| 摘要 |
<p>This dual-phase steel pipe has a chemical composition that contains, in mass%, 0.07-0.15% of C, 0.1-0.5% of Si, 0.8-1.9% of Mn and 0.020-0.10% of Nb, while limiting P to 0.05% or less, S to 0.01% or less and Al to 0.1% or less, with the balance made up of Fe and unavoidable impurities. When the content of an element X expressed in mass% is represented by [X] and it is defined that Ceq = [C] + [Mn]/6, the carbon equivalent (Ceq) calculated by the formula is 0.25-0.40 and [Nb] × [C] = 0.002 is satisfied. The metallic structure composition of the dual-phase steel pipe contains, in area ratio, 80-98% of ferrite and 2-20% in total of martensite, residual austenite or a mixed phase of martensite and residual austenite. The ferrite has an average particle diameter of 1 µm or more but less than 8 µm, and the martensite, the residual austenite or the mixed phase has an average particle diameter of 0.1-2 µm (inclusive).</p> |
