The work concerns an analysis of phase transformations in the solid state and the prediction of the structure and mechanical properties of steel using analytical models and numerical methods. The analytical models involve building simplified Continuous Cooling Transformations (CCT) diagrams for welding, predicting the microstructure based on the chemical composition of steel, as well as assessing the mechanical properties of the welded joint made of S1100QL steel. The kinetics of phase transformations and prediction of mechanical properties distribution in the cross-section of the joint are carried out based on analytical methods. The analytical models presented in this work replace the classic mathematical models of phase transformation kinetics and interpolated experimental CCT diagrams (time-temperature-transformation). The analytical relationships presented in the work are determined by the chemical composition of steel and the cooling time t_8/5. Based on the chemical composition of the steel, the CCT diagrams and the specific the volume fractions of phases as a function of the cooling time t_8/5 of the steel are determined. The numerical simulation of the welding process of a sheets made of S1100QL steel is carried out in ABAQUS software using DFLUX and HEATVAL numerical subroutines. The calculations use a mathematical model of a Goldak’s volumetric welding source power distribution. Thermal cycles and temperature field are numerically determined. The research results obtained in this work are compared with the experimental results.