Tempered martensitic steel having low yield ratio and excellent uniform elongation, and manufacturing method therefor
Provided is a tempered martensitic steel having a low yield ratio and an excellent uniform elongation, the tempered martensitic steel: comprising, by wt %, 0.2-0.6% of C, 0.01-2.2% of Si, 0.5-3.0% of Mn, 0.015% or less of P, 0.005% or less of S, 0.01-0.1% of Al, 0.01-0.1% of Ti, 0.05-0.5% of Cr, 0.0005-0.005% of B, 0.05-0.5% of Mo, 0.01% or less of N, and the balance of Fe and inevitable impurities; having a yield ratio of 0.4-0.6; having a product (TS*U-El), of a tensile strength and a uniform elongation, of 10,000 MPa % or more; and having a microstructure containing, by an area fraction, 90% or more of tempered martensite, 5% or less of ferrite and the balance of bainite.
1 . A tempered martensitic steel having a low yield ratio and an excellent uniform elongation, the tempered martensitic steel comprising:
by wt %, 0.2 to 0.6% of C, 0.01 to 2.2% of Si, 0.5 to 3.0% of Mn, 0.015% or less of P, 0.005% or less of S, 0.01 to 0.1% of Al, 0.01 to 0.1% of Ti, 0.05 to 0.5% of Cr, 0.0005 to 0.005% of B, 0.05 to 0.5% of Mo, 0.01% or less of N, and a balance of Fe and inevitable impurities;
a yield ratio of 0.4 to 0.6; and a product (TS*U-El) of a tensile strength and a uniform elongation being 10,000 MPa % or more, the uniform elongation being 5.1% to 6.8%; and
a microstructure comprising, by an area fraction, 90% or more of tempered martensite, 5% or less of ferrite and a balance of bainite,
wherein carbides are precipitated in martensite laths.
2 . The tempered martensitic steel having a low yield ratio and an excellent uniform elongation of claim 1 , further comprising: one or more of 0.05 to 0.5% of Cu, 0.05 to 0.5% of Ni, and 0.05 to 0.3% of V.
3 . The tempered martensitic steel having a low yield ratio and an excellent uniform elongation of claim 1 , wherein the microstructure is a single-phase tempered martensite.
4 . The tempered martensitic steel having a low yield ratio and an excellent uniform elongation of claim 1 , wherein the tensile strength is 1500 MPa or more.
5 . A manufacturing method of a tempered martensitic steel having a low yield ratio and an excellent uniform elongation, comprising steps of, by wt %:
preparing steel including 0.2 to 0.6% of C, 0.01 to 2.2% of Si, 0.5 to 3.0% of Mn, 0.015% or less of P, 0.005% or less of S, 0.01 to 0.1% of Al, 0.01 to 0.1% of Ti, 0.05 to 0.5% of Cr, 0.0005 to 0.005% of B, 0.05 to 0.5% of Mo, 0.01% or less of N, and the balance of Fe and inevitable impurities;
heating the steel to a temperature in a range of 850° C. to 930° C. and holding the steel for 100 to 1000 seconds; and
cooling the heated steel to a cooling stop temperature of Mf−50° C. to Mf+100° C. at a cooling rate of (a martensitic critical cooling rate) to 300° C./sec, and then holding the cooled steel for 2 to 40 minutes.
6 . The manufacturing method of a tempered martensitic steel having a low yield ratio and an excellent uniform elongation of claim 5 , wherein the steel is manufactured by steps of:
heating a slab to a temperature of 1150° C. to 1300° C.;
finish hot rolling the heated slab at Ar 3 to 950° C. to obtain a hot-rolled steel sheet; and
coiling the hot-rolled steel sheet at a temperature in a range of 500° C. to 750° C.
7 . The manufacturing method of a tempered martensitic steel having a low yield ratio and an excellent uniform elongation of claim 6 , further comprising steps of:
cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet at a temperature in a range of 750° C. to 850° C.; and
performing an over-aging treatment on the continuously annealed cold-rolled steel sheet at a temperature in a range of 400° C. to 600° C.
8 . The manufacturing method of a tempered martensitic steel having a low yield ratio and an excellent uniform elongation of claim 5 , wherein the steel further comprises, by wt %, one or more of 0.05 to 0.5% of Cu, 0.05 to 0.5% of Ni, and 0.05 to 0.3% of V.