Effect of Zr on second-phase particle and impact toughness in the heat-affected zone of high-strength steel

The effect of 0.012 4% Zr addition on second-phase particle and impact toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steel was investigated. Results shown that second phase particles were Al-Ti composite oxides and (Ti, Nb)N precipitates in the CGHAZ of Zr-free steel, whereas, Zr-Al-Ti composite oxides and (Al, Ti, Nb)N and (Ti, Nb)N precipitates were obtained in Zr-bearing steel under the simulation with 20 kJ/cm input welding thermal cycles. Quantitative data analysis revealed that the oxides and nitrides in Zr-bearing steel had higher density and were finer than that in Zr-free steel. These high density fine second phase particles effectively pinned the grain boundary movement in the welding process and inhibited austenite grain coarsening. Small and uniform austenite grains were obtained in the CGHAZ in Zr-bearing steel. Thus, the CGHAZ in Zr-bearing steel exhibited ductile fracture and excellent low temperature impact toughness.