Prediction Of Microstructural Evolution Of X120 Pipeline Steel During Cooling Process After Rolling

In recent years,natural gas has become one of the most important clean energy sources due to its such remarkable advantages as clean,efficient and convenient for storage and transportation,etc.,which leads to an ever increasing market demand of it.Due to most of the natural gas fields are located in remote areas,the pipeline for natural gas transportation will work in the harsh environment.Therefore,the development and application of high strength pipeline steel has increasingly received attention.Because the control of cooling process after rolling in the production line is determines the final microstructures and properties of the pipeline steel,the temperature field after rolling and the prediction of micro structural evolution are of great significance to the control of the microstructures and properties.This thesis established a mathematic-physical model of the temperature field during the cooling process.The temperature field of X120 pipeline steel was simulated under different conditions by using the finite element analysis method,and the evolution of microstructures predicted by using the phase change theory model.In establishing the laminar cooling model for the steel plate,a dynamic boundary condition of the convection varying with time was used to describe the changing from one to another of the cooling zone.Correspondingly,the moving steel plate can be treated as a motionless object,which simplifies simulation process and greatly decreases the time for calculation.In establishing the cooling model for steel coil,the corresponding nodes on the upper and lower closely contacted surfaces of the plate were coupled,which makes the simulation simplified and the computation time greatly saved due to reasonably taking no account of the motion of the plate.By simulating the cooling process under different water flow intensity,rolling thickness and roller speed,some interesting results are obtained.By decreasing the thickness of steel plate and increasing the water flow intensity,the temperature distribution becomes more uniform and the difference of the temperature smaller between the interior and exterior of the plate,which results in finer ferrite grains and some amount of bainite phase.Under the water flow intensity of 110L/(m2·s),the ferrite grain size is about 2.9 ?m which is about 1.0?m smaller than that under 30L/(m2·s),and the content of bainite phase increases about 19%.Keeping the thickness of steel plate and the water flow density unchanged,the coiling temperature becomes higher with increasing the rolling speed,which leads to a early start of the coiling cooling process of the steel plate undergoing the ferrite transformation.The rolling speed should be maintained in a moderate range to avoid too higher the ferrite phase content.The study of this thesis has good reference value for formulating the process parameters,controlling the organization performance and optimizing the production process for the cooling of X120 pipeline steel,and also has good academic reference value for the modeling of micro structural prediction.