STRATEGIES TO MODULATE THE INTERACTION OF NB WITH MICROSTRUCTURE DURING HOT ROLLING OF FLAT PRODUCTS
Nb microalloying in thermomechanical processes for flat products has been related to its suitability to enhance strain-induced precipitation and obtain a pancaked austenite prior to transformation. As new steel grades or lay-outs are developed, more complex metallurgical conditions need to be evaluated. In this context, Nb microalloying needs to intervene in a more controlled way in all steps. For instance, the hardenability enhancement related to Nb to achieve lower temperature transformation products demands an adequate balance of Nb precipitated during rolling and the one remaining in solid solution. Similar effects are also sought if Nb is used to increase strength through precipitation hardening. This work evaluates how changes in the rolling strategies affect the interaction between Nb(C,N) precipitation and austenite recrystallization during hot rolling. As the interactions are complex, application of microstructural models helps to define the best austenite conditioning and the convenient amount of Nb in solution as a function of final applications. Several strategies are discussed for both normal and coarse initial austenite grain sizes.
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