The Effect Of Niobium On The Normalized And Annealed Microstructures Of High Carbon Steel

As one of the most typical microalloying elements in steel,Nb can significantly improve the microstructure and properties of high carbon steel.However,the research on the influence of Nb on the microstructure of high carbon steel is still not deep,and the existence form of Nb in high carbon steel is not clear.Therefore,two alloy steels with the carbon content of 0.86%and the carbon content of 0.86%and the Nb content of 0.04%were designed as the research object.The normalized heat treatment and annealing heat treatment experiments were carried out by metallographic microscope and scanning electron microscope.Reticulated cementite and spheroidized cementite structure of high carbon steel,study the influence of Nb on pearlite transformation and the effect of Nb on cementite under traditional annealing process,and explore the microstructure of Nb on high carbon steel.Impact.Through the analysis methods of electron probes and other methods,the existence forms and laws of Nb in high carbon steel were explored,and the reasons for the segregation of Nb and the influence of the interlayer spacing of pearlite were revealed.In this paper,through the first-principles calculation,using the Materials Studio software,the distribution of strontium in high-carbon steel austenite and high-temperature cementite is calculated theoretically,and the existence form of Nb is explored.Through the combination of experiment and theory,the existence state of niobium in high carbon steel and its influence on austenite are explored.In this paper,metallographic and scanning electron microscopy observations of normalized microstructures containing Nb steel and Nb-free steel showed that a small amount of reticulated cementite appeared in the metallographic structure without Nb steel,while Nb containing steel did not appear.XRD diffraction analysis showed that the addition of 0.04%Nb changed from hypereutectoid steel to hypoeutectoid steel,and the eutectoid point moved to the right.The original secondary cementite structure disappeared and replaced by ferrite structure.When studying the spheroidizing annealing of the traditional process,it was found that the addition of Nb can significantly simplify the complexity of the traditional annealing process,and the spheroidization state at 730,760 and 790?with and without Nb is found to be free of Nb.The high carbon steel can only complete the spheroidization state of most cementite at 730?,and the Nb containing steel can show a good spheroidization state at different temperatures.When it is kept at 730?for 3 hours,cementite Behaved as the best spheroidized state.In this paper,the electron probe analysis of the quenched microstructure at 900,925 and 950°C showed that the content of Nb at the grain boundary of high carbon steel was significantly higher than that of the intragranular content,and as the temperature increased,the Nb was crystallized.The amount of segregation and the segregation positions in the boundary are first increased and then decreased.The solid solubility in steel shows that the amount of segregation of antimony is inversely proportional to the precipitation of Nb carbide.In this paper,900,925 and 950?of normalized state are analyzed.It is found that the regularity of Nb segregation is basically the same as that of quenching state,preferentially segregated at the grain boundary,but due to the slow cooling rate,there is ferrite and the cementite and the Nb atom are fully diffused and precipitated,and the amount of segregation is significantly smaller than that of the quenched state,and the amount of Nb carbide is significantly increased.In the study of the traditional annealing process,it was found that the addition of Nb can significantly simplify the complexity of the traditional annealing process.Compared with the spheroidal state at 730,760 and 790?containing antimony and antimony,it is found that the high carbon steel without Nb is only The spheroidization state of most cementites can be completed at 730?,and the Nb-containing steels can exhibit good spheroidization at different temperatures.When held at 730?for 3 hours,cementite is the best.Spheroidized state.In this paper,the first-principles calculation combined with heat treatment experiments was carried out to study the distribution behavior of Nb in high carbon steel austenite.Based on the first-principles method of density functional theory,the PW91 functional form was used by generalized gradient approximation.The distribution behavior and occupancy of Nb between?-Fe?C?unit cell and Fe₃C were calculated,and the state density,population number and differential charge density of Nb occupying the face center of?-Fe?C?cell are analyzed.The calculation results show that the Nb atom has the same probability of replacing the austenite centroid of area and the apex position.The Nb atom is easy to replace the Fe atom at the 4c position of Fe₃C.The distribution enthalpy of Nb between?-Fe?C?and Fe₃C is negative.It is indicated that Nb is easily segregated to the?-Fe?C?unit cell.The electronic structure analysis of Nb in?-Fe?C?shows that Nb atoms lose electrons,Fe atoms get electrons and lose electrons,and C atoms get electrons.It is indicated that the Nb solid solution in the?-Fe?C?unit cell has a stronger bonding bond,and this bonding enhances the stability of the?-Fe?C?unit cell.The calculation results are close to the experimental results,and the calculation is feasible.