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

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.

A Comparative Study of Acicular Ferrite Transformation Behavior between Surface and Interior in a Low C–Mn Steel by HT-LSCM

“…content, which contributed to promoting AF nucleation [27]. In the present experimental steel, Ti oxide was the main inclusion type and was considered to be effective through the MDZ mechanism.…”

“…The formation of MDZ caused a decrease in Mn content and an increase in transformation temperature, which promoted intragranular AF nucleation around the inclusion [25,26]. In addition, the driving force for ferrite formation, i.e., the Gibbs free energy change for γ→α, increased with a decrease in Mn content, which contributed to promoting AF nucleation [27]. In the present experimental steel, Ti oxide was the main inclusion type and was considered to be effective through the MDZ mechanism.…”

Comparison of Properties and Bead Geometry in MIG and CMT Single Layer Samples for WAAM Applications

“…The hardness values from existing work in the field have been observed to respond to differences in heat inputs, and consequently, cooling rates by changes to the composition of the microstructures of WAAM fabricated steels. Wang et al found that Hv values appreciated with increasing cooling rate from of acicular ferrite forming at a lower temperature with dislocation density increasing [37]. Yildiz also commented on the changes from low to high heat input with decreasing volume fraction of bainitic and martensitic microstructural components and replaced by acicular ferrite [9].…”

Effect of Heat Input on Microstructure and Impact Toughness of Coarse‐Grained Heat‐Affected Zone in Al–Ca and Ti–Ca Killed Steels

“…However, for containing Ti–Ca or Ti–Ca–Al oxide particles, the aforementioned studies mainly focus on the evolution and characterization of inclusions. Although our team determined that Ti–Ca oxides can facilitate AF nucleation and enhance mechanical properties of steel base metal, the influence of different heat input on the toughness and microstructure of CGHAZ containing Ti–Ca or Ti–Ca–Al oxide particles has not been studied. The relationship between AF nucleation and CGHAZ toughness, with increased heat input, needs to be evaluated.…”