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Experimental and Numerical Simulation Analysis of the Blocking Layer in an Electromagnetic Induction-Controlled Automated Steel Teeming System
Chunyang ShiJicheng He
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2018 Volume 59 Issue 1 Pages 39-46

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Abstract

Improving the efficiency of electromagnetic steel teeming systems have included methods to accelerate the heating process, wherein the most convenient and effective is to adjust the position of the blocking layer. Herein, numerical simulation is used to initially optimize the physical parameters (material type, shape and particle size) of the Fe-C alloy to identify the most effective induction heating area for the blocking layer. Next, 110 Mg of steel ladle from a steel mill is used as the experiment carrier to verify the numerical simulation results and the following conclusions are made: the position of the blocking layer is related to all Fe-C alloy parameters of material type, shape and particle size, whose respective optimal values are 10# steel material, cylindrical shape, and 2.0 mm particle size for 110 Mg steel ladle electromagnetic steel teeming system; and 10# steel material, cylindrical shape, and 2.5 mm particle size for a 260 Mg ladle. Furthermore, through comparative analysis of the numerical simulation models of the 110 and 260 Mg steel ladles it is found that long molten steel channels require a large amount of Fe-C alloy filling, and that the blocking layer tends to move upward and become thinner. Additionally, it is verified using a self-designed experimental device that the thickness of the blocking layer increases with higher temperatures and longer standing times of the molten steel. This work provides reference for the improvement of the efficiency of electromagnetic steel teeming systems.

1. Introduction

As competition in the iron and steel industry becomes more and more intense, it is crucial that the steel-making technology improves steel purity and quality as far as possible while doing no harm to the environment1,2). In traditional steel manufacture, stuffing sand is used in the nozzle brick as the nozzle filler. Siliceous stuffing sand is typically used in industrial production, but its poor liquidity and tendency to slip leads to a low free opening rate. Therefore, to improve the free opening rate, chrome stuffing sand containing a certain amount of chromium (Cr2O3; mass% between 32% and 35%) has been employed3,4). However, chromium is a source of harmful pollution, and its use in traditional manufacture would inevitably generate negative environmental side effects5,6). Furthermore, the steel teeming rate in traditional manufacture is only around 98%. In particular, during the steel teeming process the stuffing sand typically enters into the tundish with the molten steel, causing molten steel contamination79) and thus deterioration of the surface, anti-corrosion and anti-fatigue performances of the final product1012). Further, if automatic steel teeming does not occur in the traditional manufacture process then artificial oxygen burning will be necessary, which certainly will result in a secondary oxidation of the molten steel. In addition, oxygen burning drainage demands a 5 min production time increase compared to automatic casting, which disturbs the normal production scheme, slows down the continuous casting system, degrades the quality of the casting blank and increases the cost of production1315). To solve these problems, He et al. have proposed the method of electromagnetic steel teeming in the steel ladle steel teeming process. The principle of this method is to replace the stuffing sand with an Fe-C alloy whose composition is similar or identical to that of the steel to be smelted. The Fe-C alloy is heated in the nozzle brick by electromagnetic induction to cause it to fall with the molten steel after it is partially or completely melted, thereby completing the process of automatic steel teeming16,17). During the process, owing to the heat transmission of the upper molten steel, the up–down distribution of the Fe-C alloy in the nozzle brick and upper sliding gate is arranged to incorporate a liquid layer, solidified layer, liquid-sintered layer, solid-sintered layer and original layer; producing five layers in total. Among them the solidified layer, liquid-sintered layer and solid-sintered layer (hereafter referred as the blocking layer) play the role of blocking the molten steel to prevent it from flowing out and corroding the slide plate18,19). The results of previous experiments20,21) have shown that the best position of this blocking layer is closely related to the physical parameters of the Fe-C alloy, and the temperature and standing time of the molten steel also have an impact on the thickness of the blocking layer. Therefore, it is necessary to further adjust the physical parameters of the Fe-C alloy through 110 Mg steel ladle experiments to reduce the electromagnetic steel teeming time. To study the influence that the length of the molten steel channel and the filling amount of Fe-C alloy has on the position of the blocking layer, a separate 260 Mg steel ladle is used for comparative numerical simulation. Meanwhile, to investigate the influence of the temperature and standing time of the molten steel on the position of the blocking layer, a self-designed experimental facility is employed that also saves the huge costs inherent in massive repeated industrial experiments. This paper provides theoretical evidence for the reduction of steel teeming times.

2. Physical Parameter Experiment

2.1 Selection of Fe-C alloy

The numerical values of the external factors including the nozzle brick size, the position of its internal induction heating coil, the output parameter of the heating electric power, and the amount of Fe-C alloy filler were fixed. This fixing of values optimized the experimental process to effectively study the influences of the material, shape and particle size of the Fe-C alloy on its melting time. The purpose was to find the inherent law and optimal physical parameters of the Fe-C alloy for industrial production. The selected material, shape, and particle size are as follows:

(1) In material selection, an Fe-C alloy whose composition was identical or similar to that of the steel to be smelted was selected as the study object to avoid the impact of compositional difference. The chemical compositions of the low-carbon 10# steel and high-carbon 45# steel used are detailed in Table 1.

Table 1 Compositions of the Fe-C alloys used in the experiments.
Name Chemical composition,%
Element C Si Mn P S Cu Ni
45#
steel
0.42~0.5 0.17~0.37 0.5~0.80 ≤0.035 ≤0.035 ≤0.25 ≤0.25
10#
steel
0.07~0.13 0.17~0.37 0.35~0.65 ≤0.035 ≤0.035 ≤0.25 ≤0.25

(2) In shape selection, slice-shaped and cylindrical Fe-C alloys were chosen as study objects.

(3) In particle size selection, considering the actual molten steel channel caliber in the nozzle brick of the 110 Mg steel ladle from a steel mill, Fe-C alloys with particle sizes of 0.5, 1.0, 1.5, 2.0, and 2.5 mm were selected for the experiment.

2.2 Calculation method for blocking layer position

In the electromagnetic steel teeming experiment, the position of the blocking layer is expected to be at the center of the heating area of the electromagnetic coil embedded in the nozzle brick. Thus, according to Faraday's law of electromagnetic induction and the Joule-Lenz law, when the blocking layer is positioned such that electromagnetic heating is the most effective, the magnetic flux is maximum and the coil has the best effect in heating the blocking layer. Assuming that the height of the contact surface of the bottom slide plate is 0 mm of the residual height, and considering the nozzle brick size and the position of its internal induction heating coil (138 ± 10 mm) used for the 110 Mg steel ladle from a steel mill, we can know that, when the position of the lower surface of the blocking layer is at 138 ± 10 mm, the coil has an optimized heating effect.

2.3 Experimental process

As shown in Fig. 1, the steel ladle was placed in the steel teeming position of the ladle turret and a collecting box (600 mm diameter, 300 mm height, cylindrical shape) for the fallen Fe-C alloy was placed directly under the lower nozzle. The sliding gate was then opened and the hydraulic device was initiated and repeated three times to cause the Fe-C alloys attached to the sliding gate to shake and fall into the collecting box. Next, the collected Fe-C alloy was poured into the upper nozzle, which was the same as that used in teeming, and the height of the residual was measured. To ensure the reliability, consistency and accuracy of the experiment data, the same steel ladle, nozzle brick and type of upper nozzle were used in the 32 repeated experiments.

Fig. 1

Schematic illustration of the blocking layer experiment.

A ladle-to-ladle experiment was adopted to measure the distance between the blocking layer and the upper surface of the nozzle brick as well as its thickness, as shown in Fig. 2.

Fig. 2

Schematic diagram of the ladle-to-ladle measurement.

A 110 Mg steel ladle from a steel mill and a molten steel temperature of 1873 K were selected for the experiment. The refined steel ladle was set via 10 min of standing to cause the status of the molten steel to be exactly the same as that before teeming, whereupon the molten steel was poured into another steel ladle prepared before. After the molten steel was completely poured out the steel ladle was placed transverse to its repairing position, whereupon two measuring device setups with calibration sleeves were inserted, one at the upper surface at the inner wall of the nozzle brick and the other under the nozzle at the external wall of the ladle. Finally, the actual position and thickness of the blocking layer was measured by calculating the difference in the values given by the measuring sleeves. A schematic diagram of this process is given in Fig. 2, and the formula for calculating blocking layer thickness is given as   

\[ {\rm d} = {\rm H} + {\rm h} - {\rm D}_{1} - {\rm D}_{2}, \](1)
where, d is the thickness of the blocking layer (mm); H is the thickness of the nozzle brick (mm); h is the thickness of the bottom wall of the steel ladle (mm); and ${\rm D}_{1}$ and D2 are the difference values measured on the upper and lower surfaces (mm), respectively.

2.4 Experimental process

In the experiment, two types of Fe-C alloys comprising two different materials, two different shapes and four different particle sizes were used. The different residual heights of the Fe-C alloys were obtained as a function of material, shape and particle size and are plotted in Fig. 3.

Fig. 3

Experimental results of the residual height of Fe-C alloy with varying material types and grain parameters for a 110 Mg steel ladle.

As shown in Fig. 3, repeating the experiments 32 times revealed the optimized the physical parameters of the Fe-C alloy for the electromagnetic steel teeming system of the 110 Mg steel ladle. These parameters are 10# steel with particles that are a cylindrical shape and that are 2.0 mm in size. The repeated experiments indicated some measurement error, but the error is small and does not affect the applicability of the experimental rules.

Figure 3 indicates that, for the 110 Mg steel ladle and for fixed particle size and material, the residual height of the cylindrical Fe-C alloy is closer to the most effective heating area than that of the slice-shaped Fe-C alloy. When the particle size and shape are fixed, the residual height of the Fe-C alloy made of 10# steel is closer to the most effective heating area than that of Fe-C alloy made of 45# steel. When the shape and material are fixed, the residual height of the Fe-C alloy with 2.0 mm particle size is closer to the most effective heating area than the other particle sizes. To summarize, the residual amount of Fe-C alloy is jointly affected by material, shape and particle size. When also considering the given nozzle brick total height of 420 mm and the most effective heating area of the coil at 138 ± 10 mm, it can be concluded that the optimum physical parameters to produce the height of the Fe-C alloy residual in the most effective heating area are a material of 10# steel, a cylindrical shape and a particle size of 2.0 mm.

We posit the following theoretical analysis to explain the above results: via heat transmission from the molten steel a liquid layer is induced when the temperature of the Fe-C alloy surpasses its liquidus temperature, and with the increase of the Fe-C alloy temperature the Fe-C alloy solid phase becomes more soluble in the liquid phase. Regarding the particle shape of the Fe-C alloy, the edges and corners of the particles will dissolve first owing to their large contact area, thus the slice-shaped Fe-C alloy melts easier than the cylindrical Fe-C alloy. This signifies that the solid Fe-C alloy with slice-shaped particles reduces more quickly, causing the blocking layer to move downward more than that of the cylindrical particles. Further, the melting point of the Fe-C alloy is related to its carbon mass fraction, where a larger mass fraction lowers the melting point. Therefore, the melting point of the Fe-C alloy made of 45# steel (carbon content of 45‰) is lower than that made of 10# steel (carbon content of 10‰), signifying that the former has a greater amount of blocking layer melting. This reduces the solid Fe-C alloy and causes the blocking layer to move further downward for the 45# steel compared with the 10# steel. Via the continuous molten steel heat transmission, the temperature of the Fe-C alloy rises constantly and thereby increases the solubility of the solid phase in the liquid phase. The solubility of small particles is higher than that of large particles, so for Fe-C alloys with a relatively smaller particle size more of the alloy will be melted and the blocking layer will move down. For larger particle sizes, however, the contact area between the particles decreases as the size increases, thereby reducing the heat transmission among particles and reducing the solubility and melting speed. Therefore, with the same conduction heating time, Fe-C alloys with a large particle size have the tendency to surpass the conduction heating area. This emphasizes that a proper particle size can optimize the electromagnetic steel teeming system. In the process, the physical parameters of the material, shape and particle size of the Fe-C alloy jointly decide the height of the blocking layer to effectively block the molten steel.

3. Numerical Simulation

3.1 Establishment of the theoretical model

A three-dimensional finite element analytic model of the molten steel channel in the nozzle brick of a 110 Mg steel ladle of a certain steel mill was developed, and is shown in Fig. 4(a). To clearly observe the position and thickness of the blocking layer, the meshes around the molten steel channel and the coil were refined, and are shown in Fig. 4(b). Previous works22,23) have given the heat conductivity (24.02 W m−1 k−1) and enthalpy (1350 J·kg−1 k−1) of an Fe-C alloy changing with temperature.

Fig. 4

Three-dimensional numerical simulation (a) model and (b) its meshing.

3.2 Statement of basic assumptions

The simulation was implemented using the PROCAST finite element analyzing software. Because of the complicated nature of the electromagnetic steel teeming system, the following boundary conditions were set according to the experimental characteristics:

(1) Neglect heat dissipation of the blocking layer.

(2) Neglect corrosion of the lining material of the nozzle brick, thus deeming its compositions constant.

(3) The molten steel temperature is taken as 1873 K, and that of the Fe-C alloy in the molten steel channel as 1373 K. We note that in practical working conditions, the temperature of the blocking layer in the nozzle brick is about 1373 K18). Further, these temperatures remain unchanged.

(4) In the actual production process, the heat transfer time of the molten steel and Fe-C alloy is about 90 min for the entire process from steel teeming to casting. Therefore, the total time of the simulation is set as 90 min. The physical parameters of the stacked granular Fe-C alloy, including heat conductivity, specific heat, and density, are modified according to the experimental data.

(5) During the simulation calculation, the Fourier cylindrical solid differential equation of heat conduction is   

\[ \frac{\partial^{2}{\rm T}}{\partial {\rm r}^{2}} + \frac{1}{\rm r} \cdot \frac{\partial {\rm T}}{\partial {\rm r}} + \frac{\partial^{2}{\rm T}}{\partial {\rm z}^{2}} + \frac{\rm q_v}{\rm k} = \frac{\rm \rho'c}{\rm k} \cdot \frac{\partial {\rm T}}{\partial {\rm t}}, \](2)
where T is the thermodynamic temperature (K); r is the radial length (m); k is the heat conduction coefficient of the isotropic material (W m−1∙K−1); $\rho'$ is the material density (kg m−3); C is the specific heat of the material (J kg−1∙K−1); t is time (s); and qv is the internal heat intensity (W m−3). The boundary conditions of this model are given as   
\[ {\rm q} = {\rm L} \cdot ({\rm T}_{\rm b} - {\rm T}_{\rm a}), \](3)
where q is the heat-flow density (W m−2); L is the surface-integrated heat transfer coefficient (W m−2 K−1); and Ta and $T_{b}$ are the environment and surface temperatures, respectively (K).

3.3 Theoretical basis

Comprehensively considering the Maxwell equation set and the technological characteristics of electromagnetic steel teeming, the electromagnetic field satisfies the equation   

\[ \left\{ \begin{array}{c} \nabla \overrightarrow{\rm H} = \overrightarrow{\rm J_s} + \overrightarrow{\rm J_e}\\ \nabla \overrightarrow{\rm E} = - \frac{\partial \overrightarrow{\rm B}}{\partial {\rm t}}\\ \nabla \overrightarrow{\rm D} = 0\\ \nabla \overrightarrow{\rm B} = 0 \end{array} \right., \](4)
where $\overrightarrow{\bf H}$ is the magnetic field intensity (A m−1); $\overrightarrow{{\bf J}_{\rm S}}$ is the source current density (A m−2); $\overrightarrow{{\bf J}_{\rm e}}$ is the induced current intensity (A m−1); $\overrightarrow{\bf E}$ is the electric field intensity (V m−1); $\overrightarrow{\bf B}$ is the magnetic flux density (T); and t is time (s). The electromagnetic characteristic equations of the medium are   
\[ \overrightarrow{\rm B} = \mu_{0}\overrightarrow{\rm H}, \](5)
where $\mu_{0}$ is the permeability of vacuum (H m−1). Ohm's law within a conductor is given as   
\[ \overrightarrow{\rm J_e} = \sigma \overrightarrow{\rm E}, \](6)
where $\sigma$ represents the electrical conductivity (Ω−1·m−1). The finite element equation of harmonic electromagnetic field analysis is given as   
\[ [{\rm K} + {\rm j}\omega {\rm C}] \{{\rm u}\} = \{{\rm F}\}, \](7)
where [K] represents the coefficient matrix; j is the current density (A m−2); ω is the angular frequency (rad s−1); [C] is the magnetic damping matrix; {F} is the current load vector; and {u} is the solving variable vector. The heating intensity of the eddy current during electromagnetic induction heating is given as   
\[ {\rm q_v} = \rho \cdot \left|\, \overrightarrow{\rm j_e} \right|^2, \](8)
where qv is the heating intensity (W m−3) and $\rho$ is the resistivity of the Fe-C alloy (Ω·m).

3.4 Simulation results

In the simulation, the solidus temperature was selected for the underside of the blocking layer. Figure 5 shows that, when the match of the physical parameters of the Fe-C alloy is given as 10# steel with particles that are cylindrical and are 2.0 mm in size, the blocking layer is 156.3 mm away from the upper surface of nozzle brick, and its thickness is 128.5 mm. The residual height of Fe-C alloy is calculated to be 131.2 mm according to the total height. With the same method, the relative residual heights of Fe-C alloy with other physical parameter combinations are calculated and detailed in Table 2.

Fig. 5

Numerical simulation of the residual height of Fe-C alloy with 10# steel with particles that are cylindrical and that are 2.0 mm in size for a 110 Mg steel ladle.

Table 2 Residual height values of Fe-C alloy for varying physical parameters of material type, particle shape and particle size.
Particle size, D/mm 0.5 1.0 2.0 2.5
Residual height of
10#steel-made
slice-shaped Fe-C
alloy (mm)
35.01 37.41 49.25 40.42
Residual height of
10# steel-made
cylinder-shaped Fe-C
alloy(mm)
70.44 57.26 131.2 97.43
Residual height of
45# steel-made
slice-shaped Fe-C
alloy(mm)
18.33 30.4 41.23 48.45
Residual height of
45# steel-made
cylinder-shaped Fe-C
alloy(mm)
38.46 41.21 58.42 56.36

3.5 Numerical simulation and off-line experimental results and discussions of a 110 Mg steel ladle

A comparative analysis of the numerical simulation and off-line experimental results are shown in Fig. 6, between which slight discrepancies can be found. The main reason for these discrepancies is that, in the off-line experiment, the Fe-C alloy fills in the nozzle and, because a portion of the Fe-C alloy adheres to the collection box and is not measured Therefore, the measured value is low and causes the position of the blocking layer formed in the off-line experiment to be lower than that in the simulation. However, Fig. 6 indicates that when the material, shape and particle size remain constant, the two residual heights exhibit the same change trend, hold a small numerical difference and exhibit good conformity. Finally, when the physical parameters of the Fe-C alloy are 10# steel with particles that are cylindrical and are 2.0 mm in size, the residual height is closest to the most effective heating area (138 ± 10 mm).

Fig. 6

Comparison between experimental data and simulation data of residual Fe-C alloy for a 110 Mg steel ladle.

In the measurement of the blocking layer position and thickness, however, the experimental data are slightly different from the simulation data. The main reason for this difference is owing to a trace amount of molten steel in the experiment that will be transformed from the liquid phase to the solid–liquid or solid phase because of the temperature reduction during the pouring of molten steel. This phase transformation causes the steel to stick to the surface of the blocking layer and thicken it, causing the distance to the upper surface of the molten steel to shrink slightly.

3.6 Numerical simulation of optimized physical parameters of a 260 Mg steel ladle

The structure and material of the 260 Mg steel ladle used in the following experiment were basically the same as those of the 110 Mg steel ladle in a certain steel mill. The nozzle brick of the 260 Mg steel ladle possessed a total height of 630 mm and a molten steel channel area of φ90 mm, while all other physical parameters were the same as those of the 110 Mg steel ladle. Further, the conduction heating coils suitable for the two steel ladles were similar in size, so the simulation method and relevant parameters discussed in the previous sections could be applied in the calculation of the material, shape and particle size of the electromagnetic steel teeming system of a 260 Mg steel ladle. The calculation result is shown in Fig. 7, and the corresponding residual height and blocking layer thickness (i.e., distance between solidus and liquidus) as well as the temperature distribution nephogram are shown in Fig. 8 for an Fe-C alloy made of 10# steel with particles that are cylindrical and are 2.5 mm in size.

Fig. 7

Numerical simulation of the residual height of Fe-C alloy with varying physical parameters for a 260 Mg steel ladle.

Fig. 8

Numerical simulation of the temperature distribution in the blocking layer of Fe-C alloy with 10# steel with particles that are cylindrical and that are 2.5 mm in size for a 260 Mg ladle.

3.7 Comparative analysis of numerical simulation results of 260 and 110 Mg steel ladles

Figures 5 and 7 are compared for the analysis. Through the comparative analysis of the numerical simulations of varying tonnages and cross-sectional areas (CSAs) of the molten steel channel, it can be concluded that the position and thickness of the blocking layer are jointly affected by the steel ladle tonnage and the CSA of the molten steel channel. However, their impact is not remarkable when, compared to the 110 Mg steel ladle, the blocking layer thickness of that of a 260 Mg steel ladle increases by 4.7 mm and the blocking layer position lowers only 0.8 mm. This minimal response is owing to the fact that, in the steel ladle that contains the molten steel, the temperature of the blocking layer has a certain range. In the experiment, the 260 Mg steel ladle possesses a long molten steel channel that requires a large amount of Fe-C alloy for filling. In this channel the thermal capacity of the cold side is high, thus absorbing a great deal of heat and lowering the relative temperature of the Fe-C alloy, which causes the cold and hot sides of the blocking layer to correspondingly move up a certain degree. The influence of the temperature on the cold side is more obvious, however, and it moves upward a greater amount. Therefore, the blocking layer of the 260 Mg steel ladle is relatively thinner.

4. Molten Steel Temperature and Standing Time Experiment

4.1 Experimental methods

A schematic of the experimental facility used herein is shown in Fig. 9. The steel ingot that represents the molten steel and steel ladle was placed in an MgO crucible and the power was turned on to heat and melt the cold steel ingot. When the cold steel ingot was totally melted, the power was adjusted to set the molten steel temperature to the desired range. To measure the height of the blocking layer, a graphite rod with a diameter of 10 mm and a length of 500 mm was used, where the rod was inserted into the upper nozzle through the top side of the molten steel-filled crucible. When the front end of the graphite rod made contact with the upper surface of the blocking layer in the upper nozzle, the rod was marked at the horizontal position of the crucible top. When the required temperatures and standing time were obtained, the power was shut down and the molten steel in the crucible and upper nozzle were allowed to solidify and cool to room temperature. Finally, the upper nozzle was broken away to measure the relative parameters of the solidified steel within. On occasion, during the breaking of the upper nozzle, the sintered Fe-C alloy particles at the lower part of the blocking layer would fall out owing to the shaking, which could affect the measurement results. To prevent any loss of accuracy owing to this eventuality, we initially pulled back the slide plate and released any Fe-C alloy particles from the original layer and measured the distance between the bottom of the blocking layer and the bottom of upper nozzle with a graphite rod. The sum of the heights of the crucible and the nozzle and subtracting the length difference between the upper and lower sides measured by the graphite rod was taken as the thickness of the blocking layer. Figure 10 shows a photograph of a representative blocking layer formed in the upper nozzle; illustrating the solidified layer, shown as the glossy section below the graphite rod, and the liquid-sintered layer, shown as the section where the Fe-C alloy particles are in a semi-molten state. Because the solid-sintered layer had fallen away during the nozzle breaking process, its thickness is not evident in the photograph.

Fig. 9

Schematic of the measuring device used to obtain the blocking layer thickness.

Fig. 10

Photograph of an experimentally obtained blocking layer.

4.2 Influence of the molten steel temperature

In the electromagnetic steel teeming system of the steel ladle, the thickness of the blocking layer in the upper nozzle is closely related to the temperature of the top molten steel region. To ensure that this technology is applicable for a variety of steel types, we selected a 110 Mg steel ladle of a steel mill and various tapping temperatures (1823, 1873, and 1893 K)24), and set the Fe-C alloy filling material to be 10# steel with particles that are cylindrical and are 2.0 mm in size. The measured heights of the blocking layer as a function of the temperature are shown in Fig. 11.

Fig. 11

Experimental results of the blocking layer thickness as a function of temperature.

Figure 11 indicates that an increase of temperature can thicken the blocking layer, and that the molten steel temperature has different influences on different layers of the blocking layer. When the molten steel temperature rises from 1823 to 1873 K, the solidified layer becomes slightly thinner, while the solid-sintered and liquid-sintered layers become thicker. When the molten steel temperature rises from 1873 to 1893 K, however, noteworthy increases in the thicknesses of solidified and solid-sintered layers occur, while the liquid-sintered layer thins remarkably. When the molten steel temperature rises from 1823 to 1873 K, the solid–liquid interface moves down, while the solid layer formed at 1823 K is partially melted and thus becomes thinner. With the temperature increase, the solid-sintered and liquid-sintered layers at the lower part become thicker. When the temperature rises to 1893 K, the temperature gradient increases and a portion of the liquid-sintered layer begins to transform into the solidified layer, thereby increasing the distance between the solid–liquid interface and the molten steel in the crucible. If the temperature increases further, the transformation of the original solid-sintered layer into the liquid-sintered layer will be restrained, and the liquid-sintered layer will become thinner in contrast. The decrease in the thickness of liquid-sintered layer accelerates the heat transmission to solid-sintered layer, thus to thicken the solid-sintered layer.

4.3 Influence of the standing time

Figure 12 plots the measured blocking layer thickness as a function of the standing time, which reveals a significant relationship between the standing time of the molten steel and the thickness of the blocking layer. For the relatively short standing time of 5 min, which results in a short contacting time between the molten steel and the Fe-C alloy, the solidified layer, solid-sintered layer and liquid-sintered layer are thin. When the standing time is increased to 10 minutes, the liquid-sintered layer becomes thinner and starts to transform into the solidified layer, while the thicknesses of the other two layers begin to increase. When the standing time is increased to 15 min, the trend of change experienced by each layer remains the same, but the solid–liquid interface moves down continuously and each layer changes rapidly. With continued increase of the standing time, though the thickness of the blocking layer grows constantly the rate of increase of thickness for each layer reduces because the rate of temperature increase is reduced.

Fig. 12

Experimental results of the blocking layer thickness as a function of standing time.

5. Conclusions

(1) For the 110 Mg steel ladle, when other impact factors are held fixed the residual height of the 10# steel-fabricated Fe-C alloy is closer to the most effective heating area compared to that of 45# steel-fabricated Fe-C alloy. The residual height of cylindrical-shaped Fe-C alloy is closer to the most effective heating area compared to that of the slice-shaped Fe-C alloy. Finally, when the particle size is 2.0 mm, the residual height of the Fe-C alloy reaches a maximum value and is closer to the most effective heating area.

(2) For the 110 Mg steel ladle, the optimized physical parameters of the Fe-C alloy for the electromagnetic steel teeming system are 10# steel with particles that are cylindrical and are 2.0 mm in size.

(3) For the a 260 Mg steel ladle, the optimized physical parameters of the Fe-C alloy for the electromagnetic steel teeming system are 10# steel with particles that are cylindrical and are 2.5 mm in size.

(4) For a 110 Mg steel ladle, the distance between the blocking layer and the upper surface of the nozzle brick is 156.3 mm and its thickness is 128.5 mm. For the 260 Mg steel ladle, the distance between the blocking layer and the upper surface of the nozzle brick is 155.5 mm and its thickness is 123.8 mm.

(5) Because the molten steel channel is long and requires a large filling amount of Fe-C alloy, the blocking layer has the tendency to move up and become thinner.

(6) In the conditions of this experiment, when the material, shape and particle size of the Fe-C alloy are 10# steel, cylindrical and 2.0 mm, respectively, the thickness of the blocking layer rises with the increase of molten steel temperature.

(7) With the extension of the standing time of molten steel, the thickness of the blocking layer initially increases rapidly until 15 min has elapsed, and then increases slowly.

Acknowledgments

This work was financially supported by the steel union research fund of the National Natural Science Foundation of China and Shanghai Baosteel Group Corporation (U1560207).

REFERENCES
 
© 2017 The Japan Institute of Metals and Materials
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