Strain-ratio-dependent low-cycle fatigue and localized deformation of bake-hardened HR420 steel

Manufacturing history can substantially alter the cyclic response of automotive bake-hardened steels, yet the role of strain ratio in this processing–structure–property link remains insufficiently resolved. Here, as-received HR420 steel and specimens subjected to 4% pre-straining followed by bake hardening were examined under strain-controlled low-cycle fatigue at three strain amplitudes and at strain ratios R ε = − 1 and 0. Cyclic hysteresis, fatigue life, three-dimensional digital image correlation, and fracture-side electron backscatter diffraction were combined to identify localization and crack-path characteristics. Bake hardening produced an amplitude-dependent response: it restrained early localized plasticity and improved fatigue resistance at low amplitude, whereas at high amplitude it intensified strain localization and accelerated damage accumulation. Positive mean strain under R ε = 0 promoted directional slip accumulation and more crystallographically guided transgranular cracking than fully reversed cycling. A Chaboche-type cyclic elastoplastic damage model and EBSD-informed crystal-plasticity simulations were used to interpret the observed specimen- and grain-scale responses. The results establish a processing–microstructure–fatigue relationship for formed and bake-hardened HR420 steel.