| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel product and a steel pipe, having a low yield ratio at ordinary temperature, excellent in toughness, and also excellent in strength characteristics at a high temperature, and their manufacturing method. SOLUTION: The low yield ratio type steel product excellent in toughness has a composition which consists of, by weight, <=0.03% C, <=1% Si, 0.1-2%; Mn, <=0.02% S, 0.01-0.1% Al, 0.04-1% Nb, and the balance Fe with inevitable impurities and in which Nb content satisfies inequality Nb>=0.1+7.74C-1.94Ti+6.63N. Moreover, in this steel product, microstructure is composed of a mixed structure of polygonal ferrite and bainite-containing phase forming at a low temperature, and the area ratio of the bainite-containing phase forming at low temperature is >5 to <90% on average in plate-thickness direction. Further, this steel product has <=-40 deg.C vTrs and >=47J absorption energy at 0 deg.C.
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