Abstract: Weld thermal simulation experiments of F40 ship plates with 0.01% zirconium or not under different phase-change cooling time T_8/5 were conducted on a Gleeble3500 thermal simulation test machine.The results show that the anti-large heat input welding performance is improved significantly for F40 ship plates containing titanium by adding trace amounts of zirconium,and the impact energy reachs 238J at-60℃ when T_8/5 is 100s.The microstructure of the coarse grain heat affected zone(CGHAZ) in the F40 ship plates was studied by thermodynamic calculations and scanning electron microscopy.It is indicated that the size and mophyloy of original complex inclusions containing zirconium change with increasing T_8/5.Therefore the number of the inclusions of 1 to 3 μm including zirconium which contribute to acicular ferrite nucleation non-monotonically decreases,leading to the toughness fluctuation of the CGHAZ with the welding heat input increasing.