Evolutions of Microstructure and Mechanical Properties of the Welding Heat-affected Zone in 800 MPa-grade Complex Phase Steel

A thermomechanical simulator Gleeble 3800 was used to simulate the thermal cycles experienced by various heat-affected zones (HAZ) during the welding process. The influence of peak temperature (Tp, 500°C–1320°C) on the hardness, microstructure, precipitates, and properties of complex steel 780FB with microalloyed elements Ti, Nb, and V was systematically studied. The contributions of dislocation strengthening, precipitation strengthening, fine grain strengthening, and phase transformation strengthening increments to strength changes of samples after different thermal cycles were quantified, and the calculated results were found to be consistent with the experimental data. Compared with 780FB, there was little change in microstructure and properties when Tp was 500°C. When Tp was 650°C, the increase in VC density from 43/µm2 to 288/µm2 caused the enhancement of hardness and strength. The precipitation strengthening increment (49.84 MPa) played a dominant role in strength improvement. As partial bainite in the microstructure of 780FB transformed into ferrite at Tp of 800°C, the weakening of phase transformation strengthening (−57.5 MPa) became the main factor in strength change. The softening and strength reduction further increased when Tp was up to 980°C, as 780FB completely recrystallized and transformed into ferrite and MA islands. The phase transformation strengthening further reduced by 74.75 MPa. When Tp was 1320°C, the VC density decreased from 43/µm2 to 13/µm2, and the (Ti,Nb)C density decreased from 34/µm2 to 14/µm2, leading to severe grain growth (2.24 µm to 19.89 µm) and bainite transformation. The decrease in precipitation strengthening (−26.86 MPa) and fine grain strengthening (−87.91 MPa) counteracted with the increase in phase transformation strengthening (51.62 MPa), resulting a slight decrease in hardness and strength.