PRECIPITATION BEHAVIOR IN ULTRA-LOW-CARBON STEELS CONTAINING TITANIUMAND NIOBIUM

This work revealed-the basic mechanism for the stabilization of carbon in ultra-low-carbon (ULC) steels that contain moderate S (0.004 to 0. 010 wt pct), adequate Ti (0.060 to 0.080), and low Mn (less than or eq ual to 0.20); During cooling through the austenitic region to the ferr itic, the initially formed sulfide particles (TiS) undergo an in situ transformation into carbosulfides (H-Ti4C2S2) by absorbing C and Ti. T he transformation from TiS to H may be considered as a hybrid of shear and diffusion, i.e., faulted Ti8S9 (9R) + 10[Ti] + 9[C] --> 41/2Ti(4) C(2)S(2) (H). At low temperature (less than or equal to 930 degrees C) , the stabilization process continues through epitaxial growth of carb ides on H phase,i.e., [M] + x[C] + H --> epitaxial MCx (on H). This me chanism differs from the traditional view of stabilization, where the carbon is removed from solution by the formation of free-standing or i ndependently nucleated H and/or MCN precipitates. While these two form s of carbon stabilization are now well known, this article presents a method of predicting which mechanism of stabilization will be operativ e in a: given steel based on its bulk composition. Implications bearin g upon new ULC steel design, considering the role of S, will be discus sed.