• Citer

ON THE RELATIONSHIP BETWEEN MICROSTRUCTURE DEVELOPMENT AND MECHANICAL PROPERTIES IN Q&P STEELS

par Huyghe, Pierre
Président du jury Massart, Thierry,Jacques
Promoteur Godet, Stéphane
Publication Non publié, 2018-11-08

Thèse de doctorat

• Accès en ligne
• Détails
• Contenu
• Statistiques

Résumé :

The Quenching and Partitioning (Q&P) heat treatment has been proposed in the early 2000s to produce cold-rolled sheets combining high-strength and formability for the automotive market. Q&P consists, first, of an interrupted quench between the martensite-start temperature (Ms) and the martensite-finish temperature (Mf) from intercritical annealing or full austenitization in order to form controlled fractions of martensite. This is followed by a partitioning step in order to stabilize the untransformed austenite through carbon enrichment. In order to maximize the carbon transfer from martensite to austenite, the use of specific alloying elements and the design of appropriate Q&P parameters are required to eliminate or minimize competing phenomena such as carbide formation and austenite decomposition. The final quenched and partitioned microstructure, using full austenitization, ideally consists of carbon-depleted lath martensite and significant fractions of retained austenite providing an improved combination of strength and ductility. Hence, the transformation of retained austenite upon straining at room temperature (TRIP effect) provides supplementary work-hardening and eventually improves the ductility. In the present work, Quenching and Partitioning (Q & P) heat treatments were carried out in a quench dilatometeron a 0.2 wt% carbon steel. The microstructure evolution of the Q & P steels was characterized usingdilatometry, SEM, EBSD and XRD. The martensitic transformation profile was analyzed in order to estimate thefraction of martensite formed at a given temperature below the martensite start temperature Ms. Q & P wasshown to be an effective way to stabilize retained austenite at room temperature. However, the measuredaustenite fractions after Q & P treatments showed significant differences when compared to the calculated valuesconsidering ideal partitioning conditions. Indeed, the measured austenite fractions were found to be less sensitiveto the quench temperature and were never larger than the ideal predicted maximum fraction. Competitivereactions such as austenite decomposition into bainite and carbide precipitation were found to occur in thepresent work.Furthermore, a broad range of mechanical properties was obtained when varying the quenching temperaturesand partitioning times. The direct contributions between Q & P microstructural constituents -such as retainedaustenite as well as tempered/fresh martensite- and resulting mechanical properties were scrutinized. This wascritically discussed and compared to quenching and austempering (QAT) which is a more conventional processingroute of stabilizing retained austenite at room temperature. Finally, Q & P steels were shown to exhibit aninteresting balance between strength and ductility. The achievement of this interesting combination of mechanicalproperties was reached for much shorter processing times compared to QAT steels.