| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel pipe with a wall thickness of 40 to 100 mm which is obtained by hot forming, and which has a homogeneous whole cross-section and has mechanical properties that toughness is high without depending on the sheet thickness. SOLUTION: The steel pipe has chemical components comprising, by weight,≤0.22% C,≤0.55% Si,≤1.60% Mn,≤0.03% P,≤0.015% S,≤0.50% Cu,≤0.40% Ni,≤0.10% Cr,≤0.05% Mo and≤0.15% V, and the balance Fe with inevitable impurities. The original pipe 5 with a wall thickness t of 40 to 100 mm is heated to an A3 transformation point or higher, and is thereafter hot-formed to control the value of a yield point or proof stress to 235 to 445 N/mm<SP>2</SP>. The steel pipe produced by the hot forming is made suitable in many faces such as the yield strength and ductility of corner parts and weld zones compared with a steel pipe produced by cold forming. Further, the value of the yield point or proof stress can be made into the one similar to that of a steel pipe with a sheet thickness of≤40 mm, and the steel pipe in which the whole cross-section is homogeneous and has mechanical properties that toughness is high without depending on the sheet thickness can be obtained. COPYRIGHT: (C)2003,JPO
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