Direct quenching and tempering(DQ-T) of hot rolled steel section has been widely used in steel mill for the sake of improvement of mechanical properties and energy saving.Temperature history and microstructural evolution during hot rolling plays a major role in the properties of direct quenched and tempered products.The mathematical and physical modeling of hot forming processes is becoming a very important tool for design and development of required products as well as predicting the microstructure and the properties of the components.These models were mostly used to predict austenite grain size(AGS),dynamic,meta-dynamic and static recrystallization in the rods immediately after hot rolling and prior to DQ process.The hot compression tests were carried out on 42CrMo4 steel in the temperature range of 900-1 100 ℃ and the strain rate range of 0.05-1 s-1 in order to study the high temperature softening behavior of the steel.For the exact prediction of flow stress,the effective stress-effective strain curves were obtained from experiments under various conditions.On the basis of experimental results,the dynamic recrystallization fraction(DRX),AGS,hot deformation and activation energy behavior were investigated.It was found that the calculated results were in good agreement with the experimental flow stress and microstructure of the steel for different conditions of hot deformation. Direct quenching and tempering (DQ-T) of hot rolled steel section has been widely used in steel mill for the sake of improvement of mechanical properties and energy saving. Temperature history and microstructural evolution during hot rolling plays a major role in the properties of direct quenched and tempered products. The mathematical and physical modeling of hot forming processes is becoming a very important tool for design and development of required products as well as predicting the microstructure and the properties of the components. These models were mostly used to predict austenite grain size (AGS), dynamic, meta-dynamic and static recrystallization in the rods immediately after hot rolling and prior to DQ process. The hot compression tests were carried out on 42CrMo4 steel in the temperature range of 900-1 100 ° C and the strain rate range of 0.05-1 s-1 in order to study the high temperature softening behavior of the steel. For the exact prediction of flow stress, the effective stress-e ffective strain curves were obtained from experiments under various conditions. On the basis of experimental results, the dynamic recrystallization fraction (DRX), AGS, hot deformation and activation energy behavior were investigated. It was found that the calculated results were in good agreement with the experimental flow stress and microstructure of the steel for different conditions of hot deformation.