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Effects of Zirconium on the chemical component and size distribution of Ti-bearing inclusions, favored the grain refinement of the welding induced, coarse-grained heat affected zone (CGHAZ) with enhanced impact toughness in Ti-killed steels, which were examined based on experimental observations and thermodynamic calculations.It indicated that the chemical constituents of inclusions gradually varied from the TiO oxide to the Ti-O+Zr-O compound oxide and a single phase of the ZrO2 oxide,as the Zr content increased from zero to 0.0100%. A trace of Zr (0.0030%-0.0080%,depending on the oxygen content in liquid steel) provided a large amount of nucleating core for Ti oxide because of the larger specific density of ZrO2 oxide, and produced a small size distribution of the inclusions favorable for aeieular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of aeicular ferrite was obtained in the CGHAZ, with enhanced impact toughness. Otherwise, a high content of Zr (～0.0100%) produced a single phase ZrO2, which was impotent to nucleate acicular ferrite, and a microstructure composed of ferrite side plate and grain boundary ferrite developed in the CGHAZ. The experimental results were confirmed by thermodynamic calculations.