| 摘要 |
PROBLEM TO BE SOLVED: To provide a welded steel tube capable of improving the fatigue characteristic by reducing a softened area and a softening ratio of a weld heat affected zone of the welded steel tube manufactured of a high-strength steel plate subjected to the work reinforcement as a material. SOLUTION: A cold-rolled steel plate as a material is formed of a low-alloy steel containing Ti, Nb and B, and further containing 0.01-0.30 mass% Mo as necessary with C equivalent defined by the formula (4) C equivalent = C+1/6Mn+1/24Si+5B+1/4Mo to be 0.25-0.6 mass%. A welded steel tube is formed of the steel plate satisfying an inequalityΔHV×width of softened part≤150, whereΔHV of the weld softened part and the width of the softened part are defined as (1)ΔHV of the weld softened part = hardness of base material - hardness of the mostly softened part of the weld, and (2) the width of softened part = the width (mm) of the softened part in which its hardness is lower than that of the base material by the welding heat input. COPYRIGHT: (C)2008,JPO&INPIT
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