Study On Isothermal Decomposition Behavior Of Austenite In Q&P Process

The goal that material researchers have been pursuing is to obtain high-strength and high-plasticity materials.Q&P process is one of the important processes for the third generation of automotive high strength steels.The theoretical guidance in composition and process design will lead to more efficient development of materials.At present,the CCE model is often used in the theoretical calculation,but the peak value of retained austenite corresponding to the optimum quenching temperature predicted by the CCE model is not applicable in several alloy systems.Based on this situation,this paper aims to modify the CCE model so that it can be used in a wider range and has a guiding significance for the design of component process.The volume fraction of different phases and dynamic curves of bainite transformation under differernt heat treatment processes were obtained by using dilatometer under the alloy of Fe-0.20C-2.82Mn-1.6Si and Fe-0.21C-3.92Mn-1.6Si.And the CCE model was modified by combining the thermodynamic model of bainite transformation,such as T0/T0' model,GEB model.In addition,the bainite transformation in the isothermal partition process was also concerned,and its kinetics model was established.It was expected that the bainite transformation process can be regulated and controlled by this kinetics model.The main results of the paper were as follows:(1)In the component systems of Fe-0.20C-2.82Mn-1.6Si and Fe-0.21C-3.92Mn-1.6Si,the CCE model does not consider the austensite decomposition in the isothermal partition and the diffusion process of carbon from baninite to austenite.The predicted peak value of retained austenite does not exist,which lead to the in accuracy of the content of actual retained austensite.(2)The To model was used to correct the CCE model.The calculated and predicted values of each phase in the Fe-0.20C-2.82Mn-1.6Si alloy were consistent with the actual values.Although the calculated value of the retained austenite in the Fe-0.21C-3.92Mn-1.6Si alloy was higher than that of the CCE model,it was still lower than the actual value.The reason was that there was a deviation between the bainite content predicted by the To model and the actual value under this component system.(3)The GEB model was used to modify CCE model.The GEB model could only accurately calculated the volume fraction of bainite transformation at high temperatures in the Fe-0.20C-2.82Mn-1.6Si alloy,such as the two-step quenching temperatures of 340? and 320? and the one-step quenching temperature of 400?,but the prediction was inaccurate at low temperatures.In the Fe-0.21C-3.92Mn-1.6Si alloy,the model can be inaccurate at any heat treatment process.The application of GEB model under one-step low quenching temperature in the Fe-0.20C-2.82Mn-1.6Si alloy was modified and the modified GEB model was available to predict the volume fraction of bainite phase transformation correctly at 280? and above.(4)Based on the analysis and hypothesis of the effect of martensite on austenite decomposition,a kinetic model of bainite transformation after the introduction of martensite was established successfully.The calculated results of the model are in good agreement with the experimental values,which provides a theoretical basis for the further regulation of austenite decomposition in the isothermal partition stage.