| 摘要 |
<p>A steel pipe with dual phase structure includes, as chemical composition, by mass %, C: 0.07% to 0.15%, Si: 0.1% to 0.5%, Mn: 0.8% to 1.9%, Nb: 0.020% to 0.10%, P: limited to 0.05% or less, S: limited to 0.01% or less, Al: limited to 0.1% or less, and a balance consisting of iron and unavoidable impurities, wherein, when [X] is amount of element X in mass%, carbon equivalent Ceq defined by Equation of Ceq = [C] + [Mn] / 6 is 0.25 to 0.40 and [Nb] × [C] ‰¥ 0.002 is satisfied, and includes, as metallographic structure, by area %, ferrite of 80% to 98% and martensite, residual austenite, or mixture thereof of 2% to 20% in total, wherein average grain size of ferrite is 1 µm to less than 8 µm and average grain size of martensite, residual austenite, or mixture thereof is 0.1 µm to 2µm.</p> |
