High-strength steel sheet and method for manufacturing same
A high-strength steel sheet has a tensile strength of 1180 MPa or more. The high-strength steel sheet has a component composition containing C: 0.090 mass % or more and 0.390 mass % or less, Si: 0.01 mass % or more and 2.00 mass % or less, Mn: 2.00 mass % or more and 4.00 mass % or less, P: 0.100 mass % or less, S: 0.0200 mass % or less, Al: 1.000 mass % or less, N: 0.0100 mass % or less, and O: 0.0100 mass % or less, with a remaining part consisting of Fe and inevitable impurities; and a microstructure. In the microstructure, an area ratio of martensite is 70% or more, an area ratio of ferrite is 10% or less, an area ratio of retained austenite is 10% or less, and a proportion of the number of martensite blocks in which metastable carbides are present to the number of martensite blocks is 2% or more.
1 . A high-strength steel sheet comprising a steel sheet,
the steel sheet having: a component composition containing
C in an amount of 0.090 mass % or more and 0.390 mass % or less,
Si in an amount of 0.01 mass % or more and 2.00 mass % or less,
Mn in an amount of 2.00 mass % or more and 4.00 mass % or less,
P in an amount of 0.100 mass % or less,
S in an amount of 0.0200 mass % or less,
Al in an amount of 1.000 mass % or less,
N in an amount of 0.0100 mass % or less, and
O in an amount of 0.0100 mass % or less, with a remaining part containing Fe and inevitable impurities; and a microstructure,
wherein in the microstructure,
an area ratio of martensite is 70% or more,
an area ratio of ferrite is 10% or less,
an area ratio of retained austenite is 10% or less, and
a proportion of the number of martensite blocks in which metastable carbides are present to the number of martensite blocks is 2% or more.
2 . The high-strength steel sheet according to claim 1 , wherein a number density of the metastable carbides in the martensite blocks in which the metastable carbides are present is 1×10 6 metastable carbides/mm 2 or more.
3 . The high-strength steel sheet according to claim 1 , wherein the component composition further contains at least one element selected from the group consisting of
Ti in an amount of 0.200 mass % or less,
Nb in an amount of 0.200 mass % or less,
V in an amount of 0.200 mass % or less,
Ta in an amount of 0.10 mass % or less,
W in an amount of 0.10 mass % or less,
B in an amount of 0.0100 mass % or less,
Cr in an amount of 1.00 mass % or less,
Mo in an amount of 1.00 mass % or less,
Ni in an amount of 1.00 mass % or less,
Co in an amount of 0.010 mass % or less,
Cu in an amount of 1.00 mass % or less,
Sn in an amount of 0.200 mass % or less,
Sb in an amount of 0.200 mass % or less,
Ca in an amount of 0.0100 mass % or less,
Mg in an amount of 0.0100 mass % or less,
REM in an amount of 0.0100 mass % or less,
Zr in an amount of 0.100 mass % or less,
Te in an amount of 0.100 mass % or less,
Hf in an amount of 0.10 mass % or less, and
Bi in an amount of 0.200 mass % or less.
4 . The high-strength steel sheet according to claim 1 , wherein the steel sheet includes a soft layer in a portion from a surface of the steel sheet to 200 μm in a sheet thickness direction, and
the soft layer is a portion having Vickers hardness of 85% or less with respect to Vickers hardness at a ¼ position of a sheet thickness of the steel sheet.
5 . The high-strength steel sheet according to claim 4 , wherein when nano-hardness at the ¼ position in depth in the sheet thickness direction of the soft layer is measured at 300 or more points in an area of 50 μm×50 μm, a proportion r of measurement points having nano-hardness of 7.0 GPa or more to a total number of measurement points is 0.10 or less.
6 . The high-strength steel sheet according to claim 4 , wherein a standard deviation JA of nano-hardness at the ¼ position in depth in the sheet thickness direction of the soft layer is 1.8 GPa or less, and
a standard deviation OB of nano-hardness at a ½ position in depth in the sheet thickness direction of the soft layer is 2.2 GPa or less.
7 . The high-strength steel sheet according to claim 1 , further comprising a metal plated layer as a pre-annealing plated layer on a surface of the steel sheet.
8 . The high-strength steel sheet according to claim 1 , further comprising a plated layer on a surface of the steel sheet.
9 . The high-strength steel sheet according to claim 7 , further comprising a plated layer on a surface of the metal plated layer.
10 . A method for manufacturing the high-strength steel sheet according to claim 1 , the method comprising:
subjecting a steel slab having the component composition according to claim 1 to hot rolling, pickling, and cold rolling to obtain a cold-rolled steel sheet; and
subjecting the cold-rolled steel sheet to annealing and post-heating,
wherein in the annealing, the cold-rolled steel sheet is subjected to heating at a heating temperature of 800° C. or higher, and then to cooling to a cooling stop temperature of 150° C. or lower,
in the cooling,
a staying time t1 in a temperature range T 1 of higher than Ms° C. and 700° C. or lower is 1000 s or less,
a first average cooling rate v1 in a temperature range T 2 of (Ms-80° C.) or higher and Ms° C. or lower is 1.0° C./s or more and 40.0° C./s or less, and
a second average cooling rate v2 in a temperature range T 3 of 150° C. or higher and lower than (Ms-80° C.) is 0.3° C./s or more and less than the first average cooling rate v1, and
the post-heating is performed under a condition that a temperature X that is a maximum end-point temperature and a retention time Y at the temperature X-10° C. or higher satisfy Formula 1:
8000
≤
(
273
+
X
)
×
(
20
+
Log
(
Y
/
3600
)
)
≤
12000
,
Formula
1
where a unit of the temperature X is ° C., and a unit of the retention time Y is s.
11 . The method according to claim 10 , wherein the temperature X satisfies Formula 2:
100
≤
X
≤
400.
Formula
2
12 . The method according to claim 10 , wherein in the annealing, heating at the heating temperature is performed in an atmosphere having a dew point of −30° C. or higher to form a soft layer in a portion from a surface of the cold-rolled steel sheet to 200 μm in a sheet thickness direction, and
the soft layer is a portion having Vickers hardness of 85% or less with respect to Vickers hardness at a ¼ position of a sheet thickness of the cold-rolled steel sheet.
13 . The method according to claim 10 , wherein before being subjected to the annealing, the cold-rolled steel sheet is subjected to a metal plating treatment to form a metal plated layer as a pre-annealing plated layer on a surface of the cold-rolled steel sheet.
14 . The method according to claim 10 , wherein in the annealing, the cold-rolled steel sheet is subjected to a plating treatment to form a plated layer.
15 . The high-strength steel sheet according to claim 2 ,
wherein the component composition further contains at least one element selected from the group consisting of
Ti in an amount of 0.200 mass % or less,
Nb in an amount of 0.200 mass % or less,
V in an amount of 0.200 mass % or less,
Ta in an amount of 0.10 mass % or less,
W in an amount of 0.10 mass % or less,
B in an amount of 0.0100 mass % or less,
Cr in an amount of 1.00 mass % or less,
Mo in an amount of 1.00 mass % or less,
Ni in an amount of 1.00 mass % or less,
Co in an amount of 0.010 mass % or less,
Cu in an amount of 1.00 mass % or less,
Sn in an amount of 0.200 mass % or less,
Sb in an amount of 0.200 mass % or less,
Ca in an amount of 0.0100 mass % or less,
Mg in an amount of 0.0100 mass % or less,
REM in an amount of 0.0100 mass % or less,
Zr in an amount of 0.100 mass % or less,
Te in an amount of 0.100 mass % or less,
Hf in an amount of 0.10 mass % or less, and
Bi in an amount of 0.200 mass % or less.
16 . The high-strength steel sheet according to claim 3 ,
wherein the steel sheet includes a soft layer in a portion from a surface of the steel sheet to 200 μm in a sheet thickness direction, and
the soft layer is a portion having Vickers hardness of 85% or less with respect to Vickers hardness at a ¼ position of a sheet thickness of the steel sheet.
17 . The high-strength steel sheet according to claim 16 ,
wherein when nano-hardness at the ¼ position in depth in the sheet thickness direction of the soft layer is measured at 300 or more points in an area of 50 μm×50 μm, a proportion r of measurement points having nano-hardness of 7.0 GPa or more to a total number of measurement points is 0.10 or less.
18 . The high-strength steel sheet according to claim 16 ,
wherein a standard deviation σ A of nano-hardness at the ¼ position in depth in the sheet thickness direction of the soft layer is 1.8 GPa or less, and
a standard deviation σ B of nano-hardness at a ½ position in depth in the sheet thickness direction of the soft layer is 2.2 GPa or less.
19 . The high-strength steel sheet according to claim 3 , further comprising a metal plated layer as a pre-annealing plated layer on a surface of the steel sheet.
20 . The high-strength steel sheet according to claim 4 , further comprising a metal plated layer as a pre-annealing plated layer on a surface of the steel sheet.
21 . The high-strength steel sheet according to claim 3 , further comprising a plated layer on a surface of the steel sheet.
22 . The high-strength steel sheet according to claim 4 , further comprising a plated layer on a surface of the steel sheet.
23 . The high-strength steel sheet according to claim 19 , further comprising a plated layer on a surface of the metal plated layer.
24 . The high-strength steel sheet according to claim 20 , further comprising a plated layer on a surface of the metal plated layer.