Characterization of Compound Particles Formed during Thin Slab Direct Rolling of Ti-added Nb HSLA Steel

Abstract

The absence of a reheating stage in thin slab direct rolling of Ti-added Nb HSLA steel results in the formation of compound two-phase particles prior to and during rough rolling in an in-line strip processing line. The compound two-phase particles are composed of a cuboid Ti-rich (Ti_xNb_1–x)N (0.76≥x>0.72) core and a Nb-rich cap-shaped epitaxial deposit of (Ti_xNb_1–x)C (0.29≥x>0.09) or NbC formed on one of the {100}-type faces of the cuboid (Ti_xNb_1–x)N (0.76≥x>0.72) core. At the interface between the cuboid core and the cap-shaped deposit, the Ti/(Nb+Ti) atomic ratio was found to increase gradually from a low value of Ti/(Nb+Ti)≈0, on the cap-side of the particles, to a high value of Ti/(Nb+Ti)≈0.6, on the cuboid core side of the precipitate. The fact that the compound two-phase particles are present at 1200°C indicates that they have a greater thermodynamic stability compared to NbN or NbC. A kinetic precipitation model was used to evaluate three possible mechanisms for the formation of the compound particles: a low cap/cuboid interfacial energy and a high matrix/cuboid interfacial dislocation density.