Production of ultra-pure steel intended for forged elements

Purpose: The purpose of this research is to analyse the technology of making ultra-pure steel intended for casting forging ingots. Forging ingots, whose mass amounts to 8 Mg, are cast using the bottom pouring method after vacuum steel degassing in the ladle. Design/methodology/approach: Data from 24 melts were analysed. Researchers studied not only the final content of oxygen, sulphur and hydrogen after vacuum steel refinement but also the quantitative amount of non-metallic inclusions in forgings made of ingots. A simulation was also conducted. Its purpose was to assess the optimal share of bauxite in the production of refining slag. The simulation was performed using thermodynamic software called FactSage 5.5. Findings: Analysing the final concentration of oxygen dissolved in liquid steel led to a conclusion that approx. 90% of analysed types of steel can be categorized as ultra-pure. The simulation results concerning refining slag formation show that the use of bauxite as a slag forming additive lead to an increase in the liquid phase, and what follows, a decrease in the share of solid precipitations - including the precipitations of lime. Research limitations/implications: It was concluded that all stages of ultra-pure steel production must be conducted conscientiously and meticulously. It is also necessary to improve the conditions of vacuum steel refining process by equipping steelworks with a new vacuum device, e.g. of VOD type. Practical implications: If all stages of steel making are conducted conscientiously and meticulously and the steelworks are equipped with a device for vacuum steel refinement, then the produced steel (from the point of view of quality) can be competitive in the market of the ultra-pure steel intended for forged products. Originality/value: The simulation results concerning the production of refining slag show that the use of bauxite as a slag forming material leads to an increase in the share of liquid phase. Refining steel under a heating layer contributes to the improvement of steel purity - which is measured by analysing the share of surface non-metallic inclusions. Refining steel under refining slag with a basic character and physical properties adapted to the temperature close to the liquidus temperature of steel contributes to a decrease in the amount of non-metallic inclusions, a decrease in their size. It can aslo affect the shape of inclusions - making them almost spherical in shape. This method should be optimised and further research should be conducted into improving the purity of steel intended for forged shafts for the power industry.