US2025207215A1PendingUtilityA1
High-strength steel sheet and method for producing the same
Est. expiryMar 30, 2042(~15.7 yrs left)· nominal 20-yr term from priority
C21D 8/02C21D 8/00C22C 38/60C22C 38/38C22C 38/32C22C 38/28C22C 38/26C22C 38/16C22C 38/14C22C 38/12C22C 38/08C22C 38/06C22C 38/04C22C 38/02C22C 38/008C22C 38/005C22C 38/002C22C 38/001C21D 2211/008C21D 2211/002C21D 2211/001C21D 8/0263C21D 8/0226C21D 6/008C21D 6/005C21D 6/002C21D 6/001C21D 1/84C21D 1/18C21D 9/46C21D 8/0463C21D 8/0426C22C 38/42C22C 38/44C22C 38/46C22C 38/54C22C 38/58C22C 38/34C21D 8/0205
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Claims
Abstract
A high-strength steel sheet having a tensile strength of 980 MPa or more and a method for producing the high-strength steel sheet are disclosed. The high-strength steel sheet has a predetermined chemical composition, in a surface layer region of the high-strength steel sheet, 75% or more and less than 98.5% by area of an upper bainite phase as a primary phase, 1.5% or more and less than 25% by area of a martensite phase and/or a retained austenite phase as a secondary phase, and 2.0% or less by area of a remaining microstructure phase; wherein an average grain size of all phases included in the surface layer region is 6.0 μm or less; and wherein a dislocation density in all the phases included in the surface layer region is 8.0×10 14 /m 2 or more.
Claims
exact text as granted — not AI-modified1 . A high-strength steel sheet having a chemical composition containing, by mass,
C: 0.03% to 0.15%, Si: 0.1% to 3.0%, Mn: 0.8% to 3.0%, P: 0.001% to 0.1%, S: 0.0001% to 0.03%, Al: 0.001% to 2.0%, N: 0.001% to 0.01%, and B: 0.0002% to 0.010%, the chemical composition further containing at least one selected from, Ti: 0.01% to 0.30%, and Nb: 0.001% to 0.10%, with a balance being Fe and incidental impurities, a microstructure containing, in a surface layer region of the high-strength steel sheet extending from a surface of the steel sheet to a position 1/10 of a thickness of the steel sheet, 75% or more and less than 98.5% by area of an upper bainite phase serving as a primary phase, 1.5% or more and less than 25% by area of a martensite phase and/or a retained austenite phase as a secondary phase, and 2.0% or less by area of a remaining microstructure phase other than the upper bainite phase, the martensite phase and/or the retained austenite phases; wherein an average grain size of all phases included in the surface layer region extending from the surface of the steel sheet to the position 1/10 of the thickness of the steel sheet is 6.0 μm or less; and wherein a dislocation density in all the phases included in the surface layer region extending from the surface of the steel sheet to the position 1/10 of the thickness of the steel sheet is 8.0×10 14 /m 2 or more.
2 . The high-strength steel sheet according to claim 1 , wherein the chemical composition further contains, by mass, at least one group selected from Groups a to c below,
Group a: at least one element selected from Cu: 0.005% to 2.0%, Ni: 0.005% to 2.0%, Cr: 0.005% to 2.5%, V: 0.001% to 0.5%, and Mo: 0.005% to 1.0%, Group b: at least one element selected from Sb: 0.005% to 0.2%, and Sn: 0.001% to 0.05%, and Group c: at least one element selected from Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, and REM: 0.0005% to 0.01%.
3 . A method for producing the high-strength steel sheet according to claim 1 , the method comprising:
heating a steel material having the chemical composition to a heating temperature of 1150° C. or more; rough-rolling the heated steel material into a steel sheet; finish-rolling the steel sheet such that a total rolling reduction achieved in a temperature range of (RC1−150)° C. or more and RC1° C. or less is 35% or more, and a finish rolling delivery temperature is (RC2−100)° C. or more and (RC2+50)° C. or less; cooling the finish-rolled steel sheet such that a time interval between an end of the finish rolling to a start of the cooling is 2.0 s or less, an average cooling rate at a surface of the steel sheet is 20° C./s or more, and a cooling stop temperature is Trs°C or more and (Trs+180)° C. or less; coiling the cooled steel sheet such that a coiling temperature is Trs°C or more and (Trs+180)° C. or less; performing cooling to a temperature of (Trs−250)° C. or less at an average cooling rate of 1° C./s or less; and performing temper rolling at a rolling reduction of 0.1% or more and 5.0% or less, wherein RC1, RC2, and Trs are defined by Formulae (1), (2), and (3) below, respectively,
RC
1
(
°C
)
=
900
+
120
×
C
+
100
×
N
+
10
×
Mn
+
500
×
Ti
+
5000
×
B
+
10
×
Cr
+
50
×
Mo
+
1500
×
Nb
+
150
×
V
(
1
)
RC
2
(
°C
)
=
750
+
120
×
C
+
100
×
N
+
10
×
Mn
+
250
×
Ti
+
5000
×
B
+
10
×
Cr
+
50
×
Mo
+
750
×
Nb
+
150
×
V
(
2
)
Trs
(
°C
)
=
500
-
450
×
C
-
35
×
Mn
-
15
×
Cr
-
10
×
Ni
-
20
×
Mo
(
3
)
where each of element symbols used in Formulae (1), (2), and (3) above represents the content (% by mass) of the element and is zero when the element is absent.
4 . A method for producing the high-strength steel sheet according to claim 2 , the method comprising:
heating a steel material having the chemical composition to a heating temperature of 1150° C. or more; rough-rolling the heated steel material into a steel sheet; finish-rolling the steel sheet such that a total rolling reduction achieved in a temperature range of (RC1−150)° C. or more and RC1° C. or less is 35% or more, and a finish rolling delivery temperature is (RC2−100)° C. or more and (RC2+50)° C. or less; cooling the finish-rolled steel sheet such that a time interval between an end of the finish rolling to a start of the cooling is 2.0 s or less, an average cooling rate at a surface of the steel sheet is 20° C./s or more, and a cooling stop temperature is Trs°C or more and (Trs+180)° C. or less; coiling the cooled steel sheet such that a coiling temperature is Trs°C or more and (Trs+180)° C. or less; performing cooling to a temperature of (Trs−250)° C. or less at an average cooling rate of 1° C./s or less; and performing temper rolling at a rolling reduction of 0.1% or more and 5.0% or less, wherein RC1, RC2, and Trs are defined by Formulae (1), (2), and (3) below, respectively,
RC
1
(
°C
)
=
900
+
120
×
C
+
100
×
N
+
10
×
Mn
+
500
×
Ti
+
5000
×
B
+
10
×
Cr
+
50
×
Mo
+
1500
×
Nb
+
150
×
V
(
1
)
RC
2
(
°C
)
=
750
+
120
×
C
+
100
×
N
+
10
×
Mn
+
250
×
Ti
+
5000
×
B
+
10
×
Cr
+
50
×
Mo
+
750
×
Nb
+
150
×
V
(
2
)
Trs
(
°C
)
=
500
-
450
×
C
-
35
×
Mn
-
15
×
Cr
-
10
×
Ni
-
20
×
Mo
(
3
)
where each of element symbols used in Formulae (1), (2), and (3) above represents the content (% by mass) of the element and is zero when the element is absent.Join the waitlist — get patent alerts
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