Yoshihide Nagai

Advanced Titanium Oxide Steel With Excellent HAZ Toughness for Offshore Structures

The feature of titanium oxide steel (Ti-O steel) is that heat affected zone (HAZ) toughness is improved due to the refinement of HAZ microstructure through the formation of intragranular ferrite (IGF). This desirable microstructure, IGF, forms radially from titanium oxide particles. Recently, it has been clarified that manganese in Ti-O steel is an indispensable element for the formation of IGF. Therefore, manganese effects on Ti-O steel have been basically studied in this work, and then a new effect has been found. In Ti-O steel, manganese has the effect of suppressing the formation of ferrite side plates (FSP), which are undesirable due to their coarseness. Consequently, HAZ microstructure of Ti-O steel with high manganese content is so refined that HAZ toughness is remarkably improved. Based on the manganese effects, steel plates with excellent HAZ toughness for offshore structures have been developed and commercially mass-produced. The welded joints exhibit excellent toughness.Copyright

Production of High-Strength and High-Toughness Steel for Offshore Structures

By the utilization of new fine particles, steels refining the heat affected zone (HAZ) structure have been developed. This has made possible the mass-production of offshore structural steels which have high-strength of 460–500N/mm2 in yield strength (YS) and excellent crack-tip opening displacement (CTOD) properties. These newly developed steels for offshore structures have been mass-production to over 45,000 tons. The new particles are dispersed finely in the developed steels and show a strong pinning effect. The newly developed steels also use the intragranular ferrite (IGF) like titanium oxide bearing steel (TiO steel). In mass-production heats, γ grain size is restricted below 200 μm under conditions of 1400°C for 60 seconds. The improvement of HAZ toughness and the utilization of the thermo-mechanical control process (TMCP) has restrained the increase in carbon equivalent (Ceq,PCM ) far as possible. By application of these technologies, it has been possible to mass-produce steels with high-strength, excellent CTOD properties and good weldability. As for steel plates of YS500N/mm2 , the production of steel plates up to 70mm in thickness was carried out. The properties of welded joints for the 70mm-thick plates of YS500N/mm2 steel have been evaluated under heat input in 0.7kJ/mm of flux-cored are welding (FCAW) and 3.5kJ/mm of sub-merged arc welding (SAW). They have shown excellent CTOD and Charpy impact properties at welded joints; and have moreover indicated good weldability when PCM has been restricted to 0.22% or lower.Copyright