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External fields, such as magnetic and electric, are applied to the continuous casting mold to improve the quality of casting products. The performance of the mold flux should also be impacted when these external fields are applied in the mold. The extent of the impact is associated with the electrical conductivity. So, in this study, the electrical conductivity of the mold fluxes with various basicity was measured using the AC four-electrode method. The results show that the electrical conductivity of the mold flux increased while its activation energy decreased with the increase of basicity from 0.8 to 1.1, at 1423 K to 1573 K. In addition, the analyses of Fourier-transform infrared (FTIR) and Raman spectroscopies show that the complex structural units, such as Q3(Si), Si–O–Si, Si–O–Al, and Al–O0, reduced with the increase of basicity. So the melt structure of the mold flux was simplified by the addition of CaO and reduction of SiO2. From the variations of electrical conductivity and melt structure, it can be concluded that it is due to the simplification of melt structures of the mold flux by the increase of basicity, which promotes the diffusion of ions/ionic groups, and then leads to the increase of electrical conductivity.

The CaO-Al2O3-based mold flux is expected during the casting of aluminum containing advanced high strength steels. In this study, the rheological behavior of the CaO-Al2O3-based mold fluxes with different Na2O contents was investigated. The results show that the viscosity in the range of 1423–1573 K reduced sharply when the Na2O content increased from 0 wt % to 10 wt %, but the tendency slowed down with further increasing Na2O from 10 wt % to 15 wt %. The activation energy for viscous flow also decreased from 237.76 ± 4.88 kJ/mol to 180.37 ± 7.10 kJ/mol with increasing Na2O. The structure analyses show that the melt networks were mainly constructed by [SiO4]4--tetrahedral, [AlO4]5-tetrahedral and Si-O-Al structural units. These networks were depolymerized with the addition of Na2O since the charge compensation effect from Na+ was relativity weaker comparing with the network breaking effect from O2−. In addition, the break temperature of the mold fluxes also decreased from 1406 K to 1198 K due to the more precipitation of low melting point Ca2Al2SiO7, rather than MgAl2O4 in the mold flux, during the cooling cycle.

The crystallization behavior and melt structure of the CaO–SiO2–B2O3-based fluorine-free mold flux were investigated. The results show that the crystallization of the mold flux was first inhibited and then promoted with the increase in Al2O3 content from 4 to 12 wt.%. However, it was enhanced by MgO in the range of 2–10 wt.%. The results of Fourier transform infrared spectroscopy and Raman spectroscopy showed that Al2O3 worked as a network former in the mold flux melt when its content was in the range of 4–8 wt.%, whereas it worked as the network breaker to provide O2− when its content was in the range of 8–12 wt.%. In addition, the combined effects from the charge compensation by Mg2+ and the network broken by O2− led to the increase in some typical T–O–T (Al–O–Al, B–O–B, etc.) and simpler structural units (Q0(Si), B–O− in the [BO2O−], etc.) when the MgO content was in the range of 2–6 wt.%. The continuous increase in O2− provided by the addition of MgO from 6 to 10 wt.% further depolymerized the network of the melt and finally caused fast crystallization.

An electropulsing-assisted mold simulator (EPMS) technique was developed to investigate the effect of pulsed electric current on the initial solidification behavior of molten steel during the process of continuous casting. The results indicated that the variation of mold hot surface temperatures in the meniscus area decreased from 375.8 K to 394 K for the case without electropulsing treatment (EPT) to 363.5 K to 377.3 K for the one with EPT. The mold surface heat fluxes fluctuated around the baseline of 0.83 and 2.42 MW/m2 in the cases with and without EPT, respectively, which indicated that a higher thermal resistance between the initial shell and copper mold was introduced by employing the EPT. The crystal fraction of slag films increased from 69.8 to 77.9 pct, whereas the grain size of mold flux film decreased, when compared the case of EPT treatment with the benchmark one, which suggested that the pulsed electric current promotes the movement of molten clusters and particles, leading to the enhancement of mold flux crystallization and the improvement of thermal resistance. Moreover, the solidification structure of steel along the direction of solidification (from the mold to melt) includes a fine chilled grain layer close to the mold, a columnar dendrite layer in the middle, and equiaxed grains next to the melt core, in which a compacted microstructure with smaller secondary dendrite arm spacing was observed in the case of the EPT. The results demonstrate that the EPT could refine the as-cast solidification microstructure.

An investigation had been conducted to study the effect of CaO/SiO2 mass ratio and B2O3 on the crystallization behavior and the melt structure of fluorine-free mold fluxes. The results showed that the increase of CaO/SiO2 ratio tends to promote the crystallization behavior of the fluorine-free mold flux system, due to the depolymerization of the original complex borosilicate structure by the released free oxygen (O2−) ions with the added CaO. Small amount of B2O3 would initially inhibit the mold flux crystallization, then the further addition would promote crystallization. The melt structure analysis results suggested that with the addition of B2O3 content, the existing silicate structure would connect with each other and also link with the dissociative [BO3]-trihedral units to form borosilicate chain. Simultaneously, O2– could destroy the original diborate structure and form two simple borate structures. Subsequently, the simple borate structure would connect with the dissociative [BO3]-trihedral and the formed borosilicate chain to form a more complex borosilicate structure. Thus, the degree of polymerization of whole molten slag was getting improved, but the overall effect on the mold flux crystallization was depended on the balance between the molten slag structure and viscosity, in which 6 mass% was the critical point in the current mold flux system.

A comparison study was conducted on viscosity and heat transfer property of the traditional fluorine-bearing (F-bearing) mold flux and the newly designed fluorine-free (F-free) mold flux for casting silicon steel. The results showed that the viscosity of F-free mold flux is lower than that of F-bearing mold flux, as the apparent activation energy of F-free mold flux is also lower than that of F-bearing mold flux confirmed by the kinetics results for viscous flow, which meant that the energy barrier of F-free mold flux that the migration of ion clusters needs to overcome is smaller, and thus leads to a lower viscosity of F-free mold flux. Besides, the research results of the heat transfer suggested that the comprehensive heat transfer coefficient of F-free mold flux is lower than that of F-bearing mold flux in all directions. One reason is that the true density of F-free mold flux increases from glassy state to crystalline state, while that of F-bearing mold flux is opposite. Another reason is that the air gap gets increased due to the volume shrinkage of F-free mold flux from glassy state to crystalline state and the solidification of the molten steel. These introduce a larger thermal resistance of F-free mold flux than that of F-bearing mold flux. Through the comparison, the designed F-free mold flux shows a better lubrication and capacity to control the heat transfer than the traditional F-bearing mold flux.

In this study, the effect of naturally deposited film on the interfacial heat transfer during the sub-rapid solidification of medium manganese steel was studied by means of a droplet solidification technique. The heat transfer rates were calculated by the Inverse Heat Conduction Program (IHCP), through computing the responding temperatures’ gradient inside the cooling mold. The results showed that the contact angle between the mold and solidified droplet becomes lower (from 117.539 to 82.217 deg) with the repeat of the dropping tests, which means the wettability improves after the deposition of the natural film. Meanwhile the liquid time (the time from the start of ejection to the initiation of solidification) decreases from 1.76 to 1.28 seconds during the tests, suggesting that the steel keeps in the liquid state shorter. Therefore, the initiation of solidification is advanced, such that there would be less time for matching the scale of the shell and the mold. Consequently, the maximum heat flux increases from 2.90 to 6.35 MW/m2 with the deposition of the natural film, as the wettability is getting better leading to the reduction of the interfacial thermal resistance. However, the final maximum heat flux was decreasing to 4.98 MW/m2 due to the increased thickness of the film and corresponding interfacial thermal resistance. Moreover, it was found that the secondary dendrite arm spacing decreased from 10.84 to 5.01 μm at first, and then it increased to 8.11 μm with the variations of heat flux and cooling rate.

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.

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 O2− 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.

To compensate the negative effect caused by the absorption of chromium oxide inclusions during the casting process of Cr-contained steels, a new mold flux system has been designed and investigated. The melting temperature range of the newly designed mold flux system is from [1124 K to 1395 K (851 °C to 1122 °C)]. The viscosity at 1573 K (1300 °C) and the break temperature increase with the addition of MnO and Cr2O3 but decrease with the addition of B2O3. The crystalline fraction of mold flux decreases from 81 to 42.1 pct with the addition of MnO and Cr2O3, and then further decreases to 25.3 pct with the addition of B2O3; however, it improves from 54.4 to 81.5 pct when the basicity increases. Besides, the heat-transfer ability of mold flux is inverse to the crystallization ratio of the slag. The comprehensive study of the properties for the four designed mold fluxes suggests that the mold flux with 1.15 basicity-3.01 pct B2O3-1.10 pct MnO-2.10 pct Cr2O3 shows the best properties for the continuous casting of Cr-contained steels.

Mold flux plays very important roles in the continuous casting process, and its high-temperature properties affect the quality of the final as-cast product greatly. Investigations on the melting, isothermal and nonisothermal crystallization, and phase evolution behaviors under a simulated temperature field for the mold flux system using the single/double hot-thermocouple technique (S/DHTT) were reviewed. Meanwhile, further in situ observations on the wetting behavior and heat transfer ability of the mold flux system were also carried out using the S/DHTT. The results summarized here provide a clear understanding of both the high-temperature properties of mold flux and the detailed application of advanced real-time visual high-temperature S/DHTT to this molten slag system.

With the mold simulator technique, the effect of slag-steel reaction on the initial shell solidification as well as the heat transfer and lubrication behavior of the infiltrated mold/shell slag film was studied in this article. The results showed that the Al2O3 content, the CaO/SiO2 ratio, and the viscosity of mold flux were increased with the progress of the slag-steel reaction during casting. The slag-steel reaction has two major effects on the initial shell solidification: one is increasing the mold heat flux and shell thickness by the decrease of slag film thickness. The other is the reduction of mold heat flux by the increase of crystal fraction in slag film. Mold flux with a lower basicity, viscosity, and crystallization temperature would result in a larger liquid slag consumption and the uneven infiltration of slag into the mold and shell gap that eventually leads to the irregular solidification of initial shell with a poor surface quality, such as slag entrapment and depressions as well as glaciation marks. Conversely, mold flux with a higher viscosity, basicity, and crystallization temperature would result in a smaller liquid slag consumption, which would cause the poor mold lubrication, the longitudinal shell surface defects, and drag marks.

Mold flux plays important roles in the process of continuous casting. In this article, the performance of mold flux for the casting of Cr12MoV steel was investigated by using a mold simulator. The results showed that the slag film formed in the gap between the initial shell and mold hot surface is thin and discontinuous during the casting process with the Flux BM, due to the absorption of chromic oxide inclusions into the liquid slag, while the slag film formed in the case of the optimized Flux NEW casting process is uniform. The main precipitated crystals in Flux BM slag film are cuspidine (Ca4Si2O7F2) and Cr3O4, but only Ca4Si2O7F2 precipitated in the Flux NEW case. Besides, both the responding temperature and heat flux in the case of Flux BM are relatively higher and fluctuate in a larger amplitude. The surface of the shell obtained in the case of the Flux BM experiment is quite uneven, and many severe depressions, cracks, and entrapped slags are observed in the surface due to the lack of lubrication. However, the obtained shell surface in the case of the Flux NEW shows good surface quality due to the addition of B2O3 and the adjustment of basicity, which can compensate for the negative effects of the mold-flux properties caused by the absorption of chromic oxide during the casting process.

The slag entrapment in mold tends to cause severe defects on the slab surface, especially for casting steels containing active alloy elements such as Al, Ti, and Mn. The wetting behavior of molten mold flux on the initial solidified shell is considered to be a key factor to determine the entrapment of mold slag on the shell surface. Therefore, the wetting behavior of mold flux droplet on the steel substrate with or without interfacial reaction was investigated by the sessile drop method. The results indicated that the melting process of mold flux has a significant influence on the variation of contact angle, and the final contact angle for Flux1 droplet on 20Mn23AlV is only 15 deg, which is lower than the other two cases due to the intensive interracial reactions occurring in this case. In addition, the thickness of the interaction layer for the case of Flux1 on 20Mn23AlV is 10-μm greater than the other two cases, which confirms that the most intensive reactions occurred at the interface area. The microstructure and element distribution at the interface analyzed by a scanning electron microscope (SEM) and energy dispersive spectrum (EDS) suggested that the increase of wettability of mold flux droplet on the steel substrate is caused by the migration of Al, Mn, and Si elements occurring in the vicinity of the interface. The results obtained in this article can reveal the mechanism of flux entrapment by hook or shell and provide theoretic guidance for mold flux design and optimization.

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.

Corner cracks are one of the most widespread surface defects of continuous casting slabs, and they are especially severe for peritectic steels and low-alloy steels. Therefore, a clear understanding of molten steel initial solidification around mold corner would be of great importance for the inhibition of corner cracks. This paper has been conducted with the aim to elucidate this understanding, by using a novel mold simulator equipped with a right-angle copper mold. The responding temperatures and heat fluxes across the mold hot-face and corner were firstly calculated through a 2D-inverse heat conduction program mathematical model, and the results suggested that the cooling ability and the fluctuation of heat fluxes around the mold corner are stronger than those for mold hot-face. With the help of power spectral density analysis and fast Fourier transformation, the four characteristic signals of heat fluxes were discussed in this paper. Next, the relation between the thickness of solidified shell and solidification time was fitted with the solidification square root law; as a result, the average solidification factor $$ \bar{K} $$ for the hot-face shell is 2.32 mm/s1/2, and it is 2.77 mm/s1/2 for the shell near-corner. For the same oscillation marks (OMs), it appeared that the OMs positions on the shell corner are lower than those on the shell hot-face along the casting direction, because the stronger shrinkage of shell at the corner allows the overflowing steel to penetrate deeper into the larger gap between the shell corner and mold, which is demonstrated through the heat transfer analysis and metallographic examination. Finally, the interrelation between shell profile, mold oscillation, variation rate of heat flux, high-frequency heat flux and high-frequency temperature was discussed for above two cases, and the results suggested that meniscus conditions (heat transfer and melt flow) around the mold corner are more unsteady.

Viscosity is one of the most important properties of mold flux and affects the process of continuous casting significantly. In order to describe the variation of viscosity of mold flux accurately in a wide range of temperature occurring in the casting mold, a non-Arrhenius Vogel–Fulcher–Tammann (VFT) model was adopted in this study. The results showed that the adjusted coefficient of determination (Adj. R 2) of non-Arrhenius VFT Model ranges from 0.92 to 0.96, which suggests this model could be well adapted to predict the relationship between viscosity and temperature of mold flux. The temperature at which viscosity becomes infinite, T VFT, increased with the addition of Cr2O3 and improvement of basicity, while it decreased with the addition of B2O3, as it was determined by both the degree of polymerization of the melt structure and crystallization behavior of the melt. Also, the pseudo-activation energy, E VFT, of Samples 1 to 5 was 60.1 ± 3.6, 94.7 ± 14.9, 101.7 ± 19.0, 38.0 ± 4.8, and 32.4 ± 4.0 kJ/mol, respectively; it increased with the addition of Cr2O3 and B2O3, but deceased with the increase of basicity.

The interfacial property between liquid mold flux and steel has significant impact on the quality of casting slab, and this property is mainly determined by the chemical composition of mold flux and the reaction between the flux and steel. The effect of MnO content on the contact angle and interfacial tension between liquid mold flux and ultra-low carbon steel was investigated by sessile drop method in this article, and the results suggested that both the contact angle and interfacial tension decreased with the increase of MnO content in the mold flux. The increase of Si and Mn and the reduction of Al and Ti in the interaction layer were caused by the chemical reactions occurred in the vicinity of interface between mold flux and steel substrate. Besides, the thickness of the interaction layer increased from 4 μm to 7 μm, then to 9 μm, 11 μm and 15 μm when the MnO content was added from 1 wt% to 3 wt%, then to 5 wt%, 7 wt%, and 9 wt% due to the fact that MnO can simplify the polymerized structure of the melt and improve the penetrability of molten mold flux to make the interfacial reaction easier.

Based on the mold simulator technology, a mold-cracking simulator has been successfully developed to study the process of breakout and the shell surface crack formation during the initial solidification of molten steel inside the continuous casting mold. First, a spheroidal protrusion was installed on the mold hot surface to mimic the abnormal force that generated by mold wall deformation, and then the external force was applied to the initial solidified shell, to facilitate the formation of breakout and shell surface cracks. Second, the responding temperature and heat flux across mold hot surface were recovered by an inverse heat conduction problem. The experimental results indicated that the mold breakout occurs around the shell tip by the combined efforts from external horizontal force, ferrostatic pressure, and thermal stresses during positive strip time. The breakout tends to introduce the peak of the responding temperature and heat flux across the mold hot surface. The vertical propagation velocity of the rupture point in the solidification shell has been calculated as 0.42 m/s in this study, which is in good agreement with industrial slabs. The paper also suggested that surface transverse crack formation is related to the segregation of sulfur during the initial solidification of molten steel.

Designing and developing high-performance fluorine-free (F-free) mold flux has become a hot topic in steel continuous casting processes, with concerns of environment protection and energy saving. In conventional commercial mold flux, fluorine plays important roles on the properties as it works as a fluxing agent; however, it tends to cause serious environmental and health problems. In this paper, a new F-free mold flux based on the CaO-SiO2-B2O3 slag system has been introduced through summarizing previous works. The melting temperature range of F-free mold flux decreases with the addition of Na2O/Li2O and B2O3; the viscosity and heat flux decrease with the increase of basicity and Na2O/Li2O, as well as the decrease of B2O3 contents. Also, the crystallization temperatures of F-free mold fluxes increase with the increase of basicity and Na2O/Li2O content. The analyses of EDS and XRD show that Ca11Si4B2O22 and Ca14Mg2(SiO4)8 are the two main precipitated crystalline phases in F-free mold fluxes, and that the Ca11Si4B2O22 is a common and stable crystalline phase in the designed F-free mold fluxes system that shows the potential to replace Ca4Si2O7F2 in conventional flourine-containing mold fluxes.

The mold flux in continuous casting mold experiences a significant temperature gradient ranging from more than 1773 K (1500 °C) to room temperature, and the viscosity of the mold flux would therefore have a non-Arrhenius temperature dependency in such a wide temperature region. Three non-Arrhenius models, including Vogel–Fulcher–Tammann (VFT), Adam and Gibbs (AG), and Avramov (AV), were conducted to describe the relationship between the viscosity and temperature of mold flux in the temperature gradient existing in the casting mold. It found that the results predicted by the VFT and AG models are closer to the measured ones than those by the AV model and that they are much better than the Arrhenius model in characterizing the variation of viscosity of mold flux vs temperature. In addition, the VFT temperature and AG temperature can be considered to be key benchmarks in characterizing the lubrication ability of mold flux beyond the break temperature and glass transition temperature.

Droplet solidification technique is important with respect to the fundamental study of strip casting given the common conditions of direct contact between cooling mold and solidifying metal. In this study, an improved droplet solidification technique has been developed for the in situ observation of the sub-rapid solidification phenomena of metal droplets impinging onto the water-cooled copper substrate. The heat transfer rates were calculated by the inverse heat conduction program (IHCP), according to the responding temperatures’ gradient inside the cooling mold. Meanwhile a charge coupled device (CCD) camera was placed beside the bell jar aimed to record the whole melting and solidification process of the steel sample, which also allowed the determination of the final wetting angel, during the dipping tests. Moreover, it was found that the heat transfer rate increased with decreasing final contact angle, which means better wetting condition between the liquid sample and the copper substrate.

A thermal imaging enhanced inferred emitter technique was developed to investigate the heat transfer behavior of mold flux. Then, the phase transformation behavior, the heat transfer behavior, the temperature field evolution and the mold/slag interfacial thermal resistance evolution for a demonstration experiment of medium carbon mold flux slag disk were in situ recorded. The demonstration experiment results showed that the phase transformation behavior of mold flux significant affected the radiation heat transfer. And the phase transformation behavior also led to the change of temperature distribution on the slag. According to the in situ observation of slag temperature field, the crystallization behavior of mold flux made the high-temperature region move toward the crystalline layer. The variation of the mold/slag interfacial thermal resistance Rint also had been directly obtained with the help of thermal imager. Rint decreased with the increase of mold/slag interfacial temperature. In addition, mold/slag interfacial deformation and the decrease of interfacial temperature caused by the crystallization behavior led to an increase of Rint.

An investigation was carried out to study the influences of basicity and Li2O on the melting, crystallization, and heat transfer behavior of Fluorine-free mold flux designed for the casting of medium carbon steels using double hot thermocouple technology and infrared emitter technique. The results showed that with the addition of basicity, the melting and crystallization temperatures of the mold fluxes were increased, and the final heat transfer rate was reduced, as the basicity tends to promote the crystallization behavior of the designed mold fluxes. Besides, with the increase of Li2O content in the mold flux, the melting and crystallization temperature decreased, as the Li2O tends to inhibit the formation of high melting temperature crystal and lower the system melting temperature zone; meanwhile the crystallization capability of the mold flux was enhanced in the low-temperature region. Moreover, the results of EDS and XRD were confirmed that the main crystal phase in the Fluorine-free mold fluxes is calcium borate silicate (Ca11Si4B2O22). Those results obtained can provide guidelines for the design of new Fluorine-free mold flux for the casting medium carbon steels.

The surface quality of the continuous casting strands is closely related to the initial solidification of liquid steel in the vicinity of the mold meniscus, and thus the clear understanding of the behavior of molten steel initial solidification would be of great importance for the control of the quality of final slab. With the development of the mold simulator techniques, the complex interrelationship between the solidified shell surface profile, heat flux, shell thickness, mold level fluctuation, and the infiltrated slag film was well illustrated in our previous study. As the second part, this article investigated the effect of the mold oscillation frequency, stroke, and mold level fluctuation on the initial solidification of the molten steel through the conduction of five different experiments. Results suggested that in the case of the stable mold level, the oscillation marks (OMs) exhibit equally spaced horizon depressions on the shell surface, where the heat flux at the meniscus area raises rapidly during negative strip time (NST) period and the presence of each OMs on the shell surface is corresponding to a peak value of the heat flux variation rate. Otherwise, the shell surface is poorly defined by the existence of wave-type defects, such as ripples or deep depressions, and the heat flux variation is irregular during NST period. The rising of the mold level leads to the longer-pitch and deeper OMs formation; conversely, the falling of mold level introduces shorter-pitch and shallower OMs. With the increase of the mold oscillation frequency, the average value of the low-frequency heat flux at the meniscus increases; however, it decreases when the mold oscillation stroke increases. Additionally, the variation amplitude of the high-frequency temperature and the high-frequency heat flux decreases with the increase of the oscillation frequency and the reduction of the oscillation stroke.

Mold flux plays key roles during the continuous casting process of molten steel, which accounts for the quality of final slabs. With the development of advanced high strength steels (AHSS), certain amounts of Al have been added into steels that would introduce severe slag/metal interaction problems during process of continuous casting. The reaction is between Al and SiO2 that is the major component in the mold flux system. Intensive efforts have been conducted to optimize the mold flux and a CaO-Al2O3-based mold flux system has been proposed, which shows the potential to be applied for the casting process of AHSS. The latest developments for this new mold flux system were summarized with the aim to offer technical guidance for the design of new generation mold flux system for the casting of AHSS.

A novel method for the estimation of the mold hot surface heat flux based on the measured responding temperatures from two columns of thermocouples that embedded inside the mold during continuous casting has been developed. The method includes a Two-Dimensional Inverse transient Heat Conduction Problem (2D-IHCP) model that was solved by the conjugate gradient method with Adjoint Equation. The model was validated by comparing the results with those calculated by a robust One-Dimensional Inverse transient Heat Conduction Problem (1D-IHCP). The solution of a test problem indicated that the Mean Absolute Percentage Error of the estimated heat flux calculated by the new method is about 9 to 40 pct of those calculated by the 1D-IHCP. Then, the method is applied to compute the heat flux for a mold simulator experiment. The results indicated that the heat fluxes and temperatures across mold hot surface calculated by 2D-IHCP show the same variation tendency as those calculated by 1D-IHCP. However, the heat fluxes calculated by 2D-IHCP are about 1.2 to 2 times larger than those calculated by 1D-IHCP for the locations below the liquid mold flux surface and are about 50 to 90 pct of those calculated by 1D-IHCP for the locations above the liquid mold flux surface.

A mold simulator has been successfully used to study the initial solidification behavior of the molten low carbon steel. Coupled with 2D-IHCD calculation and PSD analysis, the variations of the responding temperatures and heat fluxes, as well as the relationship between shell surface profile, heat flux, shell thickness, mold level fluctuation, and the infiltrated slag film, were investigated in this article. The results suggested that the mold high-frequency temperatures and heat fluxes above liquid steel level vary with the oscillation of the mold, and show an opposite variation pattern as those below the shell tip. The formed shell surface profile is directly correlated to the variation of high-frequency heat fluxes, where the formation of oscillation mark is associated with a sudden increase of the heat flux during negative strip time. Mold level fluctuation contributes to the formation of the extra oscillation marks. The growth of shell thickness follows the square root law, and the instantaneous solidification factor is large near the shell tip and becomes small in the area where the deep shell surface depression is formed. The thickness of the slag film in between mold and shell is in the range of 1.4 to 2.46 mm, and the crystallization of mold flux in mold/shell gap is dynamic.

Mold flux solidification and heat transfer experiments under both non-oscillation and oscillation modes have been conducted and compared with the help of Mold Flux Heat Transfer Simulator (MFHTS) technique. The results suggested that the steady-state responding heat flux in the mode of oscillation is smaller than that in non-oscillation operation, and a transition time is observed in the responding temperature and heat flux profiles during the oscillation experiments. The oscillation of mold would introduce the roughness of slag film surface and the enlargement of air gap at the interface of mold/flux film; thus, the interfacial thermal resistance was enhanced. In addition, the thermal conductivity of solid crystalline mold flux and mold/flux film interfacial thermal resistance at steady state were calculated in this work. The thermal conductivity of crystalline mold flux was about 1.43 to 1.76 W m−1 K−1, and the interfacial thermal resistance R int in oscillation operation was calculated as 17.3 to 22.5 × 10−4 m2 (K W−1) in the measured region. The obtained interfacial thermal resistance R int in this work is higher than that in non-oscillation operations.

In order to lower the weight of automotive bodies for better fuel-efficiency and occupant safety, the demand for high Al-containing advanced high strength steel, such as transformation-induced plasticity and twinning-induced plasticity steel, is increasing. However, high aluminum content in steels would tend to significantly affect the properties of mold flux during the continuous casting process. In this paper, a kinetic study of the effect of Al2O3 content on the crystallization behavior of mold flux was conducted by using the single hot thermocouple technique and the Johnson–Mehl–Avrami model combined with the Arrhenius Equation. The results suggested that Al2O3 behaves as an amphoteric oxide in the crystallization process of mold flux. The precipitated phases of mold flux change from cuspidine (Ca4Si2O7F2) into nepheline (NaAlSiO4) and CaF2, and then into gehlenite (Ca2Al2SiO7) with the increase of Al2O3 content. The kinetics study of the isothermal crystallization process indicated that the effective crystallization rate (k) and Avrami exponent (n) also first increased and then decreased with the increase of Al2O3 content. The values for the crystallization activation energy of mold flux with different Al2O3 contents were E R0.8A7 = 150.76 ± 17.89 kJ/mol, E R0.8A20 = 136.43 ± 6.48 kJ/mol, E R0.8A30 = 108.63 ± 12.25 kJ/mol and E R0.8A40 = 116.15 ± 8.17 kJ/mol.

A Mold Flux Heat Transfer Simulator technique was developed to investigate the solidification and heat transfer behavior of mold flux in this study. The results suggested that the responding temperatures and heat fluxes increase intensively within the first second, due to the direct heating from the liquid core. It takes 1 second for the system to be heated up and to form the initial solidified mold flux shell, such that the heat fluxes and the liquid front temperature would start to reduce after that. After 2.5 seconds, the in-mold responding temperature and heat fluxes are getting attenuated with the development of the mold flux solidification and crystallization. After 15-20 seconds, the system steps into a quasi-steady state, as the cooling potential becomes identical to the heating potential due to the further development of total thermal resistance that is introduced by the further solidification and crystallization of mold flux. In addition, a mathematic model was built to calculate the interfacial thermal resistance (R int) at the mold/flux interface, and the calculated interfacial thermal resistance was around 18.9 × 10−4 m2 K W−1, which accounts for about 78.4 pct of the total thermal resistance.

The chamfered mold with a typical corner shape (angle between the chamfered face and hot face is 45 deg) was applied to the mold simulator study in this paper, and the results were compared with the previous results from a well-developed right-angle mold simulator system. The results suggested that the designed chamfered structure would increase the thermal resistance and weaken the two-dimensional heat transfer around the mold corner, causing the homogeneity of the mold surface temperatures and heat fluxes. In addition, the chamfered structure can decrease the fluctuation of the steel level and the liquid slag flow around the meniscus at mold corner. The cooling intensities at different longitudinal sections of shell are close to each other due to the similar time-average solidification factors, which are 2.392 mm/s1/2 (section A-A: chamfered center), 2.372 mm/s1/2 (section B-B: 135 deg corner), and 2.380 mm/s1/2 (section D-D: face), respectively. For the same oscillation mark (OM), the heights of OM roots at different positions\ud (profile L1 (face), profile L2 (135 deg corner), and profile L3 (chamfered center)) are very close to each other. The average value of height difference (HD) between two OMs roots for L1 and L2 is 0.22 mm, and for L2 and L3 is 0.38 mm. Finally, with the help of metallographic examination, the shapes of different hooks were also discussed.

Hot Thermocouple Technology has been developed and approved to be a novel method to study the high-temperature related properties of molten slag. In this study, it will first give the development of Hot Thermocouple Technology, and its typical application to the mold flux. One example of crystallization process of the mold flux for casting low carbon (LC flux) and medium carbon steels (MC flux) were investigated by using Double Hot Thermocouple Technology (DHTT). The results of LC flux showed that, the glass phase firstly formed at the low temperature side; then, the fine crystals precipitated at the liquid/glass interface and grew toward glass and later on to liquid phase. However, the crystals directly formed at the low temperature side when MC flux was under cooling process and grew toward the high temperature side; which indicated the crystallization ability of MC flux was stronger than LC flux. Another crystallization sample of CaO-SiO2-B2O3 based fluoride-free mold flux (F-free flux) was studied by using Single Hot Thermocouple Technology (SHTT), and the results showed the crystals first precipitated in the middle of sample and moved toward the thermocouple side, then the precipitated crystals grew up and new crystals formed in the middle of sample and moved toward the side, until the crystallization was completed and reached a steady state; the crystallization mechanism of the F-free flux was 1-dimensional growth.

With the development of advanced high-strength steel, the slag/steel reaction problems introduced by the addition of aluminum into steel become a challenge for the continuous casting process. An investigation aims to improve the crystallization property of lime–alumina-based mold flux for casting high Al-bearing steels which was carried out through the study of effects of fluorine and BaO on the crystallization behaviors of the mold flux. The single/double hot thermocouple technique and SEM, EDS were employed in the study. The results indicated that the decrease of fluorine content would promote the crystallization behaviors in the lime–alumina-based system which is different to that in the conventional lime–silica-based system, while BaO substituted for CaO can inhibit the crystallization of the lime–alumina-based mold flux. Moreover, the crystallization behavior of mold flux under simulated thermal gradient was in well accordance with TTT results, including the crystallization process and three-layered (liquid, crystalline, glassy) distribution of mold flux.

The non-reactive lime–alumina-based mold flux has been proposed to overcome the aluminum introduced slag/steel interaction problems during the casting of high aluminum bearing steels. In this article, a new mold flux with CaO/Al2O3 (C/A) ratio of 2 was designed, and the effects of BaO substituting for CaO to replace B2O3 on the mold flux crystallization behaviors have been investigated through the using of single hot thermocouple technology and double hot thermocouple technology (DHTT). The results suggested that BaO shows the stronger tendency to inhibit crystallization of mold flux comparing with B2O3; however, the synergic effect of the combination of BaO and B2O3 tends to improve the mold flux crystallization in this non-reactive mold flux system. The analysis of the crystallized phases for different samples and the crystallization behaviors under simulated thermal gradient through DHTT were also conducted to support the reliability of above conclusions.

An investigation was conducted to study the effects of Cr2O3 on the melting, viscosity, heat transfer, and crystallization behaviors of mold flux used for casting high-carbon-chromium steel (Cr12MoV) by using single/double hot thermal-couple technology (SHTT/DHTT), rotary viscosity, and infrared emitter technique (IET). The results showed that both the melting temperature and viscosity increased significantly with the addition of Cr2O3 content, while the general heat transfer rate was reduced with the increase of Cr2O3 due to the decrease of liquid layer and the reduction of radiative heat transfer. Meanwhile, results of IET and DHTT tests indicated that Cr2O3 would inhibit crystallization of mold flux. The results of this study can elucidate the effects of Cr2O3 on the properties of mold flux and provide guidelines for the new mold flux design for the casting of Cr-bearing alloy steels.

With the development of advanced continuous-casting technology, saving energy and reducing greenhouse gas emissions are crucial for its future development. Controlling the initial solidification of molten steel in the mold to improve the quality of casting products would tend to minimize extra postcast treatment like scarfing, etc., which leads to a large amount of energy savings in the continuous-casting process through the minimization of the extra labor and energy consumption. In this article, factors such as mold flux, mold oscillation, cooling potential conditions, and fluid flow in the vicinity of meniscus that correlate with the molten steel initial solidification are discussed with the aim to provide strategy and guidelines for the optimization of molten steel solidification and energy savings in continuous 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.