US2024352551A1PendingUtilityA1
Cold rolled steel sheet having excellent weldability, strength, and formability, and method for manufacturing same
Est. expiryAug 26, 2041(~15.1 yrs left)· nominal 20-yr term from priority
C21D 8/02C22C 38/38C22C 38/32C22C 38/28C22C 38/26C22C 38/22C22C 38/06C22C 38/02C22C 38/002C22C 38/001C21D 2211/008C21D 2211/005C21D 2211/002C21D 2211/001C21D 8/0278C21D 8/0263C21D 8/0236C21D 8/0226C21D 6/008C21D 6/005C21D 6/004C21D 1/84C21D 1/18C23C 2/06B21C 47/02C21D 8/0273C22C 38/14C22C 38/12C22C 38/04C21D 9/46C21D 8/0205
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Claims
Abstract
Provided is a cold-rolled steel sheet including: by weight %, C: 0.10 to 0.16%, Si: 0.3 to 0.8%, Al: 0.01 to 0.5%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.5%, B: 0.0001 to 0.001%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), a remainder of Fe, and other unavoidable impurities, as a microstructure, by area %, ferrite: 10% or less (excluding 0%), retained austenite: more than 1% to 5% or less, martensite: 25% or more but less than 50%, and bainite: 35% or more but less than 70%. An average size of martensite-austenite (MA) in the bainite is 0.35 to 0.55 μm.
Claims
exact text as granted — not AI-modified1 . A cold-rolled steel sheet comprising:
by weight %, C: 0.10 to 0.16%, Si: 0.3 to 0.8%, Al: 0.01 to 0.5%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.5%, B: 0.0001 to 0.001%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), a remainder of Fe, and other unavoidable impurities, as a microstructure comprises, by area %, ferrite: 10% or less (excluding 0%), retained austenite: more than 1% to 5% or less, martensite: 25% or more but less than 50%, and bainite: 35% or more but less than 70%, and an average size of martensite-austenite (MA) in the bainite is 0.35 to 0.55 μm.
2 . The cold-rolled steel sheet of claim 1 , wherein a value defined by the following Relationship 1 satisfies 70 or more:
2
3
4
×
[
C
]
-
29
×
[
Si
]
-
1
2
8
×
[
Al
]
+
2
9
×
[
Mn
]
+
1
0
×
[
Cr
]
-
1
7
×
[
Mo
]
-
3
7
×
[
Nb
]
-
4
9
×
[
Ti
]
+
100
×
[
B
]
[
Relationship
1
]
where [C], [Si], [Al], [Mn], [Cr], [Mo], [Nb], [Ti], and [B] represent weight percentages of the elements in parentheses, respectively.
3 . The cold-rolled steel sheet of claim 1 , wherein a value defined by the following Relationship 2 satisfies 270 or more and 330 or less:
2
7
0
×
[
C
]
+
9
0
×
[
Mn
]
+
7
0
×
[
Cr
]
+
8
0
×
[
Mo
]
[
Relationship
2
]
where [C], [Mn], [Cr], and [Mo] represent weight percentages of the elements in parentheses, respectively.
4 . The cold-rolled steel sheet of claim 1 , wherein a value defined by the following Relationship 3 satisfies 1.8 or less:
5
×
[
C
]
+
[
Si
]
+
0
.
5
×
[
Al
]
[
Relationship
3
]
where [C], [Si], and [Al] represent weight percentages of the elements in parentheses, respectively.
5 . The cold-rolled steel sheet of claim 1 , wherein the microstructure comprises, by area %, 2 to 7% of the ferrite.
6 . The cold-rolled steel sheet of claim 1 , wherein the microstructure comprises, by area %, 2 to 4% of the retained austenite.
7 . The cold-rolled steel sheet of claim 1 , wherein the microstructure comprises, by area %, 45 to 63% of the bainite.
8 . The cold-rolled steel sheet of claim 1 , wherein the microstructure comprises, by area %, 29 to 49% of the martensite.
9 . The cold-rolled steel sheet of claim 1 , wherein the cold-rolled steel sheet has a tensile strength of 980 to 1150 MPa and a yield strength of 740 to 950 MPa.
10 . The cold-rolled steel sheet of claim 1 , wherein the cold-rolled steel sheet has a hole expansion ratio (HER) of 45% or more.
11 . A method for manufacturing a cold-rolled steel sheet, comprising:
heating a steel slab including, by weight %, C: 0.10 to 0.16%, Si: 0.3 to 0.8%, Al: 0.01 to 0.5%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.5%, B: 0.0001 to 0.001%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), a remainder of Fe, and other unavoidable impurities; hot-rolling the heated slab in a finish hot rolling temperature range of 830 to 980° C. to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet in a temperature range of 450 to 700° C.; cold-rolling the coiled hot-rolled steel sheet; continuously annealing the cold-rolled steel sheet in a temperature range of 790 to 830° C.; primary cooling the continuously annealed steel sheet at an average cooling rate of less than 10° C./s to a primary cooling end temperature of 450 to 600° C.; secondary cooling the primary cooled steel sheet at an average cooling rate of 10° C./s or more to a secondary cooling end temperature of 250 to 350° C.; and reheating the secondary cooled steel sheet to a temperature in a range of 350 to 480° C., wherein the method for manufacturing the cold-rolled steel sheet satisfies the following Relationship 4:
V
1
/
V
2
×
t
>
0
.
5
[
Relationship
4
]
where V1 represents an average cooling rate during the primary cooling, V2 represents an average cooling rate during the secondary cooling, and t represents a thickness of the cold-rolled steel sheet.
12 . The method of claim 11 , wherein, during the cold-rolling, a cold-rolling reduction rate is 30 to 60%.
13 . The method of claim 11 , further comprising plating the reheated steel sheet in a zinc plating bath in a temperature range of 450 to 470° C.
14 . The method of claim 13 , further comprising alloying and heat-treating the plated steel sheet in a temperature range of 470 to 550° C.
15 . The method of claim 14 , further comprising cooling the alloy heat-treated steel sheet to room temperature and then temper-rolling the cooled alloy heat-treated steel sheet at a reduction ratio of less than 1%.
16 . The method of claim 11 , wherein an average temperature increase rate during the reheating is 0.5 to 2.5° C./s.Join the waitlist — get patent alerts
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