| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-strength, high-toughness seamless steel pipe which does not need to be tempered after having been cold-worked. SOLUTION: This seamless steel pipe has a composition including, by mass%, 0.02-0.07% C, 0.05-1.0% Si, 2.0% to 4.5% Mn, 0.030% or less P, 0.015% or less S, 0.01-0.06% Al, 0.007% or less N, 0.005% or less O, 0.05-1.0% Cr, 0.05-1.0% Mo, 0.005-0.20% Nb, 0.005% to 0.03% Ti and 0.001-0.004% B so as to satisfy the expression of (Mn+1.3Cr+1.5Mo+0.3Ni+0.6Cu)≥3.0; and has a single-phase structure of a bainitic structure, or a mixed structure including a bainitic structure of 90% or higher by an area rate and the balance a low-temperature transformation phase. The seamless steel pipe may further include one or more elements of Cu, Ni, V and Ca. Thus manufactured steel pipe shows a high tensile strength of 600 MPa or higher, high toughness and little degradation after having been cold-worked. COPYRIGHT: (C)2007,JPO&INPIT
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