Microstructure transformation and mechanical properties of EH40 shipbuilding plate steel for high heat input welding application

HBIS group developed EH40 shipbuilding plate steel for high heat input welding application by oxide metallurgy technology. The continuous cooling transformation behavior and microstructure transformation under different cooling rates of welding heat affected zone (HAZ) in EH40 shipbuilding plate steel were studied by DIL805 L quenching and phase transformation dilatometer in combination with thermal simulation technology. In addition,thermal simulation experiments were conducted on a Gleeble-3800 thermal simulation testing machine,and then the mechanical properties of HAZ were tested. Simulated HAZ continuous cooling transformation(SHCCT) curves showed that ferrite and pearlite formed when the cooling rate was less than 0.1℃/s. As the cooling rate increased to 0.2 ℃/s,bainite began to precipitate. When the cooling rate was 2～ 3 ℃/s,ferrite/pearlite and granular bainite transformation occurred. When the cooling rate was 5 ～ 10 ℃/s,ferrite/granular bainite transformation occurred. Moreover,with the increase of cooling rate,the proportion of granular bainite increased gradually. Lath bainite formed when the cooling rate increased to 15 ℃/s. When the cooling rate increased to 30 ～ 100 ℃/s,martensite began to generate and the proportion of martensite became increased. Furthermore,the results of welding thermal simulation and impact test showed that the average impact absorbed energy in HAZ of EH40 shipbuilding plate steel was 205 J at-40 ℃ after a simulated welding thermal cycle of 200 k J/cm heat input,which was much higher than the national standard value of 41 J. The scanning electron microscope and energy dispersive spectrometer were adopted to analyze particles precipitated in HAZ of EH40 shipbuilding plate steel,the results illustrated that(Ti,Mn,Si,Mg) O_x-Mn S particles were nucleation sites to promote the formation of acicular ferrite in HAZ,which could effectively improve the low temperature toughness of HAZ.