Abstract
This work addresses conflicting results in the literature regarding liquid inclusion agglomeration. To assess whether liquid calcium aluminates do agglomerate in liquid steel, laboratory experiments were performed: melting electrolytic iron, deoxidizing the melt with aluminum and subsequently calcium treating the deoxidation products (alumina and magnesia-alumina spinel inclusions). Under laboratory conditions, solid spinels and alumina inclusions were successfully modified, producing a new population of much smaller calcium aluminate inclusions. The new population of inclusions forms because the presence of calcium in the liquid steel destabilizes alumina and MgO-alumina inclusions, which then dissolve into the melt. The liquid inclusions exhibited a weak but statistically significant tendency to agglomerate. Laboratory results were assessed in the light of different collision mechanisms. Agglomeration mainly occurs by Stokes and laminar fluid flow collision when no external stirring is imposed. Monte Carlo simulations of collisions agree reasonably well with experimental results. For industrial conditions, where the liquid steel is agitated by argon bubbling and/or electromagnetic stirring, turbulent collisions dominate.
Highlights
IntroductionCalcium treatment of aluminum-killed steels is a common steelmaking practice, modifying solid non-metallic oxide inclusions (alumina and magnesia-alumina spinels) into liquid calcium aluminates
Calcium treatment of aluminum-killed steels is a common steelmaking practice, modifying solid non-metallic oxide inclusions into liquid calcium aluminates
Only one ternary diagram is shown here for samples taken after calcium treatment, and Figure 4 presents a boxplot of the Ca/Al ratio of inclusions in all samples taken after calcium treatment
Summary
Calcium treatment of aluminum-killed steels is a common steelmaking practice, modifying solid non-metallic oxide inclusions (alumina and magnesia-alumina spinels) into liquid calcium aluminates. Modifying solid alumina and spinel into liquid calcium aluminate has the benefits of improved castability, fewer surface defects and minimized susceptibility of high strength low-alloy pipe steels to hydrogen-induced cracking (HIC) (Turkdogan, 1996). Verma et al (2011) studied the early stages of calcium modification and found that initially CaS is the main reaction product (if the steel contains more than approximately 10 ppm S). Small (20 μm) can lead to catastrophic failures due to their influence on toughness (Anderson et al, 2000)
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