Thermodynamics of the Formation of MgO-Al 2 O 3 -TiO x Inclusions in Ti-Stabilized 11Cr Ferritic Stainless Steel
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ANIUM-STABILIZED ferritic stainless steels have been the subject of previous studies related to clogging of the submerged entry nozzle (SEN) during continuous casting process and control of the solidification structure through heterogeneous nucleation by the inclusions containing titanium oxide or nitride. In view of productivity of the melt shops and surface quality (sliver, flaw, or crack) of steel products, it is desirable to have the amount of inclusions as low as possible.[1–7] On the contrary, the use of inclusions is expected to help produce fine grains during primary solidification, which is preferable to other methodologies such as controlled deformation and heat treatment.[8,9] In these studies, the MgAl2O4 spinel inclusion could accelerate the formation of TiN in molten steel due to very low disregistry between them, and finally an
JOO HYUN PARK and SANG-BEOM LEE, Senior Researchers, are with the Stainless Steel Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Contact e-mail: [email protected] HENRI R. GAYE, Professor, is with the Clean Steel Laboratory, Graduate Institute of Ferrous Technology (GIFT), POSTECH, Pohang 790-784, Korea. Manuscript submitted January 20, 2008. Article published online November 11, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
equiaxed fine-grained structure was produced by the heterogeneous nucleation of delta ferrites on the TiN particles. Clogging phenomena of SEN or tundish nozzle during continuous casting of Ti-stabilized stainless steels have been investigated by several researchers. Hasegawa et al.[1] observed that the constriction of tundish nozzle becomes more severe as the ratio of Al2O3 to TiN increases and that the main origin of these particles is the reaction between aluminum in the stainless steel melt (type 321) and silica in the refractory, as well as the reaction products in deoxidation and nitrogen removal processes. They assumed that the buildup originated not only from the deposition of nonmetallic inclusions, but also from freezing of the steel. Gao and Sorimachi proposed that the globular Ti oxide could be formed by the reoxidation of TiN in type 409 stainless steel.[2] Their findings indicated that the major materials deposited in an immersed nozzle were globular Ti oxide and alumina agglomerates, whereas TiN and Al2O3 were observed in molten steel in the tundish along with a minor amount of Ti oxides. Maddalena et al.[3] reported that the presence of magnesium aluminate spinel in the stainless steels (types 321 and 409) accelerates the growth of the deposits from their observation of two distinct types of TiN-based deposits during analysis of clogged nozzles; pure TiN in VOLUME 39B, DECEMBER 2008—853
a three-dimensional array and TiN particles apparently connected via spinel phase. Recently, Nunnington and Sutcliffe reported that the main solubility product inclusions include Al2O3, Ti3O5, TiN, MgAl2O4, and Al2TiO5, and sometimes the precipitation of CaTiO3 (perovskite) in slag-type inclusions could be found in the stainle
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