Compound two‐phase particles were found in thin slab direct rolled Ti‐added Nb high strength low alloy (HSLA) steel after the rough rolling stage of an in‐line strip processing line. The compound two‐phase particles are composed of a cuboid Ti‐rich (TixNb1−x)N (0.73≥x > 0.60) core and an Nb‐rich cap‐shaped epitaxial deposit of (TixNb1−x)C (0.35≥x > 0.06) or NbC formed on the {100}‐type faces of the cuboid (TixNb1−x)N (0.73≥x > 0.60) 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) ≈ 0.7, on the cuboid core side of the precipitate. The fact that compound TiN/NbC two‐phase particles are present in the matrix indicates that NbC has a greater thermodynamic stability, i.e. a lower solubility, in the presence of TiN precipitates. A kinetic precipitation model is proposed to explain this effect. The model is based on the formation of geometrically necessary dislocation (GND) around TiN precipitates. A high TiN volume fraction leads to a high density of GND and a high density of heterogeneous nucleation sites for NbC. Consequently, the solute Nb content is lowered and the strain accumulation due to the suppression of the inter‐pass recrystallization is limited. This results in the high softening ratio observed in multi‐hit compression test.

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 (TixNb1-x)N (0.76 ≥ x>0.72) core and a Nb-rich cap-shaped epitaxial deposit of (TixNb1-x)C (0.29 ≥ x>0.09) or NbC formed on one of the {100}-type faces of the cuboid (TixNb1-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 1 200°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.