Abstract: In the current study, the effect of LF refining time on the composition of inclusions in tire cord steel was investigated to optimize the LF refining time of a domestic steel plant. Steel samples were taken during LF refining. The evolution of total oxygen and nitrogen contents, and the composition, number density, area fraction, and size of inclusions in steel samples were systematically analyzed to determine an appropriate refining time. It was found that the oxide inclusions were concentrated in the center and edge of the cross section of rolled products and the composition of part of inclusions were controlled in the region of low melting temperature. The total oxygen mass fraction gradually decreased during LF refining and then reached stable to 20×10^-6. Due to the operation of slagging-off after tapping of BOF, the molten steel was oxidized by air, resulting in the rapid increase of nitrogen content at the initial stage of LF refining. The nitrogen mass fraction was then reached a stable value of 22×10^-6during the refining process. When the calcium mass fraction in the molten steel was larger than 1.1×10^-6, the CaO in inclusions was gradually generated and the type of inclusions were transformed from SiO2-MnO to CaO-SiO₂-Al₂O₃. The Al₂O₃ in inclusions mainly came from raw materials such as alloy and deoxidizer. Besides, the MgO in inclusions came from ladle lining. Based on the variation of average composition, size fraction and area fraction of MgO-CaO-SiO₂-Al₂O₃-(MnO) inclusions, the soft blowing time during the current LF refining was suggested to shorten 20 min and the appropriate refining time was proposed as 96 min.

Key words:  tire cord steel, inclusion, steel cleanliness, refining time