Patent ID: 8801874

Claim:
A steel plate for line pipes containing, in terms of % by weight, C: 0.02 to 0.06%, Si: 0.5% or less, Mn: 0.8 to 1.6%, P: 0.008% or less, S: 0.0008% or less, Al: 0.08% or less, Nb: 0.005 to 0.035%, Ti: 0.005 to 0.025%, Cr:0.02% to 0.5% and Ca: 0.0005 to 0.0035%, with a balance of Fe and inevitable impurities, which has, as represented by the following formula, a CP value of 0.95 or less and a Ceq value of 0.30 or more: CP= 4.46 C (%)+2.37 Mn (%)/6+{1.18 Cr (%)+1.95 Mo (%)+1.74 V (%)}/5+{1.74 Cu (%)+1.7 Ni (%)}/15+22.36 P(%), Ceq=C (%)+ Mn (%)/6+{ Cr (%)+ Mo (%)+ V (%)}/5 +{ Cu (%) + Ni (%)}/15, and wherein hardness of a center segregation area is HV 250 or lower, and the length of Nb carbonitride in the center segregation area is 20 μm or less.