Phase transformation of CGHAZ and properties of welded joint of Q500qE heavy steel plate for bridges

Heavy steel plate for bridges with maximum thickness of 60mm and yield-strength larger than 500 MPa was produced by thermomechanically controlled process (TMCP). Continuous cooling transformation in the simulated heat-affected zone was investigated. The test steel was multi-pass butt welded using automatic submerged arc welding machine. The microstructure and mechanical properties of welded joint was investigated via scanning electron microscopy, electron backscatter diffraction (EBSD) and instrumented impact testing. The results show that the phase transition temperature increased with the increase of heat input. When the heat input was smaller than 50 kJ/cm, the microstructure of the coarse grained heat-affected zone (CGHAZ) was mainly lath-like Bainite (LB), which has strong impediment to crack propagate due to high fraction of large-angle grain boundaries (51.8%). Excellent combination of strength and toughness was achieved in the welded joint when the heat input was smaller than 50 kJ/cm. The micro hardness of the weld metal and heat-affected zone were higher than that of base metal. The weld metal, fusion line and CGHAZ have different crack propagation energies, which resulted in different low temperature impact performances.