C10 Deformation Mechanisms of Multiphase Steels

Dr.-Ing. D. Ponge, Dr.-Ing. S. Sandlöbes (Max-Planck Institut für Eisenforschung, Düsseldorf)

Within the 1st and 2nd period the SFB 761 showed that the high manganese steels investigated reveal a remarkable combination of high strength, high ductility and high strain hardening rate. Here, the SFB successfully developed methods to predict strain hardening and mechanical behavior of high manganese steels on the basis of austenite composition and corresponding SFE for single-phase austenite as a bulk phase.
However, these studies revealed that Mn-microsegregations have an impact on the processing and the mechanical properties of high manganese steels and that these Mn-microsegregations can be controlled but not completely be avoided.
Therefore in sub-project C10 the Mn and C contents will be reduced to 5-12 wt% and 0,1-0,3 wt%, respectively, in order to find a compromise between mechanical properties, Mn-segregations and processing. The reduction of the Mn and C content results in a multi-phase microstructure comprising reversed austenite (fcc) embedded in a martensitic matrix (bcc or bct). Possible Mn-microsegregations are assumed to support the reversion of austenite. Al is added in order to suppress the formation of cementite, modify the SFE and increase the resistance against hydrogen embrittlement.
This expansion from single-phase austenitic to multi-phase microstructures results in an important scientific question: Is it possible to transfer the recent state of knowledge of deformation and strain hardening behavior of single-phase austenite to the case of austenite films or islands embedded in martensite?
Next to the SFE the focus lies on the additional effects of grain size and grain morphology as well as the interaction of austenite with the embedding matrix on the deformation and strain hardening behavior. Therefore sub-project C10 investigates the mechanisms of deformation and strain hardening of multi-phase medium manganese steels. These multi-phase microstructures comprised of a martensitic matrix and embedded austenite islands or films can be produced by employing suitable heat treatments.
Based on the in the SFB recently investigated austenite compositions of single phase alloys the sub-project C10 varies systematically the morphology, dispersion and chemical composition of austenite islands and films to understand the effects of these microstructural features on the deformation mechanisms and strain hardening behavior and finally to design materials with superior mechanical properties in close cooperation with sub-project A3 and A5.
Suitable heat treatment parameters enable an extreme grain refinement of both phases down to nano-scale. Specific open scientific questions in multi-phase medium manganese steels are the combined effects of (i) SFE, (ii) grain refinement and (iii) austenite stability on the deformation mechanisms of austenite. Of particular importance for this sub-project is the decrease of the austenite fraction after a particular heat treatment temperature by Al. This results in a significantly increased enrichment of carbon and manganese in austenite and corresponding higher SFE and austenite stability. In the case of Al-alloyed materials higher annealing temperatures are required to produce similar austenite fractions with comparable compositions as in Al-free materials. The mechanical properties of the resulting coarse microstructures (Al-alloyed) and fine microstructures (Al-free) are compared.
In a recent investigation it was shown that by applying a suitable heat treatment very thin austenite films along martensite grain boundaries can be produced, imparting a beneficial effect on toughness and damage tolerance. This phenomenon will be further investigated in sub-project C10.
Quasi-in-situ as well as in-situ DIC (digital image correlation) methods will be employed for a systematic correlation between mechanical properties, microstructure, partitioning or localization of strain and deformation mechanisms. The kinetics of austenite reversion will be analysed by high resolution dilatometry. Within sub-project C10 and in cooperation with sub-projects C1 and C4 comprehensive microstructural characterizations will be performed by SEM (scanning electron microscopy), EBSD (electron backscatter diffraction), XRD (X-ray diffraction), ECCI (electron channelling contrast imaging) and TEM (transmission elektron microscopy).