Formation Of Intragranular Acicular Ferrite Research Articles

Nucleation of Intragranular Accicular Ferrite in a Ti-Containing Low Carbon Steel of High N Content

Formation of acicular ferrite grains by dispersing non-metallic inclusions which can act as intragranular nucleation sites within austenite grain is proposed as a method to obtain refined grain structure in steels. The microstructure of acicular ferrite steel consists of fine interlocking ferrite plates nucleated on dispersed non-metallic inclusions. This acicular ferrite structure provides a desirable combination of high strength and good toughness because of its small plate thickness and interlocking microstruture. in Ti-containing low carbon steels, researchers proposed that Ti2O3 or MnS, among inclusions, is the most effective nucleant for intragranular ferrite because of the formation of Mn depleted zone(MDZ) around Ti2O3 or MnS particles. TiN is also reported as a nucleation site owing to the coherency between ferrite and TiN crystals. But It has not been a simple task to identify which inclusions and what mechanisms are really effective in the formation of intragranular acicular ferrite.

The Formation of Intragranular Acicular Ferrite in Simulated Heat-affected Zone.

The preferential conditions of intragranular acicular ferrite (IAF) in heat-affected zone were studied by adding sulfur and calcium into a Ti-killed steel. Sulfur and calcium addition refined the austenite grain due to the increase in the number density of inclusions. An intermediate austenite grain favored the formation of IAF due to the prevention of Widmanstatten ferrite and polygonal ferrite. The inclusions changed from Ti-oxides dominant to Ti-oxysulfides dominant and Ti-Ca-oxysulfides dominant due to small addition of sulfur and calcium. The nucleation potential of inclusions was found to increase in the sequence of Ti-oxides, Ti-oxysulfides, and Ti-Ca-oxysulfides. This phenomenon could be attributed to the increase of the lattice disregistry between inclusions and ferrite matix or the decrease of strain field around inclusions. Both effects prevented the transformation of austenite to ferrite at elevated temperatures, thereby promoting the formation of IAF. The highest volume fraction of IAF was, therefore, obtained at an intermediate sulfur content with small amount of calcium due to the achievement of optimum nucleation potential and grain size.

Development of TiO x-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 x inclusions 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.