Revealing the precipitation kinetics of multi-stage and multi-scale Ti-bearing precipitation in a 460 MPa grade HSLA steel

The multi-stage and multi-scale Ti-bearing precipitates, containing TiN, Ti(C, N), Ti4C(2)S(2) and TiC, were systematically examined through experimental observations and our modified precipitation kinetic analyses. The result showed that cubic TiN (several microns) mainly precipitated in molten steel. After continuous casting, Ti(C, N) and Ti4C2S2 primarily precipitated in austenite, spanning 100-1000 nm in size. Upon subjected to severe rough rolling deformation, further precipitation of Ti(C, N) and Ti4C2S2 (tens of nanometers) appeared, accompanying with austenite recrystallization. During finishing rolling, recrystallized austenite grains were subdivided into multiple microbands, leading to an accumulation of deformation stored energy (DSE). The strain-induced precipitation (SIP) of spherical TiC particles (10-20 nm) distributed along microband walls, prior austenite grain boundaries, and even within the microbands. Subsequently, sequential matrix transformations, specifically ferrite (gamma -> alpha) and pearlite (gamma -> alpha + theta), were initiated during coiling. High-density TiC interphase precipitation occurred during the gamma -> alpha transformation, while a more dispersed, low-density TiC precipitation was observed after the gamma -> alpha + theta transformation. Kinetic analyses revealed that the nose temperature for Ti(C, N) precipitation in austenite was a minimum of 200 degrees C higher than that of Ti4C2S2. The DSE accelerated the TiC SIP with higher nose temperature in austenite during hot rolling. Moreover, kinetic analyses also implied that the migrating gamma/alpha interface promoted faster TiC nucleation and growth relative to dislocation nucleation, and the relative precipitation time shortened more than 20 orders of magnitude. Furthermore, both the gamma/alpha interface migrating velocity and the interface carbon content exhibited weak influence on the kinetics of interphase precipitation. However, the pearlite formation reduced the driving force of TiC precipitation, resulting in the TiC dispersed precipitation rather than TiC interphase precipitation.