Chemical Composition Change of Inclusions Acting as Acicular Ferrite Formation during the Ferrite Transformation Process in Low-Oxygen-Containing Weld Metals

Two types of low-carbon steels with different Al contents were used, and low-oxygen weld metals with an oxygen content of 10 ppm were fabricated. Bead-on-plate welding experiments were performed with a tungsten-inert-gas welding system and subsequent water cooling using a welding nozzle that prevents oxygen from entering the molten pool with a double shielding gas. The effect of Al content on the composition of inclusions that act as the starting point for acicular ferrite formation in the early and late stages of the austenite-to-ferrite transformation during cooling after welding was clarified. Optical micrographs and images obtained by a scanning electron microscope equipped with an energy dispersive X-ray spectroscopy detector showed that acicular ferrite formed within austenite grains on inclusions with a chemical composition and size that were considered to be ineffective for acicular ferrite formation in low-Al steel. This suggests that in low-oxygen region, Al uniformly dissolved in the austenite grains has a large effect on the formation of acicular ferrite.