Hot strip mill processing simulations on a ti-mo microalloyed steel using hot torsion testing

Abstract

Precipitation strengthened, fully ferritic microstructures in low-carbon, microalloyed steels are used in applications requiring enhanced stretch-flange formability. This work assesses the influence of thermomechanical processing on the evolution of austenite and the associated final ferritic microstructures. Hot strip mill processing simulations were performed on a low-carbon, titanium-molybdenum microalloyed steel using hot torsion testing to investigate the effects of extensive differences in austenite strain accumulation on austenite morphology and microstructural development after isothermal transformation. The gradient of imposed shear strain with respect to radial position inherent to torsion testing was utilized to explore the influence of strain on microstructural development for a given simulation, and a tangential cross-section technique was employed to quantify the amount of shear strain that accumulated within the austenite during testing. Greater austenite shear strain accumulation resulted in greater refinement of both the prior austenite and polygonal ferrite grain sizes. Further, polygonal ferrite grain diameter distributions were narrowed, and the presence of hard, secondary phase constituents was minimized, with greater amounts of austenite strain accumulation. The results indicate that extensive austenite strain accumulation before decomposition is required to achieve desirable, ferritic microstructures.