Development of a model predicting inclusions precipitation in nozzles based on chemical composition and process parameters such as casting rate, liquid temperature, nozzles design and slag composition


The research proposal aimed to achieve the following objectives:

  • increasing casting reliability by optimising calcium treatment in low sulphur and high sulphur steel grades  through predicting casting behaviour  in calcium  treated steels by means of a mathematical model;
  • investigating  the  influence  of  nozzle  design  and  casting  parameters  on  nozzle  clogging  and  steel cleanliness by means of mathematical and physical models.

The main findings of the present work as regards inclusion scenario can be thus summarized:

  • inclusion composition is strictly related to steel composition; in particular Ca, Al, Mn, S and total O content  have  been  found  as  the  elements  most  influencing  the  composition  of  inclusions;  also Nitrogen  pick-up  during  casting  has  been  found  influencing  the  final  inclusion  composition, particularly as refers to Al2O3 content;
  • the  composition  of  inclusions  found  in  slabs  is  significantly  different  from  inclusions  found  in blooms, due to the different steel composition, particularly in Al, Si and Mn content;
  • also the amount of inclusions depends on cast product: blooms show a lower inclusion amount than slabs; nevertheless, no clear evidence of  the  influence of inclusion amount on castability has been found; some links between inclusion size and castability have been evidenced;    
  • a complete modification of alumina into 12CaO.7Al2O3 has been rarely found; CaO rich aluminates tend  to  react  with  S,  resulting  in  a  CaS  phase  placed  at  the  borders  of  the  inclusions  (this modification depends on Ca amount and time, probably occurs during the late stage of casting and does not affect castability);
  • rich Al2O3 phases are often found in the central part of the aluminate inclusions, which confirms a recent kinetic  interpretation  of  the  alumina  transformation  by  means  of  calcium  treatment;  this seems any way not to affect castability;
  • heats with clogging behaviour show a higher average Al2O3 content, a wider Al2O3 rich phase an less CaS formation;
  • the presence of MgO in inclusions has always been found, in variable amounts, always associated with Al2O3  (Spinel);  some  links with  castability have been  found with  trends depending on  steel grades.

A mathematical model has been developed to predict inclusions precipitation in nozzles on the basis of chemical composition and process parameters, by selecting literature results and by use of metallurgical thermodynamic data and statistical correlations derived from trial heats data. The model is able to work both  in  predictive mode  (by  predicting  the  casting  behaviour  for  a  given  steel  composition)  and  in operator-guide mode (by suggesting Ca addition to achieve good casting performance).
The  prediction  is  based  on  melt  chemistry,  Castability  Index  calculated  by  statistical  correlations, Agglomeration  Index  calculated  by  the  ratio  between  MID  (Mean  Inclusion  Diameter)  and  MIPD (Mean Inter-Particle Distance), and Deposition Rate.
The  model  has  been  tested  with  data  from  industrial  heats.  Perfomance  was  found  satisfactory  in extreme  conditions,  while  some  refining  work  is  still  needed  to  improve  its  sensitivity  in  medium conditions.  
Attempts in use of neural networks for predictive purpose have also been performed with encouraging results, which are quoted in this report. This technique could also be included in the model in order to enhance its reliability. This work will be developed out of the frame of the present project.
Theoretical  considerations,  physical  model  trials  and  numerical  simulations  were  performed  to investigate  the  phenomena  of  clogging  in  feeding  systems.  A  simple  model  was  developed  which allows identification and quantification of clogging during casting by on-line measuring of the  control system movement. Within this model clogging was correlated to the pressure conditions in the feeding system. This  correlation was  confirmed  in physical model  trials  and numerical  computations. With  a new  numerical model,  inclusion  deposition  has  been  simulated  for  a  stopper  rod  controlled  feeding
system. Highest deposition rates were found in re-circulation zones in the upper nozzle and around the ports. It has been shown that an increasing stopper rod opening area effects smaller re-circulation areas and  therefore  less  clogging  is  expected.  A  gas  film  at  the  upper  nozzle  wall,  which  can  prevent inclusion precipitation and  therefore clogging, has been simulated with an annular gas injection in the upper nozzle. This was not achieved by gas  feeding at  the stopper  rod  tip.  It has been shown  that gas injection leads to a reduction of the low pressure in the upper nozzle and therefore less air aspiration is expected.

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