Abstract
The objective of this study is to develop heavy gauge 9% Ni steel plates for large scale LNG strage tanks. At first, influences of chemical compositions upon the strength and toughness were investigated using laboratory heated 9% Ni steels. It was clarified that lowering of C content (0.03%) and addition of Mo (0.1%) improve HAZ toughness without reduction of strength comparing with the conventional 9% Ni steels.
On the basis of the laboratory study, low-C and Mo-added type 9% Ni steel plates with thickness of 20-60mm were manufactured on a commercial scale. A series of material tests were performed and those results showed that the developed steel plates achieved the sufficient strength and toughness even in the case of 60mm in thickness. It was also confirmed that the welded joints of these steel plates had superior toughness to conventional plates especially in HAZ-end region. Gritical CTOD values in the HAZ- end exceeded 0.5mm at-196°C even if pop-in occured. The superiority in HAZ-end toughness was attributable to low C content of the steel.
