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Heat transfer enhancement in the batch annealing process for product quality improvement at Tata Steel UK Trostre Works

Schoina, Lydia 2023. Heat transfer enhancement in the batch annealing process for product quality improvement at Tata Steel UK Trostre Works. EngD Thesis, Cardiff University.
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Even though global research efforts have led to major improvements in steel manufacturing processes and energy efficiency, there still is room for improvement regarding product quality, consistency and material rejection. This research focused on the batch annealing process used for steel substrate processing in tinplate manufacturing in Trostre Works, where the inherent inefficiency of non-uniform heating of the steel coils could result in insufficient interlap grain growth and steel recrystallisation, resulting thus in lower product quality and the rough steel defect. Therefore, this project investigated potential batch annealing process performance improvements regarding the batch annealed steel quality by enhancing furnace heat transfer and altering annealing cycle parameters. This was accomplished by Computational Fluid Dynamics analysis of the current furnace and 11 proposed improvements to derive coil temperature profiles during the soaking segment of the annealing cycle for the outer lap, core lap and cold spot area of the coil in furnace position 3 and assess the temperature uniformity at the end of soaking. This was followed by a techno-economic and sensitivity analysis to examine the operational, furnace availability, rough steel material rejection performance and economic feasibility of the proposals if they were to be implemented by the company. Finally, a comparison was made between the results of this research with some existing industrial trial results regarding the real impact of some proposals on the rough steel defect occurrences. The cases examined were the increased furnace setpoint soaking temperature by 5oC and 10oC, increased hydrogen percentage in the furnace protective atmosphere from 7% to 10% and 14%, increased coiling tension before batch annealing by 10%, 50%, 100% and 200%, altered silicate layer thickness deposited on the strip in the cleaning line before annealing by +50% and -50%, increased furnace base fan speed by 50rpm, altered coil sizes, with a batch of smaller coils (13.38 tonnes each) and a batch of larger coils (16.21 tonnes each), the current furnace case with 1, 2 and 4 hours of additional soaking time, 10% increased coiling tension with 1 and 2 hours of additional soaking time, 200% increased coiling tension with 1 and 2 hours of additional soaking time, 14% hydrogen case with 1 and 2 hours of additional soaking time and large coil case with 1, 2, 4, 5 and 4.9 hours of additional soaking time. The 10oC increased soaking temperature case achieved the highest inner coil temperature and the 200% increased coiling tension case with 2 extra soaking hours exhibited the highest temperature uniformity for coil 3 at the end of soaking, both potentially leading to enhanced and uniform steel recrystallisation and grain growth reaction kinetics across the coil. The lowest expected rough steel material rejection was presented by the 200% tension case with 2 extra hours and the small coil case, while the highest rejection rate was shown by the large coil batch case. The large coil case showed the lowest total cost per heating cycle whereas the small coil case and the current furnace case with 4 extra hours showed the highest total costs. The lowest required investment payback period was shown by the 10% increased coiling tension case, followed by the 200% and 100% increased coiling tension cases. The sensitivity of the 14% hydrogen content, small coil batch and 4 hours extended soaking time current furnace cases regarding the electricity, natural gas and steel unit prices deemed them high risk options. Overall, with the early 2022 prices used for the techno-economic evaluation, the 200% increased coiling tension case is the most promising for industrial implementation to reduce the rough steel defect, followed by the large coil case with 4.9 extra hours and the 100% tension case. However, after the high energy price and inflation period between 2022-2023, the large coil case with 4.9 extra hours was deemed most promising, with the 200% tension also possible but with a long-term payback period of the necessary investment. Finally, since there is already a decreasing trend in industrial energy prices and inflation, all these three cases could become again attractive suggestions for the batch annealing process performance regarding product quality.

Item Type: Thesis (EngD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: steel coil batch annealing heat transfer non-uniform temperature profile material rejection due to defects computational fluid dynamics techno-economic analysis
Funders: M2A sponsorship
Date of First Compliant Deposit: 6 March 2024
Last Modified: 08 Mar 2024 09:50

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