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.     

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.     

Transient Thermo-fluid Model of Meniscus Behavior and Slag Consumption in Steel Continuous Casting

The behavior of the slag layer between the oscillating mold wall, the slag rim, the slag/liquid steel interface, and the solidifying steel shell, is of immense importance for the surface quality of continuous-cast steel. A computational model of the meniscus region has been developed, that includes transient heat transfer, multi-phase fluid flow, solidification of the slag, and movement of the mold during an oscillation cycle. First, the model is applied to a lab experiment done with a “mold simulator” to verify the transient temperature-field predictions. Next, the model is verified by matching with available literature and plant measurements of slag consumption. A reasonable agreement has been observed for both temperature and flow-field. The predictions show that transient temperature behavior depends on the location of the thermocouple during the oscillation relative to the meniscus. During an oscillation cycle, heat transfer variations in a laboratory frame of reference are more severe than experienced by the moving mold thermocouples, and the local heat transfer rate is increased greatly when steel overflows the meniscus. Finally, the model is applied to conduct a parametric study on the effect of casting speed, stroke, frequency, and modification ratio on slag consumption. Slag consumption per unit area increases with increase of stroke and modification ratio, and decreases with increase of casting speed while the relation with frequency is not straightforward. The match between model predictions and literature trends suggests that this methodology can be used for further investigations.     

Effects of Li2O and Na2O on the Crystallization Behavior of Lime-Alumina-Based Mold Flux for Casting High-Al Steels

With the development of advanced high strength steel (AHSS), a large amount of aluminum was added into steels. The reaction between aluminum in the molten steel and silica based mold flux in the continuous-casting process would tend to cause a series of problems and influence the quality of slabs. To solve the above problems caused by the slag–steel reaction, nonreactive lime-alumina-based mold flux system has been proposed. In this article, the effect of Li₂O and Na₂O on the crystallization behavior of the lime-alumina-silica-based mold flux has been studied by using the single hot thermocouple technology (SHTT) and double hot thermocouple technology (DHTT). The results indicated that Li₂O and Na₂O in the above mold flux system play different roles as they behaved in traditional lime-silica based mold flux, which would tend to inhibit general mold flux crystallization by lowering the initial crystallization temperature and increasing incubation time, especially in the high-temperature region. However, when their content exceeds a critical value, the crystallization process of mold fluxes in low temperature zone would be greatly accelerated by the new phase formation of LiAlO₂ and Na _x Al _y Si _z O₄ crystals, respectively. The crystalline phases precipitated in all samples during the experiments are discussed in the article.     

Volatilisation problems in the measurement of mould fluxes crystallisation by hot thermocouple technique

Since it is first verified that saturating the room temperature quartz tube with fluoride gas is not effective for inhibiting the NaF volatilization, the effects of fluorides volatilization on crystallization properties of mold fluxes are studied under the open condition of hot thermocouple technique. The volatilization ratio is analyzed by sintering the mold fluxes at an appropriate temperature to weigh the sample accurately. The results show that, at high holding temperatures (1400 - 1500°C), with the increase of fluoride losses and proportion of SiF₄ in the volatiles, the initial crystallization temperature, crystalline fraction, and relative crystallization rate reduced, which leads to the whole C curve shift to the bottom-right. At low holding temperatures (1300°C), the initial crystallization temperature increases with the increase of fluoride losses, and the crystallization could begin during the holding stage. Therefore, it is infeasible to study the crystallization behavior of mold fluxes using hot thermocouple technique.     

Mold Simulator Study of Heat Transfer Phenomenon During the Initial Solidification in Continuous Casting Mold

In this paper, mold simulator trials were firstly carried out to study the phenomena of the initial shell solidification of molten steel and the heat transfer across the initial shell to the infiltrated mold/shell slag film and mold. Second, a one-dimensional inverse heat transfer problem for solidification (1DITPS) was built to determine the temperature distribution and the heat transfer behavior through the solidifying shell from the measured shell thickness. Third, the mold wall temperature field was recovered by a 2DIHCP mathematical model from the measured in-mold wall temperatures. Finally, coupled with the measured slag film thickness and the calculations of 1DITPS and 2DIHCP, the thermal resistance and the thickness of liquid slag film in the vicinity of the meniscus were evaluated. The experiment results show that: the total mold/shell thermal resistance, the mold/slag interfacial thermal resistance, the liquid film thermal resistance, and the solid film thermal resistance is 8.0 to 14.9 × 10^?4, 2.7 to 4.8 × 10^?4, 1.5 to 4.6 × 10^?4, and 3.9 to 6.8 × 10^?4 m² K/W, respectively. The percentage of mold/slag interfacial thermal resistance, liquid film thermal resistance, and solid film thermal resistance over the total mold/shell thermal resistance is 27.5 to 34.4, 17.2 to 34.0, and 38.5 to 48.8 pct, respectively. The ratio of radiation heat flux is around 14.1 to 51.9 pct in the liquid slag film.     

Crystallization Control in Metallurgical Slags Using the Single Hot Thermocouple Technique

With the single hot thermocouple technique (SHTT) the solidification behavior of metallurgical slags has been studied by in situ observation, constructing time–temperature–transformation (TTT) or continuous‐cooling‐transformation (CCT) diagrams. The SHTT is a unique apparatus that enables measurement of the slag sample temperature using a thermocouple while the sample is heated or cooled simultaneously. Due to the low heat capacity of the system sample/thermocouple high heating or cooling rates can be easily obtained (>3000°C/min). The following findings are reported in the present paper: (i) For the CaO–Al₂O₃ slag – 44% CaO, 56% Al₂O₃ (wt%) – the CCT diagram shows large differences between liquidus and the temperature for first crystals precipitation, even at low cooling rates, for example, 168°C below the liquidus when cooling at a rate of 6°C min^?1. (ii) For the CaO–SiO₂ slag – % CaO/% SiO₂ (wt%) = 0.7 – no crystal is observed for continuous cooling, even at low cooling rates, such as 10°C min^?1. During isothermal experiments crystallization was observed only at 1000°C with an incubation time of 76 s (average of six experiments, standard deviation 27 s). However, crystallization becomes much more intense for the CaO–SiO₂ slag when increasing the temperature after reaching lower temperatures (<1000°C), where probably the conditions for nucleation are better.     

基于热丝法的连铸保护渣的结晶过程

连铸过程中,保护渣的结晶过程十分重要.笔者以保护渣结晶温度为出发点,提出了一种将双铂铑热电偶丝既作为加热元件,又作测温元件的热丝法技术.并结合计算机控制、视频显示和图像识别等技术,能在线观察、离线分析工业结晶器保护渣和实验自制渣的结晶过程,为连铸生产保护渣的选取和研究提供了更加可靠的依据.     

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.     

Analysis of shell thickness irregularity in continuously cast middle carbon steel slabs using mold thermocouple data

Thermocouples buried in the mold wall of a continuous caster are used to investigate the nature and source of shell thickness irregularity in middle carbon steel slabs. Fourier analysis is used in conjunction with digital filters to determine the power spectra of time series mold temperature and mold level measurements. Direct evidence is obtained on the physical dimension of irregularity, as well as the phase relationships between neighboring thermocouples in both the transverse and longitudinal directions. In addition, mold thermocouple readings are used to set the boundary heat flux conditions for use in self-consistent mathematical modeling of mold thermal profiles. Temperature readings—average, minimum, and maximum—allow for the calculation of an envelope of shell thicknesses around the average distribution. These techniques are used to help explain a mechanism for the occurrence of shell thickness irregularity, in terms of both meniscus disturbances and shell deflections in response to such disturbances.     

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.     

The Radial Temperature Gradient in the Gleeble<Superscript>®</Superscript> Hot-Torsion Test and Its Effect on the Interpretation of Plastic-Flow Behavior

The radial temperature gradient developed via direct-resistance heating of round-bar hot-torsion specimens in a Gleeble^® machine and its effect on the interpretation of plastic-flow behavior were established using a suite of experimental, analytical, and numerical-simulation tools. Observations of the microstructure variation developed within a γ′-strengthened nickel-base superalloy were used to infer the temperature gradient as well as differences between the temperature at the outer diameter and that indicated by thermocouples welded to the surface. At temperatures of the order of 1375 K (1102 °C), the radial variation of temperature was typically ~20 K (~20 °C). Such variations were in agreement with an analytical heat-conduction model based on the balance of input thermal energy and radiation heat loss at the free surface. Using a constitutive model for LSHR, the effect of the radial temperature gradient on plastic flow during hot torsion was assessed via numerical integration of the torque as a function of radial position for such cases as well as that corresponding to a uniformly-heated sample. These calculations revealed that the torque generated in the non-uniform case is almost identical to that developed in a sample uniformly preheated to a temperature corresponding to that experienced at a fractional radial location of 0.8 in the former case.     

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.     

210 t转炉双渣法冶炼深冲钢DC04的脱磷工艺实践

通过热力学计算得出转炉双渣法前期脱磷最佳温度为1 320~1 355℃,前期渣碱度宜为1.3~1.6,并在210 t顶底复吹转炉进行4炉DC04钢工业试验。结果表明,通过留渣量60%~80%的留渣操作,吹炼3~4 min进行倒渣操作,加入生白云石10 kg/t,球团15kg/t,前期渣样组成为33%~39%CaO,20%~25%SiO₂(碱度1.3~1.6),后期二次造渣加石灰16 kg/t,球团15kg/t和轻烧白云石10 kg/t,后期渣样组成44%~47%CaO,≤15%SiO₂(碱度2.85~3.20),使双渣法出钢平均[P]为0.014%,双渣法[Mn]收得率≤30%,前期渣中TFe含量为8.0%~12.0%,对后期渣中TFe含量影响较小。     

Nonisothermal Crystallization Kinetics of Glassy Mold Fluxes

Crystallization of the solid glassy mold flux film occurring in the gap between the initial shell and mold wall is important, as it determines the in-mold heat transfer and mold lubrication during the process of continuous casting. In order to study the nonisothermal crystallization behavior of the glassy mold flux film in the continuous casting mold, the continuous heating transformation diagram, crystallization mechanism, and precipitate phases were investigated using the single hot thermocouple technique, kinetic models, a scanning electron microscope, and an energy-dispersive spectrometer (EDS). The results show that the initial crystallization temperature for CaO-SiO₂ based flux A ranges from [1086 K to 1147 K (813 °C to 874 °C)], which is lower than the case of CaO-Al₂O₃ based flux B ranging from [1205 K to 1245 K (932 °C to 972 °C)]. The crystallization kinetics for flux A are constant nucleation rate, two-dimensional growth, and control by diffusion. For flux B, they are constant nucleation rate, three-dimensional growth, and control by interface reaction. Besides, the EDS results indicate that the precipitate crystals in fluxes A and B are CaSiO₃ and Ca₂AlSiO₄, respectively.     

保护渣晶粒尺寸对传热性能的影响

保护渣在连铸过程中具有重要的作用,其中有效控制钢液向结晶器的传热直接影响了铸坯质量;当保护渣热阻较小、不均匀传热时铸坯容易出现裂纹等缺陷.而保护渣的控热能力主要取决于保护渣的结晶性能,因此,有必要研究保护渣的结晶行为对传热性能的影响.利用双丝热电偶技术通过不同的温度制度,获得不同尺寸的单一结晶相,研究保护渣晶粒尺寸对传...     

高炉冷却壁非稳态传热热态实验分析

 冷却壁安全工作是保证高炉长寿的基础。通过设计并建造冷却壁热态实验炉,研究了高炉铸铁冷却壁热面无渣皮和有渣皮时的非稳态传热过程,考察了不同炉气温度条件下冷却壁热电偶温度的变化规律。回归得到了炉气在升温阶段、稳定阶段、降温阶段时冷却壁热电偶温度随时间的变化关系式。计算得出了冷却壁热面在有无渣皮条件下的平均热流强度,回归得出了炉气平均对流换热系数随炉温的变化关系。结果表明,冷却壁热面在有渣皮时热电偶温度的变化速率显著低于无渣皮时的变化速率,冷却壁破损的主要原因是冷却壁温度的反复变化和渣皮的频繁脱落而产生的热应力。     

连铸结晶器内渣膜形成及传热的研究现状

介绍了模拟结晶器内渣膜形成的实验方法, 综述了国内外学者在保护渣传热方面所做的研究工作, 包括固态渣膜的界面热阻、保护渣的导热系数、辐射传热以及渣膜的光学性质, 并提出了今后在渣膜形成及传热研究中有待进一步完善的内容和方向.现有的研究结果表明利用热丝法可以对渣膜的形成过程进行原位观察, 采用水冷铜探头法可以获取用于研究渣膜微观组织的固态渣膜样品.渣膜的界面热阻在0.0002~0.002 m2·K·W-1之间.在800℃以下, 保护渣的导热系数在1.0~2.0 W·m-1·K-1范围内, 且随温度的升高而逐渐增加.渣膜中的晶体一方面可以增加渣膜的界面热阻, 另一方面可以提高固态渣膜的反射率, 起到降低辐射热流的作用.此外, 过渡族金属氧化物的加入以及固态渣膜中弥散分布的微小颗粒也能改变渣膜的光学性质, 从而影响通过渣膜的辐射传热.      

Observation of the crystallization behavior of a slag containing 46 wt pct CaO, 46 wt pct SiO<Subscript>2</Subscript>, 6 wt pct Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, and 2 wt pct Na<Subscript>2</Subscript>O using the double hot thermocouple technique

In this work, isothermal crystallization of a synthetic slag containing 46 wt pct CaO, 46 wt pct SiO₂, 6 wt pct Al₂O₃, and 2 wt pct Na₂O has been investigated by means of double hot thermocouple technique (DHTT). The effect of Na₂O content on crystallization time was confirmed. Two different types of calcium silicate crystals were observed. Calcium di-silicate forms at temperatures above 1150 °C and calcium tri-silicate precipitate at temperatures below 1050 °C. A mixture of the two types of calcium silicate has been observed between the two temperatures. The tendency of crystals to become richer in calcium at low temperatures that has also been observed in previous published works has been confirmed. No effect of the cooling rate on crystallization start time was confirmed in the range of cooling rates applied in this investigation.     

无磁钢20Mn23AlV保护渣液渣和渣圈性能变化对连铸的影响

对高铝无磁钢20Mn23AIV(/%:0.14～0.20C、21.50～25.00Mn、1.50～2.50Al、0.04～0.10V)200 mm板坯连铸过程结晶器保护渣液渣和渣圈的化学组成、理化性能和结晶矿相进行了对比分析。保护渣原渣组成为(/%):31.91CaO、30.30SiO₂、6.58Al₂O₃、1.12MgO、3.02MnO、7.73Na₂0、7.10F。结果表明,连铸开浇后15 min,液渣和渣圈中的SiO₂含量分别降低至22%和18%, Al₂O₃含量分别提高至20.5%和25.5%,其碱度由原渣的1.05分别提高至1.7和2.0。此时液渣及渣圈的熔化温度和粘度大幅度增加,转折温度大幅度降低;渣圈的化学成分及理化性能的变化幅度均大于液渣。连铸开浇15 min后液渣及渣圈的成分与性能均趋于稳定。高熔点相钙铝黄长石的析出是促使渣圈形成的重要原因。