Formation and Growth Mechanism of Clusters in Liquid REM- alloyed Stainless Steels

It is well known that inclusions and clusters, which are formed in liquid steel during ladle treatment, have harmful effects on the casting process (clogging problems) as well as on the final mechanical properties of a steel product. Especially, steels deoxidized or alloyed with rare earth metals (REM) are difficult to produce due to the high tendency of REM-oxides to form cluster and clog the nozzle during casting.1,2) It was reported1,3) that the main part of the nozzle accretion in stainless steels is caused by agglomeration of Ce inclusions and clusters in the nozzle. Therefore, much attention is paid to control the formation and growth of inclusions and clusters in liquid steel during ladle treatment of stainless steels. Lindborg et al.4) showed that collisions play the most important role in the growth of inclusions after the very rapid initial nucleation and diffusion growth. Different kinds of collisions play larger role for growth of inclusions over different size ranges: Brownian collisions are effective for the inclusions in the size range 0.1–1 μm; Brownian and Turbulent collisions for 1–10 μm and Turbulent and Stokes` collisions for inclusions larger than 10 μm.5–7) These conclusions have been drawn on the basis of mathematical modeling done for spherical aluminum oxide inclusions.5) However, REM-oxide clusters mostly observed in molten stainless steels have irregular shapes and larger densities (6 500–7 200 kg/m3) in comparison to the spherical Al2O3 inclusions. Therefore, REM-oxide clusters shall Formation and Growth Mechanism of Clusters in Liquid REMalloyed Stainless Steels