WELDED STEEL PIPE FOR LINEPIPE WITH SUPERIOR COMPRESSIVE STRENGTH AND SUPERIOR TOUGHNESS, AND PROCESS FOR PRODUCING SAME

摘要

To provide a steel pipe for a linepipe having a heavy wall thickness and having excellent fracture toughness in a base material and a welded heat affected zone by suppressing lowering of yield stress caused by a Bauschinger effect by optimizing the metal microstructure of a steel plate. To be more specific, provided are a welded steel pipe for a linepipe having high compressive strength and high fracture toughness which is a steel pipe having the composition which contains by mass% 0.03 to 0.08% C, 0.10% or less Si, 1.00 to 2.00% Mn, 0.010% or less P, 0.0030% or less S, 0.06% or less Al, 0.005 to 0.020% Nb, 0.005 to 0.025% Ti, 0.0010 to 0.0060% N, and Fe and unavoidable impurities as a balance, wherein Ti(%)/N(%) is a value which falls within a range of 2 to 4, a Ceq value is 0.30 or more, and fractions and the like of bainite, M-A constituent (MA) and cementite in the metal microstructure of a base material and the metal microstructure of the welded heat affected zone are specified, and method of producing the same.