Influence of final stirring treatment on inclusion number in tool steel

Abstract

The focus was to study the elimination of non-metallic inclusions during the final part of the ladle refining of tool steel (AISI H13). More specifically, the final stirring treatment in a ladle before casting was modified to study how the number of inclusions could be decreased. The following three stirring treatments were tested: a conventional induction stirring with a 650 A current for 20 min, a short induction stirring with a 650 A current during 10 min, and a combined gas and induction stirring during 20 min (no open eye and a 600 A current). Steel samples were collected from the ladle before and after the final stirring period of the ladle treatment. Thereafter, the numbers of inclusions in steel samples were determined using a light optical microscope based on a classification according to the Swedish standard SS111116 (JK Scale II). Overall, the results showed that a combined gas and induction stirrings was the most efficient way of decreasing the number of DM, DH and DP inclusions. The decrease in the inclusion number in four heats was the following for the different size classes for a combined stirring procedure: 15-40% for DM type inclusions, 30-61% for DH type inclusions and 50-100% for DP inclusions. In addition, it should be mentioned that a stirring practice with induction stirring during 10 min resulted in a 67-100% decrease in the DH type inclusions. However, the results for a combined stirring case for DH type inclusions were more stable than those for a shorter induction stirring time.

Effect of Stirring Practice on Sulphur and Nitrogen Refining as well as Inclusion Removal in Ladle Treatment

“…The present results with respect to sulfur and nitrogen refining agree with results from a previous study . Malmberg et al have reported that a degassing time of 15 min is sufficient with respect to sulfur and nitrogen refining.…”

“…The aims of ladle treatment are many, namely refining with respective to sulfur, nitrogen and hydrogen, fine adjustment of the steel composition, homogenization as well as heating of the melt to reach optimum casting conditions, and inclusion removal. While a number of studies have been performed on the removal of hydrogen, nitrogen, and sulfur, many research works have been carried out to acquire a deeper understanding of the inclusion removal mechanisms during ladle refining …”

“…The reported results are in somewhat controversy. Some industrial trials showing a decrease of the total amount of inclusions during degassing have been reported . Cold model experiments, computational simulations and mathematical models have suggested that bubble stirring contributes to inclusion removal.…”

“…On the other hand, an increased argon flow has been reported not to enhance the removal efficiency . Soft argon stirring has been found to lead to a better inclusion removal efficiency than a strong stirring . Also, inert gas bubble stirring has been reported to be less efficient for removal of inclusions larger than approximately 10 μm It has even been reported that argon stirring during vacuum degassing might result in an increase of the number of inclusions larger than 22.4 μm …”

The role of process control on the steel cleanliness

“…Since then our group has carried out many studies focusing on determining how the inclusion characteristics vary during ladle treatment, based on plant trials [2][3][4][5][6]. Plant trials have also been made to study the following aspects of ladle refining operations: i) the effect of deoxidation praxis and ferroalloy additions on the inclusion characteristics [7][8][9], ii) the influence of the slag on the inclusion characteristics and sulphur content [10][11][12][13], iii) the influence of stirring on the inclusion characteristics [14][15][16][17] and the influence of sampling on determining the inclusion characteristics in process samples [18][19][20][21].…”