Abstract
Influence of Ti and N on strength and toughness of base plate and HAZ in 780MPa class steels with ultra-heavy thickness of over 150mm was examined using a thermal-cycle simulator.
(1) Relationship between Ti content and mechanical properties of the base plate under as-quenched condition varies depending on N content. While, toughness of medium-N steels (N = 40ppm) is improved by adding Ti due to microstructural change, toughness of low-N steels (N = 20ppm) deteriorates with increasing Ti content due to precipitation hardening by TiC.
(2) Influence of Ti and N on simulated HAZ toughness is more significant for double cycle condition than that for single cycle. Ti addition gives remarkable improvement in simulated HAZ toughness of the medium-N steels for double cycle condition. On the other hand, simulated HAZ toughness of the low-N steels deteriorates with increasing Ti content. The governing factors of the simulated HAZ toughness in double-cycle condition are the same as those of the toughness of base plate.
(3) It is concluded that the low-N steel without Ti addition and medium-N steels with Ti addition of 0.005 0.01% attain both high simulated HAZ toughness and high strength and toughness of base plate.
