Cold rolled steel sheet and method for producing same
Abstract
A cold rolled steel sheet having a predetermined chemical composition, and a steel microstructure comprising, by area %, martensite: 90.0 to 99.5%, ferrite: 0 to 5%, retained austenite: 0.5 to 7.0%, and balance: bainite wherein a ratio of tempered martensite to the total martensite is 80 to 100%, a maximum value of curvature 1/R obtained by measuring the shape at a total width×length 300 mm region and expressed by the following formula (1) is 0.010 or less, and a tensile strength of 1470 MPa or more: 1 / R = MAX { ❘ "\[LeftBracketingBar]" ρ 1 ❘ "\[RightBracketingBar]" , ❘ "\[LeftBracketingBar]" ρ 2 ❘ "\[RightBracketingBar]" } . ( 1 )
Claims
exact text as granted — not AI-modified1 . A cold rolled steel sheet having a chemical composition comprising, by mass %,
C: 0.16 to 0.40%, Si: 0.05 to 2.00%, Mn: 0.50 to 4.00%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.00%, N: 0.0100% or less, O: 0.0050% or less, Cr: 0 to 2.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, B: 0 to 0.0100%, Co: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 1.00%, Nb: 0 to 0.100%, Ti: 0 to 0.200%, V: 0 to 0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Ce: 0 to 0.0100%, Zr: 0 to 0.0100%, La: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.0100%, and balance: Fe and impurities, and a steel microstructure in a range of ⅛ thickness to ⅜ thickness centered on ¼ thickness from the surface comprising, by area %, martensite: 90.0 to 99.5%, ferrite: 0 to 5%, retained austenite: 0.5 to 7.0%, and balance: bainite, wherein a ratio of tempered martensite to the total martensite is 80 to 100%, a maximum value of curvature 1/R obtained by measuring the shape at a total width×length 300 mm region and expressed by the following formula (1) is 0.010 or less, and a tensile strength is 1470 MPa or more,
[
Equation
1
]
1
/
R
=
MAX
{
❘
"\[LeftBracketingBar]"
ρ
1
❘
"\[RightBracketingBar]"
,
❘
"\[LeftBracketingBar]"
ρ
2
❘
"\[RightBracketingBar]"
}
(
1
)
where,
1/R: curvature
ρ1 and ρ2: main curvatures on curved surface.
2 . The cold rolled steel sheet according to claim 1 , wherein the chemical composition comprises, by mass %, one or more of:
Cr: 0.001 to 2.00%, Mo: 0.001 to 1.00%, Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00%, B: 0.0001 to 0.0100%, Co: 0.001 to 1.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 1.00%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, V: 0.001 to 0.50%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Ce: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, La: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.0100%.
3 . The cold rolled steel sheet according to claim 1 , wherein no fracture occurs at sheared surfaces in a hydrogen embrittlement test of shearing the cold rolled steel sheet, then heat treating it at 170° C. for 10 minutes, and then immersing it in a concentration 0.3 g/L ammonium thiocyanate aqueous solution for 48 hours.
4 . The cold rolled steel sheet according to claim 1 , wherein the cold rolled steel sheet has any of an electrogalvanized layer, hot dip galvanized layer, and hot dip galvannealed layer on the surface thereof.
5 . A method for producing the cold rolled steel sheet according to claim 1 , comprising
(A) a hot rolling step comprising rough rolling and finish rolling a slab having a chemical composition according to claim 1 , wherein the hot rolling step satisfies the conditions of the following (A1) to (A3):
(A1) a slab heating temperature is 1150° C. or more,
(A2) width edge parts is heated so that a temperature of the width edge parts of the steel sheet after rough rolling is 10 to 150° C. higher than a temperature of a width center part, and
(A3) a coiling temperature is 450 to 650° C.,
(B) a cold rolling step comprising cold rolling the obtained hot rolled steel sheet using a tandem mill consisting of N number (N≥3) of rolling stands, wherein a cumulative cold rolling reduction is 30% or more, and the cold rolling step satisfies the following formulas (2) and (3):
[
Equation
2
]
∑
k
=
1
N
{
(
1
+
R
k
)
❘
"\[LeftBracketingBar]"
σ
k
-
1
Pb
k
-
σ
k
Pf
k
❘
"\[RightBracketingBar]"
-
1
}
<
3.
(
2
)
[
Equation
3
]
σ
k
=
(
1.667
·
σ
0
)
·
ε
k
0.1
(
3
)
R k : rolling reduction at k-th rolling stand
Pb k : backward tension at k-th rolling stand
Pf k : forward tension at k-th rolling stand
σ k−1 : flow stress of steel sheet after passing k−1-th rolling stand
σ k : flow stress of steel sheet after passing k-th rolling stand
σ 0 : yield strength of hot rolled steel sheet
ε k : cumulative strain after passing k-th rolling stand
(C) a heat treatment step comprising heat treating the obtained cold rolled steel sheet, wherein the heat treatment step satisfies the conditions of the following (C1) to (C3):
(C1) holding the cold rolled steel sheet at Ac3 to 950° C. for 10 seconds to 500 seconds (heating and holding),
(C2) performing cooling treatment satisfying the following (i) to (v):
(i) cooling stop temperature T1 is 110 to 250° C.,
(ii) average cooling rate from 300 to 700° C. is 20 to 150° C./s,
(iii) average cooling rate from T1 to 300° C. is 1.0 to 20° C./s and a gas is used as a cooling medium,
(iv) naturally cooling of 0.5 second or more each time is performed at least one time from Ms to 700° C. and from T1 to less than Ms, and
(v) tension applied to the cold rolled steel sheet is 5 to 20 MPa, and
(C3) holding from 200 to 300° C. for 100 to 1000 seconds (low temperature holding).Join the waitlist — get patent alerts
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