Influence of Ti-rare earth addition on microstructure and toughness of coarse grain heat-affected zone in C-Mn steel

The influences of rare earth (RE) and Ti-RE complex treatment on the microstructures and toughness in the coarse grain heat-affected zone (CGHAZ) of C-Mn steel were investigated using a Gleeble-1500D thermal simulator with input heat energy of 65 kJ·cm^-1. The inclusions and impact fracture morphology were observed and analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX), and the microstructure was characterized using an optical microscope (OM). The results show that the microstructures in the CGHAZs of RE-and Ti-RE-treated samples are mainly grain boundary ferrite (GBF) + polygonal ferrite (PF) + intragranular acicular ferrite (IAF) and grain boundary ferrite + intragranular acicular ferrite, respectively. Inclusions in the RE-treated specimen are La₂O₂S + Mn-Al-Si-O + MnS, while those in the Ti-RE-treated specimen are La₂O₂S + TiO_x + Mn-Al-Si-O + MnS. The complex inclusions in Ti-RE-treated steel are smaller than those in RE-treated steel, and the size of IAF in Ti-RE-treated steel is smaller. The impact properties of the CGHAZ in Ti-RE-treated C-Mn steel is improved significantly, and the material is tougher than RE-treated steel.