The feature of titanium oxide steel (Ti-O steel) is that heat affected zone (HAZ) toughness is improved due to the refinement of HAZ microstructure through the formation of intragranular ferrite (IGF). This desirable microstructure, IGF, forms radially from titanium oxide particles. Recently, it has been clarified that manganese in Ti-O steel is an indispensable element for the formation of IGF. Therefore, manganese effects on Ti-O steel have been basically studied in this work, and then a new effect has been found. In Ti-O steel, manganese has the effect of suppressing the formation of ferrite side plates (FSP), which are undesirable due to their coarseness. Consequently, HAZ microstructure of Ti-O steel with high manganese content is so refined that HAZ toughness is remarkably improved. Based on the manganese effects, steel plates with excellent HAZ toughness for offshore structures have been developed and commercially mass-produced. The welded joints exhibit excellent toughness.