| 摘要 |
PROBLEM TO BE SOLVED: To provide a high toughness steel plate for line pipes which can be suitably used as a high toughness steel plate for line pipes being excellent in base material toughness, material uniformity of base material and HAZ (weld heat-affected zone) toughness and having≥565 MPa tensile strength and also to provide its manufacturing method. SOLUTION: The steel plate is characterized in that: it has a composition consisting of, by mass, 0.03 to 0.08% C,≤0.05% Si, 1.0 to 2.0% Mn, 0.02 to 0.05% each of P, S and Al, 0.005 to 0.025% Nb, 0.005 to 0.030% Ti, 0.001 to 0.010% N, further one or more kinds among 0.10 to 0.60% Cu, 0.10 to 1.20% Ni, 0.05 to 0.40% Cr and 0.05 to 0.40% Mo and the balance Fe with inevitable inclusions, containing, if necessary, one or more elements among Zr, Ca, Mg and REM and satisfying 0.30≤Ceq≤0.45; an upper bainite structure comprises≥90% of a HAZ structure when welding is performed at 4 to 10 kJ/mm welding heat input; and island martensite contained in the upper bainite structure is≤3%. COPYRIGHT: (C)2009,JPO&INPIT
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