Kinetic Model of Decarburization and Denitrogenation in Vacuum Oxygen Decarburization Process for Ferritic Stainless Steel

The characteristics and classification of decarburization and denitrogenation in the vacuum vessel for stainless steel production are analyzed. Based on the analysis of movements of the liquid steel and bubbles, the kinetics of decarburization and denitrogenation in the vacuum oxygen decarburization (VOD) process has been studied. A kinetic model of decarburization and denitrogenation has been developed to simulate the VOD process, considering each reaction zone as oxygen blowing crater, bottom blowing plume, steel/slag interface, and plume eye. As a result, it is possible to quantify the contribution of each reaction zone in decarburization and denitrogenation rate at a different stage in the VOD process. Specific trials at a vacuum induction furnace were performed to refine stainless steel in vacuum carbon deoxidation (VCD) and VOD style, respectively. The trial results are in good agreement with the model calculation. Combining the trials and the model calculation and the influence of temperature control, critical carbon content selection on the terminal total [C] + [N] content can be discussed further to provide a reasonable proposal for high-quality ferritic stainless steel production. This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France.     

真空下钢液脱氮工艺研究

在４０ｋｇ真空感应炉上进行了真空下钢液脱氮工艺的实验研究。实验考察了真空下碳氧反应工艺、表面活性元素控制和脱氮渣系等因素对钢液脱氮动力学的影响。研究结果表明：（１）真空下强烈的碳氧反应能有效地脱除钢中的氮;（２）当钢中硫含量较低时,脱氮速率很快,随着硫含量的逐步增加,脱氮速率相应降低;（３）５０％ＳｉＯ２－４０％Ｂ２Ｏ３－１０％ＴｉＯ２渣系有明显的脱氮效果。     

钢液真空脱氮动力学研究

研究了真空度对钢液脱氮动力学的影响。结果表明,真空度为６７Ｐａ及１１０Ｐａ进,脱氮的限制性环节为氮在钢液液相边界层中的传质,真空度为２０００Ｐａ时,脱氮的限制性环节为氮在气液界面的化学反应。真空下的碳氧反应对脱氧效果有明显的影响,当有碳氧反应时,钢液脱氮速度常数增大,同时钢液内生或外来固态粒会作为气泡形核核心,加速脱氮过程。     

430铁素体不锈钢脱氮工艺

430铁素体不锈钢中的氮能恶化晶间腐蚀、低温冲击韧性、缺口敏感性以及焊接等性能。在热力学分析的基础上,研究了430铁素体不锈钢冶炼过程中初始碳质量分数、温度、氩氧比以及冶炼时间等工艺参数与对终点氮质量分数的影响。研究表明,在一定范围内,初始碳质量分数越高,氩氧比越大,脱氮效果越好;吹氩20min左右时脱氮效果最好,脱氮率可达到60%左右,继续延长冶炼时间会有回氮现象发生。     

涟钢RH—MFB试生产钢水质量分析

为充分发挥RH—MFB的精炼功能,了解RH—MFB的精炼效果,有必要对上RH的钢水质量进行分析,包括钢中[C]、[N]、T[O]及显微夹杂分析。结果表明：RH—MFB试生产钢水在中间包内增碳较为严重,出RH后钢液有轻微吸氮现象;RH—MFB精炼后,钢液中大于10μm的显微夹杂数量明显减少,T[O]的平均含量较前期生产也有明显下降。对精炼处理及中间包钢液中显微夹杂组成与形貌分析表明,基体为Al2O3的夹杂物是钢液存在的主要夹杂物,形貌特征各异,在工艺后期大多以球状和细小块状存在,此外,发现有少量铝酸盐或硅铝酸盐＋硫化物夹杂。为此,提出有针对性的改进措施,以期进一步提高钢水的质量,达到高效低耗生产的目的。     

Dynamic model for on‐line observation of the current process state during RH degassing

Based on the principles of thermodynamics and reaction kinetics, a dynamic model for the vacuum circulation (RH) process was developed. It comprises decarburisation, oxygen removal, dehydrogenation, denitrogenation and the course of steel temperature. The process model was validated by comparison of simulation results with measured process data from the RH/1 plant of Voest Alpine Stahl Linz (VASL). At this plant, it is used for on‐line observation of the contents of carbon, oxygen, hydrogen and nitrogen as well as the steel temperature. Based on cyclically measured values of vessel pressure, lift gas flow rate as well as waste gas flow rate and composition, the current heat status is calculated reliably. The accuracy of the carbon balance with measured waste gas values was improved remarkably by a model‐based correction calculation. The on‐line process observation system provides the operator with valuable information on the progress of vacuum treatment.     

超低碳汽车板钢洁净度控制

简要介绍了武钢炼钢总厂四分厂BOF→RH-MFB→CC工艺生产超低碳汽车板钢的情况,根据钢中碳、硫、氮、氧在钢水中的反应原理,分别制定了从铁水预处理、转炉、真空精炼到连铸过程的全工序控制措施,将成品碳、硫、氮、氧的平均质量分数分别降至12.7×10-6,47.2×10-6,18.2×10-6,21.3×10-6。几种元素平均值的和从121×10-6降低到99×10-6,净化了钢质,提高了板坯质量。     

VOD冶炼超纯铁素体不锈钢的脱氮研究

分析了VOD冶炼超纯铁素体不锈钢过程的脱氮热力学和动力学,得出其脱氮反应接近二级反应,可用Δ(1/w[N])来表示脱氮效果。用前期开发的脱氮模型计算并分析了VOD的脱氮过程,在VOD吹氧脱碳阶段,脱氮效果Δ(1/w[N])与脱碳量Δw[C]成正比,可用脱氮指数Δ(1/w[N])/Δw[C]来表征此阶段的脱氮能力,铬含量为11.6%、17.6%及20.5%的钢液脱氮指数计算值分别为208%-2、109%-2及81%-2,实际生产数据分析得出的对应的脱氮指数略高于计算值,而在VOD自由脱碳阶段,Δ(1/w[N])随时间变化的计算曲线基本不受初始氮含量的影响,温度是影响脱氮的主要因素。     

RH MFB精炼过程脱碳数学模型及工艺研究

 为研究RH MFB精炼工艺对脱碳过程的影响，将脱碳机理确定为钢液本体脱碳与CO克服静压力上浮、氩气泡表面脱碳和飞溅液滴脱碳，根据脱碳反应动力学和质量守恒原理建立了RH MFB脱碳数学模型。计算结果表明：降低初始碳含量、增大初始氧含量可使脱碳终点碳含量降低；提高压降速率和吹氩流量、增大浸渍管内径使得脱碳速率增大；在固定氧气流量下，随着吹氧时间的延长，脱碳终点碳含量降低，但脱碳终点氧含量升高。     

RH-MFB真空处理工艺技术

介绍了攀钢RH－MFB轻处理LCAK钢和车轴钢,自然脱碳和MFB吹拉强制脱碳处理IF钢,本处理重轨钢以及MFB加热钢水和真空室的工艺,分析讨论了真空降压方式、MFB吹拉对脱碳速度,真空度对终点碳含量影响。     

Laboratory investigation of denitrogenation of low carbon steel by gas generation in situ

Abstract

Denitrogenation of low carbon steel with carbon monoxide generated in situ by the reaction of carburised iron /graphite with dissolved oxygen in the bath has been investigated in an air induction furnace. A melt size of 50 kg was used. Experimental parameters, namely nature and size of the deoxidising reactants and bath parameters, such as dissolved carbon, nitrogen and oxygen levels in the bath and temperature of the bath, were varied to investigate effects on the extent of denitrogenation. A trend of increasing denitrogenation with increasing gas evolution in the bath was observed. The extent of denitrogenation showed a decreasing trend with increasing temperature and oxygen level in the bath. Thermodynamic consistency was checked by calculating the average partial pressure of desorbed nitrogen from the total gas generated and the total volume of nitrogen liberated at the reaction site.     

不锈钢二次精炼的转炉化

对两种不同炉型的不锈钢精炼设备的特点进行了比较,与钢包型精炼设备相比,转炉型精炼设备炉容比大,吹氧和脱碳速度大,生产率高,而且可以使用高碳铬铁,具有真空业炼功能的精炼设备优越的深脱碳,氮热力学条件,真空转炉（ＶＯＤＣ和ＶＣＲ）法在同一单元四兼有转炉型和真空精炼功能,是精炼超低碳,氮不锈钢的理想设备,特别是ＡＯＤ－ＶＣＲ法更适合于我国现有ＡＯＤ精炼炉的改造,用于生产超纯铁素体不锈钢。     

氩直流辉光等离子体钢液脱氮的研究

用25 kg真空感应炉对0.02%C钢液进行氩直流(DC)辉光等离子体条件的脱氮实验,研究了氩辉光等离子体对钢液脱氮动力学的影响。结果表明,氩辉光等离子体钢液脱氮速度和脱氮效果明显优于真空钢液脱氮,当钢液氧含量为180×10^-6,在氩辉光等离子体下钢液中氮含量可降至9×10^-6。氩辉光等离子体对钢液的脱氮效果体现在界面化学反应上,提供了加速脱氮反应的动力学条件。     

Nitrogen desorption from pure iron melts under reduced pressure

More recently, lower nitrogen contents of steel are required in view of its workability and applicability. The present study covers basic investigations on the improvement of mass transfer in denitrogenation of steel melts under vacuum. Extensive reduction of the contents of the surface-active elements oxygen and sulfur is a precondition to increase the rate of nitrogen desorption. Kinetics of nitrogen transfer is reliably described by a mixed control model which includes diffusion in the melt, interfacial reaction and diffusion in the gas phase. Particularly at low oxygen and sulfur contents, mass transfer of nitrogen during desorption from steel melts is markedly accelerated. A convincing explanation of this phenomenon has not been found yet.     

增氮析氮法生成气泡去除钢液中显微非金属夹杂物

为了进一步完善增氮析氮法生成气泡去除钢液中显微非金属夹杂物技术,研究了真空处理时间、充氮压力、气体类型等因素对钢中全氧和显微非金属夹杂物的影响.结果表明:减压处理过程中,钢液中非金属夹杂物可为过饱和气体氮气形成气泡提供非均相形核核心;增氮析氮法可有效地降低钢中全氧,去除钢中显微非金属夹杂物;真空处理时间越长,钢中全氧和显微非金属夹杂物数量越低,当真空处理时间为30 min时钢中全氧去除率达到了81.6%,而且全氧质量分数最低达到7×10-6.      

太钢VOD冶炼超纯铁素体不锈钢的工艺技术进步

阐述了超纯铁素体不锈钢的超低碳氮的特点及其熔体降碳去氮困难的原因,利用真空降碳去氮的理论结合这方面的研究成果分析和讨论了影响VOD脱碳脱氮的影响因素,并利用VOD现场冶炼的具体数据进行了这些因素的统计分析,在此基础上提出了提高真空度、加强底吹氩搅拌强度、提高入炉钢液温度、提高人炉碳含量和降低人炉氮含量、增加VOD吹氧脱碳时的供氧量、高真空吹氩纯沸腾工艺、选用无碳、或低碳还原料等工艺技术措施,最后介绍了太钢这几年在VOD冶炼超纯铁素体不锈钢采取上述措施后所取得的效果。     

On Physico‐Chemical and Technical Limits in Clean Steel Production

Great progresses in steel cleanliness have been attained during the last decades. In the measures of oxygen the lowest levels are approaching 5 ppm in O_tot i.e. close to the thermodynamic limit of Al deoxidation. As the thermodynamics of the reaction system is firmly anchored to the steel chemistry and thus to the properties of steel and the final product, it is useful to examine the thermodynamic constraints for selected steels in the refining and casting processes. In most steels Al‐O equilibrium determines the limit of deoxidation. Calculations by applying thermodynamic software showed that in selected “common” steels the equilibrium oxygen content varied from below 5 ppm up to 30 ppm or higher depending on the aluminium content, interaction effect of other alloying elements and temperature. For lower oxygen content there are several possibilities. The first one is to use stronger deoxidizing additions like Ca, Zr, Ce etc. However, they can be problematic as substitutes as they produce inclusions which influence steel properties. The second way is to intensify the deoxidizing power of certain elements e.g. Si by bringing the steel in intimate contact with a proper slag with low SiO₂ activity in ladle treatment with violent stirring. In the calculation example it was possible to decrease oxygen content from 30 ppm to the level below 10 ppm in C/Mn/Si steel at very low Al level. The third potential but unused process is vacuum deoxidation especially for medium and high carbon steels. Equilibrium oxygen contents below 1 ppm are thermodynamically easily attainable. But in practice the vacuum treatment should be designed to intensify the carbon‐oxygen reaction on the top surface of the stirred steel liquid. Also the eventual reactions with refractory materials should be suppressed.     

LF/VD过程钢液氮含量控制试验研究

结合现场试验分析了LF/VD过程影响钢液氮含量的因素。结果表明:低氧、低硫钢液,在LF精炼钢液容易吸氮,而在VD有利于脱氮;LF过程喂线可使钢液增氮;真空度小于53.3Pa,保持时间大于10min,可生产出w(N)低于40×10~(-6)的钢液。     

VD真空精炼脱氢和脱氮过程的三维数值模拟

基于质量、动量守恒计算了真空脱气装置内流场,并结合反应热力学和动力学理论建立了真空精炼过程中脱氢、脱氮与流场耦合的三维数学模型,研究了不同工艺参数对VD脱氢和脱氮过程的影响。结果表明:钢包内流场对脱气过程中氢和氮浓度空间分布具有明显影响;脱氢和脱氮速率随底吹气量的增加而增大,终点氢和氮浓度随底吹气量的增加而减小;在实际生产中,应在避免发生卷渣的前提下合理选择吹气量;不同底吹方式对脱氢和脱氮过程影响明显,其中偏心底吹效果优于中心底吹,双底吹效果最好。     

85 t VOD精炼超纯铁素体不锈钢的脱氮工艺及效果

分析了85 t VOD精炼时相关工艺参数对超纯铁素体不锈钢00Cr18Mo2和00Cr17Mo终点氮含量的影响。结果表明,随初始碳含量增加,初始氮含量降低,钢水温度提高,适当增加脱氮时间,VOD钢水终点氮含量降低;当控制钢水初始碳含量0.4%～0.9%、处理温度≥1 590℃、真空度≤70 Pa、脱氮时间15～20 min、吹氩搅拌强度8～15 L/(min·t)、初始氮含量≤0.017 0%,VOD终点钢水氮含量为0.006 4%～0.009 5%。