Evolution and Formation of Non-Metallic Inclusions during Electroslag Remelting of Ce-Bearing 15Cr-22Ni-1Nb Austenitic Heat-Resistant Steel

Abstract

The evolution of inclusions in austenitic heat-resistant steel with different Ce content during protective argon gas atmosphere electroslag remelting (ESR) was studied. All oxide inclusions in the Ce-free consumable electrode are MgO·Al₂O₃. A part of these MgO·Al₂O₃ inclusions was removed before metal droplets entered the liquid metal pool during the ESR. The soluble oxygen (arising from the reoxidation) reacted with soluble aluminum, calcium, and magnesium in liquid steel to form MgO·Al₂O₃ and CaO–Al₂O₃ inclusions in liquid steel. All oxide inclusions in the electrode with 0.016 mass% Ce are Ce₂O₂S. A portion of these Ce₂O₂S inclusions was dissociated into soluble oxygen, cerium, and sulfur in liquid steel during the ESR process, whereas the others were removed by absorbing them into molten slag. The oxide inclusions in the liquid metal pool and remelted ingot were Ce₂O₃, CeAlO₃, and Ce₂O₂S. The CeAlO₃ and Ce₂O₃ inclusions were reoxidation products formed by the chemical reaction between the soluble oxygen, soluble aluminum, and cerium. The oxide inclusions in the electrode with 0.300 mass% Ce are CeS and Ce₂O₂S. These CeS inclusions were removed by molten slag adsorption during the ESR. A part of these Ce₂O₂S inclusions was removed by slag adsorption, and the remaining entered into the liquid metal pool. The oxide inclusions in the liquid metal pool and the ingot were Ce₂O₃ and Ce₂O₂S. The Ce₂O₃ inclusions were formed through the chemical reaction between the soluble oxygen and cerium in the liquid metal pool. The Ce₂O₂S inclusions in the liquid pool originate from reoxidation products during the ESR process and the relics from the electrode.

1. Introduction

Austenitic heat-resistant steel of Ni-Cr system is extensively utilized in automotive engines and fossil fuel power generation plants applications at moderately elevated temperatures, such as fasteners and bolts, and heat exchangers, because of their superior strength, toughness, oxidation resistance, and organizational stability [1,2,3]. Except the advantages of some types of fine non-metallic inclusions through acting as the nucleation sites to generate intragranular acicular ferrites and the pinning particles to inhibit the growth of austenite grains [4], inclusions are generally detrimental to mechanical properties of steel, such as creep strength, fatigue strength, corrosion resistance, tensile properties, toughness, and ductility [5,6,7,8,9,10]. The local stress concentration induced by large inclusions often causes cracking at inclusion-steel matrix interface during working of steel [11]. The detriment degree of inclusions is strongly dependent on their number density, size, and chemical compositions.

Rare earth Ce can improve the corrosion resistance [12,13], oxidation resistance [14,15], grain refinement [16,17] and reduction of dendritic spacing [18,19] of heat-resistant steels and improve the mechanical properties of austenitic heat-resistant steels [20,21]. The generation of Ce-bearing non-metallic inclusions in Ce-containing steel was unavoidable. Ce-containing inclusions were detrimental to the fatigue strength, toughness and ductility, impact strength, tensile properties, corrosion resistance, initiation of fatigue cracking, and stress concentration cracking of steel [22,23,24]. Many studies on the evolution of Ce-bearing inclusions have been conducted in industrial-scale secondary refining (LF and RH refining) or laboratory-scale crucible experiments of slag-steel reactions [25,26]. However, no studies have been reported on the evolution of Ce-bearing inclusions in the ESR process.

In the current study, the evolution of inclusions in the steel with different Ce contents during ESR was investigated. The effect of reoxidation during the ESR on inclusion evolution was ascertained. The evolution mechanism of Ce-bearing inclusions during the ESR was elucidated.

2. Experimental

2.1. ESR Experimental Procedure

The chemical compositions of the consumable electrodes are shown in

Table 1. Chemical compositions of the consumable electrodes (mass%).

Trials No.	C	Si	Mn	Mo	Ti	Cr	Ni	Al	Nb	S	Ca	Ce	Mg	N	O
T1	0.02	0.5	0.83	2.49	0.18	15.1	22	0.15	0.98	0.0032	0.0002	0	0.0004	0.0071	0.0010
T2	0.02	0.5	0.89	2.53	0.14	14.9	22	0.10	0.99	0.0009	0.0002	0.016	0.0003	0.0120	0.0006
T3	0.02	0.5	0.78	2.50	0.21	14.6	22	0.15	1.03	0.0025	0.0002	0.300	0.0003	0.0031	0.0007

. The oxide scale on the electrode steel surface was thoroughly removed before ESR experiments. The pre-melted slag (40.92 mass% CaF₂, 27.09 mass% CaO, 0.52 mass% MgO, 29.02 mass% Al₂O₃, 0.92 mass% SiO₂, 0.21 mass% FeO) was roasted at 700 °C (973 K) for 8 h to remove moisture before the ESR trials.

Three ESR trials were conducted in protective argon gas atmosphere. The operating current, voltage, and outlet temperature of the mold cooling water were maintained at about 2500 A, 32 V, and 313 k. During electroslag remelting of the steel electrode with 0.016 mass% Ce, Al shots were added for deoxidation (addition amount: 1 kg/ton of steel). For deoxidation in ESR trial T3, Al shots were added (2 kg/ton of steel) during the ESR of the electrode with 0.300 mass% Ce. A steel sample was taken from the liquid metal pool during the ESR refining using a vacuum sampling tube made of quartz, followed by quenching in water. The remelted ingots corresponding to the ESR trials T1, T2, and T3 were designated as ingots C1, C2, and C3, respectively.

2.2. Compositional Analysis and Inclusions Characterization

The steel samples were taken from the consumable electrodes and the remelted ingots of the ESR for chemical composition analysis. The total oxygen (composed of both the soluble oxygen and the oxygen bonded as oxide inclusions) content of the steel was measured by the inert gas fusion-infrared absorptiometry. The sulfur and carbon contents of the steel were measured by the combustion-infrared absorption technique. The nitrogen content was determined by the inert gas fusion-thermal conductivity method. The contents of soluble aluminum, total calcium, magnesium, silicon, cerium, and manganese in the electrodes and ingots were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

Steel samples for metallographic observation were taken the consumable electrodes and the mid-height of each remelted ingot at the mid-radius position. To reveal the evolution of inclusions in the ESR process, steel samples collected from the liquid metal pool with vacuum sampling tubes were polished after mechanical grinding. Inclusions exposed on the cross-section of the polished sample were analyzed in terms of their chemistry, size, and morphology by scanning electron microscope (SEM; FEI Quanta-250, FEI Corp, Hillsboro, OR, USA) equipped with an energy-dispersive X-Ray spectrometer (EDS, XFlash 5030; Bruker, Karlsruhe, Germany).

Furthermore, the size of inclusions on the polished sample cross-section was calculated using the equivalent circle diameter (ECD). The equivalent circle diameter of the inclusions smaller than 1 μm is not included in analysis.

3. Results and Discussion

3.1. Steel Composition

The chemical compositions of remelted ingots are shown in

Table 2. Chemical Compositions of Remelted Ingots (mass%).

Ingot No.	C	Si	Mn	Mo	Ti	Cr	Ni	Al	Nb	S	Ca	Ce	Mg	N	O
C1	0.02	0.5	0.82	2.60	0.13	15.5	22	0.19	1.00	0.0007	0.0002	0	0.0004	0.0076	0.0016
C2	0.02	0.5	0.89	2.60	0.12	15.4	22	0.23	1.00	0.0007	0.0002	0.0055	0.0003	0.0140	0.0013
C3	0.02	0.5	0.83	2.50	0.15	14.6	22	0.15	1.02	0.0016	0.0002	0.0630	0.0003	0.0054	0.0018

. The Ce contents of the remelted ingots C2 and C3 are 0.0055 mass% and 0.063 mass%, respectively. The yields of Ce during the ESR are 34.4% and 21.0%, respectively.

The aluminum content of the steel increases from 0.15 mass% in the Ce-free electrode to 0.19 mass% in the remelted ingot C1. The aluminum content of steel in the ESR trial T2 increases from 0.10 mass% in the consumable electrode to 0.23 mass% in the ingot C2. The aluminum content of the steel in the ESR trial T3.

The slag-steel reactions during the ESR are a potential source of soluble aluminum pickup in liquid steel. The chemical reaction between Ce in liquid steel and Al₂O₃ in the slag can be expressed as follows [27]:

2[Ce] + (Al₂O₃)_slag = (Ce₂O₃)_slag + 2[Al]

(1)

$$\Delta G^{\mathrm{o}}=205703+34T (\mathrm{J}/\mathrm{mol})$$

(2)

$$K=\frac{a_{\mathrm{Al}}^2\cdot a_{\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_3}}{a_{\mathrm{Ce}}^2\cdot a_{\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3}}=\frac{{(f_{\mathrm{Al}}[\%\mathrm{Al}])}^2\cdot a_{\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_3}}{{(f_{\mathrm{Ce}}[\%\mathrm{Ce}])}^2\cdot a_{{\mathrm{Al}}_2{\mathrm{O}}_3}}$$

(3)

where $a_{\mathrm{C}{\mathrm{e}}_2{\mathrm{O}}_3}$ and $a_{\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3}$ are the activities of Ce₂O₃ and Al₂O₃ in the slag, respectively. $f_{\mathrm{Al}}$ and $f_{\mathrm{Ce}}$ are the activity coefficients of soluble aluminum and Ce in the liquid alloy, respectively, which can be calculated by the following equation [28].

$$\lg f_i={\displaystyle \sum e_i^j[\%j]}$$

(4)

where $e_i^j$ is the first-order interaction parameter.

The activities of the Al₂O₃ and Ce₂O₃ relative to pure solid standard states in the slag melts at 1873 K (1600 °C) are estimated with FactSage 8.0 (FToxid database; ThermFact/CRCT, Montreal, Canada). According to the chemical composition of the electrode with 0.300 mass% Ce combined with the first-order interaction parameters (as shown in

Table 3. First-order interaction parameters used in the current study [29,30,31,32].

	C	Si	Mn	Mo	Ti	Cr	Ni	Al	Mg	Ca	S	Ce	N	O
O	−0.421	−0.066	−0.021	0.005	−1.8	−0.033	0.006	−1.17	−300	-	−0.133	−0.57	0.057	−0.17
S	0.111	0.075	−0.026	-	−0.27	−0.0105	-	0.041	−1.82	−110	−0.046	−1.91	0.01	−0.27
Al	0.091	0.056	0.035	-	0.004	-	−0.029	0.045	−0.3	−0.047	0.035	−0.52	−0.057	−1.98
Ce	−0.077	-	-	-	−3.62	-	-	−2.25	-	-	−8.36	−0.003	−6.612	−5.03

), the Gibbs free energy change for the chemical reaction (1) was calculated to be 312.89 kJ/mol. It suggests that Ce in the electrode with 0.300 mass% Ce cannot reduce the Al₂O₃ in the slag during the ESR process, leading to the Al pickup in the steel. The Al pickup in liquid steel is caused by the Al addition in the ESR process.

In the present study, the oxygen contents of the electrodes with different Ce contents apparently increase during the ESR to 0.0016 mass%, 0.0013 mass%, and 0.0018 mass% in ingots C1, C2, and C3, respectively. It indicates that the reoxidation of the liquid steel occurred in these three ESR trials. The sulfur contents of steel decrease during the ESR to 0.0007 mass%, 0.0007 mass%, and 0.0016 mass% in ingots C1, C2, and C3, respectively.

The difference in the oxygen potential between liquid steel with low oxygen content and the molten slag contributes to soluble oxygen pickup in liquid steel during the protective argon gas atmosphere ESR [33]. The introduction of FeO (newly formed FeO during ESR and the unremoved oxide layer on the surface of the electrode steel) to the molten slag pool during ESR is difficult to be prevented, and it is the source of the increased oxygen content of liquid steel according to the chemical reaction (FeO)_slag = [Fe] + [O] (ΔG^o = −139,000 + 57.1T (J/mol)) [34].

The interaction coefficients used in the present calculation are shown in Table 3. The Gibbs free energy change for the chemical reaction (FeO)_slag = [Fe] + [O] for the ESR trials T1, T2, and T3 was calculated to be −102.75 kJ/mL, −105.99 kJ/mL, and −100.92 kJ/mL, respectively. It indicates that FeO in molten slag transfers oxygen to liquid steel, resulting in the reoxidation of liquid steel during ESR. Meanwhile, the desulfurization reaction (CaO) + [S] = (CaS)_slag + [O] is also an important source of soluble oxygen pickup in the liquid steel during the ESR [35].

3.2. Inclusions in Consumable Steel Electrode

The SEM images and EDS spectra of typical inclusions in the consumable electrodes are shown in Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5. As shown in Figure 1, the typical inclusions in the Ce-free electrode are (Ti,Nb)N, MgO·Al₂O₃ inclusions surrounded by (Ti,Nb)N and MgO·Al₂O₃ inclusions, respectively. Figure 1a shows typical (Ti,Nb)N inclusions in the electrode with near-square shape. Figure 1c,d show typical MgO·Al₂O₃ inclusions in Ce-free electrode, and the morphology of these spinel inclusions is nearly spherical. The MgO·Al₂O₃ spinel inclusion plays a nucleation role for (Ti,Nb)N.

Figure 2 and Figure 3 show the typical inclusions in the electrode with 0.016 mass% Ce. The observed inclusions are (Ti,Nb)N, and Ce₂O₂S, respectively. Figure 2a shows typical inclusions (Ti,Nb)N in the consumable electrode with a clear angular shape. Figure 2b,c show the typical inclusions of Ce₂O₂S in the consumable electrode, showing both near spherical or irregular morphologies.

The typical inclusions in the electrode with 0.300 mass% Ce are shown in Figure 4 and Figure 5. The rare-earth inclusions in the electrode are identified as CeS and Ce₂O₂S, respectively. It can be seen from SEM photographs that these inclusions are near-spherical. It is found from ESD element mappings of the Ce₂O₂S inclusion that the elements are uniformly distributed on cross-section of the inclusions.

3.3. Inclusions in the Liquid Metal Pool and Remelted Ingots

The SEM micrographs, EDS spectra, and elemental mappings of typical inclusions observed in the liquid metal pool during the ESR trial T1 and remelted ingot C1 are shown in Figure 6, Figure 7, Figure 8 and Figure 9. The inclusions in the liquid metal pool of the ESR trial T1 are (Ti,Nb)N, MgO·Al₂O₃ inclusions wrapped by (Ti,Nb)N, CaO–Al₂O₃, and MgO·Al₂O₃.

Four types of inclusions are found in ingot C1, namely (Ti,Nb)N inclusions, MgO·Al₂O₃ inclusions wrapped by (Ti,Nb)N, CaO–Al₂O₃, and MgO·Al₂O₃. Figure 7 and Figure 9 show the EDS element mappings of CaO–Al₂O₃ inclusions in the liquid metal pool of the ESR trial T1 and remelted ingot C1, respectively. CaO and Al₂O₃ are uniformly distributed over the entire cross-section of the inclusion.

SEM micrographs, EDS spectra, and elemental mappings of typical inclusions in the liquid metal pool of ESR trial T2 and remelted ingot are shown in Figure 10, Figure 11, Figure 12 and Figure 13. The types of inclusions in the liquid metal pool of the ESR trial T2 are (Ti,Nb)N, Ce₂O₂S, and CeAlO₃ encapsulated by (Ti,Nb)N, CeAlO₃, and Ce₂O₃. The types of inclusions are (Ti,Nb)N, Ce₂O₂S, CeAlO₃ surrounded by (Ti,Nb)N, and Ce₂O₃. It is observed that the types of inclusions in the remelted ingot did not change compared to the types of inclusions in the liquid metal pool.

Typical inclusions in the liquid metal pool and remelted ingot for the ESR trial T3 are shown in Figure 14, Figure 15, Figure 16 and Figure 17. The two types of rare earth inclusions in the liquid metal pool of the ESR trial T3 are observed to be Ce₂O₃ and Ce₂O₂S, and the morphology of these rare-earth inclusions is near-spherical. The results of the elements mappings of Ce₂O₂S inclusions in the liquid metal pool of ESR trial T3 and remelted ingot are shown in Figure 11 and Figure 13, respectively.

3.4. Size Distribution and Types of Inclusions

Most of inclusions in the Ce-free electrode are 2 to 5 μm in size (accounting for 72% in number proportion), followed by the inclusions larger than 5 μm (21% in the relative fraction), and the number proportion of the inclusion smaller than 2 μm is 7%. The size of inclusions in the electrode with 0.016 mass% Ce is 2 to 5 μm (accounting for 92% in number proportion). The number proportion of inclusions in the size range of 1 μm to 2 μm and the inclusions larger than 5 μm are 6% and 2%, respectively. The proportion of the inclusions in the size range of 1 μm to 2 μm in the electrode with 0.300 mass% Ce is 88%, and the number proportion of inclusions between 2 and 5 μm is 12%.

Figure 18 shows the size distribution of inclusions in liquid metal pools and remelted ingots with different Ce contents. The proportion of inclusions smaller than 2 μm in the liquid metal pool of ESR trial T1 is 67%. The proportion of the number of inclusions occupied between 2 μm and 5 μm is 31%. The size of inclusions smaller than 2 μm in ingot C1 decreased to 34%, and inclusions between 2 and 5 μm increased to 61%. The proportion of the number of inclusions in the size range of 1 μm to 2 μm in the liquid metal pool of the ESR trial T2 is 75%, and the proportion of the number of inclusions occupied between 2 and 5 μm is 25%. The proportion of inclusions in the size range of 1 μm to 2 μm in the remelted ingot decreased to 71%, and the proportion of inclusions in the size range of 2 to 5 μm increased to 27%. The proportion of inclusions larger than 5 μm is 2%. The proportion of inclusions in the liquid pool of the ESR trial T3 remained almost unchanged compared to the remelted ingot C3.

Figure 19 shows the distribution of inclusions types in electrodes, liquid metal pools, and remelted ingots with different Ce contents. All inclusions in the Ce-free electrode are identified as MgO·Al₂O₃. The type of inclusions in the liquid metal pool of the ESR trial T1 has changed compared to the electrode. The newly formed CaO–Al₂O₃ inclusions in the liquid metal pool account for 6% in the number proportion. The proportion of CaO–Al₂O₃ inclusions in the ingot account for 49% of the total inclusions.

The rare earth oxide inclusions in the electrode with 0.016 mass% Ce are identified as Ce₂O₂S. The rare earth oxide inclusions present in the liquid metal pool of ESR trial T2 are identified as Ce₂O₃, CeAlO₃, and Ce₂O₂S, accounting for 51%, 7%, and 42% in the number proportion, respectively. The Ce₂O₃ and CeAlO₃ inclusions are newly formed in the liquid metal pool. The proportion of Ce₂O₃ inclusions in the liquid metal pool and ingot reduce from 51% to 28% and CeAlO₃ inclusions increased from 7% to 39%. The Ce₂O₂S inclusions are reduced to 33% in the number proportion after ESR.

The CeS inclusions are completely removed and Ce₂O₃ inclusions are newly formed in the trial T3 during ESR. The proportion of Ce₂O₃ and Ce₂O₂S inclusions in the liquid metal pool is 46% and 54%, respectively. The relative proportions of inclusions in the remelted ingot (43% Ce₂O₃ and 57% Ce₂O₂S) do not change significantly compared to the metal pool.

3.5. Evolution Mechanism of Inclusions during Remelted Remelting

In the present study, thermodynamic calculation is performed for understanding the evolution of inclusions during the ESR with FactSage 8.0 (FSstel, FactPS, and FToxid database). Figure 20 and Figure 21 show the transformation of inclusions precipitation with temperature for electrodes with different Ce content and remelted ingots C1, C2, and C3, respectively.

According to the above experimental observations, no other types of oxide inclusions are observed in the Ce-free electrodes except for MgAl₂O₄ inclusions. The temperature of the liquid metal film steel at the electrode tip is close to the liquidus temperature during ESR [36,37]. The results showed that the part of MgAl₂O₄ inclusions are removed by the adsorption of the molten slag, and the remaining is not removed and entered into the liquid metal pool.

The predicted inclusions transformation with respect to the temperature in the electrodes is present in Figure 20. The Ce₂O₃ inclusions are already formed when the liquid steel temperature at 1800 °C. The Ce₂O₃ inclusions gradually precipitate out as the temperature decrease. when the temperature decreases to 1669 °C, the mass fraction of Ce₂O₃ inclusions decrease sharply, and the mass fraction of Ce₂O₂S inclusions increases sharply. The precipitation rate of Ce₂O₂S inclusions become slow when the temperature of the liquid steel decreases to 1593 °C. According to the thermodynamic calculation, the inclusion in the electrode containing 0.016 mass% Ce is mainly Ce₂O₂S, which is consistent with the experimental results.

It suggested that Ce₂O₂S inclusions dissociated into liquid steel as soluble oxygen, sulfur, and cerium as the liquid metal films formed on the downside of the electrode tip and collected into liquid metal droplets subsequently during ESR, whereas the others do not dissociate into liquid steel. Experimental results show that the Ce₂O₂S inclusions are not completely removed. The removal of these Ce₂O₂S inclusions is attributed to the dissociation of a portion of these original oxide inclusions into liquid steel and the absorption of the others by the molten slag phase during the ESR process.

The predicted inclusion transformation with respect to the temperature in the electrode with 0.300 mass% Ce is present in Figure 20b. The amount of the CeS inclusions is higher than that of Ce₂O₂S inclusions. The present thermodynamic calculation supports the above theoretical analysis of oxide inclusions formation and the experimental observations of oxide inclusions in the electrodes.

The electrode tip is heated to near the liquidus temperature of the steel during the ESR. As shown in Figure 20b, CeS and Ce₂O₂S inclusions in the electrode with 0.300 mass% Ce do not dissociate into liquid steel as soluble elements. The experimental results show that CeS inclusions are fully removed by the adsorption of the molten slag before liquid metal droplets collect in the liquid metal pool. According to the results of the experiment, a portion of the Ce₂O₂S inclusions are removed and the remaining enters into the liquid metal pool. The removal of these Ce₂O₂S inclusions is attributed to the absorption of the molten slag during the ESR process.

The predicted inclusions transformation with respect to the temperature in the remelted ingot C1 is present in Figure 21a. The liquid inclusions start to precipitate in the liquid steel at 1690 °C. As the temperature decreases, the mass fraction of liquid inclusions increases sharply. The precipitation rate of liquid inclusions slows down at which point MgAl₂O₄ inclusions begin to precipitate when the temperature is reduced to 1647 °C. When the temperature is reduced to 1574 °C, the mass fraction of liquid inclusions decreases and CaAl₄O₇ inclusions start to precipitate. Figure 21b shows the variation of the mass fraction of each component in the liquid inclusions with temperature. It can be seen from Figure 21b that the liquid inclusions consist of Al₂O₃, MgO, and CaO, in which Al₂O₃ is dominant in the mass fraction.

As shown in Table 1 and Table 2, the reoxidation of liquid steel occurs during the ESR process of trial T1. Thereafter, soluble oxygen reacts with calcium, aluminum and magnesium to form CaAl₄O₇ and MgAl₂O₄ inclusions in the liquid metal pool during the ESR.

All oxide inclusions in the Ce-free electrode were identified as MgAl₂O₄. The types of inclusions in the liquid metal pool after ESR are MgAl₂O₄ (accounting for 94% in number proportion) and CaAl₄O₇ (6% in number proportion). The proportion of these two types of inclusions in the remelted ingot C1 is 61% and 39%, respectively. Thermodynamic calculations support the above analysis of oxide inclusions in the remelted ingot C1.

The predicted inclusions transformation with respect to the temperature in the ingot C2 is present in Figure 21c. The Ce₂O₃ inclusions already form at 1800 °C in the liquid steel, and Ce₂O₃ inclusions precipitate slowly as the temperature gradually decreases. The mass fraction of Ce₂O₃ inclusions decreases rapidly at 1659 °C, and CeAlO₃ inclusions start to form in the liquid steel.

As listed in Table 1 and Table 2, the changes in oxygen content before and after the ESR show that reoxidation of liquid steel occurred in the trial T2 during the ESR. The soluble oxygen inside the liquid metal pool during trial T2 ESR steel reacts with cerium 2[Ce] + 3[O] = Ce₂O₃(s) (ΔG^o = −1431120 + 121T (J/mol)) [38] to form Ce₂O₃ and the Gibbs free energy change of the reaction to −14.56 kJ/mol. Once the soluble oxygen pickup takes place in liquid metal pool (arising from the reoxidation), the soluble oxygen reacts with soluble aluminum and cerium in liquid metal pool to form CeAlO₃ inclusions, [Ce] + [Al] + 3[O] = CeAlO₃(s) (ΔG^o = −1366460 + 364T (J/mol)) [39] with a Gibbs free energy change of −41.47 kJ/mol. The present thermodynamic calculation supports the above analysis of oxide inclusions formation and the experimental observations of oxide inclusions in the liquid metal pool and ingots.

The predicted inclusions transformation with respect to the temperature in ingot C3 with respect to the temperature is present in Figure 21d. The Ce₂O₃ inclusions are already formed at 1800 °C. When the temperature is reduced to 1781 °C, the mass fraction of Ce₂O₃ inclusions starts to decrease in the liquid steel and the Ce₂O₂S inclusions start to precipitate in the liquid steel.

The change of oxygen content in Table 1 and Table 2 shows that reoxidation of liquid steel occurs during ESR. The reoxidation of liquid steel leads to considerable pickup of soluble oxygen during ESR, which provides a driving force for the generation of Ce₂O₃ inclusions. 2[Ce] + 3[O] = Ce₂O₃(s) (ΔG^o = −1431120 + 121T (J/mol)), the Gibbs free energy change is −142.37 kJ/mol. Soluble oxygen in the liquid metal pool reacts with soluble sulfur and cerium 2[Ce] + 2[O] + [S] = Ce₂O₂S(s) (ΔG^o = −1353590 + 332T (J/mol)) [40] to form Ce₂O₂S inclusions. The Gibbs free energy change is −158.85 kJ/mol. According to the experimental results, the Ce₂O₂S inclusions in the electrode cannot be removed completely, the Ce₂O₂S inclusions in the liquid metal pool originate from reoxidation inside the liquid metal pool and the original inclusions that has not been removed. The thermodynamic calculations results are consistent with the experimental observations and analysis of the formation of oxide inclusions in the liquid metal pool and remelted ingot.

The liquid steel solidification progresses in a is non-equilibrium state during the ESR practice, which leads to insufficient solute diffusion in solid [41,42]. The evolution of inclusions during the solidification of liquid steel with different Ce contents is calculated using the non-equilibrium module (Scheil-Gulliver model) of the thermodynamic software FactSage 8.0.

As shown in Figure 22a, MgAl₂O₄ and CaAl₄O₇ inclusions are formed at the high-temperature stage (around 1600 °C). The mass fraction of oxide inclusions forms at temperatures below the liquidus temperature of the steel is virtually nothing during the solidification process. Figure 22b shows that the Ce₂O₃ inclusions already formed at 1600 °C in the liquid steel. The mass fraction of Ce₂O₃ inclusions no longer changes as the temperature decreases. When the temperature reaches to 1600 °C, CeAlO₃ inclusions are formed in liquid steel and the mass fraction of oxide inclusions does not change when the temperature is reduced to 1449 °C. At the temperature below the liquidus temperature of the steel, the mass fraction of oxide inclusions does not change. As shown in Figure 22c, the Ce₂O₃ and Ce₂O₂S inclusions are formed at 1600 °C in liquid steel. As the temperature of liquid steel decreases, the mass fraction of Ce₂O₃ and Ce₂O₂S inclusions no longer change.

As shown in Figure 22, amount of individual type of oxide inclusions does not change during liquid steel solidification. It can be learned that no fresh oxide inclusions are generated during the solidification of liquid steel. The types of inclusions in the liquid metal pools of the ESR trials T1, T2, and T3 are the same as those in the remelted ingots. This is consistent with the experimental observations. The differences in the number proportions of different types of inclusions between liquid metal pool and remelted ingot are attributed to the removal through floatation before full solidification of liquid steel during the ESR.

4. Conclusions

• The Al pickup in the steel is caused by the Al addition for deoxidation during the ESR process, rather than the reduction of Al₂O₃ in the slag by Ce. The soluble oxygen pickup is generated in liquid steel due to the decomposition of FeO in the slag and desulfurization during the protective argon gas atmosphere ESR.
• The oxide inclusions in Ce-free electrode are MgO·Al₂O₃, part of which are removed by molten slag absorption during the ESR. The oxide inclusions in liquid metal pool are mainly MgO·Al₂O₃ and CaO–Al₂O₃ (6% in number fraction). The soluble oxygen that arising from reoxidation of liquid steel during the ESR react with soluble calcium and aluminum to form CaO–Al₂O₃ inclusions. MgO·Al₂O₃ inclusions are originated from reoxidation products and the relics from the electrode.
• The oxide inclusions in the electrode with 0.016 mass% Ce are Ce₂O₂S. Part of Ce₂O₂S inclusions are removed during ESR in two ways: (I) dissociated into soluble oxygen and soluble elements in liquid steel, (II) absorbed by molten slag. The oxide inclusions in the liquid metal pool are Ce₂O₃, CeAlO₃, and Ce₂O₂S. CeAlO₃ inclusions are reoxidation product, and Ce₂O₂S inclusions are the relics from the electrode. The proportions of Ce₂O₃, CeAlO₃ and Ce₂O₂S inclusions in ingot are 28%, 39%, and 33%, respectively.
• The rare-earth inclusions in the electrode with 0.300 mass% Ce are Ce₂O₂S and CeS. The CeS inclusions are fully removed during ESR. Part of Ce₂O₂S inclusions are removed by slag adsorption. The oxide inclusions in liquid metal pool are Ce₂O₃ (reoxidation products, 46% in number proportion) and Ce₂O₂S (54% in number fraction). Ce₂O₂S inclusions in liquid metal pool are originated from the relics of electrode and reoxidation products. The proportion of Ce₂O₃ and Ce₂O₂S inclusions in remelted ingots are 43% and 57%, respectively.
• No fresh oxide inclusions are generated during the solidification of liquid steel. The differences in the number proportions of different types of inclusions between liquid metal pool and remelted ingot are attributed to the removal through floatation before full solidification of liquid steel during the ESR.