The effect of ferrite grain size and martensite volume fraction on the tensile properties of dual phase steel
Abstract
The yield strength and tensile strength of dual phase steel are found to follow a Hall-Petch type dependence on the mean free path in ferrite , i.e. . The Hall-Petch slope k increases with increasing volume fraction of martensite, Vm, in the dual phase in both cases, and σ0 is independent of Vm for yield strength but increases with Vm for tensile strength. The effect of ferrite grain size and the martensite volume fraction on the yield strength of a dual phase steel can be explained by extending the dislocation pile-up model to dual phase steels. Also, it is found that the effect of ferrite grain size on yield strength is much stronger than on tensile strength. This naturally leads to a decrease of the yield to tensile ratio with increasing ferrite grain size. The yield ratio approaches a constant value at large ferrite grain size and this limiting value decreases with Vm. It is demonstrated that these two observations are predictable from the Hall-Petch equations for the yield strength and tensile strength of the dual phase steel.
Résumé
Nous avons trouvé que la limite élastique et la résistance à la traction d'un acier ferritique diphasé suivent une loi du type de Hall et Petch en fonction du libre parcours moyen dans la ferrite , c'est à dire . La pente k d'Hall et Petch augmente avec la fraction volumique de martensite Vm dans les deux cas; σ0 est indépendant de Vm pour la limite élastique, mais augmente avec Vm pour la résistance à la traction. On peut expliquer l'influence de la taille des grains de la ferrite et de la fraction volumique de martensite sur la limite élastique d'un acier diphasé en étendant le modèle de l'empilement de dislocations aux aciers diphasés. Nous avons également trouvé que l'effet de la taille des grains de ferrite sur la limite élastique est plus grand que sur la résistance à la traction. Ceci conduit naturellement à une diminution du rapport de la limite élastique à la change de rupture lorsqu'on augmente la taille de grains de la ferrite. Le rapport de la limite élastique tend vers une valeur constante pour une grande taille des grains de ferrite et cette valeur limite diminue avec Vm. Nous démontrons que l'on peut prévoir ces deux observations à partir des équations de Hall et Petch pour la limite élastique et la résistance à la traction d'aciers diphasés.
Zusammenfassung
Flieβspannung und Zugfestigkeit eines zweiphasigen Stahles gehorchen einer Hall-Petch-Beziehung mit einer freien Weglänge im Ferrit, d.h. es gilt . Die Gröβe k nimmt mit ansteigendem Volumanteil des Martensits Vm zu; σ0 ist unabhängig von Vm für die Flieβspannung, nimmt aber für die Zugfestigkeit mit Vm zu. Der Einfluβ der Ferrit-Korngröβe und des Volumanteils des Martensits auf die Flieβspannung eines zweiphasigen Stahles kann erklärt werden, wenn das Modell der Versetzungsaufstauungen auf zweiphasige Stähle erweitert wird. Dazuhin wird gefunden, daβ der Einfluβ der Ferrit-Korngröβe auf die Flieβspannung viel gröβer als auf die Zugfestigkeit ist. Das führt natürlich zu einer Verkleinerung des Verhältnisses von Flieβspannung zu Zugfestigkeit mit zunehmender Ferrit-Korngröβe. Dieses Verhältnis nähert sich bei groβen Ferritkorndurchmessern einem konstanten Wert an; dieser Grenzwert sinkt mit Vm. Es wird gezeigt, daβ diese beiden Beobachtungen aus den Hall-Petch-Gleichungen für Flieβspannung und Zugfestigkeit des zweiphasigen Stahles vorausgesagt werden können.
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Present address: Materials Research Laboratory, Texas Instruments Incorporated, P.O. Box 225936, MS 147, Dallas, TX 75265, U.S.A.