2015 Volume 55 Issue 12 Pages 2589-2596
Effect of Ce addition on the interfacial reaction between alumina refractory and 25wt%Cr-20wt%Ni-4wt%Si-0.5wt%Mn stainless steel deoxidized by Al at 1873 K was investigated to understand the contribution of the refractory-steel reaction to the inclusion evolution processes. The oxygen content markedly decreased by Al deoxidation, followed by a sluggish decrease by Ce addition greater than 0.5 wt%. The Ce content continuously decreased, but the higher the initial Ce content, the lower the Ce-decreasing rate was obtained. The content of Al initially decreased due to the formation of Al-rich inclusions, followed by an abrupt increase mainly due to a reduction of alumina refractory by Ce at the steel-refractory interface. The content of Al decreased again because of the formation of CeAlO3 compound not only as inclusions but also as refractory-steel reaction products. The CeAl11O18 and CeAlO3 were formed at the refractory side, while the Ce2O3 and CeAlO3 were formed at the steel side in the 0.5 wt% and 1.0 wt% Ce added systems. From the refractory-steel reaction mechanism, the process of inclusion evolution was proposed to three steps as follows. The initial SiO2-rich oxides are reduced by Al, resulting in the formation of aluminosilicates inclusions (Step 1). The Al2O3 in the inclusions are reduced by cerium, resulting in the formation of Ce-rich oxides (Step 2). Because the Ce content continuously decreased due to the refractory-steel reaction, the inclusions transform from Ce–Al complex oxides to Al-rich aluminosilicates and from Ce2O3 to Ce–Al complex oxides in the 0.1 wt% and 0.5 wt% Ce added systems (Step 3).
Austenitic stainless steels (ASS) are the most widely used structural materials in applications which require both high strength and excellent corrosion resistance in aggressive environment. However, they are not immune to localized corrosion. The rare earth elements (REE) such as Ce have been reported to form a thermodynamically stable surface film composed of CeOx, which improves the resistance to pitting, crevice, and intergranular (stress) corrosions.1,2) Microalloying of weld metal with Ce has also been shown to significantly improve weldability and oxidation resistance of ASS with grain refinement, decreased dendrite arm spacing and lower hot cracking property due to enhanced oxide scale adherence.3,4,5) The Ce-bearing ASS has been reported to show excellent antibacterial property compared to conventional Cu-bearing ASS when the amount of Ce added is above a critical value, e.g. 0.01 to 0.1 wt%, which is attributed to the Ce-rich zone precipitated in steel matrix.6)
For duplex stainless steels (DSS), the addition of Ce led to the formation of Ce-containing oxide inclusions, which improved the resistance to pitting corrosion. This originated from a fact that inclusion-steel interfacial area, which is the preferential site for initiation of pitting corrosion, was decreased compared to conventional DSS.7) Ha et al.8) observed that addition of misch metal (52wt%Ce-26wt%La-14wt%Nd-8wt%Fe) up to 0.067 wt% in DSS led to an increase in the resistance to pitting corrosion due to a decrease in the size and density of (Mn,Cr,RE)-oxysulfide inclusions. However, further addition of misch metal degraded the resistance to pitting corrosion as a result of a change in the shape of the oxysulfide inclusions from an angular to needle-like shape.
Consequently, not only the control of the morphology, composition and size of Ce-containing inclusions in stainless steels but also the high yield of Ce during melting and casting of stainless steels are highly important for the development of high functional ASS and DSS. Nevertheless, there are a few limited experimental studies of Fe–Ce–O and Fe–Ce–O–S equilibria in molten iron.9,10,11,12,13) Jung et al.14) recently developed thermodynamic model for computation of deoxidation equilibria including REE using FactSageTM software package.
The Al–Ce complex deoxidation of molten iron was examined by Dan and Gunji,15) who found that Al–Ce addition decreased the oxygen removal rate and increased the oxygen content compared to Al single deoxidation practice. These were believed to originate from the higher density of Ce2O3-containing oxide inclusions than that of pure Al2O3. Katsumata and Todoroki also found the similar results with those by Dan and Gunji in the 25wt%Cr-6wt%Ni molten steel.16) They observed that the Ce2O3 content in the inclusion increased with increasing Ce content in molten steel and that the composition of deoxidation products took only compounds of Al2O3, CeAl11O18, CeAlO3, and Ce2O3. However, Li et al.17) reported that the oxygen and sulfur content was effectively reduced by addition of Ce in the Ni-base alloys (80wt%Ni-20wt%Cr and 43wt%Ni-35wt%Fe-13wt%Cr-3wt%Ti-0.25wt%Al) through the formation of Ce oxides, sulfides and oxysulfides. They proposed that oxygen and sulfur removal was followed by the first order kinetics and that excessive addition of Ce was deleterious to hot shortness.
Appelberg et al.18) observed in-situ particle behavior on the 20wt%Cr steel surface using Confocal Scanning Laser Microscope technique. The mostly pure alumina and Ce oxide particles were semi-globular in shape, whereas Al–Ce–O complex particles were irregular in shape. Furthermore, the agglomeration of small particles was very fast, resulting in the formation of large clusters (>20 um). The three-dimensional morphology of REE clusters in the 20 wt%Cr(-11wt%Ni) steels was recently reported by Karasev and Jonsson’s research group using electrolytic extraction method.19,20) From pilot scale casting practice of REE treated 20wt%Cr-10wt%Ni steels, they found that small single inclusions, probably formed due to steel reoxidation, resulted in faster clogging than large clusters did.21)
Recently, we investigated the effect of Al deoxidation on the inclusion evolution process in Ce-containing ASS melts.22) The initial Mn(Cr)-silicate inclusions transformed to Al2O3-rich inclusions by Al addition, and then Ce2O3 was enriched by Ce addition, resulting in the formation of AlCeO3-type inclusions. However, the Ce2O3 in the inclusions was reduced by Al due to higher chemical potential of Al than Ce in molten steel, finally forming the Al2O3–CeAl11O18 complex inclusions.
In summary, the total oxygen and Ce contents in Ce added stainless molten steel is strongly dependent not only on the composition of inclusions but also on the deoxidation conditions. That is, the higher the content of Ce2O3 in inclusions, the greater the density of inclusions, which results in the higher total oxygen content. However, some researchers reported that Ce addition enhanced the removal of oxygen. Moreover, the shape of Ce oxide inclusions, e.g. single particle or clusters, is affecting the cleanliness of molten steel. However, even though the bulk steel chemistry is possibly affected by the steel-refractory interfacial reactions for the systems containing highly reactive elements such as REE, there are few reports on this topic. Therefore, in the present study, we examined the thermodynamic effect of Ce addition on the interfacial reaction between alumina refractory and stainless molten steel at 1873 K, which contributes to the changes in steel chemistry and thus inclusion evolution process.
In order to investigate thermodynamic behavior of Ce in ASS melts reacting with an alumina refractory, the experiments were carried out using a high-frequency induction furnace. The scheme of the experimental apparatus is shown in Fig. 1. As raw materials, high purity electrolytic iron, chromium, nickel, silicon and manganese (all 4N purity) were used for adjusting specific composition, viz. Fe-25wt%Cr-20wt%Ni-4wt%Si-0.5wt%Mn. Total weight of steel was 300 g.
Schematic diagram of (a) experiment apparatus and (b) experimental procedure.
The steel sample was melted at 1873 K in a fused alumina crucible (Purity; 99.5 wt% higher, Dimension; OD-60 mm, ID-52 mm, H-120 mm, Supplied from Toda Co. Ltd., Japan) with a graphite heater under a purified Ar-3vol%H2 gas atmosphere (1 Nl/min) using an induction heating. Oxygen as an impurity in the Ar-3vol%H2 gas mixture was removed by passing the gas through Drierite® and magnesium turnings at 723 K. Before starting the experiment, the atmosphere of the induction furnace was made vacuum using mechanical rotary pump. Gas displacement was carried out at least three times. And then, the Ar-3vol%H2 gas was blown in order to maintain an inert atmosphere during the experiment. After the temperature reached at 1873 K, the system was held for 10 min for homogenization. Then, the initial (blank) steel sample was taken using a quartz tube (OD : 6 mm) connected to a syringe and rapidly quenched by dipping into brine.
A fixed amount (0.05 wt%) of high purity (4N) aluminum shot, which had been determined as an appropriate pre-deoxidation before Ce addition in our previous study,22) was added in the melt through the quartz tube under inert atmosphere. For addition of Ce into the molten steel, the 82wt%Ni-18wt%Ce alloy, which was preliminarily prepared using a vacuum arc melter, was added in order to provide 0.1, 0.5, and 1.0 wt% Ce as an initial content. After Ce addition, the steel samples were taken again at regular time intervals, i.e. 5, 10, 15, and 30 min. Detailed experimental procedures is shown in Fig. 1(b).
The composition of steel samples was determined using inductively coupled plasma – atomic emission spectroscopy. The oxygen content in the steel samples was analyzed using a combustion analyzer (TC-300, LECO) after very careful preparation through an ultrasonic cleaning. To remove the surface oxide film, the sample was polished using a grinder and fine sand papers. Then, the sample was cut for getting 0.4 to 0.5 g in order to reduce the analytical error. Finally, the fine contaminants such as dust remaining on the sample surface were removed using the ultrasonic cleaning.
The morphology, composition, and size of oxide inclusions were characterized using field emission scanning electron microscopy (MIRA3, TESCAN) combined with energy dispersive spectroscopy (FESEM-EDS) after an electrolytic extraction method (EEM) was employed. The detailed information for the inclusion characterization using EEM in conjunction with FESEM-EDS is reported in our previous article.23,24,25,26) For simplicity, a metal sample (0.4 to 0.5 g) was dissolved in 10% AA (10% Acetylacetone – 1% Tetramethylammonium chloride – Methanol) solution under a total electric charge ranging from 4000 to 5000 coulombs (500 mA current, from 2 to 3 hours). The electrolyte solution was vacuum-filtered using a membrane filter with an open pore size of 0.2 μm. Beam diameter for EDS analysis was 1 μm. The intensity of beam was 15 keV and the measuring time was 50 seconds.
The content of total oxygen changed as a function of reaction time at different Ce additions as shown in Fig. 2. The initial oxygen content was 130 (±10) ppm and markedly decreased by addition of 0.05 wt% Al. Ten minute later, the steel sample was taken, and then the Ce (0.1 to 1.0 wt%) was added to molten steel. The oxygen content remained almost constant (55–60 ppm) when Ce content was 0.1 wt%, whereas the oxygen content has further decreased from 60 (±5) ppm to 30 (±5) ppm since Ce was added greater than about 0.5 wt%, followed by constant value. Nevertheless, the oxygen content in the 1.0 wt% Ce added system is slightly (approximately 10 ppm) lower than that in the 0.5 wt% Ce added system.
Changes of oxygen content in the 25Cr-20Ni-4Si-0.5Mn molten steel at 1873 K with reaction time at different Ce levels.
The variation in the concentration of Ce is shown in Fig. 3 as a function of reaction time at different initial Ce levels. In general, the concentration of Ce in the melt continuously decreased with increasing reaction time and a decreasing rate becomes lower at higher Ce addition. A decrease in Ce content is not only due to the formation of oxide inclusions but also due to the reaction between Ce and Al2O3 refractory, which will be discussed in detail later (Section 3.2).
Changes of cerium content in the 25Cr-20Ni-4Si-0.5Mn molten steel at 1873 K with reaction time at different Ce levels.
The content of Al in the molten steel is shown in Fig. 4 as a function of reaction time at different Ce additions. It is very interesting that the Al content initially decreased due to the formation of alumina inclusions, followed by an abrupt increase after Ce addition. A rebounding tendency of Al content after Ce addition possibly originated from a reduction of alumina inclusions by Ce because the affinity between Ce and O is greater than that between Al and O, which is confirmed from a difference in the Gibbs energy of the reactions given in Eqs. (1) and (2).27)
| (1) |
| (2) |
Changes of aluminum content in the 25Cr-20Ni-4Si-0.5Mn molten steel at 1873 K with reaction time at different Ce levels.
However, the content of Al even increased over the initial content (0.05 wt%) in the 0.5 wt% and 1.0 wt% Ce added systems. In the latter, the Al content has bounced up to about 1650 ppm since 10 min after Ce addition. Because it is insufficient to consider just the reaction between Al2O3 inclusion and Ce (Eq. (3)) for understanding the phenomena observed in Fig. 4, the interfacial reaction between alumina refractory and Ce in molten steel (Eq. (4)) will be further discussed in the following section (Fig. 5).
| (3) |
| (4) |
Schematic illustration for the cerium-inclusion and cerium-refractory reactions.
In order to confirm the interfacial reaction between cerium in molten steel and alumina refractory, the post-mortem morphology of the refractory-steel interface was observed using FESEM-EDS. The morphology of the interface at refractory side, which was in contact with steel, is shown in Fig. 6. None of reaction product at the interface was formed in the 0.1 wt% Ce added system, whereas the Ce–Al complex oxides were formed at the refractory hot face in the 0.5 wt% and 1.0 wt% Ce added systems. From an EDS analysis, the ‘CeAl11O18’ layer was formed in the former and the ‘CeAl11O18–CeAlO3’ dual phase layer was formed in the latter. The formation reaction of each compound is given in Eqs. (5) and (6).28)
| (5) |
| (6) |
Secondary electron image for the reaction products at the alumina refractory side in equilibrium with molten steel containing different Ce contents.
Moreover, it is interesting that the interfacial morphology of the 0.1 wt% Ce added system is relatively flat and clear, while the interface of the 0.5 wt% and 1.0 wt% Ce added systems becomes irregular. This tendency is quite similar in the viewpoint of steel side, which was in contact with refractory, as shown in Fig. 7. Even though there was no reaction product at the steel-refractory interface in the 0.1 wt% Ce added system, there was large oxide layer, e.g. 40 to 70 μm and 100 to 200 μm in the 0.5 wt% and 1.0 wt% Ce added systems, respectively. The oxide layer was confirmed to the ‘Ce2O3–CeAlO3’ dual phase. From the shape and the position they located, it is suggested that the Ce2O3 was initially nucleated at the surface of refractory and was locally grown toward bulk steel, and then the CeAlO3 compound was formed around the root of ‘Ce2O3 tree’ and enclosed it as shown in Fig. 8. The EDS results for the identification of each oxide phase are shown in Fig. 9.
Backscattered electron image for the reaction products at the steel side in equilibrium with alumina refractory.
Backscattered electron image for the formation of the CeAlO3 compound around the Ce2O3 at the steel-refractory interface at 1873 K.
Backscattered electron image for the identification of (a) Ce2O3, (b) CeAlO3, and (c) CeAl11O18 phases.
Based on the phenomena observed in the present experiments, the reaction mechanism between the refractory and molten steel was thermodynamically analyzed as follows.24) First, the cerium reacts with alumina refractory to form Ce2O3 at the surface of alumina refractory based on Eq. (4). The Ce2O3 can react with alumina refractory in a sequence, resulting in a formation of CeAlO3, and then CeAlO3 reacts with alumina to form CeAl11O18 in the refractory side based on Eqs. (5) and (6). Both reactions mainly occurred toward the refractory side because the activity of Al2O3 is always unity. The formation of solid compounds in refractory side provides the porous layer, through which the molten steel can penetrate into. The process is schematically drawn in Fig. 10.
Schematics of the formation process of Ce2O3, CeAlO3, and CeAl11O18 compounds at the steel-refractory interface.
The oxygen content in molten 25wt%Cr-20wt%Ni-4wt%Si-0.5wt%Mn-0.05wt%Al steel, which is in equilibrium with Ce-containing oxide phase at 1873 K was computed using thermochemical software, FactSageTM6.4. The computation of multi phase equilibria, e.g. slag-steel-inclusion-refractory systems using this package has been successful in various kinds of applications.14,22,23,24,25,26,29,30,31,32,33,34) The ‘FToxid’ and ‘FTmisc’ databases were selected for estimating the oxide and molten steel phases.
The computed results are shown in Fig. 11, wherein the CeAlO3 and Ce2O3 compounds are more stable than CeAl11O18 phase through the wide Ce content ranging from 3 ppm to 10 wt%. Thus, the CeAl11O18 phase was not formed in steel side as shown in Figs. 7 and 8 but formed in refractory side as shown in Fig. 6. The CeAlO3 phase is stable at Ce content lower than about 0.03 wt%, after which the Ce2O3 is the most stable phase.
Thermodynamic calculation of oxygen-cerium equilibria in the 25Cr-20Ni-4Si-0.5Mn-0.05Al molten steel at 1873 K under conditions of the formation of CeAl11O18, CeAlO3, and Ce2O3 as equilibrium phase.
In the present conditions, i.e. when the content of Ce is greater than about 0.1 wt%, the oxygen content in the steel which is in equilibrium with Ce2O3 is much lower than that with CeAl11O18. Hence, the oxygen can transfer from the refractory side (higher activity of oxygen) to the steel side (lower activity of oxygen) through the steel stream penetrated into the refractory grain boundaries (Fig. 9), resulting in the acceleration of formation and continuous growth of Ce2O3 at the refractory-steel interface as shown in Fig. 12. The formation and growth of Ce2O3 resulted in a local depletion of Ce at the interface and thus the stable solid phase changes from Ce2O3 to CeAlO3 as discussed in Fig. 11. The CeAlO3 compound is formed based on the following reaction.27,28)
| (7) |
Formation mechanism of the growth of CeAlO3 compound around the Ce2O3 phase at the steel-refractory interface.
Consequently, the content of Al and Ce in bulk molten steel was strongly affected by the interfacial reaction mechanism between steel and refractory as mentioned above. The steel-refractory reaction also affected the composition of inclusions, which will be discussed in the following section.
3.3. Effect of Steel-Refractory Reaction on the Inclusion Evolution ProcessThe relative fraction of inclusions in molten steel is shown in Fig. 13 according to reaction time at different Ce levels. Initial inclusion was mainly silicate, followed by a transformation to aluminosilicate (or mullite-type) inclusions due to Al deoxidation. After Ce was added, the aluminosilicate inclusions were mainly modified to Ce–Al–Si oxides in the 0.1 wt% Ce added system (Fig. 13(a)). The Ce–Al–Si oxides have gradually reclaimed to aluminosilicates since 15 min after Ce was added, which originated from the following reaction.
| (8) |
Relative fractions of several types of inclusions according to reaction time at 1873 K in the (a) 0.1 wt% Ce, (b) 0.5 wt% Ce, and (c) 1.0 wt% Ce added system.
That is, the Ce2O3 in the inclusions was reduced by Al in molten steel due to very high activity ratio of Al to Ce under equilibrium conditions, which was discussed in detail in our previous research.22)
Alternatively, the Al-rich aluminosilicates, which had been formed by Al deoxidation, were fully (80 to 100%) modified to Ce2O3 in the 0.5 wt% and 1.0 wt% Ce added systems (Figs. 13(b) and 13(c)). The Ce2O3 inclusion existed over 15 min after Ce addition, followed by a partial reduction by Al in molten steel as given in Eq. (8). Finally, the Ce–Al complex oxides were formed in the 0.5 wt% Ce added system (Fig. 13(b)). However, the Ce2O3 inclusion still survived over 30 min after Ce addition, viz. during entire experiments, in the 1.0wt%Ce added system (Fig. 13(c)). The composition variation of the inclusions according to reaction time shown in Fig. 13 can be understood by the changes in the composition of molten steel, which was affected by the refractory-steel reaction as discussed in Section 3.2.
The evolution process of the inclusions can be represented by the following three steps using SiO2–Al2O3 and Al2O3–Ce2O3 phase diagrams calculated by FactSageTM6.4 (Fig. 14). First, the SiO2-rich oxide inclusions are initially formed and directly reduced by aluminum after Al deoxidation, resulting in the formation of aluminosilicates or mullite-type inclusions (Step 1). The local equilibrium reaction can be expressed as follows (Eq. (9)) and this is consistent with a decreasing tendency of aluminum and oxygen concentrations in the melt (Figs. 2 and 4).
| (9) |
Evolution process of inclusions in the present systems at 1873 K.
Second, the Al2O3 in the inclusions are (partly) reduced by cerium after Ce addition, resulting in the formation of Ce-rich oxides or Ce2O3 inclusion based on Eq. (3) (Step 2). This is such a rapid process that almost inclusions are Ce-rich oxides or Ce2O3 just after 5 min. Third, the Ce in molten steel is continuously consumed due to the refractory-steel reaction as discussed in Section 3.2 (Figs. 7, 8, 9, 10, 11, 12). Thus, the inclusions transform from Ce–Al oxides to Al-rich aluminosilicates and from Ce2O3 to Ce–Al complex oxides in the 0.1 wt% and 0.5 wt% Ce added systems, respectively (Step 3). Here, the Ce–Al complex oxides can be considered as a mixture of CeAlO3 and CeAl11O18. However, the Ce2O3 is still remained in the 1.0 wt% Ce added system.
Three dimensional morphologies of the inclusions observed after electrolytic extraction method are shown in Fig. 15. The silicate and aluminosilicate inclusions formed earlier than Ce addition were almost spherical. Although the inclusion shape did not change in the 0.1 wt% Ce added system, the inclusion shape changes from spherical to polygonal (or hexagonal), which corresponds to Ce-rich oxides or Ce2O3. The crystal structure of Ce2O3 is hexagonal (space group P3m1) with α=28.62° and β=61.38°, which was confirmed by transmission electron microscopy (TEM) analysis by the present authros.35) The crystallographic analysis using TEM will be given elsewhere.35)
Morphology of inclusions in the Ce-containing 25Cr-20Ni-4Ni-0.5Mn molten steel deoxidized by Al at 1873 K.
The effect of Ce addition on the interfacial reaction between alumina refractory and 25wt%Cr-20wt%Ni-4wt%Si-0.5wt%Mn stainless molten steel deoxidized by Al at 1873 K was investigated in order to understand the contribution of the refractory-steel reaction to the changes in steel chemistry and thus inclusion evolution processes. Major findings and related thermodynamic analysis can be summarized as follows.
(1) The oxygen content steeply decreased by Al deoxidation, followed by a gentle decrease by Ce addition greater than 0.5 wt%. The addition of 0.1 wt% Ce did not affect the content of oxygen in molten steel. The cerium content continuously decreased, but the higher the initial Ce content, the lower the Ce-decreasing rate was obtained.
(2) The content of aluminum initially decreased due to the formation of Al-rich inclusions, followed by an abrupt increase mainly due to a reduction of alumina refractory by Ce at the steel-refractory interface. When 1.0 wt% Ce was added, the Al content bounced back by a factor of three approximately than initial Al content. The content of Al decreased again since 10 min after Ce was added, which originated from the formation of CeAlO3 compound not only as inclusions but also as refractory-steel reaction products.
(3) None of the reaction product was formed at the steel-refractory interface in the 0.1 wt% Ce added system. The CeAl11O18 and CeAlO3 compounds were formed at the interface, especially at the refractory side, while the Ce2O3 and CeAlO3 compounds were formed at the steel side in the 0.5 wt% and 1.0 wt% Ce added systems. This tendency was well explained by thermodynamic assessment, from which the Ce2O3 and CeAlO3 were more stable phases compared to the CeAl11O18 phase in molten steel.
(4) From the refractory-steel interfacial reaction mechanism, the process of inclusion evolution was proposed to three steps as follows. The initial SiO2-rich oxide inclusions are reduced by Al, resulting in the formation of aluminosilicates or mullite-type inclusions (Step 1). The Al2O3 in the inclusions are (partly) reduced by cerium, resulting in the formation of Ce-rich oxides or Ce2O3 inclusion (Step 2). Because the Ce content continuously decreased due to the refractory-steel reaction, the inclusions transform from Ce–Al complex oxides to Al-rich aluminosilicates and from Ce2O3 to Ce–Al complex oxides in the 0.1 wt% and 0.5 wt% Ce added systems (Step 3). The three dimensional morphology of the inclusions exhibited good correspondences to the inclusion evolution process mentioned above.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number NRF-2012R1A1A2041774). The authors are grateful to Dr. Hidekazu TODOROKI, Nippon Yakin Kogyo, Japan, for his fruitful discussion in regard of the refractory-steel interfacial reaction mechanism.
