Prediction of deoxidation and inclusion precipitation in semikilled steel
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I. INTRODUCTION
THERE has been a considerable thrust in recent years on the production of clean steels, i.e., steels with low nonmetallic inclusions. However, it is possible to remove inclusions in a cost-effective manner only to a limited extent. An alternative complementary strategy is to modify inclusion compositions in such a way so as to make them less harmful during subsequent casting, working, fabrication, and service. This process of inclusion modification comes under the broad area of inclusion engineering. The general approach is to have soft, deformable, and preferably globular inclusions, which would not act as stress raisers during subsequent hot and cold working of steel. Liquid deoxidation products commonly promote the preceding and also impart more cleanliness due to their easy agglomeration and flotation. In contrast, solid deoxidation products (Al2O3, TiOx, MgOAl2O3, CaS) tend to deposit at the nozzle walls during casting and cause clogging. Therefore, generation of only liquid deoxidation products must be attempted to eliminate the preceding problems. Dispersed soft inclusions in steel also improve its machinability and decrease tool wear during machining. Coarse-grained steels, commonly used for deep drawing applications, are usually Mn-Si deoxidized (semikilled), and the preferred inclusions in those steels belong to the spessartite region of the MnO-Al2O3-SiO2 phase diagram (Figure 1):[2] the mineral spessartite having the chemical formula 3MnO Al2O3 3SiO2 contains 43 pct MnO, 20.6 pct Al2O3, and 36.4 pct SiO2 by weight.[3,4] Spessartite inclusions are soft and deformable, have low liquidus temperature, low viscosity, low wettability, and excellent flotation characteristics. However, in the ternary MnO-Al2O3-SiO2 phase diagram, they exist only over a restricted range of composition.
S.K. CHOUDHARY, Researcher, and S. CHANDRA, Chief, are with the Research and Development Division, Tata Steel, Jamshedpur-831 001, India. Contact e-mail: [email protected] A. GHOSH, formerly Professor, Department of Materials and Metallurgical Engineering, IIT Kanpur, Kanpur-208 016, India, is retired. Mansucript submitted November 4, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B
For the scientific control of inclusions during deoxidation, and at subsequent stages, it is desirable that the equilibrium relationship between the liquid steel and the corresponding deoxidation product be established in advance through appropriate thermodynamic calculations in order that liquid steel analysis be correlated to the corresponding equilibrium inclusion composition. Such a thermodynamic model can be used to predict the optimum treatment condition for the precipitation of only the desirable types of inclusions in steel. Though thermodynamic equilibrium is approximately attained for many reactions during steelmaking, only an analysis of the plant data can establish it for a specific process under consideration. An additional benefit of thermodynamic analysis is that the comparison between the actual and predict
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