EP4786629A1

EP4786629A1 - Zinc-based plated steel sheet and production method therefor

Info

Publication number: EP4786629A1
Application number: EP24917023.4A
Authority: EP
Inventors: Ryusuke ISHITO; Lingling Yang; Shigehiro Takajo; Masaki Tada
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2024-01-09
Filing date: 2024-12-11
Publication date: 2026-08-05
Also published as: WO2025150338A1; CN122459480A; JPWO2025150338A1; JP7786646B2

Abstract

Provided are a zinc or zinc alloy coated or plated steel sheet having excellent delayed fracture resistance in stretch flanged areas, and a method of producing same. The zinc or zinc alloy coated or plated steel sheet includes a base steel sheet and a zinc or zinc alloy coated or plated layer formed on the surface of the base steel sheet. The base steel sheet has a defined chemical composition. The base steel sheet has a defined microstructure at a 1/4 sheet thickness position. A thickness of a 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. A thickness of a bainite precipitation region directly below the carbon concentration reduced portion is 10 µm or more. A total coverage of retained austenite and fresh martensite at prior austenite grain boundaries directly below the carbon concentration reduced portion is 30.0 % or more and 60.0 % or less. A carbon concentration of retained austenite covering the prior austenite grain boundaries is less than 0.60 mass%. The tensile strength is 1470 MPa or more.

Description

TECHNICAL FIELD

The present disclosure relates to zinc or zinc alloy coated or plated steel sheets and methods of producing same.

BACKGROUND

In recent years, in the automobile industry, from the viewpoint of preserving the global environment, improving the fuel efficiency of automobiles in order to decrease CO₂ emissions has become an important issue. Decreasing the weight of automobile bodies is an effective way to improve the fuel efficiency of automobiles, but maintaining the strength of the automobile bodies is also necessary. Therefore, efforts have been made to decrease the weight of automobile bodies by increasing the strength of the steel sheets used as materials for automobile parts, thereby simplifying structure and decreasing the number of parts while maintaining strength.
However, when a high-strength steel sheet having a TS of 1470 MPa or more is formed into a part by cold pressing, bending, or the like, delayed fracture may occur due to an increase in residual stress within the part and a deterioration in the delayed fracture resistance of the steel sheet. Here, delayed fracture refers to a phenomenon in which, when a formed part is placed in a hydrogen entry environment, hydrogen enters into the steel sheet that constitutes the part, decreasing the interatomic bonding strength or causing local deformation, resulting in microcracks, which then propagate and lead to fracture of the steel sheet.
To address this, in Patent Literature (PTL) 1, a coated steel sheet is described that has excellent mechanical properties, decreases the amount of hydrogen that enters during production, and has excellent hydrogen embrittlement resistance and coating adhesion, and a method of producing same is also described. Further, in PTL 2, an ultra-high-strength steel sheet is described that has excellent hydrogen embrittlement resistance by control of the chemical composition and retained austenite of the steel sheet, and a method of producing same is also described.

CITATION LIST

Patent Literature

PTL 1: WO 2019/212047 A1
PTL 2: JP 2007-197819 A

SUMMARY

(Technical Problem)

PTL 1 and PTL 2 do not take into consideration the delayed fracture resistance of stretch-flanged portions, which are subjected to particularly severe working, and the inventors have conducted studies and found that there is room for improvement. Further, PTL 2 recommends that the carbon concentration in the retained austenite be 0.8 mass% or more, but the inventors have conducted studies and revealed that there is room for improvement in hole expansion formability.
In view of the above problems, it would be helpful to provide a zinc or zinc alloy coated or plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (E1) of 9.0 % or more, a hole expansion ratio (λ) of 25 % or more, and excellent delayed fracture resistance in a stretch-flanged portion, and a method of producing same.

(Solution to Problem)

In order to solve the aforementioned problems, the inventors conducted intensive studies and made the following discoveries.

(1) By making a total area fraction of tempered martensite and fresh martensite 70.0 % or more and an area fraction of bainite 10.0 % or less at a 1/4 thickness position of a base steel sheet, and by making a thickness of a carbon concentration reduced portion in a surface layer of the base steel sheet 150.0 µm or less, and a total coverage of retained austenite and fresh martensite at a prior austenite grain boundary directly below the carbon concentration reduced portion 30.0 % or more, a TS of 1470 MPa or more is achievable.
(2) By making a volume fraction of the retained austenite 6.0 % or more and 15.0 % or less, and by making the total coverage of the retained austenite and fresh martensite at prior austenite grain boundaries directly below the carbon concentration reduced portion 60.0 % or less, and by making a carbon concentration in the retained austenite covering the prior austenite grain boundaries less than 0.60 mass%, excellent hole expansion formability is achievable.
(3) By making the thickness of a bainite precipitation region directly below the carbon concentration reduced portion 10 µm or more, excellent delayed fracture resistance is achievable in a stretch-flanged portion.
(4) When producing a zinc or zinc alloy coated or plated steel sheet, a steel slab having a defined chemical composition is used, and holding time or cooling time and cooling rate are controlled in a holding process or each cooling process after an annealing process and a coating or plating treatment process, whereby a zinc or zinc alloy coated or plated steel sheet having a microstructure that satisfies the above (1) to (3) is obtainable.

Primary features of the present disclosure are as follows.

[1] A zinc or zinc alloy coated or plated steel sheet comprising a base steel sheet and a zinc or zinc alloy coated or plated layer formed on a surface of the base steel sheet,
- wherein the base steel sheet
- has a chemical composition containing (consisting of), in mass%,
  - C: 0.180 % or more and 0.250 % or less,
  - Si: 0.800 % or more and 1.550 % or less,
  - Mn: 2.400 % or more and 3.200 % or less,
  - P: 0.100 % or less,
  - S: 0.0200 % or less,
  - Al: 1.000 % or less,
  - N: 0.0100 % or less, and
  - O: 0.0100 % or less,
- with the balance being Fe and inevitable impurity,
- wherein, at a position where the depth from the surface of the base steel sheet is 1/4 of the sheet thickness,
  - a total area fraction of tempered martensite and fresh martensite is 70.0 % or more and 94.0 % or less,
  - a volume fraction of retained austenite is 6.0 % or more and 15.0 % or less,
  - an area fraction of bainite is 10.0 % or less, and
  - an area fraction of the residual microstructure is 10.0 % or less,
- wherein a thickness of a carbon concentration reduced portion in a surface layer of the base steel sheet is 1.0 µm or more and 150.0 µm or less,
- wherein a thickness of a bainite precipitation region directly below the carbon concentration reduced portion is 10 µm or more,
- wherein a total coverage of retained austenite and fresh martensite at prior austenite grain boundaries directly below the carbon concentration reduced portion is 30.0 % or more and 60.0 % or less,
- wherein a carbon concentration of retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduced portion is less than 0.60 mass%,
- and wherein the tensile strength is 1470 MPa or more.
[2] The zinc or zinc alloy coated or plated steel sheet according to [1], wherein the base steel sheet has a yield stress of 1000 MPa or more.
[3] The zinc or zinc alloy coated or plated steel sheet according to [1] or [2], wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of:
- 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.
[4] The zinc or zinc alloy coated or plated steel sheet according to any one of [1] to [3], wherein the zinc or zinc alloy coated or plated layer is an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed layer.
[5] A method of producing a zinc or zinc alloy coated or plated steel sheet comprising:
- a process of hot rolling a steel slab having a chemical composition containing (consisting of), in mass%,
  - C: 0.180 % or more and 0.250 % or less,
  - Si: 0.800 % or more and 1.550 % or less,
  - Mn: 2.400 % or more and 3.200 % or less,
  - P: 0.100 % or less,
  - S: 0.0200 % or less,
  - Al: 1.000 % or less,
  - N: 0.0100 % or less, and
  - O: 0.0100 % or less,
- with the balance being Fe and inevitable impurity, to obtain a hot-rolled steel sheet;
- a process of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet;
- an annealing process of holding the cold-rolled steel sheet at an annealing temperature of Ac3 (°C) or higher, where Ac3 is defined by the following expression (1), for 10 s or longer in an atmosphere having a dew point of -25 °C or higher and 35 °C or lower;
- a subsequent first cooling process of cooling the cold-rolled steel sheet to a temperature T1 of T - 20 (°C) or higher and T + 20 (°C) or lower, at a cooling rate of 7 °C/s or more, where T is defined by the following expression (2);
- a subsequent process of holding the cold-rolled steel sheet at T1 (°C) for 30 s or longer and 200 s or shorter;
- a subsequent second cooling process of cooling the cold-rolled steel sheet to a temperature T2 of lower than T - 20 (°C);
- a subsequent process of subjecting the cold-rolled steel sheet to a zinc or zinc alloy coating or plating treatment to obtain a coated or plated steel sheet;
- a subsequent third cooling process of cooling the coated or plated steel sheet to a temperature T3 of Ms - 200 (°C) or higher and Ms - 80 (°C) or lower, at a cooling rate of 5 °C/s or more, where Ms is defined by the following expression (3);
- a subsequent fourth cooling process of cooling the coated or 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;
- and a subsequent tempering process of holding the coated or plated steel sheet at a tempering temperature of higher than T4 (°C) and 350 °C or lower for 10 s or longer and 1000 s or shorter,
- wherein a total residence time from the zinc or zinc alloy coating or plating treatment to the third cooling process in which the coated or plated steel sheet is in a temperature range from Bf (°C) to T - 20 (°C), where Bf are defined by the following expression (4), is 30 s or shorter, Ac3 (°C) = 881 - 205.7 × [%C] + 53.1 × [%Si] - 15 × [%Mn] - 27 × [%Cu] - 20.1 × [%Ni] - 0.7 × [%Cr] + 41.1 × [%Mo] ...(1) $T (° C) = 743 - 90 \times [% Mn] - 37 \times [% Ni] - 70 \times [% Cr] - 83 \times [% Mo]$ 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)
- wherein [%X] indicates the content in mass% of element X in the chemical composition, and is 0 when the chemical composition does not contain element X.
[6] The method of producing a zinc or zinc alloy coated or plated steel sheet according to [5], wherein, in the tempering process, a tension with an average load tension of 2.0 kgf/mm² or more is applied, and bending and unbending are carried out one or more times, at a time at least 6 s after the start of the holding at the tempering temperature and at least 10 s before the end of the holding at the tempering temperature.
[7] The method of producing a zinc or zinc alloy coated or plated steel sheet according to [5] or [6], the chemical composition further containing, in mass%, at least one selected from the group consisting of:
- 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.
[8] The method of producing a zinc or zinc alloy coated or plated steel sheet according to any one of [5] to [7], wherein the zinc or zinc alloy coating or plating treatment is an electrogalvanization treatment, a hot-dip galvanizing treatment, or a hot-dip galvanizing treatment followed by an alloying treatment.

(Advantageous Effect)

According to the present disclosure, it is possible to provide a zinc or zinc alloy coated or plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0 % or more, a hole expansion ratio (λ) of 25 % or more, and excellent delayed fracture resistance in a stretch-flanged portion, and a method of producing same.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:
FIG. 1 is a graph illustrating a relationship between temperature and time in the method of producing a galvannealed steel sheet according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

The following describes embodiments of a zinc or zinc alloy coated or plated steel sheet according to the present disclosure and a method of producing same. The embodiments described below are example embodiments of the present disclosure, and do not limit configuration to the specific examples described.

(Chemical composition of base steel sheet)

The zinc or zinc alloy coated or plated steel sheet according to an embodiment of the present disclosure includes a base steel sheet and a zinc or zinc alloy coated or plated layer formed on the surface of the base steel sheet. The base steel sheet has a chemical composition containing, in mass%, C: 0.180 % or more and 0.250 % or less, Si: 0.800 % or more and 1.550 % or less, Mn: 2.400 % or more and 3.200 % or less, P: 0.100 % or less, S: 0.0200 % or less, Al: 1.000 % or less, N: 0.0100 % or less, and O: 0.0100 % or less, with the balance being Fe and inevitable impurity. The basic components of the base steel sheet are described below. In the following description, "%" representing the content of a component element in the base 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 disclosure in particular, is an important element that affects the total area fraction of tempered martensite and fresh martensite, and the area fraction of bainite. When C content is less than 0.180 %, 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. The C content is therefore 0.180 % or more. The C content is preferably 0.200 % or more. The C content is more preferably 0.210 % or more. On the other hand, when the C content exceeds 0.250 %, the tempered martensite and fresh martensite become embrittled, and the delayed fracture resistance of a stretch-flanged portion deteriorates. The C content is therefore 0.250 % or less. The C content is 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 TS and the volume fraction of retained austenite. When Si content is less than 0.800 %, the strength of tempered martensite and fresh martensite decreases, making it difficult to achieve a TS of 1470 MPa or more. The Si content is therefore 0.800 % or more. The Si content is preferably 0.850 % or more. The Si content is more preferably 0.900 % or more. On the other hand, when the Si content exceeds 1.550 %, the amount of retained austenite increases excessively, and the hole expansion formability decreases. The Si content is therefore 1.550 % or less. The Si content is preferably 1.500 % or less. The Si content is 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 an important element that affects the total area fraction of tempered martensite and fresh martensite, and the area fraction of bainite. When Mn content is less than 2.400 %, 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. The Mn content is therefore 2.400 % or more. The Mn content is preferably 2.500 % or more. The Mn content is more preferably 2.600 % or more. On the other hand, when the Mn content exceeds 3.200 %, the tempered martensite and fresh martensite become embrittled, and the delayed fracture resistance of a stretch-flanged portion decreases. The Mn content is therefore 3.200 % or less. The Mn content is preferably 3.100 % or less. The Mn content is more preferably 3.000 % or less.

[P: 0.100 % or less]

P segregates at prior austenite grain boundaries and embrittles the grain boundaries, thereby embrittling the base steel sheet. Therefore, when P content exceeds 0.100 %, the delayed fracture resistance of a stretch-flanged portion decreases. The P content is therefore 0.100 % or less. The P content is preferably 0.070 % or less. A lower limit of the P content is not particularly specified. P is a solid-solution-strengthening element able to increase the strength of the base steel sheet, and therefore the P content is preferably 0.001 % or more.

[S: 0.0200 % or less]

S is present as sulfides and embrittles the base steel sheet, and therefore when S content exceeds 0.0200 %, the delayed fracture resistance of a stretch-flanged portion decreases. The S content is therefore 0.0200 % or less. The S content is preferably 0.0050 % or less. A lower limit of the S content is not particularly specified. In view of production technology constraints, the S content is preferably 0.0001 % or more.

[Al: 1.000 % or less]

Al is present as oxides and embrittles the base steel sheet, and therefore when Al content exceeds 1.000 %, the delayed fracture resistance of a stretch-flanged portion decreases. The Al content is therefore 1.000 % or less. The Al content is preferably 0.500 % or less. A lower limit of the Al content is not particularly specified. Al inhibits carbide formation during continuous annealing and promotes formation of retained austenite, and therefore the Al content is preferably 0.001 % or more.

[N: 0.0100 % or less]

N is present as nitrides and embrittles the base steel sheet, and therefore when N content exceeds 0.0100 %, the delayed fracture resistance of a stretch-flanged portion decreases. The N content is therefore 0.0100 % or less. The N content is preferably 0.0050 % or less. A lower limit of the N content is not particularly specified. In view of production technology constraints, the N content is preferably 0.0001 % or more.

[O: 0.0100 % or less]

O is present as oxides and embrittles the base steel sheet, and therefore when O content exceeds 0.0100 %, the delayed fracture resistance of a stretch-flanged portion decreases. The O content is therefore 0.0100 % or less. The O content is preferably 0.0050 % or less. A lower limit of the O content is not particularly specified. In view of production technology constraints, the O content is preferably 0.0001 % or more.

(Optional components of base steel sheet)

In addition to the basic components described above, the base steel sheet may further contain, in mass%, one or more selected from the group consisting of 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.

[Ti: 0.200 % or less]
[Nb: 0.200 % or less]
[V: 0.200 % or less]

When the content of each of Ti, Nb, and V is 0.200% or less, large amounts of coarse precipitates, inclusions, and the like are not generated and the ultimate deformability of the base steel sheet is not decreased, and therefore λ and bendability are not decreased. Therefore, when one or more of Ti, Nb, or V are contained, the content of each is 0.200 % or less. The content of each is preferably 0.100 % or less. Lower limits of Ti, Nb, and V content are not particularly specified. These elements increase the strength of the base steel sheet by forming fine carbides, nitrides, and carbonitrides during hot rolling or continuous annealing, and therefore the content of each of Ti, Nb, and V is respectively preferably 0.001 % or more.

[Ta: 0.10 % or less]
[W: 0.10 % or less]

When the content of each of Ta and W is 0.10 % or less, large amounts of coarse precipitates, inclusions, and the like are not formed and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when one or both of Ta and W are contained, the content of each is 0.10 % or less. The content of each is preferably 0.08 % or less. Lower limits of Ta and W content are not particularly specified. These elements increase the strength of the base steel sheet by forming fine carbides, nitrides, and carbonitrides during hot rolling or continuous annealing, and therefore the content of each of Ta and W is respectively preferably 0.01 % or more.

[B: 0.0100 % or less]

When B content is 0.0100 % or less, cracks are not caused to form inside the steel sheet during casting or hot rolling, and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion does not decrease. Therefore, when B is contained, the content is 0.0100 % or less. The B content is preferably 0.0080 % or less. A lower limit of the B content is not particularly specified. B is an element that improves hardenability by segregating to austenite grain boundaries during annealing, and therefore the B content is preferably 0.0003 % or more.

[Cr: 1.00 % or less]
[Mo: 1.00 % or less]
[Ni: 1.00 % or less]

When the content of each of Cr, Mo, and Ni is 1.00 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when one or more of Cr, Mo, and Ni are contained, the content of each is 1.00 % or less. The content of each is preferably 0.80 % or less. Lower limits of Cr, Mo, and Ni content are not particularly specified. These elements improve hardenability, and therefore the content of each of Cr, Mo, and Ni is respectively preferably 0.01 % or more.

[Co: 0.010 % or less]

When Co content is 0.010 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when Co is contained, the content is 0.010 % or less. The Co content is preferably 0.008 % or less. A lower limit of the Co content is not particularly specified. Co improves hardenability, and therefore the Co content is preferably 0.001 % or more.

[Cu: 1.00 % or less]

When Cu content is 1.00 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when Cu is contained, the content is 1.00 % or less. The Cu content is preferably 0.80 % or less. A lower limit of the Cu content is not particularly specified. Cu improves hardenability, and therefore the Cu content is preferably 0.01 % or more.

[Sn: 0.200 % or less]

When Sn content is 0.200 % or less, cracks are not caused to form inside the steel sheet during casting or hot rolling, and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion does not decrease. Therefore, when Sn is contained, the content is 0.200 % or less. The Sn content is preferably 0.100 % or less. A lower limit of the Sn content is not particularly specified. Sn improves hardenability, and therefore the Sn content is preferably 0.001 % or more.

[Sb: 0.200 % or less]

When Sb content is 0.200 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when Sb is contained, the content is 0.200 % or less. The Sb content is preferably 0.100 % or less. A lower limit of the Sb content is not particularly specified. Sb controls surface layer softening thickness and enables strength adjustment, and therefore 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 content of each of Ca, Mg, and REM is 0.0100 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when one or more of Ca, Mg, and REM are contained, the content of each is 0.0100 % or less. The content of each is preferably 0.0050 % or less. Lower limits of the content of each of Ca, Mg, and REM are not particularly specified. These elements spheroidize the shape of nitrides, sulfides, and the like, and improve the ultimate deformability of the base steel sheet, and therefore the content of each of Ca, Mg, and REM is 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 content of each of Zr, Zn, Pb, and Te is 0.100 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when one or more of Zr, Zn, Pb, and Te are contained, the content of each is 0.100 % or less. The content of each is preferably 0.080 % or less. Lower limits of the content of each of Zr, Zn, Pb, and Te are not particularly specified. These elements spheroidize the shape of nitrides, sulfides, and the like, and improve the ultimate deformability of the base steel sheet, and therefore the content of each of Zr, Zn, Pb, and Te is 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 each of Se, Ga, Ge, and Sr is 0.020 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when one or more of Se, Ga, Ge, and Sr are contained, the content of each is 0.020 % or less. Lower limits of the content of each of Se, Ga, Ge, and Sr, are not particularly specified. These elements spheroidize the shape of nitrides, sulfides, and the like, and improve the ultimate deformability of the base steel sheet, and therefore the content of each of Se, Ga, Ge, and Sr is preferably 0.001 % or more.

[Hf: 0.10 % or less]

When Hf content is 0.10 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when Hf is contained, the content is 0.10 % or less. The Hf content is preferably 0.08 % or less. A lower limit of the Hf content is not particularly specified. Hf spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the base steel sheet, and therefore the Hf content is preferably 0.01 % or more.

[Bi: 0.200 % or less]

When Bi content is 0.200 % or less, the amount of coarse precipitates, inclusions, and the like is not increased and the base steel sheet is not embrittled, and therefore the delayed fracture resistance of a stretch-flanged portion is not decreased. Therefore, when Bi is contained, the content is 0.200 % or less. The Bi content is preferably 0.100 % or less. A lower limit of the Bi content is not particularly specified. Bi decreases segregation, and therefore the Bi content is preferably 0.001 % or more.
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 the respective preferable lower limit value, the effects described by the present disclosure are not impaired. Therefore, each content may be less than the respective preferable lower limit, in which case the content is treated as inevitable impurity. The base steel sheet according to an embodiment of the present disclosure contains the above basic components, with the balance being Fe (iron) and inevitable impurity. Here, it is preferable that the base steel sheet according to an embodiment of the present disclosure contains only the above-mentioned basic components and the balance, with the balance being Fe (iron) and inevitable impurity.

(Microstructure of base steel sheet)

The following describes microstructure of the base steel sheet. The base steel sheet has a microstructure in which, at a position at a depth from the surface of the base steel sheet that is 1/4 of the sheet thickness, the total area fraction 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, the area fraction of bainite is 10.0 % or less, and the area fraction of the residual microstructure is 10.0 % or less. Further, the thickness of a carbon concentration reduced portion in a surface layer of the base steel sheet is 1.0 µm or more and 150.0 µm or less, the thickness of a bainite precipitation region directly below the carbon concentration reduced portion is 10 µm or more, a total coverage of the retained austenite and fresh martensite at prior austenite grain boundaries directly below the carbon concentration reduced portion is 30.0 % or more and 60.0 % or less, and a carbon concentration of the retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduced portion is less than 0.60 mass%.

[Total area fraction of tempered martensite and fresh martensite of 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 is possible to achieve a TS of 1470 MPa or more. On the other hand, when the total area fraction of tempered martensite and fresh martensite exceeds 94.0 %, achieving excellent ductility becomes difficult. The total area fraction of tempered martensite and fresh martensite is therefore 70.0 % or more and 94.0 % or less.
In particular, when the area fraction of tempered martensite is 80.0 % or more, TS can be favorably obtained, which is preferable. An upper limit of the area fraction of tempered martensite is not particularly limited, but the area fraction of tempered martensite is generally 94.0 % or less. Further, when the area fraction of fresh martensite is 10.0 % or less, λ can be favorably obtained, which is preferable. A lower limit of the area fraction of fresh martensite is not particularly limited, but the area fraction of fresh martensite is generally 1.0 % or more.
The total area fraction of tempered martensite and fresh martensite can be determined as follows. After polishing an L-section of the base steel sheet, the L-section is etched with 3 vol% nital, and a 1/4 thickness position (a position corresponding to 1/4 of the sheet thickness in the depth direction from the steel sheet surface) is observed in ten fields of view using an SEM at 3000 times magnification. In the observed images, 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 microstructure having a smooth surface. The fresh martensite appears as protruding portions each having a width of 50 nm or more. In the same manner, the area fraction of tempered martensite can be determined from the observed microstructure images. Tempered martensite has a substructure (lath boundaries, block boundaries) and is a microstructure in which carbides are precipitated in multiple variants. The volume fraction of retained austenite is almost equal to the area fraction, and is therefore treated as being equivalent to the area fraction in the present disclosure.

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

When the volume fraction of retained austenite is less than 6.0 %, achieving excellent ductility becomes difficult. The volume fraction of retained austenite is therefore 6.0 % or more. The volume fraction is preferably 6.5 % or more. The volume fraction is more preferably 7.0 % or more. On the other hand, when the volume fraction of retained austenite exceeds 15.0 %, achieving excellent hole expansion formability becomes difficult. The volume fraction of retained austenite is therefore 15.0 % or less. The volume fraction is preferably 13.0 % or less. The volume fraction is more preferably 11.0 % or less.
The volume fraction of retained austenite can be determined as follows. The base steel sheet is ground to a position 0.1 mm thicker than the 1/4 thickness position. The surface is further polished by 0.1 mm by chemical polishing to the 1/4 thickness position, 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 by an X-ray diffractometer. The volume fraction of retained austenite can be determined by averaging the nine integrated intensity ratios obtained.

[Area fraction of bainite of 10.0 % or less]

When the area fraction of bainite exceeds 10.0 %, it becomes difficult to achieve a TS of 1470 MPa or more. The area fraction of bainite is therefore 10.0 % or less. A lower limit of the area fraction of bainite is not particularly limited, and the area fraction of bainite may be 0.0 %.
The area fraction of bainite can be determined as follows. After polishing an L-section of the base steel sheet, the L-section is etched with 3 vol% nital, and a 1/4 thickness position is observed in ten fields of view using an SEM at 3000 times magnification. In the observed microstructure images, bainite appears as recessed portions having a flat interior morphology. The area fraction of bainite is determined in each field of view, and the average value of these values is taken as the area fraction of bainite.

[Area fraction of residual microstructure of 10.0 % or less]

The steel microstructure of the present disclosure may contain carbides such as pearlite and cementite or other known microstructures of steel sheets as the residual microstructure. When the area fraction of the residual microstructure is 10.0 % or less, the effects described in the present disclosure are not impaired. The area fraction of the residual microstructure is therefore 10.0 % or less. A lower limit of the area fraction of the residual microstructure is not particularly limited, and the area fraction of the residual microstructure may be 0.0 %.
The area fraction of the residual microstructure can be determined as follows. After polishing an L-section of the base steel sheet, the L-section is etched with 3 vol% nital, and a 1/4 thickness position is observed in ten fields of view using an SEM at 3000 times magnification. In the observed microstructure images, the area fraction of the residual microstructure is defined as 100.0 % minus the area fractions of tempered martensite, fresh martensite, ferrite, and bainitic ferrite, and the volume fraction of retained austenite. The area fraction of the residual microstructure is determined for each field of view, and the average value of these values is taken as the area fraction of the residual microstructure.

[Thickness of carbon concentration reduced portion in surface layer of 1.0 µm or more and 150.0 µm or less]

When 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 in the surface layer where the bainite precipitation start temperature is high, and it is not possible to control the area fraction of bainite and the thickness of the bainite precipitation region described below, making it difficult to achieve excellent delayed fracture resistance in a stretch-flanged portion. The thickness of the carbon concentration reduced portion in the surface layer is therefore 1.0 µm or more. The thickness is preferably 3.0 µm or more. On the other hand, when the thickness of the carbon concentration reduced portion in the surface layer exceeds 150.0 µm, making it difficult to achieve a TS of 1470 MPa or more. The thickness of the carbon concentration reduced portion in the surface layer is therefore 150.0 µm or less. The thickness is 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 measuring 20 mm in length and 20 mm in width is taken from the zinc or zinc alloy coated or plated steel sheet. A C intensity of the test piece is measured in the depth direction from the steel sheet surface by glow discharge spectroscopy (GDS). From the measurement results, the region where the C intensity is equal to or less than (C intensity measured when GDS signal becomes constant × 0.8) is defined as the carbon concentration reduced portion of the surface layer, and the thickness of this region is defined as the thickness of the carbon concentration reduced portion. The GDS device used is GD-PROFILER 2, produced by Horiba Ltd., and the measurement conditions can be determined as follows.

Power supply: high frequency
Anode diameter: 4 mm
Measurement mode: constant power 35 W
Carrier gas: argon
Discharge gas pressure: 300 Pa
Pulse frequency: 100 Hz
Duty cycle: 50 %
Capture interval: 100 ms

[Thickness of bainite precipitation region directly below carbon concentration reduced portion of 10 µm or more]

When the thickness of the bainite precipitation region directly below the carbon concentration reduced portion is less than 10 µm, it becomes difficult to achieve excellent delayed fracture resistance in a stretch-flanged portion. The thickness of the bainite precipitation region directly below the carbon concentration reduced portion is therefore 10 µm or more. There is no particular upper limit to the thickness of the bainite precipitation region directly below the carbon concentration reduced portion. The thickness is generally 100 µm or less.
The thickness of the bainite precipitation region directly below the carbon concentration reduced portion can be determined as follows. After polishing an L-section of the zinc or zinc alloy coated or plated steel sheet, the L-section is etched with 3 vol% nital, and the above-mentioned GDS analysis is carried out on the steel sheet surface using an SEM. The region after the carbon concentration reduced portion determined by GDS analysis, where bainite precipitation is observed, is observed in ten fields of view at 3000 times magnification. In each field of view observed, bainite appears as recessed portions having a flat interior morphology. When bainite is not contained in one field of view, images of successive fields of view are taken. The area fraction of bainite is determined in each field of view, and a region where the area fraction of bainite is 20 % or more and 60 % or less 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 taken as the thickness of the bainite precipitation region.

[Total coverage of retained austenite and fresh martensite at prior austenite grain boundaries directly below carbon concentration reduced portion of 30.0 % or more and 60.0 % or less]

By making the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion 30.0 % or more, a TS of 1470 MPa or more is achievable. The total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion is therefore 30.0 % or more. The total coverage is preferably 40.0 % or more. On the other hand, when the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion exceeds 60.0 %, then among excellent formability properties, achieving excellent hole expansion formability becomes difficult. The total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion is therefore 60.0 % or less. The total coverage is preferably 55.0 % or less.
The total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion can be determined as follows. After polishing an L-section of the zinc or zinc alloy coated or plated steel sheet, the L-section is etched with 3 vol% nital, and the above-mentioned GDS analysis is carried out on the steel sheet surface using an SEM. Ten prior austenite grain boundaries that are closest to the carbon concentration reduced portion in a region following the carbon concentration reduced portion determined by GDS analysis are randomly selected. At each of the selected prior austenite grain boundaries, a convex portion having a width of 50 nm or more in a direction perpendicular to the prior austenite grain boundary and having a smooth surface is defined as a portion covered with retained austenite and fresh martensite. The sum of the circumferential lengths of the portions covered by the retained austenite and fresh martensite is divided by the circumferential length of the selected prior austenite grain boundary. The average value 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 of retained austenite covering prior austenite grain boundaries directly below carbon concentration reduced portion is less than 0.60 mass%]

When the carbon concentration of the retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduced portion is 0.60 mass% or more, a second phase having a hardness significantly different from that of the matrix phase will be present. This second phase becomes a stress concentration portion during working, thereby degrading the delayed fracture resistance. The carbon concentration in the retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduced portion is less than 0.60 mass%. The carbon concentration is preferably 0.55 mass% or less. A lower limit of the carbon concentration in the retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduced 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 reduced portion can be determined as follows. First, the surface of the steel sheet (sample) is polished with diamond paste so that a cross-section parallel to the rolling direction (L-section) becomes an observation plane, and is then polished to a mirror finish by alumina polishing. Next, in order to remove hydrocarbon contamination (carbon contamination) from the sample surface, the observation plane is cleaned using a plasma cleaner. The cleaned observation plane is measured using a field emission electron probe microanalyzer (FE-EPMA). The measurement conditions are an accelerating voltage of 7 kV and a current of 50 nA, as described in non-patent literature (T. Yamashita, Y. Tanaka, M. Nagoshi, and K. Ishida: Sci. Rep., 6 (2016), DOI: 10.1038/srep29825). Further, the sample is heated to 100 °C and maintained at this temperature during measurement to prevent contamination from adhering to the sample surface. The carbon concentration is calculated from the measurement results by a calibration method, and an elemental mapping image of carbon is obtained. In the obtained elemental mapping image, a region (high carbon region) in which the carbon concentration is equal to or more than the average of the matrix phase and less than 0.6 mass% is identified. Further, the carbon concentration reduced portion on the observation plane is identified by the above-described method, and by referring to an SEM image of the same field of view as the element mapping image, the prior austenite grains directly below the carbon concentration reduction area are identified, and the high carbon regions present on the grain boundaries of those prior austenite grains are identified. Then, a circumferential length a of the prior austenite grain and a length b of the portion of the circumferential length of the prior austenite grain that overlaps with the high carbon region are determined, and a 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 directly below the carbon concentration reduced 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 that covers the prior austenite grain boundaries is equal to or lower than the carbon concentration in the retained austenite.

(Zinc or zinc alloy coated or plated layer)

The zinc or zinc alloy coated or plated layer of the zinc or zinc alloy coated or plated steel sheet is preferably an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed layer. Specifically, a Zn-Ni alloy coated or plated layer, a zinc-aluminum-magnesium alloy coated or plated layer, or the like. The coating weight is preferably 20 g/m² to 80 g/m² per side (in the case of double-sided coating).

(Mechanical properties of zinc or zinc alloy coated or plated steel sheet)

The following describes mechanical properties of the zinc or zinc alloy coated or plated steel sheet.

[Tensile strength (TS): 1470 MPa or more]

The zinc or zinc alloy coated or plated steel sheet has a tensile strength (TS) of 1470 MPa or more. An upper limit of the tensile strength of the zinc or zinc alloy coated or 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 carried out in accordance with JIS Z 2241. A tensile test condition may be a crosshead speed of 1.67 × 10^-1 mm/s.

[Yield stress (YS) of 1000 MPa or more (preferred condition)]

The zinc or zinc alloy coated or plated steel sheet preferably has a yield stress of 1000 MPa or more. On the other hand, the yield stress of the zinc or zinc alloy coated or plated steel sheet is preferably 1350 MPa or less. The yield stress can be determined by a tensile test in the same manner as the tensile strength.

[Elongation (El) of 9.0 % or more]

The zinc or zinc alloy coated or plated steel sheet has an elongation (El) of 9.0 % or more. An upper limit of the elongation of the zinc or zinc alloy coated or plated steel sheet is not particularly limited, but is generally 15.0 % or less. The elongation (El) can be determined by a tensile test in the same manner as the tensile strength.

[Hole expansion ratio (λ) of 25 % or more]

The zinc or zinc alloy coated or plated steel sheet has a hole expansion ratio (λ) of 25 % or more. An upper limit of the hole expansion ratio of the zinc or zinc alloy coated or 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 expanding test is carried out in accordance with JIS Z 2256. The test material is sheared to a size of 100 mm × 100 mm, and then a hole having a diameter of 10 mm is punched with a clearance of 12.5 %. While the test material is held down using a die having an inner diameter of 75 mm with a blank holding force of 9 tonnes (88.26 kN), a conical punch having an apex angle of 60° is pressed into the punched hole to check for the occurrence of a crack that penetrates through the sheet thickness. The hole diameter when a crack occurs is measured, and the maximum hole expansion ratio λ (%) can be calculated using the following expression (5). $λ (%) = \{(D_{f} - D_{0}) / D_{0}\} \times 100$
Here, D_f is the hole diameter (mm) at crack initiation and D₀ is the initial hole diameter (mm).
In the case of a coated or plated steel sheet having a TS of about 1470 MPa, hydrogen is contained in the steel sheet during the production process. Further, it is known that diffusible hydrogen in steel sheets significantly deteriorates the hole expansion ratio. For this reason, when coated or plated steel sheets are shipped as products, the steel sheets are left to rest for a long period of time or are post-heated to decrease the amount of diffusible hydrogen. Therefore, the hole expanding test is preferably carried out after the diffusible hydrogen content of the steel has reached 0.01 wt% or less.

[Excellent delayed fracture resistance in stretch-flanged portion]

The zinc or zinc alloy coated or plated steel sheet according to the present disclosure has excellent delayed fracture resistance in stretch-flanged portions. The delayed fracture resistance of a stretch-flanged portion can be evaluated as follows. The above-described hole expanding test is carried out within 30 days after the production of the zinc or zinc alloy coated or plated steel sheet (test material). Immediately after the hole expanding test, a photograph of a stretch-flanged portion of the test material is taken at a magnification of 20 times using a digital microscope (RH-2000: produced by Hirox Co. Ltd.). Subsequently, the test material is left to stand at room temperature (15 °C to 25 °C) for 24 hours, and a stretch-flanged portion is observed again using a digital microscope. By comparing a photograph of a stretch-flanged portion taken immediately after the hole expanding test with a photograph of a stretch-flanged portion taken 24 hours later, when no increase or progression of cracks is observed, it can be determined that a stretch-flanged portion has excellent delayed fracture resistance.

(Method of producing zinc or zinc alloy coated or plated steel sheet)

The following describes a method of producing a zinc alloy coated steel sheet according to an embodiment of the present disclosure. FIG. 1 is a graph illustrating a relationship between temperature and time in a method of producing a galvannealed steel sheet according to an embodiment of the present disclosure. The line in the graph indicates the temperature changes of the steel sheet from the annealing process to the tempering process. A steel slab having the above-described chemical composition is subjected to hot rolling and cold rolling to obtain a cold-rolled steel sheet, and then 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 having a dew point of -25 °C or higher and 35 °C or lower. Next, a first cooling process is carried out in which the cold-rolled steel sheet is cooled to a temperature T1. Next, a holding process is carried out in which the cold-rolled steel sheet is held at T1 (°C) for 30 s or longer and 200 s or shorter. Next, a second cooling process is carried out in which the cold-rolled steel sheet is cooled to a temperature T2. Next, the cold-rolled steel sheet is subjected to a galvanizing treatment to obtain a coated steel sheet. The time required for a hot-dip galvanizing treatment is shorter than the cooling processes and the like, and therefore the time is indicated by a point at temperature T2 in FIG. 1. In the alloying treatment after the hot-dip galvanizing treatment, the coated steel sheet is heated to and held at the alloying temperature. Next, a third cooling process is carried out in which the coated steel sheet is cooled to a temperature T3. Next, a fourth cooling process is carried out in which the coated steel sheet is cooled to a cooling stop temperature T4. Next, the coated steel sheet is subjected to a tempering process in which the coated steel sheet is held at a tempering temperature higher than T4 (°C) and 350 °C or lower. In the production method, the total residence time from the coating treatment process to the third cooling process in which the temperature of the coated steel sheet is in the temperature range from Bf (°C) to T - 20 (°C) is 30 s or shorter. Further, in the tempering process, it is preferable that a tension with an average load tension of 2.0 kgf/mm² or more is applied, and bending and unbending are carried out one or more times, at a time at least 6 s after the start of the holding at the tempering temperature and at least 10 s before the end of the holding at the tempering temperature.

[Steel slab]

The steel slab used in the method of producing a zinc or zinc alloy coated or plated steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, N, and O, with the balance being Fe and inevitable impurity. Further, at least one 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 is preferably contained. The content of each element is as described above.

[Hot rolling process]

According to the present disclosure, a method of smelting the steel slab is not particularly limited, and any known smelting method such as use of a converter, an electric furnace, or the like may be applied. The steel slab is preferably produced by continuous casting to help prevent macro-segregation. The steel slab is subjected to hot rolling to obtain a hot-rolled steel sheet. The hot-rolled steel sheet may be subjected to cold rolling without other treatment, or the hot-rolled steel sheet may be subjected to pickling treatment and then cold rolling. Pickling treatment is preferable because it can remove oxides from the steel sheet surface and can provide good coating quality in the final steel sheet product. Further, the pickling treatment may be carried out once or a plurality of times.

[Cold rolling process]

The hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The rolling reduction in the cold rolling and the thickness of the sheet after rolling are not particularly limited. Further, the number of rolling passes and the rolling reduction of each pass are not particularly limited.

[Annealing process: maintaining annealing temperature of Ac3 (°C) or higher for 10 s or longer in atmosphere with a dew point of -25 °C or higher and 35 °C or lower]

The cold-rolled steel sheet is subjected to an annealing process. In the annealing process, when the dew point during annealing is lower than -25 °C, the thickness of the carbon concentration reduced portion of the surface layer formed during annealing will be less than 1.0 µm, making it impossible to provide a sufficient region in the surface layer with a high bainite precipitation start temperature, and making it impossible to achieve a thickness of 10 µm or more of the bainite precipitation region directly below the carbon concentration reduced portion. As a result, it becomes difficult to achieve excellent delayed fracture resistance in a stretch-flanged portion. Therefore, the dew point during annealing is -25 °C or higher. The dew point is preferably -20 °C or higher. On the other hand, when the dew point during annealing is higher than 35 °C, the thickness of the carbon concentration reduced portion of the surface layer formed during annealing exceeds 150.0 µm, making it difficult to achieve a TS of 1470 MPa or more. The dew point during annealing is therefore 35 °C or lower.
In the annealing process, when the annealing temperature is lower than Ac3 (°C) defined by the following expression (1), the 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. The annealing temperature is therefore Ac3 (°C) or higher. The annealing temperature is preferably Ac3 + 20 (°C) or higher. On the other hand, when the annealing temperature is 920 °C or lower, the energy efficiency does not decrease, so that it is possible to favorably prevent an increase in heating costs and also to favorably prevent damage to the furnace body. The annealing temperature is therefore preferably 920 °C or lower. 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 in mass% of element X in the chemical composition, and is 0 when the chemical composition does not contain element X.
In the annealing process, when the holding time at the annealing temperature is shorter than 10 s, the 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. The holding time at the annealing temperature is therefore 10 s or longer. The holding time is preferably 40 s or longer. On the other hand, when the holding time at the annealing temperature is 500 s or shorter, an increase in heating costs and an increase in production time can be favorably prevented, that is, a decrease in productivity can be favorably prevented. The holding time at the annealing temperature is therefore preferably 500 s or shorter.

[First cooling process: cooling to temperature T1 of T - 20 (°C) or higher and T + 20 (°C) or lower at cooling rate of 7 °C/s or more]

Following the annealing process, the cold-rolled steel sheet is subjected to a first cooling process. That is, the end of the annealing process corresponds to the start of the first cooling process. In the first cooling process, when the cooling rate is less than 7 °C/s, the 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. The cooling rate in the first cooling process is therefore 7 °C/s or more. The cooling rate is preferably 9 °C/s or more. On the other hand, when the first cooling rate is 20 °C/s or less, the cooling stop temperature can be favorably controlled. The cooling rate in the first cooling process is therefore preferably 20 °C/s or less.
When the cooling stop temperature T1 of the first cooling process is lower than T - 20 (°C), where T is defined by the following expression (2), bainite is generated not only in areas directly below the carbon concentration-reduced portion of the surface layer, the area fraction of bainite increases, and achieving a TS of 1470 MPa or more becomes difficult. T1 is therefore T - 20 (°C) or higher. On the other hand, when T1 is higher than T + 20 (°C), a bainite precipitation region is not formed directly below the carbon concentration reduced portion of the surface layer, making it difficult to achieve excellent delayed fracture resistance in a stretch-flanged portion. In addition, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion becomes excessive, making it difficult to achieve a hole expansion ratio of 25 % or more. T1 is therefore T + 20 (°C) or lower. $T (° C) = 743 - 90 \times [% Mn] - 37 \times [% Ni] - 70 \times [% Cr] - 83 \times [% Mo]$
Here, [%X] indicates the content in mass% of element X in the chemical composition, and is 0 when the chemical composition does not contain element X.

[Holding process: holding at T1 (°C) for 30 s or longer and 200 s or shorter]

Following the first cooling process, the cold-rolled steel sheet is subjected to a holding process in which the cold-rolled steel sheet is held at T1 (°C). When the holding time in the holding process is shorter than 30 s, the thickness of the bainite precipitation region directly below the carbon concentration reduced portion will be less than 10 µm, making it difficult to achieve excellent delayed fracture resistance in a stretch-flanged portion. In addition, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion becomes excessive, making it difficult to achieve a hole expansion ratio of 25 % or more. The holding time of intermediate holding is therefore 30 s or longer. The holding time is preferably 40 s or longer. On the other hand, when the holding time of the intermediate holding exceeds 200 s, the grain growth of bainite precipitated at the grain boundaries of prior austenite directly below the carbon concentration reduced portion proceeds excessively, 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 more. The holding time of the intermediate holding is therefore 200 s or lower. The holding time is preferably 180 s or lower.

[Second cooling process: cooling to temperature T2 of lower than T - 20 (°C)]

Following the holding process, the cold rolled steel sheet is subjected to a second cooling process. Accordingly, the cooling stop temperature T2 of the second cooling process is lower than T1 (°C). When the cooling stop temperature T2 of the second cooling process is T - 20 (°C) or higher, the grain growth of bainite precipitated at the grain boundaries of prior austenite directly below the carbon concentration reduced portion proceeds excessively, 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 more. The second cooling stop temperature T2 is therefore lower than T - 20 (°C). The second cooling stop temperature T2 is preferably T - 30 (°C) or lower. On the other hand, when the second cooling stop temperature T2 is equal to or higher than Ms (°C) described below, when the alloying treatment described below is carried out, precipitation of martensite before alloying is favorably prevented, and excessive tempering is not carried out during alloying, and therefore TS can be favorably obtained. The second cooling stop temperature T2 is therefore preferably Ms (°C) or higher.
When the cooling rate in the second cooling process is 1 °C/s or more, excessive nucleation and growth of bainite during cooling is favorably prevented, the total coverage of retained austenite and fresh martensite at the prior austenite grain boundaries is favorably obtained, and TS can be favorably obtained. The cooling rate in the second cooling process is therefore preferably 1 °C/s or more. The cooling rate is more preferably 2 °C/s or more. On the other hand, when the cooling rate in the second cooling process is 20 °C/s or less, the cooling stop temperature can be favorably controlled. The cooling rate in the second cooling process is therefore preferably 20 °C/s or less.

[Coating or plating treatment process]

Following the second cooling process, the cold-rolled steel sheet is subjected to a zinc or zinc alloy coating or plating treatment to obtain a coated or plated steel sheet. Examples of the zinc or zinc alloy coating or plating treatment include electrogalvanization treatment, hot-dip galvanizing treatment, or hot-dip galvanizing treatment followed by alloying treatment. Electroplating such as Zn-Ni electrolytic alloy plating may be applied, or hot-dip zinc-aluminum-magnesium alloy coating may be applied.
When the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment, the cold-rolled steel sheet is preferably immersed in a galvanizing bath at 440 °C or higher and 500 °C or lower for hot-dip galvanizing treatment, after which coating weight adjustment is carried out using gas wiping or the like. The coating weight is preferably adjusted to 20 g/m² to 80 g/m² per side (double-sided coating). For hot-dip galvanizing treatment, a galvanizing bath having an Al content from 0.10 mass% to 0.23 mass% is preferably used. The hot-dip galvanizing treatment is preferably carried out by retaining the cold rolled steel sheet in a temperature range from Ms (°C) to 700 °C, where Ms is defined by the following expression (3). Ms = 539 - 423 × [%C] - 30.4 × [%Mn] - 17.7 × [%Ni] - 12.1 × [%Cr] - 7.5 × [%Mo] ...(3)
Here, [%X] indicates the content in mass% of element X in the chemical composition, and is 0 when the chemical composition does not contain element X.
When hot-dip galvanizing treatment and subsequent alloying treatment are carried out, by setting the alloying temperature to 470 °C or higher, the Zn-Fe alloying rate is prevented from becoming excessively slow, and favorable productivity is obtainable. The alloying temperature is therefore preferably 470 °C or higher. On the other hand, by setting the alloying temperature to 600 °C or lower, it is possible to favorably prevent untransformed austenite from transforming into pearlite, which would otherwise result in a decrease in TS. The alloying temperature is therefore preferably 600 °C or lower. The alloying temperature is more preferably 560 °C or lower. Further, the galvannealed steel sheet (GA) is preferably subjected to an alloying treatment so that the Fe concentration in the coated layer is 7 mass% to 15 mass%.
The series of treatments from the annealing process to the coating or plating treatment process are not particularly limited, but from the viewpoint of productivity, they are preferably carried out in a continuous galvanizing line (CGL), which is a hot-dip galvanizing line.

[Third cooling process: cooling to temperature T3 of Ms - 200 (°C) or higher and Ms - 80 (°C) or lower at cooling rate of 5 °C/s or more]

Following the coating or plating treatment process, the coated or plated steel sheet is subjected to a third cooling process. When the cooling rate in the third cooling process is less than 5 °C/s, 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. The cooling rate in the third cooling process is therefore 5 °C/s or more. On the other hand, when the third cooling rate is 20 °C/s or less, the cooling stop temperature can be favorably controlled. The cooling rate in the third cooling process is therefore preferably 20 °C/s or less.
When the cooling stop temperature T3 in the third cooling process is lower than Ms - 200 (°C), the volume fraction of untransformed austenite decreases, the amount of retained austenite in the final microstructure decreases, and it becomes difficult to achieve excellent ductility. Further, the carbon concentration in the retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduced portion becomes 0.60 mass% or more, and the delayed fracture resistance of a stretch-flanged portion becomes inferior. T3 is therefore Ms - 200 (°C) or higher. On the other hand, when T3 is higher than Ms - 80 (°C), the total area fraction of tempered martensite and fresh martensite decreases, making it difficult to achieve a TS of 1470 MPa or more. T3 is therefore Ms - 80 (°C) or lower.
[Total residence time from coating or plating treatment process to third cooling process in which temperature of coated or plated steel sheet is in temperature range from Bf (°C) to T - 20 (°C) is 30 s or shorter]
From the coating or plating treatment process to the third cooling process described below, when the total residence time in the temperature range from Bf (°C) to T - 20 (°C) exceeds 30 s, where Bf is defined by the following expression (4), the area fraction of bainite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the total residence time from the coating or plating treatment process to the third cooling process, in which the temperature of the coated or plated steel sheet is in the temperature range from Bf (°C) to T - 20 (°C), is 30 s or shorter. A lower limit of the residence time from the coating or plating treatment process to the third cooling process, in which the temperature of the coated or plated steel sheet is in the temperature range from Bf (°C) to T - 20 (°C), is not particularly limited, but the residence time is generally 20 s or longer. Bf (°C) = 710 - 270 × [%C] - 90 × [%Mn] - 37 × [%Ni] - 70 × [%Cr] - 83 × [%Mo] ...(4)
Here, [%X] indicates the content in mass% of element X in the chemical composition, and is 0 when the chemical composition does not contain element X.

[Fourth cooling process: cooling to cooling stop temperature T4 of 100 °C or higher and lower than T3 (°C) at cooling rate of 3.0 °C/s or less]

Following the third cooling process, the coated or plated steel sheet is subjected to a fourth cooling process. When the cooling rate in the fourth cooling process exceeds 3.0 °C/s, the diffusion of carbon into untransformed austenite during the fourth cooling process does not proceed sufficiently, the volume fraction of retained austenite in the final microstructure decreases, and it becomes difficult to achieve excellent ductility and excellent delayed fracture resistance in a stretch-flanged portion. The cooling rate in the fourth cooling process is therefore 3.0 °C/s or less. The cooling rate is preferably 2.0 °C/s or less. The cooling rate is more preferably 1.5 °C/s or less. On the other hand, when the fourth cooling rate is 0.5 °C/s or more, cooling can be carried out without heating in a furnace, which is preferable. The cooling rate in the fourth cooling process is therefore preferably 0.5 °C/s or more.
When the cooling stop temperature T4 of the fourth cooling process is lower than 100 °C, the area fraction of untransformed austenite decreases, and the volume fraction of retained austenite in the final structure decreases, making it difficult to achieve excellent ductility and excellent delayed fracture resistance in a stretch-flanged portion. T4 is therefore 100 °C or higher. T4 is preferably 110 °C or higher. T4 is more preferably 120 °C or higher. The fourth cooling process is carried out following the third cooling process and therefore T4 is lower than T3 (°C). Further, from the viewpoint of decreasing the amount of fresh martensite, T4 is preferably Ms - 100 (°C) or lower.

[Tempering process: holding at tempering temperature of higher than T4 (°C) and 350 °C or lower for 10 s or longer and 1000 s or shorter]

Following the fourth cooling process, the coated or plated steel sheet is subjected to a tempering process. The untransformed austenite is stabilized by reheating and tempering the coated or plated steel sheet. When the tempering temperature is T4 (°C) or lower, the defined amount of retained austenite cannot be obtained, making it difficult to achieve excellent ductility and excellent delayed fracture resistance in a stretch-flanged portion. The tempering temperature is therefore higher than T4 (°C). The tempering temperature is preferably T4 + 50 (°C) or higher. On the other hand, when the tempering temperature is higher than 350 °C, the tempering proceeds excessively, decreasing the strength and also decomposing the retained austenite, making it difficult to achieve an El of 9 % or more. The tempering temperature is therefore 350 °C or lower. The tempering temperature is preferably 340 °C or lower.
In the tempering process, when the holding time at the tempering temperature is shorter than 10 s, the austenite does not sufficiently stabilize, and martensite transforms during final cooling, decreasing the volume fraction of retained austenite in the final microstructure, making it difficult to achieve excellent ductility. Further, the tempering of the martensite is insufficient, and therefore it becomes difficult to achieve excellent hole expansion formability. The holding time at the tempering temperature is therefore 10 s or longer. The holding time is preferably 40 s or longer. On the other hand, when the holding time at the tempering temperature exceeds 1000 s, the tempering proceeds excessively, decreasing the strength and also decomposing the retained austenite, making it difficult to achieve an El of 9 % or more. The holding time at the tempering temperature is therefore 1000 s or shorter. The holding time is preferably 800 s or shorter.
[In tempering process, tension with average load tension of 2.0 kgf/mm² or more is applied, and bending and unbending are carried out one or more times, at time at least 6 s after start of holding at tempering temperature and at least 10 s before end of holding at tempering temperature (preferred conditions)]
In the tempering process, it is preferable that a tension with an average load tension of 2.0 kgf/mm² or more is applied, and bending and unbending are carried out one or more times, at a time at least 6 s after the start of the holding at the tempering temperature and at least 10 s before the end of the holding at the tempering temperature. By applying tension and bending and unbending at least 6 s after the start of the holding at the tempering temperature, carbon is favorably concentrated in untransformed austenite after cooling is stopped, and transformation to fresh martensite can be favorably suppressed. Therefore, the volume fraction of retained austenite in the final microstructure does not decrease, and ductility is favorably obtainable. Also, the coverage of retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion is favorably obtainable, and tensile strength TS is favorably obtainable. On the other hand, in the tempering process, by applying tension and bending and unbending at least 10 s before the end of the holding, the tempering is preferably carried out after untransformed austenite is transformed into fresh martensite, and a favorable yield stress (YS) and hole expansion ratio (λ) can be obtained.
In the tempering process, when the tension applied when bending and unbending the steel sheet one or more times is 2.0 kgf/mm² or more, unstable retained austenite undergoes deformation-induced transformation during tempering, thereby favorably obtaining a high yield stress and a high hole expansion ratio. The tension applied in the tempering process is therefore preferably 2.0 kgf/mm² or more. On the other hand, when the tension applied in the tempering process is 5.0 kgf/mm² or less, the reduction of untransformed austenite during tempering is favorably prevented, and strain remaining in the steel sheet due to the application of tension is favorably prevented, so that favorable product characteristics can be obtained. The tension applied in the tempering process is therefore preferably 5.0 kgf/mm² or less.
For processes and conditions not described in the present disclosure, regular methods can be used.

EXAMPLES

Steels having the chemical compositions listed in Table 1, each with the balance consisting of Fe and inevitable impurity, were prepared by steelmaking in a converter and formed into steel slabs by continuous casting. The resulting steel slabs were then subjected to hot rolling and pickling treatment, followed by cold rolling. Next, under the conditions listed in Table 2, the annealing process, the first cooling process, the holding process, the second cooling process, the coating or plating process, the third cooling process, the fourth cooling process, and the tempering process were carried out in this order. For the examples other than No. 25, an average load tension of 2.0 kgf/mm² was applied, and bending and unbending were carried out once, at the timing listed in Table 2 in the tempering process. As a result, zinc or zinc alloy coated or plated steel sheets having thicknesses of 0.8 mm to 2.4 mm were obtained. The coating or plating treatments listed in Table 2 are represented as "GI" for hot-dip galvanizing treatment, "GA" for hot-dip galvannealing treatment, and "EG" for electrogalvanization treatment.

[Table 1]

Table 1

Steel sample ID	Chemical composition (mass%)									Classification
Steel sample ID	C	Si	Mn	P	s	Al	N	O	Other	Classification
A	0.199	1.289	2.572	0.011	0.0006	0.039	0.0041	0.0051	W:0.08, Co:0.0031, Sb:0.150, Ca:0.0006, Mg:0.0017, REM:0.0039, Bi:0.0044	Example
B	0.233	1.355	2.529	0.008	0.0013	0.056	0.0054	0.0030	Ti:0.150, B:0.0014, Mo:0.17, Cu:0.92, Sb:0.090, Ca:0.0027, REM:0.0038	Example
C	0.232	1.293	3.042	0.012	0.0011	0.043	0.0056	0.0046	Ti:0.080, Nb:0.020, Ca:0.0089, Mg:0.0026, Zr:0.038, Zn:0.031	Example
D	0.239	1.312	2.885	0.012	0.0009	0.052	0.0024	0.0018	Nb:0.017, B:0.0034, Sb:0.090, Ca:0.0039, Mg:0.0027, Pb:0.029	Example
E	0.232	1.236	2.777	0.006	0.0006	0.022	0.0038	0.0035		Example
F	0.173	1.283	2.820	0.008	0.0008	0.026	0.0050	0.0015		Comparative Example
G	0.263	1.078	2.623	0.012	0.0009	0.033	0.0067	0.0010		Comparative Example
H	0.193	0.710	2.701	0.010	0.0006	0.058	0.0014	0.0030		Comparative Example
I	0.241	1.670	2.672	0.008	0.0011	0.021	0.0022	0.0038		Comparative Example
J	0.208	1.153	2.280	0.011	0.0009	0.023	0.0027	0.0016		Comparative Example
K	0.188	1.184	3.350	0.011	0.0006	0.021	0.0065	0.0055		Comparative Example
L	0.216	1.055	2.591	0.115	0.0005	0.013	0.0057	0.0026		Comparative Example
M	0.247	1.395	2.598	0.010	0.0215	0.014	0.0061	0.0037		Comparative Example
N	0.232	1.187	2.535	0.007	0.0013	1.144	0.0015	0.0057		Comparative Example
O	0.188	1.149	2.758	0.012	0.0014	0.033	0.0121	0.0052		Comparative Example
P	0.213	1.348	2.694	0.010	0.0008	0.029	0.0067	0.0110		Comparative Example
Q	0.234	1.119	3.033	0.012	0.0014	0.056	0.0030	0.0057	V:0.026, B:0.0022, Co:0.004, Sb:0.020	Example
R	0.189	1.157	2.629	0.010	0.0011	0.014	0.0068	0.0026	W:0.07, B:0.0002, Cr:0.166, Mo:0.78, Ca:0.0032, Mg:0.0017, REM:0.0037	Example
S	0.224	1.365	2.674	0.010	0.0007	0.031	0.0048	0.0024	Cr:0.877, Mo:0.02, Ni:0.02, Co:0.003, Sb:0.070, Se:0.010	Example
T	0.242	1.210	2.870	0.005	0.0007	0.060	0.0027	0.0027	Ta:0.075, Te:0.055, Ga:0.012	Example
U	0.210	1.286	2.916	0.012	0.0015	0.041	0.0069	0.0010	Ta:0.05, Cr:0.165, Sb:0.060, Ge:0.006	Example
V	0.249	1.232	2.910	0.012	0.0009	0.026	0.0014	0.0014	Ti:0.150, Nb:0.016, B:0.0064, Sb:0.110	Example
W	0.216	1.077	3.057	0.005	0.0008	0.048	0.0038	0.0020	Ta:0.011, W:0.05, Mo:0.13, Ni:0.06, Sn:0.195, Sb:0.040, Ca:0.0096, REM:0.0027	Example
x	0.221	1.126	2.822	0.006	0.0006	0.023	0.0055	0.0033	Mo:0.11, Ca:0.0026, Mg:0.0018, Sr:0.009	Example
Y	0.237	1.030	2.519	0.013	0.0013	0.058	0.0035	0.0038	Ti:0.180, V:0.013, Sn:0.008, Sb:0.050, Ca:0.0019, REM:0.0098	Example
z	0.213	1.215	3.065	0.008	0.0009	0.037	0.0039	0.0016	B:0.0098, Ni:0.096	Example
AA	0.198	1.371	2.952	0.014	0.0008	0.036	0.0064	0.0065	V:0.173, Ta:0.03, W:0.062	Example
AB	0.222	1.056	2.736	0.006	0.0014	0.053	0.0043	0.0044	Mo:0.17, Cu:0.10, Hf:0.061	Example
AC	0.232	1.220	2.840	0.006	0.0007	0.037	0.0038	0.0019	Ti:0.021, Nb:0.018, B:0.0016, Co:0.005, Cu:0.10, Sb:0.080	Example
AD	0.188	0.811	2.540	0.085	0.0181	0.842	0.0031	0.0025	Nb:0.182,Ta:0.061, W:0.061, Ni:0.963, Sn:0.181, Zr:0.088, Zn:0.087, Pb:0.082, Ge:0.016, Sb:0.018, Bi:0.1801	Example

Underlined: indicates value outside scope of present disclosure.
Using the obtained zinc or zinc alloy coated or plated steel sheets as test materials, 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 residual microstructure were determined by the methods described above. Further, the thickness of the carbon concentration reduced portion in the surface layer of the base steel sheet, the thickness of the bainite precipitation region directly below the carbon concentration reduced portion, the total coverage of the retained austenite and fresh martensite at the prior austenite grain boundaries directly below the carbon concentration reduced portion, and the carbon concentration in the retained austenite covering the prior austenite grain boundaries were each determined. Table 3 lists the measurement results.
Next, the obtained zinc or zinc alloy coated or plated steel sheets were used as test materials and the tensile strength TS, yield stress YS, elongation El, hole expansion ratio λ, and delayed fracture resistance of a stretch-flanged portion were evaluated by the methods described above. Table 3 lists the evaluation results. In Table 3, examples for which the delayed fracture resistance of a stretch-flanged portion was excellent are indicated by "○", and examples for which the delayed fracture resistance of a stretch-flanged portion was not excellent are indicated by "×".

[Table 3]

Table 3

No.	Steel sample ID	Microstructure at 1/4 sheet thickness position				Carbon concentration reduced portion in surface layer	Directly below carbon concentration reduced portion in surface layer			Mechanical properties					Classification
No.	Steel sample ID	Total area fraction of tempered martensite and fresh martensite (%)	Volume fraction of retained austenite (%)	Area fraction of bainite (%)	Area fraction of residual microstructure (%)	Thickness (µm)	Thickness of bainite precipitation region (µm)	Total coverage of retained austenite and fresh martensite at prior austenite grain boundaries (%)	Carbon concentration in retained austenite covering prior austenite grain boundaries (mass%)	TS (MPa)	YS (MPa)	El (%)	(%)	Delayed fracture resistance of stretch-flanged portion	Classification
1	A	87.7	9.3	2.8	0.2	13.3	40	54.9	0.56	1522	1177	10.4	30	○	Example
2	B	82.1	12.7	4.3	0.9	15.2	22	40.8	0.41	1512	1148	11.2	39	○	Example
3	C	88.4	10.3	1.1	0.2	2.2	32	44.9	0.44	1537	1152	10.6	38	○	Example
4	D	83.3	13.0	3.6	0.1	9.4	35	46.0	0.54	1513	1125	11.2	29	○	Example
5	E	87.6	8.6	3.2	0.6	12.0	55	35.1	0.54	1474	1128	10.8	31	○	Example
6	E	76.8	10.7	11.3	1.2	29.4	20	48.7	0.49	1435	1099	11.7	31	○	Comparative Example
7	E	78.1	8.0	13.2	0.7	1.8	22	36.4	0.52	1409	1062	11.2	29	○	Comparative Example
8	E	86.0	9.2	4.7	0.1	0.0	4	46.0	0.59	1480	1134	10.7	28	×	Comparative Example
9	E	76.4	10.1	12.8	0.7	34.5	50	41.6	0.46	1418	1092	12.1	32	○	Comparative Example
10	E	74.1	10.2	14.1	1.6	4.4	32	49.5	0.54	1432	1088	12.2	27	○	Comparative Example
11	E	85.8	11.1	1.9	1.2	17.0	0	65.7	0.42	1619	1241	10.6	23	×	Comparative Example
12	E	84.5	10.5	4.1	0.9	15.9	8	62.5	0.54	1480	1121	11.1	22	×	Comparative Example
13	E	87.0	8.9	2.1	2.0	2.2	73	27.2	0.52	1440	1083	10.9	33	○	Comparative Example
14	E	75.8	11.4	11.3	1.5	27.2	66	48.0	0.50	1460	1092	11.5	30	○	Comparative Example
15	E	74.7	10.1	14.6	0.6	15.3	70	29.1	0.41	1207	936	13.5	35	○	Comparative Example
16	E	72.2	11.5	14.4	1.9	4.7	50	46.8	0.47	1292	978	13.1	30	○	Comparative Example
17	E	91.7	3.0	3.5	1.8	29.1	38	33.2	0.74	1558	1181	7.9	26	×	Comparative Example
18	E	67.6	23.8	7.2	1.4	42.0	48	56.1	0.52	1432	1095	14.4	22	○	Comparative Example
19	E	89.6	4.6	4.2	1.6	13.6	33	43.4	0.55	1494	1159	8.7	32	×	Comparative Example
20	E	91.8	4.3	3.2	0.7	5.5	34	53.2	0.58	1484	1129	8.5	31	×	Comparative Example
21	E	90.5	3.1	4.9	1.5	6.2	72	34.4	0.33	1498	1151	8.3	54	×	Comparative Example
22	E	86.7	5.5	4.3	0.5	7.5	72	54.4	0.55	1473	1055	8.2	31	○	Comparative Example
23	E	83.3	12.6	3.8	0.3	24.7	48	46.5	0.41	1496	1163	8.1	22	○	Comparative Example
24	E	88.8	4.2	4.4	0.5	30.6	18	50.8	0.58	1482	757	8.6	30	○	Comparative Example
25	E	85.4	10.3	2.4	1.9	16.5	50	46.9	0.48	1483	993	10.6	33	○	Example
26	E	86.1	10.0	3.8	0.1	14.8	55	37.3	0.41	1551	1166	10.3	41	○	Example
27	E	84.7	9.5	4.6	1.2	144.0	22	40.2	0.42	1486	1157	10.4	39	○	Example
28	E	87.2	8.5	3.3	1.0	160.0	54	47.7	0.45	1429	1140	9.9	37	○	Comparative Example
29	E	86.1	8.2	4.5	1.2	19.0	66	37.7	0.53	1527	1160	10.3	32	○	Example

Total area fraction of tempered martensite and fresh martensite (%)	Volume fraction of retained austenite (%)	Area fraction of bainite (%)	Area fraction of residual microstructure (%)	Thickness (µm)	Thickness of bainite precipitation region (µm)	Total coverage of retained austenite and fresh martensite at prior austenite grain boundaries (%)	Carbon concentration in retained austenite covering prior austenite grain boundaries (mass%)	TS (MPa)	YS (MPa)	El (%)	λ (% )	Delayed fracture resistance of stretch-flanged portion
30	E	87.6	9.5	2.1	0.8	39.6	46	47.2	0.53	1472	1137	10.7	27	○	Example
31	E	87.5	8.3	4.1	0.1	10.7	53	40.5	0.48	1498	1123	10.3	35	○	Example
32	E	85.3	11.9	1.3	1.5	13.4	48	36.1	0.54	1509	1170	11.0	31	○	Example
33	E	87.4	8.2	3.1	1.3	19.4	61	44.1	0.48	1520	1141	10.4	35	○	Example
34	E	88.6	9.3	2.0	0.1	18.6	32	38.1	0.55	1490	1150	10.2	31	○	Example
35	F	71.6	12.1	15.0	1.3	27.0	71	43.6	0.49	1413	1065	12.5	28	○	Comparative Example
36	G	88.7	10.0	1.2	0.1	8.4	68	51.0	0.44	1661	1284	9.6	39	×	Comparative Example
37	H	77.7	8.4	13.3	0.6	17.7	44	42.8	0.44	1443	1105	11.6	34	○	Comparative Example
38	I	78.7	18.4	2.2	0.7	7.5	39	38.2	0.47	1546	1189	12.5	23	○	Comparative Examples
39	J	76.5	8.9	13.1	1.5	2.9	19	35.0	0.50	1452	1128	11.5	30	○	Comparative Example
40	K	85.6	13.0	1.4	0.0	23.3	53	48.1	0.50	1595	1236	11.0	33	×	Comparative Example
41	L	90.0	8.2	1.1	0.7	19.7	67	51.1	0.54	1504	1163	10.1	32	×	Comparative Example
42	M	85.0	9.5	3.8	1.7	4.9	52	40.5	0.51	1531	1158	10.3	32	×	Comparative Example
43	N	82.6	11.9	4.8	0.7	4.8	29	38.8	0.58	1514	1125	11.1	27	×	Comparative Example
44	O	88.5	8.2	1.5	1.8	30.1	23	46.0	0.56	1516	1179	9.8	31	×	Comparative Example
45	P	86.6	10.0	2.7	0.7	21.7	59	35.0	0.49	1532	1170	10.8	34	×	Comparative Example
46	Q	82.0	12.3	3.9	1.8	15.3	43	38.2	0.50	1526	1150	10.8	32	○	Example
47	R	85.7	10.5	3.5	0.3	21.0	45	53.2	0.53	1524	1172	10.4	32	○	Example
48	s	83.5	11.3	3.8	1.4	16.6	20	46.4	0.53	1506	1151	11.5	31	○	Example
49	T	86.6	9.7	2.0	1.7	5.0	20	42.0	0.40	1490	1139	11.0	42	○	Example
50	U	85.2	10.1	3.0	1.7	18.4	59	49.0	0.58	1485	1107	10.6	29	○	Example
51	v	84.6	9.2	4.4	1.8	20.0	40	45.3	0.54	1540	1191	10.1	31	○	Example
52	w	86.1	11.0	2.0	0.9	3.2	62	43.7	0.59	1500	1145	11.2	29	○	Example
53	X	82.7	10.8	4.5	2.0	42.5	44	52.3	0.48	1488	1114	11.3	34	○	Example
54	Y	87.9	9.2	1.4	1.5	6.2	65	37.9	0.51	1518	1143	10.5	28	○	Example
55	Z	86.0	8.7	3.6	1.7	12.3	56	54.4	0.59	1501	1123	10.7	28	○	Example
56	AA	89.7	8.3	1.4	0.6	29.1	54	44.8	0.58	1515	1132	10.1	25	○	Example
57	AB	88.0	8.2	1.9	1.9	32.2	21	38.9	0.59	1531	1163	9.7	29	○	Example
58	AC	90.6	8.4	1.0	0.0	1.2	17	46.8	0.47	1554	1099	10.6	32	○	Example
59	AD	89.6	8.6	1.1	0.7	17.6	27	44.1	0.58	1520	1180	10.4	33	○	Example

Underlined: indicates value outside scope of present disclosure.
Underlined: indicates value outside scope of present disclosure.

As indicated in Table 3, the Examples according to the present disclosure 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 have excellent delayed fracture resistance in a stretch-flanged portion. Further, in the tempering process, for the Examples according to the present disclosure (Examples other than No. 25) for which an average load tension of 2.0 kgf/mm² or more was applied, and bending and unbending were carried out once or more, at a time at least 6 s after the start of the holding at the tempering temperature and at least 10 s before the end of the holding at the tempering temperature, the yield stress YS was 1000 MPa or more. In contrast, the Comparative Examples were inferior in one or more of TS, hole expansion formability, ductility, or delayed fracture resistance of a stretch-flanged portion.

INDUSTRIAL APPLICABILITY

According to the present disclosure, it is possible to provide a zinc or zinc alloy coated or plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (E1) of 9.0 % or more, a hole expansion ratio (λ) of 25 % or more, and excellent delayed fracture resistance in a stretch-flanged portion, and a method of producing same.

Claims

A zinc or zinc alloy coated or plated steel sheet comprising a base steel sheet and a zinc or zinc alloy coated or plated layer formed on a surface of the base steel sheet,
wherein the base steel sheet

has a chemical composition containing, in mass%,
C: 0.180 % or more and 0.250 % or less,

Si: 0.800 % or more and 1.550 % or less,

Mn: 2.400 % or more and 3.200 % or less,

P: 0.100 % or less,

S: 0.0200 % or less,

Al: 1.000 % or less,

N: 0.0100 % or less, and

O: 0.0100 % or less,

with the balance being Fe and inevitable impurity,

wherein, at a position where the depth from the surface of the base steel sheet is 1/4 of the sheet thickness,
a total area fraction of tempered martensite and fresh martensite is 70.0 % or more and 94.0 % or less,

a volume fraction of retained austenite is 6.0 % or more and 15.0 % or less,

an area fraction of bainite is 10.0 % or less, and

an area fraction of the residual microstructure is 10.0 % or less,

wherein a thickness of a carbon concentration reduced portion in a surface layer of the base steel sheet is 1.0 µm or more and 150.0 µm or less,

wherein a thickness of a bainite precipitation region directly below the carbon concentration reduced portion is 10 µm or more,

wherein a total coverage of retained austenite and fresh martensite at prior austenite grain boundaries directly below the carbon concentration reduced portion is 30.0 % or more and 60.0 % or less,

wherein a carbon concentration of retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduced portion is less than 0.60 mass%,

and wherein the tensile strength is 1470 MPa or more.
The zinc or zinc alloy coated or plated steel sheet according to claim 1, wherein the base steel sheet has a yield stress of 1000 MPa or more.
The zinc or zinc alloy coated or plated steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of:
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 or zinc alloy coated or plated steel sheet according to any one of claims 1 to 3, wherein the zinc or zinc alloy coated or plated layer is an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed layer.
A method of producing a zinc or zinc alloy coated or plated steel sheet comprising:
a process of hot rolling a steel slab having a chemical composition containing, in mass%,
C: 0.180 % or more and 0.250 % or less,

Si: 0.800 % or more and 1.550 % or less,

Mn: 2.400 % or more and 3.200 % or less,

P: 0.100 % or less,

S: 0.0200 % or less,

Al: 1.000 % or less,

N: 0.0100 % or less, and

O: 0.0100 % or less,

with the balance being Fe and inevitable impurity, to obtain a hot-rolled steel sheet;

a process of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet;

an annealing process of holding the cold-rolled steel sheet at an annealing temperature of Ac3 (°C) or higher, where Ac3 is defined by the following expression (1), for 10 s or longer in an atmosphere having a dew point of -25 °C or higher and 35 °C or lower;

a subsequent first cooling process of cooling the cold-rolled steel sheet to a temperature T1 of T - 20 (°C) or higher and T + 20 (°C) or lower, at a cooling rate of 7 °C/s or more, where T is defined by the following expression (2);

a subsequent process of holding the cold-rolled steel sheet at T1 (°C) for 30 s or longer and 200 s or shorter;

a subsequent second cooling process of cooling the cold-rolled steel sheet to a temperature T2 of lower than T - 20 (°C);

a subsequent process of subjecting the cold-rolled steel sheet to a zinc or zinc alloy coating or plating treatment to obtain a coated or plated steel sheet;

a subsequent third cooling process of cooling the coated or plated steel sheet to a temperature T3 of Ms - 200 (°C) or higher and Ms - 80 (°C) or lower, at a cooling rate of 5 °C/s or more, where Ms is defined by the following expression (3);

a subsequent fourth cooling process of cooling the coated or 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;

and a subsequent tempering process of holding the coated or plated steel sheet at a tempering temperature of higher than T4 (°C) and 350 °C or lower for 10 s or longer and 1000 s or shorter,

wherein a total residence time from the zinc or zinc alloy coating or plating treatment to the third cooling process in which the coated or plated steel sheet is in a temperature range from Bf (°C) to T - 20 (°C), where Bf are defined by the following expression (4), is 30 s or shorter, Ac3 (°C) = 881 - 205.7 × [%C] + 53.1 × [%Si] - 15 × [%Mn] - 27 × [%Cu] - 20.1 × [%Ni] - 0.7 × [%Cr] + 41.1 × [%Mo] ...(1) $T (° C) = 743 - 90 \times [% Mn] - 37 \times [% Ni] - 70 \times [% Cr] - 83 \times [% Mo]$ 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)

wherein [%X] indicates the content in mass% of element X in the chemical composition, and is 0 when the chemical composition does not contain element X.
The method of producing a zinc or zinc alloy coated or plated steel sheet according to claim 5, wherein, in the tempering process, a tension with an average load tension of 2.0 kgf/mm² or more is applied, and bending and unbending are carried out one or more times, at a time at least 6 s after the start of the holding at the tempering temperature and at least 10 s before the end of the holding at the tempering temperature.
The method of producing a zinc or zinc alloy coated or plated steel sheet according to claim 5 or 6, the chemical composition further containing, in mass%, at least one selected from the group consisting of:
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 of producing a zinc or zinc alloy coated or plated steel sheet according to any one of claims 5 to 7, wherein the zinc or zinc alloy coating or plating treatment is an electrogalvanization treatment, a hot-dip galvanizing treatment, or a hot-dip galvanizing treatment followed by an alloying treatment.