A model for the stability of dispersed austenite in low alloy triple-phase steels has been developed. The model was based on the dislocation dissociation model for classical heterogeneous martensitic nucleation by considering stress effects on the nucleation site potency distribution. The driving force for martensitic transformation has been calculated with the aid of computational thermodynamics. The model allows for the effects of chemical composition of austenite, mean austenite particle size, yield strength of the steel and stress state on austenite stability. Chemical enrichment in C and Mn, as well as size refinement of the austenite particles lead to stabilization. On the contrary, the increase in the yield strength of the steel and triaxiality of the stress state lead to destabilization. The model can be used to determine the microstructural characteristics of the austenite dispersion, l.e, chemical composition and size, for optimum transformation plasticity interactions at the particular stress state of interest and can then be useful in the design of low-alloy triple-phase steels. Development of the modelStability parameter. It has been chosen in this work to characterize the stability of dispersed austenite against mechanically induced transformation by a single parameter, the M'; temperature, as the M, temperature is used to characterize the stability of austenite against transforma-
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.
In recent years the technology of low‐alloy TRIP steels has considerably advanced. The mechanical properties are characterised by a combination of high yield strength and high uniform elongation as well as enhanced formability. In the present work an effort to correlate mechanical properties with the retained austenite stability was made. Two low‐alloy TRIP steels were investigated. The first of them represents a typical composition of the low‐alloy TRIP steels, while the other one contains aluminum as alloying element. The influence of the heat treatment on the mechanical properties and especially on the amount and stability of the retained austenite was determined. The retained austenite stability was measured with a single specimen technique, in which a tensile specimen was used to determine the MσS temperature with a loading‐unloading procedure. The results showed that there is a strong influence of the stability of the retained austenite on the mechanical properties. Increased stability combined with a high amount of retained austenite, exhibited an increase in both, yield strength and uniform elongation while increased amount of retained austenite with low stability did not show the same good combination of mechanical properties. The results clearly indicate that in order to get the maximum TRIP effect, a good combination of austenite stability and amount is required.
A coupled thermodynamic/kinetic calculation of austenite formation during intercritical annealing of low‐alloy TRIP steels is presented. The simulation was performed with the use of Dictra computational kinetics software, which employs a procedure for the numerical solution of the coupled diffusion equations involved, as well as mobility databases for the retrieval of the appropriate kinetic data. Calculated results are compared with available experimental data, in order to evaluate the model. Simulation results, regarding the amount and composition of austenite, the rate of transformation and the effect of annealing temperature and heating conditions, are presented and discussed. It is concluded that the simulation can assist the design of the intercritical annealing in these steels.
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.