| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a thick, high tensile-strength hot-rolled steel sheet, which can manufacture high strength electric resistance welded pipe having X65-grade or higher and has excellent low temperature toughness. <P>SOLUTION: There is provided the thick, high tensile-strength hot-rolled steel sheet comprising, by mass, C: 0.02-0.25%, Si: ≤1.0%, Mn: 0.3-2.3%, P: ≤0.03%, S: ≤0.03%, Al: ≤0.1%, Nb: 0.03-0.25%, Ti: 0.001-0.10% so as to satisfy (Ti+Nb/2)/C<4, and the balance Fe with inevitable impurities. A structure at 1 mm in the sheet thickness direction from the surface is a single phase composed of a bainite phase or a bainitic ferrite phase. A grain boundary cementite has a structure of ≤10% at a ratio of the length of grain boundary cementite to the total length the grain boundary and a sheet thickness is 8.7-35.4 mm. <P>COPYRIGHT: (C)2013,JPO&INPIT |
