Effect of sulfur on supersaturation in aluminum deoxidized liquid iron

The effect of sulfur addition on supersaturation observed in aluminum deoxidized liquid iron was studied at 1873 K, by using CaO-Al₂O₃ slags in an alumina or lime crucible. The contents of supersaturated oxygen for a given aluminum level were not influenced by the addition of sulfur (mass contents of 1×10^?5 ～ 0.42%) in the experiments where the sample was rapidly quenched. However, in the experiments in which the sample was cooled from 1873 to 1723 K at a cooling rate of 2.7 K/min, the effect of sulfur on supersaturation was observed for mass contents of more than 0.01% sulfur. The critical supersaturation for alumina precipitation was discussed based on the variation of the interfacial energy between liquid iron and alumina.     

Characteristics of Alumina‐Based Inclusions in Low Carbon Al‐Killed Steel under No‐Stirring Condition

Alumina‐based inclusions have a detrimental effect on castability and the surface quality of the LCAK steel sheet, thus they are expected to be removed from the steel. In order to get the detailed transient characteristics of inclusion aggregation and removal after Al addition, laboratory experiments were performed to study the formation of alumina inclusions during Al‐killed process of low carbon steel at 1873 K under no‐stirring condition. The characteristics of the alumina‐based inclusions in terms of amount, size, and morphology were investigated. The results showed that the evolution of the AF (area fraction) and average size of Al₂O₃ inclusions after deoxidation was divided into three stages: 0–9 min, aggregation was dominant; 9–22 min, floating was dominant; after 22 min, both the AF and size decreased slowly. Accordingly, the stirring was suggested to be strengthened in the initial 9 min under stirring condition to promote the removal of large inclusions. Based on the correlation between inclusion morphology and holding time, a simple precipitation and growth mechanism of alumina inclusions was proposed, which consisted of three main stages: precipitation stage of Al₂O₃ single particles, aggregation stage, and sintering stage of aggregated inclusions. And it was implied that the Ostwald‐Ripening can be finished up within 20 min.     

Structure of continuously cooled low-carbon vanadium steels

The structures of four 0.15 pct carbon steels containing vanadium, nitrogen, and aluminum separately and together were studied systematically, with the help of transmission electron microscopy, by cooling suitable steels at four different rates ranging from 120 °C/min to 3.6 °C/ min from temperatures giving a common austenite grain size of 35 μm. Except for the steel containing only vanadium and that containing only aluminum and nitrogen cooled at the fastest rate used, the observed microstructures were all essentially mixtures of polygonal ferrite and expected amounts for pearlite. For all the steels studied, except the one containing aluminum and nitrogen, it was found that general precipitation was more common than interphase precipitation, although the extent of the latter increased at lower cooling rates. Moreover, in some cases, both general and interphase precipitation were present in the same area. The presence of aluminum was observed to enhance the formation of interphase precipitates at all cooling rates, and the spacing between parallel rows of precipitates increased as the cooling rate was decreased. The dislocation density was high at all cooling rates in all the steels, but it was found to decrease with decreasing cooling rates. Very fine precipitates were found in all the steels, except the steel containing aluminum and nitrogen. At the fast cooling rates, the segregation of vanadium and interstitial elements, which led to locally lower transformation temperatures and higher supersaturations, resulted in clusters of fine particles of vanadium carbonitride, V(C, N). At the slower cooling rates, all the steels showed severe heterogeneity in precipitate morphology which was more pronounced in the steel containing aluminum and nitrogen, while a needlelike morphology of V(C, N) precipitate was occasionally found in steels containing either vanadium and nitrogen or vanadium, nitrogen, and aluminum. As the cooling rate decreased, particle coarsening and growth occurred, causing a reduction in the number of particles/unit area. The coarsening rate of V(C,N) in the presence of aluminum is considerably lower than that of vanadium carbide, VC, or of V(C, N) in the absence of aluminum. Because of the unfavorable precipitation kinetics, any aluminum nitride (A1N) formed during cooling did not nucleate separately but was deposited on the pre-existing A1N particles, thus causing them to be coarsened very rapidly with decreasing cooling rate. Formerly with the Department of Metallurgy, The University of Sheffield, Sheffield, England     

Systematic Approach to Microstructure Design of Ni-Base Alloys Using Classical Nucleation and Growth Relations Coupled with Phase Field Modeling

Absract To analyze the formation of bimodal particle size distributions during precipitation, the dynamic competition for supersaturation by growth of existing precipitates and nucleation of new particles was studied under continuous cooling conditions with constant cooling rates. The nucleation rate was calculated according to classical nucleation theory as a function of local supersaturation and temperature. The depletion of matrix supersaturation by growth of existing particles was calculated from fully diffusion-controlled precipitate growth in an infinite matrix. Phase field simulations of γ′ precipitation in a binary Ni-Al alloy were performed under continuous cooling conditions. Then the average and maximum matrix supersaturations were calculated and plotted onto the contours of nucleation rate and growth rate in concentration and temperature space. These methods were used iteratively to identify the window for bimodal particle size distributions. This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, OH, under the auspices of the TMS/ASMI Phase Transformations Committee.     

SANS and TEM studies of isothermal M2C carbide precipitation in ultrahigh strength AF1410 steels

Results are presented of SANS and TEM experimental investigations of the precipitation and growth, as a function of isothermal ageing time, of the main secondary hardening M₂C phase in ultrahigh-strength AF1410 steel. The results obtained are compared with APFIM investigations and with theoretical models. Combined use of these techniques provides valuable diagnostic data for quantifying the development of microstructures in this class of steels. It is found that the observed precipitation behaviour is consistent with the theory of precipitation at high supersaturations, but includes a surprising secondary burst of nucleation at precipitation half completion.     

The inhibition of calcium oxalate monohydrate crystal growth by maleic acid copolymers

The crystallization of calcium oxalate monohydrate (COM) was investigated at conditions of constant supersaturation both in the absence and in the presence of synthetic maleic acid copolymers at 37C, 0.15 M NaCl. The dependence of the rates of COM crystallization in the absence of inhibitors was found to be second order at low and first order at higher supersaturations suggesting a surface diffusion controlled mechanism. The presence of all copolymers tested at concentration levels up to 5 ppm retarded the rates of COM crystal growth up to 90%. The decrease of the COM crystal growth rates by the polymers depended on the nature of the comonomer polymerized with maleic acid and the order of inhibition was found to be vinyl acetate > N-vinylpyrrolidone > styrene. Taking into consideration kinetics data published in the literature concerning the inhibition of COM crystal growth, it is suggested that molecular weight also plays a role, with more inhibition at higher molecular weights. The morphology of the COM crystals grown was unaffected yet the crystals growing at lower rates in the presence of the copolymers were larger and their size more uniform. It is concluded that maleic acid copolymers are strong inhibitors of the crystallization of COM, the inhibitory activity being more pronounced in the case of the linear copolymers.     

中碳钢钙处理夹杂物变性的“液相窗口”控制模型

 系统研究了国内某钢厂生产的中碳钢Q345B钙处理前后夹杂物类型的变化,从热力学上分析铝脱氧钢中Al2O3夹杂物变性机制及夹杂物中CaS合理控制的条件,确立了夹杂物变性的“液相窗口”模型。热力学计算表明,温度为1873K,w(［Al］)为0.016%时,Al2O3转变为液态钙铝酸盐需要使钢中w(［Ca］)为0.0017%~0.0102%。生产实践表明,钙喂入量在0.0014%~0.0017%时,钙处理可以将钢中高熔点Al2O3的夹杂物转化为低熔点的12CaO·7Al2O3和CaO·Al2O3夹杂物,MnS基本转化为CaS,且无单独CaS析出。     

氧化铝种分过程的粒度分析

用粒数衡算的方法对氧化铝生产种分过程氢氧化铝粒度分布进行了分析,通过计算得到各种分槽产品氢氧化铝的晶体生长速率和净粒子生成数,由此推测了氧化铝实际生产过程中成核、生长、附聚过程发生的位置及生产条件。并分析了晶体生长速率与种分进行时间之间的关系。     

脱氧顺序对铝钛复合夹杂物的影响

 通过扫描电镜-能谱仪检测和分析了不同铝钛脱氧顺序下钢中夹杂物的形貌、成分、尺寸、数量和分布等参数,通过热力学计算分析了脱氧过程中钢液中的化学反应和夹杂物优势区图。结果表明,脱氧剂添加顺序对夹杂物形貌影响很大,先加钛后加铝的脱氧方式下,钢液中形成了较多含有铁相(“空心”)的具有浓度梯度的铝钛复合夹杂物。夹杂物径向长度增加,夹杂物也更容易偏聚。所以先加钛后加铝不利于夹杂物尺寸细小化,不利于夹杂物弥散分布。先加铝后加钛的脱氧方式下,钢中形成氧化铝夹杂,不会被溶解的钛还原,因此夹杂物内部不含有铁相。夹杂物主要为Al-Ti-O(-N)类夹杂。夹杂物尺寸和数量小于先加钛后加铝钢中的夹杂物。通过FactSage计算结果、化学反应分析和试验检测结果,探究了不同铝钛脱氧顺序下夹杂物形成和演变机理,分析了具有浓度梯度的“空心结构”的铝钛复合夹杂物形成机理,讨论了脱氧剂添加顺序对夹杂物尺寸、数量和分布等特征的影响规律。发现先加钛后加铝的脱氧方式下,钛氧化物会与金属铝反应,钛氧化物逐渐转变为“空心”氧化铝壳,同时溶解的铝、钛和氧发生氧化反应,形成了具有浓度梯度的Al₂O₃-TiO_x复合夹杂物,最终钛氧化物完全转变为氧化铝壳而消失。随着铁液的填充,形成了含有铁相的铝钛夹杂物。     

The precipitation sequence of Ni3Ti in Co-free maraging steel

TEM, microdiffraction, and X-ray microanalysis were used to study the precipitation processes in Co-free maraging steel. Austenite crystals were found to nucleate in the martensite matrix as the first step in the precipitation sequence of Ni₃Ti. The austenite reversion is a result of Ni segregation. Ni₃Ti nucleates in the austenite. The shape and orientation of Ni₃Ti is determined by the austenite precursor. The same sequence occurs for heterogeneous nucleation on dislocations and grain boundaries. At the later stages of growth Mo is incorporated in the Ni₃Ti lattice.     

Grain Growth of Deformation Induced Ferrite in V Microalloyed Low Carbon Steel During Controlled Cooling Process

The effect of vanadium on the DIFT (Deformation Induced Ferrite Transformation) microstructure coarsening in low carbon steel during the continuous cooling processes and isothermal processes at different temperatures were investigated using thermo-simulator.The results showed that the steel containing a small amount of vanadium had the similar velocity of grain growth with the vanadium free steel during the continuous cooling process,but a lower velocity of grain growth than that of vanadium free steel during isothermal processes at high temperatures.On the other hand,the vanadium remarkably inhibited grain growth in the steel containing a high amount of vanadium during both the continuous cooling and isothermal processes.Vanadium dissolved in matrix is indicated as an important factor on restraining grain growth through estimating the driving force of normal grain growth and the resistance of precipitation particles of vanadium on grain growth.The influencing mechanism of vanadium dissolved in matrix on the grain growth during the controlled cooling process is discussed.     

Morphology and growth of alumina inclusions in Fe–Al alloys at low oxygen partial pressure

Abstract

The degree of supersaturation is a factor that influences the Al₂O₃ inclusion characteristics in steel. The influence of the addition of a large amount of Al in the molten steel on the formation, growth and morphology of Al₂O₃ inclusions was investigated by laboratory scale experiments. Consecutive steel samples were taken during the deoxidation process and subjected to chemical analysis (ICP-AES), automated image analysis (AIA) and scanning electron microscopy (SEM) assessment with respect to the extracted inclusions. The characterisation and quantification of Al₂O₃ particles show different growth processes, leading to variations in particle size distribution as well as in the morphology.     

Crystallographic Analysis of Tapering of ADP Crystallites

On the basis of crystallographic characteristics of ADP (ammonium dihydrogen phosphate) crystals and the selected growth conditions, the growth habit of ADP crystals was studied. In comparison with pyramidal planes, the growth rate of prismatic faces is slower and more sensitive to the additives and impurities for ADP crystals. When the supersaturation is low, the advance of growth steps on prismatic face can be blocked by ethanol or impurities, the crystal morphology is changed from the tetragonal prism to shuttle (i.e., the tapered shape). The tapering formation of ADP crystallites was structurally studied in a novel view.     

High-temperature transmission electron microscopyin situ study of lower bainite carbide precipitation

In situ observation of the bainite carbide precipitation processes in 40CrMnSiMoV steel by means of high-temperature transmission electron microscopy (TEM) is conducted. It is evident that carbides can precipitate either in bainitic ferrite or from austenite when carbide-free bainite (meta-bainite) obtained by isothermal transformation is tempered at higher temperatures. In view of the quantity of carbides precipitated from ferrite in combination with the result of an X-ray diffraction analysis of the bainitic ferrite carbon content, it can be concluded that bainitic ferrite growth involves supersaturation of carbon content to some degree. Formerly with Northwestern Polytechnical University Formerly with Northwestern Polytechnical University This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Conditions of Liquid Steel Treatment for Near Net‐Shape Casting Processes

Slab casting for hot rolled steel and strip casting using the twin roll casting (TRC) method are compared in terms of inclusions evolution. There are differences of the processes, mainly the use of casting flux in CC of slabs, or pool shapes and sizes in terms of TRC, particularly when roll diameters vary. The inclusion evolution of alumina in a low carbon steel grade was estimated. By modelling particle growth rate, coagulation and deposition, an ‘agglomeration index’ was created to describe the probability of clogging in the SEN. Similarly the growth of secondary alumina precipitation during cooling of the melt in the pool by Stokes collision and turbulent collision was estimated in terms of forming large particles which are able to float‐up. Influences of melt superheat or caster size were taken into account.     

低碳铝镇静钢中间包浇注过程夹杂物的行为

 通过对BOF-Ar站-CC炼钢流程生产低碳铝镇静钢的中间包不同浇注时间取样及正常坯的取样,采用氧氮化学分析、光学显微镜以及扫描电镜-能谱(SEM+EDS)等多种方法研究了中间包浇注过程夹杂物特征的变化。结果表明：每炉钢包开浇时与浇注末期,钢中T[O]含量均高于浇注中期的T[O]含量,这是由于换包过程中钢水被二次氧化;中间包钢水及正常坯中的夹杂物,按照其形貌与成分可以分为以下3类：Al2O3基夹杂物,MnS基夹杂物,来自中间包覆盖剂或者钢包下渣所卷入的外来夹杂物。中间包及铸坯中的夹杂物主要以1~4 μm的Al2O3为主,同时在铸坯中发现了大量的MnS夹杂物,使铸坯中夹杂物的数量密度升高。当钢液中硫含量较高时,铸坯中气泡+Al2O3类型的夹杂物增加。在当前的工艺条件下,交换钢包之后的开浇阶段与浇注末期,钢水的二次氧化对铸坯的洁净度产生重要影响,同时应合理控制钢中的硫含量,减少铸坯中气泡+Al2O3类型的夹杂物,避免钢液在凝固过程中析出大量的MnS夹杂物。     

Modification of oxide inclusions in calcium-treated Al-killed high sulphur steels

A study has been carried out to understand the modification of alumina inclusions in Al-killed high sulphur steel with calcium treatment. For calcium treatment to be effective, a general practice is to desulphurise the steel to prevent the formation of solid CaS inclusions that are harmful to steel quality and final properties. To avoid this additional desulphurising step, the authors developed a new approach of calcium treatment of steel at an industrial scale. This approach involves treating the liquid steel with calcium treatment at low aluminium levels which enables formation of liquid calcium aluminate inclusions (C12A7) in the melt and then adding remaining amount of required aluminium. Based on this principle, Al-S diagram has been developed and calcium treatment has been modified accordingly. The inclusion transformation and morphology were studied using scanning electron microscope /energy dispersive spectroscopy analysis and. activity of CaS was calculated.     

Effect of tellurium on supersaturation in aluminum deoxidized liquid iron

The effect of tellurium addition on supersaturation observed in aluminum deoxidized liquid iron was studied at 1873 K, using CaO-Al₂O₃ slags in an alumina crucible. Supersaturated oxygen content for a given aluminum level decreased with the addition of tellurium as a result of lowering the interfacial energy between liquid iron and alumina and promoting the elimination of precipitated alumina by flotation. Furthermore, the degree of lowering of oxygen content was found to increase with a decrease in cooling rate. Tellurium distribution ratio between liquid iron and CaO-Al₂O₃ slags was determined as a function of aluminum content.     

Calcium Modification of Spinel Inclusions in Aluminum-Killed Steel: Reaction Steps

Calcium treatment is a well-established way to modify solid alumina inclusions to liquid or partially liquid calcium aluminates. Spinels (Al₂O₃·xMgO) can also form in liquid steel after aluminum deoxidation. Like alumina, the spinels can be modified readily to liquid inclusions by a calcium treatment. The modification of spinels was studied by observing the transient evolution of inclusions, in laboratory and industrial heats. Spinel modification involves the preferential reduction of MgO from the spinel, with Mg dissolving in the steel, and it proceeds through transient calcium sulfide formation, just like in the case of alumina inclusions. Because magnesium dissolves in steel after the calcium treatment of spinels, the reoxidation of the melt will produce new spinels.