Characterization of Inclusions in IF Steels from RH-OB Degasser to Mold

1. Characterization of Inclusions in IF Steelsfrom RH-OB Degasser to Mold Tsai Hwan-Tang 蔡 煥 堂

2. Purpose of this study • Characterize the in-process steel cleanliness to develop countermeasures to improve nozzle clogging and steel surface quality.

3. Different mechanisms of nozzle clogging have been proposed. • Prior formation and transport • Inclusion formation by deoxidation or reoxidation • Transport of oxides to nozzle • Adherence of oxides to nozzle and to existing build-up • In-situ formation due to cooling

4. Steel grades studied

5. RHOB x Mold Mold Steel and Slag Sampling Locations 1,2,3,4,5,6,7,8 Minutes after Kill Ladle Start Middle End Start Middle End x x x Ladle Well Well Box Start Middle End • First three heats of a sequence • Good- and bad-plugging casts After Cast Strand 1

6. Outline • Indication of origin of plugging inclusion from • Cr2O3 pick-up in tundish slag • Variation of total oxygen content • Shape and distribution of inclusions • Electrochemical method • Remelt button • Shapes • Changes during processing • In Nozzle clogs • Trials with ladle sand with less reducible oxides

7. Tundish slag picked up chrome oxide. • Pouring box • ~5% Cr2O3 • Above nozzle well • up to 9% Cr2O3

8. Otot, Avg. Each Heat Last 2 RH Samples, ppm Total oxygen decreased from ladle at the RH-OB to the tundish pouring box. 40 Higher in Ladle 30 20 10 Higher in Tundish Pour Box 0 0 10 20 30 40 Otot, Avg. Each Heat in Tundish Pouring Box, ppm

9. Otot, Avg. Each Heat in Tundish Pouring Box, ppm In contrast, total oxygen increased from the tundish pouring box to the tundish well. 40 Higher in Tundish Pour Box 30 20 10 Higher in Tundish Well 0 0 10 20 30 40 Otot, Avg. Each Heat in Tundish Well, ppm

10. Heat Avg. Mean Residence Time, min Total oxygen in the tundish well also increased with increasing residence time in the tundish. 11 10 9 8 7 6 5 0 10 20 30 40 Otot, Avg. Each Heat in Tundish Well, ppm

11. Otot, Heat Average in Well - Pouring Box, ppm The increase in total oxygen was much greater than the increase in nitrogen. 20 O:N for Air 10 0 -10 -20 -20 -10 0 10 20 Ntot, Heat Average in Well - Pouring Box , ppm

12. Total oxygen results suggested oxygen pickup in the tundish by reaction with tundish slag or ladle sand. • Total oxygen • Increased from the tundish pouring box to the tundish well. • Increased more with increasing residence time in the tundish. • Increase was greater than nitrogen increase.

13. Alumina inclusion shape - Electrochemical method

14. Alumina inclusion size - Electrochemical method

15. Alumina mass by inclusion size - Electrochemical method

16. High Surface-Area • Faceted • Spherical • Agglomeration Inclusion Classification

17. Literature review – Nippon Steel M. Akiyoshi et al. Nippon Steel Oita R&D (1991)

18. Literature review – Hoogovens (Corus) Tiekink et al. Hoogovens Ijmuiden (1994)

19. High Surface-Area • High super-saturation of O and/or Al • i.e. initial deoxidation or re-oxidation • Faceted • Formation or growth at lower super-saturation • i.e. later deoxidation or cooling • Spherical • 'Ripening' of dendrites • Compaction of agglomerated small inclusions • Local chemical variations in steel • Agglomeration • Collection of inclusions by stirring or bubbling The literature indicates that different alumina inclusions for from different conditions.

20. SEM Analysis of Inclusions on Remelt Sample

21. Dendritic High Surface Area Inclusions O, Al Starfish O, Al Gingerbread O, Al

22. Flat, Faceted Faceted, < 2 um Faceted Inclusions O, Al O, Al Globular, Faceted > 5 um O, Al

23. Spherical Inclusions Globular, Non-Faceted > 5 um Smooth Balls O, Al, Mg O, Al

24. Agglomeration Inclusions Coral > 25 um Fine Coral O, Al O, Al O, Al Lace Balloon

25. Inclusions - Steel grade & process location • There were no definite differences between grades in inclusion shape or size distribution. • But, there was a remarkable variation of shape and size distributions from ladle to mold.

26. The frequency of small, faceted inclusions (<2 um) was highest at the end of RH-OB treatment. RH Pour Well Mold Box

27. The frequency of high surface-area and coral inclusions were highest at RH-OB. RH Pour Well Mold Box

28. RHOB aO deox1, ppm The frequency of dendritic inclusions at the end of RH-OB treatment increased with decreasing aO at kill. 300 250 200 0 5 10 Avg. No. of Inc's with Sec. Arms in Last 2 RH Samples

29. The frequency of dendritic inclusions increased with increasing oxygen activity in the tundish slag. %MnO in Well Chamber 8 6 4 2 0 0 1 2 3 No. of Inc's with Secondary Arms in Well

30. The frequency of larger globular, faceted inclusions (> 5um) was highest in the tundish. RH Pour Well Mold Box

31. No. of Globular, Faceted Inc's >5 um in Pour Box The frequency of globular, faceted inclusions >5 um decreased from pouring box to well in the tundish. 100 More in Tundish Pour Box 80 60 40 20 More in Tundish Well 0 0 20 40 60 80 100 No. of Globular, Faceted Inc's >5 um in Well

32. No. of Globular, Faceted Inc's >5 um in Well The frequency of globular, faceted inclusions >5 um decreased from the tundish to the mold. 100 More in Tundish Pour Well 80 60 40 20 More in Mold 0 0 20 40 60 80 100 No. of Globular, Faceted Inc's >5 um in Mold

33. Tundish Superheat, C The number of globular faceted inclusions (>5um) increased as tundish superheat decreased. 45 40 35 30 25 20 0 20 40 60 80 100 No. of Globular, Faceted Inc's >5 um in Pour Box

34. Percent The size of globular faceted Inclusions increased during casting. 50 40 Ladle Start 30 Ladle End 20 10 0 2 3 4 5 6 7 8 9 10 >10 microns Globular, Faceted Inclusions in the Pouring Box

35. The results indicate that globular faceted inclusions grew in the ladle by cooling and were removed in the tundish. • Globular faceted inclusions > 5um • were not present in the ladle immediately after killing. • decreased from pour box to well to mold. • Increased during casting. • increased with decreasing superheat. • Globular faceted inclusions got bigger during casting.

36. Analysis of Well Nozzle Plugs Grade A

37. Analysis of Well Nozzle Plugs – Loose powder

38. Analysis of Well Nozzle Plugs - Boundary between plugged material and steel

39. Analysis of Well Nozzle Plugs - Remelt sample from boundary region

40. No. of Inclusions per Six-Photo Strip The distributions of inclusion types were similar in the tundish well, well nozzle and mold. Thousands 10 1 0.1 0.01 0.001 0.0001 0.00001 Coral High Surface Faceted <2 um Sphere Globular Faceted > 5 Well Nozzle Mold

41. Relationship of Inclusion Morphology to Clogging • The distribution of inclusion types is similar in the steel and the plugs. • Indicating that plugging comes from inclusions formed by deoxidation or reoxidation before the steel gets to the nozzle.

42. Overall, the results pointed to the reducible ladle sand as a cause of clogging. • Reduction of chrome oxide • Chrome oxide in slag • Chromium Pick-up in steel • Total oxygen • Increased from the pouring box to the well • Increased with longer time in the tundish • Lack of N Pick-up • Dendritic Inclusions • Increased with oxygen activity in the tundish slag

43. Nozzle Clogging Factor (NCF), derived from the slide-gate position, is used to quantify plugging. Plugging Higher is better

44. Ladle sand chemistry

45. Trial ladle sands with a lower percentage of reducible oxides resulted in less nozzle clogging.

46. Conclusions • Inclusion morphology in IF steels ranges from dendritic to globular depending on the degree of super-saturation of Al and O. • Inclusion morphology is similar between grades, but changes significantly from ladle to tundish. • Globular faceted inclusions are the most frequent in the tundish, nozzle clog, and mold. • At all locations, many inclusion forms coexist in the steel.

47. Conclusions • All forms of alumina inclusions clog nozzles. • The presence of dendritic inclusions in the tundish indicates either insufficient rinsing or reoxidation. • Increase of total oxygen as tundish residence time increases and as the steel flows from pouring box to well indicated that the tundish design is less optimal and needs improvement. • Ladle sand is a significant factor in nozzle clogging.

48. Acknowledgements • Co-authors: Dr. Howard Pielet of R & D and Mr. Richard Gass of Operating Technology. • Members of the “Inclusion Characterization Team”. • The chemical analysis laboratories of Quality Department.