Dissolution of carbon from alumina-carbon mixtures into liquid iron: Influence of carbonaceous materials

Due to their excellent thermal shock and wear resistance at high temperatures, alumina-carbon based refractories are used extensively in the steel industry. A clear understanding of factors affecting the dissolution of carbon from refractories is of crucial importance, as carbon depletion from the refractory can significantly deteriorate refractory performance and metal quality. Atomistic simulations on the alumina-graphite/liquid iron system have shown that nonwetting between alumina and liquid iron is an important factor inhibiting the penetration of liquid metal in the refractory matrix and limiting carbon dissolution. This study investigates the role played by the carbonaceous material in the dissolution of carbon from the refractory composite. Two carbonaceous materials, namely, petroleum coke and natural graphite, respectively, containing 0.35 and 5.26 pct ash, were used in this study. Substrates were prepared from mixtures of alumina and carbon over a wide concentration range. Using a sessile drop arrangement, carbon pickup by liquid iron from alumina-carbon mixtures was measured at 1550 °C and was compared with the carbon pickup from alumina-synthetic graphite mixtures. These studies were supplemented with wettability measurements and microscopic investigations on the interfacial region. For high alumina concentrations (>40 wt pct), carbon dissolution from refractory mixtures was found to be negligible for all carbonaceous materials under investigation. Significant differences however were observed at lower alumina concentrations. Carbon dissolution from alumina-petroleum coke mixtures was much lower than the corresponding dissolution from alumina synthetic graphite-mixtures and was attributed to poor wettability of petroleum coke with liquid iron, its structural disorder, and the presence of sulfur. Very high levels of carbon dissolution, however, were observed from alumina-natural graphite mixtures, with carbon pickup by liquid iron from mixtures with up to 30 wt pct alumina reaching saturation. A sharp reduction to near zero levels was observed in the 30 to 40 wt pct alumina range. Along with implications for commercial refractory applications, these results are discussed in terms of material characteristics, interactions between ash impurities and alumina, and formation of complexes in the interfacial region.     

Interaction of alumina inclusions in steel with calcium-containing materials

Clogging of tundish and submerged entry nozzles (SENs) adversely impacts productivity and quality in the continuous casting of aluminum-killed steels. Clogging results from an accretion layer that develops on the inside surface of the nozzle and restricts steel flow. Current nozzle refractories often react with molten steel to form solid by products that promote clogging. Nozzle materials that are inert with the liquid steel or react with accretions to form liquid reaction products could inhibit or eliminate clogging. Static experiments were conducted to investigate the stability between calcium-based materials and aluminum-killed steel. The results indicate that both calcium titanate and calcium zirconate react with alumina to form calcium aluminates. However, only the reaction between alumina and calcium titanate yielded calcium aluminate chemistries that were molten at steel casting temperatures. Liquid reaction products are preferred since they would be removed from the nozzle by the steel flow, thereby preventing accretion formation and clogging.     

Mechanism of alumina buildup in tundish nozzles during continuous casting of aluminum-killed steels

During continuous casting of aluminum-killed steels, the pouring rate through the tundish nozzle often diminishes, posing serious operating problems. This happens because a buildup of microscopic alumina particles at the nozzle orifice effectively decreases the nozzle orifice diameter and causes constriction of the liquid-steel pouring stream. There are two major aspects of the constriction problem: (1) the source of the depositing particles, and (2) the mechanism by which the particles deposit at the nozzle orifice. Detailed microscopic examination of the buildup of alumina particles in nozzles from various casting trials and petrographic examination of the nozzle refractory indicated that the particles depositing at the nozzle orifice were already present in the melt because of deoxidation and reoxidation processes. A model proposed herein explains why and how the alumina particles deposit and stay at the nozzle orifice. The model considers a microscopically thin boundary layer at the nozzle bore where the velocity of the liquid steel approaches zero. Particles passing close to the refractory surface and in the slow-moving boundary layer attach to the wall and to each other. The presence of eddy currents in and close to the turbulent boundary layer increase the particle to particle collision. Such collisions of the particles thrown from outer region of boundary layers with the already attached particles keep on dumping the particles toward the refractory surface. The high interfacial energy of alumina inclusions in steel is a driving force for particles to attach to the refractory wall and to each other, and high-température sintering then occurs to form a network of alumina particles.     

Formation and evolution of macro inclusions in IF steels during continuous casting

The evolution of macro inclusions during continuous casting was investigated by large-area inclusion characterization using ASPEX and analysis of nozzle blockage deposits. Six kinds of inclusions over 5?μm were observed in samples taken from the tundish and the slabs: single alumina particles, alumina dendrites, refractory-related alumina, alumina associated with bubbles, alumina clusters and Al–Ti complex oxides. By examining the morphology of the nozzle blockage deposit, it was concluded that the refractory-related alumina in the slabs came from the decarburization layer washed away by the steel stream. Some of the alumina clusters that came from the nozzle blockage deposit dislodged by the steel flow, were formed by the agglomeration and sintering of 5–20?μm inclusions that were carried over from the tundish. Two kinds of Al–Ti oxides were found in the tundish, and their evolution mechanisms during the casting process were proposed.     

铝碳耐火材料力学性能优化研究进展

铝碳耐火材料是冶金连铸、铁水包、中间包等部位常用的耐火材料,因其使用环境恶劣,对强度、抗热震性、抗渣侵蚀性等各项性能的要求都较高.优化铝碳耐火材料的力学性能能降低因机械应力引发的物理损毁和热应力引发的热震损毁,直接影响使用寿命.优化耐火材料的原料是易实现且高效的研究方向.从制备铝碳耐火材料的3种原料出发,分别从催化改性...     

纳米技术在镁质耐火材料中应用的研究进展

利用纳米技术制备复相镁质耐火材料,不仅可以缓解高温工业对高性能镁质材料的需求,而且又能实现镁质耐火材料的轻质化和多功能化,进而达到提高产品附加值的目的。因此,利用纳米技术制备复相镁质耐火材料具有较高的研究意义。从镁质耐火材料损毁机制的角度,综述了近年来国内外纳米技术在低碳镁碳质、镁钙质、镁铝质耐火材料中的研究现状和进展,并且分析了纳米技术在镁质耐火材料中的作用机理,最后指出了纳米技术在镁质耐火材料中应用所面临的挑战和发展方向。      

添加Al2O3--ZrO2复合粉改性氧化锆质定径水口及其损毁机理

通过添加溶胶-凝胶法制备的Al₂O₃-ZrO₂复合粉,对陶瓷型和颗粒型氧化锆质定径水口进行改性.对比研究了陶瓷型和颗粒型定径水口改性前后物理性能,矿物相组成和显微结构的变化,并通过现场连铸实验对四种定径水口损毁机理进行了探讨.结果表明:相较于颗粒型定径水口,陶瓷型定径水口的显气孔率较低,体积密度和耐压强度较高,热震稳定性较差,抗侵蚀和冲刷性能较好.通过添加Al₂O₃-ZrO₂复合粉改性后的定径水口显气孔率降低,体积密度增加,耐压强度提高,颗粒型定径水口的热震稳定性有了较大的提升,热震次数约为改性前的1.5倍以上.通过对连铸现场实际使用35 h后的残样分析发现,陶瓷型水口损毁主要是由于热震稳定性差导致使用时发生炸裂,炸裂产生的裂纹引起一定程度的剥落和扩径.未改性颗粒型定径水口由于强度低和显气孔率高,剥落和扩径更为严重,添加Al₂O₃-ZrO₂复合粉改性后生成的镁铝尖晶石增强相大幅度提高了颗粒水口的热震稳定性和抗冲刷侵蚀性能,连铸现场使用后几乎未扩径.      

Development of indices for quantification of nozzle clogging during continuous slab casting

Abstract

Deposition of alumina inclusions inside the submerge entry nozzle is a long standing problem in continuous casting of aluminium killed steel. Clogging of the casting nozzle severely affects productivity and also causes degradation of product quality. Unfortunately, very few numerical tools are available for precise quantification of the magnitude of clogging. Tools for quantifying the extent of clogging were developed during the present work and have been integrated with the control system of a running continuous slab casting shop. Reasonable match has been observed between the estimation from the developed program and the actual observations in the casting shop and subsequent products. In addition to process control, the tools were also found useful for comparing the castability of different steel grades. This feature has been utilised to study nozzle clogging during casting of steel grades that are commonly believed to have poor castability.     

连铸用浸入式水口(SEN)的发展

浸入式水口是连铸用关键功能耐火材料之一,其使用效果直接影响连铸效率和铸坯质量.随着连铸技术的发展,浸入式水口也经历了不同的发展阶段.本文介绍了浸入式水口在连铸中的位置及其作用,着重介绍了不同时期不同材质浸入式水口的优缺点和几种具有防堵功能的浸入式水口,并对浸入式水口的发展趋势进行了分析.     

以废弃滑板提取的二次颗粒代替铁沟料中高铝钒土的研究

从废弃的铝碳滑板中获取二次颗粒原料,并将其用于高炉铁沟捣打料,部分替代高铝矾土骨料。结果表明：铁沟料的耐压强度和抗折强度提高,膨胀率降低,加入量以质量分数不超过40％为宜。研制的铁沟料在广钢炼铁厂进行了两批次试用,平均通铁量达3．87万吨,使用寿命和用后残厚均与目前使用的新料相当。     

长水口渣线部位用镁碳质耐火材料的性能研究及应用

摘要：为了进一步提高长水口渣线部位用镁碳质耐火材料的服役寿命,研究了Si粉添加量（质量分数1%~5%）对镁碳质耐火材料显气孔率、体积密度、常温强度和高温抗折强度等关键性能的影响,以及与传统铝碳质耐火材料的抗渣性能对比。结果表明,随着Si粉添加量的增加,镁碳质耐火材料的显气孔率略有降低,常温强度影响不大,高温强度明显增强。其中,当Si粉添加量为5%（质量分数）时,镁碳质耐火材料高温抗折强度提高了30.2%。此外,研究发现,铝碳质耐火材料的侵蚀速率随中间包渣碱度的增大而增大,而镁碳质耐火材料在面对高碱度渣时表现出更好的抗侵蚀能力。最后,在钢厂实际环境的产品测试结果也印证了Si粉对镁碳质耐火材料使用性能的有效提升,具体为用常规铝碳质渣线浇铸SPHD、SPHE和IF钢种时的使用寿命仅为1~3炉,而用镁碳质长水口渣线浇铸的使用寿命提升到3~6炉。     

Cavity formation during tensile straining of particulate and short fibre metal matrix composites

The formation of cavities in commercially pure aluminium composites, made by both powder and casting routes and reinforced with alumina (short fibres, angular particles and spherical particles), has been monitored using periodic density measurements during tensile testing and microstructural examinations. Stable cavities always form well before final failure, usually adjacent to the reinforcement, particularly when it is elongated in the loading direction and has a relatively flat surface normal to the stress axis. Sharp corners are not favoured cavitation sites and cavities can form at spherical particles, although the incidence is somewhat less than for angular particles. Cavitation occurred earlier for higher reinforcement contents and with powder-route, as opposed to cast, material, although the void contents and composite strains at failure were similar. A simple geometrical model is proposed, allowing prediction of the failure strain as a function of the reinforcement content, aspect ratio and strain to failure of the unreinforced matrix. The data presented are in good agreement with predictions from this model.     

Effect of Nozzle Base Material on the Rate of Clogging during the Continuous Casting of Aluminum‐Killed Steels

One approach to solving the problem of nozzle clogging during the continuous casting of aluminum‐killed steels is through careful selection of nozzle materials. In this study, the rate of clogging was measured while casting steel through simulated nozzles produced from alumina, zirconia, magnesia, zirconia‐graphite, and alumina‐graphite ‐ three common base materials and two common carbon‐containing nozzle materials. Spent nozzles were characterized using optical and cathodoluminescence microscopy. Interactions between the nozzles and steel were not observed in the alumina, zirconia, and zirconia‐graphite nozzles. Slight interactions were observed in the magnesia nozzles as alumina inclusions within the steel interacted with the nozzle to produce alumina‐magnesia spinel. Greater amounts of interaction were observed with alumina‐graphite nozzles. No statistical differences in the mean rate of clogging were observed between all of the pure oxide nozzles and zirconia‐graphite nozzles. However, the alumina‐graphite nozzles clogged at a much higher rate than the other nozzles. The higher rate of clogging is thought to be due to refractory‐steel interactions.     

添加氧化铈对反应烧结制备镁铝尖晶石材料性能的影响

为了进一步提高高品质钢冶炼用镁铝尖晶石耐火材料的关键性能,以电熔镁铝尖晶石、轻烧氧化镁和白刚玉为原料,镁铝溶胶为结合剂,氧化铈为添加剂,采用反应烧结工艺成功制备了镁铝尖晶石材料.系统研究了氧化铈添加量(质量分数,0、3％,6％、9％和12％)对合成镁铝尖晶石材料显气孔率、体积密度、线收缩率、体积收缩率、常温耐压强度和抗...     

Interaction between molten steel and different kinds of MgO based tundish linings

Abstract

Interactions between the molten steel and the refractory linings are of fundamental importance for the steelmaking industry. During the casting process, the steel cleanliness can be made worse because of the reoxidation of the molten steel in contact with the atmosphere and/or other sources of oxygen (refractories, top slag, etc.). In the industrial trial periods, four tundish refractory linings (preformed boards, two kinds of gunning materials and dry vibrating material, all MgO based refractories) were used in order to evaluate the interactions between the refractories and the molten steel. Owing to the small dimension of the tundish (5 ton) used in the continuous casting at Villares Metals SA, after casting, the chilled steel was cut in order to show, in a single section, the stopper, submerged entry nozzle, MgO ramming and the working refractory lining. Cross-sections of samples from the steel/refractory interface were investigated by electron probe microanalysis (EPMA). Four major aspects were observed at the steel/refractory interface: steel infiltration into the tundish refractory lining, mainly in the gunning materials; a steel oxidised layer formed at the steel/refractory lining interface; formation of a spinel layer between steel and tundish working refractory lining; and many particles were found in the steel phase, near to the steel/refractory interface, being a potential source of non-metallic inclusions.     

Preparation of amorphous Ni-Pd-P particles by melt ejection

Amorphous spherical particles with diameters of 0.5 to 1.4 mm have been prepared by melt ejecting Ni-rich Ni-Pd-P alloys into stirred cold water with 1 vol pct surfactive agent. The sphericity of the amorphous particles defined by the difference in diameter was better than 98 pct. Particle diameters (d) are 2 to 3 times larger than the orifice diameter (2r) of the quartz nozzle. The size ratio has been interpreted by the Raplace’s relation (d = 4r) derived from the assumption that an internal pressure of molten alloy is equal between cylindrical stream and spherical droplet. From the result that no spherical amorphous particles are formed in the cold water without surfactive agent, a significant decrease in the surface tension of coolant by the addition of surfactive agent is essential in the formation of spherical liquid particles from the ejected melt stream. The amorphous particles with diameters below about 1.0 mm exhibit hardness, crystallization temperature, and heat of crys-tallization similar to those from the amorphous ribbon with about 40 μm thickness. However, the onset temperature and heat of relaxation are lower by 16 K (4.2 pct) and larger by 130 J/mol (13 pct), respectively, for the particles. It is thus concluded that the amorphous spherical particles have the same thermal stability as that of the amorphous ribbon and its structure is in a more disordered state as compared with the ribbon sample.     

Melting of γ-TiAl in the alumina crucible

Titanium aluminide alloys with unique properties as well as high strength to weight ratio, excellent oxidation resistance and acceptable high temperature mechanical properties can be used as a high temperature structural material and a competitor for super-alloys. In this research, production process of Ti-48Al-2Cr (numbers indicate atomic percent) intermetallic in an induction furnace with argon atmosphere, alumina crucibles with different purities and several ingot casting time, were investigated. Microstructure of produced ingots was studied by optical microscope and scanning electron microscope (SEM) with EDX analyzer and phase analysis were studied by XRD method. Results show that microstructures are completely lamellar and by increasing cooling rate, interlamellar space will decrease. In the structure, three morphologies of alumina particles could be seen that are: spherical, cluster and lathed. Volume fraction of Al₂O₃ particles will increase by increasing the holding time of melt and crucibles with higher SiO₂ content will react more severe with melt.     

Preparation of amorphous Ni-Pd-P particles by melt ejection into stirred water and their hardness and thermal stability

Amorphous spherical particles with diameters of 0.5 to 1.4 mm have been prepared by melt ejecting Ni-rich Ni-Pd-P alloys into stirred cold water with 1 vol pct surfactive agent. The sphericity of the amorphous particles defined by the difference in diameter was better than 98 pct. Particle diameters (d) are 2 to 3 times larger than the orifice diameter (2r) of the quartz nozzle. The size ratio has been interpreted by the Raplace’s relation (d = 4r) derived from the assumption that an internal pressure of molten alloy is equal between cylindrical stream and spherical droplet. From the result that no spherical amorphous particles are formed in the cold water without surfactive agent, a significant decrease in the surface tension of coolant by the addition of surfactive agent is essential in the formation of spherical liquid particles from the ejected melt stream. The amorphous particles with diameters below about 1.0 mm exhibit hardness, crystallization temperature, and heat of crys-tallization similar to those from the amorphous ribbon with about 40 μm thickness. However, the onset temperature and heat of relaxation are lower by 16 K (4.2 pct) and larger by 130 J/mol (13 pct), respectively, for the particles. It is thus concluded that the amorphous spherical particles have the same thermal stability as that of the amorphous ribbon and its structure is in a more disordered state as compared with the ribbon sample.     

尖晶石对刚玉-尖晶石浇注料性能的影响

以板状刚玉(6 mm～1 mm、1 mm～0.5 mm、0.5 mm～0mm)、烧结尖晶石颗粒(1mm～0.5 mm)为骨料,电熔尖晶石细粉(≤0.044 mm)、白刚玉细粉(≤0.044 mm)、α- Al2O3微粉(≤5μm,d50=2.01 μm)、纯铝酸钙水泥为基质,按骨料与基质的质量比为70∶30配料,以烧结...     

Processing and characterization of plasma-sprayed ceramic coatings on steel substrate: Part II. On coating performance

This investigation envisages the performance evaluation of a series of plasma-sprayed coatings from a few inexpensive powders, namely, alumina, plasma dissociated zircon (PDZ), zircon sand, and zircon-20 wt pct calcia. The hardness and adhesion (bond) strength of the coatings have been evaluated. All coatings offer an attractive thermal fatigue property. An added layer of yttria between the top and bond coats improves the thermal fatigue resistance appreciably. The wear performance of these coatings has also been evaluated using a “pin-on-disc” type wear testing setup. The Indian alumina coating offers a superior wear resistance and thermal fatigue property compared to an imported alumina coating. The coatings based on zirconia-silica offer an attractive thermal fatigue resistance along with an acceptable wear resistance. The latter improves considerably upon annealing of the coatings because of the removal of the residual stresses accumulated during deposition. Among these zirconia-silica-based coatings, the zircon-20 wt pct calcia is the most promising. The finishability of these coatings has been studied using a surface grinding setup equipped with a diamond wheel. The cutting forces and the surface roughness of the ground surface have been measured. The coatings have been found to possess a reasonably good grindability.