| With the rapid development of Chinese economy,electricity load in social production is rising sharply.The scale of grid construction is expanding constantly,and both the length and capacity of transmission lines are increasing constantly.Consequently,it faces the problem of line and power loss.If the steel core in the wire can improve its conductive property on the premise of ensuring the mechanical property requirements,the current loss in the process of power transmission will be greatly reduced,which is of great significance for the sustainable development of society.Therefore,it is an urgent problem to prepare a steel core with balanced mechanical properties and conductive properties.Based on this,cold-rolled low carbon Nb-microalloyed steel was annealed by different processes.The test methods of OM,EPMA,SEM and TEM were used to study the microstructure evolution,transformation texture,dislocation evolution and precipitation behavior in the test steel.The mechanical properties and conductive properties of samples under different annealing processes were studied by means of mechanical properties and conductive properties testing methods.The influence of microstructure evolution and second phase precipitation on the comprehensive properties was deeply explored.The precipitation behavior of carbides was further studied by first-principles method at atomic and electronic scales.It provides experimental guidance and theoretical basis for developing steel core with excellent comprehensive properties.After different annealing processes,the microstructure of the test steel underwent a process of recovery-primary crystallization-secondary crystallization-recrystallization.The microstructure in the matrix gradually changed from narrow fibrous grains to equiaaxial grains,and the grain size were growing and more evenly distributed.The strength of α texture decreases gradually,while the strength of γtexture increases gradually,which can improve the deep drawability of the test steel to a certain extent.According to EDS analysis and electron diffraction pattern(SAED)calibration,the second phase precipitated during the annealing stage is mainly nanometer Nb C particles,which have the function of pinning dislocation and grain boundary,so that the electrical conductivity of the test steel can be improved to the maximum without significantly reducing the strength.After different annealing processes,the mechanical properties of the test steel continue to decrease,while the conductive properties continue to improve,which is mainly affected by the recrystallization behavior,dislocation annihilation and precipitation behavior.Under the effect of precipitation strengthening,the mechanical properties and electrical conductivity reached the best equilibrium at 680℃.The Fe(100)/Nb C(100)interface model was built and three Nb substitution sites was selected for calculation.The results show that in the process of Nb atoms approaching the interface,the segregation energy is negative and the value decreases continuously,which indicates that segregation of Nb atoms towards the interface can occur in this process.When the Nb atom is closer to the interface,the interface energy is smaller and the structure is more stable.In the whole stage,Nb atoms tend to diffuse towards the Nb C side.This theoretical prediction is in good agreement with the experiment results of precipitation.The charge density and differential charge density also increase with the approach of Nb atoms to the interface,indicating that the precipitation behavior is accompanied by a large amount of charge transfer,which contributes to the improvement of electrical conductivity,It is also revealed that the interaction between the second phase and the atoms on the interface of the steel matrix is the basis for the precipitation and strengthening of the second phase. |
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