Mold Flux Research Articles

Influence of electropulsing treatment on crystallization and structure of calcium silicate-based melt

Electropulsing treatment (EPT) is a promising technology for controlling the phase transition during the solidification of melts owing to its electric and thermal effects. In this study, the influence of EPT on the crystallization and melt structure of a calcium silicate-based mold flux was investigated. The results showed that the morphology of crystals that precipitated in the mold flux changed from elongated columnar to block shape, and the equivalent grain diameter of the crystals increased with increasing voltage from 0 to 20 V. The mass fraction of Ca4Si2O7F2 precipitated in the mold flux decreased with increasing impulse voltage, whereas that of Ca2Mg0.75Al0.5Si1.75O7 increased. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses suggest that the network structure of both silicate and aluminate was simplified by electropulsing because the simpler structural units of Q0, Q1, [AlO6]9+, etc., increased with increasing impulse voltage, whereas the complex structural units of Q2, Q3, and [AlO4]5+ decreased. The extra electric field force is the repulsion force between two oppositely charged ions, which was the root of the network structure simplification and crystallization promotion. The results obtained in this study provide an innovative method for locally controlling the crystallization behavior of mold flux in a mold.

Controlling the Thermal and Rheological Properties of Silicate Glasses for Development of Advanced Mold Flux Systems

Development of the advanced mold flux is always mandatory to enhance both the quality and productivity of continuous casting of steels. Especially, the increasing demands for high alloy steels production and high speed casting reveals serious contradiction between two principal functions of mold flux: lubrication and controlling heat transfer. In order to overcome this problematic needs, some innovative research activities are being carried out. MAE (Mixed Alkali Effect) has been examined in alumino-borosilicate-based mold system to stabilize alkali cation and aluminum association, which enables chemically stable glassy mold flux film during casting of high alloy steels. Non-Newtonian mold fluxes could be developed by addition of Si3N4 or SiC due to the increase of stiffness of polymeric structure, which would be beneficial to satisfy the contradictory requirements of viscosity at mold top surface and mold wall. Some innovative ideas for controlling mold heat transfer without deteriorating the lubrication have been examined by dispersion of nano-size metallic particles and by modification of prenucleation motif. All these trials are closely related with glass science and engineering. Therefore, it should be highly beneficial to enhance the collaborative research activities between glass and metallurgy society for further development of advanced functional mold flux systems.

Improving the erosion resistance of the submerged entry nozzle by applying an external electric field

In order to improve the erosion resistance of the submerged entry nozzle (SEN) in a continuous casting process, an electric field was applied between the SEN and the carbonaceous mold flux. In view of the extensive application of carbonaceous mold flux in the high-quality steel production process, the erosion behavior under carbonaceous mold flux was explored under the application of different electric fields, in terms of amplitude. The extracted results show that the erosion process between the carbonaceous mold flux and the SEN can be changed obviously by applying an external electric field. In addition, the increase of the carbon content in mold flux will promote the manifestation of erosion resistance effects. Under the control of the negative electric field, the decarburization, dissolution, adhesion, and reaction between the SEN and mold flux will be either suppressed or eliminated. Accordingly, the SEN material remains intact without any erosion and damage after the experiment. The application of electric field is anticipated to solve the problem of serious erosion in the SEN on the continuous casting process.

Investigation of the Erosion Mechanism of the Mold Flux Containing Rare Earth Oxide on the Submerged Entry Nozzle Affected by an External Electric Field

Considering the change and unpredictability of the mold flux containing rare earth oxides, the impact of controlling the erosion behavior of the mold flux containing rare earth oxide on the submerged entry nozzle is thoroughly investigated herein by applying an electric field. The extracted results show that the rare earth is reduced, and Ce3+ ions and compounds are formed. Under the control of electrochemical reaction in the negative enforced electric field, a stable and dense protective film with a multilayer structure is formed on the surface of the electrode. The existence of this film can effectively prevent the erosion of the submerged entry nozzle. In addition, the phase composition of the mold flux is not affected by the externally applied electric field. The implementation of the external electric field is anticipated to be used as a new method for the high‐quality and high‐efficiency continuous casting production of rare earth steel.

Comprehensive Understanding of the Role of Carbon Black in the Sintering and Melting Behavior of Mold Flux

Carbon materials play an important role in controlling the sintering and melting behaviors of mold flux. However, the reactions between carbon and mold fluxes and corresponding effects on the properties of mold fluxes are ignored in previous studies. The effects of carbon black on the sintering and melting behavior of mold flux during the heating process were systematically investigated by TG-FTIR, DSC, XRD, TEM, SEM, XPS, Raman spectroscopy, and viscosity measurement. The results showed that: carbon black between the mineral particles was visualized by TEM, and Ca4Si2O7F2 as sintering phase was not formed until 1300°C in CaO–SiO2–CaF2–Na2O slag on account of the insulation of carbon black. Residual carbon black absorbed on the surface of slag droplets suppressed the melting by preventing the aggregation of droplets. Besides, SiC as production of the carbon-slag reaction significantly increased the slag viscosity by improving the polymerization degree of molten slag.

The effect of composition segregation of mold powder produced by spray granulation on the sintering performance

The composition uniformity of mold fluxes particles produced by spray granulation has an essential influence on its properties. Therefore, in the paper, the distribution of composition in mold fluxes during the heating process and the corresponding effect on the sintering properties of mold flux was comprehensively considered through TG-DSC, XPS, XRF, XRD, ICP, Infrared absorption, SEM-EDS, and sintering measurement. The results showed that: The difference in outward diffusion ability of different raw materials in the granulation process led to the enrichment of Na and C on the surface of mold flux particles. The partial aggregation of Na induced the centralized generation of sintering phase cuspidine (Ca4Si2O7F2) to inhibit the sintering of mold flux, and the sintering of mold flux can be hindered by increasing the particle size of mold flux to adjust the uniformity of mold flux composition.

Non-metallic inclusions and quality of pipe joints obtained by high-frequency electric resistance welding

An incisive analysis of studies of the most frequent defects of pipe joints obtained by high-frequency electric resistance welding (HFW) is included herein with particular focus on nonmetallic inclusions (NMIs), which are the cause of defects in such welded joints. Their composition, origin and methods of elimination under industrial conditions are discussed. In order to validate the nature of defects the interpretation procedure of the composition of NMIs detected in the discontinuities of welded joints has been developed. Examples of its application on the low-carbon line pipe steels of different grades X52–X56 are presented. The compositions of NMIs found in 64 defects of welded joints of A 516-55 (09G2S) steel pipes are summarized. These compositions significantly differ from each other and correspond to the different nature of NMIs: indigenous deoxidation products based on spinel MgO∙Al2O3 (8 %), products of steel modification by calcium (62 %), exo-indigenous NMIs with residuals of mould flux (4 %) or MgO (8 %) and indi-exogenous NMIs based on MgO (18 %). The unique variation ranges of the main elements included in these groups of NMIs are used for automated determination of their origin when carrying out metallographic examination of defects by means of the Thixomet image analyzer in the conditions of factory practice. The detailed origin of NMIs makes it possible to indicate the exact location in the technology of ladle treatment and steel casting for their improvement and to enhance on this basis the quality of line pipe joints obtained by HFW.

Study of Thermodynamic for Low-Reactive CaO-BaO-Al2O3-SiO2-CaF2-Li2O Mold Flux Based on the Model of Ion and Molecular Coexistence Theory

A thermodynamic model was proposed to calculate the activity of components in low-reactive CaO-BaO-Al2O3-SiO2-CaF2-Li2O mold flux, which was chosen to improve the castability of high Al steel, based on the ion and molecular coexistence theory. The model was indirectly validated, and the effects of the mass ratio of Al2O3/SiO2, contents of CaF2 and Li2O on the reactivity of components were discussed. The results reveal that the reactivity of mold flux attenuated with the increase in the mass ratio of Al2O3/SiO2. The decrease in reactivity was insignificant as the mass ratio was over 3.5. The steel–slag reaction experiment confirmed that the reactivity of mold flux is weakened when the content of SiO2 below 8 wt%. The reactivity of mold flux increased nearly linearly with the increase in CaF2 content, indicating that the proportion of CaF2 should be kept to a minimum in the flux. In addition, the compositional regions involving around 6 wt% Li2O should be avoided to develop low-reactive mold flux.

Structure and crystallization behavior of complex mold flux glasses in the system CaO‐Na2O‐Li2O‐CaF2‐B2O3‐SiO2: A multi‐nuclear NMR spectroscopic study

AbstractThe structure of mold flux glasses in the system CaO‐(Na,Li)2O‐SiO2‐CaF2 with unusually high modifier contents, stabilized by the addition of ∼4 mol% B2O3, is studied using 7Li, 23Na, 19F, 11B, and 29Si magic‐angle‐spinning (MAS), and 7Li{19F} and 23Na{19F} rotational echo double‐resonance (REDOR) nuclear magnetic resonance (NMR) spectroscopy. When taken together, the spectroscopic results indicate that the structure of these glasses consists primarily of dimeric [Si2O7]−6 units that are linked to the (Ca,Na,Li)‐O coordination polyhedra, and are interspersed with chains of corner‐shared BO3 units. The F atoms in the structure are exclusively bonded to Ca atoms, forming Ca(O,F)n coordination polyhedra. This structural scenario is shown to be consistent with the crystallization of cuspidine (3CaO·2SiO2·CaF2) from the parent melts on slow supercooling. The progressive addition of Li to a Na‐containing base composition results in a corresponding increase in the undercooling required for the nucleation of cuspidine in the melt, which is attributed to the frustrated local structure caused by the mixing of alkali ions.

Effect of initial solidification characteristics on longitudinal crack formation in thin slab caster

ABSTRACT Longitudinal surface crack has been one of the serious problems of thin slab casting (TSC). The present study was undertaken to comprehend the characteristics of longitudinal facial crack (LFC) and minimise the same in hot rolled low carbon steel sheets produced through thin slab casting and rolling (TSCR). Macro-structural characteristics of solidified shell has been investigated and correlated with plant observations. The centre and edges of funnel region showed maximum propensity of LFCs. TSC solidification is initiated with fine columnar dendrites which gradually coarsened towards the slab centre. This has been attributed to the characteristics of liquid steel flow instability in the upper mould region. Meniscus flow instability may lead to non-uniform heat transfer besides mould flux entrapment in the solidifying shell. Examination revealed that the mould hot faces ware covered with brass containing voids and cracks. Brass formation was found to be the most prominent around the meniscus of the wide hot face, which gradually faded out towards the mould exit. Tramp elements (zinc and lead) in liquid steel were found to be responsible for harmful brass formation during casting. The present findings were implemented in plant operation to minimise its concentration.

Effect of electropulsing on the solidification of mould flux

The effect of electropulsing on the microstructure formation in solidification of mould flux has been investigated. Experimental characterization and image analysis reveals that the electropulsing treatment reduces 46% porosity and increases the interfacial perimeter to area ratio by 100%. The latter provides strong evidence that electric current influences interfacial energy. Dendrite arm spacing is reduced considerable. Electropulsing promotes the anisotropic crystal growth and retards the formation of interconnected crystal network. The latter is favourable to the current flow, liquid refilling and lubrication. The work demonstrates that electropulsing can be used to modify the microstructure of slag and hence to control the horizontal heat transfer rate in the mould, which is desirable in continuous casting of advanced high strength steels.

Volatilization of fluorine in a CaO–SiO2–CaF2‐based system with the substitution of Na2O with K2O

AbstractCuspidine‐based systems are used to control the crystallization in mold fluxes, which is enabled by CaF2 additions. However, excess CaF2 increases the corrosion of casting machines. Therefore, Na2O and K2O are added to the mold flux system to ensure an optimized crystallization and lubrication ability of the flux with the CaF2 content. This study investigated the effect of substituting Na2O with K2O on the volatilization of fluorine in a CaO–SiO2–CaF2‐based slag system at high temperatures. The substitution of Na2O with K2O was performed at 5 mol% intervals. The volatilization was observed by thermogravimetric analysis under several isothermal conditions. The mass loss was measured at a heating rate of 5, 10, and 20 K/min. As the temperature increased, the volatilization of the mixed samples increased. The activation energy was calculated using the Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose methods. A kinetic analysis of the volatilization of fluorine was conducted using the calculated parameters and several known kinetic models. Consequently, the volatilization of the Na‐rich sample was controlled by chemical reactions and that of the K‐rich sample was identified to be controlled by a phase‐boundary‐controlled reaction. These results suggest that the addition of mixed alkali oxide promote the volatilization during the early stages of the reaction. From the post‐experimental composition analyses, the remaining Na and K in the samples suggested a different mechanism for the Na and K volatilization. The volatilization of Na increased with time, whereas K volatilized easily during the beginning of the reaction.

Mathematical Modeling of Mold Flow Field and Mold Flux Entrainment Assisted With High-Temperature Quantitative Velocity Measurement and Water Modeling

Mathematical Modeling of Mold Flow Field and Mold Flux Entrainment Assisted With High-Temperature Quantitative Velocity Measurement and Water Modeling

Influence of Mineralogical Structure of Mold Flux Film on Heat Transfer in Mold during Continuous Casting of Peritectic Steel.

The mineralogical structure of flux films is a critical factor in controlling heat transfer in the mold and avoiding the longitudinal cracking of slabs during the continuous casting of peritectic steel. In this study, the layered structure, crystallization ratio, mineralogical species, and morphology features of flux films were characterized by optical microscopy, X-ray diffraction, and electron-probe microanalysis. Microstructural observation revealed that the normal flux films for peritectic steels present a multilayered structure and high crystallization ratio (60~90 vol%), mainly composed of well-developed crystalline akermanite and cuspidine. In contrast, the films with outstanding flux characteristics with abundant longitudinal cracks on the slab surface have a low crystallization ratio (<50 vol%) or vast crystallite content (>80 vol%). Furthermore, heat transfer analysis showed that the low crystallization ratio and the vast crystallite content of flux films worsen the heat transfer rate or uniformity in the mold, whereas the appropriate thickness and cuspidine content of flux films can improve the heat transfer performance. From the above results, it is concluded that using strong crystalline flux to obtain the ideal mineral phase structure of flux film is one of the important measures for reducing longitudinal cracks during continuous casting of peritectic steel slabs.

Thickness Distributions of Mold Flux Film and Air Gap in Billet Ultra-High Speed Continuous Casting Mold Through Multiphysics Modeling

The thicknesses of mold flux film and air gap are significant factors that affect the high-efficiency heat transfer, the strand lubrication and mold taper design of billet ultra-high speed continuous casting mold. Therefore, this paper established the three-dimensional fluid flow, heat transfer and solidification model, interfacial heat transfer model and two-dimensional stress-strain model to conduct multiphysics modeling. Thereby the thickness distributions of liquid slag, solid slag and air gap in the ultra-high speed billet continuous casting mold were obtained, and analyzing the effects of melting temperature of mold flux and mold taper. The results indicate that the thicknesses of liquid slag and solid slag increase and decrease respectively along the casting direction, and air gap mainly concentrates near the mold corner. The maximum thicknesses of liquid slag, air gap, and solid slag at the mold outlet are respectively 0.18 mm at the center of the strand surface (x = 0 mm), 0.28 mm at the strand corner (x = 80 mm) and 0.67 mm at x = 74 mm. The lower melting temperature of mold flux, the greater the liquid slag thicknesses and ascend from 0.14 to 0.18 mm, and conversely the maximum air gap thicknesses descend from 0.31 to 0.28 mm and existing ranges also get smaller, which is more favorable for the strand lubrication. To eliminate the air gap, the appropriate linear mold taper is 0.45% m−1 at the 6.5 m/min in casting speed.

Development of estimation method through thermal conductivity and local structure of aluminoborosic acid melt

The development of new and improved means of steel production would be beneficial for industries across the globe. Professor Kazuki Morita, Department of Materials Engineering, The University of Tokyo, Japan, is developing a means of measuring thermal conductivity in order to clarify the properties of materials during production processes. A key focus for Morita is on the development of a prediction method for any physical or physicochemical properties of various materials. In a current project, he and the team are studying the mechanism of expression of thermal conductivity and thermodynamic properties of acid-based oxides. In order to control the heat flow process and devise improvements it's necessary to optimise the thermal conductivity of the flux system. In doing so, the team is seeking to reveal more data and better understand the thermal conductivity of the mold flux. In their work, the researchers are using high temperature furnaces in order to ensure that the materials remain extremely hot, as well as gas flowing systems to control the atmosphere and hot wire cells to measure thermal conductivity. To date, Morita and the team have successfully explained the qualitative behaviour of thermal conductivity with composition change but believe there remains more to be done to quantitatively assess the results. If they can predict the behaviour of slags and metals during cooling the researchers believe they will be able to improve the performance of the continuous casting of steel and avoid accidents such as breakout.

Effect of (BaO+CaO)/Al2O3 ratio (1.7∼2.0) on the structure and Al-Li association of BaO-CaO-Al2O3-CaF2-Li2O mold flux

Al2O3 possess properties of amphoteric oxide, so it plays different network roles in the traditional CaO-Al2O3-based melts, while, its influence mechanism in atomic scale has not yet been fully understood. Molecular dynamics (MD) simulation and MAS NMR Spectrum was conducted to gain a clear understanding about the variation of network structure and Al-Li association on the non-reactive melts structure in different (BaO+CaO)/Al2O3 ratio and temperature. The results show that [AlO4]− tetrahedra serves as the network framework of BaO-CaO-Al2O3-CaF2-Li2O non-reactive melt, and forms the chain and layered spatial network. As the (BaO+CaO)/Al2O3 ratio decreases from 2.0 to 1.7, Al2O3 converts to alkaline first and then to acidic, and the polymerization degree and viscosity of melt increase first and then decrease. In addition, the strength of Al-Li association decreases with the increase of (BaO+CaO)/Al2O3 ratio and temperature, which is conducive to the hop of Li+ between non-bridging oxygen sites and increases the diffusion rate of Li+ in the spatial network.

Effect of MgO on the Viscosity and Structure of CaO-Al2O3-B2O3-Based Non-reactive Mold Flux

Effect of MgO on the Viscosity and Structure of CaO-Al2O3-B2O3-Based Non-reactive Mold Flux

A comparison study on interfacial properties of fluorine-bearing and fluorine-free mold flux for casting advanced high-strength steels

A comparison study on interfacial properties of fluorine-bearing and fluorine-free mold flux for casting advanced high-strength steels

Crystallization characteristics and structural analysis of CaO–SiO2-based mold fluxes with different La2O3 contents

To elucidate deterioration mechanism of La2O3 on the crystallization performance of conventional mold fluxes, crystallization characteristics and structure of CaO–SiO2-based mold fluxes with various contents of La2O3 were studied. Results showed that adding 0 wt% to 15 wt% La2O3 increased initial crystallization temperature and critical cooling rate of slags. Crystalline phases changed from Ca4Si2F2O7 to CaLa2(SiO4)2 and Ca4Si2F2O7 with the increase in content of La2O3. Furthermore, crystallization incubation time was shortened gradually. Combined with structural analysis, the main reasons for enhanced crystallization characteristics of mold fluxes with La2O3 were following two factors. First, high-polymerized Q3 units were simplified by O2− released from La2O3 to form low-polymerized units Q2, Q1, and Q0. As a result, NBO/T of slags increased and the degree of polymerization decreased, resulting in the reduction of mass transfer resistance of ions and facilitating crystal nucleation. Second, with an increase in La2O3 content, La3+ exhibited strong attraction to simpler Q0 ([SiO4]4-) units due to its high field strength and large electric charge, which was conducive to the precipitation of rare earth phase. In this study, La3+ together with Ca2+ played charge balance effect and combined with Q0 units to form CaLa2(SiO4)2 phase at high temperature.