| 简介 |
PROBLEM TO BE SOLVED : To provide a steel tube having formability such as ductility, bendability and hydroform workability, and toughness high in an isotropic manner, and to provide its production method.
SOLUTION : The steel tube has a composition comprising 0.0002 to 0.70% C, 0.003 to 3.0% Si, 0.003 to 3.0% Mn, 0.002 to 2.0% Al, ≤0.15% P, ≤0.05% S and ≤0.015% N, or, further comprising 0.0002 to 0.01% B, one or more kinds of metals selected from Ti, Nb, V and Zr by 0.005 to 1% in total, one or more kinds of metals selected from Cr, Mo, Cu and Ni by 0.005 to 3% in total, 0.0001 to 0.005% Ca or 0.0001 to 0.2% rare earth metals. Further, ≥50% by the area ratio in the structure consists of ferritic phases, the average crystal grain size thereof is ≤40 μm, the aspect ratio is ≤3.0, and the ratio of the large angle grain boundaries occupied in the ferrite grain boundaries is ≥70%. The average crystal grain size of the second phases in which the area ratio is the maximum among the balance phases is ≤40 μm, and the second phases in which the distance between the closest second phases is the double of the minimum size of the second phases or above cover ≥50% in the area ratio thereof. |
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