Development of TiOx-bearing steels with superior heat-affected zone toughness

TiO_x steels with superior toughness of the heat-affected zone (HAZ) and applicable to a wide range of welding heat inputs have been developed using advance metallurgical techniques. A promising practice in industrial production is adopting a Ti-killing process in which ferro-titanium alloys are utilized as killing agents in an Al-free molten steel. This process creates Ti-oxides with various crystalline structures (referred to as TiO_x) in a steel matrix. These TiO_xinclusions improve the toughness of coarse-grained HAZ by promoting the formation of intragranular acicular ferrite (IAF), which can section an austenite grain into several colonies and refine the effective grain size to fracture resistance. The volume fraction of IAF is closely related to the number of TiO_x inclusions, inclusion diameter, austenite grain size and the hardenability of steels. The favourable conditions for the formation of IAF and the concept of process control are discussed.