Effect of Intercritical Annealing on Microstructure and Impact Toughness of Ultra-Low-carbon Steel after Welding Thermal Simulation

In this paper, the differences of the impact toughness and microstructure of specimens with (IA specimen) and without intercritical annealing (OIA specimen) after welding thermal simulation were compared to illustrate the effect of intercritical annealing. The results show that after welding thermal simulation, the impact energy of IA specimen is 96.6 J, which is much higher than 5.7 J of OIA specimen. This is because after welding thermal simulation, the high-angle (≥45°) grain boundary density of IA specimen is 0.079 μm−1, which is nearly twice as high as that of OIA specimen. On the other hand, the small-sized M-A constituents (maximum 1 μm) and more retained austenite in IA specimen can effectively deflect or prevent the propagation of the crack, helping to obtain high impact toughness.