CLAIMS
[Claim 1] A high-strength hot-rolled steel sheet
containing:
in mass%,
C: 0.02% to 0.1%;
Si: 0.001% to 3.0%;
Mn: 0.5% to 3.0%;
P: 0.1% or less;
S: 0.01% or less;
Al : 0.0 01% to 2.0%;
N: 0.02% or less;
Ti: 0.03% to 0.3%; and
Nb: 0 . 001% to 0 . 06%,
the steel sheet further containing at least one
element selected from the group consisting of:
Cu: 0.001 to 1.0%;
Cr: 0.001 to 1.0%;
Mo: 0.001 to 1.0%;
Ni: 0.001 to 1.0%; and
V: 0.01 to 0.2%,
the balance being composed of Fe and inevitable
impurities,
a parameter Q expressed by Mathematical
expression 1 below being 30.0 or more,
a microstructure being made of a ferrite
structure, a bainite structure, or a structure mixed
with the ferrite structure and the bainite structure,
an average grain size of grains included in the
micros tructure being 6 ]im or less,
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an X-ray random intensity ratio of {211} plane on
a rolled surface being 2.4 or less, and
on a cross section with a sheet width direction
set as a normal line,
with regard to inclusions having a major
diameter of 3.0 jam or more, a maximum of a major
diameter/minor diameter ratio expressed by (a major
diameter of the inclusion)/(a minor diameter of the
inclusion) being 8.0 or less,
a sum total of a rolling direction length
per 1 mm^ cross section of a predetermined inclusion
group composed of plural inclusions each having a
major diameter of 3.0 ym or more and a predetermined
extended inclusion having a length in a rolling
direction of 30 ]im or more being 0.25 mm or less,
the plural inclusions composing the
predetermined inclusion group congregating in both
the rolling direction and a direction perpendicular
to the rolling direction 50 ]im. or less apart from
each other, and
the predetermined extended inclusion being
spaced over 50 pm apart from all the inclusions each
having a major diameter of 3.0 ]im or more in at least
either the rolling direction or the direction
perpendicular to the rolling direction,
[Mathematical expression 1]
n - fTi] /[S]
U — / — . .. (Mathematical expression 1)
{[Ti] indicates the Ti content (mass%) and [S]
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indicates the S content (mass%).).
[Claim 2] A high-strength hot-rolled steel sheet
containing:
in mass%,
C: 0.02% to 0.1%;
Si: 0.001% to 3.0%;
Mn: 0.5% to 3.0%;
P: 0.1% or less;
S: 0.01% or less;
Al: 0.001% to 2.0%
N: 0.02% or less;
Ti: 0.03% to 0.3%;
Nb: 0.001% to 0.06%;
REM: 0.0001% to 0.02%; and
Ca: 0 . 0001% to 0 . 02%,
the steel sheet further containing at least one
element selected from the group consisting of:
Cu: 0.001 to 1.0%;
Cr: 0.001 to 1.0%;
Mo: 0.001 to 1.0%;
Ni: 0.001 to 1.0%; and
V: 0.01 to 0.2%, and
the balance being composed of Fe and inevitable
impurities,
a parameter Q' expressed by Mathematical
expression 1' below being 30.0 or more,
a microstructure being made of a ferrite
structure, a bainite structure, or a structure mixed
with" the ferrite structure and the bainite structure,
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an average grain size of grains included in the
microstructure being 6 ^m or less,
an X-ray random intensity ratio of {211} plane on
a rolled surface being 2.4 or less, and
on a cross section with a sheet width direction
set as a normal line,
with regard to an inclusion having a major
diameter of 3.0 ]im or more, a maximum of a major
diameter/minor diameter ratio expressed by (a major
diameter of the inclusion)/(a minor diameter of the
inclusion) being 8.0 or less,
a sum total of a rolling direction length
per 1 mm^ cross section of a predetermined inclusion
group composed of plural inclusions each having a
major diameter of 3.0 pm or more and a predetermined
extended inclusion having a length in a rolling
direction of 30 ]im or more being 0.25 mm or less,
the plural inclusions composing the
predetermined inclusion group congregating in both
the rolling direction and a direction perpendicular
to the rolling direction 50 pm or less apart from
each other, and
the predetermined extended inclusion being
spaced over 50 pm apart from all the inclusions each
having a major diameter of 3.0 \im or more in at least
either the rolling direction or the direction
perpendicular to the rolling direction,
[Mathematical expression 2]
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n . _ [Ti] /[S] , r[Ca] /[S] [REM] ,[S]) ^ . r n
U = / h 1 / /—\ X I b . U . . . ( M a t h e m a t i c al
^ 48' 32 140^32 140 ' 32j
expression 1')
([Ti] indicates the Ti content (mass%), [S] indicates
the S content (mass%), [Ca] indicates the Ca content
(mass%), and [REM] indicates the REM content
(mass%).)•
[Claim 3] The high-strength hot-rolled steel sheet
according to claim 2, wherein
Mathematical expression 2 below is satisfied, and
the maximum of the major diameter/minor diameter
ratio is 3.0 or less,
0.3 • ( [REM]/140)/( [Ca]/40) ...(Mathematical
expression 2 ).
[Claim 4] The high-strength hot-rolled steel sheet
according to claim 1, further containing, in mass%,
B: 0.0001% to 0.005% .
[Claim 5] The high-strength hot-rolled steel sheet
according to claim 2, further containing, in mass%,
B: 0.0001% to 0.005%.
[Claim 6] The high-strength hot-rolled steel sheet
according to claim 3, further containing, in mass%,
B: 0 . 0001% to 0 . 005%.
[Claim 7] The high-strength hot-rolled steel sheet
according to claim 4, wherein
a total grain boundary number density of solid
solution C and solid solution B exceeds 4.5 /nm^ and
is 12 /nm^ or less, and
a size of cementite precipitated in grain
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boundaries is 2 ijm or less.
[Claim 8] The high-strength hot-rolled steel sheet
according to claim 5, wherein
a total grain boundary number density of solid
solution C and solid solution B exceeds 4.5 /nm^ and
is 12 /nm^ or less, and
a size of cementite precipitated in grain
boundaries is 2 ^m or less.
[Claim 9] The high-strength hot-rolled steel sheet
according to claim 6, wherein
a total grain boundary number density of solid
solution C and solid solution B exceeds 4.5 /nm^ and
is 12 /nm^ or less, and
a size of cementite precipitated in grain
boundaries is 2 \im or less.
[Claim 10] A method of manufacturing a high-strength
hot-rolled steel sheet comprising:
rough-rolling a steel slab after heating the
steel slab,
the steel slab containing:
in mass%,
C: 0.02% to 0.1%;
Si: 0.001% to 3.0%;
Mn: 0.5% to 3.0%;
P: 0.1% or less;
S: 0.01% or less;
Al: 0.001% to 2.0%;
N: 0.02% o r 1e s s;
Ti: 0.03% to 0.3%; and
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#
Nb: 0. 001% to 0. 06%,
the steel slab further containing at least
one element selected from the group consisting of:
Cu: 0.0 01 to 1.0%;
Cr: 0.001 to 1.0%;
Mo: 0.001 to 1.0%;
Ni: 0.001 to 1.0%; and
V: 0.01 to 0 .2%,
the balance being composed of Fe and
inevitable impurities,
a parameter Q expressed by Mathematical
expression 1 being 30.0 or more, and
the rough-rolling being performed under a
condition in which an accumulated reduction ratio in
a temperature zone exceeding 1150°C becomes 70% or
less and an accumulated reduction ratio in a
temperature zone of 1150°C or lower becomes not less
than 10% nor more than 25%;
subsequently, finish-rolling the steel slab under
a condition in which a beginning temperature is 1050°C
or higher and a finishing temperature is not lower
than Ar3 + 130°C nor higher than Ar3 + 230°C;
subsequently, cooling the steel slab at a cooling
rate of 15°C/sec or more; and
subsequently, coiling the steel slab at 640°C or
lower,
[Mathematical expression 3]
U — / — . . . (Mathematical expression 1)
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{[Ti] indicates the Ti content (inass%) and [S]
indicates the S content (mass%).).
[Claim 11] A method of manufacturing a high-strength
hot-rolled steel sheet comprising:
rough-rolling a steel slab after heating the
steel slab,
the steel slab containing:
in mass%,
C: 0.02% to 0.1%;
Si: 0.001% to 3.0%;
Mn: 0.5% to 3.0%;
P: 0.1% or less;
S: 0.01% or less;
A l : 0 . 0 0 1 % t o 2 . 0 %;
N: 0.02% or less;
Ti: 0.03% to 0.3%;
Nb: 0.001% to 0.06%;
REM: 0.0001% to 0.02%; and
Ca: 0.0001% to 0.02%, and further
the steel slab further containing at least
one element selected from the group consisting of:
C u : 0 . 0 0 1 t o l . 0 %;
Cr: 0.0 01 to 1.0%;
Mo: 0.001 to 1.0%;
Ni : 0.001 to 1.0%, and
V: 0.01 to 0.2%; and
the balance being composed of Fe and
inevitable impurities,
a parameter Q' expressed by Mathematical
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%
expression 1' being 30.0 or more, and
the rough-rolling being performed under a
condition in which an accumulated reduction ratio in
a temperature zone exceeding 1150°C becomes 70% or
less and an accumulated reduction ratio in a
temperature zone of 1150°C or lower becomes not less
than 10% nor more than 25%;
subsequently, finish-rolling the steel slab under
a condition in which a beginning temperature is 1050°C
or higher and a finishing temperature is not lower
than Ar3 + 130°C nor higher than Ar3 + 230°C;
subsequently, cooling the steel slab at a cooling
rate of 15°C/sec or more; and
subsequently, coiling the steel slab at 640°C or
lower,
[Mathematical expression 4]
n- [Ti] AS] r[Ca] ,[S] [REM] /[S]^ ^ ^rn
U = / h 1 / /—f X Ib.U. . . Mathematical
^ 48' 32 I 40 / 32 140 ' 32)
expression 1')
([Ti] indicates the Ti content (mass%), [S] indicates
the S content (mass%), [Ca] indicates the Ca content
(mass%), and [REM] indicates the REM content
(mass%) . ) .
[Claim 12] The method of manufacturing a highstrength
hot-rolled steel sheet according to claim
11, wherein the steel slab satisfies Mathematical
expression 2 below,
0.3 D ([REM]/140)/([Ca]/40) ...(Mathematical
expression 2).
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«
[Claim 13] The method of manufacturing a highstrength
hot-rolled steel sheet according to claim
10, wherein the steel slab further contains, in
mass%, B: 0.0001% to 0.005%.
[Claim 14] The method of manufacturing a highstrength
hot-rolled steel sheet according to claim
11, wherein the steel slab further contains, in
mass%, B: 0.0001% to 0.005%.
[Claim 15] The method of manufacturing a highstrength
hot-rolled steel sheet according to claim
12, wherein the steel slab further contains, in
mass%, B: 0.0001% to 0.005%.