Coincidence of strain-induced TRIP and propagative PLC bands in Medium Mn steels

Abstract

An Fe-0.2C-5Mn-2.5Al medium manganese steel is studied for three different intercritical annealing temperatures T IA to observe the effects of variations of retained austenite stability on the mechanical work hardening in uniaxial tension. Digital image correlation (DIC) showed that both Lüders bands and type A Portevin-le-Châtelier (PLC) bands occurred in samples with a lower T IA , but both disappeared when T IA was increased. In-situ measurements of sample magnetization with a correction for stress effects on the magnetic properties of the sample allowed for continuous measurement of the retained austenite volume fraction. This method provided a complete characterization of the kinetics of transformation-induced plasticity (TRIP) in the sample over the course of tensile testing. It is shown that the martensite transformation was purely strain-induced and coincided with the passage of both Lüders and PLC bands. An Olson-Cohen model was applied to magnetic data to attempt to explain the differences in TRIP kinetics with varying T IA . and high strain rate performance of TRIP steels. 34Proper characterization of the kinetics of the TRIP effect in a particu-35 lar alloy requires the volume fraction of retained austenite to be measured 36 as a function of strain. This is notoriously difficult to do, however, as dif-37 2 ferent retained austenite volume fraction measurement techniques can give 38 very different values for similar alloys [11, 12]. Determination of the austen-39 ite content is nonetheless critical in alloy design, as it would provide the 40 metallurgist control over the strain hardening contributions from TRIP and 41 TWIP. Attempts have been made to measure the austenite volume fraction 42 as a function of strain by X-ray diffraction (XRD) [13] and magnetization 43 measurements using a ferritescope [13][14][15] in order to quantify TRIP kinet-44 ics. While in terms of the accuracy of the measurement method, magnetic 45 measurements provide no advantage over other methods [12], the primary 46 interest of the method as employed herein is the extremely high sampling 47 rate compared to, for example, XRD or EBSD. It enables one to measure 48 the retained austenite volume fraction in-situ without having to pause the 49 mechanical test, which would not be possible if one wishes to perform the 50 test at the very high strain rates used during stamping operations for exam-51 ple. Sampling rates this high are not normally obtainable by other in-situ 52 methods, making magnetic saturation measurements the unique choice for 53 such an application if the experimenter wants intermediate points between 54 the unstrained and ruptured states. Moreover, a more complete description 55 of the kinetics of TRIP using several hundreds or thousands of experimental 56 data points would make it possible to refine existing TRIP models used in 57 simulations of mechanical behavior. 58 Recent studies on Medium Mn steels have noted that strain hardening 59 is sometimes unstable and that long Lüders plateaus are often observed in ...

Influence of Intercritical Annealing Temperature on Microstructure and Mechanical Properties of a Cold-Rolled Medium-Mn Steel

“…At the same time, the intercritical annealing resulted in a softening of the material with improved ductility. In previous studies, it was pointed out the intercritical annealing temperature has a great effect on the fraction and stability of retained austenite [10,[48][49][50][51][52]. Consequently, the mechanical properties showed a strong dependence on intercritical annealing temperature ( Figure 10).…”

Effect of Deformation Temperature on the Portevin-Le Chatelier Effect in Medium-Mn Steel

“…The range of deformation when serration occurred is variable. In contrast, for the specimens deformed at 100 °C and 140 °C, the oscillations were observed in the final stage of deformation (Figure 2c Some typical features for the PLC effect [14][15][16][17][18][19][20]28] were observed for the investigated steel deformed at elevated temperatures: 60-140 • C. The characteristic serrations were observed on the stress-strain curves ( Figure 2). The specimen deformed at 20 • C showed no evidence of the PLC effect (Figure 2a).…”

“…Gibbs et al [26] proposed the theory that the characteristic serration is related to the various martensitic transformation rates during plastic deformation. Callahan et al [14] observed that the TRIP effect and the passage of both Lüders and PLC bands occur simultaneously in 0.2C-5Mn-2.5Al medium-Mn steel. Sun et al [15] reported that plastic instability in 0.26C-11.6Mn-2.7Al steel was related to discontinuous strain-induced martensite transformation.…”

Dynamic Strain Aging Phenomena and Tensile Response of Medium-Mn TRIP Steel

“…According to Medvedeva et al in the presence of Mn a Mn-C pair forms that reduces the expected increase in the fault energy that is caused by C alone. [25] Dynamic strain aging is a common observation in medium-Mn steels [4,5,7,10,12,[26][27][28][29][30] with a wide variation in processing. Gibbs et al [10] investigated a 7Mn steel annealed at temperatures varying from 848 K to 948 K (575°C to 675°C) for 168 hours.…”

“…Zhang et al reports that the starting structure of the investigated steels was primarily a-ferrite (67 vol pct) with the remainder of the microstructure being c-austenite. Recent works [27][28][29][30] with medium-Mn steels on the PLC effect have shown that there is a strain-induced transformation of c-austenite to a-martensite within the PLC band. This effect is attributed to the high localized plastic deformation leading to the strain induced transformation of cfia-martensite.…”