Study On The Short-flow Preparation Of High Quality Ferrovanadium Nitride And Reaction Mechanism

As we know,the addition of vanadium element in the steel has the functions of depositionstrengthening and grain refining,which can significantly improve the overall properties,such as strength,toughness,corrosion resistance,wear resistance and so on.With the development of theory and practice,ferrovanadium nitride is favored by more and more researchers and steel companies,and it is considered to be a most effective vanadium-bearing additive.However,at the moment,ferrovanadium nitride is mainly prepared via a high temperature self-propagation process(SHS),which has some problems to solve.These problems include heterogeneous composition,long process and so on.Moreover,in recent years,few researcher is focused on the preparation of ferrovanadium nitride.Therefore,it is necessary for us to develop a new route to improve the quality of ferrovanadium nitride.In this paper,a novel route to prepare high-qulity ferrovandium nitride via a carbonitrothermic reduction process is proposed.By using X-ray diffraction(XRD),scanning electron microscopy(SEM),carbon-sulfur analyzer,differential thermal analyzer(TG-DTA),oxygen-nitrogen-hydrogen analyzer and FactSage 7.0,the effect of C/O molar ratio,reaction temperature,heating system,reaction time and nitrogen partial pressure on the phase,microstructure,apparent density and composition of the products are studied.Combining with the thermodynamic analysis,the carbonitrothermic reaction mechanism also can be analysed in detail.From the views of thermodynamics and kinetics,the effect of Fe on the reaction rate and composition are discussed so as to study the reaction mechanism of Fe during the carbonitrothermic reduction process.Furthermore,the reaction behavior of carbon at different temperatures during the carbonitrothermic reduction process and the basic research of the CO reduction of V2O5 are studied.What’s more,a two-step process,which is an ammonia reduction nitridation process followed by a high temperature sintering process,is also propose.The research results are described as follows:First,high quality ferrovanadium nitride with high nitrogen content,low carbon content and oxygen content,are successfully prepared by the carbonitrothermic reduction process.Compared with the requirements of the national standard GB/T 30896-2014,the density and composition of the FeV45N,FeV55N and FeV65N alloys prepared by this process can perfectly meet the requirements.In addition,compared with the traditional methods,vanadium penoxides is directly used as raw material during the carbonitrothermic reduction process and the preparation of ferrovanadium is avoided,which contributes to shorten the production period.Moreover,the impurities,including aluminum,silicon and so on,can be reduced.When the C/O molar ratio is close to 80%,the composition of the ferrovanadium nitride prepared by carbonitrothermic reduction process.The higher the C/O molar ratio,the lower the nitrogen.The reaction temperature has a few effect on the composition of the ferrovanadium nitride,but it has a great effect on the density.The optimum reaction temperature is about 1773K～1823K.Furthermore,to ensure the simultaneous reaction of the carbonitrothermic reduction,the partial pressure of nitrogen should not be too low.Second,the reaction mechanism of Fe during the carbonitrothermic reduction process is studied.It is found that at a certain temperature,the iron become a liquid,which has a certain fluidity.The solid reactants are covered by it,hindering the diffusion of the gaseous reactant.Thereby,the rate of the carbonitrothermic reduction decreases.However,owing to a high reaction temperature,the hindrance of the liquid phase layer is not very obvious.Furthermore,the addition of Fe can contribute to the increase of the nitrogen content and decrease of the carbon and oxygen contents.Third,the reaction behavior of carbon at different temperatures is studied separately.It is found that at 923 K-973 K,the phase transformation occurred in the following sequential order:V2O5→V6O13→VO2.When the temperature is lower than the melting point of V2O5,the reduction reaction is a solid-solid reaction.The diffusion rate between the solid particles is low,which results in a low reduction reaction rate.Even if the reaction is carried out for 5 hours,the reaction cannot be completed completely.However,when the temperature is higher than V2O5,V2O5 begins to melt,and the reaction is converted into a solid-liquid reaction,and the reaction rate rapidly increases.At 1423 K-1473 K,the vanadium oxide is further reduced to V2O3,and the phase transformation occurred in the following sequential order:V2O5→V3O5→V2O3.At the temperature range of 1473K to 1673K,the V2O3 can be directly reacted to VN without other intermediate phase.Fourth,during the research on the reaction behavior of carbon,at a low temperature,the optimal C/O molar ratio is only 0.6,demonstrating that the CO reduction of V2O5 play a certain role during the carbonitrothermic reduction process.Therefore,the basic research of CO reduction of V2O5 is performed.It is found that during the carbonitrothermic reduction process,V2O3 cannot be further reacted to VC or other vanadium oxides by CO,even at 1623 K.At 783 K-963 K,the phase transformation occurred in the following sequential order:V2O5→V6O13→VO2→V2O3.However,the reduction mechanism at 783K and 963K are different.When the temperature is lower than the melting point of V2O5,the reduction reaction is a gas-solid reaction,and the reaction is charged by pseudomorphic transformation mechanism.Many pores and cracks are formed on the surface of V2O5 particles,and the V2O3 maintains the original microstructure.At a temperature higher than the melting point of V2O5,the reduction reaction becomes a gas-liquid reaction.The pores in the liquid phase are extremely small,resulting in the difficulty of the diffusion of the reducing gas CO,and the reduction rate drastically decreases.Fifth,ferrovanadium nitride without carbon is prepared by an ammonia reduction nitridation process,which follows by a high-temperature sintering process.It is found that during the ammonia reduction nitridation process,the phase transformation of vanadium source occurred in the following sequential order:NH4VO3→V2O5→VO2→V2O3→V(N,O).After reacting at 1373 K for 10 hours,the oxygen content of the composite powders can be decreased to 0.56%.Moreover,after sintering at 1823 K,the density of FeV55N can up to 5.4 g/cm3.