The Effect of Ladle Treatment on Steel Cleanness in Tool Steels

Detta är en avhandling från KTH Royal Institute of Technology

Sammanfattning: The aim of the present work was to get an overview of the steel cleanness in tool steel. 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 results are based on plant trials as well as on thermodynamic calculations. Trials were made on two different tool steel grades. During the trials, the top slag composition was altered and steel and slag samples were collected before and after vacuum degassing. The steel samples were analysed to determine the chemical composition of steel and slag as well as the inclusion numbers. With the aid of thermodynamic calculations, the oxygen activity for the equilibria steel/inclusion and steel/top slag was calculated. The data was compared for different calculations as well as to measured oxygen activities from the steel melts. The results showed that the top slag composition has an effect on the inclusion composition. When the CaO-content in the top slag was increased, the CaO-content in the inclusion was increased as well. The 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 for the steel/inclusion and steel/top slag equilibriums.In order to see how the inclusion characteristics evolved during vacuum degassing the vacuum treatment process was interrupted at five pre-determined time points. At each interruption of the vacuum degassing, steel and slag samples were obtained. Thereafter, the total oxygen, inclusion content and composition were determined. Before vacuum treatment the inclusion content was high in number. Thereafter, during vacuum the number of the smaller inclusions was decreased. However, for the larger inclusions (>11.2μm) the number of inclusions had increased. It was also noticed that the inclusion composition varied in samples taken before vacuum degassing. However, throughout the degassing process the inclusion composition was found to approach the top slag composition. Finally, at the end of the degassing operation only one type of inclusion composition was found in the steel melt. Also, after around 10 minutes of degassing, the inclusions reached their minimum (or plateau) values.The removal of hydrogen, nitrogen and sulphur is also of great importance during a vacuum degassing treatment in order to obtain a clean steel. The three elements were studied by using samples taken before, during (at five predetermined time points) and after vacuum degassing. The chemical steel and slag compositions were used to calculate the removal of hydrogen, nitrogen and sulphur. It was found that the removal kinetics of hydrogen and nitrogen can be described with first-order reactions. This is valid for the studied steel grade, as long as the sulphur content is below 0.003 wt-%. When 10 minutes of vacuum degassing have passed, the removal of hydrogen and nitrogen is more or less finished for the studied steel grades. In the case of sulphur it is not possible to apply a first-order reaction. Instead the sulphur content seems to follow the equilibrium sulphur content at all stages during the degassing operation. viiControlling the process is of great importance when reaching a good cleanliness of the steel. Two steel grades were studied, steel grades with similar process route and with only a few differences in steel composition. Steel and slag samples were collected and analysed. The sum of FeO and MnO was found to be a clear indicator for when reoxidation had taken place. However, the amount of carry-over slag from the electric arc furnace could not be predicted by a conclusive indicator. Also the oxygen activity was calculated and the calculations were compared with the measurements made in the liquid steel. The results 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.