Study on the Bending Formability and Strengthening–Toughening Mechanisms of Nb-Containing Ultra-High-Strength Steel for Automotive Welded Tubes

As the automotive industry accelerates its transformation toward lightweighting and high-performance design, the demand for ultra-high-strength welded tube steels in body and chassis structural components has become increasingly urgent. However, these materials face severe challenges in cold bending formability, damage resistance, and cracking resistance. In this work, a Nb-containing high-strength steel for automotive welded tubes was developed, and the effects of the composite microalloying design on the microstructure, bending formability, and strengthening–toughening mechanisms of the experimental steel were systematically investigated. Using characterisation techniques including scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction (XRD), the synergistic action and quantitative contributions of grain-refinement strengthening, dislocation strengthening, and precipitation strengthening were thoroughly elucidated. The results show that after austenitisation and tempering, the microstructure of the experimental steel is predominantly lath martensite, with martensite lath packets separated by high-angle grain boundaries, which account for 66.2% of the boundary population. Nb significantly refines the prior austenite grains and martensite blocks through grain-boundary pinning, contributing 216 MPa via grain-refinement strengthening; the high dislocation density of the martensitic structure (3.05 ± 0.0018) × 1015 m−2 provides a dislocation-strengthening contribution of 778 MPa; and nanoscale (Nb, Ti)C precipitates contribute an additional precipitation strengthening of 154 MPa. The synergistic effect of these three mechanisms enables the 1.2 mm thick welded tube steel to achieve a tensile strength exceeding 1400 MPa and an elongation above 9%. In summary, through reasonable microalloying design and an appropriate processing route, a Nb-containing high-strength steel for automotive welded tubes can be obtained that combines high strength, good ductility, and excellent formability. The elucidation of the synergistic enhancement from multiple strengthening mechanisms and the bending failure mechanism provides a valuable reference for the microalloying design of ultra-high-strength welded tube steels.