JP7786646B2 - 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 25% 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, a TS of 1,470 MPa or more can be achieved by making the total area ratio of tempered martensite and fresh martensite 70.0% or more, the area ratio of bainite 10.0% or less, the thickness of the carbon concentration reduced portion in the surface layer of the base steel sheet 150.0 μm or less, and the total coverage of retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduced portion 30.0% or more.
(2) By setting the volume fraction of the retained austenite to 6.0% or more and 15.0% or less, and by setting the total coverage rate of the retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction portion to 60.0% or less, and by setting the carbon concentration in the retained austenite covering the prior austenite grain boundary to less than 0.60 mass%, excellent hole expandability can be achieved.
(3) By making the thickness of the bainite precipitation region immediately below the surface carbon concentration reduced portion 10 μm or more, excellent delayed fracture resistance can be achieved in the stretch flanged portion.
(4) 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) to (3) 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 15.0% or less,
a structure in which the area ratio of bainite is 10.0% or less, and the area ratio of the remaining structure is 10.0% or less;
and
The thickness of the carbon concentration reduced portion in the surface layer of the base steel sheet is 1.0 μm or more and 150.0 μm or less,
the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion is 10 μm or more,
a total coverage of retained austenite and fresh martensite at the prior austenite grain boundary directly below the carbon concentration reduction portion is 30.0% or more and 60.0% or less;
the carbon concentration of the retained austenite covering the prior austenite grain boundary immediately below the carbon concentration reduced portion is less than 0.60 mass%,
A zinc-based plated steel sheet having a tensile strength of 1470 MPa or more.

[2] A zinc-based plated steel sheet as described in [1] above, wherein the yield strength of the base steel sheet is 1000 MPa or more.

[3] 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] or [2], containing at least one selected from the group consisting of:

[4] A zinc-based plated steel sheet according to any one of [1] to [3] above, wherein the zinc-based plated layer is an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer.

[5] 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 in an atmosphere having a dew point of −25°C or higher and 35°C or lower;
Next, a first cooling step in which the cold-rolled steel sheet is cooled to a temperature T1 of T-20 (°C) or more and T+20 (°C) or less at a cooling rate of 7 °C/s or more using T defined by the following formula (2):
Next, a step of holding the cold-rolled steel sheet at T1 (°C) for 30 seconds or more and 200 seconds or less;
Next, a second cooling step of cooling the cold-rolled steel sheet to a temperature T2 less than T-20 (°C);
Next, a step of subjecting the cold-rolled steel sheet to a zinc-based plating treatment to obtain a plated steel sheet;
Next, a third cooling step is performed in which the plated steel sheet is cooled to a temperature T3 of Ms-200 (°C) or more and Ms-80 (°C) or less at a cooling rate of 5°C/s or more, using Ms defined by the following formula (3):
Next, a fourth cooling step of cooling the plated steel sheet to a cooling stop temperature T4 of 100°C or higher but lower than T3 (°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 T4 (°C) and not higher than 350°C for 10 seconds or more and not higher than 1000 seconds;
and
a total residence time during the zinc-based plating treatment and the third cooling step in which the plated steel sheet is in a temperature range of Bf (°C) or more and T-20 (°C) or less, where Bf and T are defined by the following formula (4):
Ac3 (°C) = 881-205.7×[%C]+53.1×[%Si]-15×[%Mn]-27×[%Cu]-20.1×[%Ni]-0.7×[%Cr]+41.1×[%Mo]...(1)
T (°C) = 743-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo]...(2)
Ms=539-423×[%C]-30.4×[%Mn]-17.7×[%Ni]-12.1×[%Cr]-7.5×[%Mo]...(3)
Bf (°C) = 710-270×[%C]-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo]...(4)
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.

[6] The method for producing a zinc-based plated steel sheet according to the above [5], wherein in the tempering step, a tension with an average load tension of 2.0 kgf/ ^mm2 or more is applied, and bending and unbending are performed one or more times, 6 seconds or more after the start of holding at the tempering temperature and 10 seconds or more before the end of holding at the tempering temperature.

[7] 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 [5] or [6], comprising at least one selected from the group consisting of:

[8] A method for producing a zinc-based plated steel sheet according to any one of [5] to [7] 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 25% 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 area fraction of bainite. If the C content is less than 0.180%, the total area fraction of tempered martensite and fresh martensite decreases, the area fraction of bainite increases, and it becomes 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 fraction of tempered martensite and fresh martensite, as well as the area fraction of bainite. If the Mn content is less than 2.400%, the total area fraction of tempered martensite and fresh martensite decreases, the area fraction of bainite increases, and it becomes 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.

[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 that is ¼ 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 15.0%, the area fraction of bainite is 10.0% or less, and the area fraction of the remaining structure is 10.0% or less. Furthermore, at a position at a depth of 1/4 of the plate thickness from the surface of the substrate steel plate, the thickness of the carbon concentration reduced portion in the surface layer of the substrate steel plate is 1.0 μm or more and 150.0 μm or less, the thickness of the bainite precipitation region immediately below the carbon concentration reduced portion is 10 μm or more, the total coverage rate of the retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduced portion is 30.0% or more and 60.0% or less, and the carbon concentration of the retained austenite covering the prior austenite grain boundary immediately below the carbon concentration reduced portion is less than 0.60 mass%.

[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 15.0% 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 15.0%, it becomes difficult to achieve excellent hole expandability. Therefore, the volume fraction of retained austenite is set to 15.0% or less, preferably 13.0% or less, and more preferably 11.0% or less.

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.

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

The area fraction of bainite can be determined as follows. After polishing the L-section of the base 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, bainite is a concave structure with a flat interior. The area fraction of bainite is determined in each field of view, and the average of these values is taken as the area fraction of bainite.

[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.

[Thickness of carbon concentration reduced portion in surface layer is 1.0 μm or more and 150.0 μm or less]
If the thickness of the carbon concentration reduced portion in the surface layer is less than 1.0 μm, it is not possible to provide a region with a high bainite precipitation start temperature in the surface layer, and it is not possible to control the area ratio of bainite and the thickness of the bainite precipitation region (described later), making it difficult to achieve excellent delayed fracture resistance in the stretch flangeable portion. Therefore, the thickness of the carbon concentration reduced portion in the surface layer is set to 1.0 μm or more, and preferably 3.0 μm or more. On the other hand, if the thickness of the carbon concentration reduced portion in the surface layer exceeds 150.0 μm, it becomes difficult to achieve a TS of 1470 MPa or more. Therefore, the thickness of the carbon concentration reduced portion in the surface layer is set to 150.0 μm or less, and preferably 50.0 μm or less.

The thickness of the carbon concentration reduced portion in the surface layer can be determined as follows. A test piece 20 mm long and 20 mm wide is taken from a zinc-based plated steel sheet. The carbon intensity of the test piece is measured in the depth direction from the surface of the steel sheet using glow discharge spectroscopy (GDS). From the measurement results, a region where the carbon intensity is equal to or less than (the carbon intensity when the measured value in GDS becomes a constant value × 0.8) is determined to be the carbon concentration reduced portion in the surface layer, and the thickness of this region is determined to be the thickness of the carbon concentration reduced portion. The GDS device used is a GD-PROFILER 2 manufactured by Horiba, Ltd., and the measurement conditions are as follows:
Power supply: High frequency Anode diameter: 4 mm
Measurement mode: Constant power 35W
Carrier gas: Argon Discharge gas pressure: 300 Pa
Pulse frequency: 100Hz
・Ducty cycle: 50%
Capture interval: 100 ms

[Thickness of bainite precipitation region directly below carbon concentration reduction portion is 10 μm or more]
If the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion is less than 10 μm, it becomes difficult to achieve excellent delayed fracture resistance in the stretch flangeable portion. Therefore, the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion is set to 10 μm or more. On the other hand, there is no particular upper limit to the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion, and the thickness is generally 100 μm or less.

The thickness of the bainite precipitation region directly below the carbon concentration reduction area can be determined as follows. After polishing the L-section of a zinc-based plated steel sheet, it is etched with 3 vol. % nital, and the above-mentioned GDS analysis is performed on the steel sheet surface using an SEM. The area after the carbon concentration reduction area determined by the GDS analysis, where bainite precipitation is observed, is observed in 10 fields of view at 3000x magnification. In each observed field of view, the bainite is a concave structure with a flat interior. Note that if the bainite cannot be contained in one field of view, photographs of consecutive fields of view are taken. The area fraction of bainite is determined in each field of view, and the area where the bainite area fraction is 20% to 60% is defined as the bainite precipitation region. The thickness of the bainite precipitation region in the sheet thickness direction is determined in each field of view, and the average value is defined as the thickness of the bainite precipitation region.

[The total coverage rate of retained austenite and fresh martensite at the prior austenite grain boundary directly below the carbon concentration reduction portion is 30.0% or more and 60.0% or less]
By setting the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduction portion to 30.0% or more, a TS of 1470 MPa or more can be achieved. Therefore, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduction portion is set to 30.0% or more, and preferably 40.0% or more. On the other hand, if the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduction portion exceeds 60.0%, it becomes difficult to achieve the hole expandability, which is one of the excellent formability features. Therefore, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduction portion is set to 60.0% or less, and preferably 55.0% or less.

The total coverage of retained austenite and fresh martensite at the prior austenite grain boundary directly below the carbon concentration reduction zone can be determined as follows. After polishing the L-section of a zinc-based plated steel sheet, it is etched with 3 vol. % nital, and the above-described GDS analysis is performed on the steel sheet surface using an SEM. Ten prior austenite grain boundaries are randomly selected from the area following the carbon concentration reduction zone determined by the GDS analysis and closest to the carbon concentration reduction zone. On the selected prior austenite grain boundary, a convex portion with a width of 50 nm or more perpendicular to the prior austenite grain boundary and a smooth surface is defined as the area covered by retained austenite and fresh martensite. The total circumferential length of the area 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 to determine the total coverage of retained austenite and fresh martensite at the prior austenite grain boundary.

[Carbon concentration of retained austenite covering prior austenite grain boundaries directly below the carbon concentration reduced portion is less than 0.60 mass%]
If the carbon concentration of the retained austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction portion is 0.60 mass% or more, a second phase having a hardness significantly different from that of the matrix is present, and the second phase becomes a stress concentration portion during processing, deteriorating the delayed fracture resistance. Therefore, the carbon concentration of the retained austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction portion is less than 0.60 mass%, and preferably 0.55 mass% or less. On the other hand, the lower limit of the carbon concentration of the retained austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction portion is not particularly limited, and the carbon concentration is generally 0.30 mass% or more.

The carbon concentration in the retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduction zone can be determined as follows. First, the surface of the steel plate (sample) is polished using diamond paste so that 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. The cleaned observation surface 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 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, a region (high carbon region) where the carbon concentration is equal to or greater than the average of the matrix and less than 0.6 mass% is identified. Furthermore, the carbon concentration reduction area on the observation surface is identified using the above-described method, and prior austenite grains directly below the carbon concentration reduction area 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 boundary immediately below the carbon concentration reduction portion of the 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 a galvannealed 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.

[Yield strength (YS) is 1000 MPa or more (preferred condition)]
The zinc-based plated steel sheet preferably has a yield strength of 1000 MPa or more. On the other hand, the yield strength of the zinc-based plated steel sheet is preferably 1350 MPa or less. The yield strength can be determined by a tensile test, similar to the tensile strength.

[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 a tensile test, similar to the tensile strength.

[Hole expansion ratio (λ) is 25% or more]
The hole expansion ratio (λ) of the zinc-based plated steel sheet is set to 25% 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. Therefore, 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 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 production 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. After 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 heated and subjected to an annealing process in which it is held at an annealing temperature of Ac3 (°C) or higher in an atmosphere with a dew point of −25°C or higher and 35°C or lower. Next, a first cooling process is performed in which the cold-rolled steel sheet is cooled to a temperature T1. Next, a holding process is performed in which the cold-rolled steel sheet is held at T1 (°C) for a time period of 30 seconds to 200 seconds. Next, a second cooling process is performed in which the cold-rolled steel sheet is cooled to a temperature T2. 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 T2 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 third cooling step is performed in which the plated steel sheet is cooled to a temperature T3. Next, a fourth cooling step is performed in which the plated steel sheet is cooled to a cooling stop temperature T4. Next, a tempering step is performed in which the plated steel sheet is maintained at a tempering temperature above T4 (°C) and not higher than 350°C. In this manufacturing method, the total residence time of the plated steel sheet in a temperature range of not lower than Bf (°C) and not higher than T-20 (°C) during the plating treatment step and the third cooling step is not longer than 30 seconds. Furthermore, in the tempering step, it is preferable that an average load tension of not lower than 2.0 kgf/ ^mm2 is applied, and the plated steel sheet is bent and unbent one or more times, at least 6 seconds after the start of holding at the tempering temperature and at least 10 seconds before the end of holding at the tempering temperature.

[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 step: maintaining an annealing temperature of Ac3 (°C) or higher for 10 seconds or more in an atmosphere with a dew point of -25°C or higher and 35°C or lower]
The obtained cold-rolled steel sheet is subjected to an annealing process. If the dew point during annealing is less than −25°C, the thickness of the carbon concentration reduction portion of the surface layer formed during annealing will be less than 1.0 μm, and a region with a high bainite precipitation start temperature cannot be sufficiently provided in the surface layer, making it impossible to achieve a thickness of 10 μm or more of the bainite precipitation region directly below the carbon concentration reduction portion. As a result, it becomes difficult to achieve excellent delayed fracture resistance in the stretch flangeable portion. Therefore, the dew point during annealing is set to −25°C or higher, and preferably −20°C or higher. On the other hand, if the dew point during annealing exceeds 35°C, the thickness of the carbon concentration reduction portion of the surface layer formed during annealing will exceed 150.0 μm, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the dew point during annealing is set to 35°C or lower.

In the annealing step, 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 area ratio of bainite increases, making it difficult to achieve a TS of 1,470 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 (°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 ratio of tempered martensite and fresh martensite decreases and the area ratio of bainite increases, making it difficult to achieve a TS of 1470 MPa or 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 T-20 (°C) or more and T+20 (°C) or less at a cooling rate of 7°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 7°C/s, the area fractions of tempered martensite and fresh martensite decrease and the area fraction of bainite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the cooling rate in the first cooling step is set to 7°C/s or more, and preferably 9°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 T-20 (°C), where T is defined by the following formula (2), bainite formation occurs not only in areas directly below the carbon concentration reduction portion of the surface layer, the area ratio of bainite increases, and it becomes difficult to achieve a TS of 1470 MPa or more. Therefore, T1 is set to T-20 (°C) or more. On the other hand, if T1 exceeds T+20 (°C), a bainite precipitation region is not formed directly below the carbon concentration reduction portion of the surface layer, making it difficult to achieve excellent delayed fracture resistance in the stretch flangeable portion. In addition, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundary directly below the carbon concentration reduction portion becomes excessive, making it difficult to achieve a hole expansion ratio of 25% or more. Therefore, T1 is set to T+20 (°C) or less.
T (°C) = 743-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo]...(2)
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: holding at T1 (°C) for 30 s to 200 s]
Following the first cooling step, the cold-rolled steel sheet is subjected to a holding step in which it is held at T1 (°C). If the holding time in the holding step is less than 30 s, the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion becomes less than 10 μm, making it difficult to achieve excellent delayed fracture resistance in the stretch flangeable portion. In addition, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction portion becomes excessive, making it difficult to achieve a hole expansion ratio of 25% or more. Therefore, the holding time for the intermediate holding is set to 30 s or more, and preferably 40 s or more. On the other hand, if the holding time for the intermediate holding exceeds 200 s, excessive grain growth of bainite precipitated at the prior austenite grain boundary immediately below the carbon concentration reduction portion occurs, making the total coverage of retained austenite and fresh martensite at the prior austenite grain boundary less than 30%, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the holding time for the intermediate holding is set to 200 s or less, and preferably 180 s or less.

[Second cooling step: cooling to a temperature T2 lower than T-20 (°C)]
Following the holding step, the cold-rolled steel sheet is subjected to a second cooling step. Therefore, the cooling stop temperature T2 of the second cooling step is less than T1 (°C). If the cooling stop temperature T2 of the second cooling step is T-20 (°C) or higher, excessive grain growth of bainite precipitated at the prior austenite grain boundaries immediately below the carbon concentration reduction portion occurs, and the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries becomes less than 30%, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the second cooling stop temperature T2 is set to less than T-20 (°C), preferably to T-30 (°C) or lower. On the other hand, if the second cooling stop temperature T2 is set to Ms (°C) or higher, when the alloying treatment described below is performed, precipitation of martensite before alloying is suitably prevented, and excessive tempering is not performed during alloying, thereby suitably achieving a TS. Therefore, the second cooling stop temperature T2 is preferably set to Ms (°C) or higher.

When the cooling rate in the second cooling step is 1°C/s or more, excessive nucleation and growth of bainite during cooling is effectively prevented, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries is effectively obtained, and TS is effectively obtained. Therefore, the cooling rate in the second cooling step is preferably 1°C/s or more, and more preferably 2°C/s or more. On the other hand, when the cooling rate in the second cooling step is 20°C/s or less, the cooling stop temperature can be effectively controlled. Therefore, the cooling rate in the second cooling step is preferably 20°C/s or less.

[Plating process]
Following the second 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 alloying. Electroplating such as Zn—Ni electroplating or hot-dip zinc-aluminum-magnesium alloy plating may 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, as defined by the following formula (3):
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.

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.

[Third cooling step: cooling to a temperature T3 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 third cooling step. If the cooling rate in the third cooling step is less than 5°C/s, the total area fraction of tempered martensite and fresh martensite decreases, and the area fraction of bainite increases, 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. On the other hand, if the third cooling rate is 20°C/s or less, the cooling stop temperature can be suitably controlled. Therefore, the cooling rate in the third cooling step is preferably 20°C/s or less.

If the cooling stop temperature T3 of the third cooling step is less than Ms-200 (°C), the volume fraction of untransformed austenite decreases, reducing the amount of retained austenite in the final structure and making it difficult to achieve excellent ductility. Furthermore, the carbon concentration of the retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduction area will be 0.60 mass% or more, resulting in inferior delayed fracture resistance in the stretch flangeable area. Therefore, T3 should be Ms-200 (°C) or higher. On the other hand, if T3 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, T3 should be Ms-80 (°C) or lower.

[The total residence time during the plating treatment step and the third cooling step in which the temperature of the plated steel sheet is in the temperature range of Bf (°C) or more and T-20 (°C) or less is 30 seconds or less]
If the total residence time in the temperature range of Bf (°C) or more and T-20 (°C) or less, where Bf is defined by the following formula (4) and T, exceeds 30 seconds during the plating process and the third cooling process described below, the area fraction of bainite increases, making it difficult to achieve a TS of 1,470 MPa or more. Therefore, the total residence time in which the temperature of the plated steel sheet is in the temperature range of Bf (°C) or more and T-20 (°C) or less during the plating process and the third cooling process is set to 30 seconds or less. On the other hand, there is no particular limitation on the lower limit of the total residence time in which the temperature of the plated steel sheet is in the temperature range of Bf (°C) or more and T-20 (°C) or less during the plating process and the third cooling process, but the residence time is generally 20 seconds or more.
Bf (°C) = 710-270×[%C]-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo]...(4)
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.

[Fourth cooling step: cooling to a cooling stop temperature T4 of 100°C or higher and lower than T3 (°C) at a cooling rate of 3.0°C/s or less]
Following the third cooling step, the plated steel sheet is subjected to a fourth cooling step. If the cooling rate in the fourth cooling step exceeds 3.0°C/s, carbon does not sufficiently diffuse into untransformed austenite during the fourth cooling step, resulting in a decrease in the volume fraction of retained austenite in the final structure, making it difficult to achieve excellent ductility and excellent delayed fracture resistance in the stretch flangeable portion. Therefore, the cooling rate in the fourth 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, if the fourth cooling rate is 0.5°C/s or more, this is preferable because cooling can be performed without heating in a furnace. Therefore, the cooling rate in the fourth cooling step is preferably 0.5°C/s or more.

If the cooling stop temperature T4 of the fourth 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, T4 should be 100°C or higher, preferably 110°C or higher, and more preferably 120°C or higher. Note that, because the fourth cooling step is performed following the third cooling step, T4 will be less than T3 (°C). Furthermore, from the perspective of reducing fresh martensite, T4 is preferably Ms-100 (°C) or lower.

[Tempering process: holding at a tempering temperature above T4 (°C) and below 350°C for 10 seconds or more and 1000 seconds or less]
Following the fourth 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 T4 (°C) or lower, the desired amount of retained austenite cannot be obtained, making it difficult to achieve excellent ductility and excellent delayed fracture resistance in stretch flanged sections. Therefore, the tempering temperature is set to be higher than T4 (°C), preferably T4 + 50 (°C) or higher. On the other hand, if the tempering temperature exceeds 350°C, excessive tempering progresses, resulting in a decrease in strength and decomposition of the 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 10 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 10 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, resulting in a decrease in strength and 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 the tempering step, a tension of an average load tension of 2.0 kgf/ ^mm2 or more is applied, and bending and unbending are performed at least once, 6 seconds or more after the start of holding at the tempering temperature and 10 seconds or more before the end of holding at the tempering temperature (preferred conditions)]
In the tempering process, it is preferable to apply a tension with an average load tension of 2.0 kgf/ ^mm2 or more, and perform bending and unbending at least once, 6 seconds or more after the start of holding at the tempering temperature and 10 seconds or more before the end of holding at the tempering temperature. By applying tension and performing bending and unbending 6 seconds or more after the start of holding at the tempering temperature, carbon is suitably concentrated in the untransformed austenite after cooling is stopped, and transformation to fresh martensite can be suitably suppressed. As a result, the volume fraction of retained austenite in the final structure does not decrease, and ductility is suitably obtained. The coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduction portion is suitably obtained, and tensile strength TS is suitably obtained. On the other hand, in the tempering process, by applying tension and performing bending and unbending 10 seconds or more before the end of holding, tempering is suitably performed after the untransformed austenite transforms to fresh martensite, and yield strength (YS) and hole expansion ratio (λ) are suitably obtained.

In the tempering process, if the tension applied when performing the above-mentioned one or more bending and unbending processes is 2.0 kgf/ ^mm2 or more, the unstable retained austenite undergoes deformation-induced transformation during tempering, thereby favorably obtaining yield strength and hole expansion ratio. Therefore, it is preferable that the tension applied in the tempering process be 2.0 kgf/ ^mm2 or more. On the other hand, if the tension applied in the tempering process is 5.0 kgf/ ^mm2 or less, it is possible to favorably prevent a decrease in untransformed austenite during tempering and to favorably prevent strain from remaining in the steel sheet due to the application of tension, thereby favorably obtaining product characteristics. Therefore, it is preferable that the tension applied in the tempering process be 5.0 kgf/ ^mm2 or less.

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

Steels having the chemical compositions shown in Table 1, with the balance consisting of Fe and unavoidable impurities, were melted in a converter and continuously cast into steel slabs. The resulting steel slabs were then hot-rolled and pickled, followed by cold rolling. The slabs were then sequentially subjected to an annealing step, a first cooling step, a holding step, a second cooling step, a plating step, a third cooling step, a fourth cooling step, and a tempering step under the conditions shown in Table 2. In the tempering step, all samples except for No. 25 were subjected to a mean load tension of 2.0 kgf/ ^mm² at the timings shown in Table 2, and were subjected to one bending and unbending. As a result, zinc-based plated steel sheets having thicknesses of 0.8 to 2.4 mm were obtained. Regarding the plating treatments shown in Table 2, hot-dip galvanizing treatment is designated "GI," galvannealed hot-dip galvanizing treatment is designated "GA," and electrogalvanizing treatment is designated "EG."

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 area fraction of bainite, and the area fraction of the remaining structure were determined using the methods described above. Furthermore, at a position one-quarter of the test material's thickness, the thickness of the carbon-depleted area in the surface layer of the substrate steel sheet, the thickness of the bainite precipitation region immediately below the carbon-depleted area, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries immediately below the carbon-depleted area, and the carbon concentration of 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, yield strength YS, 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 25% or more, and are excellent in delayed fracture resistance of the stretch flanged portion. Furthermore, in the tempering process, an average load tension of 2.0 kgf/ ^mm2 or more is applied 6 seconds or more after the start of holding at the tempering temperature and 10 seconds or more before the end of holding at the tempering temperature, and bending and unbending are performed at least once (examples of the present invention other than No. 25), the yield strength YS is 1000 MPa or more. On the other hand, the comparative examples are inferior in one or more of TS, hole expandability, ductility, and delayed fracture resistance of the stretch flanged portion.

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 25% or more, and excellent delayed fracture resistance in stretch flanged areas.

Claims (10)

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 15.0% or less,
a structure in which the area ratio of bainite is 10.0% or less, and the area ratio of the remaining structure is 10.0% or less;
and
The thickness of the carbon concentration reduced portion in the surface layer of the base steel sheet is 1.0 μm or more and 150.0 μm or less,
the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion is 10 μm or more,
a total coverage of retained austenite and fresh martensite at the prior austenite grain boundary directly below the carbon concentration reduction portion is 30.0% or more and 60.0% or less;
the carbon concentration of the retained austenite covering the prior austenite grain boundary immediately below the carbon concentration reduced portion is less than 0.60 mass%,
A zinc-based plated steel sheet having a tensile strength of 1470 MPa or more. The zinc-based plated steel sheet according to claim 1, wherein the yield strength of the substrate steel sheet is 1000 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 or 2, 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. The zinc-based plated steel sheet according to claim 3, 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 in an atmosphere having a dew point of −25°C or higher and 35°C or lower;
Next, a first cooling step in which the cold-rolled steel sheet is cooled to a temperature T1 of T-20 (°C) or more and T+20 (°C) or less at a cooling rate of 7 °C/s or more using T defined by the following formula (2):
Next, a step of holding the cold-rolled steel sheet at T1 (°C) for 30 seconds or more and 200 seconds or less;
Next, a second cooling step of cooling the cold-rolled steel sheet to a temperature T2 less than T-20 (°C);
Next, a step of subjecting the cold-rolled steel sheet to a zinc-based plating treatment to obtain a plated steel sheet;
Next, a third cooling step is performed in which the plated steel sheet is cooled to a temperature T3 of Ms-200 (°C) or more and Ms-80 (°C) or less at a cooling rate of 5°C/s or more, using Ms defined by the following formula (3):
Next, a fourth cooling step of cooling the plated steel sheet to a cooling stop temperature T4 of 100°C or higher and lower than T3 (°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 T4 (°C) and not higher than 350°C for 10 seconds or more and not higher than 1000 seconds;
and
during the zinc-based plating treatment and the third cooling step, a total residence time during which the temperature of the plated steel sheet is in a temperature range of Bf (°C) or more and T-20 (°C) or less, using Bf and T defined by the following formula (4) is 30 s or less,
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 depth from the surface of the substrate steel sheet that is 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 15.0% or less, the area ratio of bainite is 10.0% or less, and the area ratio of a residual structure is 10.0% or less, and the thickness of a carbon concentration reduced portion in the surface layer of the substrate steel sheet is 1.0 μm or more and 150.0 μm or less the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion is 10 μm or more, the total coverage rate of retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction portion is 30.0% or more and 60.0% or less, the carbon concentration of the retained austenite covering the prior austenite grain boundary immediately below the carbon concentration reduction portion 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)
T (°C) = 743-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo]...(2)
Ms=539-423×[%C]-30.4×[%Mn]-17.7×[%Ni]-12.1×[%Cr]-7.5×[%Mo]...(3)
Bf (°C) = 710-270×[%C]-90×[%Mn]-37×[%Ni]-70×[%Cr]-83×[%Mo]...(4)
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. 7. The method for producing a zinc-based plated steel sheet according to claim 6, wherein in the tempering step, a tension with an average load tension of 2.0 kgf/ ^mm2 or more is applied, and bending and unbending are performed one or more times, 6 seconds or more after the start of holding at the tempering temperature and 10 seconds or more before the end of holding at the tempering temperature. 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 6 or 7, comprising: The method for producing zinc-based plated steel sheet according to claim 6 or 7, wherein the zinc-based plating treatment is electrogalvanizing treatment, hot-dip galvanizing treatment, or hot-dip galvanizing treatment followed by alloying treatment. The method for producing zinc-based plated steel sheet according to claim 8, 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.