JP7786644B2 - Zinc-plated steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to zinc-based plated steel sheets and methods for manufacturing the same.

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 sheets used as materials for automobile parts.

However, when high-strength steel plate with a TS of 1,470 MPa or higher is formed into a part by cold pressing, bending, or other processes, delayed fracture may occur due to increased residual stress within the part and a deterioration in the steel plate's delayed fracture resistance. Here, delayed fracture refers to a phenomenon in which, when a formed part is placed in a hydrogen penetration environment, hydrogen penetrates into the steel plate that makes up the part, reducing interatomic bonding strength or causing localized deformation, resulting in microcracks, which then propagate and lead to the destruction of the steel plate.

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

International Publication No. 2019/212047 Japanese Patent Application Laid-Open No. 2007-197819

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

In view of the above problems, the present invention aims to provide a zinc-based plated steel sheet having a tensile strength (TS) of 1,470 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 stretch flanged sections, as well as a manufacturing method thereof.

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 setting the total area ratio of tempered martensite and fresh martensite to 70.0% or more and the total area ratio of ferrite and bainitic ferrite to 10.0% or less, a TS of 1470 MPa or more can be achieved.
(2) At a quarter-point in the plate thickness, the volume fraction of retained austenite is set to 6.0% or more and 20.0% or less, and the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries is set to 20% or more and 45% or less. This makes it possible to achieve excellent workability 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 component 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.

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

[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,
A component composition containing N: 0.0100% or less and O: 0.0100% or less, with 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,
a structure in which 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
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%,
A zinc-based plated steel sheet having a tensile strength of 1470 MPa or more.

[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:

[3] A zinc-based plated steel sheet according to [1] or [2] above, wherein the zinc-based plated layer is an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed 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,
A step of hot rolling a steel slab having a component composition containing N: 0.0100% or less, O: 0.0100% or less, and the balance consisting of 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;
The method for producing a zinc-based plated steel sheet, comprising:
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] represents the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X. _dγ (μm) represents 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 [4] above, wherein the steel sheet contains at least one selected from the group consisting of:

[6] A method for producing zinc-based plated steel sheet according to [4] or [5] above, wherein the zinc-based plating treatment is an electrolytic zinc-plating treatment, a hot-dip zinc-plating treatment, or a hot-dip zinc-plating treatment followed by an alloying treatment.

The present invention provides a zinc-based plated steel sheet and its manufacturing method that has a tensile strength (TS) of 1,470 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 stretch flanged areas.

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.

The following describes an embodiment of the zinc-based plated steel sheet and its manufacturing method according to the present invention. Note that the embodiment described below is an example of how the present invention is embodied, and the specific example does not limit the configuration of the present invention.

(Composition of base steel sheet)
A zinc-based plated steel sheet according to one embodiment of the present invention includes 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 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 components 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 "mass%" unless otherwise specified.

[C: 0.180% or more and 0.250% or less]
C is one of the important basic components of the base steel sheet, and 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 decreases. Therefore, the C content is set to 0.250% or less, preferably 0.240% or less.

[Si: 0.800% or more and 1.550% or less]
Si is one of the important basic components of the base steel sheet and is an important element that affects the tensile strength and 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 tensile strength 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 retained austenite increases excessively, and the hole expandability decreases. Therefore, the Si content is set to 1.550% or less, preferably 1.500% or less, and more preferably 1.400% or less.

[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 ratio of tempered martensite and fresh martensite, and the total area ratio of ferrite and bainitic ferrite. If the Mn content is less than 2.400%, the total area ratio of tempered martensite and fresh martensite decreases, while the total area ratio 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 decreases. Therefore, the Mn content is set to 3.200% or less, preferably 3.100% or less, and more preferably 3.000% or less.

[P: 0.100% or less]
P segregates at prior austenite grain boundaries, embrittling the grain boundaries and thus embrittling the substrate steel sheet, so 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.

[S: 0.0200% or less]
S exists as sulfides 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.

[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.

[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.

[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, although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably set to 0.0001% or more.

(Optional components of the base steel sheet)
In addition to the basic components described above, the base steel sheet further contains, 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. 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.

[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 contents of Ti, Nb, and V, 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 contents of Ti, Nb, and V are preferably set to 0.001% or more.

[Ta: 0.10% or less]
[W: 0.10% or less]
If the Ta and W contents are 0.10% or less, large amounts of coarse precipitates or inclusions are not formed, and the base steel sheet is not embrittled, so 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 form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, thereby increasing the strength of the base steel sheet, so the Ta and W contents are preferably set to 0.01% or more.

[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 is preferably 0.0003% or more.

[Cr: 1.00% or less]
[Mo: 1.00% or less]
[Ni: 1.00% or less]
If 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 flanged 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.

[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 is preferably 0.001% or more.

[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.

[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, although there is no particular lower limit for the Sn content, since Sn improves hardenability, the Sn content is preferably 0.001% or more.

[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.

[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.

[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 and the base steel sheet is not embrittled, so 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.

[Se: 0.020% or less]
[Ga: 0.020% or less]
[Ge: 0.020% or less]
[Sr: 0.020% or less]
When the content of Se, Ga, Ge, and Sr is 0.020% or less, the amount of coarse precipitates or inclusions does 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 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.

[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 base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, if Hf is contained, its content should be 0.10% or less, and 0.08% or less is preferable. 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 base steel sheet, the Hf content is preferably 0.01% or more.

[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 is preferably 0.001% or more.

Note that, when the content of each 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 is less than its respective preferred lower limit, the effects of the present invention are not impaired. Therefore, each content may be less than its respective preferred lower limit, in which case it is treated as an unavoidable impurity. The base 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 preferable that the base steel sheet according to one embodiment of the present invention contains only the above basic components and the balance, with the balance consisting of Fe (iron) and unavoidable impurities.

(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 a position ¼ of the sheet thickness, the sum of the area fractions 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 sum of the area fractions 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 a position ¼ of the sheet thickness, the sum of the coverage fractions 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%. The structure described below is the structure at a depth from the surface of the substrate steel sheet at a position ¼ of the sheet thickness.

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

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 on the area fraction of tempered martensite, but the area fraction of tempered martensite is generally 94.0% or less. Furthermore, an area fraction of fresh martensite of 10.0% or less is preferable because λ can be suitably obtained. On the other hand, there is no particular lower limit on the area fraction of fresh martensite, but the area fraction of fresh martensite is generally 1.0% or more.

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. Ten fields of observation are performed at 3000x magnification using an SEM at a location 1/4 of the sheet thickness (a location corresponding to 1/4 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 represented by convex portions 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. Because the volume fraction of retained austenite is approximately equal to the area fraction, it is treated as equivalent in this invention.

[Volume fraction of retained austenite is 6.0% or more and 20.0% or less]
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.

The volume fraction of retained austenite can be determined as follows: The base steel sheet is polished to a position 0.1 mm thicker than the 1/4 position of the sheet thickness. The surface is further polished 0.1 mm using chemical polishing to the 1/4 position 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.

[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, the lower limit of the total area ratio of ferrite and bainitic ferrite is not particularly limited, and the total area ratio may be 0.0%.

The area ratios 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 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 ratios of ferrite and bainitic ferrite are determined in each field of view, and the average of these values is taken as the total area ratio of ferrite and bainitic ferrite.

[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%.

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

[The total coverage of retained austenite and fresh martensite at prior austenite grain boundaries is 20% or more and 45% or less]
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, and excellent delayed fracture resistance can be achieved 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, and 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, and preferably 40% or less.

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 ¼ 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 areas covered by retained austenite and fresh martensite. The total circumferential length of the areas 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.

[Carbon concentration in retained austenite covering prior austenite grain boundaries is less than 0.60% by 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, deteriorating 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.

The carbon concentration in the retained austenite that coats the prior austenite grain boundaries can be determined as follows. First, the surface of the steel plate (sample) is polished using diamond paste so that the cross section (L cross section) parallel to the rolling direction serves as 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 way through the plate thickness is measured using an electron beam microanalyzer (FE-EPMA) equipped with a field emission electron gun. The measurement conditions are as described in a non-patent document (T. Yamashita, Y. Tanaka, M. Nagoshi and K. Ishida: Sci. Rep., 6 (2016), DOI: 10.1038/srep29825.), with an acceleration voltage of 7 kV and a current of 50 nA. Furthermore, the sample is heated to 100°C and maintained at this temperature while the measurement is performed to prevent contamination on 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, a region (high carbon region) where the carbon concentration is equal to or greater than the average of the parent phase and less than 0.6 mass% is 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 the high carbon region present on the grain boundary of the prior austenite grain is identified. Then, the circumferential length a of the prior austenite grain and the length b of the portion of the periphery of the prior austenite grain that overlaps with the high carbon region are determined, and the ratio b/a is calculated. The above measurement is carried out 30 times for each steel sheet. The average value of the 30 measurements is taken as the carbon concentration of the retained austenite that covers the prior austenite grain boundaries of that steel sheet.

Fresh martensite is formed by the transformation of untransformed austenite during final cooling. Therefore, the carbon concentration in the fresh martensite covering the prior austenite grain boundaries is equal to or less than the carbon concentration in the retained austenite.

(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, etc. The plating coverage is preferably 20 to 80 g/m ² per side (double-sided plating).

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

[Tensile strength (TS) of 1470 MPa or more]
The zinc-based plated steel sheet has a tensile strength (TS) of 1470 MPa or more. On the other hand, the upper limit of the tensile strength of the zinc-based plated steel sheet is not particularly limited, but 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 portion 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 tensile test can be performed under the condition of a crosshead speed of 1.67 × 10 ^-1 mm/sec.

[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.

[Hole expansion ratio (λ) is 20% or more]
The hole expansion ratio (λ) of the zinc-based plated steel sheet is set to 20% or more. On the other hand, the upper limit of the hole expansion ratio of the zinc-based plated steel sheet is not particularly limited, but is generally 50% or less. The hole expansion ratio (λ) can be determined as follows. A hole expansion test is performed in accordance with JIS Z 2256. After shearing the test material to 100 mm × 100 mm, a 10 mm diameter hole is punched with a clearance of 12.5%. While holding the test material using a die with an inner diameter of 75 mm and a blank holder force of 9 tons (88.26 kN), a conical punch with an apex angle of 60° is pressed into the punched hole, and the occurrence of a crack penetrating the sheet thickness is confirmed. The hole diameter when a crack occurs is measured, and the limiting hole expansion ratio: λ (%) can be calculated using the following formula (5).
λ (%) = {(D _f - D ₀ )/D ₀ }×100 (5)
Here, _Df is the hole diameter (mm) when a crack occurs, and _D0 is the initial hole diameter (mm).

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 reduces the hole expansion ratio. For this reason, when plated steel sheets are shipped as products, the diffusible hydrogen is reduced by storing the steel sheets for long periods of time or by post-heating. Therefore, it is preferable to conduct hole expansion tests after the amount of diffusible hydrogen in the steel sheet has fallen to 0.01 wt. % or less.

[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 expansion 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 expansion 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 expansion test is compared with the photograph of the stretch flanged portion after 24 hours. If no increase or propagation of cracks is observed, the stretch flanged portion can be determined to have excellent delayed fracture resistance.

(Method for manufacturing zinc-based plated steel sheet)
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 is performed. Next, the cold-rolled steel sheet is subjected to a zinc-based plating treatment to obtain a plated steel sheet. Since the time for the hot-dip galvanizing treatment is shorter than that for the cooling process, etc., it is shown as a point at temperature T3 in Figure 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 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 more than 350°C.

[Steel slab]
The steel slab used in the method for producing a zinc-based plated 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.

[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 can remove 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.

[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.

[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 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 (°C) = 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.

If the holding time at the annealing temperature during the annealing process is less than 10 seconds, the area ratios of tempered martensite and fresh martensite decrease, while 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 should be 10 seconds or more, with 40 seconds or more being preferable. On the other hand, if the holding time at the annealing temperature is 500 seconds or less, increases in heating costs and longer manufacturing times can be effectively prevented, i.e., a decrease in productivity can be effectively prevented. Therefore, a holding time at the annealing temperature of 500 seconds or less is preferable.

[First cooling step: cooling 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]
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 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.

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 formulas (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 (3Bs - Ms)/3 (°C) or more. 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 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 0.95 × Bs (°C) or less.
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.

[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) satisfying 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.

When a second cooling step is performed, if the cooling rate exceeds 1.50°C/s, nucleation of ferrite and bainitic ferrite at the prior austenite grain boundaries will not proceed sufficiently, the coverage of retained austenite and fresh martensite at the prior austenite grain boundaries will increase, and it will be difficult to achieve excellent hole expandability. Therefore, when a second cooling step is performed, the cooling rate of the second cooling step should be 1.50°C/s or less. On the other hand, if the second cooling rate is 0.50°C/s or more, increased furnace heating costs can be effectively prevented. Therefore, a cooling rate of 0.50°C/s or more is preferred.

The holding time in the holding step or the cooling time in the second cooling step is a time t (s) in which 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, and the coverage of retained austenite and fresh martensite at the prior austenite grain boundaries increases excessively, making it difficult to achieve excellent hole expandability. Therefore, the time t (s) is a value in which f in formula (4) satisfies 0.010 or more. On the other hand, if f exceeds 0.200, the ferrite and bainitic ferrite generated at the prior austenite grain boundaries grow, the total area ratio of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the time t (s) is a time in which f in formula (4) satisfies 0.200 or less.
Here, [%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 _dγ (μm) represents the prior austenite grain size of the cold-rolled steel sheet at the end of the annealing step.

The prior austenite grain size _dγ can be determined as follows. After polishing an L-section (a vertical section parallel to the rolling direction) of a steel sheet, the steel sheet is corroded with a chemical solution that corrodes prior austenite grain boundaries (prior γ grain boundaries) (for example, a saturated picric acid aqueous solution or a solution thereof with added ferric chloride). Four random fields of view are observed at 1/4 of the thickness of the corroded steel sheet using an optical microscope at 400x magnification. The prior austenite grain size is measured using a photograph obtained by the observation by an 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 prior austenite grain size _dγ can be determined by dividing the total length of the 40 straight lines by the number of grain boundaries intersecting the lines and multiplying the obtained value by 1.13.

The prior austenite grain size _dγ 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 perform the measurement because each step is continuous. In such cases, the prior austenite grain size _dγ is estimated from mass production results and the time t (s) is set. In the present invention, the time t (s) is set with f in the range of 0.010 to less than 0.200 because errors resulting from estimating the prior austenite grain size _dγ are taken into consideration. The prior austenite grain size _dγ can be estimated as follows from the state before the annealing line and the conditions of the annealing step. A steel sheet (sample) manufactured with the same chemical composition, hot rolling conditions, and cold rolling ratio 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 _dγ of the annealed sample is measured using the above-mentioned method. The obtained measurement result can be estimated as the prior austenite grain size _dγ of the target steel sheet.

[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, 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 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.

If the cooling stop temperature T3 of the third cooling step is below Ms (°C), fresh martensite precipitates and is excessively tempered during alloying in hot-dip galvanizing, making it difficult to achieve a TS of 1,470 MPa or more. Therefore, T3 should be at least Ms (°C), preferably at least Ms + 20 (°C), and more preferably at least Ms + 40 (°C). 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 1,470 MPa or more. Therefore, T3 should be at most Bs - 20 (°C), preferably at most Bs - 25 (°C), and more preferably at most Bs - 30 (°C).

[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.

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

When hot-dip galvanizing and subsequent alloying are performed, setting the alloying temperature to 470°C or higher prevents the Zn-Fe alloying rate from becoming excessively slow, thereby ensuring favorable productivity. Therefore, an alloying temperature of 470°C or higher is preferred. On the other hand, setting the alloying temperature to 600°C or lower can favorably prevent untransformed austenite from transforming into pearlite, resulting in a decrease in TS. Therefore, an alloying temperature of 600°C or lower is preferred, and 560°C or lower is more preferred. Furthermore, it is preferable that the alloying treatment of galvannealed steel sheet (GA) be performed to achieve an Fe concentration of 7 to 15 mass% in the coating layer.

The series of processes from the annealing process to the plating process is not particularly limited, but from the standpoint of productivity, it is preferable to carry them out in a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line.

[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.

If the cooling stop temperature T4 of the fourth cooling step is less than Ms-200 (°C), the volume fraction of untransformed austenite will decrease, reducing the amount of retained austenite in the final structure and making it difficult to achieve excellent ductility. Therefore, T4 should be 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 will decrease, making it difficult to achieve a TS of 1,470 MPa or higher. Therefore, T4 should be Ms-80 (°C) or lower.

[Fifth cooling step: cooling 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]
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.

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

[Tempering process: holding at a tempering temperature above T5 (°C) and below 350°C for 5 seconds or more and 1000 seconds or less]
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 amount of retained austenite is not obtained, resulting in reduced ductility. Therefore, the tempering temperature is set to be higher than T5 (°C), preferably T5 + 50 (°C) or higher. On the other hand, if the tempering temperature exceeds 350°C, excessive tempering progresses, reducing strength and simultaneously decomposing retained austenite, making it difficult to achieve an El of 9% or higher. Therefore, the tempering temperature is set to be 350°C or lower, preferably 340°C or lower.

If the holding time at the tempering temperature during the tempering process is less than 5 seconds, austenite will not be sufficiently stabilized, will transform into martensite during final cooling, and the volume fraction of retained austenite in the final structure will decrease, making it difficult to achieve excellent ductility. Furthermore, insufficient tempering of martensite will make it difficult to achieve excellent hole expandability. Therefore, the holding time at the tempering temperature should be 5 seconds or more, with 40 seconds or more being preferable. On the other hand, if the holding time at the tempering temperature exceeds 1,000 seconds, excessive tempering will occur, reducing strength and causing decomposition of retained austenite, making it difficult to achieve an El of 9% or more. Therefore, the holding time at the tempering temperature should be 1,000 seconds or less, with 800 seconds or less being preferable.

In addition, conventional methods can be used for steps and conditions not described in this invention.

Steel having the chemical composition shown in Table 1, with the balance consisting of Fe and unavoidable impurities, was melted in a converter and continuously cast into a steel slab. The resulting steel slab was then hot-rolled and pickled, followed by cold-rolling to obtain a cold-rolled steel sheet. The resulting 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. Furthermore, a portion of the sample after the annealing step was sampled, and the prior austenite grain size d _γ was measured by the method described above. In the examples in which the holding step was performed, the cooling rate of the second cooling step was expressed as 0.00°C/s. In addition, the plating treatments shown in Table 2 are represented as "GI" for hot-dip galvanizing treatment, "GA" for alloyed hot-dip galvanizing treatment, and "EG" for electrogalvanizing treatment.

Using the resulting zinc-based plated steel sheet as the 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 using the methods described above. Furthermore, at a position one-quarter 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.

The resulting zinc-plated steel sheets were then used as test materials to evaluate the tensile strength TS, elongation El, hole expansion ratio λ, and delayed fracture resistance of the stretch-flanged portion using the methods described above. The evaluation results are shown in Table 3. In Table 3, examples with excellent delayed fracture resistance of the stretch-flanged portion are marked with "O," and examples with poor delayed fracture resistance of the stretch-flanged portion are marked with "X."

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, demonstrating excellent delayed fracture resistance in the stretch flanged portion. On the other hand, the comparative examples are inferior in one or more of the tensile strength, elongation, hole expansion ratio, and delayed fracture resistance in the stretch flanged portion.

The present invention provides a zinc-based plated steel sheet and its manufacturing method that has a tensile strength of 1,470 MPa or more, an elongation of 9.0% or more, a hole expansion ratio of 20% or more, and excellent delayed fracture resistance in stretch flanged areas.

Claims (6)

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,
A component composition containing N: 0.0100% or less and O: 0.0100% or less, with 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,
a structure in which 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
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%,
A zinc-based plated steel sheet having a tensile strength of 1470 MPa or more. 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: 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. 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,
A step of hot rolling a steel slab having a component composition containing N: 0.0100% or less, O: 0.0100% or less, and the balance consisting of 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] represents the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X. _dγ (μm) represents the prior austenite grain size of the cold-rolled steel sheet at the end of the annealing step. 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: The method for producing zinc-based plated steel sheet according to claim 4 or 5, wherein the zinc-based plating treatment is electrogalvanizing treatment, hot-dip galvanizing treatment, or hot-dip galvanizing treatment followed by alloying treatment.