Development of 550 MPa Yield Strength Steel Plate for Offshore Structures

Abstract

Offshore structure steel with high strength of YS550MPa has been investigated. As for offshore structure steel, high toughness in welded joints is required in addition to that in base metal. TMCP type steel of up to YS420MPa grade is used widely, and up to YS500MPa grade is reported in some papers. However, steel of higher strength grade with good toughness and weldability will be beneficial to structures in strict conditions. To reach the YS550MPa requirement, hardening effect by Cu precipitation was utilized. Steel plates were designed with micro-alloyed low C-Mn-Cu-Ni-Cr-Mo system. The combination of the copper precipitation and TMCP technology can increase strength without deteriorating toughness and weldability. Heat treatment for Cu precipitation was carried out to optimize the balance of strength and toughness of the base metal. The developed steel also shows good HAZ CTOD toughness up to 76.2mm thickness in several welding conditions including after PWHT. The newly developed steel has the possibility to increase the flexibility to design large-sized structures.

Effect of magnesium addition in low-carbon steel part 1: behaviour of austenite grain growth

“…To further explain the effect of Mg on the growth of austenite grains in other steels, Suito et al [10][11][12] further studied the effects of MgO and soluble Mg on the austenite grain growth in Fe-0.05%C alloys and Fe-0.20massC%-0.02mass%P; and they also found that similar results. In 2001, Kojima et al [13] compared the grain size of austenite in HAZ between the Ti-containing pipeline steel and Mg-containing new steel, and they found that the grain size of austenite after Mg addition was greatly decreased. In the Ti-containing steel, the austenite grain size is 500 μm, and in the Mg-containing steel, the average grain size of austenite is less than 200 μm.…”