Recently, the installation areas of offshore structures has expanded into deep sea areas, and as a result, strength and plate thickness of the steel plate required for the offshore structures have become higher. And the main property of offshore structure steel is the crack tip opening displacement (CTOD) property of multi-pass weld joint. Thus, the developed steel was designed to improve HAZ toughness of multi-pass weld joint. Firstly, in order to achieve both high strength of the base metal and good HAZ toughness, the TMCP technology was applied, and Ceq. was minimized. And to obtain fine HAZ microstructure, fine TiN and Ca(O,S) particles having pinning effect and nucleation site effect were used. Finally, to decrease the formation of brittle martensite-austenite constituent (M-A) in HAZ of multi-pass weld joint, Si content was decreased. By using above technologies, the YS 500MPa class thick steel plate with good weld joint CTOD property have been developed.

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