HIGH STRENGTH THICK STEEL PLATE FOR LINEPIPE HAVING EXCELLENT LOW TEMPERATURE TOUGHNESS AND DUCTILITY AS WELL AS LOW YIELD RATIO, AND METHOD THEREOF

摘要

One embodiment of the present invention provides a high strength thick steel plate for a line pipe having excellent low temperature toughness and ductility as well as a low yield ratio, and a method thereof, the steel plate comprising by weight%: 0.03 to 0.055% of C; 0.1 to 0.35% of Si; 1.7 to 2.2% of Mn; 0.01 to 0.04% of Al; 0.005 to 0.025% of Ti; 0.008% or less of N; 0.08 to 0.12% of Nb; 0.2 to 0.45% of Cr; 0.2 to 0.35% of Ni; 0.05 to 0.3% of Cu; 0.2 to 0.4% of Mo; 0.02% or less of P; 0.002% or less of S; 0.0005 to 0.004% of Ca; 0.01 to 0.04% of V; 0.0005% or less of B; and residual Fe and other unavoidable impurities, and satisfying relational expressions 1 to 4, wherein the microstructure thereof comprises by area%, 1 to 5% of polygonal ferrite; 60 to 75% of acicular ferrite; 20 to 30% of bainitic ferrite; and 5% or less of island martensite. 19≤Mo/96/P/31≤301.42≤Cr+2Mo+Cu+Ni+1000B≤1.754<3C/12+Mn/55×100≤4.9600≤400C+6800Nb+600V+800Ti+300Al≤820 (wherein the content of the alloying elements described in the relational expressions 1 to 4 is by % by weight.)