3.2. The Effect of the Cerium Addition on the Characteristics of Inclusions
Typical inclusions in 1
#~5
# steel samples were presented in
Table 5. In 1
# steel without the cerium addition, the typical inclusions were MgO·Al
2O
3, Al
2O
3, and MnS. When the content of cerium in steel was 0.0136% and 0.0277%, the main inclusions in steel were Ce–O–S, and when the content of cerium was 0.0389% or above, the inclusions in the steel were Ce–O–S, CeS, Ce–S–O–P(As), Ce–O–P, and Ce–P(As). According to the energy spectrum of the Ce–O–S inclusions in
Table 5, the S content of Ce–O–S inclusions was high, and these kinds of inclusions were judged to be Ce
2O
2S and CeS inclusions. Ce–S–O–P(As) inclusions and Ce–O–P inclusions were presumably composed of various inclusions, so these two inclusions need to be further examined for judging the composition and structure with SEM-mapping of elements.
In order to study the formation conditions of the cerium inclusions, the phase stability diagram of the inclusions bearing cerium in the C104Cr steel was founded when [%Ce] was set as 0.0277%, as shown in
Figure 3. The [%O] and [%S] of 2
#~5
# steel are labeled in
Figure 3 and [%O] was set as approximately 10% of the total oxygen. Points of the 2
#~5
# steel fall into the formation region of Ce
2O
2S. The [%O] in steel is too low to meet the formation condition of Ce
2O
3, and this is in good agreement with the experimental results.
In order to clarify the formation mechanism of the cerium composite inclusions in the 4
#~5
# steel, SEM-mappings of typical Ce–O–P inclusions and Ce–S–O–P(As) inclusions were conducted, and the results are shown in
Figure 4,
Figure 5 and
Figure 6.
According to the SEM-mappings results, there are two kinds of Ce–P–O inclusions in the 4#~5# steel:
(i) The first type of Ce–P-O inclusions is shown in
Figure 4; cerium, phosphorus, and oxygen are distributed throughout the whole inclusion, and the concentration is uniform and without fluctuation. The atomic percentages of cerium, phosphorus, and oxygen were determined to be nearly 1:4:1 (cerium/phosphorus/oxygen = 17.5%:71.9%:10.6%) using EDS. It is speculated that this indicates that they are CePO
4 inclusions. Wang et al. [
17] reported that they found LaAsO
4 inclusions in high carbon steel with a lanthanum content of 0.059%. However, there is no literature on the thermodynamic data of cerium phosphate, so the formation mechanism of this type of inclusion needs further research.
(ii) The second type of Ce–P–O inclusion is a kind of double-layer composite inclusion, as shown in
Figure 5. Cerium is distributed throughout the whole inclusion, phosphorus is distributed in the center of inclusion, and oxygen is distributed in the outer layer of the inclusion. Therefore, this type of inclusion is a two-layer composite inclusion with a core of CeP covered with Ce
2O
3.
Figure 6 presents the SEM-mappings of the typical Ce–S–O–P(As) inclusion. The results show that the cerium is distributed throughout the inclusions and the sulfur, arsenic, phosphorus, and oxygen are distributed from the inner layer to the outer layer, respectively. Thus, a possible formation order of each inclusion was obtained, and sulfide, arsenide, phosphide, and oxide formed sequentially in the steel. Therefore, this type of inclusion is a four-layer composite inclusion with a core of CeS covered with CeAs, CeP, and Ce
2O
3, successively.
3.4. Discussion on Evolution Mechanism of CeP, CeAs, and Cerium Composite Inclusions
According to the analysis and results above, many CeP, CeAs, and cerium composite inclusions (Ce–P-O inclusions with a double-layer structure and Ce–S–O–P(As) inclusion) formed in the 4#~5# steels. This result indicates that the cerium addition can combine with phosphorus and arsenic. The formation condition of CeP and CeAs inclusions and these composite inclusions will be discussed below.
Figure 8 shows that CeP and CeAs inclusions are difficult to form under steelmaking temperatures. Thus, we suppose that they may form during the cooling process, and the relationship between the formation of the Gibbs free energy of the CeP and CeAs inclusions and temperature was established as shown in
Figure 9. The formation of the Gibbs free energy and temperature of the CeP and CeAs inclusions before solidification are always positive, so CeP and CeAs inclusions are difficult to form during the cooling process of molten steel before solidification.
Referring to the work of Xin [
18] and Wang [
13], it is inferred that the distribution ratio of cerium, phosphorus, and arsenic between solid and liquid changes in the solidification process of molten steel, and the segregation of these three elements occurred, which satisfied the formation thermodynamic conditions of the CeP and CeAs inclusions. In the present experiment, the ingot cooling rate was slow, and the segregation of cerium, phosphorus, and arsenic was promoted in the solidification process. If the actual activity product (
)
ac and (
)
ac in the C104Cr liquid steel exceeds the equilibrium activity product (
)
eq and (
)
eq, respectively, then the CeP and CeAs inclusions satisfy the formation condition.
In this experiment, the composition of the 4
# steel was selected to calculate and compare the actual activity product and equilibrium activity product of the CeP and CeAs inclusions during the solidification process. The Brody–Flemings model modified by Clyne and Kurz was taken into account in determining the solute concentration in the liquid phase at the solidification front [
18,
19,
20,
21], as shown in Equations (1)–(5). The Brody–Flemings model is a commonly used model for microsegregation calculation. As Clyne and Kurz explained, when α is large, the Brody–Flemings model predicts less enrichment in the liquid phase than the Lever rule, so it is physically unreasonable, and so, Clyne and Kurz modified this model by correcting α to fit extreme cases (when the solidification parameter α is close to 0 or ∞) [
19,
20,
21]. In the present study, the approximate cooling rate of the ingot was provided on the basis of the average value of the secondary dendritic arm according to the corrosion results of the secondary dendritic arm of the as-cast steel sample, and the input value of the average secondary dendritic arm spacing was set as 48.54 μm. The liquid phase range of the C104Cr steel was calculated using Thermo-calc software. According to the calculated results, the liquidus temperature was set as 1728 K, and the solidus temperature was set as 1604 K. The segregation constants used for calculation are listed in
Table 6.
where
(mass%) is the solute concentration in the liquid phase at the solidification front,
(mass%) is the initial concentration of the solute in the liquid steel,
is the solidification fraction,
is the partition coefficient of solute,
(m
2·s
−1) is the diffusion coefficient of the solute,
(s) is the regional solidification time,
(m) is the secondary dendritic arm spacing,
(K) is the liquidus temperature,
(K) is the solidus temperature, and
(K·s
−1) is the cooling rate.
The equilibrium activity product is obtained from the chemical isothermal equations of the CeP and CeAs inclusions’ formation reactions, respectively, as shown in Equations (6) and (7), and the liquid temperature at the solidification front is obtained from Equation (8) [
26].
The actual activity product of CeP and CeAs inclusions exceeds the equilibrium activity product at the end of the solidification, as shown in
Figure 10 and
Figure 11. The formation thermodynamic conditions of CeP and CeAs inclusions are satisfied during the solidification process. In fact, the formation of CeP and CeAs inclusions during solidification faces the competitive combination of oxygen and sulfur, which indicates that a higher cerium content and lower oxygen and sulfur content are key factors for the formation of CeP and CeAs inclusions. The higher cerium content satisfies the thermodynamic and kinetic conditions of the cerium inclusions. Under the present experimental conditions, when [%Ce] was more than 0.0389 wt %, [%O] was less than 0.0001 wt %, and [%S] was less than 0.0022 wt %; CeP and CeAs inclusions can form in large quantities in C104Cr saw wire steel.
According to the SEM mapping results, as shown in
Figure 5 and
Figure 6, Ce
2O
3 inclusions concentrated in the outer layer of CeP inclusions. Therefore, it is supposed that the Ce
2O
3 inclusions layer is a product of the high-temperature solid-state reaction. It is known that the solid solubility of oxygen decreases with decreasing temperatures, and this is a significant reason for the formation of Ce
2O
3 inclusions during the cooling process. In view of the situation, two possible mechanisms regarding the formation of Ce
2O
3 inclusions were supposed:
(i) Ce
2O
3 inclusions formed from the combination of [Ce] and [O] directly, and the reaction is shown by Equation (9). The CeP inclusions precipitated during solidification served as a nucleation core for the Ce
2O
3 inclusions, and then, the CeP inclusions were covered with the later-formed Ce
2O
3 inclusions.
(ii) Ce
2O
3 formed by way of oxygen replacing the phosphorus of CeP. That means that Ce
2O
3 found in the outer layer of the CeP is the product of an oxidization reaction between CeP and [O]. This mechanism was supported by a significant phenomenon—the mixed layer of CeP and Ce
2O
3, which existed around the boundary between the two layers in the composite inclusions. The reaction equation and mechanism diagram were shown in Equation (10) and
Figure 12, respectively, and the approximate calculation result of the Gibbs free energy of the reaction at 1604 K was −813,803 J·mol
−1. After the formation of CeP, the mixed layer of CeP and Ce
2O
3 formed in the outermost layer of composite inclusions with this reaction, and the reaction in different inclusions proceeded to different extents.
Combined with the above thermodynamic and kinetic analysis, a formation mechanism of the I-type of inclusion (Ce–S–O–P (As) inclusions) and the II-type of inclusion (Ce–P-O inclusions with a double-layer structure) in C104Cr steel with high cerium content was proposed, as shown in
Figure 13.
For the I-type of inclusion, CeS inclusions precipitated at 1600 °C and then served as the nucleation core for CeP or CeAs inclusions during the solidification process, and then, Ce2O3 formed in the outer layer of CeP with the decrease of the temperature.
For the II-type of inclusion, CeP inclusions precipitated directly in molten steel, and then, the formation reaction of Ce2O3 occurred. As a result, Ce2O3 formed in the outer layer of the inclusion.