A Study of Slag/Metal Equilibrium and Inclusion Characteristics during Ladle Treatment and after Ingot Casting

Detta är en avhandling från Stockholm : KTH

Sammanfattning: Today, there is a high demand on clean steel for high performance materialproperties. Thus, steel producers try to deliver a steel product with the highestquality and cleanliness to the market. The number of parameters that affect thesteel cleanliness may vary depending on the required material properties of thefinal product. However, the non-metallic inclusion characteristics represent one ofthe most important parameters. More specifically, the composition, size, numberand morphology affect steel cleanliness. In this work, selected parameters affectingthe inclusion characteristics were studied using the following methods: i)thermodynamic calculations (including computational thermodynamiccalculations), ii) inclusion determinations using a cross sectional (CS) method (2Dinvestigations) and iii) inclusion determinations using an electrolytic extraction(EE) method (3D investigations).The computational thermodynamic calculations of the slag-steel and inclusion-steelequilibriums were carried out using the Thermo-Calc software. With the help ofthese calculations, the influence of the slag carryover on the top slag, aluminumcontent in steel and sulfur distribution ratio as well as predictions of stable phasesof inclusions were studied. In addition, inclusion determinations of tool steel gradesamples collected during various stages of the ladle treatment in a scrap-based steelplant were carried out using both 2D and 3D methods. Furthermore, inclusiondeterminations of bearing steel grade samples from a runner system after ingotcasting were performed using a 2D metallographic method (CS-method). Also, theINCAFeature software was used, when using cross sectional method, in order tocollect more statistics of the inclusion characteristics.It was found that slag carryover has a large influence on the composition of theactual top slag as well as the aluminum content in the steel as well as the sulfurdistribution ratio. In addition, steel and slag were found to be in “near”-equilibriumconditions, after the completion of the vacuum degassing operation. Furthermore,the composition of small-size inclusions in samples taken from tool steel was foundto be very scattered. Moreover, the composition of the large-size inclusions wasfound to be less scattered. Furthermore, closer to the top slag composition insamples collected after vacuum degassing. Finally, the accuracy of the inclusioncomposition determinations of tool steel samples using the electrolytic extractionmethod was found to be better than for the cross sectional method. The worseaccuracy of the CS-method is due to a considerable effect of matrix elements oninclusion composition.