STRUCTURAL STEEL HAVING EXCELLENT LOW-YIELD RATIO AND LOW-TEMPERATURE TOUGHNESS PROPERTIES AND METHOD FOR MANUFACTURING SAME

摘要

The present invention provides a structural steel that comprises, by weight%, 0.05-0.12% of C, 0.05-0.4% of Si, 1.0-2.0% of Mn, 0.01% or less of P, 0.003% or less of S, 0.015-0.04% of Sol.Al, 0.005-0.02% of Ti, 0.01% or less of Nb, 0.002-0.008% of N, and the remainder of Fe and unavoidable impurities, in which a microstructure contains, by area fraction, 90% or greater of polygonal ferrite and 10% or less of segmented pearlite as a second phase, and 2% or less of MA, wherein the polygonal ferrite includes 20-50% of polygonal ferrite having a grain size of 20 µm or greater and 40-70% of polygonal ferrite having a grain size less than 20 µm.