Ultrahigh strength cold-rolled steel sheet and manufacturing method thereof
An ultrahigh strength cold-rolled steel sheet and a manufacturing method thereof are provided. An ultrahigh strength cold-rolled steel sheet, according to a preferred aspect of the present invention, comprises, in percentage by weight: C: 0.25 to 0.4%; Si: 0.5% or less (excluding 0); Mn: 3.0 to 4.0%; P: 0.03% or less (excluding 0); S: 0.015% or less (excluding 0); Al: 0.1% or less; Cr: 1% or less (excluding 0); Ti: 48/14*[N] to 0.1% or less; Nb: 0.1% or less (excluding 0); B: 0.005% or less (excluding 0); N: 0.01% or less (excluding 0); and a balance of Fe and unavoidable impurities, and a microstructure includes 90% or more (including 100%) of martensite, and one or two kinds of 10% or less (including 0%) of ferrite and bainite.
1 . An ultrahigh strength cold-rolled steel sheet, comprising, in percentage by weight:
C: 0.25 to 0.4%;
Si: 0.5% or less, excluding 0;
Mn: 3.16 to 4.0%;
P: 0.03% or less, excluding 0;
S: 0.015% or less, excluding 0;
Al: 0.1% or less, excluding 0;
Cr: 1% or less, excluding 0;
Ti: 48/14*[N] or more and 0.1% or less;
Nb: 0.1% or less, excluding 0;
B: 0.005% or less, excluding 0;
N: 0.01% or less, excluding 0; and
a balance of Fe and other unavoidable impurities,
wherein the cold-rolled steel sheet is an as-rolled cold-rolled steel sheet,
wherein the as-rolled cold-rolled steel sheet has a microstructure consisting of 90% or more (including 100%) of martensite, and one or two of 10% or less (including 0%) of ferrite and bainite, and
wherein the as-rolled cold-rolled steel sheet has:
a tensile strength of 1700 MPa or more, and
a yield strength of 1200 MPa or more.
2 . The steel sheet of claim 1 , comprising, in percentage by weight:
Mn: 3.53 to 4.0%.
3 . The steel sheet of claim 1 , comprising, in percentage by weight:
N: 0.0042% to 0.01%.
4 . The steel sheet of claim 1 , wherein a wave height (ΔH) is 3 mm or less in an edge portion after cutting a steel sheet in the width direction thereof to provide a sample 1000 mm long in the longitudinal direction.
5 . The steel sheet of claim 1 , wherein the microstructure consists of 90% (excluding 100%) or more of martensite, and 10% or less (excluding 0%) of ferrite and bainite.
6 . An ultrahigh strength cold-rolled steel sheet, comprising, in percentage by weight:
C: 0.25 to 0.4%;
Si: 0.5% or less, excluding 0;
Mn: 3.53 to 4.0%;
P: 0.03% or less, excluding 0;
S: 0.015% or less, excluding 0;
Al: 0.1% or less, excluding 0;
Cr: 1% or less, excluding 0;
Ti: 48/14*[N] or more and 0.1% or less;
Nb: 0.1% or less, excluding 0;
B: 0.005% or less, excluding 0;
N: 0.0042% to 0.01%; and
a balance of Fe and other unavoidable impurities,
wherein a wave height (ΔH) is 3 mm or less in an edge portion after cutting a steel sheet in the width direction thereof to provide a sample 1000 mm long in the longitudinal direction,
wherein the cold-rolled steel sheet is an as-rolled cold-rolled steel sheet,
wherein the as-rolled cold-rolled steel sheet has a microstructure consisting of 90% or more (including 100%) of martensite, and one or two of 10% or less (including 0%) of ferrite and bainite, and
wherein the as-rolled cold-rolled steel sheet has:
a tensile strength of 1700 MPa or more, and
a yield strength of 1200 MPa or more.