Abstract: How to effectively predict the effect of different process conditions on microstructure and mechanical properties is one of important tasks that many iron and steel companies are rather concerned about. On the basis of the theory of free energy during the continuous cooling process, a set of unified phase transformation constitutive equations are firstly developed to predict the start and the finish of phase transformation from austenite to ferrite, pearlite or bainite and volume fraction of each phase for different cooling processes. Optimization techniques are used for the determination of material constants within constitutive equations by minimizing the errors between experimental and predicted data. The material constants of the phase transformation constitutive equations are determined for DIN SAE 5140 steel. The phase transformation of DIN SAE 5140 steel is researched in detail. Good agreement is obtained between the predicted and experimental continuous cooling transformation (CCT) diagram and volume fraction of DIN SAE 5140 steel. This unified constitutive equation is an effective tool to predict the phase transformation process.

Key words: Continuous cooling, Material model, Phase transformation, Unified constitutive equations