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The development of microstructure and mechanical properties in Q&P steels: effect of manganese

par Cadotte, Eve-Line
Président du jury Hendrick, Patrick
Promoteur Godet, Stéphane
Publication Non publié, 2024-10-11

Thèse de doctorat

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Résumé :

Med-Mn Q&P steels, as part of the 3rd generation of AHSS, exhibit promising mechanical properties. They have shown however to be very sensitive to the process parameters such as QT°. Deviations from the optimum QT° often produce undesirable M2 in the microstructure. Cases of brittle fractures were also reported. The present work aimed at investigating the effects of manganese in the development of Q&P microstructures and elucidate the relationship with the obtained mechanical properties and failure modes. Quenching at low QT° produced a microstructure with 19% retained austenite and significantly improved mechanical properties (UTS of 1600 MPa with 20% elongation) and fully ductile failure. However, the sensitivity to QT° was demonstrated and related to the absence of bainite formation during partitioning, as normally observed in Q&P with conventional TRIP compositions. The range of QT° to avoid M2 formation and maximise carbon enrichment in the austenite was thus narrowed, as only carbon partitioning contributes to austenite stabilisation. In addition, the higher retained austenite fraction found in the 5.6 wt.% manganese optimum microstructure was obtained by coarsening large austenite islands in high manganese segregation bands. An austenite reversion process was observed leading to further coarsening of these austenite grains. These larger austenite grains and associated lower carbon content were shown to transform rapidly during the first 10% of deformation. However, the higher manganese content of these grains counterbalanced the carbon and grain size effects to produce an effective TRIP effect which contributed significantly to the work hardening of the material during deformation. The presence of M2 in non-optimum microstructure severely compromised austenite stability. M2 cracking occurred and was caused by the high stress concentrations developed at M2 and M1 or RA interfaces due to the strength gradients and heterogeneous deformation between phases. When larger amounts of M2 were distributed in continuous bands, interfacial fracture was observed at PAGB, caused by the strength gradients between M2 and the surrounding phases and accelerated by the presence of a manganese peak or a continuous cementite layer at these boundaries. This type of fracture highlighted the grain boundary processes taking place in med-Mn Q&P microstructural development. This interfacial fracture was avoided by a post-tempering treatment which effectively reduced the strength gradient between the phases and allowed for a more homogeneous deformation. The fracture occurred by interfacial void nucleation and growth, followed by ductile tearing of the surrounding tough phases, similarly to the optimum microstructure. This post-treatment is a potential solution for extending the range of acceptable QT° to produce tough med-Mn Q&P microstructures.