| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength welded steel tube which has a high tensile strength of≥700 MPa and is excellent in formability and low-temperature toughness and superior in torsional fatigue resistance after cross section forming processing; and a method for manufacturing the same. SOLUTION: The high tensile strength welded steel tube has a composition comprising, by mass, 0.035-0.099% C and 2.05-2.8% Mn, where the ratio of C to Mn is≤0.04, Si, Ti, Nb, B, Cr, and Al in appropriate amounts, and P, S, N and O, as impurities, in amounts not higher than respective restricted amounts. The hot rolling conditions of a steel strip and the diameter-reduction rolling conditions of a tube body are each adjusted in an appropriate range so that such a structure that a ferrite phase is contained in an amount of≥60% by area ratio and the average crystal grain diameter in the cross section in the circumferential direction of the ferrite phase is 0.5-5.5μm is formed, and the inner and outer surfaces each have such a surface roughness that the arithmetical average roughness Ra is≤2μm, the maximum height roughness Rz is≤30μm, and the ten-point average roughness Rz<SB>JIS</SB>is≤20μm. Thereby, the welded steel having a high tensile strength of≥700 MPa and excellent in all of the formability, the low-temperature roughness and the torsional fatigue resistance after cross section forming processing can be obtained. COPYRIGHT: (C)2007,JPO&INPIT
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