0212] ·Duty cycle: 50%

[0213] · Injection interval: 100 ms

[0214] [The thickness of the bainite precipitation region immediately below the carbon concentration reduction region is 10㎛ or more]

[0215] If the thickness of the bainite precipitation region immediately below the carbon concentration reduction section is less than 10 μm, it becomes difficult to achieve excellent delayed fracture resistance characteristics in the stretch flange processing section. Therefore, the thickness of the bainite precipitation region immediately below the carbon concentration reduction section is 10 μm or more. Meanwhile, the upper limit of the thickness of the bainite precipitation region immediately below the carbon concentration reduction section is not particularly limited, and the thickness is generally 100 μm or less.

[0216] The thickness of the bainite precipitation region immediately below the carbon concentration reduction zone can be determined as follows. After polishing the L cross-section of a zinc-plated steel sheet, it is etched with 3 vol.% Nital, and the aforementioned GDS analysis is performed on the steel sheet surface using SEM. The region following the carbon concentration reduction zone identified by GDS analysis, and the region where bainite precipitation is confirmed, is observed in 10 fields of view at 3000x magnification. In each observed field of view, the bainite is a concave structure with a flat interior. Furthermore, if bainite does not appear in a single field of view, photographs of consecutive fields of view are taken. The area percentage of bainite is calculated for each field of view, and the region where the bainite area percentage is 20% or more and 60% or less is defined as the bainite precipitation region. The thickness of the bainite precipitation region in the plate thickness direction is calculated for each field of view, and the average value is taken as the thickness of the bainite precipitation region.

[0217] [Immediately below the carbon concentration reduction section, the sum of the coverage rates of retained austenite and fresh martensite at the prior austenite grain boundaries is 30.0% or more and 60.0% or less]

[0218] By making the sum of the coverage rates of retained austenite and fresh martensite at the prior austenite grain boundaries immediately below the carbon concentration reduction section 30.0% or more, a TS of 1470 MPa or more can be realized. Therefore, immediately below the carbon concentration reduction section, the sum of the coverage rates of retained austenite and fresh martensite at the prior austenite grain boundaries is 30.0% or more, and 40.0% or more is preferable. On the other hand, if the sum of the coverage rates of retained austenite and fresh martensite at the prior austenite grain boundaries immediately below the carbon concentration reduction section exceeds 60.0%, it becomes difficult to realize hole expandability among excellent formability. Therefore, immediately below the carbon concentration reduction section, the sum of the coverage rates of retained austenite and fresh martensite at the prior austenite grain boundaries is 60.0% or less, and 55.0% or less is preferable.

[0219] The sum of the coverage rates of retained austenite and fresh martensite at the prior austenite grain boundaries immediately below the carbon concentration reduction zone can be calculated as follows. After polishing the L cross-section of a zinc-plated steel sheet, it is etched with 3 vol.% Nital, and the aforementioned GDS analysis is performed on the steel sheet surface using SEM. Ten prior austenite grain boundaries adjacent to the carbon concentration reduction zone, which is the region following the carbon concentration reduction zone identified by GDS analysis, are randomly selected. On the selected prior austenite grain boundaries, a convex portion having a smooth surface and a width of 50 nm or more in a direction orthogonal to the prior austenite grain boundaries is defined as the area covered by retained austenite and fresh martensite. The sum of the perimeter lengths of the areas covered by retained austenite and fresh martensite is divided by the perimeter length of the selected prior austenite grain boundaries. The average value of the values ​​obtained at each location was calculated and taken as the sum of the coverage rates of retained austenite and fresh martensite at the prior austenite grain boundaries.

[0220] [Immediately below the carbon concentration reduction section, the carbon concentration in the residual austenite covering the prior austenite grain boundaries is less than 0.60 mass%]

[0221] If the carbon concentration in the residual austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction section is 0.60 mass% or more, a second phase with a hardness significantly different from that of the matrix phase exists, and thus, during processing, the second phase becomes a stress concentration zone, deteriorating the delayed fracture resistance. Therefore, the carbon concentration in the residual austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction section is preferably less than 0.60 mass% and 0.55 mass% or less. On the other hand, the lower limit of the carbon concentration in the residual austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction section is not particularly limited, and the carbon concentration is generally 0.30 mass% or more.

[0222] The carbon concentration in the residual austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction zone can be determined as follows. First, the surface of the steel plate (sample) is polished using diamond paste so that a cross-section (L cross-section) parallel to the rolling direction becomes the observation surface, and is further finished to a mirror surface by alumina polishing. Next, the observation surface is cleaned using a plasma cleaner to eliminate hydrocarbon contamination (carbon contamination) on the sample surface. The cleaned observation surface is measured using an electron beam micro analyzer (FE-EPMA) equipped with a field emission electron gun. As described in the non-patent literature (T. Yamashita, Y. Tanaka, M. Nagoshi and K. Ishida: Sci. Rep., 6(2016), DOI: 10.1038 / srep29825.), the measurement conditions are set with an acceleration voltage of 7 kV and a current of 50 nA. In addition, the sample is heated and maintained at 100°C to prevent contamination from occurring on the sample surface. From the measurement results, the carbon concentration is determined by a calibration method to obtain an elemental mapping image of carbon. In the obtained elemental mapping image, a region (high carbon region) in which the carbon concentration is greater than or equal to the average of the matrix phase and is less than 0.6 mass% is identified. In addition, by specifying the carbon concentration reduction area of ​​the observation surface using the method described above, and by referring to the elemental mapping image and the SEM image in the same field of view, the austenite grain located immediately below the carbon concentration reduction area is identified, and the high-carbon region existing on the grain boundary of the austenite grain is specified. Then, the length a of the perimeter of the austenite grain and the length b of the portion of the perimeter of the austenite grain that overlaps with the high-carbon region are determined, and their ratio b / a is calculated.For each steel plate, the above measurement is performed 30 times. The average value of the 30 measurements is taken as the carbon concentration in the residual austenite covering the austenite grain boundary immediately below the carbon concentration reduction portion of the steel plate.

[0223] Furthermore, fresh martensite is formed by the transformation of untransformed austenite during final cooling. Consequently, the carbon concentration in the fresh martensite covering the grain boundaries of the old austenite becomes equal to or less than the carbon concentration in the retained austenite.

[0224] (Zinc-based plating layer)

[0225] The zinc-based plating layer of the zinc-plated steel sheet is preferably an electro-galvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer. Specifically, it may be a Zn-Ni alloy plating layer, a zinc-aluminum-magnesium alloy plating layer, etc. In addition, the plating adhesion amount is preferably 20 to 80 g / m² per side (double-sided plating).

[0226] (Mechanical properties of zinc-plated steel sheets)

[0227] Next, the mechanical properties of zinc-plated steel sheets will be explained.

[0228] [Tensile strength (TS) of 1470 MPa or higher]

[0229] Zinc-plated steel sheets shall have a tensile strength (TS) of 1470 MPa or higher. Meanwhile, although the upper limit of the tensile strength of zinc-plated steel sheets is not specifically limited, it is generally 1650 MPa or lower. Tensile strength can be determined as follows. A JIS No. 5 test specimen (target point distance 50 mm, parallel section width 25 mm) is taken from the test material such that the direction perpendicular to the rolling direction is the longitudinal direction of the test specimen, and a tensile test is performed in accordance with JIS Z 2241. The conditions for the tensile test are a crosshead speed of 1.67 × 10⁻¹⁰ －1 It can be done in mm / second.

[0230] [Yield strength (YS) of 1000 MPa or higher (suitable condition)]

[0231] It is preferable that the yield strength of the zinc-plated steel sheet be 1000 MPa or higher. Meanwhile, it is preferable that the yield strength of the zinc-plated steel sheet be 1350 MPa or lower. In addition, the yield strength can be determined by a tensile test, just like the tensile strength.

[0232] [Height (El) is 9.0% or higher]

[0233] Zinc-plated steel sheets shall have an elongation (El) of 9.0% or more. Meanwhile, the upper limit of the elongation of zinc-plated steel sheets is not specifically limited, but generally it is 15.0% or less. In addition, elongation (El) can be obtained by a tensile test, just like tensile strength.

[0234] [Hole expansion rate (λ) is 25% or more]

[0235] The zinc-plated steel sheet is to have a hole expansion rate (λ) of 25% or more. Meanwhile, the upper limit of the hole expansion rate of the zinc-plated steel sheet is not specifically limited, but generally it is 50% or less. The hole expansion rate (λ) can be calculated as follows. A hole expansion test is performed in accordance with JIS Z 2256. After shearing the test specimen to a size of 100 mm × 100 mm, a hole with a diameter of 10 mm is punched with a clearance of 12.5%. Using a die with an inner diameter of 75 mm, the test specimen is pressed with a blank holding force of 9 ton (88.26 kN), and a conical punch with an apex angle of 60° is pushed into the punched hole to check for the occurrence of a crack penetrating the thickness of the sheet. By measuring the hole diameter when a crack occurs, the limit hole expansion rate: λ (%) can be calculated from the following equation (5).

[0236] λ(%)={(D f -D0) / D0}×100 ···(5)

[0237] Here, D fθ is the hole diameter at the time of crack occurrence (mm), and D0 is the initial hole diameter (mm).

[0238] In addition, for galvanized steel sheets with a TS of approximately 1470 MPa, hydrogen is incorporated into the steel sheet during the manufacturing process. Furthermore, it is known that diffusible hydrogen in the steel sheet causes deterioration with an advantage in the hole expansion rate. For this reason, when galvanized steel sheets are shipped as products, diffusible hydrogen is reduced by allowing the steel sheet to sit for a long period or by performing post-heating. Therefore, it is desirable to conduct the hole expansion test after the amount of diffusible hydrogen in the steel becomes 0.01 wt.% or less.

[0239] [Excellent delayed fracture resistance of the elongated flange processing section]

[0240] The zinc-plated steel sheet according to the present invention has excellent delayed fracture resistance characteristics of the stretched flange processing section. The delayed fracture resistance characteristics of the stretched flange processing section can be evaluated as follows. The aforementioned hole expansion test is performed within 30 days of manufacturing the zinc-plated steel sheet (test material). Immediately after the hole expansion test, a photograph of the stretched flange processing section of the test material is taken at a magnification of 20x using a digital microscope (RH-2000: manufactured by Hyrox). Afterward, the test material is left at room temperature (15–25°C) for 24 hours, and the stretched flange processing section is observed again using a digital microscope. By comparing the photograph of the stretched flange processing section taken immediately after the hole expansion test with the photograph of the stretched flange processing section after 24 hours, if no increase or progression of cracks is confirmed, it can be determined that the delayed fracture resistance characteristics of the stretched flange processing section are excellent.

[0241] (Method for manufacturing zinc-plated steel sheets)

[0242] Next, a method for manufacturing a zinc-plated steel sheet according to an embodiment of the present invention will be described. FIG. 1 shows a graph representing the relationship between temperature and time in a method for manufacturing an alloyed hot-dip galvanized steel sheet according to an embodiment of the present invention. The broken line of the graph indicates the temperature change of the steel sheet between the annealing process and the tempering process. After obtaining a cold-rolled steel sheet by performing hot rolling and cold rolling on a steel slab having the aforementioned compositional composition, the cold-rolled steel sheet is heated, and an annealing process is performed in which it is maintained at an annealing temperature of Ac3(°C) or higher under an atmosphere with a dew point of -25°C or higher and 35°C or lower. Subsequently, a first cooling process is performed to cool the cold-rolled steel sheet to a temperature T1. Subsequently, a holding process is performed in which the cold-rolled steel sheet is maintained at T1(°C) for a time of 30s or more and 200s or less. Subsequently, a second cooling process is performed to cool the cold-rolled steel sheet to a temperature T2. Next, a zinc-based plating treatment is performed on a cold-rolled steel sheet to obtain a plated steel sheet. In addition, since the time for the hot-dip galvanizing treatment is shorter compared to the cooling process, it is indicated as a point at temperature T2 in FIG. 1. In the alloying treatment after the hot-dip galvanizing treatment, the plated steel sheet is heated and maintained at the temperature of the alloying treatment. Next, a third cooling process is performed to cool the plated steel sheet to a temperature T3. Next, a fourth cooling process is performed to cool the plated steel sheet to a cooling stop temperature T4. Next, a tempering process is performed to maintain the plated steel sheet at a tempering temperature greater than T4 (°C) and less than or equal to 350°C. In addition, in the manufacturing method, the total residence time during which the temperature of the plated steel sheet is in the temperature range of Bf (°C) or higher and T-20 (°C) or lower between the plating treatment process and the third cooling process is 30s or less. In addition, in the tempering process, at least 6 seconds after the start of holding at the tempering temperature, and at least 10 seconds before the end of holding at the tempering temperature, the average load tension is 2.It is desirable to apply a tension of 0 kgf / mm² or more, and also to perform one or more bending and bending returns.

[0243] [Steel Slab]

[0244] The steel slab used in the method for manufacturing a zinc-plated steel sheet has a composition in which it contains C, Si, Mn, P, S, Al, N, and O, with the remainder consisting of Fe and unavoidable impurities. Additionally, it is preferable to contain at least one element selected from the group consisting of Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Zn, Pb, Te, Se, Ga, Ge, Sr, Hf, and Bi. Furthermore, the content of each element is as described above.

[0245] [Hot Rolling Process]

[0246] In the present invention, the method of melting the steel slab is not particularly limited, and any known melting method, such as a converter or an electric furnace, is suitable. It is preferable to manufacture the steel slab by a continuous casting method to prevent macro-segregation. Hot rolling is performed on the obtained steel slab to obtain a hot-rolled steel sheet. Cold rolling may be performed on the obtained hot-rolled steel sheet as is, or cold rolling may be performed after acid cleaning treatment on the hot-rolled steel sheet. It is preferable to perform acid cleaning treatment because oxides on the surface of the steel sheet can be removed, and the plating quality of the final product steel sheet is obtained suitably. Furthermore, the acid cleaning treatment may be performed once or divided into multiple times.

[0247] [Cold Rolling Process]

[0248] The obtained hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The reduction ratio during cold rolling and the thickness of the sheet after rolling are not particularly limited. Furthermore, the number of rolling passes and the reduction ratio of each pass are not particularly limited.

[0249] [Annealing Process: Maintain at an annealing temperature of Ac3(°C) or higher for at least 10 seconds in an atmosphere with a dew point between -25°C and 35°C]

[0250] The obtained cold-rolled steel sheet is provided to an annealing process. In the annealing process, if the dew point during annealing is less than -25°C, the thickness of the carbon concentration reduction portion of the surface layer formed during annealing becomes less than 1.0 μm, and a region with a high bainite precipitation initiation temperature cannot be sufficiently formed on the surface layer, making it impossible to achieve a bainite precipitation region thickness of 10 μm or more immediately below the carbon concentration reduction portion. As a result, it becomes difficult to achieve excellent delayed fracture resistance characteristics in the elongated flange processing portion. Therefore, the dew point during annealing is set to -25°C or higher, and -20°C or higher is preferable. On the other hand, if the dew point during annealing exceeds 35°C, the thickness of the carbon concentration reduction portion of the surface layer formed during annealing exceeds 150.0 μm, and it becomes difficult to achieve a TS of 1470 MPa or higher. Therefore, the dew point during annealing is set to 35°C or lower.

[0251] In the annealing process, if the annealing temperature is less than Ac3(°C) defined by the following equation (1), the area ratio of tempered martensite and fresh martensite decreases, and the area ratio of bainite increases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the annealing temperature should be Ac3(°C) or higher, and it is preferable to have an annealing temperature of Ac3+20(°C) or higher. On the other hand, if the annealing temperature is 920°C or lower, energy efficiency does not decrease, so it is appropriate to prevent an increase in heating costs and also appropriately prevent damage to the furnace body. Therefore, the annealing temperature should be 920°C or lower.

[0252] Ac3 (℃) = 881 - 205.7

[0253] Here, [%X] represents the content (mass%) of element X in the above component composition, and is set to 0 if the above component composition does not contain element X.

[0254] In the annealing process, if the holding time at the annealing temperature is less than 10 seconds, the area ratio of tempered martensite and fresh martensite decreases, and the area ratio of bainite increases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the holding time at the annealing temperature should be 10 seconds or more, and preferably 40 seconds or more. On the other hand, if the holding time at the annealing temperature is 500 seconds or less, it is possible to suitably prevent an increase in heating costs and a prolonged manufacturing time, that is, to suitably prevent a decrease in productivity. Therefore, the holding time at the annealing temperature is preferably 500 seconds or less.

[0255] [First cooling process: Cooling from a temperature T-20(℃) or higher to T+20(℃) or lower to T1, at a cooling rate of 7℃ / s or higher]

[0256] Following the annealing process, the cold-rolled steel sheet is provided to the first cooling process. That is, the end time of the annealing process becomes the start time of the first cooling process. In the first cooling process, if the cooling rate is less than 7°C / s, the area ratio of tempered martensite and fresh martensite decreases, and the area ratio of bainite increases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the cooling rate of the first cooling process should be 7°C / s or higher, and 9°C / s or higher is preferable. On the other hand, if the first cooling rate is 20°C / s or lower, the cooling stop temperature can be appropriately controlled. Therefore, the cooling rate of the first cooling process is 20°C / s or lower.

[0257] If the cooling stop temperature T1 of the first cooling process is less than T-20(°C) using T defined by the following equation (2), bainite is formed in areas other than immediately below the carbon concentration reduction area of ​​the surface layer, and the area ratio of bainite increases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, T1 is set to T-20(°C) or higher. On the other hand, if T1 exceeds T+20(°C), a bainite precipitation region is not formed immediately below the carbon concentration reduction area of ​​the surface layer, making it difficult to achieve excellent delayed fracture resistance characteristics in the elongation flange processing area. Furthermore, immediately below the carbon concentration reduction area, the sum of the coverage ratio of residual austenite and fresh martensite at the prior austenite grain boundary becomes excessive, making it difficult to achieve a hole expansion rate of 25% or higher. Therefore, T1 is set to T+20(°C) or lower.

[0258] T (℃) = 743 - 90

[0259] Here, [%X] represents the content (mass%) of element X in the above component composition, and is set to 0 if the above component composition does not contain element X.

[0260] [Maintenance Process: Maintain at T1(℃) for a time of 30s to 200s]

[0261] Following the first cooling process, the cold-rolled steel sheet is subjected to a holding process in which it is maintained at T1 (°C). If the holding time in the holding process is less than 30 s, the thickness of the bainite precipitation region immediately below the carbon concentration reduction section becomes less than 10 μm, making it difficult to achieve excellent delayed fracture resistance characteristics in the elongated flange processing section. Furthermore, immediately below the carbon concentration reduction section, the sum of the coverage rates of residual austenite and fresh martensite at the prior austenite grain boundaries becomes excessive, making it difficult to achieve a hole expansion rate of 25% or more. Therefore, the holding time for the intermediate holding process is 30 s or more, and 40 s or more is preferable. On the other hand, if the intermediate holding time exceeds 200s, the grain growth of bainite precipitated at the grain boundaries of the prior austenite immediately below the carbon concentration reduction region proceeds excessively, and the total coverage rate of the retained austenite and fresh martensite at the prior austenite grain boundaries becomes less than 30%, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the intermediate holding time should be 200s or less, and 180s or less is preferable.

[0262] [Second cooling process: Cooling to a temperature T2 below T-20(℃)]

[0263] Following the holding process, the cold-rolled steel sheet is provided to the second cooling process. Accordingly, the cooling stop temperature T2 of the second cooling process is less than T1 (°C). If the cooling stop temperature T2 of the second cooling process is T-20 (°C) or higher, the grain growth of bainite precipitated at the grain boundaries of the prior austenite immediately below the carbon concentration reduction section proceeds excessively, and the total coverage rate of the residual austenite and fresh martensite at the grain boundaries of the prior austenite becomes less than 30%, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the second cooling stop temperature T2 is set to be less than T-20 (°C), and preferably T-30 (°C) or lower. On the other hand, if the second cooling stop temperature T2 is Ms (°C) or higher as described later, when the alloying treatment described later is performed, the precipitation of martensite before alloying is appropriately prevented, and since excessive tempering does not occur during alloying, a suitable TS is obtained. Therefore, the second cooling stop temperature T2 is preferably Ms(°C) or higher.

[0264] If the cooling rate of the second cooling process is 1°C / s or higher, the nucleation and growth of bainite during cooling are suitably prevented from proceeding excessively, thereby suitably obtaining the sum of the coverage rates of residual austenite and fresh martensite at the prior austenite grain boundaries, and suitably obtaining TS. Therefore, the cooling rate of the second cooling process is preferably 1°C / s or higher, and more preferably 2°C / s or higher. On the other hand, if the cooling rate of the second cooling process is 20°C / s or lower, the cooling stop temperature can be suitably controlled. Therefore, the cooling rate of the second cooling process is preferably 20°C / s or lower.

[0265] [Plating Process]

[0266] Following the second cooling process, a zinc-based plating treatment is performed on the cold-rolled steel sheet to obtain a plated steel sheet. Examples of zinc-based plating treatments include electro-galvanizing, hot-dip galvanizing, or hot-dip galvanizing followed by alloying treatment. Electroplating such as Zn-Ni electro-alloy plating may be performed, or hot-dip zinc-aluminum-magnesium alloy plating may be performed.

[0267] When performing hot-dip galvanizing treatment on cold-rolled steel sheets, it is preferable to immerse the cold-rolled steel sheets in a galvanizing bath at a temperature of 440°C or higher and 500°C or lower to perform the hot-dip galvanizing treatment, and then adjust the plating adhesion amount by gas wiping, etc. In addition, it is preferable to adjust the plating adhesion amount to 20 to 80 g / m² per side (double-sided plating). For the hot-dip galvanizing treatment, it is preferable to use a galvanizing bath having an Al content of 0.10 mass% or higher and 0.23 mass% or lower. For the hot-dip galvanizing treatment, it is preferable to perform the treatment by keeping the cold-rolled steel sheets in a temperature range of Ms(°C) or higher and 700°C or lower, as defined by the following equation (3).

[0268] Ms = 539 - 423

[0269] Here, [%X] represents the content (mass%) of element X in the above component composition, and is set to 0 if the above component composition does not contain element X.

[0270] When performing a hot-dip galvanizing treatment followed by an alloying treatment, setting the alloying temperature to 470°C or higher prevents the Zn-Fe alloying rate from becoming excessively slow, thereby ensuring suitable productivity. Therefore, an alloying temperature of 470°C or higher is preferable. On the other hand, setting the alloying temperature to 600°C or lower appropriately prevents the untransformed austenite from transforming into pearlite and lowering the TS. Therefore, an alloying temperature of 600°C or lower is preferable, and 560°C or lower is more preferable. In addition, for the alloyed hot-dip galvanized steel sheet (GA), it is preferable to set the Fe concentration in the plating layer to 7 to 15 mass% by performing the alloying treatment.

[0271] In addition, while the series of processes from the annealing process to the plating process is not particularly limited, from the perspective of productivity, it is preferable to perform them using a Continuous Galvanizing Line (CGL).

[0272] [Third cooling process: Cooling to a temperature T3 between Ms-200(℃) and Ms-80(℃), at a cooling rate of 5℃ / s or more]

[0273] Following the plating process, the plated steel sheet is provided to the third cooling process. If the cooling rate of the third cooling process is less than 5℃ / s, the sum of the area ratios of tempered martensite and fresh martensite decreases, and the area ratio of bainite increases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the cooling rate of the third cooling process is set to 5℃ / s or higher. On the other hand, if the third cooling rate is 20℃ / s or lower, the cooling stop temperature can be appropriately controlled. Therefore, it is preferable that the cooling rate of the third cooling process be 20℃ / s or lower.

[0274] If the cooling stop temperature T3 of the third cooling process is less than Ms-200(°C), the volume fraction of untransformed austenite decreases, and the amount of retained austenite in the final structure decreases, making it difficult to achieve excellent ductility. In addition, immediately below the carbon concentration reduction section, the carbon concentration in the retained austenite covering the old austenite grain boundaries becomes 0.60 mass% or more, resulting in inferior resistance to delayed fracture in the stretch flange processing section. Therefore, T3 is set to Ms-200(°C) or higher. On the other hand, if T3 exceeds Ms-80(°C), the sum of the area fractions of tempered martensite and fresh martensite decreases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, T3 is set to Ms-80(°C) or lower.

[0275] [Between the plating process and the third cooling process, the total residence time when the temperature of the plated steel sheet is in the temperature range of Bf(°C) or higher and T-20(°C) or lower is 30s or less]

[0276] If the total residence time in the temperature range of Bf (°C) to T-20 (°C) between the plating process and the third cooling process described below, using Bf defined by the following equation (4) and T, exceeds 30 s, the area ratio of bainite increases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, between the plating process and the third cooling process, the total residence time in the temperature range of the plated steel sheet between Bf (°C) and T-20 (°C) is 30 s or less. Meanwhile, between the plating process and the third cooling process, the lower limit of the total residence time in the temperature range of the plated steel sheet between Bf (°C) and T-20 (°C) is not specifically limited, but the residence time is generally 20 s or more.

[0277] Bf(℃)=710-270×[%C]-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo]···(4)

[0278] Here, [%X] represents the content (mass%) of element X in the above component composition, and is set to 0 if the above component composition does not contain element X.

[0279] [4th Cooling Process: Cooling from 100℃ or higher to a cooling stop temperature T4 (below T3(℃)), at a cooling rate of 3.0℃ / s or less]

[0280] Following the third cooling process, the plated steel sheet is provided to the fourth cooling process. If the cooling rate of the fourth cooling process exceeds 3.0°C / s, the diffusion of carbon into untransformed austenite during the fourth cooling process does not proceed sufficiently, resulting in a decrease in the volume fraction of residual austenite in the final structure. Consequently, it becomes difficult to achieve excellent ductility and excellent delayed fracture resistance characteristics in the elongated flange processing section. Therefore, the cooling rate of the fourth cooling process is set to 3.0°C / s or less, preferably 2.0°C / s or less, and more preferably 1.5°C / s or less. On the other hand, if the fourth cooling rate is 0.5°C / s or more, it is suitable because cooling can be performed without heating by a furnace. Therefore, the cooling rate of the fourth cooling process is preferably 0.5°C / s or more.

[0281] If the cooling stop temperature T4 of the fourth cooling process is less than 100°C, the area percentage of untransformed austenite decreases, and the volume percentage of retained austenite in the final structure decreases, making it difficult to achieve excellent ductility and excellent delayed fracture resistance in the stretched flange processing section. Therefore, T4 should be 100°C or higher, preferably 110°C or higher, and more preferably 120°C or higher. Additionally, since the fourth cooling process is performed following the third cooling process, T4 becomes less than T3°C. Furthermore, from the perspective of reducing fresh martensite, T4 is preferably Ms-100°C or lower.

[0282] [Tempering Process: Maintain at a tempering temperature exceeding T4 (°C) and below 350°C for 10 seconds to 1000 seconds]

[0283] Following the fourth cooling process, the plated steel sheet is provided to the tempering process. By reheating the plated steel sheet and performing tempering, the untransformed austenite is stabilized. If the tempering temperature is T4 (°C) or lower, the required residual austenite is not obtained, making it difficult to achieve excellent ductility and excellent delayed fracture resistance characteristics in the elongated flange processing section. Therefore, the tempering temperature should be greater than T4 (°C), and T4 + 50 (°C) or higher is preferable. On the other hand, if the tempering temperature exceeds 350°C, tempering proceeds excessively, causing a decrease in strength and the decomposition of residual austenite, making it difficult to achieve an El of 9% or more. Therefore, the tempering temperature should be 350°C or lower, and 340°C or lower is preferable.

[0284] In the tempering process, if the holding time at the tempering temperature is less than 10 seconds, the stabilization of austenite becomes insufficient, and martensitic transformation occurs during final cooling. Consequently, the volume fraction of retained austenite in the final structure decreases, making it difficult to achieve excellent ductility. Furthermore, because the tempering of martensite is insufficient, it becomes difficult to achieve excellent hole expandability. Therefore, the holding time at the tempering temperature should be 10 seconds or more, and preferably 40 seconds or more. On the other hand, if the holding time at the tempering temperature exceeds 1000 seconds, tempering proceeds excessively, and as the strength decreases and retained austenite decomposes, it becomes difficult to achieve an El of 9% or more. Therefore, the holding time at the tempering temperature should be 1000 seconds or less, and preferably 800 seconds or less.

[0285] [In the tempering process, at least 6 seconds after the start of holding at the tempering temperature and at least 10 seconds before the end of holding at the tempering temperature, a tension with an average load tension of 2.0 kgf / mm² or more is applied, and one or more bending and bending returns are performed (suitable condition)]

[0286] In the tempering process, it is desirable to apply a tension with an average load tension of 2.0 kgf / mm² or more at least 6 seconds after the start of holding at the tempering temperature and at least 10 seconds before the end of holding at the tempering temperature, and to perform one or more bending and bending returns. By applying tension and performing bending and bending returns at least 6 seconds after the start of holding at the tempering temperature, carbon is enriched to suitably in the untransformed austenite after cooling stops, so the transformation into fresh martensite can be suitably suppressed. Therefore, the volume percentage of retained austenite in the final structure is not reduced, suitable ductility is obtained, and the coverage ratio of retained austenite and fresh martensite at the old austenite grain boundary immediately below the carbon concentration reduction section is suitably obtained, and tensile strength (TS) is suitably obtained. Meanwhile, in the tempering process, tension is applied at least 10 seconds before the end of holding to perform bending and bending return, so that tempering is performed appropriately after the untransformed austenite transforms into fresh martensite, and the yield strength (YS) and hole expansion rate (λ) are appropriately obtained.

[0287] In the tempering process, if the tension applied when performing the aforementioned one or more bending and bending returns is 2.0 kgf / mm² or higher, the unstable residual austenite undergoes work-induced transformation during tempering, thereby suitably obtaining yield strength and hole expansion rate. Therefore, it is preferable that the tension applied in the tempering process be 2.0 kgf / mm² or higher. On the other hand, if the tension applied in the tempering process is 5.0 kgf / mm² or lower, it suitably prevents the reduction of untransformed austenite during tempering and also suitably prevents deformation from remaining within the steel sheet accompanying the application of tension, thereby suitably obtaining product characteristics. Therefore, it is preferable that the tension applied in the tempering process be 5.0 kgf / mm² or lower.

[0288] In addition, for processes and conditions not described in the present invention, a fixed method may be used.

[0289] Examples

[0290] Steel having the composition shown in Table 1, with the remainder consisting of Fe and unavoidable impurities, was melted in a converter and formed into a steel slab by a continuous casting method. Subsequently, the obtained steel slab was subjected to hot rolling and acid cleaning treatment, followed by cold rolling. Then, under the conditions shown in Table 2, an annealing process, a first cooling process, a holding process, a second cooling process, a plating process, a third cooling process, a fourth cooling process, and a tempering process were performed sequentially. Furthermore, in examples other than No. 25, during the tempering process, a tension with an average load of 2.0 kgf / mm² was applied at the timing shown in Table 2, and a single bend and bend return were performed. As a result, a zinc-plated steel sheet with a thickness of 0.8 to 2.4 mm was obtained. In addition, the plating treatments shown in Table 2 are labeled as “GI” for hot-dip galvanizing treatment, “GA” for alloyed hot-dip galvanizing treatment, and “EG” for electro-galvanizing treatment.

[0291]

[0292]

[0293]

[0294] Using the obtained zinc-plated steel sheet as the test material, the sum of the area percentages of tempered martensite and fresh martensite, the volume percentage of retained austenite, the area percentage of bainite, and the area percentage of the remainder structure were each determined by the method described above. Additionally, at a position 1 / 4 of the thickness of the test material, the thickness of the carbon concentration reduction zone in the surface layer of the underlayer steel sheet, the thickness of the bainite precipitation zone immediately below the carbon concentration reduction zone, the sum of the coverage percentages of retained austenite and fresh martensite at the prior austenite grain boundaries immediately below the carbon concentration reduction zone, and the carbon concentration in the retained austenite covering the prior austenite grain boundaries were each determined. The measurement results are shown in Table 3.

[0295] Next, using the obtained zinc-plated steel sheet as a test material, the tensile strength (TS), yield strength (YS), elongation (El), hole expansion ratio (λ), and delayed fracture resistance of the stretched flange processing section were evaluated by the method described above. The evaluation results are shown in Table 3. In addition, in Table 3, examples with excellent delayed fracture resistance of the stretched flange processing section are marked “○”, and examples with poor delayed fracture resistance of the stretched flange processing section are marked “×”.

[0296]

[0297]

[0298] As shown in Table 3, in the examples of the present invention, the tensile strength is 1470 MPa or higher, the elongation is 9.0% or higher, and the hole expansion rate is 25% or higher, and the delayed fracture resistance of the elongated flange processing part is excellent. In addition, in the examples of the present invention (except No. 25) in which, during the tempering process, a tension with an average load tension of 2.0 kgf / mm² or higher is applied 6 s or more after the start of holding at the tempering temperature and 10 s or more before the end of holding at the tempering temperature, and one or more bending and bending returns are performed, the yield strength (YS) is 1000 MPa or higher. On the other hand, in the comparative examples, one or more of TS, hole expansion, ductility, and delayed fracture resistance of the elongated flange processing part are inferior.

[0299] Industrial applicability

[0300] According to the present invention, a zinc-plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0% or more, a hole expansion rate (λ) of 25% or more, and excellent delayed fracture resistance of the elongated flange processing part, and a method for manufacturing the same can be provided.

Claims

Claim 1 A zinc-plated steel sheet having a base steel sheet and a zinc-based plating layer formed on the surface of the base steel sheet, wherein the base steel sheet has a composition in mass% comprising C: 0.180% or more and 0.250% or less, Si: 0.800% or more and 1.550% or less, Mn: 2.400% or more and 3.200% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and wherein, at a depth from the surface of the base steel sheet at a position 1 / 4 of the sheet thickness, the sum of the area percentages of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less, and the volume percentage of retained austenite is 6.0% or more. A zinc-plated steel sheet characterized by having a structure in which the area ratio of bainite is 15.0% or less, the area ratio of bainite is 10.0% or less, and the area ratio of the remainder structure is 10.0% or less, the thickness of the carbon concentration reduction portion in the surface layer of the lower steel sheet is 1.0㎛ or more and 150.0㎛ or less, the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion is 10㎛ or more, the sum of the coverage ratios of residual austenite and fresh martensite in the old austenite grain boundaries immediately below the carbon concentration reduction portion is 30.0% or more and 60.0% or less, the carbon concentration in the residual austenite covering the old austenite grain boundaries immediately below the carbon concentration reduction portion is less than 0.60 mass%, and the tensile strength is 1470 MPa or more. Claim 2 A zinc-plated steel sheet according to claim 1, wherein the yield strength of the lower steel sheet is 1000 MPa or more. Claim 3 In claim 1 or 2, the above component composition further comprises, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% 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: 0.100% or less, Zn: 0.100% or less, Pb: 0.100% or less, Te: A zinc-plated steel sheet containing at least one selected from the group consisting of 0.100% or less, Se: 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less. Claim 4 A zinc-plated steel sheet according to any one of claims 1 to 3, wherein the zinc-plated layer is an electro-galvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer. Claim 5 A process of obtaining a hot-rolled steel sheet by performing hot rolling on a steel slab having a composition in mass% containing C: 0.180% or more and 0.250% or less, Si: 0.800% or more and 1.550% or less, Mn: 2.400% or more and 3.200% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities; a process of obtaining a cold-rolled steel sheet by performing cold rolling on the hot-rolled steel sheet; an annealing process in which the cold-rolled steel sheet is maintained for 10 seconds or more at an annealing temperature of Ac3(°C) or higher, defined by the following formula (1), under an atmosphere with a dew point of -25°C or more and 35°C or lower; and subsequently, the cold-rolled steel sheet, the following formula A first cooling process in which the cold-rolled steel sheet is cooled to a temperature T1 of T-20(℃) or higher and T+20(℃) or lower at a cooling rate of 7℃ / s or higher using T defined by (2), followed by a process of maintaining the cold-rolled steel sheet at T1(℃) for a time of 30s or more and 200s or less, followed by a second cooling process in which the cold-rolled steel sheet is cooled to a temperature T2 of less than T-20(℃), followed by a process of performing a zinc-based plating treatment on the cold-rolled steel sheet to obtain a plated steel sheet, followed by a third cooling process in which the plated steel sheet is cooled to a temperature T3 of Ms-200(℃) or higher and Ms-80(℃) or lower at a cooling rate of 5℃ / s or higher using Ms defined by the following equation (3), followed by cooling the plated steel sheet to a cooling stop temperature T4 of 100℃ or higher and less than T3(℃), 3.A method for manufacturing a zinc-plated steel sheet, comprising a fourth cooling process for cooling at a cooling rate of 0℃ / s or less, and subsequently a tempering process for maintaining the plated steel sheet at a tempering temperature of T4(℃) greater than 350℃ or less for 10s or more and 1000s or less, wherein between the zinc-plated treatment and the third cooling process, using Bf defined by the following equation (4) and T, the total residence time in which the temperature of the plated steel sheet is in a temperature range of Bf(℃) or more and T-20(℃) or less is 30s or less. Ac3(℃)=881-205.7×[%C]+53.1×[%Si]-15×[%Mn]-27×[%Cu]-20.1×[%Ni]-0.7×[%Cr]+41.1×[%Mo] ···(1)T(℃)=743-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo] ···(2)Ms=539-423×[%C]-30.4×[%Mn]-17.7×[%Ni]-12.1×[%Cr]-7.5×[%Mo] ···(3)Bf(℃)=710-270×[%C]-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo] ···(4)Here, [%X] represents the content (mass%) of element X in the above component composition, and is set to 0 if the above component composition does not contain element X. Claim 6 A method for manufacturing a zinc-plated steel sheet according to claim 5, wherein, in the tempering process, a tension having an average load tension of 2.0 kgf / mm² or more is applied 6 s or more after the start of holding at the tempering temperature and 10 s or more before the end of holding at the tempering temperature, and one or more bending and unbending operations are performed. Claim 7 In claim 5 or 6, the above component composition is further, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% 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: 0.100% or less, Zn: 0.100% or less, Pb: 0.100% or less, Te: A method for manufacturing a zinc-plated steel sheet containing at least one selected from the group consisting of 0.100% or less, Se: 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less. Claim 8 A method for manufacturing a zinc-plated steel sheet, wherein, in any one of claims 5 to 7, the zinc-plating treatment is an electro-galvanizing treatment, a hot-dip galvanizing treatment, or a hot-dip galvanizing treatment followed by an alloying treatment.

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