US2025197979A1PendingUtilityA1
High strength steel sheet and method for manufacturing the same
Est. expiryMar 25, 2042(~15.7 yrs left)· nominal 20-yr term from priority
C21D 8/02C23C 2/06C22C 38/06C22C 38/02C22C 38/002C22C 38/001C21D 2211/008C21D 2211/005C21D 2211/002C21D 2211/001C21D 9/46C21D 8/0278C21D 8/0273C21D 8/0236C21D 8/0226C21D 6/005C21D 1/18C22C 38/005C22C 38/008C22C 38/08C22C 38/10C22C 38/32C22C 38/34C22C 38/38C22C 38/16C22C 38/12C22C 38/14C22C 38/04C23C 2/024C23C 2/02C23C 2/28C23C 2/0224C21D 8/0263C22C 38/60C21D 8/0205
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Claims
Abstract
A high strength steel sheet having 980 MPa or higher tensile strength and 10% or more elongation and a method for manufacturing the same are disclosed. The high strength steel sheet has a specific chemical composition and is such that in a region at ¼ sheet thickness, the area fraction of martensite is 60% or more, the volume fraction of retained austenite is 3% or more and 15% or less, the area fraction of the total of ferrite and bainitic ferrite is more than 10%, and the average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.
Claims
exact text as granted — not AI-modified1 . A high strength steel sheet having a chemical composition comprising, in mass %,
C: 0.030% or more and 0.500% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.00% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, a balance being Fe and incidental impurities, the high strength steel sheet being such that in a region at ¼ sheet thickness, an area fraction of martensite is 60% or more, a volume fraction of retained austenite is 3% or more and 15% or less, an area fraction of a total of ferrite and bainitic ferrite is more than 10%, and an average of proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.
2 . The high strength steel sheet according to claim 1 , wherein the chemical composition further comprises at least one element selected from, in mass %,
Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.
3 . The high strength steel sheet according to claim 1 , which has a coated layer on a surface of the steel sheet.
4 . The high strength steel sheet according to claim 2 , which has a coated layer on a surface of the steel sheet.
5 . A method for manufacturing the high strength steel sheet according to claim 1 , the method comprising:
providing a cold rolled steel sheet produced by subjecting a steel having the chemical composition to hot rolling, pickling, and cold rolling; annealing the steel sheet by heating at an annealing temperature Ta of 700° C. or above and 900° C. or below for a holding time at the annealing temperature Ta of 10 seconds or more and 1000 seconds or less; bending and unbending the steel sheet 1 to 15 times in total with a roll having a radius of 800 mm or less during the annealing; cooling the steel sheet at an average cooling rate of 20° C./s or more in a temperature range from 700° C. to 600° C. and at an average cooling rate of less than 20° C./s in a temperature range from 499° C. to Ms; bending and unbending the steel sheet in the temperature range from 499° C. to Ms, 1 to 15 times in total with a roll having a radius of 800 mm or less; cooling the steel sheet at an average cooling rate of 150° C./s or less in a temperature range from Ms to a cooling stop temperature Tb; applying a tension to the steel sheet in the temperature range from Ms to the cooling stop temperature Tb while controlling the tension to 5 MPa or more and 100 MPa or less, the cooling stop temperature Tb being 100° C. or above and (Ms−80° C.) or below where Ms is a martensite start temperature (° C.) defined by formula (1); and tempering the steel sheet at a tempering temperature of Tb or above and 450° C. or below for a holding time at the tempering temperature of 10 seconds or more and 1000 seconds or less,
Ms
=
519
-
474
×
[
%
C
]
-
30.4
×
[
%
Mn
]
-
12.1
×
[
%
Cr
]
-
7.5
×
[
%
Mo
]
-
17.7
×
[
%
Ni
]
-
Ta
/
80
(
1
)
where [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.
6 . A method for manufacturing the high strength steel sheet according to claim 2 , the method comprising:
providing a cold rolled steel sheet produced by subjecting a steel having the chemical composition to hot rolling, pickling, and cold rolling; annealing the steel sheet by heating at an annealing temperature Ta of 700° C. or above and 900° C. or below for a holding time at the annealing temperature Ta of 10 seconds or more and 1000 seconds or less; bending and unbending the steel sheet 1 to 15 times in total with a roll having a radius of 800 mm or less during the annealing; cooling the steel sheet at an average cooling rate of 20° C./s or more in a temperature range from 700° C. to 600° C. and at an average cooling rate of less than 20° C./s in a temperature range from 499° C. to Ms; bending and unbending the steel sheet in the temperature range from 499° C. to Ms, 1 to 15 times in total with a roll having a radius of 800 mm or less; cooling the steel sheet at an average cooling rate of 150° C./s or less in a temperature range from Ms to a cooling stop temperature Tb; applying a tension to the steel sheet in the temperature range from Ms to the cooling stop temperature Tb while controlling the tension to 5 MPa or more and 100 MPa or less, the cooling stop temperature Tb being 100° C. or above and (Ms−80° C.) or below where Ms is a martensite start temperature (° C.) defined by formula (1); and tempering the steel sheet at a tempering temperature of Tb or above and 450° C. or below for a holding time at the tempering temperature of 10 seconds or more and 1000 seconds or less,
Ms
=
519
-
474
×
[
%
C
]
-
30.4
×
[
%
Mn
]
-
12.1
×
[
%
Cr
]
-
7.5
×
[
%
Mo
]
-
17.7
×
[
%
Ni
]
-
Ta
/
80
(
1
)
where [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.
7 . The method for manufacturing the high strength steel sheet according to claim 5 , further comprising performing a coating treatment.
8 . The method for manufacturing the high strength steel sheet according to claim 6 , further comprising performing a coating treatment.Join the waitlist — get patent alerts
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