Fundamental Study On The Smelting High-purity Iron And High-purity Bearing Steel Using Direct Reduced Iron Prepared By Hydrogen

The traditional Blast Furnace ironmaking process is becoming more and more mature,but its further development is restricted by the pressure of iron ore resources,coke resources and environmental protection.Compared with the Blast Furnace ironmaking process,the direct reduction ironmaking process can get rid of the dependence of the coking coal resources and greatly reduce the CO2 emissions.Therefore,the direct reduction ironmaking technology has been one of the development directions of the iron and steel industry in recent years.Direct reduction iron(DRI)has a natural advantage in element purity and is a high-quality raw material for producing high-quality steel.With the rapid growth of the output of DRI in the world,almost all of these DRI are used in the production of iron and steel products as the substitute of scrap,which is a waste of the purity of DRI.Owing to the development of the iron and steel industry,the demand for quality and performance of iron and steel products has been increasing,and the short steel material smelting process with low carbon emission,low energy consumption and environmental friendliness become the development direction of steel industry.The process of direct reduction-melting separation-refining can open up a new way for the production of high-quality steel materials.The pure DRI obtained by hydrogen reduction has the advantages of stable chemical composition and low harmful impurities.It can be used as the main raw material for smelting high-purity steel via melting separation and secondary refining.The process,which is short,low pollution and high value-added,can smelt various steels and iron-containing alloys.It only needs adding corresponding alloying elements to meet the requirements of products.This process maximizes the usage of the purity of DRI,and increases the profit of DRI,and also expands the development space of non-BF ironmaking process.In this subject,the new process of smelting high-purity iron and steel materials employing hydrogen direct reduction-melting separation-refining is exploratively studied,taking the smelting of high-purity iron and high-purity bearing steel as an example.The process mainly includes three major steps:Step 1,the iron ore after pelletizing and roasting is reduced by hydrogen,and high-purity DRI is obtained.The impurities in ore such as sulfur,silicon,manganese,titanium and aluminium cannot be reduced or get into iron in this step.Step 2,the DRI is separated into gangue(slag)and metal by melting.In this step,by utilizing the FeO in DRI,the composition of slag is adjusted to dephosphorize.Step 3,the appropriate slag is dosed to refining for deoxidation.After refining and alloying,high-purity iron and high-purity bearing steel are finally obtained.In this thesis,based on the basic principles of metallurgical physical chemistry,the fundamental study on the whole process was conducted,which provided reference for follow-up research and lay a theoretical foundation for industrial application.The main research contents and results are as follows.Study on thermodynamics of iron oxide gas-based reduction.A thermodynamic model for gas-solid reduction reactions of iron oxides was established based on Gibbs free energy minimization method.From the point of view of thermodynamic equilibrium calculation,the stepwise reduction sequence of iron oxides was verified.According to Gibbs free energy minimization method,the thermodynamic equilibrium of iron oxides reduced by hydrogen was calculated.The thermodynamic equilibrium was investigated for iron oxide reduction using CO-H2 mixtures as a reducing agent,and the three-dimensional equilibrium diagram of iron oxide reduction using CO-H2 mixtures was graphed,which provided a theoretical basis for exploring the thermodynamic mechanism of gas-based direct reduction in different reduction atmosphere.Experimental study on direct reduction by hydrogen.It was found that there are distinct stage characteristics during the reduction of oxidized pellets by hydrogen,and the stage characteristics are different under different reaction conditions.The change of rate controlling steps during the reduction was effectively analyzed by using an un-reacted shrinking kernel model of double reactions.At different stages of reduction process,the rate controlling steps gradually evolve and develop.With the reduction going on,the rate controlling step changes from chemical reaction controlling with dual-interface to internal diffusion controlling.Based on the calculation of the minimized free energy principle,the amount of gas needed for reduction of a certain amount of iron oxide pellets was calculated,and the theoretical prediction of reduction time was given,which was in good agreement with the actual completion time of reduction.The thermodynamics of dephosphorization in the DRI melting separation process was studied.Through model calculation and experiment,the proper slag used for dephosphorization was determined.The simultaneous dephosphorization was realized in the DRI melting separation process,and the steps of smelting high-purity steel materials by DRI were simplified.The P content in pure iron was reduced to 18 ppm.The content of total oxygen in high-purity iron is reduced to 10 ppm by refining with a high basicity slag.A high-purity iron with purity of 99.9868 mass pct was prepared in laboratory scale employing the process of direct reduction-melting separation-slag refining.Based on the ion and molecular coexistence theory in slag,a coupled thermodynamic equilibrium model of molten steel-slag system was established.On the basis of thermodynamic calculation,the deoxidization effect of different slags on bearing steel refining process was investigated,and the most suitable slag for refining step of smelting bearing steel using DRI was determined.A high-purity bearing steel with T.O of 4.8 ppm and Ti of 6 ppm was obtained in laboratory scale employing the process of direct reduction-melting separation-slag refining.Moreover,based on the mass transfer equation-mass conservation equation-chemical equilibrium equation,the kinetic model of deoxidization process of bearing steel refining was established.The mass transfer of[O]in molten steel is the rate controlling step for deoxidization,when using a high basicity slag to deoxidize in bearing steel refining process.And the mass transfer coefficient of[O]in molten steel is ko=7×10-5 m/s.In order to further clarify the influence of Ti content and N content in bearing steel on TiN inclusions precipitation,the precipitation thermodynamics and growth kinetics of TiN inclusions in GCr15 bearing steel during solidification were calculated in more detail.A more reasonable formula for calculating the segregation of the solute elements was adopted and the stability diagram of TiN precipitation considering solidification segregation was given.The result shows that TiN inclusions do not precipitate during the solidification of high-purity bearing steel smelted by DRI.In addition,the change of the solute element content before and after the precipitation of TiN inclusions in bearing steel produced by conventional process was calculated.Based on this,the kinetic formula of the inclusion growth was optimized.The effects of Ti content,N content and cooling rate on the size of TiN were discussed,which provided a theoretical support for reducing the size of TiN inclusions in bearing steel and reducing the damage to fatigue life.