The Effect of Solute Nb on the Austenite-to-Ferrite Transformation
- PDF / 1,023,283 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 95 Downloads / 257 Views
RODUCTION
SINCE the 1960’s microalloying with Nb has been widely used to enable thermo-mechanical controlled processing of steels. The beneficial effects of Nb are related to the fact that it forms carbides due to its strong interaction with carbon thereby producing Zener pinning on boundary motion and precipitation strengthening, while solute Nb has a strong tendency to segregate to grain or interphase boundaries because of the large lattice misfit of Nb atoms in Fe which leads to a solute drag effect on boundary motion. Regardless of Nb state in austenite, it is generally accepted that Nb delays austenite recrystallization thereby enabling controlled rolling below the recrystallization stop temperature.[1] As a result, pancaked austenite is present at the exit of finish rolling and leads upon cooling to the refinement of the final microstructure. In detail, the Nb state prior to phase transformation, however, may have for a given austenite structure two opposite effects on the austenite decomposition kinetics and resulting microstructure. When Nb forms precipitates, it can promote the ferrite or bainite transformation by removal of solute Nb thereby decreasing the solute drag effect, and possibly by providing potential nucleation sites.[2–4] On the other hand, solute Nb delays the phase transformation as has been extensively investigated.[3–8] The mechanisms of these delays have been
TAO JIA, formerly Postdoctoral Fellow with the Centre for Metallurgical Process Engineering, The University of British Columbia, 309-6350 Stores Rd., Vancouver, BC V6T1Z4, Canada, is now Associate Professor with the State Key Laboratory of Rolling and Automation, Northeastern University, P.O. Box No. 105, Shenyang, 110004 Liaoning, P.R. China. Contact e-mail: [email protected] MATTHIAS MILITZER, Professor, is with the Centre for Metallurgical Process Engineering, The University of British Columbia. Manuscript submitted March 27, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
discussed in terms of the potential effects of Nb on both nucleation and growth of ferrite. The segregation of Nb to austenite grain boundaries reduces their energy and, thus, decreases their potency as heterogeneous nucleation sites for ferrite. Further, the strong interaction of Nb with carbon decreases the diffusivity of carbon as it is partitioned from ferrite nuclei to austenite. Alternatively, the retarding effect of Nb on the ferrite transformation is primarily attributed to the solute drag mechanism.[3–5] Even though the role of Nb on phase transformation is of significant practical relevance, modeling the effect of solute Nb on the austenite decomposition kinetics has drawn comparatively little attention so far. There have been dedicated research efforts over the past decade to better understand the interaction of alloying element with migrating interfaces (Alemi).[9] For Fe-based alloys, these research efforts have primarily emphasized elements like Ni,[10–13] Cr,[14] Mo,[12,13] Si[15] and Mn.[13,16–21] Go´mez et al.[22] and Majta et al.[23] used the Johnson– Meh
Data Loading...
