Abstract: The addition of a certain amount of Zr element to marine engineering steel promoted the formation of specific non-metallic inclusions to induce the nucleation of acicular ferrite, thereby refining the grain structure and enhancing the impact toughness of the welding heat-affected zone under high-heat-input welding conditions. Scanning electron microscopy, high-temperature confocal microscopy, and optical microscopy were utilized to observe the transformation of inclusions  the nucleation of acicular ferrite on inclusion surfaces, the temperature variation during acicular ferrite nucleation, and the changes in the proportion of various microstructures after cooling in different samples under varying Zr content treatments. The electron backscatter diffraction (EBSD) system was employed to statistically analyze the distribution of grain boundary angles in each sample. Experimental results indicated that with an increase in Zr content, the predominant inclusion types in the steel shifted from Al₂O₃-TiO_x-MnO-MnS to Al₂O₃-TiO_x-ZrO₂-MnS-(TiN). When the Zr mass fraction reached 120 ×10-6, the steel exhibited a higher presence of monomeric ZrO₂. In situ observations revealed that as the Zr content in the steel increased, the onset temperature for the transformation of acicular ferrite decreased from 509 ℃ to 484 ℃, the nucleation temperature range increased from 57 ℃ to 71 ℃, and the area fraction of acicular ferrite initially increased from 26% to 36% before showing a decreasing trend. EBSD results suggested that adding of 0.002% mass fraction of Zr element could achieve a higher proportion of large-angle grain boundaries, indicating that a certain amount of Zr element can promote the formation of Zr-containing composite non-metallic inclusions to induce the nucleation of acicular ferrite and increase the temperature range for acicular ferrite nucleation.

Key words: Zr treatment, acicular ferrite, in-situ observation, EBSD