Optimisation of stirring conditions during vacuum degassing in order to lower inclusion content in tool steel

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 A+100 L min −1 Ar) and low (700 A+10 L min −1 Ar) stirring rates. By decreasing the Weber number to a value smaller than the critical value (We crit = 12·3) 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 μm) 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.