The effect of trace elements on the microstructure and properties of coarse grain heat affected zone of EH36 ship steel with super large heat input

The influence of Al, Mg, and Ti element content on the microstructure and properties of the coarse grain zone in the heat affected zone of EH36 high-strength ship steel during large heat input welding was studied through welding thermal simulation. The relationship between Al, Mg, and Ti element content and oxide type, size, quantity, and coarse grain zone phase transformation in the base material was revealed using Thermo-Calc thermodynamic calculation combined with SEM and EDS testing. The results showed that Al₂O₃ couldn`t induce the transformation of acicular ferrite. When the mass percent of Al element was below 0.005%, Ti, Mg or their composite oxides could be formed in the steel, which could promote the transformation of acicular ferrite in the coarse grain area. When Ti and Mg were added together, and the mass percent of Mg element decreased from 0.0042% to 0.0013%, the oxide type changed from MgO to Mg₂TiO₄, the number of oxides in 20 fields of view increased from 408 to 503, the average diameter of the oxide reduced from1.37 μm to 1.10 μm. The specific surface area of non-uniform nucleation was significantly increased, the formation of grain boundary ferrite was suppressed, and the impact energy of the thermal simulation sample at −20 ℃ in the coarse grain zone was increased from 43 J to 127 J at t_8/5=300 s.