EFFECT OF CARBON PICKUP ON THE SLAB WITH SLAG POOL THICKNESS IN ULTRA-LOW CARBON STEEL

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EFFECT OF CARBON PICKUP ON THE SLAB WITH SLAG POOL THICKNESS IN ULTRA-LOW CARBON STEEL

Min-Seok Park¹, Shin Yoo²

1POSCO Technical Research Laboratories 8, Pokposarang-gil, Gwangyang-si, Jeollanam-do, 57807, Republic of Korea

2Thermo-Fluid and Process Research Group, Technical Research Laboratories, POSCO, 6261, Donghaean-ro, Nam-gu, Pohang-si, Gyeongbuk, 37859, Republic of Korea

Keywords: Carbon pickup, Ultra-low carbon steel, Slag pool thickness, Slag rim Abstract

Carbon is easily picked up on the slab surface from the mold flux in the ultra low carbon steel (ULCS). We were trying to find ways to prevent the C-pickup in ULCS. Naturally, we were interested in the C-pickup with slag pool thickness. When the slag pool thickness was increased up to about 30mm, C-pickup on the slab was dramatically decreased and the deviation in the width direction of the C-pickup was also decreased. However, even if the slag pool thickness was thicker, C-pickup was increased when slag bear was formed. More details will be discussed in the paper.

Introduction

Carbon-pickup (C-pickup) on the slab surface is one of major surface quality issues in the ultra low carbon steel (ULCS) causing a black band in the hot rolling. Study on the C-pickup has been in progress since the 1980’s ^[1] and the main mechanism of C-pickup for ULCS is well known to be due to contact with unmelted mold flux or enriched carbon layer of mold flux during the continuous casting ^[1-5]. The best way to avoid the C-pickup on the surface of as-cast slab does not use a free carbon which is an important factor for controlling the melting rate of the mold flux. Because the use of carbon in the mold flux is inevitable, it has suggested ways to minimize C-pickup. By adding MnO2 in the mold flux as an oxidizing agent, casting trials showed 50%

reduction in C-pickup compared with the case of MnO2-free mold flux ^[3]. On the other hand, recarburization of a solidified shell was reduced by replacing carbon to Si3N4 in the mold flux ^[4-

5]. In other words, it is possible to reduce recarburization of the shell by using less carbon or increasing the thickness of slag pool. In addition, recarburization of the shell was reported to occur within 2mm from the surface of as-cast slab. In this study, an examination on how the recarburization of the shell occurs at ULCS in the conventional powder casting is presented, then comparison of the recarburization of a shell of new casting method having a thick slag pool thickness with that of a conventional method is considered.

Experimental

To analyze the C-pickup effect on the slab surface, ULCS casting through two strand caster using, not only a molten slag, but also a conventional powder at the same time in Gwangyang

Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016

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Steelworks, POSCO. Molten slag was prepared from the outside of the mold and it is freely adjustable thickness of the slag pool. The apparatus using the LNG gas was used as a melting device for melting mold flux. Casting conditions and Mold flux specification are listed below.

Table 1. Casting conditions for experiments

Steel grade (carbon content) Slab size Casting speed

0.001~0.002% 1800mm x 250mm 1.2m/min

Table 2. Mold flux specification

C_free basicity application

Flux A 2.5 0.89 Conventional powder method Flux B 16 0.89 molten mold flux feeding method

Flux B was applied to the new molten mold flux feeding method. After the casting, chips were made by every 1mm depth from the surface of slab for the carbon analysis. Chips were collected in pairs in all positions as shown in Fig.1. Especially, chips were sampled at the same depth from the center of the slab symmetrically. Carbon analysis was characterized by LECO CSLS600 carbon/sulfur determinator.

Every 1mm depth from the surface Center

Outside

Casting direction

5mm Thickness

250mm Width 1800mm

400mm 400mm Inside

Figure 1. Chip manufacturing schematic.

Results and discussions

There are two ways to reduce the recarburization of the shell as mentioned above. One is to reduce the amount of carbon in the mold flux, and the other is to increase the thickness of slag pool, and the latter method is applied to reduce the recarburization of the shell in this study.

Since molten mold flux is continuously fed into the mold it can freely adjust the thickness of slag pool. To increase the slag pool thickness, the new molten mold flux feeding technology is used.

In order to compare the C-pickup of the shell between the conventional powder casting and the new molten mold flux casting, a ULCS containing 0.001% carbon was cast in Gwangyang Steelworks POSCO. The two strand commercial caster, which had a 250ton ladle, was used for the casting trials. The conventional powder casting and the new molten mold flux casting were cast in the same heat for direct comparison of C-pickup. The thickness of slag pool was approximately 10mm for a conventional powder casting and it was about 30mm thick enough for the new molten mold flux casting.

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Figure 2 shows the carbon content on the inside slab surface (a), the center (b), and the outside slab surface (c) when the conventional powder casting is applied. Figure 2(b) indicates a segregation of carbon at the center. A bulk value of carbon can be determined except for the value of 1mm depth from the center. The bulk value of carbon for as-cast slab is 0.001%. Figure 2(a) and Figure 2(c) show carbon pickup on the surface of as-cast slab. The maximum C-pickup occurred at the 1mm deep from the surface. The depth is deeper as the amount of C-pickup was smaller. Interestingly, C-pickup occurred even at 5mm depth from the surface of as-cast slab. In addition, the amount of C-pickup was different in the width direction. In the case of conventional powder casting with slag pool of 10mm thick, C-pickup takes place non-uniformly depending on the thickness and the depth.

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

Carbon content (wt.%)

depth from the surface (mm) R400 Center L400

(a) Inside

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

(b) Center

depth from the center (mm) R400 Center L400

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

(c) Outside

Figure 2. C-pickup on a surface of as-cast slab in the conventional powder casting at the same trial heat.

Figure 3 shows the carbon content of the inside surface (a), the center (b), and the outside surface (c) of as-cast slab when the new molten mold flux casting is applied. Figure 3(b) indicates a segregation of carbon at the center as shown in Figure 2(b). The bulk value of carbon for as-cast slab is 0.00096%. Surprisingly, C-pickup did not occur in the new molten mold flux casting. The small C-pickup occurred only 1mm depth from the surface. 1.4ppm C-pickup occurred at R400 in inside surface. In the new method with slag pool of 30mm thick, C-pickup rarely happens on the shell. The slag rim on the mold wall was not observed in both strands. From this casting trial, it is obaserved that C-pickup can be minimized by maintaining a sufficient slag pool with 30mm thick.

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

(a) Inside

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

(b) Center

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

(c) Outside

Figure 3. C-pickup on a surface of as-cast slab in the new molten mold flux casting at the same trial heat

Figure 4 shows carbon content in different molten mold flux feeding trial. Bulk carbon is 0.0018% as shown in Figure 4(b). Despite the thick slag pool thickness, maximum C-pickup occurs 7ppm at L400 in outside surface. Nevertheless, C-pickup of shell is still less than that of

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the conventional powder casting. In this molten mold flux feeding trial, slag rim on mold wall was generated. Recarburization of the shell is thought to be the cause of slag rim generation.

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

(a) Inside

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

(b) Center

0 1 2 3 4 5 6

0.000 0.001 0.002 0.003

(c) Outside

Figure 4. C-pickup of a surface of as-cast slab in another new molten mold flux casting trial

Figure 5 shows slag rim formed in the test trial. The arrow tip indicates the surface of mold wall as shown in Figure 5(a). The size of slag rim is 15mm thick and 40mm long. Slag rim are composed of molten slag, unmelted powder, and carbon black. EDS image shows the granular type of materials trapped in the slag rim. EDS analysis also showed the same results as shown in Figure 5(b).

(a) Slag rim (b) SEM micro image

Figure 5. Optical microscopic image (a) and electron microscopic image of slag rim Conclusion

In the case of conventional powder casting with slag pool of 10mm thick, C-pickup takes place non-uniformly depending on the thickness and the depth. C-pickup occurred even at 5mm depth from the surface. On the other hand, when the slag pool thickness was increased up to about 30mm, C-pickup rarely happens on the shell. However, C-pickup occurred a little at the 1mm deep from the surface when the slag rim is formed.

Acknowledgement

This work was supported by POSCO through the PosLAB R&D program.

References

[1] H. Nagano and N. Masuo, “Mechanism of Carburiztion by Mold Flux in Continuous Casting Steel”, CAMP-ISIJ, vol. 2, 1989, p.259

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[2] K. Yamasaki et al., “Carburzation by Mold Powder in Continuous Casting Steel”, CAMP- ISIJ, vol. 3, 1990, p.181

[3] S. Terada, S. Kaneko, T. Ishikawa, and Y. Yoshida: Development of Mold Fluxes for Ultra- Low-Carbon Steels, I&SM, September 1991 pp. 41-44.

[4] C. Lefebvre, J. P. Radot, J. N. Pontoire, and Y. Roux: La Revue de Metallurgie-CIT, Avril 1997, PP. 489-496.

[5] P. Valentin, C. Bruch, K. Harste, H. Lachmund, M. Hecht, and J. Potschke: Carbon Pickup in Continuous Casting Processes, 2003, vol.74, No.3, PP. 139-146.

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