Effect of Na₂O and B₂O₃ on Heat Transfer Behavior of Low Fluorine Mold Flux for Casting Medium Carbon Steels

Abstract

An investigation was conducted to study the effects of Na₂O and B₂O₃ on heat transfer behavior of low fluorine mold flux for casting medium carbon steels by using Single Hot Thermocouple Technique (SHTT) and an advanced Infrared Emitter Technique (IET) in this paper. Results suggested that crystalline layer thickness, crystalline fraction and interfacial thermal resistance of the mold flux increased with the addition of Na₂O, while they decreased with the increase of B₂O₃. Besides, the initial crystallization temperature and steady state heat flux reduced with the addition of Na₂O, and they were getting higher with the addition of B₂O₃. Those results obtained in this research can provide fundamental guidance for designing new type of low fluorine or fluorine free mold fluxes for casting medium carbon steels.

1. Introduction

Mold flux is widely used in modern continuous casting of steel. In conventional commercial mold flux, fluorides are usually added into mold flux to reduce viscosity, melting point etc, and its content is about 7–13 mass%.^1,2) Besides, the most important function of adding fluorides is that it can form the cuspidine crystalline phase, which has significant influence on heat transfer process from shell to mold wall.^3,4) Although fluorides are important to mold flux, substitutes are still needed to be developed due to their negative impacts; such as SiF₄, NaF, AlF₃ and HF can volatilize at the operational temperatures during the continuous casting process, which will pollute the air and cooling water, hurt health of operators, and also corrode equipment (waterborne fluoride HF) in plant.^5,6)

Many studies regarding fluorine free or low fluorine mold flux have been conducted recently, which can be divided into two categories: (1) Utilizing TiO₂ to substitute the fluorides, since the precipitated crystalline phase CaO⋅SiO₂⋅TiO₂ was considered to be proper substitutes for cuspidine in commercial mold fluxes. Some works related to this topic have been done by researchers like Wen and coworkers,^7,8) Zhang et al.,⁹⁾ Nakada and Nagata¹⁰⁾ and Klug et al.¹¹⁾ However, recently published paper from Mills' research group¹²⁾ pointed out that those fluorine free mold fluxes containing TiO₂ have the risk to increase the rate of stick breakout due to the possible formation of Ti(C, N) during the melting process of mold fluxes; and the addition of TiO₂ would introduce higher crystallization temperature of mold flux that tends to bring heat transfer problem in the mold. (2) Using oxides like Na₂O, K₂O, Li₂O, MgO, MnO, B₂O₃, BaO and rare earth elements oxides to modify the negative effects of the mold flux properties caused by the absence of fluorides. Lu et al.¹³⁾ have studied the effects of Na₂CO₃ on the properties like melting temperature, viscosity and surface tension of mold flux, and suggested that partial fluorine could be replaced by Na₂CO₃, but its content should be lower than 9%. Benavidez et al.¹⁴⁾ investigated the influence of different oxides on viscosity of fluorine free mold fluxes by using viscosity models, and then validated it by DTA tests. Besides, Fox et al.¹⁵⁾ have developed fluorine free fluxes for billet casting by using B₂O₃ as alternative substitutes for CaF₂, and plant trial has been successfully tested; however, it was related to low carbon steels. Although lots of works have been done by predecessors, most of them are focused on the properties such as viscosity, melting behavior etc., and limited to the low carbon steels. The research of the development for low fluorine or fluorine free mold flux for the casting of medium carbon steels has been rarely conducted due to the difficulty of heat transfer and crystallization control. However, the crystallization and heat transfer behaviors of mold flux are important to the continuous casting process. Many surface and interior defects, such as severe oscillation marks, longitudinal cracks and breakouts of thin shell are resulted from the improper controlling of heat transfer.^16,17)

Although, the viscosity and melting properties for low/ free Fluorine mold flux could be improved by using the alkaline metal oxides, like B₂O₃ to substitute fluoride; however, it would introduce the crystallization kinetics problem, because of the fact that B₂O₃ restrains mold flux crystallization. Therefore, the in-mold heat transfer and crystallization behaviors would be affected correspondingly. As Na₂O has been regarded as network breaker to promote the crystallization of mold fluxes;¹⁸⁾ the combined effects of Na₂O and B₂O₃ may be used for the development of low/free Fluorine mold flux for casting medium carbon steels.

Consequently, the combined effects of Na₂O and B₂O₃ on crystallization and heat transfer behaviors of low fluorine (3%) mold flux for casting medium carbon steels were investigated in this article. The interactions between mold flux crystallization, heat flux, as well as interfacial (mold/ slag) thermal resistance were studied by using an advanced Infrared Emitter Technique (IET). Besides, the initial crystallization temperatures were obtained by using Single Hot Thermocouple Technique (SHTT).

2. Experimental Method and Apparatus

2.1. Heat Transfer Simulator

The heat transfer behaviors of low fluorine mold flux with different Na₂O and B₂O₃ content are investigated by using the infrared Emitter Technique (IET) which was schematically shown in Fig. 1. The details about the IET have been described elsewhere.^19,20) This experiment apparatus mainly included a power controller, an infrared radiant heater capable of emitting 2.0 MW/m² heat flux at the rate of 380 voltages, a data-acquisition system, and a commandand- control unit.

Fig. 1.

Schematic illustration of the infrared emitter.

The copper mold was simulated by a one-end, water cooled copper cylinder, which acted as the radiation target, and its schematic figure was shown in Fig. 2. As the heat flux was applied to the top surface of the copper mold, which was covered with mold flux disk, the response temperatures could be measured by the subsurface thermocouples. The thermocouples are sheathed K type with a diameter of 0.8 mm (Omega, USA), placed at 2, 5, 10 and 18 mm below the irradiated surface (Fig. 2) are recorded as T₁, T₂, T₃ and T₄ respectively. The cooling water inlet and outlet temperature are recorded as T_in and T_out.

Fig. 2.

Schematic figure of copper substrate used as the radiation target.

2.2. Single Hot Thermocouple Technique

The single hot thermocouple technique (SHTT) was used to study the initial crystallization temperatures in this article. The precipitation of crystal can be observed and recorded directly by a connected CCD onto DVD; meanwhile the corresponding temperature history can be obtained through the temperature acquisition system.

The details of SHTT apparatus have been described by Kashiwaya,²¹⁾ and Fig. 3 is the schematic of the SHTT experimental apparatus.

Fig. 3.

The schematic of SHTT experimental apparatus.

2.3. Mold Flux Disk Preparation

The design of samples is based on a commercial mold fluxes for casting medium carbon steels (Flux 0 in Table 1). The Flux 0 was decarburized by placing it into a programmable furnace at 1073 K (800°C) for about 24 h. And, Fluxes 1 to 5 are prepared by pure chemical reagents after adjusting the proportion of Na₂O and B₂O₃. The pre-melted slags are analyzed by X-ray fluoroscopy (XRF), and the results are listed in table I. It could be observed that the evaporative loss of F is small and within 1% that is because the initial designed content of F is low (3%), which introduce a relative low partial pressure of CaF₂ that in turn affects the evaporation rate of F as suggested in earlier studies.^22,23) The samples are prepared by melting in an induction furnace at 1773 K (1500°C) for 5 minutes to homogenize their chemical compositions, and then poured onto a cool steel plate to quench to achieve a fully glassy phase. Meanwhile, a new cylindrical tube-like copper mold with the same diameter as the copper substrate is used to cast the mold flux before it solidifies on the steel. After that, the cooled mold flux disks are polished by using the SiC sand papers with the grit size down to 1200 to control their surface roughness and thickness. The polished glassy samples are then placed on the top of the copper mold individually for heat transfer experiments. Otherwise, a small amount of solidified glassy mold flux is crushed and ground into powders samples for SHTT tests.

Table 1. Chemical compositions of pre-melted mold fluxes (in mass%).

	CaO	SiO2	Al2O3	MgO	Na2O	Li2O	F	B2O3	CaO/SiO2
Flux 0	43.06	34.44	4.04	2.03	7.87	1.02	7.54	0.00	1.25
Flux 1	40.28	32.19	4.02	2.04	7.62	0.97	2.85	10.03	1.25
Flux 2	38.90	31.01	4.07	2.05	10.12	0.96	2.83	10.06	1.25
Flux 3	37.79	30.20	4.03	2.04	12.08	0.97	2.82	10.07	1.25
Flux 4	41.41	33.10	4.02	2.03	7.61	0.98	2.85	8.00	1.25
Flux 5	42.51	39.99	4.01	2.01	7.60	0.98	2.87	6.03	1.25

2.4. Experiment Procedure

The measurement of initial crystallization temperature from solid glass of low fluorine mold flux, T_g, is carried out in the following way to simulate the crystallization behaviors of heat transfer tests (Stage III–IV, section 3.2) (Fig. 4): the sample powders are first heated with B type thermocouple that with a diameter of 0.5 mm at 1500°C with the rate of 15°C/s; then, they are held for 60 s to eliminate bubbles and homogenize chemical compositions. After that, they are rapidly cooled down to room temperature to achieve a solid glassy phase. About 300 s later, the glassy mold flux is heated with a rate of 0.5°C/s to get crystallized, which is an approximate value to simulate the temperature-rise rate occurred in the heat transfer experiment.²⁴⁾ Then, the initial crystallization temperature could be obtained through the in situ observation of the recorded video, where 5 Vol. pct of crystallization was defined as the begging of the crystallization. ²¹⁾

Fig. 4.

Temperature controlling profile for initial crystallization temperature measurements.

The heat transfer tests are carried out by preheating the copper mold system with individual glassy disk under a 500 kW/m² thermal energy first, and then the incident thermal energy is increased linearly to 800 kW/m² with a rate of 2 kW/(m²⋅s) and maintained for 300 seconds. After that, the incident heat flux is increased to 1400 kW/m² and maintained for 1200 seconds, which is in the magnitude of real caster as shown in Fig. 5.

Fig. 5.

The heating profile for heat transfer test.

Figure 6(a) shows the subsurface response temperatures when the heating profile in Fig. 5 is applied to a bare copper mold system. The system reached steady state after 200–300 s, and the steady state heat flux was calculated by Fourier's Law:   

$$\mathrm{q}=\frac{-\text{1}}{\text{n}}{\sum _{i}k\left(\frac{dT}{dx}\right)}_i$$(1)

where, k is thermal conductivity of copper mold, 381 W/ m⋅K,¹⁹⁾ n refers to the total number of thermocouples, i is the index number, and (dT/dx)_i means the temperature gradient measured by the i^th thermocouple.

Fig. 6.

The responding temperatures and heat flux in heat transfer test (a) The in-mold responding temperatures history, (b) Steady state subsurface responding temperatures distribution at 1800 s, (c) Heat flux for bare copper system under thermal radiation.

Figure 6(b) shows the steady state subsurface responding temperatures distribution at 1800 s, where the linear fitting coefficient is 0.99 suggesting the high degree of linear relationship between the measured temperatures and the thermocouple positions. The heat flux histories corresponding to Fig. 6(a) are given in Fig. 6(c), and it could be observed that the measured heat flux increases linearly with the addition of the output power first, and then come into a steady state in a very short time.

3. Results and Discussion

3.1. Investigation of Initial Crystallization Temperature

A typical temperature history of SHTT experiment for initial crystallization temperature measurement is shown in Fig. 7, and 8 typical images standing for each stage are given in Fig. 8.

Fig. 7.

Temperature history of SHTT experiment for initial crystallization temperature measurement.

Fig. 8.

Snapshots of crystallization process of mold flux.

Figure 8(a) shows the initial state of low fluorine mold flux powders at position I (1s) in Fig. 7. Then, those powders are sintered (Figs. 7 II and 8(b)) and melted (Figs. 7 III and 8(c)) with the increase of heating temperature. Lots of bubbles are occurring in molten slag when powders start melting. Those bubbles mainly from the decomposition of residual carbonate, low melting point compounds as well as moisture. When temperature reaches 1500°C, position IV in Fig. 7, the mold flux is totally in liquid state as shown in Fig. 8(d). Then, the temperature is kept at 1500°C for 300 s to eliminate bubbles and homogenize its chemical composition. After that, it is quenched down to room temperature (position V in Fig. 7), and transparent glassy mold flux is formed (Fig. 8(e)). From Figs. 8(f) to 8(h), the corresponding temperatures is at positions of VI to VIII in Fig. 7, and the glassy mold flux is heated with a rate of 0.5°C/s. Figure 8(f) is the time that crystals begin to precipitate, and Fig. 8(h) is the state that all solid glassy mold flux has been transformed into non-transparent crystals.

The snapshots of initial crystallization of above mold fluxes from glassy phase are shown in Fig. 9. From Figs. 9(a)–9(c), it can be observed that the initial crystallization temperatures for Flux 1, Flux 2 and Flux3 are 901°C, 879°C and 842°C respectively. It indicates that the initial crystallization temperature of low fluorine mold fluxes decreases with addition of Na₂O. The reason for that is mainly because Na⁺ belongs to silicate network breaker,^25,26) which can break the Si–O bonds in silicate network, and make the silicate structure simpler. Consequently clusters can move easily and grow into crystals when the temperature rises.

Fig. 9.

Snapshots of the initial crystallization of mold fluxes under heating rate 0.5°C/s.

The influence of B₂O₃ on initial crystallization temperature is shown in Figs. 9(d)–9(f), where initial crystallization temperature is getting higher with the increase of B₂O₃ changing from 834°C to 865°C and 901°C respectively, when B₂O₃ contents becomes from 6.03% to 8.00% and 10.03 mass%. Boron, as one of the network former,^27,28) could form the trigonal planar [BO₃]^3– or tetrahedral [BO₄]^4– structural units. Joined together via shared oxygen atoms, it could form cyclic or linear structure which is similar to silicon. Therefore, the increase of boron content will increase the size of silicate network and make its structure more complex. Those result in the fact that it needs higher temperature to loosen silicate structure to make clusters move and get crystallized when the B₂O₃ content increases.

3.2. Heat Transfer Process

The prepared mold flux disks are individually placed on the top of copper mold and subjected to the heating profile as Fig. 5 for the heat transfer tests. A typical heat transfer experiment of Flux 5 is shown in Fig. 10, and there are six typical stages appearing during the heat transfer experiment. Stage I and stage II are the pre-heating stages where the heat flux first increases linearly with the addition of thermal radiation in stage I, and then keeps constant about 300 s at stage II. The disk is kept as glassy without phase transition during the pre-heating stages. After that, the heat flux is increasing during stage III, and the deviation of heat flux occurred due to the initiation of mold flux crystallization where the opaque crystals are formed at the top of the disk when time goes to stage IV. The heat flux then reaches to its peak value and then decreases in stage V due to the further crystallization. Stage VI is the period of steady state when the crystallization completes and the heat flux keeps constant.

Fig. 10.

Typical stages during the heat transfer experiment.

3.3. Effect of Na₂O on Heat Transfer Rate

In order to study the effect of Na₂O on heat transfer behavior of low fluorine mold flux, the measured heat fluxes and final cross-section views of flux disks corresponding to different Na₂O contents are given in Fig. 11. It can be found that the final steady state heat flux decreases from about 656 kW/m² to 576 kW/m² and then to 492 kW/m² when the Na₂O content increases from 7.62% to 10.12% and 12.08%. The final cross-section views of mold flux disks show that mold flux with 12.08% Na₂O content has the thickest crystalline layer while the mold flux with 7.62% Na₂O content appears the thinnest crystalline layer.

Fig. 11.

The measured heat fluxes histories of low fluorine mold fluxes with fixed 10% B₂O₃ content and 7.62% (Flux 1), 10.12% (Flux 2), 12.08% (Flux 3) Na₂O content.

The specific thickness of crystalline layer and crystalline fraction of Flux 1, 2 and 3 are shown in Fig. 12. It suggests that the thickness of crystalline layer and crystalline fraction of disks increases with the addition of Na₂O content. For Flux 1 with 7.62% Na₂O, the thickness of crystalline layer is about 1.95 mm and 42.4% crystalline fraction. However, the crystalline layer thickness and fraction change to 3.62 mm and 78.6% when Na₂O content becomes to 12.08%. Therefore, it could be concluded that Na₂O enhances mold flux crystallization and reduces the heat flux across the disk due to the increase of crystalline layer thickness and volume fraction. With the increase of mold flux crystallization, there would be more incident radiation reflected and scattered from the crystals surface, grain boundary as well as defects, leading to less energy would be absorbed and conducted to the mold.^29,30)

Fig. 12.

The thickness of crystalline layer and crystalline fraction of low fluorine mold Flux 1(7.62% Na₂O), Flux 2 (10.12% Na₂O) and Flux 3 (12.08%Na₂O).

3.4. Effect of B₂O₃ on Heat Transfer Rate

Figure 13 shows the measured heat fluxes histories and final cross-section views of flux disks of low fluorine mold fluxes with fixed 7.6% Na₂O and varying B₂O₃, i.e. 6.03%, 8.00%, 10.03% respectively. The steady state heat flux increases from 497 kW/m² to 547 kW/m², and 656 kW/m² when B₂O₃ content is added from 6.03% to 8.00%, and to 10.03%. The variation of steady state heat flux is consistent with the change of crystalline layer thickness and crystal fraction of mold flux disks.

Fig. 13.

The measured heat fluxes histories of low fluorine mold fluxes with fixed 7.6% Na₂O content and 6.03% (Flux 5), 8.00% (Flux 4), 10.03% (Flux 1) B₂O₃ content.

The thickness of crystalline layer and crystalline fraction of Flux 1, 4, and 5 are shown in Fig. 14. Both crystalline layer thickness and crystalline fraction decreases with the increase of B₂O₃, where the crystalline layer thickness reduces from 2.75 mm to 2.13 mm and 1.95 mm when the B₂O₃ content is added from 6.03% to 8.00% and 10.03%. Hence, it could be concluded that B₂O₃ tends to inhibit the mold flux crystallization, and there would be more glassy phase remained in the structure leading to more radiation absorbed and transferred to the mold, which in turn to improve the heat transfer rate in above experiments.

Fig. 14.

The thickness of crystalline layer and crystalline fraction of low fluorine mold Flux 5 (6.03% B₂O₃), Flux 4 (8.00% B₂O₃) and Flux 1(10.03% B₂O₃).

3.5. Effect of Na₂O and B₂O₃ on Interfacial Thermal Resistance

The interfacial thermal resistance between mold wall and mold flux is a very important parameter to affect the heat flux transferring from shell to mold. In order to consider the variation of interface thermal resistance due to the crystallization of mold flux, a numerical calculation was conducted by assuming that (1) Only one-dimensional heat transfer occurred from low fluorine mold flux to the copper mold; and (2) The total heat flux is consisted of radiative and conductive heat flux. The heat flux travels from mold flux film to mold wall was schematically shown in Fig. 15.

Fig. 15.

Schematic representation of heat flux across mold flux film consisted of crystalline and glassy layers.

Therefore, the total heat flux q_tot at the steady state through the mold flux disk can be expressed as Eq. (2):   

$$q_{tot}=q_{rad}+q_{cond}$$(2)

The conductive heat flux at steady state could be expressed as following Eq. (3) according to the Fourier's law, it can be obtained by:   

$$q_{\text{cond}}=K_{cond}\frac{T_{\text{g}}-T_{gs}}{d}$$(3)

where, K_cond is the thermal conductivity of glassy mold flux, its values were employed as 1.0 to 1.2 W/(m⋅k) as previous study;²⁹⁾ T_g is the interfacial temperature between the glassy and crystalline layer which was determined as the initial crystallization (glass/crystal transformation) temperature (section 3.1) by using SHTT tests; T_gs is the bottom face temperature of mold flux disk; d is the thickness of glassy layer.

Then, the T_gs can be calculated through Eq. (4):   

$$T_{gs}=T_{ms}+R_{int}*q_{obs}$$(4)

Where, T_ms stands for the temperature of copper mold top face calculated by the in-mold temperature gradient; R_int is interface resistance; and q_obs is the measured heat flux at steady state.

For heat flux transferred through radiation, it is also convenient to use the same form as the Fourier's law, and the radiative heat flux can be calculated as Eq. (5).

  

$$q_{\text{rad}}=K_{rad}\frac{T_g-T_{gs}}{d}$$(5)

The radiative thermal conductivity K_rad of glassy mold flux can be determined by Eqs. (6) and (7) through assuming the mold flux behaves as gray gas.   

$$K_{rad}=\beta \frac{(T_g^4-T_{gs}^4)d}{T_g-T_{gs}}$$(6)

  

$$\beta =\frac{n^{\text{2}}\sigma }{0.75\alpha d+{\epsilon }_1^{-1}+{\epsilon }_2^{-1}-1}$$(7)

where, n is the refractive index that referred to be 1.6; σ is the Stefan-Boltzmann constant, 5.6705 × 10^–8 W/(m²·K⁴); ε₁ represents the emissivity of glassy mold flux, 0.92; and ε₂ is the emissivity of copper mold, 0.4;³¹⁾ and α is the absorption coefficient, 400 m^–1 for glassy mold flux.²⁴⁾

Hence, the interface thermal resistance R_int can be computed through above equations as the flow chart shown in Fig. 16. The parameters used for this calculation are listed in Table 2.

Fig. 16.

The flow chart for calculating R_int.

Table 2. Interfacial thermal resistance (R_int) for low fluorine mold fluxes.

	Tg [°C]	Tms [°C]	d (mm)	q (KW/m2)	Rint (m2K/W)
Flux 1	901	147	2.65	656	4.47 × 10–4
Flux 2	879	132	1.95	576	7.80 × 10–4
Flux 3	842	115	1.3	492	11.24 × 10–4
Flux 4	865	150	2.47	547	6.89 × 10–4
Flux 5	834	143	1.85	497	9.41 × 10–4

The calculated values for interface thermal resistance of above low fluorine mold fluxes are also listed in Table 2. It suggests that the Rint increases with the addition of Na₂O content. The values of R_int is 4.47 × 10^–4 m²⋅K/W when the Na₂O content is 7.52% (Flux 1). It increases to 7.80 × 10^–4 m²⋅K/W and 11.24 × 10^–4 m²⋅K/W when the Na₂O content changes to 10.12% (Flux 2) and 12.08% (Flux 3). While, the interface thermal resistance R_int reduces with the increase of B₂O₃. It decreases from 9.41 × 10⁻⁴ m²⋅K/W to 6.89 × 10^–4 m²⋅K/W, then to 4.47 × 10^–4 m²·K/W with the B²O³ content increased from 6.03% (Flux 5) to 8.00% (Flux 4), and to 10.03% (Flux 1). Those interface thermal resistances obtained here were consistent with other researchers' results.^32,33) The effects of Na₂O and B₂O₃ on interface thermal resistance are consistent with their influences on crystallization of low fluorine mold flux. The reason for that is because the interface thermal resistance is mainly from the air gap in-between the slag and copper mold due to the shrinkage and deformation of mold flux during crystallization. The more crystallization at the top of the mold flux, the more shrinkage and deformation of the whole solid flux disk would be and there may be more buckling occurring at the bottom side of the glassy part, which would in turn lead to a less contact between the copper mold and solid flux and therefore a higher interfacial thermal resistance. Thus, an improved crystallization due to the variation of chemical component or content would enlarge the air gap, and R_int would correspondingly increases.

4. Conclusions

The effects of Na₂O and B₂O₃ on initial crystallization (glassy/crystalline transition) temperature and heat transfer behavior of low fluorine mold flux for casting medium carbon steels are investigated by using Infrared Emitter Technique (IET) and Single Hot Thermocouple Technique (SHTT). Specific conclusions are summarized as follows:

(1) The initial crystallization temperature of low fluorine mold fluxes reduces with the increase of Na₂O content; otherwise, it increases with addition of B₂O₃ due to the fact that Na⁺ belongs to silicate network breaker promoting mold flux crystallization, but boron is network former inhibiting mold flux crystallization.

(2) The thickness of crystalline layer and the crystalline fraction of mold flux have significant impact on heat transfer process between shell and mold wall. The final steady state heat flux tends to decrease with the increase of Na₂O and reduction of B₂O₃, as the flux with higher Na₂O content would form a thicker crystalline layer, resulting in a more incident radiation reflected and scattered.

(3) The interfacial thermal resistance between the solid mold flux and copper mold increases with the addition of Na₂O and becomes lower with the increase of B₂O₃. The reason could be explained as the interfacial thermal resistance is determined by the interface air gap between the slag and copper mold, and an improved crystallization due to the addition of Na₂O would enlarge the air gap, and R_int would correspondingly increases.

Acknowledgments

The financial support from NSFC (51274244, 51250110533) and International Science & Technology Cooperation Program of China (2011DFA 71390) is greatly acknowledged.

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