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Effect of microstructure on edge ductility and fracture toughness of hot-rolled single- and multi-phase AHSS grades, The
O'Keefe, Olivia
O'Keefe, Olivia
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2024
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Hot-rolled AHSS grades are utilized in automotive parts where high formability is required, but the mechanical shearing process produces a sheared edge that reduces local ductility. Understanding the edge ductility and fracture toughness of these hot-rolled AHSS grades is critical to predict their performance in automotive applications. This study investigates the edge ductility and fracture toughness of four hot-rolled HSLA grades and four hot-rolled AHSS grades with strengths of 600 and 800 MPa achieved with single- or multi-phase matrix microstructures. The experimental and structural single-phase grades include microstructures comprised of ferritic matrices with mixed microalloyed precipitates and secondary microconstituents. The multi-phase grades include dual-phase (DP), ferritic bainitic (FB), and complex-phase (CP) steels, whose microstructures include a combination of ferrite, bainite, martensite, and mixed microalloy precipitates. Scanning electron microscopy, light optical microscopy, and electron backscatter diffraction were used to characterize the microstructure of each investigated grade. Hole expansion testing (HET) and deep draw cup (DDC) testing were performed to correlate microstructure and sheared face characteristics to edge ductility. Crack tip opening angle (CTOA) testing was performed to correlate microstructure and mechanical properties to fracture toughness.
The results of this work confirmed that the strain incompatibility present between a matrix and secondary phases or microconstituents is detrimental to the hole expansion ratio (HER). Single-phase ferritic grades that exhibited high HER values exhibited low crack susceptibility in the DDC test, suggesting that edge ductility behavior can vary based on the stress state acting on the sheared edge, i.e. microstructural features that are beneficial to edge ductility in states of tension can be detrimental to edge ductility in multiaxial stress states. The measured CTOA of all but one investigated grades was similar, suggesting that differing microstructures and material properties had little impact on fracture toughness. A steel with an anisotropic grain structure and with both coarse and fine ferritic grains distributed throughout the matrix had the lowest measured edge ductility and fracture toughness, suggesting that these features can be detrimental to both HER and CTOA. It is interpreted that edge ductility and fracture toughness did not correlate to one another because they measure different microstructural conditions in the investigated grades. Specifically, the HER measures the behavior of a damaged microstructure via the sheared edge, and the CTOA measures toughness of the base material.
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