Processing, microstructure and mechanical behavior of medium manganese steels
Notice bibliographique
Résumé
Developing high strength steels has received increasing attention in steel industry, in order to meet the body-in-white weight reduction strategies pursued by the automotive industry for fuel economy and vehicle safety. In this context, medium manganese (Mn) steels containing 3 to 12 wt.% Mn are one of the strong candidates for the so-called third-generation advanced high-strength steels (AHSS), due to the superior mechanical properties exceeding the first-generation AHSS and the potentially lower cost compared to the high Mn second-generation AHSS. The ultrafine grain size and high fraction of retained austenite, adjustable to various TRIP (transformation-induced plasticity) and/or TWIP (twinning-induced plasticity) effects, constitute the key advantages of this group of steels. This thesis aims to develop a better understanding of the processing-microstructure-property relationship of medium Mn steels with Al and Si additions. Four experimental steels with compositions of Fe-0.2C-(7~10)Mn-3Al-(0~3)Si (in wt.%) were designed and examined. The phase transformation behavior, with respect to austenite formation and decomposition, of two hot rolled samples with 3 wt.% Al and 3 wt.% Si during intercritical annealing was firstly investigated to develop a better understanding of microstructural control for such steels. The microstructure of the leaner alloy with 7 wt.% Mn was substantially influenced by the annealing temperature, whereas the richer variant 10 wt.% Mn steel exhibited a stable ferrite-austenite duplex microstructure containing a fixed amount of retained austenite, which was found to be independent of the variations of annealing temperature. From these results, controlled annealing was performed for these two hot rolled samples, followed by a further processing by subjecting them to cold rolling and examining the microstructural evolution during cold rolling. Strain partitioning, between austenite and ferrite, and deformation twinning of austenite were found to critically influence the cold rollability of the two steels. Finally, the deformation and fracture mechanisms of the cold rolled and intercritical annealed medium Mn steels were studied, with particular emphasis on the effect of different Si additions (0~3 wt.%). Results showed that the mechanical property of the steels critically depended on the austenite fraction, TRIP and TWIP effect activated in austenite, as well as the strain partitioning between austenite and ferrite during deformation. The fracture mechanisms altered with different Si levels, which can change from dimpled-type fracture driven by void formation, mainly at the ferrite/strain-induced α′-martensite interfaces, to a combined dimple and cleavage/quasi- cleavage fracture, which was related to the brittle δ-ferrite in high Si alloys. The work also revealed a strong correlation between strain-induced martensite formation and the serrated plastic flow phenomenon associated with the Portevin-Le Chatelier effect (PLC) effect. Discontinuous behavior of strain-induced transformation was observed, which was due to the localized martensite formation within the PLC bands nucleating intermittently and propagating continuously during tensile straining.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».