INFLUENCE OF SECOND--PHASE PARTICLES CONTAINING Ti ON MICROSTRUCTURE AND PROPERTIES OF WELD--HEAT--AFFECTED--ZONE OF A MICROALLOY STEEL

Abstract

The effects of Ti-containing second-phase particles on the microstructure and properties of simulated welding heat affected zone(WHAZ) in a microalloy steel have been systematically investigated.The results show that the Ti deoxidized steel has a better low temperature impact toughness of simulated WHAZ than the Al deoxidized steel even in a longer phase change cooling time (t_(8/5)=120 s,correspond to a welding heat input of 100 kJ/cm).The role of Ti-containing second-phase particles on improving the properties of WHAZ is mainly manifested in the following two aspects.First, there is a large number of Nb-containing TiO_x-MnS particles with the size about several hundreds nanometers and Ti oxide containing NbC particles with the size less than 50 nm dispersed in the Ti deoxidized steel,these two kinds of small second-phase particles restrain the growth of austenite grain and refine the microstructure of WHAZ.Second,TiO_x-MnS composite particles with the size between 1μm to 3μm can induce the nucleation of intragranular acicular ferrite(IAF) which could divide the prior austenite grain and refine the microstructure effectively.As a result of the above two effects,the low temperature toughness of HAZ in Ti deoxidized steel is significantly improved compared with the Al deoxidized steel.