Ultrahigh-strength steel sheet having excellent yield ratio, and manufacturing method therefor
One aspect of the present invention relates to an ultrahigh-strength steel sheet having an excellent yield ratio, comprising, by wt %, 0.3-0.5% of C, 2.0% (excluding 0%) of Si, 3.0-6.5% of Mn, 0.02% or less of P, 0.01% or less of S, 0.01-3.0% of Al, 0.02% or less (excluding 0%) of N, and the balance of Fe and other inevitable impurities, and a microstructure comprises 5-30% of remaining austenite by area fraction and comprises 5% or less of secondary martensite.
1. An ultrahigh-strength steel sheet having an excellent yield ratio comprising:
0.3 to 0.5 wt % of carbon (C);
0.024 wt % or less of silicon (Si)(excluding 0%);
3.0 to 6.5 wt % of manganese (Mn);
0.02 wt % or less of phosphorus (P);
0.01 wt % or less of sulfur (S);
0.01 to 3.0 wt % of aluminum (Al);
0.02 wt % or less of nitrogen(N)(excluding 0%);
a remainder of iron (Fe); and
other unavoidable impurities,
wherein a microstructure comprises:
5 to 30% of retained austenite by area fraction,
5% or less of non-tempered martensite by area fraction, ferrite, and
bainite, where a sum of the ferrite and the bainite is 20% or less by area fraction.
2. The ultrahigh-strength steel sheet having an excellent yield ratio of claim 1 , wherein the steel sheet further comprises one or more of 1.5 wt % or less of chromium (Cr)(excluding 0%),0.005 to 0.5 wt % of titanium(Ti),0.005 to 0.5 wt % of niobium (Nb), 0.005 to 0.5 wt % of vanadium (V), and 0.05 to 0.3 wt % of molybdenum (Mo).
3. The ultrahigh-strength steel sheet having an excellent yield ratio of claim 1 , wherein the steel sheet has a yield strength of 1000 MPa or more, a tensile strength of 1300 MPa or more, and a yield ratio of 0.7 or more.
4. The ultrahigh-strength steel sheet having an excellent yield ratio of claim 1 , wherein the steel sheet has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on a surface of the steel sheet.
5. The ultrahigh-strength steel sheet having an excellent yield ratio of claim 1 , comprising 6.25 to 6.5 wt % of manganese (Mn).
6. The ultrahigh-strength steel sheet having an excellent yield ratio of claim 1 , wherein Si+Al is 1.32 wt %.
7. The ultrahigh-strength steel sheet having an excellent yield ratio of claim 1 , wherein Si+Al is 1.32 wt % or less.