Experimental Determinations Of Phase Equilibria In The Fe-V-X (X: Nb, Ti) Systems And Development Of Thermodynamic Database Of Microalloyed Steel And Its Applications

Micro-alloying technology of low-carbon steel is a new field of the metallic materials in the 1970s,and is also an important method to improve the material properties.Fe-V-X(X:Nb,Ti) alloy system is the important sub-system to develop the microalloyed steel,and is also the promising hydrogen storage material system.The study of the phase diagrams of the Fe-V-X(X:Nb,Ti) system is the basis of alloy design and investigation performance in these systems.In the present work,the studies of phase equilibria in the Fe-V-X(X:Nb,Ti) system were experimentally carried out.The thermodynamic database of microalloyed steel system was initially developed under the framework of the CALPHAD,and its applications were discussed.Major research results are listed as follows:(1) This work is to firstly experimentally determine four isothermal sections at 1000℃,1100℃,1200℃and 1300℃in the Fe-V-Nb ternary system.(2) Based on the reports of the thermodynamic assessments of the Fe-V,Fe-Nb and Nb-V sub-binary systems in ternary Fe-V-Nb system,the thermodynamic optimization was carried out by combining the CALPHAD method with the experimental results in this study.Calculated results are in good agreement with the experimental data.(3) Six isothermal sections at 800℃,1000℃,1050℃,1100℃,1200℃and 1300℃in the Fe-V-Ti ternary system were experimentally established,and a new ternary compound Fe₃Ti₆V₁ with a hexagonal lattice of the MgZn₂ type,which is named as the "φ" phase in this paper,was identified by XRD measurement in the Fe-V-Ti ternary system;(4) Based on the reports of the thermodynamic assessments of the Fe-V,Fe-Ti and Ti-V sub-binary systems in ternary Fe-V-Ti system,the thermodynamic optimization was carried out by combining the CALPHAD method with the experimental results in this study.Calculated results are in good agreement with the experimental data.(5) The thermodynamic database of the Fe-V-Nb-Ti-C-N system was developed based on the present and previous thermodynamic assessments.Some vertical sections and phase fraction diagrams on the multicomponent systems were calculated from this database.These calculated results indicate that V can be substituted by Nb in some extent,and the effect of 0.01 wt.%Nb on the carbonitride precipitation in microalloyed steel is equivalent to that of 0.02-0.05 wt.%V.