Influencing mechanism of (V,Ti)@(C,N) compounds on the viscosity of hot metal

Abstract

The precipitation of the (V, Ti)@(C, N) compounds, for example, VC, VN, TiC and TiN, in the hot metal during the ironmaking process may result in high viscosity of the hot metal and cause a negative impact on the process. How to control the precipitation of the (V, Ti)@(C, N) compounds is a significant issue for the high-efficient ironmaking. In this paper, the precipitation behaviour of the (V, Ti)@(C, N) compounds and its effect on the viscosity of the hot metal were investigated. The theoretical analysis results show that the precipitation temperature of the (V, Ti)@(C, N) compounds increases with the increasing content of vanadium, titanium, silicon and manganese, while it decreases with the that of sulphur and phosphorus in hot metal. Additionally, the viscosity-measuring was carried out by a self-developed melt viscometer, and the results show that the viscosity of the hot metal can be affected significantly by the vanadium content. As the vanadium content exceeds 0.3 wt.%, the viscosity of the hot metal increases sharply with the increasing of vanadium content, which will have a serious impact on the fluidity of the hot metal. Furthermore, the precipitation product in the hot metal was confirmed to be the (V, Ti)@(C, N) compounds by using the scanning electron microscope and the energy dispersive x-ray spectroscopy, which proved that the compounds mainly consist of the V, Ti, C and N elements.

Keywords

Ironmaking hot metal viscosity high melting point phase vanadium titano-magnetite

Introduction

Vanadium titanomagnetite (VTM) is an important polymetallic mineral in industry, which riches in some valuable elements, such as iron, vanadium and titanium, and always be a raw material for extraction of high value–added metals.^1–3 According to geological survey, VTM is mainly distributed in Russia, South Africa, China and Canada.⁴ In China, the VTM mineral deposit is mostly located in Panzhihua and Chengde regions, and the main method for iron extraction from VTM ore is the blast furnace (BF)-basic oxygen furnace (BOF) process.^5–7 During the BF smelting, the iron oxides in the ore are reduced to hot metal in the high temperature, and then it flows to BOF for further treatment.^8,9 Therefore, the viscosity of the hot metal is an important factor to influence the efficiency of the smelting process, because it determines the fluidity of the hot metal.⁹ For the hot metal smelted by VTM, its viscosity would be greater than that smelted by ordinary iron ore because the former contains much more vanadium and titanium, which may form high melting point compounds, such as Ti(C, N) and V(C, N), in the hot metal.^10–12 At present, the high viscosity of the vanadium- and titanium-containing hot metal is still an obstacle to hinder the smooth smelting of VTM. Therefore, how to control the viscosity of the hot metal should be a significant issue for the efficient smelting of VTM.

Recently, many efforts have been made to investigate the precipitation of vanadium- and titanium-containing compounds in the hot metal. Gao et al.⁵ studied the effect of titanium content on the precipitation behaviours of TiC in carbon-saturated hot metal. It was found that Fe₃C precipitated first when the titanium content was less than 0.25 wt.%, while the TiC precipitated preferential as the titanium content was greater than 0.47 wt.%. Jiao et al.¹⁰ investigated the behaviours of titanium compounds generated in the BF hearth during the VTM smelting. The results show that considerable titanium compounds formed on the furnace wall and the compounds mainly consisted of Ti(C, N) crystals. Li and Fruehan¹³ discussed the formation mechanism of TiC and Ti(C, N) in carbon-saturated iron, which confirmed that the minimum content of titanium for Ti(C, N) formation in hot metal was 0.2 mass% at 1773 K and 1.0 atm of nitrogen. These finding are beneficial for further understanding the precipitation behaviour of the vanadium- and titanium-containing compounds in hot metal. However, a comprehensive investigation is still lacking. For example, the formation mechanism of V(C, N) compounds was discussed scarcely in the previous studies. Therefore, a deep investigation for precipitation of the vanadium- and titanium-containing compounds in hot metal should be necessary.

On another hand, a precise link between the precipitation of vanadium- and titanium-containing compounds and the viscosity of the melt is still lacking. This should be attributed to the fact that the precise measurement of the viscosity for hot metal is difficult. The traditional rotary method is often used for the viscosity measurement of high-temperature melt;^14–17 however, it is more suitable for molten slag with relatively high viscosity values. The torsion-pendulum method was developed to confirm the viscosity of the melt with low viscosity.^18,19 However, its application is limited because the high temperature will bring in high strengthened thermal disturbance, which will cause the measurement results differ greatly from the actual values. Additionally, the current torsion-pendulum method lacks the automatic data collection system, which may result in a low efficiency of the experiment. Therefore, a more precise method for the viscosity measurement should be necessary for deep understanding the relationship between the vanadium- and titanium-containing compounds and the viscosity of the hot metal.

In the present work, the precipitation behaviour of high melting point phase in hot metal containing vanadium and titanium was investigated employing the thermodynamic calculation and the high-temperature confocal microscopy. Additionally, the torsion pendulum vibration system for viscosity measurement was improved by using the photoelectric sensing technology and computer real-time recording system, by which the accurate online measurement of the viscosity of the hot metal was achieved. Based on this, the influence mechanism of high melting point phase precipitation on the viscosity of hot metal was explored.

Experimental

Thermodynamic analytic method

In order to clarify the precipitation behaviour of the (V, Ti)@(C, N) compounds, for example, VC, VN, TiC and TiN, in the hot metal theoretically, a thermodynamic equilibrium method was employed to calculate the solubility product, precipitation temperature and precipitation amount of high melting point phases. The formation reactions of VC, VN, TiC and TiN in hot metal and their standard Gibbs free energy were listed as follows²⁰:

[V] + [C] = VC (s) Δ G_{1}^{0} = - 103990 + 72.28 T (J / mol)

(1)

[V] + [N] = VN (s) Δ G_{2}^{0} = - 363800 + 141.17 T (J / mol)

(2)

[Ti] + [C] = TiC (s) Δ G_{3}^{0} = - 169528 + 65.31 T (J / mol)

(3)

[Ti] + [N] = TiN (s) Δ G_{4}^{0} = - 360637.2 + 53.70 T (J / mol)

(4)

When the reaction reaches equilibrium, the relationship between the standard Gibbs free energy

Δ G_{1}^{0}

, the equilibrium constant K₁ and the activity coefficient f_k is shown in equation (5):

Δ G_{1}^{0} = - {RTlnK}_{1} = - RTln \frac{a_{VC}}{a_{Va} C} = - RTln \frac{f_{VC} \cdot w [VC]}{f_{V} \cdot f_{C} \cdot w [V] \cdot w [C]}

(5)

where R represents the gas constant, J/(mol·K); K₁ represents the equilibrium constant of reaction (1); T represents the temperature, K; a represents the activity; f represents the activity coefficient; w represents the solubility.

It should be noted that the a_V and a_C in equation (5) is assumed to obey Henry's law state as the content of V and C in the hot metal is very small. Additionally, considering that VC, VN, TiC and TiN are pure solid phases, their activity can be regarded as unity. Then, the solubility product of VC was expressed as follows:

\lg (w [V] \cdot w [C]) = \lg \frac{\exp (Δ G_{1}^{0} / RT)}{f_{V} \cdot f_{C}}

(6)

It can be seen from equation (6) that the solubility product of VC, e.g., w[V]·w[C], can be calculated by the activity coefficients of [V] and [C]. This solubility product gives a saturation point with respect to composition (at a given temperature). If the concentration of [V] or [C] in hot metal exceeds its critical value (at a given temperature), it suggests that the compound, for example, VC, would precipitate. The activity coefficients of elements in melts can be characterised by the interaction coefficients between the elements, as given follows:

\lg f_{M} = \sum e_{M}^{k} \cdot w_{k}

(7)

where

e_{M}^{k}

represents the interaction coefficient; w _k represents the mass percent of element k in melts.

The interaction coefficients of the elements in hot metal at 1873 K can be obtained by literature,^21,22 which are shown in Table 1.

Table 1.

Interaction coefficients of dissolved elements in molten hot metal.^21,22

Element k	Ti	C	Si	Mn	S	P	V
$e_{V}^{k}$	6.1	−0.34	0.042	0.004	−0.028	−0.041	0.015
$e_{T i}^{k}$	0.048	−0.30	0.05	−0.05	−0.08	−0.01	6.1
$e_{C}^{k}$	−0.165	0.14	0.08	−0.012	0.046	0.051	−0.077
$e_{N}^{k}$	−0.53	0.13	0.047	−0.021	0.007	0.045	−0.093

According to the regular solution model, the interaction coefficients of elements in other temperatures can be obtained by employing equation (8):²¹

e_{m (T)}^{k} = \frac{1873}{T} \cdot e_{m (1873)}^{k}

(8)

where m denotes V, Ti, C and N. Based on equations (1)–(8), the solubility product of VC, VN, TiN and TiC in melts can be calculated by combining the above equations. As the solubility product of (V, Ti)@(C, N) was obtained, the precipitation amount and the precipitation temperature of (V, Ti)@(C, N) in melts can be calculated. The formula is shown in equation (9) to equation (11):

w_{VC} = (w_{V} - w_{[V]}) \cdot \frac{A_{V} + A_{C}}{A_{V}}

(9)

\lg (w_{[V]} \cdot w_{[C]}) = A - B / T

(10)

(w_{V} - w_{[V]}) / (w_{C} - w_{[C]}) = \frac{A_{V}}{A_{C}}

(11)

where w_[V] and w_[C] are the solubilities of [V] and [C] in melts, respectively; A and B are the fitted parameters for temperature and solubility of VC; A_V and A_C are the relative atomic masses of V and C; w_VC is the mass fraction of precipitated VC. Additionally, the precipitation amounts and precipitation temperatures of VN, TiC and TiN can be obtained by the same method.

Experimental method

To investigate the relationship between the precipitation of (V, Ti)@(C, N) compounds and the viscosity of the melt, a series of experiments were carried out. The primary pig iron used in experiment was provided by a steel company in China. The chemical composition of primary pig iron was analysed by employing inductively coupled plasma optical emission spectrometer (Optima 8000) and infrared C/S determinator (CS-8820), which is shown in Table 2. Additionally, Fe-V alloy (GB/T4319-2012, FeV80-A) and high purity titanium (99.99%) were added to the primary pig iron to adjust the composition of the hot metal.

Table 2.

Composition of pig iron.

Chemical element	C	Si	Mn	S	P	V	Ti
content(wt.%)	4.325	0.145	0.240	0.065	0.079	0.040	0.090

The pre-melting of the samples was carried out in a corundum crucible at an induction furnace (GGC-60 KW) by controlling the heating rate as the 0.04A/min. After the sample was melted, the melts were mixed well under the action of electromagnetic stirring and then they were hold for 30 min. Afterwards, the samples were cooled down under the argon gas atmosphere. There were totally 12 samples for the viscosity measurement, and their chemical compositions are shown in Table 3.

Table 3.

Composition of hot metal during the viscosity measurement (wt.%).

No.	C	Si	Mn	S	P	V	Ti
#1	4.325	0.145	0.240	0.065	0.079	0.040	0.090
#2	4.297	0.145	0.240	0.065	0.079	0.040	0.150
#3	4.295	0.15	0.240	0.065	0.079	0.040	0.200
#4	4.291	0.145	0.239	0.065	0.079	0.040	0.300
#5	4.287	0.145	0.239	0.065	0.079	0.040	0.400
#6	4.297	0.146	0.240	0.065	0.079	0.100	0.090
#7	4.294	0.146	0.240	0.065	0.079	0.150	0.090
#8	4.291	0.146	0.240	0.065	0.079	0.200	0.090
#9	4.289	0.147	0.239	0.065	0.079	0.250	0.090
#10	4.286	0.147	0.239	0.065	0.079	0.300	0.090
#11	4.284	0.148	0.239	0.065	0.079	0.350	0.090
#12	4.281	0.148	0.239	0.065	0.079	0.400	0.090

The viscosity measurement was carried out by employing a torsional pendulum vibration viscometer, which was designed and developed by our research group. The self-developed viscometer is mainly composed of the heating system, pendulum vibration system and the data processing system, and its schematic diagram is shown in Figure 1. After the sample was melted at 1450°C, the viscosity measurement began through applying an external twisting force to the rotor, by which the rotor will rotate in the testing melt to do the damping motion until it stop. During this period, the damping motion of the rotor can be detected by the photoelectric sensing system and recorded by the real-time computer recording system. The viscosity value of the molten material could be obtained by analysing the curve of the damping motion. In this experiment, three standard liquids with viscosities 1, 5 and 10 mPa·s were used to calibrate the viscosity value of the melt.

To understand the precipitation behaviour of the high melting point phase, a high-temperature confocal microscope (VL2000DX-SVF17SP, Japan) was used to observe the surface of the sample in real time during the heating and the cooling. Additionally, the morphology of the high melting point phase and its composition were confirmed by a scanning electron microscope equipped by the energy dispersive x-ray spectroscopy (SEM-EDS, TESCAN VEGA 3 LMH, Czech Republic).

Results and discussion

Thermodynamic calculation results

Effect of vanadium content on the precipitation of (V, Ti)@(C, N) compounds

Figure 2 shows the effect of vanadium content on the solubility product of (V, Ti)@(C, N) compounds under the varying temperatures. It can be seen that the solubility product of VC, VN, TiC and TiN all decreases gradually with the decreasing temperature. In addition, the solubility product of these compounds decreases with increasing the vanadium content, indicating that the vanadium dissolved in hot metal will result in the precipitation of the (V, Ti)@(C, N) compounds if the vanadium content exceeds a certain value. Under the same content of vanadium, the solubility product of TiN, TiC, VN and VC is different, which implies that the precipitation temperature of these compounds in hot metal is different.

Figure 3 demonstrates the relationship between the precipitation amount of (V, Ti)@(C, N) compounds and the temperature with the varing vanadium contents. It can be seen from Figure 3(a) and (b) that the precipitation temperatures of VC and VN gradually increase as the vanadium content increases. This should be attributed to the fact that the solubility of VC and VN decreases as the vanadium content increases (as shown in Figure 2(a) and (b)), thereby making the precipitation temperatures of VC and VN increase as the vanadium content increases. In addition, as the vanadium content increases, the precipitation amounts of VC and VN also increase. According to the results of the literature,²³ the first precipitated phase provides the core for heterogeneous nucleation and promotes the growth of the unprecipitated phase. Therefore, as the VC and VN phases begin to precipitate, the dissolved carbon, nitrogen and vanadium atoms will gather around the VC and VN phase and grow based on the precipitated cores. Meanwhile, the VC and VN will combine together to produce V(C, N) precipitation finally, because the lattice structures of VC and VN are similar.²⁴

From Figure 3(c) and (d), it can be also found that the precipitation temperature of TiC and TiN gradually increases with the increase in vanadium content. The main reason for this should be that the interaction coefficient of vanadium and titanium is greater than zero and simultaneously, the activity coefficient of titanium increases with increasing the vanadium content.²⁰ Therefore, according to the solubility equation of TiC and TiN (as shown in equation (6)), the solubility of TiC and TiN decreases with the increase of activity coefficient of titanium, which makes the precipitation temperature of TiC and TiN increases with the increase in vanadium content.

Effect of titanium content on the precipitation of (V, Ti)@(C, N) compounds

Figure 4 depicts the effect of titanium content on the solubility product of (V, Ti)@(C, N) compounds under the varying temperatures. It can be seen that the solubility product of VC and VN decreases with the increase in titanium content. The reason for this is that the interaction coefficient of titanium to vanadium is 6.2, as shown in Table 2. According to equation (7), the activity coefficient of vanadium is positively correlated with the titanium content. Meanwhile, according to equation (6), the solubility of VC and VN is negatively correlated with the activity coefficient of vanadium; therefore, the solubility product of VC and VN decreases with the increase in titanium content. Additionally, from Figure 4(c) and (d), it can be seen that the solubility product of TiC and TiN gradually decreases as the titanium content increases. The solubility product of TiN is smaller than that of TiC, which means that TiN precipitates prior to TiC at the same temperature. After the precipitation of TiN in hot metal, it will provide the core of heterogeneous nucleation for the precipitation of TiC, which leads to a lower TiC precipitation temperature as compared to the result of the theoretical calculation.

Figure 5 shows the effect of titanium content on the precipitation temperature and the amounts of (V, Ti)@(C, N) compounds. It can be found from Figure 5(a) and (b) that the precipitation temperatures of VC and VN increase with the increasing of the titanium content, respectively. The reason for this is similar to the analysis above, which is mainly because the interaction coefficient of titanium to vanadium is positive. As the titanium content in hot metal is more than 0.3 wt.%, the precipitation of VC and VN is inevitable because the actual smelting temperature of iron making is often 1437–1467°C.²⁵ Therefore, to prevent VC and VN from precipitation during the production process, the titanium content in the hot metal should be less than 0.3 wt.%. Figure 5(c) and (d) exhibits the effect of titanium content on the precipitation of TiC and TiN. It can be observed that the precipitation temperature of TiC and TiN gradually increases as the titanium content increases. Simultaneously, the precipitation amounts of TiC and TiN also increase gradually with the increasing of the titanium content. The precipitation temperature of TiC and TiN is higher than 1450°C as the titanium content is more than 0.1 wt.%, indicating that the precipitation of TiC and TiN in the actual production process is inevitable. This is consistent with the results of Jiao et al.,¹⁰ which a large amount of Ti(C_0.3, N_0.7) was found to generate in the cylinder by analysing the samples from the BF cylinder area.

Effect of other elements on the precipitation of (V, Ti)@(C, N) compounds

Apart from the vanadium and titanium, other elements such as silicon, manganese, sulphur and phosphorus dissolved in hot metal may also affect the precipitation of (V, Ti)@(C, N). According to the thermodynamic calculation of ‘Effect of vanadium content on the precipitation of (V, Ti)@(C, N) compounds’ section, the effects of silicon, manganese, sulphur and phosphorus content on the precipitation of (V, Ti)@(C, N) were obtained, and the results are shown in Figures 6–9, respectively. From Figures 6 and 7, it can be seen that the precipitation temperature and precipitation amount of (V, Ti)@(C, N) increase gradually with the increase in silicon and manganese content. The reason for this is that the interaction coefficient of silicon and manganese on vanadium and titanium at 1450°C is greater than zero,^21,22 which is consistent with the analysis above. This is also proved that the solubility of (V, Ti)@(C, N) is negatively correlated with the content of silicon and manganese, which makes the precipitation temperature and precipitation amount of (V, Ti)@(C, N) increase with the increase in silicon and manganese content. Additionally, Figures 8 and 9 show that the precipitation amount of (V, Ti)@(C, N) increase with the increase in sulphur and phosphorus content at the same temperature. This is because the interaction coefficient of phosphorus and sulphur on vanadium and titanium at 1450°C is less than zero.^21,22 In the iron-making process of BF, the content of each element in the hot metal can be regulated in an appropriate range to suppress the precipitation of high melting point phase, for example, (V, Ti)@(C, N), thereby reducing the viscosity of the hot metal and ensuring a good fluidity of the hot metal.

Effect of vanadium and titanium content on the viscosity of the hot metal

In order to investigate the effect of (V, Ti)@(C, N) compounds on the viscosity of the hot metal, a self-developed melt viscometer was employed to measure the viscosity values of the hot metal precisely. Figure 10(a) shows the viscosity-temperature curve of the hot metal with the varying vanadium content. Based on it, the melting temperature of the hot metal at the different content of vanadium was obtained by using the slope rule in the viscosity-temperature curves, and the results are shown in Figure 10(b).

As can be seen from Figure 10(a), the viscosity of the hot metal is closely related to the temperature and vanadium content. With the decreasing of temperature, the viscosity of hot metal gradually increases, which can be explained by the Arrhenius equation, for example, η=A exp [Eη/(RT)], where η is viscosity, A is constant, Eη is viscous flow activation energy, R is gas constant and T is temperature. Additionally, the viscosity of hot metal increases with the vanadium content at the same temperature. This should be attributed to the precipitation of (V, Ti)@(C, N), which is consistent with the theoretical analysis results shown in Figure 3. As the vanadium content is less than 0.30 wt.%, the effect of vanadium content on the viscosity of the hot metal is not significant; however, as the vanadium content is greater than 0.30 wt.%, the viscosity of the hot metal increases sharply with the increase in vanadium content. This is similar to the change tendency of the melting temperature, as shown in Figure 10(b), which the melting temperature of the hot metal sharply increases as the vanadium content is more than 0.30 wt.%. Therefore, it would be better to control the vanadium content less than 0.30 wt.% during the smelting of the VTM.

Figure 11(a) shows the viscosity-temperature curve of the hot metal with the varying titanium content. It can be seen that the viscosity of the hot metal gradually increases as the titanium content increases. As the titanium content is 0.09 wt.%, the viscosity of hot metal is 24.6 mPa·s (1450°C), while it increases to 124.5 mPa·s (1450°C) as the titanium content increases to 0.40 wt.%. From the calculation results of ‘Thermodynamic calculation results’ section, it is clear that the precipitation temperature of TiC and TiN in hot metal is greater than 1500°C, that is, TiC and TiN start to precipitate in hot metal at a higher temperature. With increasing the titanium content, the precipitation amount of TiC and TiN gradually increases, which makes the viscosity of the hot metal increase. Additionally, as the titanium content increases, the precipitation of VC and VN in hot metal will also happen because the interaction coefficient of titanium to vanadium is positive, which may also contribute to the increase in the viscosity.

Precipitation behaviour of (V, Ti)@(C, N) compounds in hot metal

To verify the theoretical analysis results, a high-temperature confocal in-situ observation experiment was carried out to clarify the precipitation behaviour of the (V, Ti)@(C, N) in the hot metal. Figure 12 shows the observation results of the hot metal sample containing 0.10 wt.% and 0.30 wt.% vanadium content, respectively. It can be seen that the precipitation temperature of the high melting point phase increases with the increasing of the vanadium content. The precipitation temperature of the (V, Ti)@(C, N) in hot metal was 1427°C as the vanadium content is 0.10 wt.%, as shown in Figure 12(a), while it increased to 1498°C as the vanadium content increase to 0.30 wt.%. According to the theoretical calculations, the precipitation temperatures of VC and VN are much lower than the experimental precipitation temperatures when the vanadium content is 0.10 wt.% and 0.30 wt.%; therefore, TiC and TiN would be considered as the precipitated phases because the precipitation temperature of TiC increases from 1308°C to 1610°C during the increase in vanadium content from 0.10 wt.% to 0.30 wt.%, and that TiN is higher than 1700°C. Base on this, it is reasonable that these precipitated phases observed in this experiment are TiC and TiN. In addition, considering that TiC and TiN have similar lattice constants and crystal structures, Ti(C, N) would be also a precipitation phase in the hot metal. On the another hand, by comparing the observation results in1400°C, for example, the results shown in Figure 12(b) and Figure 12(d), it can be seen that there no obvious increase in precipitated phase in hot metal as the vanadium content is 0.10 wt.%, whereas the precipitated phase gather together and increase remarkably as the vanadium content is 0.30 wt.%. This can explain the phenomenon that the viscosity of the hot metal increased slightly when the vanadium content is less than 0.30 wt.%. Therefore, the increase in the viscosity of hot metal was mainly attributed to the aggregation of the precipitated high melting point phase.

Figure 13 shows the observation results of the hot metal sample containing 0.10 wt.% and 0.30 wt.% titanium content, respectively. It can be seen that the precipitation temperature of the high melting point phase gradually increases as the titanium content increases. The precipitation temperature of the high melting point phase is 1431°C as the titanium content is 0.10 wt.%, while it increased to 1465°C as the titanium content increases to 0.30 wt.%. Additionally, from Figure 13(b) and (d), it can be seen that the precipitation of the high melting point phase gradually increases and its size also gradually increases with the decrease in temperature. According to the results of the thermodynamic calculations, the precipitation temperature and amount of TiC and TiN increases as the titanium content increases. Simultaneously, the precipitation temperature and amount of VC and VN also increase with the increasing of titanium content because the interaction coefficient of titanium to vanadium is greater than 0. Therefore, as the titanium content increases, the precipitated phase in the hot metal may not only contains the TiC and TiN but also the VC and VN.

To confirm the composition of the precipitated phase in the hot metal, a SEM-EDS analysis was carried out after the sample was cooled down in the argon atmosphere. Figure 14 illustrates the SEM-EDS results of the sample. It can be observed that the precipitated phases in hot metal contain mainly the long stripes and irregularly shaped crystals, which may be attributed to the different cooling condition in different position of the sample. The EDS analysis of points #1, #2 and #4 in Figure 14 (a) and (b) shows that the main elements in these three points contains the C, Ti, V and Fe, indicating that the precipitated phase in these three points are mainly the TiC and VC. Additionally, the EDS analysis of points #3 shows that the main elements in this point are C, N, Ti, V and Fe, implying that the precipitated phase could be composed of TiC, TiN, VC and VN. According to the theoretical calculation, TiN is the first phase to precipitate as the temperature decreases. Therefore, after the precipitated TiN is present in the hot metal, it will provide a nucleation core for the growth of TiC, VC and VN and promote the precipitation of the other phases, which will eventually result in the precipitation of (V, Ti)@(C, N) in the hot metal.

Conclusions

In this paper, the precipitation behaviour of (V, Ti)@(C, N) compounds in hot metal was investigated. Some conclusions were drawn as follows:

The theoretical precipitation temperature of (V, Ti)@(C, N) compounds in hot metal was investigated employing the thermodynamic analysis. As the content of titanium, vanadium, silicon and manganese in hot metal increases, the precipitation temperature of the (V, Ti)@(C, N) compounds increases. On the contrary, the precipitation temperature of the (V, Ti)@(C, N) compounds decreases as the sulphur and phosphorus in hot metal increases.

The viscosity of the hot metal gradually increased as the vanadium and titanium content increased. As the vanadium content is less than 0.30 wt.%, the effect of vanadium content on the viscosity of the hot metal is not significant; however, as the vanadium content is greater than 0.30 wt.%, the viscosity of the hot metal increases sharply with the increase in vanadium content.

The composition of the precipitated phase mainly contains the C, N, Ti, V and Fe, implying that the precipitated phase could be composed of TiC, TiN, VC and VN. Additionally, the (V, Ti)@(C, N) compounds starts to precipitate in hot metal as the temperature is lowered to 1400–1500°C, which leads to a significant increase of the melt viscosity.

Footnotes

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. U2003215).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China, (grant number No. U2003215).

ORCID iDs

Wenzhou Yu

Meilong Hu

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