Characteristics of HAZ Microstructure in Ti-B Treated Steel for Large Heat Input Welding

Abstract

Recently the needs for 50 kgf/mm² tensile strength steels with good HAZ toughness under the welding condition of large heat input have been increasing. A new type of 50 kgf/mm² tensile strength steel containing small amounts of Ti and B was developed for this purpose.
The steel is charactarized by the microstructure in the HAZ which consists mainly of the intragranular ferrite plates(IFP) without the high carbon martensite islands and the small amount of the grain boundary allotriomorphs.
The toughness of IFP is very good due to its fine ferrite grain. Complex precipitates of TiN-MnS-Fe₂₃(CB)₆ act as nuclei of IFP and B addition suppresses the growth of grain boundary allotriomorphs. The authors suppose a mechanism that a carbon depleted zone formed around Fe₂₃(CB)₆ assists ferrite nucleation as the reason why IFP is produced effectively by these complex precipitates.