Effects of Ti and Al on the formation of intragranular ferrites in the ultra-low-oxygen Si-Mn weld metals of low-carbon steel

Abstract

Studies on intragranular ferrite with ultra-low O content are scarce, and establishing a technology for the refinement of the microstructure of a weld metal with ultra-low O content through the formation of intragranular ferrite is challenging. In this report, the effect of addition of Ti and Al on the formation of intragranular ferrite in ultra-low-oxygen weld metals was studied. The formation process of the oxide was discussed based on the detailed observation of the oxides and thermodynamic calculations. In the weld metal of Ti added steel with the highest intragranular ferrite area fraction, it was observed that the significant Mn depleted zone (MDZ) formed around a complex oxide containing Ti, Mn and Si. The formation of the MDZ increases the transformation temperature around the oxide and promotes the formation of intragranular ferrite with the oxide as a nucleus. Since the increase in Mn content at equilibrium due to the change in the stable phase during cooling is larger for the inclusion in Ti added steel, it was considered that more Mn diffused from the matrix to the inclusion in Ti added steel and the MDZ formed at the interface between matrix and the inclusion. The promotion of intragranular ferrite formation in an ultra-low-oxygen weld metal is enhanced by oxides with a large increase in the equilibrium Mn content during cooling.