Modeling And Simulation Of The Microstructure Of Pipeline Steel

As one important product of hot rolled strip, the pipeline steel is widely used inpetroleum and natural gas transportation industry because of high strength, hightoughness, high crack, low bauschinger effect and good welding performance. Themicrostructure evolution of pipeline steel during the cooling process not only decide thefinally phase transformation organization but also decide to the finally mechanicalproperty of pipeline steel, which become the hotspot study of alloy steel to predict andcontrol the microstructure evolution during the cooling process.In the present paper, a new integrated mathematical model for prediction ofmicrostructure evolution during rolling and cooling was developed for X80 pipeline steelby considering the relationship between the transformation, latent heat and temperature,which consists of transformation thermodynamic, transformation kinetics, latent heat andtemperature field models. The influence of controlled rolling and controlled cooling onmicrostructure evolution was thoroughly investigated, which provides importanttheoretical evidence for predicting and controlling the microstructure evolution ofpipeline steel during cooling process. The innovate works and corresponding results areas follows.The phase interface concentration, transformation force and phase equilibriumtemperature of X80 pipeline steel was calculated by KRC model, and the effect of hotdeformation on the thermodynamic parameters was analyzed, which provides thenecessary calculation hypothesis for predicting the microstructure evolution during thecooling process.The transformation kinetics model of X80 pipeline steel was established, whichincluded phase incubation period model, phase transformation temperature model, phasetransformation volume fraction model and ferrite grain size model, the effect of hotdeformation and cooling rate on the transformation kinetics was analyzed. Whencalculating the ferrite phase starting temperature, the effect of the solute drag of Nbshould be considered, which improved the ferrite phase starting temperature model for the Nb micro-alloyed steel.Based on the phase transformation mechanism, a mathematical model of latent heatfor X80 pipeline steel was established, which considered the enthalpy change during thephase transition. The results given by the model were in good agreements with the DSCexperimental results. According to the variation of latent heat, combined with theory andexperiment, a new method to calculate the latent heat at any temperature was proposed,the results given by the method were in accord with the measured value.The temperature field model of X80 pipeline steel during laminar cooling wasestablished by using finite difference method, the effect of latent heat, plate thickness,conveyor speed and water discharge density on the temperature distribution. According tothe temperature field model of X80 pipeline steel, and combined with actual coolingconditions, some improvements in connection with uneven temperature strip wereproposed from the cooling method and device parameters, which achieved good results. Itcalculated the optimum water flow based on the heat transfer theory, a formula tocalculate edge masking amount was regressed through the relationship between edgemasking amount and plate width, plate thickness, water discharge density. Throughcomparing the results with field data, it proved the edge masking model reasonableness.A new model for austenite transformation volume fraction and temperature in thetwo-dim unstable temperature fields considering latent heat was established. Bycomparing volume fraction of the three phase obtained by kinetic model, temperaturecoupling model and experiment, it is indicated that the coupling model has superiorityand the calculation accuracy of the three values.The continuous cooling transformation behaviors of two different Nb content of X80pipeline steel are simulated on Gleeble-3500 thermomechanical simulator, it ananlysesthe influences of cooling rate, deformation and addition of Nb element on transformationof X80 pipeline steel with metallographic observation and verifies the veracity ofcoupling model. Overall, the coupling model for prediction of microstructure evolutionduring rolling and cooling is developed for X80 pipeline steel in the present paper, whichimproves the predicted precision of microstructure evolution during the cooling process.The effect of latent heat and alloy element Nb on the austenite transformation is studied. It has a certain theory value and practical reference sense to formulate the scientificcontrolled rolling and controlled cooling and more accurate control the cooling process.