The knowledge of the stress-and deformation-induced martensite formation in metastable austenitic steels including the formation temperatures and amounts formed is of considerable importance for the understanding of the transformation induced plasticity. For this purpose a stress-temperature-transformation (STT) and a deformation-temperature-transformation (DTT) diagram have been developed for the steel X5CrNi 18 10 (1.4301, AISI 304). It is shown that the Md-temperature for Y~E, E~a', Y~E~a' and y~a' martensite formation is defined by two stress-temperature curves which show a different temperature dependence. They specify the beginning and the end of the deformationinduced martensite formation in the range of uniform elongation. The intersection point defines the corresponding Md-temperature. The stress difference which results from the stresses for the end and the beginning of the martensite formation shows positive values below the Md-temperature. It defines the amount of martensite being formed. When the MdY-' temperature is reached and the formation of the first deformation-induced amount of e-martensite appears, an anomalous temperature dependence of the maximum uniform elongation starts. The highest values of the maximum uniform elongation are registered for the tested steel in the immediate vicinity of the Md y-a' or the Mdy-,-a' temperature -similar as in other metastable austenitic CrNi steels. At this temperature the highest amount of deformation-induced a-phase exists. The transformation plasticity in the test steel is considerably caused by the deformation-induced E and a' martensite formation. Using the new evaluation method, the increase of plasticity !1A (TRIP-effect) and strength !1R can be quantified.
Economic austenitic steels with high energy absorption capability are in the focus of worldwide research activities, whereby the steels which show TRIP, TWIP and/or SBIP effects play a crucial role.New austenitic or austenitic-martensitic stainless steels with a high cold workability and energy absorption capability are currently developed and tested in laboratory scale at the Institute of Iron and Steel Technology at the Technical University Bergakademie Freiberg. The mechanical properties of these steels are essentially influenced by the TRIP, TWIP and SBIP effect, becoming evident in hot formed and solution annealed steels as well as in as-cast steels.The TRIP/TWIP/SBIP effects have a significant impact on the toughness and the strength of stainless steels consisting of metastable austenite. The TRIP effect owns a paramount position since it serves for a simultaneous increase of toughness and strength. The influences of alloying elements like manganese or nickel on the TRIP effect are in the centre of the investigations at the Institute of Iron and Steel Technology.These austenitic or austenitc-martensitic stainless steels provide the ability for new applications fields due to their excellent mechanical properties. Exemplary, in the Collaborative Research Centre SFB 799 "TRIP-Matrix-Composites", financed through the Deutsche Forschungsgemeinschaft DFG, the suitability of this new class of steels for cast components in ductile and transformation strengthened high performance (metal) ceramic composite materials will be investigated.Key words: Stainless steels, TRIP, TWIP, SBIP
Kennzeichen der entwickelten nichtrostenden CrMnNi-Stä hleBei den entwickelten nichtrostenden CrNi-und CrMnNiStählen handelt es sich um Stähle mit austenitischem und austenitisch-martensitischem Gefüge. Unter Umständen können auch geringe d-Ferritanteile vorhanden sein. Die Stähle enthalten zwischen 12 und 18 % Chrom. Im Unterschied zu den herkömmlichen CrNi-bzw. CrNiMn-Stählen sind diese Stähle mit Silizium und Aluminium legiert. Sie weisen Aluminiumgehalte von 0,05 bis 4 % und Siliziumgehalte von 1 bis 4 % auf. Die Stähle zeigen einen temperatur-und legierungsabhängigen TWIP-und SBIP-Effekt, der von einem TRIPEffekt überlagert wird. Der Mangangehalt der Stähle liegt zwischen 2 und 20 %, der Nickelgehalt zwischen 0 und 10 %. Der Kohlenstoffgehalt ist kleiner 0,15 % und der Stickstoffgehalt ist auf 0,01 bis 0,1 % begrenzt. Besonders der Stickstoffgehalt kann zur Mischkristall-und / oder Ausscheidungshärtung genutzt werden, was Auswirkungen auf die Temperaturlage und die Höhe des TRIP-Effekts in umgeformten und gegossenen Stählen hat [1,2,3] Im Bild 2 sind die Zugfestigkeiten und Bruchdehnungen bei Raumtemperatur für gegossene austenitische und austenitisch-martensitische CrMnNi-Stähle mit TRIP-Effekt angezeigt. Es wird ersichtlich, dass vor allem aufgrund des TRIP-Effekts deutlich höhere Zugfestigkeiten und Bruchdehnungen realisiert werden als in den handelsüblichen nichtrostenden austenitischen Gussstählen. Technologisches Interesse beste...
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