Enhancing Fatigue Property of 800 MPa Grade Automotive Beam Steels via Microstructural Tailoring

Abstract

High-strength beam steels are vital in automotive body-in-white structures due to their superior mechanical and fatigue properties. These steels are often produced through thermomechanical controlled processes, yielding fine-grained ferrite microstructures reinforced by small precipitates. Coiling temperature plays a pivotal role in shaping the microstructure, including precipitation, dislocation density, grain boundaries, and substructures. However, the impact of microstructure on the fatigue properties of these steels remains incompletely understood. In this study, 800 MPa-grade automotive beam steel is developed by tweaking its chemical composition and lowering the coiling temperature to achieve distinct microstructural features. The new steel exhibits higher dislocation density and lower precipitation density within ferrite grains compared to conventional steels. Fatigue tests show that surface cracking is the primary failure mode for both steels. Despite similar tensile strengths, the new steel demonstrates enhanced fatigue properties, attributed to slower crack growth and improved fracture toughness, facilitated by a higher proportion of low-angle grain boundaries and increased dislocation density. This research offers a promising approach to boost the fatigue performance of hot-rolled automotive beam steels.