Influence of Coiling and Annealing Temperatures on Hole Expansion Property of CP780 Hot-Rolled Ultra-High Strength Automotive Steel Sheet

Achieving a balanced combination of high strength and high hole expansion ratio is a critical research objective for the application of high-strength steel in automotive manufacturing. A novel low-carbon Ti-Nb microalloyed hot-rolled multiphase steel with a high hole expansion of 780 MPa grade was designed. The thermomechanical controlled process (TMCP) was employed to investigate the influence of different coiling and annealing temperatures on its microstructure and mechanical property evolution, as well as to elucidate the mechanisms governing the high hole expansion property. The results indicate that increasing the annealing temperature enhances the precipitation strengthening effect of Ti/NbC particles, promotes grain refinement of the experimental steel, and leads to a progressive increase in both yield and tensile strengths. However, elongation after fracture and hole expansion ratio exhibit a gradual decline with rising annealing temperature. At a coiling temperature of 480 ℃, the hole expansion property is primarily governed by the bainite/ferrite ratio. Elevated annealing temperatures result in increased bainite content, which restricts crack deflection and bifurcation, thereby causing a monotonic reduction in hole expansion ratio. At a coiling temperature of 520 ℃ and an annealing temperature of 610 ℃, an optimal matching among the fine-grained microstructure of experimental steel, an appropriate bainite/ferrite phase ratio, and dispersed fine Ti/NbC precipitates was realized. This microstructural configuration effectively promotes crack deflection, bifurcation, and microvoid formation during hole expansion, enabling greater energy consumption prior to fracture and resulting in a hole expansion ratio exceeding 60%.