HIGH-STRENGTH HOT-ROLLED STEEL SHEET FOR LINE PIPE EXCELLENT IN LOW-TEMPERATURE TOUGHNESS AND DUCTILE-FRACTURE-STOPPING PERFORMANCE AND PROCESS FOR PRODUCING THE SAME.

摘要

A hot-rolled steel sheet (hot coil) for line pipes which has a high strength not lower than the API5L-X80 standard and combines low-temperature toughness with ductile-fracture-stopping performance; and a process for producing the hot-rolled steel sheet. The hot-rolled steel sheet has a composition containing C, Si, Mn, Al, N, Nb, Ti, Ca, V, Mo, Cr, Cu, and Ni in respective amounts in given ranges, the remainder being Fe and incidental impurities.  The steel sheet has a microstructure which is a continuous cooling transformation structure.  The continuous cooling transformation structure contains, dispersed therein, a niobium-containing precipitate having an average diameter of 1-3 nm and an average density of (3-30)×1022 particles per m3, and has a content of granular bainitic ferrite and/or quasi-polygonal ferrite of 50% or higher.  The transformation structure further contains a titanium nitride-containing precipitate which has a size of 0.1-3 µm in terms of average equivalent-circle diameter, at least 50% by number of the precipitate particles containing a composite oxide containing Ca, Ti, and Al.