ULTRA THICK STEEL HAVING SUPERIOR BRITTLE CRACK ARRESTABILITY AND METHOD FOR MANUFACTURING THE STEEL

摘要

The present invention provides high-strength ultra-thick steel having superior brittle crack propagation resistance and a manufacturing method thereof. According to the present invention, the ultra-thick steel having superior brittle crack propagation resistance comprises 0.03-0.09 wt% of C, 1.4-2.2 wt% of Mn, 0.2-0.9 wt% of Ni, 0.005-0.05 wt% of Nb, 0.005-0.04 wt% of Ti, 0.1-0.5 wt% of Cu, 0.05-0.5 wt% of Si, 0.01-0.05 wt% of Al, 100 ppm or lower of P, 40 ppm or lower of S, and the remainder consisting of Fe and inevitable impurities. A surface portion has a mixed structure of polygonal ferrite and bainite. A 1/2t ~ 1/4t (t is the steel thickness) thickness portion consists of 50 volume% or higher of acicular ferrite and 50 volume% or lower of bainite. A fraction of an area having a fully bainite structure in the entire steel thickness is 20% or lower. According to the present invention, high-strength ultra-thick steel having superior brittle crack propagation resistance can be provided at high productivity.