Steel Technology Conclave 2012
April 16th & 17th 2012
Hotel Leela Kempinski, Gurgaon, Delhi NCR
Durgapur Steel Plant (DSP) has in recent past installed a four-stand bloom caster of 0.75 Mt capacities in its existing concast shop to increase the share of continuously cast product. The caster was facing severe problems of rhomboidity, off-corner cracks and choking of sub-entry nozzles during casting of Al-killed grades which in turn restricted DSP in capturing market share in value added product segments. In addition, the caster was also unable to achieve its rated capacity due to frequent stand loss and low sequence length.
The problem of severe rhomboidity & off-corner cracks was experienced mainly due to asymmetric cooling of solidifying strand in mould as well as in upper zones of secondary cooling. The absence of water channel gap adjustment in mould during tube changing was found to be the main cause of asymmetric cooling of solidifying strand in mould. Chocking of secondary cooling nozzles particularly in foot-roll led to asymmetric cooling of solidifying strand.
Bloom casting of Al-killed steel grades was extremely difficult at DSP due to lower ladle nozzle and sub-entry nozzle (SEN) diameter. Deposition of solid alumina inclusions in SEN was evident in choking of nozzle leading to untimely abortion of sequence casting. The problem of choking was more prominent for low carbon Al-killed grades due to presence of higher oxygen levels. Improving the sequence length with flying tundish practice was also difficult due to automatic tail out system of the caster.
Several technological measures viz., introduction of Ar purging through mono-block stopper, modification in secondary refining process for effective de-sulphurisation, and calcium treatment to modify solid alumina inclusion to liquid calcium aluminates for eliminating nozzle choking during casting of Al-killed grades were introduced. Improvement in cast quality with respect to rhomboidity and off-corner cracks by controlling asymmetric cooling in mould and secondary cooling zones resulted increase in sequence length and enlargement of product basket. The down gradation of cast blooms because of rhomboidity was reduced by 55%. Modification of automatic stopper rod mechanism and strand withdrawal in PLC programming along with mould oscillation and secondary cooling led to introduction of flying tundish technology in bloom caster. Sequence length of 105 heats could be achieved with flying tundish operation. Successful implementation of these technological measures resulted in substantial improvement in caster productivity with various value added product through bloom caster at DSP.
K. Patwari*, B.R. Pal**, K Jha**, N. Pradhan*, KK Keshari*, N Banerjee*, AK Bandyopadhyay*, SK Sahay, S Mondal** TK Pratihar* & K Das**
* Research & Development Centre for Iron and Steel, Steel Authority of India Limited, Ranchi, Jharkhand
** Durgapur Steel Plant, Steel Authority of India Limited, Durgapur, West Bengal
Source - Global Business connect
Steel Technology Conclave 2012