US2024254585A1PendingUtilityA1
High strength hot-rolled steel sheet having excellent formability, and manufacturing method therefor
Est. expiryAug 9, 2041(~15 yrs left)· nominal 20-yr term from priority
C21D 8/02C22C 38/001C21D 2211/001C21D 2211/008C21D 2211/005C21D 2211/002C23C 2/06C22C 38/38C22C 38/32C22C 38/28C22C 38/26C22C 38/22C22C 38/06C22C 38/02C22C 38/002C21D 8/0278C21D 8/0263C21D 8/0226C21D 6/008C21D 6/005C21D 6/002C21D 1/18C21D 9/46C23G 1/02B21C 47/02C22C 38/60C22C 38/14C22C 38/12C22C 38/04C21D 8/0205
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Claims
Abstract
Provided are a hot-rolled steel sheet having excellent formability and a manufacturing method therefore. The steel sheet includes: carbon (C): 0.05 to 0.17%, silicon (Si): 0.01 to 1.5%, manganese (Mn): 1.5 to 3.0%, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 2.0% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.15%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.02%, with a balance of Fe and other unavoidable impurities, satisfies: (1.5×[Si]+1.2×[Cr]+0.7×[Mo]+8.0×[Ti])/[C]>20 where each element refers to a weight content, and a microstructure including 70 to 90% of acicular ferrite or bainitic ferrite by area fraction, as a matrix structure, and one or more second phases among low-temperature bainite, tempered martensite, and MA phase.
Claims
exact text as granted — not AI-modified1 . A high-strength hot-rolled steel sheet having excellent formability comprising, by weight:
carbon (C): 0.05 to 0.17%, silicon (Si): 0.01 to 1.5%, manganese (Mn): 1.5 to 3.0%, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 2.0% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.15%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.02%, with a balance of Fe and other unavoidable impurities, wherein the following Relational Expression 1 is satisfied, wherein a microstructure comprises 70 to 90% of acicular ferrite or bainitic ferrite by area fraction, as a matrix structure, and one or more second phases among low-temperature bainite, tempered martensite, and MA phase,
(
1.5
×
[
Si
]
+
1.2
×
[
Cr
]
+
0.7
×
[
Mo
]
+
8.
×
[
Ti
]
)
/
[
C
]
>
20
[
Relational
Expression
1
]
In Relational Expression 1, each element refers to a weight content.
2 . The high-strength hot-rolled steel sheet having excellent formability of claim 1 , wherein the hot-rolled steel sheet further comprises, one or more of niobium (Nb):
0.01 to 0.1%, and boron (B): 0.0005 to 0.005%.
3 . The high-strength hot-rolled steel sheet having excellent formability of claim 1 , wherein a ratio of a MA phase among a total fraction of the second phase is less than 30%.
4 . The high-strength hot-rolled steel sheet having excellent formability of claim 1 , wherein the hot-rolled steel sheet comprises at least one of ferrite and carbide of less than 5% by area fraction.
5 . The high-strength hot-rolled steel sheet having excellent formability of claim 1 , wherein the hot-rolled steel sheet has a yield strength of 750 MPa or more, a tensile strength of 980 MPa or more, and elongation of 9% or more.
6 . The high-strength hot-rolled steel sheet having excellent formability of claim 1 , wherein the hot-rolled steel sheet has a hole expansion rate of 30% or more.
7 . A manufacturing method for a high-strength hot-rolled steel sheet having excellent formability, comprising operations of:
reheating a steel slab including, by weight: carbon (C): 0.05 to 0.17%, silicon (Si): 0.01 to 1.5%, manganese (Mn): 1.5 to 3.0%, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 2.0% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.15%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.02%, with a balance of Fe and other unavoidable impurities, the steel slab satisfying the following Relational Expression 1 to a temperature within a range of 1100 to 1350° C.; hot rolling the reheated steel slab to manufacture a hot-rolled steel sheet; primarily cooling the hot-rolled steel sheet to a temperature of Bs or lower at a cooling rate of 70° C./s or more; secondarily cooling the steel sheet to a temperature of (Bs+Ms)/2 or higher at a cooling rate of 20° C./s or less after the primary cooling; tertiarily cooling the steel sheet to a temperature range of Ms−20° C. to 500° C. at a cooling rate of 30° C./s or more after the secondary cooling; and winding the steel sheet in the tertiarily-cooled temperature range, wherein finish hot rolling is performed to satisfy the following Relational Expression 2 within a temperature range of 750 to 1150° C. during the hot rolling, and a total reduction amount of two final passes is 10 to 40%,
(
1.5
×
[
Si
]
+
1.2
×
[
Cr
]
+
0.7
×
[
Mo
]
+
8.
×
[
Ti
]
)
/
[
C
]
>
20
[
Relational
Expression
1
]
In Relational Expression 1, each element refers to a weight content,
800≤Du≤1106 [Relational Expression 2]
In Relational Expression 2, Du is an indicator indicating an effective grain size of austenite immediately before primary cooling after hot rolling, expressed as Du=FDT+(7.35×[C])−(24.7×[Si])−(4.7×[Mn])−(3.9×[Cr])−(5.2×[Mo])−(560×[Ti])−(1110×[Nb]), where FDT refers to a rolling end temperature (° C.), and each element refers to a weight content.
8 . The manufacturing method for a high-strength hot-rolled steel sheet having excellent formability of claim 7 , wherein the steel slab further comprises, one or more of niobium (Nb): 0.01 to 0.1% and boron (B): 0.0005 to 0.005%.
9 . The manufacturing method for a high-strength hot-rolled steel sheet having excellent formability of claim 7 , wherein the secondary cooling step is performed for a time (ts) satisfying the following Relational Expression 3,
0.1
≤
exp
(
-
k
(
T
)
×
(
ts
)
2
)
≤
0.3
[
Relational
Expression
3
]
where k(T) is expressed by the following formula, and each element is a weight content, in the formula below, T1 is a primary cooling end temperature (° C.), and T2 is a secondary cooling end temperature)(° C.).
k
(
T
)
=
20
0.049
Du
-
34.2
exp
{
-
(
(
T
1
+
T
2
)
/
2
-
557
+
320
[
C
]
+
35
[
Si
]
+
90
[
Mn
]
+
70
[
Cr
]
+
120
[
Mo
]
+
7800
[
B
]
112
)
1.92
}
10 . The manufacturing method for a high-strength hot-rolled steel sheet having excellent formability of claim 7 , further comprising:
a step of finally cooling to room temperature after the winding.
11 . The manufacturing method for a high-strength hot-rolled steel sheet having excellent formability of claim 10 , further comprising:
a step of pickling and oiling after the final cooling.
12 . The manufacturing method for a high-strength hot-rolled steel sheet having excellent formability of claim 11 , further comprising:
a step of hot-dip galvanizing after the pickling and oiling.Join the waitlist — get patent alerts
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