Abstract
Through the study of a chemical composition system capable of assuring high strength without impairing HAZ toughness, 590-MPa high tensile strength steel for offshore structure was developed, and its HAZ toughness improvement mechanism was clarified. The high-copper and ultra-low niobium steel provides superior HAZ toughness with medium heat input in -60°C Charpy test and -30°C CTOD test and with high input in -60°C Charpy test. Why HAZ toughness comparable to that of 490-MPa high tensile strength steel is obtained may be considered as follows. The precipitation of copper occurs much later than other precipitation-hardening elements like niobium and vanadium. Copper precipitated by tempering for strengthening goes into solid solution in the heating process of the HAZ. Copper precipitates little on cooling (and heating) during the subsequent welding thermal cycle. The embrittlement of HAZ by the precipitation hardening of copper thus does not take place. This allows the 590-MPa steel to be welded without preheating and cracking, just like 490-MPa steel.
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