behaviour of inclusions in RH vacuum degasser

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behaviour of inclusions in RH vacuum degasserC. J. Treadgold
desired inclusion types. Progress in the development and
application of thermodynamic software packageswhich allow Vacuumdegassing processes are now an integral
rapid calculation of equilibriumconditions in complex，multi- part of secondary steelmaking operations and
component systems such as those found in steelmaking has besides the more obvious benefits of steel
been previously reviewed.1，2 This approach has， however，
chemistry control to precise specifications， they
highlighteddiVerences between actual and calculated inclusion also reduce the number of non-metallic inclusions.
compositions， suggesting that kinetic eVects are important Care has to be taken， however， that the change in
and should be considered in any modelling of steelmaking steel composition through loss of C， O， Mn， etc.
processes.
does not result in a deleterious change in the
SigniŽ cant advances have also been made in the com-
composition of the remaining inclusions. Attempts
putational modelling of  uid  ow within unit processes.
in the past to determine the effect of vacuumon
Secondary steelmaking operations have received much inclusion compositions have been through the use
attention3–5 and recently the philosophy and practice of
of thermodynamic models， following the inclusion
linking  uid  ow and chemical equilibrium to describe engineering approach. The calculated inclusion
global equilibrium between phases as a combination of local
compositions do not， however， compare well with
equilibrium conditions has been described.6，7 the inclusions as analysed in samples taken from The present paper describes the further application of the the liquid steel after the degassing operation.
approach to the recirculating (RH) degassing process. As Clearly， it is important to take into account time
in the ladle model，6 the kinetics of ferroalloy additions and dependent effects during degassing and this has
the reaction/removal of glaze on the snorkels of the degasser been achieved by the development of a combined
vessel have been considered.
fluid flow–thermodynamic model. Using the
computational fluid dynamicsmodel， CFX， to
establish the temperature， flow and species
DESCRIPTION OFMODEL contours in a two-dimensional steel ladle under the
Prior to modelling the more complex  uid  ow in a ladle influence of natural convection， the results are
of liquid steel under the in uence of vacuum degassing， the transferred as start conditions in a three-
computational  uid dynamics (CFD) package， CFX， was dimensional RH degassermodel. A body force is
used to establish the  ow and temperature contours in a then applied to simulate the argon bubbles that are
ladle under the in uence of natural convection only， after the injected into the up-leg of the degasser and
completion of Ž lling the ladle with liquid steel. During changes in flow， temperature and species
the Ž lling operation， ferroalloys and carbon were added concentrations are calculated. Allowing for
to the liquid steel stream over speciŽ c time intervals and additions made during the process， the composition
their eVects on the temperature of the liquid steel were of the top slag and any local inclusions within the
taken into account Also accounted for was the removal of steel is predicted. The influence on top slag and
the liquid fraction of the slag glaze coating on the ladle inclusion chemistry of any glaze on the snorkels of
refractories.6 The calculated  ow and temperature contours the degasser is also taken into account. I&S/1777
in the ladle were used as the basis of the modelling of the
degassing operations， an interpolation routine being used The author is at Corus Teesside Technology Centre， PO Box 11，
Grangetown， Middlesbrough， TS6 6UB， UK (chris.treadgold### to transfer the results from the essentially two-dimensional
corusgroup.com). Based on a presentation at Clean Steel 6 held at ladle model to the three-dimensional RH vacuum degasser
Balatonfu red， Hungary on 10–12 June 2002; accepted 12 August
model. The grid for the latter model， which is based on a 2002.
plant degasser， is shown in Fig. 1. The degasser and ladle
© 2003 IoM Communications Ltd. Published by Maney for the are divided into approximately 1200 cells in which the con-
Institute of Materials， Minerals and Mining centrations of all the dissolved species， elements and oxides
are known as well as the temperature and the pressure.
Many of the assumptions concerning the processes for the
ladle model7，8 are again made for the degasser model with
INTRODUCTION the exceptions/additions of the following:
(i ) the ladle/degasser is symmetrical about a line drawn The composition， quantity， and size distribution of non-
metallic inclusions are all important in in uencing the physical through the centres of the snorkels
(ii ) liquid inclusions formed on the cells adjacent to the properties of steel. The formation，modiŽ cation and removal
of these inclusions in liquid steel is controlled by the thermo- snorkel walls are removed to the top slag while solid
inclusions remain on the snorkel walls. dynamic and  uid  ow conditions within the various