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 Ti-O oxide to the Ti-O+Zr-O compound oxide and a single phase of the ZrO 2 oxide, as the Zr content increased from zero to 0.0100\%. A trace of Zr (0.0030 %--0.0080n %, 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 ZrOn 2 oxide, and produced a small size distribution of the inclusions favorable for acicular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of acicular ferrite was obtained in the CGHAZ, with enhanced impact toughness. Otherwise, a high content of Zr (~0.0100 %) produced a single phase ZrO 2, 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.
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