Addition of Titanium Oxide Inclusions into Liquid Steel to Control Nonmetallic Inclusions
- PDF / 515,044 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 70 Downloads / 245 Views
DUCTION
NONMETALLIC inclusions can affect the final mechanical properties of steel significantly. Mostly inclusions are detrimental to steel properties, but they can also be beneficial and affect the grain size by inhibiting grain growth and the final microstructure by acting as nucleation sites for ferrite. By controlling the composition and size of the nonmetallic inclusions in steelmaking, the final product quality can be improved. Titanium-containing inclusions can promote grain refinement by constitutional supercooling[1] and can inhibit grain growth by the Zener pinning effect.[2] Titanium oxides are well known to act as nucleation sites for acicular ferrite.[3–10] Several mechanisms have been proposed to be responsible for titanium oxide inclusions to act as nucleation sites for acicular ferrite. The most prevailing one seems to be the absorption of Mn by Ti2O3 resulting in a manganese depleted zone (MDZ) around the inclusions that promotes ferrite formation.[3–5,7,11–15] Shim et al.[5] even concluded that Ti2O3 in Mn-free steel is inert in terms of acicular ferrite nucleation but instead formation of (Mn,Ti)2O3 inclusions is the key phenomenon. The presence of TiOx-rich inclusions containing Mn and associated with Mn depletion in the surrounding steel have resulted in steel microstructures rich in acicular ferrite in several MIIA KIVIO¨, Doctoral Student, and LAURI HOLAPPA, Emeritus Professor, are with the Department of Materials Science and Engineering, Aalto University School of Chemical Technology, FI-00076 Aalto, Espoo, Finland. Contact e-mail: miia.kivio@aalto.fi Manuscript submitted May 18, 2011. Article published online November 17, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
studies.[3,5,7–10,16,17] The inclusions formed in Ti containing steels can be very complex depending on the steel composition (especially Ti, Mn, Al, O, N, and S) and the conditions (temperature and oxygen partial pressure).[7,10,16–25] Among TiOx, several other phases can be found in the inclusions, e.g., MnS, MnTiOx, Al2O3, AlTiOx, TiN, Ti(C,N), and Ti(O,S), which all contribute to the role of the inclusions in the steel. The main object of this work was to produce smallsized, titanium-oxide–rich inclusions uniformly distributed in steel that control the grain size and promote acicular ferrite formation. For this purpose, the different TiO2 containing ‘‘master alloys’’ were added into liquid steel before or during casting. The aim was also to study the final composition of inclusions resulting from TiO2 addition as well as the inclusion types and their size distribution in low-alloyed steel. In the experiments, Ti was added first a few minutes before the TiO2 addition in order to deoxidize the steel and to modify existing inclusions to Ti-bearing oxides. Corresponding experiments with the effect of TiO2 addition directly into liquid steel have not been reported in previously published works; however, the addition of TiO2 within the steel charge before meltdown has been reported.[3,9] In an earlier paper by the authors,[26] the inoculan
Data Loading...
