Effects of Mo Content on Low-Angle Misorientation Microstructure and Mechanical Properties of YS 550 MPa-Grade Steels for Offshore Structures

This study investigates the effects of Mo content on the low-angle misorientation microstructure and the mechanical properties of high strength steels of YS 550MPa grade. Steels of various Mo contents (0.08, 0.15, and 0.25wt%) are fabricated by controlled rolling and accelerated cooling. The Mo steels consist of acicular ferrite (AF), granular bainite (GB), a low-angle misorientation microstructure, that is, bainitic ferrite (BF), and a secondary phase of martensite-austenite (MA), regardless of the Mo content. Degenerated pearlite (DP) is observed only in the 0.08wt% Mo steel. As the Mo content increases from 0.08 to 0.25wt%, the microstructure and mechanical properties are described as follows: (i) the volume fraction of AF decreases and that of BF increases, (ii) the yield and tensile strengths increase and the elongation decreases, and (iii) the Charpy toughness at -60 degrees C decreases owing to the decrease in AF and increase in BF.