Nitrogen Enables Superior Strength–Ductility Synergy in Ultra-Low Carbon Steel via Copious Interphase Precipitation and Grain Refinement

The increasing use of electric arc furnace (EAF) in steelmaking inevitably elevates nitrogen (N) levels, which are traditionally regarded as a detrimental element to the formability of ultra-low carbon (ULC) steels due to the formation of Lüders band. Here, we demonstrate that N could act as a beneficial microalloying element in strip casting ULC steels by promoting V(C, N) precipitation and grain refinement of ferrite. Thermodynamic calculations reveal that N significantly increases both the equilibrium volume fraction and equilibrium precipitation temperature of V(C, N), enabling copious interphase nanoprecipitation during ferrite transformation. Microstructural characterization confirms the enhanced formation of V(C, N) within interphase rows in the N-containing steels, leading to greater Zener pinning effect and smaller ferrite grain size (from 7.50 μm of 0N to 4.67 μm of 96 ppm N and 3.84 μm of 139 ppm N). As a result, owing to the enhanced nanoprecipitation and grain refinement, the N-containing ULC strip casting steels exhibit a superior strength-ductility synergy, with tensile strength increased from 666 MPa (0N) to 805 MPa (96 ppm N) and 825 MPa (139 ppm N), and a slight decrease in total elongation from 29.8% (0N) to 27.3% (96 ppm N) and 22.0% (139 ppm N). In addition, no Lüders plateau was observed in the tensile stress-strain curves as the extensive formation of V(C, N) consumed the N atoms in solid solution. These findings highlight that microalloying V in the steels produced by EAF can effectively leverage the high N content for achieving superior strength-ductility synergy.