Production of Low Alloy Steels hrough MBF-EOF

M O Gupta, N J Parikh & Vijay Sharma, Hospet Steels Limited

Introduction

Hospet Steels Limited is a 300,000 TPA integrated alloy steel plant situated at Ginigera, Dist: Koppal in North Karnataka. The Plant has been jointly put up by M/s.Mukand Limited and Kalyani Steels Limited & was commissioned in 1998.

The main facilities at Hospet Steels Limited are:

• Iron ore mines at Subbarayanahalli and Jambunathanahalli.
• Two Mini Blast Furnaces of 250 M3 volume.
• Steel Melt Shop comprising of EOF, Ladle Furnaces, Vacuum Degassing unit, billet and bloom casters.
• 2 Hi Reversible Blooming Mill.

Total work force is 700 employees. The plant is running at rated capacity.

Steel making through liquid pig iron route was envisaged since long in view of the ever-increasing power tariff coupled with scarcity & high cost of scrap of appropriate quality.

The need for high productive rates on one hand & the stringent quality requirements, applicable to alloy special steels on the other, was largely responsible for opting the MBF- EOF route for steel production, a challenging proposition since it would be the first plant in the world to do so.

Today the EOF route at Hospet Steels Limited has emerged successfully to be the largest alloy steel production facility in India & the only one amongst the four operating in the world to have successfully adopted it to manufacture alloys steels.

The wide spectrum of grades ranges from very low alloy electrode grade to highly sophisticated forging quality grades, bearing steels & micro alloyed steels etc.

With the twin-shell system, it is possible to melt down and subsequently to refine and finish heats with high chromium content in a continuous manner and under optimum conditions.

High productivity reflects in the reduced tapping weight and the resulting smaller and cheaper equipment. This fact reduces significantly plant and building-related effort and costs.

The CONARC 88t process, carried out under industrial conditions, is a unique and the first process for stainless steel, which combines properties of an electric arc furnace and a converter. Clear metallurgical treatments supported with suitable process control lead in combination with in down stream placed VOD plant or an AOD converter to production of steel quality with decidedly better characteristics. It concerns especially cleanness of the metal in view on residual tramp elements.

High input of experiences and know-how in the common development of this technology by TK A8T and ACE81TA 8.A., market leaders in the production of flat stainless steel products, ensures success in their application.

In summary one can be stated, that the new production line allows stainless steel production with highest level of productivity at significant lower production and investment costs.

The CONARC SSt process completes the family of stainless steel processes and run out far in the future with its modern technological properties.

Operations:

The EOF offers distinct advantage in terms of the charge input comprising of 80% Hot Metal at 1300°C & 20% solid scrap. This ratio can be flexibly altered upto 30% solid scrap based on the permissible heat balance & the in-built scrap preheating facility.

A standardized operating regime renders predictable decarburising rate made possible by the immense stirring energy & high O2 efficiency by the submerged tuyeres. This becomes a powerful means to achieve 'catch carbon' practice leading to low [O] potential bath for downstream refining.

The submerged tap hole and fast tilt back mechanism in conjunction with alloy feeding during tapping offers minimal carryover slag and good primary deoxidation.

The typical construction of the EOF, a combination of an Arc furnace & a converter makes continuous slag flushing possible without blowing interruptions for deslagging. Thus initial dephosporisation rate is high & the low basicity slag is flushed out of the system very early in the blowing cycle, thereby enabling 'P' levels of <0.010%.

The typical blow time in the EOF is 32-35 min, with possibility of further reduction to 25 min by means of usage of supersonic lance, enabling a productivity of 45-50 MT per hour.

Maintenance:

The short cycle blow time though necessary is not obviously a sufficient condition to move towards high productivity rate without ensuring high equipment availability.

Maintaining more than 105 water-cooled elements was a critical activity. The excessive down time due to frequent water leakages & the safety hazards thereof were major hindrance. Poor refractory life of meager 300 heats forced two shut downs of 36 hours each per month for hearth change over, causing enormous loss of production time.

• This was over come by reducing the tuyere pipe internal diameter from 16 mm to 14 mm.
• Systematic maintenance of tuyere length & timely repair of refractory around the tuyere zone.
• Elimination of water leakage in the furnace.

Water leakages were minimized by introduction of spare shell thus providing ample time for the maintenance of the water-cooled panels of the spare bottom during it's idle time at home position. This resulted into increased refractory life of 650+ heats consistently.

Punctual preventive maintenance schedules helped in maintaining the health of the Gas Cleaning Plant (GCP) systems, the heart of the EOF operations. The system now runs in safe operating conditions with out interruptions.

In house innovations in the venturi scrubber unit of the GCP has enabled precise control of the furnace operating pressures, thereby minimizing Iron losses and improving the metallic yields.

Metallurgical Achievements

• Successful deployment of EOF for high productivity Alloy Special Steel production.
• High conversion ratio of Hot metal to steel without use of mixture an intermediate storage unit. Ultrasonic Soundness at 2:1 reduction ratio in High Temperature boiler tubes.
• Banding controlled within 30 microns in low alloy steel.
• Sound bloom macro to produce crank shaft quality alloy steel for OEMs with reduction ratio of 5:1. Development of High Nitrogen micro alloy steel for crank shaft application for M/s Ford Motor Co. USA
• Development of CHQ Steel through Billet Caster.
• IBR self-certification achieved.
• ISO 9001: 2000 accreditation.
• ISO 14001:1996 accreditation.

Conclusion:

The MBF EOF route for steel making at Hospet Steels Limited has proved extremely cost effective largely due to:

• Low cost of Iron Ore due to its locational advantage.
• High percentage of charge input being low cost liquid pig iron.
• Power consumption for primary steel production is mere 33 KWH/T.
• High productivity.

The EOF at Hospet Steels Limited has established the capability of producing 28,000 MT per month of finished steel blooms. This trend should surpass more than 100% utilization of the installed capacity with the capability of meeting the most stringent quality standards.

Hospet Steels Ltd has undertaken projects to step up to 400000TPA within next financial year (2004 2005).