US 2026/0201522 A1Published application

Ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties and method for manufacturing same

AssigneePosco

InventorsJong-Pan KONG, Yeon-Sang AHN, Joo-Hyun RYU

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Abstract

Provided is an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties, which can be used as a structural component for automobiles such as a member, a seat rail, and a pillar, and a method for manufacturing same. The steel sheet includes: by weight: C: 0.03 to 0.12%, Si: 0.03 to 0.30%, Mn: 2.1 to 2.9%, Al: 0.005 to 0.07%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.08%, B: 0.0005 to 0.005%, Cr: 0.7 to 1.4%, Mo: 0.005 to 0.10%, N: 0.008% or less (excluding 0%), with a balance of Fe and inevitable impurities. A microstructure thereof includes by area, fresh martensite: 4 to 19%, a sum of tempered martensite and bainite: 78 to 95%, and retained austenite: 0.2 to 2.0%. An average grain size of the microstructure is 0.5 to 6 μm.

Claims12

Dependent claims

A cold-rolled steel sheet, comprising by weight:

C:
0.03 to 0.12%, Si:
0.03 to 0.30%, Mn:
2.1 to 2.9%, Al:
0.005 to 0.07%, Nb:
0.01 to 0.08%, Ti:
0.005 to 0.08%, B:
0.0005 to 0.005%, Cr:
0.7 to 1.4%, Mo:
0.005 to 0.10%, N:
0.008% or less (excluding 0%), with a balance of Fe and inevitable impurities, satisfying the following Relational Expressions 1 to 3,
wherein a microstructure thereof includes by area,
fresh martensite:
4 to 19%, a sum of tempered martensite and bainite:
78 to 95%, and retained austenite:
0.2 to 2.0%,
wherein an average grain size of the microstructure is 0.5 to 6 μm,
0.18
≤
C
+
Si
/
30
+
Mn
/
20
+
2
⁢
P
+
4
⁢
S
≤
0.3
[
Relational
⁢
Expression
⁢
1
]
180
≤
(
48.8
+
49
⁢
log
⁢
C
+
35.1
Mn
⁢
+
25.9
Si
⁢
+
76.5
Cr
⁢
+
105.9
Mo
+
1325
⁢
Nb
)
≤
270
[
Relational
⁢
Expression
⁢
2
]
700
≤
(
48.8
+
49
⁢
log
⁢
C
+
35.1
Mn
+
25.9
Si
+
76.5
Cr
+
105.9
Mo
+
1325
⁢
Nb
)
/
(
C
+
Si
/
30
+
Mn
/
20
+
2
⁢
P
+
4
⁢
S
)
≤
1200
[
Relational
⁢
Expression
⁢
3
]
where, a content of alloy components described in the Relational Expressions 1 to 3 refers to weight %.

138 more elements

A method for manufacturing a cold-rolled steel sheet, comprising operations of:

heating a slab including, by weight:
C:
0.03 to 0.12%, Si:
0.03 to 0.30%, Mn:
2.1 to 2.9%, Al:
0.005 to 0.07%, Nb:
0.01 to 0.08%, Ti:
0.005 to 0.08%, B:
0.0005 to 0.005%, Cr:
0.7 to 1.4%, Mo:
0.005 to 0.10%, N:
0.008% or less (excluding 0%), with a balance of Fe and inevitable impurities, satisfying the following Relational Expressions 1 to 3;
finish hot rolling the heated slab so that a finish rolling exit temperature is Ar3+ 50° C. to Ar3+ 150° C.;
cooling the hot-rolled steel sheet to a temperature within a range of Ms+ 50° C. to Ms+ 300° C. and winding the same;
cold rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet at a temperature within a range of Ar3+ 10° C. to Ar3+ 70° C.;
soaking the continuously annealed cold-rolled steel sheet for 50 to 200 seconds;
primarily cooling the soaked cold-rolled steel sheet to a temperature of 620 to 700° C. at a cooling rate of 1 to 10° C./s;
secondarily cooling the primarily-cooled cold-rolled steel sheet to a temperature of 360 to 420° C. at a cooling rate of 5 to 50° C./sec;
overaging the secondarily-cooled cold-rolled steel sheet for 250 to 650 seconds, and then terminating the same at 320 to 400° C.,
wherein the following Relational Expressions 4 to 6 are satisfied during the secondary cooling and the overaging treatment,
0.18
≤
C
+
Si
/
30
+
Mn
/
20
+
2
⁢
P
+
4
⁢
S
≤
0.3
[
Relational
⁢
Expression
⁢
1
]
180
≤
(
48.8
+
49
⁢
log
⁢
C
+
35.1
Mn
⁢
+
25.9
Si
⁢
+
76.5
Cr
⁢
+
105.9
Mo
+
1325
⁢
Nb
)
≤
270
[
Relational
⁢
Expression
⁢
2
]
700
≤
(
48.8
+
49
⁢
log
⁢
C
+
35.1
Mn
+
25.9
Si
+
76.5
Cr
+
105.9
Mo
+
1325
⁢
Nb
)
/
(
C
+
Si
/
30
+
Mn
/
20
+
2
⁢
P
+
4
⁢
S
)
≤
1200
[
Relational
⁢
Expression
⁢
3
]
10
≤
A
≤
70
[
Relational
⁢
Expression
⁢
4
]
30
≤
B
≤
100
[
Relational
⁢
Expression
⁢
5
]
2.5
≤
overaging
⁢
treatment
⁢
time
/
B
≤
14
[
Relational
⁢
Expression
⁢
6
]
where, in the above Relational Expressions 1 to 3, a content of alloy components described is weight %, and in the above Relational Expressions 4 to 6, A is a Ms-secondary cooling end temperature (° C.), and B is an overaging end temperature (° C.).

184 more elements

Dependencies are inferred from the claim text.

Facts

Application: 19/566,965Filed 13 Mar 2026
Priority: KR 10-2020-017641016 Dec 2020
Published: 16 Jul 2026
Term ends: Not estimatedThe source records don’t support an estimate.

Classification27 codes

C22C 38/38
Containing chromium: with more than 1.5% by weight of manganese
C21D 1/20
By interrupted quenching: isothermal quenching, e.g. bainitic hardening
C21D 1/22
By interrupted quenching: martempering
C21D 1/84
General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering: controlled slow cooling