Effect of rare earth Ce addition on inclusions in offshore engineering steel containing arsenic

The present study investigates the impact of rare earth Ce addition and holding time on inclusions in offshore engineering steel containing arsenic. The analysis was conducted using scanning electron microscopy and energy spectrum (SEM-EDS), automatic statistics of inclusions, and thermodynamic analysis. The results indicate that prior to the addition of rare earth Ce, the typical inclusions in the steel were Al2O3, MnS and Al2O3-MnS, with an average size of approximately 2.51 μm and an irregular or stripe morphology. Upon the addition of rare earth Ce, the inclusions in the steel were transformed into Ce-S(-O), Ce-As(-O) and Ce-S-As(-O), with a smaller average size and a spherical or ellipsoidal morphology. The smallest average size of inclusions in the steel was observed when rare earth Ce was added and held for 5 min; this size was 33% smaller than that prior to the addition of Ce. The thermodynamic calculation revealed that the Ce-S(-O) inclusion is formed in molten steel, while the Ce-As(-O) inclusion is formed during the solidification stage. As element replaced parts of the S and O elements in the Ce-S(-O) inclusion and formed the Ce-S-As(-O) complex inclusion, characterized by a double-layered structure.


Introduction
For an extended period of time, global warming has emerged as a critical issue across the globe.An effective way to reduce carbon emissions is the recycling utilization of scrap steel.However, with the increasing use of scrap steel, the residual elements such as arsenic, antimony, and copper could become enriched in the steel, which are detrimental to the quality and mechanical properties of steel products [1,2].Arsenic is known to segregate and enrich at the interface, forming a low melting point phase that reduces the steel's hot workability and machinability [3][4][5][6][7].Unfortunately, it is difficult to eliminate arsenic from steel during the steelmaking process due to its lower oxidizing potential than iron.
To address this issue, rare earth (RE) materials have been found to have a remarkable influence on the cleanliness of steel [8][9][10][11][12][13][14].RE has strong chemical activity and can capture oxygen, sulfur, and other harmful residual elements in steel, forming RE inclusions.Previous studies have shown that RE can react with arsenic to form refractory compounds that eliminate arsenic from molten steel and improve the current status of arsenic in solid steel, particularly when arsenic concentrations are not high [3,4,7,15].The first compounds to form after the addition of RE to steel containing arsenic are oxides, oxysulfides, and sulfides of RE [16][17][18][19].As the concentrations of oxygen and sulfur are reduced to a certain level [20], RE reacts with arsenic to form a series of RE arsenic compounds, such as RE-As [4,15,21,22], RE-O-As [23], RE-S-As [24], and RE-O-S-As [23].
Previous research has indicated that the optimal addition of rare earth in EH36 offshore engineering steel is 0.02% [25].This study aims to analyze the modification effect and mechanism of adding 0.02% rare earth Ce metal to EH36 steel containing arsenic.The study utilizes scanning electron microscopy and energy spectrum, automatic statistics of inclusions, and thermodynamic analysis to systematically examine the impact of rare earth Ce addition and holding time on inclusions.The objective of this study is to determine the optimal holding time of rare earth Ce and clarify the formation process of arsenic RE inclusions and their distribution, to guide to reduce the harm of arsenic in EH36 steels containing arsenic.

Experimental process
The raw materials used in this study were EH36 offshore engineering steel containing arsenic produced in the industrial field, with a composition detailed in table 1.The steel was melted in an alumina crucible using a 25 kg vacuum induction furnace.The experimental process, as illustrated in figure 1, involved heating the steel to 1873 K (1600 °C) at a rate of 10 K min −1 by controlling the furnace power and holding it for 5 min, to prevent the possibility of burning out any alloy elements and ensure a consistent composition.Then, the first sample was cast into an ingot named C0.Subsequently, a certain amount of rare earth Ce was added to the melt, and steel samples were cast into ingots at 1 min, 5 min, and 10 min after the addition of rare earth Ce, named C1, C5, and C10, respectively.Each steel sample weighed approximately 6 kg.

Analysis methods
The steel's oxygen and sulfur mass fractions were determined using specialized equipment, namely the Leco ON836 oxygen and nitrogen analyzer and the Leco CS744 high frequency infrared carbon and sulfur analyzer, respectively.The steel's Ce, As, and Al contents were identified using the PerkinElmer ELAN9000 inductively coupled plasma mass spectrometry (ICP-MS).The Tescan MIRA3 LMH scanning electron microscope equipped with EDS and the Phenom Particle X automatic scanning electron microscope equipped with EDS were utilized to analyze the size, amount, morphology, and composition of inclusions present in the steel.The polished surface of the steel sample was analyzed, with a surface area of approximately 25 mm 2 , and the minimum inclusion size detected was 0.5 μm.

Results and discussion
3.1.The effect of Ce addition on the variation of the steel composition Table 2 presents the mass fractions of Ce, As, O, and S in C0, C1, C5, and C10 steels.The mass fraction of O was maintained at a low level, ranging from 11 to 15 ppm.Similarly, the mass fraction of S was between 27 and 30 ppm, while that of As ranged from 0.024% to 0.027%.It was observed that the addition of Ce had a negligible  impact on reducing the levels of O, S, and As in the steel.This was attributed to the high density of inclusions containing Ce, which made it challenging for them to float off the molten steel [26].Furthermore, the content of Ce increased significantly after its addition, followed by a gradual decrease over time.According to the findings of Ren et al [27], the introduction of Ce into steel resulted in the formation of a cerium concentration zone.However, as time elapsed, the homogenization process led to a decrease in the concentration of cerium.
Similarly, Kwon et al [28] observed that dissolved cerium in steel could react with the alumina crucible, causing a reduction in its concentration over time.These two factors contributed to the decrease in cerium content with time.It was observed that when rare earth Ce was added to steel and held for a duration of 5 min, the resultant Ce content in steel was 0.02%.

The effect of Ce addition on the characteristics of inclusions
The inclusions present in the samples were analyzed using SEM and EDS, and the results are depicted in figures 2 and 3.  As(-O) inclusions in C1, C5, and C10 steels were distributed between 1.00 and 1.20, indicating that the morphology of these inclusions was spherical or ellipsoidal and changed little with the increase in holding time, which was consistent with the observation results in figure 3.

The effect of Ce addition on the number, size, and distribution of inclusions
The present study utilized an automated scanning electron microscope to investigate the number, size, and distribution of inclusions in C0, C1, C5, and C10 steels.The results are presented in figure 5.In figure 5(a), it is indicated that the distribution of small and medium-sized inclusions (i.e., those measuring less than 2 μm) in C0, C1, C5, and C10 steels was 43%, 66%, 72%, and 60%, respectively.It can be inferred that prior to the addition of Ce, the distribution of small and medium-sized inclusions in steel was relatively low.However, the addition of Ce resulted in a significant increase in the distribution of small inclusions.The highest distribution of small and medium-sized inclusions was observed when Ce was added to steel for a duration of 5 min.Figure 5(b) illustrates the number of inclusions per unit area in C0, C1, C5, and C10 steels, which were 91, 147, 174, and 139, respectively.The results indicate a substantial increase in the number of inclusions per unit area after the addition of Ce.The highest number of inclusions per unit area was observed when Ce was added to steel for 5 min.Nevertheless, the number of inclusions per unit area experienced a decline when Ce was added to the steel for a period of 10 min.According to figure 5(c), the average inclusion size in C0, C1, C5, and C10 steels was found to be 2.51 μm, 1.82 μm, 1.68 μm, and 1.98 μm, respectively.These findings indicate a significant reduction in the average inclusion size following the addition of Ce.Notably, the smallest average inclusion size was observed when Ce was added to steel for 5 min, which was 33% smaller than the size prior to the addition of Ce.The present study focuses on the analysis of the formation and evolution of inclusions that contain Ce without As, using thermodynamic calculations.Table 3 displays the thermodynamic data regarding the reactions of Ce with O and S at a temperature of 1873 K.The formation of the Ce 2 O 3 inclusion can be described by the following chemical reaction, as per the data presented in reference [29].
where ΔG θ denotes the standard Gibbs free energy of reaction in J•mol −1 ; T represents the temperature at 1873 K; R is the gas constant at 8.314 J•mol −1 •K −1 ; K is the equilibrium constant of reaction; a C O e 2 3 is the activity of the Ce 2 O 3 inclusion, which is 1; a i a i is the activity of component i in the liquid steel, which can be calculated using equation (3) [30].The interaction coefficients utilized in the thermodynamic calculations are presented in table 4 [23].
where e i j is the interaction coefficient of component j with respect to i; w i and w i are the mass fractions of components i and j in the liquid steel, respectively.
The relation between Ce and O for Ce 2 O 3 inclusion can be represented by equation (4), which is derived from equations (1)- (3).Using the calculation method in equation (4), the relation between Ce, O, and S for Ce 2 O 2 S inclusion can be represented by equation (5) 5. Based on their thermodynamic conversion conditions, the phase stability diagram for inclusions containing Ce and without As has been established, as depicted in figure 6.The O and S contents in C1, C5, and C10 steels were marked in the phase stability diagram.As illustrated in figure 6, all specimens were located within the region where Ce 2 O 2 S inclusions are formed, indicating that such inclusions are produced in the molten steel.Despite the significantly low total oxygen levels in C1, C5, and C10 steels, there was still some dissolved oxygen in the steel.Consequently, the actual oxygen content in the inclusions is lower than the labeled data points, leading to a downward shift in the actual data points.Therefore, it can be inferred that Ce 2 O 3 inclusion does not form in molten steel.According to figure 6, the formation of CeS inclusion occurs when the S content of steel is less than 0.0055 mass%.While the actual S content in C1, C5, and C10 steels is less than 0.0030 mass%, which conforms to the conditions for the formation of CeS inclusions, it does not conform to the conditions for the formation of Ce 3 S 4 and Ce 2 S 3 inclusions.Therefore, Ce 2 O 2 S and CeS inclusions can form in molten steel in C1, C5, and C10 steels.The results of the above calculation and analysis were consistent with the SEM scanning results for inclusions.

Thermodynamic calculations of inclusions containing Ce with As
In order to investigate and explore the formation of inclusions containing Ce with As, thermodynamic calculations were conducted.The Gibbs free energy of the CeAs inclusion can be calculated using equations (3), (7), and (8) [23].
As CeAs s , 302040 237.2 7 where ΔG is the Gibbs free energy of reaction, J•mol −1 ; and a CeAs is the activity of the CeAs inclusion, and it is 1.The interaction coefficients used in thermodynamic calculations are presented in table 4. For instance, taking the composition of C5 steel as an example, the Gibbs free energy of the CeAs inclusion in steel was calculated, and the results are presented in figure 7. The positive Gibbs free energy of the CeAs inclusion before solidification indicates that the CeAs inclusion cannot be precipitated before the solidification of molten steel.
During the solidification process of molten steel, the formation of an x y A B inclusion can occur if the actual activity product ⋅ ( ) a a A x B y ac is higher than its equilibrium activity product ⋅ ( ) a a , A x B y eq as shown in equation (9).In order to determine the possibility of the formation of rare earth complex inclusions containing As forming during the process of solidification, an evaluation is conducted by comparing the actual activity product with the equilibrium activity product of the reactants.The composition of C5 steel is used to calculate and compare these values.
The segregation model [31][32][33] is a widely adopted approach for determining the concentration of solutes in various elements that exist within the liquid phase at the solidification front.
where c L is the solute concentration in the liquid phase at the solidification front; c 0 is the initial solute concentration in the liquid steel; f S is the solidification fraction; and k is the segregation coefficient, which is listed in table 6 [31,34].
The chemical isothermal equation of equation ( 9) is utilized to determine the equilibrium activity product, as demonstrated in equation (11).The temperature of the liquid at the solidification front can be calculated using equation (12) [35].The correlation between the actual activity product of the CeAs inclusion formed during the solidification phase and the equilibrium activity product is illustrated in figure 8.The findings suggest that the CeAs inclusion is generated in the latter stages of solidification.where T 0 is the melting point of pure iron, which is 1809 K; T L is the liquidus temperature; and T S is the solidus temperature.The T L and T S of C5 steel are 1791K and 1672K, respectively, which were calculated using Thermo-Calc software.

Conclusions
The present study aimed to investigate the impact of rare earth Ce addition and holding time on inclusions in offshore engineering steel containing arsenic.Based on experimental results and thermodynamic calculations, the following conclusions were drawn.
(1) Prior to the addition of rare earth Ce, the typical inclusions in the steel were Al 2 O 3 , MnS, and Al 2 O 3 -MnS, with an average size of approximately 2.51 μm and an irregular or striped morphology.Upon the addition of rare earth Ce, the inclusions in the steel were transformed into Ce-S(-O), Ce-As(-O), and Ce-S-As(-O), with a smaller average size and a spherical or ellipsoidal morphology.The smallest average size of inclusions in the steel was observed when rare earth Ce was added and held for 5 min.This size was 33% smaller than the size prior to the addition of rare earth Ce.  (2) The thermodynamic calculation revealed that the Ce-S(-O) inclusion is formed in molten steel, while the Ce-As(-O) inclusion is formed during the solidification stage.As element replaces parts of the S and O elements in the Ce-S(-O) inclusion and forms the Ce-S-As(-O) complex inclusion, characterized by a double-layered structure.

Figure 2
illustrates the typical inclusions found in C0 steel, which were irregularly shaped Al 2 O 3 inclusions, stripe-shaped MnS inclusions, and Al 2 O 3 -MnS inclusions with Al 2 O 3 at the core and MnS at the outside, and no inclusions containing arsenic were identified [25].The typical inclusions of C1, C5, and C10 steels and their map scanning are shown in figure 3.They were Ce-S(-O) inclusions, Ce-As(-O) inclusions, and Ce-S-As(-O) inclusions, which were spherical or ellipsoidal, and Al 2 O 3 , MnS, and Al 2 O 3 -MnS inclusions were disappeared.Adding rare earth Ce to steel can modify Al 2 O 3 , MnS, and Al 2 O 3 -MnS inclusions into Ce-S(-O), Ce-As(-O), and Ce-S-As(-O) rare earth inclusions effectively.And the typical rare earth inclusions in steel remained basically unchanged when Ce was added to steel for 1 min, 5 min, and 10 min.To better analyze the morphology of inclusions, an automatic scanning electron microscope was used to count the length and width of inclusions in steel, and the results were linearly fitted.The results are shown in figure 4. The slopes of the linear fitting of the length and width of Al 2 O 3 , MnS, and Al 2 O 3 -MnS inclusions in C0 steel were 1.76, 1.73, and 1.42, respectively, indicating that the morphology of Al 2 O 3 , MnS, and Al 2 O 3 -MnS inclusions in steel was stripe or irregular, which was consistent with the observation results in figure 2. The slopes of the linear fitting of the length and width of Ce-As(-O), Ce-S(-O), and Ce-S-

Figure 5 .
Figure 5.The number, size and distribution of inclusions.(a) inclusion size distribution, (b) number of inclusions, (c) inclusion average size.

Figure 6 .
Figure 6.Phase stability diagram of inclusions containing Ce without As.

Figure 7 .
Figure 7.The formation of the Gibbs free energy of CeAs before solidification.

3 .
The formation mechanism of Ce-S-As(-O) complex inclusion In the experiment, the formation mechanism of Ce-S-As(-O) complex inclusion was further analyzed by the SEM mapping results in figure3(c).The results show that the Ce element is distributed throughout the entire inclusion.The O and S elements are mainly concentrated in the center of the inclusion, while the As element is primarily distributed around the outer layer of the inclusion.In summary, the Ce-S-As(-O) complex inclusion has a double-layered structure, and the formation mechanism is shown in figure9.Firstly, Ce-S(-O) inclusion is preferentially formed in the molten steel, which can be used as the nucleation core.Then, during the solidification stage, as the As element moves towards the nucleation core, parts of the S and O elements in the rare earth inclusion are replaced, and finally, the Ce-S-As(-O) complex inclusion is formed.

Figure 8 .
Figure 8. Equilibrium activity product and actual activity product of the CeAs inclusion.(a) Whole solidification process; (b) the solidification end.

Table 1 .
Composition of EH36 offshore engineering steel containing arsenic (weight percent).

Table 2 .
The composition of different steel samples.

Table 3 .
Thermodynamic data regarding the reactions of Ce with O and S at 1873K.

Table 4 .
Interaction coefficients of elements in steel at 1873 K.

Table 6 .
Segregation coefficient of elements during solidification stage.

Table 5 .
Thermodynamic transformation conditions between inclusions containing Ce without As.