| 摘要 |
A hot-rolled steel sheet according to the present invention is a steel sheet containing a predetermined components, and satisfying O<S/Ca<0.8, N-14/48×Ti>="0" (zero) %. It is a high-strength hot-rolled steel sheet for a spiral pipe excellent in low-temperature toughness in which a pro-eutectoid ferrite fraction is 3% or more and 20% or less, and the other is a low-temperature transformation phase in a microstructure at a depth of a half thickness of a sheet thickness from a steel sheet surface, a number average crystal grain size of a whole of the microstructure is 2.5 mum or less, an area average grain size is 9 mum or less, a standard deviation of the area average grain size is 2.3 mum or less, and a reflected X-ray intensity ratio {211}/{111} in a {211} direction and in a {111} direction relative to a plane in parallel to the steel sheet surface at the depth of the half thickness of the sheet thickness from the steel sheet surface is 1.1 or more. The steel sheet has both high-toughness and strength of API5L-X80 standard or more.
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