THICK SEAMLESS STEEL PIPE FOR LINE PIPE AND ITS PRODUCTION METHOD

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PROBLEM TO BE SOLVED : To provide a thick seamless steel pipe for a line pipe having high strength and excellent toughness, and to provide a method for producing the same. SOLUTION : The thick seamless steel pipe for a line pipe having high strength and satisfactory toughness has a composition comprising 0.03 to 0.08% C, ≤0.25% Si, 0.3 to 2.5% Mn, 0.001 to 0.10% Al, 0.02 to 1.0% Cr, 0.02 to 1.0% Ni, 0.02 to 1.2% Mo, 0.004 to 0.010% Ti and 0.002 to 0.008% N, further comprising one or more kinds selected from Ca, Mg and rare earth metals by 0.0002 to 0.005% in total, comprising 0 to 0.08% V, 0 to 0.05% Nb and 0 to 1.0% Cu, and, if required, comprising 0.0003 to 0.01% B, and the balance Fe with impurities, and, in the impurities, the content of P is ≤0.05%, and the content of S is ≤0.005%. In the method for producing the steel pipe, the cooling viscosity for the slab, heating conditions for piercing, and heat treatment conditions after pipe making are characterized. COPYRIGHT : (C)2007, JPO&INPIT