Non-heat treated low-yield ratio high-tensile steel sheet and a method of manufacturing the same

摘要

PROBLEM TO BE SOLVED: To provide a non-tempered low-yield-ratio high-strength steel plate preferred for members of buildings and structures.;SOLUTION: The non-tempered low-yield-ratio high-strength steel plate has such a composition that it contains 0.05-0.10% C, 1.2-1.8% Mn, 0.0010-0.0030% S, 0.005-0.020% Ti, and 0.0030-0.0060% N so that it satisfies a Ti/N of 2.0-4.0, it further contains at least one selected from among Cu, Ni, Cr, V, and B, it furthermore contains as impurities elements Nb and Mo provided that Nb is limited to 0.004% or less, and Mo is limited to 0.04% or less, and it satisfies a Ceq of 0.35-0.48; and has such a structure that at least the surface layer has ferrite and one or more selected from among pearlite, bainite, and martensite as a hard phase, and the average crystal grain diameter of the ferrite is 4.0-18.0 μm, furthermore the average hardness of the surface layer is 225 HV or less, and the difference in the hardness between the surface layer part and the center part of the plate thickness is 60 HV or less.;COPYRIGHT: (C)2012,JPO&INPIT