Basic Research On The Preparation Of Pure Metal Raw Materials For VIM

Ultra high strength steel is the leader of structural material strength and is widely used in high-end mechanical equipment manufacturing industries such as aerospace,aerospace,high-speed rail,wind power,steam turbines and gas turbines.It always uses the most advanced pure smelting technology and represents the highest level of metallurgy in a country.At present,ultra high strength steel mostly uses VIM(vacuum induction furnace)+VAR(vacuum self-consumption furnace)double vacuum melting process.From the beginning of the development of ultra high strength steel,people noticed the relationship between purity and toughness.There is a significant gap between the control level of iron alloy impurity elements in China and the advanced level in foreign countries.This leads to impurities in the alloy(metal chromium and ferrovanadium)entering the metal bath during VIM vacuum smelting alloying and the control of ultra low Al and Ti elements is becoming very difficult.This paper proposes to use the EAF+LF+MC process to produce high-alloy pure metal raw materials.The Cr or V(according to the steel)requirements of the raw materials basically meet the requirements of finished steel.The characteristics of the process are as follows:(1)In the EAF smelting,the key points are to solve the problem of removing oxidizable elements such as Al,P,Ti,As,Sb and Bi in ordinary industrial pure iron;(2)At the early stage of LF refining,Al,Ti were oxidized into slag and Cr,V were alloyed into metal melting pool by controlling the oxygen level and utilizing the different affinity between Cr,V,Al,Ti and oxygen.Thereafter,the slag is removed oxides of A1 and Ti.The use of this process to produce high chromium or vanadium metal raw materials can provide a more pure raw material for VIM smelting,but the selective oxidation control mechanism of Al,Ti,Cr and V needs further research.In this paper,the equilibrium of oxidation reaction of aluminum,titanium and oxygen was calculated and analyzed at 1600? with 12%chromium and 0.5%vanadium at 1600? by using FactSage software.The results show that when the mass fraction of oxygen in the system is controlled in the range of 0?0.08%and A1₂O₃,Ti₂O₃ and TiO₂ can be oxidized by Al and Ti,while chromium can not be oxidized.At the same time,when the oxygen content in the system is 0.11%,the oxidation reaction of Al and Ti takes place,while vanadium exists in the form of single mass.Because the influence of slag was not considered in thermodynamic calculation,the thermal simulation experiment of slag gold reaction equilibrium was carried out in 1kg MoSi2 furnace in laboratory.The reaction equilibrium relationship between alloying elements and oxygen was investigated under the condition of top slag addition in the furnace.The experimental temperature is 1600?,and the crucible is made of high purity MgO,and slag is CaO-SiO₂-MgO-Al₂O₃-FeO slag system.The oxygen potential in the crucible molten pool is changed by adjusting the mass fraction of FeO in the slag system.Another condition is the mass ratio of slag to metal(slag to steel ratio).The experimental results show that the mass fraction of FeO and the ratio of slag to steel have obvious effects on the oxidation of elements Al,Ti,Cr and V.When the mass fraction of FeO in the initial slag is 18.4%and the ratio of slag to gold is 10%,the mass fraction of Al and Ti in the molten pool can be controlled to less than 0.005%,while the yield of Cr is up to 99.16%.When the mass fraction of FeO in the initial slag is 24%and the ratio of slag to steel is 10%,the mass fraction of A1 and Ti in the molten pool can be controlled to less than 0.005%,and the yield of V is 99.11%.Finally,the kinetic reaction model was established based on the film theory and permeation theory of liquid-liquid reaction and the law of the mass fraction of alloy elements in the molten pool was calculated and analyzed with time.The results of model calculation show that the oxidation rate of Al and Ti in the molten pool is very fast and the reaction can reach equilibrium in 15 min.The calculated results are in agreement with the experimental results.