# US 2021/0340653 A1 · High-strength steel plate having excellent formability, toughness and weldability, and production method of same

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- Number: US 2021/0340653 A1
- Document ID: US20210340653A1
- Status: Published application
- Kind: A1
- Application number: 17/312,853
- Filed: 2018-12-11
- Published: 2021-11-04
- Earliest claimed: 2018-12-11
- Assignees: Nippon Steel Corporation
- Applicants: Nippon Steel Corporation
- Inventors: Hiroyuki KAWATA; Eisaku SAKURADA; Kohichi SANO; Takafumi YOKOYAMA
- Web page: https://patentgrep.com/patent/US20210340653A1

## CPC

- C22C 38/58: with more than 1.5% by weight of manganese
- C21D 1/185: {from an intercritical temperature}
- C21D 1/19: by interrupted quenching
- C21D 1/25: Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
- C21D 1/26: Methods of annealing
- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 8/021: involving particular fabrication steps or treatments of ingots or slabs
- C21D 8/0226: {Hot rolling}
- C21D 8/0236: {Cold rolling}
- C21D 8/0263: {following hot rolling}
- C21D 8/0268: {between cold rolling steps}
- C21D 8/0273: {Final recrystallisation annealing}
- C21D 8/0426: {Hot rolling}
- C21D 8/0436: {Cold rolling}
- C21D 8/0463: {following hot rolling}
- C21D 8/0468: {between cold rolling steps}
- C21D 8/0473: {Final recrystallisation annealing}
- C21D 9/46: for sheet metals
- C21D 9/48: deep-drawing sheets
- C22C 38/001: {containing N}
- C22C 38/002: {containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60}
- C22C 38/005: {containing rare earths, i.e. Sc, Y, Lanthanides}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/08: containing nickel {(C22C38/105 takes precedence)}
- C22C 38/12: containing tungsten, tantalum, molybdenum, vanadium, or niobium
- C22C 38/14: containing titanium or zirconium
- C22C 38/16: containing copper
- C22C 38/22: with molybdenum or tungsten
- C22C 38/26: with niobium or tantalum
- C22C 38/28: with titanium or zirconium
- C22C 38/32: with boron
- C22C 38/38: with more than 1.5% by weight of manganese
- C22C 38/42: with copper
- C22C 38/44: with molybdenum or tungsten
- C22C 38/46: with vanadium
- C22C 38/48: with niobium or tantalum
- C22C 38/50: with titanium or zirconium
- C22C 38/54: with boron
- C22C 38/60: containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
- C23C 2/02: Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence)
- C23C 2/0224: {Two or more thermal pretreatments}
- C23C 2/06: Zinc or cadmium or alloys based thereon
- C23C 2/40: Plates; Strips
- C23C 30/00: Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process (C23C26/00, C23C28/00 take precedence)
- C25D 5/50: by heat-treatment
- C21D 2211/001: Austenite
- C21D 2211/002: Bainite
- C21D 2211/005: Ferrite
- C21D 2211/008: Martensite
- C22C: ALLOYS (treatment of alloys C21D, C22F)
- C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B)
- C23C: COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL (making metal-coated products by extrusion B21C23/22; covering with metal by connecting pre-existing layers to articles, see the relevant places, e.g. B21D39/00, B23K; metallising of glass C03C; metallising mortars, concrete, artificial stone, ceramics or natural stone C04B41/00; enamelling of, or applying a vitreous layer to, metals C23D; treating metal surfaces or coating of metals by electrolysis or electrophoresis C25D; single-crystal film growth C30B; by metallising textiles D06M11/83; decorating textiles by locally metallising D06Q1/04)
- C25D: PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR

## Abstract

A high-strength steel sheet excellent in formability, toughness and weldability has a chemical composition including: by mass %, C: 0.05 to 0.30%, Si: 2.50% or less, Mn: 0.50 to 3.50%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 2.500%, N: 0.0150% or less, O: 0.0050% or less, and the balance consisting of Fe and inevitable impurities. The high-strength steel sheet has a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure including: by volume %, acicular ferrite (3): 20% or more, and martensite (4):10% or more, aggregated ferrite: 20% or less, residual austenite: 2.0% or less, and the martensite satisfies a formula (A), ∑ i = 1 5 ⁢ ⁢ d i a i 1.5 ≤ 10.0 ( A )

## Claims

### Claim 1

A high-strength steel sheet excellent in formability, toughness, and weldability, the high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure, wherein
the martensite satisfies a formula (A) below,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
10.0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 2 (depends on 1)

The high-strength steel sheet excellent in formability, toughness, and weldability according to claim 1, wherein the chemical composition further comprises: by mass %, in place of a part of Fe, one or more of Ti of 0.30% or less, Nb of 0.10% or less, and V of 1.00% or less.

### Claim 3 (depends on 1)

The high-strength steel sheet excellent in formability, toughness, and weldability according to claim 1, wherein the chemical composition further comprises: by mass %, in place of a part of Fe, one or more of Cr of 2.00% or less, Ni of 2.00% or less, Cu of 2.00% or less, Mo of 1.00% or less, W of 1.00% or less, B of 0.0100% or less, Sn of 1.00% or less, and Sb of 0.20% or less.

### Claim 4 (depends on 1)

The high-strength steel sheet excellent in formability, toughness, and weldability according to claim 1, wherein
the chemical composition further comprises: by mass %, in place of a part of Fe, one or more of Ca, Ce, Mg, Zr, La, Hf, and REM at 0.0100% or less in total.

### Claim 5 (depends on 1)

The high-strength steel sheet excellent in formability, toughness, and weldability according to claim 1, wherein
martensite of the microstructure comprises, by volume %, 30% or more of tempered martensite where fine carbides having an average diameter of 1.0 m or less are precipitated with reference to the entire martensite.

### Claim 6 (depends on 1)

The high-strength steel sheet excellent in formability, toughness, and weldability according to claim 1, wherein
the high-strength steel sheet comprises a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet.

### Claim 7 (depends on 6)

The high-strength steel sheet excellent in formability, toughness, and weldability according to claim 6, wherein
the galvanized layer or the zinc alloy plated layer is an alloyed plated layer.

### Claim 8 (depends on 1)

A method of manufacturing the high-strength steel sheet excellent in formability, toughness, and weldability according to claim 1, the manufacturing method comprising:
subjecting a steel piece having the chemical composition according to claim 1 to hot rolling, completing the hot rolling at a temperature in a range from 850 degrees C. to 1050 degrees C. to provide a steel sheet after the hot rolling,
cooling the steel sheet after the hot rolling at an average cooling rate of at least 30 degrees C. per second from 850 degrees C. to 550 degrees C., winding the steel sheet at a temperature equal to or less than a Bs point that is a bainite transformation start point defined according to a formula below,
cooling the steel sheet after the hot rolling in a range from the Bs point to a point of (the Bs point −80) degrees C. under conditions satisfying a formula (1) below to provide a hot-rolled steel sheet,
subjecting or not subjecting the hot-rolled steel sheet to cold rolling at a rolling reduction of 10% or less to manufacture a steel sheet for heat treatment,
heating the steel sheet for heat treatment to a temperature in a range from (Ac1+25) degrees C. to an Ac3 point under conditions satisfying a formula (3) below for calculating by dividing an elapsed time in a temperature region from 700 degrees C. to an end point that is a lower one of a maximum heating temperature or (Ac3−20) degrees C. into 10 parts, retaining the steel sheet for 150 seconds or less in a temperature region from the maximum heating temperature minus 10 degrees C. to the maximum heating temperature,
cooling the steel sheet from a heating retention temperature at an average cooling rate of at least 25 degrees C. per second in a temperature region from 700 degrees C. to 550 degrees C., and
cooling the steel sheet in a limited range satisfying formulae (4) and (5) below for calculating by dividing a dwell time in a temperature region from a start point that is a lower one of 550 degrees C. or the Bs point to 300 degrees C. into 10 parts,
Bs point (degrees C.)=611−33·[Mn]−17·[Cr]
−17·[Ni]−21·[Mo]−11·[Si]
+30·[Al]+(24·[Cr]+15·[Mo]
+5500·[B]+240·[Nb])/(8·[C])
[element]: mass % of each element
∑
n
=
1
g
⁢
{
5.37
×
10
-
1
·
(
10
⁢
n
+
925
-
Bs
-
57
⁢
W
Cr
-
78
⁢
W
Mn
-
39
⁢
W
Si
+
⁢
⁢
56
⁢
W
Al
-
41
⁢
W
Ni
-
1598
⁢
W
B
)
2.5
·
exp
⁡
(
1.44
×
10
4
10
⁢
n
-
Bs
-
278
)
·
exp
⁡
(
-
5.5
⁢
W
Nb
-
2.0
⁢
W
Ti
-
0.2
⁢
W
Cr
-
1.1
⁢
W
Mo
)
·
Δ
⁢
⁢
t
⁡
(
n
)
1
/
3
+
1.81
×
10
1
·
(
10
⁢
n
-
5
)
1.3
·
exp
⁡
(
1.73
×
10
4
10
⁢
n
-
Bs
-
278
)
·
exp
⁡
(
-
1.1
⁢
W
Mo
-
0.6
⁢
W
Cr
-
9.0
⁢
W
B
)
·
Δ
⁢
⁢
t
⁡
(
n
)
1
/
2
}
≤
1.50
(
1
)
Bs: Bs point (degrees C.),
WM: a composition of each element (mass %),
Δt(n): an elapsed time (second) from (Bs−10× (n−1)) degrees C. to (Bs−10× n) degrees C. in a duration from the cooling after the hot rolling, through the winding, to the cooling to 400 degrees C.,
∑
n
=
1
10
⁢
8.65
×
10
2
·
(
W
Mn
+
0.51
⁢
W
Cr
+
0.51
⁢
W
Ni
-
0.64
⁢
W
Mo
-
0.33
⁢
W
Si
+
⁢
⁢
0.90
⁢
W
Al
)
0.5
·
f
Y
⁡
(
n
)
0.5
·
(
1
-
f
Y
⁡
(
n
)
)
1.8
·
exp
⁡
(
-
9.00
×
10
3
T
⁡
(
n
)
+
273
)
·
Δ
⁢
⁢
t
0.33
≤
2.0
(
3
)
Δt: one tenth (second) of the elapsed time,
WM: a composition of each elemental species (mass %),
fγ(n): an average reverse transformation ratio in the n-th section, and
T(n): an average temperature (degrees C.) in the n-th section,
⁢
∑
n
=
1
10
⁢
{
1.39
×
10
1
·
(
Bs
-
T
⁡
(
n
)
)
3
·
exp
⁡
(
-
1.44
×
10
4
T
⁡
(
n
)
+
273
)
⁢
Δ
⁢
⁢
t
0.5
}
≤
1.0
(
4
)
∑
n
=
1
10
⁢
{
1.56
×
10
2
·
(
W
Si
+
0.9
⁢
W
Al
·
(
T
⁡
(
n
)
550
)
2
+
0.3
⁢
(
W
Cr
+
W
Mo
)
·
T
⁡
(
n
)
550
)
·
exp
⁡
(
-
6.7
·
(
1
-
T
⁡
(
n
)
550
)
)
·
(
T
⁡
(
n
)
-
250
300
)
0.5
·
(
Bs
-
T
⁡
(
n
)
)
3
·
exp
⁡
(
-
1.44
×
10
4
T
⁡
(
n
)
+
273
)
·
Δ
⁢
⁢
t
0.5
}
≤
1.0
(
5
)
Δt: one tenth (second) of the elapsed time,
Bs: Bs point (degrees C.),
T(n): an average temperature (degrees C.) in the n-th section, and
WM: a composition of each elemental species (mass %).

### Claim 9 (depends on 1)

A method of manufacturing the high-strength steel sheet excellent in formability, toughness, and weldability according to claim 1, the manufacturing method comprising:
subjecting a steel piece having the chemical composition according to claim 1 to hot rolling, completing the hot rolling at a temperature in a range from 850 degrees C. to 1050 degrees C. to provide a steel sheet after the hot rolling,
cooling the steel sheet after the hot rolling from 850 degrees C. to 550 degrees C. at an average cooling rate of at least 30 degrees C. per second, winding the steel sheet at a temperature equal to or less than a Bs point that is a bainite transformation start point defined according to a formula below,
cooling the steel sheet after the hot rolling from the Bs point to a point of (the Bs point −80) degrees C. under conditions satisfying a formula (1) below to provide a hot-rolled steel sheet,
subjecting or not subjecting the hot-rolled steel sheet to a first cold rolling to manufacture a steel sheet for intermediate heat treatment,
heating the steel sheet for intermediate heat treatment to a temperature equal to or more than (Ac3−20) degrees C. under conditions satisfying a formula (2) below for calculating by dividing an elapsed time in a temperature region from 700 degrees C. to (Ac3−20) degrees C. into 10 parts,
subsequently, cooling the steel sheet for intermediate heat treatment from the heating temperature at an average cooling rate of at least 30 degrees C. per second in a temperature region from 700 degrees C. to 550 degrees C., cooling the steel sheet for intermediate heat at the average cooling rate of at least 20 degrees C. per second in a temperature region from the Bs point to (Bs−80) degrees C., and leaving the steel sheet for intermediate heat from (Bs−80) degrees C. to an Ms point for a dwell time of at most 1000 seconds and from the Ms point to (Ms −50) degrees C. at the average cooling rate of at most 100 degrees C. per second to manufacture an intermediate heat-treated steel sheet,
subjecting or not subjecting the cooled intermediate heat-treated steel sheet to a second cold rolling at a rolling reduction of 10% or less to manufacture a steel sheet for heat treatment,
heating the steel sheet for heat treatment to a temperature in a range from (Ac1+25) degrees C. to an Ac3 point under conditions satisfying a formula (3) below for calculating by dividing an elapsed time in a temperature region from 700 degrees C. to an end point that is a lower one of a maximum heating temperature or (Ac3−20) degrees C. into 10 parts, and retaining the steel sheet for 150 seconds or less in a temperature region from the maximum heating temperature minus 10 degrees C. to the maximum heating temperature,
cooling the steel sheet from a heating retention temperature at an average cooling rate of at least 25 degrees C. per second in a temperature region from 700 degrees C. to 550 degrees C., and cooling the steel sheet in a limited range satisfying formulae (4) and (5) below for calculating by dividing a dwell time in a temperature region from a start point that is a lower one of 550 degrees C. or the Bs point to 300 degrees C. into 10 parts,
Bs point (degrees C.)=611−33·[Mn]−17·[Cr]
−17·[Ni]−21·[Mo]−11·[Si]
+30·[Al]+(24·[Cr]+15·[Mo]
+5500·[B]+240·[Nb])/(8·[C])
[element]: mass % of each element,
∑
n
=
1
g
⁢
{
5.37
×
10
-
1
·
(
10
⁢
n
+
925
-
Bs
-
57
⁢
W
Cr
-
78
⁢
W
Mn
-
39
⁢
W
Si
+
⁢
⁢
56
⁢
W
Al
-
41
⁢
W
Ni
-
1598
⁢
W
B
)
2.5
·
exp
⁡
(
1.44
×
10
4
10
⁢
n
-
Bs
-
278
)
·
exp
⁡
(
-
5.5
⁢
W
Nb
-
2.0
⁢
W
Ti
-
0.2
⁢
W
Cr
-
1.1
⁢
W
Mo
)
·
Δ
⁢
⁢
t
⁡
(
n
)
1
/
3
+
1.81
×
10
1
·
(
10
⁢
n
-
5
)
1.3
·
exp
⁡
(
1.73
×
10
4
10
⁢
n
-
Bs
-
278
)
·
exp
⁡
(
-
1.1
⁢
W
Mo
-
0.6
⁢
W
Cr
-
9.0
⁢
W
B
)
·
Δ
⁢
⁢
t
⁡
(
n
)
1
/
2
}
≤
1.50
(
1
)
Bs: Bs point (degrees C.),
WM: a composition of each element (mass %),
Δt(n): an elapsed time (second) from (Bs−10×(n−1)) degrees C. to (Bs−10× n) degrees C. in a duration from the cooling after the hot rolling, through winding, to the cooling to 400 degrees C.,
Ms point (degrees C.)=561−474[C]−33·[Mn]
−17·[Cr]−17·[Ni]−21·[Mo]
−11·[Si]+30·[Al]
[element]: mass % of each element,
∑
n
=
1
10
⁢
5.92
×
10
2
·
f
Y
⁡
(
n
)
0.3
·
(
1
-
f
Y
⁡
(
n
)
)
1.4
·
exp
⁡
(
-
9.00
×
10
3
T
⁡
(
n
)
+
273
)
·
Δ
⁢
⁢
t
0.5
≤
1.0
(
2
)
Δt: one tenth (second) of the elapsed time,
fγ(n): an average reverse transformation ratio in the n-th section, and
T(n): an average temperature (degrees C.) in the n-th section,
∑
n
=
1
10
⁢
8.65
×
10
2
·
(
W
Mn
+
0.51
⁢
W
Cr
+
0.51
⁢
W
Ni
-
0.64
⁢
W
Mo
-
0.33
⁢
W
Si
+
⁢
⁢
0.90
⁢
W
Al
)
0.5
·
f
Y
⁡
(
n
)
0.5
·
(
1
-
f
Y
⁡
(
n
)
)
1.8
·
exp
⁡
(
-
9.00
×
10
3
T
⁡
(
n
)
+
273
)
·
Δ
⁢
⁢
t
0.33
≤
2.0
(
3
)
Δt: one tenth (second) of the elapsed time,
WM: a composition of each elemental species (mass %),
fγ(n): an average reverse transformation ratio in the n-th section, and
T(n): an average temperature (degrees C.) in the n-th section,
⁢
∑
n
=
1
10
⁢
{
1.39
×
10
1
·
(
Bs
-
T
⁡
(
n
)
)
3
·
exp
⁡
(
-
1.44
×
10
4
T
⁡
(
n
)
+
273
)
⁢
Δ
⁢
⁢
t
0.5
}
≤
1.0
(
4
)
∑
n
=
1
10
⁢
{
1.56
×
10
2
·
(
W
Si
+
0.9
⁢
W
Al
·
(
T
⁡
(
n
)
550
)
2
+
0.3
⁢
(
W
Cr
+
W
Mo
)
·
T
⁡
(
n
)
550
)
·
exp
⁡
(
-
6.7
·
(
1
-
T
⁡
(
n
)
550
)
)
·
(
T
⁡
(
n
)
-
250
300
)
0.5
·
(
Bs
-
T
⁡
(
n
)
)
3
·
exp
⁡
(
-
1.44
×
10
4
T
⁡
(
n
)
+
273
)
·
Δ
⁢
⁢
t
0.5
}
≤
1.0
(
5
)
Δt: one tenth (second) of the elapsed time,
Bs: Bs point (degrees C.),
T(n): an average temperature (degrees C.) in the n-th section, and
WM: a composition of each elemental species (mass %).

### Claim 10 (depends on 9)

The method according to claim 9, wherein the first cold rolling for the steel sheet for heat treatment is performed at the rolling reduction of 80% or less.

### Claim 11 (depends on 9)

The method according to claim 9, wherein the first cold rolling for the steel sheet for heat treatment is performed at the rolling reduction of more than 10%.

### Claim 12 (depends on 8)

The method according to claim 8, further comprising a tempering treatment of heating the steel sheet for heat treatment to a temperature in a range from 200 degrees C. to 600 degrees C., after cooling the steel sheet in a limited range satisfying the formulae (4) and (5) for calculating by dividing a dwell time in a temperature region from a start point that is a lower one of 550 degrees C. and the Bs point to 300 degrees C. into 10 parts.

### Claim 13 (depends on 12)

The method according to claim 12, further comprising temper rolling at a rolling reduction of 2.0% or less before the tempering treatment.

### Claim 14 (depends on 8)

The method according to claim 8 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
immersing the steel sheet in a plating bath including zinc as a main component during dwelling in a range from 550 degrees C. to 300 degrees C. to form the galvanized layer or the zinc alloy plated layer on one surface or both surfaces of the steel sheet,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
10.0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 15 (depends on 8)

The method according to claim 8 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
leaving the steel sheet dwelling in a range from 550 degrees C. to 300 degrees C., cooling the steel sheet to a room temperature, and subsequently forming the galvanized layer or the zinc alloy plated layer by electroplating on one surface or both surfaces of the steel sheet,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 16 (depends on 12)

The method according to claim 12 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%:
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
immersing the steel sheet in a plating bath including zinc as a main component during the tempering treatment to form the galvanized layer or the zinc alloy plated layer on one surface or both surfaces of the steel sheet,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 17 (depends on 12)

The method according to claim 12 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
subjecting the steel sheet to the tempering treatment, cooling the steel sheet to a room temperature, and subsequently forming the galvanized layer or the zinc alloy plated layer by electroplating on one surface or both surfaces of the steel sheet,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 18 (depends on 17)

The method according to claim 17 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising:
by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
immersing the steel sheet in a plating bath, subsequently while leaving the steel sheet dwelling from 300 degrees C. to 550 degrees C., heating the galvanized layer or the zinc alloy plated layer to a temperature in a range from 450 degrees C. to 550 degrees C. to perform an alloying treatment on the galvanized layer or the zinc alloy plated layer,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [m] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 19 (depends on 15)

The method according to claim 15 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities, the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising:
by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
setting a heating temperature of the plated layer or the zinc alloy plated layer to a temperature in a range from 450 degrees C. to 550 degrees C. in the tempering treatment to perform an alloying treatment on the galvanized layer or the zinc alloy plated layer,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 20 (depends on 9)

The method according to claim 9, further comprising a tempering treatment of heating the steel sheet for heat treatment to a temperature in a range from 200 degrees C. to 600 degrees C., after cooling the steel sheet in a limited range satisfying the formulae (4) and (5) for calculating by dividing a dwell time in a temperature region from a start point that is a lower one of 550 degrees C. and the Bs point to 300 degrees C. into 10 parts.

### Claim 21 (depends on 20)

The method according to claim 20, further comprising temper rolling at a rolling reduction of 2.0% or less before the tempering treatment.

### Claim 22 (depends on 9)

The method according to claim 9 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising:
by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet, the method comprising:
immersing the steel sheet in a plating bath including zinc as a main component during dwelling in a range from 550 degrees C. to 300 degrees C. to form the galvanized layer or the zinc alloy plated layer on one surface or both surfaces of the steel sheet,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 23 (depends on 9)

The method according to claim 9 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising:
by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet, the method comprising:
leaving the steel sheet dwelling in a range from 550 degrees C. to 300 degrees C., cooling the steel sheet to a room temperature, and subsequently forming the galvanized layer or the zinc alloy plated layer by electroplating on one surface or both surfaces of the steel sheet,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 24 (depends on 20)

The method according to claim 20 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
immersing the steel sheet in a plating bath including zinc as a main component during the tempering treatment to form the galvanized layer or the zinc alloy plated layer on one surface or both surfaces of the steel sheet,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 25 (depends on 20)

The method according to claim 20 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
subjecting the steel sheet to the tempering treatment, cooling the steel sheet to a room temperature, and subsequently forming the galvanized layer or the zinc alloy plated layer by electroplating on one surface or both surfaces of the steel sheet,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 26 (depends on 25)

The method according to claim 25 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
immersing the steel sheet in a plating bath, subsequently while leaving the steel sheet dwelling from 300 degrees C. to 550 degrees C., heating the galvanized layer or the zinc alloy plated layer to a temperature in a range from 450 degrees C. to 550 degrees C. to perform an alloying treatment on the galvanized layer or the zinc alloy plated layer,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 27 (depends on 23)

The method according to claim 23 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
setting a heating temperature of the plated layer or the zinc alloy plated layer to a temperature in a range from 450 degrees C. to 550 degrees C. in the tempering treatment to perform an alloying treatment on the galvanized layer or the zinc alloy plated layer,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 28 (depends on 16)

The method according to claim 16 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
setting a heating temperature of the plated layer or the zinc alloy plated layer to a temperature in a range from 450 degrees C. to 550 degrees C. in the tempering treatment to perform an alloying treatment on the galvanized layer or the zinc alloy plated layer,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 29 (depends on 24)

The method according to claim 24 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
setting a heating temperature of the plated layer or the zinc alloy plated layer to a temperature in a range from 450 degrees C. to 550 degrees C. in the tempering treatment to perform an alloying treatment on the galvanized layer or the zinc alloy plated layer,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 30 (depends on 18)

The method according to claim 18 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
setting a heating temperature of the plated layer or the zinc alloy plated layer to a temperature in a range from 450 degrees C. to 550 degrees C. in the tempering treatment to perform an alloying treatment on the galvanized layer or the zinc alloy plated layer,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).

### Claim 31 (depends on 26)

The method according to claim 26 of manufacturing a high-strength steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the high-strength steel sheet comprising a microstructure in a region from ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
aggregated ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure,
the martensite satisfies a formula (A) below, and
the high-strength steel sheet comprising a galvanized layer or a zinc alloy plated layer on one surface or both surfaces of the high-strength steel sheet,
the method comprising:
setting a heating temperature of the plated layer or the zinc alloy plated layer to a temperature in a range from 450 degrees C. to 550 degrees C. in the tempering treatment to perform an alloying treatment on the galvanized layer or the zinc alloy plated layer,
∑
i
=
1
5
⁢
d
i
a
i
1.5
≤
1
⁢
0
.
0
(
A
)
where: di represents a circle-equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness), and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness).