Thin Slag Research Articles

Crystallization Characteristics and In-Mold Performance of Electroslag Remelting-Type TiO2-Bearing Slag

The non-isothermal crystallization characteristics of the electroslag remelting (ESR)-type TiO2-bearing slag were studied. The effect of crystallization characteristics of the slag on the in-mold performance was verified by ESR pilot trials operating in a static mode. The results showed that increasing TiO2 content decreased the crystallization temperature and liquidus temperature of the slag and suppressed the crystallization tendency of the slag. There is no change in the types of the crystalline phases in the slag with different TiO2 contents during cooling, i.e., 11CaO·7Al2O3·CaF2, CaTiO3, and CaF2. The sequence of crystal precipitation during cooling was 11CaO·7Al2O3·CaF2 to CaTiO3, and then CaF2 with increasing TiO2 content from 4.2 to 12.6 mass pct. With adding 16.8 mass pct TiO2 in the slag, the precipitation of 11CaO·7Al2O3·CaF2 and CaTiO3 took place simultaneously, followed by CaF2. With the increase in the TiO2 content from 4.2 to 16.8 mass pct in the slag, the dominant crystalline phase changed from 11CaO·7Al2O3·CaF2 to CaTiO3. The morphology of 11CaO·7Al2O3·CaF2 and CaTiO3 changed from faceted and bonelike to elliptically faceted and blocky, respectively. The morphology of CaF2 were spherical in all cases, irrespective of TiO2 contents in the slag. The decrease in the crystallization tendency of the slag with increasing TiO2 contents was attributed to the decrease in the activity of CaO in the slag. The increasing of TiO2 content in the slag are favorable for providing thin slag skin and stable heat flux across the slag skin, which improved the surface quality of the as-cast ESR ingot as demonstrated by ESR pilot trials operating in a static mode.

Fluid Flow Turbulence in the Proximities of the Metal-Slag Interface in Ladle Stirring Operations

Three-phase interactions (metal-slag-argon) in ladle stirring operations have strong effects on the metal-slag mass transfer processes. Specifically, the thickness of the slag controls the fluid turbulence to an extent that once trespassing a critical thickness, increases of stirring strength no longer effect the flow. To analyze these conditions, a physical model considering the three phases was built to study liquid turbulence in the proximities of the metal-slag interface. A velocity probe placed close to the interface permitted the continuous monitoring and statistical analyses of any turbulence. The slag eye opening was found to be strongly dependent on the stirring conditions, and the mixing times decreased with thin slag thicknesses. Slag entrainment was enhanced with thick slag layers and high flow rates of the gas phase. A multiphase model was developed to simulate these results and was found to be a good agreement between experimental and numerical results.

Ambient weathering of steelmaking ladle slags

During cooling and permanence outside, the solidified slag involved in the refinement process taking place in the steelmaking ladles suffers attack by environmental components such as water vapor and gaseous CO2 (weathering). The reactions involved are hydration and carbonation, and as a consequence, the pulverization of the slag occurs. In the present paper, the results of a study of the degradation of a typical steelmaking ladle slag over a period of eighteen weeks (126 days) are reported. To monitor the slag evolution, several experimental techniques were used, some of them rarely employed in this context, after dividing the initial slag batch in four granulometric fractions between > 7.2 mm and < 1.4 mm: granulometry by sieving, X-ray fluorescence (XRF), X-ray diffraction (XRD), and thermogravimetric (TGA) and thermal differential (DTA) analyses. As was already known, the main elements responsible for the slag degradation are free lime, followed by calcium aluminates and magnesia. It was also found that anhydrous and hydrated calcium aluminates are concentrated in the finest granulometric fractions and contribute to the generation of fines mainly during the final stage of hydration. The high percentage of particles smaller than 1.4 mm, with cementitious properties provided mainly by the presence of anhydrous calcium aluminates, are promising characteristics for alternative reusing of the studied ladle slag. Furthermore, slag weathering mechanisms are critical for understanding other steelmaking processes in which the slag is deeply involved, such as the protective role of the remaining thin slag layer against decarburation of ladle or converter working lining refractory bricks.

Single-pass arc welding of thick plate structures of 22K steel under a thin slag layer

The efficiency of single-pass automatic arc welding under a thin slag layer (ATL) with the forced formation of weld metal of welded joints in 22K steel with a thickness of 70 mm is shown. The optimum heat input in ATL welding makes it possible to produce in a single pass a welded joint with a finer structure and higher mechanical properties in comparison with electroslag welding. Recommendations for using the ATL method in welding production are outlined.

Understanding mould powders for high-speed casting

The development and application of mould powder for high-speed continuous casting of steel is described. For thin slab casting, the main requirements are proper powder melting, undisturbed slag infiltration, adequate strand lubrication and the control of mould heat transfer. For increased casting speeds i.e. up to 8 m/min, slag infiltration and in particular the control of mould heat transfer via crystallisation of the slag film becomes even more important. It was found that a low powder consumption and hence a thin slag film is no restriction for an undisturbed thin slab casting process. Given a stable casting process and machine condition, the mould powder properties are not as critical as widely assumed.

Tundish Open Eye Formation in Inert Gas-Shrouded Tundishes: A Macroscopic Model from First Principles

Open eye formation in tundishes can result in reoxidation of liquid steel leading to the formation of harmful inclusions. Moreover, it is also a site for heat loss, gas absorption, and slag emulsification. All these factors make it necessary to understand the fundamentals of open eye formation, which in turn will allow us to prevent or control its harmful effects. In the present study, the bubble plume regions in a ladle and tundish were compared, and it was observed that there are significant differences between the two. Moreover, a simplistic model for predicting the open eye area in tundishes for ‘thin slag’ practices was derived using the principles of conservation of mass and momentum. The proposed model was able to predict open eye areas in tundish reasonably well and was compared with other models, and experimental results.

A Model for Slag Eyes in Steel Refining Ladles Covered with Thick Slag

During inert gas stirring in steel ladles, the overlying slag is pushed to the side by the rising gas-metal plume, resulting in an open eye of exposed steel. The present work is devoted to the modeling of ladle eyes in a bath covered with a thick slag layer, where the eyes formed are often smaller than the cross-section of the gas-metal plume. A mathematical model is developed from fundamental fluid flow considerations by extending a previous mechanistic-based approach for modeling eye formation under thin slag layers. The present model for the eye size correlates a dimensionless eye area to the density ratio of the liquids and an appropriate Froude number, expressed in the primary operating variables. The model is consistent with the experimental results in different gas/liquid/liquid systems. Further, a criterion is deduced for soft bubbling of Argon in the ladle without exposing the metal to the atmosphere. Finally, recommendations concerning the proper application of the thin and thick slag models are made.

Studies of Vaporization of Chromium from Thin Slag Films at Steelmaking Temperatures in Oxidizing Atmosphere

In the present work, the volatilization of chromium from thin chromium-containing slag film surfaces was studied in oxidizing atmosphere in the temperature range 1673 K to 1873 K (1400 °C to 1600 °C). The slag films on alumina rings were exposed to air or pure oxygen and the loss of Cr from the post-experiment sample films was examined by SEM/WDS analysis. The mass loss of the samples was also monitored during the heat-treatment. The results indicate that chromium loss increased with increase in temperature and oxygen partial pressure was found also to be relatively less as the sample thickness increased. The implications of chromium escape from slags during the tapping of stainless steel slags are discussed.

The Fluid Mechanics of Slag-Metal Interactions in Ladle Metallurgy

In Ladle Metallurgy operations following steelmaking, the control of the interactions between slag and metal is crucial for the production of high-quality steel. While the chemical aspects of the refining have been well studied, the fluid dynamic aspects have received less attention. Ladles are often gas-stirred by porous plugs pushing the slag to the periphery of the ladle and opening “eyes” of steel exposed to the atmosphere. The nominal slag-metal contact area is reduced, but there may be entrainment of slag in metal and vice versa which increases the effective interfacial area. Experiments were conducted with water models and the work of others in metallic systems has been included in the study. The eye size has been found to be quite different for thin and thick slag layers, and mathematical models of the controlling factors for the eye size has been determined for these two regimes. The models show that the important factor is the balance between the inertia from the gas-liquid plume and height of the slag, so the governing equations are based on Froude number control. The conditions for closure of the eye have been identified and mathematically modeled. At higher gas flow rate, droplets of slag are entrained into metal which is quantitatively linked to Kelvin-Helmholtz instability. The role that physical properties of the slag play in these phenomena is discussed.

Single hot thermocouple technique for the characterization of the crystallization behavior of transparent or translucent liquids

Both time–temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams have been established to characterize the crystallization behavior of slags in a selected temperature range. Therefore, the single hot thermocouple technique based on already existing equipment was constructed showing also some different approaches. Furthermore, the procedure was enhanced using a stretching device enabling the formation of very thin slag layers. This device permits the investigation of not only transparent but also translucent liquids with low amounts of coloring oxides, e.g. Fe2O3. For the initiation of this method, a transparent synthetic NCAS-slag showing a high crystallization tendency was used to proper adjust the control parameters. Afterward, two industrial mould slags for the continuous casting of steel were investigated and TTT- as well as CCT-diagrams created. The TTT-diagrams for both mould slags show only one nose but the shapes of crystals formed differ dependent on temperature. This is contributed to the ratio of the growth to nucleation rate which is raised at higher temperatures where dendritic crystals are formed. For the case of continuous-cooling experiments observing only the formation of dendritic crystals precipitating at temperatures close to the liquidus temperatures the same explanation is assumed. Contrary, this ratio is decreased for rather low temperatures where only fine separate crystals precipitate.

The Observation of Mold Flux Crystallization on Radiative Heat Transfer

As an important factor in the moderation of heat transfer in continuous casting mold, mold flux has been researched widely. However, the study of the effect of solid mold flux on radiative heat transfer has not been conducted widely. By using an infrared radiation emitter, which was developed at Carnegie Mellon University, a radiative heat flux was applied to a copper mold to simulate the heat transfer phenomena in continuous casting. The effect of adding a thin slag disc on top of copper mold on radiative heat transfer has been analyzed. It was found that the presence of mold flux enhanced radiative heat transfer rate. The effect of full crystallization of a slag disc was to reduce the heat transfer rate by 20% compared with a completely glassy sample. The specific effect of full crystalline and glassy parts of the mold flux on radiative heat transfer, and the influence of their properties on heat transfer rate was discussed in this paper.

A model for estimating exposed plume eye area in steel refining ladles covered with thin slag

A model for estimating exposed plume eye area in steel refining ladles covered with thin slag

A technique in confocal laser microscopy for establishing biofilm coverage and thickness

This study uses confocal laser scanning microscopy to determine the coverage and thickness of biofilms on rock types commonly used in wetland sewage treatment systems in New Zealand. Samples of scoria, greywacke and slag - with glass used as a comparison - were submerged in subsurface flow wetlands and examined after six weeks. An image analysis technique was used to quantitatively determine the coverage and thickness of each biofilm. The technique consisted of the biofilm quantification of each individual image obtained from the confocal optical sectioning. The results indicated that the biofilm coverage for the substrata types did not exceed 25%. However, there was a marked difference between the biofilm structures grown on the different substrata; that on glass formed thin spindly structures, and slag and scoria showed similar dense patches interspersed with open channel structures that followed the contours of the pocketed rock surface.

Heat transfer in an iron bath of a two-stage smelting reduction process

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On the ultrasonic detectability of thin slag inclusions in steel plates

Rayleigh's theory does not at first appear to favour the ultrasonic detection of thin slag inclusion layers in rolled steel plates. Experiments have shown, however, that there is always a very thin gas or vacuum layer present, which reflects enough energy to detect the flaw. Consequently, even very thin slag layers can be detected by ultrasonic waves.

炭酸ガス関口線材アーク熔接法について(第5報)

The C. S. Arc Welding Process does not produce so much quantity of slag as the submerged arc welding process or manual arc welding process with coated electrode, because this process needs no flux. But since the filler wire for this process contains much quantities of deoxidizers, the quantity of deoxi-dation-product is comparatively large. And the product, rised up from the molten weld metal, remains partly on the surface of bead.In case of the multi-pass-welding of thick plates, it is desirable to remove completely this small amount of slag. This slag can be removed by a chipping-hammer, and it is not easy to remove the slag by wire-brushing only. If it will be able to controll the composition of deoxidation-product, the covering and the removal-facility of slag may be improved.In this process, it is unable to controll the properties of weld metal by flux as in cases of the submerged arc welding process and manual arc welding process.Authors intended to controll the weld metal and slag obtained by the C. S. Arc Welding Process by means of adjustment of supplied gas. And they researched experimentally on the effects of oxygen mixed in the supplied gas.According to the results of experiments, they recognized the following facts ;(a) In case of using filler wires having high silicon content, with the increasing of oxygen content of the supplied gas the deoxidation-product changes from silica or silicate saturated with SiO2 to unsaturated molten solution, FeO-MnO-SiO2.(b) The addition of a suitable smount of oxygen to the supplied gas gives good effects on the stability of arc deposition, the facility of removal of thin slag and the improvement of appearance of bead.(c) It is also able to controll the mechanical properties of weld steels by the addition.