【作者】 李彬；

【导师】 郭汉杰；

【作者基本信息】 北京科技大学 ， 冶金工程， 2020， 博士

【摘要】 传统的高炉炼铁工艺日益成熟,但其进一步发展受制于铁矿资源、焦炭资源与环境保护压力。与高炉炼铁流程相比,直接还原炼铁工艺可摆脱对焦煤资源的依赖,并大量减少CO2排放,因此,直接还原炼铁技术是近年来钢铁工业发展的方向之一。直接还原铁在元素纯净性方面具有天然的优势,是生产高品质钢铁产品的优质原料。随着世界上直接还原铁的产量迅速增长,这些直接还原铁几乎全部作为废钢的替代品参与到钢铁产品的生产中,这是对直接还原铁纯净度的一种浪费。随着钢铁行业的不断发展,对钢铁产品质量、性能需求的不断提高,发展低碳排放、低能耗、环境友好的短流程钢铁材料冶炼工艺将成为钢铁行业发展的方向。铁矿石直接还原—熔分—精炼流程,可为高品质钢铁材料的生产开辟新的途径。铁矿石经氢气直接还原所获得的纯净化的直接还原铁,化学成分稳定、有害杂质含量少,将其作为主要原料,经过熔分和精炼后可以得到高纯净化的钢铁材料。该工艺流程短、污染小,产品附加值高,可以冶炼各种钢和含铁合金,只需要添加相应的合金化元素,即可满足产品的要求。这实现了直接还原铁纯净度的最大化利用,增加了直接还原铁的利润空间,同时也拓展了非高炉炼铁工艺的发展空间。本课题以氢气直接还原铁冶炼高纯铁和高纯轴承钢为例,探索性地研究了氢气直接还原—熔分—精炼流程冶炼高纯净钢铁材料的新工艺。该工艺主要包括三个步骤:第一步,用氢气还原焙烧后的铁矿石氧化球团,得到纯净化的直接还原铁。铁矿中的脉石成分,如硫、硅、锰、钛、铝,在这一步不能被还原进入铁中。第二步,直接还原铁通过熔融分离为脉石(渣)和金属。在这一步中,利用直接还原铁中的氧化亚铁,调整渣的成分以实现脱磷。第三步,对高纯铁液实施二次精炼及合金化,通过合适的渣系实现精炼脱氧,最终得到高纯铁和高纯轴承钢。本论文从冶金物理化学基本原理出发,针对整个流程中涉及到的环节开展系统的基础研究工作,为后续科研工作提供借鉴和参考依据,并为工业化应用奠定理论基础。主要的研究内容和结果如下。铁氧化物气基直接还原的热力学研究。基于最小自由能原理建立了铁氧化物气固还原反应的热力学模型。从热力学平衡计算的角度,验证了铁氧化物的逐级还原过程。根据最小自由能原理,对氢气还原铁氧化物的热力学平衡进行了计算。研究了 CO和H2混合气体还原铁氧化物的热力学平衡,作出了 CO和H2混合气体还原铁氧化物的三维平衡图,为探究气基直接还原过程不同还原气氛时的热力学机理提供了理论依据。氢气直接还原的实验研究。研究发现氢气还原氧化球团的过程存在明显的阶段性,随着反应条件的不同,还原阶段性的特征也不相同。使用双界面未反应核模型有效地分析了反应过程中速率控制步骤的变化。在还原过程的不同阶段,速率控制步骤逐渐演化和发展。随着还原过程进行,控速环节由双界面化学反应控速转变为内扩散控速。在最小自由能热力学原理的基础上,计算了还原一定数量的氧化铁球团所需要的气体量,给出了球团还原时间的理论预测,与实际还原完成时间存在良好的吻合关系。研究了直接还原铁熔分过程中脱磷的热力学,通过模型计算与实验,确定了合适的脱磷渣系,在直接还原铁熔分过程中实现了同时脱磷,简化了使用直接还原铁冶炼高纯净钢铁材料的步骤,使纯铁中的P含量降至18 ppm。通过使用高碱度炉渣精炼,使高纯铁中全氧含量降至10 ppm。采用直接还原—熔分—渣精炼流程,在实验室规模上制得了纯度为99.9868%的高纯铁。依据炉渣分子离子共存理论建立了钢渣体系耦合的热力学平衡模型。在热力学计算的基础上,探究了不同渣系对轴承钢精炼过程的脱氧效果,确定使用直接还原铁冶炼轴承钢精炼过程最适合的渣系组成。通过直接还原—熔分—精炼流程,在实验室条件下冶炼得到了全氧含量为4.8 ppm及Ti含量为6 ppm的高纯净轴承钢。基于传质方程—质量守恒方程—化学平衡方程建立了熔渣精炼脱氧过程的动力学模型,确定了使用高碱度渣系进行轴承钢精炼硅脱氧时,脱氧速率的限制环节为钢液中[O]的传质,钢液中[O]的传质系数为kO=7×10-5 m/s。为了进一步明确轴承钢中Ti含量和N含量对TiN夹杂物析出的影响,对GCr15轴承钢凝固过程中TiN夹杂物的析出热力学和长大动力学进行了详细的计算。采用了一个更合理的溶质元素偏析计算公式,给出了考虑凝固偏析的TiN析出稳定性图。结果表明,使用氢气直接还原铁冶炼的高纯净轴承钢凝固过程中不会有TiN夹杂物析出。此外,计算了采用常规流程生产的轴承钢中TiN析出过程溶质元素含量的变化,并依此优化了夹杂物长大动力学方程。讨论了钢中Ti、N含量和冷却速度对TiN尺寸的影响,为降低轴承钢中TiN夹杂物尺寸、减少其对疲劳寿命的危害提供了理论支持。更多还原

【Abstract】 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.更多还原