NRL-DWT EXTREMELY THICK STEEL HAVING EXCELLENT SURFACE PART NAVAL RESEARCH LABORATORY-DROP WEIGHT TEST PROPERTY

摘要

The present invention relates to an extremely thick steel having a high strength and a manufacturing method thereof, wherein the steel and the method provide excellent physical properties of the surface part naval research laboratory-drop weight test (NRL-DWT). According to one aspect of the present invention, the steel includes, in weight percentage, C: 0.04 to 0.1%, Mn: 1.2 to 2.0%, Ni: 0.2 to 0.9%, Nb: 0.005 to 0.04%, Ti: 0.005 to 0.03%, Cu: 0.1 to 0.4%, P: 100 ppm or less, S: 40 ppm or less, remnant Fe, and unavoidable impurities; and polygonal ferrite having 50 area% or more (inclusive of 100 area%) and bainite having 50 area% or less (inclusive of 0 area%) with the microstructure in a region up to a t/10 position right under the surface (t is the thickness of the steel, same hereafter).