High-strength hot-rolled steel sheet and method of manufacturing the same
View Patent ↗On a cross section with a sheet width direction of a high-strength hot-rolled steel sheet set as a normal line, with regard to an inclusion having a major diameter of 3.0 μm 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) is 8.0 or less, and a sum total of a rolling direction length per 1 mm 2 cross section of a predetermined inclusion group composed of plural inclusions each having a major diameter of 3.0 μm or more and a predetermined extended inclusion having a length in a rolling direction of 30 μm or more is 0.25 mm or less. The plural inclusions composing the predetermined inclusion group congregate in both the rolling direction and a direction perpendicular to the rolling direction 50 μm or less apart from each other. The predetermined extended inclusion is spaced over 50 μm apart from all the inclusions each having a major diameter of 3.0 μm or more in at least either the rolling direction or the direction perpendicular to the rolling direction.
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: 0.001% 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 microstructure being 6 μm 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 inclusions having a major diameter of 3.0 μm 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 2 cross section of an inclusion group composed of plural inclusions each having a major diameter of 3. 0 μm or more and an extended inclusion having a length in a rolling direction of 30 μm or more being 0.25 mm or less,
the plural inclusions composing the inclusion group congregating in both the rolling direction and a direction perpendicular to the rolling direction 50 μm or less apart from each other, and
the extended inclusion being spaced over 50 μm apart from all the inclusions each having a major diameter of 3.0 μm or more in at least either the rolling direction or the direction perpendicular to the rolling direction,
Q
=
[
Ti
]
48
/
[
S
]
32
(
Mathematical
expression
1
)
wherein [Ti] indicates the Ti content (mass %) and [S] indicates the S content (mass %).
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,
an average grain size of grains included in the microstructure being 6 μm 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 μm 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 2 cross section of an inclusion group composed of plural inclusions each having a major diameter of 3.0 μm or more and an extended inclusion having a length in a rolling direction of 30 μm or more being 0.25 mm or less,
the plural inclusions composing the inclusion group congregating in both the rolling direction and a direction perpendicular to the rolling direction 50 μm or less apart from each other, and
the extended inclusion being spaced over 50 μm apart from all the inclusions each having a major diameter of 3.0 μm or more in at least either the rolling direction or the direction perpendicular to the rolling direction,
Q
′
=
[
Ti
]
48
/
[
S
]
32
+
{
[
Ca
]
40
/
[
S
]
32
+
[
REM
]
140
/
[
S
]
32
}
×
15.0
(
Mathematical
expression
1
′
)
wherein [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 %).
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).
4. The high-strength hot-rolled steel sheet according to claim 1 , further containing, in mass %, B: 0.0001% to 0.005%.
5. The high-strength hot-rolled steel sheet according to claim 2 , further containing, in mass %, B: 0.0001% to 0.005%.
6. The high-strength hot-rolled steel sheet according to claim 3 , further containing, in mass %, B: 0.0001% to 0.005%.
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 2 and is 12/nm 2 or less, and
a size of cementite precipitated in grain boundaries is 2 μm or less.
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 2 and is 12/nm 2 or less, and
a size of cementite precipitated in grain boundaries is 2 μm or less.
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 2 and is 12/nm 2 or less, and
a size of cementite precipitated in grain boundaries is 2 μm or less.
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% or less;
Ti: 0.03% to 0.3%; and
Nb: 0.001% to 0.06%,
the steel slab 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 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,
Q
=
[
Ti
]
48
/
[
S
]
32
(
Mathematical
expression
1
)
wherein [Ti] indicates the Ti content (mass %) and [S] indicates the S content (mass %).
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;
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%, and further
the steel slab 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′ 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,
Q
′
=
[
Ti
]
48
/
[
S
]
32
+
{
[
Ca
]
40
/
[
S
]
32
+
[
REM
]
140
/
[
S
]
32
}
×
15.0
(
Mathematical
expression
1
′
)
wherein [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 %).
12. The method of manufacturing a high-strength hot-rolled steel sheet according to claim 11 , wherein the steel slab satisfies Mathematical expression 2 below,
0.3 ≦([REM]/140)/([Ca]/40) (Mathematical expression 2).
13. The method of manufacturing a high-strength hot-rolled steel sheet according to claim 10 , wherein the steel slab further contains, in mass %, B: 0.0001% to 0.005%.
14. The method of manufacturing a high-strength hot-rolled steel sheet according to claim 11 , wherein the steel slab further contains, in mass %, B: 0.0001% to 0.005%.
15. The method of manufacturing a high-strength hot-rolled steel sheet according to claim 12 , wherein the steel slab further contains, in mass %, B: 0.0001% to 0.005%.