The stability of retained austenite is the most important parameter controlling the transformation plasticity effects in multiphase low alloy TRIP steels. In this work the thermodynamic stability of the retained austenite has been determined experimentally by measuring the !Iff temperature as a function of bainite isothermal transformation (BIT) temperature and time in two low alloy TRIP steels. A singlespecimen temperature-variable tension test technique (SS-TV-TT) has been employed. which allowed to link the appearance of yield points in the stress-strain curve with the mechanically-induced martensitic transformation of the retained austenite. The results indicated that the !Iff temperature varies with BIT temperature and time. Higher austenite stability is associated with a BIT temperature of 400 QC rather than 375°C. In addition. the chemical stabilization of the retained austenite associated with carbon enrichment from the growing bainite is lowered at short BIT times. This stability drop is due to carbide precipitation and comes earlier in the Nb-containing steel. At longer BIT times the retained austenite dispersion becomes finer and its stability rises due to size stabilization. The experimental results are in good agreement with model predictions within the range of anticipated carbon enrichment of the retained austenite and measured austenite particle size. Experimentelle Bestimmung der Restaustenitstabilitat in nledriglegierten TRIP-Stahlen. Die Stabllltat des Restaustenits ist der wichtigste Parameter, um die umwandlungsbedingten Plastizitatseffekte mehrphasiger, niedriglegierter TRIP-Stahle zu kontrollieren. 1m Rahmen dieser Arbeit wurde die thermodynamische Stabilitat des Restaustenits experimentell bestimmt, indem die Mr; -Temperatur zweier niedriglegierter TRIP-Stahle als Funktion von Temperatur und isothermer Haltezeit in der bainitischen Transformationsstufe gemessen wurde. Dazu wurde eine Einproben-Methode (SS-TV-TT) herangezogen, bei der Spannungen bei unterschiedlichen Temperaturen aufgebracht werden. Sie ermoglicht es, das Auftreten einer Streckgrenze in der Spannungs-Dehnungs-Kurve mit der spannungsfdehnungsinduzierten Martensitumwandlung des Restaustenits zu verknOpfen. Die Ergebnisse zeigen, dal3 sich die Mr; -Temperatur in Abhangigkeit von Haltetemperaturund Haltezeit verandert. Durch eine Haltetemperatur von 400°C lal3tsich eine hohere Austenitstabilitat erreichen, als dies bei 375°C der Fall ist. Bei kurzen Haltezeiten ist die chemische Stabilisierung des Austenits aufgrund der durch die Bainitumwandlung verursachten Kohlenstoffanreicherung reduziert. Dieser Effekt tritt bei nioblegierten Stahlen frOher ein: Grund dafGr ist die Bildung von Carbiden. Langere HaltezeitenfGhren zu einer feineren Verteilung des Restaustenits und damit zu einer grol3enbedingten Stabilisierung. Die experimentellen Ergebnisse stimmen gut mit den Vorhersagen der Modelle uberein, was erwartete Kohlenstoffanreicherung und Partikalqrofia des Restaustenits betrifft.
Recently various kinds of high-strength sheet steels have been developed to meet the requirements of the automotive industry such as passive safety, weight reduction and saving energy. Usually the main problem of high-strength steels is their inferior ductility. Multiphase steels however show a very good combination of strength and formability so that the applicable region of high-strength steels has been widely enlarged. Multiphase steels have been developed for various purposes because of their ability to tailor properties by adjusting the type, the amount, and the distribution of different phases. Especially new developed triple-phase steels which make use of the TRIP effect (transformation induced Qlasticity) can further improve formability as well as strength due to the transformation of retained austenite to martensite during the deformation. In this work the transformation behaviour and the mechanical properties of low alloyed TRIP steels were investigated. The influence of the annealing parameters on transformation behaviour and on the amount of retained austenite were determined. In addition the temperature dependence of the mechanical properties and the effect of testing speed on the formability were studied. The investigation was carried out on seven different TRIP steels with different chemical compositions, especially the influence of the microalloying element niobium was considered. For reasons of comparison various mild and high-strength steels were tested parallel to the TRIP steels. It was found that the investigated TRIP steels offer very attractive combinations of elongation and strength values. An interesting temperature dependence of the mechanical properties can be observed, in such a way that the elongation values of the TRIP steels possess a maximum between +50 and +100°C. Due to its effect on grain size and on precipitation behaviour the addition of niobium leads to higher strength values without a strong decrease in ductility. In general, the mechanical properties are strongly affected by the type and the distribution of the different phases. The most important parameters, however, to influence the mechanical behaviour are the amount and the stability of the retained austenite, which are mainly controlled by the heat treatment and the chemical composition. Mechanisch-technologische Eigenschaften von niedriglegiertenTRiP Stahlen. In jOngster Zeit wurden unterschiedliche hochfeste Feinblechstahlsorten entwickelt, urn die Anforderungen der Automobilindustrie wie passive Sicherheit, Gewichtseinsparnis und verminderten Energieverbrauch zu erfOlien. Oblicherweise stellt die unzureichende Umformbarkeit das Hauptproblem von hOherfesten Stahlen dar. Mehrphasenstahle weisen hingegen eine sehr gute Kombination von Festigkeit und Umformbarkeit auf, so daB der Anwendungsbereich von hochfesten Stahlen erheblich erweitert werden konnte. Mehrphasenstahle wurden fOr unterschiedliche Zwecke entwickelt, da ihre Eigenschaften in erheblichem Umfang variiert werden konnen, indem Art. Menge und Verteilung der ...
Based on Hill's theory of plasticity and the Swift diffuse instability criterion, new theoretical models are proposed for predicting the drawing fracture load and limit drawing ratio (LDR) of an axisymmtric cup drawing. These models take into account the influence of triaxial stress state, anisotropy, strain hardening, bending, and tool geometry. By introducing both conventional and modified Hollomon's equations, the influences of these variables on the constitutive relation of sheet steels are also analyzed. It is shown that the theoretical predictions of the drawing fracture load are in good agreement with experimental results for a wide range of sheet steels currently used in the automotive industry. Specific tool geometries are found to decrease the drawing fracture load and the LDR, because of increased triaxial stress states and bending effects at the critical section of the workpiece. The optimum punchprofile radius is found to be between 5.0 and 7.0 times the thickness of the sheet. Additionally, the role of both the anisotropy and strain-hardening properties of the sheet steels in determining the drawing fracture load and the LDR are, subsequently, discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.