Slag Film Research Articles

Effect of high-frequency electromagnetic field on microstructure of mold flux

AbstractSoft-contact of molten steel can be achieved by applying a high-frequency electromagnetic field above the mold of continuous casting, which can effectively eliminate surface defects and achieve billets with no cracks and no oscillation marks. It also has some influence on the mold flux. In this study, the effect of a high-frequency electromagnetic field (20 kHz) on a mold flux flow field was simulated using a finite element software, and the slag film was extracted using a slag film simulator. The effect of the high-frequency magnetic field on the microstructure of the mold flux was analyzed using X-ray diffraction, Raman spectroscopy, and mineral phase testing. The results show that the high-frequency electromagnetic field disrupts the orderly movement and increases the movement rate of the liquid flux. The precipitate phase of the slag film did not change, but the silicate dimer Q1 decreased, the chain Q2 increased, and the network degree was increased. The slag film structure changed from the original two-layer form of crystalline layer–glass layer into a three-layer form of crystal layer–glass layer–crystal, and the crystallization ratio increased by 35% on average. The grain-size melilite granularity was reduced from the original 0.12 to 0.005 mm.

Corrosion Resistance of Partially Stabilized Zirconia Materials to Alkaline Steel Slag

Partially stabilized zirconia (PSZ) materials were fabricated using 4 wt% CaO, 3 wt% MgO, and 5.4 wt% Y2O3 as stabilizing agents together with monoclinic zirconia powder. The physical properties, phase compositions, and microstructures of the Ca-PSZ, Mg-PSZ, and Y-PSZ samples were investigated by X-ray diffraction, scanning electron microscopy, and energy spectrum analysis. A crucible method was used to explore the relationship between the stabilizing agent and erosion resistance to alkaline steel slag. The results revealed that the zirconia materials stabilized by different stabilizing agents showed obvious differences in their bulk densities, apparent porosities, microstructures, and erosion resistances to alkaline steel slag. The structure of Y-PSZ showed highest density, containing a small number of uniformly distributed pores. In terms of Mg-PSZ, the intergranular bonding in its structure was observed to not be close, and the sample contained some cracks, but no pores. A large number of intragranular pores and a small number of overall pores was observed in Ca-PSZ, resulting in this material having the lowest bulk density. The pores and cracks provide the path to penetrate and diffuse for alkaline steel slag, which weakens the corrosion resistance of PSZ materials. The phase composition of the affected layers in all of the samples after corrosion was almost completely transformed from monoclinic phase to cubic phase, and the phase transition of both the original and transition layers was not obvious due to the formation of a slag film. Y-PSZ did not react with components of the steel slag such as SiO2 and Al2O3, showing the best corrosion resistance to alkaline steel slag.

Dynamic behaviors and regime map of a molten blast furnace slag droplet impacting a solid surface

The dynamic morphologies of a molten blast furnace slag droplet impacting a quartz glass surface are studied visually for a wide range of dimensionless initial temperatures (1.604–1.951), Reynolds numbers (17.4–124.4), and Weber numbers (197–1191). Five morphological evolution modes are defined based on the final stage of the slag film as deposition, receding, prompt-splashing, retraction-deposition and retraction-splashing. An impact regime map is established with the Reynolds number and dimensionless initial temperatures as the coordinates and six boundaries dividing the different modes are identified in the regime map. The apparent temperature nephograms of the slag film are extracted by a home-developed optical calibration method. The dynamic spreading processes and temperature distributions of the different regimes are not simply determined by the initial impact parameters, the coupling effect of the flow of the high-viscosity liquid slag and the heat transfer at the interface will obstruct the consecutive morphological evolution. The regulation principle for the centrifugal granulation system is promoted based on the analysis on an energy barrier for the sufficient spreading of the droplet and the impact regime map.

Transient flow and mold flux behavior during ultra-high speed continuous casting of billet

Ultra-high casting speed is an important development feature for efficient continuous casting of billet. A mathematical model which addresses the high flow rate of molten steel in the billet mold is presented, coupling slag flow with heat flux and solidification. The model is then applied to investigate the correlation between transient steel flow, slag infiltration and heat transfer in the mold-shell gap during ultra-high casting speed. The results show that the meniscus level experiences a hump near the mold wall and a valley near the nozzle as a result of the recirculating flow in the upper mold. The liquid slag around the mold wall infiltrates preferentially into the gap to be consumed quickly, whereas those near the SEN flows to the mold with a low speed. Severe fluctuations in meniscus level will disturb the slag infiltration, which in turn causes the formation of uneven slag films, leading to intensified heat flux fluctuations in the mold-shell gap, especially in the area of 20−60 mm from the mold corner.

Effects of Transition Metal Oxides ZrO2, Y2O3, and Sc2O3 on Radiative Heat Transfer of Low-Reactive CaO-Al2O3-Based Mold Slag

To control heat transfer between the solidifying shell and the water-cooled mold during continuous casting, the transition metal oxides, ZrO2, Y2O3, and Sc2O3, were separately added into the developed low-reactive CaO-Al2O3-based mold slags to investigate the effect on radiation heat transfer. Two kinds of master mold slags were taken into account, one included 18 pct Al2O3 and 18 pct SiO2, and the other one included 28 pct Al2O3 and 8 pct SiO2. The slag viscosity and melting temperature were measured with rotating cylinder viscometer and hemisphere-point method, respectively. The optical parameters of the slag films having different contents of transition metal oxides were measured by Fourier transform infrared spectrometer, and an optical-process model was constructed to consider the radiation from the shell surface and hot and cold sides of the solid films to the mold plate. When ZrO2, Y2O3, and Sc2O3 contents up to 4 pct, the viscosity at 1573 K fluctuated in the range of 0.05 Pa s, the melting temperature varied within a range of 15 K, except for the case of 2 pct Sc2O3, for which the melting temperature was 27 K lower than that of the master mold slag with 8 pct SiO2 and 28 pct Al2O3. The incorporation of the three transition metal oxides up to 4 pct did not have a notable effect on the viscosity or melting temperature. The addition of ZrO2 and Y2O3 effectively decreased the radiation, whereas Sc2O3 had no direct relationship with radiation. When the ZrO2 (Y2O3) concentration was increased from 0 to 4 pct, the reduction rate of radiation transfer was 26.6 pct (13.3 pct) for the mold slag containing 18 pct Al2O3 and 18 pct SiO2. The current study provides a new direction for developing low-reactive mold slag and reducing heat transfer during continuous casting.

Crystallization and heat transfer of CaO–SiO2-based slag for high-Mn–high-Al steel

The crystallization and heat transfer of CaO–SiO2-based slag for high-Mn–high-Al steel have been investigated by coupling composition evolution with heat transfer and non-isothermal tests. It was found that the concentrations of Al2O3 and MnO in the slag were determined by the relative contents of Al and Mn in molten steel. Crystallization temperature of the slag rose with the increase in Al2O3 content first and then fell due to the coupled effects of basicity and Al2O3, and a small amount of MnO cannot change this trend. High viscosity of slags led to less obvious characterizations of crystallization. The crystalline phase analysis suggested that MnO accumulation suppressed the formation of fluorite crystals. As the Al2O3 content increased, the precipitation of cuspidine was replaced by the blocky gehlenite crystals, while the number and the size of nepheline and fluorite crystals both increased. According to the thickness and the crystallization structure of slags, the heat flux in the slag film first decreased and then increased with higher Al2O3 accumulation. Then, a new type of CaO–SiO2-based mold slag with high basicity was applied in plant trial, and the change in composition and properties of mold slag was studied.

Crystallization characteristics of B2O3 and TiO2-bearing glassy fluoride-free mold fluxes

To explore the effects of TiO2 and/or B2O3 on crystallization of the glassy fluoride-free slag film near the copper mould, the crystallization characteristics of glassy fluoride-free mold fluxes with fluoride being substituted by TiO2 and/or B2O3 were investigated using X- ray diffraction (XRD), scanning electron microscope (SEM) and differential thermal analysis (DTA) techniques. The glass forming ability index (Kgl) of the glassy fluoride-free mold fluxes was studied using Hruby?s method. The XRD and SEM analysis show that Ca2Al2SiO7, CaTiO3 and CaSiO3 are the dominant crystals of this fluoride-free mold fluxes system. With the content of TiO2 increasing from 0 to 7%, the crystallization of Ca2Al2SiO7 and CaSiO3 are inhibited and the formation of CaTiO3 is also weak, so crystallization tendency of the glassy fluoride-free mold fluxes weakens. But as TiO2 content reaches 10%, the crystallization tendency strengthens because of the strong crystallization of CaTiO3. An increase of B2O3 inhibits the crystallization of calcium silicate, so it weakens the crystallization tendency of the glassy fluoride-free mold fluxes. The crystallization processes of the studied fluoride-free mold fluxes correspond to the surface crystallization mechanism. This research provides important reference for further investigation on the heat transfer behavior of the TiO2 and B2O3-bearing slag between copper mould and slab to evaluate the feasibility of B2O3 and TiO2- bearing fluoride-free mold fluxes.

An Approach for Modelling Slag Infiltration and Heat Transfer in Continuous Casting Mold for High Mn–High Al Steel

To clarify the characteristics of slag infiltration and heat transfer behaviors in the meniscus region during the casting of high Mn–high Al steel, a mathematical model of a continuous casting mold that couples fluid flow with heat transfer, and solidification is developed. The model is based on the change in slag composition and properties caused by the steel/slag reaction. The formation and evolution of the meniscus profile and slag films for different mold fluxes during mold oscillation are described. The results show that the rapid growth of the slag rim with a high Al2O3 content approaches and deforms the meniscus so that a series of casting problems such as slag infiltration blocking, large fluctuations in heat flux, and even meniscus breaking occur in the continuous casting process. Predictions are in good agreement with plant measurements. These findings provide an improved understanding of the complex phenomena occurring in the meniscus region and give new insights into the evaluation and optimization of mold flux properties for high Mn–high Al steel casting.

Local corrosion mechanism of SiO2-based refractories caused by slag movement near solid–liquid–gas interface in different slag systems

Abstract Slag movement on SiO2-based prism refractories in different slag systems was observed. The cross section shape evolution mechanism was discussed. Two types of shape evolution appear. For PbO−SiO2 slag whose surface tension improves with SiO2 concentration, slag film flows up along four edges under axial Marangoni shear force and wettability. Then, it flows down along four lateral faces under gravity. Corrosion rate at edges is larger than that on lateral faces due to different SiO2 solubilities of ascending and descending flow. Prism cross section shape changes from square to round. For FetO−SiO2 slag whose surface tension reduces with the increase of SiO2 concentration, slag film flows up under the inflence of wettability. Then, it flows down under Marangoni shear force and gravity. Compared to four edges, slag is mainly up and down on four lateral faces due to larger surface tension and size. So, prism cross section shape keeps square.

Influence of Electropulsing Treatment on the Initial Solidification of Molten Steel During Continuous Casting

An electropulsing-assisted mold simulator (EPMS) technique was developed to investigate the effect of pulsed electric current on the initial solidification behavior of molten steel during the process of continuous casting. The results indicated that the variation of mold hot surface temperatures in the meniscus area decreased from 375.8 K to 394 K for the case without electropulsing treatment (EPT) to 363.5 K to 377.3 K for the one with EPT. The mold surface heat fluxes fluctuated around the baseline of 0.83 and 2.42 MW/m2 in the cases with and without EPT, respectively, which indicated that a higher thermal resistance between the initial shell and copper mold was introduced by employing the EPT. The crystal fraction of slag films increased from 69.8 to 77.9 pct, whereas the grain size of mold flux film decreased, when compared the case of EPT treatment with the benchmark one, which suggested that the pulsed electric current promotes the movement of molten clusters and particles, leading to the enhancement of mold flux crystallization and the improvement of thermal resistance. Moreover, the solidification structure of steel along the direction of solidification (from the mold to melt) includes a fine chilled grain layer close to the mold, a columnar dendrite layer in the middle, and equiaxed grains next to the melt core, in which a compacted microstructure with smaller secondary dendrite arm spacing was observed in the case of the EPT. The results demonstrate that the EPT could refine the as-cast solidification microstructure.

Effect of CeO2 on heat transfer and crystallization behavior of rare earth alloy steel mold fluxes

To improve the heat transfer capability and the crystallization property of the traditional mold flux, CaF2 was replaced with B2O3. Then, the influences of CeO2 on the heat transfer and the crystallization of the CaF2-bearing mold flux and the new mold flux with 10wt% B2O3 were studied using a slag film heat flux simulator and X-ray diffraction (XRD). The results revealed that the addition of CeO2 reduced the heat transfer by increasing the solid slag thickness and the crystallization of two mold fluxes. However, CeO2 had less effect on the B2O3-containing mold flux compared with the CaF2-bearing mold flux. According to the analyses, the CeO2 contents in the CaF2-bearing mold flux and the B2O3-containing mold flux should not exceed 8wt% and 12wt%, respectively. Therefore, these experimental results are beneficial to improve and develop the mold flux for casting rare earth alloy steels.

CFD Modeling of Melt Spreading Behavior on Spinning Discs and Cups for Centrifugal Granulation of Molten Slag

The spreading behaviors of molten slag on spinning discs and cups were studied through performing free surface flow numerical simulations by means of computational fluid dynamics (CFD) modeling technique. In this work, liquid slag film thickness at the edge of the spinning discs and cups was predicted by using the CFD model, since the slag film thickness has a predominant influence on the size of the slag granules produced after the slag film breakup. The effects of the shape of discs and cups and the operating conditions (slag flowrate and spinning speed) on the slag film thickness were examined. Flat surface disc, disc with curved surface, and cups with different sidewall height and taper angle were investigated. It was found from the modeling results that, under the same slag flowrate and spinning speed, the larger the wetting area of the slag on the discs and cups, the smaller the slag film thickness. For the same size (radius) discs and cups, the slag film thickness on the flat surface disc is larger than those on the cup and the curved surface disc. Furthermore, the film thickness on the cup is larger than that on the curved surface disc. The reason is that the cup has a sharp corner and a sidewall that impose a larger resistance to the slag flow, whereas the curved surface disc has a smooth surface that has a smaller resistance to the slag flow.

Effect of Bubbles on Crystallization Behavior of CaO–SiO2 Based Slags

Surface longitudinal cracks are a serious problem and particularly prevalent in the casting of peritectic steel (carbon content between 0.10%C and 0.18%C, non-alloyed). It is usually alleviated by controlling the horizontal heat transfer from the steel shell to the mold through increasing the crystallization performance of slags. In the actual continuous casting process, a large number of bubbles are formed in the molten slags, and the crystallization properties of the mold fluxes are affected by bubbles. Therefore, an investigation of the influence of bubbles on the crystallization performance of mold fluxes was carried out by applying the hot thermocouple technique and it is hoped that surface longitudinal cracks can be solved in this way in the peritectic steel casting process. The continuous cooling transformation (CCT) diagrams and time–temperature transformation (TTT) diagrams were constructed for an analysis of the crystallization kinetics. The results showed that the crystallization ability of mold fluxes was enhanced by adding bubbles through shortening the incubation time of crystallization, increasing the critical cooling rate, and decreasing the activation energy of crystallization. As a result, the crystalline fraction, slag film thickness, and surface roughness of the slag films were improved, but the crystalline phase was not affected by bubbles. With an increase of the bubble content remaining in the molten slag, the growth mechanism of the cuspidine crystal phase changed from a low dimension to a high dimension, and the content of the molten slag’s structure unit (Q1) needed for cuspidine in the molten slag was markedly increased.

Characteristics of Slag Infiltration in High-Mn Steel Castings

To determine the composition difference between the solid and liquid films in plant practice for high-Mn steel castings, a reaction experiment was carried out to investigate the composition profile of MnO and SiO2 in the slag layer. Based on the experimental results, a mathematical model of the mold slag for high-Mn steel was developed to describe the nonuniform infiltration behavior of mold slag during oscillation cycles. Moreover, the effects of uneven slag films on slab lubrication and heat transfer in the mold are discussed. The results show that the concentrations of MnO and SiO2 over the flux layer have an opposite stair-step distribution, which consists of a reaction layer and nonreaction layer. When the reaction layer thickness is 5 mm, a thicker liquid film is formed between the slab and the mold. The inhomogeneity of the composition in the solid film causes large fluctuations in the heat flux in the mold, resulting in irregular depressions on the shell surface. With a reacting thickness of 10 mm, the reacting slag fills in the gap to generate a thinner solid film, which increases the overall heat flux in the mold. Since mold oscillation combined with slag rim enhances the pressure flow in the vicinity of the meniscus, the liquid slag within 80 mm of the mold wall is mixed and consumed quickly. In contrast, the liquid slag beyond 80 mm from the mold wall with a low velocity stays longer in the liquid pool. The model predictions illuminate the characteristics of slag infiltration during high-Mn steel casting, showing good agreement with plant measurements.

Study of Mold Oscillation Parameters and Modes on Slag Lubrication in Slab Continuous Casting

The effects of mold oscillation parameters and modes on slag lubrication were quantified based on a verified multi-phase model for liquid slag infiltration during continuous casting. The main results indicated that the variations of slag film thickness, slag consumption, and heat flux at the meniscus were generally periodic with the mold oscillation. For sinusoidal oscillation, shorter amplitude or lower frequency consumed much more liquid slag. Non-sinusoidal mode could increase the slag consumption from 0.274 kg/m2 to 0.286 kg/m2, an approximately 4.4% increase over that of the conventional mode. The evolution of heat flux at the meniscus fluctuated severely with lower oscillation frequency, while the non-sinusoidal mode was stable, approximately 1.6 MW/m2. This study demonstrated that the positive strip time contributed more to lubrication than the negative strip time and that the slag consumption decreased with an increase of negative strip ratio in mold oscillation.

Effect of Shear Stress on Isothermal Crystallization Behavior of CaO-Al₂O₃-SiO₂-Na₂O-CaF₂ Slags.

How to coordinate the contradiction between lubrication and heat transfer in the peritectic steel casting process is the key technical difficulty in preparing mold fluxes. The mold fluxes that are required for casting are subjected to the shear stress generated by mold oscillation and slab movement, which affects the crystallization performance of slags. The quantitative effect of slags’ crystallization performance by shear stress is studied to develop a low-basicity and high-crystallization mold flux to solve the above problem. The results show that the crystallization kinetic condition is promoted, and the crystallization activation energy is reduced by the shear stress, which leads to an increase in the crystallization temperature. Concurrently, the crystal size is reduced. However, the shear stress has no effect on the crystalline phase. The influence of different shear stresses on the crystallization ability of molten slags is related to the crystal nucleation and growth mechanisms. The crystalline fraction of the slag films at 300 rpm (69 s−1) is 44.7%, which is an increase of 17.7% compared with the crystalline fraction of the slag films at 200 rpm (46 s−1). Moreover, the shear stress has little effect on the lubricating properties of the mold fluxes, although the crystallization ability is promoted by the agitation.

Prediction and Measurement of the Kinetics of Isothermal Crystallization and Devitrification of Mold Powder Slags

The rate and extent of formation of crystalline phases in the slag film that develops between the mold and the strand in steel continuous casting determine heat transfer and lubrication between them and therefore play crucial roles in shell integrity. In this work, a commercial mold powder for casting medium carbon (MC) thin steel slabs is selected for carrying out detailed comparisons between predicted and experimental results of the rates of isothermal crystallization and devitrification of liquid and glassy slag, respectively. Predictions are obtained with a kinetic model that combines the induction period (of Simon and Kolman) and the Kissinger methods to evaluate time–temperature–transformation (TTT) diagrams for the devitrification and crystallization reactions associated with the peaks appearing in non-isothermal thermo-analysis traces. To obtain experimental validation of the diagram, a high-temperature confocal laser scanning microscope (HT-CLSM) is used to rapidly heat or cool a sample, ~ 140 mg, of glassy powder or liquid slag, respectively, to selected isothermal conditions, where it is held for a prescribed length of time before quenching. Identification and quantification of the phases present in the quenched samples are done by a quantitative X-ray powder diffraction analysis (QXRPDA) method that uses a PONKCS (partial or not known crystal structure) procedure based on Le Bail and Rietveld strategies for refinement of diffraction patterns. The many experimental results indicate that the predicted TTT diagrams for devitrification and crystallization of the mold slag give a good estimation of the rate of the transformations that take place at different temperatures and reveal the differences in the reactions that occur along both paths.

Modified DHTT Equipment for Crystallization Studies of Mold Slags

The double hot thermocouple technique (DHTT) enables simulations of the temperature gradient at near-service conditions during continuous casting of steel. With the equipment applied so far, a rectangular slag film of even thickness often cannot be achieved. Further, the minimum temperature frequently lies within the slag film. Modified equipment can avoid these disadvantages via the following design features. The entire furnace chamber is heated to the selected temperature of the cold wire, and the minimum temperature is not located within the slag film. Furthermore, the shape of the heating wire is improved, which enables mounting of a thin, rectangular slag film between four platinum wires. This modification allows for investigations on transparent and translucent slags. So far, the results from DHTT investigations were represented via snapshots of the samples at certain experimental times. Therefore, appropriate methods for the graphical representation of the results were suggested: the maximum crystallinity, the time related to certain crystallinities with a dependence on the position within the slag film, and the crystal growth rate. The CaO-MgO-Al2O3-SiO2 slag investigated with this equipment was mineralogically examined additionally, and based on thermodynamic calculations, the allocation of temperatures to certain positions within the crystallized slag film was possible.

Effect of Slag-Steel Reaction on the Initial Solidification of Molten Steel during Continuous Casting

With the mold simulator technique, the effect of slag-steel reaction on the initial shell solidification as well as the heat transfer and lubrication behavior of the infiltrated mold/shell slag film was studied in this article. The results showed that the Al2O3 content, the CaO/SiO2 ratio, and the viscosity of mold flux were increased with the progress of the slag-steel reaction during casting. The slag-steel reaction has two major effects on the initial shell solidification: one is increasing the mold heat flux and shell thickness by the decrease of slag film thickness. The other is the reduction of mold heat flux by the increase of crystal fraction in slag film. Mold flux with a lower basicity, viscosity, and crystallization temperature would result in a larger liquid slag consumption and the uneven infiltration of slag into the mold and shell gap that eventually leads to the irregular solidification of initial shell with a poor surface quality, such as slag entrapment and depressions as well as glaciation marks. Conversely, mold flux with a higher viscosity, basicity, and crystallization temperature would result in a smaller liquid slag consumption, which would cause the poor mold lubrication, the longitudinal shell surface defects, and drag marks.

Xinjiang lignite ash slagging and flow under the weak reducing environment at 1300 °C – Release of sodium out of slag and its modelling from the mass transfer perspective

This study examined the Na loss upon the slagging of five typical Xinjiang lignite ashes and their blends with clay at different ratios, under the simulated cyclone combustion conditions. The ash samples were laden on a 25° – inclined plate and exposed in 1% CO (nitrogen balanced) at 1000–1300°C for maximum 2h at each temperature. Apart from experimental approach, modelling based on the mass transfer mechanism was also attempted. The results show that, the Na loss ratios achieved are lower than that has been reported for the combustion of the same coals in the pulverised – coal fired plants, varying from 40 to 70% of the entire Na that largely depends on the basicity of ash or ash – clay blend. The reaction temperatures examined here exert a marginal influence on the Na loss. The addition of clay to raw coal ash and its dosage amount are crucial. At a mild combustion temperature of 1300°C, the addition of 8–10wt% clay, on the ash mass basis to a basic lignite ash is the optimum, simultaneously accelerating both slag flow rate and Na retention in slag matrix. Additionally, the addition of clay alleviated the penetration and corrosion of Na vapour into the refractory corundum, thus extended its lifespan. The Na loss rate is dominated by the internal mass transfer rate of Na vapour inside slag film, in particular at the elevated temperatures from 1300°C when ash is partially/mostly molten, as well as when the clay is added into the original Xinjiang ash. For the original ash which is rarely molten before 1200°C, the gas-solid reaction prevailed, leading to the capture of Na vapour by Al and Fe – bearing solid species and even the refractory corundum plate. In contrast, at 1300°C, the formation of slag film decreased the Na vapour mass transfer coefficient drastically. The resistance was further enlarged by the formation of more molten species and a denser slag film upon the addition of clay. The original Na content in ash has proven insignificant in the mass diffusion control regime.