CCT and TTT Diagrams to Characterize Crystallization Behavior of Mold Fluxes

The isothermal and non-isothermal experiments were performed to construct the continuous cooling transformation （CCT） and temperature time transformation （TTT） diagrams of four industrial mold fluxes through visual observations in an experimental apparatus based on the single hot thermocouple technique （SHTT）. The results of the CCT diagrams indicate that ① the crystallization temperature of mold fluxes lowers as the cooling rate increases, ② the mold fluxes have larger critical cooling rate, higher crystallization temperature, and less onset time of crystallization when the basicity increases or the viscosity decreases, ③ the influences of the melting points of the mold fluxes on their crystallization tendency are not significant. Isothermal tests show that the onset time of crystallization decreases at first, and then increases, and finally represents a ＂C＂ shape with increasing isothermal temperature. The TTT diagrams of four industrial mold fluxes were divided into two separate ＂C＂ shape regions. The crystal phase of C20A selected was analyzed by X-ray diffraction, which is cuspidine （Ca4 Si2 O7F2 ） over I 100 ℃ and calcium silicon oxide fluoride （Ca2SiO2F2） below 1 100℃. When compared with the TTT diagram, the CCT diagram can provide a more realistic estimate of the critical cooling rate of the mold fluxes. Thus, both the CCT and TTT diagrams can unambiguously describe the crystallization phenomena of the mold fluxes.     

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.     

In-situ Crystallization of Highly Volatile Commercial Mold Flux Using an Isolated Observation System in the Confocal Laser Scanning Microscope

The in situ crystallization behavior of highly volatile commercial mold fluxes for medium carbon steels was investigated using the confocal laser scanning microscope (CLSM) equipped with an optimized isolated observation system. The highly volatile compounds of the mold flux were suppressed during heating allowing direct observation in the CLSM. Cooling rates of 25, 50, 100, 400, and 800 K/min were incorporated and continuous cooling transformation (CCT) diagrams of 4 different commercial mold fluxes for medium carbon steels were developed. Identification of the crystalline phase was conducted with XRD and SEM–EDS analysis. A cuspidine crystalline was observed in all samples at various cooling rates. With higher basicity, CaF₂, and NaF, the crystallization of the fluxes was enhanced according to the CCT diagram. As the slag structure becomes depolymerized, the diffusion rate of the cathodic ions seems to increase.     

Crystallization behavior of F-free mold fluxes

The time-temperature-transformation (TTT) diagrams of F-flee mold fluxes was constructed using single hot thermocouple technique (SHTT) and confocal scanning laser microscopy (CSLM) to study the crystallization behavior of F-free mold fluxes. The tendency of crystallization is found to increase whereas the incubation time decreases with increasing basicity. Zirconia addition enhances the crystallization tendency due to its limited solubility in the slag melt and the solid particles acting as nucleation sites. Pseudo-wollastonite is found to precipitate in the slag with low basicity (CS-1 and CS-2), kilchoanite and larnite are formed with further increasing basicity (CS-3), and larnite is finally formed as the basicity beyond unit (CS-4). The crystal morphology changes with varying compositions and isothermal temperatures. The measured growth rate is found to be linear with time under isothermal conditions and decreases with increasing isothermal temperature.     

Analysis of Crystallization Behavior of Mold Fluxes Containing TiO2 Using Single Hot Thermocouple Technique

The crystallization behavior of mold fluxes containing 0-8 mass% TiO 2 was investigated using the single hot thermocouple technique (SHTT)and X-ray diffraction (XRD)to study the possible effects on the coordination of heat transfer control and strand lubrication for casting crack-sensitive peritectic steels.Time-temperature-transforma-tion (TTT)and continuous-cooling-transformation (CCT)curves were plotted using the data obtained from SHTT to characterize the crystallization of the mold fluxes.The results showed that crystallization of the mold fluxes during isothermal and non-isothermal processes was suppressed with TiO 2 addition.From the TTT curves,it could be seen that the incubation and growth time of crystallization increased significantly with TiO 2 addition.The CCT curves showed that the crystallization temperature initially decreased,and then suddenly increased with increasing the TiO 2 content.XRD analysis suggested the presence of cuspidine in the mold fluxes with lower TiO 2 content (< 4 mass%),while both perovskite and cuspidine were detected in the mold fluxes when the TiO 2 content was increased to 8 mass%.In addition,the growth mechanisms of the crystals changed during the isothermal crystallization process from interface-controlled growth to diffusion-controlled growth with increasing the TiO 2 content.     

Crystallization Behavior in Fluoride-Free Mold Fluxes Containing TiO2/ZrO2

  The time temperature transformation (TTT) diagrams of fluoride free mold fluxes containing TiO2/ZrO2 were constructed through the confocal scanning laser microscope (CSLM) technique. It was found that the crystallization temperature of F free mold fluxes containing TiO2 or ZrO2 increases, while incubation time decreases with increasing basicity. The crystallization tendency increases with the zirconia addition in slag melt, as it may be thought that the addition solubility is limited in molten slag and the solid particles act as heterogeneous nucleation sites. Three types of crystal morphologies were observed, corresponding to different crystallization mechanisms. CaSiO3 and Ca3Si2O7 precipitate in the slag with low basicity, and Ca2SiO4 was formed with increasing basicity. The addition of TiO2 promotes the precipitation of CaTiO3. The logarithm of crystal growth rate increased with increasing isothermal temperature, suggesting that the crystal/melt interface reaction is the controlling step in the present experiment.     

Physicochemical Properties of CaO–SiO2–CaF2–NaF Slag system as a Mold Flux of Continuous Casting

The phase diagrams of CaO–SiO₂–CaF₂ and CaO–SiO₂–CaF₂–NaF systems were determined as a base system of mold fluxes. The commercial fluxes have the composition in which cuspidine first crystallizes. The thermodynamic stability of cuspidine was examined from the standard Gibbs energy of formation of cuspidine, which was measured, by two methods of the Galvanic cell method and the transpiration method. The effect of fluorine to silicate network was investigated. It is indicated that CaF₂ behaves as a neutral solvent for silicate slag. The crystallization in mold fluxes was measured by a high temperature XRD. Cuspidine predominantly appears because the construction of silicate glass with CaF₂ has CaF⁺ ion pair near non‐bridging oxygen. The over addition of Na₂O to mold flux prevents cuspidine from crystallizing.     

Heat-Transfer Behavior of Mold Fluxes for Continuous Casting of Invar Alloy

The heat-transfer behavior across mold fluxes for Invar alloy Fe-36Ni would introduce significant influence on the slab surface quality. A study on the heat-transfer property of mold flux film for Invar alloy Fe-36Ni was carried out by an interaction between laboratory simulation and field trial. The study results indicate that great effect on heat transfer across flux film is caused by chemical compositions of mold fluxes. An increase of basicity and CaF₂ content suppresses heat transfer across flux film; heat transfer across flux film increases when the Al₂O₃ content increases from 4 pct to 8 pct but decreases when Al₂O₃ content is above 8 pct. The crystalline phases of both the conventional mold fluxes and the improved mold fluxes are all cuspidine phases. However, crystallization capability of the improved mold fluxes decreases as the result of the increase of basicity and CaF₂ content as well as the decrease of Al₂O₃ content. The average thickness of flux film taken from mold is about 1.6 mm, and the crystalline fraction is only 21.4 pct. All these promote heat transfer across the flux film. The field trial of the improved mold fluxes shows that the properties of liquid slag are steady during continuous casting; comprehensive heat transfer across flux film meets the needs of continuous casting of Fe-36Ni. Border solidification structures of solidified shell are refined remarkably, and hot cracking gets avoidance eventually.     

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).     

萤石含量对连铸保护渣结晶性能的影响

通过热丝法得出保护渣连续冷却转变（CCT）和时间-温度转变（TTT）曲线,并用偏光显微分析和X-射线衍射分析方法研究了萤石含量（8%~16%）对碱度1.0的薄板坯连铸结晶器保护渣结晶性能的影响。结果表明,萤石含量每增加2%,保护渣的临界冷却速度平均增加2.5℃;同一冷却速度下,随保护渣中萤石含量的增加,结晶温度升高,结晶率增大,有利于提高界面热阻,控制渣膜传热,减少铸坯表面裂纹产生;因萤石含量超过14%时,渣膜中大量生成结晶矿物枪晶石,对连铸润滑要求较高的钢种,应控制萤石含量≤14%。     

Crystallization Behavior of the CaO-Al2O3-MgO System Studied with a Confocal Laser Scanning Microscope

The crystallization behavior of a calcium-aluminate system with various MgO content from 2.5 to 7.5?wt pct and CaO/Al₂O₃ ratios between 0.8 and 1.2 has been examined using a confocal laser scanning microscope (CLSM). CCT (continuous cooling transformation) and time temperature transformation (TTT) diagrams were constructed to identify the primary crystal phase of slag at different compositions and at cooling rates between 25 and 800?K/minutes. In the slag at a CaO/Al₂O₃ ratio of 1.0, crystallization temperature increased during isothermal and continuous cooling with higher MgO content, and the shortest incubation time was observed at 5?wt pct MgO. When MgO content was fixed to be 5?wt pct, crystallization temperature increased with lower CaO/Al₂O₃ ratio. According to the slag composition, cooling rates and temperature, the primary phase could be CA, or C₅A₃, or C₃A, or C₃MA₂, or MgO, and the crystal morphology changes from dendrites to faceted crystals to columnar crystals in this composition range.     

连铸保护渣的结晶过程

 连铸保护渣在结晶器内的结晶行为无法直接观测。为了模拟保护渣在结晶器内的结晶行为,采用热丝法研究保护渣在高温下等温和连续冷却下结晶性能,直接观察保护渣析晶的全过程,发现温度影响析出晶体的形貌和结晶率。绘制保护渣TTT曲线和CCT曲线,表征保护渣析晶行为与时间、温度的关系。在冷却速率5~25 ℃/min范围内,DSC法测定保护渣结晶温度下降110 ℃;在冷却速率6~30 ℃/min范围内,热丝法测定保护渣结晶温度下降86 ℃。     

热丝法结晶器保护渣结晶性能的控制

为控制结晶器保护渣结晶性能,采用热丝法构建保护渣的CCT与TTT曲线以探讨Na2O含量对其结晶性能的影响规律。结果表明,随Na2O含量的增加,保护渣的临界冷却速度减小,其结晶性能受到抑制;同时,保护渣的析晶开始温度升高、孕育时间减少。     

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.     

Fluorine‐Free Mould Powders for Slab Casting: Crystallization Control in the CaO–SiO2–TiO2–Na2O–Al2O3 System

The main problem related to the development of fluorine‐free mould powders for slab casting is effectively controlling the heat transfer between the steel shell and mould. In commercial mould powders crystallization of cuspidine (Ca₄Si₂O₇F₂) from mould slag has a great effect on heat‐transfer control. In industrial process the crystallization rate for a fluorine‐free mould slag should be similar to the crystallization rate of cuspidine. To evaluate the crystallization rate for slags time‐temperature‐transformation (TTT) diagrams can be constructed using the Single Hot Thermocouple Technique by in situ observation. In the present work, fundamental information related to crystallization control in the CaO–SiO₂–TiO₂–Na₂O–Al₂O₃ system was obtained. It was observed that the addition of Na₂O in CaO–SiO₂–TiO₂ slags dramatically shortens the crystals' incubation times in TTT diagrams to the range of seconds. It is possible to control the crystallization kinetics in CaO–SiO₂–TiO₂ slags by changing the Na₂O content. Some observations for the crystals' morphology are reported.     

In Situ Observation and Investigation of Mold Flux Crystallization by Using Double Hot Thermocouple Technology

The crystallization processes of mold fluxes for casting low-carbon (LC) and medium-carbon (MC) steels were investigated by using double hot thermocouple technology (DHTT) in this article. The results showed that the glass phase was first formed at the cold side thermocouple (CH-2), when the LC mold flux (mold flux for casting low-carbon steel) was exposed to the temperature gradient of 1773 K (1500 °C) to 1073 K (800 °C); then, the fine crystals were precipitated at the liquid/glass interface and grew toward glass and later on to liquid phase. However, the crystals were directly formed at CH-2 when MC flux (mold flux for casting medium-carbon steel) was under the same thermal gradient. The growth rate of MC flux crystals was much faster than that of LC ones. Scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS) analyses suggested that the crystals formed in LC mold flux were mainly dendritic cuspidine Ca₄Si₂O₇F₂, and the crystals formed from the liquid phase were larger than those from the glass. For MC mold flux, the earlier precipitated crystals were large dendritic Ca₄Si₂O₇F₂, whereas the later ones were composed of equiaxed Ca₂Al₂SiO₇ crystals. The results of DHTT measurements were consistent with the time-temperature-transformation (TTT) diagrams and X-ray diffraction (XRD) analysis.     

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.     

无氟板坯连铸结晶器保护渣的研究

针对无氟保护渣开发的关键问题一一如何有效地控制结晶器传热,发现了一种代替保护渣中枪晶石的析晶物。当析晶替代物加人量大于4％时,随着加人量的增加,熔渣析晶温度和析晶率明显增大,结晶器进出水温差减小。同时,在0～8％的析晶替代物加入量范围内,无氟渣的熔点和粘度变化均满足连铸工艺要求。在重钢2号板坯铸机上浇铸中碳包晶钢结果表明,所开发的无氟保护渣使用效果优于有氟渣。     

Observations of Crystallization in Mold Slags with Varying Al2O3/SiO2 Ratio

The present paper is an investigation into how the Al₂O₃/SiO₂ ratio in the compositions of mold slags influences the crystallization behavior of molten slags. The experimental work is based upon observing the crystallization events through a Confocal Scanning Laser Microscope equipped with a hot‐stage. The study is motivated by the variation in crystallization that might occur in mold slags due to the pickup of alumina during continuous casting of high Al containing TRIP steels. The crystallization temperature was found to increase with increasing Al₂O₃/SiO₂ ratio, and the crystal morphology was dependent upon the chemical composition and isothermal temperature. The crystallization path was complex, with CaF₂ found to precipitate first, and followed by a second precipitation event. In this second event, the precipitated phase depended on the chemical composition of mold slag and changed from cuspidine to gehlenite as the mass ratio of Al₂O₃/SiO₂ was increased beyond 0.65, and finally Al₂O₃ was observed when the alumina content was 30wt.% (corresponding to a mass ratio of = 1.42).     

Properties of High Basicity Mold Fluxes for Peritectic Steel Slab Casting

 In high speed continuous casting of peritectic steel slabs, mold fluxes with high basicity are required for less surface defect product. However, the basicity of remaining liquid slag film tends to decrease in casting process because of the crystallization of 3CaO·2SiO2·CaF2. Thus, a way is put forward to improve mold fluxes′ properties by raising the original basicity. In order to confirm the possibility of this method, the effect of rising original basicity on the properties of mold fluxes is discussed. Properties of high fluorine based mold fluxes with different basicities and contents of CaF2, Na2O, and MgO were measured, respectively. Then, properties of higher basicity mold fluxes were discussed and compared with traditional ones. The results show that increasing the basicity index can improve the melting and flow property of mold fluxes. With the increasing basicity, crystallization rate of mold fluxes increases obviously and crystallization temperature tends to decrease when the basicity exceeds 1.35. The method presented before is proved as a potential way to resolve the contradiction between horizontal heat transfer controlling and solidified shell lubricating for peritectic steel slab casting. But further study on improving the flow property of liquid slag is needed. This work can be used to guide mold fluxes design for high speed continuous casting of peritectic steel slabs.