understanding the role of mould slag and slag film understanding the role of mould slag and slag...
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UNDERSTANDING THE ROLE OF MOULD SLAG AND SLAG FILM IN
SURFACE QUALITY OF CONTINUOUSLY CAST SEMIS
B Stewart, M McDonald, M Hopkins, R Burniston,
It is accepted that the majority of surface defects in the continuous casting process originate at, or within 25 mm of, the meniscus in the mould. Whether the defects propagate into cracks is dependent on the heat transfer
down the remainder of the mould and events and conditions at and below mould exit. One major influence at the meniscus and down the mould is the performance of the mould flux in terms of its melting, lubrication,
solidification and transformations. The formation of slag film between the solidifying shell and the copper mould plate is critical in terms of lubrication and heat transfer, both of which are influenced by its thickness and degree of crystallisation. The films are usually only two to four millimetres thick, but the temperature difference between one face and the other can be 950 °C. Varying the glass/crystalline ratio of the solid part of this film, has a significant and important effect on lubrication, heat transfer and thereby surface quality.
This paper describes current and recent work to understand the role of mould slag and slag film in the surface quality of continuously cast semis within Corus UK.
KEYWORDS: mould flux, slag film, heat transfer, surface quality, microscopy
A target of all continuous casting operators is the ability to cast all grades free of surface defects. This will have two advanta- ges; the ability to hot connect a greater proportion of semis to the reheating furnaces and a reduction in the requirement for surface rectification of the as-cast semi and/or rolled product. Research to achieve the target of zero surface defects includes initial solidification, novel mould technology and optimisation of mould operations and secondary cooling. Mould powders are key to the control of initial solidification and optimisation of mould operations. Although a great deal of historical inve- stigative work has been undertaken on the factors associated with mould powders and their influence on surface quality, there is still a great deal of knowledge required on their perfor- mance, especially lubrication and heat transfer, to enable their optimisation for the achievement of a defect-free semi. Mould powders that are added to the mould melt and the re- sulting slag flows between the meniscus and the copper mould to form a thin layer or film between the solidifying steel shell and the oscillating mould. In the mould-strand gap, the film
B. Stewart, M. McDonald, M. Hopkins, R. Burniston Corus Research, Development & Technology,
Paper presented at the European Conference Continuous Casting of Steel, Riccione, 3-6 June 2008
starts to solidify and, where the chemistry dictates, transforms from a glassy to a crystalline structure. This slag film has two functions lubrication and heat transfer. Uniform and consistent lubrication and heat transfer are required for optimum surface quality, especially for crack prone steel grades, such as those in the peritectic carbon range. It is well known that the abili- ty to perform these functions depends on the mould powder properties, such as composition, crystallisation tendency, mel- ting range, melting rate and the casting conditions; steel grade, superheat, casting speed and alumina pick-up. The aim of the mould powder investigator is to tune the powder properties to suit the casting conditions. This is not always straightforward and the need for compromise is illustrated by the relationship between slag solidification temperature, heat transfer and lu- brication. Slag films with low solidification temperatures offer excellent lubrication and rapid shell growth for sticker-prone grades, but the high heat transfer rates exhibited by these films can lead to longitudinal cracking in other steel grades. Work is being carried out at Corus to improve the surface qua- lity of continuously cast semis through the development and understanding of the events and conditions at the meniscus that affect slag infiltration and conditions in the mould-strand gap resulting from the properties of the slag film. Industrial trials and laboratory studies involving the characterisation of mould powder behaviour, slag films, slag rims and surfa- ce quality are being carried out in order to define operational windows for lubrication, mould heat transfer and shell growth and thereby good surface quality. Much of the work is being
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Fig. 1 Increase in mould water temperature for
narrow plates observed in Trials 13 and 14. Aumento della temperatura dell’acqua nella lingottiera per piastre strette osservato nelle prove 13 e 14.
performed as part of a collaborative project funded by the Eu- ropean Research Fund for Coal and Steel .
A major part of the work is industrial trials on the Corus Scun- thorpe Slab Caster. The Corus Scunthorpe slab caster was en- hanced in two stages between November 2006 and February 2007 to allow an increase in casting speed of up to 30% whilst eliminating intercolumnar crack which was a problem with the old segment design at speeds in excess of 0.8 m/min when casting crack sensitive grades. The secondary cooling has been modified, with a new ‘soft cooling’ practice to minimise corner and broad face off-corner transverse cracking. The oscillation practice has also been modified so as to reduce the ‘heal time’ in line with current thinking, i.e. between 0.1 and 0.15 seconds whilst maintaining the existing 6 mm stroke length. The SEN has also been modified to facilitate the increased throughput. Further details of the Scunthorpe Slab Caster enhancement have been reported by Ludlow and Barber et. al.[2,3] One of the consequences of the enhancement is the need for new mould powders to operate at higher casting speeds. In- dustrial trials have been carried out both before and after the enhancement. The tools and techniques available for the de- velopment of mould powders at Corus were described at a previous European Conference on Continuous Casting  and include mould thermal monitoring (MTM), in-mould observa- tions, monitoring of mould powder consumption, plate dips and ‘nailboards’ for measurement of liquid slag pool depth, use of inclinometers for mould taper measurements during ca- sting and laboratory characterisation of mould powders and fluxes using high temperature thermal analysis and viscome- try techniques. To date eighteen trials have been carried out, with seven diffe- rent mould powders, A to G. Of particular interest were Trials 13 and 14, cast using Powder E. Details of Powder E given in Tab. 1. During Trial 13, the mould was being observed when, after just forty-four minutes, casting was terminated prema- turely. As the steel was drained from the mould of strand 6, it was observed that there were large holes in the slag film at the right narrow face, which gave the film a ‘lacy’ appearance.
The holes were approximately 50 mm long, 20 mm wide and were interlinked with strands of slag film. These strands of slag film were possibly similar to the ‘rails’ of film reported by Linez for billet casting . The holes were orientated with the longest dimension vertical and were of a tear drop/lozenge shape. It was not possible to see whether the slag films on the other mould plates of strand 6 or of strand 5 displayed the same structure. Casting of the remaining ladles had to be rescheduled, resulting in the two ladle sequence, Trial 14. Du- ring the cap-off procedure for Trial 14, it could be seen that the slag film was continuous. Film thickness could not be deter- mined but the film appeared to be as would be expected. Transverse slices of as-cast slabs, 200 mm deep, were obtained to determine whether the slag film observations could be lin- ked to the as-cast structures that were produced. Metallogra- phic analysis was used to investigate the shell development profile, surface quality in terms of longitudinal and transverse cracking and the nature of the oscillation marks. It was found that shell thinning was present, and may occur more, in sam- ples from Trial 13 and the first ladle of Trial 14 than in the samples from the second ladle of Trial 14. It could be inferred from these results that the slag film in the first ladle of Trial 14 was in a similar condition to that in Trial 13, but became more uniform during the casting of the second ladle. It would be expected that as the slag film becomes more uniform, control of heat transfer improves and a more uniform steel shell will develop. If it is assumed that the slag film in the first ladle of Trial 14 was indeed in a similar condition to that in Trial 13, then since the slag film observed at the end of the second ladle of Trial 14 was observed to be continuous, it follows that the slag film must become continuous at some point during casting of the second ladle. In order to determine just how long it takes for the film to become continuous, the heat transfer to the mould cooling water, ΔT, and mould thermal monitoring thermocou- ple temperatures have been analysed. Analysis of the mould water ΔT values shows that in Trial 13, ΔT is still increasing when the cast is stopped after 44 minutes, Fig. 1. An explanation for the increase in ΔT with time is pro- posed below. Where the slag film is present, heat