Research And Application Of Longitudinal Performance Uniformity Control Technology For DP980 Dual Phase Steels

Dual phase steel is widely used in modern automobile manufacturing industry,due to its comprehensive mechanical properties,such as high strength,good elongation,low yield ratio and high work hardening rate.In the hot rolling process of DP980 dual phase steel in Handan Steel,the fluctuation of head and tail performance(tensile strength)is as high as 100MPa.In order to ensure the stable production of the subsequent cold rolling process,the removal in the head and tail of the hot rolling strip is more than 70m,which reduces the yield and productivity.Therefore,it is of great significance to study and analyze the production technology of hot rolling strip,to reduce the fluctuation of longitudinal mechanical properties of hot rolling strip,and to get dual phase steel with head and tail uniform thickness size and mechanical properties.In this paper,the mechanism of microstructure evolution of DP980 dual phase steel during cooling process after rolling and the finite element numerical simulation of coil temperature field during coiling process were studied.The causes of longitudinal performance fluctuation of DP980 dual phase steel were analyzed,and the U-shaped cooling strategy was proposed.The main contents and results are as follows:(1)The cooling transformation and precipitation of DP980 dual phase steel were studied by thermal simulation.The laws of continuous cooling transformation and second phase particle precipitation were confirmed,the dynamic CCT curve and precipitation kinetics curve were established.The Strip phase transition process occurs after coiling,and the nose temperature of precipitation is 622?.(2)The cooling process of DP980 dual phase steel before and after coiling was studied by hot rolling and thermal simulation.Under different cooling process conditions,the microstructure and properties of room temperature before and after coiling were compared and analyzed respectively,and it was determined that the different cooling rates of the experimental steel after coiling leads to nonuniform longitudinal mechanical properties of the steel.(3)Finite element numerical simulation was applied,the temperature field distribution during coiling process was obtained,the different cooling path between the head,tail and the central layer during the cooling process are determined.Combining the dynamic CCT curve,determining the difference between the cooling path and the center layer is the main reason for the fluctuation of the performance of the steel coil head and tail.(4)The U-shaped cooling process is determined and optimized by finite element simulation and hot rolling test.The microstructure and mechanical properties of hot-rolled steel were analyzed after using U-shaped cooling process in the field.The performance fluctuation is controlled within ±35MPa,the shear length is less than 5m,and the thickness fluctuation is controlled within 35um.