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Aging effects on sheared edge formability
Carley-Clopton, Aiden
Carley-Clopton, Aiden
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2023
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2024-11-29
Abstract
Advanced high strength steels (AHSS) combine high strength with good formability, making them an appealing class of materials for the automotive industry. Despite good global formability, many AHSS grades have poor sheared edge formability. Additionally, previous studies have documented that sheared edge ductility, measured via hole expansion ratio (HER), changes with room temperature aging time between blanking and testing for several AHSS grades. The present study designed experiments to investigate strain aging and hydrogen embrittlement as possible mechanisms. Five steel grades were tested to evaluate effects of microstructure, tensile properties, and aging temperature on aging sensitivity of sheared edge formability. These steels include dual phase (DP), complex phase (CP), bake hardening (BH), cold rolled martensitic (CRM), and transformation induced plasticity (TRIP) aided bainitic ferrite (TBF) grades. Edge formability was measured via HER, with aging times of 0.5-240 h between punching and testing. Hydrogen embrittlement and strain aging effects were examined by: applying elevated temperature treatments during and before aging, measuring hydrogen content of different conditions, masking the sheared edge after blanking, and testing burr up and burr down. To investigate Zn coating effects on aging sensitivity, aging effects were measured for DP 800 specimens produced in an annealing/galvanizing simulator with the same thermal history and different coating and atmosphere conditions.
No consistent microstructure or tensile property effects on HER or aging sensitivity were found. Of the grades in the comparative study, the CP and CRM grades experienced aging effects on sheared edge formability. The HER of both grades increased in the first several hours of aging, followed by a decrease out to 240 h of aging. Aging temperature impacted the rate of HER loss significantly for the CP grade, but had less of an effect on the CRM grade. A heat treatment which elicited bake hardening in tensile specimens of the BH grade did not change the HER of that grade, suggesting strain aging in the sheared edge does not necessarily reduce HER. The addition of a Zn coating incited aging sensitivity in the DP 800 grade, but a Zn coating was not necessary for a steel to experience aging effects, as exemplified by the CRM grade, which likely has the highest hydrogen embrittlement susceptibility of the grades tested. Overall, the results suggest internal hydrogen is responsible for aging effects.
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