The Effect of Ladle Treatment on Steel Cleanness in Tool Steels

Author: Karin Steneholm; Pär Jönsson; Guozhu Ye; Kth; []

Keywords: ENGINEERING AND TECHNOLOGY; TEKNIK OCH TEKNOLOGIER; TEKNIK OCH TEKNOLOGIER; ENGINEERING AND TECHNOLOGY; oxygen activity; inclusion characteristics; vacuum degassing; desulphurization; slag; ladle; hydrogen; nitrogen; deslagging; Materials Science and Engineering; Teknisk materialvetenskap;

Abstract: The aim of the present work was to get an overview of the steel cleanness in tool steel. Plant trial studies of three steel grades were done with different focuses.Firstly the change of the inclusion characteristics during the vacuum degassing in the ladle was looked upon. The top slag composition was altered and sampling was made before and after vacuum degassing and the results showed that the top slag composition has an effect on the inclusion composition. Thermodynamic calculations indicated that the oxygen activity for the steel/inclusion was twice as large as that of the steel/top slag before vacuum degassing. However, after vacuum degassing the oxygen activity values were close.In order to study the inclusion characteristics during vacuum degassing the process was interrupted at five pre-determined time points for sampling. During vacuum the number of the smaller inclusions was decreased. However, for the larger inclusions (>11.2µm) the number of inclusions had increased. Throughout the degassing process the inclusion composition was found to approach the top slag composition and at the end of the process only one type of inclusion composition was found.The removal of hydrogen, nitrogen and sulphur was studied. Samples were taken before and after vacuum degassing. It was found that the removal kinetics of hydrogen and nitrogen can be described with first-order reactions. When 10 minutes of vacuum degassing have passed, the removal of hydrogen and nitrogen is more or less finished for the studied steel grades.Two steel grades with similar process route and with only a few differences in steel composition were studied. The sum of FeO and MnO was found to be a clear indicator for when reoxidation had taken place. The results from oxygen activity calculations indicate that calculations of oxygen activities with multivalence slag species such as Fe and Cr requires measurements for validations to ensure that reliable results are obtained.

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