HIGH-STRENGTH AND LOW YIELD RATIO STEEL MATERIAL WITH EXCELLENT THICKNESS DIRECTION CHARACTERISTICS, AND MANUFACTURING METHOD THEREOF

摘要

A high-strength and low yield ratio steel material with excellent thickness direction characteristics according to the present invention comprises: by weight%, C of 0.03 to 0.10%; Si of 0.01 to 0.6%; Mn of 1.5 to 3.0%; P of 0.02% (excluding 0) or less; S of 0.01% (excluding 0) or less; Al of 0.005 to 0.5%; Nb of 0.005 to 0.1%; B of 5 to 40 ppm; Ti of 0.005 to 0.1%; Cr of 0.1 to 1.0%; Ni of 0.01 to 1.5%; N of 15 to 150 ppm; and the remaining including Fe and inevitable impurities. A microstructure of the center portion includes, by area%, a second phase such as bainitic ferrite of 50 to 80%, granular bainite of 20 to 50%, and island-shaped martensite (M.A) of 5% or less (including 0%). A grain aspect ratio of the bainitic ferrite is 0.4 or less. An average grain diameter of the granular bainite (initial austenite average grain diameter) is 50 m or less. There are 100 or fewer inclusions of 1 m or more per 1 mm^2 at the center thereof. The thickness is 60 mm or more.;COPYRIGHT KIPO 2017