Q345 steel is a low-alloy high-strength steel that is widely used in production. As the solid-state phase transition of Q345 steel is very sensitive to temperature, the microstructure and hardness of joints fabricated with this steel are difficult to predict. Therefore, studying the welding metallurgy, residual welding stress, and welding deformation of Q345 steel joints is essential for improving the safety and service life of Q345 welding structures. In this study, the microstructure and distribution of hardness of a Q345-steel tungsten-inert-gas welded and re-melted joint were calculated using four models encoded in the general finite element software, ABAQUS, and FORTRAN language. Three models were based on only one of three welding continuous-cooling transformation curves of the simulated heat-affected zone (SH-CCT) of Q345 steel, with peak temperatures of 1300, 1100, and 900^oC (hereafter denoted as SH-CCT₁₃₀₀, SH-CCT₁₁₀₀, and SH-CCT₉₀₀, respectively). The final model was based on the associated diagram consisting of these curves SH-CCT diagram above. Comparing the simulation and experimental results, the capabilities and accuracies of the prediction methods based on the different models were investigated. The microstructural calculations of the single SH-CCT diagram agreed with the experimental results only in the local heat-affected zone (HAZ) and largely deviated in the other areas. In the model based on SH-CCT₉₀₀, the relative error of the ferrite volume fraction in the inter-critically HAZ (ICHAZ) was 7.7%. In the model based on SH-CCT₁₁₀₀, the relative errors of the ferrite, bainite, and martensite volume fractions in the fine-grained HAZ (FGHAZ) were 11.3%, 17.3%, and 15.5%, respectively. In the model based on SH-CCT₁₃₀₀, the relative error of the martensite volume fraction in the coarse-grained HAZ (CGHAZ) was 29.6%. In contrast, the microstructural calculations of the associated SH-CCT diagram agreed with the experimental results over the whole HAZ. The results showed that models based on the single SH-CCT diagrams met the tested hardness only in several narrow areas of the HAZ, but the hardness computed by the model, based on the associated diagram, was consistent with the tested hardness, with absolute differences ranging from 0 to 14 HV. The calculated microstructure and hardness in the fusion zone (FZ) greatly deviated from the test results in all the four models, indicating that predictions in the FZ require an accurate SH-CCT diagram of the FZ. In practical welding processes, the corresponding single SH-CCT diagram can adequately predict the microstructure and hardness during the phase transition of one HAZ area (such as the CGHAZ).