Optimal strategy to minimise central segregation during billet casting of high carbon steel grades

The main objective of this multi-partner, multi-national project was to enable the production of high carbon, quality critical wire rod grade steels through the billet route.- This was to be achieved through the investigation of selected techniques for the reduction and optimisation of central segregation and their interaction. The partners involved were Corus UK (ex British Steel plc) Teesside Technology Centre and Scunthorpe Works, ISPAT-Hamburger Stahlwerke, Sidenor I+D and Corus IJmuiden (ex Hoogovens Steal BV) and the project was co-ordinated by Corus UK. The technologies investigated for the reduction and optimisation of central segregation were: Thermal soft reduction (TSR) - Corus UK and Sidenor I+D Reduced and controlled steel casting superheat - Corus IJmuiden and ISPAT-Hamburger Stah werke (ISPAT-HSW) Intensive secondary cooling - Corus UK, Corus IJmuiden and Sidenor I+D Modified and improved secondary cooling design - Corus IJmuiden, ISPAT-HSW and Sidenor I+D Final EMS - Co.us UK (not part of the original plan but possible following an enhancement of the caster at Scunthorpe during the third semester of the project) Mechanical soft reduction - Corus UK (not part of the original plan but possible following the enhancement of the Scunthorpe caster) Besides these technologies, the investigation of the interaction of section size, casting speed and mould EMS on the chosen technologies formed a critical part of the research programme. Where the spec fic objectives of the individual partners were similar, it was the tuning of the technologies with plant practices that differed. The project highlighted the problems of application of several of the technologies to billet casting. Final EMS and both thermal and mechanical soft reduction all require specific conditions within the liquid core of the billet in terms of fraction solids, and in the case of F-EMS a minimum core diameter. In the work at Sidenor, where the TSR unit was fitted to the only possible location in the caster, the casting speed had to be reduced to achieve the required conditions of solids fraction, with a consequential result of reduced productivity and the introduction of other quality issues. Although the application of TSR at Corus UK was shown to be beneficial with respect to segregation, it was very difficult to apply during the open pour casting due to variation in casting speed. It is realised that problems of casting speed variation can be overcome by the introduction of sub-pour casting and dynamic control of the water flow and spray zones in the TSR unit Both Corus UK and Sidenor illustrated the fine balance in water requirements for the TSR, between the maximum water quantity for the reduction of segregation and the minimum water quantity to ensure that no internal reheat- type cracks are developed. With respect to F-EMS at Corus UK, its benefits were observed on 140 and 180 mm billet format, albeit with the development of a white band in the 180 mm section. For these two sections the location of the stirrer was at the correct position for the ideal conditions within the billet with respect to fraction solids etc. No benefit to segregation was noted in the 120 mm section, even at 3.8 to 4.0 m/min casting speed, at which speed the conditions within the centre of the billet were theoretically ideal.