Steel sheet and method of manufacturing the same
A steel sheet includes a predetermined composition satisfying Expression (1), in which the microstructure at the ¼ thickness position from the surface in the sheet thickness direction includes, by vol %, ferrite: 95% or more and a remainder of the microstructure: 5% or less, has a proportion of unrecrystallized ferrite in the ferrite of 5% or less, and a half width w and an X-ray wavelength λ at a peak of (200) plane of the ferrite satisfy Expression (2). 0.80≤{(Ti/48−N/14)+Nb/93}/(C/12)≤5.00 (1) w ×λ≥0.20 (2)
1. A steel sheet comprising, as a composition, by mass %:
C: 0.0003% to 0.0100%;
Si: 0.005% to 1.500%;
Mn: 0.010% to 3.000%;
Al: 0.005% to 1.000%;
P: 0.100% or less;
S: 0.0200% or less;
N: 0.0150% or less;
O: 0.0100% or less;
V: 0% to 0.50%;
Cr: 0% to 1.00%;
Ni: 0% to 1.00%;
Cu: 0% to 1.00%;
Mo: 0% to 1.00%;
W: 0% to 1.00%;
B: 0% to 0.0100%;
Sn: 0% to 1.00%;
Sb: 0% to 0.20%;
one or two or more selected from the group of Ca, Ce, Mg, Zr, La, and REM: 0% to 0.0100% in total;
one or two selected from the group of Ti: 0.010% to 0.100% and Nb: 0.005% to 0.060%; and
a remainder including Fe and impurities,
wherein Expression (1) is satisfied,
a microstructure at a ¼ thickness position from a surface in a sheet thickness direction includes, by vol %, 95% or more ferrite, and 5% or less of a remainder of the microstructure, and the ferrite has a proportion of unrecrystallized ferrite of 5% or less, and
a half width w and an X-ray wavelength λ at a peak of (200) plane of the ferrite satisfy Expression (2),
0.80≤{(Ti/48−N/14)+Nb/93}/(C/12)≤5.00 (1),
w ×λ≥0.20 (2),
wherein each of Ti, N, Nb, and C in Expression (1) represents a content by mass % of the element, and when the element is not included, 0 is substituted as the content of the element.
2. The steel sheet according to claim 1 ,
wherein the composition further includes, by mass %, one or two or more selected from the group of:
V: 0.01% to 0.50%;
Cr: 0.05% to 1.00%;
Ni: 0.05% to 1.00%;
Cu: 0.05% to 1.00%;
Mo: 0.03% to 1.00%;
W: 0.03% to 1.00%;
B: 0.0005% to 0.0100%;
Sn: 0.01% to 1.00%;
Sb: 0.005% to 0.20%; and
one or two or more selected from the group of Ca, Ce, Mg, Zr, La, and REM: 0.0001% to 0.0100% in total.
3. The steel sheet according to claim 1 ,
wherein an average grain size of the ferrite in the microstructure is 6.0 m to 15.0 m.
4. The steel sheet according to claim 1 , comprising a galvanized layer on the surface.
5. The steel sheet according to claim 1 , comprising a zinc alloy plated layer on the surface.
6. The steel sheet according to claim 4 ,
wherein a Fe content in the galvanized layer is 7.0% to 130.0% by mass %.
7. A method of manufacturing the steel sheet according to claim 1 , comprising:
a hot-rolling process of heating a steel piece having the composition according to claim 1 to 1200° C. to 1320° C., completing hot rolling such that a hot rolling completion temperature is 880° C. or higher, and cooling the steel piece to obtain a hot-rolled steel sheet such that an average cooling rate in a temperature range of the hot rolling completion temperature to 500° C. is 20° C./s or faster;
a reheating process of heating the hot-rolled steel sheet to a temperature range of 500° C. to 700° C.;
a cooling process of cooling the hot-rolled steel sheet to room temperature;
a cold rolling process of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet such that a total rolling reduction is 60% to 90% and a cold rolling completion temperature is 250° C. or lower;
an annealing process of heating the cold-rolled steel sheet to an annealing temperature of 700° C. to 850° C. and cooling to a temperature range of 80° C. or lower; and
a temper rolling process of performing temper rolling on the cold-rolled steel sheet such that a total rolling reduction is 0.05% to 2.00%,
wherein in the reheating process,
Expression (3) is satisfied in a temperature range of 500° C. to 700° C., and
in the annealing process,
Expression (4) is satisfied in a temperature range of 700° C. to the annealing temperature during heating to the annealing temperature, and
Expression (5) is satisfied in the temperature range of 500° C. to 700° C. during cooling from the annealing temperature, and
bending is performed while applying a tension of 20 MPa or higher in a temperature range of 80° C. to 500° C.,
t
n
=
10
T
n
-
1
+
273
T
n
+
273
·
log
10
t
n
-
1
-
(
1
-
T
n
-
1
+
273
T
n
+
273
)
·
21
·
(
1
+
2.5
·
C
·
(
Nb
+
4
Ti
)
)
+
Δ
t
K
K
n
=
(
T
n
+
273
)
·
{
log
10
t
n
+
21
·
(
1
+
2.5
·
C
·
(
Nb
+
4
Ti
)
)
}
K
20
≥
1.5
×
10
4
Expression
(
3
)
in Expression (3), K 20 represents an index representing a degree of progress of precipitation of a Ti and/or Nb carbonitride in a 20th period when a temperature history in the temperature range of 500° C. to 700° C. of the reheating process is divided into 20 periods with respect to time,
t n and K n are calculated when the temperature history in the temperature range of 500° C. to 700° C. of the reheating process is divided into 20 periods with respect to time and an average temperature in an n-th period is represented by T n [° C.],
Δt K represents a time [hr·] in one of 20 periods into which a total residence time in the same temperature range is divided,
each of C, Nb, and Ti represents a content [mass %] of the element, and t 1 =Δt K ,
R
m
=
A
·
Δ
t
R
0.5
K
20
·
exp
(
-
B
T
m
)
1.
≤
∑
i
=
1
10
R
i
≤
15.
Expression
(
4
)
in Expression (4), R i represents an index representing a degree of progress of recrystallization in the temperature range of 700° C. to the annealing temperature and a degree of progress of diffusion of C from a Ti and/or Nb carbonitride present in a grain boundary into crystal grains,
R m is calculated when a temperature history of the steel sheet from 700° C. to the annealing temperature during heating in the annealing process is divided into 10 periods with respect to time and an average temperature in an m-th period is represented by T m [° C.],
Δt R represents a time [s] in one of 10 periods into which a total residence time in the temperature range of 700° C. to the annealing temperature is divided,
K 20 is a value obtained by Expression (3), and
A and B represent constant terms, A represents 9.67×10 9 , and B represents 1.25×10 4 , and
P
k
=
D
·
{
E
·
(
700
-
T
k
)
1.5
+
(
700
-
T
k
)
}
·
exp
(
-
F
T
k
)
·
Δ
t
p
0.5
1.
≤
R
10
·
∑
j
=
1
10
p
j
≤
15.
Expression
(
5
)
in Expression (5), P j represents an index representing a degree of progress of precipitation of C in a temperature range of 700° C. to 500° C.,
P k is calculated when a temperature history of the steel sheet from 700° C. to 500° C. during cooling in the annealing process is divided into 10 periods with respect to time and an average temperature in a k-th period is represented by T k [° C.],
Δt p represents a time [s] in one of 10 periods into which a total residence time in the same temperature range is divided,
R 10 represents a value obtained by substituting 10 into m of R m in Expression (4), and
D, E, and F represent constant terms, D represents 4.47×10 4 , E represents 2.11×10 0 , and F represents 1.25×10 4 .
8. The method of manufacturing a steel sheet according to claim 7 ,
wherein during cooling in the annealing process, hot-dip galvanizing is performed on the cold-rolled steel sheet.
9. The method of manufacturing a steel sheet according to claim 7 ,
wherein during cooling in the annealing process, hot-dip zinc alloy plating is performed on the cold-rolled steel sheet.
10. The method of manufacturing a steel sheet according to claim 8 ,
wherein during cooling in the annealing process, alloying is performed after the hot-dip galvanizing.
11. The steel sheet according to claim 5 ,
wherein a Fe content in the zinc alloy plated layer is 7.0% to 13.0% by mass %.
12. The method of manufacturing a steel sheet according to claim 9 ,
wherein during cooling in the annealing process, alloying is performed after the hot-dip zinc alloy plating.