Steel sheet and method of manufacturing the same
A steel sheet has a predetermined chemical composition, in which a microstructure in a ¼ width portion, a microstructure in a ½ width portion, and a microstructure in a ¾ width portion, include, by area %, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, in which a proportion of unrecrystallized ferrite in the ferrite is 5% to 60%, an average grain size of carbonitrides is 6.0 nm to 30.0 nm, and Expressions (2) to (5) are satisfied. Δ SF /μ SF ≤0.10 (2) Δ dF /μ dF ≤0.20 (3) Δ SUF ≤20 (4) Δ dC /μ dC ≤0.50 (5)
1 . A steel sheet comprising, as a composition, by mass % comprising:
C: 0.035% to 0.150%;
Si: 0.010% to 1.500%;
Mn: 0.10% to 3.00%;
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%;
Nb: 0% to 0.060%;
Ti: 0% to 0.100%;
Ca: 0% to 0.0100%;
Mg: 0% to 0.0100%;
Zr: 0% to 0.0100%;
REM: 0% to 0.0100%; and
a remainder: Fe and impurities,
wherein Expressions (1-1) to (1-3) are satisfied;
a microstructure in a ¼ width portion at a ¼ width position from a sheet width direction end portion in a sheet width direction and at a ¼ thickness position from a surface in a sheet thickness direction, a microstructure in a ½ width portion at a ½ width position from the sheet width direction end portion in the sheet width direction and at the ¼ thickness position from the surface in the sheet thickness direction, and a microstructure in a ¾ width portion at a ¾ width position from the sheet width direction end portion in the sheet width direction and at the ¼ thickness position from the surface in the sheet thickness direction
include, by area %, ferrite: 80% or more, martensite: 2% or less, and residual austenite: 2% or less, and the remainder in microstructure comprises pearlite, cementite, and bainite in a total area ratio of 20% or less,
a proportion of unrecrystallized ferrite in the ferrite is 5% to 60%,
an average grain size of carbonitrides is 6.0 nm to 30.0 nm,
Expressions (2) to (5) are satisfied,
a 0.2% proof stress is 280 MPa to 600 MPa,
a tensile strength is 450 MPa to 800 MPa,
a yield ratio is 0.50 to 0.90, and
a uniform elongation is 10.0% or more,
1.5×Nb+Ti≥0.015 (1-1),
0.03≤{(Ti/48−N/14)+Nb/93}/(C/12)≤0.40 (1-2),
Ca+Mg+Zr+REM≤0.0100 (1-3),
Δ SF /μ SF ≤0.10 (2),
Δ dF /μ dF ≤0.20 (3),
Δ SUF ≤20 (4),
Δ dC /μ dC ≤0.50 (5),
each of Ti, N, Nb, C, Ca, Mg, Zr, and REM in Expressions (1-1) to (1-3) represent a content by mass % of the element, when the element is not included, 0% is substituted as the content of the element, and when a value of (Ti/48−N/14) is negative, 0 is substituted as the value of (Ti/48−N/14),
μ SF in Expression (2) represents an average value of an area ratio of ferrite in the microstructure in the ¼ width portion, an area ratio of ferrite in the microstructure in the ½ width portion, and an area ratio of ferrite in the microstructure in the ¾ width portion, and Δ SF represents a difference between a maximum value and a minimum value of area ratio of ferrite in the microstructures in the ¼ width portion, the ½ width portion, and the ¾ width portion,
μ dF in Expression (3) represents an average value of an average grain size of ferrite in the microstructure in the ¼ width portion, an average grain size of ferrite in the microstructure in the ½ width portion, and an average grain size of ferrite in the microstructure in the ¾ width portion, and A dF represents a difference between a maximum value and a minimum value of average grain size of ferrite in the microstructures in the ¼ width portion, the ½ width portion, and the ¾ width portion,
Δ SUF in Expression (4) represents a difference between a maximum value and a minimum value of area ratio of unrecrystallized ferrite in the microstructures in the ¼ width portion, the ½ width portion, and the ¾ width portion, and
μ dC in Expression (5) represents an average value of an average grain size of carbonitrides in the microstructure in the ¼ width portion, an average grain size of carbonitrides in the microstructure in the ½ width portion, and an average grain size of carbonitrides in the microstructure in the ¾ width portion, and Δ dC represents a difference between a maximum value and a minimum value of average grain size of carbonitrides in the microstructures in the ¼ width portion, the ½ width portion, and the ¾ width portion.
2 . The steel sheet according to claim 1 wherein the composition, includes Mn: 0.70% to 3.00% by mass %.
3 . The steel sheet according to claim 2 ,
wherein the average grain sizes of ferrite in the ¼ width portion, the ½ width portion, and the ¾ width portion is 5.0 μm to 15.0 μm.
4 . The steel sheet according to claim 1 ,
wherein the average grain sizes of ferrite in the ¼ width portion, the ½ width portion, and the ¾ width portion is 5.0 μm to 15.0 μm.
5 . The steel sheet according to claim 1 , comprising a galvanized layer on the surface.
6 . The steel sheet according to claim 5 ,
wherein a Fe content in the galvanized layer is 7.0% to 13.0% by mass %.
7 . The steel sheet according to claim 1 , comprising a zinc alloy plated layer on the surface.
8 . The steel sheet according to claim 7 ,
wherein a Fe content in the zinc alloy plated layer is 7.0% to 13.0% by mass %.
9 . The steel sheet according to claim 1 , wherein the proportion of unrecrystallized ferrite in the ferrite is 10% to 60%.
10 . The steel sheet according to claim 1 , wherein the proportion of unrecrystallized ferrite in the ferrite is 12% to 60%.
11 . The steel sheet according to claim 1 , wherein the steel sheet includes 0% martensite and wherein the yield ratio is 0.50 to 0.69.
12 . A method of manufacturing the steel sheet according to claim 1 , comprising:
a hot rolling process of heating a steel piece having the composition of said steel sheet to 1150° C. to 1320° C., completing hot rolling such that a hot rolling completion temperature is 850° C. to 930° C., starting cooling after 1.5 s or longer, cooling the steel piece to a temperature range of lower than 450° C. to obtain a hot-rolled steel sheet such that an average cooling rate in a temperature range of 800° C. to 450° C. is 20° C./s or higher;
a reheating process of heating the hot-rolled steel sheet to a temperature range of 450° 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 30% to 80% and a cold rolling completion temperature is 120° C. or higher; and
an annealing process of heating the cold-rolled steel sheet to an annealing temperature of 720° C. to 850° C. and cooling the cold-rolled steel sheet to a temperature range of 500° C. or lower,
wherein in the hot rolling process,
Expression (6) is satisfied in a temperature range of 1000° C. or lower,
in the reheating process,
Expression (7-1) and Expression (8) are satisfied in the temperature range of 450° C. to 700° C.,
in the annealing process,
in the process of heating to the annealing temperature,
Expression (9) is satisfied in a temperature range of 550° C. to 720° C.,
a tension of 15 MPa or higher is applied and Expression (10) is satisfied in a temperature range of 720° C. to the annealing temperature, and
in the process of cooling from the annealing temperature,
Expression (11) is satisfied in a temperature range of 720° C. to 500° C.,
g
n
=
(
0
.
5
+
h
*
h
)
·
(
1
+
a
1
Nb
+
a
2
T
i
)
(
6
)
K
n
=
(
T
n
+
273
)
·
(
a
3
+
a
4
N
b
+
a
5
Ti
)
R
n
=
(
T
n
+
273
)
·
(
a
6
+
a
7
N
b
+
a
8
Ti
)
J
n
=
(
h
*
h
)
1.5
·
(
a
9
+
a
1
0
N
b
+
a
1
1
Ti
)
f
n
=
[
f
n
-
1
·
exp
(
-
K
n
·
t
n
)
+
g
n
·
{
1
-
exp
(
-
K
n
·
t
n
)
}
]
·
exp
(
-
R
n
·
t
n
)
+
J
n
≤
1.
in Expression (6), f n represents an index representing a degree of progress of precipitation of a fine carbide in the temperature range of 1000° C. or lower of the hot rolling process, where, f n can be obtained by setting f 0 to 0 and calculating f 1 to f n in order,
reference numerals in Expression (6) are as follows,
n: the number of rolling passes at 1000° C. or lower,
h: a sheet thickness in mm before an n-th pass rolling,
h*: a sheet thickness in mm after the n-th pass rolling,
Nb and Ti: contents in mass % of Nb and Ti,
T n : an average steel sheet temperature in ° C. from the n-th pass rolling to an n+1-th pass rolling, wherein the average steel sheet temperature is a temperature in the width direction of the steel strip managed such that a temperature distribution in the width direction is within +15° C. relative to the average temperature,
t n : a shorter time among a time in s from the n-th pass rolling to the n+1-th pass rolling and a time in s taken until the steel sheet temperature decreases to 800° C. from the n-th pass rolling,
a 1 to 11 : constants (a 1 =2.28×100, a 2 =1.25×100, a 3 =7.86×10 −4 , a 4 =1.36× 10 −3 , 5=6.76×10 −4 , a 6 =7.86×10 −4 , a 7 =2.13×10 −3 , a 8 =1.14×10 −3 , a 9 =6.70×10 −2 , a 10 =1.11×10 0 , and a 11 =5.27×10 −1 ),
b
1
·
{
1
.00
-
exp
(
-
b
2
+
b
3
·
Nb
+
b
4
·
Ti
*
-
T
max
b
5
+
b
6
·
Nb
+
b
7
·
Ti
*
)
}
·
D
2
0
·
t
2
0
≥
1.
(
7
-
1
)
reference numerals in Expression (7-1) are as follows,
b 1 to 7 : constants (b 1 =6.82×10 6 , b 2 =1.00×10 3 , b 3 =8.70×10 1 , b 4 =1.25×10 2 , b 5 =1.00×10 2 , b 6 =−1.50×10 1 , and b 7 =−2.50×10 1 ),
Nb: a Nb content in mass %,
Ti*: an effective Ti content represented by Ti−42/14× N,
where each of Ti and N represents a content by mass % of the element, and when the element is not included, 0 is substituted as the content of the element,
T max : a highest heating temperature in ° C.,
t 20 : an effective heat temperature time in s in a 20th period when a residence time in the temperature range of 450° C. to 700° C. is divided into 20 periods,
D 20 : an index representing an effective diffusion rate in a 20th period when a residence time in the temperature range of 450° C. to 700° C. is divided into 20 periods,
where an m-th effective heat treatment time t m and an index D m representing an m-th effective diffusion rate are represented by Expression (7-2),
D
m
=
T
i
*
4
2
·
(
Ti
*
4
2
+
N
b
9
2
)
-
1
·
b
8
·
exp
(
-
b
9
T
m
)
+
N
b
9
2
·
(
Ti
*
4
2
+
N
b
9
2
)
-
1
·
b
1
0
·
exp
(
-
b
1
1
T
m
)
(
7
-
2
)
t
m
=
t
′
+
D
m
-
1
D
m
·
t
m
-
1
reference numerals in Expression (7-2) are as follows,
m: an integer of 1 to 20,
b 9 to 11 : constants (b 8 =6.81×10 1 , b 9 =2.61×10 5 , b 10 =5.60×10 0 , and b 11 =2.86×10 5 ),
Nb: a Nb content in mass %,
Ti*: an effective Ti content represented by Ti−42/14×N,
where each of Ti and N represents a content by mass % of the element, and when the element is not included, 0 is substituted as the content of the element,
T m : an average steel sheet temperature in ° C. in an m-th period when a residence time in the temperature range of 450° C. to 700° C. is divided into 20 periods, wherein the average steel sheet temperature is a temperature in the width direction of the steel strip managed such that a temperature distribution in the width direction is within #15° C. relative to the average temperature,
t m : an effective heat treatment time s in an m-th period when a residence time in the temperature range of 450° C. to 700° C. is divided into 20 periods,
where t 1 =t′,
t′: 1/20 s of an entire residence time in the temperature range of 450° C. to 700° C.,
K
j
=
T
j
·
(
log10
(
s
j
)
+
20
/
(
1
+
0
.
1
5
×
Si
-
0.08
×
Mn
-
0.05
×
Cr
-
0.13
×
Mo
)
)
(
8
)
s
j
=
t
′
+
10
T
j
-
1
T
j
·
log
10
s
j
-
1
+
20
T
j
·
T
j
+
T
j
-
1
1
+
0.15
Si
-
0.08
M𝔫
-
0.05
Cr
-
0.13
Mo
K
2
0
≦
2
.
0
0
×
1
0
4
in Expression (8), K 20 represents an index representing a degree of stabilization of cementite in a 20th period when a temperature history in the temperature range of 450° C. to 700° C. of the reheating process is divided into 20 periods with respect to time,
reference numerals in Expression (8) are as follows,
j: an integer of 1 to 20,
each of Si, Mn, Cr, and Mo: a content mass % of the element,
T j : an average steel sheet temperature in ° C. in a j-th period when a residence time in the temperature range of 450° C. to 700° C. is divided into 20 periods, wherein the average steel sheet temperature is a temperature in the width direction of the steel strip managed such that a temperature distribution in the width direction is within =15° C. relative to the average temperature,
s j : an effective heat treatment time s in a j-th period when a residence time in the temperature range of 450° C. to 700° C. is divided into 20 periods,
where s 1 =t′,
t′: 1/20 s of an entire residence time in the temperature range of 450° C. to 700° C.,
E
=
d
1
·
(
1
-
h
*
h
)
1.5
·
T
R
-
1
·
(
1
+
d
2
·
Nb
0.5
+
d
3
·
Ti
*
0
.
5
)
-
1
·
K
2
0
.
5
(
9
)
q
n
=
d
3
·
E
·
exp
(
-
d
4
T
n
′
+
273
)
t
n
=
Δ
t
-
ln
(
1
-
p
n
-
1
)
q
n
p
n
=
1
-
exp
(
-
q
n
·
t
n
)
0.1
≤
p
1
0
≤
1
.
0
0
in Expression (9), p 10 represents an index representing a degree of progress of recrystallization in a 10th period when a residence time in the temperature range of 550° C. to 720° C. in the process of heating in the annealing process is divided into 10 periods,
reference numerals in Expression (9) are as follows,
d 1 to 4 : constants (d 1 =4.24×10 2 , d 2 =2.10×10 0 , d 3 =1.31×10 3 , and d 4 =7.63×10 3 ),
h: a sheet thickness in mm before cold rolling,
h*: a sheet thickness in mm after cold rolling,
T R : a cold rolling completion temperature in ° C.,
Nb: a Nb content in mass %,
Ti*: an effective Ti content represented by Ti−42/14× N,
where each of Ti and N represents a content by mass % of the element, and when the element is not included, 0 is substituted as the content of the element,
K 2 : a value obtained by Expression (7-1),
n: an integer of 1 to 10,
T n ′: an average temperature in ° C. in an n-th period when a residence time in the temperature range of 550° C. to 720° C. is divided into 10 periods,
Δ t : a time in s when an elapsed time until a steel sheet temperature reaches 720° C. from 550° is divided into 10 periods,
where t 1 =Δt,
y
m
=
{
e
1
K
2
·
exp
(
-
e
2
T
m
+
2
7
3
)
·
A
c
3
-
T
m
A
c
3
-
A
c
1
·
t
m
}
1
/
2
·
e
3
·
(
T
m
-
e
4
A
c
3
-
e
4
)
3
(
10
)
t
m
=
Δ
t
+
y
m
-
1
2
·
{
e
1
K
2
·
exp
(
-
e
2
T
m
+
2
7
3
)
·
A
c
3
-
T
m
A
c
3
-
A
c
1
}
-
1
·
{
e
3
·
(
T
m
-
e
4
A
c
3
-
e
4
)
3
}
-
2
1.
0
≤
e
4
·
y
m
·
(
K
3
·
K
4
)
-
1
2
≤
5
.
0
in Expression (10), y m represents an index representing a degree of progress of reverse transformation in an m-th period when a residence time in the temperature range of 720° C. to the annealing temperature is divided into 10 periods, and y 0 =0,
reference numerals in Expression (10) are as follows,
e 1 to 4 : constants (e 1 =4.50×10 2 , e 2 =2.85×10 4 , e 3 =2.24×10 0 , and e 4 =8.56×10 −8 ),
K 2 : a value on the left side of Expression (7-1),
K 3 : a value of K 2 O obtained by Expression (8),
e 4 y m (K 3 ·K 4 ) −1/2 is a value when m=10,
K 4 : a value of p 10 obtained by Expression (9),
Ac 1 : an austenite transformation start temperature in ° C. during heating,
Ac 3 : an austenite transformation completion temperature in ° C. during heating,
T m : an average temperature in ° C. in an m-th period when a residence time in the temperature range of 720° C. to the annealing temperature is divided into 10 periods,
t m : an effective heat treatment time s in an m-th period when a residence time in the temperature range of 720° C. to the annealing temperature is divided into 10 periods, and
Δt: a time in s when an entire residence time in the temperature range of 720° C. to 500° C. is divided into 10 periods;
∑
i
=
1
1
0
(
g
1
+
g
2
·
Nb
0.5
+
g
3
·
Ti
*
0.5
)
·
(
1
+
g
4
·
Mo
0.5
)
-
1
·
K
4
1
/
3
·
(
A
c
3
-
T
max
A
c
3
-
A
c
1
)
1
/
3
·
(
Δ
i
+
g
5
·
Δ
i
0
.
5
)
·
exp
(
-
g
6
T
i
+
2
7
3
)
·
t
′0
.5
≥
1
.
0
0
(
11
)
reference numerals in Expression (11) are as follows,
i: an integer of 1 to 10,
Δ i : 750−18×Si−17×Mn−10×Cr−8×Ni+15×Al−Ti,
where each of the elements represents a content by mass % of the element, and when the element is not included, 0 is substituted as the content of the element,
when a calculated value of Δ i is a negative value, Δ i set to 0,
g 1 to 6 : constants (g 1 =1.00×10 −1 , g 2 =1.46×10 −1 , g 3 =1.14×10 −1 , g 4 =2.24×100, g 5 =4.53×10 0 , and g 6 =4.83×10 3 ),
each of Nb, Mo, Si, Mn, Cr, Ni, and Al: a content in mass % of the element,
Ti*: an effective Ti content represented by Ti−42/14× N,
where each of Ti and N represents a content by mass % of the element, and when the element is not included, 0 is substituted as the content of the element,
K 4 : a value of p 10 obtained by Expression (9),
Ac 1 : an austenite transformation start temperature in ° C. during heating,
Ac 3 : an austenite transformation completion temperature in ° C. during heating,
T max : an annealing temperature in ° C.,
T i : an average temperature in ° C. in an i-th period when a residence time in the temperature range of 720° C. to 500° C. is divided into 10 periods, and
t′: 1/20 s of an entire residence time in the temperature range of 450° C. to 700° C.
13 . The method of manufacturing a steel sheet according to claim 12 ,
wherein in the process of cooling in the annealing process, hot-dip galvanizing is performed on the cold-rolled steel sheet.
14 . The method of manufacturing a steel sheet according to claim 13 ,
wherein in the process of cooling in the annealing process, alloying is performed after the hot-dip galvanizing.
15 . The method of manufacturing a steel sheet according to claim 12 ,
wherein in the process of cooling in the annealing process, hot-dip zinc alloy plating is performed on the cold-rolled steel sheet.
16 . The method of manufacturing a steel sheet according to claim 15 ,
wherein in the process of cooling in the annealing process, alloying is performed after the hot-dip zinc alloy plating.