Direct Evidence for Phase Transition Process of VC Precipitation from (Fe,V)₃C in Low-Temperature V-Bearing Molten Iron

Abstract

V-bearing molten iron was obtained by adding Na₂CO₃ in the smelting process of vanadium titanomagnetite at low temperature. Two forms of V-rich carbides ((Fe,V)₃C, VC) were detected in the V-bearing pig iron products. Once the smelting temperature was above 1300 °C, most of the V in the raw ore was reduced into molten iron. Owning to the high content of V, the unsteady (Fe,V)₃C solid solution decomposed along with the precipitation of graphite and VC during the solidification process. The presence of VC cluster and VC precursor in (Fe,V)₃C was detected by transmission electron microscopy, which confirmed the possibility of this transition process at the atomic perspective. The transformation dramatically affected the compositions and properties of V-bearing pig iron and had important guiding significance for the actual production process.

1. Introduction

Vanadium is an important strategic metal. In 2021, 89% of the global vanadium output came from vanadium titanomagnetite [1]. V-bearing molten iron is a key intermediate product, and its composition mainly depends on the raw material and smelting process. Under traditional blast furnace smelting conditions, the reduction temperature is about 1500 °C [2,3]. A large number of high-melting-point carbides such as TiC, TiN, VC, and VN are formed in the molten iron [4], leading to a series of production difficulties and quality problems [5,6,7]. Wang [8] studied the thermodynamic properties such as the solubility and activity coefficient of V and Ti elements in the molten iron. He [9] and Zhang [10] revealed the influence mechanism of molten iron fluidity by studying viscous flow properties focusing on the carbides of titanium. Hou [11] found through thermodynamic calculation that the precipitation amount of VC and VN in V-bearing molten iron increased sharply at 1200–1250 °C, which was consistent with the change trend of hot metal viscosity. The formation of V-rich carbonitride particles was observed by a confocal laser scanning high-temperature microscope. However, the precipitation process of V-rich carbides has not been thoroughly studied.

The research on carbides of vanadium was mainly concentrated in the field of high-V medium-/low-carbon steel [12,13,14], focusing on the morphology and distribution of carbides in the alloy structure and its modification control measures. Kesri [15] studied the phase diagram of Fe–C–V ternary system and determined the thermodynamic stable regions of four phases: austenite (A, γ-Fe ), ferrite (F, α-Fe), VC_x type carbide, and M₃C type carbide in the solidification process, in which the equilibrium temperature between the VC_x and M₃C stable regions was 1120 °C. Kawalec [16] and Fras [17] calculated the eutectic transformation process of VC in white cast iron, proposed the concept of the degree of eutectic saturation (S_c) in high-vanadium cast iron, and characterized the microstructure of eutectic VC. Recently, microalloyed steel with Nb, V, and Ti has received more and more attention with the increasing demand for high-quality steel [18,19]. The precipitation mechanism of the second-phase particles and their precipitation strengthening effect on the alloy structure have been studied extensively [20,21]. Pan [22] found five forms of V carbides in V-microalloyed medium-carbon steel. Liu [23] studied the effect of V content on VC precipitation in PD3 steel. Sayed [24] studied the effect of the heat-treatment parameters on the random vanadium precipitation in low-carbon steel. Javier [25] proposed a model to simulate the precipitation of V(CN). Li [26] revealed the hetero-nucleation effect of V(CN) during γ→α phase change. Wang [27] observed an intermediate coherent crystal structure within the α-Fe matrix through TEM, revealing a two-step nucleation and growth mechanism of V(CN) in microalloyed steels. However, all the above studies focused on the eutectoid process of steel. The precipitation process and mechanism of VC in V-bearing molten iron in the solidification process have not been fully explored.

Recently, a novel low-temperature smelting technology was developed by adding soda [28] or alkali [29] in the reduction smelting process of vanadium titanomagnetite. Chen’s research [30,31] showed that the oxide components in the slag were transformed into highly active sodium salt. This slag system has a low melting point and high fluidity, and it displays excellent impurity removal performance. Thus, this process can produce high-quality V-bearing molten iron at 1150–1300 °C. However, this molten iron has its own unique properties to be further investigated due to the existence of V-rich carbides. In this paper, V-bearing pig iron with different compositions was prepared. The morphology and structure of V-rich carbides and their precipitation characteristics in the pig iron structures were studied. This study is expected to provide technical support for the industrial practice of low-temperature smelting of vanadium titanomagnetite and the high-value utilization of V-bearing pig iron.

2. Experiment

2.1. Raw Materials and Experimental Procedures

The vanadium titanomagnetite used in the experiment came from a mine in Chaoyang, Liaoning Province, China (composition as shown in

Table 1. Chemical composition of vanadium titanomagnetite concentrate (wt.%).

TFe	TiO2	SiO2	CaO	Al2O3	V2O5	MgO	MnO	S	P
47.97	19.32	6.59	2.98	2.29	1.46	0.70	0.41	0.028	0.011

, average particle size 48 μm); the reducing agent anthracite (composition as shown in

Table 2. Chemical composition of anthracite (wt.%).

FCad	Vdaf	Mad	St	P	Ad
					SiO2	Al2O3	CaO	Fe2O3	TiO2	Na2O	MnO	MgO	Total
81.87	6.32	1.66	0.35	0.028	6.93	2.79	0.95	0.43	0.33	0.21	0.1	0.06	11.81

, crushed and screened to below 150 μm) and anhydrous Na₂CO₃ (AR, 99.8 wt%) were purchased from the Sinopharm Chemical Reagents Co., Ltd.(Shanghai, China) The smelting process was carried out in a box-type resistance furnace (Tianjin Zhonghuan Electric Furnace Co., Ltd., Tianjin, China, model: SX-D64163).

Firstly, raw ore, anhydrous Na₂CO₃, and anthracite were mixed in a ratio of 10:6:3 and placed in a graphite clay crucible. When the temperature of the furnace rose to the setting value, it was held on for 10 min, and then the graphite clay crucible was quickly moved into the furnace chamber. The reaction timing began until the temperature stabilized again to the setting value. After the reaction, the crucible was taken out and cooled in the air. After cooling to room temperature, pig iron samples were separated from the slag for analysis. The flow sheet of the smelting process of vanadium titanomagnetite is shown in Figure 1.

By adding auxiliaries such as Na₂CO₃ or NaOH in the smelting process, high-quality V-bearing molten iron with a low content of Ti (<0.03 wt.%), Si (<0.03 wt.%), S (<0.003 wt.%), and P (<0.005 wt.%) was produced. In view of the fact that the contents of C (4–5 wt.%) and V (0.3–2 wt.%) elements in the pig iron, the low-temperature property of the molten iron was mainly affected by these two elements due to the formation of V-rich carbides with a high melting point.

2.2. Theory Background

The Fe–C phase diagram has been studied quite thoroughly [32,33]. However, it is still controversial in some aspects, such as the specific temperature point and C content of the eutectic and eutectoid transformation lines. In general [34,35,36], the highest content of C in the F phase is 0.0218 wt.%, in the A phase is 2.11 wt.%, and in cementite (Fe₃C) is 6.69 wt.%.

The natural cooling rate is very fast in laboratory conditions. For hypereutectic white cast iron (wt.% (C) = 4.3–6.69%), the phase transition follows the diagram of metastable equilibrium Fe–Fe₃C [33,36]. Firstly, primary cementate (Fe₃C_I) is crystallized from the liquid phase. Then, the isothermal eutectic transformation occurs at the eutectic point (1148 °C, wt.% (C) = 4.3%). Eutectic A phase is precipitated with eutectic Fe₃C; these two phase form ledeburite cast (Ld), and its form at room temperature is L’d. After the liquid phase disappears, the secondary cementate (Fe₃C_II) is precipitated from the A phase. Upon approaching the eutectoid point (723 °C, wt.% (C) = 0.77%), the eutectoid F phase is precipitated with eutectoid Fe₃C. These two phases form pearlite (P). Lastly, the third cementate (Fe₃C_III) is precipitated from the F phase. Therefore, the equilibrium microstructure of cast iron at room temperature is as follows: primary cementate (Fe₃C_I) + L’d (eutectic Fe₃C + Fe₃C_II + P (eutectoid Fe₃C + Fe₃C_III + F)).

Fe₃C is an interstitial solid solution with the broad homogeneity range [35,36]. There are still some aspects under be explored, especially the carbide section of the diagram. The situation becomes even more confusing when the V element is present in the system.

2.3. Test Method

The carbon content in pig iron was determined using a CS-2800 carbon sulfur analyzer (NCS Testing Technology CO., Ltd., Beijing, China). The content of vanadium in pig iron was determined by high-sensitivity XRF (PHECDA-PRO, Beijing Ancoren Technology Co., Ltd., Beijing, China). The morphology and composition of the various phases in the pig iron samples were analyzed using a mineral dissociation analyzer (MLA250, FEI Company, Hillsboro, Oregon, United States). Furthermore, the pig iron was thinned by focused ion beam scanning electron microscopy (Helios G4 PFIB, Thermo Scientific™, Waltham, MA, USA), and the fine structure of the V-rich carbides and pig iron matrix was characterized by high-resolution field-emission transmission electron microscopy (JEM-F200, JEOL, Tokyo, Japan).

3. Results and Discussion

3.1. C and V Content of the Pig Iron

Several samples of V-bearing pig iron were prepared under different smelting conditions. The effects of different melting temperature and reaction time on the contents of C and V elements in the samples were investigated. The results are shown in Figure 2.

As shown in Figure 2a, the C content in pig iron at different melting temperatures had little change with an average content of about 4.85 wt.%. However, the V content in pig iron increased with the increase in temperature. The V content was 0.70 wt.% at 1200 °C and 1.16 wt.% at 1300 °C.

The variation trend of V content in pig iron was almost consistent with that of C in Figure 2b. When the smelting temperature was 1300 °C, the iron was separated with the slag at 20 min, the content of C was 3.94 wt.%, and the content of V was only 0.11 wt.%. Then, the content of C and V increased rapidly. After 60 min of reaction, the content of C and V tended to change gently, and the content of C and V remained at about 4.8 wt.% and 1.1 wt.%, respectively. When the smelting temperature was 1200 °C, it took about 45 min for the separation of iron and slag to be completed. At this time, the content of V was 0.5 wt.%, and then it slowly increased to 0.7–0.9%.

It could be inferred that the solubility of V element in pig iron was greatly affected by the C content. The significant difference in V content in pig iron at 1200 °C and 1300 °C may be due to the transformation of the V-rich carbides in pig iron.

3.2. Morphology and Composition

The content of C was different in various phases; thus, it could be inferred that the content of V in different pig iron structures was also changing. For hypereutectic white cast iron, it depended on its solubility in the Fe₃C phase and the F phase, as well as the proportion of the two phases in the structure. The microstructure of V-bearing pig iron can be further analyzed in Figure 3.

As shown in Figure 3a, V-bearing pig iron at 1200 °C was mainly composed of the dark-gray long-strip Fe₃C_I phase and light and dark L’d phase. The bright color phase was the P phase, and the gray phase was a mixture of eutectic Fe₃C and Fe₃C_II. There were dark-gray particles in different L’d phases. The L’d structure almost disappeared at 1300 °C (Figure 3b), and there appeared a blacker fine flake graphite phase (G) which separated the iron matrix, while the P phase region expanded substantially. The original eutectic Fe₃C region was transformed into a large number of gray worm-like and rod-shaped particles with centripetal converging distribution, while the dark-gray cube particles were in the core. Figure 3c showed the appearance of large flake G-phase and large cube particles over 100 μm at 1400 °C. The number of gray worm-like and rod-shaped particles increased, and the distribution was more regular, indicating that the phase precipitation had better crystallinity with the increase in the smelting temperature. The components of different phases in pig iron at 1200 °C were analyzed by EDS point scanning as shown in Figure 4 and

Table 3. EDS results of the V-bearing pig iron at 1200 °C.

Element (wt.%)	1#	2#	3#	4#	5#	6#
Fe	85.82	86.94	91.68	88.78	91.38	92.26
C	12.00	11.16	8.00	9.54	8.11	7.44
V	2.18	1.92	0.33	1.68	0.51	0.31

.

The results showed that the highest content of V was 2.18 wt.% (Figure 4a), which existed in the Fe₃C_I, while the lowest V content in P was 0.31 wt.% (Figure 4c). Although EDS results had a large deviation, it was obvious that the Fe₃C phase could enrich V, and the solubility of V in the A phase was limited. Assuming that all Fe₃C had the same V content, it could be inferred that the V content in F phase was very low; the value reported in the literature [22] was only 0.09 wt.%.

According to the relevant theories in Section 2.2, Fe₃C_I was the first phase precipitated from the V-bearing molten iron; accordingly, it had the highest content of V. Then the V content in the subsequent precipitated eutectic Fe₃C gradually decreased due to the insufficient content of V. According to the lever rule, the content of Fe₃C_I in the V-bearing pig iron is presented in Equation (1).

$$w_{F{\mathrm{e}}_3{C}_I}=\frac{4.85-4.3}{6.69-4.3}=23\%.$$

(1)

Then, the LD phase accounted for 87%. The theoretical content of eutectic Fe₃C precipitated from LD is presented in Equation (2).

$$w_{F{\mathrm{e}}_{3}C}=\frac{4.3}{6.69}\times 87\%=55.9\%.$$

(2)

Theoretically, for this hypereutectic white cast iron product, the content of Fe₃C phase was 78.9% and that of F phase was 21.1%. The chemical formula of this V-rich Fe₃C could be written as (Fe_1−x,V_x)₃C, (0 ≤ x ≤ 1) (for short, (Fe,V)₃C).

The components of different phases in pig iron at 1300 °C were analyzed by EDS point scanning and mapping scanning, as shown in Figure 5 and

Table 4. EDS results of the V-bearing pig iron at 1300 °C.

Element (wt.%)	1#	2#	3#	4#
Fe	2.03	32.94	/	91.26
C	13.83	21	100	8.04
N	6.92	/	/	/
V	35.70	43.09	/	0.70
Ti	41.51	2.97	/	/

.

In Figure 5a, the content of V and Ti in regular cube particles was 35.70 wt.% and 41.51 wt.%, respectively. These secondary phase particles were (Ti,V)(C,N). The worm-like and rod-like gray particles were mainly composed of VC, with a small amount of Ti. Due to the small size of the particle, EDS scanning could inevitably identify a relatively high content of matrix Fe. After removing Fe, the content of V in VC phase reached 65 wt.%. Considering that there was no V in the G phase and (Ti,V)(C,N) particles were few, the precipitation of VC almost enriched the majority of V from the molten iron.

By further comparison of Figure 3a,c, it can be seen that the eutectic (Fe,V)₃C region originally in L’d was greatly reduced, while a large amount of VC phase appeared in the gap of the G phase, indicating that (Fe,V)₃C solid solution was in a kind of metastable phase, which would decompose into the G phase and generate VC under certain conditions. The morphology of VC was consistent with the literature [15,16,17,18]. It also showed that excessive element V, a strongly graphitized element, could reduce the stability of Fe₃C.

To sum up, the V content in molten iron was low below 1300 °C. The liquid iron had good fluidity, and the impurity content was very low. The obtained pig iron was mainly composed of (Fe,V)₃C and P phase. Above 1300 °C, the V content in molten iron increased by more than 1 wt.%; (Fe,V)₃C would lost its stability and decomposed along with the precipitation of graphite and VC during the solidification process.

3.3. Interface Structures and Phase Transformation Process

High-resolution structures of pig iron were detected by FIB-TEM. Figure 6a shows the FIB section of the pig iron sample. In Figure 6b, it can be seen that the V-bearing pig iron structure contained three phases: (Fe,V)₃C, VC, and F.

Figure 6c shows the α-Fe matrix and eutectic Fe₃C in the P phase. According to the above analysis, the eutectoid Fe₃C was also enriched with V. Surprisingly, the atomic structure of the (Fe,V)₃C solid solution was perfectly preserved due to the eutectoid transition from A to P being a solid phase transition process. We can clearly see the obvious interface undulating transition layer in the Figure 6d,e.

Figure 6f shows the high-resolution structure of VC phases, which were full of various microdefects, microtwins, semi-colattice nanostructures, and colattice nanoparticles. Figure 6g,h shows the interface structure of VC and F phases. It can be seen that there were small fluctuations of about 3–4 atomic layers thickness at the interface, from which the mismatches of semi-coherent interface micro-regions and noncoherent dislocations can be seem. The VC and A phases were eutectic precipitated from the (Fe,V)₃C matrix. When the temperature was above 723 °C, both phases had an FCC structure. Therefore, the interface should be a coherent or semi-coherent interface. When the temperature was below 723 °C, the A phase transformed into the F phase. Its structure changed from FCC to BCC, during which the atomic rearrangement led to the mismatches of the interfacial lattice. As can be seen from the Figure 6h, there was a difference of 0.005 nm in the crystal plane spacing between VC and F phase; thus, it showed periodic mismatches, forming a stepwise dislocation.

As shown in Figure 6I, there were many vacancies and defects in the complex orthogonal lattice structure of Fe₃C. When the V atoms were solidly dissolved into the Fe₃C lattice, they would partially displace the Fe atom and formed (Fe,V)₃C solid solution. In Figure 6II, we can see the VC cluster structure formed by more than six V atoms converging together. The VC precursor crystal nucleus is shown in Figure 6III. Figure 6IV shows the local structure of VC particles. The process of VC precipitation from (Fe,V)₃C is clearly displayed at the atomic level through the TEM images.

The clear images of the transition process provided by TEM helped in explaining the whole process of VC phase precipitation from the (Fe,V)₃C phase during the solidification of the V-bearing molten iron.

(a)

The nucleation induction stage. With the decrease in the temperature, the first crystallized (Fe,V)₃C_I phase was unstable. It decomposed into large G_I flakes and released V element into the liquid phase. As a result, C content decreased while V content increased in the adjacent liquid phase. Then, the A phase was precipitated out of the G_I phase as the heterogeneous nucleation interface [37]. C and V elements were released into the adjacent liquid phase due to their low solubility in A phase, which then induced the formation of (Fe,V)₃C_I and further decomposed into G_I.

(b)

The nucleation pregnant stage. With the continuous progress of the process (1), the content of C in the residual liquid phase dropped to the eutectic component, while the content of V element increased and dissolved into the eutectic (Fe,V)₃C phase. The size effect caused lattice distortion with the increase in solid solubility, leading to a gradual decrease in the stability of the (Fe,V)₃C phase. Then, many VC cluster structures were formed due to the fluctuation of V concentration and energy.

(c)

The nucleus formation stage. When the total distortion energy reached the critical value, these clusters first adsorbed to the A phase matrix interface under induction of the FCC structure. Then, they further absorbed the adjacent VC clusters. Finally, they grew into a VC precursor nucleus which started from the A phase matrix and protruded into the (Fe,V)₃C phase. For the eutectoid process, due to the difficulty of diffusion in the solid phase transition process and the absence of redundant V atoms, the VC precursor crystal nucleus could not be further grown; finally, the two-phase undulating interface structure was formed. Wang [27] made a similar discovery in V-microalloyed steels.

(d)

The precipitation and growth stage. For the eutectic process, VC precursor crystal nuclei further converged and grew, forming the VC primary crystal nuclei. With the progress of the reaction, the content of C and V elements adjacent to the crystal nuclei decreased, while the content outside was higher, thus forming a concentration gradient of C and V elements decreased to the grain center. Therefore, the growth direction of VC deviated from the center of the sphere. Under the restriction of the separation of the A phase and G phase, the VC phase was precipitated continuously and finally formed a radiating eutectic cell [13]. If there were (Ti,V)(C,N) second-phase particles or other impurity particles precipitated earlier in the molten iron, they would become the core of VC eutectic cell as the heterogeneous nucleated crystal species.

In this process, the large precipitation of (Fe,V)₃C_I and VC with a high melting point led to poor fluidity of the V-bearing molten iron. On the surface of molten iron, (Fe,V)₃C_I decomposed and a large amount of G_I precipitated due to the rapid temperature drop. The eutectic precipitation temperature of the G phase was 1154 °C [32], and the solidification temperature of the molten iron actually increased. Furthermore, G burned and released a large amount of CO₂ after contact with air, resulting in huge pores on the surface of cast iron and large number of small pores in the interior. This seriously affected the composition and quality of the pig iron.

4. Conclusions

V-bearing molten iron was obtained by adding Na₂CO₃ in the smelting process of vanadium titanomagnetite at low temperature. The morphology and structure of V-rich carbides in V-bearing pig iron were studied, and the precipitation characteristics of V-rich carbides in molten iron were discussed.

(1)

Vanadium trended to form carbide in iron and steel structure. There were two main forms of V-rich carbides in low-temperature V-bearing pig iron: (Fe,V)₃C solid solution and VC solid solution.

(2)

The microstructure of pig iron and the existence form of V-rich carbides changed at different melting temperatures. In the temperature range of 1150 °C to 1250 °C, the metallographic morphology were P and (Fe,V)₃C phase. When the temperature exceeded 1300 °C, the P, G, and VC phases were mainly present.

(3)

When V atoms were solidly dissolved into the Fe₃C lattice, they partially displaced the Fe atoms and formed (Fe,V)₃C solid solution. In the eutectic transition process, the supersaturated V element destroyed the stability of (Fe,V)₃C and promoted the precipitation of G and VC. The existence of VC clusters and VC precursor nuclei were observed by TEM, which confirmed the possibility of this transition process at the atomic perspective.

Above 1300 °C, the rapid crystallization of (Fe,V)₃C and its transition to the G and VC phases led to an increase in viscosity and poor fluidity of the V-bearing molten iron, resulting in a large number of porosity defects. The process seriously affects the composition and quality of the pig iron product. Therefore, it is recommended that the melting temperature should be controlled at no more than 1300 °C.