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.     

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。     

A Kinetic Study of the Effect of Basicity on the Mold Fluxes Crystallization

The effect of basicity on the mold fluxes crystallization was investigated in this article. The time-temperature-transformation (TTT) diagrams and continuous-cooling-transformation (CCT) diagrams of mold fluxes with different basicity were constructed by using single, hot thermocouple technology (SHTT). The results showed that with the increase of basicity, the incubation time of isothermal crystallization became shorter, the crystallization temperature was getting higher, and the critical cooling rate of continuous cooling crystallization became faster. The X-ray diffraction analysis suggested that calcium silicate (CaO·SiO₂) was precipitated at the upper part of the TTT diagram and cuspidine (Ca₄Si₂O₇F₂) was formed at the lower part, when the basicity of mold fluxes was within 1.0 to 1.2. However, when basicity was 0.8, only the cuspidine phase was formed. A kinetic study of isothermal crystallization process indicated that the increase of the basicity tended to enhance the mold flux crystallization, and the crystallization activation energy became smaller. The crystallization mechanism of cupsidine was changing from one-dimensional growth to three-dimensional growth with a constant number of nuclei, when the basicity of mold fluxes varied from 0.8 to 1.2.     

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.     

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.     

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.     

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%。     

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.     

TiO_2对连铸保护渣导热性能的影响

运用热线法测试了不同碱度（1.1、1.3）,不同TiO2含量（0～8%,质量分数,下同）的连铸保护渣在不同温度下（400～1 300℃）的热传导系数,并结合DSC和XRD分析结果,探讨了碱度和TiO2含量对保护渣导热性能的影响规律及原因。结果表明：本试验条件下液态渣（1 200～1 300℃）的导热系数为0.27～0....     

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 ℃。     

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.     

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.     

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。     

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.     

Effect of Soda Ash on Properties of Mold Flux and Mineralogical Structures of Flux Film

During continuous casting,properties of mold flux and mineralogical structures of flux film play important roles in controlling steel quality.The effect of soda ash on the physical properties and crystallization characteristics of mold flux was investigated using process mineralogy,including polarizing microscopy,X-ray diffraction,and electron microprobe,etc.Mold flux was mainly prepared using cement clinker and industrial mineral materials under laboratory conditions,and the mineralogical structures of the flux film were systematically identified and quantitatively analyzed.The results showed that,by increasing the soda ash content from 10mass% to 18mass%,the crystallization temperature,critical cooling rate and crystallization ratio of the flux film decreased,but the crystallization ratio still reached 75% after adding soda ash,and the mineral compositions and microstructures of the flux film changed slightly.When the soda ash content exceeded 14mass%,the critical cooling rate and the crystallization ratio decreased slightly,whereas the mineral content of the flux film changed evidently,with a large amount of cuspidine precipitation and a significant decrease in the wollastonite content until it reached zero,which resulted in deterioration of the lubrication of the flux film.     

Effect of Titania Absorption into a Mold Flux on the Heat Transfer Between the Mold and the Slab in the Continuous Casting of Titanium-Stabilized Stainless Steel

The heat transfer behavior between mold and slab changes when titania is absorbed into the mold flux during the continuous casting of titanium-stabilized stainless steel. The trend and the extent of this effect were investigated in the laboratory. Furthermore, to investigate the effect of titania on heat transfer, the effect of titania on the crystallization properties of the mold flux was studied. The results showed that with an increasing titania content of mold fluxes, the heat transfer coefficient and the heat flux of the slag film increases, whereas the heat transfer resistance decreases; the effect is that the growth and crystallization temperature of the cuspidine crystallites in the slag are suppressed by the absorption of titania.     

连铸保护渣中CaF_2含量对脱玻化性能的影响

通过运用SHTTⅡ型熔化结晶温度测试仪构建了TTT曲线,计算出不同CaF_2下保护渣脱玻化活化能;再通过X射线衍射仪并结合JADE软件计算出结晶率和分析结晶矿相;最后利用金相显微镜,观察多种回温处理下不同CaF_2含量的保护渣渣膜的表面形貌,探究CaF_2对保护渣脱玻化的影响。试验结果表明:随着CaF_2含量增加保护渣脱玻化的温度和活化能不断降低,渣膜表面粗糙度增大;随着玻璃体渣膜处理温度的升高,保护渣析出矿相种类单一化。     

不同温度条件下连铸保护渣矿相结构的研究

采用矿相显微镜及XRD研究了不同温度条件下连铸保护渣矿相结构的变化。结果表明,在本实验条件下的降温和升温过程中,连铸保护渣的结晶化率均随实验温度的升高而下降,结晶矿相主要为枪晶石、硅灰石、黄长石。枪晶石在1000 ℃和1200 ℃时结晶能力最强,1300 ℃时无枪晶石析出。硅灰石只在1000 ℃时析出,晶体发育程度很高。黄长石晶体在1300 ℃时发育良好且光学性质明显,1000 ℃和1200 ℃时生长缓慢,为细小的颗粒状雏晶。在相同的实验温度条件下,连铸保护渣在升温过程中结晶化率高、晶体细小、结构致密,降温过程中其晶体发育程度良好、晶体粗大     

CaO-SiO2-Al2O3-Na2O-CaF2固态渣系的再结晶性能

 含铝钢连铸时,钢水中的铝与保护渣中SiO2反应导致保护渣中Al2O3含量增加而SiO2含量减少,导致保护渣的热物理性能发生变化,从而影响了铸坯的质量。在以前的研究基础上使用X射线衍射仪(XRD)和差热分析仪(DTA)以及扫描电镜(SEM)研究了高铝保护渣固态渣膜的结晶行为与晶体显微结构的变化规律。结果表明,高铝保护渣中CaF2的析出主要是因为SiO2含量的减少和Al2O3含量的增加导致大量的Ca2+的功能由电荷补偿变为网络修饰体,温度降低时CaF2首先析出;随着保护渣中Al2O3含量的增加,在同一加热速率下析晶温度升高;当保护渣中Al2O3含量较少时,析出晶体为枪晶石(Ca4Si2O7F2)和少量的CaF2,当Al2O3含量大于5%(质量分数,下同)时,霞石(NaAlSiO4)进一步析出,且随着Al2O3含量的增加,析出晶体的尺寸减小。     

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).