The Structure Of Niobium Steels And The Effect Of Niobium On Properties Of Steels: The First Principle Study

Niobium micro alloyed steels have high strength, good toughness and plasticity,were widely used in the pipeline steels and many other fields. The effect is very obviousof the grain refinement of the niobium, the grain refinement is about50%of the share ofthe yield strength, and grain refinement can not only improve the strength of steels, canbut also well improve the toughness. We hope study the effect of niobium on propertiesof steels from atomic level in order to provide some theoretical guidance for the actualproductions. In order to study the effect of niobium in steels to austenite transformationat the atomic level, using first-principles calculations based on density functional theory(DFT) to research the two aspects.First, the total energies and binding energies of a niobium atom or a carbon atom ofinterstitial solid solution and substitutional solid solution in γ-Fe or α-Fe werecalculated, the total energies and binding energies of a niobium atom or a carbon atomin the defect-containing α-Fe were calculated.Based on the total energies and bindingenergies we determine the form of solid solution of niobium and carbon.The calculationresults indicate that in γ-Fe, niobium exists in the form of substitution solid solution andcarbon could be in the form of substitution solid solution or interstitial solid solution,the carbon atom is more likely to exist in the form of interstitial solid solution. But inα-Fe, neither niobium nor carbon exists in the form of solid solution. We calculate themechanical constants of γ-Fe and α-Fe, the mechanical constants of the twoconfigurations that include carbon of interstitial solid solution in γ-Fe and carbon andniobium of solid solution in γ-Fe, the mechanical constants of the three structures thatinclude the containing vacancy of the α-Fe and carbon or niobium in the containingvacancy of the α-Fe. The calculation results indicate that the mechanical constants ofγ-Fe are larger than α-Fe, the elastic constants of carbon of interstitial solid solution inγ-Fe and carbon and niobium of solid solution in γ-Fe are greater than γ-Fe, themechanical constants of α-Fe are larger than the containing vacancy of the α-Fe, theelastic constants of carbon or niobium in the containing vacancy of the α-Fe are huger than the containing vacancy of the α-Fe.Second, the total energies and binding energies of the three allotrope of the pureiron were calculated to study how to exercise of iron atoms and how to change of thelattice in the iron that cause the change with heterogeneous. The activation energies ofγ-Fe into α-Fe were calculated.The results show that α-Fe is the most stable structure,which is the structure of the pure iron at normal atmospheric temperature. Then, thediffusion activation energies of the carbon atom in γ-Fe were calculated and thediffusion activation energies of the carbon atom in the niobium atom of solid solution ofγ-Fe were calculated. The results show that the total energies will rise and fall heavilyand the diffusion activation energies could be increased when a carbon atom diffuses inthe niobium atom of solid solution of γ-Fe. It is this effect of niobium that leads to thedecrease of transformation temperature from austenite to ferrite and delay thetransformation of austenite to ferrite, amplifier the non-recrystallization zone,thuscausing the ferrite grain refinement and promoting the strength of the steel andimproving the toughness that improve the overall mechanical properties of the material.