Lien microstructure-comportement à rupture d'aciers de troisième génération à structure duplex pour application automobile

Abstract : To fulfil the increasing demand on lightweighting automotive vehicles, steelmakers are developing a third generation of Advanced High Strength Steels (AHSS). This work, part of the ANR project MATETPRO “MeMnAl Steels”, addressed two new families of third generation AHSS produced by ArcelorMittal which may be used for the body in white of upcoming cars. It aimed at improving our current understanding of the microstructural features allowing controlling and optimizing the fracture behavior of this steel family.Two model materials were manufactured by hot and cold rolling followed by intercritical annealing. The resulting, so-called duplex microstructure is a mixture of ferrite and retained austenite (γr, which can transform into martensite by TRIP effect). The microstructure of the first steel was made of ultra-fine grains (UFG) of ferrite and retained austenite (grain size below one micrometer), while the second steel possessed a bimodal microstructure made of coarse δ-ferrite grains and fine-grained regions of α-ferrite and retained austenite γr.The mechanical properties of the UFG microstructure were strongly sensitive to the annealing temperature, due to variations in the stability of retained austenite. On the contrary, the bimodal microstructure was very robust regarding the annealing temperature but very sensitive to the test temperature. For these two families, damage development is scarce and mainly located at ferrite-martensite interfaces. Charpy impact tests on steels with the bimodal microstructure showed that each microstructural region presents its own fracture mechanisms and a specific ductile-to-brittle transition. A transition from brittle cleavage to large-dimpled, ductile fracture was observed for coarse δ-ferrite grains, while fined-grained regions presented a transition from interfacial fracture to fine-dimpled, ductile fracture. Fracture of the UFG microstructure was ductile at room temperature and interfacial at lower temperatures. This UFG microstructure can be interpreted as a model material embodying the behavior of the fine-grained {α + γr} regions in the bimodal microstructure.For both two steels, the constitutive and fracture behavior seem to be dominated by the stability of retained austenite.
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Quentin Tonizzo. Lien microstructure-comportement à rupture d'aciers de troisième génération à structure duplex pour application automobile. Matériaux. PSL Research University, 2017. Français. ⟨NNT : 2017PSLEM073⟩. ⟨tel-02011584⟩

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