ISIJ International, Vol. 59 (2019), No. 5

© 2019 ISIJ749

ISIJ International, Vol. 59 (2019), No. 5, pp. 749–758

* Corresponding author: E-mail: ybkang@postech.ac.krDOI: https://doi.org/10.2355/isijinternational.ISIJINT-2018-672

1. Introduction

Ultra Low C (ULC) steel sheet is used as outer panels of automobile thanks to its excellent formability. Ti is often added in order to reduce interstitial elements (C, N), thus to have higher formability.1) Physical properties (total elonga-tion, normal anisotropy, strain hardening) can be enhanced in presence of Ti.2) Ti is added in the steel by charging ferro-Ti alloy or Ti sponge during a RH process.3) However, it has been a longstanding problem that increasing Ti content leads to severe clogging of Submerged Entry Nozzle (SEN) during Continuous Casting (CC).4–9)

Deposits causing a nozzle clogging during the CC of Ti-ULC steel is different to those found during CC of Ti free-ULC steel. In the absence of Ti, coral shaped alumina is mainly observed as the clog material after CC process.10) A likely mechanism of the nozzle clogging of the Ti-free

Influence of Al/Ti Ratio in Ti-ULC Steel and Refractory Components of Submerged Entry Nozzle on Formation of Clogging Deposits

Joo-Hyeok LEE,1) Myeong-Hun KANG,2) Sung-Kwang KIM,3) Janghoon KIM,4) Min-Su KIM1,5) and Youn-Bae KANG1,6)*

1) Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, 77 Cheongamro, Namgu, Pohang, Kyungbuk, 37673 Rep. of Korea.2) Gwanyang Technical Research Laboratories, POSCO, Gwangyang, Jeonnam, 57807 Rep. of Korea.3) Steelmaking Department, POSCO, Gwangyang, Jeonnam, 57807 Rep. of Korea.4) Pohang Technical Research Laboratories, POSCO, Pohang, Kyungbuk, 37859 Rep. of Korea.5) Jeonbuk Regional Division, Korean Institute of Industrial Technology, Kimje, Jeonbuk, 54325 Republic of Korea.6) Centre for Research in Computational Thermochemistry (CRCT), Department of Chemical Engineering, Polytechnique Montreal, Montreal, Quebec, H3C 3A7 Canada.

(Received on October 11, 2018; accepted on December 4, 2018)

Nozzle clogging during continuous casting of Ti-ULC (Ultra Low C) steel was investigated by inspections of plant-used SENs and laboratory scale experiments using a rotating finger method. Various Al/Ti ratios in the steel and different kinds of nozzles (with or without CaO) were employed. Clog deposits found in the used SENs were composed of complicated oxide (CaO–Al2O3–TiOx-…), Fe drops and chunks. Increas-ing Ti concentration increased amount of the deposit the metallic deposits. In the laboratory scale exper-iments, increasing Al/Ti ratios was effective to suppress the formation of the deposit and the oxidation loss of Ti in the steel. When a refractory without CaO was used, increasing Al/Ti ratio decreased the por-tion of Ti oxide in the deposit. Almost no Fe drops were observed, except for a thin layer of Fe on surface of the oxide deposit. When CaO-lined refractory was used, many numbers of Fe drops were found inside the deposit. Increasing Al/Ti ratio lowered Ti peak in the oxide deposit. The present results lend a strong support that Ti-ULC steel is oxidized by CO gas from nozzle refractory, forming FetO-containing liquid oxide and solid alumina. The liquid oxide is reduced to leave Fe drops and Al2O3–TiOx layer. If CaO presents in the nozzle, then a complicated oxide of CaO–Al2O3–TiOx forms, as was found in the SEN from the practical operation.

KEY WORDS: Ti-ULC steel casting; nozzle clogging; Al/Ti ratio; FetO–Al2O3–TiOx; nozzle refractory.

ULC steel was proposed.11) CO gas generated by a carboth-ermic reaction of constituents in the nozzle refractory (SiO2, Al2O3, C, etc.) propagates through pores inside the refrac-tory and arrives at the interface between the nozzle refrac-tory and the liquid steel. Then it oxidizes Al dissolved in the liquid steel thereby forming a thin but dense alumina layer (network alumina). Once the network alumina layer forms, suspending alumina inclusions in the liquid steel easily attach to the network alumina layer due to eddies and non-wetting character of alumina to the liquid steel (build up alumina).10) The build up alumina layer becomes thick, thereby causing nozzle clogging. Various methods were reported to be effective to minimize the nozzle clogging.10)

When Ti presents in the steel, it becomes more severe, amount of deposits increases, and constituents of the depos-its change from the alumina to mixture of frozen steel with alumina.4,7) Its formation mechanism is yet to be revealed. A number of facts may be suspected: change of inclusion evo-lution12–18) and its associated property change (agglomera-tion and binding each other, wetting to liquid steel), change

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of wetting characteristic between liquid steel and nozzle refractory,19–21) quality of ferro Ti added to the steel,15) etc. Still, any of these facts has been confirmed to be a critical.

It has been also a question whether the addition of Ti results in formation of Ti bearing inclusion/deposit or not. A few of researchers reported that Ti-bearing oxide inclusions were found either in the deposit or in the liquid steel.4,7,15,17,18) They are mostly referred to as “Al2O3–TiOx” or Al2TiO5. On the other hand, a number of thermodynamic analyses revealed that only solid alumina is a stable oxide inclusion in the liquid steel.22–25) A transient behavior of alumina inclusions (Al2O3 → Ti-containing complex inclu-sion → Al2O3) was reported,15) but it was not clear why such transient behavior happens. Final stable inclusions found in the steel is almost alumina. It should be stressed that these thermodynamic analyses assumed a condition in bulk liquid steel where oxygen content/potential is low ([ppm O] below 10 ppm).26)

Basu et al. reported that Ti-bearing alumina (Al2O3–TiOx) inclusions were found in clog materials at few locations with large amounts of frozen steel. It was not clear how such Ti-bearing inclusions could form in thermodynamically unfavorable condition.4) However, Cui et al. reported that FeO·TiO2 was observed as major clog deposits with α-Fe, Al2O3 and FeO·Al2O3 after analyzing used SEN. They men-tioned that reoxidation of Ti-ULC steel may cause the for-mation of FeO·TiO2, although it was not confirmed.7) Sasai and co-workers carried out a series of oxidation study of ULC steels using Ar + O2 gas mixtures (PO2 = 0.05 – 0.23 atm) to simulate reoxidation condition at a tundish. They found that FetO-containing liquid oxide (and some portion of solid oxides) formed at the surface of the liquid steel.27,28) These reports may be understood that oxygen potential is one of key factors playing an important role in Ti oxidation and subsequent deposit generation.

Some of the present authors recently proposed a new pos-sibility of the cause of the severe nozzle clogging, as seen in Fig. 1.9) A carbothermic reaction in a nozzle generates CO(g). The CO(g) then oxidizes not only Al, but also Fe

and Ti in the liquid steel simultaneously, thereby forming a mixture of solid alumina and a liquid FetO–Al2O3–TiOx (“FAT”), at the interface. Due to high wettability of FetO containing liquid oxide,29) it would easily spread on the nozzle inner wall, simultaneously keeping in touch with the liquid steel. Al in the liquid steel or graphite in the refractory would reduce the FetO in the liquid oxide, leaving a reduced Fe and Al2O3–TiOx, mixed with the solid alumina.

Provided that the liquid FetO–Al2O3–TiOx plays as a precursor of the deposits in the SEN, it is thought that there should be an evidence of Ti oxide in the deposit inside SEN. Also, it becomes necessary to supress the formation of “FAT” in order to suppress the nozzle clogging. One of ways to supress it is decreasing oxygen potential at the nozzle/steel interface. Therefore, in the present study, two investigations were carried out. First, used SENs for CC of ULC steels (with Ti or without Ti) were analyzed for their deposits in order to find any evidence of Ti oxidation. Sec-ond, based on the previous findings of the present authors,9) an experimental investigation was carried out to suppress the formation of FAT by controlling oxygen potential at the interface. Two different types of refractories were employed in order to observe the formation of the deposit clearly.

2. Clog Deposits Found in Used Nozzle Refractories

A number of SENs that were used for CC of ULC steels of various Ti content were collected at Gwangyang works, POSCO, Rep. of Korea. Composition of C, Ti, and Al of the steel cast are listed in Table 1. Inner wall of the SEN that was in direct contact with the liquid steel was composed of mainly CaO, ZrO2, SiO2 and graphite. Those collected SEN were cut half perpendicularly. Part of those were further cut, mounted with epoxy resin, and polished. Shape, constitu-tion, and elemental distributions were analyzed using SEM and EDS. Figure 2 shows photographs of the half-cut SEN used for CC of ULC – 0 Ti and ULC – 200 Ti, respectively. It is obvious that increasing Ti content only up to 200 ppm causes significant changes inside the nozzle.

In case of the ULC – 0 Ti, inner wall of the SEN was relatively clean, and deposit was rarely observed by an eye inspection. On the other hand, in case of ULC – 200 Ti, inner wall of the SEN was covered by almost metallic deposit. Part of it was already corroded by showing brown color. Thickness of the deposit gradually increased in the direction of steel stream flowing (red arrow in the figure), and becomes maximum near exit port.

Thickness and constitution of the deposits found near the exit port were further analyzed using SEM. The results are shown in Fig. 3 along with a photograph of each speci-men. Right hand side of the SEM images corresponds to

Fig. 1. Proposed mechanism for nozzle clogging during continu-ous casting of Ti-ULC steels (online version in color),9) relevant to a CaO-free nozzle.

Table 1. Composition of major components, relevant to the pres-ent study, in the steel cast through the collected nozzle at Gwangyang works, POSCO (in mass percent).

Type C Ti Al

ULC – 0 Ti 0.0016 0.0022 0.04

ULC – 200 Ti 0.0025 0.02 0.03

ULC – 460 Ti 0.0025 0.046 0.04

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the nozzle refractory, and left hand side of the image cor-responds to deposits formed during the casting. In all cases, a reacted layer was observed just on the refractory surface, characterized by mixture of calcium aluminate (“CA”), cal-cium zirconate (“CZ”), and calcium aluminate with silica (“CAS”). It was not possible to specify strict stoichiometry of each component in the mixture due to inhomogeneity. Next to that layer, the other mixture of alumina (Al2O3) and entrapped mold flux (Flux) was observed when Ti content in the steel was very low (ULC – 0Ti). In case of the ULC – 200 Ti, a deposit composed of CaO–Al2O3–TiOx (“CAT”) was additionally found. Increasing Ti content up to 460 ppm (ULC – 460 Ti) resulted in relatively more “CAT”. Also, some bright spots or area were observed, which was found to be steel droplets or deposits. It increased as Ti content increased. Deposits found in inner wall of the SEN (above and below meniscus level) were also examined, and the results are listed in Table 2. It is obvious that increas-ing Ti content in the steel induces deposit of frozen steel

significantly.“CAT” found in inner wall deposits of ULC – 460 Ti

specimen was analyzed in more detail by using EDS. Figure 4 shows a BSE image of magnified image of the deposit newly formed after the casting, and its elemental mapping images. In the BSE image (Fig. 4(a)), two distinc-tive constituents are found: grey region marked as “CAT” and bright region marked as “Fe” (black area is pore filled by epoxy resin). From the elemental mapping, it is appar-ent that the “CAT” consists mostly of Al, Ca, and Ti (with weak Si and Zr peaks and strong O peak). These elements are seen to be distributed almost homogeneously over the “CAT” region. It was confirmed that Ti peak does exist in the deposits in forms of oxide. It is an important finding, because Ti in the liquid steel is supposed not to oxidize due to low oxygen potential in the liquid steel.25) As mentioned in Sec. 1, only Al is oxidized in thermodynamic point of view. However, it is obvious that the only source of Ti in this system is the liquid steel. Therefore, there should have been a source of excess oxidation of the steel during the refining and the casting. If the steel was reoxidized in a ladle during the refining and is responsible for the nozzle clogging, then it is not explained why the “Fe” is seen in the middle of the “CAT”, either as a large size (~100 μm) or a small size even below 1 μm size. It is interesting to

Table 2. Deposit found in inner wall of SEN used for continuous casting of various Ti-ULC steels.

Code Item ULC – 0 Ti ULC – 200 Ti ULC – 460 Ti Meaning

A Thickness of unreacted inner layer 2–5 mm 3–5 mm 3–6 mm

Amount of inclusions reacted with the inner coating layer

B Thickness of newly formed layer 0.2–1 mm 0.2–2 mm 0.2–2 mm

C Maximum thickness of inclusion + frozen steel 1 20 >24 Wettability between the SEN

and deposit

D Relative fraction of metal droplet in A and B low medium high Wettability between the SEN

and liquid steel

Fig. 2. Cross sectional view of SENs used in Gwangyang works, POSCO for CC of ULC steels (ULC – 0Ti and ULC – 200Ti). (Online version in color.)

Fig. 3. SEM images of deposits on the SENs in Gwangyang works, POSCO for CC of ULC steels (ULC – 0Ti, ULC – 200Ti, and ULC – 460Ti). See text for the legend. (Online version in color.)

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note that, Al and Ca peaks are almost not overlapped with that of Fe, but Ti peak overlaps with that of Fe. It suggests that the “Fe” drops contains Ti, whose intensity is not significantly lower than that in “CAT”, contrary to Al and Ca. This observation may support the proposed mechanism by the present authors as shown in Fig. 1 that FetO (and partly TiOx) in the FAT is reduced and forms reduced Fe metal in the deposit.9) Remained Al2O3–TiOx then reacts with CaO in the inner lining material of the SEN to form the “CAT”. Or the FAT may react with the CaO, then FetO would be reduced thereafter. Increasing Ti content in the steel increases chance to form FAT relative to solid alumina. Therefore, there is also more possibility to form “CAT” and “Fe” subsequently. This would explain why the “CAT” and “Fe” are found together in the deposit. If these two constituents were formed by reduction process, then the produced two constituents would be bound tightly. All these considerations suggest that reoxidation of the liquid steel in contact with inner wall of SEN is responsible for growth of deposits (through formation of “FAT” to “CAT” and reduced “Fe”).

3. Decreasing Oxygen Potential to Suppress FAT by Adjusting Steel Composition

Figure 5 shows a reported oxide stability diagram of Fe–Al–Ti–O system, representing Ti-ULC steel near casting

temperature (T = 1 540°C).9) Thin lines are iso-[ppm O] in liquid steel in equilibrium with the specified oxide phases. It may be assumed in the regime of Henry’s Law that the [ppm O] is directly proportional to chemical potential of oxygen. “Region A” corresponds to a “bulk” steel composition where alumina is a stable inclusion. “Region B” corresponds to a condition where the steel is oxidized, for example by CO(g) generated from nozzle refractory,30) which yields the “FAT”. Reoxidation path of the Ti-ULC steel using an oxide stability diagram was discussed by one of the present authors.9) If the formation of “FAT” is suppressed, then sub-sequent formation of “CAT” + “Fe” could be minimized. In order to suppress the formation of “FAT”, it is clear that oxygen potential should be decreased. From the “Region B”, decreasing oxygen potential is possible by increasing Al content in the liquid steel. This would change the stable oxide from liquid “FAT” to solid alumina. Increasing Ti content is not feasible due to its low deoxidizing ability compared to that of Al, as well as its expensive character in practical point of view.

Previous study by the present authors showed that increasing Al/Ti ratio in Fe–Al–Ti liquid alloys resulted in decreasing formation of “FAT” in oxidation product by CO(g), while the oxidation product covering the liquid alloy was composed of “FAT” and alumina.9) The experimental study was limited in the reaction between the liquid alloy representing Ti-ULC steel and CO(g) which was assumed to be generated by a carbothermic reaction in a nozzle refractory. In the present study, this idea was tested in a more realistic condition: reaction between Ti-ULC steel and nozzle refractory by immersing the refractory inside the liquid Ti-ULC steel.

4. Experimental

Two types of nozzle refractories were used in the pres-ent experiment. One is a CaO containing nozzle refractory (hereafter CZ) mainly composed of CaO, SiO2, ZrO2, and

Fig. 4. Enlarged view of the deposit on the SEN used to cast ULC – 460Ti: (a) BSE image, EDS elemental mapping for (b) Al, (c) Ca, (d) Ti, and (e) Fe. (Online version in color.)

Fig. 5. Oxide stability diagram of Fe–Al–Ti–O system at 1 540°C.25) Bold-italic numbers stand for O concentration in liquid steel. “Region A” stands for a condition in bulk liquid steel, and “Region B” represents another condition at interface between the liquid steel and the nozzle. (Online version in color.)

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graphite. This is the same as the inner wall of SEN used in the practical operation, as described in Sec. 2. However, the CZ refractory would make the analysis complicated due to the presence of CaO. It forms a liquid calcium aluminate, thereby leaving the reaction interface between the refrac-tory and the steel. Therefore, an Al2O3 based nozzle refrac-tory (hereafter AG) mainly composed of Al2O3, SiO2 and graphite was additionally employed in the present study in order to observe what had been reacted and remained at the interface between the refractory and the steel. The refractory sample (AG or CZ) was machined (D 10 × H 160 (mm)), and was connected to an equipment designed for submerg-ing/rotating by stepping motors through a brass jig.

Electric iron was melted in a MgO crucible in an induc-tion melting furnace under purified Ar gas. After the elec-tric Fe was melted at 1 560°C, additives (Fe2O3, Al pellet, and Ti sponge) were added to the liquid Fe at intervals of 5 minutes. Purpose for adding Fe2O3 was to intentionally generate deoxidation inclusions that may adhere onto nozzle refractory immersed in the liquid steel. The additives were preliminarily sealed by Fe-foil, placed inside a sealed quartz tube, and dropped onto the liquid Fe one by one using a magnet. Steel compositions prepared for the laboratory scale experiments are listed in Table 3.

During the heating for the preparation of Ti-ULC steel samples, the refractory sample was located around 10 cm above the MgO crucible for preheating. Once the steel sample was homogenized, the refractory rod was dipped into the liquid steel, and was rotated at a speed of 100 rpm. During the rotating, the steel samples were periodically taken by quartz tubes. Concentrations of Al and Ti of the steel samples were obtained by using ICP-OES. After 30 minutes of the reaction, the refractory sample was with-drawn from the liquid steel, quenched in the furnace. Mass change of each refractory during the reaction was recorded by a digital weighing scale. Then, it was cut, mounted by an epoxy resin, and polished for further examination using SEM and EDS.

5. Results

5.1. Reaction with AG RefractoryVisual appearance of the reacted refractories are shown

in Fig. 6 by taking photographs: (a) AT1.6, (b) AT2.8, and (c) AT4.8 (number means mass ratio of Al over Ti in the steels). Target Ti concentration was fixed at 500 ppm, and only Al concentration was varied. By a visual inspection, it is seen that all three refractory samples were covered by “deposit” of greenish black (AT1.6) or black (AT2.8, AT4.8) color. All three samples show metallic-like shining surface. Mass change of each refractory was shown in Fig. 6(d). All the samples show mass increase, but increasing

Al concentration in the steel sample lowered the mass gain.Cross-section of the deposit was analyzed using SEM

and EDS. Results for AT1.6 and AT4.8 are shown in Fig. 7, respectively. Both samples show that the deposit is com-posed of “Fe” and oxides (covered by the “Fe”). In case of AT1.6 (lowest Al concentration), two distinctive oxide layers were observed: Al2O3 layer and Al2O3–TiOx layer. The Al2O3–TiOx layer lied on the refractory side, and the Al2O3 layer lied between the “Fe” and the Al2O3–TiOx layer. Observing the Al2O3–TiOx layer is consistent with Kaneko et al.31) and Hiraga et al.5) Its stoichiometry was not ana-lyzed further, due to small size of the layer.

On the other hand, in case of AT4.8, the deposit was com-posed of “Fe” and Al2O3 layer, only. Ti was hardly observed in the oxide layer. “Fe” was observed on the Al2O3 layer.

Maximum of layer thickness, except for “Fe”, was shown in Fig. 6(d). The thickness was high when Al concentra-tion was low (AT1.6), but those of AT2.8 and AT4.8 were almost similar.

5.2. Reaction with CZ RefractoryVisual appearance of the reacted refractories are shown in

Fig. 8. By a visual inspection, it is seen that all three refrac-tory samples were covered by “deposit”. However, contrary to the case of AG refractory, metallic-like brightness was not significant. In order to check the presence of “Fe” in the deposit, a magnet was used by touching it to the samples. Interestingly, the reacted samples with AT1.6 and AT4.8

Table 3. Al and Ti contents in the ULC steels used in the present laboratory scale test.

Name Fe Ti (wt%) Tot. Al (wt%)

AT1.6 Bal.

0.05

0.08

AT2.8 Bal. 0.14

AT4.8 Bal. 0.24

Fig. 6. Photographs of AG nozzle refractory reacted with Ti-ULC steels (a) AT1.6, (b) AT2.8, (c) AT4.8, and (d) mass change and maximum layer thickness after the reaction. (Online version in color.)

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were found to contain the “Fe”, but the other sample reacted with AT2.8 did not stick to the magnet: the deposit did not contain “Fe”. Mass change of each refractory was shown in Fig. 8(d). All the samples lost mass during the reaction, and increasing Al concentration in the steel sample first decreased the mass (AT1.6 → AT2.8) but further increas-ing Al concentration increased the mass (AT2.8 → AT4.8).

Cross-section of the deposit was analyzed using SEM and EDS. Results for AT1.6 and AT4.8 are shown in Figs. 9 and 10, respectively. Due to the presence of CaO in the refractory, oxide phase formed on the surface of the refrac-tory seems to react with the CaO. In Fig. 9(a), a thick layer of deposit (~1 mm) was found by attaching both to the refractory and a “Fe”. A magnified BSE image shows that the layer was composed of smaller “Fe” drops, Al2O3, and a complex oxide that is thought to be a liquid at the reac-tion temperature. The size of “Fe” drops varies from sub-micrometer to over 1 mm. The liquid oxide is mainly com-posed of CaO, Al2O3, TiOx. It is interesting to note that the three different phases are seen to tightly attach each other. Figure 10 shows a deposit in the case of AT4.8. Mixture of “Fe” and oxides are seen. The size of “Fe” drops also var-

Fig. 8. Photographs of CZ nozzle refractory reacted with Ti-ULC steels (a) AT1.6, (b) AT2.8, (c) AT4.8, and (d) mass change after the reaction. (Online version in color.)

Fig. 7. BSE image and EDS elemental mapping for Al and Ti in the deposit found on AG nozzle refractory reacted with Ti-ULC steels: (a) AT1.6, (b) AT4.8. (Online version in color.)

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ies from sub-micrometer to a few hundred μm. Regarding the oxide deposit, although it was not a homogeneous nor a single phase, most part of it was observed as a complex oxide of CaO, Al2O3. Intensity of Ti in the oxide looks very weak (even weaker than that in “Fe”). Contrary to the case of AT1.6, Al2O3 was not observed as separate particles, but was found to be thoroughly mixed with CaO. It indicates that the mixture of CaO and Al2O3 might be a liquid calcium aluminate or a solid calcium aluminate such as CaO.2Al2O3 (marked as CA in the figure).

6. Discussions

6.1. “Fe” Drops Found in Deposits“Frozen steel” or “skull” is often mentioned when nozzle

clogging is concerned. As seen in Fig. 2 at an eye-inspection scale, it seems that there is only one type of such metallic deposit. However, in Figs. 4, 7, 9, and 10, small size “Fe” or thin layer of “Fe” are observed frequently. In Fig. 3 for the case of ULC – 460 Ti, it may be seen that two types of metallic deposits exist: one of large scale chunk in the steel side, and the other of smaller scale inside oxide deposit in forms of drops. It is necessary to distinguish these two types of metallic deposits. In the present study, the latter was termed simply as “Fe” and the former observed in a large scale is called as “skull”.

In the Fig. 3 (real plant operation), increasing Ti content in the ULC steel increases both “skull” and “Fe”. Forma-tion mechanism of these two metallic deposits is not neces-sarily to be identical. In the light of the present authors’ finding,9) it is postulated as follows. FetO containing liquid oxide (“FAT”) can form at the interface between the nozzle refractory and the liquid steel during the casting. And the FetO would be reduced by Al in the liquid steel or C in the refractory, leaving small size “Fe” drops. The drops are clearly seen in Fig. 4. This Fe is thought to be different to that in the liquid ULC steel passing through the nozzle. It is interesting to note the reports5,10) that fine metal particles were also found in the powdery buildup alumina obtained after casting of Al-killed steel. Si was concentrated in the metal particles. It suggests that the metal particle was likely to be generated not from the liquid ULC steel directly.

In case of the present laboratory scale test with AG refrac-tory (Fig. 7), regardless of Al/Ti ratio, “Fe” drops were not seen, but thin “Fe” layer was observed. When Al/Ti ratio is low (AT1.6), oxide layers in the deposit are porous and less dense. According to the mechanism proposed by the pres-ent authors,9) the porous Al2O3–TiOx layer is thought to be a result of reduction of FetO in “FAT”, leaving Al2O3–TiOx on the refractory side. Al2O3 was an additional reaction product in the mixture of “FAT” + Al2O3, or attached alu-mina suspended in the liquid steel due to addition of Fe2O3 in the experiment. Because a contact angle between Fe and Al2O3(–TiOx) is generally high,32) the reduced Fe might be expelled out. The “Fe” layer in the figure is thought to be a mixture of the expelled/reduced Fe and solidified steel attached that was originally the liquid steel contained in the crucible. Increasing Al/Ti to 4.8 (Fig. 7(b)) resulted in only Al2O3 layer. This layer is seen to be relatively dense with almost no pores. Al2O3–TiOx layer was not observed. This is an evidence that the “FAT” did not form in this case. Only

Fig. 9. (a) BSE image and EDS elemental mapping for (b) Al, (c) Ca, (d) Ti, and (e) Fe in the deposit found on CZ nozzle refractory reacted with Ti-ULC steels (AT1.6). (Online version in color.)

Fig. 10. (a) BSE image and EDS elemental mapping for (b) Al, (c) Ca, (d) Ti, and (e) Fe in the deposit found on CZ nozzle refractory reacted with Ti-ULC steels (AT4.8). (Online version in color.)

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Al2O3 was generated by the oxidation of this steel by CO(g). This is almost the same phenomena reported by Ogibayashi referred to as network alumina.10)

In case of the present laboratory scale test with CZ refrac-tory (Figs. 9 and 10), constitution of the deposits looks more complicated. Contrary to the AG refractory cases, many numbers of small “Fe” drops were found in the deposits, similar to the finding for the real practical case (Figs. 3 and 4). Increasing Al/Ti ratio lowered Ti intensity in the oxide deposit. This is also an evidence that “FAT” once formed, but FetO in the “FAT” was reduced, leaving Al2O3 and TiOx components in the oxide, and “Fe” as separate phase. The Al2O3 and TiOx react with CaO in the refractory, thereby forming “CAT”. Increasing Al/Ti ratio is then lowering the extent of formation of “FAT”, thus lowering “Fe” and Ti intensities in the oxide deposit. Nevertheless, once such “Fe” drops are formed, these are relatively well preserved in the oxide deposit. This is due to better wetting character of the Fe and the calcium aluminate. According to the contact angle measurement by Shinozaki et al.,33) increasing CaO content in Al2O3–CaO mixture lowers the contact angle by ~20°. This favorable wetting character allows the reduced “Fe” left in the oxide deposit, contrary to the case of AG refractory.

6.2. Mixture of “Fe” and CAT in the DepositsObservation of deposits obtained from plant casting (Figs.

3 and 4) and laboratory scale experiments with the same kind of nozzle refractory (Figs. 9 and 10) shows “Fe” drops mixed with “CAT”. Alumina particles were also found in some deposit (Fig. 9). Since the only source of Ti in the deposit is Ti in the liquid steel, it is necessary to under-stand how Ti is found in the deposit in forms of dissolve oxide (TiOx) and how such mixture of “Fe” + “CAT” (+ alumina) is developed. It should be noted that the TiOx in the “CAT” is unlikely coming from non-metallic inclusion, as most of the inclusions found in Ti-ULC steels are almost alumina.12,14–17) Schematic representation of the oxidation mechanism of Ti in the ULC steel is shown in Fig. 1. A key reaction is a simultaneous oxidation of components Ti-ULC steel (Fe, Al, and Ti) by CO gas from the nozzle refractory. By the oxidation reaction, a mixture of FAT and alumina forms, and FetO in the FAT would be easily reduced to form “Fe”.9) This phenomena may be seen in Fig. 7, where only Al2O3–TiOx and “Fe” partly reduced from the FetO are seen when Al/Ti ratio is low. In a case where CaO stabilized nozzle is used, the resultant Al2O3–TiOx reacts with the CaO, thereby forming “CAT”.

This may be understood from a phase diagram shown in Fig. 11. An isothermal section of CaO–Al2O3–TiOx near casting condition (assuming T = 1 520°C, PO2 = 10 −15 atm representing reducing condition) was calculated using FactSage thermodynamic software with FTOxid database.34) At this condition, two liquid oxide regions are seen: one in low TiOx region marked by “L1” and the other in high TiOx region marked by “L2”. In case of Ti-free ULC steel, accord-ing to the Ogibayashi’s concept,10) network alumina forms at the interface between a nozzle refractory and the liquid steel. It reacts with CaO in the nozzle, thereby forming calcium aluminate. At a proper proportion of each alumina and CaO, it would form a liquid phase (“L1”). This is how

so-called “self-cleaning” nozzle works. Build up alumina would react with the CaO in a similar manner. On the other hand, in case of Ti-ULC steel, Al2O3–TiOx remains after a reduction of FetO from “FAT”. It reacts with CaO in the nozzle and forms a liquid L2, mixed with an alumina. It is to be noted that when TiOx presents in the deposit, small addition of CaO results in the formation of L2, contrary to the case of Ti-free steel where alumina transforms CA6, CA2, then L1.

It is summarized as follows. When a Ti-ULC steel is passing through CaO stabilized SEN, a mixture of “FAT” and alumina forms as a result of oxidation by CO from the carbothermic reaction inside the nozzle refraction. FetO in the “FAT” is then reduced by Al in the steel or C in the refractory, leaving Al2O3–TiOx (this is solid state) and reduced “Fe”. CaO in the nozzle refractory is then react with the Al2O3–TiOx, and part of the Al2O3–TiOx melts to form a liquid oxide (L2). As a result, a deposit is found to be composed of L2, alumina, and “Fe”, as can be seen in Fig. 9(a). This is also very similar to the observation in Fig. 4.

6.3. Ti Yield in Liquid Steel and Growing Deposits at Different Al/Ti Ratio

Figures 12 shows Al and Ti concentration changes in the Ti-ULC steel reacted with AG refractory, after adding Ti. Al concentration slightly decreased over the reaction time in all three samples. Ti concentration of the sample of the lowest Al/Ti ratio (AT1.6) kept to decrease, while that in the other samples did not change noticeably within ~10 ppm. This result seems to reflect the observation in Figs. 6 and 7 that the Ti-ULC steel was oxidized more when Al/Ti ratio was low. Not only Al but also Fe and Ti were simultaneously oxidized, leaving the porous Al2O3–TiOx layer (Fig. 7(a)). On the other hand, when the Al/Ti ratio was high, only Al was oxidized to form the dense Al2O3 layer (Fig. 7(b)). It is thought that fraction of liquid oxide phase (“FAT”) gradu-ally decreases when Al/Ti ratio increases.

Figure 13 shows a calculated phase diagram of Fe–Al–Ti–O2 system relevant to the present experimental condi-

Fig. 11. A calculated isothermal section of CaO–Al2O3–TiOx sys-tem at T = 1 520°C and PO2 = 10 −15 atm. Note two differ-ent liquid oxides: L1 in low TiOx region and L2 in high TiOx region. (Online version in color.)

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tion, using FactSage thermodynamic software with FTOxid/FTmisc databases.34) Aluminum concentration ([wt% Al]’ = 100 × WAl/(WFe + WAl + WTi) where Wi = mass of i) and oxygen partial pressure were used as axes variables at T = 1 560°C, [wt% Ti]’ = 0.05a. This phase diagram shows sta-ble phases at a given condition. Equilibrium oxygen partial pressure at CO saturation is shown by a dashed line (~10 −10 atm). The present sample composition (AT1.6, AT2.8, and AT4.8) are represented by three vertical lines. At the CO saturation condition, all the three samples should be oxi-dized to form solid alumina and liquid oxide (FAT). How-ever, increasing [wt% Al]’ (equivalent to say “increasing Al/Ti ratio”) decreases relative fraction of the liquid “FAT”.9) Increasing Al content in Ti-ULC steel naturally lowers the possibility of Ti oxidation, as can be seen in Fig. 5. From the results shown in the Figs. 5–7, it can be found that high Al/Ti ratio suppress the oxidation of Ti (associated with Fe), thereby lowering the possibility of formation of FAT. This would be beneficial to suppress the extent of nozzle deposit formation. Interestingly, there are a number of reports that increasing Al concentration (or Al/Ti ratio) generally lowers

Fig. 13. A calculated phase diagram of Fe–Al–Ti–O2 system at T = 1 560°C, [wt% Ti]’ = 0.05, showing stable phases due to oxidation. A horizontal dashed line represents an equilibrium oxygen partial pressure by CO(g). Three vertical lines are concentration of Al in steel samples employed in the present study. (Online version in color.)

a Due to the use of oxygen partial pressure in the y-axis, usual weight percent of Al ([wt% Al] = 100 × WAl/(WFe + WAl + WTi + WO)) cannot be used in the phase diagram calculation, according to the phase diagram rule.35,36) However, due to limited amount of O, [wt% Al]’ is close to [wt% Al].

Fig. 12. Concentration changes of (a) Al and (b) Ti in Ti-ULC steels reacted with AG nozzle refractory at T = 1 560°C. (Online version in color.)

Fig. 14. Concentration changes of (a) Al and (b) Ti in Ti-ULC steels reacted with CZ nozzle refractory at T = 1 560°C. (Online version in color.)

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the occurrence of nozzle clogging during continuous cast-ing of Ti-ULC steel in practical operation.37,38) Although it was not clarified what was responsible for the suppressing the nozzle clogging by the increase of Al/Ti ratio, from the finding in the present study, it is thought that increasing Al/Ti ratio in the steel suppresses the formation of the liquid “FAT”, which is effective to bind both the refractory and to the steel, and to leave “Fe”. This phenomena would not be well observed when CaO-stabilized nozzle refractory is used, because the intermediate oxidation product “FAT” is mixed with CaO, then forming a more complicated oxides. This makes the inspection of the deposits more difficult. Nevertheless, as can be seen in Fig. 14 for the concentra-tion changes of Al and Ti in Ti-ULC steels reacted with CZ refractory, Ti concentration decreases noticeably when Al/Ti ratio was low. This also lends a support that increasing Al/Ti ratio in the steel lowers possibility of Ti oxidation (associated with Fe).

7. Conclusion

Growth mechanism of clog deposits in inner wall of SEN for CC of Ti-ULC steel and its countermeasure were investigated by inspecting used SENs in practical operation and a series of laboratory scale experiments. During the observation of the used SENs, it was found that increasing Ti content in the steel increases amount of deposits. In par-ticular, amount of “Fe” drops and layer were also increased. The “Fe” drops were found in a complex oxide mixture composed mainly of CaO, Al2O3, and TiOx. The laboratory scale experiments employing ULC samples of various Al/Ti ratios and different nozzle refractory compositions showed that increasing Al/Ti ratio generally beneficial to decrease the amount of the deposits. Followings are found in the present study:

(1) When AG nozzle refractory without CaO was used, porous Al2O3–TiOx layer and additional Al2O3 layer were found, when Al/Ti ratio was low (1.6). This seems an evi-dence of formation of FAT due to simultaneous oxidation of the steel by the refractory. When Al/Ti ratio was high (4.8), only dense Al2O3 layer was found, showing selective oxidation of Al. This decreased the formation of “FAT”.

(2) When CZ nozzle refractory with CaO was used, deposits were found to be complex mixture of “Fe”, alu-mina, and “CAT”, when Al/Ti ratio was low (1.6). This constitution was found to be similar to that observed in used SEN in practical operation. A phase diagram of CaO–Al2O3–TiOx system was used to understand the constitution. When Al/Ti ration was high (4.8), Ti peaks in the deposit was low, implying suppressed oxidation of Ti.

(3) When AG nozzle refractory was used, almost no “Fe” drops were found, except for a thin “Fe” layer on the oxide deposit. On the other hand, many “Fe” drops were observed in oxide deposits when CZ nozzle refractory was used. This is due to different wetting characters of the “Fe” drops to the different oxide deposits. And this explains why many “Fe” drops were found in deposits of the used SENs, which are lined with CaO.

(4) Increasing Al/Ti ratio in ULC steel lowers oxida-tion of Ti, thereby lowering formation of FAT. This subse-

quently lowers the amount of “Fe” reduced from the “FAT” and Ti oxide content in oxide deposits. This is partly con-sistent with the report that increasing Al content in Ti-ULC steel is beneficial to reduce extent of nozzle clogging and defect index of rolled coil of the ULC steel.37,38)

AcknowledgmentThis research was financially supported by POSCO.

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