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To research the effect of Zr addition on inhibiting austenite grain growth of Ti-bearing low carbon steels,two steels with different Zr contents were prepared using a laboratory vacuum induction furnace.The performance of HAZ under weld thermal simulations was investigated.The impact toughness,microstructure and the second-phase particle performance of HAZ under weld thermal simulations were investigated.The HAZ toughness was improved from 13 J to 87 J by addition of 0.010 ％ Zr into the steel,with the fracture mechanism changing from cleavage fracture to toughness fracture,which was mainly attributed to the second-phase particles that were potent to nucleate acicular ferrite in HAZ during welding.It was concluded that the second-phase particles TiOx + MnS,ZrO2 + MnS or TiOx + ZrO2 + MnS were nucleated on ZrO2 or TiOx (x =1.5,2 ).This method can be applied to grain refinement by promoting the acicular ferrite formation and growth during large-scale welding,as in the cases of thick steel plates requiring higher heat inputs during welding.