The strength and the toughness required for steel plates used for offshore structures became higher as the installation areas move into arctic areas. The main property of offshore structure steel is the crack tip opening displacement (CTOD) property of weld joint, and CTOD testing is performed at the minimum design temperature of the structure. Thus, the demand for satisfying −40°C of CTOD test temperature specification has increased. For the improvement of HAZ toughness, coarse austenite grain is suppressed by TiN, and low-C, Ceq, Si, P, Nb design is adopted to decrease the formation of M-A constituents. Furthermore, by using Ca inclusion, which works as a pinning particle and a bainite nucleation site, very fine bainite microstructure are formed in HAZ and excellent low temperature toughness are achieved. The YP420 class plate with excellent low temperature toughness has been developed using these technology.

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