Abstract
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 °C/s because of full AF structure formation. The hardness increased significantly as the cooling rate increased from 2 to 15 °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.
Highlights
Intragranular acicular ferrite (AF) is a preferred type of microstructure that has great potential for improving steel strength and toughness because of its relatively high dislocation density, fine-grained structure, and ability in arresting cleavage crack propagation
Mechanism of AF Nucleation Promoted by Ti–Ca Oxide Inclusions
The results indicated that suitable rolling deformation and accelerated cooling can significantly improve acicular ferrite formation and steel performance
Summary
Intragranular acicular ferrite (AF) is a preferred type of microstructure that has great potential for improving steel strength and toughness because of its relatively high dislocation density, fine-grained structure, and ability in arresting cleavage crack propagation. In order to make use of the advantage, the concept of oxides metallurgy was proposed three decades ago [1] and focused on introducing particular nonmetallic inclusions in steel to promote AF transformation in welding heat-affected zones. Thereafter, oxide metallurgy technology was widely considered and researched. There have been many reported inclusion types effective for AF nucleation, such as Al-Mg-Zr-O, Ti-Al-Mn-O-S, Ti-Al-O-N, Ti-Mn-Al-Si-O-S-N, Mn-S-V-C-N, etc. Among the research relating inclusion-induced AF nucleation, effective particles have mainly been obtained by means of steel melt deoxidization [2,10], adding oxide powder into steel liquid Some comprehensive reviews were made on the influence of inclusion composition on AF formation potency [7,8,9], in which Ti- and Mn-containing complex inclusions were described as effective nucleating sites.
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