Effects of Fluorine on Solidification,Viscosity, Structure,and Heat Transfer of CaO-Al2O3-Based Mold Fluxes

Metallurgical and Materials Transactions B - To mitigate the chemical reaction between mold fluxes and high-Al steel, CaO-Al2O3-based mold fluxes were proposed to replace the conventional...     

Effects of MnO on Crystallization,Melting, and Heat Transfer of CaO-Al2O3-Based Mold Flux Used for High Al-TRIP Steel Casting

An investigation was carried out to study the effect of MnO on crystallization, melting, and heat transfer of lime-alumina-based mold flux used for high Al-TRIP steel casting, through applying the infrared emitter technique (IET) and the double hot thermocouple technique (DHTT). The results of IET tests showed that MnO could improve the general heat transfer rate through promoting the melting and inhibiting the crystallization of mold flux; meanwhile the radiative heat flux was being attenuated. DHTT experiments indicated that the crystallization fraction, melting temperature of mold flux decreased with the addition of MnO. The results of this study can further elucidate the properties of the CaO-Al₂O₃ slag system and reinforce the basis for the application of lime-alumina system mold fluxes for casting high Al steels.     

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

Metallurgical and Materials Transactions B - To control heat transfer between the solidifying shell and the water-cooled mold during continuous casting, the transition metal oxides, ZrO2, Y2O3, and...     

Dynamic Wetting of High-Al Steel by CaO-SiO2- and CaO-Al2O3-Based Mold Fluxes

Metallurgical and Materials Transactions B - Wetting of steel by mold fluxes affects the surface quality of steel products. Reaction between [Al] and mold fluxes in the continuous casting of...     

Crystallization Characteristics of CaO-Al2O3-Based Mold Flux and Their Effects on In-Mold Performance during High-Aluminum TRIP Steels Continuous Casting

Crystallization behaviors of the newly developed lime-alumina-based mold fluxes for high-aluminum transformation induced plasticity (TRIP) steels casting were experimentally studied, and compared with those of lime-silica-based mold fluxes. The effects of mold flux crystallization characteristics on heat transfer and lubrication performance in casting high-Al TRIP steels were also evaluated. The results show that the crystallization temperatures of lime-alumina-based mold fluxes are much lower than those of lime-silica-based mold fluxes. Increasing B₂O₃ addition suppresses the crystallization of lime-alumina-based mold fluxes, while Na₂O exhibits an opposite effect. In continuous cooling of lime-alumina-based mold fluxes with high B₂O₃ contents and a CaO/Al₂O₃ ratio of 3.3, faceted cuspidine precipitates first, followed by needle-like CaO·B₂O₃ or 9CaO·3B₂O₃·CaF₂. In lime-alumina-based mold flux with low B₂O₃ content (5.4 mass pct) and a CaO/Al₂O₃ ratio of 1.2, the formation of fine CaF₂ takes place first, followed by blocky interconnected CaO·2Al₂O₃ as the dominant crystalline phase, and rod-like 2CaO·B₂O₃ precipitates at lower temperature during continuous cooling of the mold flux. In B₂O₃-free mold flux, blocky interconnected 3CaO·Al₂O₃ precipitates after CaF₂ and 3CaO·2SiO₂ formation, and takes up almost the whole crystalline fraction. The casting trials show that the mold heat transfer rate significantly decreases near the meniscus during the continuous casting using lime-alumina-mold fluxes with higher crystallinity, which brings a great reduction of surface depressions on cast slabs. However, excessive crystallinity of mold flux causes poor lubrication between mold and solidifying steel shell, which induces various defects such as drag marks on cast slab. Among the studied mold fluxes, lime-alumina-based mold fluxes with higher B₂O₃ contents and a CaO/Al₂O₃ ratio of 3.3 show comparatively improved performance.     

Effect of MgO on Crystallization and Heat Transfer of Fluoride-Free Mold Fluxes

The effect of MgO on crystallization and heat transfer of fluoride-free mold fluxes was studied using single/double-hot thermocouple technique (SHTT/DHTT) and infrared emitter technique (IET), respectively. SHTT experiments demonstrated that the increase of MgO concentration promoted the crystallization tendency of mold fluxes. XRD analysis showed that the dominant phases changed from CaSiO₃ to CaSiO₃/Ca₂MgSi₂O₇/Ca₁₁Si₄B₂O₂₂, and to Ca₂MgSi₂O₇ as the MgO content was increased. The heat flux across mold flux disks was reduced from 671 to 615 kW/m² in IET experiments when MgO concentration was increased from 0.9 to 4.9 mass pct.     

Effects of Basicity and B2O3 on the Crystallization and Heat Transfer Behaviors of Low Fluorine Mold Flux for Casting Medium Carbon Steels

An investigation was carried out to study the effects of basicity (CaO/Si₂O) and B₂O₃ on the crystallization and heat transfer behaviors of low fluorine mold flux for casting medium carbon steels. The double hot thermocouple technique (DHTT) was employed to study the crystallization behavior of mold flux with a different basicity and B₂O₃ content, under the simulated thermal gradient as in a real caster. The infrared emitter technique (IET) was also applied for the study of heat transfer behavior of the above mold fluxes. By combining the results of IET and DHTT, this article indicated that the increase of basicity would decrease the general heat transfer rate of mold flux, as it tends to promote crystallization of mold flux apparently, while B₂O₃ has the opposite function. The combined effects of basicity and B₂O₃ could be used to adjust the general crystallization and heat transfer properties of low fluorine mold flux for casting medium carbon steels, which would provide an instructive way for the design of Fluorine free mold flux for casting medium carbon steels.     

Effect of Na2O and B2O3 on the Crystallization Behavior of Low Fluorine Mold Fluxes for Casting Medium Carbon Steels

An investigation has been conducted to study the effect of Na₂O and B₂O₃ on the crystallization behavior of low fluorine (F) mold powders for casting medium carbon (MC) steels in this article. The results of this study indicated that B₂O₃ tends to lower the crystallization temperature and increase crystallization incubation time of the low F powders; however, Na₂O plays an opposite role compared with that of B₂O₃. The crystalline phase of Ca₁₁Si₄B₂O₂₂ was formed in Sample D2 [F = 3 pct, Na₂O = 10 pct, B₂O₃ = 8 pct (in wt pct)], which exhibited the most similar crystallization behavior to that of cuspidine, such that Sample D2 showed closest crystallization kinetics to that of a conventional high-F mold slag for casting MC steels. The precipitated crystalline phases for all the samples have been analyzed and discussed in the article.     

Wetting and Erosion of ZrO2-Graphite Refractory by CaO-SiO2 and CaO-Al2O3-Based Mold Slags for Submerged Entry Nozzle

Metallurgical and Materials Transactions B - To moderate the erosion in a slag-line zone and extend the service life of a submerged entry nozzle during continuous casting of high-Mn and high-Al...     

Crystallization Behaviors of CaO-SiO2-Al2O3-Na2O-CaF2-(Li2O-B2O3) Mold Fluxes

The effects of basicity (CaO/SiO₂), B₂O₃, and Li₂O addition on the crystallization behaviors of lime-silica-based mold fluxes have been investigated by non-isothermal differential scanning calorimetry (DSC), field emission scanning electron microscopy, X-ray diffraction (XRD), and single hot thermocouple technique. It was found that the crystallization temperature of cuspidine increased with increasing the basicity of mold fluxes. The crystallization of wollastonite was suppressed with increasing the mold flux basicity due to the enhancement of cuspidine crystallization. The addition of B₂O₃ suppresses the crystallization of mold flux. The crystallization temperature of mold flux decreases with Li₂O addition. The size of cuspidine increases, while the number of cuspidine decreases with increasing mold flux basicity. The morphology of cuspidine in mold fluxes with lower basicity is largely dendritic. The dendritic cuspidine in mold fluxes is composed of many fine cuspidine crystals. On the contrary, in mold fluxes with higher basicity, the cuspidine crystals are larger in size with mainly faceted morphology. The crystalline phase evolution was also calculated using a thermodynamic database, and compared with the experimental results determined by DSC and XRD. The results of thermodynamic calculation of crystalline phase formation are in accordance with the results determined by DSC and XRD.     

Role of B2O3 on the Viscosity and Structure in the CaO-Al2O3-Na2O-Based System

The effect of B₂O₃ on the viscosity and structure in the calcium-aluminate melt flux system containing Na₂O was studied. An increase in the B₂O₃ content at fixed CaO/Al₂O₃ ratio lowered the viscosity. Higher CaO/Al₂O₃ ratio at fixed B₂O₃ content also decreased the viscosity. The alumino-borate structures were confirmed through Fourier transformed infrared (FTIR) and Raman spectroscopy and consisted of [AlO₄]-tetrahedral structural units, [BO₃]-triangular structural units, and [BO₄]-tetrahedral structural units, which could be correlated to the viscosity. At fixed CaO/Al₂O₃ ratio, B₂O₃ additions decreased the [AlO₄]-tetrahedral structural units and transformed the 3-D network structures such as pentaborate and tetraborate into 2-D network structures of boroxol and boroxyl rings by breaking the bridged oxygen atoms (O⁰) to produce non-bridged oxygen atoms (O^?) leading to a decrease in the molten flux viscosity. At fixed B₂O₃ contents and higher CaO/Al₂O₃ ratio, 3-D complex network structures become 3-D simple and 2-D isolated network structures, resulting in lower viscosities. The apparent activation energy for viscous flow varied from 132 to 249 kJ/mol according to the composition of B₂O₃ and CaO/Al₂O₃ ratio.     

Viscosity of Fluoride‐Free Mold Fluxes Containing B2O3 and TiO2

Viscosities of B₂O₃ and TiO₂‐bearing fluoride‐free mold fluxes have been measured by the rotating cylinder method in this work. Effects of different B₂O₃, TiO₂ content, and basicities on the viscosity characteristics have been examined. Viscosity of fluoride‐free mold fluxes containing B₂O₃ and TiO₂ was found to decrease with the increase of B₂O₃, TiO₂ content, and basicity. The values of apparent activation energy for viscous flow of slags decrease with additions of B₂O₃ and TiO₂ and the increase of basicity. Two parameters A and B in Riboud model were re‐evaluated based on the present experimental data, and the modified Riboud model was used to estimate the viscosity of fluoride‐free slag system investigated in present work. The viscosity values obtained by the experimental measurement were in good agreement with those calculated by the modified Riboud model.     

A Kinetic Study of the Effect of ZrO2 and CaO/Al2O3 Ratios on the Crystallization Behavior of a CaO-Al2O3-Based Slag System

The crystallization behavior of a CaO-Al₂O₃-based slag system with various ZrO₂ content (from 1 to 5 wt pct) and CaO/Al₂O₃ (C/A) ratio (from 0.8 to 1.2) has been studied by using single hot thermocouple technology (SHTT) in this article. The continuous-cooling-transformation (CCT) diagrams and time-temperature-transformation (TTT) diagrams of the above slag system were constructed for the analysis of the varying crystallization behaviors. The results suggested that Al₂O₃ tended to enhance the slag samples crystallization when the C/A ratio ranged from 0.8 to 1.2, and the critical cooling rate and crystallization temperature increased with the decrease of C/A ratio; meanwhile, the incubation time was also getting shorter with the reduction of C/A ratio. The addition of ZrO₂ would enhance the crystallization of slag samples because of the induced heterogeneous nucleation of molten slag. However, the general crystallization was determined by the balance between molten slag viscosity and heterogeneous nucleation, such that Sample 3 (C/A = 1.0, ZrO₂ = 3 pct, B₂O₃ = 10 pct, Li₂O = 3 pct [in wt pct]) would demonstrate the strongest crystallization kinetics in a high-temperature zone. The different crystals formed during the tests were also analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD).     

Stability of Fluorine-Free CaO-SiO2-Al2O3-B2O3-Na2O Mold Fluxes

Metallurgical and Materials Transactions B - B2O3 and Na2O are key components of fluorine-free mold fluxes for continuous casting, but both are highly volatile, which affects the flux stability....     

Effects of B2O3 and CaF2 on Melting Temperatures of CaO-SiO2-Fe2O3 System Fluxes

 Fluorite is widely employed as fluxing agent in metallurgy flux, which inevitably leads to serious fluorine pollution. B2O3 is employed as fluxing agent of CaO-SiO2-Fe2O3 steelmaking fluxes to substitute for CaF2. The effects of B2O3 and CaF2 on the melting properties of this system were investigated. The melting temperatures of fluxes including softening temperature (Ts), hemispherical temperature (Th), and flow temperature (Tf) were measured using the hemisphere method. The results indicate that the fluxing effect of B2O3 is more significant than that of CaF2. When the addition amount of B2O3 (mass percent) exceeds 6%, the melting temperatures of fluxes including Ts, Th and Tf are decreased lower than 1300 ℃. The basicity of fluxes has a significant effect on the melting temperature, and the melting temperatures of the fluxes increase with the increase of fluxes basicity. However, when B2O3 is used as fluxing agent, the melting temperature changes little with the basicity increasing from 2. 5 to 5. 0. These characteristics are suitable for steelmaking process. Moreover, Fe2O3 has an important fluxing effect on this CaO-based steelmaking fluxes. This indicates that the fluxes system is suitable for steelmaking process.     

Study of the Effects of the Mold Surface and Solid Mold Flux Crystallization on Radiative Heat Transfer Rates in Continuous Casting

Radiative heat transfer plays a crucial role in the meniscus area of a steel continuous casting machine. However, the study of radiation across the mold flux and copper molds is very difficult due to the harsh environment and highly transient nature of the phenomena in continuous casting molds. By using an infrared radiation emitter, a radiative heat flux was able to be applied to a copper mold with different top surfaces to observe their effects on the radiative heat transfer. The mold surface was covered with solid slag disk subjected to the radiative heat flux to simulate the radiative heat transfer phenomena in continuous casting. The solid slag disk could either be glass or a mixture of glass and precipitated crystals. The effect of full crystallization of a slag disk was to reduce the heat transfer rate by 20.5%, as compared with a completely glassy sample.     

Effect of Li<Subscript>2</Subscript>O on the Behavior of Melting,Crystallization, and Structure for CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Based Mold Fluxes

The effect of Li₂O content on the behavior of melting, crystallization, and molten structure for CaO-Al₂O₃-based mold fluxes was investigated in this article, through use of single hot thermocouple technology (SHTT), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray diffraction (XRD). The SHTT results showed that the melting temperature range of the designed mold fluxes decreases and the crystallization of mold fluxes is inhibited first and then becomes enhanced when the Li₂O content increases from 1 to 6 mass pct. The FTIR and Raman spectroscopy results suggested that Li₂O could release O^2? ions to break the complex Al-O-Al structural unit into Al-O^? structure. Meanwhile, Li₂O could also stabilize the structural unit of Si-O-Al by link aluminate and Q ₀ ^Si structure through providing Li⁺ ions to merge into the network and compensate for the charges between Al³⁺ and Si⁴⁺. Besides, the XRD results indicated that the precipitation of LiAlO₂ in molten slag would enhance the crystallization behavior of mold flux when Li₂O content is over 4.5 mass pct.     

冷却速率对连铸保护渣结晶性能的影响

采用 WCT-2型差热分析天平和LEITZ光学显微镜(配有 1350℃高温热台),研究了冷却速率对连铸保护渣结晶性能的影响.研究结果表明:随冷却速率提高,保护渣的结晶温度显著降低,结晶率降低,晶体尺寸减小,晶形也有很大改变.