Effect of microstructure on pre- and post-punching fatigue behavior of hot-rolled thick-plate advanced high-strength steel

Abstract

Advanced high-strength steels (AHSSs) are crucial for achieving superior strength-to-weight ratios in automotive applications, replacing traditional high-strength low-alloy (HSLA) steels. However, employing AHSSs in thick-plate configurations, e.g., heavy-duty truck chassis, presents challenges due to the potential mechanical property degradation caused by necessary sheet shearing processes, such as hole punching and trimming. This study examines the role of microstructure on the pre- and post-punching fatigue behavior of three AHSSs (800CP, 700MC, and 700MCPlus), each with distinct microstructural constituents but comparable yield and tensile strengths, and compares them with a conventional HSLA steel (500MC) commonly used in heavy-duty truck chassis. Additionally, the impact of different punching geometries on the post-punching fatigue performance of 500MC is assessed.Comprehensive microstructure characterization, tensile testing, high cycle fatigue (HCF) testing pre- and post-punching, fatigue crack growth rate (FCGR) testing, and neutron residual stress measurements are performed. The results show that punching significantly alters the microstructure, leading to microstructure refinement, sub-grain formation, defect creation, residual stresses, and the development of a work-hardened shear-affected zone around the punched edge, along with a rough fracture zone within the punched hole. At higher applied stresses and fewer load cycles (10⁵ cycles), the HCF performance is primarily determined by the fatigue crack growth resistance of the pre-punched microstructure. In this regime, 700MCPlus, with the slowest FCGR, exhibits the highest post-punching fatigue strength, while the other steels with similar FCGR show nearly identical post-punching fatigue strength. Similarly, different punching conditions of 500MC exhibit similar post-punching fatigue strength in this regime, regardless of the punching condition. The investigation into the fatigue crack propagation mechanisms reveals that the enhanced performance of 700MCPlus is due to its unique texture, which limits slip activity, and the presence of martensite at grain boundaries, contributing to crack deflection. These findings underscore the potential for optimizing FCGR behavior through texture design and the dispersion of hard constituents. At lower applied stresses and fewer load cycles (10⁶ - 2 × 10⁶ cycles), however, post-punching fatigue performance is significantly influenced by the changes induced during punching. In homogeneous microstructures (e.g., ferrite in 500MC), surface roughness and, more importantly, residual stress are key factors affecting post-punching fatigue performance. Fatigue cracks initiate at mid-thickness parallel to the punching direction, which corresponds to the location of maximum measured tensile residual stresses, with increases in the residual stress leading to greater reductions in fatigue strength. Conversely, in more heterogeneous microstructures, strain localization plays a critical role when a significant strength difference exists between microconstituents (e.g., martensite and ferrite in 700MCPlus). Strain localization promotes sub-grain formation, reducing the local threshold stress intensity factor range (∆K_th) and facilitating crack initiation. In microstructures with smaller strength differences (e.g., ferrite and bainite in 800CP and 700MC), sub-grains, along with surface roughness and residual stress, significantly contribute to the reduction in post-punching fatigue strength. These findings provide valuable insights into the mechanisms underlying punching-induced fatigue performance degradation, offering potential strategies for optimizing the fatigue properties of AHSSs for new applications.

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