Simplification of Hot Rolling Schedule in Ti-Microalloyed Steels with Optimised Ti/N Ratio

Thermomechanical simulations have been carried out on two Ti-microalloyed steels and one reference steel without Ti. The pinning forces exerted by TiN particles in the Ti-steels have been determined and compared with the driving forces for austenite grain growth and static recrystallisation between hot rolling passes. The driving forces for recrystallisation were found to be approximately two orders of magnitude higher than the pinning forces, which explains why the austenite in these steels barely experiences hardening during rolling and why the accumulated stress prior to the austenite→ferrite transformation is insufficient to refine the ferritic grain. On the other hand, austenite grain size hardly varies during hot rolling, as the TiN precipitates exert a strong control from the reheating temperature to the last pass. A Ti/N ratio close to 2 is able to control austenite grain growth at high austenitisation temperatures. So, both aspects-high driving forces for static recrystallisation and control on austenite grain size-allow reducing the number of passes applied. In this case, ferrite grain refinement should be reached by austenite strengthening and accelerated cooling during the transformation to ferrite.