Steel plate for linepipe having ultra-high strength and excellent low temperature toughness and manufacturing method of the same

A steel plate having ultra-high strength and excellent low temperature toughness, comprising, by weight %:

• 0.03˜0.10% C;
• 0˜0.6% Si;
• 1.6˜2.1% Mn;
• 0˜1.0% Cu;
• 0˜1.0% Ni;
• 0.02˜0.06% Nb;
• 0˜0.1% V;
• 0˜10.5% Mo;
• 0˜1.0% Cr;
• 0.005˜0.03% Ti;
• 0.01˜0.06% Al;
• 0.0005˜0.0025% B;
• 0.001˜0.006% N;
• 0˜0.006% Ca;
• 0.02% or less P;
• 0.005% or less S;
• and the balance of Fe and unavoidable impurities, wherein the microstructure comprises at least about 75 area percent of a mixture of bainitic ferrite and acicular ferrite.

A steel plate having ultra-high strength and excellent low temperature toughness, comprising, by weight %:

• 0.03˜0.10% C;
• 0˜0.6% Si;
• 1.6˜2.1% Mn;
• 0˜1.0% Cu;
• 0˜1.0% Ni;
• 0.20˜0.06% Nb;
• 0˜0.1 V;
• 0.1˜0.5% Mo;
• 0˜1.0 Cr;
• 0.005˜0.03% Ti;
• 0.01˜0.06% Al;
• 0.0005˜0.0025% B;
• 0.001˜0.006% N;
• 0˜0.006% Ca;
• 0.02% or less P;
• 0.005% or less S;
• and the balance of Fe and unavoidable impurities, wherein the microstructure comprises at least about 75 area percent of a mixture of bainitic ferrite and acicular ferrite, yield strength is 930 MPa or more, and Charpy impact absorbed energy at −40° C. is 230 Joules or more.

A method of manufacturing a steel plate having ultra-high strength and excellent low temperature toughness, the method comprising:

• reheating a steel slab to 1,050˜1,150° C., the slab comprising, by weight %, 0.03˜0.10% C, 0˜0.6% Si, 1.6˜2.1% Mn, 0˜1.0% Cu, 0˜1.0% Ni, 0.02˜0.06% Nb, 0˜0.1 V, 0.1˜0.5% Mo, 0˜1.0 Cr, 0.005˜0.03% Ti, 0.01˜0.06% Al, 0.0005˜0.0025% B, 0.001˜0.006% N, 0˜0.006% Ca, 0.02% or less P, 0.005% or less S, and the balance of Fe and unavoidable impurities;
• rolling the reheated steel slab by one or more hot rolling passes in an austenite recrystallization temperature region or more with a reduction ratio of 20˜80%;
• rolling the rolled steel slab by one or more hot rolling passes in the range from the austenite recrystallization temperature or less to Ar3 temperature or more with a reduction ratio of 40˜80% to provide a steel plate;
• cooling the rolled steel plate at a cooling rate of 20˜50° C./sec;
• and
• stopping the cooling of the steel plate at a temperature of 200˜400° C.