Unexpected Enhancement of High-Cycle Fatigue Property in Hot-Rolled DP600 Steel via Grain Size Tailoring

This work systematically investigates the high-cycle fatigue (HCF) properties and fatigue crack growth (FCG) behavior of hot-rolled dual-phase (DP) steels with comparable tensile strength but distinctly different yield strength (458 MPa for the FG sample and 355 MPa for the CG sample), grain sizes and morphologies. Contrary to the conventional Hall-Petch relationship, the coarse-grained (CG) sample demonstrates superior fatigue performance. This enhancement is reflected in its higher fatigue strength, combined with an elevated FCG threshold and a reduced FCG rate in the Paris regime of FCG behavior. Fracture morphologies and FCG path analyses reveal that this enhanced fatigue resistance attributes to pronounced crack path tortuosity in the CG microstructure. The tortuous crack path enhances roughness-induced crack closure effects in the near-threshold regime while promoting more frequent crack deflection during stable propagation, collectively reducing the effective driving force for crack growth. The experimental evidence confirms that properly designed CG microstructures with appropriate phase distribution can provide superior fatigue resistance in hot-rolled DP steels.