HIGH-TENSILE STEEL EXCELLENT IN WELDABILITY AND PLASTIC DEFORMABILITY, AND COLD-FORMED STEEL PIPE FORMED THEREFROM

摘要

PPROBLEM TO BE SOLVED: To provide a high-tensile steel which stably satisfies a yield ratio of 85% or less even without being subjected to an SR (stress-relieving)treatment even when subjected to severe deformation processings such as (bending working, tensile working, and compressive working) so as to be applicable to e.g., a cold-formed circular steel pipe, has good toughness and weldability, and has a tensile strength of 490 MPa or more and to provide a low-yield-ratio cold-formed steel pipe obtained therefrom. PSOLUTION: The steel is one having a chemical composition with a specified CEN value defined by the formula: CEN=[C]+A(c)*ä[Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5}, wherein the microstructure of the steel comprises 65-85% by area polygonal ferrite phase, 5-20% by area band-like pearlite phase, and 3-15% by area granular martensitic phase present along ferrite grain boundaries, and the average circle-equivalent diameter of the polygonal ferrite phase is 10-40 m. In the formula, A(c)=0.75+0.25*tanhä20([C]-0.12)}, and [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], and [V] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr, Mo, Nb, and V, respectively. PCOPYRIGHT: (C)2009,JPO&INPIT