In the production of tool steel, the control of secondary metallurgy plays an important role to meet the rapidly increasing demands for clean steel, as impurities and non-metallic inclusions can reduce its mechanical properties. This study focuses on the influence of stirring rate during the vacuum degassing of liquid steel on the inclusion characteristics. During this treatment, both gas and induction stirring are used; thus, fluid flow simulations were made for the vacuum treatment of melts at high (900 Az100 L min 21 Ar) and low (700 Az10 L min 21 Ar) stirring rates. By decreasing the Weber number to a value smaller than the critical value (We crit 512?3) at a lower stirring rate, the probability for dispersion and entrapping of slag inclusions into the liquid steel significantly decreases. Five plant heats were carried out with different rates of induction stirring and argon flow in the ladle during vacuum treatment. The results gained by light optical microscopy investigation show that the total amount of large size single inclusions (.11?3 mm) in steel samples after vacuum treatment and in the final product decreases considerably with a lowered stirring rate. Thus, the experimental results support the theoretical results based on the Weber number.

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.

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