Study on cleanliness improvement of Interstitial-free steel based on tundish structure optimisation

Abstract

Interstitial-free (IF) steel is extensively used in many fields because of its brilliant formability and timelessness properties. However, inclusions formed in steel affects these properties to a great extent. To reduce the influence of inclusions on steel quality, an industrial produced IF steel from a steel mill was taken as the research object of this study. A comprehensive assessment of the whole process of IF steel production was conducted, revealing excessive inclusions in both tundish and slab during IF steel production. This was identified tundish as the critical bottleneck limiting the cleanliness. Hence, a systematic investigation was conducted through physical simulation, numerical simulations, and industrial trials to optimise the tundish configuration. Physical simulation results indicated when the immersion depth of long nozzle was 50 mm, the relative distance between the weir and dam was 50 mm, the distance from the weir to the centre of the long nozzle was 400 mm, and the metallurgical effect of the tundish was the best. Simultaneously, numerical simulation results demonstrated that the stagnation time of molten steel in the tundish was extended after implementing the optimised scheme, and low-temperature zone ratio at the working interface was reduced from 14.74% to 9.65%, effectively promoting the flotation and removal of inclusions. Finally, the industrial test results showed that the total oxygen content in the tundish was reduced from 4.11 × 10⁻⁵ to 3.5 × 10⁻⁵, and the number density and average size of Al₂O₃ and TiN inclusions were reduced with optimisation scheme. This study improves the cleanliness of IF steel in steel mills and provides reference for other steel mills to improve the quality of steel grades.