Steel plate and steel pipe for line pipes

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=4.46C(%)+2.37Mn(%)/6+{1.18Cr(%)+1.95Mo(%)+1.74V(%)}/5+{1.74Cu(%)+1.7Ni(%)}/15+22.36P(%),
• 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.