Effect of Thermomechanical Treatment on Acicular Ferrite Formation in Ti-Ca Deoxidized Low Carbon Steel

Transformation behaviors and mechanical properties under thermomechanical treatment conditions of Ti-Ca deoxidized low carbon steel were studied in comparison to Al-Ca treated steel. A thermomechanical simulation and a hot rolling experiment were carried out. Inclusions and microstructures were characterized, and the transformation mechanism was analyzed. The results indicated that typical inclusions in Ti-Ca deoxidized steel were TiOx-MnS-Al2O3-CaO, TiOx-MnO-Al2O3-CaO, and TiOx-MnS, which were effective for acicular ferrite (AF) nucleation. Acicular ferrite formation temperature decreased with an increase in cooling rate. A fine AF dominant microstructure was formed under a high driving force for the transformation from austenite to ferrite at lower temperatures. A high deformation of 43-65% discouraged the formation of acicular ferrite because of the increase in austenite grain boundaries serving as nucleation sites. The fraction of high-angled grain boundaries that acted as obstacles to cleavage cracks was the highest in the sample cooled at 5 degrees C/s because of full AF structure formation. The hardness increased significantly as the cooling rate increased from 2 to 15 degrees C/s, whereas it decreased under the condition of deformation because of the formation of (quasi-)polygonal ferrite. By applying accelerated water cooling, the mechanical properties, particularly impact toughness, were significantly improved as a result of fine AF microstructure formation.