Aluminum-Titanium Complex Deoxidation Equilibria in Fe-Cr-Al-Ti-O Melts
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INTRODUCTION
ALUMINUM and titanium are both important alloying elements for the refining process of ferritic stainless steels and the physical properties of the final products. Al is added after the decarburization process of stainless steelmaking to deoxidize the liquid steel as well as to recover Cr by reducing the remaining Cr2O3 in the converter slag. The main purpose of Ti alloying in ferritic stainless steels is ‘‘stabilization.’’ When the Ti content exceeds the critical amount (e.g., [pct Ti] = 0.2 + 4 ([pct C] + [pct N])), it binds effectively the dissolved C and N by forming TiC and TiN precipitates in solid steel. The formation of Cr carbides and nitrides is then avoided.[1] Adding Ti following Al deoxidation is effective in minimizing excess oxidation of Ti and enhance the corrosion resistance by the improved yield of Ti, while the formation of Ti oxide inclusions cannot be completely avoided due to its relatively high affinity with O in the liquid steel. Since the various kinds of
MIN-KYU PAEK and LAURI HOLAPPA are with the Department of Chemical and Metallurgical Engineering, Aalto University, Espoo, 02150, Finland; JUNG-MOCK JANG is with the Steelmaking Research Group, Technical Research Lab., POSCO, Pohang, 37859, Korea; JONG-OH JO is with the R&D Center, Hyundai Steel Co. Ltd, Dangjin, 31719, Korea; JONG-JIN PAK is with the Department of Materials Science and Chemical Engineering, Hanyang University, ERICA, Ansan, 15588, Korea. Contact e-mail: [email protected] Manuscript submitted 19 June, 2020; Accepted October 15, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
deoxidation products such as Al2O3, Ti3O5, and Ti2O3 can be coexisting in the steel melt, these simultaneous reactions have been called ‘‘Al-Ti complex deoxidation equilibria’’ in liquid iron alloys. The Al-Ti complex deoxidation in liquid iron has been extensively studied from various points of view; inclusion evolution,[2,3] inclusion morphology and chemistry,[4–8] inclusion stability diagram,[9–11] refractory corrosion,[12] clogging of submerged entry nozzle,[13,14] re-oxidation,[15] and grain refinement.[16] However, the fundamental studies on the Al-Ti complex deoxidation equilibria in the Cr containing liquid steel are very scarce. To understand the various phenomena during the Al-Ti complex deoxidation in ferritic stainless steel melts, accurate thermodynamic information on O behavior and oxide formation in Fe-Cr-Al-Ti-O alloy melts has to be secured first. Therefore, in the present study, the Al and Ti deoxidation equilibria in Fe-Cr alloy melts were measured at 1823 K to 1923 K (1550 °C to 1650 °C) by the addition of the deoxidizers. Wagner’s formalism[17] was used to analyze the present experimental results and the available literature data. Although there are inherent limitations of the interaction parameter formalism to describe the thermodynamics of O in the entire alloy concentration,[18] it is practically acceptable for the composition range of commercial ferritic stainless steels containing Al and Ti up to 0.2 and 0.8 mass pct
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