STEEL FOR LINE PIPE

PROBLEM TO BE SOLVED: To inexpensively produce a line pipe combining excellent strength, toughness and plastic deformability with high efficiency. SOLUTION: This steel for a line pipe has a composition containing C of 0.10 to 0.20%, Si of 1.0% or less, Mn of 0.3 to 2.5%, P of 0.015% or less, S of 0.003% or less, Ti of 0.06% or less, Al of 0.001 to 0.1% and N of 0.007% or less, moreover containing, as optional elements, Cr of 1.0% or less, Mo of 1.0% or less, Cu of 1.0% or less, Ni of 1.0% or less, Nb of 0.05% or less, V of 0.10% or less, B of 0.003% or less and Ca of 0.005% or less and simultaneously satisfies the following inequality (a) and has a structure containing martensite and bainite in which the average grain size of austenite is 30 to 200 μm or has a tempered structure thereof : Al×(N-Ti/3.4)