US2025092482A1PendingUtilityA1
Ultra-high strength cold-rolled steel sheet and manufacturing method therefor
Est. expiryMay 31, 2042(~15.8 yrs left)· nominal 20-yr term from priority
C21D 8/02C22C 38/06C22C 38/04C22C 38/02C22C 38/28C22C 38/24C22C 38/38C22C 38/32C21D 8/0236C21D 1/26C21D 1/25C21D 1/19C21D 2211/008C21D 9/46C21D 8/0273C22C 38/002C21D 8/0263C21D 8/0226C21D 6/008C21D 6/005C21D 6/002C21D 1/84C21D 1/18C21D 8/0205
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Claims
Abstract
Provided is an ultra-high strength cold-rolled steel sheet, including: carbon (C): 0.23% to 0.40%; silicon (Si): 0.05% to 1.0%; manganese (Mn): 0.5% to 3.0%; vanadium (V): 0.01% to 0.12%; aluminum (Al): 0.01% to 0.3%; chromium (Cr): greater than 0% and 0.5% or less; titanium (Ti): greater than 0% and 0.1% or less; phosphorus (P): greater than 0% and 0.02% or less; sulfur(S): greater than 0% and 0.01% or less; and boron (B): 0.001% to 0.005%, based on % by weight; and a remainder being Fe and other unavoidable impurities, wherein a final microstructure includes tempered martensite having a volume fraction of 90% or more, wherein an average spacing between precipitates in the tempered martensite is 300 nm or more, an average size of the precipitates is 200 nm or less, and the number of precipitates having an average size of 40 nm or less is 25 or more based on an area of 20 μm 2 in the final microstructure.
Claims
exact text as granted — not AI-modifiedWhat is claimed:
1 . An ultra-high strength cold-rolled steel sheet, comprising: carbon (C): 0.23% to 0.40%; silicon (Si): 0.05% to 1.0%; manganese (Mn): 0.5% to 3.0%; vanadium (V): 0.01% to 0.12%; aluminum (Al): 0.01% to 0.3%; chromium (Cr): greater than 0% and 0.5% or less; titanium (Ti): greater than 0% and 0.1% or less; phosphorus (P): greater than 0% and 0.02% or less; sulfur(S): greater than 0% and 0.01% or less; and boron (B): 0.001% to 0.005%, based on % by weight; and a remainder being Fe and other unavoidable impurities,
wherein a final microstructure comprises tempered martensite having a volume fraction of 90% or more, wherein an average spacing between precipitates in the tempered martensite is 300 nm or more, an average size of the precipitates is 200 nm or less, and the number of precipitates having an average size of 40 nm or less is 25 or more based on an area of 20 μm 2 in the final microstructure.
2 . The ultra-high strength cold-rolled steel sheet according to claim 1 , wherein a surface layer of the cold-rolled steel sheet comprises a soft region having a hardness less than 85% of an average hardness of a base material, wherein a ratio of a thickness of the soft region to a thickness of the base material is 0.03 to 0.10.
3 . The ultra-high strength cold-rolled steel sheet according to claim 1 , wherein the cold-rolled steel sheet has a Prior Austenite Grain Size (PAGS) of 12 μm or less.
4 . The ultra-high strength cold-rolled steel sheet according to claim 1 , further comprising: molybdenum (Mo): 0.01% to 0.3% or niobium (Nb): 0.01% to 0.1%, based on % by weight.
5 . The ultra-high strength cold-rolled steel sheet according to claim 1 , wherein a value of [C]+ [V]+ [Cr]+ [Mo]+ [Nb] (where [C], [V], [Cr], [Mo] and [Nb] are weight % values of carbon, vanadium, chromium, molybdenum and niobium) of the cold-rolled steel sheet is smaller than 0.63.
6 . The ultra-high strength cold-rolled steel sheet according to claim 1 , wherein the ultra-high strength cold-rolled steel sheet has a yield strength (YP) of 1200 MPa or more, a tensile strength (TS) of 1500 MPa or more, an elongation (El) of 7.0% or more, a yield ratio of 70% or more, a bendability (R/t) of 2.5 or less and a hydrogen embrittlement elongation reduction rate of 35% or less.
7 . A method of manufacturing an ultra-high strength cold-rolled steel sheet, the method comprising:
hot-rolling steel comprising carbon (C): 0.23% to 0.40%; silicon (Si): 0.05% to 1.0%; manganese (Mn): 0.5% to 3.0%; vanadium (V): 0.01% to 0.12%; aluminum (Al): 0.01% to 0.3%; chromium (Cr): greater than 0% and 0.5% or less; titanium (Ti): greater than 0% and 0.1% or less; phosphorus (P): greater than 0% and 0.02% or less; sulfur(S): greater than 0% and 0.01% or less; and boron (B): 0.001% to 0.005%, based on % by weight; and a remainder being Fe and other unavoidable impurities to provide a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to provide a cold-rolled steel sheet; heating the cold-rolled steel sheet; annealing the heated cold-rolled steel sheet; cooling the annealed cold-rolled steel sheet; and reheating and tempering the cooled cold-rolled steel sheet, wherein, when annealing the cold-rolled steel sheet, an annealing temperature is gradually increased in a temperature range of 750° C. to 950° C., without a section in which a constant temperature is maintained, until cooling the cold-rolled steel sheet.
8 . The method according to claim 7 , wherein, in the annealing of the cold-rolled steel sheet, an annealing temperature (T) satisfies a relationship of Equation 1 below dependent upon time (t):
Annealing
temperature
(
T
)
=
A
×
t
2
+
B
×
t
+
C
(
where
A
,
B
and
C
constants
satisfying
-
0.007
<
A
<
-
0.005
,
2
<
B
<
3
,
and
500
<
C
<
700
)
.
[
Equation
1
]
9 . The method according to claim 7 , wherein, in the heating of the cold-rolled steel sheet, a heating rate is 3° C./sec or more, and
in the annealing of the cold-rolled steel sheet, a heating rate is less than 3° C./sec.
10 . The method according to claim 7 , wherein the hot rolling is performed under conditions of a reheating temperature (SRT) of 1180° C. to 1300° C., a finish delivery temperature (FDT) of 800° C. to 950° C., and a coiling temperature (CT) of 500° C. to 700° C.
11 . The method according to claim 7 , wherein the cooling of the annealed cold-rolled steel sheet comprises:
slow-cooling the annealed cold-rolled steel sheet up to 700 to 800° C. at a cooling rate of 3° C./sec to 15° C./sec; performing first rapid cooling for the slowly cooled cold-rolled steel sheet up to 300° C. to 350° C. at a cooling rate of 80° C./sec to 150° C./sec; and performing second rapid cooling for the cold-rolled steel sheet, which has been subjected to the first rapid cooling, up to room temperature to 300° C. at a cooling rate of 30° C./sec to 90° C./sec.
12 . The method according to claim 7 , wherein the reheating and tempering of the cooled cold-rolled steel sheet comprises reheating the cold-rolled steel sheet, which has been subjected to the second rapid cooling, and tempering by maintaining at 150° C. to 350° C. for 30 to 300 sec.
13 . The ultra-high strength cold-rolled steel sheet according to claim 2 , wherein a value of [C]+ [V]+ [Cr]+ [Mo]+ [Nb] (where [C], [V], [Cr], [Mo] and [Nb] are weight % values of carbon, vanadium, chromium, molybdenum and niobium) of the cold-rolled steel sheet is smaller than 0.63.
14 . The ultra-high strength cold-rolled steel sheet according to claim 3 , wherein a value of [C]+ [V]+ [Cr]+ [Mo]+ [Nb] (where [C], [V], [Cr], [Mo] and [Nb] are weight % values of carbon, vanadium, chromium, molybdenum and niobium) of the cold-rolled steel sheet is smaller than 0.63.
15 . The ultra-high strength cold-rolled steel sheet according to claim 4 , wherein a value of [C]+ [V]+ [Cr]+ [Mo]+ [Nb] (where [C], [V], [Cr], [Mo] and [Nb] are weight % values of carbon, vanadium, chromium, molybdenum and niobium) of the cold-rolled steel sheet is smaller than 0.63.Join the waitlist — get patent alerts
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