A study was carried out on the transformation characteristic of a newly developed Ti-O steel in comparison with that of ordinary steels. The microstructure of the weld heat affected zone (HAZ) in the Ti-O steel is characterized by the formation of intragranular ferrite plate (IFP), or so-called acicular ferrite, which was found to start from titanium sesquioxide (Ti2O3) particles dispersed in the matrix of the steel. The welding thermal cycle simulation test on the Ti-O steel showed that IFP development takes place over a wide range of conditions such as heating temperature and chemical composition. The high stability of Ti2O3 particles at high temperature explains the effect of the former condition. The effect of the latter can be well understood from the distribution of Ti2O3 particles, which is irrelevant to the microsegregation of alloying elements such as manganese and niobium. IFP formation restricts development of ferrite side plate, refines the effective grain size, and improves HAZ toughness. Various experimental results suggested that the Ti2O3 particle has an IFP formation function through the manganese sulfide (MnS) attached on it.

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