| 摘要 |
PROBLEM TO BE SOLVED: To produce a steel tube in which the lower structure is refined and excellent in a balance of strength-ductility. SOLUTION: As to this steel tube, a steel tube having a compsn. contg., by weight, 0.001 to 0.60% C, 0.01 to 3.0% Si, 0.01 to 2.0% Mn and 0.001 to 0.10% Al and furthermore contg., at need, one or >= two groups among one or >= two kinds selected from Cu, Ni, Cr and Mo, one or, >= two kinds selected from Nb, V, Ti and B and one or two kinds selected from rare earth metals and Ca is subjected to reduction rolling under prescribed conditions to form into a steel tube having a structure essentially consisting of ferrite, and in which the ferrite is furthermore devided into subgrains of <=2μm diameter. Moreover, preferably, the grain size of the ferrite is controlled to <=5μm, or, furthermore, the inverse intensity ratio in the (110) plane is controlled to >=2.0. Moreover, preferably, the diameter of 2nd phases is controlled to <=42μm, and the aspect ratio is controlled to <=3. Furthermore, the area ratio of the 2nd phases is preferably controlled to <=30%.
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