Speaker
Description
The achievement of yield strength levels above 460 MPa with ferrite-pearlite microstructures in medium carbon rebars requires, in addition to appropriate microstructural refinement, a certain contribution of precipitation strengthening. In addition to the minimum strength, a proper balance between TS and YS and good low cycle fatigue behavior can be other requirements that should be considered, mainly in seismic conditions. These multiple combinations of properties require an appropriate combination of chemistries with hot rolling/cooling strategies that include the addition of microalloying. On the other hand, the increasing trend to apply direct charging processing route, in many cases followed by short induction reheating cycles prior to hot rolling, introduces additional challenges about microalloying elements, partially precipitated or fully in solution at the entry of the first rolling pass.
The traditional approach has been to focus on the precipitation strengthening provided by V microalloying to achieve the mechanical characteristics. In this context, recent studies on small additions of Nb to V microalloyed rebars show new options to better achieve the mechanical requirements and improve the robustness of processing conditions. First, Nb modulates the austenite evolution during hot rolling and before transformation, mainly by its solute drag effect on recrystallization kinetics and grain growth. Second, Nb in solution increases hardenability during transformation. This also affects the interphase precipitation of V(C,N). The result is that there is a synergistic interaction between Nb and V that significantly increases the contribution of precipitation strengthening.
This paper focusses on these synergistic aspects of Nb and V, including how hot charging conditions, combined with tramp elements from scrap, can affect their function during processing.