Ultra-high strength cold rolled steel sheet having excellent bendability, and method of manufacturing same
AssigneePosco
InventorsJong-Pan Kong, Yeon-Sang Ahn, Joo-Hyun Ryu
Life of this patent
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Abstract
Provided are an ultra-high strength cold rolled steel sheet having excellent bendability, and a method of manufacturing the same. The steel sheet includes 0.06 to 0.17% of C, 0.1 to 0.8% of Si, 1.9 to 2.9% of Mn, 0.005 to 0.07% of Nb, 0.004 to 0.05% of Ti, 0.0004 to 0.005% of B, 0.20% or less (excluding 0%) of Cr, and 0.04 to 0.45% of Mo, with a balance of Fe and other unavoidable impurities. a microstructure of the steel sheet includes, by area: 80 to 98% of tempered martensite and a balance of fresh martensite, bainite, ferrite, and residual austenite. An average length of lath short axis of the tempered martensite is 500 nm or less.
Claims10
A cold rolled steel sheet comprising, by weight:
- 0.06 to 0.17% of C, 0.1 to 0.8% of Si, 1.9 to 2.9% of Mn, 0.005 to 0.07% of Nb, 0.004 to 0.05% of Ti, 0.0004 to 0.005% of B, 0.20% or less (excluding 0%) of Cr, and 0.04 to 0.45% of Mo, with a balance of Fe and unavoidable impurities, and satisfying the following Relations 1 to 3;
- and
- a microstructure including, by area, 80 to 98% of tempered martensite and a balance of fresh martensite, bainite, ferrite, and residual austenite, wherein an average length of lath short axis of the tempered martensite is 500 nm or less,
- 0
- .
- 4
-
- 0
- ≤
- C
- +
- Mn
- /
- 6
- +
- (
- C
-
- r
- +
- M
-
- o
- +
- V
- )
- /
- 5
- +
- (
- S
-
- i
- +
- N
-
- i
- +
- Cu
- )
- /
- 15
- ≤
- 0.7
- ,
- [
- Relation
-
- 1
- ]
- 110
- ≤
- 48.8
- +
- 49
-
- log
-
- C
- +
- 35.1
- Mn
- +
- 25.9
- Si
- +
- 76.5
- Cr
- +
- 105.9
- Mo
- +
- 1325
-
- Nb
- ≤
- 210
- ,
- [
- Relation
-
- 2
- ]
- and
- 0.2
- ≤
- Mo
- +
- 200
-
- B
- ≤
- 0.7
- ,
- [
- Relation
-
- 3
- ]
- wherein contents of alloy components described are based on % by weight.
A method of manufacturing a cold rolled steel sheet, the method comprising:
- heating a slab including, by weight:
- 0.06 to 0.17% of C, 0.1 to 0.8% of Si, 1.9 to 2.9% of Mn, 0.005 to 0.07% of Nb, 0.004 to 0.05% of Ti, 0.0004 to 0.005% of B, 0.20% or less (excluding 0%) of Cr, and 0.04 to 0.45% of Mo with a balance of Fe and unavoidable impurities, and satisfying the following Relations 1 to 3;
- finish rolling the heated slab so that a finish rolling outlet side temperature is Ar3+50° C. to Ar3+150° C., thereby obtaining a hot rolled steel sheet;
- cooling the hot rolled steel sheet to Ms+50° C. to Ms+300° C. and then winding the cooled hot rolled steel sheet;
- cold rolling the wound hot rolled steel sheet to obtain a cold rolled steel sheet;
- continuously annealing the cold rolled steel sheet in a temperature range of 820 to 860° C.;
- crack treating the continuously annealed cold rolled steel sheet for 50 to 200 seconds;
- first cooling the crack treated cold rolled steel sheet down to 620 to 700° C. at a cooling rate of 1 to 10° C./s;
- second cooling the first cooled cold rolled steel sheet down to 360 to 420° C. at a cooling rate of 5 to 50° C./s;
- and
- subjecting the second cooled cold rolled steel sheet to an overaging treatment at 370 to 420° C. or the overaging treatment after reheating,
- wherein in the second cooling and the overaging treatment, the following Relations 4 to 8 are satisfied:
- 0
- .
- 4
-
- 0
- ≤
- C
- +
- Mn
- /
- 6
- +
- (
- C
-
- r
- +
- M
-
- o
- +
- V
- )
- /
- 5
- +
- (
- S
-
- i
- +
- N
-
- i
- +
- Cu
- )
- /
- 15
- ≤
- 0.7
- ,
- [
- Relation
-
- 1
- ]
- 110
- ≤
- 48.8
- +
- 49
-
- log
-
- C
- +
- 35.1
- 25.9
- Si
-
- +
- 76.5
- Cr
- +
- 105.9
- Mo
- +
- [
- Reaction
-
- 2
- ]
- 1325
-
- Nb
- ≤
- 210
- ,
- 0.2
- ≤
- Mo
- +
- 200
-
- B
- ≤
- 0.7
- ,
- [
- Relation
-
- 3
- ]
- 0
- ≤
- A
- ≤
- 50
- ,
- [
- Relation
-
- 4
- ]
- 0
- ≤
- B
- ≤
- 40
- ,
- [
- Relation
-
- 5
- ]
- 0
- ≤
- 2.8
- A
- +
- 0.5
- B
- ≤
- 100
- ,
- [
- Relation
-
- 6
- ]
- 0
- ≤
- 3.1
- A
- +
- 2.3
- B
- ≤
- 200
- ,
- and
- [
- Relation
-
- 7
- ]
- 0.25
- ≤
- (
- 3.1
- A
- +
- 2.3
- B
- )
- /
- (
- 2.8
- A
- +
- 0.5
- B
- )
- ≤
- 3.5
- ,
- [
- Relation
-
- 8
- ]
- wherein contents of alloy components described in Relations 1 to 3 are based on % by weight, and A is Ms—a second cooling end temperature (C) and B is an overaging treatment temperature—a second cooling end temperature (° C.), in Relations 4 to 8.
Dependencies are inferred from the claim text.