Zinc-based coated steel sheet and its manufacturing method

Abstract

Provided are a galvanized steel sheet that is exceptional in terms of delayed fracture resistance characteristics of a stretch flange processed part, and a method for producing the galvanized steel sheet. This galvanized steel sheet has a base steel sheet and a zinc-based plating layer formed on the surface of the base steel sheet, wherein the galvanized steel sheet is characterized in that: the base steel sheet has a prescribed component composition; at a position amounting to one-fourth of the thickness of the base steel sheet a prescribed structure is present, the total coverage of residual austenite and fresh martensite at the old austenite grain boundaries is 20-45%, and the carbon concentration in the residual austenite that covers the old austenite grain boundaries is less than 0.60 mass%; and the tensile strength is 1470 MPa or greater.

Description

[Technical Field]

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

[0002] In recent years, improving automobile fuel efficiency has become an important issue in the automotive industry in order to reduce CO2 emissions from the perspective of preserving the global environment. While reducing the weight of automobile bodies is an effective way to improve automobile fuel efficiency, it is necessary to maintain the strength of the automobile body. Therefore, efforts have been made to reduce the weight of automobile bodies by simplifying the structure and reducing the number of parts, while maintaining strength by increasing the strength of steel plates used as materials for automobile parts.

[0003] However, when a high-strength steel sheet having a TS of 1470 MPa or more is formed into a part by cold pressing, bending, or the like, delayed fracture may occur due to an increase in residual stress within the part and a deterioration in the delayed fracture resistance of the steel sheet, etc. Here, delayed fracture refers to a phenomenon in which, when the formed part is placed in a hydrogen penetration environment, hydrogen penetrates into the steel sheet constituting the part, reducing the interatomic bonding strength or causing local deformation, resulting in microcracks, which then propagate and lead to the fracture of the steel sheet.

[0004] In contrast, Patent Document 1 describes a plated steel sheet that has excellent mechanical properties, reduces the amount of hydrogen that penetrates during production, and has excellent hydrogen embrittlement resistance and coating adhesion, and a method for manufacturing the same. Patent Document 2 describes an ultra-high strength thin steel sheet that has excellent hydrogen embrittlement resistance by controlling the chemical composition and retained austenite of the steel sheet, and a method for manufacturing the same. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2019 / 212047 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-197819 Summary of the Invention [Problem to be solved by the invention]

[0006] However, Patent Documents 1 and 2 do not consider the delayed fracture resistance of stretch flanged sections, which are subjected to particularly severe working, and the inventors' investigations have revealed that there is room for improvement. Furthermore, Patent Document 2 recommends that the carbon concentration in the retained austenite be 0.8 mass% or more, but the inventors' investigations have revealed that there is room for improvement in hole expandability.

[0007] In view of the above problems, the present invention aims to provide a zinc-based plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0% or more, a hole expansion ratio (λ) of 20% or more, and excellent delayed fracture resistance in a stretch-flanged portion, and a method for manufacturing the same. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above problems and have come to the following findings. (1) At a position 1 / 4 of the thickness of the base steel sheet, by making the total area ratio of tempered martensite and fresh martensite 70.0% or more and the total area ratio of ferrite and bainitic ferrite 10.0% or less, a TS of 1470 MPa or more can be achieved. (2) By setting the volume fraction of retained austenite at the 1 / 4 position of the plate thickness to 6.0% or more and 20.0% or less, and by setting the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries to 20% or more and 45% or less, it is possible to achieve excellent formability with an elongation (El) of 9.0% or more and a hole expansion ratio (λ) of 20% or more, and excellent delayed fracture resistance in the stretch flanged portion. (3) When producing a zinc-based plated steel sheet, a steel slab having a predetermined chemical composition is used, and the holding time or cooling time and cooling rate are controlled in the holding step or each cooling step after the annealing step and the plating step, whereby a zinc-based plated steel sheet having a structure that satisfies the above (1) and (2) can be obtained.

[0009] That is, the gist and configuration of the present invention are as follows.

[0010] [1] A zinc-based plated steel sheet having a base steel sheet and a zinc-based plating layer formed on the surface of the base steel sheet, The base steel sheet is In mass%, 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 and the remainder being Fe and unavoidable impurities; At a position where the depth from the surface of the base steel plate is 1 / 4 of the plate thickness, The total area ratio of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less, The volume fraction of retained austenite is 6.0% or more and 20.0% or less, The total area ratio of ferrite and bainitic ferrite is 10.0% or less, and a structure in which the area ratio of the remaining structure is 10.0% or less; and At the 1 / 4 position, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundary is 20% or more and 45% or less, At the 1 / 4 position, the carbon concentration in the retained austenite covering the prior austenite grain boundary is less than 0.60 mass%, Tensile strength is 1470 MPa or more A zinc-based plated steel sheet characterized by:

[0011] [2] The component composition further includes, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% 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: 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, The zinc-based plated steel sheet according to the above [1], containing at least one selected from the group consisting of:

[0012] [3] The zinc-based plated steel sheet according to the above [1] or [2], wherein the zinc-based plated layer is an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed hot-dip galvanized layer.

[0013] [4] In mass percent, 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 and hot rolling the steel slab having a composition containing Fe and unavoidable impurities to obtain a hot-rolled steel sheet. A step of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; An annealing step of holding the cold-rolled steel sheet at an annealing temperature of Ac3 (°C) or higher defined by the following formula (1) for 10 s or more; Next, a first cooling step of cooling the cold-rolled steel sheet to a temperature T1 of (3Bs-Ms) / 3 (°C) or more and 0.95 × Bs (°C) or less, using Bs and Ms defined by the following formulas (2) and (3), respectively, at a cooling rate of 5°C / s or more; Next, a holding step of holding the cold-rolled steel sheet at T1 (°C) for a time t (s) in which f in the following formula (4) satisfies 0.010 or more and 0.200 or less, or a second cooling step of cooling the cold-rolled steel sheet at a cooling rate of 1.50 °C / s or less; Next, a third cooling step of cooling the cold-rolled steel sheet to a temperature T3 of Ms (°C) or more and Bs-20 (°C) or less at a cooling rate of 5°C / s or more; Next, a step of subjecting the cold-rolled steel sheet to a zinc-based plating treatment to obtain a plated steel sheet; a fourth cooling step of cooling the plated steel sheet to a temperature T4 of not less than Ms-200 (°C) and not more than Ms-80 (°C) at a cooling rate of not less than 5°C / s; Next, a fifth cooling step of cooling the plated steel sheet to a cooling stop temperature T5 of 100°C or higher and lower than T4 (°C) at a cooling rate of 3.0°C / s or less; Next, a tempering step of holding the plated steel sheet at a tempering temperature higher than T5 (°C) and not higher than 350°C for 5 seconds or more and not higher than 1000 seconds; The method for producing a zinc-based plated steel sheet, comprising: Ac3(℃)=881-205.7×[%C]+53.1×[%Si]-15×[%Mn]-27×[%Cu]-20.1×[%Ni]-0.7×[%Cr]+41.1×[%Mo] ···(1) Bs(℃)=830-270×[%C]-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) TIFF0007786644000001.tif16157Here, [%X] indicates the content (mass%) of element X in the composition, and is set to 0 if the composition does not contain element X. γ (μm) is the prior austenite grain size of the cold-rolled steel sheet at the end of the annealing process.

[0014] [5] The component composition further includes, 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: 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, The method for producing a zinc-based plated steel sheet according to [4] above, wherein the steel sheet contains at least one selected from the group consisting of:

[0015] [6] The method for producing a zinc-based plated steel sheet according to the above [4] or [5], wherein the zinc-based plating treatment is an electrogalvanizing treatment, a hot-dip galvanizing treatment, or a hot-dip galvanizing treatment followed by an alloying treatment. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a zinc-based plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0% or more, a hole expansion ratio (λ) of 20% or more, and excellent delayed fracture resistance in a stretch flanged portion, and a manufacturing method thereof. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a graph showing the relationship between temperature and time in a method for producing a galvannealed steel sheet according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of a zinc-based coated steel sheet and a method for manufacturing the same according to the present invention will be described. Note that the embodiment described below is an example of a specific embodiment of the present invention, and the configuration of the present invention is not limited to this specific example.

[0019] (Composition of base steel sheet) A zinc-based plated steel sheet according to one embodiment of the present invention comprises a substrate steel sheet and a zinc-based plating layer formed on the surface of the substrate steel sheet. The substrate steel sheet has a chemical composition containing, by mass%, C: 0.180% to 0.250%, Si: 0.800% to 1.550%, Mn: 2.400% to 3.200%, 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 balance being Fe and unavoidable impurities. The basic chemical composition of the substrate steel sheet will be described below. In the following description, "%" representing the content of a component element in the substrate steel sheet means "% by mass" unless otherwise specified.

[0020] [C: 0.180% or more and 0.250% or less] Carbon (C) is one of the important basic components of the base steel sheet. In particular, in the present invention, it is an important element that affects the total area fraction of tempered martensite and fresh martensite, and the total area fraction of ferrite and bainitic ferrite. If the C content is less than 0.180%, the total area fraction of tempered martensite and fresh martensite decreases, while the total area fraction of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the C content is set to 0.180% or more, preferably 0.200% or more, and more preferably 0.210% or more. On the other hand, if the C content exceeds 0.250%, the tempered martensite and fresh martensite become embrittled, and the delayed fracture resistance of the stretch flanged portion deteriorates. Therefore, the C content is set to 0.250% or less, preferably 0.240% or less.

[0021] [Si:0.800% or more and 1.550% or less] Silicon (Si) is one of the important basic components of the base steel sheet and is an important element that affects the tensile strength (TS) and the volume fraction of retained austenite. If 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 more. Therefore, the Si content is set to 0.800% or more, preferably 0.850% or more, and more preferably 0.900% or more. On the other hand, if the Si content exceeds 1.550%, the amount of retained austenite increases excessively, resulting in a decrease in hole expandability. Therefore, the Si content is set to 1.550% or less, preferably 1.500% or less, and more preferably 1.400% or less.

[0022] [Mn:2.400% or more and 3.200% or less] Mn is one of the important basic components of the base steel sheet and is an important element that affects the total area fraction of tempered martensite and fresh martensite, and the total area fraction of ferrite and bainitic ferrite. If the Mn content is less than 2.400%, the total area fraction of tempered martensite and fresh martensite decreases, while the total area fraction of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the Mn content is set to 2.400% or more, preferably 2.500% or more, and more preferably 2.600% or more. On the other hand, if the Mn content exceeds 3.200%, the tempered martensite and fresh martensite become embrittled, and the delayed fracture resistance of the stretch flanged portion deteriorates. Therefore, the Mn content is set to 3.200% or less, preferably 3.100% or less, and more preferably 3.000% or less.

[0023] [P:0.100% or less] P segregates at prior austenite grain boundaries, embrittling the grain boundaries and thus embrittling the substrate steel sheet. Therefore, if the P content exceeds 0.100%, the delayed fracture resistance of the stretch flanged portion decreases. Therefore, the P content is set to 0.100% or less, and preferably 0.070% or less. On the other hand, 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 substrate steel sheet, the P content is preferably set to 0.001% or more.

[0024] [S:0.0200% or less] S exists as sulfide and embrittles the base steel sheet, so if the S content exceeds 0.0200%, the delayed fracture resistance of the stretch flanged portion decreases. Therefore, the S content is set to 0.0200% or less, and preferably 0.0050% or less. On the other hand, 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.

[0025] [Al: 1.000% or less] Since Al exists as an oxide and embrittles the base steel sheet, an Al content exceeding 1.000% reduces the delayed fracture resistance of the stretch flanged portion. Therefore, the Al content is set to 1.000% or less, and preferably 0.500% or less. On the other hand, although there is no particular lower limit for the Al content, since Al 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.

[0026] [N:0.0100% or less] N exists as a nitride and embrittles the base steel sheet, so if the N content exceeds 0.0100%, the delayed fracture resistance of the stretch flanged portion decreases. Therefore, the N content is set to 0.0100% or less, and preferably 0.0050% or less. On the other hand, there is no particular lower limit for the N content, but due to constraints on production technology, the N content is preferably set to 0.0001% or more.

[0027] [O:0.0100% or less] O exists as an oxide and embrittles the base steel sheet, so if the O content exceeds 0.0100%, the delayed fracture resistance of the stretch flanged portion decreases. Therefore, the O content is set to 0.0100% or less, and preferably 0.0050% or less. On the other hand, 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.

[0028] (Optional components of the base steel sheet) In addition to the basic components described above, the substrate steel sheet further contains, by 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, and Ca: 0. It may contain at least one selected from the group consisting of: 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: 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.

[0029] [Ti:0.200% or less] [Nb:0.200% or less] [V:0.200% or less] When the Ti, Nb, and V contents are 0.200% or less, large amounts of coarse precipitates or inclusions are not formed, and the ultimate deformability of the base steel sheet is not reduced, so that λ and bendability are not reduced. Therefore, when one or more of Ti, Nb, and V are contained, the contents thereof are each set to 0.200% or less, and preferably 0.100% or less. On the other hand, although there are no particular lower limits for the Ti, Nb, and V contents, these elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, thereby increasing the strength of the base steel sheet, so the Ti, Nb, and V contents are preferably set to 0.001% or more.

[0030] [Ta:0.10% or less] [W: 0.10% or less] When the Ta and W contents are 0.10% or less, large amounts of coarse precipitates or inclusions are not formed, the base steel sheet is not embrittled, and the delayed fracture resistance of the stretch flangeable portion is not reduced. Therefore, when at least one of Ta and W is contained, the contents thereof are each set to 0.10% or less, and preferably 0.08% or less. On the other hand, although there are no particular lower limits for the Ta and W contents, these elements increase the strength of the base steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, so the Ta and W contents are preferably set to 0.01% or more.

[0031] [B:0.0100% or less] If the B content is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flangeable portion will not be reduced. Therefore, if B is contained, its content should be 0.0100% or less, and preferably 0.0080% or less. On the other hand, although there is no particular lower limit for the B content, since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content should preferably be 0.0003% or more.

[0032] [Cr:1.00% or less] [Mo: 1.00% or less] [Ni: 1.00% or less] When the Cr, Mo, and Ni contents are each 1.00% or less, coarse precipitates or inclusions do not increase, the base steel sheet is not embrittled, and the delayed fracture resistance of the stretch flangeable portion is not reduced. Therefore, when one or more of Cr, Mo, and Ni are contained, the contents thereof are each 1.00% or less, and preferably 0.80% or less. On the other hand, although there are no particular lower limits for the Cr, Mo, and Ni contents, since these elements improve hardenability, the Cr, Mo, and Ni contents are each preferably 0.01% or more.

[0033] [Co:0.010% or less] If the Co content is 0.010% or less, the amount of coarse precipitates or inclusions will not increase, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, if Co is contained, its content should be 0.010% or less, and preferably 0.008% or less. On the other hand, although there is no particular lower limit for the Co content, since Co improves hardenability, the Co content should preferably be 0.001% or more.

[0034] [Cu:1.00% or less] If the Cu content is 1.00% or less, the amount of coarse precipitates or inclusions will not increase, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, if Cu is contained, its content should be 1.00% or less, and preferably 0.80% or less. On the other hand, although there is no particular lower limit for the Cu content, since Cu improves hardenability, the Cu content should preferably be 0.01% or more.

[0035] [Sn:0.200% or less] If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, if Sn is contained, its content should be 0.200% or less, and preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sn content, but since Sn improves hardenability, the Sn content should preferably be 0.001% or more.

[0036] [Sb:0.200% or less] If the Sb content is 0.200% or less, the amount of coarse precipitates or inclusions will not increase, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, if Sb is contained, its content should be 0.200% or less, and preferably 0.100% or less. On the other hand, although there is no particular lower limit for the Sb content, since Sb controls the softened surface thickness and enables strength adjustment, the Sb content is preferably 0.001% or more.

[0037] [Ca: 0.0100% or less] [Mg:0.0100% or less] [REM:0.0100% or less] When the Ca, Mg, and REM contents are each 0.0100% or less, coarse precipitates or inclusions do not increase, the base steel sheet is not embrittled, and the delayed fracture resistance of the stretch flangeable portion is not reduced. Therefore, when one or more of Ca, Mg, and REM are contained, the contents thereof are each 0.0100% or less, and preferably 0.0050% or less. On the other hand, although there are no particular lower limits for the Ca, Mg, and REM contents, these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the base steel sheet, so the Ca, Mg, and REM contents are preferably 0.0005% or more.

[0038] [Zr:0.100% or less] [Zn:0.100% or less] [Pb:0.100% or less] [Te:0.100% or less] When the Zr, Zn, Pb, and Te contents are each 0.100% or less, coarse precipitates or inclusions are not increased, the base steel sheet is not embrittled, and the delayed fracture resistance of the stretch flangeable portion is not reduced. Therefore, when one or more of Zr, Zn, Pb, and Te are contained, the contents thereof are each 0.100% or less, and preferably 0.080% or less. On the other hand, although there are no particular lower limits for the contents of Zr, Zn, Pb, and Te, these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the base steel sheet, so the contents of Zr, Zn, Pb, and Te are each preferably 0.001% or more.

[0039] [Se:0.020% or less] [Ga:0.020% or less] [Ge:0.020% or less] [Sr:0.020% or less] When the content of each of Se, Ga, Ge, and Sr is 0.020% or less, coarse precipitates or inclusions are not increased, the base steel sheet is not embrittled, and the delayed fracture resistance of the stretch flangeable portion is not reduced. Therefore, when one or more of Se, Ga, Ge, and Sr are contained, the content of each is set to 0.020% or less. On the other hand, although there are no particular lower limits for the contents of Se, Ga, Ge, and Sr, these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the base steel sheet, so the contents of Se, Ga, Ge, and Sr are preferably set to 0.001% or more.

[0040] [Hf:0.10% or less] If the Hf content is 0.10% or less, the amount of coarse precipitates or inclusions will not increase, and the Hf content will not embrittle the substrate steel sheet, so the delayed fracture resistance of the stretch flangeable portion will not deteriorate. Therefore, if Hf is contained, its content should be 0.10% or less, and preferably 0.08% or less. On the other hand, although there is no particular lower limit for the Hf content, since Hf spheroidizes the shape of nitrides or sulfides and improves the ultimate deformability of the substrate steel sheet, the Hf content is preferably 0.01% or more.

[0041] [Bi:0.200% or less] If the Bi content is 0.200% or less, the amount of coarse precipitates or inclusions will not increase, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, if Bi is contained, its content should be 0.200% or less, and preferably 0.100% or less. On the other hand, although there is no particular lower limit for the Bi content, since Bi reduces segregation, the Bi content should preferably be 0.001% or more.

[0042] Note that the effects of the present invention are not impaired when the contents of 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 are less than their respective preferred lower limits. Therefore, the contents of each element may be less than their respective preferred lower limits, in which case they are treated as unavoidable impurities. A substrate steel sheet according to one embodiment of the present invention contains the above basic components, with the balance consisting of Fe (iron) and unavoidable impurities. Here, it is preferred that a substrate steel sheet according to one embodiment of the present invention contains only the above basic components and the balance consisting of Fe (iron) and unavoidable impurities.

[0043] (Structure of base steel sheet) Next, the structure of the substrate steel sheet will be described. The substrate steel sheet has a structure in which, at a depth from the surface of the substrate steel sheet at 1 / 4 of the sheet thickness, the total area fraction of tempered martensite and fresh martensite is 70.0% to 94.0%, the volume fraction of retained austenite is 6.0% to 20.0%, the total area fraction of ferrite and bainitic ferrite is 10.0% or less, and the area fraction of the remaining structure is 10.0% or less. Furthermore, at a depth from the surface of the substrate steel sheet at 1 / 4 of the sheet thickness, the total coverage of retained austenite and fresh martensite at prior austenite grain boundaries is 20% to 45%, and the carbon concentration of the retained austenite covering the prior austenite grain boundaries is less than 0.60 mass%. Note that the structure described below is the structure at a depth from the surface of the substrate steel sheet at 1 / 4 of the sheet thickness.

[0044] [The total area ratio of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less] By setting the total area ratio of tempered martensite and fresh martensite to 70.0% or more, it is 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 70.0% or more and 94.0% or less.

[0045] In particular, an area fraction of tempered martensite of 80.0% or more is preferable because TS can be suitably obtained. On the other hand, there is no particular upper limit to the area fraction of tempered martensite, but the area fraction of tempered martensite is generally 94.0% or less. Also, it is preferable that the area fraction of fresh martensite is 10.0% or less because λ can be suitably obtained. On the other hand, there is no particular lower limit to the area fraction of fresh martensite, but the area fraction of fresh martensite is generally 1.0% or more.

[0046] The sum of the area fractions of tempered martensite and fresh martensite can be determined as follows. After polishing the L-section of the substrate steel sheet, it is etched with 3 vol.% nital and 10 fields of view are observed at 3000x magnification using an SEM at a position ¼ of the sheet thickness (a position corresponding to ¼ of the sheet thickness in the depth direction from the steel sheet surface). The area fraction of fresh martensite can be determined by subtracting the volume fraction of retained austenite determined by the method described below from the area fraction of the structure with a smooth surface in the observed image. Note that fresh martensite is a convex portion with a width of 50 nm or more. The area fraction of tempered martensite can be determined in a similarly observed structural image. Tempered martensite has a substructure (lath boundaries, block boundaries) and is a structure in which carbides precipitate in multiple variants. Note that the volume fraction of retained austenite is approximately equal to the area fraction, and therefore, in the present invention, it is treated as equivalent to the area fraction.

[0047] [Volume fraction of retained austenite: 6.0% to 20.0%] If 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, if the volume fraction of retained austenite exceeds 20.0%, it becomes difficult to achieve excellent hole expandability. Therefore, the volume fraction of retained austenite is set to 20.0% or less, preferably 15.0% or less, and more preferably 13.0% or less.

[0048] The volume fraction of retained austenite can be determined as follows: The substrate steel sheet is polished to a position 0.1 mm thicker than the 1 / 4 point of the sheet thickness. The surface is further polished 0.1 mm by chemical polishing to the 1 / 4 point of the sheet thickness, and the integrated intensity ratios of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron are measured using CoKα radiation in an X-ray diffractometer. The volume fraction of retained austenite can be determined by averaging the nine integrated intensity ratios obtained.

[0049] [Total area ratio of ferrite and bainitic ferrite is 10.0% or less] If the total area ratio of ferrite and bainitic ferrite exceeds 10.0%, it becomes difficult to achieve a TS of 1470 MPa or more. Therefore, the total area ratio of ferrite and bainitic ferrite is set to 10.0% or less. On the other hand, there is no particular lower limit for the total area ratio of ferrite and bainitic ferrite, and the total area ratio may be 0.0%.

[0050] The area fractions of ferrite and bainitic ferrite can be determined as follows. After polishing the L-section of the substrate steel sheet, it is etched with 3 vol.% nital, and a position at 1 / 4 of the sheet thickness is observed using an SEM at 3000x magnification in 10 fields of view. In the observed structural images, the ferrite and bainitic ferrite are recessed and the interior of the structure is flat. The area fractions of ferrite and bainitic ferrite are determined in each field of view, and the average of these values ​​is taken as the total area fraction of ferrite and bainitic ferrite.

[0051] [Area ratio of remaining structure is 10.0% or less] The steel structure of the present invention may contain carbides such as pearlite and cementite or other known structures of steel sheets as a remaining structure. If the area ratio of the remaining structure is 10.0% or less, the effects of the present invention are not impaired. Therefore, the area ratio of the remaining structure is set to 10.0% or less. On the other hand, there is no particular lower limit for the area ratio of the remaining structure, and the area ratio of the remaining structure may be 0.0%.

[0052] The area fraction of the remaining structure can be determined as follows. After polishing the L-section of the substrate steel sheet, it is corroded with 3 vol.% nital, and a position 1 / 4 of the sheet thickness is observed using an SEM at 3000x magnification in 10 fields of view. In the observed structural image, the area fraction of the remaining structure is determined by subtracting the area fractions of tempered martensite, fresh martensite, ferrite, and bainitic ferrite, as well as the volume fraction of retained austenite, from 100.0%. The area fraction of the remaining structure is determined in each field of view, and the average of these values ​​is taken as the area fraction of the remaining structure.

[0053] [The total coverage of retained austenite and fresh martensite at prior austenite grain boundaries is 20% to 45%] By setting the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries to 20% or more, hydrogen diffusion through the grain boundaries is inhibited, resulting in excellent delayed fracture resistance in stretch-flanged sections. Therefore, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries is set to 20% or more, preferably 30% or more. On the other hand, if the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries exceeds 45%, it becomes difficult to achieve excellent hole expandability. Therefore, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries is set to 45% or less, preferably 40% or less.

[0054] The total coverage of retained austenite and fresh martensite at prior austenite grain boundaries can be determined as follows. After polishing the L-section of a zinc-based coated steel sheet, it is etched with 3 vol.% nital. A position at 1 / 4 of the sheet thickness is observed using an SEM at 3000x magnification, and 10 prior austenite grain boundaries are randomly selected. On the selected prior austenite grain boundaries, convex portions with a width of 50 nm or more in the direction perpendicular to the prior austenite grain boundary and a smooth surface are defined as the regions covered by retained austenite and fresh martensite. The total circumferential length of the regions covered by retained austenite and fresh martensite is divided by the circumferential length of the selected prior austenite grain boundary. The average of the values ​​obtained at each location is calculated and used as the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries.

[0055] [Carbon concentration in the retained austenite covering the prior austenite grain boundaries is less than 0.60 mass%] If the carbon concentration in the retained austenite covering the prior austenite grain boundaries is 0.60% by mass or more, the second phase, which has a hardness significantly different from that of the matrix, becomes a stress concentration area during working, degrading the delayed fracture resistance of the stretch flanged portion. Therefore, the carbon concentration in the retained austenite covering the prior austenite grain boundaries is less than 0.60% by mass, and preferably 0.55% by mass or less. On the other hand, there is no particular lower limit for the carbon concentration in the retained austenite covering the prior austenite grain boundaries, and the carbon concentration is generally 0.30% by mass or more.

[0056] The carbon concentration in the retained austenite covering the prior austenite grain boundaries can be determined as follows. First, the surface of a steel sheet (sample) is polished using diamond paste so that the cross section (L cross section) parallel to the rolling direction becomes the observation surface, and then polished to a mirror finish using alumina polishing. Next, to remove hydrocarbon contamination (carbon contamination) from the sample surface, the observation surface is cleaned using a plasma cleaner. On the cleaned observation surface, a position 1 / 4 of the sheet thickness is measured using an electron beam microanalyzer (FE-EPMA) equipped with a field emission electron gun. The measurement conditions are an acceleration voltage of 7 kV and a current of 50 nA, as described in a non-patent document (T. Yamashita, Y. Tanaka, M. Nagoshi and K. Ishida: Sci. Rep., 6 (2016), DOI: 10.1038 / srep29825.). The sample is heated to 100°C and maintained at this temperature for measurement to prevent contamination of the sample surface. The carbon concentration is determined from the measurement results using a calibration method, and an elemental mapping image of carbon is obtained. In the obtained elemental mapping image, regions (high carbon regions) where the carbon concentration is equal to or greater than the average of the matrix and less than 0.6 mass% are identified. Furthermore, prior austenite grains are identified by referring to an SEM image of the same field of view as the elemental mapping image, and high carbon regions present on the grain boundaries of the prior austenite grains are identified. The perimeter a of the prior austenite grain and the length b of the portion of the perimeter of the prior austenite grain that overlaps with the high carbon region are then determined, and the ratio b / a is calculated. The above measurement is performed 30 times for each steel sheet. The average of the 30 measurements is taken as the carbon concentration in the retained austenite covering the prior austenite grain boundaries of that steel sheet.

[0057] Fresh martensite is formed by the transformation of untransformed austenite during final cooling, and therefore the carbon concentration in the fresh martensite covering the prior austenite grain boundaries is equal to or lower than the carbon concentration in the retained austenite.

[0058] (Zinc-based plating layer) The zinc-based plating layer of the zinc-based plated steel sheet is preferably an electrogalvanized 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, or the like. The coating weight is 20 to 80 g / m per side. 2 (Double-sided plating) is preferred.

[0059] (Mechanical properties of zinc-plated steel sheets) Next, the mechanical properties of the zinc-based plated steel sheet will be described.

[0060] [Tensile strength (TS) is 1470 MPa or more] The tensile strength (TS) of zinc-based plated steel sheets is 1470 MPa or more. On the other hand, the upper limit of the tensile strength of zinc-based plated steel sheets is not particularly limited, but it is generally 1650 MPa or less. The tensile strength can be determined as follows. A JIS No. 5 test piece (gauge length 50 mm, parallel part width 25 mm) is taken from the test material so that the longitudinal direction of the test piece is perpendicular to the rolling direction, 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 x 10 -1 It can be mm / sec.

[0061] [Elongation (El) is 9.0% or more] The zinc-based plated steel sheet has an elongation (El) of 9.0% or more. On the other hand, the upper limit of the elongation of the zinc-based plated steel sheet is not particularly limited, but is generally 15.0% or less. The elongation (El) can be determined by the tensile test described above.

[0062] [Hole expansion ratio (λ) is 20% or more] The hole expansion ratio (λ) of zinc-based coated steel sheets is set to 20% or more. While there is no particular upper limit for the hole expansion ratio of zinc-based coated steel sheets, it is generally set to 50% or less. The hole expansion ratio (λ) can be determined as follows: A hole expansion test is conducted in accordance with JIS Z 2256. After shearing the test piece to 100 mm × 100 mm, a 10 mm diameter hole is punched with a clearance of 12.5%. A conical punch with an apex angle of 60° is pressed into the punched hole while the test piece is held down with a blank holder force of 9 tons (88.26 kN) using a die with an inner diameter of 75 mm, and the occurrence of a crack penetrating the sheet thickness is confirmed. The hole diameter at which a crack occurs is measured, and the limiting hole expansion ratio (λ) (%) can be calculated using the following equation (5). λ(%)={(D f -D0) / D0}×100 (5) where D f is the hole diameter (mm) when the crack occurs, and D0 is the initial hole diameter (mm).

[0063] In addition, in plated steel sheets with a TS of approximately 1470 MPa, hydrogen is incorporated into the steel sheet during the manufacturing process. It is also known that diffusible hydrogen in steel sheets significantly deteriorates the hole expansion ratio. Therefore, when plated steel sheets are shipped as products, the diffusible hydrogen is reduced by storing the steel sheets for a long period of time or by post-heating. Therefore, it is preferable to perform a hole expansion test after the amount of diffusible hydrogen in the steel sheet has reached 0.01 wt.% or less.

[0064] [Excellent resistance to delayed fracture in stretch flanged areas] The zinc-based plated steel sheet according to the present invention has excellent delayed fracture resistance in the stretch-flanged portion. The delayed fracture resistance in the stretch-flanged portion can be evaluated as follows. The hole expanding test described above is carried out within 30 days after the preparation of the zinc-based plated steel sheet (test material). Immediately after the hole expanding test, a photograph of the stretch-flanged portion of the test material is taken at 20x magnification using a digital microscope (RH-2000: manufactured by Hirox). The test material is then left to stand at room temperature (15 to 25°C) for 24 hours, and the stretch-flanged portion is again observed using the digital microscope. The photograph of the stretch-flanged portion taken immediately after the hole expanding test is compared with the photograph of the stretch-flanged portion 24 hours later, and the stretch-flanged portion in which no increase or propagation of cracks is observed can be determined to have excellent delayed fracture resistance.

[0065] (Method of manufacturing zinc-based coated steel sheets) Next, a method for producing a zinc-based plated steel sheet according to one embodiment of the present invention will be described. FIG. 1 shows a graph illustrating the relationship between temperature and time in a method for producing a galvannealed steel sheet according to one embodiment of the present invention. The broken line in the graph indicates the temperature change of the steel sheet from the annealing process to the tempering process. A steel slab having the above-described chemical composition is hot-rolled and cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is then heated and subjected to an annealing process in which it is held at an annealing temperature of Ac3 (°C) or higher. Next, a first cooling process is performed in which the cold-rolled steel sheet is cooled to a temperature T1. Next, a holding process in which the cold-rolled steel sheet is held at T1 (°C) or a second cooling process in which the cold-rolled steel sheet is cooled to a temperature T2 is performed. Note that FIG. 1 shows the case in which the second cooling process is performed. Next, a third cooling process in which the cold-rolled steel sheet is cooled to a temperature T3. Next, the cold-rolled steel sheet is subjected to a zinc-based plating process to obtain a plated steel sheet. Note that the time for the hot-dip galvanizing process is shorter than that for the cooling process, etc., and is therefore indicated as a point at temperature T3 in FIG. 1. In the alloying treatment after the hot-dip galvanizing treatment, the plated steel sheet is heated to and maintained at the alloying treatment temperature. Next, a fourth cooling step is performed in which the plated steel sheet is cooled to a temperature T4. Next, a fifth cooling step is performed in which the plated steel sheet is cooled to a cooling stop temperature T5. Next, a tempering step is performed in which the plated steel sheet is maintained at a tempering temperature above T5 (°C) and not higher than 350°C.

[0066] [Steel slab] The steel slab used in the method for producing a zinc-based coated steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, N, and O, with the balance being Fe and unavoidable impurities. It is preferable that the steel slab further contains 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. The content of each element is as described above.

[0067] [Hot rolling process] In the present invention, the method for producing a steel slab is not particularly limited, and any known method such as a converter or an electric furnace is suitable. The steel slab is preferably produced by a continuous casting method to prevent macrosegregation. The resulting steel slab is hot-rolled to obtain a hot-rolled steel sheet. The resulting hot-rolled steel sheet may be cold-rolled as is, or may be pickled before being cold-rolled. Pickling is preferable because it removes oxides from the surface of the steel sheet, thereby ensuring good plating quality in the final steel sheet product. The pickling may be performed once or multiple times.

[0068] [Cold rolling process] The obtained hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The reduction ratio in cold rolling and the thickness of the sheet after rolling are not particularly limited. Furthermore, the number of rolling passes and the reduction ratio in each pass are not particularly limited.

[0069] [Annealing process] The obtained cold-rolled steel sheet is subjected to an annealing process. In the annealing process, if the annealing temperature is less than Ac3 (°C) defined by the following formula (1), the area ratios of tempered martensite and fresh martensite decrease and the total area ratio of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the annealing temperature is set to Ac3 (°C) or more, and preferably Ac3 + 20 (°C) or more. On the other hand, if the annealing temperature is 920°C or less, energy efficiency does not decrease, so an increase in heating costs can be suitably prevented and damage to the furnace body can be suitably prevented. Therefore, the annealing temperature is preferably 920°C or less. Ac3(℃)=881-205.7×[%C]+53.1×[%Si]-15×[%Mn]-27×[%Cu]-20.1×[%Ni]-0.7×[%Cr]+41.1×[%Mo] ···(1) Here, [%X] indicates the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X.

[0070] In the annealing step, if the holding time at the annealing temperature is less than 10 seconds, the area ratios of tempered martensite and fresh martensite decrease, and the area ratios of ferrite and bainitic ferrite increase, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the holding time at the annealing temperature is set to 10 seconds or more, and 40 seconds or more is preferable. On the other hand, if the holding time at the annealing temperature is 500 seconds or less, an increase in heating costs and an increase in manufacturing time can be suitably prevented, that is, a decrease in productivity can be suitably prevented. Therefore, the holding time at the annealing temperature is preferably 500 seconds or less.

[0071] [First cooling step: cooling to a temperature T1 of (3Bs-Ms) / 3 (°C) or higher and 0.95 x Bs (°C) or lower at a cooling rate of 5°C / s or higher] Following the annealing step, the cold-rolled steel sheet is subjected to a first cooling step. That is, the end of the annealing step corresponds to the start of the first cooling step. If the cooling rate in the first cooling step is less than 5°C / s, the area ratios of tempered martensite and fresh martensite decrease, and the area ratios of ferrite and bainitic ferrite increase, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the cooling rate in the first cooling step is set to 5°C / s or more, and preferably 7°C / s or more. On the other hand, if the first cooling rate is 20°C / s or less, the cooling stop temperature can be suitably controlled. Therefore, the cooling rate in the first cooling step is preferably 20°C / s or less.

[0072] If the cooling stop temperature T1 of the first cooling step is less than (3Bs - Ms) / 3 (°C), where Bs and Ms are defined by the following equations (2) and (3), respectively, cooling occurs at a position lower than the bainite nose. As a result, nucleation of ferrite and bainitic ferrite does not proceed, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries becomes excessive, and it becomes difficult to achieve a hole expansion ratio of 20% or more. Therefore, T1 is set to be equal to or greater than (3Bs - Ms) / 3 (°C). On the other hand, if T1 exceeds 0.95 × Bs (°C), cooling occurs at a position higher than the bainite nose. As a result, nucleation of ferrite and bainitic ferrite is slow, and the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries becomes excessive, making it difficult to achieve a hole expansion ratio of 20% or more. Therefore, T1 is set to be equal to or less than 0.95 × Bs (°C). Bs(℃)=830-270×[%C]-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) Here, [%X] indicates the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X.

[0073] [Holding step or second cooling step: holding at T1 (°C) or cooling at a cooling rate of 1.50°C / s or less for a time t (s) that satisfies f of 0.010 or more and 0.200 or less] Following the first cooling step, the cold-rolled steel sheet is subjected to a holding step in which it is held at T1 (°C), or a second cooling step in which it is cooled to a temperature T2. When the holding step is performed, by setting the holding temperature to T1 (°C), the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries is prevented from becoming excessive, and excellent hole expandability is obtained.

[0074] When the second cooling step is performed, if the cooling rate exceeds 1.50°C / s, the nucleation of ferrite and bainitic ferrite at the prior austenite grain boundaries does not proceed sufficiently, the coverage of retained austenite and fresh martensite at the prior austenite grain boundaries increases, and it becomes difficult to achieve excellent hole expandability. Therefore, when the second cooling step is performed, the cooling rate of the second cooling step is set to 1.50°C / s or less. On the other hand, if the second cooling rate is 0.50°C / s or more, an increase in furnace heating costs can be suitably prevented. Therefore, the cooling rate of the second cooling step is preferably 0.50°C / s or more.

[0075] The holding time in the holding step or the cooling time in the second cooling step is set to a time t (s) such that f in the following formula (4) satisfies 0.010 or more and 0.200 or less. If f is less than 0.010, nucleation of ferrite and bainitic ferrite does not occur at the prior austenite grain boundaries, resulting in excessive increases in the coverage of retained austenite and fresh martensite at the prior austenite grain boundaries, making it difficult to achieve excellent hole expandability. Therefore, the time t (s) is set to a value such that f in formula (4) is 0.010 or more. On the other hand, if f exceeds 0.200, the ferrite and bainitic ferrite formed at the prior austenite grain boundaries grow, increasing the total area fraction of ferrite and bainitic ferrite, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the time t (s) is set to a value such that f in formula (4) is 0.200 or less. TIFF0007786644000002.tif15157 where [%X] represents the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X, and d γ (μm) is the prior austenite grain size of the cold-rolled steel sheet at the end of the annealing process.

[0076] Prior austenite grain size d γcan be determined as follows. After polishing the L-section of a steel sheet (a vertical section parallel to the rolling direction), it is corroded with a chemical solution that corrodes prior austenite grain boundaries (prior γ grain boundaries) (for example, a saturated picric acid solution or a solution to which ferric chloride has been added). Four arbitrary fields of view are observed at 1 / 4 of the thickness of the corroded steel sheet at 400x magnification using an optical microscope. The prior austenite grain size is measured using the photograph obtained by the observation by the intercept method. That is, 20 straight lines are drawn on the photograph in the rolling direction and in the direction perpendicular to the rolling direction (thickness direction), and the number of grain boundaries intersecting these lines is counted. The total length of the 40 straight lines is divided by the number of grain boundaries intersecting the lines, and the obtained value is multiplied by 1.13 to determine the prior austenite grain size d γ can be obtained.

[0077] Prior austenite grain size d γ is preferably measured after the annealing step, i.e., immediately before the first cooling step. In laboratory tests, it can be measured immediately before the first cooling step, but in a production line, it may be difficult to measure because each step is continuous. In such cases, it is recommended to use the prior austenite grain size d γ In the present invention, the time t(s) is set by estimating f in the range of 0.010 or more and less than 0.200 because the prior austenite grain size d γ This is because the error due to the estimation of the prior austenite grain size d γ can be estimated as follows from the state before the annealing line and the conditions of the annealing process. A steel sheet (sample) manufactured with the same composition, hot rolling conditions, and cold rolling rate as the target steel sheet is prepared, and the sample is subjected to the same annealing as the target steel sheet. The prior austenite grain size d γ The measurement results are used to measure the prior austenite grain size d γ It can be estimated that:

[0078] [Third cooling step: cooling to a temperature T3 of Ms (°C) or higher and Bs - 20 (°C) or lower at a cooling rate of 5°C / s or higher] Following the holding step or the second cooling step, the cold-rolled steel sheet is subjected to a third cooling step. If the cooling rate in the third cooling step is less than 5°C / s, the ferrite and bainitic ferrite formed at the prior austenite grain boundaries grow, increasing the total area ratio of ferrite and bainitic ferrite, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the cooling rate in the third cooling step is set to 5°C / s or more, and preferably 6°C / s or more. On the other hand, if the third cooling rate is 30°C / s or less, the cooling stop temperature can be suitably controlled. Therefore, the cooling rate in the third cooling step is preferably 30°C / s or less.

[0079] If the cooling stop temperature T3 of the third cooling step is lower than Ms (°C), fresh martensite precipitates and is excessively tempered during alloying in hot-dip galvanizing, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, T3 should be set to Ms (°C) or higher, preferably Ms + 20 (°C) or higher, and more preferably Ms + 40 (°C) or higher. On the other hand, if T3 exceeds Bs - 20 (°C), ferrite and bainitic ferrite formed at the prior austenite grain boundaries grow, increasing the total area fraction of ferrite and bainitic ferrite, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, T3 should be set to Bs - 20 (°C) or lower, preferably Bs - 25 (°C) or lower, and more preferably Bs - 30 (°C) or lower.

[0080] [Plating process] Following the third cooling step, the cold-rolled steel sheet is subjected to a zinc-based plating treatment to obtain a plated steel sheet. Examples of the zinc-based plating treatment include electrogalvanizing, hot-dip galvanizing, or hot-dip galvanizing followed by an alloying treatment. Electroplating such as Zn-Ni electroplating or hot-dip zinc-aluminum-magnesium alloy plating may also be performed.

[0081] When a cold-rolled steel sheet is subjected to hot-dip galvanizing treatment, it is preferable to immerse the cold-rolled steel sheet in a galvanizing bath at a temperature of 440°C to 500°C, and then adjust the coating weight by gas wiping or the like. The coating weight is 20 to 80 g / m per side. 2 It is preferable to adjust the temperature to (double-sided plating). The hot-dip galvanizing treatment is preferably performed using a galvanizing bath having an Al content of 0.10 mass % or more and 0.23 mass % or less. The hot-dip galvanizing treatment is preferably performed by retaining the cold-rolled steel sheet in the temperature range of Ms (°C) or more and 700°C or less.

[0082] When hot-dip galvanizing treatment and subsequent alloying treatment are performed, setting the alloying temperature to 470°C or higher prevents the Zn-Fe alloying rate from becoming excessively slow, thereby favorably achieving productivity. Therefore, the alloying temperature is preferably 470°C or higher. On the other hand, setting the alloying temperature to 600°C or lower favorably prevents untransformed austenite from transforming into pearlite, which would otherwise cause a decrease in TS. Therefore, the alloying temperature is preferably 600°C or lower, more preferably 560°C or lower. Furthermore, it is preferable that the alloyed hot-dip galvannealed steel sheet (GA) be subjected to an alloying treatment to adjust the Fe concentration in the coating layer to 7 to 15 mass%.

[0083] The series of treatments from the annealing step to the plating step are not particularly limited, but from the viewpoint of productivity, they are preferably carried out in a CGL (Continuous Galvanizing Line), which is a hot dip galvanizing line.

[0084] [Fourth cooling step: cooling to a temperature T4 of Ms-200 (°C) or more and Ms-80 (°C) or less at a cooling rate of 5°C / s or more] Following the plating treatment step, the plated steel sheet is subjected to a fourth cooling step. If the cooling rate in the fourth cooling step is less than 5°C / s, the total area ratio of tempered martensite and fresh martensite decreases, and the total area ratio of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the cooling rate in the fourth cooling step is set to 5°C / s or more. On the other hand, if the fourth cooling rate is 20°C / s or less, the cooling stop temperature can be suitably controlled. Therefore, the cooling rate in the fourth cooling step is preferably 20°C / s or less.

[0085] If the cooling stop temperature T4 of the fourth cooling step 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. Therefore, T4 is set to Ms-200 (°C) or higher. On the other hand, if T4 exceeds Ms-80 (°C), the total area fraction of tempered martensite and fresh martensite decreases, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, T4 is set to Ms-80 (°C) or lower.

[0086] [Fifth cooling step: cooling to a cooling stop temperature T5 of 100°C or higher but lower than T4 (°C) at a cooling rate of 3.0°C / s or less] Following the fourth cooling step, the plated steel sheet is subjected to a fifth cooling step. If the cooling rate in the fifth cooling step exceeds 3.0°C / s, carbon does not sufficiently diffuse into untransformed austenite during the fifth cooling step, resulting in a decrease in the volume fraction of retained austenite in the final structure, making it difficult to achieve excellent ductility. Therefore, the cooling rate in the fifth cooling step 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, a fifth cooling rate of 0.5°C / s or more is preferable because cooling can be performed without heating in a furnace. Therefore, the cooling rate in the fifth cooling step is preferably 0.5°C / s or more.

[0087] If the cooling stop temperature T5 of the fifth cooling step is less than 100°C, the area fraction of untransformed austenite decreases, and the volume fraction of retained austenite in the final structure decreases, making it difficult to achieve excellent ductility and excellent delayed fracture resistance in the stretch flanged portion. Therefore, T5 is set to 100°C or higher, preferably 110°C or higher, and more preferably 120°C or higher. Note that, since the fifth cooling step is performed after the fourth cooling step, T5 is less than T4 (°C). Furthermore, from the viewpoint of reducing fresh martensite, T5 is preferably Ms-100 (°C) or lower.

[0088] [Tempering process: Hold at a tempering temperature above T5 (℃) but not exceeding 350℃ for 5 seconds or more but not exceeding 1000 seconds] Following the fifth cooling step, the plated steel sheet is subjected to a tempering step. Reheating and tempering the plated steel sheet stabilizes the untransformed austenite. If the tempering temperature is T5 (°C) or lower, the desired retained austenite cannot be obtained, resulting in reduced ductility. Therefore, the tempering temperature should be higher than T5 (°C), preferably T5 + 50 (°C) or higher. On the other hand, if the tempering temperature exceeds 350°C, excessive tempering will result in reduced strength and decomposition of the retained austenite, making it difficult to achieve an El of 9% or higher. Therefore, the tempering temperature should be 350°C or lower, preferably 340°C or lower.

[0089] In the tempering process, if the holding time at the tempering temperature is less than 5 seconds, the austenite will not be stabilized sufficiently, and will transform into martensite during final cooling, resulting in a decrease in the volume fraction of retained austenite in the final structure, making it difficult to achieve excellent ductility. Furthermore, since the martensite is not tempered sufficiently, it will be difficult to achieve excellent hole expandability. Therefore, the holding time at the tempering temperature should be 5 seconds or more, and 40 seconds or more is preferable. On the other hand, if the holding time at the tempering temperature exceeds 1000 seconds, the tempering will proceed excessively, reducing strength and simultaneously decomposing retained austenite, making it difficult to achieve an El of 9% or more. Therefore, the holding time at the tempering temperature should be 1000 seconds or less, and 800 seconds or less is preferable.

[0090] For steps and conditions not described in the present invention, conventional methods can be used. [Example]

[0091] Steel having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities, was melted in a converter and formed into a steel slab by continuous casting. The obtained steel slab was then subjected to hot rolling and pickling, and then cold rolling to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheet was sequentially subjected to an annealing step, a first cooling step, a holding step or a second cooling step, a third cooling step, a plating step, a fourth cooling step, a fifth cooling step, and a tempering step under the conditions shown in Table 2. As a result, a zinc-based plated steel sheet having a thickness of 0.6 to 2.2 mm was obtained. In addition, a portion of the sample after the annealing step was taken, and the prior austenite grain size d γ In the examples where a holding step was performed, the cooling rate of the second cooling step was expressed as 0.00°C / s. The coating treatments shown in Table 2 are expressed as "GI" for hot-dip galvanizing, "GA" for galvannealed hot-dip galvanizing, and "EG" for electrogalvanizing.

[0092] [Table 1] TIFF0007786644000004.tif233121

[0093] [Table 2] TIFF0007786644000006.tif233153

[0094] Using the obtained zinc-based plated steel sheet as a test material, the total area fraction of tempered martensite and fresh martensite, the volume fraction of retained austenite, the total area fraction of ferrite and bainitic ferrite, and the area fraction of the remaining structure were determined by the methods described above. Furthermore, at a position 1 / 4 of the way through the thickness of the test material, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries and the carbon concentration in the retained austenite covering the prior austenite grain boundaries were determined. Table 3 shows the measurement results.

[0095] Next, the obtained zinc-based plated steel sheets were used as test materials and evaluated for tensile strength TS, elongation El, hole expansion ratio λ, and delayed fracture resistance of the stretch-flanged portion by the methods described above. The evaluation results are shown in Table 3. In Table 3, examples in which the delayed fracture resistance of the stretch-flanged portion was excellent are marked with "O", and examples in which the delayed fracture resistance of the stretch-flanged portion was poor are marked with "X".

[0096] [Table 3] TIFF0007786644000008.tif233159

[0097] As shown in Table 3, the examples of the present invention have a tensile strength of 1470 MPa or more, an elongation of 9.0% or more, and a hole expansion ratio of 20% or more, and are excellent in delayed fracture resistance in the stretch flanged portion. On the other hand, the comparative examples are inferior in one or more of tensile strength, elongation, hole expansion ratio, and delayed fracture resistance in the stretch flanged portion. [Industrial Applicability]

[0098] According to the present invention, it is possible to provide a zinc-based plated steel sheet having a tensile strength of 1470 MPa or more, an elongation of 9.0% or more, a hole expansion ratio of 20% or more, and excellent delayed fracture resistance in a stretch flanged portion, and a manufacturing method thereof.

Claims

1. A zinc-based plated steel sheet having a substrate steel sheet and a zinc-based plating layer formed on a surface of the substrate steel sheet, The base steel sheet is In mass%, 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 and the balance being Fe and unavoidable impurities; At a position where the depth from the surface of the base steel plate is 1 / 4 of the plate thickness, The total area ratio of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less, The volume fraction of retained austenite is 6.0% or more and 20.0% or less, The total area ratio of ferrite and bainitic ferrite is 10.0% or less; and a structure in which the area ratio of the remaining structure is 10.0% or less; and At the quarter position, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundary is 20% or more and 45% or less, At the quarter position, the carbon concentration in the retained austenite covering the prior austenite grain boundary is less than 0.60 mass%, Tensile strength is 1470 MPa or more A zinc-based plated steel sheet characterized by:

2. The component composition further includes, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% 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: 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, The zinc-based plated steel sheet according to claim 1, comprising at least one selected from the group consisting of:

3. The zinc-based plated steel sheet according to claim 1 or 2, wherein the zinc-based plated layer is an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed hot-dip galvanized layer.

4. In mass%, 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 and hot rolling the steel slab having a composition containing Fe and unavoidable impurities to obtain a hot-rolled steel sheet. A step of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; An annealing step of holding the cold-rolled steel sheet at an annealing temperature of Ac3 (°C) or higher defined by the following formula (1) for 10 seconds or more; Next, a first cooling step in which the cold-rolled steel sheet is cooled to a temperature T1 of (3Bs-Ms) / 3 (°C) or more and 0.95 x Bs (°C) or less at a cooling rate of 5°C / s or more using Bs and Ms defined by the following formulas (2) and (3), respectively; Next, a holding step of holding the cold-rolled steel sheet at T1 (°C) for a time t (s) in which f in the following formula (4) satisfies 0.010 or more and 0.200 or less, or a second cooling step of cooling the cold-rolled steel sheet at a cooling rate of 1.50 °C / s or less; Next, a third cooling step of cooling the cold-rolled steel sheet to a temperature T3 of Ms (°C) or higher and Bs-20 (°C) or lower at a cooling rate of 5 °C / s or higher; Next, a step of subjecting the cold-rolled steel sheet to a zinc-based plating treatment to obtain a plated steel sheet; Next, a fourth cooling step of cooling the plated steel sheet to a temperature T4 of Ms-200 (°C) or more and Ms-80 (°C) or less at a cooling rate of 5°C / s or more; Next, a fifth cooling step of cooling the plated steel sheet to a cooling stop temperature T5 (°C) of 100°C or higher but lower than T4 (°C) at a cooling rate of 3.0°C / s or less; Next, a tempering step of holding the plated steel sheet at a tempering temperature higher than T5 (°C) and not higher than 350°C for 5 seconds or more and not higher than 1000 seconds; and A zinc-based plated steel sheet having a substrate steel sheet and a zinc-based plating layer formed on a surface of the substrate steel sheet, the substrate steel sheet has the above-mentioned chemical composition and a structure in which, at a position from the surface of the substrate steel sheet at a depth of 1 / 4 of the sheet thickness, the total area ratio of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less, the volume ratio of retained austenite is 6.0% or more and 20.0% or less, the total area ratio of ferrite and bainitic ferrite is 10.0% or less, and the area ratio of the remaining structure is 10.0% or less, and the total coverage ratio of retained austenite and fresh martensite at the prior austenite grain boundary at the 1 / 4 position is 20% or more and 45% or less, the carbon concentration of the retained austenite covering the prior austenite grain boundary at the 1 / 4 position is less than 0.60 mass%, and the tensile strength is 1470 MPa or more. A method for manufacturing a zinc-based plated steel sheet, which manufactures a zinc-based plated steel sheet. Ac3 (°C) = 881-205.7×[%C]+53.1×[%Si]-15×[%Mn]-27×[%Cu]-20.1×[%Ni]-0.7×[%Cr]+41.1×[%Mo]...(1) Bs (°C) = 830-270×[%C]-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) Here, [% X] indicates the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X. γ (μm) is the prior austenite grain size of the cold-rolled steel sheet at the end of the annealing step.

5. The component composition further includes, 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: 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, The method for producing a zinc-based plated steel sheet according to claim 4, wherein the zinc-based plated steel sheet contains at least one selected from the group consisting of:

6. 6. The method for producing a zinc-based plated steel sheet according to claim 4, wherein the zinc-based plating treatment is an electrogalvanizing treatment, a hot-dip galvanizing treatment, or a hot-dip galvanizing treatment followed by an alloying treatment.

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