Method of Manufacturing a Steel Plate for Linepipe having Ultra-High Strength and Excellent Low Temperature Toughness

Abstract

A steel plate for linepipes having ultra-high strength and excellent low temperature toughness, and a method for manufacturing the same are disclosed. The steel plate has a strength of 930 MPa or more and excellent toughness even with much smaller amounts of alloying elements than that of conventional steel plates, and a method for manufacturing the same. The steel includes 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 Fe and unavoidable impurities. The microstructure includes at least about 75 area percent of a mixture of bainitic ferrite and acicular ferrite.

What is claimed is: 
     
         1 . 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; 0.1-0.14% 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 Ar a  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.,   wherein a microstructure of the steel plate processed according to the method comprises bainitic ferrite and acicular ferrite and wherein the microstructure is comprised of at least 75 area percent of a mixture of bainitic ferrite and acicular ferrite, and wherein the microstructure of the steel plate further comprises initial austenite with grain size of less than 15 μm.   
     
     
         2 . The method according to  claim 1  further comprising: air cooling or room temperature-cooling the steel plate after stopping the cooling at 20-50° C./sec.