Preliminary Investigation of Mathematical Modeling of Stainless Steelmaking in an AOD Converter: Mathematical Model of the Process

Mathematical modeling of stainless steelmaking in an AOD (argon‐oxygen decarburisation) converter with side and top combined blowing has been preliminarily investigated. The actual situations of the side and top combined blowing AOD process were analysed. A mathematical model for the whole refining process of stainless steel has been proposed and developed. The model is based on the assumption that one part of the oxygen blown through a top lance reacts with CO escaping from the bath, another part of the oxygen oxidizes the elements in the molten steel droplets splashed by the oxygen jet, and the remaining oxygen penetrates and dissolves into the molten steel through the pit stroked by the jet. All the oxygen entering into the bath oxidizes C, Cr, Si, and Mn dissolved in the steel and also the Fe of the steel melt, but the FeO generated is also an oxidant of C, Cr, Si, and Mn in the steel. During the process, all possible oxidation‐reduction reactions occur simultaneously and reach their equilibria, respectively their combined equilibrium, in competition at the liquid/bubble and liquid/slag interfaces. In the simple side blowing after the top blowing operation is finished, the possible reactions take place simultaneously and reach a combined equilibrium in competition at the liquid/bubble interfaces. The overall decarburization rate in the refining process is the sum of the contributions of both the top and side blowing processes. It is also assumed that at high carbon concentrations, the oxidation rates of elements are mainly dependent upon the supplied oxygen rate, and at low carbon contents, the rate of decarburisation is primarily related to the mass transfer of carbon from the molten steel bulk to the interface. It is further assumed that the non‐reacting oxygen blown into the bath does not accumulate in the steel and will escape from the bath and react with CO in the atmosphere above the bath. The study presents calculations of the refining rate and the mass and heat balances of the system for the whole process. Additionally, the influences of the operating factors, including addition of slag materials, scrap, and alloy agents, the non‐isothermal conditions, the changes in the amounts of metal and slag during the whole refining process, and others have all been considered.     

Mathematical Modeling of Fluid Flow in Bath during Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Application of the Model and Results

The mathematical model developed for the molten steel flow in the combined side and top blowing AOD refining process of stainless steel has been used to compute and analyze the flow fields of the liquid phases in the baths of the 120 t AOD converter and its water model unit with a 1/4 linear scale. The influence of the side tuyere number and the angle between each tuyere on the flows has been examined. The results demonstrate that the mathematical model can quite reliably and well model and predict the fluid flow in an AOD bath with the combined blowing. The liquid flow in an AOD converter bath with the combined blowing is resulted from the gas side blowing streams under the influence of a gas top blowing jet. The streams play a governing role on it; and the liquid in the whole bath is in active agitation and circulatory motion during the gas blowing process. The gas jet from the top lance does not change the essential features of the gas stirring and liquid flow in the bath, but can make the local flow pattern of the bath liquid obviously vary and its turbulent kinetic energy enhance. The changes in the tuyere position and number have similarly not altered the basic characteristics and patterns of the gas agitation and liquid flow and turbulent kinetic energy distribution in the bath. At a given tuyere number and gas side blowing rate or a given angular separation between each tuyere and gas side blowing rate, however, the variation of the angle between each tuyere or the tuyere number can locally change them. Using 6 tuyeres with 27° can reach the more uniform flow field and turbulent energy distribution of the liquid in the bath than taking 7 tuyeres with 18° or 22.5° and 6 tuyeres with 22.5°.     

Preliminary Investigation of Fluid Mixing Characteristics during Side and Top Combined Blowing AOD Refining Process of Stainless Steel

The fluid mixing characteristics in the bath during the side and top combined blowing AOD (argon‐oxygen decarburization) refining process of stainless steel were preliminarily investigated on a water model unit of a 120 t AOD converter. The geometric similarity ratio between the model and its prototype (including the side tuyeres and the top lances) was 1:4. On the basis of the theoretical calculations for the parameters of the gas streams in the side tuyeres and the top lances, the gas blowing rates used for the model were more reasonably determined. The influence of the tuyere number and position arrangement, and the gas flow rates for side and top blowing on the characteristics was examined. The results demonstrated that the liquid in the bath underwent vigorous circulatory motion during gas blowing, without obvious dead zone in the bath, resulting in a high mixing effectiveness. The gas flow rate of the main tuyere had a governing role on the characteristics, a suitable increase in the gas flow rate of the subtuyere could improve mixing efficiency, and the gas jet from the top lance made the mixing time prolong. Corresponding to the oxygen top blowing rate specified by the technology, a roughly equivalent and good mixing effectiveness could be reached by using six side tuyeres with an angle of 27 degrees between each tuyere, and five side tuyeres with an angular separation of 22.5 or 27 degrees between each tuyere. The relationships of the mixing time with the gas blowing rates of main‐tuyeres and sub‐tuyeres and top lance, the angle between each tuyere, and the tuyere number were evaluated.     

Study on Mass Transfer Characteristics in an AOD Converter Bath under Conditions of Combined Side and Top Blowing

The mass transfer characteristics in a steel bath during the AOD refining process with the conditions of combined side and top blowing were investigated. The experiments were conducted on a water model unit of 1/4 linear scale for a 120‐t combined side and top blowing AOD converter. Sodium chloride powder of analytical purity was employed as the flux for blowing, and the mass transfer coefficient of solute (NaCI) in the bath was determined under the conditions of the AOD process. The effects of the gas flow rates of side and top blowing processes, the position arrangement and number of side tuyeres, the powdered flux particle (bubble) size and others on the characteristics were examined. The results indicated that, under the conditions of the present work, the mass transfer coefficient of solute in the bath liquid is in the range of (7.31×10^?5‐3.84×10^?4) m/s. The coefficient increases non‐linearly with increasing angle between each tuyere, for the simple side blowing process at a given side tuyere number and gas side blowing rate. The gas flow rate of the main tuyere has a governing influence on the characteristics, and the gas jet from the top lance decreases the mass transfer rate, the relevant coefficient being smaller than that for a simple side blowing. Also, in the range of particle (bubble) size used in the present work and with all other factors being constant, raising particle (bubble) size increases the coefficient. Excessively fine powder particle (bubble) sizes are not advantageous to strengthening the mass transfer. With the oxygen top blowing rate practiced in the industrial technology, the side tuyere arrangements of 7 and 6 tuyeres with an angular separation of 22.5° and 27° between each tuyere, as well as 5 tuyeres with an angle of 22.5° between each tuyere can provide a larger mass transfer rate in the bath. Considering the relative velocity of the particles to the liquid, the energy dissipation caused by the fluctuation in the velocity of the liquid in turbulent flow and regarding the mass transfer as that between a rigid bubble and molten steel, the related dimensionless relationships for the coefficient were obtained.     

Mathematical Modeling of Fluid Flow in Bath during Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Mathematical Model of the Fluid Flow

Considering that the liquid flow field under the conditions of the combined side and top blowing would be a combined result from the common action of the side blowing gas streams and a gas top blowing jet, as the first attempt, the three‐dimensional mathematical models for the flows of molten steel in an AOD converter bath during the simple side and top blowing processes have been proposed and developed, respectively. And the mathematical model of the flow in the bath during the combined blowing AOD refining process of stainless steel has been given by the composition and superposition of the two models. In the composed model, the gas‐liquid two‐phase flow is described and treated in terms of the two‐fluid (Eulerian‐Eulerian) model. The especially modified two‐equation k?ε model for the turbulence in the liquid phase is employed. And, the surface of the sunken pit formed by impact of the gas jet blown from a top lance at the central location of the bath liquid surface is regarded as a revolution paraboloid. The related details of the composed model are shown.     

AOD精炼不锈钢工艺发展

论述了国外AOD精炼不锈钢技术的发展。总结了太钢AOD精炼不锈钢在品种、质量、工艺研究方面的成果。对AOD炉材质进行了分析。     

顶底复合吹炼转炉冶炼不锈钢

降低不锈钢制造成本,是当前发展国内不锈钢生产的关键所在。采用转炉冶炼不锈钢是降低炼钢工序成本的合理选择。国内外已进行了大量的开发,日本川崎公司K—BOP顶底复吹转炉炼不锈钢已投入生产,太钢K—OBM转炉冶炼不锈钢在技术上是可行的。     

双流体模型在AOD熔池流场数学模拟中的应用

基于气液双流体模型和湍流的修正k-ε模型,考虑了多股气流侧吹操作对熔池流场的影响,以及AOD熔池内气液两相流的行为和两相间的动量传输,建立了AOD多股气流侧吹精炼过程中熔池内流体流动的数学模型,并对宝钢股份不锈钢事业部120 tAOD原型和线尺寸为其1/4的水模型熔池内流体的流动作了模拟,结果表明,确实可以采用双流体模型来模拟AOD精炼过程中熔池内流体的流动;用该模型计算的结果表明,整个熔池流体处于活泼的搅拌和循环运动状态。     

Numerical Simulation of Fluid Flow in Bath during Combined Top and Bottom Blowing VOD Refining Process of Stainless Steel

The fluid flow in a bath in combined top and bottom blowing vacuum‐oxygen decarburization (VOD) refining process of stainless steel has numerically been simulated. The three‐dimensional mathematical model used is essentially based on that proposed in our previous work for the flow in combined side and top blowing argon‐oxygen decarburization (AOD) process, but considering the influence of reduced ambient pressure. Applying it to the flow in the bath of a 120 t VOD vessel under the refining conditions, the results present that the model can fairly well simulate and estimate the flow phenomena. The flow pattern of molten steel in the bath with the combined blowing is a composite result under the common action of the jets from a three‐hole Laval top lance and gas bottom blowing streams. The jets have a leading role on it; the molten steel in the whole bath is in vigorous stirring and circulatory motion during the blowing process. The streams do not alter the basic features of the gas agitation and liquid flow, but can evidently change the local flow pattern of the liquid and increase its turbulent kinetic energy to a certain extent. The flow field and turbulent kinetic energy distribution in the combined blowing with three tuyeres are more uniform than those in the blowing with double tuyeres. Increasing properly the tuyere eccentricities is of advantage for improving the velocity and turbulent kinetic energy distributions, the stirring and mixing result in the practical VOD refining process.     

不锈钢AOD精炼过程数学模拟进展

综述了不锈钢AOD精炼过程的数学模拟。分析了有关模型的成功和不足,指出进一步深入研究该精炼过程具有重要的理论和实际意义。     

Fundamental Mathematical Model for AOD Process. Part I: Derivation of the Model

This paper presents a new simulation model for the AOD process that takes the local variations into account but is still computationally efficient. The new idea here was to model AOD reactor as a combination of a plug flow reactor for the plume zone and a continuously stirred tank reactor (CSTR) for the bath and surface slag. This approach adopted has many advantages compared with the previous models. At first, it offers an effective method for considering the locally varying conditions as the gas bubbles rise in the plume. The model can be built computationally very effective compared to CFD due to significantly smaller amount of variables. The validation of the model is also easier as it has features that can be experimentally determined. The model is based on the simultaneous solution of conservation equations of mass, species and energy in all the vertical cells of the plug flow reactor, and a single volume in bath and surface slag. A novel method was developed and used for solving the rates in a mass transfer controlled multi‐component reaction system. In this Part I of this paper, the model is presented and its features discussed by few illustrative examples. In the following Part II, the model is broadly validated with new full scale industrial AOD process measurements for carbon release rate, melt composition, slag composition and bath temperature rise during final stages of carbon removal.     

太钢新改造的3座AOD转炉投产及其先进的工艺控制技术

太钢3座40tAOD转炉改造后扩容到45t并于2004年4月成功投产,主要采用两步法生产铁素体和奥氏体不锈钢。介绍了该项目的主要特点以及奥钢联AOD技术特点。     

Investigating the Effect of Slag on Decarburization in an AOD Converter Using a Fundamental Model

A high‐temperature thermodynamics model has been coupled with a fundamental mathematical model describing the fluid flow, where boundary conditions were chosen based on data for an industrial AOD converter. Using this model, the effect of both slag phases (a liquid part and a solid part) on the decarburization was studied. More specifically, the separation of chromium oxide to liquid slag as well as the effect of the amount of rigid top slag (solid)on the decarburization was investigated. The liquid slag was considered with respect to the uptake of chromium oxide, while the rigid top slag was only considered with respect to the increase of the metallostatic pressure in the steel melt. The results suggest that separation of chromium oxide to liquid slag results in a decreased decarburization rate. The same conclusion can be drawn with respect to the amount of solid top slag.     

氮在炼钢及连铸过程中的变化及影响因素分析

<介绍了邯钢三炼钢厂从炼钢到连铸生产过程中[N]的变化情况,通过理论分析及生产试验研究影响[N]变化的各种因素,提出了降低[N]含量的改进措施,在生产实践中取得了明显的效果：连铸中间包钢水中[N]由平均54×10-6降低到平均43×10-6,同比降低20.4％。     

转炉预直炼法脱磷期亚音速顶吹的物理模拟

在1∶6物理模型上,通过水模型实验,对梅钢复吹转炉亚音速喷吹条件下的熔池搅拌情况进行了研究。结果表明,亚音速喷吹条件下的熔池均混时间明显长于超音速的。在亚音速喷吹条件下降低枪位、增加顶底吹气体流量可降低均混时间;A2底吹为本实验最佳布置方式,A6为适合预直炼的布置方式。     

氧气顶底复吹热模型试验的数学解析模型

对氧气复吹转炉热模型试验炼钢过程中的冶金反应进行了理论解析,并提出了一个适合于从位于炉底中心位置的透气元件向熔池内部吹入惰性气体的搅拌条件下,可以描述复吹过程中熔池内部的成分和温度变化的数学解析模型,而且模型的计算结果与热模型试验的实测结果相一致。     

Mathematical Modelling of Molten Steel Flow Process in a Whole RH Degasser during the Vacuum Circulation Refining Process: Application of the Model and Results

A three‐dimensional mathematical model for the molten steel flow during the RH refining process has been applied to the circulatory flow processes in both a practical RH degasser and its water model unit. The model was presented earlier [1] and one of its characteristics is that ladle, snorkels and vacuum vessel are regarded as a whole. Using this model, the fluid flow field and the gas holdups of liquid phases and others have been computed respectively for a 90 t RH degasser and its water model unit with a 1/5 linear scale. The results show that the mathematical model can properly describe the flow pattern of molten steel during the refining process in an RH degasser. Except in the area close to the liquid's free surface and in the zone between the two snorkels in the ladle, a strong mixing of the molten steel occurs, especially in the vacuum vessel. However, there is a boundary layer between the descending liquid stream from the down‐snorkel and its surrounding liquid, which is a typical liquid‐liquid two‐phase flow, and the molten steel in the ladle is not in a perfect mixing state. The lifting gas blown is ascending mostly near the up‐snorkel wall, which is more obvious under the conditions of a practical RH degasser, and the flow pattern of the bubbles and molten steel in the up‐snorkel is closer to an annular flow. The calculated circulation rates for the water model unit at different lifting gas rates are in good agreement with experimentally determined values.     

钢包自动底吹氩装置在炼钢厂的应用及生产工艺实践

本文通过对本厂钢包自动底吹氩装置的恢复使用,比较了自动底吹装置与传统的底吹方式的优缺点,通过生产实践及比较,钢包自动底吹氩装置结合数字吹氩系统是可靠、先进的钢包自动化吹氩技术,极具推广应用的价值。     

AOD精炼304不锈钢渗氮、脱氮的计算模型与应用

研究了AOD精炼304不锈钢过程中的渗氮和脱氮行为,建立了渗氮和脱氮计算模型。AOD渗氮模型在120 t AOD装置的验证结果显示,剔除异常数据后,氧化3期氮含量计算值与实测值之间的绝对误差在±100×10^-4%之间的约占总炉数的85%;还原期和脱硫期AOD脱氮模型计算结果与实测值之间的绝对误差在±100×10^-4%之间的约达到80%以上,AOD脱硫期的氮含量命中精度约为90%。