Project C2: Material Properties Of High Mn-Steels

C2 mechanische Eigenschaften
C2 mechanische Eigenschaften

Prof. Dr.-Ing. Bleck (Steel Institute, RWTH Aachen University)

The main focus points of the project C2 are (i) the characterization of the mechanical properties of High Mn-Steels (HMnS), (ii) the development and application of new testing methods and (iii) the simulation of mechanical properties.

During the second phase of the SFB 761, the mechanical properties of austenitic HMnS in the tensile test were examined at different strain rates and testing temperatures. The results were compared to conventional austenitic steel grades. It could be shown that HMnS exhibit an excellent combination of ductility and strength over a wide array of testing temperatures and strain rates, culminating in a significant value of the product of their ultimate tensile strength (UTS) and uniform elongation (UE), the ECO-Index. The mechanical properties are a result of a systematic utilization of the TRIP- (Transformation Induced Plasticity) and TWIP-effect (Twinning Induced Plasticity).

 

 

 

C2 mechanische Eigenschaften 2
C2 mechanische Eigenschaften 1

 

The deformation mechanism primarily is a result of the stacking fault energy (SFE), a parameter that is heavily influenced by factors such as alloying contents and temperature. However, some materials of similar SFE but differing alloying contents show different material behavior. Thus, the SFE does not seem to be the only relevant factor in determining the work hardening and deformation mechanisms.

Due to high strength and ductility, HMnS portray great potential in terms of energy absorption. As a result of that and due to the face centered cubic structure of austenite, the temperature increase due to adiabatic heating during deformation is significant and causes a continuous change in SFE and deformation behavior.

Another important material characteristic is the serrated flow curve as a result of local deformation. For some alloys, this effect can already be seen at room temperature. In order to be able to characterize the localized deformation behavior, local strain measurements and infrared-thermography have been coupled. The combination of these techniques enables the quantification of local deformation temperatures and strain rates. The gained data can be used to systematically analyze the effects of the selected testing parameters.

 

 

 

Abb 1 lokale Verformungsanalyse
Figure 1: (a) global and local flow curve (b) local strain rate distribution (c) local temperature distribution

The serrated flow of the stress-strain curve is usually explained with dynamic strain aging (DSA) at temperatures slightly above room temperature. However, this phenomenon usually can be seen in body centered cubic ferritic steels where the diffusion speed of interstitial alloying elements such as carbon or nitrogen is roughly about 100 times higher than it is in the face centered cubic structure of austenitic steels like HMnS. Nevertheless, the DSA-like effect can be correlated to the blocking of dislocation movement due to cristallograhpic short-range ordering (SRO). It is assumed that a destruction of SRO via dislocation cutting after a short time of hindered dislocation movement and dislocation pile-ups can be reversed via short jumps of carbon atoms from and to the center of different octahedral vacancies. The resulting succession of increase and drop in strength leads to the serrated flow of the stress-strain curve.

C2 Verformung
C2 Verformung

 

In the third phase, the mechanical properties for new HMnS and medium Mn-Steels (MMnS) at different strain states and temperatures are to be investigated, as well as thy behavior under cyclic loading. Those new HMnS also include materials that show pronounced planar dislocation glide at very high SFE. This effect usually is attributed to a glide plane softening as a result of a destruction of SRO. In some cases, the formation of parallel shear bands and dislocation cells during deformation could be found. Those steels previously were described as MBIP-steels (Microband Induced Plasticity).

Considering the development and application of new testing methods, it could be shown that the position-dependent analysis method of the Nakajima-test according to DIN EN ISO 12004-2 is not a viable method to determine forming limit curves (FLC) for HMnS since it does not offer reliable and viable data. This is due to the fact that HMnS show a highly localized deformation behavior which in turn influences the fracture initiation. Using the currently discussed time-dependent method, it was possible to demonstrate the deformation behavior of HMnS in multiaxial strain states.

Bild 1: a) Fließkurven der Versuchsschmelzen V15 (TWIP) und V16 (TRIP) b) Verfestigungskurven (geglättet)
Figure 2: (a) inaccurate parabolic approximation with position-dependent method
                                     (b) accurate determination of beginning of instable necking with time-dependent method

During the third phase of the project, work on the analysis of multiaxial strain states for HMnS and MMnS is to be continued.

The simulation of mechanical properties is continued in the third phase on the basis of ab initio calculations from project A1. Based on those, the effect of SRO on the yield strength (YS) can be approximated. Additionally, a constitutive model for TWIP-steels has been developed. In the third phase, the constitutive modelling approach is further elaborated on by incorporating the TRIP-effect for MMnS. Furthermore, the effects of SRO and the DSA-like effect on the work hardening are investigated.