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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 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%-0.0080%, 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 ZrO_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.
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 indicates that the chemical constituents of inclusions gradually changed 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% -0.0080%, 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 ZrO 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 co ntent 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.