High-strength steel of the 490-MPa class strength for offshore structures was developed by making the most of a TiN-MnS complex precipitate effective that is over a wide welding heat input range. At the same time, its HAZ toughness improvement mechanism was clarified. The TiN-MnS system provided superior HAZ toughness with a medium heat input in -60°C Charpy test and -50°C CTOD test and with a high heat input in -60°C Charpy test. This is probably attributable to the TiN compound that pins austenite grains and to the TiN-MnS complex precipitate that produces fine intragranular ferrite (IGF) and reduce the fracture facet unit after welding. IGF may be formed either by increase in driving energy for nucleation in the region where manganese is depleted by the MnS precipitation or decrease in interfacial energy with nucleation by the crystal coherency of TiN and ferrite. The presence of manganese-depleted zone is an indispensable mechanism and is complemented by the crystal coherency of TiN and ferrite.