Relationship Between Impact Toughness and Microstructure for the As-Rolled and Simulated HAZ of Low-Carbon Steel Containing Ti-Ca Oxide Particles

In this study, the microstructure and mechanical properties of Ti-Ca deoxidized low-carbon steel compared with Al-Ca deoxidized steel were investigated. Hot rolling experiment and high heat input welding heat-affected zone (HAZ) simulation were carried out. In situ transformation behavior and crack propagation during impact in simulated HAZ were analyzed. The results indicated that interlocking acicular ferrite (AF) was the dominant microstructure after Ti-Ca deoxidation treatment, and TiO_x-CaO-Al₂O₃-MnS-type complex inclusion with maximum proportion was effective for AF nucleation. However, Al₂O₃-CaO-(Ca, Mn)S-type inclusion that was dominant in Al-Ca steel exhibited significantly lower ability for AF nucleation. The high ratio of high-angle grain boundaries/low-angle grain boundaries in AF microstructure hindered the initiation and propagation of crack. The stress concentration in the vicinity of crack tip can be released because of superior deformability of AF structure as compared to bainite packets. By introducing Ti-Ca oxide particles, the mechanical properties of hot-rolled steel plate and HAZ, particularly impact toughness, were significantly enhanced because of the formation of fine AF.