Formation and evolution of inclusions in Q355B steel deoxidised with silicon and manganese

Q355B is extensively utilised owing to its superior mechanical properties. Nevertheless, the inclusions generated during the deoxidation process with aluminium and the treatment with calcium in steelmaking can profoundly influence these properties. Consequently, this study explores the inclusions formed during the manufacturing process of Q355B utilising silicon–manganese deoxidation alloys in an industrial setting. Furthermore, this research evaluates the deoxidation efficacy of the silicon–manganese alloy and elucidates the fluctuations in oxygen and nitrogen concentrations throughout the entire process. This study further investigates the types, number density, size, morphology and transformation processes of the inclusions generated throughout the Q355B production using silicon–manganese deoxidation alloy; additionally, electrolytic treatment was performed on the cast billets to extract large inclusions. The findings reveal that the deoxidation efficiency of the SiMn alloy is enhanced at relatively low temperatures and with a high manganese-to-silicon ratio. The predominant cause for the increase in total oxygen content in the molten steel during the smelting process is the exposure of the molten steel to secondary oxidation. Manganese–silicon deoxidation adequately fulfils the production requirements for Q355B. Oxide inclusions predominantly comprise Si–Mn-type oxides and their evolved inclusions; during the evolution process, aluminium-containing spinels generate calcium aluminium silicates in conjunction with CaO present in the slag. The number density of inclusions consistently decreases; however, it may subsequently increase due to secondary oxidation and slag entrapment. Throughout the evolution process, inclusions continuously aggregate and grow, resulting in an increasing proportion of large inclusions, while small-sized sulphide inclusions precipitate during the continuous casting process. The analysis of the isothermal ternary phase diagram of the slag system, in conjunction with the electrolytic results of the cast billets, confirms the occurrence of secondary oxidation and slag entrapment during the smelting process. Thermodynamic software calculations elucidate the formation and evolution of composite inclusions, offering critical insights for composition control to mitigate the associated hazards of composite inclusions. Thermodynamic and kinetic calculations of precipitates clarify the solidification conditions and temperature required for the precipitation of MnS, demonstrating that the size of the precipitates is negatively correlated with the cooling rate and positively correlated with the concentration of the strongly segregated element sulphur.